Regulation of limb patterning by extracellular microfibrils

Poglut2/3 double knockout in mice results in neonatal lethality with reduced levels of fibrillin in lung tissues

Fibrillin microfibrils play a critical role in the formation of elastic fibers, tissue/organ development, and cardiopulmonary function. These microfibrils not only provide structural support and flexibility to tissues, but they also regulate growth factor signaling through a plethora of microfibril-binding proteins in the extracellular space. Mutations in fibrillins are associated with human diseases affecting cardiovascular, pulmonary, skeletal, and ocular systems. Fibrillins consist of up to 47 epidermal growth factor-like repeats, of which more than half are modified by protein O-glucosyltransferase 2 (POGLUT2) and/or POGLUT3. Loss of these modifications reduces secretion of N-terminal fibrillin constructs overexpressed in vitro. Here, we investigated the role of POGLUT2 and POGLUT3 in vivo using a Poglut2/3 double knockout (DKO) mouse model. Blocking O-glucosylation caused neonatal death with skeletal, pulmonary, and eye defects reminiscent of fibrillin/elastin mutations. Proteomic analyses of DKO dermal fibroblast medium and extracellular matrix provided evidence that fibrillins were more sensitive to loss of O-glucose compared to other POGLUT2/3 substrates. This conclusion was supported by immunofluorescent analyses of late gestation DKO lungs where FBN levels were reduced and microfibrils appeared fragmented in the pulmonary arteries and veins, bronchioles, and developing saccules. Defects in fibrillin microfibrils likely contributed to impaired elastic fiber formation and histological changes observed in DKO lung blood vessels, bronchioles, and saccules. Collectively, these results highlight the importance of POGLUT2/3-mediated O-glucosylation in vivo and open the possibility that O-glucose modifications on fibrillin influence microfibril assembly and or protein interactions in the ECM environment.

Extracellular matrix protein composition dynamically changes during murine forelimb development

The extracellular matrix (ECM) is an integral part of multicellular organisms, connecting different cell layers and tissue types. During morphogenesis and growth, tissues undergo substantial reorganization. While it is intuitive that the ECM remodels in concert, little is known regarding how matrix composition and organization change during development. Here, we quantified ECM protein dynamics in the murine forelimb during appendicular musculoskeletal morphogenesis (embryonic days 11.5-14.5) using tissue fractionation, bioorthogonal non-canonical amino acid tagging, and mass spectrometry. Our analyses indicated that ECM protein (matrisome) composition in the embryonic forelimb changed as a function of development and growth, was distinct from other developing organs (brain), and was altered in a model of disease (osteogenesis imperfecta murine). Additionally, the tissue distribution for select matrisome was assessed via immunohistochemistry in the wild-type embryonic and postnatal musculoskeletal system. This resource will guide future research investigating the role of the matrisome during complex tissue development.

Genetic models of fibrillinopathies

The fibrillinopathies represent a group of diseases in which the 10-12 nm extracellular microfibrils are disrupted by genetic variants in one of the genes encoding fibrillin molecules, large glycoproteins of the extracellular matrix. The best-known fibrillinopathy is Marfan syndrome, an autosomal dominant condition affecting the cardiovascular, ocular, skeletal, and other systems, with a prevalence of around 1 in 3,000 across all ethnic groups. It is caused by variants of the FBN1 gene, encoding fibrillin-1, which interacts with elastin to provide strength and elasticity to connective tissues. A number of mouse models have been created in an attempt to replicate the human phenotype, although all have limitations. There are also natural bovine models and engineered models in pig and rabbit. Variants in FBN2 encoding fibrillin-2 cause congenital contractural arachnodactyly and mouse models for this condition have also been produced. In most animals, including birds, reptiles, and amphibians, there is a third fibrillin, fibrillin-3 (FBN3 gene) for which the creation of models has been difficult as the gene is degenerate and nonfunctional in mice and rats. Other eukaryotes such as the nematode C. elegans and zebrafish D. rerio have a gene with some homology to fibrillins and models have been used to discover more about the function of this family of proteins. This review looks at the phenotype, inheritance, and relevance of the various animal models for the different fibrillinopathies.

FOXD1 is required for 3D patterning of the kidney interstitial matrix

BACKGROUND

The interstitial extracellular matrix (ECM) is comprised of proteins and glycosaminoglycans and provides structural and biochemical information during development. Our previous work revealed the presence of transient ECM-based structures in the interstitial matrix of developing kidneys. Stromal cells are the main contributors to interstitial ECM synthesis, and the transcription factor Forkhead Box D1 (Foxd1) is critical for stromal cell function. To investigate the role of Foxd1 in interstitial ECM patterning, we combined 3D imaging and proteomics to explore how the matrix changes in the murine developing kidney when Foxd1 is knocked out.

RESULTS

We found that COL26A1, FBN2, EMILIN1, and TNC, interstitial ECM proteins that are transiently upregulated during development, had a similar distribution perinatally but then diverged in patterning in the adult. Abnormally clustered cortical vertical fibers and fused glomeruli were observed when Foxd1 was knocked out. The changes in the interstitial ECM of Foxd1 knockout kidneys corresponded to disrupted Foxd1+ cell patterning but did not precede branching dysmorphogenesis.

CONCLUSIONS

The transient ECM networks affected by Foxd1 knockout may provide support for later-stage nephrogenic structures.

Case Report: A Novel Homozygous Missense Variant of FBN3 Supporting It Is a New Candidate Gene Causative of a Bardet–Biedl Syndrome–Like Phenotype

Fibrillin proteins are extracellular matrix glycoproteins assembling into microfibrils. FBN1, FBN2, and FBN3 encode the human fibrillins and mutations in FBN1 and FBN2 cause connective tissue disorders called fibrillinopathies, affecting cardiovascular, dermal, skeletal, and ocular tissues. Recently, mutations of the less characterized fibrillin family member, FBN3, have been associated in a single family with Bardet–Biedl syndrome (BBS). Here, we report on a patient born from two first cousins and affected by developmental delay, cognitive impairment, obesity, dental and genital anomalies, and brachydactyly/syndactyly. His phenotype was very similar to that reported in the previous FBN3-mutated family and fulfilled BBS clinical diagnostic criteria, although lacking polydactyly, the most recurrent clinical feature, as the previous siblings described. A familial SNP-array and proband’s WES were performed prioritizing candidate variants on the sole patient’s runs of homozygosity. This analysis disclosed a novel homozygous missense variant in FBN3 (NM_032447:c.5434A>G; NP_115823:p.Ile1812Val; rs115948457), inherited from the heterozygous parents. This study further supports that FBN3 is a candidate gene for a BBS-like syndrome characterized by developmental delay, cognitive impairment, obesity, dental, genital, and skeletal anomalies. Anyway, additional studies are necessary to investigate the exact role of the gene and possible interactions between FBN3 and BBS proteins.

