The rights and wrongs of blood-brain barrier permeability studies: a walk through 100 years of history

Environmental Nanoparticles Reach Human Fetal Brains

Anthropogenic ultrafine particulate matter (UFPM) and industrial and natural nanoparticles (NPs) are ubiquitous. Normal term, preeclamptic, and postconceptional weeks(PCW) 8–15 human placentas and brains from polluted Mexican cities were analyzed by TEM and energy-dispersive X-ray spectroscopy. We documented NPs in maternal erythrocytes, early syncytiotrophoblast, Hofbauer cells, and fetal endothelium (ECs). Fetal ECs exhibited caveolar NP activity and widespread erythroblast contact. Brain ECs displayed micropodial extensions reaching luminal NP-loaded erythroblasts. Neurons and primitive glia displayed nuclear, organelle, and cytoplasmic NPs in both singles and conglomerates. Nanoscale Fe, Ti, and Al alloys, Hg, Cu, Ca, Sn, and Si were detected in placentas and fetal brains. Preeclamptic fetal blood NP vesicles are prospective neonate UFPM exposure biomarkers. NPs are reaching brain tissues at the early developmental PCW 8–15 stage, and NPs in maternal and fetal placental tissue compartments strongly suggests the placental barrier is not limiting the access of environmental NPs. Erythroblasts are the main early NP carriers to fetal tissues. The passage of UFPM/NPs from mothers to fetuses is documented and fingerprinting placental single particle composition could be useful for postnatal risk assessments. Fetal brain combustion and industrial NPs raise medical concerns about prenatal and postnatal health, including neurological and neurodegenerative lifelong consequences.

Modulation of gene expression on a transcriptome-wide level following human neural stem cell transplantation in aged mouse stroke brains

INTRODUCTION

Neural stem cell (NSC) transplantation offers great potential for treating ischemic stroke. Clinically, ischemia followed by reperfusion results in robust cerebrovascular injury that upregulates proinflammatory factors, disrupts neurovascular units, and causes brain cell death. NSCs possess multiple actions that can be exploited for reducing the severity of neurovascular injury. Our previous studies in young adult mice showed that human NSC transplantation during the subacute stage diminishes stroke pathophysiology and improves behavioral outcome.

METHODS

We employed a well-established and commonly used stroke model, middle cerebral artery occlusion with subsequent reperfusion (MCAO/R). Here, we assessed the outcomes of hNSC transplantation 48 h post-MCAO (24 h post-transplant) in aged mouse brains in response to stroke because aging is a crucial risk factor for cerebral ischemia. Next, we tested whether administration of the integrin α5β1 inhibitor, ATN-161, prior to hNSC transplantation further affects stoke outcome as compared with NSCs alone. RNA sequencing (RNA-seq) was used to assess the impact of hNSC transplantation on differentially expressed genes (DEGs) on a transcriptome-wide level.

RESULTS

Here, we report that hNSC-engrafted brains with or without ATN-161 showed significantly reduced infarct size, and attenuated the induction of proinflammatory factors and matrix metalloproteases. RNA-seq analysis revealed DEGs and molecular pathways by which hNSCs induce a beneficial post-stroke outcome in aged stroke brains. 811 genes were differentially expressed (651 downregulated and 160 upregulated) in hNSC-engrafted stroke brains. Functional pathway analysis identified enriched and depleted pathways in hNSC-engrafted aged mouse stroke brains. Depletion of pathways following hNSC-engraftment included signaling involving neuroinflammation, acute phase response, leukocyte extravasation, and phagosome formation. On the other hand, enrichment of pathways in hNSC-engrafted brains was associated with PPAR signaling, LXR/RXR activation, and inhibition of matrix metalloproteases. Hierarchical cluster analysis of DEGs in hNSC-engrafted brains indicate decreased expression of genes encoding TNF receptors, proinflammatory factors, apoptosis factors, adhesion and leukocyte extravasation, and Toll-like receptors.

CONCLUSIONS

Our study is the first to show global transcripts differentially expressed following hNSC transplantation in the subacute phase of stroke in aged mice. The outcome of our transcriptome study would be useful to develop new therapies ameliorating early-stage stroke injury.

Cell Interplay Model to Assess the Impact of Glioma Cells on Blood-Brain Barrier Permeability

The blood-brain barrier (BBB) is the most selective protecting layer of the central nervous system (CNS) with unique neurovascular features. The BBB is known to undergo a process of molecular alterations during disease state, such as in the case of glioma. This results in a non-uniform permeability along the BBB layer, which retains intact regions but develops focal sites of higher leakiness, especially in the surrounds of the tumor core. Although essential to guarantee brain homeostasis, the BBB has been the Achilles heel of drug delivery to the brain since the early times of the first classification as "barrier," more than a century ago. Due to the presence of the BBB, the transport of drug molecules from the bloodstream to the brain parenchyma is highly restricted, and, therefore, clinically relevant therapeutic concentrations cannot be achieved. Research efforts have focused on the development of novel tools to ameliorate drug permeability across the BBB, including drug formulation into non-invasive delivery systems with brain targeting properties and techniques that allow a temporary disruption of the BBB. To strengthen the advancement of potential drug candidates, in vitro models that recapitulate the main in vivo features of BBB are required to perform a preliminary screening of permeability, both in health and disease conditions. Herein, a protocol to assemble a BBB in vitro model to screen drug permeability in a glioma disease state is detailed. The model consists of a BBB and glioma cell co-culture and aims at exploiting the effect of the interplay between the cell constituents on the permeability of drug molecules. Although simple and straightforward, the herein in vitro model presents a high reproducibility, cost-effectiveness, and a favorable time-benefit balance.

Blood-Brain Barrier in Brain Tumors: Biology and Clinical Relevance

The presence of barriers, such as the blood–brain barrier (BBB) and brain–tumor barrier (BTB), limits the penetration of antineoplastic drugs into the brain, resulting in poor response to treatments. Many techniques have been developed to overcome the presence of these barriers, including direct injections of substances by intranasal or intrathecal routes, chemical modification of drugs or constituents of BBB, inhibition of efflux pumps, physical disruption of BBB by radiofrequency electromagnetic radiation (EMP), laser-induced thermal therapy (LITT), focused ultrasounds (FUS) combined with microbubbles and convection enhanced delivery (CED). However, most of these strategies have been tested only in preclinical models or in phase 1–2 trials, and none of them have been approved for treatment of brain tumors yet. Concerning the treatment of brain metastases, many molecules have been developed in the last years with a better penetration across BBB (new generation tyrosine kinase inhibitors like osimertinib for non-small-cell lung carcinoma and neratinib/tucatinib for breast cancer), resulting in better progression-free survival and overall survival compared to older molecules. Promising studies concerning neural stem cells, CAR-T (chimeric antigen receptors) strategies and immunotherapy with checkpoint inhibitors are ongoing.

Form and Function of the Vertebrate and Invertebrate Blood-Brain Barriers

The need to protect neural tissue from toxins or other substances is as old as neural tissue itself. Early recognition of this need has led to more than a century of investigation of the blood-brain barrier (BBB). Many aspects of this important neuroprotective barrier have now been well established, including its cellular architecture and barrier and transport functions. Unsurprisingly, most research has had a human orientation, using mammalian and other animal models to develop translational research findings. However, cell layers forming a barrier between vascular spaces and neural tissues are found broadly throughout the invertebrates as well as in all vertebrates. Unfortunately, previous scenarios for the evolution of the BBB typically adopt a classic, now discredited 'scala naturae' approach, which inaccurately describes a putative evolutionary progression of the mammalian BBB from simple invertebrates to mammals. In fact, BBB-like structures have evolved independently numerous times, complicating simplistic views of the evolution of the BBB as a linear process. Here, we review BBBs in their various forms in both invertebrates and vertebrates, with an emphasis on the function, evolution, and conditional relevance of popular animal models such as the fruit fly and the zebrafish to mammalian BBB research.

