Osteogenesis is a complex process associated with dramatic changes in gene expression. To elucidate whether modifications in chromatin structure are involved in osteoblast differentiation, we examined the expression levels of histone deacetylases (HDACs) and the degree of histone acetylation at the promoter regions of osteogenic genes. During osteogenesis, total HDAC enzymatic activity was decreased with significant reduction in HDAC1 expression. Consistently, recruitment of HDAC1 to the promoters of osteoblast marker genes, including osterix and osteocalcin, was down-regulated, whereas histone H3 and H4 were hyperacetylated at those promoters during osteoblast differentiation. Moreover, suppression of HDAC activity with a HDAC inhibitor, sodium butyrate, accelerated osteogenesis by inducing osteoblast marker genes including osteopontin and alkaline phosphatase. Consistently, knockdown of HDAC1 by the short interference RNA system stimulated osteoblast differentiation. Taken together, these data propose that down-regulation of HDAC1 is an important process for osteogenesis.

The enhancer of zeste homolog 2 (EZH2) is upregulated and has an oncogenic role in several types of human cancer. However, the abnormalities of EZH2 and its underlying mechanisms in the pathogenesis of nasopharyngeal carcinoma (NPC) remain unknown. In this study, we found that high expression of EZH2 in NPC was associated closely with an aggressive and/or poor prognostic phenotype (P<0.05). In NPC cell lines, knockdown of EZH2 by short hairpin RNA was sufficient to inhibit cell invasiveness/metastasis both in vitro and in vivo, whereas ectopic overexpression of EZH2 supported NPC cell invasive capacity with a decreased expression of E-cadherin. In addition, ablation of endogenous Snail in NPC cells virtually totally prevented the repressive activity of EZH2 to E-cadherin, indicating that Snail might be a predominant mediator of EZH2 to suppress E-cadherin. Furthermore, co-immunoprecipitation (IP), chromatin IP and luciferase reporter assays demonstrated that in NPC cells, (1) EZH2 interacted with HDAC1/HDAC2 and Snail to form a repressive complex; (2) these components interact in a linear fashion, not in a triangular fashion, that is, HDAC1 or HDAC2 bridge the interaction between EZH2 and Snail; and (3) the EZH2/HDAC1/2/Snail complex could closely bind to the E-cadherin promoter by Snail, but not YY1, to repress E-cadherin. The data provided in this report suggest a critical role of EZH2 in the control of cell invasion and/or metastasis by forming a co-repressor complex with HDAC1/HDAC2/Snail to repress E-cadherin, an activity that might be responsible, at least in part, for the development and/or progression of human NPCs.

Although histone deacetylases (HDACs) are generally viewed as corepressors, we show that HDAC1 serves as a coactivator for the glucocorticoid receptor (GR). Furthermore, a subfraction of cellular HDAC1 is acetylated after association with the GR, and this acetylation event correlates with a decrease in promoter activity. HDAC1 in repressed chromatin is highly acetylated, while the deacetylase found on transcriptionally active chromatin manifests a low level of acetylation. Acetylation of purified HDAC1 inactivates its deacetylase activity, and mutation of the critical acetylation sites abrogates HDAC1 function in vivo. We propose that hormone activation of the receptor leads to progressive acetylation of HDAC1 in vivo, which in turn inhibits the deacetylase activity of the enzyme and prevents a deacetylation event that is required for promoter activation. These findings indicate that HDAC1 is required for the induction of some genes by the GR, and this activator function is dynamically modulated by acetylation.

