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Arteriosclerosis, Thrombosis, and Vascular Biology. 1997;17:1193-1202

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(Arteriosclerosis, Thrombosis, and Vascular Biology. 1997;17:1193-1202.)
© 1997 American Heart Association, Inc.

Articles

Heterogeneity of Endothelial Cells

Specific Markers

Cecilia Garlanda; ; Elisabetta Dejana

From the Istituto di Ricerche Farmacologiche Mario Negri, Milano, Italy.

Correspondence to Cecilia Garlanda, Dipartimento di Immunologia e Biologia Cellulare, Istituto di Ricerche Farmacologiche Mario Negri, viale Eritrea 57, 20157 Milano, Italy. E-mail garlanda{at}irfmn.mnegri.it


*    Abstract
up arrowTop
 *Abstract
down arrowIntroduction
 down arrowEC Markers
 down arrowHighly Specialized Endothelium
 down arrowECs in Pathology
 down arrowEndothelium-Specific Promoters
 down arrowConclusions
 down arrowReferences
 
Abstract During embryonic development, endothelial cells differentiate from a common precursor called angioblast and acquire organ-specific properties. One of the important determinants of endothelial cell differentiation is the local environment, and especially the interaction with surrounding cells. This interaction may occur through the release of soluble cytokines, cell-to-cell adhesion and communication, and the synthesis of matrix proteins on which the endothelium adheres and grows. The acquisition and maintenance of specialized properties by endothelial cells is important in the functional homeostasis of the different organs. For instance, in the brain, alteration of the blood-brain barrier properties may have important consequences on brain functional integrity. One of the major limitations to the study of endothelial cell heterogeneity is the fact that these cells are still difficult to isolate and culture from the microcirculation of different organs, and once in culture, they tend to lose their specialized properties. This finding suggests that we have to develop new culture systems, which possibly include coculture with other cell types. An important issue is to develop tools that can help in recognizing endothelial cells and their differentiated phenotype both in vivo and in tissue culture. In this review we give a short overview of the differentiated properties of the endothelium, considering a few examples of highly specialized endothelial cells, such as the brain or bone marrow microcirculation or high endothelial venules. We made a particular effort to list the most common markers of endothelial cell phenotypes. These molecules and related antibodies may be valuable tools for endothelial cell isolation and characterization.


Key Words: endothelial cells • cytokines • angiogenesis • blood-brain barrier • tumors • promoters


*    Introduction
up arrowTop
 up arrowAbstract
 *Introduction
down arrowEC Markers
 down arrowHighly Specialized Endothelium
 down arrowECs in Pathology
 down arrowEndothelium-Specific Promoters
 down arrowConclusions
 down arrowReferences
 
The endothelium is considered a sparse organ system, due to its vast extension and ability to exert a complex array of specialized functions.1 2 A unique characteristic of ECs is that, although they present many common functional and morphological features, they also display remarkable heterogeneity in different organs. Even in the same organ, the endothelium of large and small vessels, veins, and arteries exhibits significant heterogeneity. An extreme case is the kidney, which contains different types of ECs: fenestrated in the peritubular capillaries, discontinuous in glomerular capillaries, and continuous in other regions.2

An important and still incompletely solved question is how ECs take different pathways of differentiation. One of the determinants is the local environment in which ECs differentiate, and especially their interaction with surrounding cells. This interaction may occur through the release of soluble mediators, cell-to-cell adhesion, and the synthesis and organization of matrix proteins on which the endothelium adheres and grows.

