Control of MEF2 Transcriptional Activity by Coordinated Phosphorylation and Sumoylation

Sodium Tungstate Promotes Neurite Outgrowth and Confers Neuroprotection in Neuro2a and SH-SY5Y Cells

Sodium tungstate (Na2WO4) normalizes glucose metabolism in the liver and muscle, activating the Mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK) pathway. Because this pathway controls neuronal survival and differentiation, we investigated the effects of Na2WO4 in mouse Neuro2a and human SH-SY5Y neuroblastoma monolayer cell cultures. Na2WO4 promotes differentiation to cholinergic neurites via an increased G1/G0 cell cycle in response to the synergic activation of the Phosphatidylinositol 3-kinase (PI3K/Akt) and ERK1/2 signaling pathways. In Neuro2a cells, Na2WO4 increases protein synthesis by activating the mechanistic target of rapamycin (mTOR) and S6K kinases and GLUT3-mediated glucose uptake, providing the energy and protein synthesis needed for neurite outgrowth. Furthermore, Na2WO4 increased the expression of myocyte enhancer factor 2D (MEF2D), a member of a family of transcription factors involved in neuronal survival and plasticity, through a post-translational mechanism that increases its half-life. Site-directed mutations of residues involved in the sumoylation of the protein abrogated the positive effects of Na2WO4 on the MEF2D-dependent transcriptional activity. In addition, the neuroprotective effects of Na2WO4 were evaluated in the presence of advanced glycation end products (AGEs). AGEs diminished neurite differentiation owing to a reduction in the G1/G0 cell cycle, concomitant with lower expression of MEF2D and the GLUT3 transporter. These negative effects were corrected in both cell lines after incubation with Na2WO4. These findings support the role of Na2WO4 in neuronal plasticity, albeit further experiments using 3D cultures, and animal models will be needed to validate the therapeutic potential of the compound.

Roles of Histone Deacetylase 4 in the Inflammatory and Metabolic Processes

Histone deacetylase 4 (HDAC4), a class IIa HDAC, has gained attention as a potential therapeutic target in treating inflammatory and metabolic processes based on its essential role in various biological pathways by deacetylating non-histone proteins, including transcription factors. The activity of HDAC4 is regulated at the transcriptional, post-transcriptional, and post-translational levels. The functions of HDAC4 are tissue-dependent in response to endogenous and exogenous factors and their substrates. In particular, the association of HDAC4 with non-histone targets, including transcription factors, such as myocyte enhancer factor 2, hypoxia-inducible factor, signal transducer and activator of transcription 1, and forkhead box proteins, play a crucial role in regulating inflammatory and metabolic processes. This review summarizes the regulatory modes of HDAC4 activity and its functions in inflammation, insulin signaling and glucose metabolism, and cardiac muscle development.

Triaging between post-translational modification of cell cycle regulators and their therapeutics in neurodegenerative diseases

Neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease, and Huntington's disease, present challenges in healthcare because of their complicated etiologies and absence of healing remedies. Lately, the emerging role of post-translational modifications (PTMs), in the context of cell cycle regulators, has garnered big interest as a potential avenue for therapeutic intervention. The review explores the problematic panorama of PTMs on cell cycle regulators and their implications in neurodegenerative diseases. We delve into the dynamic phosphorylation, acetylation, ubiquitination, SUMOylation, Glycation, and Neddylation that modulate the key cell cycle regulators, consisting of cyclins, cyclin-dependent kinases (CDKs), and their inhibitors. The dysregulation of these PTMs is related to aberrant cell cycle in neurons, which is one of the factors involved in neurodegenerative pathologies. Moreover, the effect of exogenous activation of CDKs and CDK inhibitors through PTMs on the signaling cascade was studied in postmitotic conditions of NDDs. Furthermore, the therapeutic implications of CDK inhibitors and associated alteration in PTMs were discussed. Lastly, we explored the putative mechanism of PTMs to restore normal neuronal function that might reverse NDDs.

The suppression of the differentiation of adipocytes with Mallotus furetianus is regulated through the posttranslational modifications of C/EBPβ

Obesity is a major risk factor for various chronic diseases, especially lifestyle-related diseases. Therefore, finding a protective substance against obesity and elucidating its molecular mechanism is one of the most important problems for improving human health. In this study, we investigated the antiobesity effect of Mallotus furetianus extract (MFE). The aim of the study was to examine the in vivo and in vitro effects of MFE on lipid synthesis. We examined the effect using an in vivo experimental system with obesity model mice and an in vitro experimental system with 3T3-L1 preadipocytes. We found that the treatment of MFE significantly suppressed the increase in body weight and adipose tissue weight and morphological changes in the liver and adipose tissue of the obesity model mice. In the in vitro experimental system, we revealed that MFE treatment suppressed the expression of transcription factors such as C/EBPα, C/EBPβ, and PPARγ, which are involved in the early differentiation of 3T3-L1 preadipocytes. As a result, the ability to synthesize triacylglycerol was suppressed. An interesting finding in this study was the clarification that MFE decreases the expression of C/EBPβ through post-translation modifications (PTMs), followed by the transcriptional suppression of PPAR𝛾 and C/EBP𝛼.

The Role of MEF2 Transcription Factor Family in Neuronal Survival and Degeneration

The family of myocyte enhancer factor 2 (MEF2) transcription factors comprises four highly conserved members that play an important role in the nervous system. They appear in precisely defined time frames in the developing brain to turn on and turn off genes affecting growth, pruning and survival of neurons. MEF2s are known to dictate neuronal development, synaptic plasticity and restrict the number of synapses in the hippocampus, thus affecting learning and memory formation. In primary neurons, negative regulation of MEF2 activity by external stimuli or stress conditions is known to induce apoptosis, albeit the pro or antiapoptotic action of MEF2 depends on the neuronal maturation stage. By contrast, enhancement of MEF2 transcriptional activity protects neurons from apoptotic death both in vitro and in preclinical models of neurodegenerative diseases. A growing body of evidence places this transcription factor in the center of many neuropathologies associated with age-dependent neuronal dysfunctions or gradual but irreversible neuron loss. In this work, we discuss how the altered function of MEF2s during development and in adulthood affecting neuronal survival may be linked to neuropsychiatric disorders.

Cytoplasmic HDAC4 recovers synaptic function in the 3×Tg mouse model of Alzheimer's disease

AIMS

Early dysfunction in Alzheimer's disease (AD) is characterised by alterations of synapse structure and function leading to dysmorphic neurites, decreased spine density, impaired synaptic plasticity and cognitive deficits. The class II member HDAC4, which recently emerged as a crucial factor in shaping synaptic plasticity and memory, was found to be altered in AD. We investigated how the modulation of HDAC4 may contribute to counteracting AD pathogenesis.

METHODS

Using a cytoplasmic HDAC4 mutant (HDAC4^SD ), we studied the recovery of synaptic function in hippocampal tissue and primary neurons from the triple-transgenic mouse model of AD (3×Tg-AD).

