Microtubule-associated protein tau gene: a risk factor in human neurodegenerative diseases

1,3,5-Triazine: A Promising Molecular Scaffold for Novel Agents for the Treatment of Alzheimer's Disease

Currently, Alzheimer's disease (AD) is one of the most frequent forms of dementia. From a molecular perspective, the molecular characteristics that better define this disease consist of abnormal protein deposits between neuronal cells, namely senile plaques (SPs) and neurofibrillary tangles (NFTs), consisting of protein aggregates of amyloid-β and hyperphosphorylated tau protein, respectively. In addition to these protein aggregates, a third molecular hallmark of AD consists of depleted neurotransmitter acetylcholine levels. To date, the treatments developed for this disease are mostly focused on the use of AChE inhibitors, presenting only a symptomatic approach against the disease instead of a cure. Triazines are nitrogen-containing heterocyclic compounds that, throughout the years, have attracted a lot of curiosity from medicinal chemists for presenting numerous biological properties and being widely present in nature. In particular, this class of compounds has been associated with inhibiting several biological targets, emerging as a promising class for developing new pharmacological agents. However, there is still a scarcity of knowledge regarding the potential of this type of compound against any of the hallmarks of AD. For this reason, this paper intends to fulfill this absence by highlighting the potential of a subclass of triazines, 1,3,5-triazines (sym-triazines), as promising molecules for developing novel AD treatments. Thus, an in-depth analysis of 1,3,5-triazine derivatives is performed regarding its inhibitory activity against AChE (cholinergic hypothesis) and its capability to inhibit amyloid-β formation and aggregation (amyloid hypothesis). Through this analysis, it is possible to indicate some structural features optimal for each described activity, a compilation that we believe to be essential for the scientific community in this never-ending pursuit.

Study liquid-liquid phase separation with optical microscopy: A methodology review

Intracellular liquid-liquid phase separation (LLPS) is a critical process involving the dynamic association of biomolecules and the formation of non-membrane compartments, playing a vital role in regulating biomolecular interactions and organelle functions. A comprehensive understanding of cellular LLPS mechanisms at the molecular level is crucial, as many diseases are linked to LLPS, and insights gained can inform drug/gene delivery processes and aid in the diagnosis and treatment of associated diseases. Over the past few decades, numerous techniques have been employed to investigate the LLPS process. In this review, we concentrate on optical imaging methods applied to LLPS studies. We begin by introducing LLPS and its molecular mechanism, followed by a review of the optical imaging methods and fluorescent probes employed in LLPS research. Furthermore, we discuss potential future imaging tools applicable to the LLPS studies. This review aims to provide a reference for selecting appropriate optical imaging methods for LLPS investigations.

Unconventional secretion of tau by VAMP8 impacts its intra- and extracellular cleavage

In Alzheimer’s disease, Tau, a microtubule-associated protein, becomes hyperphosphorylated, detaches from microtubules, and accumulates in the somato-dendritic compartment where it forms insoluble aggregates. Tau also accumulates in the CSF of patients indicating that it is released by neurons. Consistent with this, several laboratories including ours have shown that Tau is secreted by neurons through unconventional secretory pathways. Recently, we reported that VAMP8, an R-SNARE found on late endosomes, increased Tau secretion and that secreted Tau was cleaved at the C-terminal. In the present study, we examined whether the increase of Tau secretion by VAMP8 affected its intra- and extracellular cleavage. Upon VAMP8 overexpression, an increase of Tau cleaved by caspase-3 in the cell lysate and medium was observed. This was correlated to an increase of active caspase-3 in the cell lysate and medium. Using a Tau mutant not cleavable by caspase-3, we demonstrated that Tau cleavage by caspase-3 was not necessary for its secretion upon VAMP8 overexpression. By adding recombinant Tau to the culture medium, we demonstrated that extracellular Tau cleavage by caspase-3 could occur because of the release of active caspase-3, which was the highest when VAMP8 was overexpressed. When cleavage of Tau by caspase-3 was prevented by using a non-cleavable mutant, secreted Tau was still cleaved at the C-terminal, the asparagine N410 contributing to it. Lastly, we demonstrated that N-terminal of Tau regulated the secretion pattern of a Tau fragment containing the microtubule-binding domain and the C-terminal of Tau upon VAMP8 overexpression. Collectively, the above observations indicate that VAMP8 overexpression affects the intra- and extracellular cleavage pattern of Tau.

