Met5-enkephalin-Arg6-Gly7-Leu8 immunoreactivity in the human gut

Biological redundancy of endogenous GPCR ligands in the gut and the potential for endogenous functional selectivity

This review focuses on the existence and function of multiple endogenous agonists of the somatostatin and opioid receptors with an emphasis on their expression in the gastrointestinal tract. These agonists generally arise from the proteolytic cleavage of prepropeptides during peptide maturation or from degradation of peptides by extracellular or intracellular endopeptidases. In other examples, endogenous peptide agonists for the same G protein-coupled receptors can be products of distinct genes but contain high sequence homology. This apparent biological redundancy has recently been challenged by the realization that different ligands may engender distinct receptor conformations linked to different intracellular signaling profiles and, as such the existence of distinct ligands may underlie mechanisms to finely tune physiological responses. We propose that further characterization of signaling pathways activated by these endogenous ligands will provide invaluable insight into the mechanisms governing biased agonism. Moreover, these ligands may prove useful in the design of novel therapeutic tools to target distinct signaling pathways, thereby favoring desirable effects and limiting detrimental on-target effects. Finally we will discuss the limitations of this area of research and we will highlight the difficulties that need to be addressed when examining endogenous bias in tissues and in animals.

Proenkephalin-A mRNA Is Widely Expressed in Tissues of the Human Gastrointestinal Tract

BACKGROUND

It has been shown that the non-opioid effects of Met-enkephalin, which is derived from proenkephalin-A, are mediated through a specific opioid growth factor (OGF) receptor which is assumed to be involved in the control of cell growth. The expression and tissue location of proenkephalin-A mRNA in the gastrointestinal tract remains largely unknown.

METHODS

In this study we have analyzed the expression of proenkephalin-A mRNA in the human esophagus, gastrointestinal tract and surrounding organs by means of reverse-transcriptase PCR (RT-PCR).

RESULTS

The present study demonstrates proenkephalin-A mRNA expression in the human esophagus, gastrointestinal tract, pancreas, and gallbladder.

CONCLUSION

The present study demonstrates proenkephalin-A mRNA expression in regions of the human esophagus, gastrointestinal tract, pancreas, and gallbladder tissues which provides information for the future mapping of proenkephalin-A mRNA and protein expression/co-expression at the cellular level.

Slowing of intestinal transit by fat depends on naloxone-blockable efferent, opioid pathway

Slowing of transit through the proximal small intestine by fat in the distal gut is termed the ileal brake. Intravenous naloxone, an opioid receptor antagonist, abolished the fat-induced ileal brake, suggesting that an endogenous opioid pathway may be involved in this response. To test the hypothesis that slowing of intestinal transit by fat in the distal half of the gut depends on an opioid pathway located on the efferent limb of this response, we compared intestinal transit in dogs equipped with duodenal and midgut fistulas while naloxone was either compartmentalized with oleate to the distal half of the gut or with buffer to the proximal half of the gut. We found that intestinal transit depended on the perfusion conditions (P<0.00001). Specifically, compared with ileal brake (marker recovery of 35.7+/-7.4%), intestinal transit was accelerated when naloxone was delivered into the proximal half of the gut (76.2+/-5.2%) (P<0.005) but not the distal half of the gut (29.4+/-5.4%). We conclude that slowing of intestinal transit by fat in the distal half of the gut depends on an opioid pathway located on the efferent limb of the ileal brake.

