Strategies for studying opioid peptide regulation at the gene, message and protein levels

Endogenous Opioids and Their Role in Stem Cell Biology and Tissue Rescue

Opioids are considered the oldest drugs known by humans and have been used for sedation and pain relief for several centuries. Nowadays, endogenous opioid peptides are divided into four families: enkephalins, dynorphins, endorphins, and nociceptin/orphanin FQ. They exert their action through the opioid receptors (ORs), transmembrane proteins belonging to the super-family of G-protein-coupled receptors, and are expressed throughout the body; the receptors are the δ opioid receptor (DOR), μ opioid receptor (MOR), κ opioid receptor (KOR), and nociceptin/orphanin FQ receptor (NOP). Endogenous opioids are mainly studied in the central nervous system (CNS), but their role has been investigated in other organs, both in physiological and in pathological conditions. Here, we revise their role in stem cell (SC) biology, since these cells are a subject of great scientific interest due to their peculiar features and their involvement in cell-based therapies in regenerative medicine. In particular, we focus on endogenous opioids’ ability to modulate SC proliferation, stress response (to oxidative stress, starvation, or damage following ischemia–reperfusion), and differentiation towards different lineages, such as neurogenesis, vasculogenesis, and cardiogenesis.

Prognostic value of neuropeptide proenkephalin A in patients with severe traumatic brain injury

High plasma proenkephalin A levels have been associated with poor clinical outcome of aneurysmal subarachnoid hemorrhage. This prospective observatory study was designed to investigate the relationship between plasma proenkephalin A levels and 1-week mortality, 6-month mortality and 6-month unfavorable outcome (defined as Glasgow Outcome Scale score of 1-3) in patients with severe traumatic brain injury. This study recruited 128 patients and 128 sex- and age-matched healthy controls. Plasma proenkephalin A levels, as measured by chemoluminescence sandwich immunoassay, were statistically significantly higher in patients than in healthy controls (239.1±93.0 pmol/L vs.81.3±22.1 pmol/L; P<0.001) and were correlated with Glasgow Coma Scale scores (r=-0.540, P<0.001). It was identified as an independent prognostic predictor of 1-week mortality [odds ratio (OR), 1.214; 95% confidence interval (CI), 1.103-1.425; P<0.001], 6-month mortality (OR, 1.162; 95% CI, 1.101-1.372; P<0.001) and 6-month unfavorable outcome (OR, 1.116; 95% CI, 1.097-1.281; P<0.001). Moreover, it had high predictive value for 1-week mortality [area under curve (AUC), 0.852; 95% CI, 0.778-0.908], 6-month mortality (AUC, 0.841; 95% CI, 0.766-0.899) and 6-month unfavorable outcome (AUC, 0.830; 95% CI, 0.754-0.891). Furthermore, its predictive value was similar to Glasgow Coma Scale score's (all P>0.05). Yet, a combined logistic-regression model did not show that it statistically significantly improved the predictive value of Glasgow Coma Scale score (all P>0.05). Thus, it was proposed that enhanced plasma proenkephalin A could be a useful, complementary tool to predict short- or long-term clinical outcome after severe traumatic brain injury.

Proenkephalin expression and enkephalin release are widely observed in non-neuronal tissues

Enkephalins are opioid peptides that are found at high levels in the brain and endocrine tissues. Studies have shown that enkephalins play an important role in behavior, pain, cardiac function, cellular growth, immunity, and ischemic tolerance. Our global hypothesis is that enkephalins are released from non-neuronal tissues in response to brief ischemia or exercise, and that this release contributes to cardioprotection. To identify tissues that could serve as potential sources of enkephalins, we used real-time PCR, Western blot analysis, ELISA, immunofluorescence microscopy, and ex vivo models of enkephalin release. We found widespread expression of preproenkephalin (pPENK) mRNA and production of the enkephalin precursor protein proenkephalin (PENK) in rat and mouse tissues, as well as in tissues and cells from humans and pigs. Immunofluorescence microscopy with anti-enkephalin antisera demonstrated immunoreactivity in rat tissues, including heart and skeletal muscle myocytes, intestinal and kidney epithelium, and intestinal smooth muscle cells. Finally, isolated tissue studies showed that heart, skeletal muscle, and intestine released enkephalins ex vivo. Together our studies indicate that multiple non-neuronal tissues produce PENK and release enkephalins. These data support the hypothesis that non-neuronal tissues could play a role in both local and systemic enkephalin-mediated effects.

