Two endoplasmic reticulum-associated degradation (ERAD) systems for the novel variant of the mutant dysferlin: ubiquitin/proteasome ERAD(I) and autophagy/lysosome ERAD(II)

Abstract

Dysferlin is a type-II transmembrane protein and the causative gene of limb girdle muscular dystrophy type 2B and Miyoshi myopathy (LGMD2B/MM), in which specific loss of dysferlin labeling has been frequently observed. Recently, a novel mutant (L1341P) dysferlin has been shown to aggregate in the muscle of the patient. Little is known about the relationship between degradation of dysferlin and pathogenesis of LGMD2B/MM. Here, we examined the degradation of normal and mutant (L1341P) dysferlin. Wild-type (wt) dysferlin mainly localized to the ER/Golgi, associated with retrotranslocon, Sec61alpha, and VCP(p97), and was degraded by endoplasmic reticulum (ER)-associated degradation system (ERAD) composed of ubiquitin/proteasome. In contrast, mutant dysferlin spontaneously aggregated in the ER and induced eukaryotic translation initiation factor 2alpha (eIF2alpha) phosphorylation and LC3 conversion, a key step for autophagosome formation, and finally, ER stress cell death. Unlike proteasome inhibitor, E64d/pepstatin A, inhibitors of lysosomal proteases did not stimulate the accumulation of the wt-dysferlin, but stimulated aggregation of mutant dysferlin in the ER. Furthermore, deficiency of Atg5 and dephosphorylation of eIF2alpha, key molecules for LC3 conversion, also stimulated the mutant dysferlin aggregation in the ER. Rapamycin, which induces eIF2alpha phosphorylation-mediated LC3 conversion, inhibited mutant dysferlin aggregation in the ER. Thus, mutant dysferlin aggregates in the ER-stimulated autophagosome formation to engulf them via activation of ER stress-eIF2alpha phosphorylation pathway. We propose two ERAD models for dysferlin degradation, ubiquitin/proteasome ERAD(I) and autophagy/lysosome ERAD(II). Mutant dysferlin aggregates on the ER are degraded by the autophagy/lysosome ERAD(II), as an alternative to ERAD(I), when retrotranslocon/ERAD(I) system is impaired by these mutant aggregates.