Genetic ablation of one or more uroplakin genes in mice causes sellckchem severe retrograde vesicoureteral reflux, hydronephrosis, and renal failure, conditions that mirror certain human congenital diseases [1]. The ureter and trigone region of the bladder are also lined by a transitional urothelium thought to be derived from the mesoderm [19]. This is in contrast to the rest of the bladder and prostatic urethra which are patterned from the definitive endoderm, but these tissues are presumed to perform similar functions [20]. Vitamin A-mediated signaling pathways have been implicated in critical processes involved in the formation of the bladder from the urogenital sinus as well as the maintenance of a differentiated urothelial phenotype.

Batourina and colleagues demonstrated that vitamin-A-rescued transgenic mice deficient in retinaldehyde dehydrogenase-2, an enzyme crucial for embryonic retinoic acid synthesis, exhibited a high rate of urogenital sinus abnormalities ranging in severity from bladder hypoplasia to almost complete bladder agenesis in which only a rudimentary urogenital sinus was formed [21]. These defects were found to be similar to malformations previously reported in vitamin A deficiency studies in rats [22]. Vitamin A deficiency can also induce keratinizing squamous metaplasia within the rodent bladder urothelium wherein the normal transitional epithelial structure becomes morphologically similar to the epidermis [23].

The major biologically active derivative of vitamin A, all trans retinoic acid (RA), is known to promote specification of a number of endodermal phenotypes during development including lung [24], kidney [25], intestine [26], and pancreas [27], however its role in urothelium formation is currently unknown. Specific target genes of RA signaling networks such as the GATA family of zinc finger transcription factors including GATA4/6 have been shown to participate in the maturation of extra-embryonic endoderm derivatives [28], [29] as well as cardiac [30] and bronchial epithelial [31] specification. In situ hybridization studies have demonstrated that GATA factors are also highly expressed in the urogenital ridge during bladder formation [32]. These results suggest that these proteins may be targets of retinoid signaling networks which contribute to urothelial lineage progression and ultimately play key roles in bladder specification and maturation.

In vitro models of bladder urothelial specification from embryonic precursors have the potential to generate novel cell sources for tissue engineering applications as well as serve as models of development. In the present study, we demonstrate the ability of all trans retinoic acid (RA) to induce lineage progression of murine Anacetrapib embryonic stem cells (ESC) toward urothelial fate(s) in vitro.