Blockers of the delayed-rectifier potassium current in pancreatic beta-cells enhance glucose-dependent insulin secretion

Heteromeric KV2/KV8.2 Channels Mediate Delayed Rectifier Potassium Currents in Primate Photoreceptors

Silent voltage-gated potassium channel subunits (K_VS) interact selectively with members of the K_V2 channel family to modify their functional properties. The localization and functional roles of these silent subunits remain poorly understood. Mutations in the K_VS subunit, K_V8.2 (KCNV2), lead to severe visual impairment in humans, but the basis of these deficits remains unclear. Here, we examined the localization, native interactions, and functional properties of K_V8.2-containing channels in mouse, macaque, and human photoreceptors of either sex. In human retina, K_V8.2 colocalized with K_V2.1 and K_V2.2 in cone inner segments and with K_V2.1 in rod inner segments. K_V2.1 and K_V2.2 could be coimmunoprecipitated with K_V8.2 in retinal lysates indicating that these subunits likely interact directly. Retinal K_V2.1 was less phosphorylated than cortical K_V2.1, a difference expected to alter the biophysical properties of these channels. Using voltage-clamp recordings and pharmacology, we provide functional evidence for Kv2-containing channels in primate rods and cones. We propose that the presence of K_V8.2, and low levels of K_V2.1 phosphorylation shift the activation range of K_V2 channels to align with the operating range of rod and cone photoreceptors. Our data indicate a role for K_V2/K_V8.2 channels in human photoreceptor function and suggest that the visual deficits in patients with KCNV2 mutations arise from inadequate resting activation of K_V channels in rod and cone inner segments.SIGNIFICANCE STATEMENT Mutations in a voltage-gated potassium channel subunit, K_V8.2, underlie a blinding inherited photoreceptor dystrophy, indicating an important role for these channels in human vision. Here, we have defined the localization and subunit interactions of K_V8.2 channels in primate photoreceptors. We show that the K_V8.2 subunit interacts with different Kv2 channels in rods and cones, giving rise to potassium currents with distinct functional properties. Our results provide a molecular basis for retinal dysfunction in patients with mutations in the KCNV2 gene encoding K_V8.2.

Differences in ion channel phenotype and function between humans and animal models

Ion channels play crucial roles in regulating a broad range of physiological processes. They form a very large family of transmembrane proteins. Their diversity results from not only a large number of different genes encoding for ion channel subunits but also the ability of subunits to assemble into homo- or heteromultimers, the existence of splice variants, and the expression of different regulatory subunits. These characteristics and the existence of very selective modulators make ion channels very attractive targets for therapy in a wide variety of pathologies. Some ion channels are already being targeted in the clinic while many more are being evaluated as novel drug targets in both clinical and preclinical studies. Advancing ion channel modulators from the bench to the clinic requires their assessment for safety and efficacy in animal models. While extrapolating results from one species to another is tempting, doing such without careful evaluation of the ion channels in different species presents a risk as the translation is not always straightforward. Here, we discuss differences between species in terms of ion channels expressed in selected tissues, differing roles of ion channels in some cell types, variable response to pharmacological agents, and human channelopathies that cannot fully be replicated in animal models.

Venom peptides as pharmacological tools and therapeutics for diabetes

Diabetes mellitus is a chronic disease caused by a deficiency in production of insulin by the beta cells of the pancreas (type 1 diabetes, T1D), or by partial deficiency of insulin production and the ineffectiveness of the insulin produced (type 2 diabetes, T2D). Animal venoms are a unique source of compounds targeting ion channels and receptors in the nervous and cardiovascular systems. In recent years, several venom peptides have also emerged as pharmacological tools and therapeutics for T1D and T2D. Some of these peptides act directly as mimics of endogenous metabolic hormones while others act on ion channels expressed in pancreatic beta cells. Here, we provide an overview of the discovery of these venom peptides, their mechanisms of action in the context of diabetes, and their therapeutic potential for the treatment of this disease. This article is part of the Special Issue entitled 'Venom-derived Peptides as Pharmacological Tools.'

Phytochemicals genistein and capsaicin modulate Kv2.1 channel gating

BACKGROUND

Phytochemicals are a large group of plant-derived compounds that have a broad range of pharmacological effects. Some of these effects are derived from their action on transport proteins, including ion channels. The present study investigates the effects of the phytochemicals genistein and capsaicin on voltage-gated potassium Kv2.1 channels.

METHODS

The whole-cell patch clamp technique was used to explore the regulation of Kv2.1 channels expressed in HEK293 cells by genistein and capsaicin.

RESULTS

Both phytochemicals had a profound effect on the gating properties of Kv2.1 channels; the voltage dependence of activation and inactivation was shifted to hyperpolarized potentials, the closed-state inactivation was accelerated, and the recovery from inactivation was delayed. Moreover, genistein and capsaicin inhibited Kv2.1 currents in a concentration dependent manner.

CONCLUSION

This study effectively demonstrated the inhibitory effects of genistein and capsaicin on Kv2.1 channels. As Kv2.1 channels play a prominent role in glucose-stimulated insulin secretion, our findings contribute to our understanding of the putative mechanism by which these phytochemicals exert their reported hypoglycemic effects.

Reduced sensory synaptic excitation impairs motor neuron function via Kv2.1 in spinal muscular atrophy

Behavioral deficits in neurodegenerative diseases are often attributed to the selective dysfunction of vulnerable neurons via cell-autonomous mechanisms. Although vulnerable neurons are embedded in neuronal circuits, the contribution of their synaptic partners to the disease process is largely unknown. Here, we show that in a mouse model of spinal muscular atrophy (SMA), a reduction in proprioceptive synaptic drive leads to motor neuron dysfunction and motor behavior impairments. In SMA mice or after the blockade of proprioceptive synaptic transmission we observed a decrease in the motor neuron firing which could be explained by the reduction in the expression of the potassium channel Kv2.1 at the surface of motor neurons. Increasing neuronal activity pharmacologically by chronic exposure in vivo led to a normalization of Kv2.1 expression and an improvement in motor function. Our results demonstrate a key role of excitatory synaptic drive in shaping the function of motor neurons during development and the contribution of its disruption to a neurodegenerative disease.

Physiological roles of Kv2 channels in entorhinal cortex layer II stellate cells revealed by Guangxitoxin-1E

KEY POINTS

Kv2 channels underlie delayed-rectifier potassium currents in various neurons, although their physiological roles often remain elusive. Almost nothing is known about Kv2 channel functions in medial entorhinal cortex (mEC) neurons, which are involved in representing space, memory formation, epilepsy and dementia. Stellate cells in layer II of the mEC project to the hippocampus and are considered to be space-representing grid cells. We used the new Kv2 blocker Guangxitoxin-1E (GTx) to study Kv2 functions in these neurons. Voltage clamp recordings from mEC stellate cells in rat brain slices showed that GTx inhibited delayed-rectifier K+ current but not transient A-type current. In current clamp, GTx had multiple effects: (i) increasing excitability and bursting at moderate spike rates but reducing firing at high rates; (ii) enhancing after-depolarizations; (iii) reducing the fast and medium after-hyperpolarizations; (iv) broadening action potentials; and (v) reducing spike clustering. GTx is a useful tool for studying Kv2 channels and their functions in neurons.

