FHOD-1/profilin-mediated actin assembly protects sarcomeres against contraction-induced deformation in C. elegans .

Formin HOmology Domain 2-containing (FHOD) proteins are a subfamily of actin-organizing formins that are important for proper striated muscle development in many animals. We had shown previously that absence of the sole FHOD protein, FHOD-1, in C. elegans results in thin body-wall muscles with misshapen dense bodies, structures that serve as sarcomere Z-lines in body-wall muscle. However, the mechanism of FHOD-1 action was unclear. Using mutations that target the actin assembly activity of FHOD-1, we demonstrate here that actin polymerization by FHOD-1 is required for its function in muscle development. Moreover, we show FHOD-1 cooperates with profilin PFN-3 to promote dense body morphogenesis, and profilins PFN-2 and PFN-3 to promote body-wall muscle growth. This is the first demonstration that a FHOD protein works with profilin partners in striated muscle development. Utilizing worms expressing fluorescently tagged dense body proteins PAT-3/β-integrin and ATN-1/α-actinin, we demonstrate dense bodies in fhod-1 and pfn-3 mutants are less stable than in wild type animals, having a higher proportion of dynamic protein, and becoming highly distorted by prolonged muscle contraction. We also observe the accumulation of actin depolymerization factor/cofilin homolog UNC-60B/cofilin in the body-wall muscle of these mutants. We suggest such accumulations may indicate targeting of thin filaments dislodged from unstable dense bodies disassembly. Similar failures in sarcomere assembly might account for the slow growth of body-wall muscle in fhod-1 mutants compared to wild type animals, and the coincident muscle weakness that we measure in these animals. Overall, these results show the importance of FHOD protein-mediated actin assembly to forming stable sarcomere Z-lines, and identify profilin as a new contributor to FHOD activity in striated muscle development.