Subpopulation Of Pericytes Research Articles

Metabolic Coordination of Pericyte Phenotypes: Therapeutic Implications.

Pericytes are mural vascular cells found predominantly on the abluminal wall of capillaries, where they contribute to the maintenance of capillary structural integrity and vascular permeability. Generally quiescent cells in the adult, pericyte activation and proliferation occur during both physiological and pathological vascular and tissue remodeling. A considerable body of research indicates that pericytes possess attributes of a multipotent adult stem cell, as they are capable of self-renewal as well as commitment and differentiation into multiple lineages. However, pericytes also display phenotypic heterogeneity and recent studies indicate that lineage potential differs between pericyte subpopulations. While numerous microenvironmental cues and cell signaling pathways are known to regulate pericyte functions, the roles that metabolic pathways play in pericyte quiescence, self-renewal or differentiation have been given limited consideration to date. This review will summarize existing data regarding pericyte metabolism and will discuss the coupling of signal pathways to shifts in metabolic pathway preferences that ultimately regulate pericyte quiescence, self-renewal and trans-differentiation. The association between dysregulated metabolic processes and development of pericyte pathologies will be highlighted. Despite ongoing debate regarding pericyte classification and their functional capacity for trans-differentiation in vivo, pericytes are increasingly exploited as a cell therapy tool to promote tissue healing and regeneration. Ultimately, the efficacy of therapeutic approaches hinges on the capacity to effectively control/optimize the fate of the implanted pericytes. Thus, we will identify knowledge gaps that need to be addressed to more effectively harness the opportunity for therapeutic manipulation of pericytes to control pathological outcomes in tissue remodeling.

Dynamic transition of the blood-brain barrier in the development of non-small cell lung cancer brain metastases.

Invasion of the brain by non-small cell lung cancer (NSCLC) results in a shift of the blood-brain barrier (BBB) to the insufficiently characterized blood-tumor barrier (BTB). Effective drug delivery through the BTB is one of the greatest therapeutic obstacles in treating brain metastases. Using an experimental model, we defined key changes within the BTB and the BBB in the brain around the tumor (BAT) region over time. Brain-seeking NSCLC cells were delivered into the circulation of athymic-nude mice via intracardiac injection and developing brain metastases were evaluated over six-weeks. Components of the BBB and BTB were analyzed using immunofluorescence microscopy and compared using a mixed model of regression. Our results demonstrate a dynamic time-dependent BTB phenotype. Capillaries of the BAT and BTB were dilated with increased CD31 expression compared to controls. Expression of collagen IV, a pan-basement membrane component, was significantly decreased in the BTB compared to the BBB. There was also a significant increase in the desmin-positive pericyte subpopulation in the BTB compared to the BBB. The most striking changes were identified in astrocyte water channels with a 12.18-fold (p < 0.001) decrease in aquaporin-4 in the BTB; the BAT was unchanged. Analysis of NSCLC brain metastases from patient samples similarly demonstrated dilated capillaries and loss of both collagen IV and aquaporin-4. These data provide a comprehensive analysis of the BTB in NSCLC brain metastasis. Astrocytic endfeet, pericytes, and the basement membrane are potential therapeutic targets to improve efficacy of chemotherapeutic delivery into NSCLC brain metastases.

Pericytes in Chronic Lung Disease.

Pericytes are supportive mesenchymal cells located on the abluminal surface of the microvasculature, with key roles in regulating microvascular homeostasis, leukocyte extravasation, and angiogenesis. A subpopulation of pericytes with progenitor cell function has recently been identified, with evidence demonstrating the capacity of tissue-resident pericytes to differentiate into the classic MSC triad, i.e., osteocytes, chondrocytes, and adipocytes. Beyond the regenerative capacity of these cells, studies have shown that pericytes play crucial roles in various pathologies in the lung, both acute (acute respiratory distress syndrome and sepsis-related pulmonary edema) and chronic (pulmonary hypertension, lung tumors, idiopathic pulmonary fibrosis, asthma, and chronic obstructive pulmonary disease). Taken together, this body of evidence suggests that, in the presence of acute and chronic pulmonary inflammation, pericytes are not associated with tissue regeneration and repair, but rather transform into scar-forming myofibroblasts, with devastating outcomes regarding lung structure and function. It is hoped that further studies into the mechanisms of pericyte-to-myofibroblast transition and migration to fibrotic foci will clarify the roles of pericytes in chronic lung disease and open up new avenues in the search for novel treatments for human pulmonary pathologies.

