Isolation and characterization of a 115,000-dalton matrix-associated glycoprotein from chick aorta

Phylogenic Determinants of Cardiovascular Frailty, Focus on Hemodynamics and Arterial Smooth Muscle Cells

The evolution of the circulatory system from invertebrates to mammals has involved the passage from an open system to a closed in-parallel system via a closed in-series system, accompanying the increasing complexity and efficiency of life’s biological functions. The archaic heart enables pulsatile motion waves of hemolymph in invertebrates, and the in-series circulation in fish occurs with only an endothelium, whereas mural smooth muscle cells appear later. The present review focuses on evolution of the circulatory system. In particular, we address how and why this evolution took place from a closed, flowing, longitudinal conductance at low pressure to a flowing, highly pressurized and bifurcating arterial compartment. However, although arterial pressure was the latest acquired hemodynamic variable, the general teleonomy of the evolution of species is the differentiation of individual organ function, supported by specific fueling allowing and favoring partial metabolic autonomy. This was achieved via the establishment of an active contractile tone in resistance arteries, which permitted the regulation of blood supply to specific organ activities via its localized function-dependent inhibition (active vasodilation). The global resistance to viscous blood flow is the peripheral increase in frictional forces caused by the tonic change in arterial and arteriolar radius, which backscatter as systemic arterial blood pressure. Consequently, the arterial pressure gradient from circulating blood to the adventitial interstitium generates the unidirectional outward radial advective conductance of plasma solutes across the wall of conductance arteries. This hemodynamic evolution was accompanied by important changes in arterial wall structure, supported by smooth muscle cell functional plasticity, including contractility, matrix synthesis and proliferation, endocytosis and phagocytosis, etc. These adaptive phenotypic shifts are due to epigenetic regulation, mainly related to mechanotransduction. These paradigms actively participate in cardio-arterial pathologies such as atheroma, valve disease, heart failure, aneurysms, hypertension, and physiological aging.

Fibulin-4 deposition requires EMILIN-1 in the extracellular matrix of osteoblasts

Tissue microenvironments formed by extracellular matrix networks play an important role in regulating tissue structure and function. Extracellular microfibrillar networks composed of fibrillins and their associated ligands such as LTBPs, fibulins, and EMILINs are of particular interest in this regard since they provide a specialized cellular microenvironment guiding proper morphology and functional behavior of specialized cell types. To understand how cellular microenvironments composed of intricate microfibrillar networks influence cell fate decisions in a contextual manner, more information about the spatiotemporal localization, deposition, and function of their components is required. By employing confocal immunofluorescence and electron microscopy we investigated the localization and extracellular matrix deposition of EMILIN-1 and -2 in tissues of the skeletal system such as cartilage and bone as well as in in vitro cultures of osteoblasts. We found that upon RNAi mediated depletion of EMILIN-1 in primary calvarial osteoblasts and MC3T3-E1 cells only fibulin-4 matrix deposition was lost while other fibulin family members or LTBPs remained unaffected. Immunoprecipitation and ELISA-style binding assays confirmed a direct interaction between EMILIN-1 and fibulin-4. Our data suggest a new function for EMILIN-1 which implies the guidance of linear fibulin-4 matrix deposition and thereby fibulin-4 fiber formation.

Agonists and Antagonists of TGF-β Family Ligands

The discovery of the transforming growth factor β (TGF-β) family ligands and the realization that their bioactivities need to be tightly controlled temporally and spatially led to intensive research that has identified a multitude of extracellular modulators of TGF-β family ligands, uncovered their functions in developmental and pathophysiological processes, defined the mechanisms of their activities, and explored potential modulator-based therapeutic applications in treating human diseases. These studies revealed a diverse repertoire of extracellular and membrane-associated molecules that are capable of modulating TGF-β family signals via control of ligand availability, processing, ligand-receptor interaction, and receptor activation. These molecules include not only soluble ligand-binding proteins that were conventionally considered as agonists and antagonists of TGF-β family of growth factors, but also extracellular matrix (ECM) proteins and proteoglycans that can serve as "sink" and control storage and release of both the TGF-β family ligands and their regulators. This extensive network of soluble and ECM modulators helps to ensure dynamic and cell-specific control of TGF-β family signals. This article reviews our knowledge of extracellular modulation of TGF-β growth factors by diverse proteins and their molecular mechanisms to regulate TGF-β family signaling.

