Differential Regional Expression and Ultrastructural Localization of α-Actinin-2, a Putative NMDA Receptor-Anchoring Protein, in Rat Brain

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Abstract
Fast chemical neurotransmission is dependent on ionotropic receptors that are concentrated and immobilized at specific postsynaptic sites. The mechanisms of receptor clustering and anchoring in neuronal synapses are poorly understood but presumably involve molecular linkage of membrane receptor proteins to the postsynaptic cytoskeleton. Recently the actin-binding protein α-actinin-2 was shown to bind directly to the NMDA receptor subunits NR1 and NR2B (Wyszynski et al., 1997), suggesting that α-actinin-2 may function to attach NMDA receptors to the actin cytoskeleton. Here we show that α-actinin-2 is localized specifically in glutamatergic synapses in cultured hippocampal neurons. By immunogold electron microscopy, α-actinin-2 is concentrated over the postsynaptic density (PSD) of numerous asymmetric synapses where it colocalizes with NR1 immunoreactivity. Thus α-actinin-2 is appropriately positioned at the ultrastructural level to function as a postsynaptic-anchoring protein for NMDA receptors. α-Actinin-2 is not, however, exclusively found at the PSD; immunogold labeling was also associated with filaments and the spine apparatus of dendritic spines and with microtubules in dendritic shafts. α-Actinin-2 showed marked differential regional expression in rat brain. For instance, the protein is expressed at much higher levels in dentate gyrus than in area CA1 of the hippocampus. This differential regional expression implies that glutamatergic synapses in various parts of the brain differ with respect to their α-actinin-2 content and thus, potentially, the extent of possible interaction between α-actinin-2 and the NMDA receptor.