Fibrillin-1 deficiency in the outer perichondrium causes longitudinal bone overgrowth in mice with Marfan syndrome

A disproportionate tall stature is the most evident manifestation in Marfan syndrome (MFS), a multisystem condition caused by mutations in the extracellular protein and TGFβ modulator, fibrillin-1. Unlike cardiovascular manifestations, there has been little effort devoted to unravel the molecular mechanism responsible for long bone overgrowth in MFS. By combining the Cre-LoxP recombination system with metatarsal bone cultures, here we identify the outer layer of the perichondrium as the tissue responsible for long bone overgrowth in MFS mice. Analyses of differentially expressed genes in the fibrillin-1 deficient perichondrium predicted that loss of TGFβ signaling may influence chondrogenesis in the neighboring epiphyseal growth plate (GP). Immunohistochemistry revealed that fibrillin-1 deficiency in the outer perichondrium is associated with decreased accumulation of latent TGFβ-binding proteins (LTBPs)-3 and - 4, and reduced levels of phosphorylated (activated) Smad2. Consistent with these findings, mutant metatarsal bones grown in vitro were longer and released less TGFβ than the wild type counterparts. Moreover, addition of recombinant TGFβ1 normalized linear growth of mutant metatarsal bones. We conclude that longitudinal bone overgrowth in MFS is accounted for by diminished sequestration of LTBP-3 and LTBP-4 into the fibrillin-1 deficient matrix of the outer perichondrium, which results in less TGFβ signaling locally and improper GP differentiation distally.

Proteolysis of fibrillin-2 microfibrils is essential for normal skeletal development

The embryonic extracellular matrix (ECM) undergoes transition to mature ECM as development progresses, yet few mechanisms ensuring ECM proteostasis during this period are known. Fibrillin microfibrils are macromolecular ECM complexes serving structural and regulatory roles. In mice, Fbn1 and Fbn2, encoding the major microfibrillar components, are strongly expressed during embryogenesis, but fibrillin-1 is the major component observed in adult tissue microfibrils. Here, analysis of Adamts6 and Adamts10 mutant mouse embryos, lacking these homologous secreted metalloproteases individually and in combination, along with in vitro analysis of microfibrils, measurement of ADAMTS6-fibrillin affinities and N-terminomics discovery of ADAMTS6-cleaved sites, identifies a proteostatic mechanism contributing to postnatal fibrillin-2 reduction and fibrillin-1 dominance. The lack of ADAMTS6, alone and in combination with ADAMTS10 led to excess fibrillin-2 in perichondrium, with impaired skeletal development defined by a drastic reduction of aggrecan and cartilage link protein, impaired BMP signaling in cartilage, and increased GDF5 sequestration in fibrillin-2-rich tissue. Although ADAMTS6 cleaves fibrillin-1 and fibrillin-2 as well as fibronectin, which provides the initial scaffold for microfibril assembly, primacy of the protease-substrate relationship between ADAMTS6 and fibrillin-2 was unequivocally established by reversal of the defects in Adamts6^-/- embryos by genetic reduction of Fbn2, but not Fbn1.

The fibrillinopathies: new insights with focus on the paradigm of opposing phenotypes for both FBN1 and FBN2

Different pathogenic variants in the fibrillin‐1 gene (FBN1) cause Marfan syndrome and acromelic dysplasias. Whereas the musculoskeletal features of Marfan syndrome involve tall stature, arachnodactyly, joint hypermobility, and muscle hypoplasia, acromelic dysplasia patients present with short stature, brachydactyly, stiff joints, and hypermuscularity. Similarly, pathogenic variants in the fibrillin‐2 gene (FBN2) cause either a Marfanoid congenital contractural arachnodactyly or a FBN2‐related acromelic dysplasia that most prominently presents with brachydactyly. The phenotypic and molecular resemblances between both the FBN1 and FBN2‐related disorders suggest that reciprocal pathomechanistic lessons can be learned. In this review, we provide an updated overview and comparison of the phenotypic and mutational spectra of both the “tall” and “short” fibrillinopathies. The future parallel functional study of both FBN1/2‐related disorders will reveal new insights into how pathogenic fibrillin variants differently affect the fibrillin microfibril network and/or growth factor homeostasis in clinically opposite syndromes. This knowledge may eventually be translated into new therapeutic approaches by targeting or modulating the fibrillin microfibril network and/or the signaling pathways under its control.

Construction and Evaluation of Recombinant Chimeric Fibrillin and Elastin Fragment in Human Mesenchymal Stem Cells

BACKGROUND

Diverse extracellular matrix (ECM) proteins physically interact with stem cells and regulate stem cell function. However, the large molecular weight of the natural ECM renders large-scale fabrication of a similar functional structure challenging.

OBJECTIVE

The objective of this study was to construct a low molecular weight and multifunctional chimeric form of recombinant ECM to stimulate mesenchymal stem cell (MSC) for tissue repair. We engineered Fibrillin-1PF14 fused to an elastin-like polypeptide to develop a new biomimetic ECM for stem cell differentiation and investigated whether this recombinant chimeric Fibrillin-Elastin fragment (rcFE) was effective on human nasal inferior turbinate-derived mesenchymal stem cells (hTMSCs).

METHODS

hTMSCs were grown in the medium supplemented with rcFE, then the effect of the protein was confirmed through cell adhesion assay, proliferation assay, and real-time PCR.

RESULTS

rcFE enhanced the adhesion activity of hTMSCs by 2.7-fold at the optimal concentration, and the proliferation activity was 2.6-fold higher than that of the control group (non-treatment rcFE). In addition, when smooth muscle cell differentiation markers were identified by real-time PCR, Calponin increased about 6-fold, α-actin about 9-fold, and MYH11 about 10-fold compared to the control group.

