Association of Plasma Claudin-5 with Age and Alzheimer Disease

The blood-brain barrier (BBB) plays pivotal roles in synaptic and neuronal functioning by sealing the space between adjacent microvascular endothelial cells. BBB breakdown is present in patients with mild cognitive impairment (MCI) or Alzheimer disease (AD). Claudin-5 (CLDN-5) is a tetra-spanning protein essential for sealing the intercellular space between adjacent endothelial cells in the BBB. In this study, we developed a blood-based assay for CLDN-5 and investigated its diagnostic utility using 100 cognitively normal (control) subjects, 100 patients with MCI, and 100 patients with AD. Plasma CLDN-5 levels were increased in patients with AD (3.08 ng/mL) compared with controls (2.77 ng/mL). Plasma levels of phosphorylated tau (pTau181), a biomarker of pathological tau, were elevated in patients with MCI or AD (2.86 and 4.20 pg/mL, respectively) compared with control subjects (1.81 pg/mL). In patients with MCI or AD, plasma levels of CLDN-5-but not pTau181-decreased with age, suggesting some age-dependent BBB changes in MCI and AD. These findings suggest that plasma CLDN-5 may a potential biochemical marker for the diagnosis of AD.

Breaking barriers: exploring mechanisms behind opening the blood-brain barrier

The blood-brain barrier (BBB) is a selectively permeable membrane that separates the bloodstream from the brain. While useful for protecting neural tissue from harmful substances, brain-related diseases are difficult to treat due to this barrier, as it also limits the efficacy of drug delivery. To address this, promising new approaches for enhancing drug delivery are based on disrupting the BBB using physical means, including optical/photothermal therapy, electrical stimulation, and acoustic/mechanical stimulation. These physical mechanisms can temporarily and locally open the BBB, allowing drugs and other substances to enter. Focused ultrasound is particularly promising, with the ability to focus energies to targeted, deep-brain regions. In this review, we examine recent advances in physical approaches for temporary BBB disruption, describing their underlying mechanisms as well as evaluating the utility of these physical approaches with regard to their potential risks and limitations. While these methods have demonstrated efficacy in disrupting the BBB, their safety, comparative efficacy, and practicality for clinical use remain an ongoing topic of research.

Storming the gate: New approaches for targeting the dynamic tight junction for improved drug delivery

As biologics used in the clinic outpace the number of new small molecule drugs, an important challenge for their efficacy and widespread use has emerged, namely tissue penetrance. Macromolecular drugs - bulky, high-molecular weight, hydrophilic agents - exhibit low permeability across biological barriers. Epithelial and endothelial layers, for example within the gastrointestinal tract or at the blood-brain barrier, present the most significant obstacle to drug transport. Within epithelium, two subcellular structures are responsible for limiting absorption: cell membranes and intercellular tight junctions. Previously considered impenetrable to macromolecular drugs, tight junctions control paracellular flux and dictate drug transport between cells. Recent work, however, has shown tight junctions to be dynamic, anisotropic structures that can be targeted for delivery. This review aims to summarize new approaches for targeting tight junctions, both directly and indirectly, and to highlight how manipulation of tight junction interactions may help usher in a new era of precision drug delivery.

cCPE Fusion Proteins as Molecular Probes to Detect Claudins and Tight Junction Dysregulation in Gastrointestinal Cell Lines, Tissue Explants and Patient-Derived Organoids

Claudins regulate paracellular permeability, contribute to epithelial polarization and are dysregulated during inflammation and carcinogenesis. Variants of the claudin-binding domain of Clostridium perfringens enterotoxin (cCPE) are highly sensitive protein ligands for generic detection of a broad spectrum of claudins. Here, we investigated the preferential binding of YFP- or GST-cCPE fusion proteins to non-junctional claudin molecules. Plate reader assays, flow cytometry and microscopy were used to assess the binding of YFP- or GST-cCPE to non-junctional claudins in multiple in vitro and ex vivo models of human and rat gastrointestinal epithelia and to monitor formation of a tight junction barrier. Furthermore, YFP-cCPE was used to probe expression, polar localization and dysregulation of claudins in patient-derived organoids generated from gastric dysplasia and gastric cancer. Live-cell imaging and immunocytochemistry revealed cell polarity and presence of tight junctions in glandular organoids (originating from intestinal-type gastric cancer and gastric dysplasia) and, in contrast, a disrupted diffusion barrier for granular organoids (originating from discohesive tumor areas). In sum, we report the use of cCPE fusion proteins as molecular probes to specifically and efficiently detect claudin expression, localization and tight junction dysregulation in cell lines, tissue explants and patient-derived organoids of the gastrointestinal tract.