Targeting of EMILIN-1 and EMILIN-2 to Fibrillin Microfibrils Facilitates their Incorporation into the Extracellular Matrix

Elastin microfibril interface-located proteins (EMILINs) 1 and 2 belong to a family of structurally related extracellular glycoproteins with unique functions in the extracellular space, such as modulation of pro-transforming growth factor-β processing, activation of the extrinsic apoptotic pathway, and regulation of Hedgehog and Wnt ligand bioavailability. However, little is known about how EMILINs may exert their extracellular functions. We therefore investigated the spatiotemporal localization and deposition of EMILIN-1 and -2 within the extracellular space. By using immunoelectron and immunofluorescence microscopy together with biochemical extraction, we showed that EMILIN-1 and -2 are targeted to fibrillin microfibrils in the skin. In addition, during skin wound healing and in vitro matrix fiber assembly by primary dermal fibroblasts, EMILIN-1 and -2 are deposited on and coregulated with fibrillin. Analysis of wounds and mouse embryonic fibroblast cultures showed that EMILIN-1 and -2 network formation also requires the presence of fibronectin. Disruption of microfibrils in fibrillin-1-deficient mice leads to fragmentation of the EMILIN-1 and -2 networks, suggesting an involvement of EMILINs in fibrillin-related skin disorders. The addition of EMILINs to the ligand repertoire of fibrillin strengthens the concept of fibrillin microfibrils as extracellular scaffolds integrating cellular force transmission and growth factor bioactivity.

Fibulin-4 conducts proper elastogenesis via interaction with cross-linking enzyme lysyl oxidase

Great arteries, as well as lungs and skin, contain elastic fibers as important components to maintain their physiological functions. Although recent studies have revealed that a glycoprotein fibulin-4 (FBLN4) is indispensable for the assembly of mature elastic fibers, it remains to be elucidated how FBLN4 takes part in elastogenesis. Here, we report a dose-dependent requirement for FBLN4 in the development of the elastic fibers in arteries, and a specific role of FBLN4 in recruiting the elastin-cross-linking enzyme, lysyl oxidase (LOX). Reduced expression of Fbln4, which was achieved with a smooth muscle-specific Cre-mediated gene deletion, caused arterial stiffness. Electron-microscopic examination revealed disorganized thick elastic laminae with aberrant deposition of elastin. Aneurysmal dilation of the ascending aorta was found when the Fbln4 expression level was reduced to an even lower level, whereas systemic Fbln4 null mice died perinatally from rupture of the diaphragm. We also found a specific interaction between FBLN4 and the propeptide of LOX, which efficiently promotes assembly of LOX onto tropoelastin. These data suggest a mechanism of elastogenesis, in which a sufficient amount of FBLN4 is essential for tethering LOX to tropoelastin to facilitate cross-linking.

NMR-based homology model for the solution structure of the C-terminal globular domain of EMILIN1

EMILIN1 is a glycoprotein of elastic tissues that has been recently linked to the pathogenesis of hypertension. The protein is formed by different independently folded structural domains whose role has been partially elucidated. In this paper the solution structure, inferred from NMR-based homology modelling of the C-terminal trimeric globular C1q domain (gC1q) of EMILIN1, is reported. The high molecular weight and the homotrimeric structure of the protein required the combined use of highly deuterated (15)N, (13)C-labelled samples and TROSY experiments. Starting from a homology model, the protein structure was refined using heteronuclear residual dipolar couplings, chemical shift patterns, NOEs and H-exchange data. Analysis of the gC1q domain structure of EMILIN1 shows that each protomer of the trimer adopts a nine-stranded beta sandwich folding topology which is related to the conformation observed for other proteins of the family. Distinguishing features, however, include a missing edge-strand and an unstructured 19-residue loop. Although the current data do not allow this loop to be precisely defined, the available evidence is consistent with a flexible segment that protrudes from each subunit of the globular trimeric assembly and plays a key role in inter-molecular interactions between the EMILIN1 gC1q homotrimer and its integrin receptor alpha4beta1.

Deposition of tropoelastin into the extracellular matrix requires a competent elastic fiber scaffold but not live cells

The initial steps of elastic fiber assembly were investigated using an in vitro assembly model in which purified recombinant tropoelastin (rbTE) was added to cultures of live or dead cells. The ability of tropoelastin to associate with preexisting elastic fibers or microfibrils in the extracellular matrix was then assessed by immunofluorescence microscopy using species-specific tropoelastin antibodies. Results show that rbTE can associate with elastic fiber components in the absence of live cells through a process that does not depend on crosslink formation. Time course studies show a transformation of the deposited protein from an initial globular appearance early in culture to a more fibrous structure as the matrix matures. Deposition required the C-terminal region of tropoelastin and correlated with the presence of preexisting elastic fibers or microfibrils. Association of exogenously added tropoelastin to the cellular extracellular matrix was inhibited by the addition of heparan sulfate but not chondroitin sulfate sugars. Together, these results suggest that the matrix elaborated by the cell is sufficient for the initial deposition of tropoelastin in the extracellular space and that elastin assembly may be influenced by the composition of sulfated proteoglycans in the matrix.

