Revealing the Role of IGFN1 in Skeletal Muscle

The role of the skeletal muscle protein Immunoglobulin-like and fibronectin type III domain containing 1 (IGFN1) has proved elusive. There are several IGFN1 isoforms, none of which contain catalytic domains. Each isoform has a domain composition of immunoglobulin and fibronectin domains, suggesting a structural role in the sarcomere. IGFN1 was first discovered as an interacting partner of the disease-associated protein KY, and the current literature implicates IGFN1 in both atrophy and myoblast fusion. Here, characterisation of fusion and differentiation indexes. compared to wildtype, of a CRISPR/Cas9-generated, C2C12-derived, IGFN1 knockout cell line revealed fusion and differentiation defects. Furthermore, these cells display increased globular to filamentous actin ratios, indicating decreased actin polymerisation which potentially underlies the fusion defects observed. Crucially, the above phenotypes are ameliorated through expression of the IGF1_V1 isoform. Next, to identify IGFN1 interaction partners, IGFN1 fragments were purified and pull-down analysis was performed revealing the actin nucleator COBL as a potential interacting partner. This interaction subsequently validated through immunoprecipitation and colocalization. The role of COBL in myoblast fusion was investigated through overexpression experiments and the generation of a C2C12-derived knockout cell line. Initial characterisation points towards a role for COBL in myoblast fusion. Taken together, it is possible that IGFN1 influences actin remodelling, and therefore myoblast fusion, through its interaction with COBL.