Cryo-EM structures of a synthetic antibody against 22 kDa claudin-4 reveal its complex with Clostridium perfringens enterotoxin

Claudins are a family of ∼25 kDa membrane proteins that integrate into tight junctions to form molecular barriers at the paracellular spaces between endothelial and epithelial cells. Humans have 27 subtypes, which homo- and hetero-oligomerize to impart distinct properties and physiological functions to tissues and organs. As the structural and functional backbone of tight junctions, claudins are attractive targets for therapeutics capable of modulating tissue permeability to deliver drugs or treat disease. However, structures of claudins are limited due to their small sizes and physicochemical properties-these traits also make therapy development a challenge. We have developed a synthetic antibody fragment (sFab) that binds human claudin-4 and used it to resolve structures of its complex with Clostridium perfringens enterotoxin (CpE) using cryogenic electron microscopy (cryo-EM). The resolution of the structures reveals the architectures of 22 kDa claudin-4, the 14 kDa C-terminal domain of CpE, and the mechanism by which this sFab binds claudins. Further, we elucidate the biochemical and biophysical bases of sFab binding and demonstrate that this molecule exhibits subtype-selectivity by assaying homologous claudins. Our results provide a framework for developing sFabs against hard-to-target claudins and establishes the utility of sFabs as fiducial markers for determining cryo-EM structures of this small membrane protein family at resolutions that surpass X-ray crystallography. Taken together, this work highlights the ability of sFabs to elucidate claudin structure and function and posits their potential as therapeutics for modulating tight junctions by targeting specific claudin subtypes.

The CLDN5 gene at the blood-brain barrier in health and disease

The CLDN5 gene encodes claudin-5 (CLDN-5) that is expressed in endothelial cells and forms tight junctions which limit the passive diffusions of ions and solutes. The blood-brain barrier (BBB), composed of brain microvascular endothelial cells and associated pericytes and end-feet of astrocytes, is a physical and biological barrier to maintain the brain microenvironment. The expression of CLDN-5 is tightly regulated in the BBB by other junctional proteins in endothelial cells and by supports from pericytes and astrocytes. The most recent literature clearly shows a compromised BBB with a decline in CLDN-5 expression increasing the risks of developing neuropsychiatric disorders, epilepsy, brain calcification and dementia. The purpose of this review is to summarize the known diseases associated with CLDN-5 expression and function. In the first part of this review, we highlight the recent understanding of how other junctional proteins as well as pericytes and astrocytes maintain CLDN-5 expression in brain endothelial cells. We detail some drugs that can enhance these supports and are being developed or currently in use to treat diseases associated with CLDN-5 decline. We then summarise mutagenesis-based studies which have facilitated a better understanding of the physiological role of the CLDN-5 protein at the BBB and have demonstrated the functional consequences of a recently identified pathogenic CLDN-5 missense mutation from patients with alternating hemiplegia of childhood. This mutation is the first gain-of-function mutation identified in the CLDN gene family with all others representing loss-of-function mutations resulting in mis-localization of CLDN protein and/or attenuated barrier function. Finally, we summarize recent reports about the dosage-dependent effect of CLDN-5 expression on the development of neurological diseases in mice and discuss what cellular supports for CLDN-5 regulation are compromised in the BBB in human diseases.

Establishing Co-Culture Blood–Brain Barrier Models for Different Neurodegeneration Conditions to Understand Its Effect on BBB Integrity

The blood–brain barrier (BBB) is a functional interface that provides selective permeability, protection from toxic substances, transport of nutrients, and clearance of brain metabolites. Additionally, BBB disruption has been shown to play a role in many neurodegenerative conditions and diseases. Therefore, the aim of this study was to establish a functional, convenient, and efficient in vitro co-cultured BBB model that can be used for several physiological conditions related to BBB disruption. Mouse brain-derived endothelial (bEnd.3) and astrocyte (C8-D1A) cells were co-cultured on transwell membranes to establish an intact and functional in vitro model. The co-cultured model and its effects on different neurological diseases and stress conditions, including Alzheimer’s disease (AD), neuroinflammation, and obesity, have been examined by transendothelial electrical resistance (TEER), fluorescein isothiocyanate (FITC) dextran, and tight junction protein analyses. Scanning electron microscope images showed evidence of astrocyte end-feet processes passing through the membrane of the transwell. Moreover, the co-cultured model showed effective barrier properties in the TEER, FITC, and solvent persistence and leakage tests when compared to the mono-cultured model. Additionally, the immunoblot results showed that the expression of tight junction proteins such as zonula occludens-1 (ZO-1), claudin-5, and occludin-1 was enhanced in the co-culture. Lastly, under disease conditions, the BBB structural and functional integrity was decreased. The present study demonstrated that the co-cultured in vitro model mimicked the BBB’s structural and functional integrity and, under disease conditions, the co-cultured model showed similar BBB damages. Therefore, the present in vitro BBB model can be used as a convenient and efficient experimental tool to investigate a wide range of BBB-related pathological and physiological studies.

