Characterization of an associated microfibril protein through recombinant DNA techniques

Blocking Osa-miR1871 enhances rice resistance against Magnaporthe oryzae and yield

MicroRNAs (miRNAs) play vital roles in plant development and defence responses against various stresses. Here, we show that blocking miR1871 improves rice resistance against Magnaporthe oryzae and enhances grain yield simultaneously. The transgenic lines overexpressing miR1871 (OX1871) exhibit compromised resistance, suppressed defence responses and reduced panicle number resulting in slightly decreased yield. In contrast, the transgenic lines blocking miR1871 (MIM1871) show improved resistance, enhanced defence responses and significantly increased panicle number leading to enhanced yield per plant. The RNA-seq assay and defence response assays reveal that blocking miR1871 resulted in the enhancement of PAMP-triggered immunity (PTI). Intriguingly, miR1871 suppresses the expression of LOC_Os06g22850, which encodes a microfibrillar-associated protein (MFAP1) locating nearby the cell wall and positively regulating PTI responses. The mutants of MFAP1 resemble the phenotype of OX1871. Conversely, the transgenic lines overexpressing MFAP1 (OXMFAP1) or overexpressing both MFAP1 and miR1871 (OXMFAP1/OX1871) resemble the resistance of MIM1871. The time-course experiment data reveal that the expression of miR1871 and MFAP1 in rice leaves, panicles and basal internode is dynamic during the whole growth period to manipulate the resistance and yield traits. Our results suggest that miR1871 regulates rice yield and immunity via MFAP1, and the miR8171-MFAP1 module could be used in rice breeding to improve both immunity and yield.

Depletion of the MFAP1/SPP381 Splicing Factor Causes R-Loop-Independent Genome Instability

THO/TREX is a conserved complex with a role in messenger ribonucleoprotein biogenesis that links gene expression and genome instability. Here, we show that human THO interacts with MFAP1 (microfibrillar-associated protein 1), a spliceosome-associated factor. Interestingly, MFAP1 depletion impairs cell proliferation and genome integrity, increasing γH2AX foci and DNA breaks. This phenotype is not dependent on either transcription or RNA-DNA hybrids. Mutations in the yeast orthologous gene SPP381 cause similar transcription-independent genome instability, supporting a conserved role. MFAP1 depletion has a wide effect on splicing and gene expression in human cells, determined by transcriptome analyses. MFAP1 depletion affects a number of DNA damage response (DDR) genes, which supports an indirect role of MFAP1 on genome integrity. Our work defines a functional interaction between THO and RNA processing and argues that splicing factors may contribute to genome integrity indirectly by regulating the expression of DDR genes rather than by a direct role.

Microfibril-associated glycoproteins MAGP-1 and MAGP-2 in disease

Microfibril-associated glycoproteins 1 and 2 (MAGP-1, MAGP-2) are protein components of extracellular matrix microfibrils. These proteins interact with fibrillin, the core component of microfibrils, and impart unique biological properties that influence microfibril function in vertebrates. MAGPs bind active forms of TGFβ and BMPs and are capable of modulating Notch signaling. Mutations in MAGP-1 or MAGP-2 have been linked to thoracic aneurysms and metabolic disease in humans. MAGP-2 has also been shown to be an important biomarker in several human cancers. Mice lacking MAGP-1 or MAGP-2 have defects in multiple organ systems, which reflects the widespread distribution of microfibrils in vertebrate tissues. This review summarizes our current understanding of the function of the MAGPs and their relationship to human disease.

Human MFAP1 is a cryptic ortholog of the Saccharomyces cerevisiae Spp381 splicing factor

Background

Pre-mRNA splicing involves the stepwise assembly of a pre-catalytic spliceosome, followed by its catalytic activation, splicing catalysis and disassembly. Formation of the pre-catalytic spliceosomal B complex involves the incorporation of the U4/U6.U5 tri-snRNP and of a group of non-snRNP B-specific proteins. While in Saccharomyces cerevisiae the Prp38 and Snu23 proteins are recruited as components of the tri-snRNP, metazoan orthologs of Prp38 and Snu23 associate independently of the tri-snRNP as members of the B-specific proteins. The human spliceosome contains about 80 proteins that lack obvious orthologs in yeast, including most of the B-specific proteins apart from Prp38 and Snu23. Conversely, the tri-snRNP protein Spp381 is one of only five S. cerevisiae splicing factors without a known human ortholog.

Results

Using InParanoid, a state-of-the-art method for ortholog inference between pairs of species, and systematic BLAST searches we identified the human B-specific protein MFAP1 as a putative ortholog of the S. cerevisiae tri-snRNP protein Spp381. Bioinformatics revealed that MFAP1 and Spp381 share characteristic structural features, including intrinsic disorder, an elongated shape, solvent exposure of most residues and a trend to adopt α-helical structures. In vitro binding studies showed that human MFAP1 and yeast Spp381 bind their respective Prp38 proteins via equivalent interfaces and that they cross-interact with the Prp38 proteins of the respective other species. Furthermore, MFAP1 and Spp381 both form higher-order complexes that additionally include Snu23, suggesting that they are parts of equivalent spliceosomal sub-complexes. Finally, similar to yeast Spp381, human MFAP1 partially rescued a growth defect of the temperature-sensitive mutant yeast strain prp38-1.

Conclusions

Human B-specific protein MFAP1 structurally and functionally resembles the yeast tri-snRNP-specific protein Spp381 and thus qualifies as its so far missing ortholog. Our study indicates that the yeast Snu23-Prp38-Spp381 triple complex was evolutionarily reprogrammed from a tri-snRNP-specific module in yeast to the B-specific Snu23-Prp38-MFAP1 module in metazoa, affording higher flexibility in spliceosome assembly and thus, presumably, in splicing regulation.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-017-0923-1) contains supplementary material, which is available to authorized users.

