Distinct functions of alternatively spliced isoforms encoded by zebrafish mef2ca and mef2cb.
In mammals, an array of MEF2C proteins is generated by alternative splicing (AS), yet specific functions have not been ascribed to each isoform. Teleost fish possess two MEF2C paralogues, mef2ca and mef2cb. In zebrafish, the Mef2cs function to promote cardiomyogenic differentiation and myofibrillogenesis in nascent skeletal myofibers. We found that zebrafish mef2ca and mef2cb are alternatively spliced in the coding exons 4-6 region and these splice variants differ in their biological activity. Of the two, mef2ca is more abundantly expressed in developing skeletal muscle, its activity is tuned through zebrafish development by AS. By 24hpf, we found the prevalent expression of the highly active full length protein in differentiated muscle in the somites. The splicing isoform of mef2ca that lacks exon 5 (mef2ca 4-6), encodes a protein that has 50% lower transcriptional activity, and is found mainly earlier in development, before muscle differentiation. mef2ca transcripts including exon 5 (mef2ca 4-5-6) are present early in the embryo. Over-expression of this isoform alters the expression of genes involved in early dorso-ventral patterning of the embryo such as chordin, nodal related 1 and goosecoid, and induces severe developmental defects. AS of mef2cb generates a long splicing isoform in the exon 5 region (Mef2cbL) that predominates during somitogenesis. Mef2cbL contains an evolutionarily conserved domain derived from exonization of a fragment of intron 5, which confers the ability to induce ectopic muscle in mesoderm upon over-expression of the protein. Taken together, the data show that AS is a significant regulator of Mef2c activity.
University of Modena and Reggio Emilia, Department of Life Sciences, Italy; Randall Division of Cell and Molecular Biophysics, New Hunt's House, Guy's Campus, King's College London, SE1 1UL, UK.
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Ganassi
University of Modena and Reggio Emilia, Department of Life Sciences, Italy.
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Badodi
University of Modena and Reggio Emilia, Department of Life Sciences, Italy.
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Polacchini
University of Modena and Reggio Emilia, Department of Life Sciences, Italy.
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Baruffaldi
University of Modena and Reggio Emilia, Department of Life Sciences, Italy.
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Battini
Randall Division of Cell and Molecular Biophysics, New Hunt's House, Guy's Campus, King's College London, SE1 1UL, UK.
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Hughes
Randall Division of Cell and Molecular Biophysics, New Hunt's House, Guy's Campus, King's College London, SE1 1UL, UK. Electronic address: yaniv.hinits@kcl.ac.uk.
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Hinits
University of Modena and Reggio Emilia, Department of Life Sciences, Italy. Electronic address: susanna.molinari@unimore.it.
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Molinari
D017398:Alternative Splicing / Q000235:genetics; D000818:Animals; D002454:Cell Differentiation / Q000235:genetics*; D018507:Gene Expression Regulation, Developmental; D006023:Glycoproteins / Q000096:biosynthesis / Q000235:genetics; D051956:Goosecoid Protein / Q000096:biosynthesis / Q000235:genetics; D036341:Intercellular Signaling Peptides and Proteins / Q000096:biosynthesis / Q000235:genetics; D064326:MEF2 Transcription Factors / Q000235:genetics*; D024510:Muscle Development / Q000235:genetics*; D009124:Muscle Proteins / Q000235:genetics*; D055456:Nodal Signaling Ligands / Q000096:biosynthesis / Q000235:genetics; D020033:Protein Isoforms / Q000235:genetics; D012326:RNA Splicing / Q000235:genetics; D015027:Zebrafish / Q000235:genetics / Q000254:growth & development; D029961:Zebrafish Proteins / Q000096:biosynthesis / Q000235:genetics*
D006023:Glycoproteins; D051956:Goosecoid Protein; D036341:Intercellular Signaling Peptides and Proteins; D064326:MEF2 Transcription Factors; C588168:Mef2ca protein, zebrafish; C559872:Mef2cb protein, zebrafish; D009124:Muscle Proteins; D055456:Nodal Signaling Ligands; D020033:Protein Isoforms; D029961:Zebrafish Proteins; C497995:gsc protein, zebrafish; C485171:ndr1 protein, zebrafish; C054234:chordin
Alternative splicing; Development; Mef2ca; Mef2cb; Skeletal muscle; Zebrafish