Complexus adhaerentes, a new group of desmoplakin-containing junctions in endothelial cells: II. Different types of lymphatic vessels

Proteomic analysis of desmosomes reveals novel components required for epidermal integrity

Desmosomes are cell–cell adhesions necessary for the maintenance of tissue integrity in the skin and heart. While the core components of desmosomes have been identified, peripheral components that modulate canonical or noncanonical desmosome functions still remain largely unexplored. Here we used targeted proximity labeling approaches to further elaborate the desmosome proteome in epidermal keratinocytes. Quantitative mass spectrometry analysis identified all core desmosomal proteins while uncovering a diverse array of new constituents with broad molecular functions. By individually targeting the inner and outer dense plaques, we defined proteins enriched within these subcompartments. We validated a number of these novel desmosome-associated proteins and find that many are membrane proximal proteins that show a dependence on functional desmosomes for their cortical localization. We further explored the mechanism of localization and function of two novel desmosome-associated adaptor proteins enriched in the desmosome proteome, Crk and Crk-like (CrkL). These proteins interacted with Dsg1 and rely on Dsg1 and desmoplakin for robust cortical localization. Epidermal deletion of both Crk and CrkL resulted in perinatal lethality with defects in desmosome morphology and keratin organization, thus demonstrating the utility of this dataset in identifying novel proteins required for desmosome-dependent epidermal integrity.

The cell-cell junctions of mammalian testes. III. Absence of an endothelial cell layer covering the peritubular wall of the seminiferous tubules-an immunocytochemical correction of a 50-year-old error in the literature

In the molecular biological and ultrastructural studies of the peritubular wall cells encasing the seminiferous tubules of mammalian testes, we found it necessary to characterize the outermost cell layer bordering on the interstitial space in detail. For half a century, the extremely thin cells of this monolayer have in the literature been regarded as part of a lymphatic endothelium, in particular in rodents. However, our double-label immunofluorescence microscopical results have shown that in all six mammalian species examined, including three rodent ones (rat, mouse, guinea pig), this classification is not correct: the very attenuated cells of this monolayer are not of lymphatic endothelial nature as they do not contain established endothelial marker molecules. In particular, they do not contain claudin-5-positive tight junctions, VE-cadherin-positive adherens junctions, "lymph vessel endothelium hyaluronan receptor 1" (LYVE-1), podoplanin, protein myozap and "von Willebrand Factor" (vWF). By contrast and as controls, all these established marker molecules for the lymphatic endothelial cell type are found in the endothelia of the lymph and-partly also-blood vessels located nearby in the interstitial space. Thus, our results provide evidence that the monolayer cells covering the peritubular wall do not contain endothelial marker molecules and hence are not endothelial cells. We discuss possible methodological reasons for the maintenance of this incorrect cell type classification in the literature and emphasize the value of molecular analyses using multiple cell type-specific markers, also with respect to physiology and medical sciences.

Striatins as plaque molecules of zonulae adhaerentes in simple epithelia, of tessellate junctions in stratified epithelia, of cardiac composite junctions and of various size classes of lateral adherens junctions in cultures of epithelia- and carcinoma-derived cells

Proteins of the striatin family (striatins 1–4; sizes ranging from 90 to 110 kDa on SDS-polyacrylamide gel electrophoresis) are highly homologous in their amino acid sequences but can differ in their cell-type-specific gene expression patterns and biological functions. In various cell types, we have found one, two or three polypeptides of this evolutionarily old and nearly ubiquitous family of proteins known to serve as scaffold proteins for diverse protein complexes. Light and electron microscopic immunolocalization methods have revealed striatins in mammalian cell-cell adherens junctions (AJs). In simple epithelia, we have localized striatins as constitutive components of the plaques of the subapical zonulae adhaerentes of cells, including intestinal, glandular, ductal and urothelial cells and hepatocytes. Striatins colocalize with E-cadherin or E–N-cadherin heterodimers and with the plaque proteins α- and β-catenin, p120 and p0071. In some epithelia and carcinomas and in cultured cells derived therefrom, striatins are also seen in lateral AJs. In stratified epithelia and in corresponding squamous cell carcinomas, striatins can be found in plaques of some forms of tessellate junctions. Moreover, striatins are major plaque proteins of composite junctions (CJs; areae compositae) in the intercalated disks connecting cardiomyocytes, colocalizing with other CJ molecules, including plectin and ankyrin-G. We discuss the “multimodulator” scaffold roles of striatins in the initiation and regulation of the formation of various complex particles and structures. We propose that striatins are included in the diagnostic candidate list of proteins that, in the CJs of human hearts, can occur in mutated forms in the pathogeneses of hereditary cardiomyopathies, as seen in some types of genetically determined heart damage in boxer dogs.

Comparative and developmental anatomy of cardiac lymphatics

The role of the cardiac lymphatic system has been recently appreciated since lymphatic disturbances take part in various heart pathologies. This review presents the current knowledge about normal anatomy and structure of lymphatics and their prenatal development for a better understanding of the proper functioning of this system in relation to coronary circulation. Lymphatics of the heart consist of terminal capillaries of various diameters, capillary plexuses that drain continuously subendocardial, myocardial, and subepicardial areas, and draining (collecting) vessels that lead the lymph out of the heart. There are interspecies differences in the distribution of lymphatic capillaries, especially near the valves, as well as differences in the routes and number of draining vessels. In some species, subendocardial areas contain fewer lymphatic capillaries as compared to subepicardial parts of the heart. In all species there is at least one collector vessel draining lymph from the subepicardial plexuses and running along the anterior interventricular septum under the left auricle and further along the pulmonary trunk outside the heart and terminating in the right venous angle. The second collector assumes a different route in various species. In most mammalian species the collectors run along major branches of coronary arteries, have valves and a discontinuous layer of smooth muscle cells.

The role of inflammation in the pathogenesis of glaucoma

Glaucoma is an ocular disorder characterized by the progressive loss of retinal ganglion cells (RGC) and their axons. There are various hypotheses concerning the cause of RGC death. Previously, glaucoma was defined by high intraocular pressure (IOP); during the past decade, however, glaucoma specialists have acknowledged that elevated IOP is the most important risk factor for glaucoma, but does not define the disease. Other factors such as genetics, blood flow, and excitotoxicity are suggested as potential causal factors for progressive RGC death observed in glaucoma. We review recent studies elucidating a possible role of low-grade inflammation as a causal factor in the pathogenesis of glaucoma.

The role of VE-cadherin in vascular morphogenesis and permeability control

VE-cadherin is an endothelial-specific cadherin that is essential for the formation and regulation of endothelial cell junctions. The adhesive function and expression levels of VE-cadherin at endothelial contacts are central determinants of the control of vascular permeability and leukocyte recruitment into tissue. In addition to controlling junctional integrity, VE-cadherin modulates a multitude of signaling processes that influence the behavior of endothelial cells, such as proliferation, survival, migration, polarity, expression of other junctional components, and tube and lumen formation of blood vessels. This chapter highlights recent progress in understanding how VE-cadherin modulates these various cellular processes. In addition, the current knowledge about how VE-cadhern participates in the regulation of the endothelial barrier in the adult organism is discussed.