The transcription factor E2F plays a major role in cell cycle control in mammalian cells. E2F binding sites, which are present in the promoters of a variety of genes required for S phase, shift from a negative to a positive role in transcription at the commitment point, a crucial point in G1 that precedes the G1/S transition. Before the commitment point, E2F activity is repressed by members of the pocket proteins family. This repression is believed to be crucial for the proper control of cell growth. We have previously shown that Rb, the founding member of the pocket proteins family, represses E2F1 activity by recruiting the histone deacetylase HDAC1. Here, we show that the two other members of the pocket proteins family, p107 and p130, also are able to interact physically with HDAC1 in live cells. HDAC1 interacts with p107 and Rb through an "LXCXE"-like motif, similar to that used by viral transforming proteins to bind and inactivate pocket proteins. Indeed, we find that the viral transforming protein E1A competes with HDAC1 for p107 interaction. We also demonstrate that p107 is able to interact simultaneously with HDAC1 and E2F4, suggesting a model in which p107 recruits HDAC1 to repress E2F sites. Indeed, we demonstrate that histone deacetylase activity is involved in the p107- or p130-induced repression of E2F4. Taken together, our data suggest that all members of the E2F family are regulated in early G1 by similar complexes, containing a pocket protein and the histone deacetylase HDAC1.

Although mammalian MBD3 contains the mCpG-binding domain (MBD) and is highly homologous with the authentic mCpG-binding protein MBD2, it was reported that the protein does not bind to mCpG specifically. Using recombinant human wild type and mutant MBD3 proteins, we demonstrated that atypical amino acids found in MBD3 MBD, namely, His-30 and Phe-34, are responsible for the inability of MBD3 to bind to mCpG. Interestingly, although H30K/F34Y MBD3 mutant protein binds to mCpG efficiently in vitro, it was not localized at the mCpG-rich pericentromeric regions in mouse cells. We also showed that Y34F MBD2b MBD, which possesses not the mCpG-specific DNA-binding activity but the nonspecific DNA-binding activity, was localized at the pericentromeric regions. These results suggested that the mCpG-specific DNA-binding activity is largely dispensable, and another factor(s) is required for the localization of MBD proteins in vivo. MBD3 was identified as a component of the NuRD/Mi2 complex that shows chromatin remodeling and histone deacetylase activities. We demonstrated that MBD3 MBD is necessary and sufficient for binding to HDAC1 and MTA2, two components of the NuRD/Mi2 complex. It was therefore suggested that mCpG-binding-defective MBD3 has evolutionarily conserved its MBD because of the secondary role played by the MBD in protein-protein interactions.

Cancer cells exhibit a set of unique properties that distinguish them from their normal counterparts. Among these features are increased growth rates, loss of differentiation, escape from cell death pathways, evasion of anti-proliferative signals, a decreased reliance on exogenous growth factors and escape from replicative senescence. Acquisition of these features by malignant cells requires impairment of normal cellular control mechanisms. Over the past few years, it has become increasingly apparent that an important subset of the molecular changes commonly found in cancer cells involves inappropriate regulation of gene expression. This review will address regulatory pathways whose disruption contributes to the malignant phenotype. The failure to deacetylate and thus repress transcription by the Class I histone deacetylases HDAC1 and HDAC2 due to disruption of the Rb family of proteins has been firmly established as a mechanism leading to increases in growth rate and cellular proliferation. Recent data suggest that this regulatory circuit also executes G(1) checkpoint arrest downstream of DNA damage, cellular senescence and contact inhibition. In contrast to this failure to deacetylate, it now seems probable that changes in differentiation status may result in part from inappropriate deacetylation and concomitant transcriptional repression mediated by the Class II histone deacetylases. This inappropriate deacetylation by HDAC4, HDAC5 and HDAC6 follows their relocalization from the cytoplasm to the nucleus. Thus, multiple classical features of cancer cells can be manifested by improper histone deacetylation.

Breast cancer metastasis suppressor 1 (BRMS1) suppresses metastasis of multiple human and murine cancer cells without inhibiting tumorigenicity. By yeast two-hybrid and co-immunoprecipitation, BRMS1 interacts with retinoblastoma binding protein 1 and at least seven members of the mSin3 histone deacetylase (HDAC) complex in human breast and melanoma cell lines. BRMS1 co-immunoprecipitates enzymatically active HDAC proteins and represses transcription when recruited to a Gal4 promoter in vivo. BRMS1 exists in large mSin3 complex(es) of approximately 1.4-1.9 MDa, but also forms smaller complexes with HDAC1. Deletion analyses show that the carboxyl-terminal 42 amino acids of BRMS1 are not critical for interaction with much of the mSin3 complex and that BRMS1 appears to have more than one binding point to the complex. These results further show that BRMS1 may participate in transcriptional regulation via interaction with the mSin3.HDAC complex and suggest a novel mechanism by which BRMS1 might suppress cancer metastasis.

