Identification of components of the murine histone deacetylase 6 complex: link between acetylation and ubiquitination signaling pathways

The ZIKV NS5 Protein Aberrantly Alters the Tubulin Cytoskeleton, Induces the Accumulation of Autophagic p62 and Affects IFN Production: HDAC6 Has Emerged as an Anti-NS5/ZIKV Factor

Zika virus (ZIKV) infection and pathogenesis are linked to the disruption of neurogenesis, congenital Zika syndrome and microcephaly by affecting neural progenitor cells. Nonstructural protein 5 (NS5) is the largest product encoded by ZIKV-RNA and is important for replication and immune evasion. Here, we studied the potential effects of NS5 on microtubules (MTs) and autophagy flux, together with the interplay of NS5 with histone deacetylase 6 (HDAC6). Fluorescence microscopy, biochemical cell-fractionation combined with the use of HDAC6 mutants, chemical inhibitors and RNA interference indicated that NS5 accumulates in nuclear structures and strongly promotes the acetylation of MTs that aberrantly reorganize in nested structures. Similarly, NS5 accumulates the p62 protein, an autophagic-flux marker. Therefore, NS5 alters events that are under the control of the autophagic tubulin-deacetylase HDAC6. HDAC6 appears to degrade NS5 by autophagy in a deacetylase- and BUZ domain-dependent manner and to control the cytoplasmic expression of NS5. Moreover, NS5 inhibits RNA-mediated RIG-I interferon (IFN) production, resulting in greater activity when autophagy is inhibited (i.e., effect correlated with NS5 stability). Therefore, it is conceivable that NS5 contributes to cell toxicity and pathogenesis, evading the IFN-immune response by overcoming HDAC6 functions. HDAC6 has emerged as an anti-ZIKV factor by targeting NS5.

Role of Histone Deacetylase 6 and Histone Deacetylase 6 Inhibition in Colorectal Cancer

Histone deacetylase 6 (HDAC6), by deacetylation of multiple substrates and association with interacting proteins, regulates many physiological processes that are involved in cancer development and invasiveness such as cell proliferation, apoptosis, motility, epithelial to mesenchymal transition, and angiogenesis. Due to its ability to remove misfolded proteins, induce autophagy, and regulate unfolded protein response, HDAC6 plays a protective role in responses to stress and enables tumor cell survival. The scope of this review is to discuss the roles of HDCA6 and its implications for the therapy of colorectal cancer (CRC). As HDAC6 is overexpressed in CRC, correlates with poor disease prognosis, and is not essential for normal mammalian development, it represents a good therapeutic target. Selective inhibition of HDAC6 impairs growth and progression without inducing major adverse events in experimental animals. In CRC, HDAC6 inhibitors have shown the potential to reduce tumor progression and enhance the therapeutic effect of other drugs. As HDAC6 is involved in the regulation of immune responses, HDAC6 inhibitors have shown the potential to improve antitumor immunity by increasing the immunogenicity of tumor cells, augmenting immune cell activity, and alleviating immunosuppression in the tumor microenvironment. Therefore, HDAC6 inhibitors may represent promising candidates to improve the effect of and overcome resistance to immunotherapy.

A quantitative model for virus uncoating predicts influenza A infectivity

For virus infection of new host cells, the disassembly of the protective outer protein shell (capsid) is a critical step, but the mechanisms and host-virus interactions underlying the dynamic, active, and regulated uncoating process are largely unknown. Here, we develop an experimentally supported, multiscale kinetics model that elucidates mechanisms of influenza A virus (IAV) uncoating in cells. Biophysical modeling demonstrates that interactions between capsid M1 proteins, host histone deacetylase 6 (HDAC6), and molecular motors can physically break the capsid in a tug-of-war mechanism. Biochemical analysis and biochemical-biophysical modeling identify unanchored ubiquitin chains as essential and allow robust prediction of uncoating efficiency in cells. Remarkably, the different infectivity of two clinical strains can be ascribed to a single amino acid variation in M1 that affects binding to HDAC6. By identifying crucial modules of viral infection kinetics, the mechanisms and models presented here could help formulate novel strategies for broad-range antiviral treatment.

Targeting Histone Deacetylases 6 in Dual-Target Therapy of Cancer

Histone deacetylases (HDACs) are the major regulators of the balance of acetylation of histone and non-histone proteins. In contrast to other HDAC isoforms, HDAC6 is mainly involved in maintaining the acetylation balance of many non-histone proteins. Therefore, the overexpression of HDAC6 is associated with tumorigenesis, invasion, migration, survival, apoptosis and growth of various malignancies. As a result, HDAC6 is considered a promising target for cancer treatment. However, none of selective HDAC6 inhibitors are in clinical use, mainly because of the low efficacy and high concentrations used to show anticancer properties, which may lead to off-target effects. Therefore, HDAC6 inhibitors with dual-target capabilities represent a new trend in cancer treatment, aiming to overcome the above problems. In this review, we summarize the advances in tumor treatment with dual-target HDAC6 inhibitors.

Recent development of selective inhibitors targeting the HDAC6 as anti-cancer drugs: Structure, function and design

HDAC6, a member of the histone deacetylase family, mainly is a cytosolic protein and regulates cell growth by acting on non-histone substrates, such as α -tubulin, cortactin, heat shock protein HSP90, programmed death 1 (PD-1) and programmed death ligand 1 (PD-L1), that are closely related to the proliferation, invasion, immune escape and angiogenesis of cancer tissues. The approved drugs targeting the HDACs are all pan-inhibitors and have many side effects due to their lack of selectivity. Therefore, development of selective inhibitors of HDAC6 has attracted much attention in the field of cancer therapy. In this review, we will summarize the relationship between HDAC6 and cancer, and discuss the design strategies of HDAC6 inhibitors for cancer treatment in recent years.

Histone deacetylase 6's function in viral infection, innate immunity, and disease: latest advances

In the family of histone-deacetylases, histone deacetylase 6 (HDAC6) stands out. The cytoplasmic class IIb histone deacetylase (HDAC) family is essential for many cellular functions. It plays a crucial and debatable regulatory role in innate antiviral immunity. This review summarises the current state of our understanding of HDAC6's structure and function in light of the three mechanisms by which it controls DNA and RNA virus infection: cytoskeleton regulation, host innate immune response, and autophagy degradation of host or viral proteins. In addition, we summed up how HDAC6 inhibitors are used to treat a wide range of diseases, and how its upstream signaling plays a role in the antiviral mechanism. Together, the findings of this review highlight HDAC6's importance as a new therapeutic target in antiviral immunity, innate immune response, and some diseases, all of which offer promising new avenues for the development of drugs targeting the immune response.

Discovery and Characterization of a Chemical Probe Targeting the Zinc-Finger Ubiquitin-Binding Domain of HDAC6

Histone deacetylase 6 (HDAC6) inhibition is an attractive strategy for treating numerous cancers, and HDAC6 catalytic inhibitors are currently in clinical trials. The HDAC6 zinc-finger ubiquitin-binding domain (UBD) binds free C-terminal diglycine motifs of unanchored ubiquitin polymer chains and protein aggregates, playing an important role in autophagy and aggresome assembly. However, targeting this domain with small molecule antagonists remains an underdeveloped avenue of HDAC6-focused drug discovery. We report SGC-UBD253 (25), a chemical probe potently targeting HDAC6-UBD in vitro with selectivity over nine other UBDs, except for weak USP16 binding. In cells, 25 is an effective antagonist of HDAC6-UBD at 1 μM, with marked proteome-wide selectivity. We identified SGC-UBD253N (32), a methylated derivative of 25 that is 300-fold less active, serving as a negative control. Together, 25 and 32 could enable further exploration of the biological function of the HDAC6-UBD and investigation of the therapeutic potential of targeting this domain.

