Regulation of capillary formation by laminin and other components of the extracellular matrix

Notch signaling pathway induces expression of type IV collagen in angiogenesis

Mural cell adhesion is important for the localization of basement membrane components during angiogenesis, and cell-cell interactions are thought to be critical for basement membrane formation. Type IV collagen, a component of the basement membrane, and non-triple helical type IV collagen α1 chain (NTH α1(IV)) co-localize in the basement membrane of neovascular vessels. However, it remains unclear how type IV collagen and NTH α1(IV) are produced around the basement membrane. In the present study, we developed a de novo angiogenesis model using human umbilical vein endothelial cell spheroids and TIG-1 fibroblast cells and demonstrated that NTH α1(IV), probably with α1(IV) chain before forming triple helix molecule, was localized in the fibroblasts in contact with vascular endothelial cells. This localization was disrupted by DAPT, a Notch signaling inhibitor. DAPT treatment also reduced type IV collagen and NTH α1(IV) secretion in TIG-1 fibroblasts, along with diminished COL4A1 and COL4A2 gene expression. Downregulation of Notch3 in TIG-1 fibroblasts decreased the secretion of type IV collagen and NTH α1(IV). Taken together, these findings suggest that heterogeneous and homogeneous intercellular Notch signaling via Notch3 induces type IV collagen and NTH α1(IV) expression in fibroblasts and contributes to basement membrane formation in neovascular vessels.

Chromosome-level Dinobdella ferox genome provided a molecular model for its specific parasitism

BACKGROUND

Dinobdella ferox is the most frequently reported leech species parasitizing the mammalian nasal cavity. However, the molecular mechanism of this special parasitic behavior has remained largely unknown.

METHODS

PacBio long-read sequencing, next-generation sequencing (NGS), and Hi-C sequencing were employed in this study to generate a novel genome of D. ferox, which was annotated with strong certainty using bioinformatics methods. The phylogenetic and genomic alterations of D. ferox were then studied extensively alongside the genomes of other closely related species. The obligatory parasitism mechanism of D. ferox was investigated using RNA-seq and proteomics data.

RESULTS

PacBio long-read sequencing and NGS yielded an assembly of 228 Mb and contig N50 of 2.16 Mb. Along Hi-C sequencing, 96% of the sequences were anchored to nine linkage groups and a high-quality chromosome-level genome was generated. The completed genome included 19,242 protein-coding genes. For elucidating the molecular mechanism of nasal parasitism, transcriptome data were acquired from the digestive tract and front/rear ends of D. ferox. Examining secretory proteins in D. ferox saliva helped to identify intimate connections between these proteins and membrane proteins in nasal epithelial cells. These interacting proteins played important roles in extracellular matrix (ECM)-receptor interaction, tight junction, focal adhesion, and adherens junction. The interaction between D. ferox and mammalian nasal epithelial cells included three major steps of pattern recognition, mucin connection and breakdown, and repair of ECM. The remodeling of ECM between epithelial cells of the nasal mucosa and epithelial cells of D. ferox may produce a stable adhesion environment for parasitism.

CONCLUSIONS

Our study represents the first-ever attempt to propose a molecular model for specific parasitism. This molecular model may serve as a practical reference for parasitism models of other species and a theoretical foundation for a molecular process of parasitism.

Laminin as a Biomarker of Blood-Brain Barrier Disruption under Neuroinflammation: A Systematic Review

Laminin, a non-collagenous glycoprotein present in the brain extracellular matrix, helps to maintain blood–brain barrier (BBB) integrity and regulation. Neuroinflammation can compromise laminin structure and function, increasing BBB permeability. The aim of this paper is to determine if neuroinflammation-induced laminin functional changes may serve as a potential biomarker of alterations in the BBB. The 38 publications included evaluated neuroinflammation, BBB disruption, and laminin, and were assessed for quality and risk of bias (protocol registered in PROSPERO; CRD42020212547). We found that laminin may be a good indicator of BBB overall structural integrity, although changes in expression are dependent on the pathologic or experimental model used. In ischemic stroke, permanent vascular damage correlates with increased laminin expression (β and γ subunits), while transient damage correlates with reduced laminin expression (α subunits). Laminin was reduced in traumatic brain injury and cerebral hemorrhage studies but increased in multiple sclerosis and status epilepticus studies. Despite these observations, there is limited knowledge about the role played by different subunits or isoforms (such as 411 or 511) of laminin in maintaining structural architecture of the BBB under neuroinflammation. Further studies may clarify this aspect and the possibility of using laminin as a biomarker in different pathologies, which have alterations in BBB function in common.

Angiogenesis induction in breast cancer: A paracrine paradigm

Breast cancer is the most prevalent type of cancer among women globally. Angiogenesis contributes significantly to breast cancer progression and dissemination. Neovascularization is concurrent with the progression and growth of breast cancer. Breast cancer cells control angiogenesis by secreting pro-angiogenic factors like fibroblast growth factor, vascular endothelial growth factor, interleukin, transforming growth factor-β, platelet-derived growth factor and several others. These pro-angiogenic factors trigger neovascularization, and thereby lead to breast cancer development and metastasis. The hypoxia-inducible factor (HIF)-regulated angiogenesis cascade is a crucial underlying factor in breast cancer growth and metastasis. To that end, several efforts have been made to identify druggable targets within the HIF-angiogenesis components. However, escape pathways are a major hindrance for targeted therapies against angiogenesis. Thus, understanding the key factors that trigger breast cancer angiogenesis is critical in elucidating ways to inhibit breast cancer. The current review provides an overview of the key growth factors that trigger breast cancer angiogenesis.

