Reduced expression of the KATP channel subunit, Kir6.2, is associated with decreased expression of neuropeptide Y and agouti-related protein in the hypothalami of Zucker diabetic fatty rats

Loss of ATP-Sensitive Potassium Channel Expression and Function in the Nervous System Decreases Opioid Sensitivity in a High-Fat Diet-Fed Mouse Model of Diet-Induced Obesity

During diabetes progression, β-cell dysfunction due to loss of potassium channels sensitive to ATP, known as KATP channels, occurs, contributing to hyperglycemia. The aim of this study was to investigate if KATP channel expression or activity in the nervous system was altered in a high-fat diet (HFD)-fed mouse model of diet-induced obesity. Expression of two KATP channel subunits, Kcnj11 (Kir6.2) and Abcc8 (SUR1), were decreased in the peripheral and central nervous system of mice fed HFD, which was significantly correlated with mechanical paw-withdrawal thresholds. HFD mice had decreased antinociception to systemic morphine compared with control diet (CON) mice, which was expected because KATP channels are downstream targets of opioid receptors. Mechanical hypersensitivity in HFD mice was exacerbated after systemic treatment with glyburide or nateglinide, KATP channel antagonists clinically used to control blood glucose levels. Upregulation of SUR1 and Kir6.2, through an adenovirus delivered intrathecally, increased morphine antinociception in HFD mice. These data present a potential link between KATP channel function and neuropathy during early stages of diabetes. There is a need for increased knowledge of how diabetes affects structural and molecular changes in the nervous system, including ion channels, to lead to the progression of chronic pain and sensory issues.

ARTICLE HIGHLIGHTS

The relevance between abnormally elevated serum ceramide and cognitive impairment in Alzheimer's disease model mice and its mechanism

RATIONALE

The plasma ceramide levels in Alzheimer's disease (AD) patients are found abnormally elevated, which is related to cognitive decline.

OBJECTIVES

This research was aimed to investigate the mechanisms of aberrant elevated ceramides in the pathogenesis of AD.

RESULTS

The ICR mice intracerebroventricularly injected with Aβ1-42 and APP/PS1 transgenic mice were employed as AD mice. The cognitive deficiency, impaired episodic and spatial memory were observed without altered spontaneous ability. The serum levels of p-tau and ceramide were evidently elevated. The modified expressions and activities of glycogen synthase kinase-3β (GSK-3β) and protein phosphatase 2A (PP2A) influenced the serum content of p-tau. The levels of ceramide synthesis-related genes including sptlc1, sptlc2, cers2, and cers6 in the liver of AD mice were increased, while the ceramide degradation-related gene asah2 did not significantly change. The regulations of these genes were conducted by activated nuclear factor kappa-B (NF-κB) signaling. NF-κB, promoted by free fatty acid (FFA), also increased the hepatic concentrations of proinflammatory cytokines. The FFA amount was modulated by fatty acid synthesis-related genes acc1 and srebp-1c. Besides, the decreased levels of pre-proopiomelanocortin (pomc) mRNA and increased agouti-related protein (agrp) mRNA were found in the hypothalamus without significant alteration of melanocortin receptor 4 (MC4R) mRNA. The bioinformatic analyses proved the results using GEO datasets and AlzData.

CONCLUSIONS

Ceramide was positively related to the increased p-tau and impaired cognitive function. The increased generation of ceramide and endoplasmic reticulum stress in the hypothalamus was positively related to fatty acid synthesis and NF-κB signaling via brain-liver axis.

