Thr175-phosphorylated tau induces pathologic fibril formation via GSK3β-mediated phosphorylation of Thr231 in vitro

Unknown roles of tau pathology in neurological disorders. Challenges and new perspectives

Aging presents progressive changes that increase the susceptibility of the central nervous system (CNS) to suffer neurological disorders (NDs). Several studies have reported that an aged brain suffering from NDs shows the presence of pathological forms of tau protein, a microtubule accessory protein (MAP) critical for neuronal function. In this context, accumulative evidence has shown a pivotal contribution of pathological forms of tau to Alzheimer's disease (AD) and tauopathies. However, current investigations have implicated tau toxicity in other NDs that affect the central nervous system (CNS), including Parkinson's disease (PD), Huntington's disease (HD), Traumatic brain injury (TBI), Multiple sclerosis (MS), and Amyotrophic lateral sclerosis (ALS). These diseases are long-term acquired, affecting essential functions such as motor movement, cognition, hearing, and vision. Previous evidence indicated that toxic forms of tau do not have a critical contribution to the genesis or progression of these diseases. However, recent studies have shown that these tau forms contribute to neuronal dysfunction, inflammation, oxidative damage, and mitochondrial impairment events that contribute to the pathogenesis of these NDs. Recent studies have suggested that these neuropathologies could be associated with a prion-like behavior of tau, which induces a pathological dissemination of these toxic protein forms to different brain areas. Moreover, it has been suggested that this toxic propagation of tau from neurons into neighboring cells impairs the function of glial cells, oligodendrocytes, and endothelial cells by affecting metabolic function and mitochondrial health and inducing oxidative damage by tau pathology. Therefore, in this review, we will discuss current evidence demonstrating the critical role of toxic tau forms on NDs not related to AD and how its propagation and induced-bioenergetics failure may contribute to the pathogenic mechanism present in these NDs.

Proteotoxic stress and the ubiquitin proteasome system

The ubiquitin proteasome system maintains protein homeostasis by regulating the breakdown of misfolded proteins, thereby preventing misfolded protein aggregates. The efficient elimination is vital for preventing damage to the cell by misfolded proteins, known as proteotoxic stress. Proteotoxic stress can lead to the collapse of protein homeostasis and can alter the function of the ubiquitin proteasome system. Conversely, impairment of the ubiquitin proteasome system can also cause proteotoxic stress and disrupt protein homeostasis. This review examines two impacts of proteotoxic stress, 1) disruptions to ubiquitin homeostasis (ubiquitin stress) and 2) disruptions to proteasome homeostasis (proteasome stress). Here, we provide a mechanistic description of the relationship between proteotoxic stress and the ubiquitin proteasome system. This relationship is illustrated by findings from several protein misfolding diseases, mainly neurodegenerative diseases, as well as from basic biology discoveries from yeast to mammals. In addition, we explore the importance of the ubiquitin proteasome system in endoplasmic reticulum quality control, and how proteotoxic stress at this organelle is alleviated. Finally, we highlight how cells utilize the ubiquitin proteasome system to adapt to proteotoxic stress and how the ubiquitin proteasome system can be genetically and pharmacologically manipulated to maintain protein homeostasis.

Upregulation of LRRK2 following traumatic brain injury does not directly phosphorylate Thr175 tau

Phosphorylated microtubule-associated protein tau (tau) aggregates are a pathological hallmark of various neurodegenerative diseases, including chronic traumatic encephalopathy and amyotrophic lateral sclerosis with cognitive impairment. While there are many residues phosphorylated on tau, phosphorylation of threonine 175 (pThr175 tau) has been shown to initiate fibril formation in vitro and is present in pathological tau aggregates in vivo. Given this, preventing Thr175 tau phosphorylation presents a potential approach to reduce fibril formation; however, the kinase(s) acting on Thr175 are not yet fully defined. Using a single controlled cortical impact rodent model of traumatic brain injury (TBI), which rapidly induces Thr175 tau phosphorylation, we observed an upregulation and alteration in subcellular localization of leucine-rich repeat kinase 2 (LRRK2), a kinase that has been implicated in tau phosphorylation. LRRK2 upregulation was evident by one-day post-injury and persisted to day 10. The most notable changes were observed in microglia at the site of injury in the cortex. To determine if the appearance of pThr175 tau was causally related to the upregulation of LRRK2 expression, we examined the ability of LRRK2 to phosphorylate Thr175in vitro by co-transfecting 2N4R human WT-tau with either LRRK2-WT, constitutively-active LRRK2-G2019S or inactive LRRK2-3XKD. We found no significant difference in the level of pThr175 tau between the overexpression of LRRK2-WT, -G2019S or -3XKD, suggesting LRRK2 does not phosphorylate tau at Thr175. Further, downstream events known to follow Thr175 phosphorylation and known to be associated with pathological tau fibril formation (pSer9-GSK3β and pThr231 tau induction) also remained unchanged. We conclude that while LRRK2 expression is altered in TBI, it does not contribute directly to pThr175 tau generation.

