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Protein tyrosine phosphatase sigma (PTPσ) targets apical
junction complex proteins in the intestine and modulates
epithelial permeability.
by
Ryan Murchie
A thesis submitted in conformity with the requirements for the degree of Master of Science
Protein tyrosine phosphatase sigma (PTPσ) targets apical junction complex
proteins in the intestine and modulates epithelial permeability.
Ryan Murchie
Master of Science
Department of Biochemistry
University of Toronto
2012
Abstract
Protein tyrosine phosphatase sigma (PTPσ), encoded by PTPRS, was shown previously
by us to contain SNP polymorphisms that can confer susceptibility to inflammatory bowel
disease (IBD). PTPσ(-/-) mice exhibit an IBD-like phenotype and show increased susceptibility
to acute models of murine colitis. The function of PTPσ in the intestine is uncharacterized. Here,
I show an intestinal epithelial barrier defect in the PTPσ(-/-) mouse, demonstrated by a decrease
in trans-epithelial resistance and a leaky intestinal epithelium that was determined by in vivo
tracer analysis. We identified several putative PTPσ intestinal substrates; among these were
several proteins that form and regulate the apical junction complex, including ezrin. My results
show that ezrin binds to and undergoes tyrosine de-phosphorylation by PTPσ in vitro, suggesting
it is a direct substrate for this PTP. The results suggest a role for PTPσ as a positive regulator of
epithelial barrier integrity in the intestine. The proteins identified in the screen, including ezrin,
suggest that PTPσ may modulate epithelial cell adhesion through the targeting of AJC-associated
proteins, a process impaired in IBD.
ii
Acknowledgements
First and foremost, I would like to thank my supervisors Dr. Daniela Rotin and Dr. Aleixo Muise
for all of your guidance over the course of my project. Your encouragement and support were
essential to the success my endeavours. I would also like to thank my supervisory committee
members Dr. John Brumell and Dr. Christine Bear for their helpful guidance and suggestions
during my committee meetings.
To all the members of the Rotin and Muise labs, thank you for making my time in the lab a
memorable one. Your constant support always made the lab a welcoming place to work. Thank
you to Chong, Chen, Angie and Hui for all your invaluable technical support throughout my time
in the lab without which I would not have been successful. Special thanks to Melanie Gareau
from Dr. Phil Sherman’s lab for training on the Ussing Chamber experiments and to Ramzi
Fattouh from Dr. John Brumell’s lab for additional training. Lastly, thank you to my friends and
family for all of your support over the years!
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Table of Contents Acknowledgements ........................................................................................................................ iii Table of Contents ........................................................................................................................... iv List of Tables ................................................................................................................................. vi List of Figures ............................................................................................................................... vii Abbreviations ............................................................................................................................... viii
1) PTPσ(-/-) mice exhibit defects in intestinal barrier integrity ................................ 37 2) PTPRS is expressed in the crypts regions of the mouse intestine ......................... 40 3) Tyrosine phosphorylation is enriched in the crypts of PTPσ(-/-) mouse colon
and small bowel .................................................................................................... 40
iv
4) Identification of Villin and Ezrin as PTPσ binding partners by mass spectrometry .......................................................................................................... 45
5) Ezrin is a substrate of PTPσ .................................................................................. 49 6) E-cadherin and β-catenin colocalization and β-catenin localization to the
nucleus is unaffected in the PTPσ(-/-) mouse small bowel and colon .................. 54 7) Ezrin localization is altered in PTPσ(-/-) mouse small bowel after DSS
treatment ............................................................................................................... 58 8) Intestinal morphology is not disrupted in the small bowel and colon of
neonatal PTPσ (-/-) mice ....................................................................................... 61 9) Lysozyme immunostaining is reduced in the Paneth cells of the PTPσ(-/-)
mouse small bowel ................................................................................................ 64 10) Ki-67 immunostaining, a marker for cell proliferation, appears unchanged in
PTPσ(-/-) mouse small bowel and colon .............................................................. 64
Role of PTPσ in intestinal epithelia .............................................................................................. 71
1) Evidence of defects in intestinal barrier integrity ................................................. 71 2) Role for E-cadherin and β-catenin ........................................................................ 72 3) Ezrin as a colonic PTPσ substrate ......................................................................... 73
Proposed Mechanism: PTPσ regulation of adherens junction proteins ........................................ 74 Implications for PTPσ in Paneth cell function .............................................................................. 78 PTPσ in IBD pathogenesis ............................................................................................................ 78 Future Directions .......................................................................................................................... 79
1) Further characterize the intestinal barrier defect in the PTPσ(-/-) mouse............. 79 2) Investigate the intestinal ligands of PTPσ ............................................................ 79 3) Evaluate the effect of PTPσ regulation on ezrin function ..................................... 80 4) Validate and characterize other putative substrates identified in MS screen ........ 80 5) Explore the role of PTPσ in other immune cells ................................................... 81 6) Explore the role of PTPσ in Paneth cell function ................................................. 81 7) Generate tissue specific, catalytically-inactive and ΔIg3 PTPσ mouse mutants .. 82
List of Figures Figure 1: Classifications and Substrate Specificities of PTPs ....................................................... 4
Figure 2: Domain Architecture and Ectodomain Shedding of LAR-PTP Family Members ......... 7
Figure 3: Summary of IBD Susceptibility Genes and Loci identified by Genome Wide Association Studies (GWAS) ...................................................................................... 16
Figure 4: Alternative splicing of PTPσ in UC ............................................................................. 18
Figure 5: SNP polymorphisms in CDH1 associate with CD and lead to the mislocalization of a truncated E-cadherin protein. ............................................................................... 22
Figure 6: Overview of the Apical Junction Complex (AJC) ....................................................... 24
Figure 7: PTPσ(-/-) mice have defects in epithelial barrier permeability .................................... 39
Figure 8: PTPσ is expressed in epithelial cells lining the crypt regions of the mouse small bowel and colon ........................................................................................................... 42
Figure 9: Tyrosine phosphorylation is enriched in the crypt regions of PTPσ(-/-) mouse colon and small bowel ................................................................................................. 44
Figure 10: Ezrin and villin precipitate with the D1 domain of PTPσ .......................................... 51
Figure 11: Ezrin undergoes dephosphorylation in the presence of D1 domain-containing PTPσ contructs ............................................................................................................ 53
Figure 12: E-cadherin and β-catenin colocalize to the plasma membrane along the basolateral border between enterocytes ....................................................................... 56
Figure 13: Ezrin localization is altered in the small bowel of PTPσ(-/-) mice after DSS treatment ...................................................................................................................... 60
Figure 14: Normal epithelial architecture but reduced Paneth cell number in PTPσ(-/-) neonatal mouse small bowel ........................................................................................ 63
Figure 15: Lysozyme expression is reduced in the Paneth cells of the PTPσ(-/-) small bowel .. 66
Figure 16: Ki-67 immunostaining, a marker for cell proliferation, appears unchanged in PTPσ(-/-) mouse small bowel and colon ..................................................................... 68
Figure 17: Proposed mechanism for PTPσ regulation of adherens junction stability ................. 77
vii
Abbreviations AJC – apical junction complex
ATP – adenosine tri-phosphate
CAM – cell adhesion molecule
CBC – crypt base columnar
CD – Crohn’s disease
CNS – central nervous system
CSPG – chondroitin sulfate proteoglycan
D1 – first catalytic PTPase domain of PTPσ
DSP – dual specificity phosphatase
DSS – dextran sodium sulfate
FNIII – fibronectin III-like
GH – growth hormone
GST – glutathione S-transferase
GWAS – genome wide association study
HNSCC – head and neck small cell carcinoma
HSPG – heparin sulfate proteoglycan
IBD – inflammatory bowel disease
Ig – immunoglobin-like
LAR – leukocyte antigen-related
MS – mass spectrometry
NGF – nerve growth factor
PNS – peripheral nervous system
PRL – prolactin
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PTB – phosphotyrosine binding domain
PtdIns3P – phosphatidylinositol 3-phosphate
PTP – protein tyrosine phosphatase
PTPδ – protein tyrosine phosphatase delta
PTPσ – protein tyrosine phosphatase sigma
RPTP – receptor protein tyrosine phosphatase
SH2 – Src homology 2 domain
SNP – single nucleotide polymorphism
SVZ – subventricular zone
TEER – transepithelial electrical resistance
UC – ulcerative colitis
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Chapter 1: Introduction
1
1) Tyrosine Phosphorylation
Tyrosine phosphorylation is a reversible post-translational protein modification involving
the covalent attachment of inorganic phosphate derived from ATP to the terminal hydroxyl
group on the side chain of the amino acid tyrosine. This modification can modulate enzymatic
activity[1, 2], dictate subcellular localization[3], or mediate protein-protein interactions through
specialized domains such as the SH2[4] or PTB domains[5]. Since its discovery in 1979[6], a
wide range of cellular processes have been characterized which feature regulation by tyrosine
phosphorylation. These processes include: signal transduction (especially growth factor kinase
signaling); cell proliferation; cell migration and adhesion; and cell differentiation (reviewed in
[7, 8]). The tyrosine phosphorylation of proteins is regulated by the activity of two classes of
enzymes: protein tyrosine kinases (PTKs) and protein tyrosine phosphatases (PTPs). Enzymes in
these families work in opposition to modulate tyrosine phosphorylation levels in the cell. Given
the range of cellular functions mediated by tyrosine phosphorylation, it is not surprising that
dysfunction of PTKs and PTPs has been implicated in a wide range of human diseases including
cancer[9, 10], cardiovascular disease[11], obesity and diabetes[12], and inflammatory
disorders[13, 14].
