The role of calcium in the organization of fibrillin microfibrils

First report of a short in-frame biallelic deletion removing part of the EGF-like domain calcium-binding motif in LTBP4 and causing autosomal recessive cutis laxa type 1C

Cutis laxa (CL) is a rare connective tissue disorder characterized by wrinkled, abundant and sagging skin, sometimes associated with systemic impairment. Biallelic alterations in latent transforming growth factor beta-binding protein 4 gene (LTBP4) cause autosomal recessive type 1C cutis laxa (ARCL1C, MIM #613177). The present report describes the case of a 17-months-old girl with cutis laxa together with a literature review of previous ARCL1C cases. Based on proband main clinical signs (cutis laxa and pulmonary emphysema), clinical exome sequencing (CES) was performed and showed a new nine base-pairs homozygous in-frame deletion in LTBP4 gene. RT-PCR and cDNA Sanger sequencing were performed in order to clarify its impact on RNA. This report demonstrates that a genetic alteration in the EGF-like 14 domain calcium-binding motif of LTBP4 gene is likely responsible for cutis laxa in our patient.

Combined proteomic and biochemical analyses redefine the consensus sequence requirement for epidermal growth factor-like domain hydroxylation

Epidermal growth factor-like domains (EGFDs) have important functions in cell-cell signaling. Both secreted and cell surface human EGFDs are subject to extensive modifications, including aspartate and asparagine residue C3-hydroxylations catalyzed by the 2-oxoglutarate oxygenase aspartate/asparagine-β-hydroxylase (AspH). Although genetic studies show AspH is important in human biology, studies on its physiological roles have been limited by incomplete knowledge of its substrates. Here, we redefine the consensus sequence requirements for AspH-catalyzed EGFD hydroxylation based on combined analysis of proteomic mass spectrometric data and mass spectrometry-based assays with isolated AspH and peptide substrates. We provide cellular and biochemical evidence that the preferred site of EGFD hydroxylation is embedded within a disulfide-bridged macrocycle formed of 10 amino acid residues. This definition enabled the identification of previously unassigned hydroxylation sites in three EGFDs of human fibulins as AspH substrates. A non-EGFD containing protein, lymphocyte antigen-6/plasminogen activator urokinase receptor domain containing protein 6B (LYPD6B) was shown to be a substrate for isolated AspH, but we did not observe evidence for LYPD6B hydroxylation in cells. AspH-catalyzed hydroxylation of fibulins is of particular interest given their important roles in extracellular matrix dynamics. In conclusion, these results lead to a revision of the consensus substrate requirements for AspH and expand the range of observed and potential AspH-catalyzed hydroxylation in cells, which will enable future study of the biological roles of AspH.

Bioinorganic chemistry of open-angle glaucoma: A review

At present, physical methods of chemical analysis are constantly improving providing large amount of data on elemental composition of organs and tissues. However, only few works describe the correlation (or the potential connection) between the general or local bioelemental imbalances and specific biochemical reactions that are involved in pathogenesis of certain diseases. This review describes primary open-angle glaucoma (POAG) - one of the most common ophthalmic diseases - in terms of elemental chemistry. The authors look into the impact that various subgroups of elements have on passive and active processes of homeostasis regulation and hydrodynamic balance in the eye. Alkaline metals and their analogues (K, Na, Li, Rb, Cs) influence hydrostatics and hydrodynamics by means of both K-Na pumps and osmosis. Alkaline-earth elements and their analogues (Ca, Mg, Sr, Ba, Be) are involved in biomineralization and intercellular interaction in the drainage areas. Chalcophile metals and their analogues (Zn, Cu, Hg, Co, Ni, Cd, Pb, Mo, Sb) regulate redox reactions. They are the cofactors of enzymes that support structural homeostasis of the drainage area. Siderophile metals (Fe, Mn, Cr, Rh) regulate oxidation-reduction reactions, including those associated with limited nutrition of tissues in glaucoma. The role of amphoteric metals and nonmetals (Al, Si, Ga, V, TI, Sn, Ge, Zr, W) in POAG has not been described properly, but they were noted to participate in mineralization. Structure-forming non-metals and their analogues (N, S, Se, As) are directly involved in the formation of protein and non-protein aggregates that prevent aqueous humor outflow. The specific role of phosphorus in the pathogenesis of glaucoma has not been described previously. The authors analyze the involvement of phosphorus in energy-dependent processes of cellular activity, which are aimed at the reprocessing of aggregates that cause aqueous humor retention.

Calcium in Cell-Extracellular Matrix Interactions

In multicellular organisms, the cells are surrounded by persistent, dynamic extracellular matrix (ECM), the largest calcium reservoir in animals. ECM regulates several aspects of cell behavior including cell migration and adhesion, survival, gene expression and differentiation, thus playing a significant role in health and disease. Calcium is reported to be important in the assembly of ECM, where it binds to many ECM proteins. While serving as a calcium reservoir, ECM macromolecules can directly interact with cell surface receptors resulting in calcium transport across the membrane. This chapter mainly focusses on the role of cell-ECM interactions in cellular calcium regulation and how calcium itself mediates these interactions.

Nanoscale insight into the degradation mechanisms of the cartilage articulating surface preceding OA

Osteoarthritis (OA) is a degenerative disease and leading cause of disability globally. We report the a fundamental study of the mechanisms underlying deterioration of hydrated cartilage in the presence of elevated calcium content preceding OA.

Fibulin-4 exerts a dual role in LTBP-4L-mediated matrix assembly and function

Elastogenesis is a hierarchical process by which cells form functional elastic fibers, providing elasticity and the ability to regulate growth factor bioavailability in tissues, including blood vessels, lung, and skin. This process requires accessory proteins, including fibulin-4 and -5, and latent TGF binding protein (LTBP)-4. Our data demonstrate mechanisms in elastogenesis, focusing on the interaction and functional interdependence between fibulin-4 and LTBP-4L and its impact on matrix deposition and function. We show that LTBP-4L is not secreted in the expected extended structure based on its domain composition, but instead adopts a compact conformation. Interaction with fibulin-4 surprisingly induced a conformational switch from the compact to an elongated LTBP-4L structure. This conversion was only induced by fibulin-4 multimers associated with increased avidity for LTBP-4L; fibulin-4 monomers were inactive. The fibulin-4-induced conformational change caused functional consequences in LTBP-4L in terms of binding to other elastogenic proteins, including fibronectin and fibrillin-1, and of LTBP-4L assembly. A transient exposure of LTBP-4L with fibulin-4 was sufficient to stably induce conformational and functional changes; a stable complex was not required. These data define fibulin-4 as a molecular extracellular chaperone for LTBP-4L. The altered LTBP-4L conformation also promoted elastogenesis, but only in the presence of fibulin-4, which is required to escort tropoelastin onto the extended LTBP-4L molecule. Altogether, this study provides a dual mechanism for fibulin-4 in 1) inducing a stable conformational and functional change in LTBP-4L, and 2) promoting deposition of tropoelastin onto the elongated LTBP-4L.

