A Novel Mechanism for Protein Delivery

Granzyme K drives a newly-intentified pathway of complement activation

Granzymes are a family of serine proteases mainly expressed by CD8+ T cells, natural killer cells, and innate-like lymphocytes1,2. Although their major role is thought to be the induction of cell death in virally infected and tumor cells, accumulating evidence suggests some granzymes can regulate inflammation by acting on extracellular substrates2. Recently, we found that the majority of tissue CD8+ T cells in rheumatoid arthritis (RA) synovium, inflammatory bowel disease and other inflamed organs express granzyme K (GZMK)3, a tryptase-like protease with poorly defined function. Here, we show that GZMK can activate the complement cascade by cleaving C2 and C4. The nascent C4b and C2a fragments form a C3 convertase that cleaves C3, allowing further assembly of a C5 convertase that cleaves C5. The resulting convertases trigger every major event in the complement cascade, generating the anaphylatoxins C3a and C5a, the opsonins C4b and C3b, and the membrane attack complex. In RA synovium, GZMK is enriched in areas with abundant complement activation, and fibroblasts are the major producers of complement C2, C3, and C4 that serve as targets for GZMK-mediated complement activation. Our findings describe a previously unidentified pathway of complement activation that is entirely driven by lymphocyte-derived GZMK and proceeds independently of the classical, lectin, or alternative pathways. Given the widespread abundance of GZMK-expressing T cells in tissues in chronic inflammatory diseases and infection, GZMK-mediated complement activation is likely to be an important contributor to tissue inflammation in multiple disease contexts.

Reassessing granzyme B: unveiling perforin-independent versatility in immune responses and therapeutic potentials

The pivotal role of Granzyme B (GzmB) in immune responses, initially tied to cytotoxic T lymphocytes (CTLs) and natural killer (NK) cells, has extended across diverse cell types and disease models. A number of studies have challenged conventional notions, revealing GzmB activity beyond apoptosis, impacting autoimmune diseases, inflammatory disorders, cancer, and neurotoxicity. Notably, the diverse functions of GzmB unfold through Perforin-dependent and Perforin-independent mechanisms, offering clinical implications and therapeutic insights. This review underscores the multifaceted roles of GzmB, spanning immunological and pathological contexts, which call for further investigations to pave the way for innovative targeted therapies.

Murine regulatory T cells utilize granzyme B to promote tumor metastasis

Regulatory T cells (Tregs) possess a wide range of mechanisms for immune suppression. Among them, Granzyme B (GzmB) and perforin expressed by Tregs were shown to inhibit tumor clearance in previous reports, which contradicted the canonical roles of these cytotoxic molecules expressed by cytotoxic T cells and NK cells in antitumor immune responses. Given the ability of the tumor to manipulate the microenvironment, Treg-derived GzmB function may represent an important approach to aid in tumor growth as well as facilitating tumor metastasis. In this study, we utilized Treg-specific GzmB knockout (Foxp3creGzmBfl/fl) mice to test whether Treg-derived GzmB can aid in tumor progression and metastasis. Using an IL-2 complex to activate GzmB expression in the non-immunogenic B16-F10 tumor model, we provide evidence to show that GzmB produced by Tregs is important for spontaneous metastasis to the lungs. In addition, we depleted CD8 + T cells to selectively measure the impact of Treg-derived GzmB in an experimental lung metastasis model by intravenous injection of B16-F10 tumor cells; our results demonstrate that Treg-derived GzmB plays an important role in increasing the metastatic burden to the lungs.

Immune evasion by cancer stem cells

Tumor immunity represents a new avenue for cancer therapy. Immune checkpoint inhibitors have successfully improved outcomes in several tumor types. In addition, currently, immune cell-based therapy is also attracting significant attention. However, the clinical efficacy of these treatments requires further improvement. The mechanisms through which cancer cells escape the immune response must be identified and clarified. Cancer stem cells (CSCs) play a central role in multiple aspects of malignant tumors. CSCs can initiate tumors in partially immunocompromised mice, whereas non-CSCs fail to form tumors, suggesting that tumor initiation is a definitive function of CSCs. However, the fact that non-CSCs also initiate tumors in more highly immunocompromised mice suggests that the immune evasion property may be a more fundamental feature of CSCs rather than a tumor-initiating property. In this review, we summarize studies that have elucidated how CSCs evade tumor immunity and create an immunosuppressive milieu with a focus on CSC-specific characteristics and functions. These profound mechanisms provide important clues for the development of novel tumor immunotherapies.

