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1.  β1-integrin is essential for vasoregulation and smooth muscle survival in vivo 
Arteriosclerosis, thrombosis, and vascular biology  2013;33(10):10.1161/ATVBAHA.112.300648.
Objective
Integrins contribute to vascular morphogenesis through regulation of adhesion and assembly of the extracellular matrix. However the role of β1-integrin in the mature vascular wall is less clear.
Approach and Results
We sought to determine the function of β1-integrin in mature smooth muscle cells in vivo using a loss of function approach by crossing a tamoxifen-inducible sm22αCre line to a floxed β1-integrin transgenic line. Adult mice lacking smooth muscle β1-integrin survived only ten weeks post-induction. The deletion of β1-integrin resulted in profound loss of vasomotor control. Histological analysis revealed progressive fibrosis in arteries with associated apoptosis of smooth muscle cells that was not rescued by adventitial stem cells. Smooth muscle cell apoptosis was detected in arteries with dead cells replaced primarily by collagen. Despite the catastrophic effects on vascular smooth muscle, the deleted visceral smooth muscle remained viable with the exception of a short portion of the colon, indicating that vascular but not visceral smooth muscle is particularly sensitive to changes in β1-integrin.
Conclusions
This study reveals an essential function of β1-integrin in the maintenance of vasomotor control and highlights a critical role for β1-integrin in vascular, but not visceral, smooth muscle survival.
doi:10.1161/ATVBAHA.112.300648
PMCID: PMC3882010  PMID: 23887637
adhesion; apoptosis; extracellular matrix; vascular fibrosis; other vascular biology
2.  An essential requirement for β1 integrin in the assembly of extracellular matrix proteins within the vascular wall 
Developmental biology  2012;365(1):23-35.
β1 integrin has been shown to contribute to vascular smooth muscle cell differentiation, adhesion and mechanosensation in vitro. Here we showed that deletion of β1 integrin at the onset of smooth muscle differentiation resulted in interrupted aortic arch, aneurysms and failure to assemble extracellular matrix proteins. These defects result in lethality prior to birth. Our data indicates that β1 integrin is not required for the acquisition, but it is essential for the maintenance of the smooth muscle cell phenotype, as levels of critical smooth muscle proteins are gradually reduced in mutant mice. Furthermore, while deposition of extracellular matrix was not affected, its structure was disrupted. Interestingly, defects in extracellular matrix and vascular wall assembly, were restricted to the aortic arch and its branches, compromising the brachiocephalic and carotid arteries and to the exclusion of the descending aorta. Additional analysis of β1 integrin in the pharyngeal arch smooth muscle progenitors was performed using wnt1Cre. Neural crest cells deleted for β1 integrin were able to migrate to the pharyngeal arches and associate with endothelial lined arteries; but exhibited vascular remodeling defects and early lethality. This work demonstrates that β1 integrin is dispensable for migration and initiation of the smooth muscle differentiation program, however, it is essential for remodeling of the pharyngeal arch arteries and for the assembly of the vessel wall of their derivatives. It further establishes a critical role of β1 integrin in the protection against aneurysms that is particularly confined to the ascending aorta and its branches.
doi:10.1016/j.ydbio.2012.01.027
PMCID: PMC3590017  PMID: 22331032
neural crest cells; aortic arch; vascular development; vascular morphogenesis; vascular smooth muscle
3.  Notch Expression Patterns in the Retina: An Eye on Receptor-Ligand Distribution during Angiogenesis 
Gene expression patterns : GEP  2006;7(4):461-470.
The critical contribution of the Notch signaling pathway to vascular morphogenesis has been underscored by loss-of-function studies in mouse and zebrafish. Nonetheless, a comprehensive understanding as to how this signaling system influences the formation of blood vessels at the cellular and molecular level is far from reached. Here, we provide a detailed analysis of the distribution of active Notch1 in relation to its DSL (Delta, Serrate, Lag2) ligands, Jagged1, Delta-like1, and Delta-like4, during progressive stages of vascular morphogenesis and maturation. Important differences in the cellular distribution of Notch ligands were found. Jagged1 (Jag1) was detected in “stalk cells” of the leading vasculature and at arterial branch points, a site where Delta-like4 (Dll4) was clearly absent. Dll4 was the only ligand expressed in “tip cells” at the end of the growing vascular sprouts. It was also present in stalk cells, capillaries, arterial endothelium, and in mural cells of mature arteries in a homogenous manner. Delta-like1 (Dll1) was observed in both arteries and veins of the developing network, but was also excluded from mature arterial branch points. These findings support alternative and distinct roles for Notch ligands during the angiogenic process.
doi:10.1016/j.modgep.2006.11.002
PMCID: PMC3184647  PMID: 17161657
arteries; blood vessels; capillaries; delta-like1; delta-like4; endothelial; jagged1; vascular remodeling; vasculature; veins
4.  Fate tracing reveals the endothelial origin of hematopoietic stem cells 
Cell stem cell  2008;3(6):625-636.
