A Matter of Practicality
By Eric Sauter
As a new associate professor in the Department of Molecular Therapeutics on the Scripps Florida campus of The Scripps Research Institute, Laura Bohn, an internationally known researcher investigating the mysteries of G protein-coupled receptors, is focused on basic research—but keeps a highly trained eye on potential practical applications as well.
"Knowing how something works is very satisfying, but at this point I want to see something come of it," she said.
The G protein-coupled receptors (GPCR) that Bohn is studying do have a practical side—they are critical to how patients respond to various drugs, including pain treatments, and those are what she'd like to find a way to improve.
With an estimated 1,000 known genes for these receptors (perhaps as much three percent of all human genes), G protein-coupled receptors are one of the largest and most diverse protein families in the human genome. They transduce or convert extracellular stimuli into intracellular signals through a number of signaling pathways including neurotransmitters, light, hormones, lipids, and proteins. Because of their diverse signaling pathways, approximately one third, and perhaps as many as half, of currently marketed drugs are designed to target these receptors.
Bohn's work is focused on two types—the mu opioid receptor (mOR) and the serotonin 2A receptors (5-HT2ARs). The mu opioid receptor is the primary target of morphine and other prescription pain medications such as Oxycontin. Binding of morphine to the mOR is the first step to pain relief. The serotonin 2A receptors (5-HT2ARs) are molecular targets for drug-induced hallucinations, the kind that accompany certain neuropsychiatric disorders, such as schizophrenia.
"Historically, we have always looked at drug effects in the body," said Bohn, who is 38. "Since the 1970s, scientists have been focused on how a drug interacts with receptors. What has never been fully appreciated is that receptors may act differently in different organs, say in the brain versus the gut. We now realize that if we could make a drug like morphine act differently in the brain than it does in the gut, we could get pain relief without certain side effects, such as constipation, that are associated with it. By similar reasoning, there is also the hope that analgesic properties could be improved while addiction could be avoided altogether."
First, of course, you have to determine how the drug functions in various tissues and then find new drugs (or modify old ones) that will turn on the good pathways and not the problematic ones.
It's a complicated and difficult process, something that Bohn has been working on since her postdoctoral days in the late 1990s.
"There have been attempts to modify morphine since the 1900s," she said, "and these compounds turned out to have many of the same problems as morphine itself. We need to figure out how to make these potent compounds maintain potency in the brain and not produce side effects."
When activated, a G protein-coupled receptor couples to a G protein, which in turn activates a downstream signaling cascade that ultimately results in a particular biological response; any signal disruption may lead to disease, including diabetes, cardiovascular defects, and even some types of cancer.
A G protein-coupled receptor can be turned off (or turned down) by GPCR kinase (GRK)-mediated phosphorylation that is followed by the binding of beta-arrestin (barrestin) proteins; kinases add a phosphate group to molecules, changing their activity. This process desensitizes most receptors. However, some GPCRs can still signal via other pathways after binding with barrestin. Barrestin is involved in the serotonin 2A receptor activity as well.
What Bohn and her colleagues want to know is how the interaction of these two proteins, GRK and barrestin, determines the drug action response at the receptor. They have made some fundamental discoveries.
In earlier work performed at Duke University, Bohn found that mice lacking barrestin have greater analgesic responses to morphine with few side effects—in other words, their pain relief was greater, with less constipation and little respiratory suppression. The work suggests that barrestin plays an important role in the development of morphine-induced tolerance, constipation, and respiratory depression and offers one avenue for drug development.
"We still have a ways to go but we're doing pretty well," she said. "If we could find a drug that would activate the receptor but not recruit barrestin, we might have a drug with no tolerance and fewer side effects."
A Different Approach
Part of her hope for progress is pinned on her methods, which rely on transgenic mouse models with genetic deletions of GRKs and barrestin.
"We're doing our research differently than others," she said. "We're not doing it in cells but in animal models. Cell studies are limited, they tell you what can happen but not what does happen in a real tissue environment. The good thing is once we figure out what does happen we can make cellular models and use those for screening compounds."
Bohn has at least one candidate, a compound she found while an assistant professor of pharmacology at Ohio State, her academic home until she moved to Scripps Florida last March. The compound is called herkinorin, a novel mu-opioid agonist derived from salvinorin A, a hallucinogenic molecule from a type of salvia plant, a well known herb used by Mexican shamans to produce altered mental states during native religious ceremonies.
Herkinorin does not promote the recruitment of barrestin to the receptor; nor does GPCR kinase over-expression have any impact on morphine-induced beta-arrestin interactions.
"What we do is follow the physiology back to the receptor," she said. "Herkinorin looks like a good molecule in theory and in cell culture but the shape of the receptor is altered by what binds to it within the cell. So, ultimately, you have to find a way to fine tune the receptor where it normally resides in the body."
The field of pain relief is a very big place to go looking for something—and a big effort is being made.
"There are a lot of people working on this problem," she said. "When I started graduate school, I was always interested in the neurochemistry of the brain, that what you ingest can influence thought and perception. That's still the fundamental question. Right now, I'm very interested in understanding the line between these paired realities—that on one side, opiates are good because they block pain, while on the other, opiates are bad because they cause so many side effects. To me that poses an endless line of questions. It's a very deep well."
The well also includes the impact of these receptors on mental disorders. In another study, Bohn and her colleagues examined the serotonin 2A receptor, a protein on brain cells sensitive to the neurotransmitter serotonin and to hallucinogenic drugs. But when they looked closely, they found that while both compounds activated the receptor, they trigger different chemical pathways inside the neuron.
While important in and of themselves, these findings have broader implications—another case of basic research driving practical developments.
"Obviously, what we found could have a significant impact on the development of drugs affecting the serotonin 2A receptor, which is a key target in treating several mental disorders, including schizophrenia and depression," she said.
Searching for Answers
As she searches for answers, Bohn keeps in mind that the field is constantly shifting.
"Everything we do is always subject to change," she said. "For a while we thought neurons didn't divide, that you had a certain number from birth. If you operated from that belief and studied the brain, you would think you could never make a neuron grow. But that wasn't true. Biology doesn't really play by the rules as much as we think it does. Or maybe it's that the rules aren't as solid as we think they are—they're much more fluid."
In her career, Bohn has also tried to keep a fluid perspective. After graduating from Virginia Polytechnic Institute and State University in 1993 with dual bachelor's degrees in biochemistry and chemistry, Bohn got her Ph.D. from Saint Louis University in biochemistry and molecular biology. She did her postdoctoral work at Duke University in Marc Caron's laboratory; Caron in collaboration with Robert Lefkowitz, developed the genetically engineered animal models lacking the barrestin protein.
While she was a faculty member at Ohio State for a while, she ultimately became restless.
"I wanted to step my science up," she said. "I wanted to grow to the level of excellence around me—if you want to get better at tennis you challenge somebody better than you. I wanted to go where the expectations were high. While there is excellent science at Ohio State, I wanted to be where I could find more collaborations in my field of study."
The place she chose was Scripps Florida.
"It's unique because it has a stellar reputation and I get to come in on the ground floor," she said. "One of the big attractions is that I have a lot of good people to interact with—the whole Drug Discovery group, fellow researchers interested in addiction and a developing department focused on neuroscience. There are such tremendous opportunities for synergy here."
Send comments to: mikaono[at]scripps.edu