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PubMed1994

Structure of the human gene encoding the associated microfibrillar protein (MFAP1) and localization to chromosome 15q15-q21.

Microfibrils with a diameter of 10-12 nm, found either in association with elastin or independently, are an important component of the extracellular matrix of many tissues. To extend our understanding of the proteins composing these microfibrils, the cDNA and gene encoding the human associated microfibril protein (MFAP1) have been cloned and characterized. The coding portion is contained in 9 exons, and the sequence is very homologous to the previously described chick cDNA, but does not appear to share homology or domain motifs with any other known protein. Interestingly, the gene has been localized to chromosome 15q15-q21 by somatic hybrid cell and chromosome in situ analyses. This is the same chromosomal region to which the fibrillin gene, FBN1, known to be defective in the Marfan syndrome, has been mapped. MFAP1 is a candidate gene for heritable diseases affecting microfibrils.

Amino Acid Sequence↗
PubMed1998

Molecular packing of type I collagen in tendon.

X-ray diffraction of rat tail tendon shows that type I collagen fibrils contain regions of three-dimensional crystalline arrays; where molecular packing is speculated to be by a staggered sheet or microfibril arrangement. The X-ray diffraction pattern also contains a significant amount of diffuse scatter indicative of static and thermal disorder in fibrils. Removal of the diffuse scatter from the equatorial region of X-ray diffraction patterns obtained using synchrotron radiation allowed the Bragg intensities to be viewed on a flat background. Indexing of Bragg peak intensity on the 10, -10, 0 -1, 01, -11 and 1-1 row-lines of the triclinic unit cell have been used here to test possible sheet and microfibril packing arrangements. The relative translation of molecular segments in the gap and overlap regions as well as the telopeptide orientation have been investigated. A global search through combinations of molecular packing and molecular translation revealed that the sheet-type conformations cannot account for the observed low-angle off-meridional Bragg peak intensity distribution. A superior fit is obtained with D-staggered left-handed microfibril structures. The orientation of the telopeptides may indicate that there are interconnections between microfibrils that may explain the difficulty in isolating individual microfibrillar structures.

Animals↗
PubMed1996

The Cellulose System in the Cell Wall of Micrasterias

The cellulose system of the cell wall of Micrasterias denticulata and Micrasterias rotata was analyzed by diffraction contrast transmission electron microscopy, electron diffraction, and X-ray analysis. The studies, achieved on disencrusted cell ghosts, confirmed that the cellulose microfibrils occurred in crisscrossed bands consisting of a number of parallel ribbon-like microfibrils. The individual microfibrils had thicknesses of 5 nm for a width of around 20 nm, but in some instances, two or three microfibrils merged into one another to yield larger monocrystalline domains reaching up to 60 nm in lateral size. The orientation of the cellulose of Micrasterias is very unusual, as it was found that in the cell wall, the equatorial crystallographic planes of cellulose having a d-spacing of 0.60 nm [(11;0) in the Ibeta cellulose unit cell defined by Sugiyama et al., 1991, Macromolecules 24, 4168-4175] were oriented perpendicular to the cell wall surface. Up to now, such orientation has been found only in Spirogyra, another member of the Zygnemataceae group. The unusual structure of the secondary wall cellulose of Micrasterias may be tentatively correlated with the unique organization of the terminal complexes, which in this alga occur as hexagonal arrays of rosettes.

Journal Article↗
PubMed1993

Divalent cation-dependent adhesion at the myotendinous junction: ultrastructure and mechanics of failure.

Junctional microfibrils, which span the lamina lucida of the vertebrate myotendinous junction, are thought to function in force transmission at the junction. This hypothesis has been tested by disrupting junctional microfibrils through elimination of extracellular divalent cations, and determining the effects of this treatment on the ultrastructure and mechanics of whole frog skeletal muscles passively stretched to failure. Muscles incubated in divalent cation-free solution failed exclusively in the lamina lucida of the myotendinous junction, while control muscles all failed within the muscle fibres, several millimetres away from the junction. Failure sites from divalent cation-free muscles incubated with antibodies against collagen type IV, laminin, and tenascin showed no labelling of the avulsed ends of the muscle fibres, indicating that remnants of junctional microfibrils observed on the cell surface are not composed of any of these extracellular proteins. All three proteins were present on the tendon side of the failure site, confirming that the lamina densa remains attached to the tendon. Breaking stress for control muscles was 3.47 x 10(5) N m-2, and for divalent cation-free muscles, 1.84 x 10(5) N m-2, or approximately half the control value. Breaking strain averaged 1.17 for divalent cation-free muscles and 1.39 for controls, although the difference was not significant. We conclude that junctional microfibrils are components of a divalent cation-dependent adhesion mechanism at the myotendinous junction. In addition, ultrastructural analysis of divalent cation-free fibres stretched just short of failure suggests that a second, divalent cation-independent mechanism persists along the non-junctional cell surface, and can transmit substantial passive tension from myofibrils laterally to the extracellular matrix, bypassing the failed myotendinous junction.

