ß1 integrins regulate cellular behavior and cardiomyocyte organization during ventricular wall formation.

AIMS: The mechanisms regulating the cellular behavior and cardiomyocyte organization during ventricular wall morphogenesis are poorly understood. Cardiomyocytes are surrounded by extracellular matrix (ECM) and interact with ECM via integrins. This study aims to determine whether and how ß1 integrins regulate cardiomyocyte behavior and organization during ventricular wall morphogenesis in the mouse. METHODS AND RESULTS: We applied mRNA deep sequencing and immunostaining to determine the expression repertoires of α/ß integrins and their ligands in the embryonic heart. Integrin ß1 subunit (ß1) and some of its ECM ligands are asymmetrically distributed and enriched in the luminal side of cardiomyocytes, and fibronectin surrounds cardiomyocytes, creating a network for them. Itgb1, which encodes the ß1, was deleted via Nkx2.5Cre/+ to generate myocardial-specific Itgb1 knockout (B1KO) mice. B1KO hearts display an absence of a trabecular zone but a thicker compact zone. The levels of hyaluronic acid and versican, essential for trabecular initiation, were not significantly different between control and B1KO. Instead, fibronectin, a ligand of ß1, was absent in the myocardium of B1KO hearts. Furthermore, B1KO cardiomyocytes display a random cellular orientation and fail to undergo perpendicular cell division, be organized properly, and establish the proper tissue architecture to form trabeculae. Mosaic clonal lineage tracing showed that Itgb1 regulates cardiomyocyte transmural migration and proliferation autonomously. CONCLUSIONS: ß1 is asymmetrically localized in the cardiomyocytes, and some of its ECM ligands are enriched along the luminal side of the myocardium, and fibronectin surrounds cardiomyocytes. ß1 integrins are required for cardiomyocytes to attach to the ECM network. This engagement provides structural support for cardiomyocytes to maintain shape, undergo perpendicular division, and establish cellular organization. Deletion of Itgb1 leads to loss of ß1 and fibronectin and prevents cardiomyocytes from engaging the ECM network, resulting in failure to establish tissue architecture to form trabeculae.

A Small Molecule That In Vitro Neutralizes Infection of SARS-CoV-2 and Its Most Infectious Variants, Delta, and Omicron.

The COVID-19 pandemic has underscored the urgent need to develop highly potent and safe medications that are complementary to the role of vaccines. Specifically, it has exhibited the need for orally bioavailable broad-spectrum antivirals that are able to be quickly deployed against newly emerging viral pathogens. The Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV2) and its variants Delta and Omicron are still a major threat to patients of all ages. In this brief report, we describe that the small molecule CD04872SC was able to neutralize SARS-CoV2 infection with a half-maximal effective concentration (EC50) = 248 µM. Serendipitously, we also were able to observe that CD04872SC inhibited the infection of the SARS-CoV-2 variants; Delta (EC50 = 152 µM) and Omicron (EC50 = 308 µM). These properties may define CD04872SC as a potential broad-spectrum candidate lead for the development of treatments for COVID-19.

Direct reprogramming of cardiomyocytes into cardiac Purkinje-like cells.

Currently, there are no treatments that ameliorate cardiac cell death, the underlying basis of cardiovascular disease. An unexplored cell type in cardiac regeneration is cardiac Purkinje cells; specialized cells from the cardiac conduction system (CCS) responsible for propagating electrical signals. Purkinje cells have tremendous potential as a regenerative treatment because they may intrinsically integrate with the CCS of a recipient myocardium, resulting in more efficient electrical conduction in diseased hearts. This study is the first to demonstrate an effective protocol for the direct reprogramming of human cardiomyocytes into cardiac Purkinje-like cells using small molecules. The cells generated were genetically and functionally similar to native cardiac Purkinje cells, where expression of key cardiac Purkinje genes such as CNTN2, ETV1, PCP4, IRX3, SCN5a, HCN2 and the conduction of electrical signals with increased velocity was observed. This study may help to advance the quest to finding an optimized cell therapy for heart regeneration.

STEMIN and YAP5SA synthetic modified mRNAs regenerate and repair infarcted mouse hearts.