Cellular Based Strategies for Microvascular Engineering

Vascularization is a major hurdle in complex tissue and organ engineering. Tissues greater than 200 μm in diameter cannot rely on simple diffusion to obtain nutrients and remove waste. Therefore, an integrated vascular network is required for clinical translation of engineered tissues. Microvessels have been described as <150 μm in diameter, but clinically they are defined as <1 mm. With new advances in super microsurgery, vessels less than 1 mm can be anastomosed to the recipient circulation. However, this technical advancement still relies on the creation of a stable engineered microcirculation that is amenable to surgical manipulation and is readily perfusable. Microvascular engineering lays on the crossroads of microfabrication, microfluidics, and tissue engineering strategies that utilize various cellular constituents. Early research focused on vascularization by co-culture and cellular interactions, with the addition of angiogenic growth factors to promote vascular growth. Since then, multiple strategies have been utilized taking advantage of innovations in additive manufacturing, biomaterials, and cell biology. However, the anatomy and dynamics of native blood vessels has not been consistently replicated. Inconsistent results can be partially attributed to cell sourcing which remains an enigma for microvascular engineering. Variations of endothelial cells, endothelial progenitor cells, and stem cells have all been used for microvascular network fabrication along with various mural cells. As each source offers advantages and disadvantages, there continues to be a lack of consensus. Furthermore, discord may be attributed to incomplete understanding about cell isolation and characterization without considering the microvascular architecture of the desired tissue/organ.

Chronic mild hypoxia increases expression of laminins 111 and 411 and the laminin receptor α6β1 integrin at the blood-brain barrier

The laminin family of glycoproteins are major constituents of the basal lamina of blood vessels, and play a fundamental role in promoting endothelial differentiation and blood-brain barrier (BBB) stability. Chronic mild hypoxia (CMH), in which mice are exposed to 8% O2 for two weeks, induces a strong vascular remodeling response in the central nervous system (CNS) that includes endothelial proliferation, angiogenesis, arteriogenesis as well as increased expression of tight junction proteins, suggestive of enhanced vascular integrity. As previous studies highlight an important role for laminin in promoting vascular differentiation and BBB stability, the goal of this study was to determine if CMH influences the expression of the laminins and their cell surface receptors in cerebral blood vessels. Our studies revealed that over a 14 day period of CMH, blood vessels in the brain showed strong upregulation of the specific laminin subunits α1 and α4, corresponding to increased expression of laminins 111 and 411 respectively, with no discernible changes in the expression levels of the α2 or α5 laminin subunits. This was accompanied by marked endothelial upregulation of the laminin receptor α6β1 integrin but no alterations in the other laminin receptors α1β1 integrin or dystroglycan. In light of the instructive role for laminins in promoting vascular differentiation and stability, these data suggest that upregulation of the laminin-α6β1 integrin axis is part of the molecular response triggered by mild hypoxia that leads to enhanced BBB stability.

Endothelial α6β4 integrin protects during experimental autoimmune encephalomyelitis-induced neuroinflammation by maintaining vascular integrity and tight junction protein expression

Background

Extracellular matrix (ECM) proteins play critical functions regulating vascular formation and function. Laminin is a major component of the vascular basal lamina, and transgenic mice deficient in astrocyte or pericyte laminin show defective blood-brain barrier (BBB) integrity, indicating an important instructive role for laminin in cerebral blood vessels. As previous work shows that in the normal brain, vascular expression of the laminin receptor α6β4 integrin is predominantly restricted to arterioles, but induced on all vessels during neuroinflammation, it is important to define the role of this integrin in the maintenance of BBB integrity.

Methods

α6β4 integrin expression was analyzed using dual immunofluorescence (dual-IF) of brain sections taken from the mouse model of multiple sclerosis, experimental autoimmune encephalomyelitis (EAE). To investigate the role of endothelial α6β4 integrin, transgenic mice lacking β4 integrin in endothelial cells (β4-EC-KO) and wild-type (WT) littermates were subject to EAE, and clinical score and various neuropathological parameters were examined by immunofluorescence. In addition, β4 integrin null brain endothelial cells (BECs) were examined in culture for expression of tight junction proteins using immunocytochemistry and flow cytometry.

Results

Cerebrovascular expression of β4 integrin was markedly upregulated during EAE progression, such that by the acute stage of EAE (day 21), the vast majority of blood vessels expressed β4 integrin. In the EAE model, while the β4-EC-KO mice showed the same time of disease onset as the WT littermates, they developed significantly worse clinical disease over time, resulting in increased clinical score at the peak of disease and maintained elevated thereafter. Consistent with this, the β4-EC-KO mice showed enhanced levels of leukocyte infiltration and BBB breakdown and also displayed increased loss of the endothelial tight junction proteins claudin-5 and ZO-1. Under pro-inflammatory conditions, primary cultures of β4KO BECs also showed increased loss of claudin-5 and ZO-1 expression.

Conclusions

Taken together, our data suggest that α6β4 integrin upregulation is an inducible protective mechanism that stabilizes the BBB during neuroinflammatory conditions.

