In the trunk of developing zebrafish embryos, adjacent myotome blocks transmit contractile force via myoseptal junctions (MJs), which are dynamic structures that connect the actin cytoskeleton of skeletal muscle cells to extracellular matrix components via transmembrane protein complexes in the sarcolemma. Here, we report that the endolysosomal ion channel, two-pore channel type 1 (TPC1, encoded by tpcn1), generates highly localized non-propagating Ca2+ transients that play a distinct and required role in the capture and attachment of superficial slow skeletal muscle cells at MJs. Use of antisense morpholinos or CRISPR/Cas9 gene editing to disrupt tpcn1 gene expression resulted in abnormal MJ phenotypes, including slow skeletal muscle cells detaching from or crossing the myosepta. We also report that TPC1-decorated endolysosomes are dynamically associated with MJs in a microtubule-dependent manner, and that attenuating tpcn1 expression or TPC1 function disrupted endolysosomal trafficking and resulted in an abnormal distribution of β-dystroglycan (encoded by dag1; a key transmembrane component of the dystrophin-associated protein complex). Taken together, our data suggest that localized TPC1-generated Ca2+ signals facilitate essential endolysosomal trafficking and membrane contact events, which help form and maintain MJs following the onset of slow skeletal muscle cell contractile activity.

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