Calpain-dependent cytoskeletal rearrangement exploited for anthrax toxin endocytosis

Identification of molecular and cellular infection response biomarkers associated with anthrax infection through comparative analysis of gene expression data

Bacillus anthracis, a gram-positive bacillus capable of forming spores, causes anthrax in mammals, including humans, and is recognized as a potential biological weapon agent. The diagnosis of anthrax is challenging due to variable symptoms resulting from exposure and infection severity. Despite the availability of a licensed vaccines, their limited long-term efficacy underscores the inadequacy of current human anthrax vaccines, highlighting the urgent need for next-generation alternatives. Our study aimed to identify molecular biomarkers and essential biological pathways for the early detection and accurate diagnosis of human anthrax infection. Using a comparative analysis of Bacillus anthracis gene expression data from the Gene Expression Omnibus (GEO) database, this cost-effective approach enables the identification of shared differentially expressed genes (DEGs) across separate microarray datasets without additional hybridization. Three microarray datasets (GSE34407, GSE14390, and GSE12131) of B. anthracis-infected human cell lines were analyzed via the GEO2R tool to identify shared DEGs. We identified 241 common DEGs (70 upregulated and 171 downregulated) from cell lines treated similarly to lethal toxins. Additionally, 10 common DEGs (5 upregulated and 5 downregulated) were identified across different treatments (lethal toxins and spores) and cell lines. Network meta-analysis identified JUN and GATAD2A as the top hub genes for overexpression, and NEDD4L and GULP1 for underexpression. Furthermore, prognostic analysis and SNP detection of the two identified upregulated hub genes were carried out in conjunction with machine learning classification models, with SVM yielding the best classification accuracy of 87.5 %. Our comparative analysis of Bacillus anthracis infection revealed striking similarities in gene expression 241 profiles across diverse datasets, despite variations in treatments and cell lines. These findings underscore how anthrax infection activates shared genes across different cell types, emphasizing this approach in the discovery of novel gene markers. These markers offer insights into pathogenesis and may lead to more effective therapeutic strategies. By identifying these genetic indicators, we can advance the development of precise immunotherapies, potentially enhancing vaccine efficacy and treatment outcomes.

Advanced glycation end products promote the progression of chronic kidney diseases by targeting calpain 6

Advanced glycation end products (AGEs) are produced by glycosylation or oxidation of proteins and lipids and are tightly involved in the chronic kidney disease (CKD) process. Calpain 6 (CAPN6) is a non-classical calpain that has been reported to be overexpressed in CKD. This study aimed to explore the effects of AGEs in CKD progress and their correlation with CAPN6. AGEs production was measured using ELISA. The CCK-8 assay was used to test cell proliferation. mRNA and protein levels were tested using qRT-PCR and western blot. The progress of glycolysis was tested by calculating the ATP and ECAR content in HK-2 cells. The expression of AGEs and CAPN6 was significantly increased in patients with CKD3, CKD4, and CKD5. AGEs treatment inhibited cell proliferation and glycolysis and accelerated apoptosis. Additionally, CAPN6 knockdown effectively reversed the effects of AGEs in HK-2 cells. In addition, overexpressed CAPN6 played similar role to AGEs, which suppressed cell proliferation and glycolysis and facilitated apoptosis. Moreover, the administration of 2-DG, a glycolysis inhibitor, counteracted the effects of CAPN6 silencing in HK-2 cells. Mechanistically, CAPN6 interacts with NF-κB and PDTC reduced CAPN6 expression in HK-2 cells. This investigation revealed that AGEs facilitate CKD development in vitro by modulating the expression of CAPN6.

ANTXR1 as a potential sensor of extracellular mechanical cues

Cell adhesion molecules mediate cell-cell or cell-matrix interactions, some of which are mechanical sensors, such as integrins. Emerging evidence indicates that anthrax toxin receptor 1 (ANTXR1), a newly identified cell adhesion molecule, can also sense extracellular mechanical signals such as hydrostatic pressure and extracellular matrix (ECM) rigidity. ANTXR1 can interact with ECM through connecting intracellular cytoskeleton and ECM molecules (just like integrins) to regulate numerous biological processes, such as cell adhesion, cell migration or ECM homeostasis. Although with high structural similarity to integrins, its functions and downstream signal transduction are independent from those of integrins. In this perspective, based on existing evidence in literature, we analyzed the structural and functional evidence that ANTXR1 can act as a potential sensor for extracellular mechanical cues. To our knowledge, this is the first in-depth overview of ANTXR1 from the perspective of mechanobiology. STATEMENT OF SIGNIFICANCE: An overview of ANTXR1 from the perspective of mechanobiology; An analysis of mechanical sensitivity of ANTXR1 in structure and function; A summary of existing evidence of ANTXR1 as a potential mechanosensor.

