Endocytic trafficking of glycosphingolipids in sphingolipid storage diseases.

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Abstract
In this review, recent studies of membrane lipid transport in sphingolipid (SL) storage disease (SLSD) fibroblasts are summarized. Several fluorescent glycosphingolipid (GSL) analogues are internalized from the plasma membrane via caveolae and are subsequently transported to the Golgi complex of normal fibroblasts, while in 10 different SLSD cell types, these lipids accumulate in endosomes and lysosomes. Additional studies have shown that cholesterol homeostasis is perturbed in multiple SLSDs secondary to accumulation of endogenous SLs, and that mis–targeting of the GSLs is regulated by cellular cholesterol. Golgi targeting of GSLs internalized via caveolae is dependent on microtubules and phosphoinositide 3–kinase(s) and is inhibited by expression of dominant–negative rab7 and rab9 constructs. Overexpression of wild–type rab7 or rab9 (but not rab11) in Niemann–Pick C fibroblasts results in correction of lipid trafficking defects, including restoration of Golgi targeting of fluorescent lactosylceramide and endogenous GM1 ganglioside (monitored by the transport of fluorescent cholera toxin), and a dramatic reduction in accumulation of intracellular cholesterol. These results suggest an approach for restoring normal lipid trafficking in this, and perhaps other, SLSD cell types, and may provide a basis for future therapy of these diseases.