Distribution of enkephalin-like immunoreactivity in the cat digestive tract

Immunohistochemical investigations were carried out to determine the pattern of distribution of methionine- and leucine-enkephalin-like materials in the cat pylorus, duodenum, ileum and proximal and distal colon. The present results indicate that leucine-enkephalin-like materials are less densely distributed than methionine-enkephalin-like materials, but that the two patterns of distribution show some similarities. Considerable regional differences exist however in the distribution of these enkephalin-like materials in the muscular layers. In the duodenum, ileum and proximal colon, the immunoreactivity was mainly confined to the myenteric plexus and the circular muscle layer, where it was present in nerve cell bodies and in numerous fibres. In the longitudinal muscle and submucous layers, a few immunoreactive fibres were observed which sometimes surrounded blood vessels. In the pylorus and the distal colon, however, numerous immunoreactive fibres were observed in the longitudinal and circular muscle layers; the immunoreactivity was detected in the cell bodies of numerous myenteric plexus neurons but those of only a few submucous plexus neurons. In addition, the pylorus tissues contained immunoreactive plexi which were localized either within the longitudinal muscle or between the serosa and the longitudinal muscle layer. These plexi were connected to the myenteric plexus by immunoreactive nerve strands. In all the small intestinal segments studied, numerous immunoreactive varicosities were present in the deep muscular plexus, in the inner part of the circular muscle layer. Our results suggest that in cats, the nervous control of external muscular layers mediated by enkephalins shows regional differences. In the pylorus and the distal colon, it involves both the longitudinal and circular muscle layers, whereas in other intestinal segments, only the circular muscle layer is involved.

Proenkephalin gene regulation in the neuroendocrine hypothalamus: a model of gene regulation in the CNS

During the past decade, a great deal of progress has been made in studying the mechanisms by which transcription of neuropeptides is regulated by second messengers and neural activity. Such investigations, which have depended to a great extent on the use of transformed cell lines, are far from complete. Yet a major challenge for the coming decade is to understand the regulation of neuropeptide genes by physiologically and pharmacologically relevant stimuli in appropriate cell types in vivo. The proenkephalin gene, a member of the opioid gene family, has served as a model to study regulated transcription, not only in cell lines, but also in central (e.g., hypothalamic) and peripheral (e.g., adrenal) neuroendocrine tissues. Here we review regulation of proenkephalin gene expression in the hypothalamus. Several approaches, including in situ hybridization, use of transgenic mice, and the adaptation of electrophoretic mobility shift assays to complex tissues, have played critical roles in recent advances. A summary of possible future developments in this field of research is also presented.

Delta-opioid receptor agonists inhibit neuromuscular transmission in human colon

The present study was undertaken to investigate the possible role of delta-opioid receptors in the neuroregulation of human colonic motility by using a superfusion model. Spontaneous mechanical activity and responses to electrical transmural nerve stimulation of both longitudinal and circular muscle strips from the human sigmoid colon were studied. Exogenously added delta-opioid receptor agonists did not modify spontaneous contractile activities of either type of strip. Nerve stimulation induced a triphasic response composed of an initial contraction followed by a relaxation and an off-contraction. This response was mediated by cholinergic excitatory nerves and non-adrenergic, non-cholinergic excitatory and inhibitory nerves. [Met5]Enkephalin and the synthetic delta-opioid receptor agonist [D-Pen2,D-Pen5]enkephalin (DPDPE) significantly decreased the amplitude of the initial contraction and of the off-contraction. The effects of both delta-opioid receptor agonists were reduced in the presence of either the delta-opioid receptor antagonist, ICI 174864, or another delta-opioid receptor antagonist, naltrindole. ICI 174864 prevented neither the effects of a natural kappa-opioid receptor agonist, dynorphin-(1-13) nor those of the mu-opioid receptor agonist, PL017. Therefore, these data suggest that delta-opioid receptors might be involved in the neuroregulation of smooth muscle of human colon and may mediate inhibition of cholinergic and non-cholinergic excitatory transmission within the myenteric plexus.

