Ion channels form transmembrane water-filled pores that allow ions to cross membranes in a rapid and selective fashion. The amino acid residues that line these pores have been sought to reveal the mechanisms of ion conduction and selectivity. The pore (P) loop is a stretch of residues that influences single-channel-current amplitude, selectivity among ions and open-channel blockade and is conserved in potassium-channel subunits previously recognized to contribute to pore formation. To date, potassium-channel pores have been shown to form by symmetrical alignment of four P loops around a central conduction pathway. Here we show that the selectivity-determining pore region of the voltage-gated potassium channel of human heart through which the I(Ks) current passes includes the transmembrane segment of the non-P-loop protein minK. Two adjacent residues in this segment of minK are exposed in the pore on either side of a short barrier that restricts the movement of sodium, cadmium and zinc ions across the membrane. Thus, potassium-selective pores are not restricted to P loops or a strict P-loop geometry.

KCNQ1 α subunits form functionally distinct potassium channels by coassembling with KCNE ancillary subunits MinK and MiRP2. MinK-KCNQ1 channels generate the slowly activating, voltage-dependent cardiac I Ks current. MiRP2-KCNQ1 channels form a constitutively active current in the colon. The structural basis for these contrasting channel properties, and the mechanisms of α subunit modulation by KCNE subunits, are not fully understood. Here, scanning mutagenesis located a tryptophan-tolerant region at positions 338-340 within the KCNQ1 pore-lining S6 domain, suggesting an exposed region possibly amenable to interaction with transmembrane ancillary subunits. This hypothesis was tested using concomitant mutagenesis in KCNQ1 and in the membrane-localized 'activation triplet' regions of MinK and MiRP2 to identify pairs of residues that interact to control KCNQ1 activation.

MinK has neither the P region nor signature sequence that characterizes pore-forming subunits of all known K+ channels. A specific minK region has now been identified that affects external blockade by 2 common probes of K+ channel pores. When mutated to cysteine, residues in this region render minK susceptible to covalent blockade by methanethiosulfonate ethylsulfonate and alter reversible inhibition by tetraethylammonium. The 2 blockers are found to share overlapping binding site determinants and to interact. Since inhibition by external tetraethylammonium is sensitive to voltage and to the internal concentration of permeant ions, we argue that tetraethylammonium blocks by occluding the external end of a water-filled transmembrane pore. These findings support the view that minK is directly involved in forming a K+-selective ion conduction pathway.

We recently showed that activation of ATP-sensitive potassium (K ATP ) channels in PC12 cells induces protection against the neurotoxic effect of rotenone, a mitochondrial complex I inhibitor. In this study, we sought to determine the locus of the K ATP channels that mediate this protection in PC12 cells. We found that pretreatment of PC12 cells with diazoxide, a mitochondrial K ATP channel selective opener, dose-dependently increases cell viability against rotenone-induced cell death as indicated in trypan blue exclusion assays. The protective effect of this preconditioning is attenuated by 5-hydroxydecanoic acid (5-HD), a selective mitochondrial K ATP channel antagonist but not in the presence of HMR-1098, a selective plasma membrane K ATP potassium channel antagonist. In contrast, P-1075, a selective plasma membrane K ATP channel opener, does not induce protection. Using specific antibodies against SUR1 and Kir6.1, we detected immunoreactive proteins of apparent molecular masses 155 and 50 kDa, corresponding to those previously reported for SUR1 and Kir6.1, respectively, in the mitochondria-enriched fraction of PC12 cells. In addition, whole cell patch-clamp studies revealed that inward currents in PC12 cells are insensitive to P-1075, HMR-1098, glibenclamide and diazoxide, indicating that functional plasma membrane K ATP channels are negligible. Taken together, our results demonstrate for the first time that activation of mitochondrial K ATP channels elicits protection against rotenone-induced cell death.

