van Wijk Bram, van den Berg Gert, Abu-Issa Radwan, Barnett Phil, van der Velden Saskia, Schmidt Martina, Ruijter Jan M, Kirby Margaret L, Moorman Antoon F M, van den Hoff Maurice J B
Academic Medical Center, Department Anatomy and Embryology, Meibergdreef 15, 1105AZ Amsterdam, The Netherlands.
Circ Res. 2009 Aug 28;105(5):431-41. doi: 10.1161/CIRCRESAHA.109.203083. Epub 2009 Jul 23.
The epicardium contributes to the majority of nonmyocardial cells in the adult heart. Recent studies have reported that the epicardium is derived from Nkx2.5-positive progenitors and can differentiate into cardiomyocytes. Not much is known about the relation between the myocardial and epicardial lineage during development, whereas insights into these embryonic mechanisms could facilitate the design of future regenerative strategies.
Acquiring insight into the signaling pathways involved in the lineage separation leading to the differentiation of myocardial and (pro)epicardial cells at the inflow of the developing heart.
We made 3D reconstructions of Tbx18 gene expression patterns to give insight into the developing epicardium in relation to the developing myocardium. Next, using DiI tracing, we show that the (pro)epicardium separates from the same precursor pool as the inflow myocardium. In vitro, we show that this lineage separation is regulated by a crosstalk between bone morphogenetic protein (BMP) signaling and fibroblast growth factor (FGF) signaling. BMP signaling via Smad drives differentiation toward the myocardial lineage, which is inhibited by FGF signaling via mitogen-activated protein kinase kinase (Mek)1/2. Embryos exposed to recombinant FGF2 in vivo show enhanced epicardium formation, whereas a misbalance between FGF and BMP by Mek1/2 inhibition and BMP stimulation causes a developmental arrest of the epicardium and enhances myocardium formation at the inflow of the heart.
Our data show that FGF signaling via Mek1/2 is dominant over BMP signaling via Smad and is required to separate the epicardial lineage from precardiac mesoderm. Consequently, myocardial differentiation requires BMP signaling via Smad and inhibition of FGF signaling at the level of Mek1/2. These findings are of clinical interest for the development of regeneration-based therapies for heart disease.
心外膜构成了成年心脏中大多数非心肌细胞。最近的研究报道，心外膜起源于Nkx2.5阳性祖细胞，并且可以分化为心肌细胞。关于发育过程中心肌谱系和心外膜谱系之间的关系，目前所知甚少，而深入了解这些胚胎机制可能有助于未来再生策略的设计。
深入了解在发育中心脏流入区域导致心肌细胞和（前）心外膜细胞分化的谱系分离所涉及的信号通路。
我们对Tbx18基因表达模式进行了三维重建，以深入了解发育中的心外膜与发育中的心肌的关系。接下来，使用DiI追踪技术，我们发现（前）心外膜与流入心肌来自同一个前体池。在体外，我们表明这种谱系分离受骨形态发生蛋白（BMP）信号通路和成纤维细胞生长因子（FGF）信号通路之间的相互作用调控。通过Smad的BMP信号通路驱动向心肌谱系的分化，而通过丝裂原活化蛋白激酶激酶（Mek）1/2的FGF信号通路则抑制这种分化。在体内暴露于重组FGF2的胚胎显示心外膜形成增强，而通过抑制Mek1/2和刺激BMP导致FGF和BMP之间失衡，会导致心外膜发育停滞，并增强心脏流入区域的心肌形成。
我们的数据表明，通过Mek1/2的FGF信号通路比通过Smad的BMP信号通路占主导地位，并且是将心外膜谱系与心脏前体中胚层分离所必需的。因此，心肌分化需要通过Smad的BMP信号通路以及在Mek1/2水平抑制FGF信号通路。这些发现对于基于再生的心脏病治疗方法的开发具有临床意义。
