Foxf2 Is Required for Brain Pericyte Differentiation and Development and Maintenance of the Blood-Brain Barrier

Stroke Genetics: Turning Discoveries into Clinical Applications

The field of medical and population genetics in stroke is moving at a rapid pace and has led to unanticipated opportunities for discovery and clinical applications. Genome-wide association studies have highlighted the role of specific pathways relevant to etiologically defined subtypes of stroke and to stroke as a whole. They have further offered starting points for the exploration of novel pathways and pharmacological strategies in experimental systems. Mendelian randomization studies continue to provide insights in the causal relationships between exposures and outcomes and have become a useful tool for predicting the efficacy and side effects of drugs. Additional applications that have emerged from recent discoveries include risk prediction based on polygenic risk scores and pharmacogenomics. Among the topics currently moving into focus is the genetics of stroke outcome. While still at its infancy, this field is expected to boost the development of neuroprotective agents. We provide a brief overview on recent progress in these areas.

Blood-Brain Barrier and Neurodegenerative Diseases-Modeling with iPSC-Derived Brain Cells

The blood-brain barrier (BBB) regulates the delivery of oxygen and important nutrients to the brain through active and passive transport and prevents neurotoxins from entering the brain. It also has a clearance function and removes carbon dioxide and toxic metabolites from the central nervous system (CNS). Several drugs are unable to cross the BBB and enter the CNS, adding complexity to drug screens targeting brain disorders. A well-functioning BBB is essential for maintaining healthy brain tissue, and a malfunction of the BBB, linked to its permeability, results in toxins and immune cells entering the CNS. This impairment is associated with a variety of neurological diseases, including Alzheimer's disease and Parkinson's disease. Here, we summarize current knowledge about the BBB in neurodegenerative diseases. Furthermore, we focus on recent progress of using human-induced pluripotent stem cell (iPSC)-derived models to study the BBB. We review the potential of novel stem cell-based platforms in modeling the BBB and address advances and key challenges of using stem cell technology in modeling the human BBB. Finally, we highlight future directions in this area.

Cis-Repression of Foxq1 Expression Affects Foxf2-Mediated Gene Expression in Palate Development

Disruption of FOXF2, encoding a member of the Forkhead family transcription factors, has been associated with cleft palate in humans and mice. FOXF2 is located in a conserved gene cluster containing FOXQ1, FOXF2, and FOXC1. We found that expression of Foxq1 is dramatically upregulated in the embryonic palatal mesenchyme in Foxf2 ^-/- mouse embryos. We show here that the Foxf2 promoter-deletion mutation caused dramatically increased expression of the cis-linked Foxq1 allele but had little effect on the Foxq1 allele in trans. We analyzed effects of the Foxf2 mutation on the expression of other neighboring genes and compared those effects with the chromatin domain structure and recently identified enhancer-promoter associations as well as H3K27ac ChIP-seq data. We show that the Foxf2 mutation resulted in significantly increased expression of the Foxq1 and Exoc2 genes located in the same topologically associated domain with Foxf2 but not the expression of the Foxc1 and Gmds genes located in the adjacent chromatin domain. We inactivated the Foxq1 gene in mice homozygous for a Foxf2 conditional allele using CRISPR genome editing and generated (Foxf2/Foxq1)^+/- mice with loss-of-function mutations in Foxf2 and Foxq1 in cis. Whereas the (Foxf2/Foxq1)^-/- mice exhibited cleft palate at birth similar as in the Foxf2 ^-/- mice, systematic expression analyses of a large number of Foxf2-dependent genes revealed that the (Foxf2/Foxq1)^-/- embryos exhibited distinct effects on the domain-specific expression of several important genes, including Foxf1, Shox2, and Spon1, in the developing palatal shelves compared with Foxf2 ^-/- embryos. These results identify a novel cis-regulatory effect of the Foxf2 mutation and demonstrate that cis-regulation of Foxq1 contributed to alterations in palatal gene expression in Foxf2 ^-/- embryos. These results have important implications for interpretation of results and mechanisms from studies of promoter- or gene-deletion alleles. In addition, the unique mouse lines generated in this study provide a valuable resource for understanding the cross-regulation and combinatorial functions of the Foxf2 and Foxq1 genes in development and disease.

Neural crest cell-derived pericytes act as pro-angiogenic cells in human neocortex development and gliomas

Central nervous system diseases involving the parenchymal microvessels are frequently associated with a ‘microvasculopathy’, which includes different levels of neurovascular unit (NVU) dysfunction, including blood–brain barrier alterations. To contribute to the understanding of NVU responses to pathological noxae, we have focused on one of its cellular components, the microvascular pericytes, highlighting unique features of brain pericytes with the aid of the analyses carried out during vascularization of human developing neocortex and in human gliomas. Thanks to their position, centred within the endothelial/glial partition of the vessel basal lamina and therefore inserted between endothelial cells and the perivascular and vessel-associated components (astrocytes, oligodendrocyte precursor cells (OPCs)/NG2-glia, microglia, macrophages, nerve terminals), pericytes fulfil a central role within the microvessel NVU. Indeed, at this critical site, pericytes have a number of direct and extracellular matrix molecule- and soluble factor-mediated functions, displaying marked phenotypical and functional heterogeneity and carrying out multitasking services. This pericytes heterogeneity is primarily linked to their position in specific tissue and organ microenvironments and, most importantly, to their ontogeny. During ontogenesis, pericyte subtypes belong to two main embryonic germ layers, mesoderm and (neuro)ectoderm, and are therefore expected to be found in organs ontogenetically different, nonetheless, pericytes of different origin may converge and colonize neighbouring areas of the same organ/apparatus. Here, we provide a brief overview of the unusual roles played by forebrain pericytes in the processes of angiogenesis and barriergenesis by virtue of their origin from midbrain neural crest stem cells. A better knowledge of the ontogenetic subpopulations may support the understanding of specific interactions and mechanisms involved in pericyte function/dysfunction, including normal and pathological angiogenesis, thereby offering an alternative perspective on cell subtype-specific therapeutic approaches.

Neural crest cell-derived pericytes act as pro-angiogenic cells in human neocortex development and gliomas

Central nervous system diseases involving the parenchymal microvessels are frequently associated with a ‘microvasculopathy’, which includes different levels of neurovascular unit (NVU) dysfunction, including blood–brain barrier alterations. To contribute to the understanding of NVU responses to pathological noxae, we have focused on one of its cellular components, the microvascular pericytes, highlighting unique features of brain pericytes with the aid of the analyses carried out during vascularization of human developing neocortex and in human gliomas. Thanks to their position, centred within the endothelial/glial partition of the vessel basal lamina and therefore inserted between endothelial cells and the perivascular and vessel-associated components (astrocytes, oligodendrocyte precursor cells (OPCs)/NG2-glia, microglia, macrophages, nerve terminals), pericytes fulfil a central role within the microvessel NVU. Indeed, at this critical site, pericytes have a number of direct and extracellular matrix molecule- and soluble factor-mediated functions, displaying marked phenotypical and functional heterogeneity and carrying out multitasking services. This pericytes heterogeneity is primarily linked to their position in specific tissue and organ microenvironments and, most importantly, to their ontogeny. During ontogenesis, pericyte subtypes belong to two main embryonic germ layers, mesoderm and (neuro)ectoderm, and are therefore expected to be found in organs ontogenetically different, nonetheless, pericytes of different origin may converge and colonize neighbouring areas of the same organ/apparatus. Here, we provide a brief overview of the unusual roles played by forebrain pericytes in the processes of angiogenesis and barriergenesis by virtue of their origin from midbrain neural crest stem cells. A better knowledge of the ontogenetic subpopulations may support the understanding of specific interactions and mechanisms involved in pericyte function/dysfunction, including normal and pathological angiogenesis, thereby offering an alternative perspective on cell subtype-specific therapeutic approaches.

