Investigating how FHOD-family formin promotes Z-line organization and striation formation in C. elegans striated muscles

Abstract

Striated muscles are composed of basic structural and functional units called sarcomeres. The assembly of these sarcomeres is a well-studied process among vertebrates and multiple lines of evidence suggest formins as regulators of sarcomere assembly. Formins are regulators of unbranched actin networks and thus were ideal candidates to test for the initiators of thin filament assembly. We examined how Caenorhabditis elegans formins, FHOD-1 and CYK-1 regulate striated body wall muscle (BWM) growth. We found that FHOD family-related, FHOD-1 was the only formin that promoted BWM growth in a cell autonomous manner. However, the DIAPH-family related CYK-1 effect on BWMs was rather indirect. Interestingly, both these formins did not function as thin filament initiators. Our focus was to investigate the mechanisms of how FHOD-1 regulates striated muscle development. Loss of FHOD-1 however caused disorganized Z-lines in BWMs. Dense bodies (DBs) are analogous to Z-lines and are also similar to integrin-based focal adhesions. They are often arranged in rows that appear parallel in wild-type animals. We investigated how the loss of FHOD-1 affects the distribution, arrangement and morphology of the DBs. We found that loss of FHOD-1 led to the accumulation of non-parallel striations and FHOD-1 was enriched at the sites of new DB assembly as well as at sites where non-parallel striations would intersect. FHOD-1 supports the orientation of new striations. We also found that DBs from worms that lack FHOD-1 were fragile and were not able to withstand prolonged contractions. We interpret that FHOD-1 could regulate the actin dynamics or act as a linker to bundle actin filaments that are a part of this unique DB-associated cytoskeletal system, which provides structural integrity to the DBs.