Sphingosine 1-phosphate receptor regulation of N-cadherin mediates vascular stabilization

Wnt/β-catenin signaling regulates VE-cadherin-mediated anastomosis of brain capillaries by counteracting S1pr1 signaling

Canonical Wnt signaling is crucial for vascularization of the central nervous system and blood-brain barrier (BBB) formation. BBB formation and modulation are not only important for development, but also relevant for vascular and neurodegenerative diseases. However, there is little understanding of how Wnt signaling contributes to brain angiogenesis and BBB formation. Here we show, using high resolution in vivo imaging and temporal and spatial manipulation of Wnt signaling, different requirements for Wnt signaling during brain angiogenesis and BBB formation. In the absence of Wnt signaling, premature Sphingosine-1-phosphate receptor (S1pr) signaling reduces VE-cadherin and Esama at cell-cell junctions. We suggest that Wnt signaling suppresses S1pr signaling during angiogenesis to enable the dynamic junction formation during anastomosis, whereas later S1pr signaling regulates BBB maturation and VE-cadherin stabilization. Our data provides a link between brain angiogenesis and BBB formation and identifies Wnt signaling as coordinator of the timing and as regulator of anastomosis.

It Takes Two: Endothelial-Perivascular Cell Cross-Talk in Vascular Development and Disease

The formation of new blood vessels is a crucial step in the development of any new tissue both during embryogenesis and in vitro models as without sufficient perfusion the tissue will be unable to grow beyond the size where nutrition and oxygenation can be managed by diffusion alone. Endothelial cells are the primary building block of blood vessels and are capable of forming tube like structures independently however they are unable to independently form functional vasculature which is capable of conducting blood flow. This requires support from other structures including supporting perivascular cells and the extracellular matrix. The crosstalk between endothelial cells and perivascular cells is vital in regulating vasculogenesis and angiogenesis and the consequences when this is disrupted can be seen in a variety of congenital and acquired disease states. This review details the mechanisms of vasculogenesis in vivo during embryogenesis and compares this to currently employed in vitro techniques. It also highlights clinical consequences of defects in the endothelial cell-pericyte cross-talk and highlights therapies which are being developed to target this pathway. Improving the understanding of the intricacies of endothelial-pericyte signaling will inform pathophysiology of multiple vascular diseases and allow the development of effective in vitro models to guide drug development and assist with approaches in tissue engineering to develop functional vasculature for regenerative medicine applications.

Porous nanofibrous scaffold incorporated with S1P loaded mesoporous silica nanoparticles and BMP-2 encapsulated PLGA microspheres for enhancing angiogenesis and osteogenesis

Repair of bone defects remains a major clinical challenge due to inadequate or abnormal vascularization in bone substitutes, which commonly leads to inferior bone formation or bone nonunion. Therefore, healing of bone defects requires the coordinated processes of angiogenesis and osteogenesis. In this study, sphingosine-1-phosphate (S1P) was initially loaded into mesoporous silica nanoparticles (MSNs) to form angiogenic microcarriers, which were subsequently embedded into porous nanofibrous poly-l-lactide (PLLA) scaffolds during a thermally induced phase separation (TIPS) process, while bone morphogenetic protein-2 (BMP-2) was encapsulated into poly(lactic-co-glycolic acid) (PLGA) microspheres to obtain osteogenic microcarriers, which were then integrated onto a MSNs/PLLA nanofibrous scaffold by a post seeding method. The osteogenic and angiogenic activities of the resulting dual-bioactive factor containing scaffolds were evaluated both in vitro and in vivo. The simulated drug release studies indicated that both bioactive factors will be released simultaneously and continuously from the fabricated composite scaffold. Moreover, the ectopic bone formation results showed that the sustained release of S1P and BMP-2 from the composite scaffold resulted in a synergistic effect on blood vessel formation and bone regeneration. Taken together, the results showed the promising application of the dual-bioactive factor loaded nanofibrous scaffold for enhanced bone regeneration.

N-cadherin in cancer metastasis, its emerging role in haematological malignancies and potential as a therapeutic target in cancer

In many types of solid tumours, the aberrant expression of the cell adhesion molecule N-cadherin is a hallmark of epithelial-to-mesenchymal transition, resulting in the acquisition of an aggressive tumour phenotype. This transition endows tumour cells with the capacity to escape from the confines of the primary tumour and metastasise to secondary sites. In this review, we will discuss how N-cadherin actively promotes the metastatic behaviour of tumour cells, including its involvement in critical signalling pathways which mediate these events. In addition, we will explore the emerging role of N-cadherin in haematological malignancies, including bone marrow homing and microenvironmental protection to anti-cancer agents. Finally, we will discuss the evidence that N-cadherin may be a viable therapeutic target to inhibit cancer metastasis and increase tumour cell sensitivity to existing anti-cancer therapies.

The N-recognin UBR4 of the N-end rule pathway is required for neurogenesis and homeostasis of cell surface proteins

The N-end rule pathway is a proteolytic system in which single N-terminal amino acids of proteins act as a class of degrons (N-degrons) that determine the half-lives of proteins. We have previously identified a family of mammals N-recognins (termed UBR1, UBR2, UBR4/p600, and UBR5/EDD) whose conserved UBR boxes bind N-degrons to facilitate substrate ubiquitination and proteasomal degradation via the ubiquitin-proteasome system (UPS). Amongst these N-recognins, UBR1 and UBR2 mediate ubiquitination and proteolysis of short-lived regulators and misfolded proteins. Here, we characterized the null phenotypes of UBR4-deficient mice in which the UBR box of UBR4 was deleted. We show that the mutant mice die around embryonic days 9.5–10.5 (E9.5–E10.5) associated with abnormalities in various developmental processes such as neurogenesis and cardiovascular development. These developmental defects are significantly attributed to the inability to maintain cell integrity and adhesion, which significantly correlates to the severity of null phenotypes. UBR4-loss induces the depletion of many, but not all, proteins from the plasma membrane, suggesting that UBR4 is involved in proteome-wide turnover of cell surface proteins. Indeed, UBR4 is associated with and required to generate the multivesicular body (MVB) which transiently store endocytosed cell surface proteins before their targeting to autophagosomes and subsequently lysosomes. Our results suggest that the N-recognin UBR4 plays a role in the homeostasis of cell surface proteins and, thus, cell adhesion and integrity.

MicroRNA-149-5p regulates blood-brain barrier permeability after transient middle cerebral artery occlusion in rats by targeting S1PR2 of pericytes

Blood-brain barrier (BBB) disruption caused by reperfusion injury after ischemic stroke is an intractable event conducive to further injury. Brain pericytes play a vital role in maintaining BBB integrity by interacting with other components of the BBB. In this study, we found that sphingosine-1-phosphate receptor (S1PR)2 expressed in pericytes was significantly up-regulated after ischemia in vivo and in vitro. By using a S1PR2 antagonist (JTE-013), we showed that S1PR2 plays a critical role in the induction of BBB permeability of transient middle cerebral artery occlusion (tMCAO) rats and the in vitro BBB model. Furthermore, we discovered that S1PR2 may decrease N-cadherin expression and increase pericyte migration via NF-κB p65 signal and found that S1PR2 could be regulated by miR-149-5p negatively, which was decreased in the ischemic boundary zone and cultured pericytes after ischemia. Overexpression of miR-149-5p in cultured pericytes substantially increased N-cadherin expression and decreased pericyte migration, which decreased BBB leakage in the in vitro model. Up-regulating miR-149-5p by intracerebroventricular injection of agomir-149-5p attenuated BBB permeability and improved the outcomes of tMCAO rats significantly. Thus, our data suggest that miR-149-5p may serve as a potential target for treatment of BBB disruption after ischemic stroke.-Wan, Y., Jin, H.-J., Zhu, Y.-Y., Fang, Z., Mao, L., He, Q., Xia, Y.-P., Li, M., Li, Y., Chen, X., Hu, B. MicroRNA-149-5p regulates blood-brain barrier permeability after transient middle cerebral artery occlusion in rats by targeting S1PR2 of pericytes.

