Dileucine and PDZ-binding motifs mediate synaptic adhesion-like molecule 1 (SALM1) trafficking in hippocampal neurons.

Synaptic adhesion-like molecules (SALMs) are a family of cell adhesion molecules involved in neurite outgrowth and synapse formation. Of the five family members, only SALM1, -2, and -3 contain a cytoplasmic C-terminal PDZ-binding motif. We have found that SALM1 is unique among the SALMs because deletion of its PDZ-binding motif (SALM1ΔPDZ) blocks its surface expression in heterologous cells. When expressed in hippocampal neurons, SALM1ΔPDZ had decreased surface expression in dendrites and the cell soma but not in axons, suggesting that the PDZ-binding domain may influence cellular trafficking of SALMs to specific neuronal locations. Endoglycosidase H digestion assays indicated that SALM1ΔPDZ is retained in the endoplasmic reticulum (ER) in heterologous cells. However, when the entire C-terminal tail of SALM1 was deleted, SALM1 was detected on the cell surface. Using serial deletions, we identified a region of SALM1 that contains a putative dileucine ER retention motif, which is not present in the other SALMs. Mutation of this DXXXLL motif allowed SALM1 to leave the ER and enhanced its surface expression in heterologous cells and neurons. An increase in the number of protrusions at the dendrites and cell body was observed when this SALM1 mutant was expressed in hippocampal neurons. With electron microscopy, these protrusions appeared to be irregular, enlarged spines and filopodia. Thus, enrichment of SALM1 on the cell surface affects dendritic arborization, and intracellular motifs regulate its dendritic versus axonal localization.

Functional NMDA receptors at axonal growth cones of young hippocampal neurons.

NMDA receptors (NMDARs) are critical to the development of the nervous system, although their roles at axonal growth cones are unclear. We examined NMDAR localization and function at axonal growth cones of young hippocampal neurons. Our immunocytochemical data showed that native and transfected NMDAR subunits are expressed in axons and growth cones of young (days in vitro 3-6) hippocampal rat neurons. Moreover, immunogold electron microscopy showed that NR1 is expressed in growth cones of postnatal day 2 rat hippocampus. Local application of NMDAR agonists to growth cones of voltage-clamped neurons evoked inward currents that were blocked by bath application of an NMDAR antagonist (dl-APV), indicating that these NMDARs are functional. In addition, calcium imaging experiments indicated that NMDARs present in growth cones mediate calcium influx. Calcium transients in growth cones persisted despite pharmacological blockade of voltage-sensitive calcium channels and depletion of intracellular calcium stores. Our findings reveal the presence of functional NMDARs in axons and growth cones of young neurons, suggesting a role for these receptors in axonal guidance and synapse formation during neuronal development.

mPins modulates PSD-95 and SAP102 trafficking and influences NMDA receptor surface expression.

Appropriate trafficking and targeting of glutamate receptors (GluRs) to the postsynaptic density is crucial for synaptic function. We show that mPins (mammalian homologue of Drosophila melanogaster partner of inscuteable) interacts with SAP102 and PSD-95 (two PDZ proteins present in neurons), and functions in the formation of the NMDAR-MAGUK (N-methyl-D-aspartate receptor-membrane-associated guanylate kinase) complex. mPins enhances trafficking of SAP102 and NMDARs to the plasma membrane in neurons. Expression of dominant-negative constructs and short-interfering RNA (siRNA)-mediated knockdown of mPins decreases SAP102 in dendrites and modifies surface expression of NMDARs. mPins changes the number and morphology of dendritic spines and these effects depend on its Galphai interaction domain, thus implicating G-protein signalling in the regulation of postsynaptic structure and trafficking of GluRs.

SAP102 is a highly mobile MAGUK in spines.

