Broad recruitment of mGBP family members to Chlamydia trachomatis inclusions

Type-I IFNs induce GBPs and lysosomal defense in hepatocytes to control malaria

Plasmodium parasites undergo development and replication within the hepatocytes before infecting the erythrocytes and initiating clinical malaria. Although type-I interferons (IFNs) are known to hinder Plasmodium infection within the liver, the underlying mechanisms remain unclear. Here, we describe two IFN-I-driven hepatocyte antimicrobial programs controlling liver-stage malaria. First, oxidative defense by NADPH oxidases 2 and 4 triggers a pathway of lysosomal fusion with the parasitophorous vacuole (PV) to help clear Plasmodium . Second, guanylate-binding protein (GBP) 1 disruption of the PV activates caspase-1 inflammasome, inducing pyroptosis to remove the infected host cells. Remarkably, both human and mouse hepatocytes enlist these cell-autonomous immune programs to eliminate Plasmodium ; their pharmacologic or genetic inhibition led to profound malarial susceptibility, and are essential in vivo . In addition to identifying the IFN-I-mediated cell-autonomous immune circuits controlling Plasmodium infection in the hepatocytes, this study extends our understanding of how non-immune cells are integral to protective immunity against malaria.

Decoding Toxoplasma gondii virulence: the mechanisms of IRG protein inactivation

Toxoplasmosis is a common parasitic zoonosis that can be life-threatening in immunocompromised patients. About one-third of the human population is infected with Toxoplasma gondii. Primary infection triggers an innate immune response wherein IFN-γ-induced host cell GTPases, namely IRG and GBP proteins, serve as a vital component for host cell resistance. In the past decades, interest in elucidating the function of these GTPase families in controlling various intracellular pathogens has emerged. Numerous T. gondii effectors were identified to inactivate particular IRG proteins. T. gondii is re-optimizing its effectors to combat IRG function and in this way secures transmission. We discuss the IRG-specific effectors employed by the parasite in murine infections, contributing to a better understanding of T. gondii virulence.

Integrative modeling of guanylate binding protein dimers

Guanylate-binding proteins (GBPs) are essential interferon-γ-activated large GTPases that play a crucial role in host defense against intracellular bacteria and parasites. While their protective functions rely on protein polymerization, our understanding of the structural intricacies of these multimerized states remains limited. To bridge this knowledge gap, we present dimer models for human GBP1 (hGBP1) and murine GBP2 and 7 (mGBP2 and mGBP7) using an integrative approach, incorporating the crystal structure of hGBP1's GTPase domain dimer, crosslinking mass spectrometry, small-angle X-ray scattering, protein-protein docking, and molecular dynamics simulations. Our investigation begins by comparing the protein dynamics of hGBP1, mGBP2, and mGBP7. We observe that the M/E domain in all three proteins exhibits significant mobility and hinge motion, with mGBP7 displaying a slightly less pronounced motion but greater flexibility in its GTPase domain. These dynamic distinctions can be attributed to variations in the sequences of mGBP7 and hGBP1/mGBP2, resulting in different dimerization modes. Unlike hGBP1 and its close ortholog mGBP2, which exclusively dimerize through their GTPase domains, we find that mGBP7 exhibits three equally probable alternative dimer structures. The GTPase domain of mGBP7 is only partially involved in its dimerization, primarily due to an accumulation of negative charge, allowing mGBP7 to dimerize independently of GTP. Instead, mGBP7 exhibits a strong tendency to dimerize in an antiparallel arrangement across its stalks. The results of this work go beyond the sequence-structure-function relationship, as the sequence differences in mGBP7 and mGBP2/hGBP1 do not lead to different structures, but to different protein dynamics and dimerization. The distinct GBP dimer structures are expected to encode specific functions crucial for disrupting pathogen membranes.