Astragaloside IV Alleviates Brain Injury Induced by Hypoxia via the Calpain-1 Signaling Pathway

Long-term hypoxia can induce oxidative stress and apoptosis in hippocampal neurons that can lead to brain injury diseases. Astragaloside IV (AS-IV) is widely used in the antiapoptotic therapy of brain injury diseases. However, its mechanism of action is still not fully understood. In this study, we investigated the effect of AS-IV on hypoxia-induced oxidative stress and apoptosis in hippocampal neurons and explored its possible mechanism. In vivo, mice were placed in a hypoxic circulatory device containing 10% O2 and gavaged with AS-IV (60 and 120 mg/kg/d) for 4 weeks. In vitro, mouse hippocampal neuronal cells (HT22) were treated with hypoxia (1% O2) for 24 hours in the presence or absence of AS-IV, MDL-28170 (calpain-1 inhibitor), or YC-1 (HIF-1α inhibitor). The protective effect of AS-IV on brain injury was further explored by examining calpain-1 knockout mice. The results showed that hypoxia induced damage to hippocampal neurons, impaired spatial learning and memory abilities, and increased oxidative stress and apoptosis. Treatment with AS-IV or calpain-1 knockout improved the damage to hippocampal neurons and spatial learning and memory, attenuated oxidative stress and inhibited cell apoptosis. These changes were verified in HT22 cells. Overexpression of calpain-1 abolished the improvement of AS-IV on apoptosis and oxidative stress. In addition, the effects of AS-IV were accompanied by decreased calpain-1 and HIF-1α expression, and YC-1 showed a similar effect as AS-IV on calpain-1 and caspase-3 expression. In conclusion, this study demonstrates that AS-IV can downregulate the calpain-1/HIF-1α/caspase-3 pathway and inhibit oxidative stress and apoptosis of hippocampal neurons induced by hypoxia, which provides new ideas for studying the antiapoptotic activity of AS-IV.

Enterococcus faecalis alters endo-lysosomal trafficking to replicate and persist within mammalian cells

Enterococcus faecalis is a frequent opportunistic pathogen of wounds, whose infections are associated with biofilm formation, persistence, and recalcitrance toward treatment. We have previously shown that E. faecalis wound infection persists for at least 7 days. Here we report that viable E. faecalis are present within both immune and non-immune cells at the wound site up to 5 days after infection, raising the prospect that intracellular persistence contributes to chronic E. faecalis infection. Using in vitro keratinocyte and macrophage infection models, we show that E. faecalis becomes internalized and a subpopulation of bacteria can survive and replicate intracellularly. E. faecalis are internalized into keratinocytes primarily via macropinocytosis into single membrane-bound compartments and can persist in late endosomes up to 24 h after infection in the absence of colocalization with the lysosomal protease Cathepsin D or apparent fusion with the lysosome, suggesting that E. faecalis blocks endosomal maturation. Indeed, intracellular E. faecalis infection results in heterotypic intracellular trafficking with partial or absent labelling of E. faecalis-containing compartments with Rab5 and Rab7, small GTPases required for the endosome-lysosome trafficking. In addition, E. faecalis infection results in marked reduction of Rab5 and Rab7 protein levels which may also contribute to attenuated Rab incorporation into E. faecalis-containing compartments. Finally, we demonstrate that intracellular E. faecalis derived from infected keratinocytes are significantly more efficient in reinfecting new keratinocytes. Together, these data suggest that intracellular proliferation of E. faecalis may contribute to its persistence in the face of a robust immune response, providing a primed reservoir of bacteria for subsequent reinfection.