The plaque protein myozap identified as a novel major component of adhering junctions in endothelia of the blood and the lymph vascular systems

Recently the protein myozap, a 54-kD polypeptide which is not a member of any of the known cytoskeletal and junctional protein multigene families, has been identified as a constituent of the plaques of the composite junctions in the intercalated disks connecting the cardiomyocytes of mammalian hearts. Using a set of novel, highly sensitive and specific antibodies we now report that myozap is also a major constituent of the cytoplasmic plaques of the adherens junctions (AJs) connecting the endothelial cells of the mammalian blood and lymph vascular systems, including the desmoplakin-containing complexus adhaerentes of the virgultar cells of lymph node sinus. In light and electron microscopic immunolocalization experiments we show that myozap colocalizes with several proteins of desmosomal plaques as well as with AJ-specific transmembrane molecules, including VE-cadherin. In biochemical analyses, rigorous immunoprecipitation experiments have revealed N-cadherin, desmoplakin, desmoglein-2, plakophilin-2, plakoglobin and plectin as very stably bound complex partners. We conclude that myozap is a general component of cell-cell junctions not only in the myocardium but also in diverse endothelia of the blood and lymph vascular systems of adult mammals, suggesting that this protein not only serves a specific role in the heart but also a broader set of functions in the vessel systems. We also propose to use myozap as an endothelial cell type marker in diagnoses.

Desmoplakin is important for proper cardiac cell-cell interactions

Normal cardiac function is maintained through dynamic interactions of cardiac cells with each other and with the extracellular matrix. These interactions are important for remodeling during cardiac growth and pathophysiological conditions. However, the precise mechanisms of these interactions remain unclear. In this study we examined the importance of desmoplakin (DSP) in cardiac cell-cell interactions. Cell-cell communication in the heart requires the formation and preservation of cell contacts by cell adhesion junctions called desmosome-like structures. A major protein component of this complex is DSP, which plays a role in linking the cytoskeletal network to the plasma membrane. Our laboratory previously generated a polyclonal antibody (1611) against the detergent soluble fraction of cardiac fibroblast plasma membrane. In attempting to define which proteins 1611 recognizes, we performed two-dimensional electrophoresis and identified DSP as one of the major proteins recognized by 1611. Immunoprecipitation studies demonstrated that 1611 was able to directly pulldown DSP. We also demonstrate that 1611 and anti-DSP antibodies co-localize in whole heart sections. Finally, using a three-dimensional in vitro cell-cell interaction assay, we demonstrate that 1611 can inhibit cell-cell interactions. These data indicate that DSP is an important protein for cell-cell interactions and affects a variety of cellular functions, including cytokine secretion.

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.

Desmosomal molecules in and out of adhering junctions: normal and diseased States of epidermal, cardiac and mesenchymally derived cells

Current cell biology textbooks mention only two kinds of cell-to-cell adhering junctions coated with the cytoplasmic plaques: the desmosomes (maculae adhaerentes), anchoring intermediate-sized filaments (IFs), and the actin microfilament-anchoring adherens junctions (AJs), including both punctate (puncta adhaerentia) and elongate (fasciae adhaerentes) structures. In addition, however, a series of other junction types has been identified and characterized which contain desmosomal molecules but do not fit the definition of desmosomes. Of these special cell-cell junctions containing desmosomal glycoproteins or proteins we review the composite junctions (areae compositae) connecting the cardiomyocytes of mature mammalian hearts and their importance in relation to human arrhythmogenic cardiomyopathies. We also emphasize the various plakophilin-2-positive plaques in AJs (coniunctiones adhaerentes) connecting proliferatively active mesenchymally-derived cells, including interstitial cells of the heart and several soft tissue tumor cell types. Moreover, desmoplakin has also been recognized as a constituent of the plaques of the complexus adhaerentes connecting certain lymphatic endothelial cells. Finally, we emphasize the occurrence of the desmosomal transmembrane glycoprotein, desmoglein Dsg2, out of the context of any junction as dispersed cell surface molecules in certain types of melanoma cells and melanocytes. This broadening of our knowledge on the diversity of AJ structures indicates that it may still be too premature to close the textbook chapters on cell-cell junctions.

The extracellular domains of E- and N-cadherin determine the scattered punctate localization in epithelial cells and the cytoplasmic domains modulate the localization

The accumulation of classical cadherins is essential for their function, but the mechanism is poorly understood. Hence, we investigated the accumulation of E- and N-cadherin and the formation of cell junctions in epithelial cells. Immunostaining revealed a scattered dot-like accumulation of E- and N-cadherin throughout the lateral membrane in MDCK II and other epithelial cells. Mutant E-cadherin lacking the beta-catenin binding site accumulated granularly at cell-cell contact sites and showed weak cell aggregation activity in cadherin-deficient epithelial cells, MIA PaCa2 cells. Mutant E-cadherin lacking the p120-catenin binding site exhibited scattered punctate accumulation and strong cell adhesion activity in MIA PaCa2 cells. Electron microscopy demonstrated that MIA PaCa2 transfectants of E-cadherin containing beta-catenin binding site formed adherens junction, whereas E-cadherin lacking the binding site did not. Mutant N-cadherins showed accumulation properties similar to those of corresponding mutant E-cadherins. Moreover, wild type and mutant N-cadherin lacking the p120-catenin binding site showed subapical accumulation in polarized DLD-1 cells, whereas mutant N-cadherin lacking beta-catenin binding site did not. These results indicate that the extracellular domains of E- and N-cadherin determines the basic localization pattern, whereas the cytoplasmic domains modulate it thereby affects the cell adhesion activity, subapical accumulation, and the formation of adherens junction.

The junctions that don't fit the scheme: special symmetrical cell-cell junctions of their own kind

Immunocytochemical, electron-, and immunoelectron-microscopical studies have revealed that, in addition to the four major "textbook categories" of cell-cell junctions (gap junctions, tight junctions, adherens junctions, and desmosomes), a broad range of other junctions exists, such as the tiny puncta adhaerentia minima, the taproot junctions (manubria adhaerentia), the plakophilin-2-containing adherens junctions of mesenchymal or mesenchymally derived cell types including malignantly transformed cells, the composite junctions (areae compositae) of the mature mammalian myocardium, the cortex adhaerens of the eye lens, the interdesmosomal "sandwich" or "stud" junctions in the subapical layers of stratified epithelia and the tumors derived therefrom, and the complexus adhaerentes of the endothelial and virgultar cells of the lymph node sinus. On the basis of their sizes and shapes, other morphological criteria, and their specific molecular ensembles, these junctions and the genes that encode them cannot be subsumed under one of the major categories mentioned above but represent special structures in their own right, appear to serve special functions, and can give rise to specific pathological disorders.