ABSTRACT

The medial entorhinal cortex (mEC) is strongly involved in spatial navigation, memory, dementia and epilepsy. Although potassium channels shape neuronal activity, their roles in mEC are largely unknown. We used the new Kv2 blocker Guangxitoxin-1E (GTx; 10-100 nm) in rat brain slices to investigate Kv2 channel functions in mEC layer II stellate cells (SCs). These neurons project to the hippocampus and are considered to be grid cells representing space. Voltage clamp recordings from SCs nucleated patches showed that GTx inhibited a delayed rectifier K+ current activating beyond -30 mV but not transient A-type current. In current clamp, GTx (i) had almost no effect on the first action potential but markedly slowed repolarization of late spikes during repetitive firing; (ii) enhanced the after-depolarization (ADP); (iii) reduced fast and medium after-hyperpolarizations (AHPs); (iv) strongly enhanced burst firing and increased excitability at moderate spike rates but reduced spiking at high rates; and (v) reduced spike clustering and rebound potentials. The changes in bursting and excitability were related to the altered ADPs and AHPs. Kv2 channels strongly shape the activity of mEC SCs by affecting spike repolarization, after-potentials, excitability and spike patterns. GTx is a useful tool and may serve to further clarify Kv2 channel functions in neurons. We conclude that Kv2 channels in mEC SCs are important determinants of intrinsic properties that allow these neurons to produce spatial representation. The results of the present study may also be important for the accurate modelling of grid cells.

A novel epileptic encephalopathy mutation in KCNB1 disrupts Kv2.1 ion selectivity, expression, and localization

A missense mutation in the pore-forming α subunit of a delayed rectifier Kv channel is associated with epileptic encephalopathy, alters the cation selectivity of voltage-gated currents, and disrupts channel expression and localization.

The epileptic encephalopathies are a group of highly heterogeneous genetic disorders. The majority of disease-causing mutations alter genes encoding voltage-gated ion channels, neurotransmitter receptors, or synaptic proteins. We have identified a novel de novo pathogenic K⁺ channel variant in an idiopathic epileptic encephalopathy family. Here, we report the effects of this mutation on channel function and heterologous expression in cell lines. We present a case report of infantile epileptic encephalopathy in a young girl, and trio-exome sequencing to determine the genetic etiology of her disorder. The patient was heterozygous for a de novo missense variant in the coding region of the KCNB1 gene, c.1133T>C. The variant encodes a V378A mutation in the α subunit of the Kv2.1 voltage-gated K⁺ channel, which is expressed at high levels in central neurons and is an important regulator of neuronal excitability. We found that expression of the V378A variant results in voltage-activated currents that are sensitive to the selective Kv2 channel blocker guangxitoxin-1E. These voltage-activated Kv2.1 V378A currents were nonselective among monovalent cations. Striking cell background–dependent differences in expression and subcellular localization of the V378A mutation were observed in heterologous cells. Further, coexpression of V378A subunits and wild-type Kv2.1 subunits reciprocally affects their respective trafficking characteristics. A recent study reported epileptic encephalopathy-linked missense variants that render Kv2.1 a tonically activated, nonselective cation channel that is not voltage activated. Our findings strengthen the correlation between mutations that result in loss of Kv2.1 ion selectivity and development of epileptic encephalopathy. However, the strong voltage sensitivity of currents from the V378A mutant indicates that the loss of voltage-sensitive gating seen in all other reported disease mutants is not required for an epileptic encephalopathy phenotype. In addition to electrophysiological differences, we suggest that defects in expression and subcellular localization of Kv2.1 V378A channels could contribute to the pathophysiology of this KCNB1 variant.

A comparison of the delayed outward potassium current between the nucleus ambiguus and hippocampus: sensitivity to paeonol

Whole-cell patch-clamp recordings investigated the electrophysiological effects of 2'-hydroxy-4'-methoxyacetophenone (paeonol), one of the major components of Moutan Cortex, in hippocampal CA1 neurons and nucleus ambiguus (NA) neurons from neonatal rats as well as in lung epithelial H1355 cells expressing Kv2.1 or Kv1.2. Extracellular application of paeonol at 100μM did not significantly affect the spontaneous action potential frequency, whereas paeonol at 300μM increased the frequency of spontaneous action potentials in hippocampal CA1 neurons. Paeonol (300μM) significantly decreased the tetraethylammonium-sensitive outward current in hippocampal CA1 neurons, but had no effect upon the fast-inactivating potassium current (IA). Extracellular application of paeonol at 300μM did not affect action potentials or the delayed outward currents in NA neurons. Paeonol (100μM) reduced the Kv2.1 current in H1355 cells, but not the Kv1.2 current. The inhibitor of Kv2, guangxitoxin-1E, reduced the delayed outward potassium currents in hippocampal neurons, but had only minimal effects in NA neurons. We demonstrated that paeonol decreased the delayed outward current and increased excitability in hippocampal CA1 neurons, whereas these effects were not observed in NA neurons. These effects may be associated with the inhibitory effects on Kv2.1 currents.

Geniposide acutely stimulates insulin secretion in pancreatic β-cells by regulating GLP-1 receptor/cAMP signaling and ion channels

Geniposide, an iridoid glycoside, has antidiabetic effects. The present study aimed to evaluate whether geniposide has direct effects on insulin secretion from rat pancreatic islets. The results demonstrated that geniposide potentiated insulin secretion via activating the glucagon-like-1 receptor (GLP-1R) as well as the adenylyl cyclase (AC)/cAMP signaling pathway. Inhibition of protein kinase A (PKA) suppressed the insulinotropic effect of geniposide. Geniposide also inhibited voltage-dependent potassium (Kv) channels, and this effect could be attenuated by inhibition of GLP-1R or PKA. Current-clamp recording showed that geniposide prolonged action potential duration. These results collectively imply that inhibition of Kv channels is linked to geniposide-potentiated insulin secretion by acting downstream of the GLP-1R/cAMP/PKA signaling pathway. Moreover, activation of Ca(2+) channels by geniposide was observed, indicating that the Ca(2+) channel is also an important player in the geniposide effects. Together, these findings provide new insight into the mechanism underlying geniposide-regulated insulin secretion.

Differential Regulation of Action Potential Shape and Burst-Frequency Firing by BK and Kv2 Channels in Substantia Nigra Dopaminergic Neurons

Little is known about the voltage-dependent potassium currents underlying spike repolarization in midbrain dopaminergic neurons. Studying mouse substantia nigra pars compacta dopaminergic neurons both in brain slice and after acute dissociation, we found that BK calcium-activated potassium channels and Kv2 channels both make major contributions to the depolarization-activated potassium current. Inhibiting Kv2 or BK channels had very different effects on spike shape and evoked firing. Inhibiting Kv2 channels increased spike width and decreased the afterhyperpolarization, as expected for loss of an action potential-activated potassium conductance. BK inhibition also increased spike width but paradoxically increased the afterhyperpolarization. Kv2 channel inhibition steeply increased the slope of the frequency-current (f-I) relationship, whereas BK channel inhibition had little effect on the f-I slope or decreased it, sometimes resulting in slowed firing. Action potential clamp experiments showed that both BK and Kv2 current flow during spike repolarization but with very different kinetics, with Kv2 current activating later and deactivating more slowly. Further experiments revealed that inhibiting either BK or Kv2 alone leads to recruitment of additional current through the other channel type during the action potential as a consequence of changes in spike shape. Enhancement of slowly deactivating Kv2 current can account for the increased afterhyperpolarization produced by BK inhibition and likely underlies the very different effects on the f-I relationship. The cross-regulation of BK and Kv2 activation illustrates that the functional role of a channel cannot be defined in isolation but depends critically on the context of the other conductances in the cell.

