Early life lipid overload in Native American myopathy is phenocopied by stac3 knock out in zebrafish

Understanding the early stages of human congenital myopathies is critical for proposing strategies for improving skeletal muscle performance by restoring the functional integrity of cytoskeleton. SH3 and cysteine-rich domain 3 (Stac3) is a protein involved in nutrient sensing, and is an essential component of the excitation-contraction (EC) coupling machinery for Ca2+ releasing. A mutation in STAC3 causes debilitating Native American myopathy (NAM) in humans, and loss of this gene in mice and zebrafish resulted in death in early life. Previously, NAM patients demonstrated increased lipids in skeletal muscle biopsy. However, whether elevated neutral lipids could alter muscle function in NAM disease is now well understood. Here, using a CRISPR/Cas9 induced stac3 knockout (KO) zebrafish model, we determined that loss of stac3 led to muscle weakness, as evidenced by delayed larval hatching. We observed decreased whole-body Ca2+ level at 5 days post-fertilization (dpf) and defects in the skeletal muscle cytoskeleton, i.e., F-actin and slow muscle fibers at 5 and 7 dpf. To this end, myogenesis regulators such as myoD and myf5, were significantly altered in stac3−/− larvae at 5 dpf. Homozygous larvae exhibited elevated neutral lipid levels at 5 dpf, which persisted beyond 7 dpf, followed by a progressive demise of the KO larva by 11 dpf. In summary, the presented findings suggest that stac3−/− can serve as a non-mammalian model to identify lipid-lowering molecules for refining muscle function in NAM patients.