Regular ArticleFibrillin Binds Calcium and Is Coded by cDNAs That Reveal a Multidomain Structure and Alternatively Spliced Exons at the 5′ End
Abstract
Fibrillin is an important structural protein of the extracellular matrix. It is a large cysteine-rich glycoprotein with extensive intrachain disulfide bonds, likely contributed by multiple EGF-like repeats. We have previously published 6.9 kb of FBN1 cDNA sequence. FBN1 cDNA clones that extend the sequence 3089 bp in the 5′ direction are described in this report. The deduced primary structure suggests that fibrillin is composed of multiple domains. The most predominant feature is the presence of 43 calcium binding EGF-like repeats. We demonstrate here that fibrillin molecules bind calcium. In addition, three alternatively spliced exons at the 5′ end are described. Analysis of 5.8 kb of surrounding genomic sequence revealed a 1.8-kb CpG island spanning the alternatively spliced exons and the next downstream exon. Since FBN1 is the gene responsible for Marfan syndrome, the information presented here will be useful in identifying new mutations and in understanding the function of fibrillin in the pathogenesis of the disease.
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Role of fibrillin-2 in the control of TGF-β activation in tumor angiogenesis and connective tissue disorders
2020, Biochimica et Biophysica Acta - Reviews on CancerFibrillins 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
2019, Matrix BiologyFibrillin 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.
The Fibrillin-1 RGD Integrin Binding Site Regulates Gene Expression and Cell Function through microRNAs
2019, Journal of Molecular BiologyFibrillins are the major components of microfibrils in the extracellular matrix of elastic and non-elastic tissues. Fibrillin-1 contains one evolutionarily conserved RGD sequence that mediates cell–matrix interactions through cell-surface integrins. Here, we present a novel paradigm how extracellular fibrillin-1 controls cellular function through integrin-mediated microRNA regulation. Comparative mRNA studies by global microarray analysis identified growth factor activity, actin binding and integrin binding as the most important functional groups that are regulated upon fibrillin-1 binding to dermal fibroblasts. Many of these mRNAs are targets of miRNAs that were identified when RNA from the fibrillin-1-ligated fibroblasts was analyzed by a miRNA microarray. The expression profile was specific to fibrillin-1 since interaction with fibronectin displayed a partially distinct profile. The importance of selected miRNAs for the regulation of the identified mRNAs was suggested by bioinformatics prediction and the interactions between miRNAs and mRNAs were experimentally validated. Functionally, we show that miR-503 controls p-Smad2-dependent TGF-β signaling, and that miR-612 and miR-3185 are involved in the focal adhesion formation regulated by fibrillin-1. In conclusion, we demonstrate that fibrillin-1 interaction with fibroblasts regulates miRNA expression profiles which in turn control critical cell functions.
FBN1 encodes the gene for fibrillin-1, a structural macromolecule that polymerizes into microfibrils. Fibrillin microfibrils are morphologically distinctive fibrils, present in all connective tissues and assembled into tissue-specific architectural frameworks. FBN1 is the causative gene for Marfan syndrome, an inherited disorder of connective tissue whose major features include tall stature and arachnodactyly, ectopia lentis, and thoracic aortic aneurysm and dissection. More than one thousand individual mutations in FBN1 are associated with Marfan syndrome, making genotype–phenotype correlations difficult. Moreover, mutations in specific regions of FBN1 can result in the opposite features of short stature and brachydactyly characteristic of Weill–Marchesani syndrome and other acromelic dysplasias. How can mutations in one molecule result in disparate clinical syndromes? Current concepts of the fibrillinopathies require an appreciation of tissue-specific fibrillin microfibril microenvironments and the collaborative relationship between the structures of fibrillin microfibril networks and biological functions such as regulation of growth factor signaling.