2) Protein Tyrosine Phosphatases
I) Characteristics & Classifications There are 107 PTPs in the human genome[15]. These proteins can be classified
according to their specificity, molecular weight, and subcellular localization (Figure 1). The
catalytically active PTPase domains of PTPs are highly conserved and contain a signature motif
[(I/V)HCXAGXXR(S/T)G], which includes an essential cysteine residue necessary for the
2
Figure 1: Classifications and Substrate Specificities of PTPs. The four major classes of PTPs are color-coded. Class I PTPs (green) can be sub-classified into classical and VH-1 like dual specificity phosphatases (DSPs) according to substrate specificity. Classical phosphatases are tyrosine-specific and can be further classified into receptor (RPTP) and non-receptor (NRPTP) categories. The DSPs can target diverse non-tyrosine phosphorylated residues including phosphoinositols (PTENs). Class II PTPs (orange) are a family of low-molecular weight phosphatases. Class III PTPs (blue) are a group of three-related cell cycle regulators which target cyclin-dependent kinases (Cdks). Class IV PTPs (red) are unique in that they utilize an aspartic acid based catalytic mechanism and have been shown to target both phospho-tyrosine and phospho-threonine residues. (Adapted from Alonso et al. Cell. 2004)
3
Figure 1: Classifications and Substrate Specificities of PTPs
4
nucleophilic catalysis of the phosphotyrosine moiety through a cysteinyl-phosphate
intermediate[16-18]. Classical PTPs are Class I tyrosine-specific phosphatases which can be
divided into receptor or non-receptor categories according to domain architecture and subcellular
localization[15]. Receptor tyrosine phosphatases (RPTPs) are single-pass trans-membrane
proteins whose ectodomain domain can bind to ligands, transmitting extracellular signals to the
intracellular phosphatase machinery[19]. Receptor dimerization has been implicated in the
(in)activation and regulation of RPTPs[20, 21]. In addition, gatekeeping effects can be provided
by catalytically-inactive tandem PTPase domains which can bind and inhibit the active PTPase
domain[22, 23]; these domains may also play a role in dictating substrate specificity[24].
Reversible oxidation of the catalytically essential cysteine residue has also been implicated in the
regulation of PTPs[25] and specifically RPTPs[26].
II) LAR subfamily The leukocyte antigen-related (LAR) subfamily of RPTPs, comprised of LAR, PTPδ, and
PTPσ in humans, share a high degree of sequence identity (64%)[27]. Invertebrate orthologues
of this family include: hmLar1 and hmLar2[28] (H. medicinalis); DLAR[29, 30] and
DPTP69D[29, 31] (D. melanogaster); and PTP-3[32] (C. elegans). Generally, LAR family
receptors consist of a cell adhesion molecule (CAM)-like ectodomain with three immunoglobin-
like (Ig) domains and a variable number of fibronectin-III-like (FNIII) repeats; a short single
pass trans-membrane domain; and an intracellular region with two PTPase domains of which
only the first (D1) domain is catalytically active[27] (Figure 2A). Alternative splicing is known
to dictate the number of FNIII repeats[33]. These phosphatases are translated as proproteins
which undergo proteolytic processing at the membrane to form a non-covalently attached
5
Figure 2: Domain Architecture and Ectodomain Shedding of LAR-PTP Family Members. A) The domain architecture of the LAR subfamily of RPTPs. Proteins depicted include vertebrate and invertebrate orthologs. The alternatively spliced ‘short’ PTPσ isoform is depicted with FNIII repeats four through seven spliced out. B) Proteolytic processing of LAR family PTPs. After trafficking to the plasma membrane, LAR-PTPs undergo proteolytic processing to form a non-covalently attached complex consisting of a P-subunit and E-subunit. This complex can undergo further processing to disassociate the complex, thus solubilizing the E-subunit extracellularly and allowing the P-subunit to be internalized. (Adapted from Chagnon et al. Biochemistry and Cell Biology. 2004)
6
Figure 2: Domain Architecture and Ectodomain Shedding of LAR-PTP Family Members
7
complex consisting of two subunits, E-subunit and P-subunit[34, 35]. These subunits remain
bound a short distance upstream of the trans-membrane domain. This complex can undergo
further proteolytic processing to abrogate this interaction, solubilizing the E-subunit
extracellularly, thus allowing the P-subunit to be internalized: this process is known as
ectodomain shedding[34, 35] (Figure 2B). These processes may confer a regulatory effect to
these PTPs as these liberated subunits can either be redistributed within the cell, in the case of
the P-subunit, or go on to interact with other adhesion-like domains extracellularly, in the case of
the E-subunit[34].
3) Protein Tyrosine Phosphatase Sigma (PTPσ)
Protein-tyrosine phosphatase sigma (PTPσ) is encoded by the PTPRS gene in humans,
which is located at chromosome 19p13.3[15]. This phosphatase is evolutionarily conserved with
notable homologs in mice (M. musculus)[36], rats (R.norvegicus)[37], and zebrafish (D.
rerio)[38]. PTPσ has two major splice isoforms: the ‘long’ isoform, which contains 8 FNIII
repeats, is expressed in several tissue types including the thymus, colon, and small intestine,
whereas the ‘short’ isoform, which contains 4 FNIII repeats (FNIII repeats 4 through 7 are
spliced out), is expressed primarily in the nervous system[27]. Receptor homo-dimerization is
necessary for ligand binding to the PTPσ ectodomain in vitro with differential ligand binding
specificities observed for the two major splice variants[39]. Expression of PTPσ is
developmentally regulated with tightly controlled temporal and spatial distributions[37, 40]. Its
expression is strongest during embryonic development, where it is present in various epithelial
tissues and regions of the central (CNS) and peripheral (PNS) nervous systems, including the
Several protein tyrosine phosphatases have been identified as IBD susceptibility genes
through both GWAS and functional analyses. PTPN2, also known as T-cell protein tyrosine
phosphatase (TC-PTP), was first associated with Crohn’s disease in a GWAS conducted by the
Wellcome Trust Case Control Consortium[102] and subsequently replicated in several
13
independent studies[103-106]. PTPN2 is known to regulate pro-inflammatory cytokine signaling
and loss of PTPN2 expression in mice leads to systemic inflammation[107]. Furthermore,
PTPN2(-/-) mice have increased susceptibility to induced colitis following treatment with
DSS[108]. PTPN11, which encodes for SHP-2, contains polymorphisms associated with
ulcerative colitis[109] and functions to attenuate STAT3-mediated inflammation[110] and to
regulate barrier function through its role in modulating the expression of claudins in the intestinal
epithelia[111] . PTPN22 (Lymphoid protein tyrosine phosphatase), a negative regulator of T-cell
activation, has been implicated in a range of autoimmune disorders[112-115] including both
Crohn’s disease[105, 116] and ulcerative colitis[116]. The identification of these PTPs as IBD
susceptibility genes demonstrates the importance of PTPs in the pathogenesis of IBD, and
underscores how investigations into the function of PTPs in the intestine can contribute to our
understanding of the disease.
