Human fibulin-1D: molecular cloning, expression and similarity with S1-5 protein, a new member of the fibulin gene family

A single EFEMP1 mutation associated with both Malattia Leventinese and Doyne honeycomb retinal dystrophy

Malattia Leventinese (ML) and Doyne honeycomb retinal dystrophy (DHRD) refer to two autosomal dominant diseases characterized by yellow-white deposits known as drusen that accumulate beneath the retinal pigment epithelium (RPE). Both loci were mapped to chromosome 2p16-21 (refs 5,6) and this genetic interval has been subsequently narrowed. The importance of these diseases is due in large part to their close phenotypic similarity to age-related macular degeneration (AMD), a disorder with a strong genetic component that accounts for approximately 50% of registered blindness in the Western world. Just as in ML and DHRD, the early hallmark of AMD is the presence of drusen. Here we use a combination of positional and candidate gene methods to identify a single non-conservative mutation (Arg345Trp) in the gene EFEMP1 (for EGF-containing fibrillin-like extracellular matrix protein 1) in all families studied. This change was not present in 477 control individuals or in 494 patients with age-related macular degeneration. Identification of this mutation may aid in the development of an animal model for drusen, as well as in the identification of other genes involved in human macular degeneration.

EVEC, a novel epidermal growth factor-like repeat-containing protein upregulated in embryonic and diseased adult vasculature

A hallmark of vascular lesions is the phenotypic modulation of vascular smooth muscle cells (VSMCs) from a quiescent, contractile state to a more primitive, proliferative phenotype with a more fetal pattern of gene expression. Using subtraction hybridization to identify genes that may regulate this transition, we cloned a novel gene named EVEC, an acronym for its expression in the embryonic vasculature and the presence of Ca2+ binding epidermal growth factor-like repeats contained in the predicted protein structure. Although these repeats are characteristic of the extracellular matrix proteins, fibrillin, fibulin, and the latent transforming growth factor-beta binding proteins, EVEC most closely resembles the H411 and T16/S1-5 gene products, the latter of which are believed to regulate DNA synthesis in quiescent fibroblasts. Using in situ hybridization, we demonstrated that EVEC is expressed predominantly in the VSMCs of developing arteries in E11.5 through E16.5 mouse embryos. Lower levels of expression are also observed in endothelial cells, perichondrium, intestine, and mesenchyme of the face and kidney. EVEC mRNA expression is dramatically downregulated in adult arteries, except in the uterus, where cyclic angiogenesis continues; however, EVEC expression is reactivated in 2 independent rodent models of vascular injury. EVEC mRNA is observed in cellular elements of atherosclerotic plaques of LDL receptor-deficient, human apolipoprotein B transgenic mice and in VSMCs of the media and neointima of balloon-injured rat carotid arteries. These data suggest that EVEC may play an important role in the regulation of vascular growth and maturation during development and in lesions of injured vessels.

Translational control of specific genes during differentiation of HL-60 cells

Eukaryotic gene expression can be regulated through selective translation of specific mRNA species. Nevertheless, the limited number of known examples hampers the identification of common mechanisms that regulate translation of specific groups of genes in mammalian cells. We developed a method to identify translationally regulated genes. This method was used to examine the regulation of protein synthesis in HL-60 cells undergoing monocytic differentiation. A partial screening of cellular mRNAs identified five mRNAs whose translation was specifically inhibited and five others that were activated as was indicated by their mobilization onto polysomes. The specifically inhibited mRNAs encoded ribosomal proteins, identified as members of the 5'-terminal oligopyrimidine tract mRNA family. Most of the activated transcripts represented uncharacterized genes. The most actively mobilized transcript (termed TA-40) was an untranslated 1.3-kilobase polyadenylated RNA with unusual structural features, including two Alu-like elements. Following differentiation, a significant change in the cytoplasmic distribution of Alu-containing mRNAs was observed, namely, the enhancement of Alu-containing mRNAs in the polysomes. Our findings support the notion that protein synthesis is regulated during differentiation of HL-60 cells by both global and gene-specific mechanisms and that Alu-like sequences within cytoplasmic mRNAs are involved in such specific regulation.

Complete exon-intron organization of the mouse fibulin-1 gene and its comparison with the human fibulin-1 gene

Fibulin-1 is a 90 kDa calcium-binding protein present in the extracellular matrix and in the blood. Two major variants, C and D, differ in their C-termini as well as the ability to bind the basement membrane protein nidogen. Here we characterized genomic clones encoding the mouse fibulin-1 gene, which contains 18 exons spanning at least 75 kb of DNA. The two variants are generated by alternative splicing of exons in the 3' end. By searching the database we identified most of the exons encoding the human fibulin-1 gene and showed that its exon-intron organization is similar to that of the mouse gene.

Identification of chicken and C. elegans fibulin-1 homologs and characterization of the C. elegans fibulin-1 gene

Fibulin-1, a member of the emerging family of fibulin proteins, is a component of elastic extracellular matrix fibers, basement membranes and blood. Homologs of fibulin-1 have been described in man, mouse and zebrafish. In this study, we describe the isolation and sequencing of chicken fibulin-1C and D cDNA variants. We also describe identification of a C. elegans cDNA encoding fibulin-1D and cosmids containing the C. elegans fibulin-1 gene. Using the cDNA, RT-PCR and computer-based analysis of genomic sequences, the exon/intron organization of the C. elegans fibulin-1 gene was determined. The C. elegans fibulin-1 gene is located on chromosome IV, is approximately 6 kb in length, contains 16 exons and encodes fibulin-1C and D variants. Comparative analysis of the deduced amino acid sequences of nematode and chicken fibulin-1 variants with other known vertebrate fibulin-1 polypeptides showed that the number and organization of structural modules are identical. The results of this study indicate that the structure of the fibulin-1 protein has remained highly conserved over a large period of evolution, suggestive of functional conservation.

The self-association and fibronectin-binding sites of fibulin-1 map to calcium-binding epidermal growth factor-like domains

Fibulin-1 is a modular glycoprotein with amino-terminal anaphylatoxin-like modules followed by nine epidermal growth factor (EGF)-like modules and, depending on alternative splicing, four possible carboxyl termini. Fibulin-1 has been shown to self-associate as well as to bind calcium, fibronectin (FN), laminin, nidogen, and fibrinogen. To map ligand-binding sites within fibulin-1, polypeptides corresponding to various regions of fibulin-1 were expressed recombinantly and evaluated for their capacity to bind calcium, FN, or fibulin-1. A calcium-binding site(s) was mapped to EGF-like modules 5-9. A fibulin-1 self-association site was localized to EGF-like modules 5 and 6 (amino acid residues 356-440), as was a binding site for FN. The self-association interaction mediated by this pair of modules involved calcium since divalent cation chelators reduced the binding affinity of the interaction. By contrast, FN binding to EGF-like modules 5 and 6 was unaffected by the presence of divalent cation chelators. It can be concluded that EGF-like modules 5 and 6 bind calcium and mediate homotypic interaction between EGF-like modules 5 and 6 present in different fibulin-1 molecules and heterotypic interaction between EGF-like modules 5 and 6 and type III repeats 13 and 14 in FN. While additional binding sites for calcium or FN were not detected, another fibulin-1 self-association site was found within amino acid residues 30-173. However, unlike the self-association site in EGF-like modules 5 and 6, which was functional in the native protein, the amino-terminal site was cryptic and revealed only after the protein was denatured.