Embryonic ECs seem particularly "plastic." Most of the specialized characteristics of ECs are induced during development, whereas adult endothelium is not equally susceptible to differentiation factors.2 3

Despite its stable constitutive properties, the adult endothelium can reversibly change its functions on activation. In a simplified view, adult ECs might be considered like small computers that can be reprogrammed according to the transitory needs of the organism.4 For instance, exposure of ECs to inflammatory cytokines, such as IL-1 and tumor necrosis factor, or to growth factors, such as VEGF or FGF, induces a complex functional reprogramming, which implies the neosynthesis of some genes and the repression of others. ECs can be activated several times during their life span by the same or different cytokines and thereby display different and reversible phenotypes.4

Cell senescence may also influence endothelial responses. Unlike young endothelium, senescent ECs display different properties, such as an inability to properly respond to growth factors due to defective signaling pathways.5 The actual mechanism regulating cell senescence is still incompletely understood; current research suggests a correlation between senescence and intracellular IL-1 accumulation.6

ECs are also functionally different when sparse and confluent cell cultures are compared. Establishment of cell-to-cell contacts inhibits both spontaneous and growth factor–induced cell growth.7 Other properties, such as arachidonic acid metabolism or release of lytic enzymes or growth factors, are changed by the establishment of intercellular contacts.8 9 10 11 We still do not know which are the intracellular mechanisms that regulate the confluent versus sparse cell phenotype. However, molecules at intercellular junctions may transfer intracellular signals that allow the cells to "sense" their neighbors and to react accordingly.7


*    EC Markers
up arrowTop
 up arrowAbstract
 up arrowIntroduction
 *EC Markers
down arrowHighly Specialized Endothelium
 down arrowECs in Pathology
 down arrowEndothelium-Specific Promoters
 down arrowConclusions
 down arrowReferences
 
ECs express specific markers that are very helpful in identifying these cells in vivo and in culture. In many cases these molecules have been discovered through monoclonal antibodies directed to ECs. Table 1Down reports a summary of most of the identified markers (see also References 12 through 3012 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 ), while Table 2Down reports a list of the antibodies used to characterize ECs that still have unknown antigens (see References 31 through 3531 32 33 34 35 ). Interestingly, most of the markers are present on both ECs and hemopoietic precursors or mature blood cells, reinforcing the idea of a common embryonic precursor.2


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  		Table 1. Endothelial Constitutive Markers1


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  		Table 2. Endothelial Markers: Monoclonal Antibodies With Unidentified Antigen

Some of the endothelial markers are constitutive and present in essentially all types of endothelium. Other molecules are expressed only after activation by inflammatory cytokines or growth factors (see Table 3Down and References 36 through 4036 37 38 39 40 ).


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  		Table 3. Inducible Endothelial Markers

Some markers are rather specific for ECs of different origins (Table 4Down and References 41 through 4641 42 43 44 45 46 ). However, this last category is relatively scarce, possibly because EC isolation from the microvasculature of certain vascular regions is still experimentally difficult. New techniques are under development, such as the injection of phage-display peptide libraries,47 48 which detect specific surface molecules in the peripheral endothelium in vivo. Other alternatives to study microvascular endothelium are (1) the culture of ECs from different organs by implanting sponges containing angiogenic factors that promote infiltration of the ECs from that particular site49 and (2) the isolation of ECs after organ disruption through the use of magnetic beads coated with anti-endothelial antibodies.50


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  		Table 4. Markers of Specialized Endothelium

It is not unusual that the same protein may be expressed, with different degrees and types of glycosylation, in different vascular regions. This finding may lead to the development of tissue-specific monoclonals that, despite recognizing the same antigen, are capable of discriminating between tissue-specific glycosylation patterns.

EC marker expression is also related to the functional needs of the cells during vascular morphogenesis in the embryo. In an in vitro model of vasculogenesis, it was found that ECs, as the hemopoietic precursors, go through different stages of differentiation.51 Initially, they express flk-1/KDR, which is one of the receptors for VEGF, and PECAM-1, followed in sequence by Tie-2, Tie-1, and vascular endothelial cadherin. This observation implies that ECs with an intermediate phenotype may exist. Along this line, a recent study describes the presence of EC precursors in the blood of adult individuals.52 At the time of isolation, these cells express flk-1/KDR and only when kept in culture were they able to upregulate PECAM-1, Tie-2, or E-selectin.


