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

Bridging barriers: advances and challenges in modeling biological barriers and measuring barrier integrity in organ-on-chip systems

Biological barriers such as the blood-brain barrier, skin, and intestinal mucosal barrier play key roles in homeostasis, disease physiology, and drug delivery - as such, it is important to create representative in vitro models to improve understanding of barrier biology and serve as tools for therapeutic development. Microfluidic cell culture and organ-on-a-chip (OOC) systems enable barrier modelling with greater physiological fidelity than conventional platforms by mimicking key environmental aspects such as fluid shear, accurate microscale dimensions, mechanical cues, extracellular matrix, and geometrically defined co-culture. As the prevalence of barrier-on-chip models increases, so does the importance of tools that can accurately assess barrier integrity and function without disturbing the carefully engineered microenvironment. In this review, we first provide a background on biological barriers and the physiological features that are emulated through in vitro barrier models. Then, we outline molecular permeability and electrical sensing barrier integrity assessment methods, and the related challenges specific to barrier-on-chip implementation. Finally, we discuss future directions in the field, as well important priorities to consider such as fabrication costs, standardization, and bridging gaps between disciplines and stakeholders.

Reconstitution of human tissue barrier function for precision and personalized medicine

Tissue barriers in a body, well known as tissue-to-tissue interfaces represented by endothelium of the blood vessels or epithelium of organs, are essential for maintaining physiological homeostasis by regulating molecular and cellular transports. It is crucial for predicting drug response to understand physiology of tissue barriers through which drugs are absorbed, distributed, metabolized and excreted. Since the FDA Modernization Act 2.0, which prompts the inception of alternative technologies for animal models, tissue barrier chips, one of the applications of organ-on-a-chip or microphysiological system (MPS), have only recently been utilized in the context of drug development. Recent advancements in stem cell technology have brightened the prospects for the application of tissue barrier chips in personalized medicine. In past decade, designing and engineering these microfluidic devices, and demonstrating the ability to reconstitute tissue functions were main focus of this field. However, the field is now advancing to the next level of challenges: validating their utility in drug evaluation and creating personalized models using patient-derived cells. In this review, we briefly introduce key design parameters to develop functional tissue barrier chip, explore the remarkable recent progress in the field of tissue barrier chips and discuss future perspectives on realizing personalized medicine through the utilization of tissue barrier chips.

Sources and applications of endothelial seed cells: a review

Endothelial cells (ECs) are widely used as donor cells in tissue engineering, organoid vascularization, and in vitro microvascular model development. ECs are invaluable tools for disease modeling and drug screening in fundamental research. When treating ischemic diseases, EC engraftment facilitates the restoration of damaged blood vessels, enhancing therapeutic outcomes. This article presents a comprehensive overview of the current sources of ECs, which encompass stem/progenitor cells, primary ECs, cell lineage conversion, and ECs derived from other cellular sources, provides insights into their characteristics, potential applications, discusses challenges, and explores strategies to mitigate these issues. The primary aim is to serve as a reference for selecting suitable EC sources for preclinical research and promote the translation of basic research into clinical applications.

Challenges and prospects in geriatric epilepsy treatment: the role of the blood-brain barrier in pharmacotherapy and drug delivery

The blood-brain barrier (BBB) is pivotal in maintaining neuronal physiology within the brain. This review delves into the alterations of the BBB specifically in the context of geriatric epilepsy. We examine how age-related changes in the BBB contribute to the pathogenesis of epilepsy in the elderly and present significant challenges in pharmacotherapy. Subsequently, we evaluate recent advancements in drug delivery methods targeting the BBB, as well as alternative approaches that could bypass the BBB's restrictive nature. We particularly highlight the use of neurotropic viruses and various synthetic nanoparticles that have been investigated for delivering a range of antiepileptic drugs. Additionally, the advantage and limitation of these diverse delivery methods are discussed. Finally, we analyze the potential efficacy of different drug delivery approaches in the treatment of geriatric epilepsy, aiming to provide insights into more effective management of this condition in the elderly population.

A Rapid-Patterning 3D Vessel-on-Chip for Imaging and Quantitatively Analyzing Cell-Cell Junction Phenotypes

The blood-brain barrier (BBB) is a dynamic interface that regulates the molecular exchanges between the brain and peripheral blood. The permeability of the BBB is primarily regulated by the junction proteins on the brain endothelial cells. In vitro BBB models have shown great potential for the investigation of the mechanisms of physiological function, pathologies, and drug delivery in the brain. However, few studies have demonstrated the ability to monitor and evaluate the barrier integrity by quantitatively analyzing the junction presentation in 3D microvessels. This study aimed to fabricate a simple vessel-on-chip, which allows for a rigorous quantitative investigation of junction presentation in 3D microvessels. To this end, we developed a rapid protocol that creates 3D microvessels with polydimethylsiloxane and microneedles. We established a simple vessel-on-chip model lined with human iPSC-derived brain microvascular endothelial-like cells (iBMEC-like cells). The 3D image of the vessel structure can then be "unwrapped" and converted to 2D images for quantitative analysis of cell-cell junction phenotypes. Our findings revealed that 3D cylindrical structures altered the phenotype of tight junction proteins, along with the morphology of cells. Additionally, the cell-cell junction integrity in our 3D models was disrupted by the tumor necrosis factor α. This work presents a "quick and easy" 3D vessel-on-chip model and analysis pipeline, together allowing for the capability of screening and evaluating the cell-cell junction integrity of endothelial cells under various microenvironment conditions and treatments.

Rapid Quantification of Microvessels of Three-Dimensional Blood-Brain Barrier Model Using Optical Coherence Tomography and Deep Learning Algorithm

The blood-brain barrier (BBB) is a selective barrier that controls the transport between the blood and neural tissue features and maintains brain homeostasis to protect the central nervous system (CNS). In vitro models can be useful to understand the role of the BBB in disease and assess the effects of drug delivery. Recently, we reported a 3D BBB model with perfusable microvasculature in a Transwell insert. It replicates several key features of the native BBB, as it showed size-selective permeability of different molecular weights of dextran, activity of the P-glycoprotein efflux pump, and functionality of receptor-mediated transcytosis (RMT), which is the most investigated pathway for the transportation of macromolecules through endothelial cells of the BBB. For quality control and permeability evaluation in commercial use, visualization and quantification of the 3D vascular lumen structures is absolutely crucial. Here, for the first time, we report a rapid, non-invasive optical coherence tomography (OCT)-based approach to quantify the microvessel network in the 3D in vitro BBB model. Briefly, we successfully obtained the 3D OCT images of the BBB model and further processed the images using three strategies: morphological imaging processing (MIP), random forest machine learning using the Trainable Weka Segmentation plugin (RF-TWS), and deep learning using pix2pix cGAN. The performance of these methods was evaluated by comparing their output images with manually selected ground truth images. It suggested that deep learning performed well on object identification of OCT images and its computation results of vessel counts and surface areas were close to the ground truth results. This study not only facilitates the permeability evaluation of the BBB model but also offers a rapid, non-invasive observational and quantitative approach for the increasing number of other 3D in vitro models.

Recent advancements on in vitro blood-brain barrier model: A reliable and efficient screening approach for preclinical and clinical investigation

INTRODUCTION

The efficiency of brain therapeutics is greatly hindered by the blood-brain barrier (BBB). BBB's protective function, selective permeability, and dynamic functionality maintain the harmony between the brain and peripheral region. Thus, the design of any novel drug carrier system requires the complete study and investigation of BBB permeability, efflux transport, and the effect of associated cellular and non-vascular unit trafficking on BBB penetrability. The in vitro BBB models offer a most promising, and reliable mode of initial investigation of BBB permeability and associated factors as strong evidence for further preclinical and clinical investigation.

AREA COVERED

This review work covers the structure and functions of BBB components and different types of in vitro BBB models along with factors affecting BBB model development and model selection criteria.

