Neuroectodermal origin of brain pericytes and vascular smooth muscle cells

Knowns and unknowns of TiLV-associated neuronal disease

Tilapia Lake Virus (TiLV) is associated with pathological changes in the brain of infected fish, but the mechanisms driving the virus's neuropathogenesis remain poorly characterized. TiLV establishes a persistent infection in the brain of infected fish even when the virus is no longer detectable in the peripheral organs, rendering therapeutic interventions and disease management challenging. Moreover, the persistence of the virus in the brain may pose a risk for viral reinfection and spread and contribute to ongoing tissue damage and neuroinflammatory processes. In this review, we explore TiLV-associated neurological disease. We discuss the possible mechanism(s) used by TiLV to enter the central nervous system (CNS) and examine TiLV-induced neuroinflammation and brain immune responses. Lastly, we discuss future research questions and knowledge gaps to be addressed to significantly advance this field.

Meta-analysis of the make-up and properties of in vitro models of the healthy and diseased blood-brain barrier

In vitro models of the human blood-brain barrier (BBB) are increasingly used to develop therapeutics that can cross the BBB for treating diseases of the central nervous system. Here we report a meta-analysis of the make-up and properties of transwell and microfluidic models of the healthy BBB and of BBBs in glioblastoma, Alzheimer's disease, Parkinson's disease and inflammatory diseases. We found that the type of model, the culture method (static or dynamic), the cell types and cell ratios, and the biomaterials employed as extracellular matrix are all crucial to recapitulate the low permeability and high expression of tight-junction proteins of the BBB, and to obtain high trans-endothelial electrical resistance. Specifically, for models of the healthy BBB, the inclusion of endothelial cells and pericytes as well as physiological shear stresses (~10-20 dyne cm-2) are necessary, and when astrocytes are added, astrocytes or pericytes should outnumber endothelial cells. We expect this meta-analysis to facilitate the design of increasingly physiological models of the BBB.

Pericytes: jack-of-all-trades in cancer-related inflammation

Pericytes, recognized as mural cells, have long been described as components involved in blood vessel formation, playing a mere supporting role for endothelial cells (ECs). Emerging evidence strongly suggests their multifaceted roles in tissues and organs. Indeed, pericytes exhibit a remarkable ability to anticipate endothelial cell behavior and adapt their functions based on the specific cells they interact with. Pericytes can be activated by pro-inflammatory stimuli and crosstalk with immune cells, actively participating in their transmigration into blood vessels. Moreover, they can influence the immune response, often sustaining an immunosuppressive phenotype in most of the cancer types studied. In this review, we concentrate on the intricate crosstalk between pericytes and immune cells in cancer, highlighting the primary evidence regarding pericyte involvement in primary tumor mass dynamics, their contributions to tumor reprogramming for invasion and migration of malignant cells, and their role in the formation of pre-metastatic niches. Finally, we explored recent and emerging pharmacological approaches aimed at vascular normalization, including novel strategies to enhance the efficacy of immunotherapy through combined use with anti-angiogenic drugs.

Exploring the origins of neurodevelopmental proteasomopathies associated with cardiac malformations: are neural crest cells central to certain pathological mechanisms?

Neurodevelopmental proteasomopathies constitute a recently defined class of rare Mendelian disorders, arising from genomic alterations in proteasome-related genes. These alterations result in the dysfunction of proteasomes, which are multi-subunit protein complexes essential for maintaining cellular protein homeostasis. The clinical phenotype of these diseases manifests as a syndromic association involving impaired neural development and multisystem abnormalities, notably craniofacial anomalies and malformations of the cardiac outflow tract (OFT). These observations suggest that proteasome loss-of-function variants primarily affect specific embryonic cell types which serve as origins for both craniofacial structures and the conotruncal portion of the heart. In this hypothesis article, we propose that neural crest cells (NCCs), a highly multipotent cell population, which generates craniofacial skeleton, mesenchyme as well as the OFT of the heart, in addition to many other derivatives, would exhibit a distinctive vulnerability to protein homeostasis perturbations. Herein, we introduce the diverse cellular compensatory pathways activated in response to protein homeostasis disruption and explore their potential implications for NCC physiology. Altogether, the paper advocates for investigating proteasome biology within NCCs and their early cranial and cardiac derivatives, offering a rationale for future exploration and laying the initial groundwork for therapeutic considerations.

Systematic review of cardiovascular neurocristopathy-contemporary insights and future perspectives

Introduction

Neural crest cells (NCCs) are multipotent and are attributed to the combination of complex multimodal gene regulatory mechanisms. Cardiac neural crest (CNC) cells, originating from the dorsal neural tube, are pivotal architects of the cardio-neuro-vascular domain, which orchestrates the embryogenesis of critical cardiac and vascular structures. Remarkably, while the scientific community compiled a comprehensive inventory of neural crest derivatives by the early 1980s, our understanding of the CNC's role in various cardiovascular disease processes still needs to be explored. This review delves into the differentiation of NCC, specifically the CNC cells, and explores the diverse facets of non-syndromic cardiovascular neurocristopathies.

Methods

A systematic review was conducted as per the PRISMA Statement. Three prominent databases, PubMed, Scopus, and Embase, were searched, which yielded 1,840 studies. We excluded 1,796 studies, and the final selection of 44 studies formed the basis of this comprehensive review.

Results

Neurocristopathies are a group of genetic disorders that affect the development of cells derived from the NC. Cardiovascular neurocristopathy, i.e., cardiopathy and vasculopathy, associated with the NCC could occur in the form of (1) cardiac septation disorders, mainly the aortico-pulmonary septum; (2) great vessels and vascular disorders; (3) myocardial dysfunction; and (4) a combination of all three phenotypes. This could result from abnormalities in NCC migration, differentiation, or proliferation leading to structural abnormalities and are attributed to genetic, familial, sporadic or acquired causes.

Discussion

Phenotypic characteristics of cardiovascular neurocristopathies, such as bicuspid aortic valve and thoracic aortic aneurysm, share a common embryonic origin and are surprisingly prevalent in the general population, necessitating further research to identify the underlying pathogenic and genetic factors responsible for these cardiac anomalies. Such discoveries are essential for enhancing diagnostic screening and refining therapeutic interventions, ultimately improving the lives of individuals affected by these conditions.

