The ion channel TRPM8 is a direct target of the immunosuppressant rapamycin in primary sensory neurons

Background and Purpose

The mechanistic target of rapamycin (mTOR) signaling pathway is a key regulator of cell growth and metabolism. Its deregulation is implicated in several diseases. The macrolide rapamycin (RAP), a specific inhibitor of mTOR, has immunosuppressive, anti-inflammatory and antiproliferative properties. Recently, we identified tacrolimus, another macrolide immunosuppressant, as a novel activator of TRPM8 ion channels, involved in cold temperature sensing, thermoregulation, tearing and cold pain. We hypothesized that RAP may also have agonist activity on TRPM8.

Experimental approach

Using calcium imaging and electrophysiology in transfected HEK293 cells and wildtype or Trpm8 KO mouse DRG neurons, we characterized RAP effects on TRPM8. We also examined the effects of RAP on tearing in mice.

Key Results

Micromolar concentrations of RAP activate rat and mouse TRPM8 directly and potentiate cold-evoked responses. These effects were also observed in human TRPM8.

In cultured mouse DRG neurons, RAP evoked an increase in intracellular calcium almost exclusively in cold-sensitive neurons. Responses were drastically blunted in Trpm8 KO mice or by TRPM8 antagonists. Cutaneous cold thermoreceptor endings were also activated by RAP. Topical application of RAP to the eye surface evokes tearing in mice by a TRPM8-dependent mechanism.

Conclusion and implications

These results identify TRPM8 cationic channels in sensory neurons as novel molecular targets of the immunosuppressant RAP. These findings may help explain some of its therapeutic effects after topical application to the skin and the eye surface. Moreover, RAP could be used as an experimental tool in the clinic to explore cold thermoreceptors.

Bullet point summary

WHAT IS ALREADY KNOWN

• TRPM8 is a polymodal channel involved in cold detection, thermoregulation, tearing and cold pain

• Tacrolimus, a macrolide immunosupressor, is an agonist of cold-activated TRPM8 channels

WHAT THIS STUDY ADDS

• The macrolide rapamycin also activates directly TRPM8 channels in mouse sensory neurons and human TRPM8

• Rapamycin stimulates tearing in mice in a TRPM8-dependent manner CLINICAL SIGNIFICANCE

• Rapamycin, an FDA-approved drug, shows agonist activity on TRPM8 channels

• Beneficial effects of rapamycin and other macrolides on inflammatory ocular disorders may involve TRPM8 activation