In Vitro Protein Complex Formation with Cytoskeleton-anchoring Domain of Occludin Identified by Limited Proteolysis

The effect of serum starvation on tight junctional proteins and barrier formation in Caco-2 cells

Assessing the ability of pharmaceutics to cross biological barriers and reach the site-of-action requires faithful representation of these barriers in vitro. Difficulties have arisen in replicating in vivo resistance in vitro. This paper investigated serum starvation as a method to increase Caco-2 barrier stability and resistance. The effect of serum starvation on tight junction production was examined using transwell models; specifically, transendothelial electrical resistance (TEER), and the expression and localization of tight junction proteins, occludin and zonula occludens-1 (ZO-1), were studied using western blotting and immunofluorescence. Changing cells to serum-free media 2 days post-seeding resulted in TEER readings of nearly 5000 Ω cm² but the TEER rapidly declined subsequently. Meanwhile, exchanging cells to serum-free media 4–6 days post-seeding produced barriers with resistance readings between 3000 and 4000 Ω cm², which could be maintained for 18 days. This corresponded to an increase in occludin levels. Serum starvation as a means of barrier formation is simple, reproducible, and cost-effective. It could feasibly be implemented in a variety of pre-clinical pharmaceutical assessments of drug permeability across various biological barriers with the view to improving the clinical translation of novel therapeutics.

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Serum starvation increases the intracellular resistance of Caco-2 cells.

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Max TEER values of 4783 ± 610 Ω cm² were achieved in serum free conditions.

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A barrier of 3000–4000 Ω cm² could be maintained for up to 18 days.

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Serum starvation leads to a significant increase in occludin expression.

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Occludin levels correlate significantly with corresponding TEER values.

Accelerating trypsin digestion: the immobilized enzyme reactor

Sample preparation has lagged far behind the evolution of instrumentation used in mass-linked protein analysis. Trypsin digestion, for example, still takes a day, as it did 50 years ago, while mass spectral analyses are achieved in seconds. Higher order structure of proteins is frequently modified by varying digestion conditions: shifting the initial points of trypsin cleavage, changing digestion pathways, accelerating peptide bond demasking and altering the distribution of miscleaved products at the completion of proteolysis. Reduction and alkylation are even circumvented in many cases. This review focuses on immobilized enzyme reactor technology as a means to achieve accelerated trypsin digestion by exploiting these phenomena.

Cytoskeletal proteins: shaping progression of hepatitis C virus-induced liver disease

Hepatitis C virus (HCV) infection, which results in chronic hepatitis C (CHC) in most patients (70-85%), is a major cause of liver disease and remains a major therapeutic challenge. The mechanisms determining liver damage and the key factors that lead to a high rate of CHC remain imperfectly understood. The precise role of cytoskeletal (CS) proteins in HCV infection remains to be determined. Some studies including our recent study have demonstrated that changes occur in the expression of CS proteins in HCV-infected hepatocytes. A variety of host proteins interact with HCV proteins. Association between CS and HCV proteins may have implications in future design of CS protein-targeted therapy for the treatment for HCV infection. This chapter will focus on the interaction between host CS and viral proteins to signify the importance of this event in HCV entry, replication and transportation.

Occludin protein family: oxidative stress and reducing conditions

The occludin-like proteins belong to a family of tetraspan transmembrane proteins carrying a marvel domain. The intrinsic function of the occludin family is not yet clear. Occludin is a unique marker of any tight junction and is found in polarized endothelial and epithelial tissue barriers, at least in the adult vertebrate organism. Occludin is able to oligomerize and to form tight junction strands by homologous and heterologous interactions, but has no direct tightening function. Its oligomerization is affected by pro- and antioxidative agents or processes. Phosphorylation of occludin has been described at multiple sites and is proposed to play a regulatory role in tight junction assembly and maintenance and, hence, to influence tissue barrier characteristics. Redox-dependent signal transduction mechanisms are among the pathways modulating occludin phosphorylation and function. This review discusses the novel concept that occludin plays a key role in the redox regulation of tight junctions, which has a major impact in pathologies related to oxidative stress and corresponding pharmacologic interventions.

The critical role of N- and C-terminal contact in protein stability and folding of a family 10 xylanase under extreme conditions

Background

Stabilization strategies adopted by proteins under extreme conditions are very complex and involve various kinds of interactions. Recent studies have shown that a large proportion of proteins have their N- and C-terminal elements in close contact and suggested they play a role in protein folding and stability. However, the biological significance of this contact remains elusive.

