The Tight skin mouse: demonstration of mutant fibrillin-1 production and assembly into abnormal microfibrils

Aqueous humor TGFβ and fibrillin-1 in Tsk mice reveal clues to POAG pathogenesis

Aqueous humor (AH) and blood levels of transforming growth factor β (TGFβ) are elevated in idiopathic primary open angle glaucoma (POAG) representing a disease biomarker of unclear status and function. Tsk mice display a POAG phenotype and harbor a mutation of fibrillin-1, an important regulator of TGFβ bioavailability. AH TGFβ2 was higher in Tsk than wild-type (WT) mice (by 34%; p = 0.002; ELISA); similarly, AH TGFβ2 was higher in human POAG than controls (2.7-fold; p = 0.00005). As in POAG, TGFβ1 was elevated in Tsk serum (p = 0.01). Fibrillin-1 was detected in AH from POAG subjects and Tsk mice where both had similar levels relative to controls (p = 0.45). 350 kDa immunoblot bands representing WT full-length fibrillin-1 were present in human and mouse AH. A 418 kDa band representing mutant full-length fibrillin-1 was present only in Tsk mice. Lower molecular weight fibrillin-1 antibody-reactive bands were present in similar patterns in humans and mice. Certain bands (130 and 32 kDa) were elevated only in human POAG and Tsk mice (p ≤ 0.04 relative to controls) indicating discrete isoforms relevant to disease. In addition to sharing a phenotype, Tsk mice and human POAG subjects had common TGFβ and fibrillin-1 features in AH and also blood that are pertinent to understanding glaucoma pathogenesis.

Concomitant uterus agenesia and Marfan syndrome with systemic sclerosis: A rare case report

It seems that the association of two connective tissue disorders, including Marfan syndrome and systemic sclerosis, the first is associated with the loosening of the subcutaneous tissue and the second with its tightening, is a very interesting and controversial paradox at the same time and need finding possible genetic etiologies.

Systemic sclerosis skin is a primed microenvironment for soft tissue calcification-a hypothesis

Calcinosis cutis, defined as sub-epidermal deposition of calcium salts, is a major clinical problem in patients with SSc, affecting 20-40% of patients. A number of recognized factors associated with calcinosis have been identified, including disease duration, digital ischaemia and acro-osteolysis. Yet, to date, the pathogenesis of SSc-related calcinosis remains unknown, and currently there is no effective disease-modifying pharmacotherapy. Following onset of SSc, there are marked changes in the extracellular matrix (ECM) of the skin, notably a breakdown in the microfibrillar network and accumulation of type I collagen. Our hypothesis is that these pathological changes reflect a changing cellular phenotype and result in a primed microenvironment for soft tissue calcification, with SSc fibroblasts adopting a pro-osteogenic profile, and specific driving forces promoting tissue mineralization. Considering the role of the ECM in disease progression may help elucidate the mechanism(s) behind SSc-related calcinosis and inform the development of future therapeutic interventions.

Increased Susceptibility to Glaucomatous Damage in Microfibril Deficient Mice

Purpose

To test whether mice with microfibril deficiency due to the Tsk mutation of fibrillin-1 (Fbn1Tsk/+) have increased susceptibility to pressure-induced retinal ganglion cell (RGC) degeneration.

Methods

Intraocular pressure (IOP) elevation was induced in Fbn1Tsk/+ and wild type (wt) mice by injecting microbeads into the anterior chamber. Mice were then followed up for four months, with IOP measurements every three to six days. Retinas were stained for Brn3a to determine RGC number. Optic nerve cross-sections were stained with p-phenylene diamine to determine nerve area, axon number, and caliber and thickness of the pia mater.

Results

Microbead injection induced significant IOP elevation that was significantly less for Fbn1Tsk/+ mice compared with wt. The optic nerves and optic nerve axons were larger, and the elastic fiber-rich pia mater was thinner in Fbn1Tsk/+ mice. Microbead injection resulted in reduced optic nerve size, thicker pia mater, and a slight decrease in axon size. Fbn1Tsk/+ mice had significantly greater loss of RGCs and optic nerve axons compared with wt (14.8% vs. 5.8%, P = 0.002, and 17.0% vs. 7.5%, P = 0.002, respectively).

Conclusions

Fbn1Tsk/+mice had altered optic nerve structure as indicated by larger optic nerves, larger optic nerve axons and thinner pia mater, consistent with our previous findings. Despite lower IOP elevation, Fbn1Tsk/+mice had greater loss of RGCs and optic nerve axons, suggesting increased susceptibility to IOP-induced optic nerve degeneration in microfibril-deficient mice.

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.

Co-existence of Marfan syndrome and systemic sclerosis: A case report and a hypothesis suggesting a common link

FBN1 gene encodes for the connective tissue protein fibrillin-1 which can also regulate the profibrotic cytokine transforming growth factor (TGF)-ß1. Mutations in the FBN1 gene cause Marfan syndrome (MFS), a genetic condition with defective connective tissues. FBN1 haplotypes and single nucleotide polymorphisms have also been reported to be associated with systemic sclerosis (SSc), a connective tissue disease characterized by fibrosis of multiple organs. Furthermore, the duplication of the Fbn1 gene causes a SSc-like disease in the TsK1 mouse model. To the best of our knowledge, there are no reports of MFS and SSc co-existing in a patient. Here, we describe a 46-year-old woman who presented with cardiac failure. She had a family history of MFS. Physical examination revealed marfanoid habitus and scleroderma features. Echocardiography demonstrated dilated cardiomyopathy with aortic root dilatation, aortic regurgitation and mitral regurgitation. Cardiac magnetic resonance imaging was consistent with dilated cardiomyopathy, mid-wall fibrosis at basal septal wall and dilated aortic root. Extractable nuclear antigen panel detected anti-Scl 70. She fulfilled Ghent criteria for MFS and satisfied American College of Rheumatology/ European League Against Rheumatism classification criteria for SSc. Although we do not have the FBN1 sequence in our patient, the co-existence of MFS and SSc in this patient raises the possibility of co-existence of distinct mutations in the FBN1 gene that could affect TGF-β signaling differently, resulting in divergent pathologic consequences - loss of structural integrity in MFS versus increased extracellular matrix deposition in SSc, and different clinical manifestations.

Investigation of fibrillin microfibrils in the canine cruciate ligament in dogs with different predispositions to ligament rupture

Cranial cruciate ligament disease (CCLD) is the most common cause of pelvic limb lameness in dogs but its precise aetiopathogenesis is uncertain. Fibrillin microfibrils (FM) are complex macro-molecular assemblies found in many tissues including ligaments, where they are thought to play an important mechanical role. We hypothesised that FM ultrastructural variation correlates with the differing predisposition of canine breeds to CCLD. Non-diseased cranial and caudal cruciate ligaments (CCLs and CaCLs) were obtained from Greyhound (GH) and Staffordshire Bull Terrier (SBT) cadavers. Fibrillin microfibrils were extracted from the ligaments by bacterial collagenase digestion, purified by size-exclusion chromatography and subsequently visualized by atomic force microscopy (AFM). With AFM, FMs have a characteristic beads-on-a-string appearance. For each FM, periodicity (bead-bead distance) and length (number of beads/FM) was measured. Fibrillin microfibril length was found to be similar for GH and SBT, with non-significant inter-breed and inter-ligament differences. Fibrillin microfibril periodicity varied when comparing GH and SBT for CCL (GH 60.2 ± 1.4 nm; SBT 56.2 ± 0.8 nm) and CaCL (GH 55.5 ± 1.6 nm; SBT 61.2 ± 1.2 nm). A significant difference was found in the periodicity distribution when comparing CCL for both breeds (P < 0.00001), further, intra-breed differences in CCL vs CaCL were statistically significant within both breeds (P < 0.00001). The breed at low risk of CCLD exhibited a periodicity profile which may be suggestive of a repair and remodelling within the CCL.

Gelatinolytic activity in gingival crevicular fluid and saliva of growing patients with Marfan syndrome: a case-control study

Background

Aim of the study was to evaluate the gelatinolytic activity in the saliva and gingival crevicular fluid from a sample group of subjects with Marfan syndrome.

