1.Glutamate Stimulates GRIP1 Degradation By Ubiquitin-proteasome System To Regulate Surface Expression Of GluR2 2.Requirement Of Dendritic Akt Degradation By Ubiquitin Proteasome System For Neuronal Polarity

The glutamate receptor interacting protein 1 (GRIP1) is a scaffolding protein in postsynaptic density (PSD), tethering AMPA receptors to other signaling proteins. Here we report that glutamate stimulation caused a rapid reduction in protein levels of GRIP1, but not that of GluR1, 2 and PICK1 in cultured neurons. Down-regulation of GRIP1 by glutamate was blocked by MG132, a proteasome inhibitor and by expression of K48R-ubiquitin, a dominant negative form of ubiquitin. The GRIP1 reduction was inhibited by MK801, an NMDA receptor antagonist, but not by CNQX, an AMPA receptor antagonist. EGTA and BAPTA, two Ca2+ chelators, but not nifedipine, an L-type Ca2+ channel blocker, prevented GRIP1 degradation. Furthermore, MG132 prevented glutamate-stimulated reduction in surface amount of GluR2, and knockdown of GRIP1 by RNAi against GRIP1 reduced surface GluR2 in neurons. Our results suggest that glutamate induces GRIP1 degradation by proteasome through an NMDA receptor-Ca2+ pathway and that GRIP1 degradation may play an important role in regulating GluR2 surface expression. Asymmetric distribution of activities of the protein kinases, Akt and glycogen synthase kinase 3β(GSK-3β), is critical for the formation of neuronal polarity. However, the mechanisms underlying polarized regulation of this pathway remain unclear. Here we report that the instability of Akt regulated by the ubiquitin-proteasome system (UPS) was required for neuron polarity. Preferential distribution in the axons was found for Akt, but not for its target, GSK-3β. A photoactivatable GFP fused to Akt revealed preferential instability of Akt in dendrites. Akt, but not p110 and GSK-3β, was ubiquitinated. Suppressing UPS led to symmetric distribution of Akt and the formation of multiple axons. These results indicate that local protein degradation mediated by UPS is important in determining neuronal polarity.