Structure of the elastin gene

The isolation and characterization of cDNAs encompassing the full length of chicken, cow, rat and human elastin mRNA have led to the elucidation of the primary structure of the respective tropoelastins. Large segments of the sequence are conserved but there are also considerable variations which range in extent from relatively small alterations, such as conservative amino acid substitutions, to variation in the length of hydrophobic segments and largescale deletions and insertions. In general, smaller differences are found among mammalian tropoelastins and greater ones between chicken and mammalian tropoelastins. Although only a single elastin gene is found per haploid genome, the primary transcript is subject to considerable alternative splicing, resulting in multiple tropoelastin isoforms. Functionally distinct hydrophobic and cross-link domains of the protein are encoded in separate exons which alternate in the gene. The introns of the human gene are rich in Alu repetitive sequences, which may be the site of recombinational events, and there are also several dinucleotide repeats, which may exhibit polymorphism and, therefore, be effective genetic markers. The 5' flanking region is G+C rich and contains potential binding sites for numerous modulating factors, but no TATA box or functional CAAT box. The basic promoter is contained within a 136 bp segment and transcription is initiated at multiple sites. These findings suggest that the regulation of elastin gene expression is complex and takes place at several levels.

The tumorigenic phenotype of a mutated form of Fc gamma RIIB1, lacking the ability to generate soluble receptor and allowing a low-level of ligand binding

Non immunohematopoietic murine tumor cells ectopically expressing Fc gamma RIIB1 (B1) were recently shown to express a higher tumorigenicity phenotype than cells not expressing this receptor. Utilizing a genetic approach we studied the possible contribution of a soluble form of B1 to tumor enhancement. A mutated form of the B1, lacking the cleavage site responsible for the generation of soluble B1 was produced using gene splicing by overlap extension PCR. A deletion confirmed by sequence analysis from 172 to 178 residues was generated. Stable transfectants expressed the B1 deleted form (B1 Delta) both as specific RNA and as a membrane protein receptor allowing a low level of ligand binding. The soluble form of B1 was undetectable in tissue culture supernatants of Bib transfected cells while it was present in supernatants of wild type B1-transfectants. Stable B1 Delta transfectants were significantly more tumorigenic than negative control transfectants. Tumor incidence was almost as high as that of intact B1 and lagged in the latency period before the appearance of palpable tumors. It is suggested that the soluble B1 has a minimal contribution to tumor enhancement.

Calcium signalling by the high affinity macrophage Fc gamma receptor requires the cytosolic domain

Hematopoietic cells express multiple receptors which bind the Fc domain of IgG. We utilized transfection of COS-1 cells, a cell line which lacks endogenous Fc receptors, to study the expression and function of Fc gamma RI, the high affinity Fc gamma receptor in the absence of other Fc gamma receptors. Fc gamma RI was efficiently expressed in transiently transfected COS-1 cells as measured by flow cytometry and the binding of IgG sensitized RBCs (EA). In addition, analysis at the single cell level demonstrated that individually transfected COS-1 cells release cytosolic free Ca2+ [(Ca2+)i] upon activation with anti-Fc gamma RI antibody. The calcium response required Fc gamma RI cross-linking. COS-1 cells transfected with mutant Fc gamma RI lacking the cytosolic domain expressed Fc gamma receptors and bound EA as well as wild type receptors, but failed to induce an increase in [Ca2+]i. These data indicate that Fc gamma RI in the absence of other Fc gamma receptors mediates a calcium signal and that the cytoplasmic domain of Fc gamma RI contains the elements required for calcium dependent signal transduction.

Activation of Fc gamma RII induces tyrosine phosphorylation of multiple proteins including Fc gamma RII

Platelets provide a useful system for studying Fc gamma receptor-mediated signaling events because these cells express only a single class of Fc gamma receptors and because platelet aggregation and secretion can be activated through Fc gamma receptor stimulation. We report here that stimulation of platelets by cross-linking antibodies to Fc gamma RII or by treatment with an anti-CD9 monoclonal antibody, which acts through Fc gamma RII, causes an induction of tyrosine phosphorylation of multiple platelet proteins. Although the profile of tyrosine-phosphorylated proteins induced by stimulation of this Fc receptor was similar to that induced by thrombin, an additional 40-kDa phosphorylated protein was also detected. This protein co-migrated with Fc gamma RII and was immunoprecipitated with a monoclonal antibody to Fc gamma RII. In addition, after the cross-linking of Fc gamma RII in HEL cells or in COS-1 cells transfected with Fc gamma RII cDNA, the 40-kDa protein immunoprecipitated with anti-Fc gamma RII was also phosphorylated on tyrosine. These data strongly suggest that Fc gamma RII itself is a substrate for a tyrosine kinase(s) activated when Fc gamma RII is stimulated. Fc gamma RII was phosphorylated by the Src protein in vitro, suggesting that this kinase may be responsible for phosphorylation of Fc gamma RII in vivo. These studies establish that activation of platelets and human erythroleukemia cells through Fc gamma RII and CD9 involves an induction of tyrosine phosphorylation of multiple proteins including Fc gamma RII itself and suggest that these phosphorylation events may be involved in Fc gamma RII-mediated cell signaling.

