Model systems for regeneration: Drosophila

Abstract

Drosophila melanogaster has historically been a workhorse model organism for studying developmental biology. In addition, Drosophila is an excellent model for studying how damaged tissues and organs can regenerate. Recently, new precision approaches that enable both highly targeted injury and genetic manipulation have accelerated progress in this field. Here, we highlight these techniques and review examples of recently discovered mechanisms that regulate regeneration in Drosophila larval and adult tissues. We also discuss how, by applying these powerful approaches, studies of Drosophila can continue to guide the future of regeneration research.

Keywords: Drosophila; Germline; Imaginal disc; Intestine; Regeneration.

Fig. 1.

Experimental tools for identifying molecular regulators of regeneration. Many transgene systems in Drosophila employ a transgene ‘driver’ that directs transgene expression, such as an overexpression construct or a hairpin encoding a double-stranded RNA for gene knockdown, under the control of a tissue-specific promoter. The driver can also be inhibited until the desired time of expression. Transgene expression systems can be manipulated in numerous ways, including via temperature change, mosaic cassette flipping, or by feeding. These manipulations turn on the transgenes of interest and/or cause cell death. Combinations of various approaches have been employed in precision injury systems, as shown in each panel and described in detail in the text. For each system, a pre-injury, injury, and recovery state is shown, using the manipulations outlined in the figure key. For simplicity, all cells are drawn to the same scale. (A) Through use of the TARGET system, one can achieve temporally controlled cell ablation via the temperature-sensitive repressor Gal80^ts (used to control Gal4-mediated expression of both a cell death-inducing transgene and an RNAi transgene of interest), and through the shadow RNAi effect can also achieve persistent gene knockdown. (B,C) LexA (LexAop-Apoptosis) or the Q-system (QF/QUAS toxins) can be combined with the Gal4 system to manipulate tissue injury and transgene expression separately. Note that for QF/QUAS toxins, as described in the text, the system can be adapted to be activated by either temperature (to control activation of a temperature-sensitive DTA toxin) or feeding (to deliver CryA toxin). (D) The DEMISE system relies on Gal4 to drive both cell death and transgene expression, but death is limited to those cells that also express FLP.

Fig. 2.

Regenerative responses in larval Drosophila imaginal discs. Regeneration in the wing imaginal disc. (A) A tissue-level view of regeneration. Injury leads to the formation of a regeneration blastema (white), followed by full regeneration through cell proliferation. (B) During normal development, the wing imaginal disc does not appear to have stem cells or a lineage hierarchy aside from compartment restrictions. (C) During regeneration, wing cells lose commitment to particular cell fates, such as pro-vein or intervein fates, and can contribute to regeneration of multiple cell fates.

Fig. 3.

Regenerative responses in adult Drosophila tissues. (A) Two key regeneration models – the midgut and the germline – and their responses are shown. Note that only the male is shown for the germline. Both tissues can recover from acute cell loss in specific contexts (see text for details). (B) Homeostatic renewal in the midgut epithelium. (C-F) Distinct regeneration concepts revealed in these tissues are shown. In the midgut, regeneration mechanisms include: (C) accelerated homeostatic renewal and asymmetric division of intestinal stem cells; (D) symmetric division of ISCs to expand the stem cell pool; and (E) amitosis of differentiating enterocytes. Note that amitosis is a newly proposed mechanism of regeneration observed in a specific midgut region under severe starvation conditions. (F) In the germline, regeneration is accomplished following acute depletion of germline stem cells through the de-differentiation of early-stage germ cells back into stem cells through niche occupancy. The key indicates distinct cell states that participate in adult tissue regeneration. For simplicity, all cells are drawn to the same scale.