Expression of beta-nerve growth factor in cultured cells derived from the hypothalamus and cerebral cortex

Abstract

Although the synthesis of nerve growth factor (NGF) in brain regions innervated by magnocellular cholinergic neurons of the basal forebrain is well documented, the cell type(s) able to produce NGF in the central nervous system (CNS) remain only partially characterized. Moreover, little is known regarding the ability of brain areas not innervated by magnocellular cholinergic neurons to express NGF protein. The hypothalamus, which controls the endocrine system, is one of such regions. Primary culture of mixed populations of cells from the fetal hypothalamus were used to identify the presence of NGF in this brain area. Immunocytochemistry revealed that hypothalamic oligodendrocytes and a subpopulation of neurons expressed the NGF protein. In contrast, astrocytes were either immunonegative or equivocally stained. To define whether synthesis of NGF is restricted to a particular cell type, cultures of purified astrocytes, oligodendrocyte progenitor (oligoP) cells and neurons were utilized. They were obtained from the neonatal cerebral cortex to ensure an adequate yield of glial cells. Virtually the entire population of cerebral oligoP cells were found to express NGF protein. In contrast, and similar to hypothalamic astrocytes, cerebral type I astrocytes isolated at the same time as oligoP cells exhibited little or no NGF staining. When type I astrocytes were induced to differentiate in the presence of a serumless, chemically defined medium, a subpopulation of the culture became more robustly positive for the NGF protein. Contrasting with these differences in NGF immunoreactivity, Northern analysis of RNA isolated from purified cerebral type I astrocytes, oligoP cells and neurons demonstrated that NGF mRNA was expressed in each of these cell types at approximately the same levels. The results indicate that: (a) when placed in culture, each of the major cell types within the CNS has the capability of transcribing the NGF gene, and (b) despite similar NGF mRNA levels the cellular content of NGF protein is greater in a subpopulation of neurons and in oligodendrocytes than in astrocytes, suggesting differences in NGF post-transcriptional regulation between these cell types. In addition, the presence of NGF in hypothalamic cells suggests that NGF may be involved in the regulation of specific hypothalamic neuronal systems.