Fibrillin-1 regulates mesangial cell attachment, spreading, migration and proliferation

The extracellular matrix glycoprotein fibrillin-1 in health and disease

Fibrillin-1 (FBN1) is a large, cysteine-rich, calcium binding extracellular matrix glycoprotein encoded by FBN1 gene. It serves as a structural component of microfibrils and provides force-bearing mechanical support in elastic and nonelastic connective tissue. As such, mutations in the FBN1 gene can cause a wide variety of genetic diseases such as Marfan syndrome, an autosomal dominant disorder characterized by ocular, skeletal and cardiovascular abnormalities. FBN1 also interacts with numerous microfibril-associated proteins, growth factors and cell membrane receptors, thereby mediating a wide range of biological processes such as cell survival, proliferation, migration and differentiation. Dysregulation of FBN1 is involved in the pathogenesis of many human diseases, such as cancers, cardiovascular disorders and kidney diseases. Paradoxically, both depletion and overexpression of FBN1 upregulate the bioavailability and signal transduction of TGF-β via distinct mechanisms in different settings. In this review, we summarize the structure and expression of FBN1 and present our current understanding of the functional role of FBN1 in various human diseases. This knowledge will allow to develop better strategies for therapeutic intervention of FBN1 related diseases.

The fibrillin-1 RGD motif posttranscriptionally regulates ERK1/2 signaling and fibroblast proliferation via miR-1208

Fibrillin-1 is an extracellular matrix protein which contains one conserved RGD integrin-binding motif. It constitutes the backbone of microfibrils in many tissues, and mutations in fibrillin-1 cause various connective tissue disorders. Although it is well established that fibrillin-1 interacts with several RGD-dependent integrins, very little is known about the associated intracellular signaling pathways. Recent published evidence identified a subset of miRNAs regulated by fibrillin-1 RGD-cell adhesion, with miR-1208 among the most downregulated. The present study shows that the downregulated miR-1208 controls fibroblast proliferation. Inhibitor experiments revealed that fibrillin-1 RGD suppressed miR-1208 expression via c-Src kinase and the downstream JNK signaling. Bioinformatic prediction and experimental target sequence validation demonstrated four miR-1208 binding sites on the ERK2 mRNA and one on the MEK1 mRNA. ERK2 and MEK1 are critical proliferation-promoting kinases. Decreased miR-1208 levels elevated the total and phosphorylated ERK1/2 and MEK1/2 protein levels and the phosphorylated to total ERK1/2 ratio. Together, the data demonstrate a novel outside-in signaling mechanism explaining how fibrillin-1 RGD-cell binding regulates fibroblast proliferation.

Fibrillin-1 and alpha8 integrin are co-expressed in the glomerulus and interact to convey adhesion of mesangial cells

Fibrillin-1 is a microfibrillar extracellular matrix protein that was described to be a ligand for α8 integrin. α8 integrin is a matrix receptor specifically expressed in mesangial and smooth muscle cells of the kidney. In previous studies we detected glomerular expression of fibrillin-1. Moreover, fibrillin-1 promoted adhesion, migration, and proliferation of mesangial cells. We hypothesized that fibrillin-1 and α8 integrin might interact in the glomerulus, and thus, regulate mesangial cell properties. Our studies showed that fibrillin-1 and α8 integrin colocalize in the glomerular mesangium. Induction of experimental glomerulonephritis led to an increase of both fibrillin-1 and α8 integrin expression. In vitro studies revealed that mesangial cells deficient for α8 integrin adhere weaker to fibrillin-1 and migrate more easily on fibrillin-1 than wild-type mesangial cells. Baseline proliferation on fibrillin-1 is higher in α8 integrin-deficient mesangial cells, but the induction of proliferation is not different in α8 integrin-deficient and wild-type mesangial cells. We conclude that fibrillin-1 and α8 integrin interact, and thus, regulate mesangial cell adhesion and migration. The concomitant induction of both fibrillin-1 and α8 integrin in a self-limited model of glomerular injury points to a protective role of the interaction of fibrillin-1 with α8 integrin in the glomerulus resulting in reduced damage of the glomerular tuft as a consequence of firm adhesion of mesangial cells.