Structure and function of rat lymph nodes

The lymph node comprises a critical crossroad for encounters between antigen presenting cells, antigens from lymph, and lymphocytes recruited into lymph nodes from the blood. The node consists of spaces lined with lymphatic endothelial cells and parenchyma. The former spaces can be divided into the subcapsular sinuses, lymphatic labyrinths in the deep cortex, intermediate sinuses, and medullary sinuses. The sponge-like framework of the node parenchyma is composed of collagen fibers invested with reticular cells. The parenchyma can be divided into the cortex, deep cortex, and medullary cord. Lymphocytes migrate from the node parenchyma into the lymphatic labyrinths in the deep cortex. Close to the labyrinths are high endothelial venules (HEVs), through which circulating lymphocytes enter the node parenchyma. HEVs strongly express Aquaporin-1, suggesting that HEVs are involved in the net absorption of water, but not protein, from lymph coming through afferent lymphatics. Many LYVE-1 positive sinus reticular cells (i.e., lymphatic endothelial cells) with attached macrophages form a network within the lumen of the medullary sinuses. Fluids and migrating cells arriving at the node preferentially flow through the subcapsular sinuses, intermediate sinuses, and medullary sinuses in this order. Fluids and migrating cells may also enter the cortex through gaps in the floor of the subcapsular sinuses.

Endothelial and virgultar cell formations in the mammalian lymph node sinus: endothelial differentiation morphotypes characterized by a special kind of junction (complexus adhaerens)

The lymph node sinus are channel structures of unquestionable importance in immunology and pathology, specifically in the filtering of the lymph, the transport and processing of antigens, the adhesion and migration of immune cells, and the spread of metastatic cancer cells. Our knowledge of the cell and molecular biology of the sinus-forming cells is still limited, and the origin and biological nature of these cells have long been a matter of debate. Here, we review the relevant literature and present our own experimental results, in particular concerning molecular markers of intercellular junctions and cell differentiation. We show that both the monolayer cells lining the sinus walls and the intraluminal virgultar cell meshwork are indeed different morphotypes of the same basic endothelial cell character, as demonstrated by the presence of a distinct spectrum of general and lymphatic endothelial markers, and we therefore refer to these cells as sinus endothelial/virgultar cells (SEVCs). These cells are connected by unique adhering junctions, termed complexus adhaerentes, characterized by the transmembrane glycoprotein VE-cadherin, combined with the desmosomal plaque protein desmoplakin, several adherens junction plaque proteins including alpha- and beta-catenin and p120 catenin, and components of the tight junction ensemble, specifically claudin-5 and JAM-A, and the plaque protein ZO-1. We show that complexus adhaerentes are involved in the tight three-dimensional integration of the virgultar network of SEVC processes along extracellular guidance structures composed of paracrystalline collagen bundle "stays". Overall, the SEVC system might be considered as a local and specific modification of the general lymphatic vasculature system. Finally, physiological and pathological alterations of the SEVC system will be presented, and the possible value of the molecular markers described in histological diagnoses of autochthonous lymph node tumors will be discussed.

Protein p0071, a major plaque protein of non-desmosomal adhering junctions, is a selective cell-type marker

Protein p0071, which originally was introduced as a member of the p120-subfamily of armadillo proteins, common to desmosomes and adhaerens junctions (AJs) and to several other cell structures (centrosomes, midbodies), has been localized by using a series of novel mono- and polyclonal antibodies generated against various domains of the molecule. By protein analysis and immunolocalization techniques, protein p0071 has been localized as a plaque protein in AJs of diverse epithelia and certain vascular endothelia, in the composite junctions (areal compositae) of the intercalated disks of cardiomyocytes, and in the punctate or more extended AJs of the vast majority of cell culture types examined, including mitotic states. Using these antibodies, we have also shown that this AJ protein occurs only rarely or is even absent in tissues such as skeletal and smooth muscles, in a series of mesenchymal tissue cells, and in specific desmosome-rich cells such as those of the upper layers of the epidermis and certain other stratified epithelia and Hassall corpuscles of the thymus. We have also demonstrated that p0071 is absent from desmosomes. The occurrence of two major subtypes of lymphatic endothelial cells, one with AJs containing p0071 and one without detectable p0071, is emphasized. Possible structural and functional roles of p0071 are discussed in light of these new findings regarding its localization, and the addition of p0071 to the armamentarium of cytodiagnostic cell-type markers is recommended.

Organization and signaling of endothelial cell-to-cell junctions in various regions of the blood and lymphatic vascular trees

Adhesive intercellular junctions between endothelial cells are formed by tight junctions and adherens junctions. In addition to promoting cell-to-cell adhesion, these structures regulate paracellular permeability, contact inhibition of endothelial cell growth, cell survival, and maintenance of cell polarity. Furthermore, adherens junctions are required for the correct organization of new vessels during embryo development or during tissue proliferation in the adult. Extensive research on cultured epithelial and endothelial cells has resulted in the identification of many molecular components of tight junctions and adherens junctions. Such studies have revealed the complexity of these structures, which are formed by membrane-associated adhesion proteins and a network of several intracellular signaling partners. This review focuses on the structural organization of junctional structures and their functional interactions in the endothelium of blood vessels and lymphatics. We emphasize the way that these structures regulate endothelial cell homeostasis by transferring specific intracellular signals and by modulating activation and signaling of growth factor receptors.

Organization and developmental aspects of lymphatic vessels

The lymphatic system plays important roles in maintaining tissue fluid homeostasis, immune surveillance of the body, and the taking up dietary fat and fat-soluble vitamins A, D, E and K. The lymphatic system is involved in many pathological conditions, including lymphedema, inflammatory diseases, and tumor dissemination. A clear understanding of the organization of the lymphatic vessels in normal conditions would be critically important to develop new treatments for diseases involving the lymphatic vascular system. Therefore, the present paper reviews the organization of the lymphatic vascular system of a variety of organs, including the thyroid gland, lung and pleura, small intestine, cecum and colon in the rat, the diaphragm in the rat, monkey, and human, Peyer's patches and the appendix in the rabbit, and human tonsils. Methods employed include scanning electron microscopy of lymphatic corrosion casts and tissues with or without treatment of alkali maceration technique, transmission electron microscopy of intact tissues, confocal microscopy in conjunction with immunohistochemistry to some lymphatic-specific markers (i.e., LYVE-1 and VEGFR-3), and light microscopy in conjunction with enzyme-histochemistry to 5'-nucleotidase. Some developmental aspects of the lymphatic vessels and lymphedema are also discussed.