Recent progress and new challenges in modeling of human pluripotent stem cell-derived blood-brain barrier

The blood-brain barrier (BBB) is a semipermeable unit that serves to vascularize the central nervous system (CNS) while tightly regulating the movement of molecules, ions, and cells between the blood and the brain. The BBB precisely controls brain homeostasis and protects the neural tissue from toxins and pathogens. The BBB is coordinated by a tight monolayer of brain microvascular endothelial cells, which is subsequently supported by mural cells, astrocytes, and surrounding neuronal cells that regulate the barrier function with a series of specialized properties. Dysfunction of barrier properties is an important pathological feature in the progression of various neurological diseases. In vitro BBB models recapitulating the physiological and diseased states are important tools to understand the pathological mechanism and to serve as a platform to screen potential drugs. Recent advances in this field have stemmed from the use of pluripotent stem cells (PSCs). Various cell types of the BBB such as brain microvascular endothelial cells (BMECs), pericytes, and astrocytes have been derived from PSCs and synergistically incorporated to model the complex BBB structure in vitro. In this review, we summarize the most recent protocols and techniques for the differentiation of major cell types of the BBB. We also discuss the progress of BBB modeling by using PSC-derived cells and perspectives on how to reproduce more natural BBBs in vitro.

LPS tolerance prevents anxiety-like behavior and amygdala inflammation of high-fat-fed dams' adolescent offspring

Maternal high-fat diets (HFD) can generate inflammation in the offspring's amygdala, which can lead to anxiety-like behaviors. Conversely, lipopolysaccharide (LPS) tolerance can reduce neuroinflammation in the offspring caused by maternal high-fat diets. This study evaluated the combination of LPS tolerance and high-fat maternal diet on amygdala's inflammatory parameters and the anxiety-like behavior in adolescent offspring. Female pregnant Wistar rats received randomly a standard diet or a high-fat diet during gestation and lactation. On gestation days 8, 10, and 12, half of the females in each group were intraperitonially injected with LPS (0.1 mg.kg-1). After weaning, the male offspring (n = 96) were placed in individual boxes in standard conditions, and when 6 weeks-old, the animals underwent: Open-Field, Light/Dark Box, Elevated Plus-Maze, and Rotarod tests. When 50 days-old the offspring were euthanized and the amygdala removed for cytokine and redox status analysis. The offspring in the HFD group showed lower amygdala IL-10 levels, high IL-6/IL-10 ratio, and anxiety-like behaviors. These effects were attenuated in the HFD offspring submitted to LPS tolerance, which showed an anti-inflammatory compensatory response in the amygdala. Also, this group showed a higher activity of the enzyme catalase in the amygdala. In addition, receiving the combination of LPS tolerance and maternal HFD did not lead to anxiety-like behavior in the offspring. The results suggest that LPS tolerance attenuated amygdala inflammation through an anti-inflammatory compensatory response besides preventing anxiety-like behavior caused by the high-fat maternal diet.

Broadening the range of use cases for ivermectin - a review of the evidence

Ivermectin is a broad-spectrum antiparasitic agent that interferes with glutamate-gated chloride channels found in invertebrates but not in vertebrate species. Mass drug administration (MDA) with ivermectin-based regimes has been a mainstay of elimination efforts targeting onchocerciasis and lymphatic filariasis for more than 3 decades. More recently, interest in the use of ivermectin to control other neglected tropical diseases (NTDs) such as soil-transmitted helminths and scabies has grown. Interest has been further stimulated by the fact that ivermectin displays endectocidal efficacy against various Anopheles species capable of transmitting malaria. Therefore there is growing interest in using ivermectin MDA as a tool that might aid in the control of both malaria and several NTDs. In this review we outline the evidence base to date on these emerging indications for ivermectin MDA with reference to clinical and public health data and discuss the rationale for evaluating the range of impacts of a malaria ivermectin MDA on other NTDs.

The crosstalk between brain and periphery: Implications for brain health and disease

Mounting evidence indicates that signaling molecules identified primarily in the peripheral circulation can affect cognitive function in physiological and pathological conditions, including in the development of several neurological diseases. However, considering the properties of the vascular blood-brain barrier (BBB), circulating lipophobic molecules would not be expected to cross this vascular structure. Thus, if and how peripheral lipophobic molecules, such as hormones and cytokines, reach the brain to exert their reported effects remains to be better established. In this review, we will discuss evidence for and against the ability of molecules in the circulation, such as insulin, cytokines, and irisin to reach the brain and mediate the crosstalk between peripheral tissues and the central nervous system (CNS). We hypothesize that in addition to entering the brain via receptor-mediated transcytosis, these circulating molecules can have their transport facilitated by extracellular vesicles or under pathological conditions when the BBB is disrupted. We also discuss the possibility that these circulating molecules access the brain by acting directly on circumventricular organs, which lack the BBB, by local synthesis in the choroid plexus, and via activation of afferent vagal nerves. Advancing the understanding of mechanisms implicated in the transport of blood-borne molecules to the CNS will help us elucidate the contribution of peripheral factors to brain health and disease, and will enable the development of minimally invasive strategies to deliver therapeutic drugs to the brain in neurological disorders.

Neural Stem Cells for Early Ischemic Stroke

Clinical treatments for ischemic stroke are limited. Neural stem cell (NSC) transplantation can be a promising therapy. Clinically, ischemia and subsequent reperfusion lead to extensive neurovascular injury that involves inflammation, disruption of the blood-brain barrier, and brain cell death. NSCs exhibit multiple potentially therapeutic actions against neurovascular injury. Currently, tissue plasminogen activator (tPA) is the only FDA-approved clot-dissolving agent. While tPA’s thrombolytic role within the vasculature is beneficial, tPA’s non-thrombolytic deleterious effects aggravates neurovascular injury, restricting the treatment time window (time-sensitive) and tPA eligibility. Thus, new strategies are needed to mitigate tPA’s detrimental effects and quickly mediate vascular repair after stroke. Up to date, clinical trials focus on the impact of stem cell therapy on neuro-restoration by delivering cells during the chronic stroke stage. Also, NSCs secrete factors that stimulate endogenous repair mechanisms for early-stage ischemic stroke. This review will present an integrated view of the preclinical perspectives of NSC transplantation as a promising treatment for neurovascular injury, with an emphasis on early-stage ischemic stroke. Further, this will highlight the impact of early sub-acute NSC delivery on improving short-term and long-term stroke outcomes.

Focused Ultrasound Combined with Microbubbles in Central Nervous System Applications

The blood–brain barrier (BBB) protects the central nervous system (CNS) from invasive pathogens and maintains the homeostasis of the brain. Penetrating the BBB has been a major challenge in the delivery of therapeutic agents for treating CNS diseases. Through a physical acoustic cavitation effect, focused ultrasound (FUS) combined with microbubbles achieves the local detachment of tight junctions of capillary endothelial cells without inducing neuronal damage. The bioavailability of therapeutic agents is increased only in the area targeted by FUS energy. FUS with circulating microbubbles is currently the only method for inducing precise, transient, reversible, and noninvasive BBB opening (BBBO). Over the past decade, FUS-induced BBBO (FUS-BBBO) has been preclinically confirmed to not only enhance the penetration of therapeutic agents in the CNS, but also modulate focal immunity and neuronal activity. Several recent clinical human trials have demonstrated both the feasibility and potential advantages of using FUS-BBBO in diseased patients. The promising results support adding FUS-BBBO as a multimodal therapeutic strategy in modern CNS disease management. This review article explores this technology by describing its physical mechanisms and the preclinical findings, including biological effects, therapeutic concepts, and translational design of human medical devices, and summarizes completed and ongoing clinical trials.

