Protein Kinase C Signaling in Learning and Memory

Abstract

It is a generally accepted concept that highly selective changes in the strength of synaptic connections between neurons in the brain contribute to learning and memory. Storage of information in the brain appears to involve persistent, use-dependent alteration in the efficacy of synaptic transmission. Approximately a decade ago, the first experimental data in support of a putative role of protein kinase C (PKC) in synaptic plasticity and information storage were reported. PKC is a cellular second messenger involved in various neuronal signal transduction pathways through phosphorylation of specific substrate proteins by which neurons increase their excitability in response to external inputs (1,2). Protein kinases phosphorylate many cellular proteins, catalyzing the transfer of phosphate to certain amino acid residues within proteins. Phosphorylation can alter the folding of the protein and, hence, their function. The discovery of a novel, cyclic nucleotide-independent, protein kinase took place relatively recently in the late 1970s (for a historical review on PKC in learning and memory, see ref. 3 and references therein). In 1977, co-workers from the Nishizuka group at Kobe University in Japan first reported to have found a new type of kinase, and 2 yr thereafter, it was referred to as PKC. It was demonstrated by this group that the kinase could be enzymatically fully active in the presence of Ca2+ and the phospholipid phosphatidylserine. The kinase is activated in a reversible manner by attachment to membrane phospholipid in the presence of Ca2+. Further analysis showed that a small amount of diacylglycerol (DAG; a minor component of the cellular lipids) significantly increases the affinity of this enzyme for Ca2+ and phospholipid. DAG (and inositoltrisphosphate) is produced by the hydrolysis of phosphatidylinositol bisphosphate (PI turnover). Interestingly, DAG permitted activation of PKC at resting intracellular Ca2+ levels. PKC is usually present in an inactive form in the cytosol. As a result of the specific binding of PKC by DAG, which is transiently formed in the membrane, activation of PKC is accompanied by its translocation from the cytosol to the membrane. The duration and magnitude of the DAG signal determines the activation of PKC at the cellular membrane.KeywordsSpatial LearningAssociative LearningPhorbol EsterEyeblink ConditioningHippocampal Pyramidal CellThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.