Calpain regulates enterocyte brush border actin assembly and pathogenic Escherichia coli-mediated effacement

Type III Secreted Virulence Factors Manipulating Signaling to Actin Dynamics

A key aspect of bacterial pathogenesis is the colonization and persistence within the host and, later on, its dissemination to new niches. During evolution, bacteria developed a myriad of virulence mechanisms to usurp the host's sophisticated defense mechanisms in order to establish their colonization niche. Elucidation of the highly dynamic and complex interactions between host and pathogens remains an important field of study. Here, we highlight the conserved manipulation of the actin cytoskeleton by some Gram-negative gastrointestinal pathogens, addressing the role of type III secreted bacterial GEFs at the different steps of pathogenesis. As a final topic, we review cytoskeleton dynamics induced by EPEC/EHEC strains for pedestal formation.

The EHEC-host interactome reveals novel targets for the translocated intimin receptor

Enterohemorrhagic E. coli (EHEC) manipulate their human host through at least 39 effector proteins which hijack host processes through direct protein-protein interactions (PPIs). To identify their protein targets in the host cells, we performed yeast two-hybrid screens, allowing us to find 48 high-confidence protein-protein interactions between 15 EHEC effectors and 47 human host proteins. In comparison to other bacteria and viruses we found that EHEC effectors bind more frequently to hub proteins as well as to proteins that participate in a higher number of protein complexes. The data set includes six new interactions that involve the translocated intimin receptor (TIR), namely HPCAL1, HPCAL4, NCALD, ARRB1, PDE6D, and STK16. We compared these TIR interactions in EHEC and enteropathogenic E. coli (EPEC) and found that five interactions were conserved. Notably, the conserved interactions included those of serine/threonine kinase 16 (STK16), hippocalcin-like 1 (HPCAL1) as well as neurocalcin-delta (NCALD). These proteins co-localize with the infection sites of EPEC. Furthermore, our results suggest putative functions of poorly characterized effectors (EspJ, EspY1). In particular, we observed that EspJ is connected to the microtubule system while EspY1 appears to be involved in apoptosis/cell cycle regulation.

Store-operated Ca2+ entry (SOCE) regulates melanoma proliferation and cell migration

Store-operated Ca²⁺ entry (SOCE) is a major mechanism of Ca²⁺ import from extracellular to intracellular space, involving detection of Ca²⁺ store depletion in endoplasmic reticulum (ER) by stromal interaction molecule (STIM) proteins, which then translocate to plasma membrane and activate Orai Ca²⁺ channels there. We found that STIM1 and Orai1 isoforms were abundantly expressed in human melanoma tissues and multiple melanoma/melanocyte cell lines. We confirmed that these cell lines exhibited SOCE, which was inhibited by knockdown of STIM1 or Orai1, or by a pharmacological SOCE inhibitor. Inhibition of SOCE suppressed melanoma cell proliferation and migration/metastasis. Induction of SOCE was associated with activation of extracellular-signal-regulated kinase (ERK), and was inhibited by inhibitors of calmodulin kinase II (CaMKII) or Raf-1, suggesting that SOCE-mediated cellular functions are controlled via the CaMKII/Raf-1/ERK signaling pathway. Our findings indicate that SOCE contributes to melanoma progression, and therefore may be a new potential target for treatment of melanoma, irrespective of whether or not Braf mutation is present.

Calpain activation by the Shigella flexneri effector VirA regulates key steps in the formation and life of the bacterium's epithelial niche

The enteropathogen Shigella flexneri invades epithelial cells, leading to inflammation and tissue destruction. We report that Shigella infection of epithelial cells induces an early genotoxic stress, but the resulting p53 response and cell death are impaired due to the bacterium's ability to promote p53 degradation, mainly through calpain protease activation. Calpain activation is promoted by the Shigella virulence effector VirA and dependent on calcium flux and the depletion of the endogenous calpain inhibitor calpastatin. Further, although VirA-induced calpain activity is critical for regulating cytoskeletal events driving bacterial uptake, calpain activation ultimately leads to necrotic cell death, thereby restricting Shigella intracellular growth. Therefore, calpains work at multiple steps in regulating Shigella pathogenesis by disrupting the p53-dependent DNA repair response early during infection and regulating both formation and ultimate death of the Shigella epithelial replicative niche.

