Potential bioactivated neurotoxicants, N-methylated beta-carbolinium ions, are present in human brain

Nuclear DNA and Mitochondrial Damage of the Cooked Meat Carcinogen 2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine in Human Neuroblastoma Cells

Animal fat and iron-rich diets are risk factors for Parkinson's disease (PD). The heterocyclic aromatic amines (HAAs) harman and norharman are neurotoxicants formed in many foods and beverages, including cooked meats, suggesting a role for red meat in PD. The structurally related carcinogenic HAAs 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), 2-amino-3,8-dimethylmidazo[4,5-f]quinoxaline (MeIQx), and 2-amino-9H-pyrido[2,3-b]indole (AαC) also form in cooked meats. We investigated the cytotoxicity, DNA-damaging potential, and mitochondrial damage of HAAs and their genotoxic HONH-HAA metabolites in galactose-dependent SH-SY5Y cells, a human neuroblastoma cell line relevant for PD-related neurotoxicity. All HAAs and HONH-HAAs induced weak toxicity except HONH-PhIP, which was 1000-fold more potent than the other chemicals. HONH-PhIP DNA adduct formation occurred at 300-fold higher levels than adducts formed with HONH-MeIQx and HONH-AαC, assuming similar cellular uptake rates. PhIP-DNA adduct levels occurred at concentrations as low as 1 nM and were threefold or higher and more persistent in mitochondrial DNA than nuclear DNA. N-Acetyltransferases (NATs), sulfotransferases, and kinases catalyzed PhIP-DNA binding and converted HONH-PhIP to highly reactive ester intermediates. DNA binding assays with cytosolic, mitochondrial, and nuclear fractions of SH-SY5Y fortified with cofactors revealed that cytosolic AcCoA-dependent enzymes, including NAT1, mainly carried out HONH-PhIP bioactivation to form N-acetoxy-PhIP, which binds to DNA. Furthermore, HONH-PHIP and N-acetoxy-PhIP inhibited mitochondrial complex-I, -II, and -III activities in isolated SH-SY5Y mitochondria. Mitochondrial respiratory chain complex dysfunction and DNA damage are major mechanisms in PD pathogenesis. Our data support the possible role of PhIP in PD etiology.

Cytochrome P450 enzymes and metabolism of drugs and neurotoxins within the mammalian brain

Cytochrome P450 enzymes (CYPs) that metabolize xenobiotics are expressed and active in the brain. These CYPs contribute to the metabolism of many centrally acting compounds, including clinically used drugs, drugs of abuse, and neurotoxins. Although CYP levels are lower in the brain than in the liver, they may influence central substrate and metabolite concentrations, which could alter resulting centrally-mediated responses to these compounds. Additionally, xenobiotic metabolizing CYPs are highly variable due to genetic polymorphisms and regulation by endogenous and xenobiotic molecules. In the brain, these CYPs are sensitive to xenobiotic induction. As a result, CYPs in the brain vary widely, including among humans, and this CYP variation may influence central metabolism and resulting response to centrally acting compounds. It has been demonstrated, using experimental manipulation of CYP activity in vivo selectively within the brain, that CYP metabolism in the brain alters central substrate and metabolite concentrations, as well as drug response and neurotoxic effects. This suggests that variability in xenobiotic metabolizing CYPs in the human brain may meaningfully contribute to individual differences in response to, and effects of, centrally acting drugs and neurotoxins. This chapter will provide an overview of CYP expression in the brain, endogenous- and xenobiotic-mediated CYP regulation, and the functional impact of CYP-mediated metabolism of drugs and neurotoxins in the brain, with a focus on experimental approaches in mice, rats, and non-human primates, and a discussion regarding the potential role of xenobiotic metabolizing CYPs in the human brain.

Potential Role of Heterocyclic Aromatic Amines in Neurodegeneration

Heterocyclic aromatic amines (HAAs) are mainly formed in the pyrolysis process during high-temperature cooking of meat. Meat consumption is very typical of the western diet, and the amount of meat consumption in the eastern countries is growing rapidly; HAAs represents widespread exposure. HAAs are classified as possible human carcinogens; numerous epidemiological studies have demonstrated regular consumption of meat with HAAs as risk factor for cancers. Specific HAAs have received major attention. For example, 2-amino-1-methyl-6-phenylimidazo[4,5-b] pyridine has been extensively studied as a genotoxicant and mutagen, with emergent literature on neurotoxicity. Harmane has been extensively studied for a role in essential tremors and potentially Parkinson's disease (PD). Harmane levels have been demonstrated to be elevated in blood and brain in essential tremor patients. Meat consumption has been implicated in the etiology of neurodegenerative diseases; however, the role of toxicants formed during meat preparation has not been studied. Epidemiological studies are currently examining the association between HAAs and risk of neurodegenerative diseases such as essential tremors and PD. Studies from our laboratory and others have provided strong evidence that HAA exposure produces PD and Alzheimer's disease-relevant neurotoxicity in cellular and animal models. In this review, we summarize and critically evaluate previous studies on HAA-induced neurotoxicity and the molecular basis of potential neurotoxic effects of HAAs. The available studies provide strong support for the premise that HAAs may impact neurological function and that addressing gaps in understanding of adverse neurological outcomes is critical to determine whether these compounds are modifiable risk factors.

Neurotoxicity and Underlying Mechanisms of Endogenous Neurotoxins

Endogenous and exogenous neurotoxins are important factors leading to neurodegenerative diseases. In the 1980s, the discovery that 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) contributes to Parkinson's disease (PD) symptoms led to new research investigations on neurotoxins. An abnormal metabolism of endogenous substances, such as condensation of bioamines with endogenous aldehydes, dopamine (DA) oxidation, and kynurenine pathway, can produce endogenous neurotoxins. Neurotoxins may damage the nervous system by inhibiting mitochondrial activity, increasing oxidative stress, increasing neuroinflammation, and up-regulating proteins related to cell death. This paper reviews the biological synthesis of various known endogenous neurotoxins and their toxic mechanisms.

Interaction of Neuromelanin with Xenobiotics and Consequences for Neurodegeneration; Promising Experimental Models

Neuromelanin (NM) accumulates in catecholamine long-lived brain neurons that are lost in neurodegenerative diseases. NM is a complex substance made of melanic, peptide and lipid components. NM formation is a natural protective process since toxic endogenous metabolites are removed during its formation and as it binds excess metals and xenobiotics. However, disturbances of NM synthesis and function could be toxic. Here, we review recent knowledge on NM formation, toxic mechanisms involving NM, go over NM binding substances and suggest experimental models that can help identifying xenobiotic modulators of NM formation or function. Given the high likelihood of a central NM role in age-related human neurodegenerative diseases such as Parkinson’s and Alzheimer’s, resembling such diseases using animal models that do not form NM to a high degree, e.g., mice or rats, may not be optimal. Rather, use of animal models (i.e., sheep and goats) that better resemble human brain aging in terms of NM formation, as well as using human NM forming stem cellbased in vitro (e.g., mid-brain organoids) models can be more suitable. Toxicants could also be identified during chemical synthesis of NM in the test tube.

Human CYP2D6 in the Brain Is Protective Against Harmine-Induced Neurotoxicity: Evidence from Humanized CYP2D6 Transgenic Mice

CYP2D6 metabolically inactivates several neurotoxins, including beta-carbolines, which are implicated in neurodegenerative diseases. Variation in CYP2D6 within the brain may alter local inactivation of neurotoxic beta-carbolines, thereby influencing neurotoxicity. The beta-carboline harmine, which induces hypothermia and tremor, is metabolized by CYP2D6 to the non-hypothermic/non-tremorgenic harmol. Transgenic mice (TG), expressing human CYP2D6 in addition to their endogenous mouse CYP2D, experience less harmine-induced hypothermia and tremor compared with wild-type mice (WT). We first sought to elucidate the role of CYP2D in general within the brain in harmine-induced hypothermia and tremor severity. A 4-h intracerebroventricular (ICV) pretreatment with the CYP2D inhibitor propranolol increased harmine-induced hypothermia and tremor in TG and increased harmine-induced hypothermia in WT. We next sought to specifically demonstrate that human CYP2D6 expressed in TG brain altered harmine response severity. A 24-h ICV propranolol pretreatment, which selectively and irreversibly inhibits human CYP2D6 in TG brain, increased harmine-induced hypothermia. This 24-h pretreatment had no impact on harmine response in WT, as propranolol is not an irreversible inhibitor of mouse CYP2D in the brain, thus confirming no off-target effects of ICV propranolol pretreatment. Human CYP2D6 activity in TG brain was sufficient in vivo to mitigate harmine-induced neurotoxicity. These findings suggest that human CYP2D6 in the brain is protective against beta-carboline-induced neurotoxicity and that the extensive interindividual variability in CYP2D6 expression in human brain may contribute to variation in susceptibility to certain neurotoxin-associated neurodegenerative disorders.