SIGNIFICANCE STATEMENT

This work shows that BK calcium-activated potassium channels and Kv2 voltage-activated potassium channels both regulate action potentials in dopamine neurons of the substantia nigra pars compacta. Although both channel types participate in action potential repolarization about equally, they have contrasting and partially opposite effects in regulating neuronal firing at frequencies typical of bursting. Our analysis shows that this results from their different kinetic properties, with fast-activating BK channels serving to short-circuit activation of Kv2 channels, which tend to slow firing by producing a deep afterhyperpolarization. The cross-regulation of BK and Kv2 activation illustrates that the functional role of a channel cannot be defined in isolation but depends critically on the context of the other conductances in the cell.

Distinct Cell- and Layer-Specific Expression Patterns and Independent Regulation of Kv2 Channel Subtypes in Cortical Pyramidal Neurons

The Kv2 family of voltage-gated potassium channel α subunits, comprising Kv2.1 and Kv2.2, mediate the bulk of the neuronal delayed rectifier K(+) current in many mammalian central neurons. Kv2.1 exhibits robust expression across many neuron types and is unique in its conditional role in modulating intrinsic excitability through changes in its phosphorylation state, which affect Kv2.1 expression, localization, and function. Much less is known of the highly related Kv2.2 subunit, especially in forebrain neurons. Here, through combined use of cortical layer markers and transgenic mouse lines, we show that Kv2.1 and Kv2.2 are localized to functionally distinct cortical cell types. Kv2.1 expression is consistently high throughout all cortical layers, especially in layer (L) 5b pyramidal neurons, whereas Kv2.2 expression is primarily limited to neurons in L2 and L5a. In addition, L4 of primary somatosensory cortex is strikingly devoid of Kv2.2 immunolabeling. The restricted pattern of Kv2.2 expression persists in Kv2.1-KO mice, suggesting distinct cell- and layer-specific functions for these two highly related Kv2 subunits. Analyses of endogenous Kv2.2 in cortical neurons in situ and recombinant Kv2.2 expressed in heterologous cells reveal that Kv2.2 is largely refractory to stimuli that trigger robust, phosphorylation-dependent changes in Kv2.1 clustering and function. Immunocytochemistry and voltage-clamp recordings from outside-out macropatches reveal distinct cellular expression patterns for Kv2.1 and Kv2.2 in intratelencephalic and pyramidal tract neurons of L5, indicating circuit-specific requirements for these Kv2 paralogs. Together, these results support distinct roles for these two Kv2 channel family members in mammalian cortex.

SIGNIFICANCE STATEMENT

Neurons within the neocortex are arranged in a laminar architecture and contribute to the input, processing, and/or output of sensory and motor signals in a cell- and layer-specific manner. Neurons of different cortical layers express diverse populations of ion channels and possess distinct intrinsic membrane properties. Here, we show that the Kv2 family members Kv2.1 and Kv2.2 are expressed in distinct cortical layers and pyramidal cell types associated with specific corticostriatal pathways. We find that Kv2.1 and Kv2.2 exhibit distinct responses to acute phosphorylation-dependent regulation in brain neurons in situ and in heterologous cells in vitro. These results identify a molecular mechanism that contributes to heterogeneity in cortical neuron ion channel function and regulation.

Roles of specific Kv channel types in repolarization of the action potential in genetically identified subclasses of pyramidal neurons in mouse neocortex

The action potential (AP) is a fundamental feature of excitable cells that serves as the basis for long-distance signaling in the nervous system. There is considerable diversity in the appearance of APs and the underlying repolarization mechanisms in different neuronal types (reviewed in Bean BP. Nat Rev Neurosci 8: 451-465, 2007), including among pyramidal cell subtypes. In the present work, we used specific pharmacological blockers to test for contributions of Kv1, Kv2, or Kv4 channels to repolarization of single APs in two genetically defined subpopulations of pyramidal cells in layer 5 of mouse somatosensory cortex (etv1 and glt) as well as pyramidal cells from layer 2/3. These three subtypes differ in AP properties (Groh A, Meyer HS, Schmidt EF, Heintz N, Sakmann B, Krieger P. Cereb Cortex 20: 826-836, 2010; Guan D, Armstrong WE, Foehring RC. J Neurophysiol 113: 2014-2032, 2015) as well as laminar position, morphology, and projection targets. We asked what the roles of Kv1, Kv2, and Kv4 channels are in AP repolarization and whether the underlying mechanisms are pyramidal cell subtype dependent. We found that Kv4 channels are critically involved in repolarizing neocortical pyramidal cells. There are also pyramidal cell subtype-specific differences in the role for Kv1 channels. Only Kv4 channels were involved in repolarizing the narrow APs of glt cells. In contrast, in etv1 cells and layer 2/3 cells, the broader APs are partially repolarized by Kv1 channels in addition to Kv4 channels. Consistent with their activation in the subthreshold range, Kv1 channels also regulate AP voltage threshold in all pyramidal cell subtypes.

Kv5, Kv6, Kv8, and Kv9 subunits: No simple silent bystanders

Members of the electrically silent voltage-gated K(+) (Kv) subfamilies (Kv5, Kv6, Kv8, and Kv9, collectively identified as electrically silent voltage-gated K(+) channel [KvS] subunits) do not form functional homotetrameric channels but assemble with Kv2 subunits into heterotetrameric Kv2/KvS channels with unique biophysical properties. Unlike the ubiquitously expressed Kv2 subunits, KvS subunits show a more restricted expression. This raises the possibility that Kv2/KvS heterotetramers have tissue-specific functions, making them potential targets for the development of novel therapeutic strategies. Here, I provide an overview of the expression of KvS subunits in different tissues and discuss their proposed role in various physiological and pathophysiological processes. This overview demonstrates the importance of KvS subunits and Kv2/KvS heterotetramers in vivo and the importance of considering KvS subunits and Kv2/KvS heterotetramers in the development of novel treatments.

Anoctamin 1 (Ano1) is required for glucose-induced membrane potential oscillations and insulin secretion by murine β-cells

Anions such as Cl(-) and HCO3 (-) are well known to play an important role in glucose-stimulated insulin secretion (GSIS). In this study, we demonstrate that glucose-induced Cl(-) efflux from β-cells is mediated by the Ca(2+)-activated Cl(-) channel anoctamin 1 (Ano1). Ano1 expression in rat β-cells is demonstrated by reverse transcriptase-polymerase chain reaction, western blotting, and immunohistochemistry. Typical Ano1 currents are observed in whole-cell and inside-out patches in the presence of intracellular Ca(++): at 1 μM, the Cl(-) current is outwardly rectifying, and at 2 μM, it becomes almost linear. The relative permeabilities of monovalent anions are NO3 (-) (1.83 ± 0.10) > Br(-) (1.42 ± 0.07) > Cl(-) (1.0). A linear single-channel current-voltage relationship shows a conductance of 8.37 pS. These currents are nearly abolished by blocking Ano1 antibodies or by the inhibitors 2-(5-ethyl-4-hydroxy-6-methylpyrimidin-2-ylthio)-N-(4-(4-methoxyphenyl)thiazol-2-yl)acetamide (T-AO1) and tannic acid (TA). These inhibitors induce a strong decrease of 16.7-mM glucose-stimulated action potential rate (at least 87 % on dispersed cells) and a partial membrane repolarization with T-AO1. They abolish or strongly inhibit the GSIS increment at 8.3 mM and at 16.7 mM glucose. Blocking Ano1 antibodies also abolish the 16.7-mM GSIS increment. Combined treatment with bumetanide and acetazolamide in low Cl(-) and HCO3 (-) media provokes a 65 % reduction in action potential (AP) amplitude and a 15-mV AP peak repolarization. Although the mechanism triggering Ano1 opening remains to be established, the present data demonstrate that Ano1 is required to sustain glucose-stimulated membrane potential oscillations and insulin secretion.

