Transglutaminase is essential in receptor-mediated endocytosis of alpha 2-macroglobulin and polypeptide hormones

Defining the conformational states that enable transglutaminase 2 to promote cancer cell survival versus cell death

Transglutaminase 2 (TG2) is a GTP-binding/protein-crosslinking enzyme that has been investigated as a therapeutic target for Celiac disease, neurological disorders, and aggressive cancers. TG2 has been suggested to adopt two conformational states that regulate its functions: a GTP-bound, closed conformation, and a calcium-bound, crosslinking-active open conformation. TG2 mutants that constitutively adopt an open conformation are cytotoxic to cancer cells. Thus, small molecules that maintain the open conformation of TG2 could offer a new therapeutic strategy. Here, we investigate TG2, using static and time-resolved small-angle X-ray scattering (SAXS) and single-particle cryoelectron microscopy (cryo-EM), to determine the conformational states responsible for conferring its biological effects. We also describe a newly developed TG2 inhibitor, LM11, that potently kills glioblastoma cells and use SAXS to investigate how LM11 affects the conformational states of TG2. Using SAXS and cryo-EM, we show that guanine nucleotide-bound TG2 adopts a monomeric closed conformation while calcium-bound TG2 assumes an open conformational state that can form higher order oligomers. SAXS analysis also suggests how a TG2 mutant that constitutively adopts the open state binds nucleotides through an alternative mechanism to wildtype TG2. Furthermore, we use time-resolved SAXS to show that LM11 increases the ability of calcium to drive TG2 to an open conformation, which is not reversible by guanine nucleotides and is cytotoxic to cancer cells. Taken together, our findings demonstrate that the conformational dynamics of TG2 are more complex than previously suggested and highlight how conformational stabilization of TG2 by LM11 maintains TG2 in a cytotoxic conformational state.

Proteomics reveals time-dependent protein corona changes in the intracellular pathway

The intracellular protein corona has not been fully investigated in the field of nanotechnology-biology (nano-bio) interactions. To effectively understand intracellular protein corona formation and dynamics, we established a workflow to isolate the intracellular protein corona at different uptake times of two nanoparticles - magnetic hydroxyethyl starch nanoparticles (HES-NPs) and magnetic human serum albumin nanocapsules (HSA-NCs). We performed label-free quantitative LC-MS proteomics to analyze the composition of the intracellular protein corona and correlated our findings with results from conventional methods for intracellular trafficking of nanocarriers, such as flow cytometry, transmission electron microscopy (TEM), and confocal microscopy (cLSM). We determined the evolution of the intracellular protein corona. At different time stages the protein corona of the HES-NPs with a slower uptake changed, but there were fewer changes in that of the HSA-NCs with a more rapid uptake. We identified proteins that are involved in macropinocytosis (RAC1, ASAP2) as well as caveolin. This was confirmed by blocking experiments and by TEM studies. The investigated nanocarrier predominantly trafficked from early endosomes as determined by RAB5 identification in proteomics and in cLSM to late endosomes/lysosomes (RAB7, LAMP1, cathepsin K and HSP 90-beta) We further demonstrated differences between nanoparticles with slower and faster uptake kinetics and determined the associated proteome at different time points. Analysis of the intracellular protein corona provides us with effective data to examine the intracellular trafficking of nanocarriers used in efficient drug delivery and intracellular applications. STATEMENT OF SIGNIFICANCE: Many research papers focus on the protein corona on nanoparticles formed in biological fluids, but there are hardly any articles dealing with proteins that come in contact with nanoparticles inside cells. The "intracellular protein corona" studied here is a far more complex and highly demanding field. Most nanocarriers are designed to be taken up into cells. Given this, we chose two different nanocarriers to reveal changes in the proteins in dendritic cells during contact at specific times. Further studies will allow us to examine molecular target proteins using these methods. Our research is a significant addition towards the goal of understanding and thus improving the efficacy of drug nanocarriers.

The sulfonadyns: a class of aryl sulfonamides inhibiting dynamin I GTPase and clathrin mediated endocytosis are anti-seizure in animal models

We show that dansylcadaverine (1) a known in-cell inhibitor of clathrin mediated endocytosis (CME), moderately inhibits dynamin I (dynI) GTPase activity (IC50 45 μM) and transferrin (Tfn) endocytosis in U2OS cells (IC50 205 μM). Synthesis gave a new class of GTP-competitive dynamin inhibitors, the Sulfonadyns™. The introduction of a terminal cinnamyl moiety greatly enhanced dynI inhibition. Rigid diamine or amide links between the dansyl and cinnamyl moieties were detrimental to dynI inhibition. Compounds with in vitro inhibition of dynI activity <10 μM were tested in-cell for inhibition of CME. These data unveiled a number of compounds, e.g. analogues 33 ((E)-N-(6-{[(3-(4-bromophenyl)-2-propen-1-yl]amino}hexyl)-5-isoquinolinesulfonamide)) and 47 ((E)-N-(3-{[3-(4-bromophenyl)-2-propen-1-yl]amino}propyl)-1-naphthalenesulfonamide)isomers that showed dyn IC50 <4 μM, IC50(CME) <30 μM and IC50(SVE) from 12-265 μM. Both analogues (33 and 47) are at least 10 times more potent that the initial lead, dansylcadaverine (1). Enzyme kinetics revealed these sulfonamide analogues as being GTP competitive inhibitors of dynI. Sulfonadyn-47, the most potent SVE inhibitor observed (IC50(SVE) = 12.3 μM), significantly increased seizure threshold in a 6 Hz mouse psychomotor seizure test at 30 (p = 0.003) and 100 mg kg-1 ip (p < 0.0001), with similar anti-seizure efficacy to the established anti-seizure medication, sodium valproate (400 mg kg-1). The Sulfonadyn™ class of drugs target dynamin and show promise as novel leads for future anti-seizure medications.

LRP-1 links post-translational modifications to efficient presentation of celiac disease-specific T cell antigens

Celiac disease (CeD) is an autoimmune disorder in which gluten-derived antigens trigger inflammation. Antigenic peptides must undergo site-specific deamidation to be presentable to CD4+ T cells in an HLA-DQ2 or -DQ8 restricted manner. While the biochemical basis for this post-translational modification is understood, its localization in the patient's intestine remains unknown. Here, we describe a mechanism by which gluten peptides undergo deamidation and concentration in the lysosomes of antigen-presenting cells, explaining how the concentration of gluten peptides necessary to elicit an inflammatory response in CeD patients is achieved. A ternary complex forms between a gluten peptide, transglutaminase-2 (TG2), and ubiquitous plasma protein α2-macroglobulin, and is endocytosed by LRP-1. The covalent TG2-peptide adduct undergoes endolysosomal decoupling, yielding the expected deamidated epitope. Our findings invoke a pathogenic role for dendritic cells and/or macrophages in CeD and implicate TG2 in the lysosomal clearance of unwanted self and foreign extracellular proteins.

Transglutaminase 2: Development of therapeutic antibodies reveals four inhibitory epitopes and confirms extracellular function in fibrotic remodelling

BACKGROUND AND PURPOSE

Transglutaminase type 2 (TG2) catalyses formation of ε-(γ-glutamyl)-lysine bonds between proteins, including those of the extracellular matrix (ECM). Elevated extracellular TG2 leads to accelerated ECM deposition and reduced clearance that underlies tissue scarring and fibrosis. Many transglutaminase inhibitors exist and allowed for proof-of-concept studies in disease models, but their lack of specificity for the TG2 isoform, and/or poor pharmacokinetic/pharmacodynamic properties have limited their clinical application. We hypothesised that a high affinity TG2-specific antibody could be developed to specifically inhibit extracellular TG2 activity, with characteristics suitable for therapeutic development.

EXPERIMENTAL APPROACH

Individual human TG2 domains were used to immunise mice and generate hybridomas. Supernatants were screened for inhibition of recombinant human TG2 activity, with TG2 specificity determined by ELISA.

