Molecular mechanism of calcium channel regulation in the fight-or-flight response

Role of the CaV1.2 distal carboxy terminus in the regulation of L-type current

L-type calcium channels are essential for the excitation-contraction coupling in cardiac muscle. The CaV1.2 channel is the most predominant isoform in the ventricle which consists of a multi-subunit membrane complex that includes the CaV1.2 pore-forming subunit and auxiliary subunits like CaVα2δ and CaVβ2b. The CaV1.2 channel's C-terminus undergoes proteolytic cleavage, and the distal C-terminal domain (DCtermD) associates with the channel core through two domains known as proximal and distal C-terminal regulatory domain (PCRD and DCRD, respectively). The interaction between the DCtermD and the remaining C-terminus reduces the channel activity and modifies voltage- and calcium-dependent inactivation mechanisms, leading to an autoinhibitory effect. In this study, we investigate how the interaction between DCRD and PCRD affects the inactivation processes and CaV1.2 activity. We expressed a 14-amino acid peptide miming the DCRD-PCRD interaction sequence in both heterologous systems and cardiomyocytes. Our results show that overexpression of this small peptide can displace the DCtermD and replicate the effects of the entire DCtermD on voltage-dependent inactivation and channel inhibition. However, the effect on calcium-dependent inactivation requires the full DCtermD and is prevented by overexpression of calmodulin. In conclusion, our results suggest that the interaction between DCRD and PCRD is sufficient to bring about the current inhibition and alter the voltage-dependent inactivation, possibly in an allosteric manner. Additionally, our data suggest that the DCtermD competitively modifies the calcium-dependent mechanism. The identified peptide sequence provides a valuable tool for further dissecting the molecular mechanisms that regulate L-type calcium channels' basal activity in cardiomyocytes.

Tripartite interactions of PKA catalytic subunit and C-terminal domains of cardiac Ca2+ channel may modulate its β-adrenergic regulation

BACKGROUND

The β-adrenergic augmentation of cardiac contraction, by increasing the conductivity of L-type voltage-gated CaV1.2 channels, is of great physiological and pathophysiological importance. Stimulation of β-adrenergic receptors (βAR) activates protein kinase A (PKA) through separation of regulatory (PKAR) from catalytic (PKAC) subunits. Free PKAC phosphorylates the inhibitory protein Rad, leading to increased Ca2+ influx. In cardiomyocytes, the core subunit of CaV1.2, CaV1.2α1, exists in two forms: full-length or truncated (lacking the distal C-terminus (dCT)). Signaling efficiency is believed to emanate from protein interactions within multimolecular complexes, such as anchoring PKA (via PKAR) to CaV1.2α1 by A-kinase anchoring proteins (AKAPs). However, AKAPs are inessential for βAR regulation of CaV1.2 in heterologous models, and their role in cardiomyocytes also remains unclear.

RESULTS

We show that PKAC interacts with CaV1.2α1 in heart and a heterologous model, independently of Rad, PKAR, or AKAPs. Studies with peptide array assays and purified recombinant proteins demonstrate direct binding of PKAC to two domains in CaV1.2α1-CT: the proximal and distal C-terminal regulatory domains (PCRD and DCRD), which also interact with each other. Data indicate both partial competition and possible simultaneous interaction of PCRD and DCRD with PKAC. The βAR regulation of CaV1.2α1 lacking dCT (which harbors DCRD) was preserved, but subtly altered, in a heterologous model, the Xenopus oocyte.

CONCLUSIONS

We discover direct interactions between PKAC and two domains in CaV1.2α1. We propose that these tripartite interactions, if present in vivo, may participate in organizing the multimolecular signaling complex and fine-tuning the βAR effect in cardiomyocytes.

Anchored PKA synchronizes adrenergic phosphoregulation of cardiac Cav1.2 channels

Adrenergic modulation of voltage gated Ca2+ currents is a context specific process. In the heart Cav1.2 channels initiate excitation-contraction coupling. This requires PKA phosphorylation of the small GTPase Rad (Ras associated with diabetes) and involves direct phosphorylation of the Cav1.2 α1 subunit at Ser1700. A contributing factor is the proximity of PKA to the channel through association with A-kinase anchoring proteins (AKAPs). Disruption of PKA anchoring by the disruptor peptide AKAP-IS prevents upregulation of Cav1.2 currents in tsA-201 cells. Biochemical analyses demonstrate that Rad does not function as an AKAP. Electrophysiological recording shows that channel mutants lacking phosphorylation sites (Cav1.2 STAA) lose responsivity to the second messenger cAMP. Measurements in cardiomyocytes isolated from Rad-/- mice show that adrenergic activation of Cav1.2 is attenuated but not completely abolished. Whole animal electrocardiography studies reveal that cardiac selective Rad KO mice exhibited higher baseline left ventricular ejection fraction, greater fractional shortening, and increased heart rate as compared to control animals. Yet, each parameter of cardiac function was slightly elevated when Rad-/- mice were treated with the adrenergic agonist isoproterenol. Thus, phosphorylation of Cav1.2 and dissociation of phospho-Rad from the channel are local cAMP responsive events that act in concert to enhance L-type calcium currents. This convergence of local PKA regulatory events at the cardiac L-type calcium channel may permit maximal β-adrenergic influence on the fight-or-flight response.

Flight or fight: different strategies of intertidal periwinkle Littoraria sinensis coping with high temperature across populations

Intertidal organisms usually live near their upper thermal limits, and are vulnerable to future global warming. As a vital response to thermal stress, thermoregulatory strategy in physiological and behavioral performance is essential for organisms coping with thermal stress and surviving the changing world. To investigate the relationship between the thermoregulatory strategy and habitat temperature, in the present study, we comparatively investigated the thermal responsive strategy among different geographic populations of the supralittoral snail Littoraria sinensis by determining snails' cardiac function and behavioral performance. Our results indicated that populations inhabiting high ambient temperatures had higher sublethal temperatures (i.e. Arrhenius breakpoint temperatures, ABTs, the temperature at which the heart rate shapely decreases with further heating) and lethal temperatures (i.e. Flatline temperatures, FLTs, the temperature at which heart rate ceases), and behaved less actively (e.g. shorter moving distances and shorter moving time) in the face of high and rising temperatures-a physiological fight strategy. On the other hand, populations at relatively low ambient temperatures had relatively lower physiological upper thermal limits with lower ABTs and FLTs and moved more actively in the face of high and rising temperatures-a behavioral flight strategy. These results demonstrate that the thermoregulatory strategies of the snails are closely related to their habitat temperatures and are different among populations surviving divergent thermal environments.

Glucose may Contribute to Retrieval and Reconsolidation of Contextual Fear Memory Through Hippocampal Nr4a3 and Bdnf mRNA Expression and May Act Synergically with Adrenaline

Adrenaline (Ad) and glucose released into the bloodstream during stress may strengthen contextual fear memory. However, no previous studies have detached the effects of glucose from Ad in this paradigm. Using Ad-deficient mice, we aimed to evaluate the effect of glucose on contextual fear memory when endogenous Ad is absent. Fear conditioning was performed in wild-type (WT) and Ad-deficient mice (129 × 1/SvJ) administered with glucose (30 or 10 mg/kg; i.p.) or/and Ad (0.01 mg/kg; i.p.) or vehicle (0.9% NaCl; i.p.). Catecholamines were quantified using HPLC-ED. Real-time qPCR was used to assess mRNA expression of hippocampal genes. WT and Ad-deficient mice display increased contextual fear memory when administered with glucose both in acquisition and context days when compared to vehicle. Also, Nr4a3 and Bdnf mRNA expression increased in glucose-administered Ad-deficient mice. Sub-effective doses of glucose plus Ad administered simultaneously to Ad-deficient mice increased contextual fear memory, contrary to independent sub-effective doses. Concluding, glucose may be an important part of the peripheral to central pathway involved in the retrieval and reconsolidation of fear contextual memories independently of Ad, possibly due to increased hippocampal Nr4a3 and Bdnf gene expression. Furthermore, Ad and glucose may act synergically to strengthen contextual fear memory.