EXPERT OPINION

In vivo models assume to reciprocate the physiological environment to the maximum extent. However, the interspecies variability, NVUs trafficking, dynamic behavior of BBB, etc., lead to non-reproducible results. The in vitro models are comparatively less complex, and flexible, as per the study design, could generate substantial evidence and help identify suitable in vivo animal model selection.

Matrix stiffness regulates the tight junction phenotypes and local barrier properties in tricellular regions in an iPSC-derived BBB model

The blood-brain barrier (BBB) can respond to various mechanical cues such as shear stress and substrate stiffness. In the human brain, the compromised barrier function of the BBB is closely associated with a series of neurological disorders that are often also accompanied by the alteration of brain stiffness. In many types of peripheral vasculature, higher matrix stiffness decreases barrier function of endothelial cells through mechanotransduction pathways that alter cell-cell junction integrity. However, human brain endothelial cells are specialized endothelial cells that largely resist changes in cell morphology and key BBB markers. Therefore, it has remained an open question how matrix stiffness affects barrier integrity in the human BBB. To gain insight into the effects of matrix stiffness on BBB permeability, we differentiated brain microvascular endothelial-like cells from human induced pluripotent stem cells (iBMEC-like cells) and cultured the cells on extracellular matrix-coated hydrogels of varying stiffness. We first detected and quantified the junction presentation of key tight junction (TJ) proteins. Our results show matrix-dependent junction phenotypes in iBMEC-like cells, where cells on softer gels (1 kPa) have significantly lower continuous and total TJ coverages. We also determined that these softer gels also lead to decreased barrier function in a local permeability assay. Furthermore, we found that matrix stiffness regulates the local permeability of iBMEC-like cells through the balance of continuous ZO-1 TJs and no junction regions ZO-1 in tricellular regions. Together, these findings provide valuable insights into the effects of matrix stiffness on TJ phenotypes and local permeability of iBMEC-like cells. STATEMENT OF SIGNIFICANCE: Brain mechanical properties, including stiffness, are particularly sensitive indicators for pathophysiological changes in neural tissue. The compromised function of the blood-brain barrier is closely associated with a series of neurological disorders often accompanied by altered brain stiffness. In this study, we use polymeric biomaterials and provide new evidence that biomaterial stiffness regulates the local permeability in iPSC-derived brain endothelial cells in tricellular regions through the tight junction protein ZO-1. Our findings provide valuable insights into the changes in junction architecture and barrier permeability in response to different substrate stiffnesses. Since BBB dysfunction has been linked to many diseases, understanding the influence of substrate stiffness on junction presentations and barrier permeability could lead to the development of new treatments for diseases associated with BBB dysfunction or drug delivery across BBB systems.

CmPn/CmP Signaling Networks in the Maintenance of the Blood Vessel Barrier

Cerebral cavernous malformations (CCMs) arise when capillaries within the brain enlarge abnormally, causing the blood-brain barrier (BBB) to break down. The BBB serves as a sophisticated interface that controls molecular interactions between the bloodstream and the central nervous system. The neurovascular unit (NVU) is a complex structure made up of neurons, astrocytes, endothelial cells (ECs), pericytes, microglia, and basement membranes, which work together to maintain blood-brain barrier (BBB) permeability. Within the NVU, tight junctions (TJs) and adherens junctions (AJs) between endothelial cells play a critical role in regulating the permeability of the BBB. Disruptions to these junctions can compromise the BBB, potentially leading to a hemorrhagic stroke. Understanding the molecular signaling cascades that regulate BBB permeability through EC junctions is, therefore, essential. New research has demonstrated that steroids, including estrogens (ESTs), glucocorticoids (GCs), and metabolites/derivatives of progesterone (PRGs), have multifaceted effects on blood-brain barrier (BBB) permeability by regulating the expression of tight junctions (TJs) and adherens junctions (AJs). They also have anti-inflammatory effects on blood vessels. PRGs, in particular, have been found to play a significant role in maintaining BBB integrity. PRGs act through a combination of its classic and non-classic PRG receptors (nPR/mPR), which are part of a signaling network known as the CCM signaling complex (CSC). This network couples both nPR and mPR in the CmPn/CmP pathway in endothelial cells (ECs).

