Meeting Youth Where They Live: How to Use a QR Code in Safety Planning

When in extreme distress, it is difficult to remember the coping strategies and resources available to you. The purpose of a safety plan is to make it easier for individuals to make wise choices in moments of crisis. By virtue of this, we should strive to have safety plans as easy and convenient to use as possible. The Stanley-Brown safety plan is a template for this style of intervention and has been adapted and adopted in numerous institutions.1 A pillar of this intervention is ready-at-hand contact information for mental health agencies and crisis resources. Although the classic paper safety plan remains an invaluable tool, integration of digital resources may be necessary to meet our young patients "where they live."2 Generation Z are "digital natives" with great fluency and comfort with smart devices. Expansion of the audio-only National Suicide Prevention Lifeline to the audio, SMS, and Web-based instant messaging capabilities of the 988 Crisis & Suicide Lifeline recognizes this shift.3 At the University of Michigan, we developed a quick response (QR) code to save these resources directly onto youths' smartphones.

Block of Voltage-Gated Sodium Channels by Aripiprazole in a State-Dependent Manner

Aripiprazole is an atypical antipsychotic drug, which is prescribed for many psychiatric diseases such as schizophrenia and mania in bipolar disorder. It primarily acts as an agonist of dopaminergic and other G-protein coupled receptors. So far, an interaction with ligand- or voltage-gated ion channels has been classified as weak. Meanwhile, we identified aripiprazole in a preliminary test as a potent blocker of voltage-gated sodium channels. Here, we present a detailed analysis about the interaction of aripiprazole with the dominant voltage-gated sodium channel of heart muscle (hNa_v1.5). Electrophysiological experiments were performed by means of the patch clamp technique at human heart muscle sodium channels (hNa_v1.5), heterologously expressed in human TsA cells. Aripiprazole inhibits the hNa_v1.5 channel in a state- but not use-dependent manner. The affinity for the resting state is weak with an extrapolated K_r of about 55 µM. By contrast, the interaction with the inactivated state is strong. The affinities for the fast and slow inactivated state are in the low micromolar range (0.5–1 µM). Kinetic studies indicate that block development for the inactivated state must be described with a fast (ms) and a slow (s) time constant. Even though the time constants differ by a factor of about 50, the resulting affinity constants were nearly identical (in the range of 0.5 µM). Besides this, aripirazole also interacts with the open state of the channel. Using an inactivation deficit mutant, an affinity of about 1 µM was estimated. In summary, aripiprazole inhibits voltage-gated sodium channels at low micromolar concentrations. This property might add to its possible anticancer and neuroprotective properties.

Coupling the Cardiac Voltage-Gated Sodium Channel to Channelrhodopsin-2 Generates Novel Optical Switches for Action Potential Studies

Voltage-gated sodium (Na⁺) channels respond to short membrane depolarization with conformational changes leading to pore opening, Na⁺ influx, and action potential (AP) upstroke. In the present study, we coupled channelrhodopsin-2 (ChR2), the key ion channel in optogenetics, directly to the cardiac voltage-gated Na⁺ channel (Na_v1.5). Fusion constructs were expressed in Xenopus laevis oocytes, and electrophysiological recordings were performed by the two-microelectrode technique. Heteromeric channels retained both typical Na_v1.5 kinetics and light-sensitive ChR2 properties. Switching to the current-clamp mode and applying short blue-light pulses resulted either in subthreshold depolarization or in a rapid change of membrane polarity typically seen in APs of excitable cells. To study the effect of individual K⁺ channels on the AP shape, we co-expressed either K_v1.2 or hERG with one of the Na_v1.5-ChR2 fusions. As expected, both delayed rectifier K⁺ channels shortened AP duration significantly. K_v1.2 currents remarkably accelerated initial repolarization, whereas hERG channel activity efficiently restored the resting membrane potential. Finally, we investigated the effect of the LQT3 deletion mutant ΔKPQ on the AP shape and noticed an extremely prolonged AP duration that was directly correlated to the size of the non-inactivating Na⁺ current fraction. In conclusion, coupling of ChR2 to a voltage-gated Na⁺ channel generates optical switches that are useful for studying the effect of individual ion channels on the AP shape. Moreover, our novel optogenetic approach provides the potential for an application in pharmacology and optogenetic tissue-engineering.

Uncoupling of Voltage- and Ligand-Induced Activation in HCN2 Channels by Glycine Inserts

Hyperpolarization-activated cyclic nucleotide-modulated (HCN) channels are tetramers that generate electrical rhythmicity in special brain neurons and cardiomyocytes. The channels are activated by membrane hyperpolarization. The binding of cAMP to the four available cyclic nucleotide-binding domains (CNBD) enhances channel activation. We analyzed in the present study the mechanism of how the effect of cAMP binding is transmitted to the pore domain. Our strategy was to uncouple the C-linker (CL) from the channel core by inserting one to five glycine residues between the S6 gate and the A′-helix (constructs 1G to 5G). We quantified in full-length HCN2 channels the resulting functional effects of the inserted glycines by current activation as well as the structural dynamics and statics using molecular dynamics simulations and Constraint Network Analysis. We show functionally that already in 1G the cAMP effect on activation is lost and that with the exception of 3G and 5G the concentration-activation relationships are shifted to depolarized voltages with respect to HCN2. The strongest effect was found for 4G. Accordingly, the activation kinetics were accelerated by all constructs, again with the strongest effect in 4G. The simulations reveal that the average residue mobility of the CL and CNBD domains is increased in all constructs and that the junction between the S6 and A′-helix is turned into a flexible hinge, resulting in a destabilized gate in all constructs. Moreover, for 3G and 4G, there is a stronger downward displacement of the CL-CNBD than in HCN2 and the other constructs, resulting in an increased kink angle between S6 and A′-helix, which in turn loosens contacts between the S4-helix and the CL. This is suggested to promote a downward movement of the S4-helix, similar to the effect of hyperpolarization. In addition, exclusively in 4G, the selectivity filter in the upper pore region and parts of the S4-helix are destabilized. The results provide new insights into the intricate activation of HCN2 channels.

