Regulatory Architecture of Non-Apoptotic Cell Death Program in C. Elegans

Cell death is prevalent in animal development, homeostasis, and disease. While apoptotic cell death has been extensively studied, many dying cells in development do not exhibit apoptotic morphology, and mice lacking core apoptotic regulators have mostly normal rates of developmental programmed cell death. However, little is known about how alternative death programs are set in motion. In the nematode Caenorhabditis elegans, most cells fated to die by apoptosis are eliminated as young, undifferentiated cells, for no obvious functional reasons. The male nematode’s linker cell, in contrast, dies as an older, differentiated cell, whose life and death subserve precise and important functions. The linker cell first undertakes a long migration along a characteristic path, elongating the male gonad into its proper, mature shape. Once the gonad has attained its final shape and the linker cell has completed its migration, the linker cell then dies to connect the gonad to the environment and allow male fertility. Linker cell death is genetically and morphologically non-apoptotic. Instead, this death program requires the temporal regulator LIN-29, the SARM-like protein TIR-1, the mitogen-activated protein kinase kinase (MAPKK) SEK-1, and the glutamine-rich protein PQN-41. SARM and MAPKKs have been implicated in non-apoptotic degeneration of axon distal segments following axotomy, and some developmental and pathological cell death events in vertebrates resemble the morphology of the dying linker cell. Thus, the molecular mechanism governing linker cell death may be conserved; however, neither the initiating death signals nor the target/s of linker cell death regulators are known. Using classical genetics, I have investigated the initiating mechanisms of linker cell death. I have characterized the cell-autonomous involvement of a histone 3, lysine 4 methyltransferase complex centered on the Trithorax/MLL-like catalytic subunit SET-16. I then demonstrated that two opposing spatial cues, the Wnt ligands EGL-20 and LIN-44, cooperate with LIN-29 to control linker cell death initiation. I showed that the Abdominal- B-like Hox transcription factor NOB-1 likely acts upstream of these two Wnt pathways, and that the Tailless/Tlx nuclear hormone receptor NHR-67 acts in parallel to these regulators to promote linker cell death in addition to linker cell migration. Finally, I show that the Wnt pathways and all known linker cell death mediators require the heat shock factor HSF-1 for cell death. Importantly, HSF-1 function in linker cell death is distinct from, and competes with, its role in stress responses. My studies demonstrate that HSF-1, previously thought to be primarily protective, is a key downstream regulator of a nonapoptotic cell death program. I have also developed a method to isolate large numbers of linker cells from staged worms populations, to enable a comprehensive characterization of the transcriptional program driving linker cell death.