Defects in axonal elongation and neuronal migration in mice with disrupted tau and map1b genes

Cellular and pathological functions of tau

Tau protein is involved in various cellular processes, including having a canonical role in binding and stabilization of microtubules in neurons. Tauopathies are neurodegenerative diseases marked by the abnormal accumulation of tau protein aggregates in neurons, as seen, for example, in conditions such as frontotemporal dementia and Alzheimer disease. Mutations in tau coding regions or that disrupt tau mRNA splicing, tau post-translational modifications and cellular stress factors (such as oxidative stress and inflammation) increase the tendency of tau to aggregate and interfere with its clearance. Pathological tau is strongly implicated in the progression of neurodegenerative diseases, and the propagation of tau aggregates is associated with disease severity. Recent technological advancements, including cryo-electron microscopy and disease models derived from human induced pluripotent stem cells, have increased our understanding of tau-related pathology in neurodegenerative conditions. Substantial progress has been made in deciphering tau aggregate structures and the molecular mechanisms that underlie protein aggregation and toxicity. In this Review, we discuss recent insights into the diverse cellular functions of tau and the pathology of tau inclusions and explore the potential for therapeutic interventions.

The disease-causing tau V337M mutation induces tau hypophosphorylation and perturbs axon morphology pathways

Tau aggregation is a hallmark of several neurodegenerative diseases, including Alzheimer's disease and frontotemporal dementia. There are disease-causing variants of the tau-encoding gene, MAPT, and the presence of tau aggregates is highly correlated with disease progression. However, the molecular mechanisms linking pathological tau to neuronal dysfunction are not well understood due to our incomplete understanding of the normal functions of tau in development and aging and how these processes change in the context of causal disease variants of tau. To address these questions in an unbiased manner, we conducted multi-omic characterization of iPSC-derived neurons harboring the MAPT V337M mutation. RNA-seq and phosphoproteomics revealed that both V337M tau and tau knockdown consistently perturbed levels of transcripts and phosphorylation of proteins related to axonogenesis or axon morphology. Surprisingly, we found that neurons with V337M tau had much lower tau phosphorylation than neurons with WT tau. We conducted functional genomics screens to uncover regulators of tau phosphorylation in neurons and found that factors involved in axonogenesis modified tau phosphorylation in both MAPT WT and MAPT V337M neurons. Intriguingly, the p38 MAPK pathway specifically modified tau phosphorylation in MAPT V337M neurons. We propose that V337M tau might perturb axon morphology pathways and tau hypophosphorylation via a "loss of function" mechanism, which could contribute to previously reported cognitive changes in preclinical MAPT gene carriers.

The six brain-specific TAU isoforms and their role in Alzheimer's disease and related neurodegenerative dementia syndromes

INTRODUCTION

Alternative splicing of the human MAPT gene generates six brain-specific TAU isoforms. Imbalances in the TAU isoform ratio can lead to neurodegenerative diseases, underscoring the need for precise control over TAU isoform balance. Tauopathies, characterized by intracellular aggregates of hyperphosphorylated TAU, exhibit extensive neurodegeneration and can be classified by the TAU isoforms present in pathological accumulations.

METHODS

A comprehensive review of TAU and related dementia syndromes literature was conducted using PubMed, Google Scholar, and preprint server.

RESULTS

While TAU is recognized as key driver of neurodegeneration in specific tauopathies, the contribution of the isoforms to neuronal function and disease development remains largely elusive.

DISCUSSION

In this review we describe the role of TAU isoforms in health and disease, and stress the importance of comprehending and studying TAU isoforms in both, physiological and pathological context, in order to develop targeted therapeutic interventions for TAU-associated diseases.

HIGHLIGHTS

MAPT splicing is tightly regulated during neuronal maturation and throughout life. TAU isoform expression is development-, cell-type and brain region specific. The contribution of TAU to neurodegeneration might be isoform-specific. Ineffective TAU-based therapies highlight the need for specific targeting strategies.

Docking protein 6 (DOK6) selectively docks the neurotrophic signaling transduction to restrain peripheral neuropathy

The appropriate and specific response of nerve cells to various external cues is essential for the establishment and maintenance of neural circuits, and this process requires the proper recruitment of adaptor molecules to selectively activate downstream pathways. Here, we identified that DOK6, a member of the Dok (downstream of tyrosine kinases) family, is required for the maintenance of peripheral axons, and that loss of Dok6 can cause typical peripheral neuropathy symptoms in mice, manifested as impaired sensory, abnormal posture, paw deformities, blocked nerve conduction, and dysmyelination. Furthermore, Dok6 is highly expressed in peripheral neurons but not in Schwann cells, and genetic deletion of Dok6 in peripheral neurons led to typical peripheral myelin outfolding, axon destruction, and hindered retrograde axonal transport. Specifically, DOK6 acts as an adaptor protein for selectivity-mediated neurotrophic signal transduction and retrograde transport for TrkC and Ret but not for TrkA and TrkB. DOK6 interacts with certain proteins in the trafficking machinery and controls their phosphorylation, including MAP1B, Tau and Dynein for axonal transport, and specifically activates the downstream ERK1/2 kinase pathway to maintain axonal survival and homeostasis. This finding provides new clues to potential insights into the pathogenesis and treatment of hereditary peripheral neuropathies and other degenerative diseases.

The role of microtubule-associated protein tau in netrin-1 attractive signaling

Direct binding of netrin receptors with dynamic microtubules (MTs) in the neuronal growth cone plays an important role in netrin-mediated axon guidance. However, how netrin-1 (NTN1) regulates MT dynamics in axon turning remains a major unanswered question. Here, we show that the coupling of netrin-1 receptor DCC with tau (MAPT)-regulated MTs is involved in netrin-1-promoted axon attraction. Tau directly interacts with DCC and partially overlaps with DCC in the growth cone of primary neurons. Netrin-1 induces this interaction and the colocalization of DCC and tau in the growth cone. The netrin-1-induced interaction of tau with DCC relies on MT dynamics and TUBB3, a highly dynamic β-tubulin isotype in developing neurons. Netrin-1 increased cosedimentation of DCC with tau and TUBB3 in MTs, and knockdown of either tau or TUBB3 mutually blocked this effect. Downregulation of endogenous tau levels by tau shRNAs inhibited netrin-1-induced axon outgrowth, branching and commissural axon attraction in vitro, and led to defects in spinal commissural axon projection in vivo. These findings suggest that tau is a key MT-associated protein coupling DCC with MT dynamics in netrin-1-promoted axon attraction.

Signalling Pathways Involved in Microglial Activation in Alzheimer's Disease and Potential Neuroprotective Role of Phytoconstituents

Alzheimer's disease (AD) is a commonly reported neurodegenerative disorder associated with dementia and cognitive impairment. The pathophysiology of AD comprises Aβ, hyperphosphorylated tau protein formation, abrupt cholinergic cascade, oxidative stress, neuronal apoptosis, and neuroinflammation. Recent findings have established the profound role of immunological dysfunction and microglial activation in the pathogenesis of AD. Microglial activation is a multifactorial cascade encompassing various signalling molecules and pathways such as Nrf2/NLRP3/NF-kB/p38 MAPKs/ GSK-3β. Additionally, deposited Aβ or tau protein triggers microglial activation and accelerates its pathogenesis. Currently, the FDA-approved therapeutic regimens are based on the modulation of the cholinergic system, and recently, one more drug, aducanumab, has been approved by the FDA. On the one hand, these drugs only offer symptomatic relief and not a cure for AD. Additionally, no targetedbased microglial medicines are available for treating and managing AD. On the other hand, various natural products have been explored for the possible anti-Alzheimer effect via targeting microglial activation or different targets of microglial activation. Therefore, the present review focuses on exploring the mechanism and associated signalling related to microglial activation and a detailed description of various natural products that have previously been reported with anti-Alzheimer's effect via mitigation of microglial activation. Additionally, we have discussed the various patents and clinical trials related to managing and treating AD.

