Superficial cell differentiation during embryonic and postnatal development of mouse urothelium

The urothelium: a multi-faceted barrier against a harsh environment

All mucosal surfaces must deal with the challenge of exposure to the outside world. The urothelium is a highly specialized layer of stratified epithelial cells lining the inner surface of the urinary bladder, a gruelling environment involving significant stretch forces, osmotic and hydrostatic pressures, toxic substances, and microbial invasion. The urinary bladder plays an important barrier role and allows the accommodation and expulsion of large volumes of urine without permitting urine components to diffuse across. The urothelium is made up of three cell types, basal, intermediate, and umbrella cells, whose specialized functions aid in the bladder's mission. In this review, we summarize the recent insights into urothelial structure, function, development, regeneration, and in particular the role of umbrella cells in barrier formation and maintenance. We briefly review diseases which involve the bladder and discuss current human urothelial in vitro models as a complement to traditional animal studies.

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.

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.

Analysis of zebrafish periderm enhancers facilitates identification of a regulatory variant near human KRT8/18

Genome-wide association studies for non-syndromic orofacial clefting (OFC) have identified single nucleotide polymorphisms (SNPs) at loci where the presumed risk-relevant gene is expressed in oral periderm. The functional subsets of such SNPs are difficult to predict because the sequence underpinnings of periderm enhancers are unknown. We applied ATAC-seq to models of human palate periderm, including zebrafish periderm, mouse embryonic palate epithelia, and a human oral epithelium cell line, and to complementary mesenchymal cell types. We identified sets of enhancers specific to the epithelial cells and trained gapped-kmer support-vector-machine classifiers on these sets. We used the classifiers to predict the effects of 14 OFC-associated SNPs at 12q13 near KRT18. All the classifiers picked the same SNP as having the strongest effect, but the significance was highest with the classifier trained on zebrafish periderm. Reporter and deletion analyses support this SNP as lying within a periderm enhancer regulating KRT18/KRT8 expression.

Development of novel and non-invasive diagnostic markers for lower urinary tract symptoms using urothelial cells in voided urine

OBJECTIVES

We evaluated the association between lower urinary tract symptoms (LUTS) and the expression of connexin (Cx) and transient receptor potential (TRP) channel on urothelial cells non-invasively collected from voided urine in humans.

METHODS

A total of 55 patients (36 males and 19 females, median age: 71 years old), who were followed up at University of Yamanashi Hospital, were enrolled in the present study. Urothelial cells were collected from voided urine of patients, and the mRNA expression of each subtype of Cxs and TRP channels was measured using quantitive real-time reverse transcription polymerase chain reaction. We then analyzed the correlation between the expression of Cxs and TRP channels and symptom scores in International Prostate Symptom Scoreand Overactive Bladder Symptom Score, in addition to Interstitial Cystitis Symptom Index (ICSI) from only interstitial cystitis (IC) patients.

RESULTS

Non-adjusted statistical procedure using Spearman's rank-correlation showed that there were significant correlations between the following expressions and symptom scores; (positive correlations) Cx26 versus urgency score, Cx40 versus nocturia, TRPM2 versus intermittency, TRPV1 versus urge incontinence, (negative correlation) Cx40 versus intermittency, TRPM7 versus pollakisuria. However, a multiple comparison adjustment using Bonferroni correction showed that only Cx40 had a trend of correlation with nocturia in ICSI.

CONCLUSIONS

The expressions of Cxs and TRP channels on urothelial cells in voided urine could be related to LUTS. Further analysis of urothelial cells in voided urine has the potential to reveal the mechanism of the LUTS and develop new markers with non-invasive methods.

Systematic Review to Compare Urothelium Differentiation with Urethral Epithelium Differentiation in Fetal Development, as a Basis for Tissue Engineering of the Male Urethra

BACKGROUND

Tissue-engineered (TE) urethra is desirable in men with urethral disease (stricture or hypospadias) and shortage of local tissue. Although ideally a TE graft would contain urethral epithelium cells, currently, bladder epithelium (urothelium) is widely used, but morphologically different. Understanding the differences and similarities of urothelium and urethral epithelium could help design a protocol for in vitro generation of urethral epithelium to be used in TE grafts for the urethra.

PURPOSE

To understand the development toward urethral epithelium or urothelium to improve TE of the urethra.

METHODS

A literature search was done following PRISMA guidelines. Articles describing urethral epithelium and bladder urothelium development in laboratory animals and humans were selected.

RESULTS

Twenty-nine studies on development of urethral epithelium and 29 studies on development of urothelium were included. Both tissue linings derive from endoderm and although adult urothelium and urethral epithelium are characterized by different gene expression profiles, the signaling pathways underlying their development are similar, including Shh, BMP, Wnt, and FGF. The progenitor of the urothelium and the urethral epithelium is the early fetal urogenital sinus (UGS). The urethral plate and the urothelium are both formed from the p63+ cells of the UGS. Keratin 20 and uroplakins are exclusively expressed in urothelium, not in the urethral epithelium. Further research has to be done on unique markers for the urethral epithelium.