PEDF expression affects the oxidative and inflammatory state of choroidal endothelial cells

Age-related macular degeneration (AMD) is the leading cause of vision loss among the elderly population, and is associated with severe macular degeneration and choroidal neovascularization (CNV). Although the pathogenesis of AMD is associated with choroidal dysfunction and CNV, the detailed underlying mechanisms remain unresolved. Altered production of pigment epithelium-derived factor (PEDF), a neuroprotective and antiangiogenic factor, contributes to CNV. Furthermore, exogenous PEDF mitigates angiogenesis in preclinical CNV models. How PEDF expression affects choroidal endothelial cell (ChEC) function is unknown. Here we isolated ChECs from PEDF^+/+ and PEDF-deficient (PEDF^-/-) mice and determined the impact of PEDF expression on the proangiogenic and pro-inflammatory properties of ChECs. We showed that PEDF expression significantly affects the proliferation, migration, adhesion, and oxidative and inflammatory state of ChECs. The PEDF^-/- ChECs were, however, more sensitive to H₂O₂ challenge and exhibited increased rate of apoptosis and oxidative stress. We also observed a significant increase in production of cytokines with a primary role in inflammation and angiogenesis including vascular endothelial growth factor (VEGF) and osteopontin, and a reprograming of chemokines and cytokines expression profiles in PEDF^-/- ChECs. Collectively, our results indicate that PEDF expression has a significant impact on oxidative and inflammatory properties of ChECs, whose alteration could contribute to pathogenesis of chronic inflammatory diseases including exudative AMD.

Molecular Regulation of Sprouting Angiogenesis

The term angiogenesis arose in the 18th century. Several studies over the next 100 years laid the groundwork for initial studies performed by the Folkman laboratory, which were at first met with some opposition. Once overcome, the angiogenesis field has flourished due to studies on tumor angiogenesis and various developmental models that can be genetically manipulated, including mice and zebrafish. In addition, new discoveries have been aided by the ability to isolate primary endothelial cells, which has allowed dissection of various steps within angiogenesis. This review will summarize the molecular events that control angiogenesis downstream of biochemical factors such as growth factors, cytokines, chemokines, hypoxia-inducible factors (HIFs), and lipids. These and other stimuli have been linked to regulation of junctional molecules and cell surface receptors. In addition, the contribution of cytoskeletal elements and regulatory proteins has revealed an intricate role for mobilization of actin, microtubules, and intermediate filaments in response to cues that activate the endothelium. Activating stimuli also affect various focal adhesion proteins, scaffold proteins, intracellular kinases, and second messengers. Finally, metalloproteinases, which facilitate matrix degradation and the formation of new blood vessels, are discussed, along with our knowledge of crosstalk between the various subclasses of these molecules throughout the text. Compr Physiol 8:153-235, 2018.

Adropin preserves the blood-brain barrier through a Notch1/Hes1 pathway after intracerebral hemorrhage in mice

Adropin is expressed in the CNS and plays a crucial role in the development of stroke. However, little is currently known about the effects of adropin on the blood-brain barrier (BBB) function after intracerebral hemorrhage (ICH). In this study, the role of adropin in collagenase-induced ICH was investigated in mice. At 1-h post-ICH, mice were administered with recombinant human adropin by intranasal. Brain water +content, BBB permeability, and neurological function were measured at different time intervals. Proteins were quantified using western blot analysis, and the localizations of adropin and Notch1 were visualized via immunofluorescence staining. It is shown that adropin reduced brain water content and improved neurological functions. Adropin preserved the functionality of BBB by increasing N-cadherin expression and reducing extravasation of albumin. Moreover, in vivo knockdown of Notch1 and Hes1 both abolished the protective effects of adropin. Taken together, our data demonstrate that adropin constitutes a potential treatment value for ICH by preserving BBB and improving functional outcomes through the Notch1 signaling pathway.

PEDF expression affects retinal endothelial cell proangiogenic properties through alterations in cell adhesive mechanisms

Pigment epithelium-derived factor (PEDF) is an endogenous inhibitor of angiogenesis. Although various ocular cell types including retinal endothelial cells (EC) produce PEDF, we know very little about cell autonomous effects of PEDF in these cell types. Here we determined how PEDF expression affects retinal EC proangiogenic properties. Retinal EC were prepared from wild-type (PEDF+/+) and PEDF-deficient (PEDF-/-) mice. The identity of EC was confirmed by staining for specific markers including vascular endothelial cadherin, CD31, and B4-lectin. Retinal EC also expressed VEGF receptor 1 and endoglin, as well as ICAM-1, ICAM-2, and VCAM-1. PEDF-/- retinal EC were more proliferative, less apoptotic when challenged with H2O2, less migratory, and less adherent compared with PEDF+/+ EC. These changes could be associated, at least in part, with increased levels of tenascin-C, fibronectin, thrombospondin-1 and collagen IV, and lower amounts of osteopontin. PEDF-/- EC also exhibited alterations in expression of a number of integrins including α2, αv, β1, β8, and αvβ3, and cell-cell adhesion molecules including CD31, zonula occluden-1, and occludin. These observations correlated with attenuation of capillary morphogenesis and increased levels of oxidative stress in PEDF-/- EC. PEDF-/- EC also produced lower levels of VEGF compared with PEDF+/+ cells. Thus, PEDF deficiency has a significant impact on retinal EC adhesion and migration, perhaps through altered production of extracellular matrix and junctional proteins in response to increased oxidative stress affecting their proangiogenic activity.

The protective role of sphingosine-1-phosphate against the action of the vascular disrupting agent combretastatin A-4 3-O-phosphate

Solid tumours vary in sensitivity to the vascular disrupting agent combretastatin A-4 3-O-phosphate (CA4P), but underlying factors are poorly understood. The signaling sphingolipid, sphingosine-1-phosphate (S1P), promotes vascular barrier integrity by promoting assembly of VE-cadherin/β-catenin complexes. We tested the hypothesis that tumour pre-treatment with S1P would render tumours less susceptible to CA4P. S1P (1μM) pretreatment attenuated an increase in endothelial cell (HUVEC) monolayer permeability induced by 10μM CA4P. Intravenously administered S1P (8mg/kg/hr for 20 minutes then 2mg/kg/hr for 40 minutes), reduced CA4P-induced (30mg/kg) blood flow shut-down in fibrosarcoma tumours in SCID mice (n≥7 per group), as measured by tumour retention of an intravenously administered fluorescent lectin. A trend towards in vivo protection was also found using laser Doppler flowmetry. Immunohistochemical staining of tumours ex vivo revealed disrupted patterns of VE-cadherin in vasculature of mice treated with CA4P, which were decreased by pretreatment with S1P. S1P treatment also stabilized N-cadherin junctions between endothelial cells and smooth muscle cells in culture, and stabilized tubulin filaments in HUVEC monolayers. We conclude that the rapid shutdown of tumour microvasculature by CA4P is due in part to disruption of adherens junctions and that S1P has a protective effect on both adherens junctions and the endothelial cell cytoskeleton.