Pericytes Across the Lifetime in the Central Nervous System

The pericyte is a perivascular cell type that encapsulates the microvasculature of the brain and spinal cord. Pericytes play a crucial role in the development and maintenance of the blood-brain barrier (BBB) and have a multitude of important functions in the brain. Recent evidence indicates that pericyte impairment has been implicated in neurovascular pathology associated with various human diseases such as diabetes mellitus, Alzheimer's disease (AD), and stroke. Although the pericyte is essential for normal brain function, knowledge about its developmental trajectory and anatomical distribution is limited. This review article summarizes the scientific community's current understanding of pericytes' regional heterogeneity in the brain and their changes during major life stages. More specifically, this review article focuses on pericyte differentiation and migration during brain development, regional population differences in the adult brain, and changes during normal and pathological aging. Most of what is known about pericytes come from studies of the cerebral cortex and hippocampus. Therefore, we highlight the need to expand our understanding of pericyte distribution and function in the whole brain to better delineate this cell type's role in the normal brain and pathological conditions.

The regulatory roles and mechanisms of the transcription factor FOXF2 in human diseases

Many studies have focused on the relationship between transcription factors and a variety of common pathological conditions, such as diabetes, stroke, and cancer. It has been found that abnormal transcription factor regulation can lead to aberrant expression of downstream genes, which contributes to the occurrence and development of many diseases. The forkhead box (FOX) transcription factor family is encoded by the FOX gene, which mediates gene transcription and follow-up functions during physiological and pathological processes. FOXF2, a member of the FOX transcription family, is expressed in various organs and tissues while maintaining their normal structural and functional development during the embryonic and adult stages. Multiple regulatory pathways that regulate FOXF2 may also be controlled by FOXF2. Abnormal FOXF2 expression induced by uncontrollable regulatory signals mediate the progression of human diseases by interfering with the cell cycle, proliferation, differentiation, invasion, and metastasis. FOXF2 manipulates downstream pathways and targets as both a pro-oncogenic and anti-oncogenic factor across different types of cancer, suggesting it may be a new potential clinical marker or therapeutic target for cancer. However, FOXF2’s biological functions and specific roles in cancer development remain unclear. In this study, we provide an overview of FOXF2’s structure, function, and regulatory mechanisms in the physiological and pathological conditions of human body. We also discussed the possible reasons why FOXF2 performs the opposite function in the same types of cancer.

Implementation of Pericytes in Vascular Regeneration Strategies

For the survival and integration of complex large-sized tissue-engineered (TE) organ constructs that exceed the maximal nutrients and oxygen diffusion distance required for cell survival, graft (pre)vascularization to ensure medium or blood supply is crucial. To achieve this, the morphology and functionality of the microcapillary bed should be mimicked by incorporating vascular cell populations, including endothelium and mural cells. Pericytes play a crucial role in microvascular function, blood vessel stability, angiogenesis, and blood pressure regulation. In addition, tissue-specific pericytes are important in maintaining specific functions in different organs, including vitamin A storage in the liver, renin production in the kidneys and maintenance of the blood-brain-barrier. Together with their multipotential differentiation capacity, this makes pericytes the preferred cell type for application in TE grafts. The use of a tissue-specific pericyte cell population that matches the TE organ may benefit organ function. In this review, we provide an overview of the literature for graft (pre)-vascularization strategies and highlight the possible advantages of using tissue-specific pericytes for specific TE organ grafts. Impact statement The use of a tissue-specific pericyte cell population that matches the tissue-engineered (TE) organ may benefit organ function. In this review, we provide an overview of the literature for graft (pre)vascularization strategies and highlight the possible advantages of using tissue-specific pericytes for specific TE organ grafts.

si-SNHG5-FOXF2 inhibits TGF-β1-induced fibrosis in human primary endometrial stromal cells by the Wnt/β-catenin signalling pathway

Background

Intrauterine adhesions (IUAs) are manifestations of endometrial fibrosis characterized by inflammation and fibrinogen aggregation in the extracellular matrix (ECM). The available therapeutic interventions for IUA are insufficiently effective in the clinical setting for postoperative adhesion recurrence and infertility problems. In this study, we investigated whether si-SNHG5-FOXF2 can serve as a molecular mechanism for the inhibition of IUA fibrosis ex vivo.

Methods

FOXF2, TGF-β1 and collagen expression levels were measured by microarray sequencing analysis in three normal endometrium groups and six IUA patients. We induced primary human endometrial stromal cells (HESCs) into myofibroblasts (MFs) to develop an IUA cell model with various concentrations of TGF-β1 at various times. Downstream target genes of FOXF2 were screened by chromatin immunoprecipitation combined with whole-genome high-throughput sequencing (ChIP-seq). We investigated ECM formation, cell proliferation and Wnt/β-catenin signalling pathway-related proteins in primary HESCs with FOXF2 downregulation by quantitative reverse transcription-polymerase chain reaction (qRT-PCR), western blotting (WB), immunohistochemistry (IHC), flow cytometry, ethylenediurea (EdU) and CCK8 assays. We identified long noncoding RNAs (lncRNA) SNHG5 as the upstream regulatory gene of FOXF2 through RNA immunoprecipitation (RIP), RNA pulldown and fluorescence in situ hybridization (FISH). Finally, we examined FOXF2 expression, ECM formation, cell proliferation and Wnt/β-catenin signalling pathway-related proteins in primary HESCs upon FOXF2 downregulation.

Results

FOXF2 was highly expressed in the endometrium of patients with IUA. Treatment of primary HESCs with 10 ng/ml TGF-β1 for 72 h was found to be most effective for developing an IUA cell model. FOXF2 regulated multiple downstream target genes, including collagen, vimentin (VIM) and cyclin D2/DK4, by ChIP-seq and ChIP-PCR. FOXF2 downregulation inhibited TGF-β1-mediated primary HESC fibrosis, including ECM formation, cell proliferation and Wnt/β-catenin signalling pathway-related protein expression. We identified lncRNA SNHG5 as an upstream gene that directly regulates FOXF2 by RIP-seq, qRT-PCR, WB and FISH. SNHG5 downregulation suppressed FOXF2 expression in the IUA cell model, resulting in synergistic repression of the Wnt/β-catenin pathway, thereby altering TGF-β1-mediated ECM aggregation in endometrial stromal cells ex vivo.

Conclusions

Regulation of the Wnt/β-catenin signalling pathway and ECM formation by si-SNHG5-FOXF2 effectively inhibited the profibrotic effect of TGF-β1 on primary HESCs. This finding can provide a molecular basis for antagonizing TGF-β1-mediated fibrosis in primary HESCs.

