UCP4, a novel brain-specific mitochondrial protein that reduces membrane potential in mammalian cells.
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Uncoupling proteins (UCPs) are a family of mitochondrial transporter proteins that have been implicated in thermoregulatory heat production and maintenance of the basal metabolic rate. We have identified and partially characterized a novel member of the human uncoupling protein family, termed uncoupling protein-4 (UCP4). Protein sequence analyses showed that UCP4 is most related to UCP3 and possesses features characteristic of mitochondrial transporter proteins. Unlike other known UCPs, UCP4 transcripts are exclusively expressed in both fetal and adult brain tissues. UCP4 maps to human chromosome 6p11.2-q12. Consistent with its potential role as an uncoupling protein, UCP4 is localized to the mitochondria and its ectopic expression in mammalian cells reduces mitochondrial membrane potential. These findings suggest that UCP4 may be involved in thermoregulatory heat production and metabolism in the brain. (+info)
Mitochondrial regulation of the cytosolic Ca2+ concentration and the InsP3-sensitive Ca2+ store in guinea-pig colonic smooth muscle.
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1. Mitochondrial regulation of the cytosolic Ca2+ concentration ([Ca2+]c) in guinea-pig single colonic myocytes has been examined, using whole-cell recording, flash photolysis of caged InsP3 and microfluorimetry. 2. Depolarization increased [Ca2+]c and triggered contraction. Resting [Ca2+]c was virtually restored some 4 s after the end of depolarization, a time when the muscle had shortened to 50 % of its fully relaxed length. The muscle then slowly relaxed (t = 17 s). 3. The decline in the Ca2+ transient was monophasic but often undershot or overshot resting levels, depending on resting [Ca2+]c. The extent of the overshoot or undershoot increased with increasing peak [Ca2+]c. 4. Carbonyl cyanide m-chlorophenyl hydrazone (CCCP; 5 microM), which dissipates the mitochondrial proton electrochemical gradient and therefore prevents mitochondrial Ca2+ accumulation, slowed Ca2+ removal at high ( > 300 nM) but not at lower [Ca2+]c and abolished [Ca2+]c overshoots. Oligomycin B (5 microM), which prevents mitchondrial ATP production, affected neither the rate of decline nor the magnitude of the overshoot. 5. During depolarization, the global rhod-2 signal (which represents the mitochondrial matrix Ca2+ concentration, [Ca2+]m) rose slowly in a CCCP-sensitive manner during and for about 3 s after depolarization had ended. [Ca2+]m then slowly decreased over tens of seconds. 6. Inhibition of sarcoplasmic reticulum Ca2+ uptake with thapsigargin (100 nM) reduced the undershoot and increased the overshoot. 7. Flash photolysis of caged InsP3 (20 microM) evoked reproducible increases in [Ca2+]c. CCCP (5 microM) reduced the magnitude of the [Ca2+]c transients evoked by flash photolysis of caged InsP3. Oligomycin B (5 microM) did not reduce the inhibition of the InsP3-induced Ca2+ transient by CCCP thus minimizing the possibility that CCCP lowered ATP levels by reversing the mitochondrial ATP synthase and so reducing SR Ca2+ refilling. 8. While CCCP reduced the magnitude of the InsP3-evoked Ca2+ signal, the internal Ca2+ store content, as assessed by the magnitude of ionomycin-evoked Ca2+ release, did not decrease significantly. 9. [Ca2+]c decline in smooth muscle, following depolarization, may involve mitochondrial Ca2+ uptake. Following InsP3-evoked Ca2+ release, mitochondrial uptake of Ca2+ may regulate the local [Ca2+]c near the InsP3 receptor so maintaining the sensitivity of the InsP3 receptor to release Ca2+ from the SR. (+info)
Uncoupling of transfer of the presequence and unfolding of the mature domain in precursor translocation across the mitochondrial outer membrane.
