TNF-α promotes cerebral pericyte remodeling in vitro, via a switch from α1 to α2 integrins

A subpopulation of tissue remodeling monocytes stimulates revascularization of the ischemic limb

Despite decades of effort aimed at developing clinically effective cell therapies, including mixed population mononuclear cells, to revascularize the ischemic limb, there remains a paucity of patient-based studies that inform the function and fate of candidate cell types. In this study, we showed that circulating proangiogenic/arteriogenic monocytes (PAMs) expressing the FcγIIIA receptor CD16 were elevated in patients with chronic limb-threatening ischemia (CLTI), and these amounts decreased after revascularization. Unlike CD16-negative monocytes, PAMs showed large vessel remodeling properties in vitro when cultured with endothelial cells and smooth muscle cells and promoted salvage of the ischemic limb in vivo in a mouse model of hindlimb ischemia. PAMs showed a propensity to migrate toward and bind to ischemic muscle and to secrete angiogenic/arteriogenic factors, vascular endothelial growth factor A (VEGF-A) and heparin-binding epidermal growth factor. We instigated a first-in-human single-arm cohort study in which autologous PAMs were injected into the ischemic limbs of five patients with CLTI. Greater than 25% of injected cells were retained in the leg for at least 72 hours, of which greater than 80% were viable, with evidence of enhanced large vessel remodeling in the injected muscle area. In summary, we identified up-regulation of a circulatory PAM subpopulation as an endogenous response to limb ischemia in CLTI and tested a potentially clinically relevant therapeutic strategy.

β1 integrin is essential for blood-brain barrier integrity under stable and vascular remodelling conditions; effects differ with age

BACKGROUND

Maintaining a tight blood-brain barrier (BBB) is an important prerequisite for the preservation of neurological health, though current evidence suggests it declines with age. While extracellular matrix-integrin interactions play critical roles in regulating the balance between vascular stability and remodeling, it remains to be established whether manipulation of integrin function weakens or strengthens vascular integrity. Indeed, recent reports have generated conflicting outcomes in this regard.

METHODS

Here, in young (8-10 weeks) and aged (20 months) mice, we examined the impact of intraperitoneal injection of a function-blocking β1 integrin antibody, both under normoxic conditions, when the BBB is stable, and during chronic mild hypoxic (CMH; 8% O₂) conditions, when a vigorous vascular remodeling response is ongoing. Brain tissue was examined by immunofluorescence (IF) for markers of vascular remodeling and BBB disruption, and microglial activation and proliferation. Data were analyzed using one-way analysis of variance (ANOVA) followed by Tukey's multiple comparison post-hoc test.

RESULTS

In both young and aged mice, β1 integrin block greatly amplified hypoxia-induced vascular disruption, though it was much less under normoxic conditions. Interestingly, under both normoxic and hypoxic conditions, β1 integrin antibody-induced BBB disruption was greater in young mice. Enhanced BBB breakdown was associated with increased levels of the leaky BBB marker MECA-32 and with greater loss of endothelial tight junction proteins and the adherens protein VE-cadherin. Surprisingly, β1 integrin blockade did not reduce hypoxia-induced endothelial proliferation, nor did it prevent the hypoxia-associated increase in vascularity. Commensurate with the increased vascular disruption, β1 integrin blockade enhanced microglial activation both in young and aged brain, though the impact was much greater in young brain. In vitro studies revealed that β1 integrin blockade also reduced the integrity of a brain endothelial monolayer and triggered disruptions in tight junction proteins.

CONCLUSIONS

These data demonstrate that β1 integrin plays an essential role in maintaining BBB integrity, both under stable normoxic conditions and during hypoxia-induced vascular remodeling. As β1 integrin blockade had a greater disruptive effect in young brain, effectively shifting the BBB phenotype of young brain towards that of the aged, we speculate that enhancing β1 integrin function at the aged BBB may hold therapeutic potential by reverting the deteriorating BBB phenotype back towards that of the young.

Pericyte-Glioblastoma Cell Interaction: A Key Target to Prevent Glioblastoma Progression

Multiple biological processes rely on direct intercellular interactions to regulate cell proliferation and migration in embryonic development and cancer processes. Tumor development and growth depends on close interactions between cancer cells and cells in the tumor microenvironment. During embryonic development, morphogenetic signals and direct cell contacts control cell proliferation, polarity, and morphogenesis. Cancer cells communicate with cells in the tumor niche through molecular signals and intercellular contacts, thereby modifying the vascular architecture and antitumor surveillance processes and consequently enabling tumor growth and survival. While looking for cell-to-cell signaling mechanisms that are common to both brain development and cancer progression, we have studied the infiltration process in glioblastoma multiforme (GBM), which is the most malignant primary brain tumor and with the worst prognosis. Cell-to-cell contacts, by means of filopodia-like structures, between GBM cells and brain pericytes (PCs) are necessary for adequate cell signaling during cancer infiltration; similarly, contacts between embryonic regions, via cytonemes, are required for embryo regionalization and development. This GBM-PC interaction provokes two important changes in the physiological function of these perivascular cells, namely, (i) vascular co-option with changes in cell contractility and vascular malformation, and (ii) changes in the PC transcriptome, modifying the microvesicles and protein secretome, which leads to the development of an immunosuppressive phenotype that promotes tumor immune tolerance. Moreover, the GTPase Cdc42 regulates cell polarity across organisms, from yeast to humans, playing a central role in GBM cell-PC interaction and maintaining vascular co-option. As such, a review of the molecular and cellular mechanisms underlying the development and maintenance of the physical interactions between cancer cells and PCs is of particular interest.