Loss of ATP-sensitive channel expression and function decreases opioid sensitivity in a mouse model of type 2 diabetes

During diabetes, β-cell dysfunction due to loss of potassium channels sensitive to ATP, known as KATP channels occurs progressively over time contributing to hyperglycemia. KATP channels are additionally present in the central and peripheral nervous systems and are downstream targets of opioid receptor signaling. The aim of this study is to investigate if KATP channel expression or activity in the nervous system changes in diabetic mice and if morphine antinociception changes in mice fed a high fat diet (HFD) for 16 weeks compared to controls. Mechanical thresholds were also monitored before and after administration of glyburide or nateglinide, KATP channel antagonists, for four weeks. HFD mice have decreased antinociception to systemic morphine, which is exacerbated after systemic treatment with glyburide or nateglinide. HFD mice also have lower rotarod scores, decreased mobility in an open field test, and lower burrowing behavior compared to their control diet counterparts, which is unaffected by KATP channel antagonist delivery. Expression of KATP channel subunits, Kcnj11 (Kir6.2) and Abcc8 (SUR1), were decreased in the peripheral and central nervous system in HFD mice, which is significantly correlated with baseline paw withdrawal thresholds. Upregulation of SUR1 through an adenovirus delivered intrathecally increased morphine antinociception in HFD mice, whereas Kir6.2 upregulation improved morphine antinociception only marginally. Perspective: This article presents the potential link between KATP channel function and neuropathy during diabetes. There is a need for increased knowledge in how diabetes affects structural and molecular changes in the nervous system to lead to the progression of chronic pain and sensory issues.

Central KATP Channels Modulate Glucose Effectiveness in Humans and Rodents

Hyperglycemia is a potent regulator of endogenous glucose production (EGP). Loss of this "glucose effectiveness" is a major contributor to elevated plasma glucose concentrations in type 2 diabetes (T2D). K_ATP channels in the central nervous system have been shown to regulate EGP in humans and rodents. We examined the contribution of central K_ATP channels to glucose effectiveness. Under fixed hormonal conditions (studies using a pancreatic clamp), hyperglycemia suppressed EGP by ∼50% in both humans without diabetes and normal Sprague-Dawley rats. By contrast, antagonism of K_ATP channels with glyburide significantly reduced the EGP-lowering effect of hyperglycemia in both humans and rats. Furthermore, the effects of glyburide on EGP and gluconeogenic enzymes were abolished in rats by intracerebroventricular administration of the K_ATP channel agonist diazoxide. These findings indicate that about half of the suppression of EGP by hyperglycemia is mediated by central K_ATP channels. These central mechanisms may offer a novel therapeutic target for improving glycemic control in subjects with T2D.

Hypothalamic control of hepatic lipid metabolism via the autonomic nervous system

Our body is well designed to store energy in times of nutrient excess, and release energy in times of food deprivation. This adaptation to the external environment is achieved by humoral factors and the autonomic nervous system. Claude Bernard, in the 19th century, showed the importance of the autonomic nervous system in the control of glucose metabolism. In the 20th century, the discovery of insulin and the development of techniques to measure hormone concentrations shifted the focus from the neural control of metabolism to the secretion of hormones, thus functionally "decapitating" the body. Just before the end of the 20th century, starting with the discovery of leptin in 1994, the control of energy metabolism went back to our heads. Since the start of 21st century, numerous studies have reported the involvement of hypothalamic pathways in the control of hepatic insulin sensitivity and glucose production. The autonomic nervous system is, therefore, acknowledged to be one of the important determinants of liver metabolism and a possible treatment target. In this chapter, we review research to date on the hypothalamic control of hepatic lipid metabolism.

Sevoflurane postconditioning against cerebral ischemic neuronal injury is abolished in diet-induced obesity: role of brain mitochondrial KATP channels

