Lysosomes are membrane-bound organelles that function to degrade cargoes received from various sources including endocytosis, phagocytosis and autophagy. Defects in lysosome function lead to rapidly growing numbers of human disorders, indicative of its important roles in animal development. From a forward genetic screen for regulators of programmed cell death, we isolated a recessive mutation
qx42, which accumulated many refractile corpse-like objects at various embryonic stages. The appearance of corpse-like objects in
qx42 embryos is only partially suppressed by a loss-of-function mutation in the
ced-3 or
ced-4 gene which blocks almost all apoptosis in C. elegans. On the other hand, significantly higher numbers of Lysotracker Red-positive structures were observed in
qx42 mutants than in wild type, suggesting that the refractile corpse-like objects likely represent both cell corpses and abnormal lysosomes. To further examine this, we labeled lysosomes with NUC-1::mCHERRY, the C. elegans DNase II homolog that localizes to lysosomes. In wild type, NUC-1::mCHERRY stained many punctate and few tubule-like structures. In
qx42 embryos, however, the NUC-1-positive puncta were greatly enlarged, whereas tubular structures were mostly disrupted. Moreover, we partially released cells from C. elegans embryos by chitinase treatment followed by Lysotracker staining. We found that Lysotracker Red stained small punctuate structures in the cytoplasm of wild-type cells but labeled significantly enlarged compartments in
qx42 mutant cells. These data suggest that
qx42 mutants accumulate abnormally enlarged lysosomes. Consistent with this, we found that
qx42 mutants are defective in lysosomal degradation of various cargoes including apoptotic cells, yolk proteins and transmembrane cargoes like RME-2 and CAV-1, indicating that lysosome function is severely affected.
qx42 animals are viable but display a retarded embryonic development. We have cloned the gene affected in
qx42 and are in the process of characterizing its function in maintaining normal lysosome function and embryonic development.