CONCLUSION

Chimeric rcFE enhanced cellular functions such as cell adhesion, proliferation, and smooth muscle differentiation of hTMSCs, suggesting that the rcFE can facilitate the induction of tissue regeneration.

A Multi-Omics Approach Using a Mouse Model of Cardiac Malformations for Prioritization of Human Congenital Heart Disease Contributing Genes

Congenital heart disease (CHD) is the most common type of birth defect, affecting ~1% of all live births. Malformations of the cardiac outflow tract (OFT) account for ~30% of all CHD and include a range of CHDs from bicuspid aortic valve (BAV) to tetralogy of Fallot (TOF). We hypothesized that transcriptomic profiling of a mouse model of CHD would highlight disease-contributing genes implicated in congenital cardiac malformations in humans. To test this hypothesis, we utilized global transcriptional profiling differences from a mouse model of OFT malformations to prioritize damaging, de novo variants identified from exome sequencing datasets from published cohorts of CHD patients. Notch1^+/−; Nos3^−/− mice display a spectrum of cardiac OFT malformations ranging from BAV, semilunar valve (SLV) stenosis to TOF. Global transcriptional profiling of the E13.5 Notch1^+/−; Nos3^−/− mutant mouse OFTs and wildtype controls was performed by RNA sequencing (RNA-Seq). Analysis of the RNA-Seq dataset demonstrated genes belonging to the Hif1α, Tgf-β, Hippo, and Wnt signaling pathways were differentially expressed in the mutant OFT. Mouse to human comparative analysis was then performed to determine if patients with TOF and SLV stenosis display an increased burden of damaging, genetic variants in gene homologs that were dysregulated in Notch1^+/−; Nos3^−/− OFT. We found an enrichment of de novo variants in the TOF population among the 1,352 significantly differentially expressed genes in Notch1^+/−; Nos3^−/− mouse OFT but not the SLV population. This association was not significant when comparing only highly expressed genes in the murine OFT to de novo variants in the TOF population. These results suggest that transcriptomic datasets generated from the appropriate temporal, anatomic and cellular tissues from murine models of CHD may provide a novel approach for the prioritization of disease-contributing genes in patients with CHD.

Toward precision medicine in vascular connective tissue disorders

Tremendous progress has been made in understanding the etiology, pathogenesis, and treatment of inherited vascular connective tissue disorders. While new insights regarding disease etiology and pathogenesis have informed patient counseling and care, there are numerous obstacles that need to be overcome in order to achieve the full promise of precision medicine. In this review, these issues will be discussed in the context of Marfan syndrome and Loeys-Dietz syndrome, with additional emphasis on the pioneering contributions made by Victor McKusick.

A new MMP-mediated prodomain cleavage mechanism to activate bone morphogenetic proteins from the extracellular matrix

Since their discovery as pluripotent cytokines extractable from bone matrix, it has been speculated how bone morphogenetic proteins (BMPs) become released and activated from the extracellular matrix (ECM). In contrast to TGF-βs, most investigated BMPs are secreted as bioactive prodomain (PD)-growth factor (GF) complexes (CPLXs). Recently, we demonstrated that PD-dependent targeting of BMP-7 CPLXs to the extracellular fibrillin microfibril (FMF) components fibrillin-1 and -2 represents a BMP sequestration mechanism by rendering the GF latent. Understanding how BMPs become activated from ECM scaffolds such as FMF is crucial to elucidate pathomechanisms characterized by aberrant BMP activation and ECM destruction. Here, we describe a new MMP-dependent BMP-7 activation mechanism from ECM-targeted pools via specific PD degradation. Using Edman sequencing and mutagenesis, we identified a new and conserved MMP-13 cleavage site within the BMP-7 PD. A degradation screen with different BMP family PDs and representative MMP family members suggested utilization of the identified site in a general MMP-driven BMP activation mechanism. Furthermore, sandwich ELISA and solid phase cleavage studies in combination with bioactivity assays, single particle TEM, and in silico molecular docking experiments provided evidence that PD cleavage by MMP-13 leads to BMP-7 CPLX disintegration and bioactive GF release.

Controlling BMP growth factor bioavailability: The extracellular matrix as multi skilled platform

Bone morphogenetic proteins (BMPs) belong to the TGF-β superfamily of signaling ligands which comprise a family of pluripotent cytokines regulating a multitude of cellular events. Although BMPs were originally discovered as potent factors extractable from bone matrix that are capable to induce ectopic bone formation in soft tissues, their mode of action has been mostly studied as soluble ligands in absence of the physiologically relevant cellular microenvironment. This micro milieu is defined by supramolecular networks of extracellular matrix (ECM) proteins that specifically target BMP ligands, present them to their cellular receptors, and allow their controlled release. Here we focus on functional interactions and mechanisms that were described to control BMP bioavailability in a spatio-temporal manner within the respective tissue context. Structural disturbance of the ECM architecture due to mutations in ECM proteins leads to dysregulated BMP signaling as underlying cause for connective tissue disease pathways. We will provide an overview about current mechanistic concepts of how aberrant BMP signaling drives connective tissue destruction in inherited and chronic diseases.

The BMP Pathway in Blood Vessel and Lymphatic Vessel Biology

Bone morphogenetic proteins (BMPs) were originally identified as the active components in bone extracts that can induce ectopic bone formation. In recent decades, their key role has broadly expanded beyond bone physiology and pathology. Nowadays, the BMP pathway is considered an important player in vascular signaling. Indeed, mutations in genes encoding different components of the BMP pathway cause various severe vascular diseases. Their signaling contributes to the morphological, functional and molecular heterogeneity among endothelial cells in different vessel types such as arteries, veins, lymphatic vessels and capillaries within different organs. The BMP pathway is a remarkably fine-tuned pathway. As a result, its signaling output in the vessel wall critically depends on the cellular context, which includes flow hemodynamics, interplay with other vascular signaling cascades and the interaction of endothelial cells with peri-endothelial cells and the surrounding matrix. In this review, the emerging role of BMP signaling in lymphatic vessel biology will be highlighted within the framework of BMP signaling in the circulatory vasculature.