Three-dimensional biomimetic vascular model reveals a RhoA, Rac1, and N-cadherin balance in mural cell-endothelial cell-regulated barrier function

The integrity of the endothelial barrier between circulating blood and tissue is important for blood vessel function and, ultimately, for organ homeostasis. Here, we developed a vessel-on-a-chip with perfused endothelialized channels lined with human bone marrow stromal cells, which adopt a mural cell-like phenotype that recapitulates barrier function of the vasculature. In this model, barrier function is compromised upon exposure to inflammatory factors such as LPS, thrombin, and TNFα, as has been observed in vivo. Interestingly, we observed a rapid physical withdrawal of mural cells from the endothelium that was accompanied by an inhibition of endogenous Rac1 activity and increase in RhoA activity in the mural cells themselves upon inflammation. Using a system to chemically induce activity in exogenously expressed Rac1 or RhoA within minutes of stimulation, we demonstrated RhoA activation induced loss of mural cell coverage on the endothelium and reduced endothelial barrier function, and this effect was abrogated when Rac1 was simultaneously activated. We further showed that N-cadherin expression in mural cells plays a key role in barrier function, as CRISPR-mediated knockout of N-cadherin in the mural cells led to loss of barrier function, and overexpression of N-cadherin in CHO cells promoted barrier function. In summary, this bicellular model demonstrates the continuous and rapid modulation of adhesive interactions between endothelial and mural cells and its impact on vascular barrier function and highlights an in vitro platform to study the biology of perivascular-endothelial interactions.

Endothelial Progenitor Cells for the Vascularization of Engineered Tissues

Self-assembled microvasculature from cocultures of endothelial cells (ECs) and stromal cells has significantly advanced efforts to vascularize engineered tissues by enhancing perfusion rates in vivo and producing investigative platforms for microvascular morphogenesis in vitro. However, to clinically translate prevascularized constructs, the issue of EC source must be resolved. Endothelial progenitor cells (EPCs) can be noninvasively supplied from the recipient through adult peripheral and umbilical cord blood, as well as derived from induced pluripotent stem cells, alleviating antigenicity issues. EPCs can also differentiate into all tissue endothelium, and have demonstrated potential for therapeutic vascularization. Yet, EPCs are not the standard EC choice to vascularize tissue constructs in vitro. Possible reasons include unresolved issues with EPC identity and characterization, as well as uncertainty in the selection of coculture, scaffold, and culture media combinations that promote EPC microvessel formation. This review addresses these issues through a summary of EPC vascular biology and the effects of tissue engineering design parameters upon EPC microvessel formation. Also included are perspectives to integrate EPCs with emerging technologies to produce functional, organotypic vascularized tissues.

Size-selective opening of the blood-brain barrier by targeting endothelial sphingosine 1-phosphate receptor 1

The vasculature of the central nervous system (CNS) forms a selective barrier termed the blood-brain barrier (BBB). Disruption of the BBB may contribute to various CNS diseases. Conversely, the intact BBB restricts efficient penetration of CNS-targeted drugs. Here, we report the BBB-regulatory role of endothelial sphingosine 1-phosphate (S1P) receptor-1, a G protein-coupled receptor known to promote the barrier function in peripheral vessels. Endothelial-specific S1pr1 knockout mice (S1pr1^iECKO ) showed BBB breach for small-molecular-mass fluorescence tracers (<3 kDa), but not larger tracers (>10 kDa). Chronic BBB leakiness was associated with cognitive impairment, as assessed by the novel object recognition test, but not signs of brain inflammation. Brain microvessels of S1pr1^iECKO mice showed altered subcellular distribution of tight junctional proteins. Pharmacological inhibition of S1P₁ function led to transient BBB breach. These data suggest that brain endothelial S1P₁ maintain the BBB by regulating the proper localization of tight junction proteins and raise the possibility that endothelial S1P₁ inhibition may be a strategy for transient BBB opening and delivery of small molecules into the CNS.

Vascular and Immunobiology of the Circulatory Sphingosine 1-Phosphate Gradient

Vertebrates are endowed with a closed circulatory system, the evolution of which required novel structural and regulatory changes. Furthermore, immune cell trafficking paradigms adapted to the barriers imposed by the closed circulatory system. How did such changes occur mechanistically? We propose that spatial compartmentalization of the lipid mediator sphingosine 1-phosphate (S1P) may be one such mechanism. In vertebrates, S1P is spatially compartmentalized in the blood and lymphatic circulation, thus comprising a sharp S1P gradient across the endothelial barrier. Circulatory S1P has critical roles in maturation and homeostasis of the vascular system as well as in immune cell trafficking. Physiological functions of S1P are tightly linked to shear stress, the key biophysical stimulus from blood flow. Thus, circulatory S1P confinement could be a primordial strategy of vertebrates in the development of a closed circulatory system. This review discusses the cellular and molecular basis of the S1P gradients and aims to interpret its physiological significance as a key feature of the closed circulatory system.

Contribution of pericyte paracrine regulation of the endothelium to angiogenesis

During physiological development and after a stressor event, vascular cells communicate with each other to evoke new vessel formation-a process known as angiogenesis. This communication occurs via direct contact and via paracrine release of proteins and nucleic acids, both in a free form or encapsulated into micro-vesicles. In diseases with an altered angiogenic response, such as cancer and diabetic vascular complications, it becomes of paramount importance to tune the cell communication process. Endothelial cell growth and migration are essential processes for new vessel formation, and pericytes, together with some classes of circulating monocytes, are important endothelial regulators. The interaction between pericytes and the endothelium is facilitated by their anatomical apposition, which involves endothelial cells and pericytes sharing the same basement membrane. However, the role of pericytes is not fully understood. The characteristics and the function of tissue-specific pericytesis are the focus of this review. Factors involved in the cross-talk between these cell types and the opportunities afforded by micro-RNA and micro-vesicle techniques are discussed. Targeting these mechanisms in pathological conditions, in which the vessel response is altered, is considered in relation to identification of new therapies for restoring the blood flow.

Sphingosine-1-phosphate restores endothelial barrier integrity in ovarian hyperstimulation syndrome

STUDY QUESTION

Are follicular fluid (FF) sphingosine-1-phosphate (S1P) levels in patients at risk of developing ovarian hyperstimulation syndrome (OHSS) altered and in part responsible for the high vascular permeability observed in these patients.

STUDY ANSWER

FF S1P levels are lower in FF from patients at risk of OHSS and treatment with S1P may reduce vascular permeability in these patients.

WHAT IS KNOWN ALREADY

Although advances have been made in the diagnosis, and management of OHSS and in basic knowledge of its development, complete prevention has proven difficult.

STUDY DESIGN, SIZE, DURATION

A total of 40 FF aspirates were collected from patients undergoing ART. The women (aged 25-39 years old) were classified into a control group (n = 20) or a group at risk of OHSS (n = 20). The EA.hy926 endothelial cell line was used to assess the efffects of FF from patients at risk of OHSS with or without the addition of S1P. An animal model that develops OHSS in immature Sprague-Dawley rats were also used.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Migration assays, confocal microscopy analysis of actin filaments, immunoblotting and quail chorioallantoic membrane (CAM) assays of in-vivo angiogenesis were performed and statistical comparisons between groups were made.

MAIN RESULTS AND THE ROLE OF CHANCE

The S1P concentration was significantly lower in FF from patients at risk of OHSS (P = 0.03). The addition of S1P to this FF decreased cell migration (P < 0.05) and prevented VE-cadherin phosphorylation in endothelial cells (P < 0.05). S1P in the FF from patients at risk of OHSS increased the levels of VE-cadherin (P < 0.05), N-cadherin (P < 0.05) and β-catenin (P < 0.05), and partially reversed actin redistribution in endothelial cells. The addition of S1P in FF from patients at risk of OHSS also decreased the levels of vascular endothelial growth factor (VEGF₁₂₁; P < 0.01) and S1P lyase (SPL; P < 0.05) and increased the levels of S1PR1 (P < 0.05) in endothelial cells. In CAMs incubated with FF from patients at risk of OHSS with S1P, the number of vessel branch points decreased while the periendothelial cell coverage increased. Additionally, in a rat OHSS model, we demonstrated that vascular permeability and VEGF₁₂₁ and its receptor KDR expression were increased in the OHSS group compared to the control group and that S1P administration decreased these parameters.

