Embryonic heart mesenchymal cell migration in laminin

Dissecting the Role of the Extracellular Matrix in Heart Disease: Lessons from the Drosophila Genetic Model

The extracellular matrix (ECM) is a dynamic scaffold within organs and tissues that enables cell morphogenesis and provides structural support. Changes in the composition and organisation of the cardiac ECM are required for normal development. Congenital and age-related cardiac diseases can arise from mis-regulation of structural ECM proteins (Collagen, Laminin) or their receptors (Integrin). Key regulators of ECM turnover include matrix metalloproteinases (MMPs) and their inhibitors, tissue inhibitors of matrix metalloproteinases (TIMPs). MMP expression is increased in mice, pigs, and dogs with cardiomyopathy. The complexity and longevity of vertebrate animals makes a short-lived, genetically tractable model organism, such as Drosophila melanogaster, an attractive candidate for study. We survey ECM macromolecules and their role in heart development and growth, which are conserved between Drosophila and vertebrates, with focus upon the consequences of altered expression or distribution. The Drosophila heart resembles that of vertebrates during early development, and is amenable to in vivo analysis. Experimental manipulation of gene function in a tissue- or temporally-regulated manner can reveal the function of adhesion or ECM genes in the heart. Perturbation of the function of ECM proteins, or of the MMPs that facilitate ECM remodelling, induces cardiomyopathies in Drosophila, including cardiodilation, arrhythmia, and cardia bifida, that provide mechanistic insight into cardiac disease in mammals.

Collagen gel analysis of epithelial-mesenchymal transition in the embryo heart: an in vitro model system for the analysis of tissue interaction, signal transduction, and environmental effects

The cellular process of epithelial-mesenchymal cell transition (EMT) is a critical event in development that is reiterated in adult pathologies of metastasis and organ fibrosis. An initial understanding of the cellular and molecular events of this process emerged from an in vitro examination of heart valve development. Explants of the chick atrioventricular valve-forming region were placed on collagen gels and removed to show that EMT was regulated by a tissue interaction. Subsequent studies showed that specific TGFβ isoforms and receptors were required and steps of activation and invasion could be distinguished. The assay was modified for mouse hearts and has been used to explore signal transduction and gene expression in both species. The principle advantages of the system are a defined temporal window, when EMT takes place and the ability to isolate cells at various stages of the EMT process. These advantages are largely unavailable in other developmental or adult models. As the mesenchymal cells produced by EMT in the heart are involved in defects found in congenital heart disease, there is also a direct relevance of cardiac EMT to human birth defects. This relationship has been explored in relation to environmental exposures and in a number of genetic models. This review provides both an overview of the findings developed from the assay and protocols to enable the use of the assay by other laboratories. The assay provides a versatile platform to explore roles of specific gene products, drugs, and environmental agents on a critical cellular process.

Imaging cardiac extracellular matrices: a blueprint for regeneration

Once damaged, cardiac tissue does not readily repair and is therefore a primary target of regenerative therapies. One regenerative approach is the development of scaffolds that functionally mimic the cardiac extracellular matrix (ECM) to deliver stem cells or cardiac precursor populations to the heart. Technological advances in micro/nanotechnology, stem cell biology, biomaterials and tissue decellularization have propelled this promising approach forward. Surprisingly, technological advances in optical imaging methods have not been fully utilized in the field of cardiac regeneration. Here, we describe and provide examples to demonstrate how advanced imaging techniques could revolutionize how ECM-mimicking cardiac tissues are informed and evaluated.

Met-Independent Hepatocyte Growth Factor-mediated regulation of cell adhesion in human prostate cancer cells

BACKGROUND

Prostate cancer cells communicate reciprocally with the stromal cells surrounding them, inside the prostate, and after metastasis, within the bone. Each tissue secretes factors for interpretation by the other. One stromally-derived factor, Hepatocyte Growth Factor (HGF), was found twenty years ago to regulate invasion and growth of carcinoma cells. Working with the LNCaP prostate cancer progression model, we found that these cells could respond to HGF stimulation, even in the absence of Met, the only known HGF receptor. The new HGF binding partner we find on the cell surface may help to clarify conflicts in the past literature about Met expression and HGF response in cancer cells.

METHODS

We searched for Met or any HGF binding partner on the cells of the PC3 and LNCaP prostate cancer cell models, using HGF immobilized on agarose beads. By using mass spectrometry analyses and sequencing we have identified nucleolin protein as a novel HGF binding partner. Antibodies against nucleolin (or HGF) were able to ameliorate the stimulatory effects of HGF on met-negative prostate cancer cells. Western blots, RT-PCR, and immunohistochemistry were used to assess nucleolin levels during prostate cancer progression in both LNCaP and PC3 models.

