Endosomal lipid accumulation in NPC1 leads to inhibition of PKC, hypophosphorylation of vimentin and Rab9 entrapment

Abstract

Background information. Within the group of lysosomal storage diseases, NPC1 [NPC (Niemann-Pick type C) 1] disease is a lipidosis characterized by excessive accumulation of free cholesterol as well as gangliosides, glycosphingolipids and fatty acids in the late E/L (endosomal/lysosomal) system (Chen et al., 2005) due to a defect in late endosome lipid egress. We have previously demonstrated that expression of the small GTPase Rab9 in NPC1 cells can rescue the lipid transport block phenotype (Walter et al., 2003), albeit by an undefined mechanism.Results. To investigate further the mechanism by which Rab9 facilitates lipid movement from late endosomes we sought to identify novel Rab9 binding/interacting proteins. In the present study, we report that Rab9 interacts with the intermediate filament phosphoprotein vimentin and this interaction is altered by lipid accumulation in late endosomes, which results in inhibition of PKC (protein kinase C) and hypophosphorylation of vimentin, leading to late endosome dysfunction. Intermediate filament hypophosphorylation, aggregation and entrapment of Rab9 ultimately leads to transport defects and inhibition of lipid egress from late endosomes.Conclusions. These results reveal a previously unappreciated interaction between Rab proteins and intermediate filaments in regulating intracellular lipid transport.

MicroRNA-17-3p is a prostate tumor suppressor in vitro and in vivo, and is decreased in high grade prostate tumors analyzed by laser capture microdissection

“…A putative target with slightly less favorable binding energy is FAM125B (MVB12B), a subunit of the ESCRT-1 complex required for the sorting of endocytic ubiquitylated cargoes into multi-vescular bodies [44]. Vimentin has been shown to bind Rab9 and be involved in regulating intracellular lipid transport from late endosomes [45]. Changes in vesicular trafficking could affect cellular processes influencing tumor growth.…”

Secondary lipid accumulation in lysosomal disease

“…The potential pathogenic role of free sphingoid bases in Niemann-Pick type C has recently been revisited. In NPC1 fibroblasts, one group showed hypophosphorylation of vimentin as a result of PKC inhibition, leading to entrapment of rab9 and alterations in lipid trafficking [121]. In a very recent paper [122], NPC1-mutant cells were found to have a large reduction in the acidic compartment calcium store compared to wild-type cells.…”

Tracking Sphingosine Metabolism and Transport in Sphingolipidoses: NPC1 Deficiency as a Test Case

“…There are some data supporting the idea of extralysosomal Sph accumulation in NPC cells: Protein kinase C (PKC) activity is downregulated in NPC1-deficient cells and this has been linked to Sph accumulation (30,31). As Sph is known to compete with diacylglycerol for binding in the PKC regulatory domain (32) this mechanism of PKC inhibition would rely on extralysosomal Sph.…”