The Mystery of the Interstitial Cells in the Urinary Bladder

Cells and ionic conductances contributing to spontaneous activity in bladder and urethral smooth muscle

Smooth muscle organs of the lower urinary tract comprise the bladder detrusor and urethral wall, which have a reciprocal contractile relationship during urine storage and micturition. As the bladder fills with urine, detrusor smooth muscle cells (DSMCs) remain relaxed to accommodate increases in intravesical pressure while urethral smooth muscle cells (USMCs) sustain tone to occlude the urethral orifice, preventing leakage. While neither organ displays coordinated regular contractions as occurs in small intestine, lymphatics or renal pelvis, they do exhibit patterns of rhythmicity at cellular and tissue levels. In rabbit and guinea-pig urethra, electrical slow waves are recorded from USMCs. This activity is linked to cells expressing vimentin, c-kit and Ca2+-activated Cl- channels, like interstitial cells of Cajal in the gastrointestinal tract. In mouse, USMCs are rhythmically active (firing propagating Ca2+ waves linked to contraction), and this cellular rhythmicity is asynchronous across tissues and summates to form tone. Experiments in mice have failed to demonstrate a voltage-dependent mechanism for regulating this rhythmicity or contractions in vitro, suggesting that urethral tone results from an intrinsic ability of USMCs to 'pace' their own Ca2+ mobilization pathways required for contraction. DSMCs exhibit spontaneous transient contractions, increases in intracellular Ca2+ and action potentials. Consistent across numerous species, including humans, this activity relies on voltage-dependent Ca2+ influx in DSMCs. While interstitial cells are present in the bladder, they do not 'pace' the organ in an excitatory manner. Instead, specialized cells (PDGFRα+ interstitial cells) may 'negatively pace' DSMCs to prevent bladder overexcitability.

Computational modeling of inhibitory signal transduction in urinary bladder PDGFRα+ cells

A crucial aspect of bladder function is the maintenance of a normo-active detrusor during bladder filling. The physiological mechanisms and pathways underlying this function are yet to be fully elucidated. Premature detrusor contractions are a key phenotype in detrusor overactivity, a common pathophysiological condition of the urinary bladder. Recent literature has identified PDFGRα+ cells as mediators in transducing inhibitory signals to detrusor smooth muscle cells via gap junctions. We employ computational modeling to study transduction pathways via which inhibitory signals are generated in PDFGRα+ cells in response to purinergic, nitrergic and mechanical stimuli. The key focus of our study here is to explore the effect of ATP, stretch and NO on the membrane potential of PDFGRα+ cells, which is driven to hyperpolarized potentials via the activation of SK3 channels. Our results indicate that purinergic, mechanical and nitrergic inputs can induce significant membrane hyperpolarizations of 20-35 mV relative to the resting membrane potential. Given the interconnections between PDFGRα+ cells and detrusor SMCs through gap junctions, these hyperpolarizations can have significant functional implications in the maintenance of a normo-active detrusor as also in departures from this state as seen in detrusor overactivity.

Defining the molecular fingerprint of bladder and kidney fibroblasts

Fibroblasts are integral to the organization and function of all organs and play critical roles in pathologies such as fibrosis; however, we have limited understanding of the fibroblasts that populate the bladder and kidney. In this review, I describe how transcriptomics is leading to a revolution in our understanding of fibroblast biology by defining the molecular fingerprint (i.e., transcriptome) of universal and specialized fibroblast types, revealing gene signatures that allows one to resolve fibroblasts from other mesenchymal cell types, and providing a new comprehension of the fibroblast lineage. In the kidney, transcriptomics is giving us new insights into the molecular fingerprint of kidney fibroblasts, including those for cortical fibroblasts, medullary fibroblasts, and erythropoietin (EPO)-producing Norn fibroblasts, as well as new information about the gene signatures of kidney myofibroblasts and the transition of kidney fibroblasts into myofibroblasts. Transcriptomics has also revealed that the major cell type in the bladder interstitium is the fibroblast, and that multiple fibroblast types, each with their own molecular fingerprint, are found in the bladder wall. Interleaved throughout is a discussion of how transcriptomics can drive our future understanding of fibroblast identification, diversity, function, and their roles in bladder and kidney biology and physiology in health and in disease states.

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.