Single-cell transcriptomics reveals the brain evolution of web-building spiders

Spiders are renowned for their efficient capture of flying insects using intricate aerial webs. How the spider nervous systems evolved to cope with this specialized hunting strategy and various environmental clues in an aerial space remains unknown. Here we report a brain-cell atlas of >30,000 single-cell transcriptomes from a web-building spider (Hylyphantes graminicola). Our analysis revealed the preservation of ancestral neuron types in spiders, including the potential coexistence of noradrenergic and octopaminergic neurons, and many peptidergic neuronal types that are lost in insects. By comparing the genome of two newly sequenced plesiomorphic burrowing spiders with three aerial web-building spiders, we found that the positively selected genes in the ancestral branch of web-building spiders were preferentially expressed (42%) in the brain, especially in the three mushroom body-like neuronal types. By gene enrichment analysis and RNAi experiments, these genes were suggested to be involved in the learning and memory pathway and may influence the spiders' web-building and hunting behaviour. Our results provide key sources for understanding the evolution of behaviour in spiders and reveal how molecular evolution drives neuron innovation and the diversification of associated complex behaviours.

Aged mice are less susceptible to motion sickness and show decreased efferent vestibular activity compared to young adults

INTRODUCTION

The efferent vestibular system (EVS) is a feedback circuit thought to modulate vestibular afferent activity by inhibiting type II hair cells and exciting calyx-bearing afferents in the peripheral vestibular organs. In a previous study, we suggested EVS activity may contribute to the effects of motion sickness. To determine an association between motion sickness and EVS activity, we examined the effects of provocative motion (PM) on c-Fos expression in brainstem efferent vestibular nucleus (EVN) neurons that are the source of efferent innervation in the peripheral vestibular organs.

METHODS

c-Fos is an immediate early gene product expressed in stimulated neurons and is a well-established marker of neuronal activation. To study the effects of PM, young adult C57/BL6 wild-type (WT), aged WT, and young adult transgenic Chat-gCaMP6f mice were exposed to PM, and tail temperature (Ttail ) was monitored using infrared imaging. After PM, we used immunohistochemistry to label EVN neurons to determine any changes in c-Fos expression. All tissue was imaged using laser scanning confocal microscopy.

RESULTS

Infrared recording of Ttail during PM indicated that young adult WT and transgenic mice displayed a typical motion sickness response (tail warming), but not in aged WT mice. Similarly, brainstem EVN neurons showed increased expression of c-Fos protein after PM in young adult WT and transgenic mice but not in aged cohorts.

CONCLUSION

We present evidence that motion sickness symptoms and increased activation of EVN neurons occur in young adult WT and transgenic mice in response to PM. In contrast, aged WT mice showed no signs of motion sickness and no change in c-Fos expression when exposed to the same provocative stimulus.

Noncanonical Regulation of cAMP-Dependent Insulin Secretion and Its Implications in Type 2 Diabetes

Impaired glucose tolerance (IGT) and β-cell dysfunction in insulin resistance associated with obesity lead to type 2 diabetes (T2D). Glucose-stimulated insulin secretion (GSIS) from β-cells occurs via a canonical pathway that involves glucose metabolism, ATP generation, inactivation of K ATP channels, plasma membrane depolarization, and increases in cytosolic concentrations of [Ca 2+ ] c . However, optimal insulin secretion requires amplification of GSIS by increases in cyclic adenosine monophosphate (cAMP) signaling. The cAMP effectors protein kinase A (PKA) and exchange factor activated by cyclic-AMP (Epac) regulate membrane depolarization, gene expression, and trafficking and fusion of insulin granules to the plasma membrane for amplifying GSIS. The widely recognized lipid signaling generated within β-cells by the β-isoform of Ca 2+ -independent phospholipase A 2 enzyme (iPLA 2 β) participates in cAMP-stimulated insulin secretion (cSIS). Recent work has identified the role of a G-protein coupled receptor (GPCR) activated signaling by the complement 1q like-3 (C1ql3) secreted protein in inhibiting cSIS. In the IGT state, cSIS is attenuated, and the β-cell function is reduced. Interestingly, while β-cell-specific deletion of iPLA 2 β reduces cAMP-mediated amplification of GSIS, the loss of iPLA 2 β in macrophages (MØ) confers protection against the development of glucose intolerance associated with diet-induced obesity (DIO). In this article, we discuss canonical (glucose and cAMP) and novel noncanonical (iPLA 2 β and C1ql3) pathways and how they may affect β-cell (dys)function in the context of impaired glucose intolerance associated with obesity and T2D. In conclusion, we provide a perspective that in IGT states, targeting noncanonical pathways along with canonical pathways could be a more comprehensive approach for restoring β-cell function in T2D. © 2023 American Physiological Society. Compr Physiol 13:5023-5049, 2023.

Insulin enhances contextual fear memory independently of its effect in increasing plasma adrenaline

AIMS

Adrenaline enhances contextual fear memory consolidation possibly by activating liver β₂-adrenoceptors causing transient hyperglycaemia. Contrastingly, insulin-induced hypoglycaemia may culminate in blood adrenaline increment, hidering the separation of each hormone's action in contextual fear memory. Therefore, an Ad-deficient mouse model was used aiming to investigate if contextual fear memory consolidation following insulin administration requires or not subsequent increases in plasma adrenaline, which occurs in response to insulin-induced hypoglycemia.

MAIN METHODS

Fear conditioning was performed in wild-type (WT) and adrenaline-deficient (Pnmt-KO) male mice (129 × 1/SvJ) treated with insulin (2 U/kg, intraperitoneal (i.p.)) or vehicle (0.9 % NaCl (i.p.)). Blood glucose was quantified. Catecholamines were quantified using HPLC with electrochemical detection. Quantitative real-time polymerase chain reaction was used to assess mRNA expression of hippocampal Nr4a1, Nr4a2, Nr4a3, and Bdnf genes.

KEY FINDINGS

Insulin-treated WT mice showed increased freezing behaviour when compared to vehicle-treated WT mice. Also, plasma dopamine, noradrenaline, and adrenaline increased in this group. Insulin-treated Pnmt-KO animals showed increased freezing behaviour when compared with respective vehicle. However, no changes in plasma or tissue catecholamines were identified in insulin-treated Pnmt-KO mice when compared with respective vehicle. Furthermore, insulin-treated Pnmt-KO mice presented increased Bdnf mRNA expression when compared to vehicle-treated Pnmt-KO mice.

SIGNIFICANCE

Concluding, enhanced freezing behaviour after insulin treatment, even in adrenaline absence, may indicate a key role of insulin in contextual fear memory. Insulin may cause central molecular changes promoting contextual fear memory formation and/or retrieval. This work may indicate a further role of insulin in the process of contextual fear memory modulation.

Phosphorylation, compartmentalization, and cardiac function

Protein phosphorylation is a fundamental element of cell signaling. First discovered as a biochemical switch in glycogen metabolism, we now know that this posttranslational modification permeates all aspects of cellular behavior. In humans, over 540 protein kinases attach phosphate to acceptor amino acids, whereas around 160 phosphoprotein phosphatases remove phosphate to terminate signaling. Aberrant phosphorylation underlies disease, and kinase inhibitor drugs are increasingly used clinically as targeted therapies. Specificity in protein phosphorylation is achieved in part because kinases and phosphatases are spatially organized inside cells. A prototypic example is compartmentalization of the cyclic adenosine 3',5'-monophosphate (cAMP)-dependent protein kinase A through association with A-kinase anchoring proteins. This configuration creates autonomous signaling islands where the anchored kinase is constrained in proximity to activators, effectors, and selected substates. This article primarily focuses on A kinase anchoring protein (AKAP) signaling in the heart with an emphasis on anchoring proteins that spatiotemporally coordinate excitation-contraction coupling and hypertrophic responses.

Impairment of β-adrenergic regulation and exacerbation of pressure-induced heart failure in mice with mutations in phosphoregulatory sites in the cardiac CaV1.2 calcium channel

The cardiac calcium channel CaV1.2 conducts L-type calcium currents that initiate excitation-contraction coupling and serves as a crucial mediator of β-adrenergic regulation of the heart. We evaluated the inotropic response of mice with mutations in C-terminal phosphoregulatory sites under physiological levels of β-adrenergic stimulation in vivo, and we assessed the impact of combining mutations of C-terminal phosphoregulatory sites with chronic pressure-overload stress. Mice with Ser1700Ala (S1700A), Ser1700Ala/Thr1704Ala (STAA), and Ser1928Ala (S1928A) mutations had impaired baseline regulation of ventricular contractility and exhibited decreased inotropic response to low doses of β-adrenergic agonist. In contrast, treatment with supraphysiogical doses of agonist revealed substantial inotropic reserve that compensated for these deficits. Hypertrophy and heart failure in response to transverse aortic constriction (TAC) were exacerbated in S1700A, STAA, and S1928A mice whose β-adrenergic regulation of CaV1.2 channels was blunted. These findings further elucidate the role of phosphorylation of CaV1.2 at regulatory sites in the C-terminal domain for maintaining normal cardiac homeostasis, responding to physiological levels of β-adrenergic stimulation in the fight-or-flight response, and adapting to pressure-overload stress.

