Angélica Castaño

Lipopolysaccharide intranigral injection induces inflammatory reaction and damage in nigrostriatal dopaminergic system.

Abstract: The pathogenesis of Parkinsons disease is still poorly understood. To address the hypothesis that immunemediated events, such as microglial activation, may be involved in the dopaminergic neurodegeneration, we have studied the effect that intranigral injection of the immunostimulant lipopolysaccharide has on monoaminergic neurotransmitters in rats. Activation of microglial cells, visualized by immunohistochemistry with a specific monoclonal antibody, was already obvious 2 days after injection. In relation to the biochemical parameters studied, we found a significant decrease of dopamine levels in both the substantia nigra and striatum up to at least 21 days after intranigral injection of lipopolysaccharide. This result was supported by the decrease in tyrosine hydroxylase activity and the loss of tyrosine hydroxylase‐positive neuronal bodies, shown by immunohistochemistry. These alterations of the dopaminergic system did not reverse during the interval studied (21 days); conversely, the serotoninergic system suffered only transient damage. In addition, we found that the neurotoxic effect of lipopolysaccharide was not mediated by nitric oxide. Based on our results we suggest that the nigrostriatal dopaminergic system is susceptible to damage by inflammatory events and that these may be implicated in neurodegeneration processes such as Parkinsons disease.

The Single Intranigral Injection of LPS as a New Model for Studying the Selective Effects of Inflammatory Reactions on Dopaminergic System

We have injected lipopolysaccharide (LPS) into the nigrostriatal pathway of rats in order to address the role of inflammation in Parkinsons disease (PD). LPS induced a strong macrophage/microglial reaction in Substantia nigra (SN), with a characteristic clustering of macrophage cells around blood-vessels. The SN was far more sensitive than the striatum to the inflammatory stimulus. Moreover, only the dopaminergic neurons of the SN were affected, with no detectable damage to either the GABAergic or the serotoninergic neurons. The damage to the DA neurons in the SN was permanent, as observed 1 year postinjection. Unlike the direct death of dopaminergic neurons caused by agents as MPP(+) or 6-OHDA, LPS seems to cause indirect death due to inflammatory reaction. Therefore, we suggest that the injection of a single dose of LPS within the SN is an interesting model for studying the selective effects of inflammatory reaction on dopaminergic system and also potentially useful for studying PD.

The degenerative effect of a single intranigral injection of LPS on the dopaminergic system is prevented by dexamethasone, and not mimicked by rh‐TNF‐α, IL‐1β and IFN‐γ

It is becoming widely accepted that the inflammatory response is involved in neurodegenerative disease. In this context, we have developed an animal model of dopaminergic system degeneration by the intranigral injection of lipopolysaccharide (LPS), a potent inductor of inflammation. To address the importance of the inflammatory response in the LPS‐induced degeneration of nigral dopaminergic neurones, we carried out two different kinds of studies: (i) the possible protective effect of an anti‐inflammatory compound, and (ii) the effect of the intranigral injection of inflammatory cytokines (TNF‐α, IL‐1β and IFN‐γ) on dopaminergic neurones viability. Present results show that dexamethasone, a potent anti‐inflammatory drug that interferes with many of the features characterizing pro‐inflammatory glial activation, prevented the loss of catecholamine content, Tyrosine hydroxylase (TH) activity and TH immunostaining induced by LPS‐injection and also the bulk activation of microglia/macrophages. Surprisingly, injection of the pro‐inflammatory cytokines failed to reproduce the LPS effect. Taken together, our results suggest that inflammatory response is implicated in LPS‐induced neurodegeneration. This damage may be due, at least in part, to a cascade of events independent of that described for TNF‐α/IL‐1β/IFN‐γ.

Cellular environment facilitates protein accumulation in aged rat hippocampus

Aging represents the main risk factor to develop Alzheimer disease (AD) and protein aggregation constitutes a pathological hallmark thought to be involved in the etiology of this disease. Here, we show that, in basal conditions, the expression of chaperones calnexin, protein disulfide isomerase (PDI) and Grp78 was decreased in aged hippocampus, whereas the protein ubiquitination increased, suggesting the existence of age-related deficits in the systems involved in the defense against unfolded proteins. Interestingly, when cellular stress was induced by intra-hippocampal lactacystin injection, the aged rats were less efficient than young animals in alleviating the protein accumulation and, as an important factor, did not induce the expression of chaperones as young animals. However, the expression of the pro-apoptotic factor CHOP/GADD153 was induced and caspase-12 was activated in stressed aged rats but not in young animals. Current results demonstrated that unfolding protein response (UPR) is not correctly activated in aged rat hippocampus. Consequently, the up-regulation of apoptotic pathway mediators is increased in aged rats. Results might provide further understanding of the pathogenic mechanisms of age-related neurodegenerative disorders.

