Crystal structure of the apoptosis-inducing human granzyme A dimer

4-Octyl itaconate blocks GSDMB-mediated pyroptosis and restricts inflammation by inactivating granzyme A

GSDMB-mediated pyroptosis facilitates a pro-inflammatory immune microenvironment and needs to be tightly regulated to avoid excessive inflammation. Here, we provide evidence that itaconate and its cell-permeable derivative 4-octyl itaconate (4-OI) can significantly inhibit GSDMB-rendered pyroptotic activity independent of Nrf2. 4-OI interferes proteolytic process of GSDMB by directly modifying Cys54, Cys148 and Ser212 on granzyme A (GrzA), a serine protease that site-specifically cleaves the inter-domain linker of GSDMB, instead of interaction with GSDMB, thereby blocking pyroptosis and exerts anti-inflammatory effects. Moreover, 4-OI alleviates inflammation by suppressing GSDMB-induced pyroptotic cell death during acute colitis models in intestinal epithelial GSDMB conditional transgenic mice. Our data expand the role of 4-OI as a crucial immunometabolic derivative that regulates innate immunity and inflammation through a newly identified posttranslational modification, and targeting of pyroptosis by 4-OI therefore holds potent therapeutic potential for primarily inflammatory and/or autoimmune diseases.

Granzyme serine proteases in inflammation and rheumatic diseases

Granzymes (granule-secreted enzymes) are a family of serine proteases that have been viewed as redundant cytotoxic enzymes since their discovery more than 30 years ago. Predominantly produced by cytotoxic lymphocytes and natural killer cells, granzymes are delivered into the cytoplasm of target cells through immunological synapses in cooperation with the pore-forming protein perforin. After internalization, granzymes can initiate cell death through the cleavage of intracellular substrates. However, evidence now also demonstrates the existence of non-cytotoxic, pro-inflammatory, intracellular and extracellular functions that are granzyme specific. Under pathological conditions, granzymes can be produced and secreted extracellularly by immune cells as well as by non-immune cells. Depending on the granzyme, accumulation in the extracellular milieu might contribute to inflammation, tissue injury, impaired wound healing, barrier dysfunction, osteoclastogenesis and/or autoantigen generation.

Recent Advances in the Development of Non-Invasive Imaging Probes for Cancer Immunotherapy

The last two decades have witnessed a major revolution in the field of tumor immunology including clinical progress using various immunotherapy strategies. These advances have highlighted the potential for approaches that harness the power of the immune system to fight against cancer. While cancer immunotherapies have shown significant clinical successes, patient responses vary widely due to the complex and heterogeneous nature of tumors and immune responses, calling for reliable biomarkers and therapeutic strategies to maximize the benefits of immunotherapy. Especially, stratifying responding individuals from non-responders during the early stages of treatment could help avoid long-term damage and tailor personalized treatments. In efforts to develop non-invasive means for accurately evaluating and predicting tumor response to immunotherapy, multiple affinity-based agents targeting immune cell markers and checkpoint molecules have been developed and advanced to clinical trials. In addition, researchers have recently turned their attention to substrate and activity-based imaging probes that can provide real-time, functional assessment of immune response to treatment. Here, we highlight some of those recently designed probes that image functional proteases as biomarkers of cancer immunotherapy with a focus on their chemical design and detection modalities and discuss challenges and opportunities for the development of imaging tools utilized in cancer immunotherapy.

Identification of pyroptosis-related genes and long non-coding RNAs signatures in osteosarcoma

Osteosarcoma is a highly malignant tumor, with very high disability and fatality rates. However, the overall prognosis is not optimistic. Pyroptosis is a newly discovered cell death modality accompanied by inflammation, which is closely related to varieties of cancers. In this study, the RNA-seq data were downloaded from public databases, the differences in the expression of the pyroptosis-related genes (PRGs) were identified, and the six PRGs signature was established through the univariate and LASSO Cox analysis. The patients were grouped according to the PRGs signature, and the prognosis between the two groups was further compared. In addition, a ten pyroptosis-related lncRNAs (PRLs) prognostic signature was also constructed. Through functional analysis of the differentially expressed genes (DEGs), the immune-related pathways were found to be enriched. The Pearson correlation analysis showed a strong correlation between the pyroptosis-related biomarkers. Finally, we identified a promising biomarker, CHMP4C, which is highly expressed in osteosarcoma. Overexpression of CHMP4C promoted the proliferation, migration and invasion of the osteosarcoma cell. Our results thus provide new evidence for exploring prognostic biomarkers and therapeutic targets of osteosarcoma.

Direct Cytosolic Delivery of Proteins Using Lyophilized and Reconstituted Polymer-Protein Assemblies

PURPOSE

Cytosolic delivery of proteins accesses intracellular targets for chemotherapy and immunomodulation. Current delivery systems utilize inefficient endosomal pathways of uptake and escape that lead to degradation of delivered cargo. Cationic poly(oxanorbornene)imide (PONI) polymers enable highly efficient cytosolic delivery of co-engineered proteins, but aggregation and denaturation in solution limits shelf life. In the present study we evaluate polymer-protein nanocomposite vehicles as candidates for lyophilization and point-of-care resuspension to provide a transferrable technology for cytosolic protein delivery.

METHODS

Self-assembled nanocomposites of engineered poly(glutamate)-tagged (E-tagged) proteins and guanidinium-functionalized PONI homopolymers were generated, lyophilized, and stored for 2 weeks. After reconstitution and delivery, cytosolic access of E-tagged GFP cargo (GFPE15) was assessed through diffuse cytosolic and nuclear fluorescence, and cell killing with chemotherapeutic enzyme Granzyme A (GrAE10). Efficiency was quantified between freshly prepared and lyophilized samples.

RESULTS

Reconstituted nanocomposites retained key structural features of freshly prepared assemblies, with minimal loss of material. Cytosolic delivery (> 80% efficiency of freshly prepared nanocomposites) of GFPE15 was validated in several cell lines, with intracellular access validated and quantified through diffusion into the nucleus. Delivery of GrAE10 elicited significant tumorigenic cell death. Intracellular access of cytotoxic protein was validated through cell viability.

CONCLUSION

Reconstituted nanocomposites achieved efficient cytosolic delivery of protein cargo and demonstrated therapeutic applicability with delivery of GrAE10. Overall, this strategy represents a versatile and highly translatable method for cytosolic delivery of proteins.

