Prevention of peroxynitrite-induced apoptosis of motor neurons and PC12 cells by tyrosine-containing peptides

Although peroxynitrite stimulates apoptosis in many cell types, whether peroxynitrite acts directly as an oxidant or the induction of apoptosis is because of the radicals derived from peroxynitrite decomposition remains unknown. Before undergoing apoptosis because of trophic factor deprivation, primary motor neuron cultures become immunoreactive for nitrotyrosine. We show here using tyrosine-containing peptides that free radical processes mediated by peroxynitrite decomposition products were required for triggering apoptosis in primary motor neurons and in PC12 cells cultures. The same concentrations of tyrosine-containing peptides required to prevent the nitration and apoptosis of motor neurons induced by trophic factor deprivation and of PC12 cells induced by peroxynitrite also prevented peroxynitrite-mediated nitration of motor neurons, brain homogenates, and PC12 cells. The heat shock protein 90 chaperone was nitrated in both trophic factor-deprived motor neurons and PC12 cells incubated with peroxynitrite. Tyrosine-containing peptides did not affect the induction of PC12 cell death by hydrogen peroxide. Tyrosine-containing peptides should protect by scavenging peroxynitrite-derived radicals and not by direct reactions with peroxynitrite as they neither increase the rate of peroxynitrite decomposition nor decrease the bimolecular peroxynitrite-mediated oxidation of thiols. These results reveal an important role for free radical-mediated nitration of tyrosine residues, in apoptosis induced by endogenously produced and exogenously added peroxynitrite; moreover, tyrosine-containing peptides may offer a novel strategy to neutralize the toxic effects of peroxynitrite.     

Altered regulation of phosphatidylinositol 3-kinase signaling in cathepsin D-deficient brain

Cathepsin D (CD) is an essential lysosomal protease and mice lacking this enzyme exhibit neuropathology similar to that observed in brains of patients with neuronal ceroid lipofuscinosces (NCL/Batten disease), a group of autosomal recessive pediatric neurodegenerative diseases. CD-deficient (CD-/-) brains exhibit a dramatic induction of autophagic stress as defined by the aberrant accumulation of autophagosomes, which is concomitant with markers of apoptosis. However, the signaling abnormalities which lead to CD deficiency-induced neurodegeneration are poorly defined. Since phosphatidylinositol-3 kinase (PI3-K) is known to regulate both apoptosis and autophagy, PI3-K-mediated signaling events were assessed in CD-/- brain at P14 and P25-26. Compared to WT littermate controls, CD-/- cortical neurons exhibited a widespread decrease in phosphorylation of Akt (inactivation) and GSK3beta (disinhibition) at P25-26, while levels of total Akt and GSK3beta remained unchanged. This P25-26-specific decrease in phosphorylation of Akt and GSK-3beta in CD-/- brain coincided temporally with markers of apoptosis but followed the induction of autophagic stress observed at both P14 and P25-26. In addition, levels and/or activation of mTOR and Beclin were not affected by CD deficiency, suggesting that the accumulation of autophagosomes is not due to an increased synthesis of autophagosomes but rather from an inhibition of autophagosome recycling, due most likely to a compromise in lysosome function. Together these observations indicate a pronounced decrease in pro-survival PI3-K signaling in CD-/- brain that may contribute to autophagic stress-induced and apoptotic neuropathology.