LARGE SCALE DATA

N/A.

LIMITATIONS, REASONS FOR CAUTION

The results of this study were generated from an in-vitro system. This model reflects the microvasculature in vivo. Even though the ideal model would be the use of human endothelial cells from the ovary, it is obviously not possible to carry out this kind of approach in ovaries of patients from ART. More studies will be necessary to delineate the effects of S1P in the pathogenesis of OHSS. Hence, clinical studies are needed in order to choose the most appropriate method of prevention and management.

WIDER IMPLICATIONS OF THE FINDINGS

The use of bioactive sphingolipid metabolites may contribute to finding better and safer therapeutic strategies for the treatment of OHSS and other human diseases that display aberrant vascular leakage.

STUDY FUNDING/COMPETING INTERESTS

This work was supported by grants ANPCyT (PICT 2012-897), CONICET (PIP 5471), Roemmers and Baron Foundation, Argentina. The authors declare no conflict of interest.

Angiomotin like-1 is a novel component of the N-cadherin complex affecting endothelial/pericyte interaction in normal and tumor angiogenesis

Transmission of mechanical force via cell junctions is an important component that molds cells into shapes consistent with proper organ function. Of particular interest are the cadherin transmembrane proteins, which play an essential role in connecting cell junctions to the intra-cellular cytoskeleton. Understanding how these biomechanical complexes orchestrate intrinsic and extrinsic forces is important for our understanding of the underlying mechanisms driving morphogenesis. We have previously identified the Amot protein family, which are scaffold proteins that integrate polarity, junctional, and cytoskeletal cues to modulate cellular shape in endothelial as well as epithelial cells. In this report, we show that AmotL1 is a novel partner of the N-cadherin protein complex. We studied the role of AmotL1 in normal retinal as well as tumor angiogenesis using inducible endothelial-specific knock-out mice. We show that AmotL1 is essential for normal establishment of vascular networks in the post-natal mouse retina as well as in a transgenic breast cancer model. The observed phenotypes were consistent with a non-autonomous pericyte defect. We show that AmotL1 forms a complex with N-cadherin present on both endothelial cells and pericytes. We propose that AmotL1 is an essential effector of the N-cadherin mediated endothelial/pericyte junctional complex.

Cell-microenvironment interactions and architectures in microvascular systems

In the past decade, significant advances have been made in the design and optimization of novel biomaterials and microfabrication techniques to generate vascularized tissues. Novel microfluidic systems have facilitated the development and optimization of in vitro models for exploring the complex pathophysiological phenomena that occur inside a microvascular environment. To date, most of these models have focused on engineering of increasingly complex systems, rather than analyzing the molecular and cellular mechanisms that drive microvascular network morphogenesis and remodeling. In fact, mutual interactions among endothelial cells (ECs), supporting mural cells and organ-specific cells, as well as between ECs and the extracellular matrix, are key driving forces for vascularization. This review focuses on the integration of materials science, microengineering and vascular biology for the development of in vitro microvascular systems. Various approaches currently being applied to study cell-cell/cell-matrix interactions, as well as biochemical/biophysical cues promoting vascularization and their impact on microvascular network formation, will be identified and discussed. Finally, this review will explore in vitro applications of microvascular systems, in vivo integration of transplanted vascularized tissues, and the important challenges for vascularization and controlling the microcirculatory system within the engineered tissues, especially for microfabrication approaches. It is likely that existing models and more complex models will further our understanding of the key elements of vascular network growth, stabilization and remodeling to translate basic research principles into functional, vascularized tissue constructs for regenerative medicine applications, drug screening and disease models.

Neutralizing S1P inhibits intratumoral hypoxia, induces vascular remodelling and sensitizes to chemotherapy in prostate cancer

Hypoxia promotes neovascularization, increased tumor growth, and therapeutic resistance. The transcription factor, hypoxia-inducible factor 1α (HIF-1α), has been reported as the master driver of adaptation to hypoxia. We previously identified the sphingosine kinase 1/sphingosine 1-phosphate (SphK1/S1P) pathway as a new modulator of HIF-1α under hypoxia. Taking advantage of a monoclonal antibody neutralizing extracellular S1P (sphingomab), we report that inhibition of S1P extracellular signaling blocks HIF-1α accumulation and activity in several cancer cell models exposed to hypoxia. In an orthotopic xenograft model of prostate cancer, we show that sphingomab reduces hypoxia and modifies vessel architecture within 5 days of treatment, leading to increased intratumoral blood perfusion. Supporting the notion that a transient vascular normalization of tumor vessels is the mechanism by which sphingomab exerts its effects, we demonstrate that administration of the antibody for 5 days before chemotherapy is more effective at local tumor control and metastatic dissemination than any other treatment scheduling. These findings validate sphingomab as a potential new normalization agent that could contribute to successful sensitization of hypoxic tumors to chemotherapy.

Pericytes: Properties, Functions and Applications in Tissue Engineering

Mesenchymal stem cells (MSCs) are one of the most studied adult stem cells and in recent years. They have become attractive agents/cell source for cellular therapy and regenerative medicine applications. During investigations about their origin, researchers hypothesized that perivascular regions are the common anatomical regions where MSCs come from and perivascular cells like pericytes (PCs) (Rouget cells, mural cells) are in vivo counterparts of MSCs. Beside capillaries and microvessels as their most common locations, PCs are also found in large vessels (arteries and veins). They can be isolated from several tissues and organs particularly from retina and brain. There are different approaches about their isolation, characterization and culture but there has been no common protocol yet because of the lack of defined PC-specific marker. They make special contact with endothelial cells in the basement membrane and have very important functions in several tissues and organs. They participate in vascular development, stabilization, maturation, and remodeling, blood pressure control, endothelial cell proliferation and differentiation, contractility of vascular smooth muscle cells, wound healing, vasculogenesis and angiogenesis, long-term maintenance of hematopoietic stem cells in bone marrow niche. Their multipotential differentiation capacity and participation in many events in the body make PCs preferred cells in tissue engineering applications including 3D blood-brain barrier models, skeletal muscle constructs, bone tissue engineering and tissue-engineered vascular grafts.

Downregulation of Sphingosine 1-Phosphate Receptor 1 Promotes the Switch from Tangential to Radial Migration in the OB

Neuroblast migration is a highly orchestrated process that ensures the proper integration of newborn neurons into complex neuronal circuits. In the postnatal rodent brain, neuroblasts migrate long distances from the subependymal zone of the lateral ventricles to the olfactory bulb (OB) within the rostral migratory stream (RMS). They first migrate tangentially in close contact to each other and later radially as single cells until they reach their final destination in the OB. Sphingosine 1-phosphate (S1P) is a bioactive lipid that interacts with cell-surface receptors to exert different cellular responses. Although well studied in other systems and a target for the treatment of multiple sclerosis, little is known about S1P in the postnatal brain. Here, we report that the S1P receptor 1 (S1P1) is expressed in neuroblasts migrating in the RMS. Using in vivo and in vitro gain- and loss-of-function approaches in both wild-type and transgenic mice, we found that the activation of S1P1 by its natural ligand S1P, acting as a paracrine signal, contributes to maintain neuroblasts attached to each other while they migrate in chains within the RMS. Once in the OB, neuroblasts cease to express S1P1, which results in cell detachment and initiation of radial migration, likely via downregulation of NCAM1 and β1 integrin. Our results reveal a novel physiological function for S1P1 in the postnatal brain, directing the path followed by newborn neurons in the neurogenic niche.

