Isabel Pérez-Otaño

Altered synaptic physiology and reduced susceptibility to kainate-induced seizures in GluR6-deficient mice

l-glutamate, the neurotransmitter of the majority of excitatory synapses in the brain, acts on three classes of ionotropic receptors: NMDA (N-methyl-d-aspartate), AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid) and kainate receptors. Little is known about the physiological role of kainate receptors because in many experimental situations it is not possible to distinguish them from AMPA receptors,. Mice with disrupted kainate receptor genes enable the study of the specific role of kainate receptors in synaptic transmission as well as in the neurotoxic effects of kainate. We have now generated mutant mice lacking the kainate-receptor subunit GluR6. The hippocampal neurons in the CA3 region of these mutant mice are much less sensitive to kainate. In addition, a postsynaptic kainate current evoked in CA3 neurons by a train of stimulation of the mossy fibre system is absent in the mutant,. We find that GluR6-deficient mice are less susceptible to systemic administration of kainate, as judged by onset of seizures and by the activation of immediate early genes in the hippocampus. Our results indicate that kainate receptors containing the GluR6 subunit are important in synaptic transmission as well as in the epileptogenic effects of kainate.

Homeostatic plasticity and NMDA receptor trafficking

Learning, memory and brain development are associated with long-lasting modifications of synapses that are guided by specific patterns of neuronal activity. Such modifications include classical Hebbian plasticities (such as long-term potentiation and long-term depression), which are rapid and synapse-specific, and others, such as synaptic scaling and metaplasticity, that work over longer timescales and are crucial for maintaining and orchestrating neuronal network function. The cellular mechanisms underlying Hebbian plasticity have been well studied and involve rapid changes in the trafficking of highly mobile AMPA receptors. An emerging concept is that activity-dependent alterations in NMDA receptor trafficking contribute to homeostatic plasticity at central glutamatergic synapses.

Temporal and regional expression of NMDA receptor subunit NR3A in the mammalian brain

NR3A is a developmentally regulated N‐methyl‐D‐aspartate receptor (NMDAR) subunit that was previously known as NMDAR‐L or χ‐1. Unlike other NMDAR subunits, NR3A inhibits the NMDAR‐associated ion channel in a novel manner, and a role in synaptogenesis has been suggested for this subunit. Here, we report a comprehensive study to delineate the temporal and anatomic expression of NR3A protein in the mammalian brain by using a monoclonal anti‐NR3A antibody. NR3A protein was found to peak at postnatal day (P) 8, and to decrease gradually from P12 to adulthood in the rat central nervous system. Moreover, NR3A protein was heavily expressed in all areas of the isocortex, portions of the amygdaloid nuclei, and selective cell layers and nuclei of the hippocampus, thalamus, hypothalamus, brainstem, and spinal cord. NR3A protein was also expressed in the cerebellar cortex, whereas only weak signal was detected in the previous in situ studies by using riboprobes. At an ultrastructural level, NR3A was associated specifically with asymmetrical synapses and localized to postsynaptic membranes. This information will facilitate future research on NMDARs by providing clues to possible inclusion of the NR3A subunit in NMDARs in many brain regions. J. Comp. Neurol. 450:303–317, 2002.

Phenylbutyrate rescues dendritic spine loss associated with memory deficits in a mouse model of Alzheimer disease

Alzheimers disease (AD) and ageing are associated with impaired learning and memory, and recent findings point toward modulating chromatin remodeling through histone acetylation as a promising therapeutic strategy. Here we report that systemic administration of the HDAC inhibitor 4‐phenylbutyrate (PBA) reinstated fear learning in the Tg2576 mouse model of AD. Tg2576 mice develop age‐dependent amyloid pathology and cognitive decline that closely mimics disease progression in humans. Memory reinstatement by PBA was observed independently of the disease stage: both in 6‐month‐old Tg2576 mice, at the onset of the first symptoms, but also in aged, 12‐ to 16‐month‐old mice, when amyloid plaque deposition and major synaptic loss has occurred. Reversal of learning deficits was associated to a PBA‐induced clearance of intraneuronal Aβ accumulation, which was accompanied by mitigation of endoplasmic reticulum (ER) stress, and to restoration of dendritic spine densities of hippocampal CA1 pyramidal neurons to control levels. Furthermore, the expression of plasticity‐related proteins such as the NMDA receptor subunit NR2B and the synaptic scaffold SAP102 was significantly increased by PBA. Our data suggest that the beneficial effects of PBA in memory are mediated both via its chemical chaperone‐like activity and via the transcriptional activation of a cluster of proteins required for the induction of synaptic plasticity and structural remodeling.

