Macroautophagy as a Pathomechanism in Sporadic Inclusion Body Myositis

Macroautophagy as a Pathomechanism in Sporadic Inclusion Body MyositisFull Terms & Conditions of access and use can be found at https://www.tandfonline.com/action/journalInformation?journalCode=kaup20
Autophagy
Macroautophagy as a Pathomechanism in Sporadic Inclusion Body Myositis
Jan D. Lünemann, Jens C. Schmidt, Marinos C. Dalakas & Christian Münz
To cite this article: Jan D. Lünemann, Jens C. Schmidt, Marinos C. Dalakas & Christian Münz (2007) Macroautophagy as a Pathomechanism in Sporadic Inclusion Body Myositis, Autophagy, 3:4, 384-386, DOI: 10.4161/auto.4245
To link to this article: https://doi.org/10.4161/auto.4245
Published online: 13 Apr 2007.
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Addendum
[Autophagy 3:4, 384-386; July/August 2007]; ©2007 Landes Bioscience
Jan D. Lünemann1
Christian Münz1,* 1Laboratory of Viral Immunobiology; Christopher H. Browne Center for Immunology and Immune Diseases; The Rockefeller University; New York, New York USA
2Muscle Immunobiology Group; Department of Neuroimmunology and Department of Neurology; University of Göttingen, Göttingen, Germany
3Neuromuscular Diseases Section; National Institute of Neurological Disorders and Stroke; National Institutes of Health; Bethesda, Maryland USA
*Correspondence to: Christian Münz; Laboratory of Viral Immunobiology; Christopher H. Browne Center for Immunology and Immune Diseases; The Rockefeller University; Box 390; 1230 York Avenue; New York, New York 10021 USA; Tel.: +1.327.7611. Fax: +1.327.7887; Email: [email protected]
Original manuscript submitted: 03/20/07 Manuscript accepted: 04/09/07
Previously published online as an Autophagy E-publication: http://www.landesbioscience.com/journals/autophagy/abstract.php?id=4245
Key worDS
Addendum to:
β-Amyloid is a Substrate of Autophagy in Sporadic Inclusion Body Myositis
J.D. Lünemann, J. Schmidt, D. Schmid, K. Barthel, A. Wrede, M.C. Dalakas and C. Münz
Ann Neurol 2007; 61:476-83
AbStrACt Skeletal muscle fibers show a high level of constitutive and starvation-induced macro-
autophagy. Sporadic Inclusion Body Myositis (sIBM) is the most common acquired skeletal muscle disease in patients above the age of 50 years and is characterized by inflammation and intracellular accumulation of aggregate-prone proteins such as amyloid precursor protein (APP)/β-amyloid, hyperphosphorylated tau, and presenilin. In a recent study, we found that muscle fibers of sIBM patients show increased frequencies of Atg8/ LC3+ autophagosomes and that intracellular APP/β-amyloid colocalized with Atg8/LC3 in degenerating fibers. Colocalization of APP/β-amyloid with LC3+ autophagosomes was further associated with upregulation of major histocompatibility complex (MHC) class I and class II molecules and T cell infiltration. These findings indicate that APP/β-amyloid is a substrate for autophagy in skeletal muscle fibers and suggest that degradation of aggregate-prone proteins via macroautophagy can be linked with both immune-mediated and degenerative tissue damage. A better understanding of this pathway in skeletal muscle and in the inflammatory environment of sIBM might provide a rationale for novel therapeutic strategies targeting pathogenic protein aggregation.
Sporadic inclusion body myositis (sIBM) is the most commonly acquired myopathy in patients above the age of 50 years. Its prevalence is estimated between 4.3 and 9.3 per 1,000,000, rising to 35.3 per 1,000,000 for people 50 years or older. Men are 2 to 3 times more often affected than women.1 The first signs and symptoms of the disease include gait instabilities owing to quadriceps muscle weakness as well as difficulties performing certain manipulative tasks such as turning keys or tying knots, due to impairment of the finger flexors. During the further course of the disease, weakness and muscle atrophy increases and affects other skeletal muscles causing a slowly but progressively restricted mobility of the patient. There is no effective treatment for sIBM available and most patients require assistive devices such as a cane, walker, or a wheelchair within 5 to 10 years of onset.
