Pericytes modulate endothelial sprouting

TGF-β1 Drives Integrin-Dependent Pericyte Migration and Microvascular Destabilization in Fibrotic Disease

Interactions between endothelial cells (ECs) and mural pericytes (PCs) are critical in maintaining the stability and function of the microvascular wall. Abnormal interactions between these two cell types are a hallmark of progressive fibrotic diseases such as systemic sclerosis (also known as scleroderma). However, the role of PCs in signaling microvascular dysfunction remains underexplored. We hypothesized that integrin-matrix interactions contribute to PC migration from the vascular wall and conversion into interstitial myofibroblasts. Herein, pro-inflammatory tumor necrosis factor α (TNFα) or a fibrotic growth factor [transforming growth factor β1 (TGF-β1)] were used to evaluate human PC inflammatory and fibrotic phenotypes by assessing their migration, matrix deposition, integrin expression, and subsequent effects on endothelial dysfunction. Both TNFα and TGF-β1 treatment altered integrin expression and matrix protein deposition, but only fibrotic TGF-β1 drove PC migration in an integrin-dependent manner. In addition, integrin-dependent PC migration was correlated to changes in EC angiopoietin-2 levels, a marker of vascular instability. Finally, there was evidence of changes in vascular stability corresponding to disease state in human systemic sclerosis skin. This work shows that TNFα and TGF-β1 induce changes in PC integrin expression and matrix deposition that facilitate migration and reduce vascular stability, providing evidence that microvascular destabilization can be an early indicator of tissue fibrosis.

Notoginsenoside R1 decreases intraplaque neovascularization by governing pericyte-endothelial cell communication via Ang1/Tie2 axis in atherosclerosis

Atherosclerosis represents the major cause of mortality worldwide and triggers higher risk of acute cardiovascular events. Pericytes-endothelial cells (ECs) communication is orchestrated by ligand-receptor interaction generating a microenvironment which results in intraplaque neovascularization, that is closely associated with atherosclerotic plaque instability. Notoginsenoside R1 (R1) exhibits anti-atherosclerotic bioactivity, but its effect on angiogenesis in atherosclerotic plaque remains elusive. The aim of our study is to explore the therapeutic effect of R1 on vulnerable plaque and investigate its potential mechanism against intraplaque neovascularization. The impacts of R1 on plaque stability and intraplaque neovascularization were assessed in ApoE-/- mice induced by high-fat diet. Pericytes-ECs direct or non-direct contact co-cultured with VEGF-A stimulation were used as the in vitro angiogenesis models. Overexpressing Ang1 in pericytes was performed to investigate the underlying mechanism. In vivo experiments, R1 treatment reversed atherosclerotic plaque vulnerability and decreased the presence of neovessels in ApoE-/- mice. Additionally, R1 reduced the expression of Ang1 in pericytes. In vitro experiments demonstrated that R1 suppressed pro-angiogenic behavior of ECs induced by pericytes cultured with VEGF-A. Mechanistic studies revealed that the anti-angiogenic effect of R1 was dependent on the inhibition of Ang1 and Tie2 expression, as the effects were partially reversed after Ang1 overexpressing in pericytes. Our study demonstrated that R1 treatment inhibited intraplaque neovascularization by governing pericyte-EC association via suppressing Ang1-Tie2/PI3K-AKT paracrine signaling pathway. R1 represents a novel therapeutic strategy for atherosclerotic vulnerable plaques in clinical application.

Long-term day-by-day tracking of microvascular networks sprouting in fibrin gels: From detailed morphological analyses to general growth rules

Understanding and controlling of the evolution of sprouting vascular networks remains one of the basic challenges in tissue engineering. Previous studies on the vascularization dynamics have typically focused only on the phase of intense growth and often lacked spatial control over the initial cell arrangement. Here, we perform long-term day-by-day analysis of tens of isolated microvasculatures sprouting from endothelial cell-coated spherical beads embedded in an external fibrin gel. We systematically study the topological evolution of the sprouting networks over their whole lifespan, i.e., for at least 14 days. We develop a custom image analysis toolkit and quantify (i) the overall length and area of the sprouts, (ii) the distributions of segment lengths and branching angles, and (iii) the average number of branch generations-a measure of network complexity. We show that higher concentrations of vascular endothelial growth factor (VEGF) lead to earlier sprouting and more branched networks, yet without significantly affecting the speed of growth of individual sprouts. We find that the mean branching angle is weakly dependent on VEGF and typically in the range of 60°-75°, suggesting that, by comparison with the available diffusion-limited growth models, the bifurcating tips tend to follow local VEGF gradients. At high VEGF concentrations, we observe exponential distributions of segment lengths, which signify purely stochastic branching. Our results-due to their high statistical relevance-may serve as a benchmark for predictive models, while our new image analysis toolkit, offering unique features and high speed of operation, could be exploited in future angiogenic drug tests.

Biology of vascular mural cells

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

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

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

Comparison of the Behavior of Perivascular Cells (Pericytes and CD34+ Stromal Cell/Telocytes) in Sprouting and Intussusceptive Angiogenesis

Perivascular cells in the pericytic microvasculature, pericytes and CD34+ stromal cells/telocytes (CD34+SCs/TCs), have an important role in angiogenesis. We compare the behavior of these cells depending on whether the growth of endothelial cells (ECs) from the pre-existing microvasculature is toward the interstitium with vascular bud and neovessel formation (sprouting angiogenesis) or toward the vascular lumen with intravascular pillar development and vessel division (intussusceptive angiogenesis). Detachment from the vascular wall, mobilization, proliferation, recruitment, and differentiation of pericytes and CD34+SCs/TCs, as well as associated changes in vessel permeability and functionality, and modifications of the extracellular matrix are more intense, longer lasting over time, and with a greater energy cost in sprouting angiogenesis than in intussusceptive angiogenesis, in which some of the aforementioned events do not occur or are compensated for by others (e.g., sparse EC and pericyte proliferation by cell elongation and thinning). The governing mechanisms involve cell-cell contacts (e.g., peg-and-socket junctions between pericytes and ECs), multiple autocrine and paracrine signaling molecules and pathways (e.g., vascular endothelial growth factor, platelet-derived growth factor, angiopoietins, transforming growth factor B, ephrins, semaphorins, and metalloproteinases), and other factors (e.g., hypoxia, vascular patency, and blood flow). Pericytes participate in vessel development, stabilization, maturation and regression in sprouting angiogenesis, and in interstitial tissue structure formation of the pillar core in intussusceptive angiogenesis. In sprouting angiogenesis, proliferating perivascular CD34+SCs/TCs are an important source of stromal cells during repair through granulation tissue formation and of cancer-associated fibroblasts (CAFs) in tumors. Conversely, CD34+SCs/TCs have less participation as precursor cells in intussusceptive angiogenesis. The dysfunction of these mechanisms is involved in several diseases, including neoplasms, with therapeutic implications.