Clearance of intracellular tau protein from neuronal cells via VAMP8-induced secretion

In Alzheimer's disease (AD), tau, a microtubule-associated protein (MAP), becomes hyperphosphorylated, aggregates, and accumulates in the somato-dendritic compartment of neurons. In parallel to its intracellular accumulation in AD, tau is also released in the extracellular space, as revealed by its increased presence in cerebrospinal fluid (CSF). Consistent with this, recent studies, including ours, have reported that neurons secrete tau, and several therapeutic strategies aim to prevent the intracellular tau accumulation. Previously, we reported that late endosomes were implicated in tau secretion. Here, we explore the possibility of preventing intracellular tau accumulation by increasing tau secretion. Using neuronal models, we investigated whether overexpression of the vesicle-associated membrane protein 8 (VAMP8), an R-SNARE found on late endosomes, could increase tau secretion. The overexpression of VAMP8 significantly increased tau secretion, decreasing its intracellular levels in the neuroblastoma (N2a) cell line. Increased tau secretion by VAMP8 was also observed in murine hippocampal slices. The intracellular reduction of tau by VAMP8 overexpression correlated to a decrease of acetylated tubulin induced by tau overexpression in N2a cells. VAMP8 staining was preferentially found on late endosomes in N2a cells. Using total internal reflection fluorescence (TIRF) microscopy, the fusion of VAMP8-positive vesicles with the plasma membrane was correlated to the depletion of tau in the cytoplasm. Finally, overexpression of VAMP8 reduced the intracellular accumulation of tau mutants linked to frontotemporal dementia with parkinsonism and α-synuclein by increasing their secretion. Collectively, the present data indicate that VAMP8 could be used to increase tau and α-synuclein clearance to prevent their intracellular accumulation.

Regulation of human MAPT gene expression

The number of known pathologies involving deregulated Tau expression/metabolism is increasing. Indeed, in addition to tauopathies, which comprise approximately 30 diseases characterized by neuronal aggregation of hyperphosphorylated Tau in brain neurons, this protein has also been associated with various other pathologies such as cancer, inclusion body myositis, and microdeletion/microduplication syndromes, suggesting its possible function in peripheral tissues. In addition to Tau aggregation, Tau deregulation can occur at the expression and/or splicing levels, as has been clearly demonstrated in some of these pathologies. Here, we aim to review current knowledge regarding the regulation of human MAPT gene expression at the DNA and RNA levels to provide a better understanding of its possible deregulation. Several aspects, including repeated motifs, CpG island/methylation, and haplotypes at the DNA level, as well as the key regions involved in mRNA expression and stability and the splicing patterns of different mRNA isoforms at the RNA level, will be discussed.

MAPT as a predisposing gene for sporadic amyotrophic lateral sclerosis in the Chinese Han population

A previous study of European Caucasian patients with sporadic amyotrophic lateral sclerosis demonstrated that a polymorphism in the microtubule-associated protein Tau (MAPT) gene was significantly associated with sporadic amyotrophic lateral sclerosis pathogenesis. Here, we tested this association in 107 sporadic amyotrophic lateral sclerosis patients and 100 healthy controls from the Chinese Han population. We screened the mutation-susceptible regions of MAPT - the 3' and 5' untranslated regions as well as introns 9, 10, 11, and 12 - by direct sequencing, and identified 33 genetic variations. Two of these, 105788 A > G in intron 9 and 123972 T > A in intron 11, were not present in the control group. The age of onset in patients with the 105788 A > G and/or the 123972 T > A variant was younger than that in patients without either genetic variation. Moreover, the pa-tients with a genetic variation were more prone to bulbar palsy and breathing difficulties than those with the wild-type genotype. This led to a shorter survival period in patients with a MAPT genetic variant. Our study suggests that the MAPT gene is a potential risk gene for sporadic amyotrophic lateral sclerosis in the Chinese Han population.