The microfibril-associated glycoproteins (MAGPs) and the microfibrillar niche

The microfibril-associated glycoproteins MAGP-1 and MAGP-2 are extracellular matrix proteins that interact with fibrillin to influence microfibril function. The two proteins are related through a 60 amino acid matrix-binding domain but their sequences differ outside of this region. A distinguishing feature of both proteins is their ability to interact with TGFβ family growth factors, Notch and Notch ligands, and multiple elastic fiber proteins. MAGP-2 can also interact with αvβ3 integrins via a RGD sequence that is not found in MAGP-1. Morpholino knockdown of MAGP-1 expression in zebrafish resulted in abnormal vessel wall architecture and altered vascular network formation. In the mouse, MAGP-1 deficiency had little effect on elastic fibers in blood vessels and lung but resulted in numerous unexpected phenotypes including bone abnormalities, hematopoietic changes, increased fat deposition, diabetes, impaired wound repair, and a bleeding diathesis. Inactivation of the gene for MAGP-2 in mice produced a neutropenia yet had minimal effects on bone or adipose homeostasis. Double knockouts had phenotypes characteristic of each individual knockout as well as several additional traits only seen when both genes are inactivated. A common mechanism underlying all of the traits associated with the knockout phenotypes is altered TGFβ signaling. This review summarizes our current understanding of the function of the MAGPs and discusses ideas related to their role in growth factor regulation.

New tricks for "old" domains: how novel architectures and promiscuous hubs contributed to the organization and evolution of the ECM

The extracellular matrix (ECM) is a defining characteristic of metazoans and consists of a meshwork of self-assembling, fibrous proteins, and their functionally related neighbours. Previous studies, focusing on a limited number of gene families, suggest that vertebrate complexity predominantly arose through the duplication and subsequent modification of retained, preexisting ECM genes. These genes provided the structural underpinnings to support a variety of specialized tissues, as well as a platform for the organization of spatio-temporal signaling and cell migration. However, the relative contributions of ancient versus novel domains to ECM evolution have not been quantified across the full range of ECM proteins. Here, utilizing a high quality list comprising 324 ECM genes, we reveal general and clade-specific domain combinations, identifying domains of eukaryotic and metazoan origin recruited into new roles in approximately two-third of the ECM proteins in humans representing novel vertebrate proteins. We show that, rather than acquiring new domains, sampling of new domain combinations has been key to the innovation of paralogous ECM genes during vertebrate evolution. Applying a novel framework for identifying potentially important, noncontiguous, conserved arrangements of domains, we find that the distinct biological characteristics of the ECM have arisen through unique evolutionary processes. These include the preferential recruitment of novel domains to existing architectures and the utilization of high promiscuity domains in organizing the ECM network around a connected array of structural hubs. Our focus on ECM proteins reveals that distinct types of proteins and/or the biological systems in which they operate have influenced the types of evolutionary forces that drive protein innovation. This emphasizes the need for rigorously defined systems to address questions of evolution that focus on specific systems of interacting proteins.

Advances in biomimetic regeneration of elastic matrix structures

Elastin is a vital component of the extracellular matrix, providing soft connective tissues with the property of elastic recoil following deformation and regulating the cellular response via biomechanical transduction to maintain tissue homeostasis. The limited ability of most adult cells to synthesize elastin precursors and assemble them into mature crosslinked structures has hindered the development of functional tissue-engineered constructs that exhibit the structure and biomechanics of normal native elastic tissues in the body. In diseased tissues, the chronic overexpression of proteolytic enzymes can cause significant matrix degradation, to further limit the accumulation and quality (e.g., fiber formation) of newly deposited elastic matrix. This review provides an overview of the role and importance of elastin and elastic matrix in soft tissues, the challenges to elastic matrix generation in vitro and to regenerative elastic matrix repair in vivo, current biomolecular strategies to enhance elastin deposition and matrix assembly, and the need to concurrently inhibit proteolytic matrix disruption for improving the quantity and quality of elastogenesis. The review further presents biomaterial-based options using scaffolds and nanocarriers for spatio-temporal control over the presentation and release of these biomolecules, to enable biomimetic assembly of clinically relevant native elastic matrix-like superstructures. Finally, this review provides an overview of recent advances and prospects for the application of these strategies to regenerating tissue-type specific elastic matrix structures and superstructures.

The Caenorhabditis elegans gene mfap-1 encodes a nuclear protein that affects alternative splicing

RNA splicing is a major regulatory mechanism for controlling eukaryotic gene expression. By generating various splice isoforms from a single pre–mRNA, alternative splicing plays a key role in promoting the evolving complexity of metazoans. Numerous splicing factors have been identified. However, the in vivo functions of many splicing factors remain to be understood. In vivo studies are essential for understanding the molecular mechanisms of RNA splicing and the biology of numerous RNA splicing-related diseases. We previously isolated a Caenorhabditis elegans mutant defective in an essential gene from a genetic screen for suppressors of the rubberband Unc phenotype of unc-93(e1500) animals. This mutant contains missense mutations in two adjacent codons of the C. elegans microfibrillar-associated protein 1 gene mfap-1. mfap-1(n4564 n5214) suppresses the Unc phenotypes of different rubberband Unc mutants in a pattern similar to that of mutations in the splicing factor genes uaf-1 (the C. elegans U2AF large subunit gene) and sfa-1 (the C. elegans SF1/BBP gene). We used the endogenous gene tos-1 as a reporter for splicing and detected increased intron 1 retention and exon 3 skipping of tos-1 transcripts in mfap-1(n4564 n5214) animals. Using a yeast two-hybrid screen, we isolated splicing factors as potential MFAP-1 interactors. Our studies indicate that C. elegans mfap-1 encodes a splicing factor that can affect alternative splicing.

RNA splicing removes intervening intronic sequences from pre–mRNA transcripts and joins adjacent exonic sequences to generate functional messenger RNAs. The in vivo functions of numerous factors that regulate splicing remain to be understood. From a genetic screen for suppressors of the rubberband Unc phenotype caused by the Caenorhabditis elegans unc-93(e1500) mutation, we isolated a mutation that affects a highly conserved essential gene, mfap-1. MFAP-1 is a nuclear protein that is broadly expressed. MFAP-1 can affect the alternative splicing of tos-1, an endogenous reporter gene for splicing, and is required for the altered splicing at a cryptic 3′ splice site of tos-1. mfap-1 enhances the effects of the gene uaf-1 (splicing factor U2AF large subunit) in suppressing the rubberband Unc phenotype of unc-93(e1500) animals. Our studies provide in vivo evidence that MFAP-1 functions as a splicing factor.

