Molecular regulation of cardiomyocyte differentiation

Abstract

The heart is the first organ to form during embryonic development. Given the complex nature of cardiac differentiation and morphogenesis, it is not surprising that some form of congenital heart disease is present in ≈1 percent of newborns. The molecular determinants of heart development have received much attention over the past several decades. This has been driven in large part by an interest in understanding the causes of congenital heart disease coupled with the potential of using knowledge from developmental biology to generate functional cells and tissues that could be used for regenerative medicine purposes. In this review, we highlight the critical signaling pathways and transcription factor networks that regulate cardiomyocyte lineage specification in both in vivo and in vitro models. Special focus will be given to epigenetic regulators that drive the commitment of cardiomyogenic cells from nascent mesoderm and their differentiation into chamber-specific myocytes, as well as regulation of myocardial trabeculation.

Keywords: cell differentiation; embryonic development; epigenomics; myocytes, cardiac; organogenesis; transcription factors.

Figure 1. Overview of heart development

The cardiac crescent consisting of the first and second heart field cardiac progenitors is established during late gastrulation. The subsequent proliferation and differentiation of cells in the first heart field (FHF) leads to the formation of a linear heart tube, giving rise primarily to the left ventricle and a portion of the atria. The cardiac progenitors in the anterior second heart field (SHF) contribute to the right ventricle and the outflow tract while the posterior SHF cells give rise to the atria and the inflow tract. Extension and rightward looping of the linear heart tube allow cranial positioning of the atria with respect to the ventricles. Remodeling events modulate chamber formation, septation, and valve development, resulting in formation of the four-chambered heart. Transcription factors that regulate each stage of heart development are listed. FHF and its derivatives are shown in orange. SHF and its derivatives are shown in blue.

Figure 2. Regulation of cardiac mesoderm specification

Shown is a cross-section of an E7.5 mouse embryo detailing the signaling pathways that regulate cardiac specification within splanchnic mesoderm. Factors secreted by the adjacent endoderm that support cardiac mesoderm specification include FGF, BMP and Shh. Additionally, non-canonical Wnt ligands, such as Wnt11, expressed in splanchnic mesoderm also promote cardiac differentiation. Conversely, canonical Wnt ligands, including Wnt1, Wnt3a and Wnt8, secreted from the overlying neuroectoderm as well as BMP antagonists Noggin and Chordin secreted from the notochord inhibit cardiac mesoderm specification, thereby limiting the size of the cardiogenic fields. Abbreviations: FGF, fibroblast growth factor; BMP, bone morphogenetic protein; Shh, sonic hedgehog.

Figure 3. Schematic of cardiovascular lineage diversification

The specification of cardiomyocytes in the first and second heart fields is shown in the context of other cardiac cells that also derive from a common cardiogenic mesoderm progenitor. In particular, Isl1 expression distinguishes the first and second heart fields. Comparison of action potentials for mature myocytes reveals the range of function generated from each heart field. *Developmental origin of the coronary endothelium is an active topic of investigation. While some evidence points to partial contributions to the coronary endothelium from the epicardium^, , other sources such as the endocardium and the sinus venouses have also been reported.

Figure 4. Cardiac gene regulatory network

The diagram shown is a brief overview of a subset of all known transcription factor interactions and signaling pathways that drive the differentiation of FHF (orange) and SHF (blue) cardiac progenitor cells during development. Factors colored by both orange and blue represent regulators of both FHF and SHF. Arrows indicate increased expression of one transcription factor or signaling molecule due to activity of another transcription factor. Signaling pathways that activate expression of certain transcription factors are shown in red.

Figure 5. Specification of chamber myocardium

The specification of cells to the chamber myocardium is modulated by Tbx5 and Tbx20 in tandem with more broadly expressed factors Nkx2-5 and Gata4. Tbx2 and Tbx3 suppress the expression of chamber myocardium-specific genes, resulting in low proliferation rate, slow conduction velocity, and poor contractibility characteristic to the primary myocardium. The primary myocardium phenotype becomes restricted to the AVC and p-OFT. Abbreviations: AVC, atrioventricular canal; p-OFT, proximal outflow tract; d-OFT, distal outflow tract.

Figure 6. Spatiotemporal regulation of trabeculation

The development of ventricular trabeculae is governed by signal transduction involving the endocardium, myocardium, and cardiac jelly. Pathways implicated in trabeculation are depicted in approximate chronological order of expression from left to right. Beginning with establishment of the cardiac jelly and endothelium, trabeculation progresses via cellular proliferation and migration, and ends with degradation of the cardiac jelly. Factors involved in epigenetic mechanisms are underlined.