Cardiac expression of the cystic fibrosis transmembrane conductance regulator involves novel exon 1 usage to produce a unique amino-terminal protein.
Cystic fibrosis is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene, which encodes a chloride channel present in many cells. In cardiomyocytes, we report that multiple exon 1 usage and alternative splicing produces four CFTR transcripts, with different 5'-untranslated regions, CFTR(TRAD-139), CFTR(-1C/-1A), CFTR(-1C), and CFTR(-1B). CFTR transcripts containing the novel upstream exons (exons -1C, -1B, and -1A) represent more than 90% of cardiac expressed CFTR mRNA. Regulation of cardiac CFTR expression, in response to developmental and pathological stimuli, is exclusively due to the modulation of CFTR(-1C) and CFTR(-1C/-1A) expression. Upstream open reading frames have been identified in the 5'-untranslated regions of all CFTR transcripts that, in conjunction with adjacent stem-loop structures, modulate the efficiency of translation initiation at the AUG codon of the main CFTR coding region in CFTR(TRAD-139) and CFTR(-1C/-1A) transcripts. Exon -1A, only present in CFTR(-1C/-1A) transcripts, encodes an AUG codon that is in-frame with the main CFTR open reading frame, the efficient translation of which produces a novel CFTR protein isoform with a curtailed amino terminus. As the expression of this CFTR transcript parallels the spatial and temporal distribution of the cAMP-activated whole-cell current density in normal and diseased hearts, we suggest that CFTR(-1C/-1A) provides the molecular basis for the cardiac cAMP-activated chloride channel. Our findings provide further insight into the complex nature of in vivo CFTR expression, to which multiple mRNA transcripts, protein isoforms, and post-transcriptional regulatory mechanisms are now added.
Davies
Wayne L
WL
School of Biomedical Science, University of Queensland, Brisbane, Queensland 4072, Australia.
Vandenberg
Jamie I
JI
Sayeed
Rana A
RA
Trezise
Ann E O
AE
D017398:Alternative Splicing; D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D016466:CHO Cells; D006224:Cricetinae; D019005:Cystic Fibrosis Transmembrane Conductance Regulator / Q000096:biosynthesis / Q000235:genetics*; D005091:Exons; D015870:Gene Expression; D008969:Molecular Sequence Data; D009154:Mutation; D009206:Myocardium / Q000378:metabolism*; D011817:Rabbits
D019005:Cystic Fibrosis Transmembrane Conductance Regulator