CONCLUSION

This review has summarized the current knowledge about embryonic development of urothelium versus urethral epithelium and especially focuses on the influencing factors that are potentially specific for the eventual morphological differences of both cell linings, to be a basis for developmental or tissue engineering of urethral tissue.

Urothelial Defects from Targeted Inactivation of Exocyst Sec10 in Mice Cause Ureteropelvic Junction Obstructions

Most cases of congenital obstructive nephropathy are the result of ureteropelvic junction obstructions, and despite their high prevalence, we have a poor understanding of their etiology and scarcity of genetic models. The eight-protein exocyst complex regulates polarized exocytosis of intracellular vesicles in a large variety of cell types. Here we report generation of a conditional knockout mouse for Sec10, a central component of the exocyst, which is the first conditional allele for any exocyst gene. Inactivation of Sec10 in ureteric bud-derived cells using Ksp1.3-Cre mice resulted in severe bilateral hydronephrosis and complete anuria in newborns, with death occurring 6-14 hours after birth. Sec10 FL/FL;Ksp-Cre embryos developed ureteropelvic junction obstructions between E17.5 and E18.5 as a result of degeneration of the urothelium and subsequent overgrowth by surrounding mesenchymal cells. The urothelial cell layer that lines the urinary tract must maintain a hydrophobic luminal barrier again urine while remaining highly stretchable. This barrier is largely established by production of uroplakin proteins that are transported to the apical surface to establish large plaques. By E16.5, Sec10 FL/FL;Ksp-Cre ureter and pelvic urothelium showed decreased uroplakin-3 protein at the luminal surface, and complete absence of uroplakin-3 by E17.5. Affected urothelium at the UPJ showed irregular barriers that exposed the smooth muscle layer to urine, suggesting this may trigger the surrounding mesenchymal cells to overgrow the lumen. Findings from this novel mouse model show Sec10 is critical for the development of the urothelium in ureters, and provides experimental evidence that failure of this urothelial barrier may contribute to human congenital urinary tract obstructions.

Formation and regeneration of the urothelium

PURPOSE OF REVIEW

This review addresses significant changes in our understanding of urothelial development and regeneration. Understanding urothelial differentiation will be important in the push to find new methods of bladder reconstruction and augmentation, as well as identification of bladder cancer stem cells.

RECENT FINDINGS

This review will cover recent findings including the identification of novel progenitor cells in the embryo and adult urothelium, function of the urothelium, and regeneration of the urothelium. Using Cre-lox recombination with cell-type-specific Cre lines, lineage studies from our laboratory have revealed novel urothelial cell types and progenitors that are critical for formation and regeneration of the urothelium. Interestingly, our studies indicate that Keratin-5-expressing basal cells, which have previously been proposed to be urothelial stem cells, are a self-renewing unipotent population, whereas P-cells, a novel urothelial cell type, are progenitors in the embryo, and intermediate cells serve as a progenitor pool in the adult.

SUMMARY

These findings could have important implications for our understanding of cancer tumorigenesis and could move the fields of regeneration and reconstruction forward.

A population of progenitor cells in the basal and intermediate layers of the murine bladder urothelium contributes to urothelial development and regeneration

BACKGROUND

Homeostatic maintenance and repair of the bladder urothelium has been attributed to proliferation of keratin 5-expressing basal cells (K5-BC) with subsequent differentiation into superficial cells. Recent evidence, however, suggests that the intermediate cell layer harbors a population of progenitor cells. We use label-retaining cell (LRC) methodology in conjunction with a clinically relevant model of uropathogenic Escherichia coli (UPEC)-induced injury to characterize urothelial ontogeny during development and in response to diffuse urothelial injury.

RESULTS

In the developing urothelium, proliferating cells were dispersed throughout the K5-BC and intermediate cells layers, becoming progressively concentrated in the K5-BC layer with age. When 5-bromo-2-deoxyuridine (BrdU) was administered during urothelial development, LRCs in the adult were found within the K5-BC, intermediate, and superficial cell layers, the location dependent upon time of labeling. UPEC inoculation resulted in loss of the superficial cell layer followed by robust proliferation of K5-BCs and intermediate cells. LRCs within the K5-BC and intermediate cell layers proliferated in response to injury.

CONCLUSIONS

Urothelial development and regeneration following injury relies on proliferation of K5-BC and intermediate cells. The existence and proliferation of LRCs within both the K5-BC and intermediate cell layers suggests the presence of two populations of urothelial progenitor cells.