Vascular extracellular matrix and arterial mechanics

An important factor in the transition from an open to a closed circulatory system was a change in vessel wall structure and composition that enabled the large arteries to store and release energy during the cardiac cycle. The component of the arterial wall in vertebrates that accounts for these properties is the elastic fiber network organized by medial smooth muscle. Beginning with the onset of pulsatile blood flow in the developing aorta, smooth muscle cells in the vessel wall produce a complex extracellular matrix (ECM) that will ultimately define the mechanical properties that are critical for proper function of the adult vascular system. This review discusses the structural ECM proteins in the vertebrate aortic wall and will explore how the choice of ECM components has changed through evolution as the cardiovascular system became more advanced and pulse pressure increased. By correlating vessel mechanics with physiological blood pressure across animal species and in mice with altered vessel compliance, we show that cardiac and vascular development are physiologically coupled, and we provide evidence for a universal elastic modulus that controls the parameters of ECM deposition in vessel wall development. We also discuss mechanical models that can be used to design better tissue-engineered vessels and to test the efficacy of clinical treatments.

Drosophila MFAP1 is required for pre-mRNA processing and G2/M progression

The mammalian spliceosome has mainly been studied using proteomics. The isolation and comparison of different splicing intermediates has revealed the dynamic association of more than 200 splicing factors with the spliceosome, relatively few of which have been studied in detail. Here, we report the characterization of the Drosophila homologue of microfibril-associated protein 1 (dMFAP1), a previously uncharacterized protein found in some human spliceosomal fractions ( Jurica, M. S., and Moore, M. J. (2003) Mol. Cell 12, 5-14 ). We show that dMFAP1 binds directly to the Drosophila homologue of Prp38p (dPrp38), a tri-small nuclear ribonucleoprotein component ( Xie, J., Beickman, K., Otte, E., and Rymond, B. C. (1998) EMBO J. 17, 2938-2946 ), and is required for pre-mRNA processing. dMFAP1, like dPrp38, is essential for viability, and our in vivo data show that cells with reduced levels of dMFAP1 or dPrp38 proliferate more slowly than normal cells and undergo apoptosis. Consistent with this, double-stranded RNA-mediated depletion of dPrp38 or dMFAP1 causes cells to arrest in G(2)/M, and this is paralleled by a reduction in mRNA levels of the mitotic phosphatase string/cdc25. Interestingly double-stranded RNA-mediated depletion of a wide range of core splicing factors elicits a similar phenotype, suggesting that the observed G(2)/M arrest might be a general consequence of interfering with spliceosome function.

The 67 kDa spliced variant of beta-galactosidase serves as a reusable protective chaperone for tropoelastin

Numerous cell types express the 67 kDa galactolectin related to the alternatively spliced variant of beta-galactosidase. This 67 kDa protein, while present on cell surfaces, mediates cell contacts with elastin, laminin and collagen type IV. In elastin-producing tissues, the 67 kDa protein also co-localizes with intracellular tropoelastin and mature elastic fibres. We have established that this elastin binding protein (EBP) serves as a molecular chaperone for tropoelastin. The EBP binds this highly hydrophobic and unglycosylated ligand intracellularly, protecting it from intracellular self aggregation and premature proteolytic degradation, and mediates its orderly assembly upon the microfibrillar scaffold. While some of this protein is incorporated as a permanent component of elastic fibres, most of the EBP, after extracellular dissociation from its ligand, recycles back to the intracellular endosomal compartment and re-associates with the newly synthesized tropoelastin. We suggest that recycling of this reusable shuttle protein is imperative for the effective extracellular deposition of insoluble elastin.

The structure and function of fibrillin

Fibrillin is a very large molecule whose primary structure is now known from the cloning and sequencing of 10 kb of cDNA. Immunohistochemical results suggest that one of the functions of fibrillin molecules is to contribute to the structure of the microfibril. The importance of fibrillin as a structural macromolecule has been demonstrated by the identification of the gene for fibrillin (FBN1) as the disease-causing gene in Marfan's syndrome. While it is clear that fibrillin contributes to the structure of the microfibril, it is not known whether fibrillin molecules self-assemble or whether fibrillin interacts with other molecules in order to form microfibrils. In order to investigate whether particular domains of fibrillin are important to the assembly of the microfibril and to specify domains that participate in interactions with other proteins, we have produced recombinant fibrillin 1 peptides in human cells and used them in studies described here. Additionally, new information regarding the 5' end of FBN1 has been obtained from studies investigating promoter activity, and potential proteolytic cleavage sites have been identified in the N- and C-terminal domains.

The Extracellular Matrix Protein MAGP-2 Interacts with Jagged1 and Induces Its Shedding from the Cell Surface

Elastic fibers are composed of the protein elastin and a network of 10-12-nm microfibrils, which are composed of several glycoproteins, including fibrillin-1, fibrillin-2, and MAGP1/2 (microfibril-associated glycoproteins-1 and -2). Although fibrillins and MAGPs covalently associate, we find that the DSL (Delta/Serrate/LAG2) protein Jagged1, an activating ligand for Notch receptor signaling, also interacts with MAGP-2 in both yeast two-hybrid and coimmunoprecipitation studies. Interaction between Jagged1 and MAGP-2 requires the epidermal growth factor-like repeats of Jagged1. MAGP-2 was found complexed with the Jagged1 extracellular domain shed from 293T cells and COS-7 cells coexpressing full-length Jagged1 and MAGP-2. MAGP-2 shedding of the Jagged1 extracellular domain was decreased by the metalloproteinase hydroxamate inhibitor BB3103 implicating proteolysis in its release. Although MAGP-2 also interacted with the other DSL ligands, Jagged2 and Delta1, they were not found associated with MAGP-2 in the conditioned media, identifying differential effects of MAGP-2 on DSL ligand shedding. The related microfibrillar protein MAGP-1 was also found to interact with DSL ligands but, unlike MAGP-2, was unable to facilitate the shedding of Jagged1. Our findings suggest that in addition to its role in microfibrils, MAGP-2 may also affect cellular differentiation through modulating the Notch signaling pathway either by binding to cell surface DSL ligands or by facilitating release and/or stabilization of a soluble extracellular form of Jagged1.