Biology and Models of the Blood-Brain Barrier

The blood-brain barrier (BBB) is one of the most selective endothelial barriers. An understanding of its cellular, morphological, and biological properties in health and disease is necessary to develop therapeutics that can be transported from blood to brain. In vivo models have provided some insight into these features and transport mechanisms adopted at the brain, yet they have failed as a robust platform for the translation of results into clinical outcomes. In this article, we provide a general overview of major BBB features and describe various models that have been designed to replicate this barrier and neurological pathologies linked with the BBB. We propose several key parameters and design characteristics that can be employed to engineer physiologically relevant models of the blood-brain interface and highlight the need for a consensus in the measurement of fundamental properties of this barrier.

High-Resolution Insights Into the in vitro Developing Blood-Brain Barrier: Novel Morphological Features of Endothelial Nanotube Function

High-resolution electron microscopy (HREM) imaging of the in vitro blood-brain barrier (BBB), is a promising modality for investigating the dynamic morphological interplay underpinning BBB development. The successful establishment of BBB integrity is grounded in the brain endothelial cells (BEC’s) ability to occlude its paracellular spaces of brain capillaries through the expression of the intercellular tight junction (TJ) proteins. The impermeability of these paracellular spaces are crucial in the regulation of transcellular transport systems to achieve homeostasis of the central nervous system. To-date research describing morphologically, the dynamics by which TJ interaction is orchestrated to successfully construct a specialized barrier remains undescribed. In this study, the application of HREM illuminates the novel, dynamic and highly restrictive BEC paracellular pathway which is founded based on lateral membrane alignment which is the functional imperative for the mechanical juxtapositioning of TJ zones that underpin molecular bonding and sealing of the paracellular space. For the first time, we report on the secretion of a basement membrane in vitro, which allow BECs to orientate themselves into distinct basolateral and apicolateral domains and establish a 3-dimensional BEC construct. We report for the first time, on the expression of nanovesicles bound to the plasma membrane surfaces of the BECs. These membrane-bound vesicles are reported to possess an array of DNA/RNA constituents and chemotaxic properties affecting the formation of nanotubes that span the paracellular space between BECs, facilitating BBB construction, alluding to a functional role in mediating cell-to-cell communication. This study suggests that novel, ultrathin nanotubular (NT) structures are involved in functional roles in bringing into alignment the paracellular space of BECs. Immortalized mouse BECs (b.End3, b.End5) and primary rat cardiac microvascular ECs were used to further validate the in vitro BBB model by profiling variances in peripheral EC monolayer development. These cardiac capillary ECs presented with an opposite topographical profile: large fenestra and intercellular spaces, devoid of morphological ultrastructures. This comparative study alludes to the role of NT facilitation in TJ-induced hemifusion of apicolateral BEC membranes, as a structural event forming the basis for establishing a polarized BBB.

Pro- and Antiangiogenic Factors in Gliomas: Implications for Novel Therapeutic Possibilities

Angiogenesis, a complex, multistep process of forming new blood vessels, plays crucial role in normal development, embryogenesis, and wound healing. Malignant tumors characterized by increased proliferation also require new vasculature to provide an adequate supply of oxygen and nutrients for developing tumor. Gliomas are among the most frequent primary tumors of the central nervous system (CNS), characterized by increased new vessel formation. The processes of neoangiogenesis, necessary for glioma development, are mediated by numerous growth factors, cytokines, chemokines and other proteins. In contrast to other solid tumors, some biological conditions, such as the blood–brain barrier and the unique interplay between immune microenvironment and tumor, represent significant challenges in glioma therapy. Therefore, the objective of the study was to present the role of various proangiogenic factors in glioma angiogenesis as well as the differences between normal and tumoral angiogenesis. Another goal was to present novel therapeutic options in oncology approaches. We performed a thorough search via the PubMed database. In this paper we describe various proangiogenic factors in glioma vasculature development. The presented paper also reviews various antiangiogenic factors necessary in maintaining equilibrium between pro- and antiangiogenic processes. Furthermore, we present some novel possibilities of antiangiogenic therapy in this type of tumors.

Outbreak of Enterovirus Infection with Neurological Presentations in a Pediatric Population in Northern Spain: A Clinical Observational Study

OBJECTIVE

The study aimed to describe the cases of neurological disease related to the outbreak of enterovirus (EV) in three regions in Northern Spain during 2016.

MATERIALS AND METHODS

Multicenter retrospective observational study. Clinical, radiological, and microbiological data were analyzed from patients younger than 15 years with confirmed EV-associated neurological disease admitted to 10 hospitals of Asturias, Cantabria, and Castile and Leon between January 1 and December 31, 2016.

RESULTS

Fifty-five patients were included. Median age was 24 months (interquartile range = 18.5 months). Fifteen patients were classified as aseptic meningitis (27.3%). In total, 37 cases presented brainstem encephalitis (67.3%), 25 of them due to EV-A71 with excellent prognosis (84.6% asymptomatic 2 months following the onset). Three cases of acute flaccid myelitis (5.5%) by EV-D68 were reported and presented persistent paresis 2 months following the onset. Microbiological diagnosis by reverse transcriptase polymerase chain reaction was performed in all cases, finding EV in cerebrospinal fluid in meningitis, but not in brainstem encephalitis and acute flaccid myelitis, where EV was found in respiratory or rectal samples. Step therapy was administrated with intravenous immunoglobulin (IVIG; 32.7%), methylprednisolone (10%), and plasmapheresis (3.6%). Four patients received fluoxetine (7.3%). Twenty patients needed to be admitted to pediatric intensive care unit (36.4%).

CONCLUSION

Clinical, microbiological, and radiological diagnosis is essential in outbreaks of EV neurological disease, taking into account that it can be difficult to identify EV-A71 and EV-D68 in CSF, requiring throat or rectal samples. There is not specific treatment to these conditions and the efficacy and understanding of the mechanism of action of immune-modulatory treatment (IVIG, corticosteroids, and plasmapheresis) is limited.

Astrocyte-Endotheliocyte Axis in the Regulation of the Blood-Brain Barrier

The evolution of blood-brain barrier paralleled centralisation of the nervous system: emergence of neuronal masses required control over composition of the interstitial fluids. The barriers were initially created by glial cells, which employed septate junctions to restrict paracellular diffusion in the invertebrates and tight junctions in some early vertebrates. The endothelial barrier, secured by tight and adherent junctions emerged in vertebrates and is common in mammals. Astrocytes form the parenchymal part of the blood-brain barrier and commutate with endothelial cells through secretion of growth factors, morphogens and extracellular vesicles. These secreted factors control the integrity of the blood-brain barrier through regulation of expression of tight junction proteins. The astrocyte-endotheliocyte communications are particularly important in various neurological diseases associated with impairments to the blood-brain barrier. Molecular mechanisms supporting astrocyte-endotheliocyte axis in health and disease are in need of detailed characterisation.

Epigenomics and immunotherapeutic advances in pediatric brain tumors

Brain tumors are the leading cause of childhood cancer-related deaths. Similar to adult brain tumors, pediatric brain tumors are classified based on histopathological evaluations. However, pediatric brain tumors are often histologically inconsistent with adult brain tumors. Recent research findings from molecular genetic analyses have revealed molecular and genetic changes in pediatric tumors that are necessary for appropriate classification to avoid misdiagnosis, the development of treatment modalities, and the clinical management of tumors. As many of the molecular-based therapies developed from clinical trials on adults are not always effective against pediatric brain tumors, recent advances have improved our understanding of the molecular profiles of pediatric brain tumors and have led to novel epigenetic and immunotherapeutic treatment approaches currently being evaluated in clinical trials. In this review, we focus on primary malignant brain tumors in children and genetic, epigenetic, and molecular characteristics that differentiate them from brain tumors in adults. The comparison of pediatric and adult brain tumors highlights the need for treatments designed specifically for pediatric brain tumors. We also discuss the advancements in novel molecularly targeted drugs and how they are being integrated with standard therapy to improve the classification and outcomes of pediatric brain tumors in the future.