Block of Voltage-Gated Sodium Channels as a Potential Novel Anti-cancer Mechanism of TIC10

Background: Tumor therapeutics are aimed to affect tumor cells selectively while sparing healthy ones. For this purpose, a huge variety of different drugs are in use. Recently, also blockers of voltage-gated sodium channels (VGSCs) have been recognized to possess potentially beneficial effects in tumor therapy. As these channels are a frequent target of numerous drugs, we hypothesized that currently used tumor therapeutics might have the potential to block VGSCs in addition to their classical anti-cancer activity. In the present work, we have analyzed the imipridone TIC10, which belongs to a novel class of anti-cancer compounds, for its potency to interact with VGSCs.

Methods: Electrophysiological experiments were performed by means of the patch-clamp technique using heterologously expressed human heart muscle sodium channels (hNav1.5), which are among the most common subtypes of VGSCs occurring in tumor cells.

Results: TIC10 angular inhibited the hNa_v1.5 channel in a state- but not use-dependent manner. The affinity for the resting state was weak with an extrapolated K_r of about 600 μM. TIC10 most probably did not interact with fast inactivation. In protocols for slow inactivation, a half-maximal inhibition occurred around 2 µM. This observation was confirmed by kinetic studies indicating that the interaction occurred with a slow time constant. Furthermore, TIC10 also interacted with the open channel with an affinity of approximately 4 µM. The binding site for local anesthetics or a closely related site is suggested as a possible target as the affinity for the well-characterized F1760K mutant was reduced more than 20-fold compared to wild type. Among the analyzed derivatives, ONC212 was similarly effective as TIC10 angular, while TIC10 linear more selectively interacted with the different states.

Conclusion: The inhibition of VGSCs at low micromolar concentrations might add to the anti-tumor properties of TIC10.

Terahertz refractive index-based morphological dilation for breast carcinoma delineation

This paper reports investigations led on the combination of the refractive index and morphological dilation to enhance performances towards breast tumour margin delineation during conserving surgeries. The refractive index map of invasive ductal and lobular carcinomas were constructed from an inverse electromagnetic problem. Morphological dilation combined with refractive index thresholding was conducted to classify the tissue regions as malignant or benign. A histology routine was conducted to evaluate the performances of various dilation geometries associated with different thresholds. It was found that the combination of a wide structuring element and high refractive index was improving the correctness of tissue classification in comparison to other configurations or without dilation. The method reports a sensitivity of around 80% and a specificity of 82% for the best case. These results indicate that combining the fundamental optical properties of tissues denoted by their refractive index with morphological dilation may open routes to define supporting procedures during breast-conserving surgeries.

Block of Voltage-Gated Sodium Channels by Atomoxetine in a State- and Use-dependent Manner

Atomoxetine, a neuroactive drug, is approved for the treatment of attention-deficit/hyperactivity disorder (ADHD). It is primarily known as a high affinity blocker of the noradrenaline transporter, whereby its application leads to an increased level of the corresponding neurotransmitter in different brain regions. However, the concentrations used to obtain clinical effects are much higher than those which are required to block the transporter system. Thus, off-target effects are likely to occur. In this way, we previously identified atomoxetine as blocker of NMDA receptors. As many psychotropic drugs give rise to sudden death of cardiac origin, we now tested the hypothesis whether atomoxetine also interacts with voltage-gated sodium channels of heart muscle type in clinically relevant concentrations. Electrophysiological experiments were performed by means of the patch-clamp technique at human heart muscle sodium channels (hNav1.5) heterogeneously expressed in human embryonic kidney cells. Atomoxetine inhibited sodium channels in a state- and use-dependent manner. Atomoxetine had only a weak affinity for the resting state of the hNav1.5 (Kr: ∼ 120 µM). The efficacy of atomoxetine strongly increased with membrane depolarization, indicating that the inactivated state is an important target. A hallmark of this drug was its slow interaction. By use of different experimental settings, we concluded that the interaction occurs with the slow inactivated state as well as by slow kinetics with the fast-inactivated state. Half-maximal effective concentrations (2–3 µM) were well within the concentration range found in plasma of treated patients. Atomoxetine also interacted with the open channel. However, the interaction was not fast enough to accelerate the time constant of fast inactivation. Nevertheless, when using the inactivation-deficient hNav1.5_I408W_L409C_A410W mutant, we found that the persistent late current was blocked half maximal at about 3 µM atomoxetine. The interaction most probably occurred via the local anesthetic binding site. Atomoxetine inhibited sodium channels at a similar concentration as it is used for the treatment of ADHD. Due to its slow interaction and by inhibiting the late current, it potentially exerts antiarrhythmic properties.

