Pathogenesis of renal injury in the megabladder mouse: a genetic model of congenital obstructive nephropathy

3D autofluorescence imaging of hydronephrosis and renal anatomical structure using cryo-micro-optical sectioning tomography

Rationale: Mesoscopic visualization of the main anatomical structures of the whole kidney in vivo plays an important role in the pathological diagnosis and exploration of the etiology of hydronephrosis. However, traditional imaging methods cannot achieve whole-kidney imaging with micron resolution under conditions representing in vivo perfusion. Methods: We used in vivo cryofixation (IVCF) to fix acute obstructive hydronephrosis (unilateral ureteral obstruction, UUO), chronic spontaneous hydronephrosis (db/db mice), and their control mouse kidneys for cryo-micro-optical sectioning tomography (cryo-MOST) autofluorescence imaging. We quantitatively assessed the kidney-wide pathological changes in the main anatomical structures, including hydronephrosis, renal subregions, arteries, veins, glomeruli, renal tubules, and peritubular functional capillaries. Results: By comparison with microcomputed tomography imaging, we confirmed that IVCF can maintain the status of the kidney in vivo. Cryo-MOST autofluorescence imaging can display the main renal anatomical structures with a cellular resolution without contrast agents. The hydronephrosis volume reached 26.11 ± 6.00 mm3 and 13.01 ± 3.74 mm3 in 3 days after UUO and in 15-week-old db/db mouse kidneys, respectively. The volume of the cortex and inner stripe of the outer medulla (ISOM) increased while that of the inner medulla (IM) decreased in UUO mouse kidneys. Db/db mice also showed an increase in the volume of the cortex and ISOM volume but no atrophy in the IM. The diameter of the proximal convoluted tubule and proximal straight tubule increased in both UUO and db/db mouse kidneys, indicating that proximal tubules were damaged. However, some renal tubules showed abnormal central bulge highlighting in the UUO mice, but the morphology of renal tubules was normal in the db/db mice, suggesting differences in the pathology and severity of hydronephrosis between the two models. UUO mouse kidneys also showed vascular damage, including segmental artery and vein atrophy and arcuate vein dilation, and the density of peritubular functional capillaries in the cortex and IM was reduced by 37.2% and 49.5%, respectively, suggesting renal hypoxia. In contrast, db/db mouse kidneys showed a normal vascular morphology and peritubular functional capillary density. Finally, we found that the db/db mice displayed vesicoureteral reflux and bladder overactivity, which may be the cause of hydronephrosis formation. Conclusions: We observed and compared main renal structural changes in hydronephrosis under conditions representing in vivo perfusion in UUO, db/db, and control mice through cryo-MOST autofluorescence imaging. The results indicate that cryo-MOST with IVCF can serve as a simple and powerful tool to quantitatively evaluate the in vivo pathological changes in three dimensions, especially the distribution of body fluids in the whole kidney. This method is potentially applicable to the three-dimensional visualization of other tissues, organs, and even the whole body, which may provide new insights into pathological changes in diseases.

Ultrasound and Photoacoustic Imaging of the Kidney: Basic Concepts and Protocols

Noninvasive, robust, and reproducible methods to image kidneys are provided by different imaging modalities. A combination of modalities (multimodality) can give better insight into structure and function and to understand the physiology of the kidney. Magnetic resonance imaging can be complemented by a multimodal imaging approach to obtain additional information or include interventional procedures. In the clinic, renal ultrasound has been essential for the diagnosis and management of kidney disease and for the guidance of invasive procedures for a long time. Adapting ultrasound to preclinical requirements and for translational research, the combination with photoacoustic imaging expands the capabilities to obtain anatomical, functional, and molecular information from animal models. This chapter describes the basic concepts of how to image kidneys using different and most appropriate modalities.This chapter is based upon work from the COST Action PARENCHIMA, a community-driven network funded by the European Cooperation in Science and Technology (COST) program of the European Union, which aims to improve the reproducibility and standardization of renal MRI biomarkers. This introduction chapter is complemented by two separate chapters describing the experimental procedure and data analysis.