Membrane-associated guanylate kinases (MAGUKs), which are essential proteins in the postsynaptic density (PSD), cluster and anchor glutamate receptors and other proteins at synapses. The MAGUK family includes PSD-95, PSD-93, SAP102, and SAP97. Individual family members can compensate for one another in their ability to recruit and retain receptors at the postsynaptic membrane as shown through deletion and knock-down studies. SAP102 is highly expressed in both young and mature neurons; however, little is known about its localization and mobility at synapses. Here, we compared the distribution, mobility, and turnover times of SAP102 to the well studied MAGUK PSD-95. Using light and electron microscopy, we found that SAP102 shows a broader distribution as well as peak localization further away from the postsynaptic membrane than PSD-95. Using fluorescence recovery after photobleaching (FRAP), we found that 80% of SAP102 and 36% of PSD-95 are mobile in spines. Previous studies showed that PSD-95 was stabilized at the PSD by N-terminal palmitoylation. We found that stabilization of SAP102 at the PSD was dependent on its SH3/GK domains but not its PDZ interactions. Furthermore, we showed that stabilizing actin or blocking NMDA/AMPA receptors reduced the mobile pool of SAP102 but did not affect the mobile pool of PSD-95. Our results show significant differences in the localization, binding mechanism, and mobility of SAP102 and PSD-95. These differences and the compensatory properties of the MAGUKs point out an unrecognized versatility of the MAGUKs in their function in synaptic organization and plasticity.

NMDA receptor trafficking through an interaction between PDZ proteins and the exocyst complex.

NMDA (N-methyl-D-aspartate) receptors (NMDARs) are targeted to dendrites and anchored at the post-synaptic density (PSD) through interactions with PDZ proteins. However, little is known about how these receptors are sorted from the endoplasmic reticulum and Golgi apparatus to the synapse. Here, we find that synapse-associated protein 102 (SAP102) interacts with the PDZ-binding domain of Sec8, a member of the exocyst complex. Our results show that interactions between SAP102 and Sec8 are involved in the delivery of NMDARs to the cell surface in heterologous cells and neurons. Furthermore, they suggest that an exocyst-SAP102-NMDAR complex is an important component of NMDAR trafficking.

Flotillin-1 mediates neurite branching induced by synaptic adhesion-like molecule 4 in hippocampal neurons.

Proper development of neurons in the hippocampus is essential for learning and memory. Our laboratory previously discovered a family of synaptic adhesion-like molecules (SALMs) which induce neurite outgrowth in this brain region (Wang et al., 2006). Here we establish flotillin-1 (flot-1) as a molecular mediator of neurite branching for SALM4. Knockdown of flot-1 alone in cultured hippocampal neurons using siRNA from 3-7days in vitro (DIV) impaired neurite branching, whereas overexpression of flot-1 during the same time period increased the number of processes and branching. We show that induction of neurite outgrowth by flot-1 depends on amino acids 134-151 as well as lipid raft microdomains, SoHo proteins to regulate the actin cytoskeleton, and the exocyst complex to deliver new membrane proteins to growing neurites. When each of SALMs 1-5 was overexpressed, siRNA knockdown of flot-1 prevented neurite branching by SALM4. Overall, our data reveal a flot-1 signaling pathway for hippocampal neurite branching that is regulated by SALM4.

Reticulon 3 is an interacting partner of the SALM family of adhesion molecules.

Synaptic adhesion-like molecules (SALMs) are a recently discovered family of adhesion molecules that is widely distributed in the central nervous system and has been implicated in neurite outgrowth and synapse formation. To identify proteins that interact with extracellular domains of SALMs, we carried out yeast two-hybrid screening using the extracellular domain of SALM1 as bait. A clone encoding full-length reticulon 3A1 was isolated. This interaction was shown to occur through the LRR domain, which is found on all SALMs. To determine whether this relationship also occurs in brain, we performed immunoprecipitation using antibodies to SALMs 1-4. A 19-kDa band, identified as reticulon 3C, bound to all four SALMs, whereas a 90-kDa band, which did not comigrate with any known reticulon 3 variant, bound to SALMs 2 and 3. These results show that reticulon 3 may play a role in the trafficking of the SALM family of adhesion molecules.

