Autophagy and Cell Autonomous Neurodegeneration in Niemann-Pick type C Disease.

Abstract

Niemann-Pick type C disease (NPC) is an autosomal recessive lysosomal storage disorder characterized by liver dysfunction and neurodegeneration causing diverse neurologic symptoms such as ataxia, cognitive decline, and seizures, and finally leading to premature death. It is caused by mutations in the NPC1 or NPC2 genes, leading to accumulation of cholesterol and glycosphingolipids in late endosomes and lysosomes of all tissues. At present, the link between lipid storage and neurodegeneration is unknown, representing a major impediment to the development of effective therapies for NPC disease. In chapter 2, I characterize a novel conditional knockout mouse model for NPC disease and use it to determine the extent to which the degeneration of cerebellar Purkinje cells is cell autonomous. Deletion of Npc1 only in Purkinje cells was sufficient to cause their degeneration, and lead to symptoms of ataxia and tremors. However, these mice did not demonstrate the weight loss or premature death that is characteristic of global Npc1 mutants, demonstrating that these phenotypes arise from other cell types. I also noted a marked differential vulnerability to degeneration among Purkinje cell subpopulations. In chapter 3, I use bioinformatic methods to identify 16 genes whose expression patterns correlated strongly with this pattern of Purkinje cell death. One of these genes, Hsp27, promotes the survival of neurons in an in vitro model of NPC neurodegeneration. In chapter 4, I identify autophagy as a contributor to neurodegeneration in NPC. Autophagy is a pathway for delivering cytoplasmic cargoes to the lysosome via double-membrane bound organelles known as autophagosomes. It has been closely linked to the process of neurodegeneration, and the induction of autophagy along with accumulation of autophagosomes has been documented in NPC disease. Here I show that autophagy induction lies downstream of Toll-like receptor signaling through the adapter protein TRIF. Further, autophagy is a major source for stored cholesterol in NPC lysosomes. Finally, lipid storage impairs the maturation of autolysosomes via inhibition of lysosomal cathepsin activity. Inhibition of autophagy by wortmannin reduced cholesterol storage, restored lysosomal proteolysis, and rescued neurodegeneration in vitro, thus demonstrating that autophagy plays a detrimental role in NPC pathogenesis.