HDAC4 is a Class II histone deacetylase (HDAC) that is highly expressed in the brain but whose functional significance in the brain is not known. We show that forced expression of HDAC4 in cerebellar granule neurons protects them against low potassium-induced apoptosis. HDAC4 also partially protects cultured cortical neurons from 6-hydroxydopamine-induced neurotoxicity and HT22 neuroblastoma cells from death induced by oxidative stress. HDAC4-mediated neuroprotection does not require its HDAC catalytic domain and cannot be inhibited by chemical inhibitors of HDACs. Neuroprotection by HDAC4 also does not require the Raf-MEK-ERK or the PI-3 kinase -Akt signaling pathways, and occurs despite the activation of c-jun, an event that is generally believed to condemn neurons to die. The protective action of HDAC4 occurs in the nucleus and is mediated by a region that contains the nuclear localization signal. HDAC4 inhibits the activity of cyclin-dependent kinase-1 (CDK1) and the progression of proliferating HEK293T and HT22 cells through the cell cycle. Mice lacking HDAC4 have elevated CDK1 activity and display cerebellar abnormalities including a progressive loss of Purkinje neurons postnatally in posterior lobes. Surviving Purkinje neurons in these lobes have duplicated soma. Furthermore, large numbers of cells within these affected lobes incorporate BrdU, indicating cell cycle progression. These abnormalities along with the ability of HDAC4 to inhibit CDK1 and cell cycle progression in cultured cells suggest that neuroprotection by HDAC4 is mediated by preventing abortive cell cycle progression. KeywordsHDAC4; apoptosis; neuronal survival; histone deacetylase; cell cycle Histone deacetylases (HDACs) are the catalytic subunits of multiprotein complexes that deacetylate specific lysines in the tail residues of histones, resulting in the compactation of chromatin into a transcriptionally repressed state (for review, see de Ruijter et al., 2003;Verdin et al., 2003;Yang and Gregoire, 2005). Although best studied for their effects on histones and transcriptional activity, it is now known that HDACs regulate the acetylation status of a number of other non-histone proteins suggesting complex functions of HDACs (for review, see de Ruijter et al., 2003;Verdin et al., 2003). In fact, HDACs have been shown to participate, or have been implicated in a variety of cellular functions including cell transformation, proliferation, senescence, differentiation, survival, and death (for review, see Verdin et al., 2003;Yang and Gregoire, 2005 NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author ManuscriptVertebrates express at least 18 distinct HDACs which have been grouped into four classes based on their similarity with yeast HDACs (for review, see de Ruijter et al., 2003;Verdin et al., 2003;Yang and Gregoire, 2005). Class I HDACs (HDACs 1, 2, 3 and 8) are composed primarily of a catalytic domain. These HDACs are ubiquitously expressed, localized to the nucleus, and serve as transcriptional repressors. Class II HDACs ...

Apoptosis is an essential aspect of normal nervous system development, but when aberrantly activated, apoptosis leads to undesirable neuronal death, and such inappropriate neuronal loss is the hallmark of a variety of neurodegenerative diseases and neurological conditions, such as stroke or traumatic brain injury. The mechanisms underlying the regulation of apoptosis are beginning to be understood. Among the molecules that have recently been implicated are the histone deacetylases (HDACs). HDACs are the catalytic subunits of multiprotein complexes that deacetylate histones (11,42). The action of HDACs is opposed by histone acetyltransferases (HATs) such as CREB-binding protein and p300, which catalyze the transfer of an acetyl moiety from acetyl-coenzyme A to specific lysine residues of histones (25). Acetylation of histones relaxes the chromatin structure to a state that is transcriptionally active, while histone deacetylation transforms chromatin to a transcriptionally repressed state (25). Hence, gene expression is regulated, in part, by the balance of HDAC and HAT activities. Although best studied for their effects on histones and transcriptional activity, it is now known that HDACs and HATs regulate the acetylation of a number of other nonhistone proteins, such as p53, p65/RelA, E2F1, GATA1, and MyoD, suggesting complex functions of HDACs in different cellular processes (11,42). Precisely which cellular functions are involved is currently the subject of intense investigation.Vertebrates express at least 18 distinct HDACs, which have been grouped into three classes based on their similarities with Saccharomyces cerevisiae HDACs (11,42 Class I HDACs consist of little more than a deacetylase domain and function as transcriptional repressors. They generally are nuclear proteins expressed in most tissue and cell types (11,42). On the other hand, members of the class II HDAC subfamily display cell type-restricted patterns of expression and contain a large extended N-terminal extension with which a variety of signaling proteins interact, including MEF2, HP1␣, Bcl6, CtBP, calmodulin,42). Phosphorylation of conserved serine residues in class II HDACs by calcium/calmodulin-dependent kinase (CaMK) or protein kinase D in response to specific stimuli creates docking sites for the 14-3-3 family of protein chaperones (11,28,31,42). Binding of 14-3-3 results in the export of these HDACs from the nucleus and disrupts their interactions with transcriptional corepressor proteins, resulting in derepression of their target genes.Several classes of small-molecule HDAC inhibitors have been identified (11,29 (11,29). Because of their ability to induce the death of transformed cells, HDAC inhibitors are in clinical trials for the treatment of cancers. It is noteworthy, however, that while there are small differences in the sensitivities of individual class I and class II HDACs to different inhibitors, most of the commonly used inhibitors inhibit all HDACs efficiently. The significance of individual HDACs in any biological effect ha...