Access to the CNS: Strategies to overcome the BBB

The blood-brain barrier (BBB) limits the access of substances to the central nervous system (CNS) which hinders the treatment of pathologies affecting the brain and the spinal cord. Nowadays, research is focus on new strategies to overcome the BBB and can treat the pathologies affecting the CNS are needed. In this review, the different strategies that allow and increase the access of substances to the CNS are analysed and extended commented, not only invasive strategies but also non-invasive ones. The invasive techniques include the direct injection into the brain parenchyma or the CSF and the therapeutic opening of the BBB, while the non-invasive techniques include the use of alternative routes of administration (nose-to-brain route), the inhibition of efflux transporters (as it is important to prevent the drug efflux from the brain and enhance the therapeutic efficiency), the chemical modification of the molecules (prodrugs and chemical drug delivery systems (CDDS)) and the use of nanocarriers. In the future, knowledge about nanocarriers to treat CNS diseases will continue to increase, but the use of other strategies such as drug repurposing or drug reprofiling, which are cheaper and less time consuming, may limit its transfer to society. The main conclusion is that the combination of different strategies may be the most interesting approach to increase the access of substances to the CNS.

Stress and the brain transcriptome: Identifying commonalities and clusters in standardized data from published experiments

Interpretation of transcriptomic experiments is hindered by many problems including false positives/negatives inherent to big-data methods and changes in gene nomenclature. To find the most consistent effect of stress on brain transcriptome, we retrieved data from 79 studies applying animal models and 3 human studies investigating post-traumatic stress disorder (PTSD). The analyzed data were obtained either with microarrays or RNA sequencing applied to samples collected from more than 1887 laboratory animals and from 121 human subjects. Based on the initial database containing a quarter million differential expression effect sizes representing transcripts in three species, we identified the most frequently reported genes in 223 stress-control comparisons. Additionally, the analysis considers sex, individual vulnerability and contribution of glucocorticoids. We also found an overlap between gene expression in PTSD patients and animals which indicates relevance of laboratory models for human stress response. Our analysis points to genes that, as far as we know, were not specifically tested for their role in stress response (Pllp, Arrdc2, Midn, Mfsd2a, Ccn1, Htra1, Csrnp1, Tenm4, Tnfrsf25, Sema3b, Fmo2, Adamts4, Gjb1, Errfi1, Fgf18, Galnt6, Slc25a42, Ifi30, Slc4a1, Cemip, Klf10, Tom1, Dcdc2c, Fancd2, Luzp2, Trpm1, Abcc12, Osbpl1a, Ptp4a2). Provided transcriptomic resource will be useful for guiding the new research.

Opportunities and challenges in delivering biologics for Alzheimer's disease by low-intensity ultrasound

Low-intensity ultrasound combined with intravenously injected microbubbles (US^+MB) is a novel treatment modality for brain disorders, including Alzheimer's disease (AD), safely and transiently allowing therapeutic agents to overcome the blood-brain barrier (BBB) that constitutes a major barrier for therapeutic agents. Here, we first provide an update on immunotherapies in AD and how US^+MB has been applied to AD mouse models and in clinical trials, considering the ultrasound and microbubble parameter space. In the second half of the review, we compare different in vitro BBB models and discuss strategies for combining US^+MB with BBB modulators (targeting molecules such as claudin-5), and highlight the insight provided by super-resolution microscopy. Finally, we conclude with a short discussion on how in vitro findings can inform the design of animal studies, and how the insight gained may aid treatment optimization in the clinical ultrasound space.

SARS-CoV-2 disrupts respiratory vascular barriers by suppressing Claudin-5 expression

In the initial process of coronavirus disease 2019 (COVID-19), severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infects respiratory epithelial cells and then transfers to other organs the blood vessels. It is believed that SARS-CoV-2 can pass the vascular wall by altering the endothelial barrier using an unknown mechanism. In this study, we investigated the effect of SARS-CoV-2 on the endothelial barrier using an airway-on-a-chip that mimics respiratory organs and found that SARS-CoV-2 produced from infected epithelial cells disrupts the barrier by decreasing Claudin-5 (CLDN5), a tight junction protein, and disrupting vascular endothelial cadherin-mediated adherens junctions. Consistently, the gene and protein expression levels of CLDN5 in the lungs of a patient with COVID-19 were decreased. CLDN5 overexpression or Fluvastatin treatment rescued the SARS-CoV-2-induced respiratory endothelial barrier disruption. We concluded that the down-regulation of CLDN5 expression is a pivotal mechanism for SARS-CoV-2-induced endothelial barrier disruption in respiratory organs and that inducing CLDN5 expression is a therapeutic strategy against COVID-19.

A Historical Review of Brain Drug Delivery

The history of brain drug delivery is reviewed beginning with the first demonstration, in 1914, that a drug for syphilis, salvarsan, did not enter the brain, due to the presence of a blood-brain barrier (BBB). Owing to restricted transport across the BBB, FDA-approved drugs for the CNS have been generally limited to lipid-soluble small molecules. Drugs that do not cross the BBB can be re-engineered for transport on endogenous BBB carrier-mediated transport and receptor-mediated transport systems, which were identified during the 1970s-1980s. By the 1990s, a multitude of brain drug delivery technologies emerged, including trans-cranial delivery, CSF delivery, BBB disruption, lipid carriers, prodrugs, stem cells, exosomes, nanoparticles, gene therapy, and biologics. The advantages and limitations of each of these brain drug delivery technologies are critically reviewed.

