Alternative splicing of c-fos pre-mRNA: contribution of the rates of synthesis and degradation to the copy number of each transcript isoform and detection of a truncated c-Fos immunoreactive species.

Alternative splicing is a widespread mechanism of gene expression regulation. Previous analyses based on conventional RT-PCR reported the presence of an unspliced c-fos transcript in several mammalian systems. Compared to the well-defined knowledge on the alternative splicing of fosB, the physiological relevance of the unspliced c-fos transcript in regulating c-fos expression remains largely unknown. This work aimed to investigate the functional significance of the alternative splicing c-fos pre-mRNA. A set of primers was designed to demonstrate that, whereas introns 1 and 2 are regularly spliced from primary c-fos transcript, intron 3 remains unspliced in part of total transcript molecules. Here, the two species are referred to as c-fos-2 (+ intron 3) and spliced c-fos (- intron 3) transcripts. Then, we used a quantitatively rigorous approach based on real-time PCR to provide, for the first time, the actual steady-state copy numbers of the two c-fos transcripts. We tested how the mouse-organ context and mouse-gestational age, the synthesis and turnover rates of the investigated transcripts, and the serum stimulation of quiescent cells modulate their absolute-expression profiles. Intron 3 generates an in-frame premature termination codon that predicts the synthesis of a truncated c-Fos protein. This prediction was evaluated by immunoaffinity chromatography purification of c-Fos proteins. We demonstrate that: (i) The c-fos-2 transcript is ubiquitously synthesized either in vivo or in vitro, in amounts that are higher or similar to those of mRNAs coding for other Fos family members, like FosB, DeltaFosB, Fra-1 or Fra-2. (ii) Intron 3 confers to c-fos-2 an outstanding destabilizing effect of about 6-fold. (iii) Major determinant of c-fos-2 steady-state levels in cultured cells is its remarkably high rate of synthesis. (iv) Rapid changes in the synthesis and/or degradation rates of both c-fos transcripts in serum-stimulated cells give rise to rapid and transient changes in their relative proportions. Taken as a whole, these findings suggest a co-ordinated fine-tune of the two c-fos transcript species, supporting the notion that the alternative processing of the precursor mRNA might be physiologically relevant. Moreover, we detected a c-Fos immunoreactive species corresponding in mobility to the predicted truncated variant.