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 show that rapsyn plays an AChRindependent role in organizing the distribution and mobility of lysosomes. In cells devoid of AChRs, rapsyn selectively induces 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 colocalize with AChR clusters on the cell surface. In rapsyn-deficient (Rapsn −/− ) myoblasts or cells overexpressing rapsyn mutants, lysosomes are scattered within the cell and highly dynamic. The increased mobility of lysosomes in Rapsn −/− cells is associated with a significant increase in lysosomal exocytosis, as evidenced by increased release of lysosomal enzymes and 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.

DNA methylation array analysis identifies breast cancer associated RPTOR, MGRN1 and RAPSN hypomethylation in peripheral blood DNA

“…The receptor-associated protein of the synapse ( RAPSN ) gene encodes proteins that are receptors associated proteins of the synapse. Diseases associated with RAPSN include fetal akinesia deformation sequence and postsynaptic congenital myasthenic syndromes [ 34 ]. Further, RAPSN has been shown to play an important role in lysosome positioning, exocytosis and plasma membrane integrity [ 35 ].…”

Mechanisms and functions of lysosome positioning

“…This protein has a C-terminal RING finger motif with E3-ligase activity, suggesting an involvement of the ubiquitin pathway in lysosome positioning. The protein rapsyn also contains a RING finger and causes juxtanuclear lysosome clustering upon overexpression (Aittaleb et al, 2015). However, it is unknown whether Rabring7 and rapsyn function in conjunction with kinesins or dynein, and what the precise role of ubiquitylation in this process might be.…”

TFEB regulates lysosomal positioning by modulating TMEM55B expression and JIP4 recruitment to lysosomes

“…Rab34 and Rab36 localize to the Golgi/TGN and directly interact with RILP, mediating tethering or anchoring of lysosomes to the Golgi 30 , 31 . It is still unclear how Rabring7 and Rapsyn may work in combination with microtubule motors 32 , 33 .…”