Developmental expression of monoamine oxidases A and B in the central and peripheral nervous systems of the mouse

A Key Mediator and Imaging Target in Alzheimer's Disease: Unlocking the Role of Reactive Astrogliosis Through MAOB

Astrocytes primarily maintain physiological brain homeostasis. However, under various pathological conditions, they can undergo morphological, transcriptomic, and functional transformations, collectively referred to as reactive astrogliosis. Recent studies have accumulated lines of evidence that reactive astrogliosis plays a crucial role in the pathology of Alzheimer's disease (AD). In particular, monoamine oxidase B, a mitochondrial enzyme mainly expressed in astrocytes, significantly contributes to neuronal dysfunction and neurodegeneration in AD brains. Moreover, it has been reported that reactive astrogliosis precedes other pathological hallmarks such as amyloid-beta plaque deposition and tau tangle formation in AD. Due to the early onset and profound impact of reactive astrocytes on pathology, there have been extensive efforts in the past decade to visualize these cells in the brains of AD patients using positron emission tomography (PET) imaging. In this review, we summarize the recent studies regarding the essential pathological importance of reactive astrocytes in AD and their application as a target for PET imaging.

Positron Emission Tomography of Neuroimmune Responses in Humans: Insights and Intricacies

The brain's immune system plays a critical role in responding to immune challenges and maintaining homeostasis. However, dysregulated neuroimmune function contributes to neurodegenerative disease and neuropsychiatric conditions. In vivo positron emission tomography (PET) imaging of the neuroimmune system has facilitated a greater understanding of its physiology and the pathology of some neuropsychiatric conditions. This review presents an in-depth look at PET findings from human neuroimmune function studies, highlighting their importance in current neuropsychiatric research. Although the majority of human PET studies feature radiotracers targeting the translocator protein 18 kDa (TSPO), this review also considers studies with other neuroimmune targets, including monoamine oxidase B, cyclooxygenase-1 and cyclooxygenase-2, nitric oxide synthase, and the purinergic P2X7 receptor. Promising new targets, such as colony-stimulating factor 1, Sphingosine-1-phosphate receptor 1, and the purinergic P2Y12 receptor, are also discussed. The significance of validating neuroimmune targets and understanding their function and expression is emphasized in this review to better identify and interpret PET results.

Reactive astrogliosis: A friend or foe in the pathogenesis of Alzheimer's disease

Astrocytes are highly efficient homeostatic glial cells playing a crucial role in optimal brain functioning and homeostasis. Astrocytes respond to changes in brain homoeostasis following central nervous system (CNS) injury/diseased state by a specific defence mechanism called reactive astrogliosis. Recent studies have implicated and placed reactive astrogliosis in the centre of pathophysiology of Alzheimer's disease (AD) and other neurodegenerative disorders. The AD biomarker field is evolving rapidly with new findings providing strong evidence which supports the notion that a reactive astrogliosis is an early event in the time course of AD progression which may precede other pathological hallmarks of AD. Clinical/translational in vivo PET and in vitro postmortem brain imaging studies demonstrated 'a first and second wave' of reactive astrogliosis in AD with distinct close-loop relationships with other pathological biomarkers at different stages of the disease. At the end stages, reactive astrocytes are found to be associated, or in proximity, with amyloid plaque and tau pathological deposits in postmortem AD brains. Several new PET-tracers, which are being in pipeline and validated at a very fast pace for mapping and visualising reactive astrogliosis in the brain, will provide further invaluable mechanistic insights into AD and other non-AD dementia pathologies. The complementary roles of microglia and astrocyte activation in AD progression, along with the clinical value of new fluid astrocytes biomarkers in the context of existing biomarkers, are the latest avenue that needs further exploration.

Astrocyte regulation of synaptic signaling in psychiatric disorders

Over the last 15 years, the field of neuroscience has evolved toward recognizing the critical role of astroglia in shaping neuronal synaptic activity and along with the pre- and postsynapse is now considered an equal partner in tripartite synaptic transmission and plasticity. The relative youth of this recognition and a corresponding deficit in reagents and technologies for quantifying and manipulating astroglia relative to neurons continues to hamper advances in understanding tripartite synaptic physiology. Nonetheless, substantial advances have been made and are reviewed herein. We review the role of astroglia in synaptic function and regulation of behavior with an eye on how tripartite synapses figure into brain pathologies underlying behavioral impairments in psychiatric disorders, both from the perspective of measures in postmortem human brains and more subtle influences on tripartite synaptic regulation of behavior in animal models of psychiatric symptoms. Our goal is to provide the reader a well-referenced state-of-the-art understanding of current knowledge and predict what we may discover with deeper investigation of tripartite synapses using reagents and technologies not yet available.

All the brain's a stage for serotonin: the forgotten story of serotonin diffusion across cell membranes

In the conventional model of serotonin neurotransmission, serotonin released by neurons in the midbrain raphe nuclei exerts its actions on forebrain neurons by interacting with a large family of post-synaptic receptors. The actions of serotonin are terminated by active transport of serotonin back into the releasing neuron, which is mediated by the serotonin reuptake transporter (SERT). Because SERT is expressed pre-synaptically and is widely thought to be the only serotonin transporter in the forebrain, the conventional model does not include serotonin transport into post-synaptic neurons. However, a large body of evidence accumulating since the 1970s has shown that serotonin, despite having a positive charge, can cross cell membranes through a diffusion-like process. Multiple low-affinity, high-capacity, sodium-independent transporters, widely expressed in the brain, allow the carrier-mediated diffusion of serotonin into forebrain neurons. The amount of serotonin crossing cell membranes through this mechanism under physiological conditions is considerable. Most prominent textbooks fail to include this alternative method of serotonin uptake in the brain, and even most neuroscientists are unaware of it. This failure has limited our understanding of a key regulator of serotonergic neurotransmission, impeded research on the potential intracellular actions of serotonin in post-synaptic neurons and glial cells, and may have impeded our understanding of the mechanism by which antidepressant medications reduce depressive symptoms.

Curcumin improves D-galactose and normal-aging associated memory impairment in mice: In vivo and in silico-based studies

Aging-induced memory impairment is closely associated with oxidative stress. D-Galactose (D-gal) evokes severe oxidative stress and mimics normal aging in animals. Curcumin, a natural flavonoid, has potent antioxidant and anti-aging properties. There are several proteins like glutathione S-transferase A1 (GSTA1), glutathione S-transferase omega-1 (GSTO1), kelch-like ECH-associated protein 1 (KEAP1), beta-secretase 1 (BACE1), and amine oxidase [flavin-containing] A (MAOA) are commonly involved in oxidative stress and aging. This study aimed to investigate the interaction of curcumin to these proteins and their subsequent effect on aging-associated memory impairment in two robust animal models: D-Gal and normal aged (NA) mice. The aging mice model was developed by administering D-gal intraperitoneally (i.p). Mice (n = 64) were divided into the eight groups (8 mice in each group): Vehicle, Curcumin-Control, D-gal (100mg/kg; i.p), Curcumin + D-gal, Astaxanthin (Ast) + D-gal, Normal Aged (NA), Curcumin (30mg/kg Orally) + NA, Ast (20mg/kg Orally) + NA. Retention and freezing memories were assessed by passive avoidance (PA) and contextual fear conditioning (CFC). Molecular docking was performed to predict curcumin binding with potential molecular targets. Curcumin significantly increased retention time (p < 0.05) and freezing response (p < 0.05) in PA and CFC, respectively. Curcumin profoundly ameliorated the levels of glutathione, superoxide dismutase, catalase, advanced oxidation protein products, nitric oxide, and lipid peroxidation in mice hippocampi. In silico studies revealed favorable binding energies of curcumin with GSTA1, GSTO1, KEAP1, BACE1, and MAOA. Curcumin improves retention and freezing memory in D-gal and nature-induced aging mice. Curcumin ameliorates the levels of oxidative stress biomarkers in mice. Anti-aging effects of curcumin could be attributed to, at least partially, the upregulation of antioxidant enzymes through binding with GSTA1, GSTO1, KEAP1, and inhibition of oxidative damage through binding with BACE1 and MAOA.

Generation of Astrocyte-Specific MAOB Conditional Knockout Mouse with Minimal Tonic GABA Inhibition

Monoamine oxidase B (MAOB) is a key enzyme for GABA production in astrocytes in several brain regions. To date, the role of astrocytic MAOB has been studied in MAOB null knockout (KO) mice, although MAOB is expressed throughout the body. Therefore, there has been a need for genetically engineered mice in which only astrocytic MAOB is targeted. Here, we generated an astrocyte-specific MAOB conditional KO (cKO) mouse line and characterized it in the cerebellar and striatal regions of the brain. Using the CRISPR-Cas9 gene-editing technique, we generated Maob floxed mice (B6-Maob^em1Cjl/Ibs) which have floxed exons 2 and 3 of Maob with two loxP sites. By crossing these mice with hGFAP-CreER^T2, we obtained Maob floxed::hGFAP-CreER^T2 mice which have a property of tamoxifen-inducible ablation of Maob under the human GFAP (hGFAP) promoter. When we treated Maob floxed::hGFAP-CreER^T2 mice with tamoxifen for 5 consecutive days, MAOB and GABA immunoreactivity were significantly reduced in striatal astrocytes as well as in Bergmann glia and lamellar astrocytes in the cerebellum, compared to sunflower oil-injected control mice. Moreover, astrocyte-specific MAOB cKO led to a 74.6% reduction in tonic GABA currents from granule cells and a 76.8% reduction from medium spiny neurons. Our results validate that astrocytic MAOB is a critical enzyme for the synthesis of GABA in astrocytes. We propose that this new mouse line could be widely used in studies of various brain diseases to elucidate the pathological role of astrocytic MAOB in the future.

Live Tissue Imaging Reveals Distinct Transcellular Pathways for Organic Cations and Anions at the Blood-Cerebrospinal Fluid Barrier

Formed by the choroid plexus epithelial (CPE) cells, the blood-cerebrospinal fluid barrier (BCSFB) plays an active role in removing drugs, toxins, and metabolic wastes from the brain. Several organic cation and anion transporters are expressed in the CPE cells, but how they functionally mediate transepithelial transport of organic cations and anions remain unclear. In this study, we visualized the transcellular transport of fluorescent organic cation and organic anion probes using live tissue imaging in freshly isolated mouse choroid plexuses (CPs). The cationic probe, 4-[4-(dimethylamino)phenyl]-1-methylpyridinium iodide (IDT307) was transported into CPE cells at the apical membrane and highly accumulated in mitochondria. Consistent with the lack of expression of organic cation efflux transporters, there was little efflux of IDT307 into the blood capillary space. Furthermore, IDT307 uptake and intracellular accumulation was attenuated by approximately 70% in CP tissues from mice with targeted deletion of the plasma membrane monoamine transporter (Pmat). In contrast, the anionic probe fluorescein-methotrexate (FL-MTX) was rapidly transported across the CPE cells into the capillary space with little intracellular accumulation. Rifampicin, an inhibitor of organic anion transporting polypeptides (OATPs), completely blocked FL-MTX uptake into the CPE cells whereas MK-571, a pan-inhibitor of multidrug resistance associated proteins (MRPs), abolished basolateral efflux of FL-MTX. In summary, our results suggest distinct transcellular transport pathways for organic cations and anions at the BCSFB and reveal a pivotal role of PMAT, OATP and MRP transporters in organic cation and anion transport at the blood-cerebrospinal fluid interface. SIGNIFICANCE STATEMENT: Live tissue imaging revealed that while organic cations are transported from the cerebrospinal fluid (CSF) into the choroid plexus epithelial cells by plasma membrane monoamine transporter without efflux into the blood, amphipathic anions in the CSF are efficiently transported across the BCSFB through the collaborated function of apical organic anion transporting polypeptides and basolateral multidrug resistance associated proteins. These findings contribute to a mechanistic understanding of the molecular and cellular pathways for choroid plexus clearance of solutes from the brain.

Revisiting the Role of Astrocytic MAOB in Parkinson's Disease

Monoamine oxidase-B (MAOB) has been believed to mediate the degradation of monoamine neurotransmitters such as dopamine. However, this traditional belief has been challenged by demonstrating that it is not MAOB but MAOA which mediates dopamine degradation. Instead, MAOB mediates the aberrant synthesis of GABA and hydrogen peroxide (H₂O₂) in reactive astrocytes of Parkinson’s disease (PD). Astrocytic GABA tonically suppresses the dopaminergic neuronal activity, whereas H₂O₂ aggravates astrocytic reactivity and dopaminergic neuronal death. Recently discovered reversible MAOB inhibitors reduce reactive astrogliosis and restore dopaminergic neuronal activity to alleviate PD symptoms in rodents. In this perspective, we redefine the role of MAOB for the aberrant suppression and deterioration of dopaminergic neurons through excessive GABA and H₂O₂ synthesis of reactive astrocytes in PD.

Brunner syndrome associated MAOA mutations result in NMDAR hyperfunction and increased network activity in human dopaminergic neurons

Monoamine neurotransmitter abundance affects motor control, emotion, and cognitive function and is regulated by monoamine oxidases. Among these, Monoamine oxidase A (MAOA) catalyzes the degradation of dopamine, norepinephrine, and serotonin into their inactive metabolites. Loss-of-function mutations in the X-linked MAOA gene have been associated with Brunner syndrome, which is characterized by various forms of impulsivity, maladaptive externalizing behavior, and mild intellectual disability. Impaired MAOA activity in individuals with Brunner syndrome results in bioamine aberration, but it is currently unknown how this affects neuronal function, specifically in dopaminergic (DA) neurons. Here we generated human induced pluripotent stem cell (hiPSC)-derived DA neurons from three individuals with Brunner syndrome carrying different mutations and characterized neuronal properties at the single cell and neuronal network level in vitro. DA neurons of Brunner syndrome patients showed reduced synaptic density but exhibited hyperactive network activity. Intrinsic functional properties and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR)-mediated synaptic transmission were not affected in DA neurons of individuals with Brunner syndrome. Instead, we show that the neuronal network hyperactivity is mediated by upregulation of the GRIN2A and GRIN2B subunits of the N-methyl-d-aspartate receptor (NMDAR), resulting in increased NMDAR-mediated currents. By correcting a MAOA missense mutation with CRISPR/Cas9 genome editing we normalized GRIN2A and GRIN2B expression, NMDAR function and neuronal population activity to control levels. Our data suggest that MAOA mutations in Brunner syndrome increase the activity of dopaminergic neurons through upregulation of NMDAR function, which may contribute to the etiology of Brunner syndrome associated phenotypes.

Differential Susceptibilities of Catecholamines to Metabolism by Monoamine Oxidases

The endogenous catecholamines dopamine (DA), norepinephrine (NE), and epinephrine (EPI) play key roles in neurobehavioral, cardiovascular, and metabolic processes; various clinical disorders; and effects of numerous drugs. Steps in intracellular catecholamine synthesis and metabolism were delineated long ago, but there remains a knowledge gap. Catecholamines are metabolized by two isoforms of monoamine oxidase (MAO), MAO-A and MAO-B, and although the anatomic localization of MAO-A and MAO-B and substrate specificities of enzyme inhibitors are well characterized, relative susceptibilities of the endogenous catecholamines to enzymatic oxidation by MAO-A and MAO-B have not been studied systematically. MAOs catalyze the conversion of catecholamines to catecholaldehydes-3,4-dihydroxyphenylacetaldehyde (DOPAL) from DA and 3,4-dihydroxyphenylglycolaldehyde (DOPEGAL) from NE and EPI. In this study we exploited the technical ability to assay DOPAL and DOPEGAL simultaneously with the substrate catecholamines to compare DA, NE, and EPI in their metabolism by MAO-A and MAO-B. For both MAO isoforms, DA was the better substrate compared to NE or EPI, which were metabolized equally. Since catecholaminergic neurons express mainly MAO-A, the finding that MAO-A is more efficient than MAO-B in metabolizing endogenous catecholamines reinforces the view that the predominant route of intraneuronal enzymatic oxidation of catecholamines is via MAO-A. The results have implications for clinical neurochemistry, experimental therapeutics, and computational models of catecholaminergic neurodegeneration. For instance, the greater susceptibility of DA than the other catecholamines to both MAO isoforms can help explain relatively high concentrations of the deaminated DA metabolite 3,4-dihydroxyphenylacetic acid than of the NE metabolite 3,4-dihydroxyphenylglycol in human plasma and urine. SIGNIFICANCE STATEMENT: Endogenous catecholamines are metabolized by monoamine oxidase (MAO)-A and -B, yielding the catecholaldehydes 3,4-dihydroxyphenylacetaldehyde (DOPAL) from dopamine (DA) and 3,4-dihydroxyphenylglycolaldehyde (DOPEGAL) from norepinephrine (NE) and epinephrine (EPI). Based on measurements of DOPAL and DOPEGAL production, DA is a better substrate than NE or EPI for both MAO isoforms, and MAO-A is more efficient than MAO-B in metabolizing DA, NE, and EPI. MAO-A is the main route of intraneuronal metabolism of endogenous catecholamines.

