Tasty and healthy TR(i)Ps. The human quest for culinary pungency

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) signalling pathway is a key regulator of cell growth and metabolism. Its deregulation is implicated in several diseases. The macrolide rapamycin, 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 rapamycin may also have agonist activity on TRPM8 channels.

EXPERIMENTAL APPROACH

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

KEY RESULTS

Micromolar concentrations of rapamycin activated rat and mouse TRPM8 channels directly and potentiated cold-evoked responses, effects also observed in human TRPM8 channels. In cultured mouse DRG neurons, rapamycin increased intracellular calcium levels almost exclusively in cold-sensitive neurons. Responses were markedly decreased in Trpm8 KO mice or by TRPM8 channel antagonists. Cutaneous cold thermoreceptor endings were also activated by rapamycin. Topical application of rapamycin 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 rapamycin. These findings may help explain some of its therapeutic effects after topical application to the skin and the eye surface. Moreover, rapamycin could be used as an experimental tool in the clinic to explore cold thermoreceptors.

Bitter Is Better: Wild Greens Used in the Blue Zone of Ikaria, Greece

The current study reports an ethnobotanical field investigation of traditionally gathered and consumed wild greens (Chorta) in one of the five so-called Blue Zones in the world: Ikaria Isle, Greece. Through 31 semi-structured interviews, a total of 56 wild green plants were documented along with their culinary uses, linguistic labels, and locally perceived tastes. Most of the gathered greens were described as bitter and associated with members of Asteraceae and Brassicaceae botanical families (31%), while among the top-quoted wild greens, species belonging to these two plant families accounted for 50% of the wild vegetables, which were consumed mostly cooked. Cross-cultural comparison with foraging in other areas of the central-eastern Mediterranean and the Near East demonstrated a remarkable overlapping of Ikarian greens with Cretan and Sicilian, as well as in the prevalence of bitter-tasting botanical genera. Important differences with other wild greens-related food heritage were found, most notably with the Armenian and Kurdish ones, which do not commonly feature many bitter greens. The proven role of extra-oral bitter taste receptors in the modulation of gastric emptying, glucose absorption and crosstalk with microbiota opens new ways of looking at these differences, in particular with regard to possible health implications. The present study is also an important attempt to preserve and document the bio-cultural gastronomic heritage of Chorta as a quintessential part of the Mediterranean diet. The study recommends that nutritionists, food scientists, and historians, as well as policymakers and practitioners, pay the required attention to traditional rural dietary systems as models of sustainable health.

The TRPA1 Agonist Cinnamaldehyde Induces the Secretion of HCO3- by the Porcine Colon

A therapeutic potential of the TRPA1 channel agonist cinnamaldehyde for use in inflammatory bowel disease is emerging, but the mechanisms are unclear. Semi-quantitative qPCR of various parts of the porcine gastrointestinal tract showed that mRNA for TRPA1 was highest in the colonic mucosa. In Ussing chambers, 1 mmol·L^-1 cinnamaldehyde induced increases in short circuit current (ΔI_sc) and conductance (ΔG_t) across the colon that were higher than those across the jejunum or after 1 mmol·L^-1 thymol. Lidocaine, amiloride or bumetanide did not change the response. The application of 1 mmol·L^-1 quinidine or the bilateral replacement of 120 Na⁺, 120 Cl^- or 25 HCO₃^- reduced ΔG_t, while the removal of Ca²⁺ enhanced ΔG_t with ΔI_sc numerically higher. ΔI_sc decreased after 0.5 NPPB, 0.01 indometacin and the bilateral replacement of 120 Na⁺ or 25 HCO₃^-. The removal of 120 Cl^- had no effect. Cinnamaldehyde also activates TRPV3, but comparative measurements involving patch clamp experiments on overexpressing cells demonstrated that much higher concentrations are required. We suggest that cinnamaldehyde stimulates the secretion of HCO₃^- via apical CFTR and basolateral Na⁺-HCO₃^- cotransport, preventing acidosis and damage to the epithelium and the colonic microbiome. Signaling may involve the opening of TRPA1, depolarization of the epithelium and a rise in PGE2 following a lower uptake of prostaglandins via OATP2A1.