Discovery of the First Irreversible HDAC6 Isoform Selective Inhibitor with Potent Anti-Multiple Myeloma Activity

In our previous research, a series of phenylsulfonylfuroxan-based hydroxamates were developed, among which compound 1 exhibited remarkable in vitro and in vivo antitumor potency due to its histone deacetylase (HDAC) inhibitory and nitric oxide (NO)-donating activities. Herein, the in-depth study of compound 1 revealed that this HDAC inhibitor-NO donor hybrid could enduringly increase the intracellular levels of acetyl histones and acetyl α-tubulin, which could be ascribed to its irreversible inhibition toward class I HDACs and HDAC6. Structural modification of compound 1 led to a novel phenylsulfonylfuroxan-based hydroxamate 4, which exhibited considerable HDAC6 inhibitory activity and selectivity. Furthermore, compound 4 could inhibit intracellular HDAC6 both selectively and irreversibly. To the best of our knowledge, this is the first research reporting the irreversible inhibition of HDAC6. It was also demonstrated that compared with ACY-241 (a reversible HDAC6 inhibitor in clinical trials), the irreversible HDAC6 selective inhibitor 4 exhibited not only superior anti-multiple myeloma activity but also improved therapeutic index.

HDAC6 Deficiency Has Moderate Effects on Behaviors and Parkinson's Disease Pathology in Mice

Histone deacetylase 6 (HDAC6) is involved in the regulation of protein aggregation and neuroinflammation, but its role in Parkinson's disease (PD) remains controversial. In this study, Hdac6^-/- mice were generated by CRISPR-Cas9 technology for exploring the effect of HDAC6 on the pathological progression of PD. We found that male Hdac6^-/- mice exhibit hyperactivity and certain anxiety. In the acute 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced PD mice, though motor injury was slightly alleviated by HDAC6 deficiency, dopamine (DA) depletion in the striatum, the decrease in the number of DA neurons in the substantia nigra (SN) and the reduction in DA neuronal terminals were not affected. In addition, activation of glial cells and the expression of α-synuclein, as well as the levels of apoptosis-related proteins in the nigrostriatal pathway, were not changed in MPTP-injected wild-type and Hdac6^-/- mice. Therefore, HDAC6 deficiency leads to moderate alterations of behaviors and Parkinson's disease pathology in mice.

Tubulin Post-Translational Modifications: The Elusive Roles of Acetylation

Microtubules (MTs), dynamic polymers of α/β-tubulin heterodimers found in all eukaryotes, are involved in cytoplasm spatial organization, intracellular transport, cell polarity, migration and division, and in cilia biology. MTs functional diversity depends on the differential expression of distinct tubulin isotypes and is amplified by a vast number of different post-translational modifications (PTMs). The addition/removal of PTMs to α- or β-tubulins is catalyzed by specific enzymes and allows combinatory patterns largely enriching the distinct biochemical and biophysical properties of MTs, creating a code read by distinct proteins, including microtubule-associated proteins (MAPs), which allow cellular responses. This review is focused on tubulin-acetylation, whose cellular roles continue to generate debate. We travel through the experimental data pointing to α-tubulin Lys40 acetylation role as being a MT stabilizer and a typical PTM of long lived MTs, to the most recent data, suggesting that Lys40 acetylation enhances MT flexibility and alters the mechanical properties of MTs, preventing MTs from mechanical aging characterized by structural damage. Additionally, we discuss the regulation of tubulin acetyltransferases/desacetylases and their impacts on cell physiology. Finally, we analyze how changes in MT acetylation levels have been found to be a general response to stress and how they are associated with several human pathologies.

Upregulation of Profilin 2 on HDAC6 overexpression in mouse GC-1 cells and its putative role in germ cell migration in the testis

Previous reports from this laboratory have demonstrated the involvement of histone deacetylase 6 (HDAC6) in sperm motility. As the presence of HDAC6 has also been reported in the earlier stage germ cells, studies were undertaken to explore its role during these stages of spermatogenesis. HDAC6 was overexpressed in GC-1spg cells, which represent the stage between type B spermatogonia and primary spermatocyte, and its effect on germ cell transcriptome was investigated by microarray. Among the many transcripts that were differentially regulated, Profilin 2, reported previously as a neuronal specific isoform, was observed as one of the genes highly upregulated at the transcript level, which was further confirmed by real-time PCR, and the protein confirmed by indirect immunofluorescence (IIF). Profilin 2 colocalized with HDAC6, as seen both in GC-1 cells and sperm. On the sperm, the presence of Profilin 2 was detected throughout the flagella with its colocalization with HDAC6 seen conspicuously in the mid-piece region of the flagella. Co-immunoprecipitation studies confirmed Profilin 2 interaction with HDAC6. Docking studies using Z dock suggested the interaction of 8 residues of HDAC6 with 6 residues of Profilin 2. The novel observation of Profilin 2 in spermatogonial cells, its significant upregulation on HDAC6 overexpression and its interaction with HDAC6 suggests that HDAC6 in collaboration with Profilin 2 may play a role in regulating the movement of germ cells from one stage/compartment to the next.

Immunoprecipitation of HDAC6 and Interacting Proteins

The lysine deacetylase HDAC6 has unique structural and functional properties: It contains tandem catalytic domains that can deacetylate a variety of proteins and a zinc finger domain that binds ubiquitin. HDAC6 has been implicated in a variety of biological processes, normal or pathological, such as cellular motility, stress response, cancer, neurodegeneration, or viral infection. Due to this, HDAC6 is considered an attractive therapeutic target, and there is a major interest to identify small molecule inhibitors. To gain a mechanistic understanding of how HDAC6 impacts these different biological processes, there is a continued need to discover additional substrates as well as interacting proteins in different paradigms. One approach to achieve this is to perform HDAC6 immunoprecipitations to identify partner proteins. We describe here our optimized protocols to immunoprecipitate HDAC6 with the goal to identify or validate interacting proteins.

Expression and Crystallization of HDAC6 Tandem Catalytic Domains

Histone deacetylase 6 (HDAC6) is an atypical lysine deacetylase with tandem catalytic domains and an ubiquitin-binding zinc finger domain. HDAC6 is involved in various biological processes, such as cell motility or stress responses, and has been implicated in pathologies ranging from cancer to neurodegeneration. Due to this broad range of functions, there has been considerable interest in developing HDAC6-specific small molecule inhibitors, several of which are already available. The crystal structure of the tandem catalytic domains of zebrafish HDAC6 has revealed an arrangement with twofold symmetry and extensive surface interaction between the catalytic domains. Further dissection of the biochemical properties of HDAC6 and the development of novel inhibitors will benefit from being able to routinely express high-quality protein. We present here our optimized protocol for expression and crystallization of the zebrafish tandem catalytic domains.

