黑料吃瓜资源

Drugs precisely targeting portions of this receptor may have applications in Alzheimer鈥檚, depression and schizophrenia

Cold Spring Harbor, NY — The difference between mental health and mental illness can turn on changes in brain cells and their connections that are almost incomprehensibly tiny, at least in physical terms. This irony is brought to light by X-ray crystallography, a method that enables neuroscientists to map the structure of brain proteins atom by atom, using high-energy X-rays.

In research appearing today in Neuron, a team at 黑料吃瓜资源 (CSHL) reveals for the first time the structure of a portion of an important brain cell receptor, called the NMDA (N-methyl D-aspartate) receptor. The newly mapped portion is responsible for recognizing zinc, an element common throughout the brain that can inhibit a class of NMDA receptors.

Large in size compared with many other proteins, NMDA receptors are shaped like hot air balloons, ones that poke through the surface membrane of neurons that receive glutamate, an excitatory neurotransmitter.

NMDA receptors are activated when a glutamate molecule docks at a site on the receptor, setting in motion a complex series of changes. But that is only part of what is happening. Along with glutamate, some sets of neurons co-release zinc, which can also interact with NMDA receptors, playing an important role in controlling neuronal signaling. The binding of zinc to NMDA receptors is also an important factor in the transmission of the sensation of pain.

The precise structure of the NMDA receptor is critical in current efforts to design new and more effective drugs to treat a host of mental disorders. Overactive NMDA receptors are linked with neurodegenerative illness, such as Alzheimer鈥檚 disease. Underactive NMDA receptors may be a factor in schizophrenia. The receptors are also implicated in depression and epilepsy, among other illnesses.

Different sections, called domains, of the giant NMDA protein are involved in these various illnesses, and they are joined together in four major combinations, or subtypes. The subtypes, referred to by the letters A, B, C and D, have specific functions and appear at different times in life and places in the brain. Specific targeting of those subtypes is important in efforts to minimize side effects of new drugs for brain disorders now being tested in labs and clinical trials.

The subtype revealed in the new research, led by CSHL Associate Professor Hiro Furukawa, is the 鈥淎鈥� form of the NMDA receptor. The team mapped the amino terminal domain (ATD), one of the four domains of the GluN2A subunit of the receptor. The ATD鈥攖he outermost part of the NMDA structure, poking out into extracellular space鈥攊s pictured by the team as it forms a complex with zinc.

The team鈥檚 high-resolution pictures of the 鈥淎鈥� type of the NMDA receptor are the product of 3 years of work by Annabel Romero-Hernandez, a graduate student in the Furukawa lab studying at the Watson School of Biological Sciences. 鈥淎nnabel鈥檚 beautiful new pictures are important for a few reasons,鈥� says Furukawa. 鈥淭hey have enabled us to solve two puzzles, both raised by earlier pictures we made of the 鈥楤鈥� type of the receptor. First, why does zinc bind so much more readily to the 鈥楢鈥� form compared with the 鈥楤鈥� form, which is structurally almost identical? And second, why does the important candidate drug ifenprodil bind at a site in the 鈥楤鈥� form but not in the 鈥楢鈥� form?鈥�

What the new images reveal about zinc binding

Furukawa and Romero-Hernandez were intrigued by the zinc molecule鈥檚 interaction with the 鈥楢鈥� form of the receptor in part because of discoveries in recent years of zinc鈥檚 abundance and important role in the mammalian brain. When a message-transmitting NMDA receptor releases glutamate, it co-releases zinc, which interacts with the receptor at an alternate binding site, called an allosteric site. In the already-mapped 鈥楤鈥� form of the receptor, comparatively large amounts of zinc had to be present if binding was to occur; and the binding was found to be physically weak.

The new pictures of the receptor鈥檚 鈥楢鈥� form show why much less zinc is needed to achieve binding, and explains why the binding is much more robust. Zinc slips into the pivot point in a clamshell-like assembly in the ATD, and atomic-level resolution reveals that in the 鈥楢鈥� form, it is held firmly in place by bonds formed with four surrounding amino acid molecules. In previously obtained views of the 鈥楤鈥� form, only two such amino acids lie at the comparable spot in the ATD鈥檚 structure. The difference in binding affinity, says Furukawa, 鈥渋s like the difference between having two hands holding up an object vs. four hands.鈥�

Zinc鈥檚 impact on the function of the much larger full NMDA receptor is profound: its binding to the ATD is a regulator of the receptor鈥� central channel that connects the exterior environment to the cell鈥檚 interior. Zinc inhibits the ion channel, encouraging it to close up鈥攎uch more strongly in the 鈥楢鈥� type than the 鈥楤鈥� type. 鈥淶inc binding is one of the means by which NMDA receptors regulate themselves,鈥� says Romero-Hernandez. Such self-regulation is important, she notes, considering that 鈥渨hen you overactivate these receptors the consequence can be neurodegenerative.鈥�

The receptor鈥檚 ion channel flickers open and closed very rapidly鈥攊n a tenth to a hundredth of a second鈥攁s the brain performs its complex operations. Whether it is being inhibited by zinc or excited by glutamate or other binding partners, the result is significant, considered at the level of entire brain regions.

A drug binding pocket that 鈥渃ollapses鈥�

The same mapping of the ATD in the 鈥楢鈥� form of the NMDA receptor also solved the question of whether ifenprodil can bind at an allosteric site in the ATD identified clearly in the lab鈥檚 prior work on the 鈥楤鈥� form (for a depiction of the drug in its binding pocket, see the HT Summer 2013 One Experiment (pdf).

The new pictures obtained by Furukawa and Romero-Hernandez show clearly that in the 鈥楢鈥� form, 鈥渢he binding pocket, which was so large in the 鈥楤鈥� form, has now collapsed into a much smaller space,鈥� says Furukawa. 鈥淣ow we understand structurally why the drug cannot bind in the 鈥楢鈥� form.鈥� This could be crucial to drug developers, since the more specific a drug is, the less likely it will have unwanted side effects.

With recent research demonstrating the almost magical power of the drug ketamine鈥攁n NMDA inhibitor鈥攖o alleviate the worst forms of non-responsive major depression in some patients, there is great interest in finding mechanisms underlying the illness that do not involve the neurotransmitter serotonin. So-called SSRI antidepressants (drugs in the Prozac class) are taken by tens of millions, many of whom are not helped by them. Furukawa says his work on the NMDA receptor is aimed in part on exploring alternate hypotheses of depression鈥檚 causation involving NMDA or glutamate-mediated dysfunctions. His lab is pursuing this work in structural studies as well as by studying signaling pathways inside neurons鈥攁ctivated by the interaction of receptors like the NMDA receptor with the environment beyond the cell membrane.

Written by: Peter Tarr, Senior Science Writer | [email protected] | 516-367-8455

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Funding

The Furukawa lab is supported by the National Institutes of Health and the Stanley Institute of Cognitive Genomics. Annabel Romero-Hernandez is a Ph.D. student at the Watson School of Biological Sciences at CSHL and is supported by the Genentech Foundation Fellowship and the Starr Foundation.

Citation

鈥淢olecular basis for subtype-specificity and high-affinity zinc inhibition in the GluN1-GluN2A NMDA receptor amino terminal domain鈥� appears December 1, 2016 in Neuron. The authors are: Annabel Romero-Hernandez, Noriko Simorowski, Erkan Karakas and Hiro Furukawa. The paper can be obtained at: