Calcitonin gene-related peptide in the brain. Neurochemical and behavioral investigations

CGRP in the amygdala is concentrated in an area close to the central nucleus. High potassium releases CGRP, and this release, as well as the tissue concentration of CGRP in the amygdala, can be influenced by neuroleptic drugs. Both molecular forms, alpha- and beta-CGRP, are present in the amygdala in a ratio of approximately 3:1. CGRP influences social behavior in chicken. Therefore, CGRP may have a prominent role in psycho-behavioral function and may be a target for action and/or side effects of antipsychotic drugs.

Inhibition of gastric acid secretion and ulcers by calcitonin [correction of calciton] gene-related peptide

A central action of CGRP to inhibit gastric acid secretion, demonstrated in rats and dogs, is mediated at least in rats through modulation of parasympathetic outflow to the stomach. The centrally mediated protective effects of CGRP against ethanol-induced lesions is unique to this peptide and not shared by other centrally acting inhibitors of gastric function. It may be related to the increase in gastric mucosal blood flow induced by central CGRP. The presence of CGRP-like immunoreactivity and receptors in medullary nuclei receiving visceral information and influencing vagal outflow suggests a possible role of the peptide in the central regulation of gastric function. Peripheral injection of CGRP is well established to inhibit acid secretion in rats, dogs, rabbits, and humans. Its antisecretory effect is unlikely to be related to a direct action on the parietal cells. It involves specific and marked release of gastric somatostatin through an interaction with CGRP receptors characterized on D cells and coupled with cAMP. In addition, CGRP induces a decrease in acetylcholine transmission in the enteric nervous system, which may contribute to the inhibition of acid. The rich innervation of the stomach with CGRP-like immunoreactivity, which forms the major component of gastric sensory fibers, along with peptide release by sensory stimulation and potent actions on gastric secretions suggests a role of the peptide in the regulation of gastric function.

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.

Identification and partial characterization of the salmon calcitonin/CGRP gene by polymerase chain reaction

The polymerase chain reaction was used to amplify sequences encoding calcitonin and CGRP in the genomic DNA of salmon. Amplification products of the expected length were cloned and sequenced. In this way a new CGRP-coding sequence was identified. The new sequence and the known salmon calcitonin-coding sequence were shown to be part of one gene, implying that alternative gene expression takes place in fish.

The physiology of calcitonin gene-related peptide in the islet compared with that of islet amyloid polypeptide (amylin)

Following the discovery of a second gene containing a CGRP-like sequence, we demonstrated that "beta-CGRP" was indeed translated as a 37-amino acid peptide in vivo and was the predominant form of CGRP produced by the enteric nervous system. The presence of CGRP in the islet has been reported by several groups. We now show that beta-CGRP is again the major form. Another 37-amino acid peptide was recently isolated from islet amyloid deposits and found to have approximately 50% amino acid sequence homology with CGRP. Islet amyloid polypeptide, or amylin, is co-localized with insulin to the beta-cell secretory granule and is synthesized and released in parallel with insulin in response to a range of physiological and pharmacological stimuli. IAPP was subsequently shown, like CGRP, to inhibit the release of insulin pharmacologically. Interestingly, it was also shown to decrease the uptake of glucose by striated muscle, though it was considerably less potent than CGRP. This led to the suggestion that IAPP might be a circulating hormone regulating peripheral insulin sensitivity. Infusion of IAPP in human volunteers to produce plasma concentrations more than 100-fold higher than those seen physiologically, however, failed to alter peripheral glucose disposal. We conclude that beta-CGRP and IAPP are likely to play a role in local paracrine control of the islet.

Plasticity in expression of calcitonin gene-related peptide and substance P immunoreactivity in ganglia and fibres following guanethidine and/or capsaicin denervation

This study was designed to investigate the effects of multiple denervation procedures on calcitonin gene-related peptide- and substance P-immunoreactive neurons in sympathetic and sensory cranial ganglia and in selected targets. Sympathectomy by long-term guanethidine treatment induced a pronounced increase in calcitonin gene-related peptide-immunoreactive and substance P-immunoreactive nerve fibres in all the tissues investigated, in contrast to a significant reduction of immunoreactive cell bodies. Neonatal capsaicin treatment abolished substance P immunoreactivity in many targets and caused a dramatic reduction of substance P-immunoreactive sensory nerve cell bodies; calcitonin gene-related peptide-immunoreactive nerve density was decreased, but the number of immunoreactive nerve cell bodies was unchanged. Guanethidine treatment of capsaicin-injected rats reversed the loss of calcitonin gene-related peptide-immunoreactive nerves, but not that of substance P-immunoreactive neurons. In the iris, capsaicin treatment had little effect on calcitonin gene-related peptide- and substance P-immunoreactive nerves, suggesting that in rats the majority of these fibres originate from capsaicin-insensitive neurons. The results suggest that the denervation procedures used in this study alter the synthesis and transport of neuropeptides in sensory neurons in conjunction with changes in the number of nerve fibres.