Obesity is associated with increased infarct volumes and adverse outcomes following ischemic stroke. However, its effect on anesthetic postconditioning‑induced neuroprotection has not been investigated. The present study examined the effect of sevoflurane postconditioning on focal ischemic brain injury in diet‑induced obesity. Sprague‑Dawley rats were fed a high‑fat diet (HF; 45% kcal as fat) for 12 weeks to develop obesity syndrome. Rats fed a low‑fat diet (LF; 10% kcal as fat) served as controls. The HF or LF‑fed rats were subjected to focal cerebral ischemia for 60 min, followed by 24 h of reperfusion. Postconditioning was performed by exposure to sevoflurane for 15 min immediately at the onset of reperfusion. The involvement of the mitochondrial KATP (mitoKATP) channel was analyzed by the administration of a selective inhibitor of 5‑hydroxydecanoate (5‑HD) prior to sevoflurane postconditioning or by administration of diazoxide (DZX), a mitoKATP channel opener, instead of sevoflurane. The cerebral infarct volume, neurological score and motor coordination were evaluated 24 h after reperfusion. The HF‑fed rats had larger infarct volumes, and lower neurological scores than the LF‑fed rats and also failed to respond to neuroprotection by sevoflurane or DZX. By contrast, sevoflurane and DZX reduced the infarct volumes and improved the neurological scores and motor coordination in the LF‑fed rats. Pretreatment with 5‑HD inhibited sevoflurane‑induced neuroprotection in the LF‑fed rats, whereas it had no effect in the HF‑fed rats. Molecular studies demonstrated that the expression of Kir6.2, a significant mitoKATP channel component, was reduced in the brains of the HF‑fed rats compared with the LF‑fed rats. The results of this study indicate that diet‑induced obesity eliminates the ability of anesthetic sevoflurane postconditioning to protect the brain against cerebral ischemic neuronal injury, most likely due to an impaired brain mitoKATP channel.

Decreased brain K(ATP) channel contributes to exacerbating ischemic brain injury and the failure of neuroprotection by sevoflurane post-conditioning in diabetic rats

Diabetes leads to exacerbating brain injury after ischemic stroke, but the underlying mechanisms and whether therapeutic intervention with anesthetic post-conditioning can induce neuroprotection in this population are not known. We tested the hypothesis that alteration of brain mitochondrial (mito) K_ATP channels might cause exacerbating brain injury after ischemic stroke and attenuate anesthetic post-conditioning induced neuroprotection in diabetes. We also examined whether hyperglycemic correction with insulin would restore anesthetic post-conditioning in diabetes. Non-diabetic rats and diabetic rats treated with or without insulin were subjected to focal cerebral ischemia for 2 h followed by 24 h of reperfusion. Post-conditioning was performed by exposure to sevoflurane for 15 min, immediately at the onset of reperfusion. The role of the mitoK_ATP channel was assessed by administration of a selective blocker 5-hydroxydecanoate (5-HD) before sevoflurane post-conditioning or by diazoxide (DZX), a mitoK_ATP channel opener, given in place of sevoflurane. Compared with non-diabetic rats, diabetic rats had larger infarct volume and worse neurological outcome at 24 h after ischemia. Sevoflurane or DZX reduced the infarct volume and improved neurological outcome in non-diabetic rats but not in diabetic rats, and the protective effects of sevoflurane in non-diabetic rats were inhibited by pretreatment with 5-HD. Molecular studies revealed that expression of Kir6.2, an important mitoK_ATP channel component, was decreased in the brain of diabetic rats as compared to non-diabetic rats. In contrast, hyperglycemic correction with insulin in diabetic rats normalized expression of brain Kir6.2, reduced ischemic brain damage and restored neuroprotective effects of sevoflurane post-conditioning. Our findings suggest that decreased brain mitoK_ATP channel contributes to exacerbating ischemic brain injury and the failure of neuroprotection by anesthetic post-conditioning in diabetes. Insulin glycemic control in diabetes may restore the neuroprotective effects of anesthetic post-conditioning by modulation of brain mitoK_ATP channel.

CNS control of glucose metabolism: response to environmental challenges

Over the last 15 years, considerable work has accumulated to support the role of the CNS in regulating postprandial glucose levels. As discussed in the first section of this review, the CNS receives and integrates information from afferent neurons, circulating hormones, and postprandially generated nutrients to subsequently direct changes in glucose output by the liver and glucose uptake by peripheral tissues. The second major component of this review focuses on the effects of external pressures, including high fat diet and changes to the light:dark cycle on CNS-regulating glucose homeostasis. We also discuss the interaction between these different pressures and how they contribute to the multifaceted mechanisms that we hypothesize contribute to the dysregulation of glucose in type 2 diabetes mellitus (T2DM). We argue that while current peripheral therapies serve to delay the progression of T2DM, generating combined obesity and T2DM therapies targeted at the CNS, the primary site of dysfunction for both diseases, would lead to a more profound impact on the progression of both diseases.

