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Alternative splicing in the aggrecan G3 domain influences binding interactions with tenascin-C and other extracellular matrix proteins.

The proteoglycans aggrecan, versican, neurocan, and brevican bind hyaluronan through their N-terminal G1 domains, and other extracellular matrix proteins through the C-type lectin repeat in their C-terminal G3 domains. Here we identify tenascin-C as a ligand for the lectins of all these proteoglycans and map the binding site on the tenascin molecule to fibronectin type III repeats, which corresponds to the proteoglycan lectin-binding site on tenascin-R. In the G3 domain, the C-type lectin is flanked by epidermal growth factor (EGF) repeats and a complement regulatory protein-like motif. In aggrecan, these are subject to alternative splicing. To investigate if these flanking modules affect the C-type lectin ligand interactions, we produced recombinant proteins corresponding to aggrecan G3 splice variants. The G3 variant proteins containing the C-type lectin showed different affinities for various ligands, including tenascin-C, tenascin-R, fibulin-1, and fibulin-2. The presence of an EGF motif enhanced the affinity of interaction, and in particular the splice variant containing both EGF motifs had significantly higher affinity for ligands, such as tenascin-R and fibulin-2. The mRNA for this splice variant was shown by reverse transcriptase-PCR to be expressed in human chondrocytes. Our findings suggest that alternative splicing in the aggrecan G3 domain may be a mechanism for modulating interactions and extracellular matrix assembly.

Aggrecans↗

Alternative splicing of intron 23 of the human cystic fibrosis transmembrane conductance regulator gene resulting in a novel exon and transcript coding for a shortened intracytoplasmic C terminus.

The cystic fibrosis transmembrane conductance regulator (CFTR) gene, the gene responsible for the lethal hereditary disorder cystic fibrosis, codes for a membrane protein functioning as a cAMP-regulated Cl- channel. Evaluation of human CFTR mRNA transcripts from epithelial and nonepithelial cells demonstrated a CFTR cDNA containing a 260-base pair (bp) insertion between the known CFTR exons 23 and 24, introducing a premature stop codon that would result in a CFTR protein shortened by 61 amino acids at the carboxyl terminus compared to that expected from the normal reported human CFTR coding sequences. Sequence analysis of intron 23 of the CFTR gene demonstrated that the 260-bp insertion (named exon 24a), a part of the reported intron 23 and located consecutive to exon 24, is likely generated by an alternative splice acceptor site. The exon 24a+ CFTR mRNA transcripts represented 3-16% of the total CFTR transcripts in epithelial and nonepithelial cells. These observations suggest an unexpected plasticity of expression of the CFTR gene, where alternative splicing of precursor CFTR mRNA transcripts permits the use of an alternative exon derived from a genomic segment previously believed to function as an intron.

Alternative Splicing↗

Using mRNAs lengths to accurately predict the alternatively spliced gene products in Caenorhabditis elegans.

MOTIVATION: Computational gene prediction methods are an important component of whole genome analyses. While ab initio gene finders have demonstrated major improvements in accuracy, the most reliable methods are evidence-based gene predictors. These algorithms can rely on several different sources of evidence including predictions from multiple ab initio gene finders, matches to known proteins, sequence conservation and partial cDNAs to predict the final product. Despite the success of these algorithms, prediction of complete gene structures, especially for alternatively spliced products, remains a difficult task. RESULTS: LOCUS (Length Optimized Characterization of Unknown Spliceforms) is a new evidence-based gene finding algorithm which integrates a length-constraint into a dynamic programming-based framework for prediction of gene products. On a Caenorhabditis elegans test set of alternatively spliced internal exons, its performance exceeds that of current ab initio gene finders and in most cases can accurately predict the correct form of all the alternative products. As the length information used by the algorithm can be obtained in a high-throughput fashion, we propose that integration of such information into a gene-prediction pipeline is feasible and doing so may improve our ability to fully characterize the complete set of mRNAs for a genome. AVAILABILITY: LOCUS is available from http://ural.wustl.edu/software.html

Algorithms↗

Alternative splicing of the APC gene in the neural retina and retinal pigment epithelium.

