Rapid remodeling of tight junctions during paracellular diapedesis in a human model of the blood-brain barrier

Sex- and species-specific contribution of CD99 to T cell costimulation during multiple sclerosis

BACKGROUND

Differences in immune responses between women and men are leading to a strong sex bias in the incidence of autoimmune diseases that predominantly affect women, such as multiple sclerosis (MS). MS manifests in more than twice as many women, making sex one of the most important risk factor. However, it is incompletely understood which genes contribute to sex differences in autoimmune incidence. To address that, we conducted a gene expression analysis in female and male human spleen and identified the transmembrane protein CD99 as one of the most significantly differentially expressed genes with marked increase in men. CD99 has been reported to participate in immune cell transmigration and T cell regulation, but sex-specific implications have not been comprehensively investigated.

METHODS

In this study, we conducted a gene expression analysis in female and male human spleen using the Genotype-Tissue Expression (GTEx) project dataset to identify differentially expressed genes between women and men. After successful validation on protein level of human immune cell subsets, we assessed hormonal regulation of CD99 as well as its implication on T cell regulation in primary human T cells and Jurkat T cells. In addition, we performed in vivo assays in wildtype mice and in Cd99-deficient mice to further analyze functional consequences of differential CD99 expression.

RESULTS

Here, we found higher CD99 gene expression in male human spleens compared to females and confirmed this expression difference on protein level on the surface of T cells and pDCs. Androgens are likely dispensable as the cause shown by in vitro assays and ex vivo analysis of trans men samples. In cerebrospinal fluid, CD99 was higher on T cells compared to blood. Of note, male MS patients had lower CD99 levels on CD4+ T cells in the CSF, unlike controls. By contrast, both sexes had similar CD99 expression in mice and Cd99-deficient mice showed equal susceptibility to experimental autoimmune encephalomyelitis compared to wildtypes. Functionally, CD99 increased upon human T cell activation and inhibited T cell proliferation after blockade. Accordingly, CD99-deficient Jurkat T cells showed decreased cell proliferation and cluster formation, rescued by CD99 reintroduction.

CONCLUSIONS

Our results demonstrate that CD99 is sex-specifically regulated in healthy individuals and MS patients and that it is involved in T cell costimulation in humans but not in mice. CD99 could potentially contribute to MS incidence and susceptibility in a sex-specific manner.

In Vitro Blood-Brain Barrier Models for Neuroinfectious Diseases: A Narrative Review

The blood-brain barrier (BBB) is a complex, dynamic, and adaptable barrier between the peripheral blood system and the central nervous system. While this barrier protects the brain and spinal cord from inflammation and infection, it prevents most drugs from reaching the brain tissue. With the expanding interest in the pathophysiology of BBB, the development of in vitro BBB models has dramatically evolved. However, due to the lack of a standard model, a range of experimental protocols, BBB-phenotype markers, and permeability flux markers was utilized to construct in vitro BBB models. Several neuroinfectious diseases are associated with BBB dysfunction. To conduct neuroinfectious disease research effectively, there stems a need to design representative in vitro human BBB models that mimic the BBB's functional and molecular properties. The highest necessity is for an in vitro standardised BBB model that accurately represents all the complexities of an intact brain barrier. Thus, this in-depth review aims to describe the optimization and validation parameters for building BBB models and to discuss previous research on neuroinfectious diseases that have utilized in vitro BBB models. The findings in this review may serve as a basis for more efficient optimisation, validation, and maintenance of a structurally- and functionally intact BBB model, particularly for future studies on neuroinfectious diseases.

Factors regulating the differences in frequency of infiltration of Th17 and Treg of the blood-brain barrier

Controlling CD4+ immune cell infiltration of the brain is a leading aim in designing therapeutic strategies for a range of neuropathological disorders such as multiple sclerosis, Alzheimer's disease, and depression. CD4+ T cells are a highly heterogeneous and reprogrammable family, which includes various distinctive cell types such as Th17, Th1, and Treg cells. Interestingly Th17 and Treg cells share a related transcriptomic profile, where the TGFβ-SMADS pathway plays a fundamental role in regulating the differentiation of both of these cell types. However, Th17 could be highly pathogenic and was shown to promote inflammation in various neuropathological disorders. Conversely, Treg is anti-inflammatory and is known to inhibit Th17. It could be noticed that Th17 frequencies of infiltration of the blood-brain barrier in various neurological disorders are significantly upregulated. However, Treg infiltration numbers are significantly low. The reasons behind these contradicting observations are still unknown. In this perspective, we propose that the difference in the T-cell receptor repertoire diversity, diapedesis pathways, chemokine expression, and mechanical properties of these two cell types could be contributing to answering this intriguing question.

Identification of key claudin genes associated with survival prognosis and diagnosis in colon cancer through integrated bioinformatic analysis

The claudin multigene family is associated with various aberrant physiological and cellular signaling pathways. However, the association of claudins with survival prognosis, signaling pathways, and diagnostic efficacy in colon cancer remains poorly understood. Methods: Through the effective utilization of various bioinformatics methods, including differential gene expression analysis, gene set enrichment analysis protein-protein interaction (PPI) network analysis, survival analysis, single sample gene set enrichment analysis (ssGSEA), mutational variance analysis, and identifying receiver operating characteristic curve of claudins in The Cancer Genome Atlas colon adenocarcinoma (COAD). Results: We found that: CLDN2, CLDN1, CLDN14, CLDN16, CLDN18, CLDN9, CLDN12, and CLDN6 are elevated in COAD. In contrast, the CLDN8, CLDN23, CLDN5, CLDN11, CLDN7, and CLDN15 are downregulated in COAD. By analyzing the public datasets GSE15781 and GSE50760 from NCBI-GEO (https://www.ncbi.nlm.nih.gov/geo/), we have confirmed that CLDN1, CLDN2, and CLDN14 are significantly upregulated and CLDN8 and CLDN23 are significantly downregulated in normal colon, colon adenocarcinoma tumor, and liver metastasis of colon adenocarcinoma tissues from human samples. Various claudins are mutated and found to be associated with diagnostic efficacy in COAD. Conclusion: The claudin gene family is associated with prognosis, immune regulation, signaling pathway regulations, and diagnosis of COAD. These findings may provide new molecular insight into claudins in the treatment of colon cancer.

Modeling Brain Vasculature Immune Interactions In Vitro

The endothelial blood-brain barrier (BBB) protects central nervous system (CNS) neurons from the changeable milieu of the bloodstream by strictly controlling the movement of molecules and immune cells between the blood and the CNS. Immune cell migration across the vascular wall is a multistep process regulated by the sequential interaction of different signaling and adhesion molecules on the endothelium and the immune cells. Accounting for its unique barrier properties and trafficking molecule expression profile, particular adaptions in immune cell migration across the BBB have been observed. Thus, in vitro models of the BBB are desirable to explore the precise cellular and molecular mechanisms involved in immune cell trafficking across the BBB. The challenge to overcome is that barrier properties of brain microvascular endothelial cells are not intrinsic and readily lost in culture. With a focus on human in vitro BBB models, we here discuss the suitability of available in vitro models for the BBB for exploring the specific mechanisms involved in immune cell trafficking across the BBB.

Septin and actin contributions to endothelial cell-cell junctions and monolayer integrity

Septins in endothelial cells (ECs) have important roles supporting the integrity of the endothelial monolayer. Cell-cell junctions in EC monolayers are highly dynamic, with continuous retractions and protrusions. Depletion of septins in ECs leads to disruption of cell-cell junctions, which are composed of VE-cadherin and other junctional proteins. In EC monolayers, septins are concentrated at the plasma membrane at sites of cell-cell contact, in curved- and scallop-shaped patterns. These membrane-associated septin accumulations are located in regions of positive membrane curvature, and those regions are often associated with and immediately adjacent to actin-rich protrusions with negative membrane curvature. EC septins associate directly with plasma membrane lipids, based on findings with site-specific mutations of septins in ECs, which is consistent with biochemical and cell biological studies in other systems. Loss of septins leads to disruption of the EC monolayer, and gaps form between cells. The number and breadth of cell-cell contacts and junctions decreases, and the number and frequency of retractions, ruffles, and protrusions at cell edges also decreases. In addition, loss of septins leads to decreased amounts of F-actin at the cortical membrane, along with increased amounts of F-actin in stress fibers of the cytoplasm. Endothelial monolayer disruption from loss of septins is also associated with decreased transendothelial electric resistance (TEER) and increased levels of transendothelial migration (TEM) by immune and cancer cells, owing to the gaps in the monolayer. A current working model is that assembly of septin filaments at regions of positive membrane curvature contributes to a mechanical footing or base for actin-based protrusive forces generated at adjoining regions of the membrane. Specific molecular interactions between the septin and actin components of the cytoskeleton may also be important contributors. Regulators of actin assembly may promote and support the assembly of septin filaments at the membrane, as part of a molecular feedback loop between the assembly of septin and actin filaments.