The Caenorhabditis elegans CUB-like-domain containing protein RBT-1 functions as a receptor for Bacillus thuringiensis Cry6Aa toxin

Plant-parasitic nematodes cause huge agricultural economic losses. Two major families of Bacillus thuringiensis crystal proteins, Cry5 and Cry6, show nematicidal activity. Previous work showed that binding to midgut receptors is a limiting step in Cry toxin mode of action. In the case of Cry5Ba, certain Caenorhabditis elegans glycolipids were identified as receptors of this toxin. However, the receptors for Cry6 toxin remain unknown. In this study, the C. elegans CUB-like-domain containing protein RBT-1, released by phosphatidylinositol-specific phospholipase C (PI-PLC), was identified as a Cry6Aa binding protein by affinity chromatography. RBT-1 contained a predicted glycosylphosphatidylinositol (GPI) anchor site and was shown to locate in lipid rafts in the surface of the midgut cells. Western ligand blot assays and ELISA binding analysis confirmed the binding interaction between Cry6Aa and RBT-1 showing high affinity and specificity. In addition, the mutation of rbt-1 gene decreased the susceptibility of C. elegans to Cry6Aa but not that of Cry5Ba. Furthermore, RBT-1 mediated the uptake of Cry6Aa into C. elegans gut cells, and was shown to be involved in triggering pore-formation activity, indicating that RBT-1 is required for the interaction of Cry6Aa with the nematode midgut cells. These results support that RBT-1 is a functional receptor for Cry6Aa.

Bacillus thuringiensis (Bt) crystal proteins belong to pore-forming toxins (PFTs), which display virulence against target hosts by forming holes in the cell membrane. Cry6A is a nematicidal PFT, which exhibits unique protein structure and different mode of action than Cry5B, another nematicidal PFT. However, little is known about the mode of action of Cry6A. Although an intracellular nematicidal necrosis pathway of Cry6A was reported, its extracellular mode of action remains unknown. We here demonstrate that the CUB-like-domain containing protein RBT-1 acts as a functional receptor of Cry6A, which mediates the intestinal cell interaction and nematicidal activity of this toxin. RBT-1 represents a new class of crystal protein receptors. RBT-1 is dispensable for Cry5B toxicity against nematodes, consistent with that Cry6A and Cry5B have different nematicidal mechanisms. We also find that Cry6A kills nematodes by complex mechanism since rbt-1 mutation did not affect Cry6A-mediated necrosis signaling pathway. This work not only enhances the understanding of Bt crystal protein-nematode mechanism, but is also in favor for the application of Cry6A in nematode control.

A Membrane Potential- and Calpain-Dependent Reversal of Caspase-1 Inhibition Regulates Canonical NLRP3 Inflammasome

The NLRP3 inflammasome senses a range of cellular disturbances, although no consensus exists regarding a common mechanism. Canonical NLRP3 activation is blocked by high extracellular K⁺, regardless of the activating signal. We report here that canonical NLRP3 activation leads to Ca²⁺ flux and increased calpain activity. Activated calpain releases a pool of Caspase-1 sequestered by the cytoskeleton to regulate NLRP3 activation. Using electrophysiological recording, we found that resting-state eukaryotic membrane potential (MP) is required for this calpain activity, and depolarization by high extracellular K⁺ or artificial hyperpolarization results in the inhibition of calpain. Therefore, the MP/Ca²⁺/calpain/ Caspase-1 axis acts as an independent regulatory mechanism for NLRP3 activity. This finding provides mechanistic insight into high K⁺-mediated inhibition of NLRP3 activation, and it offers an alternative model of NLRP3 inflammasome activation that does not involve K⁺ efflux.

Zhang et al. find that, in canonical NLRP inflammasome activation, calpain activity is essential for releasing caspase-1 from flightless-1 and the cytoskeleton. Membrane depolarization, such as under high extracellular K⁺ or hyperpolarization, impairs this activity. This work provides insight into extracellular K⁺ -mediated inhibition of the NLRP3 inflammasome.

Converging physiological roles of the anthrax toxin receptors

The anthrax toxin receptors-capillary morphogenesis gene 2 (CMG2) and tumor endothelial marker 8 (TEM8)-were identified almost 20 years ago, although few studies have moved beyond their roles as receptors for the anthrax toxins to address their physiological functions. In the last few years, insight into their endogenous roles has come from two rare diseases: hyaline fibromatosis syndrome, caused by mutations in CMG2, and growth retardation, alopecia, pseudo-anodontia, and optic atrophy (GAPO) syndrome, caused by loss-of-function mutations in TEM8. Although CMG2 and TEM8 are highly homologous at the protein level, the difference in disease symptoms points to variations in the physiological roles of the two anthrax receptors. Here, we focus on the similarities between these receptors in their ability to regulate extracellular matrix homeostasis, angiogenesis, cell migration, and skin elasticity. In this way, we shed light on how mutations in these two related proteins cause such seemingly different diseases and we highlight the existing knowledge gaps that could form the focus of future studies.

