Pro-opiomelanocortin-related peptides in cerebrospinal fluid: a study of manic-depressive disorder

The Melanocortin System: A Promising Target for the Development of New Antidepressant Drugs

Major depression is one of the most prevalent mental disorders, causing significant human suffering and socioeconomic loss. Since conventional antidepressants are not sufficiently effective, there is an urgent need to develop new antidepressant medications. Despite marked advances in the neurobiology of depression, the etiology and pathophysiology of this disease remain poorly understood. Classical and newer hypotheses of depression suggest that an imbalance of brain monoamines, dysregulation of the hypothalamic-pituitary-adrenal axis (HPAA) and immune system, or impaired hippocampal neurogenesis and neurotrophic factors pathways are cause of depression. It is assumed that conventional antidepressants improve these closely related disturbances. The purpose of this review was to discuss the possibility of affecting these disturbances by targeting the melanocortin system, which includes adrenocorticotropic hormone-activated receptors and their peptide ligands (melanocortins). The melanocortin system is involved in the regulation of various processes in the brain and periphery. Melanocortins, including peripherally administered non-corticotropic agonists, regulate HPAA activity, exhibit anti-inflammatory effects, stimulate the levels of neurotrophic factors, and enhance hippocampal neurogenesis and neurotransmission. Therefore, endogenous melanocortins and their analogs are able to complexly affect the functioning of those body’s systems that are closely related to depression and the effects of antidepressants, thereby demonstrating a promising antidepressant potential.

The orphan G protein-coupled receptor GPR139 is activated by the peptides: Adrenocorticotropic hormone (ACTH), α-, and β-melanocyte stimulating hormone (α-MSH, and β-MSH), and the conserved core motif HFRW

GPR139 is an orphan G protein-coupled receptor that is expressed primarily in the brain. Not much is known regarding the function of GPR139. Recently we have shown that GPR139 is activated by the amino acids l-tryptophan and l-phenylalanine (EC₅₀ values of 220 μM and 320 μM, respectively), as well as di-peptides comprised of aromatic amino acids. This led us to hypothesize that GPR139 may be activated by peptides. Sequence alignment of the binding cavities of all class A GPCRs, revealed that the binding pocket of the melanocortin 4 receptor is similar to that of GPR139. Based on the chemogenomics principle “similar targets bind similar ligands”, we tested three known endogenous melanocortin 4 receptor agonists; adrenocorticotropic hormone (ACTH) and α- and β-melanocyte stimulating hormone (α-MSH and β-MSH) on CHO-k1 cells stably expressing the human GPR139 in a Fluo-4 Ca²⁺-assay. All three peptides, as well as their conserved core motif HFRW, were found to activate GPR139 in the low micromolar range. Moreover, we found that peptides consisting of nine or ten N-terminal residues of α-MSH activate GPR139 in the submicromolar range. α-MSH_1-9 was found to correspond to the product of a predicted cleavage site in the pre-pro-protein pro-opiomelanocortin (POMC). Our results demonstrate that GPR139 is a peptide receptor, activated by ACTH, α-MSH, β-MSH, the conserved core motif HFRW as well as a potential endogenous peptide α-MSH_1-9. Further studies are needed to determine the functional relevance of GPR139 mediated signaling by these peptides.

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Using GPCRdb we found that the binding cavity of GPR139 is 49% similar to MC4R.

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ACTH, α-MSH and β-MSH activate GPR139 in the low μM-range.

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We predicted a novel possible cleavage site in POMC leading to the peptide α-MSH_1-9.

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α-MSH_1-9 activates GPR139 in the high nM range.

The HPA axis in bipolar disorder: Systematic review and meta-analysis

OBJECTIVES

To provide a quantitative and qualitative synthesis of the available evidence on the role of Hypothalamic-Pituitary-Adrenal (HPA) axis in the pathophysiology of Bipolar Disorder (BD).

METHODS

Meta-analysis and meta-regression of case-control studies examining the levels of cortisol, ACTH, CRH levels. Systematic review of stress reactivity, genetic, molecular and neuroimaging studies related to HPA axis activity in BD.