Methodology

In the present study, we investigate the role of N- and C-terminal residue interaction using a family 10 xylanase (BSX) with a TIM-barrel structure that shows stability under high temperature, alkali pH, and protease and SDS treatment. Based on crystal structure, an aromatic cluster was identified that involves Phe4, Trp6 and Tyr343 holding the N- and C-terminus together; this is a unique and important feature of this protein that might be crucial for folding and stability under poly-extreme conditions.

Conclusion

A series of mutants was created to disrupt this aromatic cluster formation and study the loss of stability and function under given conditions. While the deletions of Phe4 resulted in loss of stability, removal of Trp6 and Tyr343 affected in vivo folding and activity. Alanine substitution with Phe4, Trp6 and Tyr343 drastically decreased stability under all parameters studied. Importantly, substitution of Phe4 with Trp increased stability in SDS treatment. Mass spectrometry results of limited proteolysis further demonstrated that the Arg344 residue is highly susceptible to trypsin digestion in sensitive mutants such as ΔF4, W6A and Y343A, suggesting again that disruption of the Phe4-Trp6-Tyr343 (F-W-Y) cluster destabilizes the N- and C-terminal interaction. Our results underscore the importance of N- and C-terminal contact through aromatic interactions in protein folding and stability under extreme conditions, and these results may be useful to improve the stability of other proteins under suboptimal conditions.

Brain endothelial cell-cell junctions: how to "open" the blood brain barrier

The blood-brain barrier (BBB) is a highly specialized structural and biochemical barrier that regulates the entry of blood-borne molecules into brain, and preserves ionic homeostasis within the brain microenvironment. BBB properties are primarily determined by junctional complexes between the cerebral endothelial cells. These complexes are comprised of tight and adherens junctions. Such restrictive angioarchitecture at the BBB reduces paracellular diffusion, while minimal vesicle transport activity in brain endothelial cells limits transcellular transport. Under normal conditions, this largely prevents the extravasation of large and small solutes (unless specific transporters are present) and prevents migration of any type of blood-borne cell. However, this is changed in many pathological conditions. There, BBB disruption (“opening”) can lead to increased paracellular permeability, allowing entry of leukocytes into brain tissue, but also contributing to edema formation. In parallel, there are changes in the endothelial pinocytotic vesicular system resulting in the uptake and transfer of fluid and macromolecules into brain parenchyma. This review highlights the route and possible factors involved in BBB disruption in a variety of neuropathological disorders (e.g. CNS inflammation, Alzheimer’s disease, Parkinson’s disease, epilepsy). It also summarizes proposed signal transduction pathways that may be involved in BBB “opening”.

Microbial induction of vascular pathology in the CNS

The central nervous system (CNS) is a finely tuned organ that participates in nearly every aspect of our day-to-day function. Neurons lie at the core of this functional unit and maintain an active dialogue with one another as well as their fellow CNS residents (e.g. astrocytes, oligodendrocytes, microglia). Because of this complex dialogue, it is essential that the CNS milieu be tightly regulated in order to permit uninterrupted and efficient neural chemistry. This is accomplished in part by anatomical barriers that segregate vascular components from the cerebral spinal fluid (CSF) and brain parenchyma. These barriers impede entry of noxious materials and enable the CNS to maintain requisite protein and ionic balances for constant electrochemical signaling. Under homeostatic conditions, the CNS is protected by the presence of specialized endothelium/epithelium, the blood brain barrier (BBB), and the blood-CSF barrier. However, following CNS infection these protective barriers can be comprised, sometimes resulting in severe neurological complications triggered by an imbalance or blockage of neural chemistry. In some instances, these disruptions are severe enough to be fatal. This review focuses on a selection of microbes (both viruses and parasites) that compromise vascular barriers and induce neurological complications upon gaining access to the CNS. Emphasis is placed on CNS diseases that result from a pathogenic interplay between host immune defenses and the invading microbe.

