Biodegradation of alpha-MSH and derived peptides by rat brain extracts, and by rat and human serum

The neuropeptide complement of the marine annelid Platynereis dumerilii

Background

The marine annelid Platynereis dumerilii is emerging as a powerful lophotrochozoan experimental model for evolutionary developmental biology (evo-devo) and neurobiology. Recent studies revealed the presence of conserved neuropeptidergic signaling in Platynereis, including vasotocin/neurophysin, myoinhibitory peptide and opioid peptidergic systems. Despite these advances, comprehensive peptidome resources have yet to be reported.

Results

The present work describes the neuropeptidome of Platynereis. We established a large transcriptome resource, consisting of stage-specific next-generation sequencing datasets and 77,419 expressed sequence tags. Using this information and a combination of bioinformatic searches and mass spectrometry analyses, we increased the known proneuropeptide (pNP) complement of Platynereis to 98. Based on sequence homology to metazoan pNPs, Platynereis pNPs were grouped into ancient eumetazoan, bilaterian, protostome, lophotrochozoan, and annelid families, and pNPs only found in Platynereis. Compared to the planarian Schmidtea mediterranea, the only other lophotrochozoan with a large-scale pNP resource, Platynereis has a remarkably full complement of conserved pNPs, with 53 pNPs belonging to ancient eumetazoan or bilaterian families. Our comprehensive search strategy, combined with analyses of sequence conservation, also allowed us to define several novel lophotrochozoan and annelid pNP families. The stage-specific transcriptome datasets also allowed us to map changes in pNP expression throughout the Platynereis life cycle.

Conclusion

The large repertoire of conserved pNPs in Platynereis highlights the usefulness of annelids in comparative neuroendocrinology. This work establishes a reference dataset for comparative peptidomics in lophotrochozoans and provides the basis for future studies of Platynereis peptidergic signaling.

Terminal signal: anti-inflammatory effects of α-melanocyte-stimulating hormone related peptides beyond the pharmacophore

During the last two decades a significant number of investigations has established the fact that α-Melanocyte-stimulating hormone (α-MSH) is a potent anti-inflammatory mediator. The anti-inflammatory effects of α-MSH can be elicited via melanocortin receptors (MC-Rs) broadly expressed in a number of tissues ranging from the central nervous system to cells of the immune system and on resident somatic cells of peripheral tissues. α-MSH affects various pathways regulating inflammatory responses such as NF-κB activation, expression of adhesion molecules, inflammatory cytokines, chemokine receptors, T-cell proliferation and activity and inflammatory cell migration. In vivo α-MSH has been shown to be anti-inflammatory as well in animal models of fever, irritant and allergic contact dermatitis, cutaneous vasculitis, fibrosis, in ocular, gastrointestinal, brain and allergic airway inflammation and arthritis. A broad range of effects of α-MSH exerted beyond the field of inflammation, its pigmentory capacity being only the most visible aspect, has been one of the major impediments limiting the use of α-MSH in human inflammatory disorders. Interestingly KPV, C-terminal tripeptide of α-MSH, which lacks the entire sequence motif required for binding to any of the known MC-Rs, retains almost all of the anti-inflammatory capacity of the full hormone, but in its activities display a lack of any pigmentory action. While the exact signaling mechanism utilized by KPV and related peptides currently is unknown it has been demonstrated already that significant similarities between anti-inflammatory signaling of α-MSH and those short peptides exist. These α-MSH related tripeptides thus may be useful alternatives for anti-inflammatory peptide therapy. KdPT, a derivative of KPV corresponding to IL-1β(193-195), currently is emerging as another tripeptide with potent anti-inflammatory effects. A more limited spectrum of biologic activities, potentially advantageous physicochemical, pharmacokinetic and pharmacodynamic properties as well as the expectation of low costs for pharmaceutical production make these agents interesting candidates for the treatment of immune-mediated inflammatory skin and bowel diseases, allergic asthma and arthritis.

