In vivo destabilization of dynamic microtubules by HDAC6-mediated deacetylation

Eukaryotic translation initiation factor 5A (eIF5A) is essential for HIF-1α activation in hypoxia

The eukaryotic initiation factor 5A (eIF5A) is an essential protein involved in translation elongation and cell proliferation. eIF5A undergoes several post-translational modifications including hypusination and acetylation. Hypusination is indispensable for the function of eIF5A. On the other hand, the precise function of acetylation remains unknown, but it may render the protein inactive since hypusination blocks acetylation. Here, we report that acetylation of eIF5A increases under hypoxia. During extended hypoxic periods an increase in the level of eIF5A acetylation correlated with a decrease in HIF-1α, suggesting involvement of eIF5A activity in HIF-1α expression under hypoxia. Indeed, suppression of eIF5A by siRNA oligo-mediated knockdown or treatment with GC7, a deoxyhypusine synthase inhibitor, led to significant reduction of HIF-1α activity. Furthermore, knockdown of eIF5A or GC7 treatment reduced tumor spheroid formation with a concomitant decrease in HIF-1α expression. Our results suggest that functional, hypusinated eIF5A is necessary for HIF-1α expression during hypoxia and that eIF5A is an attractive target for cancer therapy.

Epigenetic Regulation of the Blimp-1 Gene (Prdm1) in B Cells Involves Bach2 and Histone Deacetylase 3

B lymphocyte-induced maturation protein 1 (Blimp-1) encoded by Prdm1 is a master regulator of plasma cell differentiation. The transcription factor Bach2 represses Blimp-1 expression in B cells to stall terminal differentiation, by which it supports reactions such as class switch recombination of the antibody genes. We found that histones H3 and H4 around the Prdm1 intron 5 Maf recognition element were acetylated at higher levels in X63/0 plasma cells expressing Blimp-1 than in BAL17 mature B cells lacking its expression. Conversely, methylation of H3-K9 was lower in X63/0 cells than BAL17 cells. Purification of the Bach2 complex in BAL17 cells revealed its interaction with histone deacetylase 3 (HDAC3), nuclear co-repressors NCoR1 and NCoR2, transducin β-like 1X-linked (Tbl1x), and RAP1-interacting factor homolog (Rif1). Chromatin immunoprecipitation confirmed the binding of HDAC3 and Rif1 to the Prdm1 locus. Reduction of HDAC3 or NCoR1 expression by RNA interference in B cells resulted in an increased Prdm1 mRNA expression. Bach2 is suggested to cooperate with HDAC3-containing co-repressor complexes in B cells to regulate the stage-specific expression of Prdm1 by writing epigenetic modifications at the Prdm1 locus.

Loss of α-Tubulin Acetylation Is Associated with TGF-β-induced Epithelial-Mesenchymal Transition

The epithelial-to-mesenchymal transition (EMT) is a process by which differentiated epithelial cells reprogram gene expression, lose their junctions and polarity, reorganize their cytoskeleton, increase cell motility and assume a mesenchymal morphology. Despite the critical functions of the microtubule (MT) in cytoskeletal organization, how it participates in EMT induction and maintenance remains poorly understood. Here we report that acetylated α-tubulin, which plays an important role in microtubule (MT) stabilization and cell morphology, can serve as a novel regulator and marker of EMT. A high level of acetylated α-tubulin was correlated with epithelial morphology and it profoundly decreased during TGF-β-induced EMT. We found that TGF-β increased the activity of HDAC6, a major deacetylase of α-tubulin, without affecting its expression levels. Treatment with HDAC6 inhibitor tubacin or TGF-β type I receptor inhibitor SB431542 restored the level of acetylated α-tubulin and consequently blocked EMT. Our results demonstrate that acetylated α-tubulin can serve as a marker of EMT and that HDAC6 represents an important regulator during EMT process.

Synthesis and Biological Investigation of Oxazole Hydroxamates as Highly Selective Histone Deacetylase 6 (HDAC6) Inhibitors

Histone deacetylase 6 (HDAC6) catalyzes the removal of an acetyl group from lysine residues of several non-histone proteins. Here we report the preparation of thiazole-, oxazole-, and oxadiazole-containing biarylhydroxamic acids by a short synthetic procedure. We identified them as selective HDAC6 inhibitors by investigating the inhibition of recombinant HDAC enzymes and the protein acetylation in cells by Western blotting (tubulin vs histone acetylation). The most active compounds exhibited nanomolar potency and high selectivity for HDAC6. For example, an oxazole hydroxamate inhibits HDAC6 with an IC50 of 59 nM and has a selectivity index of >200 against HDAC1 and HDAC8. This is the first report showing that the nature of a heterocycle directly connected to a zinc binding group (ZBG) can be used to modulate subtype selectivity and potency for HDAC6 inhibitors to such an extent. We rationalize the high potency and selectivity of the oxazoles by molecular modeling and docking.

Ciliopathies: Does HDAC6 Represent a New Therapeutic Target?

Cilia are cellular appendages with critical roles in sensing and transducing environmental signals and guiding fluid flow. Consistent with these diverse activities, defects in ciliary structure or function have been implicated in a variety of human diseases, collectively known as 'ciliopathies'. Histone deacetylase 6 (HDAC6) is a unique cytoplasmic enzyme that regulates many biological processes through its deacetylase and ubiquitin-binding activities. There is accumulating evidence that HDAC6 is a major driver of ciliary disassembly. Small-molecule compounds that inhibit HDAC6 have been demonstrated to restore ciliary structure and function in several different ciliopathies. Here, we discuss recent findings that highlight the important role for HDAC6 in mediating ciliary disassembly and the potential for HDAC6-selective inhibitors as therapeutics for specific ciliopathies.

Altered microtubule dynamics and vesicular transport in mouse and human MeCP2-deficient astrocytes

Rett syndrome (RTT) is a rare X-linked neurodevelopmental disorder, characterized by normal post-natal development followed by a sudden deceleration in brain growth with progressive loss of acquired motor and language skills, stereotypic hand movements and severe cognitive impairment. Mutations in the methyl-CpG-binding protein 2 (MECP2) cause more than 95% of classic cases. Recently, it has been shown that the loss of Mecp2 from glia negatively influences neurons in a non-cell-autonomous fashion, and that in Mecp2-null mice, re-expression of Mecp2 preferentially in astrocytes significantly improved locomotion and anxiety levels, restored respiratory abnormalities to a normal pattern and greatly prolonged lifespan compared with globally null mice. We now report that microtubule (MT)-dependent vesicle transport is altered in Mecp2-deficient astrocytes from newborn Mecp2-deficient mice compared with control wild-type littermates. Similar observation has been made in human MECP2 p.Arg294* iPSC-derived astrocytes. Importantly, administration of Epothilone D, a brain-penetrant MT-stabilizing natural product, was found to restore MT dynamics in Mecp2-deficient astrocytes and in MECP2 p.Arg294* iPSC-derived astrocytes in vitro. Finally, we report that relatively low weekly doses of Epothilone D also partially reversed the impaired exploratory behavior in Mecp2(308/y) male mice. These findings represent a first step toward the validation of an innovative treatment for RTT.

