Matrix association of latent TGF-beta binding protein-2 (LTBP-2) is dependent on fibrillin-1

Exome sequencing identifies existing and novel variants in a South African cohort presenting with anterior segment dysgenesis

Anterior segment dysgenesis (ASD) defines a collection of congenital eye disorders that affect structures within the anterior segment of the eye. Mutations in genes that initiate and regulate the complex pathways involved in eye development can cause a spectrum of disorders such as ASD, congenital cataracts and corneal opacity. In South Africa, causes of ASD are poorly understood with few studies looking at the possible genetic basis for these disorders. In this study, we performed exome sequencing on a cohort of South African patients with ASD, focusing on a panel of genes known to regulate eye development pathways, including the PXDN gene which has recently been associated with ASD. We identified novel as well as established variants: specifically, we found a disease-causing variant in PAX6; variants that are likely to be pathogenic in GJA8, BCOR and EPHA2, as well as variants of uncertain significance in PXDN and LTBP2. In conclusion, this study is the first to show disease-causing variants in South African patients presenting with ASD, including the identification of novel variants and highlights the need to expand upon such studies in understudied populations.

Antrodia cinnamomea extract alleviates bleomycin-induced pulmonary fibrosis in mice by inhibiting the mTOR pathway

BACKGROUND

Pulmonary fibrosis is a progressive diffuse parenchymal lung disorder with a high mortality rate. Studies have indicated that injured lung tissues release various pro-inflammatory factors, and produce a large amount of nitric oxide. There is also accumulation of collagen and oxidative stress-induced injury, collectively leading to pulmonary fibrosis. Antrodia cinnamomea is an endemic fungal growth in Taiwan, and its fermented extracts exert anti-inflammatory effects to alleviate liver damages. Hence, we hypothesized and tested the feasibility of using A. cinnamomea extracts for treatment of pulmonary fibrosis.

METHODS

The TGF-β1-induced human lung fibroblast cells (MRC-5) in vitro cell assay were used to evaluate the effects of A. cinnamomea extracts on the collagen production in MRC-5. Eight-week-old ICR mice were intratracheally administered bleomycin and then fed with an A. cinnamomea extract on day 3 post-administration of bleomycin. At day 21 post-bleomycin administration, the pulmonary functional test, the expression level of inflammation- and fibrosis-related genes in the lung tissue, and the histopathological change were examined.

RESULTS

The A. cinnamomea extract significantly attenuated the expression level of collagen in the TGF-β1-induced MRC-5 cells. In the A. cinnamome-treated bleomycin-induced lung fibrotic mice, the bodyweight increased, pulmonary functions improved, the lung tissues expression level of inflammatory factor and the fibrotic indicator were decreased, and the histopathological results showed the reduction of thickening of the inter-alveolar septa.

CONCLUSIONS

The Antrodia cinnamomea extract significant protects mice against bleomycin-induced lung injuries through improvement of body weight gain and lung functions, and attenuation of expression of inflammatory and fibrotic indicators.

Microparticle Mediated Delivery of Apelin Improves Heart Function in Post Myocardial Infarction Mice

BACKGROUND

Apelin is an endogenous prepropeptide that regulates cardiac homeostasis and various physiological processes. Intravenous injection has been shown to improve cardiac contractility in patients with heart failure. However, its short half-life prevents studying its impact on left ventricular remodeling in the long term. Here, we aim to study whether microparticle-mediated slow release of apelin improves heart function and left ventricular remodeling in mice with myocardial infarction (MI).

METHODS

A cardiac patch was fabricated by embedding apelin-containing microparticles in a fibrin gel scaffold. MI was induced via permanent ligation of the left anterior descending coronary artery in adult C57BL/6J mice followed by epicardial patch placement immediately after (acute MI) or 28 days (chronic MI) post-MI. Four groups were included in this study, namely sham, MI, MI plus empty microparticle-embedded patch treatment, and MI plus apelin-containing microparticle-embedded patch treatment. Cardiac function was assessed by transthoracic echocardiography. Cardiomyocyte morphology, apoptosis, and cardiac fibrosis were evaluated by histology. Cardioprotective pathways were determined by RNA sequencing, quantitative polymerase chain reaction, and Western blot.

RESULTS

The level of endogenous apelin was largely reduced in the first 7 days after MI induction and it was normalized by day 28. Apelin-13 encapsulated in poly(lactic-co-glycolic acid) microparticles displayed a sustained release pattern for up to 28 days. Treatment with apelin-containing microparticle-embedded patch inhibited cardiac hypertrophy and reduced scar size in both acute and chronic MI models, which is associated with improved cardiac function. Data from cellular and molecular analyses showed that apelin inhibits the activation and proliferation of cardiac fibroblasts by preventing transforming growth factor-β-mediated activation of Smad2/3 (supporessor of mothers against decapentaplegic 2/3) and downstream profibrotic gene expression.

CONCLUSIONS

Poly(lactic-co-glycolic acid) microparticles prolonged the apelin release time in the mouse hearts. Epicardial delivery of the apelin-containing microparticle-embedded patch protects mice from both acute and chronic MI-induced cardiac dysfunction, inhibits cardiac fibrosis, and improves left ventricular remodeling.

Satellite glial contact enhances differentiation and maturation of human iPSC-derived sensory neurons

Sensory neurons generated from induced pluripotent stem cells (iSNs) are used to model human peripheral neuropathies, however current differentiation protocols produce sensory neurons with an embryonic phenotype. Peripheral glial cells contact sensory neurons early in development and contribute to formation of the canonical pseudounipolar morphology, but these signals are not encompassed in current iSN differentiation protocols. Here, we show that terminal differentiation of iSNs in co-culture with rodent Dorsal Root Ganglion satellite glia (rSG) advances their differentiation and maturation. Co-cultured iSNs develop a pseudounipolar morphology through contact with rSGs. This transition depends on semaphorin-plexin guidance cues and on glial gap junction signaling. In addition to morphological changes, iSNs terminally differentiated in co-culture exhibit enhanced spontaneous action potential firing, more mature gene expression, and increased susceptibility to paclitaxel induced axonal degeneration. Thus, iSNs differentiated in coculture with rSGs provide a better model for investigating human peripheral neuropathies.