Mechanisms of Gasdermin Recognition by Proteases

Members of the gasdermin family contain positively charged N-terminal domains (NTDs) capable of binding phospholipids and assembling membrane pores, and C-terminal domains (CTDs) that bind the NTDs to prevent pore formation in the resting states. The flexible NTD-CTD linker regions of gasdermins are highly variable in length and sequences, which may be attributable to gasdermin recognition by diverse proteases. In addition, protease cleavage within the NTDs is known to inactivate several gasdermin family members. Recognition and cleavage of the gasdermin family members by different proteases share common and distinct features at the protease active sites, as well as exosites recently identified for the inflammatory caspases. Utilization of exosites may strengthen enzyme-substrate interaction, improve efficiency of proteolysis, and enhance substrate selectivity. It remains to be determined if the dual site recognition of gasdermin D (GSDMD) by the inflammatory caspases is employed by other GSDMD-targeting proteases, or is involved in proteolytic processing of other gasdermins. Biochemical and structural approaches will be instrumental in revealing how potential exosites in diverse proteases engage different gasdermin substrates. Different features of gasdermin sequence, structure, expression characteristics, and post-translational modifications may dictate distinct mechanisms of protease-dependent activation or inactivation. Such diverse mechanisms may underlie the divergent physiological and pathological functions of gasdermins, and furnish opportunities for therapeutic targeting of gasdermins in infectious diseases and inflammatory disorders.

Detection of Active Granzyme A in NK92 Cells with Fluorescent Activity-Based Probe

Cytotoxic T-lymphocytes (CTLs) and natural killer cells (NKs) kill compromised cells to defend against tumor and viral infections. Both effector cell types use multiple strategies to induce target cell death including Fas/CD95 activation and the release of perforin and a group of lymphocyte granule serine proteases called granzymes. Granzymes have relatively broad and overlapping substrate specificities and may hydrolyze a wide range of peptidic epitopes; it is therefore challenging to identify their natural and synthetic substrates and to distinguish their localization and functions. Here, we present a specific and potent substrate, an inhibitor, and an activity-based probe of Granzyme A (GrA) that can be used to follow functional GrA in cells.

Granzyme A Stimulates pDCs to Promote Adaptive Immunity via Induction of Type I IFN

Granzyme A (GzmA), together with perforin, are well-known for their cytotoxic activity against tumor or virus-infected cells. In addition to this cytotoxic function, GzmA stimulates several immune cell types and induces inflammation in the absence of perforin, however, its effect on the dendritic cell (DC) is unknown. In the current study, we showed that recombinant GzmA induced the phenotypic maturation of plasmacytoid DCs (pDCs) and conventional DCs (cDCs), but not their apoptosis. Particularly, GzmA made pDCs more functional, thus leading to production of type I interferon (IFN) via the TLR9-MyD88 pathway. We also demonstrated that GzmA binds TLR9 and co-localizes with it in endosomes. When co-administered with antigen, GzmA acted as a powerful adjuvant for eliciting antigen-specific cytotoxic CD8⁺ T lymphocytes (CTLs) that protected mice from tumor challenge. The induction of CTL was completely abolished in XCR1⁺ DC-depleted mice, whereas it was reduced to less than half in pDC-depleted or IFN-α/β receptor knockout mice. Thus, CTL cross-priming was dependent on XCR1⁺cDC and also type I IFN, which was produced by GzmA-activated pDCs. These results indicate that GzmA -stimulated pDCs enhance the cross-priming activity of cDCs in situ. We also showed that the adjuvant effect of GzmA is superior to CpG-ODN and LPS. Our findings highlight the ability of GzmA to bridge innate and adaptive immune responses via pDC help and suggest that GzmA may be useful as a vaccine adjuvant.

Identification and annotation of bovine granzyme genes reveals a novel granzyme encoded within the trypsin-like locus

Granzymes are a family of serine proteases found in the lytic granules of cytotoxic T lymphocytes and natural killer (NK) cells, which are involved in killing of susceptible target cells. Most information on granzymes and their enzymatic specificities derive from studies in humans and mice. Although granzymes shared by both species show a high level of conservation, the complement of granzyme genes differs between the species. The aim of this study was to identify granzyme genes expressed in cattle, determine their genomic locations and analyse their sequences to predict likely functional specificities. Orthologues of the five granzyme genes found in humans (A, B, H, K and M) were identified, as well a novel gene designated granzyme O, most closely related to granzyme A. An orthologue of granzyme O was found in pigs and a non-function version was detected in the human genome. Use of specific PCRs demonstrated that all of these genes, including granzyme O, are expressed in activated subsets of bovine lymphocytes, with particularly high levels in CD8 T cells. Consistent with findings in humans and mice, the granzyme-encoding genes were located on three distinct genomic loci, which correspond to different proteolytic enzymatic activities, namely trypsin-like, chymotrypsin-like and metase-like. Analysis of amino acid sequences indicated that the granzyme proteins have broadly similar enzymatic specificities to their human and murine counterparts but indicated that granzyme B has a different secondary specificity. These findings provide the basis for further work to examine their role in the cytotoxic activity of bovine CD8 T cells.

Granzyme B Disrupts Central Metabolism and Protein Synthesis in Bacteria to Promote an Immune Cell Death Program

Human cytotoxic lymphocytes kill intracellular microbes. The cytotoxic granule granzyme proteases released by cytotoxic lymphocytes trigger oxidative bacterial death by disrupting electron transport, generating superoxide anion and inactivating bacterial oxidative defenses. However, they also cause non-oxidative cell death because anaerobic bacteria are also killed. Here, we use differential proteomics to identify granzyme B substrates in three unrelated bacteria: Escherichia coli, Listeria monocytogenes, and Mycobacteria tuberculosis. Granzyme B cleaves a highly conserved set of proteins in all three bacteria, which function in vital biosynthetic and metabolic pathways that are critical for bacterial survival under diverse environmental conditions. Key proteins required for protein synthesis, folding, and degradation are also substrates, including multiple aminoacyl tRNA synthetases, ribosomal proteins, protein chaperones, and the Clp system. Because killer cells use a multipronged strategy to target vital pathways, bacteria may not easily become resistant to killer cell attack.