PURPOSE: Hypertrophy and hyperplasia of the retinal pigment epithelium (RPE) is associated with an inherited predisposition to human familial adenomatous polyposis coli, suggesting that expression of the adenomatous polyposis coli (APC) tumor suppressor may regulate RPE proliferation/differentiation. Distinctive APC isoforms exist in different cell types due to alternative splicing of the APC transcripts. We hypothesize that differences in expression patterns of APC protein isoforms are critical to RPE proliferation/differentiation. METHODS: To investigate these relationships, APC gene expression was characterized in the retinas and RPE from fetal and adult human and mouse, and in the epiretinal membranes (ERM) from 5 patients with proliferative vitreoretinopathy (PVR). Expression patterns of alternative splice-forms of APC transcripts were evaluated by comparative quantitative RT-PCR. Exon 1 of APC encodes a heptad repeat that confers the ability of APC to homodimerize. APC protein isoforms containing or lacking this heptad were characterized by western blot analysis and immunohistochemistry. RESULTS: Comparative quantitative RT-PCR demonstrated a predominant exon 1 containing, conventional APC splice-form in the early developing fetal RPE and retina, and in all the tested ERM samples from patients with PVR. This method also demonstrated an increased level of exon 1 lacking APC splice-form in the mature RPE and retina. Western blot analysis and immunofluorescence microscopy demonstrated the conventional APC only in the RPE, and the APC isoform without the first heptad repeat in both the retina and RPE. Immunofluorescence microscopy also demonstrated only the conventional APC in the ERM samples tested. CONCLUSIONS: These results suggest that alternative splicing of APC leads to differential APC expression with potentially unique functions. APC isoform without the first heptad repeat may play a role in cell cycle cessation in the adult retina and RPE, and the down regulation of this APC isoform may contribute to the potential of RPE to migrate and proliferate.

Adenomatous Polyposis Coli Protein↗

Alternative splicing controls neuronal expression of v-ATPase subunit a1 and sorting to nerve terminals.

Vacuolar proton ATPase accumulates protons inside various intracellular organelles such as synaptic vesicles; its membrane domain V0 could also be involved in membrane fusion. These different functions could require vacuolar proton ATPases possessing different V0 subunit a isoforms. In vertebrates, four genes encode isoforms a1-a4, and a1 variants are also generated by alternative splicing. We identified a novel a1 splice variant a1-IV and showed that the two a1 variants containing exon C are specifically expressed in neurons. Single neurons coexpress a2, a1-I, and a1-IV, and these subunit a isoforms are targeted to different membrane compartments. Recombinant a2 was accumulated in the trans-Golgi network, and a1-I was concentrated in axonal varicosities, whereas a1-IV was sorted to both distal dendrites and axons. Our results indicate that alternative splicing of exon N controls differential sorting of a1 variants to nerve terminals or distal dendrites, whereas exon C regulates their neuronal expression.

Alternative Splicing↗

Mutation producing alternative splicing of exon 26 in the COL1A2 gene causes type IV osteogenesis imperfecta with intrafamilial clinical variability.

We have characterized a familial form of osteogenesis imperfecta (OI). Following the identification by ultrasound of short limbs and multiple fractures in a fetus at 25 weeks of gestation, the family was referred with a provisional diagnosis of severe OI. We detected subtle clinical and radiological signs of OI in the father and in the paternal grandmother of the proposita, who had never received a diagnosis of OI. Linkage analysis indicated COL1A2 as the disease locus. Heteroduplex analysis of reverse transcription-polymerase chain reaction (RT-PCR) amplification products of pro alpha2(I) mRNA from an affected member and subsequent sequencing of the candidate region demonstrated the presence of normal transcripts and a minority of transcripts lacking exon 26 (54 bp) of COL1A2. Sequencing of PCR-amplified genomic DNA identified an A --> G transition in the moderately conserved +3 position of the IVS 26 donor splice site. The mutant pre-mRNA molecules were alternatively spliced, yielding both full-length and deleted transcripts that represented less than 30% of the total pro alpha2(I) mRNA. The biochemical data on type I collagen synthesized by dermal fibroblasts showed intracellular retention of the mutant protein; failure to detect the shortened alpha2(I) chains either in the medium or in the cell layer may be the consequence of their instability at physiological temperature. These observations justified the mild resulting phenotype.

