Circulating blood cells function as a surveillance system for damaged tissue in Drosophila larvae.

Insects have an open circulatory system in which the heart pumps blood (hemolymph) into the body cavity, where it directly bathes the internal organs and epidermis. The blood contains free and tissue-bound immune cells that function in the inflammatory response. Here, we use live imaging of transgenic Drosophila larvae with fluorescently labeled blood cells (hemocytes) to investigate the circulatory dynamics of larval blood cells and their response to tissue injury. We find that, under normal conditions, the free cells rapidly circulate, whereas the tissue-bound cells are sessile. After epidermal wounding, tissue-bound cells around the wound site remain sessile and unresponsive, whereas circulating cells are rapidly recruited to the site of damage by adhesive capture. After capture, these cells distribute across the wound, appear phagocytically active, and are subsequently released back into circulation by the healing epidermis. The results demonstrate that circulating cells function as a surveillance system that monitors larval tissues for damage, and that adhesive capture, an important mechanism of recruitment of circulating cells to inflammatory sites in vertebrates, is shared by insects and vertebrates despite the vastly different architectures of their circulatory systems.

A targeted UAS-RNAi screen in Drosophila larvae identifies wound closure genes regulating distinct cellular processes.

Robust mechanisms for tissue repair are critical for survival of multicellular organisms. Efficient cutaneous wound repair requires the migration of cells at the wound edge and farther back within the epidermal sheet, but the genes that control and coordinate these migrations remain obscure. This is in part because a systematic screening approach for in vivo identification and classification of postembryonic wound closure genes has yet to be developed. Here, we performed a proof-of-principle reporter-based in vivo RNAi screen in the Drosophila melanogaster larval epidermis to identify genes required for normal wound closure. Among the candidate genes tested were kinases and transcriptional mediators of the Jun N-terminal kinase (JNK) signaling pathway shown to be required for epithelial sheet migration during development. Also targeted were genes involved in actin cytoskeletal remodeling. Importantly, RNAi knockdown of both canonical and noncanonical members of the JNK pathway caused open wounds, as did several genes involved in actin cytoskeletal remodeling. Our analysis of JNK pathway components reveals redundancy among the upstream activating kinases and distinct roles for the downstream transcription factors DJun and DFos. Quantitative and qualitative morphological classification of the open wound phenotypes and evaluation of JNK activation suggest that multiple cellular processes are required in the migrating epidermal cells, including functions specific to cells at the wound edge and others specific to cells farther back within the epidermal sheet. Together, our results identify a new set of conserved wound closure genes, determine putative functional roles for these genes within the migrating epidermal sheet, and provide a template for a broader in vivo RNAi screen to discover the full complement of genes required for wound closure during larval epidermal wound healing.