*    Highly Specialized Endothelium
up arrowTop
 up arrowAbstract
 up arrowIntroduction
 up arrowEC Markers
 *Highly Specialized Endothelium
down arrowECs in Pathology
 down arrowEndothelium-Specific Promoters
 down arrowConclusions
 down arrowReferences
 
As discussed above, there are several examples of specialized ECs. In this review, we selected three types of endothelium of particular biological relevance and on which a relatively large amount of information is available. These are (1) the high endothelium present in the postcapillary venules of lymph nodes and Peyer's patches, (2) the endothelium of the bone marrow, and (3) the endothelium of the brain.

HEVs
Lymphocytes continuously recirculate from blood to lymphatic or peripheral tissues and vice versa.53 54 Naive and memory lymphocytes follow separate pathways in this process55 : naive lymphocytes mainly recirculate by emigrating through HEVs in peripheral lymphatic tissues and then through efferent lymph and the thoracic duct back to blood. Memory lymphocytes recirculate through postcapillary venules in those peripheral tissues (for example, skin or mucosal tissues) in which they first encountered the antigen.54 The specificity of the emigration process is determined and sustained by EC specialization,56 by the expression of different combinations of adhesion molecules, and by the production of specific chemokines.57 58

HEVs are present in all peripheral lymphatic tissues, including lymph nodes and lymphatic tissues associated with the digestive tract and respiratory tree. However, in chronically inflamed tissues, such as skin, gut, or synovium, ECs from the venules may acquire HEV-like characteristics.53

In lymph nodes deprived of afferent lymph, HEVs lose their phenotype and assume a flat morphology.59 60 The inducibility and reversibility of their characteristics suggest that the local environment may be involved in HEV specialization. Antigens drained from peripheral tissues through the lymphatic circulation, along with cytokines produced by lymphocytes and interdigitating dendritic cells,59 are believed to induce HEV features. Another environmental factor involved in HEV differentiation is the extracellular matrix. Hevin is a recently cloned matrix protein61 that is highly expressed in HEVs of human lymphoid tissues. Hevin is structurally and functionally related to SPARC, an antiadhesive acidic glycoprotein. Like SPARC, hevin is antiadhesive. Hevin association with the basal, lateral, and luminal sides of HEV membranes might weaken endothelial cell-to-cell and cell-to-matrix adhesion and facilitate lymphocyte motility and emigration.61

HEVs differ from ECs of other small vessels with respect to morphology and functional properties: (1) They have an almost cuboidal cellular morphology; (2) their metabolic apparatus, Golgi complex, rough endoplasmic reticulum, and polyribosomes are particularly developed56 ; (3) they present, on their luminal surface, a prominent glycocalyx56 ; and (4) like postcapillary venules, they have discontinuous cell-to-cell junctions and essentially no or very poorly organized tight junctions.62 Most importantly, HEVs constitutively express counterreceptors for lymphocyte adhesion molecules that cannot be found elsewhere. One group of these molecules, named peripheral node addressins, was identified for its reactivity with the mAb MECA 79, which recognizes a carbohydrate epitope common among these proteins63 (Table 4Up).

Some of the molecules expressing the MECA 79 epitope are L-selectin counterreceptors. L-selectin is expressed by all normal naive T and B cells and is required for lymphocytes to enter into lymph nodes from the blood.64 MECA 79 can inhibit the interaction between lymphocytes and HEVs.63 MECA 79 immunoprecipitates 7 to 10 bands from mouse peripheral lymph node HEVs, of which only four are recognized by L-selectin.64 These are Sgp 50, or GlyCAM-1,65 Sgp 90, a specifically posttranslational modified form of CD34,66 Sgp 200, which has not been molecularly identified yet, and MAdCAM-1, which is mostly found on HEVs of mucosal lymph nodes and Peyer's patches.67

When expressed by HEVs, MECA 79 proteins are specifically glycosylated with O-linked oligosaccharides, of which the prototype is sialyl–LewisX. The presence of sialic acid residues, sulfation, and {alpha}(1,3)fucosylation of oligosaccharides are critical for L-selectin recognition.68