LTBP2 regulates cisplatin resistance in GC cells via activation of the NF-κB2/BCL3 pathway

Gastric cancer (GC) often develops resistance to cisplatin treatment, but while latent transforming growth factor β-binding protein (LTBP2) is recognized as a potential regulator in GC, its specific role in cisplatin resistance is not fully understood. This study investigated LTBP2's impact on cisplatin resistance in GC. LTBP2 expression was assessed in various GC cell lines, and its correlation with cisplatin sensitivity was determined through cell viability assays. Lentivirus-mediated LTBP2 silencing in HGC-27 cells demonstrated enhanced cisplatin sensitivity, reduced cell proliferation, and inhibition of the NF-κB2/Bcl-3/cyclin D1 pathway. Additionally, transient transfection overexpressed the NFκB2 gene in LTBP2-silenced HGC-27/DDPR cells, restoring cisplatin sensitivity and upregulating p52/Bcl-3/cyclin D1. In conclusion, silencing LTBP2 could effectively inhibit cell proliferation and mitigate cisplatin resistance via the NFKB noncanonical pathway NFKB2 p52/Bcl-3/cyclin D1. These findings propose LTBP2 as a potential therapeutic target for overcoming cisplatin resistance in GC patients.

Multi-organ phenotypes in mice lacking latent TGFβ binding protein 2 (LTBP2)

BACKGROUND

Latent TGFβ binding protein-2 (LTBP2) is a fibrillin 1 binding component of the microfibril. LTBP2 is the only LTBP protein that does not bind any isoforms of TGFβ, although it may interfere with the function of other LTBPs or interact with other signaling pathways.

RESULTS

Here, we investigate mice lacking Ltbp2 (Ltbp2-/- ) and identify multiple phenotypes that impact bodyweight and fat mass, and affect bone and skin development. The alterations in skin and bone development are particularly noteworthy since the strength of these tissues is differentially affected by loss of Ltbp2. Interestingly, some tissues that express high levels of Ltbp2, such as the aorta and lung, do not have a developmental or homeostatic phenotype.

CONCLUSIONS

Analysis of these mice show that LTBP2 has complex effects on development through direct effects on the extracellular matrix (ECM) or on signaling pathways that are known to regulate the ECM.

The prognostic significance of LTBP2 for malignant tumors: Evidence based on 11 observational studies

BACKGROUND AIMS

At present, increasing reports have shown that latent transforming growth factor-β-binding protein 2 (LTBP2) was associated with the prognosis of many types of cancer. We performed rounded analysis to comprehensively analyze and evaluate the prognostic significance of LTBP2 for patients with malignant tumors.

METHODS

We identified relevant studies by searching database including PubMed, Embase, Cochrane Library, and Web of Science. The odds ratio with its 95% confidence interval (CI) was used to assess the correlation between LTBP2 and clinicopathologic features or overall survival of patients with cancer. Hazard ratio with its 95% CI was used to explore the prognostic risk factors. The analysis was performed and assessed using Review Manager 5.2.

RESULTS

A total of 11 studies including 2322 participants were included in this systematic review. Pooled results showed that malignant tissues experienced higher incidence of high LTBP2 expression when compared with adjacent or normal tissues. Patients with high LTBP2 expression experienced significantly lower 1-year, 2-year, 3-year, and 4-year overall survival rate, with the pooled odds ratios being 0.26 (95% CI 0.13-0.53; P = .0002), 0.27 (95% CI 0.14-0.50; P < .0001), 0.26 (95% CI 0.13-0.53; P = .0002), and 0.21 (95% CI 0.06-0.73; P = .01) respectively. Univariate analysis showed high LTBP2 expression, tumor node metastasis stage, T stage, and N stage were prognostic factors of patients with tumors. Multivariate analysis indicated high LTBP2 expression was an independent prognostic factor.

CONCLUSIONS

The present analysis suggested that LTBP2 may have significant association with survival of patients with cancer. High LTBP2 expression was an independent prognostic factor and indicated poor survival.

LTBP1 promotes fibrillin incorporation into the extracellular matrix

LTBP1 is a large extracellular matrix protein and an associated ligand of fibrillin-microfibrils. Knowledge of LTBP1 functions is largely limited to its role in targeting and sequestering TGFβ growth factors within the extracellular matrix, thereby regulating their bioavailability. However, the recent description of a wide spectrum of phenotypes in multiple tissues in patients harboring LTBP1 pathogenic variants suggests a multifaceted role of the protein in the homeostasis of connective tissues. To better understand the human pathology caused by LTBP1 deficiency it is important to investigate its functional role in extracellular matrix formation. In this study, we show that LTBP1 coordinates the incorporation of fibrillin-1 and -2 into the extracellular matrix in vitro. We also demonstrate that this function is differentially exerted by the two isoforms, the short and long forms of LTBP1. Thereby our findings uncover a novel TGFβ-independent LTBP1 function potentially contributing to the development of connective tissue disorders.

Fibronectin: Molecular Structure, Fibrillar Structure and Mechanochemical Signaling

The extracellular matrix (ECM) plays a key role as both structural scaffold and regulator of cell signal transduction in tissues. In times of ECM assembly and turnover, cells upregulate assembly of the ECM protein, fibronectin (FN). FN is assembled by cells into viscoelastic fibrils that can bind upward of 40 distinct growth factors and cytokines. These fibrils play a key role in assembling a provisional ECM during embryonic development and wound healing. Fibril assembly is also often upregulated during disease states, including cancer and fibrotic diseases. FN fibrils have unique mechanical properties, which allow them to alter mechanotransduction signals sensed and relayed by cells. Binding of soluble growth factors to FN fibrils alters signal transduction from these proteins, while binding of other ECM proteins, including collagens, elastins, and proteoglycans, to FN fibrils facilitates the maturation and tissue specificity of the ECM. In this review, we will discuss the assembly of FN fibrils from individual FN molecules; the composition, structure, and mechanics of FN fibrils; the interaction of FN fibrils with other ECM proteins and growth factors; the role of FN in transmitting mechanobiology signaling events; and approaches for studying the mechanics of FN fibrils.

Latent-transforming growth factor β-binding protein 2 accelerates cardiac fibroblast apoptosis by regulating the expression and activity of caspase-3

Cardiac fibrosis is a core process in the development of heart failure. However, the underlying mechanism of cardiac fibrosis remains unclear. Recently, a study found that in an isoproterenol (ISO)-induced cardiac fibrosis animal model, there is high expression of latent-transforming growth factor β-binding protein 2 (LTBP2) in cardiac fibroblasts. Whether LTBP2 serves a role in cardiac fibrosis is currently unknown. In the present study, mouse cardiac fibroblasts (MCFs) were treated with 100 µM/l ISO for 24, 48, or 72 h, and small interfering RNAs (siRNAs) were used to knockdown LTBP2. Reverse transcription-quantitative PCR and western blotting were used to determine gene and protein expression levels, respectively. Caspase-3 serves a key role in cell apoptosis and is related to cardiac fibrosis-induced heart failure. Caspase-3 activity was therefore determined using a caspase-3 assay kit, CCK8 was used to determine the rate of cell proliferation and apoptosis rates were quantified using a cell death detection ELISA kit. The present study demonstrated that cell apoptosis and LTBP2 expression increased in MCFs treated with 100 µM/l ISO in a time-dependent manner. Expression and activity of caspase-3 also increased in MCFs treated with 100 µM/l ISO for 48 h compared with the control group. In addition, ISO stimulation-induced MCF apoptosis, along with the increased expression of caspase-3 were partly abolished when LTBP2 was knocked down. In conclusion, LTBP2 expression increased in ISO-treated MCFs and accelerated mouse cardiac fibroblast apoptosis by enhancing the expression and activity of caspase-3. LTBP2 may therefore be a potential therapeutic target for treating patients with cardiac fibrosis.

