Unexpected link between an antibiotic, pannexin channels and apoptosis

A near-infrared light-controlled, ultrasensitive one-step photoelectrochemical detection of dual cell apoptosis indicators in living cancer cells

The proposed near-infrared (NIR) light-controlled, one-step photoelectrochemical (PEC) strategy could simultaneously detect cell apoptosis indicators, phosphatidylserine (Pho) and sodium potassium adenosine triphosphatase (Sat), on living cancer cells.

Beneficial and Detrimental Remodeling of Glial Connexin and Pannexin Functions in Rodent Models of Nervous System Diseases

A variety of glial cell functions are supported by connexin and pannexin proteins. These functions include the modulation of synaptic gain, the control of excitability through regulation of the ion and neurotransmitter composition of the extracellular milieu and the promotion of neuronal survival. Connexins and pannexins support these functions through diverse molecular mechanisms, including channel and non-channel functions. The former comprise the formation of gap junction-mediated networks supported by connexin intercellular channels and the formation of pore-like membrane structures or hemichannels formed by both connexins and pannexins. Non-channel functions involve adhesion properties and the participation in signaling intracellular cascades. Pathological conditions of the nervous system such as ischemia, neurodegeneration, pathogen infection, trauma and tumors are characterized by distinctive remodeling of connexin expression and function. However, whether these changes can be interpreted as part of the pathogenesis, or as beneficial compensatory effects, remains under debate. Here we review the available evidence addressing this matter with a special emphasis in mouse models with selective manipulation of glial connexin and pannexin proteins in vivo. We postulate that the beneficial vs. detrimental effects of glial connexin remodeling in pathological conditions depend on the impact of remodeling on the different connexin and pannexin channel and non-channel functions, on the characteristics of the inflammatory environment and on the type of interaction among glial cells types.

Living on the Edge: Efferocytosis at the Interface of Homeostasis and Pathology

Nearly every tissue in the body undergoes routine turnover of cells as part of normal healthy living. The majority of these cells undergoing turnover die via apoptosis, and then are rapidly removed by phagocytes by the process of efferocytosis that is anti-inflammatory. However, a number of pathologies have recently been linked to defective clearance of apoptotic cells. Perturbed clearance arises for many reasons, including overwhelming of the clearance machinery, disruptions at different stages of efferocytosis, and responses of phagocytes during efferocytosis, all of which can alter the homeostatic tissue environment. This review covers linkages of molecules involved in the different phases of efferocytosis to disease pathologies that can arise due to their loss or altered function.

Pannexin-1 limits the production of proinflammatory cytokines during necroptosis

The activation of mixed lineage kinase-like (MLKL) by receptor-interacting protein kinase-3 (RIPK3) controls the execution of necroptosis, a regulated form of necrosis that occurs in apoptosis-deficient conditions. Active oligomerized MLKL triggers the exposure of phosphatidylserine residues on the cell surface and disrupts the plasma membrane integrity by forming lytic pores. MLKL also governs endosomal trafficking and biogenesis of small extracellular vesicles as well as the production of proinflammatory cytokines during the early steps of necroptosis; however, the molecular basis continues to be elucidated. Here, we find that MLKL oligomers activate Pannexin-1 (PANX1) channels, concomitantly to the loss of phosphatidylserine asymmetry. This plasma membrane "leakiness" requires the small GTPase RAB27A and RAB27B isoforms, which regulate intracellular vesicle trafficking, docking, and fusion with the plasma membrane. Although cells in which PANX1 is silenced or inhibited normally undergo necroptotic death, they display enhanced production of cytokines such as interleukin-8, indicating that PANX1 may tamper with inflammation. These data identify a novel signaling nexus between MLKL, RAB27, and PANX1 and propose ways to interfere with inflammation associated with necroptosis.

Death for life: a path from apoptotic signaling to tissue-scale effects of apoptotic epithelial extrusion

Apoptosis plays a crucial role in clearing old or critically compromised cells, and actively maintains epithelial homeostasis and epithelial morphogenesis during embryo development. But how is the apoptotic signaling pathway able to orchestrate such complex and dynamic multi-cellular morphological events at the tissue scale? In this review we collected the most updated knowledge regarding how apoptosis controls different cytoskeletal components. We describe how apoptosis can control epithelial homeostasis though epithelial extrusion, a highly orchestrated process based on high- order actomyosin structures and on the coordination between the apoptotic and the neighboring cells. Finally, we describe how the synergy among forces generated by multiple apoptotic cells can shape epithelia in embryo development.

Pannexin-1 promotes NLRP3 activation during apoptosis but is dispensable for canonical or noncanonical inflammasome activation

Inflammasomes are multimeric protein complex that assemble in the cytosol upon microbial infection or cellular stress. Upon activation, inflammasomes drive the maturation of proinflammatory cytokines, IL-1β and IL-18, and also activate the pore-forming protein, gasdermin D to initiate a form of lytic cell death known as "pyroptosis". Pannexin-1 is channel-forming glycoprotein that promotes membrane permeability and ATP release during apoptosis; and was implicated in canonical NLRP3 or noncanonical inflammasome activation. Here, by utilizing three different pannexin-1 channel inhibitors and two lines of Panx1^-/- macrophages, we provide genetic and pharmacological evidence that pannexin-1 is dispensable for canonical or noncanonical inflammasome activation. In contrast, we demonstrate that pannexin-1 cleavage and resulting channel activity during apoptosis promotes NLRP3 inflammasome activation.

Self-Regulation of Memory CD8 T Cell Metabolism through Extracellular ATP Signaling

Following activation, CD8 T cells transition from reliance on mitochondrial respiration to increasing utilization of aerobic glycolysis. After the effector phase, however, reversion to mitochondrial metabolism is pivotal generating memory CD8 T cells. We recently showed that sensing of extracellular ATP (eATP) through the receptor P2RX7 is crucial for both production and the long-term survival of memory CD8 T cells, evidently through promoting mitochondrial maintenance. Unexpectedly, these results indicated that sustained P2RX7 activation is required for memory CD8 T cell homeostasis, suggesting constant exposure to eATP, in contrast with the proposed role of eATP as an acute "danger" signal released by dying cells. Active release through transmembrane channels is another path for eATP export. Indeed, CD8 T cells express Pannexin 1 (Panx1) which has a reported eATP release function in vitro and is itself induced by P2RX7 and/or TCR engagement. Such a role for Panx1 could potentially provide a feed-forward mechanism for cell-autonomous P2RX7 signaling. This model envisages that memory CD8 T cells maintain themselves at the cost of reduced intracellular ATP levels, which at first glance would seem to be detrimental for sustained T cell maintenance. On the other hand, the need to tightly regulate levels of intracellular ATP may be critical for the durability and adaptability of memory CD8 T cells, hence engagement of the P2RX7/Panx1 axis may allow these cells to fine tune their metabolic status to meet changing demands. In this Perspective, we discuss how this pathway may influence memory T cell maintenance.

