Adult Apaf-1-deficient mice exhibit male infertility

Apoptotic cell death in disease-Current understanding of the NCCD 2023

Apoptosis is a form of regulated cell death (RCD) that involves proteases of the caspase family. Pharmacological and genetic strategies that experimentally inhibit or delay apoptosis in mammalian systems have elucidated the key contribution of this process not only to (post-)embryonic development and adult tissue homeostasis, but also to the etiology of multiple human disorders. Consistent with this notion, while defects in the molecular machinery for apoptotic cell death impair organismal development and promote oncogenesis, the unwarranted activation of apoptosis promotes cell loss and tissue damage in the context of various neurological, cardiovascular, renal, hepatic, infectious, neoplastic and inflammatory conditions. Here, the Nomenclature Committee on Cell Death (NCCD) gathered to critically summarize an abundant pre-clinical literature mechanistically linking the core apoptotic apparatus to organismal homeostasis in the context of disease.

Making the head: Caspases in life and death

The term apoptosis, as a way of programmed cell death, was coined a half century ago and since its discovery the process has been extensively investigated. The anatomy and physiology of the head are complex and thus apoptosis has mostly been followed in separate structures, tissues or cell types. This review aims to provide a comprehensive overview of recent knowledge concerning apoptosis-related molecules involved in the development of structures of head with a particular focus on caspases, cysteine proteases having a key position in apoptotic pathways. Since many classical apoptosis-related molecules, including caspases, are emerging in several non-apoptotic processes, these were also considered. The largest organ of the head region is the brain and its development has been extensively investigated, including the roles of apoptosis and related molecules. Neurogenesis research also includes sensory organs such as the eye and ear, efferent nervous system and associated muscles and glands. Caspases have been also associated with normal function of the skin and hair follicles. Regarding mineralised tissues within craniofacial morphogenesis, apoptosis in bones has been of interest along with palate fusion and tooth development. Finally, the role of apoptosis and caspases in angiogenesis, necessary for any tissue/organ development and maintenance/homeostasis, are discussed. Additionally, this review points to abnormalities of development resulting from improper expression/activation of apoptosis-related molecules.

Catching a killer: Mechanisms of programmed cell death and immune activation in Amyotrophic Lateral Sclerosis

In the central nervous system (CNS), execution of programmed cell death (PCD) is crucial for proper neurodevelopment. However, aberrant activation of these pathways in adult CNS leads to neurodegenerative diseases including amyotrophic lateral sclerosis (ALS). How a cell dies is critical, as it can drive local immune activation and tissue damage. Classical apoptosis engages several mechanisms to evoke "immunologically silent" responses, whereas other forms of programmed death such as pyroptosis, necroptosis, and ferroptosis release molecules that can potentiate immune responses and inflammation. In ALS, a fatal neuromuscular disorder marked by progressive death of lower and upper motor neurons, several cell types in the CNS express machinery for multiple PCD pathways. The specific cell types engaging PCD, and ultimate mechanisms by which neuronal death occurs in ALS are not well defined. Here, we provide an overview of different PCD pathways implicated in ALS. We also examine immune activation in ALS and differentiate apoptosis from necrotic mechanisms based on downstream immunological consequences. Lastly, we highlight therapeutic strategies that target cell death pathways in the treatment of neurodegeneration and inflammation in ALS.

Regulation of male germline transmission patterns by the Trp53-Cdkn1a pathway

A small number of offspring are born from the numerous sperm generated from spermatogonial stem cells (SSCs). However, little is known regarding the rules and molecular mechanisms that govern germline transmission patterns. Here we report that the Trp53 tumor suppressor gene limits germline genetic diversity via Cdkn1a. Trp53-deficient SSCs outcompeted wild-type (WT) SSCs and produced significantly more progeny after co-transplantation into infertile mice. Lentivirus-mediated transgenerational lineage analysis showed that offspring bearing the same virus integration were repeatedly born in a non-random pattern from WT SSCs. However, SSCs lacking Trp53 or Cdkn1a sired transgenic offspring in random patterns with increased genetic diversity. Apoptosis of KIT⁺ differentiating germ cells was reduced in Trp53- or Cdkn1a-deficient mice. Reduced CDKN1A expression in Trp53-deficient spermatogonia suggested that Cdkn1a limits genetic diversity by supporting apoptosis of syncytial spermatogonial clones. Therefore, the TRP53-CDKN1A pathway regulates tumorigenesis and the germline transmission pattern.

A unique form of collective epithelial migration is crucial for tissue fusion in the secondary palate and can overcome loss of epithelial apoptosis

Tissue fusion frequently requires the removal of an epithelium that intervenes distinct primordia to form one continuous structure. In the mammalian secondary palate, a midline epithelial seam (MES) forms between two palatal shelves and must be removed to allow mesenchymal confluence. Abundant apoptosis and cell extrusion support their importance in MES removal. However, genetically disrupting the intrinsic apoptotic regulators BAX and BAK within the MES results in complete loss of cell death and cell extrusion, but successful removal of the MES. Novel static- and live-imaging approaches reveal that the MES is removed through streaming migration of epithelial trails and islands to reach the oral and nasal epithelial surfaces. Epithelial trail cells that express the basal epithelial marker ΔNp63 begin to express periderm markers, suggesting that migration is concomitant with differentiation. Live imaging reveals anisotropic actomyosin contractility within epithelial trails, and genetic ablation of actomyosin contractility results in dispersion of epithelial collectives and failure of normal MES migration. These findings demonstrate redundancy between cellular mechanisms of morphogenesis, and reveal a crucial and unique form of collective epithelial migration during tissue fusion.

Summary: Multiple cellular processes mediate secondary palate fusion, including a unique form of streaming collective epithelial migration driven by pulsatile actomyosin contractility.

The effects of temperature variation treatments on embryonic development: a mouse study

Since the development of ART, embryos have been cultured at 37 °C in an attempt to mimic the in vivo conditions and the average body temperature of an adult. However, a gradient of temperatures within the reproductive tract has been demonstrated in humans and several other mammalian species. Therefore, the aim of this study was to evaluate the effects of temperature variation treatments on mouse embryo quality through morphokinetic events, blastocyst morphology, the relative gene expression of Igf2, Bax, Bcl2 and Apaf1 and the metabolomics of individual culture media. Study groups consisted of 2 circadian treatments, T1 with embryos being cultured at 37 °C during the day and 35.5 °C during the night, T2 with 38.5 °C during the day and 37 °C during the night and a control group with constant 37 °C. Our main findings are that the lower-temperature group (T1) showed a consistent negative effect on mouse embryo development with “slow” cleaving embryos, poor-quality blastocysts, a higher expression of the apoptotic gene Apaf1, and a significantly different set of amino acids representing a more stressed metabolism. On the other hand, our higher-temperature group (T2) showed similar results to the control group, with no adverse effects on blastocyst viability.

Mitochondrial regulation during male germ cell development

Mitochondria tailor their morphology to execute their specialized functions in different cell types and/or different environments. During spermatogenesis, mitochondria undergo continuous morphological and distributional changes with germ cell development. Deficiencies in these processes lead to mitochondrial dysfunction and abnormal spermatogenesis, thereby causing male infertility. In recent years, mitochondria have attracted considerable attention because of their unique role in the regulation of piRNA biogenesis in male germ cells. In this review, we describe the varied characters of mitochondria and focus on key mitochondrial factors that play pivotal roles in the regulation of spermatogenesis, from primordial germ cells to spermatozoa, especially concerning metabolic shift, stemness and reprogramming, mitochondrial transformation and rearrangement, and mitochondrial defects in human sperm. Further, we discuss the molecular mechanisms underlying these processes.

Proteolysis in Reproduction: Lessons From Gene-Modified Organism Studies

The physiological roles of proteolysis are not limited to degrading unnecessary proteins. Proteolysis plays pivotal roles in various biological processes through cleaving peptide bonds to activate and inactivate proteins including enzymes, transcription factors, and receptors. As a wide range of cellular processes is regulated by proteolysis, abnormalities or dysregulation of such proteolytic processes therefore often cause diseases. Recent genetic studies have clarified the inclusion of proteases and protease inhibitors in various reproductive processes such as development of gonads, generation and activation of gametes, and physical interaction between gametes in various species including yeast, animals, and plants. Such studies not only clarify proteolysis-related factors but the biological processes regulated by proteolysis for successful reproduction. Here the physiological roles of proteases and proteolysis in reproduction will be reviewed based on findings using gene-modified organisms.