Molecular cloning and characterization of a novel gene, EMILIN-5, and its possible involvement in skeletal development

By analyzing expression profiles of human mesenchymal stem cells incubated in osteogenic supplements, we identified and characterized a novel human cDNA, elastin microfibril interface located protein-5 (EMILIN-5), that is likely to play a significant role in the process of osteogenesis. The deduced EMILIN-5 product consists of 766 amino acids with a cysteine-rich EMI domain at the NH(2) terminus. Western blotting detected EMILIN-5 expression in a variety of osteoblastic cell lines. Immunohistochemistry of mouse embryos 13.5 days post-coitus revealed relatively high levels of EMILIN-5 protein in perichondrium cells of developing limbs. Our findings suggest that the EMILIN-5 gene plays an important role in skeletal development.

EMILIN-1 deficiency induces elastogenesis and vascular cell defects

EMILINs constitute a family of genes of the extracellular matrix with high structural similarity. Four genes have been identified so far in human and mouse. To gain insight into the function of this gene family, EMILIN-1 has been inactivated in the mouse by gene targeting. The homozygous animals were fertile and did not show obvious abnormalities. However, histological and ultrastructural examination revealed alterations of elastic fibers in aorta and skin. Formation of elastic fibers by mutant embryonic fibroblasts in culture was also abnormal. Additional alterations were observed in cell morphology and anchorage of endothelial and smooth muscle cells to elastic lamellae. Considering that EMILIN-1 is adhesive for cells and that the protein binds to elastin and fibulin-5, EMILIN-1 may regulate elastogenesis and vascular cell maintenance by stabilizing molecular interactions between elastic fiber components and by endowing elastic fibers with specific cell adhesion properties.

beta 1 Integrin-dependent cell adhesion to EMILIN-1 is mediated by the gC1q domain

EMILIN-1 (Elastin Microfibril Interface Located ProteIN), the prototype of the EMILIN family, consists of a cysteine-rich domain (EMI domain) at the N terminus, an extended region with a high potential coiled-coil structure, a short collagenous stalk, and a self-interacting globular gC1q-l domain. EMILIN-1 is an adhesive extracellular matrix constituent associated with elastic fibers, detected also in the proximity of cell surfaces. To localize the cell attachment site(s), monoclonal antibodies (mAbs) against EMILIN-1 or the gC1q-1 domain were used to inhibit cell attachment to EMILIN-1. Thus, one mAb mapping to the gC1q-1 domain caused complete inhibition of cell attachment. EMILIN-1 and gC1q-1 displayed a comparable dose-dependent ability to promote cell adhesion. Adhesion kinetics was similar to that of fibronectin (FN), reaching the maximum level of attachment at 20 min, but in the absence of cations adhesion was negligible. The relative adhesion strength to detach 50% of the cells was similar for EMILIN-1 and gC1q-1 (250-270 x g) but lower than that for FN (>>500). Cell adhesion to EMILIN-1 or gC1q-1 was completely blocked by a function-blocking beta(1) integrin subunit mAb. In contrast, adhesion to the complement C1q component was totally unaffected. Among the various function-blocking mAbs against the alpha integrin subunits only the anti-alpha(4) fully abrogated cell adhesion to gC1q-1 and up to 70% to EMILIN-1. Furthermore, only K562 cells transfected with the alpha(4) integrin chain, but not wild type K562, were able to adhere to EMILIN-1 and were specifically inhibited by anti-alpha(4) function-blocking mAb. Finally, cells attached to EMILIN-1 or gC1q-1, compared with cells plated on FN or vitronectin, which appeared well spread out on the substrate with prominent stress fibers and focal contacts, were much smaller with wide ruffles and a different organization status of the actin cytoskeleton along the cell periphery. This pattern was in accord with the ability of EMILIN-1 to promote cell movement.