A new exploration towards aminothiazolquinolone oximes as potentially multi-targeting antibacterial agents: Design, synthesis and evaluation acting on microbes, DNA, HSA and topoisomerase IV

This work did a new exploration towards aminothiazolquinolone oximes as potentially multi-targeting antimicrobial agents. A class of novel hybrids of quinolone, aminothiazole, piperazine and oxime fragments were designed for the first time, conveniently synthesized as well as characterized by ¹H NMR, ¹³C NMR and HRMS spectra. Biological activity showed that some of the synthesized compounds exhibited good antimicrobial activities in comparison with the reference drugs. Especially, O-methyl oxime derivative 10b displayed excellent inhibitory efficacy against MRSA and S. aureus 25923 with MIC values of 0.009 and 0.017 mM, respectively. Further studies indicated that the highly active compound 10b showed low toxicity toward BEAS-2B and A549 cell lines and no obvious propensity to trigger the development of bacterial resistance. Quantum chemical studies have also been conducted and rationally explained the structural features essential for activity. The preliminarily mechanism exploration revealed that compound 10b could not only exert efficient membrane permeability by interfering with the integrity of cells, bind with topoisomerase IV-DNA complex through hydrogen bonds and π-π stacking, but also form a steady biosupramolecular complex by intercalating into DNA to exert the efficient antibacterial activity. The supramolecular interaction between compound 10b and human serum albumin (HSA) was a static quenching, and the binding process was spontaneous, where hydrogen bonds and van der Waals force played vital roles in the supramolecular transportation of the active compound 10b by HSA.

Analysis of extracellular vesicles generated from monocytes under conditions of lytic cell death

Extracellular vesicles (EVs) are an important class of membrane-bound structures that have been widely investigated for their roles in intercellular communication in the contexts of tumor progression, vascular function, immunity and regenerative medicine. Much of the current knowledge on the functions of EVs pertains to those derived from viable cells (e.g. exosomes and microvesicles) or apoptotic cells (e.g. apoptotic bodies) whilst the generation of EVs from dying cells under non-apoptotic conditions remains poorly characterized. Herein, the release of EVs from THP-1 monocytes under conditions of primary necrosis, secondary necrosis and pyroptosis, was investigated. A comprehensive analysis of THP-1-derived EVs revealed that cells undergoing lytic forms of cell death generated a high number of EVs compared with viable or apoptotic cells in vitro. Differential centrifugation via 16,000 g and 100,000 g revealed that dying THP-1 cells release both medium and small EVs, respectively, consistent with the known characteristics of microvesicles and/or exosomes. In addition, large EVs isolated via 2000 g centrifugation were also present in all samples. These findings suggest that lytic cell death under both sterile and non-sterile inflammatory conditions induces monocytes to generate EVs, which could potentially act as mediators of cell-to-cell communication.

ROCK1 but not LIMK1 or PAK2 is a key regulator of apoptotic membrane blebbing and cell disassembly

Many cell types are known to undergo a series of morphological changes during the progression of apoptosis, leading to their disassembly into smaller membrane-bound vesicles known as apoptotic bodies (ApoBDs). In particular, the formation of circular bulges called membrane blebs on the surface of apoptotic cells is a key morphological step required for a number of cell types to generate ApoBDs. Although apoptotic membrane blebbing is thought to be regulated by kinases including ROCK1, PAK2 and LIMK1, it is unclear whether these kinases exhibit overlapping roles in the disassembly of apoptotic cells. Utilising both pharmacological and CRISPR/Cas9 gene editing based approaches, we identified ROCK1 but not PAK2 or LIMK1 as a key non-redundant positive regulator of apoptotic membrane blebbing as well as ApoBD formation. Functionally, we have established an experimental system to either inhibit or enhance ApoBD formation and demonstrated the importance of apoptotic cell disassembly in the efficient uptake of apoptotic materials by various phagocytes. Unexpectedly, we also noted that ROCK1 could play a role in regulating the onset of secondary necrosis. Together, these data shed light on both the mechanism and function of cell disassembly during apoptosis.

Moving beyond size and phosphatidylserine exposure: evidence for a diversity of apoptotic cell-derived extracellular vesicles in vitro

Apoptosis is a form of programmed cell death that occurs throughout life as part of normal development as well as pathologic processes including chronic inflammation and infection. Although the death of a cell is often considered as the only biological outcome of a cell committed to apoptosis, it is becoming increasingly clear that the dying cell can actively communicate with other cells via soluble factors as well as membrane-bound extracellular vesicles (EVs) to regulate processes including cell clearance, immunity and tissue repair. Compared to EVs generated from viable cells such as exosomes and microvesicles, apoptotic cell-derived EVs (ApoEVs) are less well defined and the basic criteria for ApoEV characterization have not been established in the field. In this study, we will examine the current understanding of ApoEVs, in particular, the ApoEV subtype called apoptotic bodies (ApoBDs). We described that a subset of ApoBDs can be larger than 5 μm and smaller than 1 μm based on flow cytometry and live time-lapse microscopy analysis, respectively. We also described that a subset of ApoBDs can expose a relatively low level of phosphatidylserine on its surface based on annexin A5 staining. Furthermore, we characterized the presence of caspase-cleaved proteins (in particular plasma membrane-associated or cytoplasmic proteins) in samples enriched in ApoBDs. Lastly, using a combination of biochemical-, live imaging- and flow cytometry-based approaches, we characterized the progressive lysis of ApoBDs. Taken together, these results extended our understanding of ApoBDs.

Overview of the Components of Cardiac Metabolism

Metabolism in organs other than the liver and kidneys may play a significant role in how a specific organ responds to chemicals. The heart has metabolic capability for energy production and homeostasis. This homeostatic machinery can also process xenobiotics. Cardiac metabolism includes the expression of numerous organic anion transporters, organic cation transporters, organic carnitine (zwitterion) transporters, and ATP-binding cassette transporters. Expression and distribution of the transporters within the heart may vary, depending on the patient’s age, disease, endocrine status, and various other factors. Several cytochrome P450 (P450) enzyme classes have been identified within the heart. The P450 hydroxylases and epoxygenases within the heart produce hydroxyeicosatetraneoic acids and epoxyeicosatrienoic acids, metabolites of arachidonic acid, which are critical in regulating homeostatic processes of the heart. The susceptibility of the cardiac P450 system to induction and inhibition from exogenous materials is an area of expanding knowledge, as are the metabolic processes of glucuronidation and sulfation in the heart. The susceptibility of various transcription factors and signaling pathways of the heart to disruption by xenobiotics is not fully characterized but is an area with implications for disruption of normal postnatal development, as well as modulation of adult cardiac health. There are knowledge gaps in the timelines of physiologic maturation and deterioration of cardiac metabolism. Cross-species characterization of cardiac-specific metabolism is needed for nonclinical work of optimum translational value to predict possible adverse effects, identify sensitive developmental windows for the design and conduct of informative nonclinical and clinical studies, and explore the possibilities of organ-specific therapeutics.