Network analysis of neurodegenerative disease highlights a role of Toll-like receptor signaling

Despite significant advances in the study of the molecular mechanisms altered in the development and progression of neurodegenerative diseases (NDs), the etiology is still enigmatic and the distinctions between diseases are not always entirely clear. We present an efficient computational method based on protein-protein interaction network (PPI) to model the functional network of NDs. The aim of this work is fourfold: (i) reconstruction of a PPI network relating to the NDs, (ii) construction of an association network between diseases based on proximity in the disease PPI network, (iii) quantification of disease associations, and (iv) inference of potential molecular mechanism involved in the diseases. The functional links of diseases not only showed overlap with the traditional classification in clinical settings, but also offered new insight into connections between diseases with limited clinical overlap. To gain an expanded view of the molecular mechanisms involved in NDs, both direct and indirect connector proteins were investigated. The method uncovered molecular relationships that are in common apparently distinct diseases and provided important insight into the molecular networks implicated in disease pathogenesis. In particular, the current analysis highlighted the Toll-like receptor signaling pathway as a potential candidate pathway to be targeted by therapy in neurodegeneration.

Intranasal therapeutic strategies for management of Alzheimer's disease

Alzheimer's disease (AD) is a chronic and progressive age-related irreversible neurodegenerative disorder that represents 70% of all dementia with 35 million cases worldwide. Successful treatment strategies for AD have so far been limited, and present therapy is based on cholinergic replacement therapy and inhibiting glutamate excitotoxicity. In this context, role of neuroprotective drugs has generated considerable interest in management of AD. Recently, direct intranasal (IN) delivery of drug moieties to the central nervous system (CNS) has emerged as a therapeutically viable alternative to oral and parenteral routes. IN delivery bypasses the blood-brain barrier by delivering and targeting drugs to the CNS along the olfactory and trigeminal neural pathways which are in direct contact with both the environment and the CNS. In an attempt to understand how neurotherapeutics/nanoparticulate delivery systems can be transported from the nose to the CNS, the present review sets out to discuss the mechanism of transport from nose to brain. The aim of this review is to discuss and summarize the latest findings of some of the major studies on IN drug delivery in AD models, with a focus on the potential efficacy of neuroprotective treatments.

Spreading of tau pathology in Alzheimer's disease by cell-to-cell transmission

It is well documented that neurofibrillary tangles composed of aggregated tau protein propagate in a predictable pattern in Alzheimer's disease (AD). The mechanisms underlying the propagation of tau pathology are still poorly understood. Recent studies have provided solid data demonstrating that in several neurodegenerative diseases including AD, the spreading of misfolded protein aggregates in the brain would result from prion-like cell-to-cell transmission. Consistent with this new concept, recent studies have reported that human tau can be released in the extracellular space by an active process of secretion, and can be endocytosed both in vitro and in vivo. Most importantly, it was reported that the spreading of tau pathology was observed along synaptically connected circuits in a transgenic mouse model where human tau overexpression was restricted in the entorhinal cortex. This indicates that secretion of tau by presynaptic neurons and its uptake by postsynaptic neurons could be the sequential events leading to the propagation of tau pathology in the brain.

Identification of Tau and SOD1 gene mutation in a small Chinese Han pedigree of adult amyotrophic lateral sclerosis

We report the clinical profile, and a brief investigation of SOD1 and Tau gene mutation from a small Chinese Han pedigree of adults with amyotrophic lateral sclerosis (ALS), which consisted of 32 familial members with 6 affected individuals spanning five generations, and presenting autosomal dominant genetic mode. The mean age of onset was 36.6 ± 15.9 years, and disease duration was 6 months to more than 5 years, the average survival was 16.1 ± 8.2 months. There were 5 patients with an early disease onset, rapid progressive course and short survival, and 1 patient with late onset, slow progressive course and long survival in the kindred. ALS patients began to suffer with weakness and muscle atrophy in one side of a lower extremity, which then spread to the upper extremity, the opposite side and bulbar muscles. All patients had spinal onset type. Muscle stretch reflexes were absent or weak in the upper limbs and accentuation in the lower limbs; pathological signs in the lower limbs were positive. Electromyography disclosed ongoing denervation muscle potentials in the four extremities. Brain and spinal MRI did not show any abnormal signal. A 5 exons mutation of SOD1 in all affected individuals was identified using SSCP. Polymorphisms of partial risk regions in 3',5' UTR, and in introns 9, 10, 11, 12 of the Tau gene in the affected and normal family members and in 70 healthy controls were examined by DNA sequencing. Routine exons mutation of SOD1 was not detected, but one single nucleotide polymorphism of A to G at 138278 at 3' UTR of the Tau gene was shown to significantly over-express in fALS familial members.