Expression of the EMILIN-1 gene during mouse development

Expression of EMILIN-1, the first member of a newly discovered family of extracellular matrix genes, has been investigated during mouse development. EMILIN-1 mRNA is detectable in morula and blastocyst by RT-PCR. First expression of the gene is found by in situ hybridization in ectoplacental cone in embryos of 6.5 days and in extraembryonic visceral endoderm at 7.5 days. The allantois is also labeled. Staining of ectoplacental cone-derived secondary trophoblast giant cells and spongiotrophoblast is strong up to 11.5 days and then declines. In the embryo, high levels of mRNA are initially expressed in blood vessels, perineural mesenchyme and somites at 8.5 days. Later on, intense labeling is identified in the mesenchymal component of organs anlage (i.e. lung and liver) and different mesenchymal condensations (i.e. limb bud and branchial arches). At late gestation staining is widely distributed in interstitial connective tissue and smooth muscle cell-rich tissues. The data suggest that EMILIN-1 may have a function in placenta formation and initial organogenesis and a later role in interstitial connective tissue.

Isolation and characterization of EMILIN-2, a new component of the growing EMILINs family and a member of the EMI domain-containing superfamily

EMILIN (elastin microfibril interfase located Protein) is an elastic fiber-associated glycoprotein consisting of a self-interacting globular C1q domain at the C terminus, a short collagenous stalk, an extended region of potential coiled-coil structure, and an N-terminal cysteine-rich domain (EMI domain). Using the globular C1q domain as a bait in the yeast two-hybrid system, we have isolated a cDNA encoding a novel protein. Determination of the entire primary structure demonstrated that this EMILIN-binding polypeptide is highly homologous to EMILIN. The domain organization is superimposable, one important difference being a proline-rich (41%) segment of 56 residues between the potential coiled-coil region and the collagenous domain absent in EMILIN. The entire gene (localized on chromosome 18p11.3) was isolated from a BAC clone, and it is structurally almost identical to that of EMILIN (8 exons, 7 introns with identical phases at the exon/intron boundaries) but much larger (about 40 versus 8 kilobases) than that of EMILIN. Given these findings we propose to name the novel protein EMILIN-2 and the prototype member of this family EMILIN-1 (formerly EMILIN). The mRNA expression of EMILIN-2 is more restricted compared with that of EMILIN-1; highest levels are present in fetal heart and adult lung, whereas, differently from EMILIN-1, adult aorta, small intestine, and appendix show very low expression, and adult uterus and fetal kidney are negative. Finally, the EMILIN-2 protein is secreted extracellularly by in vitro-grown cells, and in accordance with the partial coexpression in fetal and adult tissues, the two proteins shown extensive but not absolute immunocolocalization in vitro.

EMI, a novel cysteine-rich domain of EMILINs and other extracellular proteins, interacts with the gC1q domains and participates in multimerization

The N-terminal cysteine-rich domain (EMI domain) of EMILIN-1 is a new protein domain that is shared with two proteins (multimerin and EMILIN-2) and with four additional database entries. The EMI domains are always located at the N-terminus, have a common gene organization, and belong to proteins that are forming or are compatible with multimer formation. The potential role of the EMI domain in the assembly of EMILIN-1 was investigated by the two-hybrid system. No reporter gene activity was detected when EMI-1 was co-transformed with the C-terminal gC1q-1 domain excluding a head-to-tail multimerization; conversely, a strong interaction was detected when the EMI-1 domain was co-transformed with the gC1q-2 domain of EMILIN-2.

Self-assembly and supramolecular organization of EMILIN

The primary structure of human Elastin microfibril interface-located protein (EMILIN), an elastic fiber-associated glycoprotein, consists of a globular C1q domain (gC1q) at the C terminus, a short collagenous stalk, a long region with a high potential for forming coiled-coil alpha helices, and a cysteine-rich N-terminal sequence. It is not known whether the EMILIN gC1q domain is involved in the assembly process and in the supramolecular organization as shown for the similar domain of collagen X. By employing the yeast two-hybrid system the EMILIN gC1q domains interacted with themselves, proving for the first time that this interaction occurs in vivo. The gC1q domain formed oligomers running as trimers in native gels that were less stable than the comparable trimers of the collagen X gC1q domain since they did not withstand heating. The collagenous domain was trypsin-resistant and migrated at a size corresponding to a triple helix under native conditions. In reducing agarose gels, EMILIN also migrated as a trimer, whereas under non-reducing conditions it formed polymers of many millions of daltons. A truncated fragment lacking gC1q and collagenous domains assembled to a much lesser extent, thus deducing that the C-terminal domain(s) are essential for the formation of trimers that finally assemble into large EMILIN multimers.