Curcumin-mediated photodynamic inactivation (PDI) against DH5α contaminated in oysters and cellular toxicological evaluation of PDI-treated oysters

The objective of this study was to evaluate the bactericidal effect of curcumin (CUR)-mediated photodynamic inactivation (PDI) against Escherichia coli DH5α in vitro and in oysters, then further investigate the edible security of PDI-treated oysters based on cellular toxicological methods. First, DH5α cells were irradiated by a 470 nm LED light source with an energy density of 3.6 J/cm². Colony forming units (CFU) were counted and the viability of DH5α cells was calculated after treatment with CUR-mediated PDI. Intracellular production of reactive oxygen species (ROS) was studied by measuring the fluorescence of 2, 7-dichlorofluorescein (DCF) using a flow cytometry. Membrane permeability was measured using confocal laser scanning microscopy (CLSM) with propidium iodide (PI) staining. After that, the bactericidal effect of CUR-mediated PDI was evaluated in oysters which were pre-contaminated with DH5α cells. Finally, cellular toxicology of PDI-treated oysters was evaluated through morphological observation, 3-(4, 5-dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide (MTT) assay, DNA ladder assay, and nuclear staining. Results showed that the viability of DH5α was significantly decreased in a CUR concentration-dependent manner and resulted in an approximately 3.5-log reduction at the concentration of 20 μM. After treatment with CUR-mediated PDI (20 μM, 3.6 J/cm²), the ROS level in DH5α cells and the membrane permeability markedly increased. Our data demonstrated that CUR-mediated PDI had a good decontamination effect against DH5α contaminated in oysters. After incubation with PDI-treated oysters, fibroblasts L929 cell morphology, MTT absorbance and cell apoptosis had no obvious changes. Our findings preliminarily demonstrated that CUR-mediated PDI-treated oysters had no cytotoxicity to fibroblasts.

Extrinsic and intrinsic apoptosis activate pannexin-1 to drive NLRP3 inflammasome assembly

Pyroptosis is a form of lytic inflammatory cell death driven by inflammatory caspase-1, caspase-4, caspase-5 and caspase-11. These caspases cleave and activate the pore-forming protein gasdermin D (GSDMD) to induce membrane damage. By contrast, apoptosis is driven by apoptotic caspase-8 or caspase-9 and has traditionally been classified as an immunologically silent form of cell death. Emerging evidence suggests that therapeutics designed for cancer chemotherapy or inflammatory disorders such as SMAC mimetics, TAK1 inhibitors and BH3 mimetics promote caspase-8 or caspase-9-dependent inflammatory cell death and NLRP3 inflammasome activation. However, the mechanism by which caspase-8 or caspase-9 triggers cell lysis and NLRP3 activation is still undefined. Here, we demonstrate that during extrinsic apoptosis, caspase-1 and caspase-8 cleave GSDMD to promote lytic cell death. By engineering a novel Gsdmd D88A knock-in mouse, we further demonstrate that this proinflammatory function of caspase-8 is counteracted by caspase-3-dependent cleavage and inactivation of GSDMD at aspartate 88, and is essential to suppress GSDMD-dependent cell lysis during caspase-8-dependent apoptosis. Lastly, we provide evidence that channel-forming glycoprotein pannexin-1, but not GSDMD or GSDME promotes NLRP3 inflammasome activation during caspase-8 or caspase-9-dependent apoptosis.

Stem cell proliferation is induced by apoptotic bodies from dying cells during epithelial tissue maintenance

Epithelial tissues require the removal and replacement of damaged cells to sustain a functional barrier. Dying cells provide instructive cues that can influence surrounding cells to proliferate, but how these signals are transmitted to their healthy neighbors to control cellular behaviors during tissue homeostasis remains poorly understood. Here we show that dying stem cells facilitate communication with adjacent stem cells by caspase-dependent production of Wnt8a-containing apoptotic bodies to drive cellular turnover in living epithelia. Basal stem cells engulf apoptotic bodies, activate Wnt signaling, and are stimulated to divide to maintain tissue-wide cell numbers. Inhibition of either cell death or Wnt signaling eliminated the apoptosis-induced cell division, while overexpression of Wnt8a signaling combined with induced cell death led to an expansion of the stem cell population. We conclude that ingestion of apoptotic bodies represents a regulatory mechanism linking death and division to maintain overall stem cell numbers and epithelial tissue homeostasis.

Damaged epithelial tissues are known to compensate for cell death through compensatory cell divisions to maintain epithelial integrity. Here, the authors show in living epithelia that dying cells stimulate adjacent stem cells to divide through caspase-dependent production of Wnt8a-containing apoptotic bodies.

Endothelial cell apoptosis and the role of endothelial cell-derived extracellular vesicles in the progression of atherosclerosis

To maintain physiological homeostasis, cell turnover occurs every day in the body via a form of programmed cell death called apoptosis. During apoptosis, cells undergo distinct morphological changes culminating in the disassembly of the dying cell into smaller fragments known as apoptotic bodies (ApoBDs). Dysregulation of apoptosis is associated with diseases including infection, cancer and atherosclerosis. Although the development of atherosclerosis is largely attributed to the accumulation of lipids and inflammatory debris in vessel walls, it is also associated with apoptosis of macrophages, smooth muscle cells (SMCs) and endothelial cells. During cellular activation and apoptosis, endothelial cells can release several types of membrane-bound extracellular vesicles (EVs) including exosomes, microvesicles (MVs)/microparticles and ApoBDs. Emerging evidence in the field suggests that these endothelial cell-derived EVs (EndoEVs) can contribute to intercellular communication during the development of atherosclerosis via the transfer of cellular contents such as protein and microRNA, which may prevent or promote disease progression depending on the context. This review provides an up-to-date overview of the known causes and consequences of endothelial cell death during atherosclerosis along with highlighting current methodological approaches to studying EndoEVs and the potential roles of EndoEVs in atherosclerosis development.

Apoptosis and apoptotic body: disease message and therapeutic target potentials

Apoptosis is widely known as programmed cell death eliciting no inflammatory responses. The intricacy of apoptosis has been a focus of an array of researches, accumulating a wealth of knowledge which led to not only a better understanding of the fundamental process, but also potent therapies of diseases. The classic intrinsic and extrinsic signaling pathways of apoptosis, along with regulatory factors have been well delineated. Drugs and therapeutic measures designed based on current understanding of apoptosis have long been employed. Small-molecule apoptosis inducers have been clinically used for eliminating morbid cells and therefore treating diseases, such as cancer. Biologics with improved apoptotic efficacy and selectivity, such as recombinant proteins and antibodies, are being extensively researched and some have been approved by the FDA. Apoptosis also produces membrane-bound vesicles derived from disassembly of apoptotic cells, now known as apoptotic bodies (ApoBDs). These little sealed sacs containing information as well as substances from dying cells were previously regarded as garbage bags until they were discovered to be capable of delivering useful materials to healthy recipient cells (e.g., autoantigens). In this review, current understandings and knowledge of apoptosis were summarized and discussed with a focus on apoptosis-related therapeutic applications and ApoBDs.

