Whole-tissue 3D imaging reveals intra-adipose sympathetic ... · exposure (Vitali et al., 2012). However, the physiological relevance, together with the regulatory mechanism, under-lying
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RESEARCH ARTICLE
Whole-tissue 3D imaging reveals intra-adiposesympathetic plasticity regulated by NGF-TrkAsignal in cold-induced beiging
Ying Cao1,4, Huanhuan Wang5, Wenwen Zeng1,2,3&
1 Center for Life Sciences, Tsinghua University, Beijing 100084, China2 Institute for Immunology and School of Medicine, Tsinghua University, Beijing 100084, China3 Beijing Key Laboratory for Immunological Research on Chronic Diseases, Beijing 100084, China4 Academy for Advanced Interdisciplinary Studies, Peking University, Beijing 100871, China5 School of Life Sciences, Peking University, Beijing 100871, China& Correspondence: [email protected] (W. Zeng)
Received March 3, 2018 Accepted March 12, 2018
ABSTRACT
Sympathetic arborizations act as the essential efferentsignals in regulating the metabolism of peripheralorgans including white adipose tissues (WAT). However,whether these local neural structures would be of plasticnature, and how such plasticity might participate inspecific metabolic events of WAT, remains largelyuncharacterized. In this study, we exploit the new vol-ume fluorescence-imaging technique to observe thesignificant, and also reversible, plasticity of intra-adi-pose sympathetic arborizations in mouse inguinal WATin response to cold challenge. We demonstrate that thissympathetic plasticity depends on the cold-elicited sig-nal of nerve growth factor (NGF) and TrkA receptor.Blockage of NGF or TrkA signaling suppresses intra-adipose sympathetic plasticity, and moreover, the cold-induced beiging process of WAT. Furthermore, we showthat NGF expression in WAT depends on the cate-cholamine signal in cold challenge. We therefore revealthe key physiological relevance, together with the reg-ulatory mechanism, of intra-adipose sympathetic plas-ticity in the WAT metabolism.
KEYWORDS sympathetic plasticity, NGF, TrkA receptor,cold-induced beiging, whole-tissue 3D imaging
INTRODUCTION
The central nervous system exerts the indispensable controlover maintenance of the metabolic homeostasis. Brain cir-cuits, together with their distinct neuronal populations,underlying such neural regulation have been subject toextensive research. It has now been broadly accepted thatmalfunction of the neural regulation of metabolism could leadto obesity, type 2 diabetes and other profound metabolicdisorders (Friedman and Halaas, 1998; Gautron et al., 2015;Myers and Olson, 2012).
As one of the emerging focuses in the field, studies havebegun to investigate how efferent signals from the centralnervous system reach out to regulate the metabolism ofperipheral organs, e.g., white adipose tissues (WAT). WATare known as the key energy-storage depots as well as theimportant hormone-producing organ, whose essential rolesin the energy homeostasis are widely recognized. Anatomi-cal distribution and physiological function of local neuralinputs, particularly sympathetic inputs, in WAT have beenexplored to understand the mechanism of neural regulationof the WAT metabolism (Bamshad et al., 1998; Nguyen et al.,2014; Youngstrom and Bartness, 1995, 1998; Zeng et al.,2015). For instance, sympathetic fibers can form synapse-like structures onto adipocytes in WAT, and destruction ofthese sympathetic innervations inhibited the leptin-stimu-lated lipolysis of WAT (Zeng et al., 2015). In addition, accu-mulating research efforts have revealed that WAT canundergo the beiging (or browning) process, i.e., appearanceof Ucp1-positive multilocular adipocytes (or beige cells),under certain physiological conditions such as cold
Electronic supplementary material The online version of thisarticle (https://doi.org/10.1007/s13238-018-0528-5) contains sup-
plementary material, which is available to authorized users.
exposure. This metabolic event of WAT results in enhancedenergy expenditure, and therefore, has increasingly gainedattentions for its potential application in therapeutic preven-tion and treatment of obesity and type 2 diabetes (Giordanoet al., 2016; Harms and Seale, 2013; Kajimura et al., 2015;Peirce et al., 2014; Rosen and Spiegelman, 2014). Adding tothe key function of local sympathetic inputs in the WATmetabolism, it has been recently reported that the densenetwork of intra-adipose sympathetic arborizations in WAT isrequired for the cold-induced beiging process (Jiang et al.,2017).