CD146+ Pericytes Subset Isolated from Human Micro-Fragmented Fat Tissue Display a Strong Interaction with Endothelial Cells: A Potential Cell Target for Therapeutic Angiogenesis

Pericytes (PCs) are mesenchymal stromal cells (MSCs) that function as support cells and play a role in tissue regeneration and, in particular, vascular homeostasis. PCs promote endothelial cells (ECs) survival which is critical for vessel stabilization, maturation, and remodeling. In this study, PCs were isolated from human micro-fragmented adipose tissue (MFAT) obtained from fat lipoaspirate and were characterized as NG2+/PDGFRβ+/CD105+ cells. Here, we tested the fat-derived PCs for the dispensability of the CD146 marker with the aim of better understanding the role of these PC subpopulations on angiogenesis. Cells from both CD146-positive (CD146+) and negative (CD146−) populations were observed to interact with human umbilical vein ECs (HUVECs). In addition, fat-derived PCs were able to induce angiogenesis of ECs in spheroids assay; and conditioned medium (CM) from both PCs and fat tissue itself led to the proliferation of ECs, thereby marking their role in angiogenesis stimulation. However, we found that CD146+ cells were more responsive to PDGF-BB-stimulated migration, adhesion, and angiogenic interaction with ECs, possibly owing to their higher expression of NCAM/CD56 than the corresponding CD146− subpopulation. We conclude that in fat tissue, CD146-expressing cells may represent a more mature pericyte subpopulation that may have higher efficacy in controlling and stimulating vascular regeneration and stabilization than their CD146-negative counterpart.

Bioprinted microvasculature: progressing from structure to function

Three-dimensional (3D) bioprinting seeks to unlock the rapid generation of complex tissue constructs, but long-standing challenges with efficientin vitromicrovascularization must be solved before this can become a reality. Microvasculature is particularly challenging to biofabricate due to the presence of a hollow lumen, a hierarchically branched network topology, and a complex signaling milieu. All of these characteristics are required for proper microvascular-and, thus, tissue-function. While several techniques have been developed to address distinct portions of this microvascularization challenge, no single approach is capable of simultaneously recreating all three microvascular characteristics. In this review, we present a three-part framework that proposes integration of existing techniques to generate mature microvascular constructs. First, extrusion-based 3D bioprinting creates a mesoscale foundation of hollow, endothelialized channels. Second, biochemical and biophysical cues induce endothelial sprouting to create a capillary-mimetic network. Third, the construct is conditioned to enhance network maturity. Across all three of these stages, we highlight the potential for extrusion-based bioprinting to become a central technique for engineering hierarchical microvasculature. We envision that the successful biofabrication of functionally engineered microvasculature will address a critical need in tissue engineering, and propel further advances in regenerative medicine andex vivohuman tissue modeling.

Tumor perivascular cell-derived extracellular vesicles promote angiogenesis via the Gas6/Axl pathway

Aberrant angiogenesis is a hallmark of cancer and is critically associated with tumor progression. Perivascular cells are essential components of blood vessels, and the role of tumor perivascular cell-derived extracellular vesicles (TPC-EVs) in angiogenesis remains elusive. In the present study, using genetic mouse models and pharmacological inhibitors, we found that ablation of perivascular cells inhibited angiogenesis in allografted colorectal cancer tumors. Further studies demonstrated that TPC-EVs promoted the proliferation, migration, invasion, viability, and tube formation of HUVECs. They also facilitated vessel spouting in rat aortic rings and induced neovascularization in chick chorioallantoic membranes (CAMs). Silencing of Gas6 or blockade of the Axl pathway suppressed TPC-EV-induced angiogenesis in vitro and ex vivo. Moreover, inhibition of the Gas6/Axl signaling pathway impaired TPC-EV-mediated angiogenesis in vivo. Our findings present a deeper insight into the biological functions of TPCs and TPC-EVs in tumor angiogenesis and demonstrate that TPC-EV-derived Gas6 could be an attractive and innovative regulator of tumor angiogenesis.

Heterotypic Multicellular Spheroids as Experimental and Preclinical Models of Sprouting Angiogenesis

Sprouting angiogenesis is the common response of live tissues to physiological and pathological angiogenic stimuli. Its accurate evaluation is of utmost importance for basic research and practical medicine and pharmacology and requires adequate experimental models. A variety of assays for angiogenesis were developed, none of them perfect. In vitro approaches are generally less physiologically relevant due to the omission of essential components regulating the process. However, only in vitro models can be entirely non-xenogeneic. The limitations of the in vitro angiogenesis assays can be partially overcome using 3D models mimicking tissue O2 and nutrient gradients, the influence of the extracellular matrix (ECM), and enabling cell-cell interactions. Here we present a review of the existing models of sprouting angiogenesis that are based on the use of endothelial cells (ECs) co-cultured with perivascular or other stromal cells. This approach provides an excellent in vitro platform for further decoding of the cellular and molecular mechanisms of sprouting angiogenesis under conditions close to the in vivo conditions, as well as for preclinical drug testing and preclinical research in tissue engineering and regenerative medicine.

Microvascular fragment spheroids: Three-dimensional vascularization units for tissue engineering and regeneration

Adipose tissue-derived microvascular fragments (MVF) serve as vascularization units in tissue engineering and regenerative medicine. Because a three-dimensional cellular arrangement has been shown to improve cell function, we herein generated for the first time MVF spheroids to investigate whether this further increases their vascularization potential. These spheroids exhibited a morphology, size, and viability comparable to that of previously introduced stromal vascular fraction (SVF) spheroids. However, MVF spheroids contained a significantly higher number of CD31-positive endothelial cells and α-smooth muscle actin (SMA)-positive perivascular cells, resulting in an enhanced angiogenic sprouting activity. Accordingly, they also exhibited an improved in vivo vascularization and engraftment after transplantation into mouse dorsal skinfold chambers. These findings indicate that MVF spheroids are superior to SVF spheroids and, thus, may be highly suitable to improve the vascularization of tissue defects and implanted tissue constructs.

COVID-19: a closer look at the pathology in two autopsied cases. Is the pericyte at the center of the pathological process in COVID-19?

We performed autopsies on two cases of COVID-19. The microcirculations of all organs were the site of the pathological findings. Thrombotic microangiopathy was found in the brain and also the kidneys. Vasculitis was also a feature of the autopsy findings, together with portal triaditis of the liver. The major pathological findings in both cases were fibrin deposits. Within the lung, the fibrin deposits were observed in the alveolar microcirculation in sub-endothelial locations of capillaries, arterioles, post capillary venules, and the adventitia of larger vessels. These fibrin deposits in the lungs occurred at the sites where pericytes are located in these vessels. The pericyte with its high concentration of ACE-2 receptors and its procoagulant state may represent one of the primary sites of action of SARS-CoV-2. A review of pericytes in health and disease is undertaken. COVID-19 is a disease of the microcirculation.

