Urothelium update: how the bladder mucosa measures bladder filling

PIEZO ion channels: force sensors of the interoceptive nervous system

Many organs are designed to move: the heart pumps each second, the gastrointestinal tract squeezes and churns to digest food, and we contract and relax skeletal muscles to move our bodies. Sensory neurons of the peripheral nervous system detect signals from bodily tissues, including the forces generated by these movements, to control physiology. The processing of these internal signals is called interoception, but this is a broad term that includes a wide variety of both chemical and mechanical sensory processes. Mechanical senses are understudied, but rapid progress has been made in the last decade, thanks in part to the discovery of the mechanosensory PIEZO ion channels (Coste et al., 2010). The role of these mechanosensors within the interoceptive nervous system is the focus of this review. In defining the transduction molecules that govern mechanical interoception, we will have a better grasp of how these signals drive physiology.

Dynamic analysis of the individual patterns of intakes, voids, and bladder sensations reported in bladder diaries collected in the LURN study

The goal of this study was to develop the novel analytical approach and to perform an in-depth dynamic analysis of individual bladder diaries to inform which behavioral modifications would best reduce lower urinary tract symptoms, such as frequency and urgency. Three-day bladder diaries containing data on timing, volumes, and types of fluid intake, as well as timing, volumes, and bladder sensation at voids were analyzed for 197 participants with lower urinary tract symptoms. A novel dynamic analytic approach to bladder diary time series data was proposed and developed, including intra-subject correlations between time-varying variables: rates of intake, bladder filling rate, and urge growth rate. Grey-box models of bladder filling rate and multivariable linear regression models of urge growth rate were developed for individual diaries. These models revealed that bladder filling rate, rather than urine volume, was the primary determinant of urinary frequency and urgency growth rate in the majority of participants. Simulations performed with the developed models predicted that the most beneficial behavioral modifications to reduce the number of urgency episodes are those that smooth profiles of bladder filling rate, which might include behaviors such as exclusion of caffeine and alcohol and/or other measures, e.g., increasing number and decreasing volumes of intakes.

Mechanotransduction in the urothelium: ATP signalling and mechanoreceptors

The urothelium, which covers the inner surface of the bladder, is continuously exposed to a complex physical environment where it is stimulated by, and responds to, a wide range of mechanical cues. Mechanically activated ion channels endow the urothelium with functioning in the conversion of mechanical stimuli into biochemical events that influence the surface of the urothelium itself as well as suburothelial tissues, including afferent nerve fibres, interstitial cells of Cajal and detrusor smooth muscle cells, to ensure normal urinary function during the cycle of filling and voiding. However, under prolonged and abnormal loading conditions, the urothelial sensory system can become maladaptive, leading to the development of bladder dysfunction. In this review, we summarize developments in the understanding of urothelial mechanotransduction from two perspectives: first, with regard to the functions of urothelial mechanotransduction, particularly stretch-mediated ATP signalling and the regulation of urothelial surface area; and secondly, with regard to the mechanoreceptors present in the urothelium, primarily transient receptor potential channels and mechanosensitive Piezo channels, and the potential pathophysiological role of these channels in the bladder. A more thorough understanding of urothelial mechanotransduction function may inspire the development of new therapeutic strategies for lower urinary tract diseases.

Dynamic analysis of the individual patterns of intakes, voids, and bladder sensations reported in bladder diaries collected in the LURN study

The goal of this study was to perform an in-depth dynamic analysis of individual bladder diaries to inform which behavioral modifications would best reduce lower urinary tract symptoms, such as frequency and urgency. Three-day bladder diaries containing data on timing, volumes, and types of fluid intake, as well as timing, volumes, and bladder sensation at voids were analyzed for 197 participants with lower urinary tract symptoms. A novel dynamic analytic approach to bladder diary time series data was proposed and developed, including intra-subject correlations between time-varying variables: rates of intake, bladder filling rate, and urge growth rate. Grey-box models of bladder filling rate and multivariable linear regression models of urge growth rate were developed for individual diaries. These models revealed that bladder filling rate, rather than urine volume, was the primary determinant of urinary frequency and urgency growth rate in the majority of participants. Simulations performed with the developed models predicted that the most beneficial behavioral modifications to reduce the number of urgency episodes are those that smooth profiles of bladder filling rate, which might include behaviors such as exclusion of caffeine and alcohol and/or other measures, e.g., increasing number and decreasing volumes of intakes.