In silico design of HDAC6 inhibitors with neuroprotective effects

HDAC6 has emerged as a molecular target to treat neurodegenerative disorders, due to its participation in protein aggregate degradation, oxidative stress process, mitochondrial transport, and axonal transport. Thus, in this work we have designed a set of 485 compounds with hydroxamic and bulky-hydrophobic moieties that may function as HDAC6 inhibitors with a neuroprotective effect. These compounds were filtered by their predicted ADMET properties and their affinity to HDAC6 demonstrated by molecular docking and molecular dynamics simulations. The combination of in silico with in vitro neuroprotective results allowed the identification of a lead compound (FH-27) which shows neuroprotective effect that could be due to HDAC6 inhibition. Further, FH-27 chemical moiety was used to design a second series of compounds improving the neuroprotective effect from 2- to 10-fold higher (YSL-99, YSL-109, YSL-112, YSL-116 and YSL-121; 1.25 ± 0.67, 1.82 ± 1.06, 7.52 ± 1.78, 5.59 and 5.62 ± 0.31 µM, respectively). In addition, the R enantiomer of FH-27 (YSL-106) was synthesized, showing a better neuroprotective effect (1.27 ± 0.60 µM). In conclusion, we accomplish the in silico design, synthesis, and biological evaluation of hydroxamic acid derivatives with neuroprotective effect as suggested by an in vitro model. Communicated by Ramaswamy H. Sarma.

[HDAC6, a very specific deacetylase with a potential therapeutic role]

The cytoplasmic histone deacetylase 6 (HDAC6) is defined today as a new key player in the treatment of many diseases. Overexpression of HDAC6 was observed in a variety of diseases. Over the past ten years, plenty of new selective inhibitors of HDAC6 activity have been synthesized and characterized. Many studies have shown the high efficiency and beneficial effects of HDAC6 inhibitors in many diseases such as cancers, neurodegenerative, inflammatory, or neuromuscular diseases. The mechanisms of HDAC6 action that explain the benefit of its inhibition in various pathologies are still unknown. We have recently shown that HDAC6, via the regulation of the microtubule network, plays a role at the level of neuromuscular junctions by controlling acetylcholine receptor delivery.

Disrupting the HDAC6-ubiquitin interaction impairs infection by influenza and Zika virus and cellular stress pathways

The deacetylase HDAC6 has tandem catalytic domains and a zinc finger domain (ZnF) binding ubiquitin (Ub). While the catalytic domain has an antiviral effect, the ZnF facilitates influenza A virus (IAV) infection and cellular stress responses. By recruiting Ub via the ZnF, HDAC6 promotes the formation of aggresomes and stress granules (SGs), dynamic structures associated with pathologies such as neurodegeneration. IAV subverts the aggresome/HDAC6 pathway to facilitate capsid uncoating during early infection. To target this pathway, we generate designed ankyrin repeat proteins (DARPins) binding the ZnF; one of these prevents interaction with Ub in vitro and in cells. Crystallographic analysis shows that it blocks the ZnF pocket where Ub engages. Conditional expression of this DARPin reversibly impairs infection by IAV and Zika virus; moreover, SGs and aggresomes are downregulated. These results validate the HDAC6 ZnF as an attractive target for drug discovery.

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A small synthetic protein (DARPin) blocks interaction between HDAC6 and ubiquitin

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This DARPin impairs infection by influenza and Zika virus at the uncoating step

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Both viruses contain ubiquitin associated with their capsid

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The DARPin also impacts the formation of aggresomes and stress granules

HDAC6 Interacts With Poly (GA) and Modulates its Accumulation in c9FTD/ALS

The aberrant translation of a repeat expansion in chromosome 9 open reading frame 72 (C9orf72), the most common cause of frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS), results in the accumulation of toxic dipeptide repeat (DPR) proteins in the central nervous system We have found that, among the sense DPR proteins, HDAC6 specifically interacts with the poly (GA) and co-localizes with inclusions in both patient tissue and a mouse model of this disease (c9FTD/ALS). Overexpression of HDAC6 increased poly (GA) levels in cultured cells independently of HDAC6 deacetylase activity, suggesting that HDAC6 can modulate poly (GA) pathology through a mechanism that depends upon their physical interaction. Moreover, decreasing HDAC6 expression by stereotaxic injection of antisense oligonucleotides significantly reduced the number of poly (GA) inclusions in c9FTD/ALS mice. These findings suggest that pharmacologically reducing HDAC6 levels could be of therapeutic value in c9FTD/ALS.

Histone deacetylase 6 inhibition restores leptin sensitivity and reduces obesity

The adipose tissue-derived hormone leptin can drive decreases in food intake while increasing energy expenditure. In diet-induced obesity, circulating leptin levels rise proportionally to adiposity. Despite this hyperleptinemia, rodents and humans with obesity maintain increased adiposity and are resistant to leptin's actions. Here we show that inhibitors of the cytosolic enzyme histone deacetylase 6 (HDAC6) act as potent leptin sensitizers and anti-obesity agents in diet-induced obese mice. Specifically, HDAC6 inhibitors, such as tubastatin A, reduce food intake, fat mass, hepatic steatosis and improve systemic glucose homeostasis in an HDAC6-dependent manner. Mechanistically, peripheral, but not central, inhibition of HDAC6 confers central leptin sensitivity. Additionally, the anti-obesity effect of tubastatin A is attenuated in animals with a defective central leptin-melanocortin circuitry, including db/db and MC4R knockout mice. Our results suggest the existence of an HDAC6-regulated adipokine that serves as a leptin-sensitizing agent and reveals HDAC6 as a potential target for the treatment of obesity.

The role of protein acetylation in regulating mitochondrial fusion and fission

The dynamic processes of mitochondrial fusion and fission determine the shape of mitochondria, which can range from individual fragments to a hyperfused network, and influence mitochondrial function. Changes in mitochondrial shape can occur rapidly, allowing mitochondria to adapt to specific cues and changing cellular demands. Here, we will review what is known about how key proteins required for mitochondrial fusion and fission are regulated by their acetylation status, with acetylation promoting fission and deacetylation enhancing fusion. In particular, we will examine the roles of NAD+ dependant sirtuin deacetylases, which mediate mitochondrial acetylation, and how this post-translational modification provides an exquisite regulatory mechanism to co-ordinate mitochondrial function with metabolic demands of the cell.

Synthesis and biological evaluation of selective histone deacetylase 6 inhibitors as multifunctional agents against Alzheimer's disease

Histone deacetylase 6 (HDAC6) is a potential target for Alzheimer's disease (AD). In this study, a series of novel phenothiazine-, memantine-, and 1,2,3,4-tetrahydro-γ-carboline-based HDAC6 inhibitors with a variety of linker moieties were designed and synthesized. As a hydrochloride salt, the phenothiazine-based hydroxamic acid W5 with a pyridyl-containing linker motif was identified as a high potent and selective HDAC6 inhibitor. It inhibited HDAC6 with an IC₅₀ of 2.54 nM and was more than 290- to 3300-fold selective over other HDAC isoforms. In SH-SY5Y cells, W5 dose-dependently increased the acetylated α-tubulin levels and reduced the hyperphosphorylated tau proteins at Ser396. As an effective metal chelator, W5 inhibited Cu²⁺-induced Aβ_1-42 aggregation and disaggregated Cu²⁺-Aβ_1-42 oligomers, and showed protective effects on the SH-SY5Y cells against Aβ_1-42- as well as Cu²⁺-Aβ_1-42 induced cell damages, serving as a potential ligand to target AD metal dyshomeostasis. Moreover, W5 promoted the differentiated neuronal neurite outgrowth, increased the mRNA expression of the recognized neurogenesis markers, GAP43, N-myc, and MAP-2. Therefore, W5 might be a good lead for the development of novel HDAC6 inhibitors targeting multi-facets of AD.

