CO2 exposure drives a rapid pH response in live adult Drosophila.

CO2 anesthesia is the most common method for immobilizing Drosophila for research purposes. But CO2 exposure has consequences-it can impact fertility, behavior, morphogenesis, and cytoskeletal dynamics. In this respect, Drosophila is an outstanding model for studying the impact of CO2 exposure on tissues. In this study we explored the response of intracellular pH (pHi) to a one-minute CO2 pulse using a genetically encoded, ubiquitously expressed pH sensor, tpHusion, to monitor pHi within a live, intact, whole fly. We compared wild-type flies to flies lacking Imaginal disc growth factors (Idgfs), which are chitinase-like proteins that facilitate developmental processes and the innate immune response. Morphogenetic and cytoskeletal defects in Idgf-null flies are enhanced after CO2 exposure. We found that pHi drops sharply within seconds of the beginning of a CO2 pulse and recovers over several minutes. The initial profile was nearly identical in control and Idgf-null flies but diverged as the pHi returned to normal. This study demonstrates the feasibility of monitoring pH in live adult Drosophila. Studies exploring pH homeostasis are important for understanding human pathologies associated with pH dysregulation.

Imaginal disk growth factors are Drosophila chitinase-like proteins with roles in morphogenesis and CO2 response.

Chitinase-like proteins (CLPs) are members of the family 18 glycosyl hydrolases, which include chitinases and the enzymatically inactive CLPs. A mutation in the enzyme's catalytic site, conserved in vertebrates and invertebrates, allowed CLPs to evolve independently with functions that do not require chitinase activity. CLPs normally function during inflammatory responses, wound healing, and host defense, but when they persist at excessive levels at sites of chronic inflammation and in tissue-remodeling disorders, they correlate positively with disease progression and poor prognosis. Little is known, however, about their physiological function. Drosophila melanogaster has 6 CLPs, termed Imaginal disk growth factors (Idgfs), encoded by Idgf1, Idgf2, Idgf3, Idgf4, Idgf5, and Idgf6. In this study, we developed tools to facilitate characterization of the physiological roles of the Idgfs by deleting each of the Idgf genes using the CRISPR/Cas9 system and assessing loss-of-function phenotypes. Using null lines, we showed that loss of function for all 6 Idgf proteins significantly lowers viability and fertility. We also showed that Idgfs play roles in epithelial morphogenesis, maintaining proper epithelial architecture and cell shape, regulating E-cadherin and cortical actin, and remarkably, protecting these tissues against CO2 exposure. Defining the normal molecular mechanisms of CLPs is a key to understanding how deviations tip the balance from a physiological to a pathological state.

Proteomics Analysis Identifies Orthologs of Human Chitinase-Like Proteins as Inducers of Tube Morphogenesis Defects in Drosophila melanogaster.

Elevated levels of human chitinase-like proteins (CLPs) are associated with numerous chronic inflammatory diseases and several cancers, often correlating with poor prognosis. Nevertheless, there is scant knowledge of their function. The CLPs normally mediate immune responses and wound healing and, when upregulated, they can promote disease progression by remodeling tissue, activating signaling cascades, stimulating proliferation and migration, and by regulating adhesion. We identified Imaginal disc growth factors (Idgfs), orthologs of human CLPs CHI3L1, CHI3L2, and OVGP1, in a proteomics analysis designed to discover factors that regulate tube morphogenesis in a Drosophila melanogaster model of tube formation. We implemented a novel approach that uses magnetic beads to isolate a small population of specialized ovarian cells, cells that nonautonomously regulate morphogenesis of epithelial tubes that form and secrete eggshell structures called dorsal appendages (DAs). Differential mass spectrometry analysis of these cells detected elevated levels of four of the six Idgf family members (Idgf1, Idgf2, Idgf4, and Idgf6) in flies mutant for bullwinkle (bwk), which encodes a transcription factor and is a known regulator of DA-tube morphogenesis. We show that, during oogenesis, dysregulation of Idgfs (either gain or loss of function) disrupts the formation of the DA tubes. Previous studies demonstrate roles for Drosophila Idgfs in innate immunity, wound healing, and cell proliferation and motility in cell culture. Here, we identify a novel role for Idgfs in both normal and aberrant tubulogenesis processes.

Optimized RNA ISH, RNA FISH and protein-RNA double labeling (IF/FISH) in Drosophila ovaries.

