Early developmental expression pattern of retinoblastoma tumor suppressor mRNA indicates a role in the epithelial-to-mesenchyme transformation of endocardial cushion cells.

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.