Sympathetic Nervous System Regulation of Cardiac Calcium Channels

Calcium influx through voltage-gated calcium channels, Ca_v1.2, in cardiomyocytes initiates excitation-contraction coupling in the heart. The force and rate of cardiac contraction are modulated by the sympathetic nervous system, mediated substantially by changes in intracellular calcium. Norepinephrine released from sympathetic neurons innervating the heart and epinephrine secreted by the adrenal chromaffin cells bind to β-adrenergic receptors on the sarcolemma of cardiomyocytes initiating a signaling cascade that generates cAMP and activates protein kinase A, the targets of which control calcium influx. For decades, the mechanisms by which PKA regulated calcium channels in the heart were not known. Recently, these mechanisms have been elucidated. In this chapter, we will review the history of the field and the studies that led to the identification of the evolutionarily conserved process.

Spatiotemporal Control of Vascular CaV1.2 by α1C S1928 Phosphorylation

BACKGROUND

L-type CaV1.2 channels undergo cooperative gating to regulate cell function, although mechanisms are unclear. This study tests the hypothesis that phosphorylation of the CaV1.2 pore-forming subunit α1C at S1928 mediates vascular CaV1.2 cooperativity during diabetic hyperglycemia.

METHODS

A multiscale approach including patch-clamp electrophysiology, super-resolution nanoscopy, proximity ligation assay, calcium imaging' pressure myography, and Laser Speckle imaging was implemented to examine CaV1.2 cooperativity, α1C clustering, myogenic tone, and blood flow in human and mouse arterial myocytes/vessels.

RESULTS

CaV1.2 activity and cooperative gating increase in arterial myocytes from patients with type 2 diabetes and type 1 diabetic mice, and in wild-type mouse arterial myocytes after elevating extracellular glucose. These changes were prevented in wild-type cells pre-exposed to a PKA inhibitor or cells from knock-in S1928A but not S1700A mice. In addition, α1C clustering at the surface membrane of wild-type, but not wild-type cells pre-exposed to PKA or P2Y11 inhibitors and S1928A arterial myocytes, was elevated upon hyperglycemia and diabetes. CaV1.2 spatial and gating remodeling correlated with enhanced arterial myocyte Ca2+ influx and contractility and in vivo reduction in arterial diameter and blood flow upon hyperglycemia and diabetes in wild-type but not S1928A cells/mice.

CONCLUSIONS

These results suggest that PKA-dependent S1928 phosphorylation promotes the spatial reorganization of vascular α1C into "superclusters" upon hyperglycemia and diabetes. This triggers CaV1.2 activity and cooperativity, directly impacting vascular reactivity. The results may lay the foundation for developing therapeutics to correct CaV1.2 and arterial function during diabetic hyperglycemia.

The glucose tolerance test in mice: Sex, drugs and protocol

AIM

To establish the impact of sex, dosing route, fasting duration and acute habituation stress on glucose tolerance test (GTT) measurements used in the preclinical evaluation of potential glucose-modulating therapeutics.

METHODS

Adult male and female C57Bl/6J mice, implanted with HD-XG glucose telemetry devices, were fasted for 16 hours or 6 hours following acute habituation stress due to whole cage change, cage change with retention of used bedding or no cage change prior to intraperitoneal (IP) GTTs. To evaluate protocol refinement and sex on the ability of the GTT to detect drug effects, we administered 250 mg/kg oral metformin or 10 nmol/kg IP exendin-4 using optimized protocols.

RESULTS

Female mice were less sensitive to human intervention when initiating fasting. Following a 6-hour fast, retention of bedding whilst changing the cage base promotes quicker stabilization of basal blood glucose in both sexes. Prolonged fasting for 16 hours resulted in an exaggerated GTT response but induced pronounced basal hypoglycaemia. Following GTT protocol optimization the effect of exendin-4 and metformin was equivalent in both sexes, with females showing a more modest but more reproducible GTT response.

CONCLUSIONS

Variations in GTT protocol have profound effects on glucose homeostasis. Protocol refinement and/or the use of females still allows for detection of drug effects, providing evidence that more severe phenotypes are not an essential prerequisite when characterizing/validating new drugs.

Elucidating the role of the L-type calcium channel in excitability and energetics in the heart: The ISHR 2020 Research Achievement Award Lecture

Cardiovascular disease continues to be the leading health burden worldwide and with the rising rates in obesity and type II diabetes and ongoing effects of long COVID, it is anticipated that the burden of cardiovascular morbidity and mortality will increase. Calcium is essential to cardiac excitation and contraction. The main route for Ca²⁺ influx is the L-type Ca²⁺ channel (Ca_v1.2) and embryos that are homozygous null for the Ca_v1.2 gene are lethal at day 14 postcoitum. Acute changes in Ca²⁺ influx through the channel contribute to arrhythmia and sudden death, and chronic increases in intracellular Ca²⁺ contribute to pathological hypertrophy and heart failure. We use a multidisciplinary approach to study the regulation of the channel from the molecular level through to in vivo CRISPR mutant animal models. Here we describe some examples of our work from over 2 decades studying the role of the channel under physiological and pathological conditions. Our single channel analysis of purified human Ca_v1.2 protein in proteoliposomes has contributed to understanding direct molecular regulation of the channel including identifying the critical serine involved in the "fight or flight" response. Using the same approach we identified the cysteine responsible for altered function during oxidative stress. Chronic activation of the L-type Ca²⁺ channel during oxidative stress occurs as a result of persistent glutathionylation of the channel that contributes to the development of hypertrophy. We describe for the first time that activation of the channel alters mitochondrial function (and energetics) on a beat-to-beat basis via movement of cytoskeletal proteins. In translational studies we have used this response to "report" mitochondrial function in models of cardiomyopathy and to test efficacy of novel therapies to prevent cardiomyopathy.

Convergent regulation of CaV1.2 channels by direct phosphorylation and by the small GTPase RAD in the cardiac fight-or-flight response

The L-type calcium currents conducted by the cardiac Ca_V1.2 calcium channel initiate excitation-contraction coupling and serve as a key regulator of heart rate, rhythm, and force of contraction. Ca_V1.2 is regulated by β-adrenergic/protein kinase A (PKA)-mediated protein phosphorylation, proteolytic processing, and autoinhibition by its carboxyl-terminal domain (CT). The small guanosine triphosphatase (GTPase) RAD (Ras associated with diabetes) has emerged as a potent inhibitor of Ca_V1.2, and accumulating evidence suggests a key role for RAD in mediating β-adrenergic/PKA upregulation of channel activity. However, the relative roles of direct phosphorylation of Ca_V1.2 channels and phosphorylation of RAD in channel regulation remain uncertain. Here, we investigated the hypothesis that these two mechanisms converge to regulate Ca_V1.2 channels. Both RAD and the proteolytically processed distal CT (dCT) strongly reduced Ca_V1.2 activity. PKA phosphorylation of RAD and phosphorylation of Ser-1700 in the proximal CT (pCT) synergistically reversed this inhibition and increased Ca_V1.2 currents. Our findings reveal that the proteolytically processed form of Ca_V1.2 undergoes convergent regulation by direct phosphorylation of the CT and by phosphorylation of RAD. These parallel regulatory pathways provide a flexible mechanism for upregulation of the activity of Ca_V1.2 channels in the fight-or-flight response.

Cytosolic peptides encoding CaV1 C-termini downregulate the calcium channel activity-neuritogenesis coupling

L-type Ca²⁺ (Ca_V1) channels transduce channel activities into nuclear signals critical to neuritogenesis. Also, standalone peptides encoded by Ca_V1 DCT (distal carboxyl-terminus) act as nuclear transcription factors reportedly promoting neuritogenesis. Here, by focusing on exemplary Ca_V1.3 and cortical neurons under basal conditions, we discover that cytosolic DCT peptides downregulate neurite outgrowth by the interactions with Ca_V1's apo-calmodulin binding motif. Distinct from nuclear DCT, various cytosolic peptides exert a gradient of inhibitory effects on Ca²⁺ influx via Ca_V1 channels and neurite extension and arborization, and also the intermediate events including CREB activation and c-Fos expression. The inhibition efficacies of DCT are quantitatively correlated with its binding affinities. Meanwhile, cytosolic inhibition tends to facilitate neuritogenesis indirectly by favoring Ca²⁺-sensitive nuclear retention of DCT. In summary, DCT peptides as a class of Ca_V1 inhibitors specifically regulate the channel activity-neuritogenesis coupling in a variant-, affinity-, and localization-dependent manner.