Hepatitis C virus cell entry: role of lipoproteins and cellular receptors

Hepatitis C virus (HCV), a major cause of chronic liver disease, is a single-stranded positive sense virus of the family Flaviviridae. HCV cell entry is a multi-step process, involving several viral and cellular factors that trigger virus uptake into the hepatocyte. Tetraspanin CD81, human scavenger receptor SR-BI, and tight junction molecules Claudin-1 and occludin are the main receptors that mediate HCV entry. In addition, the virus may use glycosaminoglycans and/or low density receptors on host cells as initial attachment factors. A unique feature of HCV is the dependence of virus replication and assembly on host cell lipid metabolism. Most notably, during HCV assembly and release from the infected cells, virus particles associate with lipids and very-low-density lipoproteins. Thus, infectious virus circulates in patient sera in the form of triglyceride-rich particles. Consequently, lipoproteins and lipoprotein receptors play an essential role in virus uptake and the initiation of infection. This review summarizes the current knowledge about HCV receptors, mechanisms of HCV cell entry and the role of lipoproteins in this process.

The tight junction protein complex undergoes rapid and continuous molecular remodeling at steady state

The tight junction defines epithelial organization. Structurally, the tight junction is comprised of transmembrane and membrane-associated proteins that are thought to assemble into stable complexes to determine function. In this study, we measure tight junction protein dynamics in live confluent Madin–Darby canine kidney monolayers using fluorescence recovery after photobleaching and related methods. Mathematical modeling shows that the majority of claudin-1 (76 ± 5%) is stably localized at the tight junction. In contrast, the majority of occludin (71 ± 3%) diffuses rapidly within the tight junction with a diffusion constant of 0.011 μm²s⁻¹. Zonula occludens-1 molecules are also highly dynamic in this region, but, rather than diffusing within the plane of the membrane, 69 ± 5% exchange between membrane and intracellular pools in an energy-dependent manner. These data demonstrate that the tight junction undergoes constant remodeling and suggest that this dynamic behavior may contribute to tight junction assembly and regulation.

Developing limited proteolysis and mass spectrometry for the characterization of ribosome topography

An approach that combines limited proteolysis and matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) has been developed to probe protease-accessible sites of ribosomal proteins from intact ribosomes. Escherichia coli and Thermus thermophilus 70S ribosomes were subjected to limited proteolysis using different proteases under strictly controlled conditions. Intact ribosomal proteins and large proteolytic peptides were recovered and directly analyzed by MALDI-MS, which allows for the determination of proteins that are resistant to proteolytic digestion by accurate measurement of molecular weights. Larger proteolytic peptides can be directly identified by the combination of measured mass, enzyme specificity, and protein database searching. Sucrose density gradient centrifugation revealed that the majority of the 70S ribosome dissociates into intact 30S and 50S subunits after 120 min of limited proteolysis. Thus, examination of ribosome populations within the first 30 to 60 min of incubation provides insight into 70S structural features. Results from E. coli and T. thermophilus revealed that a significantly larger fraction of 50S ribosomal proteins have similar limited proteolysis behavior than the 30S ribosomal proteins of these two organisms. The data obtained by this approach correlate with information available from the high-resolution crystal structures of both organisms. This new approach will be applicable to investigations of other large ribonucleoprotein complexes, is readily extendable to ribosomes from other organisms, and can facilitate additional structural studies on ribosome assembly intermediates.

Role of tight junctions in cell proliferation and cancer

The acquisition of a cancerous phenotype by epithelial cells involves the disruption of intercellular adhesions. The reorganization of the E-cadherin/beta-catenin complex in adherens junctions during cell transformation is widely recognized. Instead the implication of tight junctions (TJs) in this process is starting to be unraveled. The aim of this article is to review the role of TJ proteins in cell proliferation and cancer.

Structure of the conserved cytoplasmic C-terminal domain of occludin: identification of the ZO-1 binding surface

Occludin is a transmembrane protein localized at tight junctions whose functions are complex yet poorly understood. Current evidence supports a role for occludin in both the formation of the paracellular barrier and in cell signaling. While the N-terminal extracellular domains of occludin mediate homotypic adhesion, the distal C-terminal cytoplasmic domain of occludin controls protein targeting and endocytosis. The C terminus can also bind to the scaffolding proteins ZO-1, ZO-2, ZO-3, cingulin, the membrane trafficking protein VAP33, and the cytoskeletal protein F-actin, suggesting an important role for this domain. This domain is highly homologous to an important functional domain in the C terminus of the ELL family of RNA polymerase II transcription factors. To explore the function of occludin, we determined the high-resolution crystal structure of its C-terminal distal cytoplasmic domain. The structure comprises three helices that form two separate anti-parallel coiled-coils and a loop that packs tightly against one of the coiled-coils. Using in vitro binding studies and site-directed mutagenesis, we have identified a large positively charged surface that contains the binding site for ZO-1, and this surface is required for proper localization of occludin to cell-cell junctions. On the basis of sequence conservation, we predict that occludin domains from different species and the C-terminal domain of the ELL transcription factors share a very similar structure. Our results provide a model to further test the function of occludin and its binding to other proteins.

