Top-down control analysis of the cadmium effects on molluscan mitochondria and the mechanisms of cadmium-induced mitochondrial dysfunction

Abstract

-Cadmium (Cd) is a toxic metal and an important environmental pollutant that can strongly affect mitochondrial function and bioenergetics in animals. We investigated the mechanisms of Cd action on mitochondrial function of a marine mollusk (the eastern oyster Crassostrea virginica) by performing a top-down control analysis of the three major mitochondrial subsystems (substrate oxidation, proton leak, and phosphorylation). Our results showed that the substrate oxidation and proton leak subsystems are the main targets for Cd toxicity in oyster mitochondria. Exposure to 12.5 M Cd strongly inhibited the substrate oxidation subsystem and stimulated the proton conductance across the inner mitochondrial membrane. Proton conductance was also elevated and substrate oxidation inhibited by Cd in the presence of a mitochondrially targeted antioxidant, MitoVitE, indicating that Cd effects on these subsystems were to a large extent ROS independent. Cd did not affect the kinetics of the phosphorylation system, indicating that it has negligible effects on F 1, FO ATP synthase and/or the adenine nucleotide transporter in oyster mitochondria. Cd exposure altered the patterns of control over mitochondrial respiration, increasing the degree of control conferred by the substrate oxidation subsystem, especially in resting (state 4) mitochondria. Taken together, these data suggest that Cd-induced decrease of mitochondrial efficiency and ATP production are predominantly driven by the high sensitivity of substrate oxidation and proton leak subsystems to this metal. cadmium; reactive oxygen species; proton leak; substrate oxidation; mitovite TOXIC METALS SUCH AS CADMIUM (Cd) are important contaminants in marine ecosystems. Most (ϳ97%) Cd in the biosphere is derived from human activities, although in some coastal areas Cd levels are naturally elevated due to the leaching of Cdenriched soils or deposition of Cd-hyperaccumulating diatoms (3,28,54). Cd is an extremely toxic metal with no known biological function in animals. Because of its rapid bioaccumulation and long residence time in the organisms, Cd represents an important health concern for humans and wildlife (54 -55).Because of its affinity to thiol groups and nitrogen-containing ligands, Cd can bind to a broad range of biological macromolecules (such as proteins and DNA), damaging their structure and interfering with their functions (5). Mitochondria are key intracellular targets for Cd due to their ability to accumulate Cd and because of the sensitivity of mitochondrial enzymes to Cd-induced damage (14,48,56,73). Because of the central role of mitochondria in critical cellular processes such as bioenergetics, redox signaling, and cell death, Cdinduced mitochondrial damage has long-ranging consequences for cellular function, energy homeostasis, and whole-organism performance and survival. Previous studies have shown that Cd suppresses respiration and leads to the partial uncoupling, as well as elevated rates of reactive oxygen species (ROS) production in animal and plant mitochondr...

Effects of temperature and cadmium exposure on the mitochondria of oysters (Crassostrea virginica) exposed to hypoxia and subsequent reoxygenation

“…Non-mitochondrial respiration and sensor drift were negligible in these experiments (data not shown). Respiratory control ratios (RCR) were calculated by the ratio of state 3 over state 4 respiration rates as described elsewhere (Kurochkin et al, 2011).…”

“…Mitochondrial membrane potentials (Δψ) were determined simultaneously with respiration using a tetraphenyl phosphonium (TPP Jenco Instruments, San Diego, CA) (Kurochkin et al, 2011). It is worth noting that mitochondrial protonmotive force (Δp) consists of the electrical membrane potential (Δψ) and the pH gradient (ΔpH) across the inner mitochondrial membrane.…”

“…The phosphorylation subsystem (including F O , F 1 -ATPase, adenylate and inorganic phosphate transporters) uses ∆p to synthesize ATP, and the proton leak subsystem involves all futile cation cycles that dissipate ∆p without ATP production. Proton leak reactions reflect inherent inefficiency of mitochondria, but can also fulfill important physiological functions such as control of ROS production (Brand, 2000;Miwa and Brand, 2003;Rofle and Brand, 1997 (Chamberlin, 2004b;Dufour et al, 1996), Cd exposure (Kesseler and Brand, 1994a;Kesseler and Brand, 1994b;Kesseler and Brand, 1994c;Kesseler and Brand, 1995;Kurochkin et al, 2011) and developmentally induced apoptosis (Chamberlin, 2004a). However, this approach has not been used to explore how rapid changes in oxygen levels affect the mitochondrial function of animals that often experience transient environmental hypoxia.…”

“…Oysters also can bioaccumulate high levels of trace metals such as Cd rendering them susceptible to their toxic effects (Roesijadi, 1996;Sokolova et al, 2005a). Both temperature and Cd can strongly affect mitochondrial function in oysters decreasing mitochondrial efficiency, negatively affecting ATP synthesis capacity and increasing oxidative stress (Cherkasov et al, 2007a;Cherkasov et al, 2006b;Kurochkin et al, 2011;Sokolova, 2004).…”

“…Due to high sensitivity of mitochondria to temperature and toxic metals (Ivanina et al, 2010a;Ivanina et al, 2011;Ivanina et al, 2010b;Kurochkin et al, 2011;Kurochkin et al, 2009), these stressors can potentially impact the ability of oysters to endure transient hypoxia and successfully recover upon reoxygenation in polluted estuaries; however, such impacts are not yet fully understood. This study investigates the combined effects of transient hypoxia, temperature and Cd stress on mitochondrial capacity and control of mitochondrial function in order to provide insights into the potential mechanisms of stress-induced injury and protection of molluscan mitochondria in the face of multiple environmental stressors.…”