Function of Ltbp-4L and fibulin-4 in survival and elastogenesis in mice

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.

The role of LTBPs in TGF beta signaling

The purpose of this review is to discuss the transforming growth factor beta (TGFB) binding proteins (LTBP) with respect to their participation in the activity of TGFB. We first describe pertinent aspects of the biology and cell function of the LTBPs. We then summarize the physiological consequences of LTBP loss in humans and mice. Finally, we consider a number of outstanding questions relating to LTBP function.

Clinical and Molecular Delineation of Cutis Laxa Syndromes: Paradigms for Homeostasis

Cutis laxa (CL) syndromes are a large and heterogeneous group of rare connective tissue disorders that share loose redundant skin as a hallmark clinical feature, which reflects dermal elastic fiber fragmentation. Both acquired and congenital-Mendelian- forms exist. Acquired forms are progressive and often preceded by inflammatory triggers in the skin, but may show systemic elastolysis. Mendelian forms are often pleiotropic in nature and classified upon systemic manifestations and mode of inheritance. Though impaired elastogenesis is a common denominator in all Mendelian forms of CL, the underlying gene defects are diverse and affect structural components of the elastic fiber or impair metabolic pathways interfering with cellular trafficking, proline synthesis, or mitochondrial functioning. In this chapter we provide a detailed overview of the clinical and molecular characteristics of the different cutis laxa types and review the latest insights on elastic fiber assembly and homeostasis from both human and animal studies.

Autosomal Recessive Cutis Laxa 1C Mutations Disrupt the Structure and Interactions of Latent TGFβ Binding Protein-4

Latent TGFβ binding protein-4 (LTBP4) is a multi-domain glycoprotein, essential for regulating the extracellular bioavailability of TGFβ and assembly of elastic fibre proteins, fibrillin-1 and tropoelastin. LTBP4 mutations are linked to autosomal recessive cutis laxa type 1C (ARCL1C), a rare congenital disease characterised by high mortality and severely disrupted connective tissues. Despite the importance of LTBP4, the structure and molecular consequences of disease mutations are unknown. Therefore, we analysed the structural and functional consequences of three ARCL1C causing point mutations which effect highly conserved cysteine residues. Our structural and biophysical data show that the LTBP4 N- and C-terminal regions are monomeric in solution and adopt extended conformations with the mutations resulting in subtle changes to their conformation. Similar to LTBP1, the N-terminal region is relatively inflexible, whereas the C-terminal region is flexible. Interaction studies show that one C-terminal mutation slightly decreases binding to fibrillin-1. We also found that the LTBP4 C-terminal region directly interacts with tropoelastin which is perturbed by both C-terminal ARCL1C mutations, whereas an N-terminal mutation increased binding to fibulin-4 but did not affect the interaction with heparan sulphate. Our results suggest that LTBP4 mutations contribute to ARCL1C by disrupting the structure and interactions of LTBP4 which are essential for elastogenesis in a range of mammalian connective tissues.

Bi-allelic premature truncating variants in LTBP1 cause cutis laxa syndrome

Latent transforming growth factor β (TGFβ)-binding proteins (LTBPs) are microfibril-associated proteins essential for anchoring TGFβ in the extracellular matrix (ECM) as well as for correct assembly of ECM components. Variants in LTBP2, LTBP3, and LTBP4 have been identified in several autosomal recessive Mendelian disorders with skeletal abnormalities with or without impaired development of elastin-rich tissues. Thus far, the human phenotype associated with LTBP1 deficiency has remained enigmatic. In this study, we report homozygous premature truncating LTBP1 variants in eight affected individuals from four unrelated consanguineous families. Affected individuals present with connective tissue features (cutis laxa and inguinal hernia), craniofacial dysmorphology, variable heart defects, and prominent skeletal features (craniosynostosis, short stature, brachydactyly, and syndactyly). In vitro studies on proband-derived dermal fibroblasts indicate distinct molecular mechanisms depending on the position of the variant in LTBP1. C-terminal variants lead to an altered LTBP1 loosely anchored in the microfibrillar network and cause increased ECM deposition in cultured fibroblasts associated with excessive TGFβ growth factor activation and signaling. In contrast, N-terminal truncation results in a loss of LTBP1 that does not alter TGFβ levels or ECM assembly. In vivo validation with two independent zebrafish lines carrying mutations in ltbp1 induce abnormal collagen fibrillogenesis in skin and intervertebral ligaments and ectopic bone formation on the vertebrae. In addition, one of the mutant zebrafish lines shows voluminous and hypo-mineralized vertebrae. Overall, our findings in humans and zebrafish show that LTBP1 function is crucial for skin and bone ECM assembly and homeostasis.