The synaptic localization of NR2B-containing NMDA receptors is controlled by interactions with PDZ proteins and AP-2.

The NMDA receptor (NMDAR) is a component of excitatory synapses and a key participant in synaptic plasticity. We investigated the role of two domains in the C terminus of the NR2B subunit--the PDZ binding domain and the clathrin adaptor protein (AP-2) binding motif--in the synaptic localization of NMDA receptors. NR2B subunits lacking functional PDZ binding are excluded from the synapse. Mutations in the AP-2 binding motif, YEKL, significantly increase the number of synaptic receptors and allow the synaptic localization of NR2B subunits lacking PDZ binding. Peptides corresponding to YEKL increase the synaptic response within minutes. In contrast, the NR2A subunit localizes to the synapse in the absence of PDZ binding and is not altered by mutations in its motif corresponding to YEKL of NR2B. This study identifies a dynamic regulation of synaptic NR2B-containing NMDARs through PDZ protein-mediated stabilization and AP-2-mediated internalization that is modulated by phosphorylation by Fyn kinase.

Masking of the endoplasmic reticulum retention signals during assembly of the NMDA receptor.

NMDA receptors are glutamate-gated ion channels that play important roles in synaptic transmission and excitotoxicity. The functional NMDA receptor is thought to be a heterotetramer composed mainly of two NR1 and two NR2 subunits. Although it is generally accepted that only correctly assembled NMDA receptors can pass the ER quality control, the mechanism underlying this process is not well understood. Using truncated and chimeric NMDA receptor subunits expressed in heterologous cells and cortical neurons, we found that the third membrane domains (M3) of both NR1 and NR2B contain signals that cause the unassembled subunits to be retained in the ER. M3 of both NR1 and NR2B and, M4 of NR1, are necessary for masking ER retention signals found in M3. Thus, our data reveal a critical role of the membrane domains in the assembly of functional NMDA receptors.

Functional studies and distribution define a family of transmembrane AMPA receptor regulatory proteins.

Functional expression of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors in cerebellar granule cells requires stargazin, a member of a large family of four-pass transmembrane proteins. Here, we define a family of transmembrane AMPA receptor regulatory proteins (TARPs), which comprise stargazin, gamma-3, gamma-4, and gamma-8, but not related proteins, that mediate surface expression of AMPA receptors. TARPs exhibit discrete and complementary patterns of expression in both neurons and glia in the developing and mature central nervous system. In brain regions that express multiple isoforms, such as cerebral cortex, TARP-AMPA receptor complexes are strictly segregated, suggesting distinct roles for TARP isoforms. TARPs interact with AMPA receptors at the postsynaptic density, and surface expression of mature AMPA receptors requires a TARP. These studies indicate a general role for TARPs in controlling synaptic AMPA receptors throughout the central nervous system.

Synaptic adhesion-like molecules (SALMs) promote neurite outgrowth.

SALMs are a family of five adhesion molecules whose expression is largely restricted to the CNS. Initial reports showed that SALM1 functions in neurite outgrowth while SALM2 is involved in synapse formation. To investigate the function of SALMs in detail, we asked if all five are involved in neurite outgrowth. Expression of epitope-tagged proteins in cultured hippocampal neurons showed that SALMs are distributed throughout neurons, including axons, dendrites, and growth cones. Over-expression of each SALM resulted in enhanced neurite outgrowth, but with different phenotypes. Neurite outgrowth could be reduced by applying antibodies targeting the extracellular leucine rich regions of SALMs and with RNAi. Through over-expression of deletion constructs, we found that the C-terminal PDZ binding domains of SALMs 1-3 are required for most aspects of neurite outgrowth. In addition, by using a chimera of SALMs 2 and 4, we found that the N-terminus is also involved in neurite outgrowth.

Narp and NP1 form heterocomplexes that function in developmental and activity-dependent synaptic plasticity.