FMRP regulates tangential neuronal migration via MAP1B

The Fragile X Syndrome (FXS) represents the most prevalent form of inherited intellectual disability and is the first monogenic cause of Autism Spectrum Disorder. FXS results from the absence of the RNA-binding protein FMRP (Fragile X Messenger Ribonucleoprotein). Neuronal migration is an essential step of brain development allowing displacement of neurons from their germinal niches to their final integration site. The precise role of FMRP in neuronal migration remains largely unexplored. Using live imaging of postnatal Rostral Migratory Stream (RMS) neurons in Fmr1-null mice, we observed that the absence of FMRP leads to delayed neuronal migration and altered trajectory, associated with defects of centrosomal movement. RNA-interference-induced knockdown of Fmr1 shows that these migratory defects are cell-autonomous. Notably, the primary FMRP mRNA target implicated in these migratory defects is MAP1B (Microtubule-Associated Protein 1B). Knocking-down MAP1B expression effectively rescued most of the observed migratory defects. Finally, we elucidate the molecular mechanisms at play by demonstrating that the absence of FMRP induces defects in the cage of microtubules surrounding the nucleus of migrating neurons, which is rescued by MAP1B knockdown. Our findings reveal a novel neurodevelopmental role for FMRP in collaboration with MAP1B, jointly orchestrating neuronal migration by influencing the microtubular cytoskeleton.

Leucine-rich repeat kinase 2 (LRRK2) inhibition upregulates microtubule-associated protein 1B to ameliorate lysosomal dysfunction and parkinsonism

Mutations in LRRK2 (encoding leucine-rich repeat kinase 2 protein, LRRK2) are the most common genetic risk factors for Parkinson's disease (PD), and increased LRRK2 kinase activity was observed in sporadic PD. Therefore, inhibition of LRRK2 has been tested as a disease-modifying therapeutic strategy using the LRRK2 mutant mice and sporadic PD. Here, we report a newly designed molecule, FL090, as a LRRK2 kinase inhibitor, verified in cell culture and animal models of PD. Using the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine mice and SNCA A53T transgenic mice, FL090 ameliorated motor dysfunctions, reduced LRRK2 kinase activity, and rescued loss in the dopaminergic neurons in the substantia nigra. Notably, by RNA-Seq analysis, we identified microtubule-associated protein 1 (MAP1B) as a crucial mediator of FL090's neuroprotective effects and found that MAP1B and LRRK2 co-localize. Overexpression of MAP1B rescued 1-methyl-4-phenylpyridinium induced cytotoxicity through rescuing the lysosomal function, and the protective effect of FL090 was lost in MAP1B knockout cells. Further studies may be focused on the in vivo mechanisms of MAP1B and microtubule function in PD. Collectively, these findings highlight the potential of FL090 as a therapeutic agent for sporadic PD and familial PD without LRRK2 mutations.

Role of Tau protein in long COVID and potential therapeutic targets

Introduction

Long COVID is an emerging public health burden and has been defined as a syndrome with common symptoms of fatigue, shortness of breath, cognitive dysfunction, and others impacting day-to-day life, fluctuating or relapsing over, occurring for at least two months in patients with a history of probable or confirmed SARS CoV-2 infection; usually three months from the onset of illness and cannot be explained by an alternate diagnosis. The actual prevalence of long-term COVID-19 is unknown, but it is believed that more than 17 million patients in Europe may have suffered from it during pandemic.

Pathophysiology

Currently, there is limited understanding of the pathophysiology of this syndrome, and multiple hypotheses have been proposed. Our literature review has shown studies reporting tau deposits in tissue samples of the brain from autopsies of COVID-19 patients compared to the control group, and the in-vitro human brain organoid model has shown aberrant phosphorylation of tau protein in response to SARS-CoV-2 infection. Tauopathies, a group of neurodegenerative disorders with the salient features of tau deposits, can manifest different symptoms based on the anatomical region of brain involvement and have been shown to affect the peripheral nervous system as well and explained even in rat model studies. Long COVID has more than 203 symptoms, with predominant symptoms of fatigue, dyspnea, and cognitive dysfunction, which tauopathy-induced CNS and peripheral nervous system dysfunction can explain. There have been no studies up till now to reveal the pathophysiology of long COVID. Based on our literature review, aberrant tau phosphorylation is a promising hypothesis that can be explored in future studies. Therapeutic approaches for tauopathies have multidimensional aspects, including targeting post-translational modifications, tau aggregation, and tau clearance through the autophagy process with the help of lysosomes, which can be potential targets for developing therapeutic interventions for the long COVID. In addition, future studies can attempt to find the tau proteins in CSF and use those as biomarkers for the long COVID.

Alzheimer's disease: an axonal injury disease?

Alzheimer's disease (AD) is the primary cause of dementia and is anticipated to impose a substantial economic burden in the future. Over a significant period, the widely accepted amyloid cascade hypothesis has guided research efforts, and the recent FDA approval of an anti- amyloid-beta (Aβ) protofibrils antibody, believed to decelerate AD progression, has further solidified its significance. However, the excessive emphasis placed on the amyloid cascade hypothesis has overshadowed the physiological nature of Aβ and tau proteins within axons. Axons, specialized neuronal structures, sustain damage during the early stages of AD, exerting a pivotal influence on disease progression. In this review, we present a comprehensive summary of the relationship between axonal damage and AD pathology, amalgamating the physiological roles of Aβ and tau proteins, along with the impact of AD risk genes such as APOE and TREM2. Furthermore, we underscore the exceptional significance of axonal damage in the context of AD.

Tau suppresses microtubule-regulated pancreatic insulin secretion

Tau protein is implicated in the pathogenesis of Alzheimer's disease (AD) and other tauopathies, but its physiological function is in debate. Mostly explored in the brain, tau is also expressed in the pancreas. We further explored the mechanism of tau's involvement in the regulation of glucose-stimulated insulin secretion (GSIS) in islet β-cells, and established a potential relationship between type 2 diabetes mellitus (T2DM) and AD. We demonstrate that pancreatic tau is crucial for insulin secretion regulation and glucose homeostasis. Tau levels were found to be elevated in β-islet cells of patients with T2DM, and loss of tau enhanced insulin secretion in cell lines, drosophila, and mice. Pharmacological or genetic suppression of tau in the db/db diabetic mouse model normalized glucose levels by promoting insulin secretion and was recapitulated by pharmacological inhibition of microtubule assembly. Clinical studies further showed that serum tau protein was positively correlated with blood glucose levels in healthy controls, which was lost in AD. These findings present tau as a common therapeutic target between AD and T2DM.

Moving beyond amyloid and tau to capture the biological heterogeneity of Alzheimer's disease

Alzheimer's disease (AD) manifests along a spectrum of cognitive deficits and levels of neuropathology. Genetic studies support a heterogeneous disease mechanism, with around 70 associated loci to date, implicating several biological processes that mediate risk for AD. Despite this heterogeneity, most experimental systems for testing new therapeutics are not designed to capture the genetically complex drivers of AD risk. In this review, we first provide an overview of those aspects of AD that are largely stereotyped and those that are heterogeneous, and we review the evidence supporting the concept that different subtypes of AD are important to consider in the design of agents for the prevention and treatment of the disease. We then dive into the multifaceted biological domains implicated to date in AD risk, highlighting studies of the diverse genetic drivers of disease. Finally, we explore recent efforts to identify biological subtypes of AD, with an emphasis on the experimental systems and data sets available to support progress in this area.