A unique death pathway keeps RIPK1 D325A mutant mice in check at embryonic day 10.5

Tumor necrosis factor receptor-1 (TNFR1) signaling, apart from its pleiotropic functions in inflammation, plays a role in embryogenesis as deficiency of varieties of its downstream molecules leads to embryonic lethality in mice. Caspase-8 noncleavable receptor interacting serine/threonine kinase 1 (RIPK1) mutations occur naturally in humans, and the corresponding D325A mutation in murine RIPK1 leads to death at early midgestation. It is known that both the demise of Ripk1D325A/D325A embryos and the death of Casp8-/- mice are initiated by TNFR1, but they are mediated by apoptosis and necroptosis, respectively. Here, we show that the defects in Ripk1D325A/D325A embryos occur at embryonic day 10.5 (E10.5), earlier than that caused by Casp8 knockout. By analyzing a series of genetically mutated mice, we elucidated a mechanism that leads to the lethality of Ripk1D325A/D325A embryos and compared it with that underlies Casp8 deletion-mediated lethality. We revealed that the apoptosis in Ripk1D325A/D325A embryos requires a scaffold function of RIPK3 and enzymatically active caspase-8. Unexpectedly, caspase-1 and caspase-11 are downstream of activated caspase-8, and concurrent depletion of Casp1 and Casp11 postpones the E10.5 lethality to embryonic day 13.5 (E13.5). Moreover, caspase-3 is an executioner of apoptosis at E10.5 in Ripk1D325A/D325A mice as its deletion extends life of Ripk1D325A/D325A mice to embryonic day 11.5 (E11.5). Hence, an unexpected death pathway of TNFR1 controls RIPK1 D325A mutation-induced lethality at E10.5.

Contribution of Apaf-1 to the pathogenesis of cancer and neurodegenerative diseases

Deregulation of apoptosis is associated with various pathologies, such as neurodegenerative disorders at one end of the spectrum and cancer at the other end. Generally speaking, differentiated cells like cardiomyocytes, skeletal myocytes and neurons exhibit low levels of Apaf-1 (Apoptotic protease activating factor 1) protein suggesting that down-regulation of Apaf-1 is an important event contributing to the resistance of these cells to apoptosis. Nonetheless, upregulation of Apaf-1 has not emerged as a common phenomenon in pathologies associated with enhanced neuronal cell death, i.e., neurodegenerative diseases. In cancer, on the other hand, Apaf-1 downregulation is a common phenomenon, which occurs through various mechanisms including mRNA hyper-methylation, gene methylation, Apaf-1 localization in lipid rafts, inhibition by microRNAs, phosphorylation, and interaction with specific inhibitors. Due to the diversity of these mechanisms and involvement of other factors, defining the exact contribution of Apaf-1 to the development of cancer in general and neurodegenerative disorders, in particular, is complicated. The current review is an attempt to provide a comprehensive image of Apaf-1's contribution to the pathologies observed in cancer and neurodegenerative diseases with the emphasis on the therapeutic aspects of Apaf-1 as an important target in these pathologies.

Evidence for the involvement of caspases in establishing proper cerebrospinal fluid hydrodynamics

A large number of cells undergo apoptosis via caspase activation during and after neural tube closure (NTC) in mammals. Apoptosis is executed by either intrinsic or extrinsic apoptotic pathways, and inhibition of each pathway causes developmental defects around NTC stages, which hampers the physiological roles of apoptosis and caspases after NTC. We generated transgenic mice in which a broad spectrum of caspases could be suppressed in a spatiotemporal manner by pan-caspase inhibitor protein p35 originating from baculovirus. Mice with nervous system-specific expression of p35 (Nestin-Cre (NCre);p35V mice) exhibited postnatal lethality within 1 month after birth. They were born at the expected Mendelian ratio, but demonstrated severe postnatal growth retardation and hydrocephalus. The flow of cerebrospinal fluid (CSF) between the third and fourth ventricles was disturbed, whereas neither stenosis nor abnormality in ciliary morphology was observed in the pathway of CSF flow. Hydrocephalus and growth retardation of NCre;p35V mice were not rescued by the deletion of RIPK3, an essential factor for necroptosis which occurs in the absence of caspase-8 activation during development. The CSF of NCre;p35V mice contained a larger amount of secreted proteins than that of the controls. These findings suggest that the establishment of proper CSF dynamics requires caspase activity during brain development after NTC.

Synergism of TNF-α and IFN-γ Triggers Inflammatory Cell Death, Tissue Damage, and Mortality in SARS-CoV-2 Infection and Cytokine Shock Syndromes

COVID-19 is characterized by excessive production of pro-inflammatory cytokines and acute lung damage associated with patient mortality. While multiple inflammatory cytokines are produced by innate immune cells during SARS-CoV-2 infection, we found that only the combination of TNF-α and IFN-γ induced inflammatory cell death characterized by inflammatory cell death, PANoptosis. Mechanistically, TNF-α and IFN-γ co-treatment activated the JAK/STAT1/IRF1 axis, inducing nitric oxide production and driving caspase-8/FADD-mediated PANoptosis. TNF-α and IFN-γ caused a lethal cytokine shock in mice that mirrors the tissue damage and inflammation of COVID-19, and inhibiting PANoptosis protected mice from this pathology and death. Furthermore, treating with neutralizing antibodies against TNF-α and IFN-γ protected mice from mortality during SARS-CoV-2 infection, sepsis, hemophagocytic lymphohistiocytosis, and cytokine shock. Collectively, our findings suggest that blocking the cytokine-mediated inflammatory cell death signaling pathway identified here may benefit patients with COVID-19 or other infectious and autoinflammatory diseases by limiting tissue damage/inflammation.

Synergism of TNF-α and IFN-γ triggers inflammatory cell death, tissue damage, and mortality in SARS-CoV-2 infection and cytokine shock syndromes

The COVID-19 pandemic has caused significant morbidity and mortality. Currently, there is a critical shortage of proven treatment options and an urgent need to understand the pathogenesis of multi-organ failure and lung damage. Cytokine storm is associated with severe inflammation and organ damage during COVID-19. However, a detailed molecular pathway defining this cytokine storm is lacking, and gaining mechanistic understanding of how SARS-CoV-2 elicits a hyperactive inflammatory response is critical to develop effective therapeutics. Of the multiple inflammatory cytokines produced by innate immune cells during SARS-CoV-2 infection, we found that the combined production of TNF-α and IFN-γ specifically induced inflammatory cell death, PANoptosis, characterized by gasdermin-mediated pyroptosis, caspase-8-mediated apoptosis, and MLKL-mediated necroptosis. Deletion of pyroptosis, apoptosis, or necroptosis mediators individually was not sufficient to protect against cell death. However, cells deficient in both RIPK3 and caspase-8 or RIPK3 and FADD were resistant to this cell death. Mechanistically, the JAK/STAT1/IRF1 axis activated by TNF-α and IFN-γ co-treatment induced iNOS for the production of nitric oxide. Pharmacological and genetic deletion of this pathway inhibited pyroptosis, apoptosis, and necroptosis in macrophages. Moreover, inhibition of PANoptosis protected mice from TNF-α and IFN-γ-induced lethal cytokine shock that mirrors the pathological symptoms of COVID-19. In vivo neutralization of both TNF-α and IFN-γ in multiple disease models associated with cytokine storm showed that this treatment provided substantial protection against not only SARS-CoV-2 infection, but also sepsis, hemophagocytic lymphohistiocytosis, and cytokine shock models, demonstrating the broad physiological relevance of this mechanism. Collectively, our findings suggest that blocking the cytokine-mediated inflammatory cell death signaling pathway identified here may benefit patients with COVID-19 or other cytokine storm-driven syndromes by limiting inflammation and tissue damage. The findings also provide a molecular and mechanistic description for the term cytokine storm. Additionally, these results open new avenues for the treatment of other infectious and autoinflammatory diseases and cancers where TNF-α and IFN-γ synergism play key pathological roles.

