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ro o to mice induced similar gene expression changes in a time-course effect (37,38) (Table 1). Here they found, via the use of the mouse U74A Gene Chip from Affymetrix, changes of genes related to cytoskeletal stability, synaptic integrity and cell cycle and apoptosis in the SN of MPTP-treated animals (37,38).

6-OHDA rat model

The 6-OHDA-lesioned rat is considered one of the most reliable animal models of PD and displays biochemical and anatomical features closely resembling those observed in parkinsonian patients (8,39). In addition, the mechanisms controlling altered striatal output in PD and in l-DOPA (the main therapy in PD)-induced dyskinesia are poorly understood. The first researchers who used this model in the context of studying gene expression were Berke et al. (40) trying to reveal the response to stimulation of striatal dopamine D1 receptors. Here, they compared the normal and 6-OHDA-lesioned rat striata of animals that were given either saline or a D1 agonist and were killed 1 or 24 h later. Gene expression study was conducted via DD, which revealed that for more than 30 genes the expression rapidly increased in response to stimulation of striatal dopamine D1 receptors. The induced mRNAs include both novel and previously described genes, with diverse time courses of expression. Some genes are expressed at near-maximal levels within 30 min, whereas others show no substantial induction until 2 h or more after stimulation. Some of the induced genes, such as CREM, CHOP and MAP-kinase phosphatase-1, may be components of a homeostatic response to excessive stimulation. Others may be part of a genetic programme involved in cellular and synaptic plasticity. The next group to investigate the gene expression profile has looked at the alteration caused by 6-OHDA-lesioning of the nigrostriatal pathway in rats and the effects of chronic l-DOPA treatment on the expression of the genes identified after 4-6 months of lesion (41). DD mRNA analysis was used to study the effects of 6-OHDA-induced lesions of the medial forebrain bundle on gene expression in the rat striatum. One up-regulated cDNA identified in two independent groups of 6-OHDA-lesioned animals was cloned and sequence analysis showed 97% homology to secretogranin II. This up-regulation of this gene was confirmed using RT-PCR, and further up-regulation was observed after chronic l-DOPA treatment. The first researchers to use microarray technology were Napolitano et al. (42) who studied the genetic correlates of the alterations produced by 6-OHDA-induced dopamine denerva-tion in the nucleus striatum. Here, they analysed gene expression changes in the striatum 2 months after lesioning of the homolateral SN of rats with 6-OHDA. They found that chronic dopamine denervation caused the modulation of 50 different genes involved in several cellular functions. In particular, products of the genes modulated by this experimental manipulation are involved both in the intracellular transduction of dopamine signal and in the regulation of glutamate transmission in striatal neurons (Table 1). Recently, Konradi et al. (43) studied, in 6-OHDA-lesioned rats, the effect of L-DOPA-induced dyskinesia on gene expression. They used unilateral nigrostriatal injections of 6-OHDA followed 5-6 weeks later with single daily injections of l-DOPA for 22 days. Rats were sacrificed 18 h after last injection and the dorsal striata were isolated for microarray analysis. In rats that developed dyskinesia, GABA neurons had an increased transcriptional activity, and genes involved in Ca2+ homeostasis, in Ca2+-dependent signalling, and structural and synaptic plasticity were up-regulated.

In a further analysis, astrocytes isolated from the striata of control and 6-OHDA-lesioned rats revealed 29 genes whose expression was up-regulated and 2 genes whose expression was down-regulated (44) using microarray methodology. Through in situ hybridization 8 genes were confirmed to alter in the astrocytes. These included GDNF, NGF, bFGF, TNF-a, MIP-1a, c-JUN, Fra-1 and Fra-2. Although, many studies on gene expression profiling of 6-OHDA-lesioned rats were conducted, unfortunately many are not comparable as a result of different treatments concerning the doses, the time length and the lesion region. Therefore, in order to have a more unified and clear results in this model it is important to continue the studies in several schemes with time courses, so as to investigate the gene expression changes also in time.

