Molecular regulation of vessel maturation

The maturation of nascent vasculature, formed by vasculogenesis or angiogenesis, requires recruitment of mural cells, generation of an extracellular matrix and specialization of the vessel wall for structural support and regulation of vessel function. In addition, the vascular network must be organized so that all the parenchymal cells receive adequate nutrients. All of these processes are orchestrated by physical forces as well as by a constellation of ligands and receptors whose spatio-temporal patterns of expression and concentration are tightly regulated. Inappropriate levels of these physical forces or molecules produce an abnormal vasculature--a hallmark of various pathologies. Normalization of the abnormal vasculature can facilitate drug delivery to tumors and formation of a mature vasculature can help realize the promise of therapeutic angiogenesis and tissue engineering.     

Fibrin mechanisms and functions in nervous system pathology

In brain physiology, cerebrovascular interactions regulate both, vascular functions, such as blood vessel branching and endothelial cell homeostasis, as well as neuronal functions, such as local synaptic activity and adult neurogenesis. In brain pathology, including stroke, HIV encephalitis, Alzheimer Disease, multiple sclerosis, bacterial meningitis, and glioblastomas, rupture of the vasculature allows the entry of blood proteins into the brain with subsequent edema formation and neuronal damage. Fibrin is a blood-derived protein that is not produced by cells of the nervous system, but accumulates only after disease associated with vasculature rupture. This review presents evidence from human disease and animal models that highlight the role of fibrin in nervous system pathology. Our review presents novel experimental data that extend the role of fibrin, from that of a blood-clotting protein in cerebrovascular pathologies, to a component of the perivascular extracellular matrix that regulates inflammatory and regenerative cellular responses in neurodegenerative diseases.     

Endothelial and steroidogenic cell migration are regulated by WNT4 in the developing mammalian gonad

The signalling molecule WNT4 has been associated with sex reversal phenotypes in mammals. Here we show that the role of WNT4 in gonad development is to pattern the sex-specific vasculature and to regulate steroidogenic cell recruitment. Vascular formation and steroid production in the mammalian gonad occur in a sex-specific manner. During testis development, endothelial cells migrate from the mesonephros into the gonad to form a coelomic blood vessel. Leydig cells differentiate and produce steroid hormones a day later. Neither of these events occurs in the XX gonad. We show that WNT4 represses mesonephric endothelial and steroidogenic cell migration in the XX gonad, preventing the formation of a male-specific coelomic blood vessel and the production of steroids. In the XY gonad, Wnt4 expression is downregulated after sex determination. Transgenic misexpression of Wnt4 in the embryonic testis did not inhibit coelomic vessel formation but vascular pattern was affected. Leydig cell differentiation was not affected in these transgenic animals and our data implies that Wnt4 does not regulate steroidogenic cell differentiation but represses the migration of steroidogenic adrenal precursors into the gonad. These studies provide a model for understanding how the same signalling molecule can act on two different cell types to coordinate sex development.     

Vasculogenesis and angiogenesis: Extracellular matrix remodeling in coronary collateral arteries and the ischemic heart

Heart failure secondary to ischemic cardiomyopathy is the primary cause of cardiovascular mortality. The promise of the collateral circulation lies in its potential to alter the course of the natural history of coronary heart disease. The collateral circulation of the heart is responsible for supplying blood and oxygen to the myocardium at ischemic risk following severe stenosis and reduced vasoelasticity function of a major coronary artery. In response to flow, stress, and pressure, collateral vessels are restructured and remodeled. Vascular remodeling by its very nature implies synthesis and degradation of extracellular matrix components in the vessel wall. Under normal physiological conditions proteinases that break down the specialized matrix are tightly regulated by antiproteinases. The balance between proteinase and antiproteinase influences is discoordinated during collateral development which leads to adaptive changes in the structure, function, and regulation of extracellular matrix components in the vessel wall. The role of extracellular matrix components in coronary collateral vessel formation in a canine model of chronic coronary artery occlusion has been demonstrated. The role of matrix proteinases and antiproteinases in the collateral vessel play a significant role in the underlying mechanisms of collateral development. This review presents new and significant information regarding the role of extracellular matrix proteinases and antiproteinases in vascular remodeling, function, and collateral development. Such information will have a significant impact on the understanding of the basic biology of the vascular extracellular matrix turnover, remodeling, and function as well as on elucidating potential avenues for pharmacological approaches designed to increase collateral formation and optimize myocardial blood flow in the treatment of ischemic heart disease. J. Cell. Biochem. 65:388–394. © 1997 Wiley-Liss, Inc.     