The impact of (ab)normal maternal environment on cortical development

The cortex in the mammalian brain is the most complex brain region that integrates sensory information and coordinates motor and cognitive processes. To perform such functions, the cortex contains multiple subtypes of neurons that are generated during embryogenesis. Newly born neurons migrate to their proper location in the cortex, grow axons and dendrites, and form neuronal circuits. These developmental processes in the fetal brain are regulated to a large extent by a great variety of factors derived from the mother - starting from simple nutrients as building blocks and ending with hormones. Thus, when the normal maternal environment is disturbed due to maternal infection, stress, malnutrition, or toxic substances, it might have a profound impact on cortical development and the offspring can develop a variety of neurodevelopmental disorders. Here we first describe the major developmental processes which generate neuronal diversity in the cortex. We then review our knowledge of how most common maternal insults affect cortical development, perturb neuronal circuits, and lead to neurodevelopmental disorders. We further present a concept of selective vulnerability of cortical neuronal subtypes to maternal-derived insults, where the vulnerability of cortical neurons and their progenitors to an insult depends on the time (developmental period), place (location in the developing brain), and type (unique features of a cell type and an insult). Finally, we provide evidence for the existence of selective vulnerability during cortical development and identify the most vulnerable neuronal types, stages of differentiation, and developmental time for major maternal-derived insults.

A historical perspective on the interactions of insulin at the blood-brain barrier

Subsequent to the discovery of insulin in 1921, the role of insulin in the brain has been investigated throughly. The ability of insulin to act within the brain to regulate peripheral glucose levels helped evolve the research surrounding the ability of insulin to be transported into the brain. Investigations aiming to determine the transport of insulin into the brain from the circulation soon followed. Once it was established that insulin could enter the brain, the ability of insulin to bind brain microvessels and regulators of this process were determined. As technology advanced, quantitative measurements to specify the transport rate of insulin across the blood-brain barrier (BBB) and the impact of physiological conditions and diseases were the logical next steps. Lastly, with the advent of genetic mouse models and high-specificity antagonists, the specific role of the insulin receptor in mediating insulin transport could begin to be explored. In this review, we summarise the main findings throughout the decades regarding the interactions of insulin at the BBB.

Vascular Dysfunction in the Brain; Implications for Heavy Metal Exposures

Normal or diseased conditions that alter the brain's requirement for oxygen and nutrients via alterations to neurovascular coupling have an impact on the level of the neurovascular unit; comprising neuronal, glial and vascular components. The communications between the components of the neurovascular unit are precise and accurate for its functions; hence a minute disturbance can result in neurovascular dysfunction. Heavy metals such as cadmium, mercury, and lead have been identified to increase the vulnerability of the neurovascular unit to damage. This review examines the role of heavy metals in neurovascular dysfunctions and the possible mechanisms by which these metals act. Risk factors ranging from lifestyle, environment, genetics, infections, and physiologic ageing involved in neurological dysfunctions were highlighted, while stroke was discussed as the prevalent consequence of neurovascular dysfunctions. Furthermore, the role of these heavy metals in the pathogenesis of stroke consequently pinpoints the importance of understanding the mechanisms of neurovascular damage in a bid to curb the occurrence of neurovascular dysfunctions.

Breakthrough of ZrO2 nanoparticles into fetal brains depends on developmental stage of maternal placental barrier and fetal blood-brain-barrier

Ingestion of nanoparticles may cause various damages to human body. However, how such ingestion by pregnant mother influences fetal development is not known because, presumably, ingested nanoparticles have to cross multiple biological barriers (such as intestinal and placental) to reach fetus. To answer this crucial question, here we investigated how a relatively biocompatible zirconia nanoparticles (ZrO₂ NPs, 16 nm) were translocated to fetal brains in three exposure models of pregnant mice: Model 1, oral exposure of nanoparticles before maternal blood-placental barrier (BPB) was fully developed; Model 2, exposures after BPB was developed, but before fetal blood-brain-barrier (BBB) was fully developed; Model 3, exposures after both maternal BPB and fetal BBB were fully developed. Our experimental results showed that translocation of ZrO₂ NPs into fetal brains was 55 % higher in Model 2 and 96 % higher in Model 1 compared with that in Model 3 after nanoparticles (50 mg/kg) were orally exposed to pregnant mice. Therefore, nanoparticles are able to cross multiple biological barriers and nanotoxicity to fetus is highly dependent on stages of pregnancy and fetal development or the maturity of multiple biological barriers. Oral exposures to nanoparticles during pregnancy are dangerous to fetal brain development, especially in early pregnancy.

Blood-Brain Barrier Dynamics to Maintain Brain Homeostasis

The blood-brain barrier (BBB) is a dynamic platform for exchange of substances between the blood and the brain parenchyma, and it is an essential functional gatekeeper for the central nervous system (CNS). While it is widely recognized that BBB disruption is a hallmark of several neurovascular pathologies, an aspect of the BBB that has received somewhat less attention is the dynamic modulation of BBB tightness to maintain brain homeostasis in response to extrinsic environmental factors and physiological changes. In this review, we summarize how BBB integrity adjusts in critical stages along the life span, as well as how BBB permeability can be altered by common stressors derived from nutritional habits, environmental factors and psychological stress.

Basic physiology of the blood-brain barrier in health and disease: a brief overview

The blood-brain barrier (BBB), a dynamic interface between blood and brain constituted mainly by endothelial cells of brain microvessels, robustly restricts the entry of potentially harmful blood-sourced substances and cells into the brain, however, many therapeutically active agents concurrently cannot gain access into the brain at effective doses in the presence of an intact barrier. On the other hand, breakdown of BBB integrity may involve in the pathogenesis of various neurodegenerative diseases. Besides, certain diseases/disorders such as Alzheimer's disease, hypertension, and epilepsy are associated with varying degrees of BBB disruption. In this review, we aim to highlight the current knowledge on the cellular and molecular composition of the BBB with special emphasis on the major transport pathways across the barrier type endothelial cells. We further provide a discussion on the innovative brain drug delivery strategies in which the obstacle formed by BBB interferes with effective pharmacological treatment of neurodegenerative diseases/disorders.

Astrocyte-derived Wnt growth factors are required for endothelial blood-brain barrier maintenance

Maintenance of the endothelial blood-brain-barrier (BBB) through Wnt/β-catenin signalling is essential for neuronal function. The cells however, providing Wnt growth factors at the adult neurovascular unit (NVU) are poorly explored. Here we show by conditionally knocking out the evenness interrupted (Evi) gene in astrocytes (Evi^ΔAC) that astrocytic Wnt release is crucial for BBB and NVU integrity. Evi^ΔAC mice developed brain oedema and increased vascular tracer leakage. While brain vascularization and endothelial junctions were not altered in 10 and 40 week-old mice, endothelial caveolin(Cav)-1-mediated vesicle formation was increased in vivo and in vitro. Moreover, astrocytic end-feet were swollen, and aquaporin-4 distribution was disturbed, coinciding with decreased astrocytic Wnt activity. Vascular permeability correlated with increased neuronal activation by c-fos staining, indicative of altered neuronal function. Astrocyte-derived Wnts thus serve to maintain Wnt/β-catenin activity in endothelia and in astrocytes, thereby controlling Cav-1 expression, vesicular abundance, and end-feet integrity at the NVU.

