Bop: a new T-cell-restricted gene located upstream of and opposite to mouse CD8b

The SMYD family proteins in immunology: An update of their obvious and non-obvious relations with the immune system

Epigenetics is an emerging field, due to its relevance in the regulation of a wide range of biological processes. The Su(Var)3–9, Enhancer-of-zeste and Trithorax (SET) and Myeloid, Nervy, and DEAF-1 (MYND) domain-containing (SMYD) proteins, named SMYD1, SMYD2, SMYD3, SMYD4 and SMYD5, are enzymes that catalyse methylation of histone and non-histone substrates, thereby playing a key role in gene expression regulation in many biological contexts, such as muscle development and physiology, haematopoiesis and many types of cancer. This review focuses on a relatively unexplored aspect of SMYD family members - their relation with immunology. Here, immunology is defined in the broadest sense of the word, including basic research on macrophage function or host immunity against pathogen infection, as well as clinical studies, most of which are centred on blood cancers.

Smyd1C Mediates CD8 T Cell Death via Regulation of Bcl2-Mediated Restriction of outer Mitochondrial Membrane Integrity

The SET and Mynd domain 1 (Smyd1) locus encodes three tissue-restricted isoforms. Two previously characterized isoforms, Smyd1A and Smyd1B, are heart and skeletal muscle-restricted histone methyl transferases. Here we report that a third, non-catalytic isoform, Smyd1C, is expressed predominantly in activated CD8 T cells. While Smyd1C- deficient CD8 T cells undergo activation-induced apoptosis, neither of two classical mechanisms activation-induced cell death nor activated cell autonomous death are utilized. Instead, Smyd1C accumulates within both mitochondria and the immunological synapse where it associates with Bcl-2, FK506-Binding Protein 8/38 (FKBP38) and Calcineurin. This complex maintains Bcl-2 phosphorylation, enhanced mitochondrial localization, and restricted apoptosis of activated CD8 T cells. We suggest that CD8 T cell death is governed, in part, by Smyd1C regulation of Bcl2-mediated restriction of outer mitochondrial membrane integrity.

SMYD proteins: key regulators in skeletal and cardiac muscle development and function

Muscle fibers are composed of myofibrils, one of the most highly ordered macromolecular assemblies in cells. Recent studies demonstrate that members of the Smyd family play critical roles in myofibril assembly of skeletal and cardiac muscle during development. The Smyd family consists of five members including Smyd1, Smyd2, Smyd3, Smyd4, and Smyd5. They share two highly conserved structural and functional domains, namely the SET and MYND domains involved in lysine methylation and protein-protein interaction, respectively. Smyd1 is specifically expressed in muscle cells under the regulation of myogenic transcriptional factors of the MyoD and Mef2 families and the serum responsive factor. Loss of function studies reveal that Smyd1 is required for cardiomyogenesis and sarcomere assembly in skeletal and cardiac muscles. Smyd2, on another hand, is dispensable for heart development in mice. However, Smyd2 appears to play a role in myofilament organization in both skeletal and cardiac muscles via Hsp90 methylation. A Drosophila Smyd4 homologue is a muscle-specific transcriptional modulator involved in the development or function of adult muscle. The molecular mechanisms by which Smyd family proteins function in muscle cells are not well understood. It has been suggested that members of the Smyd family may use multiple mechanisms to control muscle development and cell differentiation, including transcriptional regulation, epigenetic regulation via histone methylation, and methylation of proteins other than histones, such as molecular chaperone Hsp90.

The sarcomeric M-region: a molecular command center for diverse cellular processes

The sarcomeric M-region anchors thick filaments and withstands the mechanical stress of contractions by deformation, thus enabling distribution of physiological forces along the length of thick filaments. While the role of the M-region in supporting myofibrillar structure and contractility is well established, its role in mediating additional cellular processes has only recently started to emerge. As such, M-region is the hub of key protein players contributing to cytoskeletal remodeling, signal transduction, mechanosensing, metabolism, and proteasomal degradation. Mutations in genes encoding M-region related proteins lead to development of severe and lethal cardiac and skeletal myopathies affecting mankind. Herein, we describe the main cellular processes taking place at the M-region, other than thick filament assembly, and discuss human myopathies associated with mutant or truncated M-region proteins.

