The use of electron paramagnetic resonance in studies of free and bound divalent cation: the measurement of membrane potentials in mitochondria

Impact of in vitro heavy metal exposure on pancreatic β-cell function

Susceptibility to type-2 diabetes mellitus (DM) is determined, in part, by a variety of environmental factors, including exposure to metals. Heavy metals including inorganic arsenic (iAs), zinc (Zn), manganese (Mn), and cadmium (Cd) have been reported to affect glucose homeostasis or DM risk in population-based and/or laboratory studies. Previous evidence from our lab has shown that iAs can increase DM risk by impairing mitochondrial metabolism, one of the key steps in the regulation of glucose-stimulated insulin secretion (GSIS) in pancreatic β-cells. The goal of the current study was to compare the effects of iAs on GSIS and mitochondrial function in INS-1 832/13 β-cells with those of Cd, Mn, and Zn, and to evaluate effects of binary mixtures of these metals. As expected, 24-hour exposure to iAs (arsenite, ≥1 μM) significantly inhibited GSIS as did Cd (5 μM) and Mn (12.5, 25, or 50 μM). Zn had no effects on GSIS at concentrations up to 50 μM. Mitochondrial function was assessed by measuring oxygen consumption rate (OCR) after glucose stimulation and during simulated mitochondrial stress. While both iAs and Mn impaired mitochondrial function (inhibiting OCR, maximal respiration, and/or spare respiratory capacity of mitochondria), no significant effects were found in cells exposed to Cd. Interestingly, no additive or synergistic effects on GSIS or OCR were observed in binary mixtures of iAs with either Mn or Cd. These data suggest that Mn, like iAs, may inhibit GSIS by impairing mitochondrial function, whereas Cd may target other mechanisms that regulate GSIS in β-cells.

An analysis of the effects of Mn2+ on oxidative phosphorylation in liver, brain, and heart mitochondria using state 3 oxidation rate assays

Manganese (Mn) toxicity is partially mediated by reduced ATP production. We have used oxidation rate assays--a measure of ATP production--under rapid phosphorylation conditions to explore sites of Mn(2+) inhibition of ATP production in isolated liver, brain, and heart mitochondria. This approach has several advantages. First, the target tissue for Mn toxicity in the basal ganglia is energetically active and should be studied under rapid phosphorylation conditions. Second, Mn may inhibit metabolic steps which do not affect ATP production rate. This approach allows identification of inhibitions that decrease this rate. Third, mitochondria from different tissues contain different amounts of the components of the metabolic pathways potentially resulting in different patterns of ATP inhibition. Our results indicate that Mn(2+) inhibits ATP production with very different patterns in liver, brain, and heart mitochondria. The primary Mn(2+) inhibition site in liver and heart mitochondria, but not in brain mitochondria, is the F₁F₀ ATP synthase. In mitochondria fueled by either succinate or glutamate+malate, ATP production is much more strongly inhibited in brain than in liver or heart mitochondria; moreover, Mn(2+) inhibits two independent sites in brain mitochondria. The primary site of Mn-induced inhibition of ATP production in brain mitochondria when succinate is substrate is either fumarase or complex II, while the likely site of the primary inhibition when glutamate plus malate are the substrates is either the glutamate/aspartate exchanger or aspartate aminotransferase.

Characteristics and possible functions of mitochondrial Ca(2+) transport mechanisms

Mitochondria produce around 92% of the ATP used in the typical animal cell by oxidative phosphorylation using energy from their electrochemical proton gradient. Intramitochondrial free Ca(2+) concentration ([Ca(2+)](m)) has been found to be an important component of control of the rate of this ATP production. In addition, [Ca(2+)](m) also controls the opening of a large pore in the inner mitochondrial membrane, the permeability transition pore (PTP), which plays a role in mitochondrial control of programmed cell death or apoptosis. Therefore, [Ca(2+)](m) can control whether the cell has sufficient ATP to fulfill its functions and survive or is condemned to death. Ca(2+) is also one of the most important second messengers within the cytosol, signaling changes in cellular response through Ca(2+) pulses or transients. Mitochondria can also sequester Ca(2+) from these transients so as to modify the shape of Ca(2+) signaling transients or control their location within the cell. All of this is controlled by the action of four or five mitochondrial Ca(2+) transport mechanisms and the PTP. The characteristics of these mechanisms of Ca(2+) transport and a discussion of how they might function are described in this paper.

