Acetylation as a major determinant to microtubule-dependent autophagy: Relevance to Alzheimer's and Parkinson disease pathology

Protein post-translational modifications (PTMs) that potentiate protein aggregation have been implicated in several neurological disorders, including Alzheimer's (AD) and Parkinson's disease (PD). In fact, Tau and alpha-synuclein (ASYN) undergo several PTMs potentiating their aggregation and neurotoxicity.Recent data posits a role for acetylation in Tau and ASYN aggregation. Herein we aimed to clarify the role of Sirtuin-2 (SIRT2) and HDAC6 tubulin deacetylases as well as p300 acetyltransferase in AD and PD neurodegeneration. We used transmitochondrial cybrids that recapitulate pathogenic alterations observed in sporadic PD and AD patient brains and ASYN and Tau cellular models.We confirmed that Tau protein and ASYN are microtubules (MTs)-associated proteins (MAPs). Moreover, our results suggest that α-tubulin acetylation induced by SIRT2 inhibition is functionally associated with the improvement of MT dynamic determined by decreased Tau phosphorylation and by increased Tau/tubulin and ASYN/tubulin binding. Our data provide a strong evidence for a functional role of tubulin and MAPs acetylation on autophagic vesicular traffic and cargo clearance. Additionally, we showed that an accumulation of ASYN oligomers imbalance mitochondrial dynamics, which further compromise autophagy. We also demonstrated that an increase in Tau acetylation is associated with Tau phosphorylation. We found that p300, HDAC6 and SIRT2 influences Tau phosphorylation and autophagic flux in AD. In addition, we demonstrated that p300 and HDAC6 modulate Tau and Tubulin acetylation.Overall, our data disclose the role of Tau and ASYN modifications through acetylation in AD and PD pathology, respectively. Moreover, this study indicates that MTs can be a promising therapeutic target in the field of neurodegenerative disorders in which intracellular transport is altered.     

Tau aggregates improve the Purinergic receptor P2Y12-associated podosome rearrangements in microglial cells

Alzheimer's disease (AD) is a progressive neurodegenerative disease that is associated with protein misfolding, plaque accumulation, neuronal dysfunction, synaptic loss, and cognitive decline. The pathological cascade of AD includes the intracellular Tau hyperphosphorylation and its subsequent aggregation, extracellular Amyloid-β plaque formation and microglia-mediated neuroinflammation. The extracellular release of aggregated Tau is sensed by surveilling microglia through the involvement of various cell surface receptors. Among all, purinergic P2Y12R signaling is involved in microglial chemotaxis towards the damaged neurons. Microglial migration is highly linked with membrane-associated actin remodeling leading to the phagocytosis of extracellular Tau species. Here, we studied the formation of various actin structures such as podosome, lamellipodia and filopodia, in response to extracellular Tau monomers and aggregates. Microglial podosomes are colocalized with actin nucleator protein WASP, Arp2 and TKS5 adaptor protein during Tau-mediated migration. Moreover, the P2Y12 receptors were associated with F-actin-rich podosome structures, which signify the potential of Tau aggregates in microglial chemotaxis through the involvement of actin remodeling.     

Tau missorting and spastin-induced microtubule disruption in neurodegeneration: Alzheimer Disease and Hereditary Spastic Paraplegia