The effects of capsinoids and fermented red pepper paste supplementation on blood pressure: A systematic review and meta-analysis of randomized controlled trials

BACKGROUND & AIMS

The present systematic review and meta-analysis were conducted to investigate the effects of capsinoids and fermented red pepper paste (FRPP) supplementation on Systolic Blood Pressure (SBP) and Diastolic Blood Pressure (DBP).

METHODS

Relevant studies, published up to May 2020, were searched through PubMed/Medline, Scopus, ISI Web of Science, Embase, and Google Scholar. All randomized clinical trials investigating the effect of capsinoids and FRPP supplementation on blood pressure including SBP and DBP were included.

RESULTS

Out of 335 citations, 7 trials that enrolled 363 subjects were included. Capsinoids and FRPP resulted in significant reduction in DBP (Weighted mean differences (WMD): -1.90 mmHg; 95% CI, -3.72 to -0.09, P = 0.04) but no significant change in SBP (WMD: 0.55 mmHg, 95% CI: -1.45, 2.55, P = 0.588). FRPP had a significant reduction in SBP. Greater effects on SBP were detected in trials, lasted ≥12 weeks, and sample size >50. Capsinoids with dosage ≤200 and FRPP with dosage of 11.9 g significantly decreased DBP.

CONCLUSION

Overall, these data suggest that supplementation with FRPP may play a role in improving SBP and DBP but for capsinoids no effects detected in this analysis on SBP and DBP.

Mammalian Transient Receptor Potential TRPA1 Channels: From Structure to Disease

The transient receptor potential ankyrin (TRPA) channels are Ca²⁺-permeable nonselective cation channels remarkably conserved through the animal kingdom. Mammals have only one member, TRPA1, which is widely expressed in sensory neurons and in non-neuronal cells (such as epithelial cells and hair cells). TRPA1 owes its name to the presence of 14 ankyrin repeats located in the NH₂ terminus of the channel, an unusual structural feature that may be relevant to its interactions with intracellular components. TRPA1 is primarily involved in the detection of an extremely wide variety of exogenous stimuli that may produce cellular damage. This includes a plethora of electrophilic compounds that interact with nucleophilic amino acid residues in the channel and many other chemically unrelated compounds whose only common feature seems to be their ability to partition in the plasma membrane. TRPA1 has been reported to be activated by cold, heat, and mechanical stimuli, and its function is modulated by multiple factors, including Ca²⁺, trace metals, pH, and reactive oxygen, nitrogen, and carbonyl species. TRPA1 is involved in acute and chronic pain as well as inflammation, plays key roles in the pathophysiology of nearly all organ systems, and is an attractive target for the treatment of related diseases. Here we review the current knowledge about the mammalian TRPA1 channel, linking its unique structure, widely tuned sensory properties, and complex regulation to its roles in multiple pathophysiological conditions.

The Immunosuppressant Macrolide Tacrolimus Activates Cold-Sensing TRPM8 Channels

TRPM8 is a polymodal, nonselective 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. Together, 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 STATEMENT TRPM8 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 neuroimmune 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.

Enjoyment of Spicy Flavor Enhances Central Salty-Taste Perception and Reduces Salt Intake and Blood Pressure

High salt intake is a major risk factor for hypertension and is associated with cardiovascular events. Most countries exhibit a traditionally high salt intake; thus, identification of an optimal strategy for salt reduction at the population level may have a major impact on public health. In this multicenter, random-order, double-blind observational and interventional study, subjects with a high spice preference had a lower salt intake and blood pressure than subjects who disliked spicy food. The enjoyment of spicy flavor enhanced salt sensitivity and reduced salt preference. Salt intake and salt preference were related to the regional metabolic activity in the insula and orbitofrontal cortex (OFC) of participants. Administration of capsaicin-the major spicy component of chili pepper-enhanced the insula and OFC metabolic activity in response to high-salt stimuli, which reversed the salt intensity-dependent differences in the metabolism of the insula and OFC. In animal study, OFC activity was closely associated with salt preference, and salty-taste information processed in the OFC was affected in the presence of capsaicin. Thus, interventions related to this region may alter the salt preference in mice through fiber fluorometry and optogenetic techniques. In conclusion, enjoyment of spicy foods may significantly reduce individual salt preference, daily salt intake, and blood pressure by modifying the neural processing of salty taste in the brain. Application of spicy flavor may be a promising behavioral intervention for reducing high salt intake and blood pressure.