Choroid Plexus and Drug Removal Mechanisms

Timely and efficient removal of xenobiotics and metabolites from the brain is crucial in maintaining the homeostasis and normal function of the brain. The choroid plexus (CP) forms the blood-cerebrospinal fluid barrier and vitally removes drugs and wastes from the brain through several co-existing clearance mechanisms. The CP epithelial (CPE) cells synthesize and secrete the cerebrospinal fluid (CSF). As the CSF passes through the ventricular and subarachnoid spaces and eventually drains into the general circulation, it collects and removes drugs, toxins, and metabolic wastes from the brain. This bulk flow of the CSF serves as a default and non-selective pathway for the removal of solutes and macromolecules from the brain interstitium. Besides clearance by CSF bulk flow, the CPE cells express several multispecific membrane transporters to actively transport substrates from the CSF side into the blood side. In addition, several phase I and II drug-metabolizing enzymes are expressed in the CPE cells, which enzymatically inactivate a broad spectrum of reactive or toxic substances. This review summarizes our current knowledge of the functional characteristics and key contributors to the various clearance pathways in the CP-CSF system, overviewing recent developments in our understanding of CSF flow dynamics and the functional roles of CP uptake and efflux transporters in influencing CSF drug concentrations.

Astroglial tracer BU99008 detects multiple binding sites in Alzheimer's disease brain

With reactive astrogliosis being established as one of the hallmarks of Alzheimer's disease (AD), there is high interest in developing novel positron emission tomography (PET) tracers to detect early astrocyte reactivity. BU99008, a novel astrocytic PET ligand targeting imidazoline-2 binding sites (I₂BS) on astrocytes, might be a suitable candidate. Here we demonstrate for the first time that BU99008 could visualise reactive astrogliosis in postmortem AD brains and propose a multiple binding site [Super-high-affinity (SH), High-affinity (HA) and Low-affinity (LA)] model for BU99008, I₂BS specific ligands (2-BFI and BU224) and deprenyl in AD and control (CN) brains. The proportion (%) and affinities of these sites varied significantly between the BU99008, 2-BFI, BU224 and deprenyl in AD and CN brains. Regional binding studies demonstrated significantly higher ³H-BU99008 binding in AD brain regions compared to CN. Comparative autoradiography studies reinforced these findings, showing higher specific binding for ³H-BU99008 than ³H-Deprenyl in sporadic AD brain compared to CN, implying that they might have different targets. The data clearly shows that BU99008 could detect I₂BS expressing reactive astrocytes with good selectivity and specificity and hence be a potential attractive clinical astrocytic PET tracer for gaining further insight into the role of reactive astrogliosis in AD.

Brain Plasma Membrane Monoamine Transporter in Health and Disease

Precise control of monoamine neurotransmitter levels in the central nervous system (CNS) is crucial for proper brain function. Dysfunctional monoamine signaling is associated with several neuropsychiatric and neurodegenerative disorders. The plasma membrane monoamine transporter (PMAT) is a new polyspecific organic cation transporter encoded by the SLC29A4 gene. Capable of transporting monoamine neurotransmitters with low affinity and high capacity, PMAT represents a major uptake₂ transporter in the brain. Broadly expressed in multiple brain regions, PMAT can complement the high-affinity, low-capacity monoamine uptake mediated by uptake₁ transporters, the serotonin, dopamine, and norepinephrine transporters (SERT, DAT, and NET, respectively). This chapter provides an overview of the molecular and functional characteristics of PMAT together with its regional and cell-type specific expression in the mammalian brain. The physiological functions of PMAT in brain monoamine homeostasis are evaluated in light of its unique transport kinetics and brain location, and in comparison with uptake₁ and other uptake₂ transporters (e.g., OCT3) along with corroborating experimental evidences. Lastly, the possibility of PMAT's involvement in brain pathophysiological processes, such as autism, depression, and Parkinson's disease, is discussed in the context of disease pathology and potential link to aberrant monoamine pathways.

The role of monoamine oxidase A in the neurobiology of aggressive, antisocial, and violent behavior: A tale of mice and men

Over the past two decades, research has revealed that genetic factors shape the propensity for aggressive, antisocial, and violent behavior. The best-documented gene implicated in aggression is MAOA (Monoamine oxidase A), which encodes the key enzyme for the degradation of serotonin and catecholamines. Congenital MAOA deficiency, as well as low-activity MAOA variants, has been associated with a higher risk for antisocial behavior (ASB) and violence, particularly in males with a history of child maltreatment. Indeed, the interplay between low MAOA genetic variants and early-life adversity is the best-documented gene × environment (G × E) interaction in the pathophysiology of aggression and ASB. Additional evidence indicates that low MAOA activity in the brain is strongly associated with a higher propensity for aggression; furthermore, MAOA inhibition may be one of the primary mechanisms whereby prenatal smoke exposure increases the risk of ASB. Complementary to these lines of evidence, mouse models of Maoa deficiency and G × E interactions exhibit striking similarities with clinical phenotypes, proving to be valuable tools to investigate the neurobiological mechanisms underlying antisocial and aggressive behavior. Here, we provide a comprehensive overview of the current state of the knowledge on the involvement of MAOA in aggression, as defined by preclinical and clinical evidence. In particular, we show how the convergence of human and animal research is proving helpful to our understanding of how MAOA influences antisocial and violent behavior and how it may assist in the development of preventative and therapeutic strategies for aggressive manifestations.

Embracing diversity in the 5-HT neuronal system

Neurons that synthesize and release 5-hydroxytryptamine (5-HT; serotonin) express a core set of genes that establish and maintain this neurotransmitter phenotype and distinguish these neurons from other brain cells. Beyond a shared 5-HTergic phenotype, these neurons display divergent cellular properties in relation to anatomy, morphology, hodology, electrophysiology and gene expression, including differential expression of molecules supporting co-transmission of additional neurotransmitters. This diversity suggests that functionally heterogeneous subtypes of 5-HT neurons exist, but linking subsets of these neurons to particular functions has been technically challenging. We discuss recent data from molecular genetic, genomic and functional methods that, when coupled with classical findings, yield a reframing of the 5-HT neuronal system as a conglomeration of diverse subsystems with potential to inspire novel, more targeted therapies for clinically distinct 5-HT-related disorders.

Maturation of thalamocortical synapses in the somatosensory cortex depends on neocortical AKAP5 expression

Sensory cortex topographic maps consist of organized arrays of thalamocortical afferents (TCAs) that project into distinct areas of the cortex. Formation of topographic maps in sensory cortices is a prerequisite for functional maturation of the neocortex. Studies have shown that the formation of topographic maps and the maturation of thalamocortical synapses in the somatosensory cortex depend on the cyclic adenosine 5'-monophosphate-(cAMP)-protein kinase A (PKA) signaling pathway. AKAP5 is a scaffold protein (also called AKAP79 in humans or AKAP150 in rodents; AKAP79/150) that serves as a signaling hub that links cAMP and PKA signaling. Whether AKAP5 plays a role in topographic map formation and the maturation of thalamocortical synapses during development of the somatosensory cortex is still unknown. Here, we generated cortex-specific AKAP5-knockout mice (CxAKAP5KO) to examine its roles in somatosensory cortex development. We found that CxAKAP5KO mice displayed impaired cortical barrel maps. Electrophysiological recordings showed that the AMPA/NMDA ratio was reduced, and silent synapses were increased in thalamocortical synapses of CxAKAP5KO mice during postnatal development. Morphological analysis of layer IV cortical neurons demonstrated that dendritic refinement of these neurons was abnormal. These results indicate that AKAP5 is necessary for both topographic map formation and maturation of thalamocortical synapses as well as morphological development of cortical neurons in the somatosensory cortex.

