Biochemistry and genetics of monoamine oxidase.

Studies in non-neural cells have suggested that a fall in mitochondrial membrane potential (ΔΨM) is one of the earliest events in apoptosis. It is not known whether neural apoptosis caused by nerve growth factor (NGF) and serum withdrawal involves a decrease in ΔΨM. We used epifluorescence and laser confocal microscopy with the mitochondrial potentiometric dyes chloromethyl-tetramethylrosamine methyl ester and 5,5′,6,6′-tetrachloro-1,1′,3,3′-tetraethybenzimidazol carbocyanine iodide to estimate ΔΨM. PC12 cells were differentiated in media containing serum and NGF for 6 d before withdrawal of trophic support. After washing, the cells were incubated with media containing serum and NGF (M/S+N), media without serum and NGF, or media with the “trophic-like” monoamine oxidase B inhibitor, (−)-deprenyl. Mitochondria in cells without trophic support underwent a progressive shift to lower ΔΨM values that was significant by 3 hr after washing. The percentages of cells with nuclear chromatin condensation or nuclear DNA fragmentation were not significantly increased above those for cells in M/S+N until 6 hr after washing. Replacement of cells into M/S+N or treatment with (−)-deprenyl markedly reduced the proportion of mitochondria with decreased ΔΨM. Measurements of cytoplasmic peroxyl radical levels with 2′,7′-dihydrodichlorofluorescein fluorescence and intramitochondrial Ca2+ with dihydro-rhodamine-2-acetylmethyl ester indicated that cytoplasmic peroxyl radical levels were not increased until after 6 hr, whereas increases in intramitochondrial Ca2+ paralleled the decreases in ΔΨM. (−)-Deprenyl appeared to alter the relationship between intramitochondrial Ca2+ levels and ΔΨM, possibly through its reported capacity to increase the synthesis of proteins such as BCL-2.

https://www.jneurosci.org/content/jneuro/18/3/932.full.pdf

Mitochondrial Membrane Potential and Nuclear Changes in Apoptosis Caused by Serum and Nerve Growth Factor Withdrawal: Time Course and Modification by (−)-Deprenyl

(-)-Deprenyl, a monoamine oxidase (MAO)-B inhibitor, has been shown to increase neuronal survival and to alter protein synthesis and gene expression in astrocytic or PC12 cells independently of MAO-B inhibition. We used serum and nerve growth factor withdrawal to induce apoptotic death in PC12 cells to determine whether (-)-deprenyl increases neuronal survival by reducing apoptosis. (-)-Deprenyl reduced both cell death and internucleosomal DNA degradation in a concentration-dependent manner and was effective at concentrations too low to inhibit MAO (< 10(-9) M). (+)-Deprenyl did not increase PC12 cell survival, and, with the exception of pargyline, other MAO-A and MAO-B inhibitors did not alter apoptotic death. Transcriptional and translational inhibition showed that the reduction in apoptosis required the induction of new protein synthesis by (-)-deprenyl. Increased survival was induced if transcription was maintained for 4 h and translation for 6 h after (-)-deprenyl addition. The findings suggest that transcriptional induction may underlie the other MAO-independent actions of (-)-deprenyl.

Deprenyl reduces PC12 cell apoptosis by inducing new protein synthesis.

(-)-Deprenyl and structurally related propargylamines increase neuronal survival independently of monoamine oxidase B (MAO-B) inhibition, in part by decreasing apoptosis. We found that deprenyl and two other propargylamines, one of which does not inhibit monoamine oxidase B, increased survival in trophically withdrawn 6-day nerve growth factor (NGF)- and 9-day NGF-differentiated PC-12 cells but not in NGF naive or 3-day NGF-differentiated PC-12 cells. Four days of prior NGF exposure were required for the propargylamine-mediated antiapoptosis. Studies using actinomycin D, cycloheximide, and camptothecin revealed that the maintenance of both transcription and translation, particularly between 2 and 6 h after trophic withdrawal, was required for propargylamine-mediated antiapoptosis. Metabolic labeling of newly synthesized proteins for two-dimensional protein gel autoradiography and scintillation counting showed that the propargylamines either increased or reduced the levels of new synthesis or induced de novo synthesis of a number of different proteins, most notably proteins in the mitochondrial and nuclear subfractions. Western blotting for whole cell or subcellular fraction lysates showed that the timing of new protein synthesis changes or subcellular redistribution of apoptosis-related proteins induced by the propargylamines were appropriate to antiapoptosis. The apoptosis-related proteins included superoxide dismutases (SOD1 and SOD2), glutathione peroxidase, c-JUN, and glyceraldehyde-3-phosphate dehydrogenase. Most notable were the prevention of apoptotic decreases in BCL-2 levels and increases in mitochondrial BAX levels. In general, (-)-deprenyl-related propargylamines appear to reduce apoptosis by altering the levels or subcellular localization of proteins that affect mitochondrial membrane permeability, scavenge oxidative radicals, or participate in specific apoptosis signaling pathways.

E. McHugh

Papers

Contrasting monoamine oxidase activity and tyramine induced catecholamine release in PC12 and chromaffin cells.