KEY RESULTS

Thirteen TG2-specific supernatants inhibited human transamidation activity. Each hybridoma was cloned and antibody mapped to an epitope in the TG2 core domain, using phage display panning of a TG2 fragment library. Four distinct inhibitory epitopes were determined. The most effective antibodies (AB1, DC1 and BB7) bound to amino acids 313-327 (catalytic core), with an IC₅₀ of approximately 10 nM. The antibodies inhibit TG2 in human cells and block ECM accumulation in a primary human proximal tubular epithelial cell model of fibrosis, only 7 antibodies inhibited rat TG2, and all with higher IC₅₀ values.

CONCLUSIONS AND IMPLICATIONS

We identified a preferred inhibitory epitope in human TG2, developed antibodies with required characteristics for clinical development, and established that targeted inhibition of extracellular TG2 transamidation activity is sufficient to modify fibrotic remodelling.

Understanding nanoparticle endocytosis to improve targeting strategies in nanomedicine

Nanoparticles (NPs) have attracted considerable attention in various fields, such as cosmetics, the food industry, material design, and nanomedicine. In particular, the fast-moving field of nanomedicine takes advantage of features of NPs for the detection and treatment of different types of cancer, fibrosis, inflammation, arthritis as well as neurodegenerative and gastrointestinal diseases. To this end, a detailed understanding of the NP uptake mechanisms by cells and intracellular localization is essential for safe and efficient therapeutic applications. In the first part of this review, we describe the several endocytic pathways involved in the internalization of NPs and we discuss the impact of the physicochemical properties of NPs on this process. In addition, the potential challenges of using various inhibitors, endocytic markers and genetic approaches to study endocytosis are addressed along with the principal (semi) quantification methods of NP uptake. The second part focuses on synthetic and bio-inspired substances, which can stimulate or decrease the cellular uptake of NPs. This approach could be interesting in nanomedicine where a high accumulation of drugs in the target cells is desirable and clearance by immune cells is to be avoided. This review contributes to an improved understanding of NP endocytic pathways and reveals potential substances, which can be used in nanomedicine to improve NP delivery.

Tissue Transglutaminase Expression Associates With Progression of Multiple Sclerosis

Objective

The clinical course of multiple sclerosis (MS) is variable and largely unpredictable pointing to an urgent need for markers to monitor disease activity and progression. Recent evidence revealed that tissue transglutaminase (TG2) is altered in patient-derived monocytes. We hypothesize that blood cell–derived TG2 messenger RNA (mRNA) can potentially be used as biomarker in patients with MS.

Methods

In peripheral blood mononuclear cells (PBMCs) from 151 healthy controls and 161 patients with MS, TG2 mRNA was measured and correlated with clinical and MRI parameters of disease activity (annualized relapse rate, gadolinium-enhanced lesions, and T2 lesion volume) and disease progression (Expanded Disability Status Scale [EDSS], normalized brain volume, and hypointense T1 lesion volume).

Results

PBMC-derived TG2 mRNA levels were significantly associated with disease progression, i.e., worsening of the EDSS over 2 years of follow-up, normalized brain volume, and normalized gray and white matter volume in the total MS patient group at baseline. Of these, in patients with relapsing-remitting MS, TG2 expression was significantly associated with worsening of the EDSS scores over 2 years of follow-up. In the patients with primary progressive (PP) MS, TG2 mRNA levels were significantly associated with EDSS, normalized brain volume, and normalized gray and white matter volume at baseline. In addition, TG2 mRNA associated with T1 hypointense lesion volume in the patients with PP MS at baseline.

Conclusion

PBMC-derived TG2 mRNA levels hold promise as biomarker for disease progression in patients with MS.

Classification of Evidence

This study provides Class II evidence that in patients with MS, PBMC-derived TG2 mRNA levels are associated with disease progression.

Host membrane lipids are trafficked to membranes of intravacuolar bacterium Ehrlichia chaffeensis

Ehrlichia chaffeensis, a cholesterol-rich and cholesterol-dependent obligate intracellular bacterium, partially lacks genes for glycerophospholipid biosynthesis. We found here that E. chaffeensis is dependent on host glycerolipid biosynthesis, as an inhibitor of host long-chain acyl CoA synthetases, key enzymes for glycerolipid biosynthesis, significantly reduced bacterial proliferation. E. chaffeensis cannot synthesize phosphatidylcholine or cholesterol but encodes enzymes for phosphatidylethanolamine (PE) biosynthesis; however, exogenous NBD-phosphatidylcholine, Bodipy-PE, and TopFluor-cholesterol were rapidly trafficked to ehrlichiae in infected cells. DiI (3,3'-dioctadecylindocarbocyanine)-prelabeled host-cell membranes were unidirectionally trafficked to Ehrlichia inclusion and bacterial membranes, but DiI-prelabeled Ehrlichia membranes were not trafficked to host-cell membranes. The trafficking of host-cell membranes to Ehrlichia inclusions was dependent on both host endocytic and autophagic pathways, and bacterial protein synthesis, as the respective inhibitors blocked both infection and trafficking of DiI-labeled host membranes to Ehrlichia In addition, DiI-labeled host-cell membranes were trafficked to autophagosomes induced by the E. chaffeensis type IV secretion system effector Etf-1, which traffic to and fuse with Ehrlichia inclusions. Cryosections of infected cells revealed numerous membranous vesicles inside inclusions, as well as multivesicular bodies docked on the inclusion surface, both of which were immunogold-labeled by a GFP-tagged 2×FYVE protein that binds to phosphatidylinositol 3-phosphate. Focused ion-beam scanning electron microscopy of infected cells validated numerous membranous structures inside bacteria-containing inclusions. Our results support the notion that Ehrlichia inclusions are amphisomes formed through fusion of early endosomes, multivesicular bodies, and early autophagosomes induced by Etf-1, and they provide host-cell glycerophospholipids and cholesterol that are necessary for bacterial proliferation.

Intracellular Localization of Microbial Transglutaminase and Its Influence on the Transport of Gliadin in Enterocytes

OBJECTIVE

Celiac disease (CD) is a systemic inflammatory disorder, characterized by the destruction of duodenal epithelium. The CD8 T cells involved are associated with cross-presentation. In addition to other factors, the rising prevalence of CD might be induced by microbial transglutaminase (mTG) an enzyme frequently used in food production that shares enzymatic and antigenic properties of tissue transglutaminase (TG2), the autoantigen in CD. We hypothesized that mTG and gliadin are transported into the endoplasmic reticulum (ER), indicating cross-presentation of both antigens.

METHODS

Apical incubation of duodenal biopsies from CD and control patients was performed with mTG alone or with mTG and simultaneously with Frazer's fraction. Evaluation was carried out by immunofluorescence and electron microscopy.

RESULTS

Approximately 6% to 9% of the intracellular mTG and gliadin were transported to the ER of enterocytes. RACE cells (Rapid uptake of Antigen into the Cytosol of Enterocytes) displayed an enhanced antigen uptake into a dilated ER. mTG strongly localized at the basolateral membrane and the lamina propria.

CONCLUSIONS

mTG and gliadin are transported to the ER of enterocytes and to a greater extent to the ER of RACE cells, suggesting cross-presentation of exogenous antigens. The strong localization of mTG at the basolateral membrane and the lamina propria may also indicate a potential antigenic interaction with cells of the immune system. Since mTG may not only been taken up with food stuffs but could also be released by bacteria within the intestinal microbiota, further investigations are needed regarding the role of mTG in CD pathogenesis.

A pathogenic role for cystic fibrosis transmembrane conductance regulator in celiac disease

Intestinal handling of dietary proteins usually prevents local inflammatory and immune responses and promotes oral tolerance. However, in ~ 1% of the world population, gluten proteins from wheat and related cereals trigger an HLA DQ2/8‐restricted T_H1 immune and antibody response leading to celiac disease. Prior epithelial stress and innate immune activation are essential for breaking oral tolerance to the gluten component gliadin. How gliadin subverts host intestinal mucosal defenses remains elusive. Here, we show that the α‐gliadin‐derived LGQQQPFPPQQPY peptide (P31–43) inhibits the function of cystic fibrosis transmembrane conductance regulator (CFTR), an anion channel pivotal for epithelial adaptation to cell‐autonomous or environmental stress. P31–43 binds to, and reduces ATPase activity of, the nucleotide‐binding domain‐1 (NBD1) of CFTR, thus impairing CFTR function. This generates epithelial stress, tissue transglutaminase and inflammasome activation, NF‐κB nuclear translocation and IL‐15 production, that all can be prevented by potentiators of CFTR channel gating. The CFTR potentiator VX‐770 attenuates gliadin‐induced inflammation and promotes a tolerogenic response in gluten‐sensitive mice and cells from celiac patients. Our results unveil a primordial role for CFTR as a central hub orchestrating gliadin activities and identify a novel therapeutic option for celiac disease.

