Marasmius oreades lectin induces renal thrombotic microangiopathic lesions

Mineralocorticoid receptor antagonism in diabetes reduces albuminuria by preserving the glomerular endothelial glycocalyx

The glomerular endothelial glycocalyx (GEnGlx) forms the first part of the glomerular filtration barrier. Previously, we showed that mineralocorticoid receptor (MR) activation caused GEnGlx damage and albuminuria. In this study, we investigated whether MR antagonism could limit albuminuria in diabetes and studied the site of action. Streptozotocin-induced diabetic Wistar rats developed albuminuria, increased glomerular albumin permeability (Ps'alb), and increased glomerular matrix metalloproteinase (MMP) activity with corresponding GEnGlx loss. MR antagonism prevented albuminuria progression, restored Ps'alb, preserved GEnGlx, and reduced MMP activity. Enzymatic degradation of the GEnGlx negated the benefits of MR antagonism, confirming their dependence on GEnGlx integrity. Exposing human glomerular endothelial cells (GEnC) to diabetic conditions in vitro increased MMPs and caused glycocalyx damage. Amelioration of these effects confirmed a direct effect of MR antagonism on GEnC. To confirm relevance to human disease, we used a potentially novel confocal imaging method to show loss of GEnGlx in renal biopsy specimens from patients with diabetic nephropathy (DN). In addition, patients with DN randomized to receive an MR antagonist had reduced urinary MMP2 activity and albuminuria compared with placebo and baseline levels. Taken together, our work suggests that MR antagonists reduce MMP activity and thereby preserve GEnGlx, resulting in reduced glomerular permeability and albuminuria in diabetes.

Blocking matrix metalloproteinase-mediated syndecan-4 shedding restores the endothelial glycocalyx and glomerular filtration barrier function in early diabetic kidney disease

The endothelial glycocalyx is a key component of the glomerular filtration barrier. We have shown that matrix metalloproteinase (MMP)-mediated syndecan 4 shedding is a mechanism of glomerular endothelial glycocalyx damage in vitro, resulting in increased albumin permeability. Here we sought to determine whether this mechanism is important in early diabetic kidney disease, by studying streptozotocin-induced type 1 diabetes in DBA2/J mice. Diabetic mice were albuminuric, had increased glomerular albumin permeability and endothelial glycocalyx damage. Syndecan 4 mRNA expression was found to be upregulated in isolated glomeruli and in flow cytometry-sorted glomerular endothelial cells. In contrast, glomerular endothelial luminal surface syndecan 4 and Marasmium oreades agglutinin lectin labelling measurements were reduced in the diabetic mice. Similarly, syndecan 4 protein expression was significantly decreased in isolated glomeruli but increased in plasma and urine, suggesting syndecan 4 shedding. Mmp-2, 9 and 14 mRNA expression were upregulated in isolated glomeruli, suggesting a possible mechanism of glycocalyx damage and albuminuria. We therefore characterised in detail the activity of MMP-2 and 9 and found significant increases in kidney cortex, plasma and urine. Treatment with MMP-2/9 inhibitor I for 21 days, started six weeks after diabetes induction, restored endothelial glycocalyx depth and coverage and attenuated diabetes-induced albuminuria and reduced glomerular albumin permeability. MMP inhibitor treatment significantly attenuated glomerular endothelial and plasma syndecan 4 shedding and inhibited plasma MMP activity. Thus, our studies confirm the importance of MMPs in endothelial glycocalyx damage and albuminuria in early diabetes and demonstrate that this pathway is amenable to therapeutic intervention. Hence, treatments targeted at glycocalyx protection by MMP inhibition may be of benefit in diabetic kidney disease.

Uptake of Marasmius oreades agglutinin disrupts integrin-dependent cell adhesion

BACKGROUND

Fruiting body lectins have been proposed to act as effector proteins in the defense of fungi against parasites and predators. The Marasmius oreades agglutinin (MOA) is a lectin from the fairy ring mushroom with specificity for Galα1-3Gal containing carbohydrates. This lectin is composed of an N-terminal carbohydrate-binding domain and a C-terminal dimerization domain. The dimerization domain of MOA shows in addition calcium-dependent cysteine protease activity, similar to the calpain family.

METHODS

Cell detachment assay, cell viability assay, immunofluorescence, live cell imaging and Western blot using MDCKII cell line.

