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

Discoidin domain receptor-2 enhances secondary alveolar septation in mice by activating integrins and modifying focal adhesions

The extracellular matrix (ECM) of the pulmonary parenchyma must maintain the structural relationships among resident cells during the constant distortion imposed by respiration. This dictates that both the ECM and cells adapt to changes in shape, while retaining their attachment. Membrane-associated integrins and discoidin domain receptors (DDR) bind collagen and transmit signals to the cellular cytoskeleton. Although the contributions of DDR2 to collagen deposition and remodeling during osseous development are evident, it is unclear how DDR2 contributes to lung development. Using mice (smallie, Slie/Slie, DDR2Δ) bearing a spontaneous inactivating deletion within the DDR2 coding region, we observed a decrease in gas-exchange surface area and enlargement of alveolar ducts. Compared with fibroblasts isolated from littermate controls, DDR2Δ fibroblasts, spread more slowly, developed fewer lamellipodia, and were less responsive to the rigidity of neighboring collagen fibers. Activated β1-integrin (CD29) was reduced in focal adhesions (FA) of DDR2Δ fibroblasts, less phospho-zyxin localized to and fewer FA developed over ventral actin stress fibers, and the adhesions had a lower aspect ratio compared with controls. However, DDR2 deletion did not reduce cellular displacement of the ECM. Our findings indicate that DDR2, in concert with collagen-binding β1-integrins, regulates the timing and location of focal adhesion formation and how lung fibroblasts respond to ECM rigidity. Reduced rigidity sensing and mechano-responsiveness may contribute to the distortion of alveolar ducts, where the fiber cable-network is enriched and tensile forces are concentrated. Strategies targeting DDR2 could help guide fibroblasts to locations where tensile forces organize parenchymal repair.

Defining the Versican Interactome in Lung Health and Disease

The extracellular matrix (ECM) imparts critical mechanical and biochemical information to cells in the lungs. Proteoglycans are essential constituents of the ECM and play a crucial role in controlling numerous biological processes, including regulating cellular phenotype and function. Versican, a chondroitin sulfate proteoglycan required for embryonic development, is almost absent from mature, healthy lungs and is re-expressed and accumulates in acute and chronic lung disease. Studies using genetically engineered mice show that the versican-enriched matrix can be pro- or anti-inflammatory depending on the cellular source or disease process studied. The mechanisms whereby versican develops a contextual ECM remain largely unknown. The primary goal of this review is to provide an overview of the interaction of versican with its many binding partners, the "versican interactome," and how through these interactions, versican is an integrator of complex extracellular information. Hopefully, the information provided in this review will be used to develop future studies to determine how versican and its binding partners can develop contextual ECMs that control select biological processes. While this review focuses on versican and the lungs, what is described can be extended to other proteoglycans, tissues, and organs.

PRMT7 targets of Foxm1 controls alveolar myofibroblast proliferation and differentiation during alveologenesis

Although aberrant alveolar myofibroblasts (AMYFs) proliferation and differentiation are often associated with abnormal lung development and diseases, such as bronchopulmonary dysplasia (BPD), chronic obstructive pulmonary disease (COPD), and idiopathic pulmonary fibrosis (IPF), epigenetic mechanisms regulating proliferation and differentiation of AMYFs remain poorly understood. Protein arginine methyltransferase 7 (PRMT7) is the only reported type III enzyme responsible for monomethylation of arginine residue on both histone and nonhistone substrates. Here we provide evidence for PRMT7's function in regulating AMYFs proliferation and differentiation during lung alveologenesis. In PRMT7-deficient mice, we found reduced AMYFs proliferation and differentiation, abnormal elastin deposition, and failure of alveolar septum formation. We further shown that oncogene forkhead box M1 (Foxm1) is a direct target of PRMT7 and that PRMT7-catalyzed monomethylation at histone H4 arginine 3 (H4R3me1) directly associate with chromatin of Foxm1 to activate its transcription, and thereby regulate of cell cycle-related genes to inhibit AMYFs proliferation and differentiation. Overexpression of Foxm1 in isolated myofibroblasts (MYFs) significantly rescued PRMT7-deficiency-induced cell proliferation and differentiation defects. Thus, our results reveal a novel epigenetic mechanism through which PRMT7-mediated histone arginine monomethylation activates Foxm1 transcriptional expression to regulate AMYFs proliferation and differentiation during lung alveologenesis and may represent a potential target for intervention in pulmonary diseases.