Emerging Role of Pericytes and Their Secretome in the Heart

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

A dual-ink 3D printing strategy to engineer pre-vascularized bone scaffolds in-vitro

A functional vascular supply is a key component of any large-scale tissue, providing support for the metabolic needs of tissue-remodeling cells. Although well-studied strategies exist to fabricate biomimetic scaffolds for bone regeneration, success rates for regeneration in larger defects can be improved by engineering microvascular capillaries within the scaffolds to enhance oxygen and nutrient supply to the core of the engineered tissue as it grows. Even though the role of calcium and phosphate has been well understood to enhance osteogenesis, it remains unclear whether calcium and phosphate may have a detrimental effect on the vasculogenic and angiogenic potential of endothelial cells cultured on 3D printed bone scaffolds. In this study, we presented a novel dual-ink bioprinting method to create vasculature interwoven inside CaP bone constructs. In this method, strands of a CaP ink and a sacrificial template material was used to form scaffolds containing CaP fibers and microchannels seeded with vascular endothelial and mesenchymal stem cells (MSCs) within a photo-crosslinkable gelatin methacryloyl (GelMA) hydrogel material. Our results show similar morphology of growing vessels in the presence of CaP bioink, and no significant difference in endothelial cell sprouting was found. Furthermore, our initial results showed the differentiation of hMSCs into pericytes in the presence of CaP ink. These results indicate the feasibility of creating vascularized bone scaffolds, which can be used for enhancing vascular formation in the core of bone scaffolds.

A bispecific immunotoxin (IHPP) with a long half-life targeting HER2 and PDGFRβ exhibited improved efficacy against HER2-positive tumors in a mouse xenograft model

Multiple signaling pathways are usually involved in the development of tumors. Compared with monospecific antibodies, bispecific antibodies can recognize two different antigens at the same time, so they are more suitable for treating tumor diseases with complex etiology. Immunotoxins have good antitumor activity, however, single targeting limits their effectiveness. Herein, we designed a Pseudomonas exotoxin A (PE)-based bispecific immunotoxin IgBD-HER2-PDGFRβ-PE38 which could distinguish HER2 and PDGFRβ target in tumor. Meanwhile, IgG-affinity could extend the serum retention of immunotoxins after in vivo injection. In this work, we first detected the selective binding of the immunotoxins and antitumor effect in vitro. Compared with control group, IgBD-HER2-PDGFRβ-PE38 exhibited improved efficacy against HER2-positive tumors in an NCI-N87 subcutaneous xenograft model. Then, transcriptome sequencing was performed on tumor tissue originating from different treatment groups of mice bearing NCI-N87 tumors. Seven significantly differentially expressed genes were screened based on human genes, and the differential mouse genes were enriched based on the Reactome Pathway Database. At last, the RNA sequencing results were verified by real-time PCR and ELISA. Therefore, the new construct bispecific immunotoxin represents a potentially attractive therapeutic modality, and the proposed strategy make them promising for use in the development of anti-HER2 cancer therapeutics.

Pericytes improve locomotor recovery after spinal cord injury in male and female neonatal rats

OBJECTIVE

It is not known how activation of the hypoxia-inducible factor (HIF) pathway in pericytes, cells of the microvascular wall, influences new capillary growth. We tested the hypothesis that HIF-activated pericytes promote angiogenesis in a neonatal model of spinal cord injury (SCI).

METHODS

Human placental pericytes stimulated with cobalt chloride and naïve pericytes were injected into the site of a thoracic hemi-section of the spinal cord in rat pups on postnatal day three (P3). Hindlimb motor recovery and Doppler blood flow perfusion at the site of transection were measured on P10. Immunohistochemistry was used to visualize vessel and neurofilament density for quantification.

RESULTS

Injection of HIF-activated pericytes resulted in greater vascular density in males but did not result in improved motor function for males or females. Injection of non-HIF-activated pericytes resulted improved motor function recovery in both sexes (males, 2.722 ± 0.31-fold score improvement; females, 3.824 ± 0.58-fold score improvement, P < .05) but produced no significant changes in vessel density.

CONCLUSIONS

HIF-activated pericytes promote vascular density in males post-SCI. Acute delivery of non-HIF-activated pericytes at the site of injury can improve motor recovery post-SCI.

CD73+ extracellular vesicles inhibit angiogenesis through adenosine A2B receptor signalling

Pathological angiogenesis is a hallmark of several conditions including eye diseases, inflammatory diseases, and cancer. Stromal cells play a crucial role in regulating angiogenesis through the release of soluble factors or direct contact with endothelial cells. Here, we analysed the properties of the extracellular vesicles (EVs) released by bone marrow mesenchymal stromal cells (MSCs) and explored the possibility of using them to therapeutically target angiogenesis. We demonstrated that in response to pro-inflammatory cytokines, MSCs produce EVs that are enriched in TIMP-1, CD39 and CD73 and inhibit angiogenesis targeting both extracellular matrix remodelling and endothelial cell migration. We identified a novel anti-angiogenic mechanism based on adenosine production, triggering of A2B adenosine receptors, and induction of NOX2-dependent oxidative stress within endothelial cells. Finally, in pilot experiments, we exploited the anti-angiogenic EVs to inhibit tumour progression in vivo. Our results identify novel pathways involved in the crosstalk between endothelial and stromal cell and suggest new therapeutic strategies to target pathological angiogenesis.

Relevance of Microvascular Flow Assessments in Critically Ill Neonates and Children: A Systematic Review

OBJECTIVES

Resolution of impaired microvascular flow may lag the normalization of macrocirculatory variables. The significance of microcirculatory dysfunction in critically ill children and neonates is unknown, but microcirculatory variables can be measured using Doppler or videomicroscopy imaging techniques. We outline the current understanding of the role of the microcirculation in critical illness, review methods for its assessment, and perform a systematic review of how it has been monitored in critically ill neonates and children.

DESIGN

Systematic review (PROSPERO CRD42019117993).

SETTING

Not applicable.

SUBJECTS

Not applicable.

INTERVENTIONS

None.

MEASUREMENTS AND RESULTS

We systematically searched MEDLINE, EMBASE, PubMed, and Web of Science. We included studies of critically ill patients 0 to 18 years old investigating microcirculatory blood flow. Two reviewers analyzed abstracts and articles. Results were qualitatively analyzed due to study heterogeneity. A total of 2,559 abstracts met search criteria, of which 94 underwent full-text review. Of those, 36 met inclusion criteria. Seven studies investigated microcirculatory changes in critically ill children. Twenty studies investigated the microcirculatory changes in neonates with variable diagnoses compared with a diverse set of clinical endpoints. Nine studies assessed the effects of age, sex, and birth weight on microvascular flow in neonates. Across all studies, microcirculatory dysfunction was associated with poor outcomes and may not correlate with observed macrovascular function.

CONCLUSIONS

Assessment of microvascular flow in critically ill children and neonates is possible, although significant challenges remain. In many such patients, microvascular blood flow is disrupted despite medical management targeting normalized macrovascular variables. Future studies are needed to define normal pediatric microvascular flow variables and to assess the impact of patient and treatment factors on its function.