Pericytes in Muscular Dystrophies.

The muscular dystrophies are an heterogeneous group of inherited myopathies characterised by the progressive wasting of skeletal muscle tissue. Pericytes have been shown to make muscle in vitro and to contribute to skeletal muscle regeneration in several animal models, although recent data has shown this to be controversial. In fact, some pericyte subpopulations have been shown to contribute to fibrosis and adipose deposition in muscle. In this chapter, we explore the identity and the multifaceted role of pericytes in dystrophic muscle, potential therapeutic applications and the current need to overcome the hurdles of characterisation (both to identify pericyte subpopulations and track cell fate), to prevent deleterious differentiation towards myogenic-inhibiting subpopulations, and to improve cell proliferation and engraftment efficacy.

The controversial origin of pericytes during angiogenesis - Implications for cell-based therapeutic angiogenesis and cell-based therapies.

Pericytes reside within the basement membrane of small vessels and are often in direct cellular contact with endothelial cells, fulfilling important functions during blood vessel formation and homeostasis. Recently, these pericytes have been also identified as mesenchymal stem cells. Mesenchymal stem cells, and especially their specialized subpopulation of pericytes, represent promising candidates for therapeutic angiogenesis applications, and have already been widely applied in pre-clinical and clinical trials. However, cell-based therapies of ischemic diseases (especially of myocardial infarction) have not resulted in significant long-term improvement. Interestingly, pericytes from a hematopoietic origin were observed in embryonic skin and a pericyte sub-population expressing leukocyte and monocyte markers was described during adult angiogenesis in vivo. Since mesenchymal stem cells do not express hematopoietic markers, the latter cell type might represent an alternative pericyte population relevant to angiogenesis. Therefore, we sourced blood-derived angiogenic cells (BDACs) from monocytes that closely resembled hematopoietic pericytes, which had only been observed in vivo thus far. BDACs displayed many pericytic features and exhibited enhanced revascularization and functional tissue regeneration in a pre-clinical model of critical limb ischemia. Comparison between BDACs and mesenchymal pericytes indicated that BDACs (while resembling hematopoietic pericytes) enhanced early stages of angiogenesis, such as endothelial cell sprouting. In contrast, mesenchymal pericytes were responsible for blood vessel maturation and homeostasis, while reducing endothelial sprouting.Since the formation of new blood vessels is crucial during therapeutic angiogenesis or during integration of implants into the host tissue, hematopoietic pericytes (and therefore BDACs) might offer an advantageous addition or even an alternative for cell-based therapies.

Abstract P1-01-01: The blood-Tumor barrier as a therapeutic target to improve therapy of brain metastases of breast cancer