Mitochondrially targeted cytochrome P450 2D6 is involved in monomethylamine-induced neuronal damage in mouse models

Parkinson's disease (PD) is a major human disease associated with degeneration of the central nervous system. Evidence suggests that several endogenously formed 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-mimicking chemicals that are metabolic conversion products, especially β-carbolines and isoquinolines, act as neurotoxins that induce PD or enhance progression of the disease. We have demonstrated previously that mitochondrially targeted human cytochrome P450 2D6 (CYP2D6), supported by mitochondrial adrenodoxin and adrenodoxin reductase, can efficiently catalyze the conversion of MPTP to the toxic 1-methyl-4-phenylpyridinium ion. In this study, we show that the mitochondrially targeted CYP2D6 can efficiently catalyze MPTP-mimicking compounds, i.e. 2-methyl-1,2,3,4-tetrahydroisoquinoline, 2-methyl-1,2,3,4-tetrahydro-β-carboline, and 9-methyl-norharmon, suspected to induce PD in humans. Our results reveal that activity and respiration in mouse brain mitochondrial complex I are significantly affected by these toxins in WT mice but remain unchanged in Cyp2d6 locus knockout mice, indicating a possible role of CYP2D6 in the metabolism of these compounds both in vivo and in vitro These metabolic effects were minimized in the presence of two CYP2D6 inhibitors, quinidine and ajmalicine. Neuro-2a cells stably expressing predominantly mitochondrially targeted CYP2D6 were more sensitive to toxin-mediated respiratory dysfunction and complex I inhibition than cells expressing predominantly endoplasmic reticulum-targeted CYP2D6. Exposure to these toxins also induced the autophagic marker Parkin and the mitochondrial fission marker Dynamin-related protein 1 (Drp1) in differentiated neurons expressing mitochondrial CYP2D6. Our results show that monomethylamines are converted to their toxic cationic form by mitochondrially directed CYP2D6 and result in neuronal degradation in mice.

Potent inhibition of human organic cation transporter 2 (hOCT2) by β-carboline alkaloids

1. Beta-carbolines are indole alkaloids with a wide range of pharmacological and toxicological activities. Beta-carbolines are structurally related to the neurotoxin 1-methyl-4-phenylpyridinium (MPP+), a known substrate of organic cation transporters (OCTs). The goal of this study is to determine the interaction of β-carbolines with human OCT1, 2, and 3 (SLC22A1-3). 2. Dose-dependent inhibition studies were performed for five commercially available β-carbolines using a fluorescent substrate assay in HEK293 cells stably expressing hOCT1-3. The substrate potential was evaluated by uptake assays and the impact of active transport on cellular toxicity examined. 3. All tested β-carbolines potently inhibited hOCT2 with IC50 values in the sub- or low micromolar range. Harmaline is the most potent hOCT2 inhibitor (IC50 = 0.50 ± 0.08 μM). hOCT1 and hOCT3 are less sensitive to β-carboline inhibition. Harmaline, norharmanium, and 2,9-dimethyl-4,9-dihydro-3H-β-carbolinium accumulated 2- to 7-fold higher in cells expressing hOCT1-3. HEK293 cells expressing hOCT1-3 were 6.5- to 13-fold more sensitive to harmane and norharmanium toxicity. 4. Our data support a significant role of hOCT1-3 in tissue uptake and disposition of β-carbolines. Importantly, the potent inhibition of hOCT2 by β-carbolines also raises the concern of potential drug interactions between naturally occurring bioactive alkaloids and drugs eliminated by hOCT2.

Nicotinamide N-methyltransferase catalyses the N-methylation of the endogenous β-carboline norharman: evidence for a novel detoxification pathway

Nicotinamide N-methyltransferase (NNMT) is responsible for the N-methylation of nicotinamide to 1-methylnicotinamide. Our recent studies have demonstrated that NNMT regulates cellular processes fundamental to the correct functioning and survival of the cell. It has been proposed that NNMT may possess β-carboline (BC) N-methyltransferase activity, endogenously and exogenously produced pyridine-containing compounds which, when N-methylated, are potent inhibitors of Complex I and have been proposed to have a role in the pathogenesis of Parkinson's disease. We have investigated the ability of recombinant NNMT to N-methylate norharman (NH) to 2-N-methylnorharman (MeNH). In addition, we have investigated the toxicity of the BC NH, its precursor 1,2,3,4-tetrahydronorharman (THNH) and its N-methylated metabolite MeNH, using our in vitro SH-SY5Y NNMT expression model. Recombinant NNMT demonstrated NH 2N-methyltransferase activity, with a Km of 90 ± 20 µM, a kcat of 3 × 10(-4) ± 2 × 10(-5) s(-1) and a specificity constant (kcat/Km) of 3 ± 1 s(-1) M(-1) THNH was the least toxic of all three compounds investigated, whereas NH demonstrated the greatest, with no difference observed in terms of cell viability and cell death between NNMT-expressing and non-expressing cells. In NNMT-expressing cells, MeNH increased cell viability and cellular ATP concentration in a dose-dependent manner after 72 and 120 h incubation, an effect that was not observed after 24 h incubation or in non-NNNT-expressing cells at any time point. Taken together, these results suggest that NNMT may be a detoxification pathway for BCs such as NH.

Structure of Amido Pyridinium Betaines: Persistent Intermolecular C-H⋅⋅⋅N Hydrogen Bonding in Solution

A hydrogen bond of the type C-H⋅⋅⋅X (X=O or N) is known to influence the structure and function of chemical and biological systems in solution. C-H⋅⋅⋅O hydrogen bonding in solution has been extensively studied, both experimentally and computationally, whereas the equivalent thermodynamic parameters have not been enumerated experimentally for C-H⋅⋅⋅N hydrogen bonds. This is, in part, due to the lack of systems that exhibit persistent C-H⋅⋅⋅N hydrogen bonds in solution. Herein, a class of molecule based on a biologically active norharman motif that exhibits unsupported intermolecular C-H⋅⋅⋅N hydrogen bonds in solution has been described. A pairwise interaction leads to dimerisation to give bond strengths of about 7 kJ mol^-1 per hydrogen bond, which is similar to chemically and biologically relevant C-H⋅⋅⋅O hydrogen bonding. The experimental data is supported by computational work, which provides additional insight into the hydrogen bonding by consideration of electrostatic and orbital interactions and allowed a comparison between calculated and extrapolated NMR chemical shifts.

The effect of foetal bovine serum supplementation upon the lactate dehydrogenase cytotoxicity assay: Important considerations for in vitro toxicity analysis

The lactate dehydrogenase (LDH) assay is a commonly-used tool for assessing toxicity in vitro. However, anecdotal reports suggest that foetal bovine serum (FBS) may contain LDH at concentrations significant enough to interfere with the assay and thus reduce its sensitivity. A series of experiments were performed to determine whether addition of FBS to culture medium significantly elevated culture media LDH content, and whether replacement of FBS with heat inactivated foetal bovine serum (HI-FBS) reduced LDH content and interfered with cell response to cytotoxic challenge. The addition of FBS at 5, 10 and 15% final concentrations increased culture medium LDH content in a dose-dependent manner. The substitution of HI-FBS for FBS reduced culture medium LDH content and increased the dynamic range of the assay. Cell viability of the SH-SY5Y human neuroblastoma and N27 rat mesencephalic neurone cell lines were significantly reduced as measured using the MTT reduction assay, whilst HI-FBS only affected toxicity response in a cell- and toxin-specific manner, although these effects were small. Hence, for cell lines with a high FBS requirement, the use of HI-FBS or alternative toxicity assays can be considered, or the use of alternative formulations, such as chemically-defined serum-free media, be adopted.

