Background: Anomalous development of the ureterovesical junction (UVJ), which joins the ureter to the bladder, is associated with vesicoureteral reflux (VUR): the abnormal retrograde flow of urine from bladder towards the kidneys. VUR is prevented by the muscular layer of the ureters and the bladder that function like a valve to occlude the UVJ. Human studies have demonstrated that refluxing UVJs have elevated levels of fibrillary collagens and degeneration in the smooth muscle layer. This suggests that the integrity of the ECM is crucial for preventing VUR.
Hypothesis: Perturbations in the expression and assembly of fibrillary collagens and elastic fibres, as well as the smooth muscle composition of the developing urinary tract result in a UVJ that refluxes.
Results: The inbred C3H/HeJ (C3H) mouse line has been previously shown by our laboratory to be a fully penetrant model of recessively inherited VUR with a UVJ defect. We have mapped VUR in the C3H mouse to the Vurm1 locus (LOD=7.4) spanning 22Mb on the proximal end of Chromosome 12. Within this locus we have identified putative disease-causing variants in a number of ECM-related genes such as Itgb8, Itgbp1, Lmb1, Fbln5. The C3H as well as the non-VUR C57Bl/6J (B6) mouse line have been used to study the above mentioned ECM components throughout urinary tract development (embryonic day (E)15, newborn, adult) using special stains: Sirius Red (fibrillary collagens), Masson’s trichrome (collagen), Verhoeff’s Van Gieson (elastic fibres), as well as immunohistochemistry and western blot analysis. Type I and type III collagens are detected in the musculature layer of the ureter and the bladder at E15, and they are also observed in the lamina propria starting at the newborn stage. Collagen fibres grow in length and thickness with age with a preponderance of type I collagen noted in adult mice. Elastic fibres are sparse and interwoven between collagen fibres in the lamina propria and the musculature of the ureter and the bladder in newborn and adult mice. Starting at the newborn stage, the relative amount of fibrillary collagens in the ureter and the bladder of C3H mice is significantly higher than in B6 mice. The musculature of the bladder in C3H mice exhibits degenerative changes at the newborn stage that continue to adulthood. While there are no qualitative differences in elastic fibres in the ureter and bladder of C3H vs. B6 mice, quantitative analysis is ongoing. The increase in the fibrillary collagen content of the bladder and ureter in C3H mice could result in a stiffer bladder and UVJ that prevents the occlusion of the UVJ and results in VUR.
Conclusion: Our results suggest there is a link between ECM perturbations and VUR.