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Increases in brain blood flow, evoked by neuronal activity, power neural computation and form the basis of BOLD (blood-oxygen-level-dependent) functional imaging. Whether blood flow is controlled solely by arteriole smooth muscle, or also by capillary pericytes, is controversial. We demonstrate that neuronal activity and the neurotransmitter glutamate evoke the release of messengers that dilate capillaries by actively relaxing pericytes. Dilation is mediated by prostaglandin E 2 , but requires nitric oxide release to suppress vasoconstricting 20-HETE synthesis. In vivo , when sensory input increases blood flow, capillaries dilate before arterioles and are estimated to produce 84% of the blood flow increase. In pathology, ischaemia evokes capillary constriction by pericytes. We show that this is followed by pericyte death in rigor, which may irreversibly constrict capillaries and damage the blood–brain barrier. Thus, pericytes are major regulators of cerebral blood flow and initiators of functional imaging signals. Prevention of pericyte constriction and death may reduce the long-lasting blood flow decrease that damages neurons after stroke. Neuronal activity relaxes pericytes, leading to capillary dilation and increased blood flow, before arterioles dilate, suggesting that pericytes initiate blood-oxygen-level-dependent (BOLD) functional imaging signals; pericytes constrict and die in rigor in ischaemia, which will cause a long-lasting blood flow decrease after stroke, and damage the blood–brain barrier. Blood flow response to neural activity Cerebral blood flow dynamics have long been linked to neural activity, and form the basis of BOLD (blood-oxygen-level-dependent) functional imaging. But how such blood flow changes are mediated has remained controversial. Here, David Attwell and colleagues reveal how neuronal activity can hyperpolarize pericytes, leading to their relaxation and capillary dilation. Capillary dilation is responsible for 84% of the blood increase linked to neural activity, so irreversible capillary closure due to pericyte death during ischaemia can injure the blood–brain barrier and exacerbate injury. Pericyte death under pathological conditions can be reduced if glutamate receptor signalling is inhibited. This work suggests that pericytes are major regulators of cerebral blood flow and may initiate BOLD imaging signals.

The essential function of the circulatory system is to continuously and efficiently supply the O2 and nutrients necessary to meet the metabolic demands of every cell in the body, a function in which vast capillary networks play a key role. Capillary networks serve an additional important function in the central nervous system: acting as a sensory network, they detect neuronal activity in the form of elevated extracellular K⁺ and initiate a retrograde, propagating, hyperpolarizing signal that dilates upstream arterioles to rapidly increase local blood flow. Yet, little is known about how blood entering this network is distributed on a branch-to-branch basis to reach specific neurons in need. Here, we demonstrate that capillary-enwrapping projections of junctional, contractile pericytes within a postarteriole transitional region differentially constrict to structurally and dynamically determine the morphology of capillary junctions and thereby regulate branch-specific blood flow. We further found that these contractile pericytes are capable of receiving propagating K⁺-induced hyperpolarizing signals propagating through the capillary network and dynamically channeling red blood cells toward the initiating signal. By controlling blood flow at junctions, contractile pericytes within a functionally distinct postarteriole transitional region maintain the efficiency and effectiveness of the capillary network, enabling optimal perfusion of the brain.

The majority of the brain’s vasculature is composed of intricate capillary networks lined by capillary pericytes. However, it remains unclear whether capillary pericytes influence blood flow. Using two-photon microscopy to observe and manipulate brain capillary pericytes in vivo, we find that their optogenetic stimulation decreases lumen diameter and blood flow, but with slower kinetics than similar stimulation of mural cells on upstream pial and precapillary arterioles. This slow vasoconstriction was inhibited by the clinically used vasodilator fasudil, a Rho-kinase inhibitor that blocks contractile machinery. Capillary pericytes were also slower to constrict back to baseline following hypercapnia-induced dilation, and slower to dilate towards baseline following optogenetically induced vasoconstriction. Optical ablation of single capillary pericytes led to sustained local dilation and a doubling of blood cell flux selectively in capillaries lacking pericyte contact. These data indicate that capillary pericytes contribute to basal blood flow resistance and slow modulation of blood flow throughout the brain. Vast networks of capillaries feed the brain. Hartmann et al. show that pericyte contractility is critical for maintenance of enduring capillary tone, which sets an optimized rate and distribution of blood flow through brain capillary networks.

Many important cell types in adult vertebrates have a mesenchymal origin, including fibroblasts and vascular mural cells. Although their biological importance is undisputed, the level of mesenchymal cell heterogeneity within and between organs, while appreciated, has not been analyzed in detail. Here, we compare single-cell transcriptional profiles of fibroblasts and vascular mural cells across four murine muscular organs: heart, skeletal muscle, intestine and bladder. We reveal gene expression signatures that demarcate fibroblasts from mural cells and provide molecular signatures for cell subtype identification. We observe striking inter- and intra-organ heterogeneity amongst the fibroblasts, primarily reflecting differences in the expression of extracellular matrix components. Fibroblast subtypes localize to discrete anatomical positions offering novel predictions about physiological function(s) and regulatory signaling circuits. Our data shed new light on the diversity of poorly defined classes of cells and provide a foundation for improved understanding of their roles in physiological and pathological processes. To define and distinguish fibroblasts from vascular mural cells have remained challenging. Here, using single-cell RNA sequencing and tissue imaging, the authors provide a molecular basis for cell type classification and reveal inter- and intra-organ diversity of these cell types.

