More people suffer from pain than from heart disease, diabetes and cancer combined, but many of the drugs used to relieve suffering are not completely effective or have harmful side effects.
"This protein has the potential to be a groundbreaking treatment for pain and has previously not been studied in pain-sensing neurons," said lead study author Mark J. Zylka, Ph.D., assistant professor of cell and molecular physiology at UNC.
The biological basis of pain is complex. To study the transmission of painful signals throughout the body, many researchers use "marker" proteins that label pain-sensing neurons. One such marker, FRAP (fluoride-resistant acid phosphatase), has been employed for this purpose for nearly 50 years, but the gene that codes for its production was never identified. That is, until researchers at UNC found that FRAP is identical to PAP (prostatic acid phosphatase), a protein routinely used to diagnose prostate cancer whose levels increase in the blood of patients with metastatic prostate cancer.
Previous research hinted that FRAP and PAP may have a shared identity. To determine whether or not this was the case, Zylka teamed up with Dr. Pirkko Vihko, a professor from the University of Helsinki who had genetically engineered mice that were missing the gene for PAP. When Zylka and his colleagues studied tissues from these mutant mice, they were happy to see that FRAP activity was missing. This revealed that the two proteins were in fact identical.
Further, the mutant mice proved more sensitive than normal mice to inflammatory pain and neuropathic pain, two common forms of chronic pain in humans. These increased sensitivities diminished when researchers injected excess amounts of PAP into the spinal cords of the mutant mice.
"We were really blown away that a simple injection could have such a potent effect on pain," Zylka said. "Not only that, but it appeared to work much better than the commonly used drug morphine."
The new protein suppressed pain as effectively as morphine but for substantially longer. One dose of PAP lasted for up to three days, much longer than the five hours gained with a single dose of morphine.
The next question for the researchers was how PAP suppressed pain. It is already known that when pain-sensing neurons are stimulated, they release chemicals known as nucleotides, specifically adenosine triphosphate (ATP). This in turn sets off the events that invoke a painful sensation. But if ATP degrades to adenosine, that inhibits the neurons that transmit pain signals, thus relieving pain. Through a series of experiments, the UNC researchers showed that PAP removes the phosphate group, generating adenosine. Their study is the first to identify and characterize the role of such a protein in pain-sensing neurons.
Zylka and his colleagues are now searching for additional proteins that degrade nucleotides in these neurons. They are also working to develop small molecules that interact with PAP to enhance or mimic its activity.
The research undertaken at UNC was supported by grants from the Sloan Foundation, the Searle Scholars Program, the Klingenstein Foundation, the Whitehall Foundation, the Rita Allen Foundation and the National Institute of Neurological Disorders and Stroke, part of the National Institutes of Health.
The research undertaken at the University of Helsinki was supported by grants from the Sigrid Juselius Foundation, the Finnish Cancer Foundation and the Research Council for Medicine of the Academy of Finland.