Distribution of functional significance of Met-enkephalin-Arg6-Phe7- and Met-enkephalin-Arg6-Gly7-Leu8-like peptides in the blowfly Calliphora vomitoria. II. Immunocytochemical mapping of neuronal pathways in the retrocerebral complex and thoracic ganglion

Neuronal pathways in the retrocerebral complex and thoracico-abdominal ganglionic mass of the blowfly Calliphora vomitoria have been identified immunocytochemically with antisera against the extended-enkephalins, Met-enkephalin-Arg6-Phe7 (Met-7) and Met-enkephalin-Arg6-Gly7-Leu8 (Met-8). Neurons of the hypocerebral ganglion, immunoreactive to Met-8, have axons in the crop duct nerve and terminals in muscles of the crop and its duct. Certain neurons of the hypocerebral ganglion are also immunoreactive to Met-7, and axons from these cells innervate the heart. Met-8 immunoreactive nerve terminals invest the cells of the corpus allatum. The source of this material is believed to be a single pair of lateral neurosecretory cells in the brain. There is no Met-7 immunoreactive material in the corpus allatum. In the corpus cardiacum neither Met-7 nor Met-8 immunoreactivity is present in the cells. However, in the neuropil of the gland certain fibres, with their origins elsewhere, do contain Met-8 immunoreactivity. The most prominent neurons in the thoracic ganglion are the Met-7 immunoreactive ventral thoracic neurosecretory cells, axons from which project to neurohaemal areas in the dorsal neural sheath and also, via the ventral connective, to the brain. Co-localisation studies show that the perikarya of these cells are immunoreactive to antisera raised against several vertebrate-type peptides, such as Met-7, gastrin/cholecystokinin and pancreatic polypeptide. However, their axons and terminals show varying amounts of the peptides, suggesting differential transport and utilisation. Only a few cells in the thoracic ganglion are immunoreactive to Met-8 antisera. These lie close to the nerve bundles supplying the legs. In the abdominal ganglion, Met-8 immunoreactive neurons project to the muscles of the hindgut. This study suggests that the extended enkephalin-like peptides of Calliphora may have a variety of different roles: as neurotransmitter or neuromodulator substances; in the direct innervation of effector organs; and as neurohormones.

Enkephalins in the inferior mesenteric ganglion of the cat and in the area of the lower digestive tract innervated by this ganglion: quantification by radio-immunoassay and characterization by high pressure liquid chromatography

Met-enkephalin, Leu-enkephalin and Met-enkephalin-Arg-Gly-Leu were quantified and characterized in the cat inferior mesenteric ganglion and in the area of the lower digestive tract innervated by this ganglion, including the proximal colon, distal colon and internal anal sphincter. In the structures studied, the concentrations of enkephalins expressed as femtomole/mg of wet tissue ranged from 66 to 160 with Met-enkephalin, from 15 to 45 with Leu-enkephalin and from 2 to 12 for Met-enkephalin-arg-gly-leu. In the lower digestive tract, the Met- and Leu-enkephalin content decreased from the proximal colon to the internal anal sphincter. The Met-enkephalin versus Leu-enkephalin ratio of the structures investigated were as follows: inferior mesenteric ganglion 3.2, proximal colon 4.4, distal colon 5, internal and sphincter 4.5. In individual samples of all the structures assayed the results of high pressure liquid chromatography (HPLC) analysis pointed to the presence of authentic Met- and Leu-enkephalin. HPLC analysis could not be carried out on Met-enkephalin-Arg-Gly-Leu due to the very low concentrations of this peptide in all the structures assayed. Our results, combined with those of previous immunohistochemical and physiological studies, support the idea that enkephalins are involved in the nervous control of the motility of the lower digestive tract.

