Iptakalim modulates ATP-sensitive K(+) channels in dopamine neurons from rat substantia nigra pars compacta

Iptakalim alleviates synaptic damages via targeting mitochondrial ATP-sensitive potassium channel in depression

Synaptic plasticity damages play a crucial role in the onset and development of depression, especially in the hippocampus, which is more susceptible to stress and the most frequently studied brain region in depression. And, mitochondria have a major function in executing the complex processes of neurotransmission and plasticity. We have previously demonstrated that Iptakalim (Ipt), a new ATP-sensitive potassium (K-ATP) channel opener, could improve the depressive-like behavior in mice. But the underlying mechanisms are not well understood. The present study demonstrated that Ipt reversed depressive-like phenotype in vivo (chronic mild stress-induced mice model of depression) and in vitro (corticosterone-induced cellular model). Further study showed that Ipt could upregulate the synaptic-related proteins postsynaptic density 95 (PSD 95) and synaptophysin (SYN), and alleviated the synaptic structure damage. Moreover, Ipt could reverse the abnormal mitochondrial fission and fusion, as well as the reduced mitochondrial ATP production and collapse of mitochondrial membrane potential in depressive models. Knocking down the mitochondrial ATP-sensitive potassium (Mito-KATP) channel subunit MitoK partly blocked the above effects of Ipt. Therefore, our results reveal that Ipt can alleviate the abnormal mitochondrial dynamics and function depending on MitoK, contributing to improve synaptic plasticity and exert antidepressive effects. These findings provide a candidate compound and a novel target for antidepressive therapy.

Kir6.2 Deficiency Promotes Mesencephalic Neural Precursor Cell Differentiation via Regulating miR-133b/GDNF in a Parkinson's Disease Mouse Model

The loss of dopaminergic (DA) neurons in the substantia nigra (SN) is a major feature in the pathology of Parkinson's disease (PD). Using neural stem or progenitor cells (NSC/NPCs), the prospect of replacing the missing or damaged DA neurons is very attractive for PD therapy. However, little is known about the endogenous mechanisms and molecular pathways regulating the NSC/NPC proliferation and differentiation in the development of PD. Herein, using Kir6.2 knockout (Kir6.2-/-) mice, we observed that genetic deficiency of Kir6.2 exacerbated the loss of SN DA neurons relatively early in a chronic MPTP/probenecid (MPTP/p) injection course, but rescued the damage of neurons 7 days after the last MPTP/p injection. Meanwhile, we found that Kir6.2 knockout predominantly increased the differentiation of nuclear receptor-related 1 (Nurr1+) precursors to DA neurons, indicating that Kir6.2 deficiency could activate an endogenous self-repair process. Furthermore, we demonstrated in vivo and in vitro that lack of Kir6.2 promoted neuronal differentiation via inhibiting the downregulation of glia cell line-derived neurotrophic factor (GDNF), which negatively related to the level of microRNA-133b. Notably, we revealed that Gdnf is a target gene of miR-133b and transfection of miR-133b could attenuate the enhancement of neural precursor differentiation induced by Kir6.2 deficiency. Collectively, we clarify for the first time that Kir6.2/K-ATP channel functions as a novel endogenous negative regulator of NPC differentiation, and provide a promising neuroprotective target for PD therapeutics.

Subclinical Doses of ATP-Sensitive Potassium Channel Modulators Prevent Alterations in Memory and Synaptic Plasticity Induced by Amyloid-β

In addition to coupling cell metabolism and excitability, ATP-sensitive potassium channels (KATP) are involved in neural function and plasticity. Moreover, alterations in KATP activity and expression have been observed in Alzheimer's disease (AD) and during amyloid-β (Aβ)-induced pathology. Thus, we tested whether KATP modulators can influence Aβ-induced deleterious effects on memory, hippocampal network function, and plasticity. We found that treating animals with subclinical doses (those that did not change glycemia) of a KATP blocker (Tolbutamide) or a KATP opener (Diazoxide) differentially restrained Aβ-induced memory deficit, hippocampal network activity inhibition, and long-term synaptic plasticity unbalance (i.e., inhibition of LTP and promotion of LTD). We found that the protective effect of Tolbutamide against Aβ-induced memory deficit was strong and correlated with the reestablishment of synaptic plasticity balance, whereas Diazoxide treatment produced a mild protection against Aβ-induced memory deficit, which was not related to a complete reestablishment of synaptic plasticity balance. Interestingly, treatment with both KATP modulators renders the hippocampus resistant to Aβ-induced inhibition of hippocampal network activity. These findings indicate that KATP are involved in Aβ-induced pathology and they heighten the potential role of KATP modulation as a plausible therapeutic strategy against AD.