RESULTS

We have identified HGF as a major signaling component of prostate stromal-conditioned media (SCM) and have implicated the protein nucleolin in HGF signal reception by the LNCaP model prostate cancer cells. Antibodies that silence either HGF (in SCM) or nucleolin (on the cell surfaces) eliminate the adhesion-stimulatory effects of the SCM. Likewise, addition of purified HGF to control media mimics the action of SCM. C4-2, an LNCaP lineage-derived, androgen-independent human prostate cancer cell line, responds to HGF in a concentration-dependent manner by increasing its adhesion and reducing its migration on laminin substratum. These HGF effects are not due to shifts in the expression levels of laminin-binding integrins, nor can they be linked to expression of the known HGF receptor Met, as neither LNCaP nor clonally-derived C4-2 sub-line contain any detectable Met protein. Even in the absence of Met, small GTPases are activated, linking HGF stimulation to membrane protrusion and integrin activation. Membrane-localized nucelolin levels increase during cancer progression, as modeled by both the PC3 and LNCaP prostate cancer progression cell lines.

CONCLUSION

We propose that cell surface localized nucleolin protein may function in these cells as a novel HGF receptor. Membrane localized nucleolin binds heparin-bound growth factors (including HGF) and appears upregulated during prostate cancer progression. Antibodies against nucleolin are able to ameliorate the stimulatory effects of HGF on met-negative prostate cancer cells. HGF-nucleolin interactions could be partially responsible for the complexity of HGF responses and met expression reported in the literature.

gp38k (CHI3L1) is a novel adhesion and migration factor for vascular cells

gp38k (CHI3L1) is a secreted heparin-binding glycoprotein whose expression, in vitro, is associated with vascular smooth muscle cell (VSMC) migration and invasion into the underlying gelatinous matrix. gp38k is expressed at high levels in postconfluent "nodular" VSMC cultures and at low levels in subconfluent proliferating cultures. In vivo, expression of gp38k homologs is high in regions of tissue remodeling and now has been detected in atherosclerotic plaques and in the developing heart. We tested the hypothesis that gp38k functions to modulate VSMC adhesion and migration. By use of modified Boyden chambers, gp38k at a concentration as low as 1 ng/ml has profound effects on VSMC migration but little or no effect on fibroblast migration. In addition, gp38k adsorbed to polystyrene surfaces directly promotes VSMC attachment and spreading. Attachment is inhibited in the presence of affinity-purified anti-gp38k or 10 mM EDTA. These results establish that gp38k is a new vascular cell adhesion and migration factor that may have a role in processes leading to vascular occlusion and heart development. gp38k may interact with VSMC via an EDTA-sensitive mechanism consistent with integrin mediated cell-matrix interaction.

Fibulin-1 suppression of fibronectin-regulated cell adhesion and motility

Fibulin-1 is an extracellular matrix protein often associated with fibronectin (FN) in vivo. In this study, the ability of fibulin-1 to modulate adhesion, spreading and motility-promoting activities of FN was investigated. Fibulin-1 was found to have pronounced inhibitory effects on the cell attachment and spreading promoted by FN. Fibulin-1 was also found to inhibit the motility of a variety of cell types on FN substrata. For example, the FN-dependent haptotactic motility of breast carcinoma (MDA MB231) cells, epidermal carcinoma (A431), melanoma (A375 SM), rat pulmonary aortic smooth muscle cells (PAC1) and Chinese hamster ovary (CHO) cells was inhibited by the presence of fibulin-1 bound to FN-coated Boyden chamber membranes. Cells transfected to overproduce fibulin-1 displayed reduced velocity, distance of movement and persistence time on FN substrata. Similarly, the incorporation of fibulin-1 into FN-containing type I collagen gels inhibited the invasion of endocardial cushion mesenchymal cells migrating from cultured embryonic heart explants. By contrast, incorporation of fibulin-1 into collagen gels lacking FN had no effect on the migration of endocardial cushion cells.

These results suggest that the motility-suppressive effects of fibulin-1 might be FN specific. Furthermore, such effects are cell-type specific, in that the migration of gingival fibroblasts and endothelial cells on FN substrata is not responsive to fibulin-1. Additional studies found that the mechanism for the motility-suppressive effects of fibulin-1 does not involve perturbations of interactions between α5β1 or α4 integrins, or heparan sulfate proteoglycans with FN. However, fibulin-1 was found to inhibit extracellular signal regulated kinase (ERK) activation and to suppress phosphorylation of myosin heavy chain. This ability to influence signal transduction cascades that modulate the actin-myosin motor complex might be the basis for the effects of fibulin-1 on adhesion and motility.