Exploring Pro-Inflammatory Immunological Mediators: Unraveling the Mechanisms of Neuroinflammation in Lysosomal Storage Diseases

Lysosomal storage diseases are a group of rare and ultra-rare genetic disorders caused by defects in specific genes that result in the accumulation of toxic substances in the lysosome. This excess accumulation of such cellular materials stimulates the activation of immune and neurological cells, leading to neuroinflammation and neurodegeneration in the central and peripheral nervous systems. Examples of lysosomal storage diseases include Gaucher, Fabry, Tay–Sachs, Sandhoff, and Wolman diseases. These diseases are characterized by the accumulation of various substrates, such as glucosylceramide, globotriaosylceramide, ganglioside GM2, sphingomyelin, ceramide, and triglycerides, in the affected cells. The resulting pro-inflammatory environment leads to the generation of pro-inflammatory cytokines, chemokines, growth factors, and several components of complement cascades, which contribute to the progressive neurodegeneration seen in these diseases. In this study, we provide an overview of the genetic defects associated with lysosomal storage diseases and their impact on the induction of neuro-immune inflammation. By understanding the underlying mechanisms behind these diseases, we aim to provide new insights into potential biomarkers and therapeutic targets for monitoring and managing the severity of these diseases. In conclusion, lysosomal storage diseases present a complex challenge for patients and clinicians, but this study offers a comprehensive overview of the impact of these diseases on the central and peripheral nervous systems and provides a foundation for further research into potential treatments.

Implementation of blood-brain barrier on microfluidic chip: recent advance and future prospects

The complex structure of the blood-brain barrier (BBB) hinders its modeling and the treatment of brain diseases. The microfluidic technology promotes the development of BBB-on-a-chip platforms, which can be used to reproduce the complex brain microenvironment and physiological reactions. Compared with traditional transwell technology, microfluidic BBB-on-a-chip shows great technical advantages in terms of flexible control of fluid shear stress in the chip and fabrication efficiency of the chip system, which can be enhanced by the development of lithography and three-dimensional (3D) printing. It is convenient to accurately monitor the dynamic changes of biochemical parameters of individual cells in the model by integrating an automatic super-resolution imaging sensing platform. In addition, biomaterials, especially hydrogels and conductive polymers, solve the limitations of microfluidic BBB-on-a-chip by compounding onto microfluidic chip to provide a 3D space and special performance on the microfluidic chip. The microfluidic BBB-on-a-chip promotes the development of basic research, including cell migration, mechanism exploration of neurodegenerative diseases, drug barrier permeability, SARS-CoV-2 pathology. This study summarizes the recent advances, challenges and future prospects of microfluidic BBB-on-a-chip, which can help to promote the development of personalized medicine and drug discovery.

Extracellular matrix proteins in construction and function of in vitro blood-brain barrier models

The blood-brain barrier (BBB) is a highly impermeable barrier separating circulating blood and brain tissue. A functional BBB is critical for brain health, and BBB dysfunction has been linked to the pathophysiology of diseases such as stroke and Alzheimer’s disease. A variety of models have been developed to study the formation and maintenance of the BBB, ranging from in vivo animal models to in vitro models consisting of primary cells or cells differentiated from human pluripotent stem cells (hPSCs). These models must consider the composition and source of the cellular components of the neurovascular unit (NVU), including brain microvascular endothelial cells (BMECs), brain pericytes, astrocytes, and neurons, and how these cell types interact. In addition, the non-cellular components of the BBB microenvironment, such as the brain vascular basement membrane (BM) that is in direct contact with the NVU, also play key roles in BBB function. Here, we review how extracellular matrix (ECM) proteins in the brain vascular BM affect the BBB, with a particular focus on studies using hPSC-derived in vitro BBB models, and discuss how future studies are needed to advance our understanding of how the ECM affects BBB models to improve model performance and expand our knowledge on the formation and maintenance of the BBB.

