Skeletal muscle-specific variant of nascent polypeptide associated complex alpha (skNAC): Implications for a specific role in mammalian myoblast differentiation

Smyd1: Implications for novel approaches in rhabdomyosarcoma therapy

Rhabdomyosarcoma (RMS), a tumor that consists of poorly differentiated skeletal muscle cells, is the most common soft-tissue sarcoma in children. Despite considerable progress within the last decades, therapeutic options are still limited, warranting the need for novel approaches. Recent data suggest deregulation of the Smyd1 protein, a sumoylation target as well as H3K4me2/3 methyltransferase and transcriptional regulator in myogenesis, and its binding partner skNAC, in RMS cells. Here, we show that despite the fact that most RMS cells express at least low levels of Smyd1 and skNAC, failure to upregulate expression of these genes in reaction to differentiation-promoting signals can always be observed. While overexpression of the Smyd1 gene enhances many aspects of RMS cell differentiation and inhibits proliferation rate and metastatic potential of these cells, functional integrity of the putative Smyd1 sumoylation motif and its SET domain, the latter being crucial for HMT activity, appear to be prerequisites for most of these effects. Based on these findings, we explored the potential for novel RMS therapeutic strategies, employing small-molecule compounds to enhance Smyd1 activity. In particular, we tested manipulation of (a) Smyd1 sumoylation, (b) stability of H3K4me2/3 marks, and (c) calpain activity, with calpains being important targets of Smyd1 in myogenesis. We found that specifically the last strategy might represent a promising approach, given that suitable small-molecule compounds will be available for clinical use in the future.

Nascent polypeptide-Associated Complex and Signal Recognition Particle have cardiac-specific roles in heart development and remodeling

Establishing a catalog of Congenital Heart Disease (CHD) genes and identifying functional networks would improve our understanding of its oligogenic underpinnings. Our studies identified protein biogenesis cofactors Nascent polypeptide-Associated Complex (NAC) and Signal-Recognition-Particle (SRP) as disease candidates and novel regulators of cardiac differentiation and morphogenesis. Knockdown (KD) of the alpha- (Nacα) or beta-subunit (bicaudal, bic) of NAC in the developing Drosophila heart disrupted cardiac developmental remodeling resulting in a fly with no heart. Heart loss was rescued by combined KD of Nacα with the posterior patterning Hox gene Abd-B. Consistent with a central role for this interaction in cardiogenesis, KD of Nacα in cardiac progenitors derived from human iPSCs impaired cardiac differentiation while co-KD with human HOXC12 and HOXD12 rescued this phenotype. Our data suggest that Nacα KD preprograms cardioblasts in the embryo for abortive remodeling later during metamorphosis, as Nacα KD during translation-intensive larval growth or pupal remodeling only causes moderate heart defects. KD of SRP subunits in the developing fly heart produced phenotypes that targeted specific segments and cell types, again suggesting cardiac-specific and spatially regulated activities. Together, we demonstrated directed function for NAC and SRP in heart development, and that regulation of NAC function depends on Hox genes.

Stability of Smyd1 in endothelial cells is controlled by PML-dependent SUMOylation upon cytokine stimulation

Smyd1 is an epigenetic modulator of gene expression that has been well-characterized in muscle cells. It was recently reported that Smyd1 levels are modulated by inflammatory processes. Since inflammation affects the vascular endothelium, this study aimed to characterize Smyd1 expression in endothelial cells. We detected Smyd1 in human endothelial cells (HUVEC and EA.hy926 cells), where the protein was largely localized in PML nuclear bodies (PML-NBs). By transfection of EA.hy926 cells with expression vectors encoding Smyd1, PML, SUMO1, active or mutant forms of the SUMO protease SuPr1 and/or the SUMO-conjugation enzyme UBC9, as well as Smyd1- or PML-specific siRNAs, in the presence or absence of the translation blocker cycloheximide or the proteasome-inhibitor MG132, and supported by computational modeling, we show that Smyd1 is SUMOylated in a PML-dependent manner and thereby addressed for degradation in proteasomes. Furthermore, transfection with Smyd1-encoding vectors led to PML up-regulation at the mRNA level, while PML transfection lowered Smyd1 protein stability. Incubation of EA.hy926 cells with the pro-inflammatory cytokine TNF-α resulted in a constant increase in Smyd1 mRNA and protein over 24 h, while incubation with IFN-γ induced a transient increase in Smyd1 expression, which peaked at 6 h and decreased to control values within 24 h. The IFN-γ-induced increase in Smyd1 was accompanied by more Smyd1 SUMOylation and more/larger PML-NBs. In conclusion, our data indicate that in endothelial cells, Smyd1 levels are regulated through a negative feedback mechanism based on SUMOylation and PML availability. This molecular control loop is stimulated by various cytokines.