Transdifferentiation of autologous bone marrow cells on a collagen-poly(ε-caprolactone) scaffold for tissue engineering in complete lack of native urothelium

Urological reconstructive surgery is sometimes hampered by a lack of tissue. In some cases, autologous urothelial cells (UCs) are not available for cell expansion and ordinary tissue engineering. In these cases, we wanted to explore whether autologous mesenchymal stem cells (MSCs) from bone marrow could be used to create urological transplants. MSCs from human bone marrow were cultured in vitro with medium conditioned by normal human UCs or by indirect co-culturing in culture well inserts. Changes in gene expression, protein expression and cell morphology were studied after two weeks using western blot, RT-PCR and immune staining. Cells cultured in standard epithelial growth medium served as controls. Bone marrow MSCs changed their phenotype with respect to growth characteristics and cell morphology, as well as gene and protein expression, to a UC lineage in both culture methods, but not in controls. Urothelial differentiation was also accomplished in human bone marrow MSCs seeded on a three-dimensional poly(ε-caprolactone) (PCL)-collagen construct. Human MSCs could easily be harvested by bone marrow aspiration and expanded and differentiated into urothelium. Differentiation could take place on a three-dimensional hybrid PCL-reinforced collagen-based scaffold for creation of a tissue-engineered autologous transplant for urological reconstructive surgery.

Generation of bladder urothelium from human pluripotent stem cells under chemically defined serum- and feeder-free system

Human stem cells are promising sources for bladder regeneration. Among several possible sources, pluripotent stem cells are the most fascinating because they can differentiate into any cell type, and proliferate limitlessly in vitro. Here, we developed a protocol for differentiation of human pluripotent stem cells (hPSCs) into bladder urothelial cells (BUCs) under a chemically defined culture system. We first differentiated hPSCs into definitive endoderm (DE), and further specified DE cells into BUCs by treating retinoic acid under a keratinocyte-specific serum free medium. hPSC-derived DE cells showed significantly expressed DE-specific genes, but did not express mesodermal or ectodermal genes. After DE cells were specified into BUCs, they notably expressed urothelium-specific genes such as UPIb, UPII, UPIIIa, P63 and CK7. Immunocytochemistry showed that BUCs expressed UPII, CK8/18 and P63 as well as tight junction molecules, E-CADHERIN and ZO-1. Additionally, hPSCs-derived BUCs exhibited low permeability in a FITC-dextran permeability assay, indicating BUCs possessed the functional units of barrier on their surfaces. However, BUCs did not express the marker genes of other endodermal lineage cells (intestine and liver) as well as mesodermal or ectodermal lineage cells. In summary, we sequentially differentiated hPSCs into DE and BUCs in a serum- and feeder-free condition. Our differentiation protocol will be useful for producing cells for bladder regeneration and studying normal and pathological development of the human bladder urothelium in vitro.

Functional role for Piezo1 in stretch-evoked Ca²⁺ influx and ATP release in urothelial cell cultures

The urothelium is a sensory structure that contributes to mechanosensation in the urinary bladder. Here, we provide evidence for a critical role for the Piezo1 channel, a newly identified mechanosensory molecule, in the mouse bladder urothelium. We performed a systematic analysis of the molecular and functional expression of Piezo1 channels in the urothelium. Immunofluorescence examination demonstrated abundant expression of Piezo1 in the mouse and human urothelium. Urothelial cells isolated from mice exhibited a Piezo1-dependent increase in cytosolic Ca(2+) concentrations in response to mechanical stretch stimuli, leading to potent ATP release; this response was suppressed in Piezo1-knockdown cells. In addition, Piezo1 and TRPV4 distinguished different intensities of mechanical stimulus. Moreover, GsMTx4, an inhibitor of stretch-activated channels, attenuated the Ca(2+) influx into urothelial cells and decreased ATP release from them upon stretch stimulation. These results suggest that Piezo1 senses extension of the bladder urothelium, leading to production of an ATP signal. Thus, inhibition of Piezo1 might provide a promising means of treating bladder dysfunction.

Urothelial differentiation of human amniotic fluid stem cells by urothelium specific conditioned medium

Human amniotic fluid stem cells (HAFSCs) have a high proliferative capacity and a good differentiation potential, and may thus be suitable for regenerative medicine. To date, urothelial differentiation mechanisms of HAFSCs are poorly understood. We have investigated the urothelial differentiation potential of HAFSCs so that they can be therapeutically applied to cure defective diseases of bladder. To induce the stem cell differentiation, HAFSCs were cultured in a bladder cancer-derived conditioned medium. After 2 weeks of culture, HAFSCs began to express the urothelial lineage-specific markers (UPII, CK8 and FGF10). Meanwhile, pluripotency markers (Oct-4, Sox-2 and Nanog) were downregulated at both RNA and protein levels in the differentiated HAFSCs. Immunocytochemistry data revealed that differentiated HAFSCs expressed urothelial markers of UPII and CK8. We have screened the receptor tyrosine kinase arrays with the differentiated HAFSCs. The screening showed that MuSK, Tie-1 and EphA4 receptor tyrosine kinases were upregulated, whereas EphA7 and FGF R1 kinases were downregulated in HAFSCs. The data suggest that HAFSCs can be an important urothelium cell source, which can be used for urinary tract engineering.

