Neuronal death is a highly regulated event that occurs throughout the developing nervous system. In many cases, developing neurons die because they fail to receive adequate supplies of target-derived, survival promoting factors such as nerve growth factor (NGF). This type of developmentally programmed cell death is important for ensuring that the proper numbers and types of connections are established between neurons and their targets. Importantly, trophic factor deprivation is also a frequent consequence of injury and disease in the mature nervous system. In such cases, the neuronal death is pathologic and contributes to the functional deficits seen in spinal cord injury, stroke, and Alzheimer's disease. Using a well-characterized and physiologically important model for trophic factor deprivation involving NGF-dependent sympathetic neurons, we recently identified the prolyl hydroxylase EGLN3 as a mediator of cell death. While the mechanism by which EGLN3 promotes death is largely unknown, we have discovered a novel interaction between EGLN3 and the BH3-only Bcl-2 family protein BIMEL, an established regulator of cell death induced by trophic factor withdrawal. Preliminary results reveal that death induced by BIMEL is reduced in EGLN3-deficient neurons, suggesting a functional relationship exists between these two proteins. Co-immunoprecipitation experiments demonstrate that EGLN3 and BIMEL each interact with the von Hippel-Lindau tumor suppressor protein pVHL. Expression of pVHL in sympathetic neurons promotes cell death while other preliminary results suggest that pVHL may enhance EGLN3 and BIMEL protein stability. Based on these novel observations, we hypothesize that pVHL, BIMEL and EGLN3 function coordinately to regulate trophic factor deprivation-induced cell death.
In Aim 1 we will use a combination of over-expression and neurons from knockout mice to determine the functional relationship between EGLN3 and BIMEL during NGF deprivation-induced cell death. Experiments in Aim 2 will determine the biochemical significance of the interaction between EGLN3 and BIMEL for cell death. Specific experiments will determine if EGLN3 influences the function of BIMEL, if BIMEL affects the prolyl hydroxylase activity of EGLN3, or if BIMEL is a substrate for prolyl hydroxylation by EGLN3.
In Aim 3, we will test the hypothesis that pVHL regulates EGLN3 and BIMEL protein levels during trophic factor deprivation. In addition, we will test if pVHL expression is necessary for trophic factor deprivation-induced cell death. These studies will further our understanding of the mechanisms that lead to neuronal cell death during normal development and in nervous systems disorders where trophic factor deprivation contributes to neuronal loss and dysfunction.
Trophic factor deprivation-induced cell death is not only critical for proper development of the nervous system but it also contributes to the loss and dysfunction of neurons that accompanies stroke, spinal cord injury, and neurodegenerative disease. This project will characterize new mechanisms that regulate cell death caused by neurotrophic factor deprivation. Information gained from this project will further our understanding of normal development and could help identify new targets for therapies aimed at preventing pathological neuronal cell death.
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