[Elastin and microfibrils in vascular development and ageing: complementary or opposite roles?]

Large arteries allow the vascular system to be more than a simple route in which the blood circulates within the organism. The elastic fibers present in the wall endow these vessels with elasticity and are responsible for the smoothing of the blood pressure and flow, which are delivered discontinuously by the heart. This function is very important to ensure appropriate hemodynamics. Elastic fibers are composed of elastin (90%) and fibrillin-rich microfibrils (10%) which provide the vessels with elasticity and are also signals able to bind to relatively specific cell membrane receptors. Stimulation of the high affinity elastin receptor by elastin peptides or tropoelastin--the elastin precursor--triggers an increase in intracellular free calcium in vascular cells, especially endothelial cells, associated with attachment, migration or proliferation. Similar effects of the stimulation of endothelial cells by microfibrils or fibrillin-1 fragments, which bind to integrins, have been demonstrated. This dual function--mechanical and in signaling--makes the elastic fibers an important actor of the development and ageing processes taking place in blood vessels. An alteration of the elastin (Eln) or fibrillin (Fbn) gene products leads to severe genetic pathologies of the cardiovascular system, such as supravalvular aortic stenosis, or Williams Beuren syndrome--in which elastin deficiency induces aortic stenoses--or Marfan syndrome, in which on the contrary fibrillin-1 deficiency promotes the appearance of aortic aneurysms. Genetically-engineered mouse models of these pathologies (such as Eln+/- mice and Fbn-1+/mgΔ mice, Eln+/-Fbn-1+/- mice) have permitted a better understanding of the pathogenesis of these syndromes. In particular, it has been shown that elastin and fibrillin-1 roles can be complementary in some aspects, while they can be opposed in some other situations. For instance, the double heterozygosity in elastin and fibrillin-1 leads to increased arterial wall stress--compared to the level induced by one of these two deficiencies alone--while the decrease in diameter induced by Eln deficiency is partly compensated by an additional deficiency in Fbn-1. Also, it is now clear that early modifications of elastin or fibrillin-1 availability can alter the normal signaling action of these proteins and lead to long term modifications of the vascular physiology and ageing processes.

Bioinformatics analysis of proteomic profiles during the process of anti-Thy1 nephritis

Anti-Thy1 nephritis is a well-established experimental mesangial proliferative nephritis model. Exploring the molecular mechanisms of pathophysiology in anti-Thy1 nephritis may elucidate the pathogeneses of mesangial proliferation. We examined the roles and acting mechanisms of differentially expressed proteins (DEPs) by bioinformatics analysis of glomeruli proteomic profiles during the course of anti-Thy1 nephritis. In total, 108 DEPs were found by two-dimensional fluorescence difference gel electrophoresis (2D-DIGE), and 40 DEPs were identified by matrix-assisted laser desorption ionization/time of flight and liquid chromatography-MS. DEPs were classified into five clusters (Clusters 1-5), according to their expression trends using Cluster 3.0 software, involved in regulating biological processes such as the stress response, cell proliferation, apoptosis, energy metabolism, transport, and the actin cytoskeleton. The expression patterns of ten DEPs, distributed across five clusters, including AKR1A1, AGAT, ATP6V1B2, HIBADH, MDH1, MPST, NIT2, PRDX6, PSMB7, and TPI1, were validated by Western blotting. Based on Western blotting and immunohistochemistry, we also found that the DEP FHL2, which was primarily expressed in the mesangial region, was down-regulated on days 3 and 5, and up-regulated on day 10. In vitro, we found that FHL2 overexpression induced mesangial cell proliferation by increasing the number of S-phase cells and decreasing G2/M-phase cells, whereas inhibiting FHL2 had the opposite effect. This study explored novel DEPs and their expression patterns during anti-Thy1 nephritis, and elucidated FHL2's effect on mesangial cell proliferation. These results will contribute to our understanding of the pathogenesis of mesangial proliferation.

