TRPM8 is a polymodal, non-selective cation channel activated by cold temperature and cooling agents that plays a critical role in the detection of environmental cold. We found that TRPM8 is a pharmacological target of tacrolimus (FK506), a macrolide immunosuppressant with several clinical uses, including the treatment of organ rejection following transplants, treatment of atopic dermatitis and dry eye disease. Tacrolimus is an inhibitor of the phosphatase calcineurin, an action shared with cyclosporine.Tacrolimus activates TRPM8 channels in different species, including humans, and sensitizes their response to cold temperature by inducing a leftward shift in the voltage-dependent activation curve. The effects of tacrolimus on purified TRPM8 in lipid bilayers demonstrates conclusively that it has a direct gating effect. Moreover, the lack of effect of cyclosporine rules out the canonical signaling pathway involving the phosphatase calcineurin. Menthol (TRPM8-Y745H)- and icilin (TRPM8-N799A)- insensitive mutants were also activated by tacrolimus, suggesting a different binding site. In cultured mouse DRG neurons, tacrolimus evokes an increase in intracellular calcium almost exclusively in cold-sensitive neurons, and these responses were drastically blunted in TRPM8 KO mice or after the application of TRPM8 antagonists. Cutaneous and corneal cold thermoreceptor endings are also activated by tacrolimus, and tacrolimus solutions trigger blinking and cold-evoked behaviors.Altogether, our results identify TRPM8 channels in sensory neurons as molecular targets of the immunosuppressant tacrolimus. The actions of tacrolimus on TRPM8 resemble those of menthol but likely involve interactions with other channel residues.SIGNIFICANCE STATEMENTTRPM8 is a polymodal TRP channel involved in cold temperature sensing, thermoregulation and cold pain. TRPM8 is also involved in the pathophysiology of dry eye disease and TRPM8 activation has antiallodynic and antipruritic effects, making it a prime therapeutic target in several cutaneous and neural diseases.We report the direct agonist effect of tacrolimus, a potent natural immunosuppressant with multiple clinical applications, on TRPM8 activity. This interaction represents a novel neuro-immune interface. The identification of a clinically approved drug with agonist activity on TRPM8 channels could be used experimentally to probe the function of TRPM8 in humans. Our findings may explain some of the sensory and anti-inflammatory effects described for this drug in the skin and the eye surface.

Miguel arcas J, González A, Gers-barlag K, et al. The immunosuppressant macrolide tacrolimus activates cold-sensing TRPM8 channels. J Neurosci. 2018;

Transient receptor potential (TRP) channels transduce signals of chemical irritation and temperature change from the ocular surface to the brain. Dry eye disease (DED) is a multifactorial disorder wherein the eyes react to trivial stimuli with abnormal sensations, such as dryness, blurring, presence of foreign body, discomfort, irritation, and pain. There is increasing evidence of TRP channel dysfunction (i.e., TRPV1 and TRPM8) in DED pathophysiology. Here, we review some of this literature and discuss one strategy on how to manage DED using a TRPM8agonist.

Yang JM, Wei ET, Kim SJ, Yoon KC. TRPM8 Channels and Dry Eye. Pharmaceuticals (Basel). 2018;11(4)

Body temperature regulation is a fundamental homeostatic function in homeothermic animals. It is governed by the central nervous system that integrates temperature signals from internal body structures and the skin and provides efferent responses to adjust heat-exchange rates with the environment. Thermoregulation has a major influence on energy balance by regulating food intake as well as heat production and energy expenditure. Surprisingly, although almost 50% of our energy expenditure is dedicated to maintaining homeothermy, very little is yet known about the molecular aspects and the neural wiring involved in the intimate interrelationship between these two critical homeostatic systems. Some non-selective cation channels of the transient receptor potential (TRP) family work as molecular thermal sensors in sensory neurons and other cells. In this review, we discuss recent advances in our understanding of the basic mechanisms responsible for thermoregulation in the cold. We have focused our attention on the role of two cold-activated TRP channels (transient receptor potential melastatin 8 and transient receptor potential ankyrin 1) in body temperature regulation as well as their impact on energy balance and metabolism. A better understanding of the mechanisms coupling thermoregulation to energy homeostasis, including the involvement of thermosensitive TRPs, may uncover additional mechanisms underlying the pathogenesis of obesity and its metabolic consequences in humans, opening new strategies for the diagnosis, treatment, and prevention of this disease.

Señarís R, Ordás P, Reimúndez A, Viana F. Mammalian cold TRP channels: impact on thermoregulation and energy homeostasis. Pflugers Arch. 2018;470(5):761-777.

BACKGROUND: Physical cooling of the eye surface relieves ocular discomfort, but translating this event to drug treatment of dry eye discomfort not been studied. Here, we synthesized a water-soluble TRPM8 receptor agonist called cryosim-3 (C3, 1-diisopropylphosphorylnonane) which selectively activates TRPM8 (linked to cooling) but not TRPV1 or TRPA1 (linked to nociception) and tested C3 in subjects with mild forms of dry eye disease.

METHODS: A set of 1-dialkylphosphoryalkanes were tested for activation of TRPM8, TRPV1 and TRPA1 receptors in transfected cells. The bioactivity profiles were compared by perioral, topical, and intravenous delivery to anesthetized rats. The selected lead candidate C3 or vehicle (water) was applied with a cotton gauze pad to upper eyelids of patients with dry eye disease (n = 30). Cooling sensation, tear film break-up time (TBUT), basal tear secretion, and corneal staining were evaluated. C3 was then applied four times daily for 2 weeks to patients using a pre-loaded single unit applicator containing 2 mg/mL of C3 in water (n = 20) or water only. TBUT, basal tear secretion, and corneal staining, and three questionnaires surveys of ocular discomfort (VAS scale, OSDI, and CVS symptoms) were analyzed before and at 1 and 2 weeks thereafter.

RESULTS: C3 was a selective and potent TRPM8 agonist without TRPV1 or TRPA1 activity. In test animals, the absence of shaking behavior after C3 perioral administration made it the first choice for further study. C3 increased tear secretion in an animal model of dry eye disease and did not irritate when wiped on eyes of volunteers. C3 singly applied (2 mg/ml) produced significant cooling in <5 min, an effecting lasting 46 min with an increase in tear secretion for 60 min. C3 applied for 2 weeks also significantly increased basal tear secretion with questionnaire surveys of ocular discomfort indices clearly showing improvement of symptoms at 1 and 2 weeks. No complaints of irritation or pain were reported by any subject.

CONCLUSIONS: C3 is a promising candidate for study of TRPM8 function on the eye surface and for relief of dry eye discomfort.

Yang JM, Li F, Liu Q, et al. A novel TRPM8 agonist relieves dry eye discomfort. BMC Ophthalmol. 2017;17(1):101.