Key Points Malignant progression of benign tumours is typically associated with an angiogenic switch — the transition from a quiescent to a proliferative vasculature. The de novo recruitment of various innate immune cells was shown to trigger the angiogenic switch in mouse tumour models. Macrophages are important pro-angiogenic cells in the tumour microenvironment. They promote tumour angiogenesis mainly by secreting pro-angiogenic growth factors and facilitating the degradation of the perivascular extracellular matrix. Neutrophils and immature myeloid cells have important roles during the initial angiogenic switch in experimental tumour models. They were also found to sustain tumour revascularization in the context of anti-angiogenic therapy. B cells and T cells may either promote or limit tumour angiogenesis depending on the specific subtype and activation state. In the context of immunotherapy, they may induce the regression of tumour blood vessels. Tumour blood vessels typically display scant pericyte coverage. However, pericytes provide pro-survival cues to angiogenic blood vessels, and their pharmacological targeting improves tumour response to anti-angiogenic therapy. Cancer-associated fibroblasts produce the extracellular matrix and are an important source of pro-angiogenic factors and myeloid cell chemoattractants in the tumour microenvironment. Adipocytes stimulate peri-tumoural angiogenesis by secreting pro-inflammatory and pro-angiogenic cytokines, and by releasing fatty acids that are consumed by angiogenic endothelial cells. The extracellular matrix conveys both pro-angiogenic and angiostatic signals to tumour blood vessels. The metabolic properties of cancer cells and tumour-associated stromal cells influence angiogenesis in many ways (for example, by regulating glucose bioavailability to angiogenic blood vessels). Vascular heterogeneity is a hallmark of cancer and is determined by multiple factors, including the specific organ and tissue in which the tumour arises, the composition of tumour-associated stromal cells, as well as the nature, diversity and relative abundance of pro- and anti-angiogenic mediators. Tumour-associated stromal cells modulate tumour responses to anti-angiogenic therapy. This Review discusses the extrinsic regulation of angiogenesis by the tumour microenvironment, highlighting potential vulnerabilities that could be targeted to improve the applicability and reach of anti-angiogenic cancer therapies. Tumours display considerable variation in the patterning and properties of angiogenic blood vessels, as well as in their responses to anti-angiogenic therapy. Angiogenic programming of neoplastic tissue is a multidimensional process regulated by cancer cells in concert with a variety of tumour-associated stromal cells and their bioactive products, which encompass cytokines and growth factors, the extracellular matrix and secreted microvesicles. In this Review, we discuss the extrinsic regulation of angiogenesis by the tumour microenvironment, highlighting potential vulnerabilities that could be targeted to improve the applicability and reach of anti-angiogenic cancer therapies.

Functional neuroimaging, such as fMRI, is based on coupling neuronal activity and accompanying changes in cerebral blood flow (CBF) and metabolism. However, the relationship between CBF and events at the level of the penetrating arterioles and capillaries is not well established. Recent findings suggest an active role of capillaries in CBF control, and pericytes on capillaries may be major regulators of CBF and initiators of functional imaging signals. Here, using two-photon microscopy of brains in living mice, we demonstrate that stimulation-evoked increases in synaptic activity in the mouse somatosensory cortex evokes capillary dilation starting mostly at the first- or second-order capillary, propagating upstream and downstream at 5–20 μm/s. Therefore, our data support an active role of pericytes in cerebrovascular control. The gliotransmitter ATP applied to first- and second-order capillaries by micropipette puffing induced dilation, followed by constriction, which also propagated at 5–20 μm/s. ATP-induced capillary constriction was blocked by purinergic P2 receptors. Thus, conducted vascular responses in capillaries may be a previously unidentified modulator of cerebrovascular function and functional neuroimaging signals.

Pericytes are branched cells located in the wall of capillary blood vessels that are found throughout the body, embedded within the microvascular basement membrane and wrapping endothelial cells, with which they establish a strong physical contact. Pericytes regulate angiogenesis, vessel stabilization, and contribute to the formation of both the blood-brain and blood-retina barriers by Angiopoietin-1/Tie-2, platelet derived growth factor (PDGF) and transforming growth factor (TGF) signaling pathways, regulating pericyte-endothelial cell communication. Human pericytes that have been cultured for a long period give rise to multilineage progenitor cells and exhibit mesenchymal stem cell (MSC) features. We focused our attention on the roles of pericytes in brain and ocular diseases. In particular, pericyte involvement in brain ischemia, brain tumors, diabetic retinopathy, and uveal melanoma is described. Several molecules, such as adenosine and nitric oxide, are responsible for pericyte shrinkage during ischemia-reperfusion. Anti-inflammatory molecules, such as IL-10, TGFβ, and MHC-II, which are increased in glioblastoma-activated pericytes, are responsible for tumor growth. As regards the eye, pericytes play a role not only in ocular vessel stabilization, but also as a stem cell niche that contributes to regenerative processes in diabetic retinopathy. Moreover, pericytes participate in melanoma cell extravasation and the genetic ablation of the PDGF receptor reduces the number of pericytes and aberrant tumor microvessel formation with important implications for therapy efficacy. Thanks to their MSC features, pericytes could be considered excellent candidates to promote nervous tissue repair and for regenerative medicine.

