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 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 prominently involved, patho logically, in neural degeneration associated with Alzheimer's Disease. Understanding the mechanisms of neurotrophic factor action could lead to treatments for a variety of neurodegenerative conditions. The principle model used 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 have examined 1) the biochemical characteristics of NGF-stimulated phospholipase C (PLC) phosphorylation, 2) the role of protein kinase C (PKC) in mediating NGF-elicited activation of phospholipase A2 (PLA), and 3) effect of the gluccorticoid, dexamethasone (DEX) on the NGF-stimulated signal transduction. Results from collaborative studies involving NGF-stimulated, PKC-mediated activation of PLA have been published in the J. of Neurochem., 66, 1868-1875, 1996. NGF stimulates activation of important signal transducing molecules including PLC and mitogen-activated pro-tein 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 has been shown to downregulate low affinity, p75 neurotrophin bounding receptors. Preliminary studies with dexamethasone demonstrate an apparent enhancement of NGF-stimulated trkA tyrosine activity, but downstream signaling appears diminished suggesting an uncoupling of receptor activation from propagation of transducing signals. This is a particulalry significant finding as it has been suggested that DEX might be used therapeutically to boost NGF levels in the CNS.