4-isothiocyanate-2, 2, 6, 6-tetramethyl piperidinooxyl inhibits angiogenesis by suppressing VEGFR2 and Tie2 phosphorylation

Reactive oxygen species (ROS) are involved in the signaling pathway and are triggered by angiogenic factors, including vascular endothelial growth factor and angiopoietins. 4-isothiocyanate-2, 2, 6, 6-tetramethyl piperidinooxyl (4-ISO-Tempo) is one of the nitroxides that exhibits antioxidant activity. However, the anti-angiogenic effect of 4-ISO-Tempo remains unknown. The aim of this study was to investigate the effect of 4-ISO-Tempo on tumor proliferation and angiogenesis as well as its underlying mechanisms. Our results revealed that 4-ISO-Tempo significantly inhibited the viability of neoplastic and endothelial cells. Furthermore, the effective concentration of 4-ISO-Tempo on human microvascular endothelial cell 1 (HMEC-1) was lower than that on human lung adenocarcinoma A549 and human colon cancer SW620 cells. This suggested that endothelial cells were more sensitive to 4-ISO-Tempo than tumor cells. Furthermore, we demonstrated that 4-ISO-Tempo also suppressed secretion of matrix metalloproteinase (MMP)-2 and MMP-9, and migration and tube formation of HMEC-1 cells. The mechanism is attributed to the decreasing ROS generation and further phosphorylation of vascular endothelial growth factor receptor 2 and Tie2. Our findings suggest that 4-ISO-Tempo should be investigated for its usefulness in anti-angiogenesis therapies.

SILAC-based proteomics of human primary endothelial cell morphogenesis unveils tumor angiogenic markers

Proteomics has been successfully used for cell culture on dishes, but more complex cellular systems have proven to be challenging and so far poorly approached with proteomics. Because of the complexity of the angiogenic program, we still do not have a complete understanding of the molecular mechanisms involved in this process, and there have been no in depth quantitative proteomic studies. Plating endothelial cells on matrigel recapitulates aspects of vessel growth, and here we investigate this mechanism by using a spike-in SILAC quantitative proteomic approach. By comparing proteomic changes in primary human endothelial cells morphogenesis on matrigel to general adhesion mechanisms in cells spreading on culture dish, we pinpoint pathways and proteins modulated by endothelial cells. The cell-extracellular matrix adhesion proteome depends on the adhesion substrate, and a detailed proteomic profile of the extracellular matrix secreted by endothelial cells identified CLEC14A as a matrix component, which binds to MMRN2. We verify deregulated levels of these proteins during tumor angiogenesis in models of multistage carcinogenesis. This is the most in depth quantitative proteomic study of endothelial cell morphogenesis, which shows the potential of applying high accuracy quantitative proteomics to in vitro models of vessel growth to shed new light on mechanisms that accompany pathological angiogenesis. The mass spectrometry proteomics data have been deposited to the ProteomeXchange Consortium with the data set identifier PXD000359.

Angiogenesis

Extracellular matrix (ECM) is essential for all stages of angiogenesis. In the adult, angiogenesis begins with endothelial cell (EC) activation, degradation of vascular basement membrane, and vascular sprouting within interstitial matrix. During this sprouting phase, ECM binding to integrins provides critical signaling support for EC proliferation, survival, and migration. ECM also signals the EC cytoskeleton to initiate blood vessel morphogenesis. Dynamic remodeling of ECM, particularly by membrane-type matrix metalloproteases (MT-MMPs), coordinates formation of vascular tubes with lumens and provides guidance tunnels for pericytes that assist ECs in the assembly of vascular basement membrane. ECM also provides a binding scaffold for a variety of cytokines that exert essential signaling functions during angiogenesis. In the embryo, ECM is equally critical for angiogenesis and vessel stabilization, although there are likely important distinctions from the adult because of differences in composition and abundance of specific ECM components.

Celastrus orbiculatus extract inhibits tumor angiogenesis by targeting vascular endothelial growth factor signaling pathway and shows potent antitumor activity in hepatocarcinomas in Vitro and in Vivo

OBJECTIVE

Celastrus orbiculatus Thunb. has been used for thousands of years in China as a remedy against cancer and inflammatory diseases. This study aims to investigate whether C. orbiculatus extract (COE) could inhibit angiogenesis, which is the pivotal step in tumor growth, invasiveness, and metastasis.

METHODS

In this study, the extract from the stem of C. orbiculatus was used. Mouse hepatic carcinoma cells (Hepa1-6) were treated with COE in different nontoxic concentrations (10, 20, 40, 80, and 160 μg/mL). The mRNA and protein expression levels of vascular endothelial growth factor (VEGF) were detected by reverse transcription-polymerase chain reaction (RT-PCR) and Western blot, respectively; the active fractions were further tested on C57BL/6 mice and human umbilical vein endothelial cells (HUVEC) for any antiangiogenic effects.

RESULTS

COE significantly inhibited proliferation and induced apoptosis in Hepa1-6 cells and inhibited VEGF expression at both mRNA and protein levels. Furthermore, this agent inhibited the formation of the capillary-like structure in primary cultured HUVEC in a dose-dependent manner. In vivo, COE significantly reduced the volume and weight of solid tumors with low adverse effects and decreased tumor angiogenesis.

CONCLUSIONS

In summary, COE could be used to treat hepatic carcinoma. The mechanisms of the antitumor activity of COE may be due to its effects against tumor angiogenesis by targeting the VEGF protein.

Advanced glycation end-products induce basement membrane hypertrophy in endoneurial microvessels and disrupt the blood-nerve barrier by stimulating the release of TGF-β and vascular endothelial growth factor (VEGF) by pericytes

AIMS/HYPOTHESIS

The breakdown of the blood-nerve barrier (BNB) is considered to be a key step in diabetic neuropathy. Although basement membrane hypertrophy and breakdown of the BNB are characteristic features of diabetic neuropathy, the underlying pathogenesis remains unclear. The purpose of the present study was to identify the possible mechanisms responsible for inducing the hypertrophy of basement membrane and the disruption of the BNB after exposure to AGEs.