Narp is a neuronal immediate early gene that plays a role in excitatory synaptogenesis. Here, we report that native Narp in brain is part of a pentraxin complex that includes NP1. These proteins are covalently linked by disulfide bonds into highly organized complexes, and their relative ratio in the complex is dynamically dependent upon the neuron's activity history and developmental stage. Complex formation is dependent on their distinct N-terminal coiled-coil domains, while their closely homologous C-terminal pentraxin domains mediate association with AMPA-type glutamate receptors. Narp is substantially more effective in assays of cell surface cluster formation, coclustering of AMPA receptors, and excitatory synaptogenesis, yet their combined expression results in supraadditive effects. These studies support a model in which Narp can regulate the latent synaptogenic activity of NP1 by forming mixed pentraxin assemblies. This mechanism appears to contribute to both activity-independent and activity-dependent excitatory synaptogenesis.

NMDA di-heteromeric receptor populations and associated proteins in rat hippocampus.

Subunit composition of NMDA receptors (NMDARs) determines a range of physiological properties, downstream signaling effects, and binding partners. Differential localization of NR2A- or NR2B-containing NMDARs within the neuron and subunit-specific protein associations may explain differences in NR2A and NR2B contributions to synaptic plasticity and excitotoxic cell death. This question is complicated by the existence of tri-heteromeric complexes (NR1/NR2A/NR2B). To date, no quantitative biochemical determinations have been made of the relative abundance of different NMDAR populations in intact hippocampus, the region extensively correlated with NMDAR-dependent long-term potentiation. We investigated subunit composition and subunit-specific interactions in CA1/CA2 of rat hippocampus. Using sequential immunoprecipitations to deplete either NR2B or NR2A, di-heteromeric NR1/NR2A and NR1/NR2B receptor populations were isolated from postnatal day 7 (P7) hippocampus and P42 and 6-month-old CA1/CA2. Quantitative Western blot analysis revealed that 60-70% of NR2A and 70-85% of NR2B subunits were associated in NR1/NR2A or NR1/NR2B di-heteromeric complexes. Isolated di-heteromeric receptor fractions were used to examine NR2A- or NR2B-specific interactions with synapse-associated proteins. Our results indicate that NR2A- or NR2B-containing NMDARs associate similarly with postsynaptic density-95 (PSD-95), synapse-associated protein 102, and PSD-93 at P42. However, NR2A-containing receptors coimmunoprecipitated a greater proportion of the synaptic proteins neuronal nitric oxide synthase, Homer, and beta-catenin. Finally, mass spectrometry analysis of isolated di-heteromeric receptors identified a novel NMDAR interactor, collapsin response mediator protein 2, which preferentially associates with NR2B-containing di-heteromeric NMDARs. In summary, in rat hippocampus, NR2A and NR2B exist primarily in di-heteromeric complexes that interact similarly with PSD-95-related proteins but are associated with different protein complexes.

The role of the PDZ protein GIPC in regulating NMDA receptor trafficking.

The NMDA receptor is an important component of excitatory synapses in the CNS. In addition to its synaptic localization, the NMDA receptor is also present at extrasynaptic sites where it may have functions distinct from those at the synapse. Little is known about how the number, composition, and localization of extrasynaptic receptors are regulated. We identified a novel NMDA receptor-interacting protein, GIPC (GAIP-interacting protein, C terminus), that associates with surface as well as internalized NMDA receptors when expressed in heterologous cells. In neurons, GIPC colocalizes with a population of NMDA receptors on the cell surface, and changes in GIPC expression alter the number of surface receptors. GIPC is mainly excluded from the synapse, and changes in GIPC expression do not change the total number of synaptic receptors. Our results suggest that GIPC may be preferentially associated with extrasynaptic NMDA receptors and may play a role in the organization and trafficking of this population of receptors.

Rapid turnover of stereocilia membrane proteins: evidence from the trafficking and mobility of plasma membrane Ca(2+)-ATPase 2.

