NMDA RECEPTORS AND CALCINEURIN IN AGING NEURONS

University Of Kansas Lawrence, Lawrence KS

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Abstract

Aging in brain is associated with increases in intracellular free Ca2+ ([Ca2+]i), diminished excitatory synaptic transmission, enhanced long- term depression of excitatory synaptic activity and enhanced formation of reactive oxygen species (ROS). Elevations in free [Ca2+]i have an indirect inhibitory effect on N-methyl-D-aspartate (NMDA) receptors, a class of glutamate receptors involved in synaptic transmission and memory formation. Small increases in [Ca2+]i also activate protein phosphatases, especially the Ca2+- and calmodulin-dependent phosphatase calcineurin (Cn). Protein phosphatases suppress synaptic excitation induced by glutamate action on NMDA receptors. Thus, increases in [Ca2+]i are a link between NMDA receptors, Cn activation and suppression of synaptic excitation. We found that there are possibly two molecular forms of NMDA receptors in neurons and that cysteine residues in one form of the receptors are modified by superoxide (O2.-) in vitro and by aging in vivo. In the proposed work, we plan to test the hypothesis that aging in the nervous system is associated with diminution of function of a select population of NMDA receptors, resulting from either oxidative modification or protein dephosphorylation, and with a parallel dominance of phosphatase over kinase activities. We proposed to study the function of two populations of NMDA receptors in neurons that are allowed to age in primary cultures or in neurons obtained from young, adult and senescent rats (Fischer 344/BN F1). We also intend to study the oxidation and phosphorylation of select receptor protein subunits in the brain during the aging process and determine the relative levels and activity of phosphatases and kinases in nerve ending particles from brains of young, adult and senescent rats. We will measure the conductance properties of NMDA-activated ion channels that represent receptor complex composed of NR1/NR2 subunits and of the binding proteins GBP, GlyBP, CPP-BP, and TCP-BP. We will assess the extent of oxidative modification of NR1 and GBP and measure the activities and levels of the phosphatases, PP1 and Cn, and the kinases, PKC and Ca2+/calmodulin kinase II (CaMKII). The studies described above may link changes in signal transduction during aging with oxidative modification and/or altered phosphorylation of different populations of NMDA receptors, and with Ca2+ accumulation in neurons. As new therapeutic agents are developed to control select populations of NMDA receptors or phosphatases and kinases, new therapeutic interventions may be devised to prevent the losses in memory formation during aging.