The acquisition of neuronal diversity is central to the development and organization of the mammalian central nervous system (CNS). Our understanding of some of the mechanisms by which this diversity is achieved has been greatly facilitated by studies of neuronal differentiation in retina, a well characterized model of the CNS. Lineage analyses of neuronal precursors in vivo using retroviral infections and progressive lineage restriction during cell-type specification. However, our understanding of the molecular basis of these processes remains unclear for the lack of information regarding genes involved in the mammalian neurogenesis. One of the approaches is to identify and study the retina-specific expression of the mammalian homologs of Drosophila genes whose role in neurogenesis has been well established by mutation analyses. Depending upon their spatio-temporal expression and functions during early neurogenesis these genes belong to two different classes, the proneural genes of the achaete-scute (AS-C) complex that encode transcription factors of basic Helix-Loop-Helix (bHLH) class and the neurogenic genes of which Notch is a member, that encodes a membrane protein. Members of both proneural and neurogenic genes have been shown to play important roles in the development of Drosophila eye. The function of these genes in early neurogenesis in general and in the development of the eye in particular make their mammalian homologues ideal candidates for the investigation of the spatio-temporal expression of the mammalian homologs of AS-C and Notch genes in developing retina in order to know their cell-specific expression and to formulate a hypothesis regarding their function. Both genes may be involved in mediating intercellular interactions during retinal neurogenesis and cell-type specification. While AS-C homologs can activate the differentiation program by influencing the genome, Notch on the other had can link the activation f the genome with microenvironment in which the differentiation is taking place. In order to evaluate such possibilities, the expression of these genes will be studied in vitro in response to various growth factors that have been shown to modulate cell proliferation and differentiation in retina. The AS-C homologs, analogous to myogenic gene, MyoD may function as master regulatory genes involved in the activation of downstream neuron-specific genes in precursors during the early stages of neurogenesis. This hypothesis will lbs initially tested by analyzing the ability of AS-C homologs expressed in developing retina to interact with putative cis- acting elements (E-box) and evaluating their transcriptional activity by transactivation experiments. This information will be critical for the identification of downstream, neuron-specific genes and evaluation of AS- C homologs expression as the nodal point in neurogenesis. Studies of the homologs of Notch and AS-C genes during the retinal development will help us in our long term goal of obtaining a comprehensive picture of the molecular events underlying neurogenesis and cell-type specification. The information obtained from these studies will help us in understanding some of the processes involved in retinal degeneration and formulated hypotheses regarding interventions at the molecular and cellular levels to prolong and promote the survival of specific neurons by recapitulation of developmental mechanisms during retinal degeneration.
