Early events in the trafficking of N-methyl-D-aspartate (NMDA) receptors

The N-methyl-D-aspartate (NMDA) receptor plays a central role at excitatory synapses where it has been implicated in multiple functions associated with synaptic plasticity. While this receptor has been intensely studied with respect to its physiology and pharmacology, its cell-biological properties, such as subunit assembly, post-translational processing and trafficking in neurons, are only beginning to be addressed. Critical to many of the functions of the NMDA receptor are the multiple proteins with which it interacts. While these interactions have been most thoroughly studied with respect to the receptor at the synapse, the same proteins may also interact with the receptor much earlier in its biosynthetic pathway and play important roles in receptor trafficking from the endoplasmic reticulum to the synapse.

Functional expression of distinct NMDA channel subunits tagged with green fluorescent protein in hippocampal neurons in culture

We generated expression vectors for N-terminally green fluorescent protein -tagged NR2A and NR2B subunits (GFP-NR2A and GFP-NR2B). Both constructs expressed GFP and formed functional NMDA channels with similar properties to untagged controls when co-transfected with NR1 subunit partner in HEK293 cells. Primary cultured hippocampal neurons were transfected at five days in vitro with these vectors. Fifteen days after transfection, well-defined GFP clusters were observed for both GFP-NR2A and GFP-NR2B subunits being co-localized with endogenous NR1 subunit. Whole-cell recordings showed that the GFP-NR2A subunit determined the decay of NMDA-mediated miniature spontaneous excitatory postsynaptic currents (NMDA-mEPSCs) in transfected neurons. Live staining with anti-GFP antibody demonstrated the surface expression of GFP-NR2A and GFP-NR2B subunits that was partly co-localized a presynaptic marker. Localization of NMDA receptor clusters in dendrites was studied by co-transfection of CFP-actin and GFP-NR2 subunits followed by anti-GFP surface staining. Within one week after plating most surface NMDAR clusters were distributed on dendritic shafts. Later in development, a large portion of surface clusters for both GFP-NR2A and GFP-NR2B subunits were clearly localized at dendritic spines. Our report provides the basis for studies of NMDA receptor location together with dendritic dynamics in living neurons during synaptogenesis in vitro.

GABA(A) receptor alpha1 subunit deletion prevents developmental changes of inhibitory synaptic currents in cerebellar neurons

Developmental changes in miniature IPSC (mIPSC) kinetics have been demonstrated previously in cerebellar neurons in rodents. We report that these kinetic changes in mice are determined primarily by developmental changes in GABA(A) receptor subunit expression. mIPSCs were studied by whole-cell recordings in cerebellar slices, prepared from postnatal day 11 (P11) and P35 mice. Similar to reports in granule neurons, wild-type cerebellar stellate neuron mIPSCs at P11 had slow decay kinetics, whereas P35 mIPSCs decayed five times faster. When mIPSCs in cerebellar stellate neurons were compared between wild-type (+/+) and GABA(A) receptor alpha1 subunit-deficient (-/-) littermates at P35, we observed dramatically slower mIPSC decay rates in -/- animals. We took advantage of the greater potency of imidazopyridines for GABA current potentiation with alpha1 subunit-containing receptors to characterize the relative contribution of alpha1 subunits in native receptors on inhibitory synapses of cerebellar granule neurons. Zolpidem-induced prolongation of mIPSC decay was variable among distinct cells, but it increased during development in wild-type mice. Similarly, Zolpidem prolongation of mIPSC decay rate was significantly greater in adult +/+ mice than in knock-outs. We propose that an increased alpha1 subunit assembly in postsynaptic receptors of cerebellar inhibitory synapses is responsible for the fast inhibitory synaptic currents that are normally observed during postnatal development.

Increased exon 5 expression alters extrasynaptic NMDA receptors in cerebellar neurons

We investigated the ontogenic changes in expression of alternatively spliced forms of NR1 protein that contain the N1 cassette (exon 5) in comparison with the total population of NR1 within the rat cerebellum. The N1 cassette is strongly developmentally regulated in the cerebellum, with >80% of total NR1 protein in the adult rat containing the N1 cassette. In contrast, early in development, <20% of NR1 protein contained this cassette. Rat cortices from the same ages did not show an increase in the percent of NR1 protein expressing the N1 cassette, indicating that the developmental changes in the cerebellum are tissue-specific. As the N1 cassette is known to determine NMDA receptor properties, including spermine sensitivity and decay kinetics of glutamate-induced currents, changes in the characteristics of glutamate-activated currents in granule cells from cerebellar slices were compared at postnatal day 7 versus 14. Glutamate responses exhibited fast deactivation kinetics and reduced potentiation by spermine at day 14 while maintaining sensitivity to an NR2B-selective antagonist. These data are consistent with the possibility that N1 cassette expression may be a factor in the developmental changes in properties of extrasynaptic NMDA receptors in the cerebellum.

Exon 5 and spermine regulate deactivation of NMDA receptor subtypes

Deactivation of N-methyl-D-aspartate (NMDA) channels after brief agonist exposure determines the duration of their synaptic activation during excitatory neurotransmission. We performed patch-clamp recordings of L-glutamate responses from human embryonic kidney tumoral cells (HEK293) expressing NR1 subunit variants lacking exon 5 together with the NR2B subunit. These responses had deactivation components that lasted several seconds. The presence of exon 5 or spermine greatly accelerated deactivation of L-glutamate responses through alterations in desensitization. These effects were also observed at positive holding potentials and in the presence of physiological Mg(2+). Thus NR1 splicing and polyamines may have profound effects on the kinetics of NMDA receptor-mediated synaptic transmission.