In situ hybridization (ISH) is a powerful technique for detecting nucleic acids in cells and tissues. Here we describe three ISH procedures that are optimized for Drosophila ovaries: whole-mount, digoxigenin-labeled RNA ISH; RNA fluorescent ISH (FISH); and protein immunofluorescence (IF)-RNA FISH double labeling (IF/FISH). Each procedure balances conflicting requirements for permeabilization, fixation and preservation of antigenicity to detect RNA and protein expression with high resolution and sensitivity. The ISH protocol uses alkaline phosphatase-conjugated digoxigenin antibodies followed by a color reaction, whereas FISH detection involves tyramide signal amplification (TSA). To simultaneously preserve antigens for protein detection and enable RNA probe penetration for IF/FISH, we perform IF before FISH and use xylenes and detergents to permeabilize the tissue rather than proteinase K, which can damage the antigens. ISH and FISH take 3 d to perform, whereas IF/FISH takes 5 d. Probe generation takes 1 or 2 d to perform.

Destruction complex function in the Wnt signaling pathway of Drosophila requires multiple interactions between Adenomatous polyposis coli 2 and Armadillo.

The tumor suppressor Adenomatous polyposis coli (APC) negatively regulates Wnt signaling through its activity in the destruction complex. APC binds directly to the main effector of the pathway, ß-catenin (ßcat, Drosophila Armadillo), and helps to target it for degradation. In vitro studies demonstrated that a nonphosphorylated 20-amino-acid repeat (20R) of APC binds to ßcat through the N-terminal extended region of a 20R. When phosphorylated, the phospho-region of an APC 20R also binds ßcat and the affinity is significantly increased. These distinct APC-ßcat interactions suggest different models for the sequential steps of destruction complex activity. However, the in vivo role of 20R phosphorylation and extended region interactions has not been rigorously tested. Here we investigated the functional role of these molecular interactions by making targeted mutations in Drosophila melanogaster APC2 that disrupt phosphorylation and extended region interactions and deletion mutants missing the Armadillo binding repeats. We tested the ability of these mutants to regulate Wnt signaling in APC2 null and in APC2 APC1 double-null embryos. Overall, our in vivo data support the role of phosphorylation and extended region interactions in APC2's destruction complex function, but suggest that the extended region plays a more significant functional role. Furthermore, we show that the Drosophila 20Rs with homology to the vertebrate APC repeats that have the highest affinity for ßcat are functionally dispensable, contrary to biochemical predictions. Finally, for some mutants, destruction complex function was dependent on APC1, suggesting that APC2 and APC1 may act cooperatively in the destruction complex.

Cortical localization of APC2 plays a role in actin organization but not in Wnt signaling in Drosophila.

The tumor suppressor Adenomatous polyposis coli (APC) has roles in both Wnt signaling and in actin and microtubule organization. Within the cell, APC proteins have been reported to localize in the cytoplasm, at the cell cortex and in the nucleus. How these localizations relate to the functions of the protein is an aspect of APC biology that is poorly understood. Using Drosophila S2 cells, we have dissected the structural and functional requirements for the cortical localization of Drosophila APC2. Here, we show that both the Armadillo repeats and a novel C-terminal domain are necessary for the cortical localization of APC2 in S2 cells and in the embryo, and that neither domain alone is sufficient for this localization. Furthermore, we show that the Armadillo repeats mediate self-association of APC2 molecules. To test the function of the cortical localization of APC2, we asked whether an APC2 protein deleted for the C-terminal localization domain could rescue APC mutant defects in Wnt signaling and actin organization in the Drosophila embryo. We show that although cortical localization is required for the APC2 function in organizing actin, cortical localization is dispensable for its role in regulating Wnt signaling.

Apical constriction and invagination downstream of the canonical Wnt signaling pathway require Rho1 and Myosin II.

The tumor suppressor Adenomatous polyposis coli (APC) is a negative regulator of Wnt signaling and functions in cytoskeletal organization. Disruption of human APC in colonic epithelia initiates benign polyps that progress to carcinoma following additional mutations. The early events of polyposis are poorly understood, as is the role of canonical Wnt signaling in normal epithelial architecture and morphogenesis. To determine the consequences of complete loss of APC in a model epithelium, we generated APC2 APC1 double null clones in the Drosophila wing imaginal disc. APC loss leads to segregation, apical constriction, and invagination that result from transcriptional activation of canonical Wnt signaling. Further, we show that Wnt-dependent changes in cell fate can be decoupled from Wnt-dependent changes in cell shape. Wnt activation is reported to upregulate DE-cadherin in wing discs, and elevated DE-cadherin is thought to promote apical constriction. We find that apical constriction and invagination of APC null tissue are independent of DE-cadherin elevation, but are dependent on Myosin II activity. Further, we show that disruption of Rho1 suppresses apical constriction and invagination in APC null cells. Our data suggest a novel link between canonical Wnt signaling and epithelial structure that requires activation of the Rho1 pathway and Myosin II.