Mechanisms and modes of β-N-methylamino-lalanine neurotoxicity: the basis for designing therapies
- Authors: Van Onselen, Rianita
- Date: 2019
- Subjects: Cyanobacteria , Amino acids -- Toxicology , Neurotoxic agents
- Language: English
- Type: Thesis , Doctoral , DPhil
- Identifier: http://hdl.handle.net/10948/32971 , vital:32483
- Description: Since the discovery of the non-canonical amino acid β-N-methylamino-L-alanine (BMAA) and the demonstration of its acute neurotoxicity in chicks and rats, it has been postulated that BMAA might contribute to the development of neurodegenerative diseases worldwide due to its presence in numerous aquatic and terrestrial food webs. This hypothesized link was widely criticized because of the inability to reproduce symptoms in a BMAA-exposed animal model that resembled the symptoms observed in humans, and for the inability to achieve significant levels of toxicity in in vitro models via the postulated mechanisms of toxicity. The most widely described mechanism of BMAA toxicity was excitotoxicity by over-excitation of ionotropic and/or metabotropic glutamate receptors following activation by BMAA. However, the excitotoxic potency of BMAA is much lower than those of other known excitotoxins and it was not known whether BMAA could accumulate in significant concentrations in synapses to cause the said excitotoxicity. Therefore, uptake of BMAA into synaptic vesicles from where it can be released into synapses in high concentrations, was investigated and it was found that, unlike the uptake that was observed for glutamate, BMAA was not taken up into synaptic vesicles. This discovery suggests that BMAA is not released into synapses via synaptic vesicles and that excitotoxicity is an unlikely mechanism of BMAA toxicity in mammalian systems. Misincorporation of BMAA into proteins in the place of L-serine was suggested to be an important mechanism of BMAA toxicity that could lead to protein misfolding and the subsequent protein aggregates that are typically found in the central nervous system (CNS) of neurodegenerative disease patients. However, previous studies in prokaryotes and in a rat pheochromocytoma PC12 cell line showed that misincorporation of BMAA does not occur to any significant extent. However, these studies were criticized for not using human-derived model systems to show that misincorporation does not occur, and it was argued that due to differences in mitochondrial protein synthesis mechanisms, misincorporation of BMAA into human proteins could not be ruled out as a possible mechanism of toxicity. Therefore, misincorporation of BMAA was investigated in a number of human-derived non-neuronal cell lines and directly compared to the misincorporation of other known amino acid analogues. No evidence of misincorporation of BMAA into these cell lines was obtained and therefore it was concluded that misincorporation of BMAA into proteins does not occur in human-derived cell models. Although misincorporation of BMAA into proteins was refuted as a mechanism of toxicity, the strong interactions between BMAA and proteins that require extensive purification procedures to remove the associated BMAA, could not be discounted as a possible contributor to the toxicity of BMAA. Cell-free interactions between BMAA and enzymes, which resulted in reduced activity, were described previously but the nature of these interactions was never determined. Therefore, the direct interactions between BMAA and a range of commercial proteins and melanin (that is known to also have a strong affinity for BMAA) were investigated in an attempt to describe the nature of these interactions. It was discovered that BMAA has a high affinity for hydroxyl groups, and that if these hydroxyl groups in the form of hydroxyl containing amino acid residues occurred in important regulatory or active sites of proteins, BMAA reduced the enzyme activity. Catalase was subsequently selected as an important enzyme required for the maintenance of the delicate reactive oxygen species (ROS) balance in the CNS, to test the effect of BMAA on the activity of the enzyme. BMAA inhibited a human commercial extract of catalase in a cell free