Claudin-5a is essential for the functional formation of both zebrafish blood-brain barrier and blood-cerebrospinal fluid barrier

Background

Mammalian Claudin-5 is the main endothelial tight junction component maintaining blood-brain barrier (BBB) permeability, while Claudin-1 and -3 seal the paracellular space of choroid plexus (CP) epithelial cells contributing to the blood-cerebrospinal fluid barrier (BCSFB). In zebrafish, two paralogs of claudin-5a and -5b are expressed while their roles in the formation of BBB and BCSFB are unclear.

Methods

The expression patterns of Claudin-5a and -5b in zebrafish brains were systematically analyzed by immunofluorescence (IF) assay. The developmental functions of Claudin-5a and -5b were characterized by generating of claudin-5a and -5b mutants respectively. Meanwhile, the cerebral inflammation and cell apoptosis in claudin-5a^-/- were assessed by live imaging of transgenic zebrafish, RT-qPCR, IF, and TUNEL assay. The integrity of BBB and BCSFB was evaluated by in vivo angiographic and dye permeation assay. Finally, RT-qPCR, whole-mount RNA in situ hybridization (WISH), and transmission electron microscopy (TEM) analyses were performed to investigate the development of cerebral vessels and choroid plexus.

Results

We showed that Claudin-5a and -5b are both expressed in zebrafish cerebrovascular endothelial cells (ECs). In addition, Claudin-5a was strongly expressed in CP epithelial cells. Loss of Claudin-5b showed no effect on zebrafish vasculogenesis or BBB function. In contrast, the knockout of claudin-5a caused a lethal phenotype of severe whole-brain oedema, ventricular dilatation, and cerebral hernia in zebrafish larvae, although the cerebral vasculogenesis and the development of CP were not altered. In claudin-5a^-/- , although ultrastructural analysis of CP and cerebral capillary showed intact integrity of epithelial and endothelial tight junctions, permeability assay indicated a disruption of both BBB and BCSFB functions. On the molecular level, it was found that ZO-1 was upregulated in the CP epithelium of claudin-5a^-/-, while the notch and shh pathway responsible for CP development was not affected due to loss of Claudin-5a.

Conclusions

Our findings verified a non-functional role of zebrafish Claudin-5b in the BBB and identified Claudin-5a as the ortholog of mammalian Claudin-5, contributing to the development and the functional maintenance of both BBB and BCSFB.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12987-022-00337-9.

Modifying the blood-brain barrier by targeting claudin-5: Safety and risks

The blood-brain barrier is a major obstacle to the delivery of drugs to the central nervous system. In the blood-brain barrier, the spaces between adjacent brain microvascular endothelial cells are sealed by multiprotein complexes known as tight junctions. Among the many components of the tight junction, claudin-5 has received the most attention as a target for loosening the tight-junction seal and allowing drugs to be delivered to the brain. In mice, transient knockdown of claudin-5 and the use of claudin-5 binders have been shown to enhance the permeation of small molecules from the blood into the brain without apparent adverse effects. However, sustained knockdown of claudin-5 in mice is lethal within 40 days, and administration of an anti-claudin-5 antibody induced convulsions in a nonhuman primate. Here, we review the safety concerns of claudin-5-targeted technologies with respect to their clinical application.

Modulation of the Blood-Brain Barrier for Drug Delivery to Brain

The blood-brain barrier (BBB) precisely controls brain microenvironment and neural activity by regulating substance transport into and out of the brain. However, it severely hinders drug entry into the brain, and the efficiency of various systemic therapies against brain diseases. Modulation of the BBB via opening tight junctions, inhibiting active efflux and/or enhancing transcytosis, possesses the potential to increase BBB permeability and improve intracranial drug concentrations and systemic therapeutic efficiency. Various strategies of BBB modulation have been reported and investigated preclinically and/or clinically. This review describes conventional and emerging BBB modulation strategies and related mechanisms, and safety issues according to BBB structures and functions, to try to give more promising directions for designing more reasonable preclinical and clinical studies.

Modulating the Blood-Brain Barrier: A Comprehensive Review

The highly secure blood-brain barrier (BBB) restricts drug access to the brain, limiting the molecular toolkit for treating central nervous system (CNS) diseases to small, lipophilic drugs. Development of a safe and effective BBB modulator would revolutionise the treatment of CNS diseases and future drug development in the area. Naturally, the field has garnered a great deal of attention, leading to a vast and diverse range of BBB modulators. In this review, we summarise and compare the various classes of BBB modulators developed over the last five decades-their recent advancements, advantages and disadvantages, while providing some insight into their future as BBB modulators.