Loss-of-function variants in myocardin cause congenital megabladder in humans and mice

Myocardin (MYOCD) is the founding member of a class of transcriptional coactivators that bind the serum-response factor to activate gene expression programs critical in smooth muscle (SM) and cardiac muscle development. Insights into the molecular functions of MYOCD have been obtained from cell culture studies, and to date, knowledge about in vivo roles of MYOCD comes exclusively from experimental animals. Here, we defined an often lethal congenital human disease associated with inheritance of pathogenic MYOCD variants. This disease manifested as a massively dilated urinary bladder, or megabladder, with disrupted SM in its wall. We provided evidence that monoallelic loss-of-function variants in MYOCD caused congenital megabladder in males only, whereas biallelic variants were associated with disease in both sexes, with a phenotype additionally involving the cardiovascular system. These results were supported by cosegregation of MYOCD variants with the phenotype in 4 unrelated families by in vitro transactivation studies in which pathogenic variants resulted in abrogated SM gene expression and by the finding of megabladder in 2 distinct mouse models with reduced Myocd activity. In conclusion, we have demonstrated that variants in MYOCD result in human disease, and the collective findings highlight a vital role for MYOCD in mammalian organogenesis.

The uroplakin plaque promotes renal structural integrity during congenital and acquired urinary tract obstruction

Urinary tract obstruction represents a common cause of kidney injury across the human life span, resulting in chronic kidney disease and end-stage renal disease. Yet, the extent of obstructive renal damage can be heterogeneous between individuals, implying the existence of unknown mechanisms that protect against or accelerate kidney injury. In this study, we investigated the role of urothelial remodeling in renal adaptation during congenital and acquired obstruction. In the Megabladder ( Mgb-/-) model of congenital obstruction and unilateral ureteral ligation model of acute obstruction, progressive hydronephrosis is strongly associated with dynamic reorganization of the renal urothelium, which elaborates a continuous uroplakin (Upk) plaque. This led us to postulate that the Upk plaque prevents parenchymal injury during urinary tract obstruction. To test this hypothesis, we interbred Mgb-/- and Upk1b-/- mice, which lack the critical Upk1b subunit for Upk plaque formation. Upk1b-/-; Mgb-/- mice experienced an accelerated onset of bilateral hydronephrosis with severe (>67%) parenchymal loss, leading to renal failure and mortality in adolescence. To investigate the function of the renal Upk plaque during acute obstruction, we destabilized the Upk plaque by Upk1b deletion or genetically depleted Upk+ cells following unilateral ureteral obstruction. Both of these strategies accelerated renal parenchymal loss following ureteral ligation, attesting to a conserved, stabilizing role for Upk plaque deposition in the acutely obstructed kidney. In aggregate, these complementary experiments provide the first evidence that the Upk plaque confers an essential, protective adaptation to preserve renal parenchymal integrity during congenital and acquired urinary tract obstruction.

Role of renal urothelium in the development and progression of kidney disease

The clinical and financial impact of chronic kidney disease (CKD) is significant, while its progression and prognosis is variable and often poor. Studies using the megabladder (mgb ^-/- ) model of CKD show that renal urothelium plays a key role in modulating early injury responses following the development of congenital obstruction. The aim of this review is to examine the role that urothelium has in normal urinary tract development and pathogenesis. We discuss normal morphology of renal urothelium and then examine the role that uroplakins (Upks) play in its development. Histologic, biochemical, and molecular characterization of Upk1b ^RFP/RFP mice indicated Upk1b expression is essential for normal urinary tract development, apical plaque/asymmetric membrane unit (AUM) formation, and differentiation and functional integrity of the renal urothelium. Our studies provide the first evidence that Upk1b is directly associated with the development of congenital anomalies of the urinary tract (CAKUT), spontaneous age-dependent hydronephrosis, and dysplastic urothelia. These observations demonstrate the importance of proper urothelial differentiation in normal development and pathogenesis of the urinary tract and provide a unique working model to test the hypothesis that the complex etiology associated with CKD is dependent upon predetermined genetic susceptibilities that establish pathogenic thresholds for disease initiation and progression.