None of these molecules are HEV specific. MECA 79 specificity is related to posttranslational modifications of the antigens. In particular, MECA 79 recognizes serine- and/or threonine-linked oligosaccharides, which may present antigenic properties specific for HEVs. Interestingly, the expression of mouse {alpha}(1,3)fucosyltransferase correlates with the expression of L-selectin binding sites in HEVs.69 The binding specificity of such common carbohydrate residues might also be due to the diverse combinations of spacing and clustering displayed by these chains on the protein backbone.68

After the attachment of L-selectin to its counterreceptors during the rolling phase and after chemokine activation, lymphocyte arrest is mediated by activated LFA-1 and its counterreceptors ICAM-1 and ICAM-2 and by {alpha}4ß7 and its counterreceptor MAdCAM-1. MAdCAM-1 can function both as a selectin and integrin ligand as a result of its mucinlike and Ig-like domains.67 In contrast to the classical emigration process in postcapillary venules, the coupled {alpha}4ß1–VCAM-1 is not involved, due to the lack of expression of VCAM-1 by HEVs.70

VAP-1 is an endothelial molecule constitutively expressed on the surface membrane and in cytoplasmic granules of HEVs and a subset of ECs of normal and inflamed tissues, where it participates in lymphocyte recruitment.71 72 Knowledge about VAP-1 is still very scant, owing to its ligand and the stimuli able to induce its expression at sites of inflammation, which remain unidentified. VAP-1 expression in umbilical vein ECs is present only in cytoplasmic granules, where the molecule lacks sialic acid residues that are present in HEVs and essential for lymphocyte adhesion.73

Lymphocytes recirculating in peripheral tissues do not use specialized structures like HEVs. However, during chronic inflammation, microvascular structures assume HEV-like properties and allow lymphocyte infiltration.53

E-selectin plays an important role during cases of skin inflammation.74 E-selectin is transiently expressed on umbilical vein endothelium after activation by inflammatory cytokines but is more persistent in ECs from chronically inflamed tissues.64

The E-selectin counterreceptor for lymphocytes infiltrating the dermis is CLA-1.75 While only 2% to 3% of peripheral blood cells express the glycoprotein CLA-1, 80% of skin lymphocytes are CLA-1 positive, suggesting that E-selectin functions as a skin homing receptor. Besides the E-selectin–CLA-1 pair, MECA 79 reactivity63 and sialylated VAP-176 are strongly induced in the skin microvasculature in several pathological conditions and sustain lymphocyte binding and infiltration.

Bone Marrow ECs
Proliferation, differentiation, and maturation of hemopoietic cells are influenced and controlled by the bone marrow microenvironment. Bone marrow stroma is composed of different cell types, such as fibroblasts, ECs, adipocytes, osteoblasts, reticular cells, and monocytes. These cells produce cytokines and synthesize matrix proteins that can influence the growth and maturation of the hemopoietic precursors. The endothelium is located among stromal cells at the interface between the blood and bone marrow environment. These cells actively participate in hemopoiesis and in regulating the transit of both precursors and mature blood cells between the bone marrow and the circulation.77

During embryogenesis, hemopoiesis begins in the extraembryonic blood islands of the yolk sac and in the intraembryonic aorta-gonad-mesonephron region.78 Later, the fetal liver and finally the bone marrow are the sites where the hemopoietic system develops. The yolk sac blood islands are simple structures in which CD34+ hemopoietic cells in the middle are in close contact with CD34+ ECs confined at the periphery.79 These structures produce pluripotential hemopoietic stem cells.80 The endothelium of the blood islands produces important amounts of hemopoietic cytokines, such as stem cell factor; flk-2/flt-3 ligand; leukemia inhibitory factor; and IL-6 macrophage-CSF, granulocyte/macrophage-CSF, and granulocyte-CSF. This observation supports the hypothesis that these cells are major effectors in controlling hemopoiesis in the yolk sac.78

