Regulation of nerve growth factor signaling by protein phosphatase 2A

The goal of this dissertation research is to determine novel regulatory mechanisms of neurotrophin signaling mediated by protein phosphatase 2A (PP2A). PP2A is a ubiquitous Ser/Thr phosphatase that removes phosphates from proteins to switch their activity on or off. The substrate specificity and subcellular localization of PP2A is determined by almost 20 regulatory subunits that associate with a core dimer built of catalytic and scaffold subunits. Since there are more than 48 possible heterotrimers, studying the function of PP2A poses many challenges. Therefore we have devised a strategy, using scaffold subunit knockdown and mutant replacement, to discern the function of specific families of regulatory subunits. With this approach, I have identified specific PP2A holoenzymes that modulate nerve growth factor (NGF) signaling pathways by positively regulating TrkA receptor tyrosine kinase activity. Many studies have shown that NGF is required for the survival and differentiation of sensory and sympathetic neurons. Additionally, NGF is implicated in many neurodegenerative diseases including Alzheimer's disease, Parkinson's disease as well as neuropathic pain. NGF elicits its biological effect through sustained activity of the TrkA receptor and stimulated signaling cascades, including the MAP kinase pathway. Although PP2A has been shown to modulate the mitogen-activated protein (MAP) kinase pathway both positively and negatively at multiple levels, work described herein introduces yet another level of regulation. Specifically, I have shown that PP2A/B' holoenzymes complex with the TrkA neurotrophin receptor to potentiate receptor tyrosine kinase activity, downstream effector kinase activation, neurite outgrowth, and neuronal differentiation. On the other hand, extracellular signal regulated kinase (ERK), a terminal effector in the MAP kinase pathway was shown to phosphorylate a residue in the juxtamembrane region of TrkA and impose feedback inhibition of receptor activity. Collectively, these data suggest a model in which PP2A and ERK oppose each other in the regulation of TrkA receptor activity and downstream signaling cascades that govern neuronal differentiation and maintenance.