Redox regulation of endogenous substrate oxidation by cardiac mitochondria

Reactive oxygen species (ROS) play important roles in regulating mitochondrial function, as well as in ischemia-reperfusion injury and cardioprotection. Here we show that, in the absence of exogenous substrates, cardiac mitochondria have a surprisingly large capacity to phosphorylate ADP by oxidizing endogenous substrates, provided that H2O2 is removed from the extramitochondrial environment and a reduced environment is maintained in the matrix. In isolated mitochondria without exogenous substrates, addition of catalase and the membrane-permeant reducing agent N-acetylcysteine (Nac) or the ROS scavenger mercaptopropionyl glycine significantly increased the ability to phosphorylate added ADP, as demonstrated by 1) full recovery of membrane potential (Deltapsi) and matrix volume from ADP-induced dissipation and shrinkage, 2) ADP-dependent increase in O2 consumption, and 3) enhanced rate of ATP synthesis. Removal of extramitochondrial H2O2 by catalase was required to stimulate endogenous substrate oxidation, as shown by the increase in O2 consumption and Deltapsi. This effect was greatly enhanced by addition of Nac or mercaptopropionyl glycine to suppress oxidation-induced ROS increases in the matrix. Theoretical considerations, as well as reversible inhibition of O2 consumption with 3-mercaptopropionic acid and pyruvate in state 3, indicate that these substrates are fatty acids. Under in vivo conditions in which powerful antioxidant conditions are maintained, this mechanism may be important in stimulation of beta-oxidation and ATP production at low levels of extramitochondrial fatty acids. Incapacitation of this mechanism may potentially contribute to mitochondrial dysfunction during oxidative stress.

Long polyamines act as cofactors in PIP2 activation of inward rectifier potassium (Kir2.1) channels

Phosphatidylinosital-4,5-bisphosphate (PIP₂) acts as an essential factor regulating the activity of all Kir channels. In most Kir members, the dependence on PIP₂ is modulated by other factors, such as protein kinases (in Kir1), G_βγ (in Kir3), and the sulfonylurea receptor (in Kir6). So far, however, no regulator has been identified in Kir2 channels. Here we show that polyamines, which cause inward rectification by selectively blocking outward current, also regulate the interaction of PIP₂ with Kir2.1 channels to maintain channel availability. Using spermine and diamines as polyamine analogs, we demonstrate that both spontaneous and PIP₂ antibody–induced rundown of Kir2.1 channels in excised inside-out patches was markedly slowed by long polyamines; in contrast, polyamines with shorter chain length were ineffective. In K188Q mutant channels, which have a low PIP₂ affinity, application PIP₂ (10 μM) was unable to activate channel activity in the absence of polyamines, but markedly activated channels in the presence of long diamines. Using neomycin as a measure of PIP₂ affinity, we found that long polyamines were capable of strengthening either the wild type or K188Q channels' interaction with PIP₂. The negatively charged D172 residue inside the transmembrane pore region was critical for the shift of channel–PIP₂ binding affinity by long polyamines. Sustained pore block by polyamines was neither sufficient nor necessary for this effect. We conclude that long polyamines serve a dual role as both blockers and coactivators (with PIP₂) of Kir2.1 channels.

The dynamics of cardiac fibrillation

Reentry occurs when the electrical wave propagating through the atria or ventricles breaks locally and forms a rotor (also called a scroll wave or functional reentry). If the waves propagating outward from a rotor develop additional wavebreaks (which may form new rotors), fibrillation results. Tissue heterogeneity, exacerbated by electrical and structural remodeling from cardiac disease, has traditionally been considered the major factor promoting wavebreak and its degeneration to fibrillation. Recently, however, dynamic factors have also been recognized to play a key role. Dynamic factors refer to cellular properties of the cardiac action potential and Ca(i) cycling, which dynamically generate wave instability and wavebreak, even in tissue that is initially completely homogeneous. Although the latter situation can only be created in computer simulations, its relevance to real (heterogeneous) cardiac tissue has been unequivocally demonstrated. Dynamic factors are related to membrane voltage (Vm) and Ca(i). Vm factors include electrical restitution of action potential duration and conduction velocity, short-term cardiac memory, and electrotonic currents. Ca(i) factors are related to dynamic Ca(i) cycling properties. They act synergistically, as well as with tissue heterogeneity, to promote wavebreak and fibrillation. As global properties, rather than local electrophysiological characteristics, dynamic factors represent an attractive target for novel therapies to prevent ventricular fibrillation.