Adult↗

Differential activity of a variant form of the human Id-1 protein generated by alternative splicing.

Members of the Id family of helix-loop-helix proteins are ubiquitously expressed and dimerize with members of the class A and class B basic helix-loop-helix proteins. Due to the absence of a basic region, Id proteins act as dominant-negative antagonists of basic helix-loop-helix transcription factors, which regulate cell growth and differentiation in diverse cell types. Recent findings suggest that the functions of Id proteins are well regulated at both the transcriptional level and the post-transcriptional level. We show here that the alternative splicing variant of human Id-1 protein possesses a different binding specificity for basic helix-loop-helix transcription factors and is expressed in a cell cycle-dependent manner. Therefore, alternative splicing of Id-1 could provide a post-transcriptional mechanism to regulate Id-1 function.

Alternative Splicing↗

The human type VI collagen gene. mRNA and protein variants of the alpha 3 chain generated by alternative splicing of an additional 5-end exon.

The amino- and carboxyl-terminal globular domains of type VI collagen are composed of several homologous modules similar to the type A collagen-binding modules present in von Willebrand factor. The human alpha 3(VI) chain that contributes most of the amino-terminal globule appears heterogeneous in size as a result of alternative splicing of two exons (Stokes D. G., Saitta, B., Timpl, R., and Chu, M.-L. (1991) J. Biol. Chem. 266, 8626-8633). In the present study, we report a further characterization of the 5'-end of the gene of the human alpha 3(VI) chain and show that transcription initiates at multiple sites. Southern blotting and DNA sequencing indicate that there is an additional type A exon (A9/N10) at about 1.8 kilobase pairs downstream of the exon coding for the signal peptide. The open reading frame of this additional exon reveals 1 cysteine and three potential N-glycosylation sites. Polymerase chain reaction, Northern blotting, and RNase protection assays demonstrate that exon A9/N10 is subject to alternative splicing in normal and tumor cell lines and that this generates more protein variants of the alpha 3(VI) chain than expected before. A comparison with the corresponding amino-terminal globule of the chicken alpha 3(VI) chain shows the presence of 1 additional cysteine in this portion of the molecule and suggests that human type VI collagen has more possibilities for structural and functional variations compared to chicken type VI collagen.

Alternative Splicing↗

Molecular cloning of htra2-beta-1 and htra2-beta-2, two human homologs of tra-2 generated by alternative splicing.

A yeast two-hybrid screen was performed to find new factors involved in pre-mRNA splicing. Using SC35 as a bait, we isolated a human cDNA bearing high homology to the Drosophila transformer-2 (TRA-2) protein. This cDNA was named htra2-beta1. htra2-beta1 is a nuclear protein that colocalizes with SC35 in a speckled pattern. It interacts with several SR proteins tested in yeast. A second form named htra2-beta2 is generated by alternative splicing. This isoform gives rise to a truncated protein without an SR domain. Both isoforms are evenly distributed throughout adult rat tissue. The ratio of these two isoforms changes after stimulation of primary human T-cell and primary rat spleen cell cultures, indicating that alternative splicing is involved in regulation of htra2-beta activity.

Alternative Splicing↗

Differentially expressed messenger RNA isoforms of the human estrogen receptor-alpha gene are generated by alternative splicing and promoter usage.

The isolation and characterization of several new human estrogen receptor-alpha (hERalpha) mRNAs are described. Together with those previously identified, they give rise to a total of six hERalpha mRNA isoforms (A-F hERalpha mRNAs). Produced from a single hERalpha gene by multiple promoter usage, all these transcripts encode a common protein but differ in their 5'-untranslated region as a consequence of alternative splicing of five upstream exons (1B-1F). RT-PCR and S1 nuclease mapping analysis of these different hERalpha mRNA isoforms revealed a differential pattern of expression of the hERalpha gene in human tissues and cell types. The A hERalpha mRNA is the main isoform detected in mammary glands or in the tumor cell lines derived from this tissue. In endometrium, the predominant forms are the A and C hERalpha mRNA isoforms, whereas the C and F hERalpha mRNA isoforms are the major forms detected in ovary. Finally, high levels of the E hERalpha mRNA isoform are restricted to the liver with an increased expression in females. Taken together, our results demonstrate that the hERalpha gene is a complex genomic unit exhibiting alternative splicing and promoter usage in a tissue-specific manner.