Activation of TRPA1 in Bladder Suburothelial Myofibroblasts Counteracts TGF-β1-Induced Fibrotic Changes

The activation of the transient receptor potential ankyrin 1 (TRPA1) channel has anti-fibrotic effects in the lung and intestine. Suburothelial myofibroblasts (subu-MyoFBs), a specialized subset of fibroblasts in the bladder, are known to express TRPA1. However, the role of the TRPA1 in the development of bladder fibrosis remains elusive. In this study, we use the transforming growth factor-β1 (TGF-β1) to induce fibrotic changes in subu-MyoFBs and assess the consequences of TRPA1 activation utilizing RT-qPCR, western blotting, and immunocytochemistry. TGF-β1 stimulation increased α-SMA, collagen type I alpha 1 chain(col1A1), collagen type III (col III), and fibronectin expression, while simultaneously suppressing TRPA1 in cultured human subu-MyoFBs. The activation of TRPA1, with its specific agonist allylisothiocyanate (AITC), inhibited TGF-β1-induced fibrotic changes, and part of these inhibition effects could be reversed by the TRPA1 antagonist, HC030031, or by reducing TRPA1 expression via RNA interference. Furthermore, AITC reduced spinal cord injury-induced fibrotic bladder changes in a rat model. The increased expression of TGF-β1, α-SMA, col1A1 and col III, and fibronectin, and the downregulation of TRPA1, were also detected in the mucosa of fibrotic human bladders. These findings suggest that TRPA1 plays a pivotal role in bladder fibrosis, and the negative cross talk between TRPA1 and TGF-β1 signaling may represent one of the mechanisms underlying fibrotic bladder lesions.

Computational models of autonomic regulation in gastric motility: Progress, challenges, and future directions

The stomach is extensively innervated by the vagus nerve and the enteric nervous system. The mechanisms through which this innervation affects gastric motility are being unraveled, motivating the first concerted steps towards the incorporation autonomic regulation into computational models of gastric motility. Computational modeling has been valuable in advancing clinical treatment of other organs, such as the heart. However, to date, computational models of gastric motility have made simplifying assumptions about the link between gastric electrophysiology and motility. Advances in experimental neuroscience mean that these assumptions can be reviewed, and detailed models of autonomic regulation can be incorporated into computational models. This review covers these advances, as well as a vision for the utility of computational models of gastric motility. Diseases of the nervous system, such as Parkinson’s disease, can originate from the brain-gut axis and result in pathological gastric motility. Computational models are a valuable tool for understanding the mechanisms of disease and how treatment may affect gastric motility. This review also covers recent advances in experimental neuroscience that are fundamental to the development of physiology-driven computational models. A vision for the future of computational modeling of gastric motility is proposed and modeling approaches employed for existing mathematical models of autonomic regulation of other gastrointestinal organs and other organ systems are discussed.

Studies of ultrastructure, gene expression, and marker analysis reveal that mouse bladder PDGFRA+ interstitial cells are fibroblasts

Fibroblasts are crucial to normal and abnormal organ and tissue biology, yet we lack basic insights into the fibroblasts that populate the bladder wall. Candidates may include bladder interstitial cells (also referred to as myofibroblasts, telocytes, and interstitial cells of Cajal-like cells), which express the fibroblast-associated marker PDGFRA along with VIM and CD34 but whose form and function remain enigmatic. By applying the latest insights in fibroblast transcriptomics, coupled with studies of gene expression, ultrastructure, and marker analysis, we observe the following: 1) that mouse bladder PDGFRA+ cells exhibit all of the ultrastructural hallmarks of fibroblasts including spindle shape, lack of basement membrane, abundant endoplasmic reticulum and Golgi, and formation of homotypic cell-cell contacts (but not heterotypic ones); 2) that they express multiple canonical fibroblast markers (including Col1a2, CD34, LY6A, and PDGFRA) along with the universal fibroblast genes Col15a1 and Pi16 but they do not express Kit; and 3) that PDGFRA+ fibroblasts include suburothelial ones (which express ACTA2, CAR3, LY6A, MYH10, TNC, VIM, Col1a2, and Col15a1), outer lamina propria ones (which express CD34, LY6A, PI16, VIM, Col1a2, Col15a1, and Pi16), intermuscular ones (which express CD34, VIM, Col1a2, Col15a1, and Pi16), and serosal ones (which express CD34, PI16, VIM, Col1a2, Col15a1, and Pi16). Collectively, our study revealed that the ultrastructure of PDFRA+ interstitial cells combined with their expression of multiple canonical and universal fibroblast-associated gene products indicates that they are fibroblasts. We further propose that there are four regionally distinct populations of fibroblasts in the bladder wall, which likely contribute to bladder function and dysfunction.NEW & NOTEWORTHY We currently lack basic insights into the fibroblasts that populate the bladder wall. By exploring the ultrastructure of mouse bladder connective tissue cells, combined with analyses of their gene and protein expression, our study revealed that PDGRA+ interstitial cells (also referred to as myofibroblasts, telocytes, and interstitial cells of Cajal-like cells) are fibroblasts and that the bladder wall contains multiple, regionally distinct populations of these cells.

