The Enigma of the Dichotomic Pressure Response of GluN1-4a/b Splice Variants of NMDA Receptor: Experimental and Statistical Analyses

Abstract

Professional deep-water divers, exposed to hyperbaric pressure (HP) above 1.1 MPa, develop High Pressure Neurological Syndrome (HPNS), which is associated with central nervous system (CNS) hyperexcitability. It was previously reported that HP augments N-methyl-D-aspartate receptor (NMDAR) synaptic response, increases neuronal excitability and potentially causes irreversible neuronal damage. Our laboratory has reported differential current responses under HP conditions in NMDAR subtypes that contain either GluN1-1a or GluN1-1b splice variants co-expressed in Xenopus laevis oocytes with all four GluN2 subunits. Recently, we reported that the increase in ionic currents measured under HP conditions is also dependent on which of the eight splice variants of GluN1 is co-expressed with the GluN2 subunit. We now report that the NMDAR subtype that contains GluN1-4a/b splice variants exhibited “dichotomic” (either increased or decreased) responses at HP. The distribution of the results is not normal thus analysis of variance (ANOVA) test and clustering analysis were employed for statistical verification of the grouping. Furthermore, the calculated constants of alpha function distribution analysis for the two groups were similar, suggesting that the mechanism underlying the switch between an increase or a decrease of the current at HP is a single process, the nature of which is still unknown. This dichotomic response of the GluN1-4a/b splice variant may be used as a model for studying reduced response in NMDAR at HP. Successful reversal of other NMDAR subtypes response (i.e., current reduction) may allow the elimination of the reversible malfunctioning short term effects (HPNS), or even deleterious long term effects induced by increased NMDAR function during HP exposure.

Marine Mammals’ NMDA Receptor Structure: Possible Adaptation to High Pressure Environment

“…If so, it could serve as an explanation for whale capability of performing extremely deep dives of almost 2 km. It has been shown by our laboratory that the receptors containing the GluN2A subunit exhibit the biggest current increase at HP ( Mor et al, 2012 ; Bliznyuk et al, 2016 ), whereas other subtypes showed slight inhibition or no change of the currents. In other words, with the lack of Zn 2+ voltage-independent inhibition at the surface they will not experience removal of that inhibition at HP, and consequently will face much less increase of the current, resulting in less hyperexcitability of the CNS.…”

“…The choice of human as a terrestrial mammal is obvious, in order to find out whether genetic management of HP effects is feasible. We chose rat as an additional mammal because of its high gene matching to the human genome, and also because the electro-physiological research carried out in our lab was done mainly on rat NMDAR mRNAs ( Mor et al, 2008 ; Bliznyuk et al, 2015 , 2016 ). Alignment was performed by Jalview 2.10.3 program ( Waterhouse et al, 2009 ) using a web server running T-Coffee with defaults.…”

“…Electrophysiological studies in rat brain slices at He HP, preceded by the pharmacological studies, showed a significant increase in the synaptic NMDAR response followed by postsynaptic excitability changes ( Mor and Grossman, 2006 ; Mor and Grossman, 2007 ). Lately, molecular studies from our laboratory ( Mor et al, 2012 ; Bliznyuk et al, 2015 , 2016 ) have revealed that different subunit combinations of the NMDAR exhibit different current responses, an increase or a decrease, under He HP conditions. NMDAR containing GluN2A subunit showed a 25–60% current increase at HP (dependent on GluN1 subunit type), whereas the receptors containing GluN2B subunit did not change.…”

NMDA Receptor Regulation Is Involved in the Limitation of Physiological Tolerance to Both Low Temperature and High Hydrostatic Pressure

“…Hydrostatic pressure effects on membrane properties and ion channel structure (Tillman and Cascio, 2003) may affect quaternary structure, destabilizing the ligandbinding cores of glutamate receptors enhancing neurotransmitter desensitization (Sun et al, 2002). However, further exploration has revealed that high pressure effects on NMDA receptor subtypes can vary among different splice variants (Bliznyuk et al, 2015), even switching between increasing and decreasing ionic currents (Bliznyuk et al, 2017). The mechanism underlying the switch remains unknown (Bliznyuk et al, 2017).…”

“…However, further exploration has revealed that high pressure effects on NMDA receptor subtypes can vary among different splice variants (Bliznyuk et al, 2015), even switching between increasing and decreasing ionic currents (Bliznyuk et al, 2017). The mechanism underlying the switch remains unknown (Bliznyuk et al, 2017). Whether decapod crustaceans have a similar diversity of receptor subtypes is unknown, but the arthropod Drosophila spp.…”