Pain is meant to be a defense mechanism. It creates a strong sensation to get us to respond to a stimulus and prevent ourselves from further harm. But, sometimes injuries, nerve damage, or infections can cause long-lasting, severe bouts of pain that can make daily life unbearable.
What if there was a way to simply turn off pain receptors? UNC School of Medicine researchers Bryan L. Roth, MD, PhD, the Michael Hooker Distinguished Professor of Pharmacology, and Grégory Scherrer, PharmD, PhD, associate professor of cell biology and physiology and the UNC Neuroscience Center, have just proved that it is possible.
Using a tool designed by Roth in the early 2000's, the labs have created a new system that reduces acute and tissue-injury-induced inflammatory pain in mouse models. Hye Jin Kang, PhD, an alumnus of the Roth Lab and now associate professor at Yonsei University in Korea, was first author on the research paper. Their results were published in Cell.
"What we have developed is potentially a gene therapy approach for chronic pain," said Roth, who is also a member of UNC Lineberger Comprehensive Cancer Center. "The idea is that we could deliver this chemogenetic tool through a virus to the neurons that sense the pain. Then, you could just take an inert pill and turn those neurons 'off', and the pain will literally disappear."
The Humble Beginnings of Chemogenetics
Neuroscientists have been on a decades-long endeavor to build a comprehensive "map" of the human brain. If every type of cell and every neural pathway could be identified, researchers could make large strides in neurological research - including the ability to turn regions of the brain "on" and "off" to parse out their functions or mimic drug therapy.
Bryan Roth, MD, PhD. Credit: Harvard University Department of Chemistry and Chemical Biology.
In the 90s, Roth, then professor of biochemistry at Case Western Reserve University (with secondary appointments in Psychiatry, Oncology, and Neurosciences), wanted to find a way to make new, powerful therapeutics that could stop diseases without incurring dissuading side effects. It was a tall order, pharmacologically-speaking. So, Roth decided to use an up-and-coming technique called "directed molecular evolution," which essentially uses chemically engineered molecules to speed up the evolution process in nature.
"What I realized, and what a lot of people realized, is, if you could make an engineered receptor that had some of the same signaling properties as a drug of interest, and if you could put it in a particular brain region or cell type, then you could mimic the effects of the drug," said Roth, who is now the project director of the NIMH Psychoactive Drug Screening Program. "We made some several attempts in the 90s, as did other people, without a great deal of success."
The Power to Turn Neurons "On" and "Off" at Will
Roth perfected the chemogenetic technology in 2005. With yeast as his model organism, he engineered an artificial protein receptor that could only be "unlocked" by clozapine N-oxide, a synthetic drug-like compound that had been rendered inert by removing all its therapeutic qualities.
The tool, which is also termed designer receptors exclusively activated by designer drugs, or DREADDs, acts as a molecular lock and key that can only be activated when an inert drug-like compound is introduced to the body. Once activated, the technology can turn neurons "on" or shut them "off," effectively giving researchers the ability to make highly selective changes to the nervous system.
The techniques were revealed to the scientific community in March 2007 in the Proceedings of the National Academy of Sciences. Since then, Roth's technology has been used by thousands of researchers worldwide to study the functions of neurons and develop new medications to treat complex neuropsychiatric conditions - from depression and substance abuse to epilepsy and schizophrenia.
A Potential Gene Therapy for Chronic Pain
Every neuron in our body that is not part of central nervous system (CNS) belongs to the peripheral nervous system, or PNS. This division of the nervous system is responsible for relaying our five sensations to the CNS, allows our muscles to move, and aids in involuntary process such as digestion, breathing, and heart beats.
Relatively few studies have been done on the use of chemogenetics in the PNS, simply because of technical difficulty. The CNS and PNS are so intertwined on a cellular, chemical, and genetic level, that it is challenging for researchers to apply their technology solely to the PNS.
Grégory Scherrer, PharmD, PhD. Credit: NIH HEAL Initiative
"Many of the genes that are expressed in the peripheral nervous system are also expressed in the central nervous system, particularly in the brain," said Scherrer, who is also an associate professor in the UNC Department of Pharmacology. "We had to perform a multitude of analyses and tests to isolate both a receptor and drug-like compound that only operate in the periphery."
However, after seven long years, the Roth and Scherrer labs found success. Researchers based their new system off of hydroxycarboxylic acid receptor 2 (HCA2), a type of receptor implicated in anti-inflammation. HCA2 receptors are expressed in the PNS and are usually activated by vitamin B3. Using mouse models, researchers altered the HCA2 receptors so that they could only bind to FCH-2296413, an inert drug-like compound that only acts within the PNS.
The chemogenetic system, termed mHCAD, is designed to interfere with nociceptors, making it more difficult for the sensory neurons to transmit pain information to the spinal cord and brain. To be more specific, mHCAD reduces their ability to fire off their electrical and chemical messages, requiring a more intense, more painful stimulus will be needed to cause the perception of pain.
Although the technology is still far from human use, Roth and Scherrer have already thought about how the technology would best be delivered in the body: through gene therapy. Researchers successfully injected mHCAD into a mouse model using genetic technology created by colleague and gene therapy pioneer Jude Samulski, PhD, a distinguished professor of pharmacology at the UNC School of Medicine. The gene therapy leverages the infectious abilities of the adeno-associated virus (AAV), allowing researchers to deliver mHCAD into the pain neurons of interest.
Future Uses for Chemogenetics in the PNS
In 2013, the National Institutes of Health formed a partnership between Federal and non-Federal partners with a common goal of mapping every human brain cell and every neural circuit through innovative neurotechnologies called the Brain Research Through Advancing Innovative Neurotechnologies® Initiative, or BRAIN Initiative.
Roth's chemogenetic technology has played a big role in the BRAIN Initiative. To date, tens of thousands of shipments of viruses and plasmids from the Roth lab have been distributed leading to many thousand publications. Now that the technology has expanded to the peripheral nervous system, researchers can better study the neurons that produce the perception of touch, temperature, body position, pain, and more.
"There are dozens of classes of PNS neurons that we don't fully understand," said Scherrer. "By using this new innovative tool, we can then define cellular targets that we can engage with to treat diseases. It's going to be an important tool to increase our knowledge in the somatosensory field and beyond."