Enhancer of zeste homolog 2 (EZH2) is the catalytic subunit of the Polycomb-repressive complex 2 (PRC2) that represses gene transcription through histone H3 lysine 27 trimethylation (H3K27me3). Although EZH2 is abundantly present in various cancers, the molecular consequences leading to oncogenesis remain unclear. Here, we show that EZH2 concordantly silences the Wnt pathway antagonists operating at several subcellular compartments, which in turn activate Wnt/β-catenin signaling in hepatocellular carcinomas (HCC). Chromatin immunoprecipitation promoter array and gene expression analyses in HCCs revealed EZH2 occupancy and reduced expression of Wnt antagonists, including the growth-suppressive AXIN2, NKD1, PPP2R2B, PRICKLE1, and SFRP5. Knockdown of EZH2 reduced the promoter occupancy of PRC2, histone deacetylase 1 (HDAC1), and H3K27me3, whereas the activating histone marks were increased, leading to the transcriptional upregulation of the Wnt antagonists. Combinatorial EZH2 and HDAC inhibition dramatically reduced the levels of nuclear β-catenin, T-cell factor-dependent transcriptional activity, and downstream pro-proliferative targets CCND1 and EGFR. Functional analysis revealed that downregulation of EZH2 reduced HCC cell growth, partially through the inhibition of β-catenin signaling. Conversely, ectopic overexpression of EZH2 in immortalized hepatocytes activated Wnt/β-catenin signaling to promote cellular proliferation. In human HCCs, concomitant overexpression of EZH2 and β-catenin was observed in one-third (61/179) of cases and significantly correlated with tumor progression. Our data indicate that EZH2-mediated epigenetic silencing contributes to constitutive activation of Wnt/β-catenin signaling and consequential proliferation of HCC cells, thus representing a novel therapeutic target for this highly malignant tumor.

Autosomal dominant mutations of the RNA/DNA binding protein FUS are linked to familial amyotrophic lateral sclerosis (FALS); however, it is not clear how FUS mutations cause neurodegeneration. Using transgenic mice expressing a common FALS-associated FUS mutation (FUS-R521C mice), we found that mutant FUS proteins formed a stable complex with WT FUS proteins and interfered with the normal interactions between FUS and histone deacetylase 1 (HDAC1). Consequently, FUS-R521C mice exhibited evidence of DNA damage as well as profound dendritic and synaptic phenotypes in brain and spinal cord. To provide insights into these defects, we screened neural genes for nucleotide oxidation and identified brain-derived neurotrophic factor (Bdnf) as a target of FUS-R521C-associated DNA damage and RNA splicing defects in mice. Compared with WT FUS, mutant FUS-R521C proteins formed a more stable complex with Bdnf RNA in electrophoretic mobility shift assays. Stabilization of the FUS/Bdnf RNA complex contributed to Bdnf splicing defects and impaired BDNF signaling through receptor TrkB. Exogenous BDNF only partially restored dendrite phenotype in FUS-R521C neurons, suggesting that BDNF-independent mechanisms may contribute to the defects in these neurons. Indeed, RNA-seq analyses of FUS-R521C spinal cords revealed additional transcription and splicing defects in genes that regulate dendritic growth and synaptic functions. Together, our results provide insight into how gain-of-function FUS mutations affect critical neuronal functions.

BACKGROUND

Post-translational modification of histones resulting in chromatin remodelling plays a key role in the regulation of gene expression. Here we report characteristic patterns of expression of 12 members of 3 classes of chromatin modifier genes in 6 different cancer types: histone acetyltransferases (HATs)- EP300, CREBBP, and PCAF; histone deacetylases (HDACs)- HDAC1, HDAC2, HDAC4, HDAC5, HDAC7A, and SIRT1; and histone methyltransferases (HMTs)- SUV39H1and SUV39H2. Expression of each gene in 225 samples (135 primary tumours, 47 cancer cell lines, and 43 normal tissues) was analysedby QRT-PCR, normalized with 8 housekeeping genes, and given as a ratio by comparison with a universal reference RNA.