Domain motions, dimerization, and membrane interactions of the murine guanylate binding protein 2

Guanylate-binding proteins (GBPs) are a group of GTPases that are induced by interferon-[Formula: see text] and are crucial components of cell-autonomous immunity against intracellular pathogens. Here, we examine murine GBP2 (mGBP2), which we have previously shown to be an essential effector protein for the control of Toxoplasma gondii replication, with its recruitment through the membrane of the parasitophorous vacuole and its involvement in the destruction of this membrane likely playing a role. The overall aim of our work is to provide a molecular-level understanding of the mutual influences of mGBP2 and the parasitophorous vacuole membrane. To this end, we performed lipid-binding assays which revealed that mGBP2 has a particular affinity for cardiolipin. This observation was confirmed by fluorescence microscopy using giant unilamellar vesicles of different lipid compositions. To obtain an understanding of the protein dynamics and how this is affected by GTP binding, mGBP2 dimerization, and membrane binding, assuming that each of these steps are relevant for the function of the protein, we carried out standard as well as replica exchange molecular dynamics simulations with an accumulated simulation time of more than 30 μs. The main findings from these simulations are that mGBP2 features a large-scale hinge motion in its M/E domain, which is present in each of the studied protein states. When bound to a cardiolipin-containing membrane, this hinge motion is particularly pronounced, leading to an up and down motion of the M/E domain on the membrane, which did not occur on a membrane without cardiolipin. Our prognosis is that this up and down motion has the potential to destroy the membrane following the formation of supramolecular mGBP2 complexes on the membrane surface.

Toxoplasma-proximal and distal control by GBPs in human macrophages

Human guanylate binding proteins (GBPs) are key players of interferon-gamma (IFNγ)-induced cell intrinsic defense mechanisms targeting intracellular pathogens. In this study, we combine the well-established Toxoplasmagondii infection model with three in vitro macrophage culture systems to delineate the contribution of individual GBP family members to control this apicomplexan parasite. Use of high-throughput imaging assays and genome engineering allowed us to define a role for GBP1, 2 and 5 in parasite infection control. While GBP1 performs a pathogen-proximal, parasiticidal and growth-restricting function through accumulation at the parasitophorous vacuole of intracellular Toxoplasma, GBP2 and GBP5 perform a pathogen-distal, growth-restricting role. We further find that mutants of the GTPase or isoprenylation site of GBP1/2/5 affect their normal function in Toxoplasma control by leading to mis-localization of the proteins.

Interferon-induced GTPases orchestrate host cell-autonomous defence against bacterial pathogens

Interferon (IFN)-induced guanosine triphosphate hydrolysing enzymes (GTPases) have been identified as cornerstones of IFN-mediated cell-autonomous defence. Upon IFN stimulation, these GTPases are highly expressed in various host cells, where they orchestrate anti-microbial activities against a diverse range of pathogens such as bacteria, protozoan and viruses. IFN-induced GTPases have been shown to interact with various host pathways and proteins mediating pathogen control via inflammasome activation, destabilising pathogen compartments and membranes, orchestrating destruction via autophagy and the production of reactive oxygen species as well as inhibiting pathogen mobility. In this mini-review, we provide an update on how the IFN-induced GTPases target pathogens and mediate host defence, emphasising findings on protection against bacterial pathogens.

Interferons: Tug of War Between Bacteria and Their Host

Type I and III interferons (IFNs) are archetypally antiviral cytokines that are induced in response to recognition of foreign material by pattern recognition receptors (PRRs). Though their roles in anti-viral immunity are well established, recent evidence suggests that they are also crucial mediators of inflammatory processes during bacterial infections. Type I and III IFNs restrict bacterial infection in vitro and in some in vivo contexts. IFNs mainly function through the induction of hundreds of IFN-stimulated genes (ISGs). These include PRRs and regulators of antimicrobial signaling pathways. Other ISGs directly restrict bacterial invasion or multiplication within host cells. As they regulate a diverse range of anti-bacterial host responses, IFNs are an attractive virulence target for bacterial pathogens. This review will discuss the current understanding of the bacterial effectors that manipulate the different stages of the host IFN response: IFN induction, downstream signaling pathways, and target ISGs.

Human GBP1 Differentially Targets Salmonella and Toxoplasma to License Recognition of Microbial Ligands and Caspase-Mediated Death

Interferon-inducible guanylate-binding proteins (GBPs) promote cell-intrinsic defense through host cell death. GBPs target pathogens and pathogen-containing vacuoles and promote membrane disruption for release of microbial molecules that activate inflammasomes. GBP1 mediates pyroptosis or atypical apoptosis of Salmonella Typhimurium (STm)- or Toxoplasma gondii (Tg)- infected human macrophages, respectively. The pathogen-proximal detection-mechanisms of GBP1 remain poorly understood, as humans lack functional immunity-related GTPases (IRGs) that assist murine Gbps. Here, we establish that GBP1 promotes the lysis of Tg-containing vacuoles and parasite plasma membranes, releasing Tg-DNA. In contrast, we show GBP1 targets cytosolic STm and recruits caspase-4 to the bacterial surface for its activation by lipopolysaccharide (LPS), but does not contribute to bacterial vacuole escape. Caspase-1 cleaves and inactivates GBP1, and a cleavage-deficient GBP1D192E mutant increases caspase-4-driven pyroptosis due to the absence of feedback inhibition. Our studies elucidate microbe-specific roles of GBP1 in infection detection and its triggering of the assembly of divergent caspase signaling platforms.