Linking cell division to cell growth in a spatiotemporal model of the cell cycle

Cell division must be tightly coupled to cell growth in order to maintain cell size, yet the mechanisms linking these two processes are unclear. It is known that almost all proteins involved in cell division shuttle between cytoplasm and nucleus during the cell cycle; however, the implications of this process for cell cycle dynamics and its coupling to cell growth remains to be elucidated. We developed mathematical models of the cell cycle which incorporate protein translocation between cytoplasm and nucleus. We show that protein translocation between cytoplasm and nucleus not only modulates temporal cell cycle dynamics, but also provides a natural mechanism coupling cell division to cell growth. This coupling is mediated by the effect of cytoplasmic-to-nuclear size ratio on the activation threshold of critical cell cycle proteins, leading to the size-sensing checkpoint (sizer) and the size-independent clock (timer) observed in many cell cycle experiments.

ATP-sensitive K+ channels: regulation of bursting by the sulphonylurea receptor, PIP2 and regions of Kir6.2

ATP-sensitive K+ channels composed of the pore-forming protein Kir6.2 and the sulphonylurea receptor SUR1 are inhibited by ATP and activated by Phosphatidylinositol Bisphosphate (PIP2). Residues involved in binding of these ligands to the Kir6.2 cytoplasmic domain have been identified, and it has been hypothesized that gating mechanisms involve conformational changes in the regions of the bundle crossing and/or the selectivity filter of Kir6.2. Regulation of Kir6.2 by SUR1, however, is not well-understood, even though this process is ATP and PIP2 dependent. In this study, we investigated the relationship between channel regulation by SUR1 and PIP2 by comparing a number of single and double mutants known to affect open probability (P(o)), PIP2 affinity, and sulphonylurea and MgADP sensitivity. When coexpressed with SUR1, the Kir6.2 mutant C166A, which is characterized by a P(o) value close to 0.8, exhibits no sulphonylurea or MgADP sensitivity. However, when P(o) was reduced by combining mutations at the PIP2-sensitive residues R176 and R177 with C166A, sulphonylurea and MgADP sensitivities were restored. These effects correlated with a dramatic decrease in PIP2 affinity, as assessed by PIP2-induced channel reactivation and inhibition by neomycin, an antagonist of PIP2 binding. Based on macroscopic and single-channel data, we propose a model in which entry into the high-P(o) bursting state by the C166A mutation or by SUR1 depends on the interaction of PIP2 with R176 and R177 and, to a lesser extent, R54. In conjunction with this PIP2-dependent process, SUR1 also regulates channel activity via a PIP2-independent, but MgADP-dependent process.

Regulation of the ATP-sensitive K channel Kir6.2 by ATP and PIP(2)

ATP-sensitive K (K(ATP)) channels are blocked by ATP and activated by PIP(2). Both negatively-charged ligands are presumed to bind to positively-charged residues on the N-and C-termini of the channel's cytoplasmic domain. Evidence summarized here suggests that the channel's interaction with ATP and PIP(2) is regulated by separate groups of residues, involving both direct charge-charge interactions and allosteric effects. ATP interaction is regulated by R50 in the N-terminus and by K185, R192 and R201 in the C-terminus. R192 and R201 mutations decrease channel sensitivity to ATP, ADP and AMP to a similar extent, implying that they regulate interaction with either the alpha phosphate group, common to all three adenine nucleotides, or the adenosine moiety. K185 mutations, and to a lesser extent R50 mutations, decrease ATP and ADP sensitivity without markedly affecting AMP sensitivity, implying that they regulate interaction with the beta phosphate of ATP and ADP. In addition, when open probability decreases due to rundown, ATP sensitivity increases in R50, K185 and R192, but not in R201 mutants. Combining these observations with recent structural data, we hypothesize the following scenario: 1) the ATP binding site is located at the outside of the channel's cytoplasmic domain away from the pore. 2) When the channel is open, R50 and K185 interact directly with the beta phosphate of ATP, whereas R192, which appears to be removed from the ATP binding site, modulates the initial interaction with ATP allosterically. 3) When the channel closes, R201 is in position to interact with the alpha phosphate of ATP to stabilize the closed state. 4) PIP(2) also interacts with the channel's cytoplasmic domain, but at distinct positively-charged residues located above the ATP binding site and near to the plasma membrane. These residues include R54 in the N-terminus and R176, R177 and R206 in the C-terminus. Thus, the binding domains of ATP and PIP(2) in the N- and C-termini do not appear to overlap.

