Iehab
N. Talukder
2nd
Year Graduate Student
Department:
Neurobiology & Behavior
Graduate Program: Neuroscience
Advisor:
Lonnie Wollmuth
Abstract:
Title:
Coupling of ligand binding to ion channel gating in the NMDA receptor.
Iehab
Talukder, Michael Prodromou, Priya Borker, Lonnie P. Wollmuth.
The N-methyl-D-aspartate (NMDA) receptor mediates a critical component
of glutamate-induced fast synaptic transmission in the mammalian central
nervous system. Moreover, physiologic activity of the NMDA receptor
is implicated in the induction of synaptic plasticity- the cellular
parallel of learning and memory. Under normal circumstances, synapses
maintain a precise level of NMDA receptor activity. Disruption of the
balance between over- and under-activity of the NMDA receptor is evident
in several disease states, including stroke, neurodegenerative disease
and mental disorders. Maintenance of this balance has also proved to
be elusive by numerous attempts of drug design that seeks to adjust
pathologic levels of NMDA receptor activity.
The
NMDA receptor is a heterotetrameric ion channel most often composed
of two NR1 and two NR2 subunits. Each NMDA receptor subunit harbors
an N terminal zinc-binding modulatory domain and a ligand-binding domain,
both located extracellularly. The transmembrane ion channel is composed
of three membrane-spanning segments M1, M3 and M4, as well as a reentrant
M2 pore loop. M2 and M3 are the major pore-lining segments. The ligand-binding
domain (LBD) comprises lobes S1 (N terminal to M1) and S2 (between M3
and M4). The three 12-20 amino acid long segments attaching the ligand-binding
domain to the ion channel are the linker regions S1-M1, M3-S2 and S2-M4.
Additionally, there is a C terminal domain involved in intracellular
signal transduction and trafficking.
We
have used the substituted cysteine accessibility method (SCAM) on NR1-NR2C
receptors to investigate the relative dynamics of the linker regions
in the agonist bound and unbound states. Analysis of the discrete changes
in whole-cell currents carried by the modified receptors have led us
to infer about the relative movements of the linker regions during gating,
as well as possible mechanisms of these movements. We show that, in
agreement with current knowledge of the structural dynamics of the LBD
and transmembrane ion channel, the M3-S2 linker undergoes the most extensive
motion during receptor gating. We have also identified a putative helical
interface in the S2-M4 linker, which we propose is the interacting surface
for the M3-S2 linker. This interaction between the relatively immobile
S2-M4 and the dynamic M3-S2 linkers modulates the gating properties
of the NMDA receptor. We hope to exploit this interaction to stabilize
the linker regions as we try to crystallize them.