Myristoylated Neuronal Calcium Sensor-1 captures the ciliary vesicle at distal appendages

The primary cilium is a microtubule-based organelle that cycles through assembly and disassembly. In many cell types, formation of the cilium is initiated by recruitment of ciliary vesicles to the distal appendage of the mother centriole. However, the distal appendage mechanism that directly captures ciliary vesicles is yet to be identified. In an accompanying paper, we show that the distal appendage protein, CEP89, is important for thef ciliary vesicle recruitment, but not for other steps of cilium formation (Tomoharu Kanie, Love, Fisher, Gustavsson, & Jackson, 2023). The lack of a membrane binding motif in CEP89 suggests that it may indirectly recruit ciliary vesicles via another binding partner. Here, we identify Neuronal Calcium Sensor-1 (NCS1) as a stoichiometric interactor of CEP89. NCS1 localizes to the position between CEP89 and a ciliary vesicle marker, RAB34, at the distal appendage. This localization was completely abolished in CEP89 knockouts, suggesting that CEP89 recruits NCS1 to the distal appendage. Similarly to CEP89 knockouts, ciliary vesicle recruitment as well as subsequent cilium formation was perturbed in NCS1 knockout cells. The ability of NCS1 to recruit the ciliary vesicle is dependent on its myristoylation motif and NCS1 knockout cells expressing myristoylation defective mutant failed to rescue the vesicle recruitment defect despite localizing proper localization to the centriole. In sum, our analysis reveals the first known mechanism for how the distal appendage recruits the ciliary vesicles.

NCS-1 Deficiency Affects mRNA Levels of Genes Involved in Regulation of ATP Synthesis and Mitochondrial Stress in Highly Vulnerable Substantia nigra Dopaminergic Neurons

Neuronal Ca²⁺ sensor proteins (NCS) transduce changes in Ca²⁺ homeostasis into altered signaling and neuronal function. NCS-1 activity has emerged as important for neuronal viability and pathophysiology. The progressive degeneration of dopaminergic (DA) neurons, particularly within the Substantia nigra (SN), is the hallmark of Parkinson's disease (PD), causing its motor symptoms. The activity-related Ca²⁺ homeostasis of SN DA neurons, mitochondrial dysfunction, and metabolic stress promote neurodegeneration and PD. In contrast, NCS-1 in general has neuroprotective effects. The underlying mechanisms are unclear. We analyzed transcriptional changes in SN DA neurons upon NCS-1 loss by combining UV-laser microdissection and RT-qPCR-approaches to compare expression levels of a panel of PD and/or Ca²⁺-stress related genes from wildtype and NCS-1 KO mice. In NCS-1 KO, we detected significantly lower mRNA levels of mitochondrially coded ND1, a subunit of the respiratory chain, and of the neuron-specific enolase ENO2, a glycolytic enzyme. We also detected lower levels of the mitochondrial uncoupling proteins UCP4 and UCP5, the PARK7 gene product DJ-1, and the voltage-gated Ca²⁺ channel Cav2.3 in SN DA neurons from NCS-1 KO. Transcripts of other analyzed uncoupling proteins (UCPs), mitochondrial Ca²⁺ transporters, PARK genes, and ion channels were not altered. As Cav channels are linked to regulation of gene expression, metabolic stress and degeneration of SN DA neurons in PD, we analyzed Cav2.3 KO mice, to address if the transcriptional changes in NCS-1 KO were also present in Cav.2.3 KO, and thus probably correlated with lower Cav2.3 transcripts. However, in SN DA neurons from Cav2.3 KO mice, ND1 mRNA as well as genomic DNA levels were elevated, while ENO2, UCP4, UCP5, and DJ-1 transcript levels were not altered. In conclusion, our data indicate a possible novel function of NCS-1 in regulating gene transcription or stabilization of mRNAs in SN DA neurons. Although we do not provide functional data, our findings at the transcript level could point to impaired ATP production (lower ND1 and ENO2) and elevated metabolic stress (lower UCP4, UCP5, and DJ-1 levels) in SN DA neurons from NCS-1 KO mice. We speculate that NCS-1 is involved in stimulating ATP synthesis, while at the same time controlling mitochondrial metabolic stress, and in this way could protect SN DA neurons from degeneration.

Cav2.3 channels contribute to dopaminergic neuron loss in a model of Parkinson's disease

Degeneration of dopaminergic neurons in the substantia nigra causes the motor symptoms of Parkinson's disease. The mechanisms underlying this age-dependent and region-selective neurodegeneration remain unclear. Here we identify Cav2.3 channels as regulators of nigral neuronal viability. Cav2.3 transcripts were more abundant than other voltage-gated Ca2+ channels in mouse nigral neurons and upregulated during aging. Plasmalemmal Cav2.3 protein was higher than in dopaminergic neurons of the ventral tegmental area, which do not degenerate in Parkinson's disease. Cav2.3 knockout reduced activity-associated nigral somatic Ca2+ signals and Ca2+-dependent after-hyperpolarizations, and afforded full protection from degeneration in vivo in a neurotoxin Parkinson's mouse model. Cav2.3 deficiency upregulated transcripts for NCS-1, a Ca2+-binding protein implicated in neuroprotection. Conversely, NCS-1 knockout exacerbated nigral neurodegeneration and downregulated Cav2.3. Moreover, NCS-1 levels were reduced in a human iPSC-model of familial Parkinson's. Thus, Cav2.3 and NCS-1 may constitute potential therapeutic targets for combatting Ca2+-dependent neurodegeneration in Parkinson's disease.