Abstract Objectives: Brain metastases of breast cancer demonstrate low and heterogeneous levels of permeability to drugs in mouse models and human craniotomies. The Blood-Brain Barrier (BBB), the protective lining of CNS blood vessels, impedes drug entry into the normal brain. When a metastasis forms, the BBB is locally altered to a poorly characterized Blood-Tumor Barrier (BTB). Quantitative experimental models indicate that most brain metastases have increased permeability over the normal BBB, but BTB permeability is both heterogeneous and ˜2 logs less than that of systemic metastases. We have interrogated three hematogenous models of brain metastasis of breast cancer to ask (1) whether the BTB is an ordered structure or a random breakdown of the BBB; (2) among brain metastases, whether consistent differences underlie the BTBs of lesions with low- and high permeabilities to fluorescent markers and drugs; (3) if alterations in BTB composition can functionally change its permeability. Our long term goal is to enhance uptake of drugs into brain metastases to effective levels. Results: When uninvolved brain was compared with any brain metastasis, alterations in endothelial, pericytic, astrocytic, and microglial components of the BBB were observed. Both the pericyte and astrocyte components of the BTB were consistently altered with increased permeability: When metastases with relatively low and high permeability were compared, increased expression of a desmin+ subpopulation of pericytes was associated with higher permeability (231-BR6 P=0.0002; JIMT-1-BR3 P = 0.004; SUM190-BR3 P=0.008). A trend toward reduced CD13+ pericytes was observed in highly permeable metastases (231-BR6 P =0.014; JIMT-1-BR3 P =0.002, SUM190-BR3, NS). For GFAP+ astrocytes in the neuroinflammatory response surrounding metastases, no overall difference in cell number was observed between low and high permeability lesions. However, gene expression profiling of laser capture microdissected low and high permeabililty lesions demonstrated overexpression of the sphingosine-1 phosphate receptor 3 (S1P3) in the astrocytes of highly permeable lesions, which was confirmed at the protein expression level in all three models (231-BR6 P=0.034; JIMT-1-BR3 P = 0.01; SUM190-BR3 P=0.016). Inhibition of S1P3 via S1PR3 shRNA or a selective antagonist (TY-52156) functionally tightened the BTB in an in vitro model. Administration of TY-52156 to mice harboring 231-BR6 brain metastases had no effect on metastasis number, but decreased uptake of Texas Red Dextran dye into metastases (P=0.016). S1P3 mediated its effects on BTB permeability through astrocytic secretion of IL-6 and CCL2, which altered endothelial expression and localization of adhesive proteins, a potentially translatable pathway. Both desmin+ pericytes and S1P3+ astrocytes are present in human craniotomy specimens. Conclusions: These experiments demonstrate that the BTB is a structure with consistent properties, and that further consistent changes underlie the transition from a low to high permeability BTB. While proof of principle, S1P3 inhibition studies indicate that the BTB permeability can be functionally modulated in vivo. Citation Format: Steeg PS, Lyle TR, Paranjapee A, Lockman PR, Duchnowska R, Brastianos PK, Peer C, Figg WD, Pauly GT, Schneider JP, Smith QR, Gril B. The blood-Tumor barrier as a therapeutic target to improve therapy of brain metastases of breast cancer [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P1-01-01.

Pericyte Biology: Development, Homeostasis, and Disease.

In the nineteenth century, a French researcher, Charles-Marie Benjamin Rouget, revealed a population of contractile cells associated with small blood vessels, which were initially named after him as the Rouget cells. In the twentieth century, a German scientist, Karl Wilhelm Zimmermann, called these cells "pericytes" due to their anatomical position located in a perivascular position. The word pericyte was derived from "peri" meaning "around" and "cyte" from the word "kytos" (cell), illustrating a cell encircling a blood vessel. Until now, pericytes are still identified partially based on their specific anatomical location and morphology. Pericytes are present in all vascularized tissues, surrounding blood vessel walls. They encircle endothelial cells and communicate with them along the length of the blood vessels by paracrine signaling and physical contacts. Previously, the accurate distinction of pericytes from other perivascular cells was difficult, as electron and light microscopy were the sole available techniques capable to image these cells, limiting the information acquired from those works. This resulted in the misleading assumption that pericytes are merely inert supporting cells, limited exclusively to the physiological function of vascular stability. In the last 10years, the combination of fluorescent and confocal microscopy with genetic state-of-art techniques, such as fate lineage tracing, enabled remarkable progress in the discovery of multiple novel essential functions for pericytes in health and disease, before unexpected. Recently, the rapidly expanding understanding of the pathophysiological roles of pericytes drew the attention of several research groups. Now, we know, for instance, that pericytes may play immune functions: attract innate leukocytes to exit via sprouting blood vessels, regulate lymphocyte activation, and contribute to the clearance of toxic by-products, having direct phagocytic activity. Pericytes also may behave as stem cells, forming other cell populations, as well as regulate the behavior of other stem cells in their niches. Very little is known about the exact identity of pericyte ancestors within developing tissues, and there is evidence for multiple distinct developmental sources. Pericytes differ in their embryonic origins between tissues and also within the same organ. Importantly, pericytes from distinct tissues may differ in their distribution, morphology, expression of molecular markers, plasticity, and functions; and, even within the same organ, there are various pericyte subpopulations. This book describes the major contributions of pericytes to different organ biology in physiological and pathological conditions. Further insights into the biology of pericytes will have important implications for our understanding of organ development, homeostasis, and disease. This book's initial title was "Pericyte Biology: Development, Homeostasis, and Disease." However, due to the current great interest in this topic, we were able to assemble more chapters than would fit in one book, covering pericyte biology under distinct circumstances. Therefore, the book was subdivided into three volumes entitled: "Pericyte Biology: Novel Concepts," "Pericyte Biology in Different Organs," and "Pericyte Biology in Disease." Here, we present a selected collection of detailed chapters on what we know so far about pericytes. More than 30 chapters written by experts in the field summarize our present knowledge on pericyte biology.Here, we present a selected collection of detailed chapters on what we know so far about pericytes. More than 30 chapters written by experts in the field summarize our present knowledge on pericyte biology.

