Voltage-gated K+ channel modulators as neuroprotective agents

Blockade of voltage-gated potassium channels ameliorates diabetes-associated cognitive dysfunction in vivo and in vitro

The voltage-gated potassium (Kv) channel blockers tetraethylammonium (TEA) and 4-aminopyridine (4-AP) have shown beneficial effects on some neurological disorders. But their involvements in diabetes-associated cognitive dysfunction are still unknown. The present study aims to investigate whether the blockade of Kv channels by TEA and 4-AP alleviate cognitive decline in diabetes. In vivo, the effects of TEA and 4-AP (5 mg/kg body weight per day, 1 mg/kg body weight per day intraperitoneal injected for 4 weeks, respectively) were investigated in streptozotocin-induced C57BL/6 diabetic mice. In vitro study, we investigated the effects of TEA and 4-AP on the high glucose (HG) -stimulated primary cortical neurons. The results showed that TEA and 4-AP ameliorated the cognitive decline of diabetic mice in the Morris water maze test, improved the ultrastructure of pancreatic β cells, hippocampal neurons and synapses, decreased oxidative stress, modulated apoptosis-related proteins, and activated phosphatidylinositol 3-kinase (PI3K)/ Protein kinase-B (PKB or Akt) signaling pathway. In the HG-stimulated primary cultured cortical neurons, TEA and 4-AP increased the cell viability, decreased oxidative stress; prevented apoptosis and activated PI3K/Akt signaling pathway. Additionally, the PI3K inhibitor LY294002 partially abolished the effects of TEA and 4-AP. These findings indicate that the blockade of Kv channels by TEA and 4-AP ameliorates the diabetes-associated cognitive dysfunction via PI3K/Akt pathway, suggesting that targeting Kv channels could be a promising strategy for the treatments of cognitive impairments in diabetes.

Endocannabinoid 2-Arachidonoylglycerol Suppresses LPS-Induced Inhibition of A-Type Potassium Channel Currents in Caudate Nucleus Neurons Through CB1 Receptor

Inflammation plays a pivotal role in the pathogenesis of many diseases in the central nervous system. Caudate nucleus (CN), the largest nucleus in the brain, is also implicated in many neurological disorders. 2-Arachidonoylglycerol (2-AG), the most abundant endogenous cannabinoid, has been shown to exhibit neuroprotective effects through its anti-inflammatory action from some proinflammatory stimuli. However, the neuroprotective mechanism of 2-AG is complex and has not been fully understood. A-type K(+) channels critically regulate neuronal excitability and have been demonstrated to be associated with some nervous system diseases. The aim of this study was to explore whether A-type K(+) channels were involved in neurotoxicity of lipopolysaccharides (LPS) and the neuroprotective mechanism of 2-AG in CN neurons. Whole cell patch clamp recording was used to investigate the influence of LPS on the function of A-type K(+) channels and its modulation by 2-AG in primary cultured rat CN neurons. Our findings showed that in cultured CN neurons, LPS significantly decreased the A-type potassium currents (I A) in a voltage-insensitive way. The further data demonstrated that an elevation of 2-AG levels by directly applying exogenous 2-AG or inhibiting monoacylglycerol lipase (MAGL) to prevent 2-AG hydrolysis was capable of suppressing the LPS-induced inhibition of IA and the action of 2-AG is mediated through CB1 receptor-dependant way. The study provides a better understanding of inflammation-related neurological disorders and suggests the therapeutic potential for 2-AG for the treatment of these diseases.

Potassium Channels and Human Epileptic Phenotypes: An Updated Overview

Potassium (K⁺) channels are expressed in almost every cells and are ubiquitous in neuronal and glial cell membranes. These channels have been implicated in different disorders, in particular in epilepsy. K⁺ channel diversity depends on the presence in the human genome of a large number of genes either encoding pore-forming or accessory subunits. More than 80 genes encoding the K⁺ channels were cloned and they represent the largest group of ion channels regulating the electrical activity of cells in different tissues, including the brain. It is therefore not surprising that mutations in these genes lead to K⁺ channels dysfunctions linked to inherited epilepsy in humans and non-human model animals. This article reviews genetic and molecular progresses in exploring the pathogenesis of different human epilepsies, with special emphasis on the role of K⁺ channels in monogenic forms.