7) PTPσ in IBD
PTPRS resides within the IBD6 susceptibility locus[94]. Through single nucleotide
polymorphism (SNP) analysis of IBD patients, our group has shown that PTPRS is genetically
associated with ulcerative colitis[13]. The identified SNP polymorphism leads to alternative
splicing in the extracellular region of the epithelial ‘long’ isoform of PTPσ leading to a loss of
micro-exon domain B (meB) resulting in the absence of the third immunoglobin-like domain
(Ig3) in the final protein[13] (Figure 4). This splicing might potentially lead to altered ligand
recognition or may affect receptor dimerization[39]
14
Figure 3: Summary of IBD Susceptibility Genes and Loci identified by Genome Wide Association Studies (GWAS) A) Schematic representation of first identified IBD susceptibility loci. These regions, termed IBD1 through IBD9, represent genomic areas containing microsatellite markers in strong linkage disequilibrium in IBD patients compared to healthy controls. Regions in which candidate susceptibility genes were further identified and validated are colored magenta. B) Overview of GWAS-identified IBD susceptibility genes. Susceptibility genes are classified according to cellular function. Genes are color-coded according to disease classification in which association was observed (blue: ulcerative colitis, red: Crohn’s disease, black: unspecified IBD). Underlined genes indicate genes with characterized cis-acting expression quantitative trait loci (cis-eQTL) indicating allele specific gene expression changes. Asterisks indicate genes with known coding region SNPs. The labels ‘P’ and ‘G’ denote Paneth and goblet cells respectively. (Adapted from Gaya et al. The Lancet. 2005 & Khor et al. Nature. 2011)
15
Figure 3: Summary of IBD Susceptibility Genes and Loci identified by Genome Wide Association Studies (GWAS)
A
B
16
Figure 4: Alternative splicing of PTPσ in UC. A) PCR analysis demonstrating alternative splicing of isoform 1 of PTPσ in individuals with the UC-associated risk haplotype. mRNA was isolated from lymphoblast cell lines derived from three healthy controls (labeled 1 through 3) and three IBD patients possessing the risk alleles (labeled 4 through 6). A DNA fragment comprising amino acids 191 through 371 of PTPσ was amplified by PCR and separated by gel electrophoresis. The top bands represent the full 168 amino acid segment found in isoforms 2 and 3 of PTPσ. The lower bands represent the alternatively spliced isoform 1 lacking exons 8 and 9. B) Schematic representation of alternative splicing of PTPσ in UC. The meB domain (exon 8) and the third Ig domain (exon 9) are spliced out in the individuals possessing the disease haplotype. This splicing is only in frame in the ‘long’ PTPσ isoform, which is expressed in the intestine. (Adapted from Muise et al. Current Biology. 2007)
17
Figure 4: Alternative splicing of PTPσ in UC
18
8) Role of Intestinal Barrier Defence in IBD
Intestinal barrier defence refers to the coordinated action of several innate immune
pathways to protect the interior of the body from both commensal and pathogenic organisms
present in the intestinal lumen (reviewed in [117]). These pathways include: the production of
antimicrobial peptides and immunoglobins by specialized Paneth cells at the base of the
intestinal crypts; the secretion of mucins by goblet cells, and the restriction of access to the
lamina propria provided by a polarized epithelial cell monolayer. Mucosal barrier dysfunction
has been previously associated with the development of IBD[84]. In IBD patients, evidence of
impaired barrier function, in the form of increased intestinal permeability, further underscores
the importance of innate immunity to IBD pathogenesis[118-123]. Genetic studies have revealed
several IBD susceptibility genes that play important roles in mediating this defence. One of the
earliest identified IBD susceptibility factors, NOD2, functions in Paneth cells through the
activation of the NF-κB pathway to regulator defensin production in the presence of the bacterial
MAGI2[121], three proteins known to mediate tight junction dynamics, were identified in
European GWAS and replicated in North American cohorts. Our group has identified a SNP
haplotype polymorphism in CDH1 that associates with Crohn’s disease and leads to a truncated
E-cadherin protein that fails to localize to the plasma membrane[127]. The mislocalization of E-
19
cadherin and its binding partner β-catenin was observed in IBD patient biopsy samples[127]
(Figure 5). CDH1 was also identified in a recent GWAS as an UC susceptibility gene [81].
Several of these identified proteins implicate the apical junction complex as a system whose
dysfunction can contribute to IBD pathogenesis.
9) Apical Junction Complex and Epithelial Barrier Function
The apical junction complex (AJC) is essential for the formation of the epithelial cell
monolayer in the intestine as it mediates epithelial cell adhesion and polarity through two distinct
junction complexes: the tight junctions or zonula occludins and the adherens junctions or zonula
adherens (Figure 6A). These complexes form intercellular junctions through claudin-occludin
and cadherin-catenin homophilic interactions, respectively, and anchor to the actin cytoskeleton
to provide mechanical support for the epithelial cell and to maintain proper polarized
morphology (Figure 6B). In addition, these complexes restrict fluid access from the intestinal
lumen to the lamina propria except through regulated paracellular transport[128]. The defective
regulation of AJC-associated proteins creates disrupted epithelial barriers leading to permeability
defects and aberrant intestinal morphology[128, 129]. Expression of a dominant negative N-
cadherin chimera in mice was shown to induce a CD-like phenotype in the intestine[130]. The
similarity between the intestinal pathologies associated with IBD and apical junction complex
disruption suggests that AJC proteins may be potential IBD susceptibility genes.
10) PTPσ and the Apical Junction Complex
PTPσ colocalizes with plakoglobin (γ-catenin) which is known to localize to adherens
and desmosome junctions along the basolateral plasma membrane in epithelia[131]. The
localization of PTPσ to sites of intercellular junctioning, coupled with the previous identification
of N-cadherin as a neural PTPσ substrate by our group, suggest that PTPσ may regulate cell
20
Figure 5: SNP polymorphisms in CDH1 associate with CD and lead to the mislocalization of a truncated E-cadherin protein. A) Schematic representation of the truncated E-cadherin mutant found in CD patients with the disease associated haplotype. A 72 amino acid truncation at the N-terminus (Δ72) leads to a loss of the signal peptide region (S) and a portion of the precursor domain (Pre). Cad, cadherin domain; TM. transmembrane domain; ICD, intracellular domain. B) Immunofluorescence microscopy demonstrating that the Δ72 E-cadherin protein accumulates in the cytoplasm. mCherry-tagged wild-type (A) and Δ72 E-cadherin (B) plasmids were transfected into MDCK cells. Zo-1 and calnexin (green) were co-stained to act as markers for plasma membrane and cytoplasmic localization respectively. β-catenin was also co-stained and was shown to accumulate in the cytoplasm along with Δ72 E-cadherin. Scale bars = 20 μm. C) Immunohistochemical staining for E-cadherin in intestinal biopsies from patients with (E-H) or without (A-D) the CD-associated risk haplotype. DAB staining (brown) showed strong accumulation in the cytoplasm for patients with the risk allele compared to controls. Nuclei are counterstained in blue. (Adapted from Muise et al. Gut. 2009)
21
Figure 5: SNP polymorphisms in CDH1 associate with CD and lead to the mislocalization of a truncated E-cadherin protein.
A
B
C
22
Figure 6: Overview of the Apical Junction Complex (AJC). A) Schematic representation of junctioning complexes throughout the intestinal epithelial cells. Enterocytes adopt a polarized morphology as mediated by the apical junction complex partitioning the plasma membrane into distinct apical and basolateral compartments. The tight junctions restrict access to the lamina propria through claudin-occludin interactions. The adherens junctions mediate intercellular adhesion and provide structural support through interactions with the actin cytoskeleton. Microvilli protrude from the apical plasma membrane into the lumen of the small intestine. B) Schematic of principal mediators of adherens and tight junctioning. At the adherens junction, cadherins forms calcium dependent homophilic interactions with cadherin complexes on opposing cells. The cytoplasmic tail of the cadherin interacts with adapter catenin proteins, which mediates interactions with the actin cytoskeleton and other regulatory molecules. At the tight junction, claudins in conjunction with occludins and junction associated molecules (JAMs) form tight intercellular interactions and interact with the actin cytoskeleton through the tight junction proteins ZO-1 and ZO-2.
(Adapted from Guttman et al. BBA-Biomembranes. 2009 & Nyqvist et al. EMBO Reports 2008)
23
Figure 6: Overview of the Apical Junction Complex (AJC)
Biosciences), mouse anti-E-cadherin (1:400; BD Biosciences), and rabbit anti-β-catenin (1:1000;
Cell Signaling Technologies, Beverly, MA). Following washes in PBS, the slides were
incubated with secondary antibodies in SS-PBS for 1 hour at room temperature. Secondary
antibodies used were: goat anti-rabbit Alexa488 (1:1000, Invitrogen) and goat anti-mouse
Alexa555 (1:1000, Invitrogen). Slides were further stained with DAPI (1mg/mL) and coverslips
were mounted using Dako Fluorescent Mounting reagent (Dako, Glostrup, Denmar
15) Transmission Electron Microscopy
PTPσ(-/-) one day old neonates and littermates were sacrificed and the small bowel was
removed by dissection. Tissues were immediately place in aldehyde fixation buffer (2%
paraformaldehyde in 100 mM cacodylate buffer (pH 7.0) with 2 mM CaCl2) overnight at 4°C.
Samples were washed with 100 mM cacodylate buffer (pH 7.0), then post-fixed in 1% osmium
tetroxide in 100 mM cacodylate buffer (pH 7.0) for 2 h at 4°C. After a ddH2O wash, samples
34
were stained in 2.0% aqueous uranyl acetate for 2h at 4°C. Stained samples were dehydrated
through increasing percentage acetone washes followed by propylene oxide mixed in increasing
ratios of embedding resin. Embedded tissues were sectioned on an ultra-microtome and
visualized using a transmission electron microscope.
35
Chapter 3: Results
All work presented in this chapter was completed by myself with the exception of the initial
tandem MS screen which was conducted by Cong-hui Guo
36
1) PTPσ(-/-) mice exhibit defects in intestinal barrier integrity
Given the prior identification of the adherens junction components, E-cadherin and β-
catenin as colonic PTPσ substrates, and in light of their importance in mediating epithelial cell
adhesion, the integrity of the intestinal epithelial layer in our PTPσ(-/-) mice was investigated by
evaluating the permeability and electrophysiology of the small bowel and colon ex vivo and
performing in vivo tracer studies. Small bowel and colon sections from PTPσ(+/+) and
PTPσ(-/-) mice were analyzed in an Ussing Chamber. PTPσ(-/-) mouse colon and small bowel
both showed decreased trans-epithelial electrical resistance (TEER) (Figure 7A) and increased
macromolecular flux (Figure 7B) as compared to PTPσ(+/+) littermates. PTPσ(+/-) mice were
similar to wild-type controls. These data suggest that loss of PTPσ function leads to loss of
epithelial resistance that could indicate a defect in intestinal permeability. To confirm these
observations in vivo, mice were gavaged with fluorescein-isothiocyanate (FITC)-conjugated
dextran that acts as a molecular probe to evaluate epithelial barrier permeability. Two different
size dextran conjugates were used, 4.4 kDa and 40 kDa, to assess both paracellular and
macromolecular permeability, respectively. PTPσ(-/-) mouse small bowel and colon tissue
showed increased FITC serum levels for both dextran conjugates as compared to PTPσ(+/+)
littermates (Figure 7C,D). PTPσ(+/-) mouse tissue showed slightly elevated levels but were not
significantly different from their PTPσ(+/+) littermates. These data support the Ussing chamber
results and further suggests a role for PTPσ in the maintenance of epithelial barrier integrity and
defense.