Synaptic secretion from human natural killer cells is diverse and includes supramolecular attack particles

Natural killer (NK) cells form immune synapses to ascertain the state of health of cells they encounter. If a target cell triggers NK cell cytotoxicity, lytic granules containing proteins including perforin and granzyme B, are secreted into the synaptic cleft inducing target cell death. Secretion of these proteins also occurs from activated cytotoxic T lymphocytes (CTLs) where they have recently been reported to complex with thrombospondin-1 (TSP-1) in specialized structures termed supramolecular attack particles (SMAPs). Here, using an imaging method to define the position of each NK cell after removal, secretions from individual cells were assessed. NK cell synaptic secretion, triggered by ligation of NKp30 or NKG2D, included vesicles and SMAPs which contained TSP-1, perforin, and granzyme B. Individual NK cells secreted SMAPs, CD63+ vesicles, or both. A similar number of SMAPs were secreted per cell for both NK cells and CTLs, but NK cell SMAPs were larger. These data establish an unexpected diversity in NK cell synaptic secretions.

The pleiotropic role of proteoglycans in extracellular vesicle mediated communication in the tumor microenvironment

Compartmental exchange between cells through extracellular vesicles (EVs), including exosomes and microvesicles, has emerged as a central mechanism that coordinates the complex communication between malignant and stromal cells during tumor initiation and evolution. Some of the most critical processes of EV-mediated communication, including EV biogenesis and EV uptake, can be mediated by heparan sulfate proteoglycans (HSPGs) that reside on the surface of producer and recipient cells as well as on EVs. With interestingly similar, HSPG-dependent, pathways as the ones exploited by some viruses, EVs may, in an evolutionary perspective, be viewed as endogenous counterparts of viral particles. Cancer cell-derived EVs exert their protumorigenic effects by direct interactions of biologically active surface molecules, by transfer of proteins and nucleic acids into recipient cells or by transfer of metabolites that can be utilized as an energy source by the recipient cell. Here, we discuss the pleiotropic role of the HSPG family in these different contexts of EV communication with a specific focus on tumor development. We propose EV-associated PGs as dynamic reservoirs and chaperones of signaling molecules with potential implications in ligand exchange between EVs and tumor target cells. The protumorigenic consequences of EV mediated communication through HSPG should motivate the development of therapeutic approaches targeting EV-HSPG interactions as a novel strategy in cancer treatment.

Human Granzyme B Based Targeted Cytolytic Fusion Proteins

Cancer immunotherapy aims to selectively target and kill tumor cells whilst limiting the damage to healthy tissues. Controlled delivery of plant, bacterial and human toxins or enzymes has been shown to promote the induction of apoptosis in cancerous cells. The 4th generation of targeted effectors are being designed to be as humanized as possible—a solution to the problem of immunogenicity encountered with existing generations. Granzymes are serine proteases which naturally function in humans as integral cytolytic effectors during the programmed cell death of cancerous and pathogen-infected cells. Secreted predominantly by cytotoxic T lymphocytes and natural killer cells, granzymes function mechanistically by caspase-dependent or caspase-independent pathways. These natural characteristics make granzymes one of the most promising human enzymes for use in the development of fusion protein-based targeted therapeutic strategies for various cancers. In this review, we explore research involving the use of granzymes as cytolytic effectors fused to antibody fragments as selective binding domains.

Serglycin is involved in inflammatory response in articular mouse chondrocytes

Serglycin is expressed by a variety of cell types and mediates different functions in both normal and pathological conditions by interacting with different biological molecules, such as the CD44 receptor. Many studies suggest that serglycin has a crucial role in inflammatory response, but there are limited data on the functions of this proteoglycan in chondrocytes. In this study we investigated the effect of serglycin knockdown induced by a specific serglycin small interfering RNA (SRGN siRNA) in normal mouse chondrocytes stimulated with lipopolysaccharide (LPS). LPS administration in normal chondrocytes increased the expression of serglycin mRNA and related protein and the production of the pro-inflammatory mediators TNF-alpha, IL-1beta, IL-6, iNOS and MMP-9, through NF-kB activation. In addition, a marked increased expression of CD44 after LPS stimulation was observed. Notably, the CD44 expression and the inflammatory response were significantly reduced by SRGN siRNA treatment in LPS treated chondrocytes. Similar results were obtained in normal mouse chondrocytes exposed to LPS, using a specific blocking antibody against CD44. These results indicate that serglycin produced in LPS-induced inflammation in normal mouse chondrocytes is able to modulate inflammation by interacting with CD44 receptor, suggesting a possible key role in the cartilage inflammation.

The immediate protective response to microbial challenge

The innate immune system detects infection and tissue injury through different families of pattern-recognition receptors (PRRs), such as Toll-like receptors. Most PRR-mediated responses initiate elaborate processes of signaling, transcription, translation, and secretion of effector mediators, which together require time to achieve. Therefore, PRR-mediated processes are not active in the early phases of infection. These considerations raise the question of how the host limits microbial replication and invasion during this critical period. Here, we examine the crucial defense mechanisms, such as antimicrobial peptides or extracellular traps, typically activated within minutes of the initial infection phase, which we term the "immediate protective response". Deficiencies in different components of the immediate protective response are often associated with severe and recurrent infectious diseases in humans, highlighting their physiologic importance.

