Genetic Screen in Drosophila Larvae Links ird1 Function to Toll Signaling in the Fat Body and Hemocyte Motility

Abstract

To understand how Toll signaling controls the activation of a cellular immune response in Drosophila blood cells (hemocytes), we carried out a genetic modifier screen, looking for deletions that suppress or enhance the mobilization of sessile hemocytes by the gain-of-function mutation Toll10b (Tl10b). Here we describe the results from chromosome arm 3R, where five regions strongly suppressed this phenotype. We identified the specific genes immune response deficient 1 (ird1), headcase (hdc) and possibly Rab23 as suppressors, and we studied the role of ird1 in more detail. An ird1 null mutant and a mutant that truncates the N-terminal kinase domain of the encoded Ird1 protein affected the Tl10b phenotype, unlike mutations that affect the C-terminal part of the protein. The ird1 null mutant suppressed mobilization of sessile hemocytes, but enhanced other Tl10b hemocyte phenotypes, like the formation of melanotic nodules and the increased number of circulating hemocytes. ird1 mutants also had blood cell phenotypes on their own. They lacked crystal cells and showed aberrant formation of lamellocytes. ird1 mutant plasmatocytes had a reduced ability to spread on an artificial substrate by forming protrusions, which may explain why they did not go into circulation in response to Toll signaling. The effect of the ird1 mutation depended mainly on ird1 expression in hemocytes, but ird1-dependent effects in other tissues may contribute. Specifically, the Toll receptor was translocated from the cell membrane to intracellular vesicles in the fat body of the ird1 mutant, and Toll signaling was activated in that tissue, partially explaining the Tl10b-like phenotype. As ird1 is otherwise known to control vesicular transport, we conclude that the vesicular transport system may be of particular importance during an immune response.

Headcase is a Repressor of Lamellocyte Fate in Drosophila melanogaster

“…It was also observed that lamellocytes downregulate hdc expression when leaving the lymph gland [28]. These findings suggest that Hdc may exert a repressive role during hematopoiesis, which is also corroborated by the observation that hdc alleles suppress the disruption of the sessile compartment induced by Toll 10b [62].…”

Drosophilaas a Genetic Model for Hematopoiesis

“…Overexpression of Toll 10B or loss of Jumeau (Jumu), a member of the forkhead transcription factor family, throughout the lymph gland, induces lamellocyte differentiation as well as nuclear translocation of Dif (Dorsal-related immunity factor), indicating a cell autonomous role of Toll activation in lamellocyte differentiation in the lymph gland (Hao and Jin 2017). Loss of ird1 ( immune response-deficient 1 ) enhances lamellocyte formation in Toll 10B mutants and in fact induces lamellocyte formation on its own (Schmid et al 2016). Toll also plays a role in the cellular response to wasp parasitization (Louradour et al 2017).…”

“…During this process, the circulating hemocyte population also increases as the sessile pool is released into the hemolymph (Honti et al 2009). Constitutive activation of Toll signaling in Toll 10B mutants also disrupts the sessile hemocyte pools, a phenotype that is suppressed in an ird1 mutant (Schmid et al 2016).…”

Seven-Up Is a Novel Regulator of Insulin Signaling

“…Another pathway overrepresented in our RNA-seq differential expression data from Svp i fat bodies was the immune response pathway. The Drosophila fat body plays a key role in the expression of antimicrobial peptides and in hemocyte biogenesis during immune activation (Schmid et al 2014(Schmid et al , 2016Vanha-Aho et al 2015;Yang and Hultmark 2016). We found that loss of Svp led to increased expression of immune genes and improved immune function using the septic injury infection model.…”