TRPA1 receptor is upregulated in human oral lichen planus

OBJECTIVE

Oral lichen planus (OLP) is a chronic inflammatory disease of unknown etiology with antigen-specific and non-specific mechanisms. Transient receptor potential ankyrin 1 (TRPA1) is a non-selective cation channel activated by noxious stimuli such as oxidative stress products evoking pain and release of proinflammatory mediators from sensory nerve endings culminating in neurogenic inflammation. Extraneuronal TRPA1s, for example, on immune cells possess yet unknown functions.

SUBJECTS AND METHODS

We studied the buccal mRNA expression (qPCR) and protein localization (immunohistochemistry) of TRPA1 receptors and key OLP mediator transcripts in oral mucosa samples of healthy volunteers (n = 9), OLP patients (n = 43), and OLP-like hyperkeratotic patients (n = 12).

RESULTS

We measured 27.7- and 25.5-fold TRPA1 mRNA increase in OLP and OLP-like hyperkeratotic patients compared to healthy controls. TRPA1 transcripts elevated 2.4-fold in hypertensive OLP but not in hyperkeratotic patients compared to counterparts, reduced by 1.6-fold by angiotensin-convertase inhibitor intake. TRPA1 messenger RNA was more coexpressed with transcripts of tumor necrosis factor α than with interferon γ. Keratinocytes, macrophages but not T cells expressed TRPA1.

CONCLUSIONS

We provided evidence for the extraneuronal presence and upregulation of the proinflammatory TRPA1 receptor in buccal samples of patients with OLP. This may implicate the ion channel in the pathomechanism of OLP.

Phytochemicals modulate carcinogenic signaling pathways in breast and hormone-related cancers

Over the years, nutrition and environmental factors have been demonstrated to influence human health, specifically cancer. Owing to the fact that cancer is a leading cause of death worldwide, efforts are being made to elucidate molecular mechanisms that trigger or delay carcinogenesis. Phytochemicals, in particular, have been shown to modulate oncogenic processes through their antioxidant and anti-inflammatory activities and their ability to mimic the chemical structure and activity of hormones. These compounds can act not only by influencing oncogenic proteins, but also by modulating noncoding RNAs such as microRNAs and long noncoding RNAs. Although we are only beginning to understand the complete effects of many natural compounds, such as phytochemicals, researchers are motivated to combine these agents with traditional, chemo-based, or hormone-based therapies to fight against cancer. Since ongoing studies continue to prove effective, herein we exalt the importance of improving dietary choices as a chemo-preventive strategy.

TRPs in taste and chemesthesis

TRP channels are expressed in taste buds, nerve fibers, and keratinocytes in the oronasal cavity. These channels play integral roles in transducing chemical stimuli, giving rise to sensations of taste, irritation, warmth, coolness, and pungency. Specifically, TRPM5 acts downstream of taste receptors in the taste transduction pathway. TRPM5 channels convert taste-evoked intracellular Ca(2+) release into membrane depolarization to trigger taste transmitter secretion. PKD2L1 is expressed in acid-sensitive (sour) taste bud cells but is unlikely to be the transducer for sour taste. TRPV1 is a receptor for pungent chemical stimuli such as capsaicin and for several irritants (chemesthesis). It is controversial whether TRPV1 is present in the taste buds and plays a direct role in taste. Instead, TRPV1 is expressed in non-gustatory sensory afferent fibers and in keratinocytes of the oronasal cavity. In many sensory fibers and epithelial cells lining the oronasal cavity, TRPA1 is also co-expressed with TRPV1. As with TRPV1, TRPA1 transduces a wide variety of irritants and, in combination with TRPV1, assures that there is a broad response to noxious chemical stimuli. Other TRP channels, including TRPM8, TRPV3, and TRPV4, play less prominent roles in chemesthesis and no known role in taste, per se. The pungency of foods and beverages is likely highly influenced by the temperature at which they are consumed, their acidity, and, for beverages, their carbonation. All these factors modulate the activity of TRP channels in taste buds and in the oronasal mucosa.