In vivo induction of membrane damage by β-amyloid peptide oligomers

Exposure to the β-amyloid peptide (Aβ) is toxic to neurons and other cell types, but the mechanism(s) involved are still unresolved. Synthetic Aβ oligomers can induce ion-permeable pores in synthetic membranes, but whether this ability to damage membranes plays a role in the ability of Aβ oligomers to induce tau hyperphosphorylation, or other disease-relevant pathological changes, is unclear. To examine the cellular responses to Aβ exposure independent of possible receptor interactions, we have developed an in vivo C. elegans model that allows us to visualize these cellular responses in living animals. We find that feeding C. elegans E. coli expressing human Aβ induces a membrane repair response similar to that induced by exposure to the CRY5B, a known pore-forming toxin produced by B. thuringensis. This repair response does not occur when C. elegans is exposed to an Aβ Gly37Leu variant, which we have previously shown to be incapable of inducing tau phosphorylation in hippocampal neurons. The repair response is also blocked by loss of calpain function, and is altered by loss-of-function mutations in the C. elegans orthologs of BIN1 and PICALM, well-established risk genes for late onset Alzheimer's disease. To investigate the role of membrane repair on tau phosphorylation directly, we exposed hippocampal neurons to streptolysin O (SLO), a pore-forming toxin that induces a well-characterized membrane repair response. We find that SLO induces tau hyperphosphorylation, which is blocked by calpain inhibition. Finally, we use a novel biarsenical dye-tagging approach to show that the Gly37Leu substitution interferes with Aβ multimerization and thus the formation of potentially pore-forming oligomers. We propose that Aβ-induced tau hyperphosphorylation may be a downstream consequence of induction of a membrane repair process.

Anthrax protective antigen is a calcium-dependent serine protease

Bacillus anthracis secretes a three component exotoxin-complex, which contributes to anthrax pathogenesis. Formation of this complex starts with the binding of protective antigen (PA) to its cellular receptor. In this study, we report that PA is a calcium-dependent serine protease and that the protein potentially uses this proteolytic activity for receptor binding. Additionally our findings shed new light on previous research describing the inhibition of anthrax toxins and exotoxin formation. Importantly, inhibition of the proteolytic activity of protective antigen could be a novel therapeutic strategy in fighting B. anthracis-related infections.

Haemolysin Sph2 of Leptospira interrogans induces cell apoptosis via intracellular reactive oxygen species elevation and mitochondrial membrane injury

Leptospira interrogans causes widespread leptospirosis in humans and animals, with major symptoms of jaundice and haemorrhage. Sph2, a member of the sphingomyelinase haemolysins, is an important virulence factor for leptospire. In this study, the function and mechanism of Sph2 in the pathogenesis of leptospirosis were investigated to further understand the pathogenesis of leptospire. Real-time PCR analysis of expression levels during cell invasion showed that sph2 gene expression was transiently induced in human umbilical vein endothelial cells (HUVECs), human embryo liver cells (L02), and human epithelial lung cells (L132), with expression levels reaching a peak after 45 min of infection. Further functional analysis of recombinant Sph2 (rSph2) by LDH assays and confocal microscopy showed that rSph2 can be internalised by cells both by causing cell membrane damage and by a damage-independent clathrin-mediated endocytosis pathway. Subsequently, rSph2 is able to translocate to mitochondria, which led to an increase in the levels of reactive oxygen species (ROS) and a decrease of the mitochondrial membrane potential (ΔΨ_m ). Further flowcytometry analyses after rSph2 exposure showed that 28.7%, 31%, and 27.3% of the HUVEC, L02, and L132 cells, respectively, became apoptotic. Because apoptosis could be decreased with the ROS inhibitor N-acetyl cysteine, these experiments suggested that rSph2 triggers apoptosis through mitochondrial membrane damage and ROS elevation. The ability of leptospiral haemolysin rSph2 to cause apoptosis likely contributes to the pathogenesis of leptospirosis.

Targeting calpains: A novel immunomodulatory approach for microbial infections

Calpains are a family of Ca²⁺ dependent cytosolic non-lysosomal proteases with well conserved cysteine-rich domains for enzymatic activity. Due to their functional dependency on Ca²⁺ concentrations, they are involved in various cellular processes that are regulated by intracellular ca²⁺ concentration (i.e. embryo development, cell development and migration, maintenance of cellular architecture and structure etc.). Calpains are widely studied proteases in mammalian (i.e. mouse and human) physiology and pathophysiology due to their ubiquitous presence. For example, these proteases have been found to be involved in various inflammatory disorders such as neurodegeneration, cancer, brain and myocardial ischemia and infarction, cataract and muscular dystrophies etc. Besides their role in these sterile inflammatory conditions, calpains have also been shown to regulate a wide range of infectious diseases (i.e. sepsis, tuberculosis, gonorrhoea and bacillary dysentery etc.). One of these regulatory mechanisms mediated by calpains (i.e. calpain 1 and 2) during microbial infections involves the regulation of innate immune response, inflammation and cell death. Thus, the major emphasis of this review is to highlight the importance of calpains in the pathogenesis of various microbial (i.e. bacterial, fungal and viral) diseases and the use of calpain modulators as potential immunomodulators in microbial infections.

