Calcium-dependent inactivation of neuronal calcium channels

Honeybee CaV4 has distinct permeation, inactivation, and pharmacology from homologous NaV channels

DSC1, a Drosophila channel with sequence similarity to the voltage-gated sodium channel (NaV), was identified over 20 years ago. This channel was suspected to function as a non-specific cation channel with the ability to facilitate the permeation of calcium ions (Ca2+). A honeybee channel homologous to DSC1 was recently cloned and shown to exhibit strict selectivity for Ca2+, while excluding sodium ions (Na+), thus defining a new family of Ca2+ channels, known as CaV4. In this study, we characterize CaV4, showing that it exhibits an unprecedented type of inactivation, which depends on both an IFM motif and on the permeating divalent cation, like NaV and CaV1 channels, respectively. CaV4 displays a specific pharmacology with an unusual response to the alkaloid veratrine. It also possesses an inactivation mechanism that uses the same structural domains as NaV but permeates Ca2+ ions instead. This distinctive feature may provide valuable insights into how voltage- and calcium-dependent modulation of voltage-gated Ca2+ and Na+ channels occur under conditions involving local changes in intracellular calcium concentrations. Our study underscores the unique profile of CaV4 and defines this channel as a novel class of voltage-gated Ca2+ channels.

Voltage-gated Calcium Channels as Potential Therapeutic Targets in Migraine

Migraine is a complex and highly incapacitating neurological disorder that affects around 15% of the general population with greater incidence in women, often at the most productive age of life. Migraine physiopathology is still not fully understood, but it involves multiple mediators and events in the trigeminovascular system and the central nervous system. The identification of calcitonin gene-related peptide as a key mediator in migraine physiopathology has led to the development of effective and highly selective antimigraine therapies. However, this treatment is neither accessible nor effective for all migraine sufferers. Thus, a better understanding of migraine mechanisms and the identification of potential targets are still clearly warranted. Voltage-gated calcium channels (VGCCs) are widely distributed in the trigeminovascular system, and there is accumulating evidence of their contribution to the mechanisms associated with headache pain. Several drugs used in migraine abortive or prophylactic treatment target VGCCs, which probably contributes to their analgesic effect. This review aims to summarize the current evidence of VGGC contribution to migraine physiopathology and to discuss how current pharmacological options for migraine treatment interfere with VGGC function. PERSPECTIVE: Calcitonin gene-related peptide (CGRP) represents a major migraine mediator, but few studies have investigated the relationship between CGRP and VGCCs. CGRP release is calcium channel-dependent and VGGCs are key players in familial migraine. Further studies are needed to determine whether VGCCs are suitable molecular targets for treating migraine.

Asymmetric contribution of a selectivity filter gate in triggering inactivation of CaV1.3 channels

Voltage-dependent and Ca2+-dependent inactivation (VDI and CDI, respectively) of CaV channels are two biologically consequential feedback mechanisms that fine-tune Ca2+ entry into neurons and cardiomyocytes. Although known to be initiated by distinct molecular events, how these processes obstruct conduction through the channel pore remains poorly defined. Here, focusing on ultrahighly conserved tryptophan residues in the interdomain interfaces near the selectivity filter of CaV1.3, we demonstrate a critical role for asymmetric conformational changes in mediating VDI and CDI. Specifically, mutagenesis of the domain III-IV interface, but not others, enhanced VDI. Molecular dynamics simulations demonstrate that mutations in distinct selectivity filter interfaces differentially impact conformational flexibility. Furthermore, mutations in distinct domains preferentially disrupt CDI mediated by the N- versus C-lobes of CaM, thus uncovering a scheme of structural bifurcation of CaM signaling. These findings highlight the fundamental importance of the asymmetric arrangement of the pseudotetrameric CaV pore domain for feedback inhibition.

Calmodulinopathy in Japanese Children - Their Cardiac Phenotypes Are Severe and Show Early Onset in Fetal Life and Infancy

BACKGROUND

Cardiac calmodulinopathy, characterized by a life-threatening arrhythmia and sudden death in the young, is extremely rare and caused by genes encoding calmodulin, namely calmodulin 1 (CALM1), CALM2, and CALM3.Methods and Results: We screened 195 symptomatic children (age 0-12 years) who were suspected of inherited arrhythmias for 48 candidate genes, using a next-generation sequencer. Ten probands were identified as carrying variants in any of CALM1-3 (5%; median age 5 years), who were initially diagnosed with long QT syndrome (LQTS; n=5), catecholaminergic polymorphic ventricular tachycardia (CPVT; n=3), and overlap syndrome (n=2). Two probands harbored a CALM1 variant and 8 probands harbored 6 CALM2 variants. There were 4 clinical phenotypes: (1) documented lethal arrhythmic events (LAEs): 4 carriers of N98S in CALM1 or CALM2; (2) suspected LAEs: CALM2 p.D96G and D132G carriers experienced syncope and transient cardiopulmonary arrest under emotional stimulation; (3) critical cardiac complication: CALM2 p.D96V and p.E141K carriers showed severe cardiac dysfunction with QTc prolongation; and (4) neurological and developmental disorders: 2 carriers of CALM2 p.E46K showed cardiac phenotypes of CPVT. Beta-blocker therapy was effective in all cases except cardiac dysfunction, especially in combination with flecainide (CPVT-like phenotype) and mexiletine (LQTS-like).

CONCLUSIONS

Calmodulinopathy patients presented severe cardiac features, and their onset of LAEs was earlier in life, requiring diagnosis and treatment at the earliest age possible.

Asymmetric Contribution of a Selectivity Filter Gate in Triggering Inactivation of CaV1.3 Channels

Voltage-dependent and Ca2+-dependent inactivation (VDI and CDI, respectively) of CaV channels are two biologically consequential feedback mechanisms that fine-tune Ca2+ entry into neurons and cardiomyocytes. Although known to be initiated by distinct molecular events, how these processes obstruct conduction through the channel pore remains poorly defined. Here, focusing on ultra-highly conserved tryptophan residues in the inter-domain interfaces near the selectivity filter of CaV1.3, we demonstrate a critical role for asymmetric conformational changes in mediating VDI and CDI. Specifically, mutagenesis of the domain III-IV interface, but not others, enhanced VDI. Molecular dynamics simulations demonstrate that mutations in distinct selectivity filter interfaces differentially impact conformational flexibility. Furthermore, mutations in distinct domains preferentially disrupt CDI mediated by the N- versus C-lobes of CaM, thus uncovering a scheme of structural bifurcation of CaM signaling. These findings highlight the fundamental importance of the asymmetric arrangement of the pseudo-tetrameric CaV pore domain for feedback inhibition.

Belly roll, a GPI-anchored Ly6 protein, regulates Drosophila melanogaster escape behaviors by modulating the excitability of nociceptive peptidergic interneurons

Appropriate modulation of escape behaviors in response to potentially damaging stimuli is essential for survival. Although nociceptive circuitry has been studied, it is poorly understood how genetic contexts affect relevant escape responses. Using an unbiased genome-wide association analysis, we identified an Ly6/α-neurotoxin family protein, Belly roll (Bero), which negatively regulates Drosophila nociceptive escape behavior. We show that Bero is expressed in abdominal leucokinin-producing neurons (ABLK neurons) and bero knockdown in ABLK neurons resulted in enhanced escape behavior. Furthermore, we demonstrated that ABLK neurons responded to activation of nociceptors and initiated the behavior. Notably, bero knockdown reduced persistent neuronal activity and increased evoked nociceptive responses in ABLK neurons. Our findings reveal that Bero modulates an escape response by regulating distinct neuronal activities in ABLK neurons.

The Role of Ryanodine Receptors in Regulating Neuronal Activity and Its Connection to the Development of Alzheimer's Disease

Research into the early impacts of Alzheimer's disease (AD) on synapse function is one of the most promising approaches to finding a treatment. In this context, we have recently demonstrated that the Abeta42 peptide, which builds up in the brain during the processing of the amyloid precursor protein (APP), targets the ryanodine receptors (RyRs) of mouse hippocampal neurons and potentiates calcium (Ca²⁺) release from the endoplasmic reticulum (ER). The uncontrolled increase in intracellular calcium concentration ([Ca²⁺]_i), leading to the development of Ca²⁺ dysregulation events and related excitable and synaptic dysfunctions, is a consolidated hallmark of AD onset and possibly other neurodegenerative diseases. Since RyRs contribute to increasing [Ca²⁺]_i and are thought to be a promising target for AD treatment, the goal of this review is to summarize the current level of knowledge regarding the involvement of RyRs in governing neuronal function both in physiological conditions and during the onset of AD.