Lower urinary tract development and disease

Congenital anomalies of the lower urinary tract (CALUT) are a family of birth defects of the ureter, the bladder, and the urethra. CALUT includes ureteral anomaliesc such as congenital abnormalities of the ureteropelvic junction (UPJ) and ureterovesical junction (UVJ), and birth defects of the bladder and the urethra such as bladder-exstrophy-epispadias complex (BEEC), prune belly syndrome (PBS), and posterior urethral valves (PUVs). CALUT is one of the most common birth defects and is often associated with antenatal hydronephrosis, vesicoureteral reflux (VUR), urinary tract obstruction, urinary tract infections (UTI), chronic kidney disease, and renal failure in children. Here, we discuss the current genetic and molecular knowledge about lower urinary tract development and genetic basis of CALUT in both human and mouse models. We provide an overview of the developmental processes leading to the formation of the ureter, the bladder, and the urethra, and different genes and signaling pathways controlling these developmental processes. Human genetic disorders that affect the ureter, the bladder and the urethra and associated gene mutations are also presented. As we are entering the postgenomic era of personalized medicine, information in this article may provide useful interpretation for the genetic and genomic test results collected from patients with lower urinary tract birth defects. With evidence-based interpretations, clinicians may provide more effective personalized therapies to patients and genetic counseling for their families.

Correlative study of functional and structural regeneration of urothelium after chitosan-induced injury

High transepithelial electrical resistance (TEER) demonstrates a functional permeability barrier of the normal urothelium, which is maintained by a layer of highly differentiated superficial cells. When the barrier is challenged, a quick regeneration is induced. We used side-by-side diffusion chambers as an ex vivo system to determine the time course of functional and structural urothelial regeneration after chitosan-induced injury. The exposure of the urothelium to chitosan caused a 60 % decrease in TEER, the exposure of undifferentiated urothelial cells to the luminal surface and leaky tight junctions. During the regeneration period (350 min), TEER recovered to control values after approximately 200 min, while structural regeneration continued until 350 min after injury. The tight junctions are the earliest and predominant component of the barrier to appear, while complete barrier regeneration is achieved by delayed superficial cell terminal differentiation. The barrier function and the structure of untreated urothelium were unaffected in side-by-side diffusion chambers for at least 6 h. The urinary bladder tissue excised from an animal thus retains the ability to maintain and restore the transepithelial barrier and cellular ultrastructure for a sufficient period to allow for studies of regeneration in ex vivo conditions.

Oxidative status and lipofuscin accumulation in urothelial cells of bladder in aging mice

Age-related changes in various tissues have been associated with the onset of a number of age-related diseases, including inflammation and cancer. Bladder cancer, for instance, is a disease that mainly afflicts middle-aged or elderly people and is mostly of urothelial origin. Although research on age-related changes of long-lived post-mitotic cells such as neurons is rapidly progressing, nothing is known about age-related changes in the urothelium of the urinary bladder, despite all the evidence confirming the important role of oxidative stress in urinary bladder pathology. The purpose of this study was thus to investigate the oxidative status and age-related changes in urothelial cells of the urinary bladder of young (2 months) and aging (20 months) mice by means of various methods. Our results demonstrated that healthy young urothelium possesses a powerful antioxidant defence system that functions as a strong defence barrier against reactive species. In contrast, urothelial cells of aging bladder show significantly decreased total antioxidant capacity and significantly increased levels of lipid peroxides (MDA) and iNOS, markers of oxidative stress. Our study demonstrates for the first time that ultrastructural alterations in mitochondria and accumulation of lipofuscin, known to be one of the aging pigments, can clearly be found in superficial urothelial cells of the urinary bladder in aging mice. Since the presence of lipofuscin in the urothelium has not yet been reported, we applied various methods to confirm our finding. Our results reveal changes in the oxidative status and structural alterations to superficial urothelial cells similar to those of other long-lived post-mitotic cells.

Transdifferentiation of human adipose-derived stem cells into urothelial cells: potential for urinary tract tissue engineering

