Mitochondria and reactive oxygen species: physiology and pathophysiology

Abstract

The air that we breathe contains nearly 21% oxygen, most of which is utilized by mitochondria during respiration. While we cannot live without it, it was perceived as a bane to aerobic organisms due to the generation of reactive oxygen and nitrogen metabolites by mitochondria and other cellular compartments. However, this dogma was challenged when these species were demonstrated to modulate cellular responses through altering signaling pathways. In fact, since this discovery of a dichotomous role of reactive species in immune function and signal transduction, research in this field grew at an exponential pace and the pursuit for mechanisms involved began. Due to a significant number of review articles present on the reactive species mediated cell death, we have focused on emerging novel pathways such as autophagy, signaling and maintenance of the mitochondrial network. Despite its role in several processes, increased reactive species generation has been associated with the origin and pathogenesis of a plethora of diseases. While it is tempting to speculate that anti-oxidant therapy would protect against these disorders, growing evidence suggests that this may not be true. This further supports our belief that these reactive species play a fundamental role in maintenance of cellular and tissue homeostasis.

Figure 1

Mitochondria structure and generation of mitochondrial reactive species. (A) A schematic of a typical mitochondrion is represented. By virtue of its lipid bilayers, the mitochondrion can be subdivided into the outer membrane, inter-membrane space, inner membrane and matrix. The lower panel demonstrates the generation of superoxide anion through the different complexes of the electron transport chain; (B) Amplification of the free radical cycle. Superoxide generated during the electron transport chain can react with nitric oxide to form peroxy nitrite species. Alternatively, superoxide is converted by manganese superoxide dismutase to hydrogen peroxide, which is subsequently converted to water by glutathione peroxidase. In the presence of iron, hydrogen peroxide is rapidly converted to the highly reactive hydroxyl ion.

Figure 2

Autophagy process and regulation by ROS. (A) Autophagy begins with vesicle nucleation where the damaged organelles (mitochondria) are sequestered to form an autophagosome. This vesicle fuses with the lysosome to form an autolysosome where the contents are degraded by lysosomal hydrolases and nutrients are recycled to the cytoplasm; (B) ROS can regulate autophagy in two ways: direct and indirect. Direct regulation involves modification of key proteins involved in the autophagy process including Atg4, Atg5 and Beclin. Indirect regulation by ROS involves alteration of signaling pathways such as JNK, p38 that can induce autophagy. On the other hand, ROS may inhibit Akt signaling and downstream mTOR and thereby induce autophagy.

Figure 3

Changes in mitochondrial structure following hydrogen peroxide treatment. (Top panels) Transmission electron micrographs of mitochondria in untreated (A) and hydrogen peroxide treated cells (B,C,D). Electron dense granules (arrows) and fragmented mitochondria were observed. Bar = 1 μm. (Bottom panels) Mitochondria were stained with Mitotracker Red and existed as tubular (A), intermediate (B) (tubular with swollen regions) and fragmented (C). Bar = 10 μm.

Figure 3

Changes in mitochondrial structure following hydrogen peroxide treatment. (Top panels) Transmission electron micrographs of mitochondria in untreated (A) and hydrogen peroxide treated cells (B,C,D). Electron dense granules (arrows) and fragmented mitochondria were observed. Bar = 1 μm. (Bottom panels) Mitochondria were stained with Mitotracker Red and existed as tubular (A), intermediate (B) (tubular with swollen regions) and fragmented (C). Bar = 10 μm.

Figure 4

NO^• triggers mitochondrial fission. (A) 3D time-lapse microscopy of mitochondria undergoing fission in a dendritic arbor of a neuron. Neurons were transfected with Mito-DsRed2, pretreated with the pan-caspase inhibitor zVAD-fmk methyl ester (100 M), and exposed to SNOC (200 M). Images were 3D iso-surface rendered. Frames depict representative time points of the movie demonstrating mitochondrial fragmentation within 3 h of NO^• exposure (upper panels; scale bar, 15 μm) and closeup views (lower panels; scale bar = 3 μm).