RESULTS

In this study, we demonstrate in MDCKII cells that after internalization, MOA protease activity induces profound physiological cellular responses, like cytoskeleton rearrangement, cell detachment and cell death. These changes are preceded by a decrease in FAK phosphorylation and an internalization and degradation of β1-integrin, consistent with a disruption of integrin-dependent cell adhesion signaling. Once internalized, MOA accumulates in late endosomal compartments.

CONCLUSION

Our results suggest a possible toxic mechanism of MOA, which consists of disturbing the cell adhesion and the cell viability.

GENERAL SIGNIFICANCE

After being ingested by a predator, MOA might exert a protective role by diminishing host cell integrity.

The fungal chimerolectin MOA inhibits protein and DNA synthesis in NIH/3T3 cells and may induce BAX-mediated apoptosis

The Marasmius oreades mushroom agglutinin (MOA) is a blood group B-specific lectin carrying an active proteolytic domain. Its enzymatic activity has recently been shown to be critical for toxicity of MOA toward the fungivorous soil nematode Caenorhabditis elegans. Here we present evidence that MOA also induces cytotoxicity in a cellular model system (murine NIH/3T3 cells), by inhibiting protein synthesis, and that cytotoxicity correlates, at least in part, with proteolytic activity. A peptide-array screen identified the apoptosis mediator BAX as a potential proteolytic substrate and further suggests a variety of bacterial and fungal peptides as potential substrates. These findings are in line with the suggestion that MOA and related proteases may play a role for host defense.

Experimental glomerular endothelial injury in vivo

The microvascular endothelium of the kidney glomerulus is injured in Shiga-like toxigenic bacterial infection, genetic or acquired loss of complement regulatory protein function, and allo-immune responses of solid-organ or bone marrow transplantation. Existing models of diseases with glomerular endothelial cell (EC) injury, collectively grouped as thrombotic microangiopathies, are problematic, impeding investigation of the mechanisms of microvascular defense and repair. To develop a model of glomerular endothelial injury in the mouse, we conjugated the M. oreades lectin to the cytotoxin, saporin, (LS) to selectively injure the glomerular endothelium. Injury of the microvasculature was evaluated by light, immunofluorescence, and electron microscopy, and by quantitative RT-PCR of cell-type specific transcripts. Renal function was evaluated by quantitation of serum creatinine. The toxin conjugate induced apoptosis of microvascular ECs in vitro, and subtle histologic features of thrombotic microangiopathy in vivo that were enhanced by co-injection of 50 μg/kg LPS. Among LS/LPS-treated animals, loss of glomerular EC staining correlated with decreased expression of EC-specific transcripts, and impaired kidney function. Selective injury of the glomerular microvasculature with LS toxin conjugate and LPS elicits histologic features of thrombotic microangiopathy and acute kidney failure.

Nematotoxicity of Marasmius oreades agglutinin (MOA) depends on glycolipid binding and cysteine protease activity

Fruiting body lectins have been proposed to act as effector proteins in the defense of fungi against parasites and predators. The Marasmius oreades agglutinin (MOA) is a Galα1,3Gal/GalNAc-specific lectin from the fairy ring mushroom that consists of an N-terminal ricin B-type lectin domain and a C-terminal dimerization domain. The latter domain shows structural similarity to catalytically active proteins, suggesting that, in addition to its carbohydrate-binding activity, MOA has an enzymatic function. Here, we demonstrate toxicity of MOA toward the model nematode Caenorhabditis elegans. This toxicity depends on binding of MOA to glycosphingolipids of the worm via its lectin domain. We show further that MOA has cysteine protease activity and demonstrate a critical role of this catalytic function in MOA-mediated nematotoxicity. The proteolytic activity of MOA was dependent on high Ca(2+) concentrations and favored by slightly alkaline pH, suggesting that these conditions trigger activation of the toxin at the target location. Our results suggest that MOA is a fungal toxin with intriguing similarities to bacterial binary toxins and has a protective function against fungivorous soil nematodes.