NCS-1 Deficiency Is Associated With Obesity and Diabetes Type 2 in Mice

Neuronal calcium sensor-1 (NCS-1) knockout (KO) in mice (NCS-1^−/− mice) evokes behavioral phenotypes ranging from learning deficits to avolition and depressive-like behaviors. Here, we showed that with the onset of adulthood NCS-1^−/− mice gain considerable weight. Adult NCS-1^−/− mice are obese, especially when fed a high-fat diet (HFD), are hyperglycemic and hyperinsulinemic and thus develop a diabetes type 2 phenotype. In comparison to wild type (WT) NCS-1^−/− mice display a significant increase in adipose tissue mass. NCS-1^−/− adipocytes produce insufficient serum concentrations of resistin and adiponectin. In contrast to WT littermates, adipocytes of NCS-1^−/− mice are incapable of up-regulating insulin receptor (IR) concentration in response to HFD. Thus, HFD-fed NCS-1^−/− mice exhibit in comparison to WT littermates a significantly reduced IR expression, which may explain the pronounced insulin resistance observed especially with HFD-fed NCS-1^−/− mice. We observed a direct correlation between NCS-1 and IR concentrations in the adipocyte membrane and that NCS-1 can be co-immunoprecipitated with IR indicating a direct interplay between NCS-1 and IR. We propose that NCS-1 plays an important role in adipocyte function and that NCS-1 deficiency gives rise to obesity and diabetes type 2 in adult mice. Given the association of altered NCS-1 expression with behaviorial abnormalities, NCS-1^−/− mice may offer an interesting perspective for studying in a mouse model a potential genetic link between some psychiatric disorders and the risk of being obese.

Probing Conformational Changes during the Gating Cycle of a Potassium Channel in Lipid Bilayers

Ion conduction across the cellular membrane requires the simultaneous opening of activation and inactivation gates of the K⁺ channel pore. The bacterial KcsA channel has served as a powerful system for dissecting the structural changes that are related to four major functional states associated with K⁺ gating. Yet, the direct observation of the full gating cycle of KcsA has remained structurally elusive, and crystal structures mimicking these gating events require mutations in or stabilization of functionally relevant channel segments. Here, we found that changes in lipid composition strongly increased the KcsA open probability. This enabled us to probe all four major gating states in native-like membranes by combining electrophysiological and solid-state NMR experiments. In contrast to previous crystallographic views, we found that the selectivity filter and turret region, coupled to the surrounding bilayer, were actively involved in channel gating. The increase in overall steady-state open probability was accompanied by a reduction in activation-gate opening, underscoring the important role of the surrounding lipid bilayer in the delicate conformational coupling of the inactivation and activation gates.

Structural basis of a Kv7.1 potassium channel gating module: studies of the intracellular c-terminal domain in complex with calmodulin

Kv7 channels tune neuronal and cardiomyocyte excitability. In addition to the channel membrane domain, they also have a unique intracellular C-terminal (CT) domain, bound constitutively to calmodulin (CaM). This CT domain regulates gating and tetramerization. We investigated the structure of the membrane proximal CT module in complex with CaM by X-ray crystallography. The results show how the CaM intimately hugs a two-helical bundle, explaining many channelopathic mutations. Structure-based mutagenesis of this module in the context of concatemeric tetramer channels and functional analysis along with in vitro data lead us to propose that one CaM binds to one individual protomer, without crosslinking subunits and that this configuration is required for proper channel expression and function. Molecular modeling of the CT/CaM complex in conjunction with small-angle X-ray scattering suggests that the membrane proximal region, having a rigid lever arm, is a critical gating regulator.

The Low-Threshold Calcium Channel Cav3.2 Determines Low-Threshold Mechanoreceptor Function

The T-type calcium channel Cav3.2 emerges as a key regulator of sensory functions, but its expression pattern within primary afferent neurons and its contribution to modality-specific signaling remain obscure. Here, we elucidate this issue using a unique knockin/flox mouse strain wherein Cav3.2 is replaced by a functional Cav3.2-surface-ecliptic GFP fusion. We demonstrate that Cav3.2 is a selective marker of two major low-threshold mechanoreceptors (LTMRs), Aδ- and C-LTMRs, innervating the most abundant skin hair follicles. The presence of Cav3.2 along LTMR-fiber trajectories is consistent with critical roles at multiple sites, setting their strong excitability. Strikingly, the C-LTMR-specific knockout uncovers that Cav3.2 regulates light-touch perception and noxious mechanical cold and chemical sensations and is essential to build up that debilitates allodynic symptoms of neuropathic pain, a mechanism thought to be entirely A-LTMR specific. Collectively, our findings support a fundamental role for Cav3.2 in touch/pain pathophysiology, validating their critic pharmacological relevance to relieve mechanical and cold allodynia.

Tetraphenylporphyrin derivative specifically blocks members of the voltage-gated potassium channel subfamily Kv1

Tetraphenylporphyrin derivatives represent a promising class of high-affinity ligands for voltage-gated potassium (Kv) channels. Herein, we investigated the mode of Kv channel block of one tetraphenylporphyrin derivative, por3, using electrophysiological methods, structure-based mutagenesis, and solid-state NMR spectroscopy. The combined data showed that por3 specifically blocks Kv1.x channels. Unexpectedly, 2 different por3 binding modes lead to Kv1.x channel block exerted through multiple por3 binding sites: first, por3 interacts in a highly cooperative and specific manner with the voltage sensor domain stabilizing closed Kv1 channel state(s). Therefore, stronger depolarization is needed to activate Kv1.x channels in the presence of por3. Second, por3 bind to a single site at the external pore entrance to block the ion conduction pathway of activated Kv1.x channels. This block is voltage-independent. Por3 appears to have equal affinities for voltage-sensor and pore. However, at negative voltage and low por3 concentration, por3 gating modifier properties prevail due to the high cooperativity of binding. By contrast, at positive voltages, when Kv1.x channels are fully activated, por3 pore blocking properties predominate.

