Retinoic acid signaling maintains epithelial and mesenchymal progenitors in the developing mouse ureter

Abstract

The differentiated cell types of the mature ureter arise from the distal ureteric bud epithelium and its surrounding mesenchyme. Uncommitted epithelial cells first become intermediate cells from which both basal and superficial cells develop. Mesenchymal progenitors give rise to separated layers of adventitial fibrocytes, smooth muscle cells and lamina propria fibrocytes. How progenitor expansion and differentiation are balanced is poorly understood. Here, we addressed the role of retinoic acid (RA) signaling in these programs. Using expression analysis of components and target genes, we show that pathway activity is restricted to the mesenchymal and epithelial progenitor pools. Inhibition of RA signaling in ureter explant cultures resulted in tissue hypoplasia with a relative expansion of smooth muscle cells at the expense of lamina propria fibroblasts in the mesenchyme, and of superficial cells at the expense of intermediate cells in the ureteric epithelium. Administration of RA led to a slight reduction of smooth muscle cells, and almost completely prevented differentiation of intermediate cells into basal and superficial cells. We identified cellular programs and transcriptional targets of RA signaling that may account for this activity. We conclude that RA signaling is required and sufficient to maintain mesenchymal and epithelial progenitors in early ureter development.

Delayed onset of smooth muscle cell differentiation leads to hydroureter formation in mice with conditional loss of the zinc finger transcription factor gene Gata2 in the ureteric mesenchyme

“…These changes correlated with an increase of the RA target gene Rarb at E12.5 (1.6‐fold) and E14.5 (1.2‐fold) (Figure B,C and supplementary material, Table S10). Comparing the list of genes up‐regulated in the Gata2cKO microarray ( n = 219) with the list of genes up‐regulated in ureters treated with RA ( n = 548) that we established recently , we found that a highly significant ( p < 8.91 × 10 −18 ) number of genes ( n = 30) were common to the two lists (Table ), including Cd83 , Ecm1 , Hey2 , and Pou3f1 , whose increased expression in Gata2‐ deficient UM we had already confirmed by in situ hybridisation. This suggests that increased RA signalling contributes to transcriptional changes in Gata2cKO ureters and that reduced degradation of RA at least partly underlies this phenomenon.…”

“…Recently, we profiled by microarray analysis the transcriptional changes caused by manipulation of RA signalling in explant cultures of E12.5 mouse ureters. Amongst the 228 genes that were positively regulated by RA signalling was the transcription factor gene Gata2 . Here, RNA in situ hybridisation revealed a dynamic pattern of Gata2 expression in ureter development (Figure A,B).…”

“…To identify molecular changes that may cause defective SMC differentiation in Gata2cKO ureters, we screened (using in situ hybridisation) for activity of pathways and expression of genes that have been implicated in this programme: Ptch1 , target of SHH signalling ; Bmp4 ; Id2 , target of BMP signalling ; Axin2 , target of WNT signalling ; Rarb , target of RA signalling ; and the transcription factor genes Foxf1 , Tbx18 , Tszh3 , and Sox9 . At E14.5, all of these genes were expressed in the inner UM of mutant ureters as in the control except for Tbx18 , Tshz3 , and Rarb , which were up‐regulated (supplementary material, Figure S9).…”

“…Gata2 expression in the UM coincides with the temporal profile of RA signalling . Pharmacological inhibition of RA signalling largely abrogated Gata2 expression, whereas addition of RA increased Gata2 levels in the UM, identifying RA signalling as the major regulatory input for Gata2 expression in this tissue.…”

“…Interestingly, RA signalling is switched off at E14.5, i.e. slightly before the onset of the SMC differentiation programme, arguing for factors that tightly control the temporal limit of its activity .…”