Genomic Approaches Reveal Pleiotropic Effects in Crossbred Beef Cattle

Carcass and meat quality are two important attributes for the beef industry because they drive profitability and consumer demand. These traits are of even greater importance in crossbred cattle used in subtropical and tropical regions for their superior adaptability because they tend to underperform compared to their purebred counterparts. Many of these traits are challenging and expensive to measure and unavailable until late in life or after the animal is harvested, hence unrealistic to improve through traditional phenotypic selection, but perfect candidates for genomic selection. Before genomic selection can be implemented in crossbred populations, it is important to explore if pleiotropic effects exist between carcass and meat quality traits. Therefore, the objective of this study was to identify genomic regions with pleiotropic effects on carcass and meat quality traits in a multibreed Angus-Brahman population that included purebred and crossbred animals. Data included phenotypes for 10 carcass and meat quality traits from 2,384 steers, of which 1,038 were genotyped with the GGP Bovine F-250. Single-trait genome-wide association studies were first used to investigate the relevance of direct additive genetic effects on each carcass, sensory and visual meat quality traits. A second analysis for each trait included all other phenotypes as covariates to correct for direct causal effects from identified genomic regions with pure direct effects on the trait under analysis. Five genomic windows on chromosomes BTA5, BTA7, BTA18, and BTA29 explained more than 1% of additive genetic variance of two or more traits. Moreover, three suggestive pleiotropic regions were identified on BTA10 and BTA19. The 317 genes uncovered in pleiotropic regions included anchoring and cytoskeletal proteins, key players in cell growth, muscle development, lipid metabolism and fat deposition, and important factors in muscle proteolysis. A functional analysis of these genes revealed GO terms directly related to carcass quality, meat quality, and tenderness in beef cattle, including calcium-related processes, cell signaling, and modulation of cell-cell adhesion. These results contribute with novel information about the complex genetic architecture and pleiotropic effects of carcass and meat quality traits in crossbred beef cattle.

siRNA-mediated inhibition of skNAC and Smyd1 expression disrupts myofibril organization: Immunofluorescence and electron microscopy study in C2C12 cells

skNAC (skeletal and heart muscle-specific variant of nascent polypeptide-associated complex) and Smyd1 (SET and MYND domain-containing 1) form a protein dimer which is specific for striated muscle cells. Its function is largely unknown. On the one hand, skNAC-Smyd1 appears to control transcriptional processes in the nucleus, on the other hand, specifically at later stages of myogenic differentiation, both proteins translocate to the sarcoplasm and at least Smyd1 specifically associates with sarcomeric structures and might control myofibrillogenesis and/or sarcomere architecture. Here, using immunofluorescence and electron microscopy, we analyzed sarcomere formation and myofibril organization after siRNA-mediated knockdown of skNAC or Smyd1 expression in murine C2C12 skeletal muscle cells. We found that inhibition of skNAC or Smyd1 expression indeed prevents myofibrillogenesis and sarcomere formation, leading to a disorganized array of myofilaments predominantly within the region immediately beneath the plasma membrane.

skNAC and Smyd1 in transcriptional control

Skeletal and heart muscle-specific variant of the alpha subunit of nascent polypeptide associated complex (skNAC) is exclusively found in striated muscle cells. Its function, however, is largely unknown. Previous reports could demonstrate that skNAC binds to Smyd1 (SET and MYND domain containing protein 1). The facts that (a) SET domains have histone methyltransferase activity, and (b) MYND domains are known recruiters of histone deacetylases (HDACs), implicate the skNAC-Smyd1 complex in transcriptional control. To study potential target genes, we carried out cDNA microarray analysis on differentiating C2C12 myoblasts in which expression of the skNAC gene had been knocked down. We found and confirmed a series of targets, specifically genes encoding regulators of inflammation, cellular metabolism, and cell migration. Mechanistically, as shown by Western blot, ELISA, and ChIP analysis at selected promoter regions, transcriptional control by skNAC-Smyd1 appears to be exerted at least in part by affecting a series of histone modifications, specifically H3K4 di- and trimethylation and potentially also histone acetylation. Taken together, our data suggest that the skNAC-Smyd1 complex is involved in transcriptional regulation both via the control of histone methylation and histone (de)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.

Overexpression of the skNAC gene in human rhabdomyosarcoma cells enhances their differentiation potential and inhibits tumor cell growth and spreading

Skeletal and heart muscle-specific variant of the alpha subunit of nascent polypeptide complex (skNAC) is exclusively present in striated muscle cells. During skeletal muscle cell differentiation, skNAC expression is strongly induced, suggesting that the protein might be a regulator of the differentiation process. Rhabdomyosarcoma is a tumor of skeletal muscle origin. Since there is a strong inverse correlation between rhabdomyosarcoma cell differentiation status and metastatic potential, we analyzed skNAC expression patterns in a set of rhabdomyosarcoma cell lines: Whereas RD/12 and RD/18 cells showed a marked induction of skNAC gene expression upon the induction of differentiation-similarly as the one seen in nontransformed myoblasts-skNAC was not induced in CCA or Rh30 cells. Overexpressing skNAC in CCA and Rh30 cells led to a reduction in cell cycle progression and cell proliferation accompanied by an upregulation of specific myogenic differentiation markers, such as Myogenin or Myosin Heavy Chain. Furthermore, in contrast to vector-transfected controls, a high percentage of the cells formed long, Myosin Heavy Chain-positive, multinucleate myotubes. Consistently, soft agar assays revealed a drop in the metastatic potential of skNAC-overexpressing cells. Taken together, these data indicate that reconstitution of skNAC expression can enhance the differentiation potential of rhabdomyosarcoma cells and reduces their metastatic potential, a finding which might have important therapeutic implications.