Action potentials in Xenopus oocytes triggered by blue light

Voltage-gated sodium (Na⁺) channels are responsible for the fast upstroke of the action potential of excitable cells. The different α subunits of Na⁺ channels respond to brief membrane depolarizations above a threshold level by undergoing conformational changes that result in the opening of the pore and a subsequent inward flux of Na⁺. Physiologically, these initial membrane depolarizations are caused by other ion channels that are activated by a variety of stimuli such as mechanical stretch, temperature changes, and various ligands. In the present study, we developed an optogenetic approach to activate Na⁺ channels and elicit action potentials in Xenopus laevis oocytes. All recordings were performed by the two-microelectrode technique. We first coupled channelrhodopsin-2 (ChR2), a light-sensitive ion channel of the green alga Chlamydomonas reinhardtii, to the auxiliary β1 subunit of voltage-gated Na⁺ channels. The resulting fusion construct, β1-ChR2, retained the ability to modulate Na⁺ channel kinetics and generate photosensitive inward currents. Stimulation of Xenopus oocytes coexpressing the skeletal muscle Na⁺ channel Na_v1.4 and β1-ChR2 with 25-ms lasting blue-light pulses resulted in rapid alterations of the membrane potential strongly resembling typical action potentials of excitable cells. Blocking Na_v1.4 with tetrodotoxin prevented the fast upstroke and the reversal of the membrane potential. Coexpression of the voltage-gated K⁺ channel K_v2.1 facilitated action potential repolarization considerably. Light-induced action potentials were also obtained by coexpressing β1-ChR2 with either the neuronal Na⁺ channel Na_v1.2 or the cardiac-specific isoform Na_v1.5. Potential applications of this novel optogenetic tool are discussed.

Pilot study of freshly excised breast tissue response in the 300-600 GHz range

The failure to accurately define tumor margins during breast conserving surgery (BCS) results in a 20% re-excision rate. The present paper reports the investigation to evaluate the potential of terahertz imaging for breast tissue recognition within the under-explored 300 - 600 GHz range. Such a frequency window matches new BiCMOS technology capabilities and thus opens up the opportunity for near-field terahertz imaging using these devices. To assess the efficacy of this frequency band, data from 16 freshly excised breast tissue samples were collected and analyzed directly after excision. Complex refractive indices have been extracted over the as-mentioned frequency band, and amplitude frequency images show some contrast between tissue types. Principal component analysis (PCA) has also been applied to the data in an attempt to automate tissue classification. Our observations suggest that the dielectric response could potentially provide contrast for breast tissue recognition within the 300 - 600 GHz range. These results open the way for silicon-based terahertz subwavelength near field imager design, efficient up to 600 GHz to address ex vivo life-science applications.

Neurometabolic Disorders: Potentially Treatable Abnormalities in Patients With Treatment-Refractory Depression and Suicidal Behavior

OBJECTIVE

Treatment-refractory depression is a devastating condition with significant morbidity, mortality, and societal cost. At least 15% of cases of major depressive disorder remain refractory to treatment. The authors previously identified a young adult with treatment-refractory depression and multiple suicide attempts with an associated severe deficiency of CSF tetrahydrobiopterin, a critical cofactor for monoamine neurotransmitter synthesis. Treatment with sapropterin, a tetrahydrobiopterin analogue, led to dramatic and long-lasting remission of depression. This sentinel case led the authors to hypothesize that the incidence of metabolic abnormalities contributing to treatment-refractory depression is underrecognized.

METHOD

The authors conducted a case-control, targeted, metabolomic evaluation of 33 adolescent and young adult patients with well-characterized histories of treatment-refractory depression (at least three maximum-dose, adequate-duration medication treatments), and 16 healthy comparison subjects. Plasma, urine, and CSF metabolic profiling were performed by coupled gas chromatography/mass spectrometry and high-performance liquid chromatography electrospray ionization tandem mass spectrometry.

RESULTS

CSF metabolite abnormalities were identified in 21 of the 33 participants with treatment-refractory depression. Cerebral folate deficiency (N=12) was most common, with normal serum folate levels and low CSF 5-methyltetrahydrofolate (5-MTHF) levels. All patients with cerebral folate deficiency, including one with low CSF levels of 5-MTHF and tetrahydrobiopterin intermediates, showed improvement in depression symptom inventories after treatment with folinic acid; the patient with low tetrahydrobiopterin also received sapropterin. None of the healthy comparison subjects had a metabolite abnormality.

CONCLUSIONS

Examination of metabolic disorders in treatment-refractory depression identified an unexpectedly large proportion of patients with potentially treatable abnormalities. The etiology of these abnormalities remains to be determined.

Transcriptional regulation of the sodium channel gene (SCN5A) by GATA4 in human heart

Aberrant expression of the sodium channel gene (SCN5A) has been proposed to disrupt cardiac action potential and cause human cardiac arrhythmias, but the mechanisms of SCN5A gene regulation and dysregulation still remain largely unexplored. To gain insight into the transcriptional regulatory networks of SCN5A, we surveyed the promoter and first intronic regions of the SCN5A gene, predicting the presence of several binding sites for GATA transcription factors (TFs). Consistent with this prediction, chromatin immunoprecipitation (ChIP) and sequential ChIP (Re-ChIP) assays show co-occupancy of cardiac GATA TFs GATA4 and GATA5 on promoter and intron 1 SCN5A regions in fresh-frozen human left ventricle samples. Gene reporter experiments show GATA4 and GATA5 synergism in the activation of the SCN5A promoter, and its dependence on predicted GATA binding sites. GATA4 and GATA6 mRNAs are robustly expressed in fresh-frozen human left ventricle samples as measured by highly sensitive droplet digital PCR (ddPCR). GATA5 mRNA is marginally but still clearly detected in the same samples. Importantly, GATA4 mRNA levels are strongly and positively correlated with SCN5A transcript levels in the human heart. Together, our findings uncover a novel mechanism of GATA TFs in the regulation of the SCN5A gene in human heart tissue. Our studies suggest that GATA5 but especially GATA4 are main contributors to SCN5A gene expression, thus providing a new paradigm of SCN5A expression regulation that may shed new light into the understanding of cardiac disease.

