Keratin 5 Basal Cells are Temporally Regulated Developmental and Tissue Repair Progenitors in Bladder Urothelium.

Urothelium forms a distensible yet impermeable barrier, senses and transduces stimuli, and defends the urinary tract from mechanical, chemical and bacterial injuries. Biochemical and genetic labeling studies support the existence of one or more progenitor populations with the capacity to rapidly regenerate the urothelium following injury, but slow turnover, a low mitotic index, and inconsistent methodologies obscure progenitor identity. The progenitor properties of basal Keratin 5 urothelial cells (K5-UC) have been previously investigated, but those studies focused on embryonic or adult bladder urothelium. Urothelium undergoes desquamation and apoptosis after birth, which requires postnatal proliferation and restoration. Therefore, we mapped the fate of bladder K5-UCs across postnatal development/maturation and following administration of cyclophosphamide to measure homeostatic and reparative progenitor capacities, respectively. In vivo studies demonstrate that basal K5-UCs are age-restricted progenitors in neonates and juveniles, but not in adult mice. Neonatal K5-UCs retain a superior progenitor capacity in vitro, forming larger and more differentiated urothelial organoids than adult K5-UCs. Accordingly, K5-UC transcriptomes are temporally distinct, with enrichment of transcripts associated with cell proliferation and differentiation in neonates. Induction of urothelial proliferation is sufficient to restore adult K5-UC progenitor capacity. Our findings advance the understanding of urothelial progenitors and support a linear model of urothelial formation and regeneration, which may have significant impact on therapeutic development or tissue engineering strategies. Given the distinct progenitor capacities observed by neonatal and adult K5-UCs, we sorted K5-UCs from P7 and P35 bladders and used RNAseq to determine whether functional differences could be explained by gene expression.