Variability in gene-based knowledge impacts variant classification: an analysis of FBN1 missense variants in ClinVar

Gene-specific knowledge can enhance genetic variant classification, but may not be routinely incorporated into clinical laboratory practice. For example, FBN1 variants associated with Marfan syndrome may be variably classified depending on knowledge of FBN1-specific critical regions. In order to assess variability in classification of FBN1 variants, 674 FBN1 missense variants from 18 ClinVar submitters were compared and reanalyzed using FBN1-specific criteria and ACMG/AMP 2015 guidelines for variant interpretation. Conflicting variant classifications occurred in 30.7% of the missense variants that had multiple submitters. There were 451 classifications of 361 critical residue missense variants, with 80.0% (361/451) classified as likely pathogenic or pathogenic [(L)P]. Non-cysteine critical residue variants were less likely to be classified as (L)P [55.3% (78/141)] than cysteine variants [91.3% (283/310)] and were more likely to lack evidence citing the functional significance of the amino acid impacted. Application of FBN1-specific knowledge allowed for reclassification or discrepancy resolution in 65/361 (18.0%) critical residue variants. There were 522 classifications of 313 unique missense variants not known to impact a critical residue. Of these, 31.6% (165/522) were likely overclassified as either (L)P or uncertain significance (VUS), especially when minor allele frequency (MAF) was taken into account, and we reclassified or resolved classification discrepancies in 128/313 (40.9%) of these variants. Our results provide a refined framework and resource for FBN1 variant classification, and further supports the more global implications of combining gene-based knowledge with ACMG/AMP criteria and appropriate MAF cutoffs for variant classification that extend beyond FBN1.

The phenotypic heterogeneity of patients with Marfan-related disorders and their variant spectrums

Marfan syndrome (MFS) and Loeys-Dietz syndrome (LDS) are the connective tissue disorders characterized by aortic root aneurysm and/or dissection and various additional features. We evaluated the correlation of these mutations with the phenotypes and determined the clinical applicability of the revised Ghent criteria.The mutation spectrum and phenotypic heterogeneities of the 83 and 5 Korean patients with suspected MFS and LDS were investigated as a retrospective manner. In patients with suspected MFS patients, genetic testing was conducted in half of 44 patients who met the revised Ghent criteria clinically and half of 39 patients who did not meet these criteria.Fibrillin1 gene (FBN1) variants were detected in all the 22 patients (100%) who met the revised Ghent criteria and in 14 patients (77.8%) who did not meet the revised Ghent criteria (P = .0205). Patients with mutations in exons 24-32 were diagnosed at a younger age than those with mutations in other exons. Ectopia lentis was more common in patients with missense mutations than in patients with other mutations. Aortic diameter was greater in patients with missense mutations in cysteine residues than in patients with missense mutations in noncysteine residues. Five LDS patients had either TGFBR1 or TGFBR2 variants, of which 1 patient identified TGFBR1 variant uncertain significance.The revised Ghent criteria had very high clinical applicability for detecting FBN1 variants in patients with MFS and might help in selecting patients with suspected MFS for genetic testing.

Structural and compositional diversity of fibrillin microfibrils in human tissues

Elastic fibers comprising fibrillin microfibrils and elastin are present in many tissues, including the skin, lungs, and arteries, where they confer elasticity and resilience. Although fibrillin microfibrils play distinct and tissue-specific functional roles, it is unclear whether their ultrastructure and composition differ between elastin-rich (skin) and elastin-poor (ciliary body and zonule) organs or after in vitro synthesis by cultured cells. Here, we used atomic force microscopy, which revealed that the bead morphology of fibrillin microfibrils isolated from the human eye differs from those isolated from the skin. Using newly developed pre-MS preparation methods and LC-MS/MS, we detected tissue-specific regions of the fibrillin-1 primary structure that were differentially susceptible to proteolytic extraction. Comparing tissue- and culture-derived microfibrils, we found that dermis- and dermal fibroblast–derived fibrillin microfibrils differ in both bead morphology and periodicity and also exhibit regional differences in fibrillin-1 proteolytic susceptibility. In contrast, collagen VI microfibrils from the same dermal or fibroblast samples were invariant in ultrastructure (periodicity) and protease susceptibility. Finally, we observed that skin- and eye-derived microfibril suspensions were enriched in elastic fiber– and basement membrane–associated proteins, respectively. LC-MS/MS also identified proteins (such as calreticulin and protein-disulfide isomerase) that are potentially fundamental to fibrillin microfibril biology, regardless of their tissue source. Fibrillin microfibrils synthesized in cell culture lacked some of these key proteins (MFAP2 and -4 and fibrillin-2). These results showcase the structural diversity of these key extracellular matrix assemblies, which may relate to their distinct roles in the tissues where they reside.

Fell-Muir Lecture: Fibrillin microfibrils: structural tensometers of elastic tissues?

Fibrillin microfibrils are indispensable structural elements of connective tissues in multicellular organisms from early metazoans to humans. They have an extensible periodic beaded organization, and support dynamic tissues such as ciliary zonules that suspend the lens. In tissues that express elastin, including blood vessels, skin and lungs, microfibrils support elastin deposition and shape the functional architecture of elastic fibres. The vital contribution of microfibrils to tissue form and function is underscored by the heritable fibrillinopathies, especially Marfan syndrome with severe elastic, ocular and skeletal tissue defects. Research since the early 1990s has advanced our knowledge of biology of microfibrils, yet understanding of their mechanical and homeostatic contributions to tissues remains far from complete. This review is a personal reflection on key insights, and puts forward the conceptual hypothesis that microfibrils are structural 'tensometers' that direct cells to monitor and respond to altered tissue mechanics.

Fibrillin–integrin interactions in health and disease

Human fibrillin-1 is the major structural protein of extracellular matrix 10–12 nm microfibrils. It has a disulfide-rich modular organization which consists primarily of cbEGF (Ca2+-binding epidermal growth factor-like) domains and TB (transforming growth factor β-binding protein-like) domains. TB4 contains an RGD (Arg-Gly-Asp) integrin-binding motif. The atomic structure of this region has been solved by X-ray crystallography and shows the TB4 and flanking cbEGF domains to be arranged as a tetragonal pyramid with N- and C-termini exposed at opposite ends of the fragment. The RGD integrin-binding motif is located within a flexible loop. We have used a variety of biophysical, biochemical and cell biology methods to investigate the molecular properties of integrin–fibrillin-1 interactions and have demonstrated that recombinant fibrillin-1 domain fragments mediate binding to integrins αVβ3, α5β1 and αVβ6. Integrin αVβ3 is a high-affinity fibrillin-1 receptor (Kd ∼40 nM), whereas integrins αVβ6 and α5β1 show moderate-affinity (Kd ∼450 nM) and low-affinity (Kd >1 μM) binding respectively. Different patterns of α5β1 distribution are seen when human keratinocytes and fibroblasts are plated on to fibrillin domain fragments compared with those seen for fibronectin, suggesting that fibrillin may cause a lesser degree or different type of intracellular signalling. A number of disease-causing mutations which affect the TB4 domain have been identified. These are being investigated for their effects on integrin binding and/or changes in intramolecular structure.