37
Figure 7: PTPσ(-/-) mice have defects in epithelial barrier permeability. A) Trans-epithelial electrical resistance (TEER) was measured ex vivo in whole intestinal tissue sections mounted in an Ussing chamber. PTPσ(-/-) mice showed decreased resistance in both the small bowel and colon compared to wild-type littermates. B) Macromolecular flux across the epithelia was evaluated by adding horseradish peroxidase (HRP) as a tracer to the mucosal side of the Ussing chamber with tissues mounted. Measurement of the concentration of HRP in the serosal chambers at thirty minute intervals revealed an increased flux in the PTPσ(-/-) mice compared to wild-type littermates. To evaluate permeability in vivo, PTPσ(-/-) mice and controls were orogastrically gavaged with the molecular tracer FITC-dextran. C) PTPσ(-/-) mice showed increased serum levels of FITC-dextran 4kDa compared to wild-type littermates. D) PTPσ(-/-) mice showed increased serum levels of FITC-dextran 40kDa compared to wild-type littermates. * denotes p<0.05 versus PTPσ(+/+) littermate controls (n=8 to 11 mice, Student’s t-test). Error bars represent mean +/- SE.
38
Figure 7: PTPσ(-/-) mice have defects in epithelial barrier permeability
39
2) PTPRS is expressed in the crypts regions of the mouse intestine
The gene inactivating cassette used to generate our PTPσ(-/-) mice contains a β-
galactosidase reporter, which is thus expressed from the endogenous PTPRS promoter [47]. To
determine where PTPRS (PTPσ) is expressed in the intestine, we performed X-gal staining on
frozen tissue sections from PTPσ(+/-) mice and littermate controls. Dark blue/black staining
indicative of PTPσ expression was observed in the epithelial cells lining the crypt region of the
small bowel and colon in the heterozygous mice that was not present in the wild-type (Figure 8).
This suggests the PTPσ is expressed in epithelia lining the intestinal crypts, an area of rapid cell
proliferation. PTPσ expression was also observed (based on morphology) in the region at the
base of the small bowel crypts known to contain Paneth cells and crypt base columnar (CBC)
intestinal stem cells (Figure 8E). In addition, we also observed PTPσ expression in cells present
in the lamina propria regions of the small bowel and colon (Figure 8F).
3) Tyrosine phosphorylation is enriched in the crypts of PTPσ(-/-) mouse colon and small bowel
To determine how loss of PTPσ regulation affects tyrosine phosphorylation in the
intestine, we investigated the localization and extent of tyrosine phosphorylation in PTPσ(-/-)
mouse colon and small bowel using immunofluorescence microscopy. Colon and small bowel
sections from both untreated and DSS-treated PTPσ(+/+) and PTPσ(-/-) mice were
immunostained using anti-phosphotyrosine antibody (Figure 9). In the untreated mice, the villi
in the small bowel did not show significant staining for phoshotyrosine in either the PTPσ(+/+)
or PTPσ(-/-) sections. The crypt regions of the small bowel of the PTPσ(-/-) mice, however,
showed strong phosphotyrosine staining compared to PTPσ(+/+) controls (Figure 9A). A similar
crypt staining pattern was also observed in the colon of PTPσ(-/-) mice (Figure 9B). These
results support the X-gal staining data shown in Figure 6, as the increase in tyrosine
40
Figure 8: PTPσ is expressed in epithelial cells lining the crypt regions of the mouse small bowel and colon. X-gal staining was performed on frozen tissue sections from PTPσ(+/+) and PTPσ(+/-) mice. A,B) X-gal crystal formation was observed in cells in the crypt regions of the small bowel of the PTPσ(+/-) mice. Strong staining appears at the base of the intestinal crypts where the Paneth and intestinal stem cells reside. Background signal in the PTPσ(+/+) section represents non-specific staining. Scale bar = 40μm C,D) X-gal staining was present in the intestinal cells lining the crypts in the colon tissue of the PTPσ(+/-) mice but not the PTPσ(+/+) mouse. Scale bar = 40μm. E) X-gal staining is present in cells at the base of the small bowel crypts. Scale bar = 40μm. F) Cross-sections of the intestinal crypts reveal X-gal positive staining on cells inside the lamina propria region of the PTPσ(+/-) mouse.
41
Figure 8: PTPσ is expressed in epithelial cells lining the crypt regions of the mouse small bowel and colon
42
Figure 9: Tyrosine phosphorylation is enriched in the crypt regions of PTPσ(-/-) mouse colon and small bowel. Tissue sections from both naïve and DSS-treated mice were immunostained with a phosphotyrosine antibody and visualized using confocal microscopy. A,B) Tyrosine phosphorylation is enriched in the crypt regions of PTPσ(-/-) mouse small bowel and colon as compared to littermate controls. Increased tyrosine phosphorylation is also observed in cells contained within the lamina propria regions of PTPσ(-/-) mice. Scale bar = 50 μm. C,D) DSS-treatment of the PTPσ(-/-) mice and littermates leads to increased tyrosine phosphorylation in both knock-out and wild-type small bowel and colonic tissue sections. Overall tyrosine phosphorylation levels appear elevated in the PTPσ(-/-) mouse compared to the wild-type controls. Scale bar = 50 μm.
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Figure 9: Tyrosine phosphorylation is enriched in the crypt regions of PTPσ(-/-) mouse colon and small bowel
44
phosphorylation is located in regions positive for PTPσ expression. In addition, cells in the
lamina propria along the crypt-villus axis showed increased tyrosine phosphorylation in the
PTPσ(-/-) mice (Figure 9A). These cells include lymphocytes, which mediate immune function
in the intestine and are known to be present in large numbers in the lamina propria region.
In the DSS-treated mice, phosphotyrosine staining was found to be enriched in all
genotypes in both the small bowel (Figure 9C) and colon (Figure 9D). Notably, the staining was
increased in the PTPσ(-/-) mice relative to DSS-treated control littermates.
4) Identification of Villin and Ezrin as PTPσ binding partners by mass spectrometry
The altered permeability and increased tyrosine phosphorylation present in the PTPσ(-/-)
mouse intestine suggest that PTPσ is regulating proteins associated with the function and
maintenance of the epithelial barrier through its phosphatase activity. Given that E-cadherin and
β-catenin are the only intestinal substrates of PTPσ identified to date[13], we sought to
characterize the intestinal substrates of PTPσ to determine the mediators through which PTPσ
may be regulating intestinal barrier integrity. In order to identify such putative substrates, tandem
mass spectrometry was performed on phospho-tyrosine containing proteins immunoprecipitated
from colon tissue homogenate from PTPσ(-/-) and PTPσ(+/+) littermates, with the aim of
identifying hyper-tyrosine phosphorylated proteins in the PTPσ(-/-) intestine. Isolated proteins
were digested with trypsin to generate peptide fragments and analyzed by MS/MS to identify
pTyr containing fragments. Over 3600 unique spectral hits were identified in each of the tissues.
To refine this list to isolate potential PTPσ substrates, spectral hit counts were compared between
PTPσ(-/-) and PTPσ(+/+) samples, as we have demonstrated by immunofluorescence that loss of
PTPσ function leads to an increase in tyrosine phosphorylation in the intestine. Proteins were
ranked according to the difference in unique spectral counts between samples and a minimum
45
threshold of four was assigned to select specifically for phospho-proteins enriched in the PTPσ(-
/-) mouse tissue. Thirty-seven proteins which were enriched in tyrosine phosphorylation in the
PTPσ(-/-) colon and met the threshold requirement are listed in Table 1.
E-cadherin (Cdh1) and β-catenin (Cttnb1) were previously identified as colonic PTPσ
substrates by our group[13]. p130CAS (Bcar1) was recently identified as a PTPσ interacting
partner in a yeast-two hybrid screen[62]. Furthermore, EGFR was also previously shown to be
regulated by PTPσ[65] and that loss of PTPσ function leads to sustained EGFR activation[66].
The presence of these previously identified substrates in the hit list (Table 1) provided
confidence in our the MS analysis. A large proportion of the putative substrates identified in this
analysis are known to play a role in cell adhesion and cell junction pathways, specifically the
apical junction complex (AJC). In addition, many of the proteins are regulators of cytoskeletal
dynamics either by binding or processing actin directly or through downstream signaling
pathways. Given the number of cell adhesion and cell junction proteins identified in the screen,
we chose to focus on those putative substrates that are associated with epithelial barrier structure
and maintenance. Two proteins: ezrin and villin, were selected for further validation. Ezrin is a
scaffold protein implicated in the organization of the apical web region and participates in a
range of signaling pathways including adherens junction remodeling[143, 146]. Interestingly,
ezrin is known to modulate E-cadherin trafficking through activation of the Rho-GTPase Rac1 in
a tyrosine phosphorylation-mediated interaction. Furthermore, ezrin regulates villus
morphogenesis and Ezr(-/-) mice have defects in intestinal epithelial architecture including
atrophic villi[129]. Villin binds caps and severs actin filaments and regulates cell morphology
46
Table 1: Proteins with increased tyrosine phosphorylation in the intestine of the PTPσ(-/-) mice. This list was generated by comparing the unique spectral hits counts observed for each protein between the PTPσ(-/-) and PTPσ(-/-) mouse tissue samples. Hits that contained three or greater spectral counts in the PTPσ(-/-) sample compared to littermate controls were included. These represent the subset of proteins with increased tyrosine phosphorylation in the PTPσ(-/-) intestine. Proteins highlighted in red are known to be involved in the formation or regulation of the apical junction complex.