Granule Associated Serine Proteases of Hematopoietic Cells - An Analysis of Their Appearance and Diversification during Vertebrate Evolution

Serine proteases are among the most abundant granule constituents of several hematopoietic cell lineages including mast cells, neutrophils, cytotoxic T cells and NK cells. These proteases are stored in their active form in the cytoplasmic granules and in mammals are encoded from four different chromosomal loci: the chymase locus, the met-ase locus, the T cell tryptase and the mast cell tryptase locus. In order to study their appearance during vertebrate evolution we have performed a bioinformatic analysis of related genes and gene loci from a large panel of metazoan animals from sea urchins to placental mammals for three of these loci: the chymase, met-ase and granzyme A/K loci. Genes related to mammalian granzymes A and K were the most well conserved and could be traced as far back to cartilaginous fish. Here, the granzyme A and K genes were found in essentially the same chromosomal location from sharks to humans. However in sharks, no genes clearly identifiable as members of the chymase or met-ase loci were found. A selection of these genes seemed to appear with bony fish, but sometimes in other loci. Genes related to mammalian met-ase locus genes were found in bony fish. Here, the most well conserved member was complement factor D. However, genes distantly related to the neutrophil proteases were also identified in this locus in several bony fish species, indicating that this locus is also old and appeared at the base of bony fish. In fish, a few of the chymase locus-related genes were found in a locus with bordering genes other than the mammalian chymase locus and some were found in the fish met-ase locus. This indicates that a convergent evolution rather than divergent evolution has resulted in chymase locus-related genes in bony fish.

SERPINB12 Is a Slow-Binding Inhibitor of Granzyme A and Hepsin

The clade B/intracellular serpins protect cells from peptidase-mediated injury by forming covalent complexes with their targets. SERPINB12 is expressed in most tissues, especially at cellular interfaces with the external environment. This wide tissue distribution pattern is similar to that of granzyme A (GZMA). Because SERPINB12 inhibits trypsin-like serine peptidases, we determined whether it might also neutralize GZMA. SERPINB12 formed a covalent complex with GZMA and inhibited the enzyme with typical serpin slow-binding kinetics. SERPINB12 also inhibited Hepsin. SERPINB12 may function as an endogenous inhibitor of these peptidases.

On the accuracy of unit-cell parameters in protein crystallography

The availability in the Protein Data Bank (PDB) of a number of structures that are presented in space group P1 but in reality possess higher symmetry allowed the accuracy and precision of the unit-cell parameters of the crystals of macromolecules to be evaluated. In addition, diffraction images from crystals of several proteins, previously collected as part of in-house projects, were processed independently with three popular software packages. An analysis of the results, augmented by published serial crystallography data, suggests that the apparent precision of the presentation of unit-cell parameters in the PDB to three decimal points is not justified, since these parameters are subject to errors of not less than 0.2%. It was also noticed that processing data including full crystallographic symmetry does not lead to deterioration of the refinement parameters; thus, it is not beneficial to treat the crystals as belonging to space group P1 when higher symmetry can be seen.

Live cell evaluation of granzyme delivery and death receptor signaling in tumor cells targeted by human natural killer cells

Growing interest in natural killer (NK) cell-based therapy for treating human cancer has made it imperative to develop new tools to measure early events in cell death. We recently demonstrated that protease-cleavable luciferase biosensors detect granzyme B and pro-apoptotic caspase activation within minutes of target cell recognition by murine cytotoxic lymphocytes. Here we report successful adaptation of the biosensor technology to assess perforin-dependent and -independent induction of death pathways in tumor cells recognized by human NK cell lines and primary cells. Cell-cell signaling via both Fc receptors and NK-activating receptors led to measurable luciferase signal within 15 minutes. In addition to the previously described aspartase-cleavable biosensors, we report development of granzyme A and granzyme K biosensors, for which no other functional reporters are available. The strength of signaling for granzyme biosensors was dependent on perforin expression in IL-2-activated NK effectors. Perforin-independent induction of apoptotic caspases was mediated by death receptor ligation and was detectable after 45 minutes of conjugation. Evidence of both FasL and TRAIL-mediated signaling was seen after engagement of Jurkat cells by perforin-deficient human cytotoxic lymphocytes. Although K562 cells have been reported to be insensitive to TRAIL, robust activation of pro-apoptotic caspases by NK cell-derived TRAIL was detectable in K562 cells. These studies highlight the sensitivity of protease-cleaved luciferase biosensors to measure previously undetectable events in live cells in real time. Further development of caspase and granzyme biosensors will allow interrogation of additional features of granzyme activity in live cells including localization, timing, and specificity.

Substrate specificities of the granzyme tryptases A and K

The physiological roles of the granzymes A and K have been debated, especially concerning their involvement in cytotoxic and inflammatory processes. By performing N-terminal COFRADIC assisted N-terminomics on the homologous human granzymes A and K, we here provide detailed data on their substrate repertoires, their specificities, and differences in efficiency by which they cleave their substrates, all of which may aid in elucidating their key substrates. In addition, the so far uncharacterized mouse granzyme K was profiled alongside its human orthologue. While the global primary specificity profiles of these granzymes appear quite similar as they revealed only subtle differences and pointed to substrate occupancies in the P1, P1', and P2' position as the main determinants for substrate recognition, differential analyses unveiled distinguishing substrate subsite features, some of which were confirmed by the more selective cleavage of specifically designed probes.

Lassa and Marburg viruses elicit distinct host transcriptional responses early after infection

BACKGROUND

Lassa virus and Marburg virus are two causative agents of viral hemorrhagic fever. Their diagnosis is difficult because patients infected with either pathogen present similar nonspecific symptoms early after infection. Current diagnostic tests are based on detecting viral proteins or nucleic acids in the blood, but these cannot be found during the early stages of disease, before the virus starts replicating in the blood. Using the transcriptional response of the host during infection can lead to earlier diagnoses compared to those of traditional methods.

RESULTS

In this study, we use RNA sequencing to obtain a high-resolution view of the in vivo transcriptional dynamics of peripheral blood mononuclear cells (PBMCs) throughout both types of infection. We report a subset of host mRNAs, including heat-shock proteins like HSPA1B, immunoglobulins like IGJ, and cell adhesion molecules like SIGLEC1, whose differences in expression are strong enough to distinguish Lassa infection from Marburg infection in non-human primates. We have validated these infection-specific expression differences by using microarrays on a larger set of samples, and by quantifying the expression of individual genes using RT-PCR.