Blood–Brain Barrier Biomarkers before and after Kidney Transplantation

Kidney transplantation (KT) may improve the neurological status of chronic kidney disease (CKD) patients, reflected by the altered levels of circulating BBB-specific biomarkers. This study compares the levels of neuron specific enolase (NSE), brain-derived neurotrophic factor (BDNF), neurofilament light chain (NfL), and circulating plasma extracellular vesicles (EVs) in kidney-failure patients before KT and at a two-year follow up. Using ELISA, NSE, BDNF, and NfL levels were measured in the plasma of 74 living-donor KT patients. Plasma EVs were isolated with ultracentrifugation, and characterized for concentration/size and surface protein expression using flow cytometry from a subset of 25 patients. Lower NSE levels, and higher BDNF and NfL were observed at the two-year follow-up compared to the baseline (p < 0.05). Male patients had significantly higher BDNF levels compared to those of females. BBB biomarkers correlated with the baseline lipid profile and with glucose, vitamin D, and inflammation markers after KT. BBB surrogate marker changes in the microcirculation of early vascular aging phenotype patients with calcification and/or fibrosis were observed only in NSE and BDNF. CD31+ microparticles from endothelial cells expressing inflammatory markers such as CD40 and integrins were significantly reduced after KT. KT may, thus, improve the neurological status of CKD patients, as reflected by changes in BBB-specific biomarkers.

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.

Cell-specific expression and function of laminin at the neurovascular unit

Laminin, a major component of the basal lamina (BL), is a heterotrimeric protein with many isoforms. In the CNS, laminin is expressed by almost all cell types, yet different cells synthesize distinct laminin isoforms. By binding to its receptors, laminin exerts a wide variety of important functions. However, due to the reciprocal and cell-specific expression of laminin in different cells at the neurovascular unit, its functions in blood-brain barrier (BBB) maintenance and BBB repair after injury are not fully understood. In this review, we focus on the expression and functions of laminin and its receptors in the neurovascular unit under both physiological and pathological conditions. We first briefly introduce the structures of laminin and its receptors. Next, the expression and functions of laminin and its receptors in the CNS are summarized in a cell-specific manner. Finally, we identify the knowledge gap in the field and discuss key questions that need to be answered in the future. Our goal is to provide a comprehensive overview on cell-specific expression of laminin and its receptors in the CNS and their functions on BBB integrity.

New insight into neurological degeneration: Inflammatory cytokines and blood–brain barrier

Neurological degeneration after neuroinflammation, such as that resulting from Alzheimer’s disease (AD), stroke, multiple sclerosis (MS), and post-traumatic brain injury (TBI), is typically associated with high mortality and morbidity and with permanent cognitive dysfunction, which places a heavy economic burden on families and society. Diagnosing and curing these diseases in their early stages remains a challenge for clinical investigation and treatment. Recent insight into the onset and progression of these diseases highlights the permeability of the blood–brain barrier (BBB). The primary factor that influences BBB structure and function is inflammation, especially the main cytokines including IL-1β, TNFα, and IL-6, the mechanism on the disruption of which are critical component of the aforementioned diseases. Surprisingly, the main cytokines from systematic inflammation can also induce as much worse as from neurological diseases or injuries do. In this review, we will therefore discuss the physiological structure of BBB, the main cytokines including IL-1β, TNFα, IL-6, and their mechanism on the disruption of BBB and recent research about the main cytokines from systematic inflammation inducing the disruption of BBB and cognitive impairment, and we will eventually discuss the need to prevent the disruption of BBB.

Single-cell transcriptomics and surface epitope detection in human brain epileptic lesions identifies pro-inflammatory signaling

Epileptogenic triggers are multifactorial and not well understood. Here we aimed to address the hypothesis that inappropriate pro-inflammatory mechanisms contribute to the pathogenesis of refractory epilepsy (non-responsiveness to antiepileptic drugs) in human patients. We used single-cell cellular indexing of transcriptomes and epitopes by sequencing (CITE-seq) to reveal the immunotranscriptome of surgically resected epileptic lesion tissues. Our approach uncovered a pro-inflammatory microenvironment, including extensive activation of microglia and infiltration of other pro-inflammatory immune cells. These findings were supported by ligand–receptor (LR) interactome analysis, which demonstrated potential mechanisms of infiltration and evidence of direct physical interactions between microglia and T cells. Together, these data provide insight into the immune microenvironment in epileptic tissue, which may aid the development of new therapeutics.

Single-cell analysis of immune cells from surgically resected human epileptic brain tissues showed heterogeneity and pro-inflammatory signaling in microglia and evidence for direct interaction of microglia with T cells.

Lysophosphatidic acid stimulates pericyte migration via LPA receptor 1

Lysophosphatidic acid (LPA) is a bioactive compound known to regulate various vascular functions. However, despite the fact that many vascular functions are regulated by peri-vascular cells such as pericytes, the effect of LPA on brain pericytes has not been fully evaluated. Thus, we designed this study to evaluate the effects of LPA on brain pericytes. These experiments revealed that while LPA receptors (LPARs) are expressed in cultured pericytes from mouse brains, LPA treatment does not influence the proliferation of these cells but does have a profound impact on their migration, which is regulated via the expression of LPAR1. LPAR1 expression was also detected in human pericyte culture and LPA treatment of these cells also induced migration. Taken together these findings imply that LPA-LPAR1 signaling is one of the key mechanisms modulating pericyte migration, which may help to control vascular function during development and repair processes.