Variability in gene-based knowledge impacts variant classification: an analysis of FBN1 missense variants in ClinVar

Gene-specific knowledge can enhance genetic variant classification, but may not be routinely incorporated into clinical laboratory practice. For example, FBN1 variants associated with Marfan syndrome may be variably classified depending on knowledge of FBN1-specific critical regions. In order to assess variability in classification of FBN1 variants, 674 FBN1 missense variants from 18 ClinVar submitters were compared and reanalyzed using FBN1-specific criteria and ACMG/AMP 2015 guidelines for variant interpretation. Conflicting variant classifications occurred in 30.7% of the missense variants that had multiple submitters. There were 451 classifications of 361 critical residue missense variants, with 80.0% (361/451) classified as likely pathogenic or pathogenic [(L)P]. Non-cysteine critical residue variants were less likely to be classified as (L)P [55.3% (78/141)] than cysteine variants [91.3% (283/310)] and were more likely to lack evidence citing the functional significance of the amino acid impacted. Application of FBN1-specific knowledge allowed for reclassification or discrepancy resolution in 65/361 (18.0%) critical residue variants. There were 522 classifications of 313 unique missense variants not known to impact a critical residue. Of these, 31.6% (165/522) were likely overclassified as either (L)P or uncertain significance (VUS), especially when minor allele frequency (MAF) was taken into account, and we reclassified or resolved classification discrepancies in 128/313 (40.9%) of these variants. Our results provide a refined framework and resource for FBN1 variant classification, and further supports the more global implications of combining gene-based knowledge with ACMG/AMP criteria and appropriate MAF cutoffs for variant classification that extend beyond FBN1.

A novel FBN2 mutation cosegregates with congenital contractural arachnodactyly in a five-generation Chinese family

We identified a novel heterozygous mutation (c.4177T>G and p.Cys1393Gly) in FBN2 that cosegregated with congenital contractural arachnodactyly (CCA) in a five-generation Chinese family. This mutation may cause the loss of the disulfide bond between Cys 1393 and Cys 1378 residues of fibrillin-2. Our study expands the genetic profile of CCA.

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.

Platelet-derived growth factor receptor-α and Ras-related C3 botulinum toxin substrate-1 regulate mechano-responsiveness of lung fibroblasts

Platelet-derived growth factor (PDGF)-A, which only signals through PDGF-receptor-α (PDGFR-α), is required for secondary alveolar septal formation. Although PDGFR-α distinguishes mesenchymal progenitor cells during the saccular stage, PDGFR-α-expressing alveolar cells persist through adulthood. PDGF-A sustains proliferation, limits apoptosis, and maintains α-smooth muscle actin (α-SMA)-containing alveolar cells, which congregate at the alveolar entry ring at postnatal day (P)12. PDGFR-α-expressing, α-SMA-containing alveolar cells redistribute in the elongating septum, suggesting that they migrate to the alveolar entry rings, where mechanical tension is higher. We hypothesized that PDGFR-α and Ras-related C3 botulinum toxin substrate 1(Rac1) are required for mechanosensitive myofibroblast migration. Spreading of PDGFR-α-deficient lung fibroblasts was insensitive to increased rigidity, and their migration was not reduced by Rac1-guanine exchange factor (GEF)-inhibition. PDGFR-α-expressing fibroblasts migrated toward stiffer regions within two-dimensional substrates by increasing migrational persistence (durotaxis). Using a Förster resonance energy transfer (FRET) biosensor for Rac1-GTP, we observed that PDGFR-α was required for fibroblast Rac1 responsiveness to stiffness within a three-dimensional collagen substrate, which by itself increased Rac1-FRET. Rho-GTPase stabilized, whereas Rac1-GTPase increased the turnover of focal adhesions. Under conditions that increased Rac1-GTP, PDGFR-α signaled through both phosphoinositide-3-kinase (PIK) or Src to engage the Rac1 GEF dedicator of cytokinesis-1 (Dock180) and p21-activated-kinase interacting exchange factor-β (βPIX). In cooperation with collagen fibers, these signaling pathways may guide fibroblasts toward the more rigid alveolar entry ring during secondary septation. Because emphysema and interstitial fibrosis disrupt the parenchymal mechanical continuum, understanding how mechanical factors regulate fibroblast migration could elicit strategies for alveolar repair and regeneration.

