Polyploidy after tissue injury: a Drosophila model

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Metazoan tissues are diverse in organization and function. This necessitates diverse mechanisms to repair these tissues upon injury. We pioneered study of the Drosophila hindgut (large intestine) to reveal new regulation of diverse tissue injury responses. Using this model, we identified a tissue injury response whereby tissue mass is restored by increasing the DNA content (ploidy) and size of cells that survive injury. This increase in ploidy involves a conserved cell cycle with S phases but no cell division, called the endocycle. Our discovery of endocycles and polyploid cells in tissue repair has been followed by similar discoveries in multiple injured mammalian tissues, including the kidney, bladder, and cornea. Additionally, our work revealed specialized polyploid cell regulation at the injured boundary between the hindgut and adjacent midgut (small intestine). At this boundary, we identified “hybrid” cells of dual hindgut and midgut gene expression. Hybrid cells become polyploid upon injury while engaging in extensive cross-talk with stem cells in the adjacent midgut. However, if the hybrid zone is severely injured, polyploidy is suppressed, and the adjacent midgut stem cells form hyperplastic invasive tumors. Similar hybrid zones have now been discovered at mammalian organ boundaries, notably at the stem cell-enriched, cancer-prone stomach/esophagus boundary. This proposal leverages our expertise, new findings, and the genetically amenable Drosophila model to identify regulation and function of polyploidy after tissue injury. The significance of our proposed work is evident in the conservation of the injury response, the conservation of endocycles in injury, and the conservation of the molecules we study, namely JAK/STAT signaling, Dichaete/SoxB1, and fizzy-related/cdh1. Our studies are innovative because they show that tissues can regenerate by accurately controlling polyploid genome number, that hormonal signaling and Sox transcription cooperate to control injury-induced polyploidy, and that a regenerating polyploid organ boundary can suppress tumorigenesis. In Aim1, we will uncover how injury severity determines the extent of polyploidy during regeneration. To answer this question, we will identify quantitative parameters of JAK/STAT signaling at different injury strengths and identify specific pathway steps that coordinate injury level with endocycle number. Aim2 will identify the molecular mechanism by which endocycles occur after injury instead of mitosis. To answer this question, we will examine how Dichaete, a member of the conserved Sox transcription factor family, cooperates with hormone signaling to promote a switch from injury-induced mitotic cycles to endocycles. Aim3 will determine the origin and function of polyploid hybrid cells following injury. To answer this question, we will distinguish between stem cell-dependent and stem cell-independent models of organ boundary regeneration. Additionally, this aim will reveal the role of the hybrid zone and polyploidy in repressing stem cell tumor invasion. All three aims capitalize on transcriptomic data and our unique precision injury/genetics system to identify novel molecular players in an evolutionarily conserved tissue repair response.