Differential Effects of Retinoic Acid Concentrations in Regulating Blood-Brain Barrier Properties

Vitamin A deficiency compromises the barrier function of the retinal pigment epithelium

A major cause for childhood blindness worldwide is attributed to nutritional vitamin A deficiency. Surprisingly, the molecular basis of the ensuing retinal degeneration has not been well defined. Abundant expression of the retinoid transporter STRA6 in the retinal pigment epithelium (RPE) and homeostatic blood levels of retinol-binding protein delay vitamin A deprivation of the mouse eyes. Hence, genetic dissection of STRA6 makes mice susceptible to nutritional manipulation of ocular retinoid status. We performed RNA-seq analyses and complemented the data with tests of visual physiology, ocular morphology, and retinoid biochemistry to compare eyes with different vitamin A status. Mild ocular vitamin A deficiency decreased transcripts of photoreceptor transduction pathway-related genes and increased transcripts of oxidative stress pathways. The response was associated with impaired visual sensitivity and an accumulation of fluorescent debris in the retina. Severe vitamin A deficiency did not only impair visual perception but also decreased transcripts of genes encoding cell adhesion and cellular junction proteins. This response altered cell morphology, resulted in significant changes in transport pathways of small molecules, and compromised the barrier function of the RPE. Together, our analyses characterize the molecular events underlying nutritional blindness in a novel mouse model and indicate that breakdown of the outer blood-retinal barrier contributes to retinal degeneration and photoreceptor cell death in severe vitamin A deficiency.

Mouse embryonic stem cell-derived blood-brain barrier model: applicability to studying antibody triggered receptor mediated transcytosis

Blood brain barrier (BBB) models in vitro are an important tool to aid in the pre-clinical evaluation and selection of BBB-crossing therapeutics. Stem cell derived BBB models have recently demonstrated a substantial advantage over primary and immortalized brain endothelial cells (BECs) for BBB modeling. Coupled with recent discoveries highlighting significant species differences in the expression and function of key BBB transporters, the field is in need of robust, species-specific BBB models for improved translational predictability. We have developed a mouse BBB model, composed of mouse embryonic stem cell (mESC-D3)-derived brain endothelial-like cells (mBECs), employing a directed monolayer differentiation strategy. Although the mBECs showed a mixed endothelial-epithelial phenotype, they exhibited high transendothelial electrical resistance, inducible by retinoic acid treatment up to 400 Ω cm². This tight cell barrier resulted in restricted sodium fluorescein permeability (1.7 × 10^-5 cm/min), significantly lower than that of bEnd.3 cells (1.02 × 10^-3 cm/min) and comparable to human induced pluripotent stem cell (iPSC)-derived BECs (2.0 × 10^-5 cm/min). The mBECs expressed tight junction proteins, polarized and functional P-gp efflux transporter and receptor mediated transcytosis (RMT) receptors; collectively important criteria for studying barrier regulation and drug delivery applications in the CNS. In this study, we compared transport of a panel of antibodies binding species selective or cross-reactive epitopes on BBB RMT receptors in both the mBEC and human iPSC-derived BEC model, to demonstrate discrimination of species-specific BBB transport mechanisms.

Impaired retinoic acid signaling in cerebral cavernous malformations

The capillary-venous pathology cerebral cavernous malformation (CCM) is caused by loss of CCM1/Krev interaction trapped protein 1 (KRIT1), CCM2/MGC4607, or CCM3/PDCD10 in some endothelial cells. Mutations of CCM genes within the brain vasculature can lead to recurrent cerebral hemorrhages. Pharmacological treatment options are urgently needed when lesions are located in deeply-seated and in-operable regions of the central nervous system. Previous pharmacological suppression screens in disease models of CCM led to the discovery that treatment with retinoic acid improved CCM phenotypes. This finding raised a need to investigate the involvement of retinoic acid in CCM and test whether it has a curative effect in preclinical mouse models. Here, we show that components of the retinoic acid synthesis and degradation pathway are transcriptionally misregulated across disease models of CCM. We complemented this analysis by pharmacologically modifying retinoic acid levels in zebrafish and human endothelial cell models of CCM, and in acute and chronic mouse models of CCM. Our pharmacological intervention studies in CCM2-depleted human umbilical vein endothelial cells (HUVECs) and krit1 mutant zebrafish showed positive effects when retinoic acid levels were increased. However, therapeutic approaches to prevent the development of vascular lesions in adult chronic murine models of CCM were drug regiment-sensitive, possibly due to adverse developmental effects of this hormone. A treatment with high doses of retinoic acid even worsened CCM lesions in an adult chronic murine model of CCM. This study provides evidence that retinoic acid signaling is impaired in the CCM pathophysiology and suggests that modification of retinoic acid levels can alleviate CCM phenotypes.