Effects of fibrillin-1 degradation on microfibril ultrastructure

Current models of the elastic properties and structural organization of fibrillin-containing microfibrils are based primarily on microscopic analyses of microfibrils liberated from connective tissues after digestion with crude collagenase. Results presented here demonstrate that this digestion resulted in the cleavage of fibrillin-1 and loss of specific immunoreactive epitopes. The proline-rich region and regions near the second 8-cysteine domain in fibrillin-1 were easily cleaved by crude collagenase. Other sites that may also be cleaved during microfibril digestion and extraction were identified. In contrast to collagenase-digested microfibrils, guanidine-extracted microfibrils contained all fibrillin-1 epitopes recognized by available antibodies. The ultrastructure of guanidine-extracted microfibrils differed markedly from that of collagenase-digested microfibrils. Fibrillin-1 filaments splayed out, extending beyond the width of the periodic globular beads. Both guanidine-extracted and collagenase-digested microfibrils were subjected to extensive digestion by crude collagenase. Collagenase digestion of guanidine-extracted microfibrils removed the outer filaments, revealing a core structure. In contrast to microfibrils extracted from tissues, cell culture microfibrils could be digested into short units containing just a few beads. These data suggest that additional cross-links stabilize the long beaded microfibrils in tissues. Based on the microfibril morphologies observed after these experiments, on the crude collagenase cleavage sites identified in fibrillin-1, and on known antibody binding sites in fibrillin-1, a model is proposed in which fibrillin-1 molecules are staggered in microfibrils. This model further suggests that the N-terminal half of fibrillin-1 is asymmetrically exposed in the outer filaments, whereas the C-terminal half of fibrillin-1 is present in the interior of the microfibril.

The molecular genetics of Marfan syndrome and related disorders

Marfan syndrome (MFS), a relatively common autosomal dominant hereditary disorder of connective tissue with prominent manifestations in the skeletal, ocular, and cardiovascular systems, is caused by mutations in the gene for fibrillin-1 (FBN1). The leading cause of premature death in untreated individuals with MFS is acute aortic dissection, which often follows a period of progressive dilatation of the ascending aorta. Recent research on the molecular physiology of fibrillin and the pathophysiology of MFS and related disorders has changed our understanding of this disorder by demonstrating changes in growth factor signalling and in matrix-cell interactions. The purpose of this review is to provide a comprehensive overview of recent advances in the molecular biology of fibrillin and fibrillin-rich microfibrils. Mutations in FBN1 and other genes found in MFS and related disorders will be discussed, and novel concepts concerning the complex and multiple mechanisms of the pathogenesis of MFS will be explained.

Fibrillin-1 misfolding and disease

Fibrillin-1 is a 350 kDa calcium-binding protein which assembles to form 10-12 nm microfibrils in the extracellular matrix (ECM). The structure of fibrillin-1 is dominated by two types of disulfide-rich motifs, the calcium- binding epidermal growth factor-like (cbEGF) and transforming growth factor beta binding protein-like (TB) domains. Disruption of fibrillin-1 domain structure and function contributes to the pathogenic mechanisms underlying two inherited diseases with very different etiologies: Marfan syndrome (MFS) and homocystinuria (HC). MFS is a connective tissue disease caused by mutations in the fibrillin-1 gene FBN1. Many missense mutations cause variable degrees of fibrillin-1 domain misfolding, which may affect the delivery of fibrillin-1 to the ECM and/or its assembly into microfibrils. HC is a metabolic disorder which affects methionine metabolism and results in raised serum levels of the highly reactive thiol-containing amino acid homocysteine. Patients with HC often exhibit ocular and skeletal defects resembling the MFS phenotype, suggesting that elevated homocysteine levels may lead to chemical reduction of disulfide bonds within fibrillin-1 domains resulting in the loss of native structure. Protein misfolding therefore is implicated in pathogenic mechanisms underlying MFS and HC.

The mechanical function and structure of aortic microfibrils in the lobster Homarus americanus

Marfan syndrome, a connective tissue disorder affecting the cardiovascular system, is caused by mutations of fibrillin-based microfibrils. These mutations often affect the calcium-binding domains, resulting in structural changes to the proteins. It is hypothesized that these Ca+2 binding sites regulate the structure and mechanical properties of the microfibrils. The mechanical properties of fresh and extracted lobster aortic rings in calcium solutions (1, 13 and 30 mM Ca+2) were measured. Samples underwent amino acid compositional analysis. Antibodies were produced against the material comprising extracted aortic rings. The ultrastructure of strained and unstrained samples was examined using transmission electron microscopy. Calcium level altered the tangent modulus of fresh vessels. These rings were significantly stiffer when tested at 30 mM Ca+2 compared to rings tested at 1 mM Ca+2. Amino acid comparisons between extracted samples, porcine and human fibrillin showed compositional similarity. Immunohistochemical analysis showed that antibodies produced against the material in extracted samples localized to the known microfibrillar elements in the lobster aorta and cross-reacted with fibrillin microfibrils of mammalian ciliary zonules. Ultrastructurally, vessels incubated in low calcium solutions showed diffuse interbead regions while those incubated in physiological or high calcium solutions showed interbead regions with more defined lateral edges.

Ca2+-dependent interface formation in fibrillin-1

The calcium-binding epidermal growth factor-like (cbEGF) domain is a common structural motif in extracellular and transmembrane proteins. K(d) values for Ca2+ vary from the millimolar to nanomolar range; however the molecular basis for this variation is poorly understood. We have measured K(d) values for six fibrillin-1 cbEGF domains, each preceded by a transforming growth factor beta-binding protein-like (TB) domain. Using NMR and titration with chromophoric chelators, we found that K(d) values varied by five orders of magnitude. Interdomain hydrophobic contacts between TB-cbEGF domains were studied by site-directed mutagenesis and could be correlated directly with Ca2+ affinity. Furthermore, in TB-cbEGF pairs that displayed high-affinity binding, NMR studies showed that TB-cbEGF interface formation was strongly Ca2+-dependent. We suggest that Ca2+ affinity is a measure of interface formation in both homologous and heterologous cbEGF domain pairs, thus providing a measure of flexibility in proteins with multiple cbEGF domains. These data highlight the versatile role of the cbEGF domain in fine tuning the regional flexibility of proteins and provide new constraints for the organization of fibrillin-1 within 10-12-nm microfibrils of the extracellular matrix.

Fibrillin microfibrils

Fibrillin microfibrils are widely distributed extracellular matrix assemblies that endow elastic and nonelastic connective tissues with long-range elasticity. They direct tropoelastin deposition during elastic fibrillogenesis and form an outer mantle for mature elastic fibers. Microfibril arrays are also abundant in dynamic tissues that do not express elastin, such as the ciliary zonules of the eye. Mutations in fibrillin-1-the principal structural component of microfibrils-cause Marfan syndrome, a heritable disease with severe aortic, ocular, and skeletal defects. Isolated fibrillin-rich microfibrils have a complex 56 nm "beads-on-a-string" appearance; the molecular basis of their assembly and elastic properties, and their role in higher-order elastic fiber formation, remain incompletely understood.

