Z‐line‐like Structures Are Distinct from α‐actinin‐rich Dense Bodies in Nematode Obliquely Striated Muscle

Abstract

Obliquely striated muscles are present widely in soft‐bodied invertebrates and differ from cross‐striated muscles in that thin and thick filaments are staggered and obliquely aligned. Distinct Z‐lines or Z‐discs are absent from obliquely striated muscles. In general, thin filaments in obliquely striated muscles are considered to be anchored at the dense bodies, although there is no direct evidence of a linkage between the ends of thin filaments and dense bodies. In this study, we used body wall muscle of the nematode Caenorhabditis elegans as a representative obliquely striated muscle and found that the barbed ends of sarcomeric actin filaments are not localized to the dense bodies, but instead aligned to linear Z‐line‐like structures. In C. elegans, α‐actinin (ATN‐1) is one of the major components of the dense bodies. However, sarcomeric actin, as probed by anti‐actin antibody, phalloidin, or GFP‐actin, was only marginally co‐localized with ATN‐1. To specifically localize the barbed ends of sarcomeric actin filaments, we expressed GFP‐tagged CAP‐1, an α subunit of heterodimeric actin capping protein. GFP‐CAP‐1 was localized in a striated pattern filling gaps between dense bodies but was not co‐localized with ATN‐1. These “Z‐line‐like” structures of GFP‐CAP‐1 assembled normally in atn‐1‐knockout worms, indicating that ATN‐1 is not required for linear alignment of the actin barbed ends. In atn‐1‐knockout worms, major cytoplasmic portions of the dense bodies are absent, and only moderate disorganization of sarcomeric actin filaments are observed (Mulder et al., 2010). However, RNA interference of cap‐1 caused larval lethality and strong disorganization of sarcomeric actin filaments in embryonic body wall muscle cells. These results indicate that the barbed ends of sarcomeric actin filaments are aligned to Z‐line‐like structures, which assemble independently of dense bodies, and that capping protein plays an essential role in early sarcomeric assembly.Support or Funding InformationSupported by NIH 5R01AR048615‐18.