Hypothalamic serotonin-insulin signaling cross-talk and alterations in a type 2 diabetic model

Serotonin and insulin are key regulators of homeostatic mechanisms in the hypothalamus. However, in type 2 diabetes, the hypothalamic responsiveness to serotonin is not clearly established. We used a diabetic model, the Goto Kakizaki (GK) rats, to explore insulin receptor expression, insulin and serotonin efficiency in the hypothalamus and liver by means of Akt phosphorylation. Insulin or dexfenfluramine (stimulator of serotonin) treatment induced Akt phosphorylation in Wistar rats but not in GK rats that exhibit down-regulated insulin receptor. Studies in a neuroblastoma cell line showed that serotonin-induced Akt phosphorylation is PI3-kinase dependent. Finally, in response to food intake, hypothalamic serotonin release was reduced in GK rats, indicating impaired responsiveness of this neurotransmitter. In conclusion, hypothalamic serotonin as insulin efficiency is impaired in diabetic GK rats. The insulin-serotonin cross-talk and impairment observed is one potential key modification in the brain during the onset of diabetes.

Metabolic gene expression changes in the hippocampus of obese epileptic male rats in the pilocarpine model of temporal lobe epilepsy

Chronically epileptic male adult rats in the pilocarpine model of temporal lobe epilepsy (TLE), exhibited gross expansion of abdominal fat mass and significant weight gain several months after induction of status epilepticus (SE) when compared to control rats. We hypothesized that epileptogenesis can induce molecular changes in the hippocampus that may be associated with metabolism. We determined the expression levels of genes Hsd11b1, Nr3c1, Abcc8, Kcnj11, Mc4r, Npy, Lepr, Bdnf, and Drd2 that are involved in regulation of energy metabolism, in the hippocampus of age-matched control and chronic epileptic animals. Taqman-based quantitative real time polymerase chain reaction (qPCR) and the delta-delta cycle threshold (CT) methods were used for the gene expression assays. Gene expression of Hsd11b1 (cortisol generating enzyme) was significantly higher in epileptic versus control rats at 24h and 2 months, after induction of SE. Nr3c1 (glucocorticoid receptor) mRNA levels on the other hand were down-regulated at 24h, 10 days and 2 months, post SE. Abcc8 (Sur1; subunit of ATP-sensitive potassium (K(ATP)) channel) was significantly down-regulated at 10 days post SE. Kcnj11 (Kir6.2; subunit of ATP-sensitive potassium (K(ATP)) channel) was significantly up-regulated at 24h, 1 month and 2 months post SE. Thus, we demonstrated development of obesity and changes in the expression of metabolic genes in the hippocampus during epileptogenesis in male rats in the pilocarpine model of TLE.

ATP-Sensitive Potassium Channel-Deficient Mice Show Hyperphagia but Are Resistant to Obesity

BACKGROUND

The hypothalamus, the center for body weight regulation, can sense changes in blood glucose level based on ATP-sensitive potassium (K(ATP)) channels in the hypothalamic neurons. We hypothesized that a lack of glucose sensing in the hypothalamus affects the regulations of appetite and body weight.

METHODS

To evaluate this hypothesis, the responses to glucose loading and high fat feeding for eight weeks were compared in Kir6.2 knock-out (KO) mice and control C57BL/6 mice, because Kir6.2 is a key component of the K(ATP) channel.

RESULTS

The hypothalamic neuropeptide Y (NPY) analyzed one hour after glucose injection was suppressed in C57BL/6 mice, but not in Kir6.2 KO mice, suggesting a blunted hypothalamic response to glucose in Kir6.2 KO mice. The hypothalamic NPY expression at a fed state was elevated in Kir6.2 KO mice and was accompanied with hyperphagia. However, the retroperitoneal fat mass was markedly decreased in Kir6.2 KO mice compared to that in C57BL/6 mice. Moreover, the body weight and visceral fat following eight weeks of high fat feeding in Kir6.2 KO mice were not significantly different from those in control diet-fed Kir6.2 KO mice, while body weight and visceral fat mass were elevated due to high fat feeding in C57BL/6 mice.