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Neurodegenerative disease strikes millions worldwide and there is mounting evidence suggesting that underlying the onset and progression of these debilitating diseases is inappropriate neuronal apoptosis. Recent reports have implicated a family of proteins known as histone deacetylases (HDACs) in various neuronal processes including the neuronal death program. Initial headway in this field has been made largely through the use of broad-spectrum HDAC inhibitors. In fact, pharmacological inhibition of HDAC activity has been shown to protect neurons in several models of neurodegeneration. The observation that HDAC inhibitors can have opposing effects in different paradigms of neurodegeneration suggests that individual members of the HDAC protein family may play distinct roles that could depend on the specific cell type under study. The purpose of this review is to detail work involving the use of HDAC inhibitors within the context of neurodegeneration and examine the roles of individual HDAC members in the nervous system with specific focus on neuronal cell death.

Increasing evidence suggests that neuronal apoptosis is triggered by the inappropriate activation of cyclin-dependent kinases leading to an abortive re-entry of neurons into the cell cycle. Pharmacological inhibitors of cell-cycle progression may therefore have value in the treatment of neurodegenerative diseases in humans. GW8510 is a 3¢ substituted indolone that was developed recently as an inhibitor of cyclin-dependent kinase 2 (CDK2). We found that GW8510 inhibits the death of cerebellar granule neurons caused by switching them from high potassium (HK) medium to low potassium (LK) medium. Although GW8510 inhibits CDK2 and other CDKs when tested in in vitro biochemical assays, when used on cultured neurons it only inhibits CDK5, a cytoplasmic CDK that is not associated with cell-cycle progression. Treatment of cultured HEK293T cells with GW8510 does not inhibit cell-cycle progression, consistent with its inability to inhibit mitotic CDKs in intact cells. Neuroprotection by GW8510 is independent of Akt and MEK-ERK signaling. Furthermore, GW8510 does not block the LK-induced activation of Gsk3b and, while inhibiting c-jun phosphorylation, does not inhibit the increase in c-jun expression observed in apoptotic neurons. We also examined the effectiveness of other 3¢ substituted indolone compounds to protect against neuronal apoptosis. We found that like GW8510, the VEGF Receptor 2 Kinase Inhibitors [3-(1H-pyrrol-2-ylmethylene)-1,3-dihydroindol-2-one], {(Z)-3-[2,4-Dimethyl-3-(ethoxycarbonyl)pyrrol-5-yl)methylidenyl]indol-2-one} and [(Z)-5-Bromo-3-(4,5,6,6-tetrahydro-1H-indol-2-ylmethylene)-1,3-dihydroindol-2-one], the Src family kinase inhibitor SU6656 and a commercially available inactive structural analog of an RNA-dependent protein kinase inhibitor 5-Chloro-3-(3,5-dichloro-4-hydroxybenzylidene)-1,3-dihydro-indol-2-one, are all neuroprotective when tested on LK-treated neurons. Along with our recent identification of the c-Raf inhibitor GW5074 (also a 3¢ substituted indolone) as a neuroprotective compound, our findings identify the 3¢ substituted indolone as a core structure for the designing of neuroprotective drugs that may be used to treat neurodegenerative diseases in humans.

Neurodegenerative diseases affect millions of patients annually and are a significant burden on the health care systems around the world. While there are symptomatic remedies for patients suffering from various neurodegenerative diseases, there are no cures as of today. Cell death by apoptosis is a common hallmark of neurodegeneration. Therefore, deciphering the molecular pathways regulating this process is of significant value to scientists' endeavor to understand neurodegenerative disorders. Efforts along these lines have uncovered a number of molecular pathways that regulate neuronal apoptosis. Recently, a family of proteins known as histone deacetylases (HDACs) has been linked to regulation of cell survival as well as death. The focus of this review is to summarize our current understanding of the role of HDACs and in particular a subgroup of proteins in this family classified as class IIa HDACs in the regulation of neuronal cell death. It is apparent based on the information presented in this review that although very similar in their primary sequence, members of this family of proteins often have distinct roles in orchestrating apoptotic cell death in the brain.