CONCLUSIONS

These results suggest that host transcriptional signatures are correlated with specific viral infections, and that they can be used to identify highly pathogenic viruses during the early stages of disease, before standard detection methods become effective.

Leukocyte protease binding to nucleic acids promotes nuclear localization and cleavage of nucleic acid binding proteins

Killer lymphocyte granzyme (Gzm) serine proteases induce apoptosis of pathogen-infected cells and tumor cells. Many known Gzm substrates are nucleic acid binding proteins, and the Gzms accumulate in the target cell nucleus by an unknown mechanism. In this study, we show that human Gzms bind to DNA and RNA with nanomolar affinity. Gzms cleave their substrates most efficiently when both are bound to nucleic acids. RNase treatment of cell lysates reduces Gzm cleavage of RNA binding protein targets, whereas adding RNA to recombinant RNA binding protein substrates increases in vitro cleavage. Binding to nucleic acids also influences Gzm trafficking within target cells. Preincubation with competitor DNA and DNase treatment both reduce Gzm nuclear localization. The Gzms are closely related to neutrophil proteases, including neutrophil elastase (NE) and cathepsin G. During neutrophil activation, NE translocates to the nucleus to initiate DNA extrusion into neutrophil extracellular traps, which bind NE and cathepsin G. These myeloid cell proteases, but not digestive serine proteases, also bind DNA strongly and localize to nuclei and neutrophil extracellular traps in a DNA-dependent manner. Thus, high-affinity nucleic acid binding is a conserved and functionally important property specific to leukocyte serine proteases. Furthermore, nucleic acid binding provides an elegant and simple mechanism to confer specificity of these proteases for cleavage of nucleic acid binding protein substrates that play essential roles in cellular gene expression and cell proliferation.

Identification of Serpinb6b as a species-specific mouse granzyme A inhibitor suggests functional divergence between human and mouse granzyme A

The granzyme family serine proteases are key effector molecules expressed by cytotoxic lymphocytes. The physiological role of granzyme (Gzm) A is controversial, with significant debate over its ability to induce death in target cells. Here, we investigate the natural inhibitors of GzmA. We employed substrate phage display and positional proteomics to compare substrate specificities of mouse (m) and human (h) GzmA at the peptide and proteome-wide levels and we used the resulting substrate specificity profiles to search for potential inhibitors from the intracellular serpin family. We identified Serpinb6b as a potent inhibitor of mGzmA. Serpinb6b interacts with mGzmA, but not hGzmA, with an association constant of 1.9 ± 0.8 × 10(5) M(-1) s(-1) and a stoichiometry of inhibition of 1.8. Mouse GzmA is over five times more cytotoxic than hGzmA when delivered into P815 target cells with streptolysin O, whereas transfection of target cells with a Serpinb6b cDNA increases the EC50 value of mGzmA 13-fold, without affecting hGzmA cytotoxicity. Unexpectedly, we also found that Serpinb6b employs an exosite to specifically inhibit dimeric but not monomeric mGzmA. The identification of an intracellular inhibitor specific for mGzmA only indicates that a lineage-specific increase in GzmA cytotoxic potential has driven cognate inhibitor evolution.

Are all granzymes cytotoxic in vivo?

Granzymes are serine proteases mainly found in cytotoxic lymphocytes. The most-studied member of this group is granzyme B, which is a potent cytotoxin that has set the paradigm that all granzymes are cyototoxic. In the last 5 years, this paradigm has become controversial. On one hand, there is a plethora of sometimes contradictory publications showing mainly caspase-independent cytotoxic effects of granzyme A and the so-called orphan granzymes in vitro. On the other hand, there are increasing numbers of reports of granzymes failing to induce cell death in vitro unless very high (potentially supra-physiological) concentrations are used. Furthermore, experiments with granzyme A or granzyme M knock-out mice reveal little or no deficit in their cytotoxic lymphocytes’ killing ability ex vivo, but indicate impairment in the inflammatory response. These findings of non-cytotoxic effects of granzymes challenge dogma, and thus require alternative or additional explanations to be developed of the role of granzymes in defeating pathogens. Here we review evidence for granzyme cytotoxicity, give an overview of their non-cytotoxic functions, and suggest technical improvements for future investigations.

Alterations in RNA processing during immune-mediated programmed cell death

During immune-mediated death, death-inducing granzyme (Gzm) proteases concentrate in the nucleus of cells targeted for immune elimination, suggesting that nuclear processes are important targets. Here we used differential 2D proteomics of GzmA-treated nuclei to identify potential GzmA substrates. Of 44 candidates, 33 were RNA-binding proteins important in posttranscriptional RNA processing, including 14 heterogeneous nuclear ribonucleoproteins (hnRNP). Multiple hnRNPs were degraded in cells undergoing GzmA-, GzmB-, or caspase-mediated death. GzmA and caspase activation impaired nuclear export of newly synthesized RNA and disrupted pre-mRNA splicing. Expressing GzmA-resistant hnRNP A1 inhibited GzmA-mediated cell death and rescued pre-mRNA splicing, suggesting that hnRNP A1 is an important GzmA substrate. Cellular stresses are known to inhibit initiation of cap-dependent translation. Disrupting pre-mRNA processing should block further new protein synthesis and promote death by interfering with pathways induced to protect cells from death.

The substrate specificity profile of human granzyme A

The exact biological function of granzyme A, a granule-associated serine protease belonging to the tryptase family of proteases, is still a matter of debate because conflicting roles have been suggested, such as initiation of caspase-independent apoptosis-like cell death and endogenous modulation of inflammatory processes. In contrast to its well-studied family member, granzyme B, far less is known about the physiological targets of granzyme A. Using an N-terminal peptide-centric proteomics technology, the substrate specificity of human granzyme A was extensively characterized at the level of macromolecular protein substrates. Overall, more than 260 cleavage sites, almost exclusively favoring basic residues at the P1 position, in approximately 200 unique protein substrates, including the well-known in vitro substrates APEX-endonuclease 1 and different histones, were identified. Further substrate characterization was used to delineate physical properties in the substrate specificity profiles, which further highlights important aspects in protease/substrate biology.