Deciphering the function of the CNGB1b subunit in olfactory CNG channels

Olfactory cyclic nucleotide-gated (CNG) ion channels are key players in the signal transduction cascade of olfactory sensory neurons. The second messengers cAMP and cGMP directly activate these channels, generating a depolarizing receptor potential. Olfactory CNG channels are composed of two CNGA2 subunits and two modulatory subunits, CNGA4, and CNGB1b. So far the exact role of the modulatory subunits for channel activation is not fully understood. By measuring ligand binding and channel activation simultaneously, we show that in functional heterotetrameric channels not only the CNGA2 subunits and the CNGA4 subunit but also the CNGB1b subunit binds cyclic nucleotides and, moreover, also alone translates this signal to open the pore. In addition, we show that the CNGB1b subunit is the most sensitive subunit in a heterotetrameric channel to cyclic nucleotides and that it accelerates deactivation to a similar extent as does the CNGA4 subunit. In conclusion, the CNGB1b subunit participates in ligand-gated activation of olfactory CNG channels and, particularly, contributes to rapid termination of odorant signal in an olfactory sensory neuron.

Quantifying the cooperative subunit action in a multimeric membrane receptor

In multimeric membrane receptors the cooperative action of the subunits prevents exact knowledge about the operation and the interaction of the individual subunits. We propose a method that permits quantification of ligand binding to and activation effects of the individual binding sites in a multimeric membrane receptor. The power of this method is demonstrated by gaining detailed insight into the subunit action in olfactory cyclic nucleotide-gated CNGA2 ion channels.

Voltage-gated sodium channels in the mammalian heart

Mammalian species express nine functional voltage-gated Na(+) channels. Three of them, the cardiac-specific isoform Nav1.5 and the neuronal isoforms Nav1.8 and Nav1.9, are relatively resistant to the neurotoxin tetrodotoxin (TTX; IC50 ≥ 1 μM). The other six isoforms are highly sensitive to TTX with IC50 values in the nanomolar range. These isoforms are expressed in the central nervous system (Nav1.1, Nav1.2, Nav1.3, Nav1.6), in the skeletal muscle (Nav1.4), and in the peripheral nervous system (Nav1.6, Nav1.7). The isoform Nav1.5, encoded by the SCN5A gene, is responsible for the upstroke of the action potential in the heart. Mutations in SCN5A are associated with a variety of life-threatening arrhythmias, like long QT syndrome type 3 (LQT3), Brugada syndrome (BrS) or cardiac conduction disease (CCD). Previous immunohistochemical and electrophysiological assays demonstrated the cardiac expression of neuronal and skeletal muscle Na(+) channels in the heart of various mammals, which led to far-reaching speculations on their function. However, when comparing the Na(+) channel mRNA patterns in the heart of various mammalian species, only minute quantities of transcripts for TTX-sensitive Na(+) channels were detectable in whole pig and human hearts, suggesting that these channels are not involved in cardiac excitation phenomena in higher mammals. This conclusion is strongly supported by the fact that mutations in TTX-sensitive Na(+) channels were associated with epilepsy or skeletal muscle diseases, rather than with a pathological cardiac phenotype. Moreover, previous data from TTX-intoxicated animals and from cases of human tetrodotoxication showed that low TTX dosages caused at most little alterations of both the cardiac output and the electrocardiogram. Recently, genome-wide association studies identified SCN10A, the gene encoding Nav1.8, as a determinant of cardiac conduction parameters, and mutations in SCN10A have been associated with BrS. These novel findings opened a fascinating new research area in the cardiac ion channel field, and the on-going debate on how SCN10A/Nav1.8 affects cardiac conduction is very exciting.

Characterization of N-terminally mutated cardiac Na(+) channels associated with long QT syndrome 3 and Brugada syndrome

Mutations in SCN5A, the gene encoding the cardiac voltage-gated Na⁺ channel hNa_v1.5, can result in life-threatening arrhythmias including long QT syndrome 3 (LQT3) and Brugada syndrome (BrS). Numerous mutant hNa_v1.5 channels have been characterized upon heterologous expression and patch-clamp recordings during the last decade. These studies revealed functionally important regions in hNa_v1.5 and provided insight into gain-of-function or loss-of-function channel defects underlying LQT3 or BrS, respectively. The N-terminal region of hNa_v1.5, however, has not yet been investigated in detail, although several mutations were reported in the literature. In the present study we investigated three mutant channels, previously associated with LQT3 (G9V, R18W, V125L), and six mutant channels, associated with BrS (R18Q, R27H, G35S, V95I, R104Q, K126E). We applied both the two-microelectrode voltage clamp technique, using cRNA-injected Xenopus oocytes, and the whole-cell patch clamp technique using transfected HEK293 cells. Surprisingly, four out of the nine mutations did not affect channel properties. Gain-of-function, as typically observed in LQT3 mutant channels, was observed only in R18W and V125L, whereas loss-of-function, frequently found in BrS mutants, was found only in R27H, R104Q, and K126E. Our results indicate that the hNa_v1.5 N-terminus plays an important role for channel kinetics and stability. At the same time, we suggest that additional mechanisms, as e.g., disturbed interactions of the Na⁺ channel N-terminus with other proteins, contribute to severe clinical phenotypes.