Untangling the role of tau in Alzheimer’s disease: A unifying hypothesis

Recent investigations into the etiology and pathogenesis of Alzheimer’s disease (AD) in the past few years have expanded to include previously unexplored and/or disconnected aspects of AD and related conditions at both the cellular and systemic levels of organization. These include how AD-associated abnormalities affect the cell cycle and neuronal differentiation state and how they recruit signal transduction, membrane trafficking and protein transcytosis mechanisms to produce a neurotoxic syndrome capable of spreading itself throughout the brain. The recent expansion of AD research into intercellular and new aspects of cellular degenerative mechanisms is causing a systemic re-evaluation of AD pathogenesis, including the roles played by well-studied elements, such as the generation of Aβ and tau protein aggregates. It is also changing our view of neurodegenerative diseases as a whole. Here we propose a conceptual framework to account for some of the emerging aspects of the role of tau in AD pathogenesis.

Management of frontotemporal dementia: targeting symptom management in such a heterogeneous disease requires a wide range of therapeutic options

There are no US FDA-approved therapies for the management of frontotemporal dementia (FTD). Evidence-based medicine that would support a FDA indication for the treatment of FTD requires large-scale, randomized, double-blind, placebo-controlled trials that do not currently exist. Progress in obtaining approval and therapeutic indications for FTD has been severely hampered by the heterogeneity of clinical and pathological phenotypes seen in various FTD disease states. These issues are often misinterpreted by clinicians, caregivers and patients suggesting that potential treatment options are nonexistent for this devastating disease. This article discusses these issues in the context of recent studies and publications investigating therapeutic options in FTD, and further suggests a rationale for individualized therapy in FTD. Targeting the myriad of symptoms seen in FTD requires recognition of such symptoms that may play primary or secondary roles in the spectrum of deficits that lead to functional disability in FTD, and the availability of a wide range of therapeutic options that may be helpful in alleviating such symptomatology. Fortunately, agents targeting the many cognitive, behavioral, psychiatric and motor symptoms that can be seen in FTD are readily available, having been previously developed and approved for symptomatic benefit in other disease states. In contrast to the widespread belief that beneficial treatments are not available for FTD today, our therapeutic armament is stocked with pharmacological tools that may improve quality of life for those suffering from this devastating and incurable class of degenerative diseases.

Tau as a biomarker of neurodegenerative diseases

The microtubule-associated protein Tau is mainly expressed in neurons of the CNS and is crucial in axonal maintenance and axonal transport. The rationale for Tau as a biomarker of neurodegenerative diseases is that it is a major component of abnormal intraneuronal aggregates observed in numerous tauopathies, including Alzheimer's disease. The molecular diversity of Tau is very useful when analyzing it in the brain or in the peripheral fluids. Immunohistochemical and biochemical characterization of Tau aggregates in the brain allows the postmortem classification and differential diagnosis of tauopathies. As peripheral biomarkers of Alzheimer's disease in the cerebrospinal fluid, Tau proteins are now validated for diagnosis and predictive purposes. For the future, the detailed characterization of Tau in the brain and in peripheral fluids will lead to novel promising biomarkers for differential diagnosis of dementia and monitoring of therapeutics.

Alternative splicing of exon 10 in the tau gene as a target for treatment of tauopathies

Tau aggregation is one of the major features in Alzheimer's disease and in several other tauopathies, including frontotemporal dementia with Parkinsonism linked to chromosome 17 (FTDP-17), and progressive supranuclear palsy (PSP). More than 35 mutations in the tau gene have been identified from FTDP-17 patients. A group of these mutations alters splicing of exon 10, resulting in an increase in exon 10 inclusion into tau mRNA. Abnormal splicing with inclusion of exon 10 into tau mRNA has also been observed in PSP and AD patients. These results indicate that abnormal splicing of exon 10, leading to the production of tau with exon 10, is probably one of the mechanisms by which tau accumulates and aggregates in tauopathic brains. Therefore, modulation of exon 10 splicing in the tau gene could potentially be targeted to prevent tauopathies. To identify small molecules or compounds that could potentially be developed into drugs to treat tauopathies, we established a cell-based high-throughput screening assay. In this review, we will discuss how realistic, specific biological molecules can be found to regulate exon 10 splicing in the tau gene for potential treatment of tauopathies.

