The Role of Lozenge in Drosophila Hematopoiesis

Abstract

Drosophila hematopoiesis is comparable to mammalian differentiation of myeloid lineages, and therefore, has been a useful model organism in illustrating the molecular and genetic basis for hematopoiesis. Multiple novel regulators and signals have been uncovered using the tools of Drosophila genetics. A Runt domain protein, lozenge, is one of the first players recognized and closely studied in the hematopoietic lineage specification. Here, we explore the role of lozenge in determination of prohemocytes into a special class of hemocyte, namely the crystal cell, and discuss molecules and signals controlling the lozenge function and its implication in immunity and stress response. Given the highly conserved nature of Runt domain in both invertebrates and vertebrates, studies in Drosophila will enlighten our perspectives on Runx-mediated development and pathologies.

Keywords: Drosophila melanogaster; RUNX; crystal cells; hematopoiesis; lozenge; lymph gland; melanization; prophenoloxidase.

Fig. 1. Life cycle of Drosophila and constitution of the lymph gland.

(A) The fly undergoes four distinct phases of change that is commenced by the embryo which is formed after fertilization. The hatched embryo produces the first instar larva, which molts into second and then third instar larva, eventually forming a pupa. The pupa ecloses to the adult fly and the cycle repeats. (B) The lymph gland is the venue for definitive hematopoiesis. It comprises of four pairs of lobes and the primary lobe is divided into four regions: the posterior signaling center (PSC), the medullary zone (MZ), the intermediate zone (IZ), and the cortical zone (CZ). The prohemocytes of the MZ differentiate into plasmatocytes and crystal cells of the CZ. Lamellocytes are barely seen in healthy animals. The IZ contains differentiating prohemocytes that expresses both MZ and CZ markers.

Fig. 2. lozenge in crystal cell formation.

serpent-positive (srp+) prohemocytes differentiate into gcm+/gcm2+ differentiating prohemocytes or lz+ immature crystal cells. Differentiation of prohemocytes into immature crystal cells is mediated by the Notch/Serrate interaction and moderated by u-shaped. Additionally, yki and sd control the crystal cell specification in a Notch/Serrate-dependent manner. High gcm/gcm2 expression reduces the number of crystal cells, however, fated crystal cells are inhibited by klu from becoming plasmatocytes. This high gcm/gcm2 cells become plasmatocytes. Increased lz in immature crystal cells coupled with hnt/peb, DnaJ-1, Mlf and Notch leads to formation of mature crystal cells which possess crystalline inclusions and express PPO1 and PPO2. The process of mature crystal cell formation is heavily dependent on lozenge expression from the onset to the late stage. The medullary, intermediate, and cortical zones demarcate three regions of the primary lymph gland lobe. Healthy animals do not actively generate lamellocytes. Though, prohemocyte tion is lamellocyte-biased upon immune challenges.

Fig. 3. Crystal cell mediated stress responses.

When larva gets a sterile wound, crystal cells migrate towards the wound site, and rupture to initiate the melanization cascade (left). This promotes the formation of a scab at the wound site leading to a subsequent healing process. Upon wasp infestation, wasps lay eggs inside Drosophila larva that trigger innate immune responses including differentiation of lamellocytes. Lamellocytes encapsulate wasp eggs and activate PPO3 to neutralize them (right). During encapsulation, crystal cells co-opt PPO1 and PPO2 to facilitate melanization. In melanization, rupturing of crystal cells is coupled with the activation of Serine proteinase cascade which triggers the conversion of PPOs into POs initiating non-enzymatic melanin formation (middle).