males absent on the first


EVOLUTIONARY HOMOLOGS

Identification of histone acetyltransferases related to Drosophila Mof

The Tat protein of the human immunodeficiency virus (HIV) is a powerful activator of HIV gene expression. Genetic and biochemical evidence suggests that one or more cellular cofactors may be important for Tat activity. Two-hybrid interactive cloning in yeast has been used to identify a partial cDNA clone (clone 10) from a human B-lymphoblastoid library that specifically interacts with the N-terminal 31 amino acids of HIV-1 Tat, which contains the essential cysteine-rich portion of the Tat activation domain. The encoded protein also binds to purified Tat in vitro. Mutation of single essential cysteine residues in Tat abolishes interaction between Tat and clone 10, suggesting that interaction with the encoded protein is important for Tat activity. The full-length cDNA for the Tat binding protein has been identified. Overexpression of the encoded protein, Tip60 (Tat interactive protein, 60 kDa) results in a fourfold augmentation of Tat transactivation of the HIV-1 promoter in transient expression assays without increasing the basal activity of the HIV promoter or activating the heterologous RSV promoter. These data together with the genetic and in vitro binding data support the notion that Tip60 might be a cofactor of Tat involved in the regulation of HIV gene expression (Kamine, 1996).

Posttranslational acetylation of core histone amino termini has long been associated with transcriptionally active chromatin. Recent reports have demonstrated histone acetyltransferase activity in a small group of conserved transcriptional regulators directly linked to gene activation. In addition, the presence of a putative acetyltransferase domain has been discovered in a group of proteins known as the MYST family (for its founding members MOZ, YBF2/SAS3, SAS2, and Tip60). Members of this family are implicated in acute myeloid leukemia (MOZ), transcriptional silencing in yeast (SAS2 and YBF2/SAS3), HIV Tat interaction in humans (Tip60), and dosage compensation in Drosophila (MOF). A yeast ORF with homology to MYST family members, ESA1, has been shown to possess histone acetyltransferase activity. Unlike the other MYST family members in Saccharomyces cerevisiae this gene is essential for growth (Smith, 1998).

A novel human histone acetyltransferase, termed MORF (monocytic leukemia zinc finger protein-related factor) has been identified and functionally characterized. MORF is a 1781-residue protein displaying significant sequence similarity to MOZ (monocytic leukemia zinc finger protein). MORF is ubiquitously expressed in adult human tissues, and its gene is located at human chromosome band 10q22. MORF has intrinsic histone acetyltransferase activity. In addition to its histone acetyltransferase domain, MORF possesses a strong transcriptional repression domain at its N terminus and a highly potent activation domain at its C terminus. Therefore, MORF is a novel histone acetyltransferase that contains multiple functional domains and may be involved in both positive and negative regulation of transcription (Champagne, 1999).

A novel human gene product, hMof, has been identified that exhibits significant similarity to the Drosophila dosage compensation regulator, Mof. A recombinant C-terminal portion of hMof has histone acetyltransferase activity directed toward histones H3, H2A and H4, a specificity characteristic of other MYST family histone acetyltransferases (Neal, 2000).

Widespread colocalization of the Drosophila histone acetyltransferase homolog MYST5 with DREF and insulator proteins at active genes

MYST family histone acetyltransferases play important roles in gene regulation. This study has characterized the Drosophila MYST histone acetyltransferase (HAT) encoded by CG1894, whose closest homolog is Drosophila MOF, and which was termed MYST5. It localized to a large number of interbands as well as to the telomeres of polytene chromosomes, and it showed strong colocalization with the interband protein Z4/Putzig and RNA polymerase II. Accordingly, genome-wide location analysis by ChIP-seq showed co-occurrence of MYST5 with the Z4-interacting partner Chriz/Chromator. Interestingly, MYST5 bound to the promoter of actively transcribed genes, and about half of MYST5 sites colocalized with the transcription factor DNA replication-related element-binding factor (DREF), indicating a role for MYST5 in gene expression. Moreover, substantial overlap of MYST5 binding was observed with that of the insulator proteins CP190, dCTCF, and BEAF-32, which mediate the organization of the genome into functionally distinct topological domains. Altogether, these data suggest a broad role for MYST5 both in gene-specific transcriptional regulation and in the organization of the genome into chromatin domains, with the two roles possibly being functionally interconnected (Heseding, 2016).