Understanding the developmental pathways pulmonary fibroblasts may follow during alveolar regeneration

Although pulmonary alveolar interstitial fibroblasts are less specialized than their epithelial and endothelial neighbors, they play essential roles during development and in response to lung injury. At birth, they must adapt to the sudden mechanical changes imposed by the onset of respiration and to a higher ambient oxygen concentration. In diseases such as bronchopulmonary dysplasia and interstitial fibrosis, their adaptive responses are overwhelmed leading to compromised gas-exchange function. Thus, although fibroblasts do not directly participate in gas-exchange, they are essential for creating and maintaining an optimal environment at the alveolar epithelial-endothelial interface. This review summarizes new information and concepts about the ontogeny differentiation, and function of alveolar fibroblasts. Alveolar development will be emphasized, because the development of strategies to evoke alveolar repair and regeneration hinges on thoroughly understanding the way that resident fibroblasts populate specific locations in which extracellular matrix must be produced and remodeled. Other recent reviews have described the disruption that diseases cause to the fibroblast niche and so my objective is to illustrate how the unique developmental origins and differentiation pathways could be harnessed favorably to augment certain fibroblast subpopulations and to optimize the conditions for alveolar regeneration.

Fibroblast growth factor signaling in myofibroblasts differs from lipofibroblasts during alveolar septation in mice

Pulmonary alveolar fibroblasts produce extracellular matrix in a temporally and spatially regulated pattern to yield a durable yet pliable gas-exchange surface. Proliferation ensures a sufficient complement of cells, but they must differentiate into functionally distinct subtypes: contractile myofibroblasts (MF), which generate elastin and regulate air-flow at the alveolar ducts, and, in mice and rats, lipofibroblasts (LF), which store neutral lipids. PDGF-A is required but acts in conjunction with other differentiation factors arising from adjacent epithelia or within fibroblasts. We hypothesized that FGF receptor (FGFR) expression and function vary for MF and LF and contributes to their divergent differentiation. Whereas approximately half of the FGFR3 was extracellular in MF, FGFR2 and FGFR4 were primarily intracellular. Intracellular FGFR3 localized to the multivesicular body, and its abundance may be modified by Sprouty and interaction with heat shock protein-90. FGF18 mRNA is more abundant in MF, whereas FGF10 mRNA predominated in LF, which also express FGFR1 IIIb, a receptor for FGF10. FGF18 diminished fibroblast proliferation and was chemotactic for cultured fibroblasts. Although PDGF receptor-α (PDGFR-α) primarily signals through phosphoinositide 3-kinase and Akt, p42/p44 MAP kinase (Erk1/2), a major signaling pathway for FGFRs, influenced the abundance of cell-surface PDGFR-α. Observing different FGFR and ligand profiles in MF and LF is consistent with their divergent differentiation although both subpopulations express PDGFR-α. These studies also emphasize the importance of particular cellular locations of FGFR3 and PDGFR-α, which may modify their effects during alveolar development or repair.