In vivo methods for imaging blood-brain barrier function and dysfunction

The blood-brain barrier (BBB) is the interface between the central nervous system and systemic circulation. It tightly regulates what enters and is removed from the brain parenchyma and is fundamental in maintaining brain homeostasis. Increasingly, the BBB is recognised as having a significant role in numerous neurological disorders, ranging from acute disorders (traumatic brain injury, stroke, seizures) to chronic neurodegeneration (Alzheimer's disease, vascular dementia, small vessel disease). Numerous approaches have been developed to study the BBB in vitro, in vivo, and ex vivo. The complex multicellular structure and effects of disease are difficult to recreate accurately in vitro, and functional aspects of the BBB cannot be easily studied ex vivo. As such, the value of in vivo methods to study the intact BBB cannot be overstated. This review discusses the structure and function of the BBB and how these are affected in diseases. It then discusses in depth several established and novel methods for imaging the BBB in vivo, with a focus on MRI, nuclear imaging, and high-resolution intravital fluorescence microscopy.

Investigating Pathways of Ventilation Induced Brain Injury on Cerebral White Matter Inflammation and Injury After 24 h in Preterm Lambs

Initiation of respiratory support in the delivery room increases the risk and severity of brain injury in preterm neonates through two major pathways: an inflammatory pathway and a haemodynamic pathway. The relative contribution of each pathway on preterm brain injury is not known. We aimed to assess the role of the inflammatory and haemodynamic pathway on ventilation-induced brain injury (VIBI) in the preterm lamb. Fetal lambs (125 ± 1 day gestation) were exteriorised, instrumented and ventilated with a high tidal-volume (V_T) injurious strategy for 15 min either with placental circulation intact to induce the inflammatory pathway only (INJ_INF; n = 7) or umbilical cord occluded to induce both the inflammatory and haemodynamic pathways (INJ_INF+HAE; n = 7). Sham controls were exteriorised but not ventilated (SHAM; n = 5) while unoperated controls (UNOP; n = 7) did not undergo fetal instrumentation. Fetuses were returned in utero following intervention and the ewe allowed to recover. Arterial blood gases and plasma were sampled periodically. Twenty-four hours following intervention, lambs were delivered and maintained on non-injurious ventilation for ∼40 min then brains were collected post-mortem for immunohistochemistry and RT-qPCR to assess inflammation, vascular pathology and cell death within white matter regions. Compared to INJ_INF lambs, INJ_INF+HAE lambs achieved a consistently higher V_T during injurious ventilation and carotid blood flow was significantly lower than baseline by the end of ventilation. Throughout the 24 h recovery period, systemic arterial IL-6 levels of INJ_INF+HAE lambs were significantly higher than SHAM while there was no difference between INJ_INF and SHAM animals. At 24 h, mRNA expression levels of pro-inflammatory cytokines, tight junction proteins, markers of cell death, and histological injury indices of gliosis, blood vessel protein extravasation, oligodendrocyte injury and cell death were not different between groups. Injurious ventilation, irrespective of strategy, did not increase brain inflammation or injury 24 h later when compared to control animals. However, the haemodynamic pathway did influence carotid blood flow adaptations during injurious ventilation and increased systemic arterial IL-6 that may underlie long-term pathology. Future studies are required to further characterise the pathways and their long-term effects on VIBI.

Study of BBB Dysregulation in Neuropathogenicity Using Integrative Human Model of Blood-Brain Barrier

The blood-brain barrier (BBB) is a cellular and physical barrier with a crucial role in homeostasis of the brain extracellular environment. It controls the imports of nutrients to the brain and exports toxins and pathogens. Dysregulation of the blood-brain barrier increases permeability and contributes to pathologies, including Alzheimer's disease, epilepsy, and ischemia. It remains unclear how a dysregulated BBB contributes to these different syndromes. Initial studies on the role of the BBB in neurological disorders and also techniques to permit the entry of therapeutic molecules were made in animals. This review examines progress in the use of human models of the BBB, more relevant to human neurological disorders. In recent years, the functionality and complexity of in vitro BBB models have increased. Initial efforts consisted of static transwell cultures of brain endothelial cells. Human cell models based on microfluidics or organoids derived from human-derived induced pluripotent stem cells have become more realistic and perform better. We consider the architecture of different model generations as well as the cell types used in their fabrication. Finally, we discuss optimal models to study neurodegenerative diseases, brain glioma, epilepsies, transmigration of peripheral immune cells, and brain entry of neurotrophic viruses and metastatic cancer cells.

Appearance of claudin-5+ leukocyte subtypes in the blood and CNS during progression of EAE

Background

Tight junctions (TJs) are membrane specializations characteristic of barrier-forming membranes, which function to seal the aqueous pathway between endothelial cells or epithelial cells and, thereby, obstruct intercellular solute and cellular movement. However, previous work from our laboratory found that claudin-5 (CLN-5), a TJ protein prominent at the blood–brain barrier (BBB), was also detected, ectopically, on leukocytes (CLN-5⁺) in the blood and central nervous system (CNS) of mice with experimental autoimmune encephalomyelitis (EAE), a neuroinflammatory, demyelinating disease that is a model for multiple sclerosis. CLN-5 was further shown to be transferred from endothelial cells to circulating leukocytes during disease, prompting consideration this action is coupled to leukocyte transendothelial migration (TEM) into the CNS by fostering transient interactions between corresponding leukocyte and endothelial junctional proteins at the BBB.

Methods

To begin clarifying the significance of CLN-5⁺ leukocytes, flow cytometry was used to determine their appearance in the blood and CNS during EAE.

Results

Flow cytometric analysis revealed CLN-5⁺ populations among CD4 and CD8 T cells, B cells, monocytes and neutrophils, and these appeared with varying kinetics and to different extents in both blood and CNS. CLN-5 levels on circulating T cells further correlated highly with activation state. And, the percentage of CLN-5⁺ cells among each of the subtypes analyzed was considerably higher in CNS tissue than in blood, consistent with the interpretation that CLN-5⁺ leukocytes gain preferred access to the CNS.

Conclusion

Several leukocyte subtypes variably acquire CLN-5 in blood before they enter the CNS, an event that may represent a novel mechanism to guide leukocytes to sites for paracellular diapedesis across the BBB.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-021-02328-3.

Tight Junctions of the Neurovascular Unit

The homeostatic balance of the brain and retina is maintained by the presence of the blood-brain and inner blood-retinal barrier (BBB/iBRB, respectively) which are highly specialized barriers. Endothelial cells forming the lining of these blood vessels are interconnected by the presence of tight junctions which form the BBB and iBRB. These tight junctions, formed of numerous interacting proteins, enable the entry of molecules into neural tissues while restricting the entry of harmful material such as anaphylatoxins, bacteria and viruses. If the tight junction complex becomes dysregulated due to changes in expression levels of one or more of the components, this can have detrimental effects leading to brain and retinal pathology.

Deciphering the Role of Extracellular Vesicles Derived from ZIKV-Infected hcMEC/D3 Cells on the Blood-Brain Barrier System

To date, no vaccines or antivirals are available against Zika virus (ZIKV). In addition, the mechanisms underlying ZIKV-associated pathogenesis of the central nervous system (CNS) are largely unexplored. Getting more insight into the cellular pathways that ZIKV recruits to facilitate infection of susceptible cells will be crucial for establishing an effective treatment strategy. In general, cells secrete a number of vesicles, known as extracellular vesicles (EVs), in response to viral infections. These EVs serve as intercellular communicators. Here, we investigated the role of EVs derived from ZIKV-infected human brain microvascular endothelial cells on the blood–brain barrier (BBB) system. We demonstrated that ZIKV-infected EVs (IEVs) can incorporate viral components, including ZIKV RNA, NS1, and E-protein, and further transfer them to several types of CNS cells. Using label-free impedance-based biosensing, we observed that ZIKV and IEVs can temporally disturb the monolayer integrity of BBB-mimicking cells, possibly by inducing structural rearrangements of the adherent protein VE-cadherin (immunofluorescence staining). Finally, differences in the lipidomic profile between EVs and their parental cells possibly suggest a preferential sorting mechanism of specific lipid species into the vesicles. To conclude, these data suggest that IEVs could be postulated as vehicles (Trojan horse) for ZIKV transmission via the BBB.

Reviewing the Significance of Blood-Brain Barrier Disruption in Multiple Sclerosis Pathology and Treatment

The disruption of blood–brain barrier (BBB) for multiple sclerosis (MS) pathogenesis has a double effect: early on during the onset of the immune attack and later for the CNS self-sustained ‘inside-out’ demyelination and neurodegeneration processes. This review presents the characteristics of BBB malfunction in MS but mostly highlights current developments regarding the impairment of the neurovascular unit (NVU) and the metabolic and mitochondrial dysfunctions of the BBB’s endothelial cells. The hypoxic hypothesis is largely studied and agreed upon recently in the pathologic processes in MS. Hypoxia in MS might be produced per se by the NVU malfunction or secondary to mitochondria dysfunction. We present three different but related terms that denominate the ongoing neurodegenerative process in progressive forms of MS that are indirectly related to BBB disruption: progression independent of relapses, no evidence of disease activity and smoldering demyelination or silent progression. Dimethyl fumarate (DMF), modulators of S1P receptor, cladribine and laquinimode are DMTs that are able to cross the BBB and exhibit beneficial direct effects in the CNS with very different mechanisms of action, providing hope that a combined therapy might be effective in treating MS. Detailed mechanisms of action of these DMTs are described and also illustrated in dedicated images. With increasing knowledge about the involvement of BBB in MS pathology, BBB might become a therapeutic target in MS not only to make it impenetrable against activated immune cells but also to allow molecules that have a neuroprotective effect in reaching the cell target inside the CNS.