Introduction: The adult heart lacks the regenerative capacity to self-repair. Serum response factor (SRF) is essential for heart organogenesis, sarcomerogenesis, and contractility. SRF interacts with co-factors, such as NKX2.5 and GATA4, required for cardiac specified gene activity. ETS factors such as ELK1 interact with SRF and drive cell replication. To weaken SRF interactions with NKX2.5 and GATA4, one mutant, SRF153(A3) named STEMIN, did not bind CArG boxes, yet induced stem cell factors such as NANOG and OCT4, cardiomyocyte dedifferentiation, and cell cycle reentry. The mutant YAP5SA of the Hippo pathway also promotes cardiomyocyte proliferation and growth. Aim: Infarcted adult mouse hearts were injected with translatable STEMIN and YAP5SA mmRNA to evaluate their clinical potential. Methods and Results: Mice were pulsed one day later with alpha-EDU and then heart sections were DAPI stained. Replicating cells were identified by immuno-staining against members of the DNA replisome pathway that mark entry to S phase of the cell cycle. Echocardiography was used to determine cardiac function following infarcts and mRNA treatment. To monitor cardiac wall repair, microscopic analysis was performed, and the extent of myocardial fibrosis was analyzed for immune cell infiltration. Injections of STEMIN and YAP5SA mmRNA into the left ventricles of infarcted adult mice promoted a greater than 17-fold increase in the DAPI stained and alpha-EDU marked cardiomyocyte nuclei, within a day. We observed de novo expression of phospho-histone H3, ORC2, MCM2, and CLASPIN. Cardiac function was significantly improved by four weeks post-infarct, and fibrosis and immune cell infiltration were diminished in hearts treated with STEMIN and YAP5SA mmRNA than each alone. Conclusion: STEMIN and YAP5SA mmRNA improved cardiac function and myocardial fibrosis in left ventricles of infarcted adult mice. The combinatorial use of mmRNA encoding STEMIN and YAP5SA has the potential to become a powerful clinical strategy to treat human heart disease.

H19X-encoded miR-322(424)/miR-503 regulates muscle mass by targeting translation initiation factors.

BACKGROUND: Skeletal muscle atrophy is a debilitating complication of many chronic diseases, disuse conditions, and ageing. Genome-wide gene expression analyses have identified that elevated levels of microRNAs encoded by the H19X locus are among the most significant changes in skeletal muscles in a wide scope of human cachectic conditions. We have previously reported that the H19X locus is important for the establishment of striated muscle fate during embryogenesis. However, the role of H19X-encoded microRNAs in regulating skeletal mass in adults is unknown. METHODS: We have created a transgenic mouse strain in which ectopic expression of miR-322/miR-503 is driven by the skeletal muscle-specific muscle creatine kinase promoter. We also used an H19X mutant mouse strain in which transcription from the locus is interrupted by a gene trap. Animal phenotypes were analysed by standard histological methods. Underlying mechanisms were explored by using transcriptome profiling and validated in the two animal models and cultured myotubes. RESULTS: Our results demonstrate that the levels of H19X microRNAs are inversely related to postnatal skeletal muscle growth. Targeted overexpression of miR-322/miR-503 impeded skeletal muscle growth. The weight of gastrocnemius muscles of transgenic mice was only 54.5% of the counterparts of wild-type littermates. By contrast, interruption of transcription from the H19X locus stimulates postnatal muscle growth by 14.4-14.9% and attenuates the loss of skeletal muscle mass in response to starvation by 12.8-21.0%. Impeded muscle growth was not caused by impaired IGF1/AKT/mTOR signalling or a hyperactive ubiquitin-proteasome system, instead accompanied by markedly dropped abundance of translation initiation factors in transgenic mice. miR-322/miR-503 directly targets eIF4E, eIF4G1, eIF4B, eIF2B5, and eIF3M. CONCLUSIONS: Our study illustrates a novel pathway wherein H19X microRNAs regulate skeletal muscle growth and atrophy through regulating the abundance of translation initiation factors, thereby protein synthesis. The study highlights how translation initiation factors lie at the crux of multiple signalling pathways that control skeletal muscle mass.

The Spatiotemporal Expression of Notch1 and Numb and Their Functional Interaction during Cardiac Morphogenesis.

