Development of endothelial cell lines from embryonic stem cells: A tool for studying genetically manipulated endothelial cells in vitro

The gluconeogenesis enzyme PCK2 has a non-enzymatic role in proteostasis in endothelial cells

Endothelial cells (ECs) are highly glycolytic, but whether they generate glycolytic intermediates via gluconeogenesis (GNG) in glucose-deprived conditions remains unknown. Here, we report that glucose-deprived ECs upregulate the GNG enzyme PCK2 and rely on a PCK2-dependent truncated GNG, whereby lactate and glutamine are used for the synthesis of lower glycolytic intermediates that enter the serine and glycerophospholipid biosynthesis pathways, which can play key roles in redox homeostasis and phospholipid synthesis, respectively. Unexpectedly, however, even in normal glucose conditions, and independent of its enzymatic activity, PCK2 silencing perturbs proteostasis, beyond its traditional GNG role. Indeed, PCK2-silenced ECs have an impaired unfolded protein response, leading to accumulation of misfolded proteins, which due to defective proteasomes and impaired autophagy, results in the accumulation of protein aggregates in lysosomes and EC demise. Ultimately, loss of PCK2 in ECs impaired vessel sprouting. This study identifies a role for PCK2 in proteostasis beyond GNG.

Fundamental considerations for designing endothelialized in vitro models of thrombosis

Endothelialized in vitro models for cardiovascular disease have contributed greatly to our current understanding of the complex molecular mechanisms underlying thrombosis. To further elucidate these mechanisms, it is important to consider which fundamental aspects to incorporate into an in vitro model. In this review, we will focus on the design of in vitro endothelialized models of thrombosis. Expanding our understanding of the relation and interplay between the different pathways involved will rely in part on complex models that incorporate endothelial cells, blood, the extracellular matrix, and flow. Importantly, the use of tissue-specific endothelial cells will help in understanding the heterogeneity in thrombotic responses between different vascular beds. The dynamic and complex responses of endothelial cells to different shear rates underlines the importance of incorporating appropriate shear in in vitro models. Alterations in vascular extracellular matrix composition, availability of bioactive molecules, and gradients in concentration and composition of these molecules can all regulate the function of both endothelial cells and perivascular cells. Factors modulating these elements in in vitro models should therefore be considered carefully depending on the research question at hand. As the complexity of in vitro models increases, so can the variability. A bottom-up approach to designing such models will remain an important tool for researchers studying thrombosis. As new techniques are continuously being developed and new pathways are brought to light, research question-dependent considerations will have to be made regarding what aspects of thrombosis to include in in vitro models.

Junctional adhesion molecule-A is dispensable for myeloid cell recruitment and diversification in the tumor microenvironment

Junctional adhesion molecule-A (JAM-A), expressed on the surface of myeloid cells, is required for extravasation at sites of inflammation and may also modulate myeloid cell activation. Infiltration of myeloid cells is a common feature of tumors that drives disease progression, but the function of JAM-A in this phenomenon and its impact on tumor-infiltrating myeloid cells is little understood. Here we show that systemic cancer-associated inflammation in mice enhanced JAM-A expression selectively on circulating monocytes in an IL1β-dependent manner. Using myeloid-specific JAM-A-deficient mice, we found that JAM-A was dispensable for recruitment of monocytes and other myeloid cells to tumors, in contrast to its reported role in inflammation. Single-cell RNA sequencing revealed that loss of JAM-A did not influence the transcriptional reprogramming of myeloid cells in the tumor microenvironment. Overall, our results support the notion that cancer-associated inflammation can modulate the phenotype of circulating immune cells, and we demonstrate that tumors can bypass the requirement of JAM-A for myeloid cell recruitment and reprogramming.

HIF-1α is involved in blood-brain barrier dysfunction and paracellular migration of bacteria in pneumococcal meningitis

Bacterial meningitis is a deadly disease most commonly caused by Streptococcus pneumoniae, leading to severe neurological sequelae including cerebral edema, seizures, stroke, and mortality when untreated. Meningitis is initiated by the transfer of S. pneumoniae from blood to the brain across the blood-cerebrospinal fluid barrier or the blood-brain barrier (BBB). The underlying mechanisms are still poorly understood. Current treatment strategies include adjuvant dexamethasone for inflammation and cerebral edema, followed by antibiotics. The success of dexamethasone is however inconclusive, necessitating new therapies for controlling edema, the primary reason for neurological complications. Since we have previously shown a general activation of hypoxia inducible factor (HIF-1α) in bacterial infections, we hypothesized that HIF-1α, via induction of vascular endothelial growth factor (VEGF) is involved in transmigration of pathogens across the BBB. In human, murine meningitis brain samples, HIF-1α activation was observed by immunohistochemistry. S. pneumoniae infection in brain endothelial cells (EC) resulted in in vitro upregulation of HIF-1α/VEGF (Western blotting/qRT-PCR) associated with increased paracellular permeability (fluorometry, impedance measurements). This was supported by bacterial localization at cell-cell junctions in vitro and in vivo in brain ECs from mouse and humans (confocal, super-resolution, electron microscopy, live-cell imaging). Hematogenously infected mice showed increased permeability, S. pneumoniae deposition in the brain, along with upregulation of genes in the HIF-1α/VEGF pathway (RNA sequencing of brain microvessels). Inhibition of HIF-1α with echinomycin, siRNA in bEnd5 cells or using primary brain ECs from HIF-1α knock-out mice revealed reduced endothelial permeability and transmigration of S. pneumoniae. Therapeutic rescue using the HIF-1α inhibitor echinomycin resulted in increased survival and improvement of BBB function in S. pneumoniae-infected mice. We thus demonstrate paracellular migration of bacteria across BBB and a critical role for HIF-1α/VEGF therein and hence propose targeting this pathway to prevent BBB dysfunction and ensuing brain damage in infections.

MiR-155 deletion reduces ischemia-induced paralysis in an aortic aneurysm repair mouse model: Utility of immunohistochemistry and histopathology in understanding etiology of spinal cord paralysis

Spinal cord paralysis is relatively common after surgical repair of thoraco-abdominal aortic aneurysm (TAAA) and its etiology is unknown. The present study was designed to examine the histopathology of the disease and investigate whether miR-155 ablation would reduce spinal cord ischemic damage and delayed hindlimb paralysis induced by aortic cross-clamping (ACC) in our mouse model. The loss of locomotor function in ACC-paralyzed mice correlated with the presence of extensive gray matter damage and central cord edema, with minimal white matter histopathology. qRTPCR and Western blotting showed that the spinal cords of wild-type ACC mice that escaped paralysis showed lower miR-155 expression and higher levels of transcripts encoding Mfsd2a, which is implicated in the maintenance of blood-brain barrier integrity. In situ based testing demonstrated that increased miR-155 detection in neurons was highly correlated with the gray matter damage and the loss of one of its targets, Mfsd2a, could serve as a good biomarker of the endothelial cell damage. In vitro, we demonstrated that miR-155 targeted Mfsd2a in endothelial cells and motoneurons and increased endothelial cell permeability. Finally, miR-155 ablation slowed the progression of central cord edema, and reduced the incidence of paralysis by 40%. In sum, the surgical pathology findings clearly indicated that the epicenter of the ischemic-induced paralysis was the gray matter and that endothelial cell damage correlated to Mfsd2a loss is a good biomarker of the disease. MiR-155 targeting therefore offers new therapeutic opportunity for edema caused by traumatic spinal cord injury and diagnostic pathologists, by using immunohistochemistry, can clarify if this mechanism also is important in other ischemic diseases of the CNS, including stroke.

