Disfunção hepática em animais modelo para a doença de Alzheimer: o papel do stresse oxidativo e da β-amilóide
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2014-02-11
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A doença de Alzheimer é uma das principais doenças neurodegenerativas predominante entre indivíduos idosos. Clinicamente, é caracterizada por perda de memória e mudanças comportamentais, sendo uma das principais causas de demência afetando milhões de pessoas em todo o mundo. A nível celular, os marcadores patológicos da doença de Alzheimer são a acumulação extracelular de placas senis (maioritariamente formadas pela deposição do peptídeo β-amilóide) e a formação intracelular de tranças neurofibrilares constituídas pela proteína tau hiperfosforilada, associadas a uma redução da densidade sináptica e consequente perda neuronal. Considerando o papel do fígado nos processos de desintoxicação, a produção excessiva de β-amilóide associada à progressão da doença de Alzheimer pode promover a disfunção hepática.
Para investigar esta hipótese, avaliou-se o impacto fisiológico da progressão da doença de Alzheimer na atividade do fígado, usando murganhos triplo-transgénicos (3xTg-AD) com 3, 6 e 12 meses de idade, como animais modelo da doença de Alzheimer, e murganhos da estirpe selvagem (WT) com idades equivalentes, como controlo. Nestes modelos, foi investigado o efeito do envelhecimento na bioenergética mitocondrial, considerando as atividades dos complexos I, II, IV e FoF1-ATPase, bem como alguns parâmetros relacionados com o stresse oxidativo, como as atividades da catalase e Superóxido dismutase (Cu-Zn-SOD), o conteúdo total de tióis nas proteínas e os níveis de peroxidação lipídica. O efeito da β-amilóide na atividade dos complexos mitocondriais e na peroxidação lipídica foi avaliado em ensaios in vitro, para melhor compreender o papel da β-amilóide no desenvolvimento da doença de Alzheimer.
Os estudos revelaram que os animais 3xTg-AD com 3 e 6 meses de idade devem apresentar disfunção hepática, pois a atividade de enzimas chave relacionadas com bioenergética mitocondrial (complexos I, II e IV) é significativamente inferior à dos correspondentes controlos. No entanto, os efeitos tendem a desaparecer quando os animais atingem um ano de idade. Adicionalmente, nestes modelos as defesas antioxidantes das células do fígado estão comprometidas devido à menor atividade da catalase. Os estudos in vitro mostraram que a β-amilóide é tóxica para as mitocôndrias, reduzindo progressivamente a atividade do complexo I e IV até concentração de 2,5 μg/mg de proteína mitocondrial, mas concentrações superiores têm menor efeito. Estes dados sugerem que a toxicidade
mitocondrial da β-amilóide resulta apenas das formas monoméricas, tendo os aglomerados que estão na base das placas senis efeitos muito limitados.
Assim, para além da prevista degeneração do cérebro, efeitos em órgãos periféricos como o fígado parecem surgir durante a progressão da doença de Alzheimer, sugerindo que o seu estudo é fundamental não só para compreender os múltiplos aspetos relacionados com a doença como para identificar marcadores específicos para os vários estágios que caracterizam a progressão da doença de Alzheimer.