The mechanism of manganese-induced inhibition of steroidogenesis in rat primary Leydig cells

In previous studies in cultured primary rat Leydig cells, manganese was shown to inhibit hCG-stimulated steroidogenesis of Leydig cells, and the data showed that while the inhibition of StAR protein expression and/or function and mitochondrial dysfunction contribute to the acute reduction of steroidogenesis (2 and 4h manganese treatment), the enzyme activities of P450scc and 3beta-HSD were only reduced after 24h manganese treatment, we hypothesize that there were different mechanisms for its effect at later stage (24 and 48 h manganese treatment). We further our study by examining StAR mRNA level in cultured primary rat Leydig cells to understand if inhibition of StAR protein expression occurs at the level of transcription of StAR mRNA. The cellular ATP content was measured to determine the extent that manganese altered mitochondrial function. Since mitochondria are regulators of Ca(2+) homeostasis, and there are indications that manganese affects intracellular Ca(2+) levels, [Ca(2+)]i was also tested. The effects of manganese on Leydig cell apoptosis and cell cycle distribution were studied to see whether these effects contribute to the reduction of steroidogenesis by manganese at later stage of manganese treatment. In the present study, we demonstrated that manganese could increase [Ca(2+)]i and reduced ATP contents in primary Leydig cells after 4h treatment, while the effects on StAR mRNA level appeared later (24h). Manganese could also induce arrest at the G(0)/G(1) phase cell cycle after 24h manganese treatment and subsequently increased in the sub-G(1) phase DNA contents, indicating induction of apoptosis. Combined with our previous studies, the results indicate that inhibition of StAR protein expression and/or function, mitochondrial dysfunction and disturbance of calcium homeostasis contribute to the adverse effects of manganese on the Leydig cells at the early/immediate stage after treatment (2 and 4h). However, at later stages (24 and 48 h) manganese could arrest the cell cycle and induce apoptosis of primary Leydig cells, StAR mRNA and enzyme activities of P450scc and 3beta-HSD were also reduced, leading to reduced level of steroidogenesis in cultured primary Leydig cells.

Changes in the brain mitochondrial proteome of male Sprague-Dawley rats treated with manganese chloride

To probe the mitochondrial involvement in Mn intoxicity, aliquots of brain mitochondria samples from control and treated (30 mg/kg manganese chloride, ip) male Sprague-Dawley rats were separated by two-dimensional polyacrylamide gel electrophoresis (2D-PAGE) and searched for protein abundance changes induced by Mn exposure. The electrophoretic separation resolved over 300 distinct spots as visualized by colloidal Coomassie blue (CCB), of which three spots were induced and three spots were inhibited after Mn exposure in all the five brain mitochondria preparations. Analysis by matrix-assisted laser desorption/ionization time of flight (MALDI-TOF) indicated that these spots are calcium-transporting ATPase type 2C (ATP-dependent Ca(2+) pump PMR1); 60-kDa heat shock protein; Mitochondrial transmembrane GTPase FZO1B; ATP-binding cassette, sub-family b; Long-chain-fatty-acid-CoA ligase; ATP Synthase Beta Chain; and Succinate dehydrogenase flavoprotein subunit. The changes of the mitochondrial ATP synthase beta-subunit and Succinate dehydrogenase flavoprotein subunit indicate an effected level of mitochondrial ATP content and/or ATP-producing capacity. This result provides suggestion that respiratory chain complexes were implicated in the mitochondrial dysfunction induced by Mn intoxicity. And the changes of 60-kDa heat shock protein and ATP-dependent Ca(2+) pump PMR1 expression indicate that the Ca homeostasis and stress effect were involved in Mn intoxicity.

