Neurotrophins represent a family of protein factors involved in the development, sculpting and maintenance of the nervous system. Nerve growth factor (NGF) is the first-isolated and to date most thoroughly characterized member of the neurotrophin family. Among the most significant biologic effects of NGF are promotion of the survival and neuronal differentiation of distinct elements of the nervous system. These include peripheral sympathetic and sensory neurons, as well as central cholinergic neurons of the basal forebrain, a region demonstrating prominent neural degeneration associated with the pathology of Alzheimer's Disease. Understanding the mechanisms of neurotrophic factor action could lead to treatments for a variety of neurodegenerative conditions. The principle model used in this laboratory to investigate NGF signaling is the NGF-responsive, PC12 cell culture line. PC12 cells expresses the high affinity NGF receptor, p140trkA, which is a transmembrane protein with tyrosine kinase activity. Upon addition of NGF, the tyrosine kinase activity of trkA is stimulated and summarily, previously round, replicating cells cease to divide becoming, phenotypically, sympathetic neurons replete with electrically excitable neuritic processes. Investigative studies conducted in this laboratory during the cur rent reporting period have examined 1) the molecular mechanisms of NGF signal transduction, specifically, involvement of protein tyrosine phosphorylation and activation of the Ras/Raf/MEK/MAPK cascade (J. Cellular Biochem. 66:229-244, 1997) 2) cross-talk between receptor subtypes and NGF-induced heterologous down- regulation of the mitogenic epidermal growth factor (EGF) receptor (J. Biol. Chem. 272:11026-11034)and 3) the effect of the glucocorticoid, dexamethasone (DEX) on the NGF-stimulated signal transduction. NGF stimulates activation of important signal transducing molecules including PLC and mitogen-activated protein kinase (MAP) through stimulation of protein kinase cascades that promote phosphorylation of peptides on tyrosine as well as serine and threonine amino acid residues. Tyrosine phosphorylation may be mediated directly by NGF-stimulated, p140 trkA tyrosine kinase activity, or indirectly through sequential activation of enzymes such as MAP kinase kinase (MEK) which possesses dual specificity phosphorylating activity. DEX has been shown to stimulate synthesis of NGF in the CNS which appears to be associated with enhanced activation of high affinity trkA NGF receptor molecules. In PC12 cells, DEX downregulates low affinity, p75 neurotrophin receptors. Recent studies with DEX demonstrate decreased activity of the nuclear transcription factor NFkB and an apparent enhancement of NGF-stimulated trkA tyrosine kinase activity. Downstream signaling events are diminished suggesting an uncoupling of receptor activation from propagation of molecular transducing signals.
