| Lysosomes are membrane-bound acidic organelles that degrade cargoes derived from endocytosis,phagocytosis or autophagy through the action of a variety of lysosomal hydrolases.The degradation products are exported from lysosomes by membrane transporters,and are re-utilizated in cellular metabolism.Dysfunction of lysosomal enzymes or transporters has deletrious effect on cell homeostasis,causing lysosomal storage diseases.The rupture of lysosomal membrane will cause leakage of luminal hydrolases that degrade normal cellular components in the cytosol,and may cause cell death.In addition,lysosomal defects have been found to associate with a variety of neurodegenerative diseases due to defective degradation of autophagic cargoes.In C.elegans,lysosomes appear as both small puncta and thin tubules,displaying highly dynamic patterns during development,in aging process and under different culture conditions.To understand how lysosome morphology and dynamics are regulated,we perfonned genetic screens to search for mutants with altered lysosomal morphology.From a forward genetic screen that covers 10,000 haploid genomes,we have isolated 50 mutants that contain abnormal lysosomes.In this study,we identified and characterzied genes affected in two lysosome mutants qx193 and qx348.qx193 affects scav-3,which encodes the C.elegans homolog of human LIMP-2,a highly glycosylated lysosomal membrane protein.We found that lysosomes,which are highly dynamic in wild type,become static in scav-3 mutants.Further analysis revealed that loss of scav-3 caused rupture of lysosome membranes and significantly shortened the lifespan.Both phenotypes were suppressed by the overexpression of LMP-1 or LMP-2,the C.elegans homolog of human lysosome associate protein LAMP,indicating that lysosome integrity is important for maintaining adult lifespan.We found that overexpression of SCAV-3 that contained mutations in conserved glycosylation sites cannot fully rescue the damaged lysosome phenotype in scav-3 mutant.This suggests that the glycosylation of SCAV-3 may contribute to the formation of the glycocalyx that protects lysosomal membranes from lysosome luminal hydrolases’ digestion.We found that reducing insulin/IGF-1 signaling suppressed lysosomal damage and extended the lifespan in scav-3(lf)animals in a DAF-16-dependent manner.qx348 affects lpla-2,which encodes the homolog of human lysosomal phospholipase A2 in C.elegans.We found that lpla-2 mutants severely affected degradation of autophagic cargo.Future EM analysis indicates that lpla-2 lysosomes accumulate a large number of membranous cargos,consistent with function of LPLA-2 in digesting phospholipids.Interestingly,blocking autophagy pathway significantly suppressed lysosome enlargement in lpla-2 mutants,while increasing autophagy activity aggravates the defect of lysosomes.These data suggest LPLA-2 may play important roles in degradating membranous cargo derived from autophagy.lpla-2 mutants display severe developmental defects,showing high percentage of embryonic lethality.In summary,these studies reveal the role of SCAV-3 in preserving lysosomal membrane integrity and the function of LPLA-2 in degrading membraneous cargo derived from autophagy,which is essential for embryonic development in C.elegans. |