Biochemical and structural characterization of murine GBP7, a guanylate binding protein with an elongated C-terminal tail

Guanylate-binding proteins (GBPs) constitute a family of interferon-inducible guanosine triphosphatases (GTPases) that are key players in host defense against intracellular pathogens ranging from protozoa to bacteria and viruses. So far, human GBP1 and GBP5 as well as murine GBP2 (mGBP2) have been biochemically characterized in detail. Here, with murine GBP7 (mGBP7), a GBP family member with an unconventional and elongated C-terminus is analyzed. The present study demonstrates that mGBP7 exhibits a concentration-dependent GTPase activity and an apparent GTP turnover number of 20 min-1. In addition, fluorescence spectroscopy analyses reveal that mGBP7 binds GTP with high affinity (KD = 0.22 µM) and GTPase activity assays indicate that mGBP7 hydrolyzes GTP to GDP and GMP. The mGBP7 GTPase activity is inhibited by incubation with γ-phosphate analogs and a K51A mutation interfering with GTP binding. SEC-MALS analyses give evidence that mGBP7 forms transient dimers and that this oligomerization pattern is not influenced by the presence of nucleotides. Moreover, a structural model for mGBP7 is provided by homology modeling, which shows that the GTPase possesses an elongated C-terminal (CT) tail compared with the CaaX motif-containing mGBP2 and human GBP1. Molecular dynamics simulations indicate that this tail has transmembrane characteristics and, interestingly, confocal microscopy analyses reveal that the CT tail is required for recruitment of mGBP7 to the parasitophorous vacuole of Toxoplasma gondii.

Interferon inducible GBPs restrict Burkholderia thailandensis motility induced cell-cell fusion

Innate immunity responds to pathogens by producing alarm signals and activating pathways that make host cells inhospitable for pathogen replication. The intracellular bacterium Burkholderia thailandensis invades the cytosol, hijacks host actin, and induces cell fusion to spread to adjacent cells, forming multinucleated giant cells (MNGCs) which promote bacterial replication. We show that type I interferon (IFN) restricts macrophage MNGC formation during B. thailandensis infection. Guanylate-binding proteins (GBPs) expressed downstream of type I IFN were required to restrict MNGC formation through inhibition of bacterial Arp2/3-dependent actin motility during infection. GTPase activity and the CAAX prenylation domain were required for GBP2 recruitment to B. thailandensis, which restricted bacterial actin polymerization required for MNGC formation. Consistent with the effects in in vitro macrophages, Gbp2-/-, Gbp5-/-, GbpChr3-KO mice were more susceptible to intranasal infection with B. thailandensis than wildtype mice. Our findings reveal that IFN and GBPs play a critical role in restricting cell-cell fusion and bacteria-induced pathology during infection.

The Molecular Mechanism of Polymer Formation of Farnesylated Human Guanylate-binding Protein 1

The human guanylate-binding protein 1 (hGBP1) belongs to the dynamin superfamily proteins and represents a key player in the innate immune response. Farnesylation at the C-terminus is required for hGBP1's activity against microbial pathogens, as well as for its antiproliferative and antitumor activity. The farnesylated hGBP1 (hGBP1_fn) retains many characteristics of the extensively studied nonfarnesylated protein and gains additional abilities like binding to lipid membranes and formation of hGBP1_fn polymers. These polymers are believed to serve as a protein depot, making the enzyme immediately available to fight the invasion of intracellular pathogens. Here we study the molecular mechanism of hGBP1 polymer formation as it is a crucial state of this enzyme, allowing for a rapid response demanded by the biological function. We employ Förster resonance energy transfer in order to trace intra and intermolecular distance changes of protein domains. Light scattering techniques yield deep insights into the changes in size and shape. The GTP hydrolysis driven cycling between a closed, farnesyl moiety hidden state and an opened, farnesyl moiety exposed state represents the first phase, preparing the molecule for polymerization. Within the second phase of polymer growth, opened hGBP1 molecules can be incorporated in the growing polymer where the opened structure is stabilized, similar to a surfactant molecule in a micelle, pointing the farnesyl moieties into the hydrophobic center and positioning the head groups at the periphery of the polymer. We contribute the molecular mechanism of polymer formation, paving the ground for a detailed understanding of hGBP1 function.