Transient receptor potential vanilloid type 4 (TRPV4) in urinary bladder structure and function

Bladder pain syndrome (BPS)/interstitial cystitis (IC) is a urologic, chronic pelvic pain syndrome characterized by pelvic pain, pressure, or discomfort with urinary symptoms. Symptom exacerbation (flare) is common with multiple, perceived triggers including stress. Multiple transient receptor potential (TRP) channels (TRPA1, TRPV1, TRPV4) expressed in the bladder have specific tissue distributions in the lower urinary tract (LUT) and are implicated in bladder disorders including overactive bladder (OAB) and BPS/IC. TRPV4 channels are strong candidates for mechanosensors in the urinary bladder and TRPV4 antagonists are promising therapeutic agents for OAB. In this perspective piece, we address the current knowledge of TRPV4 distribution and function in the LUT and its plasticity with injury or disease with an emphasis on BPS/IC. We review our studies that extend the knowledge of TRPV4 in urinary bladder function by focusing on (i) TRPV4 involvement in voiding dysfunction, pelvic pain, and non-voiding bladder contractions in NGF-OE mice; (ii) distention-induced luminal ATP release mechanisms and (iii) involvement of TRPV4 and vesicular release mechanisms. Finally, we review our lamina propria studies in postnatal rat studies that demonstrate: (i) the predominance of the TRPV4+ and PDGFRα+ lamina propria cellular network in early postnatal rats; (ii) the ability of exogenous mediators (i.e., ATP, TRPV4 agonist) to activate and increase the number of lamina propria cells exhibiting active Ca2+ events; and (iii) the ability of ATP and TRPV4 agonist to increase the rate of integrated Ca2+ activity corresponding to coupled lamina propria network events and the formation of propagating wavefronts.

Three-dimensional analysis of interstitial cells in the lamina propria of the murine vas deferens by confocal laser scanning microscopy and FIB/SEM

The present study aimed to explore the three-dimensional (3D) ultrastructure of interstitial cells (ICs) within the lamina propria of the murine vas deferens and the spatial relationships between epithelial cells and surrounding cells. Focused ion beam scanning electron microscopy and confocal laser scanning microscopy were performed. ICs within the lamina propria had a flat, sheet-like structure of cytoplasm with multiple cellular processes. In addition, two types of 3D structures that comprised cell processes of flat, sheet-like ICs were observed: one was an accordion fold-like structure and the other was a rod-shaped structure. ICs were located parallel to the epithelium and were connected to each other via gap junctions or adherens junctions. Moreover, multiple sphere-shaped extracellular vesicle-like structures were frequently observed around the ICs. The ICs formed a complex 3D network comprising sheet-like cytoplasm and multiple cell processes with different 3D structures. From this morphological study, we noted that ICs within the lamina propria of murine vas deferens may be involved in signal transmission between the epithelium and smooth muscle cells by physical interaction and by exchanging extracellular vesicles.