Perforin oligomers form arcs in cellular membranes: a locus for intracellular delivery of granzymes

Perforin-mediated cytotoxicity is an essential host defense, in which defects contribute to tumor development and pathogenic disorders including autoimmunity and autoinflammation. How perforin (PFN) facilitates intracellular delivery of pro-apoptotic and inflammatory granzymes across the bilayer of targets remains unresolved. Here we show that cellular susceptibility to granzyme B (GzmB) correlates with rapid PFN-induced phosphatidylserine externalization, suggesting that pores are formed at a protein-lipid interface by incomplete membrane oligomers (or arcs). Supporting a role for these oligomers in protease delivery, an anti-PFN antibody (pf-80) suppresses necrosis but increases phosphatidylserine flip-flop and GzmB-induced apoptosis. As shown by atomic force microscopy on planar bilayers and deep-etch electron microscopy on mammalian cells, pf-80 increases the proportion of arcs which correlates with the presence of smaller electrical conductances, while large cylindrical pores decline. PFN appears to form arc structures on target membranes that serve as minimally disrupting conduits for GzmB translocation. The role of these arcs in PFN-mediated pathology warrants evaluation where they may serve as novel therapeutic targets.

Nuclear translocation of heparan sulfate proteoglycans and their functional significance

BACKGROUND

Heparan sulfate proteoglycans (HSPGs) are important constituents of the cell membrane and they act as co-receptors for cellular signaling. Syndecan-1, glypican and perlecan also translocate to the nucleus in a regulated manner. Similar nuclear transport of growth factors and heparanase indicate a possible co-regulation and functional significance.

SCOPE OF REVIEW

In this review we dissect the structural requirement for the nuclear translocation of HSPGs and their functional implications.s

MAJOR CONCLUSIONS

The functions of the nuclear HSPGs are still incompletely understood. Evidence point to possible functions in hampering cell proliferation, inhibition of DNA topoisomerase I activity and inhibition of gene transcription.

GENERAL SIGNIFICANCE

HSPGs influence the behavior of malignant tumors in many different ways. Modulating their functions may offer powerful tools to control fundamental biological processes and provide the basis for subsequent targeted therapies in cancer. This article is part of a Special Issue entitled Matrix-mediated cell behaviour and properties.

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.

Serglycin secretion is part of the inflammatory response in activated primary human endothelial cells in vitro

BACKGROUND

Endothelial cells have important functions in e.g. regulating blood pressure, coagulation and host defense reactions. Serglycin is highly expressed by endothelial cells, but there is limited data on the roles of this proteoglycan in immune reactions.

METHODS

Cultured primary human endothelial cells were exposed to proinflammatory agents lipopolysaccharide (LPS) and interleukin 1β (IL-1β). The response in serglycin synthesis, secretion and intracellular localization and effect on the proteoglycan binding chemokines CXCL-1 and CXCL-8 were determined by qRT-PCR, Western blotting, immunocytochemistry, ELISA and serglycin knockdown experiments.

RESULTS

Both LPS and IL-1β increased the synthesis and secretion of serglycin, while only IL-1β increased serglycin mRNA expression. Stimulation increased the number of serglycin containing vesicles, with a greater portion of large vesicles after LPS treatment. Also, increased intracellular and secreted levels of CXCL-1 and CXCL-8 were observed. The increase in CXCL-8 secretion was unchanged in serglycin knockdown cells. However, the increase in CXCL-1 secretion from IL-1β stimulation was reduced 27% in serglycin knockdown cells; while the LPS-induced secretion was not affected. In serglycin expressing cells CXCL-1 positive vesicles were evenly distributed throughout the cytoplasm, while confided to the Golgi region in serglycin knockdown cells. This was the case only for IL-1β stimulated cells. LPS-induced CXCL-1 distribution was unaffected by serglycin expression.

CONCLUSIONS

These results suggest that different signaling pathways are involved in regulating secretion of serglycin and partner molecules in activated endothelial cells.

GENERAL SIGNIFICANCE

This knowledge increases our understanding of the roles of serglycin in immune reactions. This article is part of a Special Issue entitled: Matrix-mediated cell behaviour and properties.

The role of syndecan-1 in cellular signaling and its effects on heparan sulfate biosynthesis in mesenchymal tumors

Proteoglycans (PGs) and in particular the syndecans are involved in the differentiation process across the epithelial-mesenchymal axis, principally through their ability to bind growth factors and modulate their downstream signaling. Malignant tumors have individual proteoglycan profiles, which are closely associated with their differentiation and biological behavior, mesenchymal tumors showing a different profile from that of epithelial tumors. Syndecan-1 is the main syndecan of epithelial malignancies, whereas in sarcomas its expression level is generally low, in accordance with their mesenchymal phenotype and highly malignant behavior. This proteoglycan is often overexpressed in adenocarcinoma cells, whereas mesothelioma and fibrosarcoma cells express syndecan-2 and syndecan-4 more abundantly. Increased expression of syndecan-1 in mesenchymal tumors changes the tumor cell morphology to an epithelioid direction whereas downregulation results in a change in shape from polygonal to spindle-like morphology. Although syndecan-1 plays major roles on the cell-surface, there are also intracellular functions, which are not very well studied. On the functional level, syndecan-1 affects mesenchymal tumor cell proliferation, adhesion, migration and motility, and the effect varies with the different domains of the core protein. Syndecan-1 may exert stimulatory or inhibitory effects, depending on the concentration of various mitogens, enzymes, and signaling molecules, the ratio between the shed and membrane-associated syndecan-1 and histological grade of the tumour. Growth factor signaling seems to be delicately controlled by regulatory loops involving the syndecan expression levels and their sulfation patterns. Overexpression of syndecan-1 modulates the biosynthesis and sulfation of heparan sulfate and it also affects the expression of other PGs. On transcriptomic level, syndecan-1 modulation results in profound effects on genes involved in regulation of cell growth.

