A SHH-FOXF1-BMP4 signaling axis regulating growth and differentiation of epithelial and mesenchymal tissues in ureter development

The differentiated cell types of the epithelial and mesenchymal tissue compartments of the mature ureter of the mouse arise in a precise temporal and spatial sequence from uncommitted precursor cells of the distal ureteric bud epithelium and its surrounding mesenchyme. Previous genetic efforts identified a member of the Hedgehog (HH) family of secreted proteins, Sonic hedgehog (SHH) as a crucial epithelial signal for growth and differentiation of the ureteric mesenchyme. Here, we used conditional loss- and gain-of-function experiments of the unique HH signal transducer Smoothened (SMO) to further characterize the cellular functions and unravel the effector genes of HH signaling in ureter development. We showed that HH signaling is not only required for proliferation and SMC differentiation of cells of the inner mesenchymal region but also for survival of cells of the outer mesenchymal region, and for epithelial proliferation and differentiation. We identified the Forkhead transcription factor gene Foxf1 as a target of HH signaling in the ureteric mesenchyme. Expression of a repressor version of FOXF1 in this tissue completely recapitulated the mesenchymal and epithelial proliferation and differentiation defects associated with loss of HH signaling while re-expression of a wildtype version of FOXF1 in the inner mesenchymal layer restored these cellular programs when HH signaling was inhibited. We further showed that expression of Bmp4 in the ureteric mesenchyme depends on HH signaling and Foxf1, and that exogenous BMP4 rescued cell proliferation and epithelial differentiation in ureters with abrogated HH signaling or FOXF1 function. We conclude that SHH uses a FOXF1-BMP4 module to coordinate the cellular programs for ureter elongation and differentiation, and suggest that deregulation of this signaling axis occurs in human congenital anomalies of the kidney and urinary tract (CAKUT).
Author summary The mammalian ureter is a simple tube with a specialized multi-layered epithelium, the urothelium, and a surrounding coat of fibroblasts and peristaltically active smooth muscle cells. Besides its important function in urinary drainage, the ureter represents a simple model system to study epithelial and mesenchymal tissue interactions in organ development. The differentiated cell types of the ureter coordinately arise from precursor cells of the distal ureteric bud and its surrounding mesenchyme. How their survival, growth and differentiation is regulated and coordinated within and between the epithelial and mesenchymal tissue compartments is largely unknown. Previous work identified Sonic hedgehog (SHH) as a crucial epithelial signal for growth and differentiation of the ureteric mesenchyme, but the entirety of the cellular functions and the molecular mediators of its mesenchymal signaling pathway have remained obscure. Here we showed that epithelial SHH acts in a paracrine fashion onto the ureteric mesenchyme to activate a FOXF1-BMP4 regulatory module that directs growth and differentiation of both ureteric tissue compartments. HH signaling additionally acts in outer mesenchymal cells as a survival factor. Thus, SHH is an epithelial signal that coordinates various cellular programs in early ureter development.