Methods

Two groups were analyzed in this case-control study. A group of 28 subjects with Marfan syndrome (MG) was recruited from the Centre for Rare Disease, Marfan Clinic of Tor Vergata University Hospital. The second sample, 23 subjects, with the same characteristics and without any syndrome, was the control group (CG). Saliva and gingival crevicular fluid were collected and transferred to a sterile test tube and stored frozen at − 20 °C until analysis at the Medical Chemistry Laboratory. Gelatin substrate zymography was used for the evaluation and characterization of saliva and crevicular fluid proteinases. Correlation test and Student’s t-test have been used to analyze data.

Results

In all samples different gelatin-degrading activities were observed. Two bands, which are related to the molecular weights of pro-MMP-9 and active MMP-9, respectively, were detectable in 100% of Marfan and control samples. MMP-2 activity was higher in Marfan group. Additional bands (55/48 kDa), corresponding to the activated forms of collagenase (MMP-13), were observed in saliva samples of both groups.

Conclusions

The association of an enhanced activity by MMP-13 with an increased amount of active MMP-9 might be an important biomarker for the diagnosis of Marfan syndrome.

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.

Enlarged Optic Nerve Axons and Reduced Visual Function in Mice with Defective Microfibrils

Glaucoma is a leading cause of irreversible vision loss due to retinal ganglion cell (RGC) degeneration that develops slowly with age. Elevated intraocular pressure (IOP) is a significant risk factor, although many patients develop glaucoma with IOP in the normal range. Mutations in microfibril-associated genes cause glaucoma in animal models, suggesting the hypothesis that microfibril defects contribute to glaucoma. To test this hypothesis, we investigated IOP and functional/structural correlates of RGC degeneration in mice of either sex with abnormal microfibrils due to heterozygous Tsk mutation of the fibrilin-1 gene (Fbn1^Tsk/+). Although IOP was not affected, Fbn1^Tsk/+ mice developed functional deficits at advanced age consistent with glaucoma, including reduced RGC responses in electroretinogram (ERG) experiments. While RGC density in the retina was not affected, the density of RGC axons in the optic nerve was significantly reduced in Fbn1^Tsk/+ mice. However, reduced axon density correlated with expanded optic nerves, resulting in similar numbers of axons in Fbn1^Tsk/+ and control nerves. Axons in the optic nerves of Fbn1^Tsk/+ mice were significantly enlarged and axon diameter was strongly correlated with optic nerve area, as has been reported in early pathogenesis of the DBA/2J mouse model of glaucoma. Our results suggest that microfibril abnormalities can lead to phenotypes found in early-stage glaucomatous neurodegeneration. Thinning of the elastic fiber-rich pia mater was found in Fbn1^Tsk/+ mice, suggesting mechanisms allowing for optic nerve expansion and a possible biomechanical contribution to determination of axon caliber.

Structural and compositional diversity of fibrillin microfibrils in human tissues

Elastic fibers comprising fibrillin microfibrils and elastin are present in many tissues, including the skin, lungs, and arteries, where they confer elasticity and resilience. Although fibrillin microfibrils play distinct and tissue-specific functional roles, it is unclear whether their ultrastructure and composition differ between elastin-rich (skin) and elastin-poor (ciliary body and zonule) organs or after in vitro synthesis by cultured cells. Here, we used atomic force microscopy, which revealed that the bead morphology of fibrillin microfibrils isolated from the human eye differs from those isolated from the skin. Using newly developed pre-MS preparation methods and LC-MS/MS, we detected tissue-specific regions of the fibrillin-1 primary structure that were differentially susceptible to proteolytic extraction. Comparing tissue- and culture-derived microfibrils, we found that dermis- and dermal fibroblast–derived fibrillin microfibrils differ in both bead morphology and periodicity and also exhibit regional differences in fibrillin-1 proteolytic susceptibility. In contrast, collagen VI microfibrils from the same dermal or fibroblast samples were invariant in ultrastructure (periodicity) and protease susceptibility. Finally, we observed that skin- and eye-derived microfibril suspensions were enriched in elastic fiber– and basement membrane–associated proteins, respectively. LC-MS/MS also identified proteins (such as calreticulin and protein-disulfide isomerase) that are potentially fundamental to fibrillin microfibril biology, regardless of their tissue source. Fibrillin microfibrils synthesized in cell culture lacked some of these key proteins (MFAP2 and -4 and fibrillin-2). These results showcase the structural diversity of these key extracellular matrix assemblies, which may relate to their distinct roles in the tissues where they reside.

Structural and functional failure of fibrillin‑1 in human diseases (Review)

Fibrillins (FBNs) are key relay molecules that form the backbone of microfibrils in elastic and non‑elastic tissues. Interacting with other components of the extracellular matrix (ECM), these ubiquitous glycoproteins exert pivotal roles in tissue development, homeostasis and repair. In addition to mechanical support, FBN networks also exhibit regulatory activities on growth factor signalling, ECM formation, cell behaviour and the immune response. Consequently, mutations affecting the structure, assembly and stability of FBN microfibrils have been associated with impaired biomechanical tissue properties, altered cell‑matrix interactions, uncontrolled growth factor or cytokine activation, and the development of fibrillinopathies and associated severe complications in multiple organs. Beyond a panoramic overview of structural cues of the FBN network, the present review will also describe the pathological implications of FBN disorders in the development of inflammatory and fibrotic conditions.

Proteinase activated receptor-2 counterbalances the vascular effects of endothelin-1 in fibrotic tight-skin mice

BACKGROUND AND PURPOSE

The majority of the severe vascular complications in fibrosis are a consequence of a deregulated activity of mediators controlling vasomotor tone. One of the most important of these mediators is endothelin-1 (ET-1). Here, we have investigated the role of proteinase-activated receptor 2 (PAR2) in the vascular dysfunction in a model of fibrosis, using tight-skin (Tsk) mice.

EXPERIMENTAL APPROACH

Aortas were collected from Tsk, transgenic over-expressing PAR2 (TgPAR2), PAR2 deficient (PAR2^-/- ) or the corresponding WT mice. Histological and immunohistochemistry analysis for α-smooth muscle actin, PAR2 and ET-1 receptors were performed on aorta sections. Vascular responses to phenylephrine, ET-1 and PAR2 activating peptide (PAR2-AP) were assessed on aortic rings.

KEY RESULTS

In aortas from Tsk mice, responses to phenylephrine were reduced, contractions to ET-1 were increased and vasorelaxation to PAR2-AP was enhanced. These alterations matched changes observed in whole vessel architecture such as vascular fibre re-organization, increased collagen deposition and enhanced α-smooth muscle actin expression. Expression of both ET_A receptors and PAR2 was enhanced in Tsk mice. Antagonism of PAR2 potentiated vascular effects of ET-1, whereas antagonism of ET_A receptors increased vasorelaxation induced by PAR2-AP. In TgPAR2 mice, responses to ET-1 and ET-1 plasma levels were reduced. Conversely, PAR2^-/- mice showed enhanced ET-1 induced contraction in aortic rings and higher circulating ET-1 levels.

CONCLUSIONS AND IMPLICATIONS

Our data show that PAR2 counterbalanced enhanced contractions to ET-1 in aortas from Tsk mice. PAR2 could represent a possible target for novel drugs in the treatment of vascular complications in fibrosis.

LINKED ARTICLES

This article is part of a themed section on Targeting Inflammation to Reduce Cardiovascular Disease Risk. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.22/issuetoc and http://onlinelibrary.wiley.com/doi/10.1111/bcp.v82.4/issuetoc.

Animal models of chronic obstructive pulmonary disease

Animal models of disease have always been welcomed by the scientific community because they provide an approach to the investigation of certain aspects of the disease in question. Animal models of COPD cannot reproduce the heterogeneity of the disease and usually only manage to represent the disease in its milder stages. Moreover, airflow obstruction, the variable that determines patient diagnosis, not always taken into account in the models. For this reason, models have focused on the development of emphysema, easily detectable by lung morphometry, and have disregarded other components of the disease, such as airway injury or associated vascular changes. Continuous, long-term exposure to cigarette smoke is considered the main risk factor for this disease, justifying the fact that the cigarette smoke exposure model is the most widely used. Some variations on this basic model, related to exposure time, the association of other inducers or inhibitors, exacerbations or the use of transgenic animals to facilitate the identification of pathogenic pathways have been developed. Some variations or heterogeneity of this disease, then, can be reproduced and models can be designed for resolving researchers' questions on disease identification or treatment responses.