SIGNIFICANCE STATEMENT

The function of each neuron is highly determined by the position it occupies within a neuronal circuit. Frequently, newborn neurons must travel long distances from their birthplace to their predetermined final location and, to do so, they use different modes of migration. In this study, we identify the sphingosine 1-phosphate (S1P) receptor 1 (S1P1) as one of the key players that govern the switch from tangential to radial migration of postnatally generated neuroblasts in the olfactory bulb. Of interest is the evidence that the ligand, S1P, is provided by nearby astrocytes. Finally, we also propose adhesion molecules that act downstream of S1P1 and initiate the transition from tangential chain migration to individual radial migration outside of the stream.

Composite microsphere-functionalized scaffold for the controlled release of small molecules in tissue engineering

Current tissue engineering strategies focus on restoring damaged tissue architectures using biologically active scaffolds. The ideal scaffold would mimic the extracellular matrix of any tissue of interest, promoting cell proliferation and de novo extracellular matrix deposition. A plethora of techniques have been evaluated to engineer scaffolds for the controlled and targeted release of bioactive molecules to provide a functional structure for tissue growth and remodeling, as well as enhance recruitment and proliferation of autologous cells within the implant. Recently, novel approaches using small molecules, instead of growth factors, have been exploited to regulate tissue regeneration. The use of small synthetic molecules could be very advantageous because of their stability, tunability, and low cost. Herein, we propose a chitosan-gelatin scaffold functionalized with composite microspheres consisting of mesoporous silicon microparticles and poly(dl-lactic-co-glycolic acid) for the controlled release of sphingosine-1-phospate, a small molecule of interest. We characterized the platform with scanning electron microscopy, Fourier transform infrared spectroscopy, and confocal microscopy. Finally, the biocompatibility of this multiscale system was analyzed by culturing human mesenchymal stem cells onto the scaffold. The presented strategy establishes the basis of a versatile scaffold for the controlled release of small molecules and for culturing mesenchymal stem cells for regenerative medicine applications.

N-cadherin antagonists as oncology therapeutics

The cell adhesion molecule (CAM), N-cadherin, has emerged as an important oncology therapeutic target. N-cadherin is a transmembrane glycoprotein mediating the formation and structural integrity of blood vessels. Its expression has also been documented in numerous types of poorly differentiated tumours. This CAM is involved in regulating the proliferation, survival, invasiveness and metastasis of cancer cells. Disruption of N-cadherin homophilic intercellular interactions using peptide or small molecule antagonists is a promising novel strategy for anti-cancer therapies. This review discusses: the discovery of N-cadherin, the mechanism by which N-cadherin promotes cell adhesion, the role of N-cadherin in blood vessel formation and maintenance, participation of N-cadherin in cancer progression, the different types of N-cadherin antagonists and the use of N-cadherin antagonists as anti-cancer drugs.

p38 mitogen-activated protein kinase regulates type-I vs type-II phenotyping of human vascular endothelial cells

AIM: To identify kinases involved in phenotype regulation of vascular endothelial cells (VECs): Pro-proliferative G-protein signaling 5 (RGS5)^high (type-I) vs anti-proliferative RGS5^low (type-II) VECs.

METHODS: Proteomic kinase assays were performed to identify the crucial kinase involved in the phenotype regulation of human VECs using type-I VECs, which promotes the proliferation of human vascular smooth muscle cells (VSMCs), and type-II VECs, which suppress the proliferation of human VSMCs. The assays were performed using multiple pairs of type-I and type-II VECs to obtain the least number of candidates. The involvement of the candidate kinases was verified by evaluating the effects of their specific inhibitors on the phenotype regulation of human VECs as well as the expression levels of regulator of RGS5, which is the causative gene for the “type-II to type-I” phenotype conversion of human VECs.

RESULTS: p38α mitogen-activated protein kinase (p38α MAPK) was the only kinase that showed distinctive activities between type-I and type-II VECs: p38α MAPK activities were low and high in type-I and type-II VECs, respectively. We found that an enforced expression of RGS5 indeed lowered p38α MAPK activities in type-II VECs. Furthermore, treatments with a p38α MAPK inhibitor nullified the anti-proliferative potential in type-II VECs. Interestingly, MAPK inhibitor treatments enhanced the induction of RGS5 gene. Thus, there is a vicious cycle between “RGS5 induction” and “p38α MAPK inhibition”, which can explain the unidirectional process in the stress-induced “type-II to type-I” conversions of human VECs. To understand the upstream signaling of RGS5, which is known as an inhibitory molecule against the G protein-coupled receptor (GPCR)-mediated signaling, we examined the effects of RGS5 overexpression on the signaling events from sphingosine-1-phosphate (S1P) to N-cadherin, because S1P receptors belong to the GPCR family gene and N-cadherin, one of their downstream effectors, is reportedly involved in the regulation of VEC-VSMC interactions. We found that RGS5 specifically bound with S1P₁. Moreover, N-cadherin localization at intercellular junctions in type-II VECs was abolished by “RGS5 overexpression” and “p38α MAPK inhibition”.

CONCLUSION: p38α MAPK plays crucial roles in “type-I vs type-II” phenotype regulations of human VECs at the downstream of RGS5.

Lysophosphatidic Acid and Sphingosine-1-Phosphate: A Concise Review of Biological Function and Applications for Tissue Engineering

The presentation and controlled release of bioactive signals to direct cellular growth and differentiation represents a widely used strategy in tissue engineering. Historically, work in this field has primarily focused on the delivery of large cytokines and growth factors, which can be costly to manufacture and difficult to deliver in a sustained manner. There has been a marked increase over the past decade in the pursuit of lipid mediators due to their wide range of effects over multiple cell types, low cost, and ease of scale-up. Lysophosphatidic acid (LPA) and sphingosine-1-phosphate (S1P) are two bioactive lysophospholipids (LPLs) that have gained attention for use as pharmacological agents in tissue engineering applications. While these lipids can have similar effects on cellular response, they possess distinct chemical backbones, mechanisms of synthesis and degradation, and signaling pathways using a discrete set of G-protein-coupled receptors (GPCRs). LPA and S1P predominantly act extracellularly on their GPCRs and can directly regulate cell survival, differentiation, cytokine secretion, proliferation, and migration--each of the important functions that must be considered in regenerative medicine. In addition to these potent physiological functions, these LPLs play pivotal roles in a number of pathophysiological processes. To capitalize on the promise of these molecules in tissue engineering, these lipids have been incorporated into biomaterials for in vivo delivery. Here, we survey the effects of LPA and S1P on both cellular- and tissue-level phenotypes, with an eye toward regulating stem/progenitor cell growth and differentiation. In particular, we examine work that has translational applications for cell-based tissue engineering strategies in promoting cell survival, bone and cartilage engineering, and therapeutic angiogenesis.

The evolving roles of pericyte in early brain injury after subarachnoid hemorrhage

Despite accumulated understanding on the mechanisms of early brain injury and improved management of subarachnoid hemorrhage (SAH), it is still one of the serious and refractory health problems around the world. Traditionally, pericyte, served as capillary contraction handler, is recently considered as the main participant of microcirculation regulation in SAH pathophysiology. However, accumulate evidences indicate that pericyte is much more than we already know. Therefore, we briefly review the characteristics, regulation pathways and functions of pericyte, aim to summarize the evolving new pathophysiological roles of pericyte that are implicated in early brain injury after SAH and to improve our understanding in order to explore potential novel therapeutic options for patients with SAH. This article is part of a Special Issue entitled SI: Cell Interactions In Stroke.

