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Inflammasomes are cytosolic innate immune complexes that activate caspase-1 following detection of pathogenic and endogenous dangers 1 – 5 , and NACHT-, leucine-rich repeat (LRR)- and pyrin domain (PYD)-containing protein  3 (NLRP3) is an inflammasome sensor of membrane damage highly important in regard to the induction of inflammation 2 , 6 , 7 . Here we report cryogenic electron microscopy structures of disc-shaped active NLRP3 oligomers in complex with adenosine 5′-O-(3-thio)triphosphate, the centrosomal NIMA-related kinase 7 (NEK7) and the adaptor protein ASC, which recruits caspase-1. In these NLRP3–NEK7–ASC complexes, the central NACHT domain of NLRP3 assumes an ATP-bound conformation in which two of its subdomains rotate by about 85° relative to the ADP-bound inactive conformation 8 – 12 . The fish-specific NACHT-associated domain conserved in NLRP3 but absent in most NLRPs 13 becomes ordered in its key regions to stabilize the active NACHT conformation and mediate most interactions in the disc. Mutations on these interactions compromise NLRP3-mediated caspase-1 activation. The N-terminal PYDs from all NLRP3 subunits combine to form a PYD filament that recruits ASC PYD to elicit downstream signalling. Surprisingly, the C-terminal LRR domain and the LRR-bound NEK7 do not participate in disc interfaces. Together with previous structures of an inactive NLRP3 cage in which LRR–LRR interactions play an important role 8 – 11 , we propose that the role of NEK7 is to break the inactive cage to transform NLRP3 into the active NLRP3 inflammasome disc. We report cryogenic electron microscopy structures of disc-shaped active NLRP3 oligomers in complex with NEK7 and ASC, and propose that the role of NEK7 is to transform NLRP3 into the active NLRP3 inflammasome disc.

Modulation of the biochemical composition of the tumour microenvironment is a new frontier of cancer therapy. Several immunosuppressive mechanisms operate in the milieu of most tumours, a condition that makes antitumour immunity ineffective. One of the most potent immunosuppressive factors is adenosine, which is generated in the tumour microenvironment owing to degradation of extracellular ATP. Accruing evidence over the past few years shows that ATP is one of the major biochemical constituents of the tumour microenvironment, where it acts at P2 purinergic receptors expressed on both tumour and host cells. Stimulation of P2 receptors has different effects depending on the extracellular ATP concentration, the P2 receptor subtype engaged and the target cell type. Among P2 receptors, the P2X purinergic receptor 7 (P2X7R) subtype appears to be a main player in host–tumour cell interactions. Preclinical studies in several tumour models have shown that P2X7R targeting is potentially a very effective anticancer treatment, and many pharmaceutical companies have now developed potent and selective small molecule inhibitors of P2X7R. In this Review, we report on the multiple mechanisms by which extracellular ATP shapes the tumour microenvironment and how its stimulation of host and tumour cell P2 receptors contributes to determining tumour fate.