Marasmius oreades agglutinin (MOA) is a chimerolectin with proteolytic activity

The Marasmius oreades mushroom lectin (MOA) is well known for its exquisite binding specificity for blood group B antigens. In addition to its N-terminal carbohydrate-binding domain, MOA possesses a C-terminal domain with unknown function, which structurally resembles hydrolytic enzymes. Here we show that MOA indeed has catalytic activity. It is a calcium-dependent cysteine protease resembling papain-like cysteine proteases, with Cys215 being the catalytic nucleophile. The possible importance of MOA's proteolytic activity for mushroom defense against pathogens is discussed.

Caenorhabditis elegans N-glycan core beta-galactoside confers sensitivity towards nematotoxic fungal galectin CGL2

The physiological role of fungal galectins has remained elusive. Here, we show that feeding of a mushroom galectin, Coprinopsis cinerea CGL2, to Caenorhabditis elegans inhibited development and reproduction and ultimately resulted in killing of this nematode. The lack of toxicity of a carbohydrate-binding defective CGL2 variant and the resistance of a C. elegans mutant defective in GDP-fucose biosynthesis suggested that CGL2-mediated nematotoxicity depends on the interaction between the galectin and a fucose-containing glycoconjugate. A screen for CGL2-resistant worm mutants identified this glycoconjugate as a Galbeta1,4Fucalpha1,6 modification of C. elegans N-glycan cores. Analysis of N-glycan structures in wild type and CGL2-resistant nematodes confirmed this finding and allowed the identification of a novel putative glycosyltransferase required for the biosynthesis of this glycoepitope. The X-ray crystal structure of a complex between CGL2 and the Galbeta1,4Fucalpha1,6GlcNAc trisaccharide at 1.5 A resolution revealed the biophysical basis for this interaction. Our results suggest that fungal galectins play a role in the defense of fungi against predators by binding to specific glycoconjugates of these organisms.

A new alpha-galactosyl-binding protein from the mushroom Lyophyllum decastes

A new alpha-galactosyl binding lectin was isolated from the fruiting bodies of the mushroom Lyopyllum decastes. It is a homodimer composed of noncovalently-associated monomers of molecular mass 10,276Da. The lectin's amino acid sequence was determined by cloning from a cDNA library using partial sequences determined by automated Edman sequencing and by mass spectrometry of enzyme-derived peptides. The sequence shows no significant homology to any known protein sequence. Analysis of carbohydrate binding specificity by a variety of approaches including precipitation with glycoconjugates and microcalorimetric titration reveals specificity towards galabiose (Gal alpha1,4Gal), a relatively rare disaccharide in humans. The lectin shares carbohydrate binding preference with the Shiga-like toxin, also known as verocytoxin, present in the bacteria Shigella dysenteriae and Escherichia. coli 0157:H7, both of which are causes of outbreaks of sometimes fatal food-borne illnesses.

Fungal substances as modulators of NF-κB activation pathway

MCF7 breast cancer cell line, carrying a luciferase reporter gene under the control of nuclear factor kappa B (NF-kappaB)-responsive promoter, was established and used for the screening of fungal organic extracts for their ability to interfere with the NF-kappaB activation pathway. Twenty-eight crude fungal extracts, out of 242, were found to inhibit NF-kappaB reporter activity by more than 40%. Furthermore, positive extracts were used to evaluate their antiproliferative activity as well as their ability to influence the phosphorylation and degradation levels of IkappaBa. Fungal extracts prepared from Marasmius oreades and Cyathus striatus showed significant inhibitory effects on the NF-kappaB activation pathway. Taken together, our results support the notion of the presence of novel activities that might be utilized as cancer therapeutics.

The role of endothelial cell apoptosis in inflammatory and immune diseases

The integrity of the endothelial lining of the vasculature is essential for vascular homeostasis and normal organ function. Endothelial injury or dysfunction has been implicated in the pathogenesis of diverse vascular diseases. Studies in vitro have demonstrated that a wide variety of stimuli can induce programmed cell death (apoptosis) of endothelial cells, and have suggested that apoptosis could be an important mechanism of vascular injury, resulting in vascular leak, inflammation, and coagulation. In this review, we focus on the potential role of endothelial apoptosis in the initiation and progression of inflammatory and immune disorders, reviewing human diseases and in vivo models in which endothelial cell apoptosis has been demonstrated. Although endothelial cell apoptosis is observed in many inflammatory and immune disorders, we find that there is, as yet, only limited experimental evidence demonstrating that it is critical to the pathogenesis of disease.