Graded Ca²⁺/calmodulin-dependent coupling of voltage-gated CaV1.2 channels

CaV1.3 channel clusters characterized by live-cell and isolated plasma membrane nanoscopy

A key player of excitable cells in the heart and brain is the L-type calcium channel CaV1.3. In the heart, it is required for voltage-dependent Ca2+-signaling, i.e., for controlling and modulating atrial cardiomyocyte excitation-contraction coupling. The clustering of CaV1.3 in functionally relevant channel multimers has not been addressed due to a lack of stoichiometric labeling combined with high-resolution imaging. Here, we developed a HaloTag-labeling strategy to visualize and quantify CaV1.3 clusters using STED nanoscopy to address the questions of cluster size and intra-cluster channel density. Channel clusters were identified in the plasma membrane of transfected live HEK293 cells as well as in giant plasma membrane vesicles derived from these cells that were spread on modified glass support to obtain supported plasma membrane bilayers (SPMBs). A small fraction of the channel clusters was colocalized with early and recycling endosomes at the membranes. STED nanoscopy in conjunction with live-cell and SPMB imaging enabled us to quantify CaV1.3 cluster sizes and their molecular density revealing significantly lower channel densities than expected for dense channel packing. CaV1.3 channel cluster size and molecular density were increased in SPMBs after treatment of the cells with the sympathomimetic compound isoprenaline, suggesting a regulated channel cluster condensation mechanism.

Pacemaking in the lymphatic system

Lymphatic collecting vessels exhibit spontaneous phasic contractions that are critical for lymph propulsion and tissue fluid homeostasis. This rhythmic activity is driven by action potentials conducted across the lymphatic muscle cell (LMC) layer to produce entrained contractions. The contraction frequency of a lymphatic collecting vessel displays exquisite mechanosensitivity, with a dynamic range from <1 to >20 contractions per minute. A myogenic pacemaker mechanism intrinsic to the LMCs was initially postulated to account for pressure-dependent chronotropy. Further interrogation into the cellular constituents of the lymphatic vessel wall identified non-muscle cell populations that shared some characteristics with interstitial cells of Cajal, which have pacemaker functions in the gastrointestinal and lower urinary tracts, thus raising the possibility of a non-muscle cell pacemaker. However, recent genetic knockout studies in mice support LMCs and a myogenic origin of the pacemaker activity. LMCs exhibit stochastic, but pressure-sensitive, sarcoplasmic reticulum calcium release (puffs and waves) from IP3R1 receptors, which couple to the calcium-activated chloride channel Anoctamin 1, causing depolarisation. The resulting electrical activity integrates across the highly coupled lymphatic muscle electrical syncytia through connexin 45 to modulate diastolic depolarisation. However, multiple other cation channels may also contribute to the ionic pacemaking cycle. Upon reaching threshold, a voltage-gated calcium channel-dependent action potential fires, resulting in a nearly synchronous calcium global calcium flash within the LMC layer to drive an entrained contraction. This review summarizes the key ion channels potentially responsible for the pressure-dependent chronotropy of lymphatic collecting vessels and various mechanisms of IP3R1 regulation that could contribute to frequency tuning.

Calcium- and calmodulin-dependent inhibition of NMDA receptor currents

Calcium ions (Ca2+) reduce NMDA receptor currents through several distinct mechanisms. Among these, calmodulin (CaM)-dependent inhibition (CDI) accomplishes rapid, reversible, and incomplete reduction of the NMDA receptor currents in response to elevations in intracellular Ca2+. Quantitative and mechanistic descriptions of CDI of NMDA receptor-mediated signals have been marred by variability originating, in part, from differences in the conditions and metrics used to evaluate this process across laboratories. Recent ratiometric approaches to measure the magnitude and kinetics of NMDA receptor CDI have facilitated rapid insights into this phenomenon. Notably, the kinetics and magnitude of NMDA receptor CDI depend on the degree of saturation of its CaM binding sites, which represent the bona fide calcium sensor for this type of inhibition, the kinetics and magnitude of the Ca2+ signal, which depends on the biophysical properties of the NMDA receptor or of adjacent Ca2+ sources, and on the relative distribution of Ca2+ sources and CaM molecules. Given that all these factors vary widely during development, across cell types, and with physiological and pathological states, it is important to understand how NMDA receptor CDI develops and how it contributes to signaling in the central nervous system. Here, we review briefly these recent advances and highlight remaining questions about the structural and kinetic mechanisms of NMDA receptor CDI. Given that pathologies can arise from several sources, including mutations in the NMDA receptor and in CaM, understanding how CaM responds to intracellular Ca2+ signals to initiate conformational changes in NMDA receptors, and mapping the structural domains responsible will help to envision novel therapeutic strategies to neuropsychiatric diseases, which presently have limited available treatments.

Oleic acid availability impacts thymocyte preprogramming and subsequent peripheral Treg cell differentiation

The nature of activation signals is essential in determining T cell subset differentiation; however, the features that determine T cell subset preference acquired during intrathymic development remain elusive. Here we show that naive CD4+ T cells generated in the mouse thymic microenvironment lacking Scd1, encoding the enzyme catalyzing oleic acid (OA) production, exhibit enhanced regulatory T (Treg) cell differentiation and attenuated development of experimental autoimmune encephalomyelitis. Scd1 deletion in K14+ thymic epithelia recapitulated the enhanced Treg cell differentiation phenotype of Scd1-deficient mice. The dearth of OA permitted DOT1L to increase H3K79me2 levels at the Atp2a2 locus of thymocytes at the DN2-DN3 transition stage. Such epigenetic modification persisted in naive CD4+ T cells and facilitated Atp2a2 expression. Upon T cell receptor activation, ATP2A2 enhanced the activity of the calcium-NFAT1-Foxp3 axis to promote naive CD4+ T cells to differentiate into Treg cells. Therefore, OA availability is critical for preprogramming thymocytes with Treg cell differentiation propensities in the periphery.

The formation of KV2.1 macro-clusters is required for sex-specific differences in L-type CaV1.2 clustering and function in arterial myocytes

In arterial myocytes, the canonical function of voltage-gated CaV1.2 and KV2.1 channels is to induce myocyte contraction and relaxation through their responses to membrane depolarization, respectively. Paradoxically, KV2.1 also plays a sex-specific role by promoting the clustering and activity of CaV1.2 channels. However, the impact of KV2.1 protein organization on CaV1.2 function remains poorly understood. We discovered that KV2.1 forms micro-clusters, which can transform into large macro-clusters when a critical clustering site (S590) in the channel is phosphorylated in arterial myocytes. Notably, female myocytes exhibit greater phosphorylation of S590, and macro-cluster formation compared to males. Contrary to current models, the activity of KV2.1 channels seems unrelated to density or macro-clustering in arterial myocytes. Disrupting the KV2.1 clustering site (KV2.1S590A) eliminated KV2.1 macro-clustering and sex-specific differences in CaV1.2 cluster size and activity. We propose that the degree of KV2.1 clustering tunes CaV1.2 channel function in a sex-specific manner in arterial myocytes.

α-Synuclein-dependent increases in PIP5K1γ drive inositol signaling to promote neurotoxicity

Anomalous aggregation of α-synuclein (α-Syn) is a pathological hallmark of many degenerative synucleinopathies including Lewy body dementia (LBD) and Parkinson's disease (PD). Despite its strong link to disease, the precise molecular mechanisms that link α-Syn aggregation to neurodegeneration have yet to be elucidated. Here, we find that elevated α-Syn leads to an increase in the plasma membrane (PM) phosphoinositide PI(4,5)P2, which precipitates α-Syn aggregation and drives toxic increases in mitochondrial Ca2+ and reactive oxygen species leading to neuronal death. Upstream of this toxic signaling pathway is PIP5K1γ, whose abundance and localization is enhanced at the PM by α-Syn-dependent increases in ARF6. Selective inhibition of PIP5K1γ or knockout of ARF6 in neurons rescues α-Syn aggregation and cellular phenotypes of toxicity. Collectively, our data suggest that modulation of phosphoinositide metabolism may be a therapeutic target to slow neurodegeneration for PD and other related neurodegenerative disorders.