RESULTS

Here, we report that in wild-type mice, HDAC4 is localised at synapses and interacts with postsynaptic proteins, whereas in the 3×Tg-AD, it undergoes nuclear import, reducing its interaction with synaptic proteins. Of note, HDAC4 delocalisation was induced by both amyloid-β and tau accumulation. Overexpression of the HDAC4^SD mutant in CA1 pyramidal neurons of organotypic hippocampal slices obtained from 3×Tg-AD mice increased dendritic length and promoted the enrichment of N-cadherin, GluA1, PSD95 and CaMKII proteins at the synaptic level compared with AD neurons transfected with the empty vector. Moreover, HDAC4 overexpression recovered the level of SUMO2/3ylation of PSD95 in AD hippocampal tissue, and in AD organotypic hippocampal slices, the HDAC4^SD rescued spine density and synaptic transmission.

CONCLUSIONS

These results highlight a new role of cytoplasmic HDAC4 in providing a structural and enzymatic regulation of postsynaptic proteins. Our findings suggest that controlling HDAC4 localisation may represent a promising strategy to rescue synaptic function in AD, potentially leading to memory improvement.

HDAC4 in cancer: A multitasking platform to drive not only epigenetic modifications

Controlling access to genomic information and maintaining its stability are key aspects of cell life. Histone acetylation is a reversible epigenetic modification that allows access to DNA and the assembly of protein complexes that regulate mainly transcription but also other activities. Enzymes known as histone deacetylases (HDACs) are involved in the removal of the acetyl-group or in some cases of small hydrophobic moieties from histones but also from the non-histone substrate. The main achievement of HDACs on histones is to repress transcription and promote the formation of more compact chromatin. There are 18 different HDACs encoded in the human genome. Here we will discuss HDAC4, a member of the class IIa family, and its possible contribution to cancer development.

MEF2C opposes Notch in lymphoid lineage decision and drives leukemia in the thymus

Rearrangements that drive ectopic MEF2C expression have recurrently been found in patients with human early thymocyte progenitor acute lymphoblastic leukemia (ETP-ALL). Here, we show high levels of MEF2C expression in patients with ETP-ALL. Using both in vivo and in vitro models of ETP-ALL, we demonstrate that elevated MEF2C expression blocks NOTCH-induced T cell differentiation while promoting a B-lineage program. MEF2C activates a B cell transcriptional program in addition to RUNX1, GATA3, and LMO2; upregulates the IL-7R; and boosts cell survival by upregulation of BCL2. MEF2C and the Notch pathway, therefore, demarcate opposite regulators of B- or T-lineage choices, respectively. Enforced MEF2C expression in mouse or human progenitor cells effectively blocks early T cell differentiation and promotes the development of biphenotypic lymphoid tumors that coexpress CD3 and CD19, resembling human mixed phenotype acute leukemia. Salt-inducible kinase (SIK) inhibitors impair MEF2C activity and alleviate the T cell developmental block. Importantly, this sensitizes cells to prednisolone treatment. Therefore, SIK-inhibiting compounds such as dasatinib are potentially valuable additions to standard chemotherapy for human ETP-ALL.

The Osteogenesis Imperfecta Type V Mutant BRIL/IFITM5 Promotes Transcriptional Activation of MEF2, NFATc, and NR4A in Osteoblasts

BRIL (bone restricted ifitm-like; also known as IFITM5) is a transmembrane protein expressed in osteoblasts. Although its role in skeletal development and homeostasis is unknown, mutations in BRIL result in rare dominant forms of osteogenesis imperfecta. The pathogenic mechanism has been proposed to be a gain-of or neomorphic function. To understand the function of BRIL and its OI type V mutant (MALEP BRIL) and whether they could activate signaling pathways in osteoblasts, we performed a luciferase reporter assay screen based on the activity of 26 transcription factors. When overexpressed in MC3T3-E1 and MLO-A5 cells, the MALEP BRIL activated the reporters dependent on MEF2, NFATc, and NR4A significantly more. Additional co-transfection experiments with MEF2C and NFATc1 and a number of their modulators (HDAC4, calcineurin, RCAN, FK506) confirmed the additive or synergistic activation of the pathways by MALEP, and suggested a coordinated regulation involving calcineurin. Endogenous levels of Nr4a members, as well as Ptgs2, were upregulated by MALEP BRIL. Y2H and co-immunoprecipitation indicated that BRIL interacted with CAML, but its contribution as the most upstream stimulator of the Ca²⁺-calcineurin-MEF2/NFATc cascade was not confirmed convincingly. Altogether the data presented provide the first ever readout to monitor for BRIL activity and suggest a potential gain-of-function causative effect for MALEP BRIL in OI type V, leading to perturbed signaling events and gene expression.

Systematic transition modeling analysis in the MEF2B-DNA binding interface due to Y69H and K4E variants

Transcriptional coactivator myocyte enhancer factor 2B (MEF2B) mutations are the most common cause of germinal center-derived B-cell non-Hodgkin lymphoma. Despite well-established contributions in lymphomagenesis, the structure-function paradigms of these mutations are largely unknown. Here through in silico approaches, we present structural evaluation of two reported missense variants (K4E and Y69H) in MEF2B to investigate their impact on DNA-binding through molecular dynamics simulation assays. Notably, MEF2B-specific MADs box domain (Lys23, Arg24 and Lys31) and N-terminal loop residues (Gly2, Arg3, Lys4, Lys5, Ile6 and Asn13) contribute in DNA binding, while in MEF2B^K4E, DNA binding is facilitated by Gly2, Arg3 and Arg91 (α3) residues. Conversely, in MEF2B^Y69H, Arg3, Lys5, Ser78, Arg79 and Asn81 residues mediate DNA binding. DNA binding induces pronounced conformational readjustments in MEF2B^WT-specific α1-N-terminal loop region, while MEF2B^Y69H and MEF2B^K4E exhibit fluctuations in both α1 and α3. Hydrogen (H)-bond occupancy analysis reveals a similar DNA binding behavior for MEF2^WT and MEF2B^Y69H, compared to MEF2B^K4E structure. The Anisotropic Network Model analysis depicts α1 and α3 as more fluctuant regions in MEF2B^K4E as compared to other systems. MEF2B^WT and MEF2B^K4E, Tyr69 residue is involved in p300 binding thus possible influence of Y69H variation in the functions other than DNA binding, such as p300 co-activator recruitment may explain the reduced transcriptional activation of MEF2B^Y69H. Thus, present study may provide a structural basis of DNA recognition by pinpointing the underlying conformational changes in the dynamics of MEF2B^K4E, MEF2B^Y69H, and MEF2B^WT structures that may contribute in the identification of novel therapeutic strategies for lymphomagenesis.

