Function and expression pattern of TRPM8 in bladder afferent neurons associated with bladder outlet obstruction in rats

The TRPM8 channel as a potential therapeutic target for bladder hypersensitive disorders

In the lower urinary tract, transient receptor potential (TRP) channels are primarily involved in physiological function, especially in cellular sensors responding to chemical and physical stimuli. Among TRP channels, TRP melastatin 8 (TRPM8) channels, responding to cold temperature and/or chemical agents, such as menthol or icilin, are mainly expressed in the nerve endings of the primary afferent neurons and in the cell bodies of dorsal root ganglia innervating the urinary bladder (via Aδ- and C-fibers); this suggests that TRPM8 channels primarily contribute to bladder sensory (afferent) function. Storage symptoms of overactive bladder, benign prostatic hyperplasia, and interstitial cystitis are commonly related to sensory function (bladder hypersensitivity); thus, TRPM8 channels may also contribute to the pathophysiology of bladder hypersensitivity. Indeed, it has been reported in a pharmacological investigation using rodents that TRPM8 channels contribute to the pathophysiological bladder afferent hypersensitivity of mechanosensitive C-fibers. Similar findings have also been reported in humans. Therefore, a TRPM8 antagonist would be a promising therapeutic target for bladder hypersensitive disorders, including urinary urgency or nociceptive pain. In this review article, the functional role of the TRPM8 channel in the lower urinary tract and the potential of its antagonist for the treatment of bladder disorders was described.

Activation of TRPM8 channel inhibits contraction of the isolated human ureter

AIMS

The transient receptor potential melastin-8 (TRPM8) channel is a "cooling" receptor expressed in primary sensory neurons and can be activated by compounds like menthol or icilin. TRPM8 is involved in the regulation of urinary bladder sensory function and contraction, but the role of TRPM8 in the ureter, particularly in the human ureter, is poorly understood. The aim of this study is to examine the effects of TRPM8 activation on human ureter contraction.

METHODS

Human ureters were acquired from 20 patients undergoing radical nephrectomy. Contractions of ureter strips were recorded by an isometric transducer in the organ bath. Ureteral TRPM8 expression in the human ureter was examined by immunofluorescence and western blot.

RESULTS

The two TRPM8 agonists menthol and icilin both reduced the frequency of spontaneous, electrical field stimulation, or neurokinin A-evoked ureteral contractions in a dose-dependent manner. The inhibitory effects were decreased by 10-fold in mucosa-denuded strips. The inhibitory effects of TRPM8 agonists were mimicked by calcitonin gene-related peptide (CGRP), and were blocked by KRP2579 (a TRPM8 antagonist), tetrodotoxin (a sodium channel blocker), olcegepant (BIBN, a CGRP receptor antagonist), SQ22536 (an adenylate cyclase antagonist), or H89 (a nonspecific cAMP-dependent protein kinase A inhibitor). TRPM8 was coexpressed with CGRP on the nerves located in the suburothelial and intermuscular regions and was not expressed in the urothelium.

CONCLUSIONS

The TRPM8 channel expressed on sensory nerve terminals of the human ureter is involved in the inhibitory sensory neurotransmission and modulate ureter contraction via the CGRP-adenylyl cyclase-protein kinase A pathway. TRPM8 may be involved in stone-induced changes in ureter contraction or pain.

Efficacy of the combination of KPR-5714, a novel transient receptor potential melastatin 8 (TRPM8) antagonist, and β3-adrenoceptor agonist or anticholinergic agent on bladder dysfunction in rats with bladder overactivity