Involvement of vessels and PDGFB in muscle splitting during chick limb development

Muscle formation and vascular assembly during embryonic development are usually considered separately. In this paper, we investigate the relationship between the vasculature and muscles during limb bud development. We show that endothelial cells are detected in limb regions before muscle cells and can organize themselves in space in the absence of muscles. In chick limbs, endothelial cells are detected in the future zones of muscle cleavage, delineating the cleavage pattern of muscle masses. We therefore perturbed vascular assembly in chick limbs by overexpressing VEGFA and demonstrated that ectopic blood vessels inhibit muscle formation, while promoting connective tissue. Conversely, local inhibition of vessel formation using a soluble form of VEGFR1 leads to muscle fusion. The endogenous location of endothelial cells in the future muscle cleavage zones and the inverse correlation between blood vessels and muscle suggests that vessels are involved in the muscle splitting process. We also identify the secreted factor PDGFB (expressed in endothelial cells) as a putative molecular candidate mediating the muscle-inhibiting and connective tissue-promoting functions of blood vessels. Finally, we propose that PDGFB promotes the production of extracellular matrix and attracts connective tissue cells to the future splitting site, allowing separation of the muscle masses during the splitting process.     

Genetic determinants of hyaloid and retinal vasculature in zebrafish

Background

The retinal vasculature is a capillary network of blood vessels that nourishes the inner retina of most mammals. Developmental abnormalities or microvascular complications in the retinal vasculature result in severe human eye diseases that lead to blindness. To exploit the advantages of zebrafish for genetic, developmental and pharmacological studies of retinal vasculature, we characterised the intraocular vasculature in zebrafish.

Results

We show a detailed morphological and developmental analysis of the retinal blood supply in zebrafish. Similar to the transient hyaloid vasculature in mammalian embryos, vessels are first found attached to the zebrafish lens at 2.5 days post fertilisation. These vessels progressively lose contact with the lens and by 30 days post fertilisation adhere to the inner limiting membrane of the juvenile retina. Ultrastructure analysis shows these vessels to exhibit distinctive hallmarks of mammalian retinal vasculature. For example, smooth muscle actin-expressing pericytes are ensheathed by the basal lamina of the blood vessel, and vesicle vacuolar organelles (VVO), subcellular mediators of vessel-retinal nourishment, are present. Finally, we identify 9 genes with cell membrane, extracellular matrix and unknown identity that are necessary for zebrafish hyaloid and retinal vasculature development.

Conclusion

Zebrafish have a retinal blood supply with a characteristic developmental and adult morphology. Abnormalities of these intraocular vessels are easily observed, enabling application of genetic and chemical approaches in zebrafish to identify molecular regulators of hyaloid and retinal vasculature in development and disease.     

