Fibrillins constitute the major backbone of multifunctional microfibrils in elastic and nonelastic extracellular matrices. Proper assembly mechanisms are central to the formation and function of these microfibrils, and their properties are often compromised in pathological circumstances such as in Marfan syndrome and in other fibrillinopathies. Here, we have used human dermal fibroblasts to analyze the assembly of fibrillin-1 in dependence of other matrix-forming proteins. siRNA knockdown experiments demonstrated that the assembly of fibrillin-1 is strictly dependent on the presence of extracellular fibronectin fibrils. Immunolabeling performed at the light and electron microscopic level showed colocalization of fibrillin-1 with fibronectin fibrils at the early stages of the assembly process. Protein-binding assays demonstrated interactions of fibronectin with a C-terminal region of fibrillin-1, -2, and -3 and with an N-terminal region of fibrillin-1. The C-terminal half of fibrillin-2 and -3 had propensities to multimerize, as has been previously shown for fibrillin-1. The C-terminal of all three fibrillins interacted strongly with fibronectin as multimers, but not as monomers. Mapping studies revealed that the major binding interaction between fibrillins and fibronectin involves the collagen/ gelatin-binding region between domains FNI 6 and FNI 9 . INTRODUCTIONFibrillins are extracellular matrix components with important functions in elastic and nonelastic tissues including blood vessels, bone, and the eye. Fibrillins, together with the latent transforming growth factor- (TGF-)-binding proteins (LTBPs), constitute the fibrillin-LTBP family of proteins (Hubmacher et al., 2006). Fibrillins are ϳ350 kDa in size, are disulfide-rich and glycosylated, and have a characteristic modular structure. The three human fibrillinsfibrillin-1, -2, and -3 -are encoded by different genes and are conserved at the amino acid level both in relation to one another and among species. Fibrillins are mainly composed of tandem arrays of calcium-binding epidermal growth factor-like domains interspersed with TGF--binding protein domains (TB/8-Cys) and hybrid domains Hubmacher et al., 2006). Mutations in fibrillins give rise to the so-called fibrillinopathies, which include Marfan syndrome and autosomal dominant Weill-Marchesani syndrome, both caused by mutations in fibrillin-1, and Beal's syndrome, caused by mutations in fibrillin-2 (Robinson et al., 2006).High-molecular-weight, multiprotein assemblies called microfibrils are the functional units of fibrillins, which serve as a scaffold for the biogenesis of elastic fibers, confer structural integrity to individual organ systems, regulate growth factor signaling of TGF--bone morphogenic protein (BMP) superfamily members and provide limited elasticity to tissues (Kielty et al., 2002;Charbonneau et al., 2004;Ramirez and Dietz, 2007). Extracted microfibrils from cell culture or tissues display a typical bead-on-a-string ultrastructure, having a 50 -55-nm periodicity when analyzed by ele...
Ovarian follicular granulosa cells surround and nurture oocytes, and produce sex steroid hormones. It is believed that during development the ovarian surface epithelial cells penetrate into the ovary and develop into granulosa cells when associating with oogonia to form follicles. Using bovine fetal ovaries (n = 80) we identified a novel cell type, termed GREL for Gonadal Ridge Epithelial-Like. Using 26 markers for GREL and other cells and extracellular matrix we conducted immunohistochemistry and electron microscopy and chronologically tracked all somatic cell types during development. Before 70 days of gestation the gonadal ridge/ovarian primordium is formed by proliferation of GREL cells at the surface epithelium of the mesonephros. Primordial germ cells (PGCs) migrate into the ovarian primordium. After 70 days, stroma from the underlying mesonephros begins to penetrate the primordium, partitioning the developing ovary into irregularly-shaped ovigerous cords composed of GREL cells and PGCs/oogonia. Importantly we identified that the cords are always separated from the stroma by a basal lamina. Around 130 days of gestation the stroma expands laterally below the outermost layers of GREL cells forming a sub-epithelial basal lamina and establishing an epithelial-stromal interface. It is at this stage that a mature surface epithelium develops from the GREL cells on the surface of the ovary primordium. Expansion of the stroma continues to partition the ovigerous cords into smaller groups of cells eventually forming follicles containing an oogonium/oocyte surrounded by GREL cells, which become granulosa cells, all enclosed by a basal lamina. Thus in contrast to the prevailing theory, the ovarian surface epithelial cells do not penetrate into the ovary to form the granulosa cells of follicles, instead ovarian surface epithelial cells and granulosa cells have a common precursor, the GREL cell.