The contribution of β-amyloid, Tau and α-synuclein to blood-brain barrier damage in neurodegenerative disorders

Central nervous system (CNS) accumulation of fibrillary deposits made of Amyloid β (Aβ), hyperphosphorylated Tau or α-synuclein (α-syn), present either alone or in the form of mixed pathology, characterizes the most common neurodegenerative diseases (NDDs) as well as the aging brain. Compelling evidence supports that acute neurological disorders, such as traumatic brain injury (TBI) and stroke, are also accompanied by increased deposition of toxic Aβ, Tau and α-syn species. While the contribution of these pathological proteins to neurodegeneration has been experimentally ascertained, the cellular and molecular mechanisms driving Aβ, Tau and α-syn-related brain damage remain to be fully clarified. In the last few years, studies have shown that Aβ, Tau and α-syn may contribute to neurodegeneration also by inducing and/or promoting blood-brain barrier (BBB) disruption. These pathological proteins can affect BBB integrity either directly by affecting key BBB components such as pericytes and endothelial cells (ECs) or indirectly, by promoting brain macrophages activation and dysfunction. Here, we summarize and critically discuss key findings showing how Aβ, Tau and α-syn can contribute to BBB damage in most common NDDs, TBI and stroke. We also highlight the need for a deeper characterization of the role of these pathological proteins in the activation and dysfunction of brain macrophages, pericytes and ECs to improve diagnosis and treatment of acute and chronic neurological disorders.

Notch3 directs differentiation of brain mural cells from human pluripotent stem cell-derived neural crest

Brain mural cells regulate development and function of the blood-brain barrier and control blood flow. Existing in vitro models of human brain mural cells have low expression of key mural cell genes, including NOTCH3. Thus, we asked whether activation of Notch3 signaling in hPSC-derived neural crest could direct the differentiation of brain mural cells with an improved transcriptional profile. Overexpression of the Notch3 intracellular domain (N3ICD) induced expression of mural cell markers PDGFRβ, TBX2, FOXS1, KCNJ8, SLC6A12, and endogenous Notch3. The resulting N3ICD-derived brain mural cells produced extracellular matrix, self-assembled with endothelial cells, and had functional KATP channels. ChIP-seq revealed that Notch3 serves as a direct input to relatively few genes in the context of this differentiation process. Our work demonstrates that activation of Notch3 signaling is sufficient to direct the differentiation of neural crest to mural cells and establishes a developmentally relevant differentiation protocol.

Spatially distinct otic mesenchyme cells show molecular and functional heterogeneity patterns before hearing onset

The cochlea consists of diverse cellular populations working in harmony to convert mechanical stimuli into electrical signals for the perception of sound. Otic mesenchyme cells (OMCs), often considered a homogeneous cell type, are essential for normal cochlear development and hearing. Despite being the most numerous cell type in the developing cochlea, OMCs are poorly understood. OMCs are known to differentiate into spatially and functionally distinct cell types, including fibrocytes of the lateral wall and spiral limbus, modiolar osteoblasts, and specialized tympanic border cells of the basilar membrane. Here, we show that OMCs are transcriptionally and functionally heterogeneous and can be divided into four distinct populations that spatially correspond to OMC-derived cochlear structures. We also show that this heterogeneity and complexity of OMCs commences during early phases of cochlear development. Finally, we describe the cell-cell communication network of the developing cochlea, inferring a major role for OMC in outgoing signaling.

Research advances in cochlear pericytes and hearing loss

Pericytes are specialized mural cells surrounding endothelial cells in microvascular beds. They play a role in vascular development, blood flow regulation, maintenance of blood-tissue barrier integrity, and control of angiogenesis, tissue fibrosis, and wound healing. In recent decades, understanding of the critical role played by pericytes in retina, brain, lung, and kidney has seen significant progress. The cochlea contains a large population of pericytes. However, the role of cochlear pericytes in auditory pathophysiology is, by contrast, largely unknown. The present review discusses recent progress in identifying cochlear pericytes, mapping their distribution, and defining their role in regulating blood flow, controlling the blood-labyrinth barrier (BLB) and angiogenesis, and involvement in different types of hearing loss.

Blood-spinal cord barrier disruption in degenerative cervical myelopathy

Degenerative cervical myelopathy (DCM) is the most prevalent cause of spinal cord dysfunction in the aging population. Significant neurological deficits may result from a delayed diagnosis as well as inadequate neurological recovery following surgical decompression. Here, we review the pathophysiology of DCM with an emphasis on how blood-spinal cord barrier (BSCB) disruption is a critical yet neglected pathological feature affecting prognosis. In patients suffering from DCM, compromise of the BSCB is evidenced by elevated cerebrospinal fluid (CSF) to serum protein ratios and abnormal contrast-enhancement upon magnetic resonance imaging (MRI). In animal model correlates, there is histological evidence of increased extravasation of tissue dyes and serum contents, and pathological changes to the neurovascular unit. BSCB dysfunction is the likely culprit for ischemia-reperfusion injury following surgical decompression, which can result in devastating neurological sequelae. As there are currently no therapeutic approaches specifically targeting BSCB reconstitution, we conclude the review by discussing potential interventions harnessed for this purpose.

Biology of vascular mural cells

The vasculature consists of vessels of different sizes that are arranged in a hierarchical pattern. Two cell populations work in concert to establish this pattern during embryonic development and adopt it to changes in blood flow demand later in life: endothelial cells that line the inner surface of blood vessels, and adjacent vascular mural cells, including smooth muscle cells and pericytes. Despite recent progress in elucidating the signalling pathways controlling their crosstalk, much debate remains with regard to how mural cells influence endothelial cell biology and thereby contribute to the regulation of blood vessel formation and diameters. In this Review, I discuss mural cell functions and their interactions with endothelial cells, focusing on how these interactions ensure optimal blood flow patterns. Subsequently, I introduce the signalling pathways controlling mural cell development followed by an overview of mural cell ontogeny with an emphasis on the distinguishing features of mural cells located on different types of blood vessels. Ultimately, I explore therapeutic strategies involving mural cells to alleviate tissue ischemia and improve vascular efficiency in a variety of diseases.

Single-cell analysis of shared signatures and transcriptional diversity during zebrafish development

During development, animals generate distinct cell populations with specific identities, functions, and morphologies. We mapped transcriptionally distinct populations across 489,686 cells from 62 stages during wild-type zebrafish embryogenesis and early larval development (3-120 hours post-fertilization). Using these data, we identified the limited catalog of gene expression programs reused across multiple tissues and their cell-type-specific adaptations. We also determined the duration each transcriptional state is present during development and suggest new long-term cycling populations. Focused analyses of non-skeletal muscle and the endoderm identified transcriptional profiles of understudied cell types and subpopulations, including the pneumatic duct, individual intestinal smooth muscle layers, spatially distinct pericyte subpopulations, and homologs of recently discovered human best4+ enterocytes. The transcriptional regulators of these populations remain unknown, so we reconstructed gene expression trajectories to suggest candidates. To enable additional discoveries, we make this comprehensive transcriptional atlas of early zebrafish development available through our website, Daniocell.