Brain monoamine oxidase B and A in human parkinsonian dopamine deficiency disorders

See Jellinger (doi:10.1093/awx190) for a scientific commentary on this article. The enzyme monoamine oxidases (B and A subtypes, encoded by MAOB and MAOA, respectively) are drug targets in the treatment of Parkinson's disease. Inhibitors of MAOB are used clinically in Parkinson's disease for symptomatic purposes whereas the potential disease-modifying effect of monoamine oxidase inhibitors is debated. As astroglial cells express high levels of MAOB, the enzyme has been proposed as a brain imaging marker of astrogliosis, a cellular process possibly involved in Parkinson's disease pathogenesis as elevation of MAOB in astrocytes might be harmful. Since brain monoamine oxidase status in Parkinson's disease is uncertain, our objective was to measure, by quantitative immunoblotting in autopsied brain homogenates, protein levels of both monoamine oxidases in three different degenerative parkinsonian disorders: Parkinson's disease (n = 11), multiple system atrophy (n = 11), and progressive supranuclear palsy (n = 16) and in matched controls (n = 16). We hypothesized that if MAOB is 'substantially' localized to astroglial cells, MAOB levels should be generally associated with standard astroglial protein measures (e.g. glial fibrillary acidic protein). MAOB levels were increased in degenerating putamen (+83%) and substantia nigra (+10%, non-significant) in multiple system atrophy; in caudate (+26%), putamen (+27%), frontal cortex (+31%) and substantia nigra (+23%) of progressive supranuclear palsy; and in frontal cortex (+33%), but not in substantia nigra of Parkinson's disease, a region we previously reported no increase in astrocyte protein markers. Although the magnitude of MAOB increase was less than those of standard astrocytic markers, significant positive correlations were observed amongst the astrocyte proteins and MAOB. Despite suggestions that MAOA (versus MAOB) is primarily responsible for metabolism of dopamine in dopamine neurons, there was no loss of the enzyme in the parkinsonian substantia nigra; instead, increased nigral levels of a MAOA fragment and 'turnover' of the enzyme were observed in the conditions. Our findings provide support that MAOB might serve as a biochemical imaging marker, albeit not entirely specific, for astrocyte activation in human brain. The observation that MAOB protein concentration is generally increased in degenerating brain areas in multiple system atrophy (especially putamen) and in progressive supranuclear palsy, but not in the nigra in Parkinson's disease, also distinguishes astrocyte behaviour in Parkinson's disease from that in the two 'Parkinson-plus' conditions. The question remains whether suppression of either MAOB in astrocytes or MAOA in dopamine neurons might influence progression of the parkinsonian disorders.

Monoamine Oxidase A (MAOA) Gene and Personality Traits from Late Adolescence through Early Adulthood: A Latent Variable Investigation

Very few molecular genetic studies of personality traits have used longitudinal phenotypic data, therefore molecular basis for developmental change and stability of personality remains to be explored. We examined the role of the monoamine oxidase A gene (MAOA) on extraversion and neuroticism from adolescence to adulthood, using modern latent variable methods. A sample of 1,160 male and 1,180 female participants with complete genotyping data was drawn from a British national birth cohort, the MRC National Survey of Health and Development (NSHD). The predictor variable was based on a latent variable representing genetic variations of the MAOA gene measured by three SNPs (rs3788862, rs5906957, and rs979606). Latent phenotype variables were constructed using psychometric methods to represent cross-sectional and longitudinal phenotypes of extraversion and neuroticism measured at ages 16 and 26. In males, the MAOA genetic latent variable (AAG) was associated with lower extraversion score at age 16 (β = −0.167; CI: −0.289, −0.045; p = 0.007, FDRp = 0.042), as well as greater increase in extraversion score from 16 to 26 years (β = 0.197; CI: 0.067, 0.328; p = 0.003, FDRp = 0.036). No genetic association was found for neuroticism after adjustment for multiple testing. Although, we did not find statistically significant associations after multiple testing correction in females, this result needs to be interpreted with caution due to issues related to x-inactivation in females. The latent variable method is an effective way of modeling phenotype- and genetic-based variances and may therefore improve the methodology of molecular genetic studies of complex psychological traits.

Comparative Analysis of the Neurochemical Profile and MAO Inhibition Properties of N-(Furan-2-ylmethyl)-N-methylprop-2-yn-1-amine

The regulation of brain monoamine levels is paramount for cognitive functions, and the monoamine oxidase (MAO A and B) enzymes play a central role in these processes. The aim of this study was to evaluate whether the procognitive properties exerted by propargylamine N-(furan-2-ylmethyl)-N-methylprop-2-yn-1-amine (F2MPA) are related to changes in monoamine content via MAO inhibition. In vivo microdialysis and ex vivo amine metabolite measurement demonstrated region-specific alterations in monoamine metabolism that differ from both of the classic MAO A and MAO B inhibitors, clorgyline and l-deprenyl, respectively. Although all the inhibitors (1 and 4 mg/kg) increased cortical serotonin tissue content, only F2MPA increased the levels of cortical noradrenaline. In the striatum, clorgyline (1 mg/kg), but not F2MPA (1 mg/kg), reduced extracellular levels of dopamine metabolites at rest or stimulated by the intrastriatal application of the MAO substrate 3-methoxytyramine. In vitro, F2MPA exhibited a low affinity toward MAO B and MAO A. Nonetheless, it modified the B form of MAO, forming a flavin adduct structurally similar to that with deprenyl. F2MPA was rapidly metabolized in the presence of rat but not human microsomes, producing a hydroxylated derivative. In conclusion, the effect of F2MPA on cognition may arise from monoaminergic changes in the cortex, but the role of MAO in this process is likely to be negligible, consistent with the poor affinity of F2MPA for MAO.

Pb exposure prolongs the time period for postnatal transient uptake of 5-HT by murine LSO neurons

Pb exposure is associated with cognitive deficits including Attention Deficit Hyperactivity Disorder (ADHD) in children and alters auditory temporal processing in humans and animals. Serotonin has been implicated in auditory temporal processing and previous studies from our laboratory have demonstrated that developmental Pb decreases expression of serotonin (5-HT) in the adult murine lateral superior olive (LSO). During development, certain non-serotonergic sensory neurons, including auditory LSO neurons, transiently take up 5-HT through the serotonin reuptake transporter (SERT). The uptake of 5-HT is important for development of sensory systems. This study examines the effect of Pb on the serotonergic system in the LSO of the early postnatal mouse. Mice were exposed to moderate Pb (0.01mM) or high Pb (0.1mM) throughout gestation and postnatal day 4 (P4) and P8. We found that Pb exposure prolongs the normal developmental expression of 5-HT by LSO neurons and this is correlated with expression of SERT on LSO cell bodies. The prolonged expression of 5-HT by postnatal LSO neurons is correlated with decreased synaptic immunolabeling within the LSO. This Pb-associated decrease in synaptic density within the LSO could contribute to the auditory temporal processing deficits and cognitive deficits associated with developmental Pb exposure.

The plasma membrane monoamine transporter (PMAT): Structure, function, and role in organic cation disposition

Plasma membrane monoamine transporter (PMAT) is a new polyspecific organic cation transporter that transports a variety of biogenic amines and xenobiotic cations. Highly expressed in the brain, PMAT represents a major uptake₂ transporter for monoamine neurotransmitters. At the blood-cerebrospinal fluid (CSF) barrier, PMAT is the principal organic cation transporter for removing neurotoxins and drugs from the CSF. Here I summarize our latest understanding of PMAT and its roles in monoamine uptake and xenobiotic disposition.

