Generation of neural diversity is a key question in developmental neurobiology. Studies in both vertebrates and invertebrate model organisms have shown that neural progenitors are temporally patterned to generate different neural types in a defined order, and this order can be recapitulated in progenitors cultured in vitro, suggesting an internal clock. However, the molecule mechanism is not yet clear. The Drosophila medulla is a unique system to address this question, because the neural progenitors (neuroblasts) at consecutive temporal stages can be visualized in one image. Recently a novel Temporal cascade of Transcription Factors (TTFs), Homothorax (Hth), Eyeless (Ey), Sloppy paired 1 and 2 (Slp), Dichaete (D) and Tailless (Tll), were found to be expressed sequentially in medulla neuroblasts as they age. Each gene serves as the ?master regulator? of the corresponding temporal stage, and control neuron identity. The temporal transitions between different temporal stages require that one TTF activates the next, and represses the previous one. The activation of the next TTF?s expression is a gradual process, and there is a large overlap between successive TTFs in NBs. However, there is a sharp transition in the neuronal progeny from expressing one TTF to the next with no overlap. This is critical to generate distinct neural identities in successive temporal stages. This proposal is addressing two fundamental questions of developmental timing control in neural progenitors: 1) How is the gradual turning on the next TTF in neural progenitors ?translated? into a sharp temporal transition in the progeny? 2) How is the temporal transition regulated to precisely control the number of neurons born at each stage? The preliminary data suggest a hypothesis in which the expression of the next TTF is activated gradually in NBs by its preceding TTF in a cell-cycle dependent way, but the transcription of the next TTF gene is repressed in the progeny before its protein inherited from the NB reaches a certain threshold to counteract the repression, and then the transition in the neuronal progeny occurs. Further, preliminary data show that the progression in the temporal gene cascade does not happen when NBs are arrested in the cell cycle, suggesting that the temporal progression is not dependent on the absolute time, but the cell-cycle progression. In summary cell cycle dependent oscillations could serve as the clock, and the progressive and irreversible temporal gene cascade could serve as the accumulative record of time with each cell cycle leaving its mark, providing a mechanism to control the number of neurons born at each temporal stage.
The proposed research is relevant to human health and NIH mission because it is expected to uncover conserved principles and molecular pathways for developmental timing control in neural progenitors to generate neural diversity. It will be informative for development of cell replacement therapies, in which stem cells are induced to generate certain neuron types for transplantation to treat retinal and neurodegenerative diseases.