Ketogenic diet (KD) is an effective treatment for intractable epilepsies. We recently found that KD can prevent seizure and myoclonic jerk in a rat model of post-hypoxic myoclonus. In the present study, we tested the hypothesis that KD can prevent the cerebral ischemic neurodegeneration in this animal model. Rats fed a standard diet or KD for 25 days were being subjected to mechanically induced cardiac arrest brain ischemia for 8 min 30 s. Nine days after cardiac arrest, frozen rat brains were sectioned for evaluation of ischemia-induced neurodegeneration using fluoro-jade (FJ) staining. The FJ positive degenerating neurons were counted manually. Cardiac arrest-induced cerebral ischemia in rats fed the standard diet exhibited extensive neurodegeneration in the CA1 region of the hippocampus, the number of FJ positive neurons was 822+/-80 (n=4). They also showed signs of neurodegeneration in the Purkinje cells of the cerebellum and in the thalamic reticular nucleus, the number of FJ positive neurons in the cerebellum was 55+/-27 (n=4), the number of FJ positive neurons in the thalamic reticular nucleus was 22+/-5 (n=4). In contrast, rats fed KD showed no evidence of neurodegeneration, the number of FJ positive neurons in these areas were zero. The results demonstrate that KD can prevent cardiac arrest-induced cerebral ischemic neurodegeneration in selected brain regions.

It is anticipated that further understanding of the protective mechanism induced by ischemic preconditioning will improve prognosis for patients of ischemic injury. It is not known whether preconditioning exerts beneficial actions in neurodegenerative diseases, in which ischemic injury plays a causative role. Here we show that transient activation of ATP-sensitive potassium channels, a trigger in ischemic preconditioning signaling, confers protection in PC12 cells and SH-SY5Y cells against neurotoxic effect of rotenone and MPTP, mitochondrial complex I inhibitors that have been implicated in the pathogenesis of Parkinson's disease. The degree of protection is in proportion to the bouts of exposure to an ATP-sensitive potassium channel opener, a feature reminiscent of ischemic tolerance in vivo. Protection is sensitive to a protein synthesis inhibitor, indicating the involvement of de novo protein synthesis in the protective processes. Pretreatment of PC12 cells with preconditioning stimuli FeSO(4) or xanthine/xanthine oxidase also confers protection against rotenone-induced cell death. Our results demonstrate for the first time the protective role of ATP-sensitive potassium channels in a dopaminergic neuronal cell line against rotenone-induced neurotoxicity and conceptually support the view that ischemic preconditioning-derived therapeutic strategies may have potential and feasibility in therapy for Parkinson's disease.

MinK is a transmembrane protein of 130 amino acids found in the kidney, heart, and vestibular system of mammals. Its expression in Xenopus laevis oocytes induces a voltage-dependent potassium current similar to that seen in vivo. Indirect evidence has fueled speculation that function requires association of MinK and another protein endogenous to oocytes and native tissues. In this report, we show that direct covalent modification of an oocyte membrane protein alters properties of the MinK ion conduction pore; modified channels exhibit decreased potassium conduction and increased permeability to sodium and cesium. The modifying reagents, two membrane-impermeant, sulfhydryl-specific methanethiosulfonate derivatives, react only from the extracellular solution at rates that are determined by the conformational state of the channel. These findings indicate that MinK is intimately associated with an oocyte protein whose exposure to the external solution changes during channel gating and which acts with MinK to establish ion conduction pore function.

In the present study, we evaluated the anti-seizure and anti-myoclonic activity of levetiracetam and brivaracetam in an established rat model of cardiac arrest-induced post-hypoxic myoclonus. We found that brivaracetam (0.3 mg/kg, the minimal effective dose) was more potent than levetiracetam (3 mg/kg, the minimal effective dose) against post-hypoxic seizures. The anti-seizure activity of both compounds occurred 30 min following intraperitoneal (i.p.) administration and was maintained over the entire 150 min post-dose observation period. Both brivaracetam and levetiracetam significantly reduced auditory stimulated post-hypoxic myoclonus from a dose 0.3 mg/kg. At that dose, the anti-myoclonic activity of brivaracetam was already maximal whereas it continued to increase in a dose-relation manner with levetiracetam, suggesting that brivaracetam is a more potent agent. The onset and the duration of anti-myoclonic activity of both compounds were similar. These findings demonstrate that brivaracetam possesses more potent anti-seizure and anti-myoclonic activity than levetiracetam in an established rat model of cardiac arrest-induced post-hypoxic myoclonus.