Paradoxical lower sensitivity of Locus Coeruleus than Substantia Nigra pars compacta neurons to acute actions of rotenone

Parkinson's disease (PD) is not only associated with degeneration of dopaminergic (DAergic) neurons in the Substantia Nigra, but also with profound loss of noradrenergic neurons in the Locus Coeruleus (LC). Remarkably, LC degeneration may exceed, or even precede the loss of nigral DAergic neurons, suggesting that LC neurons may be more susceptible to damage by various insults. Using a combination of electrophysiology, fluorescence imaging and electrochemistry, we directly compared the responses of LC, nigral DAergic and nigral non-dopaminergic (non-DAergic) neurons in rat brain slices to acute application of rotenone, a mitochondrial toxin used to create animal and in vitro models of PD. Rotenone (0.01-5.0μM) dose-dependently inhibited the firing of all three groups of neurons, primarily by activating K_ATP channels. The toxin also depolarised mitochondrial potential (Ψ_m) and released reactive oxygen species (H₂O₂). When K_ATP channels were blocked, rotenone (1μM) increased the firing of LC neurons by activating an inward current associated with dose-dependent increase of cytosolic free Ca²⁺ ([Ca²⁺]_i). This effect was attenuated by blocking oxidative stress-sensitive TRPM2 channels, and by pre-treatment of slices with anti-oxidants. These results demonstrate that rotenone inhibits the activity of LC neurons mainly by activating K_ATP channels, and increases [Ca²⁺]_ivia TRPM2 channels. Since the responses of LC neurons were smaller than those of nigral DAergic neurons, our study shows that LC neurons are paradoxically less sensitive to acute effects of this parkinsonian toxin.

ABCC9 gene polymorphism is associated with hippocampal sclerosis of aging pathology

Hippocampal sclerosis of aging (HS-Aging) is a high-morbidity brain disease in the elderly but risk factors are largely unknown. We report the first genome-wide association study (GWAS) with HS-Aging pathology as an endophenotype. In collaboration with the Alzheimer's Disease Genetics Consortium, data were analyzed from large autopsy cohorts: (#1) National Alzheimer's Coordinating Center (NACC); (#2) Rush University Religious Orders Study and Memory and Aging Project; (#3) Group Health Research Institute Adult Changes in Thought study; (#4) University of California at Irvine 90+ Study; and (#5) University of Kentucky Alzheimer's Disease Center. Altogether, 363 HS-Aging cases and 2,303 controls, all pathologically confirmed, provided statistical power to test for risk alleles with large effect size. A two-tier study design included GWAS from cohorts #1-3 (Stage I) to identify promising SNP candidates, followed by focused evaluation of particular SNPs in cohorts #4-5 (Stage II). Polymorphism in the ATP-binding cassette, sub-family C member 9 (ABCC9) gene, also known as sulfonylurea receptor 2, was associated with HS-Aging pathology. In the meta-analyzed Stage I GWAS, ABCC9 polymorphisms yielded the lowest p values, and factoring in the Stage II results, the meta-analyzed risk SNP (rs704178:G) attained genome-wide statistical significance (p = 1.4 × 10(-9)), with odds ratio (OR) of 2.13 (recessive mode of inheritance). For SNPs previously linked to hippocampal sclerosis, meta-analyses of Stage I results show OR = 1.16 for rs5848 (GRN) and OR = 1.22 rs1990622 (TMEM106B), with the risk alleles as previously described. Sulfonylureas, a widely prescribed drug class used to treat diabetes, also modify human ABCC9 protein function. A subsample of patients from the NACC database (n = 624) were identified who were older than age 85 at death with known drug history. Controlling for important confounders such as diabetes itself, exposure to a sulfonylurea drug was associated with risk for HS-Aging pathology (p = 0.03). Thus, we describe a novel and targetable dementia risk factor.