The EMILIN protein family

The EMILINs are a new family of glycoproteins of the extracellular matrix. The prototype of this family is the chicken EMILIN that was originally identified in extracts of aortas; it was then found to be widely distributed in several tissues associated with elastin and localized at the interface between amorphous elastin and microfibrils. Based on peptide sequences, chicken and human cDNAs coding for EMILIN were isolated by RT/PCR by screening kidney and heart cDNA libraries. By using a C-terminal fragment of human EMILIN-1 as a bait in the yeast two-hybrid system, a second family member, EMILIN-2, has also been isolated. EMILINs are characterized by a C-terminal gC1q globular domain, a short collagenous sequence, a long coiled-coil region and a new cysteine-rich N-terminal domain that can be considered a hallmark of the family being present also in multimerin. The gene for EMILIN-1 was mapped on chromosome 2p23 overlapping with the promoter region of the ketohexokinase gene. The gC1q domain of EMILIN-1 can form relatively stable and compact homotrimers and this association is then followed by a multimeric assembly of disulfide-bonded protomers. Recombinant EMILIN-1 purified from the supernatant of 293 cells represents a very efficient ligand for cell adhesion of several cell types.

Structure, chromosomal localization, and promoter analysis of the human elastin microfibril interfase located proteIN (EMILIN) gene

Elastin microfibril interfase-located protein (EMILIN) is an extracellular matrix glycoprotein abundantly expressed in elastin-rich tissues such as the blood vessels, skin, heart, and lung. It occurs with elastic fibers at the interface between amorphous elastin and microfibrils. In vitro experiments suggested a role for EMILIN in the process of elastin deposition. This multimodular protein consists of 995 amino acids; the domain organization includes a C1q-like globular domain at the C terminus, a short collagenous stalk, a region containing two leucine zippers, and at least four heptad repeats with a high potential for forming coiled-coil alpha-helices and, at the N terminus, a cysteine-rich sequence characterized by a partial epidermal growth factor-like motif and homologous to a region of multimerin. Here we report the complete characterization of the human and murine EMILIN gene, their chromosomal assignment, and preliminary functional data of the human promoter. A cDNA probe corresponding to the C terminus of EMILIN was used to isolate two genomic clones from a human BAC library. Sequencing of several derived subclones allowed the characterization of the whole gene that was found to be about 8 kilobases in size and to contain 8 exons and 7 introns. The internal exons range in size from 17 base pairs to 1929 base pairs. All internal intron/exon junctions are defined by canonical splice donor and acceptor sites, and the different domains potentially involved in the formation of a coiled-coil structure are clustered in the largest exon. The 3'-end of the EMILIN gene overlaps with the 5'-end of the promoter region of the ketohexokinase gene, whose chromosomal position is between markers D2S305 and D2S165 on chromosome 2. A 1600-base pair-long sequence upstream of the translation starting point was evaluated for its promoter activity; five deletion constructs were assayed after transfection in primary chicken fibroblasts and in a human rhabdomyosarcoma cell line. This analysis indicates the existence of two contiguous regions able to modulate luciferase expression in both cell types used, one with a strong activatory function, ranging from positions -204 to -503, and the other, ranging from positions -504 to -683, with a strong inhibitory function.

EMILIN, a component of the elastic fiber and a new member of the C1q/tumor necrosis factor superfamily of proteins

EMILIN (elastin microfibril interface located protein) is an extracellular matrix glycoprotein abundantly expressed in elastin-rich tissues such as blood vessels, skin, heart, and lung. It occurs associated with elastic fibers at the interface between amorphous elastin and microfibrils. Avian EMILIN was extracted from 19-day-old embryonic chick aortas and associated blood vessels and purified by ion-exchange chromatography and gel filtration. Tryptic peptides were generated from EMILIN and sequenced, and degenerate inosine-containing oligonucleotide primers were designed from some peptides. A set of primers allowed the amplification of a 360-base pair reverse transcription polymerase chain reaction product from chick aorta mRNA. A probe based on a human homologue selected by comparison of the chick sequence with EST data base was used to select overlapping clones from both human aorta and kidney cDNA libraries. Here we present the cDNA sequence of the entire coding region of human EMILIN encompassing an open reading frame of 1016 amino acid residues. There was a high degree of homology (76% identity and 88% similarity) between the chick C terminus and the human sequence as well as between the N terminus of the mature chick protein where 10 of 12 residues, as determined by N-terminal sequencing, were identical or similar to the deduced N terminus of human EMILIN. The domain organization of human EMILIN includes a C1q-like globular domain at the C terminus, a collagenous stalk, and a longer segment in which at least four heptad repeats and a leucine zipper can be identified with a high potential for forming coiled-coil alpha helices. At the N terminus there is a cysteine-rich sequence stretch similar to a region of multimerin, a platelet and endothelial cell component, containing a partial epidermal growth factor-like motif. The native state of the recombinantly expressed EMILIN C1q-like domain to be used in cell adhesion was determined by CD spectra analysis, which indicated a high value of beta-sheet conformation. The EMILIN C1q-like domain promoted a high cell adhesion of the leiomyosarcoma cell line SK-UT-1, whereas the fibrosarcoma cell line HT1080 was negative.