Blood-Brain Barrier Breakdown in Stress and Neurodegeneration: Biochemical Mechanisms and New Models for Translational Research

Blood-brain barrier (BBB) is a structural and functional element of the neurovascular unit (NVU), which includes cells of neuronal, glial, and endothelial nature. The main functions of NVU include maintenance of the control of metabolism and chemical homeostasis in the brain tissue, ensuring adequate blood flow in active regions, regulation of neuroplasticity processes, which is realized through intercellular interactions under normal conditions, under stress, in neurodegeneration, neuroinfection, and neurodevelopmental diseases. Current versions of the BBB and NVU models, static and dynamic, have significantly expanded research capabilities, but a number of issues remain unresolved, in particular, personification of the models for a patient. In addition, application of both static and dynamic models has an important problem associated with the difficulty in reproducing pathophysiological mechanisms responsible for the damage of the structural and functional integrity of the barrier in the diseases of the central nervous system. More knowledge on the cellular and molecular mechanisms of BBB and NVU damage in pathology is required to solve this problem. This review discusses current state of the cellular and molecular mechanisms that control BBB permeability, pathobiochemical mechanisms and manifestations of BBB breakdown in stress and neurodegenerative diseases, as well as the problems and prospects of creating in vitro BBB and NVU models for translational studies in neurology and neuropharmacology. Deciphering BBB (patho)physiology will open up new opportunities for further development in the related areas of medicine such as regenerative medicine, neuropharmacology, and neurorehabilitation.

Breast cancer brain metastasis: insight into molecular mechanisms and therapeutic strategies

Breast cancer is one of the most prevalent malignancies in women worldwide. Early-stage breast cancer is considered a curable disease; however, once distant metastasis occurs, the 5-year overall survival rate of patients becomes significantly reduced. There are four distinct metastatic patterns in breast cancer: bone, lung, liver and brain. Among these, breast cancer brain metastasis (BCBM) is the leading cause of death; it is highly associated with impaired quality of life and poor prognosis due to the limited permeability of the blood-brain barrier and consequent lack of effective treatments. Although the sequence of events in BCBM is universally accepted, the underlying mechanisms have not yet been fully elucidated. In this review, we outline progress surrounding the molecular mechanisms involved in BCBM as well as experimental methods and research models to better understand the process. We further discuss the challenges in the management of brain metastases, as well as providing an overview of current therapies and highlighting innovative research towards developing novel efficacious targeted therapies.

A urinary peptidomic profile predicts outcome in SARS-CoV-2-infected patients

BACKGROUND

COVID-19 prediction models based on clinical characteristics, routine biochemistry and imaging, have been developed, but little is known on proteomic markers reflecting the molecular pathophysiology of disease progression.

METHODS

The multicentre (six European study sites) Prospective Validation of a Proteomic Urine Test for Early and Accurate Prognosis of Critical Course Complications in Patients with SARS-CoV-2 Infection Study (Crit-COV-U) is recruiting consecutive patients (≥ 18 years) with PCR-confirmed SARS-CoV-2 infection. A urinary proteomic biomarker (COV50) developed by capillary-electrophoresis-mass spectrometry (CE-MS) technology, comprising 50 sequenced peptides and identifying the parental proteins, was evaluated in 228 patients (derivation cohort) with replication in 99 patients (validation cohort). Death and progression along the World Health Organization (WHO) Clinical Progression Scale were assessed up to 21 days after the initial PCR test. Statistical methods included logistic regression, receiver operating curve (ROC) analysis and comparison of the area under the curve (AUC).

FINDINGS

In the derivation cohort, 23 patients died, and 48 developed worse WHO scores. The odds ratios (OR) for death per 1 standard deviation (SD) increment in COV50 were 3·52 (95% CI, 2·02-6·13, p <0·0001) unadjusted and 2·73 (1·25-5·95, p = 0·012) adjusted for sex, age, baseline WHO score, body mass index (BMI) and comorbidities. For WHO scale progression, the corresponding OR were 2·63 (1·80-3·85, p<0·0001) and 3·38 (1·85-6·17, p<0·0001), respectively. The area under the curve (AUC) for COV50 as a continuously distributed variable was 0·80 (0·72-0·88) for mortality and 0·74 (0·66-0·81) for worsening WHO score. The optimised COV50 thresholds for mortality and worsening WHO score were 0·47 and 0·04 with sensitivity/specificity of 87·0 (74·6%) and 77·1 (63·9%), respectively. On top of covariates, COV50 improved the AUC, albeit borderline for death, from 0·78 to 0·82 (p = 0·11) and 0·84 (p = 0·052) for mortality and from 0·68 to 0·78 (p = 0·0097) and 0·75 (p = 0·021) for worsening WHO score. The validation cohort findings were confirmatory.

INTERPRETATION

This first CRIT-COV-U report proves the concept that urinary proteomic profiling generates biomarkers indicating adverse COVID-19 outcomes, even at an early disease stage, including WHO stages 1-3. These findings need to be consolidated in an upcoming final dataset.

FUNDING

The German Federal Ministry of Health funded the study.

Brain endothelial tricellular junctions as novel sites for T cell diapedesis across the blood-brain barrier

The migration of activated T cells across the blood–brain barrier (BBB) is a critical step in central nervous system (CNS) immune surveillance and inflammation. Whereas T cell diapedesis across the intact BBB seems to occur preferentially through the BBB cellular junctions, impaired BBB integrity during neuroinflammation is accompanied by increased transcellular T cell diapedesis. The underlying mechanisms directing T cells to paracellular versus transcellular sites of diapedesis across the BBB remain to be explored. By combining in vitro live-cell imaging of T cell migration across primary mouse brain microvascular endothelial cells (pMBMECs) under physiological flow with serial block-face scanning electron microscopy (SBF-SEM), we have identified BBB tricellular junctions as novel sites for T cell diapedesis across the BBB. Downregulated expression of tricellular junctional proteins or protein-based targeting of their interactions in pMBMEC monolayers correlated with enhanced transcellular T cell diapedesis, and abluminal presence of chemokines increased T cell diapedesis through tricellular junctions. Our observations assign an entirely novel role to BBB tricellular junctions in regulating T cell entry into the CNS.

This article has an associated First Person interview with the first author of the paper.

Highlighted Article: Ultrastructural analysis of T cell migration across the blood–brain barrier (BBB) under physiological flow identifies BBB tricellular junctions as sites of T cell diapedesis.

The Tumour Vasculature as a Target to Modulate Leucocyte Trafficking

Simple Summary

Tumour blood vessels, characterised by abnormal morphology and function, create an immunosuppressive tumour microenvironment via restricting the appropriate leucocyte subsets trafficking. Strategies to trigger phenotypic alteration in tumour vascular system to resemble normal vascular system, named vascular normalisation, promote effective trafficking of leucocytes into tumours through enhancing the interactions between leucocytes and endothelial cells. This review specifically demonstrates how targeting tumour blood vessels modulates the critical steps of leucocyte trafficking. Furthermore, selective regulation of leucocyte subsets trafficking in tumours can be achieved by vasculature-targeting strategies, contributing to improved immunotherapy and thereby delayed tumour progression.

Abstract

The effectiveness of immunotherapy against solid tumours is dependent on the appropriate leucocyte subsets trafficking and accumulating in the tumour microenvironment (TME) with recruitment occurring at the endothelium. Such recruitment involves interactions between the leucocytes and the endothelial cells (ECs) of the vessel and occurs through a series of steps including leucocyte capture, their rolling, adhesion, and intraluminal crawling, and finally leucocyte transendothelial migration across the endothelium. The tumour vasculature can curb the trafficking of leucocytes through influencing each step of the leucocyte recruitment process, ultimately producing an immunoresistant microenvironment. Modulation of the tumour vasculature by strategies such as vascular normalisation have proven to be efficient in facilitating leucocyte trafficking into tumours and enhancing immunotherapy. In this review, we discuss the underlying mechanisms of abnormal tumour vasculature and its impact on leucocyte trafficking, and potential strategies for overcoming the tumour vascular abnormalities to boost immunotherapy via increasing leucocyte recruitment.