Multifactorial Regulation of G Protein-Coupled Receptor Endocytosis

Endocytosis is a process by which cells absorb extracellular materials via the inward budding of vesicles formed from the plasma membrane. Receptor-mediated endocytosis is a highly selective process where receptors with specific binding sites for extracellular molecules internalize via vesicles. G protein-coupled receptors (GPCRs) are the largest single family of plasma-membrane receptors with more than 1000 family members. But the molecular mechanisms involved in the regulation of GPCRs are believed to be highly conserved. For example, receptor phosphorylation in collaboration with β-arrestins plays major roles in desensitization and endocytosis of most GPCRs. Nevertheless, a number of subsequent studies showed that GPCR regulation, such as that by endocytosis, occurs through various pathways with a multitude of cellular components and processes. This review focused on i) functional interactions between homologous and heterologous pathways, ii) methodologies applied for determining receptor endocytosis, iii) experimental tools to determine specific endocytic routes, iv) roles of small guanosine triphosphate-binding proteins in GPCR endocytosis, and v) role of post-translational modification of the receptors in endocytosis.

Selectivity of commonly used inhibitors of clathrin-mediated and caveolae-dependent endocytosis of G protein-coupled receptors

Among the multiple G protein-coupled receptor (GPCR) endocytic pathways, clathrin-mediated endocytosis (CME) and caveolar endocytosis are more extensively characterized than other endocytic pathways. A number of endocytic inhibitors have been used to block CME; however, systemic studies to determine the selectivity of these inhibitors are needed. Clathrin heavy chain or caveolin1-knockdown cells have been employed to determine the specificity of various chemical and molecular biological tools for CME and caveolar endocytosis. Sucrose, concanavalin A, and dominant negative mutants of dynamin blocked other endocytic pathways, in addition to CME. In particular, concanavalin A nonspecifically interfered with the signaling of several GPCRs tested in the study. Decreased pH, monodansylcadaverine, and dominant negative mutants of epsin were more specific for CME than other treatments were. A recently introduced CME inhibitor, Pitstop2™, showed only marginal selectivity for CME and interfered with receptor expression on the cell surface. Blockade of receptor endocytosis by epsin mutants and knockdown of the clathrin heavy chain enhanced the β2AR-mediated ERK activation. Overall, our studies show that previous experimental results should be interpreted with discretion if they included the use of endocytic inhibitors that were previously thought to be CME-selective. In addition, our study shows that endocytosis of β2 adrenoceptor through clathrin-mediated pathway has negative effects on ERK activation.

Cadherin-6B undergoes macropinocytosis and clathrin-mediated endocytosis during cranial neural crest cell EMT

The epithelial-to-mesenchymal transition (EMT) is important for the formation of migratory neural crest cells during development and is co-opted in human diseases such as cancer metastasis. Chick premigratory cranial neural crest cells lose intercellular contacts, mediated in part by Cadherin-6B (Cad6B), migrate extensively, and later form a variety of adult derivatives. Importantly, modulation of Cad6B is crucial for proper neural crest cell EMT. Although Cad6B possesses a long half-life, it is rapidly lost from premigratory neural crest cell membranes, suggesting the existence of post-translational mechanisms during EMT. We have identified a motif in the Cad6B cytoplasmic tail that enhances Cad6B internalization and reduces the stability of Cad6B upon its mutation. Furthermore, we demonstrate for the first time that Cad6B is removed from premigratory neural crest cells through cell surface internalization events that include clathrin-mediated endocytosis and macropinocytosis. Both of these processes are dependent upon the function of dynamin, and inhibition of Cad6B internalization abrogates neural crest cell EMT and migration. Collectively, our findings reveal the significance of post-translational events in controlling cadherins during neural crest cell EMT and migration.

Inhibitors of tissue transglutaminase

Tissue transglutaminase (TG2) catalyzes the cross-linking of proteins by the formation of isopeptide bonds between glutamine (Gln) and lysine (Lys) side chains. Although TG2 is essential for the stabilization of the extracellular matrix, its unregulated activity has been implicated in celiac disease, fibrosis, and cancer metastasis, among other disorders. Given the importance and range of TG2-related pathologies, recent work has focused on the development of potent and selective inhibitors against TG2. In this review, we present the latest and most noteworthy irreversible and reversible inhibitors of TG2, and offer perspectives for the design of future inhibitors, in the hope that lead compounds with therapeutic potential may soon be discovered.

The small GTPase Rap1 promotes cell movement rather than stabilizes adhesion in epithelial cells responding to insulin-like growth factor I

The Ras-related GTPase Rap1 promotes cell adhesion and migration. Although the significance of Rap1 contribution to cell migration is increasingly being recognized, little is known about the biochemical mechanisms driving this process. In the present study, we discovered a previously unidentified regulatory role of insulin-like growth factor type I (IGF-I) receptor (IGF-IR) in CRK Src homology 3 (SH3)-binding guanine-nucleotide-releasing protein (C3G)-Rap1-fascin-actin axis promoting cell movement. We demonstrate that a burst of Rap1 activity, rather than presumed hyperactivation, is imperative for the onset of cell movement. We show that while autophosphorylated IGF-IR signals to C3G to activate Rap1, subsequent IGF-IR internalization promotes gradual inactivation of Rap1 by putative Rap1 GTPase-activating protein (GAP). Additionally, IGF-IR signalling recruits active Rap1 at sites of cell motile protrusions. C3G depletion prevents IGF-I-induced fascin accumulation at actin microspikes and blocks protrusions. In the absence of IGF-IR activity, the wild-type (WT) Rap1 and the constitutively active V12Rap1 mutant remain in cell-cell contacts. Forced inactivation of Rap1 signalling by overexpressing dominant negative N17Rap1, Rap1GAP or by silencing C3G has a detrimental effect on filamentous (F)-actin and cell adhesion irrespective of IGF-IR signalling. We conclude that the basal levels of Rap1 activity holds up cell adhesion, whereas sequential regulation of C3G and GAP by IGF-IR reverses the labile Rap1 function from supporting adhesion to promoting migration.

Transglutaminse 2 and EGGL, the protein cross-link formed by transglutaminse 2, as therapeutic targets for disabilities of old age

Aging of the extracellular matrix (ECM), the protein matrix that surrounds and penetrates the tissues and binds the body together, contributes significantly to functional aging of tissues. ECM proteins become increasingly cross-linked with age, and this cross-linking is probably important in the decline of the ECM's function. This article reviews the role of ε-(γ-glutamyl)-lysine (EGGL), a cross-link formed by transglutaminase enzymes, and particularly the widely expressed isozyme transglutaminase 2 (TG2), in the aging ECM. There is little direct data on EGGL accumulation with age, and no direct evidence of a role of EGGL in the aging of the ECM with pathology. However, several lines of circumstantial evidence suggest that EGGL accumulates with age, and its association with pathology suggests that this might reflect degradation of ECM function. TG activity increases with age in many circumstances. ECM protein turnover is such that some EGGL made by TG is likely to remain in place for years, if not decades, in healthy tissue, and both EGGL and TG levels are enhanced by age-related diseases. If further research shows EGGL does accumulate with age, removing it could be of therapeutic benefit. Also reviewed is the blockade of TG and active removal of EGGL as therapeutic strategies, with the conclusion that both have promise. EGGL removal may have benefit for acute fibrotic diseases, such as tendinopathy, and for treating generalized decline in ECM function with old age. Extracellular TG2 and EGGL are therefore therapeutic targets both for specific and more generalized diseases of aging.

