Loss of purinergic P2X receptor innervation in human detrusor and subepithelium from adults with sensory urgency

BoNT/A in the Urinary Bladder-More to the Story than Silencing of Cholinergic Nerves

Botulinum neurotoxin (BoNT/A) is an FDA and NICE approved second-line treatment for overactive bladder (OAB) in patients either not responsive or intolerant to anti-cholinergic drugs. BoNT/A acts to weaken muscle contraction by blocking release of the neurotransmitter acetyl choline (ACh) at neuromuscular junctions. However, this biological activity does not easily explain all the observed effects in clinical and non-clinical studies. There are also conflicting reports of expression of the BoNT/A protein receptor, SV2, and intracellular target protein, SNAP-25, in the urothelium and bladder. This review presents the current evidence of BoNT/A’s effect on bladder sensation, potential mechanisms by which it might exert these effects and discusses recent advances in understanding the action of BoNT in bladder tissue.

Purinergic signalling in the urinary bladder - When function becomes dysfunction

Knowledge of the participation of ATP and related purines in urinary tract physiology has been established over the last five decades through the work of many independent groups, inspired by, and building on the pioneering studies of Professor Geoffrey Burnstock and his coworkers. As part of a series of reviews in this tribute edition, the present article summarises our current understanding of purines and purinergic signalling in modulating and regulating urinary tract function. Purinergic mechanisms underlying the origin of bladder pain; sensations of bladder filling and urinary tract motility; and regulation of detrusor smooth muscle contraction are described, encompassing the relevant history of discovery and consolidation of knowledge as methodologies and pharmacological tools have developed. We consider normal physiology, including development and ageing and then move to pathophysiology, discussing the causal and consequential contribution of purinergic signalling mechanism and their constituent components (receptors, signal transduction, effector molecules) to bladder dysfunction.

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.

Botulinum toxin in the treatment of overactive bladder

Clinical effectiveness of botulinum toxin (BTX) in the treatment of both neurogenic and idiopathic detrusor overactivity has been demonstrated in several studies. However, different protocols and techniques have been used by authors.

Methods.

Literature review on intradetrusor injection of BTX for detrusor overactivity.

Results.

The greatest clinical experience reports the use of 200 and 300 U Botox®. Available data suggest that clinical efficacy, duration, and the side effect profile is similar at these doses. Very few data, on the other hand, are available regarding the clinical outcomes using the Dysport® preparation; isolated reports support that efficacy is similar when using a dosing range of 500 to 1000 SU with increased risk of systemic side effects using 1000 SU. A variety of injection volumes was used, demonstrating similar efficacy and tolerability profile. Clinical effect duration extends six to ten months in the majority of studies. Data suggest that a repeated injection scheme proves successful in the vast majority of initial responders.

Conclusions.

Safety, effectiveness, specificity and reversibility make BTX a new attractive treatment modality for overactive bladder syndrome. However, more experience is needed to standardize the injection protocol with respect to therapeutic outcomes and adverse effects.

Effects of ischemia and oxidative stress on bladder purinoceptors expression

OBJECTIVE

To study the effects of chronic ischemia on bladder purinoceptors. A close correlation between bladder ischemia and lower urinary tract symptoms has been reported. Purinoceptors contribute to important aspects of bladder function including sensation, neural signaling, and voiding contraction. Our goal was to examine purinoceptors expression in the ischemic overactive bladder.

MATERIALS AND METHODS

Moderate bladder ischemia was produced in rabbits by creating bilateral iliac artery atherosclerosis. After 8 weeks, bladder blood flow was measured, and cystometrograms were obtained. Bladder tissues from 8-week ischemic and age-matched control bladders were processed for the analysis of oxidative stress markers, P2X and P2Y purinoceptors expression, and transmission electron microscopy.