Fibulin-2 expression marks transformed mesenchymal cells in developing cardiac valves, aortic arch vessels, and coronary vessels

Previous studies showed that extracellular matrix protein, fibulin-2, is expressed during epithelial-mesenchymal transformation in the endocardial cushion matrix during embryonic heart development. Our current study revealed that, in addition to the cardiac valvuloseptal formation, fibulin-2 is synthesized by the smooth muscle precursor cells of developing aortic arch vessels and the coronary endothelial cells that are originated from neural crest cells and epicardial cells, respectively. In the cardiac valves and the aortic arch vessels, fibulin-2 expression shows robust up-regulation when the transformed mesenchymal cells migrate into the existing extracellular matrix. In the epicardium, epicardial cells produce fibulin-2 upon their migration over the myocardial surface and its expression persists throughout coronary vasculogenesis and angiogenesis. Fibulin-2 is produced by the endothelial cells of coronary arteries and veins but not by the capillary endothelial cells in the myocardium. Thus, fibulin-2 not only uniquely marks the transformed mesenchymal cells during mouse embryonic cardiovascular development, but also indicates vascular endothelial cells of coronary arteries and veins in postnatal life.

Synthesis of extracellular matrix and adhesion through beta(1) integrins are critical for fetal ventricular myocyte proliferation

Extracellular matrix (ECM) regulates vascular smooth muscle cell proliferation. The role of ECM in myocardial growth is unexplored. We sought to determine whether human fetal ventricular myocytes (HFVMs) produce ECM and whether synthesis and attachment to ECM are necessary for their epidermal growth factor (EGF)-dependent and -independent proliferation. Cultured HFVMs proliferate in the presence but not absence of serum and EGF, as determined by increase in cell number and [(3)H]thymidine and [(14)C]leucine incorporation (measures of DNA and protein synthesis, respectively). Using a cyanogen bromide digestion technique to measure collagen and elastin and using affinity chromatography for fibronectin, we found that HFVMs synthesized collagen and fibronectin but not elastin. HFVMs grown on exogenous ECM (including fibronectin and type I collagen and laminin) demonstrated no change in proliferation or DNA and protein synthesis with or without EGF. However, inhibition of collagen synthesis using cis-4-hydroxyproline resulted in a decrease in EGF-related HFVM proliferation and DNA and protein synthesis, which was reversed by exposure to L-proline but not by growth on type I collagen. Use of beta(1) but not beta(3) integrin antibody to inhibit cell interaction with ECM resulted in a decrease in HFVM proliferation and DNA and protein synthesis in response to EGF. Furthermore, EGF-dependent proliferation was enhanced by alpha(1)beta(1) and alpha(5)beta(1) antibodies that act as functional ligands, but not alpha(3)beta(1), the only beta(1) subtype expressed in adult myocytes. In conclusion, proliferating HFVMs synthesize collagen and fibronectin. The proliferative response of HFVMs to EGF requires the synthesis of collagen as well as attachment to specific alpha/beta(1) integrin heterodimers.

Expression and potential role of the extracellular matrix in hepatic ontogenesis: a review

Studies from a number of laboratories have provided information on the temporal and spatial expression of a variety of extracellular matrix (ECM) components in the developing liver and insight into their potential roles in hepatogenesis. Collagen type IV and laminin are present in the basement membranes of the capsular mesothelium, vascular structures of the portal and hepatic vein branches, and the ductular elements of the developing liver. The mesothelial, vascular, and ductular epithelial cells synthesize laminin and type IV collagen. In contrast, fibronectin and type I collagen are restricted to the adjacent or surrounding interstitium of those ductal and vascular elements, but are not within the basement membrane proper. The hepatic perisinusoidal space (Space of Disse) of the fetal rat develops a delicate extracellular matrix by 12.5 days of gestation, which is characterized by banded collagen fibrils and bundles associated with filamentous and flocculent material. Fibronectin, laminin, and collagen types I, III, and IV are present in the developing perisinusoidal space by this early gestational date, with laminin being the most prevalent component detected. The laminin chains localized to that region in the fetal/neonatal period are alpha 2, beta 1, beta 2, and gamma 1, whereas the alpha 1 chain of laminin is absent from the developing Space of Disse. Similar data have been reported on the laminin phenotype in the perisinusoidal space during hepatic regeneration. Electron microscopy immunohistochemistry studies have demonstrated that the sinusoidal lining cells and hepatocytes synthesize these ECM proteins during hepatogenesis. By 6 to 8 weeks of postnatal life, laminin is not detectable in the perisinusoidal space. Both the transient expression of laminin and the similarity of the laminin chain phenotype expressed in the perisinusoidal space in the developing and regenerating liver suggests a role for this protein in the organization of the hepatic lobule in those forms of hepatic morphogenesis.

