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

Smyd1 is essential for myosin expression and sarcomere organization in craniofacial, extraocular, and cardiac muscles

Skeletal and cardiac muscles are striated myofibers that contain highly organized sarcomeres for muscle contraction. Recent studies revealed that Smyd1, a lysine methyltransferase, plays a key role in sarcomere assembly in heart and trunk skeletal muscles. However, Smyd1 expression and function in craniofacial muscles are not known. Here, we analyze the developmental expression and function of two smyd1 paralogous genes, smyd1a and smyd1b, in craniofacial and cardiac muscles of zebrafish embryos. Our data show that loss of smyd1a (smyd1amb5) or smyd1b (smyd1bsa15678) has no visible effects on myogenic commitment and expression of myod and myosin heavy-chain mRNA transcripts in craniofacial muscles. However, myosin heavy-chain protein accumulation and sarcomere organization are dramatically reduced in smyd1bsa15678 single mutant, and almost completely diminish in smyd1amb5; smyd1bsa15678 double mutant, but not in smyd1amb5 mutant. Similar defects are also observed in cardiac muscles of smyd1bsa15678 mutant. Defective craniofacial and cardiac muscle formation is associated with an upregulation of hsp90α1 and unc45b mRNA expression in smyd1bsa15678 and smyd1amb5; smyd1bsa15678 mutants. Together, our studies indicate that Smyd1b, but not Smyd1a, plays a key role in myosin heavy-chain protein expression and sarcomere organization in craniofacial and cardiac muscles. Loss of smyd1b results in muscle-specific stress response.

Defective sarcomere assembly in smyd1a and smyd1b zebrafish mutants

Two smyd1 paralogues, smyd1a and smyd1b, have been identified in zebrafish. Although Smyd1b function has been reported in fast muscle, its function in slow muscle and the function of Smyd1a, in general, are uncertain. In this study, we generated 2 smyd1a mutant alleles and analyzed the muscle defects in smyd1a and smyd1b single and double mutants in zebrafish. We demonstrated that knockout of smyd1a alone had no visible effect on muscle development and fish survival. This was in contrast to the smyd1b mutant, which exhibited skeletal and cardiac muscle defects, leading to early embryonic lethality. The smyd1a and smyd1b double mutants, however, showed a stronger muscle defect compared with smyd1a or smyd1b mutation alone, namely, the complete disruption of sarcomere organization in slow and fast muscles. Immunostaining revealed that smyd1a; smyd1b double mutations had no effect on myosin gene expression but resulted in a dramatic reduction of myosin protein levels in muscle cells of zebrafish embryos. This was accompanied by the up-regulation of hsp40 and hsp90-α1 gene expression. Together, our studies indicate that both Smyd1a and Smyd1b partake in slow and fast muscle development although Smyd1b plays a dominant role compared with Smyd1a.-Cai, M., Han, L., Liu, L., He, F., Chu, W., Zhang, J., Tian, Z., Du, S. Defective sarcomere assembly in smyd1a and smyd1b zebrafish mutants.

The Smyd Family of Methyltransferases: Role in Cardiac and Skeletal Muscle Physiology and Pathology

Protein methylation plays a pivotal role in the regulation of various cellular processes including chromatin remodeling and gene expression. SET and MYND domain-containing proteins (Smyd) are a special class of lysine methyltransferases whose catalytic SET domain is split by an MYND domain. The hallmark feature of this family was thought to be the methylation of histone H3 (on lysine 4). However, several studies suggest that the role of the Smyd family is dynamic, targeting unique histone residues associated with both transcriptional activation and repression. Smyd proteins also methylate several non-histone proteins to regulate various cellular processes. Although we are only beginning to understand their specific molecular functions and role in chromatin remodeling, recent studies have advanced our understanding of this relatively uncharacterized family, highlighting their involvement in development, cell growth and differentiation and during disease in various animal models. This review summarizes our current knowledge of the structure, function and methylation targets of the Smyd family and provides a compilation of data emphasizing their prominent role in cardiac and skeletal muscle physiology and pathology.