Identification of Tau protein as a novel marker for maturation and pathological changes of oligodendrocytes

Microtubule-associated protein Tau is primarily expressed in axons of neurons, but also in Olig2-positive oligodendrocytes in adult rodent and monkey brains. In this study, we sought to determine at what cell stage Tau becomes expressed in the oligodendrocyte lineage. We performed immunostaining of adult mouse brain sections using well-known markers of oligodendrocyte lineage and found that Tau is expressed in mature oligodendrocytes, but not in oligodendrocyte progenitors and immature pre-oligodendrocytes. We also investigated Tau expression in developing mouse brain. Surprisingly, Tau expression occurred after the peak of myelination and even exceeded GSTπ expression, which has been considered as a marker of myelinating oligodendrocytes. These results suggest Tau as a novel marker of oligodendrocyte maturation. We then investigated whether Tau is important for oligodendrocyte development and/or myelination and how Tau changes in demyelination. First, we found no changes in myelination and oligodendrocyte markers in Tau knockout mice, suggesting that Tau is dispensable. Next, we analyzed the proteolipid protein 1 transgenic model of Pelizaeus-Merzbacher disease, which is a rare leukodystrophy. In hemizygous transgenic mice, the number of Tau-positive cells were significantly increased as compared with wild type mice. These cells were also positive for Olig2, CC1, and GSTπ, but not PDGFRα and GPR17. In stark contrast, the expression level of Tau, as well as GSTπ, was dramatically decreased in the cuprizone-induced model of multiple sclerosis. Taken together, we propose Tau as a new marker of oligodendrocyte lineage and for investigating demyelination lesions.

Liquid-liquid phase separation of protein tau: An emerging process in Alzheimer's disease pathogenesis

Metabolic reactions within cells occur in various isolated compartments with or without borders, the latter being known as membrane-less organelles (MLOs). The MLOs show liquid-like properties and are formed by a process known as liquid-liquid phase separation (LLPS). MLOs contribute to different molecules interactions such as protein-protein, protein-RNA, and RNA-RNA driven by various factors, such as multivalency of intrinsic disorders. MLOs are involved in several cell signaling pathways such as transcription, immune response, and cellular organization. However, disruption of these processes has been found in different pathologies. Recently, it has been demonstrated that protein aggregates, a characteristic of some neurodegenerative diseases, undergo similar phase separation. Tau protein is known as a major neurofibrillary tangles component in Alzheimer's disease (AD). This protein can undergo phase separation to form a MLO known as tau droplet in vitro and in vivo, and this process can be facilitated by several factors, including crowding agents, RNA, and phosphorylation. Tau droplet has been shown to mature into insoluble aggregates suggesting that this process may precede and induce neurodegeneration in AD. Here we review major factors involved in liquid droplet formation within a cell. Additionally, we highlight recent findings concerning tau aggregation following phase separation in AD, along with the potential therapeutic strategies that could be explored in this process against the progression of this pathology.

Long COVID could become a widespread post-pandemic disease? A debate on the organs most affected

Long COVID is an emerging problem in the current health care scenario. It is a syndrome with common symptoms of shortness of breath, fatigue, cognitive dysfunction, and other conditions that have a high impact on daily life. They are fluctuating or relapsing states that occur in patients with a history of SARS-CoV-2 infection for at least 2 months. They are usually conditions that at 3 months after onset cannot be explained by an alternative diagnosis. Currently very little is known about this syndrome. A thorough review of the literature highlights that the cause is attributable to deposits of tau protein. Massive phosphorylation of tau protein in response to SARS-CoV-2 infection occurred in brain samples from autopsies of people previously affected with COVID-19. The neurological disorders resulting from this clinical condition are termed tauopathies and can give different pathological symptoms depending on the involved anatomical region of the brain. Peripheral small-fiber neuropathies are also evident among patients with Long COVID leading to fatigue, which is the main symptom of this syndrome. Certainly more research studies could confirm the association between tau protein and Long COVID by defining the main role of tau protein as a biomarker for the diagnosis of this syndrome that is widespread in the post-pandemic period.

KIF4 regulates neuronal morphology and seizure susceptibility via the PARP1 signaling pathway

Epilepsy is a common neurological disease worldwide, and one of its causes is genetic abnormalities. Here, we identified a point mutation in KIF4A, a member of kinesin superfamily molecular motors, in patients with neurological disorders such as epilepsy, developmental delay, and intellectual disability. KIF4 is involved in the poly (ADP-ribose) polymerase (PARP) signaling pathway, and the mutation (R728Q) strengthened its affinity with PARP1 through elongation of the KIF4 coiled-coil domain. Behavioral tests showed that KIF4-mutant mice exhibited mild developmental delay with lower seizure threshold. Further experiments revealed that the KIF4 mutation caused aberrant morphology in dendrites and spines of hippocampal pyramidal neurons through PARP1-TrkB-KCC2 pathway. Furthermore, supplementing NAD, which activates PARP1, could modulate the TrkB-KCC2 pathway and rescue the seizure susceptibility phenotype of the mutant mice. Therefore, these findings indicate that KIF4 is engaged in a fundamental mechanism regulating seizure susceptibility and could be a potential target for epilepsy treatment.

Impact of Copper Nanoparticles and Copper Ions on Transcripts Involved in Neural Repair Mechanisms in Rainbow Trout Olfactory Mucosa

Olfactory mucosa is well known for its lifelong ability for regeneration. Regeneration of neurons and regrowth of severed axons are the most common neural repair mechanisms in olfactory mucosa. Nonetheless, exposure to neurotoxic contaminants, such as copper nanoparticles (CuNPs) and copper ions (Cu²⁺), may alter the reparative capacity of olfactory mucosa. Here, using RNA-sequencing, we investigated the molecular basis of neural repair mechanisms that were affected by CuNPs and Cu²⁺ in rainbow trout olfactory mucosa. The transcript profile of olfactory mucosa suggested that regeneration of neurons was inhibited by CuNPs. Exposure to CuNPs reduced the transcript abundances of pro-inflammatory proteins which are required to initiate neuroregeneration. Moreover, the transcript of genes encoding regeneration promoters, including canonical Wnt/β-catenin signaling proteins and developmental transcription factors, were downregulated in the CuNP-treated fish. The mRNA levels of genes regulating axonal regrowth, including the growth-promoting signals secreted from olfactory ensheathing cells, were mainly increased in the CuNP treatment. However, the reduced transcript abundances of a few cell adhesion molecules and neural polarity genes may restrict axonogenesis in the CuNP-exposed olfactory mucosa. In the Cu²⁺-treated olfactory mucosa, both neural repair strategies were initiated at the transcript level. The stimulation of repair mechanisms can lead to the recovery of Cu²⁺-induced olfactory dysfunction. These results indicated CuNPs and Cu²⁺ differentially affected the neural repair mechanism in olfactory mucosa. Exposure to CuNP had greater effects on the expression of genes involved in olfactory repair mechanisms relative to Cu²⁺ and dysregulated the transcripts associated with stem cell proliferation and neural reconstitution.

Continuous high-frequency deep brain stimulation of the anterior insula modulates autism-like behavior in a valproic acid-induced rat model

BACKGROUND

Until now, the treatment of patients with autism spectrum disorder (ASD) remain a difficult problem. The insula is involved in empathy and sensorimotor integration, which are often impaired in individuals with ASD. Deep brain stimulation, modulating neuronal activity in specific brain circuits, has recently been considered as a promising intervention for neuropsychiatric disorders. Valproic acid (VPA) is a potential teratogenic agent, and prenatal exposure can cause autism-like symptoms including repetitive behaviors and defective sociability. Herein, we investigated the effects of continuous high-frequency deep brain stimulation in the anterior insula of rats exposed to VPA and explored cognitive functions, behavior, and molecular proteins connected to autism spectrum disorder.

