Looking for novel functions of tau.

The lack or excess of the protein tau can be deleterious for neurons. The absence of tau can result in retarded neurogenesis and neuronal differentiation, although adult mice deficient in tau are viable, probably because of the compensation of the loss of tau by other MAPs (microtubule-associated proteins). On the contrary, the overexpression of tau can be toxic for the cell. One way to reduce intracellular tau levels can be achieved by its secretion through microvesicles to the extracellular space. Furthermore, tau can be found in the extracellular space because of the neuronal cell death occurring in neurodegenerative disorders such as Alzheimer's disease. The presence of toxic extracellular tau could be the mechanism for the spreading of tau pathology in these neurodegenerative disorders.

The mixture of "ecstasy" and its metabolites impairs mitochondrial fusion/fission equilibrium and trafficking in hippocampal neurons, at in vivo relevant concentrations.

3,4-Methylenedioxymethamphetamine (MDMA; "ecstasy") is a potentially neurotoxic recreational drug of abuse. Though the mechanisms involved are still not completely understood, formation of reactive metabolites and mitochondrial dysfunction contribute to MDMA-related neurotoxicity. Neuronal mitochondrial trafficking, and their targeting to synapses, is essential for proper neuronal function and survival, rendering neurons particularly vulnerable to mitochondrial dysfunction. Indeed, MDMA-associated disruption of Ca(2+) homeostasis and ATP depletion have been described in neurons, thus suggesting possible MDMA interference on mitochondrial dynamics. In this study, we performed real-time functional experiments of mitochondrial trafficking to explore the role of in situ mitochondrial dysfunction in MDMA's neurotoxic actions. We show that the mixture of MDMA and six of its major in vivo metabolites, each compound at 10µM, impaired mitochondrial trafficking and increased the fragmentation of axonal mitochondria in cultured hippocampal neurons. Furthermore, the overexpression of mitofusin 2 (Mfn2) or dynamin-related protein 1 (Drp1) K38A constructs almost completely rescued the trafficking deficits caused by this mixture. Finally, in hippocampal neurons overexpressing a Mfn2 mutant, Mfn2 R94Q, with impaired fusion and transport properties, it was confirmed that a dysregulation of mitochondrial fission/fusion events greatly contributed to the reported trafficking phenotype. In conclusion, our study demonstrated, for the first time, that the mixture of MDMA and its metabolites, at concentrations relevant to the in vivo scenario, impaired mitochondrial trafficking and increased mitochondrial fragmentation in hippocampal neurons, thus providing a new insight in the context of "ecstasy"-induced neuronal injury.

Tau and neuron aging.

Tau protein could appear like a family of multiple isoforms rising by alternative splicing of its nuclear RNA or by different posttranslational modifications. The levels (or proportion) of these different tau isoforms could change in different neurons during development, aging or disease (tauopathies) in mammals. It is discussed that in some disorders there is a gain of toxic function of modified tau, due to the phosphorylation or aggregation of tau protein. These phenotypic changes are mainly found in aging organisms. On the other hand, loss of tau function could facilitate the appearance of some defects (related to iron toxicity) in aging animals lacking tau.

Altered expression of brain acetylcholinesterase in FTDP-17 human tau transgenic mice.

Pathological hyperphosphorylation and aggregation of the tau protein is associated with dementia and can be the central cause of neurodegeneration. Here, we examined potential alterations in the level of the cholinergic enzyme acetylcholinesterase (AChE) in the brain of transgenic mice (Tg-VLW) expressing human tau mutations. Overexpression of mutant hyperphosphorylated tau (P-tau) led to an increase in the activity of AChE in the brain of Tg-VLW mice, paralleled by an increase in AChE protein and transcripts; whereas the levels of the enzyme choline acetyltransferase remained unaffected. VLW tau overexpression in SH-SY5Y cells also increased AChE activity levels. All major molecular forms of AChE were increased in the Tg-VLW mice, including tetrameric AChE, which is the major species involved in hydrolysis of acetylcholine in the brain. Colocalization of human P-tau and AChE supports the conclusion that P-tau can act to increase AChE. This study is the first direct evidence of a modulatory effect of P-tau on brain AChE expression.

Different susceptibility to neurodegeneration of dorsal and ventral hippocampal dentate gyrus: a study with transgenic mice overexpressing GSK3ß.