Identification of histone deacetylase inhibitors with benzoylhydrazide scaffold that selectively inhibit class I histone deacetylases

Inhibitors of histone deacetylases (HDACi) hold considerable therapeutic promise as clinical anticancer therapies. However, currently known HDACi exhibit limited isoform specificity, off-target activity, and undesirable pharmaceutical properties. Thus, HDACi with new chemotypes are needed to overcome these limitations. Here, we identify a class of HDACi with a previously undescribed benzoylhydrazide scaffold that is selective for the class I HDACs. These compounds are competitive inhibitors with a fast-on/slow-off HDAC-binding mechanism. We show that the lead compound, UF010, inhibits cancer cell proliferation via class I HDAC inhibition. This causes global changes in protein acetylation and gene expression, resulting in activation of tumor suppressor pathways and concurrent inhibition of several oncogenic pathways. The isotype selectivity coupled with interesting biological activities in suppressing tumor cell proliferation support further preclinical development of the UF010 class of compounds for potential therapeutic applications.

New HDAC6-mediated deacetylation sites of tubulin in the mouse brain identified by quantitative mass spectrometry

The post-translational modifications (PTMs) occurring on microtubules have been implicated in the regulation of microtubule properties and functions. Acetylated K40 of α-tubulin, a hallmark of long-lived stable microtubules, is known to be negatively controlled by histone deacetylase 6 (HDAC6). However, the vital roles of HDAC6 in microtubule-related processes such as cell motility and cell division cannot be fully explained by the only known target site on tubulin. Here, we attempt to comprehensively map lysine acetylation sites on tubulin purified from mouse brain tissues. Furthermore, mass spectrometry-based quantitative comparison of acetylated peptides from wild-type vs HDAC6 knockout mice allowed us to identify six new deacetylation sites possibly mediated by HDAC6. Thus, adding new sites to the repertoire of HDAC6-mediated tubulin deacetylation events would further our understanding of the multi-faceted roles of HDAC6 in regulating microtubule stability and cellular functions.

4-Hydroxybenzoic acid derivatives as HDAC6-specific inhibitors modulating microtubular structure and HSP90α chaperone activity against prostate cancer

Histone deacetylase (HDAC)6 is a unique isoenzyme targeting specific substrates including α-tubulin and heat shock protein (HSP)90. HDAC6 is involved in protein trafficking and degradation, cell shape and migration. Deregulation of HDAC6 activity is associated with a variety of diseases including cancer leading to a growing interest for developing HDAC6 inhibitors. Here, we identified two new structurally related 4-hydroxybenzoic acids as selective HDAC6 inhibitors reducing proliferation, colony and spheroid formation as well as viability of prostate cancer cells. Both compounds strongly enhanced α-tubulin acetylation leading to remodeling of microtubular organization. Furthermore, 4-hydroxybenzoic acids decreased HSP90α regulation of the human androgen receptor in prostate cancer cells by increasing HSP90α acetylation levels. Collectively, our data support the potential of 4-hydroxybenzoic acid derivatives as HDAC6-specific inhibitors with anti-cancer properties.

Tubulin acetylation: responsible enzymes, biological functions and human diseases

Microtubules have important functions ranging from maintenance of cell morphology to subcellular transport, cellular signaling, cell migration, and formation of cell polarity. At the organismal level, microtubules are crucial for various biological processes, such as viral entry, inflammation, immunity, learning and memory in mammals. Microtubules are subject to various covalent modifications. One such modification is tubulin acetylation, which is associated with stable microtubules and conserved from protists to humans. In the past three decades, this reversible modification has been studied extensively. In mammals, its level is mainly governed by opposing actions of α-tubulin acetyltransferase 1 (ATAT1) and histone deacetylase 6 (HDAC6). Knockout studies of the mouse enzymes have yielded new insights into biological functions of tubulin acetylation. Abnormal levels of this modification are linked to neurological disorders, cancer, heart diseases and other pathological conditions, thereby yielding important therapeutic implications. This review summarizes related studies and concludes that tubulin acetylation is important for regulating microtubule architecture and maintaining microtubule integrity. Together with detyrosination, glutamylation and other modifications, tubulin acetylation may form a unique 'language' to regulate microtubule structure and function.

MWCNTs enhance hBMSCs spreading but delay their proliferation in the direction of differentiation acceleration

Investigating the ability of films of pristine multiwalled nanotubes (MWCNTs) to influence human mesenchymal stem cells' proliferation, morphology, and differentiation into osteoblasts, we concluded to the following: A. MWCNTs delay the proliferation of hBMS cells but increase their differentiation. The enhancement of the differentiation markers could be a result of decreased proliferation and maturation of the extracellular matrix B. Cell spread on MWCNTs toward a polygonal shape with many thin filopodia to attach to the surfaces. Spreading may be critical in supporting osteogenic differentiation in pre-osteoblastic progenitors, being related with cytoskeletal tension. C. hBMS cells prefer MWCNTs than tissue plastic to attach and grow, being non-toxic to these cells. MWCNTs can be regarded as osteoinductive biomaterial topographies for bone regenerative engineering.

Histone deacetylase inhibitors and epigenetic regulation in lymphoid malignancies

A vast majority of lymphomas and leukaemias are results of translocations. These translocations produce various genetic and epigenetic changes that lead to oncogenesis. This opens an opportunity to use a relatively new class of anti-cancer agents, inhibitors of histone deacetylases (HDACi) to target lymphoid malignancies. Surprisingly, the rational basis for treatment of lymphomas with HDACi is far from clear, although some positive results have been obtained. Here we analyze the effect of histone deacetylase (HDAC) inhibitors on lymphoid malignancies.

The Altered Hepatic Tubulin Code in Alcoholic Liver Disease

The molecular mechanisms that lead to the progression of alcoholic liver disease have been actively examined for decades. Because the hepatic microtubule cytoskeleton supports innumerable cellular processes, it has been the focus of many such mechanistic studies. It has long been appreciated that α-tubulin is a major target for modification by highly reactive ethanol metabolites and reactive oxygen species. It is also now apparent that alcohol exposure induces post-translational modifications that are part of the natural repertoire, mainly acetylation. In this review, the modifications of the "tubulin code" are described as well as those adducts by ethanol metabolites. The potential cellular consequences of microtubule modification are described with a focus on alcohol-induced defects in protein trafficking and enhanced steatosis. Possible mechanisms that can explain hepatic dysfunction are described and how this relates to the onset of liver injury is discussed. Finally, we propose that agents that alter the cellular acetylation state may represent a novel therapeutic strategy for treating liver disease.