Expression and functional characterization of Smyd1a in myofibril organization of skeletal muscles

Background

Smyd1, the founding member of the Smyd family including Smyd-1, 2, 3, 4 and 5, is a SET and MYND domain containing protein that plays a key role in myofibril assembly in skeletal and cardiac muscles. Bioinformatic analysis revealed that zebrafish genome contains two highly related smyd1 genes, smyd1a and smyd1b. Although Smyd1b function is well characterized in skeletal and cardiac muscles, the function of Smyd1a is, however, unknown.

Methodology/Principal Findings

To investigate the function of Smyd1a in muscle development, we isolated smyd1a from zebrafish, and characterized its expression and function during muscle development via gene knockdown and transgenic expression approaches. The results showed that smyd1a was strongly expressed in skeletal muscles of zebrafish embryos. Functional analysis revealed that knockdown of smyd1a alone had no significant effect on myofibril assembly in zebrafish skeletal muscles. However, knockdown of smyd1a and smyd1b together resulted in a complete disruption of myofibril organization in skeletal muscles, a phenotype stronger than knockdown of smyd1a or smyd1b alone. Moreover, ectopic expression of zebrafish smyd1a or mouse Smyd1 transgene could rescue the myofibril defects from the smyd1b knockdown in zebrafish embryos.

Conclusion/Significance

Collectively, these data indicate that Smyd1a and Smyd1b share similar biological activity in myofibril assembly in zebrafish embryos. However, Smyd1b appears to play a major role in this process.

Smyd1b is required for skeletal and cardiac muscle function in zebrafish

Myofibrillogenesis is critical for muscle cell differentiation and contraction. This study shows that Smyd1b plays a key role in myofibrillogenesis in muscle cells. Knockdown of smyd1b results in up-regulation of hsp90α1 and unc45b gene expression, increased myosin degradation, and disruption of sarcomere organization in zebrafish embryos.

Smyd1b is a member of the Smyd family that is specifically expressed in skeletal and cardiac muscles. Smyd1b plays a key role in thick filament assembly during myofibrillogenesis in skeletal muscles of zebrafish embryos. To better characterize Smyd1b function and its mechanism of action in myofibrillogenesis, we analyzed the effects of smyd1b knockdown on myofibrillogenesis in skeletal and cardiac muscles of zebrafish embryos. The results show that knockdown of smyd1b causes significant disruption of myofibril organization in both skeletal and cardiac muscles of zebrafish embryos. Microarray and quantitative reverse transcription-PCR analyses show that knockdown of smyd1b up-regulates heat shock protein 90 (hsp90) and unc45b gene expression. Biochemical analysis reveals that Smyd1b can be coimmunoprecipitated with heat shock protein 90 α-1 and Unc45b, two myosin chaperones expressed in muscle cells. Consistent with its potential function in myosin folding and assembly, knockdown of smyd1b significantly reduces myosin protein accumulation without affecting mRNA expression. This likely results from increased myosin degradation involving unc45b overexpression. Together these data support the idea that Smyd1b may work together with myosin chaperones to control myosin folding, degradation, and assembly into sarcomeres during myofibrillogenesis.

Smyd1b_tv1, a key regulator of sarcomere assembly, is localized on the M-line of skeletal muscle fibers

Background

Smyd1b is a member of the Smyd family that plays a key role in sarcomere assembly during myofibrillogenesis. Smyd1b encodes two alternatively spliced isoforms, smyd1b_tv1 and smyd1b_tv2, that are expressed in skeletal and cardiac muscles and play a vital role in myofibrillogenesis in skeletal muscles of zebrafish embryos.

Methodology/Principal Findings

To better understand Smyd1b function in myofibrillogenesis, we analyzed the subcellular localization of Smyd1b_tv1 and Smyd1b_tv2 in transgenic zebrafish expressing a myc-tagged Smyd1b_tv1 or Smyd1b_tv2. The results showed a dynamic change of their subcellular localization during muscle cell differentiation. Smyd1b_tv1 and Smyd1b_tv2 were primarily localized in the cytosol of myoblasts and myotubes at early stage zebrafish embryos. However, in mature myofibers, Smyd1b_tv1, and to a small degree of Smyd1b_tv2, exhibited a sarcomeric localization. Double staining with sarcomeric markers revealed that Smyd1b_tv1was localized on the M-lines. The sarcomeric localization was confirmed in zebrafish embryos expressing the Smyd1b_tv1-GFP or Smyd1b_tv2-GFP fusion proteins. Compared with Smyd1b_tv1, Smyd1b_tv2, however, showed a weak sarcomeric localization. Smyd1b_tv1 differs from Smyd1b_tv2 by a 13 amino acid insertion encoded by exon 5, suggesting that some residues within the 13 aa insertion may be critical for the strong sarcomeric localization of Smyd1b_tv1. Sequence comparison with Smyd1b_tv1 orthologs from other vertebrates revealed several highly conserved residues (Phe223, His224 and Gln226) and two potential phosphorylation sites (Thr221 and Ser225) within the 13 aa insertion. To determine whether these residues are involved in the increased sarcomeric localization of Smyd1b_tv1, we mutated these residues into alanine. Substitution of Phe223 or Ser225 with alanine significantly reduced the sarcomeric localization of Smyd1b_tv1. In contrast, other substitutions had no effect. Moreover, replacing Ser225 with threonine (S225T) retained the strong sarcomeric localization of Smyd1b_tv1.