ADAMTS proteins as modulators of microfibril formation and function

The ADAMTS (a disintegrin-like and metalloproteinase domain with thrombospondin-type 1 motifs) protein superfamily includes 19 secreted metalloproteases and 7 secreted ADAMTS-like (ADAMTSL) glycoproteins. The possibility of functional linkage between ADAMTS proteins and fibrillin microfibrils was first revealed by a human genetic consilience, in which mutations in ADAMTS10, ADAMTS17, ADAMTSL2 and ADAMTSL4 were found to phenocopy rare genetic disorders caused by mutations affecting fibrillin-1 (FBN1), the major microfibril component in adults. The manifestations of these ADAMTS gene disorders in humans and animals suggested that they participated in the structural and regulatory roles of microfibrils. Whereas two such disorders, Weill-Marchesani syndrome 1 and Weill-Marchesani-like syndrome involve proteases (ADAMTS10 and ADAMTS17, respectively), geleophysic dysplasia and isolated ectopia lentis in humans involve ADAMTSL2 and ADAMTSL4, respectively, which are not proteases. In addition to broadly similar dysmorphology, individuals affected by Weill-Marchesani syndrome 1, Weill-Marchesani-like syndrome or geleophysic dysplasia each show characteristic anomalies suggesting molecule-, tissue-, or context-specific functions for the respective ADAMTS proteins. Ectopia lentis occurs in each of these conditions except geleophysic dysplasia, and is due to a defect in the ciliary zonule, which is predominantly composed of FBN1 microfibrils. Together, this strongly suggests that ADAMTS proteins are involved either in microfibril assembly, stability, and anchorage, or the formation of function-specific supramolecular networks having microfibrils as their foundation. Here, the genetics and molecular biology of this subset of ADAMTS proteins is discussed from the perspective of how they might contribute to fully functional or function-specific microfibrils.

Adamtsl2 deletion results in bronchial fibrillin microfibril accumulation and bronchial epithelial dysplasia--a novel mouse model providing insights into geleophysic dysplasia

Mutations in the secreted glycoprotein ADAMTSL2 cause recessive geleophysic dysplasia (GD) in humans and Musladin–Lueke syndrome (MLS) in dogs. GD is a severe, often lethal, condition presenting with short stature, brachydactyly, stiff skin, joint contractures, tracheal-bronchial stenosis and cardiac valve anomalies, whereas MLS is non-lethal and characterized by short stature and severe skin fibrosis. Although most mutations in fibrillin-1 (FBN1) cause Marfan syndrome (MFS), a microfibril disorder leading to transforming growth factor-β (TGFβ) dysregulation, domain-specific FBN1 mutations result in dominant GD. ADAMTSL2 has been previously shown to bind FBN1 and latent TGFβ-binding protein-1 (LTBP1). Here, we investigated mice with targeted Adamtsl2 inactivation as a new model for GD (Adamtsl2^−/− mice). An intragenic lacZ reporter in these mice showed that ADAMTSL2 was produced exclusively by bronchial smooth muscle cells during embryonic lung development. Adamtsl2^−/− mice, which died at birth, had severe bronchial epithelial dysplasia with abnormal glycogen-rich inclusions in bronchial epithelium resembling the cellular anomalies described previously in GD. An increase in microfibrils in the bronchial wall was associated with increased FBN2 and microfibril-associated glycoprotein-1 (MAGP1) staining, whereas LTBP1 staining was increased in bronchial epithelium. ADAMTSL2 was shown to bind directly to FBN2 with an affinity comparable to FBN1. The observed extracellular matrix (ECM) alterations were associated with increased bronchial epithelial TGFβ signaling at 17.5 days of gestation; however, treatment with TGFβ-neutralizing antibody did not correct the epithelial dysplasia. These investigations reveal a new function of ADAMTSL2 in modulating microfibril formation, and a previously unsuspected association with FBN2. Our studies suggest that the bronchial epithelial dysplasia accompanying microfibril dysregulation in Adamtsl2^−/− mice cannot be reversed by TGFβ neutralization, and thus might be mediated by other mechanisms.

Summary: The extracellular protein ADAMTSL2 is a crucial regulator of microfibril composition in the extracellular matrix of bronchial smooth muscle cells and influences bronchial epithelial function.

Platelet-derived growth factor-A and sonic hedgehog signaling direct lung fibroblast precursors during alveolar septal formation

