Changes of bladder activity and connexin 43-derived gap junctions after partial bladder-outlet obstruction in rats

Urothelium-Specific Deletion of Connexin43 in the Mouse Urinary Bladder Alters Distension-Induced ATP Release and Voiding Behavior

Connexin43 (Cx43), the main gap junction and hemichannel forming protein in the urinary bladder, participates in the regulation of bladder motor and sensory functions and has been reported as an important modulator of day-night variations in functional bladder capacity. However, because Cx43 is expressed throughout the bladder, the actual role played by the detrusor and the urothelial Cx43 is still unknown. For this purpose, we generated urothelium-specific Cx43 knockout (uCx43KO) mice using Cre-LoxP system. We evaluated the day-night micturition pattern and the urothelial Cx43 hemichannel function of the uCx43KO mice by measuring luminal ATP release after bladder distention. In wild-type (WT) mice, distention-induced ATP release was elevated, and functional bladder capacity was decreased in the animals' active phase (nighttime) when Cx43 expression was also high compared to levels measured in the sleep phase (daytime). These day-night differences in urothelial ATP release and functional bladder capacity were attenuated in uCx43KO mice that, in the active phase, displayed lower ATP release and higher functional bladder capacity than WT mice. These findings indicate that urothelial Cx43 mediated ATP signaling and coordination of urothelial activity are essential for proper perception and regulation of responses to bladder distension in the animals' awake, active phase.

Mechanosensitivity Is a Characteristic Feature of Cultured Suburothelial Interstitial Cells of the Human Bladder

Bladder dysfunction is characterized by urgency, frequency (pollakisuria, nocturia), and dysuria and may lead to urinary incontinence. Most of these symptoms can be attributed to disturbed bladder sensitivity. There is growing evidence that, besides the urothelium, suburothelial interstitial cells (suICs) are involved in bladder afferent signal processing. The massive expansion of the bladder during the filling phase implicates mechanical stress delivered to the whole bladder wall. Little is known about the reaction of suICs upon mechanical stress. Therefore, we investigated the effects of mechanical stimulation in cultured human suICs. We used fura-2 calcium imaging as a major physiological readout. We found spontaneous intracellular calcium activity in 75 % of the cultured suICs. Defined local pressure application via a glass micropipette led to local increased calcium activity in all stimulated suICs, spreading over the whole cell. A total of 51% of the neighboring cells in a radius of up to 100 µm from the stimulated cell showed an increased activity. Hypotonic ringer and shear stress also induced calcium transients. We found an 18-times increase in syncytial activity compared to unstimulated controls, resulting in an amplification of the primary calcium signal elicited in single cells by 50%. Our results speak in favor of a high sensitivity of suICs for mechanical stress and support the view of a functional syncytium between suICs, which can amplify and distribute local stimuli. Previous studies of connexin expression in the human bladder suggest that this mechanism could also be relevant in normal and pathological function of the bladder in vivo.

Influence of Circadian Disruption Associated With Artificial Light at Night on Micturition Patterns in Shift Workers

Shift workers often experience problems associated with circadian disruption associated with artificial light at night and nocturia is commonly noted in night-shift workers. Nocturia associated with circadian disruption is due to increased urine production of the kidney and decreased storage function of the bladder. A recent discovery of peripheral clock genes in the bladder and their role in contractile property of the bladder support that micturition is closely related to the circadian rhythm. Moreover, there are clinical studies showed that shift workers more often experienced nocturia due to circadian disruption. However, comparing with other health problems, concerns on nocturia and voiding dysfunction associated with circadian disruption are insufficient. Therefore, further studies about voiding dysfunction associated with the circadian disruption in shift workers are necessary.

Evaluation of a rat model of functional urinary bladder outlet obstruction produced by chronic inhibition of nitric oxide synthase

AIMS

Ligation of the urethra to create partial bladder outlet obstruction has widely been used as an animal model of bladder obstruction, although obstructive bladder dysfunction may be due to both mechanical and functional obstruction. Previous studies in rodents have demonstrated that long-term nitric oxide (NO) deficiency can lead to detrusor overactivity, and lack of NO may thus cause impairment of bladder outlet relaxation. The aim of this study was to define the characteristics of bladder and urethral dysfunction induced by chronic NO deficiency through both in vivo and in vitro investigations.

