THE ROLE OF PEROXISOME PROLIFERATOR ACTIVATED RECEPTORS IN AMYOTROPHIC LATERAL SCLEROSIS: POTENTIAL MECHANISMS FOR NEUROPROTECTION

The vast majority of neurodegenerative disorders are adult-onset, incurable diseases. Understanding the pathogenetic mechanisms underlying these disorders and finding molecules apt to correct such processes are, therefore, among the hottest topics of biomedical research. Amyotrophic Lateral Sclerosis is one of the most common adult-onset neurodegenerative diseases characterized by progressive degeneration of upper and lower motor neurons leading to paralysis and death due to respiratory failure within 3-5 years from the onset. Only one drug, riluzole, has proved effective in extending the lifespan of patients with ALS, but only by 3-6 months. For this reason the development of effective therapies for this pathology is highly invocated, but to date all attempts to develop novel treatments have failed. In this context, two recent reports on the neuroprotective activity of the PPAR agonist Pioglitazione in ALS mice result of considerable interest: in these studies, two independent groups demonstrated that Pioglitazone, an agent which is currently used in therapy for the treatment of type II diabetes, is neuroprotective in a mouse model of Amyotrophic Lateral Sclerosis, the hSOD1-G93A transgenic mice. Pioglitazone treatment started before the appearance of the symptoms, improved the motor performance and reduced the weight loss, attenuated motor neuron death and increased the survival. In addition, Pioglitazone reduced microglial activation and gliosis in the spinal cord, decreasing the production of pro-inflammatory mediators, such as iNOS, NF-kB and COX2. On this ground, we decided to investigate the transcriptional activity of PPARs in the central nervous system of the hSOD1-G93A mouse line, a well-characterized animal model of Amyotrophic Lateral Sclerosis, with the aim of identifying the stage of the disease at which the activity of PPARs becomes relevant to the pathology. To this end, we took advantage of the transgenic mouse PPRE-Luc, available in the laboratory, in which the reporter gene luciferase is expressed under the control of a promoter responsive to PPARs. Thus, we crossed the PPRE-Luc mice with the hSOD1-G93A animals to obtain mice that are heterozygous for the PPRE-Luc transgene and heterozygous or null for the hSOD1-G93A transgene. The analysis of the enzymatic activity of luciferase in the spinal cord and the brain areas of PPRE-Luc;hSOD1-G93A mice shows an abrupt increase of PPAR activity at the end stage of the disease in the spinal cord, which is the organ principally involved in the pathology, and in all the brain areas analysed. We demonstrated that this phenomenon clearly depends on the pathology because it is not shared by the peripheral organs (e.g. kidney and liver). Furthermore, it is not dependent on the metabolic modifications induced from the starvation that the animals experience during the last days of their life when they are almost completely paralysed and, thus, unable to reach for food and water. We subsequently decided to further investigate this mechanism by identifying the isoform(s) responsible for the increase of PPARs activity at the last stage of the disease and the cell type(s) involved. We first analysed the nuclear translocation of PPARα, PPARβ/δ and PPARγ in the spinal cord of hSOD1-G93A mice with an ELISA-based Transcription Factor Assay. The results obtained from these experiments showed that the overall nuclear presence of the different isoforms of PPARs does not change during the course of the disease. In order to obtain a cell specific information about the distribution of PPARs in the spinal cord, we next analysed the localization of PPARα, PPARβ/δ and PPARγ by immunohistochemistry on sections from the lumbar spinal cord of hSOD1-G93A at the different stages of the pathology using primary antibodies for the specific isoforms of PPARs and cell specific markers. Our stainings revealed that all the three isoforms of PPARs are expressed in spinal cord motor neurons; PPARα and PPARβ/δ are localized prevalently into the nucleus but show also a cytoplasmic staining, while PPARγ is exclusively nuclear. All the three isoforms are present also in astrocytes where they are exclusively nuclear while only PPARγ was detectable in microglia, and was localized into the nucleus. Immunohistochemical analysis confirms that the increase in PPAR activity at the end stage of the disease is not dependent on the increase in the nuclear presence of the receptors in the different cell types of the spinal cord, suggesting that it possibly derives from ligand-dependent effects and/or the differential recruitment of co-regulators. To identify the specific isoform whose activity is important during the pathology we analysed the expression of isoform-specific target genes, i.e. MCAD for PPARα, Acsl6 for PPARβ/δ and LPL for PPARγ. Only the expression of LPL abruptly increases at the end stage of the disease strongly suggesting that the increase in luciferase activity detected at the later stage of ALS is due to the activation of PPARγ. To confirm this result we analysed other PPARγ target genes, i.e. Catalase, Glutathione S-tranferase alpha-2 and Peroxisome Proliferator Activated Receptor gamma coactivator 1-alpha. The RT-PCR analysis of the expression of Cat, Gsta2 and PGC1α showed that Cat and PGC1α show a similar trend of reduction till the onset of the disease, 100 days, then the levels of PGC1α slightly increase while the Cat expression increases in a significant manner. Gsta2 expression remains fairly constant till the end stage when it increases significantly. On these bases we decided to further investigate the mechanisms of PPARγ activation at the end stage of the disease by identifying the cell type involved. The analysis of the fluorescence intensity into the cellular nuclei of lumbar spinal cord sections stained for PPARγ demonstrated that the intensity of the receptor signal is greater in motor neurons than in non-neuronal cells. This data led us to hypothesize that motor neurons could be the most likely cell type involved in the activation of PPARγ at the end stage of the disease in vivo. Thus, we decided to analyse the expression of the PPARγ target genes previously analysed in the spinal cords of hSOD1-G93A mice in an immortalized motor neuronal cell line, the NSC-34 cells. The expression of LPL, Cat and PGC1α in NSC-34 cells transiently transfected with the hSOD1-G93A- encoding expression vector is significantly increased as compared to the NSC-34 cells transfected with the empty vector. These data clearly confirm the involvement of motor neurons in PPARγ activation at the last stage of the disease; on these bases future studies will be aimed to further elucidate the mechanisms of PPARγ protective activity on motor neurons in ALS.