NMDAR Flashcards
to know it
What are NMDARs?
N-Methyl-D-Aspartate (NMDA) receptors are one of the three main classes of Glutamate receptors found in CNS synapses responsible for excitatory post-synaptic potentials (EPSPs)
What makes NMDARs unusual?
Requirement of the co-agonist Glycine for activation, presence of a voltage-gated Mg2+ block, high permeability to Ca2+, receptor function is subject to modulation by a variety of extracellular and intracellular molecules, combination brings out very peculiar excitatory postsynaptic currents (EPSCs)
What can functionally distinct receptor subtypes be attributed to?
Diversity in receptor subunit composition
How many subunit families are there?
There are 3 distinct subunit families, GluN1, GluN2, and GluN3 (Traynelis et al., 2010)
What makes a functional receptor?
Two ubiquitously expressed GluN1 subunits combine with two GluN2 and/or GluN3 subunits to form di- or triheterotetrameric ionotropic receptors
Describe the structure of a NMDAR subunit.
All subunits possess a large extracellular N-terminal domain, followed by a bilobal agonist binding domain, a transmembrane region forming the cation channel, and an intracellular C-terminal domain (Traynelis et al., 2010)
Describe GluN1 subunits.
GluN1 is encoded by a single gene, GRIN1, and hosts the Glycine (or D-serine) binding site in its agonist-binding domain.
When was the GluN1 subunit first cloned?
In 1991 by Moriyoshi et al.
How many isoforms of GluN1 exist and what do they influence?
There are 8 different isoforms resulting from alternative splicing at NTD exon 5, and the CTD exons 21 and 22 (Hollman et al., 1993).
Different splice variants, via variable CTDs influence interactions with proteins and subcellular localisation, while variant NTD is subject to modulation by extracellular polyamines, ions and protons (Hollman et al., 1993)
What is the major determinant of biophysical and pharmacological features of receptor subtypes?
Glutamate-binding GluN2 subunits. Four different genes encode GluN2A, GluN2B, GluN2C and GluN2D.
What are GluN3 subunits?
Glycine binding, two genes encode GluN3A and GluN3B, which are not well defined
How do different GluN2 subunit containing NMDARs differ in their single-channel conductance?
Single-channel conductance is highest in GluN2A and GluN2B homodimer containing receptors. Conductance ranges from 38pS and 50pS in GluN2A/GluN2B containing receptors, and from19pS to 36pS in GluN2C, and 17 to 35pS in GluN2D (Stern et al., 1992 for A-C)(Wyllie et al., 1996 for D)
How do the GluN2 subunits differ in affinity to the voltage-dependet Mg2+ block?
GluN2A/GluN2B containing reeptors have greater affinitiy to Mg2+, and require lower Mg2+ concentrations for efficient block than GluN2C/GluN2D (Kuner & Schoepfer, 1996).
What determines the difference in Mg2+ block affinity?
A critical S/L residue in the third transmembrane region of the GluN2 subunits. GluN2A/B have Serine (S632 in GluN2A), GluN2C/D have Leucine.
How was the S/L residue shown to affect Mg2+ block affinity?
Substitution of the S in GluN2A/B for the characteristic L of GluN2C/D, and vice versa, led to a Mg2+ half-maximal block (IC50) of the other pair, respectively (Siegler-Retchless et al., 2012).
What are deactivation kinetics and how are they in NMDARs?
Unique characteristic of NMDARs are the unusually slow deactivation kinetics, which contributes to the slow decay of the NMDAR-mediated EPSC after agonist removal
How do deactivation time constant (T) vary between different GluN2 subunits?
GluN2A homodimer T: 40ms GluN2B homodimer T: 420ms GluN2C homodimer T: 310 ms GluN2 D homodimer T: 1200 ms (Yuan et al., 2009). Depending on the subunit, NMDARs vary greatly in the amount of Ca influx they allow
What is the channel open probability, P(o)?
P(O) defines the amount of time, as a proportion of the total recording time, in which a channel is in its open state
How do GluN2 subunits differ in their P(O)?
GluN2A: maximal P(o) of 0.35, approximately 5x greater than that of Glun2B with a peak P(o) of 0.07 (Chen et al., 1999). GluN2C mean P(o) of 0.011 (Dravid et al., 2008). GluN2D mean P(o) 0.017 (Vance et al., 2012).
What determines the variance in gating and agonist-binding characteristics of different NMDAR subtypes?
The NTD and NTD-ABD linker (Yuan et al., 2009; Gielen et al., 2009).
How do GluN1 splice variants influence kinetic behaviour of NMDARs?
Lys211 in the GluN1-1b NTD significantly increases the half-maximal concentration of Glu from 0.51um in 1a to 0.93um in 1b (Vance et al., 2012).