Human Fc gamma RII, in the absence of other Fc gamma receptors, mediates a phagocytic signal

Fc gamma receptors are important components in the binding and phagocytosis of IgG-sensitized cells. Studies on the role of these receptors have been limited by the fact that most hematopoietic cells express more than one Fc gamma receptor. We studied the role of Fc gamma RIIA in isolation on a human erythroleukemia cell line (HEL) which expresses Fc gamma RIIA as its only Fc gamma receptor. HEL cells were observed to bind and phagocytose IgG-sensitized red blood cells (RBCs) in a dose-dependent manner. We then examined the role of Fc gamma RI and Fc gamma RII in isolation and in combination, in transfected COS-1 cells. Fc gamma RIIA-transfected COS cells also mediated both the binding and phagocytosis of IgG-sensitized RBCs. In contrast, phagocytosis was not observed in Fc gamma RI-transfected cells, although these cells avidly bound IgG-sensitized RBCs. Furthermore, coexpression of both receptors by doubly transfected cells did not affect the phagocytic efficiency of Fc gamma RIIA. These studies establish that Fc gamma RIIA can mediate phagocytosis and suggest that transfected COS-1 cells provide a model for examining this process. Since HEL cells exhibit characteristics of cells of the megakaryocyte-platelet lineage, including expression of Fc gamma RII as the only Fc gamma receptor, Fc gamma RIIA on megakaryocytes and platelets may be involved in the ingestion of IgG-containing immune complexes. Furthermore, these studies indicate that Fc gamma RI and Fc gamma RIIA differ in their requirements for transduction of a phagocytic signal.

A to G polymorphism in ELN gene

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Production of recombinant human tropoelastin: characterization and demonstration of immunologic and chemotactic activity

Tropoelastin cannot readily be prepared in quantity from natural sources and this has limited research in several important areas including structure/function relationships and fiber assembly. In order to eliminate this limitation, human tropoelastin has been expressed in a recombinant bacterial system and the protein has been highly purified. The size, amino acid composition, and sequence of the amino terminus of the recombinant tropoelastin (rTE) all agree with values predicted by the nucleotide sequence of the cDNA used in the expression vector. The rTE exhibits cross-reactivity with antibodies directed against a mixture of peptides derived from human elastin as well as antibody against a defined peptide located at the carboxy terminus of the protein. In addition, the rTE is chemotactic for fetal calf ligament fibroblasts. These results suggest that rTE could be a useful reagent for many types of studies.

Structure of the elastin gene and alternative splicing of elastin mRNA: implications for human disease

The protein elastin is largely responsible for the elastic properties of vertebrate lungs, large blood vessels, and skin. The structure of the human, bovine, and chick elastin gene and protein monomer, tropoelastin, has recently been elucidated by using techniques of molecular biology. Extensive homology of amino acid sequence exists among the mammalian species and there is in addition strong conservation of nucleotide sequences in the 3' untranslated region of the gene. The translated exons are small and embedded in large expanses of introns. Sequences coding for the hydrophobic regions, responsible for the elastic properties of the molecule, and the alanine-lysine rich regions, responsible for crosslink formation between molecules, reside in separate exons and alternate for the most part in the elastin gene. S1 analyses and sequence analysis of cDNA and genomic clones have indicated that there is substantial alternative splicing of the primary elastin transcript. Variations in the structure of mRNAs resulting from alternative splicing could explain the existence of the multiple forms of tropoelastin observed electrophoretically in several species. Different kinds of splicing patterns could occur in human populations and may contribute to aging and pathological situations in the cardiovascular and pulmonary systems.