Development of selective HDAC6 inhibitors with in vitro and in vivo anti-multiple myeloma activity

Histone deacetylase 6 (HDAC6) is a promising therapeutic target for the treatment of cancers, neurodegenerative diseases and autoimmune disorders. Herein a novel series of pyrrolo[2,3-d]pyrimidine-based HDAC inhibitors were designed, synthesized and biologically evaluated, among which compounds 7a, 12a1, and 16a1 exhibited potent inhibitory activities and selectivities against HDAC6. Notably, compared with the well-known HDAC6 inhibitor Tubastatin A, our pyrrolo[2,3-d]pyrimidine-based HDAC6 inhibitors showed superior in vitro antiproliferative activity against human multiple myeloma cell lines RPMI 8226, U266 and MM.1S, while maintaining the low cytotoxicity against human breast cancer cell line MDA-MB-231 and two normal cell lines. The HDAC6 selective inhibition of one representative compound 12a1 in RPMI 8226 cells was confirmed by western blot analysis. Although pyrrolo[2,3-d]pyrimidine is a privileged structure in many kinase inhibitors, compound 12a1 showed negligible inhibition against several kinases including JAK family members and Akt1, indicating its acceptable off-target profile. Besides, compound 12a1 exhibited desirable metabolic stability in mouse liver microsome. The in vivo anti-multiple myeloma potency of 12a1, alone and in combination with bortezomib, was demonstrated in a RPMI 8226 xenograft model.

Dual Targeting Strategies On Histone Deacetylase 6 (HDAC6) And Heat Shock Protein 90 (Hsp90)

The design of multi-target drugs acting simultaneously on multiple signaling pathways is a growing field in medicinal chemistry, especially for the treatment of complex diseases such as cancer. Histone deacetylase 6 (HDAC6) is an established anticancer drug target involved in tumor cells transformation. Being an epigenetic enzyme at the interplay of many biological processes, HDAC6 has become an attractive target for polypharmacology studies aimed at improving therapeutic efficacy of anticancer drugs. For example, the molecular chaperone Heat shock protein 90 (Hsp90) is a substrate of HDAC6 deacetylation, and several lines of evidence demonstrate that simultaneous inhibition of HDAC6 and Hsp90 promote synergistic antitumor effects on different cancer cell lines, highlighting the potential benefits of developing a single molecule endowed with multi-target activity. This review will summarize the complex interplay between HDAC6 and Hsp90, providing also useful hints for multi-target drug design and discovery approaches in this field. To this end, crystallographic structures of HDAC6 and Hsp90 complexes will be extensively reviewed in the light of discussing binding pockets features and pharmacophore requirements and providing useful guidelines for the design of dual inhibitors. The few examples of multi-target inhibitors obtained so far, mostly based on chimeric approaches, will be summarized and put into context. Finally, the main features of HDAC6 and Hsp90 inhibitors will be compared, and ligand- and structure-based strategies potentially useful for the development of small molecular weight dual inhibitors will be proposed and discussed.

Histone deacetylase in neuropathology

Neuroepigenetics, a new branch of epigenetics, plays an important role in the regulation of gene expression. Neuroepigenetics is associated with holistic neuronal function and helps in formation and maintenance of memory and learning processes. This includes neurodevelopment and neurodegenerative defects in which histone modification enzymes appear to play a crucial role. These modifications, carried out by acetyltransferases and deacetylases, regulate biologic and cellular processes such as apoptosis and autophagy, inflammatory response, mitochondrial dysfunction, cell-cycle progression and oxidative stress. Alterations in acetylation status of histone as well as non-histone substrates lead to transcriptional deregulation. Histone deacetylase decreases acetylation status and causes transcriptional repression of regulatory genes involved in neural plasticity, synaptogenesis, synaptic and neural plasticity, cognition and memory, and neural differentiation. Transcriptional deactivation in the brain results in development of neurodevelopmental and neurodegenerative disorders. Mounting evidence implicates histone deacetylase inhibitors as potential therapeutic targets to combat neurologic disorders. Recent studies have targeted naturally-occurring biomolecules and micro-RNAs to improve cognitive defects and memory. Multi-target drug ligands targeting HDAC have been developed and used in cell-culture and animal-models of neurologic disorders to ameliorate synaptic and cognitive dysfunction. Herein, we focus on the implications of histone deacetylase enzymes in neuropathology, their regulation of brain function and plausible involvement in the pathogenesis of neurologic defects.

How Influenza Virus Uses Host Cell Pathways during Uncoating

Influenza is a zoonotic respiratory disease of major public health interest due to its pandemic potential, and a threat to animals and the human population. The influenza A virus genome consists of eight single-stranded RNA segments sequestered within a protein capsid and a lipid bilayer envelope. During host cell entry, cellular cues contribute to viral conformational changes that promote critical events such as fusion with late endosomes, capsid uncoating and viral genome release into the cytosol. In this focused review, we concisely describe the virus infection cycle and highlight the recent findings of host cell pathways and cytosolic proteins that assist influenza uncoating during host cell entry.

The Interplay of HIV and Autophagy in Early Infection

HIV/AIDS is still a global threat despite the notable efforts made by the scientific and health communities to understand viral infection, to design new drugs or to improve existing ones, as well as to develop advanced therapies and vaccine designs for functional cure and viral eradication. The identification and analysis of HIV-1 positive individuals that naturally control viral replication in the absence of antiretroviral treatment has provided clues about cellular processes that could interact with viral proteins and RNA and define subsequent viral replication and clinical progression. This is the case of autophagy, a degradative process that not only maintains cell homeostasis by recycling misfolded/old cellular elements to obtain nutrients, but is also relevant in the innate and adaptive immunity against viruses, such as HIV-1. Several studies suggest that early steps of HIV-1 infection, such as virus binding to CD4 or membrane fusion, allow the virus to modulate autophagy pathways preparing cells to be permissive for viral infection. Confirming this interplay, strategies based on autophagy modulation are able to inhibit early steps of HIV-1 infection. Moreover, autophagy dysregulation in late steps of the HIV-1 replication cycle may promote autophagic cell-death of CD4⁺ T cells or control of HIV-1 latency, likely contributing to disease progression and HIV persistence in infected individuals. In this scenario, understanding the molecular mechanisms underlying HIV/autophagy interplay may contribute to the development of new strategies to control HIV-1 replication. Therefore, the aim of this review is to summarize the knowledge of the interplay between autophagy and the early events of HIV-1 infection, and how autophagy modulation could impair or benefit HIV-1 infection and persistence, impacting viral pathogenesis, immune control of viral replication, and clinical progression of HIV-1 infected patients.