Differential regulation by cyclic nucleotides of the CNGA4 and CNGB1b subunits in olfactory cyclic nucleotide-gated channels

Olfactory cyclic nucleotide-gated (CNG) ion channels are essential contributors to signal transduction of olfactory sensory neurons. The activity of the channels is controlled by the cyclic nucleotides guanosine 3',5'-monophosphate (cGMP) and adenosine 3',5'-monophosphate (cAMP). The olfactory CNG channels are composed of two CNGA2 subunits, one CNGA4 and one CNGB1b subunit, each containing a cyclic nucleotide-binding domain. Using patch-clamp fluorometry, we measured ligand binding and channel activation simultaneously and showed that cGMP activated olfactory CNG channels not only by binding to the two CNGA2 subunits but also by binding to the CNGA4 subunit. In a channel in which the CNGA2 subunits were compromised for ligand binding, cGMP binding to CNGA4 was sufficient to partly activate the channel. In contrast, in heterotetrameric channels, the CNGB1b subunit did not bind cGMP, but channels with this subunit showed activation by cAMP. Thus, the modulatory subunits participate actively in translating ligand binding to activation of heterotetrameric olfactory CNG channels and enable the channels to differentiate between cyclic nucleotides.

How subunits cooperate in cAMP-induced activation of homotetrameric HCN2 channels

Hyperpolarization-activated cyclic nucleotide-modulated (HCN) channels are tetrameric membrane proteins that generate electrical rhythmicity in specialized neurons and cardiomyocytes. The channels are primarily activated by voltage but are receptors as well, binding the intracellular ligand cyclic AMP. The molecular mechanism of channel activation is still unknown. Here we analyze the complex activation mechanism of homotetrameric HCN2 channels by confocal patch-clamp fluorometry and kinetically quantify all ligand binding steps and closed-open isomerizations of the intermediate states. For the binding affinity of the second, third and fourth ligand, our results suggest pronounced cooperativity in the sequence positive, negative and positive, respectively. This complex interaction of the subunits leads to a preferential stabilization of states with zero, two or four ligands and suggests a dimeric organization of the activation process: within the dimers the cooperativity is positive, whereas it is negative between the dimers.

Alternative splicing of the cardiac sodium channel creates multiple variants of mutant T1620K channels

Alternative splicing creates several Na_v1.5 transcripts in the mammalian myocardium and in various other tissues including brain, dorsal root ganglia, breast cancer cells as well as neuronal stem cell lines. In total nine Na_v1.5 splice variants have been discovered. Four of them, namely Na_v1.5a, Na_v1.5c, Na_v1.5d, and Na_v1.5e, generate functional channels in heterologous expression systems. The significance of alternatively spliced transcripts for cardiac excitation, in particular their role in SCN5A channelopathies, is less well understood. In the present study, we systematically investigated electrophysiological properties of mutant T1620K channels in the background of all known functional Na_v1.5 splice variants in HEK293 cells. This mutation has been previously associated with two distinct cardiac excitation disorders: with long QT syndrome type 3 (LQT3) and isolated cardiac conduction disease (CCD). When investigating the effect of the T1620K mutation, we noticed similar channel defects in the background of hNa_v1.5, hNa_v1.5a, and hNa_v1.5c. In contrast, the hNa_v1.5d background produced differential effects: In the mutant channel, some gain-of-function features did not emerge, whereas loss-of-function became more pronounced. In case of hNa_v1.5e, the neonatal variant of hNa_v1.5, both the splice variant itself as well as the corresponding mutant channel showed electrophysiological properties that were distinct from the wild-type and mutant reference channels, hNa_v1.5 and T1620K, respectively. In conclusion, our data show that alternative splicing is a mechanism capable of generating a variety of functionally distinct wild-type and mutant hNa_v1.5 channels. Thus, the cellular splicing machinery is a potential player affecting genotype-phenotype correlations in SCN5A channelopathies.

Role of the S4-S5 linker in CNG channel activation

Cyclic nucleotide-gated (CNG) channels mediate sensory signal transduction in retinal and olfactory cells. The channels are activated by the binding of cyclic nucleotides to a cyclic nucleotide-binding domain (CNBD) in the C-terminus that is located at the intracellular side. The molecular events translating the ligand binding to the pore opening are still unknown. We investigated the role of the S4-S5 linker in the activation process by quantifying its interaction with other intracellular regions. To this end, we constructed chimeric channels in which the N-terminus, the S4-S5 linker, the C-linker, and the CNBD of the retinal CNGA1 subunit were systematically replaced by the respective regions of the olfactory CNGA2 subunit. Macroscopic concentration-response relations were analyzed, yielding the apparent affinity to cGMP and the Hill coefficient. The degree of functional coupling of intracellular regions in the activation gating was determined by thermodynamic double-mutant cycle analysis. We observed that all four intracellular regions, including the relatively short S4-S5 linker, are involved in controlling the apparent affinity of the channel to cGMP and, moreover, in determining the degree of cooperativity between the subunits, as derived from the Hill coefficient. The interaction energies reveal an interaction of the S4-S5 linker with both the N-terminus and the C-linker, but no interaction with the CNBD.