Heparan sulfate proteoglycan as a cell-surface endocytosis receptor

How various macromolecules are exchanged between cells and how they gain entry into recipient cells are fundamental questions in cell biology with important implications e.g. non-viral drug delivery, infectious disease, metabolic disorders, and cancer. The role of heparan sulfate proteoglycan (HSPG) as a cell-surface receptor of diverse macromolecular cargo has recently been manifested. Exosomes, cell penetrating peptides, polycation-nucleic acid complexes, viruses, lipoproteins, growth factors and morphogens among other ligands enter cells through HSPG-mediated endocytosis. Key questions that partially have been unraveled over recent years include the respective roles of HSPG core protein and HS chain structure specificity for macromolecular cargo endocytosis, the down-stream intracellular signaling events involved in HSPG-dependent membrane invagination and vesicle formation, and the biological significance of the HSPG transport pathway. Here, we discuss the intriguing role of HSPGs as a major entry pathway of macromolecules in mammalian cells with emphasis on recent in vitro and in vivo data that provide compelling evidence of HSPG as an autonomous endocytosis receptor.

Surface charge-modification prevents sequestration and enhances tumor-cell specificity of a recombinant granzyme B-TGFα fusion protein

The serine protease granzyme B (GrB) plays an important role in the immune defense mediated by cytotoxic lymphocytes. Recombinant derivatives of this pro-apoptotic protein fused to tumor-targeting ligands hold promise for cancer therapy, but their applicability may be limited by promiscuous binding to nontarget tissues via electrostatic interactions. Here, we investigated cell binding and specific cytotoxicity of chimeric molecules consisting of wild-type or surface-charge-modified human GrB and the natural EGFR ligand TGFα for tumor targeting. We mutated two cationic heparin-binding motifs responsible for electrostatic interactions of GrB with cell surface structures, and genetically fused the resulting GrBcs derivative to TGFα for expression in the yeast Pichia pastoris. Purified GrBcs-TGFα (GrBcs-T) and a corresponding fusion protein employing wild-type GrB (GrB-T) displayed similar enzymatic activity and targeted cytotoxicity against EGFR-overexpressing breast carcinoma cells in the presence of an endosomolytic reagent. However, unspecific binding of the modified GrBcs-T variant to EGFR-negative cells was dramatically reduced, preventing the sequestration by nontarget cells in mixed cell cultures and increasing tumor-cell specificity. Likewise, modification of the GrB domain alleviated unspecific extracellular effects such as cell detachment indicative of extracellular matrix degradation. Our data demonstrate improved selectivity and functionality of surface-charge-modified GrBcs, suggesting this strategy as a general approach for the development of optimized GrB fusion proteins for therapeutic applications.

Granzyme B cleaves decorin, biglycan and soluble betaglycan, releasing active transforming growth factor-β1

Objective

Granzyme B (GrB) is a pro-apoptotic serine protease that contributes to immune-mediated target cell apoptosis. However, during inflammation, GrB accumulates in the extracellular space, retains its activity, and is capable of cleaving extracellular matrix (ECM) proteins. Recent studies have implicated a pathogenic extracellular role for GrB in cardiovascular disease, yet the pathophysiological consequences of extracellular GrB activity remain largely unknown. The objective of this study was to identify proteoglycan (PG) substrates of GrB and examine the ability of GrB to release PG-sequestered TGF-β1 into the extracellular milieu.

Methods/Results

Three extracellular GrB PG substrates were identified; decorin, biglycan and betaglycan. As all of these PGs sequester active TGF-β1, cytokine release assays were conducted to establish if GrB-mediated PG cleavage induced TGF-β1 release. Our data confirmed that GrB liberated TGF-β1 from all three substrates as well as from endogenous ECM and this process was inhibited by the GrB inhibitor 3,4-dichloroisocoumarin. The released TGF-β1 retained its activity as indicated by the induction of SMAD-3 phosphorylation in human coronary artery smooth muscle cells.

Conclusion

In addition to contributing to ECM degradation and the loss of tissue structural integrity in vivo, increased extracellular GrB activity is also capable of inducing the release of active TGF-β1 from PGs.