Acetylation/deacetylation and microtubule associated proteins influence flagellar axonemal stability and sperm motility

PTMs and microtubule-associated proteins (MAPs) are known to regulate microtubule dynamicity in somatic cells. Reported literature on modulation of α-tubulin acetyl transferase (αTAT1) and histone deacetylase 6 (HDAC6) in animal models and cell lines illustrate disparity in correlating tubulin acetylation status with stability of MT. Our earlier studies showed reduced acetyl tubulin in sperm of asthenozoospermic individuals. Our studies on rat sperm showed that on inhibition of HDAC6 activity, although tubulin acetylation increased, sperm motility was reduced. Studies were therefore undertaken to investigate the influence of tubulin acetylation/deacetylation on MT dynamicity in sperm flagella using rat and human sperm. Our data on rat sperm revealed that HDAC6 specific inhibitor Tubastatin A (T) inhibited sperm motility and neutralized the depolymerizing and motility debilitating effect of Nocodazole. The effect on polymerization was further confirmed in vitro using pure MT and recHDAC6. Also polymerized axoneme was less in sperm of asthenozoosperm compared to normozoosperm. Deacetylase activity was reduced in sperm lysates and axonemes exposed to T and N+T but not in axonemes of sperm treated similarly suggesting that HDAC6 is associated with sperm axonemes or MT. Deacetylase activity was less in asthenozoosperm. Intriguingly, the expression of MDP3 physiologically known to bind to HDAC6 and inhibit its deacetylase activity remained unchanged. However, expression of acetyl α-tubulin, HDAC6 and microtubule stabilizing protein SAXO1 was less in asthenozoosperm. These observations suggest that MAPs and threshold levels of MT acetylation/deacetylation are important for MT dynamicity in sperm and may play a role in regulating sperm motility.

Tetrahydroquinoline-Capped Histone Deacetylase 6 Inhibitor SW-101 Ameliorates Pathological Phenotypes in a Charcot-Marie-Tooth Type 2A Mouse Model

Histone deacetylase 6 (HDAC6) is a promising therapeutic target for the treatment of neurodegenerative disorders. SW-100 (1a), a phenylhydroxamate-based HDAC6 inhibitor (HDAC6i) bearing a tetrahydroquinoline (THQ) capping group, is a highly potent and selective HDAC6i that was shown to be effective in mouse models of Fragile X syndrome and Charcot-Marie-Tooth disease type 2A (CMT2A). In this study, we report the discovery of a new THQ-capped HDAC6i, termed SW-101 (1s), that possesses excellent HDAC6 potency and selectivity, together with markedly improved metabolic stability and druglike properties compared to SW-100 (1a). X-ray crystallography data reveal the molecular basis of HDAC6 inhibition by SW-101 (1s). Importantly, we demonstrate that SW-101 (1s) treatment elevates the impaired level of acetylated α-tubulin in the distal sciatic nerve, counteracts progressive motor dysfunction, and ameliorates neuropathic symptoms in a CMT2A mouse model bearing mutant MFN2. Taken together, these results bode well for the further development of SW-101 (1s) as a disease-modifying HDAC6i.

HDAC6 regulates microtubule stability and clustering of AChRs at neuromuscular junctions

Microtubules (MTs) are known to be post-translationally modified at the neuromuscular junction (NMJ), hence increasing their stability. To date however, the function(s) of the dynamic MT network and its relative stability in the formation and maintenance of NMJs remain poorly described. Stabilization of the MT is dependent in part on its acetylation status, and HDAC6 is capable of reversing this post-translational modification. Here, we report that HDAC6 preferentially accumulates at NMJs and that it contributes to the organization and the stability of NMJs. Indeed, pharmacological inhibition of HDAC6 protects against MT disorganization and reduces the size of acetylcholine receptor (AChR) clusters. Moreover, the endogenous HDAC6 inhibitor paxillin interacts with HDAC6 in skeletal muscle cells, colocalizes with AChR aggregates, and regulates the formation of AChR. Our findings indicate that the focal insertion of AChRs into the postsynaptic membrane is regulated by stable MTs and highlight how an MT/HDAC6/paxillin axis participates in the regulation of AChR insertion and removal to control the structure of NMJs.

Overcome the tumor immunotherapy resistance by combination of the HDAC6 inhibitors with antitumor immunomodulatory agents

Tumor immunotherapy is currently subject of intense scientific and clinical developments. In previous decade, therapists used natural immune system from the human body to treat several diseases. Although tumor immune disease is a big challenge, combinatorial therapeutic strategy has been succeeded to show the clinical significance. In this context, we discuss the HDAC6 and tumor immune diseases relationship. Also, we summarized the current state of knowledge that based on the combination treatments of the HDAC6 inhibitors (HDAC6is) with antitumor immunomodulatory agents. We observed that, the combination therapies slow down the tumor immune diseases by blocking the aggresome and proteasome pathway. The combination therapy was able to reduce M2 macrophage and increasing PD-L1 blockade sensitivity. Most importantly, multiple combinations of HDAC6is with other agents may consider as potential strategies to treat tumor immune diseases, by reducing the side effects and improve efficacy for the future clinical development.

Upregulated Histone Deacetylase 6 Associates with Malignant Progression of Melanoma and Predicts the Prognosis of Patients

Background

Melanoma is the most malignant tumor among skin tumors, and its morbidity and mortality are increasing year by year. Although melanoma biology has been increasingly studied, no prognostic biomarkers have yet been incorporated into clinical protocols. Histone deacetylase 6 (HDAC6) has been shown to act as a prognostic biomarker in several cancers. Here, we aimed to investigate the predictive value of HDAC6 for the prognosis of cutaneous melanoma patients.

Methods

Eighty cutaneous melanoma patients were enrolled in this study. The protein and mRNA expression levels of HDAC6 were detected, and the clinical features and survival time of cutaneous melanoma patients with HDAC6 expression were analyzed.

Results

The results suggested that high HDAC6 expression was significantly associated with unfavorable clinicopathological features. High HDAC6 expression was related to melanoma metastasis and was also associated with a reduced survival time in melanoma patients, and this association remained significant in multivariate analysis adjusted for all other factors.

Conclusion

These findings validate the utility of HDAC6 expression as an independent biomarker for the prognostication of patients with cutaneous melanoma.

SMARCB1 Promotes Ubiquitination and Degradation of NR4A3 via Direct Interaction Driven by ROS in Vascular Endothelial Cell Injury

Nuclear receptor subfamily 4 group A member 3 (NR4A3) protects the vascular endothelial cell (VEC) against hypoxia stress, whose expression is primarily reported to be governed at a transcriptional level. However, the regulation of NR4A3 in the protein level is largely unknown. Here, we report that NR4A3 protein abundance is decreased immensely in VEC injury induced by reoxygenation after oxygen-glucose deprivation (OGD-R), which is significantly blocked by the administration of the antioxidative steroid TRIOL. Moreover, the notable improvement of NR4A3 and the alleviation of pulmonary endothelial barrier hyperpermeability induced by acute hypobaric hypoxia in cynomolgus monkeys are also observed after TRIOL administration. The overproduction of reactive oxygen species (ROS) decreases NR4A3 protein abundance in VEC under OGD-R condition, which is reversed by TRIOL and N-acetylcysteine (NAC). TRIOL dose-dependently increases the NR4A3 protein level by inhibiting ubiquitination and ubiquitin proteasome system- (UPS-) mediated degradation rather than promoting its transcription. Using yeast two-hybrid screening, we further identify the interaction between NR4A3 and SWI/SNF-related matrix-associated actin-dependent regulator of chromatin subfamily B member 1 (SMARCB1), and the DNA-binding domain of NR4A3 is required for this interaction. Knockdown of SMARCB1 reduces ubiquitination and degradation of NR4A3, suggesting the proubiquitylation effect of this interaction which is enhanced by ROS in VEC injury induced by OGD-R. In summary, our study here for the first time reveals a posttranslational regulation in SMARCB1-mediated NR4A3 protein degradation which is driven by ROS, providing further understanding of the impaired regulation of NR4A3-mediated prosurvival pathways under pathological condition in VEC.

