A series of robust genetic indicators for definitive identification of cardiomyocytes

Endocardial HDAC3 is required for myocardial trabeculation

Failure of proper ventricular trabeculation is often associated with congenital heart disease. Support from endocardial cells, including the secretion of extracellular matrix and growth factors is critical for trabeculation. However, it is poorly understood how the secretion of extracellular matrix and growth factors is initiated and regulated by endocardial cells. We find that genetic knockout of histone deacetylase 3 in the endocardium in mice results in early embryo lethality and ventricular hypotrabeculation. Single cell RNA sequencing identifies significant downregulation of extracellular matrix components in histone deacetylase 3 knockout endocardial cells. Secretome from cultured histone deacetylase 3 knockout mouse cardiac endothelial cells lacks transforming growth factor ß3 and shows significantly reduced capacity in stimulating cultured cardiomyocyte proliferation, which is remarkably rescued by transforming growth factor ß3 supplementation. Mechanistically, we identify that histone deacetylase 3 knockout induces transforming growth factor ß3 expression through repressing microRNA-129-5p. Our findings provide insights into the pathogenesis of congenital heart disease and conceptual strategies to promote myocardial regeneration.

Endocardial HDAC3 is required for myocardial trabeculation

BACKGROUND

Trabeculation, a key process in early heart development, is the formation of myocardial trabecular meshwork. The failure of trabeculation often leads to embryonic lethality. Support from endocardial cells, including the secretion of extracellular matrix (ECM) and growth factors is critical for trabeculation; however, it is unknown how the secretion of ECM and growth factors is initiated and regulated by endocardial cells.

METHODS

Various cellular and mouse models in conjunction with biochemical and molecular tools were employed to study the role of histone deacetylase 3 (HDAC3) in the developing endocardium.

RESULTS

We found that genetic deletion of Hdac3 in endocardial cells in mice resulted in early embryo lethality presenting as a hypotrabeculation cardiac phenotype. Single cell RNA sequencing identified several ECM components including collagens that were significantly downregulated in Hdac3 knockout (KO) endocardial cells. When cultured with supernatant from Hdac3 KO mouse cardiac endothelial cells (MCECs), wild-type mouse embryonic cardiomyocytes showed decreased proliferation, suggesting that growth signaling from Hdac3 KO MCECs is disrupted. Subsequent transcriptomic analysis revealed that transforming growth factor β3 (TGFβ3) was significantly downregulated in Hdac3 KO MCECs and Hdac3 cardiac endothelial KO hearts. Mechanistically, we identified that microRNA (miR)-129-5p was significantly upregulated in Hdac3 KO MCECs and Hdac3 cardiac endothelial KO hearts. Overexpression of miR-129-5p repressed Tgfβ3 expression in wild-type MCECs, whereas knockdown of miR-129-5p restored Tgfβ3 expression in Hdac3 KO MCECs.

CONCLUSION

Our findings reveal a critical signaling pathway in which endocardial HDAC3 promotes trabecular myocardium growth by stimulating TGFβ signaling through repressing miR-129-5p, providing novel insights into the etiology of congenital heart disease and conceptual strategies to promote myocardial regeneration.

Isolation of Embryonic Cardiomyocytes and Cell Proliferation Assay Using Genetically Engineered Reporter Mouse Model

Congenital heart disease (CHD) is often associated with myogenic defects. During heart development, cardiomyocyte growth requires essential cues from extrinsic factors such as insulin-like growth factor 2 (IGF-2). To determine whether and how growth factors account for embryonic cardiomyocyte proliferation, isolation followed by culturing of embryonic cardiomyocytes can be utilized as a useful tool for heart developmental studies. Current protocols for isolating cardiomyocytes from the heart do not include a cardiomyocyte-specific reporter to distinguish cardiomyocytes from other cell types. To optimize visualization of cardiomyocyte proliferation, our protocol utilizes a Tnnt2-promoter-driven H2B-GFP knock-in mouse model (TNNT2H2B-GFP/+) for in vitro visualization of nuclear-tagged cardiomyocyte-specific fluorescence. A cardiomyocyte-specific genetic reporter paired with an effective proliferation assay improves the reproducibility of mechanistic studies by increasing the accuracy of cell identification, proliferated cell counting, and cardiomyocyte tracking. Key features • This protocol refines previous methods of cardiomyocyte isolation to specifically target embryonic cardiomyocytes. • UsesH2B-GFP/+cardiomyocyte reporters as identified by Yan et al. (2016). • Traces cell proliferation with Phospho-Histone 3 (p-H3) assay. • Has applications in assessing the role of growth factors in cardiomyocyte proliferation.

