Yanlin Ma

Conditional Ablation of Ezh2 in Murine Hearts Reveals Its Essential Roles in Endocardial Cushion Formation, Cardiomyocyte Proliferation and Survival

Ezh2 is a histone trimethyltransferase that silences genes mainly via catalyzing trimethylation of histone 3 lysine 27 (H3K27Me3). The role of Ezh2 as a regulator of gene silencing and cell proliferation in cancer development has been extensively investigated; however, its function in heart development during embryonic cardiogenesis has not been well studied. In the present study, we used a genetically modified mouse system in which Ezh2 was specifically ablated in the mouse heart. We identified a wide spectrum of cardiovascular malformations in the Ezh2 mutant mice, which collectively led to perinatal death. In the Ezh2 mutant heart, the endocardial cushions (ECs) were hypoplastic and the endothelial-to-mesenchymal transition (EMT) process was impaired. The hearts of Ezh2 mutant mice also exhibited decreased cardiomyocyte proliferation and increased apoptosis. We further identified that the Hey2 gene, which is important for cardiomyocyte proliferation and cardiac morphogenesis, is a downstream target of Ezh2. The regulation of Hey2 expression by Ezh2 may be independent of Notch signaling activity. Our work defines an indispensible role of the chromatin remodeling factor Ezh2 in normal cardiovascular development.

Expression of Sumoylation Deficient Nkx2.5 Mutant in Nkx2.5 Haploinsufficient Mice Leads to Congenital Heart Defects

Nkx2.5 is a cardiac specific homeobox gene critical for normal heart development. We previously identified Nkx2.5 as a target of sumoylation, a posttranslational modification implicated in a variety of cellular activities. Sumoylation enhanced Nkx2.5 activity via covalent attachment to the lysine residue 51, the primary SUMO acceptor site. However, how sumoylation regulates the activity of Nkx2.5 in vivo remains unknown. We generated transgenic mice overexpressing sumoylation deficient mutant K51R (conversion of lysine 51 to arginine) specifically in mouse hearts under the control of cardiac α-myosin heavy chain (α-MHC) promoter (K51R-Tg). Expression of the Nkx2.5 mutant transgene in the wild type murine hearts did not result in any overt cardiac phenotype. However, in the presence of Nkx2.5 haploinsufficiency, cardiomyocyte-specific expression of the Nkx2.5 K51R mutant led to congenital heart diseases (CHDs), accompanied with decreased cardiomyocyte proliferation. Also, a number of human CHDs-associated Nkx2.5 mutants exhibited aberrant sumoylation. Our work demonstrates that altered sumoylation status may underlie the development of human CHDs associated with Nkx2.5 mutants.

Mechanism of fibrotic cardiomyopathy in mice expressing truncated Rho-associated coiled-coil protein kinase 1

We have previously found that in failing human hearts, Rho‐associated coiled‐coil protein kinase 1 (ROCK1) is processed by caspase‐3 into an active isoform, ROCKΔ1. The purpose of the current investigation was to elucidate the pathological consequences of truncated ROCK1 accumulation in the heart, the associated molecular mechanism of ROCKΔ1‐mediated cardiac phenotype, and the molecular signaling between Rho kinase activation in cardiomyocytes and extracellular matrix response. We generated transgenic mice expressing ROCKΔ1 in cardiomyocytes to mimic the situation observed in human heart disease, whereas an additional kinase‐deficient mouse was generated as a control. The ROCKΔ1 transgenic mice developed fibrotic cardiomyopathy with diastolic dysfunction. Transgenic hearts displayed activated TGFβ1 and NF‐κB signaling and a release of a subset of cytokines and were susceptible to angiotensin II stress. Treatment with a Rho kinase inhibitor attenuated the fibrotic phenotype. Cardiac fibroblasts differentiated into myofibroblasts when cocultured with transgenic cardiomyocytes but not with wild‐type cardiomyocytes. Inhibitors of Rho kinase as well as TGFβR1 and NF‐κB decreased these effects. The serum response factor‐dependent TGFβ1 regulation was shown to be responsible for the Rho kinase‐mediated activation of TGFβ1 signaling. We conclude that ROCKΔ1 is a novel fibrotic factor. Activation of TGFβ1 and NF‐κB signaling contributes to the Rho kinase‐mediated pathological fibrosis.—Yang, X., Li, Q., Lin, X., Ma, Y., Yue, X., Tao, Z., Wang, F., Mckeehan, W. L., Wei, L., Schwartz, R. J., Chang, J. Mechanism of fibrotic cardiomyopathy in mice expressing truncated Rho‐associated coiled‐coil protein kinase 1. FASEB J. 26, 2105‐2116 (2012). www.fasebj.org

Sumoylation regulates nuclear localization and function of zinc finger transcription factor ZIC3.