ECs from human bone marrow have been isolated and characterized.81 These cells are able to promote long-term multilineage hemopoiesis, particularly myelopoiesis and megakariocytopoiesis.82 In addition to bone marrow endothelium, umbilical vein ECs are also capable of producing cytokines that influence hemopoiesis. These include IL-1, IL-4, and IL-6 granulocyte/macrophage-CSF, granulocyte-CSF, and macrophage-CSF; stem cell factor after stimulation with inflammatory stimuli4 83 ; and FGF, transforming growth factor-ß, platelet-derived growth factor, and leukemia inhibitory factor in resting conditions.82 84 However, unlike umbilical vein ECs, bone marrow endothelium constitutively produces granulocyte-CSF, granulocyte/macrophage-CSF, IL-6, and kit ligand, which may be important in long-term hemopoiesis.82

Bone marrow ECs synthesize adhesion molecules on their surface, which could play an important role in controlling the traffic of hemopoietic cells from and to the blood. In contrast to umbilical vein ECs, where adhesion molecules for circulating cells are induced essentially after activation, bone marrow endothelium constitutively expresses VCAM-1 and E-selectin both in vitro and in vivo.85 86 87 88 This observation suggests that these structures might act as bone marrow endothelial addressins for the homing of hemopoietic progenitors.

Bone marrow capillaries have the morphology of discontinuous fenestrated sinusoids. The presence of discontinuities in the vessel wall might facilitate the traffic of the hemopoietic and mature blood cells.2

BBB
The endothelium of the brain microvasculature represents the interface between blood and the central nervous system. Due to its unique location, it has specific protective properties that strictly regulate the infiltration of plasma components and circulating cells into the brain. The BBB normally permits the passage of only small hydrophobic molecules, a limited number of specifically transported nutrients such as glucose and amino acids, and some transcytosed molecules such as transferrin (for review, see References 89 through 9189 90 91 ).

In general, the barrier activity is due to well-developed tight junctions between ECs, very selective intracellular transport systems, and a very low pinocytotic activity. Tight junctions are particularly well developed in brain vessels, most of which associate to the P phase of the cell membrane, indirectly indicating a close linkage to the cytoskeleton.92

The presence of such a developed system of tight junctions is probably responsible for the well-established cell polarization found in brain capillaries.91 Cell polarization helps to direct the transport of solutes from the apical to the basal membrane and vice versa. The ECs of the brain synthesize the multidrug-resistance protein P-glycoprotein, or mdr 1a, which actively transports a variety of low-molecular-weight molecules out of the brain to the circulation.93 94 This reverse mechanism protects the nervous tissue from the accumulation of undesired toxic molecules. Brain endothelium also has specific transport systems directing flow from the circulating blood to the brain, such as the glucose transporter Glut-1 or a well-developed set of amino acid transporters91 95 (Table 4Up).

Brain microvasculature derives from the meningeal vessels and invades the brain by angiogenesis.89 These ECs acquire the characteristics of the BBB, most likely through contact with the neuroectoderm during embryogenesis. Astrocytes contribute to the establishment of barrier properties in ECs.90 96 However, recent evidence shows that some specific markers of the brain microvasculature are already expressed by these cells as early as day 10.5 of gestation, when astrocytes are not yet detectable.94 97 This finding suggests that some forms of "early" endothelial commitment exist even in the absence of the interaction with astrocytes.98

During their differentiation, brain ECs not only acquire specific markers but also lose, or express to a low degree, molecules that are otherwise present in other types of endothelium, like MECA 32.99 100

Interestingly, the vessels that invade brain tumors, like glioblastoma multiforme, do not have BBB properties, which results in brain edema. In this type of tumor, the effect seems to be mostly due to the formation of channels through interendothelial junctions.101 This observation suggests that for maintenance of specialized barrier properties, ECs require a continuous interaction with normal nervous cells. When ECs invade a neoformed tumor, such as a glioma or glioblastoma, they come into contact with tumor cells that produce growth factors, in particular VEGF, which may be responsible not only for vascular proliferation but also for the altered permeability properties of the neoformed vessels.102 103 104


*    ECs in Pathology
up arrowTop
 up arrowAbstract
 up arrowIntroduction
 up arrowEC Markers
 up arrowHighly Specialized Endothelium
 *ECs in Pathology
down arrowEndothelium-Specific Promoters
 down arrowConclusions
 down arrowReferences
 