MAIN METHODS

Rats were divided into two groups, and one group received an NO synthase inhibitor (N_ω-nitro-L-arginine methyl ester hydrochloride: L-NAME) in the drinking water for 4 weeks. Bladder and urethral function were evaluated by continuous cystometry and isovolumetric cystometry. In vitro functional studies of detrusor strips and measurement of the mRNA and protein expression of an ischemic marker and a gap junction protein were also performed in separate rats.

KEY FINDINGS

L-NAME administration raised blood pressure and decreased plasma nitrite/nitrate level compared to the control group. L-NAME treatment increased the frequency of bladder contractions and the residual volume, and elevated urethral pressure and bladder contraction pressure. In addition, carbachol-induced contraction was reduced in isolated detrusor strips from the L-NAME group, and bladder expression of HIF-1 and connexin 43 showed upregulation.

SIGNIFICANCE

These findings suggest that chronic administration of L-NAME to rats induces bladder hyperactivity with residual urine, and may provide a useful model of functional bladder obstruction.

Structural and functional changes in gap junctional intercellular communication in a rat model of overactive bladder syndrome induced by partial bladder outlet obstruction

The aim of the present study was to investigate the association between connexin (Cx)43 levels and alterations in gap junctional mediation of intercellular communication in overactive bladder syndrome (OAB), and to examine the effects of connexin inhibitor on this condition. Adult female Wistar rats with OAB following partial bladder outlet obstruction (PBBO) (OAB group, n=37) and sham-operated rats (control group, n=17) were studied. The ultrastructure of the rat detrusor was observed by transmission electron microscopy and the protein expression levels of Cx43 were analyzed using western blot analysis. Furthermore, bladder detrusor cells in both groups were cultured and cells in the OAB group were randomly divided into ten groups. In nine of these groups, 18-β glycyrrhetinic acid (18β-GA) was administered at various doses and durations. All groups were compared using fluorescence redistribution after photobleaching and a laser scanning confocal microscope. Cystometry demonstrated that gap junctions were an abundant mechanism among adjacent cells, and Cx43 protein expression levels were increased in the OAB group following 6 weeks of obstruction, as compared with the control group. Mean fluorescence recovery rates in the OAB group were significantly increased, as compared with the control group (P<0.01). Mean fluorescence recovery rates were noted following 18β-GA administration. These results suggested that upregulation of Cx43 induces structural and functional alterations in gap junctional intercellular communication following PBOO, and connexin inhibitors may be a novel therapeutic strategy for the clinical treatment of OAB.

AMPK Suppresses Connexin43 Expression in the Bladder and Ameliorates Voiding Dysfunction in Cyclophosphamide-induced Mouse Cystitis

Bladder voiding dysfunction is closely related to local oxidation, inflammation, and enhanced channel activities. Given that the AMP-activated protein kinase (AMPK) has anti-oxidative, anti-inflammatory and channel-inhibiting properties, we examined whether and how AMPK affected bladder activity. AMPK activation in rat bladder smooth muscle cells (BSMCs) using three different AMPK agonists resulted in a decrease in connexin43 (Cx43) expression and function, which was associated with reduced CREB phosphorylation, Cx43 promoter activity and mRNA expression, but not Cx43 degradation. Downregulation of CREB with siRNA increased Cx43 expression. A functional analysis revealed that AMPK weakened BSMC contraction and bladder capacity. AMPK also counteracted the IL-1β- and TNFα-induced increase in Cx43 in BSMCs. In vivo administration of the AMPK agonist AICAR attenuated cyclophosphamide-initiated bladder oxidation, inflammation, Cx43 expression and voiding dysfunction. Further analysis comparing the responses of the wild-type (Cx43(+/+)) and heterozygous (Cx43(+/-)) Cx43 mice to cyclophosphamide revealed that the Cx43(+/-) mice retained a relatively normal micturition pattern compared to the Cx43(+/+) mice. Taken together, our results indicate that AMPK inhibits Cx43 in BSMCs and improves bladder activity under pathological conditions. We propose that strategies that target AMPK can be developed as novel therapeutic approaches for treating bladder dysfunction.