Pericytes as mediators of infiltration of macrophages in multiple sclerosis

Background

Multiple sclerosis (MS) is a neurodegenerative condition of the central nervous system (CNS). It is associated with blood–brain barrier (BBB) breakdown and intravasation of leukocytes, particularly monocyte-derived macrophages, into the CNS. Pericytes are mural cells that are encased within the basement membrane of vasculature, and they contribute functionally to the neurovascular unit. These cells play an important role in maintaining BBB integrity and CNS homeostasis. However, the critical role of pericytes in mediating inflammation in MS or its models is unclear. Whether pericytes infiltrate into the CNS parenchyma in MS also needs clarification.

Methods

CNS samples from the experimental autoimmune encephalomyelitis (EAE) mouse model of MS were collected at different time points for immunohistochemical analysis of pericytes along the inflamed vasculature. These findings were validated using MS brain specimens, and further analysis of pericyte involvement in inflammation was carried out by culturing primary pericytes and macrophages. Multiplex ELISA, transmigration assay and real-time PCR were used to study the inflammatory potential of pericytes in cultures.

Results

We found that pericytes exhibit a heterogenous morphology, with notable elongation in the inflamed perivascular cuffs of EAE. This was manifested by a decrease in pericyte density but an increase in the coverage by pericytes along the vasculature. Chondroitin sulfate proteoglycans (CSPGs), a family of extracellular matrix proteins enriched within inflamed perivascular cuffs, elevated levels of pro-inflammatory chemokines/cytokines in pericytes in culture. Importantly, pericytes stimulated with CSPGs enhanced macrophage migration. We did not detect pericytes in the CNS parenchyma during EAE, and this was corroborated in MS brain samples.

Conclusions

Our data suggest that pericytes seek to restore the BBB through increased coverage, but that their exposure to CSPGs prompt their facilitation of macrophages to enter the CNS to elevate neuroinflammation in EAE and MS.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-021-02358-x.

Estradiol Inhibits Human Brain Vascular Pericyte Migration Activity: A Functional and Transcriptomic Analysis

Stroke is the third leading cause of mortality in women and it kills twice as many women as breast cancer. A key role in the pathophysiology of stroke plays the disruption of the blood–brain barrier (BBB) within the neurovascular unit. While estrogen induces vascular protective actions, its influence on stroke remains unclear. Moreover, experiments assessing its impact on endothelial cells to induce barrier integrity are non-conclusive. Since pericytes play an active role in regulating BBB integrity and function, we hypothesize that estradiol may influence BBB by regulating their activity. In this study using human brain vascular pericytes (HBVPs) we investigated the impact of estradiol on key pericyte functions known to influence BBB integrity. HBVPs expressed estrogen receptors (ER-α, ER-β and GPER) and treatment with estradiol (10 nM) inhibited basal cell migration but not proliferation. Since pericyte migration is a hallmark for BBB disruption following injury, infection and inflammation, we investigated the effects of estradiol on TNFα-induced PC migration. Importantly, estradiol prevented TNFα-induced pericyte migration and this effect was mimicked by PPT (ER-α agonist) and DPN (ER-β agonist), but not by G1 (GPR30 agonist). The modulatory effects of estradiol were abrogated by MPP and PHTPP, selective ER-α and ER-β antagonists, respectively, confirming the role of ER-α and ER-β in mediating the anti-migratory actions of estrogen. To delineate the intracellular mechanisms mediating the inhibitory actions of estradiol on PC migration, we investigated the role of AKT and MAPK activation. While estradiol consistently reduced the TNFα-induced MAPK and Akt phosphorylation, only the inhibition of MAPK, but not Akt, significantly abrogated the migratory actions of TNFα. In transendothelial electrical resistance measurements, estradiol induced barrier function (TEER) in human brain microvascular endothelial cells co-cultured with pericytes, but not in HBMECs cultured alone. Importantly, transcriptomics analysis of genes modulated by estradiol in pericytes showed downregulation of genes known to increase cell migration and upregulation of genes known to inhibit cell migration. Taken together, our findings provide the first evidence that estradiol modulates pericyte activity and thereby improves endothelial integrity.

Targeting RGD-binding integrins as an integrative therapy for diabetic retinopathy and neovascular age-related macular degeneration

Integrins are a class of transmembrane receptors that are involved in a wide range of biological functions. Dysregulation of integrins has been implicated in many pathological processes and consequently, they are attractive therapeutic targets. In the ophthalmology arena, there is extensive evidence suggesting that integrins play an important role in diabetic retinopathy (DR), age-related macular degeneration (AMD), glaucoma, dry eye disease and retinal vein occlusion. For example, there is extensive evidence that arginyl-glycyl-aspartic acid (Arg-Gly-Asp; RGD)-binding integrins are involved in key disease hallmarks of DR and neovascular AMD (nvAMD), specifically inflammation, vascular leakage, angiogenesis and fibrosis. Based on such evidence, drugs that engage integrin-linked pathways have received attention for their potential to block all these vision-threatening pathways. This review focuses on the pathophysiological role that RGD-binding integrins can have in complex multifactorial retinal disorders like DR, diabetic macular edema (DME) and nvAMD, which are leading causes of blindness in developed countries. Special emphasis will be given on how RGD-binding integrins can modulate the intricate molecular pathways and regulate the underlying pathological mechanisms. For instance, the interplay between integrins and key molecular players such as growth factors, cytokines and enzymes will be summarized. In addition, recent clinical advances linked to targeting RGD-binding integrins in the context of DME and nvAMD will be discussed alongside future potential for limiting progression of these diseases.