Transient receptor potential melastatin 8 (TRPM8) channels may contribute to the pathophysiological bladder afferent hyperactivity, thus a TRPM8 antagonist would be a promising therapeutic target for the bladder hypersensitive disorders including urinary urgency in overactive bladder (OAB). We aimed to investigate a pharmacological effect of KPR-5714, a novel selective TRPM8 antagonist, on TRPM8 channels, M3 receptors and β3-adrenoceptors using the transfected cells of each gene at first. Then, combination effects of KPR-5714 and mirabegron, a β3-adrenoceptor agonist, or tolterodine tartrate, an anticholinergic agent, were studied on rhythmic bladder contractions (RBCs) in normal rats and bladder function in frequent-voiding rats. In vitro measurements showed that KPR-5714 acts on neither β3-adrenoceptor nor M3 receptor. In normal rats, KPR-5714 and mirabegron significantly reduced the frequency of RBCs, and a combined administration showed an additive effect. In rats with cerebral infarction, KPR-5714 and mirabegron significantly reduced the voiding frequency, and a combined administration showed an additive effect. In rats exposed to cold temperature, KPR-5714 and tolterodine tartrate significantly reduced the voiding frequency accompanied by the increased mean voided volume, and a combined administration showed additive effects. The present study demonstrated that the combined administration of KPR-5714 and mirabegron or tolterodine tartrate showed the additive effects on bladder dysfunction in different animal models, suggesting that the combination therapy of TRPM8 antagonist and β3-adrenoceptor agonist or anticholinergic agent can be the potential treatment option for obtaining additive effects in comparison with monotherapy for OAB.

The Urothelium: Life in a Liquid Environment

The urothelium, which lines the renal pelvis, ureters, urinary bladder, and proximal urethra, forms a high-resistance but adaptable barrier that surveils its mechanochemical environment and communicates changes to underlying tissues including afferent nerve fibers and the smooth muscle. The goal of this review is to summarize new insights into urothelial biology and function that have occurred in the past decade. After familiarizing the reader with key aspects of urothelial histology, we describe new insights into urothelial development and regeneration. This is followed by an extended discussion of urothelial barrier function, including information about the roles of the glycocalyx, ion and water transport, tight junctions, and the cellular and tissue shape changes and other adaptations that accompany expansion and contraction of the lower urinary tract. We also explore evidence that the urothelium can alter the water and solute composition of urine during normal physiology and in response to overdistension. We complete the review by providing an overview of our current knowledge about the urothelial environment, discussing the sensor and transducer functions of the urothelium, exploring the role of circadian rhythms in urothelial gene expression, and describing novel research tools that are likely to further advance our understanding of urothelial biology.

TRPM8 channels: A review of distribution and clinical role

Ion channels are important therapeutic targets due to their plethoric involvement in physiological and pathological consequences. The transient receptor potential cation channel subfamily M member 8 (TRPM8) is a nonselective cation channel that controls Ca²⁺ homeostasis. It has been proposed to be the predominant thermoreceptor for cellular and behavioral responses to cold stimuli in the transient receptor potential (TRP) channel subfamilies and exploited so far to reach the clinical-stage of drug development. TRPM8 channels can be found in multiple organs and tissues, regulating several important processes such as cell proliferation, migration and apoptosis, inflammatory reactions, immunomodulatory effects, pain, and vascular muscle tension. The related disorders have been expanded to new fields ranging from cancer and migraine to dry eye disease, pruritus, irritable bowel syndrome (IBS), and chronic cough. This review is aimed to summarize the distribution of TRPM8 and disorders related to it from a clinical perspective, so as to broaden the scope of knowledge of researchers to conduct more studies on this subject.

Dry eye sensitizes cool cells to capsaicin-induced changes in activity via TRPV1

Corneal cool cells are sensitive to the ocular fluid status of the corneal surface and may be responsible for the regulation of basal tear production. Previously, we have shown that dry eye, induced by lacrimal gland excision (LGE) in rats, sensitized corneal cool cells to the transient receptor potential melastatin 8 (TRPM8) agonist menthol and to cool stimulation. In the present study, we examined the effect of dry eye on the sensitivity of cool cells to the transient receptor potential vanilloid 1 (TRPV1) agonist capsaicin. Single-unit recordings in the trigeminal ganglion were performed 7-10 days after LGE. At a concentration of 0.3 μM, capsaicin did not affect ongoing or cool-evoked activity in control animals yet facilitated ongoing activity and suppressed cool-evoked activity in LGE animals. At higher concentrations (3 μM), capsaicin continued to facilitate ongoing activity in LGE animals but suppressed ongoing activity in control animals. Higher concentrations of capsaicin also suppressed cool-evoked activity in both groups of animals, with an overall greater effect in LGE animals. In addition to altering cool-evoked activity, capsaicin enhanced the sensitivity of cool cells to heat in LGE animals. Capsaicin-induced changes were prevented by the application of the TRPV1 antagonist capsazepine. With the use of fluorescent in situ hybridization, TRPV1 and TRPM8 expression was examined in retrograde tracer-identified corneal neurons. The coexpression of TRPV1 and TRPM8 in corneal neurons was significantly greater in LGE-treated animals when compared with sham controls. These results indicate that LGE-induced dry eye increases TRPV1-mediated responses in corneal cool cells at least in part through the increased expression of TRPV1. NEW & NOTEWORTHY Corneal cool cells are known to detect drying of the ocular surface. Our study is the first to report that dry eye induced alterations in cool cell response properties, including the increased responsiveness to noxious heat and activation by capsaicin. Along with the changes in cell response properties, it is possible these neurons also function differently in dry eye, relaying information related to the perception of ocular irritation in addition to regulating tearing and blinking.

