Involvement of caspase-like proteinases in apoptosis of neuronal PC12 cells and primary cultured microglia induced by 6-hydroxydopamine

Differential effects of Bcl-2 overexpression on hippocampal CA1 neurons and dentate granule cells following hypoxic ischemia in adult mice

In contrast to its known anti-apoptotic activity in sympathetic neurons, immortal neuronal cell lines, and primary cultured immature neurons of the central nervous system (CNS), the role of Bcl-2 in CNS neurons in the adult brain is poorly understood. In the present study, we examined effects of overexpression of Bcl-2 on selective neuronal death of the hippocampal CA1 neurons and the dentate granule cells induced by hypoxic ischemia in adult transgenic mice overexpressing human Bcl-2 under the control of neuron-specific enolase (NSE-hbcl-2). At the light microscopic level, numbers of TUNEL-positive cells with pyknotic nuclei were observed in the CA1 subfield of NSE-hbcl-2 transgenic mice, as well as that of wild-type mice, after hypoxic ischemic insult, although the onset of neuronal death was apparently delayed in NSE-hbcl-2 transgenic mice. The electron microscopic studies showed that morphological changes of the degenerating CA1 neurons from both groups were clearly distinct from ordinary apoptosis. In contrast, a significant amount of degenerating dentate granule cells from wild-type but not from transgenic mice had typical apoptotic nuclei by the treatment. The activation of caspase-3 was detected in the dentate granule cells but not that of the CA1 neurons. These results indicate that the overexpression of Bcl-2 effectively suppressed dentate granule cell apoptosis but only delayed cell death of the CA1 neurons induced by hypoxic ischemia, suggesting the occurrence of a non-apoptotic, caspase-3-independent mechanism for neuronal death in the CA1 subfield.

Two distinct mechanisms are involved in 6-hydroxydopamine- and MPP+-induced dopaminergic neuronal cell death: role of caspases, ROS, and JNK

In this study, we examined the possibility that MPTP and 6-hydroxydopamine (6-OHDA) act on distinct cell death pathways in a murine dopaminergic neuronal cell line, MN9D. First, we found that cells treated with 6-OHDA accompanied ultrastructural changes typical of apoptosis, whereas MPP+ treatment induced necrotic manifestations. Proteolytic cleavage of poly-(ADP-ribose)polymerase by caspase was induced by 6-OHDA, whereas it remained uncleaved up to 32 h after MPP+ treatment and subsequently disappeared. Accordingly, 6-OHDA- but not MPP(+)-induced cell death was significantly attenuated in the presence of a broad-spectrum caspase inhibitor, N-benzyloxy-carbonyl-Val-Ala-Asp-fluomethylketone (Z-VAD-fmk). As measured by fluorometric probes, the level of reactive oxygen species (ROS) significantly increased after 6-OHDA treatment. In contrast, the level of dihydroethidium-sensitive ROS following MPP+ treatment remained unchanged while a slight increase in dichlorofluorescin-sentive ROS was temporarily observed. As demonstrated by immunoblot analysis, the level of superoxide dismutase was down-regulated following 6-OHDA treatment, whereas it remained unchanged after MPP+ treatment. Cotreatment of cells with antioxidants such as N-acetylcysteine or Mn(III)tetrakis(4-benzoic acid)porphyrin chloride (MnTBAP, cell-permeable superoxide dismutase mimetic) rescued 6-OHDA- but not MPP(+)-induced cell death, whereas inclusion of catalase or N(G)-nitro-L-arginine had no effect in both cases. In addition, 6-OHDA induced ROS-mediated c-Jun N-terminal kinase (JNK) activation that was attenuated in the presence of N-acetylcysteine or MnTBAP but not catalase or Z-VAD-fmk. In contrast, MPP+ has little effect on JNK activity, indicating that ROS and/or ROS-induced cell death signaling pathway seems to play an essential role in 6-OHDA-mediated apoptosis but not in MPP(+)-induced necrosis in a mesencephalon-derived, dopaminergic neuronal cell line.

Caspase-3-dependent neuronal death in the hippocampus following kainic acid treatment

In this study, we examined the levels of activated caspase-3 in the kainic acid (KA) model of hippocampal degeneration in both sensitive (FVB/N) and resistant (129/SvEMS) strains of mice. At 30 h, 2 and 4 days following KA administration, animals were sacrificed and brains examined for pyknosis, TUNEL labeling, and activated caspase-3 immunoreactivity. Catalytically active caspase-3 was first detected 30 h following KA treatment in the sensitive, FVB/N strain. This was 18 h before the appearance of pyknosis or TUNEL labeling. The expression of activated caspase-3 continues up to 4 days post-injection. No activated caspase-3 immunoreactivity was detected in the resistant, 129/SvEMS strain, neither was there evidence of pyknosis or TUNEL staining. This suggests that activation of caspase-3 is a necessary component of KA-induced cell death.

A predominant apoptotic death pathway of neuronal PC12 cells induced by activated microglia is displaced by a non-apoptotic death pathway following blockage of caspase-3-dependent cascade

Activated microglia have been implicated in the regulation of neuronal cell death. However, the biochemical mechanism for neuronal death triggered by activated microglia is still unclear. When treated with activated microglia, neuronal PC12 cells undergo apoptosis accompanied by caspase-3-like protease activation and DNA fragmentation. Apoptotic bodies formed were subsequently phagocytosed by neighboring activated microglia. Pretreatment of the cells with the caspase-3-like protease inhibitor N-acetyl-Asp-Glu-Val-Asp-aldehyde did not reverse this cell death. Although Bcl-2 overexpression in the cells caused the inhibition of caspase-3-like protease activity and DNA fragmentation and the effective interference of apoptosis induced by deprivation of trophic factors, it could not suppress the activated microglia-induced neuronal death. At the electron microscopic level, degenerating cells with high levels of Bcl-2 were characterized by slightly condensed chromatins forming irregular-shaped masses, severely disintegrated perikarya, and marked vacuolation. Various protease inhibitors tested did not inhibit this cell death, whereas the radical oxygen species scavenger N-acetyl-L-cysteine significantly suppressed this death. Altogether, our study provides an alternative death pathway for the activated microglia-induced neuronal death by blockage of the caspase-3 protease cascade.