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

The Roles of Mast Cells in Parasitic Protozoan Infections

Protozoan parasites such as Plasmodium spp., Leishmania spp., Trypanosoma spp., and Toxoplasma gondii are major causes of parasitic diseases in both humans and animals. The immune system plays a critical role against protozoa, but their immune mechanism remains poorly understood. This highlights the need to investigate the function of immune cells involved in the process of parasite infections and the responses of host immune system to parasite infections. Mast cells (MCs) are known to be central players in allergy and anaphylaxis, and it has been demonstrated that MCs have crucial roles in host defense against a number of different pathogens, including parasites. To date, there are many studies that have examined the interaction of helminth-derived antigens and MCs. As one of the major effector cells, MCs also play an important role in the immune response against some parasitic protozoa, but their role in protozoan infections is, however, less well characterized. Herein, we review the current knowledge about the roles of MCs and their mediators during infections involving highly pathogenic protozoa including Plasmodium spp., Leishmania spp., Trypanosoma spp., and T. gondii. We offer a general review of the data from patients and experimental animal models infected with the aforementioned protozoa, which correlate MCs and MC-derived mediators with exacerbated inflammation and disease progression as well as protection against the parasitic infections in different circumstances. This review updates our current understanding of the roles of MCs during parasitic protozoan infections, and the participation of MCs in parasitic protozoan infections could be of a potential therapeutic target.

Serglycin proteoglycans limit enteropathy in Trichinella spiralis-infected mice

BACKGROUND

Serglycin proteoglycans are essential for maturation of secretory granules and for the correct granular storage of cationic proteases in hematopoietic cells, e.g. mast cells. However, little is known about the in vivo functions of serglycin proteoglycans during infection. Here we investigated the potential role of serglycin proteoglycans in host defense after infection with the nematode Trichinella spiralis.

RESULTS

Twelve days post infection lack of serglycin proteoglycans caused significantly increased enteropathy. The serglycin-deficient mice showed significantly increased intestinal worm burden, reduced recruitment of mast cells to the intestinal crypts, decreased levels of the mast cell proteases MCPT5 and MCPT6 in intestinal tissue, decreased serum levels of TNF-α, IL-1β, IL-10 and IL-13, increased levels of IL-4 and total IgE in serum, and increased intestinal levels of the neutrophil markers myeloperoxidase and elastase, as compared to wild type mice. At five weeks post infection, increased larvae burden and inflammation were seen in the muscle tissue of the serglycin-deficient mice.

CONCLUSIONS

Our results demonstrate that the serglycin-deficient mice were more susceptible to T. spiralis infection and displayed an unbalanced immune response compared to wild type mice. These findings point to an essential regulatory role of serglycin proteoglycans in immunity.

Targeting Serglycin Prevents Metastasis in Murine Mammary Carcinoma

In hematopoietic cells, serglycin proteoglycans mainly contribute to proper storage and secretion of inflammatory mediators via their negatively charged glycosaminoglycans. Serglycin proteoglycans are also expressed in cancer cells where increased expression has been linked to poor prognosis. However, the serglycin-dependent mediators promoting cancer progression remain to be determined. In the present study we report that genetic ablation of serglycin proteoglycan completely blocks lung metastasis in the MMTV-PyMT-driven mouse breast cancer model, while serglycin-deficiency did not affect primary tumour growth or number of mammary tumours. Although E-cadherin expression was higher in the serglycin-deficient primary tumour tissue, indicating reduced invasiveness, serglycin-deficient tumour cells were still detected in the circulation. These data suggest that serglycin proteoglycans play a role in extravasation as well as colonization and growth of metastatic cells. A microarray expression analysis and functional annotation of differentially expressed genes identified several biological pathways where serglycin may be important. Our results suggest that serglycin and serglycin-dependent mediators are potential drug targets to prevent metastatic disease/dissemination of cancer.

Loss of Serglycin Promotes Primary Tumor Growth and Vessel Functionality in the RIP1-Tag2 Mouse Model for Spontaneous Insulinoma Formation

The serglycin proteoglycan is mainly expressed by hematopoietic cells where the major function is to retain the content of storage granules and vesicles. In recent years, expression of serglycin has also been found in different forms of human malignancies and a high serglycin expression level has been correlated with a more migratory and invasive phenotype in the case of breast cancer and nasopharyngeal carcinoma. Serglycin has also been implicated in the development of the tumor vasculature in multiple myeloma and hepatocellular carcinoma where reduced expression of serglycin was correlated with a less extensive vasculature. To further investigate the contribution of serglycin to tumor development, we have used the immunocompetent RIP1-Tag2 mouse model of spontaneous insulinoma formation crossed into serglycin deficient mice. For the first time we show that serglycin-deficiency affects orthotopic primary tumor growth and tumor vascular functionality of late stage carcinomas. RIP1-Tag2 mice that lack serglycin develop larger tumors with a higher proliferative activity but unaltered apoptosis compared to normal RIP1-Tag2 mice. The absence of serglycin also enhances the tumor vessel functionality, which is better perfused than in tumors from serglycin wild type mice. The presence of the pro-angiogenic modulators vascular endothelial growth factor and hepatocyte growth factor were decreased in the serglycin deficient mice which suggests a less pro-angiogenic environment in the tumors of these animals. Taken together, we conclude that serglycin affects multiple aspects of spontaneous tumor formation, which strengthens the theory that serglycin acts as an important mediator in the formation and progression of tumors.