system, and this inhibition appeared to be non-competitive in nature. Subsequently, catalase in an extract from a human cell line was also shown to be inhibited by BMAA and it was concluded that this BMAA induced inhibition of catalase could be an important contributor to the toxicity of BMAA in in vivo systems. The affinity of BMAA for hydroxyl groups, especially the reactive L-tyrosine side chain hydroxyl, was recognized as a possible mechanism that can be utilized to protect against the toxicity of BMAA. It was subsequently shown that excess concentrations of L-serine and L-tyrosine could protect against the BMAA-induced enzyme inhibition and improper folding of proteins in a cell-free system. By administering an equimolar concentration of either L-phenylalanine (the soluble precursor of L-tyrosine) or L-serine an hour before administration of BMAA in a rat model, the BMAA-induced neurotoxicity was greatly reduced, especially by treatment with L-phenylalanine, which resulted in a decrease of between 60-70% in the observed neuropathologies. It was recognized that the protection offered by L-phenylalanine was greater than would be expected if protection was by virtue of direct hydroxyl binding alone and it was subsequently hypothesized that the conversion of L-phenylalanine to dopamine could have contributed to the observed protection. Subsequently, the possible protection offered by dopamine, administered as L-DOPA, against BMAA neurotoxicity was investigated in the same neonatal rat model and compared to the protection offered by L-tyrosine. It was discovered that dopamine protected against the BMAAinduced neuronal cell losses in the hippocampus, striatum and spinal cord but it was not as efficient as L-tyrosine in protection against the BMAA-induced proteinopathies, suggesting two distinct mechanisms of BMAA toxicity, one of which is a depletion of dopamine, which had not been previously described. Finally, the nature of the BMAA-induced dopamine depletion was investigated by administering BMAA in combination with other dopaminergic modifiers viz. apomorphine (a D1/D2 receptor agonist), a dopamine transporter inhibitor (GBR12783) and reserpine (a vesicular monoamine transporter -VMAT2- inhibitor) to the neonatal rat model in an attempt to describe how BMAA functions as a dopaminergic toxin. Based on these results it was concluded that BMAA inhibits uptake of dopamine into synaptic vesicles by inhibiting VMAT2-mediated uptake of dopamine, which causes neuronal loss in the hippocampus, striatum and substantia nigra pars compacta, and that the BMAA-induced inhibition of catalase contributes significantly to the toxicity of BMAA by causing an accumulation of hydrogen peroxide in the hippocampus, striatum and spinal cord, which results in extensive neuronal damage in these areas. This work was the first to thoroughly investigate the mechanisms that explain the observed pathologies caused by BMAA in an in vivo model, and was the first to suggest that BMAA can reduce the dopamine in the CNS by inhibiting VMAT2-mediated uptake of dopamine into synaptic vesicles, and increase damage by reactive oxygen species by inhibiting catalase. BMAA is therefore a multimechanistic and multimodal.
- Full Text:
- Date Issued: 2019
β-N-Methylamino-L-Alanine is a developmental neurotoxin
- Authors: Scott, Laura Louise
- Date: 2019
- Subjects: Neurotoxic agents , Nervous system -- Diseases
- Language: English
- Type: Thesis , Doctoral , DPhil
- Identifier: http://hdl.handle.net/10948/43633 , vital:36949
- Description: β-N-methylamino-L-alanine (BMAA) has been implicated in the development of the neurodegenerative diseases Amyotrophic Lateral Sclerosis/Parkinsonism Dementia Complex (ALS/PDC), Amyotrophic Lateral Sclerosis (ALS) and Alzheimer’s Disease (AD), but to date no animal model has adequately substantiated this link at environmentally relevant or even exaggerated BMAA exposure levels. The resulting controversy over a possible role for BMAA in neurodegenerative diseases was further hampered by a lack of evidence for mechanistic explanation for the disease pathology associated with these diseases However, the different responses to