Friday, May 29, 2009

TRP Channel Variations Determine What's "Too Cold"

ResearchBlogging.org In the fairy tale of Goldilocks and the Three Bears, Goldilocks encounters three bowls of porridge in the bear household. Papa Bear's porridge is too hot; Mama Bear's porridge is too cold; Baby Bear's porridge is just right. Why would Papa Bear and Mama Bear choose to heat their porridge to non-optimal temperatures? A new study by Benjamin Myers et al. published in PLoS ONE suggests that their perception of "just right" depends on their TRP channels.

Okay, it's unlikely that three members of an anthropomorphic bear family would possess significantly different temperature-sensitive ion channels. But the paper shows that warm- and cold-blooded species living in different environments have evolved ion channels with different temperature thresholds to help them maintain a body temperature that is "just right."

Transient receptor potential (TRP) ion channels are responsible for many different sensory functions. The first TRP channel was discovered in the fruit fly, where it plays a role in visual signaling. Many other TRP channels have since been found. Though these channels share a similar structure, they contribute to many different sensory systems, including touch, pain, taste, and smell. A class of TRP channels also responds specifically to temperature, allowing nerve endings in the skin to sense heat and cold. Interestingly, the channels also detect certain chemicals that create a hot or cool sensation even without a change in temperature. In 1997, we learned that the heat-sensitive TRPV1 channel also responds to capsaicin, the molecule that makes chili peppers taste "hot" (Caterina et al.). In 2002, other research showed that TRPM8, a cold-sensing ion channel, can be activated by the minty freshness of menthol (McKemy et al.; Peier et al.). But although we have learned a lot about the function of temperature-sensitive channels in mammals, not much work has been done in other model organisms.

Myers et al. examined the TRPM8 channel in a commonly used cold-blooded model organism, the South African clawed frog (Xenopus laevis). They wanted to see whether this species, with a core body temperature and preferred environmental temperature much lower than that of mammals, would have a different range of temperature sensitivity in its TRPM8 channels. They hypothesized that these animals would have channels tuned to temperatures appropriate for their ecological niche, meaning that they would require a colder temperature to become active than the TRPM8 channels of a warm-blooded mammal or bird.

To test this theory, the researchers dissected out the dorsal root ganglia (DRG) of several frogs and used calcium imaging to measure the cells' responses to different temperatures. The DRG is the portion of the spinal cord that contains the cell bodies of sensory neurons, including the cells that produce temperature-sensitive nerve fibers. Thus, the cold-sensitive cells in the DRG express the TRPM8 channel. Calcium imaging involves use of a fluorescent dye that produces light when exposed to calcium inside a neuron. Because calcium influx is related to neuronal activity, we can use the fluorescent intensity to determine which neurons respond, and how strongly, to a given stimulus.

About 23.7% of the X. laevis DRG neurons responded to menthol, the chemical activator of TRPM8. Similar percentages of menthol-sensitive neurons are seen in mammalian sensory ganglia. Frog neurons differ from those of mammals, however, when the neurons are stimulated with cold temperatures instead of menthol. While rat neurons have a thermal activation threshold of 25.4°C, frog neurons have a much colder threshold of 9.6°C. Thus, a frog TRPM8-positive sensory neuron requires a much colder temperature to become active and produce a "cold" sensation.

Myers et al. wanted to be sure that this change in temperature threshold was caused by differences in the TRPM8 channel and not some other difference between rat and frog sensory neurons. Therefore, they expressed frog, chicken, and rat TRPM8 in oocytes (egg cells which do not normally express any ion channels; this is a common experiment used for studying ion channel physiology in an isolated system). Voltage-clamp recordings were used to measure the changes in oocyte membrane potential caused by activation and opening of the TRPM8 channels. The oocytes expressing X. laevis TRPM8, as well as TRPM8 from the related frog species X. tropicalis, displayed a much lower activation temperature than the oocytes expressing chicken or rat TRPM8. This experiment also showed a slightly higher activation temperature for chicken TRPM8 than rat TRPM8, which is consistent with the elevated body temperature of birds compared to mammals.

The differences in the temperature-sensitive properties of TRPM8 channels between species occur because of slight changes in the ion channel's structure through evolution. X. laevis TRPM8 differs from rat TRPM8 in about 25% of its amino acids. The differences in protein structure allow the ion channels to exhibit slightly different responses to temperature, even though they are similar enough to retain their general TRP structure and cold sensitivity.

The researchers summarize their findings as follows:
Within visual and chemosensory systems, stimulus detectors (receptors) undergo great functional diversification as organisms evolve to inhabit a wide range of ecological niches. Our findings demonstrate that genes encoding somatosensory receptors display the same capacity for adaptation to species' environmental conditions. Specifically, we have shown that a cold receptor can be tuned to respond within a temperature range most relevant to the normal resting temperature of the primary afferent nerve terminal, whether determined by an internally regulated core body temperature or the environmental milieu.

They go on to add that it is not clear whether the X. laevis and X. tropicalis TRPM8 channels are specifically tuned to the niches of these two species, or whether they represent a universal amphibian cold-sensitive channel that does not vary between frogs inhabiting different environments. As we sequence the complete genomes of more organisms, it will become possible to search for genes homologous to TRPM8 in other cold-blooded species and compare them to these frog channels. The authors add, "It may also be interesting to examine species that experience substantial variations (short- or long-term) in environmental temperature, as there may be corresponding changes in TRPM8 expression and/or function that allow for optimal temperature detection under such circumstances." Some cold-blooded species become dormant during the winter months, and it would be informative to see whether their temperature-sensitive neurons exhibit different properties during dormant and active seasons.

This research further elucidates how evolution has shaped individual species to thrive in different environments, down to the smallest molecular details. Frogs, rats, and chickens, like Goldilocks, sense heat and cold while searching for a temperature that is "just right." But what's good for one species might not suit another. Their specialized ion channels allow them to appropriately respond to thermal stimuli -- even the ones that don't like porridge.

References

Caterina M.J., Schumacher M.A., Tominaga M., Rosen T.A., Levine J.D., et al. (1997) The capsaicin receptor: a heat-activated ion channel in the pain pathway. Nature 389 (6653): 816–24

McKemy D.D., Neuhausser W.M., Julius D. (2002) Identification of a cold receptor reveals a general role for TRP channels in thermosensation. Nature 416: 52–58.

Myers, B., Sigal, Y., & Julius, D. (2009) Evolution of Thermal Response Properties in a Cold-Activated TRP Channel PLoS ONE, 4 (5) DOI: 10.1371/journal.pone.0005741

Peier A.M., Moqrich A., Hergarden A.C., Reeve A.J., Andersson D.A., et al. (2002) A TRP channel that senses cold stimuli and menthol. Cell 108: 705–715.

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