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

Abstract

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. Upk1bRFP/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. Upk1bRFP/RFP renal urothelium became stratified with altered cellular composition. Upk1bRFP/RFP mice developed age-dependent progressive hydronephrosis. Interestingly, 16% of Upk1bRFP/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.

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

“…Loss of Upk1b expression in Upk1b RFP/RFP mice is predicted to block Upk1b/3a heterodimer production completely abrogating apical plaque/AUM formation. Our studies confirmed that Upk1b RFP/RFP mice lack Upk1b expression and do not develop apical plaques/AUMs throughout their urinary tract [12]. Interestingly, even though Upk1b is more widely expressed than its other family members (see below), we did not observe any other gross morphological or functional deficits in Upk1b RFP/RFP mice besides those associated with the urinary tract.…”

“…Superficial umbrella cells elaborating a rigid apical plaque that plays a key role in modulating membrane permeability, integrity, homeostasis and host response to pathogens [7–10]. Apical plaques are principally composed of Upk protein particles arranged into 2D crystals (Figure 2) [12]. Upk proteins are highly conserved among mammals and to date five Upk proteins have been identified as constituents of apical plaques [13, 14].…”

“…The novel identification of a duplicated collecting system in approximately 16% of Upk1b RFP/RFP mice indicates that Upk1b plays a fundamental role in normal urinary tract development [12]. The duplicated collecting system was composed of an orthotopic and ectopic ureter that drained the caudal and cranial poles of the kidney, respectively.…”

“…Upk1b ablation leads to cell-autonomous, age-dependent, progressive hydronephrosis in Upk1b RFP/RFP mice in the absence of anatomic obstruction (Figure 4) [12]. The AUM composed of apical plaques is associated with intracellular cytoskeletal elements and the tensile strength of these interactions is hypothesized to stabilize the apical membrane.…”

“…The absence of Upk1b expression and plaque/AUM formation in Upk1b RFP/RFP mice resulted in the lack of terminally differentiated umbrella cells, an increase in cellular proliferation and cell layers, and a shift towards a less mature cellular composition within the urothelium [12]. These changes caused dramatic alterations in urothelial morphology in Upk1b RFP/RFP mice that resulted in the appearance of dysplastic features in both the bladder and kidney (Figure 6) [12].…”