Intercellular communication via the endo-lysosomal system: translocation of granzymes through membrane barriers

Cytotoxic lymphocytes (CLs) are responsible for the clearance of virally infected or neoplastic cells. CLs possess specialised lysosome-related organelles called granules which contain the granzyme family of serine proteases and perforin. Granzymes may induce apoptosis in the target cell when delivered by the pore forming protein, perforin. Here we follow the perforin-granzyme pathway from synthesis and storage in the granule, to exocytosis and finally delivery into the target cell. This review focuses on the controversial subject of perforin-mediated translocation of granzymes into the target cell cytoplasm. It remains unclear whether this occurs at the cell surface with granzymes moving through a perforin pore in the plasma membrane, or if it involves internalisation of perforin and granzymes and subsequent release from an endocytic compartment. The latter mechanism would represent an example of cross talk between the endo-lysosomal pathways of individual cells. This article is part of a Special Issue entitled: Proteolysis 50 years after the discovery of lysosome.

Serglycin is a theranostic target in nasopharyngeal carcinoma that promotes metastasis

Nasopharyngeal carcinoma (NPC) is known for its high-metastatic potential. Here we report the identification of the proteoglycan serglycin as a functionally significant regulator of metastasis in this setting. Comparative genomic expression profiling of NPC cell line clones with high- and low-metastatic potential revealed the serglycin gene (SRGN) as one of the most upregulated genes in highly metastatic cells. RNAi-mediated inhibition of serglycin expression blocked serglycin secretion and the invasive motility of highly metastatic cells, reducing metastatic capacity in vivo. Conversely, serglycin overexpression in poorly metastatic cells increased their motile behavior and metastatic capacity in vivo. Growth rate was not influenced by serglycin in either highly or poorly metastatic cells. Secreted but not bacterial recombinant serglycin promoted motile behavior, suggesting a critical role for glycosylation in serglycin activity. Serglycin inhibition was associated with reduced expression of vimentin but not other epithelial-mesenchymal transition proteins. In clinical specimens, serglycin expression was elevated significantly in liver metastases from NPC relative to primary NPC tumors. We evaluated the prognostic value of serglycin by immunohistochemical staining of tissue microarrays from 263 NPC patients followed by multivariate analyses. High serglycin expression in primary NPC was found to be an unfavorable independent indicator of distant metastasis-free and disease-free survival. Our findings establish that glycosylated serglycin regulates NPC metastasis via autocrine and paracrine routes, and that it serves as an independent prognostic indicator of metastasis-free survival and disease-free survival in NPC patients.

Serglycin inhibits the classical and lectin pathways of complement via its glycosaminoglycan chains: implications for multiple myeloma

Serglycin (SG) is a proteoglycan expressed by hematopoietic cells and is constitutively secreted by multiple myeloma (MM) cells. SG participates in the regulation of various inflammatory events. We found that SG secreted by human MM cell lines inhibits both the classical and lectin pathways of complement, without influencing alternative pathway activity. The inhibitory effect of SG is due to direct interactions with C1q and mannose-binding lectin (MBL). C1q-binding is mediated through the glycosaminoglycan moieties of SG, whereas MBL binds additionally to SG protein core. Interactions between SG and C1q as well as MBL are diminished in the presence of chondroitin sulfate type E. In addition, we localized the SG-binding site to the collagen-like stalk of C1q. Interactions between SG and C1q as well as MBL are ionic in character and only the interaction with MBL was found to be partially dependent on the presence of calcium. We found the serum levels of SG to be elevated in patients with MM compared to healthy controls. Moreover, we found that SG expressed from myeloma plasma cells protects these cells from complement activation induced by treatment with anti-thymocyte immunoglobulins. This might protect myeloma cells during immunotherapy and promote survival of malignant cells.

Serglycin-independent release of active mast cell proteases in response to Toxoplasma gondii infection

Earlier studies identified serglycin proteoglycan and its heparin chains to be important for storage and activity of mast cell proteases. However, the importance of serglycin for secretion and activity of mast cell proteases in response to parasite infection has been poorly investigated. To address this issue, we studied the effects on mast cell proteases in serglycin-deficient and wild type mice after peritoneal infection with the obligate intracellular parasite Toxoplasma gondii. In line with previous results, we found severely reduced levels of cell-bound mast cell proteases in both noninfected and infected serglycin-deficient mice. However, serglycin-deficient mice secreted mast cell proteases at wild type levels at the site of infection, and enzymatic activities associated with mast cell proteases were equally up-regulated in wild type and serglycin-deficient mice 48 h after infection. In both wild type and serglycin-deficient mice, parasite infection resulted in highly increased extracellular levels of glycosaminoglycans, including hyaluronan and chondroitin sulfate A, suggesting a role of these substances in the general defense mechanism. In contrast, heparan sulfate/heparin was almost undetectable in serglycin-deficient mice, and in wild type mice, it was mainly confined to the cellular fraction and was not increased upon infection. Furthermore, the heparan sulfate/heparin population was less sulfated in serglycin-deficient than in wild type mice indicative for the absence of heparin, which supports that heparin production is dependent on the serglycin core protein. Together, our results suggest that serglycin proteoglycan is dispensable for normal secretion and activity of mast cell proteases in response to peritoneal infection with T. gondii.