Learning from BMPs and their biophysical extracellular matrix microenvironment for biomaterial design

It is nowadays well-accepted that the extracellular matrix (ECM) is not a simple reservoir for growth factors but is an organization center of their biological activity. In this review, we focus on the ability of the ECM to regulate the biological activity of BMPs. In particular, we survey the role of the ECM components, notably the glycosaminoglycans and fibrillary ECM proteins, which can be promoters or repressors of the biological activities mediated by the BMPs. We examine how a process called mechano-transduction induced by the ECM can affect BMP signaling, including BMP internalization by the cells. We also focus on the spatio-temporal regulation of the BMPs, including their release from the ECM, which enables to modulate their spatial localization as well as their local concentration. We highlight how biomaterials can recapitulate some aspects of the BMPs/ECM interactions and help to answer fundamental questions to reveal previously unknown molecular mechanisms. Finally, the design of new biomaterials inspired by the ECM to better present BMPs is discussed, and their use for a more efficient bone regeneration in vivo is also highlighted.

Cell death in the developing vertebrate limb: A locally regulated mechanism contributing to musculoskeletal tissue morphogenesis and differentiation

Our aim is to critically review current knowledge of the function and regulation of cell death in the developing limb. We provide a detailed, but short, overview of the areas of cell death observed in the developing limb, establishing their function in morphogenesis and structural development of limb tissues. We will examine the functions of this process in the formation and growth of the limb primordia, formation of cartilaginous skeleton, formation of synovial joints, and establishment of muscle bellies, tendons, and entheses. We will analyze the plasticity of the cell death program by focusing on the developmental potential of progenitors prior to death. Considering the prolonged plasticity of progenitors to escape from the death process, we will discuss a new biological perspective that explains cell death: this process, rather than secondary to a specific genetic program, is a consequence of the tissue building strategy employed by the embryo based on the formation of scaffolds that disintegrate once their associated neighboring structures differentiate.

We examine the functions of cell death in the formation and growth of the limb primordia.

We analyze the plasticity of the cell death program by focusing on the developmental potential of progenitors prior to death.

Considering the prolonged plasticity of progenitors to escape from the death process and the absence of defined genetic program in their regulation we propose that cell death is a consequence of the tissue building strategy employed by the embryo regulated by epigenetic factors .

Extracellular Matrix and Cellular Plasticity in Musculoskeletal Development

Cellular plasticity refers to the ability of cell fates to be reprogrammed given the proper signals, allowing for dedifferentiation or transdifferentiation into different cell fates. In vitro, this can be induced through direct activation of gene expression, however this process does not naturally occur in vivo. Instead, the microenvironment consisting of the extracellular matrix (ECM) and signaling factors, directs the signals presented to cells. Often the ECM is involved in regulating both biochemical and mechanical signals. In stem cell populations, this niche is necessary for maintenance and proper function of the stem cell pool. However, recent studies have demonstrated that differentiated or lineage restricted cells can exit their current state and transform into another state under different situations during development and regeneration. This may be achieved through (1) cells responding to a changing niche; (2) cells migrating and encountering a new niche; and (3) formation of a transitional niche followed by restoration of the homeostatic niche to sequentially guide cells along the regenerative process. This review focuses on examples in musculoskeletal biology, with the concept of ECM regulating cells and stem cells in development and regeneration, extending beyond the conventional concept of small population of progenitor cells, but under the right circumstances even “lineage-restricted” or differentiated cells can be reprogrammed to enter into a different fate.

Based on a cohort of 52,879 microarrays, recurrent intragenic FBN2 deletion encompassing exons 1-8 does not cause Beals syndrome

INTRODUCTION

Congenital contractural arachnodactyly (CCA) is a rare connective tissue disorder, associated with heterozygous mutations in the FBN2 gene. The objective of this study was to evaluate the prevalence of an intragenic deletion encompassing exons 1-8 of FBN2 gene in Israeli population.

MATERIALS AND METHODS

A search for intragenic FBN2 microdeletions was performed in two databases of chromosomal microarray analysis (CMA) - genetic laboratory of a tertiary medical center (the primary cohort) and one of the largest Israeli health maintenance organizations (replication cohort).

RESULTS

Overall, 52,879 microarray tests were searched for FBN2 microdeletions. The primary cohort constituted of 18,301 CMA tests, among which 33 intragenic FBN2 microdeletions in unrelated individuals were found (0.18%). Prenatal prevalence of this variant was 0.23% (28/12,604), and specifically in low risk pregnancies - 0.29% (22/7464). Of the 28 cases with known parental origin, 27 (96.4%) were of full or partial Ashkenazi Jewish ethnic background. The approximate allele incidence in the Ashkenazi Jewish origin was 0.4% (18/4961). Combined with the 34,578 CMA tests in the replication cohort, the overall frequency of FBN2 microdeletions was 0.24% (125/52,879). None of the pre- or postnatal cases had any clinical manifestations of CCA.

DISCUSSION

Intragenic FBN2 microdeletions are found in one of every 420 CMA analyses in Israeli population, and in particular one of every 340 low-risk pregnancies. Due to high allele incidence in Ashkenazi Jewish population (1:275), we suggest that FBN2 gene deletion detected by CMA among Ashkenazi Jews should be interpreted as benign copy number variant.

The tissues and regulatory pattern of limb chondrogenesis

Initial limb chondrogenesis offers the first differentiated tissues that resemble the mature skeletal anatomy. It is a developmental progression of three tissues. The limb begins with undifferentiated mesenchyme-1, some of which differentiates into condensations-2, and this tissue then transforms into cartilage-3. Each tissue is identified by physical characteristics of cell density, shape, and extracellular matrix composition. Tissue specific regimes of gene regulation underlie the diagnostic physical and chemical properties of these three tissues. These three tissue based regimes co-exist amid a background of other gene regulatory regimes within the same tissues and time-frame of limb development. The bio-molecular indicators of gene regulation reveal six identifiable patterns. Three of these patterns describe the unique bio-molecular indicators of each of the three tissues. A fourth pattern shares bio-molecular indicators between condensation and cartilage. Finally, a fifth pattern is composed of bio-molecular indicators that are found in undifferentiated mesenchyme prior to any condensation differentiation, then these bio-molecular indicators are upregulated in condensations and downregulated in undifferentiated mesenchyme. The undifferentiated mesenchyme that remains in between the condensations and cartilage, the interdigit, contains a unique set of bio-molecular indicators that exhibit dynamic behaviour during chondrogenesis and therefore argue for its own inclusion as a tissue in its own right and for more study into this process of differentiation.