Proenkephalin A-derived peptides in the human gut

The intramural distribution of the proenkephalin A-derived peptides Leu5-enkephalin, Met5-enkephalin, Met5-enkephalin-Arg6-Phe7, and Met5-enkephalin-Arg6-Gly7-Leu8 was studied throughout the human gastrointestinal tract. A parallel distribution was found of Leu5/Met5-enkephalin, measured with a Leu5-enkephalin antiserum that cross-reacts about 30% with Met5-enkephalin, and of Met5-enkephalin-Arg6-Phe7-immunoreactivity and Met5-enkephalin-Arg6-Gly7-Leu8-immunoreactivity. In each case, high tissue concentrations were present in the submucosa and muscularis corresponding to the pyloric sphincter. Taking all different regions together, a high correlation was revealed between tissue levels of Leu5/Met5-enkephalinlike peptides and Met5-enkephalin-Arg6-Gly7-Leu8-like peptides (r = 0.89), as well as between Met5-enkephalin-Arg6-Gly7-Leu8-like peptides and Met5-enkephalin-Arg6-Phe7-like peptides (r = 0.75). Met5-enkephalin-Arg6-Phe7 immunoreactivity was accounted for by a major peak (87% +/- 3% of total immunoreactivity) coeluting with the standard peptide in Sephadex G-50 chromatography and largely composed of the authentic heptapeptide, as shown by reverse-phase high-performance liquid chromatography. Leu5/Met5-enkephalin immunoreactivity was separated by high-performance liquid chromatography into peaks composed of Leu5-enkephalin and Met5-enkephalin. Allowing for Met5-enkephalin immunoreactivity in the assay used, the apparent Leu5/Met5-enkephalin molecular ratio was approximately 1:4. The high concentration of all peptides studied at the pyloric junction suggests a rich enkephalin-containing innervation at this level, in keeping with the proposed involvement of an enkephalinergic mechanism in the control of pyloric function.

Proenkephalin A processing in the upper digestive tract: isolation and characterisation of phosphorylated N-terminally extended Met-enkephalin Arg6Phe7 variants

Previous studies suggest the processing of proenkephalin A in the porcine upper digestive tract might differ from that in the brain. To characterise more precisely some of the products, we have used antibodies to Met-enkephalin Arg6Phe7 (MERF) in radioimmunoassay to monitor the isolation of immunoreactive peptides from extracts of porcine pyloric antral muscle, antral mucosa, and duodenum. Sephadex G50 gel filtration of each extract produced a single broad peak of high-molecular-weight MERF-immunoreactivity. On anion-exchange chromatography the antral muscle MERF-immunoreactivity fractionated into two major peaks, and that from the antral mucosa and duodenum each into four major peaks, suggesting tissue specific processing of proenkephalin A within the porcine gut. Reverse-phase HPLC and Edman degradation analysis revealed that the least acidic antral muscle peptide was a 31-residue N-terminally extended form of MERF that is equivalent to proenkephalin A 209-239. Alkaline phosphatase digestion of the N-terminally extended MERF variants indicated that some of these peptides were modified by phosphorylation. We conclude that there are complex patterns of proenkephalin A processing in the porcine gut, which in part are due to phosphorylation.

Characterisation of N-terminally extended met-enkephalin Arg6Gly7Leu8 variants in the porcine upper digestive tract

Proenkephalin A-derived peptides are known to occur in the gut, but their precise identity is uncertain. We report here the isolation of N-terminally extended forms of Met-enkephalin Arg6Gly7Leu8 from porcine upper digestive tract monitored by radioimmunoassay. A single major form was identified in pyloric antral muscle and mucosa, but in the duodenum two major forms were detected. Microsequence analysis together with immunological data revealed that the antral mucosal peptide and the most acidic duodenal peptide had identical amino-acid sequences, corresponding to a 5.3 kDa peptide terminating in Met-enkephalin Arg6Gly7Leu8. The data indicate that high-molecular-weight peptides may constitute a major proportion of gut opioid peptide immunoactivity.