METHODS

VPA-exposed offspring were bilaterally implanted with electrodes in the anterior insula (Day 0) with a recovery period of 1 week. (Day 0-7). High-frequency deep brain stimulation was applied from days 11 to 29. Three behavioral tests, including three-chamber social interaction test, were performed on days 7, 13, 18, 25 and 36, and several rats were used for analysis of immediate early genes and proteomic after deep brain stimulation intervention. Meanwhile, animals were subjected to a 20 day spatial learning and cognitive rigidity test using IntelliCage on day 11.

RESULTS

Deep brain stimulation improved the sociability and social novelty preference at day 18 prior to those at day 13, and the improvement has reached the upper limit compared to day 25. As for repetitive/stereotypic-like behavior, self- grooming time were reduced at day 18 and reached the upper limit, and the numbers of burried marbles were reduced at day 13 prior to those at day 18 and day 25. The improvements of sociability and social novelty preference were persistent after the stimulation had ceased. Spatial learning ability and cognitive rigidity were unaffected. We identified 35 proteins in the anterior insula, some of which were intimately linked to autism, and their expression levels were reversed upon administration of deep brain stimulation.

CONCLUSIONS

Autism-like behavior was ameliorated and autism-related proteins were reversed in the insula by deep brain stimulation intervention, these findings reveal that the insula may be a potential target for DBS in the treatment of autism, which provide a theoretical basis for its clinical application., although future studies are still warranted.

Heparan Sulfate Proteoglycans in Tauopathy

Tauopathies are a class of neurodegenerative diseases, including Alzheimer's disease, and are characterized by intraneuronal tau inclusion in the brain and the patient's cognitive decline with obscure pathogenesis. Heparan sulfate proteoglycans, a major type of extracellular matrix, have been believed to involve in tauopathies. The heparan sulfate proteoglycans co-deposit with tau in Alzheimer's patient brain, directly bind to tau and modulate tau secretion, internalization, and aggregation. This review summarizes the current understanding of the functions and the modulated molecular pathways of heparan sulfate proteoglycans in tauopathies, as well as the implication of dysregulated heparan sulfate proteoglycan expression in tau pathology and the potential of targeting heparan sulfate proteoglycan-tau interaction as a novel therapeutic option.

The organization of microtubules and Tau in oligodendrocytes: Tau pathology in damaged oligodendrocytes

Tau is abundantly expressed in neurons, however previous reports and our recent study showed tau also exist in oligodendrocytes. Also the expression levels of tau are dramatical changed in hypomyelination model rat and in demyelination region of stroke model mice. The review demonstrate microtubule and its binding partner Tau might be necessary for oligodendrocyte function based on previous reports.

The Fate of Tau Aggregates Between Clearance and Transmission

Neuronal accumulation of mis-folded tau is the pathological hallmark of multiple neurodegenerative disorders, including Alzheimer’s disease. Distinct from amyloid plaques, which appear simultaneously throughout the brain, tau pathology develops first in a specific brain region and then propagates to neuroanatomically connected brain regions, exacerbating the disease. Due to the implication in disease progression, prevention of tau transmission is recognized as an important therapeutic strategy that can halt disease progression in the brain. Recently, accumulating studies have demonstrated diverse cellular mechanisms associated with cell-to-cell transmission of tau. Once transmitted, mis-folded tau species act as a prion-like seed for native tau aggregation in the recipient neuron. In this review, we summarize the diverse cellular mechanisms associated with the secretion and uptake of tau, and highlight tau-trafficking receptors, which mediate tau clearance or cell-to-cell tau transmission.

The evolution of microtubule associated proteins - a reference proteomic perspective

Microtubule associated proteins (MAPs), defined as proteins that bind microtubules but are not molecular motors or severing enzymes, play a key role in regulating microtubule stability in neurons. Existing studies of the evolutionary relationships between these proteins are limited to genomic data from a small number of species. We therefore used a large collection of publicly available reference-quality eukaryotic proteomes to carry out a phylogenetic analysis of microtubule associated proteins in both vertebrates and invertebrates. Complete or near-complete reference quality proteomes were obtained from Uniprot. Microtubule associated proteins were identified using InterProtScan, aligned using MUSCLE and then phylogenetic trees constructed using the WAG algorithm. We identified 889 proteins with tubulin binding domains, of which 663 were in eukaryotes, including 168 vertebrates and 64 invertebrates. The vertebrate proteins separated into three families, resembling human MAP 2, MAP4 and MAPT, respectively, while invertebrate MAPs clustered separately. We found significant variation in number of microtubule associated proteins and number of microtubule binding domains between taxa, with fish and mollusks having an unexpectedly high number of MAPs and binding domains, respectively. Our findings represent a novel analysis of the evolution of microtubule associated proteins based on publicly available proteomics data sets. We were able to confirm the phylogeny of MAPs identified based on more limited genomic analyses, and in addition, derived several novel insights on the structure and function of MAPs.

Hyperphosphorylated Human Tau Accumulates at the Synapse, Localizing on Synaptic Mitochondrial Outer Membranes and Disrupting Respiration in a Mouse Model of Tauopathy

Neurogenerative disorders, such as Alzheimer’s disease (AD), represent a growing public health challenge in aging societies. Tauopathies, a subset of neurodegenerative disorders that includes AD, are characterized by accumulation of fibrillar and hyperphosphorylated forms of microtubule-associated protein tau with coincident mitochondrial abnormalities and neuronal dysfunction. Although, in vitro, tau impairs axonal transport altering mitochondrial distribution, clear in vivo mechanisms associating tau and mitochondrial dysfunction remain obscure. Herein, we investigated the effects of human tau on brain mitochondria in vivo using transgenic htau mice at ages preceding and coinciding with onset of tauopathy. Subcellular proteomics combined with bioenergetic assessment revealed pathologic forms of tau preferentially associate with synaptic over non-synaptic mitochondria coinciding with changes in bioenergetics, reminiscent of an aged synaptic mitochondrial phenotype in wild-type mice. While mitochondrial content was unaltered, mitochondrial maximal respiration was impaired in synaptosomes from htau mice. Further, mitochondria-associated tau was determined to be outer membrane-associated using the trypsin protection assay and carbonate extraction. These findings reveal non-mutant human tau accumulation at the synapse has deleterious effects on mitochondria, which likely contributes to synaptic dysfunction observed in the context of tauopathy.

Tau phosphorylation and PAD exposure in regulation of axonal growth

Introduction: Tau is a microtubule associated phosphoprotein found principally in neurons. Prevailing dogma continues to define microtubule stabilization as the major function of tau in vivo, despite several lines of evidence suggesting this is not the case. Most importantly, tau null mice have deficits in axonal outgrowth and neuronal migration while still possessing an extensive microtubule network. Instead, mounting evidence suggests that tau may have a major function in the regulation of fast axonal transport (FAT) through activation of neuronal signaling pathways. Previous studies identified a phosphatase activating domain (PAD) at the tau N-terminal that is normally sequestered, but is constitutively exposed in tauopathies. When exposed, the PAD activates a signaling cascade involving PP1 and GSK3β which affects cellular functions including release of cargo from kinesin. Furthermore, we discovered that PAD exposure can be regulated by a single phosphorylation at T205. Exposure of the PAD is an early event in multiple tauopathies and a major contributing factor to neurodegeneration associated with tau hyperphosphorylation. However, effects of tau PAD exposure on anterograde FAT raised the interesting possibility that this pathway may be a mechanism for physiological regulation of cargo delivery through site-specific phosphorylation of tau and transient activation of PP1 and GSK3β. Significantly, there is already evidence of local control of PP1 and GSK3β at sites which require cargo delivery. Methods: To investigate this hypothesis, first we evaluated cellular localization of tau PAD exposure, pT205 tau phosphorylation, and active GSK3β in primary hippocampal neurons during development. Second, we analyzed the axonal outgrowth of tau knockout neurons following transfection with full length hTau40-WT, hTau40-ΔPAD, or hTau40-T205A. Results and Discussion: The results presented here suggest that transient activation of a PP1-GSK3β signaling pathway through locally regulated PAD exposure is a mechanism for cargo delivery, and thereby important for neurite outgrowth of developing neurons.

