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.
Synaptic adhesion-like molecules (SALMs) are a newly discovered family of adhesion molecules that play roles in synapse formation and neurite outgrowth. The SALM family is comprised of five homologous molecules that are expressed largely in the central nervous system. SALMs 1-3 contain PDZ-binding domains, whereas SALMs 4 and 5 do not. We are interested in characterizing the interactions of the SALMs both among the individual members and with other binding partners. In the present study, we focused on the interactions formed by the five SALM members in rat brain and heterologous cells. In brain, we found that SALMs 1-3 strongly co-immunoprecipitated with each other, whereas SALMs 4 and 5 did not, suggesting that SALMs 4 and 5 mainly form homomeric complexes. In heterologous cells transfected with SALMs, co-immunoprecipitation studies showed that all five SALMs form heteromeric and homomeric complexes. We also determined if SALMs could form trans-cellular associations between transfected heterologous cells. Both SALMs 4 and 5 formed homophilic, but not heterophilic associations, whereas no trans associations were formed by the other SALMs. The ability of SALM4 to form trans interactions is due to its extracellular N terminus because chimeras of SALM4 N terminus and SALM2 C terminus can form trans interactions, whereas chimeras of SALM2 N terminus and SALM4 C terminus cannot. Co-culture experiments using HeLa cells and rat hippocampal neurons expressing the SALMs showed that SALM4 is recruited to points of contact between the cells. In neurons, these points of contact were seen in both axons and dendrites.Critical steps in the development of the nervous system, including cell migration, neurite outgrowth, growth cone guidance, and synapse formation, depend on cellular interactions mediated by adhesion molecules (1-4). A number of adhesion molecules that are essential to the proper development of the nervous system have been identified (5-7). The importance of these molecules is illustrated in cases of dysfunctional adhesion molecules that have been associated with specific disorders in humans, including SLITRK1 in Tourette syndrome (8) and neuroligin in autism (9, 10). Whereas our understanding of the role of adhesion molecules in neuronal development is rapidly increasing, little is known about their functions, and it is likely that many remain to be identified.Synaptic adhesion-like molecules (SALMs) 2 are a recently identified class of adhesion molecules that are highly enriched in brain (11-13). All five members of the SALM family contain extracellular leucine-rich repeats (LRR), an immunoglobulin C2-like (IgC2) domain, a fibronectin type III (FNIII) domain, a transmembrane domain, and a cytoplasmic C-terminal tail. SALMs 1-3 contain a C-terminal PDZ-binding domain (PDZ-BD), which associates with the PSD-95 family of proteins (11-13). SALMs are expressed early in the developing nervous system and persist into adulthood. They are present at the synaptic membrane as well as at non-synaptic locations. SALM1 overe...
N-methyl d-aspartate 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, DIV3-6) hippocampal rat neurons. Moreover, immunogold electron microscopy showed that NR1 is expressed in growth cones of postnatal day 2 (P2) 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.
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