Mechanisms of Lian-Gui-Ning-Xin-Tang in the treatment of arrhythmia: Integrated pharmacology and in vivo pharmacological assessment

BACKGROUND

Lian-Gui-Ning-Xin-Tang (LGNXT), a classical traditional Chinese medicine (TCM) formula, has been widely used in clinical practice and has shown satisfactory efficacy in the treatment of arrhythmias. However, its mechanism of action in the treatment of arrhythmias is still unknown. Moreover, the complex chemical composition and therapeutic targets of LGNXT pose a challenge in pharmacological research.

PURPOSE

To analyze the active compounds and action mechanisms of LGNXT for the treatment of arrhythmias.

METHODS

Here, we used an integrated pharmacology approach to identify the potential active compounds and mechanisms of action of LGNXT in treating arrhythmias. Potential active compounds in LGNXT were identified using ultra-performance liquid chromatography-quadrupole-time-of-flight mass spectrometry (UPLC-Q-TOF/MS) and the potential related targets of these compounds were predicted using an integrated in silico approach. The obtained targets were mapped onto relevant databases to identify their corresponding pathways, following the experiments that were conducted to confirm whether the presumptive results of systemic pharmacology were correct.

RESULTS

Eighty-three components were identified in herbal materials and in animal plasma using UPLC-Q-TOF/MS and were considered the potential active components of LGNXT. Thirty key targets and 57 Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways were identified as possible targets and pathways involved in LGNXT-mediated treatment using network pharmacology, with the cyclic adenosine monophosphate (cAMP)/protein kinase A (PKA)/Ca2+ system pathway being the most significantly affected. This finding was validated using an adrenaline (Adr)-induced rat model of arrhythmias. Pretreatment with LGNXT delayed the occurrence, shortened the duration, and reduced the severity of arrhythmias. LGNXT exerted antiarrhythmic effects by inhibiting cAMP, PKA, CACNA1C, and RyR2.

CONCLUSIONS

The findings of this study revealed that preventing intracellular Ca2+ overload and maintaining intracellular Ca2+ homeostasis may be the primary mechanisms of LGNXT in alleviating arrhythmias. Thus, we suggest that the β-adrenergic receptor (AR)/cAMP/PKA/Ca2+ system signaling hub may constitute a promising molecular target for the development of novel antiarrhythmic therapeutic interventions. Additionally, we believe that the approach of investigation of the biological effects of a multi-herbal formula by the combination of metabolomics and network pharmacology, as used in this study, could serve as a systematic model for TCM research.

The Oxidative Balance Orchestrates the Main Keystones of the Functional Activity of Cardiomyocytes

This review is aimed at providing an overview of the key hallmarks of cardiomyocytes in physiological and pathological conditions. The main feature of cardiac tissue is the force generation through contraction. This process requires a conspicuous energy demand and therefore an active metabolism. The cardiac tissue is rich of mitochondria, the powerhouses in cells. These organelles, producing ATP, are also the main sources of ROS whose altered handling can cause their accumulation and therefore triggers detrimental effects on mitochondria themselves and other cell components thus leading to apoptosis and cardiac diseases. This review highlights the metabolic aspects of cardiomyocytes and wanders through the main systems of these cells: (a) the unique structural organization (such as different protein complexes represented by contractile, regulatory, and structural proteins); (b) the homeostasis of intracellular Ca²⁺ that represents a crucial ion for cardiac functions and E-C coupling; and (c) the balance of Zn²⁺, an ion with a crucial impact on the cardiovascular system. Although each system seems to be independent and finely controlled, the contractile proteins, intracellular Ca²⁺ homeostasis, and intracellular Zn²⁺ signals are strongly linked to each other by the intracellular ROS management in a fascinating way to form a "functional tetrad" which ensures the proper functioning of the myocardium. Nevertheless, if ROS balance is not properly handled, one or more of these components could be altered resulting in deleterious effects leading to an unbalance of this "tetrad" and promoting cardiovascular diseases. In conclusion, this "functional tetrad" is proposed as a complex network that communicates continuously in the cardiomyocytes and can drive the switch from physiological to pathological conditions in the heart.

Adrenergic Regulation of Calcium Channels in the Heart

Each heartbeat is initiated by the action potential, an electrical signal that depolarizes the plasma membrane and activates a cycle of calcium influx via voltage-gated calcium channels, calcium release via ryanodine receptors, and calcium reuptake and efflux via calcium-ATPase pumps and sodium-calcium exchangers. Agonists of the sympathetic nervous system bind to adrenergic receptors in cardiomyocytes, which, via cascading signal transduction pathways and protein kinase A (PKA), increase the heart rate (chronotropy), the strength of myocardial contraction (inotropy), and the rate of myocardial relaxation (lusitropy). These effects correlate with increased intracellular concentration of calcium, which is required for the augmentation of cardiomyocyte contraction. Despite extensive investigations, the molecular mechanisms underlying sympathetic nervous system regulation of calcium influx in cardiomyocytes have remained elusive over the last 40 years. Recent studies have uncovered the mechanisms underlying this fundamental biologic process, namely that PKA phosphorylates a calcium channel inhibitor, Rad, thereby releasing inhibition and increasing calcium influx. Here, we describe an updated model for how signals from adrenergic agonists are transduced to stimulate calcium influx and contractility in the heart.

Post-Translational Modification of Cav1.2 and its Role in Neurodegenerative Diseases

Cav1.2 plays an essential role in learning and memory, drug addiction, and neuronal development. Intracellular calcium homeostasis is disrupted in neurodegenerative diseases because of abnormal Cav1.2 channel activity and modification of downstream Ca²⁺ signaling pathways. Multiple post-translational modifications of Cav1.2 have been observed and seem to be closely related to the pathogenesis of neurodegenerative diseases. The specific molecular mechanisms by which Cav1.2 channel activity is regulated remain incompletely understood. Dihydropyridines (DHPs), which are commonly used for hypertension and myocardial ischemia, have been repurposed to treat PD and AD and show protective effects. However, further studies are needed to improve delivery strategies and drug selectivity. Better knowledge of channel modulation and more specific methods for altering Cav1.2 channel function may lead to better therapeutic strategies for neurodegenerative diseases.

Proteolytic regulation of calcium channels - avoiding controversy

The publication of papers containing data obtained with suboptimal rigor in the experimental design and choice of key reagents, such as antibodies, can result in a lack of reproducibility and generate controversy that can both needlessly divert resources and, in some cases, damage public perception of the scientific enterprise. This exemplary paper by Buonarati et al. (2018)¹ shows how a previously published, potentially important paper on calcium channel regulation falls short of the necessary mark, and aims to resolve the resulting controversy.

Enhanced Heart Failure in Redox-Dead Cys17Ser PKARIα Knock-In Mice

Background PKARIα (protein kinase A type I-α regulatory subunit) is redox-active independent of its physiologic agonist cAMP. However, it is unknown whether this alternative mechanism of PKARIα activation may be of relevance to cardiac excitation-contraction coupling. Methods and Results We used a redox-dead transgenic mouse model with homozygous knock-in replacement of redox-sensitive cysteine 17 with serine within the regulatory subunits of PKARIα (KI). Reactive oxygen species were acutely evoked by exposure of isolated cardiac myocytes to AngII (angiotensin II, 1 µmol/L). The long-term relevance of oxidized PKARIα was investigated in KI mice and their wild-type (WT) littermates following transverse aortic constriction (TAC). AngII increased reactive oxygen species in both groups but with RIα dimer formation in WT only. AngII induced translocation of PKARI to the cell membrane and resulted in protein kinase A-dependent stimulation of I_Ca (L-type Ca current) in WT with no effect in KI myocytes. Consequently, Ca transients were reduced in KI myocytes as compared with WT cells following acute AngII exposure. Transverse aortic constriction-related reactive oxygen species formation resulted in RIα oxidation in WT but not in KI mice. Within 6 weeks after TAC, KI mice showed an enhanced deterioration of contractile function and impaired survival compared with WT. In accordance, compared with WT, ventricular myocytes from failing KI mice displayed significantly reduced Ca transient amplitudes and lack of I_Ca stimulation. Conversely, direct pharmacological stimulation of I_Ca using Bay K8644 rescued Ca transients in AngII-treated KI myocytes and contractile function in failing KI mice in vivo. Conclusions Oxidative activation of PKARIα with subsequent stimulation of I_Ca preserves cardiac function in the setting of acute and chronic oxidative stress.