Autologous urothelial cells (UCs) provide a cell source for urinary tissue engineering because they can be used safely due to their lack of immunogenicity. However, these cells cannot be harvested under the following circumstances: malignancy, infection and organ loss, etc. Human adipose-derived stem cells (HADSCs) possess the traits of high differentiation potential and ease of isolation, representing a promising resource for tissue engineering and regenerative medicine. Nevertheless, HADSCs have been poorly investigated in urology and the optimal approaches to induce HADSCs into urothelium are still under investigation. In this study, we hypothesized that the change of microenvironment by a conditioned medium was essential for the transdifferentiation of HADSCs into UCs. We then used a conditioned medium derived from urothelium to alternate the microenvironment of HADSCs. After 14 days of culture in a conditioned medium, about 25-50% HADSCs changed their morphology into polygonal epithelium-like shapes. In addition, these cells expressed up-regulating of urothelial lineage-specific markers (uroplakin 2and cytokeratin-18) and down-regulating of mesenchymal marker (vimentin) in RNA and protein level, respectively, which confirmed that HADSCs were induced into urothelial lineage cells. We also measured the growth factors in the conditioned medium in order to analyze the molecular mechanisms regulating transdifferentiation. We observed that the expression levels of PDGF-BB and VEGF were significantly higher than those of the control group after 14 days induction, suggesting they were abundantly secreted into the medium during the culturing period. In conclusion, HADSCs showed in vitro the upregulation of markers for differentiation towards urothelial cells by culturing in an urothelial-conditioned medium, which provides an alternative cell source for potential use in urinary tract tissue engineering.

Kruppel-like factor 5 is required for formation and differentiation of the bladder urothelium

Kruppel-like transcription factor 5 (Klf5) was detected in the developing and mature murine bladder urothelium. Herein we report a critical role of KLF5 in the formation and terminal differentiation of the urothelium. The Shh(GfpCre) transgene was used to delete the Klf5(floxed) alleles from bladder epithelial cells causing prenatal hydronephrosis, hydroureter, and vesicoureteric reflux. The bladder urothelium failed to stratify and did not express terminal differentiation markers characteristic of basal, intermediate, and umbrella cells including keratins 20, 14, and 5, and the uroplakins. The effects of Klf5 deletion were unique to the developing bladder epithelium since maturation of the epithelium comprising the bladder neck and urethra was unaffected by the lack of KLF5. mRNA analysis identified reductions in Pparγ, Grhl3, Elf3, and Ovol1expression in Klf5 deficient fetal bladders supporting their participation in a transcriptional network regulating bladder urothelial differentiation. KLF5 regulated expression of the mGrhl3 promoter in transient transfection assays. The absence of urothelial Klf5 altered epithelial-mesenchymal signaling leading to the formation of an ectopic alpha smooth muscle actin positive layer of cells subjacent to the epithelium and a thinner detrusor muscle that was not attributable to disruption of SHH signaling, a known mediator of detrusor morphogenesis. Deletion of Klf5 from the developing bladder urothelium blocked epithelial cell differentiation, impaired bladder morphogenesis and function causing hydroureter and hydronephrosis at birth.

The human urothelium consists of multiple clonal units, each maintained by a stem cell

Little is known about the clonal architecture of human urothelium. It is likely that urothelial stem cells reside within the basal epithelial layer, yet lineage tracing from a single stem cell as a means to show the presence of a urothelial stem cell has never been performed. Here, we identify clonally related cell areas within human bladder mucosa in order to visualize epithelial fields maintained by a single founder/stem cell. Sixteen frozen cystectomy specimens were serially sectioned. Patches of cells deficient for the mitochondrially encoded enzyme cytochrome c oxidase (CCO) were identified using dual-colour enzyme histochemistry. To show that these patches represent clonal proliferations, small CCO-proficient and -deficient areas were individually laser-capture microdissected and the entire mitochondrial genome (mtDNA) in each area was PCR amplified and sequenced to identify mtDNA mutations. Immunohistochemistry was performed for the different cell layers of the urothelium and adjacent mesenchyme. CCO-deficient patches could be observed in normal urothelium of all cystectomy specimens. The two-dimensional length of these negative patches varied from 2-3 cells (about 30 µm) to 4.7 mm. Each cell area within a CCO-deficient patch contained an identical somatic mtDNA mutation, indicating that the patch was a clonal unit. Patches contained all the mature cell differentiation stages present in the urothelium, suggesting the presence of a stem cell. Our results demonstrate that the normal mucosa of human bladder contains stem cell-derived clonal units that actively replenish the urothelium during ageing. The size of the clonal unit attributable to each stem cell was broadly distributed, suggesting replacement of one stem cell clone by another.

Anatomy and histology of the lower urinary tract

The function of the lower urinary tract is basically storage of urine in the bladder and the at-will periodic evacuation of the stored urine. Urinary incontinence is one of the most common lower urinary tract disorders in adults, but especially in the elderly female. The urethra, its sphincters, and the pelvic floor are key structures in the achievement of continence, but their basic anatomy is little known and, to some extent, still incompletely understood. Because questions with respect to continence arise from human morbidity, but are often investigated in rodent animal models, we present findings in human and rodent anatomy and histology. Differences between males and females in the role that the pelvic floor plays in the maintenance of continence are described. Furthermore, we briefly describe the embryologic origin of ureters, bladder, and urethra, because the developmental origin of structures such as the vesicoureteral junction, the bladder trigone, and the penile urethra are often invoked to explain (clinical) observations. As the human pelvic floor has acquired features in evolution that are typical for a species with bipedal movement, we also compare the pelvic floor of humans with that of rodents to better understand the rodent (or any other quadruped, for that matter) as an experimental model species. The general conclusion is that the "Bauplan" is well conserved, even though its common features are sometimes difficult to discern.