BMAA that have been observed in neonatal compared to adult rats, together with the findings of epidemiological studies that exposure to environmental factors in utero or in the early stages of life may be important for the development of ALS several years later, suggested that age of exposure might be the determining factor of BMAA neurotoxicity. This study therefore specifically addresses the developmental nature of BMAA as a neurotoxin, and investigates the pathology and progressive nature of that pathology after exposure to the toxin at the most susceptible age. This study demonstrated the importance of BMAA exposure age over total BMAA dose by showing that the administration of a single neonatal dose of BMAA to rodents on postnatal day (PND) 3, 4 and 5, and not prenatally or on PND 6, 7 and 10, caused behavioural, locomotor, emotional and long-term cognitive deficits, clinical symptoms of neurodegeneration as well as pathological hallmarks of AD, PD and ALS in the central nervous system. Furthermore, the observed behavioural deficits and distribution of neuronal loss and proteinopathies in the rodent central nervous system following exposure to BMAA on PND 3, 4 and 5 (corresponding to the developing age of an infant during the third trimester of pregnancy) is consistent with that typically associated with the disruption of normal dopamine and/or serotonin signaling in the brain and the consequent alteration in normal hippocampal and striatal neurogenesis that is modulated, in part, by dopamine. The pattern of spread and rate of propagation of pathology in this neonatal rat BMAA model provided further evidence that BMAA potentially exerts its effect by acting on neurotransmitter signaling. The observed late onset of typical ALS symptoms and pathology suggest that in this BMAA model AD and/or PD related symptoms develop first, followed by the start of ALS symptoms only after the AD and/or PD neuropathological deficits have severely progressed. This study also demonstrated that BMAA exposure at different doses and at different developmental ages resulted in the development of different combinations of either AD and/or PD and/or ALS pathology and/or symptoms in rats, and it is therefore feasible that in humans the age and/or frequency of exposure as well as the BMAA dose might similarly be a major determinant of the variant of AD, PD and/or ALS that might develop in adulthood. Based on the low BMAA dose that was able to cause AD and/or PD-like neuropathological abnormalities in rats in this study, it is feasible that a pregnant human could over the course of her pregnancy, and specifically during the third trimester of pregnancy, consume sufficient BMAA to result in her unborn child developing AD and/or PD and/or ALS up to 30-50 years later. This neonatal BMAA model is the only non-transgenic rodent model that reproduces the behavioural deficits, neuropathology and clinical symptoms that are typically associated with AD, PD and ALS in humans and that, more importantly, mimics the delayed onset of disease symptoms and typical slow progression of these neurodegenerative diseases with age. It now seems very likely that BMAA is a developmental neurotoxin that, as a result of perinatal, but probably prenatal exposure, causes or contributes significantly to the development of neurodegenerative diseases in humans.
- Full Text:
- Date Issued: 2019
The metabolism and environmental fate of the cyanobacterial neurotoxin Beta-N-methylamino-L-alanine
- Authors: Downing, Simoné
- Date: 2015
- Subjects: Cyanobacteria , Neurotoxic agents
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10948/4225 , vital:20569
- Description: The neurotoxic amino acid β-‐N-‐methylamino-Lalanine (BMAA)is present in environmentally ubiquitous cyanobacteria and bioaccumulates and biomagnifies within the environment. The implication of BMAA in the development of neurodegenerative disease has raised concerns over the potential risk of human exposure to this neurotoxin, and has focussed attention on identifying possible routes of exposure that include direct contact with cyanobacteria and the ingestion of BMAA-‐containing plant and animal products.