Mitochondria and ischemia/reperfusion injury

Cardiac ischemia/reperfusion injury results in a variable mixture of apoptotic, necrotic, and normal tissue that depends on both the duration and severity of ischemia. Injury can be abrogated by activation of protective pathways via ischemic and pharmacologic preconditioning. Mitochondria serve as final arbiters of life and death of the cell as these organelles not only are required to generate ATP but also can trigger apoptosis or necrosis. A key mechanism of mitochondrial injury is by the mitochondrial permeability transition (MPT) that has been shown to occur at reperfusion. The article hypothesizes that ischemia/reperfusion promotes MPT in two phases: (1) MPT priming during ischemia occurs as progressive inner mitochondrial membrane leak is accompanied by depressed electron transport in the setting of fatty acid accumulation and loss of cytochrome c and antioxidants; and (2) Triggering of MPT at reperfusion is determined by the interplay of mitochondrial membrane potential (DeltaPsi(m)) with mitochondrial matrix Ca, reactive oxygen species, and pH. It has been found that strategies that promote mitochondrial recovery such as pharmacologic preconditioning by diazoxide are mediated by K(+)-dependent regulation of matrix volume and DeltaPsi(m), resulting in improved efficiency of ATP synthesis as well as prevention of cytochrome c loss. If mitochondria fail to recover, then MPT and hypercontracture can result as DeltaPsi(m) depolarization waves regeneratively cross the cell (0.1 to 0.2 microm/s).

ATP sensitivity of ATP-sensitive K+ channels: role of the gamma phosphate group of ATP and the R50 residue of mouse Kir6.2

ATP-sensitive K (K(ATP)) channels are composed of Kir6, the pore-forming protein, and the sulphonylurea receptor SUR, a regulatory protein. We and others have previously shown that positively charged residues in the C terminus of Kir6.2, including R201 and K185, interact with the alpha and beta phosphate groups of ATP, respectively, to induce channel closure. A positively charged residue in the N terminus, R50, is also important, and has been proposed to interact with either the gamma or beta phosphate group of ATP. To examine this issue, we systematically mutated R50 to residues of different size, charge and hydropathy, and examined the effects on adenine nucleotide sensitivity in the absence and presence of SUR1. In the absence of SUR1, only the size of residue 50 significantly altered ATP sensitivity, with smaller side chains decreasing ATP sensitivity. In the presence of SUR1, however, hydrophathy and charge also played a role. Hydrophilic residues decreased ATP sensitivity more than hydrophobic residues for small size residues, and, surprisingly, negatively charged residues E and D preserved ATP sensitivity and increased ADP sensitivity relative to the wild-type residue R. These observations suggest that a negative charge near position 50, due to either mutation of R50 or the interaction of the gamma phosphate group of ATP with R50, facilitates closure of the ATP-dependent gate. Mutation of the nearby positively charged residue R54, known to be involved in stabilizing channel opening via electrostatic interactions with phosphatidylinositol 4,5-bisphosphate (PIP2), also caused increased ADP sensitivity as compared with ATP, suggesting a loss of function of ATP's gamma phosphate. Based on these results, we propose that a phosphate group or a negative charge at position 50 initiates channel closure by destabilizing the electrostatic interactions between negative phosphate groups of PIP2 and residues such as R54.

Intracellular calcium cycling, early afterdepolarizations, and reentry in simulated long QT syndrome

OBJECTIVES

The purpose of this study was to investigate interactions between early afterdepolarizations (EADs) and reentry in long QT (LQT) syndromes.

BACKGROUND

EADs, a characteristic feature of congenital and acquired LQT syndromes, are classically bradycardia dependent. Mechanisms by which they promote tachyarrhythmias such as torsades de pointes and ventricular fibrillation are not fully understood. Recent evidence suggests that EADs also may occur at rapid heart rates as a sequela of spontaneous sarcoplasmic reticulum (SR) Ca(2+) release related to intracellular Ca(2+) overload. Here, we performed computer simulations to explore the arrhythmogenic consequences of this phenomenon.

METHODS

We used a modified version of the Luo-Rudy dynamic model in one-dimensional and two-dimensional cardiac tissue with the time-dependent K(+) currents I(Kr) or I(Ks) reduced by 50% to simulate acquired and congenital LQT syndromes.

RESULTS

(1) Spontaneous SR Ca(2+) release prolonged action potential duration but did not induce overt EADs unless K(+) current density was reduced to simulate acquired and congenital LQT syndromes. (2) In simulated LQT syndromes, EADs were capable of both terminating and reinitiating one-dimensional reentry. (3) A similar phenomenon in simulated 2D tissue led to reinitiation of spiral wave reentry that otherwise would have self-terminated. (4) Reentry reinitiation occurred only when the L-type Ca(2+) current and SR Ca(i) cycling were potentiated to simulate moderate sympathetic stimulation, consistent with the known arrhythmogenic effects of sympathetic activation (and protection by beta-blockers) in LQT syndromes.