Apoptosis is involved in maintaining the character of the midbrain and the diencephalon roof plate after neural tube closure

Apoptosis, a major form of programmed cell death, is massively observed in neural plate border and subsequently in the roof plate (RP). While deficiency of apoptosis often results in brain malformations including exencephaly and hydrocephalus, the impact of apoptosis on RP formation and maintenance remains unclear. Here we described that mouse embryos deficient in Apaf1, a gene crucial for the intrinsic apoptotic pathway, in C57BL/6 genetic background exhibited narrow and discontinuous expression of RP marker genes in the midline of the midbrain and the diencephalon. Instead, cells positive for the neuroectodermal gene SOX1 ectopically accumulated in the midline. A lineage-tracing experiment suggests that these ectopic SOX1-positive cells began to accumulate in the midline of apoptosis-deficient embryos after E9.5. These embryos further displayed malformation of the subcommissural organ, which has been discussed in the etiology of hydrocephalus. Thus, the apoptosis machinery prevents ectopic emergence of SOX1-positive cells in the midbrain and the diencephalon RP, and helps in maintaining the character of the RP in the diencephalon and midbrain, thereby ensuring proper brain development.

Mitochondria as multifaceted regulators of cell death

Through their many and varied metabolic functions, mitochondria power life. Paradoxically, mitochondria also have a central role in apoptotic cell death. Upon induction of mitochondrial apoptosis, mitochondrial outer membrane permeabilization (MOMP) usually commits a cell to die. Apoptotic signalling downstream of MOMP involves cytochrome c release from mitochondria and subsequent caspase activation. As such, targeting MOMP in order to manipulate cell death holds tremendous therapeutic potential across different diseases, including neurodegenerative diseases, autoimmune disorders and cancer. In this Review, we discuss new insights into how mitochondria regulate apoptotic cell death. Surprisingly, recent data demonstrate that besides eliciting caspase activation, MOMP engages various pro-inflammatory signalling functions. As we highlight, together with new findings demonstrating cell survival following MOMP, this pro-inflammatory role suggests that mitochondria-derived signalling downstream of pro-apoptotic cues may also have non-lethal functions. Finally, we discuss the importance and roles of mitochondria in other forms of regulated cell death, including necroptosis, ferroptosis and pyroptosis. Collectively, these new findings offer exciting, unexplored opportunities to target mitochondrial regulation of cell death for clinical benefit.

Infertility Caused by Inefficient Apoptotic Germ Cell Clearance in Xkr8-Deficient Male Mice

During spermatogenesis, up to 75% of germ cells in the testes undergo apoptosis and are cleared by Sertoli cells. X-linked XK blood group-related 8 (Xkr8) is a plasma membrane protein that scrambles phospholipids in response to apoptotic signals, exposing phosphatidylserine (PtdSer). Here, we found that Xkr8 ^-/- male mice were infertile due to reduced sperm counts in their epididymides. Apoptotic stimuli could not induce PtdSer exposure in Xkr8 ^-/- germ cells. Consistent with the hypothesis that PtdSer functions as an "eat-me" signal to phagocytes, cells expressing phosphatidylserine receptor TIM4 and MER tyrosine kinase receptor efficiently engulfed apoptotic wild-type male germ cells but not Xkr8 ^-/- germ cells. Fluorescence and electron microscopy revealed Sertoli cells carrying engulfed and degenerated dead cells. However, many unengulfed apoptotic cells and residual bodies and much cell debris were present in Xkr8 ^-/- testes and epididymides. These results indicate that Xkr8-mediated PtdSer exposure is essential for the clearance of apoptotic germ cells by Sertoli cells. There was no apparent inflammation in Xkr8 ^-/- testes, suggesting that the unengulfed apoptotic cells may have undergone secondary necrosis, releasing noxious materials that affected the germ cells. Alternatively, failure to engulf the apoptotic germ cells may have caused the Sertoli cells to starve and lose their ability to support spermatogenesis.

Mutations of GADD45G in rabbits cause cleft lip by the disorder of proliferation, apoptosis and epithelial-mesenchymal transition (EMT)

The cleft lip with or without cleft palate (CL/P) is one of the most common congenital defects in humans. Genome-wide association studies (GWAS) have been widely used for identifying candidate genes, and different genes or chromosomal regions have shown strong evidence for the presence of causal genes in CL/P. To date, two independent GWAS have identified GADD45G as influencing risk for CL/P. However, there is no animal model evidence about GADD45G related to CL/P. Here, we reported the generation of a novel GADD45G mutated rabbit model by CRISPR/Cas9 and CRISPR-based BE4-Gam systems. The homozygous (GADD45G-/-) while not heterozygous (GADD45G+/-) pups died after birth due to severe craniofacial defects of unilateral or bilateral cleft lip (CL). Moreover, the disorder of proliferation, apoptosis and epithelial-mesenchymal transition (EMT) were also determined in the medial and lateral nasal processes (MNP and LNP) of the embryonic day 13 (E13) GADD45G-/- rabbits, which compared with the normal wild type (WT) rabbits. Thus, our study confirmed for the first time that loss of GADD45G lead to CL at the animal level and provided new insights into the crucial role of GADD45G for upper lip formation and fusion.

A new split-luciferase complementation assay identifies pentachlorophenol as an inhibitor of apoptosome formation

The expense and time required for in vivo reproductive and developmental toxicity studies have driven the development of in vitro alternatives. Here, we used a new in vitro split luciferase‐based assay to screen a library of 177 toxicants for inhibitors of apoptosome formation. The apoptosome contains seven Apoptotic Protease‐Activating Factor‐1 (Apaf‐1) molecules and induces cell death by activating caspase‐9. Apaf‐1‐dependent caspase activation also plays an important role in CNS development and spermatogenesis. In the in vitro assay, Apaf‐1 fused to an N‐terminal fragment of luciferase binds to Apaf‐1 fused to a C‐terminal fragment of luciferase and reconstitutes luciferase activity. Our assay indicated that pentachlorophenol (PCP) inhibits apoptosome formation, and further investigation revealed that PCP binds to cytochrome c. PCP is a wood preservative that reduces male fertility by ill‐defined mechanisms. Although the data show that PCP inhibited apoptosome formation, the concentration required suggests that other mechanisms may be more important for PCP's effects on spermatogenesis. Nonetheless, the data demonstrate the utility of the new assay in identifying apoptosome inhibitors, and we suggest that the assay may be useful in screening for reproductive and developmental toxicants.

Regulation of apoptosis in health and disease: the balancing act of BCL-2 family proteins

The loss of vital cells within healthy tissues contributes to the development, progression and treatment outcomes of many human disorders, including neurological and infectious diseases as well as environmental and medical toxicities. Conversely, the abnormal survival and accumulation of damaged or superfluous cells drive prominent human pathologies such as cancers and autoimmune diseases. Apoptosis is an evolutionarily conserved cell death pathway that is responsible for the programmed culling of cells during normal eukaryotic development and maintenance of organismal homeostasis. This pathway is controlled by the BCL-2 family of proteins, which contains both pro-apoptotic and pro-survival members that balance the decision between cellular life and death. Recent insights into the dynamic interactions between BCL-2 family proteins and how they control apoptotic cell death in healthy and diseased cells have uncovered novel opportunities for therapeutic intervention. Importantly, the development of both positive and negative small-molecule modulators of apoptosis is now enabling researchers to translate the discoveries that have been made in the laboratory into clinical practice to positively impact human health.

