BLMP-1 promotes developmental cell death in C. elegans by timely repression of ced-9 transcription

SQST-1/p62-regulated SKN-1/Nrf mediates a phagocytic stress response via transcriptional activation of lyst-1/LYST

Cells may be intrinsically fated to die to sculpt tissues during development or to maintain homeostasis. Cells can also die in response to various stressors, injury or pathological conditions. Additionally, cells of the metazoan body are often highly specialized with distinct domains that differ both structurally and with respect to their neighbors. Specialized cells can also die, as in normal brain development or pathological states and their different regions may be eliminated via different programs. Clearance of different types of cell debris must be performed quickly and efficiently to prevent autoimmunity and secondary necrosis of neighboring cells. Moreover, all cells, including those programmed to die, may be subject to various stressors. Some largely unexplored questions include whether predestined cell elimination during development could be altered by stress, if adaptive stress responses exist and if polarized cells may need compartment-specific stress-adaptive programs. We leveraged Compartmentalized Cell Elimination (CCE) in the nematode C. elegans to explore these questions. CCE is a developmental cell death program whereby three segments of two embryonic polarized cell types are eliminated differently. We have previously employed this in vivo genetic system to uncover a cell compartment-specific, cell non-autonomous clearance function of the fusogen EFF-1 in phagosome closure during corpse internalization. Here, we introduce an adaptive response that serves to aid developmental phagocytosis as a part of CCE during stress. We employ a combination of forward and reverse genetics, CRISPR/Cas9 gene editing, stress response assays and advanced fluorescence microscopy. Specifically, we report that, under heat stress, the selective autophagy receptor SQST-1/p62 promotes the nuclear translocation of the oxidative stress-related transcription factor SKN-1/Nrf via negative regulation of WDR-23. This in turn allows SKN-1/Nrf to transcribe lyst-1/LYST (lysosomal trafficking associated gene) which subsequently promotes the phagocytic resolution of the developmentally-killed internalized cell even under stress conditions.

CDH-3/Cadherin, YAP-1/YAP and EGL-44/TEAD promote SYX-2/Syntaxin and EFF-1 fusogen-mediated phagosome closure

Physical interactions between cells, such as cell-cell junctions, can profoundly impact cell fate. A vital cell fate for normal development and homeostasis is programmed cell death. Cells fated to die must be efficiently cleared away via phagocytosis, and defects are associated with a variety of diseased states. Whether cell-cell physical associations affect programmed cell elimination has not been well-explored. Here we describe, in vivo, a cell-cell adhesion-driven signaling pathway that ensures compartment-specific cell clearance during development. We previously described the specialized cell death program "Compartmentalized Cell Elimination" (CCE) in the C. elegans embryo. During CCE, the tail-spike cell (TSC), a polarized epithelial cell, undergoes a tripartite, ordered, and organized death sequence, allowing for the study of three distinct death modalities in a single cell setting. Prior to its demise, the TSC serves as a scaffold for the tail tip, formed by the hyp10 epithelial cell which develops along the TSC process. The hyp10 cell in turn also serves as the phagocyte for the dying TSC process. Here we present data suggesting that the physical association between the dying TSC and hyp10 phagocyte via CDH-3/cadherin mediates function of the mechanosensitive transcriptional coactivator YAP-1/YAP and its partner EGL-44/TEAD in the hyp10 phagocyte to promote localization of hyp10 SYX-2/Syntaxin around the dying TSC remnant. This pathway facilitates the phagocytic function of EFF-1/fusogen, which we have previously shown to be required for phagosome sealing during CCE. Our work sheds additional light on a poorly understood step of phagocytosis and implicates adhesive forces and signaling between cells as important in cell uptake.

EOR-1/PLZF-regulated WAH-1/AIF sequentially promotes early and late stages of non-apoptotic corpse removal

Programmed cell death (PCD) is a crucial, genetically-encoded, and evolutionarily-conserved process required for development and homeostasis. We previously identified a genetically non-apoptotic, highly ordered, and stereotyped killing program called Compartmentalized Cell Elimination (CCE) in the C. elegans tail-spike epithelial cell (TSC). Here we identify the transcription factor EOR-1/PLZF as an important coordinator of CCE. Loss of EOR-1 results in a large, persisting, un-engulfed soma with enlarged nuclei. We find that EOR-1 and its partners positively regulate the transcription of the Apoptosis Inducing Factor AIF homolog, WAH-1/AIF. We report stereotyped and sequential spatiotemporal dynamics of WAH-1/AIF1 during phagocytosis, with defined roles acting early and late, within the dying cells. Mitochondria to plasma membrane translocation within the TSC soma is required its internalization by its phagocyte, and plasma membrane to nuclear translocation is required for DNA degradation and ultimately, corpse resolution. Our study suggests that EOR-1 serves as a master regulator for the transcriptional control of DNA degradation is essential for changes in nuclear morphology required for cellular dismantling and infers that tight spatiotemporal regulation of WAH-1/AIF is required for this function.

Apoptotic and Nonapoptotic Cell Death in Caenorhabditis elegans Development

Programmed cell death (PCD) is an essential component of animal development, and aberrant cell death underlies many disorders. Understanding mechanisms that govern PCD during development can provide insight into cell death programs that are disrupted in disease. Key steps mediating apoptosis, a highly conserved cell death program employing caspase proteases, were first uncovered in the nematode Caenorhabditis elegans, a powerful model system for PCD research. Recent studies in C. elegans also unearthed conserved nonapoptotic caspase-independent cell death programs that function during development. Here, we discuss recent advances in understanding cell death during C. elegans development. We review insights expanding the molecular palette behind the execution of apoptotic and nonapoptotic cell death, as well as new discoveries revealing the mechanistic underpinnings of dying cell engulfment and clearance. A number of open questions are also discussed that will continue to propel the field over the coming years.

Apoptosis and beyond: A new era for programmed cell death in Caenorhabditis elegans

Programmed cell death (PCD) is crucial for normal development and homeostasis. Our first insights into the genetic regulation of apoptotic cell death came from in vivo studies in the powerful genetic model system of C. elegans. More recently, novel developmental cell death programs occurring both embryonically and post-embryonically, and sex-specifically, have been elucidated. Recent studies in the apoptotic setting have also shed new light on the intricacies of phagocytosis in particular. This review provides a brief historical perspective of the origins of PCD studies in C. elegans, followed by a more detailed description of non-canonical apoptotic and non-apoptotic death programs. We conclude by posing open questions and commenting on our outlook on the future of PCD studies in C. elegans, highlighting the importance of advanced imaging tools and the continued leveraging of C. elegans genetics both with classical and modern cutting-edge approaches.

Repurposing the Killing Machine: Non-canonical Roles of the Cell Death Apparatus in Caenorhabditis elegans Neurons

Here we highlight the increasingly divergent functions of the Caenorhabditis elegans cell elimination genes in the nervous system, beyond their well-documented roles in cell dismantling and removal. We describe relevant background on the C. elegans nervous system together with the apoptotic cell death and engulfment pathways, highlighting pioneering work in C. elegans. We discuss in detail the unexpected, atypical roles of cell elimination genes in various aspects of neuronal development, response and function. This includes the regulation of cell division, pruning, axon regeneration, and behavioral outputs. We share our outlook on expanding our thinking as to what cell elimination genes can do and noting their versatility. We speculate on the existence of novel genes downstream and upstream of the canonical cell death pathways relevant to neuronal biology. We also propose future directions emphasizing the exploration of the roles of cell death genes in pruning and guidance during embryonic development.