The desmosome and pemphigus

Desmosomes are patch-like intercellular adhering junctions ("maculae adherentes"), which, in concert with the related adherens junctions, provide the mechanical strength to intercellular adhesion. Therefore, it is not surprising that desmosomes are abundant in tissues subjected to significant mechanical stress such as stratified epithelia and myocardium. Desmosomal adhesion is based on the Ca(2+)-dependent, homo- and heterophilic transinteraction of cadherin-type adhesion molecules. Desmosomal cadherins are anchored to the intermediate filament cytoskeleton by adaptor proteins of the armadillo and plakin families. Desmosomes are dynamic structures subjected to regulation and are therefore targets of signalling pathways, which control their molecular composition and adhesive properties. Moreover, evidence is emerging that desmosomal components themselves take part in outside-in signalling under physiologic and pathologic conditions. Disturbed desmosomal adhesion contributes to the pathogenesis of a number of diseases such as pemphigus, which is caused by autoantibodies against desmosomal cadherins. Beside pemphigus, desmosome-associated diseases are caused by other mechanisms such as genetic defects or bacterial toxins. Because most of these diseases affect the skin, desmosomes are interesting not only for cell biologists who are inspired by their complex structure and molecular composition, but also for clinical physicians who are confronted with patients suffering from severe blistering skin diseases such as pemphigus. To develop disease-specific therapeutic approaches, more insights into the molecular composition and regulation of desmosomes are required.

Structure and function of desmosomes

Desmosomes are prominent adhesion sites that are tightly associated with the cytoplasmic intermediate filament cytoskeleton providing mechanical stability in epithelia and also in several nonepithelial tissues such as cardiac muscle and meninges. They are unique in terms of ultrastructural appearance and molecular composition with cell type-specific variations. The dynamic assembly properties of desmosomes are important prerequisites for the acquisition and maintenance of tissue homeostasis. Disturbance of this equilibrium therefore not only compromises mechanical resilience but also affects many other tissue functions as becomes evident in various experimental scenarios and multiple diseases.

Characterization of a novel type of adherens junction in meningiomas and the derived cell line HBL-52

Adhering junctions are generally grouped into desmosomes and adherens junctions based on their ultrastructural appearance and molecular composition. The armadillo-protein plakoglobin is common to both types of junctions, which are otherwise composed of mutually exclusive proteins. This view is based on observations in epithelial tissues but cannot easily be transferred to other cell types and tissues, as has become apparent during the last decade with the identification of new junctional proteins and the investigation of further non-epithelial junctions. Using a broad array of well-characterized specific antibodies against key junctional proteins in immunoblot reactions, high-resolution double-label laser scanning confocal microscopy, and immunoelectron microscopy, we describe a new type of adherens junction in human meningiomas and the human meningioma cell line HBL-52. This novel junction has a unique composition of proteins not found in any other tissue; it contains the desmosomal armadillo-protein plakophilin 2 together with the classic proteins of "epithelial" adherens junctions, i.e., E-cadherin (in some instances replaced by N-cadherin), alpha-catenin, beta-catenin, plakoglobin, and p120(ctn). Ultrastructurally, it is formed between two or three neighboring cells. For pragmatic reasons, we suggest the name "meningeal junction" for this new structure.

Endothelial adherens and tight junctions in vascular homeostasis, inflammation and angiogenesis

Endothelial cells lining the vessel wall are connected by adherens, tight and gap junctions. These junctional complexes are related to those found at epithelial junctions but with notable changes in terms of specific molecules and organization. Endothelial junctional proteins play important roles in tissue integrity but also in vascular permeability, leukocyte extravasation and angiogenesis. In this review, we will focus on specific mechanisms of endothelial tight and adherens junctions.

Lymphatic endothelial cells, tumor lymphangiogenesis and metastasis: New insights into intratumoral and peritumoral lymphatics

Lymphatic metastasis of tumor cells represents a series of extremely complex and sequential processes that include dissemination and invasion into surrounding stromal tissues from primary tumors, penetration into lymphatic walls and implantation in regional lymph nodes, and extravasation or proliferation in parenchyma of target organs. Recent developments in lymphatic biology and research, especially the application of unique molecular markers specific for lymphatic endothelial cells (LECs), LYVE-1, Prox-1 and podoplanin have provided exciting new insights into the tumor microenvironment and LEC-tumor cell interface. To date, established factors for determining the behavior and prognosis of primary tumors have been emphasized morphologically and physiologically, i.e., lymphatic impairment and vessel density, dysfunction of lymphatic valves, interstitial fluid pressure, as well as a series of lymphangiogenic growth factors including VEGF-C/-D, and other cytokines and chemokines. Increasing knowledge of the tumor biological significance in lymphatics within the tumors (intratumoral lymphatics, ITLs) and at the tumor periphery (peritumoral lymphatics, PTLs) has greatly promoted understanding of tumor access into the lymphatic system by inducing lymphangiogenesis or by co-opting preexisting lymphatics. Therefore, the targeting PTLs and ITLs, which have been proposed as an important route for antimetastatic approach, are deemed worthy of further study in various animal tumor models and human tumors.

Lymphatic Endothelial Cells, Lymphangiogenesis, and Extracellular Matrix

Exciting studies involving the molecular regulation of lymphangiogenesis in lymphatic-associated disorders (e.g., wound healing, lymphedema and tumor metastasis) have focused renewed attention on the intrinsic relationship between lymphatic endothelial cells (LECs) and extracellular matrix (ECM) microenvironment. ECM molecules and remodeling events play a key role in regulating lymphangiogenesis, and the "functionality"-relating molecules, especially hyaluronan, integrins, reelin, IL-7, and matrix metalloproteinases, provide the most fundamental and critical prerequisite for LEC growth, migration, tube formation, and survival, although lymphangiogenesis is directly or/and indirectly controlled by VEGF-C/-D/VEGFR- 3- Prox-1-, Syk/SLP76-, podoplanin/Ang-2/Nrp-2-, FOXC2-, and other signaling pathways in embryonic and pathological processes. New knowledge regarding the differentiation of initial lymphatics should enable improvements in understanding of a variety of cytokines, chemokines, and other factors. The lymphatic colocalization with histochemical staining by using the novel molecular markers (e.g., LYVE-1), along with subsequent injection technique with ferritin or some tracer, will reveal functional and structural features of newly formed and preexisting lymphatics. Growing recognition of the multiple functions of ECM and LEC molecules for important physiological and pathological events may be helpful in identifying the crucial changes in tissues subjected to lymph circulation and ultimately in the search for rational therapeutic approaches to prevent lymphatic-associated disorders.

