Dronc and LIMK1 drive F-actin polymerization and tissue overgrowth via Myo1D.

(A, A’) Apical side images of a late 3rd instar wing discs with 48-hour old mosaic clones, positively marked by GFP and labelled with PHN (magenta in A and grey in A’). Expression of Myo1DRNAi suppresses F-actin accumulation in hid-p35 clones (compared to Fig 4C and 4C’). (B) Quantification of the F-actin signal intensity in the representative clones of the indicated genotypes in the wing disc pouches. Compared to the hid-p35 clones, expression of Myo1DRNAi significantly (****p hid-p35 clones. (C, C’) Images of Z-stack projections of a late 3rd instar wing discs with 48-hour old mosaic clones, positively marked by GFP and labelled with MMP1 (magenta in C and grey in C’). Expression of Myo1DRNAi reduces the MMP1 level in hid-p35 clones (compared to Fig 5B and 5B’). (D) Quantification of the MMP1 signal intensity in the representative wing disc pouches with clones of the indicated genotypes. Expression of Myo1DRNAi significantly (****p hid-p35 clones. (E, F) Late 3rd instar eye discs of the indicated genotypes labelled with PHN. Expression of LIMK1 enhances the overgrowth phenotype in ey>hid-p35 by inducing further tissue outgrowth (E, arrow). This overgrowth phenotype is suppressed by expression of Myo1DRNAi (F). (G, H) Representative adult fly head images of the indicated genotypes. Expression of LIMK1 in ey>hid-p35 results in semi-lethality with escapers all showing severe overgrowth of head capsules and loss of eye tissues (G). These phenotypes are completed suppressed by expression of Myo1DRNAi (H). (I) Model summarizing roles of F-actin polymerization and its regulation in AiP. The data presented in this paper suggested that F-actin polymerization occurs in apoptotic signal-producing cells and is required for production of the cytosolic ROS (cROS) and its subsequent activation of JNK leading to AiP via secreting growth signals such as Dpp/TGF-β, Wg/Wnt and Spi/EGF. In response to apoptotic stress, Dronc may interact with LIMK1 via Myo1D to trigger F-actin polymerization and the initial activation of JNK. Moreover, Myo1D also mediates activation of Duox and production of the extracellular ROS (eROS) [21]. eROS work together with JNK to disrupt the basement membrane and attract hemocytes, the Drosophila macrophages, to further promote JNK activation non-cell autonomously via secretion of the ligand Eiger (Egr)/TNF [20, 22]. We propose that these mechanisms, indicated by the dashed arrows, amplifies LIMK1-mediated JNK activation leading to tissue overgrowth.