47
Table 1: Proteins with increased tyrosine phosphorylation in the intestine of the PTPσ(-/-) mice
and microvilli formation[147]. Loss of villin function has been shown to increase colonic
susceptibility to the DSS model of murine colitis[148].
To confirm these putative substrates can bind to PTPσ, an in vitro substrate trapping
assay was performed using glutathione S-transferase (GST) fusion proteins that were previously
generated by our group[63], which contain the catalytically active D1 domain of PTPσ. A site-
directed aspartic acid to alanine mutation at amino acid 1472 (GST-PTPσD1(D1472A)) was
introduced to create as a substrate trapping mutant, as described[63, 149]. Figure 10 shows that
ezrin and villin can bind to the D1 domain of PTPσ. To ensure that the binding was not GST-
mediated, a pull-down control of GST-agarose alone was used and the substrates showed no
interaction.
5) Ezrin is a substrate of PTPσ
To test whether ezrin and villin are substrates of PTPσ, an in vitro dephosphorylation
assay was performed. GST-fusion proteins containing the intracellular PTPase domains of PTPσ
were purified and their phosphatase activity was confirmed through incubation with pNPP, a
generic phosphotyrosine substrate (Figure 11A). Only the GST-fusion proteins containing the
D1 domain, namely GST-PTPσD1 and GST-PTPσD1D2, showed an increase in absorbance at
405nm, indicative of PTPase activity. This is expected considering that the D1 domain is the
catalytically active domain[150]. The GST-PTPσD1(D1472A) mutant showed no activity,
which was consistent with the substrate trapping mutation abrogating phosphatase activity.
The purified domains were incubated for 30min with an immunoprecipitated FLAG-
ezrin protein. Immunoblotting for phosphotyrosine revealed significantly decreased tyrosine
phosphorylation levels of ezrin in those incubations containing the catalytically active domain of
PTPσ (Figure 11B). No change in tyrosine phosphorylation levels was observed for the GST-
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Figure 10: Ezrin and villin precipitate with the D1 domain of PTPσ. Lysates obtained from the intestine of PTPσ(-/-) mice were incubated with wild-type (GST-PTPσD1-WT) or substrate trapping (GST-PTPσD1-(D1472A) constructs bound to GST-agarose containing the catalytic D1 domain of PTPσ. Binding partners were separated on SDS-PAGE and immunoblotted. GST-agarose alone was included as a negative control to ensure binding is not GST-dependent. A) Substrate trapping assay revealing ezrin binding to the D1 domain of PTPσ. E-cadherin was included as a positive control. B) Substrate trapping assay revealing villin binding to the D1 domain of PTPσ. For labeling, ‘σD1-WT’ and ‘σD1-DA’ refer to GST-PTPσD1-WT and GST-PTPσD2(D1472A), respectively.
50
Figure 10: Ezrin and villin precipitate with the D1 domain of PTPσ
51
Figure 11: Ezrin undergoes dephosphorylation in the presence of D1 domain-of PTPσ. GST-fusion constructs for the intracellular domains of PTPσ were incubated with FLAG-tagged ezrin. A) para-nitrophenyl phosphate (pNPP) phosphatase assay demonstrating activity of GST-fusions constructs. B) Incubation with GST-PTPσD1 or GST-PTPσD1D2 leads to decreased tyrosine phosphorylation of FLAG-ezrin. No change in tyrosine phosphorylation was observed for incubations with GST-PTPσD2 or GST-PTPσD1(D1472A). C) Incubation with GST-PTPσD1 or GST-PTPσD1D2 does not change tyrosine phosphorylation levels of FLAG-villin. No change in tyrosine phosphorylation was observed for incubations with GST-PTPσD2 or GST-PTPσD1(D1472A). D) Histogram depicting the change of in tyrosine phosphorylation of ezrin during incubation with the various GST-fusion proteins as compared to the catalytically inactive D1 mutant. All values are normalized to total protein levels by densitometry. (n=4, Student’s t-test, p<0.05). Error bars represent mean +/- SE.
52
Figure 11: Ezrin undergoes dephosphorylation in the presence of D1 domain-containing PTPσ
contructs
53
PTPσD2 or the GST-PTPσD1(D1472A) proteins. This assay was repeated and the cumulative
histogram is depicted in Figure 11D, demonstrating a statistically significant change in tyrosine
phosphorylation levels between the D1-domain containing samples and the catalytically in-active
mutant. These data suggest that ezrin can interact with and undergo tyrosine dephosphorylation
by PTPσ in vitro, supporting its role as a PTPσ substrate. No change in tyrosine phosphorylation
of villin was observed in this assay (Figure 11C), suggesting that villin is not an in vitro (direct)
substrate of PTPσ but may interact with this PTP as part of a complex.
6) E-cadherin and β-catenin colocalization and β-catenin localization to the nucleus is unaffected in the PTPσ(-/-) mouse small bowel and colon
As described earlier, tyrosine phosphorylation plays a key role in the regulation of the E-
cadherin-β-catenin interaction and increased tyrosine phosphorylation is associated with complex
disassembly[151]. To determine if the adherens junctions are disrupted by loss of PTPσ
function, the localization of E-cadherin and β-catenin was assessed in our PTPσ(-/-) mice both
naïve and after DSS treatment using immunofluorescence microscopy. Under normal
conditions, E-cadherin localized to the basolateral region of the enterocytes in PTPσ(+/+) and
PTPσ(-/-) mouse small bowel and colonic sections (Figure 12A,B). β-catenin colocalized with
the E-cadherin and was also present in the nucleus. No significant difference was found between
the PTPσ(+/+) and PTPσ(-/-) tissue sections suggesting that E-cadherin localization is not
affected by loss of PTPσ (Figure 12E). This result contradicts the permeability defect observed
earlier as we would expect a disruption of the adherens junction if E-cadherin and β-catenin are
dysregulated. This would suggest that the increased permeability observed in the PTPσ(-/-)
mouse is either independent of PTPσ mediated E-cadherin- β-catenin regulation or that PTPσ
does not regulate E-cadherin- β-catenin complex internalization. E-cadherin internalization via
endocytosis has been implicated in the regulation of cellular adhesion[152].
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Figure 10: E-cadherin and β-catenin colocalize to the plasma membrane along the basolateral border between enterocytes. E-cadherin and β-catenin localization was evaluated in the small bowel and colon of both naïve and DSS-treated PTPσ(-/-) mice and littermates using immunofluorescence microscopy. E-cadherin and β -catenin colocalized to the basolateral plasma membrane region between the enterocytes lining the villi (A) and the crypts (B) in the small bowel. β-catenin staining was also present in the nuclei of the enterocytes. DSS-treatment of the PTPσ(-/-) mice and littermates did not disrupt the colocalization of E-cadherin and β -catenin in the small bowel (C) or colon (D). Scale bars = 45μm. E) Evaluation of the colocalization of E-cadherin and β-catenin using Pearson’s coefficient revealed no significant difference between PTPσ(-/-) mice and littermate controls. (n=75, Student’s t-test, p>0.05). Error bars represent mean +/- SE. F) Evaluation of the colocalization of the nuclear marker DAPI (blue) and β-catenin revealed no significant difference between PTPσ(-/-) mice and littermate controls (n=67, Student’s t-test, p>0.05). Error bars represent mean +/- SE.
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Figure 12: E-cadherin and β-catenin colocalize to the plasma membrane along the basolateral border between enterocytes
56
E
F
57
Following treatment with DSS, E-cadherin again localized predominantly to the plasma
membrane along basolateral region between enterocytes in both PTPσ(-/-) and PTPσ(+/+) mouse
small bowel (Figure 12C) and colon tissue (Figure 12E). β-catenin immunostaining mirrored a
similar pattern to the E-cadherin in addition to being present in the nucleus. Quantification of
the colocalization between E-cadherin and β-catenin showed no significant difference between
the PTPσ(-/-) and PTPσ(+/+) samples (Figure 12E). β-catenin localization to the nucleus is
indicative of Wnt signaling pathway activation, which is known to play an important role in
enterocyte differentiation along the crypt-villus axis[153, 154]. We evaluated the colocalization
of β-catenin with the nuclear marker DAPI in immunostained tissue sections from our PTPσ(-/-)
mice and littermates (Figure 12F). There was no significant difference in nuclear localization of
β-catenin between the PTPσ(-/-) and PTPσ(+/+) tissue sections, suggesting the β-catenin
mediated Wnt signaling is not affected by loss of PTPσ.