Diagnosis and Therapy of Atrial Fibrillation: The Past, The Present and The Future

Atrial fibrillation (AF) is the most common age-related cardiac arrhythmia. It is a progressive disease, which makes treatment difficult. The progression of AF is caused by the accumulation of damage in cardiomyocytes which makes the atria more vulnerable for AF. Especially structural remodeling and electrical remodeling, together called electropathology are sustainable in the atria and impair functional recovery to sinus rhythm after cardioversion. The exact electropathological mechanisms underlying persistence of AF are at present unknown. High resolution wavemapping studies in patients with different types of AF showed that longitudinal dissociation in conduction and epicardial breakthrough were the key elements of the substrate of longstanding persistent AF. A double layer of electrically dissociated waves propagating transmurally can explain persistence of AF (Double Layer Hypothesis) but the molecular mechanism is unknown. Derailment of proteasis -defined as the homeostasis in protein synthesis, folding, assembly, trafficking, guided by chaperones, and clearance by protein degradation systems - may play an important role in remodeling of the cardiomyocyte. As current therapies are not effective in attenuating AF progression, step-by-step analysis of this process, in order to identify potential targets for drug therapy, is essential. In addition, novel mapping approaches enabling assessment of the degree of electropathology in the individual patient are mandatory to develop patient-tailored therapies. The aims of this review are to 1) summarize current knowledge of the electrical and molecular mechanisms underlying AF 2) discuss the shortcomings of present diagnostic instruments and therapeutic options and 3) to present potential novel diagnostic tools and therapeutic targets.

PTEN activation through K163 acetylation by inhibiting HDAC6 contributes to tumour inhibition

Phosphatase and tensin homologue deleted on chromosome 10 (PTEN), an important tumour-suppressor gene, is mutated, downregulated or dysfunctional in many tumours. The phosphatase activity of PTEN depends on membrane translocation (activation). As promising anti-cancer agents, histone deacetylase (HDAC) inhibitors, particularly trichostatin A (TSA), can promote PTEN membrane translocation, but the underlying mechanism remains unknown. In this study, we revealed that non-selective HDAC inhibitors, namely, TSA or suberoylanilide hydroxamic acid (SAHA), induced PTEN membrane translocation through PTEN acetylation at K163 by inhibiting HDAC6. K163 acetylation inhibited the interaction of the PTEN C-tail with the remaining part of PTEN, resulting in PTEN membrane translocation. Overexpression of wild-type PTEN, but not K163-mutated PTEN, facilitated the inhibition of cell proliferation, migration and invasion, as well as xenograft tumour growth, induced by SAHA or tubastatin A, an HDAC6-specific inhibitor. These results indicated that PTEN activation by inhibiting HDAC6 significantly contributed to tumour inhibition. Therefore, non-selective HDAC or HDAC6-specific inhibitors may be more clinically suitable to treat tumours without PTEN mutations or deletions.

Deacetylation of α-tubulin and cortactin is required for HDAC6 to trigger ciliary disassembly

Cilia play important roles in sensing extracellular signals and directing fluid flow. Ciliary dysfunction is associated with a variety of diseases known as ciliopathies. Histone deacetylase 6 (HDAC6) has recently emerged as a major driver of ciliary disassembly, but little is known about the downstream players. Here we provide the first evidence that HDAC6-mediated deacetylation of α-tubulin and cortactin is critical for its induction of ciliary disassembly. HDAC6 is localized in the cytoplasm and enriched at the centrosome and basal body. Overexpression of HDAC6 decreases the levels of acetylated α-tubulin and cortactin without affecting the expression or localization of known ciliary regulators. We also find that overexpression of α-tubulin or cortactin or their acetylation-deficient mutants enhances the ability of HDAC6 to induce ciliary disassembly. In addition, acetylation-mimicking mutants of α-tubulin and cortactin counteract HDAC6-induced ciliary disassembly. Furthermore, HDAC6 stimulates actin polymerization, and inhibition of actin polymerization abolishes the activity of HDAC6 to trigger ciliary disassembly. These findings provide mechanistic insight into the ciliary role of HDAC6 and underscore the importance of reversible acetylation in regulating ciliary homeostasis.

HDAC6 and RhoA are novel players in Abeta-driven disruption of neuronal polarity

Maintenance of neuronal polarity and regulation of cytoskeletal dynamics are vital during development and to uphold synaptic activity in neuronal networks. Here we show that soluble β-amyloid (Aβ) disrupts actin and microtubule (MT) dynamics via activation of RhoA and inhibition of histone deacetylase 6 (HDAC6) in cultured hippocampal neurons. The contact of Aβ with the extracellular membrane promotes RhoA activation, leading to growth cone collapse and neurite retraction, which might be responsible for hampered neuronal pathfinding and migration in Alzheimer's disease (AD). The inhibition of HDAC6 by Aβ increases the level of heterodimeric acetylated tubulin and acetylated tau, both of which have been found altered in AD. We also find that the loss of HDAC6 activity perturbs the integrity of axon initial segment (AIS), resulting in mislocalization of ankyrin G and increased MT instability in the AIS concomitant with loss of polarized localization of tau and impairment of action potential firing.

Accumulation of the PX domain mutant Frank-ter Haar syndrome protein Tks4 in aggresomes

Background

Cells deploy quality control mechanisms to remove damaged or misfolded proteins. Recently, we have reported that a mutation (R43W) in the Frank-ter Haar syndrome protein Tks4 resulted in aberrant intracellular localization.

Results

Here we demonstrate that the accumulation of Tks4^R43W depends on the intact microtubule network. Detergent-insoluble Tks4 mutant colocalizes with the centrosome and its aggregate is encaged by the intermediate filament protein vimentin. Both the microtubule inhibitor nocodazole and the histone deacetylase inhibitor Trichostatin A inhibit markedly the aggresome formation in cells expressing Tks4^R43W. Finally, pretreatment of cells with the proteasome inhibitor MG132 markedly increases the level of aggresomes formed by Tks4^R43W. Furthermore, two additional mutant Tks4 proteins (Tks4^1–48 or Tks4^1–341) have been investigated. Whereas the shorter Tks4 mutant, Tks4^1–48, shows no expression at all, the longer Tks4 truncation mutant accumulates in the nuclei of the cells.

Conclusions

Our results suggest that misfolded Frank-ter Haar syndrome protein Tks4^R43W is transported via the microtubule system to the aggresomes. Lack of expression of Tks4^1–48 or aberrant intracellular expressions of Tks4^R43W and Tks4^1–341 strongly suggest that these mutations result in dysfunctional proteins which are not capable of operating properly, leading to the development of FTHS.

The synergic effect of vincristine and vorinostat in leukemia in vitro and in vivo

Background

Combination therapy is a key strategy for minimizing drug resistance, a common problem in cancer therapy. The microtubule-depolymerizing agent vincristine is widely used in the treatment of acute leukemia. In order to decrease toxicity and chemoresistance of vincristine, this study will investigate the effects of combination vincristine and vorinostat (suberoylanilide hydroxamic acid (SAHA)), a pan-histone deacetylase inhibitor, on human acute T cell lymphoblastic leukemia cells.

Methods

Cell viability experiments were determined by 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide (MTT) assay, and cell cycle distributions as well as mitochondria membrane potential were analyzed by flow cytometry. In vitro tubulin polymerization assay was used to test tubulin assembly, and immunofluorescence analysis was performed to detect microtubule distribution and morphology. In vivo effect of the combination was evaluated by a MOLT-4 xenograft model. Statistical analysis was assessed by Bonferroni’s t test.