- Full Text:
- Date Issued: 2015
Potential for human exposure to Beta-N-methylamino-L-alanine in a freshwater system
- Authors: Scott, Laura Louise
- Date: 2014
- Subjects: Water quality biological assessment , Cyanobacteria , Neurotoxic agents
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10948/5159 , vital:20816
- Description: β-N-methylamino-L-alanine (BMAA) is a non-proteinogenic amino acid associated with human neurodegenerative diseases. The content of BMAA in cyanobacteria is modulated by nitrogen in laboratory cultures. In order to evaluate the potential for human exposure, the nitrogen modulation of BMAA content needed verification in a natural environment. In accordance with laboratory culture studies, data presented in this study show that combined nitrogen was the most significant modulator of both cellular microcystin (MC) and BMAA content in phytoplankton in an environmental cyanobacterial bloom. While BMAA is produced upon nitrogen deprivation, MC is only produced at a specific nitrogen threshold where the rate of increase of nitrogen in the cell exceeds the carbon fixation rate. As BMAA and MC were detected in phytoplankton sourced from the Hartbeespoort Dam reservoir, the transfer of these cyanotoxins to organisms of higher trophic levels was investigated. Both BMAA and MC were detected at high concentrations in the liver and muscle tissue of fish sourced from the Hartbeespoort Dam reservoir indicating that consumption of fish from this reservoir constitutes a serious risk of exposure to cyanotoxins. In addition to the dietary exposure route to BMAA, two recent studies reported a correlation between Amyotrophic Lateral Sclerosis (ALS) incidence and the potential for aerosol exposure to cyanobacteria. With the absence of any evidence of the systemic distribution of BMAA following inhalation, an evaluation of the potential exposure risk associated with living in close proximity to this reservoir was deemed premature. A laboratory experiment investigating the effect and systemic fate of inhaled aerosolised BMAA was therefore conducted in order to determine the feasibility of inhalation as a potential BMAA exposure route. Data from the rat inhalation exposure study, however, showed that in rats BMAA inhalation may not constitute a significant mechanism of toxicity at environmental BMAA levels.
- Full Text:
- Date Issued: 2014
Evaluation of model systems for the study of protein association / incorporation of Beta-Methylamino-L-Alanine (BMAA)
- Authors: Visser, Claire
- Date: 2011
- Subjects: Neurotoxic agents , Nervous system -- Diseases
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:10314 , http://hdl.handle.net/10948/1451 , Neurotoxic agents , Nervous system -- Diseases
- Description: β-methylamino-L-alanine (BMAA) is thought to be a contributing factor of Amyotrophic Lateral Sclerosis-Parkinsonism Dementia Complex (ALS/PDC). It has been shown that the levels of toxin ingestion by humans are too low to cause disease. However, it has recently been theorized that this toxin is bioaccumulated within cells. Via a process of slow release from this reservoir, the BMAA is able to bring about neurotoxicity. Mechanisms of uptake and bioaccumulation of BMAA have been proposed in several publications; however the mechanism of protein incorporation of BMAA has not yet been identified. Identifying suitable model systems would be a prerequisite in order for future studies on BMAA protein incorporation. Three specific models were therefore chosen for investigation; mammalian cell lines including C2C12 and HT29, a prokaryotic (E. coli) expression system and yeast cells. The cytotoxity of BMAA was established for the mammalian cell lines and further investigation of BMAA incorporation into cellular proteins was performed on all three above mentioned models. Samples were run on HPLC-MS in order to determine uptake of BMAA into cells or lack thereof. Results indicate negligible cytotoxicity as measured by MTT and CellTitre Blue assays, limited uptake and protein incorporation of BMAA within the prokaryotic model and insignificant uptake of BMAA by yeast cells. Although the uptake of BMAA in the prokaryotic model was not extensive, there was indeed uptake. BMAA was not only taken up into the cells but was also observed in inclusion body protein samples after hydrolysis. After further investigation and use, this model could very well provide researchers with information pertaining to the mechanism of association of BMAA with proteins. Although the other models provided negative results, this research was valuable in the sense that one can narrow down the number of possible model systems available. Also, in seeking models for studying protein association/incorporation, the use of the final target cell is not relevant or necessary as the purpose of the research was to identify a model system in which the mechanism of protein association/incorporation can, in future, be studied.
