Can tau filaments be both physiologically beneficial and toxic?

The physiological link between metabolic rate depression and tau phosphorylation in mammalian hibernation

Abnormal phosphorylation and aggregation of tau protein are hallmarks of a variety of neurological disorders, including Alzheimer's disease (AD). Increased tau phosphorylation is assumed to represent an early event in pathogenesis and a pivotal aspect for aggregation and formation of neurofibrillary tangles. However, the regulation of tau phosphorylation in vivo and the causes for its increased stage of phosphorylation in AD are still not well understood, a fact that is primarily based on the lack of adequate animal models. Recently we described the reversible formation of highly phosphorylated tau protein in hibernating European ground squirrels. Hence, mammalian hibernation represents a model system very well suited to study molecular mechanisms of both tau phosphorylation and dephosphorylation under in vivo physiological conditions. Here, we analysed the extent and kinetics of hibernation-state dependent tau phosphorylation in various brain regions of three species of hibernating mammals: arctic ground squirrels, Syrian hamsters and black bears. Overall, tau protein was highly phosphorylated in torpor states and phosphorylation levels decreased after arousal in all species. Differences between brain regions, hibernation-states and phosphosites were observed with respect to degree and kinetics of tau phosphorylation. Furthermore, we tested the phosphate net turnover of tau protein to analyse potential alterations in kinase and/or phosphatase activities during hibernation. Our results demonstrate that the hibernation-state dependent phosphorylation of tau protein is specifically regulated but involves, in addition, passive, temperature driven regulatory mechanisms. By determining the activity-state profile for key enzymes of tau phosphorylation we could identify kinases potentially involved in the differentially regulated, reversible tau phosphorylation that occurs during hibernation. We show that in black bears hibernation is associated with conformational changes of highly phosphorylated tau protein that are typically related to neuropathological alterations. The particular hibernation characteristics of black bears with a continuous torpor period and an only slightly decreased body temperature, therefore, potentially reflects the limitations of this adaptive reaction pattern and, thus, might indicate a transitional state of a physiological process.

Cortical and hippocampal neurons from truncated tau transgenic rat express multiple markers of neurodegeneration

Transition of protein tau from physiologically unfolded to misfolded state represent enigmatic step in the pathogenesis of tauopathies including Alzheimer's disease (AD). Major molecular events playing role in this process involve truncation and hyperphosphorylation of tau protein, which are accompanied by redox imbalance followed by functional deterioration of neuronal network. Recently we have developed transgenic rat model showing that expression of truncated tau causes neurofibrillary degeneration similar to that observed in brain of AD sufferers. Consequently we tested cortical and hippocampal neuronal cultures extracted from this model as a convenient tool for development of molecules able to target the mechanisms leading to and/or enhancing the process of neurodegeneration. Here we document three major pathological features typical for tauopathies and AD in cortical and hippocampal neurons from transgenic rat in vitro. First, an increased accumulation of human truncated tau in neurons; second, the hyperphosphorylation of truncated tau on the epitopes characteristic of AD (Ser202/Thr205 and Thr231); and third, increased vulnerability of the neurons to nitrative and oxidative stress. Our results show that primary neurons expressing human truncated tau could represent a cellular model for targeting tau related pathological events, namely, aberrant tau protein accumulation, tau hyperphosphorylation, and oxidative/nitrative damage. These characteristics make the model particularly suitable for detailed study of molecular mechanisms of tau induced neurodegeneration and easily applicable for drug screening.

Trends in the molecular pathogenesis and clinical therapeutics of common neurodegenerative disorders

The term neurodegenerative disorders, encompasses a variety of underlying conditions, sporadic and/or familial and are characterized by the persistent loss of neuronal subtypes. These disorders can disrupt molecular pathways, synapses, neuronal subpopulations and local circuits in specific brain regions, as well as higher-order neural networks. Abnormal network activities may result in a vicious cycle, further impairing the integrity and functions of neurons and synapses, for example, through aberrant excitation or inhibition. The most common neurodegenerative disorders are Alzheimer’s disease, Parkinson’s disease, Amyotrophic Lateral Sclerosis and Huntington’s disease. The molecular features of these disorders have been extensively researched and various unique neurotherapeutic interventions have been developed. However, there is an enormous coercion to integrate the existing knowledge in order to intensify the reliability with which neurodegenerative disorders can be diagnosed and treated. The objective of this review article is therefore to assimilate these disorders’ in terms of their neuropathology, neurogenetics, etiology, trends in pharmacological treatment, clinical management, and the use of innovative neurotherapeutic interventions.

The amino terminus of tau inhibits kinesin-dependent axonal transport: implications for filament toxicity

The neuropathology of Alzheimer's disease (AD) and other tauopathies is characterized by filamentous deposits of the microtubule-associated protein tau, but the relationship between tau polymerization and neurotoxicity is unknown. Here, we examined effects of filamentous tau on fast axonal transport (FAT) using isolated squid axoplasm. Monomeric and filamentous forms of recombinant human tau were perfused in axoplasm, and their effects on kinesin- and dynein-dependent FAT rates were evaluated by video microscopy. Although perfusion of monomeric tau at physiological concentrations showed no effect, tau filaments at the same concentrations selectively inhibited anterograde (kinesin-dependent) FAT, triggering the release of conventional kinesin from axoplasmic vesicles. Pharmacological experiments indicated that the effect of tau filaments on FAT is mediated by protein phosphatase 1 (PP1) and glycogen synthase kinase-3 (GSK-3) activities. Moreover, deletion analysis suggested that these effects depend on a conserved 18-amino-acid sequence at the amino terminus of tau. Interestingly, monomeric tau isoforms lacking the C-terminal half of the molecule (including the microtubule binding region) recapitulated the effects of full-length filamentous tau. Our results suggest that pathological tau aggregation contributes to neurodegeneration by altering a regulatory pathway for FAT.

