SRp55 is a regulator of calcitonin/CGRP alternative RNA splicing

Alternative splicing is an important mechanism for the regulation of gene expression. The mammalian calcitonin/calcitonin gene-related peptide (CGRP) pre-mRNA is alternatively spliced in a tissue-specific manner, leading to the production of calcitonin mRNA containing exons 1-4 in thyroid C cells and CGRP mRNA containing exons 1-3, 5, and 6 in neurons. The calcitonin-specific fourth exon contains an exonic splice enhancer (ESE) that binds SRp55. We define the RNA binding site of SRp55 in the ESE and demonstrate that base changes that decrease the level of SRp55 binding decrease the level of calcitonin splicing in vitro and calcitonin mRNA production in vivo. Base changes that increase the affinity of SRp55 for the ESE increase the level of calcitonin splicing in vitro and calcitonin mRNA levels in 293 cells. We also observe that SRp55 levels in different cell types correlate with the levels of calcitonin mRNA produced in these cells. Finally, we show that increasing the level of cellular expression of SRp55 stimulates calcitonin mRNA production in vivo. These observations suggest that SRp55 binding to a suboptimal RNA binding site in the calcitonin/CGRP pre-mRNA ESE is required for calcitonin mRNA production. Differential amounts of SRp55 present in different cell types would then control calcitonin/CGRP alternative splicing.