Permeability of the mitochondrial membrane to bicarbonate ions

Bicarbonate suppresses mitochondrial membrane depolarization induced by conventional uncouplers

Bicarbonate has been known to modulate activities of various mitochondrial enzymes such as ATPase and soluble adenylyl cyclase. Here, we found that the ability of conventional protonophoric uncouplers, such as 2,4-dinitrophenol (DNP), carbonylcyanide p-trifluoromethoxyphenylhydrazone (FCCP) and carbonyl cyanide m-chlorophenyl hydrazone (CCCP), but not that of the new popular uncoupler BAM15, to decrease mitochondrial membrane potential was significantly diminished in the presence of millimolar concentrations of bicarbonate. Thus, the depolarizing activity of DNP and FCCP in mitochondria could be sensitive to the local concentration of bicarbonate in cells and tissues. However, bicarbonate could not restore the ATP synthesis suppressed by DNP or CCCP in mitochondria. Bicarbonate neither altered the depolarizing action of DNP and FCCP on proteoliposomes with reconstituted cytochrome c oxidase, nor affected the protonophoric activity of DNP and FCCP in artificial lipid membranes as measured with pyranine-loaded liposomes, thereby showing that the bicarbonate-induced reversal of the depolarizing action of DNP and FCCP on mitochondria did not result from direct interaction of bicarbonate with the uncouplers.

New tools for an old question: dependence of ATP and bicarbonate for branched-chain keto acids oxidation

Branched-chain keto acids (BCKA) metabolism involves several well-regulated steps within mitochondria, requires cofactors, and is modulated according to the metabolic status of the cells. This regulation has made it challenging to utilize in vitro approaches to determine the contribution of branched-chain amino acid oxidation to energy production. These methodological issues were elegantly addressed in a recent publication within the Biochemical Journal. In this issue, Goldberg et al. [Biochem. J. (2019) 476, 1521-1537] demonstrated in a well-designed system the dependence of ATP and bicarbonate for BCKA full oxidation. In addition, the utilized system allowed the authors to characterize specific biochemical routes within mitochondria for each BCKA. Among them, a quantitative analysis of the participation of BCKA on mitochondrial flux was estimated between tissues. These findings are milestones with meaningful impact in several fields of metabolism.

Mitochondrial channels: ion fluxes and more

The field of mitochondrial ion channels has recently seen substantial progress, including the molecular identification of some of the channels. An integrative approach using genetics, electrophysiology, pharmacology, and cell biology to clarify the roles of these channels has thus become possible. It is by now clear that many of these channels are important for energy supply by the mitochondria and have a major impact on the fate of the entire cell as well. The purpose of this review is to provide an up-to-date overview of the electrophysiological properties, molecular identity, and pathophysiological functions of the mitochondrial ion channels studied so far and to highlight possible therapeutic perspectives based on current information.

SLC4A7 sodium bicarbonate co-transporter controls mitochondrial apoptosis in ischaemic coronary endothelial cells

AIMS

Bicarbonate transport has been shown to participate in apoptosis under ischaemic stress. However, the precise transporting mechanisms involved in ischaemic apoptosis are unknown and were thus the aim of the present study.

METHODS AND RESULTS

Rat coronary endothelial cells (EC) were exposed to simulated in vitro ischaemia for 2 h, and apoptosis was subsequently determined by chromatin staining and caspase-3 activity analysis. By examining the expression of bicarbonate transporters (BT) in EC by reverse transcriptase polymerase chain reaction and western blotting, a marked expression of the electroneutral sodium bicarbonate co-transporter (SLC4A7) was defined. To analyse the potential role of this transporter during apoptosis, a selective inhibitor (S0859, Sanofi-Aventis) was applied. Treatment with S0859 significantly increased caspase-3 activity and elevated the number of apoptotic EC. These results were comparable with an unselective inhibition of all BT due to withdrawal of bicarbonate in the anoxic medium. Knockdown of SLC4A7 in EC by transfecting appropriate siRNA similarly increased apoptosis of EC under simulated ischaemia. The initial characterization of the participating mechanisms of SLC4A7-dependent apoptosis revealed an activation of the mitochondrial pathway of apoptosis, i.e. cleavage of caspase-9 and binding of Bax to mitochondria. In contrast, no activation of the endoplasmic reticulum-dependent pathway (caspase-12 cleavage) or the extrinsic apoptotic pathway (caspase-8 cleavage) was found. Finally, a mitochondrial localization of SLC4A7 was demonstrated.

