Distinct mechanisms of apoptosis-induced compensatory proliferation in proliferating and differentiating tissues in the Drosophila eye

Abstract

In multicellular organisms, apoptotic cells induce compensatory proliferation of neighboring cells to maintain tissue homeostasis. In the Drosophila wing imaginal disc, dying cells trigger compensatory proliferation through secretion of the mitogens Decapentaplegic (Dpp) and Wingless (Wg). This process is under control of the initiator caspase Dronc, but not effector caspases. Here we show that a second mechanism of apoptosis-induced compensatory proliferation exists. This mechanism is dependent on effector caspases which trigger the activation of Hedgehog (Hh) signaling for compensatory proliferation. Furthermore, whereas Dpp and Wg signaling is preferentially employed in apoptotic proliferating tissues, Hh signaling is activated in differentiating eye tissues. Interestingly, effector caspases in photoreceptor neurons stimulate Hh signaling which triggers cell-cycle reentry of cells that had previously exited the cell cycle. In summary, dependent on the developmental potential of the affected tissue, different caspases trigger distinct forms of compensatory proliferation in an apparent nonapoptotic function.

Figure 1. GMR-hid induces a wave of compensatory proliferation

Late 3^rd instar (A–D) and mid-3^rd instar (E) eye-antennal imaginal discs. The left round-shaped structure is the antennal disc, and the right oval-shaped structure is the eye disc (see also Figure 3A). In this and all subsequent figures anterior is to the left, white arrowheads indicate the second mitotic wave (SMW), and white double arrowheads indicate the wave of compensatory proliferation induced by GMR-hid. (A,A′) Wild type eye disc labeled with anti-cleaved Caspase-3 antibodies (Cas3*) and BrdU. Posterior to the morphogenetic furrow (MF, yellow arrows), only a few cells are Cas3*-positive. The SMW is formed by a band of BrdU-positive cells. Posterior to the SMW, only a few BrdU-positive cells are visible. (B) Wild type eye disc labeled with PH3 antibodies to visualize mitotic cells. (C,C′) GMR-hid eye disc labeled with Cas3* and BrdU. GMR-hid induces massive cell death in two distinct waves posterior to the MF as indicated by Cas3* staining. A wave of compensatory proliferation is induced between the two apoptotic waves. (D) GMR-hid eye disc labeled with PH3 antibodies. A number of cells are dividing in the wave of compensatory proliferation. (E–E″) Mid-3^rd instar eye disc labeled with Cas3*, BrdU and ELAV labeling photoreceptor neurons. Around 10 columns of ommatidia are present in the disc as indicated by ELAV staining. A wave of compensatory proliferation is induced posterior to the SMW and the wave of dying cells.

Figure 2. Effector caspases DrICE and Dcp-1 are required for GMR-hid-induced compensatory proliferation

(A) Schematic outline of the apoptosis pathway in Drosophila and compensatory proliferation (CP) observed in developing wing discs when cell death is blocked by expression of P35. Activation of the pro-apoptotic genes reaper, hid and grim releases the initiator caspase Dronc from DIAP1 inhibition. Dronc can then proteolytically process the effector caspases DrICE and Dcp-1, inducing apoptosis. Dronc and the dp53 gene coordinate cell death and compensatory proliferation. (B–H) Late 3^rd instar eye-antennal imaginal discs labeled with BrdU. The wave of compensatory proliferation induced by GMR-hid is blocked by overexpression of DIAP1 (B), in dronc mutants (C), and by overexpression of P35 (D), but it persists in p53 mutants (E), drICE single mutants (F), and dcp-1 single mutants (G). The wave of compensatory proliferation is absent in dcp-1;drICE double mutants (H). (I) Number of proliferating cells posterior to the SMW in various genetic backgrounds.