From jamming to collective cell migration through a boundary induced transition

Cell monolayers provide an interesting example of active matter, exhibiting a phase transition from flowing to jammed states as they age. Here we report experiments and numerical simulations illustrating how a jammed cellular layer rapidly reverts to a flowing state after a wound. Quantitative comparison between experiments and simulations shows that cells change their self-propulsion and alignment strength so that the system crosses a phase transition line, which we characterize by finite-size scaling in an active particle model. This wound-induced unjamming transition is found to occur generically in epithelial, endothelial and cancer cells.

Endothelial-to-Mesenchymal Transition in Bone Marrow and Spleen of Primary Myelofibrosis

Primary myelofibrosis is characterized by the development of fibrosis in the bone marrow that contributes to ineffective hematopoiesis. Bone marrow fibrosis is the result of a complex and not yet fully understood interaction among megakaryocytes, myeloid cells, fibroblasts, and endothelial cells. Here, we report that >30% of the endothelial cells in the small vessels of the bone marrow and spleen of patients with primary myelofibrosis have a mesenchymal phenotype, which is suggestive of the process known as endothelial-to-mesenchymal transition (EndMT). EndMT can be reproduced in vitro by incubation of cultured endothelial progenitor cells or spleen-derived endothelial cells with inflammatory cytokines. Megakaryocytes appear to be implicated in this process, because EndMT mainly occurs in the microvessels close to these cells, and because megakaryocyte-derived supernatant fluid can reproduce the EndMT switch in vitro. Furthermore, EndMT is an early event in a JAK2-V617F knock-in mouse model of primary myelofibrosis. Overall, these data show for the first time that microvascular endothelial cells in the bone marrow and spleen of patients with primary myelofibrosis show functional and morphologic changes that are associated to the mesenchymal phenotype.

Focal Adhesion Kinase (FAK) tyrosine 397E mutation restores the vascular leakage defect in endothelium-specific FAK-kinase dead mice

Focal adhesion kinase (FAK) inhibitors have been developed as potential anticancer agents and are undergoing clinical trials. In vitro activation of the FAK kinase domain triggers autophosphorylation of Y397, Src activation, and subsequent phosphorylation of other FAK tyrosine residues. However, how FAK Y397 mutations affect FAK kinase-dead (KD) phenotypes in tumour angiogenesis in vivo is unknown. We developed three Pdgfb-iCreert -driven endothelial cell (EC)-specific, tamoxifen-inducible homozygous mutant mouse lines: FAK wild-type (WT), FAK KD, and FAK double mutant (DM), i.e. KD with a putatively phosphomimetic Y397E mutation. These ECCre+;FAKWT/WT , ECCre+;FAKKD/KD and ECCre+;FAKDM/DM mice were injected subcutaneously with syngeneic B16F0 melanoma cells. Tumour growth and tumour blood vessel functions were unchanged between ECCre+;FAKWT/WT and ECCre-;FAKWT/WT control mice. In contrast, tumour growth and vessel density were decreased in ECCre+;FAKKD/KD and ECCre+;FAKDM/DM mice, as compared with Cre - littermates. Despite no change in the percentage of perfused vessels or pericyte coverage in either genotype, tumour hypoxia was elevated in ECCre+;FAKKD/KD and ECCre+;FAKDM/DM mice. Furthermore, although ECCre+;FAKKD/KD mice showed reduced blood vessel leakage, ECCre+;FAKDM/DM and ECCre-;FAKDM/DM mice showed no difference in leakage. Mechanistically, fibronectin-stimulated Y397 autophosphorylation was reduced in Cre+;FAKKD/KD ECs as compared with Cre+;FAKWT/WT cells, with no change in phosphorylation of the known Src targets FAK-Y577, FAK-Y861, FAK-Y925, paxillin-Y118, p130Cas-Y410. Cre+;FAKDM/DM ECs showed decreased Src target phosphorylation levels, suggesting that the Y397E substitution actually disrupted Src activation. Reduced VE-cadherin-pY658 levels in Cre+;FAKKD/KD ECs were rescued in Cre+FAKDM/DM ECs, corresponding with the rescue in vessel leakage in the ECCre+;FAKDM/DM mice. We show that EC-specific FAK kinase activity is required for tumour growth, angiogenesis, and vascular permeability. The ECCre+;FAKDM/DM mice restored the KD-dependent tumour vascular leakage observed in ECCre+;FAKKD/KD mice in vivo. This study opens new fields in in vivo FAK signalling. © 2017 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.

Optimizing mesoderm progenitor selection and three-dimensional microniche culture allows highly efficient endothelial differentiation and ischemic tissue repair from human pluripotent stem cells

Background

Generation of large quantities of endothelial cells is highly desirable for vascular research, for the treatment of ischemia diseases, and for tissue regeneration. To achieve this goal, we developed a simple, chemically defined culture system to efficiently and rapidly differentiate endothelial cells from human pluripotent stem cells by going through an MESP1 mesoderm progenitor stage.

Methods

Mesp1 is a key transcription factor that regulates the development of early cardiovascular tissue. Using an MESP1-mTomato knock-in reporter human embryonic stem cell line, we compared the gene expression profiles of MESP1⁺ and MESP1⁻ cells and identified new signaling pathways that may promote endothelial differentiation. We also used a 3D scaffold to mimic the in vivo microenvironment to further improve the efficiency of endothelial cell generation. Finally, we performed cell transplantation into a critical limb ischemia mouse model to test the repairing potential of endothelial-primed MESP1⁺ cells.

Results

MESP1⁺ mesoderm progenitors, but not MESP1⁻ cells, have strong endothelial differentiation potential. Global gene expression analysis revealed that transcription factors essential for early endothelial differentiation were enriched in MESP1⁺ cells. Interestingly, MESP1 cells highly expressed Sphingosine-1-phosphate (S1P) receptor and the addition of S1P significantly increased the endothelial differentiation efficiency. Upon seeding in a novel 3D microniche and priming with VEGF and bFGF, MESP1⁺ cells markedly upregulated genes related to vessel development and regeneration. 3D microniches also enabled long-term endothelial differentiation and proliferation from MESP1⁺ cells with minimal medium supplements. Finally, we showed that transplanting a small number of endothelial-primed MESP1⁺ cells in 3D microniches was sufficient to mediate rapid repair of a mouse model of critical limb ischemia.

Conclusions

Our study demonstrates that combining MESP1⁺ mesoderm progenitor cells with tissue-engineered 3D microniche and a chemically defined endothelial induction medium is a promising route to maximizing the production of endothelial cells in vitro and augment their regenerative power in vivo.

Electronic supplementary material

The online version of this article (doi:10.1186/s13287-016-0455-4) contains supplementary material, which is available to authorized users.

Bursts of activity in collective cell migration

Dense monolayers of living cells display intriguing relaxation dynamics, reminiscent of soft and glassy materials close to the jamming transition, and migrate collectively when space is available, as in wound healing or in cancer invasion. Here we show that collective cell migration occurs in bursts that are similar to those recorded in the propagation of cracks, fluid fronts in porous media, and ferromagnetic domain walls. In analogy with these systems, the distribution of activity bursts displays scaling laws that are universal in different cell types and for cells moving on different substrates. The main features of the invasion dynamics are quantitatively captured by a model of interacting active particles moving in a disordered landscape. Our results illustrate that collective motion of living cells is analogous to the corresponding dynamics in driven, but inanimate, systems.

