We have discovered that Schwann cells, distal to an injury of sciatic nerve express high densities of nerve growth factor (NGF) receptors. We propose that Schwann cells, when deprived of axonal contact, both express NGF receptors and produce NGF. This results in an """"""""NGF-laden substratum"""""""" which provides tropic guidance and trophic support to facilitate regeneration of NGF responsive neurons. We propose to characterize the phenomenon of NGF receptor induction on Schwann cells after nerve injury, to assess the physiological significance of this phenomenon, and to determine the mechanism(s) involved in regulation of NGF receptors on Schwann cells. We shall describe by receptor quantitation and by microscopic localization the time course of receptor induction after transection or crush of sciatic nerve to determine effects of reinnervation and to provide a precise anatomical description of NGF receptor interactions during these processes. We shall determine whether a similar induction of NGF receptor occurs after various lesions within the CNS and whether NGF receptor induction only occurs on glial cells previously associated with axons of NGF responsive neurons. We have found that NGF receptors appear in high numbers when Schwann cells are cultured. We shall define the time course of receptor induction and the kinetic properties of the receptors. Once the culture system is well defined, it will be used to examine the mechanisms by which axonal interaction modulates NGF receptor density on Schwann cells. We shall examine the possibility that the NGF can directly exert effects on Schwann cells. In parallel with these experiments, NGF protein levels (two-site immunoassay) and NGF mRNA (quantitative Northern blot analysis) will be determined under similar conditions. Sites of NGF biosynthesis will be determined by in situ hybridization. We shall thus be able to correlate the phenomena and mechanisms controlling NGF receptor expression and NGF synthesis in cultured Schwann cells and in injured nerve. We shall test the hypothesis that the NGF- laden substratum is important in sympathetic axonal regeneration by assessing the effects of systemic or local NGF deprivation on regeneration after sciatic nerve crush. Lastly, we shall examine the possibility that, during development, Schwann cells bear NGF receptors by measuring NGF receptors in nerve during development and by localizing by EM immunohistochemistry NGF receptors in developing nerve. This project will provide an assessment of NGF receptors on Schwann cells and their possible role in Schwann cell/axon interactions.
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