RESULTS

Forty-one studies were included in the meta-analyses. BD was associated with significantly increased levels of cortisol (basal and post-dexamethasone) and ACTH, but not of CRH. In the meta-regression, case-control differences in cortisol levels were positively associated with the manic phase (p=0.005) and participants' age (p=0.08), and negatively with antipsychotics use (p=0.001). Reviewed studies suggest that BD is associated with abnormalities of stress-related molecular pathways in several brain areas. Variants of HPA axis-related genes seem not associated with a direct risk of developing BD, but with different clinical presentations. Also, studies on unaffected relatives suggest that HPA axis dysregulation is not an endophenotype of BD, but seems related to environmental risk factors, such as childhood trauma. Progressive HPA axis dysfunction is a putative mechanism that might underlie the clinical and cognitive deterioration of patients with BD.

CONCLUSIONS

BD is associated with dysfunction of HPA axis activity, with important pathophysiological implications. Targeting HPA axis dysfunctions might be a novel strategy to improve the outcomes of BD.

Distinct proteomic profiles in post-mortem pituitary glands from bipolar disorder and major depressive disorder patients

Disturbances of the hypothalamic-pituitary-adrenal axis have been implicated in the pathophysiology of bipolar disorder (BD) and major depressive disorder (MDD). To examine this further, we carried out proteomic profiling of post-mortem pituitaries from 13 BD and 14 MDD patients, in comparison to 15 controls. Liquid chromatography-mass spectrometry (LC-MS(E)) analysis showed that BD patients had significantly increased levels of the major pituitary hormones pro-opiomelanocortin (POMC) and galanin. BD patients also showed changes in proteins associated with gene transcription, stress response, lipid metabolism and growth signalling. In contrast, LC-MS(E) profiling revealed that MDD patients had significantly decreased levels of the prohormone-converting enzyme carboxypeptidease E and follow-up enzymatic analysis showed decreased activity of prolyl-oligopeptidase convertase. This suggested that altered prohormone processing may occur in pituitaries of MDD patients. In addition, MDD patients had significant changes in proteins involved in intracellular transport and cytoskeletal signalling. Finally, we carried out selective reaction monitoring (SRM) mass spectrometry profiling for validation of protein changes in key biological pathways. This confirmed increased POMC levels in BD patients with no change in the levels of this prohormone in MDD. This study demonstrates that proteomic profiling analysis of the pituitary can lead to new insights into the pathophysiology of BD and MDD. Also, given that the pituitary directly releases a variety of bioactive molecules into the bloodstream, many of the proteins identified here could serve as focal points in the search for peripheral biomarkers in clinical or drug treatment studies of BD and MDD patients.

Volume transmission of beta-endorphin via the cerebrospinal fluid; a review

There is increasing evidence that non-synaptic communication by volume transmission in the flowing CSF plays an important role in neural mechanisms, especially for extending the duration of behavioral effects. In the present review, we explore the mechanisms involved in the behavioral and physiological effects of β-endorphin (β-END), especially those involving the cerebrospinal fluid (CSF), as a message transport system to reach distant brain areas. The major source of β-END are the pro-opio-melano-cortin (POMC) neurons, located in the arcuate hypothalamic nucleus (ARH), bordering the 3^rd ventricle. In addition, numerous varicose β-END-immunoreactive fibers are situated close to the ventricular surfaces. In the present paper we surveyed the evidence that volume transmission via the CSF can be considered as an option for messages to reach remote brain areas. Some of the points discussed in the present review are: release mechanisms of β-END, independence of peripheral versus central levels, central β-END migration over considerable distances, behavioral effects of β-END depend on location of ventricular administration, and abundance of mu and delta opioid receptors in the periventricular regions of the brain.