Interdependence of receptor activation and ligand binding in HCN2 pacemaker channels

HCN pacemaker channels are tetramers mediating rhythmicity in neuronal and cardiac cells. The activity of these channels is controlled by both membrane voltage and the ligand cAMP, binding to each of the four channel subunits. The molecular mechanism underlying channel activation and the relationship between the two activation stimuli are still unknown. Using patch-clamp fluorometry and a fluorescent cAMP analog, we show that full ligand-induced activation appears already with only two ligands bound to the tetrameric channel. Kinetic analysis of channel activation and ligand binding suggests direct interaction between the voltage sensor and the cyclic nucleotide-binding domain, bypassing the pore. By exploiting the duality of activation in HCN2 channels by voltage and ligand binding, we quantify the increase of the binding affinity and overall free energy for binding upon channel activation, proving thus the principle of reciprocity between ligand binding and conformational change in a receptor protein.

Structure and function of splice variants of the cardiac voltage-gated sodium channel Na(v)1.5

Voltage-gated sodium channels mediate the rapid upstroke of the action potential in excitable tissues. The tetrodotoxin (TTX) resistant isoform Na(v)1.5, encoded by the SCN5A gene, is the predominant isoform in the heart. This channel plays a key role for excitability of atrial and ventricular cardiomyocytes and for rapid impulse propagation through the specific conduction system. During recent years, strong evidence has been accumulated in support of the expression of several Na(v)1.5 splice variants in the heart, and in various other tissues and cell lines including brain, dorsal root ganglia, breast cancer cells and neuronal stem cell lines. This review summarizes our knowledge on the structure and putative function of nine Na(v)1.5 splice variants detected so far. Attention will be paid to the distinct biophysical properties of the four functional splice variants, to the pronounced tissue- and species-specific expression, and to the developmental regulation of Na(v)1.5 splicing. The implications of alternative splicing for SCN5A channelopathies, and for a better understanding of genotype-phenotype correlations, are discussed.

Mutation-specific effects of polymorphism H558R in SCN5A-related sick sinus syndrome

INTRODUCTION

Mutations in SCN5A, the gene encoding alpha subunit of cardiac type sodium channel, Na(v)1.5, lead to familial sick sinus syndrome (SSS). Although several molecular mechanisms for this genetic condition have been explored, the underlying mechanisms for the variable genotype-phenotype relationships have not been well addressed. One of the important contributors to such relationships is the genetic background such as single-nucleotide polymorphisms.

METHODS AND RESULTS

To clarify the effects of a common polymorphism in SCN5A gene, H558R, on SCN5A-related SSS phenotype, we investigated the electrophysiological properties of all of the 13 known SSS-related hNa(v)1.5 mutant channels on both H558 and R558 background. Electrophysiological properties of hNa(v)1.5 mutant channels were investigated by the whole-cell patch clamp technique in HEK293 cells. When peak currents were affected by the mutation, cell surface biotinylation was performed to quantify the fraction of correctly cell membrane-targeted mutant channels. Loss-of-function defect of D1275N in SCN5A was rescued by R558 through enhancing cell surface targeting and improving steady-state activation of the mutant channels. In contrast, the defects of mutants E161K, P1298L, and R1632H were aggravated in the R558 background, mainly due to the reduced steady-state availability of mutant channels. The electrophysiological properties of the remaining SSS-related hNa(v)1.5 mutants including the missense mutants (L212P, T220I, DelF1617, T187I, R878C, G1408R), and the truncated mutants (W1421X, K1578fs/52, R1623X) were not significantly affected by H558R.

CONCLUSION

We conclude that polymorphism H558R has mutation-specific effects on SCN5A-related SSS. Our data highlight the importance of common genetic variants in modulating phenotypes of genetic diseases.

Drug-induced confusional states: the usual suspects?

BACKGROUND

Acute confusional state (ACS) is a frequent reason for hospital admission. This study examines retrospectively the frequency by which individual drugs were found responsible for ACS.

RESULTS

Drug-induced ACS was found in 65 (18.8%) of 346 hospital admissions for acute confusion. The most frequent causative substances were dopaminergic drugs in Parkinsonian patients (24.2%), diuretics (15.1%), tricyclic or tetracyclic antidepressants (13.6%) and benzodiazepines (13.6%). Almost half of the patients were demented, and in one-third of these, dementia had not been diagnosed hitherto.

CONCLUSION

The data suggest that diuretics by way of causing hyponatraemia are as relevant a cause of ACS as dopaminergic or anticholinergic substances.

[Neuroendocrine tumors of the stomach (gastric carcinoids) are on the rise: good prognosis with early detection]

Neuroendocrine tumors (NET) of the stomach are on the rise. In the United States they have increased about tenfold in the last 35 years. Prognosis has been much improved over the last three to four decades. Nowadays most of such NETs are diagnosed at an early stage. Quite often gastric NETs are found incidentally during a gastroscopy, performed for other reasons. Most of the asymptomatic, well differentiated gastric NETs are less than 2 cm in diameter. Conservative management and endoscopic surveillance is adequate for well differentiated, multifocal type 1 or type 2 gastric NETs (gastric carcinoids) of 10-20 mm , unless they are angio-invasive, have infiltrated into the muscularis propria or have metastasized. Endoscopic ultrasound is the method of choice to determine tumor size and depth of infiltration. Surgery is, however, indicated for all NETs larger than 20 mm. For optimal management tumor biology, type and stage of the neoplasm as well as the individual situation of the patient have to be taken into account. Most of the patients can be treated conservatively and be followed up with endoscopic surveillance.

Correlations between clinical and physiological consequences of the novel mutation R878C in a highly conserved pore residue in the cardiac Na+ channel

Aim:

We compared the clinical and physiological consequences of the novel mutation R878C in a highly conserved pore residue in domain II (S5-S6) of human, hNa_v1.5, cardiac Na⁺ channels.