NMR-based structural biology enhanced by dynamic nuclear polarization at high magnetic field

Dynamic nuclear polarization (DNP) has become a powerful method to enhance spectroscopic sensitivity in the context of magnetic resonance imaging and nuclear magnetic resonance spectroscopy. We show that, compared to DNP at lower field (400 MHz/263 GHz), high field DNP (800 MHz/527 GHz) can significantly enhance spectral resolution and allows exploitation of the paramagnetic relaxation properties of DNP polarizing agents as direct structural probes under magic angle spinning conditions. Applied to a membrane-embedded K(+) channel, this approach allowed us to refine the membrane-embedded channel structure and revealed conformational substates that are present during two different stages of the channel gating cycle. High-field DNP thus offers atomic insight into the role of molecular plasticity during the course of biomolecular function in a complex cellular environment.

A truncation variant of the cation channel P2RX5 is upregulated during T cell activation

Members of the P2X family of ligand-gated cation channels (P2RX) are expressed by various cell types including neurons, smooth- and cardiac muscle cells, and leukocytes. The channels mediate signalling in response to extracellular ATP. Seven subunit isoforms (P2RX1-P2RX7) have been identified and these can assemble as homo- and heterotrimeric molecules. In humans, P2RX5 exists as a natural deletion mutant lacking amino acids 328–349 of exon 10, which are part of transmembrane (TM) 2 and pre-TM2 regions in other organisms like rat, chicken and zebrafish. We show that P2RX5 gene expression of human T lymphocytes is upregulated during activation. P2RX5 is recruited to the cell surface. P2RX5-siRNA-transfected CD4⁺ T cells produced twofold more IL-10 than controls. Surface and intracellular P2RX5 expression was upregulated in activated antigen-specific CD4⁺ T cell clones. These data indicate a functional role of the human P2RX5 splice variant in T cell activation and immunoregulation.

Long QT mutations at the interface between KCNQ1 helix C and KCNE1 disrupt I(KS) regulation by PKA and PIP₂

KCNQ1 and KCNE1 co-assembly generates the I(KS) K(+) current, which is crucial to the cardiac action potential repolarization. Mutations in their corresponding genes cause long QT syndrome (LQT) and atrial fibrillation. The A-kinase anchor protein, yotiao (also known as AKAP9), brings the I(KS) channel complex together with signaling proteins to achieve regulation upon β1-adrenergic stimulation. Recently, we have shown that KCNQ1 helix C interacts with the KCNE1 distal C-terminus. We postulated that this interface is crucial for I(KS) channel modulation. Here, we examined the yet unknown molecular mechanisms of LQT mutations located at this intracellular intersubunit interface. All LQT mutations disrupted the internal KCNQ1-KCNE1 intersubunit interaction. LQT mutants in KCNQ1 helix C led to a decreased current density and a depolarizing shift of channel activation, mainly arising from impaired phosphatidylinositol-4,5-bisphosphate (PIP2) modulation. In the KCNE1 distal C-terminus, the LQT mutation P127T suppressed yotiao-dependent cAMP-mediated upregulation of the I(KS) current, which was caused by reduced KCNQ1 phosphorylation at S27. Thus, KCNQ1 helix C is important for channel modulation by PIP2, whereas the KCNE1 distal C-terminus appears essential for the regulation of IKS by yotiao-mediated PKA phosphorylation.

Smooth muscle BK channel activity influences blood pressure independent of vascular tone in mice

The large conductance voltage- and Ca(2+)-activated K(+) (BK) channel is an important determinant of vascular tone and contributes to blood pressure regulation. Both activities depend on the ancillary BKβ1 subunit. To determine the significance of smooth muscle BK channel activity for blood pressure regulation, we investigated the potential link between changes in arterial tone and altered blood pressure in BKβ1 knockout (BKβ1(-/-)) mice from three different genetically defined strains. While vascular tone was consistently increased in all BKβ1(-/-) mice independent of genetic background, BKβ1(-/-) strains exhibited increased (strain A), unaltered (strain B) or decreased (strain C) mean arterial blood pressures compared to their corresponding BKβ1(+/+) controls. In agreement with previous data on aldosterone regulation by renal/adrenal BK channel function, BKβ1(-/-) strain A mice have increased plasma aldosterone and increased blood pressure. Consistently, blockade of mineralocorticoid receptors by spironolactone treatment reversibly restored the elevated blood pressure to the BKβ1(+/+) strain A level. In contrast, loss of BKβ1 did not affect plasma aldosterone in strain C mice. Smooth muscle-restricted restoration of BKβ1 expression increased blood pressure in BKβ1(-/-) strain C mice, implying that impaired smooth muscle BK channel activity lowers blood pressure in these animals. We conclude that BK channel activity directly affects vascular tone but influences blood pressure independent of this effect via different pathways.

TRPM4 channels in the cardiovascular system

The non-selective Transient Receptor Potential Melastatin 4 (TRPM4) cation channel is abundantly expressed in cardiac cells, being involved in several aspects of cardiac rhythmicity, including cardiac conduction, pace making and action-potential repolarization. Dominantly inherited mutations in the TRPM4 gene are associated with the cardiac bundle-branch disorder progressive familial heart block type I (PFHBI) and isolated cardiac conduction disease (ICCD) giving rise to atrio-ventricular conduction block (AVB), right bundle branch block, bradycardia, and the Brugada syndrome. The mutant phenotypes closely resemble those associated with mutations in the SCN5A gene, encoding the voltage-gated Na(+) channel NaV1.5. These observations and the unexpected partnership with sulfonylurea-receptors (SURs) makes the TRPM4 channel a promising novel target for treatment of cardiac disorders.