The role of MAO in personality and drug use

Monoamine oxidases, both MAO-A and MAO-B, have been implicated in personality traits and complex behaviour, including drug use. Findings supporting the involvement of MAO-A and MAO-B in shaping personality and in the development of strategies of making behavioural choices come from a variety of studies that have examined either prevalence of gene variants in clinical groups or population-derived samples, estimates of enzyme activity in blood or, by positron emission tomography, in the brain and, most recently, measurement of methylation of the gene. Most of the studies converge in associating MAO-A and MAO-B with impulsive, aggressive or antisocial personality traits or behaviours, including alcohol-related problems, and for MAO-A available evidence strongly supports interaction with adverse environmental exposures in childhood. What is known about genotype effects, and on expression and activity of the enzyme in the brain and in blood has not yet been possible to unite into a mechanistic model of the role of monoamine systems, but the reason for this low degree of generalization is likely caused by the cross-sectional nature of investigation that has not incorporated the developmental effects of MAO-s in critical time windows, including the foetal period. The "risk variants" of both MAO-s appear to increase behavioural plasticity, as supportive environments may particularly well enhance the hidden potential of their carriers. Importantly, male and female brain and behaviours have been found very different with regard to MAO×life events interaction. Future studies need to take into consideration these developmental aspects and sex/gender, as well as to specify the role of different types of environmental factors.

Effect of voluntary alcohol consumption on Maoa expression in the mesocorticolimbic brain of adult male rats previously exposed to prolonged maternal separation

Discordant associations between monoamine oxidase A (MAOA) genotype and high alcohol drinking have been reported in human and non-human primates. Environmental influences likely moderate genetic susceptibility. The biological basis for this interplay remains elusive, and inconsistencies call for translational studies in which conditions can be controlled and brain tissue is accessible. The present study investigated whether early life stress and subsequent adult episodic alcohol consumption affect Maoa expression in stress- and reward-related brain regions in the rat. Outbred Wistar rats were exposed to rearing conditions associated with stress (prolonged maternal separation) or no stress during early life, and given free choice between alcohol and/or water in adulthood. Transcript levels of Maoa were assessed in the ventral tegmental area, nucleus accumbens (NAc), medial prefrontal cortex, cingulate cortex, amygdala and dorsal striatum (DS). Blood was collected to assess corticosterone levels. After alcohol consumption, lower blood corticosterone and Maoa expression in the NAc and DS were found in rats exposed to early life stress compared with control rats. An interaction between early life stress and voluntary alcohol intake was found in the NAc. Alcohol intake before death correlated negatively with Maoa expression in DS in high alcohol-drinking rats exposed to early life stress. Maoa expression is sensitive to adulthood voluntary alcohol consumption in the presence of early life stress in outbred rats. These findings add knowledge of the molecular basis of the previously reported associations between early life stress, MAOA and susceptibility to alcohol misuse.

Animal models of Parkinson's disease: An updated overview

Parkinson's disease (PD) is a progressive neurodegenerative disorder whose etiology, besides a minority of genetic cases, is still largely unknown. Animal models have contributed to elucidate PD etiology and pathogenesis, as well as its cellular and molecular mechanisms, leading to the general hypothesis that this neurological disorder is due to complex interactions between environmental and genetic factors. However, the full understanding of PD is still very far from being achieved, and new potential treatments need to be tested to further improve patients' quality of life and, possibly, slow down the neurodegenerative process. In this context, animal models of PD are required to address all these issues. "Classic" models are based on neurotoxins that selectively target catecholaminergic neurons (such as 6-hydroxydopamine, 1-methyl-1,2,3,6-tetrahydropiridine, agricultural pesticides, etc.), while more recent models employ genetic manipulations that either introduce mutations similar to those find in familial cases of PD (α-synuclein, DJ-1, PINK1, Parkin, etc.) or selectively disrupt nigrostriatal neurons (MitoPark, Pitx3, Nurr1, etc.). Each one of these models has its own advantages and limitations, thus some are better suited for studying PD pathogenesis, while others are more pertinent to test therapeutic treatments. Here, we provide a critical and updated review of the most used PD models.

Decrease of mRNA Editing after Spinal Cord Injury is Caused by Down-regulation of ADAR2 that is Triggered by Inflammatory Response

We recently showed that spinal cord injury (SCI) leads to a decrease in mRNA editing of serotonin receptor 2C (5-HT_2CR) contributing to post-SCI spasticity. Here we study post-SCI mRNA editing and global gene expression using massively parallel sequencing. Evidence is presented that the decrease in 5-HT_2CR editing is caused by down-regulation of adenosine deaminase ADAR2 and that editing of at least one other ADAR2 target, potassium channel Kv1.1, is decreased after SCI. Bayesian network analysis of genome-wide transcriptome data indicates that down-regulation of ADAR2 (1) is triggered by persistent inflammatory response to SCI that is associated with activation of microglia and (2) results in changes in neuronal gene expression that are likely to contribute both to post-SCI restoration of neuronal excitability and muscle spasms. These findings have broad implications for other diseases of the Central Nervous System and could open new avenues for developing efficacious antispastic treatments.

Genetic and early environmental influences on the serotonin system: consequences for brain development and risk for psychopathology

BACKGROUND

Despite more than 60 years of research in the role of serotonin (5-HT) in psychopathology, many questions still remain. From a developmental perspective, studies have provided more insight into how 5-HT dysfunctions acquired in utero or early in life may modulate brain development. This paper discusses the relevance of the developmental role of 5-HT for the understanding of psychopathology. We review developmental milestones of the 5-HT system, how genetic and environmental 5-HT disturbances could affect brain development and the potential role of DNA methylation in 5-HT genes for brain development.

METHODS

Studies were identified using common databases (e.g., PubMed, Google Scholar) and reference lists.

RESULTS

Despite the widely supported view that the 5-HT system matures in early life, different 5-HT receptors, proteins and enzymes have different developmental patterns, and development is brain region-specific. A disruption in 5-HT homeostasis during development may lead to structural and functional changes in brain circuits that modulate emotional stress responses, including subcortical limbic and (pre)frontal areas. This may result in a predisposition to psychopathology. DNA methylation might be one of the underlying physiologic mechanisms.

LIMITATIONS

There is a need for prospective studies. The impact of stressors during adolescence on the 5-HT system is understudied. Questions regarding efficacy of drugs acting on 5-HT still remain.

CONCLUSION

A multidisciplinary and longitudinal approach in designing studies on the role of 5-HT in psychopathology might help to bring us closer to the understanding of the role of 5-HT in psychopathology.

Advances in genetic studies of substance abuse in China

Summary

The importance of genetic factors in substance addiction has long been established. The rationale for this work is that understanding of the function of addiction genes and delineation of the key molecular pathways of these genes would enhance the development of novel therapeutic targets and biomarkers that could be used in the prevention and management of substance abuse. Over the past few years, there has been a substantial increase in the number of genetic studies conducted on addiction in China; these studies have primarily focused on heroin, alcohol, and nicotine dependence. Most studies of candidate genes have concentrated on the dopamine, opioid, and serotonin systems. A number of genes associated with substance abuse in Caucasians are also risk factors in Chinese, but several novel genes and genetic risk factors associated with substance abuse in Chinese subjects have also been identified. This paper reviews the genetic studies of substance abuse performed by Chinese researchers. Genotypes and alleles related to addictive behavior in Chinese individuals are discussed and the contributions of Chinese researchers to the international corpus of knowledge about the genetic understanding of substance abuse are described.

Dopamine and serotonin signaling during two sensitive developmental periods differentially impact adult aggressive and affective behaviors in mice

Pharmacologic blockade of monoamine oxidase A (MAOA) or serotonin transporter (5-HTT) has antidepressant and anxiolytic efficacy in adulthood. Yet, genetically conferred MAOA or 5-HTT hypoactivity is associated with altered aggression and increased anxiety/depression. Here we test the hypothesis that increased monoamine signaling during development causes these paradoxical aggressive and affective phenotypes. We find that pharmacologic MAOA blockade during early postnatal development (P2-P21) but not during peri-adolescence (P22-41) increases anxiety- and depression-like behavior in adult (>P90) mice, mimicking the effect of P2-21 5-HTT inhibition. Moreover, MAOA blockade during peri-adolescence, but not P2-21 or P182-201, increases adult aggressive behavior, and 5-HTT blockade from P22-P41 reduced adult aggression. Blockade of the dopamine transporter, but not the norepinephrine transporter, during P22-41 also increases adult aggressive behavior. Thus, P2-21 is a sensitive period during which 5-HT modulates adult anxiety/depression-like behavior, and P22-41 is a sensitive period during which DA and 5-HT bi-directionally modulate adult aggression. Permanently altered DAergic function as a consequence of increased P22-P41 monoamine signaling might underlie altered aggression. In support of this hypothesis, we find altered aggression correlating positively with locomotor response to amphetamine challenge in adulthood. Proving that altered DA function and aggression are causally linked, we demonstrate that optogenetic activation of VTA DAergic neurons increases aggression. It therefore appears that genetic and pharmacologic factors impacting dopamine and serotonin signaling during sensitive developmental periods can modulate adult monoaminergic function and thereby alter risk for aggressive and emotional dysfunction.