Placensin is a glucogenic hormone secreted by human placenta

FBN1 encodes asprosin, a glucogenic hormone, following furin cleavage of the C-terminus of profibrillin 1. Based on evolutionary conservation between FBN1 and FBN2, together with conserved furin cleavage sites, we identified a peptide hormone placensin encoded by FBN2 based on its high expression in trophoblasts of human placenta. In primary and immortalized murine hepatocytes, placensin stimulates cAMP production, protein kinase A (PKA) activity, and glucose secretion, accompanied by increased expression of gluconeogenesis enzymes. In situ perfusion of liver and in vivo injection with placensin also stimulate glucose secretion. Placensin is secreted by immortalized human trophoblastic HTR-8/SVneo cells, whereas placensin treatment stimulates cAMP-PKA signaling in these cells, accompanied by increases in MMP9 transcripts and activities, thereby promoting cell invasion. In pregnant women, levels of serum placensin increase in a stage-dependent manner. During third trimester, serum placensin levels of patients with gestational diabetes mellitus are increased to a bigger extent compared to healthy pregnant women. Thus, placensin represents a placenta-derived hormone, capable of stimulating glucose secretion and trophoblast invasion.

Role of fibrillin-2 in the control of TGF-β activation in tumor angiogenesis and connective tissue disorders

Fibrillins constitute a family of large extracellular glycoproteins which multimerize to form microfibrils, an important structure in the extracellular matrix. It has long been assumed that fibrillin-2 was barely present during postnatal life, but it is now clear that fibrillin-2 molecules form the structural core of microfibrils, and are masked by an outer layer of fibrillin-1. Mutations in fibrillins give rise to heritable connective tissue disorders, including Marfan syndrome and congenital contractural arachnodactyly. Fibrillins also play an important role in matrix sequestering of members of the transforming growth factor-β family, and in context of Marfan syndrome excessive TGF-β activation has been observed. TGF-β activation is highly dependent on integrin binding, including integrin αvβ8 and αvβ6, which are upregulated upon TGF-β exposure. TGF-β is also involved in tumor progression, metastasis, epithelial-to-mesenchymal transition and tumor angiogenesis. In several highly vascularized types of cancer such as hepatocellular carcinoma, a positive correlation was found between increased TGF-β plasma concentrations and tumor vascularity. Interestingly, fibrillin-1 has a higher affinity to TGF-β and, therefore, has a higher capacity to sequester TGF-β compared to fibrillin-2. The previously reported downregulation of fibrillin-1 in tumor endothelium affects the fibrillin-1/fibrillin-2 ratio in the microfibrils, exposing the normally hidden fibrillin-2. We postulate that fibrillin-2 exposure in the tumor endothelium directly stimulates tumor angiogenesis by influencing TGF-β sequestering by microfibrils, leading to a locally higher active TGF-β concentration in the tumor microenvironment. From a therapeutic perspective, fibrillin-2 might serve as a potential target for future anti-cancer therapies.

Fibrillin-2 is a key mediator of smooth muscle extracellular matrix homeostasis during mouse tracheal tubulogenesis

Epithelial tubes, comprised of polarised epithelial cells around a lumen, are crucial for organ function. However, the molecular mechanisms underlying tube formation remain largely unknown. Here, we report on the function of fibrillin (FBN)2, an extracellular matrix (ECM) glycoprotein, as a critical regulator of tracheal tube formation.We performed a large-scale forward genetic screen in mouse to identify regulators of respiratory organ development and disease. We identified Fbn2 mutants which exhibit shorter and narrowed tracheas as well as defects in tracheal smooth muscle cell alignment and polarity.We found that FBN2 is essential for elastic fibre formation and Fibronectin accumulation around tracheal smooth muscle cells. These processes appear to be regulated at least in part through inhibition of p38-mediated upregulation of matrix metalloproteinases (MMPs), as pharmacological decrease of p38 phosphorylation or MMP activity partially attenuated the Fbn2 mutant tracheal phenotypes. Analysis of human tracheal tissues indicates that a decrease in ECM proteins, including FBN2 and Fibronectin, is associated with tracheomalacia.Our findings provide novel insights into the role of ECM homeostasis in mesenchymal cell polarisation during tracheal tubulogenesis.

Targeted deletion of fibrillin-1 in the mouse eye results in ectopia lentis and other ocular phenotypes associated with Marfan syndrome

Fibrillin is an evolutionarily ancient protein that lends elasticity and resiliency to a variety of tissues. In humans, mutations in fibrillin-1 cause Marfan and related syndromes, conditions in which the eye is often severely affected. To gain insights into the ocular sequelae of Marfan syndrome, we targeted Fbn1 in mouse lens or non-pigmented ciliary epithelium (NPCE). Conditional knockout of Fbn1 in NPCE, but not lens, profoundly affected the ciliary zonule, the system of fibrillin-rich fibers that centers the lens in the eye. The tensile strength of the fibrillin-depleted zonule was reduced substantially, due to a shift toward production of smaller caliber fibers. By 3 months, zonular fibers invariably ruptured and mice developed ectopia lentis, a hallmark of Marfan syndrome. At later stages, untethered lenses lost their polarity and developed cataracts, and the length and volume of mutant eyes increased. This model thus captures key aspects of Marfan-related syndromes, providing insights into the role of fibrillin-1 in eye development and disease.

Summary: Targeted knockout of Fbn1 in the ciliary epithelium of the mouse eye undermines the structural and biomechanical integrity of the ciliary zonule and results in an ectopia lentis phenotype.