Intracellular and Extracellular Markers of Lethality in Osteogenesis Imperfecta: A Quantitative Proteomic Approach

Osteogenesis imperfecta (OI) is a heritable disorder that mainly affects the skeleton. The inheritance is mostly autosomal dominant and associated to mutations in one of the two genes, COL1A1 and COL1A2, encoding for the type I collagen α chains. According to more than 1500 described mutation sites and to outcome spanning from very mild cases to perinatal-lethality, OI is characterized by a wide genotype/phenotype heterogeneity. In order to identify common affected molecular-pathways and disease biomarkers in OI probands with different mutations and lethal or surviving phenotypes, primary fibroblasts from dominant OI patients, carrying COL1A1 or COL1A2 defects, were investigated by applying a Tandem Mass Tag labeling-Liquid Chromatography-Tandem Mass Spectrometry (TMT LC-MS/MS) proteomics approach and bioinformatic tools for comparative protein-abundance profiling. While no difference in α1 or α2 abundance was detected among lethal (type II) and not-lethal (type III) OI patients, 17 proteins, with key effects on matrix structure and organization, cell signaling, and cell and tissue development and differentiation, were significantly different between type II and type III OI patients. Among them, some non-collagenous extracellular matrix (ECM) proteins (e.g., decorin and fibrillin-1) and proteins modulating cytoskeleton (e.g., nestin and palladin) directly correlate to the severity of the disease. Their defective presence may define proband-failure in balancing aberrances related to mutant collagen.

Acromelic dysplasias: how rare musculoskeletal disorders reveal biological functions of extracellular matrix proteins

Acromelic dysplasias are a group of rare musculoskeletal disorders that collectively present with short stature, pseudomuscular build, stiff joints, and tight skin. Acromelic dysplasias are caused by mutations in genes (FBN1, ADAMTSL2, ADAMTS10, ADAMTS17, LTBP2, and LTBP3) that encode secreted extracellular matrix proteins, and in SMAD4, an intracellular coregulator of transforming growth factor-β (TGF-β) signaling. The shared musculoskeletal presentations in acromelic dysplasias suggest that these proteins cooperate in a biological pathway, but also fulfill distinct roles in specific tissues that are affected in individual disorders of the acromelic dysplasia group. In addition, most of the affected proteins directly interact with fibrillin microfibrils in the extracellular matrix and have been linked to the regulation of TGF-β signaling. Together with recently developed knockout mouse models targeting the affected genes, novel insights into molecular mechanisms of how these proteins regulate musculoskeletal development and homeostasis have emerged. Here, we summarize the current knowledge highlighting pathogenic mechanisms of the different disorders that compose acromelic dysplasias and provide an overview of the emerging biological roles of the individual proteins that are compromised. Finally, we develop a conceptual model of how these proteins may interact and form an "acromelic dysplasia complex" on fibrillin microfibrils in connective tissues of the musculoskeletal system.

A novel ADAMTS17 variant that causes Weill-Marchesani syndrome 4 alters fibrillin-1 and collagen type I deposition in the extracellular matrix

Weill-Marchesani syndrome (WMS) is a rare genetic disorder that affects the musculoskeletal system, the eye, and the cardiovascular system. Individuals with WMS present with short stature, joint contractures, thick skin, microspherophakia, small and dislocated lenses, and cardiac valve anomalies. WMS can be caused by recessive mutations in ADAMTS10 (WMS 1), ADAMTS17 (WMS 4), or LTBP2 (WMS 3), or by dominant mutations in fibrillin-1 (FBN1) (WMS 2); all genes encode secreted extracellular matrix (ECM) proteins. Individuals with WMS 4 due to ADAMTS17 mutations appear to have less severe cardiac involvement and present predominantly with the musculoskeletal and ocular features of WMS. ADAMTS17 is a member of the ADAMTS family of secreted proteases and directly binds to fibrillins. Here we report a novel pathogenic variant in ADAMTS17 that causes WMS 4 in an individual with short stature, brachydactyly, and small, spherical, and dislocated lenses. We provide biochemical and cell biological insights in the pathomechanisms of WMS 4, which also suggest potential biological functions for ADAMTS17. We show that the variant in ADAMTS17 prevents its secretion and we found intracellular accumulation of fibrillin-1 and collagen type I in patient-derived skin fibroblasts. In accordance, transmission electron microscopy revealed elastic fiber abnormalities, decreased collagen fibril diameters, and intracellular collagen accumulation in the dermis of the proband. Together, the data indicate a possible role for ADAMTS17 in the secretion of fibrillin-1 and collagen type I or in their early assembly in the pericellular matrix or the ECM.

The role of fibrillin and microfibril binding proteins in elastin and elastic fibre assembly

Fibrillin is a large evolutionarily ancient extracellular glycoprotein that assembles to form beaded microfibrils which are essential components of most extracellular matrices. Fibrillin microfibrils have specific biomechanical properties to endow animal tissues with limited elasticity, a fundamental feature of the durable function of large blood vessels, skin and lungs. They also form a template for elastin deposition and provide a platform for microfibril-elastin binding proteins to interact in elastic fibre assembly. In addition to their structural role, fibrillin microfibrils mediate cell signalling via integrin and syndecan receptors, and microfibrils sequester transforming growth factor (TGF)β family growth factors within the matrix to provide a tissue store which is critical for homeostasis and remodelling.

Expression and clinical significance of latent-transforming growth factor beta-binding protein 2 in primary hepatocellular carcinoma

The present study aimed to explore the expression of latent transforming growth factor β binding protein 2 (LTBP2) in patients with hepatocellular carcinoma (HCC) and their correlation to clinicopathologial features.Serum levels of LTBP2 in 60 patients with HCC, 35 patients with hepatocellular benign tumors, 60 patients with precancerous lesions of HCC, and 60 healthy volunteers were determined by enzyme-linked immunosorbent assay. The expression levels of LTBP2 at messenger RNA (mRNA) and protein levels in 60 cases of HCC and adjacent tissues were detected by quantitative real-time polymerase chain reaction and immunohisochemistry. Statistical analysis was used to analyze the relationship between LTBP2 and clinical characteristics of patients with HCC.The mRNA and protein levels of LTBP2 were significantly upregulated in HCC tissues compared to adjacent tissues. Additionally, higher serum LTBP2 level was also observed in HCC patients relative to normal controls. Further investigation demonstrated that LTBP2 expression was associated with malignant degree of tumor, tumor progression, tumor differentiation, tumor size, tumor stage and hepatitis virus infection, and has prognostic implications in HCC patients.LTBP2 might be served as a potential biomarker in diagnosis and treatment of HCC.