Transient receptor potential channels as drug targets: from the science of basic research to the art of medicine

The large Trp gene family encodes transient receptor potential (TRP) proteins that form novel cation-selective ion channels. In mammals, 28 Trp channel genes have been identified. TRP proteins exhibit diverse permeation and gating properties and are involved in a plethora of physiologic functions with a strong impact on cellular sensing and signaling pathways. Indeed, mutations in human genes encoding TRP channels, the so-called "TRP channelopathies," are responsible for a number of hereditary diseases that affect the musculoskeletal, cardiovascular, genitourinary, and nervous systems. This review gives an overview of the functional properties of mammalian TRP channels, describes their roles in acquired and hereditary diseases, and discusses their potential as drug targets for therapeutic intervention.

Spice up your life: adipose tissue and inflammation

Cells of the immune system are now recognized in the adipose tissue which, in obesity, produces proinflammatory chemokines and cytokines. Several herbs and spices have been in use since ancient times which possess anti-inflammatory properties. In this perspective, I discuss and propose the usage of these culinary delights for the benefit of human health.

TRP modulation by natural compounds

The use of medicinal plants or other naturally derived products to relieve illness can be traced back over several millennia, and these natural products are still extensively used nowadays. Studies on natural products have, over the years, enormously contributed to the development of therapeutic drugs used in modern medicine. By means of the use of these substances as selective agonists, antagonists, enzyme inhibitors or activators, it has been possible to understand the complex function of many relevant targets. For instance, in an attempt to understand how pepper species evoke hot and painful actions, the pungent and active constituent capsaicin (from Capsicum sp.) was isolated in 1846 and the receptor for the biological actions of capsaicin was cloned in 1997, which is now known as TRPV1 (transient receptor potential vanilloid 1). Thus, TRPV1 agonists and antagonists have currently been tested in order to find new drug classes to treat different disorders. Indeed, the transient receptor potential (TRP) proteins are targets for several natural compounds, and antagonists of TRPs have been synthesised based on the knowledge of naturally derived products. In this context, this chapter focuses on naturally derived compounds (from plants and animals) that are reported to be able to modulate TRP channels. To clarify and make the understanding of the modulatory effects of natural compounds on TRPs easier, this chapter is divided into groups according to TRP subfamilies: TRPV (TRP vanilloid), TRPA (TRP ankyrin), TRPM (TRP melastatin), TRPC (TRP canonical) and TRPP (TRP polycystin). A general overview on the naturally derived compounds that modulate TRPs is depicted in Table 1.

What do we really know and what do we need to know: some controversies, perspectives, and surprises

TRP channels comprise one of the most rapid growing research topics in ion channel research, in fields related to ion channels including channelopathies and translational medicine. We provide here a critical survey on our current knowledge of TRP channels and highlight some of the still open or controversial questions. This comprises questions related to evolution of TRP channels; biophysics, i.e., permeation; pore properties and gating; modulation; the still-elusive 3D structure; and channel subunits but also their role as general sensory channels and in human diseases. We will conclude that our knowledge on TRP channels is still at the very beginning of an exciting research journey.

Pharmacology of the capsaicin receptor, transient receptor potential vanilloid type-1 ion channel

The capsaicin receptor, transient receptor potential vanilloid type 1 ion channel (TRPV1), has been identified as a polymodal transducer molecule on a sub-set of primary sensory neurons which responds to various stimuli including noxious heat (> -42 degrees C), protons and vanilloids such as capsaicin, the hot ingredient of chilli peppers. Subsequently, TRPV1 has been found indispensable for the development of burning pain and reflex hyperactivity associated with inflammation of peripheral tissues and viscera, respectively. Therefore, TRPV1 is regarded as a major target for the development of novel agents for the control of pain and visceral hyperreflexia in inflammatory conditions. Initial efforts to introduce agents acting on TRPV1 into clinics have been hampered by unexpected side-effects due to wider than expected expression in various tissues, as well as by the complex pharmacology, of TRPV1. However, it is believed that better understanding of the pharmacological properties of TRPV1 and specific targeting of tissues may eventually lead to the development of clinically useful agents. In order to assist better understanding of TRPV1 pharmacology, here we are giving a comprehensive account on the activation and inactivation mechanisms and the structure-function relationship of TRPV1.