The Celsr3-Kif2a axis directs neuronal migration in the postnatal brain

The tangential migration of immature neurons in the postnatal brain involves consecutive migration cycles and depends on constant remodeling of the cell cytoskeleton, particularly in the leading process (LP). Despite the identification of several proteins with permissive and empowering functions, the mechanisms that specify the direction of migration remain largely unknown. Here, we report that planar cell polarity protein Celsr3 orients neuroblasts migration from the subventricular zone (SVZ) to olfactory bulb (OB). In Celsr3-forebrain conditional knockout mice, neuroblasts loose directionality and few can reach the OB. Celsr3-deficient neuroblasts exhibit aberrant branching of LP, de novo LP formation, and decreased growth rate of microtubules (MT). Mechanistically, we show that Celsr3 interacts physically with Kif2a, a MT depolymerizing protein and that conditional inactivation of Kif2a in the forebrain recapitulates the Celsr3 knockout phenotype. Our findings provide evidence that Celsr3 and Kif2a cooperatively specify the directionality of neuroblasts tangential migration in the postnatal brain.

Tau Protein Interaction Partners and Their Roles in Alzheimer's Disease and Other Tauopathies

Tau protein plays a critical role in the assembly, stabilization, and modulation of microtubules, which are important for the normal function of neurons and the brain. In diseased conditions, several pathological modifications of tau protein manifest. These changes lead to tau protein aggregation and the formation of paired helical filaments (PHF) and neurofibrillary tangles (NFT), which are common hallmarks of Alzheimer's disease and other tauopathies. The accumulation of PHFs and NFTs results in impairment of physiological functions, apoptosis, and neuronal loss, which is reflected as cognitive impairment, and in the late stages of the disease, leads to death. The causes of this pathological transformation of tau protein haven't been fully understood yet. In both physiological and pathological conditions, tau interacts with several proteins which maintain their proper function or can participate in their pathological modifications. Interaction partners of tau protein and associated molecular pathways can either initiate and drive the tau pathology or can act neuroprotective, by reducing pathological tau proteins or inflammation. In this review, we focus on the tau as a multifunctional protein and its known interacting partners active in regulations of different processes and the roles of these proteins in Alzheimer's disease and tauopathies.

Requirement of CRMP2 Phosphorylation in Neuronal Migration of Developing Mouse Cerebral Cortex and Hippocampus and Redundant Roles of CRMP1 and CRMP4

The mammalian cerebral cortex is characterized by a 6-layer structure, and proper neuronal migration is critical for its formation. Cyclin-dependent kinase 5 (Cdk5) has been shown to be a critical kinase for neuronal migration. Several Cdk5 substrates have been suggested to be involved in ordered neuronal migration. However, in vivo loss-of-function studies on the function of Cdk5 phosphorylation substrates in neuronal migration in the developing cerebral cortex have not been reported. In this study, we demonstrated that Cdk5-mediated phosphorylation of collapsing mediator protein (CRMP) 2 is critical for neuronal migration in the developing cerebral cortex with redundant functions of CRMP1 and CRMP4. The cerebral cortices of triple-mutant CRMP1 knock-out (KO); CRMP2 knock-in (KI)/KI; and CRMP4 KO mice showed disturbed positioning of layers II-V neurons in the cerebral cortex. Further experiments using bromodeoxyuridine birthdate-labeling and in utero electroporation implicated radial migration defects in cortical neurons. Ectopic neurons were detected around the CA1 region and dentate gyrus in CRMP1 KO; CRMP2 KI/KI; and CRMP4 KO mice. These results suggest the importance of CRMP2 phosphorylation by Cdk5 and redundancy of CRMP1 and CRMP4 in proper neuronal migration in the developing cerebral cortex and hippocampus.

Tau, XMAP215/Msps and Eb1 co-operate interdependently to regulate microtubule polymerisation and bundle formation in axons

The formation and maintenance of microtubules requires their polymerisation, but little is known about how this polymerisation is regulated in cells. Focussing on the essential microtubule bundles in axons of Drosophila and Xenopus neurons, we show that the plus-end scaffold Eb1, the polymerase XMAP215/Msps and the lattice-binder Tau co-operate interdependently to promote microtubule polymerisation and bundle organisation during axon development and maintenance. Eb1 and XMAP215/Msps promote each other's localisation at polymerising microtubule plus-ends. Tau outcompetes Eb1-binding along microtubule lattices, thus preventing depletion of Eb1 tip pools. The three factors genetically interact and show shared mutant phenotypes: reductions in axon growth, comet sizes, comet numbers and comet velocities, as well as prominent deterioration of parallel microtubule bundles into disorganised curled conformations. This microtubule curling is caused by Eb1 plus-end depletion which impairs spectraplakin-mediated guidance of extending microtubules into parallel bundles. Our demonstration that Eb1, XMAP215/Msps and Tau co-operate during the regulation of microtubule polymerisation and bundle organisation, offers new conceptual explanations for developmental and degenerative axon pathologies.

Differential Effects of the Six Human TAU Isoforms: Somatic Retention of 2N-TAU and Increased Microtubule Number Induced by 4R-TAU

In the adult human brain, six isoforms of the microtubule-associated protein TAU are expressed, which result from alternative splicing of exons 2, 3, and 10 of the MAPT gene. These isoforms differ in the number of N-terminal inserts (0N, 1N, 2N) and C-terminal repeat domains (3R or 4R) and are differentially expressed depending on the brain region and developmental stage. Although all TAU isoforms can aggregate and form neurofibrillary tangles, some tauopathies, such as Pick's disease and progressive supranuclear palsy, are characterized by the accumulation of specific TAU isoforms. The influence of the individual TAU isoforms in a cellular context, however, is understudied. In this report, we investigated the subcellular localization of the human-specific TAU isoforms in primary mouse neurons and analyzed TAU isoform-specific effects on cell area and microtubule dynamics in human SH-SY5Y neuroblastoma cells. Our results show that 2N-TAU isoforms are particularly retained from axonal sorting and that axonal enrichment is independent of the number of repeat domains, but that the additional repeat domain of 4R-TAU isoforms results in a general reduction of cell size and an increase of microtubule counts in cells expressing these specific isoforms. Our study points out that individual TAU isoforms may influence microtubule dynamics differentially both by different sorting patterns and by direct effects on microtubule dynamics.

LRP::FLAG Reduces Phosphorylated Tau Levels in Alzheimer's Disease Cell Culture Models

BACKGROUND

Alzheimer's disease (AD) is characterized by amyloid-β (Aβ) plaque and neurofibrillary tangle formation, respectively. Neurofibrillary tangles form as a result of the intracellular accumulation of hyperphosphorylated tau. Telomerase activity and levels of the human reverse transcriptase (hTERT) subunit of telomerase are significantly decreased in AD. Recently, it has been demonstrated that the 37 kDa/67 kDa laminin receptor (LRP/LR) interacts with telomerase and is implicated in Aβ pathology. Since both LRP/LR and telomerase are known to play a role in the Aβ facet of AD, we hypothesized that they might also play a role in tauopathy.

OBJECTIVE

This study aimed to determine if LRP/LR has a relationship with tau and whether overexpression of LRP::FLAG has an effect on tauopathy-related proteins.

METHODS

We employed confocal microscopy and FRET to determine whether LRP/LR and tau co-localize and interact. LRP::FLAG overexpression in HEK-293 and SH-SY5Y cells as well as analysis of tauopathy-related proteins was assessed by western blotting.

