Unprecedented detail of intact neuronal receptor offers blueprint for drug developers

Biologists at Cold Spring Harbor Laboratory (CSHL) report today that they have succeeded in obtaining an unprecedented view of a type of brain-cell receptor that is implicated in a range of neurological illnesses, including Alzheimer’s disease, Parkinson’s disease, depression, schizophrenia, autism, and ischemic injuries associated with stroke.

The team’s atomic-level picture of the intact NMDA (N-methyl, D-aspartate) receptor should serve as template and guide for the design of therapeutic compounds.

The NMDA receptor is a massive multi-subunit complex that integrates both chemical and electrical signals in the brain to allow neurons to communicate with one another. These conversations form the basis of memory, learning, and thought, and critically mediate brain development. The receptor’s function is tightly regulated: both increased and decreased NMDA activities are associated with neurological diseases.

Despite the importance of NMDA receptor function, scientists have struggled to understand how it is controlled. In work published today in Science, CSHL Associate Professor Hiro Furukawa and Erkan Karakas, Ph.D., a postdoctoral investigator, use a type of molecular photography known as X-ray crystallography to determine the structure of the intact receptor. Their work identifies numerous interactions between the four subunits of the receptor and offers new insight into how the complex is regulated.

“Previously, our group and others have crystallized individual subunits of the receptor – just fragments – but that simply was not enough,” says Furukawa. “To understand how this complex functions you need to see it all together, fully assembled.”

For such a large complex, this was a challenging task. Using an exhaustive array of protein purification methods, Furukawa and Karakas were able to isolate the intact receptor. Their crystal structure reveals that the receptor looks much like a hot air balloon. “The ‘basket’ is what we call the transmembrane domain. It forms an ion channel that allows electrical signals to propagate through the neuron,” explains Furukawa.

An ion channel is like a gate in the neuronal membrane. Ions, small electrically charged atoms, are unable to pass through the cell membrane. When the ion channel “gate” is closed, ions congregate outside the cell, creating an electrical potential across the cell membrane.

When the ion channel “gate” opens, ions flow in and out of the cell through the channel pores. This generates an electrical current that sums up to create pulses that rapidly propagate through the neuron. But the current can’t jump from one neuron to the next. Rather, the electrical pulse triggers the release of chemical messengers, called neurotransmitters. These molecules traverse the distance between the neurons and bind to receptors, such as the NMDA receptor, on the surface of neighboring cells. There, they act much like a key, unlocking ion channels within the receptor and propelling the electrical signal across another neuron and, ultimately, across the brain.

The “balloon” portion of the receptor that Furukawa describes is found outside the cell. This is the region that binds to neurotransmitters. The structure of the assembled multi-subunit receptor complex, including the elusive ion channel, helps to explain some of the existing data about how NMDA receptors function. “We are able to see how one domain on the exterior side of the receptor directly regulates the ion channel within the membrane,” says Furukawa. “Our structure shows why this particular domain, called the amino terminal domain, is important for the activity of the NMDA receptor, but not for other related receptors.”

This information will be critical as scientists work to develop drugs that control the NMDA receptor. “Our structure defines the interfaces where multiple subunits and domains contact one another,” says Furukawa. “In the future, these will guide the design of therapeutic compounds to treat a wide range of devastating neurological diseases.”

Picturing pain could help unlock its mysteries and lead to better treatments

Understanding the science behind pain, from a simple “ouch” to the chronic and excruciating, has been an elusive goal for centuries. But now, researchers are reporting a promising step toward studying pain in action. In a study published in the Journal of the American Chemical Society, scientists describe the development of a new technique, which they tested in rats, that could result in better ways to relieve pain and monitor healing.

Sandip Biswal, Frederick T. Chin, Justin Du Bois and colleagues note that current ways to diagnose pain basically involve asking the patient if something hurts. These subjective approaches are fraught with bias and can lead doctors in the wrong direction if a patient doesn’t want to talk about the pain or can’t communicate well. It can also be difficult to tell how well a treatment is really working. No existing method can measure pain intensity objectively or help physicians pinpoint the exact location of the pain. Past research has shown an association between pain and a certain kind of protein, called a sodium channel, that helps nerve cells transmit pain and other sensations to the brain. Certain forms of this channel are overproduced at the site of an injury, so the team set out to develop an imaging method to visualize high concentrations of this protein.

They turned to a small molecule called saxitoxin, produced naturally by certain types of microscopic marine creatures, and attached a signal to it so they could trace it by PET imaging. PET scanners are used in hospitals to diagnose diseases and injuries. When the researchers injected the molecule into rats, often a stand-in for humans in lab tests, they saw that the molecule accumulated where the rats had nerve damage. The rats didn’t show signs of toxic side effects. The work is one of the first attempts to mark these sodium channels in a living animal, they say.

How the Body’s Energy Molecule Transmits Three Types of Taste to the Brain

Saying that the sense of taste is complicated is an understatement, that it is little understood, even more so. Exactly how cells transmit taste information to the brain for three out of the five primary taste types was pretty much a mystery, until now.

A team of investigators from nine institutions discovered how ATP – the body’s main fuel source – is released as the neurotransmitter from sweet, bitter, and umami, or savory, taste bud cells. The CALHM1 channel protein, which spans a taste bud cell’s outer membrane to allow ions and molecules in and out, releases ATP to make a neural taste connection. The other two taste types, sour and salt, use different mechanisms to send taste information to the brain.