Localization of the ATP-sensitive K(+) channel regulatory subunits SUR2A and SUR2B in the rat brain

ATP-sensitive K(+) (K(ATP)) channel subunits SUR2A and SUR2B in the rat brain were investigated by RT-PCR assay, western blot analysis, in situ hybridization histochemistry, and immunohistochemical staining. The results show that the mRNA and protein of SUR2A and SUR2B are expressed in whole rat brain extracts and selected regions. SUR2 mRNA is widely expressed in many neurons and glial cells as revealed by in situ hybridization histochemistry. Immunohistochemical staining shows SUR2A to be widely expressed in neurons of the brain, especially in the large pyramidal neurons and their main dendrites in the neocortex and in the Purkinje cells of the cerebellar cortex. In contrast to SUR2A, SUR2B is potently expressed in small cells in the corpus callosum and cerebellar white matter, but is also weakly expressed in some neurons. Double immunostaining shows SUR2B to be localized in astrocytes and oligodendrocytes, while SUR2A is only localized in oligodendrocytes. These results suggest that SUR2A might be mainly a regulatory subunit of the K(ATP) channel in most neurons and part of oligodendrocytes, while SUR2B might be mainly a regulatory subunit of the K(ATP) channel in astrocytes, oligodendrocytes, and some neurons.

Mitochondrial ATP-sensitive potassium channels enhance angiotensin-induced oxidative damage and dopaminergic neuron degeneration. Relevance for aging-associated susceptibility to Parkinson's disease

Recent studies have shown that renin-angiotensin system overactivation is involved in the aging process in several tissues as well as in longevity and aging-related degenerative diseases by increasing oxidative damage and inflammation. We have recently shown that angiotensin II enhances dopaminergic degeneration by increasing levels of reactive oxygen species and neuroinflammation, and that there is an aging-related increase in angiotensin II activity in the substantia nigra in rats, which may constitute a major factor in the increased risk of Parkinson's disease with aging. The mechanisms involved in the above mentioned effects and particularly a potential angiotensin-mitochondria interaction have not been clarified. The present study revealed that activation of mitochondrial ATP-sensitive potassium channels [mitoK(ATP)] may play a major role in the angiotensin II-induced effects on aging and neurodegeneration. Inhibition of mitoK(ATP) channels with 5-hydroxydecanoic acid inhibited the increase in dopaminergic cell death induced by angiotensin II, as well as the increase in superoxide/superoxide-derived reactive oxygen species levels and the angiotensin II-induced decrease in the mitochondrial inner membrane potential in cultured dopaminergic neurons. The present study provides data for considering brain renin-angiotensin system and mitoK(ATP) channels as potential targets for protective therapy in aging-associated diseases such as Parkinson's disease.

Iptakalim enhances adult mouse hippocampal neurogenesis via opening Kir6.1-composed K-ATP channels expressed in neural stem cells

BACKGROUND AND PURPOSE

Emerging evidence indicates that stimulating adult neurogenesis provides novel strategies for central nervous system diseases. Iptakalim (Ipt), a novel ATP-sensitive potassium (K-ATP) channel opener, has been demonstrated to play multipotential neuroprotective effects in vivo and in vitro. However, it remains unknown whether Ipt could regulate the adult neurogenesis.

METHODS AND RESULTS

Based on the finding that adult neural stem cells (ANSCs) in hippocampus expressed Kir6.1/SUR1-composed K-ATP channel, Kir6.1 heterozygotic (Kir6.1(+/-) ) mice were used to investigate whether and how Ipt regulates adult hippocampal neurogenesis. We showed that administration of Ipt (10 mg/kg) or fluoxetine (Flx, 10 mg/kg) for 4 weeks significantly increased newborn ANSCs in subgranular zone (SGZ) of Kir6.1(+/+) mice but failed to affect those of Kir6.1(+/-) mice. Meanwhile, ANSCs in Kir6.1(+/-) mice exhibited decreased survival rate and impaired ability of differentiation into astrocytes. We further found that Kir6.1(+/-) mice showed lower level of brain-derived neurotrophic factor (BDNF) in hippocampus compared with Kir6.1(+/+) mice. Furthermore, Ipt increased the levels of BDNF and basic fibroblast growth factor (FGF-2) throughout the hippocampus in Kir6.1(+/+) mice but not in Kir6.1(+/-) mice. Moreover, Ipt and Flx enhanced the phosphorylation of Akt and CREB in the hippocampus of Kir6.1(+/+) mice. Notably, these effects were completely abolished in Kir6.1(+/-) mice.

