Wagner M, Miles K, Siddiqui M A
Center for Cardiovascular and Muscle Research, Department of Anatomy and Cell Biology, State University of New York Health Science Center at Brooklyn, Brooklyn, New York 11203, USA.
Dev Dyn. 2001 Mar;220(3):198-211. doi: 10.1002/1097-0177(20010301)220:3<198::AID-DVDY1103>3.0.CO;2-T.
The earliest stages of embryonic development are characterized by the generation of precursor cell populations that differentiate and coalesce into tissue and organ primordia. To provide sufficient numbers of differentiated cells for tissue and organ formation, the differentiative as well as the proliferative processes of cells must be controlled and coordinated. Potential regulators of the proliferative process include molecules that control the cell cycle, in particular, the tumor suppressor proteins. To begin to understand the role such molecules can play in development, we have studied the expression of the retinoblastoma tumor suppressor (Rb) gene in early chicken development. Our studies in early chicken embryos show that Rb is encoded by a single gene that gives rise to several Rb mRNA isoforms through alternative splicing of a primary transcript. These mRNA isoforms potentially encode Rb proteins that differ with respect to the number of sequence motifs known to target cyclin-dependent kinases to Rb, suggesting dynamic control of Rb phosphorylation and function during development. This complex expression pattern of Rb mRNA begins as early as the blastoderm stage of chicken development (stage 3) and continues through stage 18, the latest stage examined. Despite this early embryonic expression of Rb mRNA as detected by reverse transcription polymerase chain reaction, Rb mRNA levels sufficient to be detected by in situ hybridization were not expressed until after stage 14 of development. Rb mRNA was found to be localized to cells of the endocardial cushions of the early heart tube, cells of the epicardium, and myogenic cells of the somitic myotome. Interestingly, each of these cell types undergoes an epithelial-to-mesenchyme transformation to form a migratory and/or invasive population of mesenchymal cells. We have focused our studies on the expression of Rb mRNA in endocardial cells of the early heart tube, because the transition of these cells to mesenchyme initiates the important process of septation, an early step in the formation of heart valves.
胚胎发育的最早阶段的特征是产生前体细胞群体，这些细胞分化并合并形成组织和器官原基。为了为组织和器官形成提供足够数量的分化细胞，细胞的分化和增殖过程必须受到控制和协调。增殖过程的潜在调节因子包括控制细胞周期的分子，特别是肿瘤抑制蛋白。为了开始了解这些分子在发育中可能发挥的作用，我们研究了视网膜母细胞瘤肿瘤抑制（Rb）基因在鸡早期发育中的表达。我们对鸡早期胚胎的研究表明，Rb由单个基因编码，该基因通过初级转录本的可变剪接产生几种Rb mRNA异构体。这些mRNA异构体可能编码的Rb蛋白在已知将细胞周期蛋白依赖性激酶靶向Rb的序列基序数量方面有所不同，这表明在发育过程中Rb磷酸化和功能受到动态控制。Rb mRNA的这种复杂表达模式早在鸡发育的胚盘阶段（第3阶段）就开始了，并持续到第18阶段，即检查的最晚阶段。尽管通过逆转录聚合酶链反应检测到Rb mRNA在胚胎早期就有表达，但直到发育的第14阶段之后才表达出足以通过原位杂交检测到的Rb mRNA水平。发现Rb mRNA定位于早期心脏管的心内膜垫细胞、心外膜细胞和体节肌节的成肌细胞。有趣的是，这些细胞类型中的每一种都经历上皮-间充质转化，以形成迁移性和/或侵袭性的间充质细胞群体。我们将研究重点放在早期心脏管心内膜细胞中Rb mRNA的表达上，因为这些细胞向间充质的转变启动了重要的分隔过程，这是心脏瓣膜形成的早期步骤。