CONCLUSION

These results suggested that Kir6.2 KO mice showed a blunted hypothalamic response to glucose loading and elevated hypothalamic NPY expression accompanied with hyperphagia, while visceral fat mass was decreased, suggesting resistance to diet-induced obesity. Further study is needed to explain this phenomenon.

Epigenetic changes in the hypothalamic pro-opiomelanocortin gene: a mechanism linking maternal undernutrition to obesity in the offspring?

Maternal undernutrition is associated with programming of obesity in offspring. While previous evidence has linked programming to the hypothalamic, pituitary, and adrenal (HPA) axis it could also affect the hypothalamic neuropeptides which regulate food intake and energy balance. Alpha melanocyte stimulating hormone (αMSH), a key regulator of these neuronal pathways, is derived from pro-opiomelanocortin (POMC) which is therefore a prime target for the programming of obesity. Several models of maternal undernutrition have identified changes in POMC in hypothalami from foetuses or offspring at various ages. These models have also shown that the offspring go on to develop obesity and/or glucose intolerance. It is our hypothesis that programming leads to epigenetic changes in hypothalamic neuropeptide genes. Therefore when there is subsequent increased food availability, the epigenetic changes could cause dysfunctional transcriptional regulation of energy balance. We present evidence of epigenetic changes in the POMC gene promoter in foetal hypothalami after peri-conceptional undernutrition. In this model there are also epigenetic changes in the hypothalamic glucocorticoid receptor with consequent up-regulation of the receptor which could lead to alterations in the regulation of POMC and neuropeptide Y (NPY) in the hypothalamus. Thus maternal undernutrition could cause epigenetic changes in the POMC and glucocorticoid receptor genes, in the foetal hypothalamus, which may predispose the offspring to altered regulation of food intake, energy expenditure and glucose homeostasis, later in life.

Targeting type 2 diabetes

The evolving concept of how nutrient excess and inflammation modulate metabolism provides new opportunities for strategies to correct the detrimental health consequences of obesity. In this review, we focus on the complex interplay among lipid overload, immune response, proinflammatory pathways and organelle dysfunction through which excess adiposity might lead to type 2 diabetes. We then consider evidence linking dysregulated CNS circuits to insulin resistance and results on nutrient-sensing pathways emerging from studies with calorie restriction. Subsequently, recent recommendations for the management of type 2 diabetes are discussed with emphasis on prevailing current therapeutic classes of biguanides, thiazolidinediones and incretin-based approaches.

Activation of the melanocortin-4 receptor causes enhanced excitation in presympathetic paraventricular neurons in obese Zucker rats

Sympathetic nerve activity is increased in obesity-related hypertension. However, the central mechanisms involved in the increased sympathetic outflow remain unclear. The hypothalamic melanocortin system is important for regulating energy balance and sympathetic outflow. To understand the mechanisms by which the melanocortin systems regulates sympathetic outflow, we investigated the role of melanocortin 4 receptors (MC4R) in regulating presympathetic paraventricular nucleus (PVN) neurons. We performed whole-cell patch-clamp recordings on retrogradely labeled PVN neurons projecting to the rostral ventrolateral medulla in brain slices from obese zucker rats (OZRs) and lean zucker rats (LZRs). The MC4R agonists melanotan II (MTII) and α-melanocyte-stimulating hormone (α-MSH) increased the firing activity and depolarized the labeled PVN neurons from both LZRs and OZRs in a concentration-dependent manner. MTII produced significant greater increase in the firing activity in OZRs than in LZRs. Blocking MC4R with the specific antagonist SHU9119 had no effect on the basal firing rate but abolished the MTII-induced increase in the firing rate in both OZRs and LZRs. Furthermore, intracellular dialysis of guanosine 5'-O-(2-thodiphosphate), but not bath application of kynurenic acid and bicuculline, eliminated the MTII-induced increase in firing activity. In addition, MTII had no effect on the frequency and amplitude of glutamatergic excitatory postsynaptic currents and GABAergic inhibitory postsynaptic currents in labeled PVN neurons. Collectively, our findings suggest that MC4R contributes to the elevated excitability of PVN presympathetic neurons, which may be involved in obesity-related hypertension.