Association of the Tau haplotype with Parkinson's disease in the Greek population

We compared the distribution of the Tau H1 haplotype and related subhaplotypes in a group of clinically diagnosed Parkinson's disease patients (n = 133) and in control individuals (n = 113) from northern Greece. We were able to detect a statistically significant overrepresentation of the H1H1 genotype in our patient group (OR for H1H1 vs. H1H2 and H2H2: 1.73; 95% CI: 1.03-2.90; P = 0.037). The H1 subhaplotype significantly associated with the disease in our population was different from the one previously reported for a Norwegian population, suggesting that the nature of the association of Tau with Parkinson's disease is influenced by ethnic variation.

Association between glycogen synthase kinase-3beta genetic polymorphism and late-onset Alzheimer's disease

Aberrant phosphorylated tau is the major component of the neurofibrillary tangles in Alzheimer's disease (AD) brains. Glycogen synthase kinase-3beta (GSK-3beta) phosphorylates tau protein, and increased GSK-3beta expression has been associated with neurofibrillary tangles. Saitohin (STH) is a recently identified protein that shares tissue expression pattern with tau, and previous evidence in the Spanish population indicated that a polymorphism at codon 7 (Q7R) of the STH gene was associated with late-onset AD. Since both GSK-3beta and STH are related to tau, we examined the association between a polymorphism in the promoter region (-50) of the GSK-3beta gene and AD, either through an independent effect or through interaction with the STH (Q7R) polymorphism, in a well-defined group of 333 sporadic AD patients and 307 control subjects from Spain. The current study reveals that GSK-3beta (-50) TT genotype is associated with an increased risk (OR 1.99, p = 0.003) for late-onset (after the age of 72 years) AD. Our results indicate that both the GSK-3beta (-50) and STH (Q7R) polymorphisms increase the risk of late-onset (subjects >72 years) AD, although they appear to be independent and thus not to interact synergistically.

Clinical and molecular aspects of frontotemporal dementia

Frontotemporal dementia (FTD) is a neurodegenerative disease and next to Alzheimer's disease and vascular dementia, the third most common cause of early-onset progressive dementia. FTD leads to neurodegeneration in the frontal and temporal neocortex and usually encompasses both sides of the frontal and anterior temporal lobes. Psychologically, FTD is characterized by personality changes such as lack of insight, inappropriate behaviour, disinhibition, apathy, executive disabilities and a decline in cognitive functions, with large clinical and neuropathological variations among cases. Neuropathological characteristics include gliosis or microvacuolation of cortical nerve cells. Inclusions staining for tau protein and/or ubiquitin are also common findings. Both sporadic and hereditary forms of FTD have been identified and 30-50% of the FTD cases have a familial background. So far, at least three genetic loci for FTD have been identified, at human chromosomes 3, 9 and 17 in familial forms of the disease. A large number of the familial forms have been linked to chromosome 17q21 and referred to as frontotemporal dementia and Parkinsonism linked to chromosome 17. In the majority of these families, pathogenic mutations in the tau gene were identified. However, tau mutations seem to be a rare cause of disease in the general FTD population. Thus, other genes and/or environmental factors are yet to be identified, which will give further clues to this complex and heterogeneous disorder.

The H2 MAPT haplotype is associated with familial frontotemporal dementia

There is now considerable evidence that the gene encoding for tau protein (MAPT) is implicated in frontotemporal dementia (FTD). The role of MAPT haplotypes in neurodegenerative diseases has been suggested, but their contribution in familial dementia has not been extensively investigated. Here, we investigated (1) the association between the MAPT haplotypes and sporadic (sFTD) or familial FTD (FFTD) (controls n = 99, sFTD n = 53, FFTD n = 50), (2) the interactive effect between MAPT haplotypes and APOE gene. We found an overrepresentation of H2 haplotype (OR = 1.83, P = 0.029) and of H2H2 genotype in FFTD patients (OR = 6.09, P = 0.007). This association was even stronger in APOE e4 negatives FFTD (H2: OR = 2.9, P = 0.001; H2H2: OR = 12.67, P = 0.001). Our results support idea that the MAPT H2 haplotype is a risk factor for FFTD. This locus could contain this or other inheritable genetic determinants contributing to increase risk of developing dementia.