Despite those important progresses, our understanding ofneural regulation of the WAT metabolism still remainsincomplete. In particular, sympathetic arborizations in theperipheral organs have been generally viewed as beingstatic structures under physiological condition. Whether localsympathetic arborizations are in fact of the dynamic nature, i.e., plasticity, in response to certain metabolic stimuli is lar-gely uncharacterized. A previous report showed with theconventional immunohistochemistry that numbers of thesympathetic fibers in WAT would increase following coldexposure (Vitali et al., 2012). However, the physiologicalrelevance, together with the regulatory mechanism, under-lying potential intra-adipose sympathetic plasticity isunknown.
In this report, we exploit the new volume fluorescence-imaging technique to observe the significant, and alsoreversible, plasticity of sympathetic arborizations in mouseinguinal WAT in response to cold challenge. We demonstratethat this sympathetic plasticity is regulated by the cold-eli-cited signal of nerve growth factor (NGF) and TrkA receptor.Blockage of NGF-TrkA signaling suppresses intra-adiposesympathetic plasticity, and importantly, inhibits the cold-in-duced beiging process of WAT. We further show that cold-elicited NGF expression in WAT depends on the cate-cholamine signal during cold challenge. This study hastherefore documented the key physiological function as wellas the molecular mechanism of local sympathetic plasticity inWAT. These findings would provide important insights to ourunderstanding of neural control of the peripheral metabolismunder physiological and disease conditions.
RESULTS
Plasticity of intra-adipose sympathetic arborizationsin response to cold challenge
We set out to examine the sympathetic arborizations inmouse inguinal WAT (iWAT) in response to cold challenge(4 °C). We exploited the new volume fluorescence-imagingtechnique that we have recently established for adiposetissues, which enables us to easily visualize and quantifyintra-adipose neural fibers on the whole-tissue level (Jianget al., 2017). iWAT was processed through the whole-mountimmunolabeling of intended epitopes, and then subjected tothe optical clearing procedure for imaging on the lightsheet
microscope. We firstly assessed density of the total neuralfibers in iWAT of the wildtype mice by the volume fluores-cence-imaging of anti-synaptophysin, an integral protein onpre-synaptic vesicles and a pan-marker for neural structures.The significant increase of total neural fibers in iWAT couldbe observed as early as 3 days after cold challenge, andtheir density reached a plateau between 7 to 10 days(Fig. 1A and 1D). We have recently shown that the neuralfibers in iWAT are predominantly sympathetic (Jiang et al.,2017). To determine whether such cold-induced increase ofneural density in iWAT would reflect the plastic change ofsympathetic arborizations, iWAT was also visualized by thevolume fluorescence-imaging of anti-tyrosine hydroxylase, aspecific marker for sympathetic fibers. Indeed, density of thesympathetic fibers showed the significant increase as earlyas 3 days after cold challenge (Fig. 1B and 1E, and MoviesS1 and S2). Importantly, this plastic change of sympatheticarborizations in iWAT correlated with up-regulation of thebeiging-related genes including Ucp1, Dio2, Cidea andPgc1a (Fig. S1A). Of note, there was no significant alterationof the tissue size, as well as the general anatomy of sym-pathetic innervations, of the cold-challenged iWAT as visu-alized by the volume fluorescence-imaging at lowmagnification (Fig. 1C).