Clustering of CaV 1.3 L-type calcium channels by Shank3

Clustering of L-type voltage-gated Ca2+ channels (LTCCs) in the plasma membrane is increasingly implicated in creating highly localized Ca2+ signaling nanodomains. For example, neuronal LTCC activation can increase phosphorylation of the nuclear CREB transcription factor by increasing Ca2+ concentrations within a nanodomain close to the channel, without requiring bulk Ca2+ increases in the cytosol or nucleus. However, the molecular basis for LTCC clustering is poorly understood. The postsynaptic scaffolding protein Shank3 specifically associates with one of the major neuronal LTCCs, the CaV 1.3 calcium channel, and is required for optimal LTCC-dependent excitation-transcription coupling. Here, we co-expressed CaV 1.3 α1 subunits with two distinct epitope-tags with or without Shank3 in HEK cells. Co-immunoprecipitation studies using the cell lysates revealed that Shank3 can assemble complexes containing multiple CaV 1.3 α1 subunits under basal conditions. Moreover, CaV 1.3 LTCC complex formation was facilitated by CaV β subunits (β3 and β2a), which also interact with Shank3. Shank3 interactions with CaV 1.3 LTCCs and multimeric CaV 1.3 LTCC complex assembly were disrupted following the addition of Ca2+ to cell lysates, perhaps simulating conditions within an activated CaV 1.3 LTCC nanodomain. In intact HEK293T cells, co-expression of Shank3 enhanced the intensity of membrane-localized CaV 1.3 LTCC clusters under basal conditions, but not after Ca2+ channel activation. Live cell imaging studies also revealed that Ca2+ influx through LTCCs disassociated Shank3 from CaV 1.3 LTCCs clusters and reduced the CaV 1.3 cluster intensity. Deletion of the Shank3 PDZ domain prevented both binding to CaV 1.3 and the changes in multimeric CaV 1.3 LTCC complex assembly in vitro and in HEK293 cells. Finally, we found that shRNA knock-down of Shank3 expression in cultured rat primary hippocampal neurons reduced the intensity of surface-localized CaV 1.3 LTCC clusters in dendrites. Taken together, our findings reveal a novel molecular mechanism contributing to neuronal LTCC clustering under basal conditions.

NPC1-dependent alterations in KV2.1-CaV1.2 nanodomains drive neuronal death in models of Niemann-Pick Type C disease

Lysosomes communicate through cholesterol transfer at endoplasmic reticulum (ER) contact sites. At these sites, the Niemann Pick C1 cholesterol transporter (NPC1) facilitates the removal of cholesterol from lysosomes, which is then transferred to the ER for distribution to other cell membranes. Mutations in NPC1 result in cholesterol buildup within lysosomes, leading to Niemann-Pick Type C (NPC) disease, a progressive and fatal neurodegenerative disorder. The molecular mechanisms connecting NPC1 loss to NPC-associated neuropathology remain unknown. Here we show both in vitro and in an animal model of NPC disease that the loss of NPC1 function alters the distribution and activity of voltage-gated calcium channels (CaV). Underlying alterations in calcium channel localization and function are KV2.1 channels whose interactions drive calcium channel clustering to enhance calcium entry and fuel neurotoxic elevations in mitochondrial calcium. Targeted disruption of KV2-CaV interactions rescues aberrant CaV1.2 clustering, elevated mitochondrial calcium, and neurotoxicity in vitro. Our findings provide evidence that NPC is a nanostructural ion channel clustering disease, characterized by altered distribution and activity of ion channels at membrane contacts, which contribute to neurodegeneration.

Diversity of cells and signals in the cardiovascular system

This white paper is the outcome of the seventh UC Davis Cardiovascular Research Symposium on Systems Approach to Understanding Cardiovascular Disease and Arrhythmia. This biannual meeting aims to bring together leading experts in subfields of cardiovascular biomedicine to focus on topics of importance to the field. The theme of the 2022 Symposium was 'Cell Diversity in the Cardiovascular System, cell-autonomous and cell-cell signalling'. Experts in the field contributed their experimental and mathematical modelling perspectives and discussed emerging questions, controversies, and challenges in examining cell and signal diversity, co-ordination and interrelationships involved in cardiovascular function. This paper originates from the topics of formal presentations and informal discussions from the Symposium, which aimed to develop a holistic view of how the multiple cell types in the cardiovascular system integrate to influence cardiovascular function, disease progression and therapeutic strategies. The first section describes the major cell types (e.g. cardiomyocytes, vascular smooth muscle and endothelial cells, fibroblasts, neurons, immune cells, etc.) and the signals involved in cardiovascular function. The second section emphasizes the complexity at the subcellular, cellular and system levels in the context of cardiovascular development, ageing and disease. Finally, the third section surveys the technological innovations that allow the interrogation of this diversity and advancing our understanding of the integrated cardiovascular function and dysfunction.

The formation of K V 2.1 macro-clusters is required for sex-specific differences in L-type Ca V 1.2 clustering and function in arterial myocytes

In arterial myocytes, the canonical function of voltage-gated Ca V 1.2 and K V 2.1 channels is to induce myocyte contraction and relaxation through their responses to membrane depolarization, respectively. Paradoxically, K V 2.1 also plays a sex-specific role by promoting the clustering and activity of Ca V 1.2 channels. However, the impact of K V 2.1 protein organization on Ca V 1.2 function remains poorly understood. We discovered that K V 2.1 forms micro-clusters, which can transform into large macro-clusters when a critical clustering site (S590) in the channel is phosphorylated in arterial myocytes. Notably, female myocytes exhibit greater phosphorylation of S590, and macro-cluster formation compared to males. Contrary to current models, the activity of K V 2.1 channels seems unrelated to density or macro-clustering in arterial myocytes. Disrupting the K V 2.1 clustering site (K V 2.1 S590A ) eliminated K V 2.1 macro-clustering and sex-specific differences in Ca V 1.2 cluster size and activity. We propose that the degree of K V 2.1 clustering tunes Ca V 1.2 channel function in a sex-specific manner in arterial myocytes.

Regulation of Cardiac Cav1.2 Channels by Calmodulin

Cav1.2 Ca2+ channels, a type of voltage-gated L-type Ca2+ channel, are ubiquitously expressed, and the predominant Ca2+ channel type, in working cardiac myocytes. Cav1.2 channels are regulated by the direct interactions with calmodulin (CaM), a Ca2+-binding protein that causes Ca2+-dependent facilitation (CDF) and inactivation (CDI). Ca2+-free CaM (apoCaM) also contributes to the regulation of Cav1.2 channels. Furthermore, CaM indirectly affects channel activity by activating CaM-dependent enzymes, such as CaM-dependent protein kinase II and calcineurin (a CaM-dependent protein phosphatase). In this article, we review the recent progress in identifying the role of apoCaM in the channel ‘rundown’ phenomena and related repriming of channels, and CDF, as well as the role of Ca2+/CaM in CDI. In addition, the role of CaM in channel clustering is reviewed.