Abstract 4933: Desmin+pericyte subpopulations correlated with blood-tumor barrier permeability in brain metastases of breast cancer

Abstract Breast cancer brain metastases remain incurable. The blood-brain barrier (BBB) is a multicellular dynamic structure regulating exchanges between the blood and the central nervous system. As cancer cells colonize the brain, the BBB evolves into a blood-tumor barrier (BTB). The BTB limits compound penetration and therefore contributes to poor efficacy of chemotherapy. While the BBB has been well characterized in developmental and neurodegenerative disease studies, the BTB composition remains unknown. We have characterized the BTB in three model systems of brain metastasis of breast cancer developed in the laboratory: a triple negative (231-BR6), and two HER2 overexpressing (SUM190-BR3, JIMT-1-BR3) subtypes. Using Texas Red dextran (TRD) as a marker of permeability and quantitative immunofluorescence staining, we analyzed the cellular and molecular composition of: 1) unaltered BBB vs. BTB, and 2) BTB in highly permeable metastases vs. BTB in poorly permeable metastases. The BTB developed from the BBB in a series of alterations, including a neuroinflammatory reaction with astrogliosis, endothelial cell dilation, increased VEGF, reduced astrocyte endfoot polarity, and decrease in PDGFR+ pericytes. Only 10% of the metastatic lesions harbored a profound TRD exudation, which correlated with paclitaxel efficacy. We hypothesized that specific cellular and molecular changes account for the heterogeneity and increase in TRD diffusion. When metastases with relatively low- and high-TRD diffusion were compared, highly permeable metastases correlated with an increased expression of desmin+ pericytes in three models (231-BR6 p=0.0002; JIMT-1-BR3 p=0.004; SUM190-BR3 p=0.008) and a decrease in CD13+ pericytes in two model systems (231-BR6 p=0.014; JIMT-1-BR3 p=0.002). Decreased expression of laminin α2 in the parenchymal basement membrane (231-BR6 p=0.001; JIMT-1-BR3 p=0.049; SUM190-BR3 p=0.023) were associated with higher permeability. Desmin+ pericytes have been associated with pathological conditions such as fibrosis and spinal cord injury. Seven over nine human craniotomy specimens were positive for Desmin staining, validating clinically the relevance of our findings. We subsequently hypothesized that the desmin+ pericyte subpopulation functionally contributes to increased permeability. Desmin+ pericytes were produced in vitro by co-culturing primary mouse pericytes with astrocytes. When desmin+ or CD13+ pericytes were added to in vitro transendothelial electrical resistance (TEER) models of the BBB, the desmin+ pericytes exhibited less resistance, indicative of higher permeability. The data suggest that desmin+ pericytes may facilitate the permeability of the BTB. These studies show that the BTB in brain metastasis model systems involves consistent molecular changes. These data may identify new strategies to selectively permeabilize the BTB and enhance chemotherapeutic efficacy. Citation Format: Brunilde Gril, L. Tiffany Lyle, Paul R. Lockman, Chris E. Adkins, Afroz Shareef Mohammad, Emily Sechrest, Emily Hua, Diane Palmieri, David J. Liewehr, Seth M. Steinberg, Wojciech Kloc, Ewa Izycka-Swieszewska, Renata Duchnowska, Nayyar Naema, Priscilla K. Brastianos, Patricia S. Steeg. Desmin+pericyte subpopulations correlated with blood-tumor barrier permeability in brain metastases of breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 4933. doi:10.1158/1538-7445.AM2017-4933