RESULTS

This involved a total of 13,000 PCR assays allowing for rigorous analysis by fitting a linear regression model to the data. Mutation analysis of HDAC1, HDAC2, SUV39H1, and SUV39H2 revealed only two out of 181 cancer samples (both cell lines) with significant coding-sequence alterations. Supervised analysis and Independent Component Analysis showed that expression of many of these genes was able to discriminate tumour samples from their normal counterparts. Clustering based on the normalized expression ratios of the 12 genes also showed that most samples were grouped according to tissue type. Using a linear discriminant classifier and internal cross-validation revealed that with as few as 5 of the 12 genes, SIRT1, CREBBP, HDAC7A, HDAC5 and PCAF, most samples were correctly assigned.

CONCLUSIONS

The expression patterns of HATs, HDACs, and HMTs suggest these genes are important in neoplastic transformation and have characteristic patterns of expression depending on tissue of origin, with implications for potential clinical application.

Butyrate, an intestinal microbiota metabolite of dietary fiber, has been shown to exhibit protective effects toward inflammatory diseases such as ulcerative colitis (UC) and inflammation-mediated colorectal cancer. Recent studies have shown that chronic IFN-γ signaling plays an essential role in inflammation-mediated colorectal cancer development in vivo, whereas genome-wide association studies have linked human UC risk loci to IFNG, the gene that encodes IFN-γ. However, the molecular mechanisms underlying the butyrate-IFN-γ-colonic inflammation axis are not well defined. Here we showed that colonic mucosa from patients with UC exhibit increased signal transducer and activator of transcription 1 (STAT1) activation, and this STAT1 hyperactivation is correlated with increased T cell infiltration. Butyrate treatment-induced apoptosis of wild-type T cells but not Fas-deficient (Fas(lpr)) or FasL-deficient (Fas(gld)) T cells, revealing a potential role of Fas-mediated apoptosis of T cells as a mechanism of butyrate function. Histone deacetylase 1 (HDAC1) was found to bind to the Fas promoter in T cells, and butyrate inhibits HDAC1 activity to induce Fas promoter hyperacetylation and Fas upregulation in T cells. Knocking down gpr109a or slc5a8, the genes that encode for receptor and transporter of butyrate, respectively, resulted in altered expression of genes related to multiple inflammatory signaling pathways, including inducible nitric oxide synthase (iNOS), in mouse colonic epithelial cells in vivo. Butyrate effectively inhibited IFN-γ-induced STAT1 activation, resulting in inhibition of iNOS upregulation in human colon epithelial and carcinoma cells in vitro. Our data thus suggest that butyrate delivers a double-hit: induction of T cell apoptosis to eliminate the source of inflammation and suppression of IFN-γ-mediated inflammation in colonic epithelial cells, to suppress colonic inflammation.

Niemann-Pick type C (NPC) disease is predominantly caused by mutations in the NPC1 protein that affect intracellular cholesterol trafficking and cause accumulation of unesterified cholesterol and other lipids in lysosomal storage organelles. We report the use of a series of small molecule histone deacetylase (HDAC) inhibitors in tissue culture models of NPC human fibroblasts. Some HDAC inhibitors lead to a dramatic correction in the NPC phenotype in cells with either one or two copies of the NPC1(I1061T) mutation, and for several of the inhibitors, correction is associated with increased expression of NPC1 protein. Increased NPC1(I1061T) protein levels may partially account for the correction of the phenotype, because this mutant can promote cholesterol efflux if it is delivered to late endosomes and lysosomes. The HDAC inhibitor treatment is ineffective in an NPC2 mutant human fibroblast line. Analysis of the isoform selectivity of the compounds used implicates HDAC1 and/or HDAC2 as likely targets for the observed correction, although other HDACs may also play a role. LBH589 (panobinostat) is an orally available HDAC inhibitor that crosses the blood-brain barrier and is currently in phase III clinical trials for several types of cancer. It restores cholesterol homeostasis in cultured NPC1 mutant fibroblasts to almost normal levels within 72 h when used at 40 nM. The findings that HDAC inhibitors can correct cholesterol storage defects in human NPC1 mutant cells provide the potential basis for treatment options for NPC disease.