Structural and Functional Properties of the Human Notch-1 Ligand Binding Region

We present NMR structural and dynamics analysis of the putative ligand binding region of human Notch-1, comprising EGF-like domains 11-13. Functional integrity of an unglycosylated, recombinant fragment was confirmed by calcium-dependent binding of tetrameric complexes to ligand-expressing cells. EGF modules 11 and 12 adopt a well-defined, rod-like orientation rigidified by calcium. The interdomain tilt is similar to that found in previously studied calcium binding EGF pairs, but the angle of twist is significantly different. This leads to an extended double-stranded beta sheet structure, spanning the two EGF modules. Based on the conservation of residues involved in interdomain hydrophobic packing, we propose this arrangement to be prototypical of a distinct class of EGF linkages. On this premise, we have constructed a model of the 36 EGF modules of the Notch extracellular domain that enables predictions to be made about the general role of calcium binding to this region.

Microfibril-associated glycoprotein-1 binding to tropoelastin: multiple binding sites and the role of divalent cations

Microfibrils and elastin are major constituents of elastic fibers, the assembly of which is dictated by multimolecular interactions. Microfibril-associated glycoprotein-1 (MAGP-1) is a microfibrillar component that interacts with the soluble elastin precursor, tropoelastin. We describe here the adaptation of a solid-phase binding assay that defines the effect of divalent cations on the interactions between MAGP-1 and tropoelastin. Using this assay, a strong calcium-dependent interaction was demonstrated, with a dissociation constant of 2.8 +/- 0.3 nm, which fits a single-site binding model. Manganese and magnesium bestowed a weaker association, and copper did not facilitate the protein interactions. Three constructs spanning tropoelastin were used to quantify their relative contributions to calcium-dependent MAGP-1 binding. Binding to a construct spanning a region from the N-terminus to domain 18 followed a single-site binding model with a dissociation constant of 12.0 +/- 2.2 nm, which contrasted with the complex binding behavior observed for fragments spanning domains 17-27 and domain 27 to the C-terminus. To further elucidate binding sites around the kallikrein cleavage site of domains 25/26, MAGP-1 was presented with constructs containing C-terminal deletions within the region. Construct M1659, which spans a region from the N-terminus of tropoelastin to domain 26, inclusive, bound MAGP-1 with a dissociation constant of 9.7 +/- 2.0 nm, which decreased to 4.9 +/- 1.0 nm following the removal of domain 26 (M155n), thus displaying only half the total capacity to bind MAGP-1. These results demonstrate that MAGP-1 is capable of cumulative binding to distinct regions on tropoelastin, with different apparent dissociation constants and different amounts of bound protein.

Solution structure and dynamics of a calcium binding epidermal growth factor-like domain pair from the neonatal region of human fibrillin-1

Fibrillin-1 is a mosaic protein mainly composed of 43 calcium binding epidermal growth factor-like (cbEGF) domains arranged as multiple, tandem repeats. Mutations within the fibrillin-1 gene cause Marfan syndrome (MFS), a heritable disease of connective tissue. More than 60% of MFS-causing mutations identified are localized to cbEGFs, emphasizing that the native properties of these domains are critical for fibrillin-1 function. The cbEGF12-13 domain pair is within the longest run of cbEGFs, and many mutations that cluster in this region are associated with severe, neonatal MFS. The NMR solution structure of Ca(2+)-loaded cbEGF12-13 exhibits a near-linear, rod-like arrangement of domains. This observation supports the hypothesis that all fibrillin-1 (cb)EGF-cbEGF pairs, characterized by a single interdomain linker residue, possess this rod-like structure. The domain arrangement of cbEGF12-13 is stabilized by additional interdomain packing interactions to those observed for cbEGF32-33, which may help to explain the previously reported higher calcium binding affinity of cbEGF13. Based on this structure, a model of cbEGF11-15 that encompasses all known neonatal MFS missense mutations has highlighted a potential binding region. Backbone dynamics data confirm the extended structure of cbEGF12-13 and lend support to the hypothesis that a correlation exists between backbone flexibility and cbEGF domain calcium affinity. These results provide important insight into the potential consequences of MFS-associated mutations for the assembly and biomechanical properties of connective tissue microfibrils.

Elastic fibres

Elastic fibres are essential extracellular matrix macromolecules comprising an elastin core surrounded by a mantle of fibrillin-rich microfibrils. They endow connective tissues such as blood vessels, lungs and skin with the critical properties of elasticity and resilience. The biology of elastic fibres is complex because they have multiple components, a tightly regulated developmental deposition, a multi-step hierarchical assembly and unique biomechanical functions. However, their molecular complexity is at last being unravelled by progress in identifying interactions between component molecules, ultrastructural analyses and studies of informative mouse models.

Fibrillin: from microfibril assembly to biomechanical function

Fibrillins form the structural framework of a unique and essential class of extracellular microfibrils that endow dynamic connective tissues with long-range elasticity. Their biological importance is emphasized by the linkage of fibrillin mutations to Marfan syndrome and related connective tissue disorders, which are associated with severe cardiovascular, ocular and skeletal defects. These microfibrils have a complex ultrastructure and it has proved a major challenge both to define their structural organization and to relate it to their biological function. However, new approaches have at last begun to reveal important insights into their molecular assembly, structural organization and biomechanical properties. This paper describes the current understanding of the molecular assembly of fibrillin molecules, the alignment of fibrillin molecules within microfibrils and the unique elastomeric properties of microfibrils.

Effects of the N2144S mutation on backbone dynamics of a TB-cbEGF domain pair from human fibrillin-1

The calcium-binding epidermal growth factor-like (cbEGF) module and the transforming growth factor beta-binding protein-like (TB) module are the two major structural motifs found in fibrillin-1, the extracellular matrix (ECM) protein defective in the Marfan syndrome (MFS). An MFS-causing mutation, N2144S, which removes a calcium ligand in cbEGF32, does not detectably affect fibrillin-1 biosynthesis, rate of secretion, processing, or deposition of reducible fibrillin-1 into the ECM. Since the residue at position 2144 is normally engaged in calcium ligation, it is unable to mediate intermolecular interactions. We have shown previously that this mutation does not affect the folding properties of the TB or cbEGF domains in vitro, but does decrease calcium-binding in cbEGF and TB-cbEGF domain constructs. Here, we use NMR spectroscopy to probe the effects of the N2144S mutation on backbone dynamic properties of TB6-cbEGF32. Analysis of the backbone (15)N relaxation data of wild-type TB6-cbEGF32 has revealed a flexible inter-domain linkage. Parallel dynamics analysis of the N2144S mutant has shown increased flexibility in the region joining the two domains as well as in the calcium-binding site at the N terminus of cbEGF32. This research demonstrates that a small change in peptide backbone flexibility, which does not enhance proteolytic susceptibility of the domain pair, is associated with an MFS phenotype. Flexibility of the TB-cbEGF linkage is likely to contribute to the biomechanical properties of fibrillin-rich connective tissue microfibrils, and may play a role in the microfibril assembly process.