Interactive nanocluster compaction of the ELKS scaffold and Cacophony Ca2+ channels drives sustained active zone potentiation

At presynaptic active zones (AZs), conserved scaffold protein architectures control synaptic vesicle (SV) release by defining the nanoscale distribution and density of voltage-gated Ca²⁺ channels (VGCCs). While AZs can potentiate SV release in the minutes range, we lack an understanding of how AZ scaffold components and VGCCs engage into potentiation. We here establish dynamic, intravital single-molecule imaging of endogenously tagged proteins at Drosophila AZs undergoing presynaptic homeostatic potentiation. During potentiation, the numbers of α1 VGCC subunit Cacophony (Cac) increased per AZ, while their mobility decreased and nanoscale distribution compacted. These dynamic Cac changes depended on the interaction between Cac channel's intracellular carboxyl terminus and the membrane-close amino-terminal region of the ELKS-family protein Bruchpilot, whose distribution compacted drastically. The Cac-ELKS/Bruchpilot interaction was also needed for sustained AZ potentiation. Our single-molecule analysis illustrates how the AZ scaffold couples to VGCC nanoscale distribution and dynamics to establish a state of sustained potentiation.

Endoplasmic Reticulum-Plasma Membrane Junctions as Sites of Depolarization-Induced Ca2+ Signaling in Excitable Cells

Membrane contact sites between endoplasmic reticulum (ER) and plasma membrane (PM), or ER-PM junctions, are found in all eukaryotic cells. In excitable cells they play unique roles in organizing diverse forms of Ca2+ signaling as triggered by membrane depolarization. ER-PM junctions underlie crucial physiological processes such as excitation-contraction coupling, smooth muscle contraction and relaxation, and various forms of activity-dependent signaling and plasticity in neurons. In many cases the structure and molecular composition of ER-PM junctions in excitable cells comprise important regulatory feedback loops linking depolarization-induced Ca2+ signaling at these sites to the regulation of membrane potential. Here, we describe recent findings on physiological roles and molecular composition of native ER-PM junctions in excitable cells. We focus on recent studies that provide new insights into canonical forms of depolarization-induced Ca2+ signaling occurring at junctional triads and dyads of striated muscle, as well as the diversity of ER-PM junctions in these cells and in smooth muscle and neurons.

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.

Functional tuning of Vascular L-type Ca2+ channels

Vascular smooth muscle contraction is intimately tied to membrane potential and the rise in intracellular Ca2+ enabled by the opening of L-type Ca2+ channels. While voltage is often viewed as the single critical factor gating these channels, research is starting to reveal a more intricate scenario whereby their function is markedly tuned. This emerging concept will be the focus of this three-part review, the first part articulating the mechanistic foundation of contractile development in vascular smooth muscle. Part two will extend this foundational knowledge, introducing readers to functional coupling and how neighboring L-type Ca2+ channels work cooperatively through signaling protein complexes, to facilitate their open probability. The final aspect of this review will discuss the impact of L-type Ca2+ channel trafficking, a process tied to cytoskeleton dynamics. Cumulatively, this brief manuscript provides new insight into how voltage, along with channel cooperativity and number, work in concert to tune Ca2+ responses and smooth muscle contraction.

Calmodulin promotes a Ca2+ -dependent conformational change in the C-terminal regulatory domain of CaV 1.2

Calmodulin (CaM) binds to the membrane-proximal cytosolic C-terminal domain of CaV 1.2 (residues 1520-1669, CT(1520-1669)) and causes Ca2+ -induced conformational changes that promote Ca2+ -dependent channel inactivation (CDI). We report biophysical studies that probe the structural interaction between CT(1520-1669) and CaM. The recombinantly expressed CT(1520-1669) is insoluble, but can be solubilized in the presence of Ca2+ -saturated CaM (Ca4 /CaM), but not in the presence of Ca2+ -free CaM (apoCaM). We show that half-calcified CaM (Ca2 /CaM12 ) forms a complex with CT(1520-1669) that is less soluble than CT(1520-1669) bound to Ca4 /CaM. The NMR spectrum of CT(1520-1669) reveals spectral differences caused by the binding of Ca2 /CaM12 versus Ca4 /CaM, suggesting that the binding of Ca2+ to the CaM N-lobe may induce a conformational change in CT(1520-1669).

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.

Biological noise is a key determinant of the reproducibility and adaptability of cardiac pacemaking and EC coupling

Each heartbeat begins with the generation of an action potential in pacemaking cells in the sinoatrial node. This signal triggers contraction of cardiac muscle through a process termed excitation-contraction (EC) coupling. EC coupling is initiated in dyadic structures of cardiac myocytes, where ryanodine receptors in the junctional sarcoplasmic reticulum come into close apposition with clusters of CaV1.2 channels in invaginations of the sarcolemma. Cooperative activation of CaV1.2 channels within these clusters causes a local increase in intracellular Ca2+ that activates the juxtaposed ryanodine receptors. A salient feature of healthy cardiac function is the reliable and precise beat-to-beat pacemaking and amplitude of Ca2+ transients during EC coupling. In this review, we discuss recent discoveries suggesting that the exquisite reproducibility of this system emerges, paradoxically, from high variability at subcellular, cellular, and network levels. This variability is attributable to stochastic fluctuations in ion channel trafficking, clustering, and gating, as well as dyadic structure, which increase intracellular Ca2+ variance during EC coupling. Although the effects of these large, local fluctuations in function and organization are sometimes negligible at the macroscopic level owing to spatial-temporal summation within and across cells in the tissue, recent work suggests that the "noisiness" of these intracellular Ca2+ events may either enhance or counterintuitively reduce variability in a context-dependent manner. Indeed, these noisy events may represent distinct regulatory features in the tuning of cardiac contractility. Collectively, these observations support the importance of incorporating experimentally determined values of Ca2+ variance in all EC coupling models. The high reproducibility of cardiac contraction is a paradoxical outcome of high Ca2+ signaling variability at subcellular, cellular, and network levels caused by stochastic fluctuations in multiple processes in time and space. This underlying stochasticity, which counterintuitively manifests as reliable, consistent Ca2+ transients during EC coupling, also allows for rapid changes in cardiac rhythmicity and contractility in health and disease.

Studying KcsA Channel Clustering Using Single Channel Voltage-Clamp Fluorescence Imaging*

Oligomerization and complex formation play a key role for many membrane proteins and has been described to influence ion channel function in both neurons and the heart. In this study, we observed clustering of single KcsA channels in planar lipid bilayer using single molecule fluorescence, while simultaneously measuring single channel currents. Clustering coincided with cooperative opening of KcsA. We demonstrate that clustering was not caused by direct protein-protein interactions or hydrophobic mismatch with the lipid environment, as suggested earlier, but was mediated via microdomains induced by the channel in the lipid matrix. We found that single channel activity of KcsA requires conically-shaped lipids in the lamellar liquid-crystalline (L_α) phase, and the need for a negative spontaneous curvature seem to lead to the deformations in the membrane that cause the clustering. The method introduced here will be applicable to follow oligomerization of a wide range of membrane proteins.

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.

Ion Channel Partnerships: Odd and Not-So-Odd Couples Controlling Neuronal Ion Channel Function

The concerted function of the large number of ion channels expressed in excitable cells, including brain neurons, shapes diverse signaling events by controlling the electrical properties of membranes. It has long been recognized that specific groups of ion channels are functionally coupled in mediating ionic fluxes that impact membrane potential, and that these changes in membrane potential impact ion channel gating. Recent studies have identified distinct sets of ion channels that can also physically and functionally associate to regulate the function of either ion channel partner beyond that afforded by changes in membrane potential alone. Here, we review canonical examples of such ion channel partnerships, in which a Ca²⁺ channel is partnered with a Ca²⁺-activated K⁺ channel to provide a dedicated route for efficient coupling of Ca²⁺ influx to K⁺ channel activation. We also highlight examples of non-canonical ion channel partnerships between Ca²⁺ channels and voltage-gated K⁺ channels that are not intrinsically Ca²⁺ sensitive, but whose partnership nonetheless yields enhanced regulation of one or the other ion channel partner. We also discuss how these ion channel partnerships can be shaped by the subcellular compartments in which they are found and provide perspectives on how recent advances in techniques to identify proteins in close proximity to one another in native cells may lead to an expanded knowledge of other ion channel partnerships.