Imatinib Mesylate Reduces Voiding Frequency in Female Mice With Acute Cyclophosphamide-Induced Cystitis

Lamina propria interstitial cells that express the tyrosine kinase receptor, platelet-derived growth factor receptor alpha (PDGFRα) may play a role in urinary sensory signaling. Imatinib mesylate, also referred to as imatinib, is a tyrosine kinase inhibitor that can inhibit PDGFRα and has been widely used in urological research. We evaluated the functional effects of imatinib administration (via oral gavage or intravesical infusion) with two different experimental designs (prevention and treatment), in a cyclophosphamide (CYP)-induced cystitis (acute, intermediate, and chronic), male and female rodent model using conscious cystometry and somatic sensitivity testing. Imatinib significantly (0.0001 ≤ p ≤ 0.05) decreased voiding frequency and increased bladder capacity in acute CYP-induced cystitis, by the prevention (females) and treatment (females and males) designs. Imatinib was not effective in preventing or treating intermediate or chronic CYP-induced cystitis in either sex. Interestingly, in the prevention experiments, imatinib administration increased (0.0001 ≤ p ≤ 0.01) voiding frequency and decreased bladder capacity in control mice. However, in the treatment experiments, imatinib administration decreased (0.01 ≤ p ≤ 0.05) voiding frequency and increased bladder capacity in control mice. Bladder function improvements observed with imatinib treatment in acute CYP-induced cystitis mice remained and additionally improved with a second dose of imatinib 24 hours after CYP treatment. Imatinib administration did not affect pelvic somatic sensitivity in female mice with acute CYP-induced cystitis. Our studies suggest that (1) imatinib improves bladder function in mice with acute CYP-induced cystitis with a prevention and treatment design and (2) interstitial cells may be a useful target to improve bladder function in cystitis.

Imatinib Mesylate Reduces Neurotrophic Factors and pERK and pAKT Expression in Urinary Bladder of Female Mice With Cyclophosphamide-Induced Cystitis

Imatinib mesylate is a tyrosine kinase inhibitor that inhibits platelet-derived growth factor receptor (PDGFR)-α, -β, stem cell factor receptor (c-KIT), and BCR-ABL. PDGFRα is expressed in a subset of interstitial cells in the lamina propria (LP) and detrusor muscle of the urinary bladder. PDGFRα + interstitial cells may contribute to bladder dysfunction conditions such as interstitial cystitis/bladder pain syndrome (IC/BPS) or overactive bladder (OAB). We have previously demonstrated that imatinib prevention via oral gavage or treatment via intravesical infusion improves urinary bladder function in mice with acute (4 hour, h) cyclophosphamide (CYP)-induced cystitis. Here, we investigate potential underlying mechanisms mediating the bladder functional improvement by imatinib using a prevention or treatment experimental design. Using qRT-PCR and ELISAs, we examined inflammatory mediators (NGF, VEGF, BDNF, CCL2, IL-6) previously shown to affect bladder function in CYP-induced cystitis. We also examined the distribution of phosphorylated (p) ERK and pAKT expression in the LP with immunohistochemistry. Imatinib prevention significantly (0.0001 ≤ p ≤ 0.05) reduced expression for all mediators examined except NGF, whereas imatinib treatment was without effect. Imatinib prevention and treatment significantly (0.0001 ≤ p ≤ 0.05) reduced pERK and pAKT expression in the upper LP (U. LP) and deeper LP (D. LP) in female mice with 4 h CYP-induced cystitis. Although we have previously demonstrated that imatinib prevention or treatment improves bladder function in mice with cystitis, the current studies suggest that reductions in inflammatory mediators contribute to prevention benefits of imatinib but not the treatment benefits of imatinib. Differential effects of imatinib prevention or treatment on inflammatory mediators may be influenced by the route and frequency of imatinib administration and may also suggest other mechanisms (e.g., changes in transepithelial resistance of the urothelium) through which imatinib may affect urinary bladder function following CYP-induced cystitis.

Understanding Molecular Mechanisms and Identifying Key Processes in Chronic Radiation Cystitis

Chronic radiation cystitis (CRC) is a consequence of pelvic radiotherapy and affects 5–10% of patients. The pathology of CRC is without curative treatment and is characterized by incontinence, pelvic pain and hematuria, which severely degrades patients’ quality of life. Current management strategies rely primarily on symptomatic measures and have certain limitations. Thanks to a better understanding of the pathophysiology of radiation cystitis, studies targeting key manifestations such as inflammation, neovascularization and cell atrophy have emerged and are promising avenues for future treatment. However, the mechanisms of CRC are still better described in animal models than in human models. Preclinical studies conducted to elucidate the pathophysiology of CRC use distinct models and are most often limited to specific processes, such as fibrosis, vascular damage and inflammation. This review presents a synthesis of experimental studies aimed at improving our understanding of the molecular mechanisms at play and identifying key processes in CRC.