An Improved Method for Physical Separation of Cerebral Vasculature and Parenchyma Enables Detection of Blood-Brain-Barrier Dysfunction

The neurovascular niche is crucial for constant blood supply and blood-brain barrier (BBB) function and is altered in a number of different neurological conditions, making this an intensely active field of research. Brain vasculature is unique for its tight association of endothelial cells with astrocytic endfeet processes. Separation of the vascular compartment by centrifugation-based methods confirmed enrichment of astrocytic endfeet processes, making it possible to study the entire vascular niche with such methods. Several centrifugation-based separation protocols are found in the literature; however, with some constraints which limit their applicability and the scope of the studies. Here, we describe and validate a protocol for physically separating the neurovascular niche from the parenchyma, which is optimized for smaller tissue quantities. Using endothelial, neuronal, and astrocyte markers, we show that quantitative Western blot-based target detection can be performed of both the vessel-enriched and parenchymal fractions using as little as a single mouse brain hemisphere. Validation of our protocol in rodent stroke models by detecting changes in tight junction protein expression, serum albumin signals and astrocyte activation, i.e., increased glial fibrillary acidic protein expression, between the ipsilateral and the lesion-free contralateral hemisphere demonstrates this protocol as a new way of detecting BBB breakdown and astrogliosis, respectively.

The blood-brain barrier in health and disease: Important unanswered questions

The blood vessels vascularizing the central nervous system exhibit a series of distinct properties that tightly control the movement of ions, molecules, and cells between the blood and the parenchyma. This "blood-brain barrier" is initiated during angiogenesis via signals from the surrounding neural environment, and its integrity remains vital for homeostasis and neural protection throughout life. Blood-brain barrier dysfunction contributes to pathology in a range of neurological conditions including multiple sclerosis, stroke, and epilepsy, and has also been implicated in neurodegenerative diseases such as Alzheimer's disease. This review will discuss current knowledge and key unanswered questions regarding the blood-brain barrier in health and disease.

Overcoming the Blood-Brain Barrier: Functionalised Chitosan Nanocarriers

The major impediment to the delivery of therapeutics to the brain is the presence of the blood-brain barrier (BBB). The BBB allows for the entrance of essential nutrients while excluding harmful substances, including most therapeutic agents; hence, brain disorders, especially tumours, are very difficult to treat. Chitosan is a well-researched polymer that offers advantageous biological and chemical properties, such as mucoadhesion and the ease of functionalisation. Chitosan-based nanocarriers (CsNCs) establish ionic interactions with the endothelial cells, facilitating the crossing of drugs through the BBB by adsorptive mediated transcytosis. This process is further enhanced by modifications of the structure of chitosan, owing to the presence of reactive amino and hydroxyl groups. Finally, by permanently binding ligands or molecules, such as antibodies or lipids, CsNCs have showed a boosted passage through the BBB, in both in vivo and in vitro studies which will be discussed in this review.

Silencing miR-202-3p increases MMP-1 and promotes a brain invasive phenotype in metastatic breast cancer cells

BACKGROUND

Brain metastasis (BM) is a major cause of morbidity and mortality in breast cancer (BC) and its molecular mechanism remains poorly understood. Transmigration of metastatic cells through the brain endothelium is an essential step in BM. Metalloproteinase-1 (MMP-1) overexpression plays a key role in promoting trans-endothelial migration by degrading the inter-endothelial junctions and disrupting the endothelial integrity. However, little is known about the molecular mechanisms that induce MMP-1 in metastatic cells granting them a brain invasive phenotype. MiR-202-3p is downregulated in brain metastases compared to primary breast tumors and directly targets MMP-1. Here, we unraveled a critical role of miR-202-3p loss in MMP-1 upregulation promoting transmigration of metastatic cells through the brain endothelium.

METHODS

A variant of the MDA-MB-231 human BC cell line (MDA-MB-231-BrM2) selected for its propensity to form brain metastases was found to express high levels of MMP-1 and low levels of miR-202-3p compared to the parental cells. Using a gain-and-loss of function approach, we modulated levels of miR-202-3p and examined the resultant effect on MMP-1 expression. Effect of miR-202-3p modulation on integrity of the brain endothelium and the transmigrative ability of BC cells were also examined.

RESULTS

Loss of miR-202-3p in breast cancer cells enhanced their transmigration through the brain endothelium by upregulating MMP-1 and disrupting the inter-endothelial junctions (claudin-5, ZO-1 and ß-catenin). Restoring miR-202-3p exerted a metastasis-suppressive effect and preserved the endothelial barrier integrity.

CONCLUSIONS

Our study identified a critical regulatory role of miR-202-3p in brain metastasis and shed light on miR-202-3p/MMP-1 axis as a novel prognostic and therapeutic target that can be exploited to predict and prevent brain metastasis in breast cancer patients.

Deciphering the Forebrain Disorder in a Chicken Model of Cerebral Hernia

Cerebral hernia in crested chicken has been characterized as the protrusion of cerebral hemispheres into the unsealed skull for hundreds of years, since Charles Darwin. The development of deformed forebrain (telencephalon) of cerebral hernia remains largely unknown. Here, the unsealed frontal skull combined with misplaced sphenoid bone was observed and potentially associated with brain protuberance. The shifted pallidum, elongated hippocampus, expanded mesopallium and nidopallium, and reduced hyperpallium were observed in seven regions of the malformed telencephalon. The neurons were detected with nuclear pyknosis and decreased density. Astrocytes showed uneven distribution and disordered protuberances in hyperpallium and hippocampus. Transcriptome analyses of chicken telencephalon (cerebral hernia vs. control) revealed 547 differentially expressed genes (DEGs), mainly related to nervous system development, and immune system processes, including astrocyte marker gene GFAP, and neuron and astrocyte developmental gene S100A6. The upregulation of GFAP and S100A6 genes in abnormal telencephalon was correlated with reduced DNA methylation levels in the promoter regions. The morphological, cellular, and molecular variations in the shape, regional specification, and cellular states of malformed telencephalon potentially participate in brain plasticity and previously reported behavior changes. Chickens with cerebral hernia might be an interesting and valuable disease model to further explore the recognition, diagnosis, and therapy of cerebral hernia development of crested chickens and other species.

Analysis of cerebrovascular dysfunction caused by chronic social defeat in mice

Psychological stress and affective disorders are clinically associated with hypertension and vascular disease, but the biological links between the conditions have not been fully explored. To examine this relationship, we used chronic social defeat (CSD) stress, which produces anxiety-like and depressive-like behavioral declines in susceptible mice. In such mice, CSD also produces cerebrovascular microbleeds in scattered locations. Here, we showed further evidence of vascular pathology and blood-brain barrier breakdown by visualizing plasma immunoglobulins and erythrocytes within the parenchyma and perivascular spaces of CSD brains. To further characterize the impact of stress on the cerebrovasculature, brain endothelial cells (bECs) were isolated, and global gene expression profiles were generated. Bioinformatic analysis of CSD-induced transcriptional changes in bECs showed enrichment in pathways that delineate the vascular response to injury. These pathways followed a temporal sequence of inflammation, oxidative stress, growth factor signaling, and wound healing (i.e., platelet aggregation, hemostasis, fibrinogen deposition, and angiogenesis). Immunohistochemical staining for markers of fibrinogen deposition and angiogenesis confirmed the existence of the markers at the sites of vascular disruptions. Recovery after CSD cessation was marked by recruitment of leukocytes perhaps participating in vascular repair. The data suggest that co-morbidity of affective disorders and vascular diseases may be attributed in part to a common link in altered endothelial cell function.

CD99 mediates neutrophil transmigration through the bEnd.3 monolayer via the induction of oxygen-glucose deprivation

AIM/BACKGROUND

CD99 participate in neutrophil infiltration after inflammatory events; however, despite the important role of inflammation in ischemic stroke, the role of CD99 in ischemic stroke remains unclear.

METHOD

In the present study, we detected the protein expression of CD99, ICAM-1, and CD31 (PECAM-1) in oxygen-glucose deprivation (OGD)-induced bEnd.3 cells and neutrophils and explored the influence of HIF-1α and IL-1β on their expression. We also explored the role of CD99 in the OGD-induced transmigration of neutrophils.

RESULTS

Our results showed that OGD induction upregulated CD99 in bEnd.3 cells and that this effect could be abolished by the preadministration of IL-1β and was not mediated by HIF-1α. However, the activation of ICAM-1 by OGD remained activated with IL-1β treatment. No significant influence of IL-1β on OGD-induced CD31. Finally, we found a significant increase in infiltrated neutrophils after OGD induction compared with the control and OGD + anti-CD99 groups.

CONCLUSION

Our results indicated that CD99 mediates neutrophil infiltration and transmigration via OGD induction and thus constitutes a potential therapeutic target for anti-inflammatory treatment after ischemic stroke.

Immune cell trafficking across the blood-brain barrier in the absence and presence of neuroinflammation

To maintain the homeostatic environment required for proper function of CNS neurons the endothelial cells of CNS microvessels tightly regulate the movement of ions and molecules between the blood and the CNS. The unique properties of these blood vascular endothelial cells are termed blood-brain barrier (BBB) and extend to regulating immune cell trafficking into the immune privileged CNS during health and disease. In general, extravasation of circulating immune cells is a multi-step process regulated by the sequential interaction of adhesion and signalling molecules between the endothelial cells and the immune cells. Accounting for the unique barrier properties of CNS microvessels, immune cell migration across the BBB is distinct and characterized by several adaptations. Here we describe the mechanisms that regulate immune cell trafficking across the BBB during immune surveillance and neuroinflammation, with a focus on the current state-of-the-art in vitro and in vivo imaging observations.