Isolation of Type I and Type II Pericytes from Mouse Skeletal Muscles.

Pericytes are perivascular multipotent cells that show heterogeneity in different organs or even within the same tissue. In skeletal muscles, there are at least two pericyte subpopulations (called type I and type II), which express different molecular markers and have distinct differentiation capabilities. Using NG2-DsRed and Nestin-GFP double-transgenic mice, type I (NG2-DsRed+Nestin-GFP-) and type II (NG2-DsRed+Nestin-GFP+) pericytes have been successfully isolated. However, the availability of these double-transgenic mice prevents the widespread use of this purification method. This work describes an alternative protocol that allows for the easy and simultaneous isolation of type I and type II pericytes from skeletal muscles. This protocol utilizes the fluorescence-activated cell sorting (FACS) technique and targets PDGFRβ, rather than NG2, together with the Nestin-GFP signal. Following isolation, type I and type II pericytes show distinct morphologies. In addition, type I and type II pericytes isolated with this new method, like those isolated from the double-transgenic mice, are adipogenic and myogenic, respectively. These results suggest that this protocol can be used to isolate pericyte subpopulations from skeletal muscles and possibly from other tissues.

Pericytes, integral components of adult hematopoietic stem cell niches.

The interest in perivascular cells as a niche for adult hematopoietic stem cells (HSCs) is significantly growing. In the adult bone marrow (BM), perivascular cells and HSCs cohabit. Among perivascular cells, pericytes are precursors of mesenchymal stem/stromal cells (MSCs) that are capable of differentiating into osteoblasts, adipocytes and chondrocytes. In situ, pericytes are recognised by their localisation to the abluminal side of the blood vessel wall and closely associated with endothelial cells, in combination with the expression of markers such as CD146, neural glial 2 (NG2), platelet derived growth factor receptor β (PDGFRβ), α-smooth muscle actin (α-SMA), nestin (Nes) and/or leptin receptor (LepR). However, not all pericytes share a common phenotype: different immunophenotypes can be associated with distinct mesenchymal features, including hematopoietic support. In adult BM, arteriolar and sinusoidal pericytes control HSC behaviour, maintenance, quiescence and trafficking through paracrine effects. Different groups identified and characterized hematopoietic supportive pericyte subpopulations using various markers and mouse models. In this review, we summarize recent work performed by others to understand the role of the perivascular niche in the biology of HSCs in adults, as well as their importance in the development of therapies.

Alterations in Pericyte Subpopulations Are Associated with Elevated Blood-Tumor Barrier Permeability in Experimental Brain Metastasis of Breast Cancer.