7) Ezrin localization is altered in PTPσ(-/-) mouse small bowel after DSS treatment
Tyrosine phosphorylation has been implicated in the trafficking of ezrin to the plasma
membrane[155, 156]. To determine if loss of PTPσ activity disrupts ezrin localization, we
performed immunofluorescence on mouse tissue sections. Small bowel and colonic tissue was
acquired from both naïve and DSS-treated PTPσ(-/-), and PTPσ(+/+) mice. Ezrin localization in
naïve mouse colon and small bowel revealed no difference between PTPσ(-/-) and PTPσ(+/+)
sections (Figure 13A,B). However, differences in ezrin localization were observed between
PTPσ(-/-) and PTPσ(+/+) small bowel sections after DSS treatment (Figure 13C,D). The
PTPσ(+/+) small bowel tissue showed ezrin localization in both the cytoplasm and at the plasma
membrane, in contrast to the PTPσ(-/-) tissue, which showed plasma membrane staining almost
exclusively. This provides evidence that PTPσ may regulate ezrin trafficking, and that loss of
58
Figure 13: Ezrin localization is altered in the small bowel of PTPσ(-/-) mice after DSS treatment. Ezrin localization was evaluated in small bowel and colon tissue sections from both naïve and DSS-treated PTPσ(-/-) mice and littermates using immunofluorescence microscopy. A) Ezrin localization was present at the apical plasma membrane of enterocytes in the small bowel in PTPσ(-/-) mice and controls. Scale bar = 30μm. B) Ezrin localized to the apical surface of the enterocytes in the colon and there was no observed difference between PTPσ(-/-) mice and littermate controls. Scale bar = 30μm. C) Ezrin localization is disrupted in the small bowel of PTPσ(-/-) mice treated with DSS. D) DSS-treatment has no effect on the localization of ezrin in the colon. Scale bar = 30μm.
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Figure 13: Ezrin localization is altered in the small bowel of PTPσ(-/-) mice after DSS treatment
60
PTPσ may lead to accumulation of ezrin at the plasma membrane under acute inflammatory
conditions. However, given the severity of the intestinal tissue disruption in the PTPσ(-/-) mouse
after DSS treatment, it is possible that the observed changed in ezrin localization is the result of
inflammation as opposed to a contributing factor causing inflammation. Consequently,
determining if PTPσ mediates ezrin localization to the plasma membrane will require more
robust experimental techniques.
8) Intestinal morphology is not disrupted in the small bowel and colon of neonatal PTPσ (-/-) mice
Ezrin is known to play an important role in the architectural development of the intestine,
mediating villus morphogenesis and microvilli formation[129, 157]. Loss of ezrin activity has
been linked to villus blunting, a phenotype observed in the small bowel of the PTPσ knockout
mouse [129]. Given that PTPσ expression is known to be developmentally regulated and that
loss of PTPσ function has been linked to developmental delays in other tissue types, it is possible
that loss of PTPσ regulation of ezrin may affect small bowel morphology during intestinal
development in the PTPσ(-/-) mouse. To determine if intestinal morphology is altered after
birth, small bowel tissue sections from one day old PTPσ(-/-) neonates were analysed by electron
microscopy. The structure and morphology of the enterocytes lining the PTPσ(-/-) small bowel
appear normal as compared to wild-type littermates (Figure 14A,B). Microvilli and the apical
junction complex region also appear similar to wild-type controls (see indicated). However, the
number of Paneth cells present at the base of the small bowel crypts in the PTPσ(-/-) mouse
appeared reduced compared to wild-type littermates. To determine if Paneth cell function was
affected in the PTPσ(-/-) neonates, frozen small bowel neonatal tissue sections were tested for
immunoreactivity to the anti-microbial factor lysozyme, which serves as a marker for Paneth
cells. DAB staining at the base of the intestinal crypts appears reduced in the PTPσ(-/-) small
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Figure 14: Normal epithelial architecture but reduced Paneth cell lysozyme immunoreactivity in PTP(-/-) neonatal mouse small bowel. A) The morphology of the intestinal epithelial cells in the small bowel of the PTPσ(-/-) neonates and littermate controls characterized by electron microscopy. Tissue sections were fixed by aldehyde fixation, embedded, and sectioned on an ultramicrotome before visualization by TEM. Cell morphology in the PTPσ(-/-) mouse appears like wild-type. Notably, the microvilli length and distribution appears normal and junction complexes have proper size and alignment. Scale bars = 2μm B) Evaluation of lysozyme expression in the PTPσ(-/-) mouse small bowel. Frozen tissue sections from PTPσ(+/+) and PTPσ(-/-) neonates were stained for lysozyme and visualized by DAB reaction. Comparison of staining intensity in the crypt regions of the small bowel reveals reduced DAB color intensity in the PTPσ(-/-) neonates. Negative control represents DAB reaction in absence of antibody. Scale bar = 40μm.
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Figure 14: Normal epithelial architecture but reduced Paneth cell number in PTPσ(-/-) neonatal mouse small bowel
]
63
bowel compared to wild-type littermates (Figure 14C,D). This suggests that PTPσ may play a
role in the regulation of Paneth cells. .
9) Lysozyme immunostaining is reduced in the Paneth cells of the PTPσ(-/-) mouse small bowel
Paneth cells are specialized secretory epithelial cells responsible for the production of
various antimicrobials factors including α-defensins (cryptidins)[158, 159], lysozyme[160], and
phospholipase A2[161]. The observed LacZ staining pattern in the small bowel of the PTPσ(-/-)
mouse suggests that PTPσ is expressed in the Paneth cells, as evidenced by strong staining at the
base of the crypts. The reduced immunoreactivity to lysozyme observed in the PTPσ(-/-)
neonates further supports a role for PTPσ in Paneth cell function. Given that Paneth cell
dysfunction has previously been implicated in IBD, evaluating indicators of Paneth cell function
in the adult PTPσ(-/-) mice will help to determine if loss of PTPσ is impairing the innate immune
function of the Paneth cell. Using confocal microscopy, small bowel tissue sections from 2 to 3
wk old PTPσ(-/-) mice and littermates were immunostained for lysozyme. Lysozyme
immunostaining, as measured by fluorescence intensity per unit area, was reduced in the PTPσ(-
/-) mouse small bowel compared to wildtype controls (Figure 15A). To quantitate these results,
fluorescence intensity measurements were taken for approximately 95 crypts from both
PTPσ(+/+) and PTPσ(-/-) small bowel sections. These results were tabulated and the difference
in fluorescence intensity between the two genotypes was shown to be statistically significant
(Figure 15B).
10) Ki-67 immunostaining, a marker for cell proliferation, appears unchanged in PTPσ(-/-) mouse small bowel and colon
The crypts of the small bowel and colon are regions of high cell proliferation as intestinal
progenitor cells migrate toward the lumen along the crypt-villus axis. Several pathways are
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Figure 15: Lysozyme expression is reduced in the Paneth cells of the PTPσ(-/-) small bowel. A) Lysozyme immunostaining in the crypt regions of the small bowel of PTPσ(+/+) and PTPσ(-/-) mice. Paraffin-embedded small bowel tissues sections were obtained for PTPσ(-/-) mice and littermates controls and immunostained using lysozyme primary antibody. Staining was visualized using Alexa565 secondary antibody and imaged by confocal immunofluorescence microscopy. Staining intensity in the PTPσ(-/-) mouse small bowel appears reduced compared to wild-type littermates. Scale bar = 45μm. B) Quantification of lysozyme staining intensity in the PTPσ(+/+) and PTPσ(-/-) mice. Using the image processing software Velocity, areas presenting the base of the intestinal crypts were defined and the mean fluorescence intensity per unit was calculated. Statistical comparison by Student’s t-test reveals a statistically significant decrease in staining intensity in the PTPσ(-/-) mouse (n=95 crypts, Student’s t-test, p<0.05). Error bars represent mean +/- SE.
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Figure 15: Lysozyme expression is reduced in the Paneth cells of the PTPσ(-/-) small bowel
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Figure 14: Ki-67 immunostaining is similar between PTPσ(-/-) and PTPσ(+/+) mouse small bowel and colon. A) Ki-67 immunostaining in the crypt region of paraffin-embedded tissue sections from PTPσ(+/+) and PTPσ(-/-) mice small bowel reveals no difference in staining pattern or intensity. Scale bars = 90μm. B) Ki-67 immunostaining in the crypt regions of the colon of PTPσ(+/+) and PTPσ(-/-) shows no apparent difference. Scale bars = 90 μm.
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Figure 16: Ki-67 immunostaining, a marker for cell proliferation, appears unchanged in PTPσ(-/-) mouse small bowel and colon
68
implicated in modulating this proliferation including Wnt, Notch, and EGFR signaling. It is also
known that tyrosine phosphorylation is an important mediator in these pathways, and that
increased tyrosine phosphorylation is often associated with increased cell proliferation.
Furthermore, PTPσ is known to directly target key factors in these pathways, namely β -catenin
in Wnt signalling and EGFR. To determine if the observed phosphotyrosine staining pattern in
the PTPσ(-/-) mice is indicative of increased cell proliferation, immunostaining for the nuclear
proliferation marker Ki-67 was performed on small bowel and colon tissue sections from PTPσ(-
/-) mice and wild-type littermates (Figure 14). No difference in Ki-67 staining was observed
between PTPσ(+/+) and PTPσ(-/-) small bowel and colonic tissue sections. This would suggest
that PTPσ is not mediating cell proliferation in the intestinal crypts. Further analysis of cell
proliferative markers will be necessary to fully conclude that cell proliferation is unaffected.