RESULTS

We demonstrate that LRP/LR co-localizes with tau in the perinuclear cell compartment and confirmed a direct interaction between LRP/LR and tau in HEK-293 cells. Overexpression of LRP::FLAG in HEK-293 and SH-SY5Y cells decreased total and phosphorylated tau levels with a concomitant decrease in PrPc levels, a tauopathy-related protein. LRP::FLAG overexpression also resulted in increased hTERT levels.

CONCLUSION

This data suggest that LRP/LR extends its role in AD through a direct interaction with tau, and recommend LRP::FLAG as a possible alternative AD therapeutic via decreasing phosphorylated tau levels.

Beyond Neuronal Microtubule Stabilization: MAP6 and CRMPS, Two Converging Stories

The development and function of the central nervous system rely on the microtubule (MT) and actin cytoskeletons and their respective effectors. Although the structural role of the cytoskeleton has long been acknowledged in neuronal morphology and activity, it was recently recognized to play the role of a signaling platform. Following this recognition, research into Microtubule Associated Proteins (MAPs) diversified. Indeed, historically, structural MAPs—including MAP1B, MAP2, Tau, and MAP6 (also known as STOP);—were identified and described as MT-binding and -stabilizing proteins. Extensive data obtained over the last 20 years indicated that these structural MAPs could also contribute to a variety of other molecular roles. Among multi-role MAPs, MAP6 provides a striking example illustrating the diverse molecular and cellular properties of MAPs and showing how their functional versatility contributes to the central nervous system. In this review, in addition to MAP6’s effect on microtubules, we describe its impact on the actin cytoskeleton, on neuroreceptor homeostasis, and its involvement in signaling pathways governing neuron development and maturation. We also discuss its roles in synaptic plasticity, brain connectivity, and cognitive abilities, as well as the potential relationships between the integrated brain functions of MAP6 and its molecular activities. In parallel, the Collapsin Response Mediator Proteins (CRMPs) are presented as examples of how other proteins, not initially identified as MAPs, fall into the broader MAP family. These proteins bind MTs as well as exhibiting molecular and cellular properties very similar to MAP6. Finally, we briefly summarize the multiple similarities between other classical structural MAPs and MAP6 or CRMPs.In summary, this review revisits the molecular properties and the cellular and neuronal roles of the classical MAPs, broadening our definition of what constitutes a MAP.

Aging modulates the effects of ischemic injury upon mesenchymal cells within the renal interstitium and microvasculature

The renal mesenchyme contains heterogeneous cells, including interstitial fibroblasts and pericytes, with key roles in wound healing. Although healing is impaired in aged kidneys, the effect of age and injury on the mesenchyme remains poorly understood. We characterized renal mesenchymal cell heterogeneity in young vs old animals and after ischemia‐reperfusion‐injury (IRI) using multiplex immunolabeling and single cell transcriptomics. Expression patterns of perivascular cell markers (α‐SMA, CD146, NG2, PDGFR‐α, and PDGFR‐β) correlated with their interstitial location. PDGFR‐α and PDGFR‐β co‐expression labeled renal myofibroblasts more efficiently than the current standard marker α‐SMA, and CD146 was a superior murine renal pericyte marker. Three renal mesenchymal subtypes; pericytes, fibroblasts, and myofibroblasts, were recapitulated with data from two independently performed single cell transcriptomic analyzes of murine kidneys, the first dataset an aging cohort and the second dataset injured kidneys following IRI. Mesenchymal cells segregated into subtypes with distinct patterns of expression with aging and following injury. Baseline uninjured old kidneys resembled post‐ischemic young kidneys, with this phenotype further exaggerated following IRI. These studies demonstrate that age modulates renal perivascular/interstitial cell marker expression and transcriptome at baseline and in response to injury and provide tools for the histological and transcriptomic analysis of renal mesenchymal cells, paving the way for more accurate classification of renal mesenchymal cell heterogeneity and identification of age‐specific pathways and targets.

Role of Microglia in Regulating Cholesterol and Tau Pathology in Alzheimer's Disease

Cholesterol, a principal constituent of the cell membrane, plays a crucial role in the brain by regulating the synaptic transmission, neuronal signaling, as well as neurodegenerative diseases. Defects in the cholesterol trafficking are associated with enhanced generation of hyperphosphorylated Tau and Amyloid-β protein. Tau, a major microtubule-associated protein in the brain, is the key regulator of the mature neuron. Abnormally hyperphosphorylated Tau hampers the major functions related to microtubule assembly by promoting neurofibrillary tangles of paired helical filaments, twisted ribbons, and straight filaments. The observed pathological changes due to impaired cholesterol and Tau protein accumulation cause Alzheimer's disease. Thus, in order to regulate the pathogenesis of Alzheimer's disease, regulation of cholesterol metabolism, as well as Tau phosphorylation, is essential. The current review provides an overview of (1) cholesterol synthesis in the brain, neurons, astrocytes, and microglia; (2) the mechanism involved in modulating cholesterol concentration between the astrocytes and brain; (3) major mechanisms involved in the hyperphosphorylation of Tau and amyloid-β protein; and (4) microglial involvement in its regulation. Thus, the answering key questions will provide an in-depth information on microglia involvement in managing the pathogenesis of cholesterol-modulated hyperphosphorylated Tau protein.

Reward motivation and cognitive flexibility in tau null-mutation mice

The reduction of tau or hyperphosphorylated tau (p-tau) has been proposed as a therapeutic strategy for Alzheimer's disease (AD) and frontotemporal dementia (FTD). Cognitive decline and sleep-wake dysregulation seen in AD and FTD patients are mimicked in transgenic and null-mutation mouse models of tauopathy. Alterations in the reward system are additional symptoms of AD and FTD. However, the role of tau in reward processes is not well understood. The present study aimed to examine reward and reward-motivated cognitive processes in male and female tau knockout (tau-/-) and wild-type mice using progressive ratio and reversal learning tasks. Tau-/- mice were heavier, ate more in the home cage, and reached criterion in operant lever training faster than wild-type mice. Tau-/- mice had a higher breakpoint in progressive ratio but were unimpaired in reversal learning or reward sensitivity. These data indicate that tau loss of function alters reward processing. This may help to explain aberrant reward-related behaviors in tauopathy patients and highlights a potentially important area for consideration in the development of anti-tau therapies.

Tau: Enabler of diverse brain disorders and target of rapidly evolving therapeutic strategies

Several lines of evidence implicate the protein tau in the pathogenesis of multiple brain disorders, including Alzheimer's disease, other neurodegenerative conditions, autism, and epilepsy. Tau is abundant in neurons and interacts with microtubules, but its main functions in the brain remain to be defined. These functions may involve the regulation of signaling pathways relevant to diverse biological processes. Informative disease models have revealed a plethora of abnormal tau species and mechanisms that might contribute to neuronal dysfunction and loss, but the relative importance of their respective contributions is uncertain. This knowledge gap poses major obstacles to the development of truly impactful therapeutic strategies. The current expansion and intensification of efforts to translate mechanistic insights into tau-related therapeutics should address this issue and could deliver better treatments for a host of devastating conditions.

The intrinsic axon regenerative properties of mature neurons after injury

Thousands of nerve injuries occur in the world each year. Axon regeneration is a very critical process for the restoration of the injured nervous system's function. However, the precise molecular mechanism or signaling cascades that control axon regeneration are not clearly understood, especially in mammals. Therefore, there is almost no ideal treatment method to repair the nervous system's injury until now. Mammalian axonal regeneration requires multiple signaling pathways to coordinately regulate gene expression in soma and assembly of the cytoskeleton protein in the growth cone. A better understanding of their molecular mechanisms, such as axon regeneration regulatory signaling cascades, will be helpful in developing new treatment strategies for promoting axon regeneration. In this review, we mainly focus on describing these regeneration-associated signaling cascades, which regulate axon regeneration.

Can Healthy Diets, Regular Exercise, and Better Lifestyle Delay the Progression of Dementia in Elderly Individuals?