Bone Marrow Stem Cells Derived from Nerves Have Neurogenic Properties and Potential Utility for Regenerative Therapy

Neurons and glia of the peripheral nervous system are derived from progenitor cell populations, originating from embryonic neural crest. The neural crest and vasculature are intimately associated during embryonic development and in the mature central nervous system, in which they form a neurovascular unit comprised of neurons, glia, pericytes, and vascular endothelial cells that play important roles in health and disease. Our group and others have previously reported that postnatal populations of stem cells originating from glia or Schwann cells possess neural stem cell qualities, including rapid proliferation and differentiation into mature glia and neurons. Bone marrow receives sensory and sympathetic innervation from the peripheral nervous system and is known to contain myelinating and unmyelinating Schwann cells. Herein, we describe a population of neural crest-derived Schwann cells residing in a neurovascular niche of bone marrow in association with nerve fibers. These Schwann cells can be isolated and expanded. They demonstrate plasticity in vitro, generating neural stem cells that exhibit neurogenic potential and form neural networks within the enteric nervous system in vivo following transplantation to the intestine. These cells represent a novel source of autologous neural stem cells for the treatment of neurointestinal disorders.

The Active Role of Pericytes During Neuroinflammation in the Adult Brain

Microvessels in the central nervous system (CNS) have one of the highest populations of pericytes, indicating their crucial role in maintaining homeostasis. Pericytes are heterogeneous cells located around brain microvessels; they present three different morphologies along the CNS vascular tree: ensheathing, mesh, and thin-strand pericytes. At the arteriole-capillary transition ensheathing pericytes are found, while mesh and thin-strand pericytes are located at capillary beds. Brain pericytes are essential for the establishment and maintenance of the blood-brain barrier, which restricts the passage of soluble and potentially toxic molecules from the circulatory system to the brain parenchyma. Pericytes play a key role in regulating local inflammation at the CNS. Pericytes can respond differentially, depending on the degree of inflammation, by secreting a set of neurotrophic factors to promote cell survival and regeneration, or by potentiating inflammation through the release of inflammatory mediators (e.g., cytokines and chemokines), and the overexpression of cell adhesion molecules. Under inflammatory conditions, pericytes may regulate immune cell trafficking to the CNS and play a role in perpetuating local inflammation. In this review, we describe pericyte responses during acute and chronic neuroinflammation.

Approaches for the isolation and long-term expansion of pericytes from human and animal tissues

Pericytes surround capillaries in every organ of the human body. They are also present around the vasa vasorum, the small blood vessels that supply the walls of larger arteries and veins. The clinical interest in pericytes is rapidly growing, with the recognition of their crucial roles in controlling vascular function and possible therapeutic applications in regenerative medicine. Nonetheless, discrepancies in methods used to define, isolate, and expand pericytes are common and may affect reproducibility. Separating pure pericyte preparations from the continuum of perivascular mesenchymal cells is challenging. Moreover, variations in functional behavior and antigenic phenotype in response to environmental stimuli make it difficult to formulate an unequivocal definition of bona fide pericytes. Very few attempts were made to develop pericytes as a clinical-grade product. Therefore, this review is devoted to appraising current methodologies' pros and cons and proposing standardization and harmonization improvements. We highlight the importance of developing upgraded protocols to create therapeutic pericyte products according to the regulatory guidelines for clinical manufacturing. Finally, we describe how integrating RNA-seq techniques with single-cell spatial analysis, and functional assays may help realize the full potential of pericytes in health, disease, and tissue repair.

Cultured brain pericytes adopt an immature phenotype and require endothelial cells for expression of canonical markers and ECM genes

Pericytes (PCs) are essential components of the blood brain barrier. Brain PCs are critical for dynamically regulating blood flow, for maintaining vascular integrity and their dysregulation is associated with a myriad of disorders such as Alzheimer's disease. To understand their physiological and molecular functions, studies have increasingly focused on primary brain PC isolation and culture. Multiple methods for PC culture have been developed over the years, however, it is still unclear how primary PCs compare to their in vivo counterparts. To address this question, we compared cultured brain PCs at passage 5 and 20 to adult and embryonic brain PCs directly isolated from mouse brains via single cell RNA-seq. Cultured PCs were highly homogeneous, and were most similar to embryonic PCs, while displaying a significantly different transcriptional profile to adult brain PCs. Cultured PCs downregulated canonical PC markers and extracellular matrix (ECM) genes. Importantly, expression of PC markers and ECM genes could be improved by co-culture with brain endothelial cells, showing the importance of the endothelium in maintaining PC identity and function. Taken together, these results highlight key transcriptional differences between cultured and in vivo PCs which should be considered when performing in vitro experiments with brain PCs.

The Multiple Roles of Pericytes in Vascular Formation and Microglial Functions in the Brain

In the capillary walls, vascular endothelial cells are covered with mural cells, such as smooth muscle cells and pericytes. Although pericytes had been thought to play simply a structural role, emerging evidence has highlighted their multiple functions in the embryonic, postnatal, and adult brain. As the central nervous system (CNS) develops, the brain's vascular structure gradually matures into a hierarchical network, which is crucial for the proper development of neural lineage cells by providing oxygen and nutrients. Pericytes play an essential role in vascular formation and regulate blood‒brain barrier (BBB) integrity as a component of the neurovascular unit (NVU), in collaboration with other cells, such as vascular endothelial cells, astrocytes, neurons, and microglia. Microglia, the resident immune cells of the CNS, colonize the brain at embryonic day (E) 9.5 in mice. These cells not only support the development and maturation of neural lineage cells but also help in vascular formation through their extensive migration. Recent studies have demonstrated that pericytes directly contact microglia in the CNS, and their interactions have a profound effect on physiological and pathological aspects. This review summarizes the function of pericytes, focusing on the interplay between pericytes and microglia.

