Biogenesis of extracellular microfibrils: Multimerization of the fibrillin-1 C terminus into bead-like structures enables self-assembly

Microfibril-associated glycoprotein 4 forms octamers that mediate interactions with elastogenic proteins and cells

Microfibril-associated glycoprotein 4 (MFAP4) is a 36-kDa extracellular matrix glycoprotein with critical roles in organ fibrosis, chronic obstructive pulmonary disease, and cardiovascular disorders, including aortic aneurysms. MFAP4 multimerises and interacts with elastogenic proteins, including fibrillin-1 and tropoelastin, and with cells via integrins. Structural details of MFAP4 and its potential interfaces for these interactions are unknown. Here, we present a cryo-electron microscopy structure of human MFAP4. In the presence of calcium, MFAP4 assembles as an octamer, where two sets of homodimers constitute the top and bottom halves of each octamer. Each homodimer is linked together by an intermolecular disulphide bond. A C34S missense mutation prevents disulphide-bond formation between monomers but does not prevent octamer assembly. The atomic model, built into the 3.55 Å cryo-EM map, suggests that salt-bridge interactions mediate homodimer assembly, while non-polar residues form the interface between octamer halves. In the absence of calcium, an MFAP4 octamer dissociates into two tetramers. Binding studies with fibrillin-1, tropoelastin, LTBP4, and small fibulins show that MFAP4 has multiple surfaces for protein-protein interactions, most of which depend upon MFAP4 octamer assembly. The C34S mutation does not affect these protein interactions or cell interactions. MFAP4 assemblies with fibrillin-1 abrogate MFAP4 interactions with cells.

Fibrillin-1 regulates endothelial sprouting during angiogenesis

Fibrillin-1 is an extracellular matrix protein that assembles into microfibrils which provide critical functions in large blood vessels and other tissues. Mutations in the fibrillin-1 gene are associated with cardiovascular, ocular, and skeletal abnormalities in Marfan syndrome. Here, we reveal that fibrillin-1 is critical for angiogenesis which is compromised by a typical Marfan mutation. In the mouse retina vascularization model, fibrillin-1 is present in the extracellular matrix at the angiogenic front where it colocalizes with microfibril-associated glycoprotein-1, MAGP1. In Fbn1C1041G/+ mice, a model of Marfan syndrome, MAGP1 deposition is reduced, endothelial sprouting is decreased, and tip cell identity is impaired. Cell culture experiments confirmed that fibrillin-1 deficiency alters vascular endothelial growth factor-A/Notch and Smad signaling which regulate the acquisition of endothelial tip cell/stalk cell phenotypes, and we showed that modulation of MAGP1 expression impacts these pathways. Supplying the growing vasculature of Fbn1C1041G/+ mice with a recombinant C-terminal fragment of fibrillin-1 corrects all defects. Mass spectrometry analyses showed that the fibrillin-1 fragment alters the expression of various proteins including ADAMTS1, a tip cell metalloprotease and matrix-modifying enzyme. Our data establish that fibrillin-1 is a dynamic signaling platform in the regulation of cell specification and matrix remodeling at the angiogenic front and that mutant fibrillin-1-induced defects can be rescued pharmacologically using a C-terminal fragment of the protein. These findings, identify fibrillin-1, MAGP1, and ADAMTS1 in the regulation of endothelial sprouting, and contribute to our understanding of how angiogenesis is regulated. This knowledge may have critical implications for people with Marfan syndrome.

Fibrillin microfibril structure identifies long-range effects of inherited pathogenic mutations affecting a key regulatory latent TGFβ-binding site

Genetic mutations in fibrillin microfibrils cause serious inherited diseases, such as Marfan syndrome and Weill-Marchesani syndrome (WMS). These diseases typically show major dysregulation of tissue development and growth, particularly in skeletal long bones, but links between the mutations and the diseases are unknown. Here we describe a detailed structural analysis of native fibrillin microfibrils from mammalian tissue by cryogenic electron microscopy. The major bead region showed pseudo eightfold symmetry where the amino and carboxy termini reside. On the basis of this structure, we show that a WMS deletion mutation leads to the induction of a structural rearrangement that blocks interaction with latent TGFβ-binding protein-1 at a remote site. Separate deletion of this binding site resulted in the assembly of shorter fibrillin microfibrils with structural alterations. The integrin α_vβ₃-binding site was also mapped onto the microfibril structure. These results establish that in complex extracellular assemblies, such as fibrillin microfibrils, mutations may have long-range structural consequences leading to the disruption of growth factor signaling and the development of disease.

Succinylation Inhibits the Enzymatic Hydrolysis of the Extracellular Matrix Protein Fibrillin 1 and Promotes Gastric Cancer Progression

Extracellular matrix (ECM) remodeling is crucial in the regulation of gastric cancer (GC) progression. This work aims to reveal novel posttranslational modifications and their relevant mechanisms in GC. In 3D matrix culture and animal models, it is found that fibrillin 1 (FBN1) expression is increased in advanced GC and has succinylation modification. The succinylation modification of FBN1 blocks its degradation by matrix metalloproteinases (MMPs). The long-term accumulation and deposition of FBN1 enhance tumor progression by activating TGF-β1 and intracellular PI3K/Akt pathway. The FBN1 succinylation site monoclonal antibody can effectively intervene the effect of succinylation modification and inhibit GC progression. FBN1 is specifically upregulated in the progression of GC compared with other tumors. In conclusion, FBN1 is widely present in the form of K672-succinylated modifications in GC. Besides, the succinyl group of FBN1 blocks its binding to MMP2, inhibits its degradation by MMP2, and leads to the accumulation of FBN1, which poses a long-term risk to the poor prognosis of GC.

Proteolysis of fibrillin-2 microfibrils is essential for normal skeletal development

The embryonic extracellular matrix (ECM) undergoes transition to mature ECM as development progresses, yet few mechanisms ensuring ECM proteostasis during this period are known. Fibrillin microfibrils are macromolecular ECM complexes serving structural and regulatory roles. In mice, Fbn1 and Fbn2, encoding the major microfibrillar components, are strongly expressed during embryogenesis, but fibrillin-1 is the major component observed in adult tissue microfibrils. Here, analysis of Adamts6 and Adamts10 mutant mouse embryos, lacking these homologous secreted metalloproteases individually and in combination, along with in vitro analysis of microfibrils, measurement of ADAMTS6-fibrillin affinities and N-terminomics discovery of ADAMTS6-cleaved sites, identifies a proteostatic mechanism contributing to postnatal fibrillin-2 reduction and fibrillin-1 dominance. The lack of ADAMTS6, alone and in combination with ADAMTS10 led to excess fibrillin-2 in perichondrium, with impaired skeletal development defined by a drastic reduction of aggrecan and cartilage link protein, impaired BMP signaling in cartilage, and increased GDF5 sequestration in fibrillin-2-rich tissue. Although ADAMTS6 cleaves fibrillin-1 and fibrillin-2 as well as fibronectin, which provides the initial scaffold for microfibril assembly, primacy of the protease-substrate relationship between ADAMTS6 and fibrillin-2 was unequivocally established by reversal of the defects in Adamts6^-/- embryos by genetic reduction of Fbn2, but not Fbn1.