Fibrillin microfibrils

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

Codistribution analysis of elastin and related fibrillar proteins in early vertebrate development

Elastin is an extracellular matrix protein found in adult and neonatal vasculature, lung, skin and connective tissue. It is secreted as tropoelastin, a soluble protein that is cross-linked in the tissue space to form an insoluble elastin matrix. Cross-linked elastin can be found in association with several microfibril-associated proteins including fibrillin-1, fibrillin-2 and fibulin-1 suggesting that these proteins contribute to elastic fiber assembly, structure or function. To date, the earliest reported elastin expression was in the conotruncal region of the developing avian heart at 3.5 days of gestation. Here we report that elastin expression begins at significantly earlier developmental stages. Using a novel immunolabeling method, the deposition of elastin, fibrillin-1 and -2 and fibulin-1 was analyzed in avian embryos at several time points during the first 2 days of development. Elastin was found at the midline associated with axial structures such as the notochord and somites at 23 h of development. Fibrillin-1 and -2 and fibulin-1 were also expressed at the embryonic midline at this stage with fibrillin-1 and fibulin-1 showing a high degree of colocalization with elastin in fibers surrounding midline structures. The expression of these genes was confirmed by conventional immunoblotting and mRNA detection methods. Our results demonstrate that elastin polypeptide deposition occurs much earlier than was previously appreciated. Furthermore, the results suggest that elastin deposition at the early embryonic midline is accompanied by the deposition and organization of a number of extracellular matrix polypeptides. These filamentous extracellular matrix structures may act to transduce or otherwise stabilize dynamic forces generated during embryogenesis.

Ultrastructural properties of ciliary zonule microfibrils

Conventional electron microscopy and rotary shadowing techniques have provided conflicting interpretations of microfibril ultrastructure. To address this issue, we have used quick-freeze deep-etch (QFDE) microscopy to obtain 3-dimensional surface views of microfibrils that have not been fixed, dehydrated, or stained with heavy metals. By this approach, microfibrils appear as tightly packed rows of bead-like subunits that do not display the interbead filamentous links seen by other methods. At regular 50-nm intervals along the microfibril length, a larger bead is often recognized which tends to be aligned with those from adjacent microfibrils when the microfibrils are in bundles. This evidence of organized lateral associations of microfibrils is supported by the observation of small filaments that span between the adjacent microfibrils. When QFDE microscopy was used to examine microfibrils exposed to sonication, partially dissociated microfibrils with the more typical "beads on a string" appearance were observed. Beads are also seen alone, as monomers, often with an array of small thread-like filaments extending from the bead in a "crab-like" manner. Our results suggest that the beads on a string appearance of sonicated microfibrils may result from a partial loss of protein components from the interbead domains, thus leading to exposure of a filamentous substructure. It is possible, therefore, that this phenomenon might also contribute to the beads on a string appearance of microfibrils seen using other electron microscopy techniques.

Fibrillin-2 defects impair elastic fiber assembly in a homocysteinemic chick model

Homocysteinemia in humans is associated with vascular complications that increase the risk for atherosclerosis and stroke. Animal studies have shown that the disease is multifactorial and includes lesions associated with the elastin component of the extracellular matrix. In the following experiments we have used the aortas from rapidly growing chicks to assess the cause of the elastin defects resulting from homocysteinemia. Day-old chicks were fed diets containing varying amounts of DL-methionine, DL-homocysteine, homocysteine thiolactone or DL-cysteine for periods up to 9 wk. Three weeks after feeding 2% DL-methionine the plasma methionine was elevated > 20-fold, whereas plasma homocysteine was more than 3-fold normal plasma values. The aortas showed severe histopathology, evidenced by the pronounced separation of elastic lamellae with marked smooth muscle proliferation and, in some instances, aneurysms. There was no evidence of decreased desmosine content or a significant reduction in lysyl oxidase in the aortas from the treated groups compared to those from controls. Increasing other dietary factors such as the vitamins required for methionine metabolism had no effect on the development of the vascular lesions. Twenty to 30% of the chicks fed the high methionine diets exhibited severe neurological problems, expressed as tonic contractions or seizures. Electron microscopy revealed disordered aortic elastic fibrils, associated with either an absence of or disrupted assembly of microfibrils. Immunohistochemical studies demonstrated a loss of fibrillin-2 immunoreactivity in the aortas of chicks fed 2% methionine. The studies suggest that elevated plasma methionine or its metabolites disrupt normal microfibril configuration, leading to the assembly of aberrant elastic fibers.

Elastic fibres

Elastic fibres are essential extracellular matrix macromolecules comprising an elastin core surrounded by a mantle of fibrillin-rich microfibrils. They endow connective tissues such as blood vessels, lungs and skin with the critical properties of elasticity and resilience. The biology of elastic fibres is complex because they have multiple components, a tightly regulated developmental deposition, a multi-step hierarchical assembly and unique biomechanical functions. However, their molecular complexity is at last being unravelled by progress in identifying interactions between component molecules, ultrastructural analyses and studies of informative mouse models.

Versican interacts with fibrillin-1 and links extracellular microfibrils to other connective tissue networks

Fibrillin-containing microfibrils are polymeric structures that are difficult to extract from connective tissues. Proteolytic digestion of tissues has been utilized to release microfibrils for study. Few of the molecules that connect microfibrils to other elements in the matrix have been identified. In this study, electron microscopic immunolocalization of anti-versican antibodies in tissues and in extracted microfibrils demonstrated that the C-terminal region of versican is found associated with fibrillin microfibrils. Extraction of microfibrils followed by treatment of microfibrils under dissociating conditions suggested that the versican C terminus is covalently bound to microfibrils. Binding assays using recombinant fibrillin-1 polypeptides and recombinant lectican lectin domains indicated that the versican lectin domain binds to specific fibrillin-1 polypeptides. The versican lectin domain also bound to molecules comigrating with authentic fibrillin-1 monomers in an assay using cell culture medium. In assays using microfibrils, the versican lectin domain demonstrated preferential binding compared with other lecticans. Binding was calcium-dependent. The binding site for versican in microfibrils is most likely within a region of fibrillin-1 between calcium-binding epidermal growth factor-like domains 11 and 21. Human mutations in this region can result in severe forms of the Marfan syndrome ("neonatal" Marfan syndrome). The connection between versican and fibrillin microfibrils may be functionally significant, particularly in cardiovascular tissues.

