Iehab N. Talukder

2nd Year Graduate Student

Department: Neurobiology & Behavior

Graduate Program: Neuroscience

Advisor: Lonnie Wollmuth


Abstract:

Title: A novel mechanim of action underlying amantadine’s efficacy in Neurodegenerative diseases

Preceptor: Dr. Lonnie Wollmuth, Dept. of Neurobiology and Behavior, Stony Brook University

The NMDA receptor is a ligand-gated ion channel that mediates both basic neuron-to-neuron communication and higher order nervous system processes, such as learning, memory and pain perception. Pathological over-activation of NMDA receptors is neurodestructive and contributes to numerous disease states, including both acute brain pathology (e.g. stroke and epilepsy) and chronic neurodegeneration (e.g. Parkinson’s and Alzheimer’s diseases). Amantadine is one of few clinically efficacious NMDA receptor antagonists currently used in diseases where NMDA receptor over-activation is implicated. In addition to blocking deep in the ion channel pore, amantadine interacts at a more external site to alter gating steps of NMDA receptors. This gating action has been proposed to underlie amantadine’s clinical efficacy, though its precise mechanism is unknown.

Gating in the NMDA receptor is initiated in the extracellular ligand-binding domain (LBD) and is ultimately propagated via three linkers—S1-M1, M3-S2 and S2-M4—to the ion channel. Here we present evidence of gating-related interactions between two (M3-S2 and S2-M4) of the three linkers of the NR1 subunit. Cysteine-substituted positions R645C in the M3-S2 linker and S784C in the S2-M4 linker undergo spontaneous redox-dependent cross-linking, imparting a ~60 % reduction in current amplitude. Rate of reoxidation experiments reveal that the disulfide bond forms preferentially in the agonist-unbound state of NR1, suggesting that, in that state (as opposed to the agonist-bound state), the M3-S2 and S2-M4 linkers are closer together. These experiments reveal the dynamic gating-related interactions between the M3-S2 and S2-M4 linkers.

Surprisingly, previous studies have shown that amantadine’s external site of action comprises M3-S2 as well as S2-M4. We therefore propose the novel hypothesis that amantadine, by binding simultaneously to residues in gthe M3-S2 and S2-M4 linkers, constrains their gating movements and, consequently, stabilizes a closed state of the receptor. This gating effect of amantadine mimics the disulfide cross-links of the two linkers in our studies. Defining this novel mechanism of action will not only provide insight into how amantadine modulates NMDA receptor activity, but also help in the design of new more efficacious therapeutic drugs, for which amantadine can serve as a prototype.