Programmed Cell Death Protein 1 Blockade Reduces Glycogen Synthase Kinase 3β Activity and Tau Hyperphosphorylation in Alzheimer's Disease Mouse Models

Alzheimer’s disease (AD) is a central nervous system degenerative disease, with no effective treatment to date. Administration of immune checkpoint inhibitors significantly reduces neuronal damage and tau hyperphosphorylation in AD, but the specific mechanism is unclear. Here, we found that programmed cell death-receptor 1 (PD1) and its ligand PDL1 were induced by an intracerebroventricular injection of amyloid-β; they were significantly upregulated in the brains of APP/PS1, 5×FAD mice and in SH-SY5Y-APP cell line compared with control. The PD1 and PDL1 levels positively correlated with the glycogen synthase kinase 3 beta (GSK3β) activity in various AD mouse models, and the PDL1-GSK3β immune complex was found in the brain. The application of PD1-blocking antibody reduced tau hyperphosphorylation and GSK3β activity and prevented memory impairments. Mechanistically, we identified PD1 as a critical regulator of GSK3β activity. These results suggest that the immune regulation of the PD1/PDL1 axis is closely involved in AD.

Revisiting the Role of GSK3, A Modulator of Innate Immunity, in Idiopathic Inclusion Body Myositis

Idiopathic or sporadic inclusion body myositis (IBM) is the leading age-related (onset >50 years of age) autoimmune muscular pathology, resulting in significant debilitation in affected individuals. Once viewed as primarily a degenerative disorder, it is now evident that much like several other neuro-muscular degenerative disorders, IBM has a major autoinflammatory component resulting in chronic inflammation-induced muscle destruction. Thus, IBM is now considered primarily an inflammatory pathology. To date, there is no effective treatment for sporadic inclusion body myositis, and little is understood about the pathology at the molecular level, which would offer the best hopes of at least slowing down the degenerative process. Among the previously examined potential molecular players in IBM is glycogen synthase kinase (GSK)-3, whose role in promoting TAU phosphorylation and inclusion bodies in Alzheimer’s disease is well known. This review looks to re-examine the role of GSK3 in IBM, not strictly as a promoter of TAU and Abeta inclusions, but as a novel player in the innate immune system, discussing some of the recent roles discovered for this well-studied kinase in inflammatory-mediated pathology.

Redox Homeostasis and Prospects for Therapeutic Targeting in Neurodegenerative Disorders

Reactive species, such as those of oxygen, nitrogen, and sulfur, are considered part of normal cellular metabolism and play significant roles that can impact several signaling processes in ways that lead to either cellular sustenance, protection, or damage. Cellular redox processes involve a balance in the production of reactive species (RS) and their removal because redox imbalance may facilitate oxidative damage. Physiologically, redox homeostasis is essential for the maintenance of many cellular processes. RS may serve as signaling molecules or cause oxidative cellular damage depending on the delicate equilibrium between RS production and their efficient removal through the use of enzymatic or nonenzymatic cellular mechanisms. Moreover, accumulating evidence suggests that redox imbalance plays a significant role in the progression of several neurodegenerative diseases. For example, studies have shown that redox imbalance in the brain mediates neurodegeneration and alters normal cytoprotective responses to stress. Therefore, this review describes redox homeostasis in neurodegenerative diseases with a focus on Alzheimer's and Parkinson's disease. A clearer understanding of the redox-regulated processes in neurodegenerative disorders may afford opportunities for newer therapeutic strategies.

Amphiphysin I cleavage by asparagine endopeptidase leads to tau hyperphosphorylation and synaptic dysfunction

Neurofibrillary tangles composed of hyperphosphorylated tau and synaptic dysfunction are characteristics of Alzheimer's disease (AD). However, the underlying molecular mechanisms remain poorly understood. Here, we identified Amphiphysin I mediates both tau phosphorylation and synaptic dysfunction in AD. Amphiphysin I is cleaved by a cysteine proteinase asparagine endopeptidase (AEP) at N278 in the brains of AD patients. The amount of AEP-generated N-terminal fragment of Amphiphysin I (1-278) is increased with aging. Amphiphysin I (1-278) inhibits clathrin-mediated endocytosis and induces synaptic dysfunction. Furthermore, Amphiphysin I (1-278) binds p35 and promotes its transition to p25, thus activates CDK5 and enhances tau hyperphosphorylation. Overexpression of Amphiphysin I (1-278) in the hippocampus of Tau P301S mice induces synaptic dysfunction, tau hyperphosphorylation, and cognitive deficits. However, overexpression of the N278A mutant Amphiphysin I, which resists the AEP-mediated cleavage, alleviates the pathological and behavioral defects. These findings suggest a mechanism of tau hyperphosphorylation and synaptic dysfunction in AD.

