Regulation of Avian Cardiac Myogenesis by Activin/TGFβ and Bone Morphogenetic Proteins

Abstract

Previous studies have identified two signaling interactions regulating cardiac myogenesis in avians, a hypoblast-derived signal acting on epiblast and mediated by activin or a related molecule and an endoderm-derived signal acting on mesoderm and involving BMP-2. In this study, experiments were designed to investigate the temporal relationship between these signaling events and the potential role of other TGFbeta superfamily members in regulating early steps of heart muscle development. We find that while activin or TGFbeta can potently induce cardiac myogenesis in pregastrula epiblast, they show no capacity to convert noncardiogenic mesoderm toward a myocardial phenotype. Conversely, BMP-2 or BMP-4, in combination with FGF-4, can readily induce cardiac myocyte formation in posterior mesoderm, but shows no capacity to induce cardiac myogenesis in epiblast cells. Activin/TGFbeta and BMP-2/BMP-4 therefore have distinct and reciprocal cardiac-inducing capacities that mimic the tissues in which they are expressed, the pregastrula hypoblast and anterior lateral endoderm, respectively. Experiments with noggin and follistatin provide additional evidence indicating that BMP signaling lies downstream of an activin/TGFbeta signal in the cardiac myogenesis pathway. In contrast to the cardiogenic-inducing capacities of BMP-2/BMP-4 in mesoderm, however, we find that BMP-2 or BMP-4 inhibits cardiac myogenesis prior to stage 3, demonstrating multiple roles for BMPs in mesoderm induction. These and other published studies suggest a signaling cascade in which a hypoblast-derived activin/TGFbeta signal is required prior to and during early stages of gastrulation, regulated both spatially and temporally by an interplay between BMPs and their antagonists. Later cardiogenic signals arising from endoderm, and perhaps transiently from ectoderm, and mediated in part by BMPs, act on emerging mesoderm within cardiogenic regions to activate or enhance expression of cardiogenic genes such as GATA and cNkx family members, leading to cardiac myocyte differentiation.

Inhibition of Wnt activity induces heart formation from posterior mesoderm

“…It has been reported by others that FGF can respecify posterior lateral mesoderm to form heart in combination with BMP-2 (Lough et al 1996;Ladd et al 1998). However, the lateral mesoderm in that work extended considerably closer to the heart-forming region, whereas the lateral mesoderm in our assay was located at the extreme posterior of the embryo (at 75%-100% streak length).…”

Stem cells and the formation of the myocardium in the vertebrate embryo

“…These tissue explants could be either cultured under diverse conditions or inserted into various regions of a second embryo, and then assayed for various cell phenotypes. Explants of precardiac mesoderm placed in culture inevitably give rise to cardiac tissue Ladd et al, 1998;Eisenberg and Eisenberg, 1999), although the presence of the adjacent endoderm is required for the formation of a full contractile phenotype (Antin et al, 1994;Sugi and Lough, 1994). The apparent unavoidability of cardiac differentiation by cultured precardiac mesoderm does not necessarily imply that these cells are fully committed to become myocardium.…”

“…5). These cardiogenic enhancers include several members of the BMP and FGF growth factor families Ladd et al, 1998;Alsan and Schultheiss, 2002) and three distinct modulators of WNT signal transduction: WNT11, Dkk1, and crescent (Eisenberg and Eisenberg, 1999;Marvin et al, 2001;Schneider and Mercola, 2001;Pandur et al, 2002). The cardiac-promoting activities of these molecules appear to correlate with their involvement in normal cardiogenesis in the embryo.…”

“…In contrast, explants of noncardiac posterior mesendoderm will not generate cardiac tissue. However, treatment of noncardiac posterior mesendoderm with WNT11 (Eisenberg and Eisenberg, 1999), Dkk1 (Marvin et al, 2001), or BMP4 ϩ FGF2 Ladd et al, 1998) will promote the development of cardiac tissue, as exhibited by clustered fields of cells expressing cardiac proteins. Thus, despite the posterior origin of these latter explants, which correspond to regions of the embryo that do not contribute to the heart in situ, this tissue is still able to form myocardial tissue in response to changes in the extracellular environment.…”

Wnt antagonism initiates cardiogenesis in Xenopus laevis

“…Treatment of avian posterior epiblast tissue with activin-induced cardiac myogenesis (stage XI-XIV, staging according to Eyal-Giladi and Kochav 1976;Yatskievych et al 1997;Ladd et al 1998). However, the inability of this protein to induce heart muscle cells in streak stage mesodermal explants (the period when heart induction normally occurs) indicate that the role of activin in this process might be indirect, possibly by promoting the formation of precardiac mesoderm competent to respond to heartinducing signals.…”