Separation of the CaV 1.2-CaV 1.3 calcium channel duo prevents type 2 allergic airway inflammation

BACKGROUND

Voltage-gated calcium (Ca_v 1) channels contribute to T-lymphocyte activation. Ca_v 1.2 and Ca_v 1.3 channels are expressed in Th2 cells but their respective roles are unknown, which is investigated herein.

METHODS

We generated mice deleted for Ca_v 1.2 in T cells or Ca_v 1.3 and analyzed TCR-driven signaling. In this line, we developed original fast calcium imaging to measure early elementary calcium events (ECE). We also tested the impact of Ca_v 1.2 or Ca_v 1.3 deletion in models of type 2 airway inflammation. Finally, we checked whether the expression of both Ca_v 1.2 and Ca_v 1.3 in T cells from asthmatic children correlates with Th2-cytokine expression.

RESULTS

We demonstrated non-redundant and synergistic functions of Ca_v 1.2 and Ca_v 1.3 in Th2 cells. Indeed, the deficiency of only one channel in Th2 cells triggers TCR-driven hyporesponsiveness with weakened tyrosine phosphorylation profile, a strong decrease in initial ECE and subsequent reduction in the global calcium response. Moreover, Ca_v 1.3 has a particular role in calcium homeostasis. In accordance with the singular roles of Ca_v 1.2 and Ca_v 1.3 in Th2 cells, deficiency in either one of these channels was sufficient to inhibit cardinal features of type 2 airway inflammation. Furthermore, Ca_v 1.2 and Ca_v 1.3 must be co-expressed within the same CD4⁺ T cell to trigger allergic airway inflammation. Accordingly with the concerted roles of Ca_v 1.2 and Ca_v 1.3, the expression of both channels by activated CD4⁺ T cells from asthmatic children was associated with increased Th2-cytokine transcription.

CONCLUSIONS

Thus, Ca_v 1.2 and Ca_v 1.3 act as a duo, and targeting only one of these channels would be efficient in allergy treatment.

Nanoscale Organization, Regulation, and Dynamic Reorganization of Cardiac Calcium Channels

The architectural specializations and targeted delivery pathways of cardiomyocytes ensure that L-type Ca²⁺ channels (CaV1.2) are concentrated on the t-tubule sarcolemma within nanometers of their intracellular partners the type 2 ryanodine receptors (RyR2) which cluster on the junctional sarcoplasmic reticulum (jSR). The organization and distribution of these two groups of cardiac calcium channel clusters critically underlies the uniform contraction of the myocardium. Ca²⁺ signaling between these two sets of adjacent clusters produces Ca²⁺ sparks that in health, cannot escalate into Ca²⁺ waves because there is sufficient separation of adjacent clusters so that the release of Ca²⁺ from one RyR2 cluster or supercluster, cannot activate and sustain the release of Ca²⁺ from neighboring clusters. Instead, thousands of these Ca²⁺ release units (CRUs) generate near simultaneous Ca²⁺ sparks across every cardiomyocyte during the action potential when calcium induced calcium release from RyR2 is stimulated by depolarization induced Ca²⁺ influx through voltage dependent CaV1.2 channel clusters. These sparks summate to generate a global Ca²⁺ transient that activates the myofilaments and thus the electrical signal of the action potential is transduced into a functional output, myocardial contraction. To generate more, or less contractile force to match the hemodynamic and metabolic demands of the body, the heart responds to β-adrenergic signaling by altering activity of calcium channels to tune excitation-contraction coupling accordingly. Recent accumulating evidence suggests that this tuning process also involves altered expression, and dynamic reorganization of CaV1.2 and RyR2 channels on their respective membranes to control the amplitude of Ca²⁺ entry, SR Ca²⁺ release and myocardial function. In heart failure and aging, altered distribution and reorganization of these key Ca²⁺ signaling proteins occurs alongside architectural remodeling and is thought to contribute to impaired contractile function. In the present review we discuss these latest developments, their implications, and future questions to be addressed.

The Molecular Basis for the Calcium-Dependent Slow Afterhyperpolarization in CA1 Hippocampal Pyramidal Neurons

Neuronal signal transmission depends on the frequency, pattern, and timing of spike output, each of which are shaped by spike afterhyperpolarizations (AHPs). There are classically three post-spike AHPs of increasing duration categorized as fast, medium and slow AHPs that hyperpolarize a cell over a range of 10 ms to 30 s. Intensive early work on CA1 hippocampal pyramidal cells revealed that all three AHPs incorporate activation of calcium-gated potassium channels. The ionic basis for a fAHP was rapidly attributed to the actions of big conductance (BK) and the mAHP to small conductance (SK) or Kv7 potassium channels. In stark contrast, the ionic basis for a prominent slow AHP of up to 30 s duration remained an enigma for over 30 years. Recent advances in pharmacological, molecular, and imaging tools have uncovered the expression of a calcium-gated intermediate conductance potassium channel (IK, KCa3.1) in central neurons that proves to contribute to the slow AHP in CA1 hippocampal pyramidal cells. Together the data show that the sAHP arises in part from a core tripartite complex between Cav1.3 (L-type) calcium channels, ryanodine receptors, and IK channels at endoplasmic reticulum-plasma membrane junctions. Work on the sAHP in CA1 pyramidal neurons has again quickened pace, with identified contributions by both IK channels and the Na-K pump providing answers to several mysteries in the pharmacological properties of the sAHP.

Mechanisms and Physiological Implications of Cooperative Gating of Ion Channels Clusters

Ion channels play a central role in the regulation of nearly every cellular process. Dating back to the classic 1952 Hodgkin-Huxley model of the generation of the action potential, ion channels have always been thought of as independent agents. A myriad of recent experimental findings exploiting advances in electrophysiology, structural biology, and imaging techniques, however, have posed a serious challenge to this long-held axiom as several classes of ion channels appear to open and close in a coordinated, cooperative manner. Ion channel cooperativity ranges from variable-sized oligomeric cooperative gating in voltage-gated, dihydropyridine-sensitive Ca_v1.2 and Ca_v1.3 channels to obligatory dimeric assembly and gating of voltage-gated Na_v1.5 channels. Potassium channels, transient receptor potential channels, hyperpolarization cyclic nucleotide-activated channels, ryanodine receptors (RyRs), and inositol trisphosphate receptors (IP₃Rs) have also been shown to gate cooperatively. The implications of cooperative gating of these ion channels range from fine tuning excitation-contraction coupling in muscle cells to regulating cardiac function and vascular tone, to modulation of action potential and conduction velocity in neurons and cardiac cells, and to control of pace-making activity in the heart. In this review, we discuss the mechanisms leading to cooperative gating of ion channels, their physiological consequences and how alterations in cooperative gating of ion channels may induce a range of clinically significant pathologies.

Regulation of neuronal excitation-transcription coupling by Kv2.1-induced clustering of somatic L-type Ca2+ channels at ER-PM junctions

In hippocampal neurons, gene expression is triggered by electrical activity and Ca²⁺ entry via L-type Cav1.2 channels in a process called excitation–transcription coupling. We identified a domain on the voltage-gated K⁺ channel Kv2.1 that promotes the clustering of L-type Cav1.2 channels at endoplasmic reticulum–plasma membrane junctions in the soma of neurons. Importantly, we discovered by disrupting this domain that the Kv2.1-mediated clustering of Cav1.2 at this somatic microdomain is critical for depolarization-induced excitation–transcription coupling.