Elastic Fibre Proteins in Elastogenesis and Wound Healing

As essential components of our connective tissues, elastic fibres give tissues such as major blood vessels, skin and the lungs their elasticity. Their formation is complex and co-ordinately regulated by multiple factors. In this review, we describe key players in elastogenesis: fibrillin-1, tropoelastin, latent TGFβ binding protein-4, and fibulin-4 and -5. We summarise their roles in elastogenesis, discuss the effect of their mutations on relevant diseases, and describe their interactions involved in forming the elastic fibre network. Moreover, we look into their roles in wound repair for a better understanding of their potential application in tissue regeneration.

Structural studies of elastic fibre and microfibrillar proteins

Elastic tissues owe their functional properties to the composition of their extracellular matrices, particularly the range of extracellular, multidomain extensible elastic fibre and microfibrillar proteins. These proteins include elastin, fibrillin, latent TGFβ binding proteins (LTBPs) and collagens, where their biophysical and biochemical properties not only give the matrix structural integrity, but also play a vital role in the mechanisms that underlie tissue homeostasis. Thus far structural information regarding the structure and hierarchical assembly of these molecules has been challenging and the resolution has been limited due to post-translational modification and their multidomain nature leading to flexibility, which together result in conformational and structural heterogeneity. In this review, we describe some of the matrix proteins found in elastic fibres and the new emerging techniques that can shed light on their structure and dynamic properties.

Elastic fibers: formation, function, and fate during aging and disease

Elastic fibers are extracellular components of higher vertebrates and confer elasticity and resilience to numerous tissues and organs such as large blood vessels, lungs, and skin. Their formation and maturation take place in a complex multistage process called elastogenesis. It requires interactions between very different proteins but also other molecules and leads to the deposition and crosslinking of elastin's precursor on a scaffold of fibrillin-rich microfibrils. Mature fibers are exceptionally resistant to most influences and, under healthy conditions, retain their biomechanical function over the life of the organism. However, due to their longevity, they accumulate damages during aging. These are caused by proteolytic degradation, formation of advanced glycation end products, calcification, oxidative damage, aspartic acid racemization, lipid accumulation, carbamylation, and mechanical fatigue. The resulting changes can lead to diminution or complete loss of elastic fiber function and ultimately affect morbidity and mortality. Particularly, the production of elastokines has been clearly shown to influence several life-threatening diseases. Moreover, the structure, distribution, and abundance of elastic fibers are directly or indirectly influenced by a variety of inherited pathological conditions, which mainly affect organs and tissues such as skin, lungs, or the cardiovascular system. A distinction can be made between microfibril-related inherited diseases that are the result of mutations in diverse microfibril genes and indirectly affect elastogenesis, and elastinopathies that are linked to changes in the elastin gene. This review gives an overview on the formation, structure, and function of elastic fibers and their fate over the human lifespan in health and disease.

Assembly assay identifies a critical region of human fibrillin-1 required for 10-12 nm diameter microfibril biogenesis

The human FBN1 gene encodes fibrillin-1 (FBN1); the main component of the 10–12 nm diameter extracellular matrix microfibrils. Marfan syndrome (MFS) is a common inherited connective tissue disorder, caused by FBN1 mutations. It features a wide spectrum of disease severity, from mild cases to the lethal neonatal form (nMFS), that is yet to be explained at the molecular level. Mutations associated with nMFS generally affect a region of FBN1 between domains TB3-cbEGF18—the "neonatal region". To gain insight into the process of fibril assembly and increase our understanding of the mechanisms determining disease severity in MFS, we compared the secretion and assembly properties of FBN1 variants containing nMFS-associated substitutions with variants associated with milder, classical MFS (cMFS). In the majority of cases, both nMFS- and cMFS-associated neonatal region variants were secreted at levels comparable to wild type. Microfibril incorporation by the nMFS variants was greatly reduced or absent compared to the cMFS forms, however, suggesting that nMFS substitutions disrupt a previously undefined site of microfibril assembly. Additional analysis of a domain deletion variant caused by exon skipping also indicates that register in the neonatal region is likely to be critical for assembly. These data demonstrate for the first time new requirements for microfibril biogenesis and identify at least two distinct molecular mechanisms associated with disease substitutions in the TB3-cbEGF18 region; incorporation of mutant FBN1 into microfibrils changing their integral properties (cMFS) or the blocking of wild type FBN1 assembly by mutant molecules that prevents late-stage lateral assembly (nMFS).

Elastic fibers during aging and disease

Elastic fibers are essential constituents of the extracellular matrix of higher vertebrates and endow several tissues and organs including lungs, skin and blood vessels with elasticity and resilience. During the human lifespan, elastic fibers are exposed to a variety of enzymatic, chemical and biophysical influences, and accumulate damage due to their low turnover. Aging of elastin and elastic fibers involves enzymatic degradation, oxidative damage, glycation, calcification, aspartic acid racemization, binding of lipids and lipid peroxidation products, carbamylation and mechanical fatigue. These processes can trigger an impairment or loss of elastic fiber function and are associated with severe pathologies. There are different inherited or acquired pathological conditions, which influence the structure and function of elastic fibers and microfibrils predominantly in the cardiorespiratory system and skin. Inherited elastic-fiber pathologies have a direct or indirect impact on elastic-fiber formation due to mutations in the fibrillin genes (fibrillinopathies), in the elastin gene (elastinopathies) or in genes encoding proteins that are associated with microfibrils or elastic fibers. Acquired elastic-fiber pathologies appear age-related or as a result of multiple factors impairing tissue homeostasis. This review gives an overview on the fate of elastic fibers over the human lifespan in health and disease.

A guide to the composition and functions of the extracellular matrix

Extracellular matrix (ECM) is a dynamic 3-dimensional network of macromolecules that provides structural support for the cells and tissues. Accumulated knowledge clearly demonstrated over the last decade that ECM plays key regulatory roles since it orchestrates cell signaling, functions, properties and morphology. Extracellularly secreted as well as cell-bound factors are among the major members of the ECM family. Proteins/glycoproteins, such as collagens, elastin, laminins and tenascins, proteoglycans and glycosaminoglycans, hyaluronan, and their cell receptors such as CD44 and integrins, responsible for cell adhesion, comprise a well-organized functional network with significant roles in health and disease. On the other hand, enzymes such as matrix metalloproteinases and specific glycosidases including heparanase and hyaluronidases contribute to matrix remodeling and affect human health. Several cell processes and functions, among them cell proliferation and survival, migration, differentiation, autophagy, angiogenesis, and immunity regulation are affected by certain matrix components. Structural alterations have been also well associated with disease progression. This guide on the composition and functions of the ECM gives a broad overview of the matrisome, the major ECM macromolecules, and their interaction networks within the ECM and with the cell surface, summarizes their main structural features and their roles in tissue organization and cell functions, and emphasizes the importance of specific ECM constituents in disease development and progression as well as the advances in molecular targeting of ECM to design new therapeutic strategies.