Proteomic and microRNA Transcriptome Analysis revealed the microRNA-SmyD1 network regulation in Skeletal Muscle Fibers performance of Chinese perch

Fish myotomes are comprised of anatomically segregated fast and slow muscle fibers that possess different metabolic and contractile properties. Although the expression profile properties in fast and slow muscle fibers had been investigated at the mRNA levels, a comprehensive analysis at proteomic and microRNA transcriptomic levels is limited. In the present study, we first systematically compared the proteomic and microRNA transcriptome of the slow and fast muscles of Chinese perch (Siniperca chuatsi). Total of 2102 proteins were identified in muscle tissues. Among them, 99 proteins were differentially up-regulated and 400 were down-regulated in the fast muscle compared with slow muscle. MiRNA microarrays revealed that 199 miRNAs identified in the two types of muscle fibers. Compared with the fast muscle, the 32 miRNAs was up-regulated and 27 down-regulated in the slow muscle. Specifically, expression of miR-103 and miR-144 was negatively correlated with SmyD1a and SmyD1b expression in fast and slow muscles, respectively. The luciferase reporter assay further verified that the miR-103 and miR-144 directly regulated the SmyD1a and SmyD1b expression by targeting their 3′-UTR. The constructed miRNA-SmyD1 interaction network might play an important role in controlling the development and performance of different muscle fiber types in Chinese perch.

Defective myogenesis in the absence of the muscle-specific lysine methyltransferase SMYD1

The SMYD (SET and MYND domain) family of lysine methyltransferases harbor a unique structure in which the methyltransferase (SET) domain is intervened by a zinc finger protein-protein interaction MYND domain. SMYD proteins methylate both histone and non-histone substrates and participate in diverse biological processes including transcriptional regulation, DNA repair, proliferation and apoptosis. Smyd1 is unique among the five family members in that it is specifically expressed in striated muscles. Smyd1 is critical for development of the right ventricle in mice. In zebrafish, Smyd1 is necessary for sarcomerogenesis in fast-twitch muscles. Smyd1 is expressed in the skeletal muscle lineage throughout myogenesis and in mature myofibers, shuttling from nucleus to cytosol during myoblast differentiation. Because of this expression pattern, we hypothesized that Smyd1 plays multiple roles at different stages of myogenesis. To determine the role of Smyd1 in mammalian myogenesis, we conditionally eliminated Smyd1 from the skeletal muscle lineage at the myoblast stage using Myf5(cre). Deletion of Smyd1 impaired myoblast differentiation, resulted in fewer myofibers and decreased expression of muscle-specific genes. Muscular defects were temporally restricted to the second wave of myogenesis. Thus, in addition to the previously described functions for Smyd1 in heart development and skeletal muscle sarcomerogenesis, these results point to a novel role for Smyd1 in myoblast differentiation.

The Effects of Hsp90α1 Mutations on Myosin Thick Filament Organization

Heat shock protein 90α plays a key role in myosin folding and thick filament assembly in muscle cells. To assess the structure and function of Hsp90α and its potential regulation by post-translational modification, we developed a combined knockdown and rescue assay in zebrafish embryos to systematically analyze the effects of various mutations on Hsp90α function in myosin thick filament organization. DNA constructs expressing the Hsp90α1 mutants with altered putative ATP binding, phosphorylation, acetylation or methylation sites were co-injected with Hsp90α1 specific morpholino into zebrafish embryos. Myosin thick filament organization was analyzed in skeletal muscles of the injected embryos by immunostaining. The results showed that mutating the conserved D90 residue in the Hsp90α1 ATP binding domain abolished its function in thick filament organization. In addition, phosphorylation mimicking mutations of T33D, T33E and T87E compromised Hsp90α1 function in myosin thick filament organization. Similarly, K287Q acetylation mimicking mutation repressed Hsp90α1 function in myosin thick filament organization. In contrast, K206R and K608R hypomethylation mimicking mutations had not effect on Hsp90α1 function in thick filament organization. Given that T33 and T87 are highly conserved residues involved post-translational modification (PTM) in yeast, mouse and human Hsp90 proteins, data from this study could indicate that Hsp90α1 function in myosin thick filament organization is potentially regulated by PTMs involving phosphorylation and acetylation.