Lipid rafts: integrated platforms for vascular organization offering therapeutic opportunities

Research on the nanoscale membrane structures known as lipid rafts is relevant to the fields of cancer biology, inflammation and ischaemia. Lipid rafts recruit molecules critical to signalling and regulation of the invasion process in malignant cells, the leukocytes that provide immunity in inflammation and the endothelial cells that build blood and lymphatic vessels, as well as the patterning of neural networks. As angiogenesis is a common denominator, regulation of receptors and signalling molecules critical to angiogenesis is central to the design of new approaches aimed at reducing, promoting or normalizing the angiogenic process. The goal of this review is to highlight some of the key issues that indicate the involvement of endothelial cell lipid rafts at each step of so-called 'sprouting angiogenesis', from stimulation of the vascular endothelial growth factor to the choice of tip cells, activation of migratory and invasion pathways, recruitment of molecules that guide axons in vascular patterning and maturation of blood vessels. Finally, the review addresses opportunities for future studies to define how these lipid domains (and their constituents) may be manipulated to stimulate the so-called 'normalization' of vascular networks within tumors, and be identified as the main target, enabling the development of more efficient chemotherapeutics and cancer immunotherapies.

Expression of thrombospondin-1 modulates the angioinflammatory phenotype of choroidal endothelial cells

The choroidal circulation plays a central role in maintaining the health of outer retina and photoreceptor function. Alterations in this circulation contribute to pathogenesis of many eye diseases including exudative age-related macular degeneration. Unfortunately, very little is known about the choroidal circulation and its molecular and cellular regulation. This has been further hampered by the lack of methods for routine culturing of choroidal endothelial cells (ChEC), especially from wild type and transgenic mice. Here we describe a method for isolation and culturing of mouse ChEC. We show that expression of thrombospondin-1 (TSP1), an endogenous inhibitor of angiogenesis and inflammation, has a significant impact on phenotype of ChEC. ChEC from TSP1-deficient (TSP1-/-) mice were less proliferative and more apoptotic, less migratory and less adherent, and failed to undergo capillary morphogenesis in Matrigel. However, re-expression of TSP1 was sufficient to restore TSP1-/- ChEC migration and capillary morphogenesis. TSP1-/- ChEC expressed increased levels of TSP2, phosphorylated endothelial nitric oxide synthase (NOS) and inducible NOS (iNOS), a marker of inflammation, which was associated with significantly higher level of NO and oxidative stress in these cells. Wild type and TSP1-/- ChEC produced similar levels of VEGF, although TSP1-/- ChEC exhibited increased levels of VEGF-R1 and pSTAT3. Other signaling pathways including Src, Akt, and MAPKs were not dramatically affected by the lack of TSP1. Together our results demonstrate an important autocrine role for TSP1 in regulation of ChEC phenotype.

Emerging roles of pericytes in the regulation of the neurovascular unit in health and disease

Pericytes of the central nervous system (CNS) are uniquely positioned within a multicellular structure termed the neurovascular unit (NVU) to provide crucial support to blood brain barrier (BBB) formation, maintenance, and stability. Numerous CNS diseases are associated with some aspect of BBB dysfunction. A dysfunction can manifest as one or multiple disruptions to any of the following barriers: physical, metabolic, immunological and transport barrier. A breach in the BBB can notably result in BBB hyper-permeability, endothelial activation and enhanced immune-endothelial interaction. How the BBB is regulated within this integrated unit remains largely unknown, especially as it relates to pericyte-endothelial interaction. We summarize the latest findings on pericyte origin, possible marker expression, and availability within different organ systems. We highlight pericyte-endothelial cell interactions, concentrating on extra- and intra- cellular signaling mechanisms linked to platelet derived growth factor-B, transforming growth factor -β, angiopoietins, Notch, and gap junctions. We discuss the role of pericytes in the NVU under inflammatory insult, focusing on how pericytes may indirectly affect leukocyte CNS infiltration, the direct role of pericyte-mediated basement membrane modifications, and immune responses. We review new findings of pericyte actions in CNS pathologies including Alzheimer's disease, stroke, multiple sclerosis, diabetic retinopathy, and HIV-1 infection. The uncovering of the regulatory role of pericytes on the BBB will provide key insight into how barrier integrity can be re-established during neuroinflammation.

Amphiphilic degradable polymers for immobilization and sustained delivery of sphingosine 1-phosphate

Controlled delivery of the angiogenic factor sphingosine 1-phosphate (S1P) represents a promising strategy for promoting vascularization during tissue repair and regeneration. In this study, we developed an amphiphilic biodegradable polymer platform for the stable encapsulation and sustained release of S1P. Mimicking the interaction between amphiphilic S1P and its binding proteins, a series of polymers with hydrophilic poly(ethylene glycol) core and lipophilic flanking segments of polylactide and/or poly(alkylated lactide) with different alkyl chain lengths were synthesized. These polymers were electrospun into fibrous meshes, and loaded with S1P in generally high loading efficiencies (>90%). Sustained S1P release from these scaffolds could be tuned by adjusting the alkyl chain length, blockiness and lipophilic block length, achieving 35-55% and 45-80% accumulative releases in the first 8h and by 7 days, respectively. Furthermore, using endothelial cell tube formation assay and chicken chorioallantoic membrane assay, we showed that the different S1P loading doses and release kinetics translated into distinct pro-angiogenic outcomes. These results suggest that these amphiphilic polymers are effective delivery vehicles for S1P and may be explored as tissue engineering scaffolds where the delivery of lipophilic or amphiphilic bioactive factors is desired.

Monomeric C-reactive protein and Notch-3 co-operatively increase angiogenesis through PI3K signalling pathway

C-reactive protein (CRP) is the most acute-phase reactant serum protein of inflammation and a strong predictor of cardiovascular disease. Its expression is associated with atherosclerotic plaque instability and the formation of immature micro-vessels. We have previously shown that CRP upregulates endothelial-derived Notch-3, a key receptor involved in vascular development, remodelling and maturation. In this study, we investigated the links between the bioactive monomeric CRP (mCRP) and Notch-3 signalling in angiogenesis. We used in vitro (cell counting, wound-healing and tubulogenesis assays) and in vivo (chorioallantoic membrane) angiogenic assays and Western blotting to study the angiogenic signalling pathways induced by mCRP and Notch-3 activator chimera protein (Notch-3/Fc). Our results showed an additive effect on angiogenesis of mCRP stimulatory effect combined with Notch-3/Fc promoting bovine aortic endothelial cell (BAEC) proliferation, migration, tube formation in Matrigel(TM) with up-regulation of phospho-Akt expression. The pharmacological blockade of PI3K/Akt survival pathway by LY294002 fully inhibited in vitro and in vivo angiogenesis induced by mCRP/Notch-3/Fc combination while blocking Notch signalling by gamma-secretase inhibitor (DAPT) partially inhibited mCRP/Notch-3/Fc-induced angiogenesis. Using a BAEC vascular smooth muscle cell co-culture sprouting angiogenesis assay and transmission electron microscopy, we showed that activation of both mCRP and Notch-3 signalling induced the formation of thicker sprouts which were shown later by Western blotting to be associated with an up-regulation of N-cadherin expression and a down-regulation of VE-cadherin expression. Thus, mCRP combined with Notch-3 activator promote angiogenesis through the PI3K/Akt pathway and their therapeutic combination has potential to promote and stabilize vessel formation whilst reducing the risk of haemorrhage from unstable plaques.