Structural determinants of specific lipid binding to potassium channels

We have investigated specific lipid binding to the pore domain of potassium channels KcsA and chimeric KcsA-Kv1.3 on the structural and functional level using extensive coarse-grained and atomistic molecular dynamics simulations, solid-state NMR, and single channel measurements. We show that, while KcsA activity is critically modulated by the specific and cooperative binding of anionic nonannular lipids close to the channel's selectivity filter, the influence of nonannular lipid binding on KcsA-Kv1.3 is much reduced. The diminished impact of specific lipid binding on KcsA-Kv1.3 results from a point-mutation at the corresponding nonannular lipid binding site leading to a salt-bridge between adjacent KcsA-Kv1.3 subunits, which is conserved in many voltage-gated potassium channels and prevents strong nonannular lipid binding to the pore domain. Our findings elucidate how protein-lipid and protein-protein interactions modulate K(+) channel activity. The combination of MD, NMR, and functional studies as shown here may help to dissect the structural and dynamical processes that are critical for the functioning of larger membrane proteins, including Kv channels in a membrane setting.

TRPM4 cation channel mediates axonal and neuronal degeneration in experimental autoimmune encephalomyelitis and multiple sclerosis

In multiple sclerosis, an inflammatory disease of the central nervous system (CNS), axonal and neuronal loss are major causes for irreversible neurological disability. However, which molecules contribute to axonal and neuronal injury under inflammatory conditions remains largely unknown. Here we show that the transient receptor potential melastatin 4 (TRPM4) cation channel is crucial in this process. TRPM4 is expressed in mouse and human neuronal somata, but it is also expressed in axons in inflammatory CNS lesions in experimental autoimmune encephalomyelitis (EAE) in mice and in human multiple sclerosis tissue. Deficiency or pharmacological inhibition of TRPM4 using the antidiabetic drug glibenclamide resulted in reduced axonal and neuronal degeneration and attenuated clinical disease scores in EAE, but this occurred without altering EAE-relevant immune function. Furthermore, Trpm4(-/-) mouse neurons were protected against inflammatory effector mechanisms such as excitotoxic stress and energy deficiency in vitro. Electrophysiological recordings revealed TRPM4-dependent neuronal ion influx and oncotic cell swelling upon excitotoxic stimulation. Therefore, interference with TRPM4 could translate into a new neuroprotective treatment strategy.

Coupling of voltage-sensors to the channel pore: a comparative view

The activation of voltage-dependent ion channels is initiated by potential-induced conformational rearrangements in the voltage-sensor domains that propagates to the pore domain (PD) and finally opens the ion conduction pathway. In potassium channels voltage-sensors are covalently linked to the pore via S4-S5 linkers at the cytoplasmic site of the PD. Transformation of membrane electric energy into the mechanical work required for the opening or closing of the channel pore is achieved through an electromechanical coupling mechanism, which involves local interaction between residues in S4-S5 linker and pore-forming alpha helices. In this review we discuss present knowledge and open questions related to the electromechanical coupling mechanism in most intensively studied voltage-gated Shaker potassium channel and compare structure-functional aspects of coupling with those observed in distantly related ion channels. We focus particularly on the role of electromechanical coupling in modulation of the constitutive conductance of ion channels.

A molecular switch driving inactivation in the cardiac K+ channel HERG

K⁺ channels control transmembrane action potentials by gating open or closed in response to external stimuli. Inactivation gating, involving a conformational change at the K⁺ selectivity filter, has recently been recognized as a major K⁺ channel regulatory mechanism. In the K⁺ channel hERG, inactivation controls the length of the human cardiac action potential. Mutations impairing hERG inactivation cause life-threatening cardiac arrhythmia, which also occur as undesired side effects of drugs. In this paper, we report atomistic molecular dynamics simulations, complemented by mutational and electrophysiological studies, which suggest that the selectivity filter adopts a collapsed conformation in the inactivated state of hERG. The selectivity filter is gated by an intricate hydrogen bond network around residues S620 and N629. Mutations of this hydrogen bond network are shown to cause inactivation deficiency in electrophysiological measurements. In addition, drug-related conformational changes around the central cavity and pore helix provide a functional mechanism for newly discovered hERG activators.

The C-terminal tail of human neuronal calcium sensor 1 regulates the conformational stability of the Ca²⁺₋ activated state

Neuronal calcium sensor 1 (NCS-1) and orthologs are expressed in all organisms from yeast to humans. In the latter, NCS-1 plays an important role in neurotransmitter release and interacts with a plethora of binding partners mostly through a large solvent-exposed hydrophobic crevice. The structural basis behind the multispecific binding profile is not understood. To begin to address this, we applied NMR spectroscopy to determine the solution structure of calcium-bound human NCS-1. The structure in solution demonstrates interdomain flexibility and, in the absence of a binding partner, the C-terminal tail residues occupy the hydrophobic crevice as a ligand mimic. A variant with a C-terminal tail deletion shows lack of a defined structure but maintained cooperative unfolding and dramatically reduced global stability. The results suggest that the C-terminal tail is important for regulating the conformational stability of the Ca(2+)-activated state. Furthermore, a single amino acid mutation that was recently diagnosed in a patient with autistic spectrum disorder was seen to affect the C-terminal tail and binding crevice in NCS-1.

Biophysical Properties of Kv3.1 Channels in SH-SY5Y Human Neuroblastoma Cells

Biophysical properties of delayed rectifier K channels in the human neuroblastoma SH-SY5Y were established using patch clamp recordings. The whole cell K+ conductance activated at membrane potentials positive to -20 mV. The midpoint of current activation was 9.6 +/- 5.1 mV, the equivalent charge was 3.7 +/-.6. Whole-cell currents inactivated slightly with time constants of 700 ms and 5 s. The K+ currents were sensitive to micromolar concentrations of TEA and 4-aminopyridine. RT-PCR experiments amplified a cDNA fragment specific for human Kv3.1 channels. Activation gating parameters in outside-out patches were shifted by approximately 14 mV in the hyperpolarizing direction.