Potential therapeutic value of transient receptor potential channels in male urogenital system

Transient receptor potential (TRP) channels comprise a family of cation channels implicated in a variety of cellular processes including light, mechanical or chemical stimuli, temperature, pH, or osmolarity. TRP channel proteins are a diverse family of proteins that are expressed in many tissues. We debated our recent knowledge about the expression, function, and regulation of TRP channels in the different parts of the male urogenital system in health and disease. Emerging evidence suggests that dysfunction of TRP channels significantly contributes to the pathophysiology of urogenital diseases. So far, there are many efforts underway to determine if these channels can be used as drug targets to reverse declines in male urogenital function. Furthermore, developing safe and efficacious TRP channel modulators is warranted for male urogenital disorders in a clinical setting.

Benefits and limitations of animal models in partial bladder outlet obstruction for translational research

The functions of the lower urinary tract have been investigated for more than a century. Lower urinary tract symptoms, such as incomplete bladder emptying, weak urine stream, daytime urinary frequency, urgency, urge incontinence and nocturia after partial bladder outlet obstruction, is a frequent cause of benign prostatic hyperplasia in aging men. However, the pathophysiological mechanisms have not been fully elucidated. The use of animal models is absolutely imperative for understanding the pathophysiological processes involved in bladder dysfunction. Surgical induction has been used to study lower urinary tract functions of numerous animal species, such as pig, dog, rabbit, guinea pig, rat and mouse, of both sexes. Several morphological and functional modifications under partial bladder outlet obstruction have not only been observed in the bladder, but also in the central nervous system. Understanding the changes of the lower urinary tract functions induced by partial bladder outlet obstruction would also contribute to appropriate drug development for treating these pathophysiological conditions. In the present review, we discuss techniques for creating partial bladder outlet obstruction, the characteristics of several species, as well as issues of each model, and their translational value.

Hydrogen peroxide preferentially activates capsaicin-sensitive high threshold afferents via TRPA1 channels in the guinea pig bladder

BACKGROUND AND PURPOSE

There is increasing evidence suggesting that ROS play a major pathological role in bladder dysfunction induced by bladder inflammation and/or obstruction. The aim of this study was to determine the effect of H₂ O₂ on different types of bladder afferents and its mechanism of action on sensory neurons in the guinea pig bladder.

EXPERIMENTAL APPROACH

'Close-to-target' single unit extracellular recordings were made from fine branches of pelvic nerves entering the guinea pig bladder, in flat sheet preparations, in vitro.

KEY RESULTS

H₂ O₂ (300-1000 μM) preferentially and potently activated capsaicin-sensitive high threshold afferents but not low threshold stretch-sensitive afferents, which were only activated by significantly higher concentrations of hydrogen peroxide. The TRPV1 channel agonist, capsaicin, excited 86% of high threshold afferents. The TRPA1 channel agonist, allyl isothiocyanate and the TRPM8 channel agonist, icilin activated 72% and 47% of capsaicin-sensitive high threshold afferents respectively. The TRPA1 channel antagonist, HC-030031, but not the TRPV1 channel antagonist, capsazepine or the TRPM8 channel antagonist, N-(2-aminoethyl)-N-[[3-methoxy-4-(phenylmethoxy)phenyl]methyl]thiophene-2-carboxamide, significantly inhibited the H₂ O₂ -induced activation of high threshold afferents. Dimethylthiourea and deferoxamine did not significantly change the effect of H₂ O₂ on high threshold afferents.

