| The retinoblastoma gene Rb is a prototype tumor suppressor that is conserved across many organisms, including humans, mice, nematodes, and the fruit fly Drosophila. While much is known about the roles of Rb in cell proliferation and apoptosis in many of these species, relatively little is known about the mechanisms through which Rb regulates cell differentiation. In mammalian systems, while understanding the consequences of Rb inactivation on differentiation has been hampered by redundancy among RB proteins, several observations these systems do support a role for Rb in the regulation of cellular differentiation programs. RB protein is elevated in tissues that are completing terminal differentiation programs, and loss of RB impacts specific tissues including the central nervous system and eye lens in the Rb knockout mouse. However, as increased apoptosis of these tissues was also observed, it was unclear whether differentiation defects observed upon Rb removal were secondary to apoptosis, or if Rb loss also affected terminal differentiation of these tissues. In additional to high functional conservation, inactivation of Drosophila Rb (Rbf) exhibited subtle differentiation defects similar to Rb inactivation in mice, suggesting the existence of redundant mechanisms controlling differentiation in both systems. To test this possibility and to characterize the role of Rbf in cell differentiation during retina development in Drosophila, a mosaic genetic screen was conducted to identify mutations causing differentiation defects concomitant with mutation in rbf. From this screen, we found that simultaneous mutation of rbf and rhinoceros (rno) impacts photoreceptor differentiation as a result of reduced levels of the Notch-activating ligand Delta (Dl) during Drosophila larval eye development.;While rbf mutant ommatidia are largely normal and ommatidia in rno mutant clones occasionally contain more than one R8 photoreceptor, this "multiple-R8" phenotype is significantly increased in rbf; rno double mutant clones. Gene dosage reduction of Dl led to further enhancement of multiple-R8 phenotype in rno and rbf; rno clones; overexpression of Dl in rbf; rno clones suppressed multiple-R8 phenotype. In addition, Dl protein and Dl reporter levels were reduced in rno clones, with further reduction in rbf; rno clones. Interestingly, reduction of dE2F1 (a transcription factor that is normally inhibited by Rbf) in rbf; rno mutant clones reduces multiple-R8 phenotype and restores Dl protein to levels observed in rno single mutant clones, suggesting that Rbf promotes normal R8 differentiation in rno mutants via inhibition of dE2F1. The phenotypes in rbf; rno double mutant clones serve as an excellent model to study the role of Rbf in differentiation. |
