Dual role for Headcase in hemocyte progenitor fate determination in Drosophila melanogaster

The hematopoietic organ of the Drosophila larva, the lymph gland, is a simplified representation of mammalian hematopoietic compartments, with the presence of hemocyte progenitors in the medullary zone (MZ), differentiated hemocytes in the cortical zone (CZ), and a hematopoietic niche called the posterior signaling centre (PSC) that orchestrates progenitor differentiation. Our previous work has demonstrated that the imaginal cell factor Headcase (Hdc, Heca) is required in the hematopoietic niche to control the differentiation of hemocyte progenitors. However, the downstream mechanisms of Hdc-mediated hematopoietic control remained unknown. Here we show that Hdc exerts this function by negatively regulating the insulin/mTOR signaling in the niche. When Hdc is depleted in the PSC, the overactivation of this pathway triggers reactive oxygen species (ROS) accumulation and, in turn, the differentiation of effector lamellocytes non-cell-autonomously. Although overactivation of insulin/mTOR signaling normally leads to an increase in the size of the hematopoietic niche, this effect is concealed by cell death caused by hdc loss-of-function. Moreover, we describe here that hdc silencing in progenitors causes cell-autonomous ROS elevation and JNK pathway activation, resulting in decreased MZ size and differentiation of lamellocytes. Similarly to the PSC niche, knocking down hdc in the MZ also leads to caspase activation. Notably, depleting Hdc in the progenitors triggers proliferation, an opposing effect to what is observed in the niche. These findings further our understanding of how progenitor maintenance in the larval lymph gland is controlled autonomously and non-cell-autonomously, and point towards new mechanisms potentially regulating HSC maintenance across vertebrates.Author summary: Similarly to stem cell niches, in the hematopoietic organ of the Drosophila larva, the lymph gland, the maintenance of stem cells is regulated by both cell-autonomous signals from stem cells themselves, and non-cell-autonomous signals from supporting niche cells. This study describes a dual role for Headcase (Hdc) in progenitor differentiation within the lymph gland. Genetic interactions and rescue experiments revealed that in the niche, Hdc depletion leads to overactivation of the insulin/mTOR pathway, which can exhaust cells over time and lead to the formation of reactive oxygen species (ROS) acting as a non-cell-autonomous signal to induce premature differentiation of progenitors. Interestingly, within the progenitors themselves, ROS elevation due to hdc loss-of-function triggers distinct signaling pathways compared to those that function in the niche. We hope that our study contributes to a deeper understanding of regulatory mechanisms involved in progenitor maintenance in Drosophila, which would shed light on conserved processes occurring in mammalian stem cell niches.