TRPM4 is a calcium-activated, phosphatidylinositol-4,5-bisphosphate (PtdIns(4,5)P 2 ) -modulated, non-selective cation channel that belongs to the family of melastatin-related transient receptor potential (TRPM) channels. Here we present the electron cryo-microscopy structures of the mouse TRPM4 channel with and without ATP. TRPM4 consists of multiple transmembrane and cytosolic domains, which assemble into a three-tiered architecture. The N-terminal nucleotide-binding domain and the C-terminal coiled-coil participate in the tetrameric assembly of the channel; ATP binds at the nucleotide-binding domain and inhibits channel activity. TRPM4 has an exceptionally wide filter but is only permeable to monovalent cations; filter residue Gln973 is essential in defining monovalent selectivity. The S1–S4 domain and the post-S6 TRP domain form the central gating apparatus that probably houses the Ca 2+ - and PtdIns(4,5)P 2 -binding sites. These structures provide an essential starting point for elucidating the complex gating mechanisms of TRPM4 and reveal the molecular architecture of the TRPM family. Electron cryo-microscopy structures of mouse TRPM4, a calcium-activated, non-selective cation channel, in the apo and ATP-bound states. Scoping out TRPM channels Melastatin-related transient receptor potential (TRPM) ion channels are the largest group of the TRP superfamily and, as such, are widespread throughout the body with diverse physiological roles including heat and taste sensation and regulating ion homeostasis. For example, TRPM4 is a Ca 2+ -activated non-selective channel expressed in many of the central organs including the brain and heart, and is involved in the cardiac rhythm, breath pacemaking and insulin secretion. In this issue of Nature , two groups report the structure of TRPM4 by electron cryo-microscopy. Wei Lü and colleagues solved the structure of human TRPM4, which shows an umbrella-like structure, bound to Ca(ɪɪ) and decavanadate. Youxing Jiang and colleagues report the structure of mouse TRPM4 with and without ATP, which inhibits channel activity. These studies provide the first structural insights into the TRPM family.

Key Points Many different components facilitate autocrine purinergic signalling, including pannexin 1 hemichannels (which facilitate ATP release), P2X and P2Y receptors (which respond to ATP), ectonucleotidases (which hydrolyse ATP to adenosine), P1 receptors (which respond to adenosine) and nucleoside transporters and adenosine deaminase (which remove adenosine). Different immune cells express distinct purinergic signalling components, and this has an important role in providing signal amplification following cell activation. The positive autocrine feedback loops mediated by purinergic signalling are essential for gradient sensing by phagocytes and antigen recognition by T cells. When released from damaged, dying and apoptotic cells, ATP can serve as a danger signal that stimulates the NLRP3 (NOD-, LRR- and pyrin domain-containing 3) inflammasome, promotes chemotaxis of microglia, and boosts activation of other immune cell types that are recruited to sites of inflammation and tissue damage. High ATP concentrations at sites of inflammation trap neutrophils and other phagocytes by interfering with their autocrine purinergic chemotaxis signalling systems. ATP release and autocrine purinergic signalling can amplify activation signals in immune cells but can also downregulate immune cell responses, either by activating suppressive P2 receptors or through adenosine formation and activation of suppressive A2A receptors. A growing arsenal of pharmacological agents is available to modulate purinergic signalling in immune cells. The most widely investigated drugs target P1 receptors or the molecular processes that control the availability of the P1 receptor ligand adenosine. Here, Wolfgang Junger discusses the importance of purinergic receptor signalling for fine-tuning immune cell responses. Autocrine signalling through purinergic receptors can both amplify and inhibit leukocyte functions; the author explains how this is important for sensing chemotactic gradients and detecting rare antigens. Stimulation of almost all mammalian cell types leads to the release of cellular ATP and autocrine feedback through a diverse array of purinergic receptors. Depending on the types of purinergic receptors that are involved, autocrine signalling can promote or inhibit cell activation and fine-tune functional responses. Recent work has shown that autocrine signalling is an important checkpoint in immune cell activation and allows immune cells to adjust their functional responses based on the extracellular cues provided by their environment. This Review focuses on the roles of autocrine purinergic signalling in the regulation of both innate and adaptive immune responses and discusses the potential of targeting purinergic receptors for treating immune-mediated disease.