DHCR24 Knock-Down Induced Tau Hyperphosphorylation at Thr181, Ser199, Thr231, Ser262, Ser396 Epitopes and Inhibition of Autophagy by Overactivation of GSK3β/mTOR Signaling

Accumulating evidences supported that knock-down of DHCR24 is linked to the pathological risk factors of AD, suggesting a potential role of DHCR24 in AD pathogenesis. However, the molecular mechanism link between DHCR24 and tauopathy remains unknown. Here, in order to elucidate the relationship between DHCR24 and tauopathy, we will focus on the effect of DHCR24 on the tau hyperphosphorylation at some toxic sites. In present study, we found that DHCR24 knock-down significantly lead to the hyperphosphorylation of tau sites at Thr181, Ser199, Thr231, Ser262, Ser396. Moreover, DHCR24 knock-down also increase the accumulation of p62 protein, simultaneously decreased the ratio of LC3-II/LC3-I and the number of autophagosome compared to the control groups, suggesting the inhibition of autophagy activity. In contrast, DHCR24 knock-in obviously abolished the effect of DHCR24 knock-down on tau hyperphosphrylation and autophagy. In addition, to elucidate the association between DHCR24 and tauopathy, we further showed that the level of plasma membrane cholesterol, lipid raft-anchored protein caveolin-1, and concomitantly total I class PI3-K (p110α), phospho-Akt (Thr308 and Ser473) were significantly decreased, resulting in the disruption of lipid raft/caveola and inhibition of PI3-K/Akt signaling in silencing DHCR24 SH-SY5Y cells compared to control groups. At the same time, DHCR24 knock-down simultaneously decreased the level of phosphorylated GSK3β at Ser9 (inactive form) and increased the level of phosphorylated mTOR at Ser2448 (active form), leading to overactivation of GSK3β and mTOR signaling. On the contrary, DHCR24 knock-in largely increased the level of membrane cholesterol and caveolin-1, suggesting the enhancement of lipid raft/caveola. And synchronously DHCR24 knock-in also abolished the effect of DHCR24 knock-down on the inhibition of PI3-K/Akt signaling as well as the overactivation of GSK3β and mTOR signaling. Collectively, our data strongly supported DHCR24 knock-down lead to tau hyperphosphorylation and the inhibition of autophagy by a lipid raft-dependent PI3-K/Akt-mediated GSK3β and mTOR signaling. Taking together, our results firstly demonstrated that the decrease of plasma membrane cholesterol mediated by DHCR24 deficiency might contribute to the tauopathy in AD and other tauopathies.

Alterations in Tau Metabolism in ALS and ALS-FTSD

There is increasing acceptance that amyotrophic lateral sclerosis (ALS), classically considered a neurodegenerative disease affecting almost exclusively motor neurons, is syndromic with both clinical and biological heterogeneity. This is most evident in its association with a broad range of neuropsychological, behavioral, speech and language deficits [collectively termed ALS frontotemporal spectrum disorder (ALS-FTSD)]. Although the most consistent pathology of ALS and ALS-FTSD is a disturbance in TAR DNA binding protein 43 kDa (TDP-43) metabolism, alterations in microtubule-associated tau protein (tau) metabolism can also be observed in ALS-FTSD, most prominently as pathological phosphorylation at Thr¹⁷⁵ (pThr¹⁷⁵tau). pThr¹⁷⁵ has been shown to promote exposure of the phosphatase activating domain (PAD) in the tau N-terminus with the consequent activation of GSK3β mediated phosphorylation at Thr²³¹ (pThr²³¹tau) leading to pathological oligomer formation. This pathological cascade of tau phosphorylation has been observed in chronic traumatic encephalopathy with ALS (CTE-ALS) and in both in vivo and in vitro experimental paradigms, suggesting that it is of critical relevance to the pathobiology of ALS-FTSD. It is also evident that the co-existence of alterations in the metabolism of TDP-43 and tau acts synergistically in a rodent model to exacerbate the pathology of either.

JBPOS0101 regulates amyloid beta, tau, and glial cells in an Alzheimer's disease model

Alzheimer’s disease (AD) is the most prevalent neurodegenerative disease characterized by cognitive dysfunction and memory loss as the main symptoms. The deposition of amyloid beta (Aβ) and tau hyperphosphorylation are hallmarks of AD and are major therapeutic targets. However, the exact etiology has not yet been fully elucidated; thus, no drug that cures the disease has been approved. JBPOS0101 is a phenyl carbamate compound that has been tested as a drug for epileptic diseases. In our previous study, we showed that JBPOS0101 attenuated the accumulation of Aβ as well as the deficits in learning and memory in the 5xFAD mouse model. Here, we tested the dose effect (70 or 35 mg/kg) of JBPOS0101 on the memory defect and pathological markers and further investigated the underlying mechanisms in 5xFAD mice. In the behavior tests, JBPOS0101 treatment ameliorated deficits in learning and memory. Moreover, JBPOS0101 attenuated Aβ accumulation and tau phosphorylation. The elevated phosphorylation levels of the active GSK3β form (GSK3β-y216) in 5xFAD, which are responsible for tau phosphorylation, decreased in the JBPOS0101-treated groups. Furthermore, the elevation of reactive astrocytes and microglia in 5xFAD mice was attenuated in JBPOS0101-treated groups. These data suggest that JBPOS0101 may be a new drug candidate to lessen amyloid- and tau-related pathology by regulating glial cells.