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Chapter 4: Discussion
70
Role of PTPσ in intestinal epithelia
1) Evidence of defects in intestinal barrier integrity
The work presented here demonstrates a defect in the intestinal barrier integrity of the
PTPσ(-/-) mice. The reduced epithelial resistance (Figure 1A) and increased macromolecular
flux (Figure 1B) observed suggest a destabilization of the epithelial monolayer that forms the
barrier between the intestinal lumen and lamina propria. The presence of compromised barrier
integrity is further supported by the increased intestinal permeability revealed by in vivo tracer
studies (Figure 1C,D). Cell adhesion, mediated by the apical junction complex, is essential to
the barrier function of polarized epithelia[128]. Interestingly, the mass spectrometry analysis
(Table 1) identified numerous hyper-tyrosine phosphorylated proteins in the PTPσ(-/-) mouse
intestine with functions associated with the apical junction complex. These hits include proteins
that mediate intercellular junctioning (E-cadherin, β-catenin, Tjp-2), proteins which regulate
interactions with the actin cytoskeleton (villin, ezrin, gelsolin) and proteins which regulate
signaling pathways that modulate complex formation and remodeling (EGFR, ezrin). In the
context of the adherens junction specifically, 18 putative substrates were identified in our screen
that directly modulate junction formation and stability (Table 1; red highlight). Given that PTPσ
is known to localize to the adherens junctions[131], the data suggest that PTPσ may regulate
adherens junction proteins directly at sites of intercellular junctions. The observed defect in
permeability and the proteins identified in the MS screen support our hypothesis of PTPσ
functioning as a positive regulator of epithelial barrier integrity in the intestine through the
regulation of proteins associated with the apical junction complex.
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2) Role for E-cadherin and β-catenin
E-cadherin and β-catenin form the essential core of the adherens junction and were
previously validated as colonic PTPσ substrates by our group[13]. Dysregulation of E-cadherin
has been shown to decrease epithelial cell adhesion and increase paracellular permeability[162,
163]. Studies have also demonstrated that tyrosine phosphorylation mediates the cadherin-
catenin interactions and that enrichment of pTyr is associated with complex disassembly[136,
164]. Other PTPs, such as PTPµ[61], have been shown to modulate E-cadherin – β-catenin
binding at the adherens junction through regulation of these pTyr residues. The tandem MS
screen indicates increased tyrosine phosphorylation on E-cadherin and β-catenin in the PTPσ(-/-)
mouse intestine (Table 1). Interestingly, we observed increased tyrosine phosphorylation at the
plasma membrane in the PTPσ(-/-) mouse intestine compared to wild-type controls (Figure 3).
However, we did not observe a significant difference in E-cadherin- β-catenin colocalization at
the basolateral plasma membrane in the PTPσ(-/-) mouse intestine compared to wild-type
littermates (Figure 9). These results taken together could suggest several different scenarios.
PTPσ may regulate the stability of the adherens junction by targeting pTyr residues on E-
cadherin and β-catenin that modulate the cadherin-catenin interaction but not complex
internalization. This would alter cellular adhesion but would presumably not affect the
localization of E-cadherin and β-catenin to the plasma membrane. Further, immunofluorescence
microscopy may not be sensitive enough to observe complex destabilization which is evident
through functional analyses. In this scenario, loss of PTPσ regulation on E-cadherin- β-catenin
could explain the defective barrier phenotype observed in the PTPσ(-/-) mouse. However, in
contrast, it is possible that the permeability defect in the PTPσ(-/-) intestine is not related to the
adherens junction, as the colocalization data does indicate that E-cadherin and β-catenin are at
their appropriate subcellular location. The observed plasma membrane staining for pTyr in the
72
intestinal crypts may not be related to E-cadherin or β-catenin, but could represent other PTPσ
targets at the membrane. Given the multitude of AJC proteins identified in the MS screen, it is
also possible that PTPσ is influencing the function of the adherens junction through other
effectors besides E-cadherin and β-catenin.
It is important to note that these results assert that epithelial cell adhesion is not
completely abrogated by loss of PTPσ, which is clear given that total loss of AJC-mediated
adhesion would prevent epithelial layer formation, collapsing the overall morphology of the
colon and small bowel. Alternatively, it is possible that changes in epithelial permeability in the
PTPσ knockout intestines may not be the result of disrupted junctioning but are instead the result
of dysregulated paracellular trafficking, which is known to be mediated by the AJC.
The lacZ (Figure 8) and phosphotyrosine (Figure 9) immunostaining suggest that PTPσ is
expressed in cells within the lamina propria regions of the small bowel and colon. Lymphocytes
are known to be present in these regions, especially at sites of inflammation[165]. Importantly,
several T-cell secreted pro-inflammatory cytokines, including TNF-α[166, 167], IL-4[166, 168],
and IL-6[169], have been shown to modulate AJC stability which can influence the barrier
integrity of the intestine. It is possible that the disrupted barrier integrity observed in the PTPσ(-
/-) mouse may be the result of dysregulated pro-inflammatory cytokine secretion from these T-
cell populations. Further experimentation will be necessary to differentiate which effects are
immune-driven and which are adhesion-mediated.
3) Ezrin as a colonic PTPσ substrate
My results demonstrate that ezrin is a direct intestinal substrate of PTPσ (Figure 5), and
that ezrin localization is altered in the PTPσ(-/-) mouse following DSS-treatment (Figure 6). This
change in localization observed in the DSS-treated PTPσ(-/-) mouse small bowel suggests that
73
PTPσ regulates the pTyr phosphorylation status of ezrin, which is known to dictate plasma
membrane targeting. The presence of this mislocalization only during DSS-treatment does
suggest that other PTPs are likely also involved in the regulation of ezrin trafficking, given that
we still observe ezrin at the plasma membrane in the untreated mice. Tyrosine phosphorylation
is also essential to the function of ezrin as a scaffold protein. Several binding partners of ezrin,
including Rac1[143], Src[170], and p85[171] bind through SH2 domain-mediated interactions.
Once bound, these effectors can become activated through interactions with other proteins that
have also been recruited by ezrin, including guanine nucleotide exchange factors (GEFs) like
Dbl[172]. PTPσ could potentially regulate the activation of these effectors by targeting pTyr
residues that serve as SH2 binding sites, maintaining a dephosphorylated state that would prevent
recruitment of factors necessary to activate downstream signaling pathways. Interestingly,
several of these signaling pathways regulated by ezrin are important for cell adhesion. Rac1 is a
potent regulator of actin cytoskeletal dynamics and has been shown to regulate adherens junction
assembly through E-cadherin[143]. In addition, Src is a known mediator of cell adhesion and
adherens junction remodeling[170]. PTPσ may be indirectly modulating the stability of the
apical junction complex through regulating the activation of upstream effectors of epithelial cell
adhesion.
Proposed Mechanism: PTPσ regulation of adherens junction proteins We propose that PTPσ acts at the adherens junction to regulate the tyrosine
phosphorylation of AJC-associated proteins to maintain junction stability. Under normal
conditions (Figure 15A), PTPσ and associated RTKs work in opposition to modulate tyrosine
phosphorylation levels of AJC proteins according to the changing intestinal environment and the
needs of the cell. In the PTPσ(-/-) mouse (Figure 15B), loss of PTPσ regulation leads to an
74
accumulation of tyrosine phosphorylation on adherens junction proteins as the activity of the
RTKs are no longer counteracted by phosphatase activity. Key elements at the adherens
junction, including E-cadherin, β-catenin, and signal mediators such as ezrin, become hyper-
tyrosine phosphorylated, leading to a destabilization of the junction as cadherin-catenin
interactions dissociate and/or adherens junction remodeling pathways become dysregulated.
Consequently, as the intercellular interactions breakdown, paracellular permeability increases,
leading to the observed changes in TER found in the PTPσ(-/-) mice. This increased
permeability may lead to increased inflammation as pathogens in the intestinal lumen have
increased access to the lamina propria, causing an abnormal immune system response.
75
Figure 15: Proposed mechanism for PTPσ mediated regulation of the adherens junction. A) PTPσ localizes to the adherens junction and works in concert with RTKs to modulate the tyrosine phosphorylation of AJC proteins. B) In the PTPσ(-/-) mouse, PTPσ is not expressed therefore the activity of the RTK is not counteracted, leading to an accumulation of tyrosine phosphorylation of AJC associated proteins. Increased tyrosine phosphorylation destabilizes cadherin-catenin interactions, leading to complex disassembly. In addition, tyrosine phosphorylation leads to the up-regulation of adherens junction remodeling pathways through modulators such as ezrin increasing the instability of the junction. A combination of these effects leads to reduced intercellular adhesion, increasing paracellular permeability. p120 – p120-catenin; β-cat – β-catenin; α-cat – α-catenin; Vcl – vinculin; FMN – formin; orange helix – F-actin
76
Figure 17: Proposed mechanism for PTPσ regulation of adherens junction stability
A
B
77
Implications for PTPσ in Paneth cell function The X-gal staining pattern observed in the small bowel of the PTPσ(+/-) mouse (Figure
6E) demonstrates strong staining at the base of the crypts, a region known to contain secretory
Paneth cells. The reduced lysozyme immunostaining observed in the neonatal (Figure 12B) and
adult (Figure 13A) PTPσ(-/-) mice suggest that PTPσ may play a role in the regulation of Paneth
cell function. Paneth cells contribute to intestinal barrier defence through the secretion of anti-
microbial factors to mediate bacterial clearance. Paneth cell dysfunction has been associated
with inflammation and reduced microbial clearance. Interestingly, reduced bacterial clearance
and increased bacterial colonization in the intestinal crypts (following inoculation by C.
rodentium) has been observed in the PTPσ(-/-) mouse previously by our group[13]. PTPσ could
potentially regulate processes associated with Paneth cell lysozyme production or secretion.