METHODS

The newly established human peripheral nerve microvascular endothelial cell (PnMEC) and pericyte cell lines were used to elucidate which cell types constituting the BNB regulate the basement membrane and to investigate the effect of AGEs on the basement membrane of the BNB using western blot analysis.

RESULTS

Fibronectin, collagen type IV and tissue inhibitor of metalloproteinase (TIMP-1) protein were produced mainly by peripheral nerve pericytes, indicating that the basement membrane of the BNB is regulated mainly by these cells. AGEs reduced the production of claudin-5 in PnMECs by increasing autocrine signalling through vascular endothelial growth factor (VEGF) secreted by the PnMECs themselves. Furthermore, AGEs increased the amount of fibronectin, collagen type IV and TIMP-1 in pericytes through a similar upregulation of autocrine VEGF and transforming growth factor (TGF)-β released by pericytes.

CONCLUSIONS/INTERPRETATION

These results indicate that pericytes may be the main regulators of the basement membrane at the BNB. AGEs induce basement membrane hypertrophy and disrupt the BNB by increasing autocrine VEGF and TGF-β signalling by pericytes under diabetic conditions.

Immunohistochemical analysis of TIMP-2 and collagen types I and IV in experimental spinal cord ischemia-reperfusion injury in rats

BACKGROUND

Thoracic and thoracoabdominal aortic intervention carries a significant risk of spinal cord ischemia. The pathophysiologic mechanisms that cause hypoxic/ischemic injury to the spinal cord have not been totally explained. In normal spinal cord, neurons and glial cells do not express type IV collagen. Type IV collagen produced by reactive astrocytes is reported to participate in glial scar formation. Tissue inhibitors of metalloproteinases (TIMPs) are endogenous inhibitors that regulate the activity of the matrix metalloproteinases (MMPs). TIMP-2 binds strongly with MMP-2, facilitating activation by membrane-type MMP. Imbalance between TIMPs and MMPs can lead to excessive degradation of matrix components. Type IV collagen involved in the blood-brain barrier disruption and glial scar formation, TIMP-2 influences MMP-2 that controls degradation of collagen I and IV.

OBJECTIVE

To examine the immunohistochemical analysis of TIMP-2 and collagen types I-IV in experimental spinal cord ischemia-reperfusion in rats.

METHODS

Thirty-two male Wistar rats weighing 250-300 g were divided into four groups: group S: sham group (n = 8); group 0P: 30-minute occlusion without perfusion (n = 8); group 3P: 30-minute occlusion and 3-hour perfusion (n = 8); and group 24P: 30-minute occlusion and 24-hour perfusion (n = 8). Infrarenal aorta was cross-clamped at two sites by using two aneurysm clips for 30 minutes. Reperfusion was provided after removal of the clips. Lumbar spinal cord segments were removed for immunohistochemical analysis.

RESULTS

TIMP-2 and collagen staining in 3-hour perfused (3P) group were nearly the same with sham group (S). TIMP-2 and collagen staining increased in the 24-hour perfused group.

CONCLUSION

Alterations in collagen levels may relate to the biphasic breakdown of the blood-brain barrier and collagen staining in new cell types with relation to glial scar formation. Our results demonstrate that 3-hour perfusion after occlusion in hypoxic/ischemic spinal cord injury seems to be the critical reversible period.

In the hypoxic central nervous system, endothelial cell proliferation is followed by astrocyte activation, proliferation, and increased expression of the alpha 6 beta 4 integrin and dystroglycan

Cerebral hypoxia induces a profound angiogenic response in the central nervous system (CNS). Using a mouse model of chronic cerebral hypoxia, we previously demonstrated that angiogenic vessels in the hypoxic CNS show marked upregulation of the extracellular matrix (ECM) protein fibronectin, along with increased expression of its major receptor, alpha 5 beta 1 integrin on brain endothelial cells (BEC). As cerebral hypoxia also leads to glial activation, the aim of the current study was to define the temporal relationship between BEC responses and glial cell activation in this model of cerebral hypoxia. This revealed that BEC fibronectin/alpha 5 beta 1 integrin expression and proliferation both reached maximal level after 4-day hypoxia. Interestingly, up to 4-day hypoxia, all dividing cells were BEC, but at later time-points proliferating astrocytes were also observed. GFAP staining revealed that hypoxia induced marked astrocyte activation that reached maximal level between 7- and 14-day hypoxia. As newly formed cerebral capillaries require ensheathment by astrocyte end-feet to acquire mature brain endothelium characteristics, we next examined how expression of astrocyte end-feet adhesion molecules is regulated by hypoxia. This showed that the astrocyte adhesion receptors alpha 6 beta 4 integrin and dystroglycan were both markedly upregulated, with a time-course that closely resembled astrocyte activation. Taken together, this evidence shows that cerebral hypoxia promotes first an endothelial response, in which fibronectin promotes BEC proliferation. This is then followed by an astrocyte response, involving astrocyte activation, proliferation, and reorganization of astrocyte end-feet, which correlates with increased expression of astrocyte end-feet adhesion molecules.

Molecular mediators of angiogenesis

Angiogenesis, or the formation of new blood vessels from the preexisting vasculature, is a key component in numerous physiologic and pathologic responses and has broad impact in many medical and surgical specialties. In this review, we discuss the key cellular steps that lead to the neovascularization of tissues and highlight the main molecular mechanisms and mediators in this process. We include discussions on proteolytic enzymes, cell-matrix interactions, and pertinent cell signaling pathways and end with a survey of the mechanisms that lead to the stabilization and maturation of neovasculatures.