Pericytes are positioned between brain capillary endothelial cells, astrocytes and neurons. They degenerate in multiple neurological disorders. However, their role in the pathogenesis of these disorders remains debatable. Here we generate an inducible pericyte-specific Cre line and cross pericyte-specific Cre mice with iDTR mice carrying Cre-dependent human diphtheria toxin receptor. After pericyte ablation with diphtheria toxin, mice showed acute blood–brain barrier breakdown, severe loss of blood flow, and a rapid neuron loss that was associated with loss of pericyte-derived pleiotrophin (PTN), a neurotrophic growth factor. Intracerebroventricular PTN infusions prevented neuron loss in pericyte-ablated mice despite persistent circulatory changes. Silencing of pericyte-derived Ptn rendered neurons vulnerable to ischemic and excitotoxic injury. Our data demonstrate a rapid neurodegeneration cascade that links pericyte loss to acute circulatory collapse and loss of PTN neurotrophic support. These findings may have implications for the pathogenesis and treatment of neurological disorders that are associated with pericyte loss and/or neurovascular dysfunction.

The cross-talk between immune cells and blood vessel endothelial cells promotes pericyte coverage and decreases hypoxia in mouse tumour models, and correlative evidence suggests that these processes influence cancer prognosis in humans. Normalizing tumour vasculature Tumours often develop with abnormal vasculature, characterized among other things by lower pericyte coverage of blood vessels, as well as leaky vessels that result in a hypoxic environment. Abnormal vessels limit immune infiltration and CD4 T cells can regulate angiogenesis. Using mouse models, the authors further dissect this crosstalk between immune cells and blood vessels in cancer, and describe a role for immune cells in normalizing the vasculature of tumours. The crosstalk between CD4 T cells and endothelial cells promotes pericyte coverage and decreases hypoxia, and correlative evidence suggests that these processes influence cancer prognosis in humans. The authors postulate that interventions that foster CD4 T-cell function, such as immune checkpoint blockade, also have a beneficial effect by normalizing the tumour vasculature. Blockade of angiogenesis can retard tumour growth, but may also paradoxically increase metastasis 1 , 2 . This paradox may be resolved by vessel normalization 3 , which involves increased pericyte coverage, improved tumour vessel perfusion, reduced vascular permeability, and consequently mitigated hypoxia 3 . Although these processes alter tumour progression, their regulation is poorly understood. Here we show that type 1 T helper (T H 1) cells play a crucial role in vessel normalization. Bioinformatic analyses revealed that gene expression features related to vessel normalization correlate with immunostimulatory pathways, especially T lymphocyte infiltration or activity. To delineate the causal relationship, we used various mouse models with vessel normalization or T lymphocyte deficiencies. Although disruption of vessel normalization reduced T lymphocyte infiltration as expected 4 , reciprocal depletion or inactivation of CD4 + T lymphocytes decreased vessel normalization, indicating a mutually regulatory loop. In addition, activation of CD4 + T lymphocytes by immune checkpoint blockade increased vessel normalization. T H 1 cells that secrete interferon-γ are a major population of cells associated with vessel normalization. Patient-derived xenograft tumours growing in immunodeficient mice exhibited enhanced hypoxia compared to the original tumours in immunocompetent humans, and hypoxia was reduced by adoptive T H 1 transfer. Our findings elucidate an unexpected role of T H 1 cells in vasculature and immune reprogramming. T H 1 cells may be a marker and a determinant of both immune checkpoint blockade and anti-angiogenesis efficacy.

Oligodendrocytes myelinate axons in the central nervous system and develop from oligodndrocyte precursor cells (OPCs) that must first migrate extensively during brain and spinal cord development. We show that OPCs require the vasculature as a physical substrate for migration. We observed that OPCs of the embryonic mouse brain and spinal cord, as well as the human cortex, emerge from progenitor domains and associate with the abluminal endothelial surface of nearby blood vessels. Migrating OPCs crawl along and jump between vessels. OPC migration in vivo was disrupted in mice with defective vascular architecture but was normal in mice lacking pericytes. Thus, physical interactions with the vascular endothelium are required for OPC migration. We identify Wnt-Cxcr4 (chemokine receptor 4) signaling in regulation of OPC-endothelial interactions and propose that this signaling coordinates OPC migration with differentiation.