Hepatic leukemia factor-expressing paraxial mesoderm cells contribute to the developing brain vasculature

Recent genetic lineage tracing studies reveal heterogeneous origins of vascular endothelial cells and pericytes in the developing brain vasculature, despite classical experimental evidence for a mesodermal origin. Here we provide evidence through a genetic lineage tracing experiment that cephalic paraxial mesodermal cells give rise to endothelial cells and pericytes in the developing mouse brain. We show that Hepatic leukemia factor (Hlf) is transiently expressed by cephalic paraxial mesenchyme at embryonic day (E) 8.0-9.0 and the genetically-marked E8.0 Hlf-expressing cells mainly contribute to the developing brain vasculature. Interestingly, the genetically-marked E10.5 Hlf-expressing cells, which have been previously reported to contain embryonic hematopoietic stem cells, fail to contribute to the vascular cells. Combined, our genetic lineage tracing data demonstrate that a transient expression of Hlf marks a cephalic paraxial mesenchyme contributing to the developing brain vasculature.

Targeting Fibronectin to Overcome Remyelination Failure in Multiple Sclerosis: The Need for Brain- and Lesion-Targeted Drug Delivery

Multiple sclerosis (MS) is a neuroinflammatory and neurodegenerative disease with unknown etiology that can be characterized by the presence of demyelinated lesions. Prevailing treatment protocols in MS rely on the modulation of the inflammatory process but do not impact disease progression. Remyelination is an essential factor for both axonal survival and functional neurological recovery but is often insufficient. The extracellular matrix protein fibronectin contributes to the inhibitory environment created in MS lesions and likely plays a causative role in remyelination failure. The presence of the blood-brain barrier (BBB) hinders the delivery of remyelination therapeutics to lesions. Therefore, therapeutic interventions to normalize the pathogenic MS lesion environment need to be able to cross the BBB. In this review, we outline the multifaceted roles of fibronectin in MS pathogenesis and discuss promising therapeutic targets and agents to overcome fibronectin-mediated inhibition of remyelination. In addition, to pave the way for clinical use, we reflect on opportunities to deliver MS therapeutics to lesions through the utilization of nanomedicine and discuss strategies to deliver fibronectin-directed therapeutics across the BBB. The use of well-designed nanocarriers with appropriate surface functionalization to cross the BBB and target the lesion sites is recommended.

Extracellular vesicles through the blood-brain barrier: a review

Extracellular vesicles (EVs) are particles naturally released from cells that are delimited by a lipid bilayer and are unable to replicate. How the EVs cross the Blood–Brain barrier (BBB) in a bidirectional manner between the bloodstream and brain parenchyma remains poorly understood. Most in vitro models that have evaluated this event have relied on monolayer transwell or microfluidic organ-on-a-chip techniques that do not account for the combined effect of all cellular layers that constitute the BBB at different sites of the Central Nervous System. There has not been direct transcytosis visualization through the BBB in mammals in vivo, and evidence comes from in vivo experiments in zebrafish. Literature is scarce on this topic, and techniques describing the mechanisms of EVs motion through the BBB are inconsistent. This review will focus on in vitro and in vivo methodologies used to evaluate EVs transcytosis, how EVs overcome this fundamental structure, and discuss potential methodological approaches for future analyses to clarify these issues. Understanding how EVs cross the BBB will be essential for their future use as vehicles in pharmacology and therapeutics.

Cell fate determining molecular switches and signaling pathways in Pax7-expressing somitic mesoderm

During development, different cell types originate from a common progenitor at well-defined time points. Previous lineage-tracing of Pax7⁺ progenitors from the somitic mesoderm has established its developmental trajectory towards the dermis, brown adipocytes, and skeletal muscle in the dorsal trunk; yet the molecular switches and mechanisms guiding the differentiation into different lineages remain unknown. We performed lineage-tracing of Pax7-expressing cells in mouse embryos at E9.5 and profiled the transcriptomes of Pax7-progenies on E12.5, E14.5, and E16.5 at single-cell level. Analysis of single-cell transcriptomic data at multiple time points showed temporal-specific differentiation events toward muscle, dermis, and brown adipocyte, identified marker genes for putative progenitors and revealed transcription factors that could drive lineage-specific differentiation. We then utilized a combination of surface markers identified in the single-cell data, Pdgfra, Thy1, and Cd36, to enrich brown adipocytes, dermal fibroblasts, and progenitors specific for these two cell types at E14.5 and E16.5. These enriched cell populations were then used for further culture and functional assays in vitro, in which Wnt5a and Rgcc are shown to be important factors that could alter lineage decisions during embryogenesis. Notably, we found a bipotent progenitor population at E14.5, having lineage potentials towards both dermal fibroblasts and brown adipocytes. They were termed eFAPs (embryonic fibro/adipogenic progenitors) as they functionally resemble adult fibro/adipogenic progenitors. Overall, this study provides further understanding of the Pax7 lineage during embryonic development using a combination of lineage tracing with temporally sampled single-cell transcriptomics.

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.

Central Nervous System Pericytes Contribute to Health and Disease

Successful neuroprotection is only possible with contemporary microvascular protection. The prevention of disease-induced vascular modifications that accelerate brain damage remains largely elusive. An improved understanding of pericyte (PC) signalling could provide important insight into the function of the neurovascular unit (NVU), and into the injury-provoked responses that modify cell-cell interactions and crosstalk. Due to sharing the same basement membrane with endothelial cells, PCs have a crucial role in the control of endothelial, astrocyte, and oligodendrocyte precursor functions and hence blood-brain barrier stability. Both cerebrovascular and neurodegenerative diseases impair oxygen delivery and functionally impair the NVU. In this review, the role of PCs in central nervous system health and disease is discussed, considering their origin, multipotency, functions and also dysfunction, focusing on new possible avenues to modulate neuroprotection. Dysfunctional PC signalling could also be considered as a potential biomarker of NVU pathology, allowing us to individualize therapeutic interventions, monitor responses, or predict outcomes.

KCNJ8/ABCC9-containing K-ATP channel modulates brain vascular smooth muscle development and neurovascular coupling

Loss- or gain-of-function mutations in ATP-sensitive potassium channel (K-ATP)-encoding genes, KCNJ8 and ABCC9, cause human central nervous system disorders with unknown pathogenesis. Here, using mice, zebrafish, and cell culture models, we investigated cellular and molecular causes of brain dysfunctions derived from altered K-ATP channel function. We show that genetic/chemical inhibition or activation of KCNJ8/ABCC9-containing K-ATP channel function leads to brain-selective suppression or promotion of arterial/arteriolar vascular smooth muscle cell (VSMC) differentiation, respectively. We further show that brain VSMCs develop from KCNJ8/ABCC9-containing K-ATP channel-expressing mural cell progenitor and that K-ATP channel cell autonomously regulates VSMC differentiation through modulation of intracellular Ca²⁺ oscillation via voltage-dependent calcium channels. Consistent with defective VSMC development, Kcnj8 knockout mice showed deficiency in vasoconstrictive capacity and neuronal-evoked vasodilation leading to local hyperemia. Our results demonstrate a role for KCNJ8/ABCC9-containing K-ATP channels in the differentiation of brain VSMC, which in turn is necessary for fine-tuning of cerebral blood flow.