Systemic sclerosis sera affect fibrillin-1 deposition by dermal blood microvascular endothelial cells: therapeutic implications of cyclophosphamide

Introduction

Systemic sclerosis (SSc) is a connective tissue disorder characterized by endothelial cell injury, autoimmunity and fibrosis. The following three fibrillin-1 alterations have been reported in SSc. (1) Fibrillin-1 microfibrils are disorganized in SSc dermis. (2) Fibrillin-1 microfibrils produced by SSc fibroblasts are unstable. (3) Mutations in the FBN1 gene and anti-fibrillin-1 autoantibodies have been reported in SSc. Fibrillin-1 microfibrils, which are abundantly produced by blood and lymphatic microvascular endothelial cells (B-MVECs and Ly-MVECs, respectively), sequester in the extracellular matrix the latent form of the potent profibrotic cytokine transforming growth factor β (TGF-β). In the present study, we evaluated the effects of SSc sera on the deposition of fibrillin-1 and microfibril-associated glycoprotein 1 (MAGP-1) and the expression of focal adhesion molecules by dermal B-MVECs and Ly-MVECs.

Methods

Dermal B-MVECs and Ly-MVECs were challenged with sera from SSc patients who were treatment-naïve or under cyclophosphamide (CYC) treatment and with sera from healthy controls. Fibrillin-1/MAGP-1 synthesis and deposition and the expression of α_vβ₃ integrin/phosphorylated focal adhesion kinase and vinculin/actin were evaluated by immunofluorescence and quantified by morphometric analysis.

Results

Fibrillin-1 and MAGP-1 colocalized in all experimental conditions, forming a honeycomb pattern in B-MVECs and a dense mesh of short segments in Ly-MVECs. In B-MVECs, fibrillin-1/MAGP-1 production and α_vβ₃ integrin expression significantly decreased upon challenge with sera from naïve SSc patients compared with healthy controls. Upon challenge of B-MVECs with sera from CYC-treated SSc patients, fibrillin-1/MAGP-1 and α_vβ₃ integrin levels were comparable to those of cells treated with healthy sera. Ly-MVECs challenged with SSc sera did not differ from those treated with healthy control sera in the expression of any of the molecules assayed.

Conclusions

Because of the critical role of fibrillin-1 in sequestering the latent form of TGF-β in the extracellular matrix, its decreased deposition by B-MVECs challenged with SSc sera might contribute to dermal fibrosis. In SSc, CYC treatment might limit fibrosis through the maintenance of physiologic fibrillin-1 synthesis and deposition by B-MVECs.

Genetic and functional linkage between ADAMTS superfamily proteins and fibrillin-1: a novel mechanism influencing microfibril assembly and function

Tissue microfibrils contain fibrillin-1 as a major constituent. Microfibrils regulate bioavailability of TGFβ superfamily growth factors and are structurally crucial in the ocular zonule. FBN1 mutations typically cause the Marfan syndrome, an autosomal dominant disorder manifesting with skeletal overgrowth, aortic aneurysm, and lens dislocation (ectopia lentis). Infrequently, FBN1 mutations cause dominantly inherited Weill-Marchesani syndrome (WMS), isolated ectopia lentis (IEL), or the fibrotic condition, geleophysic dysplasia (GD). Intriguingly, mutations in ADAMTS [a disintegrin-like and metalloprotease (reprolysin-type) with thrombospondin type 1 motif] family members phenocopy these disorders, leading to recessive WMS (ADAMTS10), WMS-like syndrome (ADAMTS17), IEL (ADAMTSL4 and ADAMTS17) and GD (ADAMTSL2). An ADAMTSL2 founder mutation causes Musladin-Lueke syndrome, a fibrotic disorder in beagle dogs. The overlapping disease spectra resulting from fibrillin-1 and ADAMTS mutations, interaction of ADAMTS10 and ADAMTSL2 with fibrillin-1, and evidence that these ADAMTS proteins accelerate microfibril biogenesis, constitutes a consilience suggesting that some ADAMTS proteins evolved to provide a novel mechanism regulating microfibril formation and consequently cell behavior.

Molecular aspects of skin ageing

Ageing of human skin may result from both the passage of time (intrinsic ageing) and from cumulative exposure to external influences (extrinsic ageing) such as ultraviolet radiation (UVR) which promote wrinkle formation and loss of tissue elasticity. Whilst both ageing processes are associated with phenotypic changes in cutaneous cells, the major functional manifestations of ageing occur as a consequence of structural and compositional remodeling of normally long-lived dermal extracellular matrix proteins. This review briefly considers the effects of ageing on dermal collagens and proteoglycans before focusing on the mechanisms, functional consequences and treatment of elastic fibre remodeling in ageing skin. The early stages of photoageing are characterised by the differential degradation of elastic fibre proteins and whilst the activity of extracellular matrix proteases is increased in photoexposed skin, the substrate specificity of these enzymes is low. We have recently shown however, that isolated fibrillin microfibrils are susceptible to direct degradation by physiologically attainable doses of UV-B radiation and that elastic fibre proteins as a group are highly enriched in UV-absorbing amino acid residues. Functionally, elastic fibre remodeling events may adversely impact on: the mechanical properties of tissues, the recruitment and activation of immune cells, the expression of matrix metalloproteinases and cytokine signaling (by perturbing fibrillin microfibril sequestration of TGFβ). Finally, newly developed topical interventions appear to be capable of regenerating elements of the elastic fibre system in ageing skin, whilst systemic treatments may potentially prevent the pathological tissue remodeling events which occur in response to elastic fibre degradation.

Structure-function relations in an elastase-induced mouse model of emphysema

Emphysema is a progressive disease characterized by the destruction of peripheral airspaces and subsequent decline in lung function. However, the relation between structure and function during disease progression is not well understood. The objective of this study was to assess the time course of the structural, mechanical, and remodeling properties of the lung in mice after elastolytic injury. At 2, 7, and 21 days after treatment with porcine pancreatic elastase, respiratory impedance, the constituents of lung extracellular matrix, and histological sections of the lung were evaluated. In the control group, no changes were observed in the structural or functional properties, whereas, in the treatment group, the respiratory compliance and its variability significantly increased by Day 21 (P < 0.001), and the difference in parameters decreased with increasing positive end-expiratory pressure. The heterogeneity of airspace structure gradually increased over time. Conversely, the relative amounts of elastin and type I collagen exhibited a peak (P < 0.01) at Day 2, but returned to baseline levels by Day 21. Structure-function relations manifested themselves in strong correlations between compliance parameters and both mean size and heterogeneity of airspace structure (r(2) > 0.9). Similar relations were also obtained in a network model of the parenchyma in which destruction was based on the notion that mechanical forces contribute to alveolar wall rupture. We conclude that, in a mouse model of emphysema, progressive decline in lung function is sensitive to the development of airspace heterogeneity governed by local, mechanical, force-induced failure of remodeled collagen.

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.

Differential effects of alendronate and losartan therapy on osteopenia and aortic aneurysm in mice with severe Marfan syndrome

Reduced bone mineral density (osteopenia) is a poorly characterized manifestation of pediatric and adult patients afflicted with Marfan syndrome (MFS), a multisystem disorder caused by structural or quantitative defects in fibrillin-1 that perturb tissue integrity and TGFβ bioavailability. Here we report that mice with progressively severe MFS (Fbn1(mgR/mgR) mice) develop osteopenia associated with normal osteoblast differentiation and bone formation. In vivo and ex vivo experiments, respectively, revealed that adult Fbn1(mgR/mgR) mice respond more strongly to locally induced osteolysis and that Fbn1(mgR/mgR) osteoblasts stimulate pre-osteoclast differentiation more than wild-type cells. Greater osteoclastogenic potential of mutant osteoblasts was largely attributed to Rankl up-regulation secondary to improper TGFβ activation and signaling. Losartan treatment, which lowers TGFβ signaling and restores aortic wall integrity in mice with mild MFS, did not mitigate bone loss in Fbn1(mgR/mgR) mice even though it ameliorated vascular disease. Conversely, alendronate treatment, which restricts osteoclast activity, improved bone quality but not aneurysm progression in Fbn1(mgR/mgR) mice. Taken together, our findings shed new light on the pathogenesis of osteopenia in MFS, in addition to arguing for a multifaceted treatment strategy in this congenital disorder of the connective tissue.