Claudin-5: A Pharmacological Target to Modify the Permeability of the Blood-Brain Barrier

Claudin-5 is the dominant tight junction protein in brain endothelial cells and exclusively limits the paracellular permeability of molecules larger than 400 Da across the blood-brain barrier (BBB). Its pathological impairment or sustained down-regulation has been shown to lead to the progression of psychiatric and neurological disorders, whereas its expression under physiological conditions prevents the passage of drugs across the BBB. While claudin-5 enhancers could potentially act as vascular stabilizers to treat neurological diseases, claudin-5 inhibitors could function as delivery systems to enhance the brain uptake of hydrophilic small-molecular-weight drugs. Therefore, the effects of claudin-5 manipulation on modulating the BBB in different neurological diseases requires further examination. To manipulate claudin-5 expression levels and function, several claudin-5 modulating molecules have been developed. In this review, we first describe the molecular, cellular and pathological aspects of claudin-5 to highlight the mechanisms of claudin-5 enhancers/inhibitors. We then discuss recently developed claudin-5 enhancers/inhibitors and new methods to discover these molecules.

Characterization of a Primate Blood-Brain Barrier Co-Culture Model Prepared from Primary Brain Endothelial Cells, Pericytes and Astrocytes

Culture models of the blood-brain barrier (BBB) are important research tools. Their role in the preclinical phase of drug development to estimate the permeability for potential neuropharmaceuticals is especially relevant. Since species differences in BBB transport systems exist, primate models are considered as predictive for drug transport to brain in humans. Based on our previous expertise we have developed and characterized a non-human primate co-culture BBB model using primary cultures of monkey brain endothelial cells, rat brain pericytes, and rat astrocytes. Monkey brain endothelial cells in the presence of both pericytes and astrocytes (EPA model) expressed enhanced barrier properties and increased levels of tight junction proteins occludin, claudin-5, and ZO-1. Co-culture conditions also elevated the expression of key BBB influx and efflux transporters, including glucose transporter-1, MFSD2A, ABCB1, and ABCG2. The correlation between the endothelial permeability coefficients of 10 well known drugs was higher (R² = 0.8788) when the monkey and rat BBB culture models were compared than when the monkey culture model was compared to mouse in vivo data (R² = 0.6619), hinting at transporter differences. The applicability of the new non-human primate model in drug discovery has been proven in several studies.

M01 as a novel drug enhancer for specifically targeting the blood-brain barrier

Drug delivery to the brain is limited for most pharmaceuticals by the blood-brain barrier (BBB) where claudin-5 dominates the paraendothelial tightening. For circumventing the BBB, we identified the compound M01 as a claudin-5 interaction inhibitor. M01 causes transient permeabilisation of the BBB depending on the concentration of small molecules in different cell culture models within 3 to 48 h. In mice, brain uptake of fluorescein peaked within the first 3 h after M01 injection and normalised within 48 h. Compared to the cytostatic paclitaxel alone, M01 improved delivery of paclitaxel to mouse brain and reduced orthotopic glioblastoma growth. Results on interactions of M01 with claudin-5 were incorporated into a binding model which suggests association of its aromatic parts with highly conserved residues of the extracellular domain of claudin-5 and adjacent transmembrane segments. Our results indicate the following mode of action: M01 preferentially binds to the extracellular claudin-5 domain, which weakens trans-interactions between adhering cells. Further decrease in membranous claudin-5 levels due to internalization and transcriptional downregulation enables the paracellular passage of small molecules. In summary, the first small molecule is introduced here as a drug enhancer, which specifically permeabilises the BBB for a sufficient interval for allowing neuropharmaceuticals to enter the brain.

Targeting claudins in cancer: diagnosis, prognosis and therapy

Increasing evidence has linked claudins to signal transduction and tumorigenesis. The expression of claudins is frequently dysregulated in the context of neoplastic transformation, suggesting their promise as biomarkers for diagnosis and prognosis or targets for treatment. Claudin binders (Clostridium perfringens enterotoxin and monoclonal antibody) have been tested in preclinical experiments, and some of them have progressed into clinical trials involving patients with certain cancers. However, the clinical development of many of these agents has not advanced to clinical applications. Herein, I review the current status of preclinical and clinical investigations of agents targeting claudins for diagnosis, prognosis and therapy. I also discuss the potential of combining claudin binders with other currently approved therapeutic agents.

Safety and efficacy of an anti-claudin-5 monoclonal antibody to increase blood-brain barrier permeability for drug delivery to the brain in a non-human primate

Claudin-5 (CLDN-5) is an essential component of the tight junction seal in the blood-brain barrier. Previously, we showed that CLDN-5 modulation in vitro via an anti-CLDN-5 monoclonal antibody (mAb) may be useful for increasing the permeability of the blood-brain barrier for drug delivery to the brain. Based on these findings, here we examined the safety and efficacy of the anti-CLDN-5 mAb in a non-human primate. Cynomolgus monkeys were intravenously administered the anti-CLDN-5 mAb followed by fluorescein dye (376 Da), and the concentrations of the dye in the cerebrospinal fluid was examined. When the mAb was administered at 3.0 mg/kg, the concentration of dye in the cerebrospinal fluid was increased, and no behavioral changes or changes in plasma biomarkers for inflammation or liver or kidney injury were observed. However, a monkey that received the mAb at 6 mg/kg experienced convulsions, and subsequent histopathological examination of this animal revealed vasodilation in the liver, lung, and kidney; hemorrhage in the lung; and edema in the brain. Together, our data indicate that CLDN-5 might be a potential target for enhancing drug delivery to the brain, but also that the therapeutic window of the anti-CLDN-5 mAb may be narrow for separating efficacy and toxicity.

