Epidermal TRPM8 channel isoform controls the balance between keratinocyte proliferation and differentiation in a cold-dependent manner

Abstract

Deviation of the ambient temperature is one of the most ubiquitous stimuli that continuously affect mammals' skin. Although the role of the warmth receptors in epidermal homeostasis (EH) was elucidated in recent years, the mystery of the keratinocyte mild-cold sensor remains unsolved. Here we report the cloning and characterization of a new functional epidermal isoform of the transient receptor potential M8 (TRPM8) mild-cold receptor, dubbed epidermal TRPM8 (eTRPM8), which is localized in the keratinocyte endoplasmic reticulum membrane and controls mitochondrial Ca 2+ concentration ([Ca 2+ ] m ). In turn, [Ca 2+ ] m modulates ATP and superoxide (O •−2 ) synthesis in a cold-dependent manner. We report that this fine tuning of ATP and O •−2 levels by cooling controls the balance between keratinocyte proliferation and differentiation. Finally, to ascertain eTRPM8's role in EH in vivo we developed a new functional knockout mouse strain by deleting the pore domain of TRPM8 and demonstrated that eTRPM8 knockout impairs adaptation of the epidermis to low temperatures.he skin epidermis provides a protective barrier that guards the body against an uncongenial environment. Under the influence of a variety of ambient factors the skin epidermis undergoes continuous regeneration through so-called epidermal homeostasis (EH): the fine-tuning of the balance between proliferation, directional migration, differentiation, and death of keratinocytes. EH involves complex molecular and chemical pathways, regulating dynamic and continuous transition of keratinocytes from the proliferating state in the basal layer to the nonproliferating state in the suprabasal layer before the beginning of the differentiation in the stratum spinosum and stratum granulosum. The terminal differentiation step, characterized by keratinocyte death, transforms keratinocytes into corneocytes, which form the waterproof, mechanically resistant sheath of the stratum corneum (1).Deviation of the ambient temperature is one of the most important stimuli that constantly affect mammals' skin. At ambient temperatures from +10°C to +30°C, the unprotected human skin temperature settles at mean steady-state values within the range of +24°C to +33°C, respectively (2). Temperature is perceived by thermoreceptors, the ion channels that belong to the transient receptor potential (TRP) superfamily (for review see ref.3). Of these, TRPV1 and TRPV2 are activated by heat (above 42°C and above 52°C, respectively) (4), whereas TRPM8 and likely TRPA1 are activated by mild (5, 6) and noxious (7-9) cold, respectively. Heatstimulated keratinocytes have been shown to secrete ATP (10) and, taking into account that purinergic receptors are expressed in keratinocytes (11), TRPV3 is involved in a paracrine heat-

Fine-tuning of eTRPM8 expression and activity conditions keratinocyte fate

“…Thirdly, we show that icilin, mimicking cold stimulation, induces an eTRPM8 oscillatory activity lasting at least for tens of minutes. Finally, we complement our previously reported observation, that the decrease of the ratio of proliferating to the late differentiated keratinocytes dropped after a 3-days incubation at 25 C is associated with an increase of O 2 -content in an eTRPM8-dependent manner, 8,9 with demonstration that prolonged cold (25 C …”

“…Also, due to space limitations, some interesting observations were bypassed in our recent publication. 8 Here we provide these results to reinforce our hypothesis with additional insights. Firstly, we demonstrate that the modalities of eTRPM8 activation define the cell response toward proliferation or differentiation.…”

“…In two recent studies, 8,9 we reported that cold incubation of keratinocytes impairs their growth. We also cloned and characterized a novel archetype of TRP channels in epidermis, 8 dubbed eTRPM8, and we demonstrated that it was a functional channel with 4 transmembrane domains (Fig.…”

“…We also cloned and characterized a novel archetype of TRP channels in epidermis, 8 dubbed eTRPM8, and we demonstrated that it was a functional channel with 4 transmembrane domains (Fig. 1A) instead of the 6 original in full-length TRPM8.…”

“…In a recent investigation, 8 we reported the cloning and characterization of a new 4-trasmembranedomain epidermal isoform of the TRPM8 cold receptor-channel, dubbed eTRPM8. This channel is localized and functions in the keratinocyte endoplasmic reticulum (ER) membrane but not in the plasmalemma.…”