The Roles of Histone Deacetylases and Their Inhibitors in Cancer Therapy

Genetic mutations and abnormal gene regulation are key mechanisms underlying tumorigenesis. Nucleosomes, which consist of DNA wrapped around histone cores, represent the basic units of chromatin. The fifth amino group (Nε) of histone lysine residues is a common site for post-translational modifications (PTMs), and of these, acetylation is the second most common. Histone acetylation is modulated by histone acetyltransferases (HATs) and histone deacetylases (HDACs), and is involved in the regulation of gene expression. Over the past two decades, numerous studies characterizing HDACs and HDAC inhibitors (HDACi) have provided novel and exciting insights concerning their underlying biological mechanisms and potential anti-cancer treatments. In this review, we detail the diverse structures of HDACs and their underlying biological functions, including transcriptional regulation, metabolism, angiogenesis, DNA damage response, cell cycle, apoptosis, protein degradation, immunity and other several physiological processes. We also highlight potential avenues to use HDACi as novel, precision cancer treatments.

Rational Design of Suprastat: A Novel Selective Histone Deacetylase 6 Inhibitor with the Ability to Potentiate Immunotherapy in Melanoma Models

Selective inhibition of histone deacetylase 6 (HDAC6) is being recognized as a therapeutic approach for cancers. In this study, we designed a new HDAC6 inhibitor, named Suprastat, using in silico simulations. X-ray crystallography and molecular dynamics simulations provide strong evidence to support the notion that the aminomethyl and hydroxyl groups in the capping group of Suprastat establish significant hydrogen bond interactions, either direct or water-mediated, with residues D460, N530, and S531, which play a vital role in regulating the deacetylase function of the enzyme and which are absent in other isoforms. In vitro characterization of Suprastat demonstrates subnanomolar HDAC6 inhibitory potency and a hundred- to a thousand-fold HDAC6 selectivity over the other HDAC isoforms. In vivo studies reveal that a combination of Suprastat and anti-PD1 immunotherapy enhances antitumor immune response, mediated by a decrease of protumoral M2 macrophages and increased infiltration of antitumor CD8+ effector and memory T-cells.

Histone Deacetylases (HDACs): Evolution, Specificity, Role in Transcriptional Complexes, and Pharmacological Actionability

Histone deacetylases (HDACs) are evolutionary conserved enzymes which operate by removing acetyl groups from histones and other protein regulatory factors, with functional consequences on chromatin remodeling and gene expression profiles. We provide here a review on the recent knowledge accrued on the zinc-dependent HDAC protein family across different species, tissues, and human pathologies, specifically focusing on the role of HDAC inhibitors as anti-cancer agents. We will investigate the chemical specificity of different HDACs and discuss their role in the human interactome as members of chromatin-binding and regulatory complexes.

The USP10-HDAC6 axis confers cisplatin resistance in non-small cell lung cancer lacking wild-type p53

Ubiquitin-specific peptidase 10 (USP10) stabilizes both tumor suppressors and oncogenes in a context-dependent manner. However, the nature of USP10’s role in non-small cell lung cancer (NSCLC) remains unclear. By analyzing The Cancer Genome Atlas (TCGA) database, we have shown that high levels of USP10 are associated with poor overall survival in NSCLC with mutant p53, but not with wild-type p53. Consistently, genetic depletion or pharmacological inhibition of USP10 dramatically reduces the growth of lung cancer xenografts lacking wild-type p53 and sensitizes them to cisplatin. Mechanistically, USP10 interacts with, deubiquitinates, and stabilizes oncogenic protein histone deacetylase 6 (HDAC6). Furthermore, reintroducing either USP10 or HDAC6 into a USP10-knockdown NSCLC H1299 cell line with null-p53 renders cisplatin resistance. This result suggests the existence of a “USP10-HDAC6-cisplatin resistance” axis. Clinically, we have found a positive correlation between USP10 and HDAC6 expression in a cohort of NSCLC patient samples. Moreover, we have shown that high levels of USP10 mRNA correlate with poor overall survival in a cohort of advanced NSCLC patients who received platinum-based chemotherapy. Overall, our studies suggest that USP10 could be a potential biomarker for predicting patient response to platinum, and that targeting USP10 could sensitize lung cancer patients lacking wild-type p53 to platinum-based therapy.

Histone Deacetylases Inhibitors in Neurodegenerative Diseases, Neuroprotection and Neuronal Differentiation

Histone deacetylases (HADC) are the enzymes that remove acetyl group from lysine residue of histones and non-histone proteins and regulate the process of transcription by binding to transcription factors and regulating fundamental cellular process such as cellular proliferation, differentiation and development. In neurodegenerative diseases, the histone acetylation homeostasis is greatly impaired, shifting towards a state of hypoacetylation. The histone hyperacetylation produced by direct inhibition of HDACs leads to neuroprotective actions. This review attempts to elaborate on role of small molecule inhibitors of HDACs on neuronal differentiation and throws light on the potential of HDAC inhibitors as therapeutic agents for treatment of neurodegenerative diseases. The role of HDACs in neuronal cellular and disease models and their modulation with HDAC inhibitors are also discussed. Significance of these HDAC inhibitors has been reviewed on the process of neuronal differentiation, neurite outgrowth and neuroprotection regarding their potential therapeutic application for treatment of neurodegenerative diseases.

The cell biology of the retinal pigment epithelium

The retinal pigment epithelium (RPE), a monolayer of post-mitotic polarized epithelial cells, strategically situated between the photoreceptors and the choroid, is the primary caretaker of photoreceptor health and function. Dysfunction of the RPE underlies many inherited and acquired diseases that cause permanent blindness. Decades of research have yielded valuable insight into the cell biology of the RPE. In recent years, new technologies such as live-cell imaging have resulted in major advancement in our understanding of areas such as the daily phagocytosis and clearance of photoreceptor outer segment tips, autophagy, endolysosome function, and the metabolic interplay between the RPE and photoreceptors. In this review, we aim to integrate these studies with an emphasis on appropriate models and techniques to investigate RPE cell biology and metabolism, and discuss how RPE cell biology informs our understanding of retinal disease.

The Major Capsid Protein, VP1, of the Mouse Polyomavirus Stimulates the Activity of Tubulin Acetyltransferase 1 by Microtubule Stabilization

Viruses have evolved mechanisms to manipulate microtubules (MTs) for the efficient realization of their replication programs. Studying the mechanisms of replication of mouse polyomavirus (MPyV), we observed previously that in the late phase of infection, a considerable amount of the main structural protein, VP1, remains in the cytoplasm associated with hyperacetylated microtubules. VP1–microtubule interactions resulted in blocking the cell cycle in the G2/M phase. We are interested in the mechanism leading to microtubule hyperacetylation and stabilization and the roles of tubulin acetyltransferase 1 (αTAT1) and deacetylase histone deacetylase 6 (HDAC6) and VP1 in this mechanism. Therefore, HDAC6 inhibition assays, αTAT1 knock out cell infections, in situ cell fractionation, and confocal and TIRF microscopy were used. The experiments revealed that the direct interaction of isolated microtubules and VP1 results in MT stabilization and a restriction of their dynamics. VP1 leads to an increase in polymerized tubulin in cells, thus favoring αTAT1 activity. The acetylation status of MTs did not affect MPyV infection. However, the stabilization of MTs by VP1 in the late phase of infection may compensate for the previously described cytoskeleton destabilization by MPyV early gene products and is important for the observed inhibition of the G2→M transition of infected cells to prolong the S phase.