Tight Junction Modulating Bioprobes for Drug Delivery System to the Brain: A Review

The blood-brain barrier (BBB), which is composed of endothelial cells, pericytes, astrocytes, and neurons, separates the brain extracellular fluid from the circulating blood, and maintains the homeostasis of the central nervous system (CNS). The BBB endothelial cells have well-developed tight junctions (TJs) and express specific polarized transport systems to tightly control the paracellular movements of solutes, ions, and water. There are two types of TJs: bicellular TJs (bTJs), which is a structure at the contact of two cells, and tricellular TJs (tTJs), which is a structure at the contact of three cells. Claudin-5 and angulin-1 are important components of bTJs and tTJs in the brain, respectively. Here, we review TJ-modulating bioprobes that enable drug delivery to the brain across the BBB, focusing on claudin-5 and angulin-1.

A blood-brain barrier overview on structure, function, impairment, and biomarkers of integrity

The blood–brain barrier is playing a critical role in controlling the influx and efflux of biological substances essential for the brain’s metabolic activity as well as neuronal function. Thus, the functional and structural integrity of the BBB is pivotal to maintain the homeostasis of the brain microenvironment. The different cells and structures contributing to developing this barrier are summarized along with the different functions that BBB plays at the brain–blood interface. We also explained the role of shear stress in maintaining BBB integrity. Furthermore, we elaborated on the clinical aspects that correlate between BBB disruption and different neurological and pathological conditions. Finally, we discussed several biomarkers that can help to assess the BBB permeability and integrity in-vitro or in-vivo and briefly explain their advantages and disadvantages.

Blood-Brain Barrier Protein Claudin-5 Expressed in Paired Xenopus laevis Oocytes Mediates Cell-Cell Interaction

Claudin-5 determines the sealing properties of blood-brain barrier tight junctions and its function is impaired in neurodegenerative and neuroinflammatory disorders. Focusing on the contribution of claudin-5 to the trans-interaction within the tight junction seal, we used Xenopus laevis oocytes as an expression system. Cells were clustered and challenged in a novel approach for the analysis of claudin interaction. We evaluated the strengthening effect of claudin-5 to cell-cell-connection in comparison to claudin-3. Application of a hydrostatic pressure impulse on clustered control oocyte pairs revealed a reduction of contact areas. In contrast, combinations with both oocytes expressing claudins maintained an enhanced connection between the cells (cldn5-cldn5, cldn3-cldn3). Strength of interaction was increased by both claudin-3 and claudin-5. This novel approach allowed an analysis of single claudins contributing to tight junction integrity, characterizing homophilic and hetrophilic trans-interaction of claudins. To test a new screening approach for barrier effectors, exemplarily, this 2-cell model of oocytes was used to analyze the effect of the absorption enhancer sodium caprate on the oocyte pairs.

miR-331 inhibits CLDN2 expression and may alleviate the vascular endothelial injury induced by sepsis

The present study aimed to determine the expression level of claudin-2 (CLDN2) in the peripheral blood of patients with sepsis, and to investigate its potential function and mechanism of action in vascular endothelial injury. A total of 25 patients with sepsis were included in the present study. Reverse transcription-quantitative PCR was used to determine CLDN2 levels in peripheral blood. HUVECs stably expressing CLDN2 were prepared and Cell Counting Kit-8, flow cytometry and Transwell assays were performed to study the proliferation, apoptosis and migration of HUVECs, respectively. Using bioinformatics, microRNA (miR) molecules that interact with CLDN2 were predicted. A dual luciferase reporter assay was used to test whether miR-331 regulated CLDN2. Western blotting was employed to determine CLDN2 protein expression. In addition, in vitro transfection of HUVECs with miR-331 mimics was performed to test the rescue effects of miR-331 on the cell function changes induced by CLDN2. The results indicated that elevated CLDN2 expression altered the proliferation and cell cycle of peripheral vascular endothelial cells. CLDN2 overexpression inhibited HUVEC proliferation via mechanisms not associated with the cell cycle. CLDN2 mRNA levels in the peripheral blood of patients with sepsis were significantly higher than those in healthy subjects. Upregulated CLDN2 expression promoted the apoptosis of HUVECs, but reduced their proliferation and migration. Notably, miR-331 was able to bind with CLDN2 mRNA and regulate its expression. Upregulation of miR-331 expression inhibited the expression of CLDN2 and restored nearly normal proliferation, apoptosis and migration to HUVECs. The present study demonstrated that CLDN2 expression is elevated in peripheral blood from patients with sepsis, and promotes the injury of vascular endothelial cells. In addition, miR-331 participates in the direct regulation of CLDN2, and upregulation of miR-331 expression inhibits the expression of CLDN2 and restores cellular functions to HUVECs.

