Histone deacetylase 6 interacts with the microtubule-associated protein tau

Epigenetic regulation in the pathophysiology of Alzheimer's disease

With the aging of the population, the growing incidence and prevalence of Alzheimer's disease (AD) increases the burden on individuals and society as a whole. To date, the pathophysiology of AD is not yet fully understood. Recent studies have suggested that epigenetic mechanisms may play a pivotal role in its course and development. The most frequently studied epigenetic mechanisms are DNA methylation and histone modifications, and investigations relevant to aging and AD are presented in this review. Various studies on human postmortem brain samples and peripheral leukocytes, as well as transgenic animal models and cell culture studies relevant to AD will be discussed. From those, it is clear that aging and AD are associated with epigenetic dysregulation at various levels. Moreover, data on e.g. twin studies in AD support the notion that epigenetic mechanisms mediate the risk for AD. Conversely, it is still not fully clear whether the observed epigenetic changes actually represent a cause or a consequence of the disease. This is mainly due to the fact that most clinical investigations on epigenetics in AD are conducted in samples of patients already in an advanced stage of the disease. Evidently, more research is needed in order to clarify the exact role of epigenetic regulation in the course and development of AD. Research on earlier stages of the disease could provide more insight into its underlying pathophysiology, possibly contributing to the establishment of early diagnosis and the development of more effective treatment strategies.

Searching for disease modifiers-PKC activation and HDAC inhibition - a dual drug approach to Alzheimer's disease that decreases Abeta production while blocking oxidative stress

A series of benzolactam compounds were synthesized, some of which caused a concentration-dependent increase in sAPPalpha and decrease in Abeta production in the concentration range of 0.1-10 microM. Moreover, some compounds showed neuroprotective effects in the 10-20 microM range in the HCA cortical neuron model of oxidative stress and no toxicity in measurements of neuron viability by MTT assay, even at the highest concentrations tested (20 microM). Alzheimer's disease (AD) is a well-studied neurodegenerative process characterized by the presence of amyloid plaques and neurofibrillary tangles. In this study, a series of protein kinase C (PKC) activators were investigated, some of which also exhibit histone deacetylase (HDAC) inhibitory activity, under the hypothesis that such compounds might provide a new path forward in the discovery of drugs for the treatment of AD. The PKC-activating properties of these drugs were expected to enhance the alpha-secretase pathway in the processing of amyloid precursor protein (APP), while their HDAC inhibition was anticipated to confer neuroprotective activity. We found that benzolactams 9 and 11-14 caused a concentration-dependent increase in sAPPalpha and decrease in beta-amyloid (Abeta) production in the concentration range of 0.1-10 microM, consistent with a shift of APP metabolism toward the alpha-secretase-processing pathway. Moreover, compounds 9-14 showed neuroprotective effects in the 10-20 microM range in the homocysteate (HCA) cortical neuron model of oxidative stress. In parallel, we found that the most neuroprotective compounds caused increased levels of histone acetylation (H4), thus indicating their likely ability to inhibit HDAC activity. As the majority of the compounds studied also show nanomolar binding affinities for PKC, we conclude that it is possible to design, de novo, agents that combine both PKC-activating properties along with HDAC inhibitory properties. Such agents would be capable of modulating amyloid processing while showing neuroprotection. These findings may offer a new approach to therapies that exhibit disease-modifying effects, as opposed to symptomatic relief, in the treatment of AD.

Regulation of microtubule dynamics by inhibition of the tubulin deacetylase HDAC6

We studied the role of a class II histone deacetylase, HDAC6, known to function as a potent alpha-tubulin deacetylase, in the regulation of microtubule dynamics. Treatment of cells with the class I and II histone deacetylase inhibitor TSA, as well as the selective HDAC6 inhibitor tubacin, increased microtubule acetylation and significantly reduced velocities of microtubule growth and shrinkage. siRNA-mediated knockdown of HDAC6 also increased microtubule acetylation but, surprisingly, had no effect on microtubule growth velocity. At the same time, HDAC6 knockdown abolished the effect of tubacin on microtubule growth, demonstrating that tubacin influences microtubule dynamics via specific inhibition of HDAC6. Thus, the physical presence of HDAC6 with impaired catalytic activity, rather than tubulin acetylation per se, is the factor responsible for the alteration of microtubule growth velocity in HDAC6 inhibitor-treated cells. In support of this notion, HDAC6 mutants bearing inactivating point mutations in either of the two catalytic domains mimicked the effect of HDAC6 inhibitors on microtubule growth velocity. In addition, HDAC6 was found to be physically associated with the microtubule end-tracking protein EB1 and a dynactin core component, Arp1, both of which accumulate at the tips of growing microtubules. We hypothesize that inhibition of HDAC6 catalytic activity may affect microtubule dynamics by promoting the interaction of HDAC6 with tubulin and/or with other microtubule regulatory proteins.

Tau--an inhibitor of deacetylase HDAC6 function

Analysis of brain microtubule protein from patients with Alzheimer's disease showed decreased alpha tubulin levels along with increased acetylation of the alpha tubulin subunit, mainly in those microtubules from neurons containing neurofibrillary tau pathology. To determine the relationship of tau protein and increased tubulin acetylation, we studied the effect of tau on the acetylation-deacetylation of tubulin. Our results indicate that tau binds to the tubulin-deacetylase, histone deacetylase 6 (HDAC6), decreasing its activity with a consequent increase in tubulin acetylation. As expected, increased acetylation was also found in tubulin from wild-type mice compared with tubulin from mice lacking tau because of the tau-mediated inhibition of the deacetylase. In addition, we found that an excess of tau protein, as a HDAC6 inhibitor, prevents induction of autophagy by inhibiting proteasome function.