Numb family proteins (NFPs), including Numb and Numblike (Numbl), are commonly known for their role as cell fate determinants for multiple types of progenitor cells, mainly due to their function as Notch inhibitors. Previous studies have shown that myocardial NFP double knockout (MDKO) hearts display an up-regulated Notch activation and various defects in cardiac progenitor cell differentiation and cardiac morphogenesis. Whether enhanced Notch activation causes these defects in MDKO is not fully clear. To answer the question, we examined the spatiotemporal patterns of Notch1 expression, Notch activation, and Numb expression in the murine embryonic hearts using multiple approaches including RNAScope, and Numb and Notch reporter mouse lines. To further interrogate the interaction between NFPs and Notch signaling activation, we deleted both Notch1 or RBPJk alleles in the MDKO. We examined and compared the phenotypes of Notch1 knockout, NFPs double knockout, Notch1; Numb; Numbl and RBPJk; Numb; Numbl triple knockouts. Our study showed that Notch1 is expressed and activated in the myocardium at several stages, and Numb is enriched in the epicardium and did not show the asymmetric distribution in the myocardium. Cardiac-specific Notch1 deletion causes multiple structural defects and embryonic lethality. Notch1 or RBPJk deletion in MDKO did not rescue the structural defects in the MDKO but partially rescued the defects of cardiac progenitor cell differentiation, cardiomyocyte proliferation, and trabecular morphogenesis. Our study concludes that NFPs regulate progenitor cell differentiation, cardiomyocyte proliferation, and trabecular morphogenesis partially through Notch1 and play more roles than inhibiting Notch1 signaling during cardiac morphogenesis.

Transient Transgenics: An Efficient Method to Identify Gene Regulatory Elements.

We describe a novel, efficient method to identify cis-acting DNA sequences that drive cell-specific gene expression during development. We utilize transfer of Bacterial Artificial Chromosome (BAC) genomic DNAs, modified to contain a reporter gene, into fertilized mouse embryos and placing the injected embryos into pseudopregnant recipient females. The embryos are allowed to develop in utero for defined times after which they are collected for analysis. Using DNAs containing the LacZ reporter gene facilitates the analysis of gene activity through microscopy of intact embryos and subsequent sectioning of the stained embryos. With this technique cis-element activity can be identified and evaluated through further mutational analysis of the injected BAC DNA. This allows the identification of important gene regulatory domains that specify stage-specific gene expression in the developing embryo.

Smyd1 Orchestrates Early Heart Development Through Positive and Negative Gene Regulation.

SET and MYND domain-containing protein 1 (Smyd1) is a striated muscle-specific histone methyltransferase. Our previous work demonstrated that deletion of Smyd1 in either cardiomyocytes or the outflow tract (OFT) resulted in embryonic lethality at E9.5, with cardiac structural defects such as truncation of the OFT and right ventricle and impaired expansion and proliferation of the second heart field (SHF). The cardiac phenotype was accompanied by the downregulation of ISL LIM Homeobox 1 (Isl1) and upregulation of atrial natriuretic factor (ANF). However, the mechanisms of Smyd1 regulating Isl1 and ANF during embryonic heart development remain to be elucidated. Here, we employed various biochemical and molecular biological approaches including chromatin immunoprecipitation polymerase chain reaction (ChIP-PCR), pGL3 fluorescence reporter system, and co-immunoprecipitation (CoIP) and found that Smyd1 interacted with absent small homeotic-2-like protein (ASH2L) and activated the promoter of Isl1 by trimethylating H3K4. We also found that Smyd1 associated with HDAC to repress ANF expression using trichostatin A (TSA), a deacetylase inhibitor. In conclusion, Smyd1 participates in early heart development by upregulating the expression of Isl1 and downregulating the expression of ANF.

Generation of Transgenic Mice that Conditionally Overexpress Tenascin-C.