In mammalian brain neurons, membrane depolarization leads to voltage-gated Ca²⁺ channel-mediated Ca²⁺ influx that triggers diverse cellular responses, including gene expression, in a process termed excitation–transcription coupling. Neuronal L-type Ca²⁺ channels, which have prominent populations on the soma and distal dendrites of hippocampal neurons, play a privileged role in excitation–transcription coupling. The voltage-gated K⁺ channel Kv2.1 organizes signaling complexes containing the L-type Ca²⁺ channel Cav1.2 at somatic endoplasmic reticulum–plasma membrane junctions. This leads to enhanced clustering of Cav1.2 channels, increasing their activity. However, the downstream consequences of the Kv2.1-mediated regulation of Cav1.2 localization and function on excitation–transcription coupling are not known. Here, we have identified a region between residues 478 to 486 of Kv2.1’s C terminus that mediates the Kv2.1-dependent clustering of Cav1.2. By disrupting this Ca²⁺ channel association domain with either mutations or with a cell-penetrating interfering peptide, we blocked the Kv2.1-mediated clustering of Cav1.2 at endoplasmic reticulum–plasma membrane junctions and the subsequent enhancement of its channel activity and somatic Ca²⁺ signals without affecting the clustering of Kv2.1. These interventions abolished the depolarization-induced and L-type Ca²⁺ channel-dependent phosphorylation of the transcription factor CREB and the subsequent expression of c-Fos in hippocampal neurons. Our findings support a model whereby the Kv2.1-Ca²⁺ channel association domain-mediated clustering of Cav1.2 channels imparts a mechanism to control somatic Ca²⁺ signals that couple neuronal excitation to gene expression.

Photobleaching step analysis for robust determination of protein complex stoichiometries

The counting of discrete photobleaching steps in fluorescence microscopy is ideally suited to study protein complex stoichiometry in situ. The counting range of photobleaching step analysis has been significantly improved with more-sophisticated algorithms for step detection, albeit at an increasing computational cost and with the necessity for high-quality data. Here, we address concerns regarding robustness, automation, and experimental validation, optimizing both data acquisition and analysis. To make full use of the potential of photobleaching step analysis, we evaluate various labeling strategies with respect to their molecular brightness, photostability, and photoblinking. The developed analysis algorithm focuses on automation and computational efficiency. Moreover, we validate the developed methods with experimental data acquired on DNA origami labeled with defined fluorophore numbers, demonstrating counting of up to 35 fluorophores. Finally, we show the power of the combination of optimized trace acquisition and automated data analysis by counting labeled nucleoporin 107 in nuclear pore complexes of intact U2OS cells. The successful in situ application promotes this framework as a new resource enabling cell biologists to robustly determine the stoichiometries of molecular assemblies at the single-molecule level in an automated manner.

Ventricular voltage-gated ion channels: Detection, characteristics, mechanisms, and drug safety evaluation

Cardiac voltage-gated ion channels (VGICs) play critical roles in mediating cardiac electrophysiological signals, such as action potentials, to maintain normal heart excitability and contraction. Inherited or acquired alterations in the structure, expression, or function of VGICs, as well as VGIC-related side effects of pharmaceutical drug delivery can result in abnormal cellular electrophysiological processes that induce life-threatening cardiac arrhythmias or even sudden cardiac death. Hence, to reduce possible heart-related risks, VGICs must be acknowledged as important targets in drug discovery and safety studies related to cardiac disease. In this review, we first summarize the development and application of electrophysiological techniques that are employed in cardiac VGIC studies alone or in combination with other techniques such as cryoelectron microscopy, optical imaging and optogenetics. Subsequently, we describe the characteristics, structure, mechanisms, and functions of various well-studied VGICs in ventricular myocytes and analyze their roles in and contributions to both physiological cardiac excitability and inherited cardiac diseases. Finally, we address the implications of the structure and function of ventricular VGICs for drug safety evaluation. In summary, multidisciplinary studies on VGICs help researchers discover potential targets of VGICs and novel VGICs in heart, enrich their knowledge of the properties and functions, determine the operation mechanisms of pathological VGICs, and introduce groundbreaking trends in drug therapy strategies, and drug safety evaluation.

The Physiology and Pathophysiology of T-Tubules in the Heart

In cardiomyocytes, invaginations of the sarcolemmal membrane called t-tubules are critically important for triggering contraction by excitation-contraction (EC) coupling. These structures form functional junctions with the sarcoplasmic reticulum (SR), and thereby enable close contact between L-type Ca²⁺ channels (LTCCs) and Ryanodine Receptors (RyRs). This arrangement in turn ensures efficient triggering of Ca²⁺ release, and contraction. While new data indicate that t-tubules are capable of exhibiting compensatory remodeling, they are also widely reported to be structurally and functionally compromised during disease, resulting in disrupted Ca²⁺ homeostasis, impaired systolic and/or diastolic function, and arrhythmogenesis. This review summarizes these findings, while highlighting an emerging appreciation of the distinct roles of t-tubules in the pathophysiology of heart failure with reduced and preserved ejection fraction (HFrEF and HFpEF). In this context, we review current understanding of the processes underlying t-tubule growth, maintenance, and degradation, underscoring the involvement of a variety of regulatory proteins, including junctophilin-2 (JPH2), amphiphysin-2 (BIN1), caveolin-3 (Cav3), and newer candidate proteins. Upstream regulation of t-tubule structure/function by cardiac workload and specifically ventricular wall stress is also discussed, alongside perspectives for novel strategies which may therapeutically target these mechanisms.

Wenxin Keli for the Treatment of Arrhythmia-Systems Pharmacology and In Vivo Pharmacological Assessment

This study employed a systems pharmacology approach to identify the active compounds and action mechanisms of Wenxin Keli for arrhythmia treatment. Sixty-eight components identified in vivo and in vitro by UPLC/Q-TOF-MS were considered the potential active components of Wenxin Keli. Network pharmacology further revealed 33 key targets and 75 KEGG pathways as possible pathways and targets involved in WK-mediated treatment, with the CaMKII/CNCA1C/Ca²⁺ pathway being the most significantly affected. This finding was validated using an AC-induced rat arrhythmias model. Pretreatment with Wenxin Keli reduced the malignant arrhythmias and shortened RR, PR, and the QT interval. Wenxin Keli exerted some antiarrhythmic effects by inhibiting p-CaMKII and intracellular Ca²⁺ transients and overexpressing CNCA1C. Thus, suppressing SR Ca²⁺ release and maintaining intracellular Ca²⁺ balance may be the primary mechanism of Wenxin Keli against arrhythmia. In view of the significance of CaMKII and NCX identified in this experiment, we suggest that CaMKII and NCX are essential targets for treating arrhythmias.

Secondhand Smoke Exposure Impairs Ion Channel Function and Contractility of Mesenteric Arteries

Cigarette smoke, including secondhand smoke (SHS), has significant detrimental vascular effects, but its effects on myogenic tone of small resistance arteries and the underlying mechanisms are understudied. Although it is apparent that SHS contributes to endothelial dysfunction, much less is known about how this toxicant alters arterial myocyte contraction, leading to alterations in myogenic tone. The study's goal is to determine the effects of SHS on mesenteric arterial myocyte contractility and excitability. C57BL/6J male mice were randomly assigned to either filtered air (FA) or SHS (6 h/d, 5 d/wk) exposed groups for a 4, 8, or 12-weeks period. Third and fourth-order mesenteric arteries and arterial myocytes were acutely isolated and evaluated with pressure myography and patch clamp electrophysiology, respectively. Myogenic tone was found to be elevated in mesenteric arteries from mice exposed to SHS for 12 wk but not for 4 or 8 wk. These results were correlated with an increase in L-type Ca2+ channel activity in mesenteric arterial myocytes after 12 wk of SHS exposure. Moreover, 12 wk SHS exposed arterial myocytes have reduced total potassium channel current density, which correlates with a depolarized membrane potential (Vm). These results suggest that SHS exposure induces alterations in key ionic conductances that modulate arterial myocyte contractility and myogenic tone. Thus, chronic exposure to an environmentally relevant concentration of SHS impairs mesenteric arterial myocyte electrophysiology and myogenic tone, which may contribute to increased blood pressure and risks of developing vascular complications due to passive exposure to cigarette smoke.

