Calcineurin inhibition prevents synaptic plasticity deficit induced by brain-derived tau oligomers

Compelling evidence suggests that cognitive decline in Alzheimer's disease is associated with the accumulation and aggregation of tau protein, with the most toxic aggregates being in the form of oligomers. This underscores the necessity for direct isolation and analysis of brain-derived tau oligomers from patients with Alzheimer's disease, potentially offering novel perspectives into tau toxicity. Alzheimer's brain-derived tau oligomers are potent inhibitors of synaptic plasticity; however, the involved mechanism is still not fully understood. We previously reported a significantly reduced incidence of Alzheimer's disease in ageing humans chronically treated with a Food and Drug Administration-approved calcineurin inhibitor, FK506 (tacrolimus), used as an immunosuppressant after solid organ transplant. Using a combination of electrophysiological and RNA-sequencing techniques, we provide here evidence that FK506 has the potential to block the acute toxic effect of brain-derived tau oligomers on synaptic plasticity, as well as to restore the levels of some key synaptic mRNAs. These results further support FK506 as a promising novel therapeutic strategy for the treatment of Alzheimer's disease.

Evidence of mutant huntingtin and tau-related pathology within neuronal grafts in Huntington's disease cases

A number of post-mortem studies conducted in transplanted Huntington's disease (HD) patients from various trials have reported the presence of pathological and misfolded proteins, in particular mutant huntingtin (mHtt) and phosphorylated tau neuropil threads, in the healthy grafted tissue. Here, we extended these observations with histological analysis of post-mortem tissue from three additional HD patients who had received similar striatal allografts from the fetal tissue transplantation trial conducted in Los Angeles in 1998. Immunohistochemical staining was performed using anti-mHtt antibodies, EM48 and MW7, as well as anti-hyperphosphorylated tau antibodies, AT8 and CP13. Immunofluorescence was used to assess the colocalization of EM48+ mHtt aggregates with the neuronal marker MAP2 and/or the extracellular matrix protein phosphacan in both the host and grafts. We confirmed the presence of mHtt aggregates within grafts of all three cases as well as tau neuropil threads in the grafts of two of the three transplanted HD patients. Phosphorylated tau was also variably expressed in the host cerebral cortex of all three subjects. While mHtt inclusions were present within neurons (immunofluorescence co-localization of MAP2 and EM48) as well as within the extracellular matrix of the host (immunofluorescence co-localization of phosphacan and EM48), their localization was limited to the extracellular matrix in the grafted tissue. This study corroborates previous findings that both mHtt and tau pathology can be found in the host and grafts of HD patients years post-grafting.

Huntingtin CAG repeats in neuropathologically confirmed tauopathies: Novel insights

Previous studies have suggested a relationship between the number of CAG triplet repeats in the HTT gene and neurodegenerative diseases not related to Huntington's disease (HD). This study seeks to investigate whether the number of CAG repeats of HTT is associated with the risk of developing certain tauopathies and its influence as a modulator of the clinical and neuropathological phenotype. Additionally, it aims to evaluate the potential of polyglutamine staining as a neuropathological screening. We genotyped the HTT gene CAG repeat number and APOE-ℰ isoforms in a cohort of patients with neuropathological diagnoses of tauopathies (n=588), including 34 corticobasal degeneration (CBD), 98 progressive supranuclear palsy (PSP) and 456 Alzheimer's disease (AD). Furthermore, we genotyped a control group of 1070 patients, of whom 44 were neuropathologic controls. We identified significant differences in the number of patients with pathological HTT expansions in the CBD group (2.7%) and PSP group (3.2%) compared to control subjects (0.2%). A significant increase in the size of the HTT CAG repeats was found in the AD compared to the control group, influenced by the presence of the Apoliprotein E (APOE)-ℰ4 isoform. Post-mortem assessments uncovered tauopathy pathology with positive polyglutamine aggregates, with a slight predominance in the neostriatum for PSP and CBD cases and somewhat greater limbic involvement in the AD case. Our results indicated a link between HTT CAG repeat expansion with other non-HD pathology, suggesting they could share common neurodegenerative pathways. These findings support that genetic or histological screening for HTT repeat expansions should be considered in tauopathies.

Role of Tau Protein in Neurodegenerative Diseases and Development of Its Targeted Drugs: A Literature Review

Tau protein is a microtubule-associated protein that is widely distributed in the central nervous system and maintains and regulates neuronal morphology and function. Tau protein aggregates abnormally and forms neurofibrillary tangles in neurodegenerative diseases, disrupting the structure and function of neurons and leading to neuronal death, which triggers the initiation and progression of neurological disorders. The aggregation of tau protein in neurodegenerative diseases is associated with post-translational modifications, which may affect the hydrophilicity, spatial conformation, and stability of tau protein, promoting tau protein aggregation and the formation of neurofibrillary tangles. Therefore, studying the role of tau protein in neurodegenerative diseases and the mechanism of aberrant aggregation is important for understanding the mechanism of neurodegenerative diseases and finding therapeutic approaches. This review describes the possible mechanisms by which tau protein promotes neurodegenerative diseases, the post-translational modifications of tau protein and associated influencing factors, and the current status of drug discovery and development related to tau protein, which may contribute to the development of new therapeutic approaches to alleviate or treat neurodegenerative diseases.

Neuropathogenesis-on-chips for neurodegenerative diseases

Developing diagnostics and treatments for neurodegenerative diseases (NDs) is challenging due to multifactorial pathogenesis that progresses gradually. Advanced in vitro systems that recapitulate patient-like pathophysiology are emerging as alternatives to conventional animal-based models. In this review, we explore the interconnected pathogenic features of different types of ND, discuss the general strategy to modelling NDs using a microfluidic chip, and introduce the organoid-on-a-chip as the next advanced relevant model. Lastly, we overview how these models are being applied in academic and industrial drug development. The integration of microfluidic chips, stem cells, and biotechnological devices promises to provide valuable insights for biomedical research and developing diagnostic and therapeutic solutions for NDs.

Translatable plasma and CSF biomarkers for use in mouse models of Huntington's disease

Huntington's disease is an inherited neurodegenerative disorder for which a wide range of disease-modifying therapies are in development and the availability of biomarkers to monitor treatment response is essential for the success of clinical trials. Baseline levels of neurofilament light chain in CSF and plasma have been shown to be effective in predicting clinical disease status, subsequent clinical progression and brain atrophy. The identification of further sensitive prognostic fluid biomarkers is an active research area, and total-Tau and YKL-40 levels have been shown to be increased in CSF from Huntington's disease mutation carriers. The use of readouts with clinical utility in the preclinical assessment of potential therapeutics should aid in the translation of new treatments. Here, we set out to determine how the concentrations of these three proteins change in plasma and CSF with disease progression in representative, well-established mouse models of Huntington's disease. Plasma and CSF were collected throughout disease progression from R6/2 transgenic mice with CAG repeats of 200 or 90 codons (R6/2:Q200 and R6/2:Q90), zQ175 knock-in mice and YAC128 transgenic mice, along with their respective wild-type littermates. Neurofilament light chain and total-Tau concentrations were quantified in CSF and plasma using ultrasensitive single-molecule array (Quanterix) assays, and a novel Quanterix assay was developed for breast regression protein 39 (mouse homologue of YKL-40) and used to quantify breast regression protein 39 levels in plasma. CSF levels of neurofilament light chain and plasma levels of neurofilament light chain and breast regression protein 39 increased in wild-type biofluids with age, whereas total-Tau remained constant. Neurofilament light chain and breast regression protein 39 were elevated in the plasma and CSF from Huntington's disease mouse models, as compared with wild-type littermates, at presymptomatic stages, whereas total-Tau was only increased at the latest disease stages analysed. Levels of biomarkers that had been measured in the same CSF or plasma samples taken at the latest stages of disease were correlated. The demonstration that breast regression protein 39 constitutes a robust plasma biomarker in Huntington's disease mouse models supports the further investigation of YKL-40 as a CSF biomarker for Huntington's disease mutation carriers. Neurofilament light chain and Tau are considered markers of neuronal damage, and breast regression protein 39 is a marker of inflammation; the similarities and differences in the levels of these proteins between mouse models may provide future insights into their underlying pathology. These data will facilitate the use of fluid biomarkers in the preclinical assessment of therapeutic agents for Huntington's disease, providing readouts with direct relevance to clinical trials.