Soma, food of the immortals according to the Bower Manuscript (Kashmir, 6th century A.D.)

ETHNOPHARMACOLOGICAL RELEVANCE

Food is medicine and vice versa. In Hindu and Ayurvedic medicine, and among human cultures of the Indian subcontinent in general, the perception of the food-medicine continuum is especially well established. The preparation of the exhilarating, gold-coloured Soma, Amrita or Ambrosia, the elixir and food of the 'immortals'-the Hindu pantheon-by the ancient Indo-Aryans, is described in the Rigveda in poetic hymns. Different theories regarding the botanical identity of Soma circulate, but no pharmacologically and historically convincing theory exists to date. We intend to contribute to the botanical, chemical and pharmacological characterisation of Soma through an analysis of two historical Amrita recipes recorded in the Bower Manuscript. The recipes are referred therein as panaceas (clarified butter) and also as a medicine to treat nervous diseases (oil), while no exhilarating properties are mentioned. Notwithstanding this, we hypothesise, that these recipes are related to the ca. 1800 years older Rigvedic Soma. We suppose that the psychoactive Soma ingredient(s) are among the components, possibly in smaller proportions, of the Amrita recipes preserved in the Bower Manuscript.

MATERIALS AND METHODS

The Bower Manuscript is a medical treatise recorded in the 6th century A.D. in Sanskrit on birch bark leaves, probably by Buddhist monks, and unearthed towards the end of the 19th century in Chinese Turkestan. We analysed two Amrita recipes from the Bower Manuscript, which was translated by Rudolf Hoernle into English during the early 20th century. A database search with the updated Latin binomials of the herbal ingredients was used to gather quantitative phytochemical and pharmacological information.

RESULTS

Together, both Amrita recipes contain around 100 herbal ingredients. Psychoactive alkaloid containing species still important in Ayurvedic, Chinese and Thai medicine and mentioned in the recipe for 'Amrita-Prâsa clarified butter' and 'Amrita Oil' are: Tinospora cordifolia (Amrita, Guduchi), three Sida spp., Mucuna pruriens, Nelumbo nucifera, Desmodium gangeticum, and Tabernaemontana divaricata. These species contain several notorious and potential psychoactive and psychedelic alkaloids, namely: tryptamines, 2-phenylethylamine, ephedrine, aporphines, ibogaine, and L-DOPA. Furthermore, protoberberine alkaloids, tetrahydro-β-carbolines, and tetrahydroisoquinolines with monoamine oxidase inhibitor (MAO-I) activity but also neurotoxic properties are reported.

CONCLUSIONS

We propose that Soma was a combination of a protoberberine alkaloids containing Tinospora cordifolia juice with MAO-I properties mixed together with a tryptamine rich Desmodium gangeticum extract or a blending of Tinospora cordifolia with an ephedrine and phenylethylamine-rich Sida spp. extract. Tinospora cordifolia combined with Desmodium gangeticum might provide a psychedelic experience with visual effects, while a combination of Tinospora cordifolia with Sida spp. might lead to more euphoric and amphetamine-like experiences.

Elevated blood harmane (1-methyl-9H-pyrido[3,4-b]indole) concentrations in Parkinson's disease

BACKGROUND

Parkinson's disease (PD) is a late-life neurodegenerative disease. Genetic and environmental factors play an etiological role. Harmane (1-methyl-9H-pyrido[3,4-b]indole) is a potent tremor-producing neurotoxin that shows structural resemblance to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP).

OBJECTIVES

In 2002 and 2007, we demonstrated elevated blood harmane concentrations [HA] in essential tremor (ET) cases. We now assessed whether blood [HA] were elevated in Parkinson's disease (PD) as well.

METHODS

Blood [HA] were quantified by high performance liquid chromatography. Subjects comprised 113 PD cases and 101 controls.

RESULTS

Mean log blood [HA] in PD cases was double that of controls (0.59±0.63 g(-10)/ml vs. 0.27±0.63 g(-10)/ml, p<0.001). A non-parametric test on non-transformed data (median blood [HA]=3.31 g(-10)/ml in cases and 1.44 g(-10)/ml in controls) also showed this difference (p<0.001). In unadjusted and then adjusted logistic regression analyses, log blood [HA] was associated with PD (odds ratio [OR]unadjusted 2.31, 95% confidence interval [CI] 1.46-3.67, p<0.001; OR(adjusted) 2.54, 95% CI 1.55-4.16, p<0.001). In PD, log blood [HA] co-varied with family history, being lowest in PD cases with no family history (0.54±0.60 g(-10)/ml) and highest in PD cases with a family history of both ET and PD (0.84±0.68 g(-10)/ml) (p=0.06).

CONCLUSIONS

Blood harmane appears to be elevated in PD. The finding needs to be reproduced in additional cohorts to assess its generalizability. The higher concentration in familial PD suggests that the mechanism may involve genetic factors.

Elevated brain harmane (1-methyl-9H-pyrido[3,4-b]indole) in essential tremor cases vs. controls

BACKGROUND

Harmane (1-methyl-9H-pyrido[3,4-β]indole), a potent neurotoxin that has tremor-producing properties in animal models, is present in many foods; although we have demonstrated a difference in tissue harmane concentrations in ET cases vs. controls, all work to date has involved blood samples.

OBJECTIVES

We quantified harmane concentrations in human cerebellum, a brain region of particular pathogenic interest in essential tremor (ET), comparing ET to control brains.

METHODS

Cerebellar cortex was snap frozen and stored at -80°C in aliquots for biochemical analyses. Harmane concentration was assessed using high performance liquid chromatography.

RESULTS

Geometric mean brain harmane concentrations (adjusted for postmortem interval [PMI] and freezer time) were higher in ET cases than controls: 1.0824 (95% confidence interval=0.9405-1.2457) vs. 0.8037 (0.6967-0.9272), p=0.004. Geometric mean of brain harmane concentrations (adjusting for PMI and freezer time) was highest in ET cases who reported other relatives with tremor (1.2005 [0.8712-1.6541]), intermediate in ET cases without family history (1.0312 ([0.8879-1.1976]), and both were significantly higher than controls (p=0.02).

CONCLUSIONS

This study provides additional evidence of a possible etiological importance of this toxin in some cases of the human disease ET.

Metabolite profile resulting from the activation/inactivation of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine and 2-methyltetrahydro-β-carboline by oxidative enzymes

Metabolic enzymes are involved in the activation/deactivation of the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyiridine (MPTP) neurotoxin and its naturally occurring analogs 2-methyltetrahydro-β-carbolines. The metabolic profile and biotransformation of these protoxins by three enzymes, monoamine oxidase (MAO), cytochrome P450, and heme peroxidases (myeloperoxidase and lactoperoxidase), were investigated and compared. The metabolite profile differed among the enzymes investigated. MAO and heme peroxidases activated these substances to toxic pyridinium and β-carbolinium species. MAO catalyzed the oxidation of MPTP to 1-methyl-4-phenyl-2,3-dihydropyridinium cation (MPDP(+)), whereas heme peroxidases catalyzed the oxidation of MPDP(+) to 1-methyl-4-phenylpyridinium (MPP(+)) and of 2-methyltetrahydro-β-carboline to 2-methyl-3,4-dihydro-β-carbolinium cation (2-Me-3,4-DH β C(+)). These substances were inactivated by cytochrome P450 2D6 through N-demethylation and aromatic hydroxylation (MPTP) and aromatic hydroxylation (2-methyltetrahydro-β-carboline). In conclusion, the toxicological effects of these protoxins might result from a balance between the rate of their activation to toxic products (i.e., N-methylpyridinium-MPP(+) and MPDP(+)- and N-methyl--β--carbolinium- βC(+)-) by MAO and heme peroxidases and the rate of inactivation (i.e., N-demethylation, aromatic hydroxylation) by cytochrome P450 2D6.