Animals↗
PubMed1985

Electron-microscopic study of the collagen fibrils of the rat tail tendon as revealed by freeze-fracture and freeze-etching techniques.

The ultrastructure of the collagen of rat tail tendon was investigated by the freeze-fracture technique. Collagen fibers were pretreated with the digestive enzymes, alpha-amylase, elastase and collagenase to remove matrix substances. Some of the samples were etched for 20 min. Fibrils had an average diameter of 318 +/- 12 nm and a banded structure with a mean periodicity of 64.2 +/- 0.9 mm; the banding was most marked in alpha-amylase/elastase-treated specimens, although the periodicity was independent of pretreatment. Microfibrils were well-displayed following alpha-amylase/elastase and collagenase pretreatments. A difference in the diameters of microfibrils was, however, observed between etched specimens (8.3 +/- 0.3 nm) and those prepared by other experimental methods (11.4 +/- 0.5 nm). In replicas of collagenase-treated and etched specimens, the interconnecting filaments in the interfibrillar region formed a network that was continuous with the microfibrils of collagen fibrils. The diameter of the interconnecting filaments was the same as that of microfibrils. Microfibrillar bundles were observed in the interfibrillar region.

Amylases↗
PubMed1987

Detection of elastin by immunoelectronmicroscopy. A comparison of different procedures.

Elastin components have been identified in chick aorta by different immunoelectronmicroscopic procedures (peroxidase-antiperoxidase, immunoferritin and immunogold) using affinity purified antibodies to chick tropoelastin. The PAP method used in a preembedding procedure stained the outer portion of amorphous elastin and the microfibrils very intensively. The surface of the cells was also slightly stained. On the contrary immunogold labelling on Epon or Lowicryl embedded sections produced a strong decoration only of amorphous elastin, while microfibrils remained almost completely unlabelled. The result is not due to loss of antigenicity of microfibrils during embedding, since similar data were obtained with immunoferritin in a preembedding procedure. Experiments performed under different stringency conditions showed that the products of the peroxidase reaction diffuse and redistribute in the tissue, indicating that the positive staining of microfibrils and cell surface is an artifact. The value of different immunological reagents and procedures in studying the fine mapping of elastin components is discussed.

Animals↗
PubMed1987

Fine mapping of tropoelastin-derived components in the aorta of developing chick embryo.

Affinity-purified antitropoelastin antibodies have been used to localize tropoelastin-derived components in aortas from chick embryos of different age by immunoelectron microscopy. Staining in the matrix is first noted at day 3 associated with irregular bundles of filaments resembling microfibrils, in the absence of amorphous elastin deposits. Amorphous material, which rapidly accumulates at later stages, is heavily labelled, while surrounding microfibrils are only poorly labelled. By contrast, a more intense staining of microfibrils persists in regions in which amorphous material is not morphologically evident. These observations indicate that the initial accumulation of elastin requires microfibrils, while the two components are not in close association in the subsequent growth of the amorphous core of the fibre. Intracellular staining is evident in the secretory apparatus of the cell and in peripheral large vesicles. Differentiated cells also show regions of close contact with elastic fibres in which immunological staining for elastin is very close to the cell membrane.

Animals↗
PubMed1979

Structure and regeneration of the planarian basal lamina: an ultrastructural study.