CONCLUSION

The electroneutral sodium bicarbonate co-transporter SLC4A7 localizes in mitochondria and suppresses the ischaemia-induced activation of the mitochondrial pathway of apoptosis in coronary EC.

Importance of bicarbonate transport for ischaemia-induced apoptosis of coronary endothelial cells

Bicarbonate transport (BT) has been previously shown to participate in apoptosis induced by various stress factors. However, the precise role of BT in ischaemia-induced apoptosis is still unknown. To investigate this subject, rat coronary endothelial cells (EC) were exposed to simulated ischaemia (glucose free anoxia at Ph 6.4) for 2 hrs and cells undergoing apoptosis were visualized by nuclear staining or by determination of cas-pase- 3 activity. To inhibit BT, EC were either treated with the inhibitor of BT 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS, 300 mumol/l) or exposed to ischaemia in bicarbonate free, 4-(2-hydroxyethyl)-I-piperazi-neethanesulphonic acid (HEPES)-buffered medium. Simulated ischaemia in bicarbonate-buffered medium (Bic) increased caspase-3 activity and the number of apoptotic cell (23.7 + 1.4%versus 5.1 + 1.2% in control). Omission of bicarbonate during ischaemia further significantly increased caspase-3 activity and the number of apoptotic cells (36.7 1.7%). Similar proapoptotic effect was produced by DIDS treatment during ischaemia in Bic, whereas DIDS had no effect when applied in bicarbonate-free, HEPES-buffered medium (Hep). Inhibition of BT was without influence on cytosolic acidification during ischaemia and slightly reduced cytosolic Ca(2+) accumulation. Initial characterization of the underlying mechanism leading to apoptosis induced by BT inhibition revealed activation of the mitochondrial pathway of apoptosis, i.e., increase of cytochrome C release, depolarization of mitochondria and translocation of Bax protein to mitochondria. In contrast, no activation of death receptor-dependent pathway (caspase-8 cleavage) and endoplasmic reticulum- dependent pathway (caspase-12 cleavage) was detected. In conclusion, BT plays an important role in ischaemia-induced apoptosis of coronary EC by suppression of mitochondria-dependent apoptotic pathway.

An anaplerotic role for mitochondrial carbonic anhydrase in Chlamydomonas reinhardtii

Previous studies of the mitochondrial carbonic anhydrase (mtCA) of Chlamydomonas reinhardtii showed that expression of the two genes encoding this enzyme activity required photosynthetically active radiation and a low CO(2) concentration. These studies suggested that the mtCA was involved in the inorganic carbon-concentrating mechanism. We have now shown that the expression of the mtCA at low CO(2) concentrations decreases when the external NH(4)(+) concentration decreases, to the point of being undetectable when NH(4)(+) supply restricts the rate of photoautotrophic growth. The expression of mtCA can also be induced at supra-atmospheric partial pressure of CO(2) by increasing the NH(4)(+) concentration in the growth medium. Conditions that favor mtCA expression usually also stimulate anaplerosis. We therefore propose that the mtCA is involved in supplying HCO(3)(-) for anaplerotic assimilation catalyzed by phosphoenolpyruvate carboxylase, which provides C skeletons for N assimilation under some circumstances.

Electrophysiology of the inner mitochondrial membrane

The application of electrophysiological techniques to mitochondrial membranes has allowed the observation and partial characterization of several ion channels, including an ATP-sensitive K(+)-selective one, a high-conductance "megachannel", a 107 pS anionic channel and three others studied at alkaline pH's. A reliable correlation with the results of non-electrophysiological studies has been obtained so far only for the first two cases. Activities presumed to be associated with the Ca2+ uniporter and with the adenine nucleotide translocator, as well as the presence of various other conductances have also been reported. The review summarizes the main properties of these pores and their possible relationship to permeation pathways identified in biochemical studies.

Aroclor 1254 treatment and fasting influences on rat liver mitochondrial carbamoyl phosphate synthesis with ADP and ATP

