How is protein kinase C activated in CNS.
Protein kinase C (PKC) enzyme family consists of the Ca(2+)-dependent and -independent subgroups of phospholipid/diacylglycerol (DAG)-stimulated serine/threonine protein kinases. These enzymes exhibit distinct cellular and subcellular localizations in CNS and subtle differences in their biochemical characteristics and substrate specificities. It is believed that each of these isoenzymes respond differently to different input signals. However, detailed mechanism for the functioning of these enzymes in vivo is largely unknown; this is in part due to the absence of specific activator, inhibitor, or substrate for each of these enzymes. Recent advances in biochemical, biophysical, and molecular characterizations have defined certain structural features important to confer the stimulatory responses of these enzymes to Ca2+, DAG or phorbol ester, and Zn2+; other features important for the binding of anionic phospholipids, Ca2+/phospholipid complexes, and cis-unsaturated fatty acids have not yet been characterized. Activation of PKC requires the increase in [Ca2+]i and DAG and/or cis-unsaturated fatty acids. Ca2+ promotes the interactions of the Ca(2+)-dependent subgroup of PKCs with membrane phosphatidylserine (PS) and the enzymes become partially active when simultaneously associated with phosphatidylinositol 4,5-bisphosphate or fully active when DAG is available. Free fatty acids such as arachidonic acid, generated by the activation of phospholipase A2, could synergize with DAG to activate the enzyme maximally. The Ca(2+)-independent subgroup of PKCs also become active when associated with PS at elevated level of DAG. Sustained activation of PKCs leads to the conversion of these enzymes into membrane-inserted and membrane protein-associated forms, which may be responsible for certain long-term neural responses. Activation of PKC results in the phosphorylation of cellular proteins; among them, several calmodulin (CaM)-binding proteins are the prominent substrates of these kinases. Phosphorylation of these proteins by PKC favors the release of CaM, which is required for the Ca2+/CaM-dependent enzymes. Thus, activation of PKCs can lead to diverse cellular responses through such amplification steps. Future studies should be directed at the elucidation of the activation of each PKC isoform in vivo to correlate with the physiological responses.