ADAMTS proteases in cardiovascular physiology and disease

The a disintegrin-like and metalloproteinase with thrombospondin motif (ADAMTS) family comprises 19 proteases that regulate the structure and function of extracellular proteins in the extracellular matrix and blood. The best characterized cardiovascular role is that of ADAMTS-13 in blood. Moderately low ADAMTS-13 levels increase the risk of ischeamic stroke and very low levels (less than 10%) can cause thrombotic thrombocytopenic purpura (TTP). Recombinant ADAMTS-13 is currently in clinical trials for treatment of TTP. Recently, new cardiovascular roles for ADAMTS proteases have been discovered. Several ADAMTS family members are important in the development of blood vessels and the heart, especially the valves. A number of studies have also investigated the potential role of ADAMTS-1, -4 and -5 in cardiovascular disease. They cleave proteoglycans such as versican, which represent major structural components of the arteries. ADAMTS-7 and -8 are attracting considerable interest owing to their implication in atherosclerosis and pulmonary arterial hypertension, respectively. Mutations in the ADAMTS19 gene cause progressive heart valve disease and missense variants in ADAMTS6 are associated with cardiac conduction. In this review, we discuss in detail the evidence for these and other cardiovascular roles of ADAMTS family members, their proteolytic substrates and the potential molecular mechanisms involved.

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.

Identification of a Novel 19-bp Deletion Mutation in LTBP4 Using Exome Sequencing in Two Siblings with Autosomal Recessive Cutis Laxa Type 1C

Autosomal recessive type I cutis laxa is genetically heterogeneous. Biallelic mutations in latent transforming growth factor β-binding protein 4 (LTBP4; MIM*604710) lead to type 1C cutis laxa due to nonsense, frameshift, single base pair indels, or duplication mutations. In this report, we describe the first Indian family with cutis laxa as a result of a novel 19 base pair homozygous deletion leading to premature termination of short isoform LTBP-4S.

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.

Recent updates on the molecular network of elastic fiber formation

Elastic fibers confer elasticity and recoiling to tissues and organs and play an essential role in induction of biochemical responses in a cell against mechanical forces derived from the microenvironment. The core component of elastic fibers is elastin (ELN), which is secreted as the monomer tropoelastin from elastogenic cells, and undergoes self-aggregation, cross-linking and deposition on to microfibrils, and assemble into insoluble ELN polymers. For elastic fibers to form, a microfibril scaffold (primarily formed by fibrillin-1 (FBN1)) is required. Numerous elastic fiber-associated proteins are involved in each step of elastogenesis and they instruct and/or facilitate the elastogenesis processes. In this review, we designated five proteins as key molecules in elastic fiber formation, including ELN, FBN1, fibulin-4 (FBLN4), fibulin-5 (FBLN5), and latent TGFβ-binding protein-4 (LTBP4). ELN and FBN1 serve as building blocks for elastic fibers. FBLN5, FBLN4 and LTBP4 have been demonstrated to play crucial roles in elastogenesis through knockout studies in mice. Using these molecules as a platform and expanding the elastic fiber network through the generation of an interactome map, we provide a concise review of elastogenesis with a recent update as well as discuss various biological functions of elastic fiber-associated proteins beyond elastogenesis in vivo.