Enzymic activity of histone acetyltransferases

Tip60, originally isolated as an HIV-1-Tat interactive protein, contains an evolutionarily conserved domain with yeast silencing factors. This domain of Tip60 has has been shown to have histone acetyltransferase activity. The purified recombinant effectively acetylates H2A, H3, and H4 but not H2B of core histone mixtures. This substrate specificity has not been observed among histone acetyltransferases analyzed to date. These results indicate that Tip60 is a histone acetyltransferase with a novel property, suggesting that Tip60 and its related factors may introduce a distinct alteration on chromatin (Yamamoto, 1997).

Tip60, an HIV-1-Tat interactive protein, is a nuclear histone acetyltransferase (HAT) with unique histone substrate specificity. Since the acetylation of core histones at particular lysines mediates distinct effects on chromatin assembly and gene regulation, the identification of lysine site specificity of the HAT activity of Tip60 is an initial step in the analysis of its molecular function. Tip60 significantly acetylates amino-terminal tail peptides of histones H2A, H3 and H4, but not H2B, consistent with substrate preference on intact histones. Preferred acetylation sites for Tip60 are the Lys-5 of histone H2A, the Lys-14 of histone H3, and the Lys-5, -8, -12, -16 of histone H4, determined by a method that combines matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) measurements and Lys-C endopeptidase digestion, or a method detecting the incorporation of radiolabelled acetate into synthetic peptides. Thus the lysine site specificity of Tip60 in histone amino-terminal tail peptides in vitro has been characterized by either an assay measuring the molecular mass of endopeptidase digested peptides, or a previously described assay. These results agree well with the proposed classification of lysines in core histones. The classification may be useful for an analysis of the relationships between HATs and the substrates of other uncharacterized HATs (Kimura, 1998).

SAS3 was originally isolated as a gene related to SAS2, which encodes a positive regulator of transcriptional silencing in yeast. The Sas3 protein possesses an evolutionally conserved domain that is shared by a group of SAS-like factors. This conserved domain contains an atypical zinc finger motif and a putative acetyl-CoA binding motif. Recombinant Sas3 exhibits histone acetyltransferase (HAT) activity toward acetylate core histones H2A, H3, and H4. This substrate specificity is similar to those of Tip60 and Esa1. Analysis of a series of deletion mutants has revealed that the minimum region required for HAT activity is located within amino acid residues 241-577, including the domain conserved in the MYST family proteins. Amino acid substitution mutant analysis shows that both the acetyl-CoA binding motif and the zinc finger motif are required for HAT activity. These results suggest that SAS3 and its family members require the zinc finger motif for their activity (Takechi, 1999).

A stable, multisubunit human histone acetyltransferase complex (hMSL) contains homologs of the Drosophila dosage compensation proteins MOF, MSL1, MSL2, and MSL3. This complex shows strong specificity for histone H4 lysine 16 in chromatin in vitro, and RNA interference-mediated knockdown experiments reveal that it is responsible for the majority of H4 acetylation at lysine 16 in the cell. hMOF is a component of additional complexes, forming associations with host cell factor 1 and a protein distantly related to MSL1 (hMSL1v1). Two versions of hMSL3 were found in the hMSL complex that differ by the presence of the chromodomain. Lastly, it was found that reduction in the levels of hMSLs and acetylation of H4 at lysine 16 are correlated with reduced transcription of some genes and with a G2/M cell cycle arrest. This is of particular interest given the recent correlation of global loss of acetylation of lysine 16 in histone H4 with tumorigenesis (Smith, 2005).