Post-translational modifications talk and crosstalk to class IIa histone deacetylases

Epigenetic modifications, such as histone or DNA modifications are key regulators of gene transcription and changes are often associated with maladaptive processes underlying cardiovascular disease. Epigenetic regulators therefore likely play a crucial role in cardiomyocyte homeostasis and facilitate the cellular adaption to various internal and external stimuli, responding to different intercellular and extracellular cues. Class IIa histone deacetylases are a class of epigenetic regulators that possess a myriad of post-transcriptional modification sites that modulate their activity in response to oxidative stress, altered catecholamine signalling or changes in the cellular metabolism. This review summaries the known reversible, post-translational modifications (PTMs) of class IIa histone deacetylases (HDACs) that ultimately drive transcriptional changes in homeostasis and disease. We also highlight the idea of a crosstalk of various PTMs on class IIa HDACs potentially leading to compensatory or synergistic effects on the class IIa HDAC-regulated cell behavior.

MEF2 transcription factors differentially contribute to retinal ganglion cell loss after optic nerve injury

Loss of retinal ganglion cells (RGCs) in optic neuropathies results in permanent partial or complete blindness. Myocyte enhancer factor 2 (MEF2) transcription factors have been shown to play a pivotal role in neuronal systems, and in particular MEF2A knockout was shown to enhance RGC survival after optic nerve crush injury. Here we expanded these prior data to study bi-allelic, tri-allelic and heterozygous allele deletion. We observed that deletion of all MEF2A, MEF2C, and MEF2D alleles had no effect on RGC survival during development. Our extended experiments suggest that the majority of the neuroprotective effect was conferred by complete deletion of MEF2A but that MEF2D knockout, although not sufficient to increase RGC survival on its own, increased the positive effect of MEF2A knockout. Conversely, MEF2A over-expression in wildtype mice worsened RGC survival after optic nerve crush. Interestingly, MEF2 transcription factors are regulated by post-translational modification, including by calcineurin-catalyzed dephosphorylation of MEF2A Ser-408 known to increase MEF2A-dependent transactivation in neurons. However, neither phospho-mimetic nor phospho-ablative mutation of MEF2A Ser-408 affected the ability of MEF2A to promote RGC death in vivo after optic nerve injury. Together these findings demonstrate that MEF2 gene expression opposes RGC survival following axon injury in a complex hierarchy, and further support the hypothesis that loss of or interference with MEF2A expression might be beneficial for RGC neuroprotection in diseases such as glaucoma and other optic neuropathies.

Anacardic acid-mediated regulation of osteoblast differentiation involves mitigation of inflammasome activation pathways

Disruption of the finely tuned osteoblast-osteoclast balance is the underlying basis of several inflammatory bone diseases, such as osteomyelitis, osteoporosis, and septic arthritis. Prolonged and unrestrained exposure to inflammatory environment results in reduction of bone mineral density by downregulating osteoblast differentiation. Earlier studies from our laboratory have identified that Anacardic acid (AA), a constituent of Cashew nut shell liquid that is used widely in traditional medicine, has potential inhibitory effect on gelatinases (MMP2 and MMP9) which are over-expressed in numerous inflammatory conditions (Omanakuttan et al. in Mol Pharmacol, 2012 and Nambiar et al. in Exp Cell Res, 2016). The study demonstrated for the first time that AA promotes osteoblast differentiation in lipopolysaccharide-treated osteosarcoma cells (MG63) by upregulating specific markers, like osteocalcin, receptor activator of NF-κB ligand, and alkaline phosphatase. Furthermore, expression of the negative regulators, such as nuclear factor-κB, matrix metalloproteinases (MMPs), namely MMP13, and MMP1, along with several inflammatory markers, such as Interleukin-1β and Nod-like receptor protein 3 were downregulated by AA. Taken together, AA expounds as a novel template for development of potential pharmacological therapeutics for inflammatory bone diseases.

Ubiquitin and Ubiquitin-Like Proteins in the Critical Equilibrium between Synapse Physiology and Intellectual Disability

Posttranslational modifications (PTMs) represent a dynamic regulatory system that precisely modulates the functional organization of synapses. PTMs consist in target modifications by small chemical moieties or conjugation of lipids, sugars or polypeptides. Among them, ubiquitin and a large family of ubiquitin-like proteins (UBLs) share several features such as the structure of the small protein modifiers, the enzymatic cascades mediating the conjugation process, and the targeted aminoacidic residue. In the brain, ubiquitination and two UBLs, namely sumoylation and the recently discovered neddylation orchestrate fundamental processes including synapse formation, maturation and plasticity, and their alteration is thought to contribute to the development of neurological disorders. Remarkably, emerging evidence suggests that these pathways tightly interplay to modulate the function of several proteins that possess pivotal roles for brain homeostasis as well as failure of this crosstalk seems to be implicated in the development of brain pathologies. In this review, we outline the role of ubiquitination, sumoylation, neddylation, and their functional interplay in synapse physiology and discuss their implication in the molecular pathogenesis of intellectual disability (ID), a neurodevelopmental disorder that is frequently comorbid with a wide spectrum of brain pathologies. Finally, we propose a few outlooks that might contribute to better understand the complexity of these regulatory systems in regard to neuronal circuit pathophysiology.

Staurosporine and venetoclax induce the caspase-dependent proteolysis of MEF2D-fusion proteins and apoptosis in MEF2D-fusion (+) ALL cells

MEF2D-fusion (M-fusion) genes are newly discovered recurrent gene abnormalities that are detected in approximately 5 % of acute lymphoblastic leukemia (ALL) cases. Their introduction to cells has been reported to transform cell lines or increase the colony formation of bone marrow cells, suggesting their survival-supporting ability, which prompted us to examine M-fusion-targeting drugs. To identify compounds that reduce the protein expression level of MEF2D, we developed a high-throughput screening system using 293T cells stably expressing a fusion protein of MEF2D and luciferase, in which the protein expression level of MEF2D was easily measured by a luciferase assay. We screened 3766 compounds with known pharmaceutical activities using this system and selected staurosporine as a potential inducer of the proteolysis of MEF2D. Staurosporine induced the proteolysis of M-fusion proteins in M-fusion (+) ALL cell lines. Proteolysis was inhibited by caspase inhibitors, not proteasome inhibitors, suggesting caspase dependency. Consistent with this result, the growth inhibitory effects of staurosporine were stronger in M-fusion (+) ALL cell lines than in negative cell lines, and caspase inhibitors blocked apoptosis induced by staurosporine. We identified the cleavage site of MEF2D-HNRNPUL1 by caspases and confirmed that its caspase cleavage-resistant mutant was resistant to staurosporine-induced proteolysis. Based on these results, we investigated another Food and Drug Administration-approved caspase activator, venetoclax, and found that it exerted similar effects to staurosporine, namely, the proteolysis of M-fusion proteins and strong growth inhibitory effects in M-fusion (+) ALL cell lines. The present study provides novel insights into drug screening strategies and the clinical indications of venetoclax.