Tenascin-C (TNC) is an extracellular matrix glycoprotein that is expressed during embryogenesis. It is not expressed in normal adults, but is up-regulated under pathological conditions. Although TNC knockout mice do not show a distinct phenotype, analyses of disease models using TNC knockout mice combined with in vitro experiments revealed the diverse functions of TNC. Since high TNC levels often predict a poor prognosis in various clinical settings, we developed a transgenic mouse that overexpresses TNC through Cre recombinase-mediated activation. Genomic walking showed that the transgene was integrated into and truncated the Atp8a2 gene. While homozygous transgenic mice showed a severe neurological phenotype, heterozygous mice were viable, fertile, and did not exhibit any distinct abnormalities. Breeding hemizygous mice with Nkx2.5 promoter-Cre or α-myosin heavy chain promoter Cre mice induced the heart-specific overexpression of TNC in embryos and adults. TNC-overexpressing mouse hearts did not have distinct histological or functional abnormalities. However, the expression of proinflammatory cytokines/chemokines was significantly up-regulated and mortality rates during the acute stage after myocardial infarction were significantly higher than those of the controls. Our novel transgenic mouse may be applied to investigations on the role of TNC overexpression in vivo in various tissue/organ pathologies using different Cre donors.

3D Bioprinting the Cardiac Purkinje System Using Human Adipogenic Mesenchymal Stem Cell Derived Purkinje Cells.

PURPOSE: The objective of this study was to reprogram human adipogenic mesenchymal stem cells (hADMSCs) to form Purkinje cells and to use the reprogrammed Purkinje cells to bioprint Purkinje networks. METHODS: hADMSCs were reprogrammed to form Purkinje cells using a multi-step process using transcription factors ETS2 and MESP1 to first form cardiac progenitor stem cells followed by SHOX2 and TBX3 to form Purkinje cells. A novel bioprinting method was developed based on Pluronic acid as the sacrificial material and type I collagen as the structural material. The reprogrammed Purkinje cells were used in conjunction with the novel bioprinting method to bioprint Purkinje networks. Printed constructs were evaluated for retention of functional protein connexin 40 (Cx40) and ability to undergo membrane potential changes in response to physiologic stimulus. RESULTS: hADMSCs were successfully reprogrammed to form Purkinje cells based on the expression pattern of IRX3, IRX5, SEMA and SCN10. Reprogrammed purkinje cells were incorporated into a collagen type-1 bioink and the left ventricular Purkinje network was printed using anatomical images of the bovine Purkinje system as reference. Optimization studies demonstrated that 1.8 mg/mL type-I collagen at a seeding density of 300,000 cells per 200 µL resulted in the most functional bioprinted Purkinje networks. Furthermore, bioprinted Purkinje networks formed continuous syncytium, retained expression of vital functional gap junction protein Cx40 post-print, and exhibited membrane potential changes in response to electric stimulation and acetylcholine evaluated by DiBAC4(5), an electrically responsive dye. CONCLUSION: Based on the results of this study, hADMSCs were successfully reprogrammed to form Purkinje cells and bioprinted to form Purkinje networks.

A Highly Conductive 3D Cardiac Patch Fabricated Using Cardiac Myocytes Reprogrammed from Human Adipogenic Mesenchymal Stem Cells.

PURPOSE: The objective of this study was to bioengineer 3D patches from cardiac myocytes that have been reprogrammed from human adipogenic mesenchymal stem cells (hADMSCs). METHODS: Human adipogenic mesenchymal stem cells (hADMSCs) were reprogrammed to form cardiac myocytes using transcription factors ETS2 and MESP1. Reprogrammed cardiac myocytes were cultured in a fibrin gel to bioengineer 3D patch patches. The effect of initial plating density (1-25 million cells per patch), time (28-day culture period) and treatment with 1 µM isoproterenol and 1 µM epinephrine were evaluated. RESULTS: 3D patches were fabricated using cardiac myocytes that have been reprogrammed from hADMSCs. Based on optimization studies, it was determined that 10 million cells were needed to bioengineer a single patch, that measured 2 × 2 cm2. Furthermore, 3D patches fabricated 10 million cells were stable in culture for up to 28 days. Treatment of 3D patches with 1 µM isoproterenol and 1 µM epinephrine resulted in an increase in the electrical properties, as measured by electrical impulse amplitude and frequency. An increase in the expression of mTOR, KCNV1, GJA5, KCNJ16, CTNNT2, KCNV2, MYO3, FOXO1 and KCND2 was noted in response to treatment of 3D patches with isoproterenol and epinephrine. CONCLUSION: Based on the results of this study, there is evidence to support the successful fabrication of a highly functional 3D patches with measurable electrical activity using cardiac myocytes reprogrammed from hADMSCs. 3D patches fabricated using optimal conductions described in this study can be used to improve the functional properties of failing hearts. Predominantly, in case of the infarcted hearts with partial loss of electrical activity, the electrical properties of the 3D patches may restore the electrical activity of the heart.