RESULTS

Arterial atherosclerosis significantly decreased bladder blood flow. Markers of oxidative stress characterized by increased levels of advanced oxidation protein products and malondialdehyde were evident in the ischemic bladder tissues. Chronic ischemia and oxidative stress decreased the bladder capacity and increased spontaneous bladder contractions. Bladder pressure at micturition and intravesical pressure rise during contractions tended to be greater in the ischemic bladder but did not reach significance. Transmission electron microscopy showed smooth muscle cell and microvasculature structural damage and diffuse fibrosis. These changes in the ischemic bladder were associated with significant increases in purinoceptors P2X1, P2X2, P2X3, P2X4, P2X5, and P2X7 expression. The P2Y isoforms were not expressed in the rabbit bladder.

CONCLUSION

Structural and functional changes in the chronically ischemic bladder were associated with upregulation of P2X receptor isoforms. Increased P2X expression may play a role in ischemia-induced bladder overactivity and noncompliance.

Extracellular UDP enhances P2X-mediated bladder smooth muscle contractility via P2Y(6) activation of the phospholipase C/inositol trisphosphate pathway

Bladder dysfunction characterized by abnormal bladder smooth muscle (BSM) contractions is pivotal to the disease process in overactive bladder, urge incontinence, and spinal cord injury. Purinergic signaling comprises one key pathway in modulating BSM contractility, but molecular mechanisms remain unclear. Here we demonstrate, using myography, that activation of P2Y6 by either UDP or a specific agonist (MRS 2693) induced a sustained increase in BSM tone (up to 2 mN) in a concentration-dependent manner. Notably, activation of P2Y6 enhanced ATP-mediated BSM contractile force by up to 45%, indicating synergistic interactions between P2X and P2Y signaling. P2Y6-activated responses were abolished by phospholipase C (PLC) and inositol trisphosphate (IP3) receptor antagonists U73122 and xestospongin C, demonstrating involvement of the PLC/IP3 signal pathway. Mice null for Entpd1, an ectonucleotidase on BSM, demonstrated increased force generation on P2Y6 activation (150%). Thus, in vivo perturbations to purinergic signaling resulted in altered P2Y6 activity and bladder contractility. We conclude that UDP, acting on P2Y6, regulates BSM tone and in doing so selectively maximizes P2X1-mediated contraction forces. This novel neurotransmitter pathway may play an important role in urinary voiding disorders characterized by abnormal bladder motility.

The safety and efficacy of botulinum toxin-A in the management of bladder oversensitivity: a randomised double-blind placebo-controlled trial

OBJECTIVE

To assess the safety and efficacy of botulinum toxin-A (botn-A) in the management of patients with bladder oversensitivity (BO).

PATIENTS AND METHODS

Twenty-three consecutive patients with a diagnosis of BO refractory to anticholinergics were enrolled in this randomised, double blind, placebo-controlled trial. Patients were randomly allocated to receive intradetrusor injections of either botn-A (100 U Botox) or saline (placebo) via a flexible cystoscopic approach. The study was designed to have 90% power to detect a change in the maximum cystometric capacity (MCC) of 30%. It was calculated that a total cohort of 58 patients would be required. Urodynamic assessment (UDS), voiding diaries (VD) and quality of life (QoL) were assessed at baseline and at 4 and 12 weeks following intervention.

RESULTS

An interim analysis was performed and the trial halted after recruitment of 23 patients as a result of poorly perceived patient benefit. Data were analysed for 21 patients (10 botn-A; 11 placebo). In the treatment arm, there was a significant increase in MCC (mean rise 105 ml; p = 0.009). However, storage symptoms remained statistically unchanged following botn-A. Three patients in the treatment arm were required to perform clean intermittent self-catheterisation with no clinical improvement. The limitations of this trial include the small sample size and the unplanned interim analysis.

CONCLUSIONS

This is the first randomised, double blind, placebo-controlled trial examining the effects of botn-A exclusively in patients with BO. A significant increase in MCC was observed but this did not translate to clinical benefit with no change observed in the symptoms and quality of life for the majority of patients.