Delivery and therapeutic potential of human granzyme B

Granzyme B (GzmB) is used by cytotoxic lymphocytes as a molecular weapon for the defense against virus-infected and malignantly transformed host cells. It belongs to a family of small serine proteases that are stored in secretory vesicles of killer cells. After secretion of these cytolytic granules during killer cell attack, GzmB is translocated into the cytosol of target cells with the help of the pore-forming protein perforin. GzmB has adopted similar protease specificity as caspase-8, and once delivered, it activates major executioner apoptosis pathways. Since GzmB is very effective in killing human tumor cell lines that are otherwise resistant against many cytotoxic drugs and since GzmB of human origin can be recombinantly expressed, its use as part of a 'magic bullet' in tumor therapy is a very tempting idea. In this review, we emphasize the peculiar characteristics of GzmB that make it suited for use as an effector domain in potential immunoconjugates. We discuss what is known about its uptake into target cells and the trials performed with GzmB-armed immunoconjugates, and we assess the prospects of its potential therapeutic value.

Orphan granzymes find a home

Cytotoxic lymphocytes are armed with granules that are released in the granule-exocytosis pathway to kill tumor cells and virus-infected cells. Cytotoxic granules contain the pore-forming protein perforin and a family of structurally homologues serine proteases called granzymes. While perforin facilitates the entry of granzymes into a target cell, the latter initiate distinct apoptotic routes. Granzymes are also implicated in extracellular functions such as extracellular matrix degradation, immune regulation, and inflammation. The family of human granzymes consists of five members, of which granzyme A and B have been studied most extensively. Recently, elucidation of the specific characteristics of the other three human granzymes H, K, and M, also referred to as orphan granzymes, have started. In this review, we summarize and discuss what is currently known about the biology of the human orphan granzymes.

Granzyme M: characterization with sites of post-translational modification and specific sites of interaction with substrates and inhibitors

Granzymes kill cells in a variety of ways. They induce mitochondrial dysfunction through caspase dependent and caspase-independent pathways and destroy DNA and the integrity of the nucleus. For gaining a better understanding of the molecular function of granzyme M and its NK cell specificity, structural characterization of this enzyme by molecular modeling as well as its detailed comparison with other granzymes is presented in this study. The study includes mode of action of granzyme M using cationic binding sites, substrate specificity, post-translational structural modification and its functional relationship and interaction of the enzyme with inhibitor in an attempt to explore how the activity of human granzyme M is controlled under physiological conditions. It is concluded from the present study that the post-translational modification, including Oglycosylation of serine, phosphorylation of serine and threonine and myristoylation of glycine, play an important role in the interaction of enzyme with the cell surface membrane and regulate protein trafficking and stability. Phosphorylated serine and threonine also plays a role in tumor elimination, viral clearance and tissue repair. In Gzm M there are cationic sites, cs1 and cs2 that may participate in binding of Gzm M to the cell surface, thereby promoting its uptake and eventual release into the cytoplasm. Gzm M shows apoptotic activity both by caspase dependent and independent pathways. Modeling of inhibitors bound to the granzyme active site shows that the dimer also contributes to substrate specificity in a unique manner by extending the active-site cleft.

Conformational changes in redox pairs of protein structures

Disulfides are conventionally viewed as structurally stabilizing elements in proteins but emerging evidence suggests two disulfide subproteomes exist. One group mediates the well known role of structural stabilization. A second redox-active group are best known for their catalytic functions but are increasingly being recognized for their roles in regulation of protein function. Redox-active disulfides are, by their very nature, more susceptible to reduction than structural disulfides; and conversely, the Cys pairs that form them are more susceptible to oxidation. In this study, we searched for potentially redox-active Cys Pairs by scanning the Protein Data Bank for structures of proteins in alternate redox states. The PDB contains over 1134 unique redox pairs of proteins, many of which exhibit conformational differences between alternate redox states. Several classes of structural changes were observed, proteins that exhibit: disulfide oxidation following expulsion of metals such as zinc; major reorganisation of the polypeptide backbone in association with disulfide redox-activity; order/disorder transitions; and changes in quaternary structure. Based on evidence gathered supporting disulfide redox activity, we propose disulfides present in alternate redox states are likely to have physiologically relevant redox activity.

Structural basis for proteolytic specificity of the human apoptosis-inducing granzyme M

Granzyme M (GzmM), a unique serine protease constitutively expressed in NK cells, is important for granule-mediated cytolysis and can induce rapid caspase-dependent apoptosis of tumor cells. However, few substrates of GzmM have been reported to date, and the mechanism by which this enzyme recognizes and hydrolyzes substrates is unknown. To provide structural insights into the proteolytic specificity of human GzmM (hGzmM), crystal structures of wild-type hGzmM, the inactive D86N-GzmM mutant with bound peptide substrate, and the complexes with a catalytic product and with a tetrapeptide chloromethylketone inhibitor were solved to 1.96 A, 2.30 A, 2.17 A and 2.70 A, respectively. Structure-based mutagenesis revealed that the N terminus and catalytic triad of hGzmM are most essential for proteolytic function. In particular, D86N-GzmM was found to be an ideal inactive enzyme for functional studies. Structural comparisons indicated a large conformational change of the L3 loop upon substrate binding, and suggest this loop mediates the substrate specificity of hGzmM. Based on the complex structure of GzmM with its catalytic product, a tetrapeptide chloromethylketone inhibitor was designed and found to specifically block the catalytic activity of hGzmM.

The cytotoxic T lymphocyte protease granzyme A cleaves and inactivates poly(adenosine 5'-diphosphate-ribose) polymerase-1

Granzyme A (GzmA) in killer cells induces caspase-independent programmed cell death. In this study, we show that GzmA cleaves the DNA damage sensor poly(adenosine 5'-diphosphate-ribose) polymerase-1 (PARP-1) after Lys(498) in its automodification domain, separating the DNA binding domain from the catalytic domain, which interferes with repair of GzmA-induced DNA damage and enhances susceptibility to GzmA-mediated death. Overexpressing K498A PARP-1 reduces GzmA-mediated death and drives dying cells to necrosis rather than apoptosis. Conversely, inhibiting or genetically disrupting PARP-1 enhances cell vulnerability. The N-terminal GzmA cleavage fragment of PARP-1 acts as a PARP-1 dominant negative, binding to DNA and blocking DNA repair. Disrupting PARP-1, which is also a caspase target, is therefore required for efficient apoptosis by both caspase-independent and caspase-dependent pathways.