Biodistribution, cardiac and neurobehavioral assessments, and neurotransmitter quantification in juvenile rats following oral administration of aluminum oxide nanoparticles

Little is known about the uptake, biodistribution, and biological responses of nanoparticles (NPs) and their toxicity in developing animals. Here, male and female juvenile Sprague-Dawley rats received four consecutive daily doses of 10 mg/kg Al₂ O₃ NP (diameter: 24 nm [transmission electron microscope], hydrodynamic diameter: 148 nm) or vehicle control (water) by gavage between postnatal days (PNDs) 17-20. Basic neurobehavioral and cardiac assessments were performed on PND 20. Animals were sacrificed on PND 21, and selected tissues were collected, weighed, and processed for histopathology or neurotransmitter analysis. The biodistribution of Al₂ O₃ NP in tissue sections of the intestine, liver, spleen, kidney, and lymph nodes were evaluated using enhanced dark-field microscopy (EDM) and hyperspectral imaging (HSI). Liver-to-body weight ratio was significantly increased for male pups administered Al₂ O₃ NP compared with control. HSI suggested that Al₂ O₃ NP was more abundant in the duodenum and ileum tissue of the female pups compared with the male pups, whereas the abundance of NP was similar for males and females in the other tissues. The abundance of NP was higher in the liver compared with spleen, lymph nodes, and kidney. Homovanillic acid and norepinephrine concentrations in brain were significantly decreased following Al₂ O₃ NP administration in female and male pups, whereas 5-hydroxyindoleacetic acid was significantly increased in male pups. EDM/HSI indicates intestinal uptake of Al₂ O₃ NP following oral administration. Al₂ O₃ NP altered neurotransmitter/metabolite concentrations in juvenile rats' brain tissues. Together, these data suggest that orally administered Al₂ O₃ NP interferes with the brain biochemistry in both female and male pups.

Occludin regulation of blood-brain barrier and potential therapeutic target in ischemic stroke

Occludin is a key structural component of the blood–brain barrier (BBB) that has recently become an important focus of research in BBB damages. Many studies have demonstrated that occludin could regulate the integrity and permeability of the BBB. The function of BBB depends on the level of occludin protein expression in brain endothelial cells. Moreover, occludin may serve as a potential biomarker for hemorrhage transformation after acute ischemic stroke. In this review, we summarize the role of occludin in BBB integrity and the regulatory mechanisms of occludin in the permeability of BBB after ischemic stroke. Multiple factors have been found to regulate occludin protein functions in maintaining BBB permeability, such as Matrix metalloproteinas-mediated cleavage, phosphorylation, ubiquitination, and related inflammatory factors. In addition, various signaling pathways participate in regulating the occludin expression, including nuclear factor-kappa B, mitogen-activated protein kinase, protein kinase c, RhoK, and ERK1/2. Emerging therapeutic interventions for ischemic stroke targeting occludin are described, including normobaric hyperoxia, Chinese medicine, chemical drugs, genes, steroid hormones, small molecular peptides, and other therapies. Since occludin has been shown to play a critical role in regulating BBB integrity, further preclinical studies will help evaluate and validate occludin as a viable therapeutic target for ischemic stroke.

A simple novel approach for detecting blood-brain barrier permeability using GPCR internalization

Aims

Impairment of blood–brain barrier (BBB) is involved in numerous neurological diseases from developmental to aging stages. Reliable imaging of increased BBB permeability is therefore crucial for basic research and preclinical studies. Today, the analysis of extravasation of exogenous dyes is the principal method to study BBB leakage. However, these procedures are challenging to apply in pups and embryos and may appear difficult to interpret. Here we introduce a novel approach based on agonist‐induced internalization of a neuronal G protein‐coupled receptor widely distributed in the mammalian brain, the somatostatin receptor type 2 (SST2).

Methods

The clinically approved SST2 agonist octreotide (1 kDa), when injected intraperitoneally does not cross an intact BBB. At sites of BBB permeability, however, OCT extravasates and induces SST2 internalization from the neuronal membrane into perinuclear compartments. This allows an unambiguous localization of increased BBB permeability by classical immunohistochemical procedures using specific antibodies against the receptor.

Results

We first validated our approach in sensory circumventricular organs which display permissive vascular permeability. Through SST2 internalization, we next monitored BBB opening induced by magnetic resonance imaging‐guided focused ultrasound in murine cerebral cortex. Finally, we proved that after intraperitoneal agonist injection in pregnant mice, SST2 receptor internalization permits analysis of BBB integrity in embryos during brain development.

Conclusions

This approach provides an alternative and simple manner to assess BBB dysfunction and development in different physiological and pathological conditions.

A Novel Neurovascular Liaison Governing the Blood-Brain Barrier

How circulating signals enter the median eminence to trigger homeostatic hypothalamic responses is not well understood. Jiang et al. describe a neural mechanism that increases endothelial fenestrations and enhances the hypothalamic response to the circulating hormone leptin, suggesting a novel way to regulate brain entry through vascular wall remodeling.

Physiological blood-brain transport is impaired with age by a shift in transcytosis

The vascular interface of the brain, known as the blood-brain barrier (BBB), is understood to maintain brain function in part via its low transcellular permeability^1-3. Yet, recent studies have demonstrated that brain ageing is sensitive to circulatory proteins^4,5. Thus, it is unclear whether permeability to individually injected exogenous tracers-as is standard in BBB studies-fully represents blood-to-brain transport. Here we label hundreds of proteins constituting the mouse blood plasma proteome, and upon their systemic administration, study the BBB with its physiological ligand. We find that plasma proteins readily permeate the healthy brain parenchyma, with transport maintained by BBB-specific transcriptional programmes. Unlike IgG antibody, plasma protein uptake diminishes in the aged brain, driven by an age-related shift in transport from ligand-specific receptor-mediated to non-specific caveolar transcytosis. This age-related shift occurs alongside a specific loss of pericyte coverage. Pharmacological inhibition of the age-upregulated phosphatase ALPL, a predicted negative regulator of transport, enhances brain uptake of therapeutically relevant transferrin, transferrin receptor antibody and plasma. These findings reveal the extent of physiological protein transcytosis to the healthy brain, a mechanism of widespread BBB dysfunction with age and a strategy for enhanced drug delivery.

Medications for pregnant women: A balancing act between the interests of the mother and of the fetus

Drug entry into the adult brain is controlled by efflux mechanisms situated in various brain barrier interfaces. The effectiveness of these protective mechanisms in the embryo, fetus and newborn brain is less clear. The longstanding belief that "the" blood-brain barrier is absent or immature in the fetus and newborn has led to many misleading statements with potential clinical implications. Here we review the properties of brain barrier mechanisms in the context of drug entry into the developing brain and discuss the limited number of studies published on the subject. We noticed that most of available literature suffers from some experimental limitations, notably that drug levels in fetal blood and cerebrospinal fluid have not been measured. This means that the relative contribution to the overall brain protection provided by individual barriers such as the placenta (which contains similar efflux mechanisms) and the brain barriers cannot be separately ascertained. Finally, we propose that systematic studies in appropriate animal models of drug entry into the brain at different stages of development would provide a rational basis for use of medications in pregnancy and in newborns, especially prematurely born, where protection usually provided by the placenta is no longer present.

Zika virus infects pericytes in the choroid plexus and enters the central nervous system through the blood-cerebrospinal fluid barrier

Zika virus (ZIKV) can infect and cause microcephaly and Zika-associated neurological complications in the developing fetal and adult brains. In terms of pathogenesis, a critical question is how ZIKV overcomes the barriers separating the brain from the circulation and gains access to the central nervous system (CNS). Despite the importance of ZIKV pathogenesis, the route ZIKV utilizes to cross CNS barriers remains unclear. Here we show that in mouse models, ZIKV-infected cells initially appeared in the periventricular regions of the brain, including the choroid plexus and the meninges, prior to infection of the cortex. The appearance of ZIKV in cerebrospinal fluid (CSF) preceded infection of the brain parenchyma. Further the brain infection was significantly attenuated by neutralization of the virus in the CSF, indicating that ZIKV in the CSF at the early stage of infection might be responsible for establishing a lethal infection of the brain. We show that cells infected by ZIKV in the choroid plexus were pericytes. Using in vitro systems, we highlight the possibility that ZIKV crosses the blood-CSF barrier by disrupting the choroid plexus epithelial layer. Taken together, our results suggest that ZIKV might exploit the blood-CSF barrier rather than the blood-brain barrier to invade the CNS.