Expression and distribution of ectonucleotidases in mouse urinary bladder

Background

Normal urinary bladder function requires bidirectional molecular communication between urothelium, detrusor smooth muscle and sensory neurons and one of the key mediators involved in this intercellular signaling is ATP. Ectonucleotidases dephosphorylate nucleotides and thus regulate ligand exposure to P2X and P2Y purinergic receptors. Little is known about the role of these enzymes in mammalian bladder despite substantial literature linking bladder diseases to aberrant purinergic signaling. We therefore examined the expression and distribution of ectonucleotidases in the mouse bladder since mice offer the advantage of straightforward genetic modification for future studies.

Principal Findings

RT-PCR demonstrated that eight members of the ectonucleoside triphosphate diphosphohydrolase (NTPD) family, as well as 5′-nucleotidase (NT5E) are expressed in mouse bladder. NTPD1, NTPD2, NTPD3, NTPD8 and NT5E all catalyze extracellular nucleotide dephosphorylation and in concert achieve stepwise conversion of extracellular ATP to adenosine. Immunofluorescent localization with confocal microscopy revealed NTPD1 in endothelium of blood vessels in the lamina propria and in detrusor smooth muscle cells, while NTPD2 was expressed in cells localized to a region of the lamina propria adjacent to detrusor and surrounding muscle bundles in the detrusor. NTPD3 was urothelial-specific, occurring on membranes of intermediate and basal epithelial cells but did not appear to be present in umbrella cells. Immunoblotting confirmed NTPD8 protein in bladder and immunofluorescence suggested a primary localization to the urothelium. NT5E was present exclusively in detrusor smooth muscle in a pattern complementary with that of NTPD1 suggesting a mechanism for providing adenosine to P1 receptors on the surface of myocytes.

Conclusions

Ectonucleotidases exhibit highly cell-specific expression patterns in bladder and therefore likely act in a coordinated manner to regulate ligand availability to purinergic receptors. This is the first study to determine the expression and location of ectonucleotidases within the mammalian urinary bladder.

P2X receptor ligands and pain

P2X receptors belong to a superfamily of ligand-gated ion channels that conduct the influx of Ca(2+), Na(+) and K(+) cations following activation by extracellular nucleotides such as ATP. Molecular cloning studies have identified seven subunits, namely P2X(1-7), that share approximately 40 - 50% identity in amino acid sequences within the subfamily. Using gene-silencing, pharmacological and electrophysiological approaches, recent studies have revealed roles for P2X(2), P2X(3), P2X(4) and P2X(7) receptors in nociceptive signalling. Homomeric P2X(3) and heteromeric P2X(2/3) receptors are highly localised in the peripheral sensory afferent neurons that conduct nociceptive sensory information to the spinal chord and brain. The discovery of A-317491, a selective and potent non-nucleotide P2X(3) antagonist, provided a pharmacological tool to determine the site and mode of action of P2X(3)-containing receptors in different pain behaviours, including neuropathic, inflammatory and visceral pain. Other P2X receptors (P2X(4) and P2X(7)) that are predominantly expressed in microglia, macrophages and cells of immune origin can trigger the release of cytokines, such as IL-1-beta and TNF-alpha. Genetic disruption of P2X(4) and P2X(7) signalling has been demonstrated to reduce inflammatory and neuropathic pain, suggesting that these two receptors might serve as integrators of neuroinflammation and pain. This article provides an overview of recent scientific literature and patents focusing on P2X(3), P2X(4) and P2X(7) receptors, and the identification of small molecule ligands for the potential treatment of neuropathic and inflammatory pain.