The structure and regeneration of the planarian subepidermal basement membrane or basal lamina have been electron microscopically examined, particularly in relation to the changes of extracellular products at the wounded area. The intact basal lamina consists of three structural elements; namely, an electron-lucent zone, a limiting layer and a microfibrillar layer. Ultrastructural changes during wound healing have suggested that the amorphous material secreted in the interspace between the epidermal cells and blastema contains precursors of the basal lamina. Within the amorphous zone two distinct phases of the basal lamina regeneration are observed: one is a reconstitution of the limiting layer and the other is a polymerization of the microfibrils. The limiting layer arises from areas subjacent to newly developed hemidesmosomes of epidermal cells. The unit microfibrils are formed from an accumulation of the precursors through transitional smaller microfibrils. At the late stage, individual mature microfibrils are regularly lined with the limiting layer and cell membranes of the newly differentiated muscle fibres. On the basis of these observations we suggest that the planarian basal lamina is regenerated by the interaction between epidermal cells and myoblasts.

Animals↗
PubMed1983

The infrastructure of aortic elastic fibers.

Elastic fibers are composed of a central core of elastin that is amorphous and electron-lucent in conventional transmission electron micrographs and peripheral microfibrils. A complex infrastructure within the amorphous elastin of mature rat aorta is made visible by fixation and staining with a glutaraldehyde-ruthenium red mixture in phosphate buffer or osmium-ruthenium red in cacodylate buffer. The infrastructure is composed of at least two interlacing but distinct elastic structural components; a framework of circumferentially orientated microfibrils and a three-dimensional meshwork of filaments that permeate the fiber. The latter resembles a reticulum that has previously been observed in freeze-fractured and negatively stained elastin and attributed to the supramolecular organization of elastin. Microfibrils also extend from the core of the elastic fiber into the surrounding matrix where they appear to function as anchoring fibers. These observations indicate that the elastic properties of the arterial wall are an integrated function of both elastin and microfibrils.

Animals↗
PubMed1994

Immunization with undenatured bovine zonular fibrils results in monoclonal antibodies to fibrillin.

Microfibrils were dissected from the zonular apparatus of the bovine eye, homogenized and used as an immunogen to prepare monoclonal antibodies. Initial screening of hybridomas was performed by immunoblotting to a sonicate of zonular fibrils and by immunolocalization to frozen sections of the zonular apparatus. Subsequently, monoclonal antibodies with strong immunoreactivity to zonular fibrils were shown to recognize microfibrils in a wide range of connective tissues both by immunofluorescent staining and by electron microscopic immunolocalization. All antibodies were found to recognize a single protein of 350 kDa on Western blotting of the proteins secreted by bovine aortic smooth muscle cells. A protein of the same molecular weight and properties was recognized by an antibody previously prepared by another group against fibrillin. A member of the fibrillin family therefore represents the major immunogen of intact zonular fibrils, and the results support previous evidence for a close relationship between zonular fibrils and other connective tissue microfibrils. The zonular apparatus is a suitable system to obtain purified preparations of microfibrils in order to investigate their composition and structural organization.

Actin Cytoskeleton↗
PubMed2005

Cellulose metabolism in plants.

Many bacterial genomes contain a cellulose synthase operon together with a cellulase gene, indicating that cellulase is required for cellulose biosynthesis. In higher plants, there is evidence that cell growth is enhanced by the overexpression of cellulase and prevented by its suppression. Cellulase overexpression could modify cell walls not only by trimming off the paracrystalline sites of cellulose microfibrils, but also by releasing xyloglucan tethers between the microfibrils. Mutants for membrane-anchored cellulase (Korrigan) also show a typical phenotype of prevention of cellulose biosynthesis in tissues. All plant cellulases belong to family 9, which endohydrolyzes cellulose, but are not strong enough to cause the bulk degradation of cellulose microfibrils in a plant body. It is hypothesized that cellulase participates primarily in repairing or arranging cellulose microfibrils during cellulose biosynthesis in plants. A scheme for the roles of plant cellulose and cellulases is proposed.

Cell Wall↗
PubMed1998

Microfibrillar composition of umbilical cord matrix: characterization of fibrillin, collagen VI and intact collagen V.