B cells infected with Type 2 Epstein-Barr virus (EBV) have increased NFATc1/NFATc2 activity and enhanced lytic gene expression in comparison to Type 1 EBV infection

Humans are infected with two distinct strains (Type 1 (T1) and Type 2 (T2)) of Epstein-Barr virus (EBV) that differ substantially in their EBNA2 and EBNA 3A/B/C latency genes and the ability to transform B cells in vitro. While most T1 EBV strains contain the "prototype" form of the BZLF1 immediate-early promoter ("Zp-P"), all T2 strains contain the "Zp-V3" variant, which contains an NFAT binding motif and is activated much more strongly by B-cell receptor signalling. Whether B cells infected with T2 EBV are more lytic than cells infected with T1 EBV is unknown. Here we show that B cells infected with T2 EBV strains (AG876 and BL5) have much more lytic protein expression compared to B cells infected with T1 EBV strains (M81, Akata, and Mutu) in both a cord blood-humanized (CBH) mouse model and EBV-transformed lymphoblastoid cell lines (LCLs). Although T2 LCLs grow more slowly than T1 LCLs, both EBV types induce B-cell lymphomas in CBH mice. T1 EBV strains (M81 and Akata) containing Zp-V3 are less lytic than T2 EBV strains, suggesting that Zp-V3 is not sufficient to confer a lytic phenotype. Instead, we find that T2 LCLs express much higher levels of activated NFATc1 and NFATc2, and that cyclosporine (an NFAT inhibitor) and knockdown of NFATc2 attenuate constitutive lytic infection in T2 LCLs. Both NFATc1 and NFATc2 induce lytic EBV gene expression when combined with activated CAMKIV (which is activated by calcium signaling and activates MEF2D) in Burkitt Akata cells. Together, these results suggest that B cells infected with T2 EBV are more lytic due to increased activity of the cellular NFATc1/c2 transcription factors in addition to the universal presence of the Zp-V3 form of BZLF1 promoter.

Functional Inhibition of Host Histone Deacetylases (HDACs) Enhances in vitro and in vivo Anti-mycobacterial Activity in Human Macrophages and in Zebrafish

The rapid and persistent increase of drug-resistant Mycobacterium tuberculosis (Mtb) infections poses increasing global problems in combatting tuberculosis (TB), prompting for the development of alternative strategies including host-directed therapy (HDT). Since Mtb is an intracellular pathogen with a remarkable ability to manipulate host intracellular signaling pathways to escape from host defense, pharmacological reprogramming of the immune system represents a novel, potentially powerful therapeutic strategy that should be effective also against drug-resistant Mtb. Here, we found that host-pathogen interactions in Mtb-infected primary human macrophages affected host epigenetic features by modifying histone deacetylase (HDAC) transcriptomic levels. In addition, broad spectrum inhibition of HDACs enhanced the antimicrobial response of both pro-inflammatory macrophages (Mϕ1) and anti-inflammatory macrophages (Mϕ2), while selective inhibition of class IIa HDACs mainly decreased bacterial outgrowth in Mϕ2. Moreover, chemical inhibition of HDAC activity during differentiation polarized macrophages into a more bactericidal phenotype with a concomitant decrease in the secretion levels of inflammatory cytokines. Importantly, in vivo chemical inhibition of HDAC activity in Mycobacterium marinum-infected zebrafish embryos, a well-characterized animal model for tuberculosis, significantly reduced mycobacterial burden, validating our in vitro findings in primary human macrophages. Collectively, these data identify HDACs as druggable host targets for HDT against intracellular Mtb.

Functions and mechanisms of non-histone protein acetylation

Nε-lysine acetylation was discovered more than half a century ago as a post-translational modification of histones and has been extensively studied in the context of transcription regulation. In the past decade, proteomic analyses have revealed that non-histone proteins are frequently acetylated and constitute a major portion of the acetylome in mammalian cells. Indeed, non-histone protein acetylation is involved in key cellular processes relevant to physiology and disease, such as gene transcription, DNA damage repair, cell division, signal transduction, protein folding, autophagy and metabolism. Acetylation affects protein functions through diverse mechanisms, including by regulating protein stability, enzymatic activity, subcellular localization and crosstalk with other post-translational modifications and by controlling protein-protein and protein-DNA interactions. In this Review, we discuss recent progress in our understanding of the scope, functional diversity and mechanisms of non-histone protein acetylation.

Crosstalk of intracellular post-translational modifications in cancer

Post-translational modifications (PTMs) have been reported to play pivotal roles in numerous cellular biochemical and physiological processes. Multiple PTMs can influence the actions of each other positively or negatively, termed as PTM crosstalk or PTM code. During recent years, development of identification strategies for PTMs co-occurrence has revealed abundant information of interplay between PTMs. Increasing evidence demonstrates that deregulation of PTMs crosstalk is involved in the genesis and development of various diseases. Insight into the complexity of PTMs crosstalk will help us better understand etiology and provide novel targets for drug therapy. In the present review, we will discuss the important functional roles of PTMs crosstalk in proteins associated with cancer diseases.

mAKAPβ signalosomes - A nodal regulator of gene transcription associated with pathological cardiac remodeling

Striated myocytes compose about half of the cells of the heart, while contributing the majority of the heart's mass and volume. In response to increased demands for pumping power, including in diseases of pressure and volume overload, the contractile myocytes undergo non-mitotic growth, resulting in increased heart mass, i.e. cardiac hypertrophy. Myocyte hypertrophy is induced by a change in the gene expression program driven by the altered activity of transcription factors and co-repressor and co-activator chromatin-associated proteins. These gene regulatory proteins are subject to diverse post-translational modifications and serve as nuclear effectors for intracellular signal transduction pathways, including those controlled by cyclic nucleotides and calcium ion. Scaffold proteins contribute to the underlying architecture of intracellular signaling networks by targeting signaling enzymes to discrete intracellular compartments, providing specificity to the regulation of downstream effectors, including those regulating gene expression. Muscle A-kinase anchoring protein β (mAKAPβ) is a well-characterized scaffold protein that contributes to the regulation of pathological cardiac hypertrophy. In this review, we discuss the mechanisms how this prototypical scaffold protein organizes signalosomes responsible for the regulation of class IIa histone deacetylases and cardiac transcription factors such as NFAT, MEF2, and HIF-1α, as well as how this signalosome represents a novel therapeutic target for the prevention or treatment of heart failure.

Emerging roles for MEF2 in brain development and mental disorders

The MEF2 family of transcription factors regulate large programs of gene expression important for the development and maintenance of many tissues, including the brain. MEF2 proteins are regulated by neuronal synaptic activity, and they recruit several epigenetic enzymes to influence chromatin structure and gene expression during development and throughout adulthood. Here, we provide a brief review of the recent literature reporting important roles for MEF2 during early brain development and function, and we highlight emerging roles for MEF2 as a risk factor for multiple neurodevelopmental disorders and mental illnesses, such as autism, intellectual disability, and schizophrenia.

