Monoamine oxidase A and B activities in embryonic chick hepatocytes: differential regulation by retinoic acid

Identification and characterization of putative methylation targets in the MAOA locus using bioinformatic approaches

Monoamine oxidase A (MAO A) is an enzyme that catalyzes the oxidation of neurotransmitter amines. A functional polymorphism in the human MAOA gene (high- and low-MAOA) has been associated with distinct behavioral phenotypes. To investigate directly the biological mechanism whereby this polymorphism influences brain function, we recently measured the activity of the MAO A enzyme in healthy volunteers. When found no relationship between the individual's brain MAO A level and the MAOA genotype, we postulated that there are additional regulatory mechanisms that control the MAOA expression. Given that DNA methylation is linked to the regulation of gene expression, we hypothesized that epigenetic mechanisms factor into the MAOA expression. Our underplaying assumption was that the differences in an individual's genotype play a key role in the epigenetic potential of the MAOA locus and, consequently, determine the individual's level of MAO A activity in the brain. As a first step towards experimental validation of the hypothesis, we performed a comprehensive bioinformatic analysis aiming to interrogate genomic features and attributes of the MAOA locus that might modulate its epigenetic sensitivity. Major findings of our analysis are the following: (1) the extended MAOA regulatory region contains two CpG islands (CGIs), one of which overlaps with the canonical MAOA promoter and the other is located further upstream; both CGIs exhibit sensitivity to differential methylation. (2) The uVNTR's effect on the MAOA's transcriptional activity might have epigenetic nature: this polymorphic region resides within the MAOA's CGI and itself contains CpGs, thus, the number of repeating increments effectively changes the number of methylatable cytosines in the MAOA promoter. An array of in silico analyses (the nucleosome positioning, the physical properties of the local DNA, the clustering of transcription-factor binding sites) together with experimental data on histone modifications and Pol 2 sites and data from the RefSeq mRNA library suggest that the MAOA gene might have an alternative promoter. Based on our findings, we propose a regulatory mechanism for the human MAOA according to which the MAOA expression in vivo is executed by the generation of tissue-specific transcripts initiated from the alternative promoters (both CGI-associated) where transcriptional activation of a particular promoter is under epigenetic control.

Retinoic acid activates monoamine oxidase B promoter in human neuronal cells

Monoamine oxidase (MAO) B deaminates a number of biogenic and dietary amines and plays an important role in many biological processes. Among hormonal regulations of MAO B, we have recently found that retinoic acid (RA) significantly activates both MAO B promoter activity and mRNA expression in a human neuroblastoma BE(2)C cell line. RA activates MAO B promoter in both concentration- and time-dependent manners, which is mediated through retinoic acid receptor alpha (RARalpha) and retinoid X receptor alpha (RXRalpha). There are four retinoic acid response elements (RAREs) as identified in the MAO B 2-kb promoter, and mutation of the third RARE reduced RA-induced MAO B promoter activation by 50%, suggesting this element is important. Electrophoretic mobility shift analysis and chromatin immunoprecipitation assay demonstrated that RARalpha specifically binds to the third RARE both in vitro and in vivo. Moreover, transient transfection and luciferase assays revealed that Sp1 enhances but not essentially required for the RA activation of MAO B through two clusters of Sp1-binding sites in the MAO B promoter. RARalpha physically interacts with Sp1 via zinc finger domains in Sp1 as determined by co-immunoprecipitation assay. Further, RARalpha was shown to be recruited by Sp1 and to form a transcriptional regulation complex with Sp1 in the Sp1-binding sites of natural MAO B promoter. Taken together, this study provides evidence for the first time showing the stimulating effect of RA on MAO B and new insight into the molecular mechanisms of MAO B regulation by hormones.