Methods:

Full clinical evaluation of pedigree members through three generations of a Chinese family combined with SCN5A sequencing from genomic DNA was compared with patch and voltage-clamp results from two independent expression systems.

Results:

The four mutation carriers showed bradycardia, and slowed sino-atrial, atrioventricular and intraventricular conduction. Two also showed sick sinus syndrome; two had ST elevation in leads V1 and V2. Unlike WT-hNa_v1.5, whole-cell patch-clamped HEK293 cells expressing R878C-hNa_v1.5 showed no detectable Na⁺ currents (i_Na), even with substitution of a similarly charged lysine residue. Voltage-clamped Xenopus oocytes injected with either 0.04 or 1.5 μg μL⁻¹ R878C-hNa_v1.5 cRNA similarly showed no i_Na, yet WT-hNa_v1.5 cRNA diluted to 0.0004–0.0008 ng μL⁻¹resulted in expression of detectable i_Na. i_Na was simply determined by the amount of injected WT-hNa_v1.5: doubling the dose of WT-hNa_v1.5 cRNA doubled i_Na. i_Na amplitudes and activation and inactivation characteristics were similar irrespective of whether WT-hNa_v1.5 cRNA was given alone or combined with equal doses of R878C-hNa_v1.5 cRNA therefore excluding dominant negative phenotypic effects. Na⁺ channel function in HEK293 cells transfected with R878C-hNa_v1.5 was not restored by exposure to mexiletine (200 μm) and lidocaine (100 μm). Fluorescence confocal microscopy using E3-Nav1.5 antibody demonstrated persistent membrane expression of both WT and R878C-hNa_v1.5. Modelling studies confirmed that such i_Na reductions reproduced the SSS phenotype.

Conclusion:

Clinical consequences of the novel R878C mutation correlate with results of physiological studies.

Prolonged irradiation of enhanced cyan fluorescent protein or Cerulean can invalidate Forster resonance energy transfer measurements

Since its discovery, green fluorescent protein (GFP) and its variants have proven to be a good and convenient fluorescent label for proteins: GFP and other visible fluorescent proteins (VFPs) can be fused selectively to the protein of interest by simple cloning techniques and develop fluorescence without additional cofactors. Among the steadily growing collection of VFPs, several pairs can be chosen that can serve as donor and acceptor fluorophores in Forster resonance energy transfer (FRET) experiments. Among them, the cyan fluorescent proteins (ECFP/Cerulean) and the enhanced yellow fluorescent protein (EYFP) are most commonly used. We show that ECFP and Cerulean have some disadvantages despite their common use: Upon irradiation with light intensities that are commonly used for intensity- and lifetime-based FRET measurements, both the fluorescence intensity and the fluorescence lifetime of ECFP and Cerulean decrease. This can hamper both intensity- and lifetime-based FRET measurements and emphasizes the need for control measurements to exclude these artifacts.

The intracellular domain of the beta 2 subunit modulates the gating of cardiac Na v 1.5 channels

We have previously shown that the transmembrane segment plus either the extracellular or intracellular domain of the beta1 subunit are required to modify cardiac Na(v)1.5 channels. In this study, we coexpressed the intracellular domain of the beta2 subunit in a beta1/beta2 chimera with Na(v)1.5 channels in Xenopus oocytes and obtained an atypical recovery behavior of Na(v)1.5 channels not reported before for other Na(+) channels: Recovery times of up to 20 ms at -120 mV produced a similar fast recovery as observed for Na(v)1.5/beta1 channels, but the current amplitude decreased again at longer recovery times and reached a steady-state level after 1-2 s with current amplitudes of only 43 +/- 2% of the value at 20 ms. Current reduction was accompanied by slowed inactivation and by a shift of steady-state activation toward depolarized potentials by 9 mV. All effects were reversible and they were not seen when deleting the beta2 intracellular domain. These results describe the first functional effects of a beta2 subunit region on Na(v)1.5 channels and suggest a novel closed state in cardiac Na(+) channels accessible at hyperpolarized potentials.

Multi-dimensional fluorescence lifetime and FRET measurements

When and where proteins associate with each other in living cells are key questions in many biological research projects. One way to address these questions is to measure the extent of Förster resonance energy transfer (FRET) between proteins that have been labeled with appropriate donor and acceptor fluorophores. When both proteins interact, donor and acceptor fluorophores are brought into close vicinity so that the donor can transmit a part of its excitation energy to the acceptor. As a result, both the intensity and the lifetime of the donor fluorescence decrease, whereas the intensity of the acceptor emission increases. This offers different approaches to determine FRET efficiency: One is to detect changes in the intensity of donor and acceptor emission, the other is to measure changes in the lifetime of the donor molecule. One important advantage of the fluorescence lifetime approach is that it allows to distinguish between free and associated donor molecules. However, like intensity measurements it lacks an intrinsic control ensuring that changes in the measured parameters are only due to FRET and not other quenching processes. Here, we show how this limitation can be overcome by spectrally resolved fluorescence lifetime measurements in the time domain. One technique is based on a streak camera system, the other technique is based on a time-correlated-single-photon-counting approach. Both approaches allow biologists to record both donor and acceptor fluorescence emitted by the sample in a single measurement.