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.

A novel calcitonin-encoding mRNA is produced by alternative processing of calcitonin/calcitonin gene-related peptide-I pre-mRNA

The calcitonin/calcitonin gene-related peptide-I (CALC-I) gene was one of the first examples of a cellular gene exhibiting alternative, tissue-specific processing of its primary RNA transcript. Calcitonin (CT) mRNA is the predominant product in thyroid C cells, whereas calcitonin gene-related peptide-I mRNA is the main product in neurons of the central and peripheral nervous systems. In this study, we report the existence of a novel CT-like (CT*) mRNA derived from the human CALC-I gene. CT* mRNA is the result of a novel splicing reaction which was initially detected in vitro. Translation of CT* mRNA will result in a prohormone (proCT*) which only differs from proCalcitonin (proCT) in its carboxyl terminus. Proteolytic processing of proCT* predicts, beside the amino-terminal cleavage peptide and CT, the existence of a novel carboxyl-terminal protein of 21 amino acids.

Endopeptidase-24.11 cleaves a chemotactic factor from alpha-calcitonin gene-related peptide

The sequence of rat alpha-calcitonin gene-related peptide (CGRP-alpha) contains the tetrapeptide eosinophil granulocyte chemotactic factor Val32-Gly-Ser-Glu35. Peptide fragments formed following hydrolysis of rat CGRP-alpha in vitro by endopeptidase-24.11 were identified. The tetrapeptide fragment was generated following cleavage at a substrate recognition site unusual for this enzyme (-Glu-Ala-). Chemotactic activity of rat CGRP-alpha was increased following hydrolysis. Furthermore, rat CGRP-beta, which lacks the tetrapeptide sequence and is completely devoid of chemotactic activity, displayed low but measurable activity after hydrolysis. Val-Gly-Ser-Glu was identified as the principle fragment with chemotactic activity in rat CGRP-alpha. The results show that the chemotactic activity of the neuropeptide rat CGRP-alpha towards eosinophil polymorphonuclear leukocytes is increased following its hydrolysis in vitro by endopeptidase 24.11 through the formation of a previously identified eosinophil chemotactic tetrapeptide.

Nasal mucosa sensitization with toluene diisocyanate (TDI) increases preprotachykinin A (PPTA) and preproCGRP mRNAs in guinea pig trigeminal ganglion neurons

Toluene diisocyanate (TDI) induces respiratory allergy in mammals. Using immunohistochemistry and in situ hybridization histochemistry, the present study examined effects of nasal mucosa sensitization by TDI on the immunoreactivity for substance P (SP) and calcitonin gene-related peptide (CGRP) and on the expression of their mRNAs in guinea pig trigeminal ganglion and their terminals. Single intranasal application of TDI (acute experiment) did not induce nasal allergy-like behaviours and failed to cause changes of SP and CGRP immunoreactivity and in the expression of preprotachykinin A (PPTA) mRNA and preproCGRP mRNA coding for SP and CGRP respectively in the trigeminal ganglion neurons. However, repeated application of TDI (chronic experiment) caused a dramatic increase of SP and CGRP immunoreactivity in peripheral neurites of sensory nerves in the nasal mucosa but a slight increase in the spinal trigeminal nucleus, a decrease of the same immunoreactivities in the cell bodies of the trigeminal ganglion neurons, and an increase of the expression of PPTA and preproCGRP mRNA in the same neurons. These findings suggest that chronic exposure of the nasal mucosa to TDI apparently causes enhancement of both the biosynthesis of SP and CGRP and their axonal transport in the trigeminal system.