Alignment of fibrillin molecules in elastic microfibrils is defined by transglutaminase-derived cross-links

Microfibrils were extracted from human amnion in the form of a beaded filament and analyzed for the presence of transglutaminase-derived cross-links using acrylonitrile derivatization. The cross-link structure was isolated from protease hydrolysates of beaded filaments and identified as a phenylthiocarbamyl amino acid derivative by comparison to a standard. Acid hydrolysis of the isolated cross-link gave the expected lysine and glutamic acid in a 1:1 ratio. The beaded filaments were also treated with trypsin to produce a fraction that contained the bead structure and a fraction containing fragments of the interbead filaments. Cross-links were detected in the interbead filaments but not in the beads. A large tryptic peptide that contained a cross-link was isolated and sequenced. The two amino acid sequences obtained identified both of the cross-linked molecules as fibrillin-1 and enabled the approximate localization of the cross-link sites within the molecule. The locations of cross-link sites on two adjacent molecules fixed the relative positions of fibrillin monomers within the microfibrils, providing insight into the spatial organization of fibrillin within the elastic microfibrils.

Proteoglycans associated with the ciliary zonule of the rat eye: a histochemical and immunocytochemical study

The structural organization of integral and associated components of the ciliary zonule is still not fully understood. The present study is to localize and characterize the proteoglycans associated with the ciliary zonule of the rat eye by Cuprolinic blue (CB) staining and immunocytochemistry. After CB staining, the proteoglycans appeared as electron dense elongated rodlets and were localized with the zonular fibers. They were seen lying on the periphery of the zonular fibers or along the length of the individual fibrils. Most of the CB rodlets had a size of 60-170 nm long (average 130 nm) and 25 nm wide. Smaller CB rodlets measuring 25-60 nm long (average 45 nm) and 12 nm wide were sometimes found associated with the individual zonular fibrils. The CB rodlets were removed after chondroitinase ABC or chondroitinase AC treatment, but were resistant to heparitinase, nitrous acid, keratanase or Streptomyces hyaluronidase digestions. The ciliary zonule was also immunostained with three monoclonal antibodies: 2-B-6 specific for chondroitin 4-sulfate, 3-B-3 for chondroitin 6-sulfate and 1-B-5 for unsulfated chondroitin, using indirect immunoperoxidase or immuno-colloidal gold methods. The zonular fibers were immunoperoxidase stained and immunogold labeled by 2-B-6, but were not reactive to 3-B-3 and 1-B-5. The results demonstrate that chondroitin sulfate proteoglycan is associated with the ciliary zonule of the rat eye.

Fibrillin-containing microfibrils: structure and function in health and disease

Fibrillin-containing microfibrils are a unique class of connective tissue macromolecules whose critical contribution to the establishment and maintenance of diverse extracellular matrices was underlined by the recent linkage of their principal structural component fibrillin to Marfan syndrome, a heritable disorder with pleiotrophic connective tissue manifestations. The complexity of the structure: function relationships of these macromolecules was highlighted by the recent elucidation of the primary structure of fibrillin and characterisation of fibrillin mutations in Marfan patients. This review examines current understanding of the expression and assembly of fibrillin and describes new approaches which are now being applied to elucidate the many outstanding structural, organisational and functional aspects of the fibrillin-containing microfibrils.

Human scleral elastic system: an immunoelectron microscopic study

An immunocytochemical study was conducted on elastic components in the sclera of seven aged human eyes. By conventional electron microscopy, elastic tissue consists of three distinct fibre types--elastic fibres, elaunin fibres, and oxytalan fibres. The distribution of six components associated with the elastic system (elastin, amyloid P component, laminin, fibronectin, gp 115, and vitronectin) were studied by immunogold transmission electron microscopy. The codistribution of amyloid P component and laminin was further studied by double immunolabelling. Both elastic and elaunin fibres contained elastin. The microfibrillar sheaths of elastic fibres labelled for amyloid P component, those of elaunin fibres for amyloid P and laminin, and those of oxytalan fibres for laminin only. No labelling was observed for fibronectin, gp 115, and vitronectin. In terms of the proteins investigated, the biochemical profile of the three fibre types was not completely identical and was manifest as different affinities in the binding of serum amyloid P component and an association with laminin.

