The C2A domain in dysferlin is important for association with MG53 (TRIM72)

In skeletal muscle, Mitsugumin 53 (MG53), also known as muscle-specific tripartite motif 72, reportedly interacts with dysferlin to regulate membrane repair. To better understand the interactions between dysferlin and MG53, we conducted immunoprecipitation (IP) and pull-down assays. Based on IP assays, the C2A domain in dysferlin associated with MG53. MG53 reportedly exists as a monomer, a homodimer, or an oligomer, depending on the redox state. Based on pull-down assays, wild-type dysferlin associated with MG53 dimers in a Ca2+-dependent manner, but MG53 oligomers associated with both wild-type and C2A-mutant dysferlin in a Ca2+-independent manner. In pull-down assays, a pathogenic missense mutation in the C2A domain (W52R-C2A) inhibited the association between dysferlin and MG53 dimers, but another missense mutation (V67D-C2A) altered the calcium sensitivity of the association between the C2A domain and MG53 dimers. In contrast to the multimers, the MG53 monomers did not interact with wild-type or C2A mutant dysferlin in pull-down assays. These results indicated that the C2A domain in dysferlin is important for the Ca2+-dependent association with MG53 dimers and that dysferlin may associate with MG53 dimers in response to the influx of Ca2+ that occurs during membrane injury. To examine the biological role of the association between dysferlin and MG53, we co-expressed EGFP-dysferlin with RFP-tagged wild-type MG53 or RFP-tagged mutant MG53 (RFP-C242A-MG53) in mouse skeletal muscle, and observed molecular behavior during sarcolemmal repair; it has been reported that the C242A-MG53 mutant forms dimers, but not oligomers. In response to membrane wounding, dysferlin accumulated at the injury site within 1 second; this dysferlin accumulation was followed by the accumulation of wild-type MG53. However, accumulation of RFP-C242A MG53 at the wounded site was impaired relative to that of RFP-wild-type MG53. Co-transfection of RFP-C242A MG53 inhibited the recruitment of dysferlin to the sarcolemmal injury site. We also examined the molecular behavior of GFP-wild-type MG53 during sarcolemmal repair in dysferlin-deficient mice which show progressive muscular dystrophy, and found that GFP-MG53 accumulated at the wound similar to wild-type mice. Our data indicate that the coordination between dysferlin and MG53 plays an important role in efficient sarcolemmal repair.

In vivo characterization of mutant myotilins

Myofibrillar myopathy (MFM) is a group of disorders that are pathologically defined by the disorganization of the myofibrillar alignment associated with the intracellular accumulation of Z-disk-associated proteins. MFM is caused by mutations in genes encoding Z-disk-associated proteins, including myotilin. Although a number of MFM mutations have been identified, it has been difficult to elucidate the precise roles of the mutant proteins. Here, we present a useful method for the characterization of mutant proteins associated with MFM. Expression of mutant myotilins in mouse tibialis anterior muscle by in vivo electroporation recapitulated both the pathological changes and the biochemical characteristics observed in patients with myotilinopathy. In mutant myotilin-expressing muscle fibers, myotilin aggregates and is costained with polyubiquitin, and Z-disk-associated proteins and myofibrillar disorganization were commonly seen. In addition, the expressed S60C mutant myotilin protein displayed marked detergent insolubility in electroporated mouse muscle, similar to that observed in human MFM muscle with the same mutation. Thus, in vivo electroporation can be a useful method for evaluating the pathogenicity of mutations identified in MFM.

M-Ras is activated by bone morphogenetic protein-2 and participates in osteoblastic determination, differentiation, and transdifferentiation

The small GTPase M-Ras is highly expressed in the central nervous system and plays essential roles in neuronal differentiation. However, its other cellular and physiological functions remain to be elucidated. Here, we clarify the novel functions of M-Ras in osteogenesis. M-Ras was prominently expressed in developing mouse bones particularly in osteoblasts and hypertrophic chondrocytes. Its expression was elevated in C3H/10T1/2 (10T1/2) mesenchymal cells and in MC3T3-E1 preosteoblasts during differentiation into osteoblasts. Treatment of C2C12 skeletal muscle myoblasts with bone morphogenetic protein-2 (BMP-2) to bring about transdifferentiation into osteoblasts also induced M-Ras mRNA and protein expression. Moreover, the BMP-2 treatment activated the M-Ras protein. Stable expression of the constitutively active M-Ras(G22V) in 10T1/2 cells facilitated osteoblast differentiation. M-Ras(G22V) also induced transdifferentiation of C2C12 cells into osteoblasts. In contrast, knockdown of endogenous M-Ras by RNAi interfered with osteoblast differentiation in 10T1/2 and MC3T3-E1 cells. Osteoblast differentiation in M-Ras(G22V)-expressing C2C12 cells was inhibited by treatment with inhibitors of p38 MAP kinase (MAPK) and c-Jun N-terminal kinase (JNK) but not by inhibitors of MAPK and ERK kinase (MEK) or phosphatidylinositol 3-kinase. These results imply that M-Ras, induced and activated by BMP-2 signaling, participates in the osteoblastic determination, differentiation, and transdifferentiation under p38 MAPK and JNK regulation.