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.

Granzyme A activates another way to die

Granzyme A (GzmA) is the most abundant serine protease in killer cell cytotoxic granules. GzmA activates a novel programed cell death pathway that begins in the mitochondrion, where cleavage of NDUFS3 in electron transport complex I disrupts mitochondrial metabolism and generates reactive oxygen species (ROS). ROS drives the endoplasmic reticulum-associated SET complex into the nucleus, where it activates single-stranded DNA damage. GzmA also targets other important nuclear proteins for degradation, including histones, the lamins that maintain the nuclear envelope, and several key DNA damage repair proteins (Ku70, PARP-1). Cells that are resistant to the caspases or GzmB by overexpressing bcl-2 family anti-apoptotic proteins or caspase or GzmB protease inhibitors are sensitive to GzmA. By activating multiple cell death pathways, killer cells provide better protection against a variety of intracellular pathogens and tumors. GzmA also has proinflammatory activity; it activates pro-interleukin-1beta and may also have other proinflammatory effects that remain to be elucidated.

Serglycin proteoglycan deletion in mouse platelets: physiological effects and their implications for platelet contributions to thrombosis, inflammation, atherosclerosis, and metastasis

Serglycin is found in all nucleated hematopoietic cells and platelets, blood vessels, various reproductive and developmental tissues, and in chondrocytes. The serglycin knockout mouse has demonstrated that this proteoglycan is required for proper generation and function of secretory granules in several hematopoietic cells. The effects on platelets are profound, and include diminishing platelet aggregation responses and formation of platelet thrombi. This chapter will review cell-specific aspects of serglycin structure, its gene regulation, cell and tissue localization, and the effects of serglycin deletion on hematopoietic cell granule structure and function. The effects of serglycin knockout on platelets are described and discussed in detail. Rationales for further investigations into the contribution of serglycin to the known roles of platelets in thrombosis, inflammation, atherosclerosis, and tumor metastasis are presented.

Intracellular versus extracellular granzyme B in immunity and disease: challenging the dogma

The cytotoxic granzyme B (GrB)/perforin pathway has been traditionally viewed as a primary mechanism that is used by cytotoxic lymphocytes to eliminate allogeneic, virally infected and/or transformed cells. Although originally proposed to have intracellular and extracellular functions, upon the discovery that perforin, in combination with GrB, could induce apoptosis, other potential functions for this protease were, for the most part, disregarded. As there are 5 granzymes in humans and 11 granzymes in mice, many studies used perforin knockout mice as an initial screen to evaluate the role of granzymes in disease. However, in recent years, emerging clinical and biochemical evidence has shown that the latter approach may have overlooked a critical perforin-independent, pathogenic role for these proteases in disease. This review focuses on GrB, the most characterized of the granzyme family, in disease. Long known to be a pro-apoptotic protease expressed by cytotoxic lymphocytes and natural killer cells, it is now accepted that GrB can be expressed in other cell types of immune and nonimmune origin. To the latter, an emerging immune-independent role for GrB has been forwarded due to recent discoveries that GrB may be expressed in nonimmune cells such as smooth muscle cells, keratinocytes, and chondrocytes in certain disease states. Given that GrB retains its activity in the blood, can cleave extracellular matrix, and its levels are often elevated in chronic inflammatory diseases, this protease may be an important contributor to certain pathologies. The implications of sustained elevations of intracellular and extracellular GrB in chronic vascular, dermatological, and neurological diseases, among others, are developing. This review examines, for the first time, the multiple roles of GrB in disease pathogenesis.

Human and mouse granzyme A induce a proinflammatory cytokine response

Granzyme A (GzmA) is considered a major proapoptotic protease. We have discovered that GzmA-induced cell death involves rapid membrane damage that depends on the synergy between micromolar concentrations of GzmA and sublytic perforin (PFN). Ironically, GzmA and GzmB, independent of their catalytic activity, both mediated this swift necrosis. Even without PFN, lower concentrations of human GzmA stimulated monocytic cells to secrete proinflammatory cytokines (interleukin-1beta [IL-1beta], TNFalpha, and IL-6) that were blocked by a caspase-1 inhibitor. Moreover, murine GzmA and GzmA(+) cytotoxic T lymphocytes (CTLs) induce IL-1beta from primary mouse macrophages, and GzmA(-/-) mice resist lipopolysaccharide-induced toxicity. Thus, the granule secretory pathway plays an unexpected role in inflammation, with GzmA acting as an endogenous modulator.