Depolarization induces nociceptor sensitization by CaV1.2-mediated PKA-II activation

Depolarization drives neuronal plasticity. However, whether depolarization drives sensitization of peripheral nociceptive neurons remains elusive. By high-content screening (HCS) microscopy, we revealed that depolarization of cultured sensory neurons rapidly activates protein kinase A type II (PKA-II) in nociceptors by calcium influx through Ca_V1.2 channels. This effect was modulated by calpains but insensitive to inhibitors of cAMP formation, including opioids. In turn, PKA-II phosphorylated Ser1928 in the distal C terminus of Ca_V1.2, thereby increasing channel gating, whereas dephosphorylation of Ser1928 involved the phosphatase calcineurin. Patch-clamp and behavioral experiments confirmed that depolarization leads to calcium- and PKA-dependent sensitization of calcium currents ex vivo and local peripheral hyperalgesia in the skin in vivo. Our data suggest a local activity-driven feed-forward mechanism that selectively translates strong depolarization into further activity and thereby facilitates hypersensitivity of nociceptor terminals by a mechanism inaccessible to opioids.

Cyclic nucleotide signaling and pacemaker activity

The sinoatrial node (SAN) is the natural pacemaker of the heart, producing the electrical impulse that initiates every heart beat. Its activity is tightly controlled by the autonomic nervous system, and by circulating and locally released factors. Neurohumoral regulation of heart rate plays a crucial role in the integration of vital functions and influences behavior and ability to respond to changing environmental conditions. At the cellular level, modulation of SAN activity occurs through intracellular signaling pathways involving cyclic nucleotides: cyclic AMP (cAMP) and cyclic GMP (cGMP). In this Review, dedicated to Professor Dario DiFrancesco and his accomplishements in the field of cardiac pacemaking, we summarize all findings on the role of cyclic nucleotides signaling in regulating the key actors of cardiac automatism, and we provide an up-to-date review on cAMP- and cGMP-phosphodiesterases (PDEs), compellingly involved in this modulation.

Use of Proximity Labeling in Cardiovascular Research

Protein-protein interactions are of paramount importance in regulating normal cardiac physiology. Methodologies to elucidate these interactions in vivo have been limited. Recently, proximity-dependent biotinylation, with the use of BioID, TurboID, and ascorbate peroxidase, has been developed to uncover cellular neighborhoods and novel protein-protein interactions. These cutting-edge techniques have enabled the identification of subcellular localizations of specific proteins and the neighbors or interacting proteins within these subcellular regions. In contrast to classic methods such as affinity purification and subcellular fractionation, these techniques add covalently bound tags in living cells, such that spatial relationships and interaction networks are not disrupted. Recently, these methodologies have been used to identify novel protein-protein interactions relevant to the cardiovascular system. In this review, we discuss the development and current use of proximity biotin-labeling for cardiovascular research.

Reconstitution of β-adrenergic regulation of CaV1.2: Rad-dependent and Rad-independent protein kinase A mechanisms

L-type voltage-gated Ca_V1.2 channels crucially regulate cardiac muscle contraction. Activation of β-adrenergic receptors (β-AR) augments contraction via protein kinase A (PKA)-induced increase of calcium influx through Ca_V1.2 channels. To date, the full β-AR cascade has never been heterologously reconstituted. A recent study identified Rad, a Ca_V1.2 inhibitory protein, as essential for PKA regulation of Ca_V1.2. We corroborated this finding and reconstituted the complete pathway with agonist activation of β1-AR or β2-AR in Xenopus oocytes. We found, and distinguished between, two distinct pathways of PKA modulation of Ca_V1.2: Rad dependent (∼80% of total) and Rad independent. The reconstituted system reproduces the known features of β-AR regulation in cardiomyocytes and reveals several aspects: the differential regulation of posttranslationally modified Ca_V1.2 variants and the distinct features of β1-AR versus β2-AR activity. This system allows for the addressing of central unresolved issues in the β-AR-Ca_V1.2 cascade and will facilitate the development of therapies for catecholamine-induced cardiac pathologies.

The molecular basis of the inhibition of CaV1 calcium-dependent inactivation by the distal carboxy tail

Ca2+/calmodulin-dependent inactivation (CDI) of CaV channels is a critical regulatory process that tunes the kinetics of Ca2+ entry for different cell types and physiologic responses. CDI is mediated by calmodulin (CaM), which is bound to the IQ domain of the CaV carboxy tail. This modulatory process is tailored by alternative splicing such that select splice variants of CaV1.3 and CaV1.4 contain a long distal carboxy tail (DCT). The DCT harbors an inhibitor of CDI (ICDI) module that competitively displaces CaM from the IQ domain, thereby diminishing CDI. While this overall mechanism is now well described, the detailed interactions required for ICDI binding to the IQ domain are yet to be elucidated. Here, we perform alanine-scanning mutagenesis of the IQ and ICDI domains and evaluate the contribution of neighboring regions to CDI inhibition. Through FRET binding analysis, we identify functionally relevant residues within the CaV1.3 IQ domain and the CaV1.4 ICDI and nearby A region, which are required for high-affinity IQ/ICDI binding. Importantly, patch-clamp recordings demonstrate that disruption of this interaction commensurately diminishes ICDI function resulting in the re-emergence of CDI in mutant channels. Furthermore, CaV1.2 channels harbor a homologous DCT; however, the ICDI region of this channel does not appear to appreciably modulate CaV1.2 CDI. Yet coexpression of CaV1.2 ICDI with select CaV1.3 splice variants significantly disrupts CDI, implicating a cross-channel modulatory scheme in cells expressing both channel subtypes. In all, these findings provide new insights into a molecular rheostat that fine-tunes Ca2+-entry and supports normal neuronal and cardiac function.

Tissue-specific adrenergic regulation of the L-type Ca2+ channel CaV1.2

Ca²⁺ influx through the L-type Ca²⁺ channel Cav1.2 triggers each heartbeat. The fight-or-flight response induces the release of the stress response hormone norepinephrine to stimulate β-adrenergic receptors, cAMP production, and protein kinase A activity to augment Ca²⁺ influx through Ca_v1.2 and, consequently, cardiomyocyte contractility. Emerging evidence shows that Ca_v1.2 is regulated by different mechanisms in cardiomyocytes compared to neurons and vascular smooth muscle cells.

The quest to identify the mechanism underlying adrenergic regulation of cardiac Ca2+ channels

Activation of protein kinase A by cyclic AMP results in a multi-fold upregulation of CaV1.2 currents in the heart, as originally reported in the 1970's and 1980's. Despite considerable interest and much investment, the molecular mechanisms responsible for this signature modulation remained stubbornly elusive for over 40 years. A key manifestation of this lack of understanding is that while this regulation is readily apparent in heart cells, it has not been possible to reconstitute it in heterologous expression systems. In this review, we describe the efforts of many investigators over the past decades to identify the mechanisms responsible for the β-adrenergic mediated activation of voltage-gated Ca2+ channels in the heart and other tissues.

Protein kinase CK2 and ion channels (Review)

Protein kinase CK2 appears as a tetramer or higher molecular weight oligomer composed of catalytic CK2α, CK2α' subunits and non-catalytic regulatory CK2β subunits or as individual subunits. It is implicated in a variety of different regulatory processes, such as Akt signalling, splicing and DNA repair within eukaryotic cells. The present review evaluates the influence of CK2 on ion channels in the plasma membrane. CK2 phosphorylates platform proteins such as calmodulin and ankyrin G, which bind to channel proteins for a physiological transport to and positioning into the membrane. In addition, CK2 directly phosphorylates a variety of channel proteins directly to regulate opening and closing of the channels. Thus, modulation of CK2 activities by specific inhibitors, by siRNA technology or by CRISPR/Cas technology has an influence on intracellular ion concentrations and thereby on cellular signalling. The physiological regulation of the intracellular ion concentration is important for cell survival and correct intracellular signalling. Disturbance of this regulation results in a variety of different diseases including epilepsy, heart failure, cystic fibrosis and diabetes. Therefore, these effects should be considered when using CK2 inhibition as a treatment option for cancer.