Actin depolymerization disrupts tight junctions via caveolae-mediated endocytosis

The tight junction (TJ) determines epithelial barrier function. Actin depolymerization disrupts TJ structure and barrier function, but the mechanisms of this effect remain poorly understood. The goal of this study was to define these mechanisms. Madin-Darby canine kidney (MDCK) cells expressing enhanced green fluorescent protein-, enhanced yellow fluorescent protein-, or monomeric red fluorescent protein 1-fusion proteins of beta-actin, occludin, claudin-1, ZO-1, clathrin light chain A1, and caveolin-1 were imaged by time-lapse multidimensional fluorescence microscopy with simultaneous measurement of transepithelial electrical resistance (TER). Actin depolymerization was induced with latrunculin A (LatA). Within minutes of LatA addition TER began to fall. This coincided with occludin redistribution and internalization. In contrast, ZO-1 and claudin-1 redistribution occurred well after maximal TER loss. Occludin internalization and TER loss, but not actin depolymerization, were blocked at 14 degrees C, suggesting that membrane traffic is required for both events. Inhibition of membrane traffic with 0.4 M sucrose also blocked occludin internalization and TER loss. Internalized occludin colocalized with caveolin-1 and dynamin II, but not with clathrin, and internalization was blocked by dominant negative dynamin II (K44A), but not by Eps15Delta95-295 expression. Inhibition of caveolae-mediated endocytosis by cholesterol extraction prevented both LatA-induced TER loss and occludin internalization. Thus, LatA-induced actin depolymerization causes TJ structural and functional disruption by mechanisms that include caveolae-mediated endocytosis of TJ components.

Crystallization and preliminary X-ray diffraction of the ZO-binding domain of human occludin

Occludin is a tight-junction protein controlling the integrity of endothelial and epithelial cell layers. It forms complexes with the cytoplasmic proteins ZO-1, ZO-2 and ZO-3. The ZO-binding domain in the C-terminal cytoplasmic region of human occludin has previously been isolated and identified. This domain, as expressed in a bacterial system or isolated from native cellular occludin, maintains its ability to bind ZO-1 and ZO-2. The crystallization conditions of the human ZO-binding domain are reported here. The crystals diffract to 2.3 A resolution and were shown to belong to the orthorhombic space group P2(1)2(1)2(1), with unit-cell parameters a = 33.3, b = 35.4, c = 107.3 A.

The tight junction protein occludin and the adherens junction protein alpha-catenin share a common interaction mechanism with ZO-1

The exact sites, structures, and molecular mechanisms of interaction between junction organizing zona occludence protein 1 (ZO-1) and the tight junction protein occludin or the adherens junction protein alpha-catenin are unknown. Binding studies by surface plasmon resonance spectroscopy and peptide mapping combined with comparative modeling utilizing crystal structures led for the first time to a molecular model revealing the binding of both occludin and alpha-catenin to the same binding site in ZO-1. Our data support a concept that ZO-1 successively associates with alpha-catenin at the adherens junction and occludin at the tight junction. Strong spatial evidence indicates that the occludin C-terminal coiled-coil domain dimerizes and interacts finally as a four-helix bundle with the identified structural motifs in ZO-1. The helix bundle of occludin406-521 and alpha-catenin509-906 interacts with the hinge region (ZO-1591-632 and ZO-1591-622, respectively) and with (ZO-1726-754 and ZO-1756-781) in the GuK domain of ZO-1 containing coiled-coil and alpha-helical structures, respectively. The selectivity of both protein-protein interactions is defined by complementary shapes and charges between the participating epitopes. In conclusion, a common molecular mechanism of forming an intermolecular helical bundle between the hinge region/GuK domain of ZO-1 and alpha-catenin and occludin is identified as a general molecular principle organizing the association of ZO-1 at adherens and tight junctions.