Spatial Coupling Tunes NMDA Receptor Responses via Ca²⁺ Diffusion

Area of Science:

• Neuroscience
• Molecular Biology
• Cellular Signaling

Background:

• NMDA receptors are crucial for synaptic plasticity in the central nervous system (CNS).
• These receptors are clustered postsynaptically, but the functional impact of this arrangement on their signaling output remains unclear.
• Calcium-dependent inactivation (CDI) is a key regulatory mechanism involving intracellular calcium and calmodulin (CaM).

Purpose of the Study:

• To investigate whether calcium influx through one NMDA receptor affects nearby NMDA receptors.
• To determine if spatial arrangement and clustering of NMDA receptors influence their activity-dependent regulation.
• To elucidate the role of CDI in mediating signal coupling between NMDA receptors.

Main Methods:

• Cell-attached unitary current recordings from recombinant and native NMDA receptors.
• Utilized a coupled Markov model to quantify signal coupling (κ).
• Manipulated intracellular calcium levels and calmodulin activity, and altered receptor clustering via PSD-95 overexpression.

Main Results:

• NMDA receptor currents exhibit negative cooperativity (κ = 0.27) in the presence of extracellular calcium, indicating signal coupling.
• Coupling was reduced by intracellular calcium chelation or a CaM-mutant, supporting CDI as the coupling mechanism.
• Overexpression of PSD-95 significantly increased cooperativity (κ = 0.68), correlating with enhanced receptor clustering.

Conclusions:

• NMDA receptor currents are negatively coupled via calcium-dependent inactivation.
• The degree of NMDA receptor coupling is tunable by the distance between receptors.
• Channel clustering modulates activity-dependent NMDA receptor current reduction, influencing synaptic function and potentially neuropsychiatric conditions.