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Abstract: We examined the expression of SIRT1 in several experimental paradigms of human pathologies. We used a neuroblastoma cell line (B65), neuronal primary cultures (hippocampus and cerebellar granule cells) and in vivo approaches in rat and senescence murine models (SAM). Cell cultures and rats were treated with several well-know neurotoxins, i.e. rotenone, MPP+, kainate and 3-nitropropionic acid. Subsequently, SIRT1 expression was compared in these different paradigms of neurotoxicity. The pattern of expression of SIRT1 in proliferating cell cultures (B65) was different to that in quiescent cell cultures. In the murine model of senescence (senescence-accelerated mice prone, SAMP8), SIRT1 expression progressively decreased, while in the control strain (senescence-accelerated mice resistant, SAMR1) it increased. Finally, we studied human samples of Parkinson''s disease (PD), dementia with Lewy bodies (DLB) and Huntington''s diseases (HD). SIRT1 expression decreased dramatically in HD, but there were no significant changes in Parkinson-related illnesses. In conclusion, SIRT1 expression may be a good sensor of toxic neuronal processes. [Copyright &y& Elsevier]

Abstract: Increasing evidence implicates the c-Jun NH2-terminal kinase (JNK) pathway in the regulation of apoptosis in neurodegenerative diseases. In this study, we examined the neuroprotective effect of SP600125, a selective JNK inhibitor, in cerebellar granule cells (CGNs) deprived of serum and potassium (S/K withdrawal). S/K withdrawal-induced apoptosis occurs via activation of multiple pro-apoptotic pathways, including re-entry into the cell cycle, activation of glycogen synthase kinase-3 beta (GSK-3β), cyclin-dependent kinase 5 (cdk5/p35) breakdown, formation of cdk5/p25 and JNK activation. Here we demonstrate that SP600125 is able to inhibit all these pro-apoptotic pathways via the inhibition of JNK. Further, we found that JNK inhibition maintains the phosphorylation/activation of Akt after S/K withdrawal. For further confirmation of this result, we studied several targets downstream of Akt including GSK-3β, p-FOXO1, p-CREB and p35. In addition, the specific PI3K/Akt inhibitor LY294002 greatly diminished the antiapoptotic effects of SP600125 upon S/K withdrawal, confirming that Akt is involved in the neuroprotection achieved by SP600125. These results suggest that the maintenance of the PI3-kinase/Akt pathway by inhibition of JNK contributes to the prevention of apoptosis in rat cerebellar granule neurons mediated by S/K withdrawal. Furthermore, we propose that JNK may regulate the cell cycle re-entry by a novel mechanism that involves Akt, GSK-3β and Rb phosphorylation. [Copyright &y& Elsevier]

Abstract: The mechanism involved in neuronal apoptosis is largely unknown. Studies performed on neuronal cell cultures provide information about the pathways which orchestrate the process of neuronal loss and potential drugs for the treatment of neurological disorders. In the present study we select resveratrol, a natural antioxidant, as a potential drug for the treatment of neurodegenerative diseases. We evaluate the neuroprotective effects of resveratrol in two apoptotic models in rat cerebellar granule neurons (CGNs): the inhibition of mitochondrial complex I using 1-methyl-4-phenylpyridinium (MPP+) (an in vitro model of Parkinson’s disease) and serum potassium withdrawal. We study the role of the mammalian silent information regulator 2 (SIRT1) in the process of neuroprotection mediated by resveratrol. Because recent studies have demonstrated that SIRT1 is involved in cell survival and has antiaging properties, we also measured changes in the expression of this protein after the addition of these two apoptotic stimuli. MPP+-induced loss of cell viability and apoptosis in CGNs was prevented by the addition of RESV (1 μM to 100 μM). However, the neuroprotective effects were not mediated by the activation of SIRT1, since sirtinol—an inhibitor of this enzyme—did not attenuate them. Furthermore MPP+ decreases the protein expression of SIRT1. RESV did not prevent serum potassium withdrawal–induced apoptosis although it did completely attenuate oxidative stress production by these apoptotic stimuli. Furthermore, serum potassium withdrawal increases the expression of SIRT1. Our results indicate that the antiapoptotic effects of RESV in MPP+ are independent of the stimulation of SIRT1 and depend on its antioxidant properties. Furthermore, because SIRT1 is involved in neuronal survival depending on the apoptotic stimuli, changes in the expression of SIRT1 could be involved in the regulation of the apoptotic route. [Copyright &y& Elsevier]