Dorsal hippocampal regions are involved in memory and learning processes, while ventral areas are related to emotional and anxiety processes. Hippocampal dependent memory and behaviour alterations do not always come out in neurodegenerative diseases at the same time. In this study we have tested the hypothesis that dorsal and ventral dentate gyrus (DG) regions respond in a different manner to increased glycogen synthase kinase-3ß (GSK3ß) levels in GSK3ß transgenic mice, a genetic model of neurodegeneration. Reactive astrocytosis indicate tissue stress in dorsal DG, while ventral area does not show that marker. These changes occurred with a significant reduction of total cell number and with a significantly higher level of cell death in dorsal area than in ventral one as measured by fractin-positive cells. Biochemistry analysis showed higher levels of phosphorylated GSK3ß in those residues that inactivate the enzyme in hippocampal ventral areas compared with dorsal area suggesting that the observed susceptibility is in part due to different GSK3 regulation. Previous studies carried out with this animal model had demonstrated impairment in Morris Water Maze and Object recognition tests point out to dorsal hippocampal atrophy. Here, we show that two tests used to evaluate emotional status, the light-dark box and the novelty suppressed feeding test, suggest that GSK3ß mice do not show any anxiety-related disorder. Thus, our results demonstrate that in vivo overexpression of GSK3ß results in dorsal but not ventral hippocampal DG neurodegeneration and suggest that both areas do not behave in a similar manner in neurodegenerative processes.

Is tau a suitable therapeutical target in tauopathies?

Tau is an intracellular protein, found mainly in neurons, but it can also be found in the extracellular space in pathological situations. Here we discuss whether intracellular tau, in aggregated form or modified by phosphorylation, could be toxic inside a neuron. On the other hand, it has been proposed that extracellular tau could be toxic. In this review, we address the question if the elimination of tau would be a possible therapeutic method to avoid tauopathy disorder and we suggest ways to eliminate intracellular and extracellular tau as treatment.

Tau deficiency leads to the upregulation of BAF-57, a protein involved in neuron-specific gene repression.

Although tau is mainly located in the cell cytoplasm, mostly bound to tubulin, it may also be found in the nucleus of neurons. Hence, we tested whether tau might play a role in regulating the expression of certain genes by comparing gene expression in mice containing or lacking the tau protein. Our results identified a significant difference in the expression of the smarce1 gene, which codes for the BAF-57 protein, a protein involved in the repression of neuron specific genes. These data suggest a role for tau in neuron maturation.

The role of GSK3 in Alzheimer disease.

Mutations in app, ps-1 and ps-2 genes result in the appearance of Familial Alzheimer disease (FAD). Although, in many cases, those mutations result in an increase of the amount of beta amyloid peptide, there is not a clear correlation between that amount and the time of the onset of the disease. Thus, other factors may explain how mutations in those genes result in the appearance of neurodegeneration. In this minireview we propose that GSK3 could be one of those factors.

Function of tau protein in adult newborn neurons.

Levels of tau phosphorylation are high during the developmental period of intense neurite outgrowth, but decrease later. We here investigated whether tau protein plays a role in adult neurogenesis. First we demonstrate that new neurons generated in the subgranular zone express tau in a hyperphosphorylated form. Phospho-tau expression colocalized with doublecortin but not with glial fibrillary acidic protein, Ki67 or calbindin. The same was observed in the subventricular zone. Tau knockout mice did not show a significant decrease in the number of doublecortin-positive cells, although a deficit in migration was observed. These findings suggest that basal tau phosphorylation present in adult animals is in part due to neurogenesis, and from Tau knockout mice it seems that tau is involved in normal migration of new neurons.

Tau as a molecular marker of development, aging and neurodegenerative disorders.

The purpose of this work is to review the changes that take place in the microtubule associated protein tau during neuronal development, aging and neurodegeneration. Human tau protein is expressed from a single gene located on chromosome 17. The DNA is transcribed into nuclear RNA and this RNA, by alternative splicing, yields different mRNA species which are developmentally regulated. In aging, or in neurodegenerative disorders, post translational modifications of tau, such as phosphorylation, could take place, and new tau isoforms may appear. Thus, tau isoforms can be used as markers to follow neuronal development, aging or neurodegeneration.