Tau: a biomarker of Huntington's disease

Developing effective treatments for patients with Huntington's disease (HD)-a neurodegenerative disorder characterized by severe cognitive, motor and psychiatric impairments-is proving extremely challenging. While the monogenic nature of this condition enables to identify individuals at risk, robust biomarkers would still be extremely valuable to help diagnose disease onset and progression, and especially to confirm treatment efficacy. If measurements of cerebrospinal fluid neurofilament levels, for example, have demonstrated use in recent clinical trials, other proteins may prove equal, if not greater, relevance as biomarkers. In fact, proteins such as tau could specifically be used to detect/predict cognitive affectations. We have herein reviewed the literature pertaining to the association between tau levels and cognitive states, zooming in on Alzheimer's disease, Parkinson's disease and traumatic brain injury in which imaging, cerebrospinal fluid, and blood samples have been interrogated or used to unveil a strong association between tau and cognition. Collectively, these areas of research have accrued compelling evidence to suggest tau-related measurements as both diagnostic and prognostic tools for clinical practice. The abundance of information retrieved in this niche of study has laid the groundwork for further understanding whether tau-related biomarkers may be applied to HD and guide future investigations to better understand and treat this disease.

The role of glycogen synthase kinase 3 beta in neurodegenerative diseases

Neurodegenerative diseases (NDDs) pose an increasingly prevalent threat to the well-being and survival of elderly individuals worldwide. NDDs include Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD), amyotrophic lateral sclerosis (ALS), and so on. They are characterized by progressive loss or dysfunction of neurons in the central or peripheral nervous system and share several cellular and molecular mechanisms, including protein aggregation, mitochondrial dysfunction, gene mutations, and chronic neuroinflammation. Glycogen synthase kinase-3 beta (GSK-3β) is a serine/threonine kinase that is believed to play a pivotal role in the pathogenesis of NDDs. Here we summarize the structure and physiological functions of GSK3β and explore its involvement in NDDs. We also discussed its potential as a therapeutic target.

Age-dependent increase in tau phosphorylation at serine 396 in Huntington's disease pre-frontal cortex

Background

To date, it is still controversial whether tau phosphorylation plays a role in Huntington's disease (HD), as previous studies demonstrated either no alterations or increases in phosphorylated tau (pTau) in HD post-mortem brain and mouse models.

Objectives

The goal of this study was to determine whether total tau and pTau levels are altered in HD.

Methods

Immunohistochemistry, cellular fractionations, and western blots were used to measure tau and pTau levels in a large cohort of HD and control post-mortem prefrontal cortex (PFC). Furthermore, western blots were performed to assess tau, and pTau levels in HD and control isogenic embryonic stem cell (ESC)-derived cortical neurons and neuronal stem cells (NSCs). Similarly, western blots were used to assess tau and pTau in Htt Q111 and transgenic R6/2 mice. Lastly, total tau levels were assessed in HD and healthy control plasma using Quanterix Simoa assay.

Results

Our results revealed that, while there was no difference in tau or pTau levels in HD PFC compared to controls, tau phosphorylated at S396 levels were increased in PFC samples from HD patients 60 years or older at time of death. Additionally, tau and pTau levels were not changed in HD ESC-derived cortical neurons and NSCs. Similarly, tau or pTau levels were not altered in Htt Q111 and transgenic R6/2 mice compared to wild-type littermates. Lastly, tau levels were not changed in plasma from a small cohort of HD patients compared to controls.

Conclusion

Together these findings demonstrate that pTau-S396 levels increase significantly with age in HD PFC.

Implications of Tau Dysregulation in Huntington's Disease and Potential for New Therapeutics

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder. The disease, characterized by motor, cognitive, and psychiatric impairments, is caused by the expansion of a CAG repeat in the huntingtin gene. Despite the discovery of the mutation in 1993, no disease-modifying treatments are yet available. Understanding the molecular and cellular mechanisms involved in HD is therefore crucial for the development of novel treatments. Emerging research has found that HD might be classified as a secondary tauopathy, with the presence of tau insoluble aggregates in late HD. Increased total tau protein levels have been observed in both HD patients and animal models of HD. Tau hyperphosphorylation, the main feature of tau pathology, has also been investigated and our own published results suggest that the protein phosphorylation machinery is dysregulated in the early stages of HD in R6/1 transgenic mice, primarily in the cortex and striatum. Protein phosphorylation, catalysed by kinases, regulates numerous cellular mechanisms and has been shown to be dysregulated in other neurodegenerative disorders, including Alzheimer's disease. While it is still unclear how the mutation in the huntingtin gene leads to tau dysregulation in HD, several hypotheses have been explored. Evidence suggests that the mutant huntingtin does not directly interact with tau, but instead interacts with tau kinases, phosphatases, and proteins involved in tau alternative splicing, which could result in tau dysregulation as observed in HD. Altogether, there is increasing evidence that tau is undergoing pathological changes in HD and may be a good therapeutic target.

Untangling the Role of Tau in Huntington's Disease Pathology

There is increasing evidence for the presence of pathological forms of tau in tissues of both Huntington's disease (HD) patients and animal models of this condition. While cumulative studies of the past decade have led to the proposition that this disorder could also be considered a tauopathy, the implications of tau in cellular toxicity and consequent behavioral impairments are largely unknown. In fact, recent animal work has challenged the contributory role of tau in HD pathogenesis/pathophysiology. This review presents the supporting and opposing arguments for the involvement of tau in HD, highlighting the discrepancies that have emerged. Reflecting on what is known in other tauopathies, the putative mechanisms through which tau could initiate and/or contribute to pathology are discussed, shedding light on the future research directions that could be considered to confirm, or rule out, the clinical relevance of tau in HD.

Age-Dependent Increase in Tau Phosphorylation at Serine 396 in Huntington's Disease Prefrontal Cortex

BACKGROUND

To date, it is still controversial whether tau phosphorylation plays a role in Huntington's disease (HD), as previous studies demonstrated either no alterations or increases in phosphorylated tau (pTau) in HD postmortem brain and mouse models.

OBJECTIVE

The goal of this study was to determine whether total tau and pTau levels are altered in HD.

METHODS

Immunohistochemistry, cellular fractionations, and western blots were used to measure total tau and pTau levels in a large cohort of HD and control postmortem prefrontal cortex (PFC). Furthermore, western blots were performed to assess tau, and pTau levels in HD and control isogenic embryonic stem cell (ESC)-derived cortical neurons and neuronal stem cells (NSCs). Similarly, western blots were used to assess tau and pTau levels in HttQ111 and transgenic R6/2 mice. Lastly, total tau levels were assessed in HD and healthy control plasma using Quanterix Simoa assay.

RESULTS

Our results revealed that, while there was no difference in total tau or pTau levels in HD PFC compared to controls, the levels of tau phosphorylated at S396 were increased in PFC samples from HD patients 60 years or older at time of death. Additionally, tau and pTau levels were not changed in HD ESC-derived cortical neurons and NSCs. Similarly, total tau or pTau levels were not altered in HttQ111 and transgenic R6/2 mice compared to wild-type littermates. Lastly, tau levels were not changed in plasma from a small cohort of HD patients compared to controls.

CONCLUSIONS

Together these findings demonstrate that pTau-S396 levels increase significantly with age in HD PFC.