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.

Analyzing cold tolerance mechanism in transgenic zebrafish (Danio rerio)

Low temperatures may cause severe growth inhibition and mortality in fish. In order to understand the mechanism of cold tolerance, a transgenic zebrafish Tg (smyd1:m3ck) model was established to study the effect of energy homeostasis during cold stress. The muscle-specific promoter Smyd1 was used to express the carp muscle form III of creatine kinase (M3-CK), which maintained enzymatic activity at a relatively low temperature, in zebrafish skeletal muscle. In situ hybridization showed that M3-CK was expressed strongly in the skeletal muscle. When exposed to 13 °C, Tg (smyd1:m3ck) fish maintained their swimming behavior, while the wild-type could not. Energy measurements showed that the concentration of ATP increased in Tg (smyd1:m3ck) versus wild-type fish at 28 °C. After 2 h at 13 °C, ATP concentrations were 2.16-fold higher in Tg (smyd1:m3ck) than in wild-type (P<0.05). At 13 °C, the ATP concentration in Tg (smyd1:m3ck) fish and wild-type fish was 63.3% and 20.0%, respectively, of that in wild-type fish at 28 °C. Microarray analysis revealed differential expression of 1249 transcripts in Tg (smyd1:m3ck) versus wild-type fish under cold stress. Biological processes that were significantly overrepresented in this group included circadian rhythm, energy metabolism, lipid transport, and metabolism. These results are clues to understanding the mechanisms underlying temperature acclimation in fish.

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.

503unc, a small and muscle-specific zebrafish promoter

The muscle-specific UNC-45b assists in the folding of sarcomeric myosin. Analysis of the zebrafish unc-45b upstream region revealed that unc-45b promoter fragments reliably drive GFP expression after germline transmission. The muscle-specific 503-bp minimal promoter 503unc was identified to drive gene expression in the zebrafish musculature. In transgenic Tg(-503unc:GFP) zebrafish, GFP fluorescence was detected in the adaxial cells, their slow fiber descendants, and the fast muscle. At later stages, robust GFP fluorescence is eminent in the cardiac, cranial, fin, and trunk muscle, thereby recapitulating the unc-45b expression pattern. We propose that the 503unc promoter is a small and muscle-specific promoter that drives robust gene expression throughout the zebrafish musculature, making it a valuable tool for the exploration of zebrafish muscle.

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.

Multiple cis-elements in the 5'-flanking region of embryonic/larval fast-type of the myosin heavy chain gene of torafugu, MYH(M743-2), function in the transcriptional regulation of its expression

The myosin heavy chain gene, MYH(M743-2), is highly expressed in fast muscle fibers of torafugu embryos and larvae, suggesting its functional roles for embryonic and larval muscle development. However, the transcriptional regulatory mechanism involved in its expression remained unknown. Here, we analyzed the 2075bp 5'-flanking region of torafugu MYH(M743-2) to examine the spatial and temporal regulation by using transgenic and transient expression techniques in zebrafish embryos. Combining both transient and transgenic analyses, we demonstrated that the 2075bp 5'-flanking sequences was sufficient for its expression in skeletal, craniofacial and pectoral fin muscles. The immunohistochemical observation revealed that the zebrafish larvae from the stable transgenic line consistently expressed enhanced green fluorescent protein (EGFP) in fast muscle fibers. Promoter deletion analyses demonstrated that the minimum 468bp promoter region could direct MYH(M743-2) expression in zebrafish larvae. We discovered that the serum response factor (SRF)-like binding sites are required for promoting MYH(M743-2) expression and myoblast determining factor (MyoD) and myocyte enhancer factor-2 (MEF2) binding sites participate in the transcriptional control of MYH(M743-2) expression in fast skeletal muscles. We further discovered that MyoD binding sites, but not MEF2, participate in the transcriptional regulation of MYH(M743-2) expression in pectoral fin and craniofacial muscles. These results clearly demonstrated that multiple cis-elements in the 5'-flanking region of MYH(M743-2) function in the transcriptional control of its expression.