Figure 3. GMR-hid-induced compensatory proliferation occurs in undifferentiated cells and requires a short-range signal

(A) Schematic outline of a late 3^rd instar eye-antennal imaginal disc (top) and its horizontal cross section (bottom, adapted from (Wolff and Ready, 1993)). Anterior is to the left. The antennal disc is indicated by the dashed line. During larval development, the MF (yellow bar) sweeps from posterior to anterior followed by the SMW (magenta bar). Anterior to the MF, proliferating cells (red zone) are followed by a group of G1-arrested cells (white region). Posterior to the SMW, cells are rarely proliferating and begin to differentiate (green zone). As shown in the cross section, posterior to the MF, nuclei of photoreceptor neurons R1-8 (colored) and cone cells (C, orange) move toward the apical side of the eye disc during the process of differentiation. Nuclei of undifferentiated cells (gray) dominate the basal side of eye disc. (B,B′) Late 3^rd instar GMR-hid eye disc labeled with neuronal marker ELAV and BrdU. Horizontal and vertical cross sections, indicated by the green and the red lines, are shown on the right and bottom, respectively. In cross sections, apical surfaces of the eye disc are indicated by yellow lines. BrdU-positive cells do not overlap with Elav-positive photoreceptor cells and they are located at the basal side of eye discs. (C,D–D″) Late 3^rd instar GMR-hid eye disc labeled with Yan and BrdU. (D) shows an enlarged view of (C). Yan is highly expressed in all undifferentiated cells posterior to the MF. Cells in the wave of compensatory proliferation are Yan-positive (arrows). (E,F–F″) Mosaic eye disc with GMR-hid clones labeled with GFP, BrdU and Cas3* (E). GMR-hid clones (2× GMR-hid) are negatively labeled by GFP. Heterozygous tissue is 1× GMR-hid and apoptotic (yellow arrows). Enlarged views of the outlined region are shown in (F–F″). Cell death and compensatory proliferation are mainly induced within the clone, but a few adjacent wild type cells are also proliferating (white arrows in F,F′).

Figure 4. Effector Caspases trigger Hh expression in photoreceptor neurons

Late 3^rd instar eye discs. GMR-hid clones labeled by lack of GFP. (A,A′) Eye disc labeled with anti-Wg antibodies. The wild type expression of Wg at dorsal and ventral edges of eye discs (arrows) is not altered in GMR-hid mosaic eye discs. (B,B′) Eye disc labeled with a dpp-lacZ reporter which indicates Dpp expression. The wild type expression of Dpp in the MF (yellow arrow) of eye discs is not altered in GMR-hid clones. (C,C′) Eye disc labeled with a hh-lacZ reporter and the neuronal marker ELAV. Compared to surrounding wild type tissues, expression of hh-lacZ is increased in GMR-hid clones (arrows). (D,D′) Enlarged views of (C) also showing a cross section (shown in the bottom right corner) including 3 representative photoreceptor neurons. hh-lacZ expressing cells are ELAV-positive photoreceptor neurons (arrows). (E-G) Eye disc containing GMR-hid clones labeled with anti-Hh antibodies. Enlarged views of the boxed area in (E) are shown as stack of merged apical and basal confocal sections in (F) and as a single section of the basal side of the disc in (G). Hh protein accumulates in GMR-hid clones and increased levels are also seen immediately outside of GMR-hid clones (arrows). (H,H′) GMR-hid clones fail to increase hh-lacZ in the presence of the caspase inhibitor P35. Genotype: GMR-hid FRT19A/P[ubiGFP] FRT19A; GMR-P35/+; EGUF/+ (EGUF ≡ ey-Gal4 UAS-Flp).

