Mitochondrial redox control of matrix metalloproteinases

Reactive oxygen species (ROS) are constantly generated in aerobic organisms during normal metabolism and in response to both internal and external stimuli. Imbalances in the production and removal of ROS have been hypothesized to play a causative role in numerous disease pathologies such as cancer, ischemia/reperfusion injury, and degenerative diseases such as photoaging, atherosclerosis, arthritis, and neurodegeneration. A feature often associated with these diseases is a malfunctioning of the connective tissue remodeling process due to increased activity of extracellular matrix-degrading metalloproteinases (MMPs). This review summarizes the evidence that implicates ROS as key regulators of MMP production and the importance of these interactions in disease pathologies.     

Mitochondrial redox biology and homeostasis in plants

Mitochondria are key players in plant cell redox homeostasis and signalling. Earlier concepts that regarded mitochondria as secondary to chloroplasts as the powerhouses of photosynthetic cells, with roles in cell proliferation, death and ageing described largely by analogy to animal paradigms, have been replaced by the new philosophy of integrated cellular energy and redox metabolism involving mitochondria and chloroplasts. Thanks to oxygenic photosynthesis, plant mitochondria often operate in an oxygen- and carbohydrate-rich environment. This rather unique environment necessitates extensive flexibility in electron transport pathways and associated NAD(P)-linked enzymes. In this review, mitochondrial redox metabolism is discussed in relation to the integrated cellular energy and redox function that controls plant cell biology and fate.     

Mitochondrial energy metabolism and redox state in dyslipidemias

Changes in mitochondrial function are intimately associated with metabolic diseases. Here, we review recent evidence relating alterations in mitochondrial energy metabolism, ion transport and redox state in hypercholesterolemia and hypertriglyceridemia. We focus mainly on changes in mitochondrial respiration, K(+) and Ca(2+) transport, reactive oxygen species generation and susceptibility to mitochondrial permeability transition.     

Mitochondrial energy metabolism and redox responses to hypertriglyceridemia

In this work we review recent findings that explain how mitochondrial bioenergetic functions and redox state respond to a hyperlipidemic in vivo environment and may contribute to the maintenance of a normal metabolic phenotype. The experimental model utilized to evidence these adaptive mechanisms is especially useful for these studies since it exhibits genetic hypertriglyceridemia and avoids complications introduced by high fat diets. Liver from hypertrigliceridemic (HTG) mice have a greater content of glycerolipids together with increased mitochondrial free fatty acid oxidation. HTG liver mitochondria have a higher resting respiration rate but normal oxidative phosphorylation efficiency. This is achieved by higher activity of the mitochondrial potassium channel sensitive to ATP (mitoK_ATP). The mild uncoupling mediated by mitoK_ATP accelerates respiration rates and reduces reactive oxygen species generation. Although this response is not sufficient to inhibit lipid induced extra-mitochondrial oxidative stress in whole liver cells it avoids amplification of this redox imbalance. Furthermore, higher mitoK_ATP activity increases liver, brain and whole body metabolic rates. These mitochondrial adaptations may explain why these HTG mice do not develop insulin resistance and obesity even under a severe hyperlipidemic state. On the contrary, when long term high fat diets are employed, insulin resistance, fatty liver and obesity develop and mitochondrial adaptations are inefficient to counteract energy and redox imbalances.     

Mitochondrial function and redox state in mammalian embryos

Mitochondria play a central and multifaceted role in the mammalian egg and early embryo, contributing to many different aspects of early development. While the contribution of mitochondria to energy production is fundamental, other roles for mitochondria are starting to emerge. Mitochondria are central to intracellular redox metabolism as they produce reactive oxygen species (ROS, the mediators of oxidative stress) and they can generate TCA cycle intermediates and reducing equivalents that are used in antioxidant defence. A high cytosolic lactate dehydrogenase activity coupled with dynamic levels of cytosolic pyruvate is responsible for a very dynamic intracellular redox state in the oocyte and embryo. Mammalian embryos have a low glucose metabolism during the earliest stages of development, as both glycolysis and the pentose phosphate pathway are suppressed. The mitochondrial TCA cycle is therefore the major source of reducing equivalents in the cytosol so that any change in mitochondrial function in the embryo will be reflected in changes in the intracellular redox state. In the mouse, the metabolic substrates used by the oocyte and early embryo each have a different impact on the intracellular redox state. Pyruvate which oxidises the cytosolic redox state, acts as an energetic and redox substrate whereas lactate, which reduces the cytosolic redox state, acts only as a redox substrate. Mammalian early embryos are very sensitive to oxidative stress which can cause permanent developmental arrest before zygotic genome activation and apoptosis in the blastocyst. The oocyte stockpiles antioxidant defence for the early embryo to cope with exogenous and endogenous oxidant insults arising during early development. Mitochondria provide ATP for glutathione (GSH) production during oocyte maturation and also participate in the regeneration of NADPH and GSH during early development. Finally, a number of pathological conditions or environmental insults impair early development by altering mitochondrial function, illustrating the centrality of mitochondrial function in embryo development.     

