Nakajima Yuji, Yamagishi Toshiyuki, Ando Katsumi, Nakamura Hiroaki
Department of Anatomy, Graduate School of Medicine, Osaka City University, 1-4-3 Asahimachi, Abenoku, Osaka 545-8585, Japan.
Dev Biol. 2002 May 15;245(2):291-303. doi: 10.1006/dbio.2002.0637.
The heart is the first organ to form and function during vertebrate embryogenesis. Using a secreted protein, noggin, which specifically antagonizes bone morphogenetic protein (BMP)-2 and -4, we examined the role played by BMP during the initial myofibrillogenesis in chick cultured precardiac mesoendoderm (mesoderm + endoderm; ME). Conditioned medium from COS7 cells transfected with Xenopus noggin cDNA inhibited the expression of sarcomeric proteins (such as sarcomeric alpha-actinin, Z-line titin, and sarcomeric myosin), and so myofibrillogenesis was perturbed in cultured stage 4 precardiac ME; however, it did not inhibit the expression of smooth muscle alpha-actin (the first isoform of alpha-actin expressed during cardiogenesis). In cultured stage 5 precardiac ME, noggin did not inhibit either the formation of I-Z-I components or the expression of sarcomeric myosin, but it did inhibit the formation of A-bands. Although BMP4 was required to induce expressions of sarcomeric alpha-actinin, titin, and sarcomeric myosin in cultured stage 6 posterolateral mesoderm (noncardiogenic mesoderm), smooth muscle alpha-actin was expressed without the addition of BMP4. Interestingly, in cultured stage 6 posterolateral mesoderm, BMP2 induced the expressions of sarcomeric alpha-actinin and titin, but not of sarcomeric myosin. These results suggest that (1) BMP4 function lies upstream of the initial formation of I-Z-I components and A-bands separately in a stage-dependent manner, and (2) at least two signaling pathways are involved in the initial cardiac myofibrillogenesis: one is an unknown pathway responsible for the expression of smooth muscle alpha-actin; the other is BMP signaling, which is involved in the expression of sarcomeric alpha-actinin, titin, and sarcomeric myosin.
心脏是脊椎动物胚胎发育过程中第一个形成并发挥功能的器官。我们利用一种分泌蛋白——头蛋白（noggin），它能特异性拮抗骨形态发生蛋白（BMP）-2和-4，研究了BMP在鸡胚心脏前中胚层（中胚层+内胚层；ME）初始肌原纤维形成过程中所起的作用。用非洲爪蟾头蛋白cDNA转染的COS7细胞的条件培养基抑制了肌节蛋白（如肌节α-辅肌动蛋白、Z线肌联蛋白和肌节肌球蛋白）的表达，因此培养的第4阶段心脏前ME中的肌原纤维形成受到干扰；然而，它并未抑制平滑肌α-肌动蛋白（心脏发生过程中最早表达的α-肌动蛋白同工型）的表达。在培养的第5阶段心脏前ME中，头蛋白既不抑制I-Z-I组件的形成，也不抑制肌节肌球蛋白的表达，但它确实抑制了A带的形成。尽管在培养的第6阶段后外侧中胚层（非心脏中胚层）中诱导肌节α-辅肌动蛋白、肌联蛋白和肌节肌球蛋白的表达需要BMP4，但在不添加BMP4的情况下也能表达平滑肌α-肌动蛋白。有趣的是，在培养的第6阶段后外侧中胚层中，BMP2诱导了肌节α-辅肌动蛋白和肌联蛋白的表达，但未诱导肌节肌球蛋白的表达。这些结果表明：（1）BMP4的功能分别以阶段依赖的方式位于I-Z-I组件和A带初始形成的上游；（2）至少有两条信号通路参与了初始心肌原纤维形成：一条是负责平滑肌α-肌动蛋白表达的未知通路；另一条是BMP信号通路，它参与肌节α-辅肌动蛋白、肌联蛋白和肌节肌球蛋白的表达。