SUMOylation of G9a regulates its function as an activator of myoblast proliferation

The lysine methyltransferase G9a plays a role in many cellular processes. It is a potent repressor of gene expression, a function attributed to its ability to methylate histone and non-histone proteins. Paradoxically, in some instances, G9a can activate gene expression. However, regulators of G9a expression and activity are poorly understood. In this study, we report that endogenous G9a is SUMOylated in proliferating skeletal myoblasts. There are four potential SUMOylation consensus motifs in G9a. Mutation of all four acceptor lysine residues [K79, K152, K256, and K799] inhibits SUMOylation. Interestingly, SUMOylation does not impact G9a-mediated repression of MyoD transcriptional activity or myogenic differentiation. In contrast, SUMO-defective G9a is unable to enhance proliferation of myoblasts. Using complementation experiments, we show that the proliferation defect of primary myoblasts from conditional G9a-deficient mice is rescued by re-expression of wild-type, but not SUMOylation-defective, G9a. Mechanistically, SUMOylation acts as signal for PCAF (P300/CBP-associated factor) recruitment at E2F1-target genes. This results in increased histone H3 lysine 9 acetylation marks at E2F1-target gene promoters that are required for S-phase progression. Our studies provide evidence by which SUMO modification of G9a influences the chromatin environment to impact cell cycle progression.

Developing Practical Therapeutic Strategies that Target Protein SUMOylation

Post-translational modification by small ubiquitin-like modifier (SUMO) has emerged as a global mechanism for the control and integration of a wide variety of biological processes through the regulation of protein activity, stability and intracellular localization. As SUMOylation is examined in greater detail, it has become clear that the process is at the root of several pathologies including heart, endocrine, and inflammatory disease, and various types of cancer. Moreover, it is certain that perturbation of this process, either globally or of a specific protein, accounts for many instances of congenital birth defects. In order to be successful, practical strategies to ameliorate conditions due to disruptions in this post-translational modification will need to consider the multiple components of the SUMOylation machinery and the extraordinary number of proteins that undergo this modification.

Muscle A-kinase-anchoring protein-β-bound calcineurin toggles active and repressive transcriptional complexes of myocyte enhancer factor 2D

Myocyte enhancer factor 2 (MEF2) transcription factors are key regulators of the development and adult phenotype of diverse tissues, including skeletal and cardiac muscles. Controlled by multiple post-translational modifications, MEF2D is an effector for the Ca²⁺/calmodulin-dependent protein phosphatase calcineurin (CaN, PP2B, and PPP3). CaN-catalyzed dephosphorylation promotes the desumoylation and acetylation of MEF2D, increasing its transcriptional activity. Both MEF2D and CaN bind the scaffold protein muscle A-kinase-anchoring protein β (mAKAPβ), which is localized to the nuclear envelope, such that C2C12 skeletal myoblast differentiation and neonatal rat ventricular myocyte hypertrophy are inhibited by mAKAPβ signalosome targeting. Using immunoprecipitation and DNA-binding assays, we now show that the formation of mAKAPβ signalosomes is required for MEF2D dephosphorylation, desumoylation, and acetylation in C2C12 cells. Reduced MEF2D phosphorylation was coupled to a switch from type IIa histone deacetylase to p300 histone acetylase binding that correlated with increased MEF2D-dependent gene expression and ventricular myocyte hypertrophy. Together, these results highlight the importance of mAKAPβ signalosomes for regulating MEF2D activity in striated muscle, affirming mAKAPβ as a nodal regulator in the myocyte intracellular signaling network.

Chromosomal translocation-mediated evasion from miRNA induces strong MEF2D fusion protein expression, causing inhibition of PAX5 transcriptional activity

MEF2D fusion genes are newly discovered recurrent gene abnormalities that are detected in approximately 5% of acute lymphoblastic leukemia cases. We previously demonstrated that the vector-driven expression of MEF2D fusion proteins was markedly stronger than that of wild-type MEF2D; however, the underlying mechanisms and significance of this expression have yet to be clarified. We herein showed that the strong expression of MEF2D fusion proteins was caused by the loss of the target site of miRNA due to gene translocation. We identified the target region of miRNA located in the coding region and selected miR-122 as a candidate of the responsible miRNA. Mutations at a putative binding site of miR-122 increased MEF2D expression, while the transfection of its miRNA mimic reduced the expression of wild-type MEF2D, but not MEF2D fusion proteins. We also found that MEF2D fusion proteins inhibited the transcriptional activity of PAX5, a B-cell differentiation regulator in a manner that depended on fusion-specific strong expression and an association with histone deacetylase 4, which may lead to the differentiation disorders of B cells. Our results provide novel insights into the mechanisms underlying leukemia development by MEF2D fusion genes and the involvement of the deregulation of miRNA-mediated repression in cancer development.

A MADS-box transcription factor regulates a central step in sporulation of the oomycete Phytophthora infestans

MADS-box transcription factors play significant roles in eukaryotes, but have not yet been characterized in oomycetes. Here, we describe a MADS-box protein from Phytophthora infestans, which causes late blight of potato. P. infestans and most other oomycetes express a single MADS-box gene. PiMADS is not transcribed during vegetative growth, but is induced early during asexual sporulation. Its mRNA levels oscillate in response to light, which suppresses sporulation. The protein was not detected in nonsporulating mycelia, but was found in sporulating mycelia and spores. Both mRNA and protein levels decline upon spore germination. A similar expression pattern as well as nuclear localization was observed when the protein was expressed with a fluorescent tag from the native promoter. Gene silencing triggered by a construct expressing 478 nt of MADS sequences indicated that PiMADS is required for sporulation but not hyphal growth or plant colonization. A comparison of wild type to a silenced strain by RNA-seq indicated that PiMADS regulates about 3000 sporulation-associated genes, and acts before other genes previously shown to regulate sporulation. Analysis of the silenced strain also indicated that the native gene was not transcribed while the transgene was still expressed, which contradicts current models for homology-dependent silencing in oomycetes.

A CK2-RNF4 interplay coordinates non-canonical SUMOylation and degradation of nuclear receptor FXR

Farnesoid X receptor (FXR) is a ligand-activated nuclear receptor that plays a central role in regulating genes involved in bile acid homeostasis, and fat and glucose metabolism. Here, we demonstrate a post-translational interplay between FXR phosphorylation, SUMOylation, and ubiquitination that directs the receptor into an activation-degradation pathway in hepatocytes. We identify a non-canonical SUMOylation motif termed pSuM that conjugates SUMO2 at Lys-325 of FXR under the direct control of casein kinase 2 (CK2), which provides the required negative charge for Ubc9 and PIAS1 to perform SUMOylation, by phosphorylating Ser-327. Lys-325 SUMOylation is indispensable to the promotion of efficient ligand activation and transcriptional coactivation of FXR. Constitutive pSuM activation using a phospho-mimic Ser-327 mutant or catalytic CK2 expression strongly induces SUMO2 conjugation, which directs FXR ubiquitination and proteasome-dependent degradation. We also determine that such SUMOylation-dependent ubiquitination of FXR is mediated by the E3 ubiquitin ligase RNF4, which is required to achieve maximal induction of FXR and optimal up- or downregulation of responsive genes involved in bile acid homeostasis and liver regeneration. Our findings identify a highly regulated atypical SUMO conjugation motif that serves to coordinate FXR transcriptional competence, thereby expanding the intricate dynamics of the SUMOylation process used by incoming signals to govern metabolic gene regulation.