Protein interaction studies of MAGP-1 with tropoelastin and fibrillin-1

Elastic fibers consist primarily of an amorphous elastin core associated with microfibrils, 10-12 nm in diameter, containing fibrillins and microfibril-associated glycoproteins (MAGPs). To investigate the interaction of MAGP-1 with tropoelastin and fibrillin-1, we expressed human MAGP-1 as a T7-tag fusion protein in Escherichia coli. Refolding of the purified protein produced a soluble form of MAGP-1 that displayed saturable binding to tropoelastin. Fragments of tropoelastin corresponding to the N-terminal, C-terminal, and central regions of the molecule were used to characterize the MAGP-1 binding site. Cleavage of tropoelastin with kallikrein, which cleaves after Arg(515) in the central region of the molecule, disrupted the interaction, suggesting that the separated N- and C-terminal fragments were insufficient to determine MAGP-1 binding to intact tropoelastin. In addition, no evidence of an interaction was observed between MAGP-1 and a tropoelastin construct consisting of domains 17-27 that brackets the kallikrein cleavage site, suggesting a complex mechanism of interaction between the two molecules. Binding of MAGP-1 was also tested with overlapping recombinant fibrillin-1 fragments. MAGP-1 bound to a region at the N terminus of fibrillin-1 in a calcium-dependent manner. In summary, these results suggest a model for the interaction of elastin with the microfibrillar scaffold.

Role of Ca(2+) for the mechanical properties of fibrillin

Fibrillin-rich microfibrils are important structural elements widespread throughout connective tissues. Genetic defects identified in the Ca(2+) binding sites of fibrillin have severe effects and in addition Ca(2+) has a marked effect on the microfibrillar structure. We have studied the role of Ca(2+) on the mechanical behavior of fibrillin-rich microfibrils using the micro-needle technique. We find that Ca(2+)-depletion results in a 50% decrease in rest length and reduces the stiffness of fibrillin-rich microfibrils. At high strain, irreversible damage occurs. This behavior is consistent with Ca(2+) stabilization of interactions between consecutive EGF-like domains and breakdown in the quaternary structure upon over-extension.

The supramolecular organization of fibrillin-rich microfibrils

We propose a new model for the alignment of fibrillin molecules within fibrillin microfibrils. Automated electron tomography was used to generate three-dimensional microfibril reconstructions to 18.6-A resolution, which revealed many new organizational details of untensioned microfibrils, including heart-shaped beads from which two arms emerge, and interbead diameter variation. Antibody epitope mapping of untensioned microfibrils revealed the juxtaposition of epitopes at the COOH terminus and near the proline-rich region, and of two internal epitopes that would be 42-nm apart in unfolded molecules, which infers intramolecular folding. Colloidal gold binds microfibrils in the absence of antibody. Comparison of colloidal gold and antibody binding sites in untensioned microfibrils and those extended in vitro, and immunofluorescence studies of fibrillin deposition in cell layers, indicate conformation changes and intramolecular folding. Mass mapping shows that, in solution, microfibrils with periodicities of <70 and >140 nm are stable, but periodicities of approximately 100 nm are rare. Microfibrils comprise two in-register filaments with a longitudinal symmetry axis, with eight fibrillin molecules in cross section. We present a model of fibrillin alignment that fits all the data and indicates that microfibril extensibility follows conformation-dependent maturation from an initial head-to-tail alignment to a stable approximately one-third staggered arrangement.

Energy-filtering transmission electron microscopy (EFTEM) in the elemental analysis of pseudoexfoliative material

PURPOSE

To obtain more information on the basic nature of the pathological matrix product accumulating in pseudoexfoliation (PEX) syndrome by analyzing its elemental composition at the subcellular level.

METHODS

Energy-filtering transmission electron microscopy (EFTEM), combining the two microanalytical techniques of electron spectroscopic imaging (ESI) and energy-loss spectroscopy (EELS), were performed on ultrathin sections of lens specimens with PEX syndrome using a transmission electron microscope equipped with an integrated electron energy filter. EFTEM is based on inner shell ionization of elements present in the sample giving rise to characteristic signals in well-defined energy-loss regions. The EEL-spectra, demonstrating the presence of a particular element by its specific electron energy-loss edge, were recorded with an integrated scintillator-photomultiplier-system. ESI generated graphic images of elemental localization in the sections after a process of background correction with an IBAS image analysis program. Energy-dispersive X-ray (EDX) analysis of PEX deposits on hydrated lenses was conducted by variable pressure scanning electron microscopy.

RESULTS

The ESI element distribution images of both intracapsular and supracapsular PEX material displayed high signals for nitrogen, sulfur, calcium, chlorine, and zinc in clear association with the PEX fibrils. The corresponding EEL-spectra confirmed the data obtained by ESI and showed the presence of the element-specific energy-loss edges. The presence of these elements in PEX fibrils was further confirmed by EDX analysis. No specific signals were obtained for phosphorus, oxygen, or aluminum.

CONCLUSIONS

This study demonstrates the presence of nitrogen, sulfur, chlorine, zinc, and calcium both in mature and in aggregating PEX fibrils of the lens capsule. EFTEM proved to be a highly sensitive method for the microanalytical study of biological material with unknown composition, such as PEX material, at the subcellular level.

Fibrillin-rich microfibrils of the extracellular matrix: ultrastructure and assembly

Fibrillin-rich microfibrils are a unique class of extensible connective tissue macromolecules. Their critical contribution to the establishment and maintenance of diverse extracellular matrices was underlined by the linkage of their principal structural component fibrillin to Marfan syndrome, a heritable connective tissue disorder with pleiotropic manifestations. Microscopy and preparative techniques have contributed substantially to the understanding of microfibril structure and function. The supramolecular organisation of microfibrillar assemblies in tissues has been examined by tissue sectioning and X-ray diffraction methods. Published findings are discussed and new information reported on the organisation of microfibrils in the ciliary zonular fibrils by environmental scanning electron microscopy. This review summarises microscopy and X-ray diffraction studies that are informing current understanding of the ultrastructure of fibrillin-rich microfibrils.

Fibrillin-1, a calcium binding protein of extracellular matrix

Fibrillin-1 is a large extracellular matrix glycoprotein which assembles to form 10-12 nm microfibrils in extracellular matrix. Mutations in the human fibrillin-1 gene (FBN-1) cause the connective tissue disease Marfan syndrome and related disorders, which are characterised by defects in the skeletal, cardiovascular and ocular systems of the body. Fibrillin-1 has a striking modular organisation which is dominated by multiple tandem repeats of the calcium binding epidermal growth factor-like (cbEGF) domain. This review focuses on recent studies which have investigated the structural and functional role of calcium binding to cbEGF domains in fibrillin-1 and 10-12 nm microfibrils.

Fibrillin: from domain structure to supramolecular assembly

In the last 5 years, significant progress has been made in understanding the structure and function of all the major domains composing the fibrillins. A previous review [Meth. Enzymol. 245 (1994), 29] focused on the isolation of fibrillin monomers and fibrillin-containing polymers (microfibrils). In this article, information gained from recent studies which have further elucidated molecular structure and investigated effects of mutations on structural and functional properties will be summarized. In addition, studies of functional domains in fibrillins which may be important in assembling microfibrils will be discussed. Throughout this review, the authors have attempted to identify areas of research which have been controversial. In the conclusion, we raise important questions which remain unresolved.