Neurotoxic effects of berberine on long-term L-DOPA administration in 6-hydroxydopamine-lesioned rat model of Parkinson's disease

The effects of berberine on long-term administration of L-DOPA in 6-hydroxydopamine (6-OHDA)-lesioned rat model of Parkinson's disease (PD) were investigated. Rat models of PD were prepared by 6-OHDA lesions in the ipsilateral sides, and then were treated with berberine (5 and 15 mg/kg) and/or L-DOPA (10 mg/kg) once daily for 21 days. Treatments with either concentration of berberine (5 and 15 mg/kg) in 6-OHDA-lesioned groups decreased the numbers of tyrosine hydroxylase (TH)-immunopositive neurons in the substantia nigra and the levels of dopamine, norepinephrine, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the striatum as compared to 6-OHDA-lesioned groups. In addition, dopaminergic neuronal cell death of the ipsilateral sides in 6-OHDA-lesioned groups was attenuated by L-DOPA administration. However, both concentrations of berberine in 6-OHDA-lesioned groups treated with L-DOPA aggravated the numbers of TH-immunopositive neurons in the substantia nigra and the levels of dopamine, norepinephrine, DOPAC and HVA in the striatum as compared to rats not treated with berberine. These results suggest that berberine leads to the degeneration of dopaminergic neuronal cells in the substantia nigra in the rat model of PD with chronic L-DOPA administration. Long-term L-DOPA therapy that may involve possibly neurotoxic isoquinoline agents including berberine should involve monitoring for adverse symptoms.

Blood harmane (1-methyl-9H-pyrido[3,4-b]indole) concentrations in essential tremor: repeat observation in cases and controls in New York

Essential tremor (ET) is a widespread late-life neurological disease. Genetic and environmental factors are likely to play important etiological roles. Harmane (1-methyl-9H-pyrido[3,4-b]indole) is a potent tremor-producing neurotoxin. Previously, elevated blood harmane concentrations were demonstrated in ET cases compared to controls, but these observations have all been cross-sectional, assessing each subject at only one time point. Thus, no one has ever repeat-assayed blood harmane in the same subjects twice. Whether the observed case-control difference persists at a second time point, years later, is unknown. The current goal was to reassess a sample of our ET cases and controls to determine whether blood harmane concentration remained elevated in ET at a second time point. Blood harmane concentrations were quantified by a well-established high-performance liquid chromatography method in 63 ET cases and 70 controls. A mean of approximately 6 yr elapsed between the initial and this subsequent blood harmane determination. The mean log blood harmane concentration was significantly higher in cases than controls (0.30 ± 0.61 g(-10)/ml versus 0.08 ± 0.55 g(-10)/ml), and the median value in cases was double that of controls: 0.22 g(-10)/ml versus 0.11 g(-10)/ml. The log blood harmane concentration was highest in cases with a family history of ET. Blood harmane concentration was elevated in ET cases compared to controls when reassessed at a second time point several years later, indicating what seems to be a stable association between this environmental toxin and ET.

Relationship between blood harmane and harmine concentrations in familial essential tremor, sporadic essential tremor and controls

INTRODUCTION

Harmane, a potent tremor-producing β-carboline alkaloid, may play a role in the etiology of essential tremor (ET). Blood harmane concentrations are elevated in ET cases compared with controls yet the basis for this elevation remains unknown. Decreased metabolic conversion (harmane to harmine) is one possible explanation. Using a sample of >500 individuals, we hypothesized that defective metabolic conversion of harmane to harmine might underlie the observed elevated harmane concentration in ET, and therefore expected to find a higher harmane to harmine ratio in familial ET than in sporadic ET or controls.

METHODS

Blood harmane and harmine concentrations were quantified by high performance liquid chromatography.

RESULTS

There were 78 familial ET cases, 187 sporadic ET cases, and 276 controls. Blood harmane and harmine concentrations were correlated with one another (Spearman's r=0.24, p<0.001). The mean (±SD) harmane/harmine ratio=23.4±90.9 (range=0.1-987.5). The harmane/harmine ratio was highest in familial ET (46.7±140.4), intermediate in sporadic ET (28.3±108.1), and lowest in controls (13.5±50.3) (p=0.03). In familial ET cases, there was no association between this ratio and tremor severity (Spearman's r=0.08, p=0.48) or tremor duration (Spearman's r=0.14, p=0.24).

CONCLUSION

The basis for the elevated blood harmane concentration, particularly in familial ET, is not known, although the current findings (highest harmane/harmine ratio in familial ET cases) lends support to the possibility that it could be the result of a genetically-driven reduction in harmane metabolism.

9-Methyl-beta-carboline has restorative effects in an animal model of Parkinson's disease

In a previous study, a primary culture of midbrain cells was exposed to 9-methyl-beta-carboline for 48 h, which caused an increase in the number of tyrosine hydroxylase-positive cells. Quantitative RT-PCR revealed increased transcription of genes participating in the maturation of dopaminergic neurons. These in vitro findings prompted us to investigate the restorative actions of 9-methyl-beta-carboline in vivo. The compound was delivered for 14 days into the left cerebral ventricle of rats pretreated with the neurotoxin 1-methyl-4-phenyl-pyridinium ion (MPP+) for 28 days applying a dose which lowered dopamine by approximately 50%. Interestingly, 9-methyl-beta-carboline reversed the dopamine-lowering effect of the neurotoxin in the left striatum. Stereological counts of tyrosine hydroxylase-immunoreactive cells in the substantia nigra revealed that the neurotoxin caused a decrease in the number of those cells. However, when treated subsequently with 9-methyl-beta-carboline, the number reached normal values. In search of an explanation for the restorative activity, we analyzed the complexes that compose the respiratory chain in striatal mitochondria by 2-dimension gel electrophoresis followed by MALDI-TOF peptide mass fingerprinting.We found no changes in the overall composition of the complexes. However, the activity of complex I was increased by approximately 80% in mitochondria from rats treated with MPP+ and 9-methyl-beta-carboline compared to MPP+ and saline and to sham-operated rats, as determined by measurements of nicotinamide adenine dinucleotide dehydrogenase activity. Microarray technology and single RT-PCR revealed the induction of neurotrophins: brain-derived neurotrophic factor, conserved dopamine neurotrophic factor, cerebellin 1 precursor protein, and ciliary neurotrophic factor. Selected western blots yielded consistent results. The findings demonstrate restorative effects of 9-methyl-beta-carboline in an animal model of Parkinson's disease that improve the effectiveness of the respiratory chain and promote the transcription and expression of neurotrophin-related genes.

Environmental epidemiology of essential tremor

BACKGROUND

Essential tremor (ET) is one of the most common neurological disorders. Despite this, the disease mechanisms and etiology are not well understood. While susceptibility genotypes undoubtedly underlie many ET cases, no ET genes have been identified thus far. As with many other progressive, degenerative neurological disorders, it is likely that environmental factors contribute to the etiology of ET. Environmental epidemiology is the study in specific populations or communities of the effect on human health of physical, biologic and chemical factors in the external environment. The purpose of this article is to review current knowledge with regards to the environmental epidemiology of ET.

RESULTS

As will be discussed, a series of preliminary case-control studies in recent years has begun to explore several candidate toxins/exposures, including harmane (1-methyl-9H-pyrido[3,4-b]indole), lead and agricultural exposures/pesticides.

CONCLUSIONS

While several initial results are promising, as will be discussed, additional studies are needed to more definitively establish whether these exposures are associated with ET and if they are of etiological importance.

Effects of harman and norharman on dopamine biosynthesis and L-DOPA-induced cytotoxicity in PC12 cells

The effects of harman and norharman on dopamine biosynthesis and L-DOPA-induced cytotoxicity in PC12 cells were investigated. Harman and norharman at a concentration of 20 microM and 100 microM showed 49.4% and 49.5% inhibition of dopamine content for 48 h, respectively. The IC50 values of harman and norharman were 21.2 microM and 103.3 microM. Dopamine content, tyrosine hydroxylase (TH) activity and TH mRNA levels were decreased during the first 6 h, maintained for up to 48 h and then gradually recovered at 72 h after exposure to 20 microM harman and 100 microM norharman. Under the same conditions, the intracellular cyclic AMP levels and Ca2+ concentrations were also decreased by harman and norharman. In addition, harman and norharman at concentrations higher than 80 microM and 150 microM caused cytotoxicity at 48 h in PC12 cells. Non-cytotoxic ranges of 10-30 microM harman and 50-150 microM norharman inhibited L-DOPA (20-50 microM)-induced increases in dopamine content at 48 h. Harman at 20-150 microM and norharman at 100-300 microM also enhanced L-DOPA (20-100 microM)-induced cytotoxicity at 48 h with an apoptotic process. These results suggest that harman and norharman inhibit dopamine biosynthesis by reducing TH activity and enhance L-DOPA-induced cytotoxicity in PC12 cells.