Aspiration-assisted bioprinting for precise positioning of biologics

Three-dimensional (3D) bioprinting is an appealing approach for building tissues; however, bioprinting of mini-tissue blocks (i.e., spheroids) with precise control on their positioning in 3D space has been a major obstacle. Here, we unveil "aspiration-assisted bioprinting (AAB)," which enables picking and bioprinting biologics in 3D through harnessing the power of aspiration forces, and when coupled with microvalve bioprinting, it facilitated different biofabrication schemes including scaffold-based or scaffold-free bioprinting at an unprecedented placement precision, ~11% with respect to the spheroid size. We studied the underlying physical mechanism of AAB to understand interactions between aspirated viscoelastic spheroids and physical governing forces during aspiration and bioprinting. We bioprinted a wide range of biologics with dimensions in an order-of-magnitude range including tissue spheroids (80 to 600 μm), tissue strands (~800 μm), or single cells (electrocytes, ~400 μm), and as applications, we illustrated the patterning of angiogenic sprouting spheroids and self-assembly of osteogenic spheroids.

Mechanisms of endothelial cell coverage by pericytes: computational modelling of cell wrapping and in vitro experiments

Pericytes (PCs) wrap around endothelial cells (ECs) and perform diverse functions in physiological and pathological processes. Although molecular interactions between ECs and PCs have been extensively studied, the morphological processes at the cellular level and their underlying mechanisms have remained elusive. In this study, using a simple cellular Potts model, we explored the mechanisms for EC wrapping by PCs. Based on the observed in vitro cell wrapping in three-dimensional PC-EC coculture, the model identified four putative contributing factors: preferential adhesion of PCs to the extracellular matrix (ECM), strong cell-cell adhesion, PC surface softness and larger PC size. While cell-cell adhesion can contribute to the prevention of cell segregation and the degree of cell wrapping, it cannot determine the orientation of cell wrapping alone. While atomic force microscopy revealed that PCs have a larger Young's modulus than ECs, the experimental analyses supported preferential ECM adhesion and size asymmetry. We also formulated the corresponding energy minimization problem and numerically solved this problem for specific cases. These results give biological insights into the role of PC-ECM adhesion in PC coverage. The modelling framework presented here should also be applicable to other cell wrapping phenomena observed in vivo.

Pro-inflammatory macrophages coupled with glycolysis remodel adipose vasculature by producing platelet-derived growth factor-B in obesity

Adipose tissue macrophages (ATMs) play a central role in tissue remodeling and homeostasis. However, whether ATMs promote adipose angiogenesis in obesity remains unclear. We examined the impact of ATMs deletion on adipose angiogenesis and tissue expansion in the epididymal white adipose tissue (eWAT) of high-fat diet (HFD)-fed mice by using liposome-encapsulated clodronate. We further elucidated the induction mechanisms of platelet-derived growth factor (PDGF)-B in macrophages in response to obesity-associated metabolic stresses, since it plays a significant role in the regulation of pericyte behavior for the initiation of neoangiogenesis during tissue expansion. ATM depletion prevented adipose tissue expansion in HFD-fed mice by inhibiting pericyte detachment from vessels, resulting in less vasculature in eWAT. The lipopolysaccharide (LPS) stimulation and high glucose concentration augmented glucose incorporation and glycolytic capacity with the induction of Pdgfb mRNA. This effect was mediated through extracellular signal-regulated kinase (ERK) among mitogen-activated protein kinases coupled with glycolysis in RAW264.7 macrophages. The Pdgfb induction system was distinct from that of inflammatory cytokines mediated by mechanistic target of rapamycin complex 1 (mTORC1) and NFκB signaling. Thus, obesity-associated hyperglycemia and chronic inflammation fuels ERK signaling coupled with glycolysis in pro-inflammatory macrophages, which contribute to the expansion of eWAT through PDGF-B-dependent vascular remodeling.

Differential functional roles of fibroblasts and pericytes in the formation of tissue-engineered microvascular networks in vitro

Formation of a perfusable microvascular network (μVN) is critical for tissue engineering of solid organs. Stromal cells can support endothelial cell (EC) self-assembly into a μVN, but distinct stromal cell populations may play different roles in this process. Here we describe the differential effects that two widely used stromal cell populations, fibroblasts (FBs) and pericytes (PCs), have on μVN formation. We examined the effects of adding defined stromal cell populations on the self-assembly of ECs derived from human endothelial colony forming cells (ECFCs) into perfusable μVNs in fibrin gels cast within a microfluidic chamber. ECs alone failed to fully assemble a perfusable μVN. Human lung FBs stimulated the formation of EC-lined μVNs within microfluidic devices. RNA-seq analysis suggested that FBs produce high levels of hepatocyte growth factor (HGF). Addition of recombinant HGF improved while the c-MET inhibitor, Capmatinib (INCB28060), reduced μVN formation within devices. Human placental PCs could not substitute for FBs, but in the presence of FBs, PCs closely associated with ECs, formed a common basement membrane, extended microfilaments intercellularly, and reduced microvessel diameters. Different stromal cell types provide different functions in microvessel assembly by ECs. FBs support μVN formation by providing paracrine growth factors whereas PCs directly interact with ECs to modify microvascular morphology.

Microphysiological systems for recapitulating physiology and function of blood-brain barrier

Central nervous system (CNS) diseases are emerging as a major issue in an aging society. Although extensive research has focused on the development of CNS drugs, the limited transport of therapeutic agents across the blood-brain barrier (BBB) remains a major challenge. Conventional two-dimensional culture dishes do not recapitulate in vivo physiology and real-time observations of molecular transport are not possible in animal models. Recent advances in engineering techniques have enabled the generation of more physiologically relevant in vitro BBB models, and their applications have expanded from fundamental biological research to practical applications in the pharmaceutical industry. In this article, we provide an overview of recent advances in the development of in vitro BBB models, with a particular focus on the recapitulation of BBB function. The development of biomimetic BBB models is postulated to revolutionize not only fundamental biological studies but also drug screening.

Three Dimensional Bioprinting of a Vascularized and Perfusable Skin Graft Using Human Keratinocytes, Fibroblasts, Pericytes, and Endothelial Cells

Multilayered skin substitutes comprising allogeneic cells have been tested for the treatment of nonhealing cutaneous ulcers. However, such nonnative skin grafts fail to permanently engraft because they lack dermal vascular networks important for integration with the host tissue. In this study, we describe the fabrication of an implantable multilayered vascularized bioengineered skin graft using 3D bioprinting. The graft is formed using one bioink containing human foreskin dermal fibroblasts (FBs), human endothelial cells (ECs) derived from cord blood human endothelial colony-forming cells (HECFCs), and human placental pericytes (PCs) suspended in rat tail type I collagen to form a dermis followed by printing with a second bioink containing human foreskin keratinocytes (KCs) to form an epidermis. In vitro, KCs replicate and mature to form a multilayered barrier, while the ECs and PCs self-assemble into interconnected microvascular networks. The PCs in the dermal bioink associate with EC-lined vascular structures and appear to improve KC maturation. When these 3D printed grafts are implanted on the dorsum of immunodeficient mice, the human EC-lined structures inosculate with mouse microvessels arising from the wound bed and become perfused within 4 weeks after implantation. The presence of PCs in the printed dermis enhances the invasion of the graft by host microvessels and the formation of an epidermal rete. Impact Statement Three Dimensional printing can be used to generate multilayered vascularized human skin grafts that can potentially overcome the limitations of graft survival observed in current avascular skin substitutes. Inclusion of human pericytes in the dermal bioink appears to improve both dermal and epidermal maturation.

