Altered Caveolar Mediated Purinergic Signaling in Spontaneously Hypertensive Rats with Detrusor Overactivity

Decreased Expression of Urethral Caveolin-1, -2, and -3 in the Rat Model of Overactive Bladder: Potential Mediator of Functional Interaction of Urethra and Urinary Bladder

Purpose: To investigate the effect of detrusor overactivity (DO) on the urethral expression of caveolin (CAV)-1, -2, and -3 of urethra in an animal model of cyclophosphamide (CYP)-induced cystitis rat.Methods: Female Sprague-Dawley rats were divided into the control group (n=20) and the cystitis group (n=20). Cystitis was induced by intraperitoneal injection of CYP (200 mg/kg). An urodynamic study was done 3 days after the CYP injection to measure functional change of the urinary bladder and urethra. Cellular localization and expression of CAV-1, -2, and -3 in the rat urethra were determined by immunohistochemistry (IHC) and Western blot.Results: Urodynamic experiments demonstrated a decreased contraction interval in the cystitis group compared to the control (3.9±1.0 minutes vs. 6.6±1.2 minutes, P<0.05). Conversely, contraction pressure increased significantly in the cystitis group compared to the control (22.4±0.7 mmHg vs. 11.5±0.4 mmHg, P<0.05). The urethral pressure was decreased in the cystitis group compared to the control (4.05 ±2.5 mmHg vs. 5.8 ±2.8 mmHg, P <0.05). The IHC and Western blot data showed that CAV-1, -2, and -3 expression decreased significantly in the cystitis group compared control group (P<0.05).Conclusions: The decreased urethral CAV-1, -2, and -3 in the DO rats suggests that CAVs might be related with the functional change of urethra in association with DO of urinay bladder.

Myosin 5a in the Urinary Bladder: Localization, Splice Variant Expression, and Functional Role in Neurotransmission

Dysregulation of neurotransmission is a feature of several prevalent lower urinary tract conditions, but the mechanisms regulating neurotransmitter release in the bladder are not completely understood. The unconventional motor protein, Myosin 5a, transports neurotransmitter-containing synaptic vesicles along actin fibers towards the varicosity membrane, tethering them at the active zone prior to reception of a nerve impulse. Our previous studies indicated that Myosin 5a is expressed and functionally relevant in the peripheral nerves of visceral organs such as the stomach and the corpora cavernosa. However, its potential role in bladder neurotransmission has not previously been investigated. The expression of Myosin 5a was examined by quantitative PCR and restriction analyses in bladders from DBA (dilute-brown-nonagouti) mice which express a Myosin 5a splicing defect and in control mice expressing the wild-type Myosin 5a allele. Functional differences in contractile responses to intramural nerve stimulation were examined by ex vivo isometric tension analysis. Data demonstrated Myosin 5a localized in cholinergic nerve fibers in the bladder and identified several Myosin 5a splice variants in the detrusor. Full-length Myosin 5a transcripts were less abundant and the expression of splice variants was altered in DBA bladders compared to control bladders. Moreover, attenuation of neurally-mediated contractile responses in DBA bladders compared to control bladders indicates that Myosin 5a facilitates excitatory neurotransmission in the bladder. Therefore, the array of Myosin 5a splice variants expressed, and the abundance of each, may be critical parameters for efficient synaptic vesicle transport and neurotransmission in the urinary bladder.

Caveolin-1/Endothelial Nitric Oxide Synthase Interaction Is Reduced in Arteries From Pregnant Spontaneously Hypertensive Rats