Alternative Splicing of Transcription Factors Genes in Muscle Physiology and Pathology

Skeletal muscle formation is a multi-step process that is governed by complex networks of transcription factors. The regulation of their functions is in turn multifaceted, including several mechanisms, among them alternative splicing (AS) plays a primary role. On the other hand, altered AS has a role in the pathogenesis of numerous muscular pathologies. Despite these premises, the causal role played by the altered splicing pattern of transcripts encoding myogenic transcription factors in neuromuscular diseases has been neglected so far. In this review, we systematically investigate what has been described about the AS patterns of transcription factors both in the physiology of the skeletal muscle formation process and in neuromuscular diseases, in the hope that this may be useful in re-evaluating the potential role of altered splicing of transcription factors in such diseases.

The role of MEF2 transcription factors in dehydration and anoxia survival in Rana sylvatica skeletal muscle

The wood frog (Rana sylvatica) can endure freezing of up to 65% of total body water during winter. When frozen, wood frogs enter a dormant state characterized by a cessation of vital functions (i.e., no heartbeat, blood circulation, breathing, brain activity, or movement). Wood frogs utilize various behavioural and biochemical adaptations to survive extreme freezing and component anoxia and dehydration stresses, including a global suppression of metabolic functions and gene expression. The stress-responsive myocyte enhancer factor-2 (MEF2) transcription factor family regulates the selective expression of genes involved in glucose transport, protein quality control, and phosphagen homeostasis. This study examined the role of MEF2A and MEF2C proteins as well as select downstream targets (glucose transporter-4, calreticulin, and muscle and brain creatine kinase isozymes) in 40% dehydration and 24 h anoxia exposure at the transcriptional, translational, and post-translational levels using qRT-PCR, immunoblotting, and subcellular localization. Mef2a/c transcript levels remained constant during dehydration and anoxia. Total, cytoplasmic, and nuclear MEF2A/C and phospho-MEF2A/C protein levels remained constant during dehydration, whereas a decrease in total MEF2C levels was observed during rehydration. Total and phospho-MEF2A levels remained constant during anoxia, whereas total MEF2C levels decreased during 24 h anoxia and P-MEF2C levels increased during 4 h anoxia. In contrast, cytoplasmic MEF2A levels and nuclear phospho-MEF2A/C levels were upregulated during anoxia. MEF2 downstream targets remained constant during dehydration and anoxia, with the exception of glut4 which was upregulated during anoxia. These results suggest that the upregulated MEF2 response reported in wood frogs during freezing may in part stem from their cellular responses to surviving prolonged anoxia, rather than dehydration, leading to an increase in GLUT4 expression which may have an important role during anoxia survival.

Calcium Signaling and Transcriptional Regulation in Cardiomyocytes

Calcium (Ca 2+ ) is a universal regulator of various cellular functions. In cardiomyocytes, Ca 2+ is the central element of excitation–contraction coupling, but also impacts diverse signaling cascades and influences the regulation of gene expression, referred to as excitation–transcription coupling. Disturbances in cellular Ca 2+ -handling and alterations in Ca 2+ -dependent gene expression patterns are pivotal characteristics of failing cardiomyocytes, with several excitation–transcription coupling pathways shown to be critically involved in structural and functional remodeling processes. Thus, targeting Ca 2+ -dependent transcriptional pathways might offer broad therapeutic potential. In this article, we (1) review cytosolic and nuclear Ca 2+ dynamics in cardiomyocytes with respect to their impact on Ca 2+ -dependent signaling, (2) give an overview on Ca 2+ -dependent transcriptional pathways in cardiomyocytes, and (3) discuss implications of excitation–transcription coupling in the diseased heart.

Sumoylation and Its Contribution to Cancer

Post-translational modifications play an important role in regulating protein activity by altering their functions. Sumoylation is a highly dynamic process which is tightly regulated by a fine balance between conjugating and deconjugating enzyme activities. It affects intracellular localization and their interaction with their binding partners, thereby changing gene expression. Consequently, these changes in turn affect signaling mechanisms that regulate many cellular functions, such as cell growth, proliferation, apoptosis , DNA repair , and cell survival. It is becoming apparent that deregulation in the SUMO pathway contributes to oncogenic transformation by affecting sumoylation/desumoylation of many oncoproteins and tumor suppressors. Loss of balance between sumoylation and desumoylation has been reported in a number of studies in a variety of disease types including cancer. This chapter summarizes the mechanisms and functions of the deregulated SUMO pathway affecting oncogenes and tumor suppressor genes.

Quantitative Phosphoproteomic Analysis Reveals Key Mechanisms of Cellular Proliferation in Liver Cancer Cells

Understanding the mechanisms of uncontrolled proliferation in cancer cells provides valuable insights into tumor development and is benefit for discovering efficient methods in cancer treatment. In this study, we identified and quantified 2,057 phosphoproteins and 9,824 unique phosphosites in three liver cell lines with high (QGY, Hep3B) and low (L02) proliferative potentials and disclosed the wide variations in phosphorylation sites and levels among them. We found that the number of identified phosphoproteins and phosphosites in these cells were negatively correlated with their proliferative abilities. The function analysis suggested that the aberrant phosphorylation of SR proteins and activation of MAPK pathway might be two critical factors to promote cancer cell proliferation. Meanwhile, the phosphorylation status of mini-chromosome maintenance (MCM) and nuclear pore (NPC) complexes are significantly different between cell lines with high and low proliferative potentials. Furthermore, the phosphosites targeted by kinase families of CDK, STE and HIPK in the proteins coded by cancer driver genes showed distinct profiles between caner and normal cell lines. These results present key phosphorylation networks involving in abnormal proliferation of cancer cells and uncovered potential molecular markers for estimating the proliferation ability of liver cancer cells.

A single structurally conserved SUMOylation site in CRMP2 controls NaV1.7 function

The neuronal collapsin response mediator protein 2 (CRMP2) undergoes several posttranslational modifications that codify its functions. Most recently, CRMP2 SUMOylation (addition of small ubiquitin like modifier (SUMO)) was identified as a key regulatory step within a modification program that codes for CRMP2 interaction with, and trafficking of, voltage-gated sodium channel NaV1.7. In this paper, we illustrate the utility of combining sequence alignment within protein families with structural analysis to identify, from several putative SUMOylation sites, those that are most likely to be biologically relevant. Co-opting this principle to CRMP2, we demonstrate that, of 3 sites predicted to be SUMOylated in CRMP2, only the lysine 374 site is a SUMOylation client. A reduction in NaV1.7 currents was the corollary of the loss of CRMP2 SUMOylation at this site. A 1.78-Å-resolution crystal structure of mouse CRMP2 was solved using X-ray crystallography, revealing lysine 374 as buried within the CRMP2 tetramer interface but exposed in the monomer. Since CRMP2 SUMOylation is dependent on phosphorylation, we postulate that this state forces CRMP2 toward a monomer, exposing the SUMO site and consequently, resulting in constitutive regulation of NaV1.7.