A gating mechanism of the BsYetJ calcium channel revealed in an endoplasmic reticulum lipid environment

The transmembrane BAX inhibitor-1-containing motif 6 (TMBIM6) is suggested to modulate apoptosis by regulating calcium homeostasis in the endoplasmic reticulum (ER). However, the precise molecular mechanism underlying this calcium regulation remains poorly understood. To shed light on this issue, we investigated all negatively charged residues in BsYetJ, a bacterial homolog of TMBIM6, using mutagenesis and fluorescence-based functional assays. We reconstituted BsYetJ in membrane vesicles with a lipid composition similar to that of the ER. Our results show that the charged residues E49 and R205 work together as a major gate, regulating calcium conductance in these ER-like lipid vesicles. However, these residues become largely inactive when reconstituted in other lipid environments. In addition, we found that D195 acts as a minor filter compared to the E49-R205 dyad. Our study uncovers a previously unknown function of BsYetJ/TMBIM6 in the calcium-dependent inactivation of BsYetJ, providing a framework for the development of a lipid-dependent mechanistic model of BsYetJ that will facilitate our understanding of calcium-dependent apoptosis.

Modulation of Slow Desensitization (Tachyphylaxis) of Acid-Sensing Ion Channel (ASIC)1a

Among the proton-activated channels of the ASIC family, ASIC1a exhibits a specific tachyphylaxis phenomenon in the form of a progressive decrease in the response amplitude during a series of activations. This process is well known, but its mechanism is poorly understood. Here, we demonstrated a partial reversibility of this effect using long-term whole-cell recording of CHO cells transfected with rASIC1a cDNA. Thus, tachyphylaxis represents a slow desensitization of ASIC1a. Prolonged acidifications provided the same recovery from slow desensitization as short acidifications of the same frequency. Slow desensitization and steady-state desensitization are independent processes although the latter attenuates the development of the former. We found that drugs which facilitate ASIC1a activation (e.g., amitriptyline) cause an enhancement of slow desensitization, while inhibition of ASIC1a by 9-aminoacridine attenuates this process. Overall, for a broad variety of exposures, including increased calcium concentration, different pH conditions, and modulating drugs, we found a correlation between their effects on ASIC1a response amplitude and the development of slow desensitization. Thus, our results demonstrate that slow desensitization occurs only when ASIC1a is in the open state.

Mitochondrial calcium cycling in neuronal function and neurodegeneration

Mitochondria are essential for proper cellular function through their critical roles in ATP synthesis, reactive oxygen species production, calcium (Ca2+) buffering, and apoptotic signaling. In neurons, Ca2+ buffering is particularly important as it helps to shape Ca2+ signals and to regulate numerous Ca2+-dependent functions including neuronal excitability, synaptic transmission, gene expression, and neuronal toxicity. Over the past decade, identification of the mitochondrial Ca2+ uniporter (MCU) and other molecular components of mitochondrial Ca2+ transport has provided insight into the roles that mitochondrial Ca2+ regulation plays in neuronal function in health and disease. In this review, we discuss the many roles of mitochondrial Ca2+ uptake and release mechanisms in normal neuronal function and highlight new insights into the Ca2+-dependent mechanisms that drive mitochondrial dysfunction in neurologic diseases including epilepsy, Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis. We also consider how targeting Ca2+ uptake and release mechanisms could facilitate the development of novel therapeutic strategies for neurological diseases.

Calcium dysregulation and compensation in cortical pyramidal neurons of the R6/2 mouse model of Huntington's disease

Huntington's disease (HD) is a fatal, hereditary neurodegenerative disorder that predominantly affects striatal medium-sized spiny neurons and cortical pyramidal neurons (CPNs). It has been proposed that perturbations in Ca²⁺ homeostasis could play a role in CPN alterations. To test this hypothesis, we used the R6/2 mouse model of juvenile HD at different stages of disease progression; presymptomatic, early symptomatic, and late symptomatic. We combined whole-cell patch-clamp recordings of layer 2/3 CPNs with two-photon laser scanning microscopy to image somatic and dendritic Ca²⁺ transients associated with evoked action potentials (APs). We found that the amplitude of AP-induced Ca²⁺ transients recorded at the somata of CPNs was significantly reduced in presymptomatic and late symptomatic R6/2 mice compared with wild-type (WT) littermates. However, reduced amplitudes were compensated by increases in decay times, so that Ca²⁺ transient areas were similar between genotypes. AP-induced Ca²⁺ transients in CPN proximal dendrites were variable and differences did not reach statistical significance, except for reduced areas in the late symptomatic group. In late symptomatic mice, a specific store-operated Ca²⁺ channel antagonist, EVP4593, reduced somatic Ca²⁺ transient amplitude similarly in WT and R6/2 CPNs. In contrast, dantrolene, a ryanodine receptor (RyR) antagonist, and nifedipine, an L-type Ca²⁺ channel blocker, significantly reduced both somatic Ca²⁺ transient amplitude and area in R6/2 but not WT CPNs. These findings demonstrate that perturbations of Ca²⁺ homeostasis and compensation occur in CPNs before and after the onset of overt symptoms, and suggest RyRs and L-type Ca²⁺ channels as potential targets for therapeutic intervention.NEW & NOTEWORTHY We used two-photon microscopy to examine calcium influx induced by action potentials in cortical pyramidal neurons from a mouse model of Huntington's disease (HD), the R6/2. The amplitude of somatic calcium transients was reduced in R6/2 mice compared with controls. This reduction was compensated by increased decay times, which could lead to reduced calcium buffering capacity. L-type calcium channel and ryanodine receptor blockers reduced calcium transient area in HD neurons, suggesting new therapeutic avenues.

CALM1 rs3179089 polymorphism might contribute to coronary artery disease susceptibility in Chinese male: a case-control study

BACKGROUND

Calmodulin 1 (CALM1) mutations are involved in the development of coronary artery disease (CAD). However, the relationship of CALM1 rs3179089 polymorphism with CAD is unknown.

OBJECTIVE

This study aimed to identify the relationship of CALM1 rs3179089 polymorphism with CAD susceptibility, CALM1 expression, blood pressure, blood glucose, blood coagulation and serum lipid levels of CAD patients.

METHODS

550 CAD patients and 550 control subjects were genotyped for CALM1 using Sequenom MassARRAY technology. CALM1 expression level was measured by quantitative real time polymerase chain reaction (qRT-PCR).

RESULTS

CALM1 mRNA expression was higher in CAD patients than that in control subjects (P < 0.001). CAD patients with CC genotype had higher CALM1 mRNA expression level than control subjects with CC genotype (P = 0.006). Genotypic frequency of rs3179089 was different between male patients of CAD and control subjects (P = 0.045). Rs3179089 polymorphism was related to CAD risk of males in recessive model (P = 0.039). Moreover, rs3179089 polymorphism was associated with systolic blood pressure (SBP), diastolic blood pressure (DBP), fasting plasma glucose (FPG), and D-Dimer (D-D) level of patients with CAD in recessive model (P = 0.013 for SBP; P = 0.034 for DBP; P = 0.004 for FPG; P = 0.046 for D-D). In addition, rs3179089 polymorphism was correlated with low-density lipoprotein cholesterol (LDL-C) and total cholesterol (TC) serum levels of patients with CAD in both addictive (P = 0.025 for LDL-C; P = 0.001 for TC) and recessive models (P = 0.001 for LDL-C; P = 0.001 for TC).

CONCLUSION

CALM1 expression is associated with development of CAD. CALM1 rs3179089 polymorphism affects CAD susceptibility in males, and blood pressure, blood glucose, blood coagulation and serum lipid of CAD patients.

Periphery signals generated by Piezo-mediated stomach stretch and Neuromedin-mediated glucose load regulate the Drosophila brain nutrient sensor

Nutrient sensors allow animals to identify foods rich in specific nutrients. The Drosophila nutrient sensor, diuretic hormone 44 (DH44) neurons, helps the fly to detect nutritive sugar. This sensor becomes operational during starvation; however, the mechanisms by which DH44 neurons or other nutrient sensors are regulated remain unclear. Here, we identified two satiety signals that inhibit DH44 neurons: (1) Piezo-mediated stomach/crop stretch after food ingestion and (2) Neuromedin/Hugin neurosecretory neurons in the ventral nerve cord (VNC) activated by an increase in the internal glucose level. A subset of Piezo⁺ neurons that express DH44 neuropeptide project to the crop. We found that DH44 neuronal activity and food intake were stimulated following a knockdown of piezo in DH44 neurons or silencing of Hugin neurons in the VNC, even in fed flies. Together, we propose that these two qualitatively distinct peripheral signals work in concert to regulate the DH44 nutrient sensor during the fed state.