Role of tau protein in Alzheimer's disease: The prime pathological player

Alzheimer's disease (AD) is a prevalently found tauopathy characterized by memory loss and cognitive insufficiency. AD is an age-related neurodegenerative disease with two major hallmarks which includes extracellular amyloid plaques made of amyloid-β (Aβ) and intracellular neurofibrillary tangles of hyperphosphorylated tau. With population aging worldwide, there is an indispensable need for treatment strategies that can potentially manage this developing dementia. Despite broad researches on targeting Aβ in the past two decades, research findings on Aβ targeted therapeutics failed to prove efficacy in the treatment of AD. Tau protein with its extensive pathological role in several neurodegenerative diseases can be considered as a promising target candidate for developing therapeutic interventions. The abnormal hyperphosphorylation of tau plays detrimental pathological functions which ultimately lead to neurodegeneration. This review will divulge the importance of tau in AD pathogenesis, the interplay of Aβ and tau, the pathological functions of tau, and potential therapeutic strategies for an effective management of neuronal disorders.

Glycogen synthase kinase (GSK)-3 and the double-strand RNA-dependent kinase, PKR: When two kinases for the common good turn bad

Glycogen synthase kinase (GSK)-3α/β and the double-stranded RNA-dependent kinase PKR are two sentinel kinases that carry-out multiple similar yet distinct functions in both the cytosol and the nucleus. While these kinases belong to separate signal transduction cascades, they demonstrate an uncanny propensity to regulate many of the same proteins either through direct phosphorylation or by altering transcription/translation, including: c-MYC, NF-κB, p53 and TAU, as well as each another. A significant number of studies centered on the GSK3 kinases have led to the identification of the GSK3 interactome and a number of substrates, which link GSK3 activity to metabolic control, translation, RNA splicing, ribosome biogenesis, cellular division, DNA repair and stress/inflammatory signaling. Interestingly, many of these same pathways and processes are controlled by PKR, but unlike the GSK3 kinases, a clear picture of proteins interacting with PKR and a complete listing of its substrates is still missing. In this review, we take a detailed look at what is known about the PKR and GSK3 kinases, how these kinases interact to influence common cellular processes (innate immunity, alternative splicing, translation, glucose metabolism) and how aberrant activation of these kinases leads to diseases such as Alzheimer's disease (AD), diabetes mellitus (DM) and cancer.

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GSK3α/β and PKR are major regulators of cellular homeostasis and the response to stress/inflammation and infection.

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GSK3α/β and PKR interact with and/or modify many of the same proteins and affect the expression of similar genes.

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A balance between AKT and PKR nuclear signaling may be responsible for regulating the activation of nuclear GSK3β.

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GSK3α/β- and PKR-dependent signaling influence major molecular mechanisms of the cell through similar intermediates.

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Aberrant activation of GSK3α/β and PKR is highly involved in cancer, metabolic disorders, and neurodegenerative diseases.

Synergistic toxicity in an in vivo model of neurodegeneration through the co-expression of human TDP-43M337V and tauT175D protein

Although it has been suggested that the co-expression of multiple pathological proteins associated with neurodegeneration may act synergistically to induce more widespread neuropathology, experimental evidence of this is sparse. We have previously shown that the expression of Thr¹⁷⁵Asp-tau (tau^T175D) using somatic gene transfer with a stereotaxically-injected recombinant adeno-associated virus (rAAV9) vector induces tau pathology in rat hippocampus. In this study, we have examined whether the co-expression of human tau^T175D with mutant human TDP-43 (TDP-43^M337V) will act synergistically. Transgenic female Sprague-Dawley rats that inducibly express mutant human TDP-43^M337V using the choline acetyltransferase (ChAT) tetracycline response element (TRE) driver with activity modulating tetracycline-controlled transactivator (tTA) were utilized in these studies. Adult rats were injected with GFP-tagged tau protein constructs in a rAAV9 vector through bilateral stereotaxic injection into the hippocampus. Injected tau constructs were: wild-type GFP-tagged 2N4R human tau (tau^WT; n = 8), GFP-tagged tau^T175D 2N4R human tau (tau^T175D, pseudophosphorylated, toxic variant, n = 8), and GFP (control, n = 8). Six months post-injection, mutant TDP-43^M337V expression was induced for 30 days. Behaviour testing identified motor deficits within 3 weeks after TDP-43 expression irrespective of tau expression, though social behaviour and sensorimotor gating remained unchanged. Increased tau pathology was observed in the hippocampus of both tau^WT and tau^T175D expressing rats and tau^T175D pathology was increased in the presence of cholinergic neuronal expression of human TDP-43^M337V. These data indicate that co-expression of pathological TDP-43 and tau protein exacerbate the pathology associated with either individual protein.