In Alzheimer Disease (AD), the mechanistic connection of the two major pathological hallmarks, namely deposition of Amyloid-beta (Aβ) in the form of extracellular plaques, and the pathological changes of the intracellular protein Tau (such as phosphorylation, missorting, aggregation), is not well understood. Genetic evidence from AD and Down Syndrome (Trisomy 21), and animal models thereof, suggests that aberrant production of Aβ is upstream of Tau aggregation, but also points to Tau as a critical effector in the pathological process. Yet, the cascade of events leading from increased levels of Aβ to Tau-dependent toxicity remains a matter of debate.Using primary neurons exposed to oligomeric forms of Aβ, we have found that Tau becomes mislocalized (missorted) into the somatodendritic compartment. Missorting of Tau correlates with loss of microtubules and downstream consequences such as loss of mature spines, loss of synaptic activity, and mislocalization of mitochondria.In this cascade, missorting of Tau induces mislocalization of TTLL6 (Tubulin-Tyrosine-Ligase-Like 6) into the dendrites. TTLL6 induces polyglutamylation of microtubules, which acts as a trigger for spastin mediated severing of dendritic microtubules. Loss of microtubules makes cells unable to maintain transport of mitochondria, which in turn results in synaptic dysfunction and loss of mature spines. These pathological changes are absent in TauKO derived primary neurons. Thus, Tau mediated mislocalization of TTLL6 and spastin activation reveals a pathological gain of function for Tau and spastin in this cellular model system of AD.In contrast, in hereditary spastic paraplegia (HSP) caused by mutations of the gene encoding spastin (spg4 alias SPAST), spastin function in terms of microtubule severing is decreased at least for the gene product of the mutated allele, resulting in overstable microtubules in disease model systems. Whether total spastin severing activity or microtubule stability in human disease is also affected is not yet clear. No human disease has been associated so far with the long-chain polyglutamylation enzyme TTLL6, or the other TTLLs (1,5,11) possibly involved.Here we review the findings supporting a role for Tau, spastin and TTLL6 in AD and other tauopathies, HSP and neurodegeneration, and summarize possible therapeutic approaches for AD and HSP.     

Amyloidogenesis of Tau protein

The role of microtubule‐associated protein Tau in neurodegeneration has been extensively investigated since the discovery of Tau amyloid aggregates in the brains of patients with Alzheimer's disease (AD). The process of formation of amyloid fibrils is known as amyloidogenesis and attracts much attention as a potential target in the prevention and treatment of neurodegenerative conditions linked to protein aggregation. Cerebral deposition of amyloid aggregates of Tau is observed not only in AD but also in numerous other tauopathies and prion diseases. Amyloidogenesis of intrinsically unstructured monomers of Tau can be triggered by mutations in the Tau gene, post‐translational modifications, or interactions with polyanionic molecules and aggregation‐prone proteins/peptides. The self‐assembly of amyloid fibrils of Tau shares a number of characteristic features with amyloidogenesis of other proteins involved in neurodegenerative diseases. For example, in vitro experiments have demonstrated that the nucleation phase, which is the rate‐limiting stage of Tau amyloidogenesis, is shortened in the presence of fragmented preformed Tau fibrils acting as aggregation templates (“seeds”). Accordingly, Tau aggregates released by tauopathy‐affected neurons can spread the neurodegenerative process in the brain through a prion‐like mechanism, originally described for the pathogenic form of prion protein. Moreover, Tau has been shown to form amyloid strains—structurally diverse self‐propagating aggregates of potentially various pathological effects, resembling in this respect prion strains. Here, we review the current literature on Tau aggregation and discuss mechanisms of propagation of Tau amyloid in the light of the prion‐like paradigm.     

