Ca2+ influx through NMDARs (N-methyl-D-aspartate receptors) triggers long-lasting changes; in synaptic efficacy such as LTP (long-term potentiation). The NMDAR-mediated rise in postsynaptic Ca2+ activates a network of kinases and phosphatases that promotes long-lasting changes in synaptic strength. Recent findings by our laboratories indicate that Ca2+ permeability of neuronal NMDARs and NMDAR-mediated Ca2+ influx during induction of LTP are under the control of the cAMP/PKA signaling cascade. Moreover, PKA modulates NMDAR-mediated Ca2+ rises in activated dendritic spines. Our data link PKA-dependent synaptic plasticity to Ca2+ signalling in spines and thus provide a novel mechanism whereby PKA regulates induction of LTP.
SPECIFIC AIMS are as follows: 1) To examine mechanisms underlying PKA modulation of Ca2+ permeability of synaptic (NR1/NR2A) vs. extrasynaptic (NR1/NR2B) NMDARs in hippocampal neurons. Although our data show that PKA selectively modulates Ca2+ permeability of synaptic NMDARs, the impact of PKA activation on extra-synaptic NMDARs is less clear. We hypothesize that PKA differentially modulates synaptic and extra-synaptic NMDARs. We further propose that extracellular signals that modulate cAMP or protein phosphatases at postsynaptic sites will bi-directionally regulate Ca2+ permeation through synaptic NMDARs and induction of LTP. 2) To examine developmental regulation of PKA actions on NMDAR-mediated Ca+ influx in dendritic spines. Although PKA modulates Ca2+ permeability of NMDARs in mature brain, the impact of PKA activation on synaptic (NR1/NR2B) NMDARs in neonatal brain is less clear. At birth, NMDARs contain NR1 and NR2B subunits. During postnatal development, there is a progressive inclusion of the NR2A subunit. Preliminary studies with recombinant NMDARs suggest that PKA differentially regulates NR2A- vs. NR2B-containing receptors. Thus, we hypothesize that PKA modulation of NMDAR-mediated currents is less selective at early ages and becomes highly selective for NMDAR-mediated Ca2+ influx at mature synapses. 3) To identify the molecular target of PKA that controls Ca2+ permeation through NMDARs. Although the NMDAR is a known functional target of PKA, the molecular target that control Ca2+ permeation through NMDARs is unknown. We hypothesize that PKA phosphorylates one or more residues on NR1 or NR2A/B subunit and thereby alters the geometry of the channel pore; when PKA is inactive, NMDARs are rapidly dephosphorylated. The cAMP/PKA signalling cascade and calcium are crucial for synaptic plasticity. Given the widespread localization of NMDARs and PKA in the CNS, regulation of NMDA function by PKA is a powerful mechanism to modulate synaptic efficacy. Ca2+ influx via NMDARs is also implicated in the excitotoxic neuronal death associated with stroke, epilepsy, head trauma, Huntington's disease, Alzheimer's disease, AIDS/dementia, and schizophrenia. Findings from the proposed studies are relevant to our understanding of these devastating and often fatal disorders.
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