Cap-Independent mRNA Translation in Germ Cells

Cellular mRNAs in plants and animals have a 5'-cap structure that is accepted as the recognition point to initiate translation by ribosomes. Consequently, it was long assumed that the translation initiation apparatus was built solely for a cap-dependent (CD) mechanism. Exceptions that emerged invoke structural damage (proteolytic cleavage) to eukaryotic initiation factor 4 (eIF4) factors that disable cap recognition. The residual eIF4 complex is thought to be crippled, but capable of cap-independent (CI) translation to recruit viral or death-associated mRNAs begrudgingly when cells are in great distress. However, situations where CI translation coexists with CD translation are now known. In such cases, CI translation is still a minor mechanism in the major background of CD synthesis. In this review, I propose that germ cells do not fit this mold. Using observations from various animal models of oogenesis and spermatogenesis, I suggest that CI translation is a robust partner to CD translation to carry out the translational control that is so prevalent in germ cell development. Evidence suggests that CI translation provides surveillance of germ cell homeostasis, while CD translation governs the regulated protein synthesis that ushers these meiotic cells through the remarkable steps in sperm/oocyte differentiation.

RAB-35 and ARF-6 GTPases Mediate Engulfment and Clearance Following Linker Cell-Type Death

Clearance of dying cells is essential for development and homeostasis. Conserved genes mediate apoptotic cell removal, but whether these genes control non-apoptotic cell removal is a major open question. Linker cell-type death (LCD) is a prevalent non-apoptotic developmental cell death process with features conserved from C. elegans to vertebrates. Using microfluidics-based long-term in vivo imaging, we show that unlike apoptotic cells, the C. elegans linker cell, which dies by LCD, is competitively phagocytosed by two neighboring cells, resulting in cell splitting. Subsequent cell elimination does not require apoptotic engulfment genes. Rather, we find that RAB-35 GTPase is a key coordinator of competitive phagocytosis onset and cell degradation. RAB-35 binds CNT-1, an ARF-6 GTPase activating protein, and removes ARF-6, a degradation inhibitor, from phagosome membranes. This facilitates phosphatidylinositol-4,5-bisphosphate removal from phagosome membranes, promoting phagolysosome maturation. Our studies suggest that RAB-35 and ARF-6 drive a conserved program eliminating cells dying by LCD.

Interrogating the relevance of mitochondrial apoptosis for vertebrate development and postnatal tissue homeostasis

"Programmed cell death or 'apoptosis' is critical for organogenesis during embryonic development and tissue homeostasis in the adult. Its deregulation can contribute to a broad range of human pathologies, including neurodegeneration, cancer, or autoimmunity…" These or similar phrases have become generic opening statements in many reviews and textbooks describing the physiological relevance of apoptotic cell death. However, while the role in disease has been documented beyond doubt, facilitating innovative drug discovery, we wonder whether the former is really true. What goes wrong in vertebrate development or in adult tissue when the main route to apoptotic cell death, controlled by the BCL2 family, is impaired? Such scenarios have been mimicked by deletion of one or more prodeath genes within the BCL2 family, and gene targeting studies in mice exploring the consequences have been manifold. Many of these studies were geared toward understanding the role of BCL2 family proteins and mitochondrial apoptosis in disease, whereas fewer focused in detail on their role during normal development or tissue homeostasis, perhaps also due to an irritating lack of phenotype. Looking at these studies, the relevance of classical programmed cell death by apoptosis for development appears rather limited. Together, these many studies suggest either highly selective and context-dependent contributions of mitochondrial apoptosis or significant redundancy with alternative cell death mechanisms, as summarized and discussed here.

A High-Throughput Small Molecule Screen for C. elegans Linker Cell Death Inhibitors

Programmed cell death is a ubiquitous process in metazoan development. Apoptosis, one cell death form, has been studied extensively. However, mutations inactivating key mammalian apoptosis regulators do not block most developmental cell culling, suggesting that other cell death pathways are likely important. Recent work in the nematode Caenorhabditis elegans identified a non-apoptotic cell death form mediating the demise of the male-specific linker cell. This cell death process (LCD, linker cell-type death) is morphologically conserved, and its molecular effectors also mediate axon degeneration in mammals and Drosophila. To develop reagents to manipulate LCD, we established a simple high-throughput screening protocol for interrogating the effects of small molecules on C. elegans linker cell death in vivo. From 23,797 compounds assayed, 11 reproducibly block linker cell death onset. Of these, five induce animal lethality, and six promote a reversible developmental delay. These results provide proof-of principle validation of our screening protocol, demonstrate that developmental progression is required for linker cell death, and suggest that larger scale screens may identify LCD-specific small-molecule regulators that target the LCD execution machinery.

Transcriptional control of non-apoptotic developmental cell death in C. elegans

Programmed cell death is an essential aspect of animal development. Mutations in vertebrate genes that mediate apoptosis only mildly perturb development, suggesting that other cell death modes likely have important roles. Linker cell-type death (LCD) is a morphologically conserved cell death form operating during the development of Caenorhabditis elegans and vertebrates. We recently described a molecular network governing LCD in C. elegans, delineating a key role for the transcription factor heat-shock factor 1 (HSF-1). Although HSF-1 functions to protect cells from stress in many settings by inducing expression of protein folding chaperones, it promotes LCD by inducing expression of the conserved E2 ubiquitin-conjugating enzyme LET-70/UBE2D2, which is not induced by stress. Following whole-genome RNA interference and candidate gene screens, we identified and characterized four conserved regulators required for LCD. Here we show that two of these, NOB-1/Hox and EOR-1/PLZF, act upstream of HSF-1, in the context of Wnt signaling. A third protein, NHR-67/TLX/NR2E1, also functions upstream of HSF-1, and has a separate activity that prevents precocious expression of HSF-1 transcriptional targets. We demonstrate that the SET-16/mixed lineage leukemia 3/4 (MLL3/4) chromatin regulation complex functions at the same step or downstream of HSF-1 to control LET-70/UBE2D2 expression. Our results identify conserved proteins governing LCD, and demonstrate that transcriptional regulators influence this process at multiple levels.

Studies on the characteristics and mechanisms of testicular toxicity induced by Hydroxyurea

OBJECTIVE

Apoptosis plays a dominant role in both spontaneous spermatogenesis and germ cell death. This study was aimed to investigate the functions of related genes in testicular germ cell death induced by Hydroxyurea (HU).

METHOD

Wild-type (WT) and FasL transgenic (TG) DBA/C57BL mice were intraperitoneal injected with 400 mg/kg HU. Twelve hours later, testes were collected. Histomorphology of testis was observed by staining with Periodic Acid Schiff (PAS). Apoptosis was assessed by TUNEL assay. mRNA and protein levels of related genes were evaluated by quantitative RT-PCR and Western blot, respectively.

RESULTS

The 2 × 2 factorial design comparative experiments between the WT and TG mice showed that the TG mice exhibited a higher basal apoptotic index. The basal mRNA levels of Fas and FasL and protein levels of Fas, FasL, Caspase-3, Caspase-8 and Caspase-9 in the TG mice were also higher than that in the WT mice. Twelve hours after injection of HU, the testicular tubules exhibited no significantly morphological changes but apoptosis index remarkably increased in both the WT and TG mice, with the latter having the higher amplitude. Although, HU up-regulated the mRNA of apoptosis-related genes, such as Fas and FasL, in both the TG and WT mice, the increased amplitude was more obvious in the TG mice. By Western blot analysis, apoptosis-related proteins Fas, FasL Caspase-3, Caspase-8 and Caspase-9 were significantly increased in both the WT and TG mice, with the TG mice exhibiting a greater up-regulation.

CONCLUSION

Germ cell apoptosis induced by the HU treatment may be related to the FasL-mediated signal transduction pathway.