Spices: the savory and beneficial science of pungency

Spicy food does not only provide an important hedonic input in daily life, but has also been anedoctically associated to beneficial effects on our health. In this context, the discovery of chemesthetic trigeminal receptors and their spicy ligands has provided the mechanistic basis and the pharmacological means to investigate this enticing possibility. This review discusses in molecular terms the connection between the neurophysiology of pungent spices and the "systemic" effects associated to their trigeminality. It commences with a cultural and historical overview on the Western fascination for spices, and, after analysing in detail the mechanisms underlying the trigeminality of food, the main dietary players from the transient receptor potential (TRP) family of cation channels are introduced, also discussing the "alien" distribution of taste receptors outside the oro-pharingeal cavity. The modulation of TRPV1 and TRPA1 by spices is next described, discussing how spicy sensations can be turned into hedonic pungency, and analyzing the mechanistic bases for the health benefits that have been associated to the consumption of spices. These include, in addition to a beneficial modulation of gastro-intestinal and cardio-vascular function, slimming, the optimization of skeletal muscle performance, the reduction of chronic inflammation, and the prevention of metabolic syndrome and diabetes. We conclude by reviewing the role of electrophilic spice constituents on cancer prevention in the light of their action on pro-inflammatory and pro-cancerogenic nuclear factors like NFκB, and on their interaction with the electrophile sensor protein Keap1 and the ensuing Nrf2-mediated transcriptional activity. Spicy compounds have a complex polypharmacology, and just like any other bioactive agent, show a balance of beneficial and bad actions. However, at least for moderate consumption, the balance seems definitely in favour of the positive side, suggesting that a spicy diet, a caveman-era technology, could be seriously considered in addition to caloric control and exercise as a measurement to prevent and control many chronic diseases associate to malnutrition from a Western diet.

TRPV1 activation prevents nonalcoholic fatty liver through UCP2 upregulation in mice

Nonalcoholic fatty liver is characterized by the fatty deformation and lipid deposition of hepatic parenchymal cells that are associated with cardiometabolic diseases. In this study, we report the effect of capsaicin on its receptor, transient receptor potential vanilloid 1 (TRPV1) cation channel, in preventing fatty liver formation. Functional TRPV1 has been detected in hepatocytes and liver tissues. TRPV1 activation by capsaicin reduced lipid accumulation and triglyceride level in the liver from wild-type (WT) mice. However, these effects were absent in the liver from TRPV1(-/-) mice. Chronic dietary capsaicin increased the hepatic uncoupling protein 2 (UCP2) expression in WT but not in TRPV1(-/-) mice (P < 0.01). We conclude that TRPV1 long-time activation might prevent high-fat diet-induced fatty liver in mice through upregulation of hepatic UCP2. Dietary capsaicin may represent a promising intervention in populations at high risk for fatty liver.

Sub-lethal concentrations of Muscari comosum bulb extract suppress adhesion and induce detachment of sessile yeast cells

The formation of yeast biofilm on food industry equipment can lead to serious hygiene problems and economic losses due to food spoilage and equipment impairment. This study explored the ability of a sub-lethal concentration of the bulb extract of Muscari comosum to modulate adhesion of Candida albicans and subsequent biofilm development by this fungus. The HPLC profile of the ethanolic bulb extract showed phenolic constituents, which were found to undergo Folin-Ciocalteu reagent reduction. Prior to the adhesion tests, it was shown that up to 4000 mg l(-1) of natural extract did not adversely affect fungal growth nor did it act as a carbon energy source for C. albicans. Mathematical models predicted that 4000 mg l(-1) and 700 mg l(-1) of bulb extract would cause more than 98% reduction in fungal coverage on abiotic surfaces, without killing the planktonic cells. When added to C. albicans biofilm, the natural extract was shown to induce the dispersion of sessile cells in a dose-dependent manner.