Elevated blood harmane (1-methyl-9H-pyrido[3,4-b]indole) concentrations in essential tremor

Essential tremor (ET) is a widespread late-life neurological disease. Genetic and environmental factors likely play an etiological role. Harmane (1-methyl-9H-pyrido[3,4-b]indole) is a potent tremor-producing neurotoxin. In 2002, we demonstrated elevated blood harmane concentrations in an initial sample of 100 ET cases compared to 100 controls. Between 2002 and 2007, we assembled a new and larger sample of ET cases and controls. We now attempt to replicate our previous findings. Cases and controls were frequency-matched on age, gender, and race. Blood harmane concentrations were quantified by high-performance liquid chromatography. Subjects comprised 150 ET cases and 135 controls (mean age 65.3+/-15.5 vs. 65.5+/-14.2 years, p=0.94). Mean log blood harmane concentration was approximately 50% higher in cases than controls (0.50+/-0.54g(-10)/ml vs. 0.35+/-0.62g(-10)/ml, p=0.038). In a logistic regression analysis, log blood harmane concentration was associated with ET (OR(adjusted) 1.56, 95% CI 1.01-2.42, p=0.04), and odds of ET was 1.90 (95% CI 1.07-3.39, p=0.029) in the highest versus lowest log blood harmane tertile. Log blood harmane was highest in ET cases with familial ET (0.53+/-0.57g(-10)/ml), intermediate in cases with sporadic ET (0.43+/-0.45g(-10)/ml) and lowest in controls (0.35+/-0.62g(-10)/ml) (test for trend, p=0.026). Blood harmane appears to be elevated in ET. The higher concentrations in familial ET suggests that the mechanism may involve genetic factors.

Cytotoxicity of β-carbolines in dopamine transporter expressing cells: Structure–activity relationships

Some beta-carbolines (BC) are natural constituents in the human brain deriving from tryptophan, tryptamine, and serotonin. In vitro and animal experiments suggest that BC-cations may cause neurodegeneration with a higher vulnerability of dopaminergic than of other neurons. Despite the possible implication of the BC-cations in the pathogenesis of Parkinson's disease (PD), the underlying mechanisms are poorly understood. The present study further explores the structural requirements for the cytotoxic effects of BCs and searches for additional compounds involved in the pathogenesis of PD. Previous studies were now extended to serotonin-derived BCs, tetrahydro-BCs, a BC-dimer, and a BC-enantiomer to reveal possible stereoselectivity. Neutral, rather lipophilic BCs may pass the plasma membrane and the outer and inner mitochondrial membranes by diffusion whereas the cationic, more polar compounds, can be transported by the dopamine transporter (DAT). In the present study, 4 out of 17 BC-cations caused DAT-independent toxicity. This number is unexpected in view of previous findings that all BC-cations are transported by DAT. 3-Carboxylated and 6-methoxylated BCs were poor substrates. The size alone does not seem to be a limiting factor. A dimeric BC-cation was readily transported by the DAT despite its much larger structure compared to dopamine. Furthermore, (R)-enantiomers were preferentially transported. The neutral BCs were approximately one order of magnitude less toxic than the cationic BCs. There are considerable differences of the transport efficiency between the BCs. Potent cytotoxic tetrahydro-BCs were detected. Because precursor tetrahydro-BCs are present in the brain, the search for the occurrence of these compounds in human brain is warranted.

9-Methyl-beta-carboline up-regulates the appearance of differentiated dopaminergic neurones in primary mesencephalic culture

beta-Carbolines (BCs) derive from tryptophan and its derivatives. They are formed endogenously in humans and mammals and occur inter alia in cooked meat and tobacco smoke. They have been detected in human brain, cerebrospinal fluid, and plasma. Up to now they were predominantly identified as compounds exhibiting neurotoxic actions. Since significantly higher amounts are present in parkinsonian patients, they are regarded as potential pathogenetic factors in Parkinson's disease. We identified for the first time a BC (9-methyl-BC; 9-me-BC) exerting neuroprotective and neuron-differentiating effects. Treatment of primary mesencephalic dopaminergic cultures with 9-me-BC inhibited the basal release of lactate dehydrogenase and reduced the number of cells stained with propidium iodide. Caspase-3 activity was decreased, the total protein content was unchanged and ATP content was increased. Furthermore, the expression of inflammation-related genes was reduced. The number of differentiated dopaminergic neurones was significantly increased and a wide array of neurotrophic/transcription factors (Shh, Wnt1, Wnt5a, En1, En2, Nurr1, Pitx3) and marker genes (Th, Dat, Aldh1a1) decisive for dopaminergic differentiation was stimulated. Consistently, the dopamine content was slightly, although non-significantly, increased and the dopamine uptake capacity was elevated. An anti-proliferative effect was observed in human neuroblastoma SH-SY5Y cells which is consistent with a reduced incorporation of bromodesoxyuridine into the DNA of primary mesencephalic cells. Whether the additional dopaminergic neurones in primary culture derive from dopaminergic precursor cells, previously tyrosine hydroxylase negative dopaminergic neurones or are the result of a transdifferentiation process remains to be established.

Brunneins A-C, beta-carboline alkaloids from Cortinarius brunneus

Four beta-carboline alkaloids, brunneins A-C (1-3) and 3-(7-hydroxy-9H-beta-carboline-1-yl)propanoic acid (4), were isolated from fruiting bodies of the agaricoid fungus Cortinarius brunneus. The structures of 1-3 were determined by analysis of NMR and MS data, and the structure of compound 4 was determined by comparison with published data. Brunnein A (1) exhibited very low cholinesterase inhibitory effects and no cytotoxicity.

The role of Coenzyme Q in mitochondrial electron transport

In mitochondria, most Coenzyme Q is free in the lipid bilayer; the question as to whether tightly bound, non-exchangeable Coenzyme Q molecules exist in mitochondrial complexes is still an open question. We review the mechanism of inter-complex electron transfer mediated by ubiquinone and discuss the kinetic consequences of the supramolecular organization of the respiratory complexes (randomly dispersed vs. super-complexes) in terms of Coenzyme Q pool behavior vs. metabolic channeling, respectively, both in physiological and in some pathological conditions. As an example of intra-complex electron transfer, we discuss in particular Complex I, a topic that is still under active investigation.

N-methyltetrahydro-beta-carboline analogs of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) neurotoxin are oxidized to neurotoxic beta-carbolinium cations by heme peroxidases

2-Methyl-1,2,3,4-tetrahydro-beta-carboline (2-Me-THbetaC) and 2,9-dimethyl-1,2,3,4-tetrahydro-beta-carboline (2,9-diMe-THbetaC) are naturally occurring analogs of the Parkinsonian neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), whereas their corresponding aromatic 2-methyl-beta-carbolinium cations resemble 1-methyl-4-phenylpyridinium (MPP(+)) and are considered potential toxins involved in Parkinson's disease (PD). To become toxicants, 2-methyltetrahydro-beta-carbolines need to be oxidized (aromatized) by human metabolic enzymes to pyridinium-like (beta-carbolinium) cations as occur with MPTP/MPP(+) model. In contrast to MPTP, human MAO-A or -B were not able to oxidize 2-Me-THbetaC to pyridinium-like cations. Neither, cytochrome P-450 2D6 or a mixture of six P450 enzymes carried out this oxidation in a significant manner. However, 2-Me-THbetaC and 2,9-diMe-THbetaC were efficiently oxidized by horseradish peroxidase (HRP), lactoperoxidase (LPO), and myeloperoxidase (MPO) to 2-methyl-3,4-dihydro-beta-carbolinium cations (2-Me-DHbetaC(+), 2,9-diMe-DHbetaC(+)) as the main products, and detectable amount of 2-methyl-beta-carbolinium cations (2-Me-betaC(+), 2,9-diMe-betaC(+)). The apparent kinetic parameters (k(cat), k(4)) were similar for HRP and LPO and higher for MPO. Peroxidase inhibitors (hydroxylamine, sodium azide, and ascorbic acid) highly reduced or abolished this oxidation. Although MPTP was not oxidized by peroxidases; its intermediate metabolite 1-methyl-4-phenyl-2,3-dihydropyridinium cation (MPDP(+)) was efficiently oxidized to MPP(+) by heme peroxidases. It is concluded that heme peroxidases could be key catalysts responsible for the aromatization (bioactivation) of endogenous and naturally occurring N-methyltetrahydro-beta-carbolines and related protoxins to toxic pyridinium-like cations resembling MPP(+), suggesting a role for these enzymes in toxicological and neurotoxicological processes.