Summary
Hematopoietic stem cells (HSCs) originate within the aorta-gonado-mesonephros (AGM) region of the midgestation embryo, but the cell type responsible for their emergence is unknown since critical hematopoietic factors are expressed in both the AGM endothelium and its underlying mesenchyme. Here we employ a temporally restricted genetic tracing strategy to selectively label the endothelium, and separately its underlying mesenchyme, during AGM development. Lineage tracing endothelium, via an inducible VE-cadherin Cre line, reveals that the endothelium is capable of HSC emergence. The endothelial progeny migrate to the fetal liver, and later to the bone marrow, are capable of expansion, self-renewal, and multi-lineage hematopoietic differentiation. HSC capacity is exclusively endothelial, as ex vivo analyses demonstrate lack of VE-cadherin Cre induction in circulating and fetal liver hematopoietic populations. Moreover, AGM mesenchyme, as selectively traced via a myocardin Cre line, is incapable of hematopoiesis. Our genetic tracing strategy therefore reveals an endothelial origin of HSCs.
doi:10.1016/j.stem.2008.09.018
PMCID: PMC2631552  PMID: 19041779
Hematopoietic stem cells; HSC; aorta-gonado-mesonephros; AGM; VE-cadherin; Cre-recombinase; hemogenic endothelium; hematopoiesis; lineage tracing; tamoxifen
5.  Processing of VEGF-A by matrix metalloproteinases regulates bioavailability and vascular patterning in tumors 
The Journal of Cell Biology  2005;169(4):681-691.
Vascular endothelial growth factor (VEGF) is a critical mediator of blood vessel formation during development and in pathological conditions. In this study, we demonstrate that VEGF bioavailability is regulated extracellularly by matrix metalloproteinases (MMPs) through intramolecular processing. Specifically, we show that a subset of MMPs can cleave matrix-bound isoforms of VEGF, releasing soluble fragments. We have mapped the region of MMP processing, have generated recombinant forms that mimic MMP-cleaved and MMP-resistant VEGF, and have explored their biological impact in tumors. Although all forms induced similar VEGF receptor 2 phosphorylation levels, the angiogenic outcomes were distinct. MMP-cleaved VEGF promoted the capillary dilation of existent vessels but mediated a marginal neovascular response within the tumor. In contrast, MMP-resistant VEGF supported extensive growth of thin vessels with multiple and frequent branch points. Our findings support the view that matrix-bound VEGF and nontethered VEGF provide different signaling outcomes. These findings reveal a novel aspect in the regulation of extracellular VEGF that holds significance for vascular patterning.
doi:10.1083/jcb.200409115
PMCID: PMC2171712  PMID: 15911882
6.  Progesterone Receptor in the Vascular Endothelium Triggers Physiological Uterine Permeability Pre-implantation 
Cell  2014;156(3):549-562.
Summary
Vascular permeability is frequently associated with inflammation and triggered by a cohort of secreted permeability factors such as VEGF. Here we show that the physiological vascular permeability that precedes implantation is directly controlled by progesterone receptor (PR) and is independent of VEGF. Both global and endothelial-specific deletion of PR block physiological vascular permeability in the uterus whereas misexpression of PR in the endothelium of other organs results in ectopic vascular leakage. Integration of an endothelial genome-wide transcriptional profile with ChIP-sequencing revealed that PR induces a NR4A1 (Nur77/TR3)-dependent transcriptional program that broadly regulates vascular permeability in response to progesterone. Silencing of NR4A1 blocks PR-mediated permeability responses indicating a direct link between PR and NR4A1. This program triggers concurrent suppression of several junctional proteins and leads to an effective, timely and venous-specific regulation of vascular barrier function that is critical to embryo implantation.