Update on the pharmacology of selective inhibitors of MAO-A and MAO-B: focus on modulation of CNS monoamine neurotransmitter release

Inhibitors of monoamine oxidase (MAO) were initially used in medicine following the discovery of their antidepressant action. Subsequently their ability to potentiate the effects of an indirectly-acting sympathomimetic amine such as tyramine was discovered, leading to their limitation in clinical use, except for cases of treatment-resistant depression. More recently, the understanding that: a) potentiation of indirectly-acting sympathomimetic amines is caused by inhibitors of MAO-A but not by inhibitors of MAO-B, and b) that reversible inhibitors of MAO-A cause minimal tyramine potentiation, has led to their re-introduction to clinical use for treatment of depression (reversible MAO-A inhibitors and new dose form MAO-B inhibitor) and treatment of Parkinson's disease (MAO-B inhibitors). The profound neuroprotective properties of propargyl-based inhibitors of MAO-B in preclinical experiments have drawn attention to the possibility of employing these drugs for their neuroprotective effect in neurodegenerative diseases, and have raised the question of the involvement of the MAO-mediated reaction as a source of reactive free radicals. Despite the long-standing history of MAO inhibitors in medicine, the way in which they affect neuronal release of monoamine neurotransmitters is still poorly understood. In recent years, the detailed chemical structure of MAO-B and MAO-A has become available, providing new possibilities for synthesis of mechanism-based inhibitors. This review describes the latest advances in understanding the way in which MAO inhibitors affect the release of the monoamine neurotransmitters dopamine, noradrenaline and serotonin (5-HT) in the CNS, with an accent on the importance of these effects for the clinical actions of the drugs.

Non-coding-regulatory regions of human brain genes delineated by bacterial artificial chromosome knock-in mice

BACKGROUND

The next big challenge in human genetics is understanding the 98% of the genome that comprises non-coding DNA. Hidden in this DNA are sequences critical for gene regulation, and new experimental strategies are needed to understand the functional role of gene-regulation sequences in health and disease. In this study, we build upon our HuGX ('high-throughput human genes on the X chromosome') strategy to expand our understanding of human gene regulation in vivo.

RESULTS

In all, ten human genes known to express in therapeutically important brain regions were chosen for study. For eight of these genes, human bacterial artificial chromosome clones were identified, retrofitted with a reporter, knocked single-copy into the Hprt locus in mouse embryonic stem cells, and mouse strains derived. Five of these human genes expressed in mouse, and all expressed in the adult brain region for which they were chosen. This defined the boundaries of the genomic DNA sufficient for brain expression, and refined our knowledge regarding the complexity of gene regulation. We also characterized for the first time the expression of human MAOA and NR2F2, two genes for which the mouse homologs have been extensively studied in the central nervous system (CNS), and AMOTL1 and NOV, for which roles in CNS have been unclear.

CONCLUSIONS

We have demonstrated the use of the HuGX strategy to functionally delineate non-coding-regulatory regions of therapeutically important human brain genes. Our results also show that a careful investigation, using publicly available resources and bioinformatics, can lead to accurate predictions of gene expression.

Synthesis, transport, and metabolism of serotonin formed from exogenously applied 5-HTP after spinal cord injury in rats

Spinal cord transection leads to elimination of brain stem-derived monoamine fibers that normally synthesize most of the monoamines in the spinal cord, including serotonin (5-hydroxytryptamine, 5-HT) synthesized from tryptophan by enzymes tryptophan hydroxylase (TPH, synthesizing 5-hydroxytryptophan, 5-HTP) and aromatic l-amino acid decarboxylase (AADC, synthesizing 5-HT from 5-HTP). Here we examine whether spinal cord caudal to transection remains able to manufacture and metabolize 5-HT. Immunolabeling for AADC reveals that, while most AADC is confined to brain stem-derived monoamine fibers in spinal cords from normal rats, caudal to transection AADC is primarily found in blood vessel endothelial cells and pericytes as well as a novel group of neurons (NeuN positive and GFAP negative), all of which strongly upregulate AADC with injury. However, immunolabeling for 5-HT reveals that there is no detectable endogenous 5-HT synthesis in any structure in the spinal cord caudal to a chronic transection, including in AADC-containing vessels and neurons, consistent with a lack of TPH. In contrast, when we applied exogenous 5-HTP (in vitro or in vivo), AADC-containing vessels and neurons synthesized 5-HT, which contributed to increased motoneuron activity and muscle spasms (long-lasting reflexes, LLRs), by acting on 5-HT2 receptors (SB206553 sensitive) located on motoneurons (TTX resistant). Blocking monoamine oxidase (MAO) markedly increased the sensitivity of the motoneurons (LLR) to 5-HTP, more than it increased the sensitivity of motoneurons to 5-HT, suggesting that 5-HT synthesized from AADC is largely metabolized in AADC-containing neurons and vessels. In summary, after spinal cord injury AADC is upregulated in vessels, pericytes, and neurons but does not endogenously produce 5-HT, whereas when exogenous 5-HTP is provided AADC does produce functional amounts of 5-HT, some of which is able to escape metabolism by MAO, diffuse out of these AADC-containing cells, and ultimately act on 5-HT receptors on motoneurons.

Serotonin homeostasis and serotonin receptors as actors of cortical construction: special attention to the 5-HT3A and 5-HT6 receptor subtypes

Cortical circuits control higher-order cognitive processes and their function is highly dependent on their structure that emerges during development. The construction of cortical circuits involves the coordinated interplay between different types of cellular processes such as proliferation, migration, and differentiation of neural and glial cell subtypes. Among the multiple factors that regulate the assembly of cortical circuits, 5-HT is an important developmental signal that impacts on a broad diversity of cellular processes. 5-HT is detected at the onset of embryonic telencephalic formation and a variety of serotonergic receptors are dynamically expressed in the embryonic developing cortex in a region and cell-type specific manner. Among these receptors, the ionotropic 5-HT3A receptor and the metabotropic 5-HT6 receptor have recently been identified as novel serotonergic targets regulating different aspects of cortical construction including neuronal migration and dendritic differentiation. In this review, we focus on the developmental impact of serotonergic systems on the construction of cortical circuits and discuss their potential role in programming risk for human psychiatric disorders.

Distribution of monoamine oxidase proteins in human brain: implications for brain imaging studies

Positron emission tomography (PET) imaging of monoamine oxidases (MAO-A: [(11)C]harmine, [(11)C]clorgyline, and [(11)C]befloxatone; MAO-B: [(11)C]deprenyl-D2) has been actively pursued given clinical importance of MAOs in human neuropsychiatric disorders. However, it is unknown how well PET outcome measures for the different radiotracers are quantitatively related to actual MAO protein levels. We measured regional distribution (n=38) and developmental/aging changes (21 hours to 99 years) of both MAOs by quantitative immunoblotting in autopsied normal human brain. MAO-A was more abundant than MAO-B in infants, which was reversed as MAO-B levels increased faster before 1 year and, unlike MAO-A, kept increasing steadily to senescence. In adults, regional protein levels of both MAOs were positively and proportionally correlated with literature postmortem data of MAO activities and binding densities. With the exception of [(11)C]befloxatone (binding potential (BP), r=0.61, P=0.15), correlations between regional PET outcome measures of binding in the literature and MAO protein levels were good (P<0.01) for [(11)C]harmine (distribution volume, r=0.86), [(11)C]clorgyline (λk3, r=0.82), and [(11)C]deprenyl-D2 (λk3 or modified Patlak slope, r=0.78 to 0.87), supporting validity of the latter imaging measures. However, compared with in vitro data, the latter PET measures underestimated regional contrast by ∼2-fold. Further studies are needed to address cause of the in vivo vs. in vitro nonproportionality.