Mathematical Modelling of a Brain Tumour Initiation and Early Development: A Coupled Model of Glioblastoma Growth,Pre-Existing Vessel Co-Option,Angiogenesis and Blood Perfusion

We propose a coupled mathematical modelling system to investigate glioblastoma growth in response to dynamic changes in chemical and haemodynamic microenvironments caused by pre-existing vessel co-option, remodelling, collapse and angiogenesis. A typical tree-like architecture network with different orders for vessel diameter is designed to model pre-existing vasculature in host tissue. The chemical substances including oxygen, vascular endothelial growth factor, extra-cellular matrix and matrix degradation enzymes are calculated based on the haemodynamic environment which is obtained by coupled modelling of intravascular blood flow with interstitial fluid flow. The haemodynamic changes, including vessel diameter and permeability, are introduced to reflect a series of pathological characteristics of abnormal tumour vessels including vessel dilation, leakage, angiogenesis, regression and collapse. Migrating cells are included as a new phenotype to describe the migration behaviour of malignant tumour cells. The simulation focuses on the avascular phase of tumour development and stops at an early phase of angiogenesis. The model is able to demonstrate the main features of glioblastoma growth in this phase such as the formation of pseudopalisades, cell migration along the host vessels, the pre-existing vasculature co-option, angiogenesis and remodelling. The model also enables us to examine the influence of initial conditions and local environment on the early phase of glioblastoma growth.     

A novel <Emphasis Type="Italic">Flk1</Emphasis>-TVA transgenic mouse model for gene delivery to angiogenic vasculature

The genes that regulate the formation of blood vessels in adult tissues represent promising therapeutic targets because angiogenesis plays a role in many diseases, including cancer. We wished to develop a mouse model allowing characterization of gene function in adult angiogenic vasculature while minimizing effects on embryonic vasculature or adult quiescent vasculature. Here we describe a transgenic mouse model that allows expression of proteins in the endothelial cells of newly forming blood vessels in the adult using a selective retroviral gene delivery system. We generated transgenic mouse lines that express the TVA receptor for the RCAS avian-specific retrovirus from Flk1 gene regulatory elements that drive expression in proliferating endothelial cells. Several of these Flk1-TVA lines expressed TVA mRNA in the embryonic vasculature and TVA protein in new blood vessels growing into subcutaneous extracellular matrix implants in adult mice. In a Flk1-TVA line that was crossed with the MMTV-PyMT transgenic mammary tumor model, tumor endothelial cells also expressed the TVA protein. Furthermore, endothelial cells in extracellular matrix implants and the tumors of Flk1-TVA mice were susceptible to RCAS infection, as determined by expression of green fluorescent protein encoded by the virus. The Flk1-TVA mouse model in conjunction with the RCAS gene delivery system will be useful to study molecular mechanisms underlying adult forms of angiogenesis.     

Small molecule screen for compounds that affect vascular development in the zebrafish retina

Blood vessel formation in the vertebrate eye is a precisely regulated process. In the human retina, both an excess and a deficiency of blood vessels may lead to a loss of vision. To gain insight into the molecular basis of vessel formation in the vertebrate retina and to develop pharmacological means of manipulating this process in a living organism, we further characterized the embryonic zebrafish eye vasculature, and performed a small molecule screen for compounds that affect blood vessel morphogenesis. The screening of approximately 2000 compounds revealed four small molecules that at specific concentrations affect retinal vessel morphology but do not produce obvious changes in trunk vessels, or in the neuronal architecture of the retina. Of these, two induce a pronounced widening of vessel diameter without a substantial loss of vessel number, one compound produces a loss of retinal blood vessels accompanied by a mild increase of their diameter, and finally one other generates a severe loss of retinal vessels. This work demonstrates the utility of zebrafish as a screening tool for small molecules that affect eye vasculature and presents several compounds of potential therapeutic importance.     

Chondroitin sulfate proteoglycans from salmon nasal cartilage inhibit angiogenesis

Because cartilage lacks nerves, blood vessels, and lymphatic vessels, it is thought to contain factors that inhibit the growth and development of those tissues. Chondroitin sulfate proteoglycans (CSPGs) are a major extracellular component in cartilage. CSPGs contribute to joint flexibility and regulate extracellular signaling via their attached glycosaminoglycan, chondroitin sulfate (CS). CS and CSPG inhibit axonal regeneration; however, their role in blood vessel formation is largely unknown. To clarify the function of CSPG in blood vessel formation, we tested salmon nasal cartilage proteoglycan (PG), a member of the aggrecan family of CSPG, for endothelial capillary-like tube formation. Treatment with salmon PG inhibited endothelial cell adhesion and in vitro tube formation. The anti-angiogenic activity was derived from CS in the salmon PG but not the core protein. Salmon PG also reduced matrix metalloproteinase expression and inhibited angiogenesis in the chick chorioallantoic membrane. All of these data support an anti-angiogenic role for CSPG in cartilage.     

