Biology concepts – nature of science, TRPA1, thermoregulation, noxious sensor, chemical sensor, mutation, protein domains
"As we acquire more knowledge, things do not become more comprehensible, but more mysterious."
Schweitzer became a minister, but toured the world's great churches giving organ concerts for big bucks. He used the money to put himself through medical school and establish a hospital in French Equatorial Africa in the 1910’s. He expanded his hospital to over 70 buildings and treated up to 500 patients at a time, always funding his efforts with his organ concert money and the money he made from writing books.
He was awarded the Nobel Peace Prize in 1952. His quote above is true, whether he was speaking of the secrets of life in the spiritual or scientific sense. I personally phrase Schweitzer’s sentiment a little differently. The more we know, the less we know we know. Scientific knowledge is meant to do two opposite things – one, answer questions, and two, create questions. Every decent answer we think we find should bring to mind many more questions.
This naturally occurs when science works at its best, but the hardest part is knowing if we have an answer. If we aren’t sure about the answer, we have to keep looking. Of course we always keep looking, but some answers come to be so well supported that it is not the answer we question but the details of the answer – like evolution or fundamental forces.
But if the answers are tentative or not well supported, then how good are the questions that spring from them? Can our questions only be as good as our previous answers? This I where we're lucky, because bad questions can lead to good answers, if we keep our minds open to what we see and don’t find just we are expecting to find.
How does this apply to our discussion of thermoregulation and heat/cool sensing? Well, what happens when you find a thermosensor that you can’t get a good answer as to what it does? Makes it hard to design new questions doesn’t it?
TRPA1 was first described in drosophila melongaster (fruit flies). Historically, fruit flies have made great models because they eat cheap and reproduce quickly. You can read about the history of their use in genetics in a great book called The Violinist’s Thumb by Same Keene.
The scientists would induce mutations in flies (and their subsequent offspring) by giving them radiation or chemicals. They wouldn’t have any idea which flies were mutated, or what the mutations were, it was just a shotgun method. They would then study the flies, looking for abnormal anatomy, abnormal behavior, or abnormal responses to stimuli. In some cases, the mutations were spontaneous, not caused by radiation or the chemicals, but finding them was done the same way, and the ionizing radiation or mutagenic chemicals just made the mutation rate much higher. When they found a fly with a change, then they would go to work and identify the mutation.
One mutation they noticed was that some flies wouldn’t avoid things that should have been painful. Before they knew what gene was involved, they decided to call it painless. Comparing it to known genes, they found it was the drosophila homolog to a mammalian TRP called TRPA1 or ANKTM1. TRPV’s and TRPM8 were already known, and they all have ankyrin repeats, so I don’t know why TRPA1 got the “A” for ankyrin.
Ankyrin is just one of thousands of known protein domains, meaning short sequences of amino acids that are known to have specific functions. Ankyrin is often found to mediate protein folding and protein-protein interactions, even though its own folding is a little out of the ordinary. Most proteins with ankyrin domains have about 4-6 repeats of the 33 amino acid sequence, but the parasite Giardia lamblia has a protein with 34 repeats.
So, flies with broken painless (TRPA1) genes didn’t respond to pain; therefore, TRPA1 must be a gene that codes for a protein that confers a pain signal. This meshed well with the information from mammals showing that TRPA1 stimulated pain signals from some chemicals. But was this all?
This is where the controversy began and still continues. Some studies find that TRPA1 is a noxious chemical receptor, some say it's a noxious cold receptor. Some experiments show it to be both, a receptor for pain from cold and a receptor for pain from chemicals. And then there are those that show it to be a heat receptor! More on those studies in a couple of weeks – they’re cool… I mean hot!
Even within mammals, the results can sometimes be very different. Old studies suggested that TRPA1 was a noxious cold sensor (below 15˚C) in humans, but newer research (2013) shows that while TRPA1 does sense cold in rats and mice, it isn’t affected by cold in humans or monkeys. Many of the older reports suggesting that TRPA1 is a noxious cold sensor were based on studies in mice, so maybe they do act differently in humans.
The other possibility is that they don’t sense intense cold directly, but work with other TRPs to respond with pain when cold is sensed. A 2014 study showed that TRPA1 modulates TRPV1 activity. The two are often co-expressed on the same neurons, and they are also activated by many of the same chemicals. This study shows sensitization of TRPV1 by activation of TRPA1 – so maybe this is why your hands burn when you go out in to the cold for a long time.
An earlier study (2012) also showed that activity from TRPV1-4 receptors could modulate the activity of TRPA1. Gentle warm temperatures could desensitize TRPA1 and therefore keep pain from being felt. Could this be one of the ways that warm compresses work against pain?
So, if TRPA1 modulates TRPV1 and vice versa for pain sensation, maybe TRPA1 works with TRPM8 to induce noxious cold pain. There have been papers that suggest TRPM8 does sense cold temperatures below 15˚C and when those cells are lost, mice have no aversion to painfully cold stimuli. It is probable that TRPA1 works with TRPM8 and TRPV1 to elicit pain to cold temperatures.
Maybe we could get some insights into the cold sensing of TRPA1 if we could find out if it participates in warming the body when it is cold. TRPV1 is a heat sensor and initiates cooling programs. TRPM8 sense cool and starts to warm the body – so what about TRPA1?
Well, it looks like we get no help there at all. Even in the species that are most likely to have noxious cold sensation via TRPA1 (rats and mice), the channel doesn’t look to be calling for warming responses. In fact, a 2014 study suggests that when TRPA1 ion channels are knocked out in mice, cold temperatures induced physiologic changes just as if the TRPA1 was there – TRPA1 was not a participant in inducing warming activities.
Oh well, like we said at the beginning of the post, the more we know, the less we seem to know for sure. I think TRPA1 is probably involved in cold pain, even if it doesn’t sense it directly. But I can’t wait to see what they find out next.
What we do know is that TRPA1 is intimately involved with pain. Migraine headaches probably have a TRPA1 component. A 2013 paper summarized the evidence by saying that many migraine triggers are now known to be TRPA1 activators. Many of the endogenous stress activators of TRPA1, like oxidative damage, electrophilic stress, etc. also act to induce pain. Finally, many of the drugs and analgesics that work on migraines are being identified as TRPA1 antagonists.
Since it’s evident that TRPA1 doesn’t work in thermoregulation (see above), maybe we can use antagonists of TRPA1 as pain drugs without worrying about the hyperthermias and hypothermias associated with TRPV1 antagonists and TRPM8 antagonists. And wouldn’t you know it, a new antagonist for TRPA1 has just been discovered in a weird place.
Meet the Peruvian green velvet tarantula. It does have a
green hue on its legs and it is soft and velvety. But it isn’t
from Peru. It actually lives in northern Chile, south of the
Peruvian border. Its venom contains a TRPA1 antagonist,
but the problem is that even though it is not likely to bite
the hand that feeds it, it will fling urticating hairs at the
drop of a hat. This is important, as we discussed here.
One last tidbit about TRPA1 – it could save your life in the middle of the night. If you block mouse nasal activity of TRPA1, they won’t wake up in response to formalin, acrolein, or other noxious stimuli that should generate an avoidance response. Would a house fire be something you need to wake up from – you bet. Another recent study found that TRPA1 sensors in upper airway cells are important for sensing smoke from wood fires. Don’t hate the TRPA1 because it gives you pain – enjoy the pain – it’s keeping you safe.
Next week – prepare to throw TRPA1 a party; it’s saving your life in many more ways.
For more information or classroom activities, see:
Albert Schweitzer –
Protein domains/motifs –
Pervian green velvet tarantula –