LTBP2 is secreted from lung myofibroblasts and is a potential biomarker for idiopathic pulmonary fibrosis

Although differentiation of lung fibroblasts into α-smooth muscle actin (αSMA)-positive myofibroblasts is important in the progression of idiopathic pulmonary fibrosis (IPF), few biomarkers reflecting the fibrotic process have been discovered. We performed microarray analyses between FACS-sorted steady-state fibroblasts (lineage (CD45, TER-119, CD324, CD31, LYVE-1, and CD146)-negative and PDGFRα-positive cells) from untreated mouse lungs and myofibroblasts (lineage-negative, Sca-1-negative, and CD49e-positive cells) from bleomycin-treated mouse lungs. Amongst several genes up-regulated in the FACS-sorted myofibroblasts, we focussed on Ltbp2, the gene encoding latent transforming growth factor-β (TGF-β) binding protein-2 (LTBP2), because of the signal similarity to Acta2, which encodes αSMA, in the clustering analysis. The up-regulation was reproduced at the mRNA and protein levels in human lung myofibroblasts induced by TGF-β1. LTBP2 staining in IPF lungs was broadly positive in the fibrotic interstitium, mainly as an extracellular matrix (ECM) protein; however, some of the αSMA-positive myofibroblasts were also stained. Serum LTBP2 concentrations, evaluated using ELISA, in IPF patients were significantly higher than those in healthy volunteers (mean: 21.4 compared with 12.4 ng/ml) and showed a negative correlation with % predicted forced vital capacity (r = -0.369). The Cox hazard model demonstrated that serum LTBP2 could predict the prognosis of IPF patients (hazard ratio for death by respiratory events: 1.040, 95% confidence interval: 1.026-1.054), which was validated using the bootstrap method with 1000-fold replication. LTBP2 is a potential prognostic blood biomarker that may reflect the level of differentiation of lung fibroblasts into myofibroblasts in IPF.

Fibrillin microfibrils and elastic fibre proteins: Functional interactions and extracellular regulation of growth factors

Fibrillin microfibrils are extensible polymers that endow connective tissues with long-range elasticity and have widespread distributions in both elastic and non-elastic tissues. They act as a template for elastin deposition during elastic fibre formation and are essential for maintaining the integrity of tissues such as blood vessels, lung, skin and ocular ligaments. A reduction in fibrillin is seen in tissues in vascular ageing, chronic obstructive pulmonary disease, skin ageing and UV induced skin damage, and age-related vision deterioration. Most mutations in fibrillin cause Marfan syndrome, a genetic disease characterised by overgrowth of the long bones and other skeletal abnormalities with cardiovascular and eye defects. However, mutations in fibrillin and fibrillin-binding proteins can also cause short-stature pathologies. All of these diseases have been linked to dysregulated growth factor signalling which forms a major functional role for fibrillin.

Rapid Communication: High-resolution quantitative trait loci analysis identifies LTBP2 encoding latent transforming growth factor beta binding protein 2 associated with thoracic vertebrae number in a large F2 intercross between Landrace and Korean native pigs

Number of vertebrae is associated with body size and meat productivity in pigs. The aim of this study was to identify QTL and associated positional candidate genes affecting the number of thoracic vertebrae (THO). A genomewide association study was conducted in a large resource population derived from an F intercross between Landrace and Korean native pigs using the Porcine SNP 60K BeadChip and the genomewide complex trait analysis (GCTA) program based on a linear mixed-effects model. A total of 38,385 SNP markers from 1,105 F progeny were analyzed for the THO trait after filtering for quality control. A total of 90 genomewide significant SNP markers ( < 1.30 × 10) on SSC 7 covering a 20-Mb region were identified for THO in this study. Several previous studies also mapped QTL for vertebral numbers in this region. The strongest association signals were detected at ASGA0035500 (-value = 4.46 × 10; 103,574,383 bp) and DIAS0000795 (-value = 4.46 × 10; 103,594,753 bp). The QTL region on SSC 7 for THO encompasses and , which are previously described candidate genes for vertebral number variation. To refine the QTL region, a haplotype-based linkage and linkage disequilibrium (LALD) analysis using the DualPHASE program was applied because subsequent conditional association and haplotype block analyses could not resolve the region that contains the 2 loci. The LALD analysis refined the critical region to a 533.9-kb region including ; was located outside the critical region. The gene encoding latent transforming growth factor beta binding protein 2 is involved in bone metabolisms. Based on these data, we propose as a positional candidate gene for THO in pigs. After further functional studies and verification of the association in other independent populations, these results could be useful for optimizing breeding programs that improve THO and other economically important traits in pigs.

LTBPs in biology and medicine: LTBP diseases

The latent transforming growth factor (TGF) β binding proteins (LTBP) are crucial mediators of TGFβ function, as they control growth factor secretion, matrix deposition, presentation and activation. Deficiencies in specific LTBP isoforms yield discrete phenotypes representing defects in bone, lung and cardiovascular development mediated by loss of TGFβ signaling. Additional phenotypes represent loss of unique TGFβ-independent features of LTBP effects on elastogenesis and microfibril assembly. Thus, the LTBPs act as sensors for the regulation of both growth factor activity and matrix function.

Fibrillins in Tendon

Tendons among connective tissue, mainly collagen, contain also elastic fibers (EF) made of fibrillin 1, fibrillin 2 and elastin that are broadly distributed in tendons and represent 1-2% of the dried mass of the tendon. Only in the last years, studies on structure and function of EF in tendons have been performed. Aim of this review is to revise data on the organization of EF in tendons, in particular fibrillin structure and function, and on the clinical manifestations associated to alterations of EF in tendons. Indeed, microfibrils may contribute to tendon mechanics; therefore, their alterations may cause joint hypermobility and contractures which have been found to be clinical features in patients with Marfan syndrome (MFS) and Beals syndrome. The two diseases are caused by mutations in genes FBN1 and FBN2 encoding fibrillin 1 and fibrillin 2, respectively.

Identification of Novel Variants in LTBP2 and PXDN Using Whole-Exome Sequencing in Developmental and Congenital Glaucoma

Background

Primary congenital glaucoma (PCG) is the most common form of glaucoma in children. PCG occurs due to the developmental defects in the trabecular meshwork and anterior chamber of the eye. The purpose of this study is to identify the causative genetic variants in three families with developmental and primary congenital glaucoma (PCG) with a recessive inheritance pattern.

