Endoplasmic Reticulum Stress Induces Myostatin High Molecular Weight Aggregates and Impairs Mature Myostatin Secretion

Rishibha Sachdev,Karin Kappes-Horn,Jens Reimann,Philip Denner,Ina Vorberg,Yvonne Duernberger,Catharina Pleschka,Bishwajit Kundu,Lydia Paulsen

doi:10.1007/s12035-018-0997-9

Abstract

Sporadic inclusion body myositis (sIBM) is the most prevalent acquired muscle disorder in the elderly with no defined etiology or effective therapy. Endoplasmic reticulum stress and deposition of myostatin, a secreted negative regulator of muscle growth, have been implicated in disease pathology. The myostatin signaling pathway has emerged as a major target for symptomatic treatment of muscle atrophy. Here, we systematically analyzed the maturation and secretion of myostatin precursor MstnPP and its metabolites in a human muscle cell line. We find that increased MsntPP protein levels induce ER stress. MstnPP metabolites were predominantly retained within the endoplasmic reticulum (ER), also evident in sIBM histology. MstnPP cleavage products formed insoluble high molecular weight aggregates, a process that was aggravated by experimental ER stress. Importantly, ER stress also impaired secretion of mature myostatin. Reduced secretion and aggregation of MstnPP metabolites were not simply caused by overexpression, as both events were also observed in wildtype cells under ER stress. It is tempting to speculate that reduced circulating myostatin growth factor could be one explanation for the poor clinical efficacy of drugs targeting the myostatin pathway in sIBM.

Highlights

Sporadic inclusion body myositis is the most prevalent acquired muscle wasting disorder above the age of 50 with no effective treatment or cure [1,2,3]
Serial sections stained with hematoxylin-eosin confirmed typical rimmed vacuoles in Sporadic inclusion body myositis (sIBM) samples that were frequent in areas of non-atrophic muscle fibers (Fig. 1a)
Note that MstnPP was expressed under the control of the CMV promoter and might not respond to endoplasmic reticulum (ER) stress-induced transcriptional changes as demonstrated before [65]. b Quantification of fold change in BIP (n = 4). c Quantification of Mstn-N and Mstn-C (n = 4). d Reduced secretion of MstnPP metabolites upon ER stress induction

Summary

Introduction

Sporadic inclusion body myositis (sIBM) is the most prevalent acquired muscle wasting disorder above the age of 50 with no effective treatment or cure [1,2,3]. Pathological characteristics include vacuolated muscle fibers, deposition of proteinaceous material [6, 7], and T lymphocyte infiltration [8]. SIBM shares the immunological signs of an autoimmune attack by cytotoxic CD8-positive T cells in skeletal muscle fibers with polymyositis [9]. Muscle fibers show immunoreactivities for various proteins associated with inclusions in neurodegenerative diseases [12,13,14,15,16]. Protein deposits sometimes stain with Congo Red, indicative of ordered protein aggregates, so-called amyloid [17]. One aggregation-prone protein implicated in sIBM pathology is the amyloid precursor protein (APP). APP or metabolites thereof sometimes form intrafiber deposits or cluster around sIBM vacuoles, suggesting similar pathogenic events [12, 19].

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Conclusion