Therapeutic targets in Alzheimer's disease and related tauopathies

Preliminary exploration of the co-regulation of Alzheimer's disease pathogenic genes by microRNAs and transcription factors

Background

Alzheimer's disease (AD) is the most common form of age-related neurodegenerative disease. Unfortunately, due to the complexity of pathological types and clinical heterogeneity of AD, there is a lack of satisfactory treatment for AD. Previous studies have shown that microRNAs and transcription factors can modulate genes associated with AD, but the underlying pathophysiology remains unclear.

Methods

The datasets GSE1297 and GSE5281 were downloaded from the gene expression omnibus (GEO) database and analyzed to obtain the differentially expressed genes (DEGs) through the "R" language "limma" package. The GSE1297 dataset was analyzed by weighted correlation network analysis (WGCNA), and the key gene modules were selected. Next, gene ontology (GO) and Kyoto encyclopedia of genes and genomes (KEGG) pathway enrichment analysis for the key gene modules were performed. Then, the protein-protein interaction (PPI) network was constructed and the hub genes were identified using the STRING database and Cytoscape software. Finally, for the GSE150693 dataset, the "R" package "survivation" was used to integrate the data of survival time, AD transformation status and 35 characteristics, and the key microRNAs (miRNAs) were selected by Cox method. We also performed regression analysis using least absolute shrinkage and selection operator (Lasso)-Cox to construct and validate prognostic features associated with the four key genes using different databases. We also tried to find drugs targeting key genes through DrugBank database.

Results

GO and KEGG enrichment analysis showed that DEGs were mainly enriched in pathways regulating chemical synaptic transmission, glutamatergic synapses and Huntington's disease. In addition, 10 hub genes were selected from the PPI network by using the algorithm Between Centrality. Then, four core genes (TBP, CDK7, GRM5, and GRIA1) were selected by correlation with clinical information, and the established model had very good prognosis in different databases. Finally, hsa-miR-425-5p and hsa-miR-186-5p were determined by COX regression, AD transformation status and aberrant miRNAs.

Conclusion

In conclusion, we tried to construct a network in which miRNAs and transcription factors jointly regulate pathogenic genes, and described the process that abnormal miRNAs and abnormal transcription factors TBP and CDK7 jointly regulate the transcription of AD central genes GRM5 and GRIA1. The insights gained from this study offer the potential AD biomarkers, which may be of assistance to the diagnose and therapy of AD.

Our Tau Tales from Normal to Pathological Behavior

The microtubule associated protein tau in a hyperphosphorylated form was identified as the building block of the filamentous aggregates found in the neurons of Alzheimer's disease (AD) patients. In the abnormal state, hyperphosphorylated tau from AD brains (AD P-tau) was unable to promote microtubule assembly and more importantly, it could inhibit the normal activity of tau and other MAPs. AD P-tau was able to disrupt preformed microtubules and, by binding to normal tau, turn the latter into an AD P-tau like molecule. AD P-tau toxic behavior was prevalent in the soluble form and it was lost upon dephosphorylation. Mutations on tau associated with disease, e.g., R406W in frontotemporal dementia with Parkinsonism linked to chromosome 17, altered its conformation to make it a better substrate for kinases. Using phospho-mimetics, it was found that the minimum phospho-sites necessary to acquire such a toxic behavior of tau were at 199, 212, 231 and 262, and tau pseudophosphorylated at those sites in combination with R406W was named Pathological Human Tau (PH-Tau). PH-Tau expressed in cells had similar behavior to AD P-tau: disruption of the microtubule system, change in the normal subcellular localization, and gain of toxic function for cells. In animal models expressing PH-Tau, it was found that two putative mechanisms of neurodegeneration exist depending on the concentration of the toxic protein, both involving cognitive decline, due to synaptic dysfunction at lower concentration and neuronal death at higher. Studies investigating the mechanism of tau pathology and its transmission from neuron to neuron are currently ongoing.

Hyperphosphorylation of Tau Associates With Changes in Its Function Beyond Microtubule Stability

Tau is a neuronal microtubule associated protein whose main biological functions are to promote microtubule self-assembly by tubulin and to stabilize those already formed. Tau also plays an important role as an axonal microtubule protein. Tau is an amazing protein that plays a key role in cognitive processes, however, deposits of abnormal forms of tau are associated with several neurodegenerative diseases, including Alzheimer disease (AD), the most prevalent, and Chronic Traumatic Encephalopathy (CTE) and Traumatic Brain Injury (TBI), the most recently associated to abnormal tau. Tau post-translational modifications (PTMs) are responsible for its gain of toxic function. Alonso et al. (1996) were the first to show that the pathological tau isolated from AD brains has prion-like properties and can transfer its toxic function to the normal molecule. Furthermore, we reported that the pathological changes are associated with tau phosphorylation at Ser199 and 262 and Thr212 and 231. This pathological version of tau induces subcellular mislocalization in cultured cells and neurons, and translocates into the nucleus or accumulated in the perinuclear region of cells. We have generated a transgenic mouse model that expresses pathological human tau (PH-Tau) in neurons at two different concentrations (4% and 14% of the total endogenous tau). In this model, PH-Tau causes cognitive decline by at least two different mechanisms: one that involves the cytoskeleton with axonal disruption (at high concentration), and another in which the apparent neuronal morphology is not grossly affected, but the synaptic terminals are altered (at lower concentration). We will discuss the putative involvement of tau in proteostasis under these conditions. Understanding tau’s biological activity on and off the microtubules will help shed light to the mechanism of neurodegeneration and of normal neuronal function.