Determination of the oxidation states of manganese in brain, liver, and heart mitochondria

Excess brain manganese can produce toxicity with symptoms that resemble those of Parkinsonism and causes that remain elusive. Manganese accumulates in mitochondria, a major source of superoxide, which can oxidize Mn2+ to the powerful oxidizing agent Mn3+. Oxidation of important cell components by Mn3+ has been suggested as a cause of the toxic effects of manganese. Determining the oxidation states of intramitochondrial manganese could help to identify the dominant mechanism of manganese toxicity. Using X-ray absorbance near edge structure (XANES) spectroscopy, we have characterized the oxidation state of manganese in mitochondria isolated from brain, liver, and heart over concentrations ranging from physiological to pathological. Results showed that (i) spectra from different model manganese complexes of the same oxidation state were similar to each other and different from those of other oxidation states and that the position of the absorption edge increases with oxidation state; (ii) spectra from intramitochondrial manganese in isolated brain, heart and liver mitochondria were virtually identical; and (iii) under these conditions intramitochondrial manganese exists primarily as a combination of Mn2+ complexes. No evidence for Mn3+ was detected in samples containing more than endogenous manganese levels, even after incubation under conditions promoting reactive oxygen species (ROS) production. While the presence of Mn3+ complexes cannot be proven in the spectrum of endogenous mitochondrial manganese, the shape of this spectrum could suggest the presence of Mn3+ near the limit of detection, probably as MnSOD.

Effect of manganese chloride exposure on liver and brain mitochondria function in rats

Manganese (Mn) is an essential trace element found in many enzymes. As is the case for many essential trace elements, excessive Mn is toxic. Individuals suffering from manganese toxicity exhibit several symptoms, which are similar to those frequently observed in cases of Parkinson's disease. In this investigation, we studied the effect of manganese chloride (7.5, 15.0, and 30.0 mg/kg body weight) on mitochondrial function and attempted to ascertain the mechanism of manganese-induced mitochondrial dysfunction. The production of reactive oxygen species in mitochondria of rat liver and brain was assayed using 2',7'-dichlorofluorescin diacetate, and the activities of respiratory chain enzymes were examined spectrophotometrically. Monoamine oxidase (MAO) activity was assayed by measuring reduction of benzylamine. Manganese and calcium content in mitochondria were determined by atomic absorption spectrophotometry. These results indicate that manganese chloride (MnCl2) can decrease MAO activity and inhibit the respiratory chain. Manganese can accumulate in mitochondria and inhibit efflux of calcium. There is a significant inverse correlation between the amount of superoxide radicals and the specific activities of the mitochondria enzymes. Mitochondrial function was significantly affected in both males and females.

Could mitochondrial dysfunction play a role in manganese toxicity?

Individuals suffering from manganese toxicity exhibit several symptoms, including mitochondrial dysfunction, which are similar to those frequently observed in cases of Parkinson's disease. We review the literature concerning manganese toxicity and mitochondrial function, and propose a simple conceptual model of the aetiology of manganese toxicity which involves an interaction between inhibition of mitochondrial energy transduction, generation of free radicals and mutations of the mitochondrial genome. This conceptual model prompts a number of relatively simple experiments which would provide a test of the model.

Evidence for more than one Ca2+ transport mechanism in mitochondria

The active transport and internal binding of the Ca2+ analogue Mn2+ by rat liver mitochondria were monitored with electron paramagnetic resonance. The binding of transported Mn2+ depended strongly on internal pH over the range 7.7-8.9. Gradients of free Mn2+ were compared with K+ gradients measured on valinomycin-treated samples. In the steady state, the electrochemical Mn2+ activity was larger outside than inside the mitochondria. The observed gradients of free Mn2+ and of H+ could not be explained by a single "passive" uniport or antiport mechanism of divalent cation transport. This conclusion was further substantiated by observed changes in steady-state Ca2+ and Mn2+ distributions induced by La3+ and ruthenium red. Ruthenium red reduced total Ca2+ or Mn2+ uptake, and both inhibitors caused release of divalent cation from preloaded mitochondria. A model is proposed in which divalent cations are transported by at least two mechanisms: (1) a passive uniport and (2) and active pump, cation antiport or anion symport. The former is more sensitive to La3+ and ruthenium red. Under energized steady-state conditions, the net flux of Ca2+ or Mn2+ is inward over (1) and outward over (2). The need for more than one transport system inregulating cytoplasmic Ca2+ is discussed.