: During the development of the vertebrate nervous system, there is intense cell proliferation and accumulation. It is therefore somewhat surprising that a massive (50-70 percent) loss of neurons also normally occurs during development. This period of normal cell death coincides with the axonal innervation of synaptic target tissues. In addition, this process often continues through the period in which neurons also receive afferent input from cells. Unlike necrosis, development cell death requires the active expression of new mRNA or protein to initiate and complete a genetic program of cell suicide. The long term goal of this project is to elucidate the cellular and molecular mechanisms that regulate the programmed cell death (PCD) of neurons during brain development. PCD is believed to play a role in numerically matching the pre and post synaptic components of the nervous system to meet the functional demand of the organism. This proposal focuses on the mechanisms that are normally used to selectively eliminate or retain immature neurons in the construction of the vertebrate visual system. A unique aspect of this proposal is the targeted transfer of neurotropic factor genes to selected tissues or at selected period of development. We intend to regionally restrict the overexpression or deletion of these genes to test their physiological relevance in regulating PCD. The avian nervous system is ideal for these experiments because neuronal lineages can be selectively transfected and experimentally manipulated from the earliest stages of brain development in ovo. Chick chimeras will be made with a mosaic of normal and transfected brain components to experimentally control the expression of foreign genes in both presynaptic and target postsynaptic tissues. In addition, a well established model of neuronal competition for survival will also be investigated in mammals with binocular vision. Cell lines, genetically engineered to overexpress survival factors, will be implanted into the hamster visual system. Implants will be used to dissect the individual contributions of target and afferent tissues in both the intraretinal and interretinal competition of ganglion cells for survival. Lastly, in vitro assays with isolated neuronal subpopulations will provide an analysis of the direct effects of experimental manipulations in the absence of other cell types. Taken together, this integrative analysis will provide a comprehensive examination of a working hypothesis on the molecular regulation of neuronal competition and PCD. Interestingly, a similar specificity and morphology of neuronal PCD is induced by a variety of human disease (including Retinitis Pigmentosa, Alzheimer's disease, Amyotrophic Lateral Sclerosis, Parkinson's Disease and Paraplegia). Thus, an understanding of the mechanisms that determine normal developmental PCD may also be especially helpful in finding strategies to reduce or compensate for the specific neuronal losses associated with these human diseases.
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