DIFFERENTIAL REGIONAL EXPRESSION AND ULTRASTRUCTURAL-LOCALIZATION OF ALPHA-ACTININ-2, A PUTATIVE NMDA RECEPTOR-ANCHORING PROTEIN, IN RAT-BRAIN

Fast chemical neurotransmission is dependent on ionotropic receptors t hat are concentrated and immobilized at specific postsynaptic sites. T he mechanisms of receptor clustering and anchoring in neuronal synapse s are poorly understood but presumably involve molecular linkage of me mbrane receptor proteins to the postsynaptic cytoskeleton. Recently th e actin-binding protein alpha-actinin-2 was shown to bind directly to the NMDA receptor subunits NR1 and NR2B (Wyszynski et al., 1997), sugg esting that alpha-actinin-2 may function to attach NMDA receptors to t he actin cytoskeleton. Here we show that alpha-actinin-2 is localized specifically in glutamatergic synapses in cultured hippocampal neurons . By immunogold electron microscopy, alpha-actinin-2 is concentrated o ver the postsynaptic density (PSD) of numerous asymmetric synapses whe re it colocalizes with NR1 immunoreactivity. Thus alpha-actinin-2 is a ppropriately positioned at the ultrastructural level to function as a postsynaptic-anchoring protein for NMDA receptors, alpha-Actinin-2 is not, however, exclusively found at the PSD; immunogold labeling was al so associated with filaments and the spine apparatus of dendritic spin es and with microtubules in dendritic shafts, alpha-Actinin-2 showed m arked differential regional expression in rat brain. For instance, the protein is expressed at much higher levels in dentate gyrus than in a rea CA1 of the hippocampus. This differential regional expression impl ies that glutamatergic synapses in various parts of the brain differ w ith respect to their alpha-actinin-2 content and thus, potentially, th e extent of possible interaction between alpha-actinin-2 and the NMDA receptor.