Conclusion/Significance

Together, these data indicate that Phe223 and Ser225 are required for the M-line localization of Smyd1b_tv1.

Genome-wide survey and developmental expression mapping of zebrafish SET domain-containing genes

SET domain-containing proteins represent an evolutionarily conserved family of epigenetic regulators, which are responsible for most histone lysine methylation. Since some of these genes have been revealed to be essential for embryonic development, we propose that the zebrafish, a vertebrate model organism possessing many advantages for developmental studies, can be utilized to study the biological functions of these genes and the related epigenetic mechanisms during early development. To this end, we have performed a genome-wide survey of zebrafish SET domain genes. 58 genes total have been identified. Although gene duplication events give rise to several lineage-specific paralogs, clear reciprocal orthologous relationship reveals high conservation between zebrafish and human SET domain genes. These data were further subject to an evolutionary analysis ranging from yeast to human, leading to the identification of putative clusters of orthologous groups (COGs) of this gene family. By means of whole-mount mRNA in situ hybridization strategy, we have also carried out a developmental expression mapping of these genes. A group of maternal SET domain genes, which are implicated in the programming of histone modification states in early development, have been identified and predicted to be responsible for all known sites of SET domain-mediated histone methylation. Furthermore, some genes show specific expression patterns in certain tissues at certain stages, suggesting the involvement of epigenetic mechanisms in the development of these systems. These results provide a global view of zebrafish SET domain histone methyltransferases in evolutionary and developmental dimensions and pave the way for using zebrafish to systematically study the roles of these genes during development.

Muscle-specific expression of the smyd1 gene is controlled by its 5.3-kb promoter and 5'-flanking sequence in zebrafish embryos

Zebrafish SmyD1 is a SET and MYND domain-containing protein that plays an important role in myofiber maturation and muscle contraction. SmyD1 is required for myofibril organization and sarcomere assembly during myofiber maturation. Whole-mount in situ hybridization revealed that smyd1 mRNAs are specifically expressed in skeletal and cardiac muscles in zebrafish embryos. However, it is unknown if smyd1 is expressed in other striated muscles, such as cranial and fin muscles, and moreover, the regulatory elements required for its muscle-specific expression. We report here the analyses of smyd1 expression using smyd1-gfp transgenic zebrafish. smyd1-gfp transgenic zebrafish were generated using the 5.3-kb smyd1 promoter and its 5'-flanking sequence. GFP expression was found in the skeletal and cardiac muscles of smyd1-gfp transgenic embryos. GFP expression appeared stronger in slow muscles than fast muscles in transgenic zebrafish larvae. In addition, GFP expression was also detected in cranial and fin muscles of smyd1-gfp transgenic zebrafish larvae. In situ hybridization confirmed smyd1 mRNA expression in these tissues, suggesting that the expression of the smyd1-gfp transgene recapitulated that of the endogenous smyd1 gene. Deletion analysis revealed that the 0.5-kb sequence in the proximal promoter of smyd1 was essential for its muscle specificity. Together, these data indicate that smyd1 is specifically expressed in most, if not all, striated muscles, and the muscle specificity is controlled by the 5.3-kb promoter and flanking sequences.

The promoter of the rat 5alpha-reductase type 1 gene is bidirectional and Sp1-dependent

In many androgen target tissues, testosterone is reduced to the more potent androgen, dihydrotestosterone, by steroid 5alpha-reductase. Two isoforms of 5alpha-reductase, type 1 and type 2, have been cloned. They are differentially expressed and regulated. To determine the mechanisms of regulation of 5alpha-reductase type 1 expression, we have cloned its 5'upstream region and defined its promoter. The proximal 5'upstream region of 5alpha-reductase type 1 displays all the features of a CpG island and has numerous Sp1 binding sites. By transient transfection assays, we have identified a bidirectional promoter activity in this region; this activity was highest in the negative orientation, in the direction of the methyltransferase Nsun2 (predicted) gene. Promoter activity, in either orientation, was lost in Sp1 deficient cells but was rescued following co-transfection with a Sp1 expression vector. Thus, the 5'upstream region of rat 5alpha-reductase type 1 contains a bidirectional promoter with an activity that is Sp1-dependent.