Three-Dimensional Analysis of Interstitial Cells in the Smooth Muscle Layer of Murine Vas Deferens Using Confocal Laser Scanning Microscopy and FIB/SEM

The smooth muscle contraction of the vas deferens has the important function of transporting sperm. Interstitial cells (ICs) play a critical role in the pacing and modulation of various smooth muscle organs by interactions with nerves and smooth muscle. Elucidating the three-dimensional (3D) architecture of ICs is important for understanding their spatial relationship on the mesoscale between ICs, smooth muscle cells (SMCs), and nerves. In this study, the 3D ultrastructure of ICs in the smooth muscle layer of murine vas deferens and the spatial relationships between ICs, nerves, and smooth muscles were observed using confocal laser scanning microscopy and focused ion beam/scanning electron microscopy. ICs have sheet-like structures as demonstrated by 3D observation using modern analytical techniques. Sheet-like ICs have two types of 3D structures, one flattened and the other curled. Multiple extracellular vesicle (EV)-like structures were frequently observed in ICs. Various spatial relations were observed in areas between ICs, nerves, and SMCs, which formed a complex 3D network with each other. These results suggest that ICs in the smooth muscle layer of murine vas deferens may have two subtypes with different sheet-like structures and may be involved in neuromuscular signal transmission via physical interaction and EVs.

Functional Expression of Transient Receptor Potential and Piezo1 Channels in Cultured Interstitial Cells of Human-Bladder Lamina Propria

The interstitial cells in bladder lamina propria (LP-ICs) are believed to be involved in sensing/afferent signaling in bladder mucosa. Transient receptor potential (TRP) cation channels act as mechano- or chemo-sensors and may underlie some of the sensing function of bladder LP-ICs. We aimed to investigate the molecular and functional expression of TRP channels implicated in bladder sensory function and Piezo1/Piezo2 channels in cultured LP-ICs of the human bladder. Bladder tissues were obtained from patients undergoing cystectomy. LP-ICs were isolated and cultured, and used for real-time reverse transcription-quantitative polymerase chain reaction, immunocytochemistry, and calcium-imaging experiments. At the mRNA level, TRPA1, TRPV2, and Piezo1 were expressed most abundantly. Immunocytochemical staining showed protein expression of TRPA1, TRPV1, TRPV2, TRPV4, TRPM8, as well as Piezo1 and Piezo2. Calcium imaging using channel agonists/antagonists provided evidence for functional expression of TRPA1, TRPV2, TRPV4, Piezo1, but not of TRPV1 or TRPM8. Activation of these channels with their agonist resulted in release of adenosine triphosphate (ATP) from LP-ICs. Inhibition of TRPV2, TRPV4 and Piezo1 blocked the stretch induced intracellular Ca²⁺ increase. Whereas inhibition of TRPA1 blocked H₂O₂ evoked response in LP-ICs. Our results suggest LP-ICs of the bladder can perceive stretch or chemical stimuli via activation of TRPV2, TRPV4, Piezo1 and TRPA1 channels. LP-ICs may work together with urothelial cells for perception and transduction of mechanical or chemical signals in human-bladder mucosa.

Understanding of mouse and human bladder at single-cell resolution: integrated analysis of trajectory and cell-cell interactive networks based on multiple scRNA-seq datasets

Objectives

To elaborately decipher the mouse and human bladders at single‐cell levels.

Materials and Methods

We collected more than 50,000 cells from multiple datasets and created, up to date, the largest integrated bladder datasets. Pseudotime trajectory of urothelium and interstitial cells, as well as dynamic cell‐cell interactions, was investigated. Biological activity scores and different roles of signaling pathways between certain cell clusters were also identified.