[Physiopathology of overactive bladder syndrome]

The pathophysiology of OAB is complex, multifactorial and still largely unknown. Several pathophysiological mechanisms have been highlighted that may play a different role in different patient groups. There are now experimental evidences that support both the myogenic and neurogenic hypothesis, but in recent years the "integrative" hypothesis has been gaining more and more acceptance, where a disruption in the multiple interactions between different cell types (neurons, urothelium, interstitial cells, myocytes) and network functions represent a central element of lower urinary tract dysfunctions. Of utmost importance, a disorder in the urothelial sensory function and in the urothelial/suburothelial non-neural cholinergic system, favored by age and comorbidities, appears to be crucial for the development of the OAB. Neuroplastic and detrusor changes in OAB are broadly similar to those observed in bladders exposed to outlet obstruction, neuropathies, inflammation or aging, and may be driven by a common urothelial dysfunction. Several signaling substances and their receptors were found to be involved in central pathways of bidirectional communication between the different cell types in the bladder, and were shown to be modified in several animal models of OAB as well as in human models, indicating new potential therapeutic targets.

Relationship between connexin43-derived gap junction proteins in the bladder and age-related detrusor underactivity in rats

AIMS

To confirm the mechanisms of age-associated detrusor underactivity (DU), we examined the differences in bladder activity and connexin-43 (Cx43)-derived gap junctions in the bladders of young and old rats.

MAIN METHODS

Female Sprague-Dawley rats aged 3months (young) and 12months (old) were used. Continuous cystometry was performed under urethane anesthesia in both ages of rats. In addition, isovolumetric cystometry was performed in young rats during the intravesical application of carbenoxolone, a gap junction blocker, to confirm the role of gap junction proteins in the bladder. Western blotting analyses were performed to assess Cx43 protein expression in the bladders of both groups of rats. Bladders were also analyzed using Masson's trichrome staining and immunostaining for Cx43.

KEY FINDINGS

Cystometric evaluations revealed that compared with young rats, bladder contractility was reduced by 27% and residual urine volume was significantly increased in old rats. However, the intercontraction intervals did not differ between the two groups. Under isovolumetric conditions, bladder contraction was suppressed after the intravesical application of carbenoxolone. In the bladders of old rats, increase of smooth muscle cell hypertrophy and fibrous tissue was observed compared with young rats. In association with these findings, immunostaining for smooth muscle Cx43 and its protein level were decreased by 28% compared with young rats.

SIGNIFICANCE

These results suggest that age-related DU might be caused by the downregulation of gap junctional intercellular communication in the bladder. Consequently, the normal signals that contribute to voiding function might not be transported between detrusor muscles.

Myogenic bladder defects in mouse models of human oculodentodigital dysplasia

To date, over 65 mutations in the gene encoding Cx43 (connexin43) have been linked to the autosomal-dominant disease ODDD (oculodentodigital dysplasia). A subset of these patients experience bladder incontinence which could be due to underlying neurogenic deterioration or aberrant myogenic regulation. BSMCs (bladder smooth muscle cells) from wild-type and two Cx43 mutant lines (Cx43(G60S) and Cx43(I130T)) that mimic ODDD exhibit a significant reduction in total Cx43. Dye transfer studies revealed that the G60S mutant was a potent dominant-negative inhibitor of co-expressed Cx43, a property not equally shared by the I130T mutant. BSMCs from both mutant mouse strains were defective in their ability to contract, which is indicative of phenotype changes due to harbouring the Cx43 mutants. Upon stretching, Cx43 levels were significantly elevated in controls and mutants containing BSMCs, but the non-muscle myosin heavy chain A levels were only reduced in cells from control mice. Although the Cx43(G60S) mutant mice showed no difference in voided urine volume or frequency, the Cx43(I130T) mice voided less frequently. Thus, similar to the diversity of morbidities seen in ODDD patients, genetically modified mice also display mutation-specific changes in bladder function. Furthermore, although mutant mice have compromised smooth muscle contraction and response to stretch, overriding bladder defects in Cx43(I130T) mice are likely to be complemented by neurogenic changes.

Gap junction expression and the effects of gap junction inhibitors in overactive bladder models: does ovariectomy have a role?