Macrophages and microglia: the cerberus of glioblastoma

Glioblastoma (GBM) is the most aggressive and deadliest of the primary brain tumors, characterized by malignant growth, invasion into the brain parenchyma, and resistance to therapy. GBM is a heterogeneous disease characterized by high degrees of both inter- and intra-tumor heterogeneity. Another layer of complexity arises from the unique brain microenvironment in which GBM develops and grows. The GBM microenvironment consists of neoplastic and non-neoplastic cells. The most abundant non-neoplastic cells are those of the innate immune system, called tumor-associated macrophages (TAMs). TAMs constitute up to 40% of the tumor mass and consist of both brain-resident microglia and bone marrow-derived myeloid cells from the periphery. Although genetically stable, TAMs can change their expression profiles based upon the signals that they receive from tumor cells; therefore, heterogeneity in GBM creates heterogeneity in TAMs. By interacting with tumor cells and with the other non-neoplastic cells in the tumor microenvironment, TAMs promote tumor progression. Here, we review the origin, heterogeneity, and functional roles of TAMs. In addition, we discuss the prospects of therapeutically targeting TAMs alone or in combination with standard or newly-emerging GBM targeting therapies.

Pericyte mechanics and mechanobiology

Pericytes are mural cells of the microvasculature, recognized by their thin processes and protruding cell body. Pericytes wrap around endothelial cells and play a central role in regulating various endothelial functions, including angiogenesis and inflammation. They also serve as a vascular support and regulate blood flow by contraction. Prior reviews have examined pericyte biological functions and biochemical signaling pathways. In this Review, we focus on the role of mechanics and mechanobiology in regulating pericyte function. After an overview of the morphology and structure of pericytes, we describe their interactions with both the basement membrane and endothelial cells. We then turn our attention to biophysical considerations, and describe contractile forces generated by pericytes, mechanical forces exerted on pericytes, and pericyte responses to these forces. Finally, we discuss 2D and 3D engineered in vitro models for studying pericyte mechano-responsiveness and underscore the need for more evolved models that provide improved understanding of pericyte function and dysfunction.

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.

NLRP3 Is Involved in the Maintenance of Cerebral Pericytes

Pericytes play a central role in regulating the structure and function of capillaries in the brain. However, molecular mechanisms that drive pericyte proliferation and differentiation are unclear. In our study, we immunostained NACHT, LRR and PYD domains-containing protein 3 (NLRP3)-deficient and wild-type littermate mice and observed that NLRP3 deficiency reduced platelet-derived growth factor receptor β (PDGFRβ)-positive pericytes and collagen type IV immunoreactive vasculature in the brain. In Western blot analysis, PDGFRβ and CD13 proteins in isolated cerebral microvessels from the NLRP3-deficient mouse brain were decreased. We further treated cultured pericytes with NLRP3 inhibitor, MCC950, and demonstrated that NLRP3 inhibition attenuated cell proliferation but did not induce apoptosis. NLRP3 inhibition also decreased protein levels of PDGFRβ and CD13 in cultured pericytes. On the contrary, treatments with IL-1β, the major product of NLRP3-contained inflammasome, increased protein levels of PDGFRβ, and CD13 in cultured cells. The alteration of PDGFRβ and CD13 protein levels were correlated with the phosphorylation of AKT. Inhibition of AKT reduced both protein markers and abolished the effect of IL-1β activation in cultured pericytes. Thus, NLRP3 activation might be essential to maintain pericytes in the healthy brain through phosphorylating AKT. The potential adverse effects on the cerebral vascular pericytes should be considered in clinical therapies with NLRP3 inhibitors.

Pericytes in Vascular Development

Purpose of review

Pericytes are essential components of capillaries in many tissues and organs, contributing to vessel stability and integrity, with additional contributions to microvascular function still being discovered. We review current and foundational studies identifying pericyte differentiation mechanics and their roles in the earliest stages of vessel formation.

Recent findings

Recent advances in pericyte-focused tools and models have illuminated critical aspects of pericyte biology including their roles in vascular development.Pericytes likely collaborate with endothelial cells undergoing vasculogenesis, initiating direct interactions during sprouting and intussusceptive angiogenesis. Pericytes also provide important regulation of vascular growth including mechanisms underlying vessel pruning, rarefaction, and subsequent regrowth.

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 and Molecular Mechanisms of Pericytes in Regulation of Leukocyte Diapedesis in Inflamed Tissues

Leukocyte recruitment is a hallmark of the inflammatory response. Migrating leukocytes breach the endothelium along with the vascular basement membrane and associated pericytes. While much is known about leukocyte-endothelial cell interactions, the mechanisms and role of pericytes in extravasation are poorly understood and the classical paradigm of leukocyte recruitment in the microvasculature seldom adequately discusses the involvement of pericytes. Emerging evidence shows that pericytes are essential players in the regulation of leukocyte extravasation in addition to their functions in blood vessel formation and blood-brain barrier maintenance. Junctions between venular endothelial cells are closely aligned with extracellular matrix protein low expression regions (LERs) in the basement membrane, which in turn are aligned with gaps between pericytes. This forms preferential paths for leukocyte extravasation. Breaching of the layer formed by pericytes and the basement membrane entails remodelling of LERs, leukocyte-pericyte adhesion, crawling of leukocytes on pericyte processes, and enlargement of gaps between pericytes to form channels for migrating leukocytes. Furthermore, inflamed arteriolar and capillary pericytes induce chemotactic migration of leukocytes that exit postcapillary venules, and through direct pericyte-leukocyte contact, they induce efficient interstitial migration to enhance the immunosurveillance capacity of leukocytes. Given their role as regulators of leukocyte extravasation, proper pericyte function is imperative in inflammatory disease contexts such as diabetic retinopathy and sepsis. This review summarizes research on the molecular mechanisms by which pericytes mediate leukocyte diapedesis in inflamed tissues.