Fibrillin microfibrils and proteases, key integrators of fibrotic pathways

Supramolecular networks composed of multi-domain ECM proteins represent intricate cellular microenvironments which are required to balance tissue homeostasis and direct remodeling. Structural deficiency in ECM proteins results in imbalances in ECM-cell communication resulting often times in fibrotic reactions. To understand how individual components of the ECM integrate communication with the cell surface by presenting growth factors or providing fine-tuned biomechanical properties is mandatory for gaining a better understanding of disease mechanisms in the quest for new therapeutic approaches. Here we provide an overview about what we can learn from inherited connective tissue disorders caused primarily by mutations in fibrillin-1 and binding partners as well as by altered ECM processing leading to defined structural changes and similar functional knock-in mouse models. We will utilize this knowledge to propose new molecular hypotheses which should be tested in future studies.

Elastic fibers and biomechanics of the aorta: Insights from mouse studies

Elastic fibers are major components of the extracellular matrix (ECM) in the aorta and support a life-long cycling of stretch and recoil. Elastic fibers are formed from mid-gestation throughout early postnatal development and the synthesis is regulated at multiple steps, including coacervation, deposition, cross-linking, and assembly of insoluble elastin onto microfibril scaffolds. To date, more than 30 molecules have been shown to associate with elastic fibers and some of them play a critical role in the formation and maintenance of elastic fibers in vivo. Because the aorta is subjected to high pressure from the left ventricle, elasticity of the aorta provides the Windkessel effect and maintains stable blood flow to distal organs throughout the cardiac cycle. Disruption of elastic fibers due to congenital defects, inflammation, or aging dramatically reduces aortic elasticity and affects overall vessel mechanics. Another important component in the aorta is the vascular smooth muscle cells (SMCs). Elastic fibers and SMCs alternate to create a highly organized medial layer within the aortic wall. The physical connections between elastic fibers and SMCs form the elastin-contractile units and maintain cytoskeletal organization and proper responses of SMCs to mechanical strain. In this review, we revisit the components of elastic fibers and their roles in elastogenesis and how a loss of each component affects biomechanics of the aorta. Finally, we discuss the significance of elastin-contractile units in the maintenance of SMC function based on knowledge obtained from mouse models of human disease.

Fibrillin microfibrils and elastic fibre proteins: Functional interactions and extracellular regulation of growth factors

Fibrillin microfibrils are extensible polymers that endow connective tissues with long-range elasticity and have widespread distributions in both elastic and non-elastic tissues. They act as a template for elastin deposition during elastic fibre formation and are essential for maintaining the integrity of tissues such as blood vessels, lung, skin and ocular ligaments. A reduction in fibrillin is seen in tissues in vascular ageing, chronic obstructive pulmonary disease, skin ageing and UV induced skin damage, and age-related vision deterioration. Most mutations in fibrillin cause Marfan syndrome, a genetic disease characterised by overgrowth of the long bones and other skeletal abnormalities with cardiovascular and eye defects. However, mutations in fibrillin and fibrillin-binding proteins can also cause short-stature pathologies. All of these diseases have been linked to dysregulated growth factor signalling which forms a major functional role for fibrillin.

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

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

Role of LTBP4 in alveolarization, angiogenesis, and fibrosis in lungs

Deficiency of the extracellular matrix protein latent transforming growth factor-β (TGF-β)-binding protein-4 (LTBP4) results in lack of intact elastic fibers, which leads to disturbed pulmonary development and lack of normal alveolarization in humans and mice. Formation of alveoli and alveolar septation in pulmonary development requires the concerted interaction of extracellular matrix proteins, growth factors such as TGF-β, fibroblasts, and myofibroblasts to promote elastogenesis as well as vascular formation in the alveolar septae. To investigate the role of LTBP4 in this context, lungs of LTBP4-deficient (Ltbp4^-/-) mice were analyzed in close detail. We elucidate the role of LTBP4 in pulmonary alveolarization and show that three different, interacting mechanisms might contribute to alveolar septation defects in Ltbp4^-/- lungs: 1) absence of an intact elastic fiber network, 2) reduced angiogenesis, and 3) upregulation of TGF-β activity resulting in profibrotic processes in the lung.