An ADAMTSL2 founder mutation causes Musladin-Lueke Syndrome, a heritable disorder of beagle dogs, featuring stiff skin and joint contractures

Background

Musladin-Lueke Syndrome (MLS) is a hereditary disorder affecting Beagle dogs that manifests with extensive fibrosis of the skin and joints. In this respect, it resembles human stiff skin syndrome and the Tight skin mouse, each of which is caused by gene defects affecting fibrillin-1, a major component of tissue microfibrils. The objective of this work was to determine the genetic basis of MLS and the molecular consequence of the identified mutation.

Methodology and Principal Findings

We mapped the locus for MLS by genome-wide association to a 3.05 Mb haplotype on canine chromosome 9 (CFA9 (50.11–54.26; p_raw <10⁻⁷)), which was homozygous and identical-by-descent among all affected dogs, consistent with recessive inheritance of a founder mutation. Sequence analysis of a candidate gene at this locus, ADAMTSL2, which is responsible for the human TGFβ dysregulation syndrome, Geleophysic Dysplasia (GD), uncovered a mutation in exon 7 (c.660C>T; p.R221C) perfectly associated with MLS (p-value = 10⁻¹²). Murine ADAMTSL2 containing the p.R221C mutation formed anomalous disulfide-bonded dimers when transiently expressed in COS-1, HEK293F and CHO cells, and was present in the medium of these cells at lower levels than wild-type ADAMTSL2 expressed in parallel.

Conclusions/Significance

The genetic basis of MLS is a founder mutation in ADAMTSL2, previously shown to interact with latent TGF-β binding protein, which binds fibrillin-1. The molecular effect of the founder mutation on ADAMTSL2 is formation of disulfide-bonded dimers. Although caused by a distinct mutation, and having a milder phenotype than human GD, MLS nevertheless offers a new animal model for study of GD, and for prospective insights on mechanisms and pathways of skin fibrosis and joint contractures.

Mutations in fibrillin-1 cause congenital scleroderma: stiff skin syndrome

The predisposition for scleroderma, defined as fibrosis and hardening of the skin, is poorly understood. We report that stiff skin syndrome (SSS), an autosomal dominant congenital form of scleroderma, is caused by mutations in the sole Arg-Gly-Asp sequence-encoding domain of fibrillin-1 that mediates integrin binding. Ordered polymers of fibrillin-1 (termed microfibrils) initiate elastic fiber assembly and bind to and regulate the activation of the profibrotic cytokine transforming growth factor-beta (TGFbeta). Altered cell-matrix interactions in SSS accompany excessive microfibrillar deposition, impaired elastogenesis, and increased TGFbeta concentration and signaling in the dermis. The observation of similar findings in systemic sclerosis, a more common acquired form of scleroderma, suggests broad pathogenic relevance.

Radiation-induced skin injury in the animal model of scleroderma: implications for post-radiotherapy fibrosis

Background

Radiation therapy is generally contraindicated for cancer patients with collagen vascular diseases (CVD) such as scleroderma due to an increased risk of fibrosis. The tight skin (TSK) mouse has skin which, in some respects, mimics that of patients with scleroderma. The skin radiation response of TSK mice has not been previously reported. If TSK mice are shown to have radiation sensitive skin, they may prove to be a useful model to examine the mechanisms underlying skin radiation injury, protection, mitigation and treatment.

Methods

The hind limbs of TSK and parental control C57BL/6 mice received a radiation exposure sufficient to cause approximately the same level of acute injury. Endpoints included skin damage scored using a non-linear, semi-quantitative scale and tissue fibrosis assessed by measuring passive leg extension. In addition, TGF-β1 cytokine levels were measured monthly in skin tissue.

Results

Contrary to our expectations, TSK mice were more resistant (i.e. 20%) to radiation than parental control mice. Although acute skin reactions were similar in both mouse strains, radiation injury in TSK mice continued to decrease with time such that several months after radiation there was significantly less skin damage and leg contraction compared to C57BL/6 mice (p < 0.05). Consistent with the expected association of transforming growth factor beta-1 (TGF-β1) with late tissue injury, levels of the cytokine were significantly higher in the skin of the C57BL/6 mouse compared to TSK mouse at all time points (p < 0.05).

Conclusion

TSK mice are not recommended as a model of scleroderma involving radiation injury. The genetic and molecular basis for reduced radiation injury observed in TSK mice warrants further investigation particularly to identify mechanisms capable of reducing tissue fibrosis after radiation injury.

Clinical significance of serum HMGB-1 and sRAGE levels in systemic sclerosis: association with disease severity

INTRODUCTION

The high mobility group box 1 protein (HMGB-1)/advanced glycation end products (RAGE) system is recently shown to play an important part in immune/inflammatory disorders. However, the association of this system in systemic sclerosis (SSc) remains unknown.

MATERIALS AND METHODS

To determine clinical association of serum levels of HMGB-1 and soluble RAGE (sRAGE) in patients with SSc, sera from 70 patients with SSc and 25 healthy controls were examined by enzyme-linked immunosorbent assay. Sera from tight-skin mice and bleomycin-induced scleroderma mice, animal models for SSc, were also examined. Skin HMGB-1 and RAGE expression was assessed by immunohistochemistry.

RESULTS AND DISCUSSION

Serum HMGB-1 and sRAGE levels in SSc were higher than those in controls. Similarly, HMGB-1 and sRAGE levels in animal SSc models were higher than those in control mice. SSc patients with elevated HMGB-1 and sRAGE levels had more frequent involvement of several organs and immunological abnormalities compared to those with normal levels. Furthermore, HMGB-1 and sRAGE levels correlated positively with modified Rodnan total skin thickness score and negatively with pulmonary function test.

CONCLUSIONS

HMGB-1 and sRAGE expression in the sclerotic skin was more intense than normal skin. These results suggest that elevated serum HMGB-1 and sRAGE levels are associated with the disease severity and immunological abnormalities in SSc.

Aberrant fibrillin-1 expression in early emphysematous human lung: a proposed predisposition for emphysema

Parenchymal destruction, airspace enlargement, and loss of elasticity are hallmarks of pulmonary emphysema. Although the basic mechanism is unknown, there is a consensus that malfunctioning of the extracellular matrix is a major contributor to the pathogenesis of emphysema. In this study, we analyzed the expression of the elastic fiber protein fibrillin-1 in a large number (n=69) of human lung specimens with early-onset emphysema. Specimens were morphologically characterized by the Destructive Index, the Mean Linear Intercept, and the Panel Grading. We observed a strong correlation (P<0.001) of aberrant fibrillin-1 staining with the degree of destruction of lung parenchyma (r=0.71), airspace enlargement (r=0.47), and emphysema-related morphological abnormalities (r=0.69). There were no obvious correlations with age and smoking behavior. Staining for three other extracellular matrix components (type I collagen, type IV collagen, and laminin) was not affected. The aberrant fibrillin-1 staining observed in this study is similar to that observed in Marfan syndrome, a syndrome caused by mutations in the gene encoding fibrillin-1. Strikingly, emphysema is noticed in a number of Marfan patients. This, together with the notion that disruption of the fibrillin-1 gene in mice results in emphysematous lesions, makes fibrillin-1 a strong candidate to be involved in the etiology and pathogenesis of emphysema.