Molecular mechanism of prion-like tau-induced neurodegeneration

INTRODUCTION

Accumulation of hyperphosphorylated tau and the disruption of microtubules are correlated with synaptic loss and pathology of Alzheimer's disease (AD). Impaired cognitive function and pathology of AD is correlated with this lesion. This review looks at the mechanism of neurodegeneration, the prion-like behavior of tau in its interaction with normal MAPs in correlation with tau hyperphosphorylation.

METHODS

We reviewed our work in the field as well as current literature that pertains to tau phosphorylation and the biological effects.

RESULTS

Hyperphosphorylation of tau in AD, in vitro, in cells, or in animal models converts this protein into a prion-like protein that is able to propagate the altered conformation.

DISCUSSION

These findings suggest that phosphorylation of tau is a critical event in neurodegeneration. The combination of phosphorylation sites can generate a gain of toxic function for tau. The mechanism of tau toxicity might involve not only the microtubule system but also interference with other cellular compartments such as the nucleus and the actin cytoskeleton.

Revisão dos principais genes e proteínas associadas à demência frontotemporal tau-positiva

O objetivo desta revisão foi apresentar os genes APOE e MAPT e as proteínas ApoE e tau como marcadores genéticos que vêm sendo estudados na demência frontotemporal com inclusões tau-positivas, os quais poderão, futuramente, auxiliar no diagnóstico diferencial. A demência frontotemporal é um transtorno neurocognitivo marcado por disfunção dos lobos frontais e temporais, geralmente associada à atrofia dessas estruturas e relativa preservação das regiões cerebrais posteriores. Clinicamente, manifesta-se por volta dos 57 anos de idade, com igual incidência entre homens e mulheres. A demência frontotemporal tem início insidioso e caráter progressivo, com discreto comprometimento da memória episódica, mas com importantes alterações comportamentais, de personalidade e na linguagem. Devido às semelhanças possíveis entre as manifestações clínicas das demências inclusive a doença de Alzheimer, há grande dificuldade no diagnóstico diferencial, sendo necessário um exame clínico e neuropsicológico detalhado do indivíduo acometido, além de exames bioquímicos e de neuroimagem. O gene MAPT codifica a proteína tau e sua função principal é estabilizar os microtúbulos. Em células nervosas sadias, a proteína tau é normalmente encontrada nos axônios, ao contrário dos achados descritos nos transtornos neurocognitivos, em que a proteína se encontra distribuída no corpo celular e nos dendritos. A apolipoproteína E ApoE é uma glicoproteína polimórfica, codificada pelo gene APOE, que tem importante papel na absorção, transporte e redistribuição de colesterol, necessário ao reparo e manutenção do tecido nervoso. Com o aumento da expectativa de vida e controle da natalidade, o envelhecimento populacional tornou-se fato, trazendo consigo maior prevalência de doenças crônico-degenerativas, de modo que é de extrema importância conhecer melhor essas doenças, no sentido de buscar novas formas de tratamento, visto que as demências não dispõem ainda de cura. Sabe-se que o diagnóstico definitivo da maioria das síndromes demenciais depende do exame neuropatológico, mas conclui-se que, com o avanço tecnológico, bem como técnicas de biologia e genética molecular, novas perspectivas têm surgido para o diagnóstico diferencial e precoce das demências.

Tau-induced neurodegeneration: mechanisms and targets

The accumulation of hyperphosphorylated tau is a common feature of several dementias. Tau is one of the brain microtubule-associated proteins. Here we discuss tau's functions in microtubule assembly and stabilization and with regard to its interactions with other proteins. We describe and analyze important post-translational modifications: hyperphosphorylation, ubiquitination, glycation, glycosylation, nitration, polyamination, proteolysis, acetylation, and methylation. We discuss how these post-translational modifications can alter tau's biological function. We analyze the role of mitochondrial health in neurodegeneration. We propose that microtubules could be a therapeutic target and review different approaches. Finally, we consider whether tau accumulation or its conformational change is related to tau-induced neurodegeneration, and propose a mechanism of neurodegeneration.

Neurogenesis in Alzheimer´s disease: a realistic alternative to neuronal degeneration?

Neural stem cells (NSC) are cells that have the capacity to generate multiple types of differentiated brain cells. In conditions in which there is a loss of key functional cell groups, such as neurons, inducing or introducing neural stem cells to replace the function of those cells that were lost during the disease has the greatest potential therapeutic applications. Indeed, the achievement of one of the main objectives of various investigations is already on the horizon for some conditions, such as Alzheimer's disease. It is not known whether impaired neurogenesis contributes to neuronal depletion and cognitive dysfunction in Alzheimer's disease (AD). The results of the different investigations are controversial; some studies have found that neurogenesis is increased in AD brains, but others have not.

Addressing the complex etiology of Alzheimer’s disease: the role of p25/Cdk5

Alzheimer’s disease (AD) is an age-related neurodegenerative disorder characterized by the progressive loss of forebrain neurons and the deterioration of learning and memory. Therapies for AD have primarily focused upon either the inhibition of amyloid synthesis or its deposition in the brain, but clinical testing to date has not yet found an effective amelioration of cognitive symptoms. Synaptic loss closely correlates with the degree of dementia in AD patients. However, mouse AD models that target the amyloid-β pathway generally do not exhibit a profound loss of synapses, despite extensive synaptic dysfunction. The increased generation of p25, an activator of the cyclin-dependent kinase 5 (Cdk5) has been found in both human patients and mouse models of neurodegeneration. The current work reviews our knowledge, to date, on the role of p25/Cdk5 in Alzheimer’s disease, with a focus upon the interaction of amyloid-β and p25/Cdk5 in synaptic dysfunction and neuronal loss.