Modeling the activity of the dopamine signaling pathway by combination of analog electrical circuit and mathematical approaches

This paper demonstrates the application of the system biology principles on the example of the dopamine signaling pathway in neurons. Presented model is based on two approaches - cytomorphic electronic circuits and mathematical modeling. Transcription and phosphorylation of DARPP-32 was modeled by analog circuit, based on well-known approaches presented in [1]. It was shown that application of circuit helps to receive signal oscillations that close to described ones in real biological systems. This combination on the one hand gives possibility to simplify calculations, on another to show this signaling pathway dynamics. The expected effect of changes in the functioning of calcium channels is considered, and the mathematical model of the interaction of system components is proposed. The average frequency of calcium current oscillations due to the presence of dopamine was 30 Hz in presented model, that is consistent with the literature, where the frequency of such oscillations is up to several tens of Hz. All presented results shows good correlation with known data, which already published today.

A CACNA1A variant associated with trigeminal neuralgia alters the gating of Cav2.1 channels

A novel missense mutation in the CACNA1A gene that encodes the pore forming α₁ subunit of the Ca_V2.1 voltage-gated calcium channel was identified in a patient with trigeminal neuralgia. This mutation leads to a substitution of proline 2455 by histidine (P2455H) in the distal C-terminus region of the channel. Due to the well characterized role of this channel in neurotransmitter release, our aim was to characterize the biophysical properties of the P2455H variant in heterologously expressed Ca_V2.1 channels. Whole-cell patch clamp recordings of wild type and mutant Ca_V2.1 channels expressed in tsA-201 cells reveal that the mutation mediates a depolarizing shift in the voltage-dependence of activation and inactivation. Moreover, the P2455H mutant strongly reduced calcium-dependent inactivation of the channel that is consistent with an overall gain of function. Hence, the P2455H Ca_V2.1 missense mutation alters the gating properties of the channel, suggesting that associated changes in Ca_V2.1-dependent synaptic communication in the trigeminal system may contribute to the development of trigeminal neuralgia.

Calcium-induced calcium release and type 3 ryanodine receptors modulate the slow afterhyperpolarising current, sIAHP, and its potentiation in hippocampal pyramidal neurons

The slow afterhyperpolarising current, sI_AHP, is a Ca²⁺-dependent current that plays an important role in the late phase of spike frequency adaptation. sI_AHP is activated by voltage-gated Ca²⁺ channels, while the contribution of calcium from ryanodine-sensitive intracellular stores, released by calcium-induced calcium release (CICR), is controversial in hippocampal pyramidal neurons. Three types of ryanodine receptors (RyR1-3) are expressed in the hippocampus, with RyR3 showing a predominant expression in CA1 neurons. We investigated the specific role of CICR, and particularly of its RyR3-mediated component, in the regulation of the sI_AHP amplitude and time course, and the activity-dependent potentiation of the sI_AHP in rat and mouse CA1 pyramidal neurons. Here we report that enhancement of CICR by caffeine led to an increase in sI_AHP amplitude, while inhibition of CICR by ryanodine caused a small, but significant reduction of sI_AHP. Inhibition of ryanodine-sensitive Ca²⁺ stores by ryanodine or depletion by the SERCA pump inhibitor cyclopiazonic acid caused a substantial attenuation in the sI_AHP activity-dependent potentiation in both rat and mouse CA1 pyramidal neurons. Neurons from mice lacking RyR3 receptors exhibited a sI_AHP with features undistinguishable from wild-type neurons, which was similarly reduced by ryanodine. However, the lack of RyR3 receptors led to a faster and reduced activity-dependent potentiation of sI_AHP. We conclude that ryanodine receptor-mediated CICR contributes both to the amplitude of the sI_AHP at steady state and its activity-dependent potentiation in rat and mouse hippocampal pyramidal neurons. In particular, we show that RyR3 receptors play an essential and specific role in shaping the activity-dependent potentiation of the sI_AHP. The modulation of activity-dependent potentiation of sI_AHP by RyR3-mediated CICR contributes to plasticity of intrinsic neuronal excitability and is likely to play a critical role in higher cognitive functions, such as learning and memory.

Neural Cell Adhesion Molecule 2 (NCAM2)-Induced c-Src-Dependent Propagation of Submembrane Ca2+ Spikes Along Dendrites Inhibits Synapse Maturation

The neural cell adhesion molecule 2 (NCAM2) is encoded by a gene on chromosome 21 in humans. NCAM2 accumulates in synapses, but its role in regulation of synapse formation remains poorly understood. We demonstrate that an increase in NCAM2 levels results in increased instability of dendritic protrusions and reduced conversion of protrusions to dendritic spines in mouse cortical neurons. NCAM2 overexpression induces an increase in the frequency of submembrane Ca2+ spikes localized in individual dendritic protrusions and promotes propagation of submembrane Ca2+ spikes over segments of dendrites or the whole dendritic tree. NCAM2-dependent submembrane Ca2+ spikes are L-type voltage-gated Ca2+ channel-dependent, and their propagation but not initiation depends on the c-Src protein tyrosine kinase. Inhibition of initiation or propagation of NCAM2-dependent submembrane Ca2+ spikes reduces the NCAM2-dependent instability of dendritic protrusions. Synaptic boutons formed on dendrites of neurons with elevated NCAM2 expression are enriched in the protein marker of immature synapses GAP43, and the number of boutons with mature activity-dependent synaptic vesicle recycling is reduced. Our results indicate that synapse maturation is inhibited in NCAM2-overexpressing neurons and suggest that changes in NCAM2 levels and altered submembrane Ca2+ dynamics can cause defects in synapse maturation in Down syndrome and other brain disorders associated with abnormal NCAM2 expression.

Harmful Iron-Calcium Relationship in Pantothenate kinase Associated Neurodegeneration

Pantothenate Kinase-associated Neurodegeneration (PKAN) belongs to a wide spectrum of diseases characterized by brain iron accumulation and extrapyramidal motor signs. PKAN is caused by mutations in PANK2, encoding the mitochondrial pantothenate kinase 2, which is the first enzyme of the biosynthesis of Coenzyme A. We established and characterized glutamatergic neurons starting from previously developed PKAN Induced Pluripotent Stem Cells (iPSCs). Results obtained by inductively coupled plasma mass spectrometry indicated a higher amount of total cellular iron in PKAN glutamatergic neurons with respect to controls. PKAN glutamatergic neurons, analyzed by electron microscopy, exhibited electron dense aggregates in mitochondria that were identified as granules containing calcium phosphate. Calcium homeostasis resulted compromised in neurons, as verified by monitoring the activity of calcium-dependent enzyme calpain1, calcium imaging and voltage dependent calcium currents. Notably, the presence of calcification in the internal globus pallidus was confirmed in seven out of 15 genetically defined PKAN patients for whom brain CT scan was available. Moreover, we observed a higher prevalence of brain calcification in females. Our data prove that high amount of iron coexists with an impairment of cytosolic calcium in PKAN glutamatergic neurons, indicating both, iron and calcium dys-homeostasis, as actors in pathogenesis of the disease.

Ca2+/calmodulin-dependent regulation of polycystic kidney disease 2-like-1 by binding at C-terminal domain

Polycystic kidney disease 2-like-1 (PKD2L1), also known as polycystin-L or TRPP3, is a non-selective cation channel that regulates intracellular calcium concentration. Calmodulin (CaM) is a calcium binding protein, consisting of N-lobe and C-lobe with two calcium binding EF-hands in each lobe. In previous study, we confirmed that CaM is associated with desensitization of PKD2L1 and that CaM N-lobe and PKD2L1 EF-hand specifically are involved. However, the CaM-binding domain (CaMBD) and its inhibitory mechanism of PKD2L1 have not been identified. In order to identify CaM-binding anchor residue of PKD2L1, single mutants of putative CaMBD and EF-hand deletion mutants were generated. The current changes of the mutants were recorded with whole-cell patch clamp. The calmidazolium (CMZ), a calmodulin inhibitor, was used under different concentrations of intracellular. Among the mutants that showed similar or higher basal currents with that of the PKD2L1 wild type, L593A showed little change in current induced by CMZ. Co-expression of L593A with CaM attenuated the inhibitory effect of PKD2L1 by CaM. In the previous study it was inferred that CaM C-lobe inhibits channels by binding to PKD2L1 at 16 nM calcium concentration and CaM N-lobe at 100 nM. Based on the results at 16 nM calcium concentration condition, this study suggests that CaM C-lobe binds to Leu-593, which can be a CaM C-lobe anchor residue, to regulate channel activity. Taken together, our results provide a model for the regulation of PKD2L1 channel activity by CaM.

Altered β-Cell Calcium Dynamics via Electric Field Exposure

Electric field stimulation has long been investigated with results supporting its therapeutic potential; however, its effects on insulin secreting cells has yet to be fully elucidated. Herein we explored the effects of physiological direct current (DC) electric field stimulation on the intracellular calcium dynamics of mouse derived βTC-6 insulinoma cells. This electrical stimulation resulted in an elevation in intracellular calcium along with a rise in calcium spiking activity. Further investigation indicated that the rise in intracellular calcium was mediated by an influx of calcium via L-type voltage gated calcium channels. Additionally, the effects of the electric field stimulation were able to induce insulin secretion in the absence of glucose stimulation. Given these results, DC electric field stimulation could be used as a non-invasive tool to modulate intracellular calcium dynamics and insulin secretion of β-cells for therapeutic application.