Purinergic mechanisms in breast cancer support intravasation, extravasation and angiogenesis

Several advances have recently expanded models of tumor growth and promoted the concept of tumor homeostasis, the hypothesis that primary tumors exert an anti-proliferative effect on both themselves and subclinical secondary metastases. Recent trials indicate that the characterization of tumor growth as uncontrolled is inconsistent with animal models, clinical models, and epidemiological models. There is a growing body of evidence which lends support to an updated concept of tumor growth: tumor homeostasis. In the case of breast cancer, if not all metastasizing tumors, these advances suggest an inconvenient truth. That is, if breast tumor cells metastasize to distant sites early in the tumorigenesis process, then removal of a breast tumor may hasten the development of its metastases. We explore the heretofore unappreciated notion that nucleotides generated by tumor cells following the secretion of an ADP-kinase can promote metastasis and support angiogenesis. Evidence is presented that blockade of the actions of nucleotides in the setting of newly diagnosed breast cancer may provide a useful adjunct to current anti-angiogenesis treatment.

The neurovascular unit in the setting of stroke

Microvessels and neurons respond rapidly and simultaneously in focal regions of ischaemic injury in such a way as to suggest that the responses could be coordinated. The ability of neurons to modulate cerebral blood flow in regions of activation results from neurovascular coupling. But little is known about the microvessel-to-neuron direction of the relationship. The presence and participation of intervening glial cells implies the association of microvessels, glia, and neurons in a 'neurovascular unit'. The interdependent functions of the cellular and matrix components of this theoretical unit have not been rigorously explored, except under conditions of injury where, for the most part, only single components or tissue samples have been studied. Whereas maintenance or timely re-establishment of flow reduces tissue and neuron injury in both humans and animal models, protection of neuron function in humans has not prevented the evolution of injury despite the inherent mechanisms of neurovascular coupling. However, occlusion of flow to the brain rapidly identifies regions of neuron-vascular vulnerability within the vascular territory-at-risk. These coalesce to become the mature ischaemic lesion. The failure, so far, of clinical trials of neuron protectant agents to achieve detectable tissue salvage could be explained by the vulnerability (and lack of protection) of essential components of the 'unit'. This presentation summarizes evidence and thoughts on this topic. These support the need to understand component interactions within the neurovascular unit.

A biocompatible endothelial cell delivery system for in vitro tissue engineering

Engineering solid tissues, including cardiac muscle, requires the inclusion of a microvasculature. Prevascularization in vitro will likely be dependent upon coculturing parenchymal cells with vascular cells, on a matrix that is sufficiently porous to allow microvessel formation. In this study, we examined the behavior and function of endothelial cells on a highly porous elastomeric 3D poly(glycerol sebacate) (PGS) scaffold, to provide a flexible and biocompatible endothelial cell delivery system for developing cardiac engineered tissues with neovascularization potential. Both static and perfusion cell seeding methods were used, and the effects of surface treatment of the scaffold with various extracellular matrix components were examined. Endothelial cell adhesion and phenotype on the PGS scaffold under various flow conditions were also determined. Surface coating with laminin markedly improved the endothelial cell adhesion, survival, and proliferation. The anticoagulant phenotype of adhered endothelial cells was further regulated by the application of flow through regulation of nitric oxide expression. By providing a highly porous scaffolding that contains endothelium with anticoagulant properties, the endothelial cell-seeded PGS scaffold could provide a new basis for subsequent coculture studies with various cell types to develop complex engineered tissue constructs with vascularization capacity.

Relationship of neurovascular elements to neuron injury during ischemia

Occlusion of flow to the brain regions identifies regions of vulnerability within the vascular territory at risk, which coalesce to become the mature ischemic lesion. A large number of unsuccessful clinical trials have focused on neuron and extravascular targets in humans that have shown apparent salvage in preclinical models. However, the observation that microvessel and neuron responses to ischemia occur simultaneously in these regions suggest that the responses could be coordinated. This presentation examines evidence in support of the conceptual 'neurovascular unit' and its application to the setting of acute intervention trials in ischemic stroke. There are no uniform reasons for which nonvascular interventions, as a class, have not been successful in clinical trials, but both the clinical observations and the hypothesis imply the need to understand interactions with the neurovascular unit as a prelude to further neuron protectant trials.

Inflammation and the neurovascular unit in the setting of focal cerebral ischemia

Responses to focal cerebral ischemia by neurons and adjacent microvessels are rapid, simultaneous, and topographically related. Recent observations indicate the simultaneous appearance of proteases by components of nearby microvessels that are also expressed by neurons in the ischemic territory, implying that the events could be coordinated. The structural relationship of neurons to their microvascular supply, the direct functional participation of glial cells, and the observation of a highly ordered microvessel-neuron response to ischemia suggest that these elements are arranged in and behave in a unitary fashion, the neurovascular unit. Their roles as a unit in the stimulation of cellular inflammation and the generation of inflammatory mediators during focal cerebral ischemia have not been explored yet. However, components of the neurovascular unit both generate and respond to these influences under the conditions of ischemia. Here we briefly explore the potential inter-relationships of the components of the neurovascular unit with respect to their potential roles in ischemia-induced inflammatory responses.