Granzyme B delivery via perforin is restricted by size, but not by heparan sulfate-dependent endocytosis

How granzymes gain entry into the cytosol of target cells during killer cell attack has been the subject of several studies in the past, but the effective delivery mechanism during target cell encounter has not been clarified. Here we show that granzyme B (GzmB) mutants lacking binding to negatively charged, essentially heparan-sulfate-containing membrane receptors are poorly endocytosed yet are delivered to the cytosol with efficacy similar to that of WT GzmB. In a cell-based system GzmB-deficient natural killer cells provided perforin (pfn) by natural polarized secretion and synergized with externally added GzmB. Whereas receptor (heparan sulfate)-dependent endocytosis was dispensable, delivery of larger cargo like that of GzmB fusion proteins and GzmB-antibody complexes was restricted by their size. Our data support the model in which granzymes are primarily translocated through repairable membrane pores of finite size and not by the disruption of endocytosed vesicles. We conclude that structurally related translocators, i.e., perforin and cholesterol-dependent cytolysins, deliver deathly cargo across host cell membranes in a similar manner.

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 B and natural killer (NK) cell death

Granzyme B is a unique serine protease, which plays a crucial role for target cell death. Several mechanisms of delivery of granzyme B to target cells have been recently identified. Granzyme B directly activates Bid, a specific substrate for granzyme B, resulting in caspase activation. Granzyme B efficiently cleaves many prominent autoantigens, and the hypothesis that autoantibodies arise when cryptic determinants are revealed to the immune system has been proposed. Some autoantibodies directed against granzyme B-specific neoepitopes are present in serum from patients with autoimmune diseases. In the tissues from autoimmune diseases, granzyme B might play an important role for disease progression (i.e., rheumatoid arthritis synovium) or inhibition (i.e., regulatory T cells). We have identified a novel type of activation-induced cell death (granzyme B leakage-induced cell death). Activation-induced natural killer (NK) cell death is accompanied by the leakage of granzyme B from intracellular granules into the cytoplasm, and it triggers apoptosis by directing Bid to mitochondrial membranes. An excess of "leaked" granzyme B over its inhibitor, serpin proteinase inhibitor 9, is a major determinant of cell death. The role of granzyme B in autoimmunity and its influence on NK cell death are discussed.

Secretory lysosomes and their cargo in T and NK cells

Secretory lysosomes are specialized organelles that combine catabolic functions of conventional lysosomes with an inducible secretory potential. They are present in various hematopoietic cell types commonly characterized by the need for rapid mobilization and secretion of effector proteins. As an example, the cytotoxic effector function of T cells and natural killer cells strictly depends on the activation-dependent mobilization of such vesicles to the cytotoxic immunological synapse. This review focuses on some molecules that have been identified as cargo of secretory lysosomes and which play a major role in effector function of CTL and NK cells. We also briefly point to the fact that the dysregulation of formation and transport of secretory vesicles is causative for severe immunodeficiencies and autoimmunity observed in patients and also in mice that have been used as representative model systems to analyze the pathophysiological relevance of secretory vesicles in vivo.

Serglycin constitutively secreted by myeloma plasma cells is a potent inhibitor of bone mineralization in vitro

Although the biological significance of proteoglycans (PGs) has previously been highlighted in multiple myeloma (MM), little is known about serglycin, which is a hematopoietic cell granule PG. In this study, we describe the expression and highly constitutive secretion of serglycin in several MM cell lines. Serglycin messenger RNA was detected in six MM cell lines. PGs were purified from conditioned medium of four MM cell lines, and serglycin substituted with 4-sulfated chondroitin sulfate was identified as the predominant PG. Flow cytometry and confocal microscopy showed that serglycin was also present intracellularly and on the cell surface, and attachment to the cell surface was at least in part dependent on intact glycosaminoglycan side chains. Immunohistochemical staining of bone marrow biopsies showed the presence of serglycin both in benign and malignant plasma cells. Immunoblotting in bone marrow aspirates from a limited number of patients with newly diagnosed MM revealed highly increased levels of serglycin in 30% of the cases. Serglycin isolated from myeloma plasma cells was found to influence the bone mineralization process through inhibition of the crystal growth rate of hydroxyapatite. This rate reduction was attributed to adsorption and further blocking of the active growth sites on the crystal surface. The apparent order of the crystallization reaction was found to be n=2, suggesting a surface diffusion-controlled spiral growth mechanism. Our findings suggest that serglycin release is a constitutive process, which may be of fundamental biological importance in the study of MM.