Pericytes in Multiple Sclerosis

Multiple sclerosis (MS) is an autoimmune inflammatory demyelinating disease that affects the central nervous system (CNS), particularly, in young adults. Current MS treatments aim to reduce demyelination; however, these have limited efficacy, display side effects and lack of regenerative activities. Oligodendrocyte progenitor cells (OPCs) represents the major source for new myelin. Upon demyelination, OPCs get activated, proliferate, migrate towards the lesion, and differentiate into remyelinating oligodendrocytes. Although myelin repair (remyelination) represents a robust response to myelin damage, during MS, this regenerative phenomenon decays in efficiency or even fails. CNS-resident pericytes (CNS-PCs) are essential for vascular homeostasis regulating blood-brain barrier (BBB) permeability and stability as well as endothelial cells (ECs) function during angiogenesis and neovascularization. Recent studies indicate that CNS-PCs also play a crucial role regulating OPC function during remyelination, and very importantly, these cells are substantially affected in MS. This chapter summarizes important aspects of MS and CNS remyelination as well as it provides new insights supporting the contribution of CNS-PCs to myelin regeneration and to MS pathology. Currently, there is evidence arguing in favor of CNS-PCs as novel therapeutic targets for the development of future treatments for MS.

Modelling physiological and pathological conditions to study pericyte biology in brain function and dysfunction

BACKGROUND

Brain pericytes ensheathe the endothelium and contribute to formation and maintenance of the blood-brain-barrier. Additionally, pericytes are involved in several aspects of the CNS immune response including scarring, adhesion molecule expression, chemokine secretion, and phagocytosis. In vitro cultures are routinely used to investigate these functions of brain pericytes, however, these are highly plastic cells and can display differing phenotypes and functional responses depending on their culture conditions. Here we sought to investigate how two commonly used culture media, high serum containing DMEM/F12 and low serum containing Pericyte Medium (ScienCell), altered the phenotype of human brain pericytes and neuroinflammatory responses.

METHODS

Pericytes were isolated from adult human brain biopsy tissue and cultured in DMEM/F12 (D-pericytes) or Pericyte Medium (P-pericytes). Immunocytochemistry, qRT-PCR, and EdU incorporation were used to determine how this altered their basal phenotype, including the expression of pericyte markers, proliferation, and cell morphology. To determine whether culture media altered the inflammatory response in human brain pericytes, immunocytochemistry, qRT-PCR, cytometric bead arrays, and flow cytometry were used to investigate transcription factor induction, chemokine secretion, adhesion molecule expression, migration, phagocytosis, and response to inflammatory-related growth factors.

RESULTS

P-pericytes displayed elevated proliferation and a distinct bipolar morphology compared to D-pericytes. Additionally, P-pericytes displayed lower expression of pericyte-associated markers NG2, PDGFRβ, and fibronectin, with notably lower αSMA, CD146, P4H and desmin, and higher Col-IV expression. Nuclear NF-kB translocation in response to IL-1β stimulation was observed in both cultures, however, P-pericytes displayed elevated expression of the transcription factor C/EBPδ, and lower expression of the adhesion molecule ICAM-1. P-pericytes displayed elevated phagocytic and migratory ability. Both cultures responded similarly to stimulation by the growth factors TGFβ₁ and PDGF-BB.

CONCLUSIONS

Despite differences in their phenotype and magnitude of response, both P-pericytes and D-pericytes responded similarly to all examined functions, indicating that the neuroinflammatory phenotype of these cells is robust to culture conditions.

Involvement of PUMA in pericyte migration induced by methamphetamine

Mounting evidence indicates that methamphetamine causes blood-brain barrier damage, with emphasis on endothelial cells. The role of pericytes in methamphetamine-induced BBB damage remains unknown. Our study demonstrated that methamphetamine increased the migration of pericytes from the endothelial basement membrane. However, the detailed mechanisms underlying this process remain poorly understood. Thus, we examined the molecular mechanisms involved in methamphetamine-induced pericyte migration. The results showed that exposure of C3H/10T1/2 cells and HBVPs to methamphetamine increased PUMA expression via activation of the sigma-1 receptor, MAPK and Akt/PI3K pathways. Moreover, methamphetamine treatment resulted in the increased migration of C3H/10T1/2 cells and HBVPs. Knockdown of PUMA in pericytes transduced with PUMA siRNA attenuated the methamphetamine-induced increase in cell migration through attenuation of integrin and tyrosine kinase mechanisms, implicating a role of PUMA in the migration of C3H/10T1/2 cells and HBVPs. This study has demonstrated that methamphetamine-mediated pericytes migration involves PUMA up-regulation. Thus, targeted studies of PUMA could provide insights to facilitate the development of a potential therapeutic approach for alleviation of methamphetamine-induced pericyte migration.