doi:10.1016/j.cell.2013.12.025
PMCID: PMC3985399  PMID: 24485460
7.  Hybrid Photopatterned Enzymatic Reaction (HyPER) for In situ Cell Manipulation 
The ability to design artificial extracellular matrices as cell instructive scaffolds has opened the door to technologies capable of studying cell fate in vitro and to guide tissue repair in vivo. One main component of the design of artificial extracellular matrices is the incorporation of biochemical cues to guide cell phenotype and multicellular organization. The extracellular matrix is composed of a heterogeneous mixture of proteins that present a variety of spatially discrete signals to residing cell populations. In contrast, most engineered ECMs do not mimic this heterogeneity. In recent years the use of photodeprotection has been used to achieve spatial immobilization of signals. However, these approaches have been limited mostly to small peptides. Here we combine photodeprotection with enzymatic reaction to achieve spatially controlled immobilization of active bioactive signals that range from small molecules to large proteins. A peptide substrate for transglutaminase factor XIII (FXIIIa) is caged with a photodeprotectable group, which is then immobilized to the bulk of a cell compatible hydrogel. With the use of focused light the substrate can be deprotected and used to immobilize patterned bioactive signals. This approach offers an innovative strategy to immobilize delicate bioactive signals, such as growth factors, without loss of activity and enables In situ cell manipulation of encapsulated cells.
doi:10.1002/cbic.201300687
PMCID: PMC4024472  PMID: 24399784
Photopatterning; enzymatic reaction; hydrogels; ortho-nitrobenzyl; Factor XIIIa
8.  A Ligand-Independent VEGFR2 Signaling Pathway Limits Angiogenic Responses in Diabetes 
Science signaling  2014;7(307):ra1.
Although vascular complications are a hallmark of diabetes, the molecular mechanisms that underlie endothelial dysfunction are unclear. We showed that reactive oxygen species generated from hyperglycemia promoted ligand-independent phosphorylation of vascular endothelial growth factor receptor 2 (VEGFR2). This VEGFR2 signaling occurred within the Golgi compartment and resulted in progressively decreased availability of VEGFR2 at the cell surface. Consequently, the responses of endothelial cells to exogenous VEGF in a mouse model of diabetes were impaired because of a specific deficiency of VEGFR2 at the cell surface, despite a lack of change in transcript abundance. Hyperglycemia-induced phosphorylation of VEGFR2 did not require intrinsic receptor kinase activity, and was instead mediated by Src family kinases. The reduced cell surface abundance of VEGFR2 in diabetic mice was reversed by treatment with the antioxidant N-acetyl-L-cysteine, suggesting a causative role for oxidative stress. These findings uncover a mode of ligand-independent VEGFR2 signaling that can progressively lead to continuously muted responses to exogenous VEGF and limit angiogenic events.
doi:10.1126/scisignal.2004235
PMCID: PMC4030697  PMID: 24399295
9.  Selective suppression of endothelial cytokine production by progesterone receptor 
Vascular pharmacology  2013;59(0):10.1016/j.vph.2013.06.001.
Steroid hormones are well-recognized suppressors of the inflammatory response, however, their cell- and tissue-specific effects in the regulation of inflammation are far less understood, particularly for the sex-related steroids. To determine the contribution of progesterone in the endothelium, we have characterized and validated an in vitro culture system in which human umbilical vein endothelial cells constitutively express human progesterone receptor (PR). Using next generation RNA-sequencing, we identified a selective group of cytokines that are suppressed by progesterone both under physiological conditions and during pathological activation by lipopolysaccharide. In particular, IL-6, IL-8, CXCL2/3, and CXCL1 were found to be direct targets of PR, as determined by ChIP-sequencing. Regulation of these cytokines by progesterone was also confirmed by bead-based multiplex cytokine assays and quantitative PCR. These findings provide a novel role for PR in the direct regulation of cytokine levels secreted by the endothelium. They also suggest that progesterone-PR signaling in the endothelium directly impacts leukocyte trafficking in PR-expressing tissues.
doi:10.1016/j.vph.2013.06.001
PMCID: PMC3819224  PMID: 23747964
Steroid hormone; Immune cell; Reproduction; Inflammation
10.  Local acting Sticky-trap inhibits vascular endothelial growth factor dependent pathological angiogenesis in the eye 
EMBO Molecular Medicine  2014;6(5):604-623.
