We propose to investigate why central visual pathways fail to regenerate after injury and how their repair can be enhanced. We will focus on the mechanisms that normally control the survival and growth of retinal ganglion cells (RGCs) and their axons. The rat optic nerve, which mainly consists of RGC axons, astrocytes, and myelinating oligodendrocytes, will be used as a model system. The survival of RGCs and their axons is normally controlled by peptide signals such as brain-derived neurotrophic factor (BDNF), that are released by neighboring cells-tectal target neurons and optic nerve glia-and retrogradely transported to the RGC soma. When RGC axons are cut, their axons degenerate and the RGCs themselves undergo apoptosis, because they fail to get needed survival signals. In addition, we have recently found that electrical activity of RGCs is necessary for responsiveness to these peptide trophic signals and that axotomized RGCs quickly lose responsiveness to peptide trophic signals. These findings raise a question: do RGC axons fail to regenerate after injury primarily because RGCs fail to receive and respond to signals necessary to promote their survival and growth? Alternative hypotheses are that RGCs lose the ability to regenerate axons with age or that regrowing axons are inhibited by myelin and other inhibitors. We have developed methods to purify and culture to greater than 99.5 percent purity rodent RGCs, optic nerve astrocytes and oligodendrocytes. RGCs are presently the only CNS neuron that can be highly purified and cultured in defined serum-free conditions, providing us with an unusually good opportunity to investigate the signals that normally promote their survival and growth and how these mechanisms go awry after injury. We have also recently shown that bcl-2 expression is sufficient to promote the survival of purified RGCs in culture in the absence of peptide signals. We will use these methods to ask: (1) What extrinsic signals promote axon growth of surviving RGCs?, (2) Is there an effect of intrinsic neuronal age on the rate of axonal growth of surviving RGCs?, (3) Are surviving RGCs inhibited by myelin and semaphorins?, (4) How do electrical activity and CAMP elevation control trophic responsiveness of RGCs and does axotomy decrease RGC electrical activity?, and (5) Will surviving RGCs regenerate their axons in vivo? Our ultimate goal is to understand why RGCs fail to survive and regenerate after axotomy. This could suggest new ways of promoting their regeneration after injury in ocular diseases including glaucoma, retinal ischemia, optic neuritis, ischemia and neuropathy.
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