title: MIR retroposon exonization promotes evolutionary variability and generates species-specific expression of IGF-1 splice variants.
abstract: Insulin-like growth factor (IGF) -1 is a pleiotropic hormone exerting mitogenic and anti-apoptotic effects. Inclusion or exclusion of exon 5 into the IGF-1 mRNA gives rise to three transcripts, IGF-1Ea, IGF-1Eb and IGF-1Ec, which yield three different C-terminal extensions called Ea, Eb and Ec peptides. The biological significance of the IGF-1 splice variants and how the E-peptides affect the actions of mature IGF-1 are largely unknown. In this study we investigated the origin and conservation of the IGF-1 E-peptides and we compared the pattern of expression of the IGF-1 isoforms in vivo, in nine mammalian species, and in vitro using human and mouse IGF-1 minigenes. Our analysis showed that only IGF-1Ea is conserved among all vertebrates, whereas IGF-1Eb and IGF-1Ec are an evolutionary novelty originated from the exonization of a mammalian interspersed repetitive-b (MIR-b) element. Both IGF-1Eb and IGF-1Ec mRNAs were constitutively expressed in all mammalian species analyzed but their expression ratio varies greatly among species. Using IGF-1 minigenes we demonstrated that divergence in cis-acting regulatory elements between human and mouse conferred species-specific features to the exon 5 region. Finally, the protein-coding sequences of exon 5 showed low rate of synonymous mutations and contain disorder-promoting amino acids, suggesting a regulatory role for these domains. In conclusion, exonization of a MIR-b element in the IGF-1 gene determined gain of exon 5 during mammalian evolution. Alternative splicing of this novel exon added new regulatory elements at the mRNA and protein level potentially able to regulate the mature IGF-1 across tissues and species.
authors.affiliation: Department of Biomolecular Sciences, University of Urbino Carlo Bo, 61029 Urbino, Italy. Electronic address: giosue.annibalini@uniurb.it.
authors.forename: Giosuè
authors.initials: G
authors.lastname: Annibalini
authors.affiliation: Department of Biomedicine and Prevention, University of Rome Tor Vergata, 00133 Rome, Italy; Laboratory of Neuroembryology, Fondazione Santa Lucia, 00133 Rome, Italy.
authors.forename: Pamela
authors.initials: P
authors.lastname: Bielli
authors.affiliation: Department of Biomolecular Sciences, University of Urbino Carlo Bo, 61029 Urbino, Italy.
authors.forename: Mauro
authors.initials: M
authors.lastname: De Santi
authors.affiliation: Department of Biomolecular Sciences, University of Urbino Carlo Bo, 61029 Urbino, Italy.
authors.forename: Deborah
authors.initials: D
authors.lastname: Agostini
authors.affiliation: Department of Biomolecular Sciences, University of Urbino Carlo Bo, 61029 Urbino, Italy.
authors.forename: Michele
authors.initials: M
authors.lastname: Guescini
authors.affiliation: Department of Biomolecular Sciences, University of Urbino Carlo Bo, 61029 Urbino, Italy.
authors.forename: Davide
authors.initials: D
authors.lastname: Sisti
authors.affiliation: Department of Biomolecular Sciences, University of Urbino Carlo Bo, 61029 Urbino, Italy.
authors.forename: Serena
authors.initials: S
authors.lastname: Contarelli
authors.affiliation: Department of Biomolecular Sciences, University of Urbino Carlo Bo, 61029 Urbino, Italy.
authors.forename: Giorgio
authors.initials: G
authors.lastname: Brandi
authors.affiliation: Department of Preventive & Predictive Medicine, Fondazione IRCCS Istituto Nazionale dei Tumori, 20133 Milan, Italy.
authors.forename: Anna
authors.initials: A
authors.lastname: Villarini
authors.affiliation: Department of Biomolecular Sciences, University of Urbino Carlo Bo, 61029 Urbino, Italy.
authors.forename: Vilberto
authors.initials: V
authors.lastname: Stocchi
authors.affiliation: Department of Biomedicine and Prevention, University of Rome Tor Vergata, 00133 Rome, Italy; Laboratory of Neuroembryology, Fondazione Santa Lucia, 00133 Rome, Italy.
authors.forename: Claudio
authors.initials: C
authors.lastname: Sette
authors.affiliation: Department of Biomolecular Sciences, University of Urbino Carlo Bo, 61029 Urbino, Italy; IIM, Interuniversity Institute of Myology, Italy.
authors.forename: Elena
authors.initials: E
authors.lastname: Barbieri
mesh_terms: D017398:Alternative Splicing / Q000235:genetics; D000818:Animals; D019143:Evolution, Molecular; D005091:Exons / Q000235:genetics; D006801:Humans; D007334:Insulin-Like Growth Factor I / Q000235:genetics*; D008322:Mammals; D051379:Mice; D020033:Protein Isoforms / Q000235:genetics*; D018626:Retroelements / Q000235:genetics*; D013045:Species Specificity
chemical_list: D020033:Protein Isoforms; D018626:Retroelements; D007334:Insulin-Like Growth Factor I
keywords: Alternative splicing; IGF-1 isoforms; Intrinsically disordered regions; Retroposon exonization; Synonymous sites