Characterization of the complete human elastin gene. Delineation of unusual features in the 5'-flanking region

Genomic clones encompassing the entire human elastin gene, including 11 kilobases flanking the ATG translation initiation codon, have been obtained and characterized by restriction enzyme analysis and extensive DNA sequencing. These analyses demonstrated that functionally distinct hydrophobic and cross-linking domains of the protein are segregated into separate exons throughout the gene. All exons are multiples of three nucleotides, and exon:intron borders always split codons in the same way which permits cassette-like alternative splicing. The 5'-flanking region lacks a canonical TATA sequence, is G + C-rich, and contains several SP1 binding sites and an AP2 binding site. Primer extension and S1 nuclease protection experiments indicate that transcription is initiated at multiple sites in the gene.

Structure of the bovine elastin gene and S1 nuclease analysis of alternative splicing of elastin mRNA in the bovine nuchal ligament

Genomic clones encompassing all the translated sequences, the 3' untranslated sequence, and 1 kb flanking the ATG translation initiation codon of bovine tropoelastin have been obtained and characterized by restriction enzyme analysis and extensive DNA sequencing. These analyses demonstrated that functionally distinct hydrophobic and cross-linking domains of the protein are segregated into separate exons throughout the gene. The putative promoter region lacks a TATA box, has an extremely high G+C content, and contains several SP1 binding sites. Comprehensive S1 analyses using probes covering the entire mRNA and RNA isolated from the nuchal ligament of bovine fetuses of different ages, neonate calves, and adult cows demonstrated that while only a single exon is alternatively spliced at high frequency, many exons are alternatively spliced at limited, variable frequencies. The results also suggest that such limited splicing is increased in the adult tissue relative to fetal and neonate tissues.

Cloning of full-length elastin cDNAs from a human skin fibroblast recombinant cDNA library: further elucidation of alternative splicing utilizing exon-specific oligonucleotides

A human cDNA library was constructed utilizing RNA isolated from cultured skin fibroblasts. Recombinant clones containing elastin sequences were identified by plaque hybridizations with previously characterized human placental elastin cDNAs. Seven positive recombinant clones with inserts of approximately 3.2-2.2 kb were isolated. Characterization of the clones by restriction endonuclease analysis and dot-blot hybridizations with exon-specific synthetic oligonucleotides demonstrated considerable variability in the primary nucleotide sequence. Dideoxy nucleotide sequencing confirmed this finding. The variability is most likely a result of alternative splicing of exons from the primary elastin transcripts. The two largest clones contained approximately 1 kb of 3' untranslated sequence and approximately 2.2 kb of translated sequence encoding 730 amino acids. Six amino acids, encoded by exon 12A, have not been previously noted in human elastin cDNAs. In addition, these human skin fibroblast clones contained a 49 bp 5' untranslated sequence. These results demonstrate that there is considerable variability in the processed nucleotide sequence of the elastin mRNAs. These transcripts may code for isoforms of tropoelastin with different biologic properties.

Sequence variation of bovine elastin mRNA due to alternative splicing

Poly A+ RNA, isolated from a single 210 day fetal bovine nuchal ligament, was used to synthesize cDNA by the RNase H method, using AMV reverse transcriptase for first strand synthesis and DNA polymerase I for the second strand. The cDNA was inserted into lambda gt10 using EcoRI linkers, and recombinant phage containing elastin sequences were identified by hybridization with a 1.3 kb sheep elastin cDNA clone, pcSELI (Yoon, K. et al., Biochem. Biophys. Res. Comm. 118: 261-265, 1984). Three clones containing the largest inserts of 2.9, 2.8, and 2.6 kb were selected for further study. The complete sequence analysis of the 3 clones was correlated with the sequence of 10.2 kb of the bovine elastin gene. The analyses: (i) showed that the cDNA encompassed the great majority of the translated sequence, (ii) ordered the tryptic peptides of porcine tropoelastin, (iii) determined new amino acid sequences not previously found in the porcine peptides and (iv) demonstrated that alternative splicing of the primary transcript leads to significant variation in the sequence of the translated portion of the mRNA.

Alternative splicing of human elastin mRNA indicated by sequence analysis of cloned genomic and complementary DNA

Poly(A)+ RNA, isolated from a single 7-mo fetal human aorta, was used to synthesize cDNA by the RNase H method, and the cDNA was inserted into lambda gt10. Recombinant phage containing elastin sequences were identified by hybridization with cloned, exon-containing fragments of the human elastin gene. Three clones containing inserts of 3.3, 2.7, and 2.3 kilobases were selected for further analysis. Three overlapping clones containing 17.8 kilobases of the human elastin gene were also isolated from genomic libraries. Complete sequence analysis of the six clones demonstrated that: the cDNA encompassed the entire translated portion of the mRNA encoding 786 amino acids, including several unusual hydrophilic amino acid sequences not previously identified in porcine tropoelastin, exons encoding either hydrophobic or crosslinking domains in the protein alternated in the gene, and a great abundance of Alu repetitive sequences occurred throughout the introns. The data also indicated substantial alternative splicing of the mRNA. These results suggest the potential for significant variation in the precise molecular structure of the elastic fiber in the human population.

