We identified four PDZ domain-containing proteins, syntenin, PICK1, GRIP, and PSD95, as interactors with the kainate receptor (KAR) subunits GluR5(2b,) GluR5(2c), and GluR6. Of these, we show that both GRIP and PICK1 interactions are required to maintain KAR-mediated synaptic function at mossy fiber-CA3 synapses. In addition, PKC alpha can phosphorylate ct-GluR5(2b) at residues S880 and S886, and PKC activity is required to maintain KAR-mediated synaptic responses. We propose that PICK1 targets PKC alpha to phosphorylate KARs, causing their stabilization at the synapse by an interaction with GRIP. Importantly, this mechanism is not involved in the constitutive recycling of AMPA receptors since blockade of PDZ interactions can simultaneously increase AMPAR- and decrease KAR-mediated synaptic transmission at the same population of synapses.

α-Synuclein (α-Syn) is a key protein that accumulates as hyperphosphorylated aggregates in pathologic hallmark features of Parkinson’s disease (PD) and other neurodegenerative disorders. Phosphorylation of this protein at serine 129 is believed to promote its aggregation and neurotoxicity suggesting that this post-translational modification could be a therapeutic target. Here, we demonstrate that protein phosphatase 2A (PP2A) dephosphorylates α-Syn at serine 129, and that this activity is greatly enhanced by carboxyl methylation of the catalytic C subunit of PP2A. α-Syn transgenic mice raised on a diet supplemented with eicosinoyl-5-hydroxytryptamide (EHT), an agent that enhances PP2A methylation, dramatically reduced both α-Syn phosphorylation at Serine 129 and α-Syn aggregation in the brain. These biochemical changes were associated with enhanced neuronal activity, increased dendritic arborizations, reduced astroglial and microglial activation, as well as improved motor performance. These findings support the notion that serine 129 phosphorylation of α-Syn is of pathogenetic significance, and that promoting PP2A activity is a viable disease modifying therapeutic strategy for α-synucleinopathies such as PD.

The G-protein-coupled metabotropic glutamate receptor subtype 7a (mGluR7a) is a member of group III metabotropic glutamate receptors that plays an important role as a presynaptic receptor in regulating transmitter release at glutamatergic synapses. Here we report that the protein interacting with C-kinase (PICK1) binds to the C terminus (ct) of mGluR7a. In the yeast two-hybrid system, the extreme ct of mGluR7a was shown to interact with the PSD-95/Discs large/ZO-1 (PDZ) domain of PICK1. Pull-down assays indicated that PICK1 was retained by a glutathione S-transferase fusion of ct-mGluR7a. Furthermore, recombinant and native PICK1/mGluR7a complexes were coimmunoprecipitated from COS-7 cells and rat brain tissue, respectively. Confocal microscopy showed that both PICK1 and mGluR7a displayed synaptic colocalization in cultured hippocampal neurons. PICK1 has previously been shown to bind protein kinase C ␣-subunit (PKC␣), and mGluR7a is known to be phosphorylated by PKC. We show a relationship between these three proteins using recombinant PICK1, mGluR7, and PKC␣, where they were co-immunoprecipitated as a complex from COS-7 cells. In addition, PICK1 caused a reduction in PKC␣-evoked phosphorylation of mGluR7a in in vitro phosphorylation assays. These results suggest a role for PICK1 in modulating PKC␣-evoked phosphorylation of mGluR7a.

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Regulation of N-methyl-D-aspartate (NMDA) receptors is critical for the normal functioning of the central nervous system. There must be precise mechanisms to allow for changes in receptor function required for learning and normal synaptic transmission, but within tight constraints to prevent pathology. Tyrosine phosphorylation is a major means by which NMDA receptors are regulated through the equilibrium between activity of Src family kinases and tyrosine phosphatases. Identification of NMDA receptor phosphatases has been difficult, the best candidate being striatal-enriched tyrosine phosphatase (STEP). Here we demonstrate that STEP is a critical regulator of NMDA receptors and reveal that the action of this tyrosine phosphatase controls the constitutive trafficking of NMDA receptors and leads to changes in NMDA receptor activity at the neuronal surface. We show that STEP binds directly to NMDA receptors in the absence of other synaptic proteins. The activity of STEP selectively affects the expression of NMDA receptors at the neuronal plasma membrane. The result of STEP's action upon the NMDA receptor affects the functional properties of the receptor and its downstream signaling. These effects are evident when STEP levels are chronically reduced, indicating that there is no redundancy amongst phosphatases to compensate for altered STEP function in the CNS. STEP may have evolved specifically to fill a pivotal role as the NMDA receptor phosphatase, having a distinct and restricted localization and compartmentalization, and unique activity towards the NMDA receptor and its signaling pathway.

