Wallenda regulates JNK-mediated cell death in Drosophila

The c-Jun N-terminal kinase (JNK) pathway plays essential roles in regulating a variety of cellular processes including proliferation, migration and survival. Previous genetic studies in Drosophila have identified numerous cell death regulating genes, providing new insights into the mechanisms for related diseases. Despite the known role of the small GTPase Rac1 in regulating cell death, the downstream components and underlying mechanism remain largely elusive. Here, we show that Rac1 promotes JNK-dependent cell death through Wallenda (Wnd). In addition, we find that Wnd triggers JNK activation and cell death via its kinase domain. Moreover, we show that both MKK4 and Hep are critical for Wnd-induced cell death. Furthermore, Wnd is essential for ectopic Egr- or Rho1-induced JNK activation and cell death. Finally, Wnd is physiologically required for loss of scribble-induced JNK-dependent cell death. Thus, our data suggest that wnd encodes a novel essential cell death regulator in Drosophila.Programmed cell death (PCD) is a fundamental biological process required for normal organ development and tissue homeostasis in multicellular organisms.¹ Disruption of PCD would result in a variety of diseases including neurodegenerative diseases, autoimmune disorders and cancers.²Drosophila melanogaster, with its well-established genetic techniques and compact genome size, has been regarded as an excellent model organism to study PCD and its related signaling pathways.^{3, 4} The c-Jun N-terminal kinase (JNK) signaling has been implicated as one of the most important pathways that regulates various fundamental cell behaviors, such as proliferation, migration and cell death.^{5, 6}Rac1 belongs to the Rho family of small GTPase that regulates many aspects of physiological activities ranging from immune response to wound healing and migration.^{7, 8, 9, 10, 11} For instance, Rac1 has been implicated in JNK-mediated dorsal closure via Slpr (Slipper) in fly,⁷ osteoclast differentiation through TAK1-mediated NF-κB signaling¹² and myocyte hypertrophy via Ask1 (apoptotic signal-regulating kinase 1) in mammals.¹³ However, despite the reported role of Rac1 in cell death,¹⁴ its underlying mechanism and downstream components remain largely elusive.Here by using Drosophila compound eye as a model, we found Rac1 expression induces JNK-dependent cell death and identified Wallenda (Wnd), a MAPKKK (mitogen-activated protein kinase kinase kinase) member as an essential downstream mediator. Furthermore, we found that Wnd is sufficient to induce JNK-mediated cell death through both Hep and MKK4. Finally, we established Wnd as a general modulator of cell death in Drosophila by showing that it is also required for ectopic Egr or Rho1 and loss of Scribble (Scrib)-induced cell death.