Vascular targeted photodynamic therapy: A review of the efforts towards molecular targeting of tumor vasculature

The therapeutic value of vascular targeted photodynamic therapy (VTP) for cancer has already been recognized in the clinic: TOOKAD® has been clinically approved in Europe and Israel for treatment of men with low-risk prostate cancer. When light is applied shortly after intravenous administration of the photosensitizer, the damage is primarily done to the vasculature. This results in vessel constriction, blood flow stasis, and thrombus formation. Subsequently, the tumor is killed due to oxygen and nutrient deprivation. To further increase treatment specificity and to reduce undesired side effects such as damaging to the surrounding healthy tissues, efforts have been made to selectively target the PS to the tumor vasculature, an approach named molecular targeted VTP (molVTP). Several receptors have already been explored for this approach, namely CD13, CD276, Extra domains of fibronectin (A, B), Integrin αvβ3, Neuropilin-1, Nucleolin, PDGFRβ, tissue factor, and VEGFR-2, which are overexpressed on tumor vasculature. Preclinical studies have shown promising results, further encouraging the investigation and future application of molVTP, to improve selectivity and efficacy of cancer treatment. This strategy will hopefully lead to even more selective treatments for many cancer patients.

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.

The influence of osteopontin-guided collagen intrafibrillar mineralization on pericyte differentiation and vascularization of engineered bone scaffolds

Biomimetically mineralized collagen scaffolds are promising for bone regeneration, but vascularization of these materials remains to be addressed. Here, we engineered mineralized scaffolds using an osteopontin-guided polymer-induced liquid-precursor mineralization method to recapitulate bone's mineralized nanostructure. SEM images of mineralized samples confirmed the presence of collagen with intrafibrillar mineral, also EDS spectra and FTIR showed high peaks of calcium and phosphate, with a similar mineral/matrix ratio to native bone. Mineralization increased collagen compressive modulus up to 15-fold. To evaluate vasculature formation and pericyte-like differentiation, HUVECs and hMSCs were seeded in a 4:1 ratio in the scaffolds for 7 days. Moreover, we used RT-PCR to investigate the gene expression of pericyte markers ACTA2, desmin, CD13, NG2, and PDGFRβ. Confocal images showed that both nonmineralized and mineralized scaffolds enabled endothelial capillary network formation. However, vessels in the nonmineralized samples had longer vessel length, a larger number of junctions, and a higher presence of αSMA+ mural cells. RT-PCR analysis confirmed the downregulation of pericytic markers in mineralized samples. In conclusion, although both scaffolds enabled endothelial capillary network formation, mineralized scaffolds presented less pericyte-supported vessels. These observations suggest that specific scaffold characteristics may be required for efficient scaffold vascularization in future bone tissue engineering strategies. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 107B: 1522-1532, 2019.

Microvascular bioengineering: a focus on pericytes

Capillaries within the microcirculation are essential for oxygen delivery and nutrient/waste exchange, among other critical functions. Microvascular bioengineering approaches have sought to recapitulate many key features of these capillary networks, with an increasing appreciation for the necessity of incorporating vascular pericytes. Here, we briefly review established and more recent insights into important aspects of pericyte identification and function within the microvasculature. We then consider the importance of including vascular pericytes in various bioengineered microvessel platforms including 3D culturing and microfluidic systems. We also discuss how vascular pericytes are a vital component in the construction of computational models that simulate microcirculation phenomena including angiogenesis, microvascular biomechanics, and kinetics of exchange across the vessel wall. In reviewing these topics, we highlight the notion that incorporating pericytes into microvascular bioengineering applications will increase their utility and accelerate the translation of basic discoveries to clinical solutions for vascular-related pathologies.

Tunneling nanotubes evoke pericyte/endothelial communication during normal and tumoral angiogenesis

BACKGROUND

Nanotubular structures, denoted tunneling nanotubes (TNTs) have been described in recent times as involved in cell-to-cell communication between distant cells. Nevertheless, TNT-like, long filopodial processes had already been described in the last century as connecting facing, growing microvessels during the process of cerebral cortex vascularization and collateralization. Here we have investigated the possible presence and the cellular origin of TNTs during normal brain vascularization and also in highly vascularized brain tumors.

METHODS

We searched for TNTs by high-resolution immunofluorescence confocal microscopy, applied to the analysis of 20-µm, thick sections from lightly fixed, unembedded samples of both developing cerebral cortex and human glioblastoma (GB), immunolabeled for endothelial, pericyte, and astrocyte markers, and vessel basal lamina molecules.

RESULTS

The results revealed the existence of pericyte-derived TNTs, labeled by proteoglycan NG2/CSPG4 and CD146. In agreement with the described heterogeneity of these nanostructures, ultra-long (> 300 µm) and very thin (< 0.8 µm) TNTs were observed to bridge the gap between the wall of distant vessels, or were detected as short (< 300 µm) bridging cables connecting a vessel sprout with its facing vessel or two apposed vessel sprouts. The pericyte origin of TNTs ex vivo in fetal cortex and GB was confirmed by in vitro analysis of brain pericytes, which were able to form and remained connected by typical TNT structures.

CONCLUSIONS

None of the multiple roles described for TNTs can be excluded from a possible involvement during the processes of both normal and pathological vessel growth. A possible function, suggested by the pioneering studies made during cerebral cortex vascularization, is in cell searching and cell-to-cell recognition during the processes of vessel collateralization and vascular network formation. According to our results, it is definitely the pericyte-derived TNTs that seem to actively explore the surrounding microenvironment, searching for (site-to-site recognition), and connecting with (pericyte-to-pericyte and/or pericyte-to-endothelial cell communication), the targeted vessels. This idea implies that TNTs may have a primary role in the very early phases of both physiological and tumor angiogenesis in the brain.

Agent-based computational model of retinal angiogenesis simulates microvascular network morphology as a function of pericyte coverage

OBJECTIVE

Define a role for perivascular cells during developmental retinal angiogenesis in the context of EC Notch1-DLL4 signaling at the multicellular network level.

METHODS

The retinal vasculature is highly sensitive to growth factor-mediated intercellular signaling. Although EC signaling has been explored in detail, it remains unclear how PC function to modulate these signals that lead to a diverse set of vascular network patterns in health and disease. We have developed an ABM of retinal angiogenesis that incorporates both ECs and PCs to investigate the formation of vascular network patterns as a function of pericyte coverage. We use our model to test the hypothesis that PC modulate Notch1-DLL4 signaling in endothelial cell-endothelial cell interactions.

RESULTS

Agent-based model (ABM) simulations that include PCs more accurately predict experimentally observed vascular network morphologies than simulations that lack PCs, suggesting that PCs may influence sprouting behaviors through physical blockade of endothelial intercellular connections.