Reversible histone acetylation plays an important role in regulation of chromatin structure and function. The human orthologue of Drosophila melanogaster MOF, hMOF, is a histone H4 lysine K16-specific acetyltransferase. hMOF is also required for this modification in mammalian cells. Knockdown of hMOF in HeLa and HepG2 cells causes a dramatic reduction of histone H4K16 acetylation as detected by Western blot analysis and mass spectrometric analysis of endogenous histones. Evidence is provided that, similar to the Drosophila dosage compensation system, hMOF and hMSL3 form a complex in mammalian cells. hMOF and hMSL3 small interfering RNA-treated cells also show dramatic nuclear morphological deformations, depicted by a polylobulated nuclear phenotype. Reduction of hMOF protein levels by RNA interference in HeLa cells also leads to accumulation of cells in the G(2) and M phases of the cell cycle. Treatment with specific inhibitors of the DNA damage response pathway reverts the cell cycle arrest caused by a reduction in hMOF protein levels. Furthermore, hMOF-depleted cells show an increased number of phospho-ATM and gammaH2AX foci and have an impaired repair response to ionizing radiation. Taken together, these data show that hMOF is required for histone H4 lysine 16 acetylation in mammalian cells and suggest that hMOF has a role in DNA damage response during cell cycle progression (Taipale, 2005).

Physical association and coordinate function of the H3 K4 methyltransferase MLL1 and the H4 K16 acetyltransferase MOF

A stable complex containing MLL1 (Drosophila homolog, Trx) and MOF has been immunoaffinity purified from a human cell line that stably expresses an epitope-tagged WDR5 subunit. Stable interactions between MLL1 and MOF were confirmed by reciprocal immunoprecipitation, cosedimentation, and cotransfection analyses, and interaction sites were mapped to MLL1 C-terminal and MOF zinc finger domains. The purified complex has a robust MLL1-mediated histone methyltransferase activity that can effect mono-, di-, and tri-methylation of H3 K4 and a MOF-mediated histone acetyltransferase activity that is specific for H4 K16. Importantly, both activities are required for optimal transcription activation on a chromatin template in vitro and on an endogenous MLL1 target gene, Hox a9, in vivo. These results indicate an activator-based mechanism for joint MLL1 and MOF recruitment and targeted methylation and acetylation and provide a molecular explanation for the closely correlated distribution of H3 K4 methylation and H4 K16 acetylation on active genes (Dou, 2005; full text of article).

PTEN interacts with histone H1 and controls chromatin condensation

Chromatin organization and dynamics are integral to global gene transcription. Histone modification influences chromatin status and gene expression. PTEN plays multiple roles in tumor suppression, development, and metabolism. This study, performed with HeLa cells, reports on the interplay of PTEN, histone H1, and chromatin. Loss of PTEN leads to dissociation of histone H1 from chromatin and decondensation of chromatin. PTEN deletion also results in elevation of histone H4 acetylation at lysine 16, an epigenetic marker for chromatin activation. PTEN and histone H1 physically interact through their C-terminal domains. Disruption of the PTEN C terminus promotes the chromatin association of MOF acetyltransferase and induces H4K16 acetylation. Hyperacetylation of H4K16 impairs the association of PTEN with histone H1, which constitutes regulatory feedback that may reduce chromatin stability. These results demonstrate that PTEN controls chromatin condensation, thus influencing gene expression. It is proposed that PTEN regulates global gene transcription profiling through histones and chromatin remodeling (Chen, 2014: PubMed).