Figure 5. Hh signaling is required for GMR-hid-induced compensatory proliferation

(A–C) Late 3^rd instar eye discs labeled with Cas3* and BrdU. In contrast to overexpression of Ci^FL (A), overexpression of Ci⁷⁶ (B) or Ci^Ce (C) strongly reduces the compensatory proliferation in GMR-hid eye discs. GhGCi ≡ GMR-hid GMR-Gal4 UAS-ci. (D) Quantification of compensatory proliferation as indicated by number of proliferating cells posterior to the SMW in various genetic backgrounds. (E, F) Late 3^rd instar eye discs labeled with Cas3* and BrdU incubated at 18°C. In contrast to the heterozygous hh^ts2 control (E), GMR-hid-induced compensatory proliferation is strongly reduced in homozygous hh^ts2 mutants (F). Cell death is increased anterior to the MF (arrows in F). Gh ≡ GMR-hid. (G, H) Late 3^rd instar eye discs labeled with GFP and BrdU. smo³ clones (boundaries are outlined) labeled by lack of GFP. The SMW is delayed in a big smo³ clone (yellow arrows in G), while the wave of compensatory proliferation is disrupted in smo³ clones (orange arrows in G and H) with sometimes a few BrdU-positive cells (white arrows in H) remaining adjacent to the wild type tissue.

Figure 6. Expression of Dpp and Wg is induced in apoptotic proliferating eye tissue

Late 3^rd instar eye-antennal imaginal discs. ey-Gal4 drives expression of the indicated UAS-based transgenes in the anterior part of the eye disc. Mitotically active cells are labeled with PH3 which was also used to identify the SMW (yellow dashed lines in D-I). ey-p35 ≡ ey-Gal4 UASp-P35 (control discs); ey-hid-p35 ≡ ey-Gal4 UAS-hid UASp-P35 (A) ey-p35 control disc labeled with PH3 and BrdU. The boundary between antennal disc and eye disc is indicated by a dotted line (see also Figure 3A). Proliferation in the anterior part of the eye disc and in the SMW is similar to wild type eye discs (compare to Figures 1A,B). (B) ey-hid-p35 disc labeled with PH3 and BrdU. Compared to control discs (A), the anterior proliferating tissue displays overgrowth with broadly increased BrdU- and PH3-positive cells (arrows). The progression of MF is delayed or hindered as indicated by the arch-like SMW (arrowhead) and the reduced size of differentiating tissue posterior to the SMW. (C) Size measurement of various portions of ey-p35 (n=12) and ey-hid-p35 (n=15) eye-antennal discs. Asterisks indicate those portions that give a statistically significant size difference compared to controls. Although antennal discs show comparable size, anterior ey-hid-p35 eye discs are on average 61% (p<0.0001) larger than anterior ey-p35 discs due to compensatory proliferation. In contrast, posterior ey-hid-p35 eye discs are on average 26% smaller than posterior ey-p35 discs (p<0.002). (D) ey-p35 control disc labeled with PH3 and hh-lacZ. (E,E′) ey-hid-p35 eye disc labeled with PH3 and hh-lacZ. Compared to control discs (D), expression of Hh is not ectopically induced in anterior proliferating tissue although disorganization of photoreceptors (arrows) is sometimes observed. (F) ey-p35 control disc labeled with PH3 and Wg. (G,G′) ey-hid-p35 eye disc labeled with PH3 and Wg. Compared to control discs (F), expression of Wg is ectopically induced in the overgrown tissue (arrows) anterior to the SMW. (H) ey-p35 control disc labeled with PH3 and dpp-lacZ. (I,I′) ey-hid-p35 eye disc labeled with PH3 and dpp-lacZ. Compared to control discs (H), expression of dpp-lacZ is ectopically expressed in the overgrown tissue (arrows) anterior to the SMW.

Figure 7. Model of apoptosis-induced compensatory proliferation in proliferating and differentiating tissues

In proliferating tissues such as larval wing discs and the anterior part of larval eye discs, the non-apoptotic function of Dronc induces compensatory proliferation through the activation of Wg and Dpp signaling cascades. dp53 gene is also involved in this process. In differentiating tissues such as the posterior part of larval eye discs, effector caspases DrICE and Dcp-1 play key roles to induce compensatory proliferation through the activation of Hh signaling.