Vascular Stem/Progenitor Cells in Vessel Injury and Repair

Vascular repair upon vessel injury is essential for the maintenance of arterial homeostasis and function. Stem/progenitor cells were demonstrated to play a crucial role in regeneration and replenishment of damaged vascular cells during vascular repair. Previous studies revealed that myeloid stem/progenitor cells were the main sources of tissue regeneration after vascular injury. However, accumulating evidences from developing lineage tracing studies indicate that various populations of vessel-resident stem/progenitor cells play specific roles in different process of vessel injury and repair. In response to shear stress, inflammation, or other risk factors-induced vascular injury, these vascular stem/progenitor cells can be activated and consequently differentiate into different types of vascular wall cells to participate in vascular repair. In this review, mechanisms that contribute to stem/progenitor cell differentiation and vascular repair are described. Targeting these mechanisms has potential to improve outcome of diseases that are characterized by vascular injury, such as atherosclerosis, hypertension, restenosis, and aortic aneurysm/dissection. Future studies on potential stem cell-based therapy are also highlighted.

Midbrain organoids mimic early embryonic neurodevelopment and recapitulate LRRK2-p.Gly2019Ser-associated gene expression

Human brain organoid models that recapitulate the physiology and complexity of the human brain have a great potential for in vitro disease modeling, in particular for neurodegenerative diseases, such as Parkinson disease. In the present study, we compare single-cell RNA-sequencing data of human midbrain organoids to the developing human embryonic midbrain. We demonstrate that the in vitro model is comparable to its in vivo equivalents in terms of developmental path and cellular composition. Moreover, we investigate the potential of midbrain organoids for modeling early developmental changes in Parkinson disease. Therefore, we compare the single-cell RNA-sequencing data of healthy-individual-derived midbrain organoids to their isogenic LRRK2-p.Gly2019Ser-mutant counterparts. We show that the LRRK2 p.Gly2019Ser variant alters neurodevelopment, resulting in an untimely and incomplete differentiation with reduced cellular variability. Finally, we present four candidate genes, APP, DNAJC6, GATA3, and PTN, that might contribute to the LRRK2-p.Gly2019Ser-associated transcriptome changes that occur during early neurodevelopment.

Efferocytosis of vascular cells in cardiovascular disease

Cell death and the clearance of apoptotic cells are tightly regulated by various signaling molecules in order to maintain physiological tissue function and homeostasis. The phagocytic removal of apoptotic cells is known as the process of efferocytosis, and abnormal efferocytosis is linked to various health complications and diseases, such as cardiovascular disease, inflammatory diseases, and autoimmune diseases. During efferocytosis, phagocytic cells and/or apoptotic cells release signals, such as "find me" and "eat me" signals, to stimulate the phagocytic engulfment of apoptotic cells. Primary phagocytic cells are macrophages and dendritic cells; however, more recently, other neighboring cell types have also been shown to exhibit phagocytic character, including endothelial cells and fibroblasts, although they are comparatively slower in clearing dead cells. In this review, we focus on macrophage efferocytosis of vascular cells, such as endothelial cells, smooth muscle cells, fibroblasts, and pericytes, and its relation to the progression and development of cardiovascular disease. We also highlight the role of efferocytosis-related molecules and their contribution to the maintenance of vascular homeostasis.

Studying the Inflammatory Responses to Amyloid Beta Oligomers in Brain-Specific Pericyte and Endothelial Co-culture from Human Stem Cells

Background

Recently, the in vitro blood brain barrier (BBB) models derived from human pluripotent stem cells have been given extensive attention in therapeutics due to the implications it has with the health of the central nervous system. It is essential to create an accurate BBB model in vitro in order to better understand the properties of the BBB and how it can respond to inflammatory stimulation and be passed by targeted or non-targeted cell therapeutics, more specifically extracellular vesicles.

Methods

Brain-specific pericytes (iPCs) were differentiated from iPSK3 cells using dual SMAD signaling inhibitors and Wnt activation plus fibroblast growth factor 2 (FGF-2). The derived cells were characterized by immunostaining, flow cytometry and RT-PCR. In parallel, blood vessels organoids were derived using Wnt activation, BMP4, FGF2, VEGF and SB431542. The organoids were replated and treated with retinoic acid to enhance the blood brain barrier (BBB) features in the differentiated brain endothelial cells (iECs). Co-culture was performed for the iPCs and iECs in transwell system and 3-D microfluidics channels.

Results

The derived iPCs expressed common markers PDGFRb and NG2, as well as brain-specific genes FOXF2, ABCC9, KCNJ8, and ZIC1. The derived iECs expressed common endothelial cell markers CD31, VE-cadherin, as well as BBB-associated genes BRCP, GLUT-1, PGP, ABCC1, OCLN, SLC2A1. The co-culture of the two cell types responded to the stimulation of amyloid β42 oligomers by the upregulation of expression of TNFa, IL6, NFKB, Casp3, SOD2 and TP53. The co-culture also showed the property of trans-endothelial electrical resistance. The proof-of-concept vascularization strategy was demonstrated in a 3-D microfluidics-based device.

Conclusion

The derived iPCs and iECs have brain-specific properties and the co-culture of iPCs and iECs provides an in vitro BBB model that show inflammatory response. This study has significance in establishing micro-physiological systems for neurological disease modeling and drug screening.

Integrated molecular analysis identifies a conserved pericyte gene signature in zebrafish

Pericytes reside in capillary beds where they share a basement membrane with endothelial cells and regulate their function. However, little is known about embryonic pericyte development, in part, due to lack of specific molecular markers and genetic tools. Here, we applied single cell RNA-sequencing (scRNA-seq) of platelet derived growth factor beta (pdgfrb)-positive cells to molecularly characterize pericytes in zebrafish larvae. scRNA-seq revealed zebrafish cells expressing mouse pericyte gene orthologs, and comparison with bulk RNA-seq from wild-type and pdgfrb mutant larvae further refined a pericyte gene set. Subsequent integration with mouse pericyte scRNA-seq profiles revealed a core set of conserved pericyte genes. Using transgenic reporter lines, we validated pericyte expression of two genes identified in our analysis: NDUFA4 mitochondrial complex associated like 2a (ndufa4l2a), and potassium voltage-gated channel, Isk-related family, member 4 (kcne4). Both reporter lines exhibited pericyte expression in multiple anatomical locations, and kcne4 was also detected in a subset of vascular smooth muscle cells. Thus, our integrated molecular analysis revealed a molecular profile for zebrafish pericytes and allowed us to develop new tools to observe these cells in vivo.

