Mechanism of mitochondrial transport of thallous ions

Mitochondrial Oxidative Stress Is the General Reason for Apoptosis Induced by Different-Valence Heavy Metals in Cells and Mitochondria

This review analyzes the causes and consequences of apoptosis resulting from oxidative stress that occurs in mitochondria and cells exposed to the toxic effects of different-valence heavy metals (Ag+, Tl+, Hg2+, Cd2+, Pb2+, Al3+, Ga3+, In3+, As3+, Sb3+, Cr6+, and U6+). The problems of the relationship between the integration of these toxic metals into molecular mechanisms with the subsequent development of pathophysiological processes and the appearance of diseases caused by the accumulation of these metals in the body are also addressed in this review. Such apoptosis is characterized by a reduction in cell viability, the activation of caspase-3 and caspase-9, the expression of pro-apoptotic genes (Bax and Bcl-2), and the activation of protein kinases (ERK, JNK, p53, and p38) by mitogens. Moreover, the oxidative stress manifests as the mitochondrial permeability transition pore (MPTP) opening, mitochondrial swelling, an increase in the production of reactive oxygen species (ROS) and H2O2, lipid peroxidation, cytochrome c release, a decline in the inner mitochondrial membrane potential (ΔΨmito), a decrease in ATP synthesis, and reduced glutathione and oxygen consumption as well as cytoplasm and matrix calcium overload due to Ca2+ release from the endoplasmic reticulum (ER). The apoptosis and respiratory dysfunction induced by these metals are discussed regarding their interaction with cellular and mitochondrial thiol groups and Fe2+ metabolism disturbance. Similarities and differences in the toxic effects of Tl+ from those of other heavy metals under review are discussed. Similarities may be due to the increase in the cytoplasmic calcium concentration induced by Tl+ and these metals. One difference discussed is the failure to decrease Tl+ toxicity through metallothionein-dependent mechanisms. Another difference could be the decrease in reduced glutathione in the matrix due to the reversible oxidation of Tl+ to Tl3+ near the centers of ROS generation in the respiratory chain. The latter may explain why thallium toxicity to humans turned out to be higher than the toxicity of mercury, lead, cadmium, copper, and zinc.

Effects of Tl+ on the inner membrane thiol groups, respiration, and swelling in succinate-energized rat liver mitochondria were modified by thiol reagents

The effects of both Tl⁺ and thiol reagents were studied on the content of the inner membrane free SH-groups, detected with Ellman reagent, and the inner membrane potential as well as swelling and respiration of succinate-energized rat liver mitochondria in medium containing TlNO₃ and KNO₃. These effects resulted in a rise in swelling and a decrease in the content, the potential, and mitochondrial respiration in 3 and 2,4-dinitrophenol-uncoupled states. A maximal effect was seen when phenylarsine oxide reacting with thiol groups recessed into the hydrophobic regions of the membrane. Compared with phenylarsine oxide, the effective concentrations of other reagents were approximately one order of magnitude higher in experiments with mersalyl and 4,4'-diisothiocyanostilbene-2,2'-disulfonate, and two orders of magnitude higher in experiments with tert-butyl hydroperoxide and diamide. The above effects of Tl⁺ and the thiol reagents became even more pronounced with calcium overload of mitochondria. However, the effects were suppressed by inhibitors of the mitochondrial permeability transition pore (cyclosporine A, ADP, and n-ethylmaleimide). These findings suggest that opening of the pore induced by Tl⁺ in the inner membrane can be dependent on the conformation state of the adenine nucleotide translocase, which depends on the activity of its thiol groups.

Mersalyl prevents the Tl+-induced permeability transition pore opening in the inner membrane of Ca2+-loaded rat liver mitochondria

It was earlier shown that the calcium load of rat liver mitochondria in medium containing TlNO₃ and KNO₃ resulted in the Tl⁺-induced mitochondrial permeability transition pore (MPTP) opening in the inner membrane. This opening was accompanied by an increase in swelling and membrane potential dissipation and a decrease in state 3, state 4, and 2,4-dinitrophenol-uncoupled respiration. This respiratory decrease was markedly leveled by mersalyl (MSL), the phosphate symporter (PiC) inhibitor which poorly stimulated the calcium-induced swelling, but further increased the potential dissipation. All of these effects of Ca²⁺ and MSL were visibly reduced in the presence of the MPTP inhibitors (ADP, N-ethylmaleimide, and cyclosporine A). High MSL concentrations attenuated the ability of ADP to inhibit the MPTP. Our data suggest that the PiC can participate in the Tl⁺-induced MPTP opening in the inner membrane of Ca²⁺-loaded rat liver mitochondria.