Histone crosstalk between H3S10ph and H4K16ac generates a histone code that mediates transcription elongation

The phosphorylation of the serine 10 at histone H3 has been shown to be important for transcriptional activation. This study reports the molecular mechanism through which H3S10ph triggers transcript elongation of the FOSL1 gene. Serum stimulation induces the PIM1 kinase to phosphorylate the preacetylated histone H3 at the FOSL1 enhancer. The adaptor protein 14-3-3 binds the phosphorylated nucleosome and recruits the histone acetyltransferase MOF, which triggers the acetylation of histone H4 at lysine 16 (H4K16ac). This histone crosstalk generates the nucleosomal recognition code composed of H3K9acS10ph/H4K16ac determining a nucleosome platform for the bromodomain protein BRD4 binding. The recruitment of the positive transcription elongation factor b (P-TEFb) via BRD4 induces the release of the promoter-proximal paused RNA polymerase II and the increase of its processivity. Thus, the single phosphorylation H3S10ph at the FOSL1 enhancer triggers a cascade of events which activate transcriptional elongation (Zippo, 2009).

Histone acetyltransferases and gene activation and repression

The androgen receptor (AR) is a member of the nuclear hormone receptor superfamily. Recent work in this field has been focused on defining the mechanisms of transcriptional control exacted by members of this superfamily. Using a COOH-terminal region of the human AR in a yeast two-hybrid screen, Tip60 has been identified as an AR-interacting protein. Tip60, which was originally identified as a coactivator for the human immunodeficiency virus TAT protein, can enhance AR-mediated transactivation in a ligand-dependent manner in LNCaP and COS-1 cell lines. In addition, Tip60 can also enhance transactivation through the estrogen receptor and progesterone receptor in a ligand-dependent manner, thus identifying Tip60 as a nuclear hormone receptor coactivator. These studies also demonstrate that Tip60 co-immunoprecipitates with the full-length AR in vitro and that Tip60 enhances transactivation to levels observed with the coactivators steroid receptor coactivator 1, p300, and CREB-binding protein. The importance of such proteins in enhancing nuclear hormone receptor-mediated transcriptional activation is widely accepted, and this work suggests that Tip60 may have an equally important role to play (Brady, 1999).

Silencing at the cryptic mating-type loci HML and HMR of Saccharomyces cerevisiae requires regulatory sites called silencers. Mutations in the Rap1 and Abf1 binding sites of the HMR-E silencer (HMRa-e**) cause the silencer to be nonfunctional, and hence, cause derepression of HMR. Mutations in SAS2 have been isolated as second-site suppressors of the silencing defect of HMRa-e**. Silencing conferred by the removal of SAS2 (sas2 delta) depends upon the integrity of the ARS consensus sequence of the HMR-E silencer, thus arguing for an involvement of the origin recognition complex (ORC). Restoration of silencing by sas2 delta requires ORC2 and ORC5, but not SIR1 or RAP1. Furthermore, sas2 delta suppresses the temperature sensitivity, but not the silencing defect of orc2-1 and orc5-1. Moreover, sas2 delta has opposing effects on silencing of HML and HMR. The putative Sas2 protein bears similarities to known protein acetyltransferases. Several models for the role of Sas2 in silencing are discussed (Ehrenhofer-Murray, 1997).

Crosstalk between NSL histone acetyltransferase and MLL/SET complexes: NSL complex functions in promoting histone H3K4 di-methylation activity by MLL/SET complexes