Accelerated growth rate induced by neonatal high-protein milk formula is not supported by increased tissue protein synthesis in low-birth-weight piglets

Low-birth-weight neonates are routinely fed a high-protein formula to promote catch-up growth and antibiotics are usually associated to prevent infection. Yet the effects of such practices on tissue protein metabolism are unknown. Baby pigs were fed from age 2 to 7 or 28 d with high protein formula with or without amoxicillin supplementation, in parallel with normal protein formula, to determine tissue protein metabolism modifications. Feeding high protein formula increased growth rate between 2 and 28 days of age when antibiotic was administered early in the first week of life. This could be explained by the occurrence of diarrhea when piglets were fed the high protein formula alone. Higher growth rate was associated with higher feed conversion and reduced protein synthesis rate in the small intestine, muscle and carcass, whereas proteolytic enzyme activities measured in these tissues were unchanged. In conclusion, accelerated growth rate caused by high protein formula and antibiotics was not supported by increased protein synthesis in muscle and carcass.

Calpain mediates epithelial cell microvillar effacement by enterohemorrhagic Escherichia coli

A member of the attaching and effacing (AE) family of pathogens, enterohemorrhagic Escherichia coli (EHEC) induces dramatic changes to the intestinal cell cytoskeleton, including effacement of microvilli. Effacement by the related pathogen enteropathogenic E. coli (EPEC) requires the activity of the Ca⁺²-dependent host protease, calpain, which participates in a variety of cellular processes, including cell adhesion and motility. We found that EHEC infection results in an increase in epithelial (CaCo-2a) cell calpain activity and that EHEC-induced microvillar effacement was blocked by ectopic expression of calpastatin, an endogenous calpain inhibitor, or by pretreatment of intestinal cells with a cell-penetrating version of calpastatin. In addition, ezrin, a known calpain substrate that links the plasma membrane to axial actin filaments in microvilli, was cleaved in a calpain-dependent manner during EHEC infection and lost from its normal locale within microvilli. Calpain may be a central conduit through which EHEC and other AE pathogens induce enterocyte cytoskeletal remodeling and exert their pathogenic effects.

Involvement of host calpain in the invasion of Cryptosporidium parvum

Cryptosporidium parvum induces the formation of an actin-dense plaque which is essential for the successful invasion of epithelial cells. Host molecules that are involved in the regulation of this cytoskeleton reorganization are unknown. Here we identified that calcium-dependent thiol protease calpain is critical for regulating parasite-induced actin polymerization. C. parvum invasion induced activation of calpain. Inhibition of calpain activity by overexpression of the endogenous inhibitor calpastatin diminished the formation of the actin-dense plaque and decreased the initial invasion of parasites. Our data indicates a key role of calpain activity of host cell in C. parvum infection via regulating cytoskeleton reorganization.

Calpains expression during Xenopus laevis development

Calpains are cytoplasmic proteases activated by calcium, implicated in cell differentiation and apoptosis. The best characterized enzymes are calpains 1-3. The aim of this work was to localize calpains 1-3 during the development of Xenopus laevis in order to clarify the function of these three proteases. For the first time, we detected the localization of the three proteases at the protein level between one-cell stage and adult age. Their expression was weak at early stages, then increased at tadpole stage and decreased through metamorphosis and adult life. The calpain's expression was maximal during the period characterized by the appearance of organs and modelling process. These observations suggest that calpains play a crucial role during development.

The bacterial effectors EspG and EspG2 induce a destructive calpain activity that is kept in check by the co-delivered Tir effector

Bacterial pathogens deliver multiple effector proteins into eukaryotic cells to subvert host cellular processes and an emerging theme is the cooperation between different effectors. Here, we reveal that a fine balance exists between effectors that are delivered by enteropathogenic E. coli (EPEC) which, if perturbed can have marked consequences on the outcome of the infection. We show that absence of the EPEC effector Tir confers onto the bacterium a potent ability to destroy polarized intestinal epithelia through extensive host cell detachment. This process was dependent on the EPEC effectors EspG and EspG2 through their activation of the host cysteine protease calpain. EspG and EspG2 are shown to activate calpain during EPEC infection, which increases significantly in the absence of Tir – leading to rapid host cell loss and necrosis. These findings reveal a new function for EspG and EspG2 and show that Tir, independent of its bacterial ligand Intimin, is essential for maintaining the integrity of the epithelium during EPEC infection by keeping the destructive activity of EspG and EspG2 in check.