Transcription factor TLX1 controls retinoic acid signaling to ensure spleen development

The molecular mechanisms that underlie spleen development and congenital asplenia, a condition linked to increased risk of overwhelming infections, remain largely unknown. The transcription factor TLX1 controls cell fate specification and organ expansion during spleen development, and Tlx1 deletion causes asplenia in mice. Deregulation of TLX1 expression has recently been proposed in the pathogenesis of congenital asplenia in patients carrying mutations of the gene-encoding transcription factor SF-1. Herein, we have shown that TLX1-dependent regulation of retinoic acid (RA) metabolism is critical for spleen organogenesis. In a murine model, loss of Tlx1 during formation of the splenic anlage increased RA signaling by regulating several genes involved in RA metabolism. Uncontrolled RA activity resulted in premature differentiation of mesenchymal cells and reduced vasculogenesis of the splenic primordium. Pharmacological inhibition of RA signaling in Tlx1-deficient animals partially rescued the spleen defect. Finally, spleen growth was impaired in mice lacking either cytochrome P450 26B1 (Cyp26b1), which results in excess RA, or retinol dehydrogenase 10 (Rdh10), which results in RA deficiency. Together, these findings establish TLX1 as a critical regulator of RA metabolism and provide mechanistic insights into the molecular determinants of human congenital asplenia.

The actin-binding protein EPS8 binds VE-cadherin and modulates YAP localization and signaling

Vascular endothelial (VE)-cadherin transfers intracellular signals contributing to vascular hemostasis. Signaling through VE-cadherin requires association and activity of different intracellular partners. Yes-associated protein (YAP)/TAZ transcriptional cofactors are important regulators of cell growth and organ size. We show that EPS8, a signaling adapter regulating actin dynamics, is a novel partner of VE-cadherin and is able to modulate YAP activity. By biochemical and imaging approaches, we demonstrate that EPS8 associates with the VE-cadherin complex of remodeling junctions promoting YAP translocation to the nucleus and transcriptional activation. Conversely, in stabilized junctions, 14-3-3-YAP associates with the VE-cadherin complex, whereas Eps8 is excluded. Junctional association of YAP inhibits nuclear translocation and inactivates its transcriptional activity both in vitro and in vivo in Eps8-null mice. The absence of Eps8 also increases vascular permeability in vivo, but did not induce other major vascular defects. Collectively, we identified novel components of the adherens junction complex, and we introduce a novel molecular mechanism through which the VE-cadherin complex controls YAP transcriptional activity.

Excess centrosomes disrupt endothelial cell migration via centrosome scattering

Centrosome–microtubule interactions during interphase are important for centrosome clustering and cell polarity.

Supernumerary centrosomes contribute to spindle defects and aneuploidy at mitosis, but the effects of excess centrosomes during interphase are poorly understood. In this paper, we show that interphase endothelial cells with even one extra centrosome exhibit a cascade of defects, resulting in disrupted cell migration and abnormal blood vessel sprouting. Endothelial cells with supernumerary centrosomes had increased centrosome scattering and reduced microtubule (MT) nucleation capacity that correlated with decreased Golgi integrity and randomized vesicle trafficking, and ablation of excess centrosomes partially rescued these parameters. Mechanistically, tumor endothelial cells with supernumerary centrosomes had less centrosome-localized γ-tubulin, and Plk1 blockade prevented MT growth, whereas overexpression rescued centrosome γ-tubulin levels and centrosome dynamics. These data support a model whereby centrosome–MT interactions during interphase are important for centrosome clustering and cell polarity and further suggest that disruption of interphase cell behavior by supernumerary centrosomes contributes to pathology independent of mitotic effects.

Establishment and characterization of a rainbow trout heart endothelial cell line with susceptibility to viral hemorrhagic septicemia virus (VHSV)

In the current work, we have established and characterized a novel cell line from rainbow trout (Oncorhynchus mykiss). The cell line, designated as RTH (rainbow trout heart), was obtained by immortalizing heart cells with recombinant retroviruses that transduced polyoma middle T antigen. This is the first time such a strategy is used to obtain an immortalized fish cell line. The cells showed an endothelial-like morphology and characteristics, constitutively transcribing collagen, selectin and VCAM (vascular cell adhesion molecule), as well as different chemokines and chemokine receptors, but not cytokeratin. As already described for heart endothelial cells, RTH cells actively phagocytized latex beads. Furthermore, RTH cells showed a high susceptibility to viral hemorrhagic septicemia virus (VHSV). VHSV modulated the transcription of Mx, major histocompatibility complex II (MHC-II), VCAM and many of the chemokine and chemokine receptors expressed in these cells. Therefore, RTH cells constitute an excellent model to study the immune regulation of endothelial cells in fish and their role in leukocyte extravasation.

Hemogenic endothelium generates mesoangioblasts that contribute to several mesodermal lineages in vivo

The embryonic endothelium is a known source of hematopoietic stem cells. Moreover, vessel-associated progenitors/stem cells with multilineage mesodermal differentiation potential, such as the ‘embryonic mesoangioblasts’, originate in vitro from the endothelium. Using a genetic lineage tracing approach, we show that early extra-embryonic endothelium generates, in a narrow time-window and prior to the hemogenic endothelium in the major embryonic arteries, hematopoietic cells that migrate to the embryo proper, and are subsequently found within the mesenchyme. A subpopulation of these cells, distinct from embryonic macrophages, co-expresses mesenchymal and hematopoietic markers. In addition, hemogenic endothelium-derived cells contribute to skeletal and smooth muscle, and to other mesodermal cells in vivo, and display features of embryonic mesoangioblasts in vitro. Therefore, we provide new insights on the distinctive characteristics of the extra-embryonic and embryonic hemogenic endothelium, and we identify the putative in vivo counterpart of embryonic mesoangioblasts, suggesting their identity and developmental ontogeny.

Development of immortalized mouse aortic endothelial cell lines

Background

The understanding of endothelial cell biology has been facilitated by the availability of primary endothelial cell cultures from a variety of sites and species; however, the isolation and maintenance of primary mouse aortic endothelial cells (MAECs) remain a formidable challenge. Culturing MAECs is difficult as they are prone to phenotypic drift during culture. Therefore, there is a need to have a dependable in vitro culture system, wherein the primary endothelial cells retain their properties and phenotypes.

Methods

Here, we developed an effective method to prepare immortalized MAEC (iMAEC) lines. Primary MAECs, initially isolated from aortic explants, were immortalized using a retrovirus expressing polyoma middle T-antigen. Immortalized cells were then incubated with DiI-acetylated-low density lipoprotein and sorted via flow cytometry to isolate iMAECs.

Results

iMAECs expressed common markers of endothelial cells, including PECAM1, eNOS, VE-cadherin, and von Willebrand Factor. iMAECs aligned in the direction of imposed laminar shear and retained the ability to form tubes. Using this method, we have generated iMAEC lines from wild-type and various genetically modified mice such as p47^phox-/-, eNOS^-/-, and caveolin-1^-/-.