Kevin Foskett, PhD, professor of Physiology at the Perelman School of Medicine, University of Pennsylvania, and colleagues from the Monell Chemical Senses Center, the Feinstein Institute for Medical Research, and others, describe in Nature how ATP release is key to this sensory information path. They found that the calcium homeostasis modulator 1 (CALHM1) protein, recently identified by the Foskett lab as a novel ion channel, is indispensable for taste via release of ATP.  

“This is an example of a bona fide ATP ion channel with a clear physiological function,” says Foskett. “Now we can connect the molecular dots of sweet and other tastes to the brain.”

Taste buds have specialized cells that express G-protein coupled receptors (GPCRs) that bind to taste molecules and initiate a complex chain of molecular events, the final step of which Foskett and collaborators show is the opening of a pore in the cell membrane formed by CALHM1. ATP molecules leave the cell through this pore to alert nearby neurons to continue the signal to the taste centers of the brain. CALHM1 is expressed specifically in sweet, bitter, and umami taste bud cells.

Mice in which CALHM1 proteins are absent, developed by Feinstein’s Philippe Marambaud, PhD, have severely impaired perceptions of sweet, bitter and umami compounds; whereas, their recognition of sour and salty tastes remains mostly normal. The CALHM1 deficiency affects taste perception without interfering with taste cell development or overall function.

Using the CALHM1 knockout mice, team members from Monell and Feinstein tested how their taste was affected. “The mice are very unusual,” says Monell’s Michael Tordoff, PhD. “Control mice, like humans, lick avidly for sucrose and other sweeteners, and avoid bitter compounds. However, the mice without CALHM1 treat sweeteners and bitter compounds as if they were water. They can’t taste them at all.”

From all lines of evidence, the team concluded that CALHM1 is an ATP-release channel required for sweet, bitter, and umami taste perception. In addition, they found that CALHM1 was also required for  “nontraditional” Polycose, calcium, and aversive high-salt tastes, implying that the deficit displayed in the knockout animals might best be considered as a loss of all GPCR-mediated taste signals rather than simply sweet, bitter and umami taste.

Interestingly, CALHM1 was originally implicated in Alzheimer’s disease, although the link is now less clear. In 2008, co-author Marambaud identified CALHM1 as a risk gene for Alzheimer’s. They discovered that a CALHM1 genetic variant was more common among people with Alzheimer’s and they went on to show that it leads to a partial loss of function. They also found that this novel ion channel is strongly expressed in the hippocampus, a brain region necessary for learning and memory. So far, there is no connection between taste perception and Alzheimer’s risk, but Marambaud suspects that scientists will start testing this hypothesis.

Fruit flies’ eyes shrink a little to see

I spy, with my mechanical eye. It seems a simple mechanical change plays a role in sensory perception in fruit flies, and possibly in many other animals, including humans.

The eyes of the common fruit fly (Drosophila melanogaster) contain clusters of light-sensitive cells organised into rods. When light strikes one of these cells, it triggers a series of chemical reactions. These cause a protein called a transient receptor potential (TRP) ion channel to open. When it’s open, the TRP allows charged particles to flow into the cell, causing the cell to send a signal to the fly’s brain.

TRP channels play a part in sensory perception in many animals, from nematodes to humans. But nobody knew how the chemical signals make the TRP channel open.

The cellular cause of birth defects like cleft palates, missing teeth and problems with fingers and toes has been a tricky puzzle for scientists.

Professor Emily Bates and her biochemistry students at Brigham Young University studied an ion channel that regulates the electrical charge of a cell. In a new study published by the journal Development, they show that blocking this channel disrupts the work of a protein that is supposed to carry marching orders to the nucleus.

Without those instructions, cells don’t become what they were supposed to become – be that part of a palate, a tooth or a finger. Though there are various disorders that lead to birth defects, this newly discovered mechanism may be what some syndromes have in common.

Bates and her graduate student, Giri Dahal, now want to apply the findings toward the prevention of birth defects – particularly those caused by fetal alcohol syndrome and fetal alcohol spectrum disorder.

"What we think might be the case is that this is the target for a few similar disorders," Bates said. "The big thing that we have right now is that this ion channel is required for protein signaling, which means that developmental signaling pathways can sense the charge of a cell. And that’s exciting for a lot of different reasons."

For example, the new study might also have implications for the battle against cancer. With cancer, the problem is that cells are receiving a bad set of instructions that tells them to multiply and spread. If they can devise a way to block the ion channel, it may stop those cancerous instructions from getting through.

"This protein signaling pathway is the same one that tells cancer cells to metastasize," Bates said. "We’re planning to test a therapy to specifically block this channel in just the cells that we want to stop."

Smell the potassium: Surprising find in study of sex- and aggression-triggering vomeronasal organ

"We found two new ion channels—both of them potassium channels—through which VNO neurons are activated in mice," says Associate Investigator C. Ron Yu, Ph.D., senior author of the study. "This is quite unusual; potassium channels normally don’t play a direct role in the activation of sensory neurons."

Humans have shrunken, seemingly vestigial VNOs, but still exhibit instinctive, pre-programmed behaviors relating to reproduction and aggression. Scientists hope that an understanding of how the VNO works in mice and other lower mammals will provide clues to how these innate behaviors are triggered in humans.

The VNO works in much the same way as the main olfactory organ that provides the sense of smell. Its neurons and their input stalks, known as dendrites, are studded with specialized receptors that can be activated by contact with specific messenger-chemicals called pheromones, found mostly in body fluids. When activated, VNO receptors cause adjacent ion channels to open or close allowing ions to flood into or out of a neuron. These inflows and outflows of electric charge create voltage surges that can activate a VNO neuron, so that it signals to the brain to turn on a specific behavior.