Insights into the biophysical forces between proteins involved in elastic fiber assembly

Elastogenesis is a complex process beginning with transcription, translation, and extracellular release of precursor proteins leading to crosslinking, deposition, and assembly of ubiquitous elastic fibers. While the biochemical pathways by which elastic fibers are assembled are known, the biophysical forces mediating the interactions between the constituent proteins are unknown. Using atomic force microscopy, we quantified the adhesive forces among the elastic fiber components, primarily between tropoelastin, elastin binding protein (EBP), fibrillin-1, fibulin-5, and lysyl oxidase-like 2 (LOXL2). The adhesive forces between tropoelastin and other tissue-derived proteins such as insoluble elastin, laminin, and type I collagens were also assessed. The adhesive forces between tropoelastin and laminin were strong (1767 ± 126 pN; p < 10-5vs. all others), followed by forces (≥200 pN) between tropoelastin and human collagen, mature elastin, or tropoelastin. The adhesive forces between tropoelastin and rat collagen, EBP, fibrillin-1, fibulin-5, and LOXL2 coated on fibrillin-1 were in the range of 100-200 pN. The forces between tropoelastin and LOXL2, LOXL2 and fibrillin-1, LOXL2 and fibulin-5, and fibrillin-1 and fibulin-5 were less than 100 pN. Introducing LOXL2 decreased the adhesive forces between the tropoelastin monomers by ∼100 pN. The retraction phase of force-deflection curves was fitted to the worm-like chain model to calculate the rigidity and flexibility of these proteins as they unfolded. The results provided insights into how each constituent's stretching under deformation contributes to structural and mechanical characteristics of these fibers and to elastic fiber assembly.

The fibrillin microfibril/elastic fibre network: A critical extracellular supramolecular scaffold to balance skin homoeostasis

Supramolecular networks composed of fibrillins (fibrillin-1 and fibrillin-2) and associated ligands form intricate cellular microenvironments which balance skin homoeostasis and direct remodelling. Fibrillins assemble into microfibrils which are not only indispensable for conferring elasticity to the skin, but also control the bioavailability of growth factors targeted to the extracellular matrix architecture. Fibrillin microfibrils (FMF) represent the core scaffolds for elastic fibre formation, and they also decorate the surface of elastic fibres and form independent networks. In normal dermis, elastic fibres are suspended in a three-dimensional basket-like lattice of FMF intersecting basement membranes at the dermal-epidermal junction and thus conferring pliability to the skin. The importance of FMF for skin homoeostasis is illustrated by the clinical features caused by mutations in the human fibrillin genes (FBN1, FBN2), summarized as "fibrillinopathies." In skin, fibrillin mutations result in phenotypes ranging from thick, stiff and fibrotic skin to thin, lax and hyperextensible skin. The most plausible explanation for this spectrum of phenotypic outcomes is that FMF regulate growth factor signalling essential for proper growth and homoeostasis of the skin. Here, we will give an overview about the current understanding of the underlying pathomechanisms leading to fibrillin-dependent fibrosis as well as forms of cutis laxa caused by mutational inactivation of FMF-associated ligands.

Fibrillin-1 and fibrillin-1-derived asprosin in adipose tissue function and metabolic disorders

The extracellular matrix microenvironment of adipose tissue is of critical importance for the differentiation, remodeling and function of adipocytes. Fibrillin-1 is one of the main components of microfibrils and a key player in this process. Furin processing of profibrillin-1 results in mature fibrillin-1 and releases the C-terminal propeptide as a circulating hunger hormone, asprosin. Mutations in the fibrillin-1 gene lead to adipose tissue dysfunction and causes Marfan syndrome, marfanoid progeroid lipodystrophy syndrome, and neonatal progeroid syndrome. Increased TGF-β signaling, altered mechanical properties and impaired adipogenesis are potential causes of adipose tissue dysfunction, mediated through deficient microfibrils. Circulating asprosin on the other hand is secreted primarily by white adipose tissue under fasting conditions and in obesity. It increases hepatic glucose production and drives insulin secretion and appetite stimulation through inter-organ cross talk. This review discusses the metabolic consequences of fibrillin-1 and fibrillin-1-derived asprosin in pathological conditions. Understanding the dynamic role of fibrillin-1 in the adipose tissue milieu and of circulating asprosin in the body can provide novel mechanistic insights into how fibrillin-1 may contribute to metabolic syndrome. This could lead to new management regimens of patients with metabolic disease.

Role of fibrillin-2 in the control of TGF-β activation in tumor angiogenesis and connective tissue disorders

Fibrillins constitute a family of large extracellular glycoproteins which multimerize to form microfibrils, an important structure in the extracellular matrix. It has long been assumed that fibrillin-2 was barely present during postnatal life, but it is now clear that fibrillin-2 molecules form the structural core of microfibrils, and are masked by an outer layer of fibrillin-1. Mutations in fibrillins give rise to heritable connective tissue disorders, including Marfan syndrome and congenital contractural arachnodactyly. Fibrillins also play an important role in matrix sequestering of members of the transforming growth factor-β family, and in context of Marfan syndrome excessive TGF-β activation has been observed. TGF-β activation is highly dependent on integrin binding, including integrin αvβ8 and αvβ6, which are upregulated upon TGF-β exposure. TGF-β is also involved in tumor progression, metastasis, epithelial-to-mesenchymal transition and tumor angiogenesis. In several highly vascularized types of cancer such as hepatocellular carcinoma, a positive correlation was found between increased TGF-β plasma concentrations and tumor vascularity. Interestingly, fibrillin-1 has a higher affinity to TGF-β and, therefore, has a higher capacity to sequester TGF-β compared to fibrillin-2. The previously reported downregulation of fibrillin-1 in tumor endothelium affects the fibrillin-1/fibrillin-2 ratio in the microfibrils, exposing the normally hidden fibrillin-2. We postulate that fibrillin-2 exposure in the tumor endothelium directly stimulates tumor angiogenesis by influencing TGF-β sequestering by microfibrils, leading to a locally higher active TGF-β concentration in the tumor microenvironment. From a therapeutic perspective, fibrillin-2 might serve as a potential target for future anti-cancer therapies.

The role of fibrillin and microfibril binding proteins in elastin and elastic fibre assembly

Fibrillin is a large evolutionarily ancient extracellular glycoprotein that assembles to form beaded microfibrils which are essential components of most extracellular matrices. Fibrillin microfibrils have specific biomechanical properties to endow animal tissues with limited elasticity, a fundamental feature of the durable function of large blood vessels, skin and lungs. They also form a template for elastin deposition and provide a platform for microfibril-elastin binding proteins to interact in elastic fibre assembly. In addition to their structural role, fibrillin microfibrils mediate cell signalling via integrin and syndecan receptors, and microfibrils sequester transforming growth factor (TGF)β family growth factors within the matrix to provide a tissue store which is critical for homeostasis and remodelling.

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.

Biophysical Techniques to Analyze Elastic Tissue Extracellular Matrix Proteins Interacting with ADAMTS Proteins

Multidomain matrix-associated zinc extracellular proteases ADAMTS and ADAMTS-like proteins have important biological activities in cells and tissues. Beyond their traditional role in procollagen and von Willebrand factor processing and proteoglycan cleavage, ADAMTS/ADAMTSL likely participate in or at least have some role in ECM assembly as some of these proteins bind ECM proteins including fibrillins, fibronectin, and LTBPs. In this chapter, we present four biophysical techniques largely used for the characterization, multimerization, and interaction of proteins: surface plasmon resonance spectroscopy, dynamic light scattering, atomic force microscopy, and circular dichroism spectroscopy.