Results

Cell viability showed that the combination of vincristine and SAHA exhibited greater cytotoxicity with an IC₅₀ value of 0.88 nM, compared to each drug alone, 3.3 and 840 nM. This combination synergically induced G₂/M arrest, followed by an increase in cell number at the sub-G₁ phase and caspase activation. Moreover, the results of vincristine combined with an HDAC6 inhibitor (tubastatin A), which acetylated α-tubulin, were consistent with the effects of vincristine/SAHA co-treatment, thus suggesting that SAHA may alter microtubule dynamics through HDAC6 inhibition.

Conclusion

These findings indicate that the combination of vincristine and SAHA on T cell leukemic cells resulted in a change in microtubule dynamics contributing to M phase arrest followed by induction of the apoptotic pathway. These data suggest that the combination effect of vincristine/SAHA could have an important preclinical basis for future clinical trial testing.

Electronic supplementary material

The online version of this article (doi:10.1186/s13045-015-0176-7) contains supplementary material, which is available to authorized users.

Targeting Histone Deacetylases in Diseases: Where Are We?

SIGNIFICANCE

Epigenetic inactivation of pivotal genes involved in cell growth is a hallmark of human pathologies, in particular cancer. Histone acetylation balance obtained through opposing actions of histone deacetylases (HDACs) and histone acetyltransferases is one epigenetic mechanism controlling gene expression and is, thus, associated with disease etiology and progression. Interfering pharmacologically with HDAC activity can correct abnormalities in cell proliferation, migration, vascularization, and death.

RECENT ADVANCES

Histone deacetylase inhibitors (HDACi) represent a new class of cytostatic agents that interfere with the function of HDACs and are able to increase gene expression by indirectly inducing histone acetylation. Several HDACi, alone or in combination with DNA-demethylating agents, chemopreventive, or classical chemotherapeutic drugs, are currently being used in clinical trials for solid and hematological malignancies, and are, thus, promising candidates for cancer therapy.

CRITICAL ISSUES

(i) Non-specific (off-target) HDACi effects due to activities unassociated with HDAC inhibition. (ii) Advantages/disadvantages of non-selective or isoform-directed HDACi. (iii) Limited number of response-predictive biomarkers. (iv) Toxicity leading to dysfunction of critical biological processes.

FUTURE DIRECTIONS

Selective HDACi could achieve enhanced clinical utility by reducing or eliminating the serious side effects associated with current first-generation non-selective HDACi. Isoform-selective and pan-HDACi candidates might benefit from the identification of biomarkers, enabling better patient stratification and prediction of response to treatment.

The HDAC6/APOBEC3G complex regulates HIV-1 infectiveness by inducing Vif autophagic degradation

BACKGROUND

Human immunodeficiency virus type 1 (HIV-1) has evolved a complex strategy to overcome the immune barriers it encounters throughout an organism thanks to its viral infectivity factor (Vif), a key protein for HIV-1 infectivity and in vivo pathogenesis. Vif interacts with and promotes "apolipoprotein B mRNA-editing enzyme-catalytic, polypeptide-like 3G" (A3G) ubiquitination and subsequent degradation by the proteasome, thus eluding A3G restriction activity against HIV-1.

RESULTS

We found that cellular histone deacetylase 6 (HDAC6) directly interacts with A3G through its C-terminal BUZ domain (residues 841-1,215) to undergo a cellular co-distribution along microtubules and cytoplasm. The HDAC6/A3G complex occurs in the absence or presence of Vif, competes for Vif-mediated A3G degradation, and accounts for A3G steady-state expression level. In fact, HDAC6 directly interacts with and promotes Vif autophagic clearance, thanks to its C-terminal BUZ domain, a process requiring the deacetylase activity of HDAC6. HDAC6 degrades Vif without affecting the core binding factor β (CBF-β), a Vif-associated partner reported to be key for Vif- mediated A3G degradation. Thus HDAC6 antagonizes the proviral activity of Vif/CBF-β-associated complex by targeting Vif and stabilizing A3G. Finally, in cells producing virions, we observed a clear-cut correlation between the ability of HDAC6 to degrade Vif and to restore A3G expression, suggesting that HDAC6 controls the amount of Vif incorporated into nascent virions and the ability of HIV-1 particles of being infectious. This effect seems independent on the presence of A3G inside virions and on viral tropism.

CONCLUSIONS

Our study identifies for the first time a new cellular complex, HDAC6/A3G, involved in the autophagic degradation of Vif, and suggests that HDAC6 represents a new antiviral factor capable of controlling HIV-1 infectiveness by counteracting Vif and its functions.

HDAC6 mediates HIV-1 tat-induced proinflammatory responses by regulating MAPK-NF-kappaB/AP-1 pathways in astrocytes

Human immunodeficiency virus (HIV)-1 transactivator of transcription (Tat) is a viral protein that induces extensive neuroinflammation by up-regulating proinflammatory mediators, including cytokines, chemokines, and adhesion molecules. Histone deacetylase 6 (HDAC6) has been implicated in the transcriptional regulation of inflammatory genes. In this study, we investigated the possible role of HDAC6 in HIV-1 Tat-induced up-regulation of proinflammatory mediators in astrocytes. HIV-1 Tat augmented HDAC6 expression, which was correlated with a reduction in acetylated α-tubulin in CRT-MG human astroglioma cells and primary mouse astrocytes. Knockdown and pharmacological inhibition of HDAC6 significantly inhibited HIV-1 Tat-induced expression of CCL2, CXCL8, and CXCL10 chemokines; adhesion molecules; and subsequent adhesion of monocytes to astrocytes. HDAC6 knockdown attenuated HIV-1 Tat-induced activation of mitogen-activated protein kinase species, including ERK, JNK, and p38. Furthermore, HDAC6 knockdown suppressed HIV-1 Tat-induced activation of NF-κB and AP-1. Thus, HDAC6 is involved in HIV-1 Tat-induced expression of proinflammatory genes by regulating mitogen-activated protein kinase-NF-κB/AP-1 pathways and serves as a molecular target for HIV-1 Tat-mediated neuroinflammation GLIA 2015;63:1953-1965.

Effect of HDAC Inhibitors on Corneal Keratocyte Mechanical Phenotypes in 3-D Collagen Matrices

PURPOSE

Histone deacetylase inhibitors (HDAC) have been shown to inhibit the TGFβ-induced myofibroblast transformation of corneal fibroblasts in 2-D culture. However, the effect of HDAC inhibitors on keratocyte spreading, contraction, and matrix remodeling in 3-D culture has not been directly assessed. The goal of this study was to investigate the effects of the HDAC inhibitors Trichostatin A (TSA) and Vorinostat (SAHA) on corneal keratocyte mechanical phenotypes in 3-D culture using defined serum-free culture conditions.

METHODS

Rabbit corneal keratocytes were plated within standard rat tail type I collagen matrices (2.5 mg/ml) or compressed collagen matrices (~100 mg/ml) and cultured for up to 4 days in serum-free media, PDGF BB, TGFβ1, and either 50 nM TSA, 10 μM SAHA, or vehicle (DMSO). F-actin, α-SM-actin, and collagen fibrils were imaged using confocal microscopy. Cell morphology and global matrix contraction were quantified digitally. The expression of α-SM-actin was assessed using western blotting.