ZIC3, an X-linked zinc finger transcription factor, was the first identified gene involved in establishing normal left-right patterning in humans. Mutations in the Zic3 gene in patients cause heterotaxy, which includes congenital heart defects. However, very little is known about how the function of the ZIC3 protein is regulated. Sumoylation is a posttranslational modification process in which a group of small ubiquitin-like modifier (SUMO) proteins is covalently attached to targets via a series of enzymatic reactions. Here, we report for the first time that sumoylation targets human ZIC3 primarily on the consensus lysine residue K248, which is critical for the nuclear retention of ZIC3. Consequently, SUMO modification potentiates the repressive activity of ZIC3 on the promoter of its target gene cardiac α-actin, and the mutation of lysine 248 to arginine (K248R) abolishes its repressive function. We further revealed that ZIC3 variants with mutations found in human patients with congenital anomalies exhibit aberrant sumoylation activity, which at least partially accounts for their cytoplasmic diffusion. Improved sumoylation of human disease-associated ZIC3 variants reestablishes their nuclear occupancy in the presence of SUMO E3 ligase and SUMO-1. Thus, the altered sumoylation status of ZIC3 underpins the developmental abnormalities associated with these ZIC3 mutants. The SUMO targeting consensus sequence in ZIC3 is highly conserved in its paralogs and orthologs, pointing to sumoylation as a general mechanism underlying the functional control of ZIC proteins. This study provides a potential therapeutic strategy to regain the normal subcellular distribution and function of ZIC3 mutants by restoring SUMO conjugation.

Protein Tyrosine Phosphatase-Like A Regulates Myoblast Proliferation and Differentiation through MyoG and the Cell Cycling Signaling Pathway

ABSTRACT Protein tyrosine phosphatase-like A (PTPLa) has been implicated in skeletal myogenesis and cardiogenesis. Mutations in PTPLa correlated with arrhythmogenic right ventricular dysplasia in humans and congenital centronuclear myopathy with severe hypotonia in dogs. The molecular mechanisms of PTPLa in myogenesis are unknown. In this report, we demonstrate that PTPLa is required for myoblast growth and differentiation. The cells lacking PTPLa remained immature and failed to differentiate into mature myotubes. The repressed MyoG expression was responsible for the impaired myoblast differentiation. Meanwhile, impeded cell growth, with an obvious S-phase arrest and compromised G2/M transition, was observed in PTPLa-deficient myoblasts. Further study demonstrated that the upregulation of cyclin D1 and cyclin E2 complexes, along with a compromised G2/M transition due to the decreased CDK1 (cyclin-dependent kinase 1) activity and upregulated p21, contributed to the mutant cell S-phase arrest and eventually led to the retarded cell growth. Finally, the transcriptional regulation of the PTPLa gene was explored. We identified PTPLa as a new target gene of the serum response factor (SRF). Skeletal- and cardiac-muscle-specific SRF knockouts resulted in significant decreases in PTPLa expression, suggesting a conserved transcriptional regulation of the PTPLa gene in mice.

An Intragenic SRF-Dependent Regulatory Motif Directs Cardiac-Specific microRNA-1-1/133a-2 Expression

Transcriptional regulation is essential for any gene expression including microRNA expression. MiR-1-1 and miR-133a-2 are essential microRNAs (miRs) involved in cardiac and skeletal muscle development and diseases. Early studies reveal two regulatory enhancers, an upstream and an intragenic, that direct the miR-1-1 and miR-133a-2 transcripts. In this study, we identify a unique serum response factor (SRF) binding motif within the enhancer through bioinformatic approaches. This motif is evolutionarily conserved and is present in a range of organisms from yeast, flies, to humans. We provide evidence to demonstrate that this regulatory motif is SRF-dependent in vitro by electrophoretic mobility shift assay, luciferase activity assay, and endogenous chromatin immunoprecipitation assay followed by DNA sequence confirmation, and in vivo by transgenic lacZ reporter mouse studies. Importantly, our transgenic mice indicate that this motif is indispensable for the expression of miR1-1/133a-2 in the heart, but not necessary in skeletal muscle, while the enhancer is sufficient for miR1-1/133a-2 gene expression in both tissues. The mutation of the motif alone completely abolishes miR-1-1/133a-2 gene expression in the animal heart, but not in the skeletal muscle. Our findings reveal an additional architecture of regulatory complex directing miR-1-1/133a-1 gene expression, and demonstrate how this intragenic enhancer differentially manages the expression of the two miRs in the heart and skeletal muscle, respectively.