Endothelial cell calpain as a critical modulator of angiogenesis

Calpains are a family of calcium-dependent non-lysosomal cysteine proteases. In particular, calpains residing in the endothelial cells play important roles in angiogenesis. It has been shown that calpain activity can be increased in endothelial cells by growth factors, primarily vascular endothelial growth factor (VEGF). VEGF/VEGFR2 induces calpain 2 dependent activation of PI3K/AMPK/Akt/eNOS pathway, and consequent nitric oxide production and physiological angiogenesis. Under pathological conditions such as tumor angiogenesis, endothelial calpains can be activated by hypoxia. This review focuses on the molecular regulatory mechanisms of calpain activation, and the newly identified mechanistic roles and downstream signaling events of calpains in physiological angiogenesis, and in the conditions of pathological tumor angiogenesis and diabetic wound healing, as well as retinopathy and atherosclerosis that are also associated with an increase in calpain activity. Further discussed include the differential strategies of modulating angiogenesis through manipulating calpain expression/activity in different pathological settings. Targeted limitation of angiogenesis in cancer and targeted promotion of angiogenesis in diabetic wound healing via modulations of calpains and calpain-dependent signaling mechanisms are of significant translational potential. Emerging strategies of tissue-specific targeting, environment-dependent targeting, and genome-targeted editing may turn out to be effective regimens for targeted manipulation of angiogenesis through calpain pathways, for differential treatments including both attenuation of tumor angiogenesis and potentiation of diabetic angiogenesis.

Attenuation of Blood-Brain Barrier Breakdown and Hyperpermeability by Calpain Inhibition

Blood-brain barrier (BBB) breakdown and the associated microvascular hyperpermeability followed by brain edema are hallmark features of several brain pathologies, including traumatic brain injuries (TBI). Recent studies indicate that pro-inflammatory cytokine interleukin-1β (IL-1β) that is up-regulated following traumatic injuries also promotes BBB dysfunction and hyperpermeability, but the underlying mechanisms are not clearly known. The objective of this study was to determine the role of calpains in mediating BBB dysfunction and hyperpermeability and to test the effect of calpain inhibition on the BBB following traumatic insults to the brain. In these studies, rat brain microvascular endothelial cell monolayers exposed to calpain inhibitors (calpain inhibitor III and calpastatin) or transfected with calpain-1 siRNA demonstrated attenuation of IL-1β-induced monolayer hyperpermeability. Calpain inhibition led to protection against IL-1β-induced loss of zonula occludens-1 (ZO-1) at the tight junctions and alterations in F-actin cytoskeletal assembly. IL-1β treatment had no effect on ZO-1 gene (tjp1) or protein expression. Calpain inhibition via calpain inhibitor III and calpastatin decreased IL-1β-induced calpain activity significantly (p < 0.05). IL-1β had no detectable effect on intracellular calcium mobilization or endothelial cell viability. Furthermore, calpain inhibition preserved BBB integrity/permeability in a mouse controlled cortical impact model of TBI when studied using Evans blue assay and intravital microscopy. These studies demonstrate that calpain-1 acts as a mediator of IL-1β-induced loss of BBB integrity and permeability by altering tight junction integrity, promoting the displacement of ZO-1, and disorganization of cytoskeletal assembly. IL-1β-mediated alterations in permeability are neither due to the changes in ZO-1 expression nor cell viability. Calpain inhibition has beneficial effects against TBI-induced BBB hyperpermeability.