Histone acetylation alters chromatin state by modifying lysines on histone and plays an important role in modulating gene transcription. A dynamic balance of histone acetylation/deacetylation is maintained by histone acetyltransferases and histone deacetylases. Emerging evidence suggests that a family of histone deacetylases may exist to regulate diverse cellular functions, including chromatin structure, gene expression, cell cycle progression, and oncogenesis. We describe here a novel human histone deacetylase, named HDAC8, cloned from human kidney. HDAC8 encodes 377 amino acid residues and shares extensive homology to several known HDACs, in particular a histone deacetylase from Arabidopsis thaliana. Northern blot analyses revealed that HDAC8 expression pattern for HDAC8 is distinct from that for HDAC1 and HDAC3, and expression of HDAC8 mRNA occurs in multiple organs including heart, lung, kidney, and pancreas. HDAC8 mRNA was also observed in several cell lines derived from cancerous tissues. When expressed in HEK293 cells, HDAC8 exhibited deacetylase activity toward acetylated histone, indicating that this protein is a bona fide histone deacetylase. Its histone deacetylase activity was inhibited by trichostatin and other known histone deacetylase inhibitors. Furthermore, active recombinant HDAC8 was expressed and purified from Escherichia coli. When ectopically expressed in cells, HDAC8 was found to be localized to the nucleus. Co-transfection experiments demonstrated that expression of HDAC8 repressed a viral SV40 early promoter activity. These results indicate that HDAC8 is a novel member of the histone deacetylase family, which may play a role in the development of a broad range of tissues and potentially in the etiology of cancer.

Elucidation of the mechanism of transcriptional silencing of human immunodeficiency virus type 1 (HIV-1) provirus in latently infected cells is crucial to understand the pathophysiology of HIV-1 infection and to develop novel therapies. Here we demonstrate that AP-4 is responsible for the transcriptional repression of HIV-1. We found that AP-4 site within the viral long terminal repeat (LTR) is well conserved in the majority of HIV-1 subtypes and that AP-4 represses HIV-1 gene expression by recruiting histone deacetylase (HDAC) 1 as well as by masking TATA-binding protein to TATA box. AP-4-mediated transcriptional repression was inhibited by an HDAC inhibitor, tricostatin A, and could be exerted even at distant locations from the TATA box. In addition, AP-4 interacted with HDAC1 both in vivo and in vitro. Moreover, chromatin immunoprecipitation assays have revealed that AP-4 and HDAC1 are present in the HIV-1 LTR promoter in latently infected ACH2 and U1 cells, and they are dissociated from the promoter concomitantly with the association of acetylated histone H3, TBP, and RNA polymerase II upon TNF-alpha stimulation of HIV-1 replication. Furthermore, when AP-4 is knocked down by siRNA, HIV-1 production was greatly augmented in cells transfected with a full-length HIV-1 clone. These results suggest that AP-4 may be responsible for transcriptional quiescence of latent HIV-1 provirus and give a molecular basis to the reported efficacy of combination therapy of conventional anti-HIV drugs with an HDAC inhibitor in accelerating the clearance of HIV-1 from individuals infected with the virus.

MMSET, identified by its fusion to the IgH locus in t(4;14)-associated multiple myeloma, possesses domains found within chromatin regulators, including the SET domain. MMSET protein is overexpressed and highly associated with chromatin in myeloma cell lines carrying t(4;14). MMSET possesses methyltransferase activity for core histone H3 lysine 4 and histone 4 lysine 20, whereas MMSET made in cells only modified H4. Segments of MMSET fused to the Gal4 DNA binding domain repressed transcription of a chromatin-embedded Gal4 reporter gene. MMSET-mediated repression was associated with increased H4K20 methylation gene and loss of histone acetylation. Consistent with this repressive activity, MMSET could form a complex with HDAC1 and HDAC2, mSin3a, and the histone demethylase LSD1, suggesting that it is a component of corepressor complexes. Furthermore, MMSET coexpression enhances HDAC1- and HDAC2-mediated repression in transcriptional reporter assays. Finally, shRNA-mediated knockdown of MMSET compromised viability of a myeloma cell line, suggesting a biologic role for the protein in malignant cell growth. Collectively, these data suggest that, by acting directly as a modifier of chromatin as well as through binding of other chromatin-modifying enzymes, MMSET influences gene expression and potentially acts as a pathogenic agent in multiple myeloma.