Expression patterns of laminin receptor splice variants alpha 6A beta 1 and alpha 6B beta 1 suggest different roles in mouse development

The alpha 6 beta 1 integrin is a receptor for laminins and is present from early stages of mouse embryogenesis. In the present study we determined the temporal and spatial expression of the two cytoplasmic splice variants of the alpha 6 integrin subunit, alpha 6A and alpha 6B, in the early- and mid-gestation mouse postimplantation embryo using RT-PCR, in situ hybridization, and immunofluorescence. Our results show that alpha 6B is present in the embryo at all stages studied and is expressed before alpha 6A. alpha 6A expression begins in 8.5 day p.c. embryos and is initially exclusively localized to the developing heart. In 8.5 (and 9.5) day p.c. embryos alpha 6A mRNA and protein are present in a gradient in the myocardium of the heart tube from strongest expression in the sinus venosus and in the common atrial chamber to a weakening expression along the ventricle and bulbus cordis. In 10.5 day p.c. embryos this gradient is less evident and in 12.5 day p.c. embryos alpha 6A mRNA and protein are present in comparable amounts between atria and ventricles. Neither alpha 6A nor alpha 6B is present in endocardial cushion tissue. By day 12.5 p.c. alpha 6A expression is also present in the developing epidermis, dental primordia, lens, gonads, and in a few epithelia such as those of the digestive tract. alpha 6B expression is always much more widespread than alpha 6A expression. For example, only alpha 6B is present in the myotome of the somites of 9.5 day p.c. embryos, in the developing central and peripheral nervous systems, and in the nephrogenic system at all stages studied, except after the differentiation of the gonads when alpha 6A is also present. Furthermore, alpha 6B is the only splice variant present on endothelial cells. We also examined the distribution of the beta 4 integrin subunit to determine whether the alpha 6 beta 4 integrin was present during these stages of development. Beta 4 protein was absent in early postimplantation stages but was present in the epidermis and digestive tract of 12.5 day p.c. embryos. These results show a differential distribution of alpha 6A and alpha 6B during mouse development and thus strongly suggest a different function of these splice variants during embryogenesis. Our results point to a possible role for the alpha 6A beta 1 integrin in the development of the myocardium of the developing heart, but not in the migration of endocardial cushion cells, while alpha 6B beta 1 could be important in the developing nephrogenic and nervous systems.

Repair of critical size rat calvarial defects using extracellular matrix protein gels

In this study the authors examined the capacity of gels of reconstituted basement membrane, laminin, and type I collagen to mediate repair of critical size defects in rat calvaria. Although autografts are widely used to repair bone defects caused by trauma or surgical treatment of congenital malformations, neoplasms, and infections, an adequate quantity of graft is not always available. Allogenic bone is readily available, but its use is associated with an increased incidence of nonunion, fatigue fracture, and rejection. Biologically active, purified components of basement membranes, which have been shown to promote osteogenic differentiation and angiogenesis in vitro and type I collagen (the major constituent of bone extracellular matrix) can be formed into native isotonic space-filling gels. In this study critical size calvarial defects were created in retired male Sprague-Dawley rats. Thirty-six animals were divided into seven groups. Group 1 (control) received no treatment for the defects. Group 2 animals were implanted with methylcellulose. Groups 3, 4, 5, and 6 were implanted with gels of type I collagen, reconstituted basement membrane, or laminin, respectively. The last group of three animals (Group 7) was implanted with 100 micrograms of type I collagen gels (identical to Group 3) and sacrificed at 20 weeks following a single CT scan to determine if complete healing could be obtained with this method given sufficient time. Except for rats in the type I collagen group that was evaluated by multiple computerized tomography (CT) scans biweekly from 2 to 12 weeks, bone repair was evaluated using CT at 12 weeks. Healing was quantified using three-dimensional reconstruction of CT. Following the final CT scan in each experimental group, animals were sacrificed, and a sample of tissues was evaluated by conventional histology. Animals treated with type I collagen gels showed 87.5% repair of the area of the defects at 12 weeks and 92.5% repair by 20 weeks. Increasing the gel volume 1.5 x accelerated complete repair to 3 months. Murine-reconstituted basement membrane and laminin gels induced 55.5% and 46.3% repair, respectively, at 3 months. In untreated control animals 7% repair of the area of the defects showed at 3 months. Histological analysis confirmed new bone formation in partial and completely healed defects. Bioengineered native collagen gels may have wide applicability for bone repair as an alternative bone graft material alone, in combination with autograft or marrow aspirate, or as a delivery system for osteogenic growth factors.

The extracellular matrix during heart development

The embryonic extracellular matrix, which is comprised of glycosaminoglycans, glycoproteins, collagens, and proteoglycans, is believed to play multiple roles during heart morphogenesis. Some of these ECM components appear throughout development, however, certain molecules exhibit an interesting transient spatial and temporal distribution. Due to significant new data that have been gathered predominantly in the past 10 years, a comprehensive review of the literature is needed. The intent of this review is to highlight work that addresses mechanisms by which extracellular matrix influences vertebrate heart development.