Intracellular O-linked glycosylation directly regulates cardiomyocyte L-type Ca2+ channel activity and excitation-contraction coupling

Cardiomyocyte L-type Ca²⁺ channels (Ca_vs) are targets of signaling pathways that modulate channel activity in response to physiologic stimuli. Ca_v regulation is typically transient and beneficial but chronic stimulation can become pathologic; therefore, gaining a more complete understanding of Ca_v regulation is of critical importance. Intracellular O-linked glycosylation (O-GlcNAcylation), which is the result of two enzymes that dynamically add and remove single N-acetylglucosamines to and from intracellular serine/threonine residues (OGT and OGA respectively), has proven to be an increasingly important post-translational modification that contributes to the regulation of many physiologic processes. However, there is currently no known role for O-GlcNAcylation in the direct regulation of Ca_v activity nor is its contribution to cardiac electrical signaling and EC coupling well understood. Here we aimed to delineate the role of O-GlcNAcylation in regulating cardiomyocyte L-type Ca_v activity and its subsequent effect on EC coupling by utilizing a mouse strain possessing an inducible cardiomyocyte-specific OGT-null-transgene. Ablation of the OGT-gene in adult cardiomyocytes (OGTKO) reduced OGT expression and O-GlcNAcylation by > 90%. Voltage clamp recordings indicated an ~ 40% reduction in OGTKO Ca_v current (I_Ca), but with increased efficacy of adrenergic stimulation, and Ca_v steady-state gating and window current were significantly depolarized. Consistently, OGTKO cardiomyocyte intracellular Ca²⁺ release and contractility were diminished and demonstrated greater beat-to-beat variability. Additionally, we show that the Ca_v α1 and β2 subunits are O-GlcNAcylated while α2δ1 is not. Echocardiographic analyses indicated that the reductions in OGTKO cardiomyocyte Ca²⁺ handling and contractility were conserved at the whole-heart level as evidenced by significantly reduced left-ventricular contractility in the absence of hypertrophy. The data indicate, for the first time, that O-GlcNAc signaling is a critical and direct regulator of cardiomyocyte I_Ca achieved through altered Ca_v expression, gating, and response to adrenergic stimulation; these mechanisms have significant implications for understanding how EC coupling is regulated in health and disease.

Analysis of the source of aggressiveness in gamecocks

Although the fighting behaviour in gamecocks has evolved because of artificial selection, it is unknown whether the selection for aggressiveness affects neurotransmitter levels in the avian central nervous system. We sought to identify the source and origin of this trait. We collected the brain samples from 6 female Shamo gamecocks and 5 Shaver Brown chickens (control; bred for egg production). The midbrain levels of norepinephrine (NE) were significantly higher in Shamo gamecocks (P = 0.0087) than in the controls. Moreover, alleles encoding adrenergic receptors differed between the breeds in terms of response to NE. Gene mutations specific to Shamo and potentially associated with fighting behaviour were in sites T440N of ADRα1D; V296I of ADRα2A; and T44I, Q232R, and T277M of ADRβ2. The evolutionary analysis indicated that the ADRβ2 (T44I and Q232R) mutations were heritable in all Galliformes, whereas the T440N mutation of ADRα1D and V296I mutations of ADRα2A were unique to Shamo and originated by artificial selection. A high NE level may confer a selective advantage by enabling gamecocks to be aggressive and pain tolerant. Therefore, the strong fighting behaviour of Shamo has resulted from a combination of naturally inherited and mutant genes derived by artificial selection.

New aspects in cardiac L-type Ca2+ channel regulation

Cardiac excitation-contraction coupling is initiated with the influx of Ca2+ ions across the plasma membrane through voltage-gated L-type calcium channels. This process is tightly regulated by modulation of the channel open probability and channel localization. Protein kinase A (PKA) is found in close association with the channel and is one of the main regulators of its function. Whether this kinase is modulating the channel open probability by phosphorylation of key residues or via alternative mechanisms is unclear. This review summarizes recent findings regarding the PKA-mediated channel modulation and will highlight recently discovered regulatory mechanisms that are independent of PKA activity and involve protein-protein interactions and channel localization.

α-Actinin-1 promotes activity of the L-type Ca2+ channel Cav 1.2

The L-type Ca²⁺ channel Ca_V 1.2 governs gene expression, cardiac contraction, and neuronal activity. Binding of α-actinin to the IQ motif of Ca_V 1.2 supports its surface localization and postsynaptic targeting in neurons. We report a bi-functional mechanism that restricts Ca_V 1.2 activity to its target sites. We solved separate NMR structures of the IQ motif (residues 1,646-1,664) bound to α-actinin-1 and to apo-calmodulin (apoCaM). The Ca_V 1.2 K1647A and Y1649A mutations, which impair α-actinin-1 but not apoCaM binding, but not the F1658A and K1662E mutations, which impair apoCaM but not α-actinin-1 binding, decreased single-channel open probability, gating charge movement, and its coupling to channel opening. Thus, α-actinin recruits Ca_V 1.2 to defined surface regions and simultaneously boosts its open probability so that Ca_V 1.2 is mostly active when appropriately localized.

Neuronal nitric oxide synthase regulation of calcium cycling in ventricular cardiomyocytes is independent of Cav1.2 channel modulation under basal conditions

Neuronal nitric oxide synthase (nNOS) is considered a regulator of Cav1.2 L-type Ca2+ channels and downstream Ca2+ cycling in the heart. The commonest view is that nitric oxide (NO), generated by nNOS activity in cardiomyocytes, reduces the currents through Cav1.2 channels. This gives rise to a diminished Ca2+ release from the sarcoplasmic reticulum, and finally reduced contractility. Here, we report that nNOS inhibitor substances significantly increase intracellular Ca2+ transients in ventricular cardiomyocytes derived from adult mouse and rat hearts. This is consistent with an inhibitory effect of nNOS/NO activity on Ca2+ cycling and contractility. Whole cell currents through L-type Ca2+ channels in rodent myocytes, on the other hand, were not substantially affected by the application of various NOS inhibitors, or application of a NO donor substance. Moreover, the presence of NO donors had no effect on the single-channel open probability of purified human Cav1.2 channel protein reconstituted in artificial liposomes. These results indicate that nNOS/NO activity does not directly modify Cav1.2 channel function. We conclude that-against the currently prevailing view-basal Cav1.2 channel activity in ventricular cardiomyocytes is not substantially regulated by nNOS activity and NO. Hence, nNOS/NO inhibition of Ca2+ cycling and contractility occurs independently of direct regulation of Cav1.2 channels by NO.

Mechanism of adrenergic CaV1.2 stimulation revealed by proximity proteomics

Increased cardiac contractility during the fight-or-flight response is caused by β-adrenergic augmentation of CaV1.2 voltage-gated calcium channels1-4. However, this augmentation persists in transgenic murine hearts expressing mutant CaV1.2 α1C and β subunits that can no longer be phosphorylated by protein kinase A-an essential downstream mediator of β-adrenergic signalling-suggesting that non-channel factors are also required. Here we identify the mechanism by which β-adrenergic agonists stimulate voltage-gated calcium channels. We express α1C or β2B subunits conjugated to ascorbate peroxidase5 in mouse hearts, and use multiplexed quantitative proteomics6,7 to track hundreds of proteins in the proximity of CaV1.2. We observe that the calcium-channel inhibitor Rad8,9, a monomeric G protein, is enriched in the CaV1.2 microenvironment but is depleted during β-adrenergic stimulation. Phosphorylation by protein kinase A of specific serine residues on Rad decreases its affinity for β subunits and relieves constitutive inhibition of CaV1.2, observed as an increase in channel open probability. Expression of Rad or its homologue Rem in HEK293T cells also imparts stimulation of CaV1.3 and CaV2.2 by protein kinase A, revealing an evolutionarily conserved mechanism that confers adrenergic modulation upon voltage-gated calcium channels.

Calcium Channels, Synaptic Plasticity, and Neuropsychiatric Disease

Voltage-gated calcium channels couple depolarization of the cell-surface membrane to entry of calcium, which triggers secretion, contraction, neurotransmission, gene expression, and other physiological responses. They are encoded by ten genes, which generate three voltage-gated calcium channel subfamilies: Ca_V1; Ca_V2; and Ca_V3. At synapses, Ca_V2 channels form large signaling complexes in the presynaptic nerve terminal, which are responsible for the calcium entry that triggers neurotransmitter release and short-term presynaptic plasticity. Ca_V1 channels form signaling complexes in postsynaptic dendrites and dendritic spines, where their calcium entry induces long-term potentiation. These calcium channels are the targets of mutations and polymorphisms that alter their function and/or regulation and cause neuropsychiatric diseases, including migraine headache, cerebellar ataxia, autism, schizophrenia, bipolar disorder, and depression. This article reviews the molecular properties of calcium channels, considers their multiple roles in synaptic plasticity, and discusses their potential involvement in this wide range of neuropsychiatric diseases.