CONCLUSIONS

This study supports a role for PCs as a physical buffer to signal propagation during vascular network formation-a barrier that may be important for generating healthy microvascular network patterns.

Prevascularization of natural nanofibrous extracellular matrix for engineering completely biological three-dimensional prevascularized tissues for diverse applications

Self-sustainability after implantation is one of the critical obstacles facing large engineered tissues. A preformed functional vascular network provides an effective solution for solving the mass transportation problem. With the support of mural cells, endothelial cells (ECs) can form microvessels within engineered tissues. As an important mural cell, human mesenchymal stem cells (hMSCs) not only stabilize the engineered microvessel network, but also preserve their multi-potency when grown under optimal culture conditions. A prevascularized hMSC/extracellular matrix (ECM) sheet fabricated by the combination of hMSCs, ECs and a naturally derived nanofibrous ECM scaffold offers great opportunity for engineering mechanically strong and completely biological three-dimensional prevascularized tissues. The objective of this study was to create a prevascularized hMSC/ECM sheet by co-culturing ECs and hMSCs on a nanofibrous ECM scaffold. Physiologically low oxygen (2% O2 ) was introduced during the 7 day hMSC culture to preserve the stemness of hMSCs and thereby their capability to secrete angiogenic factors. The ECs were then included to form microvessels under normal oxygen (20% O2 ) for up to 7 days. The results showed that a branched and mature vascular network was formed in the co-culture condition. Angiogenic factors vascular endothelial growth factor (VEGF), basic fibroblast growth factor (bFGF) and angiopoietin-1 (Ang-1) were significantly increased by low-oxygen culture of hMSCs, which further stabilized and supported the maturation of microvessels. A differentiation assay of the prevascularized ECM scaffold demonstrated a retained hMSC multi-potency in the hypoxia cultured samples. The prevascularized hMSC/ECM sheet holds great promise for engineering three-dimensional prevascularized tissues for diverse applications.

PDGFR-β inhibitor slows tumor growth but increases metastasis in combined radiotherapy and Endostar therapy

BACKGROUND

Pericytes are pivotal mural cells of blood vessels and play an essential role in coordinating the function of endothelial cells. Previous studies demonstrated that Endostar, a novel endostatin targeting endothelial cells, can enhance the effect of radiotherapy (RT). The present study addressed whether inhibiting pericytes could potentially improve the efficacy of combined RT and Endostar therapy.

METHODS

Platelet-derived growth factor beta-receptor inhibitor (CP673451) was chosen to inhibit pericytes and RT (12 Gy) was delivered. Lewis lung carcinoma-bearing C57BL/6 mice were randomized into 3 groups: RT, RT + Endo, and RT + Endo + CP673451. Subsequently, tumor microvessel density (MVD), pericyte coverage, tumor hypoxia, and lung metastasis were monitored at different time points following different therapies.

RESULTS

Compared to the other two groups, RT + Endo + CP673451 treatment markedly inhibited tumor growth with no improvement in the overall survival. Further analyses clarified that in comparison to RT alone, RT + Endo significantly reduced the tumor MVD, with a greater decrease noted in the RT + Endo + CP673451 group. However, additional CP673451 accentuated tumor hypoxia and enhanced the pulmonary metastasis in the combined RT and Endostar treatment.

CONCLUSIONS

Tumor growth can be further suppressed by pericyte inhibitor; however, metastases are potentially enhanced. More in-depth studies are warranted to confirm the potential benefits and risks of anti-pericyte therapy.

Engineered Microvessels for the Study of Human Disease

The microvasculature is an extensive, heterogeneous, and complex system that plays a critical role in human physiology and disease. It nourishes almost all living human cells and maintains a local microenvironment that is vital for tissue and organ function. Operating under a state of continuous flow, with an intricate architecture despite its small caliber, and subject to a multitude of biophysical and biochemical stimuli, the microvasculature can be a complex subject to study in the laboratory setting. Engineered microvessels provide an ideal platform that recapitulates essential elements of in vivo physiology and allows study of the microvasculature in a precise and reproducible way. Here, we review relevant structural and functional vascular biology, discuss different methods to engineer microvessels, and explore the applications of this exciting tool for the study of human disease.

Pericytes regulate VEGF-induced endothelial sprouting through VEGFR1

Pericytes adhere to the abluminal surface of endothelial tubules and are required for the formation of stable vascular networks. Defective endothelial cell-pericyte interactions are frequently observed in diseases characterized by compromised vascular integrity such as diabetic retinopathy. Many functional properties of pericytes and their exact role in the regulation of angiogenic blood vessel growth remain elusive. Here we show that pericytes promote endothelial sprouting in the postnatal retinal vasculature. Using genetic and pharmacological approaches, we show that the expression of vascular endothelial growth factor receptor 1 (VEGFR1) by pericytes spatially restricts VEGF signalling. Angiogenic defects caused by pericyte depletion are phenocopied by intraocular injection of VEGF-A or pericyte-specific inactivation of the murine gene encoding VEGFR1. Our findings establish that pericytes promote endothelial sprouting, which results in the loss of side branches and the enlargement of vessels when pericyte function is impaired or lost.

PDGFRβ-specific affibody-directed delivery of a photosensitizer, IR700, is efficient for vascular-targeted photodynamic therapy of colorectal cancer

Vascular-targeted photodynamic therapy (PDT) is an important strategy for cancer therapy. Conventional vascular-targeted PDT has been achieved by passive photosensitizer (PS) delivery, which involves a high risk of adverse effects. Active PS delivery is urgently required for vascular-targeted PDT. Although endothelial cells and pericytes are major cellular components of tumor blood vessels, little attention has been paid to pericyte-targeted PDT for cancer therapy. PDGFRβ is abundantly expressed in the pericytes of various tumors. In this experiment, a dimeric ZPDGFRβ affibody with a 0.9 nM affinity for PDGFRβ was produced. The ZPDGFRβ affibody showed PDGFRβ-dependent pericyte binding. Intravenously injected ZPDGFRβ affibody was predominantly distributed on pericytes and thus accumulated in LS174T tumor grafts. The conjugate of the ZPDGFRβ affibody and IR700 dye, i.e. ZIR700, bound to PDGFRβ+ pericytes but not to PDGFRβ- LS174T tumor cells. Accordingly, ZIR700-mediated PDT in vitro induced the death of pericytes but not of LS174T tumor cells. In mice bearing LS174T tumor grafts, ZIR700-mediated PDT damaged tumor blood vessels, thus inducing tumor destruction by intensifying tissue hypoxia. The average mass of tumor grafts administered with ZIR700-mediated PDT was approximately 20-30% of that of the control, indicating that pericyte-targeted PDT is efficient for cancer therapy. In addition, ZIR700-mediated PDT increased the tumor uptake of TNF-related apoptosis-inducing ligand (TRAIL) injected post-illumination. Consequently, combination therapy of ZIR700-mediated PDT and TRAIL showed greater tumor suppression than ZIR700-mediated PDT- or TRAIL-based monotherapy. These results demonstrated that active vascular-targeted PDT could be achieved by using ZPDGFRβ affibody-directed delivery of PS.