- Full Text:
- Date Issued: 2011
Beta-N-methylamino-L-alanine in South African fresh water cyanobacteria : incidence, prevalence, ecotoxicological considerations and human exposure risk
- Authors: Esterhuizen-Londt, Maranda
- Date: 2010
- Subjects: Cyanobacteria , Bioaccumulation , Chromatographic analysis , Neurotoxic agents
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:10306 , http://hdl.handle.net/10948/1473 , Cyanobacteria , Bioaccumulation , Chromatographic analysis , Neurotoxic agents
- Description: β-N-methylamino-L-alanine (BMAA) is a non-proteinogenic amino acid associated with human neurodegenerative disease. Due to the cosmopolitan nature of cyanobacteria, detection of BMAA in cyanobacteria has caused concerns about human exposure risk. This study was therefore based on the hypothesis that BMAA poses a health risk to humans either by direct ingestion or by indirect exposure to BMAA from a cyanobacterial source via a freshwater food chain. A validated gas chromatography-mass spectrometry (GC-MS) BMAA analysis method and a confirmatory liquid chromatography-mass spectrometry (LC-MS) method, with improved sensitivity, were developed in addition to a LC-MS/MS method for analyte confirmation. These methods were used to quantify BMAA in South African cyanobacteria, isolated from various potable water reservoirs. The majority of the isolates tested, contained BMAA. Possible human exposure by direct consumption of BMAA released from cyanobacterial blooms was investigated by the development of a robust solid phase extraction (SPE) method used for BMAA concentration and quantification in raw and treated tap water. Despite the use of the SPE method that facilitated the concentration of BMAA from large quantities of water, no free dissolved BMAA was detected in raw or processed fresh water. The fate of exogenous BMAA was therefore investigated firstly by evaluating the efficacy of standard water treatment processes employed in South Africa and secondly by investigating the possibility of BMAA bioaccumulation and biomagnification in aquatic food chains. Standard water treatment processes proved highly efficient at removing free dissolved BMAA, explaining the absence of BMAA in treated tap water. However, the cause of the BMAA absence in raw potable water remained unknown. Uptake of BMAA by model aquatic organisms was investigated in controlled experiments. BMAA uptake was documented in both Ceratophyllum demersum and Daphnia magna, however, BMAA-protein association and biomagnification were not observed in D. magna. BMAA had an inhibitory effect on the oxidative stress enzyme acitivties of both organisms tested (as well as human S9 extracts), resulting in accumulation of detrimental reactive oxygen species (ROS) in the cells. Exposure of crop plants to BMAA in controlled experiments resulted in BMAA uptake, protein association, and subsequent inhibition of the antioxidative enzyme activities. However, BMAA was detected in neither free nor protein-associated form in natural crop plants irrigated with known BMAA-containing bloom water. Post-mortem liver samples of Clarias gariepinus (Catfish) and Crocodylus niloticus (Crocodile), from a natural fresh water ecosystem that experienced frequent cyanobacterial blooms, contained both free and protein-associated BMAA. Higher BMAA concentrations were found in crocodile liver samples compared to fish liver samples, strongly suggesting biomagnification from one trophic level to the next. BMAA concentrations corresponded to crocodile age. This is the first report of bioaccumulation and biomagnification in two trophic levels in a fresh water ecosystem. These findings strongly suggest possible human exposure via aquatic food chains of cyanobacterial origin. Direct BMAA exposure via drinking water is not plausible due to the efficiency of standard water treatment processes to remove BMAA. The use of raw water for agricultural and recreational use, however, remains a problem. The development of management strategies as well as daily tolerable levels for BMAA is urgently required.