Parkinson's disease-like syndrome in rats induced by 2,9-dimethyl-beta-carbolinium ion, a beta-carboline occurring in the human brain

Regarding the pathogenesis of Parkinson's disease, a neurotoxin hypothesis was proposed following the discovery that 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) produces a Parkinson-like syndrome in humans and primates. Since then, researchers have searched for endogenous and exogenous compounds that are structurally similar to this neurotoxin. Such compounds include beta-carbolines, formed from tryptophan and its derivatives. beta-carbolines are present naturally in the human brain and cerebrospinal fluid. The present study examined the effect of bilateral, intranigral administration of 2,9-dimethyl-beta-carbolinium ion on muscle tone, electromyographic activity, dopamine metabolism in the striatum, and the number of tyrosine hydroxylase-immunoreactive neurons and volume of the substantia nigra in rats. We found that the beta-carbolinium ion (15 or 40 nmol per side) caused a significant decrease in the striatal levels of dopamine and its metabolites, which was accompanied by an enhancement of muscle tone and electromyographic activity. Stereological counting revealed that the beta-carbolinium caused a significant decrease in the total number of tyrosine hydroxylase-immunoreactive neurons and shrinkage of the substantia nigra. The findings suggest that the methylated beta-carbolinium ion produces a dose-dependent degeneration of nigrostriatal neurons, leading to deficits in dopaminergic neurotransmission and an increase of muscle resistance and electromyographic activity, a syndrome equivalent to muscle rigidity in Parkinson's disease.

Differential effects of dopamine melanin on norharman-induced toxicity in PC12 cells

The food contaminant norharman structurally resembles MPTP a compound that selectively damages pigmented brain areas. Both compounds are sequestered and retained in melanin-containing neurons. The aim of the study was to examine whether intracellular melanin can modulate the toxicity of norharman in melanin-loaded PC12 cells. Dopamine melanin protected against norharman-induced upregulation of grp78, activation of caspase 3 and necrosis at low concentrations (5 and 50 microM). In contrast, at a high conentration (500 microM) there was a significantly increased expression of grp78, hsp90 and caspase 3 and a disassociation of melanin aggregates leading to dispersal of granules to swollen neurite terminals. In human populations, a long-term low-level exposure to toxicants with a high affinity to melanin will probably result in accumulation in melanin-containing neurons in vivo. Our data suggest that accumulation of a neurotoxicant in melanin-loaded cells may lead to increased cell stress, apoptotic signaling and disassociation of melanin aggregates.

Neurotoxic mechanisms of 2,9-dimethyl-beta-carbolinium ion in primary dopaminergic culture

beta-Carbolines are potential endogenous and exogenous neurotoxicants that may contribute to the pathogenesis of Parkinson's disease. The 2,9-dimethyl-beta-carbolinium ion (either 2,9-dimethyl-beta-norharmanium or 2,9-Me(2)NH(+)) was found to be neurotoxic in primary mesencephalic cultures and to be a potent inhibitor of mitochondrial complex I. However, the precise mechanisms of cell death remained obscure. Here, we investigated the mechanism of cell death in primary dopaminergic cultures of the mouse mesencephalon mediated by 2,9-Me(2)NH(+). The beta-carboline caused preferential death of dopaminergic neurones, which could not be attributed to cellular uptake via the dopamine transporter. Transient incubation with 2,9-Me(2)NH(+) for 48 h caused a progressive deterioration in the morphology of dopaminergic neurones during a 5-day recovery period and persistent damage to the overall culture. An increase in free radical production and caspase-3 activity, as well as a decrease of respiratory activity, mitochondrial membrane potential and ATP content, contributed to toxicity and pointed to an apoptotic mode of cell death, although a significant quantity of cells dying via necrosis were present simultaneously. These data underline the preferential susceptibility of dopaminergic neurones to 2,9-Me(2)NH(+) as a potent, oxidative stress generating neurotoxin.

2,9-Dimethyl-β-carbolinium, a neurotoxin occurring in human brain, is a potent inducer of apoptosis as 1-methyl-4-phenylpyridinium

The causes of neurodegeneration are not well understood. However, the role of environmental and endogenous toxins is receiving much attention. In this study, we compared the synthetic neurotoxin 1-methyl-4-phenyl-pyridinium with beta-carbolines occurring in human brain. Methylation of both nitrogens is necessary to convert a beta-carboline into a potent inhibitor of mitochondrial complex I. The respective beta-carboline, 2,9-dimethyl-beta-carbolinium ion is neurotoxic in rats. To investigate the underlying mechanisms, we incubated mouse neuroblastoma 2A cells with 2,9-dimethyl-beta-carbolinium ion, and compared the findings with effects of norharman, the precursor beta-carboline of methylated derivatives, and with 1-methyl-4-phenyl-pyridinium. 2,9-Dimethyl-beta-carbolinium ion caused a significant increase of reactive oxygen species (higher efficiency than 1-methyl-4-phenyl-pyridinium) and of mitochondrial membrane potential within the first minutes. After 60 min, the membrane potential dissipated. Concomitantly, the levels of glutathione increased in 2,9-dimethyl-beta-carbolinium ion but not in 1-methyl-4-phenyl-pyridinium treated cells. After 24 h effector caspases 3 and 7 were activated and the number of apoptotic cells increased as revealed by fluorescence-activated cell sorting cytometry. When incubated longer (48 h), cells underwent late apoptosis/secondary necrosis as shown by fluorescence-activated cell sorting analysis and confirmed qualitatively by an electron microscopy study. The effects of 2,9-dimethyl-beta-carbolinium ion on apoptotic changes were similar to those induced by 1-methyl-4-phenyl-pyridinium(,) while norharman showed only a weak potency at the very high doses. To investigate whether 2,9-dimethyl-beta-carbolinium ion is neurotoxic under in vivo conditions and whether only dopaminergic neurones are affected we conducted a dose-response study. Three weeks after injection of 2,9-dimethyl-beta-carbolinium ion in the substantia nigra we found a dose-dependent decrease of dopamine and its metabolites in the striatum of rats. The levels of 5-hydroxytryptamine were diminished although the decrease was less. The levels of noradrenaline increased after some doses. The findings strongly suggest an important role of endogenous beta-carbolines in neurodegeneration with apoptosis as the predominant mechanism.

Analysis of monoamine oxidase enzymatic activity by reversed-phase high performance liquid chromatography and inhibition by beta-carboline alkaloids occurring in foods and plants

Monoamine oxidase (MAO) is a flavin adenine dinucleotide (FAD)-containing enzyme located at the outer membranes of mitochondria that catalyzes the oxidative deamination of biogenic and xenobiotic amines. We have used a chromatographic method to measure MAO-enzymatic activity by using kynuramine as a non-selective substrate with its MAO-oxidation product subsequently analyzed by RP-HPLC-DAD and HPLC-mass spectrometry (MS). This method was applied to study the kinetic parameters, inhibition and reaction products of MAO recombinant enzymes in presence of tetrahydro-beta-carboline and beta-carboline alkaloids occurring in foods, plants and mammals. Analysis by HPLC showed that tetrahydro-beta-carbolines or beta-carbolines were not modified by MAO. Several beta-carbolines such as tryptoline (1,2,3,4-tetrahydro-beta-carboline) and 1-methyltryptoline (1-methyl-1,2,3,4-tetrahydro-beta-carboline) were inhibitors of MAO-A; instead their corresponding 6-hydroxy-derivatives (6-hydroxytryptoline and 6-hydroxy-1-methyltryptoline) lacked this activity. Tetrahydro-beta-carboline-3-carboxylic acids were unable to inhibit MAO enzymes. In contrast, their oxidation products, i.e. the fully aromatic beta-carbolines (norharman and harman), acted as good inhibitors of MAO. Two tetrahydro-beta-carbolines (i.e. tryptoline and 1-methyltryptoline) occurring in foods were isolated by solid-phase extraction (SPE) and RP-HPLC from selected samples of sausages and the corresponding extracts exhibited good inhibition properties over MAO-A. These results suggest that beta-carbolines from foods, plants, and mammals may exert inhibitory actions on MAO enzymes.