Conclusion

In summary, generation of iMAEC lines from various genetically modified mouse lines provides an invaluable tool to study vascular biology and pathophysiology.

Wnt activation of immortalized brain endothelial cells as a tool for generating a standardized model of the blood brain barrier in vitro

Reproducing the characteristics and the functional responses of the blood–brain barrier (BBB) in vitro represents an important task for the research community, and would be a critical biotechnological breakthrough. Pharmaceutical and biotechnology industries provide strong demand for inexpensive and easy-to-handle in vitro BBB models to screen novel drug candidates. Recently, it was shown that canonical Wnt signaling is responsible for the induction of the BBB properties in the neonatal brain microvasculature in vivo. In the present study, following on from earlier observations, we have developed a novel model of the BBB in vitro that may be suitable for large scale screening assays. This model is based on immortalized endothelial cell lines derived from murine and human brain, with no need for co-culture with astrocytes. To maintain the BBB endothelial cell properties, the cell lines are cultured in the presence of Wnt3a or drugs that stabilize β-catenin, or they are infected with a transcriptionally active form of β-catenin. Upon these treatments, the cell lines maintain expression of BBB-specific markers, which results in elevated transendothelial electrical resistance and reduced cell permeability. Importantly, these properties are retained for several passages in culture, and they can be reproduced and maintained in different laboratories over time. We conclude that the brain-derived endothelial cell lines that we have investigated gain their specialized characteristics upon activation of the canonical Wnt pathway. This model may be thus suitable to test the BBB permeability to chemicals or large molecular weight proteins, transmigration of inflammatory cells, treatments with cytokines, and genetic manipulation.

Mouse embryonic stem cells are deficient in type I interferon expression in response to viral infections and double-stranded RNA

Embryonic stem cells (ESCs) are considered to be a promising cell source for regenerative medicine because of their unlimited capacity for self-renewal and differentiation. However, little is known about the innate immunity in ESCs and ESC-derived cells. We investigated the responses of mouse (m)ESCs to three types of live viruses as follows: La Crosse virus, West Nile virus, and Sendai virus. Our results demonstrated mESCs were susceptible to viral infection, but they were unable to express type I interferons (IFNα and IFNβ, IFNα/β), which differ from fibroblasts (10T1/2 cells) that robustly express IFNα/β upon viral infections. The failure of mESCs to express IFNα/β was further demonstrated by treatment with polyIC, a synthetic viral dsRNA analog that strongly induced IFNα/β in 10T1/2 cells. Although polyIC transiently inhibited the transcription of pluripotency markers, the stem cell morphology was not significantly affected. However, polyIC can induce dsRNA-activated protein kinase in mESCs, and this activation resulted in a strong inhibition of cell proliferation. We conclude that the cytosolic receptor dsRNA-activated protein kinase is functional, but the mechanisms that mediate type I IFN expression are deficient in mESCs. This conclusion is further supported by the findings that the major viral RNA receptors are either expressed at very low levels (TLR3 and MDA5) or may not be active (retinoic acid-inducible gene I) in mESCs.

Advances in establishment and analysis of three-dimensional tumor spheroid-based functional assays for target validation and drug evaluation

Background

There is overwhelming evidence that in vitro three-dimensional tumor cell cultures more accurately reflect the complex in vivo microenvironment than simple two-dimensional cell monolayers, not least with respect to gene expression profiles, signaling pathway activity and drug sensitivity. However, most currently available three-dimensional techniques are time consuming and/or lack reproducibility; thus standardized and rapid protocols are urgently needed.

Results

To address this requirement, we have developed a versatile toolkit of reproducible three-dimensional tumor spheroid models for dynamic, automated, quantitative imaging and analysis that are compatible with routine high-throughput preclinical studies. Not only do these microplate methods measure three-dimensional tumor growth, but they have also been significantly enhanced to facilitate a range of functional assays exemplifying additional key hallmarks of cancer, namely cell motility and matrix invasion. Moreover, mutual tissue invasion and angiogenesis is accommodated by coculturing tumor spheroids with murine embryoid bodies within which angiogenic differentiation occurs. Highly malignant human tumor cells were selected to exemplify therapeutic effects of three specific molecularly-targeted agents: PI-103 (phosphatidylinositol-3-kinase (PI3K)-mammalian target of rapamycin (mTOR) inhibitor), 17-N-allylamino-17-demethoxygeldanamycin (17-AAG) (heat shock protein 90 (HSP90) inhibitor) and CCT130234 (in-house phospholipase C (PLC)γ inhibitor). Fully automated analysis using a Celigo cytometer was validated for tumor spheroid growth and invasion against standard image analysis techniques, with excellent reproducibility and significantly increased throughput. In addition, we discovered key differential sensitivities to targeted agents between two-dimensional and three-dimensional cultures, and also demonstrated enhanced potency of some agents against cell migration/invasion compared with proliferation, suggesting their preferential utility in metastatic disease.

Conclusions

We have established and validated a suite of highly reproducible tumor microplate three-dimensional functional assays to enhance the biological relevance of early preclinical cancer studies. We believe these assays will increase the translational predictive value of in vitro drug evaluation studies and reduce the need for in vivo studies by more effective triaging of compounds.

Angiogenic sprouting requires the fine tuning of endothelial cell cohesion by the Raf-1/Rok-α complex

Sprouting angiogenesis, crucial for the development of new blood vessels, is a prime example of collective migration in which endothelial cells migrate as a group joined via cadherin-containing adherens junctions (AJ). The actomyosin apparatus is connected to AJ and generates contractile forces, which, depending on their strength and duration, increase or decrease cell cohesion. Thus, appropriate spatiotemporal control of junctional myosin is critical, but the mechanisms underlying it are incompletely understood. We show that Raf-1 is an essential component of this regulatory network and that its ablation impairs endothelial cell cohesion, sprouting, and tumor-induced angiogenesis. Mechanistically, Raf-1 is recruited to VE-cadherin complexes by a mechanism involving the small G protein Rap1 and is required to bring the Rho effector Rok-α to nascent AJs. This Raf-1-mediated fine tuning of Rok-α signaling allows the activation of junctional myosin and the timely maturation of AJ essential for maintaining cell cohesion during sprouting angiogenesis.

Developmental timing of CCM2 loss influences cerebral cavernous malformations in mice

As revealed in a new model of cerebral cavernous malformations (CCM), the timing of ablation of Ccm genes determines whether or not CCM lesions arise in brain and retina venous beds.

Cerebral cavernous malformations (CCM) are vascular malformations of the central nervous system (CNS) that lead to cerebral hemorrhages. Familial CCM occurs as an autosomal dominant condition caused by loss-of-function mutations in one of the three CCM genes. Constitutive or tissue-specific ablation of any of the Ccm genes in mice previously established the crucial role of Ccm gene expression in endothelial cells for proper angiogenesis. However, embryonic lethality precluded the development of relevant CCM mouse models. Here, we show that endothelial-specific Ccm2 deletion at postnatal day 1 (P1) in mice results in vascular lesions mimicking human CCM lesions. Consistent with CCM1/3 involvement in the same human disease, deletion of Ccm1/3 at P1 in mice results in similar CCM lesions. The lesions are located in the cerebellum and the retina, two organs undergoing intense postnatal angiogenesis. Despite a pan-endothelial Ccm2 deletion, CCM lesions are restricted to the venous bed. Notably, the consequences of Ccm2 loss depend on the developmental timing of Ccm2 ablation. This work provides a highly penetrant and relevant CCM mouse model.