Single-Channel Resolution of the Interaction between C-Terminal CaV1.3 Isoforms and Calmodulin

Voltage-dependent calcium (Ca_V) 1.3 channels are involved in the control of cellular excitability and pacemaking in neuronal, cardiac, and sensory cells. Various proteins interact with the alternatively spliced channel C-terminus regulating gating of Ca_V1.3 channels. Binding of a regulatory calcium-binding protein calmodulin (CaM) to the proximal C-terminus leads to the boosting of channel activity and promotes calcium-dependent inactivation (CDI). The C-terminal modulator domain (CTM) of Ca_V1.3 channels can interfere with the CaM binding, thereby inhibiting channel activity and CDI. Here, we compared single-channel gating behavior of two natural Ca_V1.3 splice isoforms: the long Ca_V1.3₄₂ with the full-length CTM and the short Ca_V1.3_42A with the C-terminus truncated before the CTM. We found that CaM regulation of Ca_V1.3 channels is dynamic on a minute timescale. We observed that at equilibrium, single Ca_V1.3₄₂ channels occasionally switched from low to high open probability, which perhaps reflects occasional binding of CaM despite the presence of CTM. Similarly, when the amount of the available CaM in the cell was reduced, the short Ca_V1.3_42A isoform showed patterns of the low channel activity. CDI also underwent periodic changes with corresponding kinetics in both isoforms. Our results suggest that the competition between CTM and CaM is influenced by calcium, allowing further fine-tuning of Ca_V1.3 channel activity for particular cellular needs.

Coagulation factor XI induces Ca2+ response and accelerates cell migration in vascular smooth muscle cells via proteinase-activated receptor 1

Activated coagulation factor XI (FXIa) is a serine proteinase that plays a key role in the intrinsic coagulation pathway. The analysis of FXI-knockout mice has indicated the contribution of FXI to the pathogenesis of atherosclerosis. However, the underlying mechanism remains unknown. We hypothesized that FXIa exerts vascular smooth muscle effects via proteinase-activated receptor 1 (PAR₁). Fura-2 fluorometry revealed that FXIa elicited intracellular Ca²⁺ signal in rat embryo aorta smooth muscle A7r5 cells. The influx of extracellular Ca²⁺ played a greater role in generating Ca²⁺ signal than the Ca²⁺ release from intracellular stores. The FXIa-induced Ca²⁺ signal was abolished by the pretreatment with atopaxar, an antagonist of PAR₁, or 4-amidinophenylmethanesulfonyl fluoride (p-APMSF), an inhibitor of proteinase, while it was also lost in embryonic fibroblasts derived from PAR₁^-/- mice. FXIa cleaved the recombinant protein containing the extracellular region of PAR₁ at the same site (R45/S46) as that of thrombin, a canonical PAR₁ agonist. The FXIa-induced Ca²⁺ influx was inhibited by diltiazem, an L-type Ca²⁺ channel blocker, and by siRNA targeted to Ca_V1.2. The FXIa-induced Ca²⁺ influx was also inhibited by GF109203X and rottlerin, inhibitors of protein kinase C. In a wound healing assay, FXIa increased the rate of cell migration by 2.46-fold of control, which was partly inhibited by atopaxar or diltiazem. In conclusion, FXIa mainly elicits the Ca²⁺ signal via the PAR₁/Ca_V1.2-mediated Ca²⁺ influx and accelerates the migration in vascular smooth muscle cells. The present study provides the first evidence that FXIa exerts a direct cellular effect on vascular smooth muscle.

Friend and foe: β-cell Ca2+ signaling and the development of diabetes

BACKGROUND

The divalent cation Calcium (Ca2+) regulates a wide range of processes in disparate cell types. Within insulin-producing β-cells, increases in cytosolic Ca2+ directly stimulate insulin vesicle exocytosis, but also initiate multiple signaling pathways. Mediated through activation of downstream kinases and transcription factors, Ca2+-regulated signaling pathways leverage substantial influence on a number of critical cellular processes within the β-cell. Additionally, there is evidence that prolonged activation of these same pathways is detrimental to β-cell health and may contribute to Type 2 Diabetes pathogenesis.

SCOPE OF REVIEW

This review aims to briefly highlight canonical Ca2+ signaling pathways in β-cells and how β-cells regulate the movement of Ca2+ across numerous organelles and microdomains. As a main focus, this review synthesizes experimental data from in vitro and in vivo models on both the beneficial and detrimental effects of Ca2+ signaling pathways for β-cell function and health.

MAJOR CONCLUSIONS

Acute increases in intracellular Ca2+ stimulate a number of signaling cascades, resulting in (de-)phosphorylation events and activation of downstream transcription factors. The short-term stimulation of these Ca2+ signaling pathways promotes numerous cellular processes critical to β-cell function, including increased viability, replication, and insulin production and secretion. Conversely, chronic stimulation of Ca2+ signaling pathways increases β-cell ER stress and results in the loss of β-cell differentiation status. Together, decades of study demonstrate that Ca2+ movement is tightly regulated within the β-cell, which is at least partially due to its dual roles as a potent signaling molecule.

Calcium-dependent inactivation controls cardiac L-type Ca2+ currents under β-adrenergic stimulation

During a cardiac action potential, the activity of L-type Ca²⁺ channels (LTCCs) is modulated by voltage- and calcium-dependent inactivation processes. Morales et al. show that, in the context of β-adrenergic stimulation, calcium-dependent inactivation directs the regulation of LTCC activity, limiting calcium influx during the action potential.

The activity of L-type calcium channels is associated with the duration of the plateau phase of the cardiac action potential (AP) and it is controlled by voltage- and calcium-dependent inactivation (VDI and CDI, respectively). During β-adrenergic stimulation, an increase in the L-type current and parallel changes in VDI and CDI are observed during square pulses stimulation; however, how these modifications impact calcium currents during an AP remains controversial. Here, we examined the role of both inactivation processes on the L-type calcium current activity in newborn rat cardiomyocytes in control conditions and after stimulation with the β-adrenergic agonist isoproterenol. Our approach combines a self-AP clamp (sAP-Clamp) with the independent inhibition of VDI or CDI (by overexpressing Ca_Vβ_2a or calmodulin mutants, respectively) to directly record the L-type calcium current during the cardiac AP. We find that at room temperature (20–23°C) and in the absence of β-adrenergic stimulation, the L-type current recapitulates the AP kinetics. Furthermore, under our experimental setting, the activity of the sodium–calcium exchanger (NCX) does not affect the shape of the AP. We find that hindering either VDI or CDI prolongs the L-type current and the AP in parallel, suggesting that both inactivation processes modulate the L-type current during the AP. In the presence of isoproterenol, wild-type and VDI-inhibited cardiomyocytes display mismatched L-type calcium current with respect to their AP. In contrast, CDI-impaired cells maintain L-type current with kinetics similar to its AP, demonstrating that calcium-dependent inactivation governs L-type current kinetics during β-adrenergic stimulation.

Unprecedented high catecholamine production causing hair pigmentation after urinary excretion in red deer

Hormones have not been found in concentrations of orders of magnitude higher than nanograms per milliliter. Here, we report urine concentrations of a catecholamine (norepinephrine) ranging from 0.05 to 0.5 g/l, and concentrations of its metabolite DL-3,4-dihydroxyphenyl glycol (DOPEG) ranging from 1.0 to 44.5 g/l, in wild male red deer Cervus elaphus hispanicus after LC-MS analyses. The dark ventral patch of male red deer, a recently described sexually selected signal, contains high amounts of DOPEG (0.9-266.9 mg/l) stuck in the hairs, while DOPEG is not present in non-darkened hair. The formation of this dark patch is explained by the chemical structure of DOPEG, which is a catecholamine-derived o-diphenol susceptible to be oxidized by air and form allomelanins, nitrogen-free pigments similar to cutaneous melanins; by its high concentration in urine; and by the urine spraying behavior of red deer by which urine is spread through the ventral body area. Accordingly, the size of the dark ventral patch was positively correlated with the concentration of DOPEG in urine, which was in turn correlated with DOPEG absorbed in ventral hair. These findings represent catecholamine concentrations about one million higher than those previously reported for any hormone in an organism. This may have favored the evolution of the dark ventral patch of red deer by transferring information on the fighting capacity to rivals and mates. Physiological limits for hormone production in animals are thus considerably higher than previously thought. These results also unveil a novel mechanism of pigmentation based on the self-application of urine over the fur.