Sonic Hedgehog Signaling in Cranial Neural Crest Cells Regulates Microvascular Morphogenesis in Facial Development

Sonic hedgehog (Shh) pathway disruption causes craniofacial malformations including orofacial clefts (OFCs) of the lip and palate. In normal craniofacial morphogenesis, Shh signals to multipotent cranial neural crest cells (cNCCs) and was recently discovered to regulate the angiogenic transcriptome, including expression markers of perivascular cells and pericytes. The mural cells of microvasculature, pericytes in the brain and face differentiate from cNCCs, but their role in facial development is not known. Here, we examined microvascular morphogenesis in a mouse model of Shh pathway antagonist-induced cleft lip and the impact of cNCC-specific Shh pathway activation in a cNCC-endothelial cell co-culture system. During cleft pathogenesis in vivo, disrupted microvascular morphogenesis localized with attenuated tissue outgrowth in the medial nasal processes that form the upper lip. In vitro, we found that human umbilical vein endothelial cell (HUVEC) cord formation was not affected by direct Shh pathway perturbation. However, in a co-culture system in which cNCCs directly interact with endothelial cells, cNCC-autonomous Shh pathway activity significantly prolonged endothelial cord network stability. Taken together, these findings support the premise that Shh pathway activation in cNCCs promotes pericyte-like function and microvascular stability. In addition to suggesting a previously unrecognized role for Shh signaling in facial development, these studies also identify perivascular differentiation and microvascular morphogenesis as new focuses for understanding normal and abnormal craniofacial development.

Blood-brain barrier integrity in the pathogenesis of Alzheimer's disease

The blood-brain barrier (BBB) tightly controls the molecular exchange between the brain parenchyma and blood. Accumulated evidence from transgenic animal Alzheimer's disease (AD) models and human AD patients have demonstrated that BBB dysfunction is a major player in AD pathology. In this review, we discuss the role of the BBB in maintaining brain integrity and how this is mediated by crosstalk between BBB-associated cells within the neurovascular unit (NVU). We then discuss the role of the NVU, in particular its endothelial cell, pericyte, and glial cell constituents, in AD pathogenesis. The effect of substances released by the neuroendocrine system in modulating BBB function and AD pathogenesis is also discussed. We perform a systematic review of currently available AD treatments specifically targeting pericytes and BBB glial cells. In summary, this review provides a comprehensive overview of BBB dysfunction in AD and a new perspective on the development of therapeutics for AD.

Retina-specific targeting of pericytes reveals structural diversity and enables control of capillary blood flow

Pericytes are a unique class of mural cells essential for angiogenesis, maintenance of the vasculature and are key players in microvascular pathology. However, their diversity and specific roles are poorly understood, limiting our insight into vascular physiology and the ability to develop effective therapies. Here, in the mouse retina, a tractable model of the CNS, we evaluated distinct classes of mural cells along the vascular tree for both structural characterization and physiological manipulation of blood flow. To accomplish this, we first tested three inducible mural cell-specific mouse lines using a sensitive Ai14 reporter and tamoxifen application either by a systemic injection, or by local administration in the form of eye drops. The specificity and pattern of cre activation varied significantly across the three lines, under either the PDGFRβ or NG2 promoter (Pdgfrβ-CreRha, Pdgfrβ-CreCsln, and Cspg4-Cre). In particular, a mouse line with Cre under the NG2 promoter resulted in sparse TdTomato labeling of mural cells, allowing for an unambiguous characterization of anatomical features of individual sphincter cells and capillary pericytes. Furthermore, in one PDGFRβ line, we found that focal eye drop application of tamoxifen led to an exclusive Cre-activation in pericytes, without affecting arterial mural cells. We then used this approach to boost capillary blood flow by selective expression of Halorhodopsin, a highly precise hyperpolarizing optogenetic actuator. The ability to exclusively target capillary pericytes may prove a precise and potentially powerful tool to treat microcirculation deficits, a common pathology in numerous diseases.

Fgfbp1 promotes blood-brain barrier development by regulating collagen IV deposition and maintaining Wnt/β-catenin signaling

Central nervous system (CNS) blood vessels contain a functional blood-brain barrier (BBB) that is necessary for neuronal survival and activity. Although Wnt/β-catenin signaling is essential for BBB development, its downstream targets within the neurovasculature remain poorly understood. To identify targets of Wnt/β-catenin signaling underlying BBB maturation, we performed a microarray analysis that identified Fgfbp1 as a novel Wnt/β-catenin-regulated gene in mouse brain endothelial cells (mBECs). Fgfbp1 is expressed in the CNS endothelium and secreted into the vascular basement membrane during BBB formation. Endothelial genetic ablation of Fgfbp1 results in transient hypervascularization but delays BBB maturation in specific CNS regions, as evidenced by both upregulation of Plvap and increased tracer leakage across the neurovasculature due to reduced Wnt/β-catenin activity. In addition, collagen IV deposition in the vascular basement membrane is reduced in mutant mice, leading to defective endothelial cell-pericyte interactions. Fgfbp1 is required cell-autonomously in mBECs to concentrate Wnt ligands near cell junctions and promote maturation of their barrier properties in vitro Thus, Fgfbp1 is a crucial extracellular matrix protein during BBB maturation that regulates cell-cell interactions and Wnt/β-catenin activity.

Engineering Brain-Specific Pericytes from Human Pluripotent Stem Cells

Pericytes (PCs) are a type of perivascular cells that surround endothelial cells of small blood vessels. In the brain, PCs show heterogeneity depending on their position within the vasculature. As a result, PC interactions with surrounding endothelial cells, astrocytes, and neuron cells play a key role in a wide array of neurovascular functions such as regulating blood-brain barrier (BBB) permeability, cerebral blood flow, and helping to facilitate the clearance of toxic cellular molecules. Therefore, a reliable method of engineering brain-specific PCs from human induced pluripotent stem cells (hiPSCs) is critical in neurodegenerative disease modeling. This review summarizes brain-specific PC differentiation of hiPSCs through mesoderm and neural crest induction. Key signaling pathways (platelet-derived growth factor-B [PDGF-B], transforming growth factor [TGF]-β, and Notch signaling) regulating PC function, PC interactions with adjacent cells, and PC differentiation from hiPSCs are also discussed. Specifically, PDGF-BB-platelet-derived growth factor receptor β signaling promotes PC cell survival, TGF-β signal transduction facilitates PC attachment to endothelial cells, and Notch signaling is critical in vascular development and arterial-venous specification. Furthermore, current challenges facing the use of hiPSC-derived PCs are discussed, and their ongoing uses in neurodegenerative disease modeling are identified. Further investigations into PCs and surrounding cell interactions are needed to characterize the roles of brain PCs in various neurodegenerative disorders. Impact statement This article summarizes the work related to brain-specific pericytes (PCs) derived from human pluripotent stem cells (hPSCs). In particular, key signaling pathways regulating PC function, PC interactions with adjacent cells, and PC differentiation from hPSCs were discussed. Furthermore, current challenges facing the use of hPSC-derived PCs were identified, and their ongoing uses in neurodegenerative disease modeling were discussed. The review highlights the important role of cell-cell interactions in blood-brain barrier (BBB) models and neurodegeneration. The summarized findings are significant for establishing pluripotent stem cell-based BBB models toward the applications in drug screening and disease modeling.

Models of the blood-brain barrier using iPSC-derived cells

The blood-brain barrier (BBB) constitutes the interface between the blood and the brain tissue. Its primary function is to maintain the tightly controlled microenvironment of the brain. Models of the BBB are useful for studying the development and maintenance of the BBB as well as diseases affecting it. Furthermore, BBB models are important tools in drug development and support the evaluation of the brain-penetrating properties of novel drug molecules. Currently used in vitro models of the BBB include immortalized brain endothelial cell lines and primary brain endothelial cells of human and animal origin. Unfortunately, many cell lines and primary cells do not recreate physiological restriction of transport in vitro. Human-induced pluripotent stem cell (iPSC)-derived brain endothelial cells have proven a promising alternative source of brain endothelial-like cells that replicate tight cell layers with low paracellular permeability. Given the possibility to generate large amounts of human iPSC-derived brain endothelial cells they are a feasible alternative when modelling the BBB in vitro. iPSC-derived brain endothelial cells form tight cell layers in vitro and their barrier properties can be enhanced through coculture with other cell types of the BBB. Currently, many different models of the BBB using iPSC-derived cells are under evaluation to study BBB formation, maintenance, disruption, drug transport and diseases affecting the BBB. This review summarizes important functions of the BBB and current efforts to create iPSC-derived BBB models in both static and dynamic conditions. In addition, it highlights key model requirements and remaining challenges for human iPSC-derived BBB models in vitro.