Anoctamin 1 (Ano1) is required for glucose-induced membrane potential oscillations and insulin secretion by murine β-cells

Anions such as Cl⁻ and HCO₃⁻ are well known to play an important role in glucose-stimulated insulin secretion (GSIS). In this study, we demonstrate that glucose-induced Cl⁻ efflux from β-cells is mediated by the Ca²⁺-activated Cl⁻ channel anoctamin 1 (Ano1). Ano1 expression in rat β-cells is demonstrated by reverse transcriptase–polymerase chain reaction, western blotting, and immunohistochemistry. Typical Ano1 currents are observed in whole-cell and inside-out patches in the presence of intracellular Ca⁺⁺: at 1 μM, the Cl⁻ current is outwardly rectifying, and at 2 μM, it becomes almost linear. The relative permeabilities of monovalent anions are NO₃⁻ (1.83 ± 0.10) > Br⁻ (1.42 ± 0.07) > Cl⁻ (1.0). A linear single-channel current–voltage relationship shows a conductance of 8.37 pS. These currents are nearly abolished by blocking Ano1 antibodies or by the inhibitors 2-(5-ethyl-4-hydroxy-6-methylpyrimidin-2-ylthio)-N-(4-(4-methoxyphenyl)thiazol-2-yl)acetamide (T-AO1) and tannic acid (TA). These inhibitors induce a strong decrease of 16.7-mM glucose-stimulated action potential rate (at least 87 % on dispersed cells) and a partial membrane repolarization with T-AO1. They abolish or strongly inhibit the GSIS increment at 8.3 mM and at 16.7 mM glucose. Blocking Ano1 antibodies also abolish the 16.7-mM GSIS increment. Combined treatment with bumetanide and acetazolamide in low Cl⁻ and HCO₃⁻ media provokes a 65 % reduction in action potential (AP) amplitude and a 15-mV AP peak repolarization. Although the mechanism triggering Ano1 opening remains to be established, the present data demonstrate that Ano1 is required to sustain glucose-stimulated membrane potential oscillations and insulin secretion.

Cell Cycle-dependent Changes in Localization and Phosphorylation of the Plasma Membrane Kv2.1 K+ Channel Impact Endoplasmic Reticulum Membrane Contact Sites in COS-1 Cells

The plasma membrane (PM) comprises distinct subcellular domains with diverse functions that need to be dynamically coordinated with intracellular events, one of the most impactful being mitosis. The Kv2.1 voltage-gated potassium channel is conditionally localized to large PM clusters that represent specialized PM:endoplasmic reticulum membrane contact sites (PM:ER MCS), and overexpression of Kv2.1 induces more exuberant PM:ER MCS in neurons and in certain heterologous cell types. Localization of Kv2.1 at these contact sites is dynamically regulated by changes in phosphorylation at one or more sites located on its large cytoplasmic C terminus. Here, we show that Kv2.1 expressed in COS-1 cells undergoes dramatic cell cycle-dependent changes in its PM localization, having diffuse localization in interphase cells, and robust clustering during M phase. The mitosis-specific clusters of Kv2.1 are localized to PM:ER MCS, and M phase clustering of Kv2.1 induces more extensive PM:ER MCS. These cell cycle-dependent changes in Kv2.1 localization and the induction of PM:ER MCS are accompanied by increased mitotic Kv2.1 phosphorylation at several C-terminal phosphorylation sites. Phosphorylation of exogenously expressed Kv2.1 is significantly increased upon metaphase arrest in COS-1 and CHO cells, and in a pancreatic β cell line that express endogenous Kv2.1. The M phase clustering of Kv2.1 at PM:ER MCS in COS-1 cells requires the same C-terminal targeting motif needed for conditional Kv2.1 clustering in neurons. The cell cycle-dependent changes in localization and phosphorylation of Kv2.1 were not accompanied by changes in the electrophysiological properties of Kv2.1 expressed in CHO cells. Together, these results provide novel insights into the cell cycle-dependent changes in PM protein localization and phosphorylation.

Tarantula toxins use common surfaces for interacting with Kv and ASIC ion channels

Tarantula toxins that bind to voltage-sensing domains of voltage-activated ion channels are thought to partition into the membrane and bind to the channel within the bilayer. While no structures of a voltage-sensor toxin bound to a channel have been solved, a structural homolog, psalmotoxin (PcTx1), was recently crystalized in complex with the extracellular domain of an acid sensing ion channel (ASIC). In the present study we use spectroscopic, biophysical and computational approaches to compare membrane interaction properties and channel binding surfaces of PcTx1 with the voltage-sensor toxin guangxitoxin (GxTx-1E). Our results show that both types of tarantula toxins interact with membranes, but that voltage-sensor toxins partition deeper into the bilayer. In addition, our results suggest that tarantula toxins have evolved a similar concave surface for clamping onto α-helices that is effective in aqueous or lipidic physical environments.

Acredinones A and B, voltage-dependent potassium channel inhibitors from the sponge-derived fungus Acremonium sp. F9A015

Two new benzophenones, acredinones A (1) and B (2), were isolated from a marine-sponge-associated Acremonium sp. fungus. Their chemical structures were elucidated on the interpretation of spectroscopic data. The structure of 1 was confirmed by palladium-catalyzed hydrogenation, followed by spectroscopic data analysis. Acredinones A (1) and B (2) inhibited the outward K(+) currents of the insulin secreting cell line INS-1 with IC50 values of 0.59 and 1.0 μM, respectively.

Partial blockade of Kv2.1 channel potentiates GLP-1's insulinotropic effects in islets and reduces its dose required for improving glucose tolerance in type 2 diabetic male mice

Glucagon-like peptide-1 (GLP-1)-based medicines have recently been widely used to treat type 2 diabetic patients, whereas adverse effects of nausea and vomiting have been documented. Inhibition of voltage-gated K(+) channel subtype Kv2.1 in pancreatic β-cells has been suggested to contribute to mild depolarization and promotion of insulin release. This study aimed to determine whether the blockade of Kv2.1 channels potentiates the insulinotropic effect of GLP-1 agonists. Kv2.1 channel blocker guangxitoxin-1E (GxTx) and GLP-1 agonist exendin-4 at subthreshold concentrations, when combined, markedly increased the insulin release and cytosolic Ca(2+) concentration ([Ca(2+)]i) in a glucose-dependent manner in mouse islets and β-cells. Exendin-4 at subthreshold concentration alone increased islet insulin release and β-cell [Ca(2+)]i in Kv2.1(+/-) mice. The [Ca(2+)]i response to subthreshold exendin-4 and GxTx in combination was attenuated by pretreatment with protein kinase A inhibitor H-89, indicating the protein kinase A dependency of the cooperative effect. Furthermore, subthreshold doses of GxTx and GLP-1 agonist liraglutide in combination markedly increased plasma insulin and improved glucose tolerance in diabetic db/db mice and NSY mice. These results demonstrate that a modest suppression of Kv2.1 channels dramatically raises insulinotropic potency of GLP-1-based drugs, which opens a new avenue to reduce their doses and associated adverse effects while achieving the same glycemic control in type 2 diabetes.