L-type channel inactivation balances the increased peak calcium current due to absence of Rad in cardiomyocytes

The L-type Ca²⁺ channel (LTCC) provides trigger calcium to initiate cardiac contraction in a graded fashion that is regulated by L-type calcium current (I_Ca,L) amplitude and kinetics. Inactivation of LTCC is controlled to fine-tune calcium flux and is governed by voltage-dependent inactivation (VDI) and calcium-dependent inactivation (CDI). Rad is a monomeric G protein that regulates I_Ca,L and has recently been shown to be critical to β-adrenergic receptor (β-AR) modulation of I_Ca,L. Our previous work showed that cardiomyocyte-specific Rad knockout (cRadKO) resulted in elevated systolic function, underpinned by an increase in peak I_Ca,L, but without pathological remodeling. Here, we sought to test whether Rad-depleted LTCC contributes to the fight-or-flight response independently of β-AR function, resulting in I_Ca,L kinetic modifications to homeostatically balance cardiomyocyte function. We recorded whole-cell I_Ca,L from ventricular cardiomyocytes from inducible cRadKO and control (CTRL) mice. The kinetics of I_Ca,L stimulated with isoproterenol in CTRL cardiomyocytes were indistinguishable from those of unstimulated cRadKO cardiomyocytes. CDI and VDI are both enhanced in cRadKO cardiomyocytes without differences in action potential duration or QT interval. To confirm that Rad loss modulates LTCC independently of β-AR stimulation, we crossed a β₁,β₂-AR double-knockout mouse with cRadKO, resulting in a Rad-inducible triple-knockout mouse. Deletion of Rad in cardiomyocytes that do not express β₁,β₂-AR still yielded modulated I_Ca,L and elevated basal heart function. Thus, in the absence of Rad, increased Ca²⁺ influx is homeostatically balanced by accelerated CDI and VDI. Our results indicate that the absence of Rad can modulate the LTCC without contribution of β₁,β₂-AR signaling and that Rad deletion supersedes β-AR signaling to the LTCC to enhance in vivo heart function.

Mechanisms and Regulation of Cardiac CaV1.2 Trafficking

During cardiac excitation contraction coupling, the arrival of an action potential at the ventricular myocardium triggers voltage-dependent L-type Ca²⁺ (Ca_V1.2) channels in individual myocytes to open briefly. The level of this Ca²⁺ influx tunes the amplitude of Ca²⁺-induced Ca²⁺ release from ryanodine receptors (RyR2) on the junctional sarcoplasmic reticulum and thus the magnitude of the elevation in intracellular Ca²⁺ concentration and ultimately the downstream contraction. The number and activity of functional Ca_V1.2 channels at the t-tubule dyads dictates the amplitude of the Ca²⁺ influx. Trafficking of these channels and their auxiliary subunits to the cell surface is thus tightly controlled and regulated to ensure adequate sarcolemmal expression to sustain this critical process. To that end, recent discoveries have revealed the existence of internal reservoirs of preformed Ca_V1.2 channels that can be rapidly mobilized to enhance sarcolemmal expression in times of acute stress when hemodynamic and metabolic demand increases. In this review, we provide an overview of the current thinking on Ca_V1.2 channel trafficking dynamics in the heart. We highlight the numerous points of control including the biosynthetic pathway, the endosomal recycling pathway, ubiquitination, and lysosomal and proteasomal degradation pathways, and discuss the effects of β-adrenergic and angiotensin receptor signaling cascades on this process.

A Dynamic, Supramolecular View on the Multivalent Interaction between Influenza Virus and Host Cell

Understanding how influenza viruses traverse the mucus and recognize host cells is critical for evaluating their zoonotic potential, and for prevention and treatment of the disease. The surface of the influenza A virus is covered with the receptor-binding protein hemagglutinin and the receptor-cleaving enzyme neuraminidase, which jointly control the interactions between the virus and the host cell. These proteins are organized in closely spaced trimers and tetramers to facilitate multivalent interactions with sialic acid-terminated glycans. This review shows that the individually weak multivalent interactions of influenza viruses allow superselective binding, virus-induced recruitment of receptors, and the formation of dynamic complexes that facilitate molecular walking. Techniques to measure the avidity and receptor specificity of influenza viruses are reviewed, and the pivotal role of multivalent interactions with their emergent properties in crossing the mucus and entering host cells is discussed. A model is proposed for the initiation of cell entry through virus-induced receptor clustering. The multivalent interactions of influenza viruses are maintained in a dynamic regime by a functional balance between binding and cleaving.

Local hyperactivation of L-type Ca2+ channels increases spontaneous Ca2+ release activity and cellular hypertrophy in right ventricular myocytes from heart failure rats

Right ventricle (RV) dysfunction is an independent predictor of patient survival in heart failure (HF). However, the mechanisms of RV progression towards failing are not well understood. We studied cellular mechanisms of RV remodelling in a rat model of left ventricle myocardial infarction (MI)-caused HF. RV myocytes from HF rats show significant cellular hypertrophy accompanied with a disruption of transverse-axial tubular network and surface flattening. Functionally these cells exhibit higher contractility with lower Ca²⁺ transients. The structural changes in HF RV myocytes correlate with more frequent spontaneous Ca²⁺ release activity than in control RV myocytes. This is accompanied by hyperactivated L-type Ca²⁺ channels (LTCCs) located specifically in the T-tubules of HF RV myocytes. The increased open probability of tubular LTCCs and Ca²⁺ sparks activation is linked to protein kinase A-mediated channel phosphorylation that occurs locally in T-tubules. Thus, our approach revealed that alterations in RV myocytes in heart failure are specifically localized in microdomains. Our findings may indicate the development of compensatory, though potentially arrhythmogenic, RV remodelling in the setting of LV failure. These data will foster better understanding of mechanisms of heart failure and it could promote an optimized treatment of patients.

Cellular and molecular effects of hyperglycemia on ion channels in vascular smooth muscle

Diabetes affects millions of people worldwide. This devastating disease dramatically increases the risk of developing cardiovascular disorders. A hallmark metabolic abnormality in diabetes is hyperglycemia, which contributes to the pathogenesis of cardiovascular complications. These cardiovascular complications are, at least in part, related to hyperglycemia-induced molecular and cellular changes in the cells making up blood vessels. Whereas the mechanisms mediating endothelial dysfunction during hyperglycemia have been extensively examined, much less is known about how hyperglycemia impacts vascular smooth muscle function. Vascular smooth muscle function is exquisitely regulated by many ion channels, including several members of the potassium (K+) channel superfamily and voltage-gated L-type Ca2+ channels. Modulation of vascular smooth muscle ion channels function by hyperglycemia is emerging as a key contributor to vascular dysfunction in diabetes. In this review, we summarize the current understanding of how diabetic hyperglycemia modulates the activity of these ion channels in vascular smooth muscle. We examine underlying mechanisms, general properties, and physiological relevance in the context of myogenic tone and vascular reactivity.