The area composita of adhering junctions connecting heart muscle cells of vertebrates. II. Colocalizations of desmosomal and fascia adhaerens molecules in the intercalated disk

Using immunofluorescence histochemistry and immunoelectron microscopy on sections through myocardiac tissues of diverse mammalian (human, cow, rat, mouse) and fish species we show that both desmosomal and fascia adhaerens proteins identified by gel electrophoresis and immunoblot occur in the area composita, the by far major type of plaque-bearing junctions of the intercalated disks (IDs) connecting cardiomyocytes. Specifically, we demonstrate that desmoplakin and the other desmosomal proteins occur in these junctions, together with N-cadherin, cadherin-11, alpha- and beta-catenin as well as vinculin, afadin and proteins p120(ctn), ARVCF, p0071, and ZO-1, suggestive of colocalization. We conclude that the predominant type of adhering junction present in IDs is a junction sui generis, termed area composita, that is characterized by an unusually high molecular complexity and an intimate association of molecules of both ensembles, the desmosomal one and the fascia adhaerens category. We discuss possible myocardium-specific, complex-forming interactions between members of the two ensembles and the relevance of our findings for the formation and functioning of the heart and for the understanding of hereditary and other cardiomyopathies. We further propose to use this highly characteristic area composita ensemble of molecules as cardiomyocyte markers for the monitoring of cardiomyogenesis, cardiomyocyte regeneration and possible cardiomyocyte differentiation from mesenchymal stem cells.

The complexus adhaerens of mammalian lymphatic endothelia revisited: a junction even more complex than hitherto thought

The significance of a special kind of VE-cadherin-based, desmoplakin- and plakoglobin-containing adhering junction, originally identified in certain endothelial cells of the mammalian lymphatic system (notably the retothelial cells of the lymph node sinus and a subtype of lining endothelial cells of peripheral lymphatic vessels), has been widely confirmed and its importance in the formation of blood and lymph vessels has been demonstrated in vivo and in vitro. We have recently extended the molecular and structural characterization of the complexus adhaerens and can now report that it represents a rare and special combination of components known from three other major types of cell junction. It comprises zonula adhaerens proteins (VE-cadherin, alpha- and beta-catenin, protein p120(ctn), and afadin), desmosomal plaque components (desmoplakin and plakoglobin), and tight-junction proteins (claudin-5 and ZO-1) and forms junctions that vary markedly in size and shape. The special character and the possible biological roles of the complexus adhaerens and its unique ensemble of molecules in angiogenesis, immunology, and oncology are discussed. The surprising finding of claudin-5 and protein ZO-1 in substructures of retothelial cell-cell bridges, i.e. structures that do not separate different tissues or cell layer compartments, suggests that such tight-junction molecules are involved in functions other than the "fence" and "barrier" roles of zonulae occludentes.

Isolation and Characterization of A Novel Mouse Lymphatic Endothelial Cell Line: SV-LEC

BACKGROUND

The lymphatic system regulates interstitial fluid and protein balance and modulates immune responses by regulating leukocyte and antigen traffic to lymph nodes. The present article describes a stable mouse lymphatic endothelial cell line from mesenteric adventitial tissue (SV-LEC) which is distinct from blood aortic (AEC) and venous (VEC) endothelial cells, based on expression of several lymphatic markers (e.g., Prox-1, LYVE-1, Flt-4). SV-LEC also expresses MAdCAM-1 in response to TNF-alpha, an effect seen in VEC, but not AEC.

METHODS AND RESULTS

Lymphatic endothelial cells (SV-LEC) were isolated from mesenteric adventitia from mice expressing temperature-sensitive SV40 large T ('Immortomouse', H-2K(b)tsA58) selected with hypoxia culture in D-valine-substituted MEM supplemented with VEGFC in a low oxygen atmosphere (0% O2, 5% CO2, and 95% N2) with 5 mM thioglycolate. Expression of lymphatic-specific markers (Flt-4, LYVE-1, Prox-1) and the tight junction proteins (ZO-1) were examined by RT-PCR, immunoblotting, and fluorescent microscopy. MAdCAM-1 (a high endothelial venular marker) expression was also examined in response to TNF-alpha IL-1beta and IFN-gamma.

RESULTS

Message for Flt-4 and LYVE-1 was detected on SV-LEC. Immunoblotting for LYVE-1 and Prox-1 showed strong expression on SV-LEC and VEC, but not AEC. Occludin expression was seen in all cell types, junctional ZO-1 was detected at SV-LEC and VEC junctions, not AEC.

CONCLUSION

SV-LEC expresses several lymphatic endothelial markers, some of which are shared with VEC, but not AEC, and may represent a useful system for modeling lymphatic function in vitro.

Plakophilins--hard work in the desmosome, recreation in the nucleus?

The linkage of the different types of cytoskeletal proteins to cell adhesion structures at the cytoplasmic membrane and the connection of these contact sites to corresponding sites of adjacent cells is a prerequisite for integrity and stability of cells and tissues. The structurally most prominent types of such cell-cell adhesion complexes are the desmosomes (maculae adhaerentes), which are found in all epithelia and certain non-epithelial tissues. As an element of the cytoskeleton, intermediate filaments are connected to the adhesive desmosomal transmembrane proteins by the cytoplasmic desmosomal plaque proteins. At least three different types of proteins are found in the desmosomal plaque, one of which is represented by the plakophilins, a recently described sub-family of sequence-related armadillo-repeat proteins. Consisting of three isoforms, plakophilins (plakophilin 1 to 3, PKP 1 to 3) are located in all desmosomes in a differentiation-dependent manner. While PKP 2 and PKP 3 are part of almost all desmosome-bearing cell types (PKP 2 except for differentiated cells of stratified epithelia and PKP 3 for hepatocytes and cardiomyocytes), PKP 1 is restricted to desmosomes of cells of stratified and complex epithelia. Besides the architectural function that plakophilins seem to fulfill in the desmosomes, at least PKP 1 and 2 are also localized in the nucleus independently of any differentiation-related processes and with an up to now enigmatic function in this compartment. In the following article we want to summarize the current knowledge concerning structure, function and regulation of the plakophilins that has been achieved during the last decade.

Molecular and cellular mechanisms of lymphangiogenesis

Lymphangiogenesis is the growth and formation of new lymphatic vessels. It occurs in normally developing tissues and in pathological processes like inflammation, wound healing, lymphoedema and in cancer. New molecular markers that are specific to the lymphatic endothelium include: podoplanin, prox-1 and LYVE-1. The molecular mechanisms of lymphangiogenesis are not clear, but vascular endothelial growth factors (VEGF-C and VEGF-D) within tumours may simulate endothelial cells within tumour tissues to grow and generate new lymphatics. We report the current knowledge of molecular and cellular mechanisms of lymphangiogenesis.

Number 1Epithelial biology

The oral mucous membrane has features similar to skin but also differs in several ways. This paper reviews the aspects of epithelial biology necessary for an understanding of the vesiculoerosive disorders.