Differential purinergic signaling in bladder sensory neurons of naïve and bladder-inflamed mice

This study explored purinergic signaling in lumbosacral (LS) and thoracolumbar (TL) dorsal root ganglion neurons innervating the urinary bladder. In naïve mice, a greater proportion of LS (93%) than that of TL (77%) bladder neurons responded to purinergic agonists. Three types of purinergic currents were identified: 'sustained' (homomeric P2X2) currents were detected only in LS neurons, rapidly activating, 'slow' deactivating (heteromeric P2X2/3) currents predominated in both LS and TL neurons, and 'fast' activating/de-activating (homomeric P2X3) currents were detected only in TL neurons. Relative to TL bladder neurons, slow current density was greater in LS neurons, which also had a more negative action potential threshold and generated more action potentials in response to purinergic agonists (suggesting greater excitability of LS neurons). Single cell nested PCR documented P2X2 and P2X3 subunit expression in both TL and LS bladder neurons. Relative to saline treatment, bladder wall thickness and weight increased after cyclophosphamide (CYP) treatment. Both LS and TL neuron excitability increased (rheobase was decreased and responses to purinergic agonists increased) after CYP treatment. The proportion of sustained currents in LS bladder neurons increased fourfold after CYP bladder inflammation. Although proportions of slow and fast purinergic currents in TL neurons were unchanged by CYP treatment, the fast current density was greater than in saline-treated mice. These results in mouse, as previously described in rat, reveal differential purinergic signaling in TL and LS bladder neurons. The predominant currents and significant changes after inflammation, however, occur in different ganglia/sensory pathways in mouse and rat.

Botulinum toxin injection: a review of injection principles and protocols

Despite the favorable outcomes seen using botulinum toxin (BTX) for voiding dysfunction using BTX, a standardized technique and protocol for toxin injection is not defined. We reviewed the current literature on intravesical BTX injection for DO (detrusor overactivity). Specific attention was placed on defining optimal injection protocol, including dose, volume, and injection sites. In addition, we sought to describe a standard technique to BTX injection.

Age-related changes of P2X(1) receptor mRNA in the bladder detrusor from men with and without bladder outlet obstruction

The urinary bladder purinergic system is reported to change with age and with bladder dysfunction. Here, we examined the expression of purinergic P2X(1) receptors in detrusor and mucosa (urothelium+lamina propria) from male control bladder and investigated age-related P2X(1) receptor mRNA expression in control and obstructed detrusor. Biopsy specimens were obtained at cystoscopy from control patients (n=46, age range 30-86years) and patients diagnosed with outlet obstruction (n=29, 46-88years). Calponin expression (measured by RT-PCR) was similar in control and obstructed detrusor and did not change with age. Quantitative competitive RT-PCR was used to measure P2X(1) receptor and GAPDH mRNA in control and obstructed detrusor. P2X(1) receptor mRNA expression was 9-fold (p<0.0001) higher in the detrusor than in the mucosa. Expression of mRNA for the internal control GAPDH remained stable with age and across control and obstructed detrusor. No difference in P2X(1) receptor expression was observed between control and obstructed detrusor (p=0.35). However, an age-related decrease in P2X(1) mRNA expression was observed in control (n=27; p=0.0054; Spearman coefficient r=-0.520) but not obstructed detrusor (n=19; p=0.093; r=-0.396). Downregulation of P2X(1) mRNA expression might occur as a result of an increased component of neural ATP release in the aging bladder.

Mechanisms of disease: role of purinergic signaling in the pathophysiology of bladder dysfunction

Although the 'purinergic nerve hypothesis' proposed by Burnstock in the early 1970s was met with considerable skepticism, it is now accepted that certain neurons use a purine nucleotide or nucleoside such as ATP or adenosine as a neurotransmitter. Likewise, early studies indicated that the human bladder is devoid of purinergic nerves mediating contraction; however, later studies demonstrated that purinergic nerve-mediated bladder contraction is increased in pathologic conditions such as interstitial cystitis. Cloning and sequencing studies have revealed four subtypes of adenosine receptors and eight subtypes of P2Y receptors, all of which are G-protein-coupled receptors. There are no reports of the cellular location of these receptors in the human bladder. P2X receptors are ligand-gated ion channels, and seven subunits have been cloned and sequenced. Immunohistochemical studies have determined that P2X(1,2,4) subunits are on detrusor-muscle cells, P2X(1-3,5) subunits are on bladder nerves and P2X(2,3,5) subunits are on bladder urothelial cells. Development of purinergic antagonist drugs with selectivity for P2X(1) receptors on detrusor muscle cells might be useful for treatment of detrusor overactivity. Antagonists with selectivity for P2X(3) receptors on bladder sensory nerves might be clinically beneficial for treatment of urinary urgency, and perhaps chronic pelvic pain.

