Transfection of calcitonin gene regulatory elements into a cell culture model of the C cell

CGRP physiology, pharmacology, and therapeutic targets: migraine and beyond

Calcitonin gene-related peptide (CGRP) is a neuropeptide with diverse physiological functions. Its two isoforms (α and β) are widely expressed throughout the body in sensory neurons as well as in other cell types, such as motor neurons and neuroendocrine cells. CGRP acts via at least two G protein-coupled receptors that form unusual complexes with receptor activity-modifying proteins. These are the CGRP receptor and the AMY1 receptor; in rodents, additional receptors come into play. Although CGRP is known to produce many effects, the precise molecular identity of the receptor(s) that mediates CGRP effects is seldom clear. Despite the many enigmas still in CGRP biology, therapeutics that target the CGRP axis to treat or prevent migraine are a bench-to-bedside success story. This review provides a contextual background on the regulation and sites of CGRP expression and CGRP receptor pharmacology. The physiological actions of CGRP in the nervous system are discussed, along with updates on CGRP actions in the cardiovascular, pulmonary, gastrointestinal, immune, hematopoietic, and reproductive systems and metabolic effects of CGRP in muscle and adipose tissues. We cover how CGRP in these systems is associated with disease states, most notably migraine. In this context, we discuss how CGRP actions in both the peripheral and central nervous systems provide a basis for therapeutic targeting of CGRP in migraine. Finally, we highlight potentially fertile ground for the development of additional therapeutics and combinatorial strategies that could be designed to modulate CGRP signaling for migraine and other diseases.

Tissue-specific gene expression in medullary thyroid carcinoma cells employing calcitonin regulatory elements and AAV vectors

Calcitonin (CT), the major secretory product of the C cell, is also expressed in C-cell-derived neoplasia. To investigate the role of the CT gene regulatory sequence in tissue-specific gene expression, the genes coding for the herpes simplex virus thymidine kinase (HSVtk) and for the enhanced green fluorescent protein (EGFP) regulated by the CT promoter (rAAV/CT266tkneo), the CT promoter/enhancer element (rAAV/CTenhtkneo), or the cytomegalovirus (CMV) promoter (rAAV/CMVtkneo) were transduced by recombinant adenoassociated viral (AAV) vectors into the medullary thyroid carcinoma (MTC) cell lines TT and hMTC and into HeLa cells as controls. In TT cell lines and hMTC cell lines transiently infected by the rAAV/CT266tkneo viruses, a significant increase in (3)H ganciclovir uptake was observed. Upon ganciclovir treatment, TT cells infected by rAAV/CT266tkneo revealed a significant growth inhibition, which was less tissue-specific because HeLa cells were also affected by these particles (74.5%). In contrast, a minor but more tissue-specific growth inhibition (33.6%) was observed for TT cells after transient infection with the rAAV/CTenhtkneo particles. Employing EGFP controlled by CMV promoter and the individual CT regulatory sequences for transduction by rAAV particles, similar results were obtained indicating that both the CT promoter and enhancer element are required for tissue-specific gene expression in MTC.

Expression of the CT/CGRP gene and its regulation by dibutyryl cyclic adenosine monophosphate in human osteoblastic cells

There is general agreement that calcitonin (CT) inhibits bone resorption by its effects on osteoclast function. CT was also found to have direct effects on osteoblast-like cells. In this study, we investigated the expression of CT and calcitonin gene-related peptide (CGRP), the two peptides encoded by the CT/CGRP gene, in human osteosarcoma cell lines and in normal human trabecular osteoblastic cells (HOB), and we studied the modulation of CT/CGRP gene expression by dibutyryl cyclic adenosine monophosphate ((Bu)2, cAMP), a cAMP analog. We first detected by Northern blot hybridization the presence of CT and CGRP mRNAs in different osteosarcoma cell lines (OHS-4, MG-63, Saos-2, HOS-TE85) and HOB cells. In the steady state, OHS-4 cells express slightly more CT and CGRP mRNAs than other cell lines or normal human osteoblasts, in parallel with messengers of differentiated osteoblasts, such as osteocalcin (OC) and alkaline phosphatase (ALP). OHS-4 cells also express CT and CGRP proteins, as demonstrated by immunocytochemistry. Stimulation of OHS-4 cells with 1 mM (Bu)2 cAMP induced a significant increase in mRNA levels for CT (x 2.5) and CGRP (x 3), as determined by a semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR) procedure. The involvement of a transcriptional mechanism in this effect was evidenced by nuclear run-off transcription assay. In addition, (Bu)2 cAMP increased OC (x 4) and ALP (x 3) mRNA levels in OHS-4 cells. These effects were observed at 24 h and were maximal at 48 h, indicating that (Bu)2, cAMP induced cell differentiation and increased the transcription of the CT/CGRP gene in OHS-4 osteoblast-like cells. The results indicate that human osteosarcoma cells and primary human osteoblastic cells express CT and CGRP mRNA and proteins, and that (Bu)2 cAMP, an activator of protein kinase A, induces up-regulation of osteoblastic phenotypic genes and enhances CT and CGRP gene transcription, indicating that induction of osteoblastic differentiation by (Bu)2 cAMP is associated with enhanced expression of CT and CGRP in human osteoblastic cells.