Disassembly of dying cells in diverse organisms

Programmed cell death (PCD) is a conserved phenomenon in multicellular organisms required to maintain homeostasis. Among the regulated cell death pathways, apoptosis is a well-described form of PCD in mammalian cells. One of the characteristic features of apoptosis is the change in cellular morphology, often leading to the fragmentation of the cell into smaller membrane-bound vesicles through a process called apoptotic cell disassembly. Interestingly, some of these morphological changes and cell disassembly are also noted in cells of other organisms including plants, fungi and protists while undergoing 'apoptosis-like PCD'. This review will describe morphologic features leading to apoptotic cell disassembly, as well as its regulation and function in mammalian cells. The occurrence of cell disassembly during cell death in other organisms namely zebrafish, fly and worm, as well as in other eukaryotic cells will also be discussed.

Gasdermin E Does Not Limit Apoptotic Cell Disassembly by Promoting Early Onset of Secondary Necrosis in Jurkat T Cells and THP-1 Monocytes

During the progression of necroptosis and pyroptosis, the plasma membrane will become permeabilized through the activation of mixed lineage kinase domain like pseudokinase (MLKL) or gasdermin D (GSDMD), respectively. Recently, the progression of apoptotic cells into secondary necrotic cells following membrane lysis was shown to be regulated by gasdermin E (GSDME, or DFNA5), a process dependent on caspase 3-mediated cleavage of GSDME. Notably, GSDME was also proposed to negatively regulate the disassembly of apoptotic cells into smaller membrane-bound vesicles known as apoptotic bodies (ApoBDs) by promoting earlier onset of membrane permeabilisation. The presence of a process downstream of caspase 3 that would actively drive cell lysis and limit cell disassembly during apoptosis is somewhat surprising as this could favor the release of proinflammatory intracellular contents and hinder efficient clearance of apoptotic materials. In contrast to the latter studies, we present here that GSDME is not involved in regulating secondary necrosis in human T cells and monocytes, and also unlikely in epithelial cells. Furthermore, GSDME is evidently not a negative regulator of apoptotic cell disassembly in our cell models. Thus, the function of GSDME in regulating membrane permeabilization and cell disassembly during apoptosis may be more limited.

Concise Review: Intercellular Communication Via Organelle Transfer in the Biology and Therapeutic Applications of Stem Cells

The therapeutic potential of stem cell-based therapies may be largely dependent on the ability of stem cells to modulate host cells rather than on their differentiation into host tissues. Within the last decade, there has been considerable interest in the intercellular communication mediated by the transfer of cytoplasmic material and organelles between cells. Numerous studies have shown that mitochondria and lysosomes are transported between cells by various mechanisms, such as tunneling nanotubes, microvesicles, and cellular fusion. This review will focus on the known instances of organelle transfer between stem cells and differentiated cells, what effects it has on recipient cells and how organelle transfer is regulated. Stem Cells 2019;37:14-25.

The weak voltage dependence of pannexin 1 channels can be tuned by N-terminal modifications

Voltage stimulation is commonly used to study pannexin 1 (Panx1). However, whether Panx1 is a voltage-gated channel remains controversial. Michalski et al. demonstrate that Panx1 is a channel with weak voltage dependence, whose activity can be tuned by N-terminal modifications.

Pannexins are a family of ATP release channels important for physiological and pathological processes like blood pressure regulation, epilepsy, and neuropathic pain. To study these important channels in vitro, voltage stimulation is the most common and convenient tool, particularly for pannexin 1 (Panx1). However, whether Panx1 is a voltage-gated channel remains controversial. Here, we carefully examine the effect of N-terminal modification on voltage-dependent Panx1 channel activity. Using a whole-cell patch-clamp recording technique, we demonstrate that both human and mouse Panx1, with their nativeN termini, give rise to voltage-dependent currents, but only at membrane potentials larger than +100 mV. This weak voltage-dependent channel activity profoundly increases when a glycine–serine (GS) motif is inserted immediately after the first methionine. Single-channel recordings reveal that the addition of GS increases the channel open probability as well as the number of unitary conductance classes. We also find that insertions of other amino acid(s) at the same position mimics the effect of GS. On the other hand, tagging the N terminus with GFP abolishes voltage-dependent channel activity. Our results suggest that Panx1 is a channel with weak voltage dependence whose activity can be tuned by N-terminal modifications.

Divergent TCR-Initiated Calcium Signals Govern Recruitment versus Activation of Human Alloreactive Effector Memory T Cells by Endothelial Cells

Early human allograft rejection can be initiated when circulating human host versus graft Ag-specific CD8 and CD4 effector memory T cells directly recognize MHC class I and II, respectively, expressed on the luminal surface by endothelium lining graft blood vessels. TCR engagement triggers both graft entry (TCR-driven transendothelial migration or TEM) and production of proinflammatory cytokines. Both TCR-driven TEM and cytokine expression are known to depend on T cell enzymes, myosin L chain kinase, and calcineurin, respectively, that are activated by cytoplasmic calcium and calmodulin, but whether the sources of calcium that control these enzymes are the same or different is unknown. Using superantigen or anti-CD3 Ab presented by cultured human dermal microvascular cells to freshly isolated peripheral blood human effector memory T cells under conditions of flow (models of alloantigen recognition in a vascularized graft), we tested the effects of pharmacological inhibitors of TCR-activated calcium signaling pathways on TCR-driven TEM and cytokine expression. We report that extracellular calcium entry via CRAC channels is the dominant contributor to cytokine expression, but paradoxically these same inhibitors potentiate TEM. Instead, calcium entry via TRPV1, L-Type Ca_v, and pannexin-1/P2X receptors appear to control TCR-driven TEM. These data reveal new therapeutic targets for immunosuppression.

Therapeutic strategies targeting connexins

The connexin family of channel-forming proteins is present in every tissue type in the human anatomy. Connexins are best known for forming clustered intercellular channels, structurally known as gap junctions, where they serve to exchange members of the metabolome between adjacent cells. In their single-membrane hemichannel form, connexins can act as conduits for the passage of small molecules in autocrine and paracrine signalling. Here, we review the roles of connexins in health and disease, focusing on the potential of connexins as therapeutic targets in acquired and inherited diseases as well as wound repair, while highlighting the associated clinical challenges.