Partial cloning and sequencing of chick fibrillin-1 cDNA

The recent identification of numerous matrix genes and gene products has allowed a detailed examination of their roles in development. Two of these extracellular matrix proteins, fibrillin-1 and fibrillin-2, are components of the elastin-associated microfibrils. Given what is known about the distribution of the fibrillins in normal tissues and the abnormalities that result when mutations occur, a basic hypothesis has emerged: fibrillin-1 is primarily responsible for load bearing and providing structural integrity, whereas fibrillin-2 may be a director of elastogenesis. Nevertheless, examination of phenotypes in disorders caused by mutations in fibrillin-1 or fibrillin-2 suggests some common functions. To better understand these similar and diverse roles, it would be helpful to examine these proteins during chick development. To accomplish this goal, it is first necessary to characterize the chick homologs of the known fibrillins. In this study, the partial chick FBN1 cDNA was identified by polymerase chain reaction-aided cloning as a first step toward elucidating these goals. Sequence analysis indicated that there is striking conservation between chick and mammalian fibrillin-1 at the DNA and protein levels. Antisense and sense riboprobes were synthesized and used in in situ hybridization in stage 14 chick embryos and high levels of FBN1 transcripts were observed in the heart.

Microfibrils from the arterial subendothelium

Microfibrillar structures of the subendothelium are represented by either type VI collagen or elastin-associated microfibrils which are also referred to as fibrillin-containing microfibrils. These structures are present throughout the subendothelium irrespective of the presence of elastin. The localization, structure, and protein composition of microfibrils are reviewed. The arterial subendothelium is thrombogenic despite its very low content in fibrillar collagens. This thrombogenicity is linked to the microfibrillar structures, essentially to type VI collagen and to thrombospondin-containing microfibrils. Their respective ability to bind the von Willebrand factor and to activate blood platelets is discussed.

Recombinant latent transforming growth factor beta-binding protein 2 assembles to fibroblast extracellular matrix and is susceptible to proteolytic processing and release

Latent transforming growth factor beta-binding protein 2 (LTBP-2) belongs to the fibrillin-LTBP gene family and is a component of 10-nm microfibrils. LTBP-2 consists mainly of domains of 8-cysteine and EGF-like repeats linked by proline-rich regions. To characterize the biochemical properties of LTBP-2, its assembly to the extracellular matrix, and its proteolytic release from the matrix, LTBP-2 was expressed recombinantly in Chinese hamster ovary cells and purified to homogeneity under nondenaturing conditions. Purified LTBP-2 bound calcium and was glycosylated at the central domain of EGF-like repeats. Antibodies made against the recombinant LTBP-2 decorated fibrillar structures in fibroblast extracellular matrix. Treatment of matrices with plasmin or elastase released a soluble approximately 160-kDa LTBP-2 fragment. Processing of LTBP-2 was studied by treating purified LTBP-2 with plasmin or porcine pancreatic elastase. LTBP-2 was processed with these proteases initially to a approximately 160-kDa fragment, and with higher concentrations to a protease-resistant approximately 120-kDa fragment. Processing sites were localized by amino acid sequencing to proline-rich regions at the N-terminal part of LTBP-2, suggesting that the matrix binding sites locate to the N-terminal approximately 500 amino acids of LTBP-2. Purified and biotinylated LTBP-2 could be assembled to fibrillar structures in fibroblast extracellular matrix during cell cultivation, indicating that LTBP-2 assembly to the matrix is not strictly linked to cells that make it and suggesting that microfibril assembly may involve soluble intermediates.

Identification of the developmental marker, JB3-antigen, as fibrillin-2 and its de novo organization into embryonic microfibrous arrays

The monoclonal antibody JB3 was previously shown to react with a protein antigen present in the bilateral primitive heart-forming regions and septation-stage embryonic hearts; in addition, primary axial structures at primitive streak stages are JB3-immunopositive (Wunsch et al. [1994] Dev. Biol. 165:585-601). The JB3 antigen has an overlapping distribution pattern with fibrillin-1, and a similar molecular mass (Gallagher et al. [1993] Dev. Dyn. 196:70-78; Wunsch et al. [1994] Dev. Biol. 165:585-601). Here we present immunoblot and immunoprecipitation data showing that the JB3 antigen is secreted into tissue culture medium by day 10 chicken embryonic fibroblasts, from which it can be harvested using JB3-immunoaffinity chromatography. A single polypeptide (Mr = 350,000), which was not immunoreactive with an antibody to fibrillin-1, eluted from the affinity column. Mass spectroscopy peptide microsequencing determined the identity of the JB3 antigen to be an avian homologue of fibrillin-2. Live, whole-mounted, quail embryos were immunolabeled using a novel microinjection approach, and subsequently fixed. Laser scanning confocal microscopy indicated an elaborate scaffold of fibrillin-2 filaments encasing formed somites. At more caudal axial positions, discrete, punctate foci of immunofluorescent fibrillin-2 were observed; this pattern corresponded to the position of segmental plate mesoderm. Between segmental plate mesoderm and fully-formed somites, progressively longer filamentous assemblies of fibrillin-2 were observed, suggesting a developmental progression of fibrillin-2 fibril assembly across the somite-forming region of avian embryos. Extensive filaments of fibrillin-2 connect somites to the notochord. Similarly, fibrillin-2 connects the mesoderm associated with the anterior intestinal portal to the midline. Thus, fibrillin-2 fibrils are organized by a diverse group of cells of mesodermal or mesodermally derived mesenchymal origin. Fibrillin-2 microfilaments are assembled in a temporal and spatial pattern that is coincident with cranial-to-caudal segmentation, and regression of the anterior intestinal portal. Fibrillin-2 may function to impart physical stability to embryonic tissues during morphogenesis of the basic vertebrate body plan.