Epicardial HDAC3 Promotes Myocardial Growth Through a Novel MicroRNA Pathway

BACKGROUND

Establishment of the myocardial wall requires proper growth cues from nonmyocardial tissues. During heart development, the epicardium and epicardium-derived cells instruct myocardial growth by secreting essential factors including FGF (fibroblast growth factor) 9 and IGF (insulin-like growth factor) 2. However, it is poorly understood how the epicardial secreted factors are regulated, in particular by chromatin modifications for myocardial formation. The current study is to investigate whether and how HDAC (histone deacetylase) 3 in the developing epicardium regulates myocardial growth.

METHODS

Various cellular and mouse models in conjunction with biochemical and molecular tools were employed to study the role of HDAC3 in the developing epicardium.

RESULTS

We deleted Hdac3 in the developing murine epicardium, and mutant hearts showed ventricular myocardial wall hypoplasia with reduction of epicardium-derived cells. The cultured embryonic cardiomyocytes with supernatants from Hdac3 knockout (KO) mouse epicardial cells also showed decreased proliferation. Genome-wide transcriptomic analysis revealed that Fgf9 and Igf2 were significantly downregulated in Hdac3 KO mouse epicardial cells. We further found that Fgf9 and Igf2 expression is dependent on HDAC3 deacetylase activity. The supplementation of FGF9 or IGF2 can rescue the myocardial proliferation defects treated by Hdac3 KO supernatant. Mechanistically, we identified that microRNA (miR)-322 and miR-503 were upregulated in Hdac3 KO mouse epicardial cells and Hdac3 epicardial KO hearts. Overexpression of miR-322 or miR-503 repressed FGF9 and IGF2 expression, while knockdown of miR-322 or miR-503 restored FGF9 and IGF2 expression in Hdac3 KO mouse epicardial cells.

CONCLUSIONS

Our findings reveal a critical signaling pathway in which epicardial HDAC3 promotes compact myocardial growth by stimulating FGF9 and IGF2 through repressing miR-322 or miR-503, providing novel insights in elucidating the etiology of congenital heart defects and conceptual strategies to promote myocardial regeneration.

Novel Myh11 Dual Reporter Mouse Model Provides Definitive Labeling and Identification of Smooth Muscle Cells-Brief Report

OBJECTIVE

Myh11 encodes a myosin heavy chain protein that is specifically expressed in smooth muscle cells (SMCs) and is important for maintaining vascular wall stability. The goal of this study is to generate a Myh11 dual reporter mouse line for definitive visualization of MYH11+ SMCs in vivo. Approach and Results: We generated a Myh11 knock-in mouse model by inserting LoxP-nlacZ-4XpolyA-LoxP-H2B-GFP-polyA-FRT-Neo-FRT reporter cassette into the Myh11 gene locus. The nuclear (n) lacZ-4XpolyA cassette is flanked by 2 LoxP sites followed by H2B-GFP (histone 2B fused green fluorescent protein). Upon Cre-mediated recombination, nlacZ-stop cassette is removed thereby permitting nucleus localized H2B-GFP expression. Expression of the nuclear localized lacZ or H2B-GFP is under control of the endogenous Myh11 promoter. Nuclear lacZ was expressed specifically in SMCs at embryonic and adult stages. Following germline Cre-mediated deletion of nuclear lacZ, H2B-GFP was specifically expressed in the nuclei of SMCs. Comparison of nuclear lacZ expression with Wnt1Cre and Mef2cCre mediated-H2B-GFP expression revealed heterogenous origins of SMCs from neural crest and second heart field in the great arteries and coronary vessels adjacent to aortic root.

CONCLUSIONS

The Myh11 knock-in dual reporter mouse model offers an exceptional genetic tool to visualize and trace the origins of SMCs in mice.