The Ras activator RasGRP3 mediates diabetes-induced embryonic defects and affects endothelial cell migration

RATIONALE

Fetuses that develop in diabetic mothers have a higher incidence of birth defects that include cardiovascular defects, but the signaling pathways that mediate these developmental effects are poorly understood. It is reasonable to hypothesize that diabetic maternal effects are mediated by 1 or more pathways activated downstream of aberrant glucose metabolism, because poorly controlled maternal glucose levels correlate with the frequency and severity of the defects.

OBJECTIVE

We investigated whether RasGRP3 (Ras guanyl-releasing protein 3), a Ras activator expressed in developing blood vessels, mediates diabetes-induced vascular developmental defects. RasGRP3 is activated by diacylglycerol, and diacylglycerol is overproduced by aberrant glucose metabolism in diabetic individuals. We also investigated the effects of overactivation and loss of function for RasGRP3 in primary endothelial cells and developing vessels.

METHODS AND RESULTS

Analysis of mouse embryos from diabetic mothers showed that diabetes-induced developmental defects were dramatically attenuated in embryos that lacked Rasgrp3 function. Endothelial cells that expressed activated RasGRP3 had elevated Ras-ERK signaling and perturbed migration, whereas endothelial cells that lacked Rasgrp3 function had attenuated Ras-ERK signaling and did not migrate in response to endothelin-1. Developing blood vessels exhibited endothelin-stimulated vessel dysmorphogenesis that required Rasgrp3 function.

CONCLUSIONS

These findings provide the first evidence that RasGRP3 contributes to developmental defects found in embryos that develop in a diabetic environment. The results also elucidate RasGRP3-mediated signaling in endothelial cells and identify endothelin-1 as an upstream input and Ras/MEK/ERK as a downstream effector pathway. RasGRP3 may be a novel therapeutic target for the fetal complications of diabetes.

CCM1 regulates vascular-lumen organization by inducing endothelial polarity

Little is known about the molecular mechanisms that regulate the organization of vascular lumen. In this paper we show that lumen formation correlates with endothelial polarization. Adherens junctions (AJs) and VE-cadherin (VEC, encoded by CDH5) are required for endothelial apicobasal polarity in vitro and during embryonic development. Silencing of CDH5 gene expression leads to abrogation of endothelial polarity accompanied by strong alterations in lumenal structure. VEC co-distributes with members of the Par polarity complex (Par3 and PKCzeta) and is needed for activation of PKCzeta. CCM1 is encoded by the CCM1 gene, which is mutated in 60% of patients affected by cerebral cavernous malformation (CCM). The protein interacts with VEC and directs AJ organization and AJ association with the polarity complex, both in cell-culture models and in human CCM1 lesions. Both VEC and CCM1 control Rap1 concentration at cell-cell junctions. We propose that VEC, CCM1 and Rap1 form a signaling complex. In the absence of any of these proteins, AJs are dismantled, cell polarity is lost and vascular lumenal structure is severely altered.

ILK mediates LPS-induced vascular adhesion receptor expression and subsequent leucocyte trans-endothelial migration

AIMS

The inflammatory response to injurious agents is tightly regulated to avoid adverse consequences of inappropriate leucocyte accumulation or failed resolution. Lipopolysaccharide (LPS)-activated endothelium recruits leucocytes to the inflamed tissue through controlled expression of membrane-associated adhesion molecules. LPS responses in macrophages are known to be regulated by integrin-linked kinase (ILK); in this study, we investigated the role of ILK in the regulation of the LPS-elicited inflammatory response in endothelium.

METHODS AND RESULTS

This study was performed on immortalized mouse endothelial cells (EC) isolated from lung and coronary vasculature. Cells were thoroughly characterized and the role of ILK in the regulation of the LPS response was investigated by suppressing ILK expression using siRNA and shRNA technologies. Phenotypic and functional analyses confirmed that the immortalized cells behaved as true EC. LPS induced the expression of the inflammatory genes E-selectin, intercellular adhesion molecule-1 (ICAM-1) and vascular cell adhesion molecule-1 (VCAM-1). ILK knockdown impaired LPS-mediated endothelial activation by preventing the induction of ICAM-1 and VCAM-1. Blockade of the LPS-induced response inhibited the inflammatory-related processes of firm adhesion and trans-endothelial migration of leucocytes.

CONCLUSION

ILK is involved in the expression of cell adhesion molecules by EC activated with the inflammatory stimulus LPS. This reduced expression modulates leucocyte adhesion to the endothelium and the extravasation process. This finding suggests ILK as a potential anti-inflammatory target for the development of vascular-specific treatments for inflammation-related diseases.

Vasculogenesis driven by bone marrow-derived cells is essential for growth of Ewing's sarcomas

The role of vasculogenesis as opposed to angiogenesis in tumor formation has been little explored genetically. Endothelial cells that lack the MEK kinase MEKK3 cannot form vessels. In this study, we employed mice with hematopoietic deletions of the Mekk3 gene to evaluate the importance of vasculogenesis in the formation of Ewing's sarcoma tumors. Bone marrow cells (BM) from LacZ(+) Mekk3-deficient conditional knockout mice (Mekk3(Deltaflox/-) mice) were transplanted into irradiated nude mice before injection of Ewing's sarcoma cells. Because the grafted Mekk3(Deltaflox/-) BM cells cannot contribute to vessel development in the same way as the host Mekk3(+/+) endothelial cells, angiogenesis is normal in the model whereas vasculogenesis is impaired. Four weeks after BM transplant, Ewing's sarcoma TC71 or A4573 cells were injected, and tumor growth and vessel density were compared. Strikingly, chimeric mice transplanted with Mekk3(Deltaflox/-) BM exhibited a reduction in tumor growth and vessel density compared with mice transplanted with Mekk3(Deltaflox/+) BM cells. Mekk3(Deltaflox/-) cells that were LacZ positive were visualized within the tumor; however, few of the LacZ(+) cells colocalized with either CD31(+) endothelial cells or desmin(+) pericytes. Quantification of double-positive LacZ(+) and CD31(+) endothelial cells or LacZ(+) and desmin(+) pericytes confirmed that chimeric mice transplanted with Mekk3(Deltaflox/-) BM were impaired for tumor vessel formation. In contrast, siRNA-mediated knockdown of Mekk3 in TC71 Ewing's sarcoma cells had no effect on tumor growth or vessel density. Our findings indicate that vasculogenesis is critical in the expansion of the tumor vascular network.

Scaffolds with covalently immobilized VEGF and Angiopoietin-1 for vascularization of engineered tissues

The aim of this study was to engineer a biomaterial capable of supporting vascularization in vitro and in vivo. We covalently immobilized vascular endothelial growth factor (VEGF) and Angiopoietin-1 (Ang1) onto three-dimensional porous collagen scaffolds using 1-ethyl-3-[3-dimethylaminopropyl]carbodiimide hydrochloride (EDC) chemistry. Over both 3 and 7 days in vitro, seeded endothelial cells (ECs) had increased proliferation on scaffolds with immobilized VEGF and/or Ang1 compared to unmodified scaffolds and soluble growth factor controls. Notably, the group with co-immobilized VEGF and Ang1 showed significantly higher cell number (P=0.0079), higher overall lactate production rate (P=0.0044) and higher overall glucose consumption rate (P=0.0034) at Day 3, compared to its corresponding soluble control for which growth factors were added to culture medium. By Day 7, hematoxylin and eosin, live/dead, CD31, and von Willebrand factor staining all showed improved tube formation by ECs when cultivated on scaffolds with co-immobilized growth factors. Interestingly, scaffolds with co-immobilized VEGF and Ang1 showed increased EC infiltration in the chorioallantoic membrane (CAM) assay, compared to scaffolds with independently immobilized VEGF/Ang1. This study presents an alternative method for promoting the formation of vascular structures, via covalent immobilization of angiogenic growth factors that are more stable than soluble ones and have a localized effect.