CONCLUSIONS AND IMPLICATIONS

The findings show that H₂ O₂ , in the concentration range detected in inflammation or reperfusion after ischaemia, evoked long-lasting activation of the majority of capsaicin-sensitive high threshold afferents, but not low threshold stretch-sensitive afferents. The data suggest that the TRPA1 channels located on these capsaicin-sensitive afferent fibres are probable targets of ROS released during oxidative stress.

TRPM8: a potential target for cancer treatment

Transient receptor potential (TRP) cation channel superfamily plays critical roles in variety of processes, including temperature perception, pain transduction, vasorelaxation, male fertility, and tumorigenesis. One of seven families within the TRP superfamily of ion channels, the melastatin, or TRPM family comprises a group of eight structurally and functionally diverse channels. Of all the members of TRPM subfamily, TRPM8 is the most notable one. A lot of literatures have demonstrated that transient receptor potential melastatin 8 (TRPM8) could perform a myriad of functions in vertebrates and invertebrates alike. In addition to its well-known function in cold sensation, TRPM8 has an emerging role in a variety of biological systems, including thermoregulation, cancer, bladder function, and asthma. Recent studies have shown that TRPM8 is necessary to the initiation and progression of tumors, and the aberrant expression of TRPM8 was found in varieties of tumors, such as prostate tumor, melanoma, breast adenocarcinoma, bladder cancer, and colorectal cancer, making it a novel molecular target potentially useful in the diagnosis and treatment of cancer. This review outlines our current understanding on the role of TRPM8 in occurrence and development of different kinds of tumor and also includes discussion about the regulation of TRPM8 during carcinogenesis as well as therapeutic potential of targeting TRPM8 in tumor, which may be utilized for a potential pharmacological use as a target for anti-cancer therapy.

Research Findings on Overactive Bladder

Several physiopathologic conditions lead to the manifestation of overactive bladder (OAB). These conditions include ageing, diabetes mellitus, bladder outlet obstruction, spinal cord injury, stroke and brain injury, Parkinson's disease, multiple sclerosis, interstitial cystitis, stress and depression. This review has discussed research findings in human and animal studies conducted on the above conditions. Several structural and functional changes under these conditions have not only been observed in the lower urinary tract, but also in the brain and spinal cord. Significant changes were observed in the following areas: neurotransmitters, prostaglandins, nerve growth factor, Rho-kinase, interstitial cells of Cajal, and ion and transient receptor potential channels. Interestingly, alterations in these areas showed great variation in each of the conditions of the OAB, suggesting that the pathophysiology of the OAB might be different in each condition of the disease. It is anticipated that this review will be helpful for further research on new and specific drug development against OAB.

TRP channels in lower urinary tract dysfunction

Lower urinary tract dysfunction (LUTd) represents a major healthcare problem. Although it is mostly not lethal, associated social disturbance, medical costs, loss of productivity and especially diminished quality of life should not be underestimated. Although more than 15% of people suffer from a form of LUTd to some extent, pathophysiology often remains obscure. In the past 20 years, transient receptor potential (TRP) channels have become increasingly important in this field of research. These intriguing ion channels are believed to be the main molecular sensors that generate bladder sensation. Therefore, they are intensely pursued as new drug targets for both curative and symptomatic treatment of different forms of LUTd. TRPV1 was the first of its class to be investigated. Actually, even before this channel was cloned, it had already been targeted in the bladder, with clinical trials of intravesical capsaicin instillations. Several other polymodally gated TRP channels, particularly TRPM8, TRPA1 and TRPV4, also appear to play a prominent role in bladder (patho)physiology. With this review, we provide a brief overview of current knowledge on the role of these TRP channels in LUTd and their potential as molecular targets for treatment.

Transient receptor potential (TRP) channels: a clinical perspective

Transient receptor potential (TRP) channels are important mediators of sensory signals with marked effects on cellular functions and signalling pathways. Indeed, mutations in genes encoding TRP channels are the cause of several inherited diseases in humans (the so-called 'TRP channelopathies') that affect the cardiovascular, renal, skeletal and nervous systems. TRP channels are also promising targets for drug discovery. The initial focus of research was on TRP channels that are expressed on nociceptive neurons. Indeed, a number of potent, small-molecule TRPV1, TRPV3 and TRPA1 antagonists have already entered clinical trials as novel analgesic agents. There has been a recent upsurge in the amount of work that expands TRP channel drug discovery efforts into new disease areas such as asthma, cancer, anxiety, cardiac hypertrophy, as well as obesity and metabolic disorders. A better understanding of TRP channel functions in health and disease should lead to the discovery of first-in-class drugs for these intractable diseases. With this review, we hope to capture the current state of this rapidly expanding and changing field.