To involvement the conformation of the adenine nucleotide translocase in opening the Tl(+)-induced permeability transition pore in Ca(2+)-loaded rat liver mitochondria

The conformation of adenine nucleotide translocase (ANT) has a profound impact in opening the mitochondrial permeability transition pore (MPTP) in the inner membrane. Fixing the ANT in 'c' conformation by phenylarsine oxide (PAO), tert-butylhydroperoxide (tBHP), and carboxyatractyloside as well as the interaction of 4,4'-diisothiocyanostilbene-2,2'-disulfonate (DIDS) with mitochondrial thiols markedly attenuated the ability of ADP to inhibit the MPTP opening. We earlier found (Korotkov and Saris, 2011) that calcium load of rat liver mitochondria in medium containing TlNO3 and KNO3 stimulated the Tl(+)-induced MPTP opening in the inner mitochondrial membrane. The MPTP opening as well as followed increase in swelling, a drop in membrane potential (ΔΨmito), and a decrease in state 3, state 4, and 2,4-dinitrophenol-uncoupled respiration were visibly enhanced in the presence of PAO, tBHP, DIDS, and carboxyatractyloside. However, these effects were markedly inhibited by ADP and membrane-penetrant hydrophobic thiol reagent, N-ethylmaleimide (NEM) which fix the ANT in 'm' conformation. Cyclosporine A additionally potentiated these effects of ADP and NEM. Our data suggest that conformational changes of the ANT may be directly involved in the opening of the Tl(+)-induced MPTP in the inner membrane of Ca(2+)-loaded rat liver mitochondria. Using the Tl(+)-induced MPTP model is discussed in terms finding new transition pore inhibitors and inducers among different chemical and natural compounds.

Diamide accelerates opening of the Tl(+)-induced permeability transition pore in Ca(2+)-loaded rat liver mitochondria

Opening of the mitochondrial permeability transition pore (MPTP) in the inner membrane is due to matrix Ca(2+) overload and matrix glutathione loss. Fixing the 'm' conformation of the adenine nucleotide translocase (ANT) by ADP or N-ethylmaleimide (NEM) inhibits opening of the MPTP. Oxidants (diamide or tert-butylhydroperoxide (tBHP)) fix the ANT in 'c' conformation, and the ability of ADP to inhibit the MPTP is thus attenuated. Earlier we found (Korotkov and Saris, 2011) that calcium load of rat liver mitochondria resulted in Tl(+)-induced MPTP opening, which was accompanied by a decrease in state 3, state 4, and 2,4-dinitrophenol-uncoupled respiration, as well as increased swelling and membrane potential dissipation. These effects, which were increased by diamide and tBHP, were visibly reduced in the presence of the MPTP inhibitors (ADP, NEM, and cyclosporine A). Our data suggest that conformational changes of the ANT and matrix glutathione loss may be directly involved in opening the Tl(+)-induced MPTP in the inner membrane of Ca(2+)-loaded rat liver mitochondria.

Tl+ induces the permeability transition pore in Ca2+-loaded rat liver mitochondria energized by glutamate and malate

It is known that Ca2+ and heavy metals more actively induce MPTP opening in mitochondria, energized by the I complex substrates. Thus, a rise in a Tl+-induced MPTP was proposed in experiments on isolated rat liver mitochondria energized by the complex I substrate (glutamate and malate). Expose of the mitochondria to Ca2+ into a medium containing TlNO3, glutamate, and malate as well as sucrose or KNO3 resulted in a decrease in state 3, state 4, or DNP-stimulated respiration as well as an increase of both mitochondrial swelling and ΔΨmito dissipation. The MPTP inhibitors, CsA and ADP, almost completely eliminated the effect of Ca2+, which was more pronounced in the presence of the complex I substrates than the complex II substrate (succinate) and rotenone (Korotkov and Saris, 2011). The present study concludes that Tl+-induced MPTP opening is more appreciable in mitochondria energized by glutamate and malate but not succinate in the presence of rotenone. We assume that the Tl+-induced MPTP opening along with followed swelling and possible structural deformations of the complex I in Ca2+-loaded mitochondria may be a part of the thallium toxicity mechanism on mitochondria in living organisms. At the same time, oxidation of Tl+ to Tl3+ by mitochondrial oxygen reactive species is proposed for the mechanism.