A patent review of histone deacetylase 6 inhibitors in neurodegenerative diseases (2014-2019)

Introduction: Histone deacetylase 6 (HDAC6) is unique in comparison with other zinc-dependent HDAC family members. An increasing amount of evidence from clinical and preclinical research demonstrates the potential of HDAC6 inhibition as an effective therapeutic approach for the treatment of cancer, autoimmune diseases, as well as neurological disorders. The recently disclosed crystal structures of HDAC6-ligand complexes offer further means for achieving pharmacophore refinement, thus further accelerating the pace of HDAC6 inhibitor discovery in the last few years.Area covered: This review summarizes the latest clinical status of HDAC6 inhibitors, discusses pharmacological applications of selective HDAC6 inhibitors in neurodegenerative diseases, and describes the patent applications dealing with HDAC6 inhibitors from 2014-2019 that have not been reported in research articles.Expert opinion: Phenylhydroxamate has proven a very useful scaffold in the discovery of potent and selective HDAC6 inhibitors. However, weaknesses of the hydroxamate function such as metabolic instability and mutagenic potential limit its application in the neurological field, where long-term administration is required. The recent invention of oxadiazole-based ligands by pharmaceutical companies may provide a new opportunity to optimize the druglike properties of HDAC6 inhibitors for the treatment of neurodegenerative diseases.

Radiosynthesis and preliminary evaluation of an 18 F-labeled tubastatin A analog for PET imaging of histone deacetylase 6

Histone deacetylase 6 (HDAC6) is a unique member of the HDAC family because of its characteristics, namely, its cytoplasmic localization and ubiquitin binding. HDAC6 has been implicated in cancer metastasis and neurodegeneration. In the present study, we performed radiosynthesis and biological evaluation of a fluorine-18-labeled ligand [18 F]3, which is an analog of the HDAC6-selective inhibitor tubastatin A, for positron emission tomography (PET) imaging. [18 F]3 was synthesized by a two-step reaction composed of 18 F-fluorination and formation of a hydroxamic acid group. IC50 values of 3 against HDAC1 and HDAC6 activities were 996 nM and 33.1 nM, respectively. A biodistribution study in mice demonstrated low brain uptake of [18 F]3. Furthermore, bone radioactivity was stable at around 2% ID/g after injection, suggesting high tolerance to defluorination. Regarding metabolic stability, 70% of the compound was observed as the unchanged form at 30 minutes post injection in mouse plasma. A small animal PET study in mice showed that pretreatment with cyclosporine A had no effect on initial brain uptake of [18 F]3, suggesting low brain uptake of [18 F]3 was not caused by the P-glycoprotein-mediated efflux. While PET imaging using [18 F]3 has a limitation with respect to neurodegenerative diseases, further studies evaluating its utility for certain cancers are worth evaluating.

HDAC6 inhibitors: Translating genetic and molecular insights into a therapy for axonal CMT

Histone deacetylase 6 (HDAC6) plays a central role in various processes that are key for neuronal survival. In this review, we summarize the current evidence related to disease pathways in the axonal form of Charcot-Marie-Tooth disease (CMT) and highlight the role of HDAC6 in these pathways. We hypothesize that HDAC6 might in fact actively contribute to the pathogenesis of certain forms of axonal CMT. HDAC6 plays a deacetylase activity-dependent, negative role in axonal transport and axonal regeneration, which are both processes implicated in axonal CMT. On the other hand, HDAC6 coordinates a protective response during elimination of toxic misfolded proteins, but this is mostly mediated independent of its deacetylase activity. The current mechanistic insights on these functions of HDAC6 in axonal CMT, along with the selective druggability against its deacetylase activity, make the targeting of HDAC6 particularly attractive. We elaborate on the preclinical studies that demonstrated beneficial effects of HDAC6 inhibitors in axonal CMT models and outline possible modes of action. Overall, this overview ultimately provides a rationale for the use of small-molecule HDAC6 inhibitors as a therapeutic strategy for this devastating disease.

Selective Histone Deacetylase 6 Inhibition Normalizes B Cell Activation and Germinal Center Formation in a Model of Systemic Lupus Erythematosus

Autoantibody production by plasma cells (PCs) plays a pivotal role in the pathogenesis of systemic lupus erythematosus (SLE). The molecular pathways by which B cells become pathogenic PC secreting autoantibodies in SLE are incompletely characterized. Histone deactylase 6 (HDAC6) is a unique cytoplasmic HDAC that modifies the interaction of a number of tubulin- associated proteins; inhibition of HDAC6 has been shown to be beneficial in murine models of SLE, but the downstream pathways accounting for the therapeutic benefit have not been clearly delineated. In the current study, we sought to determine whether selective HDAC6 inhibition would abrogate abnormal B cell activation in SLE. We treated NZB/W lupus mice with the selective HDAC6 inhibitor, ACY-738, for 4 weeks beginning at 20 weeks-of age. After only 4 weeks of treatment, manifestation of lupus nephritis (LN) were greatly reduced in these animals. We then used RNAseq to determine the genomic signatures of splenocytes from treated and untreated mice and applied computational cellular and pathway analysis to reveal multiple signaling events associated with B cell activation and differentiation in SLE that were modulated by HDAC6 inhibition. PC development was abrogated and germinal center (GC) formation was greatly reduced. When the HDAC6 inhibitor-treated lupus mouse gene signatures were compared to human lupus patient gene signatures, the results showed numerous immune, and inflammatory pathways increased in active human lupus were significantly decreased in the HDAC6 inhibitor treated animals. Pathway analysis suggested alterations in cellular metabolism might contribute to the normalization of lupus mouse spleen genomic signatures, and this was confirmed by direct measurement of the impact of the HDAC6 inhibitor on metabolic activities of murine spleen cells. Taken together, these studies show HDAC6 inhibition decreases B cell activation signaling pathways and reduces PC differentiation in SLE and suggest that a critical event might be modulation of cellular metabolism.

MPT0G612, a Novel HDAC6 Inhibitor, Induces Apoptosis and Suppresses IFN-γ-Induced Programmed Death-Ligand 1 in Human Colorectal Carcinoma Cells

Colorectal cancer (CRC) is the third most common cancer and the leading cause of cancer-associated death worldwide. Histone deacetylases (HDACs) have been implicated in regulating complex cellular mechanisms to influence tumor biology and immunogenicity in various types of cancer. The potential of selective inhibition of HDAC6 has been widely discussed for the treatment of hematologic malignancies. We previously identified that MPT0G612 is a novel HDAC6 inhibitor exhibiting a promising antitumor activity against several solid tumors. The purpose of the present study was to evaluate the feasibility and pharmacological mechanisms of MPT0G612 as a potential therapy for CRC patients. Results revealed that MPT0G612 significantly suppresses the proliferation and viability, as well as induces apoptosis in CRC cells. Autophagy activation with LC3B-II formation and p62 degradation was observed, and the inhibition of autophagy by pharmacological inhibitor or Atg5 knockdown enhances MPT0G612-induced cell death. In addition, HDAC6 knockdown reduces MPT0G612-mediated autophagy and further potentiates apoptotic cell death. Furthermore, MPT0G612 downregulates the expression of PD-L1 induced by IFN-γ in CRC cells. These results suggest that MPT0G612 is a potent cell death inducer through inhibiting HDAC6-associated pathway, and a potential agent for combination strategy with immune checkpoint inhibitors for the treatment of CRC.