Formation and maintenance of blood-urine barrier in urothelium

Blood-urine barrier, which is formed during differentiation of superficial urothelial cells, is the tightest and most impermeable barrier in the body. In the urinary bladder, the barrier must accommodate large changes in the surface area during distensions and contractions of the organ. Tight junctions and unique apical plasma membrane of superficial urothelial cells play a critical role in the barrier maintenance. Alterations in the blood-urine barrier function accompany most of the urinary tract diseases. In this review, we discuss recent discoveries on the role of tight junctions, dynamics of Golgi apparatus and post-Golgi compartments, and intracellular membrane traffic during the biogenesis and maintenance of blood-urine barrier.

All-trans retinoic acid directs urothelial specification of murine embryonic stem cells via GATA4/6 signaling mechanisms

The urinary bladder and associated tract are lined by the urothelium, a transitional epithelium that acts as a specialized permeability barrier that protects the underlying tissue from urine via expression of a highly specific group of proteins known as the uroplakins (UP). To date, our understanding of the developmental processes responsible for urothelial differentiation has been hampered due to the lack of suitable models. In this study, we describe a novel in vitro cell culture system for derivation of urothelial cells from murine embryonic stem cells (ESCs) following cultivation on collagen matrices in the presence all trans retinoic acid (RA). Upon stimulation with micromolar concentrations of RA, ESCs significantly downregulated the pluripotency factor OCT-4 but markedly upregulated UP1A, UP1B, UP2, UP3A, and UP3B mRNA levels in comparison to naïve ESCs and spontaneously differentiating controls. Pan-UP protein expression was associated with both p63- and cytokeratin 20-positive cells in discrete aggregating populations of ESCs following 9 and 14 days of RA stimulation. Analysis of endodermal transcription factors such as GATA4 and GATA6 revealed significant upregulation and nuclear enrichment in RA-treated UP2-GFP+ populations. GATA4-/- and GATA6-/- transgenic ESC lines revealed substantial attenuation of RA-mediated UP expression in comparison to wild type controls. In addition, EMSA analysis revealed that RA treatment induced formation of transcriptional complexes containing GATA4/6 on both UP1B and UP2 promoter fragments containing putative GATA factor binding sites. Collectively, these data suggest that RA mediates ESC specification toward a urothelial lineage via GATA4/6-dependent processes.

Seven kinds of intermediate filament networks in the cytoplasm of polarized cells: structure and function

Intermediate filaments (IFs) are involved in many important physiological functions, such as the distribution of organelles, signal transduction, cell polarity and gene regulation. However, little information exists on the structure of the IF networks performing these functions. We have clarified the existence of seven kinds of IF networks in the cytoplasm of diverse polarized cells: an apex network just under the terminal web, a peripheral network lying just beneath the cell membrane, a granule-associated network surrounding a mass of secretory granules, a Golgi-associated network surrounding the Golgi apparatus, a radial network locating from the perinuclear region to the specific area of the cell membrane, a juxtanuclear network surrounding the nucleus, and an entire cytoplasmic network. In this review, we describe these seven kinds of IF networks and discuss their biological roles.

Temporally and spatially controllable gene expression and knockout in mouse urothelium

Urothelium that lines almost the entire urinary tract performs important functions and is prone to assaults by urinary microbials, metabolites, and carcinogens. To improve our understanding of urothelial physiology and disease pathogenesis, we sought to develop two novel transgenic systems, one that would allow inducible and urothelium-specific gene expression, and another that would allow inducible and urothelium-specific knockout. Toward this end, we combined the ability of the mouse uroplakin II promoter (mUPII) to drive urothelium-specific gene expression with a versatile tetracycline-mediated inducible system. We found that, when constructed under the control of mUPII, only a modified, reverse tetracycline trans-activator (rtTA-M2), but not its original version (rtTA), could efficiently trans-activate reporter gene expression in mouse urothelium on doxycycline (Dox) induction. The mUPII/rtTA-M2-inducible system retained its strict urothelial specificity, had no background activity in the absence of Dox, and responded rapidly to Dox administration. Using a reporter gene whose expression was secondarily controlled by histone remodeling, we were able to identify, colocalize with 5-bromo-2-deoxyuridine incorporation, and semiquantify newly divided urothelial cells. Finally, we established that, when combined with a Cre recombinase under the control of the tetracycline operon, the mUPII-driven rtTA-M2 could inducibly inactivate any gene of interest in mouse urothelium. The establishment of these two new transgenic mouse systems enables the manipulation of gene expression and/or inactivation in adult mouse urothelium at any given time, thus minimizing potential compensatory effects due to gene overexpression or loss and allowing more accurate modeling of urothelial diseases than previously reported constitutive systems.