Spontaneous action potentials in retinal horizontal cells of goldfish (Carassius auratus) are dependent upon L-type Ca2+ channels and ryanodine receptors

Horizontal cells (HCs) are interneurons of the outer retina that undergo graded changes in membrane potential during the light response and provide feedback to photoreceptors. We characterized spontaneous Ca²⁺-based action potentials (APs) in isolated goldfish (Carassius auratus) HCs with electrophysiological and intracellular imaging techniques. Transient changes in intracellular Ca²⁺ concentration ([Ca²⁺]_i) were observed with fura-2 and were abolished by removal of extracellular Ca²⁺ or by inhibition of Ca²⁺ channels by 50 µM Cd²⁺ or 100 µM nifedipine. Inhibition of Ca²⁺ release from stores with 20 µM ryanodine or 50 µM dantrolene abolished Ca²⁺ transients and increased baseline [Ca²⁺]_i. This increased baseline was prevented by blocking L-type Ca²⁺ channels with nifedipine, suggesting that Ca²⁺-induced Ca²⁺ release from stores may be needed to inactivate membrane Ca²⁺ channels. Caffeine (3 mM) increased the frequency of Ca²⁺ transients, and the store-operated channel antagonist 2-aminoethyldiphenylborinate (100 μM) counteracted this effect. APs were detected with voltage-sensitive dye imaging (FluoVolt) and current-clamp electrophysiology. In current-clamp recordings, regenerative APs were abolished by removal of extracellular Ca²⁺ or in the presence of 5 mM Co²⁺ or 100 µM nifedipine, and APs were amplified with 15 mM Ba²⁺. Collectively, our data suggest that during APs Ca²⁺ enters through L-type Ca²⁺ channels and that Ca²⁺ stores (gated by ryanodine receptors) contribute to the rise in [Ca²⁺]_i. This work may lead to further understanding of the possible role APs have in vision, such as transitioning from light to darkness or modulating feedback from HCs to photoreceptors.NEW & NOTEWORTHY Horizontal cells (HCs) are interneurons of the outer retina that provide inhibitory feedback onto photoreceptors. HCs respond to light via graded changes in membrane potential. We characterized spontaneous action potentials in HCs from goldfish and linked action potential generation to a rise in intracellular Ca²⁺ via plasma membrane channels and ryanodine receptors. Action potentials may play a role in vision, such as transitioning from light to darkness, or in modulating feedback from HCs to photoreceptors.

Effects of inhibiting mTOR with rapamycin on behavior, development, neuromuscular physiology and cardiac function in larval Drosophila

Rapamycin and other mTOR inhibitors are being heralded as possible treatments for many human ailments. It is currently being utilized clinically as an immunomodulator after transplantation procedures and as a treatment for certain forms of cancer, but it has numerous potential clinical indications. Some studies have shown profound effects on life cycle and muscle physiology, but these issues have not been addressed in an organism undergoing developmental processes. This paper fills this void by examining the effect of mTOR inhibition by rapamycin on several different qualities of larval Drosophila. Various dosages of the compound were fed to second instar larvae. These larvae were monitored for pupae formation to elucidate possible life cycle effects, and a delay to pupation was quantified. Behavioral deficits were documented in rapamycin-treated larvae. Electrophysiological measurements were taken to discern changes in muscle physiology and synaptic signaling (i.e. resting membrane potential, amplitude of excitatory post-synaptic potentials, synaptic facilitation). Pupation delay and effects on behavior that are likely due to synaptic alterations within the central nervous system were discovered in rapamycin-fed larvae. These results allow for several conclusions as to how mTOR inhibition by rapamycin affects a developing organism. This could eventually allow for a more informed decision when using rapamycin and other mTOR inhibitors to treat human diseases, especially in children and adolescents, to account for known side effects.

Summary: Inhibiting mTOR by rapamycin delays pupation, reduced body wall contractions and mouth-hook movements while synaptic transmission appeared normal in larval Drosophila.

Degeneracy in hippocampal physiology and plasticity

Degeneracy, defined as the ability of structurally disparate elements to perform analogous function, has largely been assessed from the perspective of maintaining robustness of physiology or plasticity. How does the framework of degeneracy assimilate into an encoding system where the ability to change is an essential ingredient for storing new incoming information? Could degeneracy maintain the balance between the apparently contradictory goals of the need to change for encoding and the need to resist change towards maintaining homeostasis? In this review, we explore these fundamental questions with the mammalian hippocampus as an example encoding system. We systematically catalog lines of evidence, spanning multiple scales of analysis that point to the expression of degeneracy in hippocampal physiology and plasticity. We assess the potential of degeneracy as a framework to achieve the conjoint goals of encoding and homeostasis without cross-interferences. We postulate that biological complexity, involving interactions among the numerous parameters spanning different scales of analysis, could establish disparate routes towards accomplishing these conjoint goals. These disparate routes then provide several degrees of freedom to the encoding-homeostasis system in accomplishing its tasks in an input- and state-dependent manner. Finally, the expression of degeneracy spanning multiple scales offers an ideal reconciliation to several outstanding controversies, through the recognition that the seemingly contradictory disparate observations are merely alternate routes that the system might recruit towards accomplishment of its goals.

Emerging Roles of Neuronal Ca2+ Sensor-1 in Cardiac and Neuronal Tissues: A Mini Review

The EF-hand calcium (Ca²⁺)-binding protein, neuronal Ca²⁺ sensor-1 (NCS-1/frequenin), is predominantly expressed in neuronal tissues and plays a crucial role in neuronal functions, including synaptic transmission and plasticity. NCS-1 has diverse functional roles, as elucidated in the past 15 years, which include the regulation of phosphatidylinositol 4-kinase IIIβ (PI-4K-β) and several ion channels such as voltage-gated K⁺ and Ca²⁺ channels, the D2 dopamine receptors, and inositol 1,4,5-trisphosphate receptors (InsP₃Rs). Functional analyses demonstrated that NCS-1 enhances exocytosis and neuronal survival after injury, as well as promotes learning and memory in mice. NCS-1 is also expressed in the heart including the Purkinje fibers (PFs) of the conduction system. NCS-1 interacts with K_V4 K⁺ channels together with dipeptidyl peptidase-like protein-6 (DPP-6), and this macromolecule then composes the transient outward current in PFs and contributes to the repolarization of PF action potential, thus being responsible for idiopathic arrhythmia. Moreover, NCS-1 expression was reported to be significantly high at the immature stage and at hypertrophy in adults. That report demonstrated that NCS-1 positively regulates cardiac contraction in immature hearts by increasing intracellular Ca²⁺ signals through interaction with InsP₃Rs. With the related signals, NCS-1 activates nuclear Ca²⁺ signals, which would be a mechanism underlying hormone-induced cardiac hypertrophy. Furthermore, NCS-1 contributes to stress tolerance in cardiomyocytes by activating mitochondrial detoxification pathways, with a key role in Ca²⁺-dependent pathways. In this review, we will discuss recent findings supporting the functional significance of NCS-1 in the brain and heart and will address possible underlying molecular mechanisms.

Stores, Channels, Glue, and Trees: Active Glial and Active Dendritic Physiology

Glial cells and neuronal dendrites were historically assumed to be passive structures that play only supportive physiological roles, with no active contribution to information processing in the central nervous system. Research spanning the past few decades has clearly established this assumption to be far from physiological realities. Whereas the discovery of active channel conductances and their localized plasticity was the turning point for dendritic structures, the demonstration that glial cells release transmitter molecules and communicate across the neuroglia syncytium through calcium wave propagation constituted path-breaking discoveries for glial cell physiology. An additional commonality between these two structures is the ability of calcium stores within their endoplasmic reticulum (ER) to support active propagation of calcium waves, which play crucial roles in the spatiotemporal integration of information within and across cells. Although there have been several demonstrations of regulatory roles of glial cells and dendritic structures in achieving common physiological goals such as information propagation and adaptability through plasticity, studies assessing physiological interactions between these two active structures have been few and far. This lacuna is especially striking given the strong connectivity that is known to exist between these two structures through several complex and tightly intercoupled mechanisms that also recruit their respective ER structures. In this review, we present brief overviews of the parallel literatures on active dendrites and active glial physiology and make a strong case for future studies to directly assess the strong interactions between these two structures in regulating physiology and pathophysiology of the brain.