Endothelial Wnt/β-catenin signaling reduces immune cell infiltration in multiple sclerosis

Disruption of the blood-brain barrier (BBB) is a defining and early feature of multiple sclerosis (MS) that directly damages the central nervous system (CNS), promotes immune cell infiltration, and influences clinical outcomes. There is an urgent need for new therapies to protect and restore BBB function, either by strengthening endothelial tight junctions or suppressing endothelial vesicular transcytosis. Although wingless integrated MMTV (Wnt)/β-catenin signaling plays an essential role in BBB formation and maintenance in healthy CNS, its role in BBB repair in neurologic diseases such as MS remains unclear. Using a Wnt/β-catenin reporter mouse and several downstream targets, we demonstrate that the Wnt/β-catenin pathway is up-regulated in CNS endothelial cells in both human MS and the mouse model experimental autoimmune encephalomyelitis (EAE). Increased Wnt/β-catenin activity in CNS blood vessels during EAE progression correlates with up-regulation of neuronal Wnt3 expression, as well as breakdown of endothelial cell junctions. Genetic inhibition of the Wnt/β-catenin pathway in CNS endothelium before disease onset exacerbates the clinical presentation of EAE, CD4⁺ T-cell infiltration into the CNS, and demyelination by increasing expression of vascular cell adhesion molecule-1 and the transcytosis protein Caveolin-1 and promoting endothelial transcytosis. However, Wnt signaling attenuation does not affect the progressive degradation of tight junction proteins or paracellular BBB leakage. These results suggest that reactivation of Wnt/β-catenin signaling in CNS vessels during EAE/MS partially restores functional BBB integrity and limits immune cell infiltration into the CNS.

Periostin Promotes Scar Formation through the Interaction between Pericytes and Infiltrating Monocytes/Macrophages after Spinal Cord Injury

Scar formation is a prominent pathological feature of traumatic central nervous system (CNS) injury, which has long been implicated as a major impediment to the CNS regeneration. However, the factors affecting such scar formation remain to be elucidated. We herein demonstrate that the extracellular matrix protein periostin (POSTN) is a key player in scar formation after traumatic spinal cord injury (SCI). Using high-throughput RNA sequencing data sets, we found that the genes involved in the extracellular region, such as POSTN, were significantly expressed in the injured spinal cord. The expression of POSTN peaked at 7 days after SCI, predominantly in the scar-forming pericytes. Notably, we found that genetic deletion of POSTN in mice reduced scar formation at the lesion site by suppressing the proliferation of the pericytes. Conversely, we found that recombinant POSTN promoted the migration capacity of the monocytes/macrophages and increased the expression of tumor necrosis factor-α from the monocytes/macrophages in vitro, which facilitated the proliferation of pericytes. Furthermore, we revealed that the pharmacological blockade of POSTN suppressed scar formation and improved the long-term functional outcome after SCI. Our findings suggest a potential mechanism whereby POSTN regulates the scar formation after SCI and provide significant evidence that POSTN is a promising therapeutic target for CNS injury.

Interferon-γ blocks signalling through PDGFRβ in human brain pericytes

BACKGROUND

Neuroinflammation and blood-brain barrier (BBB) disruption are common features of many brain disorders, including Alzheimer's disease, epilepsy, and motor neuron disease. Inflammation is thought to be a driver of BBB breakdown, but the underlying mechanisms for this are unclear. Brain pericytes are critical cells for maintaining the BBB and are immunologically active. We sought to test the hypothesis that inflammation regulates the BBB by altering pericyte biology.

METHODS

We exposed primary adult human brain pericytes to chronic interferon-gamma (IFNγ) for 4 days and measured associated functional aspects of pericyte biology. Specifically, we examined the influence of inflammation on platelet-derived growth factor receptor-beta (PDGFRβ) expression and signalling, as well as pericyte proliferation and migration by qRT-PCR, immunocytochemistry, flow cytometry, and western blotting.

RESULTS

Chronic IFNγ treatment had marked effects on pericyte biology most notably through the PDGFRβ, by enhancing agonist (PDGF-BB)-induced receptor phosphorylation, internalization, and subsequent degradation. Functionally, chronic IFNγ prevented PDGF-BB-mediated pericyte proliferation and migration.

CONCLUSIONS

Because PDGFRβ is critical for pericyte function and its removal leads to BBB leakage, our results pinpoint a mechanism linking chronic brain inflammation to BBB dysfunction.

Insights on the pathophysiology of Alzheimer's disease: The crosstalk between amyloid pathology, neuroinflammation and the peripheral immune system

Alzheimer's disease (AD) is the most common form of dementia, whose prevalence is growing along with the increased life expectancy. Although the accumulation and deposition of amyloid beta (Aβ) peptides in the brain is viewed as one of the pathological hallmarks of AD and underlies, at least in part, brain cell dysfunction and behavior alterations, the etiology of this neurodegenerative disease is still poorly understood. Noticeably, increased amyloid load is accompanied by marked inflammatory alterations, both at the level of the brain parenchyma and at the barriers of the brain. However, it is debatable whether the neuroinflammation observed in aging and in AD, together with alterations in the peripheral immune system, are responsible for increased amyloidogenesis, decreased clearance of Aβ out of the brain and/or the marked deficits in memory and cognition manifested by AD patients. Herein, we scrutinize some important traits of the pathophysiology of aging and AD, focusing on the interplay between the amyloidogenic pathway, neuroinflammation and the peripheral immune system.

Dysfunction of brain pericytes in chronic neuroinflammation

Brain pericytes are uniquely positioned within the neurovascular unit to provide support to blood brain barrier (BBB) maintenance. Neurologic conditions, such as HIV-1-associated neurocognitive disorder, are associated with BBB compromise due to chronic inflammation. Little is known about pericyte dysfunction during HIV-1 infection. We found decreased expression of pericyte markers in human brains from HIV-1-infected patients (even those on antiretroviral therapy). Using primary human brain pericytes, we assessed expression of pericyte markers (α1-integrin, α-smooth muscle actin, platelet-derived growth factor-B receptor β, CX-43) and found their downregulation after treatment with tumor necrosis factor-α (TNFα) or interleukin-1 β (IL-1β). Pericyte exposure to virus or cytokines resulted in decreased secretion of factors promoting BBB formation (angiopoietin-1, transforming growth factor-β1) and mRNA for basement membrane components. TNFα and IL-1β enhanced expression of adhesion molecules in pericytes paralleling increased monocyte adhesion to pericytes. Monocyte migration across BBB models composed of human brain endothelial cells and pericytes demonstrated a diminished rate in baseline migration compared to constructs composed only of brain endothelial cells. However, exposure to the relevant chemokine, CCL2, enhanced the magnitude of monocyte migration when compared to BBB models composed of brain endothelial cells only. These data suggest an important role of pericytes in BBB regulation in neuroinflammation.