Phosphoproteomic dysregulation in Huntington's disease mice is rescued by environmental enrichment

Huntington's disease is a fatal autosomal-dominant neurodegenerative disorder, characterized by neuronal cell dysfunction and loss, primarily in the striatum, cortex and hippocampus, causing motor, cognitive and psychiatric impairments. Unfortunately, no treatments are yet available to modify the progression of the disease. Recent evidence from Huntington's disease mouse models suggests that protein phosphorylation (catalysed by kinases and hydrolysed by phosphatases) might be dysregulated, making this major post-translational modification a potential area of interest to find novel therapeutic targets. Furthermore, environmental enrichment, used to model an active lifestyle in preclinical models, has been shown to alleviate Huntington's disease-related motor and cognitive symptoms. However, the molecular mechanisms leading to these therapeutic effects are still largely unknown. In this study, we applied a phosphoproteomics approach combined with proteomic analyses on brain samples from pre-motor symptomatic R6/1 Huntington's disease male mice and their wild-type littermates, after being housed either in environmental enrichment conditions, or in standard housing conditions from 4 to 8 weeks of age (n = 6 per group). We hypothesized that protein phosphorylation dysregulations occur prior to motor onset in this mouse model, in two highly affected brain regions, the striatum and hippocampus. Furthermore, we hypothesized that these phosphoproteome alterations are rescued by environmental enrichment. When comparing 8-week-old Huntington's disease mice and wild-type mice in standard housing conditions, our analysis revealed 229 differentially phosphorylated peptides in the striatum, compared with only 15 differentially phosphorylated peptides in the hippocampus (statistical thresholds fold discovery rate 0.05, fold change 1.5). At the same disease stage, minor differences were found in protein levels, with 24 and 22 proteins dysregulated in the striatum and hippocampus, respectively. Notably, we found no differences in striatal protein phosphorylation and protein expression when comparing Huntington's disease mice and their wild-type littermates in environmentally enriched conditions. In the hippocampus, only four peptides were differentially phosphorylated between the two genotypes under environmentally enriched conditions, and 22 proteins were differentially expressed. Together, our data indicates that protein phosphorylation dysregulations occur in the striatum of Huntington's disease mice, prior to motor symptoms, and that the kinases and phosphatases leading to these changes in protein phosphorylation might be viable drug targets to consider for this disorder. Furthermore, we show that an early environmental intervention was able to rescue the changes observed in protein expression and phosphorylation in the striatum of Huntington's disease mice and might underlie the beneficial effects of environmental enrichment, thus identifying novel therapeutic targets.

Differential Regulation of Tau Exon 2 and 10 Isoforms in Huntington's Disease Brain

Huntington's disease (HD) is an inherited neurodegenerative disorder caused by an expansion of CAG repeats in the Huntingtin (HTT) gene. Accumulating evidence suggests that the microtubule-associated tau protein participates in the pathogenesis of HD. Recently, we have identified changes in tau alternative splicing of exons 2, 3 and 10 in the putamen of HD patients (St-Amour et al, 2018). In this study, we sought to determine whether tau mis-splicing events were equally observed in other brain regions that are less prone to neurodegeneration. Using Western blot and PCR, we characterized the relationship between MAPT splicing of exons 2, 3 and 10, tauopathy and Htt pathologies, as well as neurodegeneration markers in matching putamen and cortical samples from HD (N = 48) and healthy control (N = 25) subjects. We first show that levels of 4R-tau (exon 10 inclusion) isoforms are higher in both the putamen and the cortex of individuals with HD, consistent with earlier findings. On the other hand, higher 0N-tau (exclusion of exons 2 and 3) and lower 1N-tau (exclusion of exon 3) isoforms were seen exclusively in the putamen of HD individuals. Interestingly, investigated splicing factors were deregulated in both regions whereas exon 2 differences coincided with increased tau hyperphosphorylation, aggregation and markers of neurodegeneration. Overall, these results imply a differential regulation of tau exon 2 and exon 10 alternative splicing in HD putamen that could provide a useful biomarker or therapeutic target.

Role of Ca2+/Calmodulin-Dependent Protein Kinase Type II in Mediating Function and Dysfunction at Glutamatergic Synapses

Glutamatergic synapses harbor abundant amounts of the multifunctional Ca²⁺/calmodulin-dependent protein kinase type II (CaMKII). Both in the postsynaptic density as well as in the cytosolic compartment of postsynaptic terminals, CaMKII plays major roles. In addition to its Ca²⁺-stimulated kinase activity, it can also bind to a variety of membrane proteins at the synapse and thus exert spatially restricted activity. The abundance of CaMKII in glutamatergic synapse is akin to scaffolding proteins although its prominent function still appears to be that of a kinase. The multimeric structure of CaMKII also confers several functional capabilities on the enzyme. The versatility of the enzyme has prompted hypotheses proposing several roles for the enzyme such as Ca²⁺ signal transduction, memory molecule function and scaffolding. The article will review the multiple roles played by CaMKII in glutamatergic synapses and how they are affected in disease conditions.

Zinc in Regulating Protein Kinases and Phosphatases in Neurodegenerative Diseases

Zinc is essential for human growth and development. As a trace nutrient, zinc plays important roles in numerous signal transduction pathways involved in distinct physiologic or pathologic processes. Protein phosphorylation is a posttranslational modification which regulates protein activity, degradation, and interaction with other molecules. Protein kinases (PKs) and phosphatases (PPs), with their effects of adding phosphate to or removing phosphate from certain substrates, are master regulators in controlling the phosphorylation of proteins. In this review, we summarize the disturbance of zinc homeostasis and role of zinc disturbance in regulating protein kinases and protein phosphatases in neurodegenerative diseases, with the focus of that in Alzheimer’s disease, providing a new perspective for understanding the mechanisms of these neurologic diseases.

Passive immunization against phosphorylated tau improves features of Huntington's disease pathology

Huntington's disease is classically described as a neurodegenerative disorder of monogenic aetiology. The disease is characterized by an abnormal polyglutamine expansion in the huntingtin gene, which drives the toxicity of the mutated form of the protein. However, accumulation of the microtubule-associated protein tau, which is involved in a number of neurological disorders, has also been observed in patients with Huntington's disease. In order to unravel the contribution of tau hyperphosphorylation to hallmark features of Huntington's disease, we administered weekly intraperitoneal injections of the anti-tau pS202 CP13 monoclonal antibody to zQ175 mice and characterized the resulting behavioral and biochemical changes. After 12 weeks of treatment, motor impairments, cognitive performance and general health were improved in zQ175 mice along with a significant reduction in hippocampal pS202 tau levels. Despite the lack of effect of CP13 on neuronal markers associated with Huntington's disease pathology, tau-targeting enzymes and gliosis, CP13 was shown to directly impact mutant huntingtin aggregation such that brain levels of amyloid fibrils and huntingtin oligomers were decreased, while larger huntingtin protein aggregates were increased. Investigation of CP13 treatment of Huntington's disease patient-derived induced pluripotent stem cells (iPSCs) revealed a reduction in pS202 levels in differentiated cortical neurons and a rescue of neurite length. Collectively, these findings suggest that attenuating tau pathology could mitigate behavioral and molecular hallmarks associated with Huntington's disease.

Tau Toxicity in Neurodegeneration

Tau is a microtubule-associated protein widely distributed in the central nervous system (CNS). The main function of tau is to promote the assembly of microtubules and stabilize their structure. After a long period of research on neurodegenerative diseases, the function and dysfunction of the microtubule-associated protein tau in neurodegenerative diseases and tau neurotoxicity have attracted increasing attention. Tauopathies are a series of progressive neurodegenerative diseases caused by pathological changes in tau, such as abnormal phosphorylation. The pathological features of tauopathies are the deposition of abnormally phosphorylated tau proteins and the aggregation of tau proteins in neurons. This article first describes the normal physiological function and dysfunction of tau proteins and then discusses the enzymes and proteins involved in tau phosphorylation and dephosphorylation, the role of tau in cell dysfunction, and the relationships between tau and several neurodegenerative diseases. The study of tau neurotoxicity provides new directions for the treatment of tauopathies.

Glycogen Synthase Kinase 3β Involvement in Neuroinflammation and Neurodegenerative Diseases

BACKGROUND

GSK-3β activity has been strictly related to neuroinflammation and neurodegeneration. Alzheimer's disease is the most studied neurodegenerative disease, but GSK-3β seems to be involved in almost all neurodegenerative diseases including Parkinson's disease, amyotrophic lateral sclerosis, frontotemporal dementia, Huntington's disease and the autoimmune disease multiple sclerosis.

OBJECTIVE

The aim of this review is to help researchers both working on this research topic or not to have a comprehensive overview on GSK-3β in the context of neuroinflammation and neurodegeneration.

METHOD

Literature has been searched using PubMed and SciFinder databases by inserting specific keywords. A total of more than 500 articles have been discussed.

RESULTS

First of all, the structure and regulation of the kinase were briefly discussed and then, specific GSK-3β implications in neuroinflammation and neurodegenerative diseases were illustrated also with the help of figures, to conclude with a comprehensive overview on the most important GSK-3β and multitarget inhibitors. For all discussed compounds, the structure and IC50 values at the target kinase have been reported.