We next explored whether this cold-induced plasticity ofsympathetic arborizations in iWAT would be reversible. Thewildtype mice were subject to cold challenge for 7 days, andthen recovered at thermal-neutral condition (32 °C) for up to6 weeks. Density of the sympathetic arborizations in iWATunderwent the significant decrease around 2 weeks underthermal-neutral condition, and reached the baseline level at4 weeks (Fig. 2A and 2B), as assessed by the volume flu-orescence-imaging of anti-tyrosine hydroxylase. Interest-ingly, the sympathetic density could further decline to about50% of the baseline level at 6 weeks under thermal-neutralcondition (Fig. 2A and 2B). In addition, such reversal of intra-adipose sympathetic arborizations correlated with down-regulation of the beiging-related genes Ucp1, Dio2, Cideaand Pgc1a (Fig. 2C). These results have together
Figure 1. Intra-adipose sympathetic plasticity in
response to cold challenge. (A–E) The wildtype mice
were subject to cold challenge. (A) Representative 3D
projections of iWAT harvested at indicated time points,
immunolabeled by anti-synaptophysin, and imaged at
12.6× magnification on the lightsheet microscope. (B and
C) Representative 3D projections of iWAT harvested at
indicated time points, immunolabeled by anti-tyrosine
hydroxylase, and imaged at 12.6× (B) or 1.26× (C) mag-
nification on the lightsheet microscope. (D) Density of the
neural fibers in iWAT immunolabeled by anti-synapto-
physin was quantified. n = 4, mean ± SEM. (E) Density of
the sympathetic fibers in iWAT immunolabeled by anti-
tyrosine hydroxylase was quantified. n = 5, mean ± SEM.
demonstrated that intra-adipose sympathetic arborizationshave the significant, and also reversible, plasticity inresponse to cold challenge, which might be involved in thecold-induced beiging process of WAT.
Intra-adipose sympathetic plasticity regulatedby the NGF-TrkA signal is required for cold-inducedbeiging
We then sought out to elucidate the physiological relevance,together with the regulatory mechanism, of intra-adiposesympathetic plasticity in response to cold challenge. Thevolume fluorescence-imaging revealed that there was thesignificant up-regulation of STMN2 expression (also knownas SCG10), a specific marker for axon outgrowth (Gren-ningloh et al., 2004; Mason et al., 2002), in iWAT after coldchallenge (Fig. 3A and 3B). In addition, we recently showedthat the celiac ganglia contribute to the sympathetic inner-vations in mouse iWAT (Jiang et al., 2017). Accordingly,STMN2-positive sympathetic neurons in the celiac gangliaalso dramatically increased after cold challenge (Fig. 3C).
These results have suggested that an active process ofsympathetic axon outgrowth is promoted in the cold-chal-lenged iWAT.
Axon outgrowth has been known to be under regulation ofthe neurotrophic factors (or neurotrophins) (Bothwell, 1995;Raffioni et al., 1993; Zweifel et al., 2005). We thereforeprofiled expression levels of the neurotrophin genes in iWATof the wildtype mice exposed to cold challenge, andobserved that NGF expression showed the significant up-regulation as early as 24 h after cold challenge (Fig. 3D). Incontrast, expression levels of other neurotrophin genesBDNF, NT3, and NT4 exhibited no increase in iWAT inresponse to cold challenge (Fig. 3D). Also, it is intriguing tonote that expression levels of NGF decreased significantly iniWAT of the wildtype mice shifted from room temperature to32 °C (Fig. 3E), which appeared correlating with the declineddensity of sympathetic arborizations in iWAT under thisthermal-neutral condition (Fig. 2A and 2B).
As one of the central components in neural development,genetic deletion of NGF results in severe developmentaldefects of the nervous systems, including the sympathetic
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Figure 2. Cold-induced intra-adipose sympathetic plasticity is reversible. The wildtype mice were subject to cold challenge, and
then recovered at thermal-neutral condition (32 °C). (A) Representative 3D projections of iWAT harvested at indicated time points,
immunolabeled by anti-tyrosine hydroxylase, and imaged at 12.6× magnification on the lightsheet microscope. (B) Density of the
sympathetic fibers in iWATwas quantified. n = 5, mean ± SEM. (C) Expression levels of the beiging-related genes were determined at
indicated time points by the qPCR analysis. n = 5, mean ± SEM.