Pre-enzymic origin of metabolic redox processes and of the energy storage processes

A treatment of the energetic conditions which regulate the non-equilibrium occurrence of metabolic redox processes associated with degradations of carbon chains or formations of condensed bonds shows that such processes are conditioned by the nature of the substrates and not by the preexistence of enzymic catalysts or cellular ultrastructures. An experimental program has been initiated to spectrophotometrically, electrochemically and chemically detect which redox components appear during model, studies of chemical evolution, operating from simple mixtures of CH₄, NH₃, H₂ and water and from the same mixtures with added phosphorus or sulfur derivatives and metal cations.     

Mitochondrial oxygen affinity as a function of redox and phosphate potenitals

1. The conditions under which mitochondria might catalyse a net reversal of oxidative phosphorylation are analysed.

2. Rat-liver mitochondria, incubated under such conditions, show a strongly diminished affinity for oxygen.

3. The velocity of respiration under these conditions is a hyperbolic function of the oxygen concentration.

4. The K_m for oxygen is less than 0.1 μM at low phosphate potential, irrespective of substrate, and 1–3 μM under reversal conditions.

5. The observed kinetics can be accounted for in a simple mechanism for cytochrome oxidase action.     

Mitochondrial uncoupling protein silence is compromised in photosynthesis and redox poise

Mitochondrial uncoupling proteins play important roles in generation of metabolic thermogenesis, response to stress situations, and regulation of energy metabolism. We demonstrated here that the absence of LeUCP in tomato inhibited photosynthesis using virus-induced gene-silencing approach. A significant decrease in the rate of CO₂ assimilation in LeUCP-silencing plants was observed over a range of different light intensities. Absence of LeUCP resulted in lower net photosynthetic rate, light-saturated rate of the CO₂ assimilation (A _sat), maximum carboxylation rates (V _cmax) and maximum RuBP regeneration rate (J _max). Activities of ribulose-1,5-bisphosphate carboxylase/oxygenase Rubisco and stromal fructose-1,6-bisphosphatase and genes expression levels encoded Calvin cycle enzymes of LeUCP gene-silencing plants were inhibited. However, silencing of LeUCP gene had no effect on F _v/F _m, but decreased photochemical quenching and electron transport rate. Meanwhile, non-photochemical quenching and Je (PSII), the distribution of Je (PCR) and Je (PCO), the content of AsA, NAD, and the ratios of NAD⁺/NADH, AsA/DHA were significantly reduced with increased reactive oxygen species while GSH and GSSG were unaltered. Taken together, these results strongly suggest that LeUCP gene in tomato leaves is crucial in maintaining the redox poise of the mitochondrial electron transport chain to facilitate photosynthetic metabolism.     

Mitochondrial redox systems as central hubs in plant metabolism and signaling

Plant mitochondria are indispensable for plant metabolism and are tightly integrated into cellular homeostasis. This review provides an update on the latest research concerning the organization and operation of plant mitochondrial redox systems, and how they affect cellular metabolism and signaling, plant development, and stress responses. New insights into the organization and operation of mitochondrial energy systems such as the tricarboxylic acid cycle and mitochondrial electron transport chain (mtETC) are discussed. The mtETC produces reactive oxygen and nitrogen species, which can act as signals or lead to cellular damage, and are thus efficiently removed by mitochondrial antioxidant systems, including Mn-superoxide dismutase, ascorbate–glutathione cycle, and thioredoxin-dependent peroxidases. Plant mitochondria are tightly connected with photosynthesis, photorespiration, and cytosolic metabolism, thereby providing redox-balancing. Mitochondrial proteins are targets of extensive post-translational modifications, but their functional significance and how they are added or removed remains unclear. To operate in sync with the whole cell, mitochondria can communicate their functional status via mitochondrial retrograde signaling to change nuclear gene expression, and several recent breakthroughs here are discussed. At a whole organism level, plant mitochondria thus play crucial roles from the first minutes after seed imbibition, supporting meristem activity, growth, and fertility, until senescence of darkened and aged tissue. Finally, plant mitochondria are tightly integrated with cellular and organismal responses to environmental challenges such as drought, salinity, heat, and submergence, but also threats posed by pathogens. Both the major recent advances and outstanding questions are reviewed, which may help future research efforts on plant mitochondria.

Plant mitochondria are key components of redox homeostasis and play vital roles in regulating cellular metabolism, thereby affecting development and stress tolerance at the whole plant level.

Advances

• Improved quantitative MS-based approaches have accelerated the study of mitochondrial protein abundance, turnover and PTMs.
• Mitochondrial enzymes and cellular compartments operate interactively and efficiently exchange substrates.
• Roles for mitochondrial retrograde signaling in plant growth, during physiologically relevant stress conditions and in interaction with other organelles such as the chloroplasts, have been clarified.
• Further insights into mitochondrial antioxidant and peroxidase systems and how they affect other redox systems, enzymes, and whole plant growth have been generated.
• Our understanding of how mitochondria help plants power development and cope with adversity has improved.