Neuronal regulation of the blood-brain barrier and neurovascular coupling

To continuously process neural activity underlying sensation, movement and cognition, the CNS requires a homeostatic microenvironment that is not only enriched in nutrients to meet its high metabolic demands but that is also devoid of toxins that might harm the sensitive neural tissues. This highly regulated microenvironment is made possible by two unique features of CNS vasculature absent in the peripheral organs. First, the blood-blood barrier, which partitions the circulating blood from the CNS, acts as a gatekeeper to facilitate the selective trafficking of substances between the blood and the parenchyma. Second, neurovascular coupling ensures that, following local neural activation, regional blood flow is increased to quickly supply more nutrients and remove metabolic waste. Here, we review how neural and vascular activity act on one another with regard to these two properties.

FOXF2 acts as a crucial molecule in tumours and embryonic development

As a key member of the forkhead box transcription factors, forkhead box F2 (FOXF2) serves as a transcriptional regulator and regulates downstream gene expression in embryonic development, metabolism and in some common diseases, such as stroke and gastroparesis. Recent studies have shown that aberrant expression of FOXF2 is associated with a variety of tumorigenic processes, such as proliferation, invasion and metastasis. The role of FOXF2 in the development of many different organs has been confirmed by studies and has been speculated about in case reports. We focus on the mechanisms and signal pathways of tumour development initiated by aberrant expression of FOXF2, and we summarize the diseases and signal pathways caused by aberrant expression of FOXF2 in embryogenesis. This article highlights the differences in the role of FOXF2 in different tumours and demonstrates that multiple factors can regulate FOXF2 levels. In addition, FOXF2 is considered a biomarker for the diagnosis or prognosis of various tumours. Therefore, regulating the level of FOXF2 is an ideal treatment for tumours. FOXF2 could also affect the expression of some organ-specific genes to modulate organogenesis and could serve as a biomarker for specific differentiated cells. Finally, we present prospects for the continued research focus of FOXF2.

Neuronal and glial regulation of CNS angiogenesis and barriergenesis

Neurovascular pathologies of the central nervous system (CNS), which are associated with barrier dysfunction, are leading causes of death and disability. The roles that neuronal and glial progenitors and mature cells play in CNS angiogenesis and neurovascular barrier maturation have been elucidated in recent years. Yet how neuronal activity influences these processes remains largely unexplored. Here, we discuss our current understanding of how neuronal and glial development affects CNS angiogenesis and barriergenesis, and outline future directions to elucidate how neuronal activity might influence these processes. An understanding of these mechanisms is crucial for developing new interventions to treat neurovascular pathologies.

Plasma biomarkers of astrocytic and neuronal dysfunction in early- and late-onset Alzheimer's disease

INTRODUCTION

We investigated plasma proteomic markers of astrocytopathy, brain degeneration, plasticity, and inflammation in sporadic early-onset versus late-onset Alzheimer's disease (EOAD and LOAD).

METHODS

Plasma was analyzed using ultra-sensitive immuno-based assays from 33 EOAD, 30 LOAD, and 36 functionally normal older adults.

RESULTS

Principle component analyses identified 3 factors: trophic (BDNF, VEGF, TGFβ), degenerative (GFAP, NfL), and inflammatory (TNFα, IL-6, IP-10, IL-10). Trophic factor was elevated in both AD groups and associated with cognition and gray matter volumes. Degenerative factor was elevated in EOAD, with higher levels associated with worse functioning in this group. Biomarkers of inflammation were not significantly different between groups and were only associated with age.

DISUCSSION

Plasma proteomic biomarkers provide novel means of investigating molecular processes in vivo and their contributions to clinical outcomes. We present initial investigations of several of these fluid biomarkers, capturing aspects of astrocytopathy, neuronal injury, cellular plasticity, and inflammation in EOAD versus LOAD.

Single Cell Analysis in Vascular Biology

The ability to quantify DNA, RNA, and protein variations at the single cell level has revolutionized our understanding of cellular heterogeneity within tissues. Via such analyses, individual cells within populations previously thought to be homogeneous can now be delineated into specific subpopulations expressing unique sets of genes, enabling specialized functions. In vascular biology, studies using single cell RNA sequencing have revealed extensive heterogeneity among endothelial and mural cells even within the same vessel, key intermediate cell types that arise during blood and lymphatic vessel development, and cell-type specific responses to disease. Thus, emerging new single cell analysis techniques are enabling vascular biologists to elucidate mechanisms of vascular development, homeostasis, and disease that were previously not possible. In this review, we will provide an overview of single cell analysis methods and highlight recent advances in vascular biology made possible through single cell RNA sequencing.

Identification of a combination of transcription factors that synergistically increases endothelial cell barrier resistance

Endothelial cells (ECs) display remarkable plasticity during development before becoming quiescent and functionally mature. EC maturation is directed by several known transcription factors (TFs), but the specific set of TFs responsible for promoting high-resistance barriers, such as the blood-brain barrier (BBB), have not yet been fully defined. Using expression mRNA data from published studies on ex vivo ECs from the central nervous system (CNS), we predicted TFs that induce high-resistance barrier properties of ECs as in the BBB. We used our previously established method to  generate ECs from human pluripotent stem cells (hPSCs), and then we overexpressed the candidate TFs in hPSC-ECs and measured barrier resistance and integrity using electric cell-substrate impedance sensing, trans-endothelial electrical resistance and FITC-dextran permeability assays. SOX18 and TAL1 were the strongest EC barrier-inducing TFs, upregulating Wnt-related signaling and EC junctional gene expression, respectively, and downregulating EC proliferation-related genes. These TFs were combined with SOX7 and ETS1 that together effectively induced EC barrier resistance, decreased paracellular transport and increased protein expression of tight junctions and induce mRNA expression of several genes involved in the formation of EC barrier and transport. Our data shows identification of a transcriptional network that controls barrier resistance in ECs. Collectively this data may lead to novel approaches for generation of in vitro models of the BBB.

Single-Cell Genomic Characterization Reveals the Cellular Reprogramming of the Gastric Tumor Microenvironment

PURPOSE

The tumor microenvironment (TME) consists of a heterogenous cellular milieu that can influence cancer cell behavior. Its characteristics have an impact on treatments such as immunotherapy. These features can be revealed with single-cell RNA sequencing (scRNA-seq). We hypothesized that scRNA-seq analysis of gastric cancer together with paired normal tissue and peripheral blood mononuclear cells (PBMC) would identify critical elements of cellular deregulation not apparent with other approaches.

EXPERIMENTAL DESIGN

scRNA-seq was conducted on seven patients with gastric cancer and one patient with intestinal metaplasia. We sequenced 56,167 cells comprising gastric cancer (32,407 cells), paired normal tissue (18,657 cells), and PBMCs (5,103 cells). Protein expression was validated by multiplex immunofluorescence.

RESULTS

Tumor epithelium had copy number alterations, a distinct gene expression program from normal, with intratumor heterogeneity. Gastric cancer TME was significantly enriched for stromal cells, macrophages, dendritic cells (DC), and Tregs. TME-exclusive stromal cells expressed distinct extracellular matrix components than normal. Macrophages were transcriptionally heterogenous and did not conform to a binary M1/M2 paradigm. Tumor DCs had a unique gene expression program compared to PBMC DCs. TME-specific cytotoxic T cells were exhausted with two heterogenous subsets. Helper, cytotoxic T, Treg, and NK cells expressed multiple immune checkpoint or co-stimulatory molecules. Receptor-ligand analysis revealed TME-exclusive intercellular communication.