MBD1 belongs to a family of mammalian proteins that share a methyl-CpG binding domain. Previous work has shown that MBD1 binds to methylated sites in vivo and in vitro and can repress transcription from methylated templates in transcription extracts and in cultured cells. In the present study we established by several experimental criteria that, contrary to a previous report, MBD1 is not a component of the MeCP1 repressor complex. We identified a powerful transcriptional repression domain (TRD) at the C terminus of MBD1 that can actively repress transcription at a distance. Methylation-dependent repression in vivo depends on the presence of both the TRD and the methyl-CpG binding domain. The mechanism is likely to involve deacetylation, since the deacetylase inhibitor trichostatin A can overcome MBD1-mediated repression. Accordingly, we found that endogenous MBD1 is particularly concentrated at sites of centromeric heterochromatin, where acetylated histone H4 is deficient. Unlike MBD2 and MeCP2, MBD1 is not depleted by antibodies to the histone deacetylase HDAC1. Thus, the deacetylase-dependent pathway by which MBD1 actively silences methylated genes is likely to be different from that utilized by the methylation-dependent repressors MeCP1 and MeCP2.

Chromatin remodeling through histone posttranslational modifications (PTMs) and DNA methylation has recently been implicated in cognitive functions, but the mechanisms involved in such epigenetic regulation remain poorly understood. Here, we show that protein phosphatase 1 (PP1) is a critical regulator of chromatin remodeling in the mammalian brain that controls histone PTMs and gene transcription associated with long-term memory. Our data show that PP1 is present at the chromatin in brain cells and interacts with enzymes of the epigenetic machinery including HDAC1 (histone deacetylase 1) and histone demethylase JMJD2A (jumonji domain-containing protein 2A). The selective inhibition of the nuclear pool of PP1 in forebrain neurons in transgenic mice is shown to induce several histone PTMs that include not only phosphorylation but also acetylation and methylation. These PTMs are residue-specific and occur at the promoter of genes important for memory formation like CREB (cAMP response element-binding protein) and NF-kappaB (nuclear factor-kappaB). These histone PTMs further co-occur with selective binding of RNA polymerase II and altered gene transcription, and are associated with improved long-term memory for objects and space. Together, these findings reveal a novel mechanism for the epigenetic control of gene transcription and long-term memory in the adult brain that depends on PP1.

Class I histone deacetylases (HDAC1, HDAC2, and HDAC3) are recruited by cognate corepressor proteins into specific transcriptional repression complexes that target HDAC activity to chromatin resulting in chromatin condensation and transcriptional silencing. We previously reported the structure of HDAC3 in complex with the SMRT corepressor. This structure revealed the presence of inositol-tetraphosphate [Ins(1,4,5,6)P4] at the interface of the two proteins. It was previously unclear whether the role of Ins(1,4,5,6)P4 is to act as a structural cofactor or a regulator of HDAC3 activity. Here we report the structure of HDAC1 in complex with MTA1 from the NuRD complex. The ELM2-SANT domains from MTA1 wrap completely around HDAC1 occupying both sides of the active site such that the adjacent BAH domain is ideally positioned to recruit nucleosomes to the active site of the enzyme. Functional assays of both the HDAC1 and HDAC3 complexes reveal that Ins(1,4,5,6)P4 is a bona fide conserved regulator of class I HDAC complexes.

OBJECTIVE

Medulloblastomas are the most common malignant brain tumors in childhood. Survivors suffer from high morbidity because of therapy-related side effects. Thus, therapies targeting tumors in a specific manner with small molecules such as histone deacetylase (HDAC) inhibitors are urgently warranted. This study investigated the expression levels of individual human HDAC family members in primary medulloblastoma samples, their potential as risk stratification markers, and their roles in tumor cell growth.