Results

The glucose score was significantly high in most urothelial cells, while the score of H3 acetylation was roughly equally distributed across all cell types. Several genes via a pseudotime pattern in mouse (Car3, Dkk2, Tnc, etc.) and human (FBLN1, S100A10, etc.) were discovered. S100A6, TMSB4X, and typical uroplakin genes seemed as shared pseudotime genes for urothelial cells in both human and mouse datasets. In combinational mouse (n = 16,688) and human (n = 22,080) bladders, we verified 1,330 and 1,449 interactive ligand‐receptor pairs, respectively. The distinct incoming and outgoing signaling was significantly associated with specific cell types. Collagen was the strongest signal from fibroblasts to urothelial basal cells in mouse, while laminin pathway for urothelial basal cells to smooth muscle cells (SMCs) in human. Fibronectin 1 pathway was intensely sent by myofibroblasts, received by urothelial cells, and almost exclusively mediated by SMCs in mouse bladder. Interestingly, the cell cluster of SMCs 2 was the dominant sender and mediator for Notch signaling in the human bladder, while SMCs 1 was not. The expression of integrin superfamily (the most common communicative pairs) was depicted, and their co‐expression patterns were located in certain cell types (eg, Itgb1 and Itgb4 in mouse and human basal cells).

Conclusions

This study provides a complete interpretation of the normal bladder at single‐cell levels, offering an in‐depth resource and foundation for future research.

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.

Molecular and functional characterization of detrusor PDGFRα positive cells in spinal cord injury-induced detrusor overactivity

Volume accommodation occurs via a novel mechanism involving interstitial cells in detrusor muscles. The interstitial cells in the bladder are PDGFRα+, and they restrain the excitability of smooth muscle at low levels and prevents the development of transient contractions (TCs). A common clinical manifestation of spinal cord injury (SCI)-induced bladder dysfunction is detrusor overactivity (DO). Although a myogenic origin of DO after SCI has been suggested, a mechanism for development of SCI-induced DO has not been determined. In this study we hypothesized that SCI-induced DO is related to loss of function in the regulatory mechanism provided by PDGFRα+ cells. Our results showed that transcriptional expression of Pdgfra and Kcnn3 was decreased after SCI. Proteins encoded by these genes also decreased after SCI, and a reduction in PDGFRα+ cell density was also documented. Loss of PDGFRα+ cells was due to apoptosis. TCs in ex vivo bladders during filling increased dramatically after SCI, and this was related to the loss of regulation provided by SK channels, as we observed decreased sensitivity to apamin. These findings show that damage to the mechanism restraining muscle contraction during bladder filling that is provided by PDGFRα+ cells is causative in the development of DO after SCI.

Role of Ano1 Ca2+-activated Cl- channels in generating urethral tone

Urinary continence is maintained in the lower urinary tract by the contracture of urethral sphincters, including smooth muscle of the internal urethral sphincter. These contractions occlude the urethral lumen, preventing urine leakage from the bladder to the exterior. Over the past 20 years, research on the ionic conductances that contribute to urethral smooth muscle contractility has greatly accelerated. A debate has emerged over the role of interstitial cell of Cajal (ICC)-like cells in the urethra and their expression of Ca²⁺-activated Cl^- channels encoded by anoctamin-1 [Ano1; transmembrane member 16 A (Tmem16a) gene]. It has been proposed that Ano1 channels expressed in urethral ICC serve as a source of depolarization for smooth muscle cells, increasing their excitability and contributing to tone. Although a clear role for Ano1 channels expressed in ICC is evident in other smooth muscle organs, such as the gastrointestinal tract, the role of these channels in the urethra is unclear, owing to differences in the species (rabbit, rat, guinea pig, sheep, and mouse) examined and experimental approaches by different groups. The importance of clarifying this situation is evident as effective targeting of Ano1 channels may lead to new treatments for urinary incontinence. In this review, we summarize the key findings from different species on the role of ICC and Ano1 channels in urethral contractility. Finally, we outline proposals for clarifying this controversial and important topic by addressing how cell-specific optogenetic and inducible cell-specific genetic deletion strategies coupled with advances in Ano1 channel pharmacology may clarify this area in future studies.NEW & NOTEWORTHY Studies from the rabbit have shown that anoctamin-1 (Ano1) channels expressed in urethral interstitial cells of Cajal (ICC) serve as a source of depolarization for smooth muscle cells, increasing excitability and tone. However, the role of urethral Ano1 channels is unclear, owing to differences in the species examined and experimental approaches. We summarize findings from different species on the role of urethral ICC and Ano1 channels in urethral contractility and outline proposals for clarifying this topic using cell-specific optogenetic approaches.

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.