Fibrillin-1 mutations in Marfan syndrome and other type-1 fibrillinopathies

Fibrillin is the major component of extracellular microfibrils and is widely distributed in connective tissue throughout the body. Mutations in the fibrillin-1 (FBN1) gene, on chromosome 15q21.1, have been found to cause Marfan syndrome, a dominantly inherited disorder characterised by clinically variable skeletal, ocular, and cardiovascular abnormalities. Fibrillin-1 mutations have also been found in several other related connective tissue disorders, such as severe neonatal Marfan syndrome, dominant ectopia lentis, familial ascending aortic aneurysm, isolated skeletal features of Marfan syndrome, and Shprintzen-Goldberg syndrome. Mutations are spread throughout the gene and, with the exception of neonatal Marfan syndrome, show no obvious clustering or phenotypic association.

Calcium-binding EGF-like modules in coagulation proteinases: function of the calcium ion in module interactions

Epidermal growth factor (EGF)-like modules are involved in protein-protein interactions and are found in numerous extracellular proteins and membrane proteins. Among these proteins are enzymes involved in blood coagulation, fibrinolysis and the complement system as well as matrix proteins and cell surface receptors such as the EGF precursor, the low density lipoprotein receptor and the developmentally important receptor, Notch. The coagulation enzymes, factors VII, IX and X and protein C, all have two EGF-like modules, whereas the cofactor of activated protein C, protein S, has four EGF-like modules in tandem. Certain of the cell surface receptors have numerous EGF modules in tandem. A subset of EGF modules bind one Ca(2+). The Ca(2+)-binding sequence motif is coupled to a sequence motif that brings about beta-hydroxylation of a particular Asp/Asn residue. Ca(2+)-binding to an EGF module is important to orient neighboring modules relative to each other in a manner that is required for biological activity. The Ca(2+) affinity of an EGF module is often influenced by its N-terminal neighbor, be it another EGF module or a module of another type. This can result in an increase in Ca(2+) affinity of several orders of magnitude. Point mutations in EGF modules that involve amino acids which are Ca(2+) ligands result in the biosynthesis of biologically inactive proteins. Such mutations have been identified, for instance, in factor IX, causing hemophilia B, in fibrillin, causing Marfan syndrome, and in the low density lipoprotein receptor, causing hypercholesterolemia. In this review the emphasis will be on the coagulation factors.

Backbone dynamics of a cbEGF domain pair in the presence of calcium

Calcium binding (cb) epidermal growth factor-like (EGF) domains are found in a wide variety of extracellular proteins with diverse functions. In several proteins, including the fibrillins (1 and 2), the low-density lipoprotein receptor, the Notch receptor and related molecules, these domains are organised as multiple tandem repeats. The functional importance of calcium-binding by EGF domains has been underscored by the identification of missense mutations associated with defective calcium-binding, which have been linked to human diseases. Here, we present (15)N backbone relaxation data for a pair of cbEGF domains from fibrillin-1, the defective protein in the Marfan syndrome. The data were best fit using a symmetric top model, confirming the extended conformation of the cbEGF domain pair. Our data demonstrate that calcium plays a key role in stabilising the rigidity of the domain pair on the pico- to millisecond time-scale. Strikingly, the most dynamically stable region of the construct is centred about the domain interface. These results provide important insight into the properties of intact fibrillin-1, the consequences of Marfan syndrome causing mutations, and the ultrastructure of fibrillins and other extracellular matrix proteins.

Partial cloning and sequencing of chick fibrillin-1 cDNA

The recent identification of numerous matrix genes and gene products has allowed a detailed examination of their roles in development. Two of these extracellular matrix proteins, fibrillin-1 and fibrillin-2, are components of the elastin-associated microfibrils. Given what is known about the distribution of the fibrillins in normal tissues and the abnormalities that result when mutations occur, a basic hypothesis has emerged: fibrillin-1 is primarily responsible for load bearing and providing structural integrity, whereas fibrillin-2 may be a director of elastogenesis. Nevertheless, examination of phenotypes in disorders caused by mutations in fibrillin-1 or fibrillin-2 suggests some common functions. To better understand these similar and diverse roles, it would be helpful to examine these proteins during chick development. To accomplish this goal, it is first necessary to characterize the chick homologs of the known fibrillins. In this study, the partial chick FBN1 cDNA was identified by polymerase chain reaction-aided cloning as a first step toward elucidating these goals. Sequence analysis indicated that there is striking conservation between chick and mammalian fibrillin-1 at the DNA and protein levels. Antisense and sense riboprobes were synthesized and used in in situ hybridization in stage 14 chick embryos and high levels of FBN1 transcripts were observed in the heart.

The molecular genetics of Marfan syndrome and related microfibrillopathies

Mutations in the gene for fibrillin-1 (FBN1) have been shown to cause Marfan syndrome, an autosomal dominant disorder of connective tissue characterised by pleiotropic manifestations involving primarily the ocular, skeletal, and cardiovascular systems. Fibrillin-1 is a major component of the 10-12 nm microfibrils, which are thought to play a role in tropoelastin deposition and elastic fibre formation in addition to possessing an anchoring function in some tissues. Fibrillin-1 mutations have also been found in patients who do not fulfil clinical criteria for the diagnosis of Marfan syndrome, but have related disorders of connective tissue, such as isolated ectopia lentis, familial aortic aneurysm, and Marfan-like skeletal abnormalities, so that Marfan syndrome may be regarded as one of a range of type 1 fibrillinopathies. There appear to be no particular hot spots since mutations are found throughout the entire fibrillin-1 gene. However, a clustering of mutations associated with the most severe form of Marfan syndrome, neonatal Marfan syndrome, has been noted in a region encompassing exons 24 to 32. The gene for fibrillin-2 (FBN2) is highly homologous to FBN1, and mutations in FBN2 have been shown to cause a phenotypically related disorder termed congenital contractural arachnodactyly. Since mutations in the fibrillin genes are likely to affect the global function of the microfibrils, the term microfibrillopathy may be the most appropriate to designate the spectrum of disease associated with dysfunction of these molecules. The understanding of the global and the molecular functions of the fibrillin containing microfibrils is still incomplete and, correspondingly, no comprehensive theory of the pathogenesis of Marfan syndrome has emerged to date. Many, but not all, fibrillin-1 gene mutations are expected to exert a dominant negative effect, whereby mutant fibrillin monomers impair the global function of the microfibrils. In this paper we review the molecular physiology and pathophysiology of Marfan syndrome and related microfibrillopathies.

The supramolecular organisation of fibrillin-rich microfibrils determines the mechanical properties of bovine zonular filaments

The zonular filaments from the eyes of cows are rich in microfibrils containing fibrillin. Tensile tests, stress-relaxation tests and X-ray diffraction studies were used to study the relationship between the mechanical behaviour of zonular filaments and the molecular packing and structure of the fibrillin-rich microfibrils. Zonular filaments show a non-linear (J-shaped) stress-strain curve and appreciable stress-relaxation. It is proposed that the non-linear properties are due to local variations in waviness in the microfibrils or assemblies of microfibrils, which straighten out and become more regularly aligned with strain. Previous and current X-ray diffraction results consistently show a partial ordering of microfibrils in zonular filaments into staggered aggregates which become more ordered and laterally aligned on stretching. Although the removal and re-addition of Ca(2+) is known to change the molecular structure of fibrillin, no effect was observed on the tensile properties of the zonular filaments. It is hypothesised that strain-induced deformation in the supramolecular aggregate packing may not be Ca(2+)-sensitive but could dominate the mechanical behaviour of microfibrillar arrays in zonular filaments.

