Signaling pathways in early cardiac development

Role of Wnt/β-catenin pathway in cardiac lineage commitment of human umbilical cord mesenchymal stem cells by zebularine and 2'-deoxycytidine

Wnt/β-catenin, a highly conserved signaling pathway, is involved in determining cell fate. During heart development, Wnt signaling controls specification, proliferation and differentiation of cardiac cells. This study is aimed to investigate the role of Wnt/β-catenin signaling in cardiac lineage commitment of human umbilical cord mesenchymal stem cells (hUCMSCs) after treatment with demethylating agents, zebularine and 2'-deoxycytidine (2-DC). hUCMSCs were treated with 20 µM zebularine or 2-DC for 24 h and cultured for 14 days. Control and treated MSCs were analyzed for cardiac lineage commitment at gene and protein levels. Significant upregulation of early and late cardiac markers, GATA4, Nkx2.5, cardiac myosin heavy chain (cMHC), α-actinin, cardiac troponin T (cTnT) and cardiac troponin I (cTnI) was observed in treated MSCs as compared to the untreated control. We also analyzed gene expression of key Wnt/β-catenin signaling molecules in cultures of treated and untreated hUCMSCs at 24 h, and days 3, 7 and 14. The pattern of mRNA gene expression showed that Wnt/β-catenin signaling is regulated during cardiac lineage commitment of hUCMSCs in a time-dependent manner, with the pathway being activated early but inhibited later in cardiac development. Findings of this study can lead us to identify more specific and effective strategies for cardiac lineage commitment.

Chemical Genetics Reveals a Role of Squalene Synthase in TGFβ Signaling and Cardiomyogenesis

KY02111 is a widely used small molecule that boosts cardiomyogenesis of the mesoderm cells derived from pluripotent stem cells, yet its molecular mechanism of action remains elusive. The present study resolves the initially perplexing effects of KY02111 on Wnt signaling and subsequently identifies squalene synthase (SQS) as a molecular target of KY02111 and its optimized version, KY-I. By disrupting the interaction of SQS with cardiac ER-membrane protein TMEM43, KY02111 impairs TGFβ signaling, but not Wnt signaling, and thereby recapitulates the clinical mutation of TMEM43 that causes arrhythmogenic right ventricular cardiomyopathy (ARVC), an inherited heart disease that involves a substitution of myocardium with fatty tissue. These findings reveal a heretofore undescribed role of SQS in TGFβ signaling and cardiomyogenesis. KY02111 may find its use in ARVC modeling as well as serve as a chemical tool for studying TGFβ/SMAD signaling.

Cardiomyogenesis Modeling Using Pluripotent Stem Cells: The Role of Microenvironmental Signaling

Pluripotent stem cells (PSC) can be used as a model to study cardiomyogenic differentiation. In vitro modeling can reproduce cardiac development through modulation of some key signaling pathways. Therefore, many studies make use of this strategy to better understand cardiomyogenesis complexity and to determine possible ways to modulate cell fate. However, challenges remain regarding efficiency of differentiation protocols, cardiomyocyte (CM) maturation and therapeutic applications. Considering that the extracellular milieu is crucial for cellular behavior control, cardiac niche studies, such as those identifying secreted molecules from adult or neonatal tissues, allow the identification of extracellular factors that may contribute to CM differentiation and maturation. This review will focus on cardiomyogenesis modeling using PSC and the elements involved in cardiac microenvironmental signaling (the secretome - extracellular vesicles, extracellular matrix and soluble factors) that may contribute to CM specification and maturation.