Calmodulin binds to the N-terminal domain of the cardiac sodium channel Nav1.5

The cardiac voltage-gated sodium channel Nav1.5 conducts the rapid inward sodium current crucial for cardiomyocyte excitability. Loss-of-function mutations in its gene SCN5A are linked to cardiac arrhythmias such as Brugada Syndrome (BrS). Several BrS-associated mutations in the Nav1.5 N-terminal domain (NTD) exert a dominant-negative effect (DNE) on wild-type channel function, for which mechanisms remain poorly understood. We aim to contribute to the understanding of BrS pathophysiology by characterizing three mutations in the Nav1.5 NTD: Y87C-here newly identified-, R104W, and R121W. In addition, we hypothesize that the calcium sensor protein calmodulin is a new NTD binding partner. Recordings of whole-cell sodium currents in TsA-201 cells expressing WT and variant Nav1.5 showed that Y87C and R104W but not R121W exert a DNE on WT channels. Biotinylation assays revealed reduction in fully glycosylated Nav1.5 at the cell surface and in whole-cell lysates. Localization of Nav1.5 WT channel with the ER did not change in the presence of variants, as shown by transfected and stained rat neonatal cardiomyocytes. We demonstrated that calmodulin binds the Nav1.5 NTD using in silico modeling, SPOTS, pull-down, and proximity ligation assays. Calmodulin binding to the R121W variant and to a Nav1.5 construct missing residues 80-105, a predicted calmodulin-binding site, is impaired. In conclusion, we describe the new natural BrS Nav1.5 variant Y87C and present first evidence that calmodulin binds to the Nav1.5 NTD, which seems to be a determinant for the DNE.

Modeling the Interactions Between Sodium Channels Provides Insight Into the Negative Dominance of Certain Channel Mutations

BACKGROUND

Nav1.5 cardiac Na+ channel mutations can cause arrhythmogenic syndromes. Some of these mutations exert a dominant negative effect on wild-type channels. Recent studies showed that Na+ channels can dimerize, allowing coupled gating. This leads to the hypothesis that allosteric interactions between Na+ channels modulate their function and that these interactions may contribute to the negative dominance of certain mutations.

METHODS

To investigate how allosteric interactions affect microscopic and macroscopic channel function, we developed a modeling paradigm in which Markovian models of two channels are combined. Allosteric interactions are incorporated by modifying the free energies of the composite states and/or barriers between states.

RESULTS

Simulations using two generic 2-state models (C-O, closed-open) revealed that increasing the free energy of the composite states CO/OC leads to coupled gating. Simulations using two 3-state models (closed-open-inactivated) revealed that coupled closings must also involve interactions between further composite states. Using two 6-state cardiac Na+ channel models, we replicated previous experimental results mainly by increasing the energies of the CO/OC states and lowering the energy barriers between the CO/OC and the CO/OO states. The channel model was then modified to simulate a negative dominant mutation (Nav1.5 p.L325R). Simulations of homodimers and heterodimers in the presence and absence of interactions showed that the interactions with the variant channel impair the opening of the wild-type channel and thus contribute to negative dominance.

CONCLUSION

Our new modeling framework recapitulates qualitatively previous experimental observations and helps identifying possible interaction mechanisms between ion channels.

Molecular and cellular correlates in Kv channel clustering: entropy-based regulation of cluster ion channel density

Scaffold protein-mediated ion channel clustering at unique membrane sites is important for electrical signaling. Yet, the mechanism(s) by which scaffold protein-ion channel interactions lead to channel clustering or how cluster ion channel density is regulated is mostly not known. The voltage-activated potassium channel (Kv) represents an excellent model to address these questions as the mechanism underlying its interaction with the post-synaptic density 95 (PSD-95) scaffold protein is known to be controlled by the length of the extended ‘ball and chain’ sequence comprising the C-terminal channel region. Here, using sub-diffraction high-resolution imaging microscopy, we show that Kv channel ‘chain’ length regulates Kv channel density with a ‘bell’-shaped dependence, reflecting a balance between thermodynamic considerations controlling ‘chain’ recruitment by PSD-95 and steric hindrance due to the spatial proximity of multiple channel molecules. Our results thus reveal an entropy-based mode of channel cluster density regulation that mirrors the entropy-based regulation of the Kv channel-PSD-95 interaction. The implications of these findings for electrical signaling are discussed.

Calcium Permeable Channels in Cancer Hallmarks

Cancer, the second cause of death worldwide, is characterized by several common criteria, known as the “cancer hallmarks” such as unrestrained cell proliferation, cell death resistance, angiogenesis, invasion and metastasis. Calcium permeable channels are proteins present in external and internal biological membranes, diffusing Ca²⁺ ions down their electrochemical gradient. Numerous physiological functions are mediated by calcium channels, ranging from intracellular calcium homeostasis to sensory transduction. Consequently, calcium channels play important roles in human physiology and it is not a surprise the increasing number of evidences connecting calcium channels disorders with tumor cells growth, survival and migration. Multiple studies suggest that calcium signals are augmented in various cancer cell types, contributing to cancer hallmarks. This review focuses in the role of calcium permeable channels signaling in cancer with special attention to the mechanisms behind the remodeling of the calcium signals. Transient Receptor Potential (TRP) channels and Store Operated Channels (SOC) are the main extracellular Ca²⁺ source in the plasma membrane of non-excitable cells, while inositol trisphosphate receptors (IP₃R) are the main channels releasing Ca²⁺ from the endoplasmic reticulum (ER). Alterations in the function and/or expression of these calcium channels, as wells as, the calcium buffering by mitochondria affect intracellular calcium homeostasis and signaling, contributing to the transformation of normal cells into their tumor counterparts. Several compounds reported to counteract several cancer hallmarks also modulate the activity and/or the expression of these channels including non-steroidal anti-inflammatory drugs (NSAIDs) like sulindac and aspirin, and inhibitors of polyamine biosynthesis, like difluoromethylornithine (DFMO). The possible role of the calcium permeable channels targeted by these compounds in cancer and their action mechanism will be discussed also in the review.

Elevated basal transcription can underlie timothy channel association with autism related disorders

Timothy syndrome (TS) is a neurodevelopmental disorder caused by mutations in the pore-forming subunit α₁1.2 of the L-type voltage-gated Ca²⁺-channel Cav1.2, at positions G406R or G402S. Although both mutations cause cardiac arrhythmias, only Cav1.2^G406R is associated with the autism-spectrum-disorder (ASD). We show that transcriptional activation by Cav1.2^G406R and Cav1.2^G402S is driven by membrane depolarization through the Ras/ERK/CREB pathway in a process called excitation-transcription (ET) coupling, as previously shown for wt Cav1.2. This process requires the presence of the intracellular β-subunit of the channel. We found that only the autism-associated mutant Cav1.2^G406R, as opposed to the non-autistic mutated channel Cav1.2^G402S, exhibits a depolarization-independent CREB phosphorylation, and spontaneous transcription of cFos and MeCP2. A leftward voltage-shift typical of Cav1.2^G406R activation, increases channel opening at subthreshold potentials, resulting in an enhanced channel activity, as opposed to a rightward shift in Cav1.2^G402S. We suggest that the enhanced spontaneous Cav1.2^G406R activity accounts for the increase in basal transcriptional activation. This uncontroled transcriptional activation may result in the manifestation of long-term dysregulations such as autism. Thus, gating changes provide a mechanistic framework for understanding the molecular events underlying the autistic phenomena caused by the G406R Timothy mutation. They might clarify whether a constitutive transcriptional activation accompanies other VGCC that exhibit a leftward voltage-shift of activation and are also associated with long-term cognitive disorders.