Current therapeutic antiangiogenic biologics used for the treatment of pathological ocular angiogenesis could have serious side effects due to their interference with normal blood vessel physiology. Here, we report the generation of novel antivascular endothelial growth factor-A (VEGF) biologics, termed VEGF “Sticky-traps,” with unique properties that allow for local inhibition of angiogenesis without detectable systemic side effects. Using genetic and pharmacological approaches, we demonstrated that Sticky-traps could locally inhibit angiogenesis to at least the same extent as the original VEGF-trap that also gains whole-body access. Sticky-traps did not cause systemic effects, as shown by uncompromised wound healing and normal tracheal vessel density. Moreover, if injected intravitreally, recombinant Sticky-trap remained localized to various regions of the eye, such as the inner-limiting membrane and ciliary body, for prolonged time periods, without gaining access either to the photoreceptors/choriocapillaris area or the circulation. These unique pharmacological characteristics of Sticky-trap could allow for safe treatment of pathological angiogenesis in patients with diabetic retinopathy and retinopathy of pre-maturity.
doi:10.1002/emmm.201303708
PMCID: PMC4023884  PMID: 24705878
angiogenesis; diabetic retinopathy; retinopathy of prematurity; Sticky-trap; VEGF
11.  Testosterone Levels Influence Mouse Fetal Leydig Cell Progenitors Through Notch Signaling1 
Biology of Reproduction  2013;88(4):91.
ABSTRACT
Leydig cells are the steroidogenic lineage of the mammalian testis that produces testosterone, a key hormone required throughout male fetal and adult life for virilization and spermatogenesis. Both fetal and adult Leydig cells arise from a progenitor population in the testis interstitium but are thought to be lineage-independent of one another. Genetic evidence indicates that Notch signaling is required during fetal life to maintain a balance between differentiated Leydig cells and their progenitors, but the elusive progenitor cell type and ligands involved have not been identified. In this study, we show that the Notch pathway signals through the ligand JAG1 in perivascular interstitial cells during fetal life. In the early postnatal testis, we show that circulating levels of testosterone directly affect Notch signaling, implicating a feedback role for systemic circulating factors in the regulation of progenitor cells. Between Postnatal Days 3 and 21, as fetal Leydig cells disappear from the testis and are replaced by adult Leydig cells, the perivascular population of interstitial cells active for Notch signaling declines, consistent with distinct regulation of adult Leydig progenitors.
Jagged 1-dependent Notch signaling in interstitial cells acts in conjunction with testosterone to regulate fetal Leydig cell differentiation and the transition from fetal to adult Leydig cells.
doi:10.1095/biolreprod.112.106138
PMCID: PMC4013875  PMID: 23467742
interstitium; JAG1; Leydig cell; Notch; perivascular; testis; testosterone; TNR-GFP
12.  ApoB-containing lipoproteins regulate angiogenesis by modulating expression of VEGF receptor 1 
Nature medicine  2012;18(6):967-973.
Despite the clear major contribution of hyperlipidemia to the prevalence of cardiovascular disease in the developed world, the direct effects of lipoproteins on endothelial cells have remained obscure and are under debate. Here we report a previously uncharacterized mechanism of vessel growth modulation by lipoprotein availability. Using a genetic screen for vascular defects in zebrafish, we initially identified a mutation, stalactite (stl), in the gene encoding microsomal triglyceride transfer protein (mtp), which is involved in the biosynthesis of apolipoprotein B (ApoB)-containing lipoproteins. By manipulating lipoprotein concentrations in zebrafish, we found that ApoB negatively regulates angiogenesis and that it is the ApoB protein particle, rather than lipid moieties within ApoB-containing lipoproteins, that is primarily responsible for this effect. Mechanistically, we identified downregulation of vascular endothelial growth factor receptor 1 (VEGFR1), which acts as a decoy receptor for VEGF, as a key mediator of the endothelial response to lipoproteins, and we observed VEGFR1 downregulation in hyperlipidemic mice. These findings may open new avenues for the treatment of lipoprotein-related vascular disorders.
doi:10.1038/nm.2759
PMCID: PMC3959651  PMID: 22581286
13.  Cellular and molecular regulation of vascular permeability 
Thrombosis and haemostasis  2013;109(3):407-415.
