Following activation of mGluRs GRIP stabilized AMPARs appear

Following activation of mGluRs, GRIP-stabilized AMPARs appear to be the primary target for endocytosis. Consistent with this model, we find that using siRNA to reduce GRIP1/2 expression blocks the AMPAR internalization and the synaptic depression mediated by mGluRs. Reductions in GRIP1/2 expression, however, do not interfere with the NMDAR-triggered reductions in surface GluA2s. Other studies suggest a role for GRIP1 in modulating AMPARs following their internalization by NMDAR activation by either maintaining internalized AMPARs (DeSouza et al., 2002) or in facilitating the reinsertion of the receptors (Mao et al., 2010). A recent study further indicates that the ATPase thorase can modulate AMPAR recycling though modulation of GRIP1/AMPAR interactions (Zhang et al., 2011). Our finding that NMDAR-mediated AMPAR internalization occurs despite reduced GRIP expression are not inconsistent with these results which identify a role for GRIP, not in the initial regulated TAK 165 sale of receptors, but rather in the stabilization of internalized receptors in intracellular pools. Our data does support a role for GRIP binding to AMPARs prior to LTD induction to maintain a population of surface AMPARs that can be released for endocytosis following mGluR activation. This model is consistent with that which has been proposed to occur in the cerebellum, whereby activation of the mGluR pathway mediates a dissociation of AMPARs from GRIP resulting in their enhanced endocytosis (Matsuda et al., 2000, Takamiya et al., 2008). Taken together, surface AMPARs bound to GRIP may be essential to mGluR-mediated receptor endocytosis, while AMPARs bound to GRIP after their endocytosis appear to be important for maintaining and regulating intracellular AMPAR pools following NMDAR-mediated endocytosis.
Experimental methods
Acknowledgments This work was supported by NIH/NINDSNS 049661. KGS was supported by NIH Training Grant 5T32NS007439. GU was supported by an Albert Einstein College of Medicine Neuroscience Fellowship.
Introduction AMPA receptors (AMPARs), like NMDA and kainate receptors, belong to the glutamate ion channel receptor family.1, 2, 3 AMPARs mediate the majority of fast excitatory neurotransmission in the CNS, and play crucial roles during neuronal development and synaptic plasticity.1, 2, 3 Excessive AMPAR activity has been implicated in various neurological diseases, such as amyotrophic lateral sclerosis (ALS), ischemia and epilepsy,1, 4, 5, 6 by a pathogenic mechanism known as excitotoxicity. Thus, blocking excessive AMPAR activity would be a promising therapeutic approach for the treatment of these diseases. In this context, the development of selective competitive and noncompetitive AMPAR antagonists is one of the main goals in neuropharmacology. There are a number of pharmacological agents that affect AMPAR function through interactions outside of the agonist-binding domain;7, 8, 9 these noncompetitive antagonists have the theoretical advantage to effectively counteract excitotoxicity even at high concentration of glutamate. Perampanel, a selective noncompetitive AMPAR antagonist, has recently gained FDA approval for clinical use in the treatment of partial-onset seizures and primary generalized tonic-clonic (PGTC) seizures.11, 12 Radioligand-binding studies suggest that the blocking site coincides with that of GYKI 52466, the prototype of 2,3-benzodiazepine noncompetitive AMPAR antagonists. More recently, the crystal structures of the rat AMPA-subtype GluA2 receptor in complex with three noncompetitive inhibitors have been reported. The inhibitors bind to a binding site, completely conserved between rat and human, at the interface between the ion channel and linkers connecting it to the ligand-binding domains. The authors propose that the inhibitors stabilize the AMPAR closed state by acting as wedges between the transmembrane segments, thereby preventing gating rearrangements that are necessary for ion channel opening.