Elsevier

Journal of Neuroimmunology

Volume 301, 15 December 2016, Pages 30-34
Journal of Neuroimmunology

Short communication
Caveolin-3 is aberrantly expressed in skeletal muscle cells in myasthenia gravis☆

https://doi.org/10.1016/j.jneuroim.2016.10.011Get rights and content

Highlights

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    Some myasthenia gravis (MG) muscles showed patchy caveolin-3 distribution.

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    Partial loss of caveolin-3 expression was relatively common in MG muscle.

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    Strong membrane localization of caveolin-3 was noted in 66% of MG patients.

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    Caveolin-3 mRNA/protein levels were higher in MG vs. control-patient muscles.

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    Caveolin-3 upregulation may be required after MG muscle damage.

Abstract

Caveolin-3 is a muscle-specific membrane protein that localizes to the sarcolemma and T-tubule system. Caveolin-3 is needed for muscle repair and skeletal muscle development. The objective of this study was to compare caveolin-3 expression in myasthenia gravis (MG) and control muscles. Caveolin-3 was abnormally expressed in the MG muscle membrane, with partial loss of expression and overexpression in 5/15 and 10/15 patients, respectively. Caveolin-3 mRNA and protein levels were higher in MG than in control muscles, suggesting that partial deficiency of caveolin-3 is occasionally detected in MG muscle and that caveolin-3 overexpression may be required after MG muscle damage.

Introduction

Caveolin is an integral membrane protein that exists as three isoforms—caveolin-1, -2, and -3. Caveolin-3 is expressed in the skeletal, cardiac, and smooth muscle cells (Song et al., 1996) and is an essential protein in the repair of muscle membrane damage (Cai et al., 2009, Towler et al., 2004, Volonte et al., 2003).

Mutations in caveolin-3 lead to limb-girdle muscular dystrophy 1C, hereditary rippling muscle disease (RMD), hyper-CK-emia, and distal myopathy (Woodman et al., 2004). In Duchenne muscular dystrophy, caveolin-3 is overexpressed at the muscle cell surface and in the cytoplasm (Galbiati et al., 2001, Repetto et al., 1999). Additionally, acquired RMD patients show abnormal caveolin-3 localization (Lo et al., 2011, Schoser et al., 2009).

Caveolin-3 expression is required for efficient myoblast fusion, myotube formation, muscle cell differentiation, and skeletal muscle development (Bjerregard et al., 2014, Quach et al., 2009, Towler et al., 2004, Volonte et al., 2003). It is also essential for the development of the T-tubule system through interactions with ryanodine receptor 1, dihydropyridine receptor, and other T-tubule proteins (Al-Qusairi and Laporte, 2011). Caveolin-3 is associated with various signaling molecules; caveolin-3 binds to muscle-specific receptor tyrosine kinase (MuSK) and mediates acetylcholine receptor (AChR) clustering (Hezel et al., 2010, Zhu et al., 2006). High levels of caveolin-3 are expressed in the cardiac muscle for protection against ischemia and reperfusion injury (Roth and Patel, 2011, Tsutsumi et al., 2010).

Although it is known that specific autoantibodies that destroy the post-neuromuscular junction underlie the pathogenesis of MG (Nacu et al., 2015), the mechanisms of neuromuscular junction restoration are unclear. We speculated that caveolin-3 might be needed to protect and repair the neuromuscular junction; therefore, we examined the expression of caveolin-3 in MG muscles.

Section snippets

Patients

We examined muscle biopsies from MG patients positive for anti-AChR autoantibodies (Table 1), including individuals with and without thymoma (n = 8 and 7, respectively). One patient had rippling phenomenon in the extremities. Serum creatine kinase level was normal, and the rippling phenomenon was electrically silent. All patients provided written, informed consent prior to muscle biopsy. Tissue samples were obtained from the musculus pectoralis major during thymectomy. Additionally, five control

Theory/calculation

Biopsy specimens from MG and control patients were obtained from different muscles. Muscle tissues from different anatomical locations may exhibit different histological and molecular findings. This difference may affect caveolin-3 expression with significant sampling bias. Therefore, large-scale studies need to be conducted to investigate whether damage to the neuromuscular junction induces the overexpression of caveolin-3 in MG muscle tissues.

Immunohistochemistry of caveolin-3

Caveolin-3 expression in the muscle tissue of MG and control patients was evaluated by immunohistochemistry (Fig. 1). In all control patients and in 5/15 MG patients, the membrane localization of caveolin-3 confirmed muscle cell membrane integrity (Fig. 1A). Strong membrane labeling in some muscle cells was observed in the remaining 10/15 MG patients (Fig. 1B). Patchy or partial loss of membrane expression was observed in five MG patients (Fig. 1C and D), who also showed caveolin-3

Partial deficiency of caveolin-3 in MG muscle

In 5/15 MG patients examined, there was a partial deficiency or patchy distribution of caveolin-3 at the muscle cell membrane. An autoimmune basis for RMD in MG has been reported in a previous study (Schoser et al., 2009), where partial loss of or patchy caveolin-3 expression in the skeletal muscle was observed. Caveolin-3 localizes exclusively to lipid rafts, which are lipid-rich microdomains that contain various signaling molecules, including AChR and MuSK (Hezel et al., 2010, Zhu et al., 2006

Conclusions

Partial loss of caveolin-3 expression is a relatively common occurrence in MG muscles, and caveolin-3 upregulation may be required after neuromuscular junction destruction in the MG muscle.

Contributions

KI, YF, and HY conceived the study, analyzed the data, and were involved in the preparation of the manuscript. KI and YF were responsible for the collection of muscle specimens and clinical data. MY participated in the study design and coordination, and helped draft the manuscript. All authors have read and approved the final manuscript.

Competing interests

KI, YF, HY, and MY report no disclosures relevant to the manuscript.

Ethics approval

Ethics approval was obtained from the ethics committee of the Kanazawa University School of Medicine, Kanazawa, Japan.

Acknowledgments

We thank Ms. Yumiko Kakuda for technical assistance in the muscle pathology and immunohistochemistry experiments.

References (20)

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Funding: This work was supported by the Japanese Ministry of Education, Science, Sports, and Culture (grant no. 25461273 to KI).

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