Chemoselective tarantula toxins report voltage activation of wild-type ion channels in live cells

Electrically excitable cells, such as neurons, exhibit tremendous diversity in their firing patterns, a consequence of the complex collection of ion channels present in any specific cell. Although numerous methods are capable of measuring cellular electrical signals, understanding which types of ion channels give rise to these signals remains a significant challenge. Here, we describe exogenous probes which use a novel mechanism to report activity of voltage-gated channels. We have synthesized chemoselective derivatives of the tarantula toxin guangxitoxin-1E (GxTX), an inhibitory cystine knot peptide that binds selectively to Kv2-type voltage gated potassium channels. We find that voltage activation of Kv2.1 channels triggers GxTX dissociation, and thus GxTX binding dynamically marks Kv2 activation. We identify GxTX residues that can be replaced by thiol- or alkyne-bearing amino acids, without disrupting toxin folding or activity, and chemoselectively ligate fluorophores or affinity probes to these sites. We find that GxTX-fluorophore conjugates colocalize with Kv2.1 clusters in live cells and are released from channels activated by voltage stimuli. Kv2.1 activation can be detected with concentrations of probe that have a trivial impact on cellular currents. Chemoselective GxTX mutants conjugated to dendrimeric beads likewise bind live cells expressing Kv2.1, and the beads are released by channel activation. These optical sensors of conformational change are prototype probes that can indicate when ion channels contribute to electrical signaling.

Oxidation of KCNB1 K(+) channels in central nervous system and beyond

KCNB1, a voltage-gated potassium (K(+)) channel that conducts a major delayed rectifier current in the brain, pancreas and cardiovascular system is a key player in apoptotic programs associated with oxidative stress. As a result, this protein represents a bona fide drug target for limiting the toxic effects of oxygen radicals. Until recently the consensus view was that reactive oxygen species trigger a pro-apoptotic surge in KCNB1 current via phosphorylation and SNARE-dependent incorporation of KCNB1 channels into the plasma membrane. However, new evidence shows that KCNB1 can be modified by oxidants and that oxidized KCNB1 channels can directly activate pro-apoptotic signaling pathways. Hence, a more articulated picture of the pro-apoptotic role of KCNB1 is emerging in which the protein induces cell's death through distinct molecular mechanisms and activation of multiple pathways. In this review article we discuss the diverse functional, toxic and protective roles that KCNB1 channels play in the major organs where they are expressed.

Kv2 channel regulation of action potential repolarization and firing patterns in superior cervical ganglion neurons and hippocampal CA1 pyramidal neurons

Kv2 family "delayed-rectifier" potassium channels are widely expressed in mammalian neurons. Kv2 channels activate relatively slowly and their contribution to action potential repolarization under physiological conditions has been unclear. We explored the function of Kv2 channels using a Kv2-selective blocker, Guangxitoxin-1E (GxTX-1E). Using acutely isolated neurons, mixed voltage-clamp and current-clamp experiments were done at 37°C to study the physiological kinetics of channel gating and action potentials. In both rat superior cervical ganglion (SCG) neurons and mouse hippocampal CA1 pyramidal neurons, 100 nm GxTX-1E produced near-saturating block of a component of current typically constituting ∼60-80% of the total delayed-rectifier current. GxTX-1E also reduced A-type potassium current (IA), but much more weakly. In SCG neurons, 100 nm GxTX-1E broadened spikes and voltage clamp experiments using action potential waveforms showed that Kv2 channels carry ∼55% of the total outward current during action potential repolarization despite activating relatively late in the spike. In CA1 neurons, 100 nm GxTX-1E broadened spikes evoked from -70 mV, but not -80 mV, likely reflecting a greater role of Kv2 when other potassium channels were partially inactivated at -70 mV. In both CA1 and SCG neurons, inhibition of Kv2 channels produced dramatic depolarization of interspike voltages during repetitive firing. In CA1 neurons and some SCG neurons, this was associated with increased initial firing frequency. In all neurons, inhibition of Kv2 channels depressed maintained firing because neurons entered depolarization block more readily. Therefore, Kv2 channels can either decrease or increase neuronal excitability depending on the time scale of excitation.

Deletion of the Kv2.1 delayed rectifier potassium channel leads to neuronal and behavioral hyperexcitability

The Kv2.1 delayed rectifier potassium channel exhibits high-level expression in both principal and inhibitory neurons throughout the central nervous system, including prominent expression in hippocampal neurons. Studies of in vitro preparations suggest that Kv2.1 is a key yet conditional regulator of intrinsic neuronal excitability, mediated by changes in Kv2.1 expression, localization and function via activity-dependent regulation of Kv2.1 phosphorylation. Here we identify neurological and behavioral deficits in mutant (Kv2.1(-/-) ) mice lacking this channel. Kv2.1(-/-) mice have grossly normal characteristics. No impairment in vision or motor coordination was apparent, although Kv2.1(-/-) mice exhibit reduced body weight. The anatomic structure and expression of related Kv channels in the brains of Kv2.1(-/-) mice appear unchanged. Delayed rectifier potassium current is diminished in hippocampal neurons cultured from Kv2.1(-/-) animals. Field recordings from hippocampal slices of Kv2.1(-/-) mice reveal hyperexcitability in response to the convulsant bicuculline, and epileptiform activity in response to stimulation. In Kv2.1(-/-) mice, long-term potentiation at the Schaffer collateral - CA1 synapse is decreased. Kv2.1(-/-) mice are strikingly hyperactive, and exhibit defects in spatial learning, failing to improve performance in a Morris Water Maze task. Kv2.1(-/-) mice are hypersensitive to the effects of the convulsants flurothyl and pilocarpine, consistent with a role for Kv2.1 as a conditional suppressor of neuronal activity. Although not prone to spontaneous seizures, Kv2.1(-/-) mice exhibit accelerated seizure progression. Together, these findings suggest homeostatic suppression of elevated neuronal activity by Kv2.1 plays a central role in regulating neuronal network function.

Isolation, synthesis and characterization of ω-TRTX-Cc1a, a novel tarantula venom peptide that selectively targets L-type Cav channels

Spider venoms are replete with peptidic ion channel modulators, often with novel subtype selectivity, making them a rich source of pharmacological tools and drug leads. In a search for subtype-selective blockers of voltage-gated calcium (CaV) channels, we isolated and characterized a novel 39-residue peptide, ω-TRTX-Cc1a (Cc1a), from the venom of the tarantula Citharischius crawshayi (now Pelinobius muticus). Cc1a is 67% identical to the spider toxin ω-TRTX-Hg1a, an inhibitor of CaV2.3 channels. We assembled Cc1a using a combination of Boc solid-phase peptide synthesis and native chemical ligation. Oxidative folding yielded two stable, slowly interconverting isomers. Cc1a preferentially inhibited Ba(2+) currents (IBa) mediated by L-type (CaV1.2 and CaV1.3) CaV channels heterologously expressed in Xenopus oocytes, with half-maximal inhibitory concentration (IC50) values of 825nM and 2.24μM, respectively. In rat dorsal root ganglion neurons, Cc1a inhibited IBa mediated by high voltage-activated CaV channels but did not affect low voltage-activated T-type CaV channels. Cc1a exhibited weak activity at NaV1.5 and NaV1.7 voltage-gated sodium (NaV) channels stably expressed in mammalian HEK or CHO cells, respectively. Experiments with modified Cc1a peptides, truncated at the N-terminus (ΔG1-E5) or C-terminus (ΔW35-V39), demonstrated that the N- and C-termini are important for voltage-gated ion channel modulation. We conclude that Cc1a represents a novel pharmacological tool for probing the structure and function of L-type CaV channels.