运动与阿尔茨海默病：基于Tau蛋白翻译后修饰视角的研究述评

阿尔茨海默病（Alzheimer’s disease,AD）是一种与年龄有关的神经退行性疾病,严重危害老年人的身心健康,给社会带来巨大的经济压力。但目前其发病机制尚不完全明确,临床仍无根治的有效方法。Tau蛋白是一种微管相关蛋白质,能够参与维持微管相关结构稳定,具有可溶性且不会聚集。在AD病理状态下,病人脑内Tau蛋白结构和功能异常。异常的Tau蛋白聚集成不可溶的神经纤维缠结,损害微管运输能力,导致病人认知功能障碍。Tau蛋白结构和功能的改变是由多种翻译后修饰过程来调控的,即将特定的化学修饰基团与Tau蛋白N-端或C-端结合,直接改变蛋白质的性质和功能。AD病人脑内Tau蛋白的磷酸化、糖基化、乙酰化及SUMO化等多种翻译后修饰异常,与Tau蛋白的降解和毒性物质的聚集密切相关。本文综述近年来的研究后发现,运动可以通过改善Tau蛋白翻译后的某些异常修饰来预防和改善AD,主要作用方式如下：(1）运动可通过抑制GSK 3β和MAPK等蛋白激酶活性来抑制Tau蛋白的过度磷酸化,可能通过上调PP2A活性来促进Tau蛋白去磷酸化;(2）运动可通过提高GLUT1和GLUT3蛋白质水平,可能通过调节OGA和OGT活性平衡,提高蛋白质O-GlcNAc糖基化水平;(3）运动可能通过AMPK/mTORC1途径抑制p300以及激活SIRT1,降低Tau蛋白乙酰化水平;同时运动还可能通过抑制HDAC6,改善Tau蛋白KXGS基序异常乙酰化程度;(4）运动可能通过调节磷酸化与SUMO化共定位点,改善Tau蛋白异常SUMO化水平。     

Neurotoxicity of oligomers of phosphorylated Tau protein carrying tauopathy-associated mutation is inhibited by prion protein

Tauopathies, including Alzheimer's disease (AD), are manifested by the deposition of well-characterized amyloid aggregates of Tau protein in the brain. However, it is rather unlikely that these aggregates constitute the major form of Tau responsible for neurodegenerative changes. Currently, it is postulated that the intermediates termed as soluble oligomers, assembled on the amyloidogenic pathway, are the most neurotoxic form of Tau. However, Tau oligomers reported so far represent a population of poorly characterized, heterogeneous and unstable assemblies. In this study, to obtain the oligomers, we employed the aggregation-prone K18 fragment of Tau protein with deletion of Lys280 (K18Δ280) linked to a hereditary tauopathy. We have described a new procedure of inducing aggregation of mutated K18 which leads either to the formation of nontoxic amyloid fibrils or neurotoxic globular oligomers, depending on its phosphorylation status. We demonstrate that PKA-phosphorylated K18Δ280 oligomers are toxic to hippocampal neurons, which is manifested by loss of dendritic spines and neurites, and impairment of cell-membrane integrity leading to cell death. We also show that N1, the soluble N-terminal fragment of prion protein (PrP), protects neurons from the oligomers-induced cytotoxicity. Our findings support the hypothesis on the neurotoxicity of Tau oligomers and neuroprotective role of PrP-derived fragments in AD and other tauopathies. These observations could be useful in the development of therapeutic strategies for these diseases.     

GSK3β is involved in the relief of mitochondria pausing in a Tau-dependent manner

Mitochondrial trafficking deficits have been implicated in the pathogenesis of several neurological diseases, including Alzheimer's disease (AD). The Ser/Thre kinase GSK3β is believed to play a fundamental role in AD pathogenesis. Given that GSK3β substrates include Tau protein, here we studied the impact of GSK3β on mitochondrial trafficking and its dependence on Tau protein. Overexpression of GSK3β in neurons resulted in an increase in motile mitochondria, whereas a decrease in the activity of this kinase produced an increase in mitochondria pausing. These effects were dependent on Tau proteins, as Tau (-/-) neurons did not respond to distinct GSK3β levels. Furthermore, differences in GSK3β expression did not affect other parameters like mitochondria velocity or mitochondria run length. We conclude that GSK3B activity regulates mitochondrial axonal trafficking largely in a Tau-dependent manner.     