Antiviral responses in mouse embryonic stem cells: differential development of cellular mechanisms in type I interferon production and response

We have recently reported that mouse embryonic stem cells (mESCs) are deficient in expressing type I interferons (IFNs) in response to viral infection and synthetic viral RNA analogs (Wang, R., Wang, J., Paul, A. M., Acharya, D., Bai, F., Huang, F., and Guo, Y. L. (2013) J. Biol. Chem. 288, 15926-15936). Here, we report that mESCs are able to respond to type I IFNs, express IFN-stimulated genes, and mediate the antiviral effect of type I IFNs against La Crosse virus and chikungunya virus. The major signaling components in the IFN pathway are expressed in mESCs. Therefore, the basic molecular mechanisms that mediate the effects of type I IFNs are functional in mESCs; however, these mechanisms may not yet be fully developed as mESCs express lower levels of IFN-stimulated genes and display weaker antiviral activity in response to type I IFNs when compared with fibroblasts. Further analysis demonstrated that type I IFNs do not affect the stem cell state of mESCs. We conclude that mESCs are deficient in type I IFN expression, but they can respond to and mediate the cellular effects of type I IFNs. These findings represent unique and uncharacterized properties of mESCs and are important for understanding innate immunity development and ESC physiology.

Pericyte dynamics during angiogenesis: new insights from new identities

Therapies aimed at manipulating the microcirculation require the ability to control angiogenesis, defined as the sprouting of new capillaries from existing vessels. Blocking angiogenesis would be beneficial in many pathologies (e.g. cancer, retinopathies and rheumatoid arthritis). In others (e.g. myocardial infarction, stroke and hypertension), promoting angiogenesis would be desirable. We know that vascular pericytes elongate around endothelial cells (ECs) and are functionally associated with regulating vessel stabilization, vessel diameter and EC proliferation. During angiogenesis, bidirectional pericyte-EC signaling is critical for capillary sprout formation. Observations of pericytes leading capillary sprouts also implicate their role in EC guidance. As such, pericytes have recently emerged as a therapeutic target to promote or inhibit angiogenesis. Advancing our basic understanding of pericytes and developing pericyte-related therapies are challenged, like in many other fields, by questions regarding cell identity. This review article discusses what we know about pericyte phenotypes and the opportunity to advance our understanding by defining the specific pericyte cell populations involved in capillary sprouting.

Sphingosine 1-phosphate signalling

Sphingosine 1-phosphate (S1P) is a lipid mediator formed by the metabolism of sphingomyelin. In vertebrates, S1P is secreted into the extracellular environment and signals via G protein-coupled S1P receptors to regulate cell-cell and cell-matrix adhesion, and thereby influence cell migration, differentiation and survival. The expression and localization of S1P receptors is dynamically regulated and controls vascular development, vessel stability and immune cell trafficking. In addition, crucial events during embryogenesis, such as angiogenesis, cardiogenesis, limb development and neurogenesis, are regulated by S1P signalling. Here, and in the accompanying poster, we provide an overview of S1P signalling in development and in disease.

An α-smooth muscle actin (acta2/αsma) zebrafish transgenic line marking vascular mural cells and visceral smooth muscle cells

Mural cells of the vascular system include vascular smooth muscle cells (SMCs) and pericytes whose role is to stabilize and/or provide contractility to blood vessels. One of the earliest markers of mural cell development in vertebrates is α smooth muscle actin (acta2; αsma), which is expressed by pericytes and SMCs. In vivo models of vascular mural cell development in zebrafish are currently lacking, therefore we developed two transgenic zebrafish lines driving expression of GFP or mCherry in acta2-expressing cells. These transgenic fish were used to trace the live development of mural cells in embryonic and larval transgenic zebrafish. acta2:EGFP transgenic animals show expression that largely mirrors native acta2 expression, with early pan-muscle expression starting at 24 hpf in the heart muscle, followed by skeletal and visceral muscle. At 3.5 dpf, expression in the bulbus arteriosus and ventral aorta marks the first expression in vascular smooth muscle. Over the next 10 days of development, the number of acta2:EGFP positive cells and the number of types of blood vessels associated with mural cells increases. Interestingly, the mural cells are not motile and remain in the same position once they express the acta2:EGFP transgene. Taken together, our data suggests that zebrafish mural cells develop relatively late, and have little mobility once they associate with vessels.

Innovations in studying in vivo cell behavior and pharmacology in complex tissues--microvascular endothelial cells in the spotlight

Many studies on the molecular control underlying normal cell behavior and cellular responses to disease stimuli and pharmacological intervention are conducted in single-cell culture systems, while the read-out of cellular engagement in disease and responsiveness to drugs in vivo is often based on overall tissue responses. As the majority of drugs under development aim to specifically interact with molecular targets in subsets of cells in complex tissues, this approach poses a major experimental discrepancy that prevents successful development of new therapeutics. In this review, we address the shortcomings of the use of artificial (single) cell systems and of whole tissue analyses in creating a better understanding of cell engagement in disease and of the true effects of drugs. We focus on microvascular endothelial cells that actively engage in a wide range of physiological and pathological processes. We propose a new strategy in which in vivo molecular control of cells is studied directly in the diseased endothelium instead of at a (far) distance from the site where drugs have to act, thereby accounting for tissue-controlled cell responses. The strategy uses laser microdissection-based enrichment of microvascular endothelium which, when combined with transcriptome and (phospho)proteome analyses, provides a factual view on their status in their complex microenvironment. Combining this with miniaturized sample handling using microfluidic devices enables handling the minute sample input that results from this strategy. The multidisciplinary approach proposed will enable compartmentalized analysis of cell behavior and drug effects in complex tissue to become widely implemented in daily biomedical research and drug development practice.

Sphingosine 1-phosphate receptor 3 regulates recruitment of anti-inflammatory monocytes to microvessels during implant arteriogenesis

Endothelial cells play significant roles in conditioning tissues after injury by the production and secretion of angiocrine factors. At least two distinct subsets of monocytes, CD45(+)CD11b(+)Gr1(+)Ly6C(+) inflammatory and CD45(+)CD11b(+)Gr1(-)Ly6C(-) anti-inflammatory monocytes, respond differentially to these angiocrine factors and promote pathogen/debris clearance and arteriogenesis/tissue regeneration, respectively. We demonstrate here that local sphingosine 1-phosphate receptor 3 (S1P3) agonism recruits anti-inflammatory monocytes to remodeling vessels. Poly(lactic-co-glycolic acid) thin films were used to deliver FTY720, an S1P1/3 agonist, to inflamed and ischemic tissues, which resulted in a reduction in proinflammatory cytokine secretion and an increase in regenerative cytokine secretion. The altered balance of cytokine secretion results in preferential recruitment of anti-inflammatory monocytes from circulation. The chemotaxis of these cells, which express more S1P3 than inflammatory monocytes, toward SDF-1α was also enhanced with FTY720 treatment, but not in S1P3 knockout cells. FTY720 delivery enhanced arteriolar diameter expansion and increased length density of the local vasculature. This work establishes a role for S1P receptor signaling in the local conditioning of tissues by angiocrine factors that preferentially recruit regenerative monocytes that can enhance healing outcomes, tissue regeneration, and biomaterial implant functionality.

Recruitment and retention: factors that affect pericyte migration

Pericytes are critical for vascular morphogenesis and contribute to several pathologies, including cancer development and progression. The mechanisms governing pericyte migration and differentiation are complex and have not been fully established. Current literature suggests that platelet-derived growth factor/platelet-derived growth factor receptor-β, sphingosine 1-phosphate/endothelial differentiation gene-1, angiopoietin-1/tyrosine kinase with immunoglobulin-like and EGF-like domains 2, angiopoietin-2/tyrosine kinase with immunoglobulin-like and EGF-like domains 2, transforming growth factor β/activin receptor-like kinase 1, transforming growth factor β/activin receptor-like kinase 5, Semaphorin-3A/Neuropilin, and matrix metalloproteinase activity regulate the recruitment of pericytes to nascent vessels. Interestingly, many of these pathways are directly affected by secreted protein acidic and rich in cysteine (SPARC). Here, we summarize the function of these factors in pericyte migration and discuss if and how SPARC might influence these activities and thus provide an additional layer of control for the recruitment of vascular support cells. Additionally, the consequences of targeted inhibition of pericytes in tumors and the current understanding of pericyte recruitment in pathological environments are discussed.