Multiscale Imaging Reveals the Hierarchical Organization of Fibrillin Microfibrils

Fibrillin microfibrils are evolutionarily ancient, structurally complex extracellular polymers found in mammalian elastic tissues where they endow elastic properties, sequester growth factors and mediate cell signalling; thus, knowledge of their structure and organization is essential for a more complete understanding of cell function and tissue morphogenesis. By combining multiple imaging techniques, we visualize three levels of hierarchical organization of fibrillin structure ranging from micro-scale fiber bundles in the ciliary zonule to nano-scale individual microfibrils. Serial block-face scanning electron microscopy imaging suggests that bundles of zonule fibers are bound together by circumferential wrapping fibers, which is mirrored on a shorter-length scale where individual zonule fibers are interwoven by smaller fibers. Electron tomography shows that microfibril directionality varies from highly aligned and parallel, connecting to the basement membrane, to a meshwork at the zonule fiber periphery, and microfibrils within the zonule are connected by short cross-bridges, potentially formed by fibrillin-binding proteins. Three-dimensional reconstructions of negative-stain electron microscopy images of purified microfibrils confirm that fibrillin microfibrils have hollow tubular structures with defined bead and interbead regions, similar to tissue microfibrils imaged in our tomograms. These microfibrils are highly symmetrical, with an outer ring and interwoven core in the bead and four linear prongs, each accommodating a fibrillin dimer, in the interbead region. Together these data show how a single molecular building block is organized into different levels of hierarchy from microfibrils to tissue structures spanning nano- to macro-length scales. Furthermore, the application of these combined imaging approaches has wide applicability to other tissue systems.

•

Extracellular matrix fibrillin microfibrils assemble to form ocular ligaments.

•

Individual beaded fibrillin microfibrils are highly symmetric biological polymers.

•

Zonule fibers are composed of aligned, organized arrays of fibrillin microfibrils.

•

Bundles of zonule fibers are wrapped by large fibers providing structural support.

•

Fibrillin organization shows how a single building block constructs an elastic tissue.

Truncated C-terminus of fibrillin-1 induces Marfanoid-progeroid-lipodystrophy (MPL) syndrome in rabbit

Various clinical differences have been observed between patients with the FBN1 gene mutation and those with the classical Marfan phenotype. Although FBN1 knockout (KO) or dominant-negative mutant mice are widely used as an animal model for Marfan syndrome (MFS), these mice cannot recapitulate the genotype/phenotype relationship of Marfanoid-progeroid-lipodystrophy (MPL) syndrome, which is caused by a mutation in the C-terminus of fibrillin-1, the penultimate exon of the FBN1 gene. Here, we describe the generation of a rabbit MPL model with C-terminal truncation of fibrillin-1 using a CRISPR/Cas9 system. FBN1 heterozygous (FBN1 Het) rabbits faithfully recapitulated the phenotypes of MFS, including muscle wasting and impaired connective tissue, ocular syndrome and aortic dilation. Moreover, skin symptoms, lipodystrophy, growth retardation and dysglycemia were also seen in these FBN1 Het rabbits, and have not been reported in other animal models. In conclusion, this novel rabbit model mimics the histopathological changes and functional defects of MPL syndrome, and could become a valuable model for studies of pathogenesis and drug screening for MPL syndrome.

Summary: A novel genetically engineered rabbit model of MPL syndrome, generated by CRISPR/Cas9-mediated mutation of FBN1, mimics the histopathological changes and functional defects of MPL syndrome seen in the clinic.

Fibrillins

Fibrillins are one of the major components of supramolecular fibrous structures in the extracellular matrix of elastic and nonelastic tissues, termed microfibrils. Microfibrils provide tensile strength in nonelastic tissues and scaffolds for the assembly of tropoelastin in elastic tissues, and act a regulator of growth factor bioavailability and activity in connective tissues. Mutations in fibrillins lead to a variety of connective tissue disorders including Marfan syndrome, stiff skin syndrome, dominant Weill-Marchesani syndrome, and others. Therefore, fibrillins are frequently studied to understand the pathophysiology of these diseases and to identify effective treatment strategies. Extraction of endogenous microfibrils from cells and tissues can aid in obtaining structural insights of microfibrils. Recombinant production of fibrillins is an important tool which can be utilized to study the properties of normal fibrillins and the consequences of disease causing mutations. Other means of studying the role of fibrillins in the context of various physiological settings is by knocking down the mRNA expression and analyzing its downstream consequences. It is also important to study the interactome of fibrillins by protein-protein interactions, which can be derailed in pathological situations. Interacting proteins can affect the assembly of fibrillins in cells and tissues or can affect the levels of growth factors in the matrix. This chapter describes important techniques in the field that facilitate answering relevant questions of fibrillin biology and pathophysiology.

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.

Independent multimerization of Latent TGFβ Binding Protein-1 stabilized by cross-linking and enhanced by heparan sulfate

TGFβ plays key roles in fibrosis and cancer progression, and latency is conferred by covalent linkage to latent TGFβ binding proteins (LTBPs). LTBP1 is essential for TGFβ folding, secretion, matrix localization and activation but little is known about its structure due to its inherent size and flexibility. Here we show that LTBP1 adopts an extended conformation with stable matrix-binding N-terminus, extended central array of 11 calcium-binding EGF domains and flexible TGFβ-binding C-terminus. Moreover we demonstrate that LTBP1 forms short filament-like structures independent of other matrix components. The termini bind to each other to facilitate linear extension of the filament, while the N-terminal region can serve as a branch-point. Multimerization is enhanced in the presence of heparin and stabilized by the matrix cross-linking enzyme transglutaminase-2. These assemblies will extend the span of LTBP1 to potentially allow simultaneous N-terminal matrix and C-terminal fibrillin interactions providing tethering for TGFβ activation by mechanical force.

New insights into the structure, assembly and biological roles of 10–12 nm connective tissue microfibrils from fibrillin-1 studies

The 10–12 nm diameter microfibrils of the extracellular matrix (ECM) impart both structural and regulatory properties to load-bearing connective tissues. The main protein component is the calcium-dependent glycoprotein fibrillin, which assembles into microfibrils at the cell surface in a highly regulated process involving specific proteolysis, multimerization and glycosaminoglycan interactions. In higher metazoans, microfibrils act as a framework for elastin deposition and modification, resulting in the formation of elastic fibres, but they can also occur in elastin-free tissues where they perform structural roles. Fibrillin microfibrils are further engaged in a number of cell matrix interactions such as with integrins, bone morphogenetic proteins (BMPs) and the large latent complex of transforming growth factor-β (TGFβ). Fibrillin-1 (FBN1) mutations are associated with a range of heritable connective disorders, including Marfan syndrome (MFS) and the acromelic dysplasias, suggesting that the roles of 10–12 nm diameter microfibrils are pleiotropic. In recent years the use of molecular, cellular and whole-organism studies has revealed that the microfibril is not just a structural component of the ECM, but through its network of cell and matrix interactions it can exert profound regulatory effects on cell function. In this review we assess what is known about the molecular properties of fibrillin that enable it to assemble into the 10–12 nm diameter microfibril and perform such diverse roles.