Previous studies have shown that the polychlorinated biphenyl mixture, Aroclor 1254 (ARO), -induced wasting in male rats is associated with increased permeability of hepatic mitochondria. This was correlated with hyperuremia and stimulated urea synthesis, hypoglycemia and suppressed glucogenesis after an ammonium acetate injection, and decreased retention of assimilated nitrogen and food intake. For ARO-toxic rats (100 mg/kg, ip, for 1, 2, and 4 days) versus Tween 80-treated, ad libitum-fed controls, mitochondrial carbamoyl phosphate (CP) formation (the initial step in urea synthesis from NH4+) was progressively stimulated for the duration of treatment from NH4+ and ATP but not from NH4+ and ADP. ARO maximal stimulation of CP formation also correlated with significant loss in body weight. Mitochondrial ornithine transcarbamoylase synthesis of citrulline from ornithine and carbamoyl phosphate was also stimulated. In comparison to fasted rats (24 hr), mitochondrial CP synthesis from NH4+ was enhanced with ADP but not with ATP. This ARO uncoupling of mitochondrial NH4+ metabolism and stimulation of CP formation with exogenous ATP and citrulline synthesis may have resulted from increased availability of substrates and cofactors in the matrix space, leakage of enzymes from the matrix, or a combination of these effects. These results are consistent with an increased inner membrane permeability and fragility during isolation and assays. In agreement with our previous studies, the data show that ARO exposure poises hepatic mitochondria toward the synthesis of urea intermediates.

Evidence for the existence of an inner membrane anion channel in mitochondria

Mitochondria normally exhibit very low electrophoretic permeabilities to physiologically important anions such as chloride, bicarbonate, phosphate, succinate, citrate, etc. Nevertheless, considerable evidence has accumulated which suggests that heart and liver mitochondria contain a specific anion-conducting channel. In this review, a postulated inner membrane anion channel is discussed in the context of other known pathways for anion transport in mitochondria. This anion channel exhibits the following properties. It is anion-selective and inhibited physiologically by protons and magnesium ions. It is inhibited reversibly by quinine and irreversibly by dicyclohexylcarbodiimide. We propose that the inner membrane anion channel is formed by inner membrane proteins and that this pathway is normally latent due to regulation by matrix Mg2+. The physiological role of the anion channel is unknown; however, this pathway is well designed to enable mitochondria to restore their normal volume following pathological swelling. In addition, the inner membrane anion channel provides a potential futile cycle for regulated non-shivering thermogenesis and may be important in controlled energy dissipation.

Hepatic urea synthesis and pH regulation. Role of CO2, HCO3-, pH and the activity of carbonic anhydrase

In isolated perfused rat liver, urea synthesis from ammonium ions was dependent on extracellular HCO3- and CO2 concentrations when the HCO3-/CO2 ratio in the influent perfusate was constant (pH 7.4). Urea synthesis was half-maximal at HCO3- = 4 mM, CO2 = 0.19 mM and was maximal at HCO3- and CO2 concentrations above 20 mM and 0.96 mM, respectively. At physiological HCO3- (25 mM) and CO2 (1.2 mM) concentrations in the influent perfusate, acetazolamide, the inhibitor of carbonic anhydrase, inhibited urea synthesis from ammonium ions (1 mM) by 50-60% and led to a 70% decrease in citrulline tissue levels. Acetazolamide concentrations required for maximal inhibition of urea synthesis were 0.01-0.1 mM. At subphysiological HCO3- and CO2 concentrations, inhibition of urea synthesis by acetazolamide was increased up to 90%. Inhibition of urea synthesis by acetazolamide was fully overcome in the presence of unphysiologically high HCO3- and CO2 concentrations, indicating that the inhibitory effect of acetazolamide is due to an inhibition of carbonic-anhydrase-catalyzed HCO3- supply for carbamoyl-phosphate synthetase, which can be bypassed when the uncatalyzed intramitochondrial HCO3- formation from portal CO2 is stimulated in the presence of high portal CO2 concentrations. With respect to HCO3- supply of mitochondrial carbamoyl-phosphate synthetase, urea synthesis can be separated into a carbonic-anhydrase-dependent (sensitive to acetazolamide at 0.5 mM) and a carbonic-anhydrase-independent (insensitive to acetazolamide) portion. Carbonic-anhydrase-independent urea synthesis linearly increased with the portal 'total CO2 addition' (which was experimentally determined to be CO2 addition plus 0.036 HCO3- addition) and was independent of the perfusate pH. At a constant 'total CO2 addition', carbonic-anhydrase-dependent urea synthesis was strongly affected by perfusate pH and increased about threefold when the perfusate pH was raised from 6.9 to 7.8. It is concluded that the pH dependent regulation of urea synthesis is predominantly due to mitochondrial carbonic anhydrase-catalyzed HCO3- supply for carbamoyl phosphate synthesis, whereas there is no control of urea synthesis by pH at the level of the five enzymes of the urea cycle. Because HCO3- provision for carbamoyl phosphate synthetase increases with increasing portal CO2 concentrations even in the absence of carbonic anhydrase activity, susceptibility of ureogenesis to pH decreases with increasing portal CO2 concentrations. This may explain the different response of urea synthesis to chronic metabolic and chronic respiratory acidosis in vivo.