Opening the shaker K+ channel with hanatoxin

Voltage-activated ion channels open and close in response to changes in membrane voltage, a property that is fundamental to the roles of these channels in electrical signaling. Protein toxins from venomous organisms commonly target the S1–S4 voltage-sensing domains in these channels and modify their gating properties. Studies on the interaction of hanatoxin with the Kv2.1 channel show that this tarantula toxin interacts with the S1–S4 domain and inhibits opening by stabilizing a closed state. Here we investigated the interaction of hanatoxin with the Shaker Kv channel, a voltage-activated channel that has been extensively studied with biophysical approaches. In contrast to what is observed in the Kv2.1 channel, we find that hanatoxin shifts the conductance–voltage relation to negative voltages, making it easier to open the channel with membrane depolarization. Although these actions of the toxin are subtle in the wild-type channel, strengthening the toxin–channel interaction with mutations in the S3b helix of the S1-S4 domain enhances toxin affinity and causes large shifts in the conductance–voltage relationship. Using a range of previously characterized mutants of the Shaker Kv channel, we find that hanatoxin stabilizes an activated conformation of the voltage sensors, in addition to promoting opening through an effect on the final opening transition. Chimeras in which S3b–S4 paddle motifs are transferred between Kv2.1 and Shaker Kv channels, as well as experiments with the related tarantula toxin GxTx-1E, lead us to conclude that the actions of tarantula toxins are not simply a product of where they bind to the channel, but that fine structural details of the toxin–channel interface determine whether a toxin is an inhibitor or opener.

Control of voltage-gated potassium channel Kv2.2 expression by pyruvate-isocitrate cycling regulates glucose-stimulated insulin secretion

Recent studies have shown that the pyruvate-isocitrate cycling pathway, involving the mitochondrial citrate/isocitrate carrier and the cytosolic NADP-dependent isocitrate dehydrogenase (ICDc), is involved in control of glucose-stimulated insulin secretion (GSIS). Here we demonstrate that pyruvate-isocitrate cycling regulates expression of the voltage-gated potassium channel family member Kv2.2 in islet β-cells. siRNA-mediated suppression of ICDc, citrate/isocitrate carrier, or Kv2.2 expression impaired GSIS, and the effect of ICDc knockdown was rescued by re-expression of Kv2.2. Moreover, chronic exposure of β-cells to elevated fatty acids, which impairs GSIS, resulted in decreased expression of Kv2.2. Surprisingly, knockdown of ICDc or Kv2.2 increased rather than decreased outward K(+) current in the 832/13 β-cell line. Immunoprecipitation studies demonstrated interaction of Kv2.1 and Kv2.2, and co-overexpression of the two channels reduced outward K(+) current compared with overexpression of Kv2.1 alone. Also, siRNA-mediated knockdown of ICDc enhanced the suppressive effect of the Kv2.1-selective inhibitor stromatoxin1 on K(+) currents. Our data support a model in which a key function of the pyruvate-isocitrate cycle is to maintain levels of Kv2.2 expression sufficient to allow it to serve as a negative regulator of Kv channel activity.

Molecular determinants for the tarantula toxin jingzhaotoxin-I interacting with potassium channel Kv2.1

With high binding affinity and distinct pharmacological functions, animal toxins are powerful ligands to investigate the structure-function relationships of voltage-gated ion channels. Jingzhaotoxin-I (JZTX-I) is an important neurotoxin from the tarantula Chilobrachys jingzhao venom that inhibits both sodium and potassium channels. In our previous work, JZTX-I, as a gating modifier, is able to inhibit activation of the potassium channel subtype Kv2.1. However, its binding site on Kv2.1 remains unknown. In this study, using Ala-scanning mutagenesis strategy, we demonstrated that four residues (I273, F274, E277, and K280) in S3b-S4 motif contributed to the formation of JZTX-I binding site. The mutations I273A, F274A, E277A, and K280A reduced toxin binding affinity by 6-, 10-, 8-, and 7-fold, respectively. Taken together with our previous data that JZTX-I accelerated channel deactivation, these results suggest that JZTX-I inhibits Kv2.1 activation by docking onto the voltage sensor paddle and trapping the voltage sensor in the closed state.

The role of voltage-gated potassium channels Kv2.1 and Kv2.2 in the regulation of insulin and somatostatin release from pancreatic islets

The voltage-gated potassium channels Kv2.1 and Kv2.2 are highly expressed in pancreatic islets, yet their contribution to islet hormone secretion is not fully understood. Here we investigate the role of Kv2 channels in pancreatic islets using a combination of genetic and pharmacologic approaches. Pancreatic β-cells from Kv2.1(-/-) mice possess reduced Kv current and display greater glucose-stimulated insulin secretion (GSIS) relative to WT β-cells. Inhibition of Kv2.x channels with selective peptidyl [guangxitoxin-1E (GxTX-1E)] or small molecule (RY796) inhibitors enhances GSIS in isolated wild-type (WT) mouse and human islets, but not in islets from Kv2.1(-/-) mice. However, in WT mice neither inhibitor improved glucose tolerance in vivo. GxTX-1E and RY796 enhanced somatostatin release in isolated human and mouse islets and in situ perfused pancreata from WT and Kv2.1(-/-) mice. Kv2.2 silencing in mouse islets by adenovirus-small hairpin RNA (shRNA) specifically enhanced islet somatostatin, but not insulin, secretion. In mice lacking somatostatin receptor 5, GxTX-1E stimulated insulin secretion and improved glucose tolerance. Collectively, these data show that Kv2.1 regulates insulin secretion in β-cells and Kv2.2 modulates somatostatin release in δ-cells. Development of selective Kv2.1 inhibitors without cross inhibition of Kv2.2 may provide new avenues to promote GSIS for the treatment of type 2 diabetes.

Glucose-sensitivity of the afterhyperpolarization potential: role of SK1 channel in insulin-secreting cells

The role of the small-conductance, calcium-activated SK potassium channel in regulating pancreatic β cell function remains controversial with conflicting pharmacological results. In this study, we used current clamp recordings to further characterize the function of SK channels in INS-1 cell line. We compared afterhyperpolarization potential (AHP) responses of SK1-downregulated cells with those of control INS-1 cells. They were tested with and without the presence of glucose. We found that cells in which SK1 channel subunit expression had been downregulated exhibited AHPs in the presence of 20mM glucose while control INS-1 cells had AHPs only in the absence of glucose. Our findings show that the glucose-dependence of the AHP in the rat INS-1 cell line depends only on SK1 channel subunit expression.

Identification and pharmacological characterization of multiple allosteric binding sites on the free fatty acid 1 receptor

Activation of FFA1 (GPR40), a member of G protein-coupling receptor family A, is mediated by medium- and long-chain fatty acids and leads to amplification of glucose-stimulated insulin secretion, suggesting a potential role for free fatty acid 1 (FFA1) as a target for type 2 diabetes. It was assumed previously that there is a single binding site for fatty acids and synthetic FFA1 agonists. However, using members of two chemical series of partial and full agonists that have been identified, radioligand binding interaction studies revealed that the full agonists do not bind to the same site as the partial agonists but exhibit positive heterotropic cooperativity. Analysis of functional data reveals positive functional cooperativity between the full agonists and partial agonists in various functional assays (in vitro and ex vivo) and also in vivo. Furthermore, the endogenous fatty acid docosahexaenoic acid (DHA) shows negative or neutral cooperativity with members of both series of agonists in binding assays but displays positive cooperativity in functional assays. Another synthetic agonist is allosteric with members of both agonist series, but apparently competitive with DHA. Therefore, there appear to be three allosterically linked binding sites on FFA1 with agonists specific for each of these sites. Activation of free fatty acid 1 receptor (FFAR1) by each of these agonists is differentially affected by mutations of two arginine residues, previously found to be important for FFAR1 binding and activation. These ligands with their high potencies and strong positive functional cooperativity with endogenous fatty acids, demonstrated in vitro and in vivo, have the potential to deliver therapeutic benefits.