We have investigated the role caveolae/caveolin-1 (Cav-1) plays in endothelial nitric oxide synthase (eNOS) activation and how it impacts pregnancy-induced decreased vascular reactivity in normotensive (Wistar rats) and spontaneously hypertensive rats (SHR). Wistar rats and SHR were divided into non-pregnant (NP) and pregnant (P). Nitrite levels were assessed by the Griess method in the aorta and mesenteric vascular bed. In functional studies, arteries were incubated with methyl-β-cyclodextrin (dextrin, 10mmol/L), which disrupts caveolae by depleting cholesterol, and concentration-response curves to phenylephrine (PE) and acetylcholine (ACh) were constructed. Electronic microscopy was used to determine endothelial caveolae density in the aorta and resistance mesenteric artery in the presence of vehicle or dextrin (10mmol/L). Western blot was performed to evaluate Cav-1, p-Cav-1, calmodulin (CaM), and heat shock protein 90 (Hsp90) expression. Cav-1/eNOS interaction in the aorta and mesenteric vascular bed was assessed by co-immunoprecipitation. Nitric oxide (NO) generation was greater in arteries from P groups compared to NP groups. Dextrin did not change vascular responses in the aorta from P groups or the number of caveolae in P groups compared to NP groups. Compared to NP Wistar rats, NP SHR showed smaller number of caveolae and reduced Cav-1 expression. Pregnancy did not alter Cav-1, CaM, or Hsp90 expression in the aorta or mesenteric vascular bed from Wistar rats or SHR. These results suggest that pregnancy does not alter expression of the main eNOS regulatory proteins, but it decreases Cav-1/eNOS interaction. Reduced Cav-1/eNOS interaction in the aorta and mesenteric vascular bed seems to be an important mechanism to increase eNOS activity and nitric oxide production in pregnant normotensive and hypertensive rats.

Effects of caveolae depletion and urothelial denudation on purinergic and cholinergic signaling in healthy and cyclophosphamide-induced cystitis in the rat bladder

Cholesterol rich membrane invaginations, caveolae, have important roles in various cellular activities, one of them being signal transduction. This signaling pathway seems to be affected during various bladder disorders and the current study aimed to elucidate the plausible involvement of caveolae mediated signal transduction during cyclophosphamide induced cystitis. Furthermore, the urothelial cholinergic part of ATP-evoked contractions and its possible link to caveolae were investigated. Cholinergic, as well as purinergic, contractile responses in rat urinary bladders were examined using a classic organ bath set-up with full-thickness strip preparations or a whole bladder model that enabled luminal administration of substances. Furthermore, sub groups with and without urothelium were examined. The expression of caveolin-1 was also tested using western blot and immunofluorescence. Caveolae cholesterol depletion by methyl-β-cyclodextrin entailed a significant decrease of ATP-evoked bladder contractility. Interestingly, after muscarinic blockade the ATP induced contractions were significantly reduced in the same manner. Furthermore, this atropine-sensitive part of ATP-evoked responses was absent in denuded as well as inflamed bladders. A tendency towards a reduced expression of caveolin-1 was observed in rats with experimental cystitis. The cholinergic part of ATP-induced contractile responses seemed to be affected by urothelium denudation as well as caveolae depletion. Removing one of these structures nullifies the effect of the other, suggesting an important interaction between the urothelium and the caveolar structures. These effects are absent in inflamed animals and might be one pathophysiological aspect behind BPS/IC.

Highly specific detection of muscarinic M3 receptor, G protein interaction and intracellular trafficking in human detrusor using Proximity Ligation Assay (PLA)

PURPOSE

Muscarinic acetylcholine receptors (mAChRs) regulate a number of important physiological functions. Alteration of mAChR expression or function has been associated in the etiology of several pathologies including functional bladder disorders (e.g bladder pain syndrome/interstitial cystitis - BPS/IC). In a previous study we found specific mAChR expression patterns associated with BPS/IC, while correlation between protein and gene expression was lacking. Posttranslational regulatory mechanisms, e.g. altered intracellular receptor trafficking, could explain those differences. In addition, alternative G protein (GP) coupling could add to the pathophysiology via modulation of muscarinic signaling. In our proof-of-principle study, we addressed these questions in situ. We established PLA in combination with confocal laserscanning microscopy (CLSM) and 3D object reconstruction for highly specific detection and analysis of muscarinic 3 receptors (M3), G protein (GP) coupling and intracellular trafficking in human detrusor samples.

MATERIAL AND METHODS

Paraffin sections of formalin-fixed bladder tissue (FFPE) of BPS/IC patients receiving transurethral biopsy were examined by Cy3-PLA for M3 expression, coupling of M3 to GPs (G_αq/11, G_αs, G_αi) and interaction of M3 with endocytic regulator proteins. Membranes were labeled with wheat germ agglutinin-Alexa Fluor^®488, nuclei were stained with DAPI. Object density and co-localization were analyzed in 3D-reconstruction of high resolution confocal z-stacks.