Adult neurogenic process in the subventricular zone‐olfactory bulb system is regulated by Tau protein under prolonged stress

ObjectivesThe area of the subventricular zone (SVZ) in the adult brain exhibits the highest number of proliferative cells, which, together with the olfactory bulb (OB), maintains constant brain plasticity through the generation, migration and integration of newly born neurons. Despite Tau and its malfunction is increasingly related to deficits of adult hippocampal neurogenesis and brain plasticity under pathological conditions [e.g. in Alzheimer''s disease (AD)], it remains unknown whether Tau plays a role in the neurogenic process of the SVZ and OB system under conditions of chronic stress, a well‐known sculptor of brain and risk factor for AD.Materials and methodsDifferent types of newly born cells in SVZ and OB were analysed in animals that lack Tau gene (Tau‐KO) and their wild‐type littermates (WT) under control or chronic stress conditions.ResultsWe demonstrate that chronic stress reduced the number of proliferating cells and neuroblasts in the SVZ leading to decreased number of newborn neurons in the OB of adult WT, but not Tau‐KO, mice. Interestingly, while stress‐evoked changes were not detected in OB granular cell layer, Tau‐KO exhibited increased number of mature neurons in this layer indicating altered neuronal migration due to Tau loss.ConclusionsOur findings suggest the critical involvement of Tau in the neurogenesis suppression of SVZ and OB neurogenic niche under stressful conditions highlighting the role of Tau protein as an essential regulator of stress‐driven plasticity deficits.     

N-acetylcholinesterase-induced apoptosis in Alzheimer's disease

Background

Alzheimer''s disease (AD) involves loss of cholinergic neurons and Tau protein hyper-phosphorylation. Here, we report that overexpression of an N-terminally extended “synaptic” acetylcholinesterase variant, N-AChE-S is causally involved in both these phenomena.

Methodology and Principal Findings

In transfected primary brain cultures, N-AChE-S induced cell death, morphological impairments and caspase 3 activation. Rapid internalization of fluorescently labeled fasciculin-2 to N-AChE-S transfected cells indicated membranal localization. In cultured cell lines, N-AChE-S transfection activated the Tau kinase GSK3, induced Tau hyper-phosphorylation and caused apoptosis. N-AChE-S-induced cell death was suppressible by inhibiting GSK3 or caspases, by enforced overexpression of the anti-apoptotic Bcl2 proteins, or by AChE inhibition or silencing. Moreover, inherent N-AChE-S was upregulated by stressors inducing protein misfolding and calcium imbalances, both characteristic of AD; and in cortical tissues from AD patients, N-AChE-S overexpression coincides with Tau hyper-phosphorylation.

Conclusions

Together, these findings attribute an apoptogenic role to N-AChE-S and outline a potential value to AChE inhibitor therapeutics in early AD.     

Extracellular Tau Levels Are Influenced by Variability in Tau That Is Associated with Tauopathies

Tauopathies are a class of neurodegenerative diseases marked by intracellular aggregates of hyperphosphorylated Tau. These diseases may occur by sporadic mechanisms in which genetic variants represent risk factors for disease, as is the case in Alzheimer disease (AD). In AD, cerebrospinal fluid (CSF) levels of soluble Tau/pTau-181 are higher in cases compared with controls. A subset of frontotemporal dementia (FTD) cases occur by a familial mechanism in which MAPT, the gene that encodes Tau, mutations are dominantly inherited. In symptomatic FTD patients expressing a MAPT mutation, CSF Tau levels are slightly elevated but are significantly lower than in AD patients. We sought to model CSF Tau changes by measuring extracellular Tau in cultured cells. Full-length, monomeric extracellular total Tau and pTau-181 were detectable in human neuroblastoma cells expressing endogenous Tau, in human non-neuronal cells overexpressing wild-type Tau, and in mouse cortical neurons. Tau isoforms influence the rate of Tau release, whereby the N terminus (exons 2/3) and microtubule binding repeat length contribute to Tau release from the cell. Compared with cells overexpressing wild-type Tau, cells overexpressing FTD-associated MAPT mutations produce significantly less extracellular total Tau without altering intracellular total Tau levels. This study demonstrates that cells actively release Tau in the absence of disease or toxicity, and Tau release is modified by changes in the Tau protein that are associated with tauopathies.     