Low selenium diet increases the dopamine turnover in prefrontal cortex of the rat

It has been proposed that interaction of catecholamines and indoleamines with free radicals may result in the formation of endogenous neurotoxins. In order to better understand the mechanisms involved in neurodegenerative disorders showing evidence of oxidative stress, we have studied the basal concentrations and the turnover rates of dopamine, noradrenaline, serotonin and their metabolites in the prefrontal cortex of rats that were fed on control or low selenium diets. Nutritional deficit of selenium decreases the brain antioxidant protection in experimental conditions by the decrease in glutathione peroxidase activity. The dopamine and serotonin turnover increased and noradrenaline and 5-hydroxy-3-indoleacetic acid turnover decreased compared to experimental control animals. The increase of dopamine turnover in experimental rats was accompanied by an increase in tyrosine hydroxylase activity. These results suggest that the decrease of brain protection against oxidative damage could induce brain damage by disturbing the turnover rate of some monoamines.

Molecular and cellular characterization of the age-related neuroinflammatory processes occurring in normal rat hippocampus: potential relation with the loss of somatostatin GABAergic neurons.

Increased neuroinflammatory reaction is frequently observed during normal brain aging. However, a direct link between neuroinflammation and neurodegeneration during aging has not yet been clearly shown. Here, we have characterized the age‐related hippocampal inflammatory processes and the potential relation with hippocampal neurodegeneration. The mRNA expression of the pro‐inflammatory cytokines IL‐1β and tumor necrosis factor‐α (TNF‐α), and the iNOs enzyme was significantly increased in aged hippocampus. Accordingly, numerous activated microglial cells were observed in aged rats. These cells were differentially distributed along the hippocampus, being more frequently located in the hilus and the CA3 area. The mRNA expression of somatostatin, a neuropeptide expressed by some GABAergic interneurons, and the number of somatostatin‐immunopositive cells decreased in aged rats. However, the number of hippocampal parvalbumin‐containing GABAergic interneurons was preserved. Interestingly, in aged rats, the mRNA expression of somatostatin and IL‐1β was inversely correlated and, the decrease in the number of somatostatin‐immunopositive cells was higher in the hilus of dentate gyrus than in the CA1 region. Finally, intraperitoneal chronic lipopolysaccharide (LPS) injection in young animals mimicked the age‐related hippocampal inflammation as well as the decrease of somatostatin mRNA expression. Present results strongly support the neuroinflammation as a potential factor involved in the age‐related degeneration of somatostatin GABAergic cells.

Effects of aging on the various steps of protein synthesis: fragmentation of elongation factor 2

The possible mechanism responsible for the in vivo protein synthesis decline during aging was studied. In order to determine the effect of aging on the various steps of protein synthesis, we determined the ribosomal state of aggregation and the time of assembly and release of polypeptide chains in the process of protein synthesis in rat liver. The results suggest that elongation is the most sensitive step to aging. A molecular study of the Elongation Factor 2 (EF-2), the main protein involved in the elongation step, shows that this protein has a higher content of carbonyl groups and is less active in old rats. In addition, the molecular mass analysis of EF-2 shows that this protein becomes fragmented in old rats. A similar pattern of fragmentation is found in 3-month-old rats suffering oxidative stress, in that the decline in protein synthesis is similar to that found in old rats. These data suggest that: i) oxidative stress seems to be involved in the modifications of EF-2 observed during aging, and ii) the observed modifications (oxidation and fragmentation) of EF-2 could account for the decline in protein synthesis in old animals.