IP3 and calcium signaling involved in the reorganization of the actin cytoskeleton and cell rounding induced by cigarette smoke extract in human endothelial cells

Smoking increases the risk of cardiovascular disorders and leads to damage caused by inflammation and oxidative stress. The actin cytoskeleton is a key player in the response to inflammatory stimuli and is an early target of cellular oxidative stress. The purpose of this study was to investigate the changes in actin cytoskeleton dynamics in human endothelial EA.hy926 cells exposed to cigarette smoke extract (CSE). Immunostaining revealed that CSE exposure resulted in modification of the actin cytoskeleton and led to cell rounding in a dose- and time-dependent manner. In addition, the intracellular calcium concentration was increased by treatment with CSE. Pretreatment with antioxidants (lipoic acid, glutathione, N-acetyl cysteine, aminoguanidine, α-tocopherol, and vitamin C) significantly attenuated the CSE-induced actin cytoskeleton reorganization and cell rounding. Calcium ion chelators (EGTA, BAPTA-AM AM) and a potent store-operated calcium channel inhibitor (MRS 1845) also reduced CSE-induced intracellular calcium changes and attenuated actin cytoskeleton reorganization and cell morphology change. Moreover, the CSE-induced intracellular calcium increase was suppressed by pretreatment with the inositol trisphosphate receptor (IP3R) inhibitor xestospongin C, the phospholipase C (PLC) inhibitor U-73122, and the protein kinase C (PKC) inhibitor GF109203X. These results suggest that reactive oxygen species production and intracellular calcium increase play an essential role in CSE-induced actin disorganization and cell rounding through a PLC-IP3-PKC signaling pathway. © 2015 Wiley Periodicals, Inc. Environ Toxicol 31: 1293-1306, 2016.

Acquired resistance to innate immune clearance promotes Klebsiella pneumoniae ST258 pulmonary infection

Adaptive changes in the genome of a locally predominant clinical isolate of the multidrug-resistant Klebsiella pneumoniae ST258 (KP35) were identified and help to explain the selection of this strain as a successful pulmonary pathogen. The acquisition of 4 new ortholog groups, including an arginine transporter, enabled KP35 to outcompete related ST258 strains lacking these genes. KP35 infection elicited a monocytic response, dominated by Ly6C^hi monocytic myeloid-derived suppressor cells that lacked phagocytic capabilities, expressed IL-10, arginase, and antiinflammatory surface markers. In comparison with other K. pneumoniae strains, KP35 induced global changes in the phagocytic response identified with proteomics, including evasion of Ca²⁺ and calpain activation necessary for phagocytic killing, confirmed in functional studies with neutrophils. This comprehensive analysis of an ST258 K. pneumoniae isolate reveals ongoing genetic adaptation to host microenvironments and innate immune clearance mechanisms that complements its repertoire of antimicrobial resistance genes and facilitates persistence in the lung.

Calpain-activated mTORC2/Akt pathway mediates airway smooth muscle remodelling in asthma

BACKGROUND

Allergic asthma is characterized by inflammation and airway remodelling. Airway remodelling with excessive deposition of extracellular matrix (ECM) and larger smooth muscle mass are correlated with increased airway responsiveness and asthma severity. Calpain is a family of calcium-dependent endopeptidases, which plays an important role in ECM remodelling. However, the role of calpain in airway smooth muscle remodelling remains unknown.

OBJECTIVE

To investigate the role of calpain in asthmatic airway remodelling as well as the underlying mechanism.

METHODS

The mouse asthma model was made by ovalbumin sensitization and challenge. Calpain conditional knockout mice were studied in the model. Airway smooth muscle cells (ASMCs) were isolated from smooth muscle bundles in airway of rats. Cytokines IL-4, IL-5, TNF-α, and TGF-β1, and serum from patients with asthma were selected to treated ASMCs. Collagen-I synthesis, cell proliferation, and phosphorylation of Akt in ASMCs were analysed.

RESULTS

Inhibition of calpain using calpain knockout mice attenuated airway smooth muscle remodelling in mouse asthma models. Cytokines IL-4, IL-5, TNF-α, and TGF-β1, and serum from patients with asthma increased collagen-I synthesis, cell proliferation, and phosphorylation of Akt in ASMCs, which were blocked by the calpain inhibitor MDL28170. Moreover, MDL28170 reduced cytokine-induced increases in Rictor protein, which is the most important component of mammalian target of rapamycin complex 2 (mTORC2). Blockage of the mTORC2 signal pathway prevented cytokine-induced phosphorylation of Akt, collagen-I synthesis, and cell proliferation of ASMCs and attenuated airway smooth muscle remodelling in mouse asthma models.

CONCLUSIONS AND CLINICAL RELEVANCE

Our results indicate that calpain mediates cytokine-induced collagen-I synthesis and proliferation of ASMCs via the mTORC2/Akt signalling pathway, thereby regulating airway smooth muscle remodelling in asthma.