Gain-of-Function Mutations in TRPM4 Cause Autosomal Dominant Isolated Cardiac Conduction Disease

Background—

Isolated cardiac conduction block is a relatively common condition in young and elderly populations. Genetic predisposing factors have long been suspected because of numerous familial case reports. Deciphering genetic predisposing factors of conduction blocks may give a hint at stratifying conduction block carriers in a more efficient way.

Methods and Results—

One Lebanese family and 2 French families with autosomal dominant isolated cardiac conduction blocks were used for linkage analysis. A maximum combined multipoint lod score of 10.5 was obtained on a genomic interval including more than 300 genes. After screening 12 genes of this interval for mutation, we found a heterozygous missense mutation of the TRPM4 gene in each family (p.Arg164Trp, p.Ala432Thr, and p.Gly844Asp). This gene encodes the TRPM4 channel, a calcium-activated nonselective cation channel of the transient receptor potential melastatin (TRPM) ion channel family. All 3 mutations result in an increased current density. This gain of function is due to an elevated TRPM4 channel density at the cell surface secondary to impaired endocytosis and deregulation of Small Ubiquitin MOdifier conjugation (SUMOylation). Furthermore, we showed by immunohistochemistry that TRPM4 channel signal level is higher in atrial cardiomyocytes than in common ventricular cells, but is highest in Purkinje fibers. Small bundles of highly TRPM4-positive cells were found in the subendocardium and in rare intramural bundles.

Conclusions—

the TRPM4 gene is a causative gene in isolated cardiac conduction disease with mutations resulting in a gain of function and TRPM4 channel being highly expressed in cardiac Purkinje fibers.

Identification and characterization of the ecdysterone receptor in Drosophila melanogaster by photoaffinity labeling

Salivary glands of third-instar larvae of Drosophila melanogaster as well as Drosophila K(c) tissue culture cells have been irradiated in the presence of ecdysterone. Irradiation covalently links ecdysterone to a single cellular protein, which is similar, if not identical, in salivary glands and in K(c) cells. This protein has a molecular weight of 130,000 and it has the characteristics of a typical hormone-receptor molecule in terms of hormone-binding properties, translocation into the nucleus, and sedimentation characteristics. The yield of the photoinduced bonding of ecdysterone to receptor protein is around 15%. Ponasterone A competed with ecdysterone for the bonding. Also, ponasterone A itself reacted upon photoactivation with the beta-ecdysterone receptor protein in Drosophila tissue culture cells. We have previously shown that ecdysterone can be bonded upon irradiation to specific hormone-controlled puffs of polytene chromosomes of D. melanogaster third-instar larvae [Gronemeyer, H. & Pongs, O. (1980) Proc. Natl. Acad. Sci. USA 77, 2108-2112]. Because we have now identified the molecular target of the ecdysterone photoreaction, these data show that a hormone-receptor complex translocates to the nucleus and directly binds to the genes, which are under hormonal control. A quantitative assay of hormone-receptor complex in K(c) cells before and after hormone stimulation showed that ecdysterone does not regulate the synthesis and the available amount of its receptor. It was also observed that the translocated hormone-receptor complex resides in the nucleus as long as the hormone is present in the tissue culture medium.

Ancillary subunits associated with voltage-dependent K+ channels

Since the first discovery of Kvbeta-subunits more than 15 years ago, many more ancillary Kv channel subunits were characterized, for example, KChIPs, KCNEs, and BKbeta-subunits. The ancillary subunits are often integral parts of native Kv channels, which, therefore, are mostly multiprotein complexes composed of voltage-sensing and pore-forming Kvalpha-subunits and of ancillary or beta-subunits. Apparently, Kv channels need the ancillary subunits to fulfill their many different cell physiological roles. This is reflected by the large structural diversity observed with ancillary subunit structures. They range from proteins with transmembrane segments and extracellular domains to purely cytoplasmic proteins. Ancillary subunits modulate Kv channel gating but can also have a great impact on channel assembly, on channel trafficking to and from the cellular surface, and on targeting Kv channels to different cellular compartments. The importance of the role of accessory subunits is further emphasized by the number of mutations that are associated in both humans and animals with diseases like hypertension, epilepsy, arrhythmogenesis, periodic paralysis, and hypothyroidism. Interestingly, several ancillary subunits have in vitro enzymatic activity; for example, Kvbeta-subunits are oxidoreductases, or modulate enzymatic activity, i.e., KChIP3 modulates presenilin activity. Thus different modes of beta-subunit association and of functional impact on Kv channels can be delineated, making it difficult to extract common principles underlying Kvalpha- and beta-subunit interactions. We critically review present knowledge on the physiological role of ancillary Kv channel subunits and their effects on Kv channel properties.

