The integrity of the glycine co-agonist binding site of N-methyl-D-aspartate receptors is a functional quality control checkpoint for cell surface delivery.

N-Methyl-D-aspartate receptors are a subclass of ligand-gated, heteromeric glutamatergic neurotransmitter receptors whose cell surface expression is regulated by quality control mechanisms. Functional quality control checkpoints are known to contribute to cell surface trafficking of non-N-methyl-D-aspartate glutamate receptors. Here we investigated if similar mechanisms operate for the surface delivery of NMDA receptors. Point mutations in the glycine binding domain of the NR1-1a subunit were generated: D732A, a mutation that results in an approximately 3 x 10(4) decrease in glycine binding affinity; D732E, a conservative change; and D723A, a residue in the same NR1-1a domain that has no effect on glycine binding affinity. Each NR1-1a subunit was co-expressed with NR2A in mammalian cells. Immunoblotting and immunoprecipitations showed that all mutants were expressed to similar levels as wild-type NR1-1a and associated with NR2A. Cell surface expression measured by an enzyme-linked immunosorbent assay found that whereas NR1-1a (D732E)/NR2A and NR1-1a (D723A)/NR2A trafficked as efficiently as NR1-1a/NR2A, there was a 90% decrease in surface expression for NR1-1a (D732A)/NR2A. This was confirmed by confocal microscopy imaging and cell surface biotinylation. Further imaging showed that NR1-1a (D732A) and co-transfected NR2A co-localized with an endoplasmic reticulum marker. Dichlorokynurenic acid, a competitive glycine site antagonist, partially rescued surface expression. Mutation of the NR1-1a ER retention motif showed that the ligand binding checkpoint is an early event preceding endoplasmic reticulum sorting mechanisms. These findings demonstrate that integrity of the glycine co-agonist binding site is a functional checkpoint requisite for efficient cell surface trafficking of assembled NMDA receptors.

Assembly and forward trafficking of NMDA receptors (Review).

N-Methyl-D-aspartate (NMDA) receptors are a subclass of the excitatory, ionotropic L-glutamate neurotransmitter receptors. They are important for normal brain function being both primary candidates for the molecular basis of learning and memory and in the establishment of synaptic connections during the development of the central nervous system. NMDA receptors are also implicated in neurological and psychiatric disorders. Their dysfunction which is primarily due to either hypo- or hyper-activity is pivotal to these pathological conditions. There is thus a fine balance between NMDA receptor-mediated mechanisms in normal brain and those in diseased states where receptor homeostasis is perturbed. Receptor activity is due in part to the number of surface expressed receptors. Understanding the assembly and trafficking of this complex, heteromeric, neurotransmitter receptor family may therefore, be pivotal to understanding diseases in which their altered activity is evident. This article will review the current understanding of the mechanisms of NMDA receptor assembly, how this assembly is regulated and how assembled receptors are trafficked to their appropriate sites in post-synaptic membranes where they are integral components of a macromolecular signalling complex.

Ligand concentration is a driver of divergent signaling and pleiotropic cellular responses to FGF.

Fibroblast growth factors (FGFs) are soluble ligands important for embryonic patterning, limb and brain development, and stem cell proliferation. They activate specific receptors (FGFR) to elicit changes in gene expression and cellular responses such as proliferation, differentiation, and survival, but the extent to which these pleiotropic responses are driven by FGF concentration gradients has not been systematically addressed. Here, we show that a single cell type exhibits divergent, even opposing, responses to a single FGF dependent on the exposure concentration, and that this is controlled by differential signaling with specific negative feedback inhibition. Low concentrations of FGF2 stimulate survival and differentiation but actively inhibit proliferation while intermediate concentrations stimulate proliferation in the presence of serum but apoptosis in its absence. Intriguingly, high concentrations reverse the proliferation and apoptosis effects, and mirror the low concentration effects: inhibition of proliferation and stimulation of survival and differentiation. By screening for activation of sampled signaling intermediates across the FGF2 concentration range in fibroblasts, we show that the peak in proliferation and apoptosis correlates with abrupt activation of FRS-2 and Erk that is specifically down-regulated by high concentrations of FGF2, a pattern that contrasts with an incremental increase in activation of p38 MAP kinase and the FGFR itself, across the FGF2 concentration range. Whilst proliferation stimulated by FGF2 was dependent on p38 MAP kinase, apoptosis stimulated by proliferative concentrations of FGF2 under serum-free conditions was, in contrast, dependent on Erk MAP kinase. These findings indicate that FGF exposure concentration precisely controls intracellular signaling and cellular responses to the growth factor, and have important implications for understanding how FGF gradients influence cell proliferation, survival, and differentiation during processes such as limb development.

A peptide from the first fibronectin domain of NCAM acts as an inverse agonist and stimulates FGF receptor activation, neurite outgrowth and survival.

Neural cell adhesion molecule (NCAM) contributes to axon growth and guidance during development and learning and memory in adulthood. Although the Ig domains mediate homophilic binding, outgrowth activity localizes to two membrane proximal fibronectin-like domains. The first of these contains a site identified as a potential FGF receptor (FGFR) activation motif (FRM) important for NCAM stimulation of neurite outgrowth, but its activity has hitherto remained hypothetical. Here, we have tested the effects of a domain-specific antibody and peptides corresponding to the FRM in cellular assays in vitro. The first fibronectin domain antibody inhibited NCAM-stimulated outgrowth, indicating the importance of the domain for NCAM function. Monomeric FRM peptide behaved as an inverse agonist; low concentrations specifically inhibited neurite outgrowth stimulated by NCAM and cellular responses to FGF2, while saturating concentrations stimulated FGFR-dependent neurite outgrowth equivalent to NCAM itself. Dendrimeric FRM peptide was 125-fold more active and stimulated FGFR activation, FGFR-dependent and FGF-mimetic neurite outgrowth and cell survival (but not proliferation). We conclude that the FRM peptide contains NCAM-mimetic bioactivity accounted for by stimulation of FGF signalling pathways at the level of or upstream from FGF receptors, and discuss the possibility that FRM comprises part of an FGFR activation site on NCAM.