The thermo-TRP ion channel family: properties and therapeutic implications

Abstract

The thermo-transient receptor potentials (TRPs), a recently discovered family of ion channels activated by temperature, are expressed in primary sensory nerve terminals where they provide information about thermal changes in the environment. Six thermo-TRPs have been characterised to date: TRP vanilloid (TRPV) 1 and 2 are activated by painful levels of heat, TRPV3 and 4 respond to non-painful warmth, TRP melastatin 8 is activated by non-painful cool temperatures, while TRP ankyrin (TRPA) 1 is activated by painful cold. The thermal thresholds of many thermo-TRPs are known to be modulated by extracellular mediators, released by tissue damage or inflammation, such as bradykinin, PG and growth factors. There have been intensive efforts recently to develop antagonists of thermo-TRP channels, particularly of the noxious thermal sensors TRPV1 and TRPA1. Blockers of these channels are likely to have therapeutic uses as novel analgesics, but may also cause unacceptable side effects. Controlling the modulation of thermo-TRPs by inflammatory mediators may be a useful alternative strategy in developing novel analgesics.

Figure 1

Schematic depiction of the six mammalian thermo-TRP channels. Each subunit consists of six transmembrane domains (S1–S6), a hydrophobic pore loop linking transmembrane segments five (S5) and six (S6), and large cytoplasmic N- and C- terminals (NB: not drawn to scale). All thermo-TRPs have a variable number of ankyrin repeat domains in the N-terminus (except TRPM8, which has none). Thermo-TRPs display distinct thermal thresholds from very hot (TRPV2) to cold (TRPA1). Each thermo-TRP is also activated by specific natural compounds and by synthetic substances, which are also known to induce the relevant thermal and pain sensations in humans. This figure provides a schematic overview of some of the main agonists – see Table 1 for a more complete list. BAA, bisandrographolide A.