The myosin-interacting protein SMYD1 is essential for sarcomere organization

Assembly, maintenance and renewal of sarcomeres require highly organized and balanced folding, transport, modification and degradation of sarcomeric proteins. However, the molecules that mediate these processes are largely unknown. Here, we isolated the zebrafish mutant flatline (fla), which shows disturbed sarcomere assembly exclusively in heart and fast-twitch skeletal muscle. By positional cloning we identified a nonsense mutation within the SET- and MYND-domain-containing protein 1 gene (smyd1) to be responsible for the fla phenotype. We found SMYD1 expression to be restricted to the heart and fast-twitch skeletal muscle cells. Within these cell types, SMYD1 localizes to both the sarcomeric M-line, where it physically associates with myosin, and the nucleus, where it supposedly represses transcription through its SET and MYND domains. However, although we found transcript levels of thick filament chaperones, such as Hsp90a1 and UNC-45b, to be severely upregulated in fla, its histone methyltransferase activity - mainly responsible for the nuclear function of SMYD1 - is dispensable for sarcomerogenesis. Accordingly, sarcomere assembly in fla mutant embryos can be reconstituted by ectopically expressing histone methyltransferase-deficient SMYD1. By contrast, ectopic expression of myosin-binding-deficient SMYD1 does not rescue fla mutants, implicating an essential role for the SMYD1-myosin interaction in cardiac and fast-twitch skeletal muscle thick filament assembly.

Knockdown and overexpression of Unc-45b result in defective myofibril organization in skeletal muscles of zebrafish embryos

Background

Unc-45 is a myosin chaperone and a Hsp90 co-chaperone that plays a key role in muscle development. Genetic and biochemical studies in C. elegans have demonstrated that Unc-45 facilitates the process of myosin folding and assembly in body wall muscles. Loss or overexpression of Unc-45 in C. elegans results in defective myofibril organization. In the zebrafish Danio rerio, unc-45b, a homolog of C. elegans unc-45, is expressed in both skeletal and cardiac muscles. Earlier studies indicate that mutation or knockdown of unc-45b expression in zebrafish results in a phenotype characterized by a loss of both thick and thin filament organization in skeletal and cardiac muscle. The effects of unc-45b knockdown on other sarcomeric structures and the phenotype of Unc-45b overexpression, however, are poorly understood in vertebrates.

Results

Both knockdown and overexpression provide useful tools to study gene function during animal development. Using such methods, we characterized the role of Unc-45b in myofibril assembly of skeletal muscle in Danio rerio. We showed that, in addition to thick and thin filament defects, knockdown of unc-45b expression disrupted sarcomere organization in M-lines and Z-lines of skeletal muscles in zebrafish embryos. Western blotting analysis showed that myosin protein levels were significantly decreased in unc-45b knockdown embryos. Similarly, embryos overexpressing Unc-45b also exhibited severely disorganized myosin thick filaments. Disruption of thick filament organization by Unc-45b overexpression depends on the C-terminal UCS domain in Unc-45b required for interaction with myosin. Deletion of the C-terminal UCS domain abolished the disruptive activity of Unc-45b in myosin thick filament organization. In contrast, deletion of the N-terminal TPR domain required for binding with Hsp90α had no effect.