Patents related to therapeutic activation of K(ATP) and K(2P) potassium channels for neuroprotection: ischemic/hypoxic/anoxic injury and general anesthetics

BACKGROUND

Mechanisms of neuroprotection encompass energy deficits in brain arising from insufficient oxygen and glucose levels following respiratory failure; ischemia or stroke, which produce metabolic stresses that lead to unconsciousness and seizures; and the effects of general anesthetics. Foremost among those K(+) channels viewed as important for neuroprotection are ATP-sensitive (K(ATP)) channels, which belong to the family of inwardly rectifying K(+) channels (K(ir)) and contain a sulfonylurea subunit (SUR1 or SUR2) combined with either K(ir)6.1 (KCNJ8) or K(ir)6.2 (KCNJ11) channel pore-forming alpha-subunits, and various members of the tandem two-pore or background (K(2P)) K(+) channel family, including K(2P)1.1 (KCNK1 or TWIK1), K(2P)2.1 (KCNK2 or TREK/TREK1), K(2P)3.1 (KCNK3 or TASK), K(2P)4.1 (KCNK4 or TRAAK), and K(2P)10.1 (KCNK10 or TREK2).

OBJECTIVES

This review covers patents and patent applications related to inventions of therapeutics, compound screening methods and diagnostics, including K(ATP) channel openers and blockers, as well as K(ATP) and K(2P) nucleic/amino acid sequences and proteins, vectors, transformed cells and transgenic animals. Although the focus of this patent review is on brain and neuroprotection, patents covering inventions of K(ATP) channel openers for cardioprotection, diabetes mellitus and obesity, where relevant, are addressed.

RESULTS/CONCLUSIONS

Overall, an important emerging therapeutic mechanism underlying neuroprotection is activation/opening of K(ATP) and K(2P) channels. To this end substantial progress has been made in identifying and patenting agents that target K(ATP) channels. However, current K(2P) channels patents encompass compound screening and diagnostics methodologies, reflecting an earlier 'discovery' stage (target identification/validation) than K(ATP) in the drug development pipeline; this reveals a wide-open field for the discovery and development of K(2P)-targeting compounds.

Targeting the CNS to treat type 2 diabetes

Research on the role of peripheral organs in the regulation of glucose homeostasis has led to the development of various monotherapies that aim to improve glucose uptake and insulin action in these organs for the treatment of type 2 diabetes. It is now clear that the central nervous system (CNS) also plays an important part in orchestrating appropriate glucose metabolism, with accumulating evidence linking dysregulated CNS circuits to the failure of normal glucoregulatory mechanisms. There is evidence that there is substantial overlap between the CNS circuits that regulate energy balance and those that regulate glucose levels, suggesting that their dysregulation could link obesity and diabetes. These findings present new targets for therapies that may be capable of both inducing weight loss and improving glucose regulation.

Oleic acid directly regulates POMC neuron excitability in the hypothalamus

The mammalian CNS relies on a constant supply of external glucose for its undisturbed operation. However, neurons can readily switch to using fatty acids and ketones as alternative fuels. Here, we show that oleic acid (OA) excites pro-opiomelanocortin (POMC) neurons by inhibition of ATP-activated potassium (K(ATP)) channels. The involvement of K(ATP) channels is further supported by experiments in SUR1 KO animals. Inhibition of beta-oxidation using carnitine palmitoyltransferase-1 inhibitors blocks OA-induced depolarization. The depolarizing effect of OA is specific because it is not mimicked by octanoic acid. Furthermore, OA does not regulate the excitability of agouti-related peptide neurons. High-fat feeding alters POMC neuron excitability, but not its response to OA. Thus beta-oxidation in POMC neurons may mediate the appetite-suppressing (anorexigenic) effects of OA.