A current view on Tau protein phosphorylation in Alzheimer's disease

The functions of the neuronal microtubule-associated protein Tau in the central nervous system are regulated by manifold posttranslational modifications at more than 50 sites. Tau in healthy neurons carries multiple phosphate groups, mostly in its microtubule assembly domain. Elevated phosphorylation and aggregation of Tau are widely considered pathological hallmarks in Alzheimer’s disease (AD) and other tauopathies, triggering the quest for Tau posttranslational modifications in the disease context. However, the phosphorylation patterns of physiological and pathological Tau are surprisingly similar and heterogenous, making it difficult to identify specific modifications as therapeutic targets and biomarkers for AD. We present a concise summary of - and view on - important previous and recent advances in Tau phosphorylation analysis in the context of AD.     

Propagation of Tau Misfolding from the Outside to the Inside of a Cell

Tauopathies are neurodegenerative diseases characterized by aggregation of the microtubule-associated protein Tau in neurons and glia. Although Tau is normally considered an intracellular protein, Tau aggregates are observed in the extracellular space, and Tau peptide is readily detected in the cerebrospinal fluid of patients. Tau aggregation occurs in many diseases, including Alzheimer disease and frontotemporal dementia. Tau pathology begins in discrete, disease-specific regions but eventually involves much larger areas of the brain. It is unknown how this propagation of Tau misfolding occurs. We hypothesize that extracellular Tau aggregates can transmit a misfolded state from the outside to the inside of a cell, similar to prions. Here we show that extracellular Tau aggregates, but not monomer, are taken up by cultured cells. Internalized Tau aggregates displace tubulin, co-localize with dextran, a marker of fluid-phase endocytosis, and induce fibrillization of intracellular full-length Tau. These intracellular fibrils are competent to seed fibril formation of recombinant Tau monomer in vitro. Finally, we observed that newly aggregated intracellular Tau transfers between co-cultured cells. Our data indicate that Tau aggregates can propagate a fibrillar, misfolded state from the outside to the inside of a cell. This may have important implications for understanding how protein misfolding spreads through the brains of tauopathy patients, and it is potentially relevant to myriad neurodegenerative diseases associated with protein misfolding.Tau filament deposition in Alzheimer disease (AD),² frontotemporal dementia (FTD), and other tauopathies correlates closely with cognitive dysfunction and cell death (1). Mutations in the tau gene cause autosomal dominant tauopathy, implicating Tau as the proximal cause (2–4). Specific disease phenotypes are defined by the early sites of pathology. For example, AD is characterized by memory loss that derives from involvement of hippocampal neurons, whereas FTD is characterized by personality changes that result from frontal lobe involvement (5). Pathology ultimately spreads to involve much larger regions of brain. Studies on patients with AD show a progressive, stereotyped spread of Tau deposits from the transentorhinal cortex to the hippocampus, and eventually to most cortical areas (6–8). Others have correlated the distribution of neurofibrillary tangles of Tau in AD brains with trans-synaptic distance from the affected areas (9). A similar spread affecting different subsets of neurons has been observed in other sporadic tauopathies, such as progressive supranuclear palsy (10). It is unknown why Tau misfolding progresses through the brain, whether it is a sequence of cell autonomous processes or whether a toxic factor is involved. Loss of synaptic connections and cell death may expose healthy cells to toxic factors and decrease available neurotrophins (11, 12). Another possibility is that the Tau protein itself serves as the agent of trans-cellular propagation. For example, it has been shown that extracellular Tau is toxic to cultured neuronal cells (13, 14). This is consistent with the observation that immunotherapy against Tau reduces pathology in a mouse model (15).Tau is well known as an intracellular protein that stabilizes microtubule filaments (16); however, it is readily detected in cerebrospinal fluid (17) and as extracellular aggregates, termed “ghost tangles,” in diseased brain. These are comprised predominantly of the microtubule-binding region (MTBR), the functional and pathogenic core of the Tau protein (18). We hypothesize that Tau aggregates present in the extracellular space enter naive cells and induce misfolding of intracellular Tau. We have tested this idea using cellular studies, biochemistry, and atomic force microscopy (AFM).     