Galectin-8 interacts with podoplanin and modulates lymphatic endothelial cell functions

Podoplanin is a small, mucin-like membrane glycoprotein highly expressed by lymphatic but not by blood vascular endothelial cells. Although it was shown to be indispensable for the correct formation and function of the lymphatic vasculature, its precise molecular function has remained unknown. In the present study, we identified the mammalian lectin galectin-8 as a novel, glycosylation-dependent interaction partner of podoplanin. Galectin-8 is a tandem-repeat type galectin, which interacts with cell surface glycoproteins, including certain integrins, as well as with extracellular matrix molecules such as fibronectin. Here we show that, similar to podoplanin, galectin-8 is more highly expressed by lymphatic than by blood vascular endothelial cells, and that it promotes lymphatic endothelial cell adhesion as well as haptotactic migration when immobilized onto a surface, while inhibiting the formation of tube-like structures by lymphatic endothelial cells in a collagen matrix when incorporated into the matrix. Importantly, functions of blood vascular endothelial cells, which lack podoplanin expression, are not affected by galectin-8. These data suggest a role for galectin-8 and podoplanin in supporting the connection of the lymphatic endothelium to the surrounding extracellular matrix, most likely in cooperation with other glycoproteins on the surface of lymphatic endothelial cells.     

Extra-cellular matrix in vascular networks

The vascular network is a series of linked conduits of blood vessels composed of the endothelium, a monolayer of cells that adorn the vessel lumen and surrounding layer(s) of mesenchymal cells (vascular smooth muscle, pericytes and fibroblasts). In addition to providing structural support, the mesenchymal cells are essential for vessel contractility. The extracellular matrix is a major constituent of blood vessels and provides a framework in which these various cell types are attached and embedded. The composition and organization of vascular extracellular matrix is primarily controlled by the mesenchymal cells, and is also responsible for the mechanical properties of the vessel wall, forming complex networks of structural proteins which are highly regulated. The extracellular matrix also plays a central role in cellular adhesion, differentiation and proliferation. This review examines the cellular and extracellular matrix components of vessels, with specific emphasis on the regulation of collagen type I and implications in vascular disease.     

Lymphangiogenesis in post-natal tissue remodeling: lymphatic endothelial cell connection with its environment

The main physiological function of the lymphatic vasculature is to maintain tissue fluid homeostasis. Lymphangiogenesis or de novo lymphatic formation is closely associated with tissue inflammation in adults (i.e. wound healing, allograft rejection, tumor metastasis). Until recently, research on lymphangiogenesis focused mainly on growth factor/growth factor-receptor pathways governing this process. One of the lymphatic vessel features is the incomplete or absence of basement membrane. This close association of endothelial cells with the underlying interstitial matrix suggests that cell–matrix interactions play an important role in lymphangiogenesis and lymphatic functions. However, the exploration of interaction between extracellular matrix (ECM) components and lymphatic endothelial cells is in its infancy. Herein, we describe ECM–cell and cell–cell interactions on lymphatic system function and their modification occurring in pathologies including cancer metastasis.     

Mural Cell Associated VEGF Is Required for Organotypic Vessel Formation

Background

Blood vessels comprise endothelial cells, mural cells (pericytes/vascular smooth muscle cells) and basement membrane. During angiogenesis, mural cells are recruited to sprouting endothelial cells and define a stabilizing context, comprising cell-cell contacts, secreted growth factors and extracellular matrix components, that drives vessel maturation and resistance to anti-angiogenic therapeutics.