CONCLUSION

GSK-3β is involved in several signaling pathways both in neurons as well as in glial cells and immune cells. The fine regulation and interconnection of all these pathways are at the base of the rationale use of GSK-3β inhibitors in neuroinflammation and neurodegeneration. In fact, some compounds are now under clinical trials. Despite this, pharmacodynamic and ADME/Tox profiles of the compounds were often not fully characterized and this is deleterious in such a complex system.

Quantitative Phosphoproteomics Reveals Extensive Protein Phosphorylation Dysregulation in the Cerebral Cortex of Huntington's Disease Mice Prior to Onset of Symptoms

Protein phosphorylation plays a role in many important cellular functions such as cellular plasticity, gene expression, and intracellular trafficking. All of these are dysregulated in Huntington's disease (HD), a devastating neurodegenerative disorder caused by an expanded CAG repeat in exon 1 of the huntingtin gene. However, no studies have yet found protein phosphorylation differences in preclinical HD mouse models. Our current study investigated changes occurring in the cortical phosphoproteome of 8-week-old (prior to motor deficits) and 20-week-old (fully symptomatic) R6/1 transgenic HD mice. When comparing 8-week-old HD mice with their wild-type (WT) littermates, we found 660 peptides differentially phosphorylated, which were mapped to 227 phosphoproteins. These proteins were mainly involved in synaptogenesis, cytoskeleton organization, axon development, and nervous system development. Tau protein, found hyperphosphorylated at multiple sites in early symptomatic HD mice, also appeared as a main upstream regulator for the changes observed. Surprisingly, we found fewer changes in the phosphorylation profile of HD mice at the fully symptomatic stage, with 29 peptides differentially phosphorylated compared to WT mice, mapped to 25 phosphoproteins. These proteins were involved in cAMP signaling, dendrite development, and microtubule binding. Furthermore, huntingtin protein appeared as an upstream regulator for the changes observed at the fully symptomatic stage, suggesting impacts on kinases and phosphatases that extend beyond the mutated polyglutamine tract. In summary, our findings show that the most extensive changes in the phosphorylation machinery appear at an early presymptomatic stage in HD pathogenesis and might constitute a new target for the development of treatments.

Loss-of-function and gain-of-function studies refute the hypothesis that tau protein is causally involved in the pathogenesis of Huntington's disease

Tau is a microtubule-associated protein, whose abnormal phosphorylation and deposition in the brain characterizes a range of neurodegenerative diseases called tauopathies. Recent clinical (post-mortem) and pre-clinical evidence suggests that Huntington's disease (HD), an autosomal dominant neurodegenerative disorder, could be considered as a tauopathy. Studies have found the presence of hyperphosphorylated tau, altered tau isoform ratio and aggregated tau in HD brains. However, little is known about the implication of tau in the development of HD pathophysiology, which includes motor, cognitive and affective symptoms. To shine a light on the involvement of tau in HD, our present study aimed at (i) knocking out tau expression and (ii) expressing a transgene encoding mutant human tau in the R6/1 mouse model of HD. We hypothesized that expression of the mutant human tau transgene in HD mice would worsen the HD phenotype, while knocking out endogenous mouse tau in HD mice would improve some behavioural deficits display by HD mice. Our data suggests that neither the expression of a tau transgene nor the ablation of tau expression impacted the progression of the HD motor, cognitive and affective phenotypes. Supporting these behavioural findings, we also found that modulating tau expression had no effect on brain weights in HD mice. We also report that expression of the tau transgene increased the weight of WT and HD male mice, whereas tau ablation increased the weight of HD females only. Together, our results indicate that tau might not be as important in regulating the progression of HD symptomatology.

Mitochondrial Abnormalities and Synaptic Damage in Huntington's Disease: a Focus on Defective Mitophagy and Mitochondria-Targeted Therapeutics

Huntington's disease (HD) is a fatal and pure genetic disease with a progressive loss of medium spiny neurons (MSN). HD is caused by expanded polyglutamine repeats in the exon 1 of HD gene. Clinically, HD is characterized by chorea, seizures, involuntary movements, dystonia, cognitive decline, intellectual impairment, and emotional disturbances. Several years of intense research revealed that multiple cellular changes, including defective axonal transport, protein-protein interactions, defective bioenergetics, calcium dyshomeostasis, NMDAR activation, synaptic damage, mitochondrial abnormalities, and selective loss of medium spiny neurons are implicated in HD. Recent research on mutant huntingtin (mHtt) and mitochondria has found that mHtt interacts with the mitochondrial division protein, dynamin-related protein 1 (DRP1), enhances GTPase DRP1 enzymatic activity, and causes excessive mitochondrial fragmentation and abnormal distribution, leading to defective axonal transport of mitochondria and selective synaptic degeneration. Recent research also revealed that failure to remove dead and/or dying mitochondria is an early event in the disease progression. Currently, efforts are being made to reduce abnormal protein interactions and enhance synaptic mitophagy as therapeutic strategies for HD. The purpose of this article is to discuss recent research in HD progression. This article also discusses recent developments of cell and mouse models, cellular changes, mitochondrial abnormalities, DNA damage, bioenergetics, oxidative stress, mitophagy, and therapeutics strategies in HD.

Calcineurin-nuclear factor for activated T cells (NFAT) signaling in pathophysiology of wound healing

Wound healing occurred with serial coordinated processes via coagulation-fibrinolysis, inflammation following to immune-activation, angiogenesis, granulation, and the final re-epithelization. Since the dermis forms critical physical and biological barriers, the repair system should be rapidly and accurately functioned to keep homeostasis in our body. The wound healing is impaired or dysregulated via an inappropriate microenvironment, which is easy to lead to several diseases, including fibrosis in multiple organs and psoriasis. Such a disease led to the dysregulation of several types of cells: immune cells, fibroblasts, mural cells, and endothelial cells. Moreover, recent progress in medical studies uncovers the significant concept. The calcium signaling, typically the following calcineurin-NFAT signaling, essentially regulates not only immune cell activations, but also various healing steps via coagulation, inflammation, and angiogenesis. In this review, we summarize the role of the NFAT activation pathway in wound healing and discuss its overall impact on future therapeutic ways.

Role of Phosphorylated Tau and Glucose Synthase Kinase 3 Beta in Huntington's Disease Progression

The purpose of our article is to critically assess the role of phosphorylated tau in Huntington's disease (HD) progression and pathogenesis. HD is a fatal and pure genetic disease, characterized by chorea, seizures, involuntary movements, dystonia, cognitive decline, intellectual impairment, and emotional disturbances. HD is caused by expanded polyglutamine (polyQ or CAG) repeats within the exon 1 of the HD gene. HD has an autosomal dominant pattern of inheritance with genetic anticipation. Although the HD gene was discovered 26 years ago, there is no complete understanding of how mutant huntingtin (mHTT) selectively targets medium spiny projection neurons in the basal ganglia of the brain in patients with HD. Several years of intense research revealed that multiple cellular changes are involved in disease process, including transcriptional dysregulation, mitochondrial abnormalities and impaired bioenergetics, defective axonal transport, calcium dyshomeostasis, synaptic damage and caspase, and NMDAR activations. Recent research also revealed that phosphorylated tau and defective GSK-3β signaling are strongly linked to progression of the disease. This article summarizes the recent developments of cellular and pathological changes in disease progression of HD. This article also highlights recent developments in phosphorylated tau and defective GSK-3β signaling and the involvement of calcineurin in HD progression and pathogenesis.

Hsp90-interacting Co-chaperones and their Family Proteins in Tau Regulation: Introducing a Novel Role for Cdc37L1

Tau is a microtubule-associated protein that serves as a promoter of microtubule assembly and stability in neuron cells. In a collective group of neurodegenerative diseases called tauopathies, tau processing is altered as a result of gene mutations and post-translational modifications. In particular, in Alzheimer's disease (AD) or AD-like conditions, tau becomes hyperphosphorylated and forms toxic aggregates inside the cell. The chaperone heat shock protein 90 (Hsp90) plays an important role in the proper folding, degradation, and recycling of tau proteins and tau kinases. Hsp90 has many co-chaperones that aid in tau processing. In particular, a few of these co-chaperones, such as FK506-binding protein (FKBP) 51, protein phosphatase (PP) 5, cell division cycle 37 (Cdc37), and S100A1 have family members that are reported to affect Hsp90-mediated tau processing in either a similar or an opposite manner. Here, we provide a holistic review of these selected co-chaperones and their family proteins and introduce a novel Hsp90-binding Cdc37 relative, Cdc37-like-1 (Cdc37L1 or L1) in tau regulation. Overall, the proteins discussed here highlight the importance of studying family proteins in order to fully understand the mechanism of tau pathogenesis and to establish drug targets for the treatment of tauopathies.