Dissecting the biochemical architecture and morphological release pathways of the human platelet extracellular vesiculome

Platelet extracellular vesicles (PEVs) have emerged as potential mediators in intercellular communication. PEVs exhibit several activities with pathophysiological importance and may serve as diagnostic biomarkers. Here, imaging and analytical techniques were employed to unveil morphological pathways of the release, structure, composition, and surface properties of PEVs derived from human platelets (PLTs) activated with the thrombin receptor activating peptide (TRAP). Based on extensive electron microscopy analysis, we propose four morphological pathways for PEVs release from TRAP-activated PLTs: (1) plasma membrane budding, (2) extrusion of multivesicular α-granules and cytoplasmic vacuoles, (3) plasma membrane blistering and (4) "pearling" of PLT pseudopodia. The PLT extracellular vesiculome encompasses ectosomes, exosomes, free mitochondria, mitochondria-containing vesicles, "podiasomes" and PLT "ghosts". Interestingly, a flow cytometry showed a population of TOM20+LC3+ PEVs, likely products of platelet mitophagy. We found that lipidomic and proteomic profiles were different between the small PEV (S-PEVs; mean diameter 103 nm) and the large vesicle (L-PEVs; mean diameter 350 nm) fractions separated by differential centrifugation. In addition, the majority of PEVs released by activated PLTs was composed of S-PEVs which have markedly higher thrombin generation activity per unit of PEV surface area compared to L-PEVs, and contribute approximately 60% of the PLT vesiculome procoagulant potency.

The Resting Potential and K+ Currents in Primary Human Articular Chondrocytes

Human transplant programs provide significant opportunities for detailed in vitro assessments of physiological properties of selected tissues and cell types. We present a semi-quantitative study of the fundamental electrophysiological/biophysical characteristics of human chondrocytes, focused on K⁺ transport mechanisms, and their ability to regulate to the resting membrane potential, E_m. Patch clamp studies on these enzymatically isolated human chondrocytes reveal consistent expression of at least three functionally distinct K⁺ currents, as well as transient receptor potential (TRP) currents. The small size of these cells and their exceptionally low current densities present significant technical challenges for electrophysiological recordings. These limitations have been addressed by parallel development of a mathematical model of these K⁺ and TRP channel ion transfer mechanisms in an attempt to reveal their contributions to E_m. In combination, these experimental results and simulations yield new insights into: (i) the ionic basis for E_m and its expected range of values; (ii) modulation of E_m by the unique articular joint extracellular milieu; (iii) some aspects of TRP channel mediated depolarization-secretion coupling; (iv) some of the essential biophysical principles that regulate K⁺ channel function in “chondrons.” The chondron denotes the chondrocyte and its immediate extracellular compartment. The presence of discrete localized surface charges and associated zeta potentials at the chondrocyte surface are regulated by cell metabolism and can modulate interactions of chondrocytes with the extracellular matrix. Semi-quantitative analysis of these factors in chondrocyte/chondron function may yield insights into progressive osteoarthritis.

Detection and Isolation of Apoptotic Bodies to High Purity

Apoptotic bodies (ApoBDs), microvesicles and exosomes are the key members of the extracellular vesicle family, with ApoBDs being one of the largest type. It has been proposed that ApoBDs can aid cell clearance as well as intercellular communication through trafficking biomolecules. Conventional approaches used for the identification and isolation of ApoBDs are often limited by the lack of accurate quantification and low sample purity. Here, we describe a workflow to confirm the induction of apoptosis, validate ApoBD formation, and isolate ApoBDs to high purity. We will also outline and compare fluorescence-activated cell sorting (FACS) and differential centrifugation based approaches to isolate ApoBDs. Furthermore, the purity of isolated ApoBDs will be confirmed using a previously establish flow cytometry-based staining and analytical method. Taken together, using the described approach, THP-1 monocyte apoptosis and apoptotic cell disassembly was induced and validated, and ApoBD generated from THP-1 monocytes were isolated to a purity of 97-99%.

Blockade of Pannexin-1 Channels and Purinergic P2X7 Receptors Shows Protective Effects Against Cytokines-Induced Colitis of Human Colonic Mucosa

Introduction: The pannexin-1 (Panx1) channels are found in many cell types, and ATP released from these channels can act on nearby cells activating purinergic P2X7 receptors (P2X7R) which lead to inflammation. Although Panx1 and P2X7R are implicated in the process of inflammation and cell death, few studies have looked at the role they play in inflammatory bowel disease in human. Hence, the aim of the present study was to investigate the function of Panx1 and P2X7R in an ex vivo colitis model developed from human colonic mucosal explants.

Materials and Methods: Healthy human colonic mucosal strips (4 × 10 mm) were incubated in carbogenated culture medium at 37°C for 16 h. Proinflammatory cytokines TNFα and IL-1β (each 10 ng/mL) were used to induce colitis in mucosal strips, and the effects of Panx1 and P2X7R on cytokines-induced tissue damage were determined in the presence of the Panx1 channel blocker ¹⁰Panx1 (100 μM) and P2X7R antagonist A438079 (100 μM). The effects of ¹⁰Panx1 and A438079 on cytokines-enhanced epithelial permeability were also studied using Caco-2 cells.

Results: Histological staining showed that the mucosal strips had severe structural damage in the cytokines-only group but not in the incubation-control group (P < 0.01). Compared to the cytokines-only group, crypt damage was significantly decreased in groups receiving cytokines with inhibitors (¹⁰Panx1, A438079, or ¹⁰Panx1 + A438079, P < 0.05). The immunoreactive signals of tight junction protein zonula occludens-1 (ZO-1) were abundant in all control tissues but were significantly disrupted and lost in the cytokines-only group (P < 0.01). The diminished ZO-1 immunoreactivity induced by cytokines was prevented in the presence of ¹⁰Panx1 (P = 0.04). Likewise, ¹⁰Panx1 significantly attenuated the cytokines-evoked increase in paracellular permeability of Caco-2 cells. Although the inhibition of P2X7R activity by A438079 diminished cytokines-induced crypt damage, its effect on the maintenance of ZO-1 immunoreactivity and Caco-2 epithelial cell integrity was less evident.

Conclusion: The blockade of Panx1 and P2X7R reduced the inflammatory cytokines-induced crypt damage, loss of tight junctions and increase in cell permeability. Thus, Panx1 and P2X7R may have roles in causing mucosal damage, a common clinical feature of inflammatory bowel disease.

Apoptotic Cell-Derived Extracellular Vesicles: More Than Just Debris

The many functions of extracellular vesicles (EVs) like exosomes and microvesicles released from healthy cells have been well characterized, particularly in relation to their roles in immune modulation. Apoptotic bodies, a major class of EV released as a product of apoptotic cell disassembly, and other types of EVs released from dying cells are also becoming recognized as key players in this emerging field. There is now increasing evidence to suggest that EVs produced during apoptosis have important immune regulatory roles, a concept relevant across different disease settings including autoimmunity, cancer, and infection. Therefore, this review focuses on how the formation of EVs during apoptosis could be a key mechanism of immune modulation by dying cells.