Transcriptional patterns in both host and bacterium underlie a daily rhythm of anatomical and metabolic change in a beneficial symbiosis

Mechanisms for controlling symbiont populations are critical for maintaining the associations that exist between a host and its microbial partners. We describe here the transcriptional, metabolic, and ultrastructural characteristics of a diel rhythm that occurs in the symbiosis between the squid Euprymna scolopes and the luminous bacterium Vibrio fischeri. The rhythm is driven by the host's expulsion from its light-emitting organ of most of the symbiont population each day at dawn. The transcriptomes of both the host epithelium that supports the symbionts and the symbiont population itself were characterized and compared at four times over this daily cycle. The greatest fluctuation in gene expression of both partners occurred as the day began. Most notable was an up-regulation in the host of >50 cytoskeleton-related genes just before dawn and their subsequent down-regulation within 6 h. Examination of the epithelium by TEM revealed a corresponding restructuring, characterized by effacement and blebbing of its apical surface. After the dawn expulsion, the epithelium reestablished its polarity, and the residual symbionts began growing, repopulating the light organ. Analysis of the symbiont transcriptome suggested that the bacteria respond to the effacement by up-regulating genes associated with anaerobic respiration of glycerol; supporting this finding, lipid analysis of the symbionts' membranes indicated a direct incorporation of host-derived fatty acids. After 12 h, the metabolic signature of the symbiont population shifted to one characteristic of chitin fermentation, which continued until the following dawn. Thus, the persistent maintenance of the squid-vibrio symbiosis is tied to a dynamic diel rhythm that involves both partners.

Calpastatin overexpression attenuates amyloid-beta-peptide toxicity in differentiated PC12 cells

Amyloid beta peptide (Abeta) plays a major role in the pathogenesis of Alzheimer's disease (AD). Abeta is toxic to neurons, possibly through causing initial synaptic dysfunction and neuronal membrane dystrophy, promoted by increased cellular Ca(2+). Calpain (Ca(2+)-dependent protease) and caspase have been implicated in AD. Previously, we used calpain and caspase pharmacological inhibitors to study effects of Abeta25-35 (sAbeta) on neuronal-like differentiated PC12 cells. We reported that sAbeta-treated cells exhibited calpain activation and protein degradation (due to both calpain and caspase-8). We have now found that overexpression of the calpain specific inhibitor calpastatin in differentiated PC12 cells significantly inhibited the sAbeta-induced calpain activation and decreased the protease activity. Calpastatin overexpression inhibited the sAbeta-promoted degradation of fodrin, protein kinase Cepsilon, beta-catenin (membrane structural proteins and proteins involved in signal transduction pathways), and prevented the sAbeta-induced alteration of neurite structure (manifested by varicosities). Overexpression of calpastatin also inhibited Ca(2+)-promoted calpain activation and protein degradation; this is consistent with the notion that the Abeta-induced increase in calpain activity results from a rise in cellular Ca(2+), provided the calpastatin level is not so high as to strongly inhibit calpain. Carrying out transfection without selection allowed the comparison in the same culture of calpastatin-overexpressing with non-overexpressing cells. In cultures transfected with green fluorescent protein (GFP)-calpastatin plasmid, calpastatin overexpression (indicated by GFP-labeling) led to inhibition in sAbeta-induced membrane propidium iodide (PI) permeability, whereas non-transfected, GFP-unlabeled cells exhibited PI permeability. Overall, the results demonstrate that the effects of Abeta-toxicity studied here were attenuated to a large extent by calpastatin overexpression, indicating that the protease calpain is involved in Abeta-toxicity (obviating a primary, direct role for caspases). Increased expression of calpastatin and/or decrease in calpain may serve as one of the means for ameliorating some of the early symptoms of AD.

Effects of HIV protease inhibitor ritonavir on Akt-regulated cell proliferation in breast cancer

PURPOSE

These studies were designed to determine whether ritonavir inhibits breast cancer in vitro and in vivo and, if so, how.

EXPERIMENTAL DESIGN

Ritonavir effects on breast cancer cell growth were studied in the estrogen receptor (ER)-positive lines MCF7 and T47D and in the ER-negative lines MDA-MB-436 and MDA-MB-231. Effects of ritonavir on Rb-regulated and Akt-mediated cell proliferation were studied. Ritonavir was tested for inhibition of a mammary carcinoma xenograft.

RESULTS

ER-positive estradiol-dependent lines (IC50, 12-24 micromol/L) and ER-negative (IC50, 45 micromol/L) lines exhibit ritonavir sensitivity. Ritonavir depletes ER-alpha levels notably in ER-positive lines. Ritonavir causes G1 arrest, depletes cyclin-dependent kinases 2, 4, and 6 and cyclin D1 but not cyclin E, and depletes phosphorylated Rb and Ser473 Akt. Ritonavir induces apoptosis independent of G1 arrest, inhibiting growth of cells that have passed the G1 checkpoint. Myristoyl-Akt, but not activated K-Ras, rescues ritonavir inhibition. Ritonavir inhibited a MDA-MB-231 xenograft and intratumoral Akt activity at a clinically attainable serum Cmax of 22 +/- 8 micromol/L. Because heat shock protein 90 (Hsp90) substrates are depleted by ritonavir, ritonavir effects on Hsp90 were tested. Ritonavir binds Hsp90 (K(D), 7.8 micromol/L) and partially inhibits its chaperone function. Ritonavir blocks association of Hsp90 with Akt and, with sustained exposure, notably depletes Hsp90. Stably expressed Hsp90alpha short hairpin RNA also depletes Hsp90, inhibiting proliferation and sensitizing breast cancer cells to low ritonavir concentrations.