Immunogold study of non-collagenous matrix components in normal and exfoliative iris

The present investigation was undertaken to determine if some of the components of exfoliation material in iris tissue were unique to exfoliation or were part of normal iris architecture. Eleven normal iris specimens and 10 exfoliative iris specimens were processed for cryoultramicrotomy and London resin white embedding. Immunogold electron microscopy was used to investigate the fine structural distribution of amyloid P component, elastin, entactin, fibronectin, gp115, and vitronectin in normal iris and their association with exfoliation material. Exfoliation material was positive for amyloid P component and possibly gp115, neither of which were present in normal iris tissue. Elastin and fibronectin were present in the normal iris stroma but were not associated with exfoliation material. The distribution of amyloid P component in the vessel lumen and wall led to the conclusion that amyloid P is a serum contaminant. The presence of gp115 in exfoliation material represents the synthesis of a component novel to the iris vascular cell synthetic repertoire.

The molecular basis of Marfan syndrome

The Marfan syndrome is an inherited, autosomal dominant disorder that affects the skeletal, ocular, and cardiovascular systems. Recent biochemical and genetic studies have demonstrated that this deadly genetic disorder arises from defects in the connective tissue protein fibrillin. Fibrillin is a component of microfibrils, structures found in the extracellular matrices of most tissues, including those affected in Marfan patients. The appearance of microfibrils in the matrix produced by Marfan patient fibroblasts is different from that of normal cells. Genetic linkage between the fibrillin gene and the Marfan phenotype has been established and the gene mapped to the same chromosomal position as the disease locus. In several instances, the disease has been associated with mutations in the fibrillin gene, confirming that defects in fibrillin cause the Marfan syndrome.

Mapping of binding sites for monoclonal antibodies to chick tropoelastin by recombinant DNA techniques

A fusion molecule consisting of the entire coding sequence of mature chicken tropoelastin preceded by 14 amino acids of the signal peptide and 9 amino acids of vector origin has been expressed in a recombinant bacterial system and purified. The molecule has been used as immunogen for the production of hybridomas. Monoclonal antibodies which bound specifically the immunogen were also reactive with tropoelastin purified from chick aorta and stained elastic fibers in aorta sections by immunofluorescence. The region of tropoelastin containing the antigenic determinant recognized by each antibody has been identified by a recombinant DNA expression strategy based on the use of cDNA clones spanning different portions of the coding sequence. It could be shown that several antibodies were directed against unique epitopes; among these, a group of antibodies bound specifically to the sequence (PGVGV)n. Other antibodies were found to recognize antigenic determinants present more than once in the molecule. The monoclonal antibodies thus characterized will be useful reagents in studying the function of the different domains of tropoelastin.

Mechanisms of interaction between human skin fibroblasts and elastin: differences between elastin fibres and derived peptides

3H-Labelled kappa-elastin peptides (kE:75 kDa molecular weight) were shown to bind to confluent human skin fibroblast (HSF) cultures in a time-dependent and saturable manner. Scatchard analysis indicated the presence of high affinity binding sites with kD = 2.7 x 10(-10) M and 19,000 sites per cell. Binding of kE to its receptor on HSF accelerates and intensifies the adhesion of insoluble elastin fibres (iE) to confluent HSF. Optimal effect was attained for a kE concentration of 0.3 x 10(-9) M close to kD. This stimulatory effect of kE on the binding of iE to HSF could be inhibited by neomycin, retinal and pertussis toxin, substances which act at different levels of the transduction mechanism following the activation of the receptor and the subsequent triggering of cell biological events (chemotaxis, modification of calcium fluxes). The stimulation of iE adhesion to HSF induced by kE as well as kE binding to the cells could be inhibited by lactose and laminin but not by Arg-Gly-Asp-Ser(RGDS) peptides. This indicates that the elastin peptide receptor on HSF possesses lectin-like properties and shares homology with the laminin receptor as also shown for other cell types. None of the substances tested, that is inhibitors of the transduction mechanism, lactose, laminin and Arg-Gly-Asp-Ser(RGDS) peptides were shown to interfere significantly with the binding of iE (in the absence of added kE) to confluent HSF. The proteins adhering strongly to elastin fibres were isolated by a sequential extraction procedure and the final hydrochloride guanidinium-DTT extract was analysed by SDS-PAGE under reducing conditions, Western blots using specific antibodies against several connective tissue proteins and affinity for [3H]-kE following nitrocellulose electro-transfer of proteins. Fibronectin, vitronectin, tropoelastin(s), and a 120 kDa cysteine rich glycoprotein previously designated as elastonectin were identified. Among these proteins, [3H]-kE was found to bind exclusively to a 65 kDa protein that could be eluted selectively from elastin fibres with a neutral buffer containing 100 mM lactose. Therefore the elastin peptide receptor on human skin fibroblasts shares properties with the elastin receptor characterized from other cell types. Conformational differences between elastin peptides and elastin fibres could explain the differences in the mechanisms of interactions between elastin fibres and elastin peptides with HSF in culture. The stimulatory effect of elastin-derived peptides on the adhesion of elastin fibres to HSF could have implications in the oriented biosynthesis of elastin fibres.