Connexins and Pannexins in Vascular Function and Disease

Connexins (Cxs) and pannexins (Panxs) are ubiquitous membrane channel forming proteins that are critically involved in many aspects of vascular physiology and pathology. The permeation of ions and small metabolites through Panx channels, Cx hemichannels and gap junction channels confers a crucial role to these proteins in intercellular communication and in maintaining tissue homeostasis. This review provides an overview of current knowledge with respect to the pathophysiological role of these channels in large arteries, the microcirculation, veins, the lymphatic system and platelet function. The essential nature of these membrane proteins in vascular homeostasis is further emphasized by the pathologies that are linked to mutations and polymorphisms in Cx and Panx genes.

Epithelial and Endothelial Pannexin1 Channels Mediate AKI

Background Pannexin1 (Panx1), an ATP release channel, is present in most mammalian tissues, but the role of Panx1 in health and disease is not fully understood. Panx1 may serve to modulate AKI; ATP is a precursor to adenosine and may function to block inflammation, or ATP may act as a danger-associated molecular pattern and initiate inflammation.Methods We used pharmacologic and genetic approaches to evaluate the effect of Panx1 on kidney ischemia-reperfusion injury (IRI), a mouse model of AKI.Results Pharmacologic inhibition of gap junctions, including Panx1, by administration of carbenoxolone protected mice from IRI. Furthermore, global deletion of Panx1 preserved kidney function and morphology and diminished the expression of proinflammatory molecules after IRI. Analysis of bone marrow chimeric mice revealed that Panx1 expressed on parenchymal cells is necessary for ischemic injury, and both proximal tubule and vascular endothelial Panx1 tissue-specific knockout mice were protected from IRI. In vitro, Panx1-deficient proximal tubule cells released less and retained more ATP under hypoxic stress.Conclusions Panx1 is involved in regulating ATP release from hypoxic cells, and reducing this ATP release may protect kidneys from AKI.

Apoptotic Tumor Cell-Derived Extracellular Vesicles as Important Regulators of the Onco-Regenerative Niche

Cells undergoing apoptosis produce heterogeneous populations of membrane delimited extracellular vesicles (Apo-EVs) which vary not only in size—from tens of nanometers to several microns—but also in molecular composition and cargo. Apo-EVs carry a variety of potentially biologically active components, including small molecules, proteins, and nucleic acids. Larger forms of Apo-EVs, commonly termed “apoptotic bodies,” can carry organelles, such as mitochondria and nuclear fragments. Molecules displayed on the surface of extracellular vesicles (EVs) can contribute substantially to their size, as well as their functions. Thus far, relatively little is known of the functional significance of Apo-EVs apart from their roles in fragmentation of dying cells and indicated immunomodulatory activities. Here, we discuss EV production by dying tumor cells and consider the possible roles of Apo-EVs in a cell death-driven sector of the tumor microenvironment known as the onco-regenerative niche (ORN). We propose that tumor-derived Apo-EVs are significant vehicles of the ORN, functioning as critical intercellular communicators that activate oncogenic tissue repair and regeneration pathways. We highlight important outstanding questions and suggest that Apo-EVs may harbor novel therapeutic targets.

The immune response to secondary necrotic cells

When apoptotic cells are not cleared in an efficient and timely manner, they progress to secondary necrosis and lose their membrane integrity. This results in a leakage of immunostimulatory, danger associated molecular patterns (DAMPs), similar to accidental (or primary) necrosis. However, primary necrosis is a sudden event with an inadvertent release of almost unmodified DAMPs. Secondary necrotic cells, in contrast, have gone through various modifications during the process of apoptosis. Recent research revealed that the molecules released from the cytoplasm or exposed on the cell surface differ between primary necrosis, secondary necrosis, and regulated necrosis such as necroptosis. This review gives an overview of these differences and focusses their effects on the immune response. The implications to human physiology and diseases are manifold and will be discussed in the context of cancer, neurodegenerative disorders and autoimmunity.

Communicating with the dead: lipids, lipid mediators and extracellular vesicles

Apoptosis is a key event in the control of inflammation. However, for this to be successful, dying cells must efficiently and effectively communicate their presence to phagocytes to ensure timely removal of dying cells. Here, we consider apoptotic cell-derived extracellular vesicles and the role of contained lipids and lipid mediators in ensuring effective control of inflammation. We discuss key outstanding issues in the study of cell death and cell communication, and introduce the concept of the 'active extracellular vesicle' as a metabolically active and potentially changing intercellular communicator.

Tumor-Derived Apoptotic Vesicles: With Death They Do Part

Tumor cells release lipid particles known as extracellular vesicles (EV) that contribute to cancer metastasis, to the immune response, and to thrombosis. When tumors are exposed to radiation or chemotherapy, apoptotic vesicles (ApoVs) are released in abundance as the plasma membrane delaminates from the cytoskeleton. Recent studies have suggested that ApoVs are distinct from the EVs released from living cells, such as exosomes or microvesicles. Depending on their treatment conditions, tumor-released ApoV have been suggested to either enhance or suppress anti-cancer immunity. In addition, tumor-derived ApoV possess procoagulant activity that could increase the thrombotic state in cancer patients undergoing chemotherapy or radiotherapy. Since ApoVs are one of the least appreciated type of EVs, we focus in this review on the distinctive characterization of tumor ApoVs and their proposed mechanistic effects on cancer immunity, coagulation, and metastasis.

Knockout of Pannexin-1 Induces Hearing Loss

Mutations of gap junction connexin genes induce a high incidence of nonsyndromic hearing loss. Pannexin genes also encode gap junctional proteins in vertebrates. Recent studies demonstrated that Pannexin-1 (Panx1) deficiency in mice and mutation in humans are also associated with hearing loss. So far, several Panx1 knockout (KO) mouse lines were established. In general, these Panx1 KO mouse lines demonstrate consistent phenotypes in most aspects, including hearing loss. However, a recent study reported that a Panx1 KO mouse line, which was created by Genentech Inc., had no hearing loss as measured by the auditory brainstem response (ABR) threshold at low-frequency range (<24 kHz). Here, we used multiple auditory function tests and re-examined hearing function in the Genentech Panx1 (Gen-Panx1) KO mouse. We found that ABR thresholds in the Gen-Panx1 KO mouse were significantly increased, in particular, in the high-frequency region. Moreover, consistent with the increase in ABR threshold, distortion product otoacoustic emission (DPOAE) and cochlear microphonics (CM), which reflect active cochlear amplification and auditory receptor current, respectively, were significantly reduced. These data demonstrated that the Gen-Panx1 KO mouse has hearing loss and further confirmed that Panx1 deficiency can cause deafness.