Active coacervate droplets are liquid condensates coupled to a chemical reaction that turns over their components, keeping the droplets out of equilibrium. This turnover can be used to drive active processes such as growth, and provide an insight into the chemical requirements underlying (proto)cellular behaviour. Moreover, controlled growth is a key requirement to achieve population fitness and survival. Here we present a minimal, nucleotide-based coacervate model for active droplets, and report three key findings that make these droplets into evolvable protocells. First, we show that coacervate droplets form and grow by the fuel-driven synthesis of new coacervate material. Second, we find that these droplets do not undergo Ostwald ripening, which we attribute to the attractive electrostatic interactions and translational entropy within complex coacervates, active or passive. Finally, we show that the droplet growth rate reflects experimental conditions such as substrate, enzyme and protein concentration, and that a different droplet composition (addition of RNA) leads to altered growth rates and droplet fitness. These findings together make active coacervate droplets a powerful platform to mimic cellular growth at a single-droplet level, and to study fitness at a population level. Active coacervate droplets are droplets coupled to a chemical reaction that maintains them out of equilibrium, which can be used to drive active processes, but coacervates are still subject to passive processes that compete with or mask growth. Here, the authors present a nucleotide-based model for active coacervate droplets that form and grow by fuel-driven synthesis of ATP, and, importantly, do not undergo Ostwald ripening.

Microglia, the brain’s resident macrophages, help to regulate brain function by removing dying neurons, pruning non-functional synapses, and producing ligands that support neuronal survival 1 . Here we show that microglia are also critical modulators of neuronal activity and associated behavioural responses in mice. Microglia respond to neuronal activation by suppressing neuronal activity, and ablation of microglia amplifies and synchronizes the activity of neurons, leading to seizures. Suppression of neuronal activation by microglia occurs in a highly region-specific fashion and depends on the ability of microglia to sense and catabolize extracellular ATP, which is released upon neuronal activation by neurons and astrocytes. ATP triggers the recruitment of microglial protrusions and is converted by the microglial ATP/ADP hydrolysing ectoenzyme CD39 into AMP; AMP is then converted into adenosine by CD73, which is expressed on microglia as well as other brain cells. Microglial sensing of ATP, the ensuing microglia-dependent production of adenosine, and the adenosine-mediated suppression of neuronal responses via the adenosine receptor A 1 R are essential for the regulation of neuronal activity and animal behaviour. Our findings suggest that this microglia-driven negative feedback mechanism operates similarly to inhibitory neurons and is essential for protecting the brain from excessive activation in health and disease. Microglia, the brain’s immune cells, suppress neuronal activity in response to synaptic ATP release and alter behavioural responses in mice.

Adenosine 5′ triphosphate (ATP) is a universal intracellular energy source and an evolutionarily ancient, ubiquitous extracellular signal in diverse species. Here, we report the generation and characterization of single-wavelength genetically encoded fluorescent sensors (iATPSnFRs) for imaging extracellular and cytosolic ATP from insertion of circularly permuted superfolder GFP into the epsilon subunit of F 0 F 1 -ATPase from Bacillus PS3 . On the cell surface and within the cytosol, iATPSnFR 1.0 responds to relevant ATP concentrations (30 μM to 3 mM) with fast increases in fluorescence. iATPSnFRs can be genetically targeted to specific cell types and sub-cellular compartments, imaged with standard light microscopes, do not respond to other nucleotides and nucleosides, and when fused with a red fluorescent protein function as ratiometric indicators. After careful consideration of their modest pH sensitivity, iATPSnFRs represent promising reagents for imaging ATP in the extracellular space and within cells during a variety of settings, and for further application-specific refinements. ATP has essential roles in cell signalling and energy homeostasis and biosensors to detect it have many potential applications. Here, the authors develop a new ATP sensor that can be targeted to the membrane or cytosol.

The proteasome is a sophisticated ATP-dependent molecular machine responsible for protein degradation in all known eukaryotic cells. It remains elusive how conformational changes of the AAA-ATPase unfoldase in the regulatory particle (RP) control the gating of the substrate–translocation channel leading to the proteolytic chamber of the core particle (CP). Here we report three alternative states of the ATP-γ-S-bound human proteasome, in which the CP gates are asymmetrically open, visualized by cryo-EM at near-atomic resolutions. At least four nucleotides are bound to the AAA-ATPase ring in these open-gate states. Variation in nucleotide binding gives rise to an axial movement of the pore loops narrowing the substrate-translation channel, which exhibit remarkable structural transitions between the spiral-staircase and saddle-shaped-circle topologies. Gate opening in the CP is thus regulated by nucleotide-driven conformational changes of the AAA-ATPase unfoldase. These findings demonstrate an elegant mechanism of allosteric coordination among sub-machines within the human proteasome holoenzyme. The 26S proteasome consists of a core particle that is capped at each side by a regulatory particle. Here the authors present cryo-EM structures of the activated human 26S proteasome holoenzyme in three alternative open-gate states, which provides mechanistic insights into gate opening and dynamic remodeling of the substrate–translocation pathway.