- Full Text:
- Date Issued: 2010
An investigation into the neuroprotective and neurotoxic properties of levodopa, dopamine and selegiline
- Authors: Scheepers, Mark Wesley
- Date: 2008
- Subjects: Parkinson's disease , Nervous system -- Degeneration -- Treatment , Neurotoxic agents , Neuroanatomy , Oxidative stress , Pharmacology , Dopamine , Selegiline , Dopaminergic neurons
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:3789 , http://hdl.handle.net/10962/d1003267 , Parkinson's disease , Nervous system -- Degeneration -- Treatment , Neurotoxic agents , Neuroanatomy , Oxidative stress , Pharmacology , Dopamine , Selegiline , Dopaminergic neurons
- Description: Parkinson’s disease (PD) is a neurodegenerative disorder characterized by a profound loss of dopaminergic neurons from the substantia nigra (SN). Among the many pathogenic mechanisms thought to be responsible for the demise of these cells, dopamine (DA)-dependent oxidative stress and oxidative damage has taken center stage due to extensive experimental evidence showing that DA-derived reactive oxygen species (ROS) and oxidized DA metabolites are toxic to SN neurons. Despite its being the most efficacious drug for symptom reversal in PD, there is concern that levodopa (LD) may contribute to the neuronal degeneration and progression of PD by enhancing DA concentrations and turnover in surviving dopaminergic neurons. The present study investigates the potential neurotoxic and neuroprotective effects of DA in vitro. These effects are compared to the toxicity and neuroprotective effects observed in the rat striatum after the administration of LD and selegiline (SEL), both of which increase striatal DA levels. The effects of exogenous LD and/or SEL administration on both the oxidative stress caused by increased striatal iron (II) levels and its consequences have also been investigated. 6-Hydroxydopamine (6-OHDA) is a potent neurotoxin used to mimic dopaminergic degeneration in animal models of PD. The formation of 6-OHDA in vivo could destroy central dopaminergic nerve terminals and enhance the progression of PD. Inorganic studies using high performance liquid chromatography with electrochemical detection (HPLC-ECD) show that hydroxyl radicals can react with DA to form 6-OHDA in vitro. SEL results in a significant decrease in the formation of 6-OHDA in vitro, probably as a result of its antioxidant properties. However, the exogenous administration of LD, with or without SEL, either does not lead to the formation of striatal 6-OHDA in vivo or produces concentrations below the detection limit of the assay. This is despite the fact that striatal DA levels in these rats are significantly elevated (two-fold) compared to the control group. The auto-oxidation and monoamine oxidase (MAO)-mediated metabolism of DA causes an increase in the production of superoxide anions in whole rat brain homogenate in vitro. In addition to this, DA is able to enhance the production of hydroxyl radicals by Fenton chemistry (Fe(III)-EDTA/H2O2) in a cell free environment. Treatment with systemic LD elevates the production of striatal superoxide anions, but does not lead to a detectable increase in striatal hydroxyl radical production in vivo. The co-adminstration of SEL with LD is able to prevent the LD induced rise in striatal superoxide levels. It has been found that the presence of DA or 6-OHDA is able to reduce lipid peroxidation in whole rat brain homogenate induced by Fe(II)-EDTA/H2O2 and ascorbate (Fenton system). However, DA and 6-OHDA increase protein oxidation in rat brain homogenate, which is further increased in the presence of the Fenton system. In addition to this, the incubation of rat brain homogenate with DA or 6-OHDA is also accompanied by a significant reduction in the total GSH content of the homogenate. The exogenous administration of LD and/or SEL was found to have no detrimental effects on striatal lipids, proteins or total GSH levels. Systemic LD administration actually had a neuroprotective effect in the striatum by inhibiting iron (II) induced lipid peroxidation. Inorganic studies, including electrochemistry and the ferrozine assay show that DA and 6-OHDA are able to release iron from ferritin, as iron (II), and that DA can bind iron (III), a fact that may easily impede the availability of this metal ion for participation in the Fenton reaction. The binding of iron (III) by DA appears to discard the involvement of the Fenton reaction in the increased production of hydroxyl radicals induced by the addition of DA to mixtures containing Fe(II)-EDTA and hydrogen peroxide. 6-OHDA did not form a metal-ligand complex with iron (II) or iron (III). In addition to the antioxidant activity and MAO-B inhibitory activity of SEL, the iron binding studies show that SEL has weak iron (II) chelating activity and that it can also form complexes with iron (III). This may therefore be another mechanism involved in the neuroprotective action of SEL. The results of the pineal indole metabolism study show that the systemic administration of SEL increases the production of N-acetylserotonin (NAS) by the pineal gland. NAS has been demonstrated to be a potent antioxidant in the brain and protects against 6-OHDA induced toxicity. The results of this study show that DA displays antioxidant properties in relation to lipid eroxidation and exhibits pro-oxidant properties by causing an increase in the production of hydroxyl radicals and superoxide anions, as well as protein oxidation and a loss of total GSH content. Despite the toxic effects of DA in vitro, the treatment of rats with exogenous LD does not cause oxidative stress or oxidative damage. The results also show that LD and SEL have some neuroprotective properties which make these agents useful in the treatment of PD.