Intercellular Adhesion Molecule-1 Deficiency Attenuates the Development of Skin Fibrosis in Tight-Skin Mice

The tight-skin (TSK/+) mouse, a genetic model for systemic sclerosis, develops cutaneous fibrosis. Although a fibrillin 1 gene mutation and immunological abnormalities have been demonstrated, the roles of adhesion molecules have not been investigated. To directly assess roles of adhesion molecules in skin fibrosis, TSK/+ mice lacking L-selectin and/or ICAM-1 were generated. The deficiency of ICAM-1, but not L-selectin, significantly suppressed ( approximately 48%) the development of skin sclerosis in TSK/+ mice. Similarly, ICAM-1 antisense oligonucleotides inhibited skin fibrosis in TSK/+ mice. Although T cell infiltration was modest into the skin of TSK/+ mice, ICAM-1 deficiency down-regulated this migration, which is consistent with the established roles of endothelial ICAM-1 in leukocyte infiltration. In addition, altered phenotype or function of skin fibroblasts was remarkable and dependent on ICAM-1 expression in TSK/+ mice. ICAM-1 expression was augmented on TSK/+ dermal fibroblasts stimulated with IL-4. Although growth or collagen synthesis of TSK/+ fibroblasts cultured with IL-4 was up-regulated, it was suppressed by the loss or blocking of ICAM-1. Collagen expression was dependent on the strain of fibroblasts, but not on the strain of cocultured T cells. Thus, our findings indicate that ICAM-1 expression contributes to the development of skin fibrosis in TSK/+ mice, especially via ICAM-1 expressed on skin fibroblasts.

αVβ6 Is a Novel Receptor for Human Fibrillin-1

Human fibrillin-1, the major structural protein of connective tissue 10-12 nm microfibrils, contains multiple calcium binding epidermal growth factor-like domains interspersed with transforming growth factor beta-binding protein-like (TB) domains. TB4 contains a flexible RGD loop that mediates cell adhesion via alphaVbeta3 and alpha5beta1 integrins. This study identifies integrin alphaVbeta6 as a novel cellular receptor for fibrillin-1 with a K(d) of approximately 0.45 mum. Analyses of this interaction by surface plasmon resonance and immunocytochemistry reveal different module requirements for alphaVbeta6 activation compared with those of alphaVbeta3, suggesting that a covalent linkage of an N-terminal calcium binding epidermal growth factor-like domain to TB4 can modulate alphaV integrin binding specificity. Furthermore, our data suggest alpha5beta1 is a low affinity fibrillin-1 receptor (K(d) > 1 mum), thus providing a molecular explanation for the different alpha5beta1 distribution patterns seen when human keratinocytes and fibroblasts are plated on recombinant fibrillin fragments versus those derived from the physiological ligand fibronectin. Non-focal contact distribution of alpha5beta1 suggests that its engagement by fibrillin-1 may elicit a lesser degree and/or different type of intracellular signaling compared with that seen with a high affinity ligand.

Cutaneous gene expression by DNA microarray in murine sclerodermatous graft-versus-host disease, a model for human scleroderma

The molecular mechanisms governing skin fibrosis in murine sclerodermatous graft-versus-host disease (Scl GVHD) are not known. We used Affymetrix DNA microarrays representing >14,000 mouse genes to characterize global gene expression in skin during development of Scl GVHD in lethally irradiated BALB/c (H-2d) mice transplanted with B10.D2 (H-2d) bone marrow and spleen cells. These mice develop skin thickening, whereas control mice transplanted with syngeneic BALB/c (H-2d) bone marrow and spleen cells do not develop disease. We found consistent differences between mice with Scl GVHD and controls in cytokine messenger RNAs (mRNAs) for both Th1-like (IFN-gamma) and Th2-like (IL-6, Il-10, and IL-13) inflammatory patterns. mRNAs for chemokines CCL2, CCL5, CCL17, IFN-gamma inducible chemokines (CXCL9/Mig, CXCL10/IP-10, and CXCL11/I-TAC), and for growth factors such as platelet-derived growth factor-c, connective tissue growth factor, fibroblast growth factor 1, epidermal growth factor, nerve growth factor-beta, vascular endothelial growth factor (VEGF)-alpha, and VEGF-beta were elevated, similar to human scleroderma. mRNAs for cell adhesion molecules, such as L-selectin (selectin lymphocyte), P-selectin (selectin platelet), E-selectin (selectin endothelium), and vascular cell adhesion molecule 1, were also upregulated. In separate experiments, we confirmed the increased synthesis of IFN-gamma and IL-2, unchanged IL-10, and absence of tumor necrosis factor-alpha, and IL-4 proteins by flow cytometry of isolated skin T cells. These constellations of immunologic changes provide a "fingerprint" for fibrosing autoimmune disease. They are useful to understand the pathogenesis of Scl GVHD, to identify markers for early diagnosis of disease, and to devise more effective strategies for intervention in early scleroderma and Scl GVHD.

Extracellular matrix containing mutated fibrillin-1 (Fbn1) down regulates Col1a1, Col1a2, Col3a1, Col5a1, and Col5a2 mRNA levels in Tsk/+ and Tsk/Tsk embryonic fibroblasts

It is known that the extracellular matrix (ECM) is able to signal to cells and thereby direct or modulate the transcription of certain mRNAs. This signaling plays an important role in tumor invasion and metastasis, wound healing, remodeling of the ECM and cell differentiation. There are several mechanisms whereby the ECM signals cells to change their metabolism: (1) receptor molecules binding to specific domains in the ECM, (2) direct phagocytosis of the ECM molecules or domains into the cell, (3) structural changes of the ECM domains. We report the effect of an ECM containing either mutant or normal Fbn1 on the transcription levels of several collagen mRNAs. Tsk/Tsk, Tsk/+ and +/+ mouse embryonic fibroblast cell lines were used. Tsk/Tsk cells produce only mutated fibrillin-1 which arises from mRNA containing an in-frame duplication of exons 17-40. To test the effect of the ECM containing mutant Fbn1, cells of the Tsk/Tsk, Tsk/+ and the wild-type (+/+) genotype were each grown on an ECM produced by either Tsk/Tsk, Tsk/+ cells or by wild-type cells (+/+). The embryonic cells were genotyped by Northern analyses for Fbn1 and grown to confluence. The cultures were then harvested and the cells removed, leaving the matrix in the flasks. Matrices produced from Tsk/Tsk, Tsk/+ and from +/+ cells were reseeded with Tsk/Tsk cells, Tsk/+ cells or +/+ cells. The cells were plated at a confluent concentration and incubated on the matrices for 48 h, after which total RNA was harvested and cDNA generated. Real-time PCR using cDNA or Northern analyses using RNA were performed for Fbn1 and Types I, III and V collagens. The PCR and Northern results were normalized using beta-actin and GAPDH, respectively. The Northern analyses showed that the steady state levels of mRNA for Col1a1 were depressed in both Tsk/Tsk and +/+ cells when grown on the matrix produced by Tsk/Tsk cells. Real-time PCR was then performed with primers specific for Col1a2, Col3a1, Col5a1 and Col5a2. The results showed that cells with the Tsk/Tsk, Tsk/+, and +/+ genotype all had lower steady-state levels of the above 4 collagen mRNAs when grown on the matrix produced by homozygous Tsk/Tsk cells or the matrix produced by heterozygous Tsk/+ cells compared with those grown on a matrix produced by +/+ cells. We hypothesize that the mutated Fbn1 molecules with many additional EGF-calcium binding regions and TGF-beta binding domains may (1) change the homeostasis of the ECM by binding additional growth factors and/or (2) present a radically different ECM 3-dimensional architecture. Either or both of these changes could signal the cell to produce less collagen.