Summary
Vascular permeability is a highly coordinated process that integrates vesicular trafficking, complex junctional rearrangements, and refined cytoskeletal dynamics. In response to the extracellular environment, these three cellular activities have been previously assumed to work in parallel to regulate the passage of solutes between the blood and tissues. New developments in the area of vascular permeability, however have highlighted the interdependence between trans- and para-cellular pathways, the cross-communication between adherens and tight junctions, and the instructional role of pericytes on endothelial expression of barrier-related genes. Additionally, significant effort has been placed in understanding the molecular underpinings that contribute to barrier restoration following acute permeability events and in clarifying the importance of context-dependent signaling initiated by permeability mediators. Finally, recent findings have uncovered an unpredicted role for transcription factors in the coordination of vascular permeability and clarified how junctional complexes can transmit signals to the nucleus to control barrier function. The goal of this review is to provide a concise and updated view of vascular permeability, discuss the most recent advances in molecular and cellular regulation, and introduce integrated information on the central mechanisms involved in trans-endothelial transport.
doi:10.1160/TH12-09-0678
PMCID: PMC3786592  PMID: 23389236
Adhesion molecules; endothelial cells; endothelial barrier; permeability; capillaries
14.  Real-Time Maps of Fluid Flow Fields in Porous Biomaterials 
Biomaterials  2012;34(8):1980-1986.
Mechanical forces such as fluid shear have been shown to enhance cell growth and differentiation, but knowledge of their mechanistic effect on cells is limited because the local flow patterns and associated metrics are not precisely known. Here we present real-time, noninvasive measures of local hydrodynamics in 3D biomaterials based on nuclear magnetic resonance. Microflow maps were further used to derive pressure, shear and fluid permeability fields. Finally, remodeling of collagen gels in response to precise fluid flow parameters was correlated with structural changes. It is anticipated that accurate flow maps within 3D matrices will be a critical step towards understanding cell behavior in response to controlled flow dynamics.
doi:10.1016/j.biomaterials.2012.11.030
PMCID: PMC3714210  PMID: 23245922
flow; NMR; 3D scaffold; hydrogel; fluid permeability
15.  Sox17 is indispensable for acquisition and maintenance of arterial identity 
Nature Communications  2013;4:2609.
The functional diversity of the arterial and venous endothelia is regulated through a complex system of signalling pathways and downstream transcription factors. Here we report that the transcription factor Sox17, which is known as a regulator of endoderm and hemopoietic differentiation, is selectively expressed in arteries, and not in veins, in the mouse embryo and in mouse postnatal retina and adult. Endothelial cell-specific inactivation of Sox17 in the mouse embryo is accompanied by a lack of arterial differentiation and vascular remodelling that results in embryo death in utero. In mouse postnatal retina, abrogation of Sox17 expression in endothelial cells leads to strong vascular hypersprouting, loss of arterial identity and large arteriovenous malformations. Mechanistically, Sox17 acts upstream of the Notch system and downstream of the canonical Wnt system. These data introduce Sox17 as a component of the complex signalling network that orchestrates arterial/venous specification.
The transcription factor Sox17 is required for the development of the vasculature in vertebrates. Here Corada et al. show that Sox17 acts downstream of Wnt signalling and upstream of Notch signalling in the regulation of artery and vein differentiation in mice.
doi:10.1038/ncomms3609
PMCID: PMC3826640  PMID: 24153254
16.  Cleavage of syndecan-4 by ADAMTS1 provokes defects in adhesion 
The international journal of biochemistry & cell biology  2008;41(4):10.1016/j.biocel.2008.08.014.
Syndecan-4 is a membrane-bound heparan sulfate proteoglycan that participates in cell–cell and cell–matrix interactions and modulates adhesion and migration of many cell types. Through its extracellular domain, syndecan-4 cooperates with adhesion molecules and binds matrix components relevant for cell migration. Importantly, syndecan-4 is a substrate of extracellular proteases, however the biological significance of this cleavage has not been elucidated. Here, we show that the secreted metalloprotease ADAMTS1, involved in angiogenesis and inflammatory processes, cleaves the ectodomain of syndecan-4. We further showed that this cleavage results in altered distribution of cytoskeleton components, functional loss of adhesion, and gain of migratory capacities. Using syndecan-4 null cells, we observed that ADAMTS1 proteolytic action mimics the outcome of genetic deletion of this proteoglycan with regards to focal adhesion. Our findings suggest that the shedding of syndecan-4 by ADAMTS1 disrupts cell adhesion and promotes cell migration.
doi:10.1016/j.biocel.2008.08.014
PMCID: PMC3807939  PMID: 18775505
Extracellular proteolysis; Proteoglycan; Metalloprotease; Endothelial cell
17.  Recent advances in vascular development 
Current opinion in hematology  2012;19(3):176-183.
Purpose of Review
This review offers a concise summary of the most recent experimental advances in vascular development using the mouse as a model organism.