Pathogenic Tau Protein Species: Promising Therapeutic Targets for Ocular Neurodegenerative Diseases

Tau is a microtubule-associated protein, which is highly expressed in the central nervous system as well as ocular neurons and stabilizes microtubule structure. It is a phospho-protein being moderately phosphorylated under physiological conditions but its abnormal hyperphosphorylation or some post-phosphorylation modifications would result in a pathogenic condition, microtubule dissociation, and aggregation. The aggregates can induce neuroinflammation and trigger some pathogenic cascades, leading to neurodegeneration. Taking these together, targeting pathogenic tau employing tau immunotherapy may be a promising therapeutic strategy in fighting with cerebral and ocular neurodegenerative disorders.

Structure and Functions of Microtubule Associated Proteins Tau and MAP2c: Similarities and Differences

The stability and dynamics of cytoskeleton in brain nerve cells are regulated by microtubule associated proteins (MAPs), tau and MAP2. Both proteins are intrinsically disordered and involved in multiple molecular interactions important for normal physiology and pathology of chronic neurodegenerative diseases. Nuclear magnetic resonance and cryo-electron microscopy recently revealed propensities of MAPs to form transient local structures and long-range contacts in the free state, and conformations adopted in complexes with microtubules and filamentous actin, as well as in pathological aggregates. In this paper, we compare the longest, 441-residue brain isoform of tau (tau40), and a 467-residue isoform of MAP2, known as MAP2c. For both molecules, we present transient structural motifs revealed by conformational analysis of experimental data obtained for free soluble forms of the proteins. We show that many of the short sequence motifs that exhibit transient structural features are linked to functional properties, manifested by specific interactions. The transient structural motifs can be therefore classified as molecular recognition elements of tau40 and MAP2c. Their interactions are further regulated by post-translational modifications, in particular phosphorylation. The structure-function analysis also explains differences between biological activities of tau40 and MAP2c.

Mechanism involved in insulin resistance via accumulation of β-amyloid and neurofibrillary tangles: link between type 2 diabetes and Alzheimer's disease

The pathophysiological link between type 2 diabetes mellitus (T2DM) and Alzheimer's disease (AD) has been suggested in several reports. Few findings suggest that T2DM has strong link in the development process of AD, and the complete mechanism is yet to be revealed. Formation of amyloid plaques (APs) and neurofibrillary tangles (NFTs) are two central hallmarks in the AD. APs are the dense composites of β-amyloid protein (Aβ) which accumulates around the nerve cells. Moreover, NFTs are the twisted fibers containing hyperphosphorylated tau proteins present in certain residues of Aβ that build up inside the brain cells. Certain factors contribute to the aetiogenesis of AD by regulating insulin signaling pathway in the brain and accelerating the formation of neurotoxic Aβ and NFTs via various mechanisms, including GSK3β, JNK, CamKII, CDK5, CK1, MARK4, PLK2, Syk, DYRK1A, PPP, and P70S6K. Progression to AD could be influenced by insulin signaling pathway that is affected due to T2DM. Interestingly, NFTs and APs lead to the impairment of several crucial cascades, such as synaptogenesis, neurotrophy, and apoptosis, which are regulated by insulin, cholesterol, and glucose metabolism. The investigation of the molecular cascades through insulin functions in brain contributes to probe and perceive progressions of diabetes to AD. This review elaborates the molecular insights that would help to further understand the potential mechanisms linking T2DM and AD.

Chronic Sleep Disruption Advances the Temporal Progression of Tauopathy in P301S Mutant Mice

Brainstem locus ceruleus neurons (LCn) are among the first neurons across the lifespan to evidence tau pathology, and LCn are implicated in tau propagation throughout the cortices. Yet, events influencing LCn tau are poorly understood. Activated persistently across wakefulness, LCn experience significant metabolic stress in response to chronic short sleep (CSS). Here we explored whether CSS influences LCn tau and the biochemical, neuroanatomical, and/or behavioral progression of tauopathy in male and female P301S mice. CSS in early adult life advanced the temporal progression of neurobehavioral impairments and resulted in a lasting increase in soluble tau oligomers. Intriguingly, CSS resulted in an early increase in AT8 and MC1 tau pathology in the LC. Over time tau pathology, including tangles, was evident in forebrain tau-vulnerable regions. Sustained microglial and astrocytic activation was observed as well. Remarkably, CSS resulted in significant loss of neurons in the two regions examined: the basolateral amygdala and LC. A second, distinct form of chronic sleep disruption, fragmentation of sleep, during early adult life also increased tau deposition and imparted early neurobehavioral impairment. Collectively, the findings demonstrate that early life sleep disruption has important lasting effects on the temporal progression in P301S mice, influencing tau pathology and hastening neurodegeneration, neuroinflammation, and neurobehavioral impairments.