Quantitatively relating brain endothelial cell-cell junction phenotype to global and local barrier properties under varied culture conditions via the Junction Analyzer Program

BACKGROUND

The endothelial cell-cell junctions of the blood-brain barrier (BBB) play a pivotal role in the barrier's function. Altered cell-cell junctions can lead to barrier dysfunction and have been implicated in several diseases. Despite this, the driving forces regulating junctional protein presentation remain relatively understudied, largely due to the lack of efficient techniques to quantify their presentation at sites of cell-cell adhesion. Here, we used our novel Junction Analyzer Program (JAnaP) to quantify junction phenotype (i.e., continuous, punctate, or perpendicular) in response to various substrate compositions, cell culture times, and cAMP treatments in human brain microvascular endothelial cells (HBMECs). We then quantitatively correlated junction presentation with barrier permeability on both a "global" and "local" scale.

METHODS

We cultured HBMECs on collagen I, fibronectin, collagen IV, laminin, fibronectin/collagen IV/laminin, or hyaluronic acid/gelatin for 2, 4, and 7 days with varying cAMP treatment schedules. Images of immunostained ZO-1, VE-cadherin, and claudin-5 were analyzed using the JAnaP to calculate the percent of the cell perimeter presenting continuous, punctate, or perpendicular junctions. Transwell permeability assays and resistance measurements were used to measure bulk ("global") barrier properties, and a "local" permeability assay was used to correlate junction presentation proximal to permeable monolayer regions.

RESULTS

Substrate composition was found to play little role in junction presentation, while cAMP supplements significantly increased the continuous junction architecture. Increased culture time required increased cAMP treatment time to reach similar ZO-1 and VE-cadherin coverage observed with shorter culture, though longer cultures were required for claudin-5 presentation. Prolonged cAMP treatment (6 days) disrupted junction integrity for all three junction proteins. Transwell permeability and TEER assays showed no correlation with junction phenotype, but a local permeability assay revealed a correlation between the number of discontinuous and no junction regions with barrier penetration.

CONCLUSIONS

These results suggest that cAMP signaling influences HBMEC junction architecture more than matrix composition. Our studies emphasized the need for local barrier measurement to mechanistically understand the role of junction phenotype and supported previous results that continuous junctions are indicative of a more mature/stable endothelial barrier. Understanding what conditions influence junction presentations, and how they, in turn, affect barrier integrity, could lead to the development of therapeutics for diseases associated with BBB dysfunction.

Human CD4+ T cell subsets differ in their abilities to cross endothelial and epithelial brain barriers in vitro

Background

The brain barriers establish compartments in the central nervous system (CNS) that significantly differ in their communication with the peripheral immune system. In this function they strictly control T-cell entry into the CNS. T cells can reach the CNS by either crossing the endothelial blood–brain barrier (BBB) or the epithelial blood-cerebrospinal fluid barrier (BCSFB) of the choroid plexus (ChP).

Objective

Analysis of the cellular and molecular mechanisms involved in the migration of different human CD4⁺ T-cell subsets across the BBB versus the BCSFB.

Methods

Human in vitro models of the BBB and BCSFB were employed to study the migration of circulating and CNS-entry experienced CD4⁺ T helper cell subsets (Th1, Th1*, Th2, Th17) across the BBB and BCSFB under inflammatory and non-inflammatory conditions in vitro.

Results

While under non-inflammatory conditions Th1* and Th1 cells preferentially crossed the BBB, under inflammatory conditions the migration rate of all Th subsets across the BBB was comparable. The migration of all Th subsets across the BCSFB from the same donor was 10- to 20-fold lower when compared to their migration across the BBB. Interestingly, Th17 cells preferentially crossed the BCSFB under both, non-inflamed and inflamed conditions. Barrier-crossing experienced Th cells sorted from CSF of MS patients showed migratory characteristics indistinguishable from those of circulating Th cells of healthy donors. All Th cell subsets could additionally cross the BCSFB from the CSF to ChP stroma side. T-cell migration across the BCSFB involved epithelial ICAM-1 irrespective of the direction of migration.

Conclusions

Our observations underscore that different Th subsets may use different anatomical routes to enter the CNS during immune surveillance versus neuroinflammation with the BCSFB establishing a tighter barrier for T-cell entry into the CNS compared to the BBB. In addition, CNS-entry experienced Th cell subsets isolated from the CSF of MS patients do not show an increased ability to cross the brain barriers when compared to circulating Th cell subsets from healthy donors underscoring the active role of the brain barriers in controlling T-cell entry into the CNS. Also we identify ICAM-1 to mediate T cell migration across the BCSFB.

Passing the Vascular Barrier: Endothelial Signaling Processes Controlling Extravasation

A central function of the vascular endothelium is to serve as a barrier between the blood and the surrounding tissue of the body. At the same time, solutes and cells have to pass the endothelium to leave or to enter the bloodstream to maintain homeostasis. Under pathological conditions, for example, inflammation, permeability for fluid and cells is largely increased in the affected area, thereby facilitating host defense. To appropriately function as a regulated permeability filter, the endothelium uses various mechanisms to allow solutes and cells to pass the endothelial layer. These include transcellular and paracellular pathways of which the latter requires remodeling of intercellular junctions for its regulation. This review provides an overview on endothelial barrier regulation and focuses on the endothelial signaling mechanisms controlling the opening and closing of paracellular pathways for solutes and cells such as leukocytes and metastasizing tumor cells.

Endothelial IQGAP1 regulates leukocyte transmigration by directing the LBRC to the site of diapedesis

The function of endothelial cell IQGAP1 during diapedesis requires its actin-binding domain and IQ motifs to recruit the lateral border recycling compartment. Genetic ablation of endothelial cell IQGAP1 expression in vivo causes significant disruption of diapedesis in two models of inflammation.

Transendothelial migration (TEM) of leukocytes across the endothelium is critical for inflammation. In the endothelium, TEM requires the coordination of membrane movements and cytoskeletal interactions, including, prominently, recruitment of the lateral border recycling compartment (LBRC). The scaffold protein IQGAP1 was recently identified in a screen for LBRC-interacting proteins. Knockdown of endothelial IQGAP1 disrupted the directed movement of the LBRC and substantially reduced leukocyte TEM. Expression of truncated IQGAP1 constructs demonstrated that the calponin homology domain is required for IQGAP1 localization to endothelial borders and that the IQ domain, on the same IQGAP1 polypeptide, is required for its function in TEM. This is the first reported function of IQGAP1 requiring two domains to be present on the same polypeptide. Additionally, we show for the first time that IQGAP1 in the endothelium is required for efficient TEM in vivo. These findings reveal a novel function for IQGAP1 and demonstrate that IQGAP1 in endothelial cells facilitates TEM by directing the LBRC to the site of TEM.

HIV-1 Tat and opioids act independently to limit antiretroviral brain concentrations and reduce blood-brain barrier integrity

Poor antiretroviral penetration may contribute to human immunodeficiency virus (HIV) persistence within the brain and to neurocognitive deficits in opiate abusers. To investigate this problem, HIV-1 Tat protein and morphine effects on blood-brain barrier (BBB) permeability and drug brain penetration were explored using a conditional HIV-1 Tat transgenic mouse model. Tat and morphine effects on the leakage of fluorescently labeled dextrans (10-, 40-, and 70-kDa) into the brain were assessed. To evaluate effects on antiretroviral brain penetration, Tat+ and Tat- mice received three antiretroviral drugs (dolutegravir, abacavir, and lamivudine) with or without concurrent morphine exposure. Antiretroviral and morphine brain and plasma concentrations were determined by LC-MS/MS. Morphine exposure, and, to a lesser extent, Tat, significantly increased tracer leakage from the vasculature into the brain. Despite enhanced BBB breakdown evidenced by increased tracer leakiness, morphine exposure led to significantly lower abacavir concentrations within the striatum and significantly less dolutegravir within the hippocampus and striatum (normalized to plasma). P-glycoprotein, an efflux transporter for which these drugs are substrates, expression and function were significantly increased in the brains of morphine-exposed mice compared to mice not exposed to morphine. These findings were consistent with lower antiretroviral concentrations in brain tissues examined. Lamivudine concentrations were unaffected by Tat or morphine exposure. Collectively, our investigations indicate that Tat and morphine differentially alter BBB integrity. Morphine decreased brain concentrations of specific antiretroviral drugs, perhaps via increased expression of the drug efflux transporter, P-glycoprotein.

Meningitic Escherichia coli-induced upregulation of PDGF-B and ICAM-1 aggravates blood-brain barrier disruption and neuroinflammatory response

Background

Blood-brain barrier (BBB) disruption and neuroinflammation are considered key mechanisms of pathogenic Escherichia coli invasion of the brain. However, the specific molecules involved in meningitic E. coli-induced BBB breakdown and neuroinflammatory response remain unclear. Our previous RNA-sequencing data from human brain microvascular endothelial cells (hBMECs) revealed two important host factors: platelet-derived growth factor-B (PDGF-B) and intercellular adhesion molecule-1 (ICAM-1), which were significantly upregulated in hBMECs after meningitic E. coli infection. Whether and how PDGF-B and ICAM-1 contribute to the development of E. coli meningitis are still unclear.