Alignment of fibrillin molecules in elastic microfibrils is defined by transglutaminase-derived cross-links

Microfibrils were extracted from human amnion in the form of a beaded filament and analyzed for the presence of transglutaminase-derived cross-links using acrylonitrile derivatization. The cross-link structure was isolated from protease hydrolysates of beaded filaments and identified as a phenylthiocarbamyl amino acid derivative by comparison to a standard. Acid hydrolysis of the isolated cross-link gave the expected lysine and glutamic acid in a 1:1 ratio. The beaded filaments were also treated with trypsin to produce a fraction that contained the bead structure and a fraction containing fragments of the interbead filaments. Cross-links were detected in the interbead filaments but not in the beads. A large tryptic peptide that contained a cross-link was isolated and sequenced. The two amino acid sequences obtained identified both of the cross-linked molecules as fibrillin-1 and enabled the approximate localization of the cross-link sites within the molecule. The locations of cross-link sites on two adjacent molecules fixed the relative positions of fibrillin monomers within the microfibrils, providing insight into the spatial organization of fibrillin within the elastic microfibrils.

Elastin in lung development

Elastin is a critical component of the lung interstitium, providing the property of recoil to the vascular, conducting airway, and terminal airspace compartments of the lung. Elastic fibers, consisting of soluble tropoelastin monomers cross-linked on a preexisting scaffold of microfibrils, are produced primarily during late fetal and neonatal stages of development. The factors and molecular mechanisms regulating the cell type-specific and tightly temporally regulated expression of tropoelastin are currently under investigation. The onset and inductive phase of tropoelastin expression are characterized by increased transcription of the tropoelastin gene. Glucocorticoids accelerate this induction in fetal rats during the canalicular stage of lung development. Many additional factors regulate tropoelastin expression in cultured lung fibroblasts and vascular smooth muscle cells, but the in vivo roles of such mediators are still under investigation. Cell-cell interactions may also promote elastogenesis during lung development, as localization of tropoelastin mRNA in pseudo-glandular and canalicular lungs demonstrates a close spatial relationship between epithelium and adjacent elastogenic mesenchyme. Elastin metabolism is altered in several experimental models of bronchopulmonary dysplasia, characterized by abnormal lung morphological development, suggesting that normal elastin production and deposition is necessary for proper development of alveoli. Studies employing reverse genetics may prove useful in further defining the role of elastin in lung development.

Expression of microfibrillar proteins by bovine bladder urothelium

OBJECTIVES

To determine the occurrence and potential function of proteins composing elastic microfibrils in the developing bovine bladder.

METHODS

Monospecific antibodies, generated against two well-characterized microfibrillar proteins, microfibril-associated glycoprotein (MAGP) and fibrillin-1 (FBN1), were used in immunohistochemical analysis of full-thickness frozen sections of fetal bovine bladder. The localization of these two antibodies was compared with that of anti-type IV collagen antibody. Adjacent serial sections were stained for routine light microscopy. Cultured urothelial cells were fixed in 3.7% formaldehyde and permeabilized with 0.5% Triton X-100 before immunoanalysis. Control reactions used either preimmune serum or a monoclonal antibody to a nonmatrix protein. Poly(A+) ribonucleic acid was isolated from cultured urothelial cells and subjected to Northern analysis using specific complementary deoxyribonucleic acid probes for MAGP and FBN1.

RESULTS

Both MAGP and FBN1 are expressed by the urothelium and are found in association with the underlying basement membrane, as visualized by their co-localization with type IV collagen. Furthermore, urothelial cells in culture continue to express both microfibrillar proteins.

CONCLUSIONS

The developing bovine urothelium expresses major microfibrillar protein components. The role of these microfibrils in the urothelium remains to be determined, but they may have an important anchoring function.

Lysozyme binds to elastin and protects elastin from elastase-mediated degradation

Lysozyme has been shown to be associated with damaged elastic fibers in many tissues and organs. To better characterize this interaction, binding of lysozyme to elastin was studied using solution-based binding assays. Under physiologic conditions, radio-labeled lysozyme bound specifically to elastin in a time- and concentration-dependent manner. Binding was reversible and was inhibited by unlabeled human and hen lysozyme but not by other proteins. Lysozyme had no elastolytic activity as assessed by a standard tritium-release assay, but, importantly, prevented the proteolytic degradation of elastin by human leukocyte elastase, pancreatic elastase, thermolysin, and Pseudomonas elastase. A striking feature of lysozyme's anti-elastase activity was that it did not function in the classical sense of inhibiting directly the enzymatic activity of the protease. Instead, by binding to elastin, lysozyme prevented the protease from interacting with the elastin substrate in ways that normally favor proteolysis. These results show that lysozyme binds to the elastin component of elastic fibers and that this interaction has important biological consequences for elastic fiber degradation. By preventing degradation of elastin, lysozyme can function as an important natural inhibitor that exerts a protective effect on elastic fibers at sites of tissue injury.

Proteoglycans associated with the ciliary zonule of the rat eye: a histochemical and immunocytochemical study

The structural organization of integral and associated components of the ciliary zonule is still not fully understood. The present study is to localize and characterize the proteoglycans associated with the ciliary zonule of the rat eye by Cuprolinic blue (CB) staining and immunocytochemistry. After CB staining, the proteoglycans appeared as electron dense elongated rodlets and were localized with the zonular fibers. They were seen lying on the periphery of the zonular fibers or along the length of the individual fibrils. Most of the CB rodlets had a size of 60-170 nm long (average 130 nm) and 25 nm wide. Smaller CB rodlets measuring 25-60 nm long (average 45 nm) and 12 nm wide were sometimes found associated with the individual zonular fibrils. The CB rodlets were removed after chondroitinase ABC or chondroitinase AC treatment, but were resistant to heparitinase, nitrous acid, keratanase or Streptomyces hyaluronidase digestions. The ciliary zonule was also immunostained with three monoclonal antibodies: 2-B-6 specific for chondroitin 4-sulfate, 3-B-3 for chondroitin 6-sulfate and 1-B-5 for unsulfated chondroitin, using indirect immunoperoxidase or immuno-colloidal gold methods. The zonular fibers were immunoperoxidase stained and immunogold labeled by 2-B-6, but were not reactive to 3-B-3 and 1-B-5. The results demonstrate that chondroitin sulfate proteoglycan is associated with the ciliary zonule of the rat eye.