Disparate spinal and supraspinal opioid antinociceptive responses in beta-endorphin-deficient mutant mice

The role of endogenous opioid systems in the analgesic response to exogenous opiates remains controversial. We previously reported that mice lacking the peptide neurotransmitter beta-endorphin, although unable to produce opioid-mediated stress-induced antinociception, nevertheless displayed intact antinociception after systemic administration of the exogenous opiate morphine. Morphine administered by a peripheral route can activate opioid receptors in both the spinal cord and brain. However, beta-endorphin neuronal projections are confined predominantly to supraspinal nociceptive nuclei. Therefore, we questioned whether the absence of beta-endorphin would differentially affect antinociceptive responses depending on the route of opiate administration. Time- and dose-response curves were obtained in beta-endorphin-deficient and matched wild-type C57BL/6 congenic control mice using the tail-immersion/withdrawal assay. Null mutant mice were found to be more sensitive to supraspinal (i.c.v.) injection of the micro-opioid receptor-selective agonists, morphine and D-Ala(2)-MePhe(4)-Gly-ol(5) enkephalin. In contrast, the mutant mice were less sensitive to spinal (i.t.) injection of these same drugs. Quantitative receptor autoradiography revealed no differences between genotypes in the density of mu, delta, or kappa opioid receptor binding sites in either the spinal cord or pain-relevant supraspinal areas. Thus we report that the absence of a putative endogenous ligand for the mu-opioid receptor results in opposite changes in morphine sensitivity between discrete areas of the nervous system, which are not simply caused by changes in opioid receptor expression.

Species-dependent pharmacological properties of the melanocortin-5 receptor

The genes encoding the melanocortin-3 receptor and melanocortin-5 receptor have been cloned from rhesus monkey. Heterologous expression in CHO cells indicated species dependent in vitro pharmacological properties for the human and rhesus melanocortin-5 receptors. Several peptides including NDP-alpha-MSH, alpha-MSH, MT-II and ACTH1-24 are more potent at the rhesus melanocortin-5 receptor than the human melanocortin-5 receptor by more than 10-fold. In contrast, we found no species difference in pharmacological properties between the human and rhesus melanocortin-3 receptors. Such a species-dependent pharmacological difference for melanocortin-5 receptor appears to be an exception compared to other G protein-coupled receptors from human and rhesus monkey.

Met5-enkephalin protects isolated adult rabbit cardiomyocytes via delta-opioid receptors

In rats and rabbits, endogenous opioid peptides participate in ischemic preconditioning. However, it is not known which endogenous opioid(s) can trigger cardioprotection. We examined preconditioning-induced and opioid-induced limitation of cell death in isolated, calcium-tolerant, adult rabbit cardiomyocytes. Cells were subjected to simulated ischemia by pelleting and normothermic hypoxic incubation. Preconditioning was elicited with 15 min of simulated ischemia followed by 15 min of resuspension and reoxygenation. All cells underwent 180 min of simulated ischemia. Cell death was assessed by trypan blue permeability. Morphine protected cells, as did preconditioning; naloxone blocked the preconditioning-induced protection. Exogenous Met5-enkephalin (ME) induced protection, but exogenous beta-endorphin did not. ME-induced protection was blocked by the delta-selective antagonist naltrindole. Additionally, two other proenkephalin products, Leu5-enkephalin and Met5-enkephalin-Arg-Phe, provided protection equipotent to ME. These data suggest that one or more proenkephalin products interact with delta-opioid receptors to endogenously trigger opioid-mediated protection.

Regulation of beta-endorphin receptor expression in mouse spleen cells with Con A and rIL-2

The expression of the beta-endorphin receptor on both activated and unstimulated mouse spleen cells was studied. Results showed that unstimulated cells have only one type of beta-endorphin receptor with a specific low affinity (Kd = 1.034 +/- 0.0237 x 10(-7) M, 25,000 sites/cell). After Con A stimulation, cells express two types of receptors, one with a low affinity (Kd = 1.034 +/- 0.024 x 10(-7) M, 320,000 sites/cell) and the other with a high affinity (Kd = 1.052 +/- 0.033 x 10(-9) M, 49,000 sites/cell). The kinetic experiments during 4 days after Con A activation indicated that the receptor of high affinity emerged from 24 to 72 h, while the low affinity one increased in number after stimulation. The receptor numbers of both high and low affinity ones reached a maximum peak at 72 h, then began to decline. The addition of exogenous rIL-2 depressed the Con A-induced increment of the receptor numbers of both the high and low affinity ones, but enhanced the proliferative response of the cells. It is suggested that the degree of the expression of the receptors does not simply depend on the mitogenic degree of the cells. In addition, our experiment demonstrated that splenocytes cultured in medium with or without Con A or Con A + rIL-2 for 96 h did not secrete any detectable amount of beta-endorphin with use of the RIA assay, which is sensitive enough to detect the much lower levels of beta-endorphin than that necessary for biological effects. We suggest that the expression of the high affinity beta-endorphin receptor on the activated T-lymphocytes may have to precede the production of IL-2 to potentiate the T-cell proliferative response. The mechanisms and modes of interaction between the neuroendocrine system and the immune system were discussed.