Cross-inhibition of pathogenic agents and the host proteins they exploit

The major limitations of pathogen-directed therapies are the emergence of drug-resistance and their narrow spectrum of coverage. A recently applied approach directs therapies against host proteins exploited by pathogens in order to circumvent these limitations. However, host-oriented drugs leave the pathogens unaffected and may result in continued pathogen dissemination. In this study we aimed to discover drugs that could simultaneously cross-inhibit pathogenic agents, as well as the host proteins that mediate their lethality. We observed that many pathogenic and host-assisting proteins belong to the same functional class. In doing so we targeted a protease component of anthrax toxin as well as host proteases exploited by this toxin. We identified two approved drugs, ascorbic acid 6-palmitate and salmon sperm protamine, that effectively inhibited anthrax cytotoxic protease and demonstrated that they also block proteolytic activities of host furin, cathepsin B, and caspases that mediate toxin's lethality in cells. We demonstrated that these drugs are broad-spectrum and reduce cellular sensitivity to other bacterial toxins that require the same host proteases. This approach should be generally applicable to the discovery of simultaneous pathogen and host-targeting inhibitors of many additional pathogenic agents.

Calpain-6 confers atherogenicity to macrophages by dysregulating pre-mRNA splicing

Macrophages contribute to the development of atherosclerosis through pinocytotic deposition of native LDL-derived cholesterol in macrophages in the vascular wall. Inhibiting macrophage-mediated lipid deposition may have protective effects in atheroprone vasculature, and identifying mechanisms that potentiate this process may inform potential therapeutic interventions for atherosclerosis. Here, we report that dysregulation of exon junction complex-driven (EJC-driven) mRNA splicing confers hyperpinocytosis to macrophages during atherogenesis. Mechanistically, we determined that inflammatory cytokines induce an unconventional nonproteolytic calpain, calpain-6 (CAPN6), which associates with the essential EJC-loading factor CWC22 in the cytoplasm. This association disturbs the nuclear localization of CWC22, thereby suppressing the splicing of target genes, including those related to Rac1 signaling. CAPN6 deficiency in LDL receptor-deficient mice restored CWC22/EJC/Rac1 signaling, reduced pinocytotic deposition of native LDL in macrophages, and attenuated macrophage recruitment into the lesions, generating an atheroprotective phenotype in the aorta. In macrophages, the induction of CAPN6 in the atheroma interior limited macrophage movements, resulting in a decline in cell clearance from the lesions. Consistent with this finding, we observed that myeloid CAPN6 contributed to atherogenesis in a murine model of bone marrow transplantation. Furthermore, macrophages from advanced human atheromas exhibited increased CAPN6 induction and impaired CWC22 nuclear localization. Together, these results indicate that CAPN6 promotes atherogenicity in inflamed macrophages by disturbing CWC22/EJC systems.

The Ins and Outs of Anthrax Toxin

Anthrax is a severe, although rather rare, infectious disease that is caused by the Gram-positive, spore-forming bacterium Bacillus anthracis. The infectious form is the spore and the major virulence factors of the bacterium are its poly-γ-D-glutamic acid capsule and the tripartite anthrax toxin. The discovery of the anthrax toxin receptors in the early 2000s has allowed in-depth studies on the mechanisms of anthrax toxin cellular entry and translocation from the endocytic compartment to the cytoplasm. The toxin generally hijacks the endocytic pathway of CMG2 and TEM8, the two anthrax toxin receptors, in order to reach the endosomes. From there, the pore-forming subunit of the toxin inserts into endosomal membranes and enables translocation of the two catalytic subunits. Insertion of the pore-forming unit preferentially occurs in intraluminal vesicles rather than the limiting membrane of the endosome, leading to the translocation of the enzymatic subunits in the lumen of these vesicles. This has important consequences that will be discussed. Ultimately, the toxins reach the cytosol where they act on their respective targets. Target modification has severe consequences on cell behavior, in particular on cells of the immune system, allowing the spread of the bacterium, in severe cases leading to host death. Here we will review the literature on anthrax disease with a focus on the structure of the toxin, how it enters cells and its immunological effects.