Intracellular Clusterin Interacts with Brain Isoforms of the Bridging Integrator 1 and with the Microtubule-Associated Protein Tau in Alzheimer's Disease

Sporadic or late-onset Alzheimer''s disease (AD) is expected to affect 50% of individuals reaching 85 years of age. The most significant genetic risk factor for late-onset AD is the e4 allele of APOE gene encoding apolipoprotein E, a lipid carrier shown to modulate brain amyloid burden. Recent genome-wide association studies have uncovered additional single nucleotide polymorphisms (SNPs) linked to AD susceptibility, including those in the CLU and BIN1 genes encoding for clusterin (CLU) and the bridging integrator 1 (BIN1) proteins, respectively. Because CLU has been implicated in brain amyloid-β (Aβ) clearance in mouse models of amyloid deposition, we sought to investigate whether an AD-linked SNP in the CLU gene altered Aβ42 biomarker levels in the cerebrospinal fluid (CSF). Instead, we found that the CLU rs11136000 SNP modified CSF levels of the microtubule-associated protein Tau in AD patients. We also found that an intracellular form of CLU (iCLU) was upregulated in the brain of Tau overexpressing Tg4510 mice, but not in Tg2576 amyloid mouse model. By overexpressing iCLU and Tau in cell culture systems we discovered that iCLU was a Tau-interacting protein and that iCLU associated with brain-specific isoforms of BIN1, also recently identified as a Tau-binding protein. Through expression analysis of CLU and BIN1 variants, we found that CLU and BIN1 interacted via their coiled-coil motifs. In co-immunoprecipitation studies using human brain tissue, we showed that iCLU and the major BIN1 isoform expressed in neurons were associated with modified Tau species found in AD. Finally, we showed that expression of certain coding CLU variants linked to AD risk led to increased levels of iCLU. Together, our findings suggest that iCLU and BIN1 interaction might impact Tau function in neurons and uncover potential new mechanisms underlying the etiology of Tau pathology in AD.     

Tau reduction does not prevent motor deficits in two mouse models of Parkinson's disease

Many neurodegenerative diseases are increasing in prevalence and cannot be prevented or cured. If they shared common pathogenic mechanisms, treatments targeting such mechanisms might be of benefit in multiple conditions. The tau protein has been implicated in the pathogenesis of diverse neurodegenerative disorders, including Alzheimer's disease (AD) and Parkinson's disease (PD). Tau reduction prevents cognitive deficits, behavioral abnormalities and other pathological changes in multiple AD mouse models. Here we examined whether tau reduction also prevents motor deficits and pathological alterations in two mouse models of PD, generated by unilateral striatal injection of 6-hydroxydopamine (6-OHDA) or transgene-mediated neuronal expression of human wildtype α-synuclein. Both models were evaluated on Tau(+/+), Tau(+/-) and Tau(-/-) backgrounds in a variety of motor tests. Tau reduction did not prevent motor deficits caused by 6-OHDA and slightly worsened one of them. Tau reduction also did not prevent 6-OHDA-induced loss of dopaminergic terminals in the striatum. Similarly, tau reduction did not prevent motor deficits in α-synuclein transgenic mice. Our results suggest that tau has distinct roles in the pathogeneses of AD and PD and that tau reduction may not be of benefit in the latter condition.     