The epidermal differentiation-associated Grainyhead gene Get1/Grhl3 also regulates urothelial differentiation

Skin and bladder epithelia form effective permeability barriers through the activation of distinct differentiation gene programs. Using a genome-wide gene-expression study, we identified transcriptional regulators whose expression correlates highly with that of differentiation markers in both the bladder and skin, including the Grainyhead factor Get1/Grhl3, which is already known to be important for epidermal barrier formation. In the bladder, Get1 is most highly expressed in the differentiated umbrella cells and its mutation in mice leads to a defective bladder epithelial barrier formation due to the failure of apical membrane specialization. Genes encoding components of the specialized urothelial membrane, the uroplakins, were downregulated in Get1(-/-) mice. At least one of these genes, uroplakin II, is a direct target of Get1. The urothelial-specific activation of the uroplakin II gene is due to selective binding of Get1 to the uroplakin II promoter in urothelial cells, which is most likely regulated by histone modifications. These results show a crucial role for Get1 in urothelial differentiation and barrier formation.

The TRPV4 cation channel mediates stretch-evoked Ca2+ influx and ATP release in primary urothelial cell cultures

Transient receptor potential channels have recently been implicated in physiological functions in a urogenital system. In this study, we investigated the role of transient receptor potential vanilloid 4 (TRPV4) channels in a stretch sensing mechanism in mouse primary urothelial cell cultures. The selective TRPV4 agonist, 4alpha-phorbol 12,13-didecanoate (4alpha-PDD) evoked Ca(2+) influx in wild-type (WT) urothelial cells, but not in TRPV4-deficient (TRPV4KO) cells. We established a cell-stretch system to investigate stretch-evoked changes in intracellular Ca(2+) concentration and ATP release. Stretch stimulation evoked intracellular Ca(2+) increases in a stretch speed- and distance-dependent manner in WT and TRPV4KO cells. In TRPV4KO urothelial cells, however, the intracellular Ca(2+) increase in response to stretch stimulation was significantly attenuated compared with that in WT cells. Stretch-evoked Ca(2+) increases in WT urothelium were partially reduced in the presence of ruthenium red, a broad TRP channel blocker, whereas that in TRPV4KO cells did not show such reduction. Potent ATP release occurred following stretch stimulation or 4alpha-PDD administration in WT urothelial cells, which was dramatically suppressed in TRPV4KO cells. Stretch-dependent ATP release was almost completely eliminated in the presence of ruthenium red or in the absence of extracellular Ca(2+). These results suggest that TRPV4 senses distension of the bladder urothelium, which is converted to an ATP signal in the micturition reflex pathway during urine storage.

Apoptosis and desquamation of urothelial cells in tissue remodeling during rat postnatal development

Postnatal rat urothelium was studied from day 0 to day 14, when intense cell loss as part of tissue remodeling was expected. The morphological and biochemical characteristics of urothelial cells in the tissue and released cells were investigated by light and electron microscopy, by terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) assay, by annexin V/propidium iodide assay, and by immunofluorescent detection of active caspases and tight-junction protein occludin. Intense apoptosis and massive desquamation were detected between postnatal days 7 and 10. During this period, active caspases and TUNEL-positive cells were found in the urothelium. Disassembled cell-cell junctions were detected between cells. The majority of desquamated cells expressed no apoptotic cell morphology, but were active caspase positive and TUNEL positive. Ann+/PI- apoptotic bodies and desquamated Ann+/PI+ cells were detected in the lumen. These results indicate that apoptosis and desquamation participate in urothelial cell loss in the rat early postnatal period, indispensable for fast urothelial remodeling during development.

Differential localizations of the transient receptor potential channels TRPV4 and TRPV1 in the mouse urinary bladder

We studied the localization and physiological functions of the transient receptor potential (TRP) channels TRPV1 (TRP vanilloid 1) and TRPV4 (TRP vanilloid 4) in the mouse bladder, because both channels are thought to be mechanosensors for bladder distention. RT-PCR specifically amplified TRPV4 transcripts from the urothelial cells, whereas TRPV1 transcripts were barely detectable. ISH experiments showed that TRPV4 transcripts were abundantly expressed in the urothelium, whereas TRPV1 transcripts were not detectable in the urothelial cells. Immunoblotting and IHC studies showed that TRPV4 proteins were mainly localized at the basal plasma membrane domains of the basal urothelial cells. In contrast, TRPV1-immunoreactivities were found not in the urothelial cells but in the nerve fibers that innervate the urinary bladder. In Ca(2+)-imaging experiments, 4alpha-phorbol 12,13-didecanoate, a TRPV4 agonist, and hypotonic stimuli induced significant increases in intracellular calcium ion concentration ([Ca(2+)](i)) in isolated urothelial cells, whereas capsaicin, a TRPV1 agonist, showed no marked effect on the cells. These findings raise the possibility that, in mouse urothelial cells, TRPV4 may contribute to the detection of increases in intravesical pressure related to the micturition reflex.