Muscle cells, nerves, fibroblasts and vessels in the detrusor of the rat urinary bladder

All the cells of rat detrusor muscle fall into one of five ultrastructural types: muscle cells, fibroblasts, axons and glia, and vascular cells (endothelial cells and pericytes). The tissue is ~79% cellular and 21% non-cellular. Muscle cells occupy 72%, nerves ~4% (1/3 axons, 2/3 glia), and fibroblast >3% of space. Muscle cells (up to 6 µm across and ~600 µm long, packed to almost 100,000 per mm²) have surface-to-volume ratio of 2.4 µm²/µm³ ~93% of cell volume is contractile apparatus, 3.1% mitochondria and 2.5% nucleus. Cell profiles are irregular but sectional area decreases regularly towards either end of the cell. Muscle cells are gathered into bundles (the mechanical units of detrusor), variable in length and size, but of constant width. The musculature is highly compact (without fascia or capsule) with smooth outer surfaces and extensive association and adhesion between its cells. Among many types of intercellular contact and junction, digitations are very common, each muscle cell issuing minute finger-like processes that abut on adjacent cells. Sealed apposition are wide areas of specialized contact, possibly forming a chamber between two muscle cells, distinct from the extracellular space at large (stromal space). The innervation is very dense, virtually all intramuscular axons being varicose (including afferent ones). There are identifiable neuro-muscular junctions on each muscle cell, often several junctions on a single cell. There are also unattached terminals. Fibroblasts (involved in the production of collagen), ~1% of the total number of cells, do not make specialized contacts.

Single-Cell Transcriptomic Map of the Human and Mouse Bladders

BACKGROUND

Having a comprehensive map of the cellular anatomy of the normal human bladder is vital to understanding the cellular origins of benign bladder disease and bladder cancer.

METHODS

We used single-cell RNA sequencing (scRNA-seq) of 12,423 cells from healthy human bladder tissue samples taken from patients with bladder cancer and 12,884 cells from mouse bladders to classify bladder cell types and their underlying functions.

RESULTS

We created a single-cell transcriptomic map of human and mouse bladders, including 16 clusters of human bladder cells and 15 clusters of mouse bladder cells. The homology and heterogeneity of human and mouse bladder cell types were compared and both conservative and heterogeneous aspects of human and mouse bladder evolution were identified. We also discovered two novel types of human bladder cells. One type is ADRA2A ⁺ and HRH2 ⁺ interstitial cells which may be associated with nerve conduction and allergic reactions. The other type is TNNT1 ⁺ epithelial cells that may be involved with bladder emptying. We verify these TNNT1 ⁺ epithelial cells also occur in rat and mouse bladders.

CONCLUSIONS

This transcriptomic map provides a resource for studying bladder cell types, specific cell markers, signaling receptors, and genes that will help us to learn more about the relationship between bladder cell types and diseases.

Targetable purinergic receptors P2Y12 and A2b antagonistically regulate bladder function

Abnormalities in purine availability or purinergic receptor density are commonly seen in patients with lower urinary tract symptoms (LUTS), but the underlying mechanisms relating altered receptor function to LUTS are unknown. Here we provide extensive evidence for the reciprocal interplay of multiple receptors responding to ATP, ADP (adenosine diphosphate), and adenosine, agonists that regulate bladder function significantly. ADP stimulated P2Y12 receptors, causing bladder smooth muscle (BSM) contraction, whereas adenosine signaling through potentially newly defined A2b receptors, actively inhibited BSM purinergic contractility. The modulation of adenylyl cyclase-cAMP signaling via A2b and P2Y12 interaction actively regulated bladder contractility by modulating intracellular calcium levels. KO mice lacking the receptors display diametrically opposed bladder phenotypes, with P2Y12-KO mice exhibiting an underactive bladder (UAB) phenotype with increased bladder capacity and reduced voiding frequency, whereas A2b-KO mice have an overactive bladder (OAB), with decreased capacity and increased voiding frequency. The opposing phenotypes in P2Y12-KO and A2b-KO mice not only resulted from dysregulated BSM contractility, but also from abnormal BSM cell growth. Finally, we demonstrate that i.p. administration of drugs targeting P2Y12 or A2b receptor rescues these abnormal phenotypes in both KO mice. These findings strongly indicate that P2Y12 and A2b receptors are attractive therapeutic targets for human patients with LUTS.