Monoamine oxidases in development

Monoamine oxidases (MAOs) are flavoproteins of the outer mitochondrial membrane that catalyze the oxidative deamination of biogenic and xenobiotic amines. In mammals there are two isoforms (MAO-A and MAO-B) that can be distinguished on the basis of their substrate specificity and their sensitivity towards specific inhibitors. Both isoforms are expressed in most tissues, but their expression in the central nervous system and their ability to metabolize monoaminergic neurotransmitters have focused MAO research on the functionality of the mature brain. MAO activities have been related to neurodegenerative diseases as well as to neurological and psychiatric disorders. More recently evidence has been accumulating indicating that MAO isoforms are expressed not only in adult mammals, but also before birth, and that defective MAO expression induces developmental abnormalities in particular of the brain. This review is aimed at summarizing and critically evaluating the new findings on the developmental functions of MAO isoforms during embryogenesis.

Impaired monoamine and organic cation uptake in choroid plexus in mice with targeted disruption of the plasma membrane monoamine transporter (Slc29a4) gene

The choroid plexus (CP) forms the blood-cerebrospinal fluid (CSF) barrier and protects the brain from circulating metabolites, drugs, and toxins. The plasma membrane monoamine transporter (PMAT, SLC29A4) is a new polyspecific organic cation transporter that transports a wide variety of organic cations including biogenic amines, cationic drugs, and neurotoxins. PMAT is known to be expressed in the CP, but its specific role in CP transport of organic cations has not been clearly defined. Here we showed that PMAT transcript is highly expressed in human and mouse CPs, whereas transcripts of other functionally related transporters are minimally expressed in the CPs. Immunofluorescence staining further revealed that PMAT protein is localized to the apical (CSF-facing) membrane of the CP epithelium, consistent with a role of transporting organic cations from the CSF into CP epithelial cells. To further evaluate the role of PMAT in the CP, mice with targeted deletion of the Slc29a4 gene were generated and validated. Although Pmat(-/-) mice showed no overt abnormalities, the uptake of monoamines and the neurotoxin 1-methyl-4-phenylpyridinium was significantly reduced in CP tissues isolated from the knock-out mice. Together, our data demonstrated that PMAT is a major transporter for CP uptake of bioactive amines and xenobiotic cations. By removing its substrates from the CSF, PMAT may play an important role in protecting the brain from cationic neurotoxins and other potentially toxic organic cations.

Severe serotonin depletion after conditional deletion of the vesicular monoamine transporter 2 gene in serotonin neurons: neural and behavioral consequences

The vesicular monoamine transporter type 2 gene (VMAT2) has a crucial role in the storage and synaptic release of all monoamines, including serotonin (5-HT). To evaluate the specific role of VMAT2 in 5-HT neurons, we produced a conditional ablation of VMAT2 under control of the serotonin transporter (slc6a4) promoter. VMAT2(sert-cre) mice showed a major (-95%) depletion of 5-HT levels in the brain with no major alterations in other monoamines. Raphe neurons contained no 5-HT immunoreactivity in VMAT2(sert-cre) mice but developed normal innervations, as assessed by both tryptophan hydroxylase 2 and 5-HT transporter labeling. Increased 5-HT(1A) autoreceptor coupling to G protein, as assessed with agonist-stimulated [(35)S]GTP-γ-S binding, was observed in the raphe area, indicating an adaptive change to reduced 5-HT transmission. Behavioral evaluation in adult VMAT2(sert-cre) mice showed an increase in escape-like reactions in response to tail suspension and anxiolytic-like response in the novelty-suppressed feeding test. In an aversive ultrasound-induced defense paradigm, VMAT2(sert-cre) mice displayed a major increase in escape-like behaviors. Wild-type-like defense phenotype could be rescued by replenishing intracellular 5-HT stores with chronic pargyline (a monoamine oxidase inhibitor) treatment. Pargyline also allowed some form of 5-HT release, although in reduced amounts, in synaptosomes from VMAT2(sert-cre) mouse brain. These findings are coherent with the notion that 5-HT has an important role in anxiety, and provide new insights into the role of endogenous 5-HT in defense behaviors.

Monoamine oxidase a expression is vital for embryonic brain development by modulating developmental apoptosis

Monoamine oxidases (MAO-A, MAO-B) metabolize biogenic amines and have been implicated in neuronal apoptosis. Although apoptosis is an important process in embryo development, the role of MAO isoenzymes has not been investigated in detail. We found that expression of MAO-A and MAO-B can be detected early on during embryo development. Expression levels remained constant until around midgestation but then dropped to almost undetectable levels toward birth. Similar expression kinetics were observed in the brain. Isoform-specific expression silencing of MAO-A mediated by siRNA during in vitro embryogenesis induced developmental defects, as indicated by a reduction of the crown rump length and impaired cerebral development. These alterations were paralleled by elevated serotonin levels. Similar abnormalities were observed when embryos were cultured in the presence of the MAO-A inhibitor clorgyline or when the transcriptional inhibitor of MAO-A expression R1 was overexpressed. In contrast, no such alterations were detected when expression of MAO-B was knocked down. To explore the underlying mechanisms for the developmental abnormalities in MAO-A knockdown embryos, we quantified the degree of developmental apoptosis in the developing brain. MAO-A knockdown reduced the number of apoptotic cells in the neuroepithelium, which coincided with impaired activation of caspases 3 and 9. Moreover, we observed reduced cyclin D1 levels as an indicator of impaired cell proliferation in MAO-A knockdown embryos. This data highlights MAO-A as a vital regulator of embryonic brain development.

Down-regulated GABAergic expression in the olfactory bulb layers of the mouse deficient in monoamine oxidase B and administered with amphetamine

This study explores primarily the role of the activity of monoamine oxidase B (MAOB) in the regulation of glutamic acid decarboxylase(67) (GAD(67)) expression in distinct layers of main olfactory bulb (OlfB), which links the limbic system. Moreover, the response of GAD(67) was investigated to amphetamine perturbation in the absence of MAOB activity. Immunocytochemical analysis was performed on OlfB sections prepared from the adult wild type (WT) and the MAOB gene-knocked-out (KO) mice after receiving repeated intraperitoneal injections (two doses per day, total seven doses) of saline or amphetamine, 5 mg/kg. The levels of the GAD(67) immunoreactivity were approximate 25 and 38% lower in respective glomerular (GloL) and mitral cell layers (ML) of saline-treated KO mice than that of WT, whereas similar in the external plexiform or granule cell layers (GraL) of the KO and WT. In the GloL, the level of tyrosine hydroxylase was 39% lower in the KO mice than WT, implicating different dopamine content in the KO from WT. The amphetamine exposure down-regulated the levels of GAD(67) in the WT layers by 46 to 52%, and in KO layers 65 to 71%, except ML. The GraL GAD(67) level may be regulated by the activation of CREB, as the phosphorylated (p) CREB coexisted with GAD(67), and the percentage of GAD(67)-expressing pCREB neurons was decreased by the amphetamine exposure. The data indicate that the activity of MAOB could modulate the regular and amphetamine-perturbed expression of GAD(67) and pCREB. Thus, interactions are suggested among the MAOB activity, GABA content of OlfB, and olfaction.

Association of MAOA, 5-HTT, and NET promoter polymorphisms with gene expression and protein activity in human placentas

Monoamine oxidase A (MAOA) and the transporters for serotonin (5-HTT) and norepinephrine (NET) may play important roles in regulating maternal monoamine neurotransmitters transferred across the placenta to the fetus. We investigated whether promoter polymorphisms in MAOA (uVNTR), 5-HTT (5-HTTLPR), and NET (NETpPR AAGG(4)) could influence gene expression and protein activity in human placentas. Normal term human placentas (n = 73) were collected, and placental MAOA, 5-HTT, and NET mRNA levels and protein activity were determined. The mRNA levels or protein activities were compared between different genotype groups. Placentas hemizygous (male fetus) or homozygous (female fetus) for MAOA uVNTR 4-repeat allele had significantly higher MAOA mRNA levels than those hemizygous or homozygous for the 3-repeat allele (P = 0.001). However, no significant difference in MAOA enzyme activity was found for these two groups of genotypes (P = 0.161). Placentas with the 5-HTTLPR short (S)-allele (S/S+S/L) had significantly lower 5-HTT mRNA levels and serotonin uptake rate than those homozygous for the long (L)-allele (L/L) (mRNA: P < 0.001; serotonin transporting activity: P < 0.001). Placentas homozygous for the NET AAGG(4) L(4) allele had significantly higher NET mRNA levels, as well as dopamine and norepinephrine uptake rates, than those with the S(4)/L(4) genotype (mRNA: P < 0.001; dopamine transporting activity: P = 0.012; norepinephrine transporting activity: P = 0.011). These findings suggest that the three promoter polymorphisms of MAOA, 5-HTT, and NET influence gene expression levels and protein activity of these genes in human placentas, potentially leading to different fetal levels of maternal monoamine neurotransmitters, which may have an impact on fetal neurodevelopment.