Structure of the 3' region of the human elastin gene: great abundance of Alu repetitive sequences and few coding sequences

Two overlapping clones encompassing 8.5 kb of the human elastin gene were isolated from two genomic libraries constructed by partial digestion with either HaeIII/AluI or Sau3A and contained in lambda Charon 4A or EMBL3, respectively. The 6 kb of DNA comprising the most 3' portion of the gene were sequenced demonstrating an extremely low coding ratio since only three exons containing a total of 134 translated nucleotides were identified. Two exons totaling 78 bp of translated sequences which were previously found in the bovine gene were absent in the human gene. The 3' most exon encoded the unusual amino acid sequence, GGACLGKACGRKRK. The human gene was terminated by 1.2 kb of untranslated sequence which contained two polyadenylation attachment signals. The remainder of the 6 kb was composed of intervening sequences which were abundantly rich in Alu family repetitive sequences found in both orientations. This first report of the characterization of the human elastin gene suggests that significant variation in the gene may exist between species and raises the possibility of consequential polymorphism, mediated by recombination between Alu sequences, in the human population.

Newly determined carboxy terminal sequences in tropoelastin: immunologic identification in insoluble elastin

The carboxy terminal sequence of sheep, bovine and human tropoelastin (GFPGGACLGKA/SCGRKRK) has been inferred in earlier studies from sequencing of cloned complementary and genomic DNA. However, this putative carboxy terminal sequence was not found previously in peptides recovered from tryptic digests of tropoelastin. In order to determine whether the amino acid sequence described above is found in insoluble elastin, antibodies were raised against the chemically synthesized peptides with the appropriate sequences and the antibodies were shown to react with peptides derived from human, bovine, porcine, dog and hamster insoluble elastins. These results strongly suggest that the sequence (GFPGGACLGKA/SCGRKRK) at the carboxy terminus of tropoelastin is found in the elastins of many species.

Analysis of the 3' region of the sheep elastin gene

The nucleotide sequences of a 1279-bp sheep elastin cDNA clone, pcSEL1 [Yoon et al. (1984) Biochem. Biophys. Res. Commun. 118, 261-269], and a 1230-bp sheep elastin genomic subclone, pSS1 [Davidson et al. (1984) Biochem. J. 220, 643-652], corresponding to a portion of the cDNA clone, were determined. These analyses permitted determination of the 100 amino acids at the carboxy terminus of sheep tropoelastin. A portion of this sequence showed strong homology to known sequences of pig tropoelastin, but most of the sequence had not been previously determined through protein sequencing. Novel aspects of the tropoelastin molecule which have been revealed by the present analyses are (i) the presence of an unusual sequence, KPPKP, which may contribute to crosslink formation; and (ii) the finding of cysteine within a sequence, CLGKSCGRKRK, at the putative carboxy terminus of tropoelastin. Because of the presence of these sequences, it is speculated that the carboxy-terminal region may be of importance in crosslinking tropoelastin molecules to themselves or to other matrix macromolecules. The nucleotide analyses revealed that sheep elastin mRNA contains a 974-bp untranslated sequence at the 3' end, which appears to be strongly conserved among species.

Structure of the 3' portion of the bovine elastin gene

A bovine genomic library constructed by partial Sau3A digestion and contained in lambda Charon 30 was screened by in situ hybridization with a 1.3-kilobase (kb) sheep elastin cDNA clone [Yoon, K., May, M., Goldstein, N., Indik, Z., Oliver, L., Boyd, C., & Rosenbloom, J. (1984) Biochem. Biophys. Res. Commun. 118, 261-269]. Three clones encompassing 10 kb of the bovine elastin gene were identified and characterized by restriction mapping and DNA sequencing of the 6.2 kb of the most 3' region of the gene. These analyses have permitted localization of eight exons in the 6.2 kb in which the translated exons vary in size from 27 to 69 base pairs, and there is an approximately 1-kb untranslated region at the 3' end. In addition to identification of sequences homologous to those found in porcine tropoelastin, the analyses defined a 58 amino acid sequence that forms the carboxy-terminal region of tropoelastin, and this sequence, which contains two cysteine residues, was previously not observed in the protein sequence data. The analyses also suggest that functionally distinct cross-link and hydrophobic domains of the protein are encoded in separate exons.