P.5.11 LGMD1D mutations in DNAJB6 disrupt disaggregation of TDP-43

Abstract

Heat shock proteins (HSPs) facilitate the folding or degradation of misfolded, damaged and aggregated proteins. Disruptions in HSP function may underlie the molecular basis of many degenerative disorders including some myopathies. The pathogenic mechanism of these chaperonopathies is unclear. We recently identified mutations in DNAJB6, an HSP40 co-chaperone, as the cause of a hereditary IBM also named LGMD1D. One feature of LGMD1D muscle is the accumulation of protein inclusions that contain TDP-43. TDP-43 is an RNA binding protein with a prion-like domain (PrLD) that is mutated in familial amyotrophic lateral sclerosis (ALS). LGMD1D mutations in DNAJB6 reside within the highly conserved G/F domain. Although the role of the G/F domain in DNAJB6 is unclear, studies in S.cerevisiae, have shown that the homologous G/F domain in Sis1 (a DNAJB6 ortholog) is required for the propagation of select yeast prions. Yeast prions contain Q/N rich PrLDs, a feature they share with TDP-43 and other RNA binding proteins. Consistent with this, homologous LGMD1D mutation in the G/F domain of Sis1 abrogate its ability to modulate yeast prion propagation. In mammalian cell culture DNAJB6 associates with TDP-43 in the nucleus upon heat shock suggesting that TDP-43 is indeed a DNAJB6 client protein. DNAJB6 expression reduces the formation and enhances the dissolution of TDP-43 positive nuclear bodies. LGMD1D mutant DNAJB6 expression increases TDP-43 granule formation and slows their dissolution upon heat shock recovery. This effect is more pronounced in cells expressing DNAJB6 that lacks the G/F domain. We hypothesize that LGMD1D mutant DNAJB6 affects localization, aggregation and toxicity of TDP43. Characterization of a transgenic mouse model of LGMD1D recently generated in our laboratory will help to elucidate the role of DNAJB6 and other HSPs in skeletal muscle disease and the complex interplay between RNA binding protein aggregation and disaggregation.