Fibrillin-containing microfibrils are key signal relay stations for cell function

Fibrillins constitute the backbone of microfibrils in the extracellular matrix of elastic and non-elastic tissues. Mutations in fibrillins are associated with a wide range of connective tissue disorders, the most common is Marfan syndrome. Microfibrils are on one hand important for structural stability in some tissues. On the other hand, microfibrils are increasingly recognized as critical mediators and drivers of cellular signaling. This review focuses on the signaling mechanisms initiated by fibrillins and microfibrils, which are often dysregulated in fibrillin-associated disorders. Fibrillins regulate the storage and bioavailability of growth factors of the TGF-β superfamily. Cells sense microfibrils through integrins and other receptors. Fibrillins potently regulate pathways of the immune response, inflammation and tissue homeostasis. Emerging evidence show the involvement of microRNAs in disorders caused by fibrillin deficiency. A thorough understanding of fibrillin-mediated cell signaling pathways will provide important new leads for therapeutic approaches of the underlying disorders.

Corneal stroma microfibrils

Elastic tissue was first described well over a hundred years ago and has since been identified in nearly every part of the body. In this review, we examine elastic tissue in the corneal stroma with some mention of other ocular structures which have been more thoroughly described in the past. True elastic fibers consist of an elastin core surrounded by fibrillin microfibrils. However, the presence of elastin fibers is not a requirement and some elastic tissue is comprised of non-elastin-containing bundles of microfibrils. Fibers containing a higher relative amount of elastin are associated with greater elasticity and those without elastin, with structural support. Recently it has been shown that the microfibrils, not only serve mechanical roles, but are also involved in cell signaling through force transduction and the release of TGF-β. A well characterized example of elastin-free microfibril bundles (EFMBs) is found in the ciliary zonules which suspend the crystalline lens in the eye. Through contraction of the ciliary muscle they exert enough force to reshape the lens and thereby change its focal point. It is believed that the molecules comprising these fibers do not turn-over and yet retain their tensile strength for the life of the animal. The mechanical properties of the cornea (strength, elasticity, resiliency) would suggest that EFMBs are present there as well. However, many authors have reported that, although present during embryonic and early postnatal development, EFMBs are generally not present in adults. Serial-block-face imaging with a scanning electron microscope enabled 3D reconstruction of elements in murine corneas. Among these elements were found fibers that formed an extensive network throughout the cornea. In single sections these fibers appeared as electron dense patches. Transmission electron microscopy provided additional detail of these patches and showed them to be composed of fibrils (∼10 nm diameter). Immunogold evidence clearly identified these fibrils as fibrillin EFMBs and EFMBs were also observed with TEM (without immunogold) in adult mammals of several species. Evidence of the presence of EFMBs in adult corneas will hopefully pique an interest in further studies that will ultimately improve our understanding of the cornea's biomechanical properties and its capacity to repair.

C-terminal propeptide is required for fibrillin-1 secretion and blocks premature assembly through linkage to domains cbEGF41-43

Fibrillin microfibrils are 10-12 nm diameter, extracellular matrix assemblies that provide dynamic tissues of metazoan species with many of their biomechanical properties as well as sequestering growth factors and cytokines. Assembly of fibrillin monomers into microfibrils is thought to occur at the cell surface, with initial steps including proprotein processing, multimerization driven by the C terminus, and the head-to-tail alignment of adjacent molecules. At present the mechanisms that regulate microfibril assembly are still to be elucidated. We have used structure-informed protein engineering to create a recombinant, GFP-tagged version of fibrillin-1 (GFP-Fbn) to study this process. Using HEK293T cells transiently transfected with GFP-Fbn constructs, we show that (i) the C-terminal propeptide is an essential requirement for the secretion of full-length fibrillin-1 from cells; (ii) failure to cleave off the C-terminal propeptide blocks the assembly of fibrillin-1 into microfibrils produced by dermal fibroblasts; and (iii) the requirement of the propeptide for secretion is linked to the presence of domains cbEGF41-43, because either deletion or exchange of domains in this region leads to cellular retention. Collectively, these data suggest a mechanism in which the propeptide blocks a key site at the C terminus to prevent premature microfibril assembly.

Homocysteine in renovascular complications: hydrogen sulfide is a modulator and plausible anaerobic ATP generator

Homocysteine (Hcy) is a non-protein amino acid derived from dietary methionine. High levels of Hcy, known as hyperhomocysteinemia (HHcy) is known to cause vascular complications. In the mammalian tissue, Hcy is metabolized by transsulfuration enzymes to produce hydrogen sulfide (H2S). H2S, a pungent smelling gas was previously known for its toxic effects in the central nervous system, recent studies however has revealed protective effects in a variety of diseases including hypertension, diabetes, inflammation, atherosclerosis, and renal disease progression and failure. Interestingly, under stress conditions including hypoxia, H2S can reduce metabolic demand and also act as a substrate for ATP production. This review highlights some of the recent advances in H2S research as a potential therapeutic agent targeting renovascular diseases associated with HHcy.

Early fibrillin-1 assembly monitored through a modifiable recombinant cell approach

Fibrillin proteins constitute the backbone of extra-cellular macromolecular microfibrils. Mutations in fibrillins cause heritable connective tissue disorders, including Marfan syndrome, dominant Weill-Marchesani syndrome, and stiff skin syndrome. Fibronectin provides a critical scaffold for microfibril assembly in cell culture models. Full length recombinant fibrillin-1 was expressed by HEK 293 cells, which deposited the secreted protein in a punctate pattern on the cell surface. Cocultured fibroblasts consistently triggered assembly of recombinant fibrillin-1, which was dependent on a fibronectin network formed by the fibroblasts. Deposition of recombinant fibrillin-1 on fibronectin fibers occurred first in discrete packages that subsequently extended along fibronectin fibers. Mutant fibrillin-1 harboring either a cysteine 204 to serine mutation or a RGD to RGA mutation which prevents integrin binding, did not affect fibrillin-1 assembly. In conclusion, we developed a modifiable recombinant full-length fibrillin-1 assembly system that allows for rapid analysis of critical roles in fibrillin assembly and functionality. This system can be used to study the contributions of specific residues, domains, or regions of fibrillin-1 to the biogenesis and functionality of microfibrils. It provides also a method to evaluate disease-causing mutations, and to produce microfibril-containing matrices for tissue engineering applications, for example, in designing novel vascular grafts or stents.