Gastrulation in the sea urchin, Strongylocentrotus purpuratus, is disrupted by the small laminin peptides YIGSR and IKVAV

Laminin is present on the apical and basolateral sides of epithelial cells of very early sea urchin blastulae. We investigated whether small laminin-peptides, known to have cell binding activities, alter the development of sea urchin embryos. The peptide YIGSR-NH2 (850 microM) and the peptide PA22-2 (5 microM), which contains the peptide sequence IKVAV (Tashiro et al., J. Biol. Chem. 264, 16174, 1989), typically blocked archenteron formation when added to the sea water soon after fertilization. At lower doses, the YIGSR peptide allowed invagination of the archenteron but blocked archenteron extension and differentiation and evagination of the feeding arms. The effect of YIGSR and PA22-2 peptides declined when added to progressively older stages until no effect was seen when added at the mesenchyme blastula stage (24 hours after fertilization). Control peptides GRGDS, YIGSE, and SHA22, a dodeca-peptide with a scrambled IKVAV sequence, had no effect on development. The YIGSK peptide containing a conserved amino acid modification had only a small effect on gastrulation. The results suggest that YIGSR and IKVAV peptides specifically disrupt cell/extracellular matrix interactions required for normal development of the archenteron and feeding arms. Our recent finding that YTGIR is at the cell binding site of the B1 chain of S. purpuratus laminin supports this conclusion. Evidently, laminin or other laminin-like molecules are among the many extracellular matrix components needed for the invagination and extension of the archenteron during the gastrulation movements of these embryos.

Expression of beta 1 integrins changes during transformation of avian lens epithelium to mesenchyme in collagen gels

Remarkably, a number of definitive epithelia, such as that of the anterior lens, give rise when suspended within 3D gels of type I collagen, to elongate, bipolar shaped cells that exhibit the ultrastructure, polarity, and migratory ability of mesenchymal cells. They begin producing type I collagen and stop producing crystallins, type IV collagen, and laminin. Here, we investigated changes in beta 1 integrins and their extracellular matrix (ECM) ligands during this transdifferentiation. The former free surface of the lens epithelium that is now in contact with collagen begins within a day to stain intensely for beta 1 and it is this surface rather than the surface facing the basement membrane that gives rise to mesenchymal cells. Immunoprecipitation experiments reveal a large increase in the beta 1 integrin subunit on mesenchymal cells as compared to the epithelium of origin. The alpha 5 integrin subunit, which is barely detectable in the lens, increases in the mesenchymal cells and alpha 3 continues to be expressed at about the same level as in the epithelium. alpha 6, the epithelial integrin subunit, and laminin, its ECM ligand, are not detected immunohistochemically or biochemically in the mesenchyme. Rather, the mesenchymal cells secrete abundant fibronectin, the major ECM ligand for alpha 5 beta 1. RGD peptides do not inhibit the transformation but antibodies to beta 1 do perturb the emigration of mesenchymal cells from the lens apical surface. We conclude that the beta 1 integrins newly expressed on the apical epithelial surface interact with the surrounding 3D collagen gel to help bring about this unusual epithelial-mesenchymal transition.

Migratory interaction of amphibian epidermal cells with components of the basement membrane

In adult newts, basal epidermal cells adjacent to a fresh wound move toward the damaged area by migrating over the epidermal basement membrane. In an attempt to determine which basement membrane components mediate this migration, small pieces of glass coated with various natural matrices, purified proteins, or fragments of proteins were implanted into skin wounds such that epidermal cells attempting to form a wound epithelium would encounter the implants. Laminin derived from a cell line (M1536-B3) that produces no type IV collagen was inactive as a migration substrate. Migration on recombinant entactin was somewhat better than on laminin but was still only approximately 14% of that on type I collagen. M15 matrix, a laminin and entactin-containing product of M1536-B3 cells, was no better than entactin alone. Type IV collagen was an excellent substrate, producing slightly more migration than corresponding concentrations of type I collagen at nearly all concentrations tested. Migration on type IV lacking the NC1 domain was at least as good as on intact type IV. All the activity in type IV was present in a 95 kD fragment (alpha 1(IV)95) from the carboxy terminal two-thirds of the alpha 1 chain. Approximately 60% of the activity on alpha 1(IV)95 was obtained on implants coated with a 110 amino acid fragment of the alpha 1 chain derived from the carboxy terminal half of alpha 1(IV)95. Adding the synthetic peptide, arg-gly-asp-ser (RGDS) to the medium, blocked migration on fibronectin-coated implants but had no effect on implants coated with type IV, suggesting that migration on type IV involves different cell surface receptors than those mediating migration over fibronectin. Matrigel, a commercial product containing most basement membrane components, was a poor migration substrate. Thus if type IV mediates basal cell migration toward a wound in vivo, there may have to be some alterations in basement membrane structure to allow epidermal receptors to access type IV active site(s).

