Frazzled/DCC directs spatial progenitor integration ensuring steady-state intestinal turnover.

Adult epithelial organs undergo continual steady-state turnover that is achieved by tight coupling of stem cell production with replacement of worn-out epithelial cells by local intercellular signalling(1,2). Like many eukaryotic epithelia, absorptive enterocytes (EC) of the adult Drosophila midgut are arranged in a hexagonal, honeycomb-like pattern. On tricellular nexuses of EC, intestinal stem cells (ISC) are scattered in a way so that around two thirds of EC can be renewed directly by adjacent ISC. However, the mechanism for replacement of the remaining third of remotely located EC is unknown.Here, we show that a conserved axonal guidance cue directs enteroblasts (EB), the immediate ISC daughters, to selectively replace worn-out adjacent and remote EC with identical frequency. Worn-out EC express Netrin-B ligands that attract Frazzled/DCC-receptor dependent EB protrusions and subsequent EB migration towards the Netrin-B expressing EC. Our newly developed 'Hamelin' assay confirms Frazzled-dependent EB migration towards Netrin-B sources and hints to invasive progenitor behaviour as midgut progenitors cross the organ boundary into the hindgut. Together, we establish spatially directed EB migration and integration as essential for intestinal homeostasis and provide first mechanistic support for recent findings resuscitating conserved Netrins and Frazzled/DCC-signalling as therapeutic target in metastasis.