Cholesterol Accumulation in NPC1-Deficient Neurons Is Ganglioside Dependent

Abstract

Niemann-Pick type C (NPC) disease is a lysosomal disorder commonly caused by a recessive mutation in NPC1, which encodes an integral membrane protein with regions of homology to the morphogen receptor, Patched, and to 3-hydroxy-3-methylglutaryl coenzyme A reductase [1, 2]. Neurons in NPC disease exhibit extensive storage of free cholesterol and glycosphingolipids (GSLs), including GM2 and GM3 gangliosides [3, 4, 5]. Most studies have viewed cholesterol storage as primary, with NPC1 functioning as a retroendocytic transporter for regulation of cholesterol homeostasis [3, 6, 7, 8]. Here, we analyze the effects of genetically depriving NPC neurons of complex gangliosides by creating mice doubly deficient in both NPC1 and the GSL synthetic enzyme, GM2/GD2 synthase (GalNAcT). Ganglioside and cholesterol expression in neurons of NPC1−/−/GalNAcT+/+, NPC1−/−/GalNAcT−/−, NPC1+/+/GalNAcT−/−, and WT mice was examined in situ by immunocytochemical and histochemical methods. Neurons in double-deficient mice lacked intraneuronal GM2 accumulation as expected, but remarkably also exhibited absence or dramatic reduction in free cholesterol. Neurons storing cholesterol consistently showed GM3 accumulation but some GM3-positive neurons lacked cholesterol storage. These findings provide a compelling argument that cholesterol sequestration in NPC1-deficient neurons is ganglioside dependent and suggest that the function of NPC1 in these cells may be more closely linked to homeostatic control of GSLs than cholesterol.