CONCLUSIONS

Ritonavir inhibits breast cancer growth in part by inhibiting Hsp90 substrates, including Akt. Ritonavir may be of interest for breast cancer therapeutics and its efficacy may be increased by sustained exposure or Hsp90 RNA interference.

Regulation of proteolysis by cytokines in the human intestinal epithelial cell line HCT-8: role of IFNgamma

Protein metabolism contributes in the regulation of gut barrier function, which may be altered during inflammatory states. There are three major proteolytic pathways in mammalian cells: lysosomal, Ca(2+)-activated and ubiquitin-proteasome. The regulation of proteolytic activities during inflammation remains unknown in intestine. Intestinal epithelial cells, HCT-8, were stimulated by IL-1beta, IFNgamma and TNFalpha each alone or in combination (Cytomix). Proteolytic activities were assessed using fluorogenic substrates and specific inhibitors, protein expressions by Western blot. Lysosomal and Ca(2+)-activated pathways were not significantly altered by any treatment. In contrast, the activity of ubiquitin-proteasome system was stimulated by IFNgamma and Cytomix (155, 160 versus 100, P<0.05, respectively) but remained unaffected by IL-1beta and TNFalpha. Free ubiquitin expression, but not ubiquitinated proteins, was enhanced by IFNgamma and Cytomix. The expression of proteasome 20S alpha1 subunit, a constitutive proteasome 20S subunit, was not altered, beta5 subunit expression was weakly decreased by Cytomix and inducible beta5i subunit expression was markedly increased in response to IFNgamma and to Cytomix (202, 206 versus 100, P<0.05, respectively). In conclusion, lysosomal, Ca(2+)-activated and constitutive proteasome activities were not affected by IL-1beta, IFNgamma and TNFalpha alone or in combination, in HCT-8 cells. These results suggest that IFNgamma, but not IL-1beta and TNFalpha, increases immunoproteasome, which might contribute to enhanced antigen presentation during inflammatory bowel diseases.

Overexpression of calpastatin inhibits L8 myoblast fusion

The formation of skeletal muscle fibers involves cessation of myoblast division, myoblast alignment, and fusion to multinucleated myofibers. Calpain is one of the factors shown to be involved in myoblast fusion. Using L8 rat myoblasts, we found that calpain levels did not change significantly during myoblast differentiation, whereas calpastatin diminished prior to myoblast fusion and reappeared after fusion. The transient diminution in calpastatin allows the Ca2+-promoted activation of calpain and calpain-induced membrane proteolysis, which is required for myoblast fusion. Here we show that calpastatin overexpression in L8 myoblasts does not inhibit cell proliferation and alignment, but prevents myoblast fusion and fusion-associated protein degradation. In addition, calpastatin appears to modulate myogenic gene expression, as indicated by the lack of myogenin (a transcription factor expressed in differentiating myoblasts) in myoblasts overexpressing calpastatin. These results suggest that, in addition to the role in membrane disorganization in the fusing myoblasts, the calpain-calpastatin system may also modulate the levels of factors required for myoblast differentiation.

Ritonavir does not inhibit calpain in vitro

Ritonavir, an inhibitor of HIV-1 protease, has been reported to also inhibit the Ca2+-dependent cysteine protease, calpain. We have investigated these claims with an in vitro study of the effect of ritonavir on the m-calpain and mu-calpain isoforms. Ritonavir failed to block either autolytic or hydrolytic calpain activity, but remained fully capable of inhibiting the HIV-1 protease. Any calpain-related effects of ritonavir in cells must, therefore, arise by a mechanism other than direct inhibition of calpains.

