Model for tissue specific Calcitonin/CGRP-I RNA processing from in vitro experiments

The Calcitonin/CGRP-I (CALC-I) gene is known to be expressed in a tissue specific fashion resulting in the production of Calcitonin mRNA in thyroid C-cells and CGRP-I mRNA in particular nerve cells. The alternative RNA processing reactions include splicing of exons 1, 2 and 3 to exon 4 and poly (A) addition at exon 4 (Calcitonin mRNA) or splicing of exons 1, 2 and 3 to exons 5 and 6 and poly (A) addition at exon 6 (CGRP-I mRNA). Using a model precursor RNA containing the exon 3 to exon 5 region of the human CALC-I gene we have investigated the Calcitonin- and CGRP-I mRNA-specific processing reactions in vitro, in nuclear extracts of Hela, PC12 and Ewing-1B cells, respectively. Extracts of PC12- and Ewing-1B cells were expected to perform CGRP mRNA-specific splicing, whereas Calcitonin mRNA specific processing was expected to occur in Hela cell extracts. Surprisingly, CGRP mRNA-specific splicing of exon 3 to exon 5 was the predominant reaction in all three extracts. Significant Calcitonin mRNA-specific splicing of exon 3 to exon 4 only took place upon elimination of the dominant downstream 3' splice site used in CGRP mRNA-specific splicing. This elimination occurs most definitively by cleavage at the Calcitonin mRNA specific poly (A) site at exon 4 which may then be the major regulatory mechanism for tissue-specific expression of the CALC-I gene.