Recent Findings
Recent mouse studies have revealed a spread of phenotypic diversity between endothelia of distinct developmental origins and organs. For example, expression of unique transcription factors distinguishes hemogenic from non-hemogenic endothelium within the same vessel. Vasculature of the brain is particularly susceptible to endothelial malformations due to combinatorial germline and somatic mutations; surprisingly these mutations can afflict the endothelium by either cell autonomous or paracrine effects. Mutant mice have been used to understand how multiple signaling pathways integrate and refine cellular responses. In particular, we learned how VEGFR3 regulates Notch signaling and EphrinB2 coordinates VEGFR2 responses. The regulation of Prox1 by miR181 highlighted the contribution of microRNAs in the induction of lymphatic endothelium. Information gained on heterotypic interactions has further clarified the influence of blood vessels on the morphogenesis of parenchyma and contributed to our understanding of organ-specific endothelial differentiation. Finally, mouse models have uncovered endothelial cell polarity as a keystone for successful vascular lumenization.
Summary
Our understanding of the process of vascular development has gained significant refinement in the last two years and has clarified the origin of several disorders rooted in development.
doi:10.1097/MOH.0b013e3283523e90
PMCID: PMC3666551  PMID: 22406825
Alagille syndrome; cerebral cavernous malformations; hemogenic endothelium; macrophage; pericyte; vascular development; vascular lumen
18.  VEGF internalization is not required for VEGFR-2 phosphorylation in bioengineered surfaces with covalently linked VEGF 
Vascular endothelial growth factor (VEGF) is known to activate proliferation, migration, and survival pathways in endothelial cells through phosphorylation of VEGF receptor-2 (VEGFR-2). VEGF has been incorporated into biomaterials through encapsulation, electrostatic sequestration, and covalent attachment, but the effect of these immobilization strategies on VEGF signaling has not been thoroughly investigated. Further, although growth factor internalization along with the receptor generally occurs in a physiological setting, whether this internalization is needed for receptor phosphorylation is not entirely clear. Here we show that VEGF covalently bound through a modified heparin molecule elicits an extended response of pVEGFR-2 in human umbilical vein endothelial cells (HUVECs) and that the covalent linkage reduces internalization of the growth factor during receptor endocytosis. Optical tweezer measurements show that the rupture force required to disrupt the heparin-VEGF-VEGFR-2 interaction increases from 3–8 pN to 6–12 pN when a covalent bond is introduced between VEGF and heparin. Importantly, by covalently binding VEGF to a heparin substrate, the stability (half-life) of VEGF is extended over three-fold. Here, mathematical models support the biological conclusions, further suggesting that VEGF internalization is significantly reduced when covalently bound, and indicating that VEGF is available for repeated phosphorylation events.
doi:10.1039/c1ib00037c
PMCID: PMC3621282  PMID: 21826315
19.  Trophoblasts regulate the placental hematopoietic niche through PDGF-B signaling 
Developmental Cell  2012;22(3):651-659.
Summary
The placenta is a hematopoietic organ that supports hematopoietic stem/progenitor cell (HSPC) generation and expansion without promoting differentiation. We identified PDGF-B signaling in trophoblasts as a key component of the unique placental hematopoietic microenvironment that protects HSPCs from premature differentiation. Loss of PDGF-B or its receptor, PDGFRβ, induced definitive erythropoiesis in placental labyrinth vasculature. This was evidenced by accumulation of CFU-Es and actively proliferating definitive erythroblasts that clustered around central macrophages, highly reminiscent of erythropoiesis in the fetal liver. Ectopic erythropoiesis was not due to a requirement of PDGF-B signaling in hematopoietic cells but rather in placental trophoblasts, which upregulated Epo in the absence of PDGF-B signaling. Furthermore, overexpression of hEPO specifically in the trophoblasts in vivo was sufficient to convert the placenta into an erythropoietic organ. These data provide genetic evidence of a signaling pathway that is required to restrict erythroid differentiation to specific anatomical niches during development.
doi:10.1016/j.devcel.2011.12.022
PMCID: PMC3395466  PMID: 22387002
20.  Fibulin-1 is required during cardiac ventricular morphogenesis for versican cleavage, suppression of ErbB2 and Erk1/2 activation and to attenuate trabecular cardiomyocyte proliferation 
Background
Trabeculation is an integral component of cardiac ventricular morphogenesis and is dependent on the matrix metalloproteinase, ADAMTS1. A substrate of ADAMTS1 is the proteoglycan versican which is expressed in the developing ventricle and which has been implicated in trabeculation. Fibulin-1 is a versican and ADAMTS1-binding extracellular matrix protein required for ventricular morphogenesis. Here we investigated the involvement of fibulin-1 in ADAMTS1-mediated cleavage of versican in vitro, and the involvement of fibulin-1 in versican cleavage in ventricular morphogenesis.
