Autophagy and tau in Niemann-Pick Type C disease

Abstract

Niemann-Pick type C disease (NPC) is an autosomal recessive lipid storage disorder characterized by disrupted sphingolipid and cholesterol trafficking that produces cognitive impairment, ataxia and death, often in childhood. Most cases are caused by loss of function mutations in the Npc1 gene. We first demonstrated that NPC1 deficient primary human fibroblasts and npc1 -/- mice showed increased autophagy, a bulk protein degradation pathway that has been implicated in the pathogenesis of several neurodegenerative disorders. Autophagy due to NPC1 deficiency was associated with increased expression of Beclin-1, and siRNA knock-down of Beclin-1 decreased the rate of protein degradation. Our data defined a critical role for Beclin-1 in the activation of autophagy in NPC.

Efficient protein degradation by autophagy requires trafficking along microtubules. NPC is characterized by the accumulation of hyperphosphorylated and aggregated tau, a microtubule binding protein implicated in the regulation of intracellular transport. To determine whether these changes in tau contribute to NPC pathogenesis, we decreased tau expression in cellular and mouse models of NPC. In NPC1 deficient primary human fibroblasts, siRNA knock-down of tau decreased autophagy. NPC1/tau double null mutant mice died significantly earlier and were generated in significantly smaller litters than NPC1 single null mutants, demonstrating a genetic interaction between Mapt and Npc1. Surviving double null mutants exhibited an enhanced systemic phenotype that included mild facial dysmorphia, kyphosis and an abnormal, toe-walking gait. Our data established that tau modifies the severity of the NPC phenotype through a loss-of-function mechanism that does not involve neurofibrillary tangle formation or tau protein aggregation, and is associated with impaired activation of autophagy.

The study of NPC pathogenesis has been limited by the lack of mammalian model systems in which Npc1 gene expression can be manipulated in a cell type or temporally controlled manner. To address this problem, in the final part of my thesis I used gene targeting to generate a conditional null mutant mouse in which Npc1 exon 9 is flanked by loxP sites. Studies utilizing these mice are expected to provide insights into the mechanism of neurodegeneration in NPC and lipid trafficking in the CNS.