Targeting Tau to Treat Clinical Features of Huntington's Disease

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder characterized by severe motor, cognitive and psychiatric impairments. While motor deficits often confirm diagnosis, cognitive dysfunctions usually manifest early in the disease process and are consistently ranked among the leading factors that impact the patients' quality of life. The genetic component of HD, a mutation in the huntingtin (HTT) gene, is traditionally presented as the main contributor to disease pathology. However, accumulating evidence suggests the implication of the microtubule-associated tau protein to the pathogenesis and therefore, proposes an alternative conceptual framework where tau and mutant huntingtin (mHTT) act conjointly to drive neurodegeneration and cognitive dysfunction. This perspective on disease etiology offers new avenues to design therapeutic interventions and could leverage decades of research on Alzheimer's disease (AD) and other tauopathies to rapidly advance drug discovery. In this mini review, we examine the breadth of tau-targeting treatments currently tested in the preclinical and clinical settings for AD and other tauopathies, and discuss the potential application of these strategies to HD.

How Do Post-Translational Modifications Influence the Pathomechanistic Landscape of Huntington's Disease? A Comprehensive Review

Huntington’s disease (HD) is an autosomal dominant inherited neurodegenerative disorder characterized by the loss of motor control and cognitive ability, which eventually leads to death. The mutant huntingtin protein (HTT) exhibits an expansion of a polyglutamine repeat. The mechanism of pathogenesis is still not fully characterized; however, evidence suggests that post-translational modifications (PTMs) of HTT and upstream and downstream proteins of neuronal signaling pathways are involved. The determination and characterization of PTMs are essential to understand the mechanisms at work in HD, to define possible therapeutic targets better, and to challenge the scientific community to develop new approaches and methods. The discovery and characterization of a panoply of PTMs in HTT aggregation and cellular events in HD will bring us closer to understanding how the expression of mutant polyglutamine-containing HTT affects cellular homeostasis that leads to the perturbation of cell functions, neurotoxicity, and finally, cell death. Hence, here we review the current knowledge on recently identified PTMs of HD-related proteins and their pathophysiological relevance in the formation of abnormal protein aggregates, proteolytic dysfunction, and alterations of mitochondrial and metabolic pathways, neuroinflammatory regulation, excitotoxicity, and abnormal regulation of gene expression.

Altered Levels and Isoforms of Tau and Nuclear Membrane Invaginations in Huntington's Disease

Since the early reports of neurofibrillary Tau pathology in brains of some Huntington’s disease (HD) patients, mounting evidence of multiple alterations of Tau in HD brain tissue has emerged in recent years. Such Tau alterations range from increased total levels, imbalance of isoforms generated by alternative splicing (increased 4R-/3R-Tau ratio) or by post-translational modifications such as hyperphosphorylation or truncation. Besides, the detection in HD brains of a new Tau histopathological hallmark known as Tau nuclear rods (TNRs) or Tau-positive nuclear indentations (TNIs) led to propose HD as a secondary Tauopathy. After their discovery in HD brains, TNIs have also been reported in hippocampal neurons of early Braak stage AD cases and in frontal and temporal cortical neurons of FTD-MAPT cases due to the intronic IVS10+16 mutation in the Tau gene (MAPT) which results in an increased 4R-/3R-Tau ratio similar to that observed in HD. TNIs are likely pathogenic for contributing to the disturbed nucleocytoplasmic transport observed in HD. A key question is whether correction of any of the mentioned Tau alterations might have positive therapeutic implications for HD. The beneficial effect of decreasing Tau expression in HD mouse models clearly implicates Tau in HD pathogenesis. Such beneficial effect might be exerted by diminishing the excess total levels of Tau or specifically by diminishing the excess 4R-Tau, as well as any of their downstream effects. In any case, since gene silencing drugs are under development to attenuate both Huntingtin (HTT) expression for HD and MAPT expression for FTD-MAPT, it is conceivable that the combined therapy in HD patients might be more effective than HTT silencing alone.

A soluble truncated tau species related to cognitive dysfunction and caspase-2 is elevated in the brain of Huntington's disease patients

Huntington’s disease (HD) is a progressive neurodegenerative disease. Involuntary movements, cognitive impairment and psychiatric disturbance are the major clinical manifestations, and gradual atrophy and selective neuronal loss in the striatum and cerebral cortex are the pathologic hallmarks. HD is caused by expanded CAG trinucleotide repeats at the N-terminus of IT15 that encodes the huntingtin (HTT) protein, though the molecular mechanisms through which the mutant HTT (mHTT) exerts toxic effects remain obscure. Members of the caspase family, including caspase-2 (Casp2), play an important role in HD pathogenesis. Genetic ablation of Casp2 ameliorates cognitive and motor deficits of HD mice, though the molecular targets of Casp2 are still unclear. It is well established that the microtubule-associated protein tau potentiates cognitive dysfunction in a variety of neurodegenerative disorders, including HD. Our recent study indicates that Casp2-catalyzed tau cleavage at aspartate 314 (tau 2N4R isoform numbering system) mediates synaptotoxicity, cognitive deficits and neurodegeneration in cellular and mouse models of frontotemporal dementia; further, levels of Δtau314, the soluble, N-terminal cleavage product, are elevated in individuals with mild cognitive impairment and Alzheimer’s disease, compared with cognitively normal individuals. Here, we identified the presence of Δtau314 proteins in the striatum (caudate nucleus) and prefrontal cortex (Brodmann’s area 8/9) of human subjects, and showed that in both structures, levels of Casp2 and Δtau314 proteins correlate well, and both proteins are higher in HD patients than non-HD individuals. Our findings advance our understanding of the contribution of Casp2-mediated Δtau314 production to HD pathogenesis.

Novel proteomic changes in brain mitochondria provide insights into mitochondrial dysfunction in mouse models of Huntington's disease

Huntington's disease (HD) is a progressive ultimately fatal disorder caused by a glutamine-encoding CAG expansion in the huntingtin (HTT) gene that results in degeneration mainly in striatal and cerebro-cortical brain regions. Mitochondrial dysfunction is one important facet of HD pathogenesis. Here we used R6/2 and YAC128 HD mouse models of human HD, that express different HTT transgenes and have different progression rates, to identify HD brain mitochondrial proteomic signatures. Cerebral cortical mitochondrial preparations from HD and wild-type litter mate mice were compared by two-dimensional SDS-PAGE electrophoresis and MALDI-TOF/TOF mass spectrometry. Proteomic analyses inferred 17 and 12 differentially expressed proteins, respectively in 12 week R6/2 and 15 month YAC128 HD mice, compared to controls. Peroxiredoxin 3, stress-70, DJ-1, isocitrate dehydrogenase [NAD] α subunit and ATP synthase subunit D were differentially expressed in both models. Using the PANTHER (Protein ANalysis THrough Evolutionary Relationships) classification system we show that the inferred proteins are involved in oxidative stress defense, oxidative phosphorylation, the citric acid cycle, pyruvate metabolism, apoptosis, protein folding and iron metabolism. Common mitochondrial proteomic changes are significant in mouse models of middle (YAC128) and advanced (R6/2) HD despite differences in the HTT transgenes, age, genetic background and disease stage. The findings identify a proteomic signature of HD mitochondria in mouse models that includes previously unrecognized proteins.

Are immunotherapies for Huntington's disease a realistic option?

There is compelling evidence that the pathophysiology of many neurodegenerative diseases includes dysregulation of the immune system, with some elements that precede disease onset. However, if these alterations are prominent, why have clinical trials targeting this system failed to translate into long-lasting meaningful benefits for patients? This review focuses on Huntington's disease, a genetic disorder marked by notable cerebral and peripheral inflammation. We summarize ongoing and completed clinical trials that have involved pharmacological approaches to inhibit various components of the immune system and their pre-clinical correlates. We then discuss new putative treatment strategies using more targeted immunotherapies such as vaccination and intrabodies and how these may offer new hope in the treatment of Huntington's disease as well as other neurodegenerative diseases.