Methods

The western blot, real-time PCR, enzyme-linked immunosorbent assay, immunohistochemistry, and immunofluorescence were applied to verify the significant induction of PDGF-B and ICAM-1 by meningitic E. coli in vivo and in vitro. Evan’s blue assay and electric cell-substrate impedance sensing assay were combined to identify the effects of PDGF-B on BBB permeability. The CRISPR/Cas9 technology, cell-cell adhesion assay, and electrochemiluminescence assay were used to investigate the role of ICAM-1 in neuroinflammation subversion.

Results

We verified the significant induction of PDGF-B and ICAM-1 by meningitic E. coli in mouse as well as monolayer hBMECs models. Functionally, we showed that the increase of PDGF-B may directly enhance the BBB permeability by decreasing the expression of tight junction proteins, and the upregulation of ICAM-1 contributed to neutrophils or monocytes recruitment as well as neuroinflammation subversion in response to meningitic E. coli infection.

Conclusions

Our findings demonstrated the roles of PDGF-B and ICAM-1 in mediating bacterial-induced BBB damage as well as neuroinflammation, providing new concepts and potential targets for future prevention and treatment of bacterial meningitis.

Quantitative Phenotyping of Cell-Cell Junctions to Evaluate ZO-1 Presentation in Brain Endothelial Cells

The selective permeability of the blood-brain barrier (BBB) is controlled by tight junction-expressing brain endothelial cells. The integrity of these junctional proteins, which anchor to actin via zonula occludens (e.g., ZO-1), plays a vital role in barrier function. While disrupted junctions are linked with several neurodegenerative diseases, the mechanisms underlying disruption are not fully understood. This is largely due to the lack of appropriate models and efficient techniques to quantify edge-localized protein. Here, we developed a novel junction analyzer program (JAnaP) to semi-automate the quantification of junctional protein presentation. Because significant evidence suggests a link between myosin-II mediated contractility and endothelial barrier properties, we used the JAnaP to investigate how biochemical and physical cues associated with altered contractility influence ZO-1 presentation in brain endothelial cells. Treatment with contractility-decreasing agents increased continuous ZO-1 presentation; however, this increase was greatest on soft gels of brain-relevant stiffness, suggesting improved barrier maturation. This effect was reversed by biochemically inhibiting protein phosphatases to increase cell contractility on soft substrates. These results promote the use of brain-mimetic substrate stiffness in BBB model design and motivates the use of this novel JAnaP to provide insight into the role of junctional protein presentation in BBB physiology and pathologies.

PECAM-1 Stabilizes Blood-Brain Barrier Integrity and Favors Paracellular T-Cell Diapedesis Across the Blood-Brain Barrier During Neuroinflammation

Breakdown of the blood-brain barrier (BBB) and increased immune cell trafficking into the central nervous system (CNS) are hallmarks of the pathogenesis of multiple sclerosis (MS). Platelet endothelial cell adhesion molecule-1 (PECAM-1; CD31) is expressed on cells of the vascular compartment and regulates vascular integrity and immune cell trafficking. Involvement of PECAM-1 in MS pathogenesis has been suggested by the detection of increased levels of soluble PECAM-1 (sPECAM-1) in the serum and CSF of MS patients. Here, we report profound upregulation of cell-bound PECAM-1 in initial (pre-phagocytic) white matter as well as active cortical gray matter MS lesions. Using a human in vitro BBB model we observed that PECAM-1 is not essential for the transmigration of human CD4+ T-cell subsets (Th1, Th1*, Th2, and Th17) across the BBB. Employing an additional in vitro BBB model based on primary mouse brain microvascular endothelial cells (pMBMECs) we show that the lack of endothelial PECAM-1 impairs BBB properties as shown by reduced transendothelial electrical resistance (TEER) and increases permeability for small molecular tracers. Investigating T-cell migration across the BBB under physiological flow by in vitro live cell imaging revealed that absence of PECAM-1 in pMBMECs did not influence arrest, polarization, and crawling of effector/memory CD4+ T cells on the pMBMECs. Absence of endothelial PECAM-1 also did not affect the number of T cells able to cross the pMBMEC monolayer under flow, but surprisingly favored transcellular over paracellular T-cell diapedesis. Taken together, our data demonstrate that PECAM-1 is critically involved in regulating BBB permeability and although not required for T-cell diapedesis itself, its presence or absence influences the cellular route of T-cell diapedesis across the BBB. Upregulated expression of cell-bound PECAM-1 in human MS lesions may thus reflect vascular repair mechanisms aiming to restore BBB integrity and paracellular T-cell migration across the BBB as it occurs during CNS immune surveillance.

Claudin-5: gatekeeper of neurological function

Tight junction proteins of the blood–brain barrier are vital for maintaining integrity of endothelial cells lining brain blood vessels. The presence of these protein complexes in the space between endothelial cells creates a dynamic, highly regulated and restrictive microenvironment that is vital for neural homeostasis. By limiting paracellular diffusion of material between blood and brain, tight junction proteins provide a protective barrier preventing the passage of unwanted and potentially damaging material. Simultaneously, this protective barrier hinders the therapeutic effectiveness of central nervous system acting drugs with over 95% of small molecule therapeutics unable to bypass the blood–brain barrier. At the blood–brain barrier, claudin-5 is the most enriched tight junction protein and its dysfunction has been implicated in neurodegenerative disorders such as Alzheimer’s disease, neuroinflammatory disorders such as multiple sclerosis as well as psychiatric disorders including depression and schizophrenia. By regulating levels of claudin-5, it is possible to abrogate disease symptoms in many of these disorders. This review will give an overview of the blood–brain barrier and the role of tight junction complexes in maintaining blood–brain barrier integrity before focusing on the role of claudin-5 and its regulation in homeostatic and pathological conditions. We will also summarise therapeutic strategies to restore integrity of cerebral vessels by targeting tight junction protein complexes.

Lack of junctional adhesion molecule (JAM)-B ameliorates experimental autoimmune encephalomyelitis

In multiple sclerosis (MS) and its animal model experimental autoimmune encephalomyelitis (EAE) autoaggressive CD4⁺ T cells cross the blood-brain barrier (BBB) and cause neuroinflammation. Therapeutic targeting of CD4⁺ T-cell trafficking into the CNS by blocking α4-integrins has proven beneficial for the treatment of MS but comes with associated risks, probably due to blocking CD8⁺ T cell mediated CNS immune surveillance. Our recent observations show that CD8⁺ T cells also rely on α4β1-integrins to cross the BBB. Besides vascular cell adhesion molecule-1 (VCAM-1), we identified junctional adhesion molecule-B (JAM-B) as a novel vascular α4β1-integrin ligand involved in CD8⁺ T-cell migration across the BBB. This prompted us to investigate, if JAM-B also mediates CD4⁺ T-cell migration across the BBB. We first ensured that encephalitogenic T cells can bind to JAM-B in vitro and next compared EAE pathogenesis in JAM-B^-/- C57BL/6J mice and their wild-type littermates. Following immunization with MOG_aa35-55 peptide, JAM-B^-/- mice developed ameliorated EAE compared to their wild-type littermates. At the same time, we isolated higher numbers of CD45⁺ infiltrating immune cells from the CNS of JAM-B^-/- C57BL/6J mice suffering from EAE. Immunofluorescence staining revealed that the majority of CD45⁺ inflammatory cells accumulated in the leptomeningeal and perivascular spaces of the CNS behind the BBB but do not gain access to the CNS parenchyma. Trapping of CNS inflammatory cells was not due to increased inflammatory cell proliferation. Neither a loss of BBB integrity or BBB polarity potentially affecting local chemokine gradients nor a lack of focal gelatinase activation required for CNS parenchymal immune cell entry across the glia limitans could be detected in JAM-B^-/- mice. Lack of a role for JAM-B in the effector phase of EAE was supported by the observation that we did not detect any role for JAM-B in EAE pathogenesis, when EAE was elicited by in vitro activated MOG _aa35-55-specific CD4⁺ effector T cells. On the other hand, we also failed to demonstrate any role of JAM-B in in vivo priming, proliferation or polarization of MOG_aa35-55-specific CD4⁺ T cells in peripheral immune organs. Finally, our study excludes expression of and thus a role for JAM-B on peripheral and CNS infiltrating myeloid cells. Taken together, although endothelial JAM-B is not required for immune cell trafficking across the BBB in EAE, in its absence accumulation of inflammatory cells mainly in CNS leptomeningeal spaces leads to amelioration of EAE.

