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The tauopathies are a diverse class of devastating neurodegenerative disorders, characterized by the hyperphosphorylation and aggregation of the microtubule binding protein tau. Niemann-Pick type C disease (NPC) is a tauopathy that affects children, and is caused by mutations in intracellular lipid and cholesterol trafficking proteins. Loss-of-function mutations in the NPC1 gene are responsible for 95% of all NPC cases, and lead to progressive neurodegeneration and early death. To assess the extent to which tau affects NPC pathology, we generated mice that lack both NPC1 and tau. NPC1/tau double-null mutants exhibit an exacerbated NPC phenotype, including severe systemic manifestations, and die significantly earlier than NPC1 single-null mutants. Since autophagy has been previously implicated in NPC pathogenesis, we investigated the impact of tau deletion on this pathway. Acute reductions of tau in NPC1-deficient fibroblasts significantly decrease autophagic induction and flux, while having no effect on the autophagic pathway in control cells. Here we propose a model in which tau’s normal function is critical to the induction of autophagy in NPC1 deficiency, and suggest that this novel mechanism contributes to cellular dysfunction in the tauopathies.
A diverse group of neurodegenerative disorders are characterized as tauopathies, diseases demonstrating profound accumulations of the microtubule-associated protein tau. Under normal conditions, tau is primarily bound to microtubules, where it is believed to act to promote their stabilization.1 Emerging evidence also supports the concept that tau serves additional cellular functions, such as regulating anterograde and retrograde trafficking.2,3 In the tauopathies, tau becomes aberrantly hyperphosphorylated and dissociates from microtubules, leading to a loss of its normal function at this site. Hyperphosphorylated tau eventually forms paired helical filaments, which tend to aggregate into large, cytoplasmic inclusions termed neurofibrillary tangles. Although these protein aggregates are present in all tauopathies, data linking the pathogenesis of tauopathies to a fundamental toxic gain-of-function versus a loss-of-function mechanism4 are controversial and are in need of clarification.
Niemann-Pick type C disease (NPC) is a fatal and progressive neurodegenerative disorder that typically begins in childhood5 and is classified as a tauopathy.6 Approximately 95% of all NPC cases are caused by recessive loss-of-function mutations in the NPC1 gene,7 which encodes a multipass transmembrane protein localized to late endosomes and lysosomes.8–11 This protein normally regulates the transit of LDL-derived cholesterol from the endosomal/lysosomal system to other organelles, and its functional absence leads to marked cholesterol and sphingolipid accumulations. Mice with loss of function mutations in their orthologous Npc1 gene demonstrate striking phenotypic similarities to human NPC patients, including cellular accumulations of glycosphingolipids and cholesterol, hepatosplenomegaly, ataxia and premature death.12–15 Npc1−/− mice also accumulate hyperphosphorylated tau in the central nervous system, but fail to generate the neurofibrillary tangles seen in human patients.16,17 In order to assess whether tau influences NPC pathology in vivo, we generated NPC1/tau double-null mutant mice.
Whereas tau null mice (Mapt−/−) exhibit no overt abnormalities,18 Npc1−/− mice additionally lacking tau (NPC1/tau double-null mutants) display a much more severe phenotype than NPC1 single-null mutants.19 Nearly 50% of NPC1/tau double-null mutants die before 5 weeks of age, whereas NPC1 single-null mutants survive between 10 – 12 weeks. NPC1/tau double-null mutants were generated in significantly smaller litters, suggesting that tau deletion impaired fertility of NPC1 heterozygotes. Additionally, nearly all of the NPC1/tau double-null mutants exhibited curvature of the cervical spine, small size, and an abnormal gait, and most males that survived longer than 5 weeks developed penile prolapse. This constellation of findings strongly suggests a genetic relationship between NPC1 and tau that affects the severity of the disease phenotype.
Our laboratory and others have previously demonstrated that NPC1 deficiency induces macroautophagy (hereafter referred to simply as autophagy) in mice 20–22 and in primary human fibroblasts.22 Prior work established that increased flux through the autophagic pathway in NPC1 deficiency is mediated by Beclin 1, an autophagy regulator that is responsive to the lipid trafficking defects that occur in this disorder.22,23 Despite this robust induction of autophagy, ubiquitinated proteins accumulate in the brains of NPC patients and mouse models, suggesting that flux through this pathway is not sufficient to handle the quantity of proteins targeted for degradation. Situations such as this, where autophagic induction and flux are imbalanced, may lead to autophagic stress, a possible mediator of neuronal dysfunction and a precursor to cell death.24
In order to determine the mechanism by which tau loss-of-function exacerbates the NPC phenotype, we began by studying the microtubule-dependent protein degradation pathways including autophagy. We initially treated NPC1-deficient human fibroblasts with siRNA pools targeted to tau, and then measured the degradation of long-lived proteins, since this assay provides a quantitative read-out of flux through the autophagic pathway.25–27 Tau knockdown reduces degradation of long-lived proteins by ~25% in NPC1-deficient fibroblasts, and causes a marked decrease in LC3-II expression. In contrast, tau knockdown in control fibroblasts has no effect on either the degradation of long-lived proteins or the expression of LC3-II. These data demonstrate that acute reductions of tau in NPC1-deficient cells decrease induction and flux through the autophagic pathway. In contrast to the robust effects observed in cell culture, tau deletion in NPC mice produces more subtle effects on the autophagic pathway, manifest primarily by the accumulation of LC3-immunoreactive vesicles in hepatocytes.19 Whereas tau deletion impairs autophagy in NPC model systems, our analyses revealed no tau-dependent changes in the ubiquitin-proteasome pathway in NPC1-deficient mice19 or cells (M. Elrick, unpublished). We conclude that tau deletion primarily impairs protein degradation through autophagy in NPC1 deficiency.
Our studies demonstrate that tau loss-of-function exacerbates the NPC phenotype in mice, and that reduction of tau decreases activity of the autophagic pathway in NPC1-deficient fibroblasts. Although we were unable to detect quantitative differences in the levels of LC3-II or p62 between NPC1 single-null and NPC1/tau double-null mutant mice, our in vitro evidence suggests a clear role for tau in the regulation of autophagy due to NPC1 deficiency. These data lead to our working hypothesis that, in the context of NPC1 deficiency, the induction of autophagy is dependent upon functional tau, and that depletion of tau impairs this beneficial response (Fig. 1). Our in vitro experiments support this hypothesis, as acute reductions of tau in NPC1-deficient cells decrease LC3-II levels and autophagic flux. We propose that in the course of NPC disease pathogenesis, tau hyperphosphorylation leads to a progressive loss of tau’s normal function, diminishing the ability of cells to induce autophagy in response to NPC1 deficiency (Fig. 1b). We hypothesize that the loss of this protective autophagic response contributes to the accelerated disease progression exhibited by double-null mutant mice. We speculate that loss of functional tau may similarly contribute to the pathogenesis of other neurodegenerative tauopathies by impairing induction and flux through the autophagic pathway. Our studies are a critical first step linking tau to autophagy, and may lead to a better understanding of the relationship between tau and a diverse array of neurodegenerative diseases.
Tau deletion exacerbates the phenotype of Niemann-Pick type C mice and implicates autophagy in pathogenesis.
C.D. Pacheco, M.J. Elrick, A.P. Lieberman
Human Molecular Genetics 18:956-965, 2009.