Spatiotemporal Proteomic Profiling of Huntington's Disease Inclusions Reveals Widespread Loss of Protein Function

Aggregation of polyglutamine-expanded huntingtin exon 1 (HttEx1) in Huntington’s disease (HD) proceeds from soluble oligomers to late-stage inclusions. The nature of the aggregates and how they lead to neuronal dysfunction is not well understood. We employed mass spectrometry (MS)-based quantitative proteomics to dissect spatiotemporal mechanisms of neurodegeneration using the R6/2 mouse model of HD. Extensive remodeling of the soluble brain proteome correlated with insoluble aggregate formation during disease progression. In-depth and quantitative characterization of the aggregates uncovered an unprecedented complexity of several hundred proteins. Sequestration to aggregates depended on protein expression levels and sequence features such as low-complexity regions or coiled-coil domains. In a cell-based HD model, overexpression of a subset of the sequestered proteins in most cases rescued viability and reduced aggregate size. Our spatiotemporally resolved proteome resource of HD progression indicates that widespread loss of cellular protein function contributes to aggregate-mediated toxicity.

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Spatiotemporally resolved brain proteome of wild-type and HD mice

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Quantitative characterization of huntingtin inclusion bodies in vivo

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Sequestration correlates with protein expression levels and specific sequence features

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Resupplying sequestered proteins ameliorates HTT-induced toxicity and inclusion size

A Friend or Foe: Calcineurin across the Gamut of Neurological Disorders

The serine/threonine phosphatase calcineurin (CaN) is a unique but confounding calcium/calmodulin-mediated enzyme. CaN has shown to play essential roles from regulating calcium homeostasis to being an intricate part of learning and memory formation. Neurological disorders, despite differing in their etiology, share similar pathological outcomes, such as mitochondrial dysfunction and apoptotic signaling brought about by excitotoxic elements. CaN, being deeply integrated in vital neuronal functions, may be implicated in various neurological disorders. Understanding the enzyme and its physiological niche in the nervous system is vital in uncovering its roles in the spectrum of brain disorders. By reviewing the crosstalk in different neurological pathologies, a possible grasp of CaN's complex signaling may lead to forming better neurotherapy. This Outlook attempts to explore the various neuronal functions of CaN and investigate its pervasive role through the gamut of neurological disorders.

Tau: A Common Denominator and Therapeutic Target for Neurodegenerative Disorders

There is compelling evidence that a number of neurodegenerative diseases share common pathogenic mechanisms. Better understanding these mechanisms will allow us to develop new therapeutic strategies. This commentary follows up on our recent findings that tau pathology can be found in healthy fetal tissue transplanted into the brain of patients with either Huntington or Parkinson disease. We will examine how tau appears to be shared in a number of different conditions and how its expression relates to cognitive decline and disease progression. We will further review pathogenic mechanisms and especially the relevance of the possible prion-like behavior of tau. We will conclude by discussing how all this work opens up novel therapeutic approaches to treating the cognitive impairments related to neurodegenerative diseases using a common strategy.

Co-occurrence of mixed proteinopathies in late-stage Huntington's disease

Accumulating evidence highlights the potential role of mixed proteinopathies (i.e., abnormal protein aggregation) in the development of clinical manifestations of neurodegenerative diseases (NDD). Huntington's disease (HD) is an inherited NDD caused by autosomal-dominant expanded CAG trinucleotide repeat mutation in the gene coding for Huntingtin (Htt). Previous studies have suggested the coexistence of phosphorylated-Tau, α-synuclein (α-Syn) and TAR DNA-binding protein 43 (TDP-43) inclusions in HD. However, definite evidence that HD pathology in humans can be accompanied by other proteinopathies is still lacking. Using human post-mortem putamen samples from 31 controls and 56 HD individuals, we performed biochemical analyses of the expression, oligomerization and aggregation of Tau, α-Syn, TDP-43, and Amyloid precursor protein (APP)/Aβ. In HD brain, we observed reduced soluble protein (but not mRNA) levels of Htt, α-Syn, and Tau. Our results also support abnormal phosphorylation of Tau in more advanced stages of disease. Aberrant splicing of Tau exons 2, 3 (exclusion) and 10 (inclusion) was also detected in HD patients, leading to higher 0N4R and lower 1N3R isoforms. Finally, following formic acid extraction, we observed increased aggregation of TDP-43, α-Syn, and phosphorylated-Tau during HD progression. Notably, we observed that 88% of HD patients with Vonsattel grade 4 neuropathology displayed at least one non-Htt proteinopathy compared to 29% in controls. Interestingly, α-Syn aggregation correlated with Htt, TDP-43 and phosphorylated-Tau in HD but not in controls. The impact of this work is twofold: (1) it provides compelling evidences that Tau, α-Syn and TDP-43 proteinopathies are increased in HD, and (2) it suggests the involvement of common mechanisms leading to abnormal accumulation of aggregation-prone proteins in NDD. Further studies will be needed to decipher the impact of these proteinopathies on clinical manifestation of HD.

Calcineurin is an important factor involved in glucose uptake in human adipocytes

Calcineurin inhibitors are used in immunosuppressive therapy applied after transplantation, but they are associated with major metabolic side effects including the development of new onset diabetes. Previously, we have shown that the calcineurin inhibiting drugs tacrolimus and cyclosporin A reduce adipocyte and myocyte glucose uptakes by reducing the amount of glucose transporter type 4 (GLUT4) at the cell surface, due to an increased internalization rate. However, this happens without alteration in total protein and phosphorylation levels of key proteins involved in insulin signalling or in the total amount of GLUT4. The present study evaluates possible pathways involved in the altered internalization of GLUT4 and consequent reduction of glucose uptake provoked by calcineurin inhibitors in human subcutaneous adipose tissue. Short- and long-term treatments with tacrolimus, cyclosporin A or another CNI deltamethrin (herbicide) decreased basal and insulin-dependent glucose uptake in adipocytes, without any additive effects observed when added together. However, no tacrolimus effects were observed on glucose uptake when gene transcription and protein translation were inhibited. Investigation of genes potentially involved in GLUT4 trafficking showed only a small effect on ARHGEF11 gene expression (p < 0.05). In conlusion, the specific inhibition of calcineurin, but not that of protein phosphatases, decreases glucose uptake in human subcutaneous adipocytes, suggesting that calcineurin is an important regulator of glucose transport. This inhibitory effect is mediated via gene transcription or protein translation; however, expression of genes potentially involved in GLUT4 trafficking and endocytosis appears not to be involved in these effects.

Thermoregulatory disorders in Huntington disease

Huntington disease (HD) is a paradigmatic autosomal-dominant adult-onset neurodegenerative disease. Since the identification of an abnormal expansion of a trinucleotide repeat tract in the huntingtin gene as the underlying genetic defect, a broad range of transgenic animal models of the disease has become available and these have helped to unravel the relevant molecular pathways in unprecedented detail. Of note, some of the most informative of these models develop thermoregulatory defects such as hypothermia, problems with adaptive thermogenesis, and an altered circadian temperature rhythm. Both central, e.g., in the hypothalamus and peripheral, i.e., the brown adipose tissue and skeletal muscle, problems contribute to the phenotype. Importantly, these structures and pathways are also affected in human HD. Yet, currently the evidence for bona fide thermodysregulation in human HD patients remains anecdotal. This may be due to a lack of reliable tools for monitoring body temperature in an outpatient setting. Regardless, study of the temperature phenotype has contributed to the identification of unexpected molecular targets, such as the PGC-1α pathway.