Tumor-derived mural-like cells coordinate with endothelial cells: role of YKL-40 in mural cell-mediated angiogenesis

Tumor neo-vasculature is characterized by spatial coordination of endothelial cells with mural cells, which delivers oxygen and nutrients. Here, we explored a key role of the secreted glycoprotein YKL-40, a mesenchymal marker, in the interaction between endothelial cells and mesenchymal mural-like cells for tumor angiogenesis. Xenotransplantation of tumor-derived mural-like cells (GSDCs) expressing YKL-40 in mice developed extensive and stable blood vessels covered with more GSDCs than those in YKL-40 gene knockdown tumors. YKL-40 expressed by GSDCs was associated with increased interaction of neural cadherin/β-catenin/smooth muscle alpha actin; thus, mediating cell-cell adhesion and permeability. YKL-40 also induced the interaction of vascular endothelial cadherin/β-catenin/actin in endothelial cells (HMVECs). In cell co-culture systems, YKL-40 enhanced both GSDC and HMVEC contacts, restricted vascular leakage, and stabilized vascular networks. Collectively, the data inform new mechanistic insights into the cooperation of mural cells with endothelial cells induced by YKL-40 during tumor angiogenesis, and also enhance our understanding of YKL-40 in both mural and endothelial cell biology.

Ophthalmic evaluations in clinical studies of fingolimod (FTY720) in multiple sclerosis

PURPOSE

To report outcomes of ophthalmic evaluations in clinical studies of patients receiving fingolimod (Gilenya; Novartis Pharma AG, Basel, Switzerland) for multiple sclerosis (MS).

DESIGN

Analysis done on pooled safety data (N = 2615, all studies group) from 3 double-masked, randomized, parallel-group clinical trials (phase 2 core and extension >5 years, and phase 3 FREEDOMS and TRANSFORMS core and extension studies).

PARTICIPANTS

Patients aged 18 to 55 years (18-60 years in phase 2 study) diagnosed with relapsing-remitting MS were included. Patients with diabetes mellitus or macular edema (ME) at screening were excluded.

INTERVENTION

Participants received fingolimod (0.5/1.25 mg), placebo, or interferon beta for the respective study durations. Ophthalmic examination included detailed eye history (at screening), visual acuity (VA) assessment, dilated ophthalmoscopy, optical coherence tomography (OCT), and fluorescein angiography (FA).

MAIN OUTCOME MEASURES

Extensive ophthalmic monitoring was performed for all patients. While being studied, patients with abnormal findings on dilated ophthalmoscopy and OCT compatible with ME were further studied by FA. All locally diagnosed ME cases were centrally reviewed by the retina specialist (M.A.Z.) on the Data and Safety Monitoring Board.

RESULTS

Among 2615 patients assessed, 19 confirmed ME cases were observed in fingolimod-treated groups (0.5 mg: n = 4, 0.3%; 1.25 mg: n = 15, 1.2%). Most patients (n = 13, 68%) presented with blurred vision, decreased VA, or eye pain. Macular edema was diagnosed within 3 to 4 months of treatment initiation in most cases (n = 13, 68%); 2 patients had late onset (>12 months) ME. Of the 19 patients with ME, 5 (26%), all treated with fingolimod 1.25 mg, had a history of uveitis compared with 26 (1%) in the all studies group. In most cases (n = 16, 84%), ME resolved after discontinuing the study drug. Eleven patients required topical anti-inflammatory medications. No patient had further vision deterioration.

CONCLUSIONS

Fingolimod 0.5 mg is associated with a low incidence of ME in MS studies. Patients with a history of uveitis may be at an increased risk of developing ME. An ophthalmic examination before initiating fingolimod therapy and regular follow-up eye examinations during fingolimod therapy are recommended.

S1P and the birth of platelets

Recent work has highlighted the multitude of biological functions of sphingosine 1-phosphate (S1P), which include roles in hematopoietic cell trafficking, organization of immune organs, vascular development, and neuroinflammation. Indeed, a functional antagonist of S1P(1) receptor, FTY720/Gilenya, has entered the clinic as a novel therapeutic for multiple sclerosis. In this issue of the JEM, Zhang et al. highlight yet another function of this lipid mediator: thrombopoiesis. The S1P(1) receptor is required for the growth of proplatelet strings in the bloodstream and the shedding of platelets into the circulation. Notably, the sharp gradient of S1P between blood and the interstitial fluids seems to be essential to ensure the production of platelets, and S1P appears to cooperate with the CXCL12-CXCR4 axis. Pharmacologic modulation of the S1P(1) receptor altered circulating platelet numbers acutely, suggesting a potential therapeutic strategy for controlling thrombocytopenic states. However, the S1P(4) receptor may also regulate thrombopoiesis during stress-induced accelerated platelet production. This work reveals a novel physiological action of the S1P/S1P(1) duet that could potentially be harnessed for clinical translation.

RACK1 is involved in endothelial barrier regulation via its two novel interacting partners

Background

RACK1, receptor for activated protein kinase C, serves as an anchor in multiple signaling pathways. TIMAP, TGF-β inhibited membrane-associated protein, is most abundant in endothelial cells with a regulatory effect on the endothelial barrier function. The interaction of TIMAP with protein phosphatase 1 (PP1cδ) was characterized, yet little is known about its further partners.

Results

We identified two novel interacting partners of RACK1, namely, TGF-β inhibited membrane-associated protein, TIMAP, and farnesyl transferase. TIMAP is most abundant in endothelial cells where it is involved in the regulation of the barrier function. WD1-4 repeats of RACK1 were identified as critical regions of the interaction both with TIMAP and farnesyl transferase. Phosphorylation of TIMAP by activation of the cAMP/PKA pathway reduced the amount of TIMAP-RACK1 complex and enhanced translocation of TIMAP to the cell membrane in vascular endothelial cells. However, both membrane localization of TIMAP and transendothelial resistance were attenuated after RACK1 depletion. Farnesyl transferase, the enzyme responsible for prenylation and consequent membrane localization of TIMAP, is present in the RACK1-TIMAP complex in control cells, but it does not co-immunoprecipitate with TIMAP after RACK1 depletion.

Conclusions

Transient parallel linkage of TIMAP and farnesyl transferase to RACK1 could ensure prenylation and transport of TIMAP to the plasma membrane where it may attend in maintaining the endothelial barrier as a phosphatase regulator.

The sphingosine-1-phosphate receptor S1PR1 restricts sprouting angiogenesis by regulating the interplay between VE-cadherin and VEGFR2

Angiogenesis, the process by which new blood vessels arise from preexisting ones, is critical for embryonic development and is an integral part of many disease processes. Recent studies have provided detailed information on how angiogenic sprouts initiate, elongate, and branch, but less is known about how these processes cease. Here, we show that S1PR1, a receptor for the blood-borne bioactive lipid sphingosine-1-phosphate (S1P), is critical for inhibition of angiogenesis and acquisition of vascular stability. Loss of S1PR1 leads to increased endothelial cell sprouting and the formation of ectopic vessel branches. Conversely, S1PR1 signaling inhibits angiogenic sprouting and enhances cell-to-cell adhesion. This correlates with inhibition of vascular endothelial growth factor-A (VEGF-A)-induced signaling and stabilization of vascular endothelial (VE)-cadherin localization at endothelial junctions. Our data suggest that S1PR1 signaling acts as a vascular-intrinsic stabilization mechanism, protecting developing blood vessels against aberrant angiogenic responses.