The ER-Bound RING Finger Protein 5 (RNF5/RMA1) Causes Degenerative Myopathy in Transgenic Mice and Is Deregulated in Inclusion Body Myositis

Abstract

Growing evidence supports the importance of ubiquitin ligases in the pathogenesis of muscular disorders, although underlying mechanisms remain largely elusive. Here we show that the expression of RNF5 (aka RMA1), an ER-anchored RING finger E3 ligase implicated in muscle organization and in recognition and processing of malfolded proteins, is elevated and mislocalized to cytoplasmic aggregates in biopsies from patients suffering from sporadic-Inclusion Body Myositis (sIBM). Consistent with these findings, an animal model for hereditary IBM (hIBM), but not their control littermates, revealed deregulated expression of RNF5. Further studies for the role of RNF5 in the pathogenesis of s-IBM and more generally in muscle physiology were performed using RNF5 transgenic and KO animals. Transgenic mice carrying inducible expression of RNF5, under control of β-actin or muscle specific promoter, exhibit an early onset of muscle wasting, muscle degeneration and extensive fiber regeneration. Prolonged expression of RNF5 in the muscle also results in the formation of fibers containing congophilic material, blue-rimmed vacuoles and inclusion bodies. These phenotypes were associated with altered expression and activity of ER chaperones, characteristic of myodegenerative diseases such as s-IBM. Conversely, muscle regeneration and induction of ER stress markers were delayed in RNF5 KO mice subjected to cardiotoxin treatment. While supporting a role for RNF5 Tg mice as model for s-IBM, our study also establishes the importance of RNF5 in muscle physiology and its deregulation in ER stress associated muscular disorders.

De Novo Prion Aggregates Trigger Autophagy in Skeletal Muscle

“…Notably, the myopathy in mice is characterized by internal nuclei, split fibers, central rimmed vacuoles, cell death and replacement by adipocytes, and the accumulation of intracellular aggregates. These lesions are similar to many other protein aggregate myopathies in patients and in mouse models, including gelsolin, amyloid ␤, and phosphorylated tau aggregate myopathies (46,(57)(58)(59). PrP inclusions were in type I and type II fibers, suggesting that aggregation was influenced neither by the oxidative capacity nor by the glycolytic ability of the cell.…”

Regulation of Endoplasmic Reticulum-associated Degradation by RNF5-dependent Ubiquitination of JNK-associated Membrane Protein (JAMP)

“…Antibodies detecting Lys-63 chain ubiquitin were kindly obtained from Dr. V. Dixit at Genentech. Antibodies against RNF5 were generated as described (14) and used for immunoblot analysis (1:2000). Immunoblots were visualized using goat anti-mouse or anti-rabbit Alexa-Fluor 680 secondary antibodies (Molecular Probes) followed by detection with the Odyssey Infrared Imaging System (LI-COR Biosciences).…”

“…Gene Silencing by RNA Interference-Silencing of RNF5 in HeLa cells and generation of stable cell lines were performed using the pSuper and pCMS3-cherry construct (kind gift from Dr. Dan Billadeau (24)) as described previously (14). Levels of RNF5 expression were monitored by Western blot and reverse transcription-PCR analysis using the corresponding primers.…”

“…Consistent with our observations in C. elegans, work performed using RNF5 transgenic and knockout mice confirmed a role of RNF5 in skeletal muscle. Based on histology and pathology criteria, inducible RNF5 expression resulted in degenerative myopathy with characteristic pheno-types seen in inclusion body myocytis (IBM (14)), including high levels of ER stress. In humans, sporadic IBM is the more common myopathy affecting older patients and is marked by progressive muscle weakness associated with inflammatory response, vacuolar degeneration, and aggregate formation (15,16).…”

Emerging Role of Ubiquitination in Antiviral RIG-I Signaling

“…The activity of MITA, a crucial adaptor protein promoting the recruitment of TBK1 and IKK to MAVS, was recently shown to be modulated by the RING domain-containing E3 ligase RNF5, which specifically promotes MITA degradation at the mitochondria (183). RNF5 localized at the ER has been implicated in protein quality control (18,143). Interestingly RNF5 translocates to the mitochondria following viral infection, where it induces proteasomal degradation of MITA by targeting it for K48-linked polyubiquitination, hence inhibiting downstream signaling.…”