Human GBP1 is a microbe-specific gatekeeper of macrophage apoptosis and pyroptosis

The guanylate binding protein (GBP) family of interferon-inducible GTPases promotes antimicrobial immunity and cell death. During bacterial infection, multiple mouse Gbps, human GBP2, and GBP5 support the activation of caspase-1-containing inflammasome complexes or caspase-4 which trigger pyroptosis. Whether GBPs regulate other forms of cell death is not known. The apicomplexan parasite Toxoplasma gondii causes macrophage death through unidentified mechanisms. Here we report that Toxoplasma-induced death of human macrophages requires GBP1 and its ability to target Toxoplasma parasitophorous vacuoles through its GTPase activity and prenylation. Mechanistically, GBP1 promoted Toxoplasma detection by AIM2, which induced GSDMD-independent, ASC-, and caspase-8-dependent apoptosis. Identical molecular determinants targeted GBP1 to Salmonella-containing vacuoles. GBP1 facilitated caspase-4 recruitment to Salmonella leading to its enhanced activation and pyroptosis. Notably, GBP1 could be bypassed by the delivery of Toxoplasma DNA or bacterial LPS into the cytosol, pointing to its role in liberating microbial molecules. GBP1 thus acts as a gatekeeper of cell death pathways, which respond specifically to infecting microbes. Our findings expand the immune roles of human GBPs in regulating not only pyroptosis, but also apoptosis.

Cell-autonomous immunity by IFN-induced GBPs in animals and plants

Inside host cells, guanylate binding proteins (GBPs) rapidly assemble into large antimicrobial defense complexes that combat a wide variety of bacterial pathogens. These massive nanomachines often completely coat targeted microbes where they act as recruitment platforms for downstream effectors capable of direct bactericidal activity. GBP-containing platforms also serve as sensory hubs to activate inflammasome-driven responses in the mammalian cytosol while in plants like Arabidopsis, GBP orthologues may facilitate intranuclear signaling for immunity against invasive phytopathogens. Together, this group of immune GTPases serve as a major defensive repertoire to protect the host cell interior from bacterial colonization across plant and animal kingdoms.

Interferon-induced guanylate-binding proteins: Guardians of host defense in health and disease

Guanylate-binding proteins (GBPs) have recently emerged as central orchestrators of immunity to infection, inflammation, and neoplastic diseases. Within numerous host cell types, these IFN-induced GTPases assemble into large nanomachines that execute distinct host defense activities against a wide variety of microbial pathogens. In addition, GBPs customize inflammasome responses to bacterial infection and sepsis, where they act as critical rheostats to amplify innate immunity and regulate tissue damage. Similar functions are becoming evident for metabolic inflammatory syndromes and cancer, further underscoring the importance of GBPs within infectious as well as altered homeostatic settings. A better understanding of the basic biology of these IFN-induced GTPases could thus benefit clinical approaches to a wide spectrum of important human diseases.

Safe haven under constant attack-The Chlamydia-containing vacuole

Chlamydia belong to the group of obligate intracellular bacteria that reside in a membrane bound vacuole during the entire intracellular phase of their life cycle. This vacuole called inclusion shields the bacteria from adverse influences in the cytosol of the host cell like the destructive machinery of the cell-autonomous defence system. The inclusion thereby prevents the digestion and eradication in specialised compartments of the intact and viable cell called phagolysosomes or autophagolysosomes. It is becoming more and more evident that keeping the inclusion intact also prevents the onset of cell intrinsic cell death programmes that are activated upon damage of the inclusion and direct the cell to destruct itself and the pathogen inside. Chlamydia secrete numerous proteins into the inclusion membrane to protect and stabilise their unique niche inside the host cell. We will focus in this review on the diverse attack strategies of the host aiming at the destruction of the Chlamydia-containing inclusion and will summarise the current knowledge on the protection mechanisms elaborated by the bacteria to maintain the integrity of their replication niche.