Methods

DNA samples were obtained from consanguineous families of Pakistani ancestry. The CYP1B1 gene was sequenced in the affected probands by conventional Sanger DNA sequencing. Whole exome sequencing (WES) was performed in DNA samples of four individuals belonging to three different CYP1B1-negative families. Variants identified by WES were validated by Sanger sequencing.

Results

WES identified potentially causative novel mutations in the latent transforming growth factor beta binding protein 2 (LTBP2) gene in two PCG families. In the first family a novel missense mutation (c.4934G>A; p.Arg1645Glu) co-segregates with the disease phenotype, and in the second family a novel frameshift mutation (c.4031_4032insA; p.Asp1345Glyfs*6) was identified. In a third family with developmental glaucoma a novel mutation (c.3496G>A; p.Gly1166Arg) was identified in the PXDN gene, which segregates with the disease.

Conclusions

We identified three novel mutations in glaucoma families using WES; two in the LTBP2 gene and one in the PXDN gene. The results will not only enhance our current understanding of the genetic basis of glaucoma, but may also contribute to a better understanding of the diverse phenotypic consequences caused by mutations in these genes.

The Extracellular Matrix in Bronchopulmonary Dysplasia: Target and Source

Bronchopulmonary dysplasia (BPD) is a common complication of preterm birth that contributes significantly to morbidity and mortality in neonatal intensive care units. BPD results from life-saving interventions, such as mechanical ventilation and oxygen supplementation used to manage preterm infants with acute respiratory failure, which may be complicated by pulmonary infection. The pathogenic pathways driving BPD are not well-delineated but include disturbances to the coordinated action of gene expression, cell-cell communication, physical forces, and cell interactions with the extracellular matrix (ECM), which together guide normal lung development. Efforts to further delineate these pathways have been assisted by the use of animal models of BPD, which rely on infection, injurious mechanical ventilation, or oxygen supplementation, where histopathological features of BPD can be mimicked. Notable among these are perturbations to ECM structures, namely, the organization of the elastin and collagen networks in the developing lung. Dysregulated collagen deposition and disturbed elastin fiber organization are pathological hallmarks of clinical and experimental BPD. Strides have been made in understanding the disturbances to ECM production in the developing lung, but much still remains to be discovered about how ECM maturation and turnover are dysregulated in aberrantly developing lungs. This review aims to inform the reader about the state-of-the-art concerning the ECM in BPD, to highlight the gaps in our knowledge and current controversies, and to suggest directions for future work in this exciting and complex area of lung development (patho)biology.

LTBP-2 Has a Single High-Affinity Binding Site for FGF-2 and Blocks FGF-2-Induced Cell Proliferation

Latent transforming growth factor-beta-1 binding protein-2 (LTBP-2) belongs to the fibrillin-LTBP superfamily of extracellular matrix proteins. LTBPs and fibrillins are involved in the sequestration and storage of latent growth factors, particularly transforming growth factor β (TGF-β), in tissues. Unlike other LTBPs, LTBP-2 does not covalently bind TGF-β, and its molecular functions remain unclear. We are screening LTBP-2 for binding to other growth factors and have found very strong saturable binding to fibroblast growth factor-2 (FGF-2) (Kd = 1.1 nM). Using a series of recombinant LTBP-2 fragments a single binding site for FGF-2 was identified in a central region of LTBP-2 consisting of six tandem epidermal growth factor-like (EGF-like) motifs (EGFs 9–14). This region was also shown to contain a heparin/heparan sulphate-binding site. FGF-2 stimulation of fibroblast proliferation was completely negated by the addition of 5-fold molar excess of LTBP-2 to the assay. Confocal microscopy showed strong co-localisation of LTBP-2 and FGF-2 in fibrotic keloid tissue suggesting that the two proteins may interact in vivo. Overall the study indicates that LTBP-2 is a potent inhibitor of FGF-2 that may influence FGF-2 bioactivity during wound repair particularly in fibrotic tissues.

Targeting latent TGFβ release in muscular dystrophy

Latent transforming growth factor-β (TGFβ) binding proteins (LTBPs) bind to inactive TGFβ in the extracellular matrix. In mice, muscular dystrophy symptoms are intensified by a genetic polymorphism that changes the hinge region of LTBP, leading to increased proteolytic susceptibility and TGFβ release. We have found that the hinge region of human LTBP4 was also readily proteolysed and that proteolysis could be blocked by an antibody to the hinge region. Transgenic mice were generated to carry a bacterial artificial chromosome encoding the human LTBP4 gene. These transgenic mice displayed larger myofibers, increased damage after muscle injury, and enhanced TGFβ signaling. In the mdx mouse model of Duchenne muscular dystrophy, the human LTBP4 transgene exacerbated muscular dystrophy symptoms and resulted in weaker muscles with an increased inflammatory infiltrate and greater LTBP4 cleavage in vivo. Blocking LTBP4 cleavage may be a therapeutic strategy to reduce TGFβ release and activity and decrease inflammation and muscle damage in muscular dystrophy.

NF-κB p65 Subunit Is Modulated by Latent Transforming Growth Factor-β Binding Protein 2 (LTBP2) in Nasopharyngeal Carcinoma HONE1 and HK1 Cells

NF-κB is a well-characterized transcription factor, widely known as a key player in tumor-derived inflammation and cancer development. Herein, we present the functional and molecular relevance of the canonical NF-κB p65 subunit in nasopharyngeal carcinoma (NPC). Loss- and gain-of-function approaches were utilized to reveal the functional characteristics of p65 in propagating tumor growth, tumor-associated angiogenesis, and epithelial-to-mesenchymal transition in NPC cells. Extracellular inflammatory stimuli are critical factors that trigger the NF-κB p65 signaling; hence, we investigated the components of the tumor microenvironment that might potentially influence the p65 signaling pathway. This led to the identification of an extracellular matrix (ECM) protein that was previously reported as a candidate tumor suppressor in NPC. Our studies on the Latent Transforming Growth Factor-β Binding Protein 2 (LTBP2) protein provides substantial evidence that it can modulate the p65 transcriptional activity. Re-expression of LTBP2 elicits tumor suppressive effects that parallel the inactivation of p65 in NPC cells. LTBP2 was able to reduce phosphorylation of p65 at Serine 536, inhibit nuclear localization of active phosphorylated p65, and impair the p65 DNA-binding ability. This results in a consequential down-regulation of p65-related gene expression. Therefore, the data suggest that the overall up-regulation of p65 expression and the loss of this candidate ECM tumor suppressor are milestone events contributing to NPC development.