Calmodulin mutant in central linker reduces the binding affinity with PreIQ and IQ while interacting with CaV1.2 channels

Calmodulin (CaM) was reported to interact with PreIQ and IQ of Ca_V1.2 channels, but to date, no explicit binding sites of CaM were illustrated. Therefore, in the present study, we firstly used MOE (Molecular Operating Environment) for protein-protein docking and we found that the most likely residues of CaM that play an important role in the interface are concentrated in central linker region. Next we examined the binding properties of CaM and its mutants to PreIQ and IQ by GST pull-down assays. Here we confirmed that CaM binds to PreIQ and IQ in a concentration-dependent and [Ca²⁺]-dependent manner. However, silencing the effect of N-lobe and C-lobe by mutating two Ca²⁺ binding sites of each lobe abolished [Ca²⁺]-dependence of CaM binding, but could not influence the combination. And the mutant in central linker reduced the binding of CaM/PreIQ and CaM/IQ especially at low [Ca²⁺]. We confirmed that N-lobe and C-lobe play vital role in sensing the change of Ca²⁺, and found that the central linker of CaM is involved in the binding of CaM to Ca_V1.2 channels in particular at low [Ca²⁺], not only participates in the combination with PreIQ, but also with IQ.

The LQT-associated calmodulin mutant E141G induces disturbed Ca2+-dependent binding and a flickering gating mode of the CaV1.2 channel

Calmodulin (CaM) mutations are associated with congenital long QT (LQT) syndrome (LQTS), which may be related to the dysregulation of the cardiac-predominant Ca²⁺ channel isoform Ca_V1.2. Among various mutants, CaM-E141G was identified as a critical missense variant. However, the interaction of this CaM mutant with the Ca_V1.2 channel has not been determined. In this study, by utilizing a semiquantitative pull-down assay, we explored the interaction of CaM-E141G with CaM-binding peptide fragments of the Ca_V1.2 channel. Using the patch-clamp technique, we also investigated the electrophysiological effects of the mutant on Ca_V1.2 channel activity. We found that the maximum binding (B_max) of CaM-E141G to the proximal COOH-terminal region, PreIQ-IQ, PreIQ, IQ, and NT (an NH₂-terminal peptide) was decreased (by 17.71-59.26%) compared with that of wild-type CaM (CaM-WT). In particular, the Ca²⁺-dependent increase in B_max became slower with the combination of CaM-E141G + PreIQ and IQ but faster in the case of NT. Functionally, CaM-WT and CaM-E141G at 500 nM Ca²⁺ decreased Ca_V1.2 channel activity to 24.88% and 55.99%, respectively, compared with 100 nM Ca²⁺, showing that the inhibitory effect was attenuated in CaM-E141G. The mean open time of the Ca_V1.2 channel was increased, and the number of blank traces with no channel opening was significantly decreased. Overall, CaM-E141G exhibits disrupted binding with the Ca_V1.2 channel and induces a flickering gating mode, which may result in the dysfunction of the Ca_V1.2 channel and, thus, the development of LQTS. The present study is the first to investigate the detailed binding properties and single-channel gating mode induced by the interaction of CaM-E141G with the Ca_V1.2 channel.

Modulation of Function, Structure and Clustering of K+ Channels by Lipids: Lessons Learnt from KcsA

KcsA, a prokaryote tetrameric potassium channel, was the first ion channel ever to be structurally solved at high resolution. This, along with the ease of its expression and purification, made KcsA an experimental system of choice to study structure–function relationships in ion channels. In fact, much of our current understanding on how the different channel families operate arises from earlier KcsA information. Being an integral membrane protein, KcsA is also an excellent model to study how lipid–protein and protein–protein interactions within membranes, modulate its activity and structure. In regard to the later, a variety of equilibrium and non-equilibrium methods have been used in a truly multidisciplinary effort to study the effects of lipids on the KcsA channel. Remarkably, both experimental and “in silico” data point to the relevance of specific lipid binding to two key arginine residues. These residues are at non-annular lipid binding sites on the protein and act as a common element to trigger many of the lipid effects on this channel. Thus, processes as different as the inactivation of channel currents or the assembly of clusters from individual KcsA channels, depend upon such lipid binding.

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.

Clusters of cooperative ion channels enable a membrane-potential-based mechanism for short-term memory

Across biological systems, cooperativity between proteins enables fast actions, supra-linear responses, and long-lasting molecular switches. In the nervous system, however, the function of cooperative interactions between voltage-dependent ionic channels remains largely unknown. Based on mathematical modeling, we here demonstrate that clusters of strongly cooperative ion channels can plausibly form bistable conductances. Consequently, clusters are permanently switched on by neuronal spiking, switched off by strong hyperpolarization, and remain in their state for seconds after stimulation. The resulting short-term memory of the membrane potential allows to generate persistent firing when clusters of cooperative channels are present together with non-cooperative spike-generating conductances. Dynamic clamp experiments in rodent cortical neurons confirm that channel cooperativity can robustly induce graded persistent activity - a single-cell based, multistable mnemonic firing mode experimentally observed in several brain regions. We therefore propose that ion channel cooperativity constitutes an efficient cell-intrinsic implementation for short-term memories at the voltage level.

Kv2.1 channels play opposing roles in regulating membrane potential, Ca2+ channel function, and myogenic tone in arterial smooth muscle

The accepted role of the protein Kv2.1 in arterial smooth muscle cells is to form K⁺ channels in the sarcolemma. Opening of Kv2.1 channels causes membrane hyperpolarization, which decreases the activity of L-type Ca_V1.2 channels, lowering intracellular Ca²⁺ ([Ca²⁺]_i) and causing smooth muscle relaxation. A limitation of this model is that it is based exclusively on data from male arterial myocytes. Here, we used a combination of electrophysiology as well as imaging approaches to investigate the role of Kv2.1 channels in male and female arterial myocytes. We confirmed that Kv2.1 plays a canonical conductive role but found it also has a structural role in arterial myocytes to enhance clustering of Ca_V1.2 channels. Less than 1% of Kv2.1 channels are conductive and induce membrane hyperpolarization. Paradoxically, by enhancing the structural clustering and probability of Ca_V1.2-Ca_V1.2 interactions within these clusters, Kv2.1 increases Ca²⁺ influx. These functional impacts of Kv2.1 depend on its level of expression, which varies with sex. In female myocytes, where expression of Kv2.1 protein is higher than in male myocytes, Kv2.1 has conductive and structural roles. Female myocytes have larger Ca_V1.2 clusters, larger [Ca²⁺]_i, and larger myogenic tone than male myocytes. In contrast, in male myocytes, Kv2.1 channels regulate membrane potential but not Ca_V1.2 channel clustering. We propose a model in which Kv2.1 function varies with sex: in males, Kv2.1 channels control membrane potential but, in female myocytes, Kv2.1 plays dual electrical and Ca_V1.2 clustering roles. This contributes to sex-specific regulation of excitability, [Ca²⁺]_i, and myogenic tone in arterial myocytes.

Bridging the Molecular-Cellular Gap in Understanding Ion Channel Clustering

The clustering of many voltage-dependent ion channel molecules at unique neuronal membrane sites such as axon initial segments, nodes of Ranvier, or the post-synaptic density, is an active process mediated by the interaction of ion channels with scaffold proteins and is of immense importance for electrical signaling. Growing evidence indicates that the density of ion channels at such membrane sites may affect action potential conduction properties and synaptic transmission. However, despite the emerging importance of ion channel density for electrical signaling, how ion channel-scaffold protein molecular interactions lead to cellular ion channel clustering, and how this process is regulated are largely unknown. In this review, we emphasize that voltage-dependent ion channel density at native clustering sites not only affects the density of ionic current fluxes but may also affect the conduction properties of the channel and/or the physical properties of the membrane at such locations, all changes that are expected to affect action potential conduction properties. Using the concrete example of the prototypical Shaker voltage-activated potassium channel (Kv) protein, we demonstrate how insight into the regulation of cellular ion channel clustering can be obtained when the molecular mechanism of ion channel-scaffold protein interaction is known. Our review emphasizes that such mechanistic knowledge is essential, and when combined with super-resolution imaging microscopy, can serve to bridge the molecular-cellular gap in understanding the regulation of ion channel clustering. Pressing questions, challenges and future directions in addressing ion channel clustering and its regulation are discussed.