Physiological consequences of complex II inhibition for aging, disease, and the mKATP channel.

In recent years, it has become apparent that there exist several roles for respiratory complex II beyond metabolism. These include: (i) succinate signaling, (ii) reactive oxygen species (ROS) generation, (iii) ischemic preconditioning, (iv) various disease states and aging, and (v) a role in the function of the mitochondrial ATP-sensitive K(+) (mKATP) channel. This review will address the involvement of complex II in each of these areas, with a focus on how complex II regulates or may be involved in the assembly of the mKATP. This article is part of a Special Issue entitled: Respiratory complex II: Role in cellular physiology and disease.

Sex modifies genetic effects on residual variance in urinary calcium excretion in rat (Rattus norvegicus).

Conventional genetics assumes common variance among alleles or genetic groups. However, evidence from vertebrate and invertebrate models suggests that residual genotypic variance may itself be under partial genetic control. Such a phenomenon would have great significance: high-variability alleles might confound the detection of "classically" acting genes or scatter predicted evolutionary outcomes among unpredicted trajectories. Of the few works on this phenomenon, many implicate sex in some aspect of its control. We found that female genetic hypercalciuric stone-forming (GHS) rats (Rattus norvegicus) had higher coefficients of variation (CVs) for urinary calcium (CV = 0.14) than GHS males (CV = 0.06), and the reverse in normocalciuric Wistar-Kyoto rats (WKY) (CV(♂) = 0.14; CV(♀) = 0.09), suggesting sex-by-genotype interaction on residual variance. We therefore investigated the effect of sex on absolute-transformed residuals in urinary calcium in an F(2) GHS × WKY mapping cohort. Absolute residuals were associated with genotype at two microsatellites, D3Rat46 (RNO3, 33.9 Mb) and D4Mgh1 (RNO4, 84.8 MB) at Bonferroni thresholds across the entire cohort, and with the microsatellites D3Rat46, D9Mgh2 (RNO9, 84.4 Mb), and D12Rat25 (RNO12, 40.4 Mb) in females (P < 0.05) but not males. In GHS chromosome 1 congenic lines bred onto a WKY genomic background, we found that congenic males had significantly (P < 0.0001) higher CVs for urinary calcium (CV = 0.25) than females (CV = 0.15), supporting the hypothesis of the inheritance of sex-by-genotype interaction on this effect. Our findings suggest that genetic effects on residual variance are sex linked; heritable, sex-specific residuals might have great potential implications for evolution, adaptation, and genetic analysis.

The mitochondrial complex II and ATP-sensitive potassium channel interaction: quantitation of the channel in heart mitochondria.

The mitochondrial ATP-sensitive potassium channel (mK(ATP)) is important in cardioprotection, although the channel remains molecularly undefined. Several studies have demonstrated that mitochondrial complex II inhibitors activate the mK(ATP), suggesting a potential role for complex II in channel composition or regulation. However, these inhibitors activate mK(ATP) at concentrations which do not affect bulk complex II activity. Using the potent complex II inhibitor Atpenin A5, this relationship was investigated using tight-binding inhibitor theory, to demonstrate that only 0.4 % of total complex II molecules are necessary to activate the mK(ATP). These results estimate the mK(ATP) content at 15 channels per mitochondrion.

A novel mitochondrial K(ATP) channel assay.

RATIONALE: The mitochondrial ATP sensitive potassium channel (mK(ATP)) is implicated in cardioprotection by ischemic preconditioning (IPC), but the molecular identity of the channel remains controversial. The validity of current methods to assay mK(ATP) activity is disputed. OBJECTIVE: We sought to develop novel methods to assay mK(ATP) activity and its regulation. METHODS AND RESULTS: Using a thallium (Tl(+))-sensitive fluorophore, we developed a novel Tl(+) flux based assay for mK(ATP) activity, and used this assay probe several aspects of mK(ATP) function. The following key observations were made. (1) Time-dependent run down of mK(ATP) activity was reversed by phosphatidylinositol-4,5-bisphosphate (PIP(2)). (2) Dose responses of mK(ATP) to nucleotides revealed a UDP EC(50) of approximately 20 micromol/L and an ATP IC(50) of approximately 5 micromol/L. (3) The antidepressant fluoxetine (Prozac) inhibited mK(ATP) (IC(50)=2.4 micromol/L). Fluoxetine also blocked cardioprotection triggered by IPC, but did not block protection triggered by a mK(ATP)-independent stimulus. The related antidepressant zimelidine was without effect on either mK(ATP) or IPC. CONCLUSIONS: The Tl(+) flux mK(ATP) assay was validated by correlation with a classical mK(ATP) channel osmotic swelling assay (R(2)=0.855). The pharmacological profile of mK(ATP) (response to ATP, UDP, PIP(2), and fluoxetine) is consistent with that of an inward rectifying K(+) channel (K(IR)) and is somewhat closer to that of the K(IR)6.2 than the K(IR)6.1 isoform. The effect of fluoxetine on mK(ATP)-dependent cardioprotection has implications for the growing use of antidepressants in patients who may benefit from preconditioning.

The C. elegans mitochondrial K+(ATP) channel: a potential target for preconditioning.

Ischemic preconditioning (IPC) is an evolutionarily conserved endogenous mechanism whereby short periods of non-lethal exposure to hypoxia alleviate damage caused by subsequent ischemia reperfusion (IR). Pharmacologic targeting has suggested that the mitochondrial ATP-sensitive potassium channel (mK(ATP)) is central to IPC signaling, despite its lack of molecular identity. Here, we report that isolated Caenorhabditis elegans mitochondria have a K(ATP) channel with the same physiologic and pharmacologic characteristics as the vertebrate channel. Since C. elegans also exhibit IPC, our observations provide a framework to study the role of mK(ATP) in IR injury in a genetic model organism.