Altered expression of basement membrane-related molecules in rat brain pericyte, endothelial, and astrocyte cell lines after transforming growth factor-beta1 treatment

The basement membrane at the blood-brain barrier (BBB) plays important roles in maintaining the structure and function of capillary vessels. The BBB is constructed from endothelial cells, astrocytes and pericytes, but their interactions in the formation or maintenance of basement membrane have not been established. Transforming growth factor-beta1 (TGF-beta1) is known to increase fibronectin in brain capillary basement membrane with deposition of beta-amyloid. We previously reported that the mRNA level of alpha-smooth muscle actin in a brain capillary pericyte cell line TR-PCT1 was increased by treatment with TGF-beta1. In this study, expression of mRNAs encoding basement membrane-related molecules in TR-PCT1, a rat endothelial cell line TR-BBB13, and a type 2 astrocyte cell line TR-AST4 was evaluated by RT-PCR. The effects of TGF-beta1 on expression of basement membrane-related genes in these cell lines were also examined. Fibronectin, MMP-9, tPA, TIMP-1, and PAI-l in TR-PCT1 were higher than in TR-BBB13 and TR-AST4. In TR-PCT1 treated with TGF-beta1, collagen type IV, PAI-1, and MMP-9 were increased, and TIMP-2 was reduced. The change in PAI-1 mRNA was faster than those in MMP-9, TIMP-2, collagen type IV mRNAs. These results suggest that pericytes may be key cells in the maintenance of the basement membrane at the BBB.

Angiostatin's molecular mechanism: aspects of specificity and regulation elucidated

Tumor growth requires the development of new vessels that sprout from pre-existing normal vessels in a process known as "angiogenesis" [Folkman (1971) N Engl J Med 285:1182-1186]. These new vessels arise from local capillaries, arteries, and veins in response to the release of soluble growth factors from the tumor mass, enabling these tumors to grow beyond the diffusion-limited size of approximately 2 mm diameter. Angiostatin, a naturally occurring inhibitor of angiogenesis, was discovered based on its ability to block tumor growth in vivo by inhibiting the formation of new tumor blood vessels [O'Reilly et al. (1994a) Cold Spring Harb Symp Quant Biol 59:471-482]. Angiostatin is a proteolytically derived internal fragment of plasminogen and may contain various members of the five plasminogen "kringle" domains, depending on the exact sites of proteolysis. Different forms of angiostatin have measurably different activities, suggesting that much remains to be elucidated about angiostatin biology. A number of groups have sought to identify the native cell surface binding site(s) for angiostatin, resulting in at least five different binding sites proposed for angiostatin on the surface of endothelial cells (EC). This review will consider the data supporting all of the various reported angiostatin binding sites and will focus particular attention on the angiostatin binding protein identified by our group: F(1)F(O) ATP synthase. There have been several developments in the quest to elucidate the mechanism of action of angiostatin and the regulation of its receptor. The purpose of this review is to describe the highlights of research on the mechanism of action of angiostatin, its' interaction with ATP synthase on the EC surface, modulators of its activity, and issues that should be explored in future research related to angiostatin and other anti-angiogenic agents.

TEM8 interacts with the cleaved C5 domain of collagen alpha 3(VI)

Tumor endothelial marker (TEM)8 was uncovered as a gene expressed predominantly in tumor endothelium, and its protein product was recently identified as the receptor for anthrax toxin. Here, we demonstrate that TEM8 protein is preferentially expressed in endothelial cells of neoplastic tissue. We used the extracellular domain of TEM8 to search for ligands and identified the alpha 3 subunit of collagen VI as an interacting partner. The TEM8-interacting region on collagen alpha 3(VI) was mapped to its COOH-terminal C5 domain. Remarkably, collagen alpha 3(VI) is also preferentially expressed in tumor endothelium in a pattern concordant with that of TEM8. These results suggest that the TEM8/C5 interaction may play an important biological role in tumor angiogenesis.

Angiogenic role of adrenomedullin through activation of Akt, mitogen-activated protein kinase, and focal adhesion kinase in endothelial cells

Adrenomedullin (AM) is a multifunctional peptide in human pheochromocytoma. To evaluate whether AM could be an angiogenic factor, we examined its effect on kinases and angiogenic processes. AM induced tyrosine phosphorylation of Akt and mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase1/2 (ERK1/2) by using distinct signaling pathways in human umbilical vein endothelial cells (HUVECs). AM also phosphorylated focal adhesion kinase, and phosphatidylinositol 3'-kinase inhibitor inhibited AM-induced focal adhesion kinase phosphorylation. Pretreatment with high concentrations of AM22-52, a putative AM receptor antagonist, partially suppressed AM-induced phosphorylation of Akt, ERK1/2, and focal adhesion kinase. AM and vascular endothelial growth factor produced increases in DNA synthesis and migration in HUVECs. AM induced tube formation in HUVECs, and its effect was inhibited by pretreatment with phosphatidylinositol 3'-kinase inhibitor or ERK1/2 inhibitor. AM induced sprouting in porcine pulmonary arterial endothelial cells and promoted neovessel formation in a mouse Matrigel plug assay. Inhibitors of phosphatidylinositol 3'-kinase and ERK1/2 inhibited AM-induced endothelial sprouting in vitro and angiogenesis in vivo. AM exerts angiogenic activity through activation of Akt, MAPK, and focal adhesion kinase in endothelial cells.