The blood-brain barrier (BBB) is modified to a blood-tumor barrier (BTB) as a brain metastasis develops from breast or other cancers. We (i) quantified the permeability of experimental brain metastases, (ii) determined the composition of the BTB, and (iii) identified which elements of the BTB distinguished metastases of lower permeability from those with higher permeability. A SUM190-BR3 experimental inflammatory breast cancer brain metastasis subline was established. Experimental brain metastases from this model system and two previously reported models (triple-negative MDA-231-BR6, HER2+ JIMT-1-BR3) were serially sectioned; low- and high-permeability lesions were identified with systemic 3-kDa Texas Red dextran dye. Adjoining sections were used for quantitative immunofluorescence to known BBB and neuroinflammatory components. One-sample comparisons against a hypothesized value of one were performed with the Wilcoxon signed-rank test. When uninvolved brain was compared with any brain metastasis, alterations in endothelial, pericytic, astrocytic, and microglial components were observed. When metastases with relatively low and high permeability were compared, increased expression of a desmin+ subpopulation of pericytes was associated with higher permeability (231-BR6 P = 0.0002; JIMT-1-BR3 P = 0.004; SUM190-BR3 P = 0.008); desmin+ pericytes were also identified in human craniotomy specimens. Trends of reduced CD13+ pericytes (231-BR6 P = 0.014; JIMT-1-BR3 P = 0.002, SUM190-BR3, NS) and laminin α2 (231-BR6 P = 0.001; JIMT-1-BR3 P = 0.049; SUM190-BR3 P = 0.023) were also observed with increased permeability. We provide the first account of the composition of the BTB in experimental brain metastasis. Desmin+ pericytes and laminin α2 are potential targets for the development of novel approaches to increase chemotherapeutic efficacy. Clin Cancer Res; 22(21); 5287-99. ©2016 AACR.

Recovery from Toxic-Induced Demyelination Does Not Require the NG2 Proteoglycan.

NG2 cells are defined as CNS cells expressing chondroitin sulfate proteoglycan nerve/glia antigen. The vast majority of NG2-positive cells also express platelet-derived growth factor receptor alpha (PDGFRα) and are oligodendroglial progenitors (OPC). In addition a subpopulation of pericytes expresses NG2, but is positive for PDGF receptor beta (PDGFRβ) [1]. NG2-positive OPC comprise approximately 5% of the cells in the CNS where they are evenly distributed in grey and white matter [2, 3]. NG2-positive OPC form synapses with neurons [4–6] and react to brain injury with proliferation, as has been shown in several animal models as well as in human demyelinating and degenerative diseases [7–9]. In vitro, NG2 positive cells can give rise to oligodendrocytes, astrocytes and occasional neurons depending on cell culture conditions [10–12]. In vivo, NG2 cells generate mostly oligodendrocytes as well as small populations of astrocytes as has been demonstrated in fate mapping studies [9, 13–16]. The developmental fate switch from the oligodendroglial into the astrocytic lineage is regulated by Olig2 [17]. A large percentage of NG2 positive cells persists as a self-renewing population in the adult CNS [18–21]. Although NG2 has been extensively used as a marker for OPC, relatively little is known about the functional role of the NG2 proteoglycan. NG2 consists of a small intracellular and a large extracellular domain. The extracellular domain is cleaved by proteases such as ADAM 10 in an activity-dependent fashion, which regulates glutamate signalling at nearby neurons. The NG2 extracellular domain binds receptors, growth factors, extracellular matrix components and proteases (for review see [22, 23]). Lack of NG2 expression in NG2 deficient (NG2-/-) mice or pharmacological inhibition of NG2 ectodomain shedding in wild type OPC results in NMDA and AMPA receptor-dependent reduction of neuronal current amplitudes and an altered behaviour of NG2-/- mice in tests measuring sensorimotor function. These results demonstrate a bidirectional cross-talk between OPC and the surrounding neuronal network [24, 25]. The intracellular domain can be cleaved by the gamma-secretase and may influence the expression of genes, such as prostaglandin D2 synthase which has neuromodulatory properties [26]. NG2 has been reported to promote migration and proliferation in oligodendroglial and neoplastic glioma cells [27–30]. In OPC the effect on migration is mediated via modulation of Rho GTPases and RAC activity and an influence on cell polarity via selective subcellular localization [31, 32]. Contradictory results have been published with respect to the effect of NG2 on de- and remyelination, as well as on inflammation in inflammatory and/or demyelinating animal models. Mice lacking NG2 were reported to show reduced inflammation as well as reduced myelin damage and repair after injection of lysolecithin [33]. In contrast, in EAE experiments using this same NG2-/- mouse line [34] no differences in disease course or extent of de- and remyelination or inflammation was observed [35]. We hypothesized that the effect of a lack of NG2 might be amplified by extended time periods of demyelination. Furthermore, the initial NG2-/- mouse line [34] was generated by insertion of a neo cassette that may affect the function of nearby genes. We thus utilized mice lacking NG2 in which eYFP was inserted in the endogenous NG2 locus [36]. When bred to homozygosity these mice lack expression of NG2. We fed homozygous NG2-/- mice and their wild type littermates (NG2+/+) for 10 weeks with the copper chelator cuprizone which leads to oligodendroglial death and compared the extent of de- and remyelination as well as the degree of inflammation as measured by the numbers of Mac3 (+) microglia/macrophages. In addition, we isolated OPC from NG2-/- as well NG2+/+ mice to compare and analyze oligodendroglial properties prerequisite for remyelination, namely proliferation, migration and differentiation. In vitro, NG2-/- OPC demonstrated an increased migratory capacity in PDGF-AA, but not FGF-elicited chemotaxis; however lack of NG2 did not affect proliferation or differentiation of isolated OPC. No effect of NG2 deficiency on de- or remyelination, numbers of myelinated axons, oligodendrocytes or microglia/macrophages was observed.