Tau蛋白在阿尔茨海默病中的作用

阿尔茨海默病(AD)是非常普遍的神经变性性疾病并且是老年人痴呆的主要原因。AD患者的症状特点包括进行性的认知障碍、记忆丧失和行为障碍,与大脑中的病理变化密切相关。AD现成为全球最严重的健康和社会经济问题。在AD患者脑中神经纤维网或神经营养障碍的过程中存在tau蛋白的异常。tau蛋白丧失其促微管组装的生物学功能,导致细胞骨架的破坏、丝状物形成和神经缠结,轴突运输损害,进而导致突触蛋白失去功能和神经退行性病变。其数量和结构的改变将会影响其功能而且会出现异常聚集。调节Tau蛋白的异常聚集的分子机制主要是一些翻译后修饰使其结构及构象发生变化。因此,异常磷酸化和截断的tau蛋白作为tau蛋白病理过程的关键机制而引起学者关注。本文描述了tau蛋白的结构和功能及其在AD中的主要病理变化,同时在本文中还涉及到磷酸化的tau蛋白是神经元对氧化应激的代偿反应这一观点。对tau蛋白进行更加全面的解读。     

Tau Oligomers Impair Artificial Membrane Integrity and Cellular Viability

The microtubule-associated protein Tau is mainly expressed in neurons, where it binds and stabilizes microtubules. In Alzheimer disease and other tauopathies, Tau protein has a reduced affinity toward microtubules. As a consequence, Tau protein detaches from microtubules and eventually aggregates into β-sheet-containing filaments. The fibrillization of monomeric Tau to filaments is a multistep process that involves the formation of various aggregates, including spherical and protofibrillar oligomers. Previous concepts, primarily developed for Aβ and α-synuclein, propose these oligomeric intermediates as the primary cytotoxic species mediating their deleterious effects through membrane permeabilization. In the present study, we thus analyzed whether this concept can also be applied to Tau protein. To this end, viability and membrane integrity were assessed on SH-SY5Y neuroblastoma cells and artificial phospholipid vesicles, treated with Tau monomers, Tau aggregation intermediates, or Tau fibrils. Our findings suggest that oligomeric Tau aggregation intermediates are the most toxic compounds of Tau fibrillogenesis, which effectively decrease cell viability and increase phospholipid vesicle leakage. Our data integrate Tau protein into the class of amyloidogenic proteins and enforce the hypothesis of a common toxicity-mediating mechanism for amyloidogenic proteins.     

综述:Tau蛋白过度磷酸化机制及其在阿尔茨海默病神经元变性中的作用

Tau蛋白是神经元中含量最高的微管相关蛋白,其经典生物学功能是促进微管组装和维持微管的稳定性．在阿尔茨海默病(Alzheimer's disease,AD)患者,异常过度磷酸化的Tau蛋白以配对螺旋丝结构形成神经原纤维缠结并在神经元内聚积．大量研究提示,Tau蛋白异常在AD患者神经变性和学习记忆障碍的发生发展中起重要作用．本课题组对Tau蛋白异常磷酸化的机制及其对细胞的影响进行了系列研究,发现Tau蛋白表达和磷酸化具有调节细胞生存命运的新功能,并由此对AD神经细胞变性的本质提出了新见解．本文主要综述作者实验室有关Tau蛋白的部分研究结果．     

Down-regulation of WW domain-containing oxidoreductase induces Tau phosphorylation in vitro. A potential role in Alzheimer's disease

Numerous enzymes hyperphosphorylate Tau in vivo, leading to the formation of neurofibrillary tangles (NFTs) in the neurons of Alzheimer's disease (AD). Compared with age-matched normal controls, we demonstrated here that the protein levels of WW domain-containing oxidoreductase WOX1 (also known as WWOX or FOR), its Tyr33-phosphorylated form, and WOX2 were significantly down-regulated in the neurons of AD hippocampi. Remarkably knock-down of WOX1 expression by small interfering RNA in neuroblastoma SK-N-SH cells spontaneously induced Tau phosphorylation at Thr212/Thr231 and Ser515/Ser516, enhanced phosphorylation of glycogen synthase kinase 3beta (GSK-3beta) and ERK, and enhanced NFT formation. Also an increased binding of phospho-GSK-3beta with phospho-Tau was observed in these WOX1 knock-down cells. In comparison, increased phosphorylation of Tau, GSK-3beta, and ERK, as well as NFT formation, was observed in the AD hippocampi. Activation of JNK1 by anisomycin further increased Tau phosphorylation, and SP600125 (a JNK inhibitor) and PD-98059 (an MEK1/2 inhibitor) blocked Tau phosphorylation and NFT formation in these WOX1 knock-down cells. Ectopic or endogenous WOX1 colocalized with Tau, JNK1, and GSK-3beta in neurons and cultured cells. 17Beta-estradiol, a neuronal protective hormone, increased the binding of WOX1 and GSK-3beta with Tau. Mapping analysis showed that WOX1 bound Tau via its COOH-terminal short-chain alcohol dehydrogenase/reductase domain. Together WOX1 binds Tau via its short-chain alcohol dehydrogenase/reductase domain and is likely to play a critical role in regulating Tau hyperphosphorylation and NFT formation in vivo.     