Endocytosis of Mycobacterium tuberculosis heat shock protein 60 is required to induce interleukin-10 production in macrophages

Understanding the signaling pathways involved in the regulation of anti-inflammatory and pro-inflammatory responses in tuberculosis is extremely important in tailoring a macrophage innate response to promote anti-tuberculosis immunity in the host. Although the role of toll-like receptors (TLRs) in the regulation of anti-inflammatory and pro-inflammatory responses is known, the detailed molecular mechanisms by which the Mycobacterium tuberculosis bacteria modulate these innate responses are not clearly understood. In this study, we demonstrate that M. tuberculosis heat shock protein 60 (Mtbhsp60, Cpn60.1, and Rv3417c) interacts with both TLR2 and TLR4 receptors, but its interaction with TLR2 leads to clathrin-dependent endocytosis resulting in an increased production of interleukin (IL)-10 and activated p38 MAPK. Blockage of TLR2-mediated endocytosis inhibited IL-10 production but induced production of tumor necrosis factor (TNF)-α and activated ERK1/2. In contrast, upon interaction with TLR4, Mtbhsp60 remained predominantly localized on the cell surface due to poorer endocytosis of the protein that led to decreased IL-10 production and p38 MAPK activation. The Escherichia coli homologue of hsp60 was found to be retained mainly on the macrophage surface upon interaction with either TLR2 or TLR4 that triggered predominantly a pro-inflammatory-type immune response. Our data suggest that cellular localization of Mtbhsp60 upon interaction with TLRs dictates the type of polarization in the innate immune responses in macrophages. This information is likely to help us in tailoring the host protective immune responses against M. tuberculosis.

Interactions among secretory immunoglobulin A, CD71, and transglutaminase-2 affect permeability of intestinal epithelial cells to gliadin peptides

BACKGROUND & AIMS

The transferrin receptor (CD71) is up-regulated in duodenal biopsy samples from patients with active celiac disease and promotes retrotransport of secretory immunoglobulin A (SIgA)-gliadin complexes. We studied intestinal epithelial cell lines that overexpress CD71 to determine how interactions between SIgA and CD71 promote transepithelial transport of gliadin peptides.

METHODS

We analyzed duodenal biopsy specimens from 8 adults and 1 child with active celiac disease. Caco-2 and HT29-19A epithelial cell lines were transfected with fluorescence-labeled small interfering RNAs against CD71. Interactions among IgA, CD71, and transglutaminase 2 (Tgase2) were analyzed by flow cytometry, immunoprecipitation, and confocal microscopy. Transcytosis of SIgA-CD71 complexes and intestinal permeability to the gliadin 3H-p31-49 peptide were analyzed in polarized monolayers of Caco-2 cells.

RESULTS

Using fluorescence resonance energy transfer and in situ proximity ligation assays, we observed physical interactions between SIgA and CD71 or CD71 and Tgase2 at the apical surface of enterocytes in biopsy samples and monolayers of Caco-2 cells. CD71 and Tgase2 were co-precipitated with SIgA, bound to the surface of Caco-2 cells. SIgA-CD71 complexes were internalized and localized in early endosomes and recycling compartments but not in lysosomes. In the presence of celiac IgA or SIgA against p31-49, transport of intact 3H-p31-49 increased significantly across Caco-2 monolayers; this transport was inhibited by soluble CD71 or Tgase2 inhibitors.

CONCLUSIONS

Upon binding to apical CD71, SIgA (with or without gliadin peptides) enters a recycling pathway and avoids lysosomal degradation; this process allows apical-basal transcytosis of bound peptides. This mechanism is facilitated by Tgase2 and might be involved in the pathogenesis of celiac disease.

Massive calcium-activated endocytosis without involvement of classical endocytic proteins

We describe rapid massive endocytosis (MEND) of >50% of the plasmalemma in baby hamster kidney (BHK) and HEK293 cells in response to large Ca transients. Constitutively expressed Na/Ca exchangers (NCX1) are used to generate Ca transients, whereas capacitance recording and a membrane tracer dye, FM 4–64, are used to monitor endocytosis. With high cytoplasmic adenosine triphosphate (ATP; >5 mM), Ca influx causes exocytosis followed by MEND. Without ATP, Ca transients cause only exocytosis. MEND can then be initiated by pipette perfusion of ATP, and multiple results indicate that ATP acts via phosphatidylinositol-bis 4,5-phosphate (PIP₂) synthesis: PIP₂ substitutes for ATP to induce MEND. ATP-activated MEND is blocked by an inositol 5-phosphatase and by guanosine 5′-[γ-thio]triphosphate (GTPγS). Block by GTPγS is overcome by the phospholipase C inhibitor, U73122, and PIP₂ induces MEND in the presence of GTPγS. MEND can occur in the absence of ATP and PIP₂ when cytoplasmic free Ca is clamped to 10 µM or more by Ca-buffered solutions. ATP-independent MEND occurs within seconds during Ca transients when cytoplasmic solutions contain polyamines (e.g., spermidine) or the membrane is enriched in cholesterol. Although PIP₂ and cholesterol can induce MEND minutes after Ca transients have subsided, polyamines must be present during Ca transients. MEND can reverse over minutes in an ATP-dependent fashion. It is blocked by brief β-methylcyclodextrin treatments, and tests for involvement of clathrin, dynamins, calcineurin, and actin cytoskeleton were negative. Therefore, we turned to the roles of lipids. Bacterial sphingomyelinases (SMases) cause similar MEND responses within seconds, suggesting that ceramide may be important. However, Ca-activated MEND is not blocked by reagents that inhibit SMases. MEND is abolished by the alkylating phospholipase A₂ inhibitor, bromoenol lactone, whereas exocytosis remains robust, and Ca influx causes MEND in cardiac myocytes without preceding exocytosis. Thus, exocytosis is not prerequisite for MEND. From these results and two companion studies, we suggest that Ca promotes the formation of membrane domains that spontaneously vesiculate to the cytoplasmic side.

Neuronal precursor cell-expressed developmentally down-regulated 4-1 (NEDD4-1) controls the sorting of newly synthesized Ca(V)1.2 calcium channels

Neuronal precursor cell-expressed developmentally down-regulated 4 (Nedd4) proteins are ubiquitin ligases, which attach ubiquitin moieties to their target proteins, a post-translational modification that is most commonly associated with protein degradation. Nedd4 ubiquitin ligases have been shown to down-regulate both potassium and sodium channels. In this study, we investigated whether Nedd4 ubiquitin ligases also regulate Ca(v) calcium channels. We expressed three Nedd4 family members, Nedd4-1, Nedd4-2, and WWP2, together with Ca(v)1.2 channels in tsA-201 cells. We found that Nedd4-1 dramatically decreased Ca(v) whole-cell currents, whereas Nedd4-2 and WWP2 failed to regulate the current. Surface biotinylation assays revealed that Nedd4-1 decreased the number of channels inserted at the plasma membrane. Western blots also showed a concomitant decrease in the total expression of the channels. Surprisingly, however, neither the Ca(v) pore-forming α1 subunit nor the associated Ca(v)β and Ca(v)α(2)δ subunits were ubiquitylated by Nedd4-1. The proteasome inhibitor MG132 prevented the degradation of Ca(v) channels, whereas monodansylcadaverine and chloroquine partially antagonized the Nedd4-1-induced regulation of Ca(v) currents. Remarkably, the effect of Nedd4-1 was fully prevented by brefeldin A. These data suggest that Nedd4-1 promotes the sorting of newly synthesized Ca(v) channels for degradation by both the proteasome and the lysosome. Most importantly, Nedd4-1-induced regulation required the co-expression of Ca(v)β subunits, known to antagonize the retention of the channels in the endoplasmic reticulum. Altogether, our results suggest that Nedd4-1 interferes with the chaperon role of Ca(v)β at the endoplasmic reticulum/Golgi level to prevent the delivery of Ca(v) channels at the plasma membrane.