Trovafloxacin attenuates neuroinflammation and improves outcome after traumatic brain injury in mice

Background

Trovafloxacin is a broad-spectrum antibiotic, recently identified as an inhibitor of pannexin-1 (Panx1) channels. Panx1 channels are important conduits for the adenosine triphosphate (ATP) release from live and dying cells that enhances the inflammatory response of immune cells. Elevated extracellular levels ATP released upon injury activate purinergic pathways in inflammatory cells that promote migration, proliferation, phagocytosis, and apoptotic signals. Here, we tested whether trovafloxacin administration attenuates the neuroinflammatory response and improves outcomes after brain trauma.

Methods

The murine controlled cortical impact (CCI) model was used to determine whether in vivo delivery of trovafloxacin has anti-inflammatory and neuroprotective actions after brain trauma. Locomotor deficit was assessed using the rotarod test. Levels of tissue damage markers and inflammation were measured using western blot, qPCR, and immunofluorescence. In vitro assays were used to evaluate whether trovafloxacin blocks ATP release and cell migration in a chemotactic-stimulated microglia cell line.

Results

Trovafloxacin treatment of CCI-injured mice significantly reduced tissue damage markers and improved locomotor deficits. In addition, trovafloxacin treatment significantly reduced mRNA levels of several pro-inflammatory cytokines (IL-1β, IL-6, and TNF-α), which correlates with an overall reduction in the accumulation of inflammatory cell types (neutrophils, microglia/macrophages, and astroglia) at the injury zone. To determine whether trovafloxacin exerted these effects by direct action on immune cells, we evaluated its effect on ATP release and cell migration using a chemotactic-stimulated microglial cell line. We found that trovafloxacin significantly inhibited both ATP release and migration of these cells.

Conclusion

Our results show that trovafloxacin administration has pronounced anti-inflammatory and neuroprotective effects following brain injury. These findings lay the foundation for future studies to directly test a role for Panx1 channels in pathological inflammation following brain trauma.

Electronic supplementary material

The online version of this article (10.1186/s12974-018-1069-9) contains supplementary material, which is available to authorized users.

Pannexin 1 Channels as an Unexpected New Target of the Anti-Hypertensive Drug Spironolactone

RATIONALE

Resistant hypertension is a major health concern with unknown cause. Spironolactone is an effective antihypertensive drug, especially for patients with resistant hypertension, and is considered by the World Health Organization as an essential medication. Although spironolactone can act at the mineralocorticoid receptor (MR; NR3C2), there is increasing evidence of MR-independent effects of spironolactone.

OBJECTIVE

Here, we detail the unexpected discovery that Panx1 (pannexin 1) channels could be a relevant in vivo target of spironolactone.

METHODS AND RESULTS

First, we identified spironolactone as a potent inhibitor of Panx1 in an unbiased small molecule screen, which was confirmed by electrophysiological analysis. Next, spironolactone inhibited α-adrenergic vasoconstriction in arterioles from mice and hypertensive humans, an effect dependent on smooth muscle Panx1, but independent of the MR NR3C2. Last, spironolactone acutely lowered blood pressure, which was dependent on smooth muscle cell expression of Panx1 and independent of NR3C2. This effect, however, was restricted to steroidal MR antagonists as a nonsteroidal MR antagonist failed to reduced blood pressure.

CONCLUSIONS

These data suggest new therapeutic modalities for resistant hypertension based on Panx1 inhibition.

Revisiting multimodal activation and channel properties of Pannexin 1

Pannexin 1 (Panx1) forms plasma membrane ion channels that are widely expressed throughout the body. Panx1 activation results in the release of nucleotides such as adenosine triphosphate and uridine triphosphate. Thus, these channels have been implicated in diverse physiological and pathological functions associated with purinergic signaling, such as apoptotic cell clearance, blood pressure regulation, neuropathic pain, and excitotoxicity. In light of this, substantial attention has been directed to understanding the mechanisms that regulate Panx1 channel expression and activation. Here we review accumulated evidence for the various activation mechanisms described for Panx1 channels and, where possible, the unitary channel properties associated with those forms of activation. We also emphasize current limitations in studying Panx1 channel function and propose potential directions to clarify the exciting and expanding roles of Panx1 channels.

Determining the contents and cell origins of apoptotic bodies by flow cytometry

Over 200 billion cells undergo apoptosis every day in the human body in order to maintain tissue homeostasis. Increased apoptosis can also occur under pathological conditions including infection and autoimmune disease. During apoptosis, cells can fragment into subcellular membrane-bound vesicles known as apoptotic bodies (ApoBDs). We recently developed a flow cytometry-based method to accurately differentiate ApoBDs from other particles (e.g. cells and debris). In the present study, we aim to further characterize subsets of ApoBDs based on intracellular contents and cell type-specific surface markers. Utilizing a flow cytometry-based approach, we demonstrated that intracellular contents including nuclear materials and mitochondria are distributed to some, but not all ApoBDs. Interestingly, the mechanism of ApoBD formation could affect the distribution of intracellular contents into ApoBDs. Furthermore, we also showed that ApoBDs share the same surface markers as their cell of origin, which can be used to distinguish cell type-specific ApoBDs from a mixed culture. These studies demonstrate that ApoBDs are not homogeneous and can be divided into specific subclasses based on intracellular contents and cell surface markers. The described flow cytometry-based method to study ApoBDs could be used in future studies to better understand the function of ApoBDs.

Plasma membrane changes during programmed cell deaths

Ruptured and intact plasma membranes are classically considered as hallmarks of necrotic and apoptotic cell death, respectively. As such, apoptosis is usually considered a non-inflammatory process while necrosis triggers inflammation. Recent studies on necroptosis and pyroptosis, two types of programmed necrosis, revealed that plasma membrane rupture is mediated by MLKL channels during necroptosis but depends on non-selective gasdermin D (GSDMD) pores during pyroptosis. Importantly, the morphology of dying cells executed by MLKL channels can be distinguished from that executed by GSDMD pores. Interestingly, it was found recently that secondary necrosis of apoptotic cells, a previously believed non-regulated form of cell lysis that occurs after apoptosis, can be programmed and executed by plasma membrane pore formation like that of pyroptosis. In addition, pyroptosis is associated with pyroptotic bodies, which have some similarities to apoptotic bodies. Therefore, different cell death programs induce distinctive reshuffling processes of the plasma membrane. Given the fact that the nature of released intracellular contents plays a crucial role in dying/dead cell-induced immunogenicity, not only membrane rupture or integrity but also the nature of plasma membrane breakdown would determine the fate of a cell as well as its ability to elicit an immune response. In this review, we will discuss recent advances in the field of apoptosis, necroptosis and pyroptosis, with an emphasis on the mechanisms underlying plasma membrane changes observed on dying cells and their implication in cell death-elicited immunogenicity.

