Fixation Induces Mislocalization of Thin Filament Components in Caenorhabditis elegans Striated Muscle

The simple nematode Caenorhabditis elegans has long served as a powerful genetic model system for studying muscle structure and function, with sarcomeres of the striated body wall muscle (BWM) that are considered largely homologous to those in vertebrates. However, we have observed a population of F-actin in BWM separate from I-bands and centered at the sarcomere M-line, something not seen in vertebrate sarcomeres. Here, we investigate whether M-line-associated F-actin is a true feature of worm muscle, or an artifact of sample preparation. Using thin filament components tagged with fluorescent proteins, we confirm that living C. elegans BWM has a traditional sarcomere organization with actin restricted to I-bands, whereas two methods of fixation resulted in varying degrees of mislocalization of actin as well as barbed end capping protein/CapZ and tropomodulin. Additionally, we observe in live animals that α-actinin-rich dense bodies are flanked by discrete patches of CapZ, something we cannot observe in fixed animals, but which provides some support for a model in which dense bodies anchor thin filaments in a manner similar to vertebrate Z-discs. Overall, our work highlights the susceptibility of thin filaments in worm BWM to fixation artifacts and the potential utility in studying sarcomere structure in live animals.