An epistatic effect of apaf-1 and caspase-9 on chlamydial infection

Chlamydia is an obligate intracellular bacterial pathogen that replicates solely within a membrane-bound vacuole termed an inclusion. Chlamydia seems to perturb multiple cellular processes of the host, such as, rearrangement of the membrane trafficking system for its intracellular multiplication, and inhibition of host cell apoptosis for persistent infection. In an attempt to clarify host factor involvement in apoptosis regulation, we found that inhibition of Caspase-9 restricted, while Apaf-1 promoted, Chlamydia pneumoniae infection in HEp-2, HeLa, and mouse epithelial fibroblast (MEF) cells. These opposition contributions to the chlamydial infection were confirmed using caspase-9^−/− and apaf-1^−/− MEFs. Similar phenomena also appeared in the case of infection with Chlamydia trachomatis. Interestingly, caspase-9 in apaf-1^−/− MEFs was activated by chlamydial infection but during the infection caspase-3 was not activated. That is, caspase-9 was activated without support for multiplication and activation by Apaf-1, and the activated caspase-9 may be physically disconnected from the caspase cascade. This may be partially explained by the observation of caspase-9 accumulation within chlamydial inclusions. The sequestration of caspase-9 by chlamydia seems to result in apoptosis repression, which is crucial for the chlamydial development cycle. Because Apaf-1 shares domains with intracellular innate immune receptor NOD1, it may play a key role in the strategy to regulate chlamydial infection.

HSF-1 activates the ubiquitin proteasome system to promote non-apoptotic developmental cell death in C. elegans

Apoptosis is a prominent metazoan cell death form. Yet, mutations in apoptosis regulators cause only minor defects in vertebrate development, suggesting that another developmental cell death mechanism exists. While some non-apoptotic programs have been molecularly characterized, none appear to control developmental cell culling. Linker-cell-type death (LCD) is a morphologically conserved non-apoptotic cell death process operating in Caenorhabditis elegans and vertebrate development, and is therefore a compelling candidate process complementing apoptosis. However, the details of LCD execution are not known. Here we delineate a molecular-genetic pathway governing LCD in C. elegans. Redundant activities of antagonistic Wnt signals, a temporal control pathway, and mitogen-activated protein kinase kinase signaling control heat shock factor 1 (HSF-1), a conserved stress-activated transcription factor. Rather than protecting cells, HSF-1 promotes their demise by activating components of the ubiquitin proteasome system, including the E2 ligase LET-70/UBE2D2 functioning with E3 components CUL-3, RBX-1, BTBD-2, and SIAH-1. Our studies uncover design similarities between LCD and developmental apoptosis, and provide testable predictions for analyzing LCD in vertebrates.

DOI:http://dx.doi.org/10.7554/eLife.12821.001

Embryos make numerous new cells as they develop, but also destroy many cells to remove the faulty ones and to ensure that tissues grow to the right size and shape. This deliberate form of cell death must be precisely regulated to prevent too many cells or healthy cells, from being destroyed. Understanding the molecular mechanisms that govern cell death is therefore important for understanding normal development and also human disease.

One well-studied process that leads to cell death is called apoptosis. This process carefully dismantles and breaks down the components of a cell, but does not seem to account for all cell death that occurs during animal development. Recently another developmental cell-death pathway, called the linker-cell-type death, was discovered in a small roundworm called Caenorhabditis elegans. This pathway appears to work in mammalian cells as well, and may help to break down nerve fibers that are not needed. However, many of this pathway’s component parts remained unknown.

Kinet, Malin et al. have now used a combination of genetics and cell biology in C. elegans to uncover the components of linker-cell-type death and to investigate how they interact. The results of these studies revealed a hierarchy of genetic interactions that governs this pathway in C. elegans. One protein called HSF-1 plays a particularly important role. This protein is a transcription factor and it binds to, and regulates, the activities of various genes. HSF-1 usually works in cells to protect them from stress, but Kinet, Malin et al. showed that it instead promotes linker-cell-type death by activating a molecular machine, called the proteasome, that breaks down proteins. The experiments also revealed two proteins (called BTBD-2 and SIAH-1) that may be important for shuttling specific proteins for degradation by the proteasome.

Three signalling pathways that regulate important developmental processes also regulate the activation of linker-cell-type death. Kinet, Malin et al. propose that these signalling pathways do so by working together to activate HSF-1, which in turn activates the genes that lead to the destruction of cells by the proteasome.

A future challenge is to understand in more detail how the more recently discovered cell death pathway actually kills cells. Further work could also explore how HSF-1, a protein that normally protects cells, is transformed into a cell-killing protein.

DOI:http://dx.doi.org/10.7554/eLife.12821.002

Molecular Cell Biology of Apoptosis and Necroptosis in Cancer

Cell death is a major mechanism to eliminate cells in which DNA is damaged, organelles are stressed, or oncogenes are overexpressed, all events that would otherwise predispose cells to oncogenic transformation. The pathways that initiate and execute cell death are complex, genetically encoded, and subject to significant regulation. Consequently, while these pathways are often mutated in malignancy, there is considerable interest in inducing cell death in tumor cells as therapy. This chapter addresses our current understanding of molecular mechanisms contributing to two cell death pathways, apoptotic cell death and necroptosis, a regulated form of necrotic cell death. Apoptosis can be induced by a wide variety of signals, leading to protease activation that dismantles the cell. We discuss the physiological importance of each apoptosis pathway and summarize their known roles in cancer suppression and the current efforts at targeting each pathway therapeutically. The intricate mechanistic link between death receptor-mediated apoptosis and necroptosis is described, as well as the potential opportunities for utilizing necroptosis in the treatment of malignancy.

Fas-Activated Mitochondrial Apoptosis Culls Stalled Embryonic Stem Cells to Promote Differentiation

The intrinsic (mitochondrial) apoptotic pathway is a conserved cell death program crucial for eliminating superfluous, damaged, or incorrectly specified cells, and the multi-domain pro-death BCL-2 family proteins BAX and BAK are required for its activation. In response to internal damage or developmental signals, BAX and/or BAK permeabilize the mitochondrial outer membrane, resulting in cytochrome c release and activation of effector caspases such as Caspase-3 (Casp3). While the mitochondrial apoptotic pathway plays a critical role during late embryonic development in mammals, its role during early development remains controversial. Here, we show that Bax(-/-)Bak(-/-) murine embryonic stem cells (ESCs) display defects during the exit from pluripotency, both in culture and during teratoma formation. Specifically, we find that when ESCs are stimulated to differentiate, a subpopulation fails to do so and instead upregulates FAS in a p53-dependent manner to trigger Bax/Bak-dependent apoptosis. Blocking this apoptotic pathway prevents the removal of these poorly differentiated cells, resulting in the retention of cells that have not exited pluripotency. Taken together, our results provide further evidence for heterogeneity in the potential of ESCs to successfully differentiate and reveal a novel role for apoptosis in promoting efficient ESC differentiation by culling cells that are slow to exit pluripotency.

Cell Death in C. elegans Development

Cell death is a common and important feature of animal development, and cell death defects underlie many human disease states. The nematode Caenorhabditis elegans has proven fertile ground for uncovering molecular and cellular processes controlling programmed cell death. A core pathway consisting of the conserved proteins EGL-1/BH3-only, CED-9/BCL2, CED-4/APAF1, and CED-3/caspase promotes most cell death in the nematode, and a conserved set of proteins ensures the engulfment and degradation of dying cells. Multiple regulatory pathways control cell death onset in C. elegans, and many reveal similarities with tumor formation pathways in mammals, supporting the idea that cell death plays key roles in malignant progression. Nonetheless, a number of observations suggest that our understanding of developmental cell death in C. elegans is incomplete. The interaction between dying and engulfing cells seems to be more complex than originally appreciated, and it appears that key aspects of cell death initiation are not fully understood. It has also become apparent that the conserved apoptotic pathway is dispensable for the demise of the C. elegans linker cell, leading to the discovery of a previously unexplored gene program promoting cell death. Here, we review studies that formed the foundation of cell death research in C. elegans and describe new observations that expand, and in some cases remodel, this edifice. We raise the possibility that, in some cells, more than one death program may be needed to ensure cell death fidelity.

The Roles of Hedgehog Signaling in Upper Lip Formation

Craniofacial development consists of a highly complex sequence of the orchestrated growth and fusion of facial processes. It is also known that craniofacial abnormalities can be detected in 1/3 of all patients with congenital diseases. Within the various craniofacial abnormalities, orofacial clefting is one of the most common phenotypic outcomes associated with retarded facial growth or fusion. Cleft lip is one of the representative and frequently encountered conditions in the spectrum of orofacial clefting. Despite various mechanisms or signaling pathways that have been proposed to be the cause of cleft lip, a detailed mechanism that bridges individual signaling pathways to the cleft lip is still elusive. Shh signaling is indispensable for normal embryonic development, and disruption can result in a wide spectrum of craniofacial disorders, including cleft lip. This review focuses on the current knowledge about the mechanisms of facial development and the etiology of cleft lip that are related to Shh signaling.