CONCLUSIONS

Our findings demonstrate that Ipt stimulates the adult hippocampal neurogenesis via activation of Akt and CREB signal following the opening of Kir6.1-composed K-ATP channels, which gives us an insight into the therapeutic implication of Ipt in the diseases with adult neurogenesis deficiency, such as major depression.

Paraquat- and rotenone-induced models of Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disorder mainly characterized by a loss of dopaminergic (DA) neurons in the substantia nigra pars compacta. In recent years, several new genes and environmental factors have been implicated in PD, and their impact on DA neuronal cell death is slowly emerging. However, PD etiology remains unknown, whereas its pathogenesis begins to be clarified as a multifactorial cascade of deleterious factors. Recent epidemiological studies have linked exposure to environmental agents, including pesticides, with an increased risk of developing the disease. As a result, over the last two decades the "environmental hypothesis" of PD has gained considerable interest. This speculates that agricultural chemicals in the environment, by producing selective dopaminergic cell death, can contribute to the development of the disease. However, a causal role for pesticides in the etiology of PD has yet to be definitively established. Importantly, most insights into PD pathogenesis came from investigations performed in experimental models of PD, especially those produced by neurotoxins. This review presents data obtained in our laboratories along with current views on the neurotoxic actions induced by the two most popular parkinsonian pesticide neurotoxins, namely paraquat and rotenone. Although confined to these two chemicals, mechanistic studies underlying dopaminergic cell death are of the utmost importance to identify new drug targets for the treatment of PD.

Glucosensing in parvocellular neurons of the rat hypothalamic paraventricular nucleus

Specialized hypothalamic neurons responding to rising extracellular glucose via increases or decreases in their electrical activity [glucose-excited (GE) and glucose-inhibited (GI) cells, respectively] have been reported in the hypothalamic arcuate, ventromedial and lateral nuclei. The hypothalamic paraventricular nucleus (PVN) is an important neurosecretory and preautonomic output nucleus. We tested whether parvocellular PVN neurons also possess glucosensing properties, using patch-clamp recording and immunocytochemistry. Putative neurosecretory (p-NS) and preautonomic (p-PA) cells were identified electrophysiologically. Although parvocellular neurons were insensitive to transitions from 10 to 2.5 mm glucose, approximately 68% of p-PA cells responded directly to glucopenia (mimicked by a step to 0.2 mm glucose) with an increased membrane conductance. Of these, approximately 24% hyperpolarized (accompanied by an outward current) and thus were GE, approximately 26% depolarized (with an inward current, thus GI) and approximately 18% did not change membrane potential. The concentration dependence of the glucose response was similar for both GE and GI cells (EC(50) of 0.67-0.7 mm), but was steep, with Hill slopes of 3-4. The K(ATP) channel blockers glibenclamide and tolbutamide did not prevent, while the K(ATP) channel opener diazoxide did not mimic, the effects of low glucose on GE neurons. Moreover, the K(ATP) sulfonylurea receptor SUR1 was not detected in glucosensitive neurons. We conclude that the PVN contains previously unknown GE and GI cells that could participate in regulation of autonomic functions. GE neurons in the PVN sense ambient glucose via a unique mechanism, probably independent of K(ATP) channels, in contrast to neurons in other hypothalamic nuclei.