Targeted Delivery to Tumor-associated Pericytes via an Affibody with High Affinity for PDGFRβ Enhances the in vivo Antitumor Effects of Human TRAIL

Human tumor necrosis factor-related apoptosis-inducing ligand (hTRAIL) has exhibited superior in vitro cytotoxicity in a variety of tumor cells. However, hTRAIL showed a disappointing anticancer effect in clinical trials, although hTRAIL-based regimens were well tolerated. One important reason might be that hTRAIL was largely trapped by its decoy receptors, which are ubiquitously expressed on normal cells. Tumor-targeted delivery might improve the tumor uptake and thus enhance the antitumor effect of hTRAIL. Platelet-derived growth factor receptor β (PDGFRβ)-expressing pericytes are enriched in tumor tissues derived both from patients with colon cancer and from mice bearing colorectal tumor xenografts. A ZPDGFRβ affibody showed high affinity (nM) for PDGFRβ and was predominantly distributed on tumor-associated PDGFRβ-positive pericytes. Co-administration with the ZPDGFRβ affibody did not significantly enhance the antitumor effect of hTRAIL in mice bearing tumor xenografts. Fusion to the ZPDGFRβ affibody endows hTRAIL with PDGFRβ-binding ability but does not interfere with its death receptor binding and activation. The fused ZPDGFRβ affibody mediated PDGFRβ-dependent binding of hTRAIL to pericytes. In addition, hTRAIL bound on pericytes could kill tumor cells through juxtatropic activity or exhibit cytotoxicity in tumor cells after being released from pericytes. Intravenously injected hTRAIL fused to ZPDGFRβ affibody initially accumulated on tumor-associated pericytes and then diffused to the tumor parenchyma over time. Fusion to the ZPDGFRβ affibody increased the tumor uptake of hTRAIL, thus enhancing the antitumor effect of hTRAIL in mice bearing tumor xenografts. These results demonstrate that pericyte-targeted delivery mediated by a ZPDGFRβ affibody is an alternative strategy for tumor-targeted delivery of anticancer agents.

A short discourse on vascular tissue engineering

Vascular tissue engineering has significant potential to make a major impact on a wide array of clinical problems. Continued progress in understanding basic vascular biology will be invaluable in making further advancements. Past and current achievements in tissue engineering of microvasculature to perfuse organ specific constructs, small vessels for dialysis grafts, and modified synthetic and pediatric large caliber-vessel grafts will be discussed. An emphasis will be placed on clinical trial results with small and large-caliber vessel grafts. Challenges to achieving engineered constructs that satisfy the physiologic, immunologic, and manufacturing demands of engineered vasculature will be explored.

Discovering cardiac pericyte biology: From physiopathological mechanisms to potential therapeutic applications in ischemic heart disease

Microvascular pericytes and the more recently discovered adventitial pericyte-like progenitor cells are a subpopulation of vascular stem cells closely associated with small and large blood vessels respectively. These populations of perivascular cells are remarkably abundant in the heart. Pericytes control important physiological processes such as angiogenesis, blood flow and vascular permeability. In the heart, this pleiotropic activity makes pericytes extremely interesting for applications in regenerative medicine. On the other hand, dysfunction of pericytes could participate in the pathogenesis of cardiovascular disease, such as arterial hypertension, fibro-calcific cardiovascular remodeling, myocardial edema and post-ischemic coronary no-reflow. On a therapeutic standpoint, preclinical studies in small animal models of myocardial infarction have demonstrated the healing potential of pericytes transplantation, which has been ascribed to direct vascular incorporation and paracrine pro-angiogenic and anti-apoptotic activities. These promising findings open the door to the clinical use of pericytes for the treatment of cardiovascular diseases.

Endothelial exocytosis of angiopoietin-2 resulting from CCM3 deficiency contributes to cerebral cavernous malformation

Cerebral cavernous malformations (CCMs) are vascular malformations that affect the central nervous system and result in cerebral hemorrhage, seizure and stroke. CCMs arise from loss-of-function mutations in one of three genes: KRIT1 (also known as CCM1), CCM2 or PDCD10 (also known as CCM3). PDCD10 mutations in humans often result in a more severe form of the disease relative to mutations in the other two CCM genes, and PDCD10-knockout mice show severe defects, the mechanistic basis for which is unclear. We have recently reported that CCM3 regulates exocytosis mediated by the UNC13 family of exocytic regulatory proteins. Here, in investigating the role of endothelial cell exocytosis in CCM disease progression, we found that CCM3 suppresses UNC13B- and vesicle-associated membrane protein 3 (VAMP3)-dependent exocytosis of angiopoietin 2 (ANGPT2) in brain endothelial cells. CCM3 deficiency in endothelial cells augments the exocytosis and secretion of ANGPT2, which is associated with destabilized endothelial cell junctions, enlarged lumen formation and endothelial cell-pericyte dissociation. UNC13B deficiency, which blunts ANGPT2 secretion from endothelial cells, or treatment with an ANGPT2-neutralizing antibody normalizes the defects in the brain and retina caused by endothelial-cell-specific CCM3 deficiency, including the disruption of endothelial cell junctions, vessel dilation and pericyte dissociation. Thus, enhanced secretion of ANGPT2 in endothelial cells contributes to the progression of CCM disease, providing a new therapeutic approach for treating this devastating pathology.

A Hypothesis Concerning the Biphasic Dose-response of Tumors to Angiostatin and Endostatin

This manuscript proposes a hypothesis to explain the U-shaped dose-response observed for angiostatin and other high-molecular-weight drugs in various anti-cancer bio-assays. The dose-response curves for angiostatin and endostatin (measured as suppression of tumor growth) go through an optimum (i.e., minimum tumor growth) and then becomes less effective at higher doses. The literature suggests that at lower doses the primary action of these high-molecular-weight drugs is to counteract the angiogenic effects of vascular endothelial growth factor (VEGF). To do this, the drugs must pass out of the blood vessel and enter the extra-cellular matrix (ECM) where VEGF induces the growth and fusion of tip cells. Ironically, VEGF actually facilitates access of the drugs to the ECM by making the vascular endothelium leaky. At higher doses, the high-molecular-weight drugs seem to reverse VEGF-induced permeability of the endothelium. Thus, at high dose rates, it is hypothesized that the drugs are not able to enter the ECM and block the angiogenic effects of VEGF there. As a result, high doses of the drugs do not suppress vascularization of the tumor or tumor growth. Moreover, if the permeability of the vessels is suppressed, the VEGF released by the stroma is concentrated in the ECM where it amplifies the angiogenic activity around the tumor.

Driving the Hypoxia-Inducible Pathway in Human Pericytes Promotes Vascular Density in an Exosome-Dependent Manner

OBJECTIVES

The mechanisms involved in activating pericytes, cells that ensheath capillaries, to engage in the formation of new capillaries, angiogenesis, remain unknown. In this study, the hypothesis was tested that pericytes could be stimulated to promote angiogenesis by driving the HIF pathway.

