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METHYLMERCURY IMPAIRS NEURONAL DIFFERENTIATION BY ALTERING NEUROTROPHIN SIGNALING.
Citation:
Parran, D. K., S Barone, AND W R. Mundy. METHYLMERCURY IMPAIRS NEURONAL DIFFERENTIATION BY ALTERING NEUROTROPHIN SIGNALING. Presented at Society of Toxicology, Nashville, TN, March 17-21, 2002.
Description:
In previous in vivo studies, we observed that developmental exposure to CH3Hg can alter neocortical morphology and neurotrophin signaling. Using primed PC12 cells as a model system for neuronal differentiation, we examined the hypothesis that the developmental effects of CH3Hg may result from inhibition of neurotrophin signaling through the Trk receptor. Stimulation of PC12 cells with nerve growth factor (NGF) for 24 hr resulted in robust neurite outgrowth, which was inhibited by CH3Hg in a concentration-dependent manner (EC50=0.03 mM). Whole cell binding assays using 125I-NGF revealed a single binding site with Kd of ~1 nM. Exposure of PC12 cells to CH3Hg (0.001 - 3 mM) had no effects on NGF binding. Following NGF binding, TrkA receptor autophosphorylation peaked at 2.5 min of NGF stimulation and was sustained up to 60 min. Concurrent exposure to CH3Hg for 2.5 min resulted in a concentration-dependent decrease in TrkA autophosphorylation, which was significant at 0.1 mM CH3Hg (~50%). To determine whether inhibition of TrkA activation affected downstream signaling, activation of MAP kinase (ERK1/2) and Protein Kinase C (PKC) were examined. Similar to TrkA, NGF-stimulation resulted in a time-dependent activation of ERK1/2, which peaks at 5 min and was sustained up to 60 min. Concurrent exposure to CH3Hg for 2.5 min resulted in a concentration-dependent decrease in ERK1/2 activation, which was significant at 0.1 and 0.3 mM (~75%). While CH3Hg inhibited PKC activity for several recombinant PKC isoforms (d, e, z) and in PC12 cell homogenates, there was no effect on NGF-stimulated PKC activity in whole cells. To correlate the neurochemical and morphological effects, PC12 cells were exposed to specific inhibitors of TrkA (K252A) and MEK (U0126) at concentrations selected to produce 50-75% inhibition. Both K252A and U0126 significantly inhibited neurite outgrowth. These data suggest that the inhibition of neurite outgrowth by CH3Hg may be related to a disruption of the NGF signaling through the TrkA receptor.(This abstract does not reflect EPA policy)