Renal epithelial miR-205 expression correlates with disease severity in a mouse model of congenital obstructive nephropathy

BACKGROUND

Congenital obstructive nephropathy (CON) is a leading cause of pediatric chronic kidney disease (CKD). Despite optimal surgical and medical care, there is a high rate of CKD progression. Better understanding of molecular and cellular changes is needed to facilitate development of improved biomarkers and novel therapeutic approaches in CON.

METHODS

The megabladder (mgb) mouse is an animal model of CKD with impaired bladder emptying, hydronephrosis, and progressive renal injury. In this study, we characterize a particular microRNA, miR-205, whose expression changes with the degree of hydronephrosis in the mgb(-/-) kidney.

RESULTS

Expression of miR-205 is progressively increased in the adult mgb(-/-) mouse with worsening severity of hydronephrosis. miR-205 expression is correlated with altered expression of cytokeratins and uroplakins, which are markers of cellular differentiation in urothelium. We describe the spatial pattern of miR-205 expression, including increased expression in renal urothelium and novel miR-205 expression in medullary collecting duct epithelium in the congenitally obstructed kidney.

CONCLUSION

miR-205 is increased with severity of CON and CKD in the mgb(-/-) mouse and may regulate urothelial differentiation.

Uroplakin 1b is critical in urinary tract development and urothelial differentiation and homeostasis

Proper development and maintenance of urothelium is critical to its function. Uroplakins are expressed in developing and mature urothelium where they establish plaques associated with the permeability barrier. Their precise functional role in development and disease is unknown. Here, we disrupted Upk1b in vivo where its loss resulted in urothelial plaque disruption in the bladder and kidney. Upk1b(RFP/RFP) bladder urothelium appeared dysplastic with expansion of the progenitor cell markers, Krt14 and Krt5, increased Shh expression, and loss of terminal differentiation markers Krt20 and uroplakins. Upk1b(RFP/RFP) renal urothelium became stratified with altered cellular composition. Upk1b(RFP/RFP) mice developed age-dependent progressive hydronephrosis. Interestingly, 16% of Upk1b(RFP/RFP) mice possessed unilateral duplex kidneys. Our study expands the role of uroplakins, mechanistically links plaque formation to urinary tract development and function, and provides a tantalizing connection between congenital anomalies of the kidney and urinary tract along with functional deficits observed in a variety of urinary tract diseases. Thus, kidney and bladder urothelium are regionally distinct and remain highly plastic, capable of expansion through tissue-specific progenitor populations. Furthermore, Upk1b plays a previously unknown role in early kidney development representing a novel genetic target for congenital anomalies of the kidney and urinary tract.

Urine Stasis Predisposes to Urinary Tract Infection by an Opportunistic Uropathogen in the Megabladder (Mgb) Mouse

PURPOSE

Urinary stasis is a risk factor for recurrent urinary tract infection (UTI). Homozygous mutant Megabladder (Mgb-/-) mice exhibit incomplete bladder emptying as a consequence of congenital detrusor aplasia. We hypothesize that this predisposes Mgb-/- mice to spontaneous and experimental UTI.

METHODS

Mgb-/-, Mgb+/-, and wild-type female mice underwent serial ultrasound and urine cultures at 4, 6, and 8 weeks to detect spontaneous UTI. Urine bacterial isolates were analyzed by Gram stain and speciated. Bladder stones were analyzed by x-ray diffractometry. Bladders and kidneys were subject to histologic analysis. The pathogenicity of coagulase-negative Staphylococcus (CONS) isolated from Mgb-/- urine was tested by transurethral administration to culture-negative Mgb-/- or wild-type animals. The contribution of urinary stasis to CONS susceptibility was evaluated by cutaneous vesicostomy in Mgb-/- mice.