Tumor Vasculature
With some exceptions, such as the female reproductive cycle and wound healing, angiogenesis in the adult occurs only in response to pathogenic stimuli (for review see Reference 105105 ). The release of growth factors from tumor cells induces ECs of adjacent vessels to migrate, proliferate, and invade the developing tumor focus.106 107

An important question is whether these new vessels are different from the rest of the vasculature and whether they can be distinguished by the use of specific markers. This issue has many practical implications. One can link cytotoxic agents to monoclonal antibodies or ligands able to bind the tumor vasculature and induce its destruction without affecting other vessels or tissues.108

Tumor vessels present specific morphology, relatively uncontrolled permeability, fenestrae, and a well-developed vesicular system. This phenotype seems to be induced, at least in part, by VEGF,109 110 which is released by tumor cells (see also above).

In general, the ECs that vascularize tumors upregulate antigens, such as the growth factor receptors flk-1/KDR, Tie-1, Tie-2,111 112 113 the integrin {alpha}vß3,114 or the alternative spliced variant of fibronectin, ED-B,115 which otherwise are present only in the vessels of the developing embryo. These molecules are markers of endothelial proliferation and as such are not strictly specific for the tumor vasculature but can be found in other angiogenic vessels.

Tumor ECs express different levels of adhesive molecules for circulating leukocytes. More specifically, activation of ECs with VEGF depresses and with FGF upregulates ICAM-1 and VCAM-1 levels.116 The level of E-selectin was also found to be high in tumor capillaries, possibly due to high levels of tumor necrosis factor-{alpha}.108 Therefore, the nature of the local environment may influence EC susceptibility to cytotoxic leukocytes.

Leukocyte adhesive molecules are present in many types of ECs on activation with inflammatory cytokines, and they cannot be considered strictly specific for tumor vessels. More specific markers for tumor endothelium are still scant and only partially characterized (Table 4Up).

It is possible that not all tumor vessels are the same. It is conceivable that angiogenic ECs retain the properties of the vessels of origin and that the tumor tissue influences the functional and morphological characteristics of the invading vasculature. This raises the question of whether one can expect to find markers for the vessels of a given tumor that are not necessarily present in the vasculature of another type of tumor.

EC Tumors
ECs can acquire malignant properties. For instance, they can form tumors such as angiosarcomas, which are highly invasive and usually connote a lethal prognosis (for review, see Reference 117117 ). Angiosarcomas and hemangioendotheliomas retain most of the endothelial markers but express low amounts of vascular endothelial cadherin.118 This is interesting, since cadherins in general are downregulated in other types of malignant tumors.119

The majority of data available strongly suggest that Kaposi's sarcoma has an endothelial origin. However, it expresses markers for both ECs and macrophages,118 120 suggesting that it might originate from a special type of EC similar to macrophages present in lymph nodes.

Kaposi's sarcoma cells have the specificity of producing a large set of cytokines.121 These cells express KDR but not flt-1; this finding is of interest, since flt-1 is important in vascular morphogenesis.111 122

Other types of hemangiomas have a more benign outcome. They are characterized by rapid EC growth and the formation of abnormal vascular structures. In some cases they are likely to represent malformations.117 The antigenic profile of a large variety of hemangiomas is similar to that of normal ECs,118 but hemangiomas express E-selectin constitutively.123 The abnormal vascular proliferation could be related to locally released growth factors and fibrinolytic agents rather than to structural alterations of ECs.124

For some hereditary hemangiomas, the vascular anomalies seem to be related to altered interactions between ECs and mesenchymal cells.

In two familiar mucocutaneous venous malformations,125 the endothelial tyrosine kinase receptor Tie-2 is mutated. Tie-2 is the receptor for angiopoietin,126 127 which is produced by mesenchymal cells and is important for normal vessel morphogenesis.

Hereditary hemorrhagic telangiectasia (Rendu-Osler-Weber syndrome) has been found to be due to the mutation of two transforming growth factor-ß binding proteins: endoglin and activin-receptor–like kinase.128 129 These changes might be related to abnormal mesenchymal cell proliferation and organization around the vascular structures.