METHODS

Gene expression arrays were used to screen for <em>HDAC1</em> through <em>HDAC1</em>1. Using quantitative real time reverse transcriptase-PCR and immunohistochemistry, we studied the expression of HDAC5 and HDAC9 in primary medulloblastoma samples. In addition, we conducted functional studies using siRNA-mediated knockdown of HDAC5 and HDAC9 in medulloblastoma cells.

RESULTS

HDAC5 and HDAC9 showed the highest expression in prognostically poor subgroups. This finding was validated in an independent set of medulloblastoma samples. High HDAC5 and HDAC9 expression was significantly associated with poor overall survival, with high HDAC5 and HDAC9 expression posing an independent risk factor. Immunohistochemistry revealed a strong expression of HDAC5 and HDAC9 proteins in most of all primary medulloblastomas investigated. siRNA-mediated knockdown of HDAC5 or HDAC9 in medulloblastoma cells resulted in decreased cell growth and cell viability.

CONCLUSIONS

HDAC5 and HDAC9 are significantly upregulated in high-risk medulloblastoma in comparison with low-risk medulloblastoma, and their expression is associated with poor survival. Thus, HDAC5 and HDAC9 may be valuable markers for risk stratification. Because our functional studies point toward a role in medulloblastoma cell growth, HDAC5 and HDAC9 may potentially be novel drug targets.

Histone deacetylase 2 (HDAC2) is a member of a large family of enzymes that alter gene expression by catalyzing the removal of acetyl groups from core histones. Originally isolated as a transcriptional co-repressor, HDAC2 possesses extensive amino acid sequence homology to HDAC1 (the founding member and most extensively studied HDAC enzyme). Because of this high degree of sequence similarity between HDAC1 and HDAC2, coupled with the fact that the two always co-exist in the same complexes, it is difficult to assess whether different properties exist between these two proteins. We report here that HDAC2 is a phosphoprotein similar to HDAC1. In addition, like HDAC1, the phospho-acceptor sites in HDAC2 are located in the C-terminal portion of the protein. However, unlike HDAC1, which can be phosphorylated by protein kinase CK2, cAMP-dependent protein kinase, and protein kinase G, HDAC2 is phosphorylated uniquely by protein kinase CK2 in vitro. Studies using unfractionated cell extracts with CK2 inhibitors suggest that protein kinase CK2 is the major source of HDAC2 kinase. Finally, and perhaps most interesting, HDAC2 phosphorylation promotes enzymatic activity, selectively regulates complex formation, but has no effect on transcriptional repression. Together, our data indicate that like many HDACs, HDAC2 is regulated by post-translational modification, particularly phosphorylation. Furthermore, we demonstrate for the first time that there are similarities and differences in the regulation of HDAC1 and HDAC2 by phosphorylation.

Regulation of cell differentiation programs requires complex interactions between transcriptional and epigenetic networks. Elucidating the principal molecular events responsible for the establishment and maintenance of cell fate identities will provide important insights into how cell lineages are specified and maintained and will improve our ability to recapitulate cell differentiation events in vitro. In this study, we demonstrate that Nkx2.2 is part of a large repression complex in pancreatic β cells that includes DNMT3a, Grg3, and HDAC1. Mutation of the endogenous Nkx2.2 tinman (TN) domain in mice abolishes the interaction between Nkx2.2 and Grg3 and disrupts β-cell specification. Furthermore, we demonstrate that Nkx2.2 preferentially recruits Grg3 and HDAC1 to the methylated Aristaless homeobox gene (Arx) promoter in β cells. The Nkx2.2 TN mutation results in ectopic expression of Arx in β cells, causing β-to-α-cell transdifferentiation. A corresponding β-cell-specific deletion of DNMT3a is also sufficient to cause Arx-dependent β-to-α-cell reprogramming. Notably, subsequent removal of Arx in the β cells of Nkx2.2(TNmut/TNmut) mutant mice reverts the β-to-α-cell conversion, indicating that the repressor activities of Nkx2.2 on the methylated Arx promoter in β cells are the primary regulatory events required for maintaining β-cell identity.