Old but Gold: Tracking the New Guise of Histone Deacetylase 6 (HDAC6) Enzyme as a Biomarker and Therapeutic Target in Rare Diseases

Epigenetic regulation orchestrates many cellular processes and greatly influences key disease mechanisms. Histone deacetylase (HDAC) enzymes play a crucial role either as biomarkers or therapeutic targets owing to their involvement in specific pathophysiological pathways. Beyond their well-characterized role as histone modifiers, HDACs also interact with several nonhistone substrates and their increased expression has been highlighted in specific diseases. The HDAC6 isoform, due to its unique cytoplasmic localization, modulates the acetylation status of tubulin, HSP90, TGF-β, and peroxiredoxins. HDAC6 also exerts noncatalytic activities through its interaction with ubiquitin. Both catalytic and noncatalytic functions of HDACs are being actively studied in the field of specific rare disorders beyond the well-established role in carcinogenesis. This Perspective outlines the application of HDAC(6) inhibitors in rare diseases, such as Rett syndrome, inherited retinal disorders, idiopathic pulmonary fibrosis, and Charcot-Marie-Tooth disease, highlighting their therapeutic potential as innovative and targeted disease-modifying agents.

Dysregulation of autophagy and stress granule-related proteins in stress-driven Tau pathology

Imbalance of neuronal proteostasis associated with misfolding and aggregation of Tau protein is a common neurodegenerative feature in Alzheimer's disease (AD) and other Tauopathies. Consistent with suggestions that lifetime stress may be an important AD precipitating factor, we previously reported that environmental stress and high glucocorticoid (GC) levels induce accumulation of aggregated Tau; however, the molecular mechanisms for such process remain unclear. Herein, we monitor a novel interplay between RNA-binding proteins (RBPs) and autophagic machinery in the underlying mechanisms through which chronic stress and high GC levels impact on Tau proteostasis precipitating Tau aggregation. Using molecular, pharmacological and behavioral analysis, we demonstrate that chronic stress and high GC trigger mTOR-dependent inhibition of autophagy, leading to accumulation of Tau aggregates and cell death in P301L-Tau expressing mice and cells. In parallel, we found that environmental stress and GC disturb cellular homeostasis and trigger the insoluble accumulation of different RBPs, such as PABP, G3BP1, TIA-1, and FUS, shown to form stress granules (SGs) and Tau aggregation. Interestingly, an mTOR-driven pharmacological stimulation of autophagy attenuates the GC-driven accumulation of Tau and SG-related proteins as well as the related cell death, suggesting a critical interface between autophagy and the response of the SG-related protein in the neurodegenerative potential of chronic stress and GC. These studies provide novel insights into the RNA-protein intracellular signaling regulating the precipitating role of environmental stress and GC on Tau-driven brain pathology.

MPT0G413, A Novel HDAC6-Selective Inhibitor, and Bortezomib Synergistically Exert Anti-tumor Activity in Multiple Myeloma Cells

In multiple myeloma (MM), homeostasis is largely maintained by misfolded protein clearance via the proteasomal and aggresomal pathways. Histone deacetylase 6 (HDAC6) binds polyubiquitinated proteins and dynein motors and transports this protein cargo to the aggresome for further degradation. Accordingly, a combination of an HDAC6 inhibitor and bortezomib (BTZ) could increase ubiquitinated protein accumulation, leading to further apoptosis. Here we evaluated the anti-MM activity of MPT0G413, a novel specific HDAC6 inhibitor, using in vitro and in vivo models. MPT0G413 treatment more significantly inhibited cell growth in MM cells than in normal bone marrow cells. Furthermore, the combination of MPT0G413 and BTZ enhanced polyubiquitinated protein accumulation and synergistically reduced MM viability, increased caspase-3, caspase-8, caspase-9 levels, and cleaved poly (ADP) ribosome polymerase and also inhibited adherence of MM cells to bone marrow stromal cells (BMSC) and reduced VEGF and IL-6 levels and cell growth in a co-culture system. The combination treatment disturbed the bone marrow microenvironment and induced synergic, caspase-dependent apoptosis. Xenograft tumor growth significantly decreased in combination-treated SCID mice. In conclusion, MPT0G413 and BTZ synergistically inhibit MM viability, providing a framework for the clinical evaluation of combined therapies for MM.

Genetic mechanisms underlying spermatic and testicular traits within and among cattle breeds: systematic review and prioritization of GWAS results

Reduced bull fertility imposes economic losses in bovine herds. Specifically, testicular and spermatic traits are important indicators of reproductive efficiency. Several genome-wide association studies (GWAS) have identified genomic regions associated with these fertility traits. The aims of this study were as follows: 1) to perform a systematic review of GWAS results for spermatic and testicular traits in cattle and 2) to identify key functional candidate genes for these traits. The identification of functional candidate genes was performed using a systems biology approach, where genes shared between traits and studies were evaluated by a guilt by association gene prioritization (GUILDify and ToppGene software) in order to identify the best functional candidates. These candidate genes were integrated and analyzed in order to identify overlapping patterns among traits and breeds. Results showed that GWAS for testicular-related traits have been developed for beef breeds only, whereas the majority of GWAS for spermatic-related traits were conducted using dairy breeds. When comparing traits measured within the same study, the highest number of genes shared between different traits was observed, indicating a high impact of the population genetic structure and environmental effects. Several chromosomal regions were enriched for functional candidate genes associated with fertility traits. Moreover, multiple functional candidate genes were enriched for markers in a species-specific basis, taurine (Bos taurus) or indicine (Bos indicus). For the different candidate regions identified in the GWAS in the literature, functional candidate genes were detected as follows: B. Taurus chromosome X (BTX) (TEX11, IRAK, CDK16, ATP7A, ATRX, HDAC6, FMR1, L1CAM, MECP2, etc.), BTA17 (TRPV4 and DYNLL1), and BTA14 (MOS, FABP5, ZFPM2). These genes are responsible for regulating important metabolic pathways or biological processes associated with fertility, such as progression of spermatogenesis, control of ciliary activity, development of Sertoli cells, DNA integrity in spermatozoa, and homeostasis of testicular cells. This study represents the first systematic review on male fertility traits in cattle using a system biology approach to identify key candidate genes for these traits.

Pharmacophore-based virtual screening for identification of potential selective inhibitors of human histone deacetylase 6

Histone deacetylase (HDAC) 6 plays a role in oncogenic transformation and cancer metastasis via tubulin deacetylation, making it a critical target for anticancer drug design. However, lack of selectivity shown by many of the current HDAC6 inhibitors in clinical use and trials prompts the continuous search for selective inhibitors. Here, 10 pharmacophore hypotheses were developed based on the 3D common features of training set of 20 HDAC inhibitors in clinical use and trials. The hypotheses were validated using a test set of another 20 HDAC inhibitors along with 400 inactive (decoys) molecules based on Güner-Henry pharmacophore scoring method. Hypothesis 1 consisting of 1 H-bond donor, 1 H-bond acceptor and 2 hydrophobic features, was used to screen "DruglikeDiverse" database using Biovia Discovery Studio 4.5. The top 10 hit compounds were selected based on the pharmacophore fit values (>3.00). Their binding affinity against HDAC6 compared to class I HDACs (1, 2, 3 & 8) and a class IIa member (HDAC7), was calculated by molecular docking using AutoDock4. The stability of binding modes of 2 potential HDAC6-selective inhibitors (ENA501965 and IBS399024) was examined by 30 ns-molecular dynamics (MD) simulation using nanoscale MD (NAMD) software. Both ligands showed potential stability in HDAC6 active site over time. Therefore, these may provide additional scaffolds for further optimization towards the design of safe, potent and selective HDAC6 inhibitors.