Cinnamamide Derivatives for Central and Peripheral Nervous System Disorders--A Review of Structure-Activity Relationships

The cinnamamide scaffold has been incorporated in to the structure of numerous organic compounds with therapeutic potential. The scaffold enables multiple interactions, such as hydrophobic, dipolar, and hydrogen bonding, with important molecular targets. Additionally, the scaffold has multiple substitution options providing the opportunity to optimize and modify the pharmacological activity of the derivatives. In particular, cinnamamide derivatives have exhibited therapeutic potential in animal models of both central and peripheral nervous system disorders. Some have undergone clinical trials and were introduced on to the pharmaceutical market. The diverse activities observed in the nervous system included anticonvulsant, antidepressant, neuroprotective, analgesic, anti-inflammatory, muscle relaxant, and sedative properties. Over the last decade, research has focused on the molecular mechanisms of action of these derivatives, and the data reported in the literature include targeting the γ-aminobutyric acid type A (GABAA ) receptors, N-methyl-D-aspartate (NMDA) receptors, transient receptor potential (TRP) cation channels, voltage-gated potassium channels, histone deacetylases (HDACs), prostanoid receptors, opioid receptors, and histamine H3 receptors. Here, the literature data from reports evaluating cinnamic acid amide derivatives for activity in target-based or phenotypic assays, both in vivo and in vitro, relevant to disorders of the central and peripheral nervous systems are analyzed and structure-activity relationships discussed.

Spinocerebellar ataxia type 19/22 mutations alter heterocomplex Kv4.3 channel function and gating in a dominant manner

The dominantly inherited cerebellar ataxias are a heterogeneous group of neurodegenerative disorders caused by Purkinje cell loss in the cerebellum. Recently, we identified loss-of-function mutations in the KCND3 gene as the cause of spinocerebellar ataxia type 19/22 (SCA19/22), revealing a previously unknown role for the voltage-gated potassium channel, Kv4.3, in Purkinje cell survival. However, how mutant Kv4.3 affects wild-type Kv4.3 channel functioning remains unknown. We provide evidence that SCA19/22-mutant Kv4.3 exerts a dominant negative effect on the trafficking and surface expression of wild-type Kv4.3 in the absence of its regulatory subunit, KChIP2. Notably, this dominant negative effect can be rescued by the presence of KChIP2. We also found that all SCA19/22-mutant subunits either suppress wild-type Kv4.3 current amplitude or alter channel gating in a dominant manner. Our findings suggest that altered Kv4.3 channel localization and/or functioning resulting from SCA19/22 mutations may lead to Purkinje cell loss, neurodegeneration and ataxia.

The large conductance calcium-activated potassium channel affects extrinsic and intrinsic mechanisms of apoptosis

The large-conductance calcium-activated K(+) or BK channel underlies electrical signals in a number of different cell types. Studies show that BK activity can also serve to regulate cellular homeostasis by protecting cells from apoptosis resulting from events such as ischemia. Recent coimmunoprecipitation studies, combined with mass spectrometry, suggest putative protein partners that interact with BK to regulate intrinsic and extrinsic apoptotic pathways. This study tests two of those partners to determine the effects on these two signaling pathways. Through reciprocal coimmunoprecipitation (coIP) experiments, we show that BK interacts with p53 and fas-associated protein with death domain (FADD) in mouse brain and when overexpressed in a heterologous expression system, such as HEK293 cells. Moreover, coIP experiments with N- and C-terminal fragments reveal that FADD interacts with the C-terminus of BK, whereas p53 interacts with either the N- or the C-terminus. Immunolocalization studies show that BK colocalizes with p53 and FADD in the mitochondrion and plasmalemma, respectively. HEK cells that stably express BK are more resistant to apoptosis when p53 or FADD is overexpressed or when their intrinsic and extrinsic pathways are stimulated via mitomycin C or tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), respectively. Moreover, when stimulating with TRAIL, caspase-8 activation decreases in BK-expressing cells. These data suggest that BK is part of a larger complex of proteins that protects against apoptosis by interacting with proapoptotic proteins, such as p53 and FADD.