Restriction fragment length polymorphism at the CALCA locus identified by the probe pEMBL36 in immigrant populations of Australia

Restriction fragment length polymorphisms detected by the cDNA probe, pEMBL36, at the CALCA locus (calcitonin gene and calcitonin gene related peptide) on TaqI blots were examined in samples from Italian, Greek and Vietnamese migrants to Melbourne, Australia and in a sample of residents from the island of Tasmania, Australia. The frequency of the rarer of the two alleles of this polymorphism, A2 (8.0kb) varied between a low of 6% in Vietnamese to a maximum of 38% in Tasmanians. The frequency range of the A2 allele in European populations, however, was considerably less. Analysis revealed no significant heterogeneity for this polymorphism among either the European or European derived populations, and these combined data exhibited a frequency of 33% for the A2 allele. Though based on a very small Vietnamese sample this study suggests that the A2 allele is less frequent in those of Asian ancestry.

Degeneration of motor nerve fibers enhances the expression of calcitonin gene-related peptide in rat sensory neurons

Calcitonin gene-related peptide (CGRP) in skeletal muscle is of neural origin. After degeneration of sensory fibers induced by removal of the lumbar dorsal root ganglia (DRG), the CGRP content in the rat soleus muscle was reduced to about 5% of the normal level. In contrast, degeneration of motor fibers induced by sectioning of the lumbar ventral roots markedly increased the CGRP content in the soleus muscle and CGRP levels in the lumbar DRG. It is suggested that the expression of CGRP in sensory neurons is up-regulated by some humoral factor induced by degeneration of motor nerve fibers.

The role of target muscles in the expression of calcitonin gene-related peptide mRNA in the spinal nucleus of the bulbocavernosus

Previous studies have shown that calcitonin gene-related peptide (CGRP) mRNA steady-state levels and CGRP immunoreactivity in motoneurons of the spinal nucleus of the bulbocavernosus (SNB) are inversely related to the gonadal steroid environment in male rats. Since both the SNB motoneurons and their target muscles, the bulbocavernosus and the levator ani, are steroid sensitive, gonadal steroids may act at either site to regulate CGRP expression. In the present study, we tested the hypothesis that gonadal steroids influence CGRP expression in SNB motoneurons through their effects on the bulbocavernosus and levator ani muscles. We determined the levels of alpha-CGRP mRNA and immunoreactive CGRP in SNB motoneurons of adult male rats following injection of the bulbocavernosus with muscle extracts from bulbocavernosus/levator ani of castrated rats, paralysis of the bulbocavernosus or pudendal nerve cuts. Following injection of the bulbocavernosus/levator ani with extracts from castrated rats, the level of CGRP expression and the number of SNB motoneurons with alpha-CGRP message were increased. These studies suggest that the bulbocavernosus/levator ani muscles from castrated rats produce a factor that increases levels of CGRP. Injections of extract prepared from the bulbocavernosus and levator ani muscles of gonadally intact rats did not change the expression of alpha-CGRP mRNA in the SNB. Paralysis of the bulbocavernosus/levator ani with a local anesthetic increased the number of SNB motoneurons expressing alpha-CGRP mRNA and CGRP immunoreactive neurons. To determine whether nerve damage accounted for the observed effects following injection of anesthetic, the pudendal nerves were cut bilaterally.(ABSTRACT TRUNCATED AT 250 WORDS)

Cooperative regulation of calcitonin gene-related peptide levels in rat sensory neurons via their central and peripheral processes

Calcitonin gene-related peptide (CGRP) is present in both motor and sensory neurons and transported in the somatofugal direction. CGRP levels in sensory neurons are assumed to be regulated by NGF supplied from their peripheral targets. In cultured sensory neurons, however, a basal level of CGRP persists even without NGF. This suggests that some additional factors may be involved in regulation of CGRP levels of sensory neurons. The present study shows that chronic section of the sciatic nerve in the rat reduces CGRP levels in the lumbar dorsal root ganglia (DRG), whereas section of dorsal roots increases CGRP levels in the DRG. This increased CGRP level by dorsal rhizotomy was associated with enhancement of the CGRP mRNA expression in the DRG. Thus, CGRP expression in DRG appears to be regulated reciprocally via their central and peripheral processes. When the sciatic nerve had been cut 1 week previously, however, dorsal rhizotomy no longer increased CGRP levels in the lumbar DRG. Therefore, stimulation of CGRP synthesis in the DRG by dorsal rhizotomy may require the integrity of the peripheral processes. When NGF had been infused into the central stump of the cut sciatic nerve, dorsal rhizotomy again increased CGRP levels in the DRG, despite prior section of the peripheral processes. We conclude that CGRP expression in sensory neurons may be regulated by cooperative action of some factors derived via their central processes and NGF supplied from the peripheral targets.

