Specific localization of the annexin II heterotetramer in brain lipid raft fractions and its changes in spatial learning

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Abstract
Annexin-II (AII) is a Ca(2+)-dependent phospholipid-binding protein that is present in both intracellular and extracellular compartments. In the present study AII immunoreactivity was found in a subpopulation of neurons in specific brain regions, including the cerebral cortex and the surface of hippocampal pyramidal neurons from adult rats. AII from synaptic membranes was detected by immunoblotting as multiple species containing the monomer (AII36) and heterotetramer (AIIt). AIIt was resistant to beta-mercaptoethanol and dithiothreitol in sodium dodecyl sulfate-polyacrylamide gel electrophoresis, but was completely reduced to monomers (36 kDa) by two-dimensional electrophoresis. AIIt resided exclusively in the detergent-resistant lipid rafts concentrated in neuronal dendrites, and its recruitment to those structures was enhanced by antibody cross-link. AII abundantly distributed on the outer leaflet of neuronal membranes and between spaces of neurons appeared to be neuronal adhesive. The formation of AIIt required synthesis of sphingolipids and cholesterol, and its stability depended on Ca2+. Increases in neuronal activities such as depolarization and learning were shown to promote formation of AIIt. Our results suggest that, via a dynamic association with dendritic lipid rafts, AII may play a role in synaptic signal transduction and remodeling. This probably involves focal adhesion and interactions with actin that are associated with brain development and memory consolidation.