hMOF (MYST1), a histone acetyltransferase (HAT), forms at least two distinct multiprotein complexes in human cells. The male specific lethal (MSL) HAT complex plays a key role in dosage compensation in Drosophila and is responsible for histone H4K16ac in vivo. A second hMOF-containing HAT complex has been described, the non-specific lethal (NSL) HAT complex. The NSL complex has a broader substrate specificity, can acetylate H4 on K16, K5, and K8. The WD (tryptophan-aspartate) repeat domain 5 (WDR5) and host cell factor 1 (HCF1) are shared among members of the MLL/SET (mixed-lineage leukemia/set-domain containing) family of histone H3K4 methyltransferase complexes. The presence of these shared subunits raises the possibility that there are functional links between these complexes and the histone modifications they catalyze; however, the degree to which NSL and MLL/SET influence one another's activities remains unclear. This study presents evidence from biochemical assays and knockdown/overexpression approaches arguing that the NSL HAT promotes histone H3K4me2 by MLL/SET complexes by an acetylation-dependent mechanism. In genomic experiments, a set of genes was identified, including ANKRD2, that are affected by knockdown of both NSL and MLL/SET subunits, suggested they are co-regulated by NSL and MLL/SET complexes. In ChIP assays, it was observed that depletion of the NSL subunits hMOF or NSL1 resulted in a significant reduction of both H4K16ac and H3K4me2 in the vicinity of the ANKRD2 transcriptional start site proximal region. However, depletion of RbBP5 (a core component of MLL/SET complexes) only reduced H3K4me2 marks, but not H4K16ac in the same region of ANKRD2, consistent with the idea that NSL acts upstream of MLL/SET to regulate H3K4me2 at certain promoters, suggesting coordination between NSL and MLL/SET complexes is involved in transcriptional regulation of certain genes. Taken together, these results suggest a crosstalk between the NSL and MLL/SET complexes in cells (Zhao, 2013)

Other interactions of histone acetyltransferases

Tip60 (Tat interactive protein, 60 kDa), specifically interacts with the Tat (transactivating transcriptional regulator) protein of the human immunodeficiency virus-1 (HIV-1). To identify cellular functions of Tip, the effects of Tip on cellular pathways that Tat has been reported to affect have been examined. Overexpression of Tip results in an almost complete block in activation of a Gal4-CREB (cAMP response element binding protein) fusion protein by cyclic AMP dependent protein kinase A. This inhibition appears to be mediated through direct interaction of Tip and CREB, since Tip directly binds to CREB protein in vitro. Amino acid substitutions of two conserved amino acids found in the putative acetyl coenzyme A binding motif of Tip completely abolishes the histone acetyltransferase (HAT) activity of recombinant Tip. Inhibition of CREB activation by Tip is not diminished in a HAT negative Tip mutant, indicating that Tip can negatively regulate gene expression independent of HAT activity. Recently, Tip has also been shown to be a transcriptional coactivator of nuclear hormone receptors; therefore, Tip can both activate transcription factors of one signaling pathway (nuclear hormone receptors) and bind to a different transcription factor (CREB) and inhibit activation of another signaling pathway (Gavaravarapu, 2000).

Interleukin-9 (IL-9) exerts its pleiotropic effects through the IL-9 receptor (IL-9R) complex that consists of the ligand specific IL-9R alpha-chain, and the IL-2R gamma-chain. A modified yeast two-hybrid system was used to isolate cDNAs encoding proteins that interact with the intracellular domain of the human IL-9R alpha-chain (hIL-9Ralpha). Tip60, an HIV-1 Tat transcription cofactor, has been identified as an hIL-9Ralpha interacting protein. The interaction between hIL-9Ralpha and Tip60 was confirmed by coimmunoprecipitation and colocalization studies. This is the first demonstration that Tip60 associates with a membrane receptor. Amino acids 411-423 in hIL-9Ralpha and amino acids 100-147 in Tip60 are important for interaction. Interestingly, the region in hIL-9alpha that binds Tip60 is adjacent to the site previously shown to interact with Stat3. Tip60 binds HIV-Tat and mediates Tat-dependent transactivation possibly through its histone acetyltransferase activity. These results therefore suggest that Tip60 may act as a cofactor of Stat3 or as an adaptor protein for molecules that are important for IL-9 signaling (Sliva, 1999).