Effect of inhibitors of NADPH oxidase complex and mitochondrial ATP-sensitive potassium channels on generation of dopaminergic neurons from neurospheres of mesencephalic precursors

Reactive oxygen species signaling has been suggested to regulate stem cell development. In the present study, we treated neurospheres of rat mesencephalic precursors with inhibitors of the NADPH oxidase complex and mitochondrial ATP-sensitive potassium (mitoKATP) channel blockers during the proliferation and/or the differentiation periods to study the effects on generation of dopaminergic neurons. Treatment with low doses (100 or 250 μM) of the NADPH inhibitor apocynin during the proliferation period increased the generation of dopaminergic neurons. However, higher doses (1 mM) were necessary during the differentiation period to induce the same effect. Treatment with general (glibenclamide) or mitochondrial (5-hydroxydecanoate) KATP channel blockers during the proliferation and differentiation periods increased the number of dopaminergic neurons. Furthermore, neither increased proliferation rate nor apoptosis had a major role in the observed increase in generation of dopaminergic neurons, which suggests that the redox state is able to regulate differentiation of precursors into dopaminergic neurons.

Tetrahydroberberine blocks ATP-sensitive potassium channels in dopamine neurons acutely-dissociated from rat substantia nigra pars compacta

Tetrahydroberberine (THB) exhibits neuroprotective effects but its targets and underlying mechanisms are largely unknown. Emerging evidence indicates that ATP-sensitive potassium (K(ATP)) channels in the substantia nigra pars compacta (SNc) promote Parkinson disease (PD) pathogenesis, thus blocking K(ATP) channels may protect neurons against neuronal degeneration. In the present study, we tested a hypothesis that THB blocks K(ATP) channels in dopaminergic (DA) neurons acutely dissociated from rat SNc. Using perforated patch-clamp recording in current-clamp mode, the functional K(ATP) channels can be opened by persistent perfusion of rotenone, an inhibitor of complex I of the mitochondrial respiratory chain. Bath-application of THB reversibly blocks opened K(ATP) channels in a concentration-dependent manner, which is comparable to a classical K(ATP) channel blocker, Tol. Compared to THB analogs, l-stepholidine (l-SPD) or l-tetrahydropalmatine (l-THP), THB exhibits more profound blockade in K(ATP) channels. In addition, exposure of THB alone to the recorded neuron increases action potential firing, and THB also restores rotenone-induced membrane hyperpolarization in the presence of dopamine D2 receptor antagonist (sulpiride), suggesting that THB exhibits an excitatory effect on SNc DA neurons through the block of K(ATP) channels. Collectively, the blockade of neuronal K(ATP) channels by THB in SNc DA neurons is a novel pharmacological mechanism of THB, which may contribute to its neuroprotective effects in PD.

Effects of nicorandil on sympathetic neurotransmission in rat caudal artery

1. We examined the effects of nicorandil, an ATP-sensitive potassium (K(ATP)) channel opener and nitric oxide donor, on the release of noradrenaline from vascular sympathetic nerves. This effect was compared to the effect on vascular smooth muscle. 2. Caudal artery preparations from Wistar rats were electrically stimulated (1 Hz, 0.5-ms duration) and noradrenaline release in the artery was detected using an high-pressure liquid chromatography-electrochemical detection technique. The pharmacological properties of the prejunctional effect of nicorandil were determined using the nonselective K(ATP) channel blocker glibenclamide, the pancreatic beta-cell and brain-type K(ATP) channel blocker tolbutamide, and the smooth muscle-type K(ATP) channel blocker PNU-37883A. 3. Nicorandil inhibited the electrical stimulation-evoked noradrenaline release in a concentration-dependent manner. This inhibitory effect was abolished by 1 micromol/L glibenclamide and 10 micromol/L tolbutamide, but was not affected by 10 micromol/L PNU-37883A or 0.3 micromol/L ODQ. Nicorandil did not affect the noradrenaline transporter uptake 1 in the adrenergic nerve terminals. 4. Nicorandil produced a relaxation response in a concentration-dependent manner in the caudal artery pre-contracted with 0.3 micromol/L noradrenaline. This relaxation response was significantly diminished in the presence of 1 micromol/L glibenclamide, 10 micromol/L PNU-37883A and 0.3 micromol/L ODQ but not by 10 micromol/L tolbutamide. 5. These findings suggest that nicorandil inhibits noradrenaline release via the K(ATP) channels of sympathetic nerves. These channels may be pharmacologically different from those of vascular smooth muscle.

