Transgenic model for the study of oxidative damage in Huntington's disease

Study of oxidative damage and antioxidant systems in two Huntington's disease rodent models

Huntington's disease (HD) is a hereditary neurodegenerative disorder, characterized by motor, psychiatric, and cognitive symptoms. The genetic defect responsible for the onset of the disease, expansion of CAG repeats in exon 1 of the gene that codes for huntingtin, has been unambiguously identified. The mechanisms by which the mutation causes the disease are not completely understood yet. However, defects in the energy metabolism of affected cells, which may cause oxidative damage, have been proposed as underlying molecular mechanisms that participate in the etiology of the disease. In this chapter, we describe biochemical methods that allow us to determine striatal oxidative damage in transgenic mice and in the quinolinic acid-induced excitotoxicity model in rat, and establish the status of protective cellular systems. The excitotoxic model is acute, easier and faster to perform than the transgenic model, and can within a short period provide valuable data to try new therapeutic strategies. The methods described in this chapter permit us to link the kynurenine pathway with the cascade of toxic and harmful reactions that cause the damage observed in HD. We consider that determining the mechanisms inducing oxidative damage in two different models of HD will allow the testing of drugs or other therapeutic strategies with antioxidant activities.

Glutamate toxicity in the striatum of the R6/2 Huntington's disease transgenic mice is age-dependent and correlates with decreased levels of glutamate transporters

Glutamate excitotoxicity has been implicated in the neuropathology of Huntington's disease (HD), due to the toxicity of glutamate receptor agonists on striatal medium spiny neurons (MSN), the most affected neuronal population in HD. Previous studies showed functional alterations of NMDA glutamate receptors and decreased expression of glutamate transporters in transgenic models and HD patients, suggesting the presence of excitotoxic damage. We have studied the vulnerability of the striatum to glutamate toxicity in R6/2 mice at 10 and 14 weeks of age. At 10 weeks R6/2 and wild-type mice are equally vulnerable to glutamate toxicity, while at 14 weeks transgenic mice show increased damage, as assessed by Nissl and Fluoro Jade staining. In addition, increased electrical brain activity is observed after glutamate administration in transgenic mice, as monitored electroencephalographically. According to western blot analysis, increased vulnerability to glutamate toxicity correlates with decreased levels of GLT-1 and GLAST glutamate transporters in the striatum.

Selective protection against oxidative damage in brain of mice with a targeted disruption of the neuronal nitric oxide synthase gene

Nitric oxide (NO) is an essential messenger molecule in brain, where it is produced in neurons mostly by the activity of the neuronal isoform of nitric oxide synthase (nNOS). To understand the participation of the different isoforms of NOS in physiological functioning and in pathological processes, mice with null mutations for each of the NOS isoforms have been generated. In the present paper, we report that there is a selective protection from oxidative damage in the brain of mice with a targeted disruption of the nNOS gene. The cerebellum of these mice shows reduced levels of lipid peroxidation (LP) at the different ages tested, compared with wild-type mice, and also a reduction in the formation of reactive oxygen species (ROS). We observed a decrease of LP in cortex, and no effect on either LP or ROS formation was observed in striatum of knockout mice compared with wild type. We also report increased spontaneous motor activity of knockout mice. The expression and activity of nNOS are crucial to maintain redox status in brain, and we consider that the alteration in oxidative damage may help us to explain the phenotypical characteristics of nNOS knockout mice and their differential susceptibility to brain insults.

Effects of alpha-tocopherol on an animal model of tauopathies

We have reported that transgenic (Tg) mice overexpressing human tau protein develop filamentous tau aggregates in the CNS. We overexpressed the smallest human tau isoform (T44) in the mouse CNS to model tauopathies. These tau Tg mice acquire age-dependent CNS pathologies, including insoluble, hyperphosphorylated tau and argyrophilic intraneuronal inclusions formed by tau-immunoreactive filaments. Therefore, these Tg mice are a model that can be exploited for drug discovery in studies that target amelioration of tau-induced neurodegeneration as well as for elucidating mechanisms of tau pathology in various neurodegenerative tauopathies. Oxidative stress has been implicated in the pathogenesis of various neurodegenerative diseases, including tauopathies, and many epidemiological, clinical, and basic studies have suggested the neuroprotective effects of vitamin E in neurodegenerative diseases. To elucidate the role of oxidative damage in the pathological mechanisms of these Tg mice, we fed them alpha-tocopherol, the major component of antioxidant vitamin E. Supplementation of alpha-tocopherol suppressed and/or delayed the development of tau pathology, which correlated with improvement in the health and attenuation of motor weakness in the Tg mice. These results suggest that oxidative damage is involved in the pathological mechanisms of the tau Tg mice and that treatment with antioxidative agents like alpha-tocopherol may prevent neurodegenerative tauopathies.

Age-dependent changes in nitric oxide synthase activity and protein expression in striata of mice transgenic for the Huntington's disease mutation

Huntington's disease (HD) is an autosomal hereditary neurodegenerative disorder caused by an abnormal expansion of the CAG repeats that code for a polyglutamine tract in a novel protein called huntingtin (htt). Both patients and experimental animals exhibit oxidative damage in specific areas of the brain, particularly the striatum. Nitric oxide (NO) is involved in many different physiological processes, and under pathological conditions it may promote oxidative damage through the formation of the highly reactive metabolite peroxynitrite; however, it may also play a role protecting cells from oxidative damage. We previously showed a correlation between the progression of the neurological phenotype and striatal oxidative damage in a line of transgenic mice, R6/1, which expresses a human mutated htt exon 1 with 116 CAG repeats. The purpose of the present work was to explore the participation of NO in the progressive oxidative damage that occurs in the striata of R6/1 mice. We analyzed the role of NO by measuring the activity of nitric oxide synthase (NOS) in the striata of transgenic and control mice at different ages. There was no difference in NOS activity between transgenic and wild-type mice at 11 weeks of age. In contrast, 19-week-old transgenic mice showed a significant increase in NOS activity, compared with same age controls. By 35 weeks of age, there was a decrease in NOS activity in transgenic mice when compared with wild-type controls. NOS protein expression was also determined in 11-, 19- and 35-week-old transgenic mice and wild-type littermates. Our results show increased neuronal NOS expression in 19-week-old transgenic mice, followed by a decreased level in 35-week-old mice, compared with controls, a phenomenon that parallels the changes in NOS enzyme activity. The present results suggest that NO is involved in the process leading to striatal oxidative damage and that it is associated with the onset of the progressive neurological phenotype in mice transgenic for the HD mutation.