Role of ion transport in control of apoptotic cell death

Cell shrinkage is a hallmark and contributes to signaling of apoptosis. Apoptotic cell shrinkage requires ion transport across the cell membrane involving K(+) channels, Cl(-) or anion channels, Na(+)/H(+) exchange, Na(+),K(+),Cl(-) cotransport, and Na(+)/K(+)ATPase. Activation of K(+) channels fosters K(+) exit with decrease of cytosolic K(+) concentration, activation of anion channels triggers exit of Cl(-), organic osmolytes, and HCO3(-). Cellular loss of K(+) and organic osmolytes as well as cytosolic acidification favor apoptosis. Ca(2+) entry through Ca(2+)-permeable cation channels may result in apoptosis by affecting mitochondrial integrity, stimulating proteinases, inducing cell shrinkage due to activation of Ca(2+)-sensitive K(+) channels, and triggering cell-membrane scrambling. Signaling involved in the modification of cell-volume regulatory ion transport during apoptosis include mitogen-activated kinases p38, JNK, ERK1/2, MEKK1, MKK4, the small G proteins Cdc42, and/or Rac and the transcription factor p53. Osmosensing involves integrin receptors, focal adhesion kinases, and tyrosine kinase receptors. Hyperosmotic shock leads to vesicular acidification followed by activation of acid sphingomyelinase, ceramide formation, release of reactive oxygen species, activation of the tyrosine kinase Yes with subsequent stimulation of CD95 trafficking to the cell membrane. Apoptosis is counteracted by mechanisms involved in regulatory volume increase (RVI), by organic osmolytes, by focal adhesion kinase, and by heat-shock proteins. Clearly, our knowledge on the interplay between cell-volume regulatory mechanisms and suicidal cell death is still far from complete and substantial additional experimental effort is needed to elucidate the role of cell-volume regulatory mechanisms in suicidal cell death.

The effect of nicotinamide on gene expression in a traumatic brain injury model

Microarray-based transcriptional profiling was used to determine the effect of nicotinamide on gene expression in an experimental traumatic brain injury (TBI) model. Ingenuity Pathway Analysis (IPA) was used to evaluate the effect on relevant functional categories and canonical pathways. At 24 h, 72 h, and 7 days, respectively, 70, 58, and 76%, of the differentially expressed genes were up-regulated in the vehicle treated compared to the sham animals. At 24 h post-TBI, there were 150 differentially expressed genes in the nicotinamide treated animals compared to vehicle; the majority (82%) down-regulated. IPA analysis identified a significant effect of nicotinamide on the functional categories of cellular movement, cell-to-cell-signaling, antigen presentation and cellular compromise, function, and maintenance and cell death. The canonical pathways identified were signaling pathways primarily involved with the inflammatory process. At 72 h post-cortical contusion injury, there were 119 differentially expressed genes in the nicotinamide treated animals compared to vehicle; the majority (90%) was up-regulated. IPA analysis identified a significant effect of nicotinamide on cell signaling pathways involving neurotransmitters, neuropeptides, growth factors, and ion channels with little to no effect on inflammatory pathways. At 7 days post-TBI, there were only five differentially expressed genes with nicotinamide treatment compared to vehicle. Overall, the effect of nicotinamide on counteracting the effect of TBI resulted in significantly decreased number of genes differentially expressed by TBI. In conclusion, the mechanism of the effect of nicotinamide on secondary injury pathways involves effects on inflammatory response, signaling pathways, and cell death.

Taurine activates delayed rectifier Kv channels via a metabotropic pathway in retinal neurons

Taurine is one of the most abundant amino acids in the retina, throughout the CNS, and in heart and muscle cells. In keeping with its broad tissue distribution, taurine serves as a modulator of numerous basic processes, such as enzyme activity, cell development, myocardial function and cytoprotection. Despite this multitude of functional roles, the precise mechanism underlying taurine's actions has not yet been identified. In this study we report findings that indicate a novel role for taurine in the regulation of voltage-gated delayed rectifier potassium (K(V)) channels in retinal neurons by means of a metabotropic receptor pathway. The metabotropic taurine response was insensitive to the Cl(-) channel blockers, picrotoxin and strychnine, but it was inhibited by a specific serotonin 5-HT(2A) receptor antagonist, MDL11939. Moreover, we found that taurine enhanced K(V) channels via intracellular protein kinase C-mediated pathways. When 5-HT(2A) receptors were expressed in human embryonic kidney cells, taurine and AL34662, a non-specific 5-HT(2) receptor activator, produced a similar regulation of K(IR) channels. In sum, this study provides new evidence that taurine activates a serotonin system, apparently via 5-HT(2A) receptors and related intracellular pathways.

Redox-active Cu(II)-Aβ causes substantial changes in axonal integrity in cultured cortical neurons in an oxidative-stress dependent manner

BACKGROUND

The beta-amyloid (Aβ) peptide comprises the amyloid plaques that characterise Alzheimer's disease (AD), and is thought to significantly contribute towards disease pathogenesis. Oxidative stress is elevated in the AD brain, and there is substantial evidence that the interaction between Aβ and redox-active copper is a major contributing factor towards oxidative stress in AD.