Expression and accumulation of interstitial collagen in the neonatal rat heart

Significant physiological changes occur in the heart following birth including increased arterial blood pressure and heart rate. Concurrently, biochemical and structural alterations are evident in the neonatal heart in response to these dynamic physiological properties. Prominent among these is the elaborate development of the cardiac extracellular matrix, composed primarily of interstitial collagen. The collagenous fibers, together with other matrix components, form an elastic, stress-tolerant network which functions in the dissipation of force throughout the heart wall. The present studies have used biochemical and molecular techniques to show the temporal and spatial patterns of interstitial collagen accumulation and expression during late fetal and neonatal development of the rat heart. The use of biochemical and particularly molecular methodologies allows the analysis of the expression of matrix components at a resolution previously not attained by structural studies alone. These data show relative increases in interstitial collagen immediately following birth as well as spatial differences in collagen mRNAs within the heart. The data presented provide further evidence for a role of mechanical stimulation in the regulation of collagen gene expression during this period of heart development.

The extracellular matrix glycoproteins BM-90 and tenascin are expressed in the mesenchyme at sites of endothelial-mesenchymal conversion in the embryonic mouse heart

BM-90 is a novel glycoprotein initially isolated from the extracellular matrix of a mouse tumor. We here studied the expression of BM-90 during embryonic development of the mouse heart and compared its expression pattern with that of tenascin and laminin. Distribution was studied by immunofluorescence using antibodies specifically raised against mouse BM-90, laminin and tenascin. Some expression of BM-90 was seen in myocardial basement membranes at early developmental stages, but expression abruptly decreased from these sites at day 12 of embryogenesis. Laminin B chains were also found in the muscle basement membranes early but did not decrease with advancing development. The most striking observation was the markedly enriched expression of BM-90 in the endocardial cushion tissue (ECT). The ECT is derived from mesenchymal cells converted from endothelium and they will form the cardiac valves and septa. In the ECT, BM-90 showed considerable co-distribution with tenascin, but tenascin expression was more focal and did not mark all areas of the ECT. Northern blot data show that BM-90 and tenascin were produced by the developing heart. With antibodies detecting A, B1 and B2 chains of mouse laminin, no immunoreactivity was seen in the ECT. Our data thus show clear-cut differences in the molecular composition of the ECT and muscle basement membranes in the developing heart. The focal expression of BM-90 in the ECT suggests that BM-90 could be involved in epithelial-mesenchymal transitions.

Functional domains of cell adhesion molecules

A number of molecules involved in cell adhesion (e.g. fibronectin, laminin, collagens I and IV, thrombospondin, entactin) have now been identified and the consequent roles that they play in the processes of growth, migration, differentiation and tumor spread have been described. Active sequences of the molecules have been identified using synthetic peptides derived from specific domains. Several adhesive molecules contain multiple active domains with different biological activities.

Role of vitronectin in embryonic rat endocardial cell migration in vitro

Vitronectin is one of the extracellular matrices that mediate cell spreading and attachment in vitro. In the present paper, we demonstrate the involvement of vitronectin in the migration of cushion mesenchymal cells of the embryonic rat heart. Immunohistochemistry established the localization of vitronectin in the myocardial cells and in some of the cushion mesenchymal cells of the truncus arteriosus and atrioventricular canal. In vitro, vitronectin, fibronectin, and collagen type-I revealed significant stimulating activity for cushion mesenchymal cell migration. The distance migrated by cushion mesenchymal cells cultured on vitronectin, collagen type-I, or both vitronectin and fibronectin was similar, but that on fibronectin was significantly shorter. Following the addition of anti-vitronectin IgG to the medium, the migration distance of cushion mesenchymal cells on fibronectin was remarkably increased. Most explants on vitronectin or on both vitronectin and fibronectin became detached from dishes after the addition of the antivitronectin antibody. Immunostaining revealed that cushion mesenchymal cells cultured on substrata other than vitronectin synthesized vitronectin. From these results, it is suggested that vitronectin is synthesized by myocardial cells and some cushion mesenchymal cells, and that vitronectin inhibits cell movement on fibronectin. This feature of vitronectin may be important in the regulation of the migration of cushion mesenchymal cell in vivo.

Distribution and expression of two interactive extracellular matrix proteins, cytotactin and cytotactin-binding proteoglycan, during development of Xenopus laevis. I. Embryonic development

An immunohistochemical study of the localization of cytotactin and cytotactin-binding (CTB) proteoglycan throughout embryonic development of the anuran Xenopus laevis reveals that both appear in a restricted pattern related to specific morphogenetic events. CTB proteoglycan expression is first detected during gastrulation at the blastopore lip. Later, it is seen in the archenteron roof around groups of cells forming the notochord, somites and neural plate. Cytotactin first appears after neurulation, and is restricted to the intersomitic regions. Both molecules appear along the migratory pathways of neural crest cells in the trunk and tail. Later, cytotactin is present at sites where neural crest cells differentiate, around the aorta and in the smooth muscle coat of the gut; CTB proteoglycan is absent from these sites. In the head, cytotactin is initially restricted to the regions between cranial somites, while CTB proteoglycan is distributed throughout the cranial mesenchyme. The expression of both molecules is later associated with key events in chondrogenesis during the development of the skull. After chondrogenesis, CTB proteoglycan is distributed throughout the cartilage matrix, while cytotactin is restricted to a thin perichondrial deposit. Both molecules are expressed in developing brain. These findings are compared to studies of the chick embryo and although distinct anatomical differences exist between frog and chick, the expression of these molecules is associated with similar developmental processes in both species. These include mesoderm segmentation, neural crest cell migration and differentiation, cartilage development, and central nervous system histogenesis.