SIGNIFICANCE STATEMENT Chronic short sleep (CSS) is pervasive in modern society. Here, we found that early life CSS influences behavioral, biochemical, and neuroanatomic aspects of the temporal progression of tauopathy in a mouse model of the P301S tau mutation. Specifically, CSS hastened the onset of motor impairment and resulted in a greater loss of neurons in both the locus ceruleus and basolateral/lateral amygdala. Importantly, despite a protracted recovery opportunity after CSS, mice evidenced a sustained increase in pathogenic tau oligomers, and increased pathogenic tau in the locus ceruleus and limbic system nuclei. These findings unveil early life sleep habits as an important determinant in the progression of tauopathy.

Phosphorylation of different tau sites during progression of Alzheimer's disease

Alzheimer’s disease is characterized by accumulation of amyloid plaques and tau aggregates in several cortical brain regions. Tau phosphorylation causes formation of neurofibrillary tangles and neuropil threads. Phosphorylation at tau Ser202/Thr205 is well characterized since labeling of this site is used to assign Braak stage based on occurrence of neurofibrillary tangles. Only little is known about the spatial and temporal phosphorylation profile of other phosphorylated tau (ptau) sites. Here, we investigate total tau and ptau at residues Tyr18, Ser199, Ser202/Thr205, Thr231, Ser262, Ser396, Ser422 as well as amyloid-β plaques in human brain tissue of AD patients and controls. Allo- and isocortical brain regions were evaluated applying rater-independent automated quantification based on digital image analysis. We found that the level of ptau at several residues, like Ser199, Ser202/Thr205, and Ser422 was similar in healthy controls and Braak stages I to IV but was increased in Braak stage V/VI throughout the entire isocortex and transentorhinal cortex. Quantification of ThioS-stained plaques showed a similar pattern. Only tau phosphorylation at Tyr18 and Thr231 was already significantly increased in the transentorhinal region at Braak stage III/IV and hence showed a progressive increase with increasing Braak stages. Additionally, the increase in phosphorylation relative to controls was highest at Tyr18, Thr231 and Ser199. By contrast, Ser396 tau and Ser262 tau showed only a weak phosphorylation in all analyzed brain regions and only minor progression. Our results suggest that the ptau burden in the isocortex is comparable between all analyzed ptau sites when using a quantitative approach while levels of ptau at Tyr18 or Thr231 in the transentorhinal region are different between all Braak stages. Hence these sites could be crucial in the pathogenesis of AD already at early stages and therefore represent putative novel therapeutic targets.

Phosphorylation of Threonine 175 Tau in the Induction of Tau Pathology in Amyotrophic Lateral Sclerosis-Frontotemporal Spectrum Disorder (ALS-FTSD). A Review

Approximately 50–60% of all patients with amyotrophic lateral sclerosis (ALS) will develop a deficit of frontotemporal function, ranging from frontotemporal dementia (FTD) to one or more deficits of neuropsychological, speech or language function which are collectively known as the frontotemporal spectrum disorders of ALS (ALS-FTSD). While the neuropathology underlying these disorders is most consistent with a widespread alteration in the metabolism of transactive response DNA-binding protein 43 (TDP-43), in both ALS with cognitive impairment (ALSci) and ALS with FTD (ALS-FTD; also known as MND-FTD) there is evidence for alterations in the metabolism of the microtubule associated protein tau. This alteration in tau metabolism is characterized by pathological phosphorylation at residue Thr¹⁷⁵ (pThr¹⁷⁵ tau) which in vitro is associated with activation of GSK3β (pTyr²¹⁶GSK3β), phosphorylation of Thr²³¹tau, and the formation of cytoplasmic inclusions with increased rates of cell death. This putative pathway of pThr¹⁷⁵ induction of pThr²³¹ and the formation of pathogenic tau inclusions has been recently shown to span a broad range of tauopathies, including chronic traumatic encephalopathy (CTE) and CTE in association with ALS (CTE-ALS). This pathway can be experimentally triggered through a moderate traumatic brain injury, suggesting that it is a primary neuropathological event and not secondary to a more widespread neuronal dysfunction. In this review, we discuss the neuropathological underpinnings of the postulate that ALS is associated with a tauopathy which manifests as a FTSD, and examine possible mechanisms by which phosphorylation at Thr¹⁷⁵tau is induced. We hypothesize that this might lead to an unfolding of the hairpin structure of tau, activation of GSK3β and pathological tau fibril formation through the induction of cis-Thr²³¹ tau conformers. A potential role of TDP-43 acting synergistically with pathological tau metabolism is proposed.

