Peptide diversity from exon choice: RNA processing regulation in the neuroendocrine system

Molecular cloning and characterisation of a novel membrane receptor gene from the lobster Jasus edwardsii

The eyestalk of the lobster, Jasus edwardsii, is an important source for hormones involved in the regulation of growth and reproduction. How these hormones transfer their messages to the cell and nucleus is not known. This paper describes the cloning, characterization and expression analyses of two genes that code for two membrane-associated peptides that may be involved in signal transduction. These genes, peJK2 and peJK3, were isolated from a cDNA library derived from lobster eyestalk mRNAs. The two clones shared 96.6 % sequence homology, and code for putative proteins of 110 and 113 amino acids, respectively. These were likely to be two allelic forms of the same gene. Northern blot analysis using these clones as probes detected the same mRNA from eyestalk, muscle and epithelial extracts, but with greater intensity in the eyestalk extract. In situ hybridisation also indicated the predominant expression of these genes in the eyestalk. Analysis of the putative protein sequences showed that they contained two transmembrane (TM) helices, a short amino acid sequence sharing high homology with the G-protein-coupled receptor (GPCR) motif in the second TM, a signal sequence between the TMs, and a protein kinase phosphorylation site at the C termini. Sequence analyses therefore suggested that the deduced peptides may function in signal transduction.

Colocalization of immunoreactivities to serotonin, calcitonin and CGRP in endocrine pulmonary cells of the iberian lizard Podarcis hispanica (Reptilia)

The coexistence of immunoreactivities to serotonin (5HT), calcitonin (CT) and calcitonin gene-related peptide (CGRP) was studied in pulmonary endocrine cells of the Iberian lizard by immunocytochemistry and in semithin/thin sections under light and electron microscope. Immunostaining of serial sections revealed coexistence of 5HT/CT/CGRP immunoreactivities in some cells, while in others only 5HT/CT or CGRP immunoreactivities were found. Appropriate absorption controls excluded crossreactivity between the antisera used. Ultrastructurally, cells immunoreactive to 5HT/CT and CGRP share similar features, with round or slightly ovoid secretory granules of mean diameter from 165 to 180 nm. The possible functional significance of the copresence of 5HT, CT and CGRP is discussed.

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.

A novel human cytochrome P450 gene (P450IIB): chromosomal localization and evidence for alternative splicing

We have isolated from a single human liver cDNA library two clones which are highly homologous (78% over the coding region) to the major phenobarbital-inducible P450 from rat (P450IIB1). This is the first direct demonstration of the presence of the P450IIB gene subfamily in humans. This subfamily is much less extensive than the rodent homologues, but does appear to contain at least two genes. Of the cDNA clones isolated one is apparently normally spliced, whereas the other lacks exon 8 and retains all or part of intron 5. Both clones contain transcribed Alu sequences. The human P450IIB gene has been located to chromosome 19q12----19q13.2 using a probe derived from intron 5, and is close to the CYP 2A locus encoding cytochrome P450IIA2. Restriction fragment length polymorphisms have been found with the enzymes BamHI and MspI which will enable linkage to be determined between these two loci.

Molecular cloning and expression of salmon prolactin cDNA

Prolactin was purified from chum salmon pituitaries. It was resolved into two variants by reverse-phase high-performance liquid chromatography. A cDNA library was prepared from Pacific chinook salmon pituitaries. Salmon prolactin gene was screened using a synthetic oligonucleotide based on partial protein sequence. A positive clone (PRL-10) was identified and sequenced. It is a full-size clone containing 1.1 kb and coding for a preprolactin of 211 amino acids. A modified prolactin plasmid (PRL-10A), in which the 5' untranslated sequence and the nucleotide sequence coding for the signal peptide of prolactin were deleted, was reconstructed into an expression vector using the heat-inducible lambda pL promotor. Mature prolactin, a single polypeptide of 22 kDa, was efficiently expressed in the bacteria at an elevated temperature.

A nonsense mutation causes hereditary goitre in the Afrikander cattle and unmasks alternative splicing of thyroglobulin transcripts

The hereditary goitre of Afrikander cattle is an autosomal recessive disease characterized in homozygotes by the production of abnormal thyroglobulin (Tg) and the coexistence in the thyroid of normal-sized 8.4-kilobase (kb) Tg mRNA with a misspliced 7.3-kb message having lost exon 9. We have cloned and sequenced the cDNA segment corresponding to the abnormal exon 8-exon 10 junction and the relevant genomic DNA region. The mutation responsible for the disease is a cytosine to thymine transition creating a stop codon at position 697 in exon 9. The original reading frame is maintained in the 7.3-kb mRNA, which, as it lacks the mutated exon, is translatable into a potentially functional protein. This puzzling phenotype in which a mutated exon is apparently removed selectively from transcripts by alternative splicing leads us to suggest that the 7.3-kb transcript could be present in normal animals. Using a sensitive oligonucleotide hybridization assay, we have demonstrated that a 7.3-kb mRNA lacking exon 9 does exist in normal thyroids as a minor mRNA species. As it is fully translatable, the 7.3-kb mRNA is expected to be more stable than the normal-sized 8.4-kb message. This probably accounts for the higher proportion of 7.3-kb transcript found in the goitre.

Model for alternative RNA processing in human calcitonin gene expression

The alternative RNA processing pathways in human calcitonin gene (CALC-I gene) expression were investigated using steady state RNA isolated from human medullary thyroid carcinoma (MTC) and from a culture line derived from this tumor. On Northern blots the mature 1.0 kilobases (Kb) calcitonin (CT) - and 1.1 Kb calcitonin gene-related peptide (CGRP) mRNAs were detected with CALCI gene specific probes as well as high molecular weight poly (A) containing RNAs of 2.1, 2.3, 3.3, 4.2, 5.0 and 5.7 Kb. The 5.7 Kb RNA was identified as the poly(A) tailed primary transcript containing sequences corresponding to all 6 exons and 5 introns of the CALC-I gene. From the composition of the other RNAs the splicing order of the different introns could be deduced. The results suggest the following model. First all introns not involved in alternative processing (introns 1, 2 and 5) are spliced from the 5.7 Kb RNA in rapid successive reactions yielding a 3.3 Kb RNA, which accumulates. From this 3.3 Kb RNA, the last common intermediate in the alternative processing pathway, CT mRNA is formed by splicing of intron 3 and poly(A) addition at exon 4, in this order or the reverse order via 2.3 Kb or 2.1 Kb RNA intermediates respectively. Alternatively, the whole intron 3-exon 4-intron 4 region is spliced from the 3.3 Kb RNA yielding CGRP mRNA. The temporal sequence of poly(A) addition at exons 4 and 6 may relate to the observed structural differences between the poly(A) addition signals at these sites. The ratio of CT- to CGRP mRNA may relate also to the differences in the primary structures of the intron 3- and intron 4 splice acceptor sites.