Intimal cushions and endothelial nuclear elongation around mouse aortic branches and their spatial correspondence with patterns of lipid deposition

Spatial variation in hemodynamic stresses acting on the arterial wall may explain the nonuniform distribution of atherosclerosis. In thoracic aortas of LDL receptor/apolipoprotein E double knockout mice, lesions develop preferentially around the entire circumference of intercostal branch ostia, regardless of age, with the highest prevalence occurring upstream. Additional chevron-shaped lesions occur further upstream of the ostia. This pattern differs from the age-related ones occurring in people and rabbits. In the present study, patterns of near-wall blood flow around intercostal ostia in wild-type mice were estimated from the morphology of endothelial nuclei, which were shown in vitro to elongate in response to elevated shear stress and to align with the flow, and wall structure was assessed from confocal and scanning electron microscopy. A triangular intimal cushion surrounded the upstream part of most ostia. Nuclear length-to-width ratios were lowest over this cushion and highest at the sides of branches, regardless of age. Nuclear orientations were consistent with flow diverging around the branch. The pattern of nuclear morphology differed from the age-related ones observed in rabbits. The intimal cushion and the distribution of shear stress inferred from these observations can partly account for the pattern of lesions observed in knockout mice. Nuclear elongation in nonbranch regions was approximately constant across animals of different size, demonstrating the existence of a mechanism by which endothelial cells compensate for the dependence of mean aortic wall shear stress on body mass.

Microfluidic platform for controlling the differentiation of embryoid bodies

Embryonic stem (ES) cells are pluripotent cells, which can differentiate into any cell type. This cell type has often been implicated as an eminent source of renewable cells for tissue regeneration and cellular replacement therapies. Studies on manipulation of the various differentiation pathways have been at the forefront of research. There are many ways in which ES cells can be differentiated. One of the most common techniques is to initiate the development of embryoid bodies (EBs) by in vitro aggregation of ES cells. Thereafter, EBs can be induced to undergo differentiation into various cell lineages. In this article, we present a microfluidic platform using biocompatible materials, which is suitable for culturing EBs. The platform is based on a Y-channel device with two inlets for two different culturing media. An EB is located across both streams. Using the laminar characteristics at low Reynolds number and high Peclet numbers, we have induced cell differentiation on half of the EB while maintaining the other half in un-induced stages. The results prove the potential of using microfluidic technology for manipulation of EBs and ES cells in tissue engineering.

Sox7 and Sox17 are strain-specific modifiers of the lymphangiogenic defects caused by Sox18 dysfunction in mice

Developmental defects caused by targeted gene inactivation in mice are commonly subject to strain-specific modifiers that modulate the severity of the phenotype. Although several genetic modifier loci have been mapped in mice, the gene(s) residing at these loci are mostly unidentified, and the molecular mechanisms of modifier action remain poorly understood. Mutations in Sox18 cause a variable phenotype in the human congenital syndrome hypotrichosis-lymphedema-telangiectasia, and the phenotype of Sox18-null mice varies from essentially normal to completely devoid of lymphatic vasculature and lethal, depending on the strain of the mice, suggesting a crucial role for strain-specific modifiers in this system. Here we show that two closely related Group F Sox factors, SOX7 and SOX17, are able to functionally substitute for SOX18 in vitro and in vivo. SOX7 and SOX17 are not normally expressed during lymphatic development, excluding a conventional redundancy mechanism. Instead, these genes are activated specifically in the absence of SOX18 function, and only in certain strains. Our studies identify Sox7 and Sox17 as modifiers of the Sox18 mutant phenotype, and reveal their mechanism of action as a novel mode of strain-specific compensatory upregulation.

Transforming growth factor-beta 1 (TGF-beta1) induces angiogenesis through vascular endothelial growth factor (VEGF)-mediated apoptosis

VEGF and TGF-beta1 induce angiogenesis but have opposing effects on endothelial cells. VEGF protects endothelial cells from apoptosis; TGF-beta1 induces apoptosis. We have previously shown that VEGF/VEGF receptor-2 (VEGFR2) signaling mediates TGF-beta1 induction of apoptosis. This finding raised an important question: Does this mechanism stimulate or inhibit angiogenesis? Here we report that VEGF-mediated apoptosis is required for TGF-beta1 induction of angiogenesis. In vitro the apoptotic effect of TGF-beta1 on endothelial cells is rapid and followed by a long period in which the cells are refractory to apoptosis induction by TGF-beta1. Inhibition of VEGF/VEGFR2 signaling abrogates formation of cord-like structures by TGF-beta1 with an effect comparable to that of z-VAD, an apoptosis inhibitor. Similarly, genetic deficiency of VEGF abolishes TGF-beta1 upregulation of endothelial cell differentiation and formation of vascular structures in embryoid bodies. In vivo TGF-beta1 induces endothelial cell apoptosis as rapidly as in vitro. Inhibition of VEGF blocks TGF-beta1 induction of both apoptosis and angiogenesis, an effect similar to that of z-VAD. Thus, TGF-beta1 induction of angiogenesis requires a rapid and transient apoptotic effect mediated by VEGF/VEGFR2. This novel, unexpected role of VEGF and VEGFR2 indicates VEGF-mediated apoptosis as a potential target to control angiogenesis.

JAM-A promotes neutrophil chemotaxis by controlling integrin internalization and recycling

The membrane-associated adhesion molecule JAM-A is required for neutrophil infiltration in inflammatory or ischemic tissues. JAM-A expressed in both endothelial cells and neutrophils has such a role, but the mechanism of action remains elusive. Here we show that JAM-A has a cell-autonomous role in neutrophil chemotaxis both in vivo and in vitro, which is independent of the interaction of neutrophils with endothelial cells. On activated neutrophils, JAM-A concentrates in a polarized fashion at the leading edge and uropod. Surprisingly, a significant amount of this protein is internalized in intracellular endosomal-like vesicles where it codistributes with integrin beta1. Clustering of beta1 integrin leads to JAM-A co-clustering, whereas clustering of JAM-A does not induce integrin association. Neutrophils derived from JAM-A-null mice are unable to correctly internalize beta1 integrins upon chemotactic stimuli and this causes impaired uropod retraction and cell motility. Consistently, inhibition of integrin internalization upon treatment with BAPTA-AM induces a comparable phenotype. These data indicate that JAM-A is required for the correct internalization and recycling of integrins during cell migration and might explain why, in its absence, the directional migration of neutrophils towards an inflammatory stimulus is markedly impaired.