PURPOSE

In this study, gap junction expression and the effects of estrogen deficiency and gap junction inhibitors were investigated in overactive bladder models which were created by bladder outlet obstruction.

METHODS

In our study, we created four groups as control, ovariectomy, bladder outlet obstruction (BOO), and ovariectomy + BOO. We investigated the effects of oxybutynin and 18-alpha glycyrrhetinic acid (18-α-GA) which is a gap junction blocker on isolated detrusor strips. Western blot method was used to measure the level of connexin-43 in detrusor.

RESULTS

Bladder weights were significantly increased in the BOO and ovariectomy + BOO groups (p < 0.05). There was no statistically significant difference in the maximal contraction responses to carbachol between ovariectomy and control groups. In BOO and ovariectomy + BOO groups, contractile responses were significantly prominent with higher doses of carbachol. Oxybutynin-induced relaxant responses of BOO and ovariectomy + BOO groups were significantly higher than control group (p < 0.05). The relaxation effect of 18-a-GA was more effective in the obstruction groups. Among those two groups, the relaxation observed in BOO group was higher than ovariectomy + BOO group in higher doses of 18-a-GA. Connexin-43 expression was increased in BOO group compared with the control group (p = 0.006). Ovariectomy did not change connexin-43 expression alone; however, when combined with BOO, connexin-43 expression decreased significantly (p = 0.023).

CONCLUSIONS

Ovariectomy had no effect on the gap junctions in the bladder and bladder overactivity alone. Therefore, obstruction is the main factor that increases the amount of gap junctions, and gap junction blockers are thus more effective in obstruction. However, ovariectomy was shown to decrease the expression of gap junctions and relaxation effect of gap junction blockers, when combined with BOO.

Effects of mirodenafil, a phosphodiesterase-5 inhibitor, on female rat bladder in a partial bladder outlet obstruction model: physiological and immunohistochemical aspects

Purpose

We investigated the effects of mirodenafil, a phosphodiesterase-5 inhibitor developed in South Korea, on the female rat bladder in a partial bladder outlet obstruction (BOO) model.

Materials and Methods

Thirty-six female Sprague-Dawley rats were divided into four groups: the control group, BOO without medication group, BOO with mirodenafil 1 mg/kg group, and BOO with mirodenafil 4 mg/kg group. Mirodenafil was administered orally for 2 weeks after the induction of BOO. Two weeks after BOO, the rats in each group underwent cystometry under urethane anesthesia. After cystometry, the bladder was excised to perform immunohistochemical staining for connexin 43.

Results

The three BOO groups showed significant increases in mean bladder weight compared with the control group. Baseline pressure, threshold pressure, and maximum contraction pressure were not significantly different between the four groups. Although the contraction interval was decreased in all BOO groups compared with the control group, it was prolonged in the two groups treated with mirodenafil compared with the untreated BOO group. In the immunohistochemical examination, connexin 43 staining intensity in the lamina propria increased in the three BOO groups compared with the control group. The two groups treated with mirodenafil, however, showed decreased connexin 43 staining compared with the untreated BOO group.

Conclusions

Mirodenafil may increase the contraction intervals of female rat bladders in a partial BOO model. Decreasing bladder overactivity by mirodenafil may be related to intracellular communication mechanisms involving connexin 43.

Characterization of gap junction proteins in the bladder of Cx43 mutant mouse models of oculodentodigital dysplasia

Oculodentodigital dysplasia (ODDD) is a rare developmental disease resulting from germline mutations in the GJA1 gene that encodes the gap junction protein connexin43 (Cx43). In addition to the classical ODDD symptoms that affect the eyes, teeth, bone and digits, in some cases ODDD patients have reported bladder impairments. Thus, we chose to characterize the bladder in mutant mouse models of ODDD that harbor two distinct Cx43 mutations, G60S and I130T. Histological assessment revealed no difference in bladder detrusor wall thickness in mutant compared to littermate control mice. The overall localization of Cx43 in the lamina propria and detrusor also appeared to be similar in the bladders of mutant mice with the exception that the G60S mice had more instances of intracellular Cx43. However, both mutant mouse lines exhibited a significant reduction in the phosphorylated P1 and P2 isoforms of Cx43, while only the I130T mice exhibited a reduction in total Cx43 levels. Interestingly, Cx26 levels and distribution were not altered in mutant mice as it was localized to intracellular compartments and restricted to the basal cell layers of the urothelium. Our studies suggest that these two distinct genetically modified mouse models of ODDD probably mimic patients who lack bladder defects or other factors, such as aging or co-morbidities, are necessary to reveal a bladder phenotype.