Endocytosis and synaptic removal of NR3A-containing NMDA receptors by PACSIN1/syndapin1

A key step in glutamatergic synapse maturation is the replacement of developmentally expressed N-methyl-D-aspartate receptors (NMDARs) with mature forms that differ in subunit composition, electrophysiological properties and propensity to elicit synaptic plasticity. However, the mechanisms underlying the removal and replacement of synaptic NMDARs are poorly understood. Here we demonstrate that NMDARs containing the developmentally regulated NR3A subunit undergo rapid endocytosis from the dendritic plasma membrane in cultured rat hippocampal neurons. This endocytic removal is regulated by PACSIN1/syndapin1, which directly and selectively binds the carboxy-terminal domain of NR3A through its NPF motifs and assembles a complex of proteins including dynamin and clathrin. Endocytosis of NR3A by PACSIN1 is activity dependent, and disruption of PACSIN1 function causes NR3A accumulation at synaptic sites. Our results reveal a new activity-dependent mechanism involved in the regulation of NMDAR expression at synapses during development, and identify a brain-specific endocytic adaptor that confers spatiotemporal and subunit specificity to NMDAR endocytosis.

Downregulation of NR3A-Containing NMDARs Is Required for Synapse Maturation and Memory Consolidation

NR3A is the only NMDA receptor (NMDAR) subunit that downregulates sharply prior to the onset of sensitive periods for plasticity, yet the functional importance of this transient expression remains unknown. To investigate whether removal/replacement of juvenile NR3A-containing NMDARs is involved in experience-driven synapse maturation, we used a reversible transgenic system that prolonged NR3A expression in the forebrain. We found that removal of NR3A is required to develop strong NMDAR currents, full expression of long-term synaptic plasticity, a mature synaptic organization characterized by more synapses and larger postsynaptic densities, and the ability to form long-term memories. Deficits associated with prolonged NR3A were reversible, as late-onset suppression of transgene expression rescued both synaptic and memory impairments. Our results suggest that NR3A behaves as a molecular brake to prevent the premature strengthening and stabilization of excitatory synapses and that NR3A removal might thereby initiate critical stages of synapse maturation during early postnatal neural development.

The Role of RNA Editing of Kainate Receptors in Synaptic Plasticity and Seizures

The ionotropic glutamate receptor subunit GluR6 undergoes developmentally and regionally regulated Q/R site RNA editing that reduces the calcium permeability of GluR6-containing kainate receptors. To investigate the functional significance of this editing in vivo, we engineered mice deficient in GluR6 Q/R site editing. In these mutant mice but not in wild types, NMDA receptor-independent long-term potentiation (LTP) could be induced at the medial perforant path-dentate gyrus synapse. This indicates that kainate receptors with unedited GluR6 subunits can mediate LTP. Behavioral analyses revealed no differences from wild types, but mutant mice were more vulnerable to kainate-induced seizures. Together, these results suggest that GluR6 Q/R site RNA editing may modulate synaptic plasticity and seizure vulnerability.

Learning from NMDA receptor trafficking: clues to the development and maturation of glutamatergic synapses.

Activity-dependent changes in excitatory transmission allow the brain to develop, mature, learn and retain memories, and underlie many pathological states of the central nervous system. A principal mechanism by which neurons regulate excitatory transmission is by altering the number and composition of glutamate receptors at the postsynaptic plasma membrane. The dynamic trafficking of glutamate receptors to and from synaptic sites involves a complex series of events including receptor assembly, trafficking through secretory compartments, membrane insertion and endocytic cycling. While these events have become widely appreciated as critical processes regulating AMPA-type glutamate receptors during synaptic plasticity, the mechanisms that control the trafficking of NMDA-type glutamate receptors (NMDARs) are only now beginning to be understood. Until recently, NMDARs were considered immobile receptors, tightly anchored to the postsynaptic membrane. Here, we review recent evidence that challenges this view, focusing on the role that activity plays in altering NMDAR trafficking and how such dynamic regulation of NMDARs may impact on the plasticity of neural circuits.

NR3A-containing NMDARs promote neurotransmitter release and spike timing–dependent plasticity

Recent evidence suggests that presynaptic-acting NMDA receptors (preNMDARs) are important for neocortical synaptic transmission and plasticity. We found that unique properties of the NR3A subunit enable preNMDARs to enhance spontaneous and evoked glutamate release and that NR3A is required for spike timing–dependent long-term depression in the juvenile mouse visual cortex. In the mature cortex, NR2B-containing preNMDARs enhanced neurotransmission in the absence of magnesium, indicating that presynaptic NMDARs may function under depolarizing conditions throughout life. Our findings indicate that NR3A relieves preNMDARs from the dual-activation requirement of ligand-binding and depolarization; the developmental removal of NR3A limits preNMDAR functionality by restoring this associative property.

Influence of the NR3A subunit on NMDA receptor functions.

Various combinations of subunits assemble to form the NMDA-type glutamate receptor (NMDAR), generating diversity in its functions. Here we review roles of the unique NMDAR subunit, NR3A, which acts in a dominant-negative manner to suppress receptor activity. NR3A-containing NMDARs display striking regional and temporal expression specificity, and, unlike most other NMDAR subtypes, they have a low conductance, are only modestly permeable to Ca(2+), and pass current at hyperpolarized potentials in the presence of magnesium. While glutamate activates triheteromeric NMDARs composed of NR1/NR2/NR3A subunits, glycine is sufficient to activate diheteromeric NR1/NR3A-containing receptors. NR3A dysfunction may contribute to neurological disorders involving NMDARs, and the subunit offers an attractive therapeutic target given its distinct pharmacological and structural properties.