Y3+, La3+, and some bivalent metals inhibited the opening of the Tl+-induced permeability transition pore in Ca2+-loaded rat liver mitochondria

We showed earlier that diminution of 2,4-dinitrophenol (DNP)-stimulated respiration and increase of both mitochondrial swelling and electrochemical potential (ΔΨmito) dissipation in medium containing TlNO3 and KNO3 were caused by opening of Tl(+)-induced mitochondrial permeability transition pore (MPTP) in the inner membrane of Ca(2+)-loaded rat liver mitochondria. The MPTP opening was studied in the presence of bivalent metal ions (Sr(2+), Ba(2+), Mn(2+), Co(2+) and Ni(2+)), trivalent metal ions (Y(3+) and La(3+)), and ruthenium red. We found that these metal ions (except Ba(2+) and Co(2+)) as well as ruthenium red inhibited to the MPTP opening that manifested in preventing both diminution of the DNP-stimulated respiration and increase of the swelling and of the ΔΨmito dissipation in medium containing TlNO3, KNO3, and Ca(2+). Inhibition of the MPTP opening by Sr(2+) and Mn(2+) is suggested because of their interaction with high affinity Ca(2+) sites, facing the matrix side and participating in the MPTP opening. The inhibitory effects of metal ions (Y(3+), La(3+), and Ni(2+)), and ruthenium red are accordingly discussed in regard to competitive and noncompetitive inhibition of the mitochondrial Ca(2+)-uniporter. High concentrations (50μM) of Y(3+) and La(3+) favored of MPTP opening in the inner membrane of rat liver mitochondria in Ca(2+) free medium containing TlNO3. The latter MPTP opening was markedly eliminated by MPTP inhibitors (cyclosporine A and ADP).

Tl(+) showed negligible interaction with inner membrane sulfhydryl groups of rat liver mitochondria, but formed complexes with matrix proteins

The effects of Tl(+) on protein sulfhydryl (SH) groups, swelling, and respiration of rat liver mitochondria (RLM) were studied in a medium containing TlNO3 and sucrose, or TlNO3 and KNO3 as well as glutamate plus malate, or succinate plus rotenone. Detected with Ellman's reagent, an increase in the content of the SH groups was found in the inner membrane fraction, and a simultaneous decline was found in the content of the matrix-soluble fraction for RLM, incubated and frozen in 25-75 mM TlNO3 . This increase was greater in the medium containing KNO3 regardless of the presence of Ca(2+) . It was eliminated completely for RLM injected in the medium containing TlNO3 and then washed and frozen in the medium containing KNO3 . Calcium-loaded RLM showed increased swelling and decreased respiration. These results suggest that a ligand interaction of Tl(+) with protein SH groups, regardless of the presence of calcium, may underlie the mechanism of thallium toxicity.

Influence of Tl(+) on mitochondrial permeability transition pore in Ca(2+)-loaded rat liver mitochondria

The Tl(+)-induced opening of the MPTP in Ca(2+)-loaded rat liver mitochondria energized by respiration on the substrates succinate or glutamate plus malate was recorded as increased swelling and dissipation of mitochondrial membrane potential as well as decreased state 4, or state 3, or 2,4-dinitrophenol-stimulated respiration. These effects of Tl(+) increased in nitrate media containing monovalent cations in the order of Li(+) < NH (4) (+) ≤ Na(+) < K(+). They were potentiated by inorganic phosphate and diminished by the MPTP inhibitors (ADP, CsA, Mg(2+), Li(+), rotenone, EGTA, and ruthenium red) both individually and more potently in their combinations. Maximal swelling of both non-energized and energized Ca(2+)-loaded mitochondria in rotenone-free media is an indication of Ca(2+) uptake driven by respiration on mitochondrial endogenous substrates. It is suggested that Tl(+) (distinct from Cd(2+), Hg(2+), and other heavy metals and regardless of the used respiratory substrates) can stimulate opening of the MPTP only in the presence of Ca(2+). We discuss the possible participation of Ca(2+)-binding sites, located near the respiratory complex I and the adenine nucleotide translocase, in inducing opening of the MPTP.

Increase in the toxic effects of Tl+ on isolated rat liver mitochondria in the presence of nonactin

The effects of Tl(+) ions on isolated rat liver mitochondria were studied in the presence of nonactin, a cyclic ionophore. Nonenergized rat liver mitochondria were increasingly swollen at an elevated concentration of Tl(+) in the 160 mOsm medium containing 0-150 mM sucrose and 0-75 mM TlNO(3) or 0-50 mM Tl acetate. On the contrary, mitochondria in experiments with nonactin were contracted in the medium with 5-25 mM Tl(+) and were swollen only in the medium with 50-75 mM TlNO(3) or 50 mM Tl acetate. State 4 respiration along with swelling of succinate-energized mitochondria followed contraction after their deenergization was further enhanced at increasing concentration of Tl acetate in a medium containing nonactin. Regardless of the presence of nonactin, State 3 and 2,4-dinitrophenol (DNP)-stimulated respiration and the monoamine oxidase (MAO) activity were not affected in the medium with 0-25 mM Tl acetate and sucrose. DNP-stimulated respiration decreased and the MAO activity somewhat increased in the medium containing 50 mM Tl acetate and nonactin. Uptake of (86)Rb(+) by energized mitochondria in the presence of valinomycin was considerably decreased when Tl(+) and nonactin were simultaneously present in the medium. An increase of the toxic effect of Tl(+) on rat liver mitochondria in the presence of nonactin is accounted for by disruption of mitochondria due to their more extensive swelling and uncoupling of mitochondria, resulting in the stimulation of State 4 and depletion of their energy store.