Mast cells form antibody-dependent degranulatory synapse for dedicated secretion and defence

Mast cells are tissue-resident immune cells that play a key role in inflammation and allergy. Here we show that interaction of mast cells with antibody-targeted cells induces the polarized exocytosis of their granules resulting in a sustained exposure of effector enzymes, such as tryptase and chymase, at the cell-cell contact site. This previously unidentified mast cell effector mechanism, which we name the antibody-dependent degranulatory synapse (ADDS), is triggered by both IgE- and IgG-targeted cells. ADDSs take place within an area of cortical actin cytoskeleton clearance in the absence of microtubule organizing centre and Golgi apparatus repositioning towards the stimulating cell. Remarkably, IgG-mediated degranulatory synapses also occur upon contact with opsonized Toxoplasma gondii tachyzoites resulting in tryptase-dependent parasite death. Our results broaden current views of mast cell degranulation by revealing that human mast cells form degranulatory synapses with antibody-targeted cells and pathogens for dedicated secretion and defence.

Serglycin: at the crossroad of inflammation and malignancy

Serglycin has been initially characterized as an intracellular proteoglycan expressed by hematopoietic cells. All inflammatory cells highly synthesize serglycin and store it in granules, where it interacts with numerous inflammatory mediators, such as proteases, chemokines, cytokines, and growth factors. Serglycin is implicated in their storage into the granules and their protection since they are secreted as complexes and delivered to their targets after secretion. During the last decade, numerous studies have demonstrated that serglycin is also synthesized by various non-hematopoietic cell types. It has been shown that serglycin is highly expressed by tumor cells and promotes their aggressive phenotype and confers resistance against drugs and complement system attack. Apart from its direct beneficial role to tumor cells, serglycin may promote the inflammatory process in the tumor cell microenvironment thus enhancing tumor development. In the present review, we discuss the role of serglycin in inflammation and tumor progression.

Toxoplasma gondii inhibits mast cell degranulation by suppressing phospholipase Cγ-mediated Ca(2+) mobilization

Toxoplasma gondii is well-known to subvert normal immune responses, however, mechanisms are incompletely understood. In particular, its capacity to alter receptor-activated Ca²⁺-mediated signaling processes has not been well-characterized. In initial experiments, we found evidence that T. gondii infection inhibits Ca²⁺ responses to fMetLeuPhe in murine macrophages. To further characterize the mechanism of inhibition of Ca²⁺ mobilization by T. gondii, we used the well-studied RBL mast cell model to probe the capacity of T. gondii to modulate IgE receptor-activated signaling within the first hour of infection. Ca²⁺ mobilization that occurs via IgE/FcεRI signaling leads to granule exocytosis in mast cells. We found that T. gondii inhibits antigen-stimulated degranulation in infected cells in a strain-independent manner. Under these conditions, we found that cytoplasmic Ca²⁺ mobilization, particularly antigen-mediated Ca²⁺ release from intracellular stores, is significantly reduced. Furthermore, stimulation-dependent activation of Syk kinase leading to tyrosine phosphorylation and activation of phospholipase Cγ is inhibited by infection. Therefore, we conclude that inhibitory effects of infection are likely due to parasite-mediated inhibition of the tyrosine kinase signaling cascade that results in reduced hydrolysis of phosphatidylinositol 4,5-bisphosphate. Interestingly, inhibition of IgE/FcεRI signaling persists when tachyzoite invasion is arrested via cytochalasin D treatment, suggesting inhibition is mediated by a parasite-derived factor secreted into the cells during the invasion process. Our study provides direct evidence that immune subversion by T. gondii is initiated concurrently with invasion.

Serglycin: a structural and functional chameleon with wide impact on immune cells

Among the different proteoglycans expressed by mammals, serglycin is in most immune cells the dominating species. A unique property of serglycin is its ability to adopt highly divergent structures, because of glycosylation with variable types of glycosaminoglycans when expressed by different cell types. Recent studies of serglycin-deficient animals have revealed crucial functions for serglycin in a diverse array of immunological processes. However, its exact function varies to a large extent depending on the cellular context of serglycin expression. Based on these findings, serglycin is emerging as a structural and functional chameleon, with radically different properties depending on its exact cellular and immunological context.

Engineering of routes to heparin and related polysaccharides

Anticoagulant heparin has been shown to possess important biological functions that vary according to its fine structure. Variability within heparin's structure occurs owing to its biosynthesis and animal tissue-based recovery and adds another dimension to its complex polymeric structure. The structural variations in chain length and sulfation patterns mediate its interaction with many heparin-binding proteins, thereby eliciting complex biological responses. The advent of novel chemical and enzymatic approaches for polysaccharide synthesis coupled with high throughput combinatorial approaches for drug discovery have facilitated an increased effort to understand heparin's structure-activity relationships. An improved understanding would offer potential for new therapeutic development through the engineering of polysaccharides. Such a bioengineering approach requires the amalgamation of several different disciplines, including carbohydrate synthesis, applied enzymology, metabolic engineering, and process biochemistry.