Distribution and expression of two interactive extracellular matrix proteins, cytotactin and cytotactin-binding proteoglycan, during development of Xenopus laevis. II. Metamorphosis

During metamorphosis of Xenopus laevis the extracellular matrix (ECM) proteins cytotactin and cytotactin-binding (CTB) proteoglycan and the cell adhesion molecules N-CAM and Ng-CAM, appear in highly restricted patterns determined by immunofluorescence histology. During limb development, cytotactin appears from the earliest stages in a meshwork of ECM fibrils associated with migrating mesenchymal cells forming the limb bud. Cytotactin also appears in the ECM between the apical limb ectoderm and mesenchyme. Later, both cytotactin and CTB proteoglycan appear co-localized within the central (prechondrogenic) limb mesenchyme. During chondrogenesis in these areas, cytotactin becomes restricted to perichondrium, while CTB proteoglycan is expressed throughout the cartilage matrix. The premyogenic mesenchyme surrounding the chondrogenic areas expressed N-CAM. Later, N-CAM is concentrated at the myogenic foci where cytotactin appears at sites of nerve/muscle contact and in tendons. Expression of these molecules in the blastemas of regenerating limbs was also studied, and during development of the central nervous system, stomach, and small intestine. Analysis of the expression patterns of cytotactin and CTB proteoglycan throughout development and metamorphosis reveals several consistent themes. The expression of these molecules is highly dynamic, often transient, and associated with key morphogenetic events. Cytotactin appears at multiple sites where cells undergo a transition from an undifferentiated, migratory phenotype to a differentiated phenotype. One or both molecules appear at several sites of border formation between disparate cell collectives, and CTB proteoglycan expression is associated with chondrogenesis.

Expression of adhesion molecules during the formation and differentiation of the avian endocardial cushion tissue

The expression of cytotactin, the cytotactin-binding (CTB) proteoglycan, and the neural cell adhesion molecule, N-CAM, was examined during the development of the avian endocardial cushion tissue (ECT). N-CAM was present in the cardiac mesoderm from its earliest time of development. At the time when endothelial cells converted to mesenchyme and began to migrate, they ceased their expression of N-CAM. Cytotactin and CTB proteoglycan were present in the cardiac jelly (into which the ECT cells migrate) in patterns that were correlated with cell migration. At early times of migration (stage 18), the region of the cardiac jelly near the endocardium contained cytotactin in the vicinity of the migrating cells. During later migration (stage 22), cytotactin remained associated with the leading zone of cell migration, but its expression began to decrease in areas where cells had accumulated. After ECT cell migration had ceased, cytotactin expression decreased, remaining high only in the peripheral portion of the aorticopulmonary septum and absent from its ridges. CTB proteoglycan was expressed during early migration at high levels in and adjacent to the myocardium. By stage 22, its distribution had become more uniform throughout the ECT regions and in the myocardium. The combined results of this study suggest that cytotactin, CTB proteoglycan, and N-CAM each play a distinct, critical role in pattern formation in the early heart.

Avian vasculogenesis and the distribution of collagens I, IV, laminin, and fibronectin in the heart primordia

The heart-forming regions of the early embryo are composed of splanchnic mesoderm, endoderm, and the associated ECM. The ECM of the heart-forming regions in stage 7-9 chicken embryos was examined using immunofluorescence. Affinity purified antibodies to chicken collagens type I and IV, chicken fibronectin, and mouse laminin were used as probes. We report that (1) the basement membrane of the endoderm contains immunoreactive laminin and collagen IV; (2) the nascent basement membrane of the heart splanchnic mesoderm contains immunoreactive laminin, but not type IV collagen, and (3) the prominent ECM between the splanchnic mesoderm and the endoderm (the primitive-heart ECM) contains collagen IV, collagen I, fibronectin, but not laminin. In addition, we describe microscopic observations on the spatial relationship of cardiogenic cells to the primitive-heart ECM and the endodermal basement membrane.