- Full Text:
- Date Issued: 2008
An investigation into the neuroprotective effects of dehydroepiandrosterone
- Authors: Palvie, Stefanie Michelle
- Date: 2006
- Subjects: Aging -- Physiological aspects , Nervous system -- Degeneration -- Treatment , Steroid hormones , Dehydroepiandrosterone , Dehydroepiandrosterone -- Therapeutic use , Neurosciences , Neuroanatomy , Apoptosis , Pineal gland -- Physiology , Neurotoxic agents , Free radicals (Chemistry) -- Physiological effect
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:3782 , http://hdl.handle.net/10962/d1003260 , Aging -- Physiological aspects , Nervous system -- Degeneration -- Treatment , Steroid hormones , Dehydroepiandrosterone , Dehydroepiandrosterone -- Therapeutic use , Neurosciences , Neuroanatomy , Apoptosis , Pineal gland -- Physiology , Neurotoxic agents , Free radicals (Chemistry) -- Physiological effect
- Description: Dehydroepiandrosterone, a C-19 steroid, is found endogenously with the highest circulating serum levels. It is converted to important steroids such as the sex hormones oestrogen and testosterone. DHEA has come under the spotlight as a purported “fountain of youth” due to its well-characterised age-related decline. The supplementation of DHEA in both the elderly and those with a pathophysiological deficiency has been shown to be of benefit, particularly with regard to wellbeing and depression. The role of DHEA in the periphery has not been elucidated beyond its role as a precursor hormone in sex steroid biosynthesis, though it has been established as a neuroactive neurosteroid, capable of exerting neuroprotective effects in the brain. Since the importance of free radicals in aging and neurodegeneration is well established, investigations were conducted on the ability of DHEA to inhibit free radical generation or scavenge existing free radicals. DHEA was able to significantly inhibit quinolinic acid-induced lipid peroxidation, a measure of membrane damage, over a range of concentrations, although the reduction did not appear to be dose-dependent. This was observed in both in vitro and in vivo studies. Thus, the ability of a compound to reduce the degree of lipid peroxidation may indicate its value as a neuroprotectant. However, DHEA did not significantly reduce cyanide induced generation of the superoxide free radical, suggesting that DHEA is not an effective free radical scavenger of the superoxide anion and that the reduction in lipid peroxidation does not occur through a scavenging mechanism. Apoptosis is a physiological process which is necessary for development and homeostasis. However, this form of programmed cell death can be initiated through various mechanisms and too much apoptotic cell death results in deleterious effects in the body. DHEA was shown not to induce apoptosis. Even the lowest concentration of DHEA investigated in this thesis shows a remarkable decrease in the degree of apoptosis caused by intrahippocampal chemical insult by the neurotoxin quinolinic acid. Cresyl violet was used to visualise tissue for histological examination which revealed that DHEA is able to preserve the normal healthy morphology of hippocampal cells which have been exposed to quinolinic acid. Cells maintained their integrity and showed little evidence of swelling associated with necrosis. Organ culture studies were performed by assessing the impact of DHEA on several pineal metabolites. The study revealed that DHEA exerted an effect on the metabolism of indoleamines in the pineal gland. Melatonin, the chief pineal hormone, did not appear to be affected while the concentrations of N-acetylserotonin, serotonin and methoxytryptamine showed significant alterations. Thus, the neuroprotective mechanism of DHEA does not appear to be mediated by an increase in the presence of melatonin. The biological importance of metal ions in neurodegeneration is also well established and thus the potential