Adolescent↗

Cloning and characterization of alternatively spliced isoforms of rat tenascin. Platelet-derived growth factor-BB markedly stimulates expression of spliced variants of tenascin mRNA in arterial smooth muscle cells.

To understand the alteration of extracellular matrix composition evoked by chemotactic factors, we have studied the expression of adhesive (fibronectin) and anti-adhesive (tenascin) proteins in response to platelet-derived growth factor-BB (PDGF-BB), a potent chemoattractant for rat aortic smooth muscle cells (ASMC). PDGF-BB markedly induced two major tenascin mRNA transcripts, whereas fibronectin mRNA levels did not change. The results of immunoprecipitation studies paralleled Northern blot data. Since alternative splicing is responsible for the generation of multiple tenascin mRNAs in other cell types, we studied the effect of chemotactic factors on the relative abundance of tenascin isoforms. The alternatively spliced region of ASMC-derived rat tenascin was amplified and the identity of the products confirmed by sequencing. Three major polymerase chain reaction products were detected: a 1727-base pair unspliced form which was maximal at 2 h and 635- and 362-base pair products which were more abundant at 8 h after treatment with PDGF-BB or angiotensin II. Functional studies showed that the unspliced isoform of human tenascin inhibited attachment of both human and rat ASMC to fibronectin. These results suggest that PDGF-BB markedly up-regulates the expression of tenascin variants, which may lead to destabilization of cell-matrix interactions and promotion of cell migration.

Alternative Splicing↗

Induced nitric oxide synthesis is dependent on induced alternatively spliced CAT-2 encoding L-arginine transport in brain astrocytes.

The inducible isoform II of nitric-oxide synthase (iNOS) was recently cloned from brain and identified in astroglial cells. Induced nitric oxide biosynthesis occurs in brain cells only if extracellular cerebrospinal fluid contains -arginine. This study demonstrates for the first time that induced iNOS activity is strictly dependent on concomitant induction of an alternatively spliced transcript of the cat-2 gene encoding high affinity -arginine transporter System y+ in cultured rat astrocytes. Inhibition profiles of radiolabeled -arginine and -leucine uptake identified the dominance of Na+-independent transport System y+ serving cationic amino acids, with insignificant activities of Systems y+L, bo,+, or Bo,+. A reverse transcription-polymerase chain reaction/sequencing/cloning strategy was used to identify a single 123-base nucleotide sequence coding the high affinity domain of alternatively spliced CAT-2 (not CAT-2a) in astrocytes activated by lipopolysaccharide/interferon-gamma. Using this sequence as a cDNA probe, it was determined that CAT-2 mRNA, iNOS mRNA, and System y+ activity were concomitantly and strongly induced in astrocytes. Constitutive CAT-1 mRNA was weakly present in neurons and astrocytes, was not inducible in either cell type, and contributed <3% to total System y+ activity. Although astroglial iNOS Km approximately 10 microM L-arginine for intracellular substrate, hyperbolic kinetics of inducible iNOS activity measured as a function of extracellular L-arginine concentration gave Km approximately 50 microM L-arginine with intact cells. The same Km approximately 50 microM was obtained for induced membrane transport System y+ activity. iNOS activity was reduced to zero in the absence of extracellular L-arginine uptake via System y+. These findings expand the current understanding of NO biosynthesis modulation and implicate a coordinated regulation of intracellular iNOS enzyme activity with membrane L-arginine transport in brain.

Alternative Splicing↗

Alternate splicing at the 3'-end of the human pancreatic tumor-associated mucin MUC4 cDNA.