RESULTS

The major findings of this study are that redox-active Cu(II)-Aβ causes pronounced axonal pathology in long-term neuronal cultures, including axonal fragmentation and the formation of hyperphosphorylated tau-immunoreactive axonal swellings. Notably, MAP-2 expressing dendritic processes remain largely un-affected by Cu(II)-Aβ treatment. These dystrophic axonal manifestations resemble some of the characteristic neuritic pathology of the AD brain. We show that Cu(II)-Aβ directly causes formation of intra-axonal swellings via the generation of free radicals and subsequent efflux of K+ out of neurons.

CONCLUSION

In summary, we report that redox-active Cu(II)-Aβ can induce substantial neurodegenerative changes in mature neurons, and may have an important role to play in the slowly progressing pathogenesis of AD.

Inhibitory effects of magnolol on voltage-gated Na+ and K+ channels of NG108-15 cells

Magnolol, a polyphenolic compound isolated from Houpu, a Chinese herb from the bark of Magnolia officinalis, has been reported to have in vitro and in vivo neuroprotective effects. In spite of these reported beneficial effects, studies on the direct impact of magnolol on neuronal ion channels have been scarce. Whether magnolol affects voltage-gated Na(+) channels (VGSC) and voltage-gated K(+) (Kv) channels is unknown. Using the whole-cell voltage-clamp method, we studied the effects of magnolol on voltage-gated ion channels in neuronal NG108-15 cells. Magnolol inhibited VGSC channels with mild state-dependence (IC(50) of 15 and 30 μM, at holding potentials of -70 and -100 mV, respectively). No frequency-dependence was observed in magnolol block. Magnolol caused a left-shift of 18 mV in the steady-state inactivation curve but did not affect the voltage-dependence of activation. Magnolol inhibited Kv channels with an IC(50) of 21 μM, and it caused a 20-mV left-shift in the steady-state inactivation curve without affecting the voltage-dependence of activation. In conclusion, magnolol is an inhibitor of both VGSC and Kv channels and these inhibitory effects may in part contribute to some of the reported neuroprotective effects of magnolol.

Identification of putative potassium channel homologues in pathogenic protozoa

K⁺ channels play a vital homeostatic role in cells and abnormal activity of these channels can dramatically alter cell function and survival, suggesting that they might be attractive drug targets in pathogenic organisms. Pathogenic protozoa lead to diseases such as malaria, leishmaniasis, trypanosomiasis and dysentery that are responsible for millions of deaths each year worldwide. The genomes of many protozoan parasites have recently been sequenced, allowing rational design of targeted therapies. We analyzed the genomes of pathogenic protozoa and show the existence within them of genes encoding putative homologues of K⁺ channels. These protozoan K⁺ channel homologues represent novel targets for anti-parasitic drugs. Differences in the sequences and diversity of human and parasite proteins may allow pathogen-specific targeting of these K⁺ channel homologues.

Apocynum venetum leaf aqueous extract inhibits voltage-gated sodium channels of mouse neuroblastoma N2A cells

ETHNOPHARMACOLOGICAL RELEVANCE

Apocynum venetum Linn. (Apocynaceae family), also called Luobuma, is a shrub which grows widely in the Xinjiang Autonomous Region of China. Its leaves are used in herbal tea for the treatment of hypertension, anxiety and depression. Animal studies have also shown that Apocynum venetum leaf extract (AVLE) also exerts anti-depressant and anti-anxiety activities. The effects of AVLE on neuronal tissues in vitro are not fully understood.

MATERIALS AND METHODS

Using the whole-cell voltage-clamp method, we studied the effects of AVLE on ion channels in cultured mouse neuroblastoma N2A cells.

RESULTS

AVLE inhibited voltage-gated inward Na(+) current in a reversible and concentration-dependent manner (half-inhibitory concentration was 18 μg/ml and maximum inhibition at 100 μg/ml). AVLE specifically promoted steady-state inactivation of Na(+) channels but did not affect voltage-dependence of activation. The inhibitory effect was not use-dependent and was not affected by 300μM L-NAME, suggesting that NO was not involved in the action of AVLE in neuronal cells. AVLE also had a mild inhibitory effect on voltage-gated K(+) channels, but did not affect ATP-sensitive K(+) channels.

CONCLUSIONS

Since voltage-gated Na(+) and K(+) channels are associated with neuronal excitability and therefore affect neurotransmission, the modulation of neuronal ion channels by AVLE may exert neuropharmacological effects. In particular, the inhibition of voltage-gated Na(+) currents by AVLE may in part account for the psychopharmacological effects of this herbal remedy.