RESULTS

Confocal image stack processing resulted in well demarcated objects. Calculated receptor densities correlated significantly with existing confocal expression data, while significantly improved specificity of M3 detection by PLA was verified using bladder tissue samples from transgenic mice. 50-60% of the M3 receptor complexes were plasma membrane associated in human bladder detrusor. Application of PLA for M3 and GPs allowed visualization of M3-GP interactions and revealed individual GP-subtype coupling patterns. Detection of M3 interactions with endocytic trafficking proteins by PLA resulted in object sizes correlating with well-documented vesicle sizes of the endocytosis pathway.

CONCLUSION

PLA enabled highly specific detection of M3 receptor expression, demonstration of M3/GP differential coupling and intracellular M3 trafficking in human detrusor smooth muscle cells. This new approach minimized background fluorescence and antibody cross-reactions resulting from single antibody application, and enhanced specificity due to the use of two primary antibodies. Use of subcellular markers allowed visualization of subcellular receptor location. PLA/CLSM allows analyses of muscarinic "receptor - G protein - promiscuity" and intracellular trafficking even in bladder paraffin sections and may give new insights into the etiology and pathology of BPS/IC.

Microvascular Injury in Ketamine-Induced Bladder Dysfunction

The pathogenesis of ketamine-induced cystitis (KC) remains unclear. In this study, bladder microvascular injury was investigated as a possible contributing mechanism. A total of 36 KC patients with exposure to ketamine for more than 6 months, and 9 control subjects, were prospectively recruited. All participants completed questionnaires, including the O'Leary-Sant interstitial cystitis symptom index (ICSI) and the interstitial cystitis problem index (ICPI). All KC patients received a urodynamic study and radiological exams. Bladder tissues were obtained from cystoscopic biopsies in the control group and after hydrodistention in the KC group. Double-immunofluorescence staining of N-methyl-d-aspartate receptor subunit 1 (NMDAR1) and the endothelial marker, cluster of differentiation 31 (CD31), was performed to reveal the existence of NMDAR1 on the endothelium. Electron microscopy (EM) was applied to assess the microvascular change in the urinary bladder and to measure the thickening of the basement membrane (BM). A proximity ligation assay (PLA) was used to quantify the co-localization of the endothelial CD31 receptor and the mesenchymal marker [fibroblast-specific protein 1 (FSP-1)]. The Mann-Whitney U test and Spearman's correlation coefficient were used for statistical analysis. The mean ICSI [14.38 (± 4.16)] and ICPI [12.67 (± 3.54)] scores of the KC group were significantly higher than those (0 and 0, respectively) of the control group (both p < 0.001). The KC patients had decreasing cystometric bladder capacity (CBC) with a mean volume of 65.38 (± 48.67) mL. NMDAR1 was expressed on endothelial cells in both groups under immunofluorescence staining. Moreover, KC patients had significant BM duplication of microvessels in the mucosa of the urinary bladder under EM. The co-expression of the endothelial marker CD31 and mesenchymal marker FSP1 was significantly stained and calculated under PLA. In conclusion, microvascular injury and mesenchymal phenotypic alteration of endothelial cells can potentially contribute to KC-induced bladder dysfunction.

Interaction of Caveolin-3 and HCN is involved in the pathogenesis of diabetic cystopathy

A growing body of research suggests that impaired bladder Cajal-like interstitial cells (ICCs) are a important component in the pathogenesis of diabetes-induced bladder dysfunction, although the molecular mechanisms have not been illustrated completely. The purpose of this study was to examine whether the hyperpolarization-activated cyclic nucleotide-gated (HCN) channels in ICCs-DM were responsible for the detrusor weak contractility of Diabetic cystopathy (DCP) and to study the possible mechanism of regulating the expression and function of HCN channels. HCN channels expression were decreased at the mRNA and protein levels. Forskolin (FSK), which can elevate intracellular cAMP levels, increased the density of the hyperpolarization-activated current and intracellular calcium concentration in both normal control (NC) rats and DCP rats, but the sensitivity of FSK on HCN channels was clearly down-regulated in DCP rats. The loss of caveolae and caveolin was in accordance with the decrease in HCN channels. Caveolin-3 co-localizes with and affects the expression and function of HCN. Taken together, these results indicate that the loss of caveolae and HCN channels in ICCs-DM is important in the pathogenesis of DCP. Increasing the number of caveolae to enhance the function of HCN channels may represent a viable target for the pharmacological treatment of DCP.