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Translocation of mitochondrial precursor proteins across the mitochondrial outer membrane is facilitated by the translocase of the outer membrane (TOM) complex. By using site-specific photocrosslinking, we have mapped interactions between TOM proteins and a mitochondrial precursor protein arrested at two distinct stages, stage A (accumulated at 0 degrees C) and stage B (accumulated at 30 degrees C), in the translocation across the outer membrane at high resolution not achieved previously. Although the stage A and stage B intermediates were assigned previously to the forms bound to the cis site and the trans site of the TOM complex, respectively, the results of crosslinking indicate that the presequence of the intermediates at both stage A and stage B is already on the trans side of the outer membrane. The mature domain is unfolded and bound to Tom40 at stage B whereas it remains folded at stage A. After dissociation from the TOM complex, translocation of the stage B intermediate, but not of the stage A intermediate, across the inner membrane was promoted by the intermembrane-space domain of Tom22. We propose a new model for protein translocation across the outer membrane, where translocation of the presequence and unfolding of the mature domain are not necessarily coupled. (+info)
Excretion of taurocholate from isolated hepatocytes.
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Efflux of taurocholate from isolated rat hepatocytes was studied to characterize the mechanism of bile acid secretion. Cells were incubated with taurocholate for 15 min. The amount of the intracellularly accumulated bile acid was directly related to the concentration in the medium. Transfer of the loaded cells from the incubation medium to a medium without taurocholate led to taurocholate efflux. Efflux was saturable, its activation energy amounted to 12 kcal/mol (50 kJ). It was strongly inhibited by the metabolic inhibitor antimycin A and to a lesser extend by the uncoupler carbonylcyanide-m-chlorophenylhydrazone. Dinitrofluorobenzene and mersalyl, reagents which react with amino acids, inhibited efflux by about 30% when applied at concentrations of 50 muM. Ouabain increased the rate of efflux. The observations indicate that secretory functions are maintained in isolated liver cells. (+info)
Light-induced oxidation-reduction reactions of cytochromes in the green sulfur photosynthetic bacterium Prosthecochloris aesturarii.
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The light-induced oxidation-reduction reactions of cytochromes in intact cells, starved cells, and chlorobium vesicle fractions of the green sulfur photosynthetic bacterium Prosthecochloris aesturarii were studied under anaerobic conditions. On the basis of both kinetic and spectral properties, at least three cytochrome species were found to be involved in the light-induced oxidation-reduction reactions of intact cells. These cytochromes were designated according to the positions of alpha-band maxima as C555 (rapid and slow components) and C552 (intermediate). By comparing the light-minus-dark difference spectra with the reduced-minus-oxidized difference spectra of purified cytochromes of this organism, rapid component C555 and intermediate component C552 are suggested to correspond to the purified cytochromes c-555(550) and c-551.5, respectively. Although the identity of the slow-phase component is uncertain, one possibility is that the slow phase is due to the bound form of c-555(550). In substrate-depleted (starved) cells, only one cytochrome species, C555 remained in the reduced state in the dark and oxidized upon actinic illumination. This corresponds to the rapid C555 component in intact cells. In the case of chlorobium vesicle fractions, one cytochrome species having an alpha-band maximum at 554 nm was oxidized by actinic light. The effects of several inhibitors on the absorbance changes of intact cells were studied. Antimycin A decreased the rate of the dark reduction of rapid C555 component. The complex effects of CCCP (carbonyl cyanide m-chlorophenylhydrazone) on the oxidation-reduction reactions of cytochromes were interpreted as the results of inhibition of the electron donation to oxidized C552 and C555 (slow), and a shift of the dark steady-state redox levels of cytochromes. Based on these findings, it is suggested that the rapid C555 component is located in a cyclic electron transfer pathway. The other two cytochromes, C552 and C555 (slow), may be located in non-cyclic electron transfer pathways and receive electrons from exogenous substrates such as sodium sulfide. A tentative scheme for the electron transfer system in Prosthecochloris aestuarii is presented and its nature is discussed. (+info)
cAMP-mediated catabolite repression and electrochemical potential-dependent production of an extracellular amylase in Vibrio alginolyticus.
