Mucolipidosis Type IV Protein TRPML1‐Dependent Lysosome Formation

Abstract

Lysosomes are dynamic organelles that undergo cycles of fusion and fission with themselves and with other organelles. Following fusion with late endosomes to form hybrid organelles, lysosomes are reformed as discrete organelles. This lysosome reformation or formation is a poorly understood process that has not been systematically analyzed and that lacks known regulators. In this study, we quantitatively define the multiple steps of lysosome formation and identify the first regulator of this process.

The lysosomal Ca2+ release channel TRPML1 regulates lysosome size by activating calmodulin

“…Given that lysosome fission is implicated in lysosome biogenesis and reformation (7,45), this finding suggests that TRPML1 may function as the key lysosomal Ca 2ϩ channel regulating autophagic lysosome reformation 4 (44,45) or lysosome biogenesis (7,30,32). In general, our results are in agreement with previous reports, including that 1) TRPML1-null mutant cells display enlarged lysosomes and defects in lysosome biogenesis (10,29,30,32); and 2) an increase in PI(3,5)P2, an endogenous agonist of TRPML1, promotes vacuole fission (33), whereas the deficiency in PI(3,5)P2 causes vacuole enlargement in both yeast (33,34) and mammalian cells (14).…”

ESCRT-Dependent Cell Death in a Caenorhabditis elegans Model of the Lysosomal Storage Disorder Mucolipidosis Type IV

“…It is not known why developing intestinal cells die in C. elegans or why neurons die in MLIV patients. In C. elegans cup-5(null) mutants, the embryonic lethality is not solely due to cells undergoing apoptosis from starvation; when ATP levels are restored or when apoptosis is blocked, embryonic lethality is only partially rescued (14% of embryos hatch and arrest at the L1 larval stage) (Hersh et al 2002;Schaheen et al 2006a).We have shown that C. elegans CUP-5 and mammalian TRPML1 likely function in lysosome formation, which involves the budding of nascent lysosomes from endosomes and scission to release the nascent lysosomes (Treusch et al 2004;Miller et al 2015). In contrast to this outward budding event, Endosomal Sorting Complex Required for Transport (ESCRT) proteins are required for the sequestration of integral membrane proteins in intraluminal vesicles through an inward budding and scission event in late endosomes/multivesicular bodies (Henne et al 2011).…”

“…We have shown that C. elegans CUP-5 and mammalian TRPML1 likely function in lysosome formation, which involves the budding of nascent lysosomes from endosomes and scission to release the nascent lysosomes (Treusch et al 2004;Miller et al 2015). In contrast to this outward budding event, Endosomal Sorting Complex Required for Transport (ESCRT) proteins are required for the sequestration of integral membrane proteins in intraluminal vesicles through an inward budding and scission event in late endosomes/multivesicular bodies (Henne et al 2011).…”

Regulators of Lysosome Function and Dynamics inCaenorhabditis elegans

“…In the endocytic pathway, mature lysosomes fuse with late endosomes to form hybrid organelles using a molecular machinery that includes Rab7, HOPS complex, and SNARES (Luzio et al 2005; Balderhaar and Ungermann 2013; Yasuda et al 2016). Lysosomes are formed, or reformed, from hybrid organelles in complex eukaryotes by the budding of a small nascent lysosome from the hybrid organelle, movement of the nascent lysosome away from the hybrid organelle while maintaining a membrane bridge, and scission of the membrane bridge to release a discrete primary lysosome; molecules destined for lysosomes are concentrated in the nascent lysosome during lysosome formation/reformation (Treusch et al 2004; Bright et al 2005; Thompson et al 2007; Miller et al 2015). Autophagic lysosome reformation following fusion of autophagosomes with lysosomes is similar, perhaps identical, to lysosome formation in the endocytic pathway (Yu et al 2010; Rong et al 2011, 2012).…”

“…Autophagic lysosome reformation following fusion of autophagosomes with lysosomes is similar, perhaps identical, to lysosome formation in the endocytic pathway (Yu et al 2010; Rong et al 2011, 2012). Lysosome formation requires the Mucolipidosis type IV protein TRPML1 (CUP-5 in Caenorhabditis elegans ), Ca 2+ efflux from hybrid organelles, and the C. elegans small Rab GTPase 2-like UNC-108 protein (Pryor et al 2000; Chun et al 2008; Lu et al 2008; Miller et al 2015); autophagic lysosome reformation is activated by mTOR signaling, and requires clathrin and phosphatidylinositol-4,5-bisphosphate kinases (Yu et al 2010; Rong et al 2012). Primary lysosomes that are formed are thought to undergo cycles of homotypic fusion with each other using the HOPS complex and SNARES, and fission reactions that yield mature lysosomes that can fuse with late endosomes (Ward et al 2000; Wang et al 2003).…”