Tyrosine Phosphorylation Regulates the Endocytosis and Surface Expression of GluN3A-Containing NMDA Receptors

Selective control of receptor trafficking provides a mechanism for remodeling the receptor composition of excitatory synapses, and thus supports synaptic transmission, plasticity, and development. GluN3A (formerly NR3A) is a nonconventional member of the NMDA receptor (NMDAR) subunit family, which endows NMDAR channels with low calcium permeability and reduced magnesium sensitivity compared with NMDARs comprising only GluN1 and GluN2 subunits. Because of these special properties, GluN3A subunits act as a molecular brake to limit the plasticity and maturation of excitatory synapses, pointing toward GluN3A removal as a critical step in the development of neuronal circuitry. However, the molecular signals mediating GluN3A endocytic removal remain unclear. Here we define a novel endocytic motif (YWL), which is located within the cytoplasmic C-terminal tail of GluN3A and mediates its binding to the clathrin adaptor AP2. Alanine mutations within the GluN3A endocytic motif inhibited clathrin-dependent internalization and led to accumulation of GluN3A-containing NMDARs at the cell surface, whereas mimicking phosphorylation of the tyrosine residue promoted internalization and reduced cell-surface expression as shown by immunocytochemical and electrophysiological approaches in recombinant systems and rat neurons in primary culture. We further demonstrate that the tyrosine residue is phosphorylated by Src family kinases, and that Src-activation limits surface GluN3A expression in neurons. Together, our results identify a new molecular signal for GluN3A internalization that couples the functional surface expression of GluN3A-containing receptors to the phosphorylation state of GluN3A subunits, and provides a molecular framework for the regulation of NMDAR subunit composition with implications for synaptic plasticity and neurodevelopment

Surprising features of plastid ndhD transcripts: addition of non-encoded nucleotides and polysome association of mRNAs with an unedited start codon

RNA editing in higher plant plastids is a post- transcriptional RNA maturation process changing single cytidine nucleotides into uridine. In the ndhD transcript of tobacco and several other plant species, editing of an ACG codon to a standard AUG initiator codon is believed to be a prerequisite for translation. In order to test this assumption experimentally, we have analyzed the editing status of ndhD mRNA species in the process of translation. We show that unedited ndhD transcripts are also associated with polysomes in vivo, suggesting that they are translated. This surprising finding challenges the view that ACG to AUG editing is strictly required to make the ndhD message translatable and raises the possibility that ACG can be utilized as an initiator codon in chloroplasts. In addition, we have mapped the termini of the ndhD transcript and discovered a novel form of RNA processing. Unexpectedly, we find that highly specific sequences are added to the 3′ end of the ndhD mRNA at high frequency. We propose a model in which these sequences are added by the successive action of a CCA-adding enzyme (tRNA nucleotidyltransferase) and an RNA-dependent RNA polymerase (RdRp) activity. The presence of an RdRp activity may have general implications also for other steps in plastid gene expression

Distinct isoforms of ADPglucose pyrophosphatase and ADPglucose pyrophosphorylase occur in the suspension-cultured cells of sycamore (Acer pseudoplatanus L.)

AbstractThe intracellular localizations of ADPglucose pyrophosphatase (AGPPase) and ADPglucose pyrophosphorylase (AGPase) have been studied using protoplasts prepared from suspension-cultured cells of sycamore (Acer pseudoplatanus L.). Subcellular fractionation studies revealed that all the AGPPase present in the protoplasts is associated with amyloplasts, whereas more than 60% of AGPase is in the extraplastidial compartment. Immunoblots of amyloplast- and extraplastid-enriched extracts further confirmed that AGPase is located mainly outside the amyloplast. Experiments carried out to identify possible different isoforms of AGPPase in the amyloplast revealed the presence of soluble and starch granule-bound isoforms. We thus propose that ADPglucose levels linked to starch biosynthesis in sycamore cells are controlled by enzymatic reactions catalyzing the synthesis and breakdown of ADPglucose, which take place both inside and outside the amyloplast

GluN3A promotes dendritic spine pruning and destabilization during postnatal development

Synaptic rearrangements during critical periods of postnatal brain development rely on the correct formation, strengthening, and elimination of synapses and associated dendritic spines to form functional networks. The correct balance of these processes is thought to be regulated by synapse-specific changes in the subunit composition of NMDA-type glutamate receptors (NMDARs). Among these, the nonconventional NMDAR subunit GluN3A has been suggested to play a role as a molecular brake in synaptic maturation. We tested here this hypothesis using confocal time-lapse imaging in rat hippocampal organotypic slices and assessed the role of GluN3A-containing NMDARs on spine dynamics. We found that overexpressing GluN3A reduced spine density over time, increased spine elimination, and decreased spine stability. The effect of GluN3A overexpression could be further enhanced by using an endocytosis-deficient GluN3A mutant and reproduced by silencing the adaptor protein PACSIN1, which prevents the endocytosis of endogenous GluN3A. Conversely, silencing of GluN3A reduced spine elimination and favored spine stability. Moreover, reexpression of GluN3A in more mature tissue reinstated an increased spine pruning and a low spine stability. Mechanistically, the decreased stability in GluN3A overexpressing neurons could be linked to a failure of plasticity-inducing protocols to selectively stabilize spines and was dependent on the ability of GluN3A to bind the postsynaptic scaffold GIT1. Together, these data provide strong evidence that GluN3A prevents the activity-dependent stabilization of synapses thereby promoting spine pruning, and suggest that GluN3A expression operates as a molecular signal for controlling the extent and timing of synapse maturation