Nostocarboline: isolation and synthesis of a new cholinesterase inhibitor from Nostoc 78-12A

A new quaternary beta-carboline alkaloid, nostocarboline, was isolated from the freshwater cyanobacterium Nostoc 78-12A, and its constitution was assigned by 2D-NMR methods. The structure was proven by its total synthesis starting from norharmane via chlorination at C-6 and methylation at N-2. Nostocarboline was found to be a potent butyrylcholinesterase (BChE) inhibitor, with an IC(50) of 13.2 microM. The related 2-methylnorharmane, which is present in the human brain and might be relevant to Parkinson's disease (PD), was also determined to be a BChE inhibitor (11.2 microM). These inhibitory concentrations are comparable to galanthamine, an approved drug for the treatment of Alzheimer's disease (AD). Nostocarboline can thus be considered as a lead for the development of novel neurochemicals.

Norharman-induced motoric impairment in mice: neurodegeneration and glial activation in substantia nigra

The beta-carboline norharman is present in cooked food and tobacco smoke and show structural resemblance to the neurotoxicant 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. C57BL/6 mice were injected subcutaneously with norharman (3 and 10 mg/kg) twice per day for five consecutive days. Eighteen hours after the last dose an increased expression of glial fibrillary acidic protein and fluoro-jade staining were demonstrated whereas the number of tyrosine hydroxylase positive cells were unchanged in the substantia nigra. Two weeks after the last treatment a decreased motor activity was observed whereas cognitive functions remained intact. In cultured PC12 cells norharman treatment induced mitochondrial dysfunction and increased the number of caspase-3 and TUNEL-positive cells. The results demonstrate that norharman induced apoptosis in cultured cells as well as early neurodegeneration, glial activation and sustained motor deficits in mice and suggest that exposure to norharman may contribute to idiopathic Parkinson's disease.

Antitumor and neurotoxic effects of novel harmine derivatives and structure-activity relationship analysis

Beta-carboline alkaloids such as harmine are present in medicinal plants such as Peganum harmala that have been used as folk medicine in anticancer therapy. In our study, 9 harmine derivatives (including harmine) were investigated for their antitumor effects and acute toxicities in mice, and the structure-activity relationship (SAR) was also analyzed. Administration of these compounds resulted in tumor inhibition rates of 15.3-49.5% in mice bearing Lewis Lung Cancer, sarcoma180 or HepA tumor, with the highest value of 49.5% from compound 6. Acute toxicity studies showed that all these compounds except compounds 2 and 5 caused remarkable acute neurotoxicities manifested by tremble, twitch and jumping. SAR analysis indicated that the formate substitution at R3 of the tricyclic skeleton reduced their neurotoxicity, while the short alkyl or aryl substitution at R9 increased the antitumor activity. The harmine and its derivatives resulted in in vitro cytotoxicity (IC50) values of 0.011-0.021 micromol/ml in HepG2 cells, with compound 8 being the most potent among all agents tested. Compounds 1, 6, 7 and 8 induced apoptosis in HepG2 cells, with the highest apoptotic rate (55.34%) from compound 6. Western blotting analysis demonstrated that compound 6 completely inhibited the expression of Bcl-2 gene, and compounds 1 and 8 produced a significant inhibition by 40 and 60%, respectively, compared to the control, while compound 7 did not alter the level of Bcl-2. Compounds 1, 6, 7 and 8 upregulated the expression of death receptor Fas by approximately 50-120%. All these findings indicate that compounds with both substitutions at R3 and R9 (such as compound 5) have high antitumor activity and low toxicity, which might be chosen as lead molecules for further development. Further studies on the effects of harmine derivatives on key regulators for tumor cell apoptosis are needed.

Studies on the pathogenesis of Parkinson's disease in Japan

Studies on the pathogenesis of nigral cell death in Parkinson's disease (PD) are reviewed. Discussions are focused mainly on studies performed by Japanese investigators because of the purpose of this issue. We and other groups found a decrease in complex I of the mitochondrial electron transfer complex in the substantia nigra of patients with PD, and in addition to complex I deficiency, we reported loss of alpha-ketoglutarate dehydrogenase complex of the tricarboxylic acid cycle (TCA cycle) by immunohistochemistry. Thus mitochondrial respiratory failure and resultant energy crisis appear to be one of the most important mechanisms that lead nigral neurons to cell death. The primary cause of mitochondrial respiratory failure has not been elucidated yet; however, environmental neurotoxins such as 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) may be responsible for nigral cell death in PD; in this respect a number of candidate toxins including tetrahydroisoquinolines and beta-carbolines have extensively been studied for nigral as well as mitochondrial toxicity. Recent progress in this field is also reviewed. Even if an environmental neurotoxin is involved in PD, exposure to such a neurotoxin alone may not account for its pathogenesis, as most of us are probably being exposed to the same toxin. Therefore, genetic predisposition appears to be essential for the development of PD. The genetic predisposition may involve hepatic detoxifying enzymes for such neurotoxins, the transport mechanism of those toxins to the brain, bioactivation of those toxins in the brain, the uptake mechanism to the nigral neurons, and the activity levels of target enzymes or proteins; all of these factors are being extensively studied in many laboratories at a molecular level.

Human monoamine oxidase is inhibited by tobacco smoke: beta-carboline alkaloids act as potent and reversible inhibitors

Monoamine oxidase (MAO) is a mitochondrial outer-membrane flavoenzyme involved in brain and peripheral oxidative catabolism of neurotransmitters and xenobiotic amines, including neurotoxic amines, and a well-known target for antidepressant and neuroprotective drugs. Recently, positron emission tomography imaging has shown that smokers have a much lower activity of peripheral and brain MAO-A (30%) and -B (40%) isozymes compared to non-smokers. This MAO inhibition results from a pharmacological effect of smoke, but little is known about its mechanism. Working with mainstream smoke collected from commercial cigarettes we confirmed that cigarette smoke is a potent inhibitor of human MAO-A and -B isozymes. MAO inhibition was partly reversible, competitive for MAO-A, and a mixed-type inhibition for MAO-B. Two beta-carboline alkaloids, norharman (beta-carboline) and harman (1-methyl-beta-carboline), were identified by GC-MS, quantified, and isolated from the mainstream smoke by solid phase extraction and HPLC. Kinetics analysis revealed that beta-carbolines from cigarette smoke were competitive, reversible, and potent inhibitors of MAO enzymes. Norharman was an inhibitor of MAO-A (K(i)=1.2+/-0.18 microM) and MAO-B (K(i)=1.12+/-0.19 microM), and harman of MAO-A (K(i)=55.54+/-5.3nM). Beta-carboline alkaloids are psychopharmacologically active compounds that may occur endogenously in human tissues, including the brain. These results suggest that beta-carboline alkaloids from cigarette smoke acting as potent reversible inhibitors of MAO enzymes may contribute to the MAO-reduced activity produced by tobacco smoke in smokers. The presence of MAO inhibitors in smoke like beta-carbolines and others may help us to understand some of the purported neuropharmacological effects associated with smoking.