Molecular targets of lithium action

Lithium is an effective drug for both the treatment and prophylaxis of bipolar disorder. However, the precise mechanism of lithium action is not yet well understood. Extensive research aiming to elucidate the molecular mechanisms underlying the therapeutic effects of lithium has revealed several possible targets. The behavioral and physiological manifestations of the illness are complex and are mediated by a network of interconnected neurotransmitter pathways. Thus, lithium's ability to modulate the release of serotonin at presynaptic sites and modulate receptor-mediated supersensitivity in the brain remains a relevant line of investigation. However, it is at the molecular level that some of the most exciting advances in the understanding of the long-term therapeutic action of lithium will continue in the coming years. The lithium cation possesses the selective ability, at clinically relevant concentrations, to alter the PI second-messenger system, potentially altering the activity and dynamic regulation of receptors that are coupled to this intracellular response. Subtypes of muscarinic receptors in the limbic system may represent particularly sensitive targets in this regard. Likewise, preclinical data have shown that lithium regulates arachidonic acid and the protein kinase C signaling cascades. It also indirectly regulates a number of factors involved in cell survival pathways, including cAMP response element binding protein, brain-derived neurotrophic factor, bcl-2 and mitogen-activated protein kinases, and may thus bring about delayed long-term beneficial effects via under-appreciated neurotrophic effects. Identification of the molecular targets for lithium in the brain could lead to the elucidation of the pathophysiology of bipolar disorder and the discovery of a new generation of mood stabilizers, which in turn may lead to improvements in the long-term outcome of this devastating illness (1).

Beta-endorphin in pituitary intraglandular colloid of intermediate lobe origin

Radioimmunoassay (RIA) detected the presence of beta-endorphin in the intraglandular colloid (IGC) of bovine pituitary intermediate lobe origin. The amount of beta-endorphin recovered in each of twelve samples ranged from 0.15 to 218.30 pmol/mg protein. A second group of assays [amino acid analysis, high performance liquid chromatography (HPLC) and mass spectral analysis] confirmed the RIA findings in another series of colloid samples. Approximately 75 pmol was collected from eight pooled glands. beta-Endorphin is an addition to the list of peptide hormones (e.g., methionine-enkephalin, adrenocorticotropin, arginine-vasopressin, alpha-melanocyte-stimulating hormone, beta-lipotropin and somatostatin) previously discovered in IGC by this laboratory.

Intravenous physostigmine increases cerebrospinal fluid neuropeptide-Y

The ability to measure directly central nervous system (CNS) neurotransmitter changes after an acute pharmacological challenge would be a useful clinical tool in psychiatric research. As one approach to this possibility, we attempted to measure cerebrospinal fluid (CSF) neuropeptide changes produced by an intravenous infusion of the indirect cholinergic agonist physostigmine. Six rhesus monkeys, with indwelling CSF catheters, had serial CSF samples removed before and after a 15 micrograms/kg physostigmine infusion. Five of six monkeys studied showed at least a 50% increase in CSF neuropeptide-Y (NPY) levels. Normal human subjects (n = 27) had CSF sampled before and 15, 30, and 45 min after an acute intravenous infusion of physostigmine (either 0, 5, or 15 micrograms/kg). An Analysis of Variance revealed a significant (p = 0.04) dose-time interaction, suggesting that physostigmine increased CSF NPY at the 15 micrograms/kg dose. CSF levels of seven other neuropeptides remained unchanged. These results suggest that the pharmacological challenge paradigm can be adapted to CSF neuropeptides, providing new measures of CNS stimulus-induced response beyond the peripheral plasma determinations usually employed.

Characteristics of cerebrospinal fluid neuropeptides relevant to clinical research

Studies of human cerebrospinal fluid (CSF) peptides were conducted in an attempt to broaden the utility of CSF peptide determinations in psychiatric research. Healthy volunteers had two lumbar punctures, at least 3 weeks apart, to assess reproducibility within subjects. CSF levels of eight peptides were reliably reproducible, indicating that longitudinal studies of these CSF neuropeptides are feasible. Levels of 10 peptides were determined in four sequential 8 ml aliquots of CSF. CSF rostrocaudal gradients were not found for any of these 10 peptides. Neuropeptide Y (NPY), growth hormone releasing factor (GHRF), and corticotropin releasing factor (CRF) were measured in CSF from twins and brothers. CSF NPY levels were heritable, while CRF and GHRF levels were influenced more by environment. CSF levels of CRF, beta-lipotropin, vasoactive intestinal peptide, adrenocorticotropic hormone, and somatostatin were highly correlated with one another, suggesting that a common factor is responsible for a significant proportion of the observed variance in their CSF levels. These results suggest that CSF peptide measurements may have a broad range of applicability to clinical psychiatric research.