system (Crowley et al., 1994). To explore the potentialfunction of NGF in intra-adipose sympathetic plasticity with-out the developmental complications, we set out to acutelyblock the NGF signal by NGF neutralization. NGF-neutral-izing antibody could strongly suppress the NGF-stimulatedaxon outgrowth of cultured sympathetic neurons of thesuperior cervical ganglia (SCG), demonstrating the efficacyof this neutralization strategy (Fig. 4A). The wildtype micewere intravenously administrated with NGF-neutralizingantibody (i.e., NGF-blocked mice) or isotype control IgG. Thevolume fluorescence-imaging revealed no
detectable alteration of the tissue size and the generalanatomy of sympathetic innervations (Fig. S2A), as well asdensity of the sympathetic arborizations (Fig. 4B and 4C), ofiWAT of the NGF-blocked mice vs. control mice maintainedat room temperature for up to 1 week. However, while den-sity of the sympathetic arborizations increased significantlyin control mice after cold challenge, such intra-adiposesympathetic plasticity was largely inhibited in the NGF-blocked mice (Fig. 4B and 4C). More importantly, this inhi-bition of sympathetic plasticity led to the diminished beigingprocess of iWAT, as assessed by the histochemical
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Figure 3. Cold-elicited NGF expression for intra-adipose sympathetic plasticity. (A–D) The wildtype mice were subject to cold
challenge. (A and B) iWATwas processed for the whole-mount immunolabeling of STMN2. (A) Representative 3D projections of iWAT
imaged at 12.6× magnification on the lightsheet microscope. (B) Density of the STMN2-positive neural fibers was quantified. n = 4,
mean ± SEM, *P < 0.01. (C) STMN2 expression in sympathetic neurons of the celiac ganglia was examined by conventional
immunostaining. (D) Expression levels of the neurotrophin genes in iWAT were determined by the qPCR analysis. n = 4,
mean ± SEM, *P < 0.01. (E) The wildtype mice were subject to thermal-neutral condition (32 °C), and expression levels of NGF in
iWAT were determined by the qPCR analysis. n = 4, mean ± SEM, *P < 0.01.
Intra-adipose sympathetic plasticity in cold-induced beiging RESEARCH ARTICLE
examination of multilocular beige cells (Fig. 4D) and theqPCR analysis of beiging-related genes Ucp1, Dio2, Cideaand Pgc1a (Fig. 4E). These results have demonstrated thatNGF regulates intra-adipose sympathetic plasticity inresponse to cold challenge, which is critical for the cold-induced beiging process.
Sympathetic neurons in the celiac ganglia that innervateiWAT predominantly express TrkA, the high-affinity receptor
for NGF (Bothwell, 1995; Raffioni et al., 1993; Zweifel et al.,2005). Consistent with the up-regulated NGF expression iniWAT after cold challenge, significant activation of the TrkAsignal was observed in sympathetic neurons of the celiacganglia, as assessed by p-TrkA immunostaining (Fig. 5A and5B). Of note, p-TrkA exhibited the vesicular localization inneuronal cell bodies (Fig. 5A), reflecting the retrogradevesicular transport of TrkA receptor (Zweifel et al., 2005).
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Figure 4. Intra-adipose sympathetic plasticity regulated by NGF is essential for cold-induced beiging process. (A) Repre-
sentative images of cultured sympathetic SCG neurons treated with NGF-neutralizing antibody or control IgG. (B–E) The wildtype
mice administrated with NGF-neutralizing antibody or control IgG were subject to cold challenge. (B) iWAT was processed for the
volume fluorescence-imaging of anti-tyrosine hydroxylase. Representative 3D projections of iWAT imaged at 12.6× magnification on
the lightsheet microscope. (C) Density of the sympathetic fibers in iWAT was quantified. n = 5, mean ± SEM, *P < 0.01.
(D) Appearance of multilocular beige cells in iWATwas examined by H&E staining. (E) Expression levels of the beiging-related genes
in iWAT were determined by the qPCR analysis. n = 5, mean ± SEM, *P < 0.01.
