Evidence suggests that parallel biochemical and regulatory processes occur during normal development and following various forms of central nervous system (CNS) injury. Among these, areas of particular interest are: (1) identification of novel neurotrophic factors; and (2) the analysis of the regulation of neurotrophic factor and neuropeptide gene expression during development and in response to injury. Since astrocytes can synthesize a number of neurotrophic factors, primary cultures of astrocytes are being used to determine factors which regulate production of trophic factors such as nerve growth factor (NGF) in response to 6-OHDA lesion of rat substantia nigra, a Parkinsonian-like model. Reactive astrocytes are prepared from 6-OHDA-lesioned brain: monoclonal antibodies raised against epitopes expressed only by reactive astrocytes in vivo distinguish between normal adult and reactive astrocytes in culture. These reactive astrocytes also express significantly more glial fibrillary acidic protein, 5-10013, and vimentin. Astrocytes from newborn animals more closely resemble reactive astrocytes in terms of expression of all of these markers. Both cytokines and (3-adrenergic agonists increase synthesis of NGF by these reactive astrocytes but have no effect in control astrocytes from normal adult brain. Cytokines, produced at high levels in injured brain, induce expression of neurotrophic factors as well as of nitric oxide synthase, which may be responsible for some of the neuronal damage. Depletion of the cytokine interleukin-3 (IL-3) peripherally by an antisense construct results in transgenic mice overexpressing IL-3 in the CNS. These mice develop a neurologic syndrome with a lesion resulting from migration/activation of microglia in the cerebellar peduncle and astrocyte expression of IL-3. The neuropeptide somatostatin, produced only by cerebellar astrocytes and only until the second week postnatally, stimulates both neurite outgrowth and phenotypic differentiation of cerebellar granule neurons in culture. Transgenic female mice overexpressing somatostatin in astrocytes are hyperactive, apparently due to an alteration of the dopamine system. Only the females show increased content of somatostatin, in cerebellum and striatum: the excess somatostatin alters normal developmental regulation of somatostatin receptor expression. Retinal pigment epithelium-derived factor (PEDF) not only functions as a survival factor for cerebellar granule cell neurons but also can protect them against both glutamate toxicity and apoptotic cell death. The protection involves modulation of changes in intracellular calcium. In addition, PEDF activates microglia, which produce an as yet unidentified factor which inhibits astrocyte proliferation and may thus be useful when brain injury results in gliosis due to astrocyte division. The factor also inhibits proliferation of gliomas.
NIH ExPorter
NIH ExPorter