Abstract: The biochemical pathways involved in neuronal cell death in Parkinson’s disease are not completely characterized. Mitochondrial dysfunction, specifically alteration of the mitochondrial complex I, is the primary target of the parkinsonian neurotoxin 1-methyl-4-phenylpyridinium (MPP+) induced apoptosis in neurons. In the present study, we examine the role of caspase-dependent and -independent routes in MPP+-induced apoptosis in rat cerebellar granule neurons (CGNs). We show a distinct increase in the expression of the cell cycle proteins cyclin D, cyclin E, cdk2, cdk4 and the transcription factor E2F-1 following a MPP+ treatment of CGNs. Flavopiridol (FLAV), a broad inhibitor of cyclin-dependent kinases (CDKs), attenuated the neurotoxic effects of MPP+ and significantly attenuates apoptosis mediated by MPP+ 200 μM. Likewise, the antioxidant vitamin E (vit E) increases neuronal cell viability and attenuates apoptosis induced by MPP+. Moreover, the expression levels of cyclin D and E2F-1 induced by this parkinsonian neurotoxin were also attenuated by vit E. Since, the broad-spectrum caspase inhibitor zVAD-fmk did not attenuate MPP+-induced apoptosis in CGNs, our data provide a caspase-independent mechanism mediated by neuronal reentry in the cell cycle and increased expression of the pro-apoptotic transcription factor E2F-1. Our results also suggest a potential role of oxidative stress in neuronal reentry in the cell cycle mediated by MPP+. Finally, our data further support the therapeutic potential of flavopiridol, for the treatment of Parkinson’s disease. [Copyright &y& Elsevier]

Abstract: Although numerous studies have demonstrated a neuroprotective and anti-apoptotic role of lithium in neuronal cell cultures, the precise mechanism by which this occurs, remains to be elucidated. In this study, we evaluated the lithium-mediated neuroprotection against colchicine-induced apoptosis in cultured cerebellar granule neurons. Previously, it has been demonstrated that colchicine mediates apoptosis in cerebellar granule neurons through cytoskeletal alteration and activation of an intrinsic pro-apoptotic pathway. Recently we also demonstrated a potential role of cyclin-dependent kinase 5 (cdk5) in this pathway. Here we report that colchicine induces dephosphorylation in Ser-9 and phosphorylation in Tyr-216, and thus activation, of glycogen synthase kinase-3β in cerebellar granule neurons, and that this modification is inhibited by the presence of 5mM lithium. However, the selective glycogen synthase kinase-3β inhibitors SB-415286 and SB-216763 were unable to prevent colchicine-induced apoptosis in these cells, suggesting that the anti-apoptotic activity of lithium is not mediated by glycogen synthase kinase-3β under these conditions. On the other hand, 5mM lithium prevented the colchicine-induced increase in cdk5 expression and breakdown of cdk5/p35 to cdk5/p25. In addition, we show that up-regulation of cdk5/p25 is unrelated to inhibition of the activity of myocyte enhancer factor 2, a pro-survival transcription factor. These data suggest a previously undescribed neuroprotective mechanism of lithium associated with the modulation of cdk5/p35 or cdk5/p25 expression. [Copyright &y& Elsevier]

The mechanisms underlying selective neuronal cell death in kainic acid-mediated neurodegeneration are not fully understood. We have recently demonstrated that in cerebellar granule neurons, kainic acid induces the expression of proteins associated with cell-cycle progression. In the present study we show that 3-amino thioacridone (3-ATA), a selective cyclin-dependent kinase 4 inhibitor, attenuates kainic acid-induced apoptosis in cerebellar granule neurons. When neurons were pre-treated with 3-ATA 10 μM for 24 h, they were less susceptible to damage induced by kainic acid 500 μM, since the number of dead cells decreased significantly. In flow cytometry studies using propidium iodide staining, 3-ATA also reduced the ratio of apoptotic cells induced by kainic acid. Moreover, 3-ATA decreased the proportion of cells with a condensed nucleus from 55% to 22%. Our data suggest that the cell cycle pathway is involved in the mechanism of apoptosis mediated by kainic acid and that cyclin-dependent kinase 4 plays a prominent role in this process. 3-ATA may to prevent the apoptosis associated with neurodegenerative disorders without the over-activation of excitatory amino acid receptors. [Copyright &y& Elsevier]