Desmosomal cell adhesion in mammalian development

Defects in desmosome-mediated cell-cell adhesion can lead to tissue fragility syndromes. Both inherited and acquired diseases caused by desmosomal defects have been described. The two organs that appear most vulnerable to these defects are the skin with its appendages, and the heart. Furthermore, the analysis of genetically engineered mice has led to the discovery that desmosomal proteins are also required for normal embryonic development. Knockout mice for several desmosomal proteins die in utero. Depending on the protein studied, death occurs either around the time of implantation, at mid-gestation or shortly before birth. So far, it appears that structural defects leading to abnormal histo-architecture and tissue fragility are the main cause of death, i.e. there is no evidence that loss of a desmosomal protein would abort specific cell lineages or differentiation programs. Nevertheless, we are only beginning to understand the functions of individual desmosomal proteins during development. This review focuses on the role of desmosomes during mouse embryonic development.

The second valve system in lymphatics

The mechanism for interstitial fluid uptake into the lymphatics of the microcirculation remains speculative and uncertain. There exists a system of intralymphatic valves that prevent reflow along the length of the lymphatic channels, but these valves are insufficient to provide unidirectional flow at the level of the initial lymphatics. We propose that initial lymphatics have a two-valve system: a (primary) valve system at the level of the endothelium in addition to the classical (secondary) intralymphatic valves. The primary valves, in conjunction with the secondary valves, provide a mechanism that facilitates the unidirectional flow during periodic compression and expansion of initial lymphatics.

Interleukin-1β Reduces Transcellular Monocyte Diapedesis and Compromises Endothelial Adherens Junction Integrity

OBJECTIVE

Diapedesis occurs through endothelial cell-cell junctions (paracellular) or through individual endothelial cells without disrupting junctions (transcellular). While in vitro studies have provided considerable insight into mechanisms controlling paracellular diapedesis, little is known about what regulates transcellular diapedesis. The authors investigated whether transcellular diapedesis is susceptible to IL-1beta exposure of the endothelium.

METHODS

Laser scanning confocal microscopy and biochemical analysis were used to determine the effect of IL-1beta pretreatment of the endothelium on adherens junctional morphology and monocyte transcellular diapedesis in cocultures of human peripheral blood monocytes and coronary artery endothelial cells.

RESULTS

IL-1beta pretreatment caused a 40% decrease in the number of migrating monocytes that used a transcellular route of diapedesis, and resulted in elongate endothelial cell morphology, a reorganization of the F-actin cytoskeleton, and a significant decrease in transendothelial electrical resistance. In IL-1beta treated monolayers, VE-cadherin and its associated catenins were distributed in a punctate pattern in comparison to the lacy pattern seen in control monolayers. Coimmunoprecipitation of VE-cadherin molecular assemblies revealed that IL-1beta-mediated changes in distribution were associated with a decrease in the presence of cadherin/catenin complexes in the detergent insoluble fraction.

CONCLUSIONS

IL-1beta-induced rearrangement of interendothelial adherens junctions facilitates paracellular diapedesis at the expense of transcellular diapedesis.

Requirement of plakophilin 2 for heart morphogenesis and cardiac junction formation

Plakophilins are proteins of the armadillo family that function in embryonic development and in the adult, and when mutated can cause disease. We have ablated the plakophilin 2 gene in mice. The resulting mutant mice exhibit lethal alterations in heart morphogenesis and stability at mid-gestation (E10.5–E11), characterized by reduced trabeculation, disarrayed cytoskeleton, ruptures of cardiac walls, and blood leakage into the pericardiac cavity. In the absence of plakophilin 2, the cytoskeletal linker protein desmoplakin dissociates from the plaques of the adhering junctions that connect the cardiomyocytes and forms granular aggregates in the cytoplasm. By contrast, embryonic epithelia show normal junctions. Thus, we conclude that plakophilin 2 is important for the assembly of junctional proteins and represents an essential morphogenic factor and architectural component of the heart.

Endothelial cell-to-cell junctions: molecular organization and role in vascular homeostasis

Intercellular junctions mediate adhesion and communication between adjoining endothelial and epithelial cells. In the endothelium, junctional complexes comprise tight junctions, adherens junctions, and gap junctions. The expression and organization of these complexes depend on the type of vessels and the permeability requirements of perfused organs. Gap junctions are communication structures, which allow the passage of small molecular weight solutes between neighboring cells. Tight junctions serve the major functional purpose of providing a "barrier" and a "fence" within the membrane, by regulating paracellular permeability and maintaining cell polarity. Adherens junctions play an important role in contact inhibition of endothelial cell growth, paracellular permeability to circulating leukocytes and solutes. In addition, they are required for a correct organization of new vessels in angiogenesis. Extensive research in the past decade has identified several molecular components of the tight and adherens junctions, including integral membrane and intracellular proteins. These proteins interact both among themselves and with other molecules. Here, we review the individual molecules of junctions and their complex network of interactions. We also emphasize how the molecular architectures and interactions may represent a mechanistic basis for the function and regulation of junctions, focusing on junction assembly and permeability regulation. Finally, we analyze in vivo studies and highlight information that specifically relates to the role of junctions in vascular endothelial cells.

Transendothelial transport and migration in vessels of the apparatus lymphaticus periphericus absorbens (ALPA)

The vessel of the apparatus lymphaticus periphericus absorbens (ALPA) represents the sector with high absorption capacity of the canalization of the lymphatic vascular system. It plays a basic role in preserving tissue homeostasis and in directing interstitial capillary filtrate back to the bloodstream. ALPA lymphatic endothelium differs from the endothelia of conduction and flowing vessels (precollectors, prelymph nodal and postlymph nodal collectors, main trunks), since it presents a discontinuous basement membrane, which is often absent, and lacks pores and fenestrations. The mesenchymal origin of the ALPA lymphatic vessel, morphological and ultrastructural aspects, intrinsic contractile properties, the presence of valves, innervation, and specific lymphatic markers that reliably distinguish it from blood capillaries are studied. Furthermore, its role in lymph formation through different mechanisms (hydrostatic pressure and colloidal osmotic-reticular mechanisms, vesicular pathway, and intraendothelial channel) is investigated. We have studied morphological and biomolecular mechanisms that control the transendothelial migration, from the extracellular interstitial matrix into the lumen of the lymphatic vessel, of cells involved in immune response and resistance (lymphocyte recirculation, etc.) and in the tumoral metastatic process via the lymphatic system. Finally, future research prospects, clinical implications, and therapeutic strategies are considered.

Desmoplakin and Plakoglobin - Specific Markers of Lymphatic Vessels in the Skin?