Hammerhead ribozyme-mediated silencing of the mutant fibrillin-1 of tight skin mouse: insight into the functional role of mutant fibrillin-1

The tight skin (Tsk/+) mouse is a model for fibrotic disorders. The genetic defect in the Tsk/+ is an in-frame duplication between exons 17 and 40 of the fibrillin-1 gene which gives rise to a large transcript and protein. Mice homozygous for the mutation die in utero, whereas heterozygotes survive and spontaneously develop connective tissue disease. In this study, we generated hammerhead ribozymes directed against the mutant fibrillin-1 transcript. A partially mispairing ribozyme was the most effective vehicle to cleave the mutant transcript without undesired cleavage of wild type transcripts, as shown by cell-free RNA cleavage and cleavage in cell lines harboring the ribozyme, by RT-PCR, Northern and Western Blotting. Global gene expression profiling using oligonucleotide microarrays showed the expected reduction in fibrillin-1 mRNA, and down-regulation of several gene cohorts in ribozyme harboring TskR1 cells compared to Tsk/+ cells. Two of the functional clusters included genes regulating extracellular matrix such as connective tissue growth factor, serpine-1 (plasminogen activator inhibitor-1) and TIMP-1 and TIMP-3, and those involved in cytoskeletal organization and myofibroblast formation including calponins and transgelin. Ribozyme-mediated inhibition was confirmed by Western Blot and functional analysis using cell-reporter systems and remodeling of three dimensional collagen gels. Our results underline the therapeutic potential of hammerhead ribozymes in dominant negative defects and suggest that changes in microfibril architecture brought about by fibrillin-1 mutation lead to a complex disease phenotype.

Early emphysema in the tight skin and pallid mice: roles of microfibril-associated glycoproteins, collagen, and mechanical forces

The nature of the development of emphysema in the tight skin (Tsk) and the pallid (Pa) mice are not well understood. We assessed the mechanical and nonlinear properties of the respiratory system, the alveolar structure, and the levels of microfibril-associated glycoproteins (MAGP) 1 and 2 in Tsk mice with developmental emphysema; in Pa mice, which are thought to develop adult onset emphysema; and their background, the C57BL/6 mice, at an age of 7 wk. Minor differences between collagen-related elastic properties of the lungs of the Pa and C57BL/6 mice were seen at this early age. The lungs of the Tsk mice were significantly softer yet more nonlinear than those of the Pa and C57BL/6 mice. The MAGP-1 levels were similar in all three groups. However, the level of MAGP-2, which is associated with both fibrillin-1 and collagen, was higher in the Tsk than in the Pa mice, which also had more MAGP-2 than the C57BL/6. Both the mean and the variance of alveolar diameters were larger in the Tsk than in the other two groups, while the variance in the Pa was larger than in the C57BL/6 mice, implying early development of heterogeneity. Using a network model of the parenchyma, we linked the pathophysiologic changes in the Tsk mice to mechanical forces and failure of the alveolar walls. Our findings suggest the possibility that MAGP-2-related abnormal collagen assembly, combined with mechanical forces, is involved in the progression of emphysema in the Tsk mice.

Fibrillins: from biogenesis of microfibrils to signaling functions

Fibrillins are large proteins that form extracellular microfibril suprastructures ubiquitously found in elastic and nonelastic tissues. Mutations in fibrillin-1 and -2 lead to a number of heritable connective tissue disorders generally termed fibrillinopathies. Clinical symptoms in fibrillinopathies manifest in the skeletal, ocular, and cardiovascular systems and highlight the importance of fibrillins in development and homeostasis of tissues and organs, including blood vessels, bone, and eye. Microfibrils appear to have dual roles in (1) conferring mechanical stability and limited elasticity to tissues, and (2) modulating the activity of growth factors of the transforming growth factor beta (TGF-beta) superfamily. This chapter's focus is on the biogenesis of microfibrils, developmental expression patterns of fibrillins, signaling functions of microfibrils, and mouse models deficient in fibrillins.

Animal models in scleroderma

Scleroderma or systemic sclerosis is an insidious connective tissue disease with no known cure. A hallmark feature of scleroderma is the excess synthesis and deposition of collagen resulting in a fibrotic state. In scleroderma, fibrosis is not confined only to the skin but impacts internal organs as well. In an effort to better understand the pathophysiology of this disease, researchers have developed a variety of animal models that display features of the human condition. This paper focuses on mouse models of scleroderma and summarizes work conducted with these experimental paradigms that is focused on understanding the cellular and molecular events associated with the onset and maintenance of fibrosis.

Autoantibodies to a collagen-specific molecular chaperone, heat-shock protein 47, in systemic sclerosis

Heat-shock proteins are highly conserved and immunogenic proteins, which may be involved in the initiation and perpetuation of autoimmune diseases. Heat-shock protein 47 (HSP47) is expressed by collagen-secreting cells such as fibroblasts and serves as a collagen-specific molecular chaperone that plays a crucial role in collagen metabolism. Abnormal collagen accumulation and autoimmunity are characteristics of systemic sclerosis (SSc). We determined the presence and prevalence of autoantibodies to HSP47 in patients with SSc and also in tight-skin (TSK/+) mice, an animal model for SSc. Anti-HSP47 autoantibodies were present in SSc patients with a frequency of 26%, while patients with systemic lupus erythematosus, those with dermatomyositis, those with keloid and healthy subjects did not have anti-HSP47 antibodies. IgG1 and IgG2 were the major Ig isotypes of the autoantibodies. Patients positive for anti-HSP47 had a significantly shorter duration of disease than those who were negative. Anti-HSP47 autoantibodies were also positive in 79% of TSK/+ mice. Thus, autoantibodies to HSP47 were present in the sera from SSc patients as well as those from TSK mice, and may be associated with the pathogenesis of SSc.

Fibulin-2 and fibulin-5 alterations in tsk mice associated with disorganized hypodermal elastic fibers and skin tethering

The Tight skin (Tsk) mouse is an important model of skin fibrosis that occurs in systemic sclerosis. These mice develop skin tethering and thickening associated with expression of a mutant fibrillin-1 gene. We show that Tsk fibrillin-1 leads to marked alterations in elastic fibers of the hypodermis of Tsk animals. In Tsk mice, a prominent elastic fiber layer found normally at the interface between hypodermal muscle and connective tissue was absent from an early age. The lack of elastic fibers at the hypodermal muscle-connective tissue (M-CT) interface was associated with a loss of staining for fibulin-5 in the same region. These mice also formed disorganized elastic fibers throughout hypodermal connective tissue as they aged. The increased elastic fibers in Tsk hypodermal connective tissue was associated with increased fibrillin-1 and fibulin-2 matrices. These results suggest that Tsk fibrillin-1 causes skin tethering by altering matrix protein composition in Tsk hypodermal connective tissues. The closely parallel alterations in elastogenesis associated with increased fibulin-2 in hypodermal connective tissues and decreased fibulin-5 at the hypodermal M-CT interface suggest that these proteins mediate the effect of Tsk-fibrillin-1 on elastogenesis.

Fibrillin microfibrils

Fibrillin microfibrils are widely distributed extracellular matrix assemblies that endow elastic and nonelastic connective tissues with long-range elasticity. They direct tropoelastin deposition during elastic fibrillogenesis and form an outer mantle for mature elastic fibers. Microfibril arrays are also abundant in dynamic tissues that do not express elastin, such as the ciliary zonules of the eye. Mutations in fibrillin-1-the principal structural component of microfibrils-cause Marfan syndrome, a heritable disease with severe aortic, ocular, and skeletal defects. Isolated fibrillin-rich microfibrils have a complex 56 nm "beads-on-a-string" appearance; the molecular basis of their assembly and elastic properties, and their role in higher-order elastic fiber formation, remain incompletely understood.

Transgenic analysis of scleroderma: understanding key pathogenic events in vivo

Modern molecular genetic methods have allowed better understanding of established mouse models of scleroderma and also facilitated the development of new and better defined mouse strains for investigating the pathogenesis of the disease. The best characterized scleroderma animal model is the type 1 tight skin mouse (Tsk1). Backcrossing these animals with other mutant strains has been informative. These experiments implicate the IL-4 ligand-receptor axis in the development of skin fibrosis. Parallel expression analysis of genes using microarrays has provided insight into novel mediators of fibrosis including the C-C chemokine MCP-3. Other experiments suggest that embryonically defined fibroblast-specific regulatory elements may be targets for activation in this model. The same lineage-specific elements have been used to selectively activate TGF beta signaling pathways in fibrosis to generate a novel model for scleroderma and also have been used to develop systems for ligand-dependent fibroblast-specific genetic recombination that will allow further analysis key candidate genes implicated in scleroderma pathogenesis. Better mouse models will improve understanding of this intractable rheumatic disease and can be expected to ultimately lead to improved treatments and outcome.