RESULTS

Corneal keratocytes in 3-D matrices had a quiescent mechanical phenotype, as indicated by a dendritic morphology, a lack of stress fibers, and minimal cell-induced matrix remodeling. This phenotype was generally maintained following the addition of TSA or SAHA. TGFβ1 induced a contractile phenotype, as indicated by a loss of dendritic cell processes, the development of stress fibers, and significant matrix compaction. In contrast, cells cultured in TGFβ1 plus TSA or SAHA remained dendritic and did not form stress fibers or induce ECM compaction. Western blotting showed that the expression of α-SM actin after treatment with TGFβ1 was inhibited by TSA and SAHA. PDGF BB stimulated the elongation of keratocytes and the extension of dendritic processes within 3-D matrices without inducing stress fiber formation or collagen reorganization. This spreading response was maintained in the presence of TSA or SAHA.

CONCLUSIONS

Overall, HDAC inhibitors appear to mitigate the effects of TGFβ1 on the transformation of corneal keratocytes to a contractile, myofibroblast phenotype in both compliant and rigid 3-D matrices while preserving normal cell spreading and their ability to respond to the pro-migratory growth factor PDGF.

Selective inhibitor of histone deacetylase 6 (tubastatin A) suppresses proliferation of hepatitis C virus replicon in culture of human hepatocytes

Acetylation of α-tubulin was studied in cultures of human hepatocytes under the influence of selective inhibitors of histone deacetylases HDAC6 and SIRT-2 - tubastatin A and 2-(3-phenethoxyphenylamino)benzamide, respectively. It was found that in hepatocyte cell line HepG2 acetylated α-tubulin is accumulated preferentially on inhibition of HDAC6 but not of SIRT-2. Under the same conditions, no acetylation of α-tubulin was observed in hepatocyte cell line Huh7. However, the inhibition of HDAC6 with tubastatin A led to hyperacetylation of α-tubulin and simultaneously to decrease in viral RNA concentration in hepatocyte cell line Huh7-luc/neo, which supports propagation of the full genome replicon of hepatitis C virus. The correlation between these two processes points to HDAC6 as a promising cellular target for therapy of hepatitis C.

The potential of histone deacetylase inhibitors in breast cancer therapy

SUMMARY 

Breast cancer is the second leading cause of cancer-related mortality in women. Despite improvements in prevention, detection and treatment, breast cancer will be responsible for nearly 40,000 deaths in 2014. The function of histone deacetylases (HDACs) and their potential as therapeutic targets has become an area of intense investigation and small molecule inhibitors of HDACs (HDACi) are now being investigated as potential chemotherapeutics for breast cancer. In addition to altering chromatin structure through stabilization of histone acetylation, HDACi treatment induces the accumulation of acetylated isoforms of many nonhistone proteins, altering their structure and function. These structural changes influence protein–protein interactions and cellular processes including cell cycle arrest, apoptosis, autophagy, induction of reactive oxygen species and mitotic catastrophe. While the usefulness of these compounds as single agents for treatment of breast cancer is still under investigation, cotreatment with other therapies is being evaluated in a number of clinical trials.

Histone acetylation inhibitors promote axon growth in adult dorsal root ganglia neurons

Intrinsic mechanisms that guide damaged axons to regenerate following spinal cord injury remain poorly understood. Manipulation of posttranslational modifications of key proteins in mature neurons could reinvigorate growth machinery after injury. One such modification is acetylation, a reversible process controlled by two enzyme families, the histone deacetylases (HDACs) and the histone acetyl transferases (HATs), acting in opposition. Whereas acetylated histones in the nucleus are associated with upregulation of growth-promoting genes, deacetylated tubulin in the axoplasm is associated with more labile microtubules, conducive to axon growth. This study investigates the effects of HAT and HDAC inhibitors on cultured adult dorsal root ganglia (DRG) neurons and shows that inhibition of HATs by anacardic acid or CPTH2 improves axon outgrowth, whereas inhibition of HDACs by TSA or tubacin inhibits axon growth. Anacardic acid increased the number of axons able to cross an inhibitory chondroitin sulfate proteoglycan border. Histone acetylation but not tubulin acetylation level was affected by HAT inhibitors, whereas tubulin acetylation levels were increased in the presence of the HDAC inhibitor tubacin. Although the microtubule-stabilizing drug taxol did not have an effect on the lengths of DRG axons, nocodazole decreased axon lengths. Determining the mechanistic basis will require future studies, but this study shows that inhibitors of HAT can augment axon growth in adult DRG neurons, with the potential of aiding axon growth over inhibitory substrates produced by the glial scar.

Methamphetamine promotes α-tubulin deacetylation in endothelial cells: the protective role of acetyl-l-carnitine

Methamphetamine (METH) is a powerful psychostimulant drug used worldwide for its reinforcing properties. In addition to the classic long-lasting monoaminergic-disrupting effects extensively described in the literature, METH has been consistently reported to increase blood brain barrier (BBB) permeability, both in vivo and in vitro, as a result of tight junction and cytoskeleton disarrangement. Microtubules play a critical role in cell stability, which relies on post-translational modifications such as α-tubulin acetylation. As there is evidence that psychostimulants drugs modulate the expression of histone deacetylases (HDACs), we hypothesized that in endothelial cells METH-mediation of cytoplasmatic HDAC6 activity could affect tubulin acetylation and further contribute to BBB dysfunction. To validate our hypothesis, we exposed the bEnd.3 endothelial cells to increasing doses of METH and verified that it leads to an extensive α-tubulin deacetylation mediated by HDACs activation. Furthermore, since we recently reported that acetyl-l-carnitine (ALC), a natural occurring compound, prevents BBB structural loss in a context of METH exposure, we reasoned that ALC could also preserve the acetylation of microtubules under METH action. The present results confirm that ALC is able to prevent METH-induced deacetylation providing effective protection on microtubule acetylation. Although further investigation is still needed, HDACs regulation may become a new therapeutic target for ALC.

Biological evaluation of 4,5-diarylimidazoles with hydroxamic acid appendages as novel dual mode anticancer agents

PURPOSE

New (4-aryl-1-methylimidazol-5-yl)cinnamoylhydroxamic acids were prepared as potential dual mode anticancer agents combining the antivascular effect of the 4,5-diarylimidazole moiety and the histone deacetylases (HDAC) inhibition by the cinnamoyl hydroxamate.

METHODS

Their antiproliferative activity against a panel of primary cells and cancer cell lines was determined by MTT assays and their apoptosis induction by caspase-3 activation. Their ability to reduce the activity of HDAC was measured by enzymatic assays and Western blot analyses of cellular HDAC substrates. Additional effects on cancer cell migration were ascertained via immunofluorescence staining of cytoskeleton components and three-dimensional migration assays. The chorioallantoic membrane assay was used as an in vivo model to assess their antiangiogenic properties.

