Failure of lysosome clustering and positioning in the juxtanuclear region in cells deficient in rapsyn

Abstract

Rapsyn, a scaffold protein, is required for the clustering of acetylcholine receptors (AChRs) at contacts between motor neurons and differentiating muscle cells. Rapsyn is also expressed in cells that do not express AChRs. However, its function in these cells remains unknown. Here, we showed that rapsyn plays an AChR-independent role in organizing the distribution and mobility of lysosomes. In cells devoid of AChRs, rapsyn selectively induced the clustering of lysosomes at high density in the juxtanuclear region without affecting the distribution of other intracellular organelles. However, when the same cells overexpress AChRs, rapsyn is recruited away from lysosomes to co-localize with AChR clusters on the cell surface. In rapsyn-deficient (rapsyn−/−) myoblasts or cells overexpressing rapsyn mutants, lysosomes are scattered within the cell and highly dynamic. The increased mobility of lysosomes in rapsyn−/− cells is associated with a significant increase of lysosomal exocytosis as evidenced by increased release of lysosomal enzymes and the plasma membrane damage when cells were challenged with the bacterial pore-forming toxin streptolysin-O. These findings uncover a new link between rapsyn, lysosome positioning, exocytosis, and plasma membrane integrity.