Voltage-independent GluN2A-type NMDA receptor Ca2+ signaling promotes audiogenic seizures, attentional and cognitive deficits in mice

Ilaria Bertocchi,Georg Köhr,Thorsten Bus,J N P Rawlins ,Nail Burnashev,Ahmed Eltokhi,Shibin Li ,Martin Both,Peter Gass,Hannah Sonntag,Michael Briese,Vivan Nguyễn ,Andrei Rozov ,David M Bannerman,Bo Yang ,Burkhard Niewoehner,Verena Pawlak,Horst A Obenhaus,Vidar R Jensen ,Frank Nicolai Single ,Marta Serafino,Thomas Boerner,Rolf Sprengel

doi:10.1038/s42003-020-01538-4

Abstract

The NMDA receptor-mediated Ca2+ signaling during simultaneous pre- and postsynaptic activity is critically involved in synaptic plasticity and thus has a key role in the nervous system. In GRIN2-variant patients alterations of this coincidence detection provoked complex clinical phenotypes, ranging from reduced muscle strength to epileptic seizures and intellectual disability. By using our gene-targeted mouse line (Grin2aN615S), we show that voltage-independent glutamate-gated signaling of GluN2A-containing NMDA receptors is associated with NMDAR-dependent audiogenic seizures due to hyperexcitable midbrain circuits. In contrast, the NMDAR antagonist MK-801-induced c-Fos expression is reduced in the hippocampus. Likewise, the synchronization of theta- and gamma oscillatory activity is lowered during exploration, demonstrating reduced hippocampal activity. This is associated with exploratory hyperactivity and aberrantly increased and dysregulated levels of attention that can interfere with associative learning, in particular when relevant cues and reward outcomes are disconnected in space and time. Together, our findings provide (i) experimental evidence that the inherent voltage-dependent Ca2+ signaling of NMDA receptors is essential for maintaining appropriate responses to sensory stimuli and (ii) a mechanistic explanation for the neurological manifestations seen in the NMDAR-related human disorders with GRIN2 variant-meidiated intellectual disability and focal epilepsy.

Highlights

The NMDA receptor-mediated Ca2+ signaling during simultaneous pre- and postsynaptic activity is critically involved in synaptic plasticity and has a key role in the nervous system
The Grin2a cDNA sequence analysis of total brain mRNA of heterozygous Grin2a+/S mice together with the comparable GluN2A immunosignals in forebrain extracts of Grin2a+/+ and Grin2aS/S littermates verified that adult mice expressed the Grin2a(N615S) and Grin2a+ alleles at the same level (Fig. 1d, e)
We observed in forebrain extracts statistically comparable levels of the GluA1 subunit of the amino-3-hydroxy-5-methyl-4isoxazolepropionic acid receptor (AMPAR), the postsynaptic marker protein PSD95 and the phosphorylated form of αCaMKII in Grin2aS/S, Grin2a+/s, and Grin2a+/+ mice

Summary

Introduction

The NMDA receptor-mediated Ca2+ signaling during simultaneous pre- and postsynaptic activity is critically involved in synaptic plasticity and has a key role in the nervous system. By introducing the well-characterized GluN2A(N615S) mutation (previously called N596)[14,15] into the mouse genome, we were able to study the effects of an inappropriate glutamate-induced Ca2+ influx through GluN2A-type NMDARs, even at resting potentials, on synaptic plasticity, activity-induced c-Fos expression, neuronal network activity in the hippocampus and, lastly, on behavior. This analysis had not been possible in previous studies with gene-targeted Grin2a(N614Q) mice that died for unknown reasons 2 weeks after birth[12]. In these NMDAR subtypes, an asparagine amino acid residue in the GluN1 subunits (N614, labeled previously N598 (ref. 20)) and two neighboring N residues in the GluN2 subunits (N614 and N615, labeled previously N595 and N596 (ref. 15)) located at the tip of the P-loops, build the narrow constriction of the ion channel pore, and differentially modulate Mg2+ block and Ca2+ permeability[14,15,21]

Methods

Results

Conclusion