Antiretroviral protease inhibitors prevent l6 muscle cell fusion by reducing calpain activity

The antiretroviral protease inhibitors indinavir (IDV) and ritonavir (RTV) are used in highly active antiretroviral therapies (HAART). Side effects from long-term HAART therapy include loss of muscle mass. Myoblasts when cultured in media low in growth factors withdraw from the cell cycle, express muscle-specific differentiation inducers and proteins, and fuse to form myotubes. The neutral protease, calpain, is required for myotube formation and RTV decreased calpain activity in vitro. We found lower calpain activity, but not protein, in homogenates of RTV-treated L6 cells than in control cultures. Importantly, L6 and C2C12 myoblasts did not form myotubes when cultured with 10 or 20 microM IDV or RTV. Control and drug-related L6 myoblasts showed identical decreases in proliferating cell nuclear antigens expression indicating proliferation arrest. Similarly, muscle differentiation inducers MyoD and myogenin and their downstream target, myosin heavy chain, were expressed at similar levels in control and drug-treated cells. Thus, whereas muscle differentiation was unaffected by protease inhibitors, calpain activity was reduced and myotube formation prevented. We conclude that RTV and IDV reduced myotube formation by reducing calpain activity. Our data suggest that protease inhibitors included in HAART might be directly involved in muscle wasting by reducing muscle remodeling.

m-Calpain implication in cell cycle during muscle precursor cell activation

Milli-calpain, a member of the ubiquitous cysteine protease family, is known to control late events of cell-cell fusion in skeletal muscle tissue through its involvement in cell membrane and cytoskeleton component reorganization. In this report, we describe the characterization of m-calpain compartmentalization and activation during the initial steps of muscle precursor cell recruitment and differentiation. By immunofluorescence analysis, we show that m-calpain is present throughout the cell cycle in the nucleus of proliferating myoblast C2 cells. However, when myoblasts enter a quiescent/G0 stage, m-calpain staining is detected only in the cytoplasm. Moreover, comparison of healthy and injured muscle shows distinct m-calpain localization in satellite stem cells. Indeed, m-calpain is not found in quiescent satellite cells, but following muscle injury, when satellite cells start to proliferate, m-calpain appears in the nucleus. To determine the implication of m-calpain during the cell cycle progression, quiescent myoblasts were forced to re-enter the cell cycle in the presence or not of the specific calpain inhibitor MDL 28170. We demonstrate that this calpain inhibitor blocks the cell cycle, prevents accumulation of MyoD in the G1 phase and enhances Myf5 expression. These data support an important new role for m-calpain in the control of muscle precursor cell activation and thus suggest its possible implication during the initial events of muscle regeneration.

Calpain as a multi-site regulator of cell cycle

Calpain has long been implicated in the regulation of cell cycle, mostly based on studies with inhibitors that lack strict specificity toward the enzyme. Further, previous work has primarily focused on one particular point, the G(1) checkpoint, and made no attempt at dissecting the full cycle in terms of calpain action. To extend and complement these findings, we tested the effect of a specific inhibitor, PD 150606, on granulocyte-macrophage-colony stimulating factor (GM-CSF)-stimulated human TF-1 cells by flow cytometry following single- and double labelling by propidium iodide and bromodeoxyuridine. Using a new algorithm of analysis, we determined the time-dependence of the absolute number of cells leaving G(1), S and G(2)M phases following the application of the inhibitor. Our results point to the simultaneous involvement of calpain activity in promoting the cycle at the G(1) checkpoint and somewhere in the G(2)M compartment. Furthermore, the inhibitor significantly impedes the progress of cells through the S phase, indicating calpain activity in S phase checkpoint signalling. Overall, our analysis suggests that calpain regulates the cell cycle at more points than previously thought.

Proteomic analysis of the intestinal epithelial cell response to enteropathogenic Escherichia coli

We present the first large scale proteomic analysis of a human cellular response to a pathogen. Enteropathogenic Escherichia coli (EPEC) is an enteric human pathogen responsible for much childhood morbidity and mortality worldwide. EPEC uses a type III secretion system (TTSS) to inject bacterial proteins into the cytosol of intestinal epithelial cells, resulting in diarrhea. We analyzed the host response to TTSS-delivered EPEC effector proteins by infecting polarized intestinal epithelial monolayers with either wild-type or TTSS-deficient EPEC. Host proteins were isolated and subjected to quantitative profiling using isotope-coded affinity tagging (ICAT) combined with electrospray ionization tandem mass spectrometry. We identified over 2000 unique proteins from infected Caco-2 monolayers, of which approximately 13% are expressed differentially in the presence of TTSS-delivered EPEC effector proteins. We validated these data in silico and through immunoblotting and immunofluorescence microscopy. The identified changes extend cytoskeletal observations made in less relevant cell types and generate testable hypotheses with regard to host proteins potentially involved in EPEC-induced diarrhea. These data provide a framework for future biochemical analyses of host-pathogen interactions.