Monoclonal antibodies against Desmoplakin and Plakoglobin were tested for their suitability as specific markers of lymphatic vessels. The tissue samples were taken from horse skin in an attempt to establish the horse as a model for human lymphatic diseases. To obtain a clear, positive identification of blood and lymphatic vessels, immunohistochemical staining with antibodies against vascular endothelial growth factor receptor 3 (VEGFR-3) and platelet endothelial adhesion molecule (PECAM-1, CD31), was compared with Desmoplakin and Plakoglobin. Because anti-VEGFR-3 is specific for lymphatic vessels in the skin while anti-CD31 stains blood and lymphatic vessels as well, it can be concluded that VEGFR-3(-)/CD31(+) vessels are blood vessels and VEGFR-3(+)/CD31(+) vessels are lymphatic vessels. It was documented on serial sections that Plakoglobin stains both blood and lymphatic vessels. However, Desmoplakin did not stain several positively identified lymphatic vessels. Therefore, Desmoplakin and Plakoglobin antibodies are not specific markers of lymphatic vessels in the skin and the staining pattern is tissues and species dependent.

Lymphatics at the crossroads of angiogenesis and lymphangiogenesis

The lymphatic system is implicated in interstitial fluid balance regulation, immune cell trafficking, oedema and cancer metastasis. However, the sequence of events that initiate and coordinate lymphatic vessel development (lymphangiogenesis) remains obscure. In effect, the understanding of physiological regulation of lymphatic vasculature has been overshadowed by the greater emphasis focused on angiogenesis, and delayed by a lack of specific markers, thereby limiting this field to no more than a descriptive characterization. Recently, new insights into lymphangiogenesis research have been due to the discovery of lymphatic-specific markers and growth factors of vascular endothelial growth factor (VEGF) family, such as VEGF-C and VEGF-D. Studies using transgenic mice overexpressing VEGF-C and VEGF-D have demonstrated a crucial role for these factors in tumour lymphangiogenesis. Knowledge of lymphatic development has now been redefined at the molecular level, providing an interesting target for innovative therapies. This review highlights the recent insights and advances into the field of lymphatic vascular research, outlining the most important aspects of the embryo development, structure, specific markers and methods applied for studying lymphangiogenesis. Finally, molecular mechanisms involved in the regulation of lymphangiogenesis are described.

VE-cadherin: adhesion at arm's length

VE-cadherin was first identified in the early 1990s and quickly emerged as an important endothelial cell adhesion molecule. The past decade of research has revealed key roles for VE-cadherin in vascular permeability and in the morphogenic events associated with vascular remodeling. The details of how VE-cadherin functions in adhesion became apparent with structure-function analysis of the cadherin extracellular domain and with the identification of the catenins, a series of cytoplasmic proteins that bind to the cadherin tail and mediate interactions between cadherins and the cytoskeleton. Whereas early work focused on the armadillo family proteins beta-catenin and plakoglobin, more recent investigations have identified p120-catenin (p120(ctn)) and a related group of armadillo family members as key binding partners for the cadherin tail. Furthermore, a series of new studies indicate a key role for p120(ctn) in regulating cadherin membrane trafficking in mammalian cells. These recent studies place p120(ctn) at the hub of a cadherin-catenin regulatory mechanism that controls cadherin plasma membrane levels in cells of both epithelial and endothelial origin.

Lymphatic vasculature development

Although the process of blood vasculature formation has been well documented, little is known about lymphatic vasculature development, despite its importance in normal and pathological conditions. The lack of specific lymphatic markers has hampered progress in this field. However, the recent identification of genes that participate in the formation of the lymphatic vasculature denotes the beginning of a new era in which better diagnoses and therapeutic treatment(s) of lymphatic disorders could become a reachable goal.

The conditional inactivation of the beta-catenin gene in endothelial cells causes a defective vascular pattern and increased vascular fragility

Using the Cre/loxP system we conditionally inactivated beta-catenin in endothelial cells. We found that early phases of vasculogenesis and angiogenesis were not affected in mutant embryos; however, vascular patterning in the head, vitelline, umbilical vessels, and the placenta was altered. In addition, in many regions, the vascular lumen was irregular with the formation of lacunae at bifurcations, vessels were frequently hemorrhagic, and fluid extravasation in the pericardial cavity was observed. Cultured beta-catenin -/- endothelial cells showed a different organization of intercellular junctions with a decrease in alpha-catenin in favor of desmoplakin and marked changes in actin cytoskeleton. These changes paralleled a decrease in cell-cell adhesion strength and an increase in paracellular permeability. We conclude that in vivo, the absence of beta-catenin significantly reduces the capacity of endothelial cells to maintain intercellular contacts. This may become more marked when the vessels are exposed to high or turbulent flow, such as at bifurcations or in the beating heart, leading to fluid leakage or hemorrhages.

Submucosal territory of the direct lymphatic drainage system to the thoracic duct in the human esophagus

OBJECTIVE

To investigate how large submucosal drainage territory extends in lymphatic drainage vessels of the esophagus with and without nodal delay and which morphologies are shown when passing through the muscularis propria.

METHODS

Submucosal territories of the 22 highly selected direct drainage vessels of 17 esophagi were histologically examined using transverse or sagittal serial sections. Afferent vessels from the esophagus to the subcarinal (6 esophagi) and para-esophageal (5 esophagi) nodes were also examined to identify their courses and drainage territories.

RESULTS

We found the direct drainage vessel from the esophagus in 17 of 75 cadavers macroscopically (22.7%). A single submucosal drainage unit gave off 1-3 thick drainage vessels passing through a complete muscle gap of the 2 muscular layers. The unit extended longitudinally for >40 mm but was restricted to the right and/or dorsal quadrants of the esophagus. In contrast, drainage routes with a nodal relay originated from the intermuscular area, except 1 case when the adjacent or concomitant esophageal artery and vein provided the complete muscle gap.

CONCLUSIONS

Due to the extended longitudinal but restricted transverse territory of the direct drainage system without a nodal relay and because of the suggested much more frequent occurrence in patients than in cadavers, when superficial carcinoma is found in the dorsal and/or right quadrants of the esophagus, we recommend detailed presurgical investigations of cervical nodes. In contrast, afferents from the esophagus to the first regional node usually seemed to be less responsible for early nodal metastasis than the direct drainage route because of their intermuscular origins.

A model for mechanics of primary lymphatic valves

Recent experimental evidence indicates that lymphatics have two valve systems, a set of primary valves in the wall of the endothelial cells of initial lymphatics and a secondary valve system in the lumen of the lymphatics. While the intralymphatic secondary valves are well described, no analysis of the primary valves is available. We propose a model for primary lymphatics valves at the junctions between lymphatic endothelial cells. The model consists of two overlapping endothelial extensions at a cell junction in the initial lymphatics. One cell extension is firmly attached to the adjacent connective tissue while the other cell extension is not attached to the interstitial collagen. It is free to bend into the lumen of the lymphatic when the lymphatic pressure falls below the adjacent interstitial fluid pressure. Thereby the cell junction opens a gap permitting entry of interstitial fluid into the lymphatic lumen. When the lymphatic fluid pressure rises above the adjacent interstitial fluid pressure, the endothelial extensions contact each other and the junction is closed preventing fluid reflow into the interstitial space. The model illustrates the mechanics of valve action and provides the first time a rational analysis of the mechanisms underlying fluid collection in the initial lymphatics and lymph transport in the microcirculation.