RESULTS

The 4-phenyl- and 4-(p-methoxyphenyl)-imidazole derivatives had a greater antiproliferative and apoptosis inducing effect in a variety of cancer cell lines when compared with the approved HDAC inhibitor SAHA, and most distinctly so in non-malignant human umbilical vein endothelial cells. Like SAHA, both compounds acted as pan-HDAC inhibitors. In 518A2 melanoma cells, they led to hyperacetylation of histones and of the cytoplasmic HDAC6 substrate alpha-tubulin. As a consequence, they inhibited the migration and invasion of these cells in transwell invasion assays. In keeping with its pronounced impact on endothelial cells, the 4-phenyl-imidazole derivative also inhibited the growth and sprouting of blood vessels in the chorioallantoic membrane of fertilized hen eggs.

CONCLUSIONS

The 4-phenyl- and 4-(p-methoxyphenyl)-imidazole compounds combine the antivascular effects of 4,5-diarylimidazoles with HDAC inhibition by cinnamoyl hydroxamates and show additional antimetastatic activity. They are promising candidates for pleiotropic HDAC inhibitors.

Circovirus transport proceeds via direct interaction of the cytoplasmic dynein IC1 subunit with the viral capsid protein

Microtubule transport of circovirus from the periphery of the cell to the nucleus is essential for viral replication in early infection. How the microtubule is recruited to the viral cargo remains unclear. In this study, we observed that circovirus trafficking is dependent on microtubule polymerization and that incoming circovirus particles colocalize with cytoplasmic dynein and endosomes. However, circovirus binding to dynein was independent of the presence of microtubular α-tubulin and translocation of cytoplasmic dynein into the nucleus. The circovirus capsid (Cap) subunit enhanced microtubular acetylation and directly interacted with intermediate chain 1 (IC1) of dynein. N-terminal residues 42 to 100 of the Cap viral protein were required for efficient binding to the dynein IC1 subunit and for retrograde transport. Knockdown of IC1 decreased virus transport and replication. These results demonstrate that Cap is a direct ligand of the cytoplasmic dynein IC1 subunit and an inducer of microtubule α-tubulin acetylation. Furthermore, Cap recruits the host dynein/microtubule machinery to facilitate transport toward the nucleus by an endosomal mechanism distinct from that used for physiological dynein cargo.

IMPORTANCE

Incoming viral particles hijack the intracellular trafficking machinery of the host in order to migrate from the cell surface to the replication sites. Better knowledge of the interaction between viruses and virus proteins and the intracellular trafficking machinery may provide new targets for antiviral therapies. Currently, little is known about the molecular mechanisms of circovirus transport. Here, we report that circovirus particles enter early endosomes and utilize the microtubule-associated molecular motor dynein to travel along microtubules. The circovirus capsid subunit enhances microtubular acetylation, and N-terminal residues 42 to 100 directly interact with the dynein IC1 subunit during retrograde transport. These findings highlight a mechanism whereby circoviruses recruit dynein for transport to the nucleus via the dynein/microtubule machinery.

The nphp-2 and arl-13 genetic modules interact to regulate ciliogenesis and ciliary microtubule patterning in C. elegans

Cilia are microtubule-based cellular organelles that mediate signal transduction. Cilia are organized into several structurally and functionally distinct compartments: the basal body, the transition zone (TZ), and the cilia shaft. In vertebrates, the cystoprotein Inversin localizes to a portion of the cilia shaft adjacent to the TZ, a region termed the "Inversin compartment" (InvC). The mechanisms that establish and maintain the InvC are unknown. In the roundworm C. elegans, the cilia shafts of amphid channel and phasmid sensory cilia are subdivided into two regions defined by different microtubule ultrastructure: a proximal doublet-based region adjacent to the TZ, and a distal singlet-based region. It has been suggested that C. elegans cilia also possess an InvC, similarly to mammalian primary cilia. Here we explored the biogenesis, structure, and composition of the C. elegans ciliary doublet region and InvC. We show that the InvC is conserved and distinct from the doublet region. nphp-2 (the C. elegans Inversin homolog) and the doublet region genes arl-13, klp-11, and unc-119 are redundantly required for ciliogenesis. InvC and doublet region genes can be sorted into two modules-nphp-2+klp-11 and arl-13+unc-119-which are both antagonized by the hdac-6 deacetylase. The genes of this network modulate the sizes of the NPHP-2 InvC and ARL-13 doublet region. Glutamylation, a tubulin post-translational modification, is not required for ciliary targeting of InvC and doublet region components; rather, glutamylation is modulated by nphp-2, arl-13, and unc-119. The ciliary targeting and restricted localization of NPHP-2, ARL-13, and UNC-119 does not require TZ-, doublet region, and InvC-associated genes. NPHP-2 does require its calcium binding EF hand domain for targeting to the InvC. We conclude that the C. elegans InvC is distinct from the doublet region, and that components in these two regions interact to regulate ciliogenesis via cilia placement, ciliary microtubule ultrastructure, and protein localization.

Inhibition of HDAC6 modifies tau inclusion body formation and impairs autophagic clearance

Proteinaceous inclusions in nerve cells and glia are a defining neuropathological hallmark in a variety of neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease, progressive supranuclear palsy (PSP), and corticobasal degeneration (CBD). Their occurrence may be related to malfunctions of the proteolytic degradation systems. In cultured oligodendrocytes, proteasomal inhibition leads to protein aggregate formation resembling coiled bodies, which are characteristic for PSP and CBD. Large protein aggregates are excluded from the proteasome and can only be degraded by autophagy, a lysosomal pathway. Autophagy is a highly selective process, which requires a variety of receptor proteins for ubiquitinated proteins, such as p62 and histone deacetylase 6 (HDAC6). HDAC6 is mainly localized in the cytoplasm, and alpha-tubulin is its major substrate. HDAC6 is considered as a sensor of proteasomal stress; it is involved in the autophagosomal pathway and can mediate the retrograde transport of ubiquitinated proteins along the microtubules. As we have shown recently, HDAC6 is present in oligodendrocytes and its inhibition leads to morphological alterations, microtubule bundling, modulation of acetylation, and phosphorylation of the microtubule-associated protein tau. The present study was undertaken to investigate whether HDAC6 is involved in protein aggregate formation in oligodendrocytes and whether its inhibition modifies the consequences of MG-132-induced inhibition of the ubiquitin proteasome system (UPS). The data show that HDAC6 and acetylated tau are recruited to protein aggregates after proteasomal inhibition. Pharmacological inhibition of HDAC6 by the selective inhibitor tubastatin A (TST) and its small hairpin RNA (shRNA)-mediated downregulation alters the assembly of MG-132-induced compact protein aggregates. After TST treatment, they appear more diffusely dispersed throughout the cytoplasm. This is not a protective means but promotes the onset of apoptotic cell death. Furthermore, the heat shock response is altered, and TST suppresses the MG-132-stimulated induction of HSP70. To test whether the alteration of protein aggregate formation is related to the influence of HDAC6 on the autophagic degradation system, an oligodendroglial cell line, i.e., OLN-93 cells stably expressing green fluorescent protein (GFP)-microtubule associated protein light chain 3 (LC3) and tau, was used. During autophagosome formation, endogenous LC3 is processed to LC3-I, which is then converted to LC3-II. An increase of LC3-II is used as a reliable marker for autophagosome formation and abundance. It is demonstrated that inhibition of HDAC6 leads to the accumulation of LC3-positive autophagosomal vacuoles and an increase in LC3-II immunoreactivity, but the autophagic flux is rather impaired. Hence, the inhibition or dysregulation of HDAC6 contributes to stress responses and pathological processes in oligodendrocytes.