Role of proopiomelanocortin-derived peptides and their receptors in the osteoarticular system: from basic to translational research

Proopiomelanocortin (POMC)-derived peptides such as melanocortins and β-endorphin (β-ED) exert their pleiotropic effects via binding to melanocortin receptors (MCR) and opioid receptors (OR). There is now compelling evidence for the existence of a functional POMC system within the osteoarticular system. Accordingly, distinct cell types of the synovial tissue and bone have been identified to generate POMC-derived peptides like β-ED, ACTH, or α-MSH. MCR subtypes, especially MC1R, MC2R (the ACTH receptor), MC3R, and MC4R, but also the μ-OR and δ-OR, have been detected in various cells of the synovium, cartilage, and bone. The respective ligands of these POMC-derived peptide receptors mediate an increasing number of newly recognized biological effects in the osteoarticular system. These include bone mineralization and longitudinal growth, cell proliferation and differentiation, extracellular matrix synthesis, osteoprotection, and immunomodulation. Importantly, bone formation is also regulated by the central melanocortin system via a complex hormonal interplay with other organs and tissues involved in energy metabolism. Among the POMC-derived peptides examined in cell culture systems from osteoarticular tissue and in animal models of experimentally induced arthritis, α-MSH, ACTH, and MC3R-specific agonists appear to have the most promising antiinflammatory actions. The effects of these melanocortin peptides may be exploited in future for the treatment of patients with inflammatory and degenerative joint diseases.

Alpha-melanocyte-stimulating hormone and related tripeptides: biochemistry, antiinflammatory and protective effects in vitro and in vivo, and future perspectives for the treatment of immune-mediated inflammatory diseases

Alpha-MSH is a tridecapeptide derived from proopiomelanocortin. Many studies over the last few years have provided evidence that alpha-MSH has potent protective and antiinflammatory effects. These effects can be elicited via centrally expressed melanocortin receptors that orchestrate descending neurogenic antiinflammatory pathways. alpha-MSH can also exert antiinflammatory and protective effects on cells of the immune system and on peripheral nonimmune cell types expressing melanocortin receptors. At the molecular level, alpha-MSH affects various pathways implicated in regulation of inflammation and protection, i.e., nuclear factor-kappaB activation, expression of adhesion molecules and chemokine receptors, production of proinflammatory cytokines and mediators, IL-10 synthesis, T cell proliferation and activity, inflammatory cell migration, expression of antioxidative enzymes, and apoptosis. The antiinflammatory effects of alpha-MSH have been validated in animal models of experimentally induced fever; irritant and allergic contact dermatitis, vasculitis, and fibrosis; ocular, gastrointestinal, brain, and allergic airway inflammation; and arthritis, but also in models of organ injury. One obstacle limiting the use of alpha-MSH in inflammatory disorders is its pigmentary effect. Due to its preserved antiinflammatory effect but lack of pigmentary action, the C-terminal tripeptide of alpha-MSH, KPV, has been delineated as an alternative for antiinflammatory therapy. KdPT, a derivative of KPV corresponding to amino acids 193-195 of IL-1beta, is also emerging as a tripeptide with antiinflammatory effects. The physiochemical properties and expected low costs of production render both agents suitable for the future treatment of immune-mediated inflammatory skin and bowel disease, fibrosis, allergic and inflammatory lung disease, ocular inflammation, and arthritis.

Effect of S 17092, a novel prolyl endopeptidase inhibitor, on substance P and alpha-melanocyte-stimulating hormone breakdown in the rat brain

In the present study, we have investigated the effects of a novel prolyl endopeptidase (EC 3.4.21.26, PEP) inhibitor, compound S 17092, on substance P (SP) and alpha-melanocyte-stimulating hormone (alpha-MSH) metabolism in the rat brain. In vitro experiments revealed that S 17092 inhibits in a dose-dependent manner PEP activity in rat cortical extracts (IC50 = 8.3 nm). In addition, S 17092 totally abolished the degradation of SP and alpha-MSH induced by bacterial PEP. In vivo, a significant decrease in PEP activity was observed in the medulla oblongata after a single oral administration of S 17092 at doses of 10 and 30 mg/kg (-78% and -82%, respectively) and after chronic oral treatment with S 17092 at doses of 10 and 30 mg/kg per day (-75% and -88%, respectively). Concurrently, a single administration of S 17092 (30 mg/kg) caused a significant increase in SP- and alpha-MSH-like immunoreactivity (LI) in the frontal cortex (+41% and +122%, respectively) and hypothalamus (+84% and +49%, respectively). In contrast, chronic treatment with S 17092 did not significantly modify SP- and alpha-MSH-LI in the frontal cortex and hypothalamus. Collectively, the present results show that S 17092 elevates SP and alpha-MSH concentrations in the rat brain by inhibiting PEP activity. These data suggest that the effect of S 17092 on memory impairment can be accounted for, at least in part, by inhibition of catabolism of promnesic neuropeptides such as SP and alpha-MSH.