The extracellular matrix and transforming growth factor-β1: Tale of a strained relationship

Physiological tissue repair aims at restoring the mechano-protective properties of the extracellular matrix. Consequently, redundant regulatory mechanisms are in place ensuring that tissue remodeling terminates once matrix homeostasis is re-established. If these mechanisms fail, stromal cells become continuously activated, accumulate excessive amounts of stiff matrix, and fibrosis develops. In this mini-review, I develop the hypothesis that the mechanical state of the extracellular matrix and the pro-fibrotic transforming growth factor (TGF)-β1 cooperate to regulate the remodeling activities of stromal cells. TGF-β1 is stored in the matrix as part of a large latent complex and can be activated by cell contractile force that is transmitted by integrins. Matrix straining and stiffening lower the threshold for TGF-β1 activation by increasing the mechanical resistance to cell pulling. Different elements of this mechanism can be pharmacologically targeted to interrupt the mechanical positive feedback loop of fibrosis, including specific integrins and matrix protein interactions.

Latent TGF-β-binding proteins

The LTBPs (or latent transforming growth factor β binding proteins) are important components of the extracellular matrix (ECM) that interact with fibrillin microfibrils and have a number of different roles in microfibril biology. There are four LTBPs isoforms in the human genome (LTBP-1, -2, -3, and -4), all of which appear to associate with fibrillin and the biology of each isoform is reviewed here. The LTBPs were first identified as forming latent complexes with TGFβ by covalently binding the TGFβ propeptide (LAP) via disulfide bonds in the endoplasmic reticulum. LAP in turn is cleaved from the mature TGFβ precursor in the trans-golgi network but LAP and TGFβ remain strongly bound through non-covalent interactions. LAP, TGFβ, and LTBP together form the large latent complex (LLC). LTBPs were originally thought to primarily play a role in maintaining TGFβ latency and targeting the latent growth factor to the extracellular matrix (ECM), but it has also been shown that LTBP-1 participates in TGFβ activation by integrins and may also regulate activation by proteases and other factors. LTBP-3 appears to have a role in skeletal formation including tooth development. As well as having important functions in TGFβ regulation, TGFβ-independent activities have recently been identified for LTBP-2 and LTBP-4 in stabilizing microfibril bundles and regulating elastic fiber assembly.

Osteopontin and latent-TGF β binding-protein 2 as potential diagnostic markers for HBV-related hepatocellular carcinoma

Chronic Hepatitis B (HB) is the main risk factor for chronic liver disease (CLD) and hepatocellular carcinoma (HCC) in many low-resource countries, where diagnosis is constrained by lack of clinical, histopathological and biomarker resources. We have used proteomics to detect plasma biomarkers that outperform α-Fetoprotein (AFP), the most widely used biomarker for HCC diagnosis in low-resource contexts. Deep-plasma proteome analysis was performed in HCC patients, patients with CLD and in HB-carrier controls from Thailand (South-East Asia) and The Gambia (West-Africa). Mass spectrometry profiling identified latent-transforming growth factor β binding-protein 2 (LTBP2) and Osteopontin (OPN) as being significantly elevated in HCC versus CLD and controls. These two proteins were further analyzed by ELISA in a total of 684 plasma samples, including 183 HCC, 274 CLD and 227 asymptomatic controls. When combined, LTBP2 and OPN showed an area under the receiver operating curve of 0.85 in distinguishing HCC from CLD in subjects with AFP <20 ng/mL. In a prospective cohort of 115 CLD patients from Korea, increased plasma levels of LTBP2 and/or OPN were detected in plasma collected over 2 years prior to diagnosis in 21 subjects who developed HCC. Thus, the combination of LTBP2 and OPN outperformed AFP for diagnosis and prediction of HCC and may therefore improve biomarker-based detection of HBV-related HCC.

Confocal microscopy demonstrates association of LTBP-2 in fibrillin-1 microfibrils and colocalisation with perlecan in the disc cell pericellular matrix

Comparative immunolocalisations of latent transforming growth factor-beta-1 binding protein (LTBP)-2, fibrillin-1, versican and perlecan were undertaken in foetal human and wild type C57BL/6 mouse and Hspg2 exon 3 null HS deficient mouse intervertebral discs (IVDs). LTBP-2 was a prominent pericellular component of annular fibrochondrocytes in the posterior annulus fibrosus (AF), interstitial matrix adjacent to nucleus pulposus (NP) cells and to fibrillar and cell associated material in the anterior AF of the human foetal IVD and also displayed a pericellular localisation pattern in murine IVDs. Perlecan and LTBP-2 displayed strong pericellular colocalisation patterns in the posterior AF and to fibrillar material in the outer anterior AF in the foetal human IVD. Versican was a prominent fibril-associated component in the posterior and anterior AF, localised in close proximity to fibrillin-1 in fibrillar arrangements in the cartilaginous vertebral rudiments around paraspinal blood vessels, to major collagen fibre bundles in the anterior and posterior AF and shorter fibres in the NP. Fibrillin-1 was prominent in the outer anterior AF of the human foetal IVD and in fibres extending from the AF into the cartilaginous vertebral rudiments. LTBP-2 was prominently associated with annular fibrils containing fibrillin-1, versican was localised in close proximity to these but not specifically with LTBP-2. The similar deposition levels of LTBP-2 observed in the AF of the Hspg2 exon 3 null and wild type murine IVDs indicated that perlecan HS was not essential for LTBP-2 deposition but colocalisation of LTBP-2 with perlecan in the foetal human IVD was consistent with HS mediated interactions which have already been demonstrated in-vitro.

LTBP-2 competes with tropoelastin for binding to fibulin-5 and heparin, and is a negative modulator of elastinogenesis

Latent transforming growth factor-beta-1 binding protein-2 (LTBP-2) is a protein of ill-defined function associated with elastic fibers during elastinogenesis. Although LTBP-2 binds fibrillin-1, fibulin-5, and heparin/heparan sulfate, molecules critical for normal elastic fiber assembly, it does not interact directly with elastin or its precursor, tropoelastin. We investigated the modulating effect of LTBP-2 on two key interactions of tropoelastin during elastinogenesis a) with fibulin-5 and b) with heparan sulfate (using heparin). Firstly, using solid phase assays we showed that LTBP-2 bound fibulin-5 (Kd=26.47±5.68 nM) with an affinity similar to that of the tropoelastin-fibulin-5 interaction (Kd=24.66±5.64 nM). Then using a competitive binding assay we showed that LTBP-2 inhibited the tropoelastin-fibulin-5 interaction in a dose dependent manner with almost complete inhibition obtained with 5-fold molar excess of LTBP-2. Interestingly, a fragment of LTBP-2 containing the fibulin-5 binding sequence only partially inhibited the tropoelasin-fibulin-5 interaction suggesting that LTBP-2 was directly blocking only the C-terminal tropoelastin binding site on fibulin-5 and indirectly blocking tropoelastin binding to the N-terminal region. In parallel experiments heparin was shown to have minor inhibitory effects on fibulin-5 interactions with tropoelastin and LTBP-2. However, LTBP-2 was shown to significantly inhibit the binding of heparin to tropoelastin with 50% inhibition achieved with 10 fold molar excess of LTBP-2. Confocal microscopy of fibroblast matrix showed strong co-distribution of LTBP-2 with fibulin-5 and fibrillin-1 and partial co-distribution with heparan sulfate proteoglycans, perlecan and syndecan-4. Also addition of exogenous LTBP-2 to ear cartilage chondrocyte cultures blocked elastinogenesis in a concentration-dependent manner. Overall the results indicate that LTBP-2 may have a negative regulatory role during elastic fiber assembly, perhaps in displacing elastin microassemblies from complexes with fibulin-5 and/or cell surface heparan sulfate proteoglycans.