MEF2 transcription factors: developmental regulators and emerging cancer genes

The MEF2 transcription factors have roles in muscle, cardiac, skeletal, vascular, neural, blood and immune system cell development through their effects on cell differentiation, proliferation, apoptosis, migration, shape and metabolism. Altered MEF2 activity plays a role in human diseases and has recently been implicated in the development of several cancer types. In particular, MEF2B, the most divergent and least studied protein of the MEF2 family, has a role unique from its paralogs in non-Hodgkin lymphomas. The use of genome-scale technologies has enabled comprehensive MEF2 target gene sets to be identified, contributing to our understanding of MEF2 proteins as nodes in complex regulatory networks. This review surveys the molecular interactions of MEF2 proteins and their effects on cellular and organismal phenotypes. We include a discussion of the emerging roles of MEF2 proteins as oncogenes and tumor suppressors of cancer. Throughout this article we highlight similarities and differences between the MEF2 family proteins, including a focus on functions of MEF2B.

A class of their own: exploring the nondeacetylase roles of class IIa HDACs in cardiovascular disease

Histone deacetylases (HDACs) play integral roles in many cardiovascular biological processes ranging from transcriptional and translational regulation to protein stabilization and localization. There are 18 known HDACs categorized into 4 classes that can differ on the basis of substrate targets, subcellular localization, and regulatory binding partners. HDACs are classically known for their ability to remove acetyl groups from histone and nonhistone proteins that have lysine residues. However, despite their nomenclature and classical functions, discoveries from many research groups over the past decade have suggested that nondeacetylase roles exist for class IIa HDACs. This is not surprising given that class IIa HDACs have, for example, relatively poor deacetylase capabilities and are often shuttled in and out of nuclei upon specific pathological and nonpathological cardiac events. This review aims to consolidate and elucidate putative nondeacetylase roles for class IIa HDACs and, where possible, highlight studies that provide evidence for their noncanonical roles, especially in the context of cardiovascular maladies. There has been great interest recently in exploring the pharmacological regulators of HDACs for use in therapeutic interventions for treating cardiovascular diseases and inflammation. Thus it is of interest to earnestly consider nonenzymatic and or nondeacetylase roles of HDACs that might be key in potentiating or abrogating pathologies. These noncanonical HDAC functions may possibly yield new mechanisms and targets for drug discovery.

MEF2C-MYOCD and Leiomodin1 Suppression by miRNA-214 Promotes Smooth Muscle Cell Phenotype Switching in Pulmonary Arterial Hypertension

BACKGROUND

Vascular hyperproliferative disorders are characterized by excessive smooth muscle cell (SMC) proliferation leading to vessel remodeling and occlusion. In pulmonary arterial hypertension (PAH), SMC phenotype switching from a terminally differentiated contractile to synthetic state is gaining traction as our understanding of the disease progression improves. While maintenance of SMC contractile phenotype is reportedly orchestrated by a MEF2C-myocardin (MYOCD) interplay, little is known regarding molecular control at this nexus. Moreover, the burgeoning interest in microRNAs (miRs) provides the basis for exploring their modulation of MEF2C-MYOCD signaling, and in turn, a pro-proliferative, synthetic SMC phenotype. We hypothesized that suppression of SMC contractile phenotype in pulmonary hypertension is mediated by miR-214 via repression of the MEF2C-MYOCD-leiomodin1 (LMOD1) signaling axis.

METHODS AND RESULTS

In SMCs isolated from a PAH patient cohort and commercially obtained hPASMCs exposed to hypoxia, miR-214 expression was monitored by qRT-PCR. miR-214 was upregulated in PAH- vs. control subject hPASMCs as well as in commercially obtained hPASMCs exposed to hypoxia. These increases in miR-214 were paralleled by MEF2C, MYOCD and SMC contractile protein downregulation. Of these, LMOD1 and MEF2C were directly targeted by the miR. Mir-214 overexpression mimicked the PAH profile, downregulating MEF2C and LMOD1. AntagomiR-214 abrogated hypoxia-induced suppression of the contractile phenotype and its attendant proliferation. Anti-miR-214 also restored PAH-PASMCs to a contractile phenotype seen during vascular homeostasis.

CONCLUSIONS

Our findings illustrate a key role for miR-214 in modulation of MEF2C-MYOCD-LMOD1 signaling and suggest that an antagonist of miR-214 could mitigate SMC phenotype changes and proliferation in vascular hyperproliferative disorders including PAH.

Sumoylation in Synaptic Function and Dysfunction

Sumoylation has recently emerged as a key post-translational modification involved in many, if not all, biological processes. Small Ubiquitin-like Modifier (SUMO) polypeptides are covalently attached to specific lysine residues of target proteins through a dedicated enzymatic pathway. Disruption of the SUMO enzymatic pathway in the developing brain leads to lethality indicating that this process exerts a central role during embryonic and post-natal development. However, little is still known regarding how this highly dynamic protein modification is regulated in the mammalian brain despite an increasing number of data implicating sumoylated substrates in synapse formation, synaptic communication and plasticity. The aim of this review is therefore to briefly describe the enzymatic SUMO pathway and to give an overview of our current knowledge on the function and dysfunction of protein sumoylation at the mammalian synapse.

Metal-dependent Deacetylases: Cancer and Epigenetic Regulators

Epigenetic regulation is a key factor in cellular homeostasis. Post-translational modifications (PTMs) are a central focus of this regulation as they function as signaling markers within the cell. Lysine acetylation is a dynamic, reversible PTM that has garnered recent attention due to alterations in various types of cancer. Acetylation levels are regulated by two opposing enzyme families: lysine acetyltransferases (KATs) and histone deacetylases (HDACs). HDACs are key players in epigenetic regulation and have a role in the silencing of tumor suppressor genes. The dynamic equilibrium of acetylation makes HDACs attractive targets for drug therapy. However, substrate selectivity and biological function of HDAC isozymes is poorly understood. This review outlines the current understanding of the roles and specific epigenetic interactions of the metal-dependent HDACs in addition to their roles in cancer.

Regulation of class IIa HDAC activities: it is not only matter of subcellular localization

In response to environmental cues, enzymes that influence the functions of proteins, through reversible post-translational modifications supervise the coordination of cell behavior like orchestral conductors. Class IIa histone deacetylases (HDACs) belong to this category. Even though in vertebrates these deacetylases have discarded the core enzymatic activity, class IIa HDACs can assemble into multiprotein complexes devoted to transcriptional reprogramming, including but not limited to epigenetic changes. Class IIa HDACs are subjected to variegated and interconnected layers of regulation, which reflect the wide range of biological responses under the scrutiny of this gene family. Here, we discuss about the key mechanisms that fine tune class IIa HDACs activities.