Ultrastructural immuno-localization of tropoelastin in the chick eye

Immuno-gold labeling at the electron-microscopy level was used to investigate the distribution of tropoelastin in the chick eye. Intense staining was found in the amorphous part of mature elastic fibers in different regions of the organ. In elaunin fibers, both the amorphous core and the surrounding microfibrils were clearly labeled. In addition, reactive sites were detected in the oxitalan fibers of the stroma- of the cornea and in Descemet's membrane, which showed a gradient of reactive sites increasing from the center toward the periphery. Oxitalan fibers of the stroma often fused with Descemet's membrane; the pattern of immunological staining suggested a continuity between the two structures. In the ciliary zonule, labeling for tropoelastin was observed in discrete areas on the bundles of microfibrils. The results show a complex structural organization of elastic tissue; this may be important in endowing the various parts of the eye with different mechanical properties.

Widespread codistribution of glycoprotein gp 115 and elastin in chick eye and other tissues

Frozen sections of chick tissues were exposed to affinity-purified monoclonal antibodies raised against chick gp 115 and to affinity-purified antibodies raised against chick tropoelastin to study the distribution pattern of the corresponding antigens by the avidin-biotin immunoperoxidase technique. Laminin and fibronectin antibodies were used for comparison. Gp 115 and tropoelastin antibodies localized to the same structure in several of the tissues examined. The endothelial membrane of the cornea and Bruch's membrane in the choroid were positive, while the corneal epithelial membrane was negative. Both antibodies displayed a peculiar punctate reactivity in the corneal stroma and a very fine fibrillar pattern in the conjunctiva and at the corneal-scleral junction. Liver, heart and large vessels, striated muscle and skin showed a similar pattern both for tropoelastin and gp 115 antibodies. Few differences were seen in the distribution of the reactivity: the pericellular matrix of intestinal smooth muscle cells was stained by gp 115 but not by tropoelastin antibodies. However, the reactivity of gp 115 and tropoelastin antibodies was similarly distributed in the lung smooth muscle cell clusters. The peritubular matrix in the kidney did also not react with tropoelastin antibodies as did the brain intraparenchymal vessels; whereas gp 115 antibody reactivity was present in both sites. We interpret these lack of apparent codistribution in some tissues as a variation in the relative availability of the target antigen for the reaction with the antibody and not as a consequence of a qualitative difference in the distribution of gp 115 and tropoelastin. By the use of anti gp 115 monoclonal antibodies that do not cross-react, and presumably recognize different epitopes, it was shown that some but not all antibodies, react with brain intraparenchymal blood vessels; whereas the pattern of distribution in other tissues was the same. This suggests that in vessels with an undetectable level of elastin, certain epitopes of gp 115 molecule might not be recognized as a result of being masked by other components or by a different conformation of the molecule.

Monoclonal antibodies against chick gp 115, a matrix glycoprotein with broad distribution

Hybridoma cell lines were generated producing monoclonal antibodies to chick gp 115, a 115,000-dalton glycoprotein widely distributed in the connective tissue. The specificity of the antibodies was determined by indirect radioimmunobinding: the extent of binding was a function of i) antigen and ii) antibody concentration; iii) inhibition of binding of radiolabelled antibody by unlabelled antibody and iv) among many known extracellular collagenous or noncollagenous glycoproteins tested only gp 115 gave a strong positive binding reaction. The antibodies were used for indirect immunofluorescence and a strong staining reaction was detected in all blood vessels, around smooth muscle cells in several organs, and in the connective matrix of other tissues such as the liver, and the lung. Based on the competition of binding of [125I]-labeled purified antibody by unlabeled antibodies, two separate epitopes were identified on gp 115. Further analysis of the localization of the epitope was obtained by CNBr cleavage and partial digestion of gp 115 with Staphylococcus aureus V8 protease and alpha-chymotrypsin digestion. Following CNBr cleavage a major fragment of Mr = 35,000 was recognized by 4 monoclonal antibodies, and fragments of comparable Mr were detected following V8 protease and alpha-chymotrypsin digestion.