Underlying mechanisms of urine storage dysfunction in rats with salt-loading hypertension

AIMS

Spontaneous hypertensive rats provide a genetic model for exploring the pathogenesis of urine storage dysfunction related to hypertension (HT). In humans, however, HT develops by both genetic and environmental factors including lifestyle factors such as a high-calorie diet, excessive salt intake and stress. We investigated the influence of salt-loading on bladder function and the underlying mechanisms of storage dysfunction related to HT.

MAIN METHODS

Six-week-old male Dahl salt-sensitive (DS) and Dahl salt-resistant (DR) rats were fed with a normal or high-salt diet for 12weeks. Micturition parameters were obtained from a metabolic cage. Whole bladders were excised from 18-week-old rats and distended in an organ bath. The releases of adenosine triphosphoric acid (ATP) and prostaglandin E2 (PGE2) from the distended bladder epithelia were measured. Changes in bladder blood flow (BBF) were determined with a laser-speckle-blood-flow imaging system.

KEY FINDINGS

An increase in mean blood pressure (BP) was noted only in DS rats after salt-loading. During the inactive (sleeping) period, voided volume per micturition gradually increased in DR rats fed a normal or high-salt diet and normal-diet DS rats, while it did not change in the DS rats fed a high-salt diet. Bladder distension significantly increased ATP and PGE2 release from the urothelium in DS rats fed a high-salt diet. BBF was significantly decreased in high-salt-diet DS rats.

SIGNIFICANCE

One mechanism behind the relationship between salt-sensitive HT and urine storage dysfunction may be an increase in ATP and PGE2 release from the urothelium via suppression of BBF.

CaMKIIα and caveolin-1 cooperate to drive ATP-induced membrane delivery of the P2X3 receptor

The P2X3 receptor plays a vital role in sensory processing and transmission. The assembly and trafficking of the P2X3 receptor are important for its function in primary sensory neurons. As an important inflammation mediator, ATP is released from different cell types around primary sensory neurons, especially under pathological pain conditions. Here, we show that α, β-MeATP dramatically promoted membrane delivery of the P2X3 receptor both in HEK293T cells expressing recombinant P2X3 receptor and in rat primary sensory neurons. α, β-MeATP induced P2X3 receptor-mediated Ca²⁺ influx, which further activated Ca²⁺/calmodulin-dependent protein kinase IIα (CaMKIIα). The N terminus of the P2X3 receptor was responsible for CaMKIIα binding, whereas Thr³⁸⁸ in the C terminus was phosphorylated by CaMKIIα. Thr³⁸⁸ phosphorylation increased P2X3 receptor binding to caveolin-1. Caveolin-1 knockdown abrogated the α, β-MeATP-induced membrane insertion of the P2X3 receptor. Moreover, α, β-MeATP drove the CaMKIIα-mediated membrane coinsertion of the P2X2 receptor with the P2X3 receptor. The increased P2X3 receptors on the cell membrane that are due to Thr³⁸⁸ phosphorylation facilitated P2X3 receptor-mediated signal transduction. Together, our data indicate that CaMKIIα and caveolin-1 cooperate to drive ligand-induced membrane delivery of the P2X3 receptor and may provide a mechanism of P2X3 receptor sensitization in pain development.

Purinergic signalling in the urinary tract in health and disease

Purinergic signalling is involved in a number of physiological and pathophysiological activities in the lower urinary tract. In the bladder of laboratory animals there is parasympathetic excitatory cotransmission with the purinergic and cholinergic components being approximately equal, acting via P2X1 and muscarinic receptors, respectively. Purinergic mechanosensory transduction occurs where ATP, released from urothelial cells during distension of bladder and ureter, acts on P2X3 and P2X2/3 receptors on suburothelial sensory nerves to initiate the voiding reflex, via low threshold fibres, and nociception, via high threshold fibres. In human bladder the purinergic component of parasympathetic cotransmission is less than 3 %, but in pathological conditions, such as interstitial cystitis, obstructed and neuropathic bladder, the purinergic component is increased to 40 %. Other pathological conditions of the bladder have been shown to involve purinoceptor-mediated activities, including multiple sclerosis, ischaemia, diabetes, cancer and bacterial infections. In the ureter, P2X7 receptors have been implicated in inflammation and fibrosis. Purinergic therapeutic strategies are being explored that hopefully will be developed and bring benefit and relief to many patients with urinary tract disorders.