Scientists have discovered how salt acts as a key regulator for drugs used to treat a variety of brain diseases including chronic pain, Parkinson’s disease, and depression.
The finding clears the way for more precisely targeted therapies with the potential of fewer side effects.
“There’s a reason why certain drugs, for instance, work well for some people but not others and why those drugs can cause serious side effects, such as seizures, addiction, and death due to overdose,” says Patrick Giguere, pharmacology postdoctoral fellow at University of North Carolina School of Medicine. “The reason is that we haven’t known the precise biological markers for those drugs.”
Markers represent the biological abnormalities that drugs aim to treat. Currently, many approved drugs, including morphine, oxycodone, and heroin, target opioid receptors, which use a variety of pathways to transmit chemical signals in the brain.
“These drugs activate all of the receptor pathways,” Giguere says. “None of them modulates just one pathway.”
This ability of opioid medications to indiscriminately target receptor pathways is likely responsible for the beneficial and harmful effects associated with these addictive and commonly abused medications.
But now, Giguere and colleagues found a way to modulate just one pathway. They discovered that tweaking specific amino acids, the building blocks of receptors, can drastically change how opioid receptors control chemical signals.
How does salt do it?
Published online in the journal Nature, the finding offers a way to create more precisely targeted therapies with the potential for enhanced beneficial actions and fewer side effects, researchers say.
And since the opioid system is key for many brain processes, drugs that target these receptors could be useful for many diseases, including depression, chronic pain and Parkinson’s disease.
At the heart of the finding is a simple element—sodium—the main component of table salt. Forty years ago scientists figured out that altering sodium concentrations in the brain changed the activity of opioid receptors.
But since then, no one had figured out precisely how sodium did that. That’s because no one could create a clear picture of what the receptor looked like; researchers couldn’t see what sodium was doing. What they needed was a high-resolution crystal structure of the delta-opioid receptor. None existed until two years ago.
A crystal structure of a tiny piece of brain anatomy is similar to a snow crystal. Both have adopted a solid form that is the most stable form. Water, under specific circumstances, is most stable as a snow crystal. A protein, under certain circumstances, is most stable in a crystal form.
Yet, snow and protein crystals are formed very differently. A snow flake forms when water freezes onto a dust particle. To create a crystal structure of a protein, such as a receptor, scientists have to use x-rays and liquid nitrogen to keep the protein stable. For years, this proved difficult in the case of the delta opioid receptor protein because it’s an extremely fragile part of a brain cell’s membrane.
Sodium in a pocket
But scientists at the Scripps Research Institute developed a novel technique that allowed them to create the first ever high-resolution 3D crystal structure of the delta-opioid receptor. This sharp image revealed a sodium ion at the heart of the receptor.
“Sodium is not everywhere in the receptor,” Giguere says. “It fits in a pocket within the receptor’s structure.”
With that information, Giguere and colleagues in the lab of Professor Bryan Roth, created unique experimental procedures to show how specific amino acids hold the sodium ion in place. They also showed how the amino acids and sodium interact in order to modulate brain signals.
“The amino acids control the sodium ion,” Giguere says. “This control is like a trigger; it has a specific function on the opioid receptor.”
When researchers mutated the amino acids, they saw extreme changes in how the delta-opioid receptor responds to chemical signals.
In one experiment, Roth’s team tweaked an amino acid to cause a major change in the signaling response of the receptor’s beta-arrestin pathway, which is responsible for shutting down chemical signaling.
These findings suggest that it’s possible to create a drug that targets specific pathways inside the delta-opioid receptor.
Current medications either turn on the opioid receptor or turn it off. The new work shows it’s possible to fine-tune the receptor so it functions optimally.
“This is a new field of research, which we call functional selectivity,” Giguere says. “There are very few crystal structures that show how this sort of pathway selectivity can work. This is why we think our findings will lead to another, potentially better class of drugs.”
The research was funded by the National Institute of Drug Abuse, the National Cancer Institute, the National Institute for General Medical Sciences, and the National Institute of Mental Health Psychoactive Drug Screening Program.
Source: UNC-Chapel Hill