¹H, ¹³C and ¹⁵N assignments of the four N-terminal domains of human fibrillin-1

Fibrillins are extracellular, disulphide-rich glycoproteins that form 10-12 nm diameter microfibrils in connective tissues. They are found in the majority of higher animals, from jellyfish to humans. Fibrillin microfibrils confer properties of elasticity and strength on connective tissue and regulate growth factor availability in the extracellular matrix (ECM). Mutations in FBN1, the human gene encoding the fibrillin-1 isoform, are linked to several inherited connective tissue disorders. The fibrillin-1 N-terminus forms many functionally-important interactions, both with other fibrillin molecules and various ECM components. In particular, the first four domains, the fibrillin unique N-terminal (FUN) and three epidermal growth factor (EGF)-like domains (FUN-EGF3), are implicated in microfibril assembly and growth factor sequestration. The structure of these domains, which comprise 134 residues, is unknown. We have produced a recombinant fragment corresponding to this region of human fibrillin-1. Here, we report (1)H, (13)C and (15)N resonance assignments of the FUN-EGF3 fragment. Assignments will facilitate structure determination, analysis of interdomain dynamics and the mapping of interaction surfaces.

Complex contributions of fibronectin to initiation and maturation of microfibrils

Fibrillins constitute the backbone of extracellular multifunctional assemblies present in elastic and non-elastic matrices, termed microfibrils. Assembly of fibrillins into microfibrils and their homoeostasis is poorly understood and is often compromised in connective tissue disorders such as Marfan syndrome and other fibrillinopathies. Using interaction mapping studies, we demonstrate that fibrillins require the complete gelatin-binding region of fibronectin for interaction, which comprises domains FNI6-FNI9. However, the interaction of fibrillin-1 with the gelatin-binding domain of fibronectin is not involved in fibrillin-1 network assembly mediated by human skin fibroblasts. We show further that the fibronectin network is essential for microfibril homoeostasis in early stages. Fibronectin is present in extracted mature microfibrils from tissue and cells as well as in some in situ microfibrils observed at the ultrastructural level, indicating an extended mechanism for the involvement of fibronectin in microfibril assembly and maturation.

Structure of the fibrillin-1 N-terminal domains suggests that heparan sulfate regulates the early stages of microfibril assembly

The human extracellular matrix glycoprotein fibrillin-1 is the primary component of the 10- to 12-nm-diameter microfibrils, which perform key structural and regulatory roles in connective tissues. Relatively little is known about the molecular mechanisms of fibrillin assembly into microfibrils. Studies using recombinant fibrillin fragments indicate that an interaction between the N- and C-terminal regions drives head-to-tail assembly. Here, we present the structure of a fibrillin N-terminal fragment comprising the fibrillin unique N-terminal (FUN) and the first three epidermal growth factor (EGF)-like domains (FUN-EGF3). Two rod-like domain pairs are separated by a short, flexible linker between the EGF1 and EGF2 domains. We also show that the binding site for the C-terminal region spans multiple domains and overlaps with a heparin interaction site. These data suggest that heparan sulfate may sequester fibrillin at the cell surface via FUN-EGF3 prior to aggregation of the C terminus, thereby regulating microfibril assembly.

Elastic fibres in health and disease

Elastic fibres are insoluble components of the extracellular matrix of dynamic connective tissues such as skin, arteries, lungs and ligaments. They are laid down during development, and comprise a cross-linked elastin core within a template of fibrillin-based microfibrils. Their function is to endow tissues with the property of elastic recoil, and they also regulate the bioavailability of transforming growth factor β. Severe heritable elastic fibre diseases are caused by mutations in elastic fibre components; for example, mutations in elastin cause supravalvular aortic stenosis and autosomal dominant cutis laxa, mutations in fibrillin-1 cause Marfan syndrome and Weill–Marchesani syndrome, and mutations in fibulins-4 and -5 cause autosomal recessive cutis laxa. Acquired elastic fibre defects include dermal elastosis, whereas inflammatory damage to fibres contributes to pathologies such as pulmonary emphysema and vascular disease. This review outlines the latest understanding of the composition and assembly of elastic fibres, and describes elastic fibre diseases and current therapeutic approaches.

Fibrillin-1 mutations causing Weill-Marchesani syndrome and acromicric and geleophysic dysplasias disrupt heparan sulfate interactions

The extracellular glycoprotein fibrillin-1 forms microfibrils that act as the template for elastic fibers. Most mutations in fibrillin-1 cause Marfan syndrome with severe cardiovascular and ocular symptoms, and tall stature. This is in contrast to mutations within a heparin-binding TB domain (TB5), which is downstream of the arg-gly-asp cell adhesion domain, which can cause Weill-Marchesani syndrome (WMS) or Acromicric (AD) and Geleophysic Dysplasias (GD). WMS is characterized by short limbs, joint stiffness and ocular defects, whilst fibrillin-1 AD and GD have severe short stature, joint defects and thickened skin. We previously showed that TB5 binds heparin. Here, we show that the corresponding region of fibrillin-2 binds heparin very poorly, highlighting a novel functional difference between the two isoforms. This finding enabled us to map heparin/heparan sulfate binding to two sites on fibrillin-1 TB5 using a mutagenesis approach. Once these sites were mapped, we were able to investigate whether disease-causing mutations in this domain disrupt binding to HS. We show that a WMS deletion mutant, and five AD and GD point mutants all have disrupted heparin binding to TB5. These data provide insights into the biology of fibrillins and the pathologies of WMS, AD and GD.

Comparative immunolocalisation of fibrillin-1 and perlecan in the human foetal, and HS-deficient hspg2 exon 3 null mutant mouse intervertebral disc

The aim of this study was to examine the comparative localisations of fibrillin-1 and perlecan in the foetal human, wild-type C57BL/6 and HS-deficient hspg2Δ³⁻/Δ³⁻ exon 3 null mouse intervertebral disc (IVD) using fluorescent laser scanning confocal microscopy. Fibrillin-1 fibrils were prominent components of the outer posterior and anterior annulus fibrosus (AF) of the foetal human IVD. Finer fibrillin-1 fibrils were evident in the inner AF where they displayed an arcade-type arrangement in the developing lamellae. Relatively short but distinct fibrillin-1 fibrils were evident in the central region of the IVD and presumptive cartilaginous endplate and defined the margins of the nuclear sheath in the developing nucleus pulposus (NP). Fibrillin-1 was also demonstrated in the AF of C57BL/6 wild-type mice but to a far lesser extent in the HS-deficient hspg2Δ³⁻/Δ³⁻ exon 3 null mouse. This suggested that the HS chains of perlecan may have contributed to fibrillin-1 assembly or its deposition in the IVD. The cell-matrix interconnections provided by the fibrillin fibrils visualised in this study may facilitate communication between disc cells and their local biomechanical microenvironment in mechanosensory processes which regulate tissue homeostasis. The ability of fibrillin-1 to sequester TGF-β a well-known anabolic growth factor in the IVD also suggests potential roles in disc development and/or remodelling.

Dissecting the fibrillin microfibril: structural insights into organization and function

Force-bearing tissues such as blood vessels, lungs, and ligaments depend on the properties of elasticity and flexibility. The 10 to 12 nm diameter fibrillin microfibrils play vital roles in maintaining the structural integrity of these highly dynamic tissues and in regulating extracellular growth factors. In humans, defective microfibril function results in several diseases affecting the skin, cardiovascular, skeletal, and ocular systems. Despite the discovery of fibrillin-1 having occurred more than two decades ago, the structure and organization of fibrillin monomers within the microfibrils are still controversial. Recent structural data have revealed strategies by which fibrillin is able to maintain its architecture in dynamic tissues without compromising its ability to interact with itself and other cell matrix components. This review summarizes our current knowledge of microfibril structure, from individual fibrillin domains and the calcium-dependent tuning of pairwise interdomain interactions to microfibril dynamics, and how this relates to microfibril function in health and disease.