Is sensory urgency part of the same spectrum of bladder dysfunction as detrusor overactivity?

It has been suggested that the urogynecological diagnosis of sensory urgency is an early form of detrusor overactivity and may be just earlier in the spectrum of disease. The former term is generally defined as increased perceived bladder sensation during filling, a low first desire to void and low bladder capacity in the absence of recorded urinary tract infection (UTI) or detrusor overactivity. The aims of this study are to determine the prevalence and associations of sensory urgency in comparison with detrusor overactivity, and whether sensory urgency is shown to be in the same spectrum of bladder dysfunction as detrusor overactivity. Five hundred and ninety-two women attending for an initial urogynecological/urodynamic assessment took part in this prospective study. In addition to a full clinical assessment, all women underwent free uroflowmetry, residual urine volume measurement (by vaginal ultrasound) and multichannel filling and voiding cystometry. Data were separated into those having (1) sensory urgency or (2) detrusor overactivity. Apart from prevalence figures, comparative associations were sought for (3) age; (4) parity; (5) presenting symptoms; (6) presence of at least one (medically) documented UTI in the previous 12 months; (7) two or more (recurrent) documented UTIs in the previous 12 months; (8) prior hysterectomy; (9) prior continence surgery; (10) menopause; (11) menopause and HRT use; (12) sign of clinical stress leakage; (13) retroverted uterus; (14) anterior vaginal wall prolapse; (15) uterine prolapse; (16) posterior vaginal wall prolapse; (17) apical vaginal prolapse; (18, 19) maximum, average urine flow rate (MUFR, AUFR) centiles, Liverpool Nomograms; (20) median residual urine volume (RUV) in milliliters; (21, 22) voiding difficulty: VD1,VD2 (MUFR, AUFR under 10th centile Liverpool Nomogram and/or RUV >30 ml); (23) diagnosis of urodynamic stress incontinence and (24) diagnosis of uterine and/or vaginal prolapse (grade >0). The prevalence of sensory urgency was 13%. The only differences in the clinical and urodynamic profiles of it and detrusor overactivity were (1) significantly increased prevalence of the symptom of urge incontinence and (2) (by definition) abnormal detrusor contractions during filling cystometry in women with detrusor overactivity. Overall, sensory urgency and detrusor overactivity appear to be part of the same spectrum of bladder dysfunction.

Botulinum toxin type a inhibits calcitonin gene-related peptide release from isolated rat bladder

PURPOSE

Increasing evidence suggests that sensory nerve dysfunction may underlie several urological disorders, including interstitial cystitis and sensory urgency. We determined the effect of botulinum toxin type A (Allergan, Irvine, California) on baseline and chemically evoked release of the sensory neuropeptide, calcitonin gene-related peptide in an isolated bladder preparation.

MATERIALS AND METHODS

Whole rat bladders were incubated in a series of tissue baths containing physiological salt solution. Following bladder equilibration in PSS sequential incubation was performed and this sample was used to measure baseline CGRP release. To evoke CGRP release tissue was subsequently incubated in PSS containing capsaicin (30 nM) and adenosine triphosphate (10 microM). To measure the effect of BTX-A on baseline and evoked CGRP release bladders were incubated for 6 hours in an organ bath containing BTX-A (50 microM) or vehicle prior to bladder equilibration. CGRP release was determined by radioimmunoassay.