RESULTS

Mgb-/- mice develop spontaneous bacteriuria (42%) and struvite bladder stones (31%) by 8 weeks, findings absent in Mgb+/- and wild-type controls. CONS was cultured as a solitary isolate from Mgb-/- bladder stones. Bladders and kidneys from mice with struvite stones exhibit mucosal injury, inflammation, and fibrosis. These pathologic features of cystitis and pyelonephritis are replicated by transurethral inoculation of CONS in culture-negative Mgb-/- females, whereas wild-type animals are less susceptible to CONS colonization and organ injury. Cutaneous vesicostomy prior to CONS inoculation significantly reduces the quantity of CONS recovered from Mgb-/- urine, bladders, and kidneys.

CONCLUSIONS

CONS is an opportunistic uropathogen in the setting of urinary stasis, leading to enhanced UTI incidence and severity in Mgb-/- mice.

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.

Megabladder mouse model of congenital obstructive nephropathy: genetic etiology and renal adaptation

Congenital obstructive nephropathy remains one of the leading causes of chronic renal failure in children. The direct link between obstructed urine flow and abnormal renal development and subsequent dysfunction represents a central paradigm of urogenital pathogenesis that has far-reaching clinical implications. Even so, a number of diagnostic, prognostic, and therapeutic quandaries still exist in the management of congenital obstructive nephropathy. Studies in our laboratory have characterized a unique mutant mouse line that develops in utero megabladder, variable hydronephrosis, and progressive renal failure. Megabladder mice represent a valuable functional model for the study of congenital obstructive nephropathy. Recent studies have begun to shed light on the genetic etiology of mgb (-/-) mice as well as the molecular pathways controlling disease progression in these animals.

Molecular basis of renal adaptation in a murine model of congenital obstructive nephropathy

Congenital obstructive nephropathy is a common cause of chronic kidney disease and a leading indication for renal transplant in children. The cellular and molecular responses of the kidney to congenital obstruction are incompletely characterized. In this study, we evaluated global transcription in kidneys with graded hydronephrosis in the megabladder (mgb^−/−) mouse to better understand the pathophysiology of congenital obstructive nephropathy. Three primary pathways associated with kidney remodeling/repair were induced in mgb^−/− kidneys independent of the degree of hydronephrosis. These pathways included retinoid signaling, steroid hormone metabolism, and renal response to injury. Urothelial proliferation and the expression of genes with roles in the integrity and maintenance of the renal urothelium were selectively increased in mgb^−/− kidneys. Ngal/Lcn2, a marker of acute kidney injury, was elevated in 36% of kidneys with higher grades of hydronephrosis. Evaluation of Ngal^high versus Ngal^low kidneys identified the expression of several novel candidate markers of renal injury. This study indicates that the development of progressive hydronephrosis in mgb^−/− mice results in renal adaptation that includes significant changes in the morphology and potential functionality of the renal urothelium. These observations will permit the development of novel biomarkers and therapeutic approaches to progressive renal injury in the context of congenital obstruction.

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.

Struvite urolithiasis and chronic urinary tract infection in a murine model of urinary diversion

OBJECTIVE

To characterize the clinical course after cutaneous vesicostomy (CV) in megabladder (mgb(-/-)) mice with functional urinary bladder obstruction.

MATERIALS AND METHODS

A total of 45 mgb(-/-) male mice underwent CV at a median age of 25 days. The 34 mice that survived >3 days after CV were evaluated by serial observation and renal ultrasonography. The moribund mice were killed. The urinary bladders and kidneys were analyzed by histopathologic analysis, and urine biochemical studies were performed.