Conclusion

Collectively, these studies indicate that the expression levels of Unc-45b must be precisely regulated to ensure normal myofibril organization. Loss or overexpression of Unc-45b leads to defective myofibril organization.

Cardiac deletion of Smyd2 is dispensable for mouse heart development

Chromatin modifying enzymes play a critical role in cardiac differentiation. Previously, it has been shown that the targeted deletion of the histone methyltransferase, Smyd1, the founding member of the SET and MYND domain containing (Smyd) family, interferes with cardiomyocyte maturation and proper formation of the right heart ventricle. The highly related paralogue, Smyd2 is a histone 3 lysine 4- and lysine 36-specific methyltransferase expressed in heart and brain. Here, we report that Smyd2 is differentially expressed during cardiac development with highest expression in the neonatal heart. To elucidate the functional role of Smyd2 in the heart, we generated conditional knockout (cKO) mice harboring a cardiomyocyte-specific deletion of Smyd2 and performed histological, functional and molecular analyses. Unexpectedly, cardiac deletion of Smyd2 was dispensable for proper morphological and functional development of the murine heart and had no effect on global histone 3 lysine 4 or 36 methylation. However, we provide evidence for a potential role of Smyd2 in the transcriptional regulation of genes associated with translation and reveal that Smyd2, similar to Smyd3, interacts with RNA Polymerase II as well as to the RNA helicase, HELZ.

SMYD1, the myogenic activator, is a direct target of serum response factor and myogenin

SMYD1 is a heart and muscle specific SET-MYND domain containing protein, which functions as a histone methyltransferase and regulates downstream gene transcription. We demonstrated that the expression of SMYD1 is restricted in the heart and skeletal muscle tissues in human. To reveal the regulatory mechanisms of SMYD1 expression during myogenesis and cardiogenesis, we cloned and characterized the human SMYD1 promoter, which contains highly conserved serum response factor (SRF) and myogenin binding sites. Overexpression of SRF and myogenin significantly increased the endogenous expression level of Smyd1 in C2C12 cells, respectively. Deletion of Srf in the heart of mouse embryos dramatically decreased the expression level of Smyd1 mRNA and the expression of Smyd1 can be rescued by exogenous SRF introduction in SRF null ES cells during differentiation. Furthermore, we demonstrated that SRF binds to the CArG site and myogenin binds to the E-box element on Smyd1 promoter region using EMSA and ChIP assays. Moreover, forced expression of SMYD1 accelerates myoblast differentiation and myotube formation in C2C12 cells. Taken together, these studies demonstrated that SMYD1 is a key regulator of myogenic differentiation and acts as a downstream target of muscle regulatory factors, SRF and myogenin.

Implanted microvessels progress through distinct neovascularization phenotypes

We have previously demonstrated that implanted microvessels form a new microcirculation with minimal host-derived vessel investment. Our objective was to define the vascular phenotypes present during neovascularization in these implants and identify post-angiogenesis events. Morphological, functional and transcriptional assessments identified three distinct vascular phenotypes in the implants: sprouting angiogenesis, neovascular remodeling, and network maturation. A sprouting angiogenic phenotype appeared first, characterized by high proliferation and low mural cell coverage. This was followed by a neovascular remodeling phenotype characterized by a perfused, poorly organized neovascular network, reduced proliferation, and re-associated mural cells. The last phenotype included a vascular network organized into a stereotypical tree structure containing vessels with normal perivascular cell associations. In addition, proliferation was low and was restricted to the walls of larger microvessels. The transition from angiogenesis to neovascular remodeling coincided with the appearance of blood flow in the implant neovasculature. Analysis of vascular-specific and global gene expression indicates that the intermediate, neovascular remodeling phenotype is transcriptionally distinct from the other two phenotypes. Therefore, this vascular phenotype likely is not simply a transitional phenotype but a distinct vascular phenotype involving unique cellular and vascular processes. Furthermore, this neovascular remodeling phase may be a normal aspect of the general neovascularization process. Given that this phenotype is arguably dysfunctional, many of the microvasculatures present within compromised or diseased tissues may not represent a failure to progress appropriately through a normally occurring neovascularization phenotype.