Methamphetamine mouse model

Methamphetamine is an amphetamine analog that causes damage to dopaminergic nerve endings of the striatum. Methamphetamine can access the nerve ending as a substrate for the dopamine transporter. Once inside it causes inhibition of the vesicle monoamine transporter, allowing dopamine to leak into the cytoplasm and eventually into the synaptic space by reverse transport through the dopamine transporter. Several studies have implicated OS involvement in methamphetamine-induced neurotoxicity (8,9,45). Actually, these oxygen-free radicals are thought to be secondary to methamphetamine neurotoxicity, as dopamine turnover itself produces ROS (9,46). This animal model was first tested for its influence on gene expression in CD-1 mice treated with a single dose of 40 mg/kg methamphetamine. These mice were sacrificed 2, 4 and 16 h later to analyse cortical mRNA expression on a cDNA expression array (47). In this study, the early gene expression changes observed in the cortex were of genes involved in transcription factor and DNA-binding proteins, oncogenes and tumour suppressors, stress response proteins apoptosis-associated proteins and growth factors. In the later time course, more genes of the transcription factors, oncogenes and apoptosis were involved in the inflammatory system (Table 1). This observation provided further evidence that OS may be an important culprit in the manifestations of the deleterious effects of methamphetamine. In addition, a recent gene expression profiling study of C57BL mice treated with single (i.p.) injection of 40 mg/kg methamphetamine showed similar results in the striatum (Table 1) (48). In this study, mRNA expression was analysed after 3, 6, 12 and 24 h after treatment on the GeneChip microarray from Affymetrix. Genes of functional groups of cell cycle, cell growth, cell structure, inflammation, protein turnover, signal transduction and transcription factors were altered in a specific time course. These results suggest a strong link between methamphetamine and microglia, the CNS equivalent of systemic macrophages. Cytokines and their receptors play a major role in the defence of the CNS. Cytokines, such as BDNF, inhibits intracellular oxy-radical stress triggered by dopamine and partially blocks basal and dopamine-induced apoptosis. The involvement of microglia in other models such as MPTP and 6-OHDA may imply a specific response to OS.

Gad mouse model

An altered ubiquitin-proteasomal protein degradation pathway was postulated to be one of the events in the pathogenesis of PD (49,50). Efficient targeting for degradation by the 26S proteasome requires polyubiquitination. In addition to the isopeptide linkages made to lysines, the ubiquitin C-terminus can also form peptide bonds to a-linked polyubi-quitin or ubiquitin followed by a C-terminal peptide extension. This step is catalysed by a family of ubiquitin C-terminal hydrolases which are tissue specific and likely to target distinct substrates. Of these, ubiquitin C-terminal hydrolase-Ll (UCHL1) is highly expressed in the brain (51) and has recently been implicated in PD by the identification of a missense mutation in autosomal-dominant PD with reduced penetrance (52,53). Additionally, inhibitors of the proteasomal pathway in cultured neurons by ubiquitin aldehyde, which is an UCH inhibitor cause the formation of protein aggregates and cell death (54). Most interestingly, UCH-L1 is also a part of the Lewy bodies (55). Recently in birds it was found that the UCH-L1 mRNA expression in replaceable high vocal centre neurons is higher compared to non-replaceable neurons (56). Therefore, it could be postulated that reduced UCH-L1 function may jeopardize the survival of CNS neurons. In a recent study, the UCH-Ll-deficient gracile axonal dystrophy (gad) mice were investigated using microarray expression analyses. The gad mouse is an autosomal recessive spontaneous neurological mutant. In these mice, the gad mutation is caused by an intragenic deletion of the UCH-L1 gene including exons 7 and 8. Subsequent studies have shown that the mutant lacks the expression of UCH-L1 protein (57,58). Pathologically, the gad mouse displays dying-back type of axonal degeneration of the gracile tract. Most interestingly, gad mice accumulate amyloid precursor protein (APP) in the form of ubiquitin-positive deposits along the sensory and motor nervous systems and thereby cause staining, another indication that the gad mutation affects protein turnover. Therefore, direct involvement of an altered ubiquitin system in neurodegeneration has been indicated by this model.

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