Generation of Pecam1 endothelial specific dual reporter mouse model

Endothelial cells are specialized epithelium lining the interior surface of vessels and play fundamental roles in angiogenesis, vascular permeability, and immune response. To identify endothelial cells in vivo, we constructed a Pecam1^{nlacZ-H2B-GFP/+} knock-in mouse model in which the endothelial cells are labeled by nuclear LacZ (nlacZ) expression. When Pecam1^{nlacZ-H2B-GFP/+} mice are bred with germline Cre deleter mice, Pecam1^H2B-GFP/+ line is created with native nuclear GFP (H2B-GFP) expression in the endothelium of various organs. This dual reporter mouse provides us with a powerful genetic tool for definitive identification of endothelial cells and monitoring this important cell population throughout development, homeostasis, and disease conditions in mammals.

Cardiac Sca-1+ Cells Are Not Intrinsic Stem Cells for Myocardial Development, Renewal, and Repair

BACKGROUND

For more than a decade, Sca-1⁺ cells within the mouse heart have been widely recognized as a stem cell population with multipotency that can give rise to cardiomyocytes, endothelial cells, and smooth muscle cells in vitro and after cardiac grafting. However, the developmental origin and authentic nature of these cells remain elusive.

METHODS

Here, we used a series of high-fidelity genetic mouse models to characterize the identity and regenerative potential of cardiac resident Sca-1⁺ cells.

RESULTS

With these novel genetic tools, we found that Sca-1 does not label cardiac precursor cells during early embryonic heart formation. Postnatal cardiac resident Sca-1⁺ cells are in fact a pure endothelial cell population. They retain endothelial properties and exhibit minimal cardiomyogenic potential during development, normal aging and upon ischemic injury.

CONCLUSIONS

Our study provides definitive insights into the nature of cardiac resident Sca-1⁺ cells. The observations challenge the current dogma that cardiac resident Sca-1⁺ cells are intrinsic stem cells for myocardial development, renewal, and repair, and suggest that the mechanisms of transplanted Sca-1⁺ cells in heart repair need to be reassessed.

Secondary Placental Defects in Cxadr Mutant Mice

The Coxsackie virus and adenovirus receptor (CXADR) is an adhesion molecule known for its role in virus-cell interactions, epithelial integrity, and organogenesis. Loss of Cxadr causes numerous embryonic defects in mice, notably abnormal development of the cardiovascular system, and embryonic lethality. While CXADR expression has been reported in the placenta, the precise cellular localization and function within this tissue are unknown. Since impairments in placental development and function can cause secondary cardiovascular abnormalities, a phenomenon referred to as the placenta-heart axis, it is possible placental phenotypes in Cxadr mutant embryos may underlie the reported cardiovascular defects and embryonic lethality. In the current study, we determine the cellular localization of placental Cxadr expression and whether there are placental abnormalities in the absence of Cxadr. In the placenta, CXADR is expressed specifically by trophoblast labyrinth progenitors as well as cells of the visceral yolk sac (YS). In the absence of Cxadr, we observed altered expression of angiogenic factors coupled with poor expansion of trophoblast and fetal endothelial cell subpopulations, plus diminished placental transport. Unexpectedly, preserving endogenous trophoblast Cxadr expression revealed the placental defects to be secondary to primary embryonic and/or YS phenotypes. Moreover, further tissue-restricted deletions of Cxadr suggest that the secondary placental defects are likely influenced by embryonic lineages such as the fetal endothelium or those within the extraembryonic YS vascular plexus.

A real-time monitoring platform of myogenesis regulators using double fluorescent labeling

Real-time, quantitative measurement of muscle progenitor cell (myoblast) differentiation is an important tool for skeletal muscle research and identification of drugs that support skeletal muscle regeneration. While most quantitative tools rely on sacrificial approach, we developed a double fluorescent tagging approach, which allows for dynamic monitoring of myoblast differentiation through assessment of fusion index and nuclei count. Fluorescent tagging of both the cell cytoplasm and nucleus enables monitoring of cell fusion and the formation of new myotube fibers, similar to immunostaining results. This labeling approach allowed monitoring the effects of Myf5 overexpression, TNFα, and Wnt agonist on myoblast differentiation. It also enabled testing the effects of surface coating on the fusion levels of scaffold-seeded myoblasts. The double fluorescent labeling of myoblasts is a promising technique to visualize even minor changes in myogenesis of myoblasts in order to support applications such as tissue engineering and drug screening.