Postnatal development of cerebrovascular structure and the neurogliovascular unit

The unceasing metabolic demands of brain function are supported by an intricate three-dimensional network of arterioles, capillaries, and venules, designed to effectively distribute blood to all neurons and to provide shelter from harmful molecules in the blood. The development and maturation of this microvasculature involves a complex interplay between endothelial cells with nearly all other brain cell types (pericytes, astrocytes, microglia, and neurons), orchestrated throughout embryogenesis and the first few weeks after birth in mice. Both the expansion and regression of vascular networks occur during the postnatal period of cerebrovascular remodeling. Pial vascular networks on the brain surface are dense at birth and are then selectively pruned during the postnatal period, with the most dramatic changes occurring in the pial venular network. This is contrasted to an expansion of subsurface capillary networks through the induction of angiogenesis. Concurrent with changes in vascular structure, the integration and cross talk of neurovascular cells lead to establishment of blood-brain barrier integrity and neurovascular coupling to ensure precise control of macromolecular passage and metabolic supply. While we still possess a limited understanding of the rules that control cerebrovascular development, we can begin to assemble a view of how this complex process evolves, as well as identify gaps in knowledge for the next steps of research. This article is categorized under: Nervous System Development > Vertebrates: Regional Development Vertebrate Organogenesis > Musculoskeletal and Vascular Nervous System Development > Vertebrates: General Principles.

Enterovirus and Encephalitis

Enterovirus-induced infection of the central nervous system (CNS) results in acute inflammation of the brain (encephalitis) and constitutes a significant global burden to human health. These viruses are thought to be highly cytolytic, therefore normal brain function could be greatly compromised following enteroviral infection of the CNS. A further layer of complexity is added by evidence showing that some enteroviruses may establish a persistent infection within the CNS and eventually lead to pathogenesis of certain neurodegenerative disorders. Interestingly, enterovirus encephalitis is particularly common among young children, suggesting a potential causal link between the development of the neuroimmune system and enteroviral neuroinvasion. Although the CNS involvement in enterovirus infections is a relatively rare complication, it represents a serious underlying cause of mortality. Here we review a selection of enteroviruses that infect the CNS and discuss recent advances in the characterization of these enteroviruses with regard to their routes of CNS infection, tropism, virulence, and immune responses.

Association of Adverse Hearing, Growth, and Discharge Age Outcomes With Postnatal Cytomegalovirus Infection in Infants With Very Low Birth Weight

IMPORTANCE

Studies suggest that postnatal cytomegalovirus (CMV) infection can lead to long-term morbidity in infants with very low birth weight (VLBW; <1500 g), including bronchopulmonary dysplasia (BPD), necrotizing enterocolitis (NEC), and neurodevelopmental impairment. However, to date, the association of postnatal CMV with hearing, growth, and length of stay among VLBW infants is unknown.

OBJECTIVES

To determine the risk for failed hearing screen, increased postnatal age at discharge, or decreased growth at discharge in VLBW infants with postnatal CMV infection compared with CMV-uninfected infants and to compare the risk for other major outcomes of prematurity, including BPD and NEC, in infants with and without postnatal CMV infection.

PARTICIPANTS

This multicenter retrospective cohort study included VLBW infants from 302 neonatal intensive care units managed by the Pediatrix Medical Group from January 1, 2002, through December 31, 2016. Infants hospitalized on postnatal day 21 with a diagnosis of postnatal CMV and hearing screen results after a postmenstrual age of 34 weeks were included in the study population. Data were analyzed from December 11, 2017, to June 14, 2019.

MAIN OUTCOMES AND MEASURES

Infants with and without postnatal CMV infection were matched using propensity scores. Poisson and linear regression were used to examine the association between postnatal CMV and the risk of failed hearing screen, postnatal age at discharge, growth, BPD, and NEC.

RESULTS

A total of 304 infants with postnatal CMV were identified, and 273 of these infants (89.8%; 155 boys [56.8%]) were matched with 273 infants without postnatal CMV (148 boys [54.2%]). Hearing screen failure occurred in 45 of 273 infants (16.5%) with postnatal CMV compared with 25 of 273 infants (9.2%) without postnatal CMV (risk ratio [RR], 1.80; 95% CI, 1.14 to 2.85; P = .01). Postnatal CMV was also associated with an increased postnatal age at discharge of 11.89 days (95% CI, 6.72 to 17.06 days; P < .001) and lower weight-for-age z score (-0.23; 95% CI, -0.39 to -0.07; P = .005). Analysis confirmed an increased risk of BPD (RR, 1.30; 95% CI, 1.17 to 1.44; P < .001), previously reported on infants from this cohort from 1997 to 2012, but not an increased risk of NEC after postnatal day 21 (RR, 2.00; 95% CI, 0.18 to 22.06; P = .57).

CONCLUSIONS AND RELEVANCE

These data suggest that postnatal CMV infection is associated with lasting sequelae in the hearing and growth status of VLBW infants and with prolonged hospitalization. Prospective studies are needed to determine the full effects of postnatal CMV infection and whether antiviral treatment reduces the associated morbidity.

Chronic periodontitis induces microbiota-gut-brain axis disorders and cognitive impairment in mice

Epidemiological studies suggest that chronic periodontitis (CP) is closely associated with the incidence and progression of cognitive impairment. The present study investigated the causal relationship between CP and cognitive decline and the underlying mechanism in mice. Long-term ligature around the left second maxillary molar tooth was used to induce CP in mice. Severe alveolar bone loss and inflammatory changes were observed in gingival tissues, accompanied by progressive cognitive deficits during a 12-month period. We also observed cerebral neuronal and synaptic injury and glial activation in this mouse model of CP. Furthermore, CP mice exhibited significant dysbiosis of the oral and gut microbiota, disruption of the intestinal barrier and blood-brain barrier, increases in the serum contents of proinflammatory cytokines and lipopolysaccharide (LPS), and increases in brain LPS levels, Toll-like receptor 4 (TLR4) expression, nuclear factor-κB (NF-κB) nuclear translocation and proinflammatory cytokine mRNA levels. These results indicate that CP may directly induce progressive cognitive decline and its mechanism is probably related to microbiota-gut-brain axis disorders, LPS/TLR4/NF-κB signaling activation and neuroinflammatory responses in mice. Therefore, the microbiota-gut-brain axis may provide the potential strategy for the prevention and treatment of CP-associated cognitive impairment.

The FDP/FIB Ratio and Blood FDP Level May Be Related to Seizures After Fever in Young Children

Objective: To evaluate the relationship of the blood fibrinogen (FIB) degradation product (FDP) level and FDP/FIB ratio with seizure in young children with fever. Methods: A total of 35 children with simple febrile seizures and 80 children with fever but no seizure were selected. First, the differences in white blood cell (WBC), platelets (PLT), prothrombin time (PT), activated partial thromboplastin time (APTT), thrombin time (TT), FIB, FDP, FDP/FIB ratio, and C-reactive protein (CRP) between 35 children with simple febrile seizures and 40 randomly selected children with fever but no seizure were retrospectively analyzed. Then, an ROC curve was used to determine the diagnostic utility of the FDP level, FDP/FIB ratio, and FDP+FDP/FIB ratio, and the best diagnostic cutoff points were selected. Finally, the diagnostic specificities of the three diagnostic indicators were verified by comparison with the results of all 80 children with fever but no seizure. Results: The FDP level and FDP/FIB ratio were significantly different between the two groups (P < 0.0001) and there was a positive correlation between the FDP and FIB levels. Both the FDP level and FDP/FIB ratio had good diagnostic value. An FDP ≥ 2.0 mg/L and FDP/FIB ratio ≥ 0.5 had good diagnostic specificities. Combined application of an FDP ≥ 2.0 mg/L and FDP/FIB ratio ≥ 0.5 improved the diagnostic power. Conclusions: The blood FDP level and FDP/FIB ratio may be related to seizures after fever, and an FDP ≥ 2.0 mg/L + FDP/FIB ratio ≥ 0.5 has good diagnostic specificity.