Endocytosis in the mouse oocyte and its contribution to cAMP signaling during meiotic arrest

Mammalian oocytes are arrested at prophase I of meiosis until a preovulatory surge of LH stimulates them to resume meiosis. Prior to the LH surge, high levels of cAMP within the oocyte maintain meiotic arrest; this cAMP is generated in the oocyte through the activity of the constitutively active, G(s)-coupled receptor, G-protein-coupled receptor 3 (GPR3) or GPR12. Activated GPRs are typically targeted for desensitization through receptor-mediated endocytosis, but a continuously high level of cAMP is needed for meiotic arrest. The aim of this study was to examine whether receptor-mediated endocytosis occurs in the mouse oocyte and whether this could affect the maintenance of meiotic arrest. We found that constitutive endocytosis occurs in the mouse oocyte. Inhibitors of receptor-mediated endocytosis, monodansylcadaverine and dynasore, inhibited the formation of early endosomes and completely inhibited spontaneous meiotic resumption. A red fluorescent protein-tagged GPR3 localized in the plasma membrane and within early endosomes in the oocyte, demonstrating that GPR3 is endocytosed. However, overexpression of G-protein receptor kinase 2 and β-arrestin-2 had only a modest effect on stimulating meiotic resumption, suggesting that these proteins do not play a major role in GPR3 endocytosis. Inhibition of endocytosis elevated cAMP levels within oocytes, suggesting that there is an accumulation of GPR3 at the plasma membrane. These results show that endocytosis occurs in the oocyte, leading to a decrease in cAMP production, and suggest that there is a balance between cAMP production and degradation in the arrested oocyte that maintains cAMP levels at an appropriate level during the maintenance of meiotic arrest.

Fluorescent probes of tissue transglutaminase reveal its association with arterial stiffening

Tissue transglutaminase (TG2) catalyzes the crosslinking of proteins. TG2 has been implicated in fibrosis and vascular calcification, both of which lead to a common feature of aging known as arterial stiffness. In order to probe the role of TG2 in arterial rigidification, we have prepared a fluorescent irreversible inhibitor as a probe for TG2 activity (RhodB-PGG-K(Acr)-LPF-OH). This probe was synthesized on solid support, characterized kinetically (k(inact) = 0.68 min⁻¹, K(I) = 79 μM), and then used to stain the aorta from rats used as a model of isolated systolic hypertension (ISH). Interestingly, TG2 activity was thus shown to increase over 4 weeks of the hypertension model, corresponding with the previously observed increase in arterial stiffness. These results clearly suggest an association between TG2 and the phenomenon of arterial rigidification.

Proteomic analysis for the anti-apoptotic effects of cystamine on apoptosis-prone macrophage

Increased macrophage vulnerability is associated with progression of systemic lupus erythematosus. Our previous studies have shown that cystamine, an inhibitor of transglutaminase 2 (TG2), alleviated the apoptosis of hepatocyte and brain cell in lupus-prone mice NZB/W-F1. In present study, we further investigated the effects of cystamine on apoptosis-prone macrophages (APMs) in the lupus mice. Using two-dimensional gel electrophoresis (2-DE) analysis, we found that cystamine induced a differential protein expression pattern of APM as comparing to the PBS control. The protein spots presenting differential level between cystamine and PBS treatment were then identified by peptide-mass fingerprinting (PMF). After bioinformatic analysis, these identified proteins were found involved in mitochondrial apoptotic pathway, oxidative stress, and mitogen-activated protein (MAP) kinase-mediated pathway. Further investigation revealed that cystamine significantly decreased the levels of apoptotic Bax and Apaf-1 and the activity of caspase-3, and increased the levels of anti-apoptotic Bcl-2 in APM. We also found that these apoptotic mediators were up-regulated in a correlation with the progression of lupus severity in NZB/W-F1, which were little affected in BALB/c mice. We also found that the reduced serum glutathione was restored by cystamine in NZB/W-F1. Interestingly, the phosphorylation of extracellular signal-regulated kinase 1/2 (ERK1/2) in APM and the phagocytic ability was diminished in presence of cystamine. In conclusion, our findings indicate that cystamine significantly inhibited mitochondrial pathway, induced antioxidant proteins, and diminished phosphorylation of extracellular ERK1/2, which may alleviate the apoptosis and the phagocytic ability of APM.

Celiac anti-tissue transglutaminase antibodies interfere with the uptake of alpha gliadin peptide 31-43 but not of peptide 57-68 by epithelial cells

Celiac disease is characterized by the secretion of IgA-class autoantibodies that target tissue transglutaminase (tTG). It is now recognized that anti-tTG antibodies are functional and not mere bystanders in the pathogenesis of celiac disease. Here we report that interaction between anti-tTG antibodies and extracellular membrane-bound tTG inhibits peptide 31-43 (but not peptide 57-68) uptake by cells, thereby impairing the ability of p31-43 to drive Caco-2 cells into S-phase. This effect did not involve tTG catalytic activity. Because anti-tTG antibodies interfered with epidermal growth factor endocytosis, we assume that they exert their effect by reducing peptide 31-43 endocytosis. Our results suggest that cell-surface tTG plays a hitherto unknown role in the regulation of gliadin peptide uptake and endocytosis.

Transglutaminase 2: a multi-functional protein in multiple subcellular compartments

Transglutaminase 2 (TG2) is a multifunctional protein that can function as a transglutaminase, G protein, kinase, protein disulfide isomerase, and as an adaptor protein. These multiple biochemical activities of TG2 account for, at least in part, its involvement in a wide variety of cellular processes encompassing differentiation, cell death, inflammation, cell migration, and wound healing. The individual biochemical activities of TG2 are regulated by several cellular factors, including calcium, nucleotides, and redox potential, which vary depending on its subcellular location. Thus, the microenvironments of the subcellular compartments to which TG2 localizes, such as the cytosol, plasma membrane, nucleus, mitochondria, or extracellular space, are important determinants to switch on or off various TG2 biochemical activities. Furthermore, TG2 interacts with a distinct subset of proteins and/or substrates depending on its subcellular location. In this review, the biological functions and molecular interactions of TG2 will be discussed in the context of the unique environments of the subcellular compartments to which TG2 localizes.

Endocytosis and membrane potential are required for HeLa cell uptake of R.I.-CKTat9, a retro-inverso Tat cell penetrating peptide

Cell-penetrating peptides (CPPs) can enter many types of cells and have become useful tools for introducing a variety of cargo such as exogenous peptides, proteins, and nucleic acids into cultured cells in vitro. Tat CPPs derived from the HIV-1 Tat protein are the most widely used among the arginine-rich CPPs. Even though CPPs hold considerable promise for drug delivery, poor biological stability and high in vivo clearance may limit their effectiveness for delivering cargo. Therefore, we utilize a retro-inverso form of a Tat peptide, R.I.-CKTat9, which is proteolytically stable. In the current study, the cellular entry mechanism of this arginine-rich CPP is investigated. Fluorescently labeled R.I.-CKTat9 entered HeLa cells in a concentration- and energy-dependent manner demonstrating both diffuse and punctate (vesicular) appearance inside the cells. The labeled R.I.-CKTat9 colocalized with labeled transferrin in the punctate structure, suggesting that the peptide enters HeLa cells by clathrin-dependent endocytosis. Incubation of cells with an isotonic/high K(+) buffer (KPBS) or an NH(4)Cl solution abolished the diffuse but not the punctate fluorescence, suggesting that membrane potential plays a critical role. This result also suggests that the flux originates from the endosome, not the extracellular space, and relies on the acidity of the endosome. Impairment of clathrin-mediated endocytosis by RNAi with clathrin heavy chain function and endocytosis inhibitors greatly reduced or completely abolished both diffuse and punctate fluorescence, further supporting a single route of endocytosis and subsequent endosomal escape. Since cells in the mitotic (M) phase shut down endocytosis but maintain plasma membrane potential, this property was used to further confirm the endocytic mechanism. Direct measurement of plasma membrane potential confirmed its persistence in M phase arrested HeLa cells. Consistent with our working hypothesis, these cells did not show any vesicular nor diffuse fluorescence of labeled R.I.-CKTat9, providing compelling evidence for the sequential steps of endocytosis and endosomal escape. Binding of labeled R.I.-CKTat9 to the surface of HeLa cells at 0 degrees C was reduced under the mildly acidic conditions of early endosomes, suggesting an acidity-dependent endosomal escape mechanism. Overall, these results indicate that both endocytosis and membrane potential are required for R.I.-CKTat9 entry into HeLa cells and suggest that translocation occurs at the endosomal membrane.