CRISPR-Cas9-induced IGF1 gene activation as a tool for enhancing muscle differentiation via multiple isoform expression.

Muscle wasting, or muscle atrophy, can occur with age, injury, and disease; it affects the quality of life and complicates treatment. Insulin-like growth factor 1 (IGF1) is a key positive regulator of muscle mass. The IGF1/Igf1 gene encodes multiple protein isoforms that differ in tissue expression, potency, and function, particularly in cellular proliferation and differentiation, as well as in systemic versus localized signaling. Genome engineering is a novel strategy for increasing gene expression and has the potential to recapitulate the diverse biology seen in IGF1 signaling through the overexpression of multiple IGF1 isoforms. Using a CRISPR-Cas9 gene activation approach, we showed that the expression of multiple IGF1 or Igf1 mRNA variants can be increased in human and mouse skeletal muscle myoblast cell lines using a single-guide RNA (sgRNA). We found increased IGF1 protein levels in the cell culture media and increased cellular phosphorylation of AKT1, the main effector of IGF1 signaling. We also showed that the expression of Class 1 or Class 2 mRNA variants can be selectively increased by changing the sgRNA target location. The expression of multiple IGF1 or Igf1 mRNA transcript variants in human and mouse skeletal muscle myoblasts promoted myotube differentiation and prevented dexamethasone-induced atrophy in myotubes in vitro. Our findings suggest that this novel approach for enhancing IGF1 signaling has potential therapeutic applications for treating skeletal muscle atrophy.

ß-Adrenergic stimuli and rotating suspension culture enhance conversion of human adipogenic mesenchymal stem cells into highly conductive cardiac progenitors.

Clinical trials using human adipogenic mesenchymal stem cells (hAdMSCs) for the treatment of cardiac diseases have shown improvement in cardiac function and were proven safe. However, hAdMSCs do not convert efficiently into cardiomyocytes (CMs) or vasculature. Thus, reprogramming hAdMSCs into myocyte progenitors may fare better in future investigations. To reprogramme hAdMSCs into electrically conductive cardiac progenitor cells, we pioneered a three-step reprogramming strategy that uses proven MESP1/ETS2 transcription factors, ß-adrenergic and hypoxic signalling induced in three-dimensional (3D) cardiospheres. In Stage 1, ETS2 and MESP1 activated NNKX2.5, TBX5, MEF2C, dHAND, and GATA4 during the conversion of hAdMSCs into cardiac progenitor cells. Next, in Stage 2, ß2AR activation repositioned cardiac progenitors into de novo immature conductive cardiac cells, along with the appearance of RYR2, CAV2.1, CAV3.1, NAV1.5, SERCA2, and CX45 gene transcripts and displayed action potentials. In Stage 3, electrical conduction that was fostered by 3D cardiospheres formed in a Synthecon®, Inc. rotating bioreactor induced the appearance of hypoxic genes: HIF-1α/ß, PCG 1α/ß, and NOS2, which coincided with the robust activation of adult contractile genes including MLC2v, TNNT2, and TNNI3, ion channel genes, and the appearance of hyperpolarization-activated and cyclic nucleotide-gated channels (HCN1-4). Conduction velocities doubled to ~200 mm/s after hypoxia and doubled yet again after dissociation of the 3D cell clusters to ~400 mm/s. By comparison, normal conduction velocities within working ventricular myocytes in the whole heart range from 0.5 to 1 m/s. Epinephrine stimulation of stage 3 cardiac cells in patches resulted in an increase in amplitude of the electrical wave, indicative of conductive cardiac cells. Our efficient protocol that converted hAdMSCs into highly conductive cardiac progenitors demonstrated the potential utilization of stage 3 cells for tissue engineering applications for cardiac repair.