CONCLUSIONS

Single-cell gene expression studies revealed widespread reprogramming across multiple cellular elements in the gastric cancer TME. Cellular remodeling was delineated by changes in cell numbers, transcriptional states, and intercellular interactions. This characterization facilitates understanding of tumor biology and enables identification of novel targets including for immunotherapy.

Overview of Crosstalk Between Multiple Factor of Transcytosis in Blood Brain Barrier

Blood brain barrier (BBB) conserves unique regulatory system to maintain barrier tightness while allowing adequate transport between neurovascular units. This mechanism possess a challenge for drug delivery, while abnormality may result in pathogenesis. Communication between vascular and neural system is mediated through paracellular and transcellular (transcytosis) pathway. Transcytosis itself showed dependency with various components, focusing on caveolae-mediated. Among several factors, intense communication between endothelial cells, pericytes, and astrocytes is the key for a normal development. Regulatory signaling pathway such as VEGF, Notch, S1P, PDGFβ, Ang/Tie, and TGF-β showed interaction with the transcytosis steps. Recent discoveries showed exploration of various factors which has been proven to interact with one of the process of transcytosis, either endocytosis, endosomal rearrangement, or exocytosis. As well as providing a hypothetical regulatory pathway between each factors, specifically miRNA, mechanical stress, various cytokines, physicochemical, basement membrane and junctions remodeling, and crosstalk between developmental regulatory pathways. Finally, various hypotheses and probable crosstalk between each factors will be expressed, to point out relevant research application (Drug therapy design and BBB-on-a-chip) and unexplored terrain.

New Insights in the Complexity and Functionality of the Neurovascular Unit

The neurovascular unit (NVU) encompasses all brain cells and underlines that neurons, glia and brain vasculature are in intimate physical and functional association. Brain function is dependent on blood flow and local increases in blood flow in response to neural activity - functional hyperaemia takes place at the NVU. Although this is a vital function of the NVU, many studies have demonstrated that the NVU also performs other tasks. Blood vessels in the brain, which are composed of multiple cell types, are essential for correct brain development. They constitute the niche for brain stem cells, sense the environment and communicate changes to neural tissue, and control the immune quiescence of the CNS. In this brief chapter we will discuss new insights into the biology of NVU, which have further revealed the heterogeneity and complexity of the vascular tree and its neurovascular associations.

Emerging links between cerebrovascular and neurodegenerative diseases-a special role for pericytes

Neurodegenerative and cerebrovascular diseases cause considerable human suffering, and therapy options for these two disease categories are limited or non-existing. It is an emerging notion that neurodegenerative and cerebrovascular diseases are linked in several ways, and in this review, we discuss the current status regarding vascular dysregulation in neurodegenerative disease, and conversely, how cerebrovascular diseases are associated with central nervous system (CNS) degeneration and dysfunction. The emerging links between neurodegenerative and cerebrovascular diseases are reviewed with a particular focus on pericytes-important cells that ensheath the endothelium in the microvasculature and which are pivotal for blood-brain barrier function and cerebral blood flow. Finally, we address how novel molecular and cellular insights into pericytes and other vascular cell types may open new avenues for diagnosis and therapy development for these important diseases.

Challenges and opportunities in stroke genetics

Stroke, ischaemic stroke and subtypes of ischaemic stroke display substantial heritability. When compared with related vascular conditions, the number of established risk loci reaching genome-wide significance for association with stroke is still in the lower range, particularly for aetiological stroke subtypes such as large artery atherosclerotic stroke or small vessel stroke. Nevertheless, for individual loci substantial progress has been made in determining the specific mechanisms mediating stroke risk. In this review, we present a roadmap for functional follow-up of common risk variants associated with stroke. First, we discuss in silico strategies for characterizing signals in non-coding regions and highlight databases providing information on quantitative trait loci for mRNA and protein expression, as well as methylation, focussing on those with presumed relevance for stroke. Next, we discuss experimental strategies for following up on non-coding risk variants and regions such as massively parallel reporter assays, proteome-wide association studies, and chromatin conformation capture (3C) assays. These and other approaches are relevant for gaining insight into the specific variants and mechanisms mediating genetic stroke risk. Finally, we discuss how genetic findings could influence clinical practice by adding to diagnostic algorithms and eventually improve treatment options for stroke.

Development and Cell Biology of the Blood-Brain Barrier

The vertebrate vasculature displays high organotypic specialization, with the structure and function of blood vessels catering to the specific needs of each tissue. A unique feature of the central nervous system (CNS) vasculature is the blood-brain barrier (BBB). The BBB regulates substance influx and efflux to maintain a homeostatic environment for proper brain function. Here, we review the development and cell biology of the BBB, focusing on the cellular and molecular regulation of barrier formation and the maintenance of the BBB through adulthood. We summarize unique features of CNS endothelial cells and highlight recent progress in and general principles of barrier regulation. Finally, we illustrate why a mechanistic understanding of the development and maintenance of the BBB could provide novel therapeutic opportunities for CNS drug delivery.

FOXF2 is required for cochlear development in humans and mice

Molecular mechanisms governing the development of the human cochlea remain largely unknown. Through genome sequencing, we identified a homozygous FOXF2 variant c.325A>T (p.I109F) in a child with profound sensorineural hearing loss (SNHL) associated with incomplete partition type I anomaly of the cochlea. This variant is not found in public databases or in over 1000 ethnicity-matched control individuals. I109 is a highly conserved residue in the forkhead box (Fox) domain of FOXF2, a member of the Fox protein family of transcription factors that regulate the expression of genes involved in embryogenic development as well as adult life. Our in vitro studies show that the half-life of mutant FOXF2 is reduced compared to that of wild type. Foxf2 is expressed in the cochlea of developing and adult mice. The mouse knockout of Foxf2 shows shortened and malformed cochleae, in addition to altered shape of hair cells with innervation and planar cell polarity defects. Expressions of Eya1 and Pax3, genes essential for cochlear development, are reduced in the cochleae of Foxf2 knockout mice. We conclude that FOXF2 plays a major role in cochlear development and its dysfunction leads to SNHL and developmental anomalies of the cochlea in humans and mice.

Stroke genetics: discovery, biology, and clinical applications

Stroke, a leading cause of long-term disability and death worldwide, has a heritable component. Recent gene discovery efforts have expanded the number of known single-gene disorders associated with stroke and have linked common variants at approximately 35 genetic loci to stroke risk. These discoveries have highlighted novel mechanisms and pathways implicated in stroke related to large artery atherosclerosis, cardioembolism, and small vessel disease, and defined shared genetic influences with related vascular traits. Genetics has also successfully established causal relationships with risk factors and holds promise for prioritising targets for exploration in clinical trials. Genome-wide polygenic scores enable the identification of high-risk individuals before the emergence of vascular risk factors. Challenges ahead include a better understanding of rare variants and ancestral differences for integration of genetics into precision medicine, integration with other omics data, uncovering the genetic factors that govern stroke recurrence and stroke outcome, and the conversion of genetic discoveries to novel therapies.

