Evidence for existence of an apoptosis-inducing BH3-only protein, sayonara, in Drosophila

Insidious chromatin change with a propensity to exhaust intestinal stem cells during aging

During aging, tissue stem cells can demonstrate two opposing phenotypes of tissue homeostasis disruption: proliferation and exhaustion. Stem cells can exhaust as a result of excessive cell proliferation or independently of cell proliferation. There are many silent changes in chromatin structures and gene expression that are not necessarily reflected in manifested phenotypes during aging. Here through analyses of chromatin accessibility and gene expression in intestinal progenitor cells during aging, we discovered changes of chromatin accessibility and gene expression that have a propensity to exhaust intestinal stem cells (ISCs). During aging, Trithorax-like (Trl) target genes, ced-6 and ci, close their chromatin structures and decrease their expression in intestinal progenitor cells. Inhibition of Trl, ced-6, or ci exhausts ISCs. This study provides new insight into changes of chromatin accessibility and gene expression that have a potential to exhaust ISCs during aging.

A feedback loop that drives cell death and proliferation and its defect in intestinal stem cells

Cell death and proliferation are at a glance dichotomic events, but occasionally coupled. Caspases, traditionally known to execute apoptosis, play non-apoptotic roles, but their exact mechanism remains elusive. Here, using Drosophila intestinal stem cells (ISCs), we discovered that activation of caspases induces massive cell proliferation rather than cell death. We elucidate that a positive feedback circuit exists between caspases and JNK, which can simultaneously drive cell proliferation and cell death. In ISCs, signalling from JNK to caspases is defective, which skews the balance towards proliferation. Mechanistically, two-tiered regulation of the DIAP1 inhibitor rpr, through its transcription and its protein localization, exists. This work provides a conceptual framework that explains how caspases perform apoptotic and non-apoptotic functions in vivo and how ISCs accomplish their resistance to cell death.

Understanding Developmental Cell Death Using Drosophila as a Model System

Cell death plays an essential function in organismal development, wellbeing, and ageing. Many types of cell deaths have been described in the past 30 years. Among these, apoptosis remains the most conserved type of cell death in metazoans and the most common mechanism for deleting unwanted cells. Other types of cell deaths that often play roles in specific contexts or upon pathological insults can be classed under variant forms of cell death and programmed necrosis. Studies in Drosophila have contributed significantly to the understanding and regulation of apoptosis pathways. In addition to this, Drosophila has also served as an essential model to study the genetic basis of autophagy-dependent cell death (ADCD) and other relatively rare types of context-dependent cell deaths. Here, we summarise what is known about apoptosis, ADCD, and other context-specific variant cell death pathways in Drosophila, with a focus on developmental cell death.

Cell death

Cell death supports morphogenesis during development and homeostasis after birth by removing damaged or obsolete cells. It also curtails the spread of pathogens by eliminating infected cells. Cell death can be induced by the genetically programmed suicide mechanisms of apoptosis, necroptosis, and pyroptosis, or it can be a consequence of dysregulated metabolism, as in ferroptosis. Here, we review the signaling mechanisms underlying each cell-death pathway, discuss how impaired or excessive activation of the distinct cell-death processes can promote disease, and highlight existing and potential therapies for redressing imbalances in cell death in cancer and other diseases.

Necrosensor: a genetically encoded fluorescent sensor for visualizing necrosis in Drosophila

Historically, necrosis has been considered a passive process, which is induced by extreme stress or damage. However, recent findings of necroptosis, a programmed form of necrosis, shed a new light on necrosis. It has been challenging to detect necrosis reliably in vivo, partly due to the lack of genetically encoded sensors to detect necrosis. This is in stark contrast with the availability of many genetically encoded biosensors for apoptosis. Here we developed Necrosensor, a genetically encoded fluorescent sensor that detects necrosis in Drosophila, by utilizing HMGB1, which is released from the nucleus as a damage-associated molecular pattern (DAMP). We demonstrate that Necrosensor is able to detect necrosis induced by various stresses in multiple tissues in both live and fixed conditions. Necrosensor also detects physiological necrosis that occurs during spermatogenesis in the testis. Using Necrosensor, we discovered previously unidentified, physiological necrosis of hemocyte progenitors in the hematopoietic lymph gland of developing larvae. This work provides a new transgenic system that enables in vivo detection of necrosis in real time without any intervention.

Programmed Cell Death in Unicellular Versus Multicellular Organisms

Programmed cell death (self-induced) is intrinsic to all cellular life forms, including unicellular organisms. However, cell death research has focused on animal models to understand cancer, degenerative disorders, and developmental processes. Recently delineated suicidal death mechanisms in bacteria and fungi have revealed ancient origins of animal cell death that are intertwined with immune mechanisms, allaying earlier doubts that self-inflicted cell death pathways exist in microorganisms. Approximately 20 mammalian death pathways have been partially characterized over the last 35 years. By contrast, more than 100 death mechanisms have been identified in bacteria and a few fungi in recent years. However, cell death is nearly unstudied in most human pathogenic microbes that cause major public health burdens. Here, we consider how the current understanding of programmed cell death arose through animal studies and how recently uncovered microbial cell death mechanisms in fungi and bacteria resemble and differ from mechanisms of mammalian cell death.

Sayonara to some evolutionary puzzles in the Bcl‐2 family

BH3-only proteins are key regulators of Bcl-2 family members to activate apoptosis. The absence of a BH3-only protein in Drosophila has complicated the understanding of how Bcl-2 family members contribute to cell death in this model organism. Recent work published in The EMBO Journal reports on the identification of a BH3-only protein in flies. The reported findings may help to clarify the functional role and molecular mechanisms of the highly conserved Bcl-2 pathway in divergent organisms.