MUC4 is a membrane-bound mucin and is considered as the human homologue of the rat sialomucin complex (SMC). The deduced structural organization of the wild type-MUC4 cDNA (WT-MUC4) sequence revealed two subunits: a large amino mucin type subunit (MUC4alpha) and a transmembrane subunit (MUC4beta). MUC4beta is a membrane-bound growth factor like subunit and contains three EGF-like domains. The MUC4 gene is expressed in several normal tissues like trachea, lung, and testis. It is not expressed in a normal human pancreas; however, its dysregulation results in high levels of expression in pancreatic tumors and tumor cell lines. Recently, we have demonstrated the presence of alternative splice events in the 3'-end of the MUC4 cDNA that generated new putative variants (sv1-sv10) in normal human testis and in a pancreatic tumor cell line (HPAF). In search of MUC4 variant(s) that are specific to pancreatic adenocarcinoma, we investigated the splicing phenomena in the MUC4 cDNA sequence by using a large panel of pancreatic tumor cell lines. We have identified ten alternative splice events located downstream to the central large tandem repeat domain. These splice events generated 12 variant species (sv4, sv9, sv10-18, and sv21) of MUC4 cDNAs. The deduced amino acid sequence of these variant MUC4 cDNAs revealed two distinct types: a family of secreted and a membrane-associated variant form. Among the members of MUC4 secreted variant family, three (sv4, sv12, and sv13) of ten showed a short 144 residue COOH-terminus compared to 1154 residues in WT-MUC4. The variants with this short COOH-terminus (144 residues) was found in 37% (4/11) of the tumor lines. The putative membrane-bound variant sv10 was detected in 37% (4/11) pancreatic tumor cell lines but not in any normal human tissues. In conclusion, we have identified novel splice variant(s) of MUC4 in pancreatic adenocarcinoma.

3' Untranslated Regions↗

Multiple protein forms of the human breast tumor-associated epithelial membrane antigen (EMA) are generated by alternative splicing and induced by hormonal stimulation.

Cloning and sequencing of epithelial membrane antigen (EMA) has demonstrated the existence of a variable number of tandem repeats (VNTR) flanked by unique sequences, and alternative splicing has been proposed to result in secreted and membrane-bound antigenic forms. Antisense oligonucleotides, specific for the VNTR region and various alternative splice forms, were used as probes to define EMA transcripts in poly A+ RNA from a mucinous breast tumor cell line. The BT549 line has been shown to exhibit enhanced expression and secretion of EMA when the cells are cultivated in a medium supplemented with hydrocortisone and insulin, and Northern blot analysis demonstrated that EMA-related RNA transcripts are commensurately enhanced. As a result of the large increase in EMA RNA levels, two major transcripts in BT549 have been identified as coding for either the secreted or transmembrane EMA forms and two antigenic forms have been immunoprecipitated from BT549 cell layer and medium translation products.

Antigens, Neoplasm↗

Cloning of Caenorhabditis U2AF65: an alternatively spliced RNA containing a novel exon.

The U2 small nuclear ribonucleoprotein particle (snRNP) auxiliary factor, U2AF, is an essential splicing factor required for recognition of the polypyrimidine tract and subsequent U2 snRNP assembly at the branch point. Because Caenorhabditis elegans introns lack both polypyrimidine tract and branch point consensus sequences but have a very highly conserved UUUUCAG/R consensus at their 3' splice sites, we hypothesized that U2AF might serve to recognize this sequence and thus promote intron recognition in C. elegans. Here we report the cloning of the gene for the large subunit of U2AF, uaf-1. Three classes of cDNA were identified. In the most abundant class the open reading frame is similar to that for the U2AF65 from mammals and flies. The remaining two classes result from an alternative splicing event in which an exon containing an in-frame stop codon is inserted near the beginning of the second RNA recognition motif. However, this alternative mRNA is apparently not translated. Interestingly, the inserted exon contains 10 matches to the 3' splice site consensus. To determine whether this feature is conserved, we sequenced uaf-1 from the related nematode Caenorhabditis briggsae. It is composed of six exons, including an alternatively spliced third exon interrupting the gene at the same location as in C. elegans. uaf-1 is contained in an operon with the rab-18 gene in both species. Although the alternative exons from the two species are not highly conserved and would not encode related polypeptides, the C. briggsae alternative exon has 18 matches to the 3' splice site consensus. We hypothesize that the array of 3' splice site-like sequences in the pre-mRNA and alternatively spliced exon may have a regulatory role. The alternatively spliced RNA accumulates at high levels following starvation, suggesting that this RNA may represent an adaption for reducing U2AF65 levels when pre-mRNA levels are low.

Alternative Splicing↗

Fibrillin binds calcium and is coded by cDNAs that reveal a multidomain structure and alternatively spliced exons at the 5' end.