Human pluripotent stem cell-derived brain pericyte-like cells induce blood-brain barrier properties

Brain pericytes play important roles in the formation and maintenance of the neurovascular unit (NVU), and their dysfunction has been implicated in central nervous system disorders. While human pluripotent stem cells (hPSCs) have been used to model other NVU cell types, including brain microvascular endothelial cells (BMECs), astrocytes, and neurons, hPSC-derived brain pericyte-like cells have not been integrated into these models. In this study, we generated neural crest stem cells (NCSCs), the embryonic precursor to forebrain pericytes, from hPSCs and subsequently differentiated NCSCs to brain pericyte-like cells. These cells closely resembled primary human brain pericytes and self-assembled with endothelial cells. The brain pericyte-like cells induced blood-brain barrier properties in BMECs, including barrier enhancement and reduced transcytosis. Last, brain pericyte-like cells were incorporated with iPSC-derived BMECs, astrocytes, and neurons to form an isogenic human model that should prove useful for the study of the NVU.

Organizational hierarchy and structural diversity of microvascular pericytes in adult mouse cortex

Smooth muscle cells and pericytes, together called mural cells, coordinate many distinct vascular functions. Canonically, smooth muscle cells are ring-shaped and cover arterioles with circumferential processes, whereas pericytes extend thin processes that run longitudinally along capillaries. In between these canonical mural cell types are cells with features of both smooth muscle cells and pericytes. Recent studies suggest that these transitional cells are critical for controlling blood flow to the capillary bed during health and disease, but there remains confusion on how to identify them and where they are located in the brain microvasculature. To address this issue, we measured the morphology, vascular territory, and α-smooth muscle actin content of structurally diverse mural cells in adult mouse cortex. We first imaged intact 3D vascular networks to establish the locations of major gradations in mural cell appearance as arterioles branched into capillaries. We then imaged individual mural cells occupying the regions within these gradations. This revealed two transitional cells that were often similar in appearance, but with sharply contrasting levels of α-smooth muscle actin. Our findings highlight the diversity of mural cell morphologies in brain microvasculature, and provide guidance for identification and categorization of mural cell types.

Induction of Mesoderm and Neural Crest-Derived Pericytes from Human Pluripotent Stem Cells to Study Blood-Brain Barrier Interactions

In the CNS, perivascular cells (“pericytes”) associate with endothelial cells to mediate the formation of tight junctions essential to the function of the blood-brain barrier (BBB). The BBB protects the CNS by regulating the flow of nutrients and toxins into and out of the brain. BBB dysfunction has been implicated in the progression of Alzheimer's disease (AD), but the role of pericytes in BBB dysfunction in AD is not well understood. In the developing embryo, CNS pericytes originate from two sources: mesoderm and neural crest. In this study, we report two protocols using mesoderm or neural crest intermediates, to generate brain-specific pericyte-like cells from induced pluripotent stem cell (iPSC) lines created from healthy and AD patients. iPSC-derived pericytes display stable expression of pericyte surface markers and brain-specific genes and are functionally capable of increasing vascular tube formation and endothelial barrier properties.

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Mesoderm and neural crest-derived pericytes (PCs) from hPSCs termed mPCs and ncPCs

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mPCs and ncPCs express general and brain-specific pericyte genes

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mPCs and ncPCs associate with and promote barrier properties of endothelial cells

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WNT is important, but not necessary for neural crest-derived PC specification

Transcytosis at the blood-brain barrier

The blood-brain barrier (BBB) is a functional interface separating the brain from the circulatory system and is essential for homeostasis of the central nervous system (CNS). The BBB regulates molecular flux to maintain an optimal environment for neuronal function and protects the brain from toxins and pathogens. Endothelial cells forming the walls of CNS blood vessels constitute the BBB. CNS endothelial cells exhibit two features that underlie the restrictive properties of the BBB: specialized tight junctions that prevent paracellular passage between the blood and the brain, and unusually low levels of vesicle trafficking that limit transcellular transport or transcytosis. While the prevailing view in the field was that specialized tight junctions contributed to CNS barrier properties, recent findings have revealed the importance of maintaining low rates of transcytosis at the BBB. It is now clear that suppression of transcytosis at the BBB is an active process and CNS-specific genetic programs inhibit this pathway to maintain a functional barrier.

Hedgehog signaling patterns the oral-aboral axis of the mandibular arch

Development of vertebrate jaws involves patterning neural crest-derived mesenchyme cells into distinct subpopulations along the proximal-distal and oral-aboral axes. Although the molecular mechanisms patterning the proximal-distal axis have been well studied, little is known regarding the mechanisms patterning the oral-aboral axis. Using unbiased single-cell RNA-seq analysis followed by in situ analysis of gene expression profiles, we show that Shh and Bmp4 signaling pathways are activated in a complementary pattern along the oral-aboral axis in mouse embryonic mandibular arch. Tissue-specific inactivation of hedgehog signaling in neural crest-derived mandibular mesenchyme led to expansion of BMP signaling activity to throughout the oral-aboral axis of the distal mandibular arch and subsequently duplication of dentary bone in the oral side of the mandible at the expense of tongue formation. Further studies indicate that hedgehog signaling acts through the Foxf1/2 transcription factors to specify the oral fate and pattern the oral-aboral axis of the mandibular mesenchyme.

Pericytes in Glioblastomas: Multifaceted Role Within Tumor Microenvironments and Potential for Therapeutic Interventions

Glioblastoma (GBM) is an aggressive and lethal disease that often results in a poor prognosis. Unlike most solid tumors, GBM is characterized by diffuse infiltrating margins, extensive angiogenesis, hypoxia, necrosis, and clonal heterogeneity. Recurrent disease is an unavoidable consequence for many patients as standard treatment options such as surgery, radiotherapy, and chemotherapy have proven to be insufficient in causing long-term survival benefits. Systemic delivery of promising drugs is hindered due to the blood-brain barrier and non-uniform perfusion within GBM tissue. In recent years, many investigations have highlighted the role of GBM stem cells (GSCs) and their microenvironment in the initiation and maintenance of tumor tissue. Preclinical and early clinical studies to target GSCs and microenvironmental components are currently underway. Of these strategies, immunotherapy using checkpoint inhibitors and redirected cytotoxic T cells have shown promising results in early investigations. But, GBM microenvironment is heterogenous and recent investigations have shown cell populations within this microenvironment to be plastic. These studies underline the importance of identifying the role of and targeting multiple cell populations within the GBM microenvironment which could have a synergistic effect when combined with novel therapies. Pericytes are multipotent perivascular cells that play a vital role within the GBM microenvironment by assisting in tumor initiation, survival, and progression. Due to their role in regulating the blood-brain barrier permeability, promoting angiogenesis, tumor growth, clearing extracellular matrix for infiltrating GBM cells and in helping GBM cells evade immune surveillance, pericytes could be ideal therapeutic targets for stymieing or exploiting their role within the GBM microenvironment. This chapter will introduce hallmarks of GBM and elaborate on the contributions of pericytes to these hallmarks by examining recent findings. In addition, the chapter also highlights the therapeutic value of targeting pericytes, while discussing conventional and novel GBM therapies and obstacles to their efficacy.