Region-specific alterations of global protein expression in the remodelled rat myocardium

We applied the novel ProteinChip technology (SELDI-MS) to investigate and identify differentially regulated proteins upon myocardial remodelling in different heart regions. Tissue samples were isolated from the atria, the interventricular septum, and the right and left ventricles three months after surgically-induced myocardial infarction (MI) in rats. Corresponding protein extracts from control versus MI hearts were analysed on two different ProteinChip surfaces. In each of the functionally distinct cardiac regions, we obtained specific protein profile alterations upon myocardial remodelling. Most alterations were observed in the non-infarcted right ventricle, where down-regulation occurred more frequently than up-regulation of protein expression. Three of the differentially regulated proteins were identified: the metabolic enzyme triosephosphate isomerase (TIM), the cell signalling protein Raf-1 kinase inhibitory protein (RKIP), also known as phosphatidylethanolamine binding protein (PEBP), and the small heat shock protein alphaB-crystallin. These proteins showed a pronounced tissue-dependent regulation. TIM was down-regulated only in the atrium and in the left ventricle, RKIP/PEBP was down-regulated only in the right ventricle and in the interventricular septum, and alphaB-crystallin was up-regulated only in the right and in the left ventricle. A simple correlation of peak intensity changes using two of the identified peaks demonstrated the diagnostic potential of SELDI-MS. We conclude that this novel proteomic method is a powerful high-throughput tool for the fast detection of region-specific cardiac protein profiles in small biopsy samples, and that SELDI-MS may become a useful complementary technique for the diagnosis and prognosis of cardiac diseases.

Modulation of Nav1.5 Channel Function by an Alternatively Spliced Sequence in the DII/DIII Linker Region

In the present study, we identified a novel splice variant of the human cardiac Na(+) channel Na(v)1.5 (Na(v)1.5d), in which a 40-amino acid sequence of the DII/DIII intracellular linker is missing due to a partial deletion of exon 17. Expression of Na(v)1.5d occurred in embryonic and adult hearts of either sex, indicating that the respective alternative splicing is neither age-dependent nor gender-specific. In contrast, Na(v)1.5d was not detected in the mouse heart, indicating that alternative splicing of Na(v)1.5 is species-dependent. In HEK293 cells, splice variant Na(v)1.5d generated voltage-dependent Na(+) currents that were markedly reduced compared with wild-type Na(v)1.5. Experiments with mexiletine and 8-bromo-cyclic AMP suggested that the trafficking of Na(v)1.5d channels was not impaired. However, single-channel recordings showed that the whole-cell current reduction was largely due to a significantly reduced open probability. Additionally, steady-state activation and inactivation were shifted to depolarized potentials by 15.9 and 5.1 mV, respectively. Systematic mutagenesis analysis of the spliced region provided evidence that a short amphiphilic region in the DII/DIII linker resembling an S4 voltage sensor of voltage-gated ion channels is an important determinant of Na(v)1.5 channel gating. Moreover, the present study identified novel short sequence motifs within this amphiphilic region that specifically affect the voltage dependence of steady-state activation and inactivation and current amplitude of human Na(v)1.5.

K(ATP) channel current increases in postinfarction remodeled cardiomyocytes

Adenosintriphosphate-sensitive potassium channels (K(ATP) channels) are an important linkage between the metabolic state of a cell and electrophysiological membrane properties. In this study, K(ATP) channels were studied in myocytes of normal and remodeled myocardium of the rat. Myocardial infarction was induced by ligature of the left anterior descending artery. Remodeled myocytes were obtained from the hypertrophied posterior left ventricular wall and interventricular septum 3 months after infarction. The current through K(ATP) channels was measured in whole-cell and inside-out patches by using the patch-clamp technique. After myocardial infarction, the heart weight/body weight ratio was doubled and the myocytes were hypertrophied yielding a cell capacitance of 266+/-16 pF compared to 122+/-12 pF in control cells. The amount of Kir6.2 protein was indistinguishable in corresponding regions of control and remodeled hearts. The ATP sensitivity of K(ATP) channels in remodeled cells was significantly lower than in control cells (half maximum block at 115 micromol/l ATP in remodeled and at 71 mumol/l ATP in control cells). The maximum I (KATP) density induced by metabolic inhibition was higher in small remodeled (176+/-15 pA/pF) than in control cells (127+/-11 pA/pF), but was unchanged in large remodeled cells. Both, the higher I (KATP) density and the lower sensitivity of the K(ATP) channels to ATP suggest that remodeled cardiomyocytes develop an improved tolerance to ischemia by stabilizing the resting potential and decreasing excitability.

Activation of olfactory-type cyclic nucleotide-gated channels is highly cooperative

Cyclic nucleotide-gated (CNG) ion channels play a key role in the sensory transduction of vision and olfaction. The channels are opened by the binding of cyclic nucleotides. Native olfactory CNG channels are heterotetramers of CNGA2, CNGA4, and CNGB1b subunits. Upon heterologous expression, only CNGA2 subunits can form functional homotetrameric channels. It is presently not known how the binding of the ligands to the four subunits is translated to channel opening. We studied activation of olfactory CNG channels by photolysis-induced jumps of cGMP or cAMP, two cyclic nucleotides with markedly different apparent affinity. It is shown that at equal degree of activation, the activation time course of homotetrameric channels is similar with cGMP and cAMP and it is also similar in homo- and heterotetrameric channels with the same cyclic nucleotide. Kinetic models were globally fitted to activation time courses of homotetrameric channels. While all models containing equivalent binding sites failed, a model containing three binding sites with a ligand affinity high-low-high described the data adequately. Only the second binding step switches from a very low to a very high open probability. We propose a unique gating mechanism for homotetrameric and heterotetrameric channels that involves only three highly cooperative binding steps.