Neuronal properties of a thyroid C-cell line: partial repression by dexamethasone and retinoic acid

We have analyzed the effect of extracellular stimuli on the differentiation state of the CA77 thyroid C-cell line as a model to understand the control of neural crest cell differentiation. In contrast to the endocrine C-cell phenotype, we found that CA77 cells have a neuronal phenotype characterized by laminin-induced neurites, neuronal antigens, and calcitonin gene-related peptide (CGRP) mRNA expression. Treatment with dexamethasone and retinoic acid reversibly repressed some of these neuronal characteristics to induce features more characteristic of the parental C-cells. In the case of dexamethasone treatment, there was a partial retraction and thinning of neurites, an increased number of secretory vesicles in the cell bodies, and about a 10-fold decrease in DNA synthesis. Treatment with retinoic acid alone or in combination with dexamethasone caused decreased cell adhesion and an even more extensive retraction of the neurites. Dexamethasone also biased the steady state levels of the alternatively spliced transcripts from the calcitonin/CGRP gene to favor calcitonin relative to CGRP mRNA. While retinoic acid treatment decreased both calcitonin and CGRP mRNA levels, the combination of dexamethasone and retinoic acid still yielded the increase in calcitonin relative to CGRP mRNA. These results suggest that glucocorticoids and retinoic acid may contribute to a late and reversible differentiation of thyroid C-cells by partly repressing neuronal properties.

Maxadilan. Cloning and functional expression of the gene encoding this potent vasodilator peptide

Maxadilan is a potent vasodilator peptide released into the skin when the sand fly Lutzomyia longipalpis, an important vector of leishmania, probes for a blood meal. As several lines of evidence suggest that this peptide may play a critical role in the enhancement of leishmania infectivity attributed to sand fly saliva, the peptide has been proposed as a candidate antigen for a leishmanial vaccine. Although maxadilan is the most potent vasodilator peptide known and shares several properties with calcitonin gene-related peptide (CGRP), studies of its structure, physiological effects, and biological roles have been limited by the miniscule quantities available. Here we report the isolation of cDNA and genomic DNA clones that encode maxadilan. The predicted translation product shows no significant homology with any previously isolated proteins. The coding DNA has been expressed in Escherichia coli and the purified recombinant peptide is biologically active with a specific activity comparable to the natural peptide. Recombinant maxadilan will be useful in studies of vascular biology and could lead to novel therapeutic and prophylactic agents.

Expression of galanin in rat primary sensory afferents after moxibustion to the skin

We examined the effects of moxibustion on primary sensory neurons in the skin of rats using immunocytochemistry combined with a fluorescent retrograde tracer dye, fluoro gold (FG). Galanin-like immunoreactive (IR) fibers were often observed in the dermis of treated skin at 18 hours after moxibustion, while such fibers were rarely detected in untreated (control) skin. Moreover, most of galanin-IR fibers also displayed substance P (SP)-like immunoreactivity. About 20-30% of the dorsal root ganglion (DRG) neurons labeled when FG was injected intradermally into the moxibustion-treated skin showed galanin-like immunoreactivity, while the proportion of FG-labeled neurons with such immunoreactivity was less than 10% in control DRGs. These results show that moxibustion induced galanin expression by primary sensory neurons containing SP. The possible functions of this peptide are discussed in relation to the effects of moxibustion.

Amylin

Amylin is a 37 amino-acid peptide which is secreted from the pancreatic islets of Langerhans. It has major sequence homology with calcitonin gene related peptide. Amylin can precipitate out in these cells to form amyloid. Amylin is secreted by similar stimuli to those that secrete insulin. Amylin has a number of effects that may counteract the effect of secreted insulin, i.e., decreased second phase insulin secretion, increased hepatic glucose output, and inhibition of insulin effects on skeletal muscle. It must, however, be recognized that in many cases the doses necessary to produce these effects appear to be supraphysiological. The putative role of amylin in the hyperglycemia of aging and Type II diabetes mellitus therefore remains controversial. Amylin has a number of other effects including inhibition of osteoclastic activity, vasodilatation, anorectic effects and enhanced memory retention. This review postulates a role for amylin in the pathogenesis of a number of age-related changes.