Retinoid-mediated regulation of mood: possible cellular mechanisms

Vitamin A and its derivatives, the retinoids, have long been studied for their ability to alter central nervous system (CNS) development. Increasingly, it is recognized that sufficient levels of retinoids may also be required for adult CNS function. However, excess dietary vitamin A, due to the consumption of supplements or foods rich in vitamin A, has been reported to induce psychosis. In addition, 13-cis-retinoic acid (13-cis-RA, isotretinoin), the active ingredient in the acne treatment Accutane, has been reported to cause adverse psychiatric events, including depression and suicidal ideation. Nevertheless, epidemiological studies have reported no consistent link between Accutane use and clinical depression in humans. Using an animal model, we have recently shown that 13-cis-RA induces an increase in depression-related behavior. Impairments in spatial learning and memory have also been demonstrated following 13-cis-RA treatment in mice. This review focuses on the behavioral and possible cellular effects of retinoid deficiency or excess in the adult brain in relation to altered mood. Specifically, we discuss the effect of retinoids on depression-related behaviors and whether norepinephrinergic, dopaminergic, or serotonergic neurotransmitter systems may be impaired. In addition, we consider the evidence that adult neurogenesis, a process implicated in the pathophysiology of depression, is reduced by retinoid signaling. We suggest that 13-cis-RA treatment may induce depression-related behaviors by decreasing adult neurogenesis and/or altering the expression of components of serotonergic neurotransmitter system, thereby leading to impaired serotonin signaling.

Platelet MAO activity during treatment with pegylated interferon-alfa in melanoma patients

Depression and cognitive disturbance are well-known neuropsychiatric side effects of therapy with interferon-alfa (IFN-alfa). Aggression and irritability are also reported as side effects. Probably, central nervous system (CNS) serotonergic dysfunction is one of the underlying pathophysiological mechanisms of IFN-alfa-induced neuropsychiatric toxicity. Platelet activity of monoamine oxidase-B (MAO; EC1.4.3.4) is a possible indicator of central serotonergic function. Moreover, low platelet MAO activity is linked to impulsiveness, addiction and personality disorder. In this exploratory study in 17 high-risk melanoma patients, platelet counts, whole blood MAO, and platelet MAO activity were measured before and during therapy with IFN-alfa. Patients were randomized to treatment either with pegylated IFN-alfa (PEG-IFN-alfa) once a week at a dose of 6 microg/kg/week subcuteanously (s.c.) during 8 weeks, followed by a maintenance treatment of 3 microg/kg/week s.c. for a total of 5 years, or to observation only. Blood samples were taken at baseline, 4 and 8 weeks and 3 months. During treatment with IFN-alfa, platelet counts decreased at 4 and 8 weeks and 3 months, while platelet MAO activity increased, both compared to baseline and compared to non-treated controls. Compared to non-treated controls, platelet MAO activity increased with 86.4% (95 CI: 52.9-127.2). No significant changes in platelet MAO activity were observed in the control group. This indicates that platelet MAO activity is influenced by IFN-alfa. Since platelet MAO activity is a model for CNS MAO-B activity, it may be speculated that CNS MAO-B activity will also be increased. This could influence serotonin (5-HT) metabolism and thereby contribute to the development of psychiatric disturbance. However, a preferential inhibition of platelet production cannot be ruled out. Hypothetically, the antiproliferative effects of IFN-alfa could interfere more strongly with the synthesis of platelets than with the synthesis of mitochondria. In that case, increased platelet MAO activity reflects an increased number of mitochondria per platelet.

Serotonin transporter and MAO-B levels in monoamine nuclei of the human brainstem are normal in major depression

Neurochemical imbalance between noradrenergic and serotonergic systems has been postulated to underlie the pathophysiology of psychiatric illnesses involving mood disorders. The present study was designed to examined the possibility that serotonergic innervation of the locus coeruleus (LC) is abnormal in major depression, by measuring two proteins expressed by serotonergic neurons, but not by noradrenergic neurons, in the region of the LC. The specific binding of [(3)H]paroxetine to serotonin transporter (SERT) and of [(3)H]lazabemide to monoamine oxidase (MAO-B) were measured autoradiographically in tissue sections cut transversely at multiple levels along the rostro-caudal extent of the LC, as well as in the caudal portion of the dorsal raphe nucleus, from psychiatrically normal subjects and age-matched subjects with major depression. Under the conditions of the assays, [(3)H]paroxetine binding in the LC was specific for the SERT, based on the rank order of affinity of compounds for inhibiting [(3)H]paroxetine binding in the LC, i.e. citalopram > imipramine > desipramine > mazindol. The binding of [(3)H]paroxetine to SERT and [(3)H]lazabemide to MAO-B were higher in the raphe nuclei than in the LC. Comparison of control subjects to major depressive subjects revealed no differences in the amount of [(3)H]paroxetine binding to SERT and [(3)H]lazabemide to MAO-B in the LC, as well as in the raphe nuclei. These findings imply that serotonergic innervation of the LC is intact in major depression.