An ex vivo bladder model with detrusor smooth muscle removed to analyse biologically active mediators released from the suburothelium

KEY POINTS

Studies of urothelial cells, bladder sheets or lumens of filled bladders have suggested that mediators released from urothelium into suburothelium (SubU)/lamina propria (LP) activate mechanisms controlling detrusor excitability. None of these approaches, however, has enabled direct assessment of availability of mediators at SubU/LP during filling. We developed an ex vivo mouse bladder preparation with intact urothelium and SubU/LP but no detrusor, which allows direct access to the SubU/LP surface of urothelium during filling. Pressure-volume measurements during filling demonstrated that bladder compliance is governed primarily by the urothelium. Measurements of purine mediators in this preparation demonstrated asymmetrical availability of purines in lumen and SubU/LP, suggesting that interpretations based solely on intraluminal measurements of mediators may be inaccurate. The preparations are suitable for assessments of release, degradation and transport of mediators in SubU/LP during bladder filling, and are superior to experimental approaches previously used for urothelium research.

ABSTRACT

The purpose of this study was to develop a decentralized (ex vivo) detrusor smooth muscle (DSM)-denuded mouse bladder preparation, a novel model that enables studies on availability of urothelium-derived mediators at the luminal and anti-luminal aspects of the urothelium during filling. Urinary bladders were excised from C57BL6/J mice and the DSM was removed by fine-scissor dissection without touching the mucosa. Morphology and cell composition of the preparation wall, pressure-volume relationships during filling, and fluorescent dye permeability of control, protamine sulfate- and lipopolysaccharide-treated denuded bladders were characterized. The preparation wall contained intact urothelium and suburothelium (SubU)/lamina propria (LP) and lacked the DSM and the serosa. The utility of the model for physiological research was validated by measuring release, metabolism and transport of purine mediators at SubU/LP and in bladder lumen during filling. We determined asymmetrical availability of purines (e.g. ATP, ADP, AMP and adenosine) in lumen and at SubU/LP during filling, suggesting differential mechanisms of release, degradation and bilateral transurothelial transport of purines during filling. Some observations were validated in DSM-denuded bladder of the cynomolgus monkey (Macaca fascicularis). The novel model was superior to current models utilized to study properties of the urothelium (e.g. cultured urothelial cells, bladder mucosa sheets mounted in Ussing chambers or isolated bladder strips in organ baths) in that it enabled direct access to the vicinity of SubU/LP during authentic bladder filling. The model is particularly suitable for understanding local mechanisms of urothelium-DSM connectivity and for broad understanding of the role of urothelium in regulating continence and voiding.

Changes in the Neuronal Control of the Urinary Bladder in a Model of Radiation Cystitis

Currently, we have assessed the neuronal control of the urinary bladder in radiation cystitis and whether interstitial cells contribute to the condition. Fourteen days after bladder irradiation (20 Gy), rats were sedated and the urinary bladder was cut into two sagittal halves where electrical field stimulation (EFS; 5-20 Hz) was applied on the pelvic nerve afferents or stretch (80 mN) on one-half of the bladder, while contractions were registered on the contralateral half of the bladder in the absence and presence of increasing doses of imatinib (1-10 mg/kg; inhibitor of c-kit-positive interstitial cells), atropine (1 mg/kg; to block muscarinic M3 receptors), or pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (2 mg/kg; P2X1 purinoceptor antagonist). Urinary bladders were also excised for organ bath experiments, Western blot, quantitative polymerase chain reaction, and immunohistochemistry. In vivo, EFS contractions were enhanced after irradiation, and imatinib (1-10 mg/mg) inhibited contractions by EFS and stretched dose-dependently in controls but not in irradiated bladders. In the irradiated bladder in vitro, atropine resistance was increased and imatinib (100 µM) inhibited contractions by EFS and agonists (ATP, methacholine) in irradiated bladders and controls. The urinary bladder expressions of P2X1 purinoceptors, muscarinic M3 receptor, choline acetyltransferase, c-kit, and the agonist of c-kit, stem cell factor, were not changed by irradiation. In conclusion, bladder irradiation affects several levels of neuronal control of the urinary bladder. Interstitial cells may contribute to some of the symptoms associated with radiation cystitis.