Understanding the role of the perivascular space in cerebral small vessel disease

Small vessel diseases (SVDs) are a group of disorders that result from pathological alteration of the small blood vessels in the brain, including the small arteries, capillaries and veins. Of the 35-36 million people that are estimated to suffer from dementia worldwide, up to 65% have an SVD component. Furthermore, SVD causes 20-25% of strokes, worsens outcome after stroke and is a leading cause of disability, cognitive impairment and poor mobility. Yet the underlying cause(s) of SVD are not fully understood. Magnetic resonance imaging has confirmed enlarged perivascular spaces (PVS) as a hallmark feature of SVD. In healthy tissue, these spaces are proposed to form part of a complex brain fluid drainage system which supports interstitial fluid exchange and may also facilitate clearance of waste products from the brain. The pathophysiological signature of PVS and what this infers about their function and interaction with cerebral microcirculation, plus subsequent downstream effects on lesion development in the brain has not been established. Here we discuss the potential of enlarged PVS to be a unique biomarker for SVD and related brain disorders with a vascular component. We propose that widening of PVS suggests presence of peri-vascular cell debris and other waste products that form part of a vicious cycle involving impaired cerebrovascular reactivity, blood-brain barrier dysfunction, perivascular inflammation and ultimately impaired clearance of waste proteins from the interstitial fluid space, leading to accumulation of toxins, hypoxia, and tissue damage. Here, we outline current knowledge, questions and hypotheses regarding understanding the brain fluid dynamics underpinning dementia and stroke through the common denominator of SVD.

NCK-dependent pericyte migration promotes pathological neovascularization in ischemic retinopathy

Pericytes are mural cells that surround capillaries and control angiogenesis and capillary barrier function. During sprouting angiogenesis, endothelial cell-derived platelet-derived growth factor-B (PDGF-B) regulates pericyte proliferation and migration via the platelet-derived growth factor receptor-β (PDGFRβ). PDGF-B overexpression has been associated with proliferative retinopathy, but the underlying mechanisms remain poorly understood. Here we show that abnormal, α-SMA-expressing pericytes cover angiogenic sprouts and pathological neovascular tufts (NVTs) in a mouse model of oxygen-induced retinopathy. Genetic lineage tracing demonstrates that pericytes acquire α-SMA expression during NVT formation. Pericyte depletion through inducible endothelial-specific knockout of Pdgf-b decreases NVT formation and impairs revascularization. Inactivation of the NCK1 and NCK2 adaptor proteins inhibits pericyte migration by preventing PDGF-B-induced phosphorylation of PDGFRβ at Y1009 and PAK activation. Loss of Nck1 and Nck2 in mural cells prevents NVT formation and vascular leakage and promotes revascularization, suggesting PDGFRβ-Y1009/NCK signaling as a potential target for the treatment of retinopathies.

Pericytes are perivascular cells that regulate blood vessel formation and function. Here Dubrac et al. show that pericyte recruitment contributes to pathological neovascularisation in a mouse model of ischemic retinopathy, and that this depends on the regulation of PDGF-B signaling by NCK adaptor proteins.

Tissue Specific Origin, Development, and Pathological Perspectives of Pericytes

Pericytes are mural cells surrounding blood vessels, adjacent to endothelial cells. Pericytes play critical roles in maturation and maintenance of vascular branching morphogenesis. In the central nervous system (CNS), pericytes are necessary for the formation and regulation of the blood-brain barrier (BBB) and pericyte deficiency accompanies CNS diseases including multiple sclerosis, diabetic retinopathy, neonatal intraventricular hemorrhage, and neurodegenerative disorders. Despite the importance of pericytes, their developmental origins and phenotypic diversity remain incompletely understood. Pericytes express multiple markers and the origin of pericytes differs by tissue, which may cause difficulty for the identification and understanding of the ontogeny of pericytes. Also, pericytes have the potential to give rise to different tissues in vitro but this is not clear in vivo. These studies indicate that pericytes are heterogeneous in a tissue- and context- dependent manner. This short review focuses on recent studies about identification of pericytes, heterogeneous origin of pericytes during development and in adults, and the differentiation capacity of pericytes, and pericytes in pathological settings.

Bridging barriers: a comparative look at the blood-brain barrier across organisms

This review by O'Brown et al. discusses the cellular nature of the blood–brain barrier (BBB) and the conservation and variation of BBB function across taxa. It compares the BBB across organisms in order to provide insight into the human BBB both under normal physiological conditions and in neurological diseases.

The blood–brain barrier (BBB) restricts free access of molecules between the blood and the brain and is essential for regulating the neural microenvironment. Here, we describe how the BBB was initially characterized and how the current field evaluates barrier properties. We next detail the cellular nature of the BBB and discuss both the conservation and variation of BBB function across taxa. Finally, we examine our current understanding of mouse and zebrafish model systems, as we expect that comparison of the BBB across organisms will provide insight into the human BBB under normal physiological conditions and in neurological diseases.

Free water determines diffusion alterations and clinical status in cerebral small vessel disease

INTRODUCTION

Diffusion tensor imaging detects early tissue alterations in Alzheimer's disease and cerebral small vessel disease (SVD). However, the origin of diffusion alterations in SVD is largely unknown.

METHODS

To gain further insight, we applied free water (FW) imaging to patients with genetically defined SVD (Cerebral Autosomal Dominant Arteriopathy with Subcortical Infarcts and Leukoencephalopathy [CADASIL], n = 57), sporadic SVD (n = 444), and healthy controls (n = 28). We modeled freely diffusing water in the extracellular space (FW) and measures reflecting fiber structure (tissue compartment). We tested associations between these measures and clinical status (processing speed and disability).

RESULTS

Diffusion alterations in SVD were mostly driven by increased FW and less by tissue compartment alterations. Among imaging markers, FW showed the strongest association with clinical status (R2 up to 34%, P < .0001). Findings were consistent across patients with CADASIL and sporadic SVD.

DISCUSSION

Diffusion alterations and clinical status in SVD are largely determined by extracellular fluid increase rather than alterations of white matter fiber organization.

Roles of Pericytes in Stroke Pathogenesis

Stroke is a cerebrovascular disorder that affects many people worldwide. In addition to the well-established functions of astrocytes and microglia in stroke pathogenesis, pericytes also play an important role in stroke progression and recovery. As perivascular multi-potent cells and an important component of the blood–brain barrier (BBB), pericytes have been shown to exert a large variety of functions, including serving as stem/progenitor cells and maintaining BBB integrity. Here in this review, we summarize the roles of pericytes in stroke pathogenesis, with a focus on their effects in cerebral blood flow, BBB integrity, angiogenesis, immune responses, scar formation and fibrosis.

The physiological functions of central nervous system pericytes and a potential role in pain

Central nervous system (CNS) pericytes regulate critical functions of the neurovascular unit in health and disease. CNS pericytes are an attractive pharmacological target for their position within the neurovasculature and for their role in neuroinflammation. Whether the function of CNS pericytes also affects pain states and nociceptive mechanisms is currently not understood. Could it be that pericytes hold the key to pain associated with CNS blood vessel dysfunction? This article reviews recent findings on the important physiological functions of CNS pericytes and highlights how these neurovascular functions could be linked to pain states.

Pericyte Biology in Zebrafish

The zebrafish is an outstanding model for studying vascular biology in vivo. Pericytes and vascular smooth muscle cells can be imaged as they associate with vessels and provide stability and integrity to the vasculature. In zebrafish, pericytes associate with the cerebral and trunk vasculature on the second day of development, as assayed by pdgfrβ and notch3 markers. In the head, cerebral pericytes are neural crest derived, except for the pericytes of the hindbrain vasculature, which are mesoderm derived. Similar to the hindbrain, pericytes on the trunk vasculature are also mesoderm derived. Regardless of their location, pericyte development depends on a complex interaction between blood flow and signalling pathways, such as Notch, SONIC HEDGEHOG and BMP signalling, all of which positively regulate pericyte numbers.Pericyte numbers rapidly increase as development proceeds in order to stabilize both the blood-brain barrier and the vasculature and hence, prevent haemorrhage. Consequently, compromised pericyte development results in compromised vascular integrity, which then evolves into detrimental pathologies. Some of these pathologies have been modelled in zebrafish by inducing mutations in the notch3, foxc1 and foxf2 genes. These zebrafish models provide insights into the mechanisms of disease as associated with pericyte biology. Going forward, these models may be key contributors in elucidating the role of vascular mural cells in regulating vessel diameter and hence, blood flow.