The strong inhibition of triosephosphate isomerase by the natural beta-carbolines may explain their neurotoxic actions

The natural beta-carbolines (BC) closely resemble the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in structure. The N-methylated beta-carbolinium ions (BC+) are potent inhibitors of mitochondrial respiration and are nigrostriatal neurotoxins. Utilizing [3H]BC, we have identified several proteins to which BC binds with high affinity (e.g. the chaperone member glucose regulated protein 78, the enzyme carboxylesterase, the cytochrome P450 2E1, the enzyme monoamine oxidase B and a small G-protein of the Rho subfamily). In the present study we isolated a protein from bovine brain to which [3H]BC binds with high affinity and identified it being the enzyme triosephosphate isomerase (TPI; EC 5.3.1.1.). 2,9-Dimethyl-BC+ was the most potent inhibitor of TPI, clearly more potent than the known inhibitors. TPI deficiency is a rare disorder in humans characterized by a severe progressive extrapyramidal course. Thus, TPI inhibition could contribute to neurodegeneration observed after injection of BCs into substantia nigra. Furthermore, the findings fit into the hypothesis of BCs as endogenous toxins responsible for neurodegeneration.

Exposure to beta-carbolines norharman and harman

The aromatic beta-carbolines norharman and harman have been implicated in a number of human diseases including Parkinson's disease, tremor, addiction and cancer. It has been shown that these compounds are normal body constituents formed endogenously but external sources have been identified. Here, we summarise literature data on levels of norharman and harman in fried meat and fish, meat extracts, alcoholic drinks, and coffee brews. Other sources include edible and medicinal plants but tobacco smoke has been identified as a major source. Exposure levels from these different dietary sources are estimated to a maximum of 4 microg norharman per kg body weight (bw) per day and 1 microg harman per kg bw per day. Exposure via tobacco smoke depends on smoking habits and type of cigarettes but can be estimated to 1.1 microg/kg bw for norharman and 0.6 microg/kg bw for harman per package of cigarettes smoked. Studies on toxicokinetics indicate that inhalative exposure leads to a rapid increase in plasma levels and high bioavailability of norharman and harman. Oral bioavailability is lower but there are indications that sublingual absorption may increase dietary uptake of beta-carbolines. Endogenous formation can be estimated to be 50-100 ng/kg bw per day for norharman and about 20 ng/kg bw per day for harman but these rates may increase with high intake of precursors. Biomarker studies on plasma levels of beta-carbolines reported on elevated levels of norharman, harman or both in diseased patients, alcoholics and following tobacco smoking or consumption of beta-carboline-containing food. Cigarette smoking has been identified as major influence but dietary exposure may contribute to exposure.

N-methylated beta-carbolines protect PC12 cells from cytotoxic effect of MPP+ by attenuation of mitochondrial membrane permeability change

Opening of the mitochondrial permeability transition pore has been recognized to be involved in cell death. The present study investigated the effect of beta-carbolines (harmaline and harmalol) on the MPP(+)-induced change in the mitochondrial membrane permeability and cell death in differentiated PC12 cells. beta-Carbolines and antioxidants (superoxide dismutase, catalase, ascorbate or rutin) prevented the loss of cell viability in PC12 cells treated with 250 microM MPP(+), while the effects of N-acetylcysteine and dithiothreitol were not observed. beta-Carbolines reduced the condensation and fragmentation of nuclei caused by MPP(+) in PC12 cells. beta-Carbolines alone did not exhibit a significant cytotoxic effect on PC12 cells. beta-Carbolines (50 microM) inhibited the decrease in mitochondrial transmembrane potential, cytochrome c release, activation of caspase-3, formation of reactive oxygen species (ROS) and depletion of GSH caused by MPP(+) in PC12 cells. beta-Carbolines reduced the hydrogen peroxide- or SIN-1-induced cell death in PC12 cells. The results suggest that beta-carbolines may attenuate the MPP(+)-induced viability loss in PC12 cells by inhibition of change in the mitochondrial membrane permeability and by antioxidant effect.

Contribution of individual cytochrome P450 isozymes to the O-demethylation of the psychotropic beta-carboline alkaloids harmaline and harmine

The psychotropic beta-carboline alkaloids, showing high affinity for 5-hydroxytryptamine, dopamine, benzodiazepine, and imidazoline receptors and the stimulation of locus coeruleus neurons, are formed endogenously from tryptophan-derived indolealkylamines through the Pictet-Spengler condensation with aldehydes in both plants and mammals. Cytochromes P450 1A1 (18.5), 1A2 (20), and 2D6 (100) catalyzed the O-demethylation of harmaline, and CYP1A1 (98.5), CYP1A2 (35), CYP2C9 (16), CYP2C19 (30), and CYP2D6 (115) catalyzed that of harmine (relative activities). The dehydrogenation/aromatization of harmaline to harmine was not carried out by aromatase (CYP19), CYP1A2, CYP2C9, CYP2D6, CYP3A4, pooled recombinant cytochromes P450, or human liver microsomes (HLMs). Kinetic parameters were calculated for the O-demethylations mediated by each isozyme and by pooled HLMs. K(cat) (min(-1)) and K(m) Awake M) values for harmaline were: CYP1A1, 10.8 and 11.8; CYP1A2, 12.3 and 13.3; CYP2C9, 5.3 and 175; CYP2C19, 10.3 and 160; and CYP2D6, 39.9 and 1.4. Values for harmine were: CYP1A1, 45.2 and 52.2; CYP1A2, 9.2 and 14.7; CYP2C9, 11.9 and 117; CYP2C19, 21.4 and 121; and CYP2D6, 29.7 and 7.4. Inhibition studies using monoclonal antibodies confirmed that CYP1A2 and CYP2D6 were the major isozymes contributing to both harmaline (20% and 50%, respectively) and harmine (20% and 30%) O-demethylations in pooled HLMs. The turnover numbers for CYP2D6 are among the highest ever reported for a CYP2D6 substrate. Finally, CYP2D6-transgenic mice were found to have increased harmaline and harmine O-demethylase activities as compared with wild-type mice. These findings suggest a role for polymorphic CYP2D6 in the pharmacology and toxicology of harmine and harmaline.

Isolation and identification of two [(3)H]norharman- ([(3)H]beta-carboline)-binding proteins from rat liver

Norharman (9H-pyrido-[3,4-b]indol) represents a member of the mammalian alkaloids with the group name beta-carbolines. In mammals, it exhibits psychotropic and co-mutagenic actions. Highly specific [(3)H]norharman binding sites have been detected in the liver of rats (B(max): 11 pmol mg(-1) protein; K(D): lower nanomolar range). Two [(3)H]norharman binding proteins with apparent molecular masses of 60 and 80 kDa (SDS-PAGE) were isolated from rat liver crude membrane fraction and identified as the enzyme carboxylesterase (EC 3.1.1.1; 60 kDa) and the stress protein glucose-regulated protein 78 (GRP78; 78 kDa). Possible functional consequences of the interaction of norharman with these two proteins are discussed.

Identification of brain proteins that interact with 2-methylnorharman. An analog of the parkinsonian-inducing toxin, MPP+

N-Methylated beta-carbolines, including 2-methylnorharman, are structural and functional analogs of the parkinsonian-inducing toxin, MPP+. We are investigating N-methylated beta-carbolines, including 2-methylnorharman, as possible etiologic factors in the pathogenesis of Parkinson's disease. The cellular targets of N-methylated beta-carboline-mediated cytotoxicity are unknown; therefore, we used the T7Select Phage Display System in a novel approach to identify brain proteins that bind to 2-methylnorharman. We incubated (biopanned) immobilized 2-methylnorharman with a phage display cDNA library that expressed a library of human brain proteins on the surface of bacteriophage T7. We washed off unbound phage, amplified the phage that were bound to 2-methylnorharman, and enriched for toxin-interacting phage by repeating the biopanning and amplification steps. The cDNA sequences from the toxin-interacting phage were used to derive the amino acid sequences of the phage-displayed proteins. Five of the six 2-methylnorharman-interacting proteins may have relevance to Parkinson's disease: alpha-tubulin, paraoxonase, dorfin, fatty acid binding protein, and platelet-activating factor acetylhydrolase. Dorfin has sequence homology with parkin, which is interesting because mutations in the parkin gene associate with early-onset Parkinson's disease. Our findings are the basis for future studies aimed at determining whether 2-methylnorharman affects the function of these specific proteins in vitro and in vivo.