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

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

Integrative analysis of the human brain mural cell transcriptome

Brain mural cells, including pericytes and vascular smooth muscle cells, are important for vascular development, blood-brain barrier function, and neurovascular coupling, but the molecular characteristics of human brain mural cells are incompletely characterized. Single cell RNA-sequencing (scRNA-seq) is increasingly being applied to assess cellular diversity in the human brain, but the scarcity of mural cells in whole brain samples has limited their molecular profiling. Here, we leverage the combined power of multiple independent human brain scRNA-seq datasets to build a transcriptomic database of human brain mural cells. We use this combined dataset to determine human-mouse species differences in mural cell transcriptomes, culture-induced dedifferentiation of human brain pericytes, and human mural cell organotypicity, with several key findings validated by RNA fluorescence in situ hybridization. Together, this work improves knowledge regarding the molecular constituents of human brain mural cells, serves as a resource for hypothesis generation in understanding brain mural cell function, and will facilitate comparative evaluation of animal and in vitro models.

EBF1 is expressed in pericytes and contributes to pericyte cell commitment

Early B-cell factor-1 (EBF1) is a transcription factor with an important role in cell lineage specification and commitment during the early stage of cell maturation. Originally described during B-cell maturation, EBF1 was subsequently identified as a crucial molecule for proper cell fate commitment of mesenchymal stem cells into adipocytes, osteoblasts and muscle cells. In vessels, EBF1 expression and function have never been documented. Our data indicate that EBF1 is highly expressed in peri-endothelial cells in both tumor vessels and in physiological conditions. Immunohistochemistry, quantitative reverse transcription polymerase chain reaction (RT-qPCR) and fluorescence-activated cell sorting (FACS) analysis suggest that EBF1-expressing peri-endothelial cells represent bona fide pericytes and selectively express well-recognized markers employed in the identification of the pericyte phenotype (SMA, PDGFRβ, CD146, NG2). This observation was also confirmed in vitro in human placenta-derived pericytes and in human brain vascular pericytes (HBVP). Of note, in accord with the key role of EBF1 in the cell lineage commitment of mesenchymal stem cells, EBF1-silenced HBVP cells showed a significant reduction in PDGFRβ and CD146, but not CD90, a marker mostly associated with a prominent mesenchymal phenotype. Moreover, the expression levels of VEGF, angiopoietin-1, NG2 and TGF-β, cytokines produced by pericytes during angiogenesis and linked to their differentiation and activation, were also significantly reduced. Overall, the data suggest a functional role of EBF1 in the cell fate commitment toward the pericyte phenotype.

Pericytes for Therapeutic Approaches to Ischemic Stroke

Pericytes are perivascular multipotent cells located on capillaries. Although pericytes are discovered in the nineteenth century, recent studies have found that pericytes play an important role in maintaining the blood—brain barrier (BBB) and regulating the neurovascular system. In the neurovascular unit, pericytes perform their functions by coordinating the crosstalk between endothelial, glial, and neuronal cells. Dysfunction of pericytes can lead to a variety of diseases, including stroke and other neurological disorders. Recent studies have suggested that pericytes can serve as a therapeutic target in ischemic stroke. In this review, we first summarize the biology and functions of pericytes in the central nervous system. Then, we focus on the role of dysfunctional pericytes in the pathogenesis of ischemic stroke. Finally, we discuss new therapies for ischemic stroke based on targeting pericytes.

Distinct Fibroblast Lineages Give Rise to NG2+ Pericyte Populations in Mouse Skin Development and Repair

We have examined the developmental origins of Ng2+ perivascular cell populations that adhere to the basement membrane of blood vessels, and their contribution to wound healing. Neural/glial antigen 2 (Ng2) labeled most perivascular cells (70-80%) in developing and adult mouse back skin, a higher proportion than expressed by other pericyte markers Tbx18, Nestin and Pdgfrβ. In adult mouse back skin Ng2+ perivascular cells could be categorized into 4 populations based on whether they expressed Pdgfrα and Pdgfrβ individually or in combination or were Pdgfr-negative. Lineage tracing demonstrated that although Ng2+ cells in embryonic and neonatal back skin contributed to multiple cell types they did not give rise to interfollicular fibroblasts within the dermis. Lineage tracing of distinct fibroblast populations during skin development showed that papillary fibroblasts (Lrig1+) gave rise to Ng2+ perivascular cells in the upper dermis, whilst Ng2+ perivascular cells in the lower dermis were primarily derived from reticular Dlk1+ fibroblasts. Following wounding of adult skin, Ng2+ dermal cells only give rise to Ng2+ blood vessel associated cells and did not contribute to other fibroblast lineages. The relative abundance of Ng2+ Pdgfrβ+ perivascular populations was comparable in wounded and non-wounded skin, indicating that perivascular heterogeneity was maintained during full thickness skin repair. In the wound bed Ng2+ perivascular populations were primarily derived from Lrig1+ papillary or Dlk1+ reticular fibroblast lineages, according to the location of the regenerating blood vessels. We conclude that Ng2+ perivascular cells represent a heterogeneous lineage restricted population that is primarily recruited from the papillary or reticular fibroblast lineages during tissue regeneration.

ATP-binding cassette transporters at the zebrafish blood-brain barrier and the potential utility of the zebrafish as an in vivo model

The brain is protected from toxins by a tightly regulated network of specialized cells, including endothelial cells, pericytes, astrocyes, and neurons, known collectively as the blood-brain barrier (BBB). This selectively permeable barrier permits only the most crucial molecules essential for brain function to enter and employs a number of different mechanisms to prevent the entry of potentially harmful toxins and pathogens. In addition to a physical barrier comprised of endothelial cells that form tight junctions to restrict paracellular transport, there is an active protective mechanism made up of energy-dependent transporters that efflux compounds back into the bloodstream. Two of these ATP-binding cassette (ABC) transporters are highly expressed at the BBB: P-glycoprotein (P-gp, encoded by the ABCB1 gene) and ABCG2 (encoded by the ABCG2 gene). Although a number of in vitro and in vivo systems have been developed to examine the role that ABC transporters play in keeping compounds out of the brain, all have inherent advantages and disadvantages. Zebrafish (Danio rerio) have become a model of interest for studies of the BBB due to the similarities between the zebrafish and mammalian BBB systems. In this review, we discuss what is known about ABC transporters in zebrafish and what information is still needed before the zebrafish can be recommended as a model to elucidate the role of ABC transporters at the BBB.

