The activity-dependent removal of AMPA receptors from synapses underlies long-term depression in hippocampal excitatory synapses. In this study, we have investigated the role of the small GTPase Rab5 during this process. We propose that Rab5 is a critical link between the signaling cascades triggered by LTD induction and the machinery that executes the activity-dependent removal of AMPA receptors. We have found that Rab5 activation drives the specific internalization of synaptic AMPA receptors in a clathrin-dependent manner and that this activity is required for LTD. Interestingly, Rab5 does not participate in the constitutive cycling of AMPA receptors. Rab5 is able to remove both GluR1 and GluR2 AMPA receptor subunits, leading to GluR1 dephosphorylation. Importantly, NMDA receptor-dependent LTD induction produces a rapid and transient increase of active (GTP bound) Rab5. We propose a model in which synaptic activity leads to Rab5 activation, which in turn drives the removal of AMPA receptors from synapses.

The delivery of neurotransmitter receptors into synapses is essential for synaptic function and plasticity. In particular, AMPA-type glutamate receptors (AMPA receptors) reach excitatory synapses according to two distinct routes: a regulated pathway, which operates transiently during synaptic plasticity, and a constitutive pathway, which maintains synaptic function under conditions of basal transmission. However, the specific mechanisms that distinguish these two trafficking pathways are essentially unknown. Here, we evaluate the role of the molecular chaperone hsp90 (heat shock protein 90) in excitatory synaptic transmission in the hippocampus. On one hand, we found that hsp90 is necessary for the efficient neurotransmitter release at the presynaptic terminal. In addition, we identified hsp90 as a critical component of the cellular machinery that delivers AMPA receptors into the postsynaptic membrane. Using the hsp90-specific inhibitors radicicol and geldanamycin, we show that hsp90 is required for the constitutive trafficking of AMPA receptors into synapses during their continuous cycling between synaptic and nonsynaptic sites. In contrast, hsp90 function is not required for either the surface delivery of AMPA receptors into the nonsynaptic plasma membrane or for the acute, regulated delivery of AMPA receptors into synapses during plasticity induction (long-term potentiation). The synaptic cycling of AMPA receptors was also blocked by an hsp90-binding tetratricopeptide repeat (TPR) domain, suggesting that the role of hsp90 in AMPA receptor trafficking is mediated by a TPR domaincontaining protein. These results demonstrate new roles for hsp90 in synaptic function by controlling neurotransmitter release and, independently, by mediating the continuous cycling of synaptic AMPA receptors.