Tau deletion exacerbates the phenotype of Niemann-Pick type C mice and implicates autophagy in pathogenesis

Amyloid-β/Fyn-induced synaptic, network, and cognitive impairments depend on tau levels in multiple mouse models of Alzheimer's disease

Alzheimer's disease (AD), the most common neurodegenerative disorder, is a growing public health problem and still lacks effective treatments. Recent evidence suggests that microtubule-associated protein tau may mediate amyloid-β peptide (Aβ) toxicity by modulating the tyrosine kinase Fyn. We showed previously that tau reduction prevents, and Fyn overexpression exacerbates, cognitive deficits in human amyloid precursor protein (hAPP) transgenic mice overexpressing Aβ. However, the mechanisms by which Aβ, tau, and Fyn cooperate in AD-related pathogenesis remain to be fully elucidated. Here we examined the synaptic and network effects of this pathogenic triad. Tau reduction prevented cognitive decline induced by synergistic effects of Aβ and Fyn. Tau reduction also prevented synaptic transmission and plasticity deficits in hAPP mice. Using electroencephalography to examine network effects, we found that tau reduction prevented spontaneous epileptiform activity in multiple lines of hAPP mice. Tau reduction also reduced the severity of spontaneous and chemically induced seizures in mice overexpressing both Aβ and Fyn. To better understand these protective effects, we recorded whole-cell currents in acute hippocampal slices from hAPP mice with and without tau. hAPP mice with tau had increased spontaneous and evoked excitatory currents, reduced inhibitory currents, and NMDA receptor dysfunction. Tau reduction increased inhibitory currents and normalized excitation/inhibition balance and NMDA receptor-mediated currents in hAPP mice. Our results indicate that Aβ, tau, and Fyn jointly impair synaptic and network function and suggest that disrupting the copathogenic relationship between these factors could be of therapeutic benefit.

Molecular signaling how do axons die?

Axons depend critically on axonal transport both for supplying materials and for communicating with cell bodies. This chapter looks at each activity, asking what aspects are essential for axon survival. Axonal transport declines in neurodegenerative disorders, such as Alzheimer's disease, amyotrophic lateral sclerosis, and multiple sclerosis, and in normal ageing, but whether all cargoes are equally affected and what limits axon survival remains unclear. Cargoes can be differentially blocked in some disorders, either individually or in groups. Each missing protein cargo results in localized loss-of-function that can be partially modeled by disrupting the corresponding gene, sometimes with surprising results. The axonal response to losing specific proteins also depends on the rates of protein turnover and on whether the protein can be locally synthesized. Among cargoes with important axonal roles are components of the PI3 kinase, Mek/Erk, and Jnk signaling pathways, which help to communicate with cell bodies and to regulate axonal transport itself. Bidirectional trafficking of Bdnf, NT-3, and other neurotrophic factors contribute to intra- and intercellular signaling, affecting the axon's cellular environment and survival. Finally, several adhesion molecules and gangliosides are key determinants of axon survival, probably by mediating axon-glia interactions. Thus, failure of long-distance intracellular transport can deprive axons of one, few, or many cargoes. This can lead to axon degeneration either directly, through the absence of essential axonal proteins, or indirectly, through failures in communication with cell bodies and nonneuronal cells.

Lessons learnt from animal models: pathophysiology of neuropathic lysosomal storage disorders

Approximately 50 inborn errors of metabolism known as lysosomal storage disorders have been discovered to date, most of which are due to a single mutation in a gene encoding a soluble lysosomal enzyme. Consequently, inadequate enzyme activity results in the accumulation of substrates for that enzyme, invariably accompanied by a wide variety of secondary pathological changes. Many of these conditions remain untreatable, and therefore, research into pathogenic processes and potential treatment strategies is intense. A key tool for researchers in this area is the availability of clinically relevant animal models in which to study disease manifestation and evaluate therapeutic outcomes. Large numbers of both naturally occurring and genetically modified animal models of neurodegenerative lysosomal storage disorders are in existence, with spontaneous models occurring in both large domestic (e.g., cat, dog, sheep) and small (e.g., mouse) animal species. Many have undergone rigorous phenotypic characterization and are now providing us with insights into neurological disease processes. The purpose of this review is to highlight some of the major lessons learnt from these studies.

Autophagy gone awry in neurodegenerative diseases

Autophagy is essential for neuronal homeostasis, and its dysfunction has been directly linked to a growing number of neurodegenerative disorders. The reasons behind autophagic failure in degenerating neurons can be very diverse because of the different steps required for autophagy and the characterization of the molecular players involved in each of them. Understanding the step(s) affected in the autophagic process in each disorder could explain differences in the course of these pathologies and will be essential to developing targeted therapeutic approaches for each disease based on modulation of autophagy. Here we present examples of different types of autophagic dysfunction described in common neurodegenerative disorders and discuss the prospect of exploring some of the recently identified autophagic variants and the interactions among autophagic and non-autophagic proteolytic systems as possible future therapeutic targets.

Defective autophagy in neurons and astrocytes from mice deficient in PI(3,5)P2

Mutations affecting the conversion of PI3P to the signaling lipid PI(3,5)P₂ result in spongiform degeneration of mouse brain and are associated with the human disorders Charcot–Marie–Tooth disease and amyotrophic lateral sclerosis (ALS). We now report accumulation of the proteins LC3-II, p62 and LAMP-2 in neurons and astrocytes of mice with mutations in two components of the PI(3,5)P₂ regulatory complex, Fig4 and Vac14. Cytoplasmic inclusion bodies containing p62 and ubiquinated proteins are present in regions of the mutant brain that undergo degeneration. Co-localization of p62 and LAMP-2 in affected cells indicates that formation or recycling of the autolysosome is impaired. These results establish a role for PI(3,5)P₂ in autophagy in the mammalian central nervous system (CNS) and demonstrate that mutations affecting PI(3,5)P₂ can contribute to inclusion body disease.

Metabolic adaptations to interrupted glycosaminoglycan recycling

Lysosomal storage diseases (LSD) are metabolic disorders characterized by accumulation of undegraded material. The mucopolysaccharidoses (MPS) are LSDs defined by the storage of glycosaminoglycans. Previously, we hypothesized that cells affected with LSD have increased energy expenditure for biosynthesis because of deficiencies of raw materials sequestered within the lysosome. Thus, LSDs can be characterized as diseases of deficiency as well as overabundance (lysosomal storage). In this study, metabolite analysis identified deficiencies in simple sugars, nucleotides, and lipids in the livers of MPSI mice. In contrast, most amino acids, amino acid derivatives, dipeptides, and urea were elevated. These data suggest that protein catabolism, perhaps because of increased autophagy, is at least partially fulfilling intermediary metabolism. Thus, maintaining glycosaminoglycan synthesis in the absence of recycled precursors results in major shifts in the energy utilization of the cells. A high fat diet increased simple sugars and some fats and lowered the apparent protein catabolism. Interestingly, autophagy, which is increased in several LSDs, is responsive to dietary intervention and is reduced in MPSVII and MPSI mice fed a high fat diet. Although long term dietary treatment improved body weight in MPSVII mice, it failed to improve life span or retinal function. In addition, the ventricular hypertrophy and proximal aorta dilation observed in MPSVII mice were unchanged by a high fat, simple sugar diet. As the mechanism of this energy imbalance is better understood, a more targeted nutrient approach may yet prove beneficial as an adjunct therapy to traditional approaches.

Tau normal function influences Niemann-Pick type C disease pathogenesis in mice and modulates autophagy in NPC1-deficient cells

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 NPC1gene are responsible for 95 percent 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.