Elastic system fibers in rat incisor periodontal ligament--immunohistochemical study using sections of fresh-frozen un-demineralized tissues

The aim of this study was to examine the localization and distribution of the components of elastic system fibers in the periodontal ligament of continuously erupting rat incisors in an effort to understand the mechanism of the eruption of the tooth. Sections of fresh-frozen, un-demineralized incisors of the rat mandible were prepared for immunohistochemical localization of elastin, fibrillin-2 and microfibril-associated glycoprotein-1 (MAGP-1). The structure of the periodontal ligament was well preserved in sections of fresh-frozen tissues. At the basal region of the ligament, intense immunolabelling for fibrillin-2 and MAGP-1 was observed as dot-like structures (transversely sectioned fibers) mainly on the tooth side of the ligament close to the cementum. These dot-like structures gradually increased in number towards the incisal area and were distributed throughout the tooth side of the ligament. This pattern of distribution was the same as that of reported oxytalan fibers. Elastin-immunopositive fibers were also detected in the ligament, although the labelling was limited and distribution was sparse. In conclusion, both fibrillin-2 and MAGP-1 immunopositive fibers may serve as a scaffold for deposition of tropoelastin during elastogenesis in the periodontal ligament. They may also provide guidance for the migration of fibroblasts to the occlusive side, which generates contractile forces for the movement of the tooth for continuous eruption of incisors.

Proteoglycans in health and disease: novel regulatory signaling mechanisms evoked by the small leucine-rich proteoglycans

The small leucine-rich proteoglycans (SLRPs) are involved in many aspects of mammalian biology, both in health and disease. They are now being recognized as key signaling molecules with an expanding repertoire of molecular interactions affecting not only growth factors, but also various receptors involved in controlling cell growth, morphogenesis and immunity. The complexity of SLRP signaling and the multitude of affected signaling pathways can be reconciled with a hierarchical affinity-based interaction of various SLRPs in a cell- and tissue-specific context. Here, we review this interacting network, describe new relationships of the SLRPs with tyrosine kinase and Toll-like receptors and critically assess their roles in cancer and innate immunity.

Microfibrils and fibrillin-1 induce integrin-mediated signaling, proliferation and migration in human endothelial cells

Microfibrils are macromolecular complexes associated with elastin to form elastic fibers that endow extensible tissues, such as arteries, lungs, and skin, with elasticity property. Fibrillin-1, the main component of microfibrils, is a 350-kDa glycoprotein for which genetic haploinsufficiency in humans can lead to Marfan syndrome, a severe polyfeatured pathology including aortic aneurysms and dissections. Microfibrils and fibrillin-1 fragments mediate adhesion of several cell types, including endothelial cells, while fibrillin-1 additionally triggers lung and mesangial cell migration. However, fibrillin-1-induced intracellular signaling is unknown. We have studied the signaling events induced in human umbilical venous endothelial cells (HUVECs) by aortic microfibrils as well as recombinant fibrillin-1 Arg-Gly-Asp (RGD)-containing fragments PF9 and PF14. Aortic microfibrils and PF14, not PF9, substantially and dose dependently increased HUVEC cytoplasmic and nuclear calcium levels measured using the fluorescent dye Fluo-3. This effect of PF14 was confirmed in bovine aortic endothelial cells. PF14 action in HUVECs was mediated by αvβ3 and α5β1 integrins, phospholipase-C, inosital 1,4,5-trisphosphate, and mobilization of intracellular calcium stores, whereas membrane calcium channels were not or only slightly implicated, as shown in patch-clamp experiments. Finally, PF14 enhanced endothelial cell proliferation and migration. Hence, fibrillin-1 sequences may physiologically activate endothelial cells. Genetic fibrillin-1 deficiency could alter normal endothelial signaling and, since endothelium dysfunction is an important contributor to Marfan syndrome, participate in the arterial anomalies associated with this developmental disease.