HDAC6 deacetylates alpha tubulin in sperm and modulates sperm motility in Holtzman rat

Histone deacetylase 6 (HDAC6) is an alpha (α)-tubulin deacetylase and its over-expression has been demonstrated to promote chemotactic cell movement. Motility in sperm is driven by the flagella, the cytoskeletal structure comprising the microtubules, which are heterodimers of α- and β-tubulins. We have hypothesized that HDAC6, by virtue of being an α-tubulin deacetylase, might modulate sperm motility. However, the presence of HDAC6 on sperm has hitherto not been reported. In this study, we have demonstrated, for the first time, the presence of HDAC6 transcript and protein in the testicular and caudal sperm of rat. We have observed a significantly overlapping expression of HDAC6 with acetyl α-tubulin (Ac α-tubulin) in the mid-piece and principal piece of sperm flagella, and the co-precipitation of α-tubulin and Ac α-tubulin together with HDAC6 and vice versa in sperm lysates. This indicates that HDAC6 interacts with α-tubulin. The HDAC6 activity of sperm, sperm motility and status of Ac α-tubulin investigated in the presence of HDAC inhibitors Trichostatin A, Tubastatin A and sodium butyrate demonstrate that HDAC6 in sperm is catalytically active and that inhibitors of HDAC6 increase acetylation and restrict sperm motility. Thus, we show that (1) active HDAC6 enzyme is present in sperm, (2) HDAC6 in sperm is able to deacetylate α-tubulin, (3) inhibition of HDAC6 results in increased Ac α-tubulin expression and (4) HDAC6 inhibition affects sperm motility. This evidence suggests that HDAC6 is involved in modulating sperm movement.

Towards selective inhibition of histone deacetylase isoforms: what has been achieved, where we are and what will be next

Histone deacetylases (HDACs) are widely studied targets for the treatment of cancer and other diseases. Up to now, over twenty HDAC inhibitors have entered clinical studies and two of them have already reached the market, namely the hydroxamic acid derivative SAHA (vorinostat, Zolinza) and the cyclic depsipeptide FK228 (romidepsin, Istodax) that have been approved for the treatment of cutaneous T-cell lymphoma (CTCL). A common aspect of the first HDAC inhibitors is the absence of any particular selectivity towards specific isozymes. Some of molecules resulted to be “pan”-HDAC inhibitors, while others are class I selective. In the meantime, the knowledge of HDAC biology has continuously progressed. Key advances in the structural biology of various isozymes, reliable molecular homology models as well as suitable biological assays have provided new tools for drug discovery activities. This Minireview aims at surveying these recent developments as well as the design, synthesis and biological characterization of isoform-selective derivatives.

Exome-wide rare variant analysis identifies TUBA4A mutations associated with familial ALS

Exome sequencing is an effective strategy for identifying human disease genes. However, this methodology is difficult in late-onset diseases where limited availability of DNA from informative family members prohibits comprehensive segregation analysis. To overcome this limitation, we performed an exome-wide rare variant burden analysis of 363 index cases with familial ALS (FALS). The results revealed an excess of patient variants within TUBA4A, the gene encoding the Tubulin, Alpha 4A protein. Analysis of a further 272 FALS cases and 5,510 internal controls confirmed the overrepresentation as statistically significant and replicable. Functional analyses revealed that TUBA4A mutants destabilize the microtubule network, diminishing its repolymerization capability. These results further emphasize the role of cytoskeletal defects in ALS and demonstrate the power of gene-based rare variant analyses in situations where causal genes cannot be identified through traditional segregation analysis.

Paxillin inhibits HDAC6 to regulate microtubule acetylation, Golgi structure, and polarized migration

Polarized cell migration is essential for normal organism development and is also a critical component of cancer cell invasion and disease progression. Directional cell motility requires the coordination of dynamic cell-extracellular matrix interactions as well as repositioning of the Golgi apparatus, both of which can be controlled by the microtubule (MT) cytoskeleton. In this paper, we have identified a new and conserved role for the focal adhesion scaffold protein paxillin in regulating the posttranslational modification of the MT cytoskeleton through an inhibitory interaction with the α-tubulin deacetylase HDAC6. We also determined that through HDAC6-dependent regulation of the MT cytoskeleton, paxillin regulates both Golgi organelle integrity and polarized cell invasion and migration in both three-dimensional and two-dimensional matrix microenvironments. Importantly, these data reveal a fundamental role for paxillin in coordinating MT acetylation-dependent cell polarization and migration in both normal and transformed cells.

Primary cilium: an elaborate structure that blocks cell division?

A primary cilium is a microtubule-based membranous protrusion found in almost all cell types. A primary cilium has a "9+0" axoneme that distinguishes this ancient organelle from the canonical motile "9+2" cilium. A primary cilium is the sensory center of the cell that regulates cell proliferation and embryonic development. The primary ciliary pocket is a specialized endocytic membrane domain in the basal region. The basal body of a primary cilium exists as a form of the centriole during interphase of the cell cycle. Although conventional thinking suggests that the cell cycle regulates centrosomal changes, recent studies suggest the opposite, that is, centrosomal changes regulate the cell cycle. In this regard, centrosomal kinase Aurora kinase A (AurA), Polo-like kinase 1 (Plk1), and NIMA related Kinase (Nek or Nrk) propel cell cycle progression by promoting primary cilia disassembly which indicates a non-mitotic function. However, the persistence of primary cilia during spermatocyte division challenges the dominate idea of the incompatibility of primary cilia and cell division. In this review, we demonstrate the detailed structure of primary cilia and discuss the relationship between primary cilia disassembly and cell cycle progression on the background of various mitotic kinases.

Erucin, the major isothiocyanate in arugula (Eruca sativa), inhibits proliferation of MCF7 tumor cells by suppressing microtubule dynamics

Consumption of cruciferous vegetables is associated with reduced risk of various types of cancer. Isothiocyanates including sulforaphane and erucin are believed to be responsible for this activity. Erucin [1-isothiocyanato-4-(methylthio)butane], which is metabolically and structurally related to sulforaphane, is present in large quantities in arugula (Eruca sativa, Mill.), kohlrabi and Chinese cabbage. However, its cancer preventive mechanisms remain poorly understood. We found that erucin inhibits proliferation of MCF7 breast cancer cells (IC50 = 28 µM) in parallel with cell cycle arrest at mitosis (IC50 = 13 µM) and apoptosis, by a mechanism consistent with impairment of microtubule dynamics. Concentrations of 5-15 µM erucin suppressed the dynamic instability of microtubules during interphase in the cells. Most dynamic instability parameters were inhibited, including the rates and extents of growing and shortening, the switching frequencies between growing and shortening, and the overall dynamicity. Much higher erucin concentrations were required to reduce the microtubule polymer mass. In addition, erucin suppressed dynamic instability of microtubules reassembled from purified tubulin in similar fashion. The effects of erucin on microtubule dynamics, like those of sulforaphane, are similar qualitatively to those of much more powerful clinically-used microtubule-targeting anticancer drugs, including taxanes and the vinca alkaloids. The results suggest that suppression of microtubule dynamics by erucin and the resulting impairment of critically important microtubule-dependent cell functions such as mitosis, cell migration and microtubule-based transport may be important in its cancer preventive activities.