Results
We show that fibulin-1 is a cofactor for ADAMTS1-dependent in vitro cleavage of versican V1, yielding a 70-kDa amino-terminal fragment. Furthermore, fibulin-1-deficiency in mice was found to cause a significant reduction (>90%) in ventricular levels of the 70-kDa versican V1 cleavage product and a 2-fold increase in trabecular cardiomyocyte proliferation. Decreased versican V1 cleavage and augmented trabecular cardiomyocyte proliferation in fibulin-1 null hearts is accompanied by increased ventricular activation of ErbB2 and Erk1/2. By contrast, versican deficiency was found to lead to decreased cardiomyocyte proliferation and reduced ventricular trabeculation.
Conclusion
We conclude that fibulin-1 regulates versican-dependent events in ventricular morphogenesis by promoting ADAMTS1 cleavage of versican leading to suppression of trabecular cardiomyocyte proliferation mediated by the ErbB2-Map kinase pathway.
doi:10.1002/dvdy.23716
PMCID: PMC3489172  PMID: 22183742
trabeculation; ventricular noncompaction; versican; fibulin-1; ADAMTS-1; ErbB2; Brg1; Erk1/2; DPEAAE; cardiomyocyte; knockout
21.  Selective Decline of Synaptic Protein Levels in the Frontal Cortex of Female Mice Deficient in the Extracellular Metalloproteinase ADAMTS1 
PLoS ONE  2012;7(10):e47226.
The chondroitin sulfate-bearing proteoglycans, also known as lecticans, are a major component of the extracellular matrix (ECM) in the central nervous system and regulate neural plasticity. Growing evidence indicates that endogenous, extracellular metalloproteinases that cleave lecticans mediate neural plasticity by altering the structure of ECM aggregates. The bulk of this in vivo data examined the matrix metalloproteinases, but another metalloproteinase family that cleaves lecticans, a disintegrin and metalloproteinase with thrombospondin motifs (ADAMTS), modulates structural plasticity in vitro, although few in vivo studies have tested this concept. Thus, the purpose of this study was to examine the neurological phenotype of a mouse deficient in ADAMTS1. Adamts1 mRNA was absent in the ADAMTS1 null mouse frontal cortex, but there was no change in the abundance or proteolytic processing of the prominent lecticans brevican and versican V2. However, there was a marked increase in the perinatal lectican neurocan in juvenile ADAMTS1 null female frontal cortex. More prominently, there were declines in synaptic protein levels in the ADAMTS1 null female, but not male, frontal cortex beginning at postnatal day 28. These synaptic marker declines did not affect learning or memory in the adult female ADAMTS1 null mice when tested with the radial-arm water maze. These results indicate that in vivo Adamts1 knockout leads to sexual dimorphism in frontal cortex synaptic protein levels. Since changes in lectican abundance and proteolytic processing did not accompany the synaptic protein declines, ADAMTS1 may play a nonproteolytic role in regulating neural plasticity.
doi:10.1371/journal.pone.0047226
PMCID: PMC3469530  PMID: 23071766
22.  LUMENating Blood Vessels 
Developmental cell  2011;20(4):412-414.
The acquisition of a lumen is an essential step in vascular morphogenesis. In this issue of Developmental Cell, Xu et al. (2011) show that the small GTPase Rasip is a critical regulator of cytoskeleton dynamics and cell adhesion, which together drive the emergence of vascular lumens.
doi:10.1016/j.devcel.2011.03.020
PMCID: PMC3099250  PMID: 21497753
23.  CCN2/Connective Tissue Growth Factor Is Essential for Pericyte Adhesion and Endothelial Basement Membrane Formation during Angiogenesis 
PLoS ONE  2012;7(2):e30562.
