Control of cardiomyocyte differentiation timing by intercellular signaling pathways

Abstract

Congenital Heart Disease (CHD), malformations of the heart present at birth, is the most common class of life-threatening birth defect (Hoffman (1995) [1], Gelb (2004) [2], Gelb (2014) [3]). A major research challenge is to elucidate the genetic determinants of CHD and mechanistically link CHD ontogeny to a molecular understanding of heart development. Although the embryonic origins of CHD are unclear in most cases, dysregulation of cardiovascular lineage specification, patterning, proliferation, migration or differentiation have been described (Olson (2004) [4], Olson (2006) [5], Srivastava (2006) [6], Dunwoodie (2007) [7], Bruneau (2008) [8]). Cardiac differentiation is the process whereby cells become progressively more dedicated in a trajectory through the cardiac lineage towards mature cardiomyocytes. Defects in cardiac differentiation have been linked to CHD, although how the complex control of cardiac differentiation prevents CHD is just beginning to be understood. The stages of cardiac differentiation are highly stereotyped and have been well-characterized (Kattman et al. (2011) [9], Wamstad et al. (2012) [10], Luna-Zurita et al. (2016) [11], Loh et al. (2016) [12], DeLaughter et al. (2016) [13]); however, the developmental and molecular mechanisms that promote or delay the transition of a cell through these stages have not been as deeply investigated. Tight temporal control of progenitor differentiation is critically important for normal organ size, spatial organization, and cellular physiology and homeostasis of all organ systems (Raff et al. (1985) [14], Amthor et al. (1998) [15], Kopan et al. (2014) [16]). This review will focus on the action of signaling pathways in the control of cardiomyocyte differentiation timing. Numerous signaling pathways, including the Wnt, Fibroblast Growth Factor, Hedgehog, Bone Morphogenetic Protein, Insulin-like Growth Factor, Thyroid Hormone and Hippo pathways, have all been implicated in promoting or inhibiting transitions along the cardiac differentiation trajectory. Gaining a deeper understanding of the mechanisms controlling cardiac differentiation timing promises to yield insights into the etiology of CHD and to inform approaches to restore function to damaged hearts.

Keywords: Bone morphogenic protein; Cardiac progenitor; Cardiac regeneration; Cardiomyocyte; Developmental timing; Differentiation; Fibroblast growth factor; Hedgehog; Hippo; Insulin-like growth factor; Signaling; Thyroid hormone; Wnt.

Fig. 1.

Cardiomyocyte differentiation timing varies across model systems. Researchers use a variety of model systems to study cardiomyocyte differentiation, including zebrafish and mouse embryos and in vitro models such as mouse or human embryonic stem cells (ESCs) or human induced pluripotent stem cells (hiPSCs). The broad stages of cardiomyocyte differentiation are shown, and the developmental timepoints at which select models transition from one differentiation stage to the next is noted. HPF, hours post-fertilization; DPC, days post-conception.

Fig. 2.

Overview of the signaling pathways that control cardiomyocyte differentiation timing. A schematic of the developmental stages that a cell transitions through during cardiomyocyte differentiation is shown from left to right. The pro-cardiac differentiation TFs or signaling pathways that promote the forward progression of a cell from one stage to the next are depicted with forward arrows. The factors that inhibit the transition of a cell to the next stage, and thus maintaining the current stage, are depicted with looped arrows. Generally, TGF-β/BMP signaling promotes the transition to the next differentiation stage, while Hh and FGF signaling inhibit or delay cardiomyocyte differentiation. The effects of Wnt signaling on cardiac differentiation timing are stage-dependent. FGF, Fibroblast Growth Factor; Hh, Hedgehog; BMP, Bone Morphogenetic Protein; IGF, Insulin-like Growth Factor; TF, transcription factor.

Fig. 3.

Evolving signaling and transcriptional networks during cardiac differentiation and regeneration. A differentiation trajectory, focused on the transitions occurring between the cardiac progenitor and mature cardiomyocyte stages, is shown from left to right. Cardiac progenitors, immature cardiomyocytes and mature cardiomyocytes all express cardiogenic transcription factors (TFs) that promote cardiac differentiation transitions and maintain the expression of cardiac differentiation products. Signaling pathway-dependent TFs are also active in the nucleus of differentiating progenitors and can facilitate, or counteract, the activity of the cardiogenic TFs. Examples of signal-dependent transcriptional regulation active during cardiomyocyte differentiation are shown. Early cardiac progenitors experience high levels of Wnt and Hippo signaling, and their TFs β-catenin and YAP facilitate the activation of pro-proliferation genes, a hallmark of the progenitor state. As cardiac progenitors begin to differentiate, β-catenin is inhibited by BMP signaling SMAD TFs and YAP is phosphorylated and shuttled out of the nucleus, resulting in the de-activation of proliferation genes and the preponderance of pro-differentiation gene expression. Re-activation of the Hippo pathway in terminally differentiated cardiomyocytes can increase their proliferation rate, thereby promoting cardiac regeneration after injury. BMP, Bone Morphogenetic Protein; YAP, Yes Associated Protein; TEAD, TEA Domain.