Thallium Induces Apoptosis in Jurkat Cells

Thallium induces swelling in mitochondria and apoptosis in Jurkat cells. Both the swelling and the apoptosis are inhibited by means of cyclosporine A (CsA), thus suggesting that these phenomena result from the opening of the membrane transition pore (MTP) in mitochondria. Therefore, apoptosis can be explained as due to the opening of the MTP in mitochondria.

Subcellular distribution of thallium: morphological and quantitative study in rat myocardium

The purpose of this study is to determine the subcellular distribution of thallium (SDTl) by electron microscopy and a newly designed fixation method that makes insoluble grains of Tl visible.

METHODS

To obtain the high dose necessary for electron microscopic visualization, we employed TlCl instead of 201TlCl. EM was performed in fixed rat myocardium resected at 20 min (early phase) and 3 hr (delay phase) after intravenous injection of TlCl. To fix Tl in the cell, we used orthovanadate in our fixative. Atomic absorption spectroscopy (AAS) of Tl and quantification of subcellular distribution of 201Tl (SD201Tl) were studied to prove the propriety of our fixation.

RESULTS

AAS detected Tl in the Tl-loaded specimen but not in the control, indicating that Tl was the origin of the grains observed in the former. In the early phase, numerous grains were observed in mitochondria, sarcoplasmic reticulum (SR), myofibrils, and nuclei, but no such grains were visible in controls. In the delay phase, grains were retained in mitochondria, SR and nuclei, but not in myofibrils. Electron microscopic SDTl(%) correlated with SD201Tl(%) calculated from isolated fractions.

CONCLUSION

In both the early and delay phases, mitochondria are the major site of Tl and 201Tl uptake.

Magnesium transport in murine S49 lymphoma cells: pharmacology and divalent cation selectivity

The murine S49 lymphoma cell transports Mg2+ by a system distinct from systems responsible for Ca2+ influx (J. Physiol. London 337: 351-371, 1983). We have now determined the ability of various cations, anions, and drugs to modulate Mg2+ influx. Neither sulfate, nitrate, phosphate, nor bicarbonate altered Mg2+ influx. Among cations only T1+, Ba2+, Zn2+, Mn2+, Sc3+, and La3+ potently inhibited Mg2+ influx without causing obvious cell toxicity. Seventeen other cations were ineffective at maximal nontoxic concentrations. T1+ inhibition (Ki = 300 micron) is noncompetitive and apparently derives from its ability to dissipate membrane potential. The noncompetitive nature of and the rather poor inhibition constants for Ca2+ (Ki approximately equal to 5 mM) and Mn2+ (Ki = 200 micron) indicate that neither cation is an effective physiological antagonist of Mg2+ influx. Only Ba2+ exhibited competitive inhibition of Mg2+ influx (Ki = 1 mM). Cisplatin and Ca2+ channel antagonists also did not inhibit Mg2+ influx. These data further differentiate Mg2+ transport systems from those for Ca2+. In addition, the selectivity series for group IIa cation inhibition of influx (Mg2+ greater than Ba2+ much greater than Ca2+ greater than or equal to Sr2+) has not been observed previously in biological systems and is indicative of a very high anionic field strength at the Mg2+ recognition site.

The action of thallium acetate on spontaneous transmitter release in the rat neuromuscular junction

Frequencies and amplitudes of miniature endplate potentials (MEPP's) were recorded from neuromuscular junctions of the rat phrenic nerve-diaphragm preparation in vitro. Superfusion of the preparations with Ringer solution containing thallium acetate (Tlac) gradually increased the frequency of MEPP's by a factor of 10 within 30 min (1 X 10(-3) mol/l Tlac) and 180 min (5 X 10(-4) mol/l Tlac), whereas the amplitude of MEPP's remained unchanged. Frequency of MEPP's fitted a Poisson-distribution which persisted during superfusion with Tl+-Ringer. Sub-MEPP amplitudes remained unaffected by the action of thallium. It is concluded that thallium interferes presynaptically with spontaneous transmitter release.

Bidirectional axonal transport of thallium in frog sciatic nerve

Bidirectional axonal transport of radioactivity was demonstrated in frog sciatic nerve in vitro and in vivo after local application of the thallium isotope 204Tl+ to the nerve. The transport rate was similar in both directions and about 30 min/day at 18 degrees C. The transport was depressed by 2,4-DNP, low-temperature and vinblastine. A somewhat larger amount of radioactivity was transported in the retrograde than in the anterograde direction. The possibility that K+ is in part replaced by Tl+, which is transported bound to organelles, e.g. mitochondria, will be considered. Thallium might be a useful tool for future studies of axonal transport.