Association study of polymorphisms in LRP1, tau and 5-HTT genes and Alzheimer’s disease in a sample of Colombian patients

Analysis of genetic susceptibility factors for Alzheimer's disease (AD) in populations with different genetic and environmental background may be useful to understand AD etiology. There are few genetic association studies of AD in Latin America. In the present work, we analyzed polymorphisms in 3 candidate genes; the LDL receptor related protein-1, the microtubule-associated protein Tau and the serotonin transporter genes in a sample of 106 Colombian AD patients and 97 control subjects. We did not find a significant allelic or genotypic association with any of the three polymorphisms analyzed using different statistical analysis, including a neural network model or different sample stratifications. To date, APOE polymorphisms are the only genetic risk factors identified for AD in the Colombian population. It may be factible that future combination of high-throughput genotyping platforms and multivariate analysis models may lead to the identification of other genetic susceptibility factors for AD in the Colombian population.

Characteristics of two distinct clinical phenotypes in pathologically proven progressive supranuclear palsy: Richardson's syndrome and PSP-parkinsonism

The clinical diagnosis of progressive supranuclear palsy (PSP) relies on the identification of characteristic signs and symptoms. A proportion of pathologically diagnosed cases do not develop these classic features, prove difficult to diagnose during life and are considered as atypical PSP. The aim of this study was to examine the apparent clinical dichotomy between typical and atypical PSP, and to compare the biochemical and genetic characteristics of these groups. In 103 consecutive cases of pathologically confirmed PSP, we have identified two clinical phenotypes by factor analysis which we have named Richardson's syndrome (RS) and PSP-parkinsonism (PSP-P). Cases of RS syndrome made up 54% of all cases, and were characterized by the early onset of postural instability and falls, supranuclear vertical gaze palsy and cognitive dysfunction. A second group of 33 (32%) were characterized by asymmetric onset, tremor, a moderate initial therapeutic response to levodopa and were frequently confused with Parkinson's disease (PSP-P). Fourteen cases (14%) could not be separated according to these criteria. In RS, two-thirds of cases were men, whereas the sex distribution in PSP-P was even. Disease duration in RS was significantly shorter (5.9 versus 9.1 years, P < 0.001) and age at death earlier (72.1 versus 75.5 years, P = 0.01) than in PSP-P. The isoform composition of insoluble tangle-tau isolated from the basal pons also differed significantly. In RS, the mean four-repeat:three-repeat tau ratio was 2.84 and in PSP-P it was 1.63 (P < 0.003). The effect of the H1,H1 PSP susceptibility genotype appeared stronger in RS than in PSP-P (odds ratio 13.2 versus 4.5). The difference in genotype frequencies between the clinical subgroups was not significant. There were no differences in apolipoprotein E genotypes. The classic clinical description of PSP, which includes supranuclear gaze palsy, early falls and dementia, does not adequately describe one-third of cases in this series of pathologically confirmed cases. We propose that PSP-P represents a second discrete clinical phenotype that needs to be clinically distinguished from classical PSP (RS). The different tau isoform deposition in the basal pons suggests that this may ultimately prove to be a discrete nosological entity.

Rare genetically defined causes of dementia

Several genetic disorders, though rare, are associated or present with dementia. Developments in the field of genetics are contributing to clarify and expand our knowledge of the complex physiopathological mechanisms leading to neurodegeneration and cognitive decline. Disorders associated with misfolded and aggregated proteins and lipid, metal or energy metabolism are examples of the multifarious disease processes converging in the clinical features of dementia, either as its predominant feature, as in cases of Alzheimer's disease (AD) or frontotemporal dementia (FTD), or as part of a cohort of accompanying or late-developing symptoms, as in Parkinson's disease (PD) or amyotrophic lateral sclerosis with dementia (ALS-D). Awareness of these disorders, allied with recent advances in genetic, biochemical and neuroimaging techniques, may lead to early diagnosis, successful treatment and better prognosis.