Methods and Findings

To better understand the basis for mural cell regulation of angiogenesis, we conducted high content imaging analysis on a microtiter plate format in vitro organotypic blood vessel system comprising primary human endothelial cells co-cultured with primary human mural cells. We show that endothelial cells co-cultured with mural cells undergo an extensive series of phenotypic changes reflective of several facets of blood vessel formation and maturation: Loss of cell proliferation, pathfinding-like cell migration, branching morphogenesis, basement membrane extracellular matrix protein deposition, lumen formation, anastamosis and development of a stabilized capillary-like network. This phenotypic sequence required endothelial-mural cell-cell contact, mural cell-derived VEGF and endothelial VEGFR2 signaling. Inhibiting formation of adherens junctions or basement membrane structures abrogated network formation. Notably, inhibition of mural cell VEGF expression could not be rescued by exogenous VEGF.

Conclusions

These results suggest a unique role for mural cell-associated VEGF in driving vessel formation and maturation.     

Haemodynamics-driven developmental pruning of brain vasculature in zebrafish

The brain blood vasculature consists of a highly ramified vessel network that is tailored to meet its physiological functions. How the brain vasculature is formed has long been fascinating biologists. Here we report that the developing vasculature in the zebrafish midbrain undergoes not only angiogenesis but also extensive vessel pruning, which is driven by changes in blood flow. This pruning process shapes the initial exuberant interconnected meshwork into a simplified architecture. Using in vivo long-term serial confocal imaging of the same zebrafish larvae during 1.5-7.5 d post-fertilization, we found that the early formed midbrain vasculature consisted of many vessel loops and higher order segments. Vessel pruning occurred preferentially at loop-forming segments via a process mainly involving lateral migration of endothelial cells (ECs) from pruned to unpruned segments rather than EC apoptosis, leading to gradual reduction in the vasculature complexity with development. Compared to unpruned ones, pruned segments exhibited a low and variable blood flow, which further decreased irreversibly prior to the onset of pruning. Local blockade of blood flow with micro-bead obstruction led to vessel pruning, whereas increasing blood flow by noradrenergic elevation of heartbeat impeded the pruning process. Furthermore, the occurrence of vessel pruning could be largely predicted by haemodynamics-based numerical simulation of vasculature refinement. Thus, changes of blood flow drive vessel pruning via lateral migration of ECs, leading to the simplification of the vasculature and possibly efficient routing of blood flow in the developing brain.     

Method for the decellularization of intact rat liver

We have developed a method for the decellularization of whole rat livers by perfusion with increasing concentrations of detergents. This procedure resulted in an intact, decellularized organ with an intact liver capsule. These decellularized organs were analyzed by immunohistochemistry, and retained an appropriate distribution of extracellular matrix components. The laminin basement membranes of the liver vasculature also remain intact. These acellular vessel remnants were strong enough to be cannulated, providing a convenient means for the delivery of cells to areas deep within the decellularized organ. Cannulation of the extrahepatic vessel remnants allow for media to be circulated through the decellularized organ. These decellularized livers provide a natural matrix for research in the fields of bio-artificial livers and liver engineering.     

Method for the decellularization of intact rat liver

We have developed a method for the decellularization of whole rat livers by perfusion with increasing concentrations of detergents. This procedure resulted in an intact, decellularized organ with an intact liver capsule. These decellularized organs were analyzed by immunohistochemistry, and retained an appropriate distribution of extracellular matrix components. The laminin basement membranes of the liver vasculature also remain intact. These acellular vessel remnants were strong enough to be cannulated; providing a convenient means for the delivery of cells to areas deep within the decellularized organ. Cannulation of the extrahepatic vessel remnants allow for media to be circulated through the decellularized organ. These decellularized livers provide a natural matrix for research in the fields of bio-artificial livers and liver engineering.