Bilirubin Induced Encephalopathy

Hyperbilirubinemia is one of the most common neonatal disorders. Delayed diagnosis and treatment of the pathologic and progressive indirect hyperbilirubinemia lead to neurological deficits, defined as bilirubin induced encephalopathy (BIE) (2). The incidence of this disorder in underdeveloped countries is much more than developed areas. All neonates with the risk factors for increased the blood level of indirect bilirubin are at risk for BIE, especially preterm neonates which are prone to low bilirubin kernicterus . BIE can be transient and acute (with early, intermediate and advanced phases)or be permanent, chronic and lifelong ( with tetrad of symptoms including visual (upward gaze palsy), auditory (sensory neural hearing loss), dental dysplasia abnormalities, and extrapyramidal disturbances (choreoathetosis cerebral palsy).Beside the abnormal neurologic manifestations of the jaundiced neonates ,brain MRI is the best imaging modality for the confirmation of the diagnosis. Although early treatment of extreme hyperbilirubinemia by phototherapy and exchange transfusion can prevent the BIE, unfortunately the chronic bilirubin encephalopathy does not have definitive treatment.

In Vitro Models of the Blood-Brain Barrier

Knowledge about the transport of active compounds across the blood-brain barrier is of essential importance for drug development. Systemically applied drugs for the central nervous system (CNS) must be able to cross the blood-brain barrier in order to reach their target sites, whereas drugs that are supposed to act in the periphery should not permeate the blood-brain barrier so that they do not trigger any adverse central adverse effects. A number of approaches have been pursued, and manifold in silico, in vitro, and in vivo animal models were developed in order to be able to make a better prediction for humans about the possible penetration of active substances into the CNS. In this particular case, however, in vitro models play a special role, since the data basis for in silico models is usually in need of improvement, and the predictive power of in vivo animal models has to be checked for possible species differences. The blood-brain barrier is a dynamic, highly selective barrier formed by brain capillary endothelial cells. One of its main tasks is the maintenance of homeostasis in the CNS. The function of the barrier is regulated by cells of the microenvironment and the shear stress mediated by the blood flow, which makes the model development most complex. In general, one could follow the credo "as easy as possible, as complex as necessary" for the usage of in vitro BBB models for drug development. In addition to the description of the classical cell culture models (transwell, hollow fiber) and guidance how to apply them, the latest developments (spheroids, microfluidic models) will be introduced in this chapter, as it is attempted to get more in vivo-like and to be applicable for high-throughput usage with these models. Moreover, details about the development of models based on stem cells derived from different sources with a special focus on human induced pluripotent stem cells are presented.

The blood-brain barrier and blood-tumour barrier in brain tumours and metastases

For a blood-borne cancer therapeutic agent to be effective, it must cross the blood vessel wall to reach cancer cells in adequate quantities, and it must overcome the resistance conferred by the local microenvironment around cancer cells. The brain microenvironment can thwart the effectiveness of drugs against primary brain tumours as well as brain metastases. In this Review, we highlight the cellular and molecular components of the blood-brain barrier (BBB), a specialized neurovascular unit evolved to maintain brain homeostasis. Tumours are known to compromise the integrity of the BBB, resulting in a vasculature known as the blood-tumour barrier (BTB), which is highly heterogeneous and characterized by numerous distinct features, including non-uniform permeability and active efflux of molecules. We discuss the challenges posed by the BBB and BTB for drug delivery, how multiple cell types dictate BBB function and the role of the BTB in disease progression and treatment. Finally, we highlight emerging molecular, cellular and physical strategies to improve drug delivery across the BBB and BTB and discuss their impact on improving conventional as well as emerging treatments, such as immune checkpoint inhibitors and engineered T cells. A deeper understanding of the BBB and BTB through the application of single-cell sequencing and imaging techniques, and the development of biomarkers of BBB integrity along with systems biology approaches, should enable new personalized treatment strategies for primary brain malignancies and brain metastases.

Transport Studies Using Blood-Brain Barrier In Vitro Models: A Critical Review and Guidelines

Permeation is one of the most evaluated parameters using preclinical in vitro blood-brain barrier models, as it has long been considered to be one of the major factors influencing central nervous system drug delivery. Blood-brain barrier permeability can be defined as the speed at which a compound crosses the brain endothelial cell barrier and is employed to assess barrier tightness, which is a crucial feature of brain capillaries in vivo. In addition, it is used to assess brain drug penetration. We review traditionally used methods to assess blood-brain barrier permeability in vitro and summarize often neglected in vivo (e.g., plasma protein and brain tissue binding) or in vitro (e.g., culture insert materials or methodology) factors that influence this property. These factors are crucial to consider when performing BBB permeability assessments, and especially when comparing permeability data obtained from different models, since model diversification significantly complicates inter-study comparisons. Finally, measuring transendothelial electrical resistance can be used to describe blood-brain barrier tightness; however, several parameters should be considered while comparing these measurements to the blood-brain barrier permeability to paracellular markers.

The blood-brain barrier: Physiology and strategies for drug delivery

The blood-brain barrier (BBB) is a dynamic structure that functions as a gatekeeper, reflecting the unique requirements of the brain. In this review, following a brief historical overview of how the concepts of the BBB and the neurovascular unit (NVU) developed, we describe its physiology and architecture, which pose a particular challenge to therapeutic intervention. We then discuss how the restrictive nature of this barrier can be overcome for the delivery of therapeutic agents. Alterations to drug formulation offer one option, in part by utilizing distinct transport modes; another is invasive or non-invasive strategies to bypass the BBB. An emerging non-invasive technology for targeted drug delivery is focused ultrasound that allows for the safe and reversible disruption of the BBB. We discuss the underlying mechanisms and provide an outlook, emphasizing the need for more research into the NVU and investment in innovative technologies to overcome the BBB for drug delivery.

Tight Junctions in Cell Proliferation

Tight junction (TJ) proteins form a continuous intercellular network creating a barrier with selective regulation of water, ion, and solutes across endothelial, epithelial, and glial tissues. TJ proteins include the claudin family that confers barrier properties, members of the MARVEL family that contribute to barrier regulation, and JAM molecules, which regulate junction organization and diapedesis. In addition, the membrane-associated proteins such as MAGUK family members, i.e., zonula occludens, form the scaffold linking the transmembrane proteins to both cell signaling molecules and the cytoskeleton. Most studies of TJ have focused on the contribution to cell-cell adhesion and tissue barrier properties. However, recent studies reveal that, similar to adherens junction proteins, TJ proteins contribute to the control of cell proliferation. In this review, we will summarize and discuss the specific role of TJ proteins in the control of epithelial and endothelial cell proliferation. In some cases, the TJ proteins act as a reservoir of critical cell cycle modulators, by binding and regulating their nuclear access, while in other cases, junctional proteins are located at cellular organelles, regulating transcription and proliferation. Collectively, these studies reveal that TJ proteins contribute to the control of cell proliferation and differentiation required for forming and maintaining a tissue barrier.