Fibrillin-containing microfibrils: structure and function in health and disease

Fibrillin-containing microfibrils are a unique class of connective tissue macromolecules whose critical contribution to the establishment and maintenance of diverse extracellular matrices was underlined by the recent linkage of their principal structural component fibrillin to Marfan syndrome, a heritable disorder with pleiotrophic connective tissue manifestations. The complexity of the structure: function relationships of these macromolecules was highlighted by the recent elucidation of the primary structure of fibrillin and characterisation of fibrillin mutations in Marfan patients. This review examines current understanding of the expression and assembly of fibrillin and describes new approaches which are now being applied to elucidate the many outstanding structural, organisational and functional aspects of the fibrillin-containing microfibrils.

Cysteine-to-arginine point mutation in a ‘hybrid’ eight-cysteine domain of FBN1: consequences for fibrillin aggregation and microfibril assembly

Mutations in the FBN1 gene encoding the microfibrillar glycoprotein fibrillin cause Marfan syndrome, a relatively common autosomal dominant connective tissue disease. Causative FBN1 mutations appear to be dispersed throughout the coding frame, and to date no predictable genotype: phenotype correlations have emerged. We have identified a point mutation within an eight-cysteine ‘hybrid’ motif of the fibrillin polypeptide which results in the substitution of an arginine for a cysteine, in a patient severely affected in the cardiovascular, skeletal and ocular systems. We have utilised cell cultures from various tissues of this patient to investigate the effects of this mutation on fibrillin expression and deposition, and the consequences in terms of microfibril assembly and organisation. We have established that there is no difference in the expression of normal and mutant alleles, and fibrillin synthesis, secretion and deposition are also normal. However, the rate of fibrillin aggregation is reduced and microfibrillar assemblies are both remarkably scarce and morphologically abnormal. These data clearly demonstrate that the mutated allele interferes with normal assembly, and strongly implicate this particular region of the fibrillin-1 molecule in stabilising microfibrillar assemblies.

Molecular cloning of the microfibrillar protein MFAP3 and assignment of the gene to human chromosome 5q32-q33.2

Microfibrils having a diameter of 10-12 nm, found either in association with elastin or independently, are an important component of the extracellular matrix of many tissues, but characterization of these microfibrils is incomplete. To further our understanding of the gene structure of proteins composing the microfibrils and to identify their chromosomal location, we have cloned and characterized another microfibril protein, designated microfibril-associated protein-3 (MFAP3). The human gene encoding MFAP3 has a very simple structure, containing only two translated exons encoding a protein of 362 amino acids. Monospecific antibodies prepared against the recombinantly expressed protein reacted with the microfibrils found in ocular zonules. MFAP3 does not appear to share homology with any other known protein. The gene was found to be located on chromosome 5q32-q33.2, near the locus 5q21-q31 reported for the fibrillin gene, FBN2, which has been linked to congenital contractural arachnodactyly. MFAP3 is a candidate gene for heritable diseases affecting microfibrils.

Characterization of the human gene for microfibril-associated glycoprotein (MFAP2), assignment to chromosome 1p36.1-p35, and linkage to D1S170

Microfibril-associated glycoprotein, MAGP (gene symbol MFAP2), is a component of connective tissue microfibrils and a candidate for involvement in the etiology of inherited connective tissue diseases. We have cloned a human MAGP cDNA that is highly homologous to the previously characterized bovine and murine genes. Like the bovine and murine loci, the human gene has eight coding exons, but it contains two alternatively used 5' untranslated exons, whereas only one untranslated exon was described in the bovine and murine Magp genes. By using rodent x human somatic cell hybrid panels and fluorescence chromosomal in situ hybridization, we have assigned the locus to human chromosome 1p36.1-p35. An insertion/deletion polymorphism has been identified within intron 7. Linkage analysis between this polymorphism and markers on distal chromosome 1 revealed that MAGP is tightly linked to the anonymous marker D1S170. Physical mapping revealed a distance of < 100 kb between the two markers. This information can be used to screen for linkage in families with microfibrillar abnormalities that are not linked to the fibrillin genes on chromosomes 15 or 5.

Primary structure and developmental expression of Fbn-1, the mouse fibrillin gene

Previous studies have reported > 10 kilobases of human fibrillin-1 cDNA sequence, but a consensus regarding the 5' end of the transcript remains to be worked out. One approach to developing a clear consensus would be to search for regions of evolutionary conservation in transcripts from a related species such as mouse. As reported here, the mouse fibrillin-1 transcript encodes a highly conserved polypeptide of 2,871 amino acids. The upstream sequence that flanks the ATG is considerably less well conserved, however. Indeed, the ATG codon (which occurs in the context of a Kozak consensus sequence and is located just upstream of a consensus signal peptide) signals the point where human and mouse fibrillin-1 sequences cease to be nearly identical. Together, these results are consistent with previous efforts by Pereira et al. (Pereira, L., D'Alessio, M., Ramirez, F., Lynch, J. R., Sykes, B., Pangilinan, T., and Bonadio, J. (1993) Human Mol. Genet. 2, 961-968) to identify the human fibrillin-1 translational start site. Sequences immediately upstream of the ATG are GC-rich and devoid of TATA and CCAAT boxes, which suggests that the mouse fibrillin-1 gene will be broadly expressed. A survey of expression in mouse embryo tissues is consistent with this hypothesis and suggests two novel functions for fibrillin-associated microfibrils in non-elastic connective tissues.

