Amyloid ␤ (A␤) is involved in the etiology of Alzheimer's disease (AD) and may contribute to cognitive deficits by increasing internalization of ionotropic glutamate receptors. Striatal-enriched protein tyrosine phosphatase 61 (STEP 61 ), which is targeted in part to the postsynaptic terminal, has been implicated in this process. Here we show that STEP 61 levels are progressively increased in the cortex of Tg2576 mice over the first year, as well as in prefrontal cortex of human AD brains. The increased STEP 61 was associated with greater STEP activity, dephosphorylation of phospho-tyr 1472 of the NR2B subunit, and decreased NR1 and NR2B subunits on neuronal membranes. Treatment with A␤-enriched medium also increased STEP 61 levels and decreased NR1/NR2B abundance in mouse cortical cultures as determined by biotinylation experiments. In STEP knock-out cultures, A␤ treatment failed to induce NMDA receptor internalization. The mechanism for the increase in STEP 61 levels appears to involve the ubiquitin proteasome system. Blocking the proteasome resulted in elevated levels of STEP 61 . Moreover, STEP 61 -ubiquitin conjugates were increased in wild-type cortical slices upon A␤ treatment as well as in 12 month Tg2576 cortex. These findings reveal a novel mechanism by which A␤-mediated accumulation of STEP 61 results in increased internalization of NR1/NR2B receptor that may contribute to the cognitive deficits in AD.

Although it is well established that AMPA receptor (AMPAR) trafficking is a central event in several forms of synaptic plasticity, the mechanisms that regulate the surface expression of AMPARs are poorly understood. Previous work has shown that striatal-enriched protein tyrosine phosphatase (STEP) mediates NMDAR endocytosis. This protein tyrosine phosphatase is enriched in the synapses of the striatum, hippocampus, cerebral cortex, and other brain regions. In the present investigation, we have explored whether STEP also regulates AMPAR internalization. We found that (RS)-3,5-dihydroxyphenylglycine (DHPG) stimulation triggered a dose-dependent increase in STEP translation in hippocampal slices and synaptoneurosomes, a process that requires stimulation of mGluR5 (metabotropic glutamate receptor 5) and activation of mitogen-activated protein kinases and phosphoinositide-3 kinase pathways. DHPGinduced AMPAR internalization and tyrosine dephosphorylation of GluR2 (glutamate receptor 2) was blocked by a substrate-trapping TAT-STEP [C/S] protein in hippocampal slices and cultures. Moreover, DHPG-triggered AMPAR internalization was abolished in STEP knock-out mice and restored after replacement of wild-type STEP. These results suggest a role for STEP in the regulation of AMPAR trafficking.

Background-Formation of long-term memories is critically dependent on extracellular-regulated kinase (ERK) signaling. Activation of the ERK pathway by the sequential recruitment of mitogenactivated protein kinases is well understood. In contrast, the proteins that inactivate this pathway are not as well characterized.

STriatal Enriched protein tyrosine Phosphatase (STEP) is a brain-specific protein that is thought to play a role in synaptic plasticity. This hypothesis is based on previous findings demonstrating a role for STEP in the regulation of the extracellular signal-regulated kinase1/2 (ERK1/2). We have now generated a STEP knockout mouse and investigated the effect of knocking out STEP in the regulation of ERK1/2 activity. Here, we show that the STEP knockout mice are viable and fertile and have no detectable cytoarchitectural abnormalities in the brain. The homozygous knockout mice lack the expression of all STEP isoforms, whereas the heterozygous mice have reduced STEP protein levels when compared with the wild-type mice. The STEP knockout mice show enhanced phosphorylation of ERK1/2 in the striatum, CA2 region of the hippocampus, as well as central and lateral nuclei of the amygdala. In addition, the cultured neurons from KO mice showed significantly higher levels of pERK1/2 following synaptic stimulation when compared with wild-type controls. These data demonstrate more conclusively the role of STEP in the regulation of ERK1/2 activity.

STEP is a brain-specific phosphatase that opposes synaptic strengthening by the regulation of key synaptic signaling proteins. Previous studies suggest a possible role for STriatal-Enriched protein tyrosine Phosphatase (STEP) in learning and memory. To demonstrate the functional importance of STEP in learning and memory, we generated STEP knockout (KO) mice and examined the effect of deletion of STEP on behavioral performance, as well as the phosphorylation and expression of its substrates. Here we report that loss of STEP leads to significantly enhanced performance in hippocampal-dependent learning and memory tasks. In addition, STEP KO mice displayed greater dominance behavior, although they were normal in their motivation, motor coordination, visual acuity and social interactions. STEP KO mice displayed enhanced tyrosine phosphorylation of extracellular-signal regulated kinase 1/2 (ERK1/2), the NR2B subunit of the N-methyl-D-aspartate receptor (NMDAR), Proline-rich tyrosine kinase (Pyk2), as well as an increased phosphorylation of ERK1/2 substrates. Concomitant to the increased phosphorylation of NR2B, synaptosomal expression of NR1/NR2B NMDARs was increased in STEP KO mice, as was the GluR1/GluR2 containing α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptors (AMPAR), providing a potential molecular mechanism for the improved cognitive performance. The data support a role for STEP in the regulation of synaptic strengthening. The absence of STEP improves cognitive performance, and may do so by the regulation of downstream effectors necessary for synaptic transmission.