Caveolin1 Is Required for Th1 Cell Infiltration, but Not Tight Junction Remodeling, at the Blood-Brain Barrier in Autoimmune Neuroinflammation

Lymphocytes cross vascular boundaries via either disrupted tight junctions (TJs) or caveolae to induce tissue inflammation. In the CNS, Th17 lymphocytes cross the blood-brain barrier (BBB) before Th1 cells; yet this differential crossing is poorly understood. We have used intravital two-photon imaging of the spinal cord in wild-type and caveolae-deficient mice with fluorescently labeled endothelial tight junctions to determine how tight junction remodeling and caveolae regulate CNS entry of lymphocytes during the experimental autoimmune encephalomyelitis (EAE) model for multiple sclerosis. We find that dynamic tight junction remodeling occurs early in EAE but does not depend upon caveolar transport. Moreover, Th1, but not Th17, lymphocytes are significantly reduced in the inflamed CNS of mice lacking caveolae. Therefore, tight junction remodeling facilitates Th17 migration across the BBB, whereas caveolae promote Th1 entry into the CNS. Moreover, therapies that target both tight junction degradation and caveolar transcytosis may limit lymphocyte infiltration during inflammation.

Blast exposure elicits blood-brain barrier disruption and repair mediated by tight junction integrity and nitric oxide dependent processes

Mild blast-induced traumatic brain injury (TBI) is associated with blood-brain barrier (BBB) disruption. However, the mechanisms whereby blast disrupts BBB integrity are not well understood. To address this issue BBB permeability to peripherally injected ¹⁴C-sucrose and ^99mTc-albumin was quantified in ten brain regions at time points ranging from 0.25 to 72 hours. In mice, repetitive (2X) blast provoked BBB permeability to ¹⁴C-sucrose that persisted in specific brain regions from 0.25 to 72 hours. However, ^99mTc-albumin revealed biphasic BBB disruption (open-closed-open) over the same interval, which was most pronounced in frontal cortex and hippocampus. This indicates that blast initiates interacting BBB disruption and reparative processes in specific brain regions. Further investigation of delayed (72 hour) BBB disruption revealed that claudin-5 (CLD5) expression was disrupted specifically in the hippocampus, but not in dorsal striatum, a brain region that showed no blast-induced BBB permeability to sucrose or albumin. In addition, we found that delayed BBB permeability and disrupted CLD5 expression were blocked by the nitric oxide synthase inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME). These data argue that latent nitric oxide-dependent signaling pathways initiate processes that result in delayed BBB disruption, which are manifested in a brain-region specific manner.

Identification of brain metastasis genes and therapeutic evaluation of histone deacetylase inhibitors in a clinically relevant model of breast cancer brain metastasis

Breast cancer brain metastases remain largely incurable. Although several mouse models have been developed to investigate the genes and mechanisms regulating breast cancer brain metastasis, these models often lack clinical relevance since they require the use of immunocompromised mice and/or are poorly metastatic to brain from the mammary gland. We describe the development and characterisation of an aggressive brain metastatic variant of the 4T1 syngeneic model (4T1Br4) that spontaneously metastasises to multiple organs, but is selectively more metastatic to the brain from the mammary gland than parental 4T1 tumours. As seen by immunohistochemistry, 4T1Br4 tumours and brain metastases display a triple-negative phenotype, consistent with the high propensity of this breast cancer subtype to spread to brain. In vitro assays indicate that 4T1Br4 cells have an enhanced ability to adhere to or migrate across a brain-derived endothelial monolayer and greater invasive response to brain-derived soluble factors compared to 4T1 cells. These properties are likely to contribute to the brain selectivity of 4T1Br4 tumours. Expression profiling and gene set enrichment analyses demonstrate the clinical relevance of the 4T1Br4 model at the transcriptomic level. Pathway analyses implicate tumour-intrinsic immune regulation and vascular interactions in successful brain colonisation, revealing potential therapeutic targets. Evaluation of two histone deacetylase inhibitors, SB939 and 1179.4b, shows partial efficacy against 4T1Br4 metastasis to brain and other sites in vivo, and potent radio-sensitising properties in vitro. The 4T1Br4 model provides a clinically relevant tool for mechanistic studies and to evaluate novel therapies against brain metastasis.

This article has an associated First Person interview with Soo-Hyun Kim, joint first author of the paper.

Summary: The authors introduce a new syngeneic mouse model of spontaneous breast cancer brain metastasis, demonstrate its phenotypic, functional and transcriptomic relevance to human TNBC brain metastasis, and test novel therapies.

CD99L2 deficiency inhibits leukocyte entry into the central nervous system and ameliorates neuroinflammation

Leukocyte entry into the CNS is a crucial step in the development of multiple sclerosis and its animal model experimental autoimmune encephalomyelitis (EAE). Adhesion molecules mediating the docking of leukocytes to the endothelium of the blood-brain barrier (BBB) represent valuable targets for interference with the disease. However, little is known about the adhesion and signaling mechanisms in endothelial cells that mediate the diapedesis through the BBB. Here, we show that conditional Tie-2-Cre driven gene inactivation of CD99L2 inhibits leukocyte entry into the CNS during active MOG_35-55 -induced EAE and alleviates severity of the disease. No detrimental effect on the immune response was observed. The number of perivascular cuffs around vessels of the CNS was reduced, as was the number of inflammatory foci, sites of demyelination and expression levels of pro-inflammatory cytokines. Three-dimensional analysis of vibratome sections of the CNS revealed an accumulation of leukocytes between endothelial cells and the underlying basement membrane, whereas leukocyte docking to the luminal surface of the endothelium of the BBB was unaffected. Collectively, these results suggest that CD99L2 participates in the development of EAE by supporting diapedesis of leukocytes through the endothelial basement membrane of blood vessels of the BBB in the CNS.

Induction of VEGFA and Snail-1 by meningitic Escherichia coli mediates disruption of the blood-brain barrier

Escherichia coli is the most common Gram-negative bacterium that possesses the ability to cause neonatal meningitis, which develops as circulating bacteria penetrate the blood-brain barrier (BBB). However, whether meningitic E. coli could induce disruption of the BBB and the underlying mechanisms are poorly understood. Our current work highlight for the first time the participation of VEGFA and Snail-1, as well as the potential mechanisms, in meningitic E. coli induced disruption of the BBB. Here, we characterized a meningitis-causing E. coli PCN033, and demonstrated that PCN033 invasion could increase the BBB permeability through downregulating and remodeling the tight junction proteins (TJ proteins). This process required the PCN033 infection-induced upregulation of VEGFA and Snail-1, which involves the activation of TLR2-MAPK-ERK1/2 signaling cascade. Moreover, production of proinflammatory cytokines and chemokines in response to infection also promoted the upregulation of VEGFA and Snail-1, therefore further mediating the BBB disruption. Our observations reported here directly support the involvement of VEGFA and Snail-1 in meningitic E. coli induced BBB disruption, and VEGFA and Snail-1 would therefore represent the essential host targets for future prevention of clinical E. coli meningitis.

Functional morphology of the blood-brain barrier in health and disease

The adult quiescent blood-brain barrier (BBB), a structure organised by endothelial cells through interactions with pericytes, astrocytes, neurons and microglia in the neurovascular unit, is highly regulated but fragile at the same time. In the past decade, there has been considerable progress in understanding not only the molecular pathways involved in BBB development, but also BBB breakdown in neurological diseases. Specifically, the Wnt/β-catenin, retinoic acid and sonic hedgehog pathways moved into the focus of BBB research. Moreover, angiopoietin/Tie2 signalling that is linked to angiogenic processes has gained attention in the BBB field. Blood vessels play an essential role in initiation and progression of many diseases, including inflammation outside the central nervous system (CNS). Therefore, the potential influence of CNS blood vessels in neurological diseases associated with BBB alterations or neuroinflammation has become a major focus of current research to understand their contribution to pathogenesis. Moreover, the BBB remains a major obstacle to pharmaceutical intervention in the CNS. The complications may either be expressed by inadequate therapeutic delivery like in brain tumours, or by poor delivery of the drug across the BBB and ineffective bioavailability. In this review, we initially describe the cellular and molecular components that contribute to the steady state of the healthy BBB. We then discuss BBB alterations in ischaemic stroke, primary and metastatic brain tumour, chronic inflammation and Alzheimer's disease. Throughout the review, we highlight common mechanisms of BBB abnormalities among these diseases, in particular the contribution of neuroinflammation to BBB dysfunction and disease progression, and emphasise unique aspects of BBB alteration in certain diseases such as brain tumours. Moreover, this review highlights novel strategies to monitor BBB function by non-invasive imaging techniques focussing on ischaemic stroke, as well as novel ways to modulate BBB permeability and function to promote treatment of brain tumours, inflammation and Alzheimer's disease. In conclusion, a deep understanding of signals that maintain the healthy BBB and promote fluctuations in BBB permeability in disease states will be key to elucidate disease mechanisms and to identify potential targets for diagnostics and therapeutic modulation of the BBB.

Strain-dependent effects of clinical echovirus 30 outbreak isolates at the blood-CSF barrier

BACKGROUND

Echovirus (E) 30 (E-30) meningitis is characterized by neuroinflammation involving immune cell pleocytosis at the protective barriers of the central nervous system (CNS). In this context, infection of the blood-cerebrospinal fluid barrier (BCSFB), which has been demonstrated to be involved in enteroviral CNS pathogenesis, may affect the tight junction (TJ) and adherens junction (AJ) function and morphology.

