Elevated levels of cyclin D1 mRNA in the undifferentiated chick retina

The retina is derived from precursor neuroectodermal cells that differentiate into six classes of neuronal cells and one class of glial cells (Müller). To gain insight into the molecular events underlying retinal differentiation, we used the differential display polymerase chain reaction (DD-PCR) technique to identify transcripts preferentially expressed in precursor retinal cells prior to their differentiation. One of the cDNAs that we selected using this technique encoded cyclin D1, a G1 cyclin shown to bind to the retinoblastoma protein (pRB) and which is involved in the phosphorylation of pRB during mid to late G1. Similar to what has been reported recently in the mouse retina, we found cyclin D1 mRNA to be highly expressed in the undifferentiated chick retina. Tissue maturation was accompanied by a substantial reduction in cyclin D1 mRNA levels. A similar temporal pattern of expression was observed in the developing brain although transcript levels were lower than in the retina. In contrast, cyclin D1 mRNA levels increased with differentiation in the kidney. These results suggest that the proliferating cells of the developing chick retina require exceptionally high levels of cyclin D1 mRNA, perhaps to promote progression through the cell cycle by countering the effect of molecules with a negative role in the cell cycle such as pRB.