Summary: A new structural biology study resolves a decades-old question in neuroscience: how the brain’s learning receptors distinguish calcium from magnesium to enable memory formation. Using single-particle cryo-electron microscopy (cryo-EM) combined with powerful computational analysis, researchers assembled and analyzed more than 50,000 high-resolution microscopic movies of the NMDA receptor (NMDAR) channel to capture ions and water molecules in action.
The structures reveal a specialized constriction, the Asn cage, that forces calcium to shed part of its water shell so it can pass through the pore, while magnesium remains tightly hydrated and becomes lodged outside the filter, physically blocking the channel and controlling synaptic plasticity.
Key Facts
- The periodic table challenge: Calcium (Ca2+) and magnesium (Mg2+) are adjacent elements with the same charge, so distinguishing them by size or charge alone is difficult for proteins.
- Dehydration matters: Magnesium binds water molecules much more strongly than calcium, making it harder to strip off its hydration shell before entering a narrow pore.
- The Asn cage filter: High-resolution imaging shows a region of the NMDAR pore — the Asn cage — acting like a molecular sieve. Partially dehydrated calcium can fit through, while fully hydrated magnesium is too bulky and becomes a blocker.
- Over 50,000 cryo-EM movies: Because water and ions move rapidly, the team recorded millions of images and combined them into more than 50,000 movie frames to reach the resolution needed to visualize water coordination and ion positions. Electrophysiology confirmed ion transit and channel behavior in real time.
- Clinical relevance for GRIN disorders: The Asn cage region is vulnerable to spontaneous mutations linked to GRIN disorders, a set of severe neurodevelopmental conditions characterized by intellectual disability, motor impairment, and in many cases treatment-resistant seizures.
Source: CSHL
How do we form memories? At the molecular level, it comes down to ions and electrical signals. Calcium and magnesium play opposing roles at NMDA-type glutamate receptors (NMDARs). At resting potential, magnesium blocks the receptor pore and prevents current flow. When the neuron is depolarized and the magnesium block is relieved, calcium enters through the channel. That calcium influx is a critical trigger for synaptic changes underlying learning and memory.
This functional distinction has been known for decades, but the structural basis for how NMDARs discriminate Ca2+ from Mg2+ was unclear. Cold Spring Harbor Laboratory (CSHL) Professor Hiro Furukawa, postdoctoral researcher Ruben Steigerwald, and collaborators used advances in cryo-EM and high-performance computing to visualize the process and identify the decisive role of dehydration and the Asn cage.
Although calcium and magnesium are chemically similar, their interactions with surrounding water differ. “Magnesium attracts water more strongly than calcium,” says Furukawa, making magnesium’s hydration shell far harder to remove. That difference in dehydration energy has long been suspected to underlie selectivity, but direct observation required much higher resolution.
Using single-particle cryo-EM, the team focused on the narrowest region of the channel — the Asn cage. The cryo-EM reconstructions show magnesium remaining surrounded by water molecules outside the constriction, effectively blocking the pore, while calcium occupies binding positions consistent with partial dehydration as it squeezes through the filter. Furukawa likens the Asn cage to a sieve: it allows smaller or partially dehydrated species to pass while trapping bulkier hydrated ions.
Capturing this behavior demanded extraordinary data. Water molecules are dynamic and diffuse, so the researchers collected tens of thousands of movie-frame images from many orientations to achieve the spatial and temporal resolution necessary to resolve ion coordination and water positions. The CSHL cryo-EM and high-performance computing infrastructure were essential, and complementary electrophysiology experiments validated the structural findings by showing how ion flow changes with mutations or voltage.
Why is this important beyond basic science? The Asn cage sits at the heart of the receptor’s ion selectivity and is a hotspot for spontaneous mutations that cause GRIN disorders. These mutations can deform the filter and impair its ability to distinguish ions, producing severe developmental impairments and refractory seizures. By providing the clearest structural blueprint yet of the ion filter in action, this work paves the way for rational design of therapies that target altered channel mechanics in affected patients.
Key Questions Answered:
A: NMDA receptors act as molecular gatekeepers for synaptic plasticity. At rest, Mg2+ plugs the channel to prevent random activity from producing spurious changes. When a meaningful electrical signal arrives, the block is removed and Ca2+ flows in to trigger biochemical processes that stabilize synaptic change. If the receptor could not distinguish the two ions, the gate would either be permanently open or permanently closed, undermining reliable memory formation.
A: In solution, ions are surrounded by a shell of water molecules. Calcium’s hydration shell is easier to partially remove, so Ca2+ can shed some water and enter the narrow Asn cage. Magnesium binds water more tightly and remains fully hydrated, making it too large to fit through the constriction and therefore an effective blocker.
A: Mutations in the residues that form the Asn cage produce a spectrum of GRIN disorders by altering the ion filter’s shape and function. With a detailed structural model showing how ions and water interact at the filter, researchers can design more precise molecules to restore normal channel mechanics or compensate for the altered function, offering a clearer path toward next-generation, targeted therapies.
Editorial Notes:
- This article was edited by a Neuroscience News editor.
- The journal paper was reviewed in full by the editorial team.
- Additional context and explanation were added by staff to clarify technical points.
About this learning and memory research news
Author: Samuel Diamond
Source: CSHL
Contact: Samuel Diamond – CSHL
Image: The image is credited to Neuroscience News
Original Research: Open access.
“Molecular mechanism of calcium permeability and magnesium block in NMDA receptors” by Ruben Steigerwald, Max Epstein, Tsung-Han Chou, Noriko Simorowski & Hiro Furukawa. Nature Neuroscience
DOI: 10.1371/journal.pmed.1005063
Abstract
Molecular mechanism of calcium permeability and magnesium block in NMDA receptors
Hebbian neuroplasticity, a leading model for the cellular basis of learning and memory, depends on coincident detection of presynaptic neurotransmitter release and postsynaptic Ca2+ influx during membrane depolarization.
This process is mediated by N-methyl-D-aspartate (NMDA) type glutamate receptors, which bind glutamate and glycine and permit Ca2+ influx when the Mg2+ channel block is relieved during depolarization.
Despite its importance, the structural mechanism behind Ca2+ permeability and Mg2+ blockade in NMDA receptors has remained incompletely understood.
Using single-particle cryo-electron microscopy, we show that Ca2+ permeation through the narrow constriction of the selectivity filter involves partial dehydration, consistent with multiple Ca2+ binding sites detected within the filter.
By contrast, Mg2+ binds outside the selectivity filter via a network of water molecules and remains hydrated, acting as a physical channel blocker.
We also find that the surrounding lipid environment affects the stability of Mg2+ binding in a voltage-dependent manner. Together, these results define the transmembrane chemistry that initiates neuroplasticity and clarify how ion coordination and hydration determine selectivity in NMDA receptors.