Ultrastructural studies made on human umbilical cord revealed that the striated collagen fibrils of the Wharton's jelly matrix are mixed with many microfibrillar structures. Microfibrils were found with a tubular cross-section of 10-12 nm diameter and were organized as beaded filaments characteristic of fibrillin-rich microfibrils. Beads had an average diameter of 25 nm and were spaced at about 50-80 nm. This ultrastructural observation was confirmed by indirect immunofluorescent staining of the jelly matrix using monoclonal antibody to fibrillin. Another constituent of the microfibrillar network was present as typical 100-nm periodic filaments of type VI collagen. Indirect immunofluorescent staining using antibodies to collagen VI showed for the first time that this collagen appeared to be distributed largely in the jelly matrix. In addition, other microfibrils with no specific banding pattern were observed. These microfibrils may constitute an organization of type V collagen different from the one which is generally assembled in heterotypic fibrils with collagen I. Among the latter heterotypic fibrils, type V collagen was studied using an anti-peptide antibody to the most N-terminal non-collagenous region of its alpha 2(V) chain. This antibody recognized a filamentous mesh decorating the bundles of collagen fibrils by immunofluorescent staining. This indicates that at least this part of alpha 2(V) chain may be accessible to the antibody at the surface of the fibrils.

Actin Cytoskeleton↗
PubMed1994

Abnormalities of fibrillin in acquired cutis laxa.

BACKGROUND: Degeneration of elastic tissue in acquired cutis laxa has been previously described, but microfibrils have not been adequately studied. OBJECTIVE: We determined whether the microfibrillar component of elastic tissue is affected in skin of a patient with acquired cutis laxa. METHODS: Lesional skin was examined with indirect immunofluorescence and immunoelectron microscopy with antibodies to fibrillin. RESULTS: Indirect immunofluorescence showed a reduction in the distribution of fibrillin in the papillary dermis, where there was loss of the usual pattern of microfibrils perpendicular to the epidermis. Immunoelectron microscopy showed a typical distribution of elastic microfibrils around elastin of normal skin. In skin affected by cutis laxa microfibrils appeared morphologically normal but appeared less frequently in selected sites. CONCLUSION: The microfibrillar component of elastic fibers was reduced in the papillary dermis of this patient with acquired cutis laxa.

Actin Cytoskeleton↗
PubMed2003

Structural and morphological diversity of (1-->3)-beta-D-glucans synthesized in vitro by enzymes from Saprolegnia monoïca. Comparison with a corresponding in vitro product from blackberry (Rubus fruticosus).

Detergent extracts of microsomal fractions from Saprolegnia monoïca and blackberry (Rubus fruticosus) cells were incubated with UDP-glucose to yield in vitro (1-->3)-beta-d-glucans. The insoluble products were analyzed by conventional and cryo transmission electron microscopy, X-ray diffraction, and (13)C CP/MAS NMR, and their molecular weights were determined by light scattering experiments. All the products were microfibrillar, but for the detergent extracts from S. monoïca, important morphological differences were observed when the pH of the synthesizing medium was modified. At pH 6, the product had a weight average degree of polymerization () exceeding 20 000 and consisted of endless ribbon-like microfibrils. The microfibrils obtained at pH 9 had a length of only 200-300 nm, and their was approximately 5000. Of all the in vitro (1-->3)-beta-d-glucans, the one from R. fruticosus had the shortest length and the smallest. Crystallographic and spectroscopic data showed that the three in vitro samples consisted of triple helices of (1-->3)-beta-d-glucans and contained substantial amounts of water molecules in their structure, the shortest microfibrils being more hydrated. In addition, the long microfibrils from S. monoïca synthesized at pH 6 were more resistant toward the action of an endo-(1-->3)-beta-d-glucanase than the shorter ones obtained at pH 9. These results are discussed in terms of molecular biosynthetic mechanisms of fungal and plant (1-->3)-beta-d-glucans, and in relation with the possible existence of several (1-->3)-beta-d-glucan synthases in a given organism. The interpretation and discussion of these observations integrate the current knowledge of the structure and function of (1-->3)-beta-d-glucans.

Crystallography, X-Ray↗
PubMed2005

Coacervation is promoted by molecular interactions between the PF2 segment of fibrillin-1 and the domain 4 region of tropoelastin.