Transient receptor potential channels in bladder function

The transient receptor potential (TRP) superfamily of cationic ion channels includes proteins involved in the transduction of several physical and chemical stimuli to finely tune physiological functions. In the urinary bladder, they are highly expressed in, but not restricted to, primary afferent neurons. The urothelium and some interstitial cells also express several TRP channels. In this review, we describe the expression and the known roles of some members of TRP subfamilies, namely TRPV, TRPM and TRPA, in the urinary bladder. The therapeutic interest of modulating the activity of TRP channels to treat bladder dysfunctions is also discussed.

Function of the Cold Receptor (TRPM8) Associated with Voiding Dysfunction in Bladder Outlet Obstruction in Rats

Purpose

Bladder outlet obstruction (BOO) causes storage and voiding dysfunction in the lower urinary tract. We investigated the expression of transient receptor potential cation channel subfamily M member 8 (TRPM8) to evaluate the relationship between TRPM8 expression and overactive bladder (OAB) in a rat model of BOO.

Methods

Fifty female Sprague-Dawley rats were divided into 4 groups; normal (n=10), normal-menthol (n=10), BOO (n=15), BOO-menthol (n=15). After 3 weeks, cystometry was performed by infusing physiological saline and menthol (3 mM) into the bladder at a slow infusion rate. The histological changes and expression of TRPM8 in the bladder were investigated by Masson's trichrome staining, immunofluorescence and reverse transcription-polymerase chain reaction.

Results

Cystometry showed that the intercontraction interval (ICI; 428.2±23.4 vs. 880.4±51.2, P<0.001), micturition pressure (MP; 25.7±1.01 vs. 71.80±3.01, P<0.001), and threshold pressure (2.9±0.25 vs. 9.2±1.58, P<0.01) were significantly increased in BOO rats. The bladder wall was significantly dilated compared with the control. Detrusor muscle hypertrophy and a thick mucosa layer were observed in BOO bladder. After menthol treatment, ICIs were decreased and MPs were increased in the menthol treatment groups. TRPM8-positive cells and mRNA were predominantly increased in the bladder and dorsal root ganglia of all groups compared with the normal group.

Conclusions

Increased bladder wall thickness and proportion of collagen probably affect voiding dysfunction. Furthermore, an increase of TRPM8 expression in BOO may induce entry of Ca²⁺ from the extracellular space or stores. The increase of Ca²⁺ probably causes contraction of smooth muscle in BOO. However, OAB symptoms were not observed after menthol treatment although the expression of TRPM8 was abundant in the bladder epithelium after menthol treatment. Although OAB in BOO models may be caused by complex pathways, regulation of TRPM8 presents possibilities for OAB treatment.

Cold Stress and Urinary Frequency

There have been few studies regarding the onset of urinary sensations and frequent urination induced by sudden whole-body cooling. In this article, we review the relationship between cold stress and urinary frequency based mainly on our previous studies. A recent study showed that cold stress induces bladder overactivity in conscious rats, and these effects were mediated, at least in part, by α1A -adrenergic receptor (AR) and α1D -AR. Another study suggested that the resiniferatoxin-sensitive nerves present in the urinary bladder may also be involved in the regulation of detrusor activity associated with cold stress. The mammalian transient receptor potential (TRP) channel family consists of 28 channels subdivided into five different classes: TRPV (vanilloid), TRPC (canonical), TRPM (melastatin), TRPML (mucolipin), and TRPA (ankyrin). TRP channels function as multifunctional sensors at the cellular level. They can be activated by physical (voltage, heat, cold, mechanical stress) or chemical stimuli and binding of specific ligands. In 2002, it was reported that a nonselective cation channel, TRPM8, could be activated by both menthol and thermal stimuli (8-28 °C). We demonstrated the presence of TRPM8 in the skin from the legs and back of rats by immunofluorescence staining and that stimulation of this receptor by menthol causes urinary frequency. There have been other reports demonstrating roles of TRPM8 not related to its thermosensory function. Further studies are needed to clarify the mechanism of cold stress-induced urinary frequency, and the roles of TRPM8 in the micturition control system.