ADAMTSL6β protein rescues fibrillin-1 microfibril disorder in a Marfan syndrome mouse model through the promotion of fibrillin-1 assembly

Marfan syndrome (MFS) is a systemic disorder of the connective tissues caused by insufficient fibrillin-1 microfibril formation and can cause cardiac complications, emphysema, ocular lens dislocation, and severe periodontal disease. ADAMTSL6β (A disintegrin-like metalloprotease domain with thrombospondin type I motifs-like 6β) is a microfibril-associated extracellular matrix protein expressed in various connective tissues that has been implicated in fibrillin-1 microfibril assembly. We here report that ADAMTSL6β plays an essential role in the development and regeneration of connective tissues. ADAMTSL6β expression rescues microfibril disorder after periodontal ligament injury in an MFS mouse model through the promotion of fibrillin-1 microfibril assembly. In addition, improved fibrillin-1 assembly in MFS mice following the administration of ADAMTSL6β attenuates the overactivation of TGF-β signals associated with the increased release of active TGF-β from disrupted fibrillin-1 microfibrils within periodontal ligaments. Our current data thus demonstrate the essential contribution of ADAMTSL6β to fibrillin-1 microfibril formation. These findings also suggest a new therapeutic strategy for the treatment of MFS through ADAMTSL6β-mediated fibrillin-1 microfibril assembly.

Classical and neonatal Marfan syndrome mutations in fibrillin-1 cause differential protease susceptibilities and protein function

Mutations in fibrillin-1 give rise to Marfan syndrome (MFS) characterized by vascular, skeletal, and ocular abnormalities. Fibrillins form the backbone of extracellular matrix microfibrils in tissues including blood vessels, bone, and skin. They are crucial for regulating elastic fiber biogenesis and growth factor bioavailability. To compare the molecular consequences of mutations causing the severe neonatal MFS with mutations causing the milder classical MFS, we introduced representative point mutations from each group in a recombinant human fibrillin-1 fragment. Structural effects were analyzed by circular dichroism spectroscopy and analytical gel filtration chromatography. Proteolytic susceptibility was probed with non-physiological and physiological proteases, including plasmin, thrombin, matrix metalloproteinases, and cathepsins. All mutant proteins showed a similar gross secondary structure and no differences in heat stability as compared with the wild-type protein. Proteins harboring neonatal mutations were typically more susceptible to proteolytic cleavage compared with those with classical mutations and the wild-type protein. Proteolytic neo-cleavage sites were found both in close proximity and distant to the mutations, indicating small but significant structural changes exposing cryptic cleavage sites. We also report for the first time that cathepsin K and V cleave non-mutated fibrillin-1 at several domain boundaries. Compared with the classical mutations and the wild type, the group of neonatal mutations more severely affected the ability of fibrillin-1 to interact with heparin/heparan sulfate, which plays a role in microfibril assembly. These results suggest differential molecular pathogenetic concepts for neonatal and classical MFS including enhanced proteolytic susceptibility for physiologically relevant enzymes and loss of function for heparin binding.

FBN1 isoform expression varies in a tissue and development-specific fashion

Mutations in FBN1 cause Marfan syndrome, a heritable disorder of connective tissue. FBN1 encodes the extracellular matrix protein, fibrillin. Our objective was to elucidate the extent that variation in RNA splicing contributes to FBN1 isoforms. To identify FBN1 splice variants, we scanned each of its 64 internal exons in a set of pooled human brain cDNA samples. FBN1 splicing is generally efficient as we identified only two variants. Neither variant has previously been reported in the literature and include (i) an isoform which contains a cryptic 105 basepair exon between exons 54 and 55 (54A-FBN1) and (ii) an isoform which contains a cryptic 62 basepair exon between exons 57 and 58 (57A-FBN1). We compared 57A-FBN1 and FBN1 expression in multiple human tissues, including adult skeletal muscle and brain, as well as fetal skeletal muscle, brain, liver, aorta, lung, skin, and heart. 57A-FBN1 represents 8-44% of FBN1 mRNA and varies in a tissue- and development-specific fashion. In adult brain, 57A-FBN1 represented 39±3 (%, mean±SD) of total FBN1 expression. In contrast, 57A-FBN1 represented 19±2 (%, mean±SD) of FBN1 expression in skeletal muscle. In fetal tissue, the 57A-FBN1 proportion was highest in brain (27%) and low elsewhere, e.g., skin, aorta and lung (9-13%). In summary, a significant proportion of FBN1 is expressed as 57A-FBN1 and this proportion varies in a tissue- and development-specific fashion. Since the 57A insertion creates a premature stop codon that mimics Marfan-associated mutations, the protein encoded by 57A-FBN1 is likely to not be functional. These results suggest that altered splicing may modulate disease severity, regulate FBN1 expression, and potentially represent a therapeutic target.

Homocysteine modifies structural and functional properties of fibronectin and interferes with the fibronectin-fibrillin-1 interaction

Homocystinuria is a genetic disorder resulting in elevated levels of homocysteine in plasma and tissues. Some of the skeletal and ocular symptoms such as long bone overgrowth, scoliosis, and ectopia lentis overlap with symptoms seen in Marfan syndrome. Marfan syndrome is caused by mutations in the extracellular matrix protein fibrillin-1. We previously showed that fibrillin-1 is a target for homocysteine and that the deposition of homocysteinylated fibrillin-1 in the extracellular matrix is compromised. Since the assembly of fibrillin-1 is critically dependent on fibronectin, we analyzed the consequences of fibronectin homocysteinylation and its interaction with fibrillin-1. Cellular fibronectin and proteolytic fragments were homocysteinylated and tested in various interaction assays with recombinant fibrillin-1 and heparin. Fibronectin homocysteinylation consistently compromised the fibronectin-fibrillin-1 interaction, while the interaction with heparin was not affected. Fibronectin homocysteinylation, but not cysteinylation, reduced the fibronectin dimers to monomers as shown by Western blotting. ELISA analyses of homocysteinylated fibronectin with three monoclonal antibodies demonstrated structural changes in the disulfide-containing FNI domains FNI(2), FNI(4), and FNI(9). Using fluorescently labeled fibronectin, we studied the consequence of fibronectin homocysteinylation on assembly in cell culture. Modified fibronectin showed deficiencies in denovo matrix incorporation and initial assembly. In conclusion, we define here characteristic structural changes of fibronectin upon homocysteinylation that translate into functional deficiencies in the fibronectin-fibrillin-1 interaction and in fibronectin assembly. Since fibronectin is a major organizer of various extracellular protein networks, these structural and functional alterations may contribute to the pathogenesis of homocystinuria and Marfan syndrome.