Clinical relevance of endorphins in psychiatry

1. 13 years after the discovery of endorphins, the relationships between these peptides and mental illness remain unclear. 2. Current concepts are controversial and convincing hypotheses have not been elaborated yet. 3. Endorphin measurements in CSF or serum of psychiatric patients did not strongly support that schizophrenia or depression is related to a deficit or an excess of these peptides. Moreover, treatment with endorphins or opiate antagonists did not result in a distinct change of psychiatric symptoms. Recently, research focused on the relationship between endorphins and addictive behavior, but substantial evidence for clinical relevance is still missing. 4. The physiological relevance of many newly discovered endorphins is barely known and several subtypes of the opiate receptor have been found. Thus, it will still require a substantial effort in basic and clinical studies before, if at all, endorphins turn out to be helpful in the treatment of mental illness.

Reduced cerebrospinal fluid levels of immunoreactive pro-opiomelanocortin related peptides (including beta-endorphin) in anorexia nervosa

The discovery that the endogenous opioid peptides contribute to the modulation of appetitive behavior and neuroendocrine function has raised questions as to whether disturbances of opioids contributes to the pathophysiology of eating disorders. To assess central nervous system (CNS) beta-endorphin in patients with anorexia nervosa we measured cerebrospinal fluid (CSF) beta-endorphin concentrations before, and at intervals after weight correction. In addition, we measured three sister peptides (beta-lipotropin, adrenocorticotropic hormone (ACTH), and the N-terminal fragment) derived from the same precursor molecule, pro-opiomelanocortin (POMC) to determine whether possible disturbances might extend to sister peptides. Underweight anorectics (58 +/- 5% of average body weight (ABW), n = 10) had significantly lower CSF concentrations of all 4 peptides compared to healthy controls (102 +/- 10% ABW, n = 11). CSF concentrations of all 4 POMC-related peptides were found to be significantly increased when the same anorectics were restudied 4 to 6 weeks after weight gain (83 +/- 4% ABW). After weight gain, levels of CSF beta-endorphin, beta-lipotropin, and ACTH were similar to controls, whereas levels of CSF N-POMC remained significantly less than controls. Another group of women, previously underweight with anorexia nervosa, but weight-restored (93 +/- 11% ABW, n = 12) for greater than 1 year had CSF concentrations of all 4 POMC-related peptides that were similar to controls. We conclude that underweight anorectics have state-associated disturbances of CNS beta-endorphin as well as other POMC-related peptides. These abnormalities are part of the neurobiological syndrome of anorexia nervosa and may contribute to the characteristic alterations in behavior and neuroendocrine function.

Growth hormone releasing factor in human cerebrospinal fluid

Growth hormone releasing factor (GHRF) was determined by radioimmunoassay in cerebrospinal fluid (CSF) from 37 unmedicated healthy volunteers (mean +/- SD = 23.4 +/- 7.2 pg/ml). A second lumbar puncture in 11 of these subjects suggested that the CSF GHRF levels were stable over time. High performance liquid chromatography revealed that the CSF immunoreactivity co-eluted with the 1-44NH2 and 1-4ONH2 forms of GHRF. Measurements of GHRF in sequential CSF aliquots revealed no evidence of a rostro-caudal gradient, an observation that is compatible with the hypothesis that GHRF has a long half-life in CSF. A significant negative correlation with body weight was found in the 37 subjects. Levels of CSF GHRF were determined in 24 lithium-treated euthymic bipolar patients, 13 of whom provided a second CSF sample during the unmedicated state. No group differences or effects of lithium were noted.