Quantification and characterization of enkephalins in the upper part of the cat digestive tract and the coeliac ganglia

The [Met]enkephalin, [Leu]enkephalin and [Met]enkephalin-arg-gly-leu contents of the upper part of the digestive tract (lower oesophageal sphincter, fundus, antrum, pylorus, duodenum, ileum) and coeliac ganglia of the cat were determined and identified. The enkephalin content of all the structures studied, expressed in femtomole/mg of wet tissue, was found to range from 83 to 446 with [Met]enkephalin; 19 to 63 with [Leu]enkephalin; 2.5 to 13 with [Met]enkephalin-arg-gly-leu. In the muscular and plexus layers the [Met]- and [Leu]enkephalin contents increase gradually from the lower oesophageal sphincter to the pylorus and then decrease from the duodenum to the ileum. The [Met]enkephalin versus [Leu]enkephalin ratio is 2.7 in the coeliac ganglia and ranges from 4.3 to 8.1 in the areas of the digestive tract investigated. In addition, the presence of authentic [Met]- and [Leu]enkephalin was confirmed in all the structures assayed by high pressure liquid chromatography. Owing to the low amounts of [Met]enkephalin-arg-gly-leu detected in individual samples of the coeliac ganglia and in the areas of the digestive tract investigated, it was not possible to characterize this peptide using high pressure liquid chromatography and therefore to confirm the presence of authentic [Met]enkephalin-arg-gly-leu in these structures. The differences in the enkephalin concentrations observed among these various areas of the digestive tract suggest that these peptides may act differently from one area to another, thus playing a complex integrative role in the nervous control of gastrointestinal tract motility.

Regional distribution of immunoreactive dynorphin A in the human gastrointestinal tract

Immunoreactive dynorphin A (ir-Dyn A) was detected throughout the human gastrointestinal tract by a validated radioimmunoassay. Moreover, the stability of 125I-Dyn A during extraction procedures was confirmed by high performance liquid chromatography. Levels of ir-Dyn A were higher in the stomach and in the small bowel. In tissue samples separated into the main layers composing the gut wall (muscularis externa, submucosa and mucosa) ir-Dyn A was uniformly distributed. An exception was the colon, where concentrations were higher in the muscular portion. Gel permeation chromatography on samples of mucosa and muscularis externa extracts of ileum and gastric fundus, showed immunoreactive material eluting in several forms of apparently higher molecular weight than Dyn A, while only a minor peak was found to coelute with authentic Dyn A.

Intramural distribution of Met5-enkephalin-Arg6-Gly7-Leu8 in sphincter regions of the human gut

The intramural distribution of Met5-enkephalin-Arg6-Gly7-Leu8 (MERGL) was studied in the oesophago-cardiac, pyloric, ileo-caecal and sigmoid-recto-anal regions of the human digestive tract. Serial samples encompassing each area were separated into mucosa, submucosa and muscularis externa and extracted for radioimmunoassay. Comparatively low levels of MERGL immunoreactivity were measured throughout the cardiac junction. Conversely, a remarkable peak of MERGL concentration was detected at the pyloric junction, in both submucosa and muscularis. A progressive decrease in tissue levels of the same peptide, most evident in the submucosa, was detected on the proximal side of the ileo-caecal region. In the distal sigmoid colon and rectum MERGL concentrations showed a rapid decline, down to very low levels in the anal canal. The results may suggest the involvement of an enkephalinergic mechanism in the control of the human pylorus.

Opioids and opioid receptors in peripheral tissues

Opioid peptides belonging to the enkephalin, beta-endorphin or dynorphin family, acting on specific opiate receptors may be found in peripheral tissues. Enkephalins have a widespread peripheral distribution, while beta-endorphin and dynorphin may be found locally in the enteric nervous system. The peptides of the various families are formed from specific precursor molecules. Apart from the enteric nervous system, opioids are also found in the adrenal medulla as well as in several autonomic ganglia. There is some evidence of three different classes of opioid receptors in peripheral tissues, i.e. mu-, delta- and kappa-receptors. These receptors are not only found on enteric nervous and mucosa cells but also on various cells in the immune system where opioid peptides seem to have important actions and appear to link the neuroendocrine and immune systems to control immunological functions. The physiological as well as the pathophysiological role of opioid peptides in the periphery is gradually being elucidated and, based on such knowledge, new therapeutic implications in gastrointestinal or immune diseases may be developed.