Elastic fibres in health and disease

Elastic fibres are insoluble components of the extracellular matrix of dynamic connective tissues such as skin, arteries, lungs and ligaments. They are laid down during development, and comprise a cross-linked elastin core within a template of fibrillin-based microfibrils. Their function is to endow tissues with the property of elastic recoil, and they also regulate the bioavailability of transforming growth factor β. Severe heritable elastic fibre diseases are caused by mutations in elastic fibre components; for example, mutations in elastin cause supravalvular aortic stenosis and autosomal dominant cutis laxa, mutations in fibrillin-1 cause Marfan syndrome and Weill–Marchesani syndrome, and mutations in fibulins-4 and -5 cause autosomal recessive cutis laxa. Acquired elastic fibre defects include dermal elastosis, whereas inflammatory damage to fibres contributes to pathologies such as pulmonary emphysema and vascular disease. This review outlines the latest understanding of the composition and assembly of elastic fibres, and describes elastic fibre diseases and current therapeutic approaches.

Elastic fibres are broadly distributed in tendon and highly localized around tenocytes

Elastic fibres have the unique ability to withstand large deformations and are found in numerous tissues, but their organization and structure have not been well defined in tendon. The objective of this study was to characterize the organization of elastic fibres in tendon to understand their function. Immunohistochemistry was used to visualize elastic fibres in bovine flexor tendon with fibrillin-1, fibrillin-2 and elastin antibodies. Elastic fibres were broadly distributed throughout tendon, and highly localized longitudinally around groups of cells and transversely between collagen fascicles. The close interaction of elastic fibres and cells suggests that elastic fibres are part of the pericellular matrix and therefore affect the mechanical environment of tenocytes. Fibres present between fascicles are likely part of the endotenon sheath, which enhances sliding between adjacent collagen bundles. These results demonstrate that elastic fibres are highly localized in tendon and may play an important role in cellular function and contribute to the tissue mechanics of the endotenon sheath.

Specificity of latent TGF-β binding protein (LTBP) incorporation into matrix: role of fibrillins and fibronectin

Fibrillin microfibrils are extracellular matrix structures with essential functions in the development and the organization of tissues including blood vessels, bone, limbs and the eye. Fibrillin-1 and fibrillin-2 form the core of fibrillin microfibrils, to which multiple proteins associate to form a highly organized structure. Defining the components of this structure and their interactions is crucial to understand the pathobiology of microfibrillopathies associated with mutations in fibrillins and in microfibril-associated molecules. In this study, we have analyzed both in vitro and in vivo the role of fibrillin microfibrils in the matrix deposition of latent TGF-β binding protein 1 (LTBP-1), -3 and -4; the three LTBPs that form a complex with TGF-β. In Fbn1(-/-) ascending aortas and lungs, LTBP-3 and LTBP-4 are not incorporated into a matrix lacking fibrillin-1 microfibrils, whereas LTBP-1 is still deposited. In addition, in cultures of Fbn1(-/-) smooth muscle cells or lung fibroblasts, LTBP-3 and LTBP-4 are not incorporated into a matrix lacking fibrillin-1 microfibrils, whereas LTBP-1 is still deposited. Fibrillin-2 is not involved in the deposition of LTBP-1 in Fbn1(-/-) extracellular matrix as cells deficient for both fibrillin-1 and fibrillin-2 still incorporate LTBP-1 in their matrix. However, blocking the formation of the fibronectin network in Fbn1(-/-) cells abrogates the deposition of LTBP-1. Together, these data indicate that LTBP-3 and LTBP-4 association with the matrix depends on fibrillin-1 microfibrils, whereas LTBP-1 association depends on a fibronectin network.

LTBPs, more than just an escort service

Latent transforming growth factor beta (TGF-β) binding proteins (LTBPs) are large extracellular glycoproteins structurally similar to fibrillins. They perform intricate and important roles in the extracellular matrix (ECM) and perturbations of their function manifest as a wide range of diseases. LTBPs are major regulators of TGF-β bioavailability and action. In addition, LTBPs interact with other ECM proteins-from cytokines to large multi-factorial aggregates like microfibrils and elastic fibers, affecting their genesis, structure, and performance. In the present article, we review recent advancements in the field and relate the complex roles of LTBP in development and homeostasis.

The distribution of fibrillin-2 and LTBP-2, and their co-localisation with fibrillin-1 in adult bovine tail disc

We investigated the distribution of fibrillin-2 and LTBP-2 (latent TGF-β binding protein-2) in the intervertebral disc of the adult bovine tail. The association of fibrillin-2 and of LTBP-2 with fibrillin-1 was examined by dual immunofluorescence staining. Both fibrillin-2 and LTBP-2 were found extensively distributed in all regions of the disc with the organisation of the network varying significantly region to region. In the outer annulus fibrosus (OAF) both fibrillin-2 and LTBP-2 co-localised with fibrillin-1 forming fibres running parallel to the collagen fibres of the lamellae with the microfibrillar network staining densely in between the adjacent lamellae and also at the boundaries of the collagen bundle compartments. In the inner annulus fibrosus (IAF) and nucleus pulposus (NP), co-localised fibrillin-1,2 and LTBP-2 formed a chondron-like structure around the cell. By contrast, the inter-territorial matrix of the IAF and NP contained a dense network of fibrillin-2 but only sparse/filamentous fibres of fibrillin-1 and LTBP-2. Dual immunostaining revealed that in this region, fibrillin-2 was highly colocalised with elastin. The LTBP-2 network co-localised well with that of fibrillin-1 in all regions and indeed is reported to bind strongly to fibrillin-1. However, interestingly LTBP-2 but not fibrillin-1 or fibrillin-2 was removed by hyaluronidase but not collagenase pre-digestion. Our results suggest that fibrillin-2 and LTBP-2 could play an important role in disc function.