Ion channel drug discovery: challenges and future directions

Ion channels are targets of many therapeutically useful agents, and worldwide sales of ion channel-targeted drugs are estimated to be approximately US$12 billion. Nevertheless, considering that over 400 genes encoding ion channel subunits have been identified, ion channels remain significantly under-exploited as therapeutic targets. This is at least partly due to limitations in high-throughput assay technologies that support screening and lead optimization. Will the recent developments in automated electrophysiology rectify this situation? What are the other major limitations and can they be overcome? In this article, we review the status of ion channel drug discovery, discuss current challenges and propose alternative approaches that may facilitate the discovery of new drugs in the future.

Cardiotoxin-I: an unexpectedly potent insulinotropic agent

Insulin secretion from pancreatic β-cells is a complex process, involving the integration and interaction of multiple external and internal stimuli, in which glucose plays a major role. Understanding the physiology leading to insulin release is a crucial step toward the identification of new targets. In this study, we evaluated the presence of insulinotropic metabolites in Naja kaouthia snake venom. Only one fraction, identified as cardiotoxin-I (CTX-I) was able to induce insulin secretion from INS-1E cells without affecting cell viability and integrity, as assessed by MTT and LDH assays. Interestingly, CTX-I was also able to stimulate insulin secretion from INS-1E cells even in the absence of glucose. Although cardiotoxins have been characterized as potent hemolytic agents and vasoconstrictors, CTX-I was unable to induce direct hemolysis of human erythrocytes or to induce potent vasoconstriction. As such, this newly identified insulin-releasing toxin will surely enrich the pool of existing tools to study β-cell physiology or even open a new therapeutic avenue.

Block of Kv1.7 potassium currents increases glucose-stimulated insulin secretion

Glucose-stimulated insulin secretion (GSIS) relies on repetitive, electrical spiking activity of the beta cell membrane. Cyclic activation of voltage-gated potassium channels (K_v) generates an outward, ‘delayed rectifier’ potassium current, which drives the repolarizing phase of each spike and modulates insulin release. Although several K_v channels are expressed in pancreatic islets, their individual contributions to GSIS remain incompletely understood. We take advantage of a naturally occurring cone-snail peptide toxin, Conkunitzin-S1 (Conk-S1), which selectively blocks K_v1.7 channels to provide an intrinsically limited, finely graded control of total beta cell delayed rectifier current and hence of GSIS. Conk-S1 increases GSIS in isolated rat islets, likely by reducing K_v1.7-mediated delayed rectifier currents in beta cells, which yields increases in action potential firing and cytoplasmic free calcium. In rats, Conk-S1 increases glucose-dependent insulin secretion without decreasing basal glucose. Thus, we conclude that K_v1.7 contributes to the membrane-repolarizing current of beta cells during GSIS and that block of this specific component of beta cell K_v current offers a potential strategy for enhancing GSIS with minimal risk of hypoglycaemia during metabolic disorders such as Type 2 diabetes.

Solution structure of kurtoxin: a gating modifier selective for Cav3 voltage-gated Ca(2+) channels

Kurtoxin is a 63-amino acid polypeptide isolated from the venom of the South African scorpion Parabuthus transvaalicus. It is the first and only peptide ligand known to interact with Cav3 (T-type) voltage-gated Ca(2+) channels with high affinity and to modify the voltage-dependent gating of these channels. Here we describe the nuclear magnetic resonance (NMR) solution structure of kurtoxin determined using two- and three-dimensional NMR spectroscopy with dynamical simulated annealing calculations. The molecular structure of the toxin was highly similar to those of scorpion α-toxins and contained an α-helix, three β-strands, and several turns stabilized by four disulfide bonds. This so-called "cysteine-stabilized α-helix and β-sheet (CSαβ)" motif is found in a number of functionally varied small proteins. A detailed comparison of the backbone structure of kurtoxin with those of the scorpion α-toxins revealed that three regions [first long loop (Asp(8)-Ile(15)), β-hairpin loop (Gly(39)-Leu(42)), and C-terminal segment (Arg(57)-Ala(63))] in kurtoxin significantly differ from the corresponding regions in scorpion α-toxins, suggesting that these regions may be important for interacting with Cav3 (T-type) Ca(2+) channels. In addition, the surface profile of kurtoxin shows a larger and more focused electropositive patch along with a larger hydrophobic surface compared to those seen on scorpion α-toxins. These distinct surface properties of kurtoxin could explain its binding to Cav3 (T-type) voltage-gated Ca(2+) channels.

Repression of a potassium channel by nuclear hormone receptor and TGF-β signaling modulates insulin signaling in Caenorhabditis elegans

Transforming growth factor β (TGF-β) signaling acts through Smad proteins to play fundamental roles in cell proliferation, differentiation, apoptosis, and metabolism. The Receptor associated Smads (R-Smads) interact with DNA and other nuclear proteins to regulate target gene transcription. Here, we demonstrate that the Caenorhabditis elegans R-Smad DAF-8 partners with the nuclear hormone receptor NHR-69, a C. elegans ortholog of mammalian hepatocyte nuclear factor 4α HNF4α), to repress the exp-2 potassium channel gene and increase insulin secretion. We find that NHR-69 associates with DAF-8 both in vivo and in vitro. Functionally, daf-8 nhr-69 double mutants show defects in neuropeptide secretion and phenotypes consistent with reduced insulin signaling such as increased expression of the sod-3 and gst-10 genes and a longer life span. Expression of the exp-2 gene, encoding a voltage-gated potassium channel, is synergistically increased in daf-8 nhr-69 mutants compared to single mutants and wild-type worms. In turn, exp-2 acts selectively in the ASI neurons to repress the secretion of the insulin-like peptide DAF-28. Importantly, exp-2 mutation shortens the long life span of daf-8 nhr-69 double mutants, demonstrating that exp-2 is required downstream of DAF-8 and NHR-69. Finally, animals over-expressing NHR-69 specifically in DAF-28–secreting ASI neurons exhibit a lethargic, hypoglycemic phenotype that is rescued by exogenous glucose. We propose a model whereby DAF-8/R-Smad and NHR-69 negatively regulate the transcription of exp-2 to promote neuronal DAF-28 secretion, thus demonstrating a physiological crosstalk between TGF-β and HNF4α-like signaling in C. elegans. NHR-69 and DAF-8 dependent regulation of exp-2 and DAF-28 also provides a novel molecular mechanism that contributes to the previously recognized link between insulin and TGF-β signaling in C. elegans.

All animals must ensure metabolic homeostasis; if they fail to do so, diseases such as obesity and diabetes can develop. To maintain glucose balance, insulin is secreted upon glucose intake in a highly regulated and coordinated process. Previous studies suggested that the transforming growth factor beta (TGF-β) signaling pathway regulates insulin secretion in mammals. In the genetically tractable roundworm Caenorhabditis elegans, TGF-β and insulin signaling modulate larval development and aging, although the molecular link between insulin and TGF-β signaling remains poorly understood. In this study, we show that the TGF-β signaling component DAF-8 partners with NHR-69, a nuclear hormone receptor, to control the expression of the potassium channel exp-2, which in turn modulates the secretion of an insulin-like peptide. A loss-of-function exp-2 mutant exhibits increased insulin secretion and a shortened life span, whereas a gain-of-function mutant exhibits decreased insulin secretion. We also show that tissue-specific expression of nhr-69 in a pair of neurons that secrete neuropeptides causes reduced glucose content, increased insulin-like peptide levels and a lethargic phenotype. Because insulin and TGF-β signaling are linked to numerous diseases, our data may provide novel insights into the mechanisms contributing to pathophysiological changes.