Although not often discussed, the ovaries of women with polycystic ovary syndrome (PCOS) show all the hallmarks of increased TGF-β activity, with increased amounts of fibrous tissue and collagen in the ovarian capsule or tunica albuginea and ovarian stroma. Recent studies suggest that PCOS could have fetal origins. Genetic studies of PCOS have also found linkage with a microsatellite located in intron 55 of the extracellular matrix protein fibrillin 3. Fibrillins regulate TGF-β bioactivity in tissues by binding latent TGF-β binding proteins. We therefore examined expression of fibrillins 1-3, latent TGF-β binding proteins 1-4, and TGF-β 1-3 in bovine and human fetal ovaries at different stages of gestation and in adult ovaries. We also immunolocalized fibrillins 1 and 3. The results indicate that TGF-β pathways operate during ovarian fetal development, but most important, we show fibrillin 3 is present in the stromal compartments of fetal ovaries and is highly expressed at a critical stage early in developing human and bovine fetal ovaries when stroma is expanding and follicles are forming. These changes in expression of fibrillin 3 in the fetal ovary could lead to a predisposition to develop PCOS in later life.
Fibrillin proteins are the major components of extracellular microfibrils found in many connective tissues. Fibrillin-1 and fibrillin-2 are well studied and mutations in these proteins cause a number of fibrillinopathies including Marfan syndrome and congenital contractural arachnodactyly, respectively. Fibrillin-3 was more recently discovered and is much less well characterized. Fibrillin-1 is expressed throughout life, whereas fibrillin-2 and -3 are thought to be primarily present during development. Here, we report detailed fibrillin-3 expression patterns in early human development.A polyclonal antiserum against a C-terminal recombinant half of human fibrillin-3 was produced in rabbit. Anti-fibrillin-3 antibodies were affinity-purified and antibodies cross-reacting with the other fibrillins were removed by absorption resulting in specific anti-fibrillin-3 antibodies. Immunohistochemical analyses with these purified antibodies demonstrate that fibrillin-3 is temporally expressed in numerous tissues relatively evenly from the 6 th to the 12 th gestational week. Fibrillin-3 was found spatially expressed in perichondrium, perineurium, perimysium, skin, developing bronchi, glomeruli, pancreas, kidney, heart and testis and at the prospective basement membranes in developing epithelia and endothelia. Double immunohistochemical analyses showed that all fibrillins are globally expressed in the same organs, with a number of differences on the tissue level in cartilage, perichondrium and developing bronchi. These results suggest that fibrillin-3, compared to the other fibrillins, fulfills both overlapping and distinct functions in human development.
Semilunar valve leaflets have a well-described trilaminar histoarchitecture, with a sophisticated elastic fiber network. It was previously proposed that elastin-containing fibers play a subordinate role in early human cardiac valve development; however, this assumption was based on data obtained from mouse models and human second and third trimester tissues. Here, we systematically analyzed tissues from human fetal first (4-12 weeks) and second (13-18 weeks) trimester, adolescent (14-19 years) and adult (50-55 years) hearts to monitor the temporal and spatial distribution of elastic fibers, focusing on semilunar valves. Global expression analyses revealed that the transcription of genes essential for elastic fiber formation starts early within the first trimester. These data were confirmed by quantitative PCR and immunohistochemistry employing antibodies that recognize fibronectin, fibrillin 1, 2 and 3, EMILIN1 and fibulin 4 and 5, which were all expressed at the onset of cardiac cushion formation (~week 4 of development). Tropoelastin/elastin protein expression was first detectable in leaflets of 7-week hearts. We revealed that immature elastic fibers are organized in early human cardiovascular development and that mature elastin-containing fibers first evolve in semilunar valves when blood pressure and heartbeat accelerate. Our findings provide a conceptual framework with the potential to offer novel insights into human cardiac valve development and disease.
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