Mechanisms of VE-cadherin processing and degradation in microvascular endothelial cells

VE-cadherin is an endothelial-specific cadherin that plays important roles in vascular morphogenesis and growth control. To investigate the mechanisms by which endothelial cells regulate cadherin cell surface levels, a VE-cadherin mutant containing the non-adhesive interleukin-2 (IL-2) receptor extracellular domain and the VE-cadherin cytoplasmic tail (IL-2R-VE-cadcyto) was expressed in microvascular endothelial cells. Expression of the IL-2R-VE-cadcyto mutant resulted in the internalization of endogenous VE-cadherin and in a dramatic decrease in endogenous VE-cadherin levels. The internalized VE-cadherin co-localized with early endosomes, and the lysosomal inhibitor chloroquine dramatically inhibited the down-regulation of VE-cadherin in cells expressing the IL-2R-VE-cadcyto mutant. Chloroquine treatment also resulted in the accumulation of a VE-cadherin fragment lacking the beta-catenin binding domain of the VE-cadherin cytoplasmic tail. The formation of the VE-cadherin fragment could be prevented by treating endothelial cells with proteasome inhibitors. Furthermore, inhibition of the proteasome prevented VE-cadherin internalization and inhibited the disruption of endothelial intercellular junctions by the IL-2RVE-cadcyto mutant. These results provide new insights into the mechanisms of VE-cadherin processing and degradation in microvascular endothelial cells.

De novo formation of desmosomes in cultured cells upon transfection of genes encoding specific desmosomal components

Desmosomes are cell junctions and cytoskeleton-anchoring structures of epithelia, the myocardium, and dendritic reticulum cells of lymphatic follicles whose major components are known. Using cultured HT-1080 SL-1 fibrosarcoma-derived cells and transfection of cDNAs encoding specific desmosomal components, we have determined a minimum ensemble of proteins sufficient to introduce de novo structures, which, by morphology and functional competence, are indistinguishable from authentic desmosomes. In a more refined analysis, the influence of the desmosomal proteins desmoplakin (Dp), plakoglobin (Pg), and plakophilin 2 (Pp2) on the lateral clustering of the desmosomal transmembrane-glycoprotein desmoglein 2 (Dsg) was examined. We found that for efficient clustering of desmoglein 2 and desmosome structure formation, all three major plaque proteins-desmoplakin, plakoglobin, and plakophilin 2- were necessary. Furthermore, in this cell model, plakophilin 2 was capable of directing desmoplakin to adhaerens junctions (AJ), whereas plakoglobin was crucial for the segregation of desmosomal and AJ components. These results are discussed with respect to the variability in cell junction composition observed in various nonepithelial tissues.

The Armadillo family protein p0071 is a VE-cadherin- and desmoplakin-binding protein

p0071, a member of the armadillo protein family, localizes to both adherens junctions and desmosomes in epithelial cells and exhibits homology to the adherens junction protein p120 and the desmosomal protein plakophilin-1. p0071 is also present at dermal microvascular endothelial intercellular junctions and colocalizes with VE-cadherin, an endothelium-specific cadherin that associates with both actin and intermediate filament networks. To define the role of p0071 in junction assembly, p0071 was tested for interactions with other components of the endothelial junctional complex. In transient expression assays, p0071 colocalized with and formed complexes with both VE-cadherin and desmoplakin. Deletion analysis using the yeast two-hybrid system revealed that the armadillo repeat domain of p0071 bound directly to VE-cadherin. Site-directed mutagenesis experiments demonstrated that p0071 and p120 bound to the same region on the cytoplasmic tail of VE-cadherin and that overexpression of p0071 could displace p120 from intercellular junctions. In contrast to VE-cadherin, desmoplakin was found to associate with the non-armadillo head domain of p0071. Cotransfections and triple-label immunofluorescence analysis revealed that VE-cadherin colocalization with desmoplakin in transfected COS cells required p0071, suggesting that p0071 may couple VE-cadherin to desmoplakin. Based on previous findings that both VE-cadherin and desmoplakin play central roles in vasculogenesis, these new results suggest that p0071 may play an important role in endothelial junction assembly and in the morphogenic events associated with vascular remodeling.

Molecular characterization of lymphatic endothelial cells

The lymphatic microvasculature is uniquely adapted for the continuous removal of interstitial fluid and proteins and is an important entry point for leukocytes and tumor cells. Specialized functions of lymphatics suggest differences in the molecular composition of the lymphatic and blood vascular endothelium. However, the extent to which the two cell types differ is still unclear, and few molecules that are truly specific to lymphatic endothelial cells have been identified to date. We have isolated primary lymphatic and blood microvascular endothelial cells from human skin by immunoselection with the lymphatic marker LYVE-1 and demonstrate that the two cell lineages express distinct sets of vascular markers and respond differently to growth factors and extracellular matrix. Comparative microarray analysis of gene-expression profiles revealed a number of unique molecular properties that distinguish lymphatic and blood vascular endothelium. The molecular profile of lymphatic endothelium seems to reflect characteristic functional and structural features of the lymphatic capillaries. Classification of the differentially expressed genes into functional groups revealed particularly high levels of genes implicated in protein sorting and trafficking, indicating a more active role of lymphatic endothelium in uptake and transport of molecules than previously anticipated. The identification of a large number of genes selectively expressed by lymphatic endothelium should facilitate the discovery of hitherto unknown lymphatic vessel markers and provide a basis for the analysis of the molecular mechanisms accounting for the characteristic functions of lymphatic capillaries.

Regulation of endothelial barrier function and growth by VE-cadherin, plakoglobin, and beta-catenin

VE-cadherin is an endothelial-specific cadherin that plays a central role in vascular barrier function and angiogenesis. The cytoplasmic domain of VE-cadherin is linked to the cytoskeleton through interactions with the armadillo family proteins beta-catenin and plakoglobin. Growing evidence indicates that beta-catenin and plakoglobin play important roles in epithelial growth and morphogenesis. To test the role of these proteins in vascular cells, a replication-deficient retroviral system was used to express intercellular junction proteins and mutants in the human dermal microvascular endothelial cell line (HMEC-1). A mutant VE-cadherin lacking an adhesive extracellular domain disrupted endothelial barrier function and inhibited endothelial growth. In contrast, expression of exogenous plakoglobin or metabolically stable mutants of beta-catenin stimulated HMEC-1 cell growth, which suggests that the beta-catenin signaling pathway was active in HMEC-1 cells. This possibility was supported by the finding that a dominant-negative mutant of the transcription factor TCF-4, designed to inhibit beta-catenin signaling, also inhibited HMEC-1 cell growth. These observations suggest that intercellular junction proteins function as components of an adhesion and signaling system that regulates vascular barrier function and growth.