Background Mitochondrial transplantation (MTx) is an emerging but poorly understood technology with the potential to mitigate severe ischemia–reperfusion injuries after cardiac arrest (CA). To address critical gaps in the current knowledge, we test the hypothesis that MTx can improve outcomes after CA resuscitation. Methods This study consists of both in vitro and in vivo studies. We initially examined the migration of exogenous mitochondria into primary neural cell culture in vitro. Exogenous mitochondria extracted from the brain and muscle tissues of donor rats and endogenous mitochondria in the neural cells were separately labeled before co-culture. After a period of 24 h following co-culture, mitochondrial transfer was observed using microscopy. In vitro adenosine triphosphate (ATP) contents were assessed between freshly isolated and frozen-thawed mitochondria to compare their effects on survival. Our main study was an in vivo rat model of CA in which rats were subjected to 10 min of asphyxial CA followed by resuscitation. At the time of achieving successful resuscitation, rats were randomly assigned into one of three groups of intravenous injections: vehicle, frozen-thawed, or fresh viable mitochondria. During 72 h post-CA, the therapeutic efficacy of MTx was assessed by comparison of survival rates. The persistence of labeled donor mitochondria within critical organs of recipient animals 24 h post-CA was visualized via microscopy. Results The donated mitochondria were successfully taken up into cultured neural cells. Transferred exogenous mitochondria co-localized with endogenous mitochondria inside neural cells. ATP content in fresh mitochondria was approximately four times higher than in frozen-thawed mitochondria. In the in vivo survival study, freshly isolated functional mitochondria, but not frozen-thawed mitochondria, significantly increased 72-h survival from 55 to 91% ( P  = 0.048 vs. vehicle). The beneficial effects on survival were associated with improvements in rapid recovery of arterial lactate and glucose levels, cerebral microcirculation, lung edema, and neurological function. Labeled mitochondria were observed inside the vital organs of the surviving rats 24 h post-CA. Conclusions MTx performed immediately after resuscitation improved survival and neurological recovery in post-CA rats. These results provide a foundation for future studies to promote the development of MTx as a novel therapeutic strategy to save lives currently lost after CA.

Adenosine triphosphate (ATP) plays fundamental roles in cellular biochemistry and was recently discovered to function as a biological hydrotrope. Here, we use mass spectrometry to interrogate ATP-mediated regulation of protein thermal stability and protein solubility on a proteome-wide scale. Thermal proteome profiling reveals high affinity interactions of ATP as a substrate and as an allosteric modulator that has widespread influence on protein complexes and their stability. Further, we develop a strategy for proteome-wide solubility profiling, and discover ATP-dependent solubilization of at least 25% of the insoluble proteome. ATP increases the solubility of positively charged, intrinsically disordered proteins, and their susceptibility for solubilization varies depending on their localization to different membrane-less organelles. Moreover, a few proteins, exhibit an ATP-dependent decrease in solubility, likely reflecting polymer formation. Our data provides a proteome-wide, quantitative insight into how ATP influences protein structure and solubility across the spectrum of physiologically relevant concentrations. ATP can function as a biological hydrotrope, but its global effects on protein solubility have not yet been characterized. Here, the authors quantify the effect of ATP on the thermal stability and solubility of the cellular proteome, providing insights into protein solubility regulation by ATP.