Suppression of thecup-5mucolipidosis type IV-related lysosomal dysfunction by the inactivation of an ABC transporter inC. elegans

Abstract

Mutations in MCOLN1, which encodes the protein mucolipin 1, result in the lysosomal storage disease mucolipidosis Type IV. Studies on human mucolipin 1 and on CUP-5, the Caenorhabditis elegans ortholog of mucolipin 1, have shown that these proteins are required for lysosome biogenesis/function. Loss of CUP-5 results in a defect in lysosomal degradation, leading to embryonic lethality. We have identified a mutation in the ABC transporter MRP-4 that rescues the degradation defect and the corresponding lethality, owing to the absence of CUP-5. MRP-4 localizes to endocytic compartments and its levels are elevated in the absence of CUP-5. These results indicate that the lysosomal degradation defect is exacerbated in some cells because of the accumulation of MRP-4 in lysosomes rather than the loss of CUP-5 per se. We also show that under some conditions, loss of MRP-4 rescues the embryonic lethality caused by the loss of the cathepsin L protease, indicating that the accumulation of ABC transporters may be a more general mechanism whereby an initial lysosomal dysfunction is more severely compromised.

Role of the Caenorhabditis elegans Multidrug Resistance Gene, mrp-4, in Gut Granule Differentiation

“…Both mrp-4(ok1095) and mrp-4(cd8) (Schaheen et al 2006) embryos contain differentiated intestinal cells that lacked birefringent material in bean and 1.5-fold-stage embryos ( Figure 1A, c-h, and data not shown). The majority of pretzel-stage mrp-4(À) embryos lacked birefringent material ( Figure 1A, k and l, and Table 1).…”

“…While the transport specificity of C. elegans MRPs are unclear, they are known to be necessary for heavy metal resistance (Broeks et al 1996), dauer formation (Yabe et al 2005), and RNAi (Sundaram et al 2006a). C. elegans mrp-4 has recently been shown to contribute to the cellular defects and embryonic lethality resulting from loss of cup-5 function (Schaheen et al 2006). CUP-5, a C. elegans lysosomal protein functionally orthologous to human mucolipin-1 (Fares and Greenwald 2001;Hersh et al 2002), regulates a terminal step of lysosomal trafficking (Treusch et al 2004).…”

“…nonfunctional late endosomal compartments that contribute to the embryonic lethality of cup-5(À) (Schaheen et al 2006).…”

“…Recently, two ABC transporters, PGP-2 and MRP-4, were identified as functioning in the endolysosomal system of C. elegans intestinal cells (Nunes et al 2005;Schaheen et al 2006;Schroeder et al 2007). PGP-2 is associated with the gut granule, a cell-type-specific lysosome-related organelle that functions in fat storage, where it plays an essential role in its biogenesis (Schroeder et al 2007).…”

“…pgp-2(À) embryos improperly form gut granules and mislocalize birefringent material normally found only within the gut granule into the intestinal lumen. Disrupting the function of mrp-4 suppresses the formation of enlarged, fat-containing, late endosomal/lysosomal compartments formed in the intestinal cells of cup-5(À) mutant embryos (Schaheen et al 2006). CUP-5, which is orthologous to human mucolipin-1, functions in trafficking from late endosomes to lysosomes (Treuschet al 2004).…”