Photolabeling of tissue transglutaminase reveals the binding mode of potent cinnamoyl inhibitors

We have recently developed a new class of cinnamoyl derivatives as potent tissue transglutaminase (TG2) inhibitors. Herein, we report the synthesis of a diazirine derivative of these inhibitors and its application to the photolabeling of its binding site on guinea pig liver transglutaminase. Two novel homology models were generated for this commonly studied TG2, which differ in the conformational state they represent. Tryptic digest and mass spectrometric analysis of the photolabeling experiment showed that only residue Cys230 was labeled, and our homology models were used to visualize these results. This visualization suggested that Cys230 is somewhat more solvent-exposed in the "closed" conformation of TG2, compared to the "open" conformation. Docking experiments suggested binding modes consistent with the labeling pattern that would block access to the tunnel leading to the active site, consistent with the observed mode of inhibition. However, while these modeling simulations favored the closed conformation as the target of our cinnamoyl inhibitors, native PAGE experiments indicated the open conformation of the enzyme in fact predominates in the presence of our photolabeling derivative. These results are important for understanding the binding modes of TG2 inhibitors in general and will be critical for the structure-based design of future inhibitors.

Cooperation of salivary protein histatin 3 with heat shock cognate protein 70 relative to the G1/S transition in human gingival fibroblasts

Histatins, a family of salivary proteins, have antimicrobial activity. Candida albicans, which is killed by histatins, induces oral candidiasis in individuals with compromised immune systems. Although the functional significance of histatins has been documented, their biological and physiological functions against host cells have not been clarified. In this study, we found that histatin 3, a member of the histatin family, binds to heat shock cognate protein 70 (HSC70). These proteins were co-localized in the cytoplasm and nucleus in human gingival fibroblasts following non-heat and heat shock. Histatin 3 induced stimulation of DNA synthesis and cell survival in human gingival fibroblasts in a dose-dependent manner. This DNA synthesis was found to be dependent on HSC70 by knockdown experiments. The effect of heat shock on DNA synthesis induced by histatin 3 was approximately 2-fold higher than that of non-heat shock. When the histatin 3 uptake into cells was inhibited by monodansylcadaverine or when histatin 3 binding to HSC70 was precluded by 15-deoxyspergualin, DNA synthesis by histatin 3 was approximately 2-fold less than that without monodansylcadaverine or 15-deoxyspergualin. Although HSC70 directly bound to p27(Kip1) (a cyclin-dependent kinase inhibitor), histatin 3 increased the binding between those proteins but not with a peptide capable of binding to HSC70. Moreover histatin 3 prevented ATP-dependent dissociation of HSC70-p27(Kip1). ATP was unable to form a histatin 3-HSC70(D10N)-p27(Kip1) complex (HSC70(D10N) is a mutant attenuating ATPase activity). These findings suggest that histatin 3 may be involved in cell proliferation through the regulation of HSC70 and p27(Kip1) in oral cells.

Integrin trafficking regulated by Rab21 is necessary for cytokinesis

Adherent cells undergo remarkable changes in shape during cell division. However, the functional interplay between cell adhesion turnover and the mitotic machinery is poorly understood. The endo/exocytic trafficking of integrins is regulated by the small GTPase Rab21, which associates with several integrin alpha subunits. Here, we show that targeted trafficking of integrins to and from the cleavage furrow is required for successful cytokinesis, and that this is regulated by Rab21. Rab21 activity, integrin-Rab21 association, and integrin endocytosis are all necessary for normal cytokinesis, which becomes impaired when integrin-mediated adhesion at the cleavage furrow fails. We also describe a chromosomal deletion and loss of Rab21 gene expression in human cancer, which leads to the accumulation of multinucleate cells. Importantly, reintroduction of Rab21 rescued this phenotype. In conclusion, Rab21-regulated integrin trafficking is essential for normal cell division, and its defects may contribute to multinucleation and genomic instability, which are hallmarks of cancer.

Shigella toxin and the pathogenesis of shigellosis

Shigella dysenteriae 1 produces a periplasmic protein with multiple toxic effects in vivo and in vitro. These include neurotoxicity, cytotoxicity and enterotoxicity, as well as the ability to inhibit cell-free protein synthesis. The purified toxin is a protein of relative molecular mass (Mr) 64 000. It is composed of one catalytically active A subunit (Mr = 32 000) that inhibits protein synthesis, and a complex of five B monomers (Mr approximately 6500 each). Studies using subunit-specific antibodies demonstrate that the B subunit mediates the binding of toxin to toxin receptors in the cell membrane. In a model system in HeLa cell culture, the surface membrane receptor has been shown to be a glycoprotein, most probably asparagine-linked, and to contain oligomeric beta 1----4 linked N-acetyl-D-glucosamine. Studies with metabolic inhibitors and agents that disrupt the cytoskeleton, and/or alter the pH and function of acidic cytoplasmic vesicles, provide indirect evidence that toxin is transported from the cell surface to the cell interior. This process is probably receptor-mediated endocytosis, since it is also inhibited by amines that prevent receptor-mediated uptake of other ligands in well-characterized systems. The toxic action in the HeLa cell is due to the subsequent inhibition of protein synthesis which results from catalytic inactivation of the 60S ribosomal subunit and the cessation of polypeptide chain elongation. Inhibition of protein synthesis by toxin produced subsequent to bacterial invasion of colonic epithelial cells could explain the destructive lesions found in shigellosis. Although toxin can induce jejunal secretion in animal models, there is at present no clear explanation for the secretory response of the gut mucosa in shigella infection.

Chapter 6 Protein Sorting in the Secretory Pathway

This chapter focuses on protein sorting in the secretory pathway. From primary and secondary biosynthetic sites in the cytosol and mitochondrial matrix, respectively, proteins and lipids are distributed to more than 30 final destinations in membranes or membrane-bound spaces, where they carry out their programmed function. Molecular sorting is defined, in its most general sense, as the sum of the mechanisms that determine the distribution of a given molecule from its site of synthesis to its site of function in the cell. The final site of residence of a protein in a eukaryotic cell is determined by a combination of various factors, acting in concert: (1) site of synthesis, (2) sorting signals or zip codes, (3) signal recognition or decoding mechanisms, (4) cotranslational or posttranslational mechanisms for translocation across membranes, (5) specific fusion-fission interactions between intracellular vesicular compartments, and (6) restrictions to the lateral mobility in the plane of the bilayer. Improvements in cell fractionation, protein separation, and immune precipitation procedures in the past decade have made them possible. Very little is known about the mechanisms that mediate the localization and concentration of specific proteins and lipids within organelles. Various experimental model systems have become available for their study. The advent of recombinant DNA technology has shortened the time needed for obtaining the primary structure of proteins to a few months.

Pharmacological inhibition of endocytic pathways: is it specific enough to be useful?

Eukaryotic cells constantly form and internalize plasma membrane vesicles in a process known as endocytosis. Endocytosis serves a variety of housekeeping and specialized cellular functions, and it can be mediated by distinct molecular pathways. Among them, internalization via clathrin-coated pits, lipid raft/caveolae-mediated endocytosis and macropinocytosis/phagocytosis are the most extensively characterized. The major endocytic pathways are usually distinguished on the basis of their differential sensitivity to pharmacological/chemical inhibitors, although the possibility of nonspecific effects of such inhibitors is frequently overlooked. This review provides a critical evaluation of the selectivity of the most widely used pharmacological inhibitors of clathrin-mediated, lipid raft/caveolae-mediated endocytosis and macropinocytosis/phagocytosis. The mechanisms of actions of these agents are described with special emphasis on their reported side effects on the alternative internalization modes and the actin cytoskeleton. The most and the least-selective inhibitors of each major endocytic pathway are highlighted.