Emerging molecular mechanisms and genetic targets for developing novel therapeutic strategies for treating bladder diseases

Bladder diseases affect millions of patients worldwide and compromise their quality of life with a substantial economic impact. The not fully understood aetiologies of bladder diseases limit the current diagnosis and therapeutic options to primarily symptomatic treatment. In addition, bladder targeted drug delivery is challenging due to its unique anatomical features and its natural physiological function of urine storage and frequent voiding. Therefore, current treatment options often fail to provide a highly effective, precisely targeted and long-lasting treatment. With the growing maturity of gene therapy, comprehensive studies are needed to provide a better understanding of the molecular mechanisms underpinning bladder diseases and help to identify novel gene therapeutic targets and biomarkers for treating bladder diseases. In this review, molecular mechanisms involved in pathology of bladder cancer, interstitial cystitis and overactive bladder syndrome are reviewed, with focus on establishing potential novel treatment options. Proposed novel therapies, including gene therapy combined with nanotechnology, localised drug delivery by nanoparticles, and probiotics, are discussed in regard to their safety profiles, efficacy, treatment lenght, precise targeting, and in comparison to conventional treatment methods.

The effect of amino acids on the bladder cycle: a concise review

The human bladder maintains a cycle of filling, storing, and micturating throughout an individual's lifespan. The cycle relies on the ability of the bladder to expand without increasing the intravesical pressure, which is only possible with the controlled relaxation of well-complaint muscles and the congruously organized construction of the bladder wall. A competent bladder outlet, which functions in a synchronous fashion with the bladder, is also necessary for this cycle to be completed successfully without deterioration. In this paper, we aimed to review the contemporary physiological findings on bladder physiology and examine the effects of amino acids on clinical conditions affecting the bladder, with special emphasis on the available therapeutic evidence and possible future roles of the amino acids in the treatment of the bladder-related disorders.

Urothelial Calcium-Sensing Receptor Modulates Micturition Function via Mediating Detrusor Activity and Ameliorates Bladder Hyperactivity in Rats

The urothelium displays mechano- and chemosensory functions via numerous receptors and channels. The calcium-sensing receptor (CaSR) detects extracellular calcium and modulates several physiological functions. Nonetheless, information about the expression and the role of CaSR in lower urinary tract has been absent. We aimed to determine the existence of urothelial CaSR in urinary bladder and its effect on micturition function. We utilized Western blot to confirm the expression of CaSR in bladder and used immunofluorescence to verify the location of the CaSR in the bladder urothelium via colocalization with uroplakin III A. The activation of urothelial CaSR via the CaSR agonist, AC-265347 (AC), decreased urinary bladder smooth muscle (detrusor) activity, whereas its inhibition via the CaSR antagonist, NPS-2143 hydrochloride (NPS), increased detrusor activity in in vitro myography experiments. Cystometry, bladder nerve activities recording, and bladder surface microcirculation detection were conducted to evaluate the effects of the urothelial CaSR via intravesical administrations. Intravesical AC inhibited micturition reflex, bladder afferent and efferent nerve activities, and reversed cystitis-induced bladder hyperactivity. The urothelial CaSR demonstrated a chemosensory function, and modulated micturition reflex via regulating detrusor activity. This study provided further evidence of how the urothelial CaSR mediated micturition and implicated the urothelial CaSR as a potential pharmacotherapeutic target in the intervention of bladder disorders.

The nitric oxide-cyclic guanosine monophosphate pathway inhibits the bladder ATP release in response to a physiological or pathological stimulus

The release of ATP from the epithelium of the urinary bladder (urothelium) in response to mechanical/chemical stimuli contributes to the visceral sensation in the micturition reflex. The nitric oxide (NO)-mediated induction of cyclic guanosine monophosphate (cGMP) has been detected in urothelial cells and may inhibit the micturition reflex. However, the function of the NO-cGMP pathway in the regulation of urothelial ATP release remains poorly understood in contrast to its effects on smooth muscles or primary afferent nerves. Therefore, we investigated the relevance of the NO-cGMP pathway to ATP release on the mucosal side in the present study. The administration of l-arginine (NO precursor) or NOC 12 (NO donor) significantly reduced ATP release to the mucosal side at a physiologically normal urine storage pressure (5 cmH2 O). L-NAME (NO synthase inhibitor) significantly increased the distention-induced release of ATP. The phosphodiesterase-5 inhibitor, sildenafil, which increases cGMP levels, inhibited distention-induced ATP release. Furthermore, sildenafil significantly reduced ATP release in response to the administration of lipopolysaccharide. These results suggest that the NO-cGMP pathway inhibited urothelial ATP release during the storage phase under both physiological and pathological conditions.