Endothelial expression of the alpha6beta4 integrin is negatively regulated during angiogenesis

Development and homeostasis of the vascular system requires integrin-facilitated cellular adhesion, migration, proliferation and survival. A specific role for the alpha6beta4 integrin in the vasculature, however, has not been identified. Using immunohistochemistry, we observed alpha6beta4 expression on the dermal microvasculature of human foreskin. Analysis of individual cells isolated from trypsin-disrupted foreskin tissue indicated that alpha6beta4 was expressed by a subset of epithelial and endothelial cells, and not by smooth muscle cells. Expression of alpha6beta4 was also analyzed during new vessel growth using explants of human saphenous vein cultured in fibrinogen gels. The results indicate that alpha6beta4 is not expressed by outgrowing endothelial cells, and is downregulated by the original alpha6beta4-positive endothelial cells of the explant. To determine whether alpha6beta4 is expressed during angiogenesis in vivo, the expression of the beta4 subunit was analyzed during the development of the mouse mystacial (whisker) pad. Immunohistochemical staining of the whisker pad indicates that beta4 is expressed by the adult vasculature. To identify when and where beta4 is turned on in the vasculature, we examined the whisker pads from the developing embryo (E19.5 pc), and from postnatal days zero (P0), three (P3) and seven (P7) pups. The expression of alpha6beta4 was found to be turned on spatially and temporally from caudal to rostral regions and from the deep to superficial vasculature, correlating with the maturation of the whisker pad and its corresponding vasculature. Together, these findings suggest a potential role for alpha6beta4 as a negative component of the angiogenic switch, whereas expression of alpha6beta4 on the adult vasculature may indicate regions requiring additional adhesive mechanisms.

Angiostatin induces intracellular acidosis and anoikis in endothelial cells at a tumor-like low pH

Angiostatin inhibits angiogenesis by binding to endothelial cells (ECs) lining the vasculature of growing tumors. These cells are in a dynamic state during angiogenesis and are thus not firmly attached to the extracellular matrix. This makes them more vulnerable to anoikis, a process resulting in cell death initiated by or promoted by loss of attachment. Another potential source of EC vulnerability during tumor angiogenesis is that tumor extracellular pH is typically lower than in normal tissues. This presents an additional challenge to ECs in terms of maintaining ionic homeostasis. We report here that the lethality of angiostatin is significantly enhanced both by reduced matrix attachment during exposure and lowered extracellular pH (pH(e)). Another effect of angiostatin at reduced pH(e) is a decreased intracellular pH (pH(i)). These effects were observed in three model systems: aortic ring sprouts, ECs during tube formation, and ECs in a scratch/migration assay. In these three dynamic assays, angiostatin-induced cell death and intracellular acidification were clearly seen when pH(e) was reduced to 6.7. The intracellular acidification was far greater than that induced by pH(e) reduction alone. In contrast, the effect of angiostatin on pH(i) and on viability were not observed in a subconfluent monolayer in which the cells were allowed to attach to substrate for 48 h prior to exposure to angiostatin. These data suggest that low pH(e) and reduced adhesion to matrix play a role in the specificity of angiostatin for tumor neovasculature in contrast to wound healing and other normal angiogenic processes. The results also implicate roles for both pH(e) and pH(i) regulation in the mechanism of angiostatin action.

Gene structure and functional analysis of the mouse nidogen-2 gene: nidogen-2 is not essential for basement membrane formation in mice

Nidogens are highly conserved proteins in vertebrates and invertebrates and are found in almost all basement membranes. According to the classical hypothesis of basement membrane organization, nidogens connect the laminin and collagen IV networks, so stabilizing the basement membrane, and integrate other proteins. In mammals two nidogen proteins, nidogen-1 and nidogen-2, have been discovered. Nidogen-2 is typically enriched in endothelial basement membranes, whereas nidogen-1 shows broader localization in most basement membranes. Surprisingly, analysis of nidogen-1 gene knockout mice presented evidence that nidogen-1 is not essential for basement membrane formation and may be compensated for by nidogen-2. In order to assess the structure and in vivo function of the nidogen-2 gene in mice, we cloned the gene and determined its structure and chromosomal location. Next we analyzed mice carrying an insertional mutation in the nidogen-2 gene that was generated by the secretory gene trap approach. Our molecular and biochemical characterization identified the mutation as a phenotypic null allele. Nidogen-2-deficient mice show no overt abnormalities and are fertile, and basement membranes appear normal by ultrastructural analysis and immunostaining. Nidogen-2 deficiency does not lead to hemorrhages in mice as one may have expected. Our results show that nidogen-2 is not essential for basement membrane formation or maintenance.

VEGF receptor signal transduction

The family of vascular endothelial growth factors (VEGFs) currently includes VEGF-A, -B, -C, -D, -E, and placenta growth factor (PlGF). Several of these factors, notably VEGF-A, exist as different isoforms, which appear to have unique biological functions. The VEGF family proteins bind in a distinct pattern to three structurally related receptor tyrosine kinases, denoted VEGF receptor-1, -2, and -3. Neuropilins, heparan-sulfated proteoglycans, cadherins, and integrin alphavbeta3 serve as coreceptors for certain but not all VEGF proteins. Moreover, the angiogenic response to VEGF varies between different organs and is dependent on the genetic background of the animal. Inactivation of the genes for VEGF-A and VEGF receptor-2 leads to embryonal death due to the lack of endothelial cells. Inactivation of the gene encoding VEGF receptor-1 leads to an increased number of endothelial cells, which obstruct the vessel lumen. Inactivation of VEGF receptor-3 leads to abnormally organized vessels and cardiac failure. Although VEGF receptor-3 normally is expressed only on lymphatic endothelial cells, it is up-regulated on vascular as well as nonvascular tumors and appears to be involved in the regulation of angiogenesis. A large body of data, such as those on gene inactivation, indicate that VEGF receptor-1 exerts a negative regulatory effect on VEGF receptor-2, at least during embryogenesis. Recent data imply a positive regulatory role for VEGF receptor-1 in pathological angiogenesis. The VEGF proteins are in general poor mitogens, but binding of VEGF-A to VEGF receptor-2 leads to survival, migration, and differentiation of endothelial cells and mediation of vascular permeability. This review outlines the current knowledge about the signal transduction properties of VEGF receptors, with focus on VEGF receptor-2.