Synergistic actions of blocking angiopoietin-2 and tumor necrosis factor-α in suppressing remodeling of blood vessels and lymphatics in airway inflammation

Remodeling of blood vessels and lymphatics are prominent features of sustained inflammation. Angiopoietin-2 (Ang2)/Tie2 receptor signaling and tumor necrosis factor-α (TNF)/TNF receptor signaling are known to contribute to these changes in airway inflammation after Mycoplasma pulmonis infection in mice. We determined whether Ang2 and TNF are both essential for the remodeling on blood vessels and lymphatics, and thereby influence the actions of one another. Their respective contributions to the initial stage of vascular remodeling and sprouting lymphangiogenesis were examined by comparing the effects of function-blocking antibodies to Ang2 or TNF, given individually or together during the first week after infection. As indices of efficacy, vascular enlargement, endothelial leakiness, venular marker expression, pericyte changes, and lymphatic vessel sprouting were assessed. Inhibition of Ang2 or TNF alone reduced the remodeling of blood vessels and lymphatics, but inhibition of both together completely prevented these changes. Genome-wide analysis of changes in gene expression revealed synergistic actions of the antibody combination over a broad range of genes and signaling pathways involved in inflammatory responses. These findings demonstrate that Ang2 and TNF are essential and synergistic drivers of remodeling of blood vessels and lymphatics during the initial stage of inflammation after infection. Inhibition of Ang2 and TNF together results in widespread suppression of the inflammatory response.

Engineering of a Biomimetic Pericyte-Covered 3D Microvascular Network

Pericytes enveloping the endothelium play an important role in the physiology and pathology of microvessels, especially in vessel maturation and stabilization. However, our understanding of fundamental pericyte biology is limited by the lack of a robust in vitro model system that allows researchers to evaluate the interactions among multiple cell types in perfusable blood vessels. The present work describes a microfluidic platform that can be used to investigate interactions between pericytes and endothelial cells (ECs) during the sprouting, growth, and maturation steps of neovessel formation. A mixture of ECs and pericytes was attached to the side of a pre-patterned three dimensional fibrin matrix and allowed to sprout across the matrix. The effects of intact coverage and EC maturation by the pericytes on the perfused EC network were confirmed using a confocal microscope. Compared with EC monoculture conditions, EC-pericyte co-cultured vessels showed a significant reduction in diameter, increased numbers of junctions and branches and decreased permeability. In response to biochemical factors, ECs and pericytes in the platform showed the similar features with previous reports from in vivo experiments, thus reflect various pathophysiological conditions of in vivo microvessels. Taken together, these results support the physiological relevancy of our three-dimensional microfluidic culture system but also that the system can be used to screen drug effect on EC-pericyte biology.

CD49f Acts as an Inflammation Sensor to Regulate Differentiation, Adhesion, and Migration of Human Mesenchymal Stem Cells

The advent of mesenchymal stem cell (MSC)-based therapies has been an exciting innovation for the treatment of degenerative and inflammatory diseases. However, the surface markers that accurately reflect the self-renewal and differentiation potential of MSCs and their sensitivity to environmental cues remain poorly defined. Here, we studied the role of CD49f in bone marrow MSCs (BMSCs) and the mechanism by which it regulates the behavior of BMSCs under inflammatory conditions. We found that CD49f is preferentially expressed in fetal cells rather than adult cells, CD49f-positive BMSCs possess higher CFU-F formation ability and differentiation potential than CD49f negative cells, and the CD49f expression of BMSCs gradually decreases during in vitro passaging. CD49f knockdown dramatically decreased the differentiation of BMSCs and isoform A was demonstrated to be the main functional form that enhanced the differentiation ability of BMSCs. The influences of inflammatory cytokines on BMSCs revealed that TNF-α downregulated CD49f in BMSCs with impaired differentiation, decreased adhesion to laminins, and increased migration. Moreover, tissue transglutaminase was found to work together with CD49f to regulate the behavior of BMSCs. Finally, we showed that mTOR signaling rather than NF-κB activation mediated CD49f downregulation induced by TNF-α and maintained CD49f homeostasis in BMSCs. Our findings suggest that CD49f is a stemness marker of BMSCs and is tightly correlated with the behavioral changes of BMSCs under inflammatory conditions. These data demonstrate a novel role for CD49f in sensing inflammation through mTOR pathway to further modulate the behavior of MSCs to fulfill the requirements of the body.