The Physical and Biochemical Properties of the Extracellular Matrix Regulate Cell Fate

The extracellular matrix is a complex network of hydrated macromolecular proteins and sugars that, in concert with bound soluble factors, comprise the acellular stromal microenvironment of tissues. Rather than merely providing structural information to cells, the extracellular matrix plays an instructive role in development and is critical for the maintenance of tissue homeostasis. In this chapter, we review the composition of the extracellular matrix and summarize data illustrating its importance in embryogenesis, tissue-specific development, and stem cell differentiation. We discuss how the biophysical and biochemical properties of the extracellular matrix ligate specific transmembrane receptors to activate intracellular signaling that alter cell shape and cytoskeletal dynamics to modulate cell growth and viability, and direct cell migration and cell fate. We present examples describing how the extracellular matrix functions as a highly complex physical and chemical entity that regulates tissue organization and cell behavior through a dynamic and reciprocal dialogue with the cellular constituents of the tissue. We suggest that the extracellular matrix not only transmits cellular and tissue-level force to shape development and tune cellular activities that are key for coordinated tissue behavior, but that it is itself remodeled such that it temporally evolves to maintain the integrated function of the tissue. Accordingly, we argue that perturbations in extracellular matrix composition and structure compromise key developmental events and tissue homeostasis, and promote disease.

Three-dimensional visualization of extracellular matrix networks during murine development

The extracellular matrix (ECM) plays a crucial role in embryogenesis, serving both as a substrate to which cells attach and as an active regulator of cell behavior. However, little is known about the spatiotemporal expression patterns and 3D structure of ECM proteins during embryonic development. The lack of suitable methods to visualize the embryonic ECM is largely responsible for this gap, posing a major technical challenge for biologists and tissue engineers. Here, we describe a method of viewing the 3D organization of the ECM using a polyacrylamide-based hydrogel to provide a 3D framework within developing murine embryos. After removal of soluble proteins using sodium dodecyl sulfate, confocal microscopy was used to visualize the 3D distribution of independent ECM networks in multiple developing tissues, including the forelimb, eye, and spinal cord. Comparative analysis of E12.5 and E14.5 autopods revealed proteoglycan-rich fibrils maintain connections between the epidermis and the underlying tendon and cartilage, indicating a role for the ECM during musculoskeletal assembly and demonstrating that our method can be a powerful tool for defining the spatiotemporal distribution of the ECM during embryogenesis.

Optimizing Genomic Methods for Mapping and Identification of Candidate Variants in ENU Mutagenesis Screens Using Inbred Mice

Positional cloning of ENU-induced mutations has traditionally relied on analysis of polymorphic variation between two strains. In contrast, the application of whole-genome sequencing (WGS) has enabled gene discovery in mutant lines maintained on an inbred genetic background. This approach utilizes genetic variation derived from ENU-induced variants for mapping and reduces the likelihood of phenotypic variation, making it an ideal method for genetic modifier screening. Here, we describe the results of such a screen, wherein we determined the minimal number of mutant genomic DNA samples to include in our analyses and improved the sensitivity of our screen by individually barcoding each genomic DNA library. We present several unique cases to illustrate this approach's efficacy, including the discovery of two distinct mutations that generate essentially identical mutant phenotypes, the ascertainment of a non-ENU-induced candidate variant through homozygosity mapping, and an approach for the identification of putative dominant genetic modifiers.

TGF-β Family Signaling in Connective Tissue and Skeletal Diseases

The transforming growth factor β (TGF-β) family of signaling molecules, which includes TGF-βs, activins, inhibins, and numerous bone morphogenetic proteins (BMPs) and growth and differentiation factors (GDFs), has important functions in all cells and tissues, including soft connective tissues and the skeleton. Specific TGF-β family members play different roles in these tissues, and their activities are often balanced with those of other TGF-β family members and by interactions with other signaling pathways. Perturbations in TGF-β family pathways are associated with numerous human diseases with prominent involvement of the skeletal and cardiovascular systems. This review focuses on the role of this family of signaling molecules in the pathologies of connective tissues that manifest in rare genetic syndromes (e.g., syndromic presentations of thoracic aortic aneurysm), as well as in more common disorders (e.g., osteoarthritis and osteoporosis). Many of these diseases are caused by or result in pathological alterations of the complex relationship between the TGF-β family of signaling mediators and the extracellular matrix in connective tissues.

Exome Sequencing Identified a Novel FBN2 Mutation in a Chinese Family with Congenital Contractural Arachnodactyly

Congenital contractural arachnodactyly (CCA) is an autosomal dominant disorder of connective tissue. CCA is characterized by arachnodactyly, camptodactyly, contrature of major joints, scoliosis, pectus deformities, and crumpled ears. The present study aimed to identify the genetic cause of a three-generation Chinese family with CCA. We successfully identified a novel missense mutation p.G1145D in the fibrillin-2 (FBN2) gene as the pathogenic mutation by whole exome sequencing (WES). The p.G1145D mutation occurs in the 12th calcium-binding epidermal growth factor-like (cbEGF) domain. The p.G1145D mutation caused a hydrophobic to hydrophilic substitution, altering the amino acid property from neutral to acidic. Three-dimensional structural analysis showed that this mutation could alter the conformation of the residue side chain, thereby producing steric clashes with spatially adjacent residues, disrupting the formation of H bonds and causing folding destabilization. Therefore, this amino acid appears to play an important role in the structure and function of FBN2. Our results may also provide new insights into the cause and diagnosis of CCA and may have implications for genetic counseling and clinical management.

Whole exome sequencing identifies a novel missense FBN2 mutation co-segregating in a four-generation Chinese family with congenital contractural arachnodactyly

BACKGROUND

Congenital contractural arachnodactyly (CCA) is an autosomal dominant rare genetic disease, estimated to be less than 1 in 10,000 worldwide. People with this condition often have permanently bent joints (contractures), like bent fingers and toes (camptodactyly).

CASE PRESENTATION

In this study, we investigated the genetic aetiology of CCA in a four-generation Chinese family. The blood samples were collected from 22 living members of the family in the Yangquan County, Shanxi Province, China. Of those, eight individuals across 3 generations have CCA. Whole exome sequencing (WES) identified a missense mutation involving a T-to-G transition at position 3229 (c.3229 T > G) in exon 25 of the FBN2 gene, resulting in a Cys 1077 to Gly change (p.C1077G). This previously unreported mutation was found in all 8 affected individuals, but absent in 14 unaffected family members. SIFT/PolyPhen prediction and protein conservation analysis suggest that this novel mutation is pathogenic. Our study extended causative mutation spectrum of FBN2 gene in CCA patients.