Latent TGF-beta binding proteins: extracellular matrix association and roles in TGF-beta activation

Transforming growth factor betas (TGF-betas) are multifunctional and pleiotropic growth factors. Their major effects include inhibition of cell proliferation and enhancement of extracellular matrix production. TGF-betas are secreted from cells as latent complexes, consisting of mature dimeric growth factor, the latency-associated propeptide (LAP), and a distinct gene product, latent TGF-beta binding protein LTBP. The secreted complex is targeted to specific locations in the extracellular matrix by the appropriate LTBP. The latent complex needs subsequently to be activated. Most studies describing biological effects of TGF-beta have been carried out in cell cultures using high concentrations of active, soluble TGF-beta, where appropriate targeting of the growth factor is missing. However, TGF-beta is produced and secreted in vivo as a latent complex in a specific and targeted manner. Various experimental approaches have convincingly shown the importance of the activation of latent TGF-beta, as well as the importance of LTBPs as targeting molecules of the effects of TGF-beta. Essential steps in the activation appear to be cellular recognition of extracellular matrix-associated LTBPs and subsequent recognition of the associated latent TGF-beta. Cell recognition by specific molecules like integrins and proteolytic events involving plasminogen activation evidently play multifaceted roles in the regulation of TGF-beta activation.

Altered B lymphocyte function induces systemic autoimmunity in systemic sclerosis

Systemic sclerosis (SSc) is a connective tissue disease characterized by excessive extracellular matrix deposition in the skin and visceral organs. SSc is associated with immune activation characterized by autoantibody production, lymphocyte activation, and release of various cytokines. The presence of autoantibodies is a central feature of immune activation in SSc. Although autoantibodies are thought to be closely linked to the pathogenesis of SSc, the pathogenic relationship between systemic autoimmunity and the clinical manifestations of SSc, including skin fibrosis, remains unknown. Recent studies have revealed that B cells play a critical role in systemic autoimmunity and disease expression through various functions, including cytokine production in addition to autoantibody production. The B cell signaling thresholds are regulated by response regulators that augment or diminish B cell signals during responses to self and foreign antigens. Abnormal regulation of the response regulator function and expression may result in autoantibody production. Among these response regulators, CD19, which is a critical cell-surface signal transduction molecule of B cells, is the most potent positive regulator. Transgenic mice that overexpress CD19 by approximately 3-fold lose tolerance and generate autoantibodies spontaneously. B cells from SSc patients exhibit a 20%-increase in CD19 expression that induces SSc-specific autoantibody production in transgenic mice. Furthermore, SSc patients have intrinsic B cell abnormalities characterized by expanded naive B cells, activated but diminished memory B cells, and chronic hyper-reactivity of memory B cells, possibly due to CD19 overexpression. Similarly, B cells from a tight-skin mouse, a model of SSc, show augmented CD19 signaling and chronic hyper-reactivity. Remarkably, CD19 loss results in inhibition of chronic B cell hyper-reactivity and elimination of autoantibody production, which is associated with improvement in skin fibrosis and a parallel decrease in IL-6 production by B cells. Thus, chronic B cell activation resulting from augmented CD19 signaling leads to skin fibrosis possibly through IL-6 overproduction, as well as autoantibody production, in tight-skin mice and SSc patients.

B Lymphocyte signaling established by the CD19/CD22 loop regulates autoimmunity in the tight-skin mouse

Systemic sclerosis (SSc) is characterized by fibrosis and autoimmmunity. Peripheral blood B cells from SSc patients specifically overexpress CD19, a critical cell-surface signal transduction molecule in B cells. CD19 deficiency in B cells also attenuates skin fibrosis in the tight-skin (TSK/+) mouse, a genetic model for SSc. Herein we analyzed two transgenic mouse lines that overexpress CD19. Remarkably, 20% increase of CD19 expression in mice spontaneously induced SSc-specific anti-DNA topoisomerase I (topo I) antibody (Ab) production, which was further augmented by 200% overexpression. In TSK/+ mice overexpressing CD19, skin thickness did not increase, although anti-topo I Ab levels were significantly augmented, indicating that abnormal CD19 signaling influences autoimmunity in TSK/+ mice and also that anti-topo I Ab does not have a pathogenic role. The molecular mechanisms for abnormal CD19 signaling were further assessed. B-cell antigen receptor crosslinking induced exaggerated calcium responses and augmented activation of extracellular signal-regulated kinase in TSK/+ B cells. CD22 function was specifically impaired in TSK/+ B cells. Consistently, CD19, a major target of CD22-negative regulation, was hyperphosphorylated in TSK/+ B cells. These findings indicate that reduced inhibitory signal provided by CD22 results in abnormal activation of signaling pathways including CD19 in TSK/+ mice and also suggest that this disrupted B cell signaling contribute to specific autoantibody production.

Mutant fibrillin 1 from tight skin mice increases extracellular matrix incorporation of microfibril-associated glycoprotein 2 and type I collagen

OBJECTIVE

Skin fibrosis in the TSK mouse, a model of skin fibrosis seen in systemic sclerosis (SSc), is caused by a large in-frame duplication in the Fbn1 gene, tsk-Fbn1. We investigated whether tsk-Fbn1 might cause dermal fibrosis by affecting Fbn1 and associated extracellular matrices. We also studied whether deposition of microfibril-associated glycoprotein 2 (MAGP-2), a protein that is associated with fibrillin 1, was altered in the skin of patients with SSc.

METHODS

An in vitro model of the TSK mouse was created by conditionally expressing tsk-Fbn1 in mouse embryonic fibroblasts (MEFs). Cell cultures were examined by immunofluorescence and Western and Northern blotting to determine the effect of tsk-Fbn1 on the structure, expression, and deposition of fibrillin 1 (Fbn-1), type I collagen, and MAGP-2. The skin of TSK mice and SSc patients was analyzed by immunohistochemistry for MAGP-2 expression.

RESULTS

Expression of tsk-Fbn1 in cultured MEF cells altered the morphology of Fbn-1 fibers and increased the deposition of type I collagen into the extracellular matrix (ECM) without concomitantly changing messenger RNA expression, secretion, or processing of type I procollagen. Moreover, MEF cells expressing tsk-Fbn1 showed increased MAGP-2 matrix. MAGP-2 was increased in the dermis of TSK mice. Fibrotic SSc skin also showed higher levels of MAGP-2 in the dermis than nonfibrotic SSc skin and normal skin.

CONCLUSION

Tsk-Fbn1 altered ECM organization and caused fibrosis by affecting the deposition of MAGP-2 or other Fbn-1-associated proteins. Alterations in microfibril structure or deposition might contribute to fibrosis in SSc.

Latent TGF-beta binding protein LTBP-2 decreases fibroblast adhesion to fibronectin

We have analyzed the effects of latent TGF-beta binding protein 2 (LTBP-2) and its fragments on lung fibroblast adhesion. Quantitative cell adhesion assays indicated that fibroblasts do not adhere to full-length LTBP-2. Interestingly, LTBP-2 had dominant disrupting effects on the morphology of fibroblasts adhering to fibronectin (FN). Fibroblasts plated on LTBP-2 and FN substratum exhibited less adherent morphology and displayed clearly decreased actin stress fibers than cells plated on FN. These cells formed, instead, extensive membrane ruffles. LTBP-2 had no effects on cells adhering to collagen type I. Fibroblasts adhered weakly to the NH2-terminal fragment of LTBP-2. Unlike FN, this fragment did not augment actin stress fiber formation. Interestingly, the adhesion-mediating and cytoskeleton-disrupting effects were localized to the same NH2-terminal proline-rich region of LTBP-2. LTBP-2 and its antiadhesive fragment bound to FN in vitro, and the antiadhesive fragment associated with the extracellular matrix FN fibrils. These observations reveal a potentially important role for LTBP-2 as an antiadhesive matrix component.