Presence of tau pathology within foetal neural allografts in patients with Huntington's and Parkinson's disease

Cell replacement has been explored as a therapeutic strategy to repair the brain in patients with Huntington's and Parkinson's disease. Post-mortem evaluations of healthy grafted tissue in such cases have revealed the development of Huntington- or Parkinson-like pathology including mutant huntingtin aggregates and Lewy bodies. An outstanding question remains if tau pathology can also be seen in patients with Huntington's and Parkinson's disease who had received foetal neural allografts. This was addressed by immunohistochemical/immunofluorescent stainings performed on grafted tissue of two Huntington's disease patients, who came to autopsy 9 and 12 years post-transplantation, and two patients with Parkinson's disease who came to autopsy 18 months and 16 years post-transplantation. We show that grafts also contain tau pathology in both types of transplanted patients. In two patients with Huntington's disease, the grafted tissue showed the presence of hyperphosphorylated tau [both AT8 (phospho-tau Ser202 and Thr205) and CP13 (pSer202) immunohistochemical stainings] pathological inclusions, neurofibrillary tangles and neuropil threads. In patients with Parkinson's disease, the grafted tissue was characterized by hyperphosphorylated tau (AT8; immunofluorescent staining) pathological inclusions, neurofibrillary tangles and neuropil threads but only in the patient who came to autopsy 16 years post-transplantation. Abundant tau-related pathology was observed in the cortex and striatum of all cases studied. While the striatum of the grafted Huntington's disease patient revealed an equal amount of 3-repeat and 4-repeat isoforms of tau, the grafted tissue showed elevated 4-repeat isoforms by western blot. This suggests that transplants may have acquired tau pathology from the host brain, although another possibility is that this was due to acceleration of ageing. This finding not only adds to the recent reports that tau pathology is a feature of these neurodegenerative diseases, but also that tau pathology can manifest in healthy neural tissue transplanted into the brains of patients with two distinct neurodegenerative disorders.

Dysregulation of Corticostriatal Connectivity in Huntington's Disease: A Role for Dopamine Modulation

Aberrant communication between striatum, the main information processing unit of the basal ganglia, and cerebral cortex plays a critical role in the emergence of Huntington’s disease (HD), a fatal monogenetic condition that typically strikes in the prime of life. Although both striatum and cortex undergo substantial cell loss over the course of HD, corticostriatal circuits become dysfunctional long before neurons die. Understanding the dysfunction is key to developing effective strategies for treating a progressively worsening triad of motor, cognitive, and psychiatric symptoms. Cortical output neurons drive striatal activity through the release of glutamate, an excitatory amino acid. Striatal outputs, in turn, release γ-amino butyric acid (GABA) and exert inhibitory control over downstream basal ganglia targets. Ample evidence from transgenic rodent models points to dysregulation of corticostriatal glutamate transmission along with corresponding changes in striatal GABA release as underlying factors in the HD behavioral phenotype. Another contributor is dysregulation of dopamine (DA), a modulator of both glutamate and GABA transmission. In fact, pharmacological manipulation of DA is the only currently available treatment for HD symptoms. Here, we review data from animal models and human patients to evaluate the role of DA in HD, including DA interactions with glutamate and GABA within the context of dysfunctional corticostriatal circuitry.

Transcriptome analyses of chronic traumatic encephalopathy show alterations in protein phosphatase expression associated with tauopathy

Chronic traumatic encephalopathy (CTE) is a progressive neurodegenerative disorder that is associated with repetitive head injury and has distinctive neuropathological features that differentiate this disease from other neurodegenerative diseases. Intraneuronal tau aggregates, although they occur in different patterns, are diagnostic neuropathological features of CTE, but the precise mechanism of tauopathy is not known in CTE. We performed whole RNA sequencing analysis of post-mortem brain tissue from patients with CTE and compared the results to normal controls to determine the transcriptome signature changes associated with CTE. The results showed that the genes related to the MAP kinase and calcium-signaling pathways were significantly downregulated in CTE. The altered expression of protein phosphatases (PPs) in these networks further suggested that the tauopathy observed in CTE involves common pathological mechanisms similar to Alzheimer's disease (AD). Using cell lines and animal models, we also showed that reduced PPP3CA/PP2B phosphatase activity is directly associated with increases in phosphorylated (p)-tau proteins. These findings provide important insights into PP-dependent neurodegeneration and may lead to novel therapeutic approaches to reduce the tauopathy associated with CTE.

Roles of tau protein in health and disease

Tau is well established as a microtubule-associated protein in neurons. However, under pathological conditions, aberrant assembly of tau into insoluble aggregates is accompanied by synaptic dysfunction and neural cell death in a range of neurodegenerative disorders, collectively referred to as tauopathies. Recent advances in our understanding of the multiple functions and different locations of tau inside and outside neurons have revealed novel insights into its importance in a diverse range of molecular pathways including cell signalling, synaptic plasticity, and regulation of genomic stability. The present review describes the physiological and pathophysiological properties of tau and how these relate to its distribution and functions in neurons. We highlight the post-translational modifications of tau, which are pivotal in defining and modulating tau localisation and its roles in health and disease. We include discussion of other pathologically relevant changes in tau, including mutation and aggregation, and how these aspects impinge on the propensity of tau to propagate, and potentially drive neuronal loss, in diseased brain. Finally, we describe the cascade of pathological events that may be driven by tau dysfunction, including impaired axonal transport, alterations in synapse and mitochondrial function, activation of the unfolded protein response and defective protein degradation. It is important to fully understand the range of neuronal functions attributed to tau, since this will provide vital information on its involvement in the development and pathogenesis of disease. Such knowledge will enable determination of which critical molecular pathways should be targeted by potential therapeutic agents developed for the treatment of tauopathies.

Regulation of motor proteins, axonal transport deficits and adult-onset neurodegenerative diseases

Neurons affected in a wide variety of unrelated adult-onset neurodegenerative diseases (AONDs) typically exhibit a "dying back" pattern of degeneration, which is characterized by early deficits in synaptic function and neuritic pathology long before neuronal cell death. Consistent with this observation, multiple unrelated AONDs including Alzheimer's disease, Parkinson's disease, Huntington's disease, and several motor neuron diseases feature early alterations in kinase-based signaling pathways associated with deficits in axonal transport (AT), a complex cellular process involving multiple intracellular trafficking events powered by microtubule-based motor proteins. These pathogenic events have important therapeutic implications, suggesting that a focus on preservation of neuronal connections may be more effective to treat AONDs than addressing neuronal cell death. While the molecular mechanisms underlying AT abnormalities in AONDs are still being analyzed, evidence has accumulated linking those to a well-established pathological hallmark of multiple AONDs: altered patterns of neuronal protein phosphorylation. Here, we present a short overview on the biochemical heterogeneity of major motor proteins for AT, their regulation by protein kinases, and evidence revealing cell type-specific AT specializations. When considered together, these findings may help explain how independent pathogenic pathways can affect AT differentially in the context of each AOND.

Exosomes and Homeostatic Synaptic Plasticity Are Linked to Each other and to Huntington's, Parkinson's, and Other Neurodegenerative Diseases by Database-Enabled Analyses of Comprehensively Curated Datasets

Huntington's disease (HD) is a progressive and autosomal dominant neurodegeneration caused by CAG expansion in the huntingtin gene (HTT), but the pathophysiological mechanism of mutant HTT (mHTT) remains unclear. To study HD using systems biological methodologies on all published data, we undertook the first comprehensive curation of two key PubMed HD datasets: perturbation genes that impact mHTT-driven endpoints and therefore are putatively linked causally to pathogenic mechanisms, and the protein interactome of HTT that reflects its biology. We perused PubMed articles containing co-citation of gene IDs and MeSH terms of interest to generate mechanistic gene sets for iterative enrichment analyses and rank ordering. The HD Perturbation database of 1,218 genes highly overlaps the HTT Interactome of 1,619 genes, suggesting links between normal HTT biology and mHTT pathology. These two HD datasets are enriched for protein networks of key genes underlying two mechanisms not previously implicated in HD nor in each other: exosome synaptic functions and homeostatic synaptic plasticity. Moreover, proteins, possibly including HTT, and miRNA detected in exosomes from a wide variety of sources also highly overlap the HD datasets, suggesting both mechanistic and biomarker links. Finally, the HTT Interactome highly intersects protein networks of pathogenic genes underlying Parkinson's, Alzheimer's and eight non-HD polyglutamine diseases, ALS, and spinal muscular atrophy. These protein networks in turn highly overlap the exosome and homeostatic synaptic plasticity gene sets. Thus, we hypothesize that HTT and other neurodegeneration pathogenic genes form a large interlocking protein network involved in exosome and homeostatic synaptic functions, particularly where the two mechanisms intersect. Mutant pathogenic proteins cause dysfunctions at distinct points in this network, each altering the two mechanisms in specific fashion that contributes to distinct disease pathologies, depending on the gene mutation and the cellular and biological context. This protein network is rich with drug targets, and exosomes may provide disease biomarkers, thus enabling drug discovery. All the curated datasets are made available for other investigators. Elucidating the roles of pathogenic neurodegeneration genes in exosome and homeostatic synaptic functions may provide a unifying framework for the age-dependent, progressive and tissue selective nature of multiple neurodegenerative diseases.

