Growth factor-mediated regulation of cardiac myogenesis during early avian embryogenesis

Abstract

Previous studies have identified two signaling interactions regulating cardiac myogenesis in avians, a hypoblast-derived signal acting on epiblast and an endoderm-derived signal acting on mesoderm. In this study, experiments were designed to investigate the potential role of TGFβ superfamily members in regulating these early steps of heart muscle cell development. While activin or TGFβ can potently induce cardiac myogenesis in pregastrula epiblast, they show no capacity to convert noncardiogenic mesoderm toward a myocardial phenotype. Conversely, BMP-2/BMP-4 can induce cardiac myocyte formation in mesoderm in a variety of contexts, but show no capacity to induce cardiac myogenesis in epiblast cells. Activin/TGFβ and BMP-2/BMP-4 therefore have distinct and reciprocal inducing capacities that mimic the tissues in which they are expressed, the pregastrula hypoblast and the anterior lateral endoderm, respectively. Experiments with follistatin and noggin provide additional evidence that BMP signaling lies downstream of activin signaling in the cardiac myogenesis pathway. BMP-2 or BMP-4 inhibit cardiac myogenesis prior to stage 3, demonstrating a dual role for BMPs in mesoderm induction. These and other published studies suggest a signaling cascade in which a hypoblast-derived activin/TGFβ 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, act on emerging mesoderm within cardiogenic regions. These signals, mediated in part by BMPs, activate or enhance expression of cardiogenic genes such as GATA and cNkx family members, leading to cardiac myocyte differentiation. Members of the FGF and the EGF-related CFC families may also participate in this pathway. FGF-2/FGF-4 can induce posterior region epiblast to form heart muscle cells, and the CFC family member Cripto can convert posterior lateral mesoderm to a myocardial phenotype. The role of these factors in the cardiac myogenesis pathway is unclear.