METHODS

Pericytes were stimulated with CoCl2 to activate the HIF pathway. Stimulated pericytes were cocultured with endothelial cells in a wound healing assay and in a 3D collagen matrix assay of angiogenesis. A culture system of spinal cord tissue was used to assess microvascular outcomes after treatment with stimulated pericytes. Pharmaceutical inhibition of exosome production was also performed.

RESULTS

Treatment with stimulated pericytes resulted in faster wound healing (1.92 ± 0.18 fold increase, p < 0.05), greater endothelial cord formation (2.9 ± 0.14 fold increase, p < 0.05) in cell culture assays, and greater vascular density (1.78 ± 0.23 fold increase, p < 0.05) in spinal cord tissue. Exosome secretion and the physical presence of stimulated pericytes were necessary in the promotion of angiogenic outcomes.

CONCLUSIONS

These results elucidate a mechanism that may be exploited to enhance features of angiogenesis in the CNS.

Tissue-Engineered Microvasculature to Reperfuse Isolated Renal Glomeruli

Kidney transplantation is often the most effective therapy for end-stage renal disease, but there are not enough donor organs to meet the rising demand. Tissue engineering of kidneys is a potential solution to this organ shortage. Achieving microvascular perfusion has been a major barrier to engineering tissues beyond thin muscularized sheets such as the bladder wall. Our laboratory has previously reported that human umbilical vein endothelial cells (ECs) transduced with the antiapoptotic protein Bcl-2 will spontaneously organize into perfused microvessels within type I collagen gels when implanted in immunodeficient mice. To test if this system can be used to perfuse more complex structures, we combined Bcl-2-transduced ECs (Bcl-2-ECs) with renal glomeruli, the specialized vascular filtration units of the kidney. Microdissected green fluorescent protein-expressing rat glomeruli suspended in type I collagen gels were implanted within immunodeficient mice with or without the inclusion of Bcl-2-ECs. Survival of rat glomeruli was enhanced by coimplantation with Bcl-2-ECs. Intravital rhodamine dextran injections demonstrated that surviving glomeruli were perfused through Bcl-2-EC-derived microvessels. Perfused glomeruli maintained podocin staining, but transmission electron microscopy revealed endothelial swelling and podocyte foot process effacement. Anastomosis of microvessels derived from Bcl-2-ECs with glomerular capillaries provides proof of concept that self-assembled microvessels can perfuse specialized organ structures such as glomeruli, but that perfusion alone may be insufficient to maintain normal structure.

Functional Effects of a Tissue-Engineered Cardiac Patch From Human Induced Pluripotent Stem Cell-Derived Cardiomyocytes in a Rat Infarct Model

A tissue-engineered cardiac patch provides a method to deliver cardiomyoctes to the injured myocardium with high cell retention and large, controlled infarct coverage, enhancing the ability of cells to limit remodeling after infarction. The patch environment can also yield increased survival. In the present study, we sought to assess the efficacy of a cardiac patch made from human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) to engraft and limit left ventricular (LV) remodeling acutely after infarction. Cardiac patches were created from hiPSC-CMs and human pericytes (PCs) entrapped in a fibrin gel and implanted acutely onto athymic rat hearts. hiPSC-CMs not only remained viable after in vivo culture, but also increased in number by as much as twofold, consistent with colocalization of human nuclear antigen, cardiac troponin T, and Ki-67 staining. CM+PC patches led to reduced infarct sizes compared with myocardial infarction-only controls at week 4, and CM+PC patch recipient hearts exhibited greater fractional shortening over all groups at both 1 and 4 weeks after transplantation. However, a decline occurred in fractional shortening for all groups over 4 weeks, and LV thinning was not mitigated. CM+PC patches became vascularized in vivo, and microvessels were more abundant in the host myocardium border zone, suggesting a paracrine mechanism for the improved cardiac function. PCs in a PC-only control patch did not survive 4 weeks in vivo. Our results indicate that cardiac patches containing hiPSC-CMs engraft onto acute infarcts, and the hiPSC-CMs survive, proliferate, and contribute to a reduction in infarct size and improvements in cardiac function.

SIGNIFICANCE

In the present study, a cardiac patch was created from human induced pluripotent stem cell-derived cardiomyocytes and human pericytes entrapped in a fibrin gel, and it was transplanted onto infarcted rat myocardium. It was found that a patch that contained both cardiomyocytes and pericytes survived transplantation and resulted in improved cardiac function and a reduced infarct size compared with controls.

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.

Biology of colorectal cancer

Colorectal cancer is a serious health problem, a challenge for research, and a model for studying the molecular mechanisms involved in its development. According to its incidence, this pathology manifests itself in three forms: family, hereditary, and most commonly sporadic, apparently not associated with any hereditary or familial factor. For the types having inheritance patterns and a family predisposition, the tumours develop through defined stages ranging from adenomatous lesions to the manifestation of a malignant tumour. It has been established that environmental and hereditary factors contribute to the development of colorectal cancer, as indicated by the accumulation of mutations in oncogenes, genes which suppress and repair DNA, signaling the existence of various pathways through which the appearance of tumours may occur. In the case of the suppressive and mutating tracks, these are characterised by genetic disorders related to the phenotypical changes of the morphological progression sequence in the adenoma/carcinoma. Moreover, alternate pathways through mutation in BRAF and KRAS genes are associated with the progression of polyps to cancer. This review surveys the research done at the cellular and molecular level aimed at finding specific alternative therapeutic targets for fighting colorectal cancer.

Sustained delivery of proangiogenic microRNA-132 by nanoparticle transfection improves endothelial cell transplantation

Transplantation of endothelial cells (ECs) for therapeutic vascularization or tissue engineering is a promising method for increasing tissue perfusion. Here, we report on a new approach for enhanced EC transplantation using targeted nanoparticle transfection to deliver proangiogenic microRNA-132 (miR-132) to cultured ECs before their transplantation, thereby sensitizing cells to the effects of endogenous growth factors. We synthesized biodegradable PLGA polymer nanoparticles (NPs) that were loaded with miR-132 and coated with cyclic RGD (cRGD) peptides that target integrin αvβ3 expressed on cultured human umbilical vein ECs (HUVECs), increasing NP uptake through clathrin-coated pits. Unlike previously reported NPs for miR delivery, these NPs slowly release RNA for several weeks. The endocytosed NPs remain in clathrin-coated vesicles from which they mediate intracellular delivery of siRNA or miRNA. Transfection of HUVECs with miR-132 enhances growth factor-induced proliferation and migration in 2D culture, producing a 1.8- and 5-fold increase, respectively. However, while the effects of conventional transfection were short-lived, NP transfection produced protein knockdown and biological effects that were significantly longer in duration (≥ 6 d). Transfection of HUVECs with miR-132 NP resulted in a 2-fold increase in the number of microvessels per square millimeter compared to lipid after transplantation into immunodeficient mice and led to a higher number of mural cell-invested vessels than control transfection. These data suggest that sustained delivery of miR-132 encapsulated in a targeted biodegradable polymer NP is a safe and efficient strategy to improve EC transplantation and vascularization.