Histone acetyltransferases and cell cycle regulation

Histones are dynamically modified during chromatin assembly, as specific transcriptional patterns are established, and during mitosis and development. Modifications include acetylation, phosphorylation, ubiquitination, methylation, and ADP-ribosylation, but the biological significance of each of these is not well understood. For example, distinct acetylation patterns correlate with nucleosome formation and with transcriptionally activated or silenced chromatin, yet mutations in genes encoding several yeast histone acetyltransferase (HAT) activities result in either no cellular phenotype or only modest growth defects. ESA1, an essential gene that is a member of the MYST family has been characterized. Esa1p acetylates histones in a pattern distinct from those of other yeast enzymes, and temperature-sensitive mutant alleles abolish enzymatic activity in vitro and result in partial loss of an acetylated isoform of histone H4 in vivo. Strains carrying these mutations are also blocked in the cell cycle such that at restrictive temperatures, esa1 mutants succeed in replicating their DNA but fail to proceed normally through mitosis and cytokinesis. Recent studies show that Esa1p enhances transcription in vitro and thus may modulate expression of genes important for cell cycle control. These observations therefore link an essential HAT activity to cell cycle progression, potentially through discrete transcriptional regulatory events (Clarke, 1999).

The origin recognition complex (ORC) is an initiator protein for DNA replication, but also affects transcriptional silencing in Saccharomyces cerevisiae and heterochromatin function in Drosophila. It is not known, however, whether any of these functions of ORC are conserved in mammals. A novel protein, HBO1 (histone acetyltransferase binding to ORC), has been identified that interacts with human ORC1 protein, the largest subunit of ORC. HBO1 exists as part of a multisubunit complex that possesses histone H3 and H4 acetyltransferase activities. A fraction of the relatively abundant HBO1 protein associates with ORC1 in human cell extracts. HBO1 is a member of the MYST domain family that includes S. cerevisiae Sas2p, a protein involved in control of transcriptional silencing that also has been genetically linked to ORC function. Thus the interaction between ORC and a MYST domain acetyltransferase is widely conserved. Roles are suggested for ORC-mediated acetylation of chromatin in control of both DNA replication and gene expression (Iizuka, 1999).

MOF-associated complexes ensure stem cell identity and Xist repression

Histone acetyl transferases (HATs) play distinct roles in many cellular processes and are frequently misregulated in cancers. This study examined the regulatory potential of MYST1-(MOF)-containing MSL and NSL HAT complexes in mouse embryonic stem cells (ESCs) and neuronal progenitors. Both complexes influence transcription by targeting promoters and TSS-distal enhancers. In contrast to flies, the MSL complex is not exclusively enriched on the X chromosome, yet it is crucial for mammalian X chromosome regulation as it specifically regulates Tsix, the major repressor of Xist lncRNA. MSL depletion leads to decreased Tsix expression, reduced REX1 recruitment, and consequently, enhanced accumulation of Xist and variable numbers of inactivated X chromosomes during early differentiation. The NSL complex provides additional, Tsix-independent repression of Xist by maintaining pluripotency. MSL and NSL complexes therefore act synergistically by using distinct pathways to ensure a fail-safe mechanism for the repression of X inactivation in ESCs (Chelmicki, 2014).

MOF-associated complexes have overlapping and unique roles in regulating pluripotency in embryonic stem cells and during differentiation

The histone acetyltransferase (HAT) Mof is essential for mouse embryonic stem cells (mESC) pluripotency and early development. Mof is the enzymatic subunit of two different HAT complexes, male specific lethal (MSL) and non-specific lethal (NSL). The individual contribution of MSL and NSL to transcription regulation in mESCs is not well understood. A genome-wide analysis show that (1) MSL and NSL bind to specific and common sets of expressed genes; (2) NSL binds exclusively at promoters, while (3) MSL binds in gene bodies. Nsl1 regulates proliferation and cellular homeostasis of mESCs. MSL is the main HAT acetylating H4K16 in mESCs, is enriched at many mESC-specific and bivalent genes. MSL is important to keep a subset of bivalent genes silent in mESCs, while developmental genes require MSL for expression during differentiation. Thus, NSL and MSL HAT complexes differentially regulate specific sets of expressed genes in mESCs and during differentiation (Ravens, 2014).