Fibrillin-1 and fibrillin-2 show temporal and tissue-specific regulation of expression in developing elastic tissues

The recent characterization of multiple fibrillin genes raises the question of whether each of the fibrillin proteins is a component of elastic fiber microfibrils and whether their expression during development of elastic tissues is consistent with a function associated with elastogenesis. To address these possibilities, the expression of fibrillin-1 and fibrillin-2 was compared with expression of MAGP and tropoelastin in two elastogenic tissues that undergo different developmental programs. For both fibrillins, the greatest increase in expression occurred during the last half of fetal development when elastin production is highest. In fetal bovine nuchal ligament, mRNA levels for fibrillin-1 and fibrillin-2 increased approximately threefold during this period, whereas tropoelastin increased 20-fold. Although the relative increase in expression of both fibrillins was equivalent, the basal level of fibrillin-1 expression was greater than fibrillin-2. In developing bovine aorta, fibrillin mRNA levels again paralleled tropoelastin expression although, compared to ligament, elastin synthesis began at an earlier fetal age in this tissue. Furthermore, the relative increase in aortic fibrillin-2 expression was greater than that for fibrillin-1 and the ratio of fibrillin-2 to fibrillin-1 was higher than in the ligament. In contrast to the fibrillins, MAGP expression in nuchal ligament and aorta remained at a constant high level throughout the fetal period. Indirect immunofluorescent staining and immunoelectron microscopy localized both fibrillins as well as MAGP to elastic fiber microfibrils in these developing tissues. The coordinate upregulation of fibrillin-1 and fibrillin-2 expression with the onset of tropoelastin production is consistent with a role in elastic fiber assembly. Our findings also suggest temporal and tissue-specific regulation for the fibrillins during development.

Expression patterns of novel genes during mouse preimplantation embryogenesis

Little is known about the repertoire of genes expressed following zygotic gene activation, which occurs during the two-cell stage in the mouse. As an initial attempt to isolate novel genes, we used previously prepared two-cell and two-cell subtraction cDNA libraries (Rothstein et al., Genes Dev 6:1190-1201, 1992) to isolate a panel of seven cDNA clones. Three cDNAs had no match in the current DNA sequence data banks and three others revealed sequence homology to portions of sequences in the data banks. One cDNA was 90% homologous to the ras-related gene Krev/rap 1A. The temporal patterns of expression of these genes during oocyte maturation and preimplantation development were analyzed by a reverse transcription-polymerase chain reaction (RT-PCR) assay developed to measure relative levels of mRNAs. Three distinct temporal patterns of expression, designated Classes 1-3, were found. The two Class 1 genes displayed an actin-like pattern, with a gradual decline in expression during oocyte maturation and through the two-cell stage, followed by increases at the eight-cell and/or blastocyst stages. The four genes in Class 2 were expressed at relatively high levels during oocyte maturation and through the one-cell stage and then declined abruptly between the one- and two-cell stages; an increase then occurred at the eight-cell and/or blastocyst stages. The expression of the gene in Class 3 declined during oocyte maturation, but then showed a transient increase at the one-cell stage, with only a very slight increase in synthesis at either the eight-cell or blastocyst stage.

Immunization with undenatured bovine zonular fibrils results in monoclonal antibodies to fibrillin

Microfibrils were dissected from the zonular apparatus of the bovine eye, homogenized and used as an immunogen to prepare monoclonal antibodies. Initial screening of hybridomas was performed by immunoblotting to a sonicate of zonular fibrils and by immunolocalization to frozen sections of the zonular apparatus. Subsequently, monoclonal antibodies with strong immunoreactivity to zonular fibrils were shown to recognize microfibrils in a wide range of connective tissues both by immunofluorescent staining and by electron microscopic immunolocalization. All antibodies were found to recognize a single protein of 350 kDa on Western blotting of the proteins secreted by bovine aortic smooth muscle cells. A protein of the same molecular weight and properties was recognized by an antibody previously prepared by another group against fibrillin. A member of the fibrillin family therefore represents the major immunogen of intact zonular fibrils, and the results support previous evidence for a close relationship between zonular fibrils and other connective tissue microfibrils. The zonular apparatus is a suitable system to obtain purified preparations of microfibrils in order to investigate their composition and structural organization.

The fibrillin-Marfan syndrome connection

A few years ago no one would have suspected that the well-known disorder of connective tissue, Marfan syndrome, could be caused by mutations in a recently discovered extracellular component, fibrillin. Likewise, nobody would have predicted that fibrillin represents a small family of proteins that are associated with several phenotypically overlapping disorders. The fibrillins are integral constituents of the non-collagenous microfibrils, with an average diameter of 10 nm. These aggregates are distributed in the extracellular matrix of virtually every tissue. Microfibrillar bundles provide the external coating to elastin in elastic fibers, and serve an anchoring function in non-elastic tissues. At higher resolution, individual microfibrils have a "beads-on-a-string" appearance resulting from the head-to-tail polymerization of multiple fibrillin aggregates. Structurally, fibrillin contains a series of repeated sequences homologous to the epidermal growth factor calcium-binding motif. Characterization of fibrillin mutations in Marfan syndrome patients, together with the elucidation of the structure of the fibrillin proteins, have provided new insights, and raised new questions, about the function of the 10 nm microfibrils. For example, it is possible that the fibrillins, in addition to serving a structural function, might also be involved in regulating cellular activities and morphogenetic programs. It is fitting that the long search for the Marfan syndrome gene has brought a novel group of proteins to the forefront of extracellular matrix biology.

The molecular basis of Marfan syndrome

The Marfan syndrome is an inherited, autosomal dominant disorder that affects the skeletal, ocular, and cardiovascular systems. Recent biochemical and genetic studies have demonstrated that this deadly genetic disorder arises from defects in the connective tissue protein fibrillin. Fibrillin is a component of microfibrils, structures found in the extracellular matrices of most tissues, including those affected in Marfan patients. The appearance of microfibrils in the matrix produced by Marfan patient fibroblasts is different from that of normal cells. Genetic linkage between the fibrillin gene and the Marfan phenotype has been established and the gene mapped to the same chromosomal position as the disease locus. In several instances, the disease has been associated with mutations in the fibrillin gene, confirming that defects in fibrillin cause the Marfan syndrome.