Calcium requirements for barium stimulation of enkephalin and vasoactive intestinal peptide biosynthesis in adrenomedullary chromaffin cells

Region-specific central nervous system expression and axotomy-induced regulation in sympathetic neurons of a VIP-beta-galactosidase fusion gene in transgenic mice

To assess the activity of cis-acting elements that direct human vasoactive intestinal peptide (VIP) expression in vivo, two independent transgenic mouse lines were created using a transgene comprised of 1.9 kb of 5'-flanking sequence of the human VIP gene joined to the Escherichia coli beta-galactosidase reporter gene. Transgene expression in brain was assessed using beta-galactosidase histochemistry and compared to the distribution of endogenous VIP expression. Transgene expression was observed in most central and peripheral nervous system sites in which endogenous VIP is expressed. We investigated whether the VIP-beta-galactosidase transgene was regulated in sympathetic neurons in experimental paradigms in which VIP regulation is dependent on the release of leukemia inhibitory factor (LIF). After dissociation in vitro and postganglionic axotomy in vivo there were parallel increases in endogenous VIP and transgene expression in superior cervical ganglia. These results indicate that the 1.9 kb region of 5'-flanking sequence of the human VIP gene includes genomic elements important for cell-specific expression and LIF-dependent regulation in neurons.

Neuropeptide genes: targets of activity-dependent signal transduction

Neuroendocrine cells respond to hormones and synaptic input by increasing or decreasing their own electrical activity and secretory output, and by changes in the repertoire of expression of neuronal genes. Neuropeptide genes are among those whose transcription rates can be dramatically up-and downregulated when neuronal activity is altered. In the last decade or so, our understanding of neuropeptide gene regulation has evolved from the concept of calcium-dependent coupling of neuropeptide secretion and biosynthesis to the current perspective of neuropeptide genes as the targets of multiple intracellular signaling pathways, entrained by intrinsic electrical activity and by transsynaptic influences. This review describes our current understanding of neuropeptide gene regulation in the adrenal gland as well as in the peripheral and central nervous systems. Particular emphasis is placed on the molecular mechanisms that allow unique patterns of expression of neuropeptide genes within specific types of neuroendocrine cells that contribute to the remarkable anatomical specificity of neuropeptide gene expression.

VIP as a cell-growth and differentiation neuromodulator role in neurodevelopment

In addition to its commonly recognized status as a neuromodulator of virtually all vital functions, including neurobiological, the neuropeptide VIP plays a role in the control of cell growth and differentiation and of neuronal survival. Through these actions, VIP, whose impact appears early in ontogeny, may possess developmental functions. VIP can be stimulatory or inhibitory on cell growth in function of the model considered. The growth regulatory actions of VIP, which are often independent of cAMP, are most likely significant when mitogenic or trophic factors, eventually released by nontarget cells, are simultaneously present in the extracellular medium. The intracellular mechanisms that mediate these actions of VIP may involve different transduction cascades triggered by subsets of VIP binding sites that may coexist in the same tissue.

Enkephalin biosynthesis is coupled to secretory activity via transcription of the proenkephalin A gene

The molecular mechanisms regulating neuropeptide and secretory protein biosynthesis in neuroendocrine cells were examined using the prototype neuropeptide and secretory proteins enkephalin and chromogranin A (CGA). Treatment with the secretogogue nicotine results in the calcium-dependent secretion of enkephalin peptides from bovine chromaffin cells in primary culture and a concomitant increase in enkephalin peptide biosynthesis. Both secretion and biosynthesis are also stimulated by cell depolarization with elevated potassium. Elevation of intracellular cyclic AMP, on the other hand, results in stimulation of enkephalin biosynthesis and long-term, but not acute, secretion of enkephalin peptides. Coupling of enkephalin biosynthesis to calcium influx occurs at the level of transcription of the enkephalin gene. Thus, potassium depolarization causes a calcium-dependent elevation of enkephalin mRNA which is preceded by an increase in the rate of transcription of the enkephalin gene in the chromaffin cell. The accumulation of enkephalin message or peptide by potassium depolarization or treatment with nicotine is prevented by D600 or hexamethonium respectively, added 1 h after addition of nicotine or KCl and following acute release, suggesting that calcium acts as a continuous rather than triggering stimulus for enkephalin biosynthesis. Sequence analysis of the bovine enkephalin promoter identified sequence conservation of three enhancers previously reported in the human gene which are required for regulation of the gene by calcium, cAMP, and phorbol ester in vitro. In contrast to the regulation of the enkephalin system, no increase in either CGA or CGB mRNA or gene transcription attended depolarization-induced secretion from chromaffin cells. Since enkephalin and CGA are co-stored in and co-released from the same secretory vesicles in these cells, the results imply that a surplus of CGA is constitutively synthesized in chromaffin cells such that compensatory up-regulation during changes in the secretory state of the cell, such as occurs for enkephalin, is not required for the secretory proteins.