Cysteine substitutions in epidermal growth factor-like domains of fibrillin-1: distinct effects on biochemical and clinical phenotypes

Fibrillin-1 (FBN1) contains 47 epidermal growth factor (EGF)-like domains characterized by six conserved cysteine residues. Cysteine substitutions that disrupt one of the three disulfide bonds are frequent causes of Marfan syndrome (MFS). We identified 19 new substitutions involving cysteine residues in each of the six positions of EGF-like domains. Allele-specific mRNA assays revealed equal abundance of mutant and normal FBN1 transcripts in all 10 individuals studied. Quantitative pulse-chase analysis of fibrillin protein was performed on 25 mutant fibroblast strains with substitutions of 22 different cysteine residues in 18 different EGF-like domains spanning the entire gene. Normal synthesis and stability of mutant fibrillin molecules was seen in 20/25 individuals, 11 of whom showed delayed intracellular processing and/or secretion. In the remaining five cases, the mutant protein was apparently unstable. In four of these five cases, the second or third disulfide bond of EGF-like domains immediately preceding an 8-cysteine or hybrid domain was affected. All but two mutations caused severe reduction of matrix deposition, which was attributed to a dominant-negative effect of mutant molecules. For genotype/phenotype comparisons, clinical data on 25 probands and 19 mutation-positive family members were analyzed. Ocular manifestations were among the most consistent features (ectopia lentis in 86%, myopia in 80%). Nine mutations encoded by exons 26-32 resulted in early-onset classic MFS and, in one case, neonatal-lethal MFS. Mutations outside this region were associated with variable clinical phenotypes, including individuals with fibrillinopathies not meeting diagnostic criteria for MFS.

Microfibrils from the arterial subendothelium

Microfibrillar structures of the subendothelium are represented by either type VI collagen or elastin-associated microfibrils which are also referred to as fibrillin-containing microfibrils. These structures are present throughout the subendothelium irrespective of the presence of elastin. The localization, structure, and protein composition of microfibrils are reviewed. The arterial subendothelium is thrombogenic despite its very low content in fibrillar collagens. This thrombogenicity is linked to the microfibrillar structures, essentially to type VI collagen and to thrombospondin-containing microfibrils. Their respective ability to bind the von Willebrand factor and to activate blood platelets is discussed.

EGF-like domain calcium affinity modulated by N-terminal domain linkage in human fibrillin-1

Calcium binding epidermal growth factor-like domains (cbEGFs) are present in many extracellular proteins, including fibrillin-1, Notch-3, protein S, factor IX and the low density lipoprotein (LDL) receptor, which perform a diverse range of functions. Genetic mutations that cause amino acid changes within these proteins have been linked to the Marfan syndrome (MFS), CADASIL, protein S deficiency, haemophilia B and familial hypercholesterolaemia, respectively. A number of these mutations disrupt calcium binding to cbEGFs, emphasising the critical functional role of calcium in these proteins. We have determined the calcium binding affinity of two sites within a cbEGF pair (cbEGF12-13) from human fibrillin-1 using two-dimensional nuclear magnetic resonance (NMR) and fluorescence techniques. Fibrillin-1 is a mosaic protein containing 43 cbEGF domains, mainly arranged as tandem repeats. Our results show that the cbEGF13 site in the cbEGF12-13 pair possesses the highest calcium affinity of any cbEGF investigated from fibrillin-1. A comparative analysis of these and previously reported calcium binding data from fibrillin-1 demonstrate that the affinity of cbEGF13 is enhanced more than 70-fold by the linkage of an N-terminal cbEGF domain. In contrast, comparison of calcium binding by cbEGF32 in isolation relative to when linked to a transforming growth factor beta-binding protein-like domain (TB6-cbEGF32) reveals that the same enhancement is not observed for this heterologous domain pair. Taken together, these results indicate that fibrillin-1 cbEGF Ca2+ affinity can be significantly modulated by the type of domain which is linked to its N terminus. The cbEGF12-13 pair is located within the longest contiguous section of cbEGFs in fibrillin-1, and a number of mutations in this region are associated with the most severe neonatal form of MFS. The affinities of cbEGF domains 13 and 14 in this region are substantially higher than in the C-terminal region of fibrillin-1. This increased affinity may be important for fibrillin assembly into 10-12 nm connective tissue microfibrils and/or may contribute to the biomechanical properties of the microfibrillar network.

Multiple molecular mechanisms underlying subdiagnostic variants of Marfan syndrome

Mutations in the FBN1 gene, which encodes fibrillin-1, cause Marfan syndrome (MFS) and have been associated with a wide range of milder, overlap phenotypes. The factors that modulate phenotypic severity, both between and within families, remain to be determined. This study examines the relationship between the FBN1 genotype and phenotype in families with extremely mild phenotypes and in those that show striking clinical variation among apparently affected individuals. In one family, clinically similar but etiologically distinct disorders are segregating independently. In another, somatic mosaicism for a mutant FBN1 allele is associated with subdiagnostic manifestations, whereas germ-line transmission of the identical mutation causes severe and rapidly progressive disease. A third family cosegregates mild mitral valve prolapse syndrome with a mutation in FBN1 that can be functionally distinguished from those associated with the classic MFS phenotype. These data have immediate relevance for the diagnostic and prognostic counseling of patients and their family members.

Calcium determines the supramolecular organization of fibrillin-rich microfibrils

Microfibrils are ubiquitous fibrillin-rich polymers that are thought to provide long-range elasticity to extracellular matrices, including the zonular filaments of mammalian eyes. X-ray diffraction of hydrated bovine zonular filaments demonstrated meridional diffraction peaks indexing on a fundamental axial periodicity (D) of approximately 56 nm. A Ca2+-induced reversible change in the intensities of the meridional Bragg peaks indicated that supramolecular rearrangements occurred in response to altered concentrations of free Ca2+. In the presence of Ca2+, the dominant diffracting subspecies were microfibrils aligned in an axial 0.33-D stagger. The removal of Ca2+ caused an enhanced regularity in molecular spacing of individual microfibrils, and the contribution from microfibrils not involved in staggered arrays became more dominant. Scanning transmission electron microscopy of isolated microfibrils revealed that Ca2+ removal or addition caused significant, reversible changes in microfibril mass distribution and periodicity. These results were consistent with evidence from x-ray diffraction. Simulated meridional x-ray diffraction profiles and analyses of isolated Ca2+-containing, staggered microfibrillar arrays were used to interpret the effects of Ca2+. These observations highlight the importance of Ca2+ to microfibrils and microfibrillar arrays in vivo.