Previous studies have documented the essential roles ofTrkA receptor in neural development, and genetic deletion ofTrkA severely impairs establishment of the sympatheticsystem (Smeyne et al., 1994). Therefore, to investigate theTrkA signal in intra-adipose sympathetic plasticity withoutperturbing the normal sympathetic development, we exploi-ted the chemical-genetic approach of TrkAF592A/F592A mice.In this knock-in mouse line, Phe592 residue at the ATP-binding pocket of TrkA has been replaced by Ala, enablingthe efficient inhibition of TrkAF592A activity by the chemical1-NaPP1, which otherwise has little effect on the wildtypeversion of TrkA protein (Chen et al., 2005). TrkAF592A/F592A
mice were treated daily with 1-NaPP1 or vehicle control. Thevolume fluorescence-imaging revealed no significant alter-ation of the tissue size and the general anatomy of sympa-thetic innervations (Fig. S3A), or density of the sympatheticarborizations (Fig. 5E and 5F), of iWAT of the 1-NaPP1-treated vs. vehicle-treated TrkAF592A/F592A mice maintainedat room temperature for up to 1 week. However, when themice were subject to cold challenge, axon outgrowthresponse of sympathetic neurons in the celiac ganglia wassignificantly inhibited by 1-NaPP1, as assessed by anti-STMN2 immunostaining (Fig. 5C and 5D). Accordingly, thesympathetic plasticity in iWAT was strongly suppressed inthe 1-NaPP1-treated TrkAF592A/F592A mice, compared to thatnormally occurring in the vehicle-treated TrkAF592A/F592A
mice (Fig. 5E and 5F). Moreover, such inhibition of intra-adipose sympathetic plasticity suppressed the beiging pro-cess of iWAT in the 1-NaPP1-treated TrkAF592A/F592A mice,as determined by the histochemical examination of multi-locular beige cells (Fig. 5G) and the qPCR analysis ofbeiging-related genes (Fig. 5H). Of importance, 1-NaPP1treatment of the cold-challenged wildtype mice had no effecton the sympathetic plasticity in iWAT (Fig. S3B), appearanceof multilocular beige cells (Fig. S3C), or expression of thebeiging-related genes (Fig. S3D), confirming the exquisitespecificity of this chemical-genetic strategy. Taken together,the results have shown that intra-adipose sympatheticplasticity regulated by the NGF-TrkA signal has the keyinvolvement in the cold-induced beiging process.
The catecholamine signal regulates NGF expressionfor intra-adipose sympathetic plasticity
We next examined the mechanism regulating NGF expres-sion in response to cold challenge. Studies in the field havedocumented the central role of catecholamines (i.e., epi-nephrine and norepinephrine) in the beiging process. Inparticular, genetic deletion of key enzymes in the cate-cholamine synthesis or β-adrenergic receptors for cate-cholamines could strongly suppress the cold-inducedbeiging process (Barbatelli et al., 2010; Fischer et al., 2017;Jiang et al., 2017; Jimenez et al., 2003; Qiu et al., 2014). Wetherefore explored whether the catecholamine signal wouldbe involved in the cold-elicited NGF expression. We
observed that treatment with norepinephrine (NE) efficientlyincreased the expression levels of NGF in iWAT of thewildtype mice maintained at room temperature (Fig. 6A).Moreover, the in vitro treatment of acutely-dissected iWAT ofcontrol mice (i.e., Adrb1+/−; Adrb2+/−; Adrb3+/+) with NEenhanced the NGF expression levels, but such effect wasabolished in iWAT of Adrb1−/−; Adrb2−/−; Adrb3−/− mice(Fig. 6B), in which all the three genes of β-adrenergicreceptors are deleted. To examine whether NE-treatmentwould stimulate the production of functional NGF protein byiWAT, we tested the conditioned media of in vitro treatediWAT on cultured sympathetic neurons. Indeed, the condi-tioned media of NE-treated iWAT of Adrb1+/−; Adrb2+/−;Adrb3+/+ control mice, but not Adrb1−/−; Adrb2−/−; Adrb3−/−
mice, exhibited the increased ability to promote the sympa-thetic axon outgrowth in vitro (Fig. 6C and Fig. S4A).
To further determine the regulatory function of cate-cholamine signal in intra-adipose sympathetic plasticity,Adrb1−/−; Adrb2−/−; Adrb3−/− mice were examined in thecold-challenge condition. Supporting the in vitro observation,the cold-elicited NGF up-regulation was absent in iWAT ofAdrb1−/−; Adrb2−/−; Adrb3−/− mice, compared to that normallyoccurring in iWAT of Adrb1+/−; Adrb2+/−; Adrb3+/+ controlmice (Fig. 6E). More importantly, in line with this bluntedNGF response, density of the sympathetic arborizationsremained unchanged in Adrb1−/−; Adrb2−/−; Adrb3−/− miceafter cold challenge as assessed by the volume fluores-cence-imaging of anti-tyrosine hydroxylase (Fig. 6D and 6F),suggesting the complete loss of sympathetic plasticity withthis simultaneous deletion of β-adrenergic receptors. Theresults have together revealed that the catecholamine signalregulates NGF expression for intra-adipose sympatheticplasticity in response to cold challenge.