interaction between DHEA and metal ions was considered as a mechanism of action. Spectroscopic and electrochemical analyses were performed to determine whether DHEA is able to interact with metal ions as a ligand. These reveal that DHEA does not form a strong bond with the metals investigated, namely copper (II) and iron (III), but that a weak interaction is evident. These investigations were conducted in a rodent model, which has neither large amounts of endogenous DHEA, nor the enzymatic infrastructure present in humans. Thus, the theory that DHEA exerts its effects through downstream metabolic products is unlikely. However, these investigations reveal that there is merit in the statement that DHEA itself is a neuroprotective molecule, and confirm that the further investigation of DHEA is an advisable strategy in the war against neurodegeneration and aging.
- Full Text:
- Date Issued: 2006
An investigation into the possible neuroprotective or neurotoxic properties of metrifonate
- Authors: Ramsunder, Adrusha
- Date: 2005 , 2013-06-11
- Subjects: Nervous system -- Degeneration -- Treatment , Neurotoxic agents , Alzheimer's disease -- Treatment , Metrifonate
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:3833 , http://hdl.handle.net/10962/d1007560 , Nervous system -- Degeneration -- Treatment , Neurotoxic agents , Alzheimer's disease -- Treatment , Metrifonate
- Description: Alzheimer's disease is a progressive neurodegenerative disorder, in which there is a marked decline in neurotransmitters, especially those of the cholinergic pathways. One of the approaches to the symptomatic treatment of Alzheimer's disease is the inhibition of the breakdown of the neurotransmitter acetylcholine, using an acetylcholinesterase inhibitor. One such drug tested, is the organophosphate, metrifonate. Any drug used for the treatment of neurodegenerative disorders should preferably not induce further neurological damage. Thus, in the present study, we investigated whether or not metrifonate is neuroprotective. The in vivo and in vitro effect of this drug on free radicals generation shows that metrifonate increases the level ofthese reactive species. Lipid peroxidation induced using quinolinic acid (QA) and iron (II) and show that metrifonate increased the peroxidative damage induced by using quinolinic acid. Metrifonate is also able to induce lipid peroxidation both in vivo and in vitro. This was reduced in vitro in the presence of melatonin. Using iron (II), in vi/ro, there was no significant difference in the level of lipid peroxidation in the presence of this drug. An investigation of the activity of the mitochondrial electron transport chain and complex I of the electron transport chain in the presence of metrifonate revealed that metrifonate reduces the activity of the electron transport chain at the level of complex I. The activity of the mitochondrial electron transport chain was restored in the presence of melatonin. Pineal organ culture showed that metrifonate does not increase melatonin production. Histological and apoptosis studies show that tissue necrosis and apoptosis respectively, occur in the presence of this agent, which is reduced in the presence of melatonin. Metal binding studies were performed USing ultraviolet spectroscopy, and electrochemical analysis to examine the interaction of metrifonate with iron (II) and iron (III). No shift in the peak was observed in the ultraviolet spectrum when iron (ll) was added to metrifonate. Electrochemical studies show that there may be a very weak or no ligand formed between the metal and drug. This study shows that while drugs such as metrifonate may be beneficial in restoring cognitive function in Alzheimer's disease, it could also have the potential to enhance neurodegeneration, thus worsening the condition, in the long term. , KMBT_363 , Adobe Acrobat 9.54 Paper Capture Plug-in
- Full Text:
- Date Issued: 2005