Recurrent Urinary Tract Infection: A Mystery in Search of Better Model Systems

Urinary tract infections (UTIs) are among the most common infectious diseases worldwide but are significantly understudied. Uropathogenic E. coli (UPEC) accounts for a significant proportion of UTI, but a large number of other species can infect the urinary tract, each of which will have unique host-pathogen interactions with the bladder environment. Given the substantial economic burden of UTI and its increasing antibiotic resistance, there is an urgent need to better understand UTI pathophysiology - especially its tendency to relapse and recur. Most models developed to date use murine infection; few human-relevant models exist. Of these, the majority of in vitro UTI models have utilized cells in static culture, but UTI needs to be studied in the context of the unique aspects of the bladder's biophysical environment (e.g., tissue architecture, urine, fluid flow, and stretch). In this review, we summarize the complexities of recurrent UTI, critically assess current infection models and discuss potential improvements. More advanced human cell-based in vitro models have the potential to enable a better understanding of the etiology of UTI disease and to provide a complementary platform alongside animals for drug screening and the search for better treatments.

Sensations Reported During Urodynamic Bladder Filling in Spinal Cord Injury Patients Give Additional Important Information

Purpose

To study sensations reported during filling cystometry in patients with different levels and completeness of spinal cord lesion (SCL).

Methods

A retrospective cohort study. Patient age and sex, cause of SCL, American Spinal Injury Association Impairment Scale (AIS), and lower urinary tract -related sensations in daily life were gathered. Filling cystometry (video-urodynamics) was performed following ICS good urodynamic practice guidelines. Beside bladder filling sensations (first sensation of bladder filling, first desire to void, strong desire to void), other sensations as detrusor overactivity related sensation and pain were noted.

Results

170 patients were included, age 45 ± 17 years, 114 male and 56 female, 92 complete and 78 incomplete SCL. The test was done 6 ± 4 years post SCL. Sensation was reported by 57% of all patients. In complete SCL half (46/92) had sensation, while 36% with incomplete SCL (28/78) reported no sensation. Bladder awareness was not predictable by the AIS. The filling sensations reported were equal to those given in the terminology of ICS. Pain was seldom present 10/170 (6%), Detrusor overactivity contraction was felt by 45/78 ( 58%). Very few patients used sensory information for bladder management at home.

Conclusion

After SCL, most patients retain the ability to be aware of the LUT, assessable and gradable, during urodynamic testing. Filling sensations were not different from those described in healthy, but the number and sequence of the sensations were altered in the minority. Pain and sensation of unstable contractions gave additional important information. As different sensations relate to different spinal afferent pathways, the sensory evaluation during cystometry provided additional important information on the spinal cord's condition.

Cell Deformation at the Air-Liquid Interface Evokes Intracellular Ca2+ Increase and ATP Release in Cultured Rat Urothelial Cells

Urothelial cells have been implicated in bladder mechanosensory transduction, and thus, initiation of the micturition reflex. Cell deformation caused by tension forces at an air-liquid interface (ALI) can induce an increase in intracellular Ca²⁺ concentration ([Ca²⁺]_i) and ATP release in some epithelial cells. In this study, we aimed to examine the cellular mechanisms underlying ALI-induced [Ca²⁺]_i increase in cultured urothelial cells. The ALI was created by stopping the influx of the perfusion but maintaining efflux. The [Ca²⁺]_i increase was measured using the Ca²⁺ imaging method. The ALI evoked a reversible [Ca²⁺]_i increase and ATP release in urothelial cells, which was almost abolished by GdCl₃. The specific antagonist of the transient receptor potential vanilloid (TRPV4) channel (HC0674) and the antagonist of the pannexin 1 channel (¹⁰panx) both diminished the [Ca²⁺]_i increase. The blocker of Ca²⁺-ATPase pumps on the endoplasmic reticulum (thapsigargin), the IP3 receptor antagonist (Xest-C), and the ryanodine receptor antagonist (ryanodine) all attenuated the [Ca²⁺]_i increase. Degrading extracellular ATP with apyrase or blocking ATP receptors (P2X or P2Y) with pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS) significantly attenuated the [Ca²⁺]_i increase. Our results suggest that both Ca²⁺ influx via TRPV4 or pannexin 1 and Ca²⁺ release from intracellular Ca²⁺ stores via IP3 or ryanodine receptors contribute to the mechanical responses of urothelial cells. The release of ATP further enhances the [Ca²⁺]_i increase by activating P2X and P2Y receptors via autocrine or paracrine mechanisms.