Alveolar septal formation is required to support the respiration of growing mammals; in humans effacement of the alveolar surface and impaired gas exchange are critical features of emphysema and pulmonary fibrosis. Platelet-derived growth factor-A (PDGF-A) and its receptor PDGF-receptor-α (PDGFRα) are required for secondary septal elongation in mice during postnatal days 4 through 12 and they regulate the proliferation and septal location of interstitial fibroblasts. We examined lung fibroblasts (LF) to learn whether PDGFRα expression distinguished a population of precursor cells, with enhanced proliferative and migratory capabilities. We identified a subpopulation of LF that expresses sonic hedgehog (Shh) and stem cell antigen-1 (Sca1). PDGF-A and Shh both increased cytokinesis and chemotaxis in vitro, but through different mechanisms. In primary LF cultures, Shh signaled exclusively through a noncanonical pathway involving generation of Rac1-GTP, whereas both the canonical and noncanonical pathways were used by the Mlg neonatal mouse LF cell line. LF preferentially oriented their primary cilia toward their anterior pole during migration. Furthermore, a larger proportion of PDGFRα-expressing LF, which are more abundant at the septal tips, bore primary cilia compared with other alveolar cells. In pulmonary emphysema, destroyed alveolar septa do not regenerate, in part because cells fail to assume a configuration that allows efficient gas exchange. Better understanding how LF are positioned during alveolar development could identify signaling pathways, which promote alveolar septal regeneration.

The oxytalan fibre network in the periodontium and its possible mechanical function

The biomechanical character of the periodontal ligament (PDL) is crucial in its response to functional and orthodontic forces. Collagen has been the primary subject of investigations in this field. Several studies, however, indicate that oxytalan fibres, which belong to the elastic fibre family, also contribute to the biomechanical character and behaviour of the PDL. In order to elucidate this, we have evaluated the available literature on the oxytalan fibre network within the PDL and supra-alveolar tissues with respect to development, morphology and distribution, and response to mechanical stimulation. To this end, we have combined the classical histological studies with more recent in vitro studies. Oxytalan fibres develop simultaneously with the root and the vascular system within the PDL. A close association between oxytalan fibres and the vascular system also remains later in life, suggesting a role in vascular support. Mechanical loading of the PDL, through orthodontic force application, appears to induce an increase in the number, size, and length of oxytalan fibres. In line with this, in vitro stretching of PDL fibroblasts (PDLFs) results in an increased production of fibrillin, a major structural component of the microfibrils that make up oxytalan fibres. The available data suggest a mechanical function for oxytalan, but to date experimental data are limited. Further research is required to clarify their exact mechanical function and possible role in orthodontic tooth movement.

Colocalization in vivo and association in vitro of perlecan and elastin

We have colocalized elastin and fibrillin-1 with perlecan in extracellular matrix of tensional and weight-bearing connective tissues. Elastin and fibrillin-1 were identified as prominent components of paraspinal blood vessels, and posterior longitudinal ligament in the human fetal spine and outer annulus fibrosus of the fetal intervertebral disc. We also colocalized perlecan with a synovial elastic basal lamina, where the attached synovial cells were observed to produce perlecan. Elastin, fibrillin-1 and perlecan were co-localized in the intima and media of small blood vessels in the synovium and in human fetal paraspinal blood vessels. Elastic fibers were observed at the insertion point of the anterior cruciate ligament to bone in the ovine stifle joint where they colocalized with perlecan. Elastin has not previously been reported to be spatially associated with perlecan in these tissues. Interactions between the tropoelastin and perlecan heparan sulfate chains were demonstrated using quartz crystal microbalance with dissipation solid phase binding studies. Electrostatic interactions through the heparan sulfate chains of perlecan and core protein mediated the interactions with tropoelastin, and were both important in the coacervation of tropoelastin and deposition of elastin onto perlecan immobilized on the chip surface. This may help us to understand the interactions which are expected to occur in vivo between the tropoelastin and perlecan to facilitate the deposition of elastin and formation of elastic microfibrils in situ and would be consistent with the observed distributions of these components in a number of connective tissues.

Induction of macrophage chemotaxis by aortic extracts from patients with Marfan syndrome is related to elastin binding protein