Histone acetyltransferases and viral pathogenicity

Tip60, a cellular histone-acetyltransferase, is known to interact with the HIV-1-encoded transactivator protein, Tat. The interaction of Tat with Tip60 efficiently inhibits the Tip60 histone-acetyltransferase activity. Besides its histone-acetyltransferase activity, Tip60 can undergo an autoacetylation that is not affected by Tat interaction. These data show that Tip60 does not significantly influence Tat-dependent transcriptional activation of the 5'-LTR of HIV, suggesting that its interaction with Tat affects some intrinsic cellular process. A cellular gene, Mn-dependent superoxide dismutase (Mn-SOD), has a Tip60-dependent transcriptional activity. Interestingly, the simultaneous expression of Tat and Tip60 abolishes the effect of Tip60 on the activity of the Mn-SOD promoter. It is postulated that in targeting Tip60, the HIV-1 transactivator, Tat, hinders the expression of cellular genes (such as Mn-SOD), which normally interfere with the efficient replication and propagation of the virus (Creaven, 1999).

Histone acetyltransferases and cancer

Chromosomal abnormalities of band 8p11 are associated with a distinct subtype of acute myeloid leukemia with French-American-British M4/5 morphology and prominent erythrophagocytosis by the blast cells. This subtype is usually associated with the t(8;16)(p11;p13), a translocation that has recently been shown to result in a fusion between the MOZ and CBP genes. The inv(8)(p11q13), an abnormality associated with the same leukemia phenotype, has been cloned and a novel fusion between MOZ and the nuclear receptor transcriptional coactivator TIF2/GRIP-1/NCoA-2 has been found. This gene has not previously been implicated in the pathogenesis of leukemia or other malignancies. MOZ-TIF2 retains the histone acetyltransferase homology domains of both proteins and also the CBP binding domain of TIF2. It is speculated that the apparently identical leukemia cell phenotype observed in cases with the t(8;16) and the inv(8) arises by recruitment of CBP by MOZ-TIF2, resulting in modulation of the transcriptional activity of target genes by a mechanism involving abnormal histone acetylation (Carapeti, 1998).

MOF maintains transcriptional programs regulating cellular stress response

MOF (MYST1, KAT8) is the major H4K16 lysine acetyltransferase (KAT) in Drosophila and mammals and is essential for embryonic development. However, little is known regarding the role of MOF in specific cell lineages. This study analyzed the differential role of MOF in mice, in proliferating and terminally differentiated tissues at steady state and under stress conditions. In proliferating cells, MOF directly binds and maintains the expression of genes required for cell cycle progression. In contrast, MOF is dispensable for terminally differentiated, postmitotic glomerular podocytes under physiological conditions. However, in response to injury, MOF is absolutely critical for podocyte maintenance in vivo. Consistently, defective nuclear, endoplasmic reticulum and Golgi structures, as well as presence of multivesicular bodies, were detected in vivo in podocytes lacking Mof following injury. Undertaking genome-wide expression analysis of podocytes, several MOF-regulated pathways required for stress response were uncovered. It was found that MOF, along with the members of the non-specific lethal but not the male-specific lethal complex, directly binds to genes encoding the lysosome, endocytosis and vacuole pathways, which are known regulators of podocyte maintenance. Thus, this work identifies MOF as a key regulator of cellular stress response in glomerular podocytes (Sheikh, 2015).

This study found that MOF is required for cell cycle progression in MEFs and cultured podocytes. Indeed, MOF binds directly to genes required for cell cycle progression and maintains their transcription. Although MOF was dispensable for the maintenance of postmitotic glomerular podocytes at steady state, MOF appeared to be a key regulator of podocyte stress response and adaptation. At a mechanistic level, MOF was required in podocytes to maintain transcription of genes of the lysosome, endocytosis and vacuole pathways, which are key regulators of cellular homeostasis in podocytes and other cells. Together, this work establishes a novel role for MOF in maintaining cellular homeostasis in response to stress (Sheikh, 2015).