Angiopoietin 2 induces pericyte apoptosis via α3β1 integrin signaling in diabetic retinopathy

Pericyte loss is an early characteristic change in diabetic retinopathy (DR). Despite accumulating evidence that hyperglycemia-induced angiopoietin 2 (Ang2) has a central role in pericyte loss, the precise molecular mechanism has not been elucidated. This study investigated the role of Ang2 in pericyte loss in DR. We demonstrated that pericyte loss occurred with Ang2 increase in the diabetic mouse retina and that the source of Ang2 could be the endothelial cell. Ang2 induced pericyte apoptosis via the p53 pathway under high glucose, whereas Ang2 alone did not induce apoptosis. Integrin, not Tie-2 receptor, was involved for Ang2-induced pericyte apoptosis under high glucose as an Ang2 receptor. High glucose changed the integrin expression pattern, which increased integrin α3 and β1 in the pericyte. Furthermore, Ang2-induced pericyte apoptosis in vitro was effectively attenuated via p53 suppression by blocking integrin α3 and β1. Although intravitreal injection of Ang2 induced pericyte loss in C57BL/6J mice retina in vivo, intravitreal injection of anti-integrin α3 and β1 antibodies attenuated Ang2-induced pericyte loss. Taken together, Ang2 induced pericyte apoptosis under high glucose via α3β1 integrin. Glycemic control or blocking Ang2/integrin signaling could be a potential therapeutic target to prevent pericyte loss in early DR.

Pericytes in chronic lung disease

Pericytes are mesenchymal cells embedded within the abluminal surface of the endothelium of microvessels such as capillaries, pre-capillary arterioles, post-capillary and collecting venules, where they maintain microvascular homeostasis and participate in angiogenesis. In addition to their roles in supporting the vasculature and facilitating leukocyte extravasation, pericytes have been recently investigated as a subpopulation of mesenchymal stem cells (MSCs) due to their capacity to differentiate into numerous cell types including the classic MSC triad, i.e. osteocytes, chondrocytes and adipocytes. Other studies in models of fibrotic inflammatory disease of the lung have demonstrated a vital role of pericytes in myofibroblast activation, collagen deposition and microvascular remodelling, which are hallmark features of chronic lung diseases such as asthma, chronic obstructive pulmonary disorder, pulmonary fibrosis and pulmonary hypertension. Further studies into the mechanisms of the pericyte-to-myofibroblast transition and migration to fibrotic foci will hopefully clarify the role of these cells in chronic lung disease and confirm the importance of pericytes in human fibrotic pulmonary disease.

Pericytes: brain-immune interface modulators

The premise that the central nervous system is immune-privileged arose from the fact that direct contact between immune and nervous cells is hindered by the blood-brain barrier. However, the blood-brain barrier also comprises the interface between the immune and nervous systems by secreting chemo-attractant molecules and by modulating immune cell entry into the brain. The majority of published studies on the blood-brain barrier focus on endothelial cells (ECs), which are a critical component, but not the only one; other cellular components include astroglia, microglia, and pericytes. Pericytes are poorly studied in comparison with astrocytes or ECs; they are mesenchymal cells that can modify their ultrastructure and gene expression in response to changes in the central nervous system microenvironment. Pericytes have a unique synergistic relationship with brain ECs in the regulation of capillary permeability through secretion of cytokines, chemokines, nitric oxide, matrix metalloproteinases, and by means of capillary contraction. Those pericyte manifestations are related to changes in blood-brain barrier permeability by an increase in endocytosis-mediated transport and by tight junction disruption. In addition, recent reports demonstrate that pericytes control the migration of leukocytes in response to inflammatory mediators by up-regulating the expression of adhesion molecules and releasing chemo-attractants; however, under physiological conditions they appear to be immune-suppressors. Better understanding of the immune properties of pericytes and their participation in the effects of brain infections, neurodegenerative diseases, and sleep loss will be achieved by analyzing pericyte ultrastructure, capillary coverage, and protein expression. That knowledge may provide a mechanism by which pericytes participate in the maintenance of the proper function of the brain-immune interface.

Pericytes and adaptive angioplasticity: the role of tumor necrosis factor-like weak inducer of apoptosis (TWEAK)

The TNF superfamily member TWEAK has emerged as a pleiotropic cytokine that regulates many cellular functions that include immune/inflammatory activity, angiogenesis, cell proliferation, and fate. TWEAK through its inducible receptor, FGF-inducible molecule 14 (Fn14), can induce both beneficial and deleterious activity that has a profound effect on cell survival. Thus it is highly likely that TWEAK and Fn14 expressed by cells of the neurovascular unit help regulate and maintain vascular and tissue homeostasis. In this chapter we discuss the expression of TWEAK and Fn14 signaling in the cerebral microvascular pericyte. Pericytes are a highly enigmatic population of microvascular cells that are important in regulatory pathways that modulate physiological angiogenesis in response to chronic mild hypoxic stress. A brief introduction will identify the microvascular pericyte. A more detailed discussion of pericyte TWEAK signaling during adaptive angioplasticity will follow.

Analysis of TNF-alpha-mediated cerebral pericyte remodeling

As well as being a central regulator of inflammatory and immune-mediated events, TNF-α also influences vascular remodeling, resulting in alterations in the structure and function of blood vessels. In addition to endothelial cells, pericytes are another type of vascular cell that significantly contribute to the development, maturation, stabilization, and remodeling of blood vessels. To investigate the regulatory influence of different factors on pericyte behavior, we recently described a novel yet simple approach of isolating and culturing highly pure, high density cultures of mouse brain pericytes. In this chapter, we briefly describe this culture system, as well as methods for examining different aspects of pericyte behavior, including cell adhesion, cell migration, and cell proliferation. These assays can be used to examine the influence of TNF-α or any other factor on pericyte behavior.

Could pericytic mimicry represent another type of melanoma cell plasticity with embryonic properties?

We hypothesize that the interaction between angiotropic melanoma cells and the abluminal vascular surface can induce or sustain embryonic and/or stem cell migratory properties in these tumor cells. As a result, such angiotropic melanoma cells may migrate along the abluminal vascular surface, demonstrating pericytic mimicry. Through these cellular interactions, melanoma cells may migrate toward secondary sites.