Death by a thousand cuts: granzyme pathways of programmed cell death

The granzymes are cell death-inducing enzymes, stored in the cytotoxic granules of cytotoxic T lymphocytes and natural killer cells, that are released during granule exocytosis when a specific virus-infected or transformed target cell is marked for elimination. Recent work suggests that this homologous family of serine esterases can activate at least three distinct pathways of cell death. This redundancy likely evolved to provide protection against pathogens and tumors with diverse strategies for evading cell death. This review discusses what is known about granzyme-mediated pathways of cell death as well as recent studies that implicate granzymes in immune regulation and extracellular proteolytic functions in inflammation.

Granzyme A cleaves a mitochondrial complex I protein to initiate caspase-independent cell death

The killer lymphocyte protease granzyme A (GzmA) triggers caspase-independent target cell death with morphological features of apoptosis. We previously showed that GzmA acts directly on mitochondria to generate reactive oxygen species (ROS) and disrupt the transmembrane potential (DeltaPsi(m)) but does not permeabilize the mitochondrial outer membrane. Mitochondrial damage is critical to GzmA-induced cell death since cells treated with superoxide scavengers are resistant to GzmA. Here we find that GzmA accesses the mitochondrial matrix to cleave the complex I protein NDUFS3, an iron-sulfur subunit of the NADH:ubiquinone oxidoreductase complex I, after Lys56 to interfere with NADH oxidation and generate superoxide anions. Target cells expressing a cleavage site mutant of NDUFS3 are resistant to GzmA-mediated cell death but remain sensitive to GzmB.

Characterization of Novel Forms of Coagulation Factor XIa: independence of factor XIa subunits in factor IX activation

Factor XI is the zymogen of a dimeric plasma protease, factor XIa, with two active sites. In solution, and during contact activation in plasma, conversion of factor XI to factor XIa proceeds through an intermediate with one active site (1/2-FXIa). Factor XIa and 1/2-FXIa activate the substrate factor IX, with similar kinetic parameters in purified and plasma systems. During hemostasis, factor IX is activated by factors XIa or VIIa, by cleavage of the peptide bonds after Arg145 and Arg180. Factor VIIa cleaves these bonds sequentially, with accumulation of factor IX alpha, an intermediate cleaved after Arg145. Factor XIa also cleaves factor IX preferentially after Arg145, but little intermediate is detected. It has been postulated that the two factor XIa active sites cleave both factor IX peptide bonds prior to releasing factor IX abeta. To test this, we examined cleavage of factor IX by four single active site factor XIa proteases. Little intermediate formation was detected with 1/2-FXIa, factor XIa with one inhibited active site, or a recombinant factor XIa monomer. However, factor IX alpha accumulated during activation by the factor XIa catalytic domain, demonstrating the importance of the factor XIa heavy chain. Fluorescence titration of active site-labeled factor XIa revealed a binding stoichiometry of 1.9 +/- 0.4 mol of factor IX/mol of factor XIa (Kd = 70 +/- 40 nm). The results indicate that two forms of activated factor XI are generated during coagulation, and that each half of a factor XIa dimer behaves as an independent enzyme with respect to factor IX.

Natural killer cell-derived human granzyme H induces an alternative, caspase-independent cell-death program

Granzyme H (GzmH) belongs to a family of 5 human serine proteases that are expressed by cytotoxic immune effector cells. Although GzmH is most closely related to the caspase-activating granzyme B (GzmB), neither a natural substrate nor a role in immune defense reactions has been demonstrated for this orphan granzyme. In rodents, multiple related genes exist, but none of these can be regarded as functional homologs. Here we show that host cells are efficiently killed by GzmH after perforin and streptolysin O-mediated delivery into the cytosol. Dying cells show typical hallmarks of programmed cell death, such as mitochondrial depolarization, reactive oxygen species (ROS) generation, DNA degradation, and chromatin condensation. Contrary to GzmB, cell death by GzmH does not involve the activation of executioner caspases, the cleavage of Bid or inhibitor of caspase-activated DNase (ICAD), or the release of cytochrome c. The high expression levels of GzmH in naive natural killer (NK) cells and its potent killing ability strongly support the role of the protease in triggering an alternative cell-death pathway in innate immunity.

Expression profile of novel members of the rat mast cell protease (rMCP)-2 and (rMCP)-8 families, and functional analyses of mouse mast cell protease (mMCP)-8

Four hematopoietic serine proteases are common to the mast cell chymase locus of all analyzed mammals: alpha-chymase, cathepsin G, granzyme B, and granzyme C/H. Apart from these common genes, the mouse and rat loci hold additional granzyme-, beta-chymase-, and Mcpt8-like genes. To better understand the functional consequences of these additional enzymes and to be able to compare human and rodent immune functions, we have analyzed the expression of novel beta-chymase- and Mcpt8-like genes in the rat. Four novel genes, i.e., Mcpt2-rs2a, Mcpt2-rs2c, Mcpt8-rs1, and Mcpt8-rs4 were transcribed in tissues holding mucosal mast cells (MMC), where also the classical MMC protease Mcpt2 was expressed. We also found transcripts of rat vascular chymase (rVch) in some of these tissues. RVch is a beta-chymase that converts angiotensin I, like the human chymase. Rat MMC may therefore have similar angiotensin-converting properties as chymase-positive human mast cells, although these are mostly regarded the counterpart of rat connective tissue mast cells. The human mast cells that are considered the counterpart of rat MMC express, however, only tryptase, whereas rat MMC express various proteases, but no tryptase. We further studied the proteolytic activity of mMCP-8 as a first representative for the Mcpt8-subfamily. Based on sequence comparison and molecular modeling, mMCP-8 may prefer aspartic acid in substrate P1 position. However, we could not detect hydrolysis of chromogenic substrates or phage-displayed random nonapeptides despite numerous trials. On the other hand, we have obtained evidence that the function of the Mcpt8-like proteases depends on proteolytic activity. Namely, the expression of the only Mcpt8-family member with a mutation in the catalytic triad, Mcpt8-rs3, was strongly reduced. Thus, the substrate specificity of mMCP-8 may be too narrow to be detected with the employed methods, or the enzyme may require a substrate conformation that is not provided by the analyzed peptides.