Emerging roles of the TRPV4 channel in bladder physiology and dysfunction

The transient receptor potential vanilloid type 4, TRPV4, is a polymodal cation channel which can be activated by diverse stimuli including mechanical, thermal and chemical cues. In the urinary bladder, TRPV4 is not only abundantly expressed in the urothelium but may also be localized in subepithelium, detrusor smooth muscles and afferent neurons. Emerging evidence indicates that the TRPV4 channel plays a sensory role in the uroepithelium, where it may regulate the release of sensory mediators such as ATP, which in turn modulates afferent nerve activity in response to bladder filling during the urination cycle. TRPV4 may also directly regulate detrusor contractility and the urothelial barrier function. Altered TRPV4 expression has been detected in various pathological bladder conditions. As such, TRPV4 may be a promising therapeutic target for bladder dysfunctions.

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.

Urinary dysfunction in transgenic sickle cell mice: model of idiopathic overactive bladder syndrome

Voiding abnormalities are common among the sickle cell disease (SCD) population, among which overactive bladder (OAB) syndrome is observed at rates as high as 39%. Although detrusor overactivity is the most common cause of OAB, its molecular pathophysiology is not well elucidated. The nitric oxide (NO) signaling pathway has been implicated in the regulation of lower genitourinary tract function. In the present study, we evaluated the role of the NO signaling pathway in voiding function of transgenic SCD mice compared with combined endothelial and neuronal NO synthase gene-deficient mice, both serving as models of NO deficiency. Mice underwent void spot assay and cystometry, and bladder and urethral specimens were studied using in vitro tissue myography. Both mouse models exhibited increased void volumes; increased nonvoiding and voiding contraction frequencies; decreased bladder compliance; increased detrusor smooth muscle contraction responses to electrical field stimulation, KCl, and carbachol; and increased urethral smooth muscle relaxation responses to sodium nitroprusside compared with WT mice. In conclusion, our comprehensive behavioral and functional study of the SCD mouse lower genitourinary tract, in correlation with that of the NO-deficient mouse, reveals NO effector actions in voiding function and suggests that NO signaling derangements are associated with an OAB phenotype. These findings may allow further study of molecular targets for the characterization and evaluation of OAB.

TRP Channels as Lower Urinary Tract Sensory Targets

Several members of the transient receptor potential (TRP) superfamily, including TRPV1, TRPV2, TRPV4, TRM4, TRPM8 and TRPA1, are expressed in the lower urinary tract (LUT), not only in neuronal fibers innervating the bladder and urethra, but also in the urothelial and muscular layers of the bladder and urethral walls. In the LUT, TRP channels are mainly involved in nociception and mechanosensory transduction. Animal studies have suggested the therapeutic potential of several TRP channels for the treatment of both bladder over- and underactivity and bladder pain disorders,; however translation of this finding to clinical application has been slow and the involvement of these channels in normal human bladder function, and in various pathologic states have not been established. The development of selective TRP channel agonists and antagonists is ongoing and the use of such agents can be expected to offer new and important information concerning both normal physiological functions and possible therapeutic applications.

Aging increases the expression of vasopressin receptors in both the kidney and urinary bladder

AIMS

The goal of this study was to determine whether aging effects the expression of V_1a and V₂ vasopressin receptors in the urinary bladder mucosa (UBM) and kidney.

METHODS

UBM and kidneys were obtained from young (3 months-of-age) and old (25-30 months-of-age) female Fisher 344 rats. Tissue samples were analyzed by western blotting for V_1a and V₂ receptor expression, and rat plasma levels of vasopressin levels were measured by ELISA.

RESULTS

V_1a and V₂ receptors were detected in both the UBM and kidneys. Aging significantly (P < 0.05) increased the expression of V₂ receptors by 2.80 ± 0.52 and 6.52 ± 1.24-fold in the UBM and kidneys, respectively. Aging also increased V_1a receptor expression in the kidneys (5.52 ± 1.05 fold; P < 0.05), but not in the UBM. To the best of our knowledge, because this is the first detection of V₂ receptors in the mammalian bladder mucosa, we also probed human UBM for V₂ receptors and observed high expression in human UBM. Unlike V_1a and V₂ receptors, aging had only a minor effect on plasma vasopressin levels (8% increase).

CONCLUSIONS

V₂ receptors are substantially increased in the aging UBM. The role of these receptors in UBM is as yet undefined, but given their presence and action in the kidneys, the possible effect of these receptors in free water regulation should be considered. The large age-related increase in the expression of V₂ receptors in both the UBM and kidney may contribute to the effectiveness of desmopressin in age-related nocturia.

