Limits...
Bcl-xL regulates mitochondrial energetics by stabilizing the inner membrane potential.

Chen YB, Aon MA, Hsu YT, Soane L, Teng X, McCaffery JM, Cheng WC, Qi B, Li H, Alavian KN, Dayhoff-Brannigan M, Zou S, Pineda FJ, O'Rourke B, Ko YH, Pedersen PL, Kaczmarek LK, Jonas EA, Hardwick JM - J. Cell Biol. (2011)

Bottom Line: Computational, biochemical, and genetic evidence indicated that Bcl-x(L) reduces futile ion flux across the inner mitochondrial membrane to prevent a wasteful drain on cellular resources, thereby preventing an energetic crisis during stress.Given that F(1)F(O)-ATP synthase directly affects mitochondrial membrane potential and having identified the mitochondrial ATP synthase β subunit in a screen for Bcl-x(L)-binding partners, we tested and found that Bcl-x(L) failed to protect β subunit-deficient yeast.Thus, by bolstering mitochondrial energetic capacity, Bcl-x(L) may contribute importantly to cell survival independently of other Bcl-2 family proteins.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Pharmacology and Molecular Sciences, Johns Hopkins School of Medicine, Baltimore, MD 21205, USA.

ABSTRACT
Mammalian Bcl-x(L) protein localizes to the outer mitochondrial membrane, where it inhibits apoptosis by binding Bax and inhibiting Bax-induced outer membrane permeabilization. Contrary to expectation, we found by electron microscopy and biochemical approaches that endogenous Bcl-x(L) also localized to inner mitochondrial cristae. Two-photon microscopy of cultured neurons revealed large fluctuations in inner mitochondrial membrane potential when Bcl-x(L) was genetically deleted or pharmacologically inhibited, indicating increased total ion flux into and out of mitochondria. Computational, biochemical, and genetic evidence indicated that Bcl-x(L) reduces futile ion flux across the inner mitochondrial membrane to prevent a wasteful drain on cellular resources, thereby preventing an energetic crisis during stress. Given that F(1)F(O)-ATP synthase directly affects mitochondrial membrane potential and having identified the mitochondrial ATP synthase β subunit in a screen for Bcl-x(L)-binding partners, we tested and found that Bcl-x(L) failed to protect β subunit-deficient yeast. Thus, by bolstering mitochondrial energetic capacity, Bcl-x(L) may contribute importantly to cell survival independently of other Bcl-2 family proteins.

Show MeSH

Related in: MedlinePlus

Bcl-xL stabilizes the mitochondrial membrane potentialto conserve energy. (A) A simplified model of ion flux acrossthe inner mitochondrial membrane. Ions may enter or leave the matrixthrough leakage channels at point a (e.g., theF1FO ATP synthase through which protons enterthe mitochondrial matrix), and overall stability and membrane potentialare maintained by active pumps at point b (e.g., the ETC). The doublearrow represents the source of fluctuation in potential. (B) Numericalsimulations of the additional ion flux that occurs as a result offluctuations in membrane potential using the 1-µm membranevesicle model in A. An external perturbation with a fixed amplitudebetween 0 and 10 pA (5-ms duration) was allowed to occur randomly with amean interval of 1 s, and the total ion flux was integrated over 20-speriods. The graph plots the increase in total integrated flux thatresults from fluctuations of increasing amplitude. (C) Numericalsimulations of changes in mitochondrial inner membrane potentialproduced by stochastic opening of a nonselective cation channel. Noopenings occur when probability of channel opening (Po) equals 0, andthe membrane is maintained at a steady level (−180 mV; dashedlines). Indicated opening rates of the channel produce fluctuations inmembrane potential accompanied by net hyperpolarization (more negativepotentials). (D) Mean ± SD of membrane potentials for traces inC.
© Copyright Policy - openaccess
Related In: Results  -  Collection

License 1 - License 2
getmorefigures.php?uid=PMC3198165&req=5

fig5: Bcl-xL stabilizes the mitochondrial membrane potentialto conserve energy. (A) A simplified model of ion flux acrossthe inner mitochondrial membrane. Ions may enter or leave the matrixthrough leakage channels at point a (e.g., theF1FO ATP synthase through which protons enterthe mitochondrial matrix), and overall stability and membrane potentialare maintained by active pumps at point b (e.g., the ETC). The doublearrow represents the source of fluctuation in potential. (B) Numericalsimulations of the additional ion flux that occurs as a result offluctuations in membrane potential using the 1-µm membranevesicle model in A. An external perturbation with a fixed amplitudebetween 0 and 10 pA (5-ms duration) was allowed to occur randomly with amean interval of 1 s, and the total ion flux was integrated over 20-speriods. The graph plots the increase in total integrated flux thatresults from fluctuations of increasing amplitude. (C) Numericalsimulations of changes in mitochondrial inner membrane potentialproduced by stochastic opening of a nonselective cation channel. Noopenings occur when probability of channel opening (Po) equals 0, andthe membrane is maintained at a steady level (−180 mV; dashedlines). Indicated opening rates of the channel produce fluctuations inmembrane potential accompanied by net hyperpolarization (more negativepotentials). (D) Mean ± SD of membrane potentials for traces inC.