Ventral midbrain astrocytes display unique physiological features and sensitivity to dopamine D2 receptor signaling

Astrocytes are ubiquitous CNS cells that support tissue homeostasis through ion buffering, neurotransmitter recycling, and regulation of CNS vasculature. Yet, despite the essential functional roles they fill, very little is known about the physiology of astrocytes in the ventral midbrain, a region that houses dopamine-releasing neurons and is critical for reward learning and motivated behaviors. Here, using a combination of whole-transcriptome sequencing, histology, slice electrophysiology, and calcium imaging, we performed the first functional and molecular profiling of ventral midbrain astrocytes and observed numerous differences between these cells and their telencephalic counterparts, both in their gene expression profile and in their physiological properties. Ventral midbrain astrocytes have very low membrane resistance and inward-rectifying potassium channel-mediated current, and are extensively coupled to surrounding oligodendrocytes through gap junctions. They exhibit calcium responses to glutamate but are relatively insensitive to norepinephrine. In addition, their calcium activity can be dynamically modulated by dopamine D2 receptor signaling. Taken together, these data indicate that ventral midbrain astrocytes are physiologically distinct from astrocytes in cortex and hippocampus. This work provides new insights into the extent of functional astrocyte heterogeneity within the adult brain and establishes the foundation for examining the impact of regional astrocyte differences on dopamine neuron function and susceptibility to degeneration.

A mathematical and computational model of the calcium dynamics in Caenorhabditis elegans ASH sensory neuron

We propose a mathematical and computational model that captures the stimulus-generated Ca²⁺ transients in the C. elegans ASH sensory neuron. The rationale is to develop a tool that will enable a cross-talk between modeling and experiments, using modeling results to guide targeted experimental efforts. The model is built based on biophysical events and molecular cascades known to unfold as part of neurons' Ca²⁺ homeostasis mechanism, as well as on Ca²⁺ signaling events. The state of ion channels is described by their probability of being activated or inactivated, and the remaining molecular states are based on biochemically defined kinetic equations or known biochemical motifs. We estimate the parameters of the model using experimental data of hyperosmotic stimulus-evoked Ca²⁺ transients detected with a FRET sensor in young and aged worms, unstressed and exposed to oxidative stress. We use a hybrid optimization method composed of a multi-objective genetic algorithm and nonlinear least-squares to estimate the model parameters. We first obtain the model parameters for young unstressed worms. Next, we use these values of the parameters as a starting point to identify the model parameters for stressed and aged worms. We show that the model, in combination with experimental data, corroborates literature results. In addition, we demonstrate that our model can be used to predict ASH response to complex combinations of stimulation pulses. The proposed model includes for the first time the ASH Ca²⁺ dynamics observed during both "on" and "off" responses. This mathematical and computational effort is the first to propose a dynamic model of the Ca²⁺ transients' mechanism in C. elegans neurons, based on biochemical pathways of the cell's Ca²⁺ homeostasis machinery. We believe that the proposed model can be used to further elucidate the Ca²⁺ dynamics of a key C. elegans neuron, to guide future experiments on C. elegans neurobiology, and to pave the way for the development of more mathematical models for neuronal Ca²⁺ dynamics.

A Friend or Foe: Calcineurin across the Gamut of Neurological Disorders

The serine/threonine phosphatase calcineurin (CaN) is a unique but confounding calcium/calmodulin-mediated enzyme. CaN has shown to play essential roles from regulating calcium homeostasis to being an intricate part of learning and memory formation. Neurological disorders, despite differing in their etiology, share similar pathological outcomes, such as mitochondrial dysfunction and apoptotic signaling brought about by excitotoxic elements. CaN, being deeply integrated in vital neuronal functions, may be implicated in various neurological disorders. Understanding the enzyme and its physiological niche in the nervous system is vital in uncovering its roles in the spectrum of brain disorders. By reviewing the crosstalk in different neurological pathologies, a possible grasp of CaN's complex signaling may lead to forming better neurotherapy. This Outlook attempts to explore the various neuronal functions of CaN and investigate its pervasive role through the gamut of neurological disorders.

A Sialylated Voltage-Dependent Ca2+ Channel Binds Hemagglutinin and Mediates Influenza A Virus Entry into Mammalian Cells

Influenza A virus (IAV) infection is initiated by the attachment of the viral glycoprotein hemagglutinin (HA) to sialic acid on the host cell surface. However, the sialic acid-containing receptor crucial for IAV infection has remained unidentified. Here, we show that HA binds to the voltage-dependent Ca²⁺ channel Ca_v1.2 to trigger intracellular Ca²⁺ oscillations and subsequent IAV entry and replication. IAV entry was inhibited by Ca²⁺ channel blockers (CCBs) or by knockdown of Ca_v1.2. The CCB diltiazem also inhibited virus replication in vivo. Reintroduction of wild-type but not the glycosylation-deficient mutants of Ca_v1.2 restored Ca²⁺ oscillations and virus infection in Ca_v1.2-depleted cells, demonstrating the significance of Ca_v1.2 sialylation. Taken together, we identify Ca_v1.2 as a sialylated host cell surface receptor that binds HA and is critical for IAV entry.

The CaV2α1 EF-hand F helix tyrosine, a highly conserved locus for GPCR inhibition of CaV2 channels

The sensory neuron of Aplysia californica participates in several forms of presynaptic plasticity including homosynaptic depression, heterosynaptic depression, facilitation and the reversal of depression. The calcium channel triggering neurotransmitter release at most synapses is Ca_V2, consisting of the pore forming α1 subunit (Ca_V2α1), and auxiliary Ca_Vβ, and Ca_Vα2δ subunits. To determine the role of the Ca_V2 channel in presynaptic plasticity in Aplysia, we cloned Aplysia Ca_V2α1, Ca_Vβ, and Ca_Vα2δ and over-expressed the proteins in Aplysia sensory neurons (SN). We show expression of exogenous Ca_V2α1 in the neurites of cultured Aplysia SN. One proposed mechanism for heterosynaptic depression in Aplysia is through inhibition of Ca_V2. Here, we demonstrate that heterosynaptic depression of the Ca_V2 calcium current is inhibited when a channel with a Y-F mutation at the conserved Src phosphorylation site is expressed, showing the strong conservation of this mechanism over evolution. We also show that the Y-F mutation reduces heterosynaptic inhibition of neurotransmitter release, highlighting the physiological importance of this mechanism for the regulation of synaptic efficacy. These results also demonstrate our ability to replace endogenous Ca_V2 channels with recombinant channels allowing future examination of the structure function relationship of Ca_V2 in the regulation of transmitter release in this system.

Calmodulin regulates Cav3 T-type channels at their gating brake

Calcium (Cav1 and Cav2) and sodium channels possess homologous CaM-binding motifs, known as IQ motifs in their C termini, which associate with calmodulin (CaM), a universal calcium sensor. Cav3 T-type channels, which serve as pacemakers of the mammalian brain and heart, lack a C-terminal IQ motif. We illustrate that T-type channels associate with CaM using co-immunoprecipitation experiments and single particle cryo-electron microscopy. We demonstrate that protostome invertebrate (LCav3) and human Cav3.1, Cav3.2, and Cav3.3 T-type channels specifically associate with CaM at helix 2 of the gating brake in the I-II linker of the channels. Isothermal titration calorimetry results revealed that the gating brake and CaM bind each other with high-nanomolar affinity. We show that the gating brake assumes a helical conformation upon binding CaM, with associated conformational changes to both CaM lobes as indicated by amide chemical shifts of the amino acids of CaM in 1H-15N HSQC NMR spectra. Intact Ca2+-binding sites on CaM and an intact gating brake sequence (first 39 amino acids of the I-II linker) were required in Cav3.2 channels to prevent the runaway gating phenotype, a hyperpolarizing shift in voltage sensitivities and faster gating kinetics. We conclude that the presence of high-nanomolar affinity binding sites for CaM at its universal gating brake and its unique form of regulation via the tuning of the voltage range of activity could influence the participation of Cav3 T-type channels in heart and brain rhythms. Our findings may have implications for arrhythmia disorders arising from mutations in the gating brake or CaM.

Syringaresinol suppresses excitatory synaptic transmission and picrotoxin-induced epileptic activity in the hippocampus through presynaptic mechanisms

Many neuromodulating drugs acting on the nervous system originate from botanical sources. These plant-derived substances modulate the activity of receptors, ion channels, or transporters in neurons. Their properties make the substances useful for medicine and research. Here, we show that the plant lignan (+)-syringaresinol (SYR) suppresses excitatory synaptic transmission via presynaptic modulation. Bath application of SYR rapidly reduced the slopes of the field excitatory postsynaptic potentials (fEPSPs) at the hippocampal Schaffer collateral (SC)-CA1 synapse in a dose-dependent manner. SYR preferentially affected excitatory synapses, while inhibitory synaptic transmission remained unchanged. SYR had no effect on the conductance or the desensitization of AMPARs but increased the paired-pulse ratios of synaptic responses at short (20-200 ms) inter-stimulus intervals. These presynaptic changes were accompanied by a reduction of the readily releasable pool size. Pretreatment of hippocampal slices with the G_i/o protein inhibitor N-ethylmaleimide (NEM) abolished the effect of SYR on excitatory synaptic transmission, while the application of SYR significantly decreased Ca²⁺ currents and hyperpolarized the resting membrane potentials of hippocampal neurons. In addition, SYR suppressed picrotoxin-induced epileptiform activity in hippocampal slices. Overall, our study identifies SYR as a new neuromodulating agent and suggests that SYR suppresses excitatory synaptic transmission by modulating presynaptic transmitter release.