Inhibitory action of Wnt target gene osteopontin on mitochondrial cytochrome c release determines renal ischemic resistance

Certain determinants of ischemic resistance in the Brown Norway rat strain have been proposed, but no studies to date have focused on the role of the Wnt pathway in the ischemic resistance mechanism. We performed a comparative genomic study in Brown Norway vs. Sprague-Dawley rats. Selective manipulations of the Wnt pathway in vivo and in vitro allowed us to study whether the action of the Wnt pathway on apoptosis through the regulation of osteopontin was critical to the maintenance of inherent ischemic resistance mechanisms. The results revealed a major gene upregulation of the Wnt family in Brown Norway rats after renal ischemia-reperfusion. Manipulation of the Wnt signaling cascade by selective antibodies increased mitochondrial cytochrome c release and caspase 3 activity. The antiapoptotic role of Wnt was mediated by osteopontin, a direct Wnt target gene. Osteopontin was reduced by Wnt antibody administration in vivo, and osteopontin gene silencing in vitro significantly increased mitochondrial cytochrome c release. The overexpression of Wnt pathway genes detected in Brown Norway rats is critical in the maintenance of their inherent ischemic resistance. Activation of the Wnt signaling cascade reduces mitochondrial cytochrome c release and caspase 3 activity through the action of osteopontin.

Myocilin promotes substrate adhesion, spreading and formation of focal contacts in podocytes and mesangial cells

Myocilin, a secreted glycoprotein of the olfactomedin family, is constitutively expressed in podocytes of the rat kidney and induced in mesangial cells during mesangioproliferative glomerulonephritis. As myocilin has been found to be associated with fibrillar components of the extracellular matrix, and adhesive properties have been shown for other members of the olfactomedin family, we hypothesized that myocilin might play a role in cell-matrix interactions in the glomerulus. To elucidate functional properties of myocilin, recombinant myocilin was expressed in 293 EBNA cells and purified by Ni-chelate and heparin chromatography. Culture plates were coated with myocilin, and primary rat mesangial cells and cells from an immortal murine podocyte cell line were seeded onto the plates in serum free conditions. Both cell types showed concentration-dependant attachment to myocilin, an effect that was statistically significant and could be blocked with specific antibodies. When compared to equal amounts of fibronectin or collagen 1, myocilin was less effective in promoting substrate adhesion. Synergistic effects in substrate adhesion were observed when myocilin was added to low concentrations of fibronectin. Twenty-five percent of cells that had attached to myocilin substrates showed spreading and expressed focal contacts which were labeled by vinculin/phalloidin staining. Comparable findings were observed when human or murine trabecular meshwork cells were seeded on myocilin substrates. Adhesive properties of myocilin required multimer formation, and were not observed when culture plates were coated with a C-terminal fragment of myocilin, containing the olfactomedin domain. We conclude that myocilin promotes substrate adhesion of podocytes and mesangial cells, and might contribute to cell-matrix adhesion of both cell types in vivo.

Arg-Gly-Asp-containing domains of fibrillins-1 and -2 distinctly regulate lung fibroblast migration

Development of the extracellular matrix is a critical feature of alveolar formation and actively involves pulmonary interstitial fibroblasts. The elastic fiber network is an interconnected system of load-bearing fibers that also influences the behavior of adjacent cells, particularly the interstitial lung fibroblasts (LF). We hypothesized that discrete domains of fibrillins-1 and -2 interact with LF integrins and direct their migration in the presence of platelet-derived growth factor (PDGF)-A. Surfaces coated with recombinant peptides lacking or including an arginine-glycine-aspartic acid (RGD) motif were used to study LF migration across porous filters and on protein-coated glass. Exon 24 of fibrillin-2 (Fib2 24), which encodes for an RGD-containing transforming growth factor-beta-binding (TB) domain, stimulated migration with greater directional persistence and more effectively stimulated trans-filter migration at low concentrations. Exons 36-44 of fibrillin-1 (Fib1 36-44), which include epidermal growth factor-like domains and an RGD-containing TB domain, induce more lamlellipodia and more widespread remodeling of the leading edge, resulting in greater migration velocity than did Fib2 24. Distinct structural features in regions that surround the RGD motifs may differentially regulate how the PDGF receptor-alpha promotes integrin distribution and actin filament remodeling at the cell's leading edge. Understanding how fibrillins regulate LF migration may help elucidate how the elastic fiber system could be restored as an interconnected unit, which fails to occur in emphysematous lungs.