Oxidative stress and mitochondrial dysfunction as determinants of ischemic neuronal death and survival

Abstract

Mitochondria are the powerhouse of the cell. Their primary physiological function is to generate adenosine triphosphate through oxidative phosphorylation via the electron transport chain. Reactive oxygen species generated from mitochondria have been implicated in acute brain injuries such as stroke and neurodegeneration. Recent studies have shown that mitochondrially-formed oxidants are mediators of molecular signaling, which is implicated in the mitochondria-dependent apoptotic pathway that involves pro- and antiapoptotic protein binding, the release of cytochrome c, and transcription-independent p53 signaling, leading to neuronal death. Oxidative stress and the redox state of ischemic neurons are also implicated in the signaling pathway that involves phosphatidylinositol 3-kinase/Akt and downstream signaling, which lead to neuronal survival. Genetically modified mice or rats that over-express or are deficient in superoxide dismutase have provided strong evidence in support of the role of mitochondrial dysfunction and oxidative stress as determinants of neuronal death/survival after stroke and neurodegeneration.

Fig. 1

Involvement of p53 signaling after ROS generation. After ROS generation from mitochondria, p53 transcriptionally generates pro-apoptotic proteins such as Bax, Noxa, p53AIP1, PUMA, and Bid. These products act directly on mitochondria. Mitochondrial translocation of Bax is promoted by JNK through transcriptional activation of Bim. Full-length PIDD (PIDD-FL) is also transcriptionally upregulated by p53. PIDD-CC, a fragment of PIDD-FL cleaved by autoproteolysis, activates caspase-2 through the formation of the PIDDosome, which precedes Bid truncation and translocation to mitochondria. Moreover, p53 translocates to the mitochondrial membrane and activates the mitochondria-dependent apoptotic pathway in a transcription-independent manner. BH3-only proteins and p53 interact with both pro-apoptotic Bax and anti-apoptotic Bcl-X_L on the mitochondrial membrane. This interaction causes Bax oligomerization and activation, which triggers cytochrome c release, leading to neuronal death. tBid, truncated Bid.

Fig. 2

Mitochondrial PUMA upregulation after tGCI. (A) Western blot analysis shows that mitochondrial PUMA increased 4 and 24 h after tGCI, followed by cytosolic upregulation of cleaved caspase-9 and cytochrome c release. β-actin and cytochrome oxidase subunit IV (COX IV) analyses are shown as internal controls. c, control. (B) Coimmunoprecipitation analyses show that PUMA immunoreactivity precipitated by Bcl-X_L or Bax increased after tGCI. Bcl-X_L precipitated by Bcl-X_L, and Bax precipitated by Bax were used to show equal precipitation. IP, immunoprecipitation; IB, immunoblotting. (Data modified from Niizuma et al. 2009.)