TB domain proteins: evolutionary insights into the multifaceted roles of fibrillins and LTBPs

Fibrillins and LTBPs [latent TGFβ (transforming growth factor β)-binding proteins] perform vital and complex roles in the extracellular matrix and are relevant to a wide range of human diseases. These proteins share a signature 'eight cysteine' or 'TB (TGFβ-binding protein-like)' domain that is found nowhere else in the human proteome, and which has been shown to mediate a variety of protein-protein interactions. These include covalent binding of the TGFβ propeptide, and RGD-directed interactions with a repertoire of integrins. TB domains are found interspersed with long arrays of EGF (epidermal growth factor)-like domains, which occur more widely in extracellular proteins, and also mediate binding to a large number of proteins and proteoglycans. In the present paper, newly available protein sequence information from a variety of sources is reviewed and related to published findings on the structure and function of fibrillins and LTBPs. These sequences give valuable insight into the evolution of TB domain proteins and suggest that the fibrillin domain organization emerged first, over 600 million years ago, prior to the divergence of Cnidaria and Bilateria, after which it has remained remarkably unchanged. Comparison of sequence features and domain organization in such a diverse group of organisms also provides important insights into how fibrillins and LTBPs might perform their roles in the extracellular matrix.

Fibrillin assemblies: extracellular determinants of tissue formation and fibrosis

The extracellular matrix (ECM) plays a key role in tissue formation, homeostasis and repair, mutations in ECM components have catastrophic consequences for organ function and therefore, for the fitness and survival of the organism. Collagen, fibrillin and elastin polymers represent the architectural scaffolds that impart specific mechanic properties to tissues and organs. Fibrillin assemblies (microfibrils) have the additional function of distributing, concentrating and modulating local transforming growth factor (TGF)-β and bone morphogenetic protein (BMP) signals that regulate a plethora of cellular activities, including ECM formation and remodeling. Fibrillins also contain binding sites for integrin receptors, which induce adaptive responses to changes in the extracellular microenvironment by reorganizing the cytoskeleton, controlling gene expression, and releasing and activating matrix-bound latent TGF-β complexes. Genetic evidence has indicated that fibrillin-1 and fibrillin-2 contribute differently to the organization and structural properties of non-collagenous architectural scaffolds, which in turn translate into discrete regulatory outcomes of locally released TGF-β and BMP signals. Additionally, the study of congenital dysfunctions of fibrillin-1 has yielded insights into the pathogenesis of acquired connective tissue disorders of the connective tissue, such as scleroderma. On the one hand, mutations that affect the structure or expression of fibrillin-1 perturb microfibril biogenesis, stimulate improper latent TGF-β activation, and give rise to the pleiotropic manifestations in Marfan syndrome (MFS). On the other hand, mutations located around the integrin-binding site of fibrillin-1 perturb cell matrix interactions, architectural matrix assembly and extracellular distribution of latent TGF-β complexes, and lead to the highly restricted fibrotic phenotype of Stiff Skin syndrome. Understanding the molecular similarities and differences between congenital and acquired forms of skin fibrosis may therefore provide new therapeutic tools to mitigate or even prevent disease progression in scleroderma and perhaps other fibrotic conditions.

Fibrillin-1 and -2 differentially modulate endogenous TGF-β and BMP bioavailability during bone formation

Extracellular microfibrils composed of fibrillin-1 and -2 regulate bone formation through modulation of TGF-β and BMP signaling.

Extracellular regulation of signaling by transforming growth factor (TGF)–β family members is emerging as a key aspect of organ formation and tissue remodeling. In this study, we demonstrate that fibrillin-1 and -2, the structural components of extracellular microfibrils, differentially regulate TGF-β and bone morphogenetic protein (BMP) bioavailability in bone. Fibrillin-2–null (Fbn2^−/−) mice display a low bone mass phenotype that is associated with reduced bone formation in vivo and impaired osteoblast maturation in vitro. This Fbn2^−/− phenotype is accounted for by improper activation of latent TGF-β that selectively blunts expression of osterix, the transcriptional regulator of osteoblast maturation, and collagen I, the structural template for bone mineralization. Cultured osteoblasts from Fbn1^−/− mice exhibit improper latent TGF-β activation as well, but mature faster because of increased availability of otherwise matrix-bound BMPs. Additional in vitro evidence excludes a direct role of microfibrils in supporting mineral deposition. Together, these findings identify the extracellular microfibrils as critical regulators of bone formation through the modulation of endogenous TGF-β and BMP signaling.

Assembly of fibrillin microfibrils governs extracellular deposition of latent TGF beta

Control of the bioavailability of the growth factor TGFbeta is essential for tissue formation and homeostasis, yet precisely how latent TGFbeta is incorporated into the extracellular matrix is unknown. Here, we show that deposition of a large latent TGFbeta complex (LLC), which contains latent TGFbeta-binding protein 1 (LTBP-1), is directly dependent on the pericellular assembly of fibrillin microfibrils, which interact with fibronectin during higher-order fibrillogenesis. LTBP-1 formed pericellular arrays that colocalized with microfibrils, whereas fibrillin knockdown inhibited fibrillar LTBP-1 and/or LLC deposition. Blocking alpha5beta1 integrin or supplementing cultures with heparin, which both inhibited microfibril assembly, disrupted LTBP-1 deposition and enhanced Smad2 phosphorylation. Full-length LTBP-1 bound only weakly to N-terminal pro-fibrillin-1, but this association was strongly enhanced by heparin. The microfibril-associated glycoprotein MAGP-1 (MFAP-2) inhibited LTBP-1 binding to fibrillin-1 and stimulated Smad2 phosphorylation. By contrast, fibulin-4, which interacted strongly with full-length LTBP-1, did not induce Smad2 phosphorylation. Thus, LTBP-1 and/or LLC deposition is dependent on pericellular microfibril assembly and is governed by complex interactions between LTBP-1, heparan sulfate, fibrillin-1 and microfibril-associated molecules. In this way, microfibrils control TGFbeta bioavailability.

Recent molecular biological progress in Marfan syndrome and Marfan-associated disorders

Marfan syndrome (MFS) is a connective tissue disorder with autosomal dominant inheritance. Advances in medicine and surgery have increased the average lifespan of classically affected patients. Serious visual and/or musculoskeletal impairment often has detrimental effects on day-to-day activities and quality of life. MFS patients suffer from many problems at younger ages and with higher frequencies than the general population because of the degenerative nature of the genetic condition. In classical MFS, changes are caused by mutations in the fibrillin-1 gene (FBN1). Mutations in the fibrillin-2 gene were discovered in individuals with a phenotypically related disorder, congenital contractural arachnodactyly. Some of the clinical manifestations of MFS cannot be explained by mechanical properties alone. Recently, mutations in the genes required for transforming growth factor-beta signaling (TGFBR1 and TGFBR2) have been found in several disorders with varying degrees of overlap with classical MFS, including Loeys-Dietz syndrome and familial thoracic aortic aneurysms and dissections. MFS is a disorder that is variable in its phenotypic expression. Specific information about mutations in the large FBN1 gene will give rise to more information about the phenotype-genotype correlations. Possible molecular mechanisms for the pathogenesis of MFS will be discussed which may assist healthcare professionals to control environmental factors that provoke individual complications in MFS.