Molecular mechanisms of cell-specific and regulated expression of the calcitonin/alpha-CGRP and beta-CGRP genes.

The calcitonin/CGRP gene family utilizes several fundamental mechanisms for regulation of gene expression. The structural diversity of the family depends both on tissue-specific RNA processing and the presence of multiple, independently regulated genes. Our laboratory has been studying the structure and expression of the rat calcitonin/alpha-CGRP and beta-CGRP genes. We have studied the processing of transcripts from these genes by introducing a variety of mutated and hybrid genes into several cell lines to identify sequences critical for processing regulation. These mutant genes have ranged from point mutations to exchanges of entire splice sites, as well as chimeric constructs between the calcitonin/alpha-CGRP and beta-CGRP genes. The beta-CGRP gene provides a unique insight into the role of cis-acting sequences in tissue-specific splicing events. The rat beta-CGRP gene has an overall structure similar to that of the calcitonin/alpha-CGRP gene, but the former lacks an exon encoding a calcitonin-like hormone. Although the beta-CGRP gene contains splice junction sequences analogous to those utilized for alternative splicing in the calcitonin/alpha-CGRP gene, alternatively spliced products from regions within the beta-CGRP gene are not observed. Substitution of specific domains from the calcitonin/alpha-CGRP gene into the beta-gene can reconstitute some, but not all, aspects of alternative RNA processing. The results of transfection studies suggest that multiple regions within these genes contribute to alternative RNA splicing.