SmyD1, a histone methyltransferase, is required for myofibril organization and muscle contraction in zebrafish embryos

Histone modification has emerged as a fundamental mechanism for control of gene expression and cell differentiation. Recent studies suggest that SmyD1, a novo SET domain-containing protein, may play a critical role in cardiac muscle differentiation. However, its role in skeletal muscle development and its mechanism of actions remains elusive. Here we report that SmyD1a and SmyD1b, generated by alternative splicing of SmyD1 gene, are histone methyltransferases that play a key role in skeletal and cardiac muscle contraction. SmyD1a and SmyD1b are specifically expressed in skeletal and cardiac muscles of zebrafish embryos. Knockdown of SmyD1a and SmyD1b expression by morpholino antisense oligos resulted in malfunction of skeletal and cardiac muscles. The SmyD1 morphant embryos (embryos injected with morpholino oligos) could not swim and had no heartbeat. Myofibril organization in the morphant embryos was severely disrupted. The affected myofibers appeared as immature fibers with centrally located nuclei. Together, these data indicate that SmyD1a and SmyD1b are histone methyltransferases and play a critical role in myofibril organization during myofiber maturation.

BOP, a regulator of right ventricular heart development, is a direct transcriptional target of MEF2C in the developing heart

The vertebrate heart is assembled during embryogenesis in a modular manner from different populations of precursor cells. The right ventricular chamber and outflow tract are derived primarily from a population of progenitors known as the anterior heart field. These regions of the heart are severely hypoplastic in mutant mice lacking the myocyte enhancer factor 2C (MEF2C) and BOP transcription factors, suggesting that these cardiogenic regulatory factors may act in a common pathway for development of the anterior heart field and its derivatives. We show that Bop expression in the developing heart depends on the direct binding of MEF2C to a MEF2-response element in the Bop promoter that is necessary and sufficient to recapitulate endogenous Bop expression in the anterior heart field and its cardiac derivatives during mouse development. The Bop promoter also directs transcription in the skeletal muscle lineage, but only cardiac expression is dependent on MEF2. These findings identify Bop as an essential downstream effector gene of MEF2C in the developing heart, and reveal a transcriptional cascade involved in development of the anterior heart field and its derivatives.

The ETO (MTG8) gene family

Cloning and characterization of the 8;21 chromosomal breakpoint identified AML1 on chromosome 21 and ETO (MTG8) on chromosome 8, and the resultant chimeric gene product, AML-1/ETO. The ETO gene family now includes three human members encoding proteins composed of four evolutionarily conserved domains termed nervy homology regions (NHR) 1-4. ETO associates with N-CoR/Sin3a/HDAC complexes in vivo and acts as a corepressor for the promyelocytic zinc finger protein. Moreover, ETO is nuclear matrix attached at sites coincident with histone deacetylase enzymes and mSin3a. These data suggest that ETO proteins function as transcriptional corepressors. This review focuses on the ETO gene family in terms of expression and function. Specifically, the role of ETO as a co-repressor will be detailed. Additionally, the impact of this recent discovery on treatment of t(8;21)-containing leukemia will be discussed.

Bop encodes a muscle-restricted protein containing MYND and SET domains and is essential for cardiac differentiation and morphogenesis

Many transcription factors regulate specific temporal-spatial events during cardiac differentiation; however, the mechanisms that regulate such events are largely unknown. Using a modified subtractive hybridization method to identify specific genes that influence early cardiac development, we found that Bop is expressed specifically in cardiac and skeletal muscle precursors before differentiation of these lineages. Bop encodes a protein containing MYND and SET domains, which have been shown to regulate transcription by mediating distinct chromatin modifications. We show that m-Bop is a histone deacetylase-dependent transcriptional repressor. Targeted deletion of Bop in mice disrupted maturation of ventricular cardiomyocytes and interfered with formation of the right ventricle. Normal expression of Hand2, a transcription factor essential for right ventricular development, in cardiomyocyte precursors is dependent upon m-Bop. These results indicate that m-Bop is essential for cardiomyocyte differentiation and cardiac morphogenesis.