     

Mitochondrial redox state as a potential detector of liver dysoxia in vivo

Dysoxia canbe defined as ATP flux decreasing in proportion toO₂ availability with preserved ATPdemand. Hepatic venous -hydroxybutyrate-to-acetoacetate ratio(-OHB/AcAc) estimates liver mitochondrial NADH/NAD and may detectthe onset of dysoxia. During partial dysoxia (as opposed to anoxia),however, flow may be adequate in some liver regions, diluting effluentfrom dysoxic regions, thereby rendering venous -OHB/AcAc unreliable.To address this concern, we estimated tissue ATP whilegradually reducing liver blood flow of swine to zero in a nuclearmagnetic resonance spectrometer. ATP flux decreasing withO₂ availability was taken asO₂ uptake(O₂) decreasing inproportion to O₂ delivery(O₂);and preserved ATP demand was taken as increasingP_i/ATP.O₂, tissueP_i/ATP, and venous -OHB/AcAcwere plotted againstO₂to identify critical inflection points. Tissue dysoxia required meanO₂for the group to be critical for bothO₂ and forP_i/ATP. CriticalO₂values for O₂ andP_i/ATP of 4.07 ± 1.07 and 2.39 ± 1.18 (SE) ml · 100 g¹ · min¹,respectively, were not statistically significantly different but notclearly the same, suggesting the possibility that dysoxia might havecommenced after O₂ begandecreasing, i.e., that there could have been"O₂ conformity." CriticalO₂for venous -OHB/AcAc was 2.44 ± 0.46 ml · 100 g¹ · min¹(P = NS), nearly the same as that forP_i/ATP, supporting venous -OHB/AcAc as a detector of dysoxia. All issues considered, tissue mitochondrial redox state seems to be an appropriate detector ofdysoxia in liver.

     

On compartmentalization

The notion of a compartment is discussed in terms of the Markovian process. From the stochastic matrix (the elements of which are state transition probabilities between different states of a particle of a chemical element), one may find a (generally) nonstochastic matrix; the elements of this second matrix are probabilities that, starting from some initial state, the particle will reach another seleced state (W. Feller, 1962,An Introduction to Probability Theory). Forming equivalence classes of states it can be shown that the equivalence classes based on an equivalence relation, which holds for the elements of the above-mentioned nonstochastic matrix, are essential for the notion of a compartment. From this procedure it is also obvious that a rigorous definition of a physically realizable compartment is impossible. Some conclusions on the practical use of compartmental analysis are drawn.     

Intra- and intermolecular electron transfer processes in redox proteins

Initial velocity and product inhibition experiments were performed to characterize the kinetic mechanism of branched chain ketoacid dehydrogenase (the branched chain complex) activity. The results were directly compared to predicted patterns for a three-site ping-pong mechanism. Product inhibition experiments confirmed that NADH is competitive versus NAD+ and isovaleryl CoA is competitive versus CoA. Furthermore, both NADH and isovaleryl CoA were uncompetitive versus ketoisovaleric acid. These results are consistent with a ping-pong mechanism and are similar to pyruvate dehydrogenase and alpha-ketoglutarate dehydrogenase. However, inhibition patterns for isovaleryl CoA versus NAD+ and NADH versus CoA are not consistent with a ping-pong mechanism. These patterns may result from a steric interaction between the flavoprotein and transacetylase subunits of the complex. To determine the kinetic mechanism of the substrates and feedback inhibitors (NADH and isovaleryl CoA) of the branched chain complex, it was necessary to define the interaction of the inhibitors at nonsaturating fixed substrate (CoA and NAD+) concentrations. While the competitive inhibition patterns were maintained, slope replots for NADH versus NAD+ at nonsaturating CoA concentrations were parabolic. This unexpected finding resembles a linear mixed type of inhibition where the inhibition is a combination of pure competitive and noncompetitive inhibition.     

The carbon cycle and associated redox processes through time

Earth's biogeochemical cycle of carbon delivers both limestones and organic materials to the crust. In numerous, biologically catalysed redox reactions, hydrogen, sulphur, iron, and oxygen serve prominently as electron donors and acceptors. The progress of these reactions can be reconstructed from records of variations in the abundance of 13C in sedimentary carbonate minerals and organic materials. Because the crust is always receiving new CO2 from the mantle and a portion of it is being reduced by photoautotrophs, the carbon cycle has continuously released oxidizing power. Most of it is represented by Fe3+ that has accumulated in the crust or been returned to the mantle via subduction. Less than 3% of the estimated, integrated production of oxidizing power since 3.8 Gyr ago is represented by O2 in the atmosphere and dissolved in seawater. The balance is represented by sulphate. The accumulation of oxidizing power can be estimated from budgets summarizing inputs of mantle carbon and rates of organic-carbon burial, but levels of O2 are only weakly and indirectly coupled to those phenomena and thus to carbon-isotopic records. Elevated abundances of 13C in carbonate minerals ca 2.3 Gyr old, in particular, are here interpreted as indicating the importance of methanogenic bacteria in sediments rather than increased burial of organic carbon.