Lineage tradeoffs during injury-accelerated intestinal cell differentiation

About This Grant

PROJECT SUMMARY Barrier epithelia face continual damage from environmental insults, and successful injury repair is crucial for organismal health. A prime case study is the one-cell-thick intestinal epithelium, which forms a leakproof bar- rier between the gut lumen and the body cavity. To replace damaged cells, the intestine mobilizes stem cells to divide rapidly; to restore intestinal form and function, these new daughter cells must also differentiate rapidly. Indeed, injury-born intestinal cells acquire their mature identity twice as fast as their normal counterparts. Using the Drosophila adult intestine, we recently discovered this injury-accelerated differentiation arises through disruption of Notch-Delta lateral inhibition circuitry that normally specifies stem versus terminal fate. During injury, many newly born cells exhibit >10x faster Notch signaling speed, which propels faster intestinal differentiation to restore the breached epithelial barrier. Yet this strategy comes with risks for long-term tissue health: For stem cells, loss of Notch-Delta feedback during injury skews daughter fates toward dead-end, ter- minal:terminal outcomes, which depletes the organ’s stem cells and culminates in stem cell exhaustion. For terminal progeny, accelerated differentiation yields provisional ‘stopgap’ cells—mature cells with digestive and barrier-forming functions but altered morphology and a supercompetitor-like transcriptomic profile. Here, we will investigate how the organ copes with these two tradeoffs. We combine physiological injury of the fly gut with in vivo live imaging and cutting-edge cell lineage tracing to elucidate how these ‘side effects’ of accelerated differentiation are managed at the organ-scale for post-injury tissue homeostasis. The fly gut com- bines conserved intestinal cell lineages, fate signals, and digestive physiology with supreme experimental trac- tability: A single Notch receptor and Delta ligand, an unparalleled wealth of genetic tools, and long-term in vivo live imaging—pioneered by our lab—provide the technical bases for deep mechanistic investigation. Leveraging these strengths, in Aim 1 we will define how some, ‘escaper’ stem cells persist after injury, de- spite disrupted Notch-Delta feedback that should force all cells to differentiate. We will test if escaper stem cells inherit an intracellular Notch inhibitor, autonomously override how Notch and Delta interact, or lose con- tact with their signaling partners via injury-induced epithelial fluidization. In Aim 2, we will ascertain the time- evolution and function of stopgap cells during and after injury. We will determine their ultimate fates in the tis- sue during recovery, e.g., they may evolve into normal cells, arrest in an abnormal state, or simply be shed. We will parse these scenarios using longitudinal live imaging, whole-population analyses, and single-cell tran- scriptomics. Finally, we will examine how stopgap cells shape the tissue post-injury by ‘purging’ unfit, toxin- exposed cells during injury or by exerting selective pressure on new cells during post-injury recovery. By probing these lineage tradeoffs of accelerated cell differentiation during injury, our work will suggest new strategies to promote intestinal regeneration and combat chronic intestinal disease.