Neural Sensing of Organ Volume

Many internal organs change volume periodically. For example, the stomach accommodates ingested food and drink, the bladder stores urine, the heart fills with blood, and the lungs expand with every breath. Specialized peripheral sensory neurons function as mechanoreceptors that detect tissue stretch to infer changes in organ volume and then relay this information to the brain. Central neural circuits process this information and evoke perceptions (satiety, nausea), control physiology (breathing, heart rate), and impact behavior (feeding, micturition). Yet, basic questions remain about how neurons sense organ distension and whether common sensory motifs are involved across organs. Here, we review candidate mechanosensory receptors, cell types, and neural circuits, focusing on the stomach, bladder, and airways. Understanding mechanisms of organ stretch sensation may provide new ways to treat autonomic dysfunction.

The time-dependent variation of ATP release in mouse primary-cultured urothelial cells is regulated by the clock gene

AIMS

The sensation of bladder fullness (SBF) is triggered by the release of ATP. Therefore, the aim of this study was to investigate whether time-dependent changes in the levels of stretch-released ATP in mouse primary-cultured urothelial cells (MPCUCs) is regulated by circadian rhythm via clock genes.

METHODS

MPCUCs were derived from wild-type and Clock mutant mice (Clock^Δ19/Δ19 ), presenting a nocturia phenotype. They were cultured in elastic silicone chambers. Stretch-released ATP was quantified every 4 h by ATP photon count. An experiment was also performed to determine whether ATP release correlated with the rhythm of the expression of Piezo1, TRPV4, VNUT, and Connexin26 (Cx26) in MPCUCs regulated by clock genes with circadian rhythms. MPCUCs were treated with carbenoxolone, an inhibitor of gap junction protein; were derived from VNUT-KO mice; or treated with Piezo1-siRNA, TRPV4-siRNA, and Cx26-siRNA.

RESULTS

Stretch-released ATP showed time-dependent changes in wild-type mice and correlated with the rhythm of the expression of Piezo1, TRPV4, VNUT, and Cx26. However, these rhythms were disrupted in Clock^Δ19/Δ19 mice. Carbenoxolone eliminated the rhythmicity of ATP release in wild-type mice. However, time-dependent ATP release changes were maintained when a single gene was deficient such as VNUT-KO, Piezo1-, TRPV4-, and Cx26-siRNA.

CONCLUSIONS

ATP release in the bladder urothelium induces SBF and may have a circadian rhythm regulated by the clock genes. In the bladder urothelium, clock gene abnormalities may disrupt circadian ATP release by inducing Piezo1, TRPV4, VNUT, and Cx26. All these genes can trigger nocturia.

Neuronal Activation in the Periaqueductal Gray Matter Upon Electrical Stimulation of the Bladder

Reflexes, that involve the spinobulbospinal pathway control both storage and voiding of urine. The periaqueductal gray matter (PAG), a pontine structure is part of the micturition pathway. Alteration in this pathway could lead to micturition disorders and urinary incontinence, such as the overactive bladder symptom complex (OABS). Although different therapeutic options exist for the management of OABS, these are either not effective in all patients. Part of the pathology of OABS is faulty sensory signaling about the filling status of the urinary bladder, which results in aberrant efferent signaling leading to overt detrusor contractions and the sensation of urgency and frequent voiding. In order to identify novel targets for therapy (i.e., structures in the central nervous system) and explore novel treatment modalities such as neuromodulation, we aimed at investigating which areas in the central nervous system are functionally activated upon sensory afferent stimulation of the bladder. Hence, we designed a robust protocol with multiple readout parameters including immunohistological and behavioral parameters during electrical stimulation of the rat urinary bladder. Bladder stimulation induced by electrical stimulation, below the voiding threshold, influences neural activity in: (1) the caudal ventrolateral PAG, close to the aqueduct; (2) the pontine micturition center and locus coeruleus; and (3) the superficial layers of the dorsal horn, sacral parasympathetic nucleus and central canal region of the spinal cord. In stimulated animals, a higher voiding frequency was observed but was not accompanied by increase in anxiety level and locomotor deficits. Taken together, this work establishes a critical role for the vlPAG in the processing of sensory information from the urinary bladder and urges future studies to investigate the potential of neuromodulatory approaches for urological diseases.

ls clinical practice aligned with the latest scientific evidence on GAG therapy?

The role of glycosaminoglycans (GAGs) as key components of the protective bladder barrier is well accepted. Replenishment of the GAG layer could restore the normal protective barrier function of the damaged bladder urothelium and re-establish normal permeability. A number of bladder diseases, including interstitial cystitis/bladder pain syndrome, recurrent urinary tract infections, radiation cystitis, and other forms of cystitis may benefit from GAG therapy.