Alzheimer's disease (AD) is a progressive neurodegenerative disease characterized by memory loss and multiple cognitive impairments. Current healthcare costs for over 50 million people afflicted with AD are about $818 million and are projected to be $2 billion by 2050. Unfortunately, there are no drugs currently available that can delay and/or prevent the progression of disease in elderly individuals and in AD patients. Loss of synapses and synaptic damage are largely correlated with cognitive decline in AD patients. Women are at a higher lifetime risk of developing AD encompassing two-thirds of the total AD afflicted population. Only about 1-2% of total AD patients can be explained by genetic mutations in APP, PS1, and PS2 genes. Several risk factors have been identified, such as Apolipoprotein E4 genotype, type 2 diabetes, traumatic brain injury, depression, and hormonal imbalance, are reported to be associated with late-onset AD. Strong evidence reveals that antioxidant enriched diets and regular exercise reduces toxic radicals, enhances mitochondrial function and synaptic activity, and improves cognitive function in elderly populations. Current available data on the use of antioxidants in mouse models of AD and antioxidant(s) supplements in diets of elderly individuals were investigated. The use of antioxidants in randomized clinical trials in AD patients was also critically assessed. Based on our survey of current literature and findings, we cautiously conclude that healthy diets, regular exercise, and improved lifestyle can delay dementia progression and reduce the risk of AD in elderly individuals and reverse subjects with mild cognitive impairment to a non-demented state.

Neuregulin-1 inhibits CoCl2-induced upregulation of excitatory amino acid carrier 1 expression and oxidative stress in SH-SY5Y cells and the hippocampus of mice

Excitatory amino acid carrier 1 (EAAC1) is an important subtype of excitatory amino acid transporters (EAATs) and is the route for neuronal cysteine uptake. CoCl₂ is not only a hypoxia-mimetic reagent but also an oxidative stress inducer. Here, we found that CoCl₂ induced significant EAAC1 overexpression in SH-SY5Y cells and the hippocampus of mice. Transient transfection of EAAC1 reduced CoCl₂-induced cytotoxicity in SH-SY5Y cells. Based on this result, upregulation of EAAC1 expression by CoCl₂ is thought to represent a compensatory response against oxidative stress in an acute hypoxic state. We further demonstrated that pretreatment with Neuregulin-1 (NRG1) rescued CoCl₂-induced upregulation of EAAC1 and tau expression. NRG1 plays a protective role in the CoCl₂-induced accumulation of reactive oxygen species (ROS) and reduction in antioxidative enzyme (SOD and GPx) activity. Moreover, NRG1 attenuated CoCl₂-induced apoptosis and cell death. NRG1 inhibited the CoCl₂-induced release of cleaved caspase-3 and reduction in Bcl-X_L levels. Our novel finding suggests that NRG1 may play a protective role in hypoxia through the inhibition of oxidative stress and thereby maintain normal EAAC1 expression levels.

Microtubule-associated proteins (MAPs) in microtubule cytoskeletal dynamics and spermatogenesis

The microtubule (MT) cytoskeleton in Sertoli cells, a crucial cellular structure in the seminiferous epithelium of adult mammalian testes that supports spermatogenesis, was studied morphologically decades ago. However, its biology, in particular the involving regulatory biomolecules and the underlying mechanism(s) in modulating MT dynamics, are only beginning to be revealed in recent years. This lack of studies in delineating the biology of MT cytoskeletal dynamics undermines other studies in the field, in particular the plausible therapeutic treatment and management of male infertility and fertility since studies have shown that the MT cytoskeleton is one of the prime targets of toxicants. Interestingly, much of the information regarding the function of actin-, MT- and intermediate filament-based cytoskeletons come from studies using toxicant models including some genetic models. During the past several years, there have been some advances in studying the biology of MT cytoskeleton in the testis, and many of these studies were based on the use of pharmaceutical/toxicant models. In this review, we summarize the results of these findings, illustrating the importance of toxicant/pharmaceutical models in unravelling the biology of MT dynamics, in particular the role of microtubule-associated proteins (MAPs), a family of regulatory proteins that modulate MT dynamics but also actin- and intermediate filament-based cytoskeletons. We also provide a timely hypothetical model which can serve as a guide to design functional experiments to study how the MT cytoskeleton is regulated during spermatogenesis through the use of toxicants and/or pharmaceutical agents.

Dysregulation of Neurite Outgrowth and Cell Migration in Autism and Other Neurodevelopmental Disorders

Despite decades of study, elucidation of the underlying etiology of complex developmental disorders such as autism spectrum disorder (ASD), schizophrenia (SCZ), intellectual disability (ID), and bipolar disorder (BPD) has been hampered by the inability to study human neurons, the heterogeneity of these disorders, and the relevance of animal model systems. Moreover, a majority of these developmental disorders have multifactorial or idiopathic (unknown) causes making them difficult to model using traditional methods of genetic alteration. Examination of the brains of individuals with ASD and other developmental disorders in both post-mortem and MRI studies shows defects that are suggestive of dysregulation of embryonic and early postnatal development. For ASD, more recent genetic studies have also suggested that risk genes largely converge upon the developing human cerebral cortex between weeks 8 and 24 in utero. Yet, an overwhelming majority of studies in autism rodent models have focused on postnatal development or adult synaptic transmission defects in autism related circuits. Thus, studies looking at early developmental processes such as proliferation, cell migration, and early differentiation, which are essential to build the brain, are largely lacking. Yet, interestingly, a few studies that did assess early neurodevelopment found that alterations in brain structure and function associated with neurodevelopmental disorders (NDDs) begin as early as the initial formation and patterning of the neural tube. By the early to mid-2000s, the derivation of human embryonic stem cells (hESCs) and later induced pluripotent stem cells (iPSCs) allowed us to study living human neural cells in culture for the first time. Specifically, iPSCs gave us the unprecedented ability to study cells derived from individuals with idiopathic disorders. Studies indicate that iPSC-derived neural cells, whether precursors or "matured" neurons, largely resemble cortical cells of embryonic humans from weeks 8 to 24. Thus, these cells are an excellent model to study early human neurodevelopment, particularly in the context of genetically complex diseases. Indeed, since 2011, numerous studies have assessed developmental phenotypes in neurons derived from individuals with both genetic and idiopathic forms of ASD and other NDDs. However, while iPSC-derived neurons are fetal in nature, they are post-mitotic and thus cannot be used to study developmental processes that occur before terminal differentiation. Moreover, it is important to note that during the 8-24-week window of human neurodevelopment, neural precursor cells are actively undergoing proliferation, migration, and early differentiation to form the basic cytoarchitecture of the brain. Thus, by studying NPCs specifically, we could gain insight into how early neurodevelopmental processes contribute to the pathogenesis of NDDs. Indeed, a few studies have explored NPC phenotypes in NDDs and have uncovered dysregulations in cell proliferation. Yet, few studies have explored migration and early differentiation phenotypes of NPCs in NDDs. In this chapter, we will discuss cell migration and neurite outgrowth and the role of these processes in neurodevelopment and NDDs. We will begin by reviewing the processes that are important in early neurodevelopment and early cortical development. We will then delve into the roles of neurite outgrowth and cell migration in the formation of the brain and how errors in these processes affect brain development. We also provide review of a few key molecules that are involved in the regulation of neurite outgrowth and migration while discussing how dysregulations in these molecules can lead to abnormalities in brain structure and function thereby highlighting their contribution to pathogenesis of NDDs. Then we will discuss whether neurite outgrowth, migration, and the molecules that regulate these processes are associated with ASD. Lastly, we will review the utility of iPSCs in modeling NDDs and discuss future goals for the study of NDDs using this technology.