Syndecan-4 regulates subcellular localization of mTOR Complex2 and Akt activation in a PKCalpha-dependent manner in endothelial cells

Mammalian target of rapamycin (mTOR) activity is regulated by assembly of two functionally distinct complexes, mTORC1 and mTORC2. In syndecan-4 (S4) null endothelial cells, mTORC2 activity is reduced, resulting in decreased Akt activation, while mTORC1 activity is increased. Levels of rictor, mLST8, and mSin-1 are unchanged in total cell lysates but decreased in the rafts of S4(-/-) endothelial cells, as is the level of PKCalpha. Expression of myristoylated-PKCalpha in S4(-/-) cells restores rictor, mLST8, and mSin-1 presence in the rafts and rescues Akt phosphorylation. PKCalpha knockdown mimics the effect of S4 deletion on mTORC2 localization and Akt activation. Reduced mTORC2 activity in S4(-/-) endothelial cells results in decreased FoxO1/3a and eNOS phosphorylation, decreased endothelial cell size, and increased arterial blood pressure in S4(-/-) mice. Thus, S4-dependent targeting of PKCalpha to the plasma membrane is required for recruitment of mTORC2 components to the rafts and Akt activation.

Sox18 induces development of the lymphatic vasculature in mice

The lymphatic system plays a key role in tissue fluid regulation and tumour metastasis, and lymphatic defects underlie many pathological states including lymphoedema, lymphangiectasia, lymphangioma and lymphatic dysplasia. However, the origins of the lymphatic system in the embryo, and the mechanisms that direct growth of the network of lymphatic vessels, remain unclear. Lymphatic vessels are thought to arise from endothelial precursor cells budding from the cardinal vein under the influence of the lymphatic hallmark gene Prox1 (prospero homeobox 1; ref. 4). Defects in the transcription factor gene SOX18 (SRY (sex determining region Y) box 18) cause lymphatic dysfunction in the human syndrome hypotrichosis-lymphoedema-telangiectasia, suggesting that Sox18 may also play a role in lymphatic development or function. Here we use molecular, cellular and genetic assays in mice to show that Sox18 acts as a molecular switch to induce differentiation of lymphatic endothelial cells. Sox18 is expressed in a subset of cardinal vein cells that later co-express Prox1 and migrate to form lymphatic vessels. Sox18 directly activates Prox1 transcription by binding to its proximal promoter. Overexpression of Sox18 in blood vascular endothelial cells induces them to express Prox1 and other lymphatic endothelial markers, while Sox18-null embryos show a complete blockade of lymphatic endothelial cell differentiation from the cardinal vein. Our findings demonstrate a critical role for Sox18 in developmental lymphangiogenesis, and suggest new avenues to investigate for therapeutic management of human lymphangiopathies.

Characterizing endothelial cells derived from the murine embryonic stem cell line CCE

Embryonic stem cells (ESC) are defined by two main properties of self-renewal and their multipotency to differentiate into virtually all cell types of the body, including endothelial cells. ESCs have been widely regarded as an unlimited source of cells in regeneration medicine and also an ideal in vitro model to investigate complex developmental processes. Here, we report a simple and efficient in vitro model to derive a nearly pure population of endothelial cells from a murine ESC line. CCE ES cells are exposed to alpha-MEM medium containing 10% FBS for 4 days and then cultured in endothelial basal-2 medium containing vascular endothelial growth factor (VEGF), basic fibroblast growth factor (bFGF), insulin-like growth factor (IGF), epidermal growth factor (EGF), and 2% FBS for 42 days. The cells acquired a relatively uniform endothelial cell morphology and were able to propagate and expand in culture. When murine ES cell-derived endothelial cells (MESDECs) were cultured on Matrigel and incubated for 48 h, vessel-like tube structures consisting of CD31 (PECAM-1) or BS-1 immunoreactive cells were developed. Immunocytochemistry and RT-PCR analyses revealed that MESDECs express endothelial cell-specific marker proteins such as Flk-1, PECAM-1, Tie-1, and Tie-2, in which the expressions persist for long periods of time after differentiation. The cells were also capable of taking up acetylated low-density lipoprotein (LDL) in culture. Our data suggest that MESDECs could provide a suitable in vitro model to study molecular events involved in vascular development and open up a new therapeutic strategy in regeneration medicine of cardiovascular disorders.

Cell-autonomous requirement for beta1 integrin in endothelial cell adhesion, migration and survival during angiogenesis in mice

beta1 integrin (encoded by Itgb1) is established as a regulator of angiogenesis based upon the phenotypes of complete knockouts of beta1 heterodimer partners or ligands and upon antibody inhibition studies in mice. Its direct function in endothelial cells (ECs) in vivo has not been determined because Itgb1(-/-) embryos die before vascular development. Excision of Itgb1 from ECs and a subset of hematopoietic cells, using Tie2-Cre, resulted in abnormal vascular development by embryonic day (e) 8.5 and lethality by e10.5. Tie1-Cre mediated a more restricted excision of Itgb1 from ECs and hematopoietic cells and resulted in embryonic lethal vascular defects by e11.5. Capillaries of the yolk sacs were disorganized, and the endothelium of major blood vessels and of the heart was frequently discontinuous in mutant embryos. We also found similar vascular morphogenesis defects characterized by EC disorganization in embryonic explants and isolated ECs. Itgb1-null ECs were deficient in adhesion and migration in a ligand-specific fashion, with impaired responses to laminin and collagens, but not to fibronectin. Deletion of Itgb1 reduced EC survival, but did not affect proliferation. Our findings demonstrate that beta1 integrin is essential for EC adhesion, migration and survival during angiogenesis, and further validate that therapies targeting beta1 integrins may effectively impair neovascularization.

Development of a new method for isolation and long-term culture of organ-specific blood vascular and lymphatic endothelial cells of the mouse

Endothelial cells are indispensable components of the vascular system, and play pivotal roles during development and in health and disease. Their properties have been studied extensively by in vivo analysis of genetically modified mice. However, further analysis of the molecular and cellular phenotypes of endothelial cells and their heterogeneity at various developmental stages, in vascular beds and in various organs has often been hampered by difficulties in culturing mouse endothelial cells. In order to overcome these difficulties, we developed a new transgenic mouse line expressing the SV40 tsA58 large T antigen (tsA58T Ag) under the control of a binary expression system based on Cre/loxP recombination. tsA58T Ag-positive endothelial cells in primary cultures of a variety of organs proliferate continuously at 33 degrees C without undergoing cell senescence. The resulting cell population consists of blood vascular and lymphatic endothelial cells, which could be separated by immunosorting. Even when cultured for two months, the cells maintained endothelial cell properties, as assessed by expression of endothelium-specific markers and intracellular signaling through the vascular endothelial growth factor receptors VEGFR-2 and VEGFR-3, as well as their physiological characteristics. In addition, lymphatic vessel endothelial hyaluronan receptor-1 (Lyve-1) expression in liver sinusoidal endothelial cells in vivo was retained in vitro, suggesting that an organ-specific endothelial characteristic was maintained. These results show that our transgenic cell culture system is useful for culturing murine endothelial cells, and will provide an accessible method and applications for studying endothelial cell biology.