An efficient cellular response is needed for ensuring that homeostasis within a cell is maintained and stress does not induce overt damage and eventually cell death. The importance of MOF in responding to stress has been particularly well studied in the context of induced DNA damage. Following DNA damage, there is normally an increase in H4K16ac.16 In the absence of MOF, DNA response proteins including MDC1, 53BP1 and BRCA1 are not recruited to DNA damage foci in a timely manner. The response to DNA damage is at least partially imparted by the interaction between MOF and the ataxia telangiectasia mutated kinase, whereby the ataxia telangiectasia mutated kinase phosphorylates MOF at T392. This process is particularly important for DNA damage repair in the S, G2 and M phases of the cell cycle, and potentially explains the observed G2/M cell cycle arrest in Mof knockout cells. Although the importance of MOF in responding to DNA damage, especially in proliferating cells is well studied, there are currently no reports investigating the importance of MOF in responding to stress in non-proliferating cells. This study provides strong evidence that MOF is required to maintain the lysosome, endocytosis and vacuole pathways, as well as the ER, Golgi and nuclear structure in response to stress. This is particularly true for postmitotic, long-lived podocytes, where alterations in each of these pathways directly underlies the susceptibility to acquired and genetic diseases. Although ths study provides clear evidence for the importance of MOF in the transcriptional regulation of organelle pathways in podocytes, an additional direct role for MOF in maintaining non-nuclear organelles needs to be addressed in the future (Sheikh, 2015).

Chronic kidney disease encompasses a set of debilitating diseases whose incidence is rising in the Western world, especially in elderly individuals. Hallmarks of chronic kidney disease include glomerulosclerosis, inflammation and fibrosis, and are associated with the continuous loss of kidney parenchyma and functional nephrons. The kidney normally compensates for this loss by demanding further work from remaining glomeruli, which in turn induces further stress on these components of the kidney. This study found that Mof deletion in podocytes had no adverse functional side effects at steady state at least on a pure C57BL/6 background. However, once low-level damage was induced through Adriamycin administration, Mof-deleted podocytes were not able to cope with the added stress. Mechanistic analyses revealed that this was in part because of the inability of Mof-deleted podocytes to activate components of the lysosome, vacuole and endocytosis pathways. Interestingly, these three pathways have a key role in autophagy. A large body of literature has recently shown the importance of autophagy in protecting kidney cells including podocytes from damage. Strikingly, Mof deletion in podocytes phenocopies the effects of autophagy-deficient podocytes in terms of the markedly increased susceptibility toward Adriamycin, suggesting that at least in the context of podocytes, MOF is critical for activating part of the autophagy response in reply to cellular stress. Intriguingly, this study found that autophagy-related genes were expressed in opposing directions in Adriamycin-treated Mof-deficient podocytes and MEFs. This is likely due to inherent differences between the two cell types. Although autophagy is required to protect against kidney disease, autophagy becomes activated in fibroblasts once they come senescent, or once senescence is induced. Consistently, it has recently been suggested that downregulation of MOF and H4K16ac are critical for MEFs to induce autophagy and avoid cell death. Together with the current results, these observations suggest that MOF has a highly context-specific role and is required for different functions depending on the particular circumstances (Sheikh, 2015).

In summary, this study provides evidence for MOF-dependent cell-specific regulation of stress adaptive gene regulatory networks. In postmitotic podocytes, MOF directly contributes to the maintenance of the lysosome, endosome and vacuole systems in response to stress, whereas in fibroblasts and undifferentiated podocytes, MOF drives cell cycle progression. Taken together, this work uncovers a novel and critical context-specific role for MOF in protecting cells against cellular stress (Sheikh, 2015).


males absent on the first: Biological Overview | Regulation | Developmental Biology | References

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