SUMMARY Many psychiatric and neurological disorders are characterized by learning and memory deficits, for which cognitive enhancement is considered a valid treatment strategy. The N-methyl-D-aspartate receptor (NMDAR) is a prime target for the development of cognitive enhancers due to its fundamental role in learning and memory. In particular, the NMDAR subunit NR2B improves synaptic plasticity and memory when over-expressed in neurons. However, NR2B regulation is not well understood and no therapies potentiating NMDAR function have been developed. Here, we show that serine 1116 of NR2B is phosphorylated by cyclin-dependent kinase 5 (Cdk5). Cdk5-dependent NR2B phosphorylation is regulated by neuronal activity and controls the receptor’s cell surface expression. Disrupting NR2B-Cdk5 interaction using a small interfering peptide (siP) increases NR2B surface levels, facilitates synaptic transmission, and improves memory formation in vivo. Our results reveal a novel regulatory mechanism critical to NR2B function that can be targeted for the development of cognitive enhancers.

NHE-RF, a regulatory cofactor for NHE (Na(+)-H(+) exchanger) type 3, interacts with ion transporters and receptors through its PDZ domains and with the MERM proteins (merlin, ezrin, radixin and moesin) via its carboxyl terminus. Thus, NHE-RF may act as a multifunctional adaptor protein and play a role in the assembly of signal transduction complexes, linking ion channels and receptors to the actin cytoskeleton. NHE-RF expression is up-regulated in response to estrogen in estrogen receptor-positive breast carcinoma cell lines, suggesting that it may be involved in estrogen signaling. To further understand NHE-RF function and its possible role in estrogen signaling, we analyzed NHE-RF expression in normal human tissues, including cycling endometrium, and in breast carcinomas, tissues in which estrogen plays an important role in regulating cell growth and proliferation. NHE-RF is expressed in many epithelia, especially in cells specialized in ion transport or absorption, and is often localized to apical (luminal) membranes. NHE-RF expression varies markedly in proliferative versus secretory endometrium, with high expression in proliferative (estrogen-stimulated) endometrium. Furthermore, estrogen receptor status and NHE-RF expression correlate closely in breast carcinoma specimens. These findings support a role for NHE-RF in estrogen signaling.

Merlin, the neurofibromatosis 2 tumor suppressor protein, has two major isoforms with alternate C termini and is related to the ERM (ezrin, radixin, moesin) proteins. Regulation of the ERMs involves intramolecular and/or intermolecular head-to-tail associations between family members. We have determined whether merlin undergoes similar interactions, and our findings indicate that the C terminus of merlin isoform 1 is able to associate with its N-terminal domain in a head-to-tail fashion. However, the C terminus of isoform 2 lacks this property. Similarly, the N terminus of merlin can also associate with C terminus of moesin. We have also explored the effect of merlin self-association on binding to the regulatory cofactor of Na Merlin is the tumor suppressor protein deficient in neurofibromatosis 2 (NF2), 1 a dominantly inherited disorder characterized by bilateral vestibular schwannomas and other brain tumors (1, 2). Merlin has a striking similarity in sequence and structure with ezrin, radixin, and moesin, commonly referred to as the ERM proteins. The ERM proteins share ϳ78% amino acid identity with each other, and all three are 45-47% identical to merlin (3). Like the ERM proteins and protein 4.1, merlin possesses a FERM (protein 4.1, ezrin, radixin, moesin) domain (ϳ270 amino acids defining membership in the protein 4.1 superfamily) in its N-terminal half, followed by a long ␣-helical segment and a charged C-terminal domain (4). The NF2 gene comprises 17 exons with alternative splicing of the penultimate exon producing two major merlin isoforms. Isoform 1 is a 595-amino acid protein produced from exons 1-15 and exon 17. Isoform 2 results from the presence of the alternatively spliced exon 16, which alters the C terminus of the protein to produce a 590-amino acid protein identical to isoform 1 over the first 579 residues (5-7). Mutational analysis has revealed a wide variety of mutations in the germline and tumors of NF2 patients as well as in sporadic schwannomas and meningiomas, confirming merlin's tumor suppressor function (8).ERM proteins act as linkers between integral membrane proteins and the actin cytoskeleton (9). Proteins identified as ligands for ERM proteins include CD44, CD43, ICAM1, ICAM2, and actin (9 -13). We and others have recently identified the human homologue of a regulatory cofactor for Na ϩ -H ϩ exchanger (NHE-RF) as a novel interactor for the conserved N terminus of merlin and ERM proteins (14,15). In addition to interacting with many binding partners, the ERM proteins are capable of forming homo-and heterotypic associations between family members (16, 17). Indeed, several recent studies performed on the regulation of ERM proteins suggest that the availability of ERM domains to binding partners is controlled by self-association of the N-terminal and C-terminal regions (13,18,19). Thus the ERM proteins can exist in the "closed" state, where the N-and C-terminal regions undergo an intramolecular interaction, masking the respective ligand-binding site. This closed state can be converted to...

Membrane transport proteins are directed toward and inserted into specific cell surface domains by an elaborate series of sorting mechanisms (1). A relatively recent development in understanding how some of these proteins are concentrated into functionally differentiated regions of the plasma membrane has been the discovery of the so-called PDZ domain family of proteins and associated PDZ-binding proteins. Named after the initial three members of the family (PSD-95, Drosophila discs large protein, and ZO-1), PDZ proteins contain 80 -