This laboratory investigates the signal transduction in synaptic transmission and plasticity by biochemical, behavioral, and electrophysiological approaches using a genetically modified mice model. A strain of mutant mice was established by deleting the gene coding for a neural-specific protein, neurogranin (Ng). This protein is normally expressed at high levels in cerebral cortex, hippocampus, and amygdala, and has been implicated in the modulation of synaptic plasticity. Ng is a small molecular weight protein (78 amino acids) and its concentration in the neuronal soma and dendrites is much greater than CaM. This high level of Ng has sufficient capacity to sequester the entire CaM at basal physiological calcium concentration and releases it after calcium influx resulting from opening of ligand-gated or voltage-sensitive ion channels. Upon dissociation from CaM, Ng is phosphorylated by PKC and (or) oxidized by nitric oxide and other oxidants. The phosphorylated and oxidized Ngs are poor binding partners of CaM. These multifaceted regulations of Ng provide a fine tune mechanism to set the levels of free calcium and calcium-CaM depending on the strength of stimulation to the neurons and, thus, gate the output response. Because calcium and calcium-CaM regulate a myriad of signaling components leading to the modification of synaptic connection, it is not surprising that deletion of Ng gene in mice causes deficits in cognitive function and expression of certain abnormal behaviors.? ? Synaptic responses triggering long-term potentiation (LTP) or long-term depression (LTD) depend on the amplitude of calcium influx and the sensitivity of the transduction machinery to amplify the signal. Ng knockout (KO) mice performed poorly in cognitive function tasks and exhibited deficits in LTP and activation of CaMKII by autophosphorylation. Provision of short-term (3 weeks) environmental enrichment (SEE) for the mutant mice did not improve their performance, even though it was beneficial to the wild type and heterozygous mice, whose hippocampal Ng levels and LTP were elevated under SEE as compared to the control mice. Interestingly, for Ng KO mice, SEE caused a negligible effect on their LTP in spite of the fact that other important signaling components for synaptic plasticity, including CaMKII and cAMP responsive element-binding protein (CREB), were elevated to the same levels as the wild type and heterozygous mice. These findings suggest that Ng gates the neuronal signaling reactions involved in learning and memory and during EE those Ng-regulated reactions are critical for the enhancement of synaptic plasticity and cognitive functions. In contrast, a long-term environmental enrichment (LEE) for the aging mice was beneficial to Ng KO as well as wild type and heterozygous mice in preventing age-related cognitive decline. LEE also caused an increase in the hippocampal CREB level of all three genotypes and Ng level of wild type and heterozygous mice, but not that of CaMKII or ERK. Hippocampal slices of these enriched aging Ng KO mice, unlike those of wild type and heterozygous mice, didnt show enhancement in LTP. It appears that the learning and memory processes in these enriched aging Ng KO mice do not correlate with the LTP, which is facilitated by Ng. These results indicate that LEE for the aging Ng KO mice may improve their cognitive function through an Ng-independent plasticity pathway. ? ? Electrophysiological experiments showed that the tetanus-frequency response curve of Ng KO mice was shifted to the right compared to that of the wild type mice; low frequency stimulation (5-10 Hz) induced LTD in the former and modest LTP in the latter. Measurement of intracellular calcium induced by high-frequency stimulation (HFS) confirmed our hypothesis that Ng is involved in the potentiation of calcium-transients amplitude through a mass-action mechanism, which predicts that increasing Ng levels cann potentiate the synaptic responses by raising free calcium at any given calcium influx. An increase in calcium favors the activation of PKC-, cAMPPKA-, and calcium-CaM-regulated signaling pathways. Because Ng KO mice exhibit deficits in the HFS-induced LTP, which is mediated by the calcium influx through NMDA receptors, we investigated if direct stimulation of the down-stream signaling components of these pathways could rescue their deficits. Bath application of activator of PKC, phorbol 12,13-dibutyrate (PDBu), or cAMPPKA, forskolin, to the hippocampal slices was effective to induce LTP in the Ng KO mice, albeit, less effective than their wild type counterpart. Similarly, application of a histone deacetylase inhibitor, trichostatin A, could augment the HFS-induced LTP in Ng KO mice, likely resulting from chromatin remodeling-mediated gene transcription. These findings suggest that drugs that stimulate these signaling components will be beneficial to correct the behavioral deficits of the mutant mice.? ? Synaptic stimulation results in the generation of several oxidants, including nitric oxide, superoxide, and hydrogen peroxide, and these oxidants serve as messenger molecules under normal physiological conditions. However, during oxidative stress resulting from excessive synaptic stimulation associated with seizure, ischemia, and neurodegenerative diseases, high levels of these oxidants can cause cellular damage by modification of the various cellular components. Formation of protein disulfide or mixed disulfide is a common occurrence during oxidative stress. The aggregation of CaMKII in the hippocampal slices ischemia model was found to be, partly, due to the formation of intermolecular disulfide. The proximal mediators for this event appears to be glutathione disulfide S-oxides (GS-DSOs), which oxidize CaMKII to form large aggregates and a unique product not produced by other oxidants, including hydrogen peroxide, sodium nitroprusside, and diamide. These GS-DSOs can be synthesized by oxidation of glutathione with hydrogen peroxide in the presence of metal catalyst; however, the cellular mechanism for the formation of these compounds has yet to be elucidated. Ischemia causes oxidation of CaMKII and suppression of basal synaptic transmission in the hippocampal CA1 region. Although reperfusion with oxygenated fluid restored the basal synaptic transmission, the extent of HFS-induced LTP in the tissues undergo ischemia-reperfusion cycle was significant lower than that of the control without ischemia. These findings suggest that ischemia causes a deleterious effect on the signaling components for the enhancement of synaptic plasticity.
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