Mentions: It is known that when any chemical system is not at thermodynamic equilibrium, asis the case for respiring mitochondria, the occurrence of persistentfluctuations or oscillations can only be maintained by expending energy (Nicolis and Prigogine, 1977). Moreover,the additional time-dependent flux of ions across the inner membrane that drivesthese fluctuations in potential can result in an overall ion flux (both inwardand outward directions) that is greater than what is required simply to maintaina nonfluctuating membrane potential at a steady negative value. Thus, thefluctuations in mitochondrial membrane potential in bcl-xknockouts imply that more energy is required to maintain ion gradients acrossthe inner membrane. To illustrate this concept, we constructed a simplenumerical model to investigate the effect of fluctuations on the dissipation ofion gradients across the mitochondrial membrane. A vesicle (1 µm indiameter) was used to represent a mitochondrion (Fig. 5 A). This vesicle was equipped with active ion pumps (Fig. 5 A, b) capable of pumping outprotons/ions (approximating the respiratory chain) to build a negative potential(−180 mV) and with ion channels (Fig. 5A, a) that can partially dissipate this potential by allowing ions toreenter the vesicle (approximating the F1FO ATP synthaseand nonproductive leaks). We first modeled steady-state conditions in which theinward flux and outward flux of ions are exactly matched in time, and themembrane potential does not fluctuate in amplitude. These conditions approximatethe steady-state conditions of mitochondria in wild-type cells. Next, we modeledfluctuations in membrane potential by introducing small currents across thevesicle membrane (Fig. 5 A, c). Thesesmall currents (set arbitrarily at 5 ms with a fixed amplitude between 0 and 10pA) were allowed to occur randomly (averaging 1/s) to drive fluctuations in thepotential across the vesicle membrane. To assess the effects of these externalcurrent amplitudes (Fig. 5 A, c), wemeasured the magnitude of total ion flux through the pumps (Fig. 5 A, a) and the channels (Fig. 5 A, b). In all cases, the total amount of ion flux(measured in picocoulombs) was increased when current fluctuations wereintroduced and was further increased with increasing external current amplitude(Fig. 5 B). The additional amount ofion movement (Fig. 5 A, a and b) producedby the small transient current fluctuations (Fig. 5 A, c) represents a futile dissipation of the ion gradientthat has to be balanced by pump activity to restore the mean membrane potential.Thus, Bcl-xL could improve mitochondrial energetics simply bypreventing futile ion flux.


Bcl-xL regulates mitochondrial energetics by stabilizing the inner membrane potential.

Chen YB, Aon MA, Hsu YT, Soane L, Teng X, McCaffery JM, Cheng WC, Qi B, Li H, Alavian KN, Dayhoff-Brannigan M, Zou S, Pineda FJ, O'Rourke B, Ko YH, Pedersen PL, Kaczmarek LK, Jonas EA, Hardwick JM - J. Cell Biol. (2011)

Bcl-xL stabilizes the mitochondrial membrane potentialto conserve energy. (A) A simplified model of ion flux acrossthe inner mitochondrial membrane. Ions may enter or leave the matrixthrough leakage channels at point a (e.g., theF1FO ATP synthase through which protons enterthe mitochondrial matrix), and overall stability and membrane potentialare maintained by active pumps at point b (e.g., the ETC). The doublearrow represents the source of fluctuation in potential. (B) Numericalsimulations of the additional ion flux that occurs as a result offluctuations in membrane potential using the 1-µm membranevesicle model in A. An external perturbation with a fixed amplitudebetween 0 and 10 pA (5-ms duration) was allowed to occur randomly with amean interval of 1 s, and the total ion flux was integrated over 20-speriods. The graph plots the increase in total integrated flux thatresults from fluctuations of increasing amplitude. (C) Numericalsimulations of changes in mitochondrial inner membrane potentialproduced by stochastic opening of a nonselective cation channel. Noopenings occur when probability of channel opening (Po) equals 0, andthe membrane is maintained at a steady level (−180 mV; dashedlines). Indicated opening rates of the channel produce fluctuations inmembrane potential accompanied by net hyperpolarization (more negativepotentials). (D) Mean ± SD of membrane potentials for traces inC.
© Copyright Policy - openaccess
Related In: Results  -  Collection