Understanding the Heterogeneity of Human Pericyte Subsets in Blood-Brain Barrier Homeostasis and Neurological Diseases

Pericytes are increasingly recognized as being important in the control of blood–brain barrier permeability and vascular flow. Research on this important cell type has been hindered by widespread confusion regarding the phenotypic identity and nomenclature of pericytes and other perivascular cell types. In addition, pericyte heterogeneity and mouse–human species differences have contributed to confusion. Herein we summarize our present knowledge on the identification of pericytes and pericyte subsets in humans, primarily focusing on recent findings in humans and nonhuman primates. Precise identification and definition of pericytes and pericyte subsets in humans may help us to better understand pericyte biology and develop new therapeutic approaches specifically targeting disease-associated pericyte subsets.

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.

Emerging Role of Pericytes and Their Secretome in the Heart

Pericytes, as mural cells covering microvascular capillaries, play an essential role in vascular remodeling and maintaining vascular functions and blood flow. Pericytes are crucial participants in the physiological and pathological processes of cardiovascular disease. They actively interact with endothelial cells, vascular smooth muscle cells (VSMCs), fibroblasts, and other cells via the mechanisms involved in the secretome. The secretome of pericytes, along with diverse molecules including proinflammatory cytokines, angiogenic growth factors, and the extracellular matrix (ECM), has great impacts on the formation, stabilization, and remodeling of vasculature, as well as on regenerative processes. Emerging evidence also indicates that pericytes work as mesenchymal cells or progenitor cells in cardiovascular regeneration. Their capacity for differentiation also contributes to vascular remodeling in different ways. Previous studies primarily focused on the roles of pericytes in organs such as the brain, retina, lung, and kidney; very few studies have focused on pericytes in the heart. In this review, following a brief introduction of the origin and fundamental characteristics of pericytes, we focus on pericyte functions and mechanisms with respect to heart disease, ending with the promising use of cardiac pericytes in the treatment of ischemic heart failure.

Differentiation of Brain Pericyte-Like Cells from Human Pluripotent Stem Cell-Derived Neural Crest

Brain pericytes regulate diverse aspects of neurovascular development and function, including blood-brain barrier (BBB) induction and maintenance. Primary brain pericytes have been widely employed in coculture-based in vitro models of the BBB, and a method to generate brain pericytes from human pluripotent stem cells (hPSCs) could provide a renewable, genetically tractable source of cells for BBB modeling and studying pericyte roles in development and disease. Here, we describe a protocol to differentiate hPSCs to NG2+ PDGFRβ+ αSMAlow brain pericyte-like cells in 22-25 days through a p75-NGFR+ HNK-1+ neural crest intermediate, which mimics the developmental origin of forebrain pericytes. The resulting brain pericyte-like cells have molecular and functional attributes of brain pericytes. We also provide protocols for maintenance, cryopreservation, and recovery of the neural crest intermediate, and for molecular and functional characterization of the resulting cells. © 2021 Wiley Periodicals LLC. Basic Protocol 1: Differentiation of hPSCs to neural crest Basic Protocol 2: Differentiation of neural crest to brain pericyte-like cells Support Protocol 1: Flow cytometry analysis of neural crest cells Support Protocol 2: Maintenance, cryopreservation, and recovery of neural crest cells Support Protocol 3: Molecular characterization of brain pericyte-like cells Support Protocol 4: Cord formation assay with endothelial cells and brain pericyte-like cells.

Sox10-cre BAC transgenes reveal temporal restriction of mesenchymal cranial neural crest and identify glandular Sox10 expression

Diversity of neural crest derivatives has been studied with a variety of approaches during embryonic development. In mammals Cre-LoxP lineage tracing is a robust means to fate map neural crest relying on cre driven from regulatory elements of early neural crest genes. Sox10 is an essential transcription factor for normal neural crest development. A variety of efforts have been made to label neural crest derivatives using partial Sox10 regulatory elements to drive cre expression. To date published Sox10-cre lines have focused primarily on lineage tracing in specific tissues or during early fetal development. We describe two new Sox10-cre BAC transgenes, constitutive (cre) and inducible (cre/ER^T2), that contain the complete repertoire of Sox10 regulatory elements. We present a thorough expression profile of each, identifying a few novel sites of Sox10 expression not captured by other neural crest cre drivers. Comparative mapping of expression patterns between the Sox10-cre and Sox10-cre/ER^T2 transgenes identified a narrow temporal window in which Sox10 expression is present in mesenchymal derivatives prior to becoming restricted to neural elements during embryogenesis. In more caudal structures, such as the intestine and lower urinary tract, our Sox10-cre BAC transgene appears to be more efficient in labeling neural crest-derived cell types than Wnt1-cre. The analysis reveals consistent expression of Sox10 in non-neural crest derived glandular epithelium, including salivary, mammary, and urethral glands of adult mice. These Sox10-cre and Sox10-cre/ER^T2 transgenic lines are verified tools that will enable refined temporal and cell-type specific lineage analysis of neural crest derivatives as well as glandular tissues that rely on Sox10 for proper development and function.

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.

Ischemia-Induced Multipotent Stem Cells Isolated from Stroke Patients Exhibit Higher Neurogenic Differentiation Potential than Bone Marrow-Derived Mesenchymal Stem Cells

Perivascular areas of the brain harbor multipotent stem cells. We recently demonstrated that after a stroke, brain pericytes exhibit features of multipotent stem cells. Moreover, these ischemia-induced multipotent stem cells (iSCs) are present within ischemic areas of the brain of patients diagnosed with stroke. Although increasing evidence shows that iSCs have traits similar to those of mesenchymal stem cells (MSCs), the phenotypic similarities and differences between iSCs and MSCs remain unclear. In this study, we used iSCs extracted from stroke patients (h-iSCs) and compared their neurogenic potential with that of human MSCs (h-MSCs) in vitro. Microarray analysis, fluorescence-activated cell sorting, immunohistochemistry, and multielectrode array were performed to compare the characteristics of h-iSCs and h-MSCs. Although h-iSCs and h-MSCs had similar gene expression profiles, the percentage expressing the neural stem/progenitor cell marker nestin was significantly higher in h-iSCs than in h-MSCs. Consistent with these findings, h-iSCs, but not h-MSCs, differentiated into electrophysiologically functional neurons. In contrast, although both h-iSCs and h-MSCs were able to differentiate into several mesodermal lineages, including adipocytes, osteocytes, and chondrocytes, the potential of h-iSCs to differentiate into adipocytes and osteocytes was relatively low. These results suggest that compared with h-MSCs, h-iSCs predominantly exhibit neural rather than mesenchymal lineages. In addition, these results indicate that h-iSCs have the potential to repair the injured brain of patients with stroke by directly differentiating into neuronal lineages.