Beta-endorphin's modulation of lymphocyte proliferation is dose, donor, and time dependent

We find that beta-endorphin (Bend) can have, positive, negative, or neutral dose-dependent effects on the mitogen-stimulated proliferation of human peripheral blood lymphocytes. The distribution of positive, negative, or neutral responses was nonrandom. In studies carried out over a year, we show that an individual's mitogen-stimulated lymphocyte proliferative response to Bend can change with time. We show that the inhibition induced by cortisol can be, in part, relieved by Bend. On the basis of our results and those of others in the field, we put forward a model that can qualitatively account for many of the observations we and other investigators have made.

Detection of Met-enkephalin and Leu-enkephalin in the brain of the hagfish, Eptatretus stouti, and the lamprey, Petromyzon marinus

Acid extracts of the brain of the pacific hagfish, Eptatretus stouti, and the marine lamprey, Petromyzon marinus, were each fractionated by gel filtration chromatography and aliquots of column fractions were screened with radioimmunoassays (RIAs) specific for pro-dynorphin-related end products and for pro-enkephalin-related end products. Only pro-enkephalin-related immunoreactive forms were detected. The enkephalin-sized immunoreactive material, isolated for each species, was separately fractionated by reverse-phase high-performance liquid chromatography (HPLC). Aliquots of column fractions were screened with RIAs specific for Met-enkephalin, Leu-enkephalin, Met-enkephalin-Arg-Phe, and Met-enkephalin-Arg-Gly-Leu. In the hagfish brain, immunoreactive forms with the same retention times as synthetic Met-enkephalin and Leu-enkephalin were detected in a ratio of approximately 2:1. In addition, an immunoreactive form was detected with the Met-enkephalin-Arg-Phe-specific RIA. This form had the same chromatographic properties as synthetic Met-enkephalin-Arg-Phe. Analyses with the Met-enkephalin-Arg-Gly-Leu RIA were negative. HPLC analysis of the lamprey enkephalin-related material revealed the presence of authentic Met-enkaphalin and Leu-enkephalin in a molar ratio of 3:1. C-terminally extended forms of Met-enkaphalin were not detected in the lamprey extracts. Collectively these observations indicate that pro-enkephalin-related opioid peptides are present in the brain of cyclostomes.

Regulation of pro-opiomelanocortin biosynthesis and processing by transplantation immunity

It is more than thirty years since Billingham and Medawar showed that adrenocorticotrophic hormone (ACTH) and cortisol can prolong the survival of skin allografts. It has since become clear that glucocorticoid hormones are critically involved in the regulation of immunity. The level of glucocorticoids secreted in response to antigenic challenge corresponds to the magnitude of the immune response and in general reaches immunosuppressive levels. Interestingly, not all immune responses enhance ACTH and glucocorticoid hormone production. In transplantation immunity, the reverse seems to be true: circulating glucocorticoid levels at the time of skin graft rejection are lower than control levels. Because beta-endorphin and ACTH originate from the same prohormone, pro-opiomelanocortin (POMC), and are closely related in their tissue-specific processing and coordinate release, we have investigated the role of pituitary beta-endorphin in transplantation immunity. We report here that POMC biosynthesis and processing in the pars intermedia, but not in the anterior pituitary, can be regulated by T cell-specific factors secreted in animals undergoing transplantation immunity.

Beta-endorphin genetics in the etiology of alcoholism

"Reinforcing" effects are ascribed to endogenous opioids, particularly to the pro-opiomelanocortin (POMC)-derived beta-endorphin 1-31, the most potent opiate-active substance. Alcohol induces variations in the genetic processing of the precursor POMC and of beta-endorphin at different levels. Studies focused on changes in POMC gene expression (mRNA quantitation) and post-translational processing. Chronic alcohol intake significantly reduces POMC mRNA in the lobes of the pituitary. In inbred strains of mice, genotypic differences are seen in post-translational processing of hypothalamic beta-endorphin, thus inducing differences in alcohol sensitivity. Clinical studies show a disproportion of POMC cleavage products in the CSF of chronic alcoholics (reduced beta-endorphin versus increased ACTH contents), together with remarkable indications for baseline differences in beta-endorphin levels. Errors within the genetic sequence of POMC are suggested to underlie alcohol-seeking behavior.

