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. One novel neurotrophic factor under investigation is retinal pigment epithelium-derived factor (PEDF). 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. A lentiviral construct carrying the PEDF cDNA provides sufficient PEDF to infected cerebellar granule cells for both survival and anti-glutamate protection. PEDF's ability to block glutamate toxicity involves modulation of the changes in intracellular calcium caused by glutamate. Recent data support the idea that PEDF acts via a Gi-linked receptor, and that a decrease in cyclic AMP may mediate some of its actions. PEDF also 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. 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) and glial-derived neurotrophic factor (GDNF) 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, S-100-beta, and vimentin. Astrocytes from newborn animals more closely resemble reactive astrocytes in terms of expression of all of these markers. Both cytokines and beta-adrenergic agonists increase synthesis of NGF, but decrease synthesis of GDNF, by the reactive astrocytes, but have no effect on 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. Current studies are focussed on changes in extracellular matrix (ECM) molecules and cell adhesion molecules in response to injury. Although comparable differences are seen between what occurs in vivo and in the cultures of reactive astrocytes, the changes do not all parallel those of the classical markers of reactive gliosis. Neurons cultured on top of reactive astrocytes produce significantly longer and more fasciculated processes compared with those cultured on normal adult astrocytes, which presumably is a reflection of the differential expression of ECM molecules.