RESULTS

At a median duration of 11 weeks after CV, 35% of mgb(-/-) male mice (12 of 34) had become moribund with pelvic masses, which were identified as bladder stones at necropsy. The urine pH was alkaline, and microscopic examination demonstrated struvite crystals. The urine samples contained Gram-positive cocci, and the urine cultures were polymicrobial. The stone composition was chiefly struvite (88%-94%) admixed with calcium phosphate. In 40% of cases (2 of 5), retained intravesical polypropylene suture was identified as the presumed nidus. No stones were detected in >100 male mice before CV or in 25 cases when CV was performed using polydioxanone suture. The kidneys from 33% of the mice (4/12) with bladder stones contained staghorn calculi. The histopathologic findings from the mice with struvite stones demonstrated active cystitis, pyelitis, and chronic pyelonephritis.

CONCLUSION

These findings attest to the importance of the nidus in lithogenesis and provide a novel murine model for struvite urolithiasis and chronic infection of the diverted urinary tract.

Investigating mechanisms of chronic kidney disease in mouse models

Animal models of chronic kidney disease (CKD) are important experimental tools that are used to investigate novel mechanistic pathways and to validate potential new therapeutic interventions prior to pre-clinical testing in humans. Over the past several years, mouse CKD models have been extensively used for these purposes. Despite significant limitations, the model of unilateral ureteral obstruction (UUO) has essentially become the high-throughput in vivo model, as it recapitulates the fundamental pathogenetic mechanisms that typify all forms of CKD in a relatively short time span. In addition, several alternative mouse models are available that can be used to validate new mechanistic paradigms and/or novel therapies. Here, we review several models-both genetic and experimentally induced-that provide investigators with an opportunity to include renal functional study end-points together with quantitative measures of fibrosis severity, something that is not possible with the UUO model.

Urinary diversion via cutaneous vesicostomy in the megabladder mouse

Lower urinary tract obstruction in mice can lead to end-stage renal disease and death. We have developed a surgical technique to create a cutaneous vesicostomy in mice providing an external outlet for drainage of urine, thereby relieving the obstruction and slowing and/or preventing the development of end-stage renal disease and death.

Ultrasound imaging of the murine kidney

Ultrasound (US) is the most common and least invasive modality for clinical imaging of the kidney. One important application of US in nephrology is the detection and monitoring of structural changes in the kidney. Recent advances in US technology have facilitated the application of similar techniques to animal models of human disease. We have developed a simple US-based method of detection and quantitation of hydronephrosis in a mouse model of congenital obstructive nephropathy, the megabladder (mgb) mouse.

Fetal genitourinary imaging

The fetal urinary tract is routinely evaluated sonographically beginning in the first trimester with documentation of fetal bladder visualization. Fetal MR might be indicated to further clarify abnormalities found sonographically. The primary imaging modality for evaluation of the fetal kidney is US, which plays an important role in the detection of collecting system dilatation and parenchymal diseases that influence counseling and postnatal care. A commonly seen birth defect affecting the fetal kidney is pyelectasis. The significance of this finding has been extensively evaluated by a number of fetal imaging centers and will be presented in this review. Further topics of interest within the fetal genitourinary system include fetal renal parenchymal disorders and fetal bladder abnormalities. Characteristic imaging features, as well as developmental pathology and differential considerations, are discussed here.

Current perspectives on congenital obstructive nephropathy

Congenital obstructive nephropathy is the leading cause of chronic renal disease in children. As a result, it represents a tremendous societal burden in terms of morbidity and mortality, as well as in health care expenses of caring for children with chronic kidney disease and end-stage renal disease. The various diagnostic, prognostic, and therapeutic challenges associated with congenital obstructive nephropathy highlight the importance of developing effective experimental models for studying this disease process. In this review, we define the clinical entity that is congenital obstructive nephropathy, outline the current standards of diagnosis and care, and discuss the utilization of current experimental models designed to help clarify some of the clinical conundrums associated with this important disease.

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.