Jae-Young Koh

Blockade of glutamate receptors unmasks neuronal apoptosis after oxygen-glucose deprivation in vitro.

Mouse cortical cell cultures exposed to transient oxygen-glucose deprivation developed marked acute cell body swelling followed by neurodegeneration, consistent with necrosis-type death. This death was not attenuated by the protein synthesis inhibitor, cycloheximide, but was attenuated by addition of the N-methyl-D-asparate antagonist, MK-801 (dizocilpine maleate), and the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid/kainate antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione. If the deprivation insult was extended to overcome the protective effect of glutamate antagonists, neuronal death resulted that was associated with cell body shrinkage and DNA fragmentation, and was attenuated by cycloheximide. These data suggest that oxygen-glucose deprivation can induce in cortical neurons both excitotoxic necrosis, and apoptosis dependent on new macromolecule synthesis.

Staurosporine-induced neuronal apoptosis

Staurosporine, a nonselective protein kinase inhibitor, has been shown to induce apoptosis in several different nonneuronal cell types. We tested the hypothesis that staurosporine would also induce apoptosis in central neurons. Exposure of murine cortical cell cultures to 30-100 nM staurosporine induced concentration-dependent selective neuronal degeneration over the following day; at higher concentrations, staurosporine damaged glial cells as well. Staurosporine-induced neuronal death was accompanied by cell body shrinkage, chromatin condensation, and DNA laddering. In contrast, NMDA-induced neuronal death was accompanied by acute cell body swelling without DNA laddering. Staurosporine-induced neuronal death, unlike excitotoxic death, was markedly attenuated by the protein synthesis inhibitor cycloheximide; this protective effect was not reversed by a glutathione synthesis inhibitor, buthionine sulfoximine. Interestingly, the glial cell death induced by 1 microM staurosporine was markedly potentiated by cycloheximide. Staurosporine-induced neuronal death was not accompanied by an increase in intracellular free Ca2+ and was attenuated by 30 mM K+; this protective effect of high K+ was blocked by nimodipine or Co2+. Present data suggest that staurosporine can induce apoptosis in cultured cortical neurons and that this apoptosis can be blocked by raising intracellular Ca2+ or by blocking protein synthesis. Staurosporine exposure may be useful as a model for studying central neuronal apoptosis in vitro.

Slowly triggered excitotoxicity occurs by necrosis in cortical cultures

This study examined the possibility that the excitotoxin-induced death of cultured cortical neurons might occur by apoptosis, specifically focusing on the slowly triggered death induced by low concentrations of excitotoxin. Cultured murine cortical neurons (days in vitro 10-12) were exposed continuously to N-methyl-D-aspartate (10-15 microM), alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (3-100 microM) or kainate (30-60 microM) over 24 h. Within 2 h of exposure onset, neuronal cell body swelling was visible under phase-contrast optics. At this point, transmission electron microscopy revealed disruption of cell membranes and organelles, mitochondrial swelling and scattered chromatin condensation at the periphery of nuclei. By 8 h after exposure onset, many neurons were devoid of cytoplasmic structures, but nuclear membranes remained relatively intact. This excitotoxic degeneration was not blocked by the protein synthesis inhibitor, cycloheximide, or the growth factors, brain-derived neurotrophic factor or insulin-like growth factor-1, agents that did block serum deprivation-induced apoptosis death in other cultures. DNA agarose gel electrophoresis, however, revealed the transient occurrence of internucleosomal DNA fragmentation, appearing 4-8 h after exposure onset, but absent 24 h after exposure onset. The present results suggest that even slowly triggered excitotoxicity occurs by necrosis, and raise a cautionary note in interpreting internucleosomal DNA fragmentation in isolation as evidence for apoptosis.

Calcium ionophores can induce either apoptosis or necrosis in cultured cortical neurons

Cultured cortical neurons exposed for 24 h to low concentrations of the Ca2+ ionophores, ionomycin (250 nM) or A-23187 (100 nM), underwent apoptosis, accompanied by early degeneration of neurites, cell body shrinkage, chromatin condensation and internucleosomal DNA fragmentation. This death could be blocked by protein synthesis inhibitors, as well as by the growth factors brain-derived neurotrophic factor or insulin-like growth factor I. If the ionomycin concentration was increased to 1-3 microM, then neurons underwent necrosis, accompanied by early cell body swelling without DNA laddering, or sensitivity to cycloheximide or growth factors. Calcium imaging with Fura-2 suggested a possible basis for the differential effects of low and high concentrations of ionomycin. At low concentrations, ionomycin induced greater increases in intracellular Ca2+ concentration in neurites than in neuronal cell bodies, whereas at high concentrations, ionomycin produced large increases in intracellular Ca2+ concentration in both neurites and cell bodies. We hypothesize that the ability of low concentrations of Ca2+ ionophores to raise intracellular Ca2+ concentration preferentially in neurites caused early neurite degeneration, leading to loss of growth factor availability to the cell body and consequent apoptosis, whereas high concentrations of ionophores produced global cellular Ca2+ overload and consequent necrosis.

Prevention of neuronal apoptosis by phorbol ester-induced activation of protein kinase C: blockade of p38 mitogen-activated protein kinase

Consistent with previous studies on cell lines and non-neuronal cells, specific inhibitors of protein kinase C induced mouse primary cultured neocortical neurons to undergo apoptosis. To examine the complementary hypothesis that activating protein kinase C would attenuate neuronal apoptosis, the cultures were exposed for 1 h to phorbol-12-myristate-13-acetate, which activated protein kinase C as evidenced by downstream enhancement of the mitogen-activated protein kinase pathway. Exposure to phorbol-12-myristate-13-acetate, or another active phorbol ester, phorbol-12,13-didecanoate, but not to the inactive ester, 4alpha-phorbol-12,13-didecanoate, markedly attenuated neuronal apoptosis induced by serum deprivation. Phorbol-12-myristate-13-acetate also attenuated neuronal apoptosis induced by exposure to beta-amyloid peptide 1-42, or oxygen-glucose deprivation in the presence of glutamate receptor antagonists. The neuroprotective effects of phorbol-12-myristate-13-acetate were blocked by brief (non-toxic) concurrent exposure to the specific protein kinase C inhibitors, but not by a specific mitogen-activated protein kinase 1 inhibitor. Phorbol-12-myristate-13-acetate blocked the induction of p38 mitogen-activated protein kinase activity and specific inhibition of this kinase by SB 203580 attenuated serum deprivation-induced apoptosis. c-Jun N-terminal kinase 1 activity was high at rest and not modified by phorbol-12-myristate-13-acetate treatment. These data strengthen the idea that protein kinase C is a key modulator of several forms of central neuronal apoptosis, in part acting through inhibition of p38 mitogen-activated protein kinase regulated pathways.