P450 oxidoreductase regulates barrier maturation by mediating retinoic acid metabolism in a model of the human BBB

The blood-brain barrier (BBB) selectively regulates the entry of molecules into the central nervous system (CNS). A crosstalk between brain microvascular endothelial cells (BMECs) and resident CNS cells promotes the acquisition of functional tight junctions (TJs). Retinoic acid (RA), a key signaling molecule during embryonic development, is used to enhance in vitro BBB models’ functional barrier properties. However, its physiological relevance and affected pathways are not fully understood. P450 oxidoreductase (POR) regulates the enzymatic activity of microsomal cytochromes. POR-deficient (PORD) patients display impaired steroid homeostasis and cognitive disabilities. Here, we used both patient-specific POR-deficient and CRISPR-Cas9-mediated POR-depleted induced pluripotent stem cell (iPSC)-derived BMECs (iBMECs) to study the role of POR in the acquisition of functional barrier properties. We demonstrate that POR regulates cellular RA homeostasis and that POR deficiency leads to the accumulation of RA within iBMECs, resulting in the impaired acquisition of TJs and, consequently, to dysfunctional development of barrier properties.

•

Retinoic acid (RA) promotes functional barrier properties

•

POR-deficient iPS-brain endothelial-like cells display impaired barrier development

•

POR mediates CYP26-dependent cellular RA catabolism

•

RA accumulation induces a pro-inflammatory response

Signaling Pathways in Neurovascular Development

During development, the central nervous system (CNS) vasculature grows to precisely meet the metabolic demands of neurons and glia. In addition, the vast majority of the CNS vasculature acquires a unique set of molecular and cellular properties-collectively referred to as the blood-brain barrier-that minimize passive diffusion of molecules between the blood and the CNS parenchyma. Both of these processes are controlled by signals emanating from neurons and glia. In this review, we describe the nature and mechanisms-of-action of these signals, with an emphasis on vascular endothelial growth factor (VEGF) and beta-catenin (canonical Wnt) signaling, the two best-understood systems that regulate CNS vascular development. We highlight foundational discoveries, interactions between different signaling systems, the integration of genetic and cell biological studies, advances that are of clinical relevance, and questions for future research.

Developmental regulation of barrier- and non-barrier blood vessels in the CNS

The blood-brain barrier (BBB) is essential for creating and maintaining tissue homeostasis in the central nervous system (CNS), which is key for proper neuronal function. In most vertebrates, the BBB is localized to microvascular endothelial cells that acquire barrier properties during angiogenesis of the neuroectoderm. Complex and continuous tight junctions, and the lack of fenestrae combined with low pinocytotic activity render the BBB endothelium a tight barrier for water-soluble molecules that may only enter the CNS via specific transporters. The differentiation of these unique endothelial properties during embryonic development is initiated by endothelial-specific flavours of the Wnt/β-catenin pathway in a precise spatiotemporal manner. In this review, we summarize the currently known cellular (neural precursor and endothelial cells) and molecular (VEGF and Wnt/β-catenin) mechanisms mediating brain angiogenesis and barrier formation. Moreover, we introduce more recently discovered crosstalk with cellular and acellular elements within the developing CNS such as the extracellular matrix. We discuss recent insights into the downstream molecular mechanisms of Wnt/β-catenin in particular, the recently identified target genes like Foxf2, Foxl2, Foxq1, Lef1, Ppard, Zfp551, Zic3, Sox17, Apcdd1 and Fgfbp1 that are involved in refining and maintaining barrier characteristics in the mature BBB endothelium. Additionally, we elute to recent insight into barrier heterogeneity and differential endothelial barrier properties within the CNS, focussing on the circumventricular organs as well as on the neurogenic niches in the subventricular zone and the hippocampus. Finally, open questions and future BBB research directions are highlighted in the context of taking benefit from understanding BBB development for strategies to modulate BBB function under pathological conditions.

Historical and current perspectives on blood endothelial cell heterogeneity in the brain

Dynamic brain activity requires timely communications between the brain parenchyma and circulating blood. Brain-blood communication is facilitated by intricate networks of brain vasculature, which display striking heterogeneity in structure and function. This vascular cell heterogeneity in the brain is fundamental to mediating diverse brain functions and has long been recognized. However, the molecular basis of this biological phenomenon has only recently begun to be elucidated. Over the past century, various animal species and in vitro systems have contributed to the accumulation of our fundamental and phylogenetic knowledge about brain vasculature, collectively advancing this research field. Historically, dye tracer and microscopic observations have provided valuable insights into the anatomical and functional properties of vasculature across the brain, and these techniques remain an important approach. Additionally, recent advances in molecular genetics and omics technologies have revealed significant molecular heterogeneity within brain endothelial and perivascular cell types. The combination of these conventional and modern approaches has enabled us to identify phenotypic differences between healthy and abnormal conditions at the single-cell level. Accordingly, our understanding of brain vascular cell states during physiological, pathological, and aging processes has rapidly expanded. In this review, we summarize major historical advances and current knowledge on blood endothelial cell heterogeneity in the brain, and discuss important unsolved questions in the field.

Liver X Receptor as a Possible Drug Target for Blood-Brain Barrier Integrity

Purpose: blood-brain barrier (BBB) is made of specialized cells that are responsible for the selective passage of substances directed to the brain. The integrated BBB is essential for precise controlling of the different substances passage as well as protecting the brain from various damages. In this article, we attempted to explain the role of liver X receptor (LXR) in maintaining BBB integrity as a possible drug target.

Methods: In this study, various databases, including PubMed, Google Scholar, and Scopus were searched using the following keywords: blood-brain barrier, BBB, liver X receptor, and LXR until July, 2020. Additionally, contents close to the subject of our study were surveyed.

Results: LXR is a receptor the roles of which in various diseases have been investigated. LXR can affect maintaining BBB by affecting various ways such as ATP-binding cassette transporter A1 (ABCA1), matrix metalloproteinase-9 (MMP9), insulin-like growth factor 1 (IGF1), nuclear factor-kappa B (NF-κB) signaling, mitogen-activated protein kinase (MAPK), tight junction molecules, both signal transducer and activator of transcription 1 (STAT1), Wnt/β-catenin Signaling, transforming growth factor beta (TGF-β) signaling, and expressions of Smad 2/3 and Snail.

Conclusion: LXR could possibly be used either as a target for drug delivery to brain tissue or as a target for maintaining the BBB integrity in different diseases; thereby the drug will be conducted to tissues, other than the brain. If it is verified that only LXRα is necessary for protecting BBB, some specific LXRα ligands must be found and then used in medication.

Drug Development for Central Nervous System Diseases Using In vitro Blood-brain Barrier Models and Drug Repositioning

An important goal of biomedical research is to translate basic research findings into practical clinical implementation. Despite the advances in the technology used in drug discovery, the development of drugs for central nervous system diseases remains challenging. The failure rate for new drugs targeting important central nervous system diseases is high compared to most other areas of drug discovery. The main reason for the failure is the poor penetration efficacy across the blood-brain barrier. The blood-brain barrier represents the bottleneck in central nervous system drug development and is the most important factor limiting the future growth of neurotherapeutics. Meanwhile, drug repositioning has been becoming increasingly popular and it seems a promising field in central nervous system drug development. In vitro blood-brain barrier models with high predictability are expected for drug development and drug repositioning. In this review, the recent progress of in vitro BBB models and the drug repositioning for central nervous system diseases will be discussed.

Human induced pluripotent stem cells (BIONi010-C) generate tight cell monolayers with blood-brain barrier traits and functional expression of large neutral amino acid transporter 1 (SLC7A5)

The barrier properties of the brain capillary endothelium, the blood-brain barrier (BBB) restricts uptake of most small and all large molecule drug compounds to the CNS. There is a need for predictive human in vitro models of the BBB to enable studies of brain drug delivery. Here, we investigated whether human induced pluripotent stem cell (hiPSC) line (BIONi010-C) could be differentiated to brain capillary endothelial- like cells (BCEC) and evaluated their potential use in drug delivery studies. BIONi010-C hIPSCs were differentiated according to established protocols. BCEC monolayers displayed transendothelial electrical resistance (TEER) values of 5,829±354 Ω∙cm², a P_app,_mannitol of 1.09±0.15 ∙ 10^-6 cm∙s^-1 and a P_app,diazepam of 85.7 ± 5.9 ∙ 10^-6 cm ∙s^-1. The P_diazepam/P_mannitol ratio of ~80, indicated a large dynamic passive permeability range. Monolayers maintained their integrity after medium exchange. Claudin-5, Occludin, Zonulae Occludens 1 and VE-Cadherin were expressed at the cell-cell contact zones. Efflux transporters were present at the mRNA level, but functional efflux of substrates was not detected. Transferrin-receptor (TFR), Low density lipoprotein receptor-related protein 1 (LRP1) and Basigin receptors were expressed at the mRNA-level. The presence and localization of TFR and LRP1 were verified at the protein level. A wide range of BBB-expressed solute carriers (SLC's) were detected at the mRNA level. The presence and localization of SLC transporters GLUT1 and LAT1 was verified at the protein level. Functional studies revealed transport of the LAT1 substrate [³H]-L-Leucine and the LRP1 substrate angiopep-2. In conclusion, we have demonstrated that BIONi010-C-derived BCEC monolayers exhibited, BBB properties including barrier tightness and integrity, a high dynamic range, expression of some of the BBB receptor and transporter expression, as well as functional transport of LAT1 and LRP1 substrates. This suggests that BIONi010-C-derived BCEC monolayers may be useful for studying the roles of LAT-1 and LRP1 in brain drug delivery.

Xiao-Yao-San reduces blood-brain barrier injury induced by chronic stress in vitro and vivo via glucocorticoid receptor-mediated upregulation of Occludin

Blood-brain barrier (BBB) is a barrier which maintains the material exchange balance of brain microenvironment and could be destroyed by chronic stress (CS). Glucocorticoids (GCs) can mimic the chronic stress induced damage to BBB. GCs induced BBB trauma models in vitro and in vivo to explore the effects of the traditional medicine Xiao-Yao-San (XYS). In this research, we found CS could injure the BBB to change the biochemical index, which could be reversed by XYS in vitro. The abilities of cell proliferation, invasion, and the expression of tight junction related genes (Occludin, Claudin, JAM-1 and ZO-1) were suppressed by CS and the trauma could be reversed by XYS partly. It was showed that GRs interacted with Occludin directly and inhibited Occluding expression. In rats BBB trauma model, the GC content was deceased and BBB permeability was repaired by XYS. The expression of Occludin, Claudin, JAM-1 and ZO-1 were increased in the treatment of XYS. In our research, it shown that XYS affect the content of the GC and GR which interacted with Occludin directly for the first time. In addition, we also found that XYS could reduce BBB injury induced by CS via GR in BBB model in vitro. Therefore, it proves that XYS is a potential BBB repair medicine and may help to elucidate mechanism of brain pathology.

Structure and Function of the Blood-Brain Barrier (BBB)

The blood-brain barrier (BBB) protects the vertebrate central nervous system from harmful blood-borne, endogenous and exogenous substances to ensure proper neuronal function. The BBB describes a function that is established by endothelial cells of CNS vessels in conjunction with pericytes, astrocytes, neurons and microglia, together forming the neurovascular unit (NVU). Endothelial barrier function is crucially induced and maintained by the Wnt/β-catenin pathway and requires intact NVU for proper functionality. The BBB and the NVU are characterized by a specialized assortment of molecular specializations, providing the basis for tightening, transport and immune response functionality.The present chapter introduces state-of-the-art knowledge of BBB structure and function and highlights current research topics, aiming to understanding in more depth the cellular and molecular interactions at the NVU, determining functionality of the BBB in health and disease, and providing novel potential targets for therapeutic BBB modulation. Moreover, we highlight recent advances in understanding BBB and NVU heterogeneity within the CNS as well as their contribution to CNS physiology, such as neurovascular coupling, and pathophysiology, is discussed. Finally, we give an outlook onto new avenues of BBB research.

Retinoic acid signaling in vascular development

Formation of the vasculature is an essential developmental process, delivering oxygen and nutrients to support cellular processes needed for tissue growth and maturation. Retinoic acid (RA) and its downstream signaling pathway is vital for normal pre- and post-natal development, playing key roles in the specification and formation of many organs and tissues. Here, we review the role of RA in blood and lymph vascular development, beginning with embryonic yolk sac vasculogenesis and remodeling and discussing RA's organ-specific roles in angiogenesis and vessel maturation. In particular, we highlight the multi-faceted role of RA signaling in CNS vascular development and acquisition of blood-brain barrier properties.

Retinoic Acid Regulates Endothelial β-catenin Expression and Pericyte Numbers in the Developing Brain Vasculature

The acquisition of brain vascular properties, like tight junctions and pericytes, to form the blood-brain barrier (BBB) is crucial for a properly functioning central nervous system (CNS). Endothelial WNT signaling is a known driver of brain vascular development and BBB properties, however, it is unclear how endothelial WNT signaling is regulated. We recently showed that mouse embryos with disruptions in endothelial retinoic acid (RA) signaling have ectopic WNT signaling in the brain vasculature. Using immunohistochemistical analysis, we show that increased vascular WNT signaling in RA mutants (Pdgfbi^cre; dnRAR403-flox and Rdh10 mutants) is associated with elevated expression of the WNT transcriptional effector, β-catenin, in the brain endothelium. In vitro immunocytochemistry and proximity ligation studies in brain endothelial cells reveal that RA, through its receptor RARα, regulates β-catenin expression in brain endothelial cells via transcriptional suppression and phosphorylation events that targets β-catenin for proteasomal degradation, the latter dependent on PKCα. We find that one function of RA in regulating vascular WNT signaling is to modulate the pericyte numbers in the developing brain vasculature. RA-mediated regulation of vascular WNT signaling could be needed to prevent over-recruitment of pericytes that might impair endothelial-pericyte interactions crucial for vascular stability.

Retinoic Acid, Leaky Gut, and Autoimmune Diseases

A leaky gut has been observed in a number of autoimmune diseases including type 1 diabetes, multiple sclerosis, inflammatory bowel disease, and systemic lupus erythematosus. Previous studies from our laboratory have shown that lupus mice also bear a leaky gut and that the intestinal barrier function can be enhanced by gut colonization of probiotics such as Lactobacillus spp. Retinoic acid (RA) can increase the relative abundance of Lactobacillus spp. in the gut. Interestingly, RA has also been shown to strengthen the barrier function of epithelial cells in vitro and in the absence of probiotic bacteria. These reports bring up an interesting question of whether RA exerts protective effects on the intestinal barrier directly or through regulating the microbiota colonization. In this review, we will discuss the roles of RA in immunomodulation, recent literature on the involvement of a leaky gut in different autoimmune diseases, and how RA shapes the outcomes of these diseases.

Retinoic acid signaling is essential for maintenance of the blood-retinal barrier

The predominant function of the blood-retinal barrier (BRB) is to maintain retinal homeostasis by regulating the influx and efflux between the blood and retina. Breakdown of the BRB occurs in a number of ocular diseases that result in vision loss. Understanding the molecular and cellular pathways involved in the development and maintenance of the BRB is critical to developing therapeutics for these conditions. To visualize the BRB in vivo, we used the transgenic Tg(l-fabp:DBP-EGFP:flk1:mCherry) zebrafish model that expresses vitamin D binding protein (a member of the albumin gene family) tagged to green fluorescent protein. Retinoic acid (RA) plays a number of important roles in vertebrate development and has been shown to play a protective role during inflammation-induced blood-brain barrier disruption. The role of RA in BRB development and maintenance remains unknown. To disrupt RA signaling, Tg(l-fabp:DBP-EGFP:flk1:mCherry) zebrafish were treated with N, N-diethylaminobenzaldehyde and 4-[(1 E)-2-[5,6-dihydro-5,5-dimethyl-8-(2-phenylethynyl)-2-naphthalenyl]ethenyl]benzoic acid, which are antagonists of retinal dehydrogenase and the RA receptor, respectively. Treatment with either compound resulted in BRB disruption and reduced visual acuity, whereas cotreatment with all- trans RA effectively rescued BRB integrity. Additionally, transgenic overexpression of Cyp26a1, which catalyzes RA degradation, resulted in breakdown of the BRB. Our results demonstrate that RA signaling is critical for maintenance of the BRB and could play a role in diseases such as diabetic macular edema.-Pollock, L. M., Xie, J., Bell, B. A., Anand-Apte, B. Retinoic acid signaling is essential for maintenance of the blood-retinal barrier.