The tight regulation of hematopoietic progenitors is required to continuously supply blood cells during the lifespan of an organism and to make cells rapidly available in response to stress conditions, such as injury or infection. In Drosophila, a large pool of multipotent blood progenitors are maintained in the larval hematopoietic organ, the lymph gland (LG), by a complex network of signaling pathways that are mediated by niche-, progenitor-, or differentiated hemocyte-derived signals. Despite extensive studies in the LG during recent years, several important questions remain unanswered. In particular, the nature of the signals required to regulate the stereotypical proliferation and differentiation of LG blood progenitors during larval development remain unidentified.
In this dissertation we identify the Target of Rapamycin (TOR) and Fibroblast Growth Factor Receptor (FGFR) signaling pathways as critical regulators of growth and differentiation of the Drosophila LG. In particular, we demonstrate that the tumor suppressors TSC and PTEN control blood progenitor proliferation through a common TOR- and 4EBP-dependent pathway. Tsc or Pten deficiency in progenitors increases TOR signaling and causes LG overgrowth by increasing the number of actively dividing cells during a critical window of growth. These phenotypes are associated with increased reactive oxygen species (ROS) levels in the LG, and scavenging ROS in progenitors is sufficient to rescue overgrowth. Differences between Tsc and Pten function become apparent at later stages. While loss of Tsc specifically expands the number of intermediate progenitors and limits terminal differentiation, absence of PTEN induces the myeloproliferative expansion of terminally differentiated blood lineages. This increased malignancy is associated with non-autonomous TOR activation within peripheral differentiating hemocytes, culminating in their premature release into circulation.
Since reduced TOR signaling in progenitors also induces increased differentiation, our findings demonstrate that although TOR signaling is sufficient, it is not required for inducing differentiation in the LG during development. In contrast, we identified the Drosophila FGFR, Heartless (Htl), and its two ligands, Pyramus and Thisbe, to be both required and sufficient for inducing blood progenitor differentiation in the LG. This Htl-mediated differentiation response of progenitors is dependent on two transcriptional regulators, the ETS protein, Pointed, and the Friend-of-GATA protein, U-Shaped, as well as TOR signaling, which is required specifically downstream of Thisbe-mediated Htl activation. Finally, we identify the Drosophila heparan sulfate proteoglycan (HSPG), Trol, as a critical negative regulator of FGF signaling in the LG, and suggest that sequestration of this potent differentiation signal by the extracellular matrix is a unique mechanism employed in blood progenitor maintenance in the Drosophila LG.