CCN2/Connective Tissue Growth Factor (CTGF) is a matricellular protein that regulates cell adhesion, migration, and survival. CCN2 is best known for its ability to promote fibrosis by mediating the ability of transforming growth factor β (TGFβ) to induce excess extracellular matrix production. In addition to its role in pathological processes, CCN2 is required for chondrogenesis. CCN2 is also highly expressed during development in endothelial cells, suggesting a role in angiogenesis. The potential role of CCN2 in angiogenesis is unclear, however, as both pro- and anti-angiogenic effects have been reported. Here, through analysis of Ccn2-deficient mice, we show that CCN2 is required for stable association and retention of pericytes by endothelial cells. PDGF signaling and the establishment of the endothelial basement membrane are required for pericytes recruitment and retention. CCN2 induced PDGF-B expression in endothelial cells, and potentiated PDGF-B-mediated Akt signaling in mural (vascular smooth muscle/pericyte) cells. In addition, CCN2 induced the production of endothelial basement membrane components in vitro, and was required for their expression in vivo. Overall, these results highlight CCN2 as an essential mediator of vascular remodeling by regulating endothelial-pericyte interactions. Although most studies of CCN2 function have focused on effects of CCN2 overexpression on the interstitial extracellular matrix, the results presented here show that CCN2 is required for the normal production of vascular basement membranes.
doi:10.1371/journal.pone.0030562
PMCID: PMC3282727  PMID: 22363445
24.  Molecular Mechanisms of Tumor Angiogenesis 
Genes & Cancer  2011;2(12):1085-1096.
Tumors have been recently recognized as aberrant organs composed of a complex mixture of highly interactive cells that in addition to the cancer cell include stroma (fibroblasts, adipocytes, and myofibroblasts), inflammatory (innate and adaptive immune cells), and vascular cells (endothelial and mural cells). While initially cancer cells co-opt tissue-resident vessels, the tumor eventually recruits its own vascular supply. The process of tumor neovascularization proceeds through the combined output of inductive signals from the entire cellular constituency of the tumor. During the last two decades, the identification and mechanistic outcome of signaling pathways that mediate tumor angiogenesis have been elucidated. Interestingly, many of the genes and signaling pathways activated in tumor angiogenesis are identical to those operational during developmental vascular growth, but they lack feedback regulatory control and are highly affected by inflammatory cells and hypoxia. Consequently, tumor vessels are abnormal, fragile, and hyperpermeable. The lack of hierarchy and inconsistent investment of mural cells dampen the ability of the vessels to effectively perfuse the tumor, and the resulting hypoxia installs a vicious cycle that continuously perpetuates a state of vascular inefficiency. Pharmacological targeting of blood vessels, mainly through the VEGF signaling pathway, has proven effective in normalizing tumor vessels. This normalization improves perfusion and distribution of chemotherapeutic drugs with resulting tumor suppression and moderate increase in overall survival. However, resistance to antiangiogenic therapy occurs frequently and constitutes a critical barrier in the inhibition of tumor growth. A concrete understanding of the chief signaling pathways that stimulate vascular growth in tumors and their cross-talk will continue to be essential to further refine and effectively abort the angiogenic response in cancer.
doi:10.1177/1947601911432334
PMCID: PMC3411131  PMID: 22866200
endothelial cells; neovascularization; tumor angiogenesis; Notch; Delta; Jagged; Ephrin; Eph; angiopoietins; Tie; VEGF; VEGFR; Robo; Slit
25.  β1 integrin establishes endothelial cell polarity and arteriolar lumen formation via a Par3-dependent mechanism 
Developmental cell  2010;18(1):39-51.
Summary
Maintenance of single layered endothelium, squamous endothelial cell shape, and formation of a patent vascular lumen all require defined endothelial cell polarity. Loss of β1 integrin (Itgb1) in nascent endothelium leads to disruption of arterial endothelial cell polarity and lumen formation. The loss of polarity is manifested as cuboidal shaped endothelial cells, dysregulated levels and mis-localization of normally polarized cell-cell adhesion molecules, as well as decreased expression of the polarity gene Par3 (pard3). β1 integrin and Par3 are both localized to the endothelial layer, with preferential expression of Par3 in arterial endothelium. Luminal occlusion is also exclusively noted in arteries, and is partially rescued by replacement of Par3 protein in β1 deficient vessels. Combined, our findings demonstrate that β1 integrin functions upstream of Par3 as part of a molecular cascade required for endothelial cell polarity and lumen formation.
doi:10.1016/j.devcel.2009.12.006
PMCID: PMC3178410  PMID: 20152176
β1 integrin; Itgb1; endothelium; VE-cadherin; vasculature; lumen formation; polarity; Par3; pard3; Cre; lox

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