Endocytosis and recycling of AMPA receptors lacking GluR2/3.

Excitatory synapses in the mammalian brain contain two types of ligand-gated ion channels: AMPA receptors (AMPARs) and NMDA receptors (NMDARs). AMPARs are responsible for generating excitatory synaptic responses, whereas NMDAR activation triggers long-lasting changes in these responses by modulating the trafficking of AMPARs toward and away from synapses. AMPARs are tetramers composed of four subunits (GluR1-GluR4), which current models suggest govern distinct AMPAR trafficking behavior during synaptic plasticity. Here, we address the roles of GluR2 and GluR3 in controlling the recycling- and activity-dependent endocytosis of AMPARs by using cultured hippocampal neurons prepared from knockout (KO) mice lacking these subunits. We find that synapses and dendritic spines form normally in cells lacking GluR2/3 and that upon NMDAR activation, GluR2/3-lacking AMPARs are endocytosed in a manner indistinguishable from GluR2-containing AMPARs in wild-type (WT) neurons. AMPARs lacking GluR2/3 also recycle to the plasma membrane identically to WT AMPARs. However, because of their permeability to calcium, GluR2-lacking but not WT AMPARs exhibited robust internalization throughout the dendritic tree in response to AMPA application. Dendritic endocytosis of AMPARs also was observed in GABAergic neurons, which express a high proportion of GluR2-lacking AMPARs. These results demonstrate that GluR2 and GluR3 are not required for activity-dependent endocytosis of AMPARs and suggest that the most important property of GluR2 in the context of AMPAR trafficking may be its influence on calcium permeability.

Interactions between drebrin and Ras regulate dendritic spine plasticity.

Dendritic spines are major sites of morphological plasticity in the CNS, but the molecular mechanisms that regulate their dynamics remain poorly understood. Here we show that the association of drebrin with actin filaments plays a major role in regulating dendritic spine stability and plasticity. Overexpressing drebrin or the internal actin-binding site of drebrin in rat hippocampal neurons destabilized mature dendritic spines so that they lost synaptic contacts and came to resemble immature dendritic filopodia. Drebrin-induced spine destabilization was dependent on Ras activation: expression of constitutively active Ras destabilized spine morphology whereas drebrin-induced spine destabilization was rescued by co-expressing dominant negative Ras. Conversely, RNAi-mediated drebrin knockdown prevented Ras-induced destabilization and promoted spine maturation in developing neurons. Together these data demonstrate a novel mechanism in which the balance between stability and plasticity in dendritic spines depends on binding of drebrin to actin filaments in a manner that is regulated by Ras.

A calcineurin/AKAP complex is required for NMDA receptor-dependent long-term depression.

AKAP79/150 is a protein scaffold that is thought to position specific kinases (protein kinase A and C) and phosphatases (calcineurin) in appropriate synaptic domains so that their activities can regulate excitatory synaptic strength. Using a viral-mediated molecular replacement strategy in rat hippocampal slices, we found that AKAP is required for NMDA receptor-dependent long-term depression solely because of its interaction with calcineurin.

A critical role for PSD-95/AKAP interactions in endocytosis of synaptic AMPA receptors.

The endocytosis of AMPA receptors (AMPARs) underlies several forms of synaptic plasticity, including NMDA receptor (NMDAR)-dependent long-term depression (LTD), but the molecular mechanisms responsible for this trafficking remain unknown. We found that PSD-95, a major postsynaptic density protein, is important for NMDAR-triggered endocytosis of synaptic AMPARs in rat neuron cultures because of its binding to A kinase-anchoring protein 150 (AKAP150), a scaffold for specific protein kinases and phosphatases. Knockdown of PSD-95 with shRNA blocked NMDAR-triggered, but not constitutive or mGluR-triggered, endocytosis of AMPARs. Deletion of PSD-95's Src homology 3 and guanylate kinase-like domains, as well as a point mutation (L460P), both of which inhibit binding of PSD-95 to AKAP150, also blocked NMDAR-triggered AMPAR endocytosis. Furthermore, expression of a mutant AKAP150 that does not bind calcineurin inhibited this NMDAR-triggered trafficking event. Our results suggest that PSD-95's interaction with AKAP150 is critical for NMDAR-triggered AMPAR endocytosis and LTD, possibly because these scaffolds position calcineurin in the appropriate subsynaptic domain.

Transfecting Cultured Hippocampal Neurons with an Actin-GFP Plasmid.

INTRODUCTIONThis protocol describes two transfection methods for expressing GFP-tagged actin in primary neurons. The lipid reagent DOTAP (Roche Diagnostics) method produces actin-GFP-expressing hippocampal neurons that survive well during long periods in culture. The calcium phosphate method can be used to transfect neurons that have already been growing on coverslips in vitro. Transfected cells suitable for imaging can be obtained in cultures up to 15 days in vitro. One to two percent transfected cells is a typical result. A disadvantage of the calcium phosphate method is that hippocampal neurons become "fragile" after treatment.