skNAC (skeletal Naca), a muscle-specific isoform of Naca (nascent polypeptide-associated complex alpha), is required for myofibril organization

Myofibrillogenesis, the precise assembly of sarcomeric proteins into the highly organized sarcomeres, is essential for muscle cell differentiation and function. Myofibrillogenesis requires proper folding and assembly of newly synthesized sarcomeric proteins. sknac (skeletal naca) is an alternatively spliced isoform of naca, which encodes the nascent polypeptide-associated complex alpha polypeptide that binds to newly synthesized polypeptides emerging from the ribosome. sknac is specifically expressed in skeletal and cardiac muscles. However, little is known about the function of skNAC in muscle development in vivo. To determine skNAC function, we have isolated and characterized the sknac gene from zebrafish. Zebrafish sknac cDNA differs from naca by containing an extra large exon that encodes 815 aa. Knockdown of sknac expression by antisense oligos resulted in zebrafish embryos with skeletal muscle defects. The sknac-knockdown embryos showed a paralyzed phenotype with little muscle contraction. In contrast, injection of a control oligo had no effect. Immunostaining and histological analyses revealed that sknac-knockdown embryos contained disorganized thick and thin filaments. Western blot analysis revealed that myosin protein levels were significantly reduced. Collectively, these results demonstrate that skNAC plays a vital role in myofibril assembly and function during muscle cell differentiation.

smyd1 and smyd2 are expressed in muscle tissue in Xenopus laevis

Epigenetic modifications of histone play important roles for regulation of cell activity, such as cell division, cell death, and cell differentiation. A SET domain consisting of about 130 amino acids has lysine methyltransferase activity in the presence of the cosubstrate S-adenosyl-methionine. More than 60 SET domain-containing proteins have been predicted in various organisms. One of them, the SMYD family genes which contain a SET domain and a zinc-finger MYND domain are reported to regulate cell cycle and muscle formation. Here we examined the expression and function of smyd1 and 2 in Xenopus. smyd1 and 2 were expressed in various muscle tissues. While smyd1 expression was observed mainly in cardiac muscle and skeletal muscle, smyd2 expression was done abundantly in skeletal muscle and face region. Moreover, by loss-of-function experiments using antisense morpholino oligonucleotides, it was suggested that smyd1 and 2 related to muscle cells differentiation.

Heat-shock protein 90alpha1 is required for organized myofibril assembly in skeletal muscles of zebrafish embryos

Heat-shock protein 90alpha (Hsp90alpha) is a member of the molecular chaperone family involved in protein folding and assembly. The role of Hsp90alpha in the developmental process, however, remains unclear. Here we report that zebrafish contains two Hsp90alpha genes, Hsp90alpha1, and Hsp90alpha2. Hsp90alpha1 is specifically expressed in developing somites and skeletal muscles of zebrafish embryos. We have demonstrated that Hsp90alpha1 is essential for myofibril organization in skeletal muscles of zebrafish embryos. Knockdown of Hsp90alpha1 resulted in paralyzed zebrafish embryos with poorly organized myofibrils in skeletal muscles. In contrast, knockdown of Hsp90alpha2 had no effect on muscle contraction and myofibril organization. The filament defects could be rescued in a cell autonomous manner by an ectopic expression of Hsp90alpha1. Biochemical analyses revealed that knockdown of Hsp90alpha1 resulted in significant myosin degradation and up-regulation of unc-45b gene expression. These results indicate that Hsp90alpha1 plays an important role in muscle development, likely through facilitating myosin folding and assembly into organized myofibril filaments.