We studied the spatial distribution, mobility, and trafficking of plasma membrane Ca2+ATPase-2 (PMCA2), a protein enriched in the hair cell apical membrane and essential for hair cell function. Using immunofluorescence, we determined that PMCA2 is enriched in the stereocilia and present at a relatively low concentration in the kinocilium and in the remaining apical membrane. Using an antibody to the extracellular domain of PMCA2 as a probe, we observed that PMCA2 diffuses laterally from the stereocilia membrane and is internalized at the apical cell border maintaining an estimated half-life of residency in the stereocilia of approximately 5-7 h. A computer simulation of our data indicates that PMCA2 has an estimated global diffusion coefficient of 0.01-0.005 microm2/s. Using a green fluorescent protein tag, we observed that PMCA2 is rapidly delivered to the apical cell border from where it diffuses to the entire stereocilia surface. Fluorescence recovery after photobleaching experiments show that approximately 60% of PMCA2 in the stereocilia exhibit high mobility with a diffusion coefficient of 0.1-0.2 microm2/s, whereas the remaining pool represents a relatively immobile fraction. These results suggest that PMCA2 molecules maintain transient interactions with other components of the stereocilia, and the mobile pool of PMCA2 mediates the exchange between the stereocilia and the removal and delivery sites at the periphery of the apical cell surface. This rapid turnover of a major stereocilia membrane protein matches the previously described rapid turnover of proteins of the stereocilia actin core, further demonstrating that these organelles undergo rapid continuous renewal.

A novel family of adhesion-like molecules that interacts with the NMDA receptor.

We have identified a novel family of synaptic adhesion-like molecules (SALMs). The family members, SALM1-SALM4, have a single transmembrane (TM) domain and contain extracellular leucine-rich repeats, an Ig C2 type domain, a fibronectin type III domain, and an intracellular postsynaptic density-95 (PSD-95)/Discs large/zona occludens-1 (PDZ) binding domain, which is present on all members except SALM4. SALM1 interacts with PSD-95, synapse-associated protein 102 (SAP102), and SAP97 based on coimmunoprecipitation of detergent-solubilized brain. Distribution studies show that SALM1 is present in synaptic membrane and postsynaptic density fractions but is also distributed in axons and dendrites. Transfection of hippocampal neurons for 4 d in vitro (DIV) with SALM1 more than doubles the dendritic lengths of neurons after 48 h, whereas transfection of neurons 14 DIV has no significant effect on neurite outgrowth. Overexpression of SALM1 in 14 DIV neurons recruits NMDA receptors (NR) and PSD-95 to dendritic puncta. This effect is dependent on the PDZ-binding domain of SALM1. SALM1 also enhances surface expression of transfected NR2A subunit. Immunoprecipitation of detergent-solubilized brain membranes with anti-SALM1 antibodies shows coimmunoprecipitation of NR1 and NR2 subunits. After transfection of heterologous cells with NR1 and NR2 cDNAs, through coimmunoprecipitation analyses, we find that SALM1 also interacts with the NMDA receptor NR1 subunit through its extracellular or TM1 domains.

Asymmetric localization of Vangl2 and Fz3 indicate novel mechanisms for planar cell polarity in mammals.

Planar cell polarity (PCP) is a process in which cells develop with uniform orientation within the plane of an epithelium. To begin to elucidate the mechanisms of PCP in vertebrates, the localization of the protein Vangl2 (Van Gogh-like) was determined during the development of the mammalian cochlea. Results indicate that Vangl2 becomes asymmetrically localized to specific cell-cell boundaries along the axis of polarization and that this asymmetry is lost in PCP mutants. In addition, PDZ2 (postsynaptic density/Discs large/zona occludens 1), PDZ3, and PDZ4 of the PCP protein Scrb1 (Scribble) are shown to bind to the C-terminal PDZ binding domain of Vangl2, suggesting that Scrb1 plays a direct role in asymmetric targeting of Vangl2. Finally, Fz3 (Frizzled), a newly demonstrated mediator of PCP, is also asymmetrically localized in a pattern that matches that of Vangl2. The presence and asymmetry of Fz3 at the membrane is shown to be dependent on Vangl2. This result suggests a role for Vangl2 in the targeting or anchoring of Fz3, a hypothesis strengthened by the existence of a physical interaction between the two proteins. Together, our data support the idea that protein asymmetry plays an important role in the development of PCP, but the colocalization and interaction of Fz3 and Vangl2 suggests that novel PCP mechanisms exist in vertebrates.