Isozyme-Specific Role of SAD-A in Neuronal Migration During Development of Cerebral Cortex

SAD kinases regulate presynaptic vesicle clustering and neuronal polarization. A previous report demonstrated that Sada-/- and Sadb-/- double-mutant mice showed perinatal lethality with a severe defect in axon/dendrite differentiation, but their single mutants did not. These results indicated that they were functionally redundant. Surprisingly, we show that on a C57BL/6N background, SAD-A is essential for cortical development whereas SAD-B is dispensable. Sada-/- mice died within a few days after birth. Their cortical lamination pattern was disorganized and radial migration of cortical neurons was perturbed. Birth date analyses with BrdU and in utero electroporation using pCAG-EGFP vector showed a delayed migration of cortical neurons to the pial surface in Sada-/- mice. Time-lapse imaging of these mice confirmed slow migration velocity in the cortical plate. While the neurites of hippocampal neurons in Sada-/- mice could ultimately differentiate in culture to form axons and dendrites, the average length of their axons was shorter than that of the wild type. Thus, analysis on a different genetic background than that used initially revealed a nonredundant role for SAD-A in neuronal migration and differentiation.

The physiological roles of tau and Aβ: implications for Alzheimer's disease pathology and therapeutics

Tau and amyloid beta (Aβ) are the prime suspects for driving pathology in Alzheimer's disease (AD) and, as such, have become the focus of therapeutic development. Recent research, however, shows that these proteins have been highly conserved throughout evolution and may have crucial, physiological roles. Such functions may be lost during AD progression or be unintentionally disrupted by tau- or Aβ-targeting therapies. Tau has been revealed to be more than a simple stabiliser of microtubules, reported to play a role in a range of biological processes including myelination, glucose metabolism, axonal transport, microtubule dynamics, iron homeostasis, neurogenesis, motor function, learning and memory, neuronal excitability, and DNA protection. Aβ is similarly multifunctional, and is proposed to regulate learning and memory, angiogenesis, neurogenesis, repair leaks in the blood-brain barrier, promote recovery from injury, and act as an antimicrobial peptide and tumour suppressor. This review will discuss potential physiological roles of tau and Aβ, highlighting how changes to these functions may contribute to pathology, as well as the implications for therapeutic development. We propose that a balanced consideration of both the physiological and pathological roles of tau and Aβ will be essential for the design of safe and effective therapeutics.

The Cell Biology of Tau Secretion

The progressive accumulation and spread of misfolded tau protein in the nervous system is the hallmark of tauopathies, progressive neurodegenerative diseases with only symptomatic treatments available. A growing body of evidence suggests that spreading of tau pathology can occur via cell-to-cell transfer involving secretion and internalization of pathological forms of tau protein followed by templated misfolding of normal tau in recipient cells. Several studies have addressed the cell biological mechanisms of tau secretion. It now appears that instead of a single mechanism, cells can secrete tau via three coexisting pathways: (1) translocation through the plasma membrane; (2) membranous organelles-based secretion; and (3) ectosomal shedding. The relative importance of these pathways in the secretion of normal and pathological tau is still elusive, though. Moreover, glial cells contribute to tau propagation, and the involvement of different cell types, as well as different secretion pathways, complicates the understanding of prion-like propagation of tauopathy. One of the important regulators of tau secretion in neuronal activity, but its mechanistic connection to tau secretion remains unclear and may involve all three secretion pathways of tau. This review article summarizes recent advancements in the field of tau secretion with an emphasis on cell biological aspects of the secretion process and discusses the role of neuronal activity and glial cells in the spread of pathological forms of tau.

Tau Gene Deletion does not Influence Axonal Regeneration and Retinal Neuron Survival in the Injured Mouse Visual System

In the present study, we hypothesized that the microtubule-associated protein Tau may influence retinal neuron survival and axonal regeneration after optic nerve injury. To test this hypothesis, the density of retinal ganglion cells was evaluated by immunostaining retinal flat-mounts for RNA-binding protein with multiple splicing (RBPMS) two weeks after optic nerve micro-crush lesion in Tau-deprived (Tau knock-out (KO)) and wild-type (WT) mice. Axon growth was determined on longitudinal sections of optic nerves after anterograde tracing. Our results showed that the number of surviving retinal ganglion cells and growing axons did not significantly vary between WT and Tau KO animals. Moreover, sustained activation of the neuronal growth program with ciliary neurotrophic factor (CNTF) resulted in a similar increase in surviving neurons and in growing axons in WT and Tau KO mice. Taken together, our data suggest that Tau does not influence axonal regeneration or neuronal survival.

Molecular mechanisms of cell polarity in a range of model systems and in migrating neurons

Cell polarity is defined as the asymmetric distribution of cellular components along an axis. Most cells, from the simplest single-cell organisms to highly specialized mammalian cells, are polarized and use similar mechanisms to generate and maintain polarity. Cell polarity is important for cells to migrate, form tissues, and coordinate activities. During development of the mammalian cerebral cortex, cell polarity is essential for neurogenesis and for the migration of newborn but as-yet undifferentiated neurons. These oriented migrations include both the radial migration of excitatory projection neurons and the tangential migration of inhibitory interneurons. In this review, I will first describe the development of the cerebral cortex, as revealed at the cellular level. I will then define the core molecular mechanisms - the Par/Crb/Scrib polarity complexes, small GTPases, the actin and microtubule cytoskeletons, and phosphoinositides/PI3K signaling - that are required for asymmetric cell division, apico-basal and front-rear polarity in model systems, including C elegans zygote, Drosophila embryos and cultured mammalian cells. As I go through each core mechanism I will explain what is known about its importance in radial and tangential migration in the developing mammalian cerebral cortex.

Neuro-taxis: Neuronal movement in gradients of chemical and physical environments

Environmental chemical and physical cues dynamically interact with migrating neurons and sprouting axons, and in particular, the gradients of environmental cues are regarded as one of the factors intimately involved in the neuronal movement. Since a growth cone was first described by Cajal, more than one century ago, chemical gradients have been suggested as one of the mechanisms by which the neurons determine proper paths and destinations. However, the gradients of physical cues, such as stiffness and topography, which also interact constantly with the neurons and their axons as a component of the extracellular environments, have rarely been noted regarding the guidance of neurons, despite their gradually increasingly reported influences in the case of nonneuronal-cell migration. In this review, we discuss chemical (i.e., chemo- and hapto-) and physical (i.e., duro-) taxis phenomena on the movement of neurons including axonal elongation. In addition, we suggest topotaxis, the most recently proposed physical-taxis phenomenon, as another potential mechanism in the neuronal movement, based on the reports of neuronal recognition of and responses to nanotopography.

Proteostasis Failure in Neurodegenerative Diseases: Focus on Oxidative Stress

Protein homeostasis or proteostasis is an essential balance of cellular protein levels mediated through an extensive network of biochemical pathways that regulate different steps of the protein quality control, from the synthesis to the degradation. All proteins in a cell continuously turn over, contributing to development, differentiation, and aging. Due to the multiple interactions and connections of proteostasis pathways, exposure to stress conditions may cause various types of protein damage, altering cellular homeostasis and disrupting the entire network with additional cellular stress. Furthermore, protein misfolding and/or alterations during protein synthesis results in inactive or toxic proteins, which may overload the degradation mechanisms. The maintenance of a balanced proteome, preventing the formation of impaired proteins, is accomplished by two major catabolic routes: the ubiquitin proteasomal system (UPS) and the autophagy-lysosomal system. The proteostasis network is particularly important in nondividing, long-lived cells, such as neurons, as its failure is implicated with the development of neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis. These neurological disorders share common risk factors such as aging, oxidative stress, environmental stress, and protein dysfunction, all of which alter cellular proteostasis, suggesting that general mechanisms controlling proteostasis may underlay the etiology of these diseases. In this review, we describe the major pathways of cellular proteostasis and discuss how their disruption contributes to the onset and progression of neurodegenerative diseases, focusing on the role of oxidative stress.