Pathologic Thr175 tau phosphorylation in CTE and CTE with ALS

Objective

To investigate whether chronic traumatic encephalopathy (CTE) and CTE with amyotrophic lateral sclerosis (CTE-ALS) exhibit features previously observed in other tauopathies of pathologic phosphorylation of microtubule-associated protein tau at Thr¹⁷⁵ (pThr¹⁷⁵ tau) and Thr²³¹ (pThr²³¹ tau), and glycogen synthase kinase–3β (GSK3β) activation, and whether these pathologic features are a consequence of traumatic brain injury (TBI).

Methods

Tau isoform expression was assayed by western blot in 6 stage III CTE cases. We also used immunohistochemistry to analyze 5 cases each of CTE, CTE-ALS, and 5 controls for expression of activated GSK3β, pThr¹⁷⁵ tau, pThr²³¹ tau, and oligomerized tau within spinal cord tissue and hippocampus. Using a rat model of moderate TBI, we assessed tau pathology and phospho-GSK3β expression at 3 months postinjury.

Results

CTE and CTE-ALS are characterized by the presence of all 6 tau isoforms in both soluble and insoluble tau isolates. Activated GSK3β, pThr¹⁷⁵ tau, pThr²³¹ tau, and oligomerized tau protein expression was observed in hippocampal neurons and spinal motor neurons. We observed tau neuronal pathology (fibrillar inclusions and axonal damage) and increased levels of pThr¹⁷⁵ tau and activated GSK3β in moderate TBI rats.

Conclusions

Pathologic phosphorylation of tau at Thr¹⁷⁵ and Thr²³¹ and activation of GSK3β are features of the tauopathy of CTE and CTE-ALS. These features can be replicated in an animal model of moderate TBI.

Cotinine improves visual recognition memory and decreases cortical Tau phosphorylation in the Tg6799 mice

Alzheimer's disease (AD) is associated with the progressive aggregation of hyperphosphorylated forms of the microtubule associated protein Tau in the central nervous system. Cotinine, the main metabolite of nicotine, reduced working memory deficits, synaptic loss, and amyloid β peptide aggregation into oligomers and plaques as well as inhibited the cerebral Tau kinase, glycogen synthase 3β (GSK3β) in the transgenic (Tg)6799 (5XFAD) mice. In this study, the effect of cotinine on visual recognition memory and cortical Tau phosphorylation at the GSK3β sites Serine (Ser)-396/Ser-404 and phospho-CREB were investigated in the Tg6799 and non-transgenic (NT) littermate mice. Tg mice showed short-term visual recognition memory impairment in the novel object recognition test, and higher levels of Tau phosphorylation when compared to NT mice. Cotinine significantly improved visual recognition memory performance increased CREB phosphorylation and reduced cortical Tau phosphorylation. Potential mechanisms underlying theses beneficial effects are discussed.

Synthesis and anti-diabetic activity of new N,N-dimethylphenylenediamine-derivatized nitrilotriacetic acid vanadyl complexes

Vanadium compounds are promising anti-diabetic agents. However, reducing the metal toxicity while keeping/improving the hypoglycemic effect is still a big challenge towards the success of anti-diabetic vanadium drugs. To improve the therapeutic potency using the anti-oxidative strategy, we synthesized new N,N-dimethylphenylenediamine (DMPD)-derivatized nitrilotriacetic acid vanadyl complexes ([VO(dmada)]). The in vitro biological evaluations revealed that the DMPD-derivatized complexes showed improved antioxidant capacity and lowered cytotoxicity on HK-2 cells than bis(maltolato)oxidovanadium (IV) (BMOV). In type II diabetic mice, [VO(p-dmada)] (0.15mmolkg^-1/day) exhibited better hypoglycemic effects than BMOV especially on improving glucose tolerance and alleviating the hyperglycemia-induced liver damage. These insulin enhancement effects were associated with increased expression of peroxisome proliferator-activated receptor α and γ (PPARα/γ) in fat, activation of Akt (v-Akt murine thymoma viral oncogene)/PKB (protein kinase-B) in fat and liver, and inactivation of c-Jun NH₂-terminal protein kinases (JNK) in liver. Moreover, [VO(p-dmada)] showed no tissue toxicity at the therapeutic dose in diabetic mice and the oral acute toxicity (LD50) was determined to be 1640mgkg^-1. Overall, the experimental results indicated that [VO(p-dmada)] can be a potent insulin enhancement agent with improved efficacy-over- toxicity index for further drug development. In addition, the results on brain Tau phosphorylation suggested necessary investigation on the effects of vanadyl complexes on the pathology of the Alzheimer's disease in the future.

C9orf72 mutations do not influence the tau signature of amyotrophic lateral sclerosis with cognitive impairment (ALSci)

OBJECTIVE

C9orf72 mutations are associated with amyotrophic lateral sclerosis (ALS), frontotemporal dementia (FTD) and ALS-FTD. In addition to ALS-FTD, ALS patients may develop a spectrum of neuropsychological and neuropsychiatric deficits including ALS with cognitive impairment (ALSci). Here we examine the extent to which C9orf72 mutations are associated with ALSci and whether this alters the tau molecular signature.