Furthermore, autophagy is known to play an important role in mediating Paneth cell granule
formation, which is necessary for its secretory function. Given the recently identified role for
PTPσ in the regulation of autophagy, it may be possible that loss of PTPσ is affecting the
maturation of secretory granules in the PTPσ(-/-) Paneth cell. Although the underlying effect is
unknown, the evidence suggests that PTPσ may influence Paneth cell function, although it is a
role that will require further investigation.
PTPσ in IBD pathogenesis In the context of IBD pathogenesis, it is possible that the defective barrier integrity
observed in the PTPσ(-/-) mouse intestine may contribute to the development of intestinal
enteropathies by increasing the exposure of the lamina propria to luminal pathogens. A ‘leaky’
intestinal epithelium has been previously implicated in IBD[128, 173]. Interestingly, barrier
defects have been observed in IBD patients[174] but the underlying genetic basis for these
effects were not well characterized. Our work presented here suggests a novel function for PTPσ
78
as a positive regulator of epithelial barrier integrity, which may help to explain the underlying
genetic risk in IBD associated with defects in permeability.
Future Directions
1) Further characterize the intestinal barrier defect in the PTPσ(-/-) mouse
The barrier defect observed in the PTPσ(-/-) mouse requires further characterization to
understand the underlying defects causing the destabilization of the epithelial monolayer.
Biochemical analysis of binding affinities between E-cadherin and its various catenin adapters in
the PTPσ(-/-) mouse would provide more evidence for the direct regulation of the cadherin-
catenin complex by PTPσ. If the members of the complex have reduced binding affinities
correlated with increased tyrosine phosphorylation, it would support the hypothesis that PTPσ is
a positive regulator of junction stability. The role and function of the tight junctions has also not
been evaluated in the PTPσ(-/-) mouse. Tjp-2 (ZO-2), an important mediator of tight junction
anchorage to the actin cytoskeleton, was identified in the MS screen as well as other modulators
of TJ formation. Given that tight junctions play an important role in mediating paracellular
transport, it would be important to characterize the stability of this junction in the PTPσ(-/-)
mouse.
2) Investigate the intestinal ligands of PTPσ
To date, the intestinal ligands for PTPσ have not been characterized. In order to fully
understand the extracellular signals that modulate PTPσ phosphatase activity, it will be necessary
to characterize which ligands bind to the ectodomain in the intestine. In addition to determining
the intestinal ligands, binding experiments can be conducted with PTPσ ectodomain mutants that
mimic the alternatively spliced variant implicated in UC pathogenesis (PTPσΔmeB, ie
79
PTPσΔIg3). Binding affinities could be measured to determine if the splice variant alters ligand
binding or the ability of the receptor to dimerize. This information will contribute to our
understanding of how this splice variant leads to IBD pathogenesis.
3) Evaluate the effect of PTPσ regulation on ezrin function
The effect of PTPσ regulation on ezrin function is not clear. Preliminary evidence does
suggest that PTPσ regulation may regulate ezrin localization, however, this effect will need
further validation using mutational analysis of tyrosine residues implicated in ezrin function.
First, it will be necessary to determine which specific phosphotyrosine residues in ezrin are
targeted by PTPσ. Mass spectrometry can be performed on intestinal epithelial cells both naïve
and transfected with PTPσ-targeted siRNA. Comparison of the tyrosine phosphorylation profile
of ezrin under these different conditions will provide evidence as to which residues are targeted.
Once these residues have been determined, site-directed ezrin mutants, with tyrosine mutated to
phospho-mimetic (Glu) or phospho-ablated (Phe) residues, can be generated. These mutants
will provide deeper understanding of how tyrosine phosphorylation affects ezrin function and
how this, in turn, influences downstream signalling which could affect epithelial cell function.
As a corollary to the mutagenesis, the signalling of ezrin can also be evaluated through
analysis of the activation of downstream effectors molecules such as Src or Rac1. This will
determine if adherens junction remodeling pathways are being activated due to loss of signal
attenuation by PTPσ.
4) Validate and characterize other putative substrates identified in MS screen
Several other potent AJC effector proteins were identified in the MS screen and their
dysregulation could contribute to the phenotype observed in the PTPσ(-/-) mouse. To determine
which of these putative substrates are real in vivo PTPσ targets, substrate trapping and
80
dephosphorylation experiments need to be repeated. Validated interactors should be examined
using immunofluorescence microscopy and other functional experiments to determine if their
function is altered in the PTPσ(-/-) mouse. Identification of more intestinal substrates for PTPσ
will provide a clearer picture of the downstream regulatory targets of PTPσ and how these may
influence intestinal cell function.
5) Explore the role of PTPσ in other immune cells
Evidence from the lacZ staining identifies X-gal positive cells present in the lamina
propria region of the PTPσ(+/-) intestine. As this region is known to contain lymphocytes,
which mediate immune function, it is possible that PTPσ may play a role in adaptive immunity.
PTPσ expression in T-cell lineages has been previously described[175]. Defective T-cell
function has been linked to inflammation and to IBD specifically. Furthermore, specific
cytokines, namely IL-6[169] and IL-10[176], have been implicated in the regulation of epithelial
barrier defence through the modulation of barrier permeability[177] Evaluating the secretion of
pro-inflammatory cytokines by T-cells in the PTPσ(-/-) mouse intestine by ELISA and
characterizing the subsets of T-cell populations present in the mice by FACS sorting would help
to determine if PTPσ plays a role in modulating T-cell mediated immune function.
6) Explore the role of PTPσ in Paneth cell function
The reduced lysozyme immunostaining observed in the PTPσ(-/-) mouse suggests that
Paneth cell function may be influenced by loss of PTPσ. To determine if this effect is due to
direct PTPσ regulation, the function of Paneth cells can be evaluated in the PTPσ(-/-) mouse
using functional assays and immunofluorescence microscopy. Expression levels of key
antimicrobial factors can be evaluated by Western blotting, while the role of autophagy in the
81
Paneth cells can be characterized by identifying the presence of autophagic markers by
immunofluorescence.
7) Generate tissue specific, catalytically-inactive and ΔIg3 PTPσ mouse mutants
The PTPσ(-/-) mouse line used for the experiments outlined in this report is a whole-
tissue knockout. This system cannot separate which effects are intestinal-specific and which are
caused by dysfunctions in other organs influencing intestinal function through mediators such as
hormones or cytokines. Given the range of processes affected by loss of PTPσ function, the
phenotype observed in the mice may be the result of a combination of these effects. To discern
to what degree intestinal PTPσ function contributes to the observed colitis phenotype, a tissue-
specific knock-out mouse generated using the loxP-cre recombinase system crossed against an
intestinal-specific Villin-Cre mouse would isolate the knock-out to the intestine to better
understand the phenotype. In the context of IBD pathogenesis, the effect of the Ig3 deletion
observed in UC patients on the function of the PTPσ has not been characterized. In order to
determine if this alternative splicing affects PTPσ signaling, an ΔIg3 PTPσ knock-in mouse
model can be generated. These mice would be more representative of the putative phenotype
expected in the UC patients as this model would reflect the specific disease associated variant,
isolating the effects caused by the polymorphism from the phenotype associated with the
complete PTPσ knock-out.
Another in vivo model that would be informative would be to generate a catalytically
inactive PTPσ mutant (PTPσ-CI) by mutating the catalytically essential cysteine residue within
the full-length protein. This would help to differentiate which effects are caused by loss of the
CAM-like function of the ectodomain of PTPσ versus its intracellular phosphatase activity.
82
Summary The intestinal barrier integrity of the PTPσ(-/-) mouse was disrupted as observed through
both ex vivo and in vivo assays, suggesting that PTPσ plays a role in modulating barrier defence.
Tandem mass spectrometry identified thirty-seven hyper-tyrosine phosphorylated proteins in the
PTPσ(-/-) intestine, a large proportion of which are known to regulate the formation and
maintenance of the apical junction complex, a key mediator of cellular adhesion. One of the
identified hits, the cytosketelal linker protein ezrin, was demonstrated to be a direct PTPσ
substrate and its cellular localization was found to be disrupted in the PTPσ(-/-) mouse small
bowel under DSS conditions, suggesting it is an in vivo PTPσ substrate. Paneth cell function
also appeared to be altered in the PTPσ(-/-) small bowel. Taken together, these results suggest
that PTPσ regulates the tyrosine phosphorylation of apical junction complex proteins, which in
turn modulates the permeability of the epithelial monolayer.
Conclusion The impaired barrier function observed in the PTPσ(-/-) mouse implicates PTPσ as
a positive regulator of barrier integrity. The hyper-tyrosine phosphorylated proteins
identified in the mass spectrometry screen suggest that PTPσ may target apical junction
complex proteins. My results also suggest that PTPσ may play a role in Paneth cell
function.
83
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