How Shigella Utilizes Ca(2+) Jagged Edge Signals during Invasion of Epithelial Cells

Shigella, the causative agent of bacillary dysentery invades intestinal epithelial cells using a type III secretion system (T3SS). Through the injection of type III effectors, Shigella manipulates the actin cytoskeleton to induce its internalization in epithelial cells. At early invasion stages, Shigella induces atypical Ca(2+) responses confined at entry sites allowing local cytoskeletal remodeling for bacteria engulfment. Global Ca(2+) increase in the cell triggers the opening of connexin hemichannels at the plasma membrane that releases ATP in the extracellular milieu, favoring Shigella invasion and spreading through purinergic receptor signaling. During intracellular replication, Shigella regulates inflammatory and death pathways to disseminate within the epithelium. At later stages of infection, Shigella downregulates hemichannel opening and the release of extracellular ATP to dampen inflammatory signals. To avoid premature cell death, Shigella activates cell survival by upregulating the PI3K/Akt pathway and downregulating the levels of p53. Furthermore, Shigella interferes with pro-apoptotic caspases, and orients infected cells toward a slow necrotic cell death linked to mitochondrial Ca(2+) overload. In this review, we will focus on the role of Ca(2+) responses and their regulation by Shigella during the different stages of bacterial infection.

Repurposing FDA approved drugs against the human fungal pathogen, Candida albicans

BACKGROUND

The high cost and prolonged timeline of new drug discovery and development are major roadblocks to creating therapies for infectious diseases. Candida albicans is an opportunistic fungal pathogen that is the most common cause of fatal fungal infections in humans and costs $2-4 billion dollars to treat in the US alone.

METHODS

To accelerate drug discovery, we screened a library of 1581 existing FDA approved drugs, as well as drugs approved abroad, for inhibitors of C. albicans. The screen was done on YPD yeast growth media as well as on the serum plate assay developed in this study.

RESULTS

We discovered that fifteen drugs, all which were originally approved for treating various infectious and non-infectious diseases, were able to kill Candida albicans. Additionally, one of those drugs, Octodrine, displays wide-spectrum anti-microbial activity. Compared to other selected anti-Candida drugs, Octodrine was shown to be one of the most effective drugs in killing serum-grown Candida albicans without significantly affecting the survival of host macrophages and skin cells.

CONCLUSIONS

This approach is useful for the discovery of economically viable new therapies against infectious diseases.

Bidirectional effect of Wnt signaling antagonist DKK1 on the modulation of anthrax toxin uptake

LRP6, a co-receptor for the morphogen Wnt, aids endocytosis of anthrax complexes. Here we report that Dickkopf1 (DKK1) protein, a secreted LRP6 ligand and antagonist, is also a modulator of anthrax toxin sensitivity. shRNA-mediated gene silencing or TALEN-mediated gene knockout of DKK1 reduced sensitivity of cells to PA-dependent hybrid toxins. However, unlike the solely inhibitory effect on Wnt signaling, the effects of DKK1 overexpression on anthrax toxicity were bidirectional, depending on its endogenous expression and cell context. Fluorescence microscopy and biochemical analyses showed that DKK1 facilitates internalization of anthrax toxins and their receptors, an event mediated by DKK1-LRP6-Kremen2 complex. Monoclonal antibodies against DKK1 provided dose-dependent protection to macrophages from killing by anthrax lethal toxin (LT). Our discovery that DKK1 forms ternary structure with LRP6 and Kremen2 in promoting PA-mediated toxin internalization provides a paradigm for bacterial exploitation of mechanisms that host cells use to internalize signaling proteins.

Designing inhibitors of anthrax toxin

INTRODUCTION

Present-day rational drug design approaches are based on exploiting unique features of the target biomolecules, small- or macromolecule drug candidates and physical forces that govern their interactions. The 2013 Nobel Prize in chemistry awarded 'for the development of multiscale models for complex chemical systems' once again demonstrated the importance of the tailored drug discovery that reduces the role of the trial-and-error approach to a minimum. The intentional dissemination of Bacillus anthracis spores in 2001 via the so-called anthrax letters has led to increased efforts, politically and scientifically, to develop medical countermeasures that will protect people from the threat of anthrax bioterrorism.

AREAS COVERED

This article provides an overview of the recent rational drug design approaches for discovering inhibitors of anthrax toxin. The review also directs the readers to the vast literature on the recognized advances and future possibilities in the field.

EXPERT OPINION

Existing options to combat anthrax toxin lethality are limited. With the only anthrax toxin inhibiting therapy (protective antigen-targeting with a monoclonal antibody, raxibacumab) approved to treat inhalational anthrax, the situation, in our view, is still insecure. Further, the FDA's animal rule for drug approval, which clears compounds without validated efficacy studies on humans, creates a high level of uncertainty, especially when a well-characterized animal model does not exist. Better identification and validation of anthrax toxin therapeutic targets at the molecular level as well as elucidation of the parameters determining the corresponding therapeutic windows are still necessary for more effective therapeutic options.