The Effect of Head-to-Head Competition on Behavioural Thermoregulation, Thermophysiological Strain and Performance During Exercise in the Heat

Background

It has been suggested that pacing is a thermoregulatory behaviour. We investigated the effect of competition on pacing, performance and thermophysiological strain during exercise in the heat and the psychological factors mediating competition effects.

Method

Eighteen males (maximum oxygen uptake [VO_2max] 3.69 [0.44] L min⁻¹) undertook a preliminary 20-km cool (wet-bulb globe temperature [WBGT] 12 °C) cycling time trial (TT) and three experimental 20-km trials (balanced order): (i) cool TT (CoolSolo); (ii) hot (WBGT 26 °C) TT (HotSolo); (iii) hot head-to-head competition (HotH2H). During TTs, an avatar of the participant’s performance was visible. During HotH2H, participants believed they were competing against another participant, but the competitor’s avatar replicated their own preliminary (cool) TT.

Results

TTs (min:sec [SD]) slowed with increased ambient temperature [CoolSolo 35:31 (2:11) versus HotSolo 36:10 (2:26); p = 0.011]. This effect was negated by competition; performances were not different between HotH2H [35:17 (1:52)] and CoolSolo (p = 0.160) and were quicker in HotH2H versus HotSolo (p = 0.001). End-exercise rectal temperature, mean body temperature and physiological strain index were (p < 0.05) higher in HotH2H than either solo condition. Despite faster performance and greater thermophysiological strain, rating of perceived exertion (RPE), thermal comfort and sensation, and perceptual strain index were not different between HotH2H and HotSolo. The difference in end-exercise rectal temperature between HotH2H and HotSolo was related to pre-exercise anticipatory heart rate response (r = 0.608, p = 0.010) and participants’ propensity for deliberate risk-taking (B = 0.12, p < 0.001), whereas self-reported resilience predicted change in performance times between HotH2H versus HotSolo (B = − 9.40, p = 0.010).

Conclusion

Competition changes the relationship between perceived and actual thermophysiological state, altering behavioural thermoregulation and increasing thermophysiological strain; this could increase heat-illness risk. Psychophysiological and psychological measures may identify susceptible individuals.

Voltage-Dependent Sarcolemmal Ion Channel Abnormalities in the Dystrophin-Deficient Heart

Mutations in the gene encoding for the intracellular protein dystrophin cause severe forms of muscular dystrophy. These so-called dystrophinopathies are characterized by skeletal muscle weakness and degeneration. Dystrophin deficiency also gives rise to considerable complications in the heart, including cardiomyopathy development and arrhythmias. The current understanding of the pathomechanisms in the dystrophic heart is limited, but there is growing evidence that dysfunctional voltage-dependent ion channels in dystrophin-deficient cardiomyocytes play a significant role. Herein, we summarize the current knowledge about abnormalities in voltage-dependent sarcolemmal ion channel properties in the dystrophic heart, and discuss the potentially underlying mechanisms, as well as their pathophysiological relevance.

Epinephrine Released During Traumatic Events May Strengthen Contextual Fear Memory Through Increased Hippocampus mRNA Expression of Nr4a Transcription Factors

Epinephrine (EPI) strengthens contextual fear memories by acting on peripheral β₂-adrenoceptors. Phenylethanolamine-N-methyltransferase-knockout (Pnmt-KO) mice are EPI-deficient mice and have reduced contextual fear learning. Our aim was to evaluate the molecular mechanisms by which peripheral EPI strengthens contextual fear memory and if a β₂-adrenoceptor antagonist can erase contextual fear memories. Pnmt-KO and wild-type (WT) mice were submitted to fear conditioning (FC) procedure after treatment with EPI, norepinephrine (NE), EPI plus ICI 118,551 (selective β₂-adrenoceptor antagonist), ICI 118,551 or vehicle (NaCl 0.9%). Catecholamines were separated and quantified by high performance liquid chromatography-electrochemical detection (HPLC-ED). Blood glucose was measured by coulometry. Real-time polymerase chain reaction (qPCR) was used to evaluate mRNA expression of nuclear receptor 4a1 (Nr4a1), Nr4a2 and Nr4a3 in hippocampus samples. In WT mice, plasma EPI concentration was significantly higher after fear acquisition (FA) compared with mice without the test. NE did not increase in plasma after FA and did not strengthen contextual fear memory, contrary to EPI. Freezing induced by EPI was blocked by ICI 118,551 in Pnmt-KO mice. In WT mice, ICI 118,551 blocked blood glucose release into the bloodstream after FA and decreased contextual fear memory. Nr4a1, Nr4a2 and Nr4a3 mRNA expression decreased in Pnmt-KO mice compared with WT mice after FC procedure. In Pnmt-KO mice, EPI induced an increase in mRNA expression of Nr4a2 compared to vehicle. In conclusion, EPI increases in plasma after an aversive experience, possibly improving long-term and old memories, by acting on peripheral β₂-adrenoceptors. Glucose could be the mediator of peripheral EPI in the central nervous system, inducing the expression of Nr4a transcription factor genes involved in consolidation of contextual fear memories.

Voltage-gated calcium channels: their discovery, function and importance as drug targets

This review will first describe the importance of Ca²⁺ entry for function of excitable cells, and the subsequent discovery of voltage-activated calcium conductances in these cells. This finding was rapidly followed by the identification of multiple subtypes of calcium conductance in different tissues. These were initially termed low- and high-voltage activated currents, but were then further subdivided into L-, N-, PQ-, R- and T-type calcium currents on the basis of differing pharmacology, voltage-dependent and kinetic properties, and single channel conductance. Purification of skeletal muscle calcium channels allowed the molecular identification of the pore-forming and auxiliary α₂δ, β and ϒ subunits present in these calcium channel complexes. These advances then led to the cloning of the different subunits, which permitted molecular characterisation, to match the cloned channels with physiological function. Studies with knockout and other mutant mice then allowed further investigation of physiological and pathophysiological roles of calcium channels. In terms of pharmacology, cardiovascular L-type channels are targets for the widely used antihypertensive 1,4-dihydropyridines and other calcium channel blockers, N-type channels are a drug target in pain, and α₂δ-1 is the therapeutic target of the gabapentinoid drugs, used in neuropathic pain. Recent structural advances have allowed a deeper understanding of Ca²⁺ permeation through the channel pore and the structure of both the pore-forming and auxiliary subunits. Voltage-gated calcium channels are subject to multiple pathways of modulation by G-protein and second messenger regulation. Furthermore, their trafficking pathways, subcellular localisation and functional specificity are the subjects of active investigation.

PKA phosphorylation of Cav1.2 channel modulates the interaction of calmodulin with the C terminal tail of the channel

Activity of cardiac Cav1.2 channels is enhanced by cyclic AMP-PKA signaling. In this study, we studied the effects of PKA phosphorylation on the binding of calmodulin to the fragment peptide of the proximal C-terminal tail of α1C subunit (CT1, a.a. 1509-1789 of guinea-pig). In the pull-down assay, in vitro PKA phosphorylation significantly decreased calmodulin binding to CT1 (61%) at high [Ca²⁺]. The phosphoresistant (CT1_SA) and phosphomimetic (CT1_SD) CT1 mutants, in which three PKA sites (Ser1574, 1626, 1699) were mutated to Ala and Asp, respectively, bound with calmodulin with 99% and 65% amount, respectively, compared to that of wild-type CT1. In contrast, at low [Ca²⁺], calmodulin-binding to CT1_SD was higher (33-35%) than that to CT1_SA. The distal C-terminal region of α1C (CT3, a.a. 1942-2169) is known to interact with CT1 and inhibit channel activity. CT3 bound to CT1_SD was also significantly less than that to CT1_SA. In inside-out patch, PKA catalytic subunit (PKAc) facilitated Ca²⁺ channel activity at both high and low Ca²⁺ condition. Altogether, these results support the hypothesis that PKA phosphorylation may enhance channel activity and attenuate the Ca²⁺-dependent inactivation, at least partially, by modulating calmodulin-CT1 interaction both directly and indirectly via CT3-CT1 interaction.