Holes in mitochondrial inner membranes

Acetyl-CoA the key factor for survival or death of cholinergic neurons in course of neurodegenerative diseases

Glucose-derived pyruvate is a principal source of acetyl-CoA in all brain cells, through pyruvate dehydogenase complex (PDHC) reaction. Cholinergic neurons like neurons of other transmitter systems and glial cells, utilize acetyl-CoA for energy production in mitochondria and diverse synthetic pathways in their extramitochondrial compartments. However, cholinergic neurons require additional amounts of acetyl-CoA for acetylcholine synthesis in their cytoplasmic compartment to maintain their transmitter functions. Characteristic feature of several neurodegenerating diseases including Alzheimer’s disease and thiamine diphosphate deficiency encephalopathy is the decrease of PDHC activity correlating with cholinergic deficits and losses of cognitive functions. Such conditions generate acetyl-CoA deficits that are deeper in cholinergic neurons than in noncholinergic neuronal and glial cells, due to its additional consumption in the transmitter synthesis. Therefore, any neuropathologic conditions are likely to be more harmful for the cholinergic neurons than for noncholinergic ones. For this reason attempts preserving proper supply of acetyl-CoA in the diseased brain, should attenuate high susceptibility of cholinergic neurons to diverse neurodegenerative conditions. This review describes how common neurodegenerative signals could induce deficts in cholinergic neurotransmission through suppression of acetyl-CoA metabolism in the cholinergic neurons.

Inner membrane anion channel and dicarboxylate carrier in brown adipose tissue mitochondria

In brown adipose tissue mitochondria, the anion transport proteins should respond to regulatory mechanisms controlling the thermogenic or resting state. We re-evaluated the role of transport of organic/metabolite anions in these mitochondria, namely with regards to delta pH-regulation and substrate specificity. Valinomycin-induced osmotic swelling in potassium salts indicated by light scattering either directly on a fluorometer, or as the reciprocal absorbance, was used to characterize the anion uniport. A delta pH "jump" was thus created in respiring mitochondria and the delta pH-driven transport was studied. The two major features are reported: (1) existence of the inner membrane anion channel exhibiting the same full spectrum of anion and inhibitor specificity as in liver; and (2) existence of dicarboxylate carrier, so far disputed in brown adipose tissue mitochondria. The inner membrane anion channel was activated either by elevating delta pH in respiring mitochondria or by depleting matrix Mg2+ at alkaline pH. Dicarboxylate carrier was activated by elevated delta pH under conditions when the channel was blocked. A specific delta pH regulation could explain this activation and silence of the carrier in early studies. In conclusion, wide substrate specificity makes the inner membrane anion channel suitable for the regulation of volume homeostasis and a feed-back control between the delta psi-driven and the delta pH-driven transport. The delta pH-activated dicarboxylate carrier is essential in the coupled state for malate uptake which enables fatty acid synthesis, while, in the uncoupled state, inaccessibility of dicarboxylates favors oxidation of fatty acids or pyruvate.

Cardiolipins and biomembrane function

Evidence is discussed for roles of cardiolipins in oxidative phosphorylation mechanisms that regulate State 4 respiration by returning ejected protons across and over bacterial and mitochondrial membrane phospholipids, and that regulate State 3 respiration through the relative contributions of proteins that transport protons, electrons and/or metabolites. The barrier properties of phospholipid bilayers support and regulate the slow proton leak that is the basis for State 4 respiration. Proton permeability is in the range 10(-3)-10(-4) cm s-1 in mitochondria and in protein-free membranes formed from extracted mitochondrial phospholipids or from stable synthetic phosphatidylcholines or phosphatidylethanolamines. The roles of cardiolipins in proton conductance in model phospholipid membrane systems need to be assessed in view of new findings by Hübner et al. [313]: saturated cardiolipins form bilayers whilst natural highly unsaturated cardiolipins form nonlamellar phases. Mitochondrial cardiolipins apparently participate in bilayers formed by phosphatidylcholines and phosphatidylethanolamines. It is not yet clear if cardiolipins themselves conduct protons back across the membrane according to their degree of fatty acyl saturation, and/or modulate proton conductance by phosphatidylcholines and phosphatidylethanolamines. Mitochondrial cardiolipins, especially those with high 18:2 acyl contents, strongly bind many carrier and enzyme proteins that are involved in oxidative phosphorylation, some of which contribute to regulation of State 3 respiration. The role of cardiolipins in biomembrane protein function has been examined by measuring retained phospholipids and phospholipid binding in purified proteins, and by reconstituting delipidated proteins. The reconstitution criterion for the significance of cardiolipin-protein interactions has been catalytical activity; proton-pumping and multiprotein interactions have yet to be correlated. Some proteins, e.g., cytochrome c oxidase are catalytically active when dimyristoylphosphatidylcholine replaces retained cardiolipins. Cardiolipin-protein interactions orient membrane proteins, matrix proteins, and on the outerface receptors, enzymes, and some leader peptides for import; activate enzymes or keep them inactive unless the inner membrane is disrupted; and modulate formation of nonbilayer HII-phases. The capacity of the proton-exchanging uncoupling protein to accelerate thermogenic respiration in brown adipose tissue mitochondria of cold-adapted animals is not apparently affected by the increased cardiolipin unsaturation; this protein seems to take over the protonophoric role of cardiolipins in other mitochondria. Many in vivo influences that affect proton leakage and carrier rates selectively alter cardiolipins in amount per mitochondrial phospholipids, in fatty acyl composition and perhaps in sidedness; other mitochondrial membrane phospholipids respond less or not at all.(ABSTRACT TRUNCATED AT 400 WORDS)

