Summary: New structural work reveals how tiny electrical gates in the brain—NMDA receptors (NMDARs)—are regulated to control learning, memory and neuron survival. Using high-resolution cryo-electron microscopy, researchers captured atomic-scale images showing how a natural neurosteroid, 24S-hydroxycholesterol (24S‑HC), stabilizes a fully open channel, while a synthetic modulator locks the channel into a partially open state.
The study shows that fully open NMDAR channels permit large flows of calcium and sodium into neurons, increasing electrical signaling. Partially open channels, by contrast, allow sodium but restrict calcium entry, a balance that may support normal signaling while reducing calcium-driven neuronal damage. These findings point to ways to design therapies that precisely tune brain signaling for conditions such as Alzheimer’s disease, stroke, and other neurodegenerative disorders.
Key Facts
- Molecular gatekeepers: NMDA receptors govern electrical signaling by controlling calcium and sodium ion flow across neuronal membranes.
- Precision control: Endogenous neurosteroids and synthetic regulators can fine-tune these ion gates, maintaining calcium levels needed for learning while preventing excessive calcium that damages cells.
- Therapeutic potential: Targeting NMDAR conductance states could enable safer treatments for memory loss, neurodegeneration and recovery after stroke.
Source: CSHL
Electrical impulses shuttle information across the brain by moving ions through tiny protein channels.
At Cold Spring Harbor Laboratory (CSHL), structural biologist Hiro Furukawa and colleagues focused on the pore-forming elements of NMDARs and the molecules that modulate their opening. These ion channels respond to neurotransmitters and drugs, and their opening must be tightly regulated: excessive or insufficient activity can impair learning and memory and contribute to neurodegeneration.
Using single-particle cryo-electron microscopy (cryo-EM), Furukawa, postdoctoral researcher Hyunook Kang and collaborators visualized the NMDAR in different functional states. They found that the natural neurosteroid 24S‑HC binds to a juxtamembrane pocket in the GluN2B subunit and stabilizes a fully open gate. In that conformation, four pore-forming helices bend outward, dilating the channel to allow maximal ion conductance.
By contrast, a synthetic positive allosteric modulator named EU1622‑240 binds the same GluN2B pocket and an additional pocket in GluN1a, stabilizing a sub‑open state where only one of the pore helices bends. That partial opening produces a distinct subconductance state with lower ion throughput than the fully open conformation.
To correlate structure with function, the team measured single-channel currents with electrophysiological recordings. Those measurements confirmed that 24S‑HC promotes full-conductance openings while EU1622‑240 favors subconductance events. Importantly, the partially open state permits sodium passage but more strongly restricts calcium. Because calcium influx supports synaptic plasticity and memory formation but can also trigger neurotoxicity when excessive, the ability to reduce calcium entry without abolishing electrical signaling has clear therapeutic implications.
Furukawa commented that preserving normal electrical activity while limiting harmful calcium overload could be a viable strategy for treating neurodegenerative disease and mitigating damage after stroke. The brain contains many NMDAR subtypes and a diverse set of neurosteroids; mapping how each regulator binds and shifts channel conformation will be essential for translating these structural insights into safe, targeted drugs.
Beyond defining specific binding pockets, the study highlights juxtamembrane sites as structural hubs that set conductance levels in NMDARs. The work also shows that different neurosteroids follow varied recognition patterns—pregnenolone sulfate, for example, can occupy the GluN2B pocket with two molecules bound simultaneously—revealing a complexity that researchers can exploit when designing modulators with desired conductance profiles.
Key Questions Answered:
A: They visualized how NMDARs adopt fully open and sub-open gate structures when bound to natural and synthetic regulators, linking these conformations to specific ion conductance behaviors.
A: Calcium is essential for learning and memory but can cause neuronal damage in excess. Controlling calcium entry while preserving sodium-driven electrical signaling can help maintain brain function without triggering degeneration.
A: Designing molecules that act like adjustable “doorstops” for NMDARs could fine-tune synaptic signaling, supporting cognition and protecting neurons in neurodegenerative conditions and after stroke.
About this neuroscience research news
Author: Samuel Diamond
Source: CSHL
Contact: Samuel Diamond – CSHL
Image: The image is credited to Neuroscience News
Original Research: Closed access.
Title: Mechanism of conductance control and neurosteroid binding in NMDA receptors — Hiro Furukawa et al., Nature
Abstract
Mechanism of conductance control and neurosteroid binding in NMDA receptors
Ion-channel activity is determined by both open probability and unitary conductance. Many channels adopt subconductance states that tune signaling strength, but the structural basis for these conductance levels has been unclear. This study shows that conductance in the heterotetrameric neuronal GluN1a‑2B NMDAR is governed by bending patterns in the pore-forming transmembrane helices.
Single-particle cryo‑EM reveals that the endogenous neurosteroid 24S‑hydroxycholesterol (24S‑HC) binds a juxtamembrane pocket in GluN2B and stabilizes a fully open gate in which both GluN1a M3 and GluN2B M3′ helices bend to dilate the pore. In contrast, the synthetic PAM EU1622‑240 occupies the GluN2B pocket and an additional GluN1a pocket to favor a sub‑open state in which only the GluN2B M3′ helix bends. Electrophysiological single‑channel recordings are consistent with these structural states: 24S‑HC predominantly produces full‑conductance openings, whereas EU1622‑240 promotes subconductance events.
A different neurosteroid class, exemplified by pregnenolone sulfate, engages the GluN2B pocket with two molecules binding simultaneously, indicating diverse recognition modes. Overall, the findings identify juxtamembrane pockets as critical structural hubs for modulating NMDAR conductance and provide a framework for developing modulators that precisely tune synaptic signaling.