Ependymal cells: biology and pathology

The literature was reviewed to summarize the current understanding of the role of ciliated ependymal cells in the mammalian brain. Previous reviews were summarized. Publications from the past 10 years highlight interactions between ependymal cells and the subventricular zone and the possible role of restricted ependymal populations in neurogenesis. Ependymal cells provide trophic support and possibly metabolic support for progenitor cells. Channel proteins such as aquaporins may be important for determining water fluxes at the ventricle wall. The junctional and anchoring proteins are now fairly well understood, as are proteins related to cilia function. Defects in ependymal adhesion and cilia function can cause hydrocephalus through several different mechanisms, one possibility being loss of patency of the cerebral aqueduct. Ependymal cells are susceptible to infection by a wide range of common viruses; while they may act as a line of first defense, they eventually succumb to repeated attacks in long-lived organisms. Ciliated ependymal cells are almost certainly important during brain development. However, the widespread absence of ependymal cells from the adult human lateral ventricles suggests that they may have only regionally restricted value in the mature brain of large size.

Gene expression down-regulation in CD90+ prostate tumor-associated stromal cells involves potential organ-specific genes

Background

The prostate stroma is a key mediator of epithelial differentiation and development, and potentially plays a role in the initiation and progression of prostate cancer. The tumor-associated stroma is marked by increased expression of CD90/THY1. Isolation and characterization of these stromal cells could provide valuable insight into the biology of the tumor microenvironment.

Methods

Prostate CD90⁺ stromal fibromuscular cells from tumor specimens were isolated by cell-sorting and analyzed by DNA microarray. Dataset analysis was used to compare gene expression between histologically normal and tumor-associated stromal cells. For comparison, stromal cells were also isolated and analyzed from the urinary bladder.

Results

The tumor-associated stromal cells were found to have decreased expression of genes involved in smooth muscle differentiation, and those detected in prostate but not bladder. Other differential expression between the stromal cell types included that of the CXC-chemokine genes.

Conclusion

CD90⁺ prostate tumor-associated stromal cells differed from their normal counterpart in expression of multiple genes, some of which are potentially involved in organ development.

Effects of MAOA-genotype, alcohol consumption, and aging on violent behavior

BACKGROUND

Environmental factors appear to interact with a functional polymorphism (MAOA-LPR) in the promoter region of the monoamine oxidase A gene (MAOA) in determining some forms of antisocial behavior. However, how MAOA-LPR modulates the effects of other factors such as alcohol consumption related to antisocial behavior is not completely understood.

METHODS

This study examines the conjunct effect of MAOA-LPR, alcohol consumption, and aging on the risk for violent behavior. Recidivism in severe impulsive violent behavior was assessed after 7 to 15 years in a sample of 174 Finnish alcoholic offenders, the majority of whom exhibited antisocial or borderline personality disorder or both, and featured impulsive temperament traits.

RESULTS

The risk for committing new acts of violence increased by 2.3% for each kilogram of increase in yearly mean alcohol consumption (p = 0.004) and decreased by 7.3% for every year among offenders carrying the high activity MAOA genotype. In contrast, alcohol consumption and aging failed to affect violent behavior in the low activity MAOA genotyped offenders. MAOA-LPR showed no main effect on the risk for recidivistic violence.

CONCLUSIONS

Violent offenders carrying the high activity MAOA genotype differ in several ways from carriers with the low activity MAOA risk allele previously associated with antisocial behavior. Finnish high activity MAOA genotyped risk alcoholics exhibiting antisocial behavior, high alcohol consumption, and abnormal alcohol-related impulsive and uncontrolled violence might represent an etiologically distinct alcohol dependence subtype.

Regional changes in the cholinergic system in mice lacking monoamine oxidase A

Elevated brain monoamine concentrations resulting from monoamine oxidase A genetic ablation (MAOA knock-out mice) lead to changes in other neurotransmitter systems. To investigate the consequences of MAOA deficiency on the cholinergic system, we measured ligand binding to the high-affinity choline transporter (CHT1) and to muscarinic and nicotinic receptors in brain sections of MAOA knock-out (KO) and wild-type mice. A twofold increase in [(3)H]-hemicholinium-3 ([(3)H]-HC-3) binding to CHT1 was observed in the caudate putamen, nucleus accumbens, and motor cortex in MAOA KO mice as compared with wild-type (WT) mice. There was no difference in [(3)H]-HC-3 labeling in the hippocampus (dentate gyrus) between the two genotypes. Binding of [(125)I]-epibatidine ([(125)I]-Epi), [(125)I]-alpha-bungarotoxin ([(125)I]-BGT), [(3)H]-pirenzepine ([(3)H]-PZR), and [(3)H]-AFDX-384 ([(3)H]-AFX), which respectively label high- and low-affinity nicotinic receptors, M1 and M2 muscarinic cholinergic receptors, was not modified in the caudate putamen, nucleus accumbens, and motor cortex. A small but significant decrease of 19% in M1 binding densities was observed in the hippocampus (CA1 field) of KO mice. Next, we tested acetylcholinesterase activity and found that it was decreased by 25% in the striatum of KO mice as compared with WT mice. Our data suggest that genetic deficiency in MAOA enzyme is associated with changes in cholinergic activity, which may account for some of the behavioral alterations observed in mice and humans lacking MAOA.

Cortical enlargement in autism is associated with a functional VNTR in the monoamine oxidase A gene

Monoamine oxidase A (MAOA) is an enzyme expressed in the brain that metabolizes dopamine, norepinephrine, epinephrine, and serotonin. Abnormalities of serotonin neurotransmission have long been implicated in the psychopathology of autism. A polymorphism exists within the promoter region of the MAOA gene that influences MAOA expression levels so that "low activity" alleles are associated with increased neurotransmitter levels in the brain. Individuals with autism often exhibit elevated serotonin levels. Additional studies indicate that the "low activity" allele may be associated with lower IQ and more severe autistic symptoms. In this study we genotyped the MAOA promoter polymorphism in a group of 29 males (age 2-3 years) with autism and a group of 39 healthy pediatric controls for whom brain MRI data was available. We found a consistent association between the "low activity" allele and larger brain volumes for regions of the cortex in children with autism but not in controls. We did not find evidence for over-transmission of the "low activity" allele in a separate sample of 114 affected sib pair families. Nor did we find any unknown SNPs in yet another sample of 96 probands. Future studies will determine if there is a more severe clinical phenotype associated with both the "low activity" genotype and the larger brain volumes in our sample.

Embryonic depletion of serotonin affects cortical development

Compelling evidence suggests that serotonin (5-HT) is necessary for the refined organization of the cerebral cortex. Here we sought to analyse the short- and long-term consequences of embryonic 5-HT depletion on the development of the cerebral neocortex of the rat. We focused on the migration and differentiation of the pyramidal (projection) and nonpyramidal (interneuron) neuronal populations. Our paradigm used daily injection of DL-P-chlorophenylalanine (PCPA), a reversible inhibitor of 5-HT synthesis, during the E12-17 stage of embryonic development, when major events in corticogenesis take place. We monitored the 5-HT depletion induced by this treatment and showed that it led to subtle alterations in both the pyramidal and nonpyramidal neuronal populations. We found that E12-17 PCPA treatment altered the maturation of pyramidal neurons of layers III and V of the somatosensory cortex, with these cells displaying reduced dendritic arborization and complexity. These long-lasting alterations were not associated with modification of cortical BDNF levels at postnatal stages. We also showed that PCPA treatment transiently altered the incorporation in the cortical plate of interneurons derived from the caudal ganglionic eminence, and persistently affected the differentiation of a subpopulation expressing calretinin and/or cholecystokinin.