Bombesin receptor subtype-3 (BRS-3) regulates glucose-stimulated insulin secretion in pancreatic islets across multiple species

Bombesin receptor subtype-3 (BRS-3) regulates energy homeostasis, and BRS-3 agonism is being explored as a possible therapy for obesity. Here we study the role of BRS-3 in the regulation of glucose-stimulated insulin secretion (GSIS) and glucose homeostasis. We quantified BRS-3 mRNA in pancreatic islets from multiple species and examined the acute effects of Bag-1, a selective BRS-3 agonist, on GSIS in mouse, rat, and human islets, and on oral glucose tolerance in mice. BRS-3 is highly expressed in human, mouse, rhesus, and dog (but not rat) pancreatic islets and in rodent insulinoma cell lines (INS-1 832/3 and MIN6). Silencing BRS-3 with small interfering RNA or pharmacological blockade with a BRS-3 antagonist, Bantag-1, reduced GSIS in 832/3 cells. In contrast, the BRS-3 agonist (Bag-1) increased GSIS in 832/3 and MIN6 cells. The augmentation of GSIS by Bag-1 was completely blocked by U73122, a phospholipase C inhibitor. Bag-1 also enhanced GSIS in islets isolated from wild-type, but not Brs3 knockout mice. In vivo, Bag-1 reduced glucose levels during oral glucose tolerance test in a BRS-3-dependent manner. BRS-3 agonists also increased GSIS in human islets. These results identify a potential role for BRS-3 in islet physiology, with agonism directly promoting GSIS. Thus, in addition to its potential role in the treatment of obesity, BRS-3 may also regulate blood glucose levels and have a role in the treatment of diabetes mellitus.

Cholesterol elevation impairs glucose-stimulated Ca(2+) signaling in mouse pancreatic β-cells

Recent studies have demonstrated that cholesterol elevation in pancreatic islets is associated with a reduction in glucose-stimulated insulin secretion, but the underlying cellular mechanisms remain elusive. Here, we show that cholesterol enrichment dramatically reduced the proportion of mouse β-cells that exhibited a Ca(2+) signal when stimulated by high glucose. When cholesterol-enriched β-cells were challenged with tolbutamide, there was a decrease in the amplitude of the Ca(2+) signal, and it was associated with a reduction in the cell current density of voltage-gated Ca(2+) channels (VGCC). Although the cell current densities of the ATP-dependent K(+) channels and the delayed rectifier K(+) channels were also reduced in the cholesterol-enriched β-cells, glucose evoked only a small depolarization in these cells. In cholesterol-enriched cells, the glucose-mediated increase in cellular ATP content was dramatically reduced, and this was related to a decrease in glucose uptake via glucose transporter 2 and an impairment of mitochondrial metabolism. Thus, cholesterol enrichment impaired glucose-stimulated Ca(2+) signaling in β-cells via two mechanisms: a decrease in the current density of VGCC and a reduction in glucose-stimulated mitochondrial ATP production, which in turn led to a smaller glucose-evoked depolarization. The decrease in VGCC-mediated extracellular Ca(2+) influx in cholesterol-enriched β-cells was associated with a reduction in the amount of exocytosis. Our findings suggest that defect in glucose-stimulated Ca(2+) signaling is an important mechanism underlying the impairment of glucose-stimulated insulin secretion in islets with elevated cholesterol level.

Arylnaphthalene lignans from Taiwania cryptomerioides as novel blockers of voltage-gated K+ channels

Lignans are natural phytochemicals which exhibit multiple pharmacological effects such as anti-inflammation, antivirus and anti-tumor activities. Whether they have effects on neural tissues and ion channels is still unknown. The effects of several arylnaphathalene lignans purified from Taiwania cryptomerioides on voltage-gated K(+) (Kv) channels in mouse neuroblastoma N2A cells were examined. These lignans included Taiwanin E, helioxanthin (HXT) and diphyllin. All lignans showed inhibitory effects on Kv channels and HXT was the most potent compound (IC(50)=1.7 μM). The mechanism of HXT block was further investigated. Its action was found to be extracellular but not intracellular. HXT accelerated current decay, caused a left-shift in steady-state inactivation curve but had no effect on voltage-dependence of activation. HXT block was unaffected by intracellular K(+) concentrations. Further, it did not affect ATP-sensitive K(+) channels. Our data therefore suggest that HXT is a potent and specific blocker of Kv channels, possibly with an inhibitory mechanism involving acceleration of slow inactivation.

Solution structure of GxTX-1E, a high-affinity tarantula toxin interacting with voltage sensors in Kv2.1 potassium channels

GxTX-1E is a neurotoxin recently isolated from Plesiophrictus guangxiensis venom that inhibits the Kv2.1 channel in pancreatic beta-cells. The sequence of the toxin is related to those of previously studied tarantula toxins that interact with the voltage sensors in Kv channels, and GxTX-1E interacts with the Kv2.1 channel with unusually high affinity, making it particularly useful for structural and mechanistic studies. Here we determined the three-dimensional solution structure of GxTX-1E using NMR spectroscopy and compared it to that of several related tarantula toxins. The molecular structure of GxTX-1E is similar to those of tarantula toxins that target voltage sensors in Kv channels in that it contains an ICK motif, composed of beta-strands, and contains a prominent cluster of solvent-exposed hydrophobic residues surrounded by polar residues. When compared with the structure of SGTx1, a toxin for which mutagenesis data are available, the residue compositions of the two toxins are distinct in regions that are critical for activity, suggesting that their modes of binding to voltage sensors may be different. Interestingly, the structural architecture of GxTX-1E is also similar to that of JZTX-III, a tarantula toxin that interacts with Kv2.1 with low affinity. The most striking structural differences between GxTX-1E and JZTX-III are found in the orientation between the first and second cysteine loops and the C-terminal region of the toxins, suggesting that these regions of GxTX-1E are responsible for its high affinity.

Calcium-activated and voltage-gated potassium channels of the pancreatic islet impart distinct and complementary roles during secretagogue induced electrical responses

Glucose-induced β-cell action potential (AP) repolarization is regulated by potassium efflux through voltage gated (Kv) and calcium activated (K(Ca)) potassium channels. Thus, ablation of the primary Kv channel of the β-cell, Kv2.1, causes increased AP duration. However, Kv2.1(-/-) islet electrical activity still remains sensitive to the potassium channel inhibitor tetraethylammonium. Therefore, we utilized Kv2.1(-/-) islets to characterize Kv and K(Ca) channels and their respective roles in modulating the β-cell AP. The remaining Kv current present in Kv2.1(-/-) β-cells is inhibited with 5 μM CP 339818. Inhibition of the remaining Kv current in Kv2.1(-/-) mouse β-cells increased AP firing frequency by 39.6% but did not significantly enhance glucose stimulated insulin secretion (GSIS). The modest regulation of islet AP frequency by CP 339818 implicates other K(+) channels, possibly K(Ca) channels, in regulating AP repolarization. Blockade of the K(Ca) channel BK with slotoxin increased β-cell AP amplitude by 28.2%, whereas activation of BK channels with isopimaric acid decreased β-cell AP amplitude by 30.6%. Interestingly, the K(Ca) channel SK significantly contributes to Kv2.1(-/-) mouse islet AP repolarization. Inhibition of SK channels decreased AP firing frequency by 66% and increased AP duration by 67% only when Kv2.1 is ablated or inhibited and enhanced GSIS by 2.7-fold. Human islets also express SK3 channels and their β-cell AP frequency is significantly accelerated by 4.8-fold with apamin. These results uncover important repolarizing roles for both Kv and K(Ca) channels and identify distinct roles for SK channel activity in regulating calcium- versus sodium-dependent AP firing.