METHODS

We identified 16 ALSci cases within a post-mortem archive of 94 fully genotyped ALS cases, eight of which harboured a C9orf72 mutation, in addition to three cognitively-intact ALS cases with a C9orf72 mutation. Tau was fractionated into soluble and insoluble fractions, with or without dephosphorylation, and immunoblots for tau phospho-isoforms performed.

RESULTS

Regardless of cognitive state or the presence of C9orf72 mutation, all ALS cases demonstrated six tau isoforms in both soluble and insoluble tau isolates. This pattern was unaffected by dephosphorylation. pThr¹⁷⁵tau isoforms, a molecular signature of ALSci, were present regardless of C9orf72 genetic status. The pathognomic paired helical triplet in the insoluble tau fraction of Alzheimer's disease was not observed, regardless of cognitive or C9orf72 status.

CONCLUSIONS

These findings suggest that the presence of a C9orf72 mutation does not influence the tau signature of ALS or ALSci.

Threonine175, a novel pathological phosphorylation site on tau protein linked to multiple tauopathies

Microtubule associated protein tau (tau) deposition is associated with a spectrum of neurodegenerative diseases collectively termed tauopathies. We have previously shown that amyotrophic lateral sclerosis (ALS) with cognitive impairment (ALSci) is associated with tau phosphorylation at Thr¹⁷⁵ and that this leads to activation of GSK3β which then induces phosphorylation at tau Thr²³¹. This latter step leads to dissociation of tau from microtubules and pathological tau fibril formation. To determine the extent to which this pathway is unique to ALS, we have investigated the expression of pThr¹⁷⁵ tau and pThr²³¹ tau across a range of frontotemporal degenerations. Representative sections from the superior frontal cortex, anterior cingulate cortex (ACC), amygdala, hippocampal formation, basal ganglia, and substantia nigra were selected from neuropathologically confirmed cases of Alzheimer’s disease (AD; n = 3), vascular dementia (n = 2), frontotemporal lobar degeneration (FTLD; n = 4), ALS (n = 5), ALSci (n = 6), Parkinson’s disease (PD; n = 5), corticobasal degeneration (CBD; n = 2), diffuse Lewy body dementia (DLBD; n = 2), mixed DLBD (n = 3), multisystem atrophy (MSA; n = 6) and Pick’s disease (n = 1) and three neuropathologically-normal control groups aged 50–60 (n = 6), 60–70 (n = 6) and 70–80 (n = 8). Sections were examined using a panel of phospho-tau antibodies (pSer^208,210, pThr²¹⁷, pThr¹⁷⁵, pThr²³¹, pSer²⁰² and T22 (oligomeric tau)). Across diseases, phospho-tau load was most prominent in layers II/III of the entorhinal cortex, amygdala and hippocampus. This is in contrast to the preferential deposition of phospho-tau in the ACC and frontal cortex in ALSci. Controls showed pThr¹⁷⁵ tau expression only in the 7^th decade of life and only in the presence of tau pathology and tau oligomers. With the exception of DLBD, we observed pThr¹⁷⁵ co-localizing with pThr²³¹ in the same cell populations as T22 positivity. This suggests that this pathway may be a common mechanism of toxicity across the tauopathies.

New Features about Tau Function and Dysfunction

Tau is a brain microtubule-associated protein that directly binds to a microtubule and dynamically regulates its structure and function. Under pathological conditions, tau self-assembles into filamentous structures that end up forming neurofibrillary tangles. Prominent tau neurofibrillary pathology is a common feature in a number of neurodegenerative disorders, collectively referred to as tauopathies, the most common of which is Alzheimer’s disease (AD). Beyond its classical role as a microtubule-associated protein, recent advances in our understanding of tau cellular functions have revealed novel insights into their important role during pathogenesis and provided potential novel therapeutic targets. Regulation of tau behavior and function under physiological and pathological conditions is mainly achieved through post-translational modifications, including phosphorylation, glycosylation, acetylation, and truncation, among others, indicating the complexity and variability of factors influencing regulation of tau toxicity, all of which have significant implications for the development of novel therapeutic approaches in various neurodegenerative disorders. A more comprehensive understanding of the molecular mechanisms regulating tau function and dysfunction will provide us with a better outline of tau cellular networking and, hopefully, offer new clues for designing more efficient approaches to tackle tauopathies in the near future.

Frontotemporal Dysfunction and Dementia in Amyotrophic Lateral Sclerosis

Although amyotrophic lateral sclerosis (ALS) is classically considered a disorder exclusively affecting motor neurons, there is substantial clinical, neuroimaging, and neuropathologic evidence that more than half of patients have an associated syndrome of frontotemporal dysfunction. These syndromes range from frontotemporal dementia to behavioral or cognitive syndromes. Neuroimaging and neuropathologic findings are consistent with frontotemporal lobar degeneration that underpins alterations in network connectivity. Future clinical trials need to be stratified based on the presence or absence of frontotemporal dysfunction on the disease course of ALS.