Proteostasis failure and cellular senescence in long‐term cultured postmitotic rat neurons

Cellular senescence, a stress‐induced irreversible cell cycle arrest, has been defined for mitotic cells and is implicated in aging of replicative tissues. Age‐related functional decline in the brain is often attributed to a failure of protein homeostasis (proteostasis), largely in postmitotic neurons, which accordingly is a process distinct by definition from senescence. It is nevertheless possible that proteostasis failure and cellular senescence have overlapping molecular mechanisms. Here, we identify postmitotic cellular senescence as an adaptive stress response to proteostasis failure. Primary rat hippocampal neurons in long‐term cultures show molecular changes indicative of both senescence (senescence‐associated β‐galactosidase, p16, and loss of lamin B1) and proteostasis failure relevant to Alzheimer's disease. In addition, we demonstrate that the senescent neurons exhibit resistance to stress. Importantly, treatment of the cultures with an mTOR antagonist, protein synthesis inhibitor, or chemical compound that reduces the amount of protein aggregates relieved the proteotoxic stresses as well as the appearance of senescence markers. Our data propose mechanistic insights into the pathophysiological brain aging by establishing senescence as a primary cell‐autonomous neuroprotective response.     

Histone deacetylase mediates the decrease in drebrin cluster density induced by amyloid beta oligomers

Dendritic spine defects are found in a number of cognitive disorders, including Alzheimer’s disease (AD). Amyloid beta (Aβ) toxicity is mediated not only by the fibrillar form of the protein, but also by the soluble oligomers (Aβ-derived diffusible ligands, ADDLs). Drebrin is an actin-binding protein that is located at mature dendritic spines. Because drebrin expression is decreased in AD brains and in cultured neurons exposed to Aβ, it is thought that drebrin is closely associated with cognitive functions. Recent studies show that histone deacetylase (HDAC) activity is elevated in the AD mouse model, and that memory impairments in these animals can be ameliorated by HDAC inhibitors. In addition, spine loss and memory impairment in HDAC2 over-expressing mice are ameliorated by chronic HDAC inhibitor treatment. Therefore, we hypothesized that the regulation of histone acetylation/deacetylation is critical to synaptic functioning. In this study, we examined the relationship between HDAC activity and synaptic defects induced by ADDLs using an HDAC inhibitor, suberoylanilide hydroxamic acid (SAHA). We show that ADDLs reduce the cluster density of drebrin along dendrites without reducing drebrin expression. SAHA markedly increased the acetylation of histone proteins, and it simultaneously attenuated the ADDL-induced decrease in drebrin cluster density. In comparison, SAHA treatment did not affect the density of drebrin clusters or dendritic protrusions in control neurons. Therefore, SAHA likely inhibits ADDL-induced drebrin loss from dendritic spines by stabilizing drebrin in these structures, rather than by increasing drebrin clusters or dendritic protrusions. Taken together, our findings suggest that HDAC is involved in ADDL-induced synaptic defects, and that the regulation of histone acetylation plays an important role in modulating actin cytoskeletal dynamics in dendritic spines under cellular stress conditions, such as ADDL exposure.