The C-terminal domain promotes the hemorrhagic damage caused by Vibrio vulnificus metalloprotease

Vibrio vulnificus, an opportunistic human pathogen, produces a 45-kDa zinc metalloprotease (V. vulnificus protease; VVP) as an important virulence determinant. VVP injected intradermally into the dorsal skin causes the hemorrhagic damage through specific degradation of type IV collage in the vascular basement membrane. The N-terminal 35-kDa polypeptide (VVP-N), the catalytic domain, also evoked the hemorrhagic skin reaction within minutes. However, the hemorrhagic activity of VVP-N was one-third of that of VVP. Besides, the proteolytic activity of VVP-N toward the reconstituted basement membrane or type IV collagen was found to be about 50 % of VVP. VVP-N, like VVP, was quickly inactivated by an equimolar amount of alpha(2)-macroglobulin, a broad-spectrum plasma protease inhibitor. These findings indicate that the C-terminal 10-kDa polypeptide, the substrate-binding domain mediating the effective binding to protein substrates, functions to augment the hemorrhagic reaction of VVP.

Halofuginone: a potent inhibitor of critical steps in angiogenesis progression

We have previously demonstrated that halofuginone, a low molecular weight quinazolinone alkaloid, is a potent inhibitor of collagen alpha1(I) and matrix metalloproteinase 2 (MMP-2) gene expression. Halofuginone also effectively suppresses tumor progression and metastasis in mice. These results together with the well-documented role of extracellular matrix (ECM) components and matrix degrading enzymes in formation of new blood vessels led us to investigate the effect of halofuginone on the angiogenic process. In a variety of experimental system, representing sequential events in the angiogenic cascade, halofuginone treatment resulted in profound inhibitory effect. Among these are the abrogation of endothelial cell MMP-2 expression and basement membrane invasion, capillary tube formation, and vascular sprouting, as well as deposition of subendothelial ECM. The most conclusive anti-angiogenic activity of halofuginone was demonstrated in vivo (mouse corneal micropocket assay) by showing a marked inhibition of basic fibroblast growth factor (bFGF) -induced neovascularization in response to systemic administration of halofuginone, either i.p. or in the diet. The ability of halofuginone to interfere with key events in neovascularization, together with its oral bioavailability and safe use as an anti-parasitic agent, make it a promising drug for further evaluation in the treatment of a wide range of diseases associated with pathological angiogenesis.

Cationic colloidal silica membrane perturbation as a means of examining changes at the sinusoidal surface during liver regeneration

By employing the cationic colloidal silica membrane density perturbation technique, we examined growth factor receptor and extracellular matrix (ECM) changes at the sinusoidal surface during rat liver regeneration 72 hours after 70% partial hepatectomy (PHx). At this time after PHx, hepatocyte division has mostly subsided, while sinusoidal endothelial cell (SEC) proliferation is initiating, resulting in avascular hepatocyte islands. Because of the discontinuous nature of the surface of liver SEC, ECM proteins underlying the SEC, as well as SEC luminal membrane proteins, are available to absorption to the charged silica beads when the liver is perfused with the colloid. Subsequent liver homogenization and density centrifugation yield two separate fractions, enriched in SECs as well as hepatocyte basolateral membrane-specific proteins up to 50-fold over whole liver lysates. This technique facilitates examination of changes in protein composition that influence or occur as a result of SEC mitogenesis and migration during regeneration of the liver. When ECM and receptor proteins from SEC-enriched fractions were examined by Western immunoblotting, urokinase plasminogen activator receptor, fibronectin, and plasmin increased at the SEC surface 72 hours after PHx. Epidermal growth factor receptor, plasminogen, SPARC (secreted protein, acidic and rich in cysteine, also called osteonectin or BM40), and collagen IV decreased, and fibrinogen subunits and c-Met expression remained constant 72 hours after PHx when compared to control liver. These results display the usefulness of the cationic colloidal silica membrane isolation protocol. They also show considerable modulation of surface components that may regulate angiogenic processes at the end stage of liver regeneration during the reformation of sinusoids.

Characterization of the hemorrhagic reaction caused by Vibrio vulnificus metalloprotease, a member of the thermolysin family

Vibrio vulnificus is an opportunistic human pathogen causing wound infections and septicemia, characterized by hemorrhagic and edematous damage to the skin. This human pathogen secretes a metalloprotease (V. vulnificus protease [VVP]) as an important virulence determinant. When several bacterial metalloproteases including VVP were injected intradermally into dorsal skin, VVP showed the greatest hemorrhagic activity. The level of the in vivo hemorrhagic activity of the bacterial metalloproteases was significantly correlated with that of the in vitro proteolytic activity for the reconstituted basement membrane gel. Of two major basement membrane components (laminin and type IV collagen), only type IV collagen was easily digested by VVP. Additionally, the immunoglobulin G antibody against type IV collagen, but not against laminin, showed sufficient protection against the hemorrhagic reaction caused by VVP. Capillary vessels are known to be stabilized by binding of the basal surface of vascular endothelial cells to the basement membrane. Therefore, specific degradation of type IV collagen may cause destruction of the basement membrane, breakdown of capillary vessels, and leakage of blood components including erythrocytes.