A central role for calcineurin in protein misfolding neurodegenerative diseases

Accumulation of misfolded/unfolded aggregated proteins in the brain is a hallmark of many neurodegenerative diseases affecting humans and animals. Dysregulation of calcium (Ca2+) and disruption of fast axonal transport (FAT) are early pathological events that lead to loss of synaptic integrity and axonal degeneration in early stages of neurodegenerative diseases. Dysregulated Ca2+ in the brain is triggered by accumulation of misfolded/unfolded aggregated proteins in the endoplasmic reticulum (ER), a major Ca2+ storing organelle, ultimately leading to neuronal dysfunction and apoptosis. Calcineurin (CaN), a Ca2+/calmodulin-dependent serine/threonine phosphatase, has been implicated in T cells activation through the induction of nuclear factor of activated T cells (NFAT). In addition to the involvement of several other signaling cascades, CaN has been shown to play a role in early synaptic dysfunction and neuronal death. Therefore, inhibiting hyperactivated CaN in early stages of disease might be a promising therapeutic strategy for treating patients with protein misfolding diseases. In this review, we briefly summarize the structure of CaN, inhibition mechanisms by which immunosuppressants inhibit CaN, role of CaN in maintaining neuronal and synaptic integrity and homeostasis and the role played by CaN in protein unfolding/misfolding neurodegenerative diseases.

The protein serine/threonine phosphatases PP2A, PP1 and calcineurin: A triple threat in the regulation of the neuronal cytoskeleton

The microtubule, F-actin and neurofilament networks play a critical role in neuronal cell morphogenesis, polarity and synaptic plasticity. Significantly, the assembly/disassembly and stability of these cytoskeletal networks is crucially modulated by protein phosphorylation and dephosphorylation events. Herein, we aim to more closely examine the role played by three major neuronal Ser/Thr protein phosphatases, PP2A, PP1 and calcineurin, in the homeostasis of the neuronal cytoskeleton. There is strong evidence that these enzymes interact with and dephosphorylate a variety of cytoskeletal proteins, resulting in major regulation of neuronal cytoskeletal dynamics. Conversely, we also discuss how multi-protein cytoskeletal scaffolds can also influence the regulation of these phosphatases, with important implications for neuronal signalling and homeostasis. Not surprisingly, deregulation of these cytoskeletal scaffolds and phosphatase dysfunction are associated with many neurological diseases.

Faulty splicing and cytoskeleton abnormalities in Huntington's disease

Huntington's disease (HD) is caused by a CAG-repeat encoding a polyglutamine (polyQ) tract in the huntingtin protein. There is plenty of evidence of polyQ-driven toxicity. However, CAG repeat RNA-driven alteration of splicing has recently been proposed in analogy to CUG-repeat diseases. Here we review the reported alteration of the CAG-repeat associated splicing factor SRSF6 in brains of HD patients and mouse models and how this correlates with altered splicing of, at least, two microtubule-associated proteins in HD, namely MAPT (tau) and MAP2. Regarding tau, altered splicing of exon 10 has been reported, along with increased levels and 4R/3R-tau ratio and detection of tau in a new nuclear rod-shaped histopathological hallmark termed tau nuclear rod (TNR) or tau nuclear indentation (TNI). These findings, together with an attenuation of HD phenotype in R6/1 mice with tau deficiency and subsequent studies showing increased phosphorylation in mouse models and increased levels in CSF of patients, has led to proposing HD as a tauopathy. Regarding MAP2, an increase in its juvenile form and a decrease in total MAP2 together with redistribution from dendrites to soma is observed in HD patients, which may contribute to the dendritic atrophy in HD. Furthermore, MAP2 positive structures filling nuclear indentations have occasionally been found and co-localized with tau. Therefore, altered MAP function with imbalance in tau/MAP2 content could contribute to HD striatal atrophy and dysfunction. Besides, TNIs might be indicative of such MAP abnormalities. TNIs are also found in early pathology Alzheimer's disease and in tauopathy mice over-expressing mutant 4R-tau. This indicates that tau alteration is sufficient for TNI detection, which becomes a marker of increased total tau and/or altered 4R/3R-tau ratio and reporter of pathology-associated nuclear indentations. Altogether, these recent studies suggest that correcting the SRSF6-driven missplicing and/or microtubule-associated imbalance might be of therapeutic value in HD.

The Emerging Roles of the Calcineurin-Nuclear Factor of Activated T-Lymphocytes Pathway in Nervous System Functions and Diseases

The ongoing epidemics of metabolic diseases and increase in the older population have increased the incidences of neurodegenerative diseases. Evidence from murine and cell line models has implicated calcineurin-nuclear factor of activated T-lymphocytes (NFAT) signaling pathway, a Ca²⁺/calmodulin-dependent major proinflammatory pathway, in the pathogenesis of these diseases. Neurotoxins such as amyloid-β, tau protein, and α-synuclein trigger abnormal calcineurin/NFAT signaling activities. Additionally increased activities of endogenous regulators of calcineurin like plasma membrane Ca²⁺-ATPase (PMCA) and regulator of calcineurin 1 (RCAN1) also cause neuronal and glial loss and related functional alterations, in neurodegenerative diseases, psychotic disorders, epilepsy, and traumatic brain and spinal cord injuries. Treatment with calcineurin/NFAT inhibitors induces some degree of neuroprotection and decreased reactive gliosis in the central and peripheral nervous system. In this paper, we summarize and discuss the current understanding of the roles of calcineurin/NFAT signaling in physiology and pathologies of the adult and developing nervous system, with an emphasis on recent reports and cutting-edge findings. Calcineurin/NFAT signaling is known for its critical roles in the developing and adult nervous system. Its role in physiological and pathological processes is still controversial. However, available data suggest that its beneficial and detrimental effects are context-dependent. In view of recent reports calcineurin/NFAT signaling is likely to serve as a potential therapeutic target for neurodegenerative diseases and conditions. This review further highlights the need to characterize better all factors determining the outcome of calcineurin/NFAT signaling in diseases and the downstream targets mediating the beneficial and detrimental effects.

MAP2 Splicing is Altered in Huntington's Disease

Dendritic alteration of striatal medium spiny neurons is one of the earliest morphological abnormalities in Huntington's disease (HD). The main microtubule-associated protein in dendrites is MAP2. The low-molecular weight isoforms of MAP2 (LMW-MAP2) are the juvenile forms resulting from exclusion of the sequence encoded by exons E7-E9 and are downregulated after the early stages of neuronal development when E7-E9 exon-including high-molecular weight isoforms (HMW-MAP2) are favored. Splicing alteration has recently been proposed to contribute to HD in view of two pathogenic missplicing events resulting in a highly toxic N-terminal version of mutant huntingtin and in a detrimental imbalance in MAP Tau isoforms with three or four tubulin-binding repeats. Both splicing events are postulated targets of the SR splicing factor SRSF6 which has recently been reported to be dramatically altered in HD. SR proteins often regulate functionally related sets of genes and SRSF6 targets are enriched in genes involved in brain organogenesis including several actin-and tubulin-binding proteins. Here we hypothesized that MAP2 might be target of SRSF6 and altered in HD. By SRSF6 knockdown in neuroblastoma cells, we demonstrate that splicing of MAP2 E7-E9 exons is affected by SRSF6. We then show a disbalance in LMW and HMW MAP2 mRNA isoforms in HD striatum in favor of the juvenile LMW forms together with a decrease in total MAP2 mRNA. This is accompanied by a global decrease in total MAP2 protein due to almost total disappearance of HMW-MAP2 isoforms with preservation of LMW-MAP2 isoforms. Accordingly, the predominant dendritic MAP2 staining in striatal neuropil of control subjects is absent in HD cases. In these, MAP2-immunoreactivity is faint and restricted to neuronal cell bodies often showing a sharp boundary at the base of dendrites. Together, our results highlight the importance of splicing alteration in HD and suggest that MAP2 alteration contributes to dendritic atrophy.