Direct visualization of interaction between calmodulin and connexin45

Calmodulin (CaM) is an intracellular Ca²⁺ transducer involved in numerous activities in a broad Ca²⁺ signaling network. Previous studies have suggested that the Ca²⁺/CaM complex may participate in gap junction regulation via interaction with putative CaM-binding motifs in connexins; however, evidence of direct interactions between CaM and connexins has remained elusive to date due to challenges related to the study of membrane proteins. Here, we report the first direct interaction of CaM with Cx45 (connexin45) of γ-family in living cells under physiological conditions by monitoring bioluminescence resonance energy transfer. The interaction between CaM and Cx45 in cells is strongly dependent on intracellular Ca²⁺ concentration and can be blocked by the CaM inhibitor, N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide hydrochloride (W7). We further reveal a CaM-binding site at the cytosolic loop (residues 164-186) of Cx45 using a peptide model. The strong binding (K_d ∼ 5 nM) observed between CaM and Cx45 peptide, monitored by fluorescence-labeled CaM, is found to be Ca²⁺-dependent. Furthermore, high-resolution nuclear magnetic resonance spectroscopy reveals that CaM and Cx45 peptide binding leads to global chemical shift changes of ¹⁵N-labeled CaM, but does not alter the size of the structure. Observations involving both N- and C-domains of CaM to interact with the Cx45 peptide differ from the embraced interaction with Cx50 from another connexin family. Such interaction further increases Ca²⁺ sensitivity of CaM, especially at the N-terminal domain. Results of the present study suggest that both helicity and the interaction mode of the cytosolic loop are likely to contribute to CaM's modulation of connexins.

Allele-specific ablation rescues electrophysiological abnormalities in a human iPS cell model of long-QT syndrome with a CALM2 mutation

Calmodulin is a ubiquitous Ca2+ sensor molecule encoded by three distinct calmodulin genes, CALM1-3. Recently, mutations in CALM1-3 have been reported to be associated with severe early-onset long-QT syndrome (LQTS). However, the underlying mechanism through which heterozygous calmodulin mutations lead to severe LQTS remains unknown, particularly in human cardiomyocytes. We aimed to establish an LQTS disease model associated with a CALM2 mutation (LQT15) using human induced pluripotent stem cells (hiPSCs) and to assess mutant allele-specific ablation by genome editing for the treatment of LQT15. We generated LQT15-hiPSCs from a 12-year-old boy with LQTS carrying a CALM2-N98S mutation and differentiated these hiPSCs into cardiomyocytes (LQT15-hiPSC-CMs). Action potentials (APs) and L-type Ca2+ channel (LTCC) currents in hiPSC-CMs were analyzed by the patch-clamp technique and compared with those of healthy controls. Furthermore, we performed mutant allele-specific knockout using a CRISPR-Cas9 system and analyzed electrophysiological properties. Electrophysiological analyses revealed that LQT15-hiPSC-CMs exhibited significantly lower beating rates, prolonged AP durations, and impaired inactivation of LTCC currents compared with control cells, consistent with clinical phenotypes. Notably, ablation of the mutant allele rescued the electrophysiological abnormalities of LQT15-hiPSC-CMs, indicating that the mutant allele caused dominant-negative suppression of LTCC inactivation, resulting in prolonged AP duration. We successfully recapitulated the disease phenotypes of LQT15 and revealed that inactivation of LTCC currents was impaired in CALM2-N98S hiPSC model. Additionally, allele-specific ablation using the latest genome-editing technology provided important insights into a promising therapeutic approach for inherited cardiac diseases.

The Potential Role of Gap Junctional Plasticity in the Regulation of State

Electrical synapses are the functional correlate of gap junctions and allow transmission of small molecules and electrical current between coupled neurons. Instead of static pores, electrical synapses are actually plastic, similar to chemical synapses. In the thalamocortical system, gap junctions couple inhibitory neurons that are similar in their biochemical profile, morphology, and electrophysiological properties. We postulate that electrical synaptic plasticity among inhibitory neurons directly interacts with the switching between different firing patterns in a state-dependent and type-dependent manner. In neuronal networks, electrical synapses may function as a modifiable resonance feedback system that enables stable oscillations. Furthermore, the plasticity of electrical synapses may play an important role in regulation of state, synchrony, and rhythmogenesis in the mammalian thalamocortical system, similar to chemical synaptic plasticity. Based on their plasticity, rich diversity, and specificity, electrical synapses are thus likely to participate in the control of consciousness and attention.

Ca2+ Regulation of Cav3.3 T-type Ca2+ Channel Is Mediated by Calmodulin

Calcium-dependent inactivation of high voltage-activated Ca2+ channels plays a crucial role in limiting rises in intracellular calcium (Ca2+i). A key mediator of these effects is calmodulin, which has been found to bind the C-terminus of the pore-forming α subunit. In contrast, little is known about how Ca2+i can regulate low voltage-activated T-type Ca2+ channels. Using whole cell patch clamp, we examined the biophysical properties of Ca2+ current through the three T-type Ca2+ channel isoforms, Cav3.1, Cav3.2, or Cav3.3, comparing internal solutions containing 27 nM and l μM free Ca2+ Both activation and inactivation kinetics of Cav3.3 current in l μM Ca2+i solution were more rapid than those in 27 nM Ca2+i solution. In addition, both activation and steady-state inactivation curves of Cav3.3 were negatively shifted in the higher Ca2+i solution. In contrast, the biophysical properties of Cav3.1 and Cav3.2 isoforms were not significantly different between the two internal solutions. Overexpression of CaM1234 (a calmodulin mutant that doesn't bind Ca2+) occluded the effects of l μM Ca2+i on Cav3.3, implying that CaM is involved in the Ca2+i regulation effects on Cav3.3. Yeast two-hybrid screening and co-immunoprecipitation experiments revealed a direct interaction of CaM with the carboxyl terminus of Cav3.3. Taken together, our results suggest that Cav3.3 T-type channel is potently regulated by Ca2+i via interaction of Ca2+/CaM with the carboxyl terminus of Cav3.3.

Human induced pluripotent stem cells for modelling neurodevelopmental disorders

We currently have a poor understanding of the pathogenesis of neurodevelopmental disorders, owing to the fact that postmortem and imaging studies can only measure the postnatal status quo and offer little insight into the processes that give rise to the observed outcomes. Human induced pluripotent stem cells (hiPSCs) should, in principle, prove powerful for elucidating the pathways that give rise to neurodevelopmental disorders. hiPSCs are embryonic-stem-cell-like cells that can be derived from somatic cells. They retain the unique genetic signature of the individual from whom they were derived, and thus enable researchers to recapitulate that individual's idiosyncratic neural development in a dish. In the case of individuals with disease, we can re-enact the disease-altered trajectory of brain development and examine how and why phenotypic and molecular abnormalities arise in these diseased brains. Here, we review hiPSC biology and possible experimental designs when using hiPSCs to model disease. We then discuss existing hiPSC models of neurodevelopmental disorders. Our hope is that, as some studies have already shown, hiPSCs will illuminate the pathophysiology of developmental disorders of the CNS and lead to therapeutic options for the millions that are affected by these conditions.

Ionic currents influencing spontaneous firing and pacemaker frequency in dopamine neurons of the ventrolateral periaqueductal gray and dorsal raphe nucleus (vlPAG/DRN): A voltage-clamp and computational modelling study

Dopamine (DA) neurons of the ventrolateral periaqueductal gray (vlPAG) and dorsal raphe nucleus (DRN) fire spontaneous action potentials (APs) at slow, regular patterns in vitro but a detailed account of their intrinsic membrane properties responsible for spontaneous firing is currently lacking. To resolve this, we performed a voltage-clamp electrophysiological study in brain slices to describe their major ionic currents and then constructed a computer model and used simulations to understand the mechanisms behind autorhythmicity in silico. We found that vlPAG/DRN DA neurons exhibit a number of voltage-dependent currents activating in the subthreshold range including, a hyperpolarization-activated cation current (I_H), a transient, A-type, potassium current (I_A), a background, ‘persistent’ (I_NaP) sodium current and a transient, low voltage activated (LVA) calcium current (I_CaLVA). Brain slice pharmacology, in good agreement with computer simulations, showed that spontaneous firing occurred independently of I_H, I_A or calcium currents. In contrast, when blocking sodium currents, spontaneous firing ceased and a stable, non-oscillating membrane potential below AP threshold was attained. Using the DA neuron model we further show that calcium currents exhibit little activation (compared to sodium) during the interspike interval (ISI) repolarization while, any individual potassium current alone, whose blockade positively modulated AP firing frequency, is not required for spontaneous firing. Instead, blockade of a number of potassium currents simultaneously is necessary to eliminate autorhythmicity. Repolarization during ISI is mediated initially via the deactivation of the delayed rectifier potassium current, while a sodium background ‘persistent’ current is essentially indispensable for autorhythmicity by driving repolarization towards AP threshold.

