In a new study, published on June 28, 2011 in Nature Communications, researchers from The Scripps Research Institute have recognized a remarkable new molecular pathway in skin cells that is involved in wound-healing and sensory communication.
Scientists have identified the ability of skin cells to produce nitric oxide via an oxygen independent mechanism earlier believed to take place only outside cells. Nitric oxide is a signaling molecule that plays significant roles in sensing of temperature and the healing of body wounds. It also plays a role as a neurotransmitter in the brain.
Ardem Patapoutian, a professor at the Dorris Neuroscience Center at Scripps Research and the senior author of the study said:
“This alternative nitric oxide production process could prove to be crucial in the clinic,” “The usual nitric oxide production process requires oxygen, so drugs that target that process might not work when oxygen availability is low after blood supply disruption.”
Molecular biology of skin-based sensory pathways is the main focus of the laboratory where Dr. Patapoutian spends most of his time conducting research. Nerve ends contain numerous stimulus sensing receptors which are generally the starting points of these sensory pathways. One such class of receptors is the TRPV (transient receptor potential vanilloid) which enable us to perceive various temperature- and pain-related stimuli. Although TRPV receptors are of various types, it is the TRPV3 receptors that are of importance in the current study. In addition to being found on nerve cells and nerve endings, they are also found on outer skin cells known as keratinocytes.
TRPV3 receptors appear to be responsible for perception of heat, since mice bred without them lacked a normal sensitivity to moderately warm stimuli. This was published in 2005 by Patapoutian’s lab in the journal Science.
Patapoutian said:
“That and previous findings made us suspect that TRPV3-expressing keratinocytes are somehow involved in sending thermosensory signals to local nerve ends.”
In the current study, Patapoutian et al proved that nitric oxide is produced by outer skin cells upon activation of the TRPV3 receptors. This is highly suggestive of nitric oxide acting as a carrier of thermosensory signals from skin cells to nearby nerve ends.
Miyamoto, a graduate student of Patapoutian and the first author of the research study said:
“Nitric oxide was high on our list of possibilities because it is known to be produced in keratinocytes when they are warmed.”
Upon application of substances that are known to activate TRPV3 to cultured mouse skin cells, Miyamoto witnessed an appreciable increase in their production of nitric oxide.
He said:
“The surprise was that I couldn’t find evidence that the nitric oxide was being produced in the normal way, with nitric oxide synthase (NOS) enzymes. The keratinocytes turned out to be producing nitric oxide through a different process, which is known to occur in saliva and other bodily fluids, but hadn’t yet been seen in cells.”
Our body receives nitrites from the food we eat and removal of oxygen atoms from them results in the production of nitric oxide. This is the alternate mechanism of how nitric oxide gets produced in the body without the use of NOS enzymes. Miyamoto confirmed this mechanism by depriving the cultured skin cells of nitrites which resulted in an immediate cessation of their TRPV3-triggered production of nitric oxide.
To ascertain that nitrites played a key role in this pathway, Miyamoto compared the mice bred without TRPV3 – which don’t distinguish two different innocuous warm temperatures – to those with no-nitrite diets. He said, “The behavior of the no-nitrite mice was basically the same as that of the TRPV3-knockout mice,” “Feeding TRPV3-knockout mice with no-nitrite diets had no additive effect, which again suggests that the two work on the same pathway.”
Following this, the researchers wondered, “If nitric oxide is a messenger that delivers temperature-sense signals from skin cells to nearby nerve ends, then to what nerve-end receptor does it bind?” The answer to this question came from a research that was previously conducted in their lab and was published in 2009. The results of this study had demonstrated that TRPV1 receptors on nerve endings get activated by nitric oxide. These receptors enable us to perceive pain and temperature. In the current study the scientists used a chemical to stop the activity of TRPV1 receptors in mice. The results showed that absence of TRPV3 receptors or lack of nitrites in mice made no difference in the way they react to different innocuous warm temperatures. This observation was completely in line with the idea that TRPV1 is the main nerve-end receptor on this thermosensory pathway, acting directly or indirectly.
Considering how versatile nitric oxide is as a signaling molecule, scientists have also suspected other pathways that may be mediated by TRPV3. Miyamoto stated:
“We found evidence that the nitric oxide produced by this pathway makes a partial contribution to wound-healing and also specifically to the keratinocyte migration that occurs during wound healing.”
The researchers now plan to detail the elements of the TRPV3-activated nitric oxide pathway in temperature sensing. They will also look for evidence that a similar nitrite-dependent pathway is involved in other nitric oxide-producing cells throughout the body.
Miyamoto said:
“The dogma has been that nitric oxide can be produced in cells only with NOS enzymes, but this study hints that nitrite-based nitric oxide production could potentially be just as important.”
Takashi Miyamoto, Matt J. Petrus, Adrienne E. Dubin & Ardem Patapoutian
Nature Communications 2, Article number: 369 doi:10.1038/ncomms1371
Written by Anne Hudsmith