Leptin and leptin receptor gene polymorphisms are associated with clinical and metabolic parameters in resistant hypertension

The interest in perivascular cells as a niche for adult hematopoietic stem cells (HSCs) is significantly growing. In the adult bone marrow (BM), perivascular cells and HSCs cohabit. Among perivascular cells, pericytes are precursors of mesenchymal stem/stromal cells (MSCs) that are capable of differentiating into osteoblasts, adipocytes and chondrocytes. In situ, pericytes are recognised by their localisation to the abluminal side of the blood vessel wall and closely associated with endothelial cells, in combination with the expression of markers such as CD146, neural glial 2 (NG2), platelet derived growth factor receptor β (PDGFRβ), α-smooth muscle actin (α-SMA), nestin (Nes) and/or leptin receptor (LepR). However, not all pericytes share a common phenotype: different immunophenotypes can be associated with distinct mesenchymal features, including hematopoietic support. In adult BM, arteriolar and sinusoidal pericytes control HSC behaviour, maintenance, quiescence and trafficking through paracrine effects. Different groups identified and characterized hematopoietic supportive pericyte subpopulations using various markers and mouse models. In this review, we summarize recent work performed by others to understand the role of the perivascular niche in the biology of HSCs in adults, as well as their importance in the development of therapies.

ABCG2pos lung mesenchymal stem cells are a novel pericyte subpopulation that contributes to fibrotic remodeling.

Genesis of myofibroblasts is obligatory for the development of pathology in many adult lung diseases. Adult lung tissue contains a population of perivascular ABCG2(pos) mesenchymal stem cells (MSC) that are precursors of myofibroblasts and distinct from NG2 pericytes. We hypothesized that these MSC participate in deleterious remodeling associated with pulmonary fibrosis (PF) and associated hypertension (PH). To test this hypothesis, resident lung MSC were quantified in lung samples from control subjects and PF patients. ABCG2(pos) cell numbers were decreased in human PF and interstitial lung disease compared with control samples. Genetic labeling of lung MSC in mice enabled determination of terminal lineage and localization of ABCG2 cells following intratracheal administration of bleomycin to elicit fibrotic lung injury. Fourteen days following bleomycin injury enhanced green fluorescent protein (eGFP)-labeled lung MSC-derived cells were increased in number and localized to interstitial areas of fibrotic and microvessel remodeling. Finally, gene expression analysis was evaluated to define the response of MSC to bleomycin injury in vivo using ABCG2(pos) MSC isolated during the inflammatory phase postinjury and in vitro bleomycin or transforming growth factor-β1 (TGF-β1)-treated cells. MSC responded to bleomycin treatment in vivo with a profibrotic gene program that was not recapitulated in vitro with bleomycin treatment. However, TGF-β1 treatment induced the appearance of a profibrotic myofibroblast phenotype in vitro. Additionally, when exposed to the profibrotic stimulus, TGF-β1, ABCG2, and NG2 pericytes demonstrated distinct responses. Our data highlight ABCG2(pos) lung MSC as a novel cell population that contributes to detrimental myofibroblast-mediated remodeling during PF.