Cross talk among TGF-β signaling pathways, integrins, and the extracellular matrix

The growth factor TGF-β is secreted in a latent complex consisting of three proteins: TGF-β, an inhibitor (latency-associated protein, LAP, which is derived from the TGF-β propeptide) and an ECM-binding protein (one of the latent TGF-β binding proteins, or LTBPs). LTBPs interact with fibrillins and other ECM components and thus function to localize latent TGF-β in the ECM. LAP contains an integrin-binding site (RGD), and several RGD-binding integrins are able to activate latent TGF-β through binding this site. Mutant mice defective in integrin-mediated activators, and humans and mice with fibrillin gene mutations, show the critical role of ECM and integrins in regulating TGF-β signaling.

Comparative immunolocalization of the elastin fiber-associated proteins fibrillin-1, LTBP-2, and MAGP-1 with components of the collagenous and proteoglycan matrix of the fetal human intervertebral disc

STUDY DESIGN

A comparative immunolocalization study of elastin-associated proteins and established intervertebral disc (IVD) extracellular matrix (ECM) components.

OBJECTIVE

To localize for the first time, elastic fiber–associated proteins with structural fibrillar components in the annulus fibrosus (AF) of the fetal IVD.

SUMMARY OF BACKGROUND DATA

Elastin has been identified histochemically in adult bovine, human, and immature rat IVDs, and in fetal human IVDs using electron microscopy; however, no immunolocalization studies have been undertaken for associated components in human fetal IVDs.

METHODS

En-bloc fixation of thoracolumbar spinal segments in formalin and Histochoice followed by standard histochemical processing, paraffin embedding, microtome sectioning, and identification of IVD ECM components using a range of specific mono- and polyclonal antibodies and bright-field and laser scanning confocal microscopy.

RESULTS

The elastic fiber-associated proteins fibrillin-1, LTBP-2, and MAGP-1 were prominently immunolocalized in the outer lamellar layers of the AF of the human fetal IVD. Dual localization of selected components by confocal microscopy demonstrated that versican and LTBP-2 were colocalized with fibrillin-1 microfibrils in the AF lamellae with a similar distribution to the elastin fibers. LTBP-2 was also associated with pericellular perlecan in the outer AF. These interconnections between elastin-associated proteins resulted in an elastic network, which connected the AF cells with the adjacent cartilaginous vertebral bodies.

CONCLUSION

Specific immunolocalization of fibrillin-1, MAGP-1, and versican with elastin in the outer AF of the fetal human IVD has been demonstrated. We deduce from the established distributions of the elastin-associated proteins and their known interactivities with matrix components that these stabilize and aid in the integration of the elastic fibers in the annular lamellae and may be responsible for the generation of tensional forces in the outer AF, which direct the assembly of this tissue.

Dilated capillaries, disorganized collagen fibers and differential gene expression in periodontal ligaments of hypomorphic fibrillin-1 mice

The periodontal ligaments (PDLs) are soft connective tissue between the cementum covering the tooth root surface and alveolar bone. PDLs are composed of collagen and elastic system fibers, blood vessels, nerves, and various types of cells. Elastic system fibers are generally formed by elastin and microfibrils, but PDLs are mainly composed of the latter. Compared with the well-known function of collagen fibers to support teeth, little is known about the role of elastic system fibers in PDLs. To clarify their role, we examined PDLs of mice under-expressing fibrillin-1 (mgR mice), which is one of the major microfibrillar proteins. The PDLs of homozygous mgR mice showed one-quarter of the elastic system fibers of wild-type (WT) mice. A close association between the elastic system fibers and the capillaries was noted in WT, homozygous and heterozygous mgR mice. Interestingly, capillaries in PDLs of homozygous mice were dilated or enlarged compared with those of WT mice. A comparable level of type I collagen, which is the major collagen in PDLs, was expressed in PDL-cells of mice with three genotypes. However, multi-oriented collagen fiber bundles with a thinner appearance were noted in homozygous mice, whereas well-organized collagen fiber bundles were seen in WT mice. Moreover, there was a marked decrease in periostin expression, which is known to regulate the fibrillogenesis and crosslinking of collagen. These observations suggest that the microfibrillar protein, fibrillin-1, is indispensable for normal tissue architecture and gene expression of PDLs.

Assembly of fibrillin microfibrils governs extracellular deposition of latent TGF beta

Control of the bioavailability of the growth factor TGFbeta is essential for tissue formation and homeostasis, yet precisely how latent TGFbeta is incorporated into the extracellular matrix is unknown. Here, we show that deposition of a large latent TGFbeta complex (LLC), which contains latent TGFbeta-binding protein 1 (LTBP-1), is directly dependent on the pericellular assembly of fibrillin microfibrils, which interact with fibronectin during higher-order fibrillogenesis. LTBP-1 formed pericellular arrays that colocalized with microfibrils, whereas fibrillin knockdown inhibited fibrillar LTBP-1 and/or LLC deposition. Blocking alpha5beta1 integrin or supplementing cultures with heparin, which both inhibited microfibril assembly, disrupted LTBP-1 deposition and enhanced Smad2 phosphorylation. Full-length LTBP-1 bound only weakly to N-terminal pro-fibrillin-1, but this association was strongly enhanced by heparin. The microfibril-associated glycoprotein MAGP-1 (MFAP-2) inhibited LTBP-1 binding to fibrillin-1 and stimulated Smad2 phosphorylation. By contrast, fibulin-4, which interacted strongly with full-length LTBP-1, did not induce Smad2 phosphorylation. Thus, LTBP-1 and/or LLC deposition is dependent on pericellular microfibril assembly and is governed by complex interactions between LTBP-1, heparan sulfate, fibrillin-1 and microfibril-associated molecules. In this way, microfibrils control TGFbeta bioavailability.

Unraveling the mechanism of elastic fiber assembly: The roles of short fibulins

Evolution of elastic fibers is associated with establishment of the closed circulation system. Primary roles of elastic fibers are to provide elasticity and recoiling to tissues and organs and to maintain the structural integrity against mechanical strain over a lifetime. Elastic fibers are comprised of an insoluble elastin core and surrounding mantle of microfibrils. Elastic fibers are formed in a regulated, stepwise manner, which includes the formation of a microfibrillar scaffold, deposition and integration of tropoelastin monomers into the scaffold, and cross-linking of the monomers to form an insoluble, functional polymer. In recent years, an increasing number of glycoproteins have been identified and shown to be located on or surrounding elastic fibers. Among them, the short fibulins-3, -4 and -5 particularly drew attention because of their potent elastogenic activity. Fibulins-3, -4 and -5 are characterized by tandem repeats of calcium binding EGF-like motifs and a C-terminal fibulin module, which is conserved throughout fibulin family members. Initial biochemical characterization and gene expression studies predicted that fibulins might be involved in structural support and/or matrix-cell interactions. Recent analyses of short fibulin knockout mice have revealed their critical roles in elastic fiber development in vivo. We review recent findings on the elastogenic functions of short fibulins and discuss the molecular mechanism underlying their activity in vitro and in vivo.