Molecular Basis for Modulation of Metabotropic Glutamate Receptors and Their Drug Actions by Extracellular Ca2

Metabotropic glutamate receptors (mGluRs) associated with the slow phase of the glutamatergic signaling pathway in neurons of the central nervous system have gained importance as drug targets for chronic neurodegenerative diseases. While extracellular Ca²⁺ was reported to exhibit direct activation and modulation via an allosteric site, the identification of those binding sites was challenged by weak binding. Herein, we review the discovery of extracellular Ca²⁺ in regulation of mGluRs, summarize the recent developments in probing Ca²⁺ binding and its co-regulation of the receptor based on structural and biochemical analysis, and discuss the molecular basis for Ca²⁺ to regulate various classes of drug action as well as its importance as an allosteric modulator in mGluRs.

Degeneracy in the regulation of short-term plasticity and synaptic filtering by presynaptic mechanisms

KEY POINTS

We develop a new biophysically rooted, physiologically constrained conductance-based synaptic model to mechanistically account for short-term facilitation and depression, respectively through residual calcium and transmitter depletion kinetics. We address the specific question of how presynaptic components (including voltage-gated ion channels, pumps, buffers and release-handling mechanisms) and interactions among them define synaptic filtering and short-term plasticity profiles. Employing global sensitivity analyses (GSAs), we show that near-identical synaptic filters and short-term plasticity profiles could emerge from disparate presynaptic parametric combinations with weak pairwise correlations. Using virtual knockout models, a technique to address the question of channel-specific contributions within the GSA framework, we unveil the differential and variable impact of each ion channel on synaptic physiology. Our conclusions strengthen the argument that parametric and interactional complexity in biological systems should not be viewed from the limited curse-of-dimensionality standpoint, but from the evolutionarily advantageous perspective of providing functional robustness through degeneracy.

ABSTRACT

Information processing in neurons is known to emerge as a gestalt of pre- and post-synaptic filtering. However, the impact of presynaptic mechanisms on synaptic filters has not been quantitatively assessed. Here, we developed a biophysically rooted, conductance-based model synapse that was endowed with six different voltage-gated ion channels, calcium pumps, calcium buffer and neurotransmitter-replenishment mechanisms in the presynaptic terminal. We tuned our model to match the short-term plasticity profile and band-pass structure of Schaffer collateral synapses, and performed sensitivity analyses to demonstrate that presynaptic voltage-gated ion channels regulated synaptic filters through changes in excitability and associated calcium influx. These sensitivity analyses also revealed that calcium- and release-control mechanisms were effective regulators of synaptic filters, but accomplished this without changes in terminal excitability or calcium influx. Next, to perform global sensitivity analysis, we generated 7000 randomized models spanning 15 presynaptic parameters, and computed eight different physiological measurements in each of these models. We validated these models by applying experimentally obtained bounds on their measurements, and found 104 (∼1.5%) models to match the validation criteria for all eight measurements. Analysing these valid models, we demonstrate that analogous synaptic filters emerge from disparate combinations of presynaptic parameters exhibiting weak pairwise correlations. Finally, using virtual knockout models, we establish the variable and differential impact of different presynaptic channels on synaptic filters, underlining the critical importance of interactions among different presynaptic components in defining synaptic physiology. Our results have significant implications for protein-localization strategies required for physiological robustness and for degeneracy in long-term synaptic plasticity profiles.

Cooperative and acute inhibition by multiple C-terminal motifs of L-type Ca2+ channels

Inhibitions and antagonists of L-type Ca²⁺ channels are important to both research and therapeutics. Here, we report C-terminus mediated inhibition (CMI) for Ca_V1.3 that multiple motifs coordinate to tune down Ca²⁺ current and Ca²⁺ influx toward the lower limits determined by end-stage CDI (Ca²⁺-dependent inactivation). Among IQ_V (preIQ₃-IQ domain), PCRD and DCRD (proximal or distal C-terminal regulatory domain), spatial closeness of any two modules, e.g., by constitutive fusion, facilitates the trio to form the complex, compete against calmodulin, and alter the gating. Acute CMI by rapamycin-inducible heterodimerization helps reconcile the concurrent activation/inactivation attenuations to ensure Ca²⁺ influx is reduced, in that Ca²⁺ current activated by depolarization is potently (~65%) inhibited at the peak (full activation), but not later on (end-stage inactivation, ~300 ms). Meanwhile, CMI provides a new paradigm to develop Ca_V1 inhibitors, the therapeutic potential of which is implied by computational modeling of Ca_V1.3 dysregulations related to Parkinson’s disease.

DOI:http://dx.doi.org/10.7554/eLife.21989.001

All cells need calcium ions to stay healthy, but having too many calcium ions can interfere with important processes in the cell and cause severe problems. Proteins known as calcium channels on the cell surface allow calcium ions to flow into the cell from the surrounding environment. Cells carefully control the opening and closing of these channels to prevent too many calcium ions entering the cell at once. Ca_V1.3 channels are a type of calcium channel that are important for the heart and brain to work properly. Defects in Ca_V1.3 channels can lead to irregular heart rhythms and neurodegenerative diseases such as Parkinson’s disease.

Studies have shown that part of the Ca_V1.3 channel that sits inside the cell – known as the “tail” – responds to increases in the levels of calcium ions inside the cell by closing the channel. The tail region of Ca_V1.3 contains three modules, but how these modules work together to regulate channel activity is not clear.

Liu, Yang et al. investigated whether the three modules need to be physically connected to each other in the channel protein. For the experiments, several versions of the protein were constructed with different combinations of tail modules being directly linked as part of the same molecule or present as separate molecules. When any two modules were directly linked, the third module could bind to them and this was enough to close the Ca_V1.3 channel. However, the channel did not close if the modules were totally isolated from each other as three separate molecules.

Certain types of neurons in the brain produce electrical signals in a rhythmic fashion that depends on Ca_V1.3 channels. In Parkinson’s disease, increased movement of calcium ions into these neurons via Ca_V1.3 channels interferes with the rhythms of the signals and can cause these cells to die. Liu, Yang et al. performed computer simulations to analyse the effects of closing Ca_V1.3 channels in these neurons. The results suggest that this can restore normal rhythms of electrical activity and prevent these cells from dying.

The next step is to understand the molecular details of how the tail region closes Ca_V1.3 channels and its role in healthy and diseased cells. This may lead to new ways to block Ca_V1.3 channels in different types of diseases.

DOI:http://dx.doi.org/10.7554/eLife.21989.002

Decrease of high voltage Ca2+ currents in the dentate gyrus granule cells by entorhinal amyloidopathy is reversed by calcium channel blockade

In the Alzheimer's disease (AD), entorhinal-hippocampal circuit is one of the earliest affected networks. There are some evidences indicating abnormal neuronal excitability and impaired synaptic plasticity in the dentate gyrus (DG) of AD animal model. However, the underlying mechanism leading to DG dysfunction particularly in the early phase of AD is not known. Since calcium dyshomeostasis has a critical role in the etiology of AD, it is possible that this phenomenon precedes electrophysiological alteration in the DG. Here, the effect of the amyloid pathogenesis in the entorhinal cortex (EC) on high activated Ca²⁺ currents in the DG granule cells was investigated. One week after bilaterally injection of amyloid beta (Aβ) 1-42 into the EC, Ca²⁺ currents in the DG granule cells were assessed by whole cell patch clamp. Voltage clamp recording showed the amplitude of high voltage calcium currents in the DG granule cells was decreased following EC amyloidopathy. However, the Ca²⁺ current decay was slower than control. Double-pulse recording revealed that Ca²⁺-dependent inactivation of calcium current (CDI) was more pronounced in the EC-Aβ group compared to the control group. However, chronic treatment by calcium channel blocker (CCBs), isradipine or nimodipine, reverse the Ca²⁺ currents toward the control level. On the other hand, there was no significant difference in the calbindin level in the DG of different groups. In conclusion, our results suggest that Aβ in the EC independent of calbindin level triggers a decreased Ca²⁺ currents along with increased CDI in the DG granule cells which may lead to further electrophysiological alterations in these cells, and treatment by CCBs could preserve normal calcium current and may ultimately normal function against the Aβ toxicity.

Biophysical characterization of the honeybee DSC1 orthologue reveals a novel voltage-dependent Ca2+ channel subfamily: CaV4

Insect DSC1 channels have sequences that are intermediate between voltage-gated Na⁺ and Ca²⁺ channels but have hitherto been classified as the former. Gosselin-Badaroudine et al. clone and characterize honeybee DSC1, revealing high selectivity for Ca²⁺ and suggesting reclassification of DSC1 homologues as Ca²⁺ channels.