Tissue Myeloid Progenitors Differentiate into Pericytes through TGF-β Signaling in Developing Skin Vasculature

Mural cells (pericytes and vascular smooth muscle cells) are essential for the regulation of vascular networks and maintenance of vascular integrity, but their origins are diverse in different tissues and not known in the organs that arise from the ectoderm, such as skin. Here, we show that tissue-localized myeloid progenitors contribute to pericyte development in embryonic skin vasculature. A series of in vivo fate-mapping experiments indicates that tissue myeloid progenitors differentiate into pericytes. Furthermore, depletion of tissue myeloid cells and their progenitors in PU.1 (also known as Spi1) mutants results in defective pericyte development. Fluorescence-activated cell sorting (FACS)-isolated myeloid cells and their progenitors from embryonic skin differentiate into pericytes in culture. At the molecular level, transforming growth factor-β (TGF-β) induces pericyte differentiation in culture. Furthermore, type 2 TGF-β receptor (Tgfbr2) mutants exhibit deficient pericyte development in skin vasculature. Combined, these data suggest that pericytes differentiate from tissue myeloid progenitors in the skin vasculature through TGF-β signaling.

Gradual Suppression of Transcytosis Governs Functional Blood-Retinal Barrier Formation

Blood-central nervous system (CNS) barriers partition neural tissues from the blood, providing a homeostatic environment for proper neural function. The endothelial cells that form blood-CNS barriers have specialized tight junctions and low rates of transcytosis to limit the flux of substances between blood and CNS. However, the relative contributions of these properties to CNS barrier permeability are unknown. Here, by studying functional blood-retinal barrier (BRB) formation in mice, we found that immature vessel leakage occurs entirely through transcytosis, as specialized tight junctions are functional as early as vessel entry into the CNS. A functional barrier forms only when transcytosis is gradually suppressed during development. Mutant mice with elevated or reduced levels of transcytosis have delayed or precocious sealing of the BRB, respectively. Therefore, the temporal regulation of transcytosis governs the development of a functional BRB, and suppression of transcytosis is a principal contributor for functional barrier formation.

Association of FOXF2 gene polymorphisms with ischemic stroke in Chinese Han population

Recently, a novel locus at chromosome 6p25 (rs12204590, near FOXF2) associated with an increased risk of stroke in European populations was identified. However, whether polymorphisms in FOXF2 are also associated with the incidence of ischemic stroke in other populations remains unknown. In this case-control study, 803 Chinese Han patients with ischemic stroke and 803 matched control individuals were enrolled. Four tag SNPs and rs12204590 located in or near FOXF2 were selected, and the associations between genotypes/alleles and ischemic stroke were analyzed. In our study, we did not detect an association between the previously reported locus rs12204590 and ischemic stroke. By the genotype analysis, a novel SNP rs1711972, near FOXF2, was observed to be associated with an increased risk of ischemic stroke(CA genotype, adjusted OR = 1.35; 95% CI, 1.07 to 1.70), but not significantly after Bonferroni corrections for multiple tests. However, in the subgroup analysis, we discovered that rs1711972 was associated with an increased risk of large-artery atherosclerotic stroke in the additive model (P = 0.020; CA genotype, adjusted OR = 1.50; 95%CI, 1.09 to 2.07) and dominant model (P = 0.010; OR = 1.47; 95%CI, 1.09 to 1.99). Collectively, these results indicate that a novel SNP near FOXF2 may influence the risk of large-artery atherosclerotic stroke in Chinese Han population.

Pericytes as Inducers of Rapid, Matrix Metalloproteinase-9-Dependent Capillary Damage during Ischemia

Blood-brain barrier disruption (BBB) and release of toxic blood molecules into the brain contributes to neuronal injury during stroke and other cerebrovascular diseases. While pericytes are builders and custodians of the BBB in the normal brain, their impact on BBB integrity during ischemia remains unclear. We imaged pericyte-labeled transgenic mice with in vivo two-photon microscopy to examine the relationship between pericytes and blood plasma leakage during photothrombotic occlusion of cortical capillaries. Upon cessation of capillary flow, we observed that plasma leakage occurred with three times greater frequency in regions where pericyte somata adjoined the endothelium. Pericyte somata covered only 7% of the total capillary length in cortex, indicating that a disproportionate amount of leakage occurred from a small fraction of the capillary bed. Plasma leakage was preceded by rapid activation of matrix metalloproteinase (MMP) at pericyte somata, which was visualized at high resolution in vivo using a fluorescent probe for matrix metalloproteinase-2/9 activity, fluorescein isothiocyanate (FITC)-gelatin. Coinjection of an MMP-9 inhibitor, but not an MMP-2 inhibitor, reduced pericyte-associated FITC-gelatin fluorescence and plasma leakage. These results suggest that pericytes contribute to rapid and localized proteolytic degradation of the BBB during cerebral ischemia.

SIGNIFICANCE STATEMENT

Pericytes are a key component of the neurovascular unit and are essential for normal BBB function. However, during acute ischemia, we find that pericytes are involved in creating rapid and heterogeneous BBB disruption in the capillary bed. The mechanism by which pericytes contribute to BBB damage warrants further investigation, as it may yield new therapeutic targets for acute stroke injury and other neurological diseases involving capillary flow impairment.

Pericytes of the neurovascular unit: key functions and signaling pathways

Pericytes are vascular mural cells embedded in the basement membrane of blood microvessels. They extend their processes along capillaries, pre-capillary arterioles and post-capillary venules. CNS pericytes are uniquely positioned in the neurovascular unit between endothelial cells, astrocytes and neurons. They integrate, coordinate and process signals from their neighboring cells to generate diverse functional responses that are critical for CNS functions in health and disease, including regulation of the blood-brain barrier permeability, angiogenesis, clearance of toxic metabolites, capillary hemodynamic responses, neuroinflammation and stem cell activity. Here we examine the key signaling pathways between pericytes and their neighboring endothelial cells, astrocytes and neurons that control neurovascular functions. We also review the role of pericytes in CNS disorders including rare monogenic diseases and complex neurological disorders such as Alzheimer's disease and brain tumors. Finally, we discuss directions for future studies.

Dual role of pericyte α6β1-integrin in tumour blood vessels

The α6β1-integrin is a major laminin receptor, and formation of a laminin-rich basement membrane is a key feature in tumour blood vessel stabilisation and pericyte recruitment, processes that are important in the growth and maturation of tumour blood vessels. However, the role of pericyte α6β1-integrin in angiogenesis is largely unknown. We developed mice where the α6-integrin subunit is deleted in pericytes and examined tumour angiogenesis and growth. These mice had: (1) reduced pericyte coverage of tumour blood vessels; (2) reduced tumour blood vessel stability; (3) increased blood vessel diameter; (4) enhanced blood vessel leakiness, and (5) abnormal blood vessel basement membrane architecture. Surprisingly, tumour growth, blood vessel density and metastasis were not altered. Analysis of retinas revealed that deletion of pericyte α6-integrin did not affect physiological angiogenesis. At the molecular level, we provide evidence that pericyte α6-integrin controls PDGFRβ expression and AKT-mTOR signalling. Taken together, we show that pericyte α6β1-integrin regulates tumour blood vessels by both controlling PDGFRβ and basement membrane architecture. These data establish a novel dual role for pericyte α6-integrin as modulating the blood vessel phenotype during pathological angiogenesis.