Chemical suppression of specific C-C chemokine signaling pathways enhances cardiac reprogramming

Reprogramming of fibroblasts into induced cardiomyocytes (iCMs) is a potentially promising strategy for regenerating a damaged heart. However, low fibroblast-cardiomyocyte conversion rates remain a major challenge in this reprogramming. To this end, here we conducted a chemical screen and identified four agents, insulin-like growth factor-1, Mll1 inhibitor MM589, transforming growth factor-β inhibitor A83-01, and Bmi1 inhibitor PTC-209, termed IMAP, which coordinately enhanced reprogramming efficiency. Using α-muscle heavy chain-GFP-tagged mouse embryo fibroblasts as a starting cell type, we observed that the IMAP treatment increases iCM formation 6-fold. IMAP stimulated higher cardiac troponin T and α-actinin expression and increased sarcomere formation, coinciding with up-regulated expression of many cardiac genes and down-regulated fibroblast gene expression. Furthermore, IMAP promoted higher spontaneous beating and calcium transient activities of iCMs derived from neonatal cardiac fibroblasts. Intriguingly, we also observed that the IMAP treatment repressed many genes involved in immune responses, particularly those in specific C-C chemokine signaling pathways. We therefore investigated the roles of C-C motif chemokine ligand 3 (CCL3), CCL6, and CCL17 in cardiac reprogramming and observed that they inhibited iCM formation, whereas inhibitors of C-C motif chemokine receptor 1 (CCR1), CCR4, and CCR5 had the opposite effect. These results indicated that the IMAP treatment directly suppresses specific C-C chemokine signaling pathways and thereby enhances cardiac reprogramming. In conclusion, a combination of four chemicals, named here IMAP, suppresses specific C-C chemokine signaling pathways and facilitates Mef2c/Gata4/Tbx5 (MGT)-induced cardiac reprogramming, providing a potential means for iCM formation in clinical applications.

Boosters and barriers for direct cardiac reprogramming

Heart disease is currently the most significant cause of morbidity and mortality worldwide, which accounts for approximately 33% of all deaths. Recently, a promising and alchemy-like strategy has been developed called direct cardiac reprogramming, which directly converts somatic cells such as fibroblasts to cardiac lineage cells such as cardiomyocytes (CMs), termed induced CMs or iCMs. The first in vitro cardiac reprogramming study, mediated by cardiac transcription factors (TFs)-Gata4, Tbx5 and Mef2C-, was not enough efficient to produce an adequate number of fully reprogrammed, functional iCMs. As a result, numerous combinations of cardiac TFs exist for direct cardiac reprogramming of mouse and human fibroblasts. However, the efficiency of direct cardiac reprogramming remains low. Recently, a number of cellular and molecular mechanisms have been identified to increase the efficiency of direct cardiac reprogramming and the quality of iCMs. For example, microgrooved substrate, cardiogenic growth factors [VEGF, FGF, BMP4 and Activin A], and an appropriate stoichiometry of TFs boost the direct cardiac reprogramming. On the other hand, serum, TGFβ signaling, activators of epithelial to mesenchymal transition, and some epigenetic factors (Bmi1 and Ezh2) are barriers for direct cardiac reprogramming. Manipulating these mechanisms by the application of boosters and removing barriers can increase the efficiency of direct cardiac reprogramming and possibly make iCMs reliable for cell-based therapy or other potential applications. In this review, we summarize the latest trends in cardiac TF- or miRNA-based direct cardiac reprogramming and comprehensively discuses all molecular and cellular boosters and barriers affecting direct cardiac reprogramming.

Irx4 Marks a Multipotent, Ventricular-Specific Progenitor Cell

While much progress has been made in the resolution of the cellular hierarchy underlying cardiogenesis, our understanding of chamber-specific myocardium differentiation remains incomplete. To better understand ventricular myocardium differentiation, we targeted the ventricle-specific gene, Irx4, in mouse embryonic stem cells to generate a reporter cell line. Using an antibiotic-selection approach, we purified Irx4⁺ cells in vitro from differentiating embryoid bodies. The isolated Irx4⁺ cells proved to be highly proliferative and presented Cxcr4, Pdgfr-alpha, Flk1, and Flt1 on the cell surface. Single Irx4⁺ ventricular progenitor cells (VPCs) exhibited cardiovascular potency, generating endothelial cells, smooth muscle cells, and ventricular myocytes in vitro. The ventricular specificity of the Irx4⁺ population was further demonstrated in vivo as VPCs injected into the cardiac crescent subsequently produced Mlc2v⁺ myocytes that exclusively contributed to the nascent ventricle at E9.5. These findings support the existence of a newly identified ventricular myocardial progenitor. This is the first report of a multipotent cardiac progenitor that contributes progeny specific to the ventricular myocardium. Stem Cells 2016;34:2875-2888.