Secretory IgA mediates retrotranscytosis of intact gliadin peptides via the transferrin receptor in celiac disease

Celiac disease (CD) is an enteropathy resulting from an abnormal immune response to gluten-derived peptides in genetically susceptible individuals. This immune response is initiated by intestinal transport of intact peptide 31-49 (p31-49) and 33-mer gliadin peptides through an unknown mechanism. We show that the transferrin receptor CD71 is responsible for apical to basal retrotranscytosis of gliadin peptides, a process during which p31-49 and 33-mer peptides are protected from degradation. In patients with active CD, CD71 is overexpressed in the intestinal epithelium and colocalizes with immunoglobulin (Ig) A. Intestinal transport of intact p31-49 and 33-mer peptides was blocked by polymeric and secretory IgA (SIgA) and by soluble CD71 receptors, pointing to a role of SIgA–gliadin complexes in this abnormal intestinal transport. This retrotranscytosis of SIgA–gliadin complexes may promote the entry of harmful gliadin peptides into the intestinal mucosa, thereby triggering an immune response and perpetuating intestinal inflammation. Our findings strongly implicate CD71 in the pathogenesis of CD.

Polarized signaling endosomes coordinate BDNF-induced chemotaxis of cerebellar precursors

During development, neural precursors migrate in response to positional cues such as growth factor gradients. However, the mechanisms that enable precursors to sense and respond to such gradients are poorly understood. Here we show that cerebellar granule cell precursors (GCPs) migrate along a gradient of brain-derived neurotrophic factor (BDNF), and we demonstrate that vesicle trafficking is critical for this chemotactic process. Activation of TrkB, the BDNF receptor, stimulates GCPs to secrete BDNF, thereby amplifying the ambient gradient. The BDNF gradient stimulates endocytosis of TrkB and associated signaling molecules, causing asymmetric accumulation of signaling endosomes at the subcellular location where BDNF concentration is maximal. Thus, regulated BDNF exocytosis and TrkB endocytosis enable precursors to polarize and migrate in a directed fashion along a shallow BDNF gradient.

Cytoskeleton and motor proteins are required for the transcytosis of Neisseria gonorrhoeae through polarized epithelial cells

Neisseria gonorrhoeae interact with polarized T84 epithelial cells by engaging carcinoembryonic antigen-related cellular adhesion molecule (CEACAM) receptors. Adherent bacteria that are taken up by the cells are able to traverse the epithelial layer from the apical to the basal side. Herein, we demonstrate that the actin cytoskeleton of the cells is not required for the initial adherence of the bacteria, however, it is essential for invasion into and traversal through T84 cells. Furthermore, microtubule inhibitors blocked the traversal, but not the adherence and invasion of the bacteria. Inhibition of the motor activity of myosins reduced invasion and traversal, but not bacterial adherence. Immunofluorescence confocal laser scanning microscopy revealed the colocalization of the microtubule-based kinesin and dynein motors, and the actin-based motor myosin with adherent and intracellular gonococci. Transcytosis was reduced by blocking kinesin and myosin with specific antibodies. This underlines the importance of these motor proteins for the transcytosis of epithelial monolayers by N. gonorrhoeae.

LIGHT signals directly to intestinal epithelia to cause barrier dysfunction via cytoskeletal and endocytic mechanisms

BACKGROUND & AIMS

LIGHT (lymphotoxin-like inducible protein that competes with glycoprotein D for herpes virus entry on T cells) is a tumor necrosis factor core family member that regulates T-cell activation and causes experimental inflammatory bowel disease. Additional data suggest that LIGHT may be involved in the pathogenesis of human inflammatory bowel disease. The aim of this study was to determine if LIGHT is capable of signaling directly to intestinal epithelia and to define the mechanisms and consequences of such signaling.

METHODS

The effects of LIGHT and interferon-gamma on barrier function, cytoskeletal regulation, and tight junction structure were assessed in mice and intestinal epithelial monolayers.

RESULTS

LIGHT induced barrier loss in cultured epithelia via myosin II regulatory light chain (MLC) phosphorylation; both barrier loss and MLC phosphorylation were reversed by MLC kinase (MLCK) inhibition. Pretreatment with interferon-gamma, which induced lymphotoxin beta receptor (LT beta R) expression, was required for these effects, and neither barrier dysfunction nor intestinal epithelial MLC phosphorylation occurred in LT beta R knockout mice. In cultured monolayers, endocytosis of the tight junction protein occludin correlated with barrier loss. Internalized occludin colocalized with caveolin-1. LIGHT-induced occludin endocytosis and barrier loss were both prevented by inhibition of caveolar endocytosis.

CONCLUSIONS

T cell-derived LIGHT activates intestinal epithelial LT beta R to disrupt barrier function. This requires MLCK activation and caveolar endocytosis. These data suggest a novel role for LIGHT in disease pathogenesis and suggest that inhibition of MLCK-dependent caveolar endocytosis may represent an approach to restoring barrier function in inflammatory bowel disease.

Coagulation factor XIII Val34Leu gene polymorphism and Alzheimer's disease

Blood coagulation factor XIII (FXIII) plays a role in inflammatory processes and a pathogenetic role of inflammation in neurodegenerative disorders has been proposed. FXIIIa subunit was immunohistochemically detected in a subpopulation of reactive microglia in Alzheimer's disease (AD). Aim of the present study is to evaluate whether a common polymorphism of the FXIII gene is associated with sporadic AD. We examined 90 patients affected by sporadic AD and 139 age- and sex-matched controls to assess the distribution of V/L alleles and genotypes of the FXIIIa-subunit gene. The LL genotype showed a significantly higher frequency in AD patients (p<0.05) with a significantly increased risk of AD in the presence of LL genotype at the logistic regression analysis [odds ratio: 3.6 (1.36-9.44), p<0.01]. This study shows for the first time an association between FXIII Val34Leu polymorphism and AD.

Cross-linking transglutaminases with G protein-coupled receptor signaling

Transglutaminases are a family of calcium- and thiol-dependent acyl transferases that catalyze the formation of an amide bond between the gamma-carboxamide groups of peptide-bound glutamine residues and the primary amino groups in various compounds, including the epsilon-amino group of lysines in certain proteins. As a result, these enzymes effect posttranslational modification of proteins by amine incorporation, or stabilization of protein assemblies by their cross-linking; such actions profoundly influence critical biological processes such as blood clotting and protection from infection and dehydration by establishing the barrier function of skin. In addition, transglutaminases have other more diverse actions, including involvement in signaling by the superfamily of heterotrimeric guanine nucleotide-binding protein (G protein)-coupled receptors (GPCRs) in one of three ways: (i) through actions as guanosine triphosphate-binding proteins that activate intracellular effectors, such as phospholipase C; (ii) by cross-linking GPCR monomers to enhance signaling as a result of covalent dimer formation; or (iii) by interacting with an apparent growth inhibitory orphan GPCR, GPR56, to limit metastatic spread of melanoma cells. The implications of these receptor-coupled actions of transglutaminases are discussed.

Exploitation of the endocytic pathway by Orientia tsutsugamushi in nonprofessional phagocytes

Orientia tsutsugamushi, a causative agent of scrub typhus, is an obligate intracellular bacterium that requires the exploitation of the endocytic pathway in the host cell. We observed the localization of O. tsutsugamushi with clathrin or adaptor protein 2 within 30 min after the infection of nonprofessional phagocytes. We have further confirmed that the infectivity of O. tsutsugamushi is significantly reduced by drugs that block clathrin-mediated endocytosis but not by filipin III, an inhibitor that blocks caveola-mediated endocytosis. In the present study, with a confocal microscope, O. tsutsugamushi was sequentially colocalized with the early and late endosomal markers EEA1 and LAMP2, respectively, within 1 h after infection. The colocalization of O. tsutsugamushi organisms with EEA1 and LAMP2 gradually disappeared until 2 h postinfection, and then free O. tsutsugamushi organisms were found in the cytoplasm. When the acidification of endocytic vesicles was blocked by treating the cells with NH(4)Cl or bafilomycin A, the escape of O. tsutsugamushi organisms from the endocytic pathway was severely impaired, and the infectivity of O. tsutsugamushi was drastically reduced. To our knowledge, this is the first report that the invasion of O. tsutsugamushi is dependent on the clathrin-dependent endocytic pathway and the acidification process of the endocytic vesicles in nonprofessional phagocytes.