CONCLUSIONS

This study has identified a novel missense mutation in FBN2 gene (p.C1077G) resulting in CCA in a family of China.

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.

Regulation of the Bioavailability of TGF-β and TGF-β-Related Proteins

The bioavailability of members of the transforming growth factor β (TGF-β) family is controlled by a number of mechanisms. Bona fide TGF-β is sequestered into the matrix in a latent state and must be activated before it can bind to its receptors. Here, we review the molecules and mechanisms that regulate the bioavailability of TGF-β and compare these mechanisms with those used to regulate other TGF-β family members. We also assess the physiological significance of various latent TGF-β activators, as well as other extracellular modulators of TGF-β family signaling, by examining the available in vivo data from knockout mouse models and other biological systems.

Regulation of the Bioavailability of TGF-β and TGF-β-Related Proteins

The bioavailability of members of the transforming growth factor β (TGF-β) family is controlled by a number of mechanisms. Bona fide TGF-β is sequestered into the matrix in a latent state and must be activated before it can bind to its receptors. Here, we review the molecules and mechanisms that regulate the bioavailability of TGF-β and compare these mechanisms with those used to regulate other TGF-β family members. We also assess the physiological significance of various latent TGF-β activators, as well as other extracellular modulators of TGF-β family signaling, by examining the available in vivo data from knockout mouse models and other biological systems.

Extracellular Regulation of Bone Morphogenetic Protein Activity by the Microfibril Component Fibrillin-1

Since the discovery of bone morphogenetic proteins (BMPs) as pluripotent cytokines extractable from bone matrix, it has been speculated how targeting of BMPs to the extracellular matrix (ECM) modulates their bioavailability. Understanding these processes is crucial for elucidating pathomechanisms of connective tissue disorders characterized by ECM deficiency and growth factor dysregulation. Here, we provide evidence for a new BMP targeting and sequestration mechanism that is controlled by the ECM molecule fibrillin-1. We present the nanoscale structure of the BMP-7 prodomain-growth factor complex using electron microscopy, small angle x-ray scattering, and circular dichroism spectroscopy, showing that it assumes an open V-like structure when it is bioactive. However, upon binding to fibrillin-1, the BMP-7 complex is rendered into a closed ring shape, which also confers latency to the growth factor, as demonstrated by bioactivity measurements. BMP-7 prodomain variants were used to map the critical epitopes for prodomain-growth factor and prodomain-prodomain binding. Together, these data show that upon prodomain binding to fibrillin-1, the BMP-7 complex undergoes a conformational change, which denies access of BMP receptors to the growth factor.

Forelimb contractures and abnormal tendon collagen fibrillogenesis in fibulin-4 null mice

Fibulin-4 is an extracellular matrix glycoprotein essential for elastic fiber formation. Mice deficient in fibulin-4 die perinatally because of severe pulmonary and vascular defects associated with the lack of intact elastic fibers. Patients with fibulin-4 mutations demonstrate similar defects, and a significant number die shortly after birth or in early childhood from cardiopulmonary failure. The patients also demonstrate skeletal and other systemic connective tissue abnormalities, including joint laxity and flexion contractures of the wrist. A fibulin-4 null mouse strain was generated and used to analyze the roles of fibulin-4 in tendon fibrillogenesis. This mouse model displayed bilateral forelimb contractures, in addition to pulmonary and cardiovascular defects. The forelimb and hindlimb tendons exhibited disruption in collagen fibrillogenesis in the absence of fibulin-4 as analyzed by transmission electron microscopy. Fewer fibrils were assembled, and fibrils were disorganized compared with wild-type controls. The organization of developing tenocytes and compartmentalization of the extracellular space was also disrupted. Fibulin-4 was co-localized with fibrillin-1 and fibrillin-2 in limb tendons by using immunofluorescence microscopy. Thus, fibulin-4 seems to play a role in regulating tendon collagen fibrillogenesis, in addition to its essential function in elastogenesis.

Fibrillin-containing microfibrils are key signal relay stations for cell function

Fibrillins constitute the backbone of microfibrils in the extracellular matrix of elastic and non-elastic tissues. Mutations in fibrillins are associated with a wide range of connective tissue disorders, the most common is Marfan syndrome. Microfibrils are on one hand important for structural stability in some tissues. On the other hand, microfibrils are increasingly recognized as critical mediators and drivers of cellular signaling. This review focuses on the signaling mechanisms initiated by fibrillins and microfibrils, which are often dysregulated in fibrillin-associated disorders. Fibrillins regulate the storage and bioavailability of growth factors of the TGF-β superfamily. Cells sense microfibrils through integrins and other receptors. Fibrillins potently regulate pathways of the immune response, inflammation and tissue homeostasis. Emerging evidence show the involvement of microRNAs in disorders caused by fibrillin deficiency. A thorough understanding of fibrillin-mediated cell signaling pathways will provide important new leads for therapeutic approaches of the underlying disorders.

Fibrillin microfibrils in bone physiology

The severe skeletal abnormalities associated with Marfan syndrome (MFS) and congenital contractural arachnodactyly (CCA) underscore the notion that fibrillin assemblies (microfibrils and elastic fibers) play a critical role in bone formation and function in spite of representing a low abundance component of skeletal matrices. Studies of MFS and CCA mice have correlated the skeletal phenotypes of these mutant animals with distinct pathophysiological mechanisms that reflect the contextual contribution of fibrillin-1 and -2 scaffolds to TGFβ and BMP signaling during bone patterning, growth and metabolism. Illustrative examples include the unique role of fibrillin-2 in regulating BMP-dependent limb patterning and the distinct impact of the two fibrillin proteins on the commitment and differentiation of marrow mesenchymal stem cells. Collectively, these findings have important implication for our understanding of the pathophysiological mechanisms that drive age- and injury-related processes of bone degeneration.