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Vibrio alginolyticus, a halophilic marine bacterium, produced an extracellular amylase with a molecular mass of approximately 56,000, and the amylase appeared to be subject to catabolite repression mediated by cAMP. The production of amylase at pH 6.5, at which the respiratory chain-linked H+ pump functions, was inhibited about 75% at 24 hours following the addition of 2 microM carbonyl cyanide m-chlorophenylhydrazone (CCCP), while the production at pH 8.5, at which the respiratory chain-linked Na+ pump functions, was only slightly inhibited by the addition of 2 microM CCCP. In contrast, the production of amylase in a mutant bacterium defective in the Na+ pump was almost completely inhibited even at pH 8.5 as well as pH 6.5 by the addition of 2 microM CCCP. (+info)
Uncouplers of oxidative phosphorylation can enhance a Fas death signal.
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Recent work suggests a participation of mitochondria in apoptotic cell death. This role includes the release of apoptogenic molecules into the cytosol preceding or after a loss of mitochondrial membrane potential DeltaPsim. The two uncouplers of oxidative phosphorylation carbonyl cyanide m-chlorophenylhydrazone (CCCP) and 2, 4-dinitrophenol (DNP) reduce DeltaPsim by direct attack of the proton gradient across the inner mitochondrial membrane. Here we show that both compounds enhance the apoptosis-inducing capacity of Fas/APO-1/CD95 signaling in Jurkat and CEM cells without causing apoptotic changes on their own account. This amplification occurred upstream or at the level of caspases and was not inhibited by Bcl-2. The effect could be blocked by the cowpox protein CrmA and is thus likely to require caspase 8 activity. Apoptosis induction by staurosporine in Jurkat cells as well as by Fas in SKW6 cells was unaffected by CCCP and DNP. The role of cytochrome c during Fas-DNP signaling was investigated. No early cytochrome c release from mitochondria was detected by Western blotting. Functional assays with cytoplasmic preparations from Fas-DNP-treated cells also indicated that there was no major contribution by cytochrome c or caspase 9 to the activation of effector caspases. Furthermore, an increase of rhodamine-123 uptake into intact cells, which has been explained by mitochondrial swelling, occurred considerably later than the caspase activation and was blocked by Z-VAD-fmk. These data show that uncouplers of oxidative phosphorylation can presensitize some but not all cells for a Fas death signal and provide information about the existence of separate pathways in the induction of apoptosis. (+info)
Secretagogues modulate the calcium concentration in the endoplasmic reticulum of insulin-secreting cells. Studies in aequorin-expressing intact and permeabilized ins-1 cells.
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The precise regulation of the Ca2+ concentration in the endoplasmic reticulum ([Ca2+]er) is important for protein processing and signal transduction. In the pancreatic beta-cell, dysregulation of [Ca2+]er may cause impaired insulin secretion. The Ca2+-sensitive photoprotein aequorin mutated to lower its Ca2+ affinity was stably expressed in the endoplasmic reticulum (ER) of rat insulinoma INS-1 cells. The steady state [Ca2+]er was 267 +/- 9 microM. Both the Ca2+-ATPase inhibitor cyclopiazonic acid and 4-chloro-m-cresol, an activator of ryanodine receptors, caused an almost complete emptying of ER Ca2+. The inositol 1,4,5-trisphosphate generating agonists, carbachol, and ATP, reduced [Ca2+]er by 20-25%. Insulin secretagogues that raise cytosolic [Ca2+] by membrane depolarization increased [Ca2+]er in the potency order K+ >> glucose > leucine, paralleling their actions in the cytosolic compartment. Glucose, which augmented [Ca2+]er by about 25%, potentiated the Ca2+-mobilizing effect of carbachol, explaining the corresponding observation in cytosolic [Ca2+]. The filling of ER Ca2+ by glucose is not directly mediated by ATP production as shown by the continuous monitoring of cytosolic ATP in luciferase expressing cells. Both glucose and K+ increase [Ca2+]er, but only the former generated whereas the latter consumed ATP. Nonetheless, drastic lowering of cellular ATP with a mitochondrial uncoupler resulted in a marked decrease in [Ca2+]er, emphasizing the requirement for mitochondrially derived ATP above a critical threshold concentration. Using alpha-toxin permeabilized cells in the presence of ATP, glucose 6-phosphate did not change [Ca2+]er, invalidating the hypothesis that glucose acts through this metabolite. Therefore, insulin secretagogues that primarily stimulate Ca2+ influx, elevate [Ca2+]er to ensure beta-cell homeostasis. (+info)