In forming elastic fibers, microfibrils act as the scaffold sites for depositing the elastin precursor tropoelastin. We examined key binding interactions that promote massive tropoelastin association through coacervation. Using a segment of the microfibril protein fibrillin-1, PF2, known to bind full-length tropoelastin, we mapped its interaction site to the N-terminal region of tropoelastin bounded by domains 2 and 18. Precise contact residues between domain 4 of tropoelastin and domain 16 of fibrillin-1 were discovered through a novel combination of transglutaminase cross-linking and mass spectroscopy, with contact sites at residues K38 of tropoelastin and Q669 of fibrillin-1. This is the first report of a role for this region of tropoelastin in microfibril interactions. The addition of PF2 thermodynamically facilitated the coacervation of tropoelastin, resulting in smaller changes in entropy and enthalpy values for the coacervating system. A novel multicomponent in vitro tropoelastin assembly reaction system demonstrated that amassed tropoelastin was spatially and preferentially directed to surfaces coated with PF2 as expected for organized three-dimensional distribution during tissue elastogenesis. This study underscores the role of this part of fibrillin-1 as an anchor point for tropoelastin at the microfibril-elastin junction during the initial stages of elastic fiber assembly.

Amino Acid Sequence↗
PubMed2005

Atomic force microscopy studies on heat-induced gelation of curdlan.

Heat-induced gelation of a cold-water insoluble polysaccharide, Curdlan, was investigated using atomic force microscopy (AFM). Curdlan dissolved into NaOH aqueous solutions exhibited a spectral transition around 0.2 mol/L NaOH, which is an indicative of conformational transitions from a single helix at a lower alkali concentration to a disordered chain at a higher concentration. Nevertheless, AFM images of Curdlan solubilized in 0.01 mol/L NaOH revealed the presence of heterogeneous supramolecular assemblies of Curdlan: the majority of the molecules were in the form of microfibrils, the lengths of which were on the order of micrometers and the cross-sectional heights of which were approximately 2-3 nm, whereas single molecular chains, partially dissociated from these microfibrils, were also observed. Heating such a sol resulted in the formation of densely cross-linked microgel networks. Heat-induced gelation of Curdlan appears to be initiated by partial dissociation of single chains from supramolecular microfibrils and followed by cross-linking of microfibrils via hydrophobic interactions among these partially dissociated chains.

Gels↗
PubMed1991

Partial sequence of a candidate gene for the Marfan syndrome.

Fibrillin is a large (relative molecular mass 350,000) glycoprotein which can be isolated from fibroblast cell cultures and is a component of the microfibrils that are ubiquitous in the connective tissue space. The microfibrils of the suspensory ligament of the lens as well as the elastic fibre microfibrils of the blood vessel wall are composed of fibrillin. The ocular and cardiovascular manifestations of the Marfan syndrome are consistent with a defect in the gene coding for a structural constituent of these connective tissues. Immunohistological experiments have recently implicated fibrillin microfibrils in the pathogenesis of the Marfan syndrome. Genetic linkage data localizing the Marfan gene to chromosome 15 and the in situ hybridization of fibrillin complementary DNA to 15q21.1 together support fibrillin as a candidate Marfan gene. As a first step towards investigating the function of fibrillin in the architecture and development of connective tissues and its relationship to the Marfan syndrome, we report the cloning and partial sequencing of fibrillin cDNA.

Amino Acid Sequence↗
PubMed1997

Targetting of the gene encoding fibrillin-1 recapitulates the vascular aspect of Marfan syndrome.

Aortic aneurysm and dissection account for about 2% of all deaths in industrialized countries; they are also components of several genetic diseases, including Marfan syndrome (MFS). The vascular phenotype of MFS results from mutations in fibrillin-1 (FBN1), the major constituent of extracellular microfibrils. Microfibrils, either associated with or devoid of elastin, give rise to a variety of extracellular networks in elastic and non-elastic tissues. It is believed that microfibrils regulate elastic fibre formation by guiding tropo-elastin deposition during embryogenesis and early post-natal life. Hence, vascular disease in MFS is thought to result when FBN1 mutations preclude elastic fibre maturation by disrupting microfibrillar assembly. Here we report a gene-targetting experiment in mice that indicates that fibrillin-1 microfibrils are predominantly engaged in tissue homeostasis rather than elastic matrix assembly. This finding, in turn, suggests that aortic dilation is due primarily to the failure by the microfibrillar array of the adventitia to sustain physiological haemodynamic stress, and that disruption of the elastic network of the media is a secondary event.

Aortic Dissection↗