METHODS

We used an in vitro human choroid plexus epithelial (HIBCPP) cell model to investigate the effect of three clinical outbreak strains (13-311, 13-759, and 14-397) isolated in Germany in 2013, and compared them to E-30 Bastianni. Conducting transepithelial electrical resistance (TEER), paracellular dextran flux measurement, quantitative real-time polymerase chain reaction (qPCR), western blot, and immunofluorescence analysis, we investigated TJ and AJ function and morphology as well as strain-specific E-30 infection patterns. Additionally, transmission electron and focused ion beam microscopy electron microscopy (FIB-SEM) was used to evaluate the mode of leukocyte transmigration. Genome sequencing and phylogenetic analyses were performed to discriminate potential genetic differences among the outbreak strains.

RESULTS

We observed a significant strain-dependent decrease in TEER with strains E-30 Bastianni and 13-311, whereas paracellular dextran flux was only affected by E-30 Bastianni. Despite strong similarities among the outbreak strains in replication characteristics and particle distribution, strain 13-311 was the only outbreak isolate revealing comparable disruptive effects on TJ (Zonula Occludens (ZO) 1 and occludin) and AJ (E-cadherin) morphology to E-30 Bastianni. Notwithstanding significant junctional alterations upon E-30 infection, we observed both para- and transcellular leukocyte migration across HIBCPP cells. Complete genome sequencing revealed differences between the strains analyzed, but no explicit correlation with the observed strain-dependent effects on HIBCPP cells was possible.

CONCLUSION

The findings revealed distinct E-30 strain-specific effects on barrier integrity and junctional morphology. Despite E-30-induced barrier alterations leukocyte trafficking did not exclusively occur via the paracellular route.

Characterization of cell-cell junction changes associated with the formation of a strong endothelial barrier

A principal function of endothelial cells is the formation of a barrier between the blood and tissues.  This barrier arises from the physical connections at cell-cell junctions, which includes cytoskeletal tight junction and adherens junction proteins. Methods that alter barrier function must therefore affect these cell-cell connections. The blood brain barrier (BBB) represents perhaps the most selective endothelial barrier, which arises from endothelial cell interactions with astrocytes and pericytes. Even in non-central nervous system (CNS) endothelial cells, barrier properties can be enhanced, mimicking the BBB, through induction of intercellular junctions, by either direct co-culture with astrocytes, supplementation with astrocyte conditioned medium (ACM) and/or pharmacologic enhancement of cAMP. To understand how cell-cell junctions change during endothelial barrier enhancement, we examined the effects of ACM and/or cAMP donors added to standard media on human umbilical vein endothelial cells (HUVEC). HUVEC cultured with cAMP-elevating agents had the most enhanced barrier function as measured by Electric Cell-substrate Impedance Sensing (ECIS®), a real-time, label-free, impedance based method of studying cell barrier properties. However, subtle differences in actin and cell-cell junction proteins were seen across all four culture conditions. cAMP-elevating agents also triggered the redistribution of ZO-1 and VE-cadherin to cell-cell junctions, and intensified the actin microfilament network at the cell cortex.  Using a VE-cadherin FRET-force sensor, we observed a decrease in VE-cadherin force in HUVEC cultured with ACM with cAMP donors. Our data indicate cAMP elevation induces both junctional strengthening and reduced VE-cadherin forces. Additionally, treatment with an inhibitor of formin, which reduced actin stress fibers, enhanced barrier function. These data suggest that barrier function is modulated both through the trafficking of proteins to cell-cell junctions, and through the modulation and a relaxation of mechanical force through adherens junctions as intercellular junctional complexes become established.

One-week exposure to a free-choice high-fat high-sugar diet does not disrupt blood-brain barrier permeability in fed or overnight fasted rats

Objectives: The hypothalamus lies adjacent to the third ventricle and is in close proximity with the median eminence (ME), a circumventricular organ with an incomplete blood-brain barrier (BBB) which controls direct entry of nutrients into the brain. The blood-CSF barrier of the hypothalamus shows dynamic changes upon neuroendocrine events and adjusts permeability with the tight junction (TJ) complex. It has been shown that chronic exposure to a high-fat diet (HFD) affects BBB permeability. HFD also induces leptin resistance and alters neuropeptide expression in the arcuate nucleus (Arc) of the hypothalamus starting early during overnutrition. We hypothesized altered integrity of the BBB to occur after exposing rats to a free-choice high-fat high-sugar (fcHFHS) diet for 1 week. Methods: We measured diffusion of Evans blue dye over the ME and assessed expression of the TJ proteins ZO-1, claudin-5, and occludin in the tanycytic wall of the third ventricle. Furthermore, we assessed protein expression of glucose transporter 1 (GLUT-1), which is highly expressed in the Arc-ME complex and facilitates glucose transport over the BBB. Results: fcHFHS-fed rats increased caloric intake compared to control, however, there was no effect of the fcHFHS diet on permeability of the BBB, nor changes in protein expression of tight TJ proteins or GLUT-1. Fasting acutely affects the BBB and we hypothesized that exposure to the fcHFHS diet affects the BBB differently compared to chow after fasting. We did not, however, find any differences in Evans blue diffusion nor protein expression between chow- and fcHFHS-fed rats when fasted overnight. Conclusions: We conclude that short-term consumption of a fcHFHS diet does not change permeability or diffusion in the hypothalamus barrier in ad libitum fed or fasted rats.

Differentiation and Transmigration of CD4 T Cells in Neuroinflammation and Autoimmunity

CD4⁺ T cells play a central role in orchestrating protective immunity and autoimmunity. The activation and differentiation of myelin-reactive CD4⁺ T cells into effector (Th1 and Th17) and regulatory (Tregs) subsets at the peripheral tissues, and their subsequent transmigration across the blood–brain barrier (BBB) into the central nervous system (CNS) parenchyma are decisive events in the pathogenesis of multiple sclerosis and experimental autoimmune encephalomyelitis. How the Th1, Th17, and regulatory Tregs transmigrate across the BBB into the CNS and cause CNS inflammation is not clearly understood. Studies with transgenic and gene knockout mice have unraveled that Th1, Th17, and Tregs play a critical role in the induction and resolution of neuroinflammation. However, the plasticity of these lineages and functional dichotomy of their cytokine products makes it difficult to understand what role CD4⁺ T cells in the peripheral lymphoid organs, endothelial BBB, and the CNS parenchyma play in the CNS autoimmune response. In this review, we describe some of the recent findings that shed light on the mechanisms behind the differentiation and transmigration of CD4⁺ T cells across the BBB into the CNS parenchyma and also highlight how these two processes are interconnected, which is crucial for the outcome of CNS inflammation and autoimmunity.

Effects of HIV-1 Tat and Methamphetamine on Blood-Brain Barrier Integrity and Function In Vitro

Human immunodeficiency (HIV) infection results in neurocognitive deficits in about one half of infected individuals. Despite systemic effectiveness, restricted antiretroviral penetration across the blood-brain barrier (BBB) is a major limitation in fighting central nervous system (CNS)-localized infection. Drug abuse exacerbates HIV-induced cognitive and pathological CNS changes. This study's purpose was to investigate the effects of the HIV-1 protein Tat and methamphetamine on factors affecting drug penetration across an in vitro BBB model. Factors affecting paracellular and transcellular flux in the presence of Tat and methamphetamine were examined. Transendothelial electrical resistance, ZO-1 expression, and lucifer yellow (a paracellular tracer) flux were aspects of paracellular processes that were examined. Additionally, effects on P-glycoprotein (P-gp) and multidrug resistance protein 1 (MRP-1) mRNA (via quantitative PCR [qPCR]) and protein (via immunoblotting) expression were measured; Pgp and MRP-1 are drug efflux proteins. Transporter function was examined after exposure of Tat with or without methamphetamine using the P-gp substrate rhodamine 123 and also using the dual P-gp/MRP-1 substrate and protease inhibitor atazanavir. Tat and methamphetamine elicit complex changes affecting transcellular and paracellular transport processes. Neither Tat nor methamphetamine significantly altered P-gp expression. However, Tat plus methamphetamine exposure significantly increased rhodamine 123 accumulation within brain endothelial cells, suggesting that treatment inhibited or impaired P-gp function. Intracellular accumulation of atazanavir was not significantly altered after Tat or methamphetamine exposure. Atazanavir accumulation was, however, significantly increased by simultaneous inhibition of P-gp and MRP. Collectively, our investigations indicate that Tat and methamphetamine alter aspects of BBB integrity without affecting net flux of paracellular compounds. Tat and methamphetamine may also affect several aspects of transcellular transport.

Endothelial cell-cell adhesion and signaling

Endothelial cells line blood vessels and provide a dynamic interface between the blood and tissues. They remodel to allow leukocytes, fluid and small molecules to enter tissues during inflammation and infections. Here we compare the signaling networks that contribute to endothelial permeability and leukocyte transendothelial migration, focusing particularly on signals mediated by small GTPases that regulate cell adhesion and the actin cytoskeleton. Rho and Rap GTPase signaling is important for both processes, but they differ in that signals are activated locally under leukocytes, whereas endothelial permeability is a wider event that affects the whole cell. Some molecules play a unique role in one of the two processes, and could therefore be targeted to selectively alter either endothelial permeability or leukocyte transendothelial migration.