Alzheimer’s disease (AD) is one of the major neurodegenerative diseases prevalent among older individuals. Clinically, it is characterized by memory loss and behavioral changes, being a major cause of dementia affecting millions of people worldwide. At cellular level, the pathological hallmarks of Alzheimer's disease are the extracellular accumulation of senile plaques (mostly formed by deposition of β-amyloid peptide) and the formation of intracellular neurofibrillary tangles composed by hyperphosphorylated tau protein, associated with a reduction in synaptic density and consequent neuronal loss. Considering the role of liver in detoxification processes, excessive production of β-amyloid associated with Alzheimer's disease progression can promote liver dysfunction. To investigate this hypothesis, the liver physiological impact of Alzheimer's disease progression was evaluated using triple transgenic mice (3xTg-AD) with 3, 6 and 12 months-old, as Alzheimer's disease animal model, and wild type (WT) mice, as age-matched controls. In these models, the age-dependent effects on liver bioenergetics were evaluated considering the effects on complex I, II, IV and FoF1-ATPase activities, as well as on key parameters related to oxidative stress, including catalase and superoxide dismutase (Cu-Zn -SOD) activities, total thiols proteins contents and lipid peroxidation. In vitro assays, β-amyloid effects on the activity of complex I and IV of liver mitochondria isolated from health animals were assessed to evaluate its putative influence on liver mitochondria dysfunction. Additionally, the role of β-amyloid on lipid peroxidation process was evaluated in synthetic and native membranes. Our studies suggest that 3xTg-AD animals with 3 and 6 months-old may present hepatic dysfunction, since key mitochondrial enzymes related with liver bioenergetics (complexes I, II and IV) exhibit significantly lower activity than in the age-matched controls. However, these effects are vanished when the animals reach 12 months-old. On the other hand, the liver antioxidant defenses in 3xTg-AD animals may be committed due to the low catalase activity. In vitro studies show β-amyloid mitochondrial toxic effects mainly at low concentration, since a progressive reduction of both complexes I and IV activities were detected up to 2.5 μg β-amyloid/mg of mitochondrial protein, while higher concentrations did not have significant additional effects. These data suggests that the mitochondrial β-amyloid toxicity results mainly from its high soluble monomeric structure. Therefore, besides the intended brain degeneration, peripheral organs dysfunction (e.g. liver) may contribute to Alzheimer's disease phenotype. Thus, studies in peripheral organs may be essential not only to understand specific aspects related with the molecular mechanism underlay disease progression, but also to identify novel biomarkers of each stage of disease.
Alzheimer’s disease (AD) is one of the major neurodegenerative diseases prevalent among older individuals. Clinically, it is characterized by memory loss and behavioral changes, being a major cause of dementia affecting millions of people worldwide. At cellular level, the pathological hallmarks of Alzheimer's disease are the extracellular accumulation of senile plaques (mostly formed by deposition of β-amyloid peptide) and the formation of intracellular neurofibrillary tangles composed by hyperphosphorylated tau protein, associated with a reduction in synaptic density and consequent neuronal loss. Considering the role of liver in detoxification processes, excessive production of β-amyloid associated with Alzheimer's disease progression can promote liver dysfunction. To investigate this hypothesis, the liver physiological impact of Alzheimer's disease progression was evaluated using triple transgenic mice (3xTg-AD) with 3, 6 and 12 months-old, as Alzheimer's disease animal model, and wild type (WT) mice, as age-matched controls. In these models, the age-dependent effects on liver bioenergetics were evaluated considering the effects on complex I, II, IV and FoF1-ATPase activities, as well as on key parameters related to oxidative stress, including catalase and superoxide dismutase (Cu-Zn -SOD) activities, total thiols proteins contents and lipid peroxidation. In vitro assays, β-amyloid effects on the activity of complex I and IV of liver mitochondria isolated from health animals were assessed to evaluate its putative influence on liver mitochondria dysfunction. Additionally, the role of β-amyloid on lipid peroxidation process was evaluated in synthetic and native membranes. Our studies suggest that 3xTg-AD animals with 3 and 6 months-old may present hepatic dysfunction, since key mitochondrial enzymes related with liver bioenergetics (complexes I, II and IV) exhibit significantly lower activity than in the age-matched controls. However, these effects are vanished when the animals reach 12 months-old. On the other hand, the liver antioxidant defenses in 3xTg-AD animals may be committed due to the low catalase activity. In vitro studies show β-amyloid mitochondrial toxic effects mainly at low concentration, since a progressive reduction of both complexes I and IV activities were detected up to 2.5 μg β-amyloid/mg of mitochondrial protein, while higher concentrations did not have significant additional effects. These data suggests that the mitochondrial β-amyloid toxicity results mainly from its high soluble monomeric structure. Therefore, besides the intended brain degeneration, peripheral organs dysfunction (e.g. liver) may contribute to Alzheimer's disease phenotype. Thus, studies in peripheral organs may be essential not only to understand specific aspects related with the molecular mechanism underlay disease progression, but also to identify novel biomarkers of each stage of disease.
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Dissertação de Mestrado em Biologia Clínica Laboratorial
Palavras-chave
Doença de Alzheimer , Disfunção hepática , Toxicidade da B-amilóide