STriatal Enriched tyrosine Phosphatase (STEP) has recently been identified as a critical player in the regulation of synaptic function. It is highly restricted to neurons within the CNS and acts by downregulating the activity of the MAP kinases, the tyrosine kinase Fyn, and NMDA receptors. By modulating these substrates, STEP acts on several parallel pathways that impact upon the progression of synaptic plasticity. Recent advances have demonstrated the importance of STEP in normal cognitive function and its possible involvement in cognitive disorders, such as Alzheimer's disease.

␣-Amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor (AMPAR) stability and movement at synapses are important factors controlling synaptic strength. Here, we study the roles of proteins [N-ethylmaleimide-sensitive fusion protein (NSF), glutamate receptor AMPAR binding protein (ABP)-interacting protein (GRIP)͞(ABP), and protein interacting with C-kinase-1 (PICK1) that interact with the GluR2 subunit in the control of the surface expression and cycling of AMPARs. Epitope-tagged GluR2 formed functional receptors that exhibited targeting to synaptic sites. Constructs in which binding to NSF, PDZ proteins (GRIP͞ABP and PICK1), or GRIP͞ABP alone was eliminated each exhibited normal surface targeting and constitutive cycling. The lack of NSF binding, however, resulted in receptors that were endocytosed to a greater extent than wild-type receptors in response to application of AMPA or N-methyl-D-aspartate (NMDA). Conversely, the behavior of the GluR2 mutants incapable of binding to GRIP͞ABP suggests that these PDZ proteins play a role in the stabilization of an intracellular pool of AMPARs that have been internalized on stimulation, thus inhibiting their recycling to the synaptic membrane. These results provide further evidence for distinct functional roles of GluR2-interacting proteins in AMPAR trafficking.T he activity-dependent insertion and endocytosis of ␣-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptors (AMPARs) play an important role in modulating synaptic strength, such as occurs during certain forms of long-term potentiation (LTP; refs. 1 and 2) and long-term depression (LTD; ref.3). Thus, the trafficking of AMPARs must be subject to regulation at several steps allowing precise control of the number of AMPARs in the synaptic membrane. AMPARs are heteromeric assemblies of which GluR2 is a particularly important subunit in that it is a component of AMPARs expressed on principal (e.g., pyramidal) cells in adult brain (4) and controls their biophysical properties (5). GluR2 interacts with a number of intracellular proteins that have potential roles in regulating AMPAR trafficking (6-8), including N-ethylmaleimide sensitive fusion protein (NSF), a protein involved in membrane fusion events (9). Interfering with this interaction by using blocking peptides reduces AMPAR expression at synapses (10-13). It is unclear, however, whether this loss of AMPARs is due to a block of their surface delivery or because NSF stabilizes AMPARs in the plasma membrane and limits their endocytosis. The PDZ domain-containing proteins protein interacting with C-kinase-1 (PICK1), glutamate receptor-interacting protein (GRIP), and AMPAR binding protein (ABP), also interact with GluR2 (6-8) and have been suggested to play roles in the clustering and surface expression of AMPARs (14-16). Preventing the interaction of GluR2 with these proteins impairs LTD in both the hippocampus and cerebellum (17)(18)(19)(20). These studies, however, suggest conflicting roles for GRIP͞ABP and PICK1 in the induction and maintenance of LTD....

Alzheimer’s Disease (AD) is characterized by progressive loss of cognitive function, linked to marked neuronal loss. Pathological hallmarks of the disease are the accumulation of the amyloid-β (Aβ) peptide in the form of amyloid plaques and the intracellular formation of neurofibrillary tangles (NFTs). Accumulating evidence supports a key role for protein phosphorylation in both the normal and pathological actions of Aβ as well as the formation of NFTs. NFTs contain hyperphosphorylated forms of the microtubule-binding protein tau, and phosphorylation of tau by several different kinases leads to its aggregation. The protein kinases involved in the generation and/or actions of tau or Aβ are viable drug targets to prevent or alleviate AD pathology. However, it has also been recognized that the protein phosphatases that reverse the actions of these protein kinases are equally important. Here, we review recent advances in our understanding of serine/threonine and tyrosine protein phosphatases in the pathology of AD.