DISCUSSION
In summary, aided by the new volume fluorescence-imagingtechnique, our study has revealed that intra-adipose sym-pathetic plasticity, regulated by the cold-elicited NGF-TrkAsignal, exerts a key role in the beiging process of WAT. Toour knowledge, this research work represents among thefirst examples demonstrating the physiological relevance,together with the molecular mechanism, of local sympatheticplasticity in neural regulation of the peripheral metabolism.
Intriguingly, the catecholamine signal derived from thelocal sympathetic arborizations in WAT promotes NGFexpression in response to cold challenge. This cold-elicitedNGF in turn stimulates the sympathetic axon outgrowth. Thisplastic change appears to have evolved as a positive-feed-back mechanism of locally enhancing the sympatheticefferent outputs to ensure effectiveness of the beiging pro-cess (Fig. 6G). Our findings have therefore implicated anadditional layer of neural regulation in the WAT metabolism,which involves the crosstalk between the neural-metabolicsystems. However, the cellular source(s) of NGF protein inWAT remains to be determined, and the volume
Intra-adipose sympathetic plasticity in cold-induced beiging RESEARCH ARTICLE
fluorescence-imaging technique would serve to provide theimportant clues. In addition, the molecular mechanismunderlying the catecholamine-stimulated NGF expressionrequires further detailed characterization, which likelyengages the protein kinase A (PKA) signal downstream of β-adrenergic receptors. The future research efforts have beenwarranted to explore these important questions.
Our current work has been focused on the sympatheticplasticity in mouse inguinal WAT. Importantly, a recent studyin the field has systematically characterized the adiposetissues in mice, revealing the heterogeneity of beigingcapacity among different depots (Zhang et al., 2018). Thevolume fluorescence-imaging technique is readily applicableto WATand BAT (Jiang et al., 2017), which makes it possibleto explore whether the NGF/TrkA-dictated sympatheticplasticity might also occur in other adipose depots, e.g.,epididymal WAT or BAT, in response to cold challenge. Ofnote, our findings that density of the sympathetic arboriza-tions increased in the cold-challenged iWAT are in accor-dance with the previous study done with the conventionalimmunohistochemistry (Vitali et al., 2012). However, ourresults are in disagreement with a recent report suggestingthat the sympathetic density remained unchanged in iWATafter cold challenge (Chi et al., 2018). Such discrepancymight be due to the different procedure of whole-mountimmunostaining used in this recent report (Chi et al., 2018).
This current study would provide new insights to neuralcontrol of the peripheral metabolism not only under physio-logical condition but also in metabolic disease. In fact, ourprevious work has reported that density of the sympatheticarborizations in WAT dramatically decreased in the obesecondition, e.g., in the high-fat diet-fed mice or ob/ob mice(Jiang et al., 2017). Whether this phenomenon could reflectdys-regulation of intra-adipose sympathetic plasticity, andwhether impairment of the NGF-TrkA signal would be cau-sative, then needs to be determined. Conversely, harnessingthis sympathetic plasticity to regenerate the intra-adiposearborizations afflicted in metabolic disorders might provide anovel entry point for therapeutic strategy to restore the localsympathetic efferent outputs, and as the result, the metabolichomeostasis of WAT.
MATERIALS AND METHODS
Animal information
All the experimental procedures in mice were performed in compli-
ance with the protocol approved by the Institutional Animal Care and
Use Committee (IACUC) of Tsinghua University.
Animals were maintained on the 12-h light/12-h dark cycles with
the chow diet and water available ad libitum. Mice utilized in the
experiments were females at the age of 2 to 4 months. Wildtype
C57BL/6 mice were purchased from the Charles River International.