As a mainstay of the disease pathology, aggregate-prone proteins such as β-amyloid as well as hyperphosphorylated tau and presenilin are overexpressed and accumulate in muscle fibers of sIBM patients.2 The frequency of overexpression of these proteins and their association with the formation of inclusion bodies and vacuoles in myofibers suggests that the accumulation of aggregate-prone proteins is a central component of the degeneration of skeletal muscle fibers. In line with this assumption, transgenic mice with overexpression of APP in skeletal muscle develop extensive weakness and atrophy with a histopathology that resembles the vacuolar degeneration seen in sIBM.3
Accumulation of aggregate-prone proteins is also considered a central mechanism in the pathogenesis of various neurodegenerative diseases of the central nervous system (CNS) and morphological evidence of autophagy has been reported in some of them, including Alzheimer’s disease. In addition, in animals lacking essential autophagy genes, accumulation of diffusely ubiquitinated proteins as well as inclusion bodies are observed in neuronal cells, which leads to spontaneous neurodegenerative disease phenotypes.4,5 These studies indicated that, in addition to the ubiquitin-proteasome system, autophagy constitutively contributes to the degradation of cytosolic proteins and, thereby, protects cells from intracellular protein aggregation.5
In vivo analysis of autophagy in transgenic mice expressing a fluorescent autophago- some marker, i.e., GFP-Atg8/LC3, shows that regulation of autophagy is organ-dependent and that some tissues constitutively produce Atg8/LC3+ autophagosomes even in the absence of nutrient starvation.6 Such constitutive autophagosomal activity has been observed in metabolically active tissues such as liver, skeletal muscle as well as thymus. The main tissue subtypes within the skeletal muscle compartment are slow/type I- and
www.landesbioscience.com Autophagy 385
Macroautophagy in sIBM
fast/type II-twitching muscle fibers. In the LC3-reporter mice, autophagosomes were predominantly found in type II fibers.6
The finding that aggregations of aberrant proteins are susceptible to autophagic degradation in neurodegenerative diseases of the CNS and the observation that macroautophagy is a constitutively active process in muscle tissue, led us to investigate the role of autophagy in skeletal muscle as a possible pathomechanism operative in sIBM.7
First, we stably transfected a human muscle cell line (CCL136) with a GFP-Atg8/LC3 fusion construct in order to visualize autophagosomes by fluorescence microscopy. As expected, GFP-Atg8/LC3 strongly accumulated in cytosolic vesicles after treatment with the lysosomal acidification inhibitor chloroquine (CQ), suggesting that large numbers of GFP-Atg8/LC3-labeled autophagosomes had formed and fused with lysosomes. Next, we analyzed the degradation behavior of endogenous APP in these cells. After CQ-treatment, APP strongly accumulated in perinuclear vesicles and frequently colocalized with GFP-Atg8/LC3 in large autophagosomal structures, indicating that APP is at least partially degraded via lysosomal proteolysis after autophagy.
Extending our findings to human skeletal muscle, biopsies from patients with sIBM, myopathies with or without vacuoles and nonmyopathic muscle were analyzed for the presence of Atg8/LC3-positive autophagosomes. By confocal microscopy, we could observe accumulation of large Atg8/LC3-positive vesicular structures in degenerating muscle fibers. Most of these contained β-amyloid and most β-amyloid-positive fibers were double-positive for Atg8/LC3. In line with a previous study, we found that nearly all degenerating and β-amyloid- containing muscle fibers in sIBM were fast-twitching type II fibers and only these displayed APP/β-amyloid and Atg8/LC3 double-positive compartments.3 Colocalization was not observed in other vacuolated and nonvacuolated controls, indicating that, after transport by autophagosomes, a substantial proportion of this APP/ β-amyloid is targeted for lysosomal degradation in sIBM.
We have previously shown that autophagosomes continuously fuse with multivesicular MHC class II-loading compartments and that antigens targeted into the autophagy pathway are presented to CD4+ T cells via MHC class II.8,9 In sIBM, we found that a subset of degenerating and β-amyloid-positive muscle fibers expressed MHC class II and were surrounded by infiltrating CD4+ and CD8+ T cells. These observations suggest that upregulated autophagy in degener- ating muscle fibers might even contribute to antigen processing for MHC class II presentation to CD4+ T cells in the proinflammatory environment of sIBM.