Isolation of immunoreactive beta-endorphin-related and Met-enkephalin-related peptides from the posterior pituitary of the amphibian, Xenopus laevis

Acid extracts of the posterior pituitary of the amphibian, Xenopus laevis, were analyzed with two heterologous region specific beta-endorphin RIAs. Following gel filtration chromatography and cation exchange chromatography four peaks of immunoreactivity were detected. All four peaks were detected with a N-acetyl specific beta-endorphin RIA. Peak I represented 92% of the total immunoreactivity isolated following cation exchange chromatography. This peak had a net positive charge at pH 2.5 of +1 and an apparent molecular weight of 1.4 Kd. Following reverse phase HPLC, Peak I fractionated into two peaks: Peak Ia and Peak Ib. Both peaks were detected with the N-acetyl specific beta-endorphin RIA and a Met-enkephalin RIA, however, neither peak co-migrated with either Met-enkephalin or N-acetyl-beta-endorphin(1-16). At present it is not clear whether Peak I is derived from pro-opiomelanocortin or one of the other opioid polyproteins. Peaks II, III, and IV represented 8% of the total immunoreactivity recovered following cation exchange chromatography. These peaks had net positive charges of +3, +4, and +5, respectively, and apparent molecular weights of 2.8, 3.2, and 3.5 Kd, respectively. These apparently N-acetylated beta-endorphin-sized forms are minor end products of the pro-opiomelanocortin biosynthetic pathway.

Brain opioids and autism: an updated analysis of possible linkages

Considerable clinical evidence suggests that autistic children lack the normal ability or desire to engage others socially, as indicated by their poor social skills and inappropriate use of language for communicative purposes. Specifically, these children seem to lack normal amounts of social-emotional interest in other people, leading perhaps to a decreased initiative to communicate. This paper summarizes experimental evidence supporting a neurological theory, which posits that autism, at least partially, represents in the brain, such as brain opioids. These substances modulate social-emotional processes, and the possibility that blockade of opioid activity in the brain may be therapeutic for early childhood autism is discussed.

In vivo effects of beta-endorphin on lymphocyte proliferation and interleukin 2 production

Experiments were undertaken in rats to investigate the effects of in vivo infusion of beta-endorphin (BEP) on subsequent Con A-induced proliferation and interleukin 2 (IL-2) production by spleen cells in vitro. BEP administration induced a dose-dependent enhancement of the proliferative response to Con A. Infusion of the opiate antagonist naloxone (NAL) inhibited the Con A response and infusion of NAL prior to BEP resulted in even further inhibition. None of these treatments resulted in detectable alterations in IL-2 production after 48 h in culture. To demonstrate a direct interaction between BEP and lymphocytes, spleen cells were incubated in vitro with varying concentrations of BEP and/or NAL. Enhanced Con A-induced proliferation was observed following incubation with BEP in the range 10(-12) to 10(-9) M (levels comparable to the effective in vivo doses) and this effect was abrogated by NAL pretreatment (10(-6) M). These data indicate a role for BEP in enhancing lymphocyte reactivity which is to some extent dependent on opiate receptors on the cell surface. This report extends the evidence obtained from in vitro experiments implicating endogenous opioids in modulation of host immunity by demonstrating that these effects can be obtained in vivo.

Postnatal development of ACTH and alpha-MSH in the medulla oblongata of rat: alpha-MSH is the predominant peptide

ACTH and alpha-MSH levels were measured by radioimmunoassays in extracts of the caudal medulla oblongata of developing rats on postnatal (P) days 1-42 at 7 day intervals, and in adult rats. From P1 to adulthood, ACTH increased greater than 11-fold from 7.2 +/- 1.9 fmol to 82.4 +/- 12.6 fmol per medulla section (mean +/- S.E.M.). In comparison, alpha-MSH increased greater than 7-fold from 68.75 +/- 11.0 fmol to 491 +/- 97.8 fmol during this time period. ACTH/microgram of soluble protein decreased during postnatal development from 0.006 +/- 0.01 to 0.005 +/- 0.001 fmol/microgram of protein and alpha-MSH increased from 0.06 +/- 0.01 fmol/microgram of protein to 0.11 +/- 0.009 fmol/microgram of protein between P1 and P7, decreased to 0.015 +/- 0.003 fmol/microgram of protein by P42 and increased to 0.03 +/- 0.006 fmol/protein per unit protein by adulthood. These data indicate a significant shift in the levels of alpha-MSH detected during development with a decrease in the concentration of material occurring from early postnatal development (P1-P7) to adulthood, which does not appear to be solely related to a regional increase in protein. These studies, as well as radioimmunoassays for ACTH and alpha-MSH in combination with sizing chromatography of pooled extracts at P1, P7 and the adult, demonstrated the predominance of alpha-MSH at all ages.