Laminin promotes formation of cord-like structures by Sertoli cells in vitro

Basement membranes are thin extracellular matrices which contact epithelial cells and promote their adhesion, migration, differentiation, and morphogenesis. These matrices are composed of collagen IV, heparan sulfate proteoglycan, laminin, and entactin as well as other minor components. Sertoli cells, like most epithelial cells, are in contact at their basal surface with a basement membrane. When cultured within three-dimensional basement membrane gels (Matrigel), Sertoli cells reorganize into cords that resemble testicular seminiferous cords found in the in vivo differentiating testis. Anti-laminin and anti-entactin antisera inhibit this cord morphogenesis by Sertoli cells whereas antisera against type IV and type I collagen, heparan sulfate proteoglycan, fibronectin, and preimmune sera had no effect. The RGD (RGDS-NH2) sequence, found in the cell binding domain of the integrin family of cell adhesion molecules as well as in the A chain of laminin and in entactin, effectively inhibited Sertoli cell cord formation at a concentration of 1.0 mg/ml but was unable to prevent Sertoli cell attachment at concentrations as high as 2.0 mg/ml. A synthetic pentapeptide from a cell-binding domain of the B1 chain of laminin. YIGSR-NH2, inhibited cord formation at a concentration of 0.25 mg/ml, but Sertoli cells were still adherent to the basement membrane matrix. At concentrations greater than 0.50 mg/ml, Sertoli cells detached. Antiserum against the YIGSR-NH2-containing sequence was also effective in inhibiting cord formation by Sertoli cells. Ligand (YIGSR-NH2 peptide) blot analysis of Sertoli cell lysates revealed an interaction with a major band at 60 kDa and with minor bands at 39 and 127 kDa. Furthermore, in Western blot analysis the anti-67-kDa laminin-binding protein antibody recognized a 59- to 60-kDa protein in Sertoli cells. The data indicate that laminin is involved in both Sertoli cell attachment and migration during formation of histotypic cord structures by these cells in culture. Two separate laminin cell-binding domains appear to be involved in Sertoli cell cord morphogenesis in vitro and are likely to participate in the formation of seminiferous cords in vivo.

A comparison of fibronectin, laminin, and galactosyltransferase adhesion mechanisms during embryonic cardiac mesenchymal cell migration in vitro

Embryonic hearts contain a homogeneous population of mesenchymal cells which migrate through an extensive extracellular matrix (ECM) to become the earliest progenitors of the cardiac valves. Since these cells normally migrate through an ECM containing several adhesion substrates, this study was undertaken to examine and compare three ECM binding mechanisms for mesenchymal cell migration in an in vitro model. Receptor mechanisms for the ECM glycoproteins fibronectin (FN) and laminin (LM) and the cell surface receptor galactosyltransferase (GalTase), which binds an uncharacterized ECM substrate, were compared. Primary cardiac explants from stage 17 chick embryos were cultured on three-dimensional collagen gels. Mesenchymal cell outgrowth was recorded every 24 hr and is reported as a percentage of control. Migration was perturbed using specific inhibitors for each of the three receptor mechanisms. These included the hexapeptide GRGDSP (300-1000 micrograms/ml), which mimics a cell binding domain of FN, the pentapeptide YIGSR (300-1000 micrograms/ml), which mimics a binding domain of LM, and alpha-lactalbumin (1-10 mg/ml), a protein modifier of GalTase activity. The functional role of these adhesion mechanisms was further tested using antibodies to avian integrin (JG22) and avian GalTase. While the FN-related peptide had no significant effect on cell migration it did produce a rounded cellular morphology. The LN-related peptide inhibited mesenchymal migration 70% and alpha-lactalbumin inhibited cell migration 50%. Antibodies against integrin and GalTase inhibited mesenchymal cell migration by 80 and 50%, respectively. The substrate for GalTase was demonstrated to be a single high molecular weight substrate which was not LM or FN. Control peptides, proteins and antibodies demonstrated the specificity of these effects. These data demonstrate that multiple adhesion mechanisms, including cell surface GalTase, are potentially functional during cardiac mesenchymal cell migration. The sensitivity of cell migration to the various inhibitors suggests that occupancy of specific ECM receptors can modulate the activity of other, unrelated, ECM adhesion mechanisms utilized by these cells.

The influence of cell-matrix interactions on the development of quail chorioallantoic vascular system

The extracellular matrix-component fibronectin (FN) was detected in close localisation to the vascular system (VS) of the quail chorioallantoic membrane (CAM). We have examined the role of cell-fibronectin interactions within the developing CAM. In two series of experiments the CAM was directly exposed to (1) an antibody against the cell-binding fragment of FN, and to (2) RGD-containing synthetic peptides which are recognized by the FN receptor. For controls an antibody against tubulin and a SHLVE-pentapeptide that does not interfere with the FN-receptor were applied. In the presence of anti-FN antibodies and RGD-sequences the CAM could not establish a normal vascular system. We observed hypo- and partially avascular regions; the resulting vascular pattern was atypically lacunar. None of the control substances affected the regular development of the chorioallantoic vascular system. These results demonstrate the essential role of FN in CAM angiogenesis.