An intron enhancer recognized by splicing factors activates polyadenylation

Alternative processing of the pre-messenger RNA encoding calcitonin/calcitonin gene-related peptide (CT/CGRP) involves alternative inclusion of a 3'-terminal exon (exon 4) embedded within a six exon primary transcript. Expression of CT/CGRP in transgenic mice indicates that inclusion of exon 4 occurs in a wide variety of tissues, suggesting that the factors responsible for exon 4 inclusion are widely distributed. Inclusion of exon 4 requires an enhancer sequence located within the intron downstream of the poly(A) site of exon 4. Here we show that the intron enhancer activated in vitro polyadenylation cleavage of precursor RNAs containing the CT/CGRP exon 4 poly(A) site or heterologous poly(A) sites. To our knowledge this is the first example of an intron-located enhancer that facilitates polyadenylation. Within the enhancer sequence is a 5' splice site sequence immediately preceded by a pyrimidine tract. This 5' splice site sequence was required for enhanced polyadenylation and was recognized by both U1 small nuclear ribonucleoproteins (snRNPs) and alternative splicing factor/splicing factor 2 (ASF/SF2). Enhancement of polyadenylation required U1 RNA, suggesting that the 5' splice site sequence within the enhancer mediates enhancement via interaction with factors normally associated with functional 5' splice sites. Mutation of the polypyrimidine track of the enhancer also inhibited in vitro polyadenylation cleavage. Oligonucleotide competitions and UV cross-linking indicated that the enhancer pyrimidine track binds the polypyrimidine tract binding protein (PTB), but not U2 snRNP auxiliary factor (U2AF), and that binding of PTB was required for maximal enhancer-mediated polyadenylation. These results suggest that the enhancer binds known splicing factors, and that binding of these factors activates polyadenylation cleavage. Furthermore, these results suggest that regulation of alternative processing of CT/CGRP could occur at the level of polyadenylation, rather than splicing.

An intron enhancer containing a 5' splice site sequence in the human calcitonin/calcitonin gene-related peptide gene

Regulation of calcitonin (CT)/calcitonin gene-related peptide (CGRP) RNA processing involves the use of alternative 3' terminal exons. In most tissues and cell lines, the CT terminal exon is recognized. In an attempt to define regulatory sequences involved in the utilization of the CT-specific terminal exon, we performed deletion and mutation analyses of a mini-gene construct that contains the CT terminal exon and mimics the CT processing choice in vivo. These studies identified a 127-nucleotide intron enhancer located approximately 150 nucleotides downstream of the CT exon poly(A) cleavage site that is required for recognition of the exon. The enhancer contains an essential and conserved 5' splice site sequence. Mutation of the splice site resulted in diminished utilization of the CT-specific terminal exon and increased skipping of the CT exon in both the mini-gene and in the natural CT/CGRP gene. Other components of the intron enhancer modified utilization of the CT-specific terminal exon and were necessary to prevent utilization of the 5' splice site within the intron enhancer as an actual splice site directing cryptic splicing. Conservation of the intron enhancer in three mammalian species suggests an important role for this intron element in the regulation of CT/CGRP processing and an expanded role for intronic 5' splice site sequences in the regulation of RNA processing.

Dexamethasone and activators of the protein kinase A and C signal transduction pathways regulate neuronal calcitonin gene-related peptide expression and release

Primary cultures of adult rat dorsal root ganglia (DRG) neurons were used to determine if activation of either the protein kinase A or C signal transduction pathways or treatment with the synthetic glucocorticoid dexamethasone modulate neuronal calcitonin gene-related peptide (CGRP) synthesis and release. DRG are the sites of neuronal cell bodies known to produce abundant CGRP levels, and to send axons peripherally to blood vessels and centrally to the spinal cord. Using immunocytochemical techniques, we confirmed that synthesis of immunoreactive CGRP (iCGRP) is restricted to a subpopulation of DRG neurons. Subsequently, we determined that treatment (24 h) of the neurons with either dibutyryl cAMP (1 mM) or phorbol 12-myristate 13-acetate (2 microM) increased CGRP mRNA content 2.2 +/- 0.4 (n = 6, p < 0.03) and 3.0 +/- 0.6-fold (n = 6, P < 0.02) respectively, while secreted iCGRP levels were increased 1.8 +/- 0.2 (n = 14, P < 0.005) and 4.5 +/- 1.0 (n = 14, P < 0.001)-fold over control levels. Treatment of the neurons with dexamethasone alone had no effect on CGRP expression; however, this agent was able to significantly attenuate the stimulatory effects of NGF on both CGRP mRNA accumulation and release of iCGRP. Time course studies demonstrated that in the phorbol ester treated neurons CGRP mRNA levels continued to increase at 48 h, while maximal induction with dibutyryl cAMP occurred at approximately 12 h. These results indicate that local and/or circulating factors which act through the protein kinase A and C signal transduction pathways upregulate both CGRP expression and release, while glucocorticoids attenuate the stimulatory effects of NGF.