Apoptotic death concurrent with CD3 stimulation in primary human CD8+ T lymphocytes: a role for endogenous granzyme B

We exposed primary CD8(+) T cells to soluble CD3 mAb plus IL-2 and limited numbers of monocytes (3%). These cells were activated but concurrently subjected to ongoing apoptosis ( approximately 25% were apoptotic from day 2 of culture). However, their costimulated CD4(+) counterparts were much less prone to apoptosis. The apoptotic signaling pathway bypassed Fas and TNFRs, and required the activity of cathepsin C, a protease which performs the proteolytic maturation of granzyme (Gr) A and GrB proenzymes within the cytolytic granules. Silencing the GrB gene by RNA interference in activated CD8(+) T cells prevented the activation of procaspase-3 and Bid, and indicated that GrB was the upstream death mediator. A GrB-specific mAb immunoprecipitated a approximately 70-kDa molecular complex from cytolytic extracts of activated CD8(+) (but not resting) T cells, that was specifically recognized by a nucleocytoplasmic protease inhibitor 9 (PI-9) specific mAb. This complex was also detected after reciprocal immunoprecipitation of PI-9. It coexisted in the cytosol with the 32-kDa form of GrB. As neither were detected in the cytosol of CD4(+) bystander T cells (which poorly synthesized GrB), and as silencing the perforin (Pf) gene had no effect in our system, endogenous GrB was likely implicated. Immunoprecipitation experiments failed to reveal Pf in the cytosol of CD8(+) T cells, and only a tiny efflux of granular GrA was detected by ELISA. We propose that some GrB is released from cytolytic granules to the cytosol of CD8(+) T lymphocytes upon CD3/TCR stimulation and escapes PI-9, thereby mediating apoptotic cell death.

Extracellular granzymes: current perspectives

Granzyme A (GrA) and granzyme B (GrB) play key roles in the induction of target cell death induced by cytotoxic lymphocytes. Whilst these roles have been extensively studied, it is becoming apparent that both granzymes also possess extracellular activities. Soluble granzymes are found extracellularly in normal plasma and are elevated in a number of diseases, ranging from viral and bacterial infections to autoimmune diseases. Here, we discuss the current knowledge of extracellular granzyme substrates, inhibitors and functions; and the pathological consequences of extracellular granzymes in disease. In addition, we provide new evidence for the role of glycosaminoglycan-binding sites of granzymes in extracellular matrix remodeling.

Survey of the year 2005 commercial optical biosensor literature

We identified 1113 articles (103 reviews, 1010 primary research articles) published in 2005 that describe experiments performed using commercially available optical biosensors. While this number of publications is impressive, we find that the quality of the biosensor work in these articles is often pretty poor. It is a little disappointing that there appears to be only a small set of researchers who know how to properly perform, analyze, and present biosensor data. To help focus the field, we spotlight work published by 10 research groups that exemplify the quality of data one should expect to see from a biosensor experiment. Also, in an effort to raise awareness of the common problems in the biosensor field, we provide side-by-side examples of good and bad data sets from the 2005 literature.

Granule-mediated killing by granzyme B and perforin requires a mannose 6-phosphate receptor and is augmented by cell surface heparan sulfate

During granule-mediated killing by cytotoxic T lymphocytes or natural killer cells, the serine protease granzyme B enters the target cell by endocytosis and induces apoptosis. Previous studies suggested a role for the mannose 6-phosphate receptor, but further experiments with purified granzyme B indicated this was not essential. Additionally, it is now clear that grB is exocytosed from killer cells in a high-molecular-weight complex with the proteoglycan serglycin. Here granzyme B was delivered as a purified monomer, or in complex with either glycosaminoglycans or serglycin, and killing was evaluated. When granzyme B was a monomer, soluble mannose 6-phosphate had a limited impact, whereas apoptosis induced by the complexed grB was effectively inhibited by mannose 6-phosphate. Most importantly, when granzyme B and perforin were delivered together from granules, inhibition by mannose 6-phosphate was also observed. In pulldown assays mediated by the cation-independent mannose 6-phosphate receptor, granzyme B bound to the receptor more intensely in the presence of immobilized heparan sulfate. We therefore propose the model that under physiological conditions serglycin-bound granzyme B is critically endocytosed by a mannose 6-phosphate receptor, and receptor binding is enhanced by cell surface heparan sulfate.

Cationic sites on granzyme B contribute to cytotoxicity by promoting its uptake into target cells

Granzyme B (GrB) is a key effector of cytotoxic lymphocyte-mediated cell death. It is delivered to target cells bound to the proteoglycan serglycin, but how it crosses the plasma membrane and accesses substrates in the cytoplasm is poorly understood. Here we identify two cationic sequences on GrB that facilitate its binding and uptake. Mutation of cationic sequence 1 (cs1) prevents accumulation of GrB in a distinctive intracellular compartment and reduces cytotoxicity 20-fold. Mutation of cs2 reduces accumulation in this intracellular compartment and cytotoxicity two- to threefold. We also show that GrB-mediated cytotoxicity is abrogated by heparin and that target cells deficient in cell surface sulfate or glycosaminoglycans resist GrB. However, heparin does not completely prevent GrB internalization and chondroitin 4-sulfate does not inhibit cytotoxicity, suggesting that glycosaminoglycans are not essential GrB receptors. We propose that GrB enters cells by nonselective adsorptive pinocytosis, exchanging from chondroitin sulfate on serglycin to anionic components of the cell surface. In this electrostatic "exchange-adsorption" model, cs1 and cs2 participate in binding of GrB to the cell surface, thereby promoting its uptake and eventual release into the cytoplasm.