Coordinate and differential regulation of proenkephalin A and PNMT mRNA expression in cultured bovine adrenal chromaffin cells: responses to secretory stimuli

The expression of proenkephalin A (ProEnk A) mRNA and phenylethanolamine N-methyltransferase (PNMT) mRNA in response to nicotine and to a number of secretagogues was examined in cultured bovine adrenal chromaffin cells. Prolonged incubation with nicotine (10 microM) resulted in a 2-fold increase in ProEnk A mRNA but had no significant effect on the level of PNMT mRNA. Similarly, prolonged stimulation with high K+ (56 mM) induced a time-dependent elevation in the level of ProEnk A mRNA reaching 4-fold basal level after 24 h incubation. By contrast, the level of PNMT mRNA was not changed by treatment with high K+. The increase in the level of ProEnk A mRNA by high K+ was abolished by the presence of 10 microM D600, a calcium channel blocker. Unlike the effects of high K+, treatment of the cells with the sodium channel activator veratridine significantly elevated the levels of both ProEnk A and PNMT mRNA. This increase in ProEnk A and PNMT mRNA levels was however less affected by D600. Stimulation of the cells with Ba2+ (1.1 mM) also stimulated the levels of ProEnk A and PNMT mRNA and this action required the presence of extracellular Ca2+. This was in contrast to the effect of Ba2+ in stimulating catecholamine secretion, which was inhibited by Ca2+ and enhanced in Ca2(+)-free buffer. The results of the present study indicate that membrane depolarization and entry of extracellular Ca2+ play an important role on the regulation of ProEnk A and PNMT mRNAs, in addition to their well-known actions on hormone secretion. Furthermore, these results suggest that the expression of ProEnk A mRNA and PNMT mRNA are under independent regulation in response to secretory stimulation.

Expression of a chimeric VIP gene is targeted to the intestine in transgenic mice

We showed previously that a gene construction that consisted of 5.2 kb of 5' flanking sequence, the first exon, and part of the first intron of the human gene encoding vasoactive intestinal peptide (VIP) fused to the reporter gene chloramphenicol acetyltransferase (CAT) fully mimicked the diverse behavior of the endogenous VIP gene when transfected into subclones of the human neuroblastoma cell line SK-N-SH (Waschek et al., 1988). To determine if the same sequences were sufficient to target expression of a reporter to VIP-producing tissues in the mouse, we initiated a pilot study in which we generated four transgenic mice or mouse lines that contained the VIPCAT fusion gene. Detectable levels of CAT were found in the ileum of either founder or offspring of each of the transgenic mouse lines. In all other tissues tested, CAT activity was either below the level of detection or the transgene was not expressed, with the exception of one mouse in which ectopic expression in the cerebellum was observed. The results indicate that the VIP sequences utilized were sufficient to direct expression of the transgene to the intestine, but not necessarily to other sites of VIP expression. To investigate what specific DNA sequences might confer VIP expression in the intestine and other sites, we analyzed further the VIP gene in SK-N-SH subclones using VIP/luciferase fusion gene constructions. A 0.6 kb DNA fragment located between 4.0 kb and 4.6 kb upstream from the VIP transcriptional start site was found to impart a high level of expression in one subclone and an increased degree of phorbol ester induction in another. These and other data indicate that multiple transcriptional elements control VIP expression in neuroblastoma cells and are candidates as mediators of VIP gene expression in the intact animal.

VIP: molecular biology and neurobiological function

In the mammalian brain, a major regulatory peptide is vasoactive intestinal peptide (VIP). This 28 amino acid peptide, originally isolated from the porcine duodenum, was later found in the central and peripheral nervous systems and in endocrine cells, where it exhibits neurotransmitter and hormonal roles. Increasing evidence points to VIP's importance as a mediator or a modulator of several basic functions. Thus, VIP is a major factor in brain activity, neuroendocrine functions, cardiac activity, respiration, digestion, and sexual potency. In view of this peptide's importance, the mechanisms controlling its production and the pathways regulating its functions have been reviewed. VIP is a member of a peptide family, including peptides such as glucagon, secretin, and growth hormone releasing hormone. These peptides may have evolved by exon duplication coupled with gene duplication. The human VIP gene contains seven exons, each encoding a distinct functional domain on the protein precursor or the mRNA. VIP gene transcripts are mainly found in neurons or neuron-related cells. VIP gene expression is regulated by neuronal and endocrine signals that contribute to its developmental control. VIP exerts its function via receptor-mediated systems, activating signal transduction pathways, including cAMP. It can act as a neurotransmitter, neuromodulator, and a secretagog. As a growth and developmental regulator, VIP may have a crucial effect as a neuronal survival factor. We shall proceed from the gene to its multiple functions.