The E3 SUMO ligase Nse2 regulates sumoylation and nuclear-to-cytoplasmic translocation of skNAC-Smyd1 in myogenesis

Skeletal and heart muscle-specific variant of the α subunit of nascent polypeptide associated complex (skNAC; encoded by NACA) is exclusively found in striated muscle cells. Its function, however, is largely unknown. Previous reports have demonstrated that skNAC binds to m-Bop/Smyd1, a multi-functional protein that regulates myogenesis both through the control of transcription and the modulation of sarcomerogenesis, and that both proteins undergo nuclear-to-cytoplasmic translocation at the later stages of myogenic differentiation. Here, we show that skNAC binds to the E3 SUMO ligase mammalian Mms21/Nse2 and that knockdown of Nse2 expression inhibits specific aspects of myogenic differentiation, accompanied by a partial blockade of the nuclear-to-cytoplasmic translocation of the skNAC-Smyd1 complex, retention of the complex in promyelocytic leukemia (PML)-like nuclear bodies and disturbed sarcomerogenesis. In addition, we show that the skNAC interaction partner Smyd1 contains a putative sumoylation motif and is sumoylated in muscle cells, with depletion of Mms21/Nse2 leading to reduced concentrations of sumoylated Smyd1. Taken together, our data suggest that the function, specifically the balance between the nuclear and cytosolic roles, of the skNAC-Smyd1 complex might be regulated by sumoylation.

skNAC depletion stimulates myoblast migration and perturbs sarcomerogenesis by enhancing calpain 1 and 3 activity

skNAC (skeletal and heart muscle specific variant of nascent polypeptide-associated complex α) is a skeletal and heart muscle-specific protein known to be involved in the regulation of sarcomerogenesis. The respective mechanism, however, is largely unknown. In the present paper, we demonstrate that skNAC regulates calpain activity. Specifically, we show that inhibition of skNAC gene expression leads to enhanced, and overexpression of the skNAC gene to repressed, activity of calpain 1 and, to a lesser extent, calpain 3 in myoblasts. In skNAC siRNA-treated cells, enhanced calpain activity is associated with increased migration rates, as well as with perturbed sarcomere architecture. Treatment of skNAC-knockdown cells with the calpain inhibitor ALLN (N-acetyl-leucyl-leucyl-norleucinal) reverts both the positive effect on myoblast migration and the negative effect on sarcomere architecture. Taken together, our data suggest that skNAC controls myoblast migration and sarcomere architecture in a calpain-dependent manner.

αNAC interacts with histone deacetylase corepressors to control Myogenin and Osteocalcin gene expression

In the nucleus of differentiated osteoblasts, the DNA-binding αNAC protein acts as a transcriptional coactivator of the Osteocalcin gene. Chromatin immunoprecipitation-microarray assays (ChIP-chip) showed that αNAC binds the Osteocalcin promoter but also identified the Myogenin promoter as an αNAC target. Here, we confirm these array data using quantitative ChIP and further detected that αNAC binds to these promoters in myoblasts. Since these genes are differentially regulated during osteoblastogenesis or myogenesis, these results suggest cell- and promoter-context specific functions for αNAC. We hypothesized that αNAC dynamically recruits corepressors to inhibit Myogenin expression in cells committing to the osteoblastic lineage or to inhibit Osteocalcin transcription in differentiating myoblasts. Using co-immunoprecipitation assays, we detected complexes between αNAC and the corepressors HDAC1 and HDAC3, in myoblasts and osteoblasts. Sequential ChIP confirmed HDAC1 recruitment by αNAC at the Osteocalcin and Myogenin promoters. Interaction with the corepressors was detectable in pre-osteoblasts and in myoblasts but disappeared as the cells differentiate. Treatment with an HDAC inhibitor caused de-repression of Osteocalcin expression in myoblasts. Overexpression of αNAC in myoblasts inhibits expression of Myogenin and differentiation. However, overexpression of an N-terminus truncated αNAC mutant allowed myoblasts to express Myogenin and differentiate, and this mutant did not interact with HDAC1 or HDAC3. This study identified an additional DNA-binding target and novel protein-protein interactions for αNAC. We propose that αNAC plays a role in regulating gene transcription during mesenchymal cell differentiation by differentially recruiting corepressors at target promoters.