VE-cadherin is a critical endothelial regulator of TGF-beta signalling

VE-cadherin is an endothelial-specific transmembrane protein concentrated at cell-to-cell adherens junctions. Besides promoting cell adhesion and controlling vascular permeability, VE-cadherin transfers intracellular signals that contribute to vascular stabilization. However, the molecular mechanism by which VE-cadherin regulates vascular homoeostasis is still poorly understood. Here, we report that VE-cadherin expression and junctional clustering are required for optimal transforming growth factor-beta (TGF-beta) signalling in endothelial cells (ECs). TGF-beta antiproliferative and antimigratory responses are increased in the presence of VE-cadherin. ECs lacking VE-cadherin are less responsive to TGF-beta/ALK1- and TGF-beta/ALK5-induced Smad phosphorylation and target gene transcription. VE-cadherin coimmunoprecipitates with all the components of the TGF-beta receptor complex, TbetaRII, ALK1, ALK5 and endoglin. Clustered VE-cadherin recruits TbetaRII and may promote TGF-beta signalling by enhancing TbetaRII/TbetaRI assembly into an active receptor complex. Taken together, our data indicate that VE-cadherin is a positive and EC-specific regulator of TGF-beta signalling. This suggests that reduction or inactivation of VE-cadherin may contribute to progression of diseases where TGF-beta signalling is impaired.

Transcriptional profiling of CD31(+) cells isolated from murine embryonic stem cells

Identification of genes involved in endothelial differentiation is of great interest for the understanding of the cellular and molecular mechanisms involved in the development of new blood vessels. Mouse embryonic stem (mES) cells serve as a potential source of endothelial cells for transcriptomic analysis. We isolated endothelial cells from 8-days old embryoid bodies by immuno-magnetic separation using platelet endothelial cell adhesion molecule-1 (also known as CD31) expressed on both early and mature endothelial cells. CD31(+) cells exhibit endothelial-like behavior by being able to incorporate DiI-labeled acetylated low-density lipoprotein as well as form tubular structures on matrigel. Quantitative and semi-quantitative PCR analysis further demonstrated the increased expression of endothelial transcripts. To ascertain the specific transcriptomic identity of the CD31(+) cells, large-scale microarray analysis was carried out. Comparative bioinformatic analysis reveals an enrichment of the gene ontology categories angiogenesis, blood vessel morphogenesis, vasculogenesis and blood coagulation in the CD31(+) cell population. Based on the transcriptomic signatures of the CD31(+) cells, we conclude that this ES cell-derived population contains endothelial-like cells expressing a mesodermal marker BMP2 and possess an angiogenic potential. The transcriptomic characterization of CD31(+) cells enables an in vitro functional genomic model to identify genes required for angiogenesis.

Genetic modification of cells for transplantation

Progress in gene therapy has produced promising results that translate experimental research into clinical treatment. Gene modification has been extensively employed in cell transplantation. The main barrier is an effective gene delivery system. Several viral vectors were utilized in end-stage differentiated cells. Recently, successful applications were described with adenovirus-associated vectors. As an alternative, embryonic stem cell- and stem cell-like systems were established for generation of tissue-specified gene-modified cells. Owing to the feasibility for genetic manipulations and the self-renewing potency of these cells they can be used in a way enabling large-scale in vitro production. This approach offers the establishment of in vitro cell culture systems that will deliver sufficient amounts of highly purified, immunoautologous cells suitable for application in regenerative medicine. In this review, the current technology of gene delivery systems to cells is recapitulated and the latest developments for cell transplantation are discussed.

Angiomotin regulates endothelial cell migration during embryonic angiogenesis

The development of the embryonic vascular system into a highly ordered network requires precise control over the migration and branching of endothelial cells (ECs). We have previously identified angiomotin (Amot) as a receptor for the angiogenesis inhibitor angiostatin. Furthermore, DNA vaccination targeting Amot inhibits angiogenesis and tumor growth. However, little is known regarding the role of Amot in physiological angiogenesis. We therefore investigated the role of Amot in embryonic neovascularization during zebrafish and mouse embryogenesis. Here we report that knockdown of Amot in zebrafish reduced the number of filopodia of endothelial tip cells and severely impaired the migration of intersegmental vessels. We further show that 75% of Amot knockout mice die between embryonic day 11 (E11) and E11.5 and exhibit severe vascular insufficiency in the intersomitic region as well as dilated vessels in the brain. Furthermore, using ECs differentiated from embryonic stem (ES) cells, we demonstrate that Amot-deficient cells have intact response to vascular endothelial growth factor (VEGF) in regard to differentiation and proliferation. However, the chemotactic response to VEGF was abolished in Amot-deficient cells. We provide evidence that Amot is important for endothelial polarization during migration and that Amot controls Rac1 activity in endothelial and epithelial cells. Our data demonstrate a critical role for Amot during vascular patterning and endothelial polarization.

Cell isolation and expansion using Dynabeads

This chapter describes the use of Dynabeads for cell isolation and expansion. Dynabeads are uniform polystyrene spherical beads that have been made magnetisable and superparamagnetic, meaning they are only magnetic in a magnetic field. Due to this property, the beads can easily be resuspended when the magnetic field is removed. The invention of Dynabeads made, by Professor John Ugelstad, has revolutionized the separation of many biological materials. For example, the attachment of target-specific antibodies to the surface of the beads allows capture and isolation of intact cells directly from a complex suspension such as blood. This is all accomplished under the influence of a simple magnetic field without the need for column separation techniques or centrifugation. In general, magnetic beads coated with specific antibodies can be used either for isolation or depletion of various cell types. Positive or negative cell isolation can be performed depending on the nature of the starting sample, the cell surface markers and the downstream application in question. Positive cell isolation is the method of choice for unprocessed samples, such as whole blood, and for downstream molecular applications. Positive cell isolation can also be used for any downstream application after detachment and removal of the beads. Negative cell isolation is the method of choice when it is critical that cells of interest remain untouched, i.e., no antibodies have been bound to any cell surface markers on the cells of interest. Some cell populations can only be defined by multiple cell surface markers. Such populations of cells can be isolated by the combination of negative and positive cell isolation. By coupling Dynabeads with antibodies directed against cell surface activation molecules, the beads can be used both for isolation and expansion of the cells. Dynabeads are currently used in two major clinical applications: 1) In the Isolex 300i Magnetic Cell Selection System for CD34 Stem Cell Isolation--2) For ex vivo T cell isolation and expansion using Dynabeads ClinExVivo CD3/CD28 for clinical trials in novel adoptive immunotherapy.

Effective isolation and culture of endothelial cells in embryoid body differentiated from human embryonic stem cells

We describe the isolation of endothelial cells from the center region of the attached embryoid body (EB) by a two-step enzyme treatment. The isolated cells from the center and outgrowth region of the EB were characterized separately. As human embryonic stem (hES) cells differentiated in EB, they lost expression of the undifferentiated marker Oct-4, whereas expression levels of endothelial-specific markers were increased. Using RT-PCR, fluorescence-activated cell sorting (FACS) analysis, and immunofluorescence, we have shown that various endothelial cell markers, including platelet/endothelial cell adhesion molecule (PECAM), von Willebrand factor (vWF), Flk-1, and Tie2, were expressed on the attached EB. Compared with the outgrowth region of EB, the center region had a higher population of cells with these endothelial cell markers. Once isolated by the twostep enzyme treatment, cells from the center region continued to differentiate into endothelial cell lineage while expression level of endothelial cell markers in cells from the outgrowth region decreased in subcultures. This study has demonstrated that the isolation of EB by a two-step enzyme treatment is a useful technique to obtain endothelial cell marker-positive cells with high yield. Furthermore, a similar approach can be taken to identify the location and distribution of specific cell types in EB and thereby allow us to isolate and expand specific cell types.