Die another way--non-apoptotic mechanisms of cell death

Regulated, programmed cell death is crucial for all multicellular organisms. Cell death is essential in many processes, including tissue sculpting during embryogenesis, development of the immune system and destruction of damaged cells. The best-studied form of programmed cell death is apoptosis, a process that requires activation of caspase proteases. Recently it has been appreciated that various non-apoptotic forms of cell death also exist, such as necroptosis and pyroptosis. These non-apoptotic cell death modalities can be either triggered independently of apoptosis or are engaged should apoptosis fail to execute. In this Commentary, we discuss several regulated non-apoptotic forms of cell death including necroptosis, autophagic cell death, pyroptosis and caspase-independent cell death. We outline what we know about their mechanism, potential roles in vivo and define outstanding questions. Finally, we review data arguing that the means by which a cell dies actually matters, focusing our discussion on inflammatory aspects of cell death.

Convergence and extrusion are required for normal fusion of the mammalian secondary palate

The fusion of two distinct prominences into one continuous structure is common during development and typically requires integration of two epithelia and subsequent removal of that intervening epithelium. Using confocal live imaging, we directly observed the cellular processes underlying tissue fusion, using the secondary palatal shelves as a model. We find that convergence of a multi-layered epithelium into a single-layer epithelium is an essential early step, driven by cell intercalation, and is concurrent to orthogonal cell displacement and epithelial cell extrusion. Functional studies in mice indicate that this process requires an actomyosin contractility pathway involving Rho kinase (ROCK) and myosin light chain kinase (MLCK), culminating in the activation of non-muscle myosin IIA (NMIIA). Together, these data indicate that actomyosin contractility drives cell intercalation and cell extrusion during palate fusion and suggest a general mechanism for tissue fusion in development.

A study of the mouse palate shows that the fusion of tissues during development involves convergence and displacement of epithelial cells, coupled with cell extrusion driven by the contractile activity of actomyosin.

Tissue fusion, the process by which two independent prominences become united to form one continuous structure, is common during development, and its failure leads to multiple structural birth defects. In this study, we directly examine the cellular and molecular mechanisms by which tissue fusion occurs using the mouse secondary palate as a model. Using live imaging, we find that fusion of the secondary palatal shelves proceeds by a progression of previously undescribed cell behaviors. Cellular protrusions and establishment of contacts between palatal shelves leads to the formation of a transient multicellular epithelial structure that then converges toward the midline, driven by cell intercalation. This convergence occurs together with displacement of the epithelium and epithelial cell extrusions that squeeze epithelial cells out from between the palatal shelves and mediate continuity of the structure. We show that in mice this morphogenesis requires an actomyosin contractility pathway culminating in non-muscle myosin IIA activation. Altogether, these data support a new model for tissue fusion during mouse embryogenesis in which convergence, displacement, and cell extrusion drive the union of independent structures.

Cell death in development: Signaling pathways and core mechanisms

Programmed cell death eliminates unneeded and dangerous cells in a timely and effective manner during development. In this review, we examine the role cell death plays during development in worms, flies and mammals. We discuss signaling pathways that regulate developmental cell death, and describe how they communicate with the core cell death pathways. In most organisms, the majority of developmental cell death is seen in the nervous system. Therefore we focus on what is known about the regulation of developmental cell death in this tissue. Understanding how the cell death is regulated during development may provide insight into how this process can be manipulated in the treatment of disease.

Tug of war--the influence of opposing physical forces on epithelial cell morphology

The shape of a single animal cell is determined both by its internal cytoskeleton and through physical interactions with its environment. In a tissue context, this extracellular environment is made up largely of other cells and the extracellular matrix. As a result, the shape of cells residing within an epithelium will be determined both by forces actively generated within the cells themselves and by their deformation in response to forces generated elsewhere in the tissue as they propagate through cell-cell junctions. Together these complex patterns of forces combine to drive epithelial tissue morphogenesis during both development and homeostasis. Here we review the role of both active and passive cell shape changes and mechanical feedback control in tissue morphogenesis in different systems.

Caspase-9 is required for normal hematopoietic development and protection from alkylator-induced DNA damage in mice

Apoptosis and the DNA damage responses have been implicated in hematopoietic development and differentiation, as well as in the pathogenesis of myelodysplastic syndromes (MDS) and leukemia. However, the importance of late-stage mediators of apoptosis in hematopoiesis and leukemogenesis has not been elucidated. Here, we examine the role of caspase-9 (Casp9), the initiator caspase of the intrinsic apoptotic cascade, in murine fetal and adult hematopoiesis. Casp9 deficiency resulted in decreased erythroid and B-cell progenitor abundance and impaired function of hematopoietic stem cells after transplantation. Mouse bone marrow chimeras lacking Casp9 or its cofactor Apaf1 developed low white blood cell counts, decreased B-cell numbers, anemia, and reduced survival. Defects in apoptosis have also been previously implicated in susceptibility to therapy-related leukemia, a disease caused by exposure to DNA-damaging chemotherapy. We found that the burden of DNA damage was increased in Casp9-deficient cells after exposure to the alkylator, N-ethyl-nitrosourea (ENU). Furthermore, exome sequencing revealed that oligoclonal hematopoiesis emerged in Casp9-deficient bone marrow chimeras after alkylator exposure. Taken together, these findings suggest that defects in apoptosis could be a key step in the pathogenesis of alkylator-associated secondary malignancies.

Putting the pieces together: How is the mitochondrial pathway of apoptosis regulated in cancer and chemotherapy?

In order to solve a jigsaw puzzle, one must first have the complete picture to logically connect the pieces. However, in cancer biology, we are still gaining an understanding of all the signaling pathways that promote tumorigenesis and how these pathways can be pharmacologically manipulated by conventional and targeted therapies. Despite not having complete knowledge of the mechanisms that cause cancer, the signaling networks responsible for cancer are becoming clearer, and this information is serving as a solid foundation for the development of rationally designed therapies. One goal of chemotherapy is to induce cancer cell death through the mitochondrial pathway of apoptosis. Within this review, we present the pathways that govern the cellular decision to undergo apoptosis as three distinct, yet connected puzzle pieces: (1) How do oncogene and tumor suppressor pathways regulate apoptosis upstream of mitochondria? (2) How does the B-cell lymphoma 2 (BCL-2) family influence tumorigenesis and chemotherapeutic responses? (3) How is post-mitochondrial outer membrane permeabilization (MOMP) regulation of cell death relevant in cancer? When these pieces are united, it is possible to appreciate how cancer signaling directly impacts upon the fundamental cellular mechanisms of apoptosis and potentially reveals novel pharmacological targets within these pathways that may enhance chemotherapeutic success.

Essential versus accessory aspects of cell death: recommendations of the NCCD 2015

Cells exposed to extreme physicochemical or mechanical stimuli die in an uncontrollable manner, as a result of their immediate structural breakdown. Such an unavoidable variant of cellular demise is generally referred to as ‘accidental cell death' (ACD). In most settings, however, cell death is initiated by a genetically encoded apparatus, correlating with the fact that its course can be altered by pharmacologic or genetic interventions. ‘Regulated cell death' (RCD) can occur as part of physiologic programs or can be activated once adaptive responses to perturbations of the extracellular or intracellular microenvironment fail. The biochemical phenomena that accompany RCD may be harnessed to classify it into a few subtypes, which often (but not always) exhibit stereotyped morphologic features. Nonetheless, efficiently inhibiting the processes that are commonly thought to cause RCD, such as the activation of executioner caspases in the course of apoptosis, does not exert true cytoprotective effects in the mammalian system, but simply alters the kinetics of cellular demise as it shifts its morphologic and biochemical correlates. Conversely, bona fide cytoprotection can be achieved by inhibiting the transduction of lethal signals in the early phases of the process, when adaptive responses are still operational. Thus, the mechanisms that truly execute RCD may be less understood, less inhibitable and perhaps more homogeneous than previously thought. Here, the Nomenclature Committee on Cell Death formulates a set of recommendations to help scientists and researchers to discriminate between essential and accessory aspects of cell death.