NADPH Oxidase and Guanylate Binding Protein 5 Restrict Survival of Avirulent Type III Strains of Toxoplasma gondii in Naive Macrophages

Toxoplasma infections in humans and other mammals are largely controlled by IFN-γ produced by the activated adaptive immune system. However, we still do not completely understand the role of cell-intrinsic functions in controlling Toxoplasma or other apicomplexan infections. The present work identifies intrinsic activities in naive macrophages in counteracting T. gondii infection. Using an avirulent strain of T. gondii, we highlight the importance of Nox complexes in conferring protection against parasite infection both in vitro and in vivo. We also identify Gbp5 as a novel macrophage factor involved in limiting intracellular infection by avirulent strains of T. gondii. The rarity of human infections caused by type III strains suggests that these mechanisms may also be important in controlling human toxoplasmosis. These findings further extend our understanding of host responses and defense mechanisms that act to control parasitic infections at the cellular level.

Phagocytic cells are the first line of innate defense against intracellular pathogens, and yet Toxoplasma gondii is renowned for its ability to survive in macrophages, although this paradigm is based on virulent type I parasites. Surprisingly, we find that avirulent type III parasites are preferentially cleared in naive macrophages, independent of gamma interferon (IFN-γ) activation. The ability of naive macrophages to clear type III parasites was dependent on enhanced activity of NADPH oxidase (Nox)-generated reactive oxygen species (ROS) and induction of guanylate binding protein 5 (Gbp5). Macrophages infected with type III parasites (CTG strain) showed a time-dependent increase in intracellular ROS generation that was higher than that induced by type I parasites (GT1 strain). The absence of Nox1 or Nox2, gp91 subunit isoforms of the Nox complex, reversed ROS-mediated clearance of CTG parasites. Consistent with this finding, both Nox1^−/− and Nox2^−/− mice showed higher susceptibility to CTG infection than wild-type mice. Additionally, Gbp5 expression was induced upon infection and the enhanced clearance of CTG strain parasites was reversed in Gbp5^−/− macrophages. Expression of a type I ROP18 allele in CTG prevented clearance in naive macrophages, suggesting that it plays a role counteracting Gbp5. Although ROS and Gbp5 have been linked to activation of the NLRP3 inflammasome, clearance of CTG parasites did not rely on induction of pyroptosis. Collectively, these findings reveal that not all strains of T. gondii are adept at avoiding clearance in macrophages and define new roles for ROS and Gbps in controlling this important intracellular pathogen.

Regulation of innate immune functions by guanylate-binding proteins

Guanylate-binding proteins (GBP) are a family of dynamin-related large GTPases which are expressed in response to interferons and other pro-inflammatory cytokines. GBPs mediate a broad spectrum of innate immune functions against intracellular pathogens ranging from viruses to bacteria and protozoa. Several binding partners for individual GBPs have been identified and several different mechanisms of action have been proposed depending on the organisms, the cell type and the pathogen used. Many of these anti-pathogenic functions of GBPs involve the recruitment to and the subsequent destruction of pathogen containing vacuolar compartments, the assembly of large oligomeric innate immune complexes such as the inflammasome, or the induction of autophagy. Furthermore, GBPs often cooperate with immunity-related GTPases (IRGs), another family of dynamin-related GTPases, to exert their anti-pathogenic function, but since most IRGs have been lost in the evolution of higher primates, the anti-pathogenic function of human GBPs seems to be IRG-independent. GBPs and IRGs share biochemical and structural properties with the other members of the dynamin superfamily such as low nucleotide affinity and a high intrinsic GTPase activity which can be further enhanced by oligomerisation. Furthermore, GBPs and IRGs can interact with lipid membranes. In the case of three human and murine GBP isoforms this interaction is mediated by C-terminal isoprenylation. Based on cell biological studies, and in analogy to the function of other dynamins in membrane scission events, it has been postulated that both GBPs and IRGs might actively disrupt the outer membrane of pathogen-containing vacuole leading to the detection and destruction of the pathogen by the cytosolic innate immune system of the host. Recent evidence, however, indicates that GBPs might rather function by mediating membrane tethering events similar to the dynamin-related atlastin and mitofusin proteins, which mediate fusion of the ER and mitochondria, respectively. The aim of this review is to highlight the current knowledge on the function of GBPs in innate immunity and to combine it with the recent progress in the biochemical characterisation of this protein family.