NMDA receptors: neural map designers and refiners?

Evolution of NMDA receptor cytoplasmic interaction domains: implications for organisation of synaptic signalling complexes

Background

Glutamate gated postsynaptic receptors in the central nervous system (CNS) are essential for environmentally stimulated behaviours including learning and memory in both invertebrates and vertebrates. Though their genetics, biochemistry, physiology, and role in behaviour have been intensely studied in vitro and in vivo, their molecular evolution and structural aspects remain poorly understood. To understand how these receptors have evolved different physiological requirements we have investigated the molecular evolution of glutamate gated receptors and ion channels, in particular the N-methyl-D-aspartate (NMDA) receptor, which is essential for higher cognitive function. Studies of rodent NMDA receptors show that the C-terminal intracellular domain forms a signalling complex with enzymes and scaffold proteins, which is important for neuronal and behavioural plasticity

Results

The vertebrate NMDA receptor was found to have subunits with C-terminal domains up to 500 amino acids longer than invertebrates. This extension was specific to the NR2 subunit and occurred before the duplication and subsequent divergence of NR2 in the vertebrate lineage. The shorter invertebrate C-terminus lacked vertebrate protein interaction motifs involved with forming a signaling complex although the terminal PDZ interaction domain was conserved. The vertebrate NR2 C-terminal domain was predicted to be intrinsically disordered but with a conserved secondary structure.

Conclusion

We highlight an evolutionary adaptation specific to vertebrate NMDA receptor NR2 subunits. Using in silico methods we find that evolution has shaped the NMDA receptor C-terminus into an unstructured but modular intracellular domain that parallels the expansion in complexity of an NMDA receptor signalling complex in the vertebrate lineage. We propose the NR2 C-terminus has evolved to be a natively unstructured yet flexible hub organising postsynaptic signalling. The evolution of the NR2 C-terminus and its associated signalling complex may contribute to species differences in behaviour and in particular cognitive function.

Modeling early cortical serotonergic deficits in autism

Autism is a developmental brain disorder characterized by deficits in social interaction, language and behavior. Brain imaging studies demonstrate increased cerebral cortical volumes and micro- and macro-scopic neuroanatomic changes in children with this disorder. Alterations in forebrain serotonergic function may underlie the neuroanatomic and behavioral features of autism. Serotonin is involved in neuronal growth and plasticity and these actions are likely mediated via serotonergic and glutamatergic receptors. Few animal models of autism have been described that replicate both etiology and pathophysiology. We report here on a selective serotonin (5-HT) depletion model of this disorder in neonatal mice that mimics neurochemical and structural changes in cortex and, in addition, displays a behavioral phenotype consistent with autism. Newborn male and female mice were depleted of forebrain 5-HT with injections of the serotonergic neurotoxin, 5,7-dihydroxytryptamine (5,7-DHT), into the bilateral medial forebrain bundle (mfb). Behavioral testing of these animals as adults revealed alterations in social, sensory and stereotypic behaviors. Lesioned mice showed significantly increased cortical width. Serotonin immunocytochemistry showed a dramatic long-lasting depletion of 5-HT containing fibers in cerebral cortex until postnatal day (PND) 60. Autoradiographic binding to high affinity 5-HT transporters was significantly but transiently reduced in cerebral cortex of 5,7-DHT-depleted mice. AMPA glutamate receptor binding was decreased at PND 15. We hypothesize that increased cerebral cortical volume and sensorimotor, cognitive and social deficits observed in both 5-HT-depleted animals and in individuals with autism, may be the result of deficiencies in timely axonal pruning to key cerebral cortical areas.

Inhibitory synapses in the developing auditory system are glutamatergic

Activity-dependent synapse refinement is crucial for the formation of precise excitatory and inhibitory neuronal circuits. Whereas the mechanisms that guide refinement of excitatory circuits are becoming increasingly clear, the mechanisms guiding inhibitory circuits have remained obscure. In the lateral superior olive (LSO), a nucleus in the mammalian sound localization system that receives inhibitory input from the medial nucleus of the trapezoid body (MNTB), specific elimination and strengthening of synapses that are both GABAergic and glycinergic (GABA/glycinergic synapses) is essential for the formation of a precise tonotopic map. We provide evidence that immature GABA/glycinergic synapses in the rat LSO also release the excitatory neurotransmitter glutamate, which activates postsynaptic NMDA receptors (NMDARs). Immunohistochemical studies demonstrate synaptic colocalization of the vesicular glutamate transporter 3 with the vesicular GABA transporter, indicating that GABA, glycine and glutamate are released from single MNTB terminals. Glutamatergic transmission at MNTB-LSO synapses is most prominent during the period of synapse elimination. Synapse-specific activation of NMDARs by glutamate release at GABAergic and glycinergic synapses could be important in activity-dependent refinement of inhibitory circuits.