Conversion of human cardiac progenitor cells into cardiac pacemaker-like cells.

We used a screening strategy to test for reprogramming factors for the conversion of human cardiac progenitor cells (CPCs) into Pacemaker-like cells. Human transcription factors SHOX2, TBX3, TBX5, TBX18, and the channel protein HCN2, were transiently induced as single factors and in trio combinations into CPCs, first transduced with the connexin 30.2 (CX30.2) mCherry reporter. Following screens for reporter CX30.2 mCherry gene activation and FACS enrichment, we observed the definitive expression of many pacemaker specific genes; including, CX30.2, KCNN4, HCN4, HCN3, HCN1, and SCN3b. These findings suggest that the SHOX2, HCN2, and TBX5 (SHT5) combination of transcription factors is a much better candidate in driving the CPCs into Pacemaker-like cells than other combinations and single transcription factors. Additionally, single-cell RNA sequencing of SHT5 mCherry+ cells revealed cellular enrichment of pacemaker specific genes including TBX3, KCNN4, CX30.2, and BMP2, as well as pacemaker specific potassium and calcium channels (KCND2, KCNK2, and CACNB1). In addition, similar to human and mouse sinoatrial node (SAN) studies, we also observed the down-regulation of NKX2.5. Patch-clamp recordings of the converted Pacemaker-like cells exhibited HCN currents demonstrated the functional characteristic of pacemaker cells. These studies will facilitate the development of an optimal Pacemaker-like cell-based therapy within failing hearts through the recovery of SAN dysfunction.

Aberrant expression of embryonic mesendoderm factor MESP1 promotes tumorigenesis.

BACKGROUND: Mesoderm Posterior 1 (MESP1) belongs to the family of basic helix-loop-helix transcription factors. It is a master regulator of mesendoderm development, leading to formation of organs such as heart and lung. However, its role in adult pathophysiology remains unknown. Here, we report for the first time a previously-unknown association of MESP1 with non-small cell lung cancer (NSCLC). METHODS: MESP1 mRNA and protein levels were measured in NSCLC-derived cells by qPCR and immunoblotting respectively. Colony formation assay, colorimetric cell proliferation assay and soft agar colony formation assays were used to assess the effects of MESP1 knockdown and overexpression in vitro. RNA-sequencing and chromatin immunoprecipitation (ChIP)-qPCR were used to determine direct target genes of MESP1. Subcutaneous injection of MESP1-depleted NSCLC cells in immuno-compromised mice was done to study the effects of MESP1 mediated tumor formation in vivo. FINDINGS: We found that MESP1 expression correlates with poor prognosis in NSCLC patients, and is critical for proliferation and survival of NSCLC-derived cells, thus implicating MESP1 as a lung cancer oncogene. Ectopic MESP1 expression cooperates with loss of tumor suppressor ARF to transform murine fibroblasts. Xenografts from MESP1-depleted cells showed decreased tumor growth in vivo. Global transcriptome analysis revealed a MESP1 DNA-binding-dependent gene signature associated with various hallmarks of cancer, suggesting that transcription activity of MESP1 is most likely responsible for its oncogenic abilities. INTERPRETATION: Our study demonstrates MESP1 as a previously-unknown lineage-survival oncogene in NSCLC which may serve as a potential prognostic marker and therapeutic target for lung cancer in the future.

C.B-17 SCID mice develop epicardial calcinosis with unaltered cardiac function.

Inbred mouse strains are the most widely used mammalian model organism in biomedical research owing to ease of genetic manipulation and short lifespan; however, each inbred strain possesses a unique repertoire of deleterious homozygous alleles that can make a specific strain more susceptible to a particular disease. In the current study, we report dystrophic cardiac calcinosis (DCC) in C.B-17 SCID male mice at 10 weeks of age with no significant change in cardiac function. Acquisition of DCC was characterized by myocardial injury, fibrosis, calcification, and necrosis of the tissue. At 10 weeks of age, 38% of the C.B-17 SCID mice from two different commercial colonies exhibited significant calcinosis on the ventricular epicardium, predominantly on the right ventricle. The frequency of calcinosis was more than 50% for mice obtained from Taconic's Cambridge City colony and 25% for mice obtained from Taconic's German Town colony. Interestingly, the DCC phenotype did not affect cardiac function at 10 weeks of age. No differences in echocardiography or electrocardiography were observed between the calcinotic and non-calcinotic mice from either colony. Our findings suggest that C.B-17 SCID mice exhibit DCC as early as 10 weeks of age with no significant impact on cardiac function. This strain of mice should be cautiously considered for the study of cardiac physiology.