Tight junction modulation at the blood-brain barrier: Current and future perspectives

The blood-brain barrier (BBB) is the one of the most robust physical barriers in the body, comprised of tight junction (TJ) proteins in brain microvascular endothelial cells. The need for drugs to treat central nervous systems diseases is ever increasing, however the presence of the BBB significantly hampers the uptake of drugs into the brain. To overcome or circumvent the barrier, many kinds of techniques are being developed. Modulating the paracellular route by disruption of the TJ complex has been proposed as a potential drug delivery system to treat brain diseases, however, it has several limitations and is still in a developmental stage. However, recent significant advance in medical equipment /tools such as targeted ultra-sound technologies may resolve these limitations. In this review, we introduce recent advances in site- or molecular size-selective BBB disruption/modulation technologies and we include details on pharmacological inhibitory molecules against intercellular TJ proteins to modulate the BBB.

Targeting claudin-overexpressing thyroid and lung cancer by modified Clostridium perfringens enterotoxin

Clostridium perfringens enterotoxin (CPE) can be used to eliminate carcinoma cells that overexpress on their cell surface CPE receptors - a subset of claudins (e.g., Cldn3 and Cldn4). However, CPE cannot target tumors expressing solely CPE-insensitive claudins (such as Cldn1 and Cldn5). To overcome this limitation, structure-guided modifications were used to generate CPE variants that can strongly bind to Cldn1, Cldn2 and/or Cldn5, while maintaining the ability to bind Cldn3 and Cldn4. This enabled (a) targeting of the most frequent endocrine malignancy, namely, Cldn1-overexpressing thyroid cancer, and (b) improved targeting of the most common cancer type worldwide, non-small-cell lung cancer (NSCLC), which is characterized by high expression of several claudins, including Cldn1 and Cldn5. Different CPE variants, including the novel mutant CPE-Mut3 (S231R/S313H), were applied on thyroid cancer (K1 cells) and NSCLC (PC-9 cells) models. In vitro, CPE-Mut3, but not CPEwt, showed Cldn1-dependent binding and cytotoxicity toward K1 cells. For PC-9 cells, CPE-Mut3 improved claudin-dependent cytotoxic targeting, when compared to CPEwt. In vivo, intratumoral injection of CPE-Mut3 in xenograft models bearing K1 or PC-9 tumors induced necrosis and reduced the growth of both tumor types. Thus, directed modification of CPE enables eradication of tumor entities that cannot be targeted by CPEwt, for instance, Cldn1-overexpressing thyroid cancer by using the novel CPE-Mut3.

Potential for Tight Junction Protein-Directed Drug Development Using Claudin Binders and Angubindin-1

The tight junction (TJ) is an intercellular sealing component found in epithelial and endothelial tissues that regulates the passage of solutes across the paracellular space. Research examining the biology of TJs has revealed that they are complex biochemical structures constructed from a range of proteins including claudins, occludin, tricellulin, angulins and junctional adhesion molecules. The transient disruption of the barrier function of TJs to open the paracellular space is one means of enhancing mucosal and transdermal drug absorption and to deliver drugs across the blood–brain barrier. However, the disruption of TJs can also open the paracellular space to harmful xenobiotics and pathogens. To address this issue, the strategies targeting TJ proteins have been developed to loosen TJs in a size- or tissue-dependent manner rather than to disrupt them. As several TJ proteins are overexpressed in malignant tumors and in the inflamed intestinal tract, and are present in cells and epithelia conjoined with the mucosa-associated lymphoid immune tissue, these TJ-protein-targeted strategies may also provide platforms for the development of novel therapies and vaccines. Here, this paper reviews two TJ-protein-targeted technologies, claudin binders and an angulin binder, and their applications in drug development.

[Transition of the Field from Biochemical Engineering to Pharmaceutical Sciences during 40 Years of the Research]

This review reflects back over almost 40 years of the author's basic research conducted at Graduate School of Pharmaceutical Sciences, Osaka University, Japan. After performing postdoctoral research in USA, the author became a research associate at Prof. Yoshiharu Miura's lab and started research on Biochemical Engineering in 1984. At that time, the main research purpose was to solve global environmental issues for maintaining human health. The author's achievements included novel useful material production system under inorganic conditions and genetically engineered whole-cell bacterial sensors detecting arsenite by naked eye without a detecting device. Another theme in the lab was to construct bioartificial liver support system. Various scaffolds for hepatocytes were newly prepared for constructing the compact reactor. Besides the bioreactor study, the author conducted cell transplantation research for the treatment of chronic liver diseases. It was shown that mesenchymal stem cells derived from third molars (wisdom teeth) could differentiate into hepatocytes and exhibit therapeutic effects in liver-damaged animals. After 2006, the lab started research on drug delivery systems, including noninvasive delivery of drugs such as peptides and nucleic acids by regulating epithelial tight junctions. Many substances enabling drug delivery through "paracellular" route were newly prepared. The author started basic research on Biochemical Engineering in the 1970s. Although these studies eventually shifted into the pharmaceutical field, the underlying concept was based on "engineering" throughout a 40-year research period. The author cordially thanks all colleagues for supporting engineering research in our lab.