CONCLUSIONS

These computer simulations suggest that EADs related to spontaneous SR Ca(2+) release can enhance arrhythmogenesis in LQT syndromes by reinitiating reentry.

Effects of Na(+) and K(+) channel blockade on vulnerability to and termination of fibrillation in simulated normal cardiac tissue

Na(+) and K(+) channel-blocking drugs have anti- and proarrhythmic effects. Their effects during fibrillation, however, remain poorly understood. We used computer simulation of a two-dimensional (2-D) structurally normal tissue model with phase I of the Luo-Rudy action potential model to study the effects of Na(+) and K(+) channel blockade on vulnerability to and termination of reentry in simulated multiple-wavelet and mother rotor fibrillation. Our main findings are as follows: 1) Na(+) channel blockade decreased, whereas K(+) channel blockade increased, the vulnerable window of reentry in heterogeneous 2-D tissue because of opposing effects on dynamical wave instability. 2) Na(+) channel blockade increased the cycle length of reentry more than it increased refractoriness. In multiple-wavelet fibrillation, Na(+) channel blockade first increased and then decreased the average duration or transient time (<T(s)>) of fibrillation. In mother rotor fibrillation, Na(+) channel blockade caused peripheral fibrillatory conduction block to resolve and the mother rotor to drift, leading to self-termination or sustained tachycardia. 3) K(+) channel blockade increased dynamical instability by steepening action potential duration restitution. In multiple-wavelet fibrillation, this effect shortened <T(s)> because of enhanced wave instability. In mother rotor fibrillation, this effect converted mother rotor fibrillation to multiple-wavelet fibrillation, which then could self-terminate. Our findings help illuminate, from a theoretical perspective, the possible underlying mechanisms of termination of different types of fibrillation by antiarrhythmic drugs.

K+-dependent regulation of matrix volume improves mitochondrial function under conditions mimicking ischemia-reperfusion

To delineate the role of mitochondrial K+ fluxes in cardioprotection, we investigated the effect of extramitochondrial K+ on the ability of mitochondria to support membrane potential (DeltaPsi), regulate matrix volume, consume oxygen, and phosphorylate ADP under conditions mimicking key elements of ischemia-reperfusion. Isolated energized mitochondria responded to ADP addition with depolarization, increased O2 consumption, and matrix shrinkage. The time required for full recovery of DeltaPsi, signaling the completion of ADP phosphorylation, was used to evaluate the rate of ATP synthesis during repeated ADP pulses. In mitochondria with a decreased ability to support DeltaPsi, the rate of ADP phosphorylation was significantly improved by extramitochondrial K+ > Na+ > Li+, especially at higher buffer osmolarity, which promotes matrix shrinkage. K+-induced improvement in DeltaPsi recovery after ADP pulses was accompanied by more rapid and complete matrix volume recovery and enhanced O2 consumption. Manipulations expected to affect matrix swelling by regulating K+ fluxes or water distribution indicate that matrix volume regulation by external factors becomes increasingly important in mitochondria with decreased ability to support DeltaPsi in the face of a high ADP load. Under these conditions, opening of K+ influx pathways improved mitochondrial function and delayed failure. This may be an important factor in the mechanism of diaxozide-induced cardioprotection.

Action Potential Duration Restitution and Alternans in Rabbit Ventricular Myocytes

Action potential duration (APD) restitution properties and repolarization alternans are thought to be important arrhythmogenic factors. We investigated the role of intracellular calcium (Ca 2+ i ) cycling in regulating APD restitution slope and repolarization (APD) alternans in patch-clamped rabbit ventricular myocytes at 34 to 36°C, using the perforated or ruptured patch clamp techniques with Fura-2-AM to record Ca 2+ i . When APD restitution was measured by either the standard extrastimulus (S1S2) method or the dynamic rapid pacing method, the maximum APD restitution slope exceeded 1 by both methods, but was more shallow with the dynamic method. These differences were associated with greater Ca 2+ i accumulation during dynamic pacing. The onset of APD alternans occurred at diastolic intervals at which the APD restitution slope was significantly <1 and was abolished by suppressing sarcoplasmic reticulum (SR) Ca 2+ i cycling with thapsigargin and ryanodine, or buffering the global Ca 2+ i transient with BAPTA-AM or BAPTA. Thapsigargin and ryanodine flattened APD restitution slope to <1 when measured by the dynamic method, but not by the S1S2 method. BAPTA-AM or BAPTA failed to flatten APD restitution slope to <1 by either method. In conclusion, APD alternans requires intact Ca 2+ i cycling and is not reliably predicted by APD restitution slope when Ca 2+ i cycling is suppressed. Ca 2+ i cycling may contribute to differences between APD restitution curves measured by S1S2 versus dynamic pacing protocols by inducing short-term memory effects related to pacing-dependent Ca 2+ i accumulation.