Bilaterian voltage-gated Na⁺ channels (Na_V) evolved from voltage-gated Ca²⁺ channels (Ca_V). The Drosophila melanogaster Na⁺ channel 1 (DSC1), which features a D-E-E-A selectivity filter sequence that is intermediate between Ca_V and Na_V channels, is evidence of this evolution. Phylogenetic analysis has classified DSC1 as a Ca²⁺-permeable Na⁺ channel belonging to the Na_V2 family because of its sequence similarity with Na_V channels. This is despite insect Na_V2 channels (DSC1 and its orthologue in Blatella germanica, BSC1) being more permeable to Ca²⁺ than Na⁺. In this study, we report the cloning and molecular characterization of the honeybee (Apis mellifera) DSC1 orthologue. We reveal several sequence variations caused by alternative splicing, RNA editing, and genomic variations. Using the Xenopus oocyte heterologous expression system and the two-microelectrode voltage-clamp technique, we find that the channel exhibits slow activation and inactivation kinetics, insensitivity to tetrodotoxin, and block by Cd²⁺ and Zn²⁺. These characteristics are reminiscent of Ca_V channels. We also show a strong selectivity for Ca²⁺ and Ba²⁺ ions, marginal permeability to Li⁺, and impermeability to Mg²⁺ and Na⁺ ions. Based on current ion channel nomenclature, the D-E-E-A selectivity filter, and the properties we have uncovered, we propose that DSC1 homologues should be classified as Ca_V4 rather than Na_V2. Indeed, channels that contain the D-E-E-A selectivity sequence are likely to feature the same properties as the honeybee’s channel, namely slow activation and inactivation kinetics and strong selectivity for Ca²⁺ ions.

Towards a Unified Theory of Calmodulin Regulation (Calmodulation) of Voltage-Gated Calcium and Sodium Channels

Voltage-gated Na and Ca(2+) channels represent two major ion channel families that enable myriad biological functions including the generation of action potentials and the coupling of electrical and chemical signaling in cells. Calmodulin regulation (calmodulation) of these ion channels comprises a vital feedback mechanism with distinct physiological implications. Though long-sought, a shared understanding of the channel families remained elusive for two decades as the functional manifestations and the structural underpinnings of this modulation often appeared to diverge. Here, we review recent advancements in the understanding of calmodulation of Ca(2+) and Na channels that suggest a remarkable similarity in their regulatory scheme. This interrelation between the two channel families now paves the way towards a unified mechanistic framework to understand vital calmodulin-dependent feedback and offers shared principles to approach related channelopathic diseases. An exciting era of synergistic study now looms.

Ca2+ controls gating of voltage-gated calcium channels by releasing the β2e subunit from the plasma membrane

Voltage-gated calcium (Cav) channels, which are regulated by membrane potential, cytosolic Ca(2+), phosphorylation, and membrane phospholipids, govern Ca(2+) entry into excitable cells. Cav channels contain a pore-forming α1 subunit, an auxiliary α2δ subunit, and a regulatory β subunit, each encoded by several genes in mammals. In addition to a domain that interacts with the α1 subunit, β2e and β2a also interact with the cytoplasmic face of the plasma membrane through an electrostatic interaction for β2e and posttranslational acylation for β2a. We found that an increase in cytosolic Ca(2+) promoted the release of β2e from the membrane without requiring substantial depletion of the anionic phospholipid phosphatidylinositol 4,5-bisphosphate (PIP2) from the plasma membrane. Experiments with liposomes indicated that Ca(2+) disrupted the interaction of the β2e amino-terminal peptide with membranes containing PIP2 Ca(2+) binding to calmodulin (CaM) leads to CaM-mediated inactivation of Cav currents. Although Cav2.2 coexpressed with β2a required Ca(2+)-dependent activation of CaM for Ca(2+)-mediated reduction in channel activity, Cav2.2 coexpressed with β2e exhibited Ca(2+)-dependent inactivation of the channel even in the presence of Ca(2+)-insensitive CaM. Inducible depletion of PIP2 reduced Cav2.2 currents, and in cells coexpressing β2e, but not a form that lacks the polybasic region, increased intracellular Ca(2+) further reduced Cav2.2 currents. Many hormone- or neurotransmitter-activated receptors stimulate PIP2 hydrolysis and increase cytosolic Ca(2+); thus, our findings suggest that β2e may integrate such receptor-mediated signals to limit Cav activity.

Population Density and Moment-based Approaches to Modeling Domain Calcium-mediated Inactivation of L-type Calcium Channels

We present a population density and moment-based description of the stochastic dynamics of domain [Formula: see text]-mediated inactivation of L-type [Formula: see text] channels. Our approach accounts for the effect of heterogeneity of local [Formula: see text] signals on whole cell [Formula: see text] currents; however, in contrast with prior work, e.g., Sherman et al. (Biophys J 58(4):985-995, 1990), we do not assume that [Formula: see text] domain formation and collapse are fast compared to channel gating. We demonstrate the population density and moment-based modeling approaches using a 12-state Markov chain model of an L-type [Formula: see text] channel introduced by Greenstein and Winslow (Biophys J 83(6):2918-2945, 2002). Simulated whole cell voltage clamp responses yield an inactivation function for the whole cell [Formula: see text] current that agrees with the traditional approach when domain dynamics are fast. We analyze the voltage-dependence of [Formula: see text] inactivation that may occur via slow heterogeneous domain [[Formula: see text]]. Next, we find that when channel permeability is held constant, [Formula: see text]-mediated inactivation of L-type channels increases as the domain time constant increases, because a slow domain collapse rate leads to increased mean domain [[Formula: see text]] near open channels; conversely, when the maximum domain [[Formula: see text]] is held constant, inactivation decreases as the domain time constant increases. Comparison of simulation results using population densities and moment equations confirms the computational efficiency of the moment-based approach, and enables the validation of two distinct methods of truncating and closing the open system of moment equations. In general, a slow domain time constant requires higher order moment truncation for agreement between moment-based and population density simulations.

Impaired Inactivation of L-Type Ca2+ Current as a Potential Mechanism for Variable Arrhythmogenic Liability of HERG K+ Channel Blocking Drugs

The proarrhythmic effects of new drugs have been assessed by measuring rapidly activating delayed-rectifier K⁺ current (I_Kr) antagonist potency. However, recent data suggest that even drugs thought to be highly specific I_Kr blockers can be arrhythmogenic via a separate, time-dependent pathway such as late Na⁺ current augmentation. Here, we report a mechanism for a quinolone antibiotic, sparfloxacin-induced action potential duration (APD) prolongation that involves increase in late L-type Ca²⁺ current (I_CaL) caused by a decrease in Ca²⁺-dependent inactivation (CDI). Acute exposure to sparfloxacin, an I_Kr blocker with prolongation of QT interval and torsades de pointes (TdP) produced a significant APD prolongation in rat ventricular myocytes, which lack I_Kr due to E4031 pretreatment. Sparfloxacin reduced peak I_CaL but increased late I_CaL by slowing its inactivation. In contrast, ketoconazole, an I_Kr blocker without prolongation of QT interval and TdP produced reduction of both peak and late I_CaL, suggesting the role of increased late I_CaL in arrhythmogenic effect. Further analysis showed that sparfloxacin reduced CDI. Consistently, replacement of extracellular Ca²⁺ with Ba²⁺ abolished the sparfloxacin effects on I_CaL. In addition, sparfloxacin modulated I_CaL in a use-dependent manner. Cardiomyocytes from adult mouse, which is lack of native I_Kr, demonstrated similar increase in late I_CaL and afterdepolarizations. The present findings show that sparfloxacin can prolong APD by augmenting late I_CaL. Thus, drugs that cause delayed I_CaL inactivation and I_Kr blockage may have more adverse effects than those that selectively block I_Kr. This mechanism may explain the reason for discrepancies between clinically reported proarrhythmic effects and I_Kr antagonist potencies.

Optimization of a High-Throughput Assay for Calcium Channel Modulators on IonWorks Barracuda

Voltage-gated calcium channels represent important drug targets. The implementation of higher throughput electrophysiology assays is necessary to characterize the interaction of test compounds with several conformational states of the channel, but has presented significant challenges. We report on the development of a high-throughput, automated electrophysiology assay for Cav2.2 on the IonWorks Barracuda™ platform. The assay provides an assessment of the potency of the test compound on the resting/closed and inactivated states of the channel in the same assay run. Inclusion of the heavy metal chelator 2,3-bis(sulfanyl)propane-1-sulfonate in the assay solutions improved the data quality by reversing a loss of current seen in wells directly above the ground electrodes. We hypothesize that the loss of current is caused by block of Cav2.2 currents by silver ions originating from the electrodes.