Insulin in the cerebrospinal fluid

The presence, distribution and specific localization of insulin and its receptors in the central nervous system (CNS) have been described in numerous reports. Insulin in the CNS appears to be similar to pancreatic insulin by biochemical and immunological criteria. While the presence of insulin in the cerebrospinal fluid (CSF)--an essential neurohumoral transport system--has been widely reported, the available information is fragmented and therefore it is difficult to determine the significance of insulin in the CSF and to establish future research directions. This paper presents an integrative view of the studies concerning insulin in the CSF of various species including the human. Evidence suggests that insulin in the CSF and brain may be the result of local synthesis in the CNS, and uptake from the peripheral blood through the blood-brain barrier and circumventricular organs. The passage of insulin from the peripheral blood through the blood-brain barrier may be mediated by a specific transport system coupled to insulin receptors in cerebral microvessels. The transfer of insulin from the peripheral blood through the circumventricular organs is not specific and may depend on simple diffusion. Slow access of insulin to brain interstitial fluid adjacent to the blood-brain barrier and circumventricular organs may be followed by selective transport to other brain sites and into the ventricular-subarachnoideal CSF. It has been hypothesized that the choroid plexuses, which constitute the blood-CSF interface, might be a nonspecific pathway for rapid insulin transport into the CSF. Insulin may also pass from the CSF into the peripheral blood via absorption into the arachnoid villi. This evidence indicates that insulin may be transported in both directions between the CSF-brain and the peripheral blood. Evidence also suggests that the presence of insulin in the CSF is of pivotal importance for its neurophysiological or neuropathophysiological significance.

Peptide transmitters of primary sensory neurons: similar actions of tachykinins and bombesin-like peptides

Previous studies have demonstrated that two peptides, substance P (SP) and substance K (SK), are contained in a common prohormone--beta-preprotachykinin. Both peptides are cleaved from the prohormone and appear to coexist throughout the brain. This study evaluated the behavioral activity of SK and compared it to the activities of SP, bombesin (BN), and structurally related peptides. After intraspinal injection, all of the peptides induced "bite/scratch" behaviors, which differed in durations of action. The specific rank order of these durations of action were: BN greater than gastrin releasing peptide (GRP) = ranatensin (RT) greater than neuromedin B (NMB) greater than kassinin (KASS) = SK = SP and ranged from dose-dependent maxima of approximately 2 min (SP) to approximately 100 min (BN). To examine the possibility that differences in durations of action are due to differences in rates of proteolytic degradation, each peptide was incubated in spinal cord homogenates at 37 degrees C, and the degradation rates were monitored by radioimmunoassay (RIA) and by bioassay. The lengths of incubation time required to produce approximately 90% degradation of peptide immunoreactivity varied across peptides from less than 5 min (SP) to more than 60 min (BN and RT). Degradation of bioactivity generally paralleled degradation of immunoreactivity. The results of this study suggest that durations of nociceptive effects produced by the peptides tested are due, in part, to their resistance to proteolytic degradation.

Short and long term inhibitory actions of alpha-melanocyte stimulating hormone on lordosis in rats

The effects of 200 ng of intracerebroventricularly (ICV) and 20 micrograms of subcutaneously (SC) administered alpha-melanocyte stimulating hormone (MSH) on lordosis in rats were examined. Previous research, employing crossover designs, has revealed significant effects of MSH on lordosis. The results of Experiments 1a and 1b suggest that similar designs produce significant effects even in the absence of MSH. Thus, it is not clear that previous results were due exclusively to an action of MSH. Experiment 2 employed a modification of previous procedures and indicated that MSH administered either SC or ICV inhibited receptivity in subjects displaying high levels of responding. Moreover, MSH administered SC was also found to facilitate receptivity in subjects displaying low levels of responding. However, a possible long term inhibitory action of MSH on receptivity was also revealed. Because animals were tested repeatedly, this brought into question the results of Experiment 2. Procedures were revised accordingly and the effects of MSH re-examined. The results of Experiment 3 indicated that MSH administered SC facilitated receptivity while MSH administered ICV inhibited receptivity. In addition, MSH administered ICV exerted an inhibitory effect one week after administration. Therefore, MSH appears to exert both short and long-acting effects on sexual receptivity.