License 1 - License 2
Show All Figures
getmorefigures.php?uid=PMC3198165&req=5

fig5: Bcl-xL stabilizes the mitochondrial membrane potentialto conserve energy. (A) A simplified model of ion flux acrossthe inner mitochondrial membrane. Ions may enter or leave the matrixthrough leakage channels at point a (e.g., theF1FO ATP synthase through which protons enterthe mitochondrial matrix), and overall stability and membrane potentialare maintained by active pumps at point b (e.g., the ETC). The doublearrow represents the source of fluctuation in potential. (B) Numericalsimulations of the additional ion flux that occurs as a result offluctuations in membrane potential using the 1-µm membranevesicle model in A. An external perturbation with a fixed amplitudebetween 0 and 10 pA (5-ms duration) was allowed to occur randomly with amean interval of 1 s, and the total ion flux was integrated over 20-speriods. The graph plots the increase in total integrated flux thatresults from fluctuations of increasing amplitude. (C) Numericalsimulations of changes in mitochondrial inner membrane potentialproduced by stochastic opening of a nonselective cation channel. Noopenings occur when probability of channel opening (Po) equals 0, andthe membrane is maintained at a steady level (−180 mV; dashedlines). Indicated opening rates of the channel produce fluctuations inmembrane potential accompanied by net hyperpolarization (more negativepotentials). (D) Mean ± SD of membrane potentials for traces inC.
Mentions: It is known that when any chemical system is not at thermodynamic equilibrium, asis the case for respiring mitochondria, the occurrence of persistentfluctuations or oscillations can only be maintained by expending energy (Nicolis and Prigogine, 1977). Moreover,the additional time-dependent flux of ions across the inner membrane that drivesthese fluctuations in potential can result in an overall ion flux (both inwardand outward directions) that is greater than what is required simply to maintaina nonfluctuating membrane potential at a steady negative value. Thus, thefluctuations in mitochondrial membrane potential in bcl-xknockouts imply that more energy is required to maintain ion gradients acrossthe inner membrane. To illustrate this concept, we constructed a simplenumerical model to investigate the effect of fluctuations on the dissipation ofion gradients across the mitochondrial membrane. A vesicle (1 µm indiameter) was used to represent a mitochondrion (Fig. 5 A). This vesicle was equipped with active ion pumps (Fig. 5 A, b) capable of pumping outprotons/ions (approximating the respiratory chain) to build a negative potential(−180 mV) and with ion channels (Fig. 5A, a) that can partially dissipate this potential by allowing ions toreenter the vesicle (approximating the F1FO ATP synthaseand nonproductive leaks). We first modeled steady-state conditions in which theinward flux and outward flux of ions are exactly matched in time, and themembrane potential does not fluctuate in amplitude. These conditions approximatethe steady-state conditions of mitochondria in wild-type cells. Next, we modeledfluctuations in membrane potential by introducing small currents across thevesicle membrane (Fig. 5 A, c). Thesesmall currents (set arbitrarily at 5 ms with a fixed amplitude between 0 and 10pA) were allowed to occur randomly (averaging 1/s) to drive fluctuations in thepotential across the vesicle membrane. To assess the effects of these externalcurrent amplitudes (Fig. 5 A, c), wemeasured the magnitude of total ion flux through the pumps (Fig. 5 A, a) and the channels (Fig. 5 A, b). In all cases, the total amount of ion flux(measured in picocoulombs) was increased when current fluctuations wereintroduced and was further increased with increasing external current amplitude(Fig. 5 B). The additional amount ofion movement (Fig. 5 A, a and b) producedby the small transient current fluctuations (Fig. 5 A, c) represents a futile dissipation of the ion gradientthat has to be balanced by pump activity to restore the mean membrane potential.Thus, Bcl-xL could improve mitochondrial energetics simply bypreventing futile ion flux.

Bottom Line: Computational, biochemical, and genetic evidence indicated that Bcl-x(L) reduces futile ion flux across the inner mitochondrial membrane to prevent a wasteful drain on cellular resources, thereby preventing an energetic crisis during stress.Given that F(1)F(O)-ATP synthase directly affects mitochondrial membrane potential and having identified the mitochondrial ATP synthase β subunit in a screen for Bcl-x(L)-binding partners, we tested and found that Bcl-x(L) failed to protect β subunit-deficient yeast.Thus, by bolstering mitochondrial energetic capacity, Bcl-x(L) may contribute importantly to cell survival independently of other Bcl-2 family proteins.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Pharmacology and Molecular Sciences, Johns Hopkins School of Medicine, Baltimore, MD 21205, USA.

ABSTRACT
Mammalian Bcl-x(L) protein localizes to the outer mitochondrial membrane, where it inhibits apoptosis by binding Bax and inhibiting Bax-induced outer membrane permeabilization. Contrary to expectation, we found by electron microscopy and biochemical approaches that endogenous Bcl-x(L) also localized to inner mitochondrial cristae. Two-photon microscopy of cultured neurons revealed large fluctuations in inner mitochondrial membrane potential when Bcl-x(L) was genetically deleted or pharmacologically inhibited, indicating increased total ion flux into and out of mitochondria. Computational, biochemical, and genetic evidence indicated that Bcl-x(L) reduces futile ion flux across the inner mitochondrial membrane to prevent a wasteful drain on cellular resources, thereby preventing an energetic crisis during stress. Given that F(1)F(O)-ATP synthase directly affects mitochondrial membrane potential and having identified the mitochondrial ATP synthase β subunit in a screen for Bcl-x(L)-binding partners, we tested and found that Bcl-x(L) failed to protect β subunit-deficient yeast. Thus, by bolstering mitochondrial energetic capacity, Bcl-x(L) may contribute importantly to cell survival independently of other Bcl-2 family proteins.

Show MeSH
Related in: MedlinePlus