The role of E3 ligases in the ubiquitin-dependent regulation of spermatogenesis

The ubiquitination of proteins is a post-translational modification that was first described as a means to target misfolded or unwanted proteins for degradation by the proteasome. It is now appreciated that the ubiquitination of proteins also serves as a mechanism to modify protein function and cellular functions such as protein trafficking, cell signaling, DNA repair, chromatin modifications, cell-cycle progression and cell death. The ubiquitination of proteins occurs through the hierarchal transfer of ubiquitin from an E1 ubiquitin-activating enzyme to an E2 ubiquitin-conjugating enzyme and finally to an E3 ubiquitin ligase that transfers the ubiquitin to its target protein. It is the final E3 ubiquitin ligase that confers the substrate specificity for ubiquitination and is the focus of this review. Spermatogenesis is a complex and highly regulated process by which spermatogonial stem cells undergo mitotic proliferation and expansion of the diploid spermatogonial population, differentiate into spermatocytes and progress through two meiotic divisions to produce haploid spermatids that proceed through a final morphogenesis to generate mature spermatozoa. The ubiquitination of proteins in the cells of the testis occurs in many of the processes required for the progression of mature spermatozoa. Since it is the E3 ubiquitin ligase that recognizes the target protein and provides the specificity and selectivity for ubiquitination, this review highlights known examples of E3 ligases in the testis and the differing roles that they play in maintaining functional spermatogenesis.

Autophagy and apoptosis dysfunction in neurodegenerative disorders

Autophagy and apoptosis are basic physiologic processes contributing to the maintenance of cellular homeostasis. Autophagy encompasses pathways that target long-lived cytosolic proteins and damaged organelles. It involves a sequential set of events including double membrane formation, elongation, vesicle maturation and finally delivery of the targeted materials to the lysosome. Apoptotic cell death is best described through its morphology. It is characterized by cell rounding, membrane blebbing, cytoskeletal collapse, cytoplasmic condensation, and fragmentation, nuclear pyknosis, chromatin condensation/fragmentation, and formation of membrane-enveloped apoptotic bodies, that are rapidly phagocytosed by macrophages or neighboring cells. Neurodegenerative disorders are becoming increasingly prevalent, especially in the Western societies, with larger percentage of members living to an older age. They have to be seen not only as a health problem, but since they are care-intensive, they also carry a significant economic burden. Deregulation of autophagy plays a pivotal role in the etiology and/or progress of many of these diseases. Herein, we briefly review the latest findings that indicate the involvement of autophagy in neurodegenerative diseases. We provide a brief introduction to autophagy and apoptosis pathways focusing on the role of mitochondria and lysosomes. We then briefly highlight pathophysiology of common neurodegenerative disorders like Alzheimer's diseases, Parkinson's disease, Huntington's disease and Amyotrophic lateral sclerosis. Then, we describe functions of autophagy and apoptosis in brain homeostasis, especially in the context of the aforementioned disorders. Finally, we discuss different ways that autophagy and apoptosis modulation may be employed for therapeutic intervention during the maintenance of neurodegenerative disorders.

Gene expression and reproductive abilities of male Drosophila melanogaster subjected to ELF-EMF exposure

Extremely low frequency electromagnetic field (ELF-EMF) exposure is attracting increased attention as a possible disease-inducing factor. The in vivo effects of short-term and long-term ELF-EMF exposure on male Drosophila melanogaster were studied using transcriptomic analysis for preliminary screening and QRT-PCR for further verification. Transcriptomic analysis indicated that 439 genes were up-regulated and 874 genes were down-regulated following short-term exposures and that 514 genes were up-regulated and 1206 genes were down-regulated following long-term exposures (expression >2- or <0.5-fold, respectively). In addition, there are 238 up-regulated genes and 598 down-regulated genes in the intersection of short-term and long-term exposure (expression >2- or <0.5-fold). The DEGs (differentially expressed genes) in D. melanogaster following short-term exposures were involved in metabolic processes, cytoskeletal organization, mitotic spindle organization, cell death, protein modification and proteolysis. Long-term exposure let to changes in expression of genes involved in metabolic processes, response to stress, mitotic spindle organization, aging, cell death and cellular respiration. In the intersection of short-term and long-term exposure, a series of DEGs were related to apoptosis, aging, immunological stress and reproduction. To check the ELF-EMF effects on reproduction, some experiments on male reproduction ability were performed. Their results indicated that short-term ELF-EMF exposure may decrease the reproductive ability of males, but long-term exposures had no effect on reproductive ability. Down-regulation of ark gene in the exposed males suggests that the decrease in reproductive capacity may be induced by the effects of ELF-EMF exposure on spermatogenesis through the caspase pathway. QRT-PCR analysis confirmed that jra, ark and decay genes were down regulated in males exposed for 1 Generation (1G) and 72 h, which suggests that apoptosis may be inhibited in vivo. ELF-EMF exposure may have accelerated cell senescence, as suggested by the down-regulation of both cat and jra genes and the up-regulation of hsp22 gene. Up-regulation of totA and hsp22 genes during exposure suggests that exposed flies might induce an in vivo immune response to counter the adverse effects encountered during ELF-EMF exposure. Down-regulation of cat genes suggests that the partial oxidative protection system might be restrained, especially during short-term exposures. This study demonstrates the bioeffects of ELF-EMF exposure and provides evidence for understanding the in vivo mechanisms of ELF-EMF exposure on male D. melanogaster.

Mitochondria and mammalian reproduction

Mitochondria are cellular organelles with crucial roles in ATP synthesis, metabolic integration, reactive oxygen species (ROS) synthesis and management, the regulation of apoptosis (namely via the intrinsic pathway), among many others. Additionally, mitochondria in different organs or cell types may have distinct properties that can decisively influence functional analysis. In terms of the importance of mitochondria in mammalian reproduction, and although there are species-specific differences, these aspects involve both energetic considerations for gametogenesis and fertilization, control of apoptosis to ensure the proper production of viable gametes, and ROS signaling, as well as other emerging aspects. Crucially, mitochondria are the starting point for steroid hormone biosynthesis, given that the conversion of cholesterol to pregnenolone (a common precursor for all steroid hormones) takes place via the activity of the cytochrome P450 side-chain cleavage enzyme (P450scc) on the inner mitochondrial membrane. Furthermore, mitochondrial activity in reproduction has to be considered in accordance with the very distinct strategies for gamete production in the male and female. These include distinct gonad morpho-physiologies, different types of steroids that are more prevalent (testosterone, estrogens, progesterone), and, importantly, the very particular timings of gametogenesis. While spermatogenesis is complete and continuous since puberty, producing a seemingly inexhaustible pool of gametes in a fixed environment; oogenesis involves the episodic production of very few gametes in an environment that changes cyclically. These aspects have always to be taken into account when considering the roles of any common element in mammalian reproduction.

Apaf1 apoptotic function critically limits Sonic hedgehog signaling during craniofacial development

Apaf1 is an evolutionarily conserved component of the apoptosome. In mammals, the apoptosome assembles when cytochrome c is released from mitochondria, binding Apaf1 in an ATP-dependent manner and activating caspase 9 to execute apoptosis. Here we identify and characterize a novel mouse mutant, yautja, and find it results from a leucine-to-proline substitution in the winged-helix domain of Apaf1. We show that this allele of Apaf1 is unique, as the yautja mutant Apaf1 protein is stable, yet does not possess apoptotic function in cell culture or in vivo assays. Mutant embryos die perinatally with defects in craniofacial and nervous system development, as well as reduced levels of apoptosis. We further investigated the defects in craniofacial development in the yautja mutation and found altered Sonic hedgehog (Shh) signaling between the prechordal plate and the frontonasal ectoderm, leading to increased mesenchymal proliferation in the face and delayed or absent ossification of the skull base. Taken together, our data highlight the time-sensitive link between Shh signaling and the regulation of apoptosis function in craniofacial development to sculpt the face. We propose that decreased apoptosis in the developing nervous system allows Shh-producing cells to persist and direct a lateral outgrowth of the upper jaw, resulting in the craniofacial defects we see. Finally, the novel yautja Apaf1 allele offers the first in vivo understanding of a stable Apaf1 protein that lacks a function, which should make a useful tool with which to explore the regulation of programmed cell death in mammals.