TRPM4‐dependent contraction of retinal pericytes (LB846)

Pericytes are an integral part of the neurovascular unit and likely contribute to the neuronal‐dependent regulation of capillary blood flow. Like vascular smooth muscle cells (VSMCs), pericytes are thought to contract through membrane depolarization and activation of voltage‐dependent calcium channels which then causes focal constrictions of capillaries. TRPM4, a melastatin transient receptor potential channel, is essential for VSMC membrane depolarization and contraction. Its role in pericyte contraction is not known. We used immunohistochemistry and confocal microscopy to examine the role of TRPM4 channels in pericyte contraction within the mouse retina. We observed TRPM4‐specific immunolabeling in the smooth muscle cells within precapillary arterioles (&lt;10 µm) and in a sub‐population of pericytes surrounding the capillaries most proximal to the arterioles in the mouse retinal vasculature. In addition, we observed constriction in response to the thromboxane A2 analog, 9,11‐dideoxy‐9α,11α‐methanoepoxy prostaglandin F2α (U46619) in pre‐capillary arterioles and in distal and proximal pericyte‐containing capillaries. This response was blocked by the TRPM4 inhibitor 9‐phenanthrol in pre‐capillary arterioles and proximal pericyte‐containing capillaries, but not in pericyte‐containing capillaries distal to feeding arteriole. These findings suggest that TRPM4 plays a role in pericyte constriction within retinal capillary system.Grant Funding Source: HL044455, R37 DK053832, and 1PO1HL095488

Role of Pericytes in Skeletal Muscle Regeneration and Fat Accumulation

Stem cells ensure tissue regeneration, while overgrowth of adipogenic cells may compromise organ recovery and impair function. In myopathies and muscle atrophy associated with aging, fat accumulation increases dysfunction, and after chronic injury, the process of fatty degeneration, in which muscle is replaced by white adipocytes, further compromises tissue function and environment. Some studies suggest that pericytes may contribute to muscle regeneration as well as fat formation. This work reports the presence of two pericyte subpopulations in the skeletal muscle and characterizes their specific roles. Skeletal muscle from Nestin-GFP/NG2-DsRed mice show two types of pericytes, Nestin-GFP-/NG2-DsRed+ (type-1) and Nestin-GFP+/NG2-DsRed+ (type-2), in close proximity to endothelial cells. We also found that both Nestin-GFP-/NG2-DsRed+ and Nestin-GFP+/NG2-DsRed+ cells colocalize with staining of two pericyte markers, PDGFRβ and CD146, but only type-1 pericyte express the adipogenic progenitor marker PDGFRα. Type-2 pericytes participate in muscle regeneration, while type-1 contribute to fat accumulation. Transplantation studies indicate that type-1 pericytes do not form muscle in vivo, but contribute to fat deposition in the skeletal muscle, while type-2 pericytes contribute only to the new muscle formation after injury, but not to the fat accumulation. Our results suggest that type-1 and type-2 pericytes contribute to successful muscle regeneration which results from a balance of myogenic and nonmyogenic cells activation.

Msx1 and Msx2 are expressed in sub‐populations of vascular smooth muscle cells

Using an nlacZ reporter gene inserted at the Msx1 and Msx2 loci, we could analyze the expression of these homeogenes in the adult mouse. We observed that Msx genes are prominently expressed in a subset of blood vessels. The Msx2nlacZ allele is mainly expressed in a restricted population of mural cells in peripheral arteries and veins. Msx1nlacZ is expressed to a lesser extent by vascular smooth muscle cells of peripheral arteries, but is highly expressed in arterioles and capillaries, making Msx1 a novel marker for a subpopulation of pericytes. Expression is set up early in developing vessels and maintained throughout life. In addition, expression of both genes is observed in a few endothelial cells of the aorta at fetal stages, and only Msx2 continues to be expressed in this layer at the adult stage. These results suggest major functions for Msx genes in vascular mural cell formation and remodeling.