Science Signaling Podcast: 15 March 2011

Technical Details

Length: 14 min

File size: 8.4 MB

File Format: mp3

RSS Feed: http://stke.sciencemag.org/rss/podcast.xml

Listen to Podcast: http://podcasts.aaas.org/science_signaling/ScienceSignaling_110315.mp3

Educational Details

Learning Resource Type: Audio

Context: High school upper division 11-12, undergraduate lower division 13-14, undergraduate upper division 15-16, graduate, professional, general public and informal education

Intended Users: Teacher, learner

Intended Educational Use: Learn, teach

Discipline: Biochemistry, cell biology, developmental biology, evolutionary biology, invertebrate biology, molecular biology

Keywords: Science Signaling, Drosophila melanogaster, fruit fly, light, photoreception, rhodopsin, temperature, thermosensation, thermotaxis, transient receptor potential channel, TRP

Transcript

Host – Annalisa VanHookWelcome to the Science Signaling Podcast for March 15, 2011. I’m Annalisa VanHook, and today I’ll be speaking with Craig Montell, senior author of a Research Report in the March 11th issue of Science (1). His group reports an unexpected role for rhodopsin independent of its well-known role in sensing light.

Rhodopsin is a G protein–coupled receptor that functions as a light sensor in the visual systems of animals. It’s a transmembrane protein that initiates intracellular signaling in response to light and has been thought to function exclusively in light sensation. In a paper published in the current issue of Science, Montell’s group reports an unexpected role for rhodopsin in thermosensation. Specifically, they report that rhodopsin is required for fruit fly larvae to crawl towards an area of their preferred temperature, a phenomenon known as thermotaxis.

Montell joined me on the phone from his lab at The Johns Hopkins University School of Medicine in Baltimore.

Interviewer – Annalisa VanHookWelcome, Dr. Montell.

Interviewee – Craig MontellWell, thanks for asking me to participate in this podcast.

Interviewer – Annalisa VanHookWhat’s currently known about the molecular machinery that’s required for thermotaxis in fruit flies?

Interviewee – Craig MontellSo, in flies the molecular sensors that respond to changes in temperatures are members of a large family of proteins that are called TRP channels, which we originally studied some years ago due to their role in visual transduction. And thermosensory TRP channels were actually first identified in mammals back in ’97 by Caterina and Julius (2), and as with the mammalian thermo-TRPs, as we call them, the Drosophila TRPs that function in the sensation of uncomfortably warm temperatures are directly activated by changes in temperature.

Interviewer – Annalisa VanHookFor a fruit fly, what is a comfortable temperature, and what’s an uncomfortably warm temperature?

Interviewee – Craig MontellSo, fruit flies find any temperature between 18 and 24 degrees optimal. Larvae prefer 18, but the adult flies prefer 24. And the study that we did was focusing on larvae, so they prefer 18 over any other temperature. But, even very small differences in temperature in this comfortable range—18 to 24—can have really a profound effect on their rate of development and on their life span. And that’s true for many other cold-blooded animals, as well.

Interviewer – Annalisa VanHookWhat prompted you to consider that rhodopsin might have a role in thermosensation?

Interviewee – Craig MontellWell, it actually started about three years ago. We were interested in trying to figure out the mechanism by which fly larvae identify their favorite temperature within the comfort range, which I just defined as 18 to 24 degrees, and they prefer, of course, 18. And so, we found that larvae can really discriminate very small differences in this comfortable temperature range through TRP channels, but not through direct activation of TRP channels. Rather, they required a thermosensory signaling cascade, and this culminated with the activation of a channel called TRPA1. And so, what we found in this earlier study was that the other molecular players that were required upstream of TRPA1 were molecules that function subsequent to the activation of G-coupled receptors, or GPCRs, as we refer to them. And this was really the earliest example of a thermosensory signaling cascade culminating with the activation of a TRP channel. But, in any case, that made it likely that some GPCR initiated the thermosensory signaling cascade. And, of course, the problem is that there are hundreds of GPCRs in flies, and no GPCR had previously been shown to be required for thermotaxis. So, the classical GPCR rhodopsin, which, of course, is a light sensor, was a good candidate since we found that the other molecules known to function downstream of rhodopsin and upstream of TRPA1 were required for discrimination of, of temperature in this comfortable range.

Interviewer – Annalisa VanHookIn this current study, then, you looked at flies that were lacking the gene that encodes rhodopsin. What effect did that have on their thermotaxis?

Interviewee – Craig MontellThe rhodopsin mutants had really a very specific thermotactic defect—they were unable to choose 18 over any other temperature in the comfortable range. But, they had really a normal ability to avoid uncomfortably cool or uncomfortably warm temperatures—that was just like wild-type. The, the rhodopsin mutant larvae are absolutely normal in avoiding uncomfortably warm and uncomfortably cool temperatures. And this, in a way, we think, makes sense. Before this work, we only knew of a direct mode for activating TRP channels, and, of course, now there’s this indirect, this thermosensory signaling cascade. And we think it sort of makes sense that you would have a thermosensory signaling cascade in the comfortable range because one of the advantages of having signaling cascades is for amplification of the signal. So, we think this thermosensory signaling cascade is specifically required in the comfortable range to enable these animals to amplify very small differences in temperature in the comfortable range. But, we think having this thermosensory signaling cascade specifically in the comfortable range has a second and maybe even more important role, and that’s for thermal adaptation. So, if you’re a larvae, say, at 30 degrees or 32 degrees, and you’re there for very long periods of time, you’ll eventually die. You don’t want to adapt to that temperature. If you’re a larvae, say, at 20 degrees, and you can’t find your favorite temperature, which is 18 degrees, well, 20 degrees is still permissive for growth and survival, so it’s okay to adapt to that temperature. And so, these GPCR signaling cascades are very well suited for, for adaptation. So, we think the direct activation of TRP channels—say, in the noxious heat range or the uncomfortably warm range—is really an issue of survival. But, we think that having a rhodopsin-initiated thermosensory signaling cascade is really more of a quality-of-life issue. If you can find your optimal temperature—18—in a reasonable period of time, that’s great. But, if you can’t, why keep avoiding other permissive temperatures like 20 or 22? It’s okay to adapt.