Intravenous alpha-MSH reduces fever in the squirrel monkey

alpha-MSH reduces fever in rabbits when administered IV, ICV, or by gavage; however, the applicability of this finding to higher species, specifically to primates, has not been determined. In this study, we chose the squirrel monkey as an appropriate primate model since it responds reliably to peripheral administration of bacterial endotoxins that cause fever in man. From pilot studies, doses of S. typhosa endotoxin necessary to produce maximum fever and doses of alpha-MSH which did not cause hypothermia were determined for each animal. In the main experiments endotoxin was given via an indwelling catheter in the saphenous vein, followed by alpha-MSH injections when the rectal temperature increased 0.3 degrees C. alpha-MSH (100-400 micrograms) reduced the area under the fever curve an average of 50.0%, but had no effect on afebrile temperature. Molar equivalent amounts of the antipyretic drug acetaminophen had little effect on fever. These findings support the idea, based on research on rabbits, that alpha-MSH has a role in central modulation of fever.

Relative stability of alpha-melanotropin and related analogues to rat brain homogenates

alpha-Melanotropin (alpha-MSH) retains less than 1% of its original activity after a 60 min incubation with 10% rat brain homogenate. [Nle4,D-Phe7]-alpha-MSH is nonbiodegradable in rat serum (240 min incubation) and still maintains 10% of its original activity in 10% rat brain homogenate (240 min incubation). The related fragment analogue, Ac-[Nle4,D-Phe7]-alpha-MSH4-10-NH2, retains 50% of its activity after a 240 min incubation in rat brain homogenate, whereas Ac-[Nle4,D-Phe7]-alpha-MSH4-11-NH2 is totally resistant to inactivation by rat brain homogenate. Both [Nle4,D-Phe7]-fragments are resistant to degradation by rat serum, but [Nle4]-alpha-MSH, Ac-[Nle4]-alpha-MSH4-10-NH2 and Ac-[Nle4]-alpha-MSH4-11-NH2 are rapidly inactivated under both conditions. The cyclic melanotropin, [Cys4,Cys10]-alpha-MSH, is inactivated in rat brain homogenate as is the shorter Ac-[Cys4,Cys10]-alpha-MSH4-10-NH2 analogue, but neither cyclic melanotropin is inactivated upon incubation in serum from rats. Ac-[Cys4,D-Phe7,Cys10]-alpha-MSH4-10-NH2 is resistant to inactivation by either rat serum or a brain homogenate. Some of these melanotropin analogues may provide useful probes for the localization and characterization of putative melanotropin receptors in both the central nervous system and peripheral tissues.

Conversion of a primary amine to a labeled secondary amine by the addition of phenolic group and radioiodination

To preserve the nucleophilicity of amino compounds during conjugative radioiodination, a new method for converting primary amines to phenolic secondary amines was developed. Amino acids were used as model compounds for establishing optimal conditions for the reductive amination. In the first step of the reaction, the aldehyde group of 4-hydroxybenzaldehyde (formylphenol) was reacted reversibly with an amino group to form an imine. The irreversible attachment of formylphenol to the amino group was accomplished by reduction of the imine with sodium cyanoborohydride. The pH optimum for the reaction was 5.0. Higher temperature has favorable effects on the rate and extent of the conjugation. Phenolic derivatives of amino compounds suitable for radioiodination are produced by the reactions described.

Absorption of alpha-MSH from subcutaneous and intraperitoneal sites in the rat

Pentobarbitone anesthetized rats were injected with 30 nmol (50 micrograms) alpha-MSH administered intraperitoneally (IP) and subcutaneously (SC) in an acid-saline vehicle, or SC in a zinc phosphate vehicle. Concentrations of alpha-MSH in plasma were measured by radioimmunoassay. The pharmacokinetic parameters for the three modes of administration were determined by fitting a one-compartment open model to the plasma level data. The t1/2 for absorption using the saline vehicle was 7.3 and 5.6 min from the IP and SC sites, respectively. The t1/2 for absorption from the zinc phosphate complex of 17.7 min was significantly longer. Five percent of the IP dose was absorbed into the systemic circulation giving a peak plasma level of 14.1 nmol/l. The absorption of 2-3 percent was significantly lower following SC administration; peak plasma levels were 8.3 and 4.8 nmol/l for the saline and zinc phosphate vehicles, respectively. The low percentage absorption values indicated a high degree of metabolism of the peptide by peripheral tissues on its passage from the injection sites into the circulation.

