Pain
afflicts at least 1.5 billion people worldwide, and despite the
availability of various painkilling drugs, not all forms of pain are
treatable. Moreover, pain medications can have side-effects such as
dependence and tolerance, especially in the case of morphine and other
opioids.
In
search of novel painkillers, researchers studied TRPV1, a sensory
neuron receptor that captures noxious stimuli, including heat and the
burning sensation conveyed by chili peppers, and discovered a potential
pain insensitivity mutation in the gene that encodes this protein. They
report their findings in an article published in the Journal of Clinical Investigation.
The
researchers analyzed a number of mutations in humans and also benefited
from existing knowledge of birds, which unlike mammals have a TRPV1
receptor that is naturally resistant to noxious insults and even peppery
food, yet can still perceive pain.
“There
are more than 1,000 TRPV1 mutations in humans, and there’s nothing
novel about trying to switch the receptor off in order to relieve pain,
but these attempts haven’t been successful until now,” said a
co-first author of the article. “First, many drugs resulting from this
process interfere with body temperature regulation. Second, TRPV1 is an
important channel for signaling heat, and completely altering its
activity cancels out physiological pain, interfering with the sensation
of burning heat, which has a protective function.”
The
researchers began by exploring a genome database to compare the genetic
sequences of avian and human TRPV1. Using a computational approach,
they identified five avian mutations they believed to be linked to
resistance to pain. Cryogenic electron microscopy (which does not
require large sample sizes or crystallization and is therefore suited to
the visualization of structures at near-atomic resolution) showed that
the five avian mutations were located in K710, an amino acid residue
believed to control gating (opening and closing) of the TRPV1 channel.
The
mutations can also be present in humans, but they are very rare, so the
researchers decided to find out what would happen if they were
“transplanted” into mammals. When they tested these variants in
genetically modified cells, they found that the function of the channel
was indeed altered. Next, they used the CRISPR/Cas9 gene editing
technique to create mice with the mutation K710N, which they had
previously found to reduce the receptor’s reaction to capsaicin in
cells. Capsaicin is the active principle in pepper.
The
researchers did not observe nociceptive behavior (suggesting avoidance
of pain) in mice with the K710N mutation injected with capsaicin and
given peppery chicken feed, in contrast with the behavior of normal
mice, which lifted their paws to avoid touching the capsaicin,
presumably because even skin contact caused pain.
The
mice with the K710N mutation also showed less hypersensitivity to nerve
injury, while their response to noxious heat remained intact.
Furthermore, blocking the K710 region in normal mice limited acute
behavioral responses to noxious stimuli and returned pain
hypersensitivity induced by nerve injury to baseline levels.
In
addition to modulating pain, TRPV1 also plays an important role in
protection against other stimuli. For example, recent evidence suggests
that it serves in non-neuronal cells as an intracellular molecular
sensor that protects against glucose-induced cellular stress or tissue
ischemia. Additional tests performed as part of this study involving
cardiomyocytes (heart muscle cells) insulted with hydrogen peroxide,
high levels of glucose and a cerebral ischemia model confirmed the
protective effect even with the mutation.
The
second part of the study consisted of an attempt to reduce the
receptor’s function pharmacologically. To this end, the researchers
developed a peptide, V1-cal, which acted selectively on the K710 region.
Mice treated with V1-cal and given capsaicin displayed less nociceptive
behavior and diminished release of neuropeptides leading to neurogenic
inflammation and edema without altering temperature. Lastly, chronic
pain also improved considerably.
“We
now want to add value to this study by validating the results under
best-practice laboratory conditions [required by regulatory agencies],
identify other small molecules besides the peptide that can more easily
be synthesized, conduct preclinical trials and, if these are successful,
begin a clinical trial,” the author said.
https://www.jci.org/articles/view/163735
http://sciencemission.com/site/index.php?page=news&type=view&id=health-science%2Fa-human-trpv1-genetic&filter=22