How to form and close the brain: insight into the mechanism of cranial neural tube closure in mammals

The development of the embryonic brain critically depends on successfully completing cranial neural tube closure (NTC). Failure to properly close the neural tube results in significant and potentially lethal neural tube defects (NTDs). We believe these malformations are caused by disruptions in normal developmental programs such as those involved in neural plate morphogenesis and patterning, tissue fusion, and coordinated cell behaviors. Cranial NTDs include anencephaly and craniorachischisis, both lethal human birth defects. Newly emerging methods for molecular and cellular analysis offer a deeper understanding of not only the developmental NTC program itself but also mechanical and kinetic aspects of closure that may contribute to cranial NTDs. Clarifying the underlying mechanisms involved in NTC and how they relate to the onset of specific NTDs in various experimental models may help us develop novel intervention strategies to prevent NTDs.

The nuclear receptor NR4A1 induces a form of cell death dependent on autophagy in mammalian cells

The control of cell death is a biological process essential for proper development, and for preventing devastating pathologies like cancer and neurodegeneration. On the other hand, autophagy regulation is essential for protein and organelle degradation, and its dysfunction is associated with overlapping pathologies like cancer and neurodegeneration, but also for microbial infection and aging. In the present report we show that two evolutionarily unrelated receptors--Neurokinin 1 Receptor (NK(1)R,) a G-protein coupled receptor, and Insulin-like Growth Factor 1 Receptor (IGF1R), a tyrosine kinase receptor--both induce non-apoptotic cell death with autophagic features and requiring the activity of the autophagic core machinery proteins PI3K-III, Beclin-1 and Atg7. Remarkably, this form of cell death occurs in apoptosis-competent cells. The signal transduction pathways engaged by these receptors both converged on the activation of the nuclear receptor NR4A1, which has previously been shown to play a critical role in some paradigms of apoptosis and in NK(1)R-induced cell death. The activity of NR4A1 was necessary for IGF1R-induced cell death, as well as for a canonical model of cell death by autophagy induced by the presence of a pan-caspase inhibitor, suggesting that NR4A1 is a general modulator of this kind of cell death. During cell death by autophagy, NR4A1 was transcriptionally competent, even though a fraction of it was present in the cytoplasm. Interestingly, NR4A1 interacts with the tumor suppressor p53 but not with Beclin-1 complex. Therefore the mechanism to promote cell death by autophagy might involve regulation of gene expression, as well as protein interactions. Understanding the molecular basis of autophagy and cell death mediation by NR4A1, should provide novel insights and targets for therapeutic intervention.

A perspective on mammalian caspases as positive and negative regulators of inflammation

Members of the caspase family of cysteine proteases coordinate the morphological and biochemical events that typify apoptosis. However, neutralization of caspase activity in mammals fails to block death in response to most proapoptotic stimuli. This is because many cell death triggers provoke mitochondrial dysfunction upstream of caspase activation as a consequence of BAX/BAK channel opening. Although genetic or pharmacological inactivation of caspases fails to block cell death in most instances, it does convert the phenotype from apoptosis to necrosis. This has important implications for how the immune system responds to such cells, as necrotic cells provoke inflammation whereas apoptotic cells typically do not. Here, we propose an alternative perspective on apoptosis-associated caspase function by suggesting that these proteases are activated, not to kill, but to extinguish the proinflammatory properties of dying cells. This perspective unifies the mammalian caspase family as either positive or negative regulators of inflammation.

The apoptotic function analysis of p53, Apaf1, Caspase3 and Caspase7 during the spermatogenesis of the Chinese fire-bellied newt Cynops orientalis

BACKGROUND

Spontaneous and stress-induced germ cell apoptosis during spermatogenesis of multicellular organisms have been investigated broadly in mammals. Spermatogenetic process in urodele amphibians was essentially like that in mammals in spite of morphological differences; however, the mechanism of germ cell apoptosis in urodele amphibians remains unknown. The Chinese fire-belly newt, Cynops orientalis, was an excellent organism for studying germ cell apoptosis due to its sensitiveness to temperature, strong endurance of starvation, and sensitive skin to heavy metal exposure.

METHODOLOGY/PRINCIPAL FINDINGS

TUNEL result showed that spontaneous germ cell apoptosis took place in normal newt, and severe stress-induced apoptosis occurred to spermatids and sperm in response to heat shock (40°C 2 h), cold exposure (4°C 12 h), cadmium exposure (Cd 36 h), and starvation stress. Quantitative reverse transcription polymerase chain reactions (qRT-PCR) showed that gene expression of Caspase3 or Caspase7 was obviously elevated after stress treatment. Apaf1 was not altered at its gene expression level, and p53 was significantly decreased after various stress treatment. Caspase assay demonstrated that Caspase-3, -8, -9 enzyme activities in newt testis were significantly elevated after heat shock (40°C 2 h), cold exposure (4°C 12 h), and cadmium exposure (Cd 36 h), while Caspase3 and Caspase8 activities were increased with Caspase9 significantly decreased after starvation treatment.

CONCLUSIONS/SIGNIFICANCE

Severe germ cell apoptosis triggered by heat shock, cold exposure, and cadmium exposure was Caspase3 dependent, which probably involved both extrinsic and intrinsic pathways. Apaf1 may be involved in this process without elevating its gene expression. But starvation-induced germ cell apoptosis was likely mainly through extrinsic pathway. p53 was probably not responsible for stress-induced germ cell apoptosis in newt testis. The intriguing high occurrence of spermatid and sperm apoptosis probably resulted from the sperm morphology and unique reproduction policy of Chinese fire-belly newt, Cynops orientalis.

Discovery Genetics - The History and Future of Spontaneous Mutation Research

Historically, spontaneous mutations in mice have served as valuable models of heritable human diseases, contributing substantially to our understanding of both disease mechanisms and basic biological pathways. While advances in molecular technologies have improved our ability to create mouse models of human disease through targeted mutagenesis and transgenesis, spontaneous mutations continue to provide valuable research tools for discovery of novel genes and functions. In addition, the genetic defects caused by spontaneous mutations are molecularly similar to mutations in the human genome and, therefore often produce phenotypes that more closely resemble those characteristic of human disease than do genetically engineered mutations. Due to the rarity with which spontaneous mutations arise and the animal intensive nature of their genetic analysis, large-scale spontaneous mutation analysis has traditionally been limited to large mammalian genetics institutes. More recently, ENU mutagenesis and new screening methods have increased the rate of mutant strain discovery, and high-throughput DNA sequencing has enabled rapid identification of the underlying genes and their causative mutations. Here, we discuss the continued value of spontaneous mutations for biomedical research.

Control of nonapoptotic developmental cell death in Caenorhabditis elegans by a polyglutamine-repeat protein

Death is a vital developmental cell fate. In Caenorhabditis elegans, programmed death of the linker cell, which leads gonadal elongation, proceeds independently of caspases and apoptotic effectors. To identify genes promoting linker-cell death, we performed a genome-wide RNA interference screen. We show that linker-cell death requires the gene pqn-41, encoding an endogenous polyglutamine-repeat protein. pqn-41 functions cell-autonomously and is expressed at the onset of linker-cell death. pqn-41 expression is controlled by the mitogen-activated protein kinase kinase SEK-1, which functions in parallel to the zinc-finger protein LIN-29 to promote cellular demise. Linker-cell death is morphologically similar to cell death associated with normal vertebrate development and polyglutamine-induced neurodegeneration. Our results may therefore provide molecular inroads to understanding nonapoptotic cell death in metazoan development and disease.