A Pulmonary Perspective on GASPIDs: Granule-Associated Serine Peptidases of Immune Defense

Airways are protected from pathogens by forces allied with innate and adaptive immunity. Recent investigations establish critical defensive roles for leukocyte and mast cell serine-class peptidases garrisoned in membrane-bound organelles-here termed Granule-Associated Serine Peptidases of Immune Defense, or GASPIDs. Some better characterized GASPIDs include neutrophil elastase and cathepsin G (which defend against bacteria), proteinase-3 (targeted by antineutrophil antibodies in Wegener's vasculitis), mast cell beta-tryptase and chymase (which promote allergic inflammation), granzymes A and B (which launch apoptosis pathways in infected host cells), and factor D (which activates complement's alternative pathway). GASPIDs can defend against pathogens but can harm host cells in the process, and therefore become targets for pharmaceutical inhibition. They vary widely in specificity, yet are phylogenetically similar. Mammalian speciation supported a remarkable flowering of these enzymes as they co-evolved with specialized immune cells, including mast cells, basophils, eosinophils, cytolytic T-cells, natural killer cells, neutrophils, macrophages and dendritic cells. Many GASPIDs continue to evolve rapidly, providing some of the most conspicuous examples of divergent protein evolution. Consequently, students of GASPIDs are rewarded not only with insights into their roles in lung immune defense but also with clues to the origins of cellular specialization in vertebrate immunity.

Molecular characterization and expression of a granzyme of an ectothermic vertebrate with chymase-like activity expressed in the cytotoxic cells of Nile tilapia (Oreochromis niloticus)

We have identified the gene coding for a novel serine protease with close similarities to mammalian granzymes from nonspecific cytotoxic cells of a teleost fish Oreochromis niloticus. The genomic organization of tilapia granzyme-1 (TLGR-1) has the signature five-exon-four-intron structure shared by all granzymes and similar hematopoietic Ser proteases. Molecular modeling studies suggested a granzyme-like structure for this protein with four disulfide linkages and two additional Cys residues. The expression of this gene is found to be restricted to cytotoxic cell populations with a low level of constitutive expression when compared to similar granzymes in other teleost species. High levels of transcriptional activation of TLGR-1 with different stimuli suggested that this gene is highly induced during immune reactions. Triplet residues around the active site Ser of TLGR, which determines the primary substrate specificity of granzymes, differ significantly from that of other granzymes. Recombinant TLGR-1 was expressed in the mature and proenzyme forms using pPICZ-alpha vector in the Pichia pastoris expression system. Recombinant TLGR-1 was used to determine the primary substrate specificity of this protease using various synthetic thiobenzyl ester substrates. In vitro enzyme kinetics assays suggested a preference for residues with bulky side chains at the P1 site, indicating a chymase-like activity for this protease. These results indicate the presence of novel granzymes in cytotoxic cells from ectothermic vertebrates.

Granzyme A, which causes single-stranded DNA damage, targets the double-strand break repair protein Ku70

Granzyme A (GzmA) induces caspase-independent cell death with morphological features of apoptosis. Here, we show that GzmA at nanomolar concentrations cleaves Ku70, a key double-strand break repair (DSBR) protein, in target cells. Ku70 is cut after Arg(301), disrupting Ku complex binding to DNA. Cleaving Ku70 facilitates GzmA-mediated cell death, as silencing Ku70 by RNA interference increases DNA damage and cell death by GzmB cluster-deficient cytotoxic T lymphocytes or by GzmA and perforin, whereas Ku70 overexpression has the opposite effect. Ku70 has two known antiapoptotic effects-facilitating DSBR and sequestering bax to prevent its translocation to mitochondria. However, GzmA triggers single-stranded, not double-stranded, DNA damage, and GzmA-induced cell death does not involve bax. Therefore, Ku70 has other antiapoptotic functions in GzmA-induced cell death, which are blocked when GzmA proteolyses Ku70.

Differential expression of human granzymes A, B, and K in natural killer cells and during CD8+ T cell differentiation in peripheral blood

NK cells and cytotoxic T lymphocytes can induce apoptosis in virus-infected and transformed target cells via the granule exocytosis pathway. The key components of the cytolytic granules are perforin and several serine esterases, termed granzymes. While the cellular distribution of human granzymes A (GrA) and B (GrB) has been well characterized much less is known about the expression pattern of human granzyme K (GrK). In this study GrA, GrB, and GrK expression was analyzed in human peripheral blood lymphocytes using flow cytometry. There was a distinct population of GrK expressing CD8+ T cells with a CD27+/CD28+/CCR5high/CCR7-/perforin-/low/IFN-gamma+ memory-like phenotype, while all CD56bright NK cells were also positive for GrK. In addition, GrK was also expressed in subpopulations of CD56+ T cells, CD4+ T cells, and TCRgammadelta+ T cells. In contrast, GrB was primarily expressed in CD56dim NK cells and differentiated memory CD8+ T cells with the CD27-/low/CD28-/low/CCR5-/low/CCR7-/CD11b+/perforinhigh phenotype. Only few CD8+ T cells expressed both GrB and GrK. GrA was found to be co-expressed in all GrB- and GrK-expressing T cells. Our findings suggest that granzyme expression during the differentiation process of memory CD8+ T cells might be as follows: GrA+/GrB-/GrK+ --> GrA+/GrB+/GrK+ --> GrA+/GrB+/GrK-.

Serglycin-deficient cytotoxic T lymphocytes display defective secretory granule maturation and granzyme B storage

Cytotoxic T lymphocytes eliminate infected and tumor cells mainly by perforin/granzyme-induced apoptosis. Earlier studies suggested that serglycin-proteoglycans form macromolecular complexes with granzymes and perforin in the cytotoxic granule. Serglycin-proteoglycans may also be involved in the delivery of the cytolytic machinery into target cells. We have developed a serglycin-deficient mouse strain, and here we studied the importance of serglycin-proteoglycans for various aspects of cytotoxic T lymphocyte function. 35SO4(2-) radiolabeling of serglycin-deficient cells demonstrated a dramatic reduction of incorporated label as compared with wild type cells, indicating that serglycin is by far the dominating proteoglycan species produced by the cytotoxic T lymphocyte. Moreover, lack of serglycin resulted in impaired ability of cytotoxic T lymphocytes to produce secretory granule of high electron density, although granule of lower electron density were produced both in wild type and serglycin-deficient cells. The serglycin deficiency did not affect the mRNA expression for granzyme A, granzyme B, or perforin. However, the storage of granzyme B, but not granzyme A, Fas ligand, or perforin, was severely defective in serglycin-deficient cells. Serglycin-deficient cells did not display defects in late cytotoxicity toward target cell lines. Taken together, these results point to a key role for serglycin in the storage of granzyme B and for secretory granule maturation but argue against a major role for serglycin in the apoptosis mediated by cytotoxic T lymphocytes.