Trafficking of Streptococcus uberis in bovine mammary epithelial cells

Results from our laboratory showed that Streptococcus uberis internalized bovine mammary epithelial cells by exploiting host cell cytoskeleton and signal transduction mechanisms. It was also shown that S. uberis survived intracellularly for up to 120 h and capable of transcytose bovine mammary epithelial cells. To define mechanisms and strategies used by S. uberis to move through host cells and survive intracellularly, internalization studies using specific inhibitors, double immunofluorescence labeling and confocal laser microscopy were conducted. When bovine mammary epithelial cells were treated with inhibitors of endocytic vesicle acidification, the number of intracellular S. uberis was similar to untreated controls. When selective inhibitors of lipid rafts/caveolae or receptor-mediated endocytosis were used, a significantly lower number of intracellular S. uberis was detected compared with untreated controls. However, when the effect of inhibitors of receptor-mediated endocytosis and lipid rafts/caveolae were compared, the latter induced the lowest S. uberis internalization values suggesting a preferential exploitation of caveolae-mediated endocytosis. Since caveloae-dependent intracellular trafficking does not include intravesicular acidification or lysosome fusion; these results suggest that by exploiting preferential intracellular trafficking pathways in bovine mammary epithelial cells, S. uberis avoids intracellular bactericidal mechanisms. Such a strategy would allow S. uberis to persist intracellularly and may explain how persistent intramammary infections occur.

Hhip regulates zebrafish muscle development by both sequestering Hedgehog and modulating localization of Smoothened

Sharp borders between cells with different developmental fates are important for patterning of invertebrates, but are not well understood in vertebrates. Zebrafish slow muscle cells develop from adaxial cells, a one-cell-diameter-thick pseudo-epithelium immediately adjacent to the notochord. Hedgehog (Hh) signals from notochord specify adaxial cells to form slow muscle cells. Cells next to adaxial cells form fast muscle. This suggests that Hh signaling is locally regulated to produce a sharp border that separates slow and fast muscle precursors. To understand how Hh activity is locally regulated, we characterized the dynamic roles of Hhip, a protein that binds Hedgehog at the cell surface. Hhip is strongly expressed by adaxial cells and, together with Patched, the Hedgehog receptor, limits transduction of the Hedgehog signaling by Smoothened to adaxial cells. Hhip protein lacking its membrane associated domain still suppresses Hh activity but no longer acts synergistically with Patched. Hhip and Smoothened colocalize at the cell surface and, in response to Hedgehog, internalize together. Knockdown of Hhip blocks Smoothened internalization while increasing Hedgehog signaling and slow muscle formation. These data support a model in which Hhip regulates muscle development both by sequestering Hedgehog and by modulating localization of Smoothened.

Cytosolic Entry of Bisphosphonate Drugs Requires Acidification of Vesicles after Fluid-Phase Endocytosis

Bisphosphonates such as alendronate and zoledronate are blockbuster drugs used to inhibit osteoclast-mediated bone resorption. Although the molecular mechanisms by which bisphosphonates affect osteoclasts are now evident, the exact route by which they are internalized by cells is not known. To clarify this, we synthesized a novel, fluorescently labeled analog of alendronate (AF-ALN). AF-ALN was rapidly internalized into intracellular vesicles in J774 macrophages and rabbit osteoclasts; uptake of AF-ALN or [14C]zoledronate was stimulated by the presence of Ca2+ and Sr2+ and could be inhibited by addition of EGTA or clodronate, both of which chelate calcium ions. Both EGTA and clodronate also prevented the bisphosphonate-induced inhibition of Rap1A prenylation, an effect that was reversed by addition of Ca2+. In J774 cells and osteoclasts, vesicular AF-ALN colocalized with dextran (but not wheat germ agglutinin or transferrin), and uptake of AF-ALN or [14C]zoledronate was inhibited by dansylcadaverine, indicating that fluid-phase endocytosis is involved in the initial internalization of bisphosphonate into vesicles. Endosomal acidification then seems to be absolutely required for exit of bisphosphonate from vesicles and entry into the cytosol, because monensin and bafilomycin A1, both inhibitors of endosomal acidification, did not inhibit vesicular uptake of AF-ALN or internalization of [14C]zoledronate but prevented the inhibitory effect of alendronate or zoledronate on Rap1A prenylation. Taken together, these results demonstrate that cellular uptake of bisphosphonate drugs requires fluid-phase endocytosis and is enhanced by Ca2+ ions, whereas transfer from endocytic vesicles into the cytosol requires endosomal acidification.

Expression and Activity of Transglutaminase II in Spontaneous Tumours of Dogs and Cats

Tissue transglutaminase II (TGase II) is a dual function protein with both transamidating and guanidine triphosphate (GTP)-binding capabilities. Previous studies have implicated TGase as a pro-apoptotic molecule; however, our recent findings indicate that TGase II may act as a survival factor in various cell types. The purpose of this study was to survey TGase II expression in normal tissue and spontaneous tumours of dogs and cats, by Western blotting and immunohistochemistry. Bladder, liver and adrenal gland exhibited prominent expression of TGase II while other tissues, including mammary gland, displayed limited expression and activity. TGase II GTP-binding in normal tissues was proportional to the level of expression in all tissues examined. Normal mammary tissue and that showing benign hyperplasia did not express TGase II. However, 11/25 (44%) of canine mammary carcinomas and 10/12 (83%) of feline mammary carcinomas strongly expressed TGase II in either a stromal, cellular or combined pattern. The pattern of expression was not related to the classification of mammary carcinoma (solid, tubulopapillary, complex or anaplastic), except that two anaplastic canine mammary carcinomas showed prominent TGase II expression. Two canine mammary carcinoma cell lines showed prominent TGase expression, and when the TGase activity was inhibited, the cells became more sensitive to doxorubicin-induced cell death. Thus, TGase II was significantly expressed in mammary cancers from dogs and cats and immunoreactivity of TGase II was similar to that reported in humans beings. The pro-survival effect of TGase II in canine mammary carcinoma cell lines was similar to that previously reported in humans patients.

The PilC adhesin of the Neisseria type IV pilus - binding specificities and new insights into the nature of the host cell receptor

Type IV pili of Neisseria gonorrhoeae and Neisseria meningitidis mediate the first contact to human mucosal epithelial cells, an interaction which is also critical for the interaction with vascular endothelial cells. The PilC proteins have been characterized as the principal pilus-associated adhesin. Here we show that PilC2 exhibits a defined cell and tissue tropism, as it binds to human epithelial and endothelial cell lines, but not to human T cells or fibroblasts. Piliated gonococci and PilC2 exhibit similar patterns of binding to human epithelial and endothelial cells, supporting the function of PilC as the key pilus adhesin. Although CD46 has previously been suggested to be a pilus receptor, several observations indicate that neisserial type IV pili and the pilus adhesin PilC2 interact with epithelial cells in a CD46 independent manner. Biochemical approaches were used to characterize the nature of host cell factors mediating binding of piliated gonococci and PilC2 protein. Our data indicate that the putative host cell receptor for gonococcal pili and the PilC2 pilus adhesin is a surface protein. Glycostructures were found to not be involved in binding. Moreover, we observed the uptake of purified PilC2 protein together with its receptor via receptor-mediated endocytosis and subsequent receptor re-exposure on the cell surface. Our data support the existence of a specific pilus receptor and provide intriguing information on the nature of the receptor.

Mammalian transglutaminases. Identification of substrates as a key to physiological function and physiopathological relevance

Transglutaminases form a large family of intracellular and extracellular enzymes that catalyse the Ca2+-dependent post-translational modification of proteins. Despite significant advances in our understanding of the biological role of most mammalian transglutaminase isoforms, recent findings suggest new scenarios, most notably for the ubiquitous tissue transglutaminase. It is becoming apparent that some transglutaminases, normally expressed at low levels in many tissue types, are activated and/or overexpressed in a variety of diseases, thereby resulting in enhanced concentrations of cross-linked proteins. As applies to all enzymes that exert their metabolic function by modifying the properties of target proteins, the identification and characterization of the modified proteins will cast light on the functions of transglutaminases and their involvement in human diseases. In this paper we review data on the properties of mammalian transglutaminases, particularly as regards their protein substrates and the relevance of transglutaminase-catalysed reactions in physiological and disease conditions.