Marfan syndrome is an autosomal dominantly inherited disorder of connective tissue with prominent skeletal, ocular, and cardiovascular manifestations. Aortic aneurysm and dissection are the major determinants of premature death in untreated patients. In previous work, we showed that extracts of aortic tissues from the mgR mouse model of Marfan syndrome showed increased chemotactic stimulatory activity related to the elastin-binding protein. Aortic samples were collected from 6 patients with Marfan syndrome and 8 with isolated aneurysms of the ascending aorta. Control samples were obtained from 11 organ donors without known vascular or connective tissue diseases. Soluble proteins extracted from the aortic samples of the two patient groups were compared against buffer controls and against the aortic samples from controls with respect to the ability to induce macrophage chemotaxis as measured using a modified Boyden chamber, as well as the reactivity to a monoclonal antibody BA4 against bioactive elastin peptides using ELISA. Samples from Marfan patients displayed a statistically significant increase in chemotactic inductive activity compared to control samples. Additionally, reactivity to BA4 was significantly increased. Similar statistically significant increases were identified for the samples from patients with idiopathic thoracic aortic aneurysm. There was a significant correlation between the chemotactic index and BA4 reactivity, and the increases in chemotactic activity of extracts from Marfan patients could be inhibited by pretreatment with lactose, VGVAPG peptides, or BA4, which indicates the involvement of EBP in mediating the effects. Our results demonstrate that aortic extracts of patients with Marfan syndrome can elicit macrophage chemotaxis, similar to our previous study on aortic extracts of the mgR mouse model of Marfan syndrome (Guo et al., Circulation 2006; 114:1855-62).

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.

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.

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.

Biogenesis and function of fibrillin assemblies

Fibrillin-1 and fibrillin-2 are large cysteine-rich glycoproteins that serve two key physiological functions: as supporting structures that impart tissue integrity and as regulators of signaling events that instruct cell performance. The structural role of fibrillins is exerted through the temporal and hierarchical assembly of microfibrils and elastic fibers, whereas the instructive role reflects the ability of fibrillins to sequester transforming growth factor beta (TGFbeta) and bone morphogenetic protein (BMP) complexes in the extracellular matrix. Characterization of fibrillin mutations in human patients and in genetically engineered mice has demonstrated that perturbation of either function manifests in disease. More generally, these studies have indicated that fibrillins are integral components of a broader biological network of extracellular, cell surface, and signaling molecules that orchestrate morphogenetic and homeostatic programs in multiple organ systems. They have also suggested that the relative composition of fibrillin-rich microfibrils imparts contextual specificity to TGFbeta and BMP signaling by concentrating the ligands locally so as to regulate cell differentiation within a spatial context during organ formation (positive regulation) and by restricting their bioavailability so as to modulate cell performance in a timely fashion during tissue remodeling/repair (negative regulation). Correlative evidence suggests functional coupling of the cell-directed assembly of microfibrils and targeting of TGFbeta and BMP complexes to fibrillins. Hence, the emerging view is that fibrillin-rich microfibrils are molecular integrators of structural and instructive signals, with TGFbeta and BMPs as the nodal points that convert extracellular inputs into discrete and context-dependent cellular responses.

Neuraminidase-1 is required for the normal assembly of elastic fibers

The assembly of elastic fibers in tissues that undergo repeated cycles of extension and recoil, such as the lungs and blood vessels, is dependent on the proper interaction and alignment of tropoelastin with a microfibrillar scaffold. Here, we describe in vivo histopathological effects of neuraminidase-1 (Neu1) deficiency on elastin assembly in the lungs and aorta of mice. These mice exhibited a tight-skin phenotype very similar to the Tsk mouse. Normal septation of Neu1-null mice did not occur in neonatal mice, resulting in enlarged alveoli that were maintained in adults. The abnormal development of elastic fibers was remarkable under electron microscopy and confirmed by the overlapping distribution of elastin, fibrillin-1, fibrillin-2, and fibulin-5 (Fib-5) by the light microscopy immunostainings. Fib-5 fibers appeared diffuse and unorganized around the alveolar walls and the apex of developing secondary septal crests. Fibrillin-2 deposition was also abnormal in neonatal and adult lungs. Dispersion of myofibroblasts appeared abnormal in developing lungs of Neu1-null mice, with a random distribution of myofibroblast around the alveolar walls, rather than concentrating at sites of elastin synthesis. The elastic lamellae in the aorta of the Neu1-null mice were thinner and separated by hypertrophic smooth muscle cells that were surrounded by an excess of the sialic acid-containing moieties. The concentration of elastin, as measure by desmosine levels, was significantly reduced in the aorta of Neu1-null mice. Message levels for tropoelastin and Fib-5 were normal, suggesting the elastic fiber defects in Neu1-null mice result from impaired extracellular assembly.