Clade F AAVHSCs cross the blood brain barrier and transduce the central nervous system in addition to peripheral tissues following intravenous administration in nonhuman primates

The biodistribution of AAVHSC7, AAVHSC15, and AAVHSC17 following systemic delivery was assessed in cynomolgus macaques (Macaca fascicularis). Animals received a single intravenous (IV) injection of a self-complementary AAVHSC-enhanced green fluorescent protein (eGFP) vector and tissues were harvested at two weeks post-dose for anti-eGFP immunohistochemistry and vector genome analyses. IV delivery of AAVHSC vectors produced widespread distribution of eGFP staining in glial cells throughout the central nervous system, with the highest levels seen in the pons and lateral geniculate nuclei (LGN). eGFP-positive neurons were also observed throughout the central and peripheral nervous systems for all three AAVHSC vectors including brain, spinal cord, and dorsal root ganglia (DRG) with staining evident in neuronal cell bodies, axons and dendritic arborizations. Co-labeling of sections from brain, spinal cord, and DRG with anti-eGFP antibodies and cell-specific markers confirmed eGFP-staining in neurons and glia, including protoplasmic and fibrous astrocytes and oligodendrocytes. For all capsids tested, 50 to 70% of glial cells (S100-β+) and on average 8% of neurons (NeuroTrace+) in the LGN were positive for eGFP expression. In the DRG, 45 to 62% of neurons and 8 to 12% of satellite cells were eGFP-positive for the capsids tested. eGFP staining was also observed in peripheral tissues with abundant staining in hepatocytes, skeletal- and cardio-myocytes and in acinar cells of the pancreas. Biodistribution of AAVHSC vector genomes in the central and peripheral organs generally correlated with eGFP staining and were highest in the liver for all AAVHSC vectors tested. These data demonstrate that AAVHSCs have broad tissue tropism and cross the blood-nerve and blood-brain-barriers following systemic delivery in nonhuman primates, making them suitable gene editing or gene transfer vectors for therapeutic application in human genetic diseases.

Computational Nanoscopy of Tight Junctions at the Blood-Brain Barrier Interface

The selectivity of the blood-brain barrier (BBB) is primarily maintained by tight junctions (TJs), which act as gatekeepers of the paracellular space by blocking blood-borne toxins, drugs, and pathogens from entering the brain. The BBB presents a significant challenge in designing neurotherapeutics, so a comprehensive understanding of the TJ architecture can aid in the design of novel therapeutics. Unraveling the intricacies of TJs with conventional experimental techniques alone is challenging, but recently developed computational tools can provide a valuable molecular-level understanding of TJ architecture. We employed the computational methods toolkit to investigate claudin-5, a highly expressed TJ protein at the BBB interface. Our approach started with the prediction of claudin-5 structure, evaluation of stable dimer conformations and nanoscale assemblies, followed by the impact of lipid environments, and posttranslational modifications on these claudin-5 assemblies. These led to the study of TJ pores and barriers and finally understanding of ion and small molecule transport through the TJs. Some of these in silico, molecular-level findings, will need to be corroborated by future experiments. The resulting understanding can be advantageous towards the eventual goal of drug delivery across the BBB. This review provides key insights gleaned from a series of state-of-the-art nanoscale simulations (or computational nanoscopy studies) performed on the TJ architecture.

A Single Injection of Human Neutralizing Antibody Protects against Zika Virus Infection and Microcephaly in Developing Mouse Embryos

Zika virus (ZIKV) is a mosquito-transmitted flavivirus that is generally benign in humans. However, an emergent strain of ZIKV has become widespread, causing severe pre- and post-natal neurological defects. There is now an urgent need for prophylactic and therapeutic agents. To address this, we investigated six human monoclonal antibodies with ZIKV epitope specificity and neutralizing activity in mouse models of ZIKV infection and microcephaly. A single intraperitoneal injection of these antibodies conveyed distinct levels of adult and in utero protection from ZIKV infection, which closely mirrored their respective in vitro neutralizing activities. One antibody, ZK2B10, showed the most potent neutralization activity, completely protected uninfected mice, and markedly reduced tissue pathology in infected mice. Thus, ZK2B10 is a promising candidate for the development of antibody-based interventions and informs the rational design of ZIKV vaccine.

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Human antibodies against ZIKV are tested in mouse models

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In vitro neutralizing activity correlates with in vivo protection

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The most potent antibody, ZK2B10, provides protection against infection

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ZK2B10 markedly delays mortality

The Evolving Concept of the Blood Brain Barrier (BBB): From a Single Static Barrier to a Heterogeneous and Dynamic Relay Center

The blood–brain barrier (BBB) helps maintain a tightly regulated microenvironment for optimal central nervous system (CNS) homeostasis and facilitates communications with the peripheral circulation. The brain endothelial cells, lining the brain’s vasculature, maintain close interactions with surrounding brain cells, e.g., astrocytes, pericytes and perivascular macrophages. This function facilitates critical intercellular crosstalk, giving rise to the concept of the neurovascular unit (NVU). The steady and appropriate communication between all components of the NVU is essential for normal CNS homeostasis and function, and dysregulation of one of its constituents can result in disease. Among the different brain regions, and along the vascular tree, the cellular composition of the NVU varies. Therefore, differential cues from the immediate vascular environment can affect BBB phenotype. To support the fluctuating metabolic and functional needs of the underlying neuropil, a specialized vascular heterogeneity is required. This is achieved by variances in barrier function, expression of transporters, receptors, and adhesion molecules. This mini-review will take you on a journey through evolving concepts surrounding the BBB, the NVU and beyond. Exploring classical experiments leading to new approaches will allow us to understand that the BBB is not merely a static separation between the brain and periphery but a closely regulated and interactive entity. We will discuss shifting paradigms, and ultimately aim to address the importance of BBB endothelial heterogeneity with regard to the function of the BBB within the NVU, and touch on its implications for different neuropathologies.

Cerebrovascular plasticity: Processes that lead to changes in the architecture of brain microvessels

The metabolic demands of the brain are met by oxygen and glucose, supplied by a complex hierarchical network of microvessels (arterioles, capillaries, and venules). Transient changes in neural activity are accommodated by local dilation of arterioles or capillaries to increase cerebral blood flow and hence nutrient availability. Transport and communication between the circulation and the brain is regulated by the brain microvascular endothelial cells that form the blood-brain barrier. Under homeostatic conditions, there is very little turnover in brain microvascular endothelial cells, and the cerebrovascular architecture is largely static. However, changes in the brain microenvironment, due to environmental factors, disease, or trauma, can result in additive or subtractive changes in cerebrovascular architecture. Additions occur by angiogenesis or vasculogenesis, whereas subtractions occur by vascular pruning, injury, or endothelial cell death. Here we review the various processes that lead to changes in the cerebrovascular architecture, including sustained changes in the brain microenvironment, development and aging, and injury, disease, and repair.

Improved Classification of Blood-Brain-Barrier Drugs Using Deep Learning

Blood-Brain-Barrier (BBB) is a strict permeability barrier for maintaining the Central Nervous System (CNS) homeostasis. One of the most important conditions to judge a CNS drug is to figure out whether it has BBB permeability or not. In the past 20 years, the existing prediction approaches are usually based on the data of the physical characteristics and chemical structure of drugs. However, these methods are usually only applicable to small molecule compounds based on passive diffusion through BBB. To deal this problem, one of the most famous methods is multi-core SVM method, which is based on clinical phenotypes about Drug Side Effects and Drug Indications to predict drug penetration of BBB. This paper proposed a Deep Learning method to predict the Blood-Brain-Barrier permeability based on the clinical phenotypes data. The validation result on three datasets proved that Deep Learning method achieves better performance than the other existing methods. The average accuracy of our method reaches 0.97, AUC reaches 0.98, and the F1 score is 0.92. The results proved that Deep Learning methods can significantly improve the prediction accuracy of drug BBB permeability and it can help researchers to reduce clinical trials and find new CNS drugs.