Feedback regulation of steady-state epithelial turnover and organ size

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Steady-state turnover of the Drosophila midgut arises through an intercellular, E-cadherin-EGFR relay that couples the death of individual enterocytes to the divisions of nearby stem cells.Epithelial organs undergo steady-state turnover throughout adult life, with old cells being continually replaced by the progeny of stem cell divisions1. To avoid hyperplasia or atrophy, organ turnover demands strict equilibration of cell production and loss2,3,4. However, the mechanistic basis of this equilibrium is unknown. Here we show that robustly precise turnover of the adult Drosophila intestine5 arises through a coupling mechanism in which enterocyte apoptosis breaks feedback inhibition of stem cell division. Healthy enterocytes inhibit stem cell division through E-cadherin, which prevents secretion of mitogenic epidermal growth factors (EGFs) by repressing transcription of the EGF maturation factor rhomboid. Individual apoptotic enterocytes promote divisions by loss of E-cadherin, which releases cadherin-associated β-catenin (Armadillo in Drosophila) and p120-catenin to induce rhomboid. Induction of rhomboid in the dying enterocyte triggers activation of the EGF receptor (Egfr) in stem cells within a discrete radius. When we blocked apoptosis, E-cadherin-controlled feedback suppressed divisions, and the organ retained the same number of cells. When we disrupted feedback, apoptosis and divisions were uncoupled, and the organ developed either hyperplasia or atrophy. Together, our results show that robust cellular balance hinges on the obligate coupling of divisions to apoptosis, which limits the proliferative potential of a stem cell to the precise time and place at which a replacement cell is needed. In this way, localized cell-cell communication gives rise to tissue-level homeostatic equilibrium and constant organ size.

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