Inner mitochondrial membrane anion channel is present in brown adipocytes but is not identical with the uncoupling protein

Vesicles of inner mitochondrial membrane, mitoplasts, from rat brown adipose tissue were prepared by osmotic swelling and studied using the patch-clamp technique. Current events of a 107.8 +/- 8.7 pS (n = 16, 21 degrees C) channel were recorded in the mitoplast-attached mode. This channel was selective for anions and its kinetics resembled those of channels previously found in liver and heart mitochondria of mouse and ox. In whole-mitoplast mode each of five purine nucleotides (20 microM) blocked the channel. This is the first demonstration of pharmacological blockade of this type of channel. Although a similar anion channel in mouse and ox mitochondria was suggested to be the uncoupling protein (UCP) associated with nonshivering thermogenesis, we present several arguments against this possibility. Thus we describe a high-conductance, purine-nucleotide-binding, anion selective mitochondrial channel, that is not the UCP.

Respiratory inhibitors and uncouplers prevent the aeration-induced increase in mitochondrial anion conductivity

1. When mitochondria are stirred in air the rate of anion conductivity increases, this effect being enhanced by the addition of respiratory substrate. 2. This effect is reversible if the mitochondria are stored for a period of time under N2. 3. The aeration-induced increase in mitochondrial anion conductivity can also be prevented by the addition of respiratory inhibitors rotenone and antimycin A, as well as by 30 microM-cyanide. 4. A decrease in this aeration-induced anion conductivity can also be observed upon the addition of the uncouplers carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone (2 microM) and 2,4-dinitrophenol (100 microM). 5. Simultaneous measurements of mitochondrial anion conductivity and membrane potential show a relationship between the level of membrane potential and anion conductivity. 6. It is suggested that the level of membrane potential is either directly or indirectly responsible for the level of mitochondrial anion conductivity.

Evidence for two distinct chloride uniport pathways in brown adipose tissue mitochondria

Chloride permeability of the inner membrane of brown adipose tissue mitochondria was analyzed by monitoring mitochondrial swelling in KCl salts in the presence of K+ ionophores. The results indicate that the high anion conductance observed in these mitochondria is due to the presence of two separate pathways: 1) a Cl-conducting pathway that is inhibited by guanosine 5'-diphosphate (GDP) but neither by N,N'-dicyclohexylcarbodiimide (DCCD) nor by amphiphilic amines and that is found uniquely in brown adipose tissue mitochondria and 2) an inner membrane anion uniport channel that is inhibited both by DCCD and by amphiphilic amines but not by GDP and that is opened either by depletion of matrix Mg2+ or by alkalinization of the matrix.

Cold-induced mitochondrial degradation and cryoprotectant synthesis in freeze-tolerant arctic caterpillars

The larvae of Gynaephora groenlandica, a long-lived moth endemic to the high arctic, are perennially freeze-tolerant and able to increase their freeze-tolerance by synthesizing glycerol. Cold-induced mitochondrial changes were correlated (using electron microscopy, DNA staining, cytochrome c assay, and oxygen uptake) with glycerol production (using NMR spectroscopy) in larvae under different acclimations and in the field. Hypometabolism in summer- or warm-acclimated larvae led to glycerol accumulation. Extended exposure to near-zero or freezing temperatures caused mitochondrial degradation and glycerol accumulation. Rapid freezing of warm-acclimated larvae did not result in mitochondrial breakdown. Mitochondrial reconstitution upon warm-acclimation occurred much more rapidly (less than 1 week) than did degradation (greater than 2 months). Concomitant with mitochondrial breakdown was reduced oxidative metabolism, but the cytochrome c concentration remained independent of acclimation temperature. The adaptive response to cold by mitochondrial degradation and glycerol accumulation by G. groenlandica may be linked to diapause in other species of ectotherms.