Molecular and Functional Characterization of Histone Deacetylase 4 (HDAC4)

Histone deacetylases (HDACs) regulate various nuclear and cytoplasmic processes. In mammals, these enzymes are divided into four classes, with class II further divided into two subclasses: IIa (HDAC4, HDAC5, HDAC7, HDAC9) and IIb (HDAC6 and HDAC10). While HDAC6 is mainly cytoplasmic and HDAC10 is pancellular, class IIa HDACs are dynamically shuttled between the nucleus and cytoplasm in a signal-dependent manner, indicating that they are unique signal transducers able to transduce signals from the cytoplasm to chromatin in the nucleus. Once inside the nucleus, class IIa HDACs interact with MEF2 and other transcription factors, mainly acting as transcriptional corepressors. Although class IIa HDACs share many molecular properties in vitro, they play quite distinct roles in vivo. This chapter lists methods that we have used for molecular and biochemical characterization of HDAC4, including development of regular and phospho-specific antibodies, deacetylase activity determination, reporter gene assays, analysis of subcellular localization, and determination of interaction with 14-3-3 and MEF2. Although described specifically for HDAC4, the protocols should be adaptable for analysis to the other three class IIa members, HDAC5, HDAC7, and HDAC9, as well as for other proteins with related properties.

Evidence for a non-canonical role of HDAC5 in regulation of the cardiac Ncx1 and Bnp genes

Class IIa histone deacetylases (HDACs) are very important for tissue specific gene regulation in development and pathology. Because class IIa HDAC catalytic activity is low, their exact molecular roles have not been fully elucidated. Studies have suggested that class IIa HDACs may serve as a scaffold to recruit the catalytically active class I HDAC complexes to their substrate. Here we directly address whether the class IIa HDAC, HDAC5 may function as a scaffold to recruit co-repressor complexes to promoters. We examined two well-characterized cardiac promoters, the sodium calcium exchanger (Ncx1) and the brain natriuretic peptide (Bnp) whose hypertrophic upregulation is mediated by both class I and IIa HDACs. Selective inhibition of class IIa HDACs did not prevent adrenergic stimulated Ncx1 upregulation, however HDAC5 knockout prevented pressure overload induced Ncx1 upregulation. Using the HDAC5((-/-)) mouse we show that HDAC5 is required for the interaction of the HDAC1/2/Sin3a co-repressor complexes with the Nkx2.5 and YY1 transcription factors and critical for recruitment of the HDAC1/Sin3a co-repressor complex to either the Ncx1 or Bnp promoter. Our novel findings support a non-canonical role of class IIa HDACs in the scaffolding of transcriptional regulatory complexes, which may be relevant for therapeutic intervention for pathologies.

The Transcription Factor MEF2 Is a Novel Regulator of Gsta Gene Class in Mouse MA-10 Leydig Cells

Testosterone is essential for spermatogenesis and the development of male sexual characteristics. However, steroidogenesis produces a significant amount of reactive oxygen species (ROS), which can disrupt testosterone production. The myocyte enhancer factor 2 (MEF2) is an important regulator of organogenesis and cell differentiation in various tissues. In the testis, MEF2 is present in Sertoli and Leydig cells throughout fetal and adult life. MEF2-deficient MA-10 Leydig cells exhibit a significant decrease in steroidogenesis concomitant with a reduction in glutathione S-transferase (GST) activity and in the expression of the 4 Gsta members (GST) that encode ROS inactivating enzymes. Here, we report a novel role for MEF2 in ROS detoxification by directly regulating Gsta expression in Leydig cells. Endogenous Gsta1-4 mRNA levels were decreased in MEF2-deficient MA-10 Leydig cells. Conversely, overexpression of MEF2 increased endogenous Gsta1 levels. MEF2 recruitment to the proximal Gsta1 promoter and direct binding on the -506-bp MEF2 element were confirmed by chromatin immunoprecipitation and DNA precipitation assays. In MA-10 Leydig cells, MEF2 activates the Gsta1 promoter and cooperates with Ca(2+)/calmodulin-dependent kinases I to further enhance Gsta1 promoter activity. These effects were lost when the -506-bp MEF2 element was mutated or when a MEF2-Engrailed dominant negative protein was used. Similar results were obtained on the Gsta2, Gsta3, and Gsta4 promoters, suggesting a global role for MEF2 factors in the regulation of all 4 Gsta genes. Altogether, our results identify a novel role for MEF2 in the expression of genes involved in ROS detoxification, a process essential for adequate testosterone production in Leydig cells.

SUMOylation of TARBP2 regulates miRNA/siRNA efficiency

Small RNA-induced gene silencing is essential for post-transcriptional regulation of gene expression; however, it remains unclear how miRNA/siRNA efficiency is regulated. Here we show that TARBP2 is SUMOylated at K52, which can be enhanced by its phosphorylation. This modification can stabilize TARBP2 via repressing its K⁴⁸-linked ubiquitination. We find that TARBP2 SUMOylation does not influence the overall production of mature miRNAs, but it regulates miRNA/siRNA efficiency. SUMOylated TARBP2 recruits Ago2 to constitute the RNA-induced silencing complex (RISC)-loading complex (RLC), and simultaneously promotes more pre-miRNAs to load into the RLC. Consequently, Ago2 is stabilized and miRNAs/siRNAs bound by TARBP2/Dicer is effectively transferred to Ago2. Thus, these processes lead to the formation of the effective RISC for RNA interference (RNAi). Collectively, our data suggest that SUMOylation of TARBP2 is required for regulating miRNA/siRNA efficiency, which is a general mechanism of miRNA/siRNA regulation.

As part of the miRNA-generating machinery, TARBP2 stabilizes the RNA-induced silencing complex (RISC) loading complex (RLC). Here, Chen et al. show that sumoylation of TARBP2 regulates RNAi efficiency by increasing precursor miRNAs loaded on RLC.

Sequential posttranslational modifications regulate PKC degradation

PKC phosphorylation promotes its sumoylation, which in turn inhibits its ubiquitination and ultimately reduces its degradation via the ubiquitin-proteasome pathway. These findings provide a molecular explanation for the activation-induced down-regulation of PKC proteins.

Cross-talk among different types of posttranslational modifications (PTMs) has emerged as an important regulatory mechanism for protein function. Here we elucidate a mechanism that controls PKCα stability via a sequential cascade of PTMs. We demonstrate that PKCα dephosphorylation decreases its sumoylation, which in turn promotes its ubiquitination and ultimately enhances its degradation via the ubiquitin-proteasome pathway. These findings provide a molecular explanation for the activation-induced down-regulation of PKC proteins.