ATP-sensitive potassium channels: novel potential roles in Parkinson's disease

The ATP-sensitive potassium (K(ATP)) channels which extensively distribute in diverse tissues (e.g. vascular smooth muscle, cardiac cells, and pancreas) are well-established for characteristics like vasodilatation, myocardial protection against ischemia, and insulin secretion. The aim of this review is to get insight into the novel roles of K(ATP) channels in Parkinson's disease (PD), with consideration of the specificities K(ATP) channels in the central nervous system (CNS), such as the control of neuronal excitability, action potential, mitochondrial function and neurotransmitter release.

Iptakalim, a vascular ATP-sensitive potassium (KATP) channel opener, closes rat pancreatic beta-cell KATP channels and increases insulin release

Sulfonylureas have been the leading oral antihyperglycemic agents, and they presently continue to be the most popular antidiabetic drugs prescribed for treatment of type 2 diabetes. However, concern has arisen over the side effects of sulfonylureas on the cardiovascular system. Here, we tested the hypothesis that iptakalim, a novel vascular ATP-sensitive potassium (K(ATP)) channel opener, closes rat pancreatic beta-cell K(ATP) channels and increases insulin release. Rat pancreatic beta-cell K(ATP) channels and heterologously expressed K(ATP) channels in both human embryonic kidney (HEK) 293 cells and Xenopus oocytes were used to test the pharmacological effects of iptakalim. Patch-clamp recordings, Ca(2+) imaging, and measurements of insulin release were applied. Patch-clamp whole-cell recordings revealed that iptakalim depolarized beta-cells, induced action potential firing, and reduced K(ATP) channel-mediated currents. Single-channel recordings revealed that iptakalim reduced the open probability of K(ATP) channels without changing channel sensitivity to ATP. By closing beta-cell K(ATP) channels, iptakalim elevated intracellular Ca(2+) concentrations and increased insulin release. In addition, iptakalim decreased the open probability of recombinant Kir6.2FL4A (a trafficking mutant of the Kir6.2) K(ATP) channels heterologously expressed in HEK 293 cells, suggesting that iptakalim suppressed the function of beta-cell K(ATP) channels by directly inhibiting the Kir6.2 subunit. Finally, iptakalim inhibited Kir6.2/SUR1, but it activated Kir6.1/SUR2B (vascular-type), K(ATP) channels heterologously expressed in Xenopus oocytes. Iptakalim bidirectionally regulated pancreatic-type and vascular-type K(ATP) channels, and this unique pharmacological property suggests the potential use of iptakalim as a new therapeutic strategy for treating type 2 diabetes with the additional benefit of alleviating vascular disorders.

ATP-sensitive potassium channel opener iptakalim protects against MPP+-induced astrocytic apoptosis via mitochondria and mitogen-activated protein kinase signal pathways

Inhibition of astrocytic apoptosis has been regarded as a novel prospective strategy for treating neurodegenerative disorders such as Parkinson's disease. In the present study, we demonstrated that iptakalim (IPT), an ATP-sensitive potassium channel (K(ATP) channel) opener, exerted protective effect on MPP(+)-induced astrocytic apoptosis, which was reversed by selective mitochondrial K(ATP) channel blocker 5-hydroxydecanoate. Further study revealed that IPT inhibited glutathione (GSH) depletion, mitochondrial membrane potential loss and subsequent release of pro-apoptotic factors (cytochrome c and apoptosis-inducing factor (AIF), and c-Jun NH(2)-terminal kinase/mitogen-activated protein kinases (MAPK) phosphorylation induced by MPP(+). Meanwhile, extracellular signal-regulated kinase (ERK) 1/2 inhibitor PD98059 inhibited the protective effect of IPT on MPP(+)-induced astrocytic apoptosis. Furthermore, IPT could also activate ERK/MAPK and maintain increased phospho-ERK1/2 level after MPP(+) exposure. Taken together, these findings reveal for the first time that IPT protects against MPP(+)-induced astrocytic apoptosis via inhibition of mitochondria apoptotic pathway and regulating the MAPK signal transduction pathways by opening mitochondrial ATP-sensitive potassium (mitoK(ATP)) channels in astrocytes. And targeting K(ATP) channels expressed in astrocytes may provide a novel therapeutic strategy for neurodegenerative disorders.