An Unexpected Link Between Notch Signaling and ROS in Restricting the Differentiation of Hematopoietic Progenitors in Drosophila

Abstract

A fundamental question in hematopoietic development is how multipotent progenitors achieve precise identities, while the progenitors themselves maintain quiescence. In Drosophila melanogaster larvae, multipotent hematopoietic progenitors support the production of three lineages, exhibit quiescence in response to cues from a niche, and from their differentiated progeny. Infection by parasitic wasps alters the course of hematopoiesis. Here we address the role of Notch (N) signaling in lamellocyte differentiation in response to wasp infection. We show that Notch activity is moderately high and ubiquitous in all cells of the lymph gland lobes, with crystal cells exhibiting the highest levels. Wasp infection reduces Notch activity, which results in fewer crystal cells and more lamellocytes. Robust lamellocyte differentiation is induced even in N mutants. Using RNA interference knockdown of N, Serrate, and neuralized (neur), and twin clone analysis of a N null allele, we show that all three genes inhibit lamellocyte differentiation. However, unlike its cell-autonomous function in crystal cell development, Notch's inhibitory influence on lamellocyte differentiation is not cell autonomous. High levels of reactive oxygen species in the lymph gland lobes, but not in the niche, accompany N RNAi -induced lamellocyte differentiation and lobe dispersal. Our results define a novel dual role for Notch signaling in maintaining competence for basal hematopoiesis: while crystal cell development is encouraged, lamellocytic fate remains repressed. Repression of Notch signaling in fly hematopoiesis is important for host defense against natural parasitic wasp infections. These findings can serve as a model to understand how reactive oxygen species and Notch signals are integrated and interpreted in vivo.

The EBF transcription factor Collier directly promotes Drosophila blood cell progenitor maintenance independently of the niche

“…Thus, we suggest that the PSC does not function as a niche required for blood cell progenitor maintenance. Several lines of evidence supporting the niche function of the PSC are based on ectopic or overexpression of various proteins in the PSC to alter its size and activity (11,13,15,17,21,29). These manipulations may not always reflect the real function of the PSC as they can cause sustained activation of abnormal programs that indirectly impinge on prohemocytes.…”

Reactive Oxygen Species Affect Transglutaminase Activity and Regulate Hematopoiesis in a Crustacean

“…In addition, increased levels of ROS have induced the differentiation of hematopoietic stem cells in Drosophila and mammals (21,22). In Drosophila, high ROS levels were detected in hematopoietic progenitor cells, and increased ROS levels were detected in the posterior signaling center of the lymph gland after a wasp infestation (23). In the fly, ROS serve as an important signal for progenitor cell differentiation into lamellocytes, a hemocyte type that encapsulates wasp eggs (24,25).…”

The Genetics of Immunity

“…Infection by parasitoid wasps also results in proliferation of lamellocytes in the larval lymph gland. Unexpectedly, Small et al (2014) found that Notch signaling regulates this lamellocyte differentiation. It appears that Notch signaling acts in a non-cell-autonomous manner in the lymphatic organ to hold lamelloctye precursor cells in quiescence, but inhibition of Notch by RNAi or parasitoid infection allows lamellocyte differentiation to proceed.…”

“…Tiller and Garsin (2014) identified a novel peroxidase, skpo1, that controls ROS production in the C. elegans hypodermal epithelia and correspondingly determines defense against infection by the bacterium Enterococcus faecalis. Small et al (2014) found an unexpected pleiotropy between ROS production and immune cell differentiation mediated by Notch signaling in Drosophila. Notch signaling promotes the embryonic differentiation of crystal cells, which are responsible for ROS production in larvae attacked by parasitoid wasps.…”