The notochord: structure and functions

The notochord is an embryonic midline structure common to all members of the phylum Chordata, providing both mechanical and signaling cues to the developing embryo. In vertebrates, the notochord arises from the dorsal organizer and it is critical for proper vertebrate development. This evolutionary conserved structure located at the developing midline defines the primitive axis of embryos and represents the structural element essential for locomotion. Besides its primary structural function, the notochord is also a source of developmental signals that patterns surrounding tissues. Among the signals secreted by the notochord, Hedgehog proteins play key roles during embryogenesis. The Hedgehog signaling pathway is a central regulator of embryonic development, controlling the patterning and proliferation of a wide variety of organs. In this review, we summarize the current knowledge on notochord structure and functions, with a particular emphasis on the key developmental events that take place in vertebrates. Moreover, we discuss some genetic studies highlighting the phenotypic consequences of impaired notochord development, which enabled to understand the molecular basis of different human congenital defects and diseases.

Abnormal Activation of BMP Signaling Causes Myopathy in Fbn2 Null Mice

Fibrillins are large extracellular macromolecules that polymerize to form the backbone structure of connective tissue microfibrils. Mutations in the gene for fibrillin-1 cause the Marfan syndrome, while mutations in the gene for fibrillin-2 cause Congenital Contractural Arachnodactyly. Both are autosomal dominant disorders, and both disorders affect musculoskeletal tissues. Here we show that Fbn2 null mice (on a 129/Sv background) are born with reduced muscle mass, abnormal muscle histology, and signs of activated BMP signaling in skeletal muscle. A delay in Myosin Heavy Chain 8, a perinatal myosin, was found in Fbn2 null forelimb muscle tissue, consistent with the notion that muscle defects underlie forelimb contractures in these mice. In addition, white fat accumulated in the forelimbs during the early postnatal period. Adult Fbn2 null mice are already known to demonstrate persistent muscle weakness. Here we measured elevated creatine kinase levels in adult Fbn2 null mice, indicating ongoing cycles of muscle injury. On a C57Bl/6 background, Fbn2 null mice showed severe defects in musculature, leading to neonatal death from respiratory failure. These new findings demonstrate that loss of fibrillin-2 results in phenotypes similar to those found in congenital muscular dystrophies and that FBN2 should be considered as a candidate gene for recessive congenital muscular dystrophy. Both in vivo and in vitro evidence associated muscle abnormalities and accumulation of white fat in Fbn2 null mice with abnormally activated BMP signaling. Genetic rescue of reduced muscle mass and accumulation of white fat in Fbn2 null mice was accomplished by deleting a single allele of Bmp7. In contrast to other reports that activated BMP signaling leads to muscle hypertrophy, our findings demonstrate the exquisite sensitivity of BMP signaling to the fibrillin-2 extracellular environment during early postnatal muscle development. New evidence presented here suggests that fibrillin-2 can sequester BMP complexes in a latent state.

New strategies for treating congenital muscular dystrophies are needed. Current treatments are limited and aim to prolong ambulation and survival. Since most of the genes responsible for congenital muscular dystrophies are still unknown, elucidation of these genes may provide new insights that can lead to novel treatments. Fibrillin-2 null mice are born with myopathy and contractures and demonstrate accumulation of white fat during the early postnatal period. Both the histological features of myopathy and the accumulation of fat are rescued by inhibiting BMP signaling. Results indicate that FBN2 is a candidate gene for congenital muscular dystrophy and that strategies aimed at inhibition of abnormal BMP signaling may be applicable to muscular dystrophies. Furthermore, results reveal the importance of extracellular control of BMP signaling in skeletal muscle.

ADAMTS proteins as modulators of microfibril formation and function

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

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

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

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

Simultaneous rather than ordered cleavage of two sites within the BMP4 prodomain leads to loss of ligand in mice

ProBMP4 is generated as a latent precursor that is sequentially cleaved at two sites within the prodomain to generate an active ligand. An initial cleavage occurs adjacent to the ligand domain, which generates a non-covalently associated prodomain/ligand complex that is subsequently dissociated by cleavage at an upstream site. An outstanding question is whether the two sites need to be cleaved sequentially and in the correct order to achieve proper control of BMP4 signaling during development. In the current studies, we demonstrate that mice carrying a knock-in point mutation that causes simultaneous rather than sequential cleavage of both prodomain sites show loss of BMP4 function and die during mid-embryogenesis. Levels of mature BMP4 are severely reduced in mutants, although levels of precursor and cleaved prodomain are unchanged compared with wild type. Our biochemical analysis supports a model in which the transient prodomain/ligand complex that forms during sequential cleavage plays an essential role in prodomain-mediated stabilization of the mature ligand until it can acquire protection from degradation by other means. By contrast, simultaneous cleavage causes premature release of the ligand from the prodomain, leading to destabilization of the ligand and loss of signaling in vivo.

Fibrillin-1 directly regulates osteoclast formation and function by a dual mechanism

Mutations in the fibrillin-1 gene give rise to a number of heritable disorders, which are all characterized by various malformations of bone as well as manifestations in other tissues. However, the role of fibrillin-1 in the development and homeostasis of bone is not well understood. Here, we examined the role of fibrillin-1 in regulating osteoclast differentiation from primary bone-marrow-derived precursors and monocytic RAW 264.7 cells. The soluble N-terminal half of fibrillin-1 (rFBN1-N) strongly inhibited osteoclastogenesis, whereas the C-terminal half (rFBN1-C) did not. By contrast, when rFBN1-N was immobilized on calcium phosphate, it did not affect osteoclastogenesis but modulated osteoclast resorptive activity, which was evident by a larger number of smaller resorption pits. Using a panel of recombinant sub-fragments spanning rFBN1-N, we localized an osteoclast inhibitory activity to the 63 kDa subfragment rF23 comprising the N-terminal region of fibrillin-1. Osteoclastic resorption led to the generation of small fibrillin-1 fragments that were similar to those identified in human vertebral bone extracts. rF23, but not rFBN1-N, was found to inhibit the expression of cathepsin K, matrix metalloproteinase 9 and Dcstamp in differentiating osteoclasts. rFBN1-N, but not rF23, exhibited interaction with RANKL. Excess RANKL rescued the inhibition of osteoclastogenesis by rFBN1-N. By contrast, rF23 disrupted RANKL-induced Ca(2+) signaling and activation of transcription factor NFATc1. These studies highlight a direct dual inhibitory role of N-terminal fibrillin-1 fragments in osteoclastogenesis, the sequestration of RANKL and the inhibition of NFATc1 signaling, demonstrating that osteoclastic degradation of fibrillin-1 provides a potent negative feedback that limits osteoclast formation and function.