A neurovascular unit-on-a-chip: culture and differentiation of human neural stem cells in a three-dimensional microfluidic environment

Biological studies typically rely on a simple monolayer cell culture, which does not reflect the complex functional characteristics of human tissues and organs, or their real response to external stimuli. Microfluidic technology has advantages of high-throughput screening, accurate control of the fluid velocity, low cell consumption, long-term culture, and high integration. By combining the multipotential differentiation of neural stem cells with high throughput and the integrated characteristics of microfluidic technology, an in vitro model of a functionalized neurovascular unit was established using human neural stem cell-derived neurons, astrocytes, oligodendrocytes, and a functional microvascular barrier. The model comprises a multi-layer vertical neural module and vascular module, both of which were connected with a syringe pump. This provides controllable conditions for cell inoculation and nutrient supply, and simultaneously simulates the process of ischemic/hypoxic injury and the process of inflammatory factors in the circulatory system passing through the blood-brain barrier and then acting on the nerve tissue in the brain. The in vitro functionalized neurovascular unit model will be conducive to central nervous system disease research, drug screening, and new drug development.

Pramipexole Protects Against Traumatic Brain Injury-Induced Blood-Brain Barrier (BBB) Dysfunction

Traumatic brain injury (TBI) is a severe disease of brain damage accompanied by blood-brain barrier (BBB) dysfunction. The BBB is composed of brain microvascular endothelial cells (BMECs), astrocyte terminus, pericytes, and a basement membrane. Tight junction proteins expressed by BMECs play important roles in preserving BBB integrity. Pramipexole is a selective dopamine agonist applied for treating Parkinson's disease and has been recently claimed with neuroprotective capacity. This study will further explore the impact of Pramipexole on tight junctions and BBB integrity to provide the potential treatment strategy for TBI-induced BBB damage. The TBI model was established in mice and was identified by the promoted brain water content, declined Garcia scores, reduced latency of the rotarod test, aggravated pathological changes in the brain cortex, and excessively released inflammatory factors. After treatment with Pramipexole, the neurofunctional deficits, behavioral disability, and aggravated pathological changes were dramatically reversed, accompanied by the alleviated BBB permeability, and upregulated occludin, an important tight junction protein. TBI model cells were established by the scratching bEnd.3 cells method. Cells were stimulated with 10 and 20 μM Pramipexole, followed by exposure to TBI. Increased fluorescence intensity of FITC-dextran, reduced value of TEER, and downregulated occludin and KLF2 were observed in TBI-exposed cells, all of which were greatly reversed by 10 and 20 μM Pramipexole. Furthermore, in KLF2-silenced bEnd.3 cells, the protective ability of Pramipexole against endothelial permeability and the expression level of occludin were dramatically abolished. Collectively, our results suggest that Pramipexole protected against TBI-induced BBB dysfunction by mediating KLF2.

Experimental Comparison of Primary and hiPS-Based In Vitro Blood–Brain Barrier Models for Pharmacological Research

In vitro model systems of the blood–brain barrier (BBB) play an essential role in pharmacological research, specifically during the development and preclinical evaluation of new drug candidates. Within the past decade, the trend in research and further development has moved away from models based on primary cells of animal origin towards differentiated models derived from human induced pluripotent stem cells (hiPSs). However, this logical progression towards human model systems from renewable cell sources opens up questions about the transferability of results generated in the primary cell models. In this study, we have evaluated both models with identical experimental parameters and achieved a directly comparable characterisation showing no significant differences in protein expression or permeability even though the achieved transendothelial electrical resistance (TEER) values showed significant differences. In the course of this investigation, we also determined a significant deviation of both model systems from the in vivo BBB circumstances, specifically concerning the presence or absence of serum proteins in the culture media. Thus, we have further evaluated both systems when confronted with an in vivo-like distribution of serum and found a notable improvement in the differential permeability of hydrophilic and lipophilic compounds in the hiPS-derived BBB model. We then transferred this model into a microfluidic setup while maintaining the differential serum distribution and evaluated the permeability coefficients, which showed good comparability with values in the literature. Therefore, we have developed a microfluidic hiPS-based BBB model with characteristics comparable to the established primary cell-based model.