RERE deficiency leads to decreased expression of GATA4 and the development of ventricular septal defects.

Deletions of chromosome 1p36 are associated with a high incidence of congenital heart defects (CHDs). The arginine-glutamic acid dipeptide repeats gene (RERE) is located in a critical region for CHD on chromosome 1p36 and encodes a cardiac-expressed nuclear receptor co-regulator. Mutations affecting RERE cause atrial and ventricular septal defects (VSDs) in humans, and RERE-deficient mice also develop VSDs. During cardiac development, mesenchymal cells destined to form part of the atrioventricular (AV) septum are generated when endocardial cells in the AV canal undergo epithelial-to-mesenchymal transition (EMT) and migrate into the space between the endocardium and the myocardium. These newly generated mesenchymal cells then proliferate to fill the developing AV endocardial cushions. Here, we demonstrate that RERE-deficient mouse embryos have reduced numbers of mesenchymal cells in their AV endocardial cushions owing to decreased levels of EMT and mesenchymal cell proliferation. In the endocardium, RERE colocalizes with GATA4, a transcription factor required for normal levels of EMT and mesenchymal cell proliferation. Using a combination of in vivo and in vitro studies, we show that Rere and Gata4 interact genetically in the development of CHDs, RERE positively regulates transcription from the Gata4 promoter and GATA4 levels are reduced in the AV canals of RERE-deficient embryos. Tissue-specific ablation of Rere in the endocardium leads to hypocellularity of the AV endocardial cushions, defective EMT and VSDs, but does not result in decreased GATA4 expression. We conclude that RERE functions in the AV canal to positively regulate the expression of GATA4, and that deficiency of RERE leads to the development of VSDs through its effects on EMT and mesenchymal cell proliferation. However, the cell-autonomous role of RERE in promoting EMT in the endocardium must be mediated by its effects on the expression of proteins other than GATA4.This article has an associated First Person interview with the first author of the paper.

Cardiac-specific developmental and epigenetic functions of Jarid2 during embryonic development.

Epigenetic regulation is critical in normal cardiac development. We have demonstrated that the deletion of Jarid2 (Jumonji (Jmj) A/T-rich interaction domain 2) in mice results in cardiac malformations recapitulating human congenital cardiac disease and dysregulation of gene expression. However, the precise developmental and epigenetic functions of Jarid2 within the developing heart remain to be elucidated. Here, we determined the cardiac-specific functions of Jarid2 and the genetic networks regulated by Jarid2. Jarid2 was deleted using different cardiac-specific Cre mice. The deletion of Jarid2 by Nkx2.5-Cre mice (Jarid2Nkx) caused cardiac malformations including ventricular septal defects, thin myocardium, hypertrabeculation, and neonatal lethality. Jarid2Nkx mice exhibited elevated expression of neural genes, cardiac jelly, and other key factors including Isl1 and Bmp10 in the developing heart. By employing combinatorial genome-wide approaches and molecular analyses, we showed that Jarid2 in the myocardium regulates a subset of Jarid2 target gene expression and H3K27me3 enrichment during heart development. Specifically, Jarid2 was required for PRC2 occupancy and H3K27me3 at the Isl1 promoter locus, leading to the proper repression of Isl1 expression. In contrast, Jarid2 deletion in differentiated cardiomyocytes by cTnt-Cre mice caused no gross morphological defects or neonatal lethality. Thus, the early deletion of Jarid2 in cardiac progenitors, prior to the differentiation of cardiac progenitors into cardiomyocytes, results in morphogenetic defects manifested later in development. Our studies reveal that there is a critical window during early cardiac progenitor differentiation when Jarid2 is crucial to establish the epigenetic landscape at later stages of development.