Hybridocytochemical and immuno-ultrastructural study of calcitonin gene expression in cultured medullary carcinoma cells

The study was aimed at a morphological demonstration of calcitonin (CT) gene expression in cultured TT cells, or, more specifically, hybridocytochemical detection of CT mRNA and calcitonin gene-related peptide (CGRP) mRNA and ultrastructural localization of the two hormones. The TT cells originated from medullary carcinoma of human thyroid gland. Ultrastructural studies of TT cells demonstrated a well-developed rough endoplasmic reticulum, large Golgi apparatus and low number of secretory granules. Hybridocytochemical studies showed the presence of mRNAs for CT and CGRP in all TT cells. At the ultrastructural level, double immunolabelling demonstrated that the two hormones were always expressed together in the same secretory granules. Our results provide a significant addition to the biochemical studies performed up to now and indicate that all TT cells produce both mRNAs and both hormones in parallel.

Regulation of human calcitonin gene transcription by cyclic AMP

Transcription of the human calcitonin (CT) gene is markedly increased by cAMP in the TT line of medullary thyroid carcinoma. This response is conferred by 5' flanking DNA sequences located between -132 and -252 relative to the transcription initiation site. Within this region are an upstream cyclic AMP response element (CRE), a downstream CRE flanked by two octamer motifs, and two adjacent C-rich AP2-like sequences. In transfection experiments in TT cells, the downstream CRE, combined with CT promoter sequences, generated 70% of the maximal cAMP response. The upstream CRE and the C-rich elements conferred 10 and 30% of this response, respectively. In gel mobility shift assays, specific TT cell proteins bound to each of these sequences. Therefore, the cAMP response of the CT gene is complex, requiring multiple elements acting in concert.

Identification of exon sequences and an exon binding protein involved in alternative RNA splicing of calcitonin/CGRP

Transcripts derived from the 6 exon CALC I gene are differentially processed in a tissue-specific fashion to include or exclude a calcitonin-specific exon 4. All cell types which transcribe a second calcitonin/CGRP gene, CALC II, exclude exon 4. Substitution of the first 30 nucleotides of CALC I exon 4 with analogous CALC II sequence was sufficient to prevent recognition of exon 4 in in vitro or in vivo RNA splicing systems. UV crosslinking detected a approximately 66 kDa RNA-binding protein in HeLa nuclear extract which interacted with CALC I proximal exon sequence, but not CALC II or mutant sequences. UV crosslinking of this protein was inhibited by addition of nuclear extract from a cell type which normally causes exclusion of exon 4. These results identify an important regulatory element within exon 4 and support a model in which calcitonin production requires protein interaction with this sequence to facilitate exon recognition.

Deregulation of alternative processing of Calcitonin/CGRP-I pre-mRNA by a single point mutation

The Calcitonin/CGRP-I (CALC-I) gene was one of the first examples of a cellular gene exhibiting alternative, tissue-specific processing of its primary transcript. Calcitonin (CT) mRNA is the predominant product in thyroid C-cells, whereas CGRP-I (Calcitonin Gene Related Peptide-I) mRNA is the main product in neurons of the central and peripheral nervous systems. Investigating the molecular mechanism underlying the alternative processing events, we have demonstrated that the CT-specific splice acceptor site is an intrinsical weak site due to usage of a uridine branch acceptor. The data presented in this report show that a single point mutation changing the uridine branch acceptor into a commonly preferred adenosine residue results in the predominant production of CT mRNA in otherwise CGRP-I mRNA-producing F9 cells. The results of the experiments implicate that the low efficiency of CT splicing, caused by usage of a uridine branch acceptor, allows the production of CGRP-I mRNA in neural cells.

Validation of an in vitro RNA processing system for CT/CGRP precursor mRNA

The pre-mRNA encoding calcitonin (CT) and calcitonin gene-related peptide (CGRP) is differentially processed in a tissue-specific fashion to include or exclude the calcitonin-specific exon 4. A minigene containing a viral first exon and exons 4, 5, and 6 from the human CT/CGRP gene was correctly processed in transfected HeLa or F9 teratocarcinoma cells to produce mRNA that included or excluded exon 4, respectively. This processing decision could be reproduced in vitro using nuclear extracts from these two cell lines and an RNA precursor from a similar minigene. Supplementation of extract from HeLa cells with extract from F9 cells resulted in the F9 splicing pattern in which exon 4 was excluded. This model system may be useful for the purification of splicing factors important in the regulation of this splice choice.