Limitations to effect of alpha-MSH on permeability of blood-brain barrier to IV 99mTc-pertechnetate

The effects of several variables on the permeability of the blood-brain barrier (BBB) to 99mTc-labeled sodium pertechnetate after IV administration of alpha-MSH were investigated. Doses of alpha-MSH of about 200 micrograms/kg were generally more effective in increasing the brain:blood ratio of radioactivity than the smaller doses that had previously been shown to affect behavior and the EEG. Pulsatile administration of a total of 200 micrograms/kg alpha-MSH over 90 min did not change the permeability of the BBB to the pertechnetate anion. Infusion of the same dose over 90 min significantly increased the brain:blood ratio of radioactivity in one of two experiments: no significant effects were seen with infusion for shorter times, lower concentrations, or with a 4-9 analog (Org 2766). In another experiment, bolus injection of 200 micrograms/kg alpha-MSH resulted in a significantly increased ratio 90 min later as compared with controls. Although the effects of a peptide on the permeability of the BBB to other compounds remains intriguing, limitations appear to exist in experiments with 99mTc-pertechnetate.

Evidence that N-acetylation regulates the behavioral activity of alpha-MSH in the rat and human central nervous system

alpha-MSH immunoreactive peptides were fractionated and characterized in rat and human brain and rat pituitary by reversed phase high pressure liquid chromatographic techniques. alpha-MSH and deacetylated alpha-MSH were two major naturally existing peptides in both brain and pituitary gland. Subsequent experiments examined the roles of these two peptides in neuronal function. The alpha-MSH was clearly more effective than deacetylated alpha-MSH in improving performance on a visual discrimination task after intraperitoneal administration and in inducing excessive grooming after intraventricular administration. The difference in behavioral potency may be explained by the fact that alpha-MSH was much more resistant to peptidase degradation than was deacetylated alpha-MSH. N-acetylation of alpha-MSH may be an effective regulatory process for modulating the behavioral potency of the secretory product of alpha-MSH-containing pituitary cells and neurons.

Evidence for homologous actions of pro-opiocortin products

alpha-Melanocyte-stimulating hormone (alpha-MSH), a modified fragment of adrenocorticotropic hormone, derives from the same biosynthetic route as beta-endorphin and is stored by the same arcuate neurons. Microinjection of alpha-melanocyte-stimulating hormone and several related peptides into the periaqueductal gray matter significantly reduced responsiveness to pain and had a behavioral profile similar to that produced by beta-endorphin.

Disapearance and excretion of labeled alpha-MSH in man

Despite the considerable evidence for the CNS actions of alpha-melanocyte-stimulating hormone (alpha-MSH) in man, little attention has been given to its half-time disappearance from plasma and urinary excretion in normal individuals. In the first experiment, a healthy man was given 15 muCi of tritiated (3H)-alpha-MSH as a rapid IV injection. A plot of the disappearance time in plasma was characteristic of a multiexponential curve, the linear components of which were resolved by the subtraction method and half-time disappearance calculated directly from the slope of the regression line. The half-time disappearance was 1 min for the first component and 25 min for the second component. After the IV adminisration of 50 muCi of 125I-alpha-MSH in the second experiment, the two components showed halftime disappearances of 1 min and 4.8 min respectively. These times were not changed by precipitation of the plasma with 10% trichloroacetic acid. Thirty-eight percent and 42% of the label appeared in the urine 4 hr after the injection of either 3H-alpha-MSH or 125I-alpha-MSH. The results suggest that the persistence of high levels of alpha-MSH in the blood after injection in man may be too short to fully explain the CNS effects of alpha-MSH.

Enkephalin and a potent analog facilitate maze performance after intraperitoneal administration in rats

Met-enkephalin and its analog [D-Ala2]-Met-enkephalin-NH2 were administered intraperitoneally (IP) at a dose of 80 microng/kg body weight to hungry rats which were tested over 3 days for their ability to run a complex, 12 choice Warden maze for a reward of food. Animals receiving either peptide negotiated the maze significantly (p less than 0.01) faster (74.1 and 73.5 vs. 128.8 sec) and made significantly (p less than 0.01) fewer errors (5.5 and 5.4 vs 9.1) than animals receiving the diluent vehicle. These findings did not seem to be explained by differences in appetite, thirst, olfaction, or general activity. Rats injected in a preliminary study with an analog, [D-Phe4]-Met-enkephalin, which has essentially no opiate activity appeared to run the maze as fast as rats injected with [D-Ala2]-Met-enkephalin-NH2 and with just as few errors. Injection of morphine seemed to result in slower running times and more errors in the maze. These results demonstrate that enkephalin and some of its analogs can exert significant behavioral changes after IP administration and that these behavioral effects probably can be dissociated from the opiate effects.