Interplay between HDAC6 and its interacting partners: essential roles in the aggresome-autophagy pathway and neurodegenerative diseases

Cytoplasmic localization and possession of two deacetylase domains and a ubiquitin-binding domain make histone deacetylase 6 (HDAC6) a unique histone deacetylase. HDAC6 interacts with a number of proteins in the cytoplasm. Some of these proteins can be deacetylated by HDAC6 deacetylase activity. Others can affect HDAC6 functions by modulating its catalytic activity or ubiquitin-binding capability. Over the last decade, HDAC6 has been shown to play important roles in the aggresome-autophagy pathway, which selectively targets on protein aggregates or damaged organelles for their accumulation and clearance in cells. HDAC6-interacting partners are integral components in this pathway with regard to their regulatory roles through interaction with HDAC6. The aggresome-autophagy pathway appears to be an attractive therapeutic target for the treatment of neurodegenerative diseases as accumulation of protein aggregates are hallmarks in these diseases. In the current review, I discuss the molecular details of how HDAC6 and its interacting partners regulate each individual step in the aggresome-autophagy pathway and also provide perspectives of how HDAC6 can be targeted in treating neurodegenerative diseases.

Early axonal loss accompanied by impaired endocytosis, abnormal axonal transport, and decreased microtubule stability occur in the model of Krabbe's disease

In Krabbe's disease (KD), a leukodystrophy caused by β-galactosylceramidase deficiency, demyelination and a myelin-independent axonopathy contributes to the severe neuropathology. Beyond axonopathy, we show that in Twitcher mice, a model of KD, a decreased number of axons both in the PNS and in the CNS, and of neurons in dorsal root ganglia (DRG), occurred before the onset of demyelination. Despite the early axonal loss, and although in vitro Twitcher neurites degenerated over time, Twitcher DRG neurons displayed an initial neurite overgrowth and, following sciatic nerve injury, Twitcher axons were regeneration-competent, at a time point where axonopathy was already ongoing. Psychosine, the toxic substrate that accumulates in KD, induced lipid raft clustering. At the mechanistic level, TrkA recruitment to lipid rafts was dysregulated in Twitcher neurons, and defective activation of the ERK1/2 and AKT pathways was identified. Besides defective recruitment of signaling molecules to lipid rafts, the early steps of endocytosis and the transport of endocytic and synaptic vesicles were impaired in Twitcher DRG neurons. Defects in axonal transport, specifically in the retrograde component, correlated with decreased levels of dynein, abnormal levels of post-translational tubulin modifications and decreased microtubule stability. The identification of the axonal defects that precede demyelination in KD, together with the finding that Twitcher axons are regeneration-competent when axonopathy is already installed, opens new windows of action to effectively correct the neuropathology that characterizes this disorder.

Regulation of SIRT2-dependent α-tubulin deacetylation by cellular NAD levels

Acetylation of α-tubulin on lysine 40 is one of the major posttranslational modifications of microtubules. The acetylation reaction is catalyzed by alpha-tubulin N-acetyltransferase and the modification can be reversed by either the NAD-independent class II histone deacetylase HDAC6 or the NAD-dependent deacetylase SIRT2. In this study, we assessed to what extent cellular NAD levels are involved in the regulation of the α-tubulin acetylation state. Cells were subjected to different treatments known to influence cellular NAD content. In response to NAD depletion caused by inhibition of NAD synthesis from nicotinamide, α-tubulin was hyperacetylated. Under these conditions, the normal tubulin acetylation state could be restored by providing the cells with alternative NAD precursors. Likewise, decreasing the rate of endogenous NAD consumption using an inhibitor of poly-ADP-ribosylation also stabilized the acetylation of α-tubulin. Conversely, the level of acetylated α-tubulin decreased when NAD synthesis was enhanced by overexpression of an NAD biosynthetic enzyme. Combined, these results show that the tubulin acetylation status is reciprocally regulated by cellular NAD levels. Furthermore, we provide evidence confirming that the NAD-dependent regulation of tubulin acetylation is mediated by SIRT2.

Septins promote dendrite and axon development by negatively regulating microtubule stability via HDAC6-mediated deacetylation

Neurite growth requires two guanine nucleotide-binding protein polymers of tubulins and septins. However, whether and how those cytoskeletal systems are coordinated was unknown. Here we show that the acute knockdown or knockout of the pivotal septin subunit SEPT7 from cerebrocortical neurons impairs their interhemispheric and cerebrospinal axon projections and dendritogenesis in perinatal mice, when the microtubules are severely hyperacetylated. The resulting hyperstabilization and growth retardation of microtubules are demonstrated in vitro. The phenotypic similarity between SEPT7 depletion and the pharmacological inhibition of α-tubulin deacetylase HDAC6 reveals that HDAC6 requires SEPT7 not for its enzymatic activity, but to associate with acetylated α-tubulin. These and other findings indicate that septins provide a physical scaffold for HDAC6 to achieve efficient microtubule deacetylation, thereby negatively regulating microtubule stability to an optimal level for neuritogenesis. Our findings shed light on the mechanisms underlying the HDAC6-mediated coupling of the two ubiquitous cytoskeletal systems during neural development.

Septins are a family of heteropolymerizing GTP/GDP-binding proteins and are implicated in neuritogenesis in nematodes. Ageta-Ishihara et al. show that septins also facilitate this process in the developing mouse brain as scaffolds that coordinate HDAC6-mediated deacetylation of microtubules.

Erasers of histone acetylation: the histone deacetylase enzymes

Histone deacetylases (HDACs) are enzymes that catalyze the removal of acetyl functional groups from the lysine residues of both histone and nonhistone proteins. In humans, there are 18 HDAC enzymes that use either zinc- or NAD(+)-dependent mechanisms to deacetylate acetyl lysine substrates. Although removal of histone acetyl epigenetic modification by HDACs regulates chromatin structure and transcription, deacetylation of nonhistones controls diverse cellular processes. HDAC inhibitors are already known potential anticancer agents and show promise for the treatment of many diseases.

Dido3-dependent HDAC6 targeting controls cilium size

Primary cilia are involved in a variety of physiological processes such as sensing of the environment, cell growth and development. Numerous developmental disorders and pathologies arise from defects in these organelles. Multiple proteins that promote formation and disassembly of the primary cilium have been identified, but little is known about the mechanisms that control steady-state cilium size. Here, we show that death inducer obliterator (Dido3)-dependent targeting of histone deacetylase 6 (HDAC6) is a key determinant of cilium size in growth-arrested cells. The amount of either protein negatively correlates with cilium size. Dido3 availability at the centrosome governs ciliary HDAC6 levels, and redistribution of the two proteins controls tubulin acetylation. In turn, basal body localization of Dido3 and HDAC6 depends on the actin network, previously shown to limit cilium size independent of the cell cycle. These results show that not only kinase-dependent activation of a deacetylase but also its subcellular distribution controls substrate selection.