ROLE OF p38 AND INDUCIBLE NITRIC OXIDE SYNTHASE IN THE IN VIVO DOPAMINERGIC CELLS' DEGENERATION INDUCED BY INFLAMMATORY PROCESSES AFTER LIPOPOLYSACCHARIDE INJECTION

Accumulating evidences suggest that neuroinflammation is involved in the progressive death of dopaminergic neurons in Parkinsons disease. Several studies have shown that intranigral injection of lipopolysaccharide induces inflammation in the substantia nigra leading to death of tyrosine hydroxylase-positive cells. To better understand how the inflammatory response gives rise to neurotoxicity we induced inflammation in substantia nigra by injecting lipopolysaccharide. The damage of substantia nigra dopaminergic neurons was evaluated by immunohistochemistry, reverse transcription-PCR and Western blot analysis of tyrosine hydroxylase. In parallel, activation of microglial cells, a hallmark of inflammation in CNS, was revealed by immunohistochemistry. Similarly the expression of molecules involved in the inflammatory response and apoptotic pathway was also tested, such as cytokines (tumor necrosis factor-alpha, interleukin-1beta, interleukin-6), inducible nitric oxide synthase and caspase-11. Tyrosine hydroxylase expression (both mRNA and protein) started to decrease around 3 days post-injection. At the mRNA level, our results showed that the cytokines expression peaked shortly (3-6 h) after lipopolysaccharide injection, followed by the induction of inducible nitric oxide synthase and caspase-11 (14 h). However, inducible nitric oxide synthase protein peaked at 24 h and lasted for 14 days. The lipopolysaccharide-induced loss of substantia nigra dopaminergic neurons was partially inhibited by co-injection of lipopolysaccharide with S-methylisothiourea, an inducible nitric oxide synthase inhibitor. Co-injections of lipopolysaccharide with SB203580, a p38 MAP kinase inhibitor, reduced inducible nitric oxide synthase and caspase-11 mRNA expression, and also rescued dopaminergic neurons in substantia nigra. In summary, this is the first report to describe in vivo the temporal profile of the expression of these inflammatory mediators and proteins involved in dopaminergic neuronal death after intranigral injection of lipopolysaccharide. Moreover data strongly support that lipopolysaccharide-induced dopaminergic cellular death in substantia nigra could be mediated, at least in part, by the p38 signal pathway leading to activation of inducible nitric oxide synthase and caspase-11.

Low selenium diet affects monoamine turnover differentially in substantia nigra and striatum.

Abstract: Turnover of dopamine, noradrenaline. serotonin, and their metabolites has been measured in striatum and substantia nigra of adult female rats that were fed control or selenium‐deficient diets for 15 days. In addition, the glutathione peroxidase activity has been studied. The most striking result was the increase of dopamine turnover (63%) and 3‐ methoxytyramine turnover (55%) in substantia nigra between control and experimental animals. On the other hand, no changes were found in the turnover rate of dopamine and its metabolites in the striatum. Likewise, no changes were found in noradrenaline turnover in substantia nigra. In the striatum, there was a significant increase of serotonin turnover versus no change for 5‐hydroxy‐3‐indoleacetic acid. However, in the substantia nigra, serotonin turnover did not show significant changes, whereas 5‐hydroxy‐3‐indoleacetic acid turnover decreased. At the same time, glutathione peroxidase activity significantly decreased in both structures after selenium‐deficient diets. These results suggest that a selenium‐deficient diet for a short period of time decreases brain protection. principally in the substantia nigra, against oxidative damage.

Age‐related increase in the immunoproteasome content in rat hippocampus: molecular and functional aspects

Alterations in the proteasome activity in the CNS have been described during aging. However, a detailed study of all proteasome subunits is actually lacking. We have analyzed, in vivo, the age‐related modifications in the molecular composition of hippocampal proteasomes. We found that the immunoproteasome/proteasome ratio was increased in aged hippocampus. The processing of the low‐molecular‐mass protein (LMP)7/β5i subunit, practically absent in young hippocampus, was increased in aged animals. Among the potential factors underlying these modifications we evaluated the neuroinflammation and the transcription factor Zif268. Lipopolysaccharide (LPS)‐induced neuroinflammation in young rats, up‐regulated the expression of immunoproteasome subunits and increased the processing of the LMP7/β5i protein. Moreover, the hydrophobicity of cellular peptides, analyzed by liquid chromatography, increased in both, young LPS‐injected animals and aged rats, suggesting that immunoproteasomes including the LMP7/β5i subunit could, at least in part, account for this modification. Also, the mRNA expression of the transcription factor Zif268, which down‐regulates the immunoproteasome subunit LMP7/β5i by binding to sequences within the promoter regions, was decreased in both, aged hippocampus and young LPS‐injected animals. Finally, we found that spatial memory training in young animals, a situation in which the expression of Zif268 is increased, modified the mRNA expression of the constitutive and catalytic subunits in an opposite manner. Based on present data, we propose that the age‐related increases in the content of hippocampal immunoproteasome is mostly because of neuroinflammatory processes associated to aging.