G.P.33 Protein turnover mechanisms in M-band titinopathies

Abstract: The titinopathies tibial muscular dystrophy (TMD) and limb-girdle muscular dystrophy 2J (LGMD2J) are caused by mutations in the C-terminus of titin, residing in the sarcomeric M-band. Mutations identified so far affect the last Ig domain M10 or the preceding is 7 region. The FINmaj mutation, underlying TMD/LGMD2J in patients of Finnish origin, causes the exchange of four amino acids in M10 and presumably leads to domain unfolding. The other known mutations cause missense changes of single amino acids or truncation of the ultimate C-terminus. Loss of C-terminal titin epitopes in IF microscopy and reduced amount of C-terminal titin fragments in western blotting suggest that increased or abnormal proteolytic turnover of mutant titin may contribute to the pathomechanism. However, the detailed molecular mechanisms responsible for titin proteolysis and downstream processing of the fragments remain unknown. We recently identified mutations in the co-chaperone DNAJB6 as the molecular cause of LGMD1D, a dominant form of limb-girdle muscular dystrophy with myofibrillar and rimmed vacuolar pathology. Our findings also suggested that DNAJB6 is associated with CASA (chaperone-assisted selective authophagy), a protein turnover pathway important for Z-disk maintenance. Interestingly, DNAJB6 and the CASA proteins BAG3 and HSPB8 also show occasional localization to the M-band, suggesting that the CASA pathway might have additional functions there. To assess whether CASA could be involved in mediating the normal and pathological turnover of M-band titin, we have characterized the expression and localization of these proteins in muscles of titinopathy patients and the FINmaj knock-in mouse. We have also studied the effects of overexpression and knockdown of CASA components on the behaviour of C-terminal titin fragments. Our preliminary results suggest there might be cross-talk between regulatory maintenance functions of the Z-disk and the M-band of the sarcomere. [Copyright &y& Elsevier]