*    Endothelium-Specific Promoters
up arrowTop
 up arrowAbstract
 up arrowIntroduction
 up arrowEC Markers
 up arrowHighly Specialized Endothelium
 up arrowECs in Pathology
 *Endothelium-Specific Promoters
down arrowConclusions
 down arrowReferences
 
The presence of EC-specific markers is related to the presence of specific expression systems.

Several endothelium-specific promoters have been described so far (a few examples are flk-1,130 Flt-1,131 Tie-2,132 von Willebrand factor,133 and endothelin-1134 ). In many cases, the promoters of endothelial genes contain the consensus motifs for Sp1, EGR-1, ets, and GATA transcription factors (eg, see Reference 135135 ). These factors are not cell specific and not in all cases have these sites been proven necessary for the expression of the gene.

Many efforts have been directed toward the definition of endothelium-specific transcription factors. It is likely that the combination of different factors confers "cell-specific" expression of genes.

Promoter sequences for endothelium-specific genes have important applications. They can be used for gene targeting in ECs, both in gene therapy and in transgenic mice.108 The possibility of constructing promoters containing endothelium-specific and inducible sequences allows the induction of the expression of a given gene at the time and condition desired.

Only a few examples exist that analyze promoter sequences specific for the endothelium of selected regions of the vascular tree.133 One of these is the von Willebrand factor promoter sequence. Using different stretches of the 5' end of the gene, one can obtain promoter sequences that act only in some regions of the vascular tree but not in others.133


*    Conclusions
up arrowTop
 up arrowAbstract
 up arrowIntroduction
 up arrowEC Markers
 up arrowHighly Specialized Endothelium
 up arrowECs in Pathology
 up arrowEndothelium-Specific Promoters
 *Conclusions
down arrowReferences
 
Understanding the molecular basis of EC heterogeneity is an important task. ECs might strongly influence the homeostasis of the different organs, and any pathological conditions in which they lose their specialized properties might have dramatic consequences. As discussed above, a typical example is the brain endothelium, whose differentiated properties are of fundamental importance in protecting the central nervous system from noxious agents.

Most of our knowledge about ECs comes from the study of human umbilical vein endothelium. These cells might not be an ideal model, since they are close to senescence and are cultured from hypoxic and possibly activated vessels.

Hopefully, in the future it will be easier to culture the endothelium from the microvasculature of different organs and to maintain their specialized properties in vitro. To this end, it is possible that complex coculture systems with different cell types will be required. The development of immortalized EC lines from different origins could also be an important tool, providing that immortalization would alter only partially the tissue-specific characteristics.


*    Selected Abbreviations and Acronyms
 
BBB = blood-brain barrier
CAM = cell adhesion molecule
CLA = cutaneous lymphocyte-associated antigen
CSF = colony-stimulating factor
EC = endothelial cell
FGF = fibroblast growth factor
HEV = high endothelial venule
ICAM = intercellular adhesion molecule
IL = interleukin
MAdCAM = mucosal addressin CAM
PECAM = platelet endothelial CAM
VAP = vascular adhesion protein
VCAM = vascular CAM
VEGF = vascular endothelial growth factor


*    Acknowledgments
 
This work was supported by Associazione Italiana per la Ricerca sul Cancro, the European Community (projects CHRX-CT9.940593, Biomed 2 PL 950669, and BMH4-CT95-0875; and Biotech 960036), and Istituto Superiore di Sanità project "Sostituzioni funzionali, organi artificiali e trapianti di organo" and Special Projects AIDS. C. Garlanda is recipient of a fellowship from the Istituto Superiore di Sanità for the research on AIDS. We thank A. Mantovani, A. Vecchi, and I. Martin-Padura for many helpful discussions and suggestions.

Received March 16, 1997; accepted April 1, 1997.


*    References
up arrowTop
 up arrowAbstract
 up arrowIntroduction
 up arrowEC Markers
 up arrowHighly Specialized Endothelium
 up arrowECs in Pathology
 up arrowEndothelium-Specific Promoters
 up arrowConclusions
 *References
 

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