Dissociation of Membrane Potential and Intracellular Calcium during Ventricular Fibrillation

Membrane potential and intracellular calcium during VF.

INTRODUCTION

The cardiac action potential (AP) and the intracellular Ca transient (CaT) are closely associated under normal physiological conditions, but not during ventricular fibrillation (VF). The purpose of this study was to determine whether this dissociation is directly related to the fast activation rate during VF.

METHODS AND RESULTS

We optically mapped AP and CaT simultaneously in nine isolated rabbit hearts. Pinacidil, a K(ATP) channel opener, was used to shorten the action potential duration (APD) in order to capture tissue at fast pacing rates or to induce ventricular tachycardia (VT) comparable to VF activation rates. Mutual information (MI) was used to calculate the degree of AP and CaT coupling. Pinacidil (40 microM) infusion significantly shortened APD. The CL of VF without pinacidil averaged 77+/-13 ms, whereas the shortest CL achieved during VT under pinacidil infusion was 76 ms. MIs during fast pacing (1.13+/-0.15 bits) and fast VT (0.88+/-0.18 bits) were higher than those during baseline VF (0.39+/-0.11 bits), VF with pinacidil infusion (0.21+/-0.07 bits) and VF after pinacidil washout (0.36+/-0.15 bits). MIs during fast pacing or fast VT were higher than that of VFs at comparable dominant frequencies.

CONCLUSIONS

CaT is closely associated with the AP during fast pacing and fast VT, but not during VF. The reduced MI during VF is not secondary to the fast rate of activation.

Multisite phosphorylation and network dynamics of cyclin-dependent kinase signaling in the eukaryotic cell cycle

Multisite phosphorylation of regulatory proteins has been proposed to underlie ultrasensitive responses required to generate nontrivial dynamics in complex biological signaling networks. We used a random search strategy to analyze the role of multisite phosphorylation of key proteins regulating cyclin-dependent kinase (CDK) activity in a model of the eukaryotic cell cycle. We show that multisite phosphorylation of either CDK, CDC25, wee1, or CDK-activating kinase is sufficient to generate dynamical behaviors including bistability and limit cycles. Moreover, combining multiple feedback loops based on multisite phosphorylation do not destabilize the cell cycle network by inducing complex behavior, but rather increase the overall robustness of the network. In this model we find that bistability is the major dynamical behavior of the CDK signaling network, and that negative feedback converts bistability into limit cycle behavior. We also compare the dynamical behavior of several simplified models of CDK regulation to the fully detailed model. In summary, our findings suggest that multisite phosphorylation of proteins is a critical biological mechanism in generating the essential dynamics and ensuring robust behavior of the cell cycle.

Hysteresis and cell cycle transitions: how crucial is it?

Recently, experiments have shown that cyclin-dependent kinase (CDK) activity exhibits hysteresis in its response to total cyclin when cyclin is made nondegradable and controlled externally. This observation was taken to support mathematical modeling predictions regarding the underlying dynamics of the cell cycle. However, cell cycle dynamics can also be generated by other nonhysteretic mechanisms. To examine the robustness of the hysteretic response of CDK activity to total cyclin, we simulated various cell cycle signal transduction networks, and correlated the dynamics to the response function of CDK activity versus total cyclin. By randomly searching the parameter space, we assessed robustness by estimating the frequency of hysteretic versus nonhysteretic dynamical mechanisms. When the dynamical instabilities were caused by feedback loops in CDK phosphorylation and dephosphorylation or by feedback between cyclin and the CDK inhibitor, the response function of CDK activity versus total cyclin correlated well with the dynamical instabilities. However, when the dynamical instabilities originated from feedback between cyclin and APC-CDH1 or RB-E2F, the response function did not correlate with dynamical instabilities. Thus, although a hysteretic response is neither necessary nor sufficient, it is in general a much more robust mechanism for generating cell cycle dynamics than nonhysteretic mechanisms.