Membrane receptors are not required to deliver granzyme B during killer cell attack

Granzyme B (GzmB), a serine protease of cytotoxic T lymphocytes and natural killer (NK) cells, induces apoptosis by caspase activation after crossing the plasma membrane of target cells. The mechanism of this translocation during killer cell attack, however, is not understood. Killer cells release GzmB and the membrane-disturbing perforin at the contact site after target recognition. Receptor-mediated import of glycosylated GzmB and release from endosomes were suggested, but the role of the cation-independent mannose 6-phosphate receptor was recently refuted. Using recombinant nonglycosylated GzmB, we observed binding of GzmB to cellular membranes in a cell type–dependent manner. The basis and functional impact of surface binding were clarified. GzmB binding was correlated with the surface density of heparan sulfate chains, was eliminated on treatment of target cells with heparinase III or sodium chlorate, and was completely blocked by an excess of catalytically inactive GzmB or GzmK. Although heparan sulfate–bound GzmB was taken up rapidly into intracellular lysosomal compartments, neither of the treatments had an inhibitory influence on apoptosis induced by externally added streptolysin O and GzmB or by natural killer cells. We conclude that membrane receptors for GzmB on target cells are not crucial for killer cell–mediated apoptosis.

Killing of target cells by redirected granzyme B in the absence of perforin

Granzyme B (GzmB) is a potent apoptosis-inducing serine protease of cytotoxic lymphocytes. Following receptor-mediated endocytosis, GzmB is supposed to enter the cytosol through perforin-mediated membrane disruption. We investigated whether retargeting of GzmB to Lewis Y positive surface receptors could lead to perforin-independent target cell death. We coupled recombinant GzmB to the Lewis Y-binding antibody dsFv-B3. Targeting of GzmB to Lewis Y positive cells triggered cell death with similar efficacy as dsFv-B3 targeted Pseudomonas exotoxin fragment 38 (PE38). Since GzmB was only weakly inhibited by plasma proteins, GzmB-based immunoconjugates should be useful as a new class of immunotoxins with low immunogenicity utilizing programmed cell death for therapeutic purposes.

The three-dimensional structure of the N-acetylglucosamine-6-phosphate deacetylase, NagA, from Bacillus subtilis: a member of the urease superfamily

The enzyme N-acetylglucosamine-6-phosphate deacetylase, NagA, catalyzes the hydrolysis of the N-acetyl group of GlcNAc-6-P to yield glucosamine 6-phosphate and acetate, the first committed step in the biosynthetic pathway to amino-sugar-nucleotides. It is classified into carbohydrate esterase family CE-9 (see afmb.cnrs-mrs.fr/CAZY/). Here we report the cloning, expression, and three-dimensional structure (Protein Data Bank code 1un7) determination by x-ray crystallography of the Bacillus subtilis NagA at a resolution of 2.0 A. The structure presents two domains, a (beta/alpha)(8) barrel enclosing the active center and a small beta barrel domain. The structure is dimeric, and the substrate phosphate coordination at the active center is provided by an Arg/His pair contributed from the second molecule of the dimer. Both the overall structure and the active center bear a striking similarity to the urease superfamily with two metals involved in substrate binding and catalysis. PIXE (Proton-Induced x-ray Emission) data show that iron is the predominant metal in the purified protein. We propose a catalytic mechanism involving proton donation to the leaving group by aspartate, nucleophilic attack by an Fe-bridged hydroxide, and stabilization of the carbonyl oxygen by one of the two Fe atoms of the pair. We believe that this is the first sugar deacetylase to utilize this fold and catalytic mechanism.

Nuclear war: the granzyme A-bomb

Granzyme A, a serine protease in the cytotoxic granules of natural killer cells and cytotoxic T lymphocytes, induces caspase-independent cell death when introduced into target cells by perforin. Granzyme A induces single-stranded DNA damage as well as rapid loss of cell membrane integrity and mitochondrial transmembrane potential through unknown mechanisms. Granzyme A destroys the nuclear envelope by targeting lamins and opens up DNA for degradation by targeting histones. A special target of the granzyme A cell death pathway is an endoplasmic reticulum-associated complex, called the SET complex, which contains three granzyme A substrates, the nucleosome assembly protein SET, the DNA bending protein HMG-2, and the base excision repair endonuclease Ape1. The SET complex also contains the tumor suppressor protein pp32 and the granzyme A-activated DNase NM23-H1, which is inhibited by SET. Granzyme A cleavage of SET releases the inhibition and unleashes NM23-H1. Cleavage of Ape1 by granzyme A interferes with the ability of the target cell to repair itself. The novel cell death pathway initiated by granzyme A provides a parallel pathway for apoptosis, important in destroying targets that overexpress bcl-2 or are otherwise invulnerable to the caspases.

The oligomeric structure of human granzyme A is a determinant of its extended substrate specificity

The cell death-inducing serine protease granzyme A (GzmA) has a unique disulfide-linked quaternary structure. The structure of human GzmA bound to a tripeptide CMK inhibitor, determined at a resolution of 2.4 A, reveals that the oligomeric state contributes to substrate selection by limiting access to the active site for potential macromolecular substrates and inhibitors. Unlike other serine proteases, tetrapeptide substrate preferences do not correlate well with natural substrate cleavage sequences. This suggests that the context of the cleavage sequence within a macromolecular substrate imposes another level of selection not observed with the peptide substrates. Modeling of inhibitors bound to the GzmA active site shows that the dimer also contributes to substrate specificity in a unique manner by extending the active-site cleft. The crystal structure, along with substrate library profiling and mutagenesis, has allowed us to identify and rationally manipulate key components involved in GzmA substrate specificity.