A Method to Prepare Claudin-Modulating Recombinant Proteins

The epithelium forms tight junctions by sealing the paracellular space, and tight junctions prevent the free movement of solutes. Claudin is an important structural and functional component of tight junctions and contributes to the formation of paracellular pathways for different populations of size- and charge-selective solutes. Therefore, modulation of tight junctions is important to develop drug delivery strategies. Clostridium perfringens enterotoxin (CPE) causes food poisoning in humans and is a 35-kDa polypeptide, consisting of 319 amino acids and two functional regions. The C-terminal region of CPE (C-CPE) is not cytotoxic and binds to its receptor claudin, which in turn modulates the epithelial tight junction barrier. Thus, claudin binders, such as C-CPE, are useful tools for drug delivery targeting tight junctions. Here, we provide a protocol for the expression and purification of recombinant C-CPE proteins as claudin binders, an analysis method for C-CPE binding affinity, and a procedure for assessing the effect of modulating tight junction integrity.

Brain endothelial cell junctions after cerebral hemorrhage: Changes, mechanisms and therapeutic targets

Vascular disruption is the underlying cause of cerebral hemorrhage, including intracerebral, subarachnoid and intraventricular hemorrhage. The disease etiology also involves cerebral hemorrhage-induced blood-brain barrier (BBB) disruption, which contributes an important component to brain injury after the initial cerebral hemorrhage. BBB loss drives vasogenic edema, allows leukocyte extravasation and may lead to the entry of potentially neurotoxic and vasoactive compounds into brain. This review summarizes current information on changes in brain endothelial junction proteins in response to cerebral hemorrhage (and clot-related factors), the mechanisms underlying junction modification and potential therapeutic targets to limit BBB disruption and, potentially, hemorrhage occurrence. It also addresses advances in the tools that are now available for assessing changes in junctions after cerebral hemorrhage and the potential importance of such junction changes. Recent studies suggest post-translational modification, conformational change and intracellular trafficking of junctional proteins may alter barrier properties. Understanding how cerebral hemorrhage alters BBB properties beyond changes in tight junction protein loss may provide important therapeutic insights to prevent BBB dysfunction and restore normal function.

Engineered membrane protein antigens successfully induce antibodies against extracellular regions of claudin-5

The production of antibodies against the extracellular regions (ECR) of multispanning membrane proteins is notoriously difficult because of the low productivity and immunogenicity of membrane proteins due to their complex structure and highly conserved sequences among species. Here, we introduce a new method to generate ECR-binding antibodies utilizing engineered liposomal immunogen prepared using a wheat cell-free protein synthesis system. We used claudin-5 (CLDN-5) as the target antigen, which is a notoriously difficult to produce and poorly immunogenic membrane protein with two highly conserved extracellular loops. We drastically improved the productivity of CLDN-5 in the cell-free system after suppressing and normalizing mRNA GC content. To overcome its low immunogenicity, two engineered antigens were designed and synthesized as proteoliposomes: a human/mouse chimeric CLDN-5, and a CLDN-5-based artificial membrane protein consisting of symmetrically arranged ECRs. Intraperitoneal immunization of both engineered CLDN-5 ECR antigens induced ECR-binding antibodies in mice with a high success rate. We isolated five monoclonal antibodies that specifically recognized CLDN-5 ECR. Antibody clone 2B12 showed high affinity (<10 nM) and inhibited CLDN-5-containing tight junctions. These results demonstrate the effectiveness of the methods for monoclonal antibody development targeting difficult-to-produce membrane proteins such as CLDNs.

Angubindin-1 opens the blood-brain barrier in vivo for delivery of antisense oligonucleotide to the central nervous system

Within the field of RNA therapeutics, antisense oligonucleotide-based therapeutics are a potentially powerful means of treating intractable diseases. However, if these therapeutics are used for the treatment of neurological disorders, safe yet efficient methods of delivering antisense oligonucleotides across the blood-brain barrier to the central nervous system must be developed. Here, we examined the use of angubindin-1, a binder to the tricellular tight junction, to modulate paracellular transport between brain microvascular endothelial cells in the blood-brain barrier for the delivery of antisense oligonucleotides to the central nervous system. This proof-of-concept study demonstrated that intravenously injected angubindin-1 increased the permeability of the blood-brain barrier and enabled transient delivery of subsequently administered antisense oligonucleotides into the mouse brain and spinal cord, leading to silencing of a target RNA without any overt adverse effects. We also found that two bicellular tight junction modulators did not produce such a silencing effect, suggesting that the tricellular tight junction is likely a better target for the delivery of antisense oligonucleotides than the bicellular tight junction. Our delivery strategy of modulating the tricellular tight junction in the blood-brain barrier via angubindin-1 provides a novel avenue of research for the development of antisense oligonucleotide-based therapeutics for the treatment of neurological disorders.

Progress in brain barriers and brain fluid research in 2017

The past year, 2017, has seen many important papers published in the fields covered by Fluids and Barriers of the CNS. This article from the Editors highlights some.