Multifaceted roles of pericytes in central nervous system homeostasis and disease

Pericytes, the mural cells surrounding microcirculation, are gaining increasing attention for their roles in health and disease of the central nervous system (CNS). As an essential part of the neurovascular unit (NVU), pericytes are actively engaged in interactions with neighboring cells and work in synergy with them to maintain homeostasis of the CNS, such as maintaining the blood-brain barrier (BBB), regulating cerebral blood flow (CBF) and the glymphatic system as well as mediating immune responses. However, the dysfunction of pericytes may contribute to the progression of various pathologies. In this review, we discuss: (1) origin of pericytes and different pericyte markers; (2) interactions of pericytes with endothelial cells (ECs), astrocytes, microglia, oligodendrocytes, and neurons; (3) physiological roles of pericytes in the CNS; (4) effects of pericytes in different CNS diseases; (5) relationship of pericytes with extracellular vesicles (EVs) and microRNAs (miRs); (6) recent advances in pericytes studies and future perspective.

Research advances in pericyte function and their roles in diseases

Pericyte, a kind of pluripotent cell, may regulate the irrigation flow and permeability of microcirculation. Pericytes are similar to the smooth muscle cells, which express several kinds of contractile proteins and have contractility. The dysfunction of pericytes is related to many microvascular diseases, including hypoxia, hypertension, diabetic retinopathy, fibrosis, inflammation, Alzheimer's disease, multiple sclerosis, and tumor formation. For a long time, their existence and function have been neglected. The distribution, structure, biomarker, related signaling pathways as well as the roles of pericytes on vascular diseases will be introduced in this review.

The pericyte microenvironment during vascular development

Vascular pericytes provide critical contributions to the formation and integrity of the blood vessel wall within the microcirculation. Pericytes maintain vascular stability and homeostasis by promoting endothelial cell junctions and depositing extracellular matrix (ECM) components within the vascular basement membrane, among other vital functions. As their importance in sustaining microvessel health within various tissues and organs continues to emerge, so does their role in a number of pathological conditions including cancer, diabetic retinopathy, and neurological disorders. Here, we review vascular pericyte contributions to the development and remodeling of the microcirculation, with a focus on the local microenvironment during these processes. We discuss observations of their earliest involvement in vascular development and essential cues for their recruitment to the remodeling endothelium. Pericyte involvement in the angiogenic sprouting context is also considered with specific attention to crosstalk with endothelial cells such as through signaling regulation and ECM deposition. We also address specific aspects of the collective cell migration and dynamic interactions between pericytes and endothelial cells during angiogenic sprouting. Lastly, we discuss pericyte contributions to mechanisms underlying the transition from active vessel remodeling to the maturation and quiescence phase of vascular development.

Role of iPSC-derived pericytes on barrier function of iPSC-derived brain microvascular endothelial cells in 2D and 3D

Background

Pericytes of the blood–brain barrier (BBB) are embedded within basement membrane between brain microvascular endothelial cells (BMECs) and astrocyte end-feet. Despite the direct cell–cell contact observed in vivo, most in vitro BBB models introduce an artificial membrane that separates pericytes from BMECs. In this study, we investigated the effects of pericytes on BMEC barrier function across a range of in vitro platforms with varied spatial orientations and levels of cell–cell contact.

Methods

We differentiated RFP-pericytes and GFP-BMECs from hiPSCs and monitored transendothelial electrical resistance (TEER) across BMECs on transwell inserts while pericytes were either directly co-cultured on the membrane, indirectly co-cultured in the basolateral chamber, or embedded in a collagen I gel formed on the transwell membrane. We then incorporated pericytes into a tissue-engineered microvessel model of the BBB and measured pericyte motility and microvessel permeability.

Results

We found that BMEC monolayers did not require co-culture with pericytes to achieve physiological TEER values (> 1500 Ω cm²). However, under stressed conditions where TEER values for BMEC monolayers were reduced, indirectly co-cultured hiPSC-derived pericytes restored optimal TEER. Conversely, directly co-cultured pericytes resulted in a decrease in TEER by interfering with BMEC monolayer continuity. In the microvessel model, we observed direct pericyte-BMEC contact, abluminal pericyte localization, and physiologically-low Lucifer yellow permeability comparable to that of BMEC microvessels. In addition, pericyte motility decreased during the first 48 h of co-culture, suggesting progression towards pericyte stabilization.

Conclusions

We demonstrated that monocultured BMECs do not require co-culture to achieve physiological TEER, but that suboptimal TEER in stressed monolayers can be increased through co-culture with hiPSC-derived pericytes or conditioned media. We also developed the first BBB microvessel model using exclusively hiPSC-derived BMECs and pericytes, which could be used to examine vascular dysfunction in the human CNS.

Electronic supplementary material

The online version of this article (10.1186/s12987-019-0136-7) contains supplementary material, which is available to authorized users.

SOX10 Immunostaining in granulomatous dermatoses and benign reactive lymph nodes

BACKGROUND

SOX10 immunostaining has been considered a highly sensitive and specific marker for melanoma. But there is evidence suggesting that SOX10 positive cells can be present in dermal scars. Therefore, we investigated whether non-melanocytic cell types present in chronic inflammatory processes or benign lymph nodes express SOX10.

METHODS

We retrospectively selected 20 benign lymph nodes and 20 cutaneous granulomatous dermatoses. SOX10, CD68, and Melan-A immunohistochemistry was performed in all cases.

RESULTS

Scattered SOX10 positivity was found in 85% of lymph nodes, specifically in subcapsular and medullary sinuses and in 85% of granulomatous dermatoses. In granulomatous dermatoses, the Melan-A stain did not label the scattered SOX10 positive cells and it was difficult to determine if CD68 was co-expressed on the SOX10 positive cells. In the lymph nodes, the SOX10 positive cells did not co-express Melan-A or CD68.

CONCLUSIONS

We report SOX10 positive cells detected in granulomatous dermatoses and benign lymph nodes. In lymph nodes, SOX10 positive cells were exclusively in subcapsular and medullary sinuses. Therefore, SOX10 is an excellent stain for evaluation of metastatic melanoma with the caveat that positivity in subcapsular and medullary sinuses can be of non-melanocytic origin; the use of additional melanocytic markers is recommended in this situations.