PSD-95 stabilizes NMDA receptors by inducing the degradation of STEP ₆₁

Abstract

Phosphorylation regulates surface and synaptic expression of NMDA receptors (NMDARs). Both the tyrosine kinase Fyn and the tyrosine phosphatase striatal-enriched protein tyrosine phosphatase (STEP) are known to target the NMDA receptor subunit GluN2B on tyrosine 1472, which is a critical residue that mediates NMDAR endocytosis. STEP reduces the surface expression of NMDARs by promoting dephosphorylation of GluN2B Y1472, whereas the synaptic scaffolding protein postsynaptic density protein 95 (PSD-95) stabilizes the surface expression of NMDARs. However, nothing is known about a potential functional interaction between STEP and PSD-95. We now report that STEP 61 binds to PSD-95 but not to other PSD-95 family members. We find that PSD-95 expression destabilizes STEP 61 via ubiquitination and degradation by the proteasome. Using subcellular fractionation, we detect low amounts of STEP 61 in the PSD fraction. However, STEP 61 expression in the PSD is increased upon knockdown of PSD-95 or in vivo as detected in PSD-95-KO mice, demonstrating that PSD-95 excludes STEP 61 from the PSD. Importantly, only extrasynaptic NMDAR expression and currents were increased upon STEP knockdown, as is consistent with low STEP 61 localization in the PSD. Our findings support a dual role for PSD-95 in stabilizing synaptic NMDARs by binding directly to GluN2B but also by promoting synaptic exclusion and degradation of the negative regulator STEP 61 .PSD-95 | NMDA receptor | STEP | ubiquitination N MDA receptors (NMDARs) are ionotropic glutamate receptors that are expressed throughout the nervous system and play crucial roles in neuronal development, synaptic plasticity, and learning and memory (1-4). Functional NMDARs are heterotetrameric complexes that are composed of homologous subunits (GluN1, GluN2A-D, and GluN3A-B). Two GluN1 subunits typically combine with two GluN2 subunits, which modulate channel activity and receptor properties (5-8). GluN2A and GluN2B are the predominant GluN2 subunits in hippocampus and cortex, and the subunit composition varies during neuronal development (9)(10)(11)(12). Therefore the precise regulation of NMDAR subunit expression, composition, trafficking, and localization is critical for proper neuronal function. NMDAR activity is dynamically regulated by protein phosphorylation, e.g. NMDAR currents are potentiated by tyrosine kinases and suppressed by tyrosine phosphatases (13). In fact, GluN2B is the most prominent tyrosine phosphorylated protein within postsynaptic densities (PSDs) (14), and phosphorylation is increased during long-term potentiation (LTP) in CA1 hippocampus (15). Moreover, tyrosine phosphorylation of GluN2B has been shown to increase in several pathological conditions, including ischemia and seizures (16)(17)(18)(19).Striatal-enriched protein tyrosine phosphatase (STEP, also known as "PTPN5") is a brain-specific protein phosphatase that is expressed in the striatum, hippocampus, and cortex (20, 21). The STEP family of protein tyrosine phosphatases includes both membrane-associated [str...

MAGUKs: multifaceted synaptic organizers

“…Current studies are now elucidating the functional differences between MAGUKs hinted at by this differential expression. Examples include SAP102’s unanticipated role in clearing NMDARs [23*], PSD-95’s role in triggering STEP degradation [29**], and the divergent roles of PSD-93 and PSD-95 in both Hebbian [52] and non-Hebbian plasticity [9]. Ongoing research will almost certainly continue to reveal the mechanisms underlying differential roles [53–55*] of the MAGUKs.…”

“…In addition to the many roles that MAGUKs play in receptor trafficking, a recent study demonstrates that PSD-95 acts in an unexpected way to regulate the expression of the tyrosine phosphatase PTPN5, also known as STEP ( ST riatal- E nriched protein tyrosine P hosphatase), in the PSD [29**]. STEP is known to regulate surface expression of NMDARs by dephosphorylating GluN2B Y1472 within the endocytic motif, thereby increasing internalization [30].…”

Alcohol‐dependent molecular adaptations of the NMDA receptor system

“…PSD-95 associates with Src PTKs (Kalia and Salter, 2003), and the phosphorylation of GluN2B enhances the binding of PSD-95 to the subunit (Rong et al, 2001). PSD-95 also associates with STEP and the association destabilizes STEP leading to its degradation (Won et al, 2016). Interestingly, PSD-95 KO consume less alcohol compared to WT (Camp et al, 2011), however, PSD-95 KO mice consume more alcohol compared to baseline after deprivation (Camp et al, 2011).…”

The STEP 61 interactome reveals subunit-specific AMPA receptor binding and synaptic regulation

“…All STEP splice variants (STEP 61 , STEP 46 , STEP 38 , and STEP 20 ) are highly expressed in striatum. In contrast, the fulllength gene product, STEP 61 , is the only variant expressed in cortex and hippocampus (26)(27)(28). The brain-specific tyrosine phosphatase STEP has been implicated in neuronal diseases (29) and neuropsychiatric disorders caused by its dysregulation (30)(31)(32)(33)(34).…”