alpha7 Nicotinic receptor gene delivery into mouse hippocampal neurons leads to functional receptor expression, improved spatial memory-related performance, and tau hyperphosphorylation

Brain alpha7 nicotinic receptors have become therapeutic targets for Alzheimer's disease (AD) based on their memory-enhancing and neuroprotective actions. This study investigated the feasibility of increasing neuronal alpha7 receptor functions using a gene delivery approach based on neuron-selective recombinant adeno-associated virus (rAAV)-derived vectors. In order to determine whether alpha7 receptor-mediated cytotoxicity was dependent on receptor density, rat alpha7 nicotinic receptors were expressed at high concentrations in GH4C1 cells as measured with nicotine-displaceable [3H]methyllycaconitine (MLA) binding. The potency of GTS-21 (an alpha7 receptor agonist) to induce cell loss was similar in these cells to that seen in pheochromocytoma (PC12) cells expressing nine-times-lower receptor levels, suggesting that cytotoxicity was more dependent on agonist concentration than receptor density. Hippocampal transduction with rat alpha7 nicotinic receptors increased [3H]MLA binding in this region in wild type and alpha7 receptor-knockout (KO) mice without apparent cytotoxicity. No difference was observed in Kd values for MLA binding between endogenous and transgenic receptors. Single cell recordings demonstrated that dentate granule cells that normally have no alpha7 receptor response did so following alpha7 receptor gene delivery in wild type mice. Recovery of alpha7 function was also observed in stratum oriens and stratum radiatum neurons of KO mice following gene delivery. Wild type mice exhibited improved acquisition performance in the Morris water task 1 month after bilateral hippocampal transductions with the rat alpha7 receptor gene compared with green fluorescent protein-transduced controls. However, both groups reached similar training levels and there was no difference in subsequent probe performance. Finally, this gene delivery approach was used to test whether alpha7 receptors affect tau-phosphorylation. Chronic (i.e. 2 month but not 2 week) expression of high levels of alpha7 receptors in hippocampus increased AT8 staining characteristic of hyperphosphorylated tau in that region, indicating that endogenous agonist-mediated receptor activation may be able to modulate this process.

Cholesterol, statins and tau

Many of the known risk factors for Alzheimer's disease (AD) are associated with cholesterol metabolism. Interestingly, it seems as if higher doses of statins, i.e. inhibitors of the cholesterol biosynthesis by blocking formation of mevalonate, might lower the progression of AD. The mechanisms, however, by which statins or cholesterol levels exert their influence are unknown. A hereditary cholesterol-storage disorder, Niemann Pick C, shows Alzheimer-like tau-pathology in youth or adolescence but with no amyloid plaques. This gives rise to the possibility that disturbances in cholesterol metabolism induce changes in tau without interposition of Abeta-protein aggregates. Experimental data suggest that manipulation of cholesterol levels may lead to changes in tau phosphorylation. These changes vary depending on how cholesterol metabolism is manipulated. Effects seem to be either mild and transient, or drastic and related to neurodegeneration, or independent of the mevalonate pathway.

Misregulation of tau alternative splicing in neurodegeneration and dementia

Tau is a microtubule-associated protein that fulfills several functions critical for neuronal formation and health. Tau discharges its functions by producing multiple isoforms via intricately regulated alternative splicing. These isoforms modulate tau function in normal brain by altering the domains of the protein, thereby influencing its conformation and post-translational modifications and hence its affinity for microtubules and other ligands. Disturbances in tau expression result in disruption of the neuronal cytoskeleton and formation of pathological tau structures (neurofibrillary tangles) found in brains of dementia sufferers. More specifically, aberrations in tau splicing regulation directly cause several neurodegenerative diseases that lead to dementia. This review briefly presents our cumulative knowledge of tau splicing regulation in connection with the alterations in tau splicing seen in neurodegeneration.

Beta-site APP cleaving enzyme 1 (BACE1) is increased in remaining neurons in Alzheimer's disease brains

Alzheimer's disease (AD) is characterized by the extensive deposition of amyloid beta protein (Abeta) in the brain cortex. Abeta is produced from beta-amyloid precursor protein (APP) by beta-secretase and gamma-secretase. beta-Secretase has been identified as beta-site APP cleaving enzyme1 (BACE1). We produced rabbit polyclonal antibodies against the amino and the carboxyl terminals of BACE1. Using these antibodies, BACE1 was characterized in temporal lobe cortices by Western blotting and immunohistochemistry. Immunohistochemical studies employing anti-GFAP and anti-MAP2 antibodies as well as anti-BACE1 antibodies showed that BACE1 was expressed exclusively in neurons but not in glial cells. Brain samples were directly extracted by 0.5% SDS and analyzed by Western blotting and densitometer. Although the mean level of BACE1/mg protein in AD brains was not increased, the ratio of BACE1 to MAP2 or to NSE was significantly increased compared with that in control brains. Taken together, these findings suggest that those neurons that survive in AD brains might generate more BACE1 than normal neurons in control brains, indicating that increased BACE1 activity could be one of the causes of AD. This could justify the development of anti-BACE1 drugs for AD treatment.