The transient receptor potential vanilloid-4 channel: detecting body temperatures that drive defences against mild warmth.

Abstract

Over the last two decades, many channels, including several of the transient receptor potential (TRP) channels, have been shown to be highly sensitive to temperature and proposed to serve as thermosensors for physiological functions and behaviours that depend on temperature. However, it is still largely unknown which TRP channels detect skin, brain and other body temperatures that drive physiological (autonomic) and behavioural cold and heat defences in mammals (Romanovsky 2014). Arguably, the only exception is the TRP channel melastatin-8 (M8), which has been shown to sense a decreased skin temperature to drive tail skin vasoconstriction, non-shivering thermogenesis and cold-avoidance (warmth-seeking) behaviour in rodents (Almeida et al. 2012). In the current issue of Acta Physiologica, Vizin et al. (2015) report the results of a study aimed at establishing whether another TRP channel, vanilloid-4 (V4), plays a similarly critical role in heat defences. TRPV4 is known to be activated by warmth (25–34 °C) in vitro (Guler et al. 2002, Watanabe et al. 2002), but the earlier in vivo studies of its thermoregulatory role produced contradictory results. On one side, several groups showed that body temperature of TRPV4-knockout (Trpv4−/−) mice did not differ from that of wild-type littermates, whether under thermoneutral conditions or during cold or warmth exposure (Liedtke & Friedman 2003, Lee et al. 2005). These negative results seem to suggest that TRPV4 is not essential for thermoregulation. On the other hand, based on the increased withdrawal latencies in a tail-immersion test and on the increased preference for warmth observed in Trpv4−/− mice, Lee et al. (2005) proposed that TRPV4 plays important roles in the avoidance responses caused by innocuous warmth (a thermoregulatory response) or noxious heat (a response to thermal pain). Yet, in a subsequent work, the same group argued against such a role for TRPV4 in either response (Huang et al. 2011). It should be noted, however, that thermophysiological studies in mice are notoriously tricky to conduct (Rudaya et al. 2005) and that any studies in knockout animals (typically mice) are often difficult to interpret due to the development of compensation for the knocked-out gene. To circumvent both the methodological and interpretational difficulties of studying genetically modified mice, pharmacological antagonists are often used in rat experiments. This is exactly the approach taken by Vizin et al. (2015). The authors showed that the intravenous administration of HC-067047, a potent TRPV4 antagonist (Everaerts et al. 2010), caused a rise in deep body temperature at an ambient temperature of 26 °C (which was neutral in the experimental setup used) or 30 °C (a low supraneutral temperature). No body temperature rise occurred at a subneutral ambient temperature of 22 °C (at which TRPV4 channels in the skin were presumably not activated) or at a higher supraneutral ambient temperature of 32 °C (at which multiple additional central and/or peripheral thermosensors can be speculated to be involved, thus compensating for the blockade of TRPV4 channels). Unfortunately, the authors did not confirm in direct experiments (using Trpv4−/− animals) whether the hyperthermic effect of HC-067047 was indeed due to an action on TRPV4 channels. The lack of such a direct confirmation is the major shortcoming of the Vizin et al. (2015) study, as it leaves the door open for the possibility of an off-target action, especially in view of the contradictory and largely negative results of the earlier studies discussed above (Liedtke & Friedman 2003, Lee et al. 2005, Huang et al. 2011). Another issue that needs clarification is how the magnitude of the body temperature response to HC-067047 (when the response occurs) depends on basal (before HC-067047 administration) peripheral body temperatures, especially the temperature of hairy skin, which plays a major role in driving thermoeffectors (Romanovsky 2014). Clarifying this would help to determine whether HC-067047 affects thermoregulation by blocking non-thermal (e.g. osmotic) or thermal signals. Examples of TRP channel antagonists affecting body temperature by blocking non-thermal vs. thermal activation of the corresponding channels are TRPV1 vs. TRPM8 antagonists, respectively. TRPV1 antagonists typically cause hyperthermia at all ambient and body temperatures studied by blocking non-thermal (proton) activation of abdominal visceral TRPV1 channels; this effect does not depend on basal temperatures (Steiner et al. 2007, Garami et al. 2010). Hence, TRPV1 is not a thermosensor used by the rodent thermoregulation system (Romanovsky et al. 2009). On the other hand, TRPM8 antagonists cause hypothermia only in a cold environment, and they do so by blocking thermal activation of cutaneous TRPM8 channels; the effect is the greatest when basal body temperatures are the lowest (Almeida et al. 2012). Hence, TRPM8 is a thermosensor used by the rodent thermoregulation system (Almeida et al. 2012). The study by Vizin et al. (2015) did not allow the authors to determine definitively the nature of those signals that were blocked by HC-067047 to cause hyperthermia. However, the fact that HC-067047 raised deep body temperature in rats only in a certain range of ambient temperatures suggests that the signals blocked by HC-067047 could be thermal. The spectrum of thermoeffectors driven by the TRPV4-mediated presumably thermal signals blocked by HC-067047 deserves a separate discussion. In general, different effectors are driven by different combinations of peripheral and central body temperatures and, therefore, are likely to utilize different combinations of thermosensors (Romanovsky 2014). We applaud Vizin et al. (2015) for running labour-intensive experiments to clarify which thermoeffectors were recruited in the response to HC-067047 under different conditions. Using a variety of experimental setups and conditions, the authors showed that HC-067047 affected the activity of all autonomic and behavioural thermoeffector mechanisms studied, viz., tail skin vasodilation, non-shivering thermogenesis and the search for preferred ambient temperature. Vizin et al. (2015) also studied the effects of RN-1747, a TRPV4 agonist. Importantly, the experiments with RN-1747 suggested a cutaneous location of TRPV4 channels that drive thermoeffectors. However, we find any experiments with agonists less revealing. Indeed, an agonist of a TRP channel can cause a response in vivo, but this does not mean that the channels involved are naturally exposed to this (or any other) agonist. Unlike an effect of an agonist, an on-target effect of an antagonist almost for sure reveals a physiological mechanism – the targeted channels should be already activated via a naturally occurring mechanism in order for the antagonist to have an effect. Furthermore, it is not totally unusual for agonists and antagonists of the same receptor to affect a physiological function via different mechanisms and via receptors at different locations. For example, the hyperthermic responses to systemic administration of TRPV1 antagonists originate in the periphery, whereas the hypothermic responses to systemic administration of TRPV1 agonists likely originate in the brain (Romanovsky et al. 2009). Sometimes, agonists and antagonists of the same receptor can even cause the same effect on body temperature (Packman et al. 1953)! In summary, using pharmacological modulation of the TRPV4 channel in an extensive and expertly performed thermophysiological study, Vizin et al. (2015) came to a conclusion that TRPV4 detects body temperature (perhaps superficial skin temperature) signals that drive both autonomic and behavioural warmth-defence thermoeffectors. Even though several aspects of this conclusion remain to be confirmed in direct experiments, the authors’ story about the role of TRPV4 in thermoregulation is compelling. The significance of the present work may be limited by the fact that the hyperthermic effect of the TRPV4 antagonist used was observed in a very narrow range of ambient temperatures. Does this mean that other thermosensors, perhaps both peripheral and central, are more important outside this narrow range? If so, a search for thermosensors that drive autonomic and behavioural heat defences should continue. A. Garami discloses no conflict of interest. A. A. Romanovsky has consulted for TRP programs at Amgen, Abbott Laboratories and several other pharmaceutical companies, and his research related to the thermosensory roles of TRP channels has been funded by Amgen., Abbott Laboratories and AbbVie.