Novel insights into the mechanisms mediating the local antihypertrophic effects of cardiac atrial natriuretic peptide: role of cGMP-dependent protein kinase and RGS2

Cardiac atrial natriuretic peptide (ANP) locally counteracts cardiac hypertrophy via the guanylyl cyclase-A (GC-A) receptor and cGMP production, but the downstream signalling pathways are unknown. Here, we examined the influence of ANP on β-adrenergic versus Angiotensin II (Ang II)-dependent (G_s vs. G_αq mediated) modulation of Ca²⁺_i-handling in cardiomyocytes and of hypertrophy in intact hearts. L-type Ca²⁺ currents and Ca²⁺_i transients in adult isolated murine ventricular myocytes were studied by voltage-clamp recordings and fluorescence microscopy. ANP suppressed Ang II-stimulated Ca²⁺ currents and transients, but had no effect on isoproterenol stimulation. Ang II suppression by ANP was abolished in cardiomyocytes of mice deficient in GC-A, in cyclic GMP-dependent protein kinase I (PKG I) or in the regulator of G protein signalling (RGS) 2, a target of PKG I. Cardiac hypertrophy in response to exogenous Ang II was significantly exacerbated in mice with conditional, cardiomyocyte-restricted GC-A deletion (CM GC-A KO). This was concomitant to increased activation of the Ca²⁺/calmodulin-dependent prohypertrophic signal transducer CaMKII. In contrast, β-adrenoreceptor-induced hypertrophy was not enhanced in CM GC-A KO mice. Lastly, while the stimulatory effects of Ang II on Ca²⁺-handling were absent in myocytes of mice deficient in TRPC3/TRPC6, the effects of isoproterenol were unchanged. Our data demonstrate a direct myocardial role for ANP/GC-A/cGMP to antagonize the Ca²⁺_i-dependent hypertrophic growth response to Ang II, but not to β-adrenergic stimulation. The selectivity of this interaction is determined by PKG I and RGS2-dependent modulation of Ang II/AT₁ signalling. Furthermore, they strengthen published observations in neonatal cardiomyocytes showing that TRPC3/TRPC6 channels are essential for Ang II, but not for β-adrenergic Ca²⁺_i-stimulation in adult myocytes.

Impaired endocytosis of the ion channel TRPM4 is associated with human progressive familial heart block type I

Progressive familial heart block type I (PFHBI) is a progressive cardiac bundle branch disease in the His-Purkinje system that exhibits autosomal-dominant inheritance. In 3 branches of a large South African Afrikaner pedigree with an autosomal-dominant form of PFHBI, we identified the mutation c.19G-->A in the transient receptor potential cation channel, subfamily M, member 4 gene (TRPM4) at chromosomal locus 19q13.3. This mutation predicted the amino acid substitution p.E7K in the TRPM4 amino terminus. TRPM4 encodes a Ca2+-activated nonselective cation (CAN) channel that belongs to the transient receptor potential melastatin ion channel family. Quantitative analysis of TRPM4 mRNA content in human cardiac tissue showed the highest expression level in Purkinje fibers. Cellular expression studies showed that the c.19G-->A missense mutation attenuated deSUMOylation of the TRPM4 channel. The resulting constitutive SUMOylation of the mutant TRPM4 channel impaired endocytosis and led to elevated TRPM4 channel density at the cell surface. Our data therefore revealed a gain-of-function mechanism underlying this type of familial heart block.

Coupling of activation and inactivation gate in a K+-channel: potassium and ligand sensitivity

Potassium (K(+))-channel gating is choreographed by a complex interplay between external stimuli, K(+) concentration and lipidic environment. We combined solid-state NMR and electrophysiological experiments on a chimeric KcsA-Kv1.3 channel to delineate K(+), pH and blocker effects on channel structure and function in a membrane setting. Our data show that pH-induced activation is correlated with protonation of glutamate residues at or near the activation gate. Moreover, K(+) and channel blockers distinctly affect the open probability of both the inactivation gate comprising the selectivity filter of the channel and the activation gate. The results indicate that the two gates are coupled and that effects of the permeant K(+) ion on the inactivation gate modulate activation-gate opening. Our data suggest a mechanism for controlling coordinated and sequential opening and closing of activation and inactivation gates in the K(+)-channel pore.

Protein dynamics detected in a membrane-embedded potassium channel using two-dimensional solid-state NMR spectroscopy

We report longitudinal (15)N relaxation rates derived from two-dimensional ((15)N, (13)C) chemical shift correlation experiments obtained under magic angle spinning for the potassium channel KcsA-Kv1.3 reconstituted in multilamellar vesicles. Thus, we demonstrate that solid-state NMR can be used to probe residue-specific backbone dynamics in a membrane-embedded protein. Enhanced backbone mobility was detected for two glycine residues within the selectivity filter that are highly conserved in potassium channels and that are of core relevance to the filter structure and ion selectivity.

Intracellular domains interactions and gated motions of I(KS) potassium channel subunits

Voltage-gated K(+) channels co-assemble with auxiliary beta subunits to form macromolecular complexes. In heart, assembly of Kv7.1 pore-forming subunits with KCNE1 beta subunits generates the repolarizing K(+) current I(KS). However, the detailed nature of their interface remains unknown. Mutations in either Kv7.1 or KCNE1 produce the life-threatening long or short QT syndromes. Here, we studied the interactions and voltage-dependent motions of I(KS) channel intracellular domains, using fluorescence resonance energy transfer combined with voltage-clamp recording and in vitro binding of purified proteins. The results indicate that the KCNE1 distal C-terminus interacts with the coiled-coil helix C of the Kv7.1 tetramerization domain. This association is important for I(KS) channel assembly rules as underscored by Kv7.1 current inhibition produced by a dominant-negative C-terminal domain. On channel opening, the C-termini of Kv7.1 and KCNE1 come close together. Co-expression of Kv7.1 with the KCNE1 long QT mutant D76N abolished the K(+) currents and gated motions. Thus, during channel gating KCNE1 is not static. Instead, the C-termini of both subunits experience molecular motions, which are disrupted by the D76N causing disease mutation.