Interviewer – Annalisa VanHookDoes light play any role in this rhodopsin-mediated thermotaxis towards the most comfortable range, or is it completely light-independent?

Interviewee – Craig MontellSo, that, that’s really, I think, a very important question. And, in fact, a very related question is “Since rhodopsin is a light sensor, one might wonder, well, wouldn’t light interfere with the ability to sense temperature?” So, it turns out that in photoreceptor cells, rhodopsin is expressed at incredibly high levels, and that’s necessary for efficient photon capture because a single photon is hitting a very discrete place in the photoreceptor cell and will be missed unless you had rhodopsin expressed at incredibly high levels in the membrane. The opposite situation is in the thermosensory neurons. It turns out that in these neurons, rhodopsin is expressed at incredibly low levels, so low you can barely detect it, and this is to avoid efficient photon capture. So, in fact, the rhodopsin in the thermosensory neurons is not sensing light, and you don’t want it to sense light, so this way you won’t get interference with light.

Interviewer – Annalisa VanHookObviously, rhodopsin is not activated the same way—that activating it by changes in temperature is not the same as activating it by hitting it with a photon.

Interviewee – Craig MontellRight.

Interviewer – Annalisa VanHookBut, in terms of the molecular conformation changes that are happening in rhodopsin in response to light, in response to temperature differentials, are the molecular changes in rhodopsin similar between the two stimuli?

Interviewee – Craig MontellWell, the final activation of the opsin may be similar. So, the conformational change that rhodopsin has to undergo to activate its effector, the trimeric G protein, may actually be the same regardless of whether the initiation is light activation of the chromophore, which, in turn, activates the opsin, or temperature, which more directly activates the opsin. Either way, the opsin part needs to activate the trimeric G protein. So, the final activation step of the opsin is probably quite similar.

Interviewer – Annalisa VanHookCould rhodopsin act as both as a light sensor and as a thermal sensor in the same cell types?

Interviewee – Craig MontellSo, we don’t think so because—at least not in the thermosensory neurons, because if light was activating the rhodopsin, then it would interfere with thermosensation, and, again, we really don’t think that’s happening in the thermosensory neurons because the rhodopsin is expressed at such low levels that it wouldn’t effectively be absorbing photons. Now, in the photoreceptor cells, you could imagine that the light response might differ at different temperatures, and we actually tested that. And we don’t see—in the photoreceptor cells—a different response either, say, at 18 degrees or 25 degrees. The electrical response of the photoreceptor cells is the same. Well, it turns out that there’s at least one protein that we know of in the photoreceptor cells, and this is not work that we’ve described in this paper, but we do know of at least one protein that—in the photoreceptor cells—that when you take it away, it increases the activity of rhodopsin by temperature. And so, we think that the thermal activation of rhodopsin in the photoreceptor cells is being actively suppressed.

Interviewer – Annalisa VanHookDo you think that this function of rhodopsin has evolved specifically in insects or arthropods, or do you think that it could act as a thermosensor in other species?

Interviewee – Craig MontellSo, that’s that’s a really interesting question. And it turns out that in mammals—in addition to the rhodopsin proteins, the light receptors in rods and cones—that a bit more than 10 years ago, there was a third class of photoreceptor cell identified. About 1% of the retinal ganglion cells in the mammalian retina is intrinsically photosensitive. And its primary role is not in image formation but in allowing animals like mice, and presumably people, to respond to light over their circadian cycle. It turns out that the opsin-like molecule in these new class of photoreceptor cells in mammals—which is called melanopsin—both at a biophysical level and at a sequence level, is far more similar to Drosophila rhodopsin than it is to the light receptors in mammalian rods and cones. So, to test the idea that rhodopsins may have some role in thermosensation in mammals, we looked to see if the phenotype—the inability to sense temperature in the comfortable range that results from eliminating the Drosophila rhodopsin—can now be rescued by putting mammalian melanopsin back into the flies. And we found it did largely rescue it. So, we think the concept that opsins respond to changes in temperature is evolutionarily conserved. And you could imagine that it might have roles in functions such as allowing mammals to sense very small differences in core body temperature. Given that the rhodopsin in flies, and possibly also melanopsin, may also function in thermosensation in addition to light sensation, it really makes you wonder what the original role of these rhodopsin-like molecules were. Were they originally functioning in light sensation or in thermosensation? And that’s obviously an unresolved question.

Interviewer – Annalisa VanHookThank you, Dr. Montell.

Interviewee – Craig MontellIt was my pleasure.

Host – Annalisa VanHookThat was Craig Montell, senior author of a Research Article published in the March 11th issue of Science. That paper is by Shen and colleagues, and it’s titled “Function of Rhodopsin in Temperature Discrimination in Drosophila” (1).

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And that concludes this Science Signaling Podcast. If you have any questions or suggestions, please write to us at sciencesignalingeditors{at}aaas.org. This show is a production of Science Signaling and AAAS—Advancing Science, Serving Society. I'm Annalisa VanHook, and on behalf of Science Signaling and its publisher, the American Association for the Advancement of Science, thanks for listening.