Distribution of kainate receptor subunits at hippocampal mossy fiber synapses.

Kainate receptors function as mediators of postsynaptic currents and as presynaptic modulators of synaptic transmission at mossy fiber synapses. Despite intense research into the physiological properties of mossy fiber kainate receptors, their subunit composition in the presynaptic and postsynaptic compartments is unclear. Here we describe the distribution of kainate receptor subunits in mossy fiber synapses using subunit-selective antibodies and knock-out mice. We provide morphological evidence for the presynaptic localization of KA1 and KA2 receptor subunits at mossy fiber synapses. Immunogold staining for KA1 and KA2 was commonly seen at synaptic contacts and in vesicular structures. Postsynaptic labeling in dendritic spines was also observed. Although KA1 predominantly showed presynaptic localization, KA2 was concentrated to a greater degree on postsynaptic membranes. Both subunits coimmunoprecipitated from hippocampal membrane extracts with GluR6 but not GluR7 subunits. These results demonstrate that KA1 and KA2 subunits are localized presynaptically and postsynaptically at mossy fiber synapses where they most likely coassemble with GluR6 subunits to form functional heteromeric kainate receptor complexes.

Aberrant formation of glutamate receptor complexes in hippocampal neurons of mice lacking the GluR2 AMPA receptor subunit.

The number and type of receptors present at the postsynaptic membrane determine the response to the neurotransmitter released from the presynaptic terminal. Because most neurons receive multiple and distinct synaptic inputs and contain several different subtypes of receptors stimulated by the same neurotransmitter, the assembly and trafficking of receptors in neurons is a complex process involving many levels of regulation. To investigate the mechanism that neurons use to regulate the assembly of receptor subunits, we studied a GluR2 knock-out mouse. GluR2 is a critical subunit that controls calcium permeability of AMPA receptors and is present in most native AMPA receptors. Our data indicate that in the absence of GluR2, aberrant receptor complexes composed of GluR1 and GluR3 are formed in the hippocampus, and that there is an increased number of homomeric GluR1 and GluR3 receptors. We also show that these homomeric and heteromeric receptors are less efficiently expressed at the synapse. Our results show that GluR2 plays a critical role in controlling the assembly of AMPA receptors, and that the assembly of subunits may reflect the affinity of one subunit for another or the stability of intermediates in the assembly process. Therefore, GluR1 may have a greater preference for GluR2 than it does for GluR3.

Relationship between availability of NMDA receptor subunits and their expression at the synapse.

The effect of increasing the expression of NMDA subunits in cerebellar granule cells (CGCs) by transfection was studied to determine how the availability of various NMDA subunits controls both the total pool of functional receptors and the synaptic pool. Overexpression of either NR2A or NR2B, but not splice variants of NR1, by transfection caused a significant increase in the total number of functional NMDA receptors and in surface NR1 subunit cluster density in CGCs in primary culture. These data solidify the central role of NR2 subunit availability in determining the number of cell surface receptors. Overexpression of either NR2A or NR2B significantly altered the deactivation kinetics of NMDA-mediated miniature EPSCs (NMDA-mEPSCs). However, there was no significant effect of NR2 subunit overexpression on the mEPSC amplitude or single-channel conductance. NR2 subunit overexpression did not change the rate of block by MK-801 of NMDA-mediated currents in excised patches from CGCs, indicating that subunit composition does not regulate peak open probability of the channel in CGCs. With the overexpression of a mutant of NR2B lacking the PDZ binding domain, there was an increase in the total number of NMDA receptors without a change in mEPSC kinetics. Therefore, the entry of NMDA receptors into the synapse requires a PDZ binding domain and is limited by means other than receptor subunit availability.