Ins and outs of cardiac voltage-gated potassium channels

Voltage-gated potassium (Kv) channels play an important role in regulating cardiac muscle excitability by controlling action potential duration and frequency. Essential for this activity is proper localization and organization of the cardiac Kv channels in specific microdomains of the plasma membrane. The underlying processes involve tight control of anterograde and retrograde Kv channel trafficking into and out of the plasma membrane. Thus, cardiac Kv channel density at the cell surface is regulated by a dynamic interplay of endocytotic and recycling pathways, the mechanisms of which are mostly unknown. Recent studies have indicated that the lipid composition in the Kv channel surround profoundly influences these processes. Local differences in lipid composition altering the mechanic state of the lipid bilayer or a specific interaction with an important domain of the Kv channel markedly affect voltage-sensitive gating, clustering, and mobility of cardiac Kv channels and, thereby, the excitability in the healthy and diseased heart muscles.

The molecular mechanism of toxin-induced conformational changes in a potassium channel: relation to C-type inactivation

Recently, a solid-state NMR study revealed that scorpion toxin binding leads to conformational changes in the selectivity filter of potassium channels. The exact nature of the conformational changes, however, remained elusive. We carried out all-atom molecular dynamics simulations that enabled us to cover the complete pathway of toxin approach and binding, and we validated our simulation results by using solid-state NMR data and electrophysiological measurements. Our structural model revealed a mechanism of cooperative toxin-induced conformational changes that accounts both for the signal changes observed in solid-state NMR and for the tight interaction between KcsA-Kv1.3 and Kaliotoxin. We show that this mechanism is structurally and functionally closely related to recovery from C-type inactivation. Furthermore, our simulations indicate heterogeneity in the binding modes of Kaliotoxin, which might serve to enhance its affinity for KcsA-Kv1.3 further by entropic stabilization.

High-resolution 3D structure determination of kaliotoxin by solid-state NMR spectroscopy

High-resolution solid-state NMR spectroscopy can provide structural information of proteins that cannot be studied by X-ray crystallography or solution NMR spectroscopy. Here we demonstrate that it is possible to determine a protein structure by solid-state NMR to a resolution comparable to that by solution NMR. Using an iterative assignment and structure calculation protocol, a large number of distance restraints was extracted from ¹H/¹H mixing experiments recorded on a single uniformly labeled sample under magic angle spinning conditions. The calculated structure has a coordinate precision of 0.6 Å and 1.3 Å for the backbone and side chain heavy atoms, respectively, and deviates from the structure observed in solution. The approach is expected to be applicable to larger systems enabling the determination of high-resolution structures of amyloid or membrane proteins.

Regulation of excitability by potassium channels

Neurons express a large number of different voltage-gated potassium (Kv) channels with distinct biophysical and biochemical properties. Possibly, this diversity reflects the need to regulate and fine-tune neuronal excitability at various levels of complexity in space and time. In this context, Kv channels operating in the subthreshold range of action- potential firing are of particular interest. It is likely that these Kv channels play a prominent role in both propagating and integrating dendritic signaling, as well as axonal action-potential firing and propagation.

Two-dimensional solid-state NMR applied to a chimeric potassium channel

Solid-state NMR (ssNMR) represents a spectroscopic method to study membrane protein structure and dynamics in lipid bilayers. We present two-dimensional correlation experiments conducted on a fully [13C,15N] labeled version of a chimeric potassium (KcsA-Kv1.3) channel. Data obtained by using two different ion concentrations suggest a structural conservation of the selectivity filter region. SsNMR experiments conducted at two different temperatures point to differential molecular dynamics of the channel.

Toxin binding to chimeric K+ channels immobilised on a solid nitrocellulose support

In this work, we used a panel prokaryote/eukaryote K+ channel chimeras to generate K+ channel arrays. Their behaviour in solution was compared with that when spotted on a nitrocellulose-supported film and their responses to selective high affinity ligands, including polypeptide toxins and TEA, were studied.

C-terminal HERG (LQT2) mutations disrupt IKr channel regulation through 14-3-3ϵ

Beta-adrenergic receptor-mediated cAMP or protein kinase A (PKA)-dependent modulation of cardiac potassium currents controls ventricular action potential duration (APD) at faster heart rates. HERG (KCNH2) gene mutations are associated with congenital long-QT syndrome (LQT2) and affect IKr activity, a key determinant in ventricular repolarization. Physical activity or emotional stress often triggers lethal arrhythmias in LQT2 patients. Beta-adrenergic stimulation of HERG channel activity is amplified and prolonged in vitro by the adaptor protein 14-3-3epsilon. In LQT2 families, we identified three novel heterozygous HERG mutations (G965X, R1014PfsX39, V1038AfsX21) in the C-terminus that led to protein truncation and loss of a PKA phosphorylation site required for binding of 14-3-3epsilon. When expressed in CHO cells, the mutants produced functional HERG channels with normal kinetic properties. We now provide evidence that HERG channel regulation by 14-3-3epsilon is of physiological significance in humans. Upon co-expression with 14-3-3epsilon, mutant channels still bound 14-3-3epsilon but did not respond with a hyperpolarizing shift in voltage dependence as seen in wild-type channels. Co-expression experiments of wild-type and mutant channels revealed dominant-negative behavior of all three HERG mutations. Simulations of the effects of sympathetic stimulation of HERG channel activity on the whole-cell action potential suggested a role in rate-dependent control of APD and an impaired ability of mutant cardiac myocytes to respond to a triggered event or an ectopic beat. In summary, the attenuated functional effects of 14-3-3epsilon on C-terminally truncated HERG channels demonstrate the physiological importance of coupling beta-adrenergic stimulation and HERG channel activity.

NADPH oxidase accounts for enhanced superoxide production and impaired endothelium-dependent smooth muscle relaxation in BKbeta1-/- mice

OBJECTIVE

Nitric oxide (NO)-induced vasorelaxation involves activation of large conductance Ca2+-activated K+ channels (BK). A regulatory BKbeta1 subunit confers Ca2+, voltage, and NO/cGMP sensitivity to the BK channel. We investigated whether endothelial function and NO/cGMP signaling is affected by a deletion of the beta1-subunit.