Fine tuning of growth factor signals depends on fibrillin microfibril networks

Growth factors, potent regulators of cell differentiation, tissue morphogenesis, tissue homeostasis, and cellular response to injury, reside in the extracellular matrix. Genetic evidence in humans and mice as well as biochemical data implicate fibrillins and LTBPs in the extracellular control of TGFbeta and BMP signaling. Fibrillins and LTBPs form tissue-specific and temporally regulated microfibril networks. In the developing embryo, three fibrillins and four LTBPs contribute molecular heterogeneity to microfibril networks, and provide different templates upon which TGFbeta-related growth factors can be positioned. By accommodating this molecular heterogeneity, microfibril architecture can orchestrate a variety of different signals in very specific tissue locations. Human fibrillinopathies display a broad phenotypic spectrum from tall to short stature, from hypermobile joints to joint contractures and stiffness, and from severe to mild or no cardiovascular manifestations. A spectrum of growth factor dysregulation may be caused by differential effects of mutations in fibrillins on microfibril architecture, thus altering appropriate targeting or positioning of growth factors within microfibril networks. Growth factor dysregulation may help to explain the broad phenotypic spectrum of the fibrillinopathies.

Pulmonary inflammatory response to inhaled ultrafine particles is modified by age, ozone exposure, and bacterial toxin

Epidemiological studies demonstrate associations between increasing levels of ambient particles and morbidity in the elderly with cardiopulmonary disease. Such findings have been challenged partly because particles may not act alone to cause these effects. We hypothesized that carbonaceous ambient ultrafine particles and ozone can act together to induce greater oxidative stress and inflammation in the lung than when administered alone and that these effects would be amplified in the compromised, aging lung. Two models of a compromised lung were used: endotoxin priming and old-age emphysema (TSK mice). Young (10 wk) and old (22 mo) male F344 rats and male TSK mice (14-17 mo) were exposed to ultrafine carbon particles (count median diameter 25 nm, 110 micrograms/m3) and to ozone (1 ppm) alone and in combination for 6 h. Inhalation of low-dose endotoxin (70 and 7.5 units estimated alveolar deposited dose in rats and mice, respectively) was used to model respiratory-tract infection. Cellular and biochemical lavage parameters and oxidant release from lung lavage cells were assessed 24 h after exposure. Inflammatory cell influx into the alveolar space was observed for both species and age groups: The combination of inhaled ultrafine carbon and ozone after endotoxin priming resulted in the greatest increase in lavage-fluid neutrophils. In general, the unstimulated and stimulated release of reactive oxygen species (ROS) from lavage inflammatory cells correlated well with the neutrophil response. There were significant effects of carbon particles as well as a consistent interaction between carbon and ozone as determined by analysis of variance (ANOVA). However, this interaction was in the opposite direction in young rats versus old rats and old TSK mice: Carbon and ozone interacted such that ROS activity was depressed in young rats, whereas it was enhanced in old rats and old TSK mice, indicating age-dependent functional differences in elicited pulmonary inflammatory cells. These results demonstrate that ultrafine carbonaceous particles inhaled for short periods of time can induce significant pulmonary inflammation and oxidative stress that are modified by age, copollutants, and a compromised respiratory tract.

Animal models of emphysema and their relevance to studies of particle-induced disease

Emphysema is a pulmonary disease that may be exacerbated by inhaled particles. Over the years, many animal models of emphysema have been developed that may be useful in studying the effects of inhaled particles on humans with emphysema. Models have been described in many species, and many approaches have been described for inducing emphysema. Emphysema in humans is a parenchymal component of chronic obstructive pulmonary disease and frequently coexists in a complex with disease of the airways such as bronchitis. Animal models of emphysema usually recapitulate only one or a few aspects of this complex disease. Thus, the emphysema model must be selected carefully in order to answer specific questions about the interactive effects of particles and emphysema.

Systemic sclerosis: the susceptible host (genetics and environment)

It is becoming evident that several genetic factors participate in modulating susceptibility to SSc and its clinical manifestations. Some genes that specifically affect ECM metabolism and vascular function may be unique to SSc and scleroderma-related disorders; others, such as those genes involved in regulating immune tolerance, are likely shared with other autoimmune diseases. The effect of genetic variations (or polymorphisms) that are found in most of these genes taken individually will likely have only a small or modest effect on disease risk; only a few genetic variations are expected to be highly penetrant. Moreover, genetic studies in SSc have to deal with the additional issues of heterogeneous phenotypes, low disease prevalence in the general population, and an even greater paucity of multiplex families that makes traditional linkage studies difficult, if not impossible. Alternative approaches include allelic association studies, but conventional case-controls designs may be subject to selection bias and will require large sample sizes if the genes that are under investigation confer only modest (OR = 1.5-2.0) disease risk (Fig. 2). The simultaneous examination of several genes that are biologically relevant to a specific disease process to attain higher aggregate ORs, is one approach that was used in several reports that were cited in this review. The use of family-based controls, such as in the transmission-disequilibrium test (based on assessment of the transmitted or nontransmitted alleles that are associated with disease from heterozygous parents to affected offspring), would provide more robustness to spurious associations from population stratification, but is actually less powerful and efficient than case-control designs. Furthermore, for many late adult-onset diseases the effort required to obtain samples from living parents are for a variety of reasons not trivial. The success of these allelic association-based approaches depends on the identification of likely candidate disease genes (or at least markers in disequilibrium with disease genes), careful definition/ascertainment of disease phenotypes to minimize genetic heterogeneity, and for case-control designs, strategies to account for population stratification or admixture. The identification of candidate genes will be aided by rapid progress in the Human Genome Project and other genome efforts that will eventually identify all human genetic variations. Although this will lead to better understanding of the genes that might be involved in complex diseases, much work is required to understand the basic biology of how disease genotypes become clinical phenotypes. This is especially daunting in complex diseases, such as SSc, where the phenotype (including disease susceptibility and clinical presentation) is influenced by dynamic interactions between genetic variations and environment. Multi-center collaborative efforts with research paradigms that integrate genetic and environmental factors (including sociodemographic variables) will be required to elucidate the contribution of environment and genetics in the pathogenesis of SSc.

Endothelial dysfunction in murine model of systemic sclerosis: tight-skin mice 1

We conducted this study to analyze endothelial cell function within intact thoracic aorta of the systemic sclerosis murine model, the heterozygous tight-skin mice 1: (i) assessing the distribution and activation intensity of endothelial cells, responsive to endothelium-dependent vasodilators (acetylcholine, adenosine triphosphate, bradykinin, and substance P) and Iloprost, using laser line confocal microscopy in combination with two Ca2+ fluorescent dyes; (ii) evaluating en-dothelium-dependent vasodilator- and Iloprostinduced relaxation, using isometric tension measurement; and (iii) investigating the role of nitric oxide in mediating relaxation to acetylcholine and adenosine triphosphate. The number of activated endothelial cells was significantly lower in heterozygous tight-skin mice 1, compared with controls, for adenosine triphosphate and Iloprost. Maximal increase of Ca2+ fluorescence intensity ratio in activated endothelial cells was decreased for adenosine triphosphate, bradykinin, and Iloprost, in heterozygous tight-skin mice 1. Adenosine triphosphate- and Iloprost-mediated aortic relaxation was further impaired in heterozygous tight-skin mice 1. Finally, aortic relaxation to acetylcholine and adenosine triphosphate was markedly decreased by nitric oxide synthase inhibitor in heterozygous tight-skin mice 1. This study suggests that endothelial cell receptors for endothelium-dependent vasodilators and Iloprost may not be homogeneously distributed or continuously expressed in thoracic aorta of heterozygous tight-skin mice 1, resulting in endothelium-dependent vasodilatation dysfunction. Moreover, because endothelium-dependent relaxation was highly dependent on nitric oxide release in heterozygous tight-skin mice 1, endothelium-dependent relaxation may differ from that of controls by increased production of nitric oxide. In turn, in heterozygous tight-skin mice 1, the resulting elevated nitric oxide levels may contribute to nitric oxide-mediated free radical endothelial cytotoxicity, although endothelium impairment may be related to other factors, particularly: Fbn-1 gene mutation and transforming growth factor-beta.