RESULTS

Mean baseline release of CGRP +/- SEM was 346 +/- 44 pg/gm. Adenosine triphosphate/capsaicin application increased CGRP release by 75% over baseline (606 +/- 98 pg/gm, p < 0.005). BTX-A application resulted in a 19% decrease in baseline release of CGRP, although this difference did not achieve statistical significance. BTX-A application significantly decreased evoked CGRP by 62% vs control (606 +/- 98 vs 229 +/- 21 pg/gm, p < 0.005).

CONCLUSIONS

BTX-A application inhibits the evoked release of CGRP from afferent nerve terminals in isolated rat bladder. This finding suggests a potential clinical benefit of BTX-A for the treatment of interstitial cystitis or sensory urgency.

Alterations in connexin expression in the bladder of patients with urge symptoms

OBJECTIVE

To compare the formation of gap junctions between detrusor smooth muscle cells in situ and the distribution of connexin (Cx)40, Cx43 and Cx45 expressions in bladder biopsies from a control group (with bladder tumour) and from patients with urge symptoms, as smooth muscle cells of the human detrusor muscle communicate via gap junctions and express several connexin subtypes, alterations of which may be involved in the causes of lower urinary tract symptoms.

MATERIALS AND METHODS

Connexin expression is prominent in myofibroblast-like cells, supposedly involved in afferent signalling pathways of the bladder. Their strategic position directly beneath the urothelium suggests they are a link between urothelial ATP signalling during bladder filling and afferent Adelta-fibre stimulation for co-ordination of bladder tonus and initialization of the micturition reflex. Modification of their coupling characteristics may have profound impact on bladder sensation. Bladder tissue probes of patients undergoing cystectomy or transurethral tumour resection for bladder cancer were used as controls. Tissue samples from patients with severe idiopathic urge symptoms were taken for exclusion diagnostics of interstitial cystitis (IC) and carcinoma in situ. The formation of functional syncytia between detrusor smooth muscle cells were examined in dye-coupling experiments by injecting with Lucifer Yellow. The morphology and structure of gap junctions were assessed by transmission electron microscopy and immunogold labelling of Cx43 and Cx45. The expression of connexin subtypes Cx40, Cx43 and Cx45 was compared by indirect immunofluorescence, and confocal laser scanning microscopy used for semiquantitative analysis.

RESULTS

There was dye coupling between smooth muscle cells of the detrusor in situ. Electron microscopy and immunogold labelling showed very small gap junctional plaques. These findings were confirmed by confocal immunofluorescence. Semiquantitative analyses showed significantly higher Cx43 expression in the detrusor muscle, and a tendency to higher Cx45 expression in the suburothelial layer associated with urge symptoms, whereas Cx40 expression was unaffected.

CONCLUSIONS

Smooth muscle cells of the human detrusor muscle are coupled by classical gap junctions, forming limited local functional syncytia. Both Cx43 and Cx45 are expressed at low levels in normal detrusor. Up-regulation of Cx43 in patients with urge incontinence supports the possibility of functional changes in the syncytial properties of detrusor smooth muscle cells in this condition. In addition, the observed increase of Cx45 in the myofibroblast cell layer supports the idea that alterations in sensory signalling are also involved. Comparison with previous reports implies that the pathophysiology of urgency is distinct from that of the unstable bladder and other forms of incontinence.

A Role for the P2X Receptor in Urinary Tract Physiology and in the Pathophysiology of Urinary Dysfunction

OBJECTIVE

We provide a historical perspective of the P2X receptor class in bladder physiology and the pathophysiology of urinary dysfunction.

METHODS

A literature search was performed using the MEDLINE database.

RESULTS

Evidence suggests that P2X receptors serve a combined function in sensory and motor activity of human bladder. P2X receptors mediate excitation of sensory neurons and evoke muscle contraction in response to ATP release. Anatomical and functional defects in the P2X receptor signaling are associated with a variety of urologic diseases.

CONCLUSION

Current research underscores the importance of P2X receptors in urologic physiology. Potential applications exist in relation to the diagnosis and treatment of urinary dysfunction. However, the detailed mechanism of P2X receptor function in bladder physiology and in urinary tract disease remains unknown and warrants further investigation.