Fibrillin is an important structural protein of the extracellular matrix. It is a large cysteine-rich glycoprotein with extensive intrachain disulfide bonds, likely contributed by multiple EGF-like repeats. We have previously published 6.9 kb of FBN1 cDNA sequence. FBN1 cDNA clones that extend the sequence 3089 bp in the 5' direction are described in this report. The deduced primary structure suggests that fibrillin is composed of multiple domains. The most predominant feature is the presence of 43 calcium binding EGF-like repeats. We demonstrate here that fibrillin molecules bind calcium. In addition, three alternatively spliced exons at the 5' end are described. Analysis of 5.8 kb of surrounding genomic sequence revealed a 1.8-kb CpG island spanning the alternatively spliced exons and the next downstream exon. Since FBN1 is the gene responsible for Marfan syndrome, the information presented here will be useful in identifying new mutations and in understanding the function of fibrillin in the pathogenesis of the disease.

Alternative Splicing↗

Alternatively spliced juxtamembrane domain of a tyrosine kinase receptor is a multifunctional regulatory site. Deletion alters cellular tyrosine phosphorylation pattern and facilitates binding of phosphatidylinositol-3-OH kinase to the hepatocyte growth factor receptor.

The hepatocyte growth factor (HGF) receptor is a tyrosine kinase receptor that mediates signal transduction upon ligand stimulation. This receptor is present in mouse tissues as two major isoforms differing by a 47-amino acid segment in the juxtamembrane domain, an alternatively spliced cytoplasmic region adjacent to the transmembrane domain of the receptor. We report here that the juxtamembrane domain of the receptor is involved in the regulation of downstream signal transduction. The two receptor isoforms were transiently expressed in COS-7 cells. Both exogenous receptors underwent autophosphorylation and subsequently stimulated a set of protein tyrosine phosphorylations that were not present in control cells. Comparisons of phosphotyrosine profiles of transfected cell lysates induced by receptor isoforms demonstrated that at least three phosphorylated proteins of approximately 62, approximately 35, and approximately 30 kDa were differentially induced by the receptor isoforms, suggesting that the juxtamembrane domain of a kinase receptor can play a role in selective signal transduction. Furthermore, the p85 subunit of phosphatidylinositol-3-OH kinase (PI3 kinase) co-precipitated with the small isoform of the HGF receptor, and this association was dramatically inhibited by treatment with 12-O-tetradecanoylphorbol-13-acetate. Since removal of the juxtamembrane domain facilitates the binding of p85 to the receptor, it is likely that the juxtamembrane region plays a role in negative regulation of the binding of PI3 kinase to the HGF receptor. Our study establishes novel molecular sequelae of alternative splicing of an intracellular domain of the HGF receptor.

Alternative Splicing↗

Missense mutation in the alternative splice region of the PAX6 gene in eye anomalies.

The PAX6 gene is involved in ocular morphogenesis, and PAX6 mutations have been detected in various types of ocular anomalies, including aniridia, Peters anomaly, corneal dystrophy, congenital cataract, and foveal hypoplasia. The gene encodes a transcriptional regulator that recognizes target genes through its paired-type DNA-binding domain. The paired domain is composed of two distinct DNA-binding subdomains, the N-terminal subdomain (NTS) and the C-terminal subdomain (CTS), which bind respective consensus DNA sequences. The human PAX6 gene produces two alternative splice isoforms that have the distinct structure of the paired domain. The insertion, into the NTS, of 14 additional amino acids encoded by exon 5a abolishes the DNA-binding activity of the NTS and unmasks the DNA-binding ability of the CTS. Thus, exon 5a appears to function as a molecular switch that specifies target genes. We ascertained a novel missense mutation in four pedigrees with Peters anomaly, congenital cataract, Axenfeldt anomaly, and/or foveal hypoplasia, which, to our knowledge, is the first mutation identified in the splice-variant region. A T-->A transition at the 20th nucleotide position of exon 5a results in a Val-->Asp (GTC-->GAC) substitution at the 7th codon of the alternative splice region. Functional analyses demonstrated that the V54D mutation slightly increased NTS binding and decreased CTS transactivation activity to almost half.

Abnormalities, Multiple↗