2023 journal article
A new role of TRPM8 in circadian rhythm and molecular clock
Over the past decades, studies of the TRPM8 channel, a non-selective cation channel, have provided much insight into the fundamental mechanisms of sensory neuron function that lead to the detection of cold sensors. In the current issue of Acta Physiologica, Reimundez et al1 provide evidence of a novel role of TRPM8 in circadian function in mice. The TRPM (transient receptor potential ion channels, where “m” stands for melastatin) is a family of eight different channels, TRPM1-TRPM8.2 The TRPM8 is a cold sensor and is also activated by chemical ligands such as menthol and icilin, but recently, their role in maintaining core body temperature has also been recognized.3, 4 TRPM8 is expressed in sensory neurons whose axonal afferents innervate peripheral tissues such as skin and oral cavity. Still, it has not been detected in the brain and spinal cord. Besides nerve tissues, TRPM8 is expressed in tissues such as the prostate, bladder, lungs, and urogenital tract. However, other than the sensory nervous system, the functional role of TRPM8 channels is not well understood. In this issue of Acta Physiologica, Reimundez et al1 provide solid experimental evidence for such a novel mechanism—where they demonstrate that TRPM8 might regulate Period 2 (Per2) mRNA levels in central clock (SCN) and peripheral clock (liver and white adipose tissue) and the circadian regulation of core body temperature. Their publication is among the most significant advances in the field of circadian regulation using cold sensors—it will probably set up the stage for several future research activities in which Reimundez and colleagues' ideas will be rigorously dissected and probed in circadian biology research. Previously, Ordás et al5 demonstrated the presence of TRPM8 fibers in the suprachiasmatic nucleus of the hypothalamus (SCN) of the brain, which is the principal circadian pacemaker in mammals and host a range of other physiological processes such as circadian oscillation, autonomic/peripheral and central nervous system function, regulating core body temperature and sleep–wake cycle. Since SCN receives axonal projections mainly from intrinsically photosensitive retinal ganglion cells (ipRGCs) that are responsible for resetting the circadian clock through SCN neuron activity with light as a primary zeitgeber.6 Here, authors used conventional PCR and mouse reporter lines to show the expression of TRPM8 in the inner retina, specifically in the ganglion cell layer (GC) and in the inner nuclear layer (INL). Next, they used the colocalization technique to demonstrate that melanopsin, a marker of ipRGCs7 and using mouse reporter lines, that TRPM8 expressing cells are indeed expressed in a subset of ipRGCs cells. Furthermore, they showed that TRPM8 is also expressed in cholinergic amacrine cells, characterized by releasing two neurotransmitters, GABA and acetylcholine. Injection of the fluorescent anterograde tracer cholera toxin subunit B (CTB-594) into both eyes in two TRPM8 reporter lines indicates the existence of ipRGCs expressing TRPM8 and projecting to the SCN (Figure 1). The choroid is an essential high blood flow vascular structure in the eye and is known to regulate ocular and retinal temperature8 and is richly innervated by sensory trigeminal nerve fibers; therefore, the authors next examined the role of TRPM8 in regulation of eye temperature (Teye) using infrared (IR) thermography. Interestingly, they discovered mice lacking TRPM8 started to decline at temperatures below 25°C, and Trpm8−/− mice displayed a significantly lower Teye than WT littermates when the temperature plate was around 15°C. This suggests a function of TRPM8 and ambient temperature in regulating choroid and ciliary body blood flow and, therefore, in controlling internal ocular temperature. The ability of mice to regulate homeostatic temperature control and the expression of TRPM8 in the ipRGCs is indicative but does not directly link that to central clock regulation. The authors performed expression and functional studies to investigate a direct link between TRPM8 and circadian clockwork at the SCN. First, they determined the expression of the Per2 gene, which is one of the essential components of core circadian clocks in the SCN.9 Authors found in the TRPM8 deficient mouse SCN a significant increase of ~40% in Per2 mRNA content in relation to WT littermates during the daytime (ZT4). Next, they examined one of the most relevant clock-controlled neuropeptide Arginine Vasopressin (AVP) in the SCN. AVP-expressing neurons in the SCN constitute the main output of this nucleus and are known to be involved in core temperature (Tc) circadian regulation.10 Like Per2, Trpm8−/− mice displayed greater AVP expression than control mice during the lights-on phase (~50% higher) but conserved the oscillation between day and nighttime. Briefly, these results suggest that the absence of TRPM8 does not prevent oscillatory expression of Per2 and AVP but involves modifying clockwork and neuropeptide levels in this hypothalamic nucleus, contributing to the regulation of the circadian oscillation of Tc. Second, Reimundez et al1 found that TRPM8 plays a significant role in regulating core body temperature where TRPM8 deficient mice show a significantly reduced temperature and increased amplitude both in light/dark (LD) and dark/dark (DD) conditions compared with its wild-type controls. Future studies investigating TRPM8 regulation on circadian rhythm properties such as period, acrophase, and mesor would be interesting. Third, authors extended the TRPM8 role in peripheral clocks liver and gonadal WAT by measuring Per2 mRNA levels during the day (ZT4) and night (ZT16). TRPM8 deficient mice liver has a significantly elevated Per2 levels during day and dampened levels during night compared to WT control. Interestingly, in gonadal WAT tissue, Per2 levels were significantly lower both in day and night compared to WT control. Future studies, investigating with more circadian timepoints (every 3 hours in a 24-hour cycle) on whether Per2 levels were significantly rhythmic would be interesting. The authors provide strong evidence of the TRPM8 channel as a connecting link between temperature and central and peripheral clocks, regulating the circadian oscillations of body temperature. However, the presented evidence is scant and indirect, especially using the global TRPM8 deficient mice and knowing that TRPM8 is also a peripheral sensor in the skin, which might play an indirect, yet a meaningful role in mediating this effect. Most importantly, extending the TRPM8 role to other core clock genes such as Cry1, Cry2, Per1, Bmal1, Clock, and Npas2 oscillation and circadian rhythmicity will be interesting. No conflict of interest to declare.