Ah Receptor Activation by Dioxin Disrupts Activin, BMP, and WNT Signals During the Early Differentiation of Mouse Embryonic Stem Cells and Inhibits Cardiomyocyte Functions

The AHR is a ligand-activated transcription factor that mediates gene-environment interactions. Genome-wide expression profiling during differentiation of mouse ES cells into cardiomyocytes showed that AHR activation by 2,3,7,8-tetrachlorodibenzo-p-dioxin; Dioxin (TCDD), its prototypical ligand, disrupted the expression of multiple homeobox transcription factors and inhibited cardiomyocyte contractility. Here we treated ES cells with TCDD at daily differentiation intervals to investigate whether TCDD-induced loss of contractility had a developmental window of sensitivity. Surprisingly, contractility was an AHR-dependent TCDD target solely between differentiation days 0 and 3 during the period of panmesoderm development, when TCDD also disrupted expression of genes in the TGFβ/BMP2/4 and wingless-type MMTV integration site (WNT)signaling pathways, suppressed the secretion of bone morphogenetic protein (BMP4), WNT3a, and WNT5a and elevated the secretion of Activin A, as determined by ELISA of the secreted proteins in the culture medium. Supplementing the culture medium with BMP4, WNT3a, or WNT5a during the first 3 days of differentiation successfully countered TCDD-induced impairment of contractility, while anti-WNT3a, or anti-WNT5a antibodies or continuous Noggin (a BMP4 antagonist) or Activin A treatment inhibited the contractile phenotype. In Ahr(+/+), but not in Ahr(-) (/) (-) ES cells, TCDD treatment significantly increased mitochondrial copy number, suggestive of mitochondrial stress and remodeling. Sustained AHR activation during ES cell differentiation appears to disrupt the expression of signals critical to the ontogeny of cardiac mesoderm and cause the loss of contractility in the resulting cardiomyocyte lineage.

Tmem88a mediates GATA-dependent specification of cardiomyocyte progenitors by restricting WNT signaling

Biphasic control of WNT signaling is essential during cardiogenesis, but how the pathway switches from promoting cardiac mesoderm to restricting cardiomyocyte progenitor fate is unknown. We identified genes expressed in lateral mesoderm that are dysregulated in zebrafish when both gata5 and gata6 are depleted, causing a block to cardiomyocyte specification. This screen identified tmem88a, which is expressed in the early cardiac progenitor field and was previously implicated in WNT modulation by overexpression studies. Depletion of tmem88a results in a profound cardiomyopathy, secondary to impaired cardiomyocyte specification. In tmem88a morphants, activation of the WNT pathway exacerbates the cardiomyocyte deficiency, whereas WNT inhibition rescues progenitor cells and cardiogenesis. We conclude that specification of cardiac fate downstream of gata5/6 involves activation of the tmem88a gene to constrain WNT signaling and expand the number of cardiac progenitors. Tmem88a is a novel component of the regulatory mechanism controlling the second phase of biphasic WNT activity essential for embryonic cardiogenesis.

DMH1, a novel BMP small molecule inhibitor, increases cardiomyocyte progenitors and promotes cardiac differentiation in mouse embryonic stem cells

The possibility of using cell-based therapeutics to treat cardiac failure has generated significant interest since the initial introduction of stem cell-based technologies. However, the methods to quickly and robustly direct stem cell differentiation towards cardiac cell types have been limited by a reliance on recombinant growth factors to provide necessary biological cues. We report here the use of dorsomorphin homologue 1 (DMH1), a second-generation small molecule BMP inhibitor based on dorsomorphin, to efficiently induce beating cardiomyocyte formation in mouse embryonic stem cells (ESCs) and to specifically upregulate canonical transcriptional markers associated with cardiac development. DMH1 differs significantly from its predecessor by its ability to enrich for pro-cardiac progenitor cells that respond to late-stage Wnt inhibition using XAV939 and produce secondary beating cardiomyocytes. Our study demonstrates the utility of small molecules to complement existing in vitro cardiac differentiation protocols and highlights the role of transient BMP inhibition in cardiomyogenesis.