Calcium dysregulation, functional calpainopathy, and endoplasmic reticulum stress in sporadic inclusion body myositis

Abstract

Sporadic inclusion body myositis (IBM) is the most common primary myopathy in the elderly, but its pathoetiology is still unclear. Perturbed myocellular calcium (Ca2+) homeostasis can exacerbate many of the factors proposed to mediate muscle degeneration in IBM, such as mitochondrial dysfunction, protein aggregation, and endoplasmic reticulum stress. Ca2+ dysregulation may plausibly be initiated in IBM by immune-mediated membrane damage and/or abnormally accumulating proteins, but no studies to date have investigated Ca2+ regulation in IBM patients. We first investigated protein expression via immunoblot in muscle biopsies from IBM, dermatomyositis, and non-myositis control patients, identifying several differentially expressed Ca2+-regulatory proteins in IBM. Next, we investigated the Ca2+-signaling transcriptome by RNA-seq, finding 54 of 183 (29.5%) genes from an unbiased list differentially expressed in IBM vs. controls. Using an established statistical approach to relate genes with causal transcription networks, Ca2+ abundance was considered a significant upstream regulator of observed whole-transcriptome changes. Post-hoc analyses of Ca2+-regulatory mRNA and protein data indicated a lower protein to transcript ratio in IBM vs. controls, which we hypothesized may relate to increased Ca2+-dependent proteolysis and decreased protein translation. Supporting this hypothesis, we observed robust (4-fold) elevation in the autolytic activation of a Ca2+-activated protease, calpain-1, as well as increased signaling for translational attenuation (eIF2α phosphorylation) downstream of the unfolded protein response. Finally, in IBM samples we observed mRNA and protein under-expression of calpain-3, the skeletal muscle-specific calpain, which broadly supports proper Ca2+ homeostasis. Together, these data provide novel insight into mechanisms by which intracellular Ca2+ regulation is perturbed in IBM and offer evidence of pathological downstream effects.Electronic supplementary materialThe online version of this article (doi:10.1186/s40478-017-0427-7) contains supplementary material, which is available to authorized users.

Machine learning algorithms reveal unique gene expression profiles in muscle biopsies from patients with different types of myositis

“…RNA-sequencing (RNA-seq) was performed as previously described. 13 Briefly, RNA was prepared using TRIzol. Libraries were prepared using the NeoPrep system according to the TruSeq Stranded mRNA Library Prep protocol (Illumina) and sequenced using the Illumina HiSeq 2500 or 3000.…”

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

“…Misfolded proteins are exported from the ER and subsequently degraded via the ubiquitin proteasome system (UPS) or autophagy [ 24 ]. Calcium dysregulation [ 25 ], upregulation of ER-resident chaperones [ 21 , 26 – 28 ] and UPR transcription factors [ 22 ], suppressed translation [ 25 ] and altered clearance of proteins by UPR and the autophagosomal-lysosomal pathway are prominent features of sIBM [ 29 – 31 ].…”

Tang-Luo-Ning, a Traditional Chinese Medicine, Inhibits Endoplasmic Reticulum Stress-Induced Apoptosis of Schwann Cells under High Glucose Environment

“…ER is the main storage organelle of calcium and intracellular Ca 2+ is indispensable for the development of ER. Overload calcium is identified to be a factor for cell apoptosis [ 14 , 15 ] and participates in ER stress [ 20 , 21 ]. In this study, Ca 2+ level in 150 mM glucose group increased significantly compared with 25 mM group; it suggested that intracellular environmental homeostasis was destroyed and led to apoptosis.…”

“…The accumulation of unfolded proteins and misfolded proteins in the ER resulted in defect in the ER homeostasis and then induced ER stress [ 20 , 21 ]. In order to restore homeostasis, UPR, which is involved in three ER-transmembrane transducers, PERK, IRE1 α , and ATF6, and ER chaperone GRP78 were activated [ 8 ].…”