METHODS AND RESULTS

Vascular superoxide in BKbeta1-/- was measured using the fluorescent dye hydroethidine and lucigenin-enhanced chemiluminescence. Vascular NO formation was analyzed using electron paramagnetic resonance (EPR), expression of NADPH oxidase subunits, the endothelial NO synthase (eNOS), the soluble guanylyl cyclase (sGC), as well as the activity and expression of the cyclic GMP-dependent kinase I (cGK-I) were assessed by Western blotting technique. eNOS, sGC, cGK-I expression and acetylcholine-induced NO production were unaltered in Bkbeta1-/- animals, whereas endothelial function was impaired and the activity of the cGK-I was reduced. Vascular O2- and expression of the NADPH oxidase subunits p67phox and Nox1 were increased. Endothelial dysfunction was normalized by the NADPH oxidase inhibitor apocynin. Potassium chloride- and iberiotoxin-induced depolarization mimicked the effect of BKbeta1-deletion by increasing vascular O2- in an NADPH-dependent fashion.

CONCLUSIONS

The deletion of BKbeta1 causes endothelial dysfunction by increasing O2- formation via increasing activity and expression of the vascular NADPH oxidase.

Toxin-induced conformational changes in a potassium channel revealed by solid-state NMR

The active site of potassium (K+) channels catalyses the transport of K+ ions across the plasma membrane--similar to the catalytic function of the active site of an enzyme--and is inhibited by toxins from scorpion venom. On the basis of the conserved structures of K+ pore regions and scorpion toxins, detailed structures for the K+ channel-scorpion toxin binding interface have been proposed. In these models and in previous solution-state nuclear magnetic resonance (NMR) studies using detergent-solubilized membrane proteins, scorpion toxins were docked to the extracellular entrance of the K+ channel pore assuming rigid, preformed binding sites. Using high-resolution solid-state NMR spectroscopy, here we show that high-affinity binding of the scorpion toxin kaliotoxin to a chimaeric K+ channel (KcsA-Kv1.3) is associated with significant structural rearrangements in both molecules. Our approach involves a combined analysis of chemical shifts and proton-proton distances and demonstrates that solid-state NMR is a sensitive method for analysing the structure of a membrane protein-inhibitor complex. We propose that structural flexibility of the K+ channel and the toxin represents an important determinant for the high specificity of toxin-K+ channel interactions.

Calmodulin Is Essential for Cardiac I KS Channel Gating and Assembly

The slow I KS K + channel plays a major role in repolarizing the cardiac action potential and consists of the assembly of KCNQ1 and KCNE1 subunits. Mutations in either KCNQ1 or KCNE1 genes produce the long-QT syndrome, a life-threatening ventricular arrhythmia. Here, we show that long-QT mutations located in the KCNQ1 C terminus impair calmodulin (CaM) binding, which affects both channel gating and assembly. The mutations produce a voltage-dependent macroscopic inactivation and dramatically alter channel assembly. KCNE1 forms a ternary complex with wild-type KCNQ1 and Ca 2+ -CaM that prevents inactivation, facilitates channel assembly, and mediates a Ca 2+ -sensitive increase of I KS- current, with a considerable Ca 2+ -dependent left-shift of the voltage-dependence of activation. Coexpression of KCNQ1 or I KS channels with a Ca 2+ -insensitive CaM mutant markedly suppresses the currents and produces a right shift in the voltage-dependence of channel activation. KCNE1 association to KCNQ1 long-QT mutants significantly improves mutant channel expression and prevents macroscopic inactivation. However, the marked right shift in channel activation and the subsequent decrease in current amplitude cannot restore normal levels of I KS channel activity. Our data indicate that in healthy individuals, CaM binding to KCNQ1 is essential for correct channel folding and assembly and for conferring Ca 2+ -sensitive I KS -current stimulation, which increases the cardiac repolarization reserve and hence prevents the risk of ventricular arrhythmias.

Contribution of N- and C-terminal Kv4.2 channel domains to KChIP interaction [corrected]

Association of Shal gene-related voltage-gated potassium (Kv4) channels with cytoplasmic Kv channel interacting proteins (KChIPs) influences inactivation gating and surface expression. We investigated both functional and biochemical consequences of mutations in cytoplasmic N and C-terminal Kv4.2 domains to characterize structural determinants for KChIP interaction. We performed a lysine-scanning mutagenesis within the proximal 40 amino acid portion and a structure-based mutagenesis in the tetramerization 1 (T1) domain of Kv4.2. In addition, the cytoplasmic Kv4.2 C-terminus was truncated at various positions. Wild-type and mutant Kv4.2 channels were coexpressed with KChIP2 isoforms in mammalian cell lines. The KChIP2-induced modulation of Kv4.2 currents was studied with whole-cell patch clamp and the binding of KChIP2 isoforms to Kv4.2 channels with coimmunoprecipitation experiments. Our results define one major interaction site for KChIPs, including amino acids in the proximal N-terminus between residues 11 and 23, where binding and functional modulation are essentially equivalent. A further interaction site includes residues in the T1 domain. Notably, C-terminal deletions also had marked effects on KChIP2-dependent gating modulation and KChIP2 binding, revealing a previously unknown involvement of domains within the cytoplasmic Kv4.2 C-terminus in KChIP interaction. Less coincidence of binding and functional modulation indicates a more loose 'anchoring' at T1- and C-terminal interaction sites. Our results refine and extend previously proposed structural models for Kv4.2/KChIP complex formation.