What do these findings have in common and how are they different from the observations in neurodegenerative disorders? In Alzheimer’s disease (AD), for which sIBM is considered to be a “skeletal muscle counterpart”, LC3/Atg8-positive autophagosomes and prelysosomal late autophagosomes accumulate in dystrophic neuronal dendrites and are a major reservoir of intracellular β-amyloid.10,11 Inducing or inhibiting macroautophagy in neuronal and nonneuronal cells by modulating the mammalian target of rapamycin (mTOR) kinase elicited parallel changes in autophagosome and β-amyloid levels and indicated that the autophagic degradation pathway is dysfunc- tional in AD and that aberrant autophagic degradation without efficient breakdown of autophagic cargo in lysosomes contributes to the accumulation of intra- and extracellular β-amyloid plaques.11 These observations differ from what has been reported in animal
models of Huntington’s disease, in which induction of autophagy via inhibition of mTOR by rapamycin attenuated huntingtin accumula- tion and beneficially influenced the clinical phenotype.12 Similar mechanisms as in neurodegeneration may well be crucial events during the pathogenesis of sIBM, and could either enhance or prevent accumulation of β-amyloid and its degradation (Fig. 1, parts 1 and 2). A more detailed analysis of the autophagic pathway and, in particular, the maturation process of autophagosomes in sIBM would be necessary to address these important questions.
Different from all neurodegenerative diseases of the CNS, sIBM is considered an immune-mediated disease of the skeletal muscle. In the steady state, muscle cells express only low levels of MHC anti- gens. In sIBM, MHC class I is ubiquitously upregulated on muscle fibers, even in the absence of evidence of morphological changes. MHC class II is expressed at much lower levels and is more confined to myofibers in areas of inflammation. Accordingly, cellular infiltrates predominantly consist of CD8+, and to a lesser extent of CD4+ T cells, and affected muscle fibers produce several proinflammatory mediators including cytokines such as IL-1β and chemokines such as CXCL-9 and CCL-3/-4.13,14 Whether the degenerative or the immune-mediated process predominates in the pathogenesis of sIBM, whether they occur in parallel and whether and to which extent both processes are linked, is still a matter of debate. However, since macroautophagy has been linked to intracellular antigen processing for MHC class II presentation to CD4+ T cells, it can be hypothesized that an enhancement of this catabolic pathway might lead to increased autoantigen presentation on MHC class II, and thereby sustain inflammation via CD4+ T cell activation (Fig. 1, part 3).
Figure 1. Increased macroautophagy in degenerating muscle fibers of sporadic inclusion body myositis could enhance β-amyloid deposition (1), degrade β-amyloid aggregates (2), and/or lead to MHC class II presentation of autoantigens (3), thereby initiating or sustaining the inflammatory environment of this muscle disease. Inflammation and especially CD4+ T cells, recognizing intracellular antigen after processing via autophagy, could further stimulate autophagy (4) and the upregula- tion of antigen presentation via MHC class I and II, as well as maintain the activation of infiltrating CD8+ T cells via cytokine secretion (5). MIIC, MHC class II loading compartments.
Macroautophagy in sIBM
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Since inflammatory cytokines, which are abundantly expressed in the inflammatory environment of sIBM, have been shown to regulate autophagy and cell survival in various mammalian cell types, it will be important to determine whether skeletal muscle fibers are specifically susceptible to cytokine-mediated regulation of autophagy. Along these lines, infiltrating T cells could increase macroautophagy via secretion of IFNg and TNFa (Fig. 1, part 4). IFNg has been shown to upregulate autophagic turnover of intracellular bacteria in macrophages15 and TNFa was suggested to enhance autophago- some formation in epithelial cell lines after NFkB inactivation.16 Furthermore, CD4+ T cells could maintain an inflammatory envi- ronment in sIBM by IFNg-mediated upregulation of both MHC class I and II antigen presentation,17 as well as maintain CD8+ T cell persistence in the inflamed tissue via IL-218 (Fig. 1, part 5).
Enhanced autophagy might promote or ameliorate degenerative protein aggregation and inflammation in sIBM. Studies that inves- tigate autophagy in skeletal muscle cells and in sIBM in more detail are underway. A better understanding of this pathway in a tissue with high autophagic activity and in the inflammatory environment of sIBM might provide exciting insights into the general regulation of macroautophagy in mammalian cells and could provide a rationale for novel therapeutic strategies targeting pathogenic protein aggrega- tion in sIBM.
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