Metal ion dependency of microfibrils supports a rod-like conformation for fibrillin-1 calcium-binding epidermal growth factor-like domains

The effects of the removal and replacement of divalent cations on the ultrastructure of 10 to 12 nm fibrillin-1-containing microfibrils have been studied, in order to investigate the conformation of fibrillin-1 calcium-binding epidermal growth factor-like (cbEGF-like) domains within the microfibril. The NMR structure of a covalently linked pair of cbEGF-like domains from fibrillin-1 recently identified a rigid rod-like conformation for the domain pair stabilised by interdomain calcium binding. This suggested that tandem arrays of fibrillin-1 cbEGF-like domains may adopt an extended conformation within a microfibril. If correct, then removal of bound calcium from fibrillin-1 would be expected to increase the flexibility of each cbEGF-like interdomain linkage, resulting in a decrease in the length of the interbead region of the microfibril (and thus a decrease in bead to bead periodicity), a concomitant increase in its diameter, and an overall increase in the flexibility of the microfibril. Our results show that removal of calcium by treatment with EGTA causes a large alteration of the microfibril structure, resulting in microfibrils with a reduced beaded periodicity, a disrupted interbead region and an increased overall flexibility. These effects are readily reversible by the re-addition of calcium (in the form of CaCl2), but not by the addition of magnesium (MgCl2). This is consistent with conformational changes in cbEGF-like domains causing the major structural effects on the microfibril. These results provide the first direct experimental evidence to support an extended rod-like conformation for multiple tandem repeats of fibrillin-1 cbEGF-like domains within the microfibril, as predicted by the NMR structure of an isolated fibrillin-1 cbEGF-like domain pair.

The Tight skin mouse: demonstration of mutant fibrillin-1 production and assembly into abnormal microfibrils

Mice carrying the Tight skin (Tsk) mutation harbor a genomic duplication within the fibrillin-1 (Fbn 1) gene that results in a larger than normal in-frame Fbn 1 transcript. In this study, the consequences of the Tsk mutation for fibrillin-containing microfibrils have been examined. Dermal fibroblasts from Tsk/+ mice synthesized and secreted both normal fibrillin (approximately 330 kD) and the mutant oversized Tsk fibrillin-1 (approximately 450 kD) in comparable amounts, and Tsk fibrillin-1 was stably incorporated into cell layers. Immunohistochemical and ultrastructural analyses of normal and Tsk/+ mouse skin highlighted differences in the gross organization and distribution of microfibrillar arrays. Rotary shadowing of high Mr preparations from Tsk/+ skin demonstrated the presence of abundant beaded microfibrils. Some of these had normal morphology and periodicity, but others were distinguished by diffuse interbeads, longer periodicity, and tendency to aggregate. The presence of a structurally abnormal population of microfibrils in Tsk/+ skin was unequivocally demonstrated after calcium chelation and in denaturating conditions. Scanning transmission electron microscopy highlighted the presence of more mass in Tsk/+ skin microfibrils than in normal mice skin microfibrils. These data indicate that Tsk fibrillin-1 polymerizes and becomes incorporated into a discrete population of beaded microfibrils with altered molecular organization.

B-cell deficiency does not abrogate development of cutaneous hyperplasia in mice inheriting the defective fibrillin-1 gene

Tight-skin (TSK) mouse, the experimental model for scleroderma, develops cutaneous hyperplasia, cardiac hypertrophy, pulmonary emphysema and autoimmunity against scleroderma target autoantigens. The cutaneous hyperplasia is associated with the accumulation of microfibrils and elastic fibers in the middle and deep dermis. Fibrillin-1 (Fbn-1) is a major component of the 10-12 nm microfibrils found in the extracellular matrix. In this study we report the identification of a genetic marker in the Fbn-1 gene that can distinguish the mutant phenotype. TSK mice exhibit an unique polymorphism in the Fbn-1 gene. RNA analysis, PCR analysis and sequence determination of the mutant gene showed that the Fbn-1 gene polymorphism is due to intragenic duplication of a segment of the gene coding for 3.0 Kb of mRNA sequence (10 Kb of the genome). Histological analysis of skin samples from F1 progeny obtained by crossing TSK mice with JH-/-, RAG2-/- or vit/vit showed a significant correlation between the inheritance of the defective Fbn-1 gene and the development of cutaneous hyperplasia. Further, our results also show that in mice deficient in mature B cells inheriting the defective Fbn-1 gene, development of cutaneous hyperplasia is not abrogated. Thus, production of autoantibodies or the presence of mature B lymphocytes do not play an integral role in the pathogenesis of cutaneous hyperplasia.

Binding of fibulin-1 to nidogen depends on its C-terminal globular domain and a specific array of calcium-binding epidermal growth factor-like (EG) modules

The calcium-binding basement membrane protein fibulin-1C was shown to bind nidogen in a calcium-dependent fashion. Fibulin-1C consists of small N (domain 1) and C-terminal (domain III) globular structures connected by a central rod (domain II) composed of nine epidermal growth factor (EG) modules, eight of which possess a consensus sequence for calcium binding. Several point and deletion mutants and chimeric protein constructs were used to define the nidogen binding epitope of fibulin-1C by surface plasmon resonance and solid phase assays. All recombinant products were obtained from transfected kidney cells in a folded form as shown by CD spectroscopy, electron microscopy and proteolysis. They were used to demonstrate that calcium-binding is essentially due to the EG modules possessing the consensus binding sequence. Deletion of domain III caused a 30-fold reduction in nidogen binding, whereas deletion of domain I had no effect, yet domain III alone was also inactive. Successive deletions of two to seven EG modules of domain II also caused partial of complete inactivation of binding depending on how many were deleted or their position relative to domain III. Site-directed mutagenesis within the calcium binding consensus sequences demonstrated a similar dependence. Replacement of seven of the calcium-binding modules by a similar tandem array from a related protein showed a distinct (fibulin-2) to almost complete loss of binding (fibrillin-1). This indicates a complex epitope structure involving domains II and III, which each may provide binding epitopes or stabilize each other.

Fibrillin-rich microfibrils: an X-ray diffraction study of the fundamental axial periodicity

Microfibrils are ubiquitous matrix polymers which are thought to provide elastic properties in all extracellular matrix structures. The major component of the elastic microfibrils is the protein fibrillin; its molecular structure is unknown. In electron microscopy, microfibrils appear as beaded structures exhibiting a variable periodicity, indicating that they may be elastomeric. The X-ray diffraction of fibrillin-rich microfibrils in the form of zonular filaments from bovine eyes exhibits meridional diffraction peaks indexing on a fundamental periodicity of 55 nm in the relaxed state. The application of a 40% extension produced a lengthening of the periodicity by 3% as judged by alteration of the D spacing of the principal peaks. This effect was shown to be reversible. Changes in the periodicity of the meridional reflections indicate changes in the fundamental structure of the microfilaments, but cannot account for all long range elastomeric properties of fibrillin-containing microfibrils.

Microfibrillar elements of the dermal matrix

Connective tissue microfibrils are key structural elements of the dermal matrix which play major roles in establishing and maintaining the structural and mechanical integrity of this complex tissue. Type VI collagen microfibrils form extensive microfibrillar networks which intercalate between the major collagen fibrils and are juxtaposed to cellular basement membranes, blood vessels and other interstitial structures. Fibrillin microfibrils define the continuous elastic network of skin, and are present in dermis as microfibril bundles devoid of measureable elastin extending from the dermal-epithelial junction and as components of the thick elastic fibres present in the deep reticular dermis. Electron microscopic analyses have revealed both classes of microfibrils to have complex ultrastructures. The ability to isolate intact native microfibrils from skin has enabled a combination of high resolution and biochemical techniques to be applied to elucidate their structure:function relationships. These approaches have generated new information about their molecular organisation and physiological interactions in health and disease.