β-Catenin signaling contributes to platelet derived growth factor elicited bladder smooth muscle cell contraction through up-regulation of Cx43 expression

PURPOSE

Increased gap junctions contribute to bladder overactivity but the factors and mechanisms involved in gap junction regulation in the bladder are not well established. We examined whether and how platelet derived growth factor regulates connexin43 in bladder smooth muscle cells.

MATERIALS AND METHODS

Cultured rat bladder smooth muscle cells were treated with growth factors with or without agents that interfere with phosphatidylinositol 3-kinase, mitogen activated protein kinase and β-catenin signaling pathways. Connexin43 expression was examined by Western and Northern blot, and immunochemistry. Functional gap junctions were evaluated by scrape-loading dye transfer assay. Bladder smooth muscle cell contraction was measured by collagen gel contraction.

RESULTS

1) Platelet derived growth factor induced phosphatidylinositol 3-kinase and mitogen activated protein kinase dependent accumulation of nuclear β-catenin. This was followed by increased connexin43 expression. 2) Down-regulation of β-catenin by specific siRNA abolished the connexin43 increasing effect of platelet derived growth factor while β-catenin stimulation due to glycogen synthase kinase inhibition mimicked that effect. 3) Basic fibroblast growth factor and epidermal growth factor also induced connexin43 expression. Their effects were potentiated by platelet derived growth factor. 4) Gap junction inhibition attenuated the bladder smooth muscle cell contraction induced by platelet derived growth factor. Consistently fibroblasts from connexin43 knockout (Cx43-/-) mice showed a much weaker contractile response to platelet derived growth factor than cells from connexin43-wild (Cx43+/+) litter mates.

CONCLUSIONS

Platelet derived growth factor induces connexin43 expression and bladder smooth muscle cell contraction by activating β-catenin signaling. As a convergence point for many signal pathways, β-catenin may be targeted to treat bladder overactivity.

Regulation of connexin 43 by basic fibroblast growth factor in the bladder: transcriptional and behavioral implications

PURPOSE

Basic fibroblast growth factor is a candidate causative factor of detrusor overactivity in bladder outlet obstruction cases through up-regulation of the gap junction protein connexin 43. We addressed the transcriptional and behavioral implications of this axis.

MATERIALS AND METHODS

Cx43 and Cx45 mRNA expression was assessed by real-time reverse transcriptase-polymerase chain reaction in the bladder of a rat bladder outlet obstruction model and in cultured rat bladder smooth muscle cells with and without basic fibroblast growth factor treatment. Involvement of the extracellular signal regulated kinase 1/2-activator protein-1 pathway was evaluated by immunofluorescence study and a promoter-reporter assay in bladder smooth muscle cells. The effect of basic fibroblast growth factor on micturition behavior was measured in unrestrained rats under a 12-hour light/dark cycle using a controlled release system from gelatin hydrogels fixed on the bladder. The expression of extracellular signal regulated kinase 1/2 and connexin 43 protein was assessed by Western blotting of rat bladder protein.

RESULTS

Cx43 but not Cx45 mRNA expression was increased in the bladder of the obstruction model and in bladder smooth muscle cells treated with basic fibroblast growth factor. The mitogen-activated and extracellular signal-regulated kinase kinase inhibitor PD98059 blocked the stimulatory effect of basic fibroblast growth factor on connexin 43 protein expression and promoter activity, which was also decreased by mutation or deletion of an activator protein-1 cis-element of the connexin 43 promoter. In vivo application of basic fibroblast growth factor on the bladder increased urinary frequency during the latter half of the dark phase, ie the late active phase of rats (F = 5.1, 2-way ANOVA p <0.05). The expression of phospho-extracellular signal regulated kinase 1/2 and connexin 43 protein was increased in the bladder.

CONCLUSIONS

The extracellular signal regulated kinase 1/2-activator protein-1-connexin 43 axis could be a potential therapeutic target for increased urinary frequency.