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.     

Histological detection of messenger RNAs with biotinylated synthetic oligonucleotide probes

We achieved histological detection of the messenger RNAs coding for vasopressin, calcitonin, or calcitonin gene-related peptide by using biotinylated synthetic oligonucleotides, and defined the technical parameters enabling optimal detection of these mRNAs. Oligonucleotides labeled by fixation of one biotin at their 5' end or by addition of a biotin-11-dUTP tail at their 3' end can be used to detect mRNAs, although the latter are more sensitive. Streptavidin-alkaline phosphatase revealed with nitroblue tetrazolium-bromo-chloro-indolyl phosphate as substrate makes possible detection of the biotinylated oligonucleotides. Increasing formaldehyde concentration in the fixative decreases the signal intensity; 1% formaldehyde fixation provides the most intense signal. Several controls, including those with addition of unlabeled oligonucleotides to the hybridization buffer, confirm the specificity of mRNA detection. The sensitivity of the biotinylated probes is identical or lower as compared to the corresponding radiolabeled oligonucleotides. Histological and subcellular resolution is greatly enhanced with biotinylated probes. The rat vasopressin probes stain magnocellular neurons in the supraoptic and paraventricular nuclei and, under optimal conditions, parvocellular neurons in the suprachiasmatic nucleus. Vasopressin mRNA is present in the cytoplasm of the cell bodies and in the roots of certain processes. Calcitonin and calcitonin gene-related peptide mRNA are found co-localized in the cytoplasm of the same tumor cells in human medullary thyroid carcinoma.     

Regulation of Alternative Polyadenylation by U1 snRNPs and SRp20

Although considerable information is currently available about the factors involved in constitutive vertebrate polyadenylation, the factors and mechanisms involved in facilitating communication between polyadenylation and splicing are largely unknown. Even less is known about the regulation of polyadenylation in genes in which 3′-terminal exons are alternatively recognized. Here we demonstrate that an SR protein, SRp20, affects recognition of an alternative 3′-terminal exon via an effect on the efficiency of binding of a polyadenylation factor to an alternative polyadenylation site. The gene under study codes for the peptides calcitonin and calcitonin gene-related peptide. Its pre-mRNA is alternatively processed by the tissue-specific inclusion or exclusion of an embedded 3′-terminal exon, exon 4, via factors binding to an intronic enhancer element that contains both 3′ and 5′ splice site consensus sequence elements. In cell types that preferentially exclude exon 4, addition of wild-type SRp20 enhances exon 4 inclusion via recognition of the intronic enhancer. In contrast, in cell types that preferentially include exon 4, addition of a mutant form of SRp20 containing the RNA-binding domain but missing the SR domain inhibits exon 4 inclusion. Inhibition is likely at the level of polyadenylation, because the mutant SRp20 inhibits binding of CstF to the exon 4 poly(A) site. This is the first demonstration that an SR protein can influence alternative polyadenylation and suggests that this family of proteins may play a role in recognition of 3′-terminal exons and perhaps in the communication between polyadenylation and splicing.