Connexins in Cardiovascular and Neurovascular Health and Disease: Pharmacological Implications

Connexins are ubiquitous channel forming proteins that assemble as plasma membrane hemichannels and as intercellular gap junction channels that directly connect cells. In the heart, gap junction channels electrically connect myocytes and specialized conductive tissues to coordinate the atrial and ventricular contraction/relaxation cycles and pump function. In blood vessels, these channels facilitate long-distance endothelial cell communication, synchronize smooth muscle cell contraction, and support endothelial-smooth muscle cell communication. In the central nervous system they form cellular syncytia and coordinate neural function. Gap junction channels are normally open and hemichannels are normally closed, but pathologic conditions may restrict gap junction communication and promote hemichannel opening, thereby disturbing a delicate cellular communication balance. Until recently, most connexin-targeting agents exhibited little specificity and several off-target effects. Recent work with peptide-based approaches has demonstrated improved specificity and opened avenues for a more rational approach toward independently modulating the function of gap junctions and hemichannels. We here review the role of connexins and their channels in cardiovascular and neurovascular health and disease, focusing on crucial regulatory aspects and identification of potential targets to modify their function. We conclude that peptide-based investigations have raised several new opportunities for interfering with connexins and their channels that may soon allow preservation of gap junction communication, inhibition of hemichannel opening, and mitigation of inflammatory signaling.

Possible roles for ATP release from RBCs exclude the cAMP-mediated Panx1 pathway

Red blood cell (RBC)-derived adenosine triphosphate (ATP) has been proposed as an integral component in the regulation of oxygen supply to skeletal muscle. In ex vivo settings RBCs have been shown to release ATP in response to a number of stimuli, including stimulation of adrenergic receptors. Further evidence suggested that ATP release from RBCs was dependent on activation of adenylate cyclase (AC)/cyclic adenosine monophosphate (cAMP)-dependent pathways and involved the pannexin 1 (Panx1) channel. Here we show that RBCs express Panx1 and confirm its absence in Panx1 knockout (^-/-) RBCs. However, Panx1^-/- mice lack any decrease in exercise performance, challenging the assumptions that Panx1 plays an essential role in increased blood perfusion to exercising skeletal muscle and therefore in ATP release from RBCs. We therefore tested the role of Panx1 in ATP release from RBCs ex vivo in RBC suspensions. We found that stimulation with hypotonic potassium gluconate buffer resulted in a significant increase in ATP in the supernatant, but this was highly correlated with RBC lysis. Next, we treated RBCs with a stable cAMP analog, which did not induce ATP release from wild-type or Panx1^-/- mice. Similarly, multiple pharmacological treatments activating AC in RBCs increased intracellular cAMP levels (as measured via mass spectrometry) but did not induce ATP release. The data presented here question the importance of Panx1 for exercise performance and dispute the general assumption that ATP release from RBCs via Panx1 is regulated via cAMP.

Clearance of Dying Cells by Phagocytes: Mechanisms and Implications for Disease Pathogenesis

The efficient clearance of apoptotic cells is an evolutionarily conserved process crucial for homeostasis in multicellular organisms. The clearance involves a series of steps that ultimately facilitates the recognition of the apoptotic cell by the phagocytes and the subsequent uptake and processing of the corpse. These steps include the phagocyte sensing of "find-me" signals released by the apoptotic cell, recognizing "eat-me" signals displayed on the apoptotic cell surface, and then intracellular signaling within the phagocyte to mediate phagocytic cup formation around the corpse and corpse internalization, and the processing of the ingested contents. The engulfment of apoptotic cells by phagocytes not only eliminates debris from tissues but also produces an anti-inflammatory response that suppresses local tissue inflammation. Conversely, impaired corpse clearance can result in loss of immune tolerance and the development of various inflammation-associated disorders such as autoimmunity, atherosclerosis, and airway inflammation but can also affect cancer progression. Recent studies suggest that the clearance process can also influence antitumor immune responses. In this review, we will discuss how apoptotic cells interact with their engulfing phagocytes to generate important immune responses, and how modulation of such responses can influence pathology.

Multiple and complex influences of connexins and pannexins on cell death

Cell death is a fundamental process for organogenesis, immunity and cell renewal. During the last decades a broad range of molecular tools were identified as important players for several different cell death pathways (apoptosis, pyroptosis, necrosis, autosis…). Aside from these direct regulators of cell death programs, several lines of evidence proposed connexins and pannexins as potent effectors of cell death. In the present review we discussed the potential roles played by connexins, pannexins and innexins in the different cell death programs at different scales from gap junction intercellular communication to protein-protein interactions. This article is part of a Special Issue entitled: Gap Junction Proteins edited by Jean Claude Herve.

Up-regulated Ectonucleotidases in Fas-Associated Death Domain Protein- and Receptor-Interacting Protein Kinase 1-Deficient Jurkat Leukemia Cells Counteract Extracellular ATP/AMP Accumulation via Pannexin-1 Channels during Chemotherapeutic Drug-Induced Apoptosis

Pannexin-1 (Panx1) channels mediate the efflux of ATP and AMP from cancer cells in response to induction of extrinsic apoptosis by death receptors or intrinsic apoptosis by chemotherapeutic agents. We previously described the accumulation of extracellular ATP /AMP during chemotherapy-induced apoptosis in Jurkat human leukemia cells. In this study, we compared how different signaling pathways determine extracellular nucleotide pools in control Jurkat cells versus Jurkat lines that lack the Fas-associated death domain (FADD) or receptor-interacting protein kinase 1 (RIP1) cell death regulatory proteins. Tumor necrosis factor-α induced extrinsic apoptosis in control Jurkat cells and necroptosis in FADD-deficient cells; treatment of both lines with chemotherapeutic drugs elicited similar intrinsic apoptosis. Robust extracellular ATP/AMP accumulation was observed in the FADD-deficient cells during necroptosis, but not during apoptotic activation of Panx1 channels. Accumulation of extracellular ATP/AMP was similarly absent in RIP1-deficient Jurkat cells during apoptotic responses to chemotherapeutic agents. Apoptotic activation triggered equivalent proteolytic gating of Panx1 channels in all three Jurkat cell lines. The differences in extracellular ATP/AMP accumulation correlated with cell-line-specific expression of ectonucleotidases that metabolized the released ATP/AMP. CD73 mRNA, and αβ-methylene-ADP-inhibitable ecto-AMPase activity were elevated in the FADD-deficient cells. In contrast, the RIP1-deficient cells were defined by increased expression of tartrate-sensitive prostatic acid phosphatase as a broadly acting ectonucleotidase. Thus, extracellular nucleotide accumulation during regulated tumor cell death involves interplay between ATP/AMP efflux pathways and different cell-autonomous ectonucleotidases. Differential expression of particular ectonucleotidases in tumor cell variants will determine whether chemotherapy-induced activation of Panx1 channels drives accumulation of immunostimulatory ATP versus immunosuppressive adenosine within the tumor microenvironment.