The structural assembly of synapses can be accomplished in a rapid time frame, although most nascent synapses formed during early development are not fully functional and respond poorly to presynaptic action potentials. The mechanisms that are responsible for this delay in synapse maturation are unknown. Histone deacetylases (HDACs) regulate the activity state of chromatin and repress gene expression through the removal of acetyl groups from histones. Class I HDACs, which include HDAC1 and HDAC2, are expressed in the CNS, although their specific role in neuronal function has not been studied. To delineate the contribution of HDAC1 and HDAC2 in the brain, we have used pharmacological inhibitors of HDACs and mice with conditional alleles to HDAC1 and HDAC2. We found that a decrease in the activities of both HDAC1 and HDAC2 during early synaptic development causes a robust facilitation of excitatory synapse maturation and a modest increase in synapse numbers. In contrast, in mature neurons a decrease in HDAC2 levels alone was sufficient to attenuate basal excitatory neurotransmission without a significant change in the numbers of detectable nerve terminals. Therefore, we propose that HDAC1 and HDAC2 form a developmental switch that controls synapse maturation and function acting in a manner dependent on the maturational states of neuronal networks.

SK-N-SH cells exposed to low ratios of ICP0-null (DeltaICP0) mutants of herpes simplex virus per cell express the viral alpha proteins, but the progression to beta and gamma gene expression does not ensue. In these restrictive cells, post-alpha gene expression can be induced after exposure of the infected cells to sodium butyrate, an indication that VP16 brought into cells by the virus and the alpha gene products made after infection cannot block the silencing of viral post-alpha genes by histone deacetylases (HDACs). This observation is consistent with evidence reported earlier that ICP0 dissociates HDAC1/2 from the CoREST/REST complex. In permissive U2OS cells, replication is independent of the ratio of DeltaICP0 mutant per cell. To determine whether other viral genes are involved in blocking HDACs, we used a surrogate system consisting of baculoviruses carrying viral or cellular genes driven by CMV immediate-early promoter. Expression of these genes requires blocking of histone deacetylation. We report that (i) cotransduced U(S)3 or U(S)3.5 protein kinase substitutes for sodium butyrate in enabling the expression of a reporter gene in restrictive cells and enhancing it in permissive cells; (ii) HDAC1 is phosphorylated concomitant with the expression of reporter genes; and (iii) the amounts and appearance of HDAC1 are altered in transduced cells expressing U(S)3 protein kinase in the absence of other viral proteins. We conclude that the U(S)3 protein kinase blocks histone deacetylation by a mechanism distinct from that of ICP0 and that debilitated histone deacetylation contributes to the permissiveness of U2OS cells for DeltaICP0 mutants.

In the developing vertebrate retina, progenitor cells initially proliferate but begin to produce postmitotic neurons when neuronal differentiation occurs. However, the mechanism that determines whether retinal progenitor cells continue to proliferate or exit from the cell cycle and differentiate is largely unknown. Here, we report that histone deacetylase 1 (Hdac1) is required for the switch from proliferation to differentiation in the zebrafish retina. We isolated a zebrafish mutant, ascending and descending (add), in which retinal cells fail to differentiate into neurons and glial cells but instead continue to proliferate. The cloning of the add gene revealed that it encodes Hdac1. Furthermore, the ratio of the number of differentiating cells to that of proliferating cells increases in proportion to Hdac activity, suggesting that Hdac proteins regulate a crucial step of retinal neurogenesis in zebrafish. Canonical Wnt signaling promotes the proliferation of retinal cells in zebrafish, and Notch signaling inhibits neuronal differentiation through the activation of a neurogenic inhibitor, Hairy/Enhancer-of-split (Hes). We found that both the Wnt and Notch/Hes pathways are activated in the add mutant retina. The cell-cycle progression and the upregulation of Hes expression in the add mutant retina can be inhibited by the blockade of Wnt and Notch signaling, respectively. These data suggest that Hdac1 antagonizes these pathways to promote cell-cycle exit and the subsequent neurogenesis in zebrafish retina. Taken together, these data suggest that Hdac1 functions as a dual switch that suppresses both cell-cycle progression and inhibition of neurogenesis in the zebrafish retina.