Deletion of the transient receptor potential cation channel TRPV4 impairs murine bladder voiding

Here we provide evidence for a critical role of the transient receptor potential cation channel, subfamily V, member 4 (TRPV4) in normal bladder function. Immunofluorescence demonstrated TRPV4 expression in mouse and rat urothelium and vascular endothelium, but not in other cell types of the bladder. Intracellular Ca2+ measurements on urothelial cells isolated from mice revealed a TRPV4-dependent response to the selective TRPV4 agonist 4alpha-phorbol 12,13-didecanoate and to hypotonic cell swelling. Behavioral studies demonstrated that TRPV4-/- mice manifest an incontinent phenotype but show normal exploratory activity and anxiety-related behavior. Cystometric experiments revealed that TRPV4-/- mice exhibit a lower frequency of voiding contractions as well as a higher frequency of nonvoiding contractions. Additionally, the amplitude of the spontaneous contractions in explanted bladder strips from TRPV4-/- mice was significantly reduced. Finally, a decreased intravesical stretch-evoked ATP release was found in isolated whole bladders from TRPV4-/- mice. These data demonstrate a previously unrecognized role for TRPV4 in voiding behavior, raising the possibility that TRPV4 plays a critical role in urothelium-mediated transduction of intravesical mechanical pressure.

Neurturin regulates postnatal differentiation of parasympathetic pelvic ganglion neurons, initial axonal projections, and maintenance of terminal fields in male urogenital organs

We have investigated the development of autonomic nerves in the urogenital tract of male mice and the effect of neurturin gene deletion on this process. At birth, autonomic innervation of the reproductive organs was sparse, but urinary bladder smooth muscle was well innervated. Further innervation of reproductive tissues occurred until P21, but noradrenergic axons established their complete terminal field later than nitrergic cholinergic axons: in adults the former are more prevalent, yet this became apparent only at P7 (vas deferens, seminal vesicles), P14 (prostate) or after P14 (penis). Neurturin was essential for initial projection of axons (mucosa of vas deferens), maintenance of terminal fields (prostate and seminal vesicles), or both functions (cavernosum of penis). In contrast, some targets (e.g., bladder muscle and suburothelium, vas deferens smooth muscle) were unaffected by neurturin gene deletion. Pelvic ganglion neurons more than doubled between birth and adulthood, probably as aresult of continued maturation of p75-positive undifferentiated neuronal precursors rather than cell division. The adult number of neurons was achieved by P7 (sympathetic) or P21 (parasympathetic). In adult neurturin knockout mice, there were approximately 25% fewer parasympathetic neurons compared with wild types, because of failure of differentiation after P14. This study revealed the complexity of postnatal maturation of urogenital innervation, with each organ showing a distinct chronology of innervation and different requirement for neurturin. Our results also indicate that in adults there will be distinct differences in neurturin dependence between organs, such that proregenerative therapies may have to be tailored specifically for the nerve pathway of interest.

Two clonal cell lines of immortalized human corneal endothelial cells show either differentiated or precursor cell characteristics

Access to primary human corneal endothelial cells (HCEC) is limited and donor-derived differences between cultures exacerbate the issue of data reproducibility, whereas cell lines can provide sufficient numbers of homogenous cells for multiple experiments. An immortalized HCEC population was adapted to serum-free culture medium and repeated cloning was performed. Clonally grown cells were propagated under serum-free conditions and growth curves were recorded. Cells were characterized immunocytochemically for junctional proteins, collagens, Na,K-ATPase and HCEC-specific 9.3.E-antigen. Ultrastructure was monitored by scanning and transmission electron microscopy. Two clonal cell lines, HCEC-B4G12 and HCEC-H9C1, could be isolated and expanded, which differed morphologically: B4G12 cells were polygonal, strongly adherent and formed a strict monolayer, H9C1 cells were less adherent and formed floating spheres. The generation time of B4G12 cells was 62.26 +/- 14.5 h and that of H9C1 cells 44.05 +/- 5.05 h. Scanning electron microscopy revealed that B4G12 cells had a smooth cell surface, while H9C1 cells had numerous thin filopodia. Both cell lines expressed ZO-1 and occludin adequately, and little but well detectable amounts of connexin-43. Expression of HCEC-specific 9.3.E-antigen was found commensurately in both cell lines, while expression of Na,K-ATPase alpha1 was higher in H9C1 cells than in B4G12 cells. B4G12 cells expressed collagen IV abundantly and almost no collagen III, while H9C1 cells expressed both collagens at reasonable amounts. It is concluded that the clonal cell line B4G12 represents an ideal model of differentiated HCEC, while H9C1 may reflect features of developing or transitional HCEC.