Summary: A new structural biology study has solved a decades-old neuroscience mystery: how the brain’s learning receptors distinguish between calcium and magnesium ions to form memories. Researchers utilized single-particle cryo-electron microscopy (cryo-EM) and advanced computational cores to capture over 50,000 microscopic movies of the NMDA receptor (NMDAR) channel.
The structural data demonstrates that a specialized filter called the Asn cage forces calcium to shed its surrounding water molecules to pass through, while magnesium remains trapped in a hydrated state, effectively blocking the channel and regulating synaptic plasticity.
Key Facts
- The Periodic Table Paradox: Calcium ($Ca^{2+}$) and magnesium ($Mg^{2+}$) sit close together on the periodic table and carry the exact same electrical charge, making them structurally difficult for brain receptors to tell apart.
- The Dehydration Secret: Magnesium attracts water molecules significantly more strongly than calcium, making it far more difficult to strip away its surrounding water jacket.
- The Asn Cage Filter: Advanced imaging revealed that a specific region inside the NMDAR pore, known as the Asn cage, acts as a selective molecular sieve. Dehydrated calcium is small enough to slip through, whereas fully hydrated magnesium gets stuck, acting like a backed-up sieve.
- 50,000 High-Resolution Movies: Because water molecules are fluid and constantly in motion, researchers captured millions of cryo-EM images from different angles to compile 50,000 movies, paired with electrophysiology to verify the ion transit in real time.
- Clinical Implications for GRIN Disorders: The Asn cage is highly susceptible to spontaneous mutations linked to GRIN disorders—severe neurodevelopmental conditions that leave patients non-verbal, unable to walk, and prone to treatment-resistant seizures.
Source: CSHL
How do we learn to remember? At the most fundamental level, it’s all about chemicals and electricity. Beyond their roles in diet and nutrition, calcium and magnesium work as ions, or charged particles, in the brain. Magnesium can block a channel found within brain receptors known as NMDARs. When the blockade lifts, calcium can pass through the channel.
These processes enable the brain to perform essential functions, like learning and remembering.
Scientists have known all of this for a while. What they couldn’t figure out was how NMDARs tell calcium from magnesium. Now, Cold Spring Harbor Laboratory (CSHL) Professor Hiro Furukawa, postdoc Rubin Steigerwald, and colleagues have found an answer that could have implications for brain development and disease. It involves water, dehydration, and a molecular cage captured across 50,000 movies.
If you think back to chemistry class, you might remember that calcium and magnesium sit close together on the periodic table. They also carry the same electrical charge. That makes it hard to tell them apart. One key difference is that “magnesium attracts water more strongly than calcium,” Furukawa says.
“It’s more difficult to take out water molecules surrounding magnesium than calcium.”
Since the 1980s, scientists have thought this might explain why calcium passes through the NMDAR channel more easily. It made sense. However, it was impossible to observe. It took decades for imaging technology and computing power to catch up with the theory. But now, using a method called single-particle cryo-EM, Steigerwald and his colleagues have demonstrated how dehydration enables calcium to pass through the NMDAR channel.
Steigerwald focused his attention on a part of the channel known as the Asn cage. This molecular cage acts as a filter, allowing only molecules that are small enough to pass. Outside the filter, the team saw magnesium surrounded by water, blocking the channel. If you’re picturing a backed-up spaghetti strainer, you’re right. “It’s a sieve,” Furukawa explains.
So that covers water, dehydration, and the molecular cage. But how do 50,000 movies fit into the picture? “It’s all about resolution,” Furukawa says.
Think about water’s fluid nature. It’s constantly in motion. Tracking the movement of a few water molecules requires high resolution. Single-particle cryo-EM images get you part of the way there. But to really see what’s going on, you need to take millions of images from different angles.
Therein lies the power of CSHL’s cryo-EM and high-performance computing cores. Additionally, Furukawa’s team confirmed their observations using electrophysiology.
Why go through all this trouble? Remember, we’re not just talking about chemicals. We’re viewing one of the key molecular features of learning and memory. Furthermore, the Asn cage is susceptible to spontaneous mutations linked to GRIN disorders, which cause severe developmental disabilities.
Many patients with these mutations are non-verbal and unable to walk. They often experience severe seizures. To understand the effects of these mutations, you need to know what you’re looking at. This study gives scientists the clearest picture yet.
Key Questions Answered:
A: NMDA receptors are the molecular gatekeepers of learning and memory. To prevent your brain from being overwhelmed by random background noise, magnesium acts as a natural plug, blocking the channel. When an important electrical signal arrives, the block is lifted, allowing calcium to rush in and lock down the memory. If the receptor couldn’t tell them apart, your brain’s memory gateway would either be permanently stuck open or permanently locked shut.
A: In your body, ions don’t float around naked; they are surrounded by a shell of water molecules. Because calcium holds onto water loosely, it can easily strip off its “water jacket” (dehydrate) right before entering the ultra-narrow Asn cage filter. Magnesium holds onto its water shell with a chemical grip that is far too tight, leaving it too bulky to squeeze through the sieve.
A: Spontaneous mutations in the genes that build the Asn cage cause a spectrum of devastating neurodevelopmental conditions known as GRIN disorders. Children with GRIN mutations are often non-verbal, struggle with mobility, and experience debilitating seizures because their ion filters are malformed. By capturing the first clear, 50,000-movie blueprint of this filter in action, scientists can now design highly targeted, next-generation drugs to physically correct the altered channel mechanics.
Editorial Notes:
- This article was edited by a Neuroscience News editor.
- Journal paper reviewed in full.
- Additional context added by our staff.
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, which is thought to be a cellular substrate of learning and memory, can occur by means of coincidental detection of presynaptic neurotransmitter release and Ca2+ influx upon postsynaptic depolarization.
This is mediated at a molecular level by N-methyl-D-aspartate-type glutamate receptors, which bind glutamate and glycine and facilitate Ca2+ influx upon relief of Mg2+ channel block during membrane depolarization.
However, the structural mechanism underlying Ca2+ permeability and Mg2+ blockade in N-methyl-D-aspartate-type glutamate receptors has yet to be fully elucidated.
Here we demonstrate using single-particle cryo-electron microscopy that Ca2+ permeation through the narrow constriction of the cation selectivity filter involves partial dehydration, as evidenced by several Ca2+ binding sites.
In contrast, Mg2+ binds outside of the selectivity filter through a water network and remains hydrated, thereby acting as a channel blocker.
Furthermore, the lipid network around the selectivity filter influences the stability of Mg2+ binding in a voltage-dependent manner. Our study details the transmembrane chemistry essential for initiating neuroplasticity.
