BIN1 knockdown rescues systolic dysfunction in aging male mouse hearts

Cardiac dysfunction is a hallmark of aging in humans and mice. Here we report that a two-week treatment to restore youthful Bridging Integrator 1 (BIN1) levels in the hearts of 24-month-old mice rejuvenates cardiac function and substantially reverses the aging phenotype. Our data indicate that age-associated overexpression of BIN1 occurs alongside dysregulated endosomal recycling and disrupted trafficking of cardiac CaV1.2 and type 2 ryanodine receptors. These deficiencies affect channel function at rest and their upregulation during acute stress. In vivo echocardiography reveals reduced systolic function in old mice. BIN1 knockdown using an adeno-associated virus serotype 9 packaged shRNA-mBIN1 restores the nanoscale distribution and clustering plasticity of ryanodine receptors and recovers Ca2+ transient amplitudes and cardiac systolic function toward youthful levels. Enhanced systolic function correlates with increased phosphorylation of the myofilament protein cardiac myosin binding protein-C. These results reveal BIN1 knockdown as a novel therapeutic strategy to rejuvenate the aging myocardium.

The Piezo channel is a mechano-sensitive complex component in the mammalian inner ear hair cell

The inner ear is the hub where hair cells (HCs) transduce sound, gravity, and head acceleration stimuli to the brain. Hearing and balance rely on mechanosensation, the fastest sensory signals transmitted to the brain. The mechanoelectrical transducer (MET) channel is the entryway for the sound-balance-brain interface, but the channel-complex composition is not entirely known. Here, we report that the mouse utilizes Piezo1 (Pz1) and Piezo2 (Pz2) isoforms as MET-complex components. The Pz channels, expressed in HC stereocilia, and cell lines are co-localized and co-assembled with MET complex partners. Mice expressing non-functional Pz1 and Pz2 at the ROSA26 locus have impaired auditory and vestibular traits that can only be explained if the Pzs are integral to the MET complex. We suggest that Pz subunits constitute part of the MET complex and that interactions with other MET complex components yield functional MET units to generate HC MET currents.

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.

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.

Cytoskeletal disarray increases arrhythmogenic vulnerability during sympathetic stimulation in a model of hypertrophic cardiomyopathy

Familial hypertrophic cardiomyopathy (FHC) patients are advised to avoid strenuous exercise due to increased risk of arrhythmias. Mice expressing the human FHC-causing mutation R403Q in the myosin heavy chain gene (MYH6) recapitulate the human phenotype, including cytoskeletal disarray and increased arrhythmia susceptibility. Following in vivo administration of isoproterenol, mutant mice exhibited tachyarrhythmias, poor recovery and fatigue. Arrhythmias were attenuated with the β-blocker atenolol and protein kinase A inhibitor PKI. Mutant cardiac myocytes had significantly prolonged action potentials and triggered automaticity due to reduced repolarization reserve and connexin 43 expression. Isoproterenol shortened cycle length, and escalated electrical instability. Surprisingly isoproterenol did not increase CaV1.2 current. We found alterations in CaV1.2-β1 adrenergic receptor colocalization assessed using super-resolution nanoscopy, and increased CaV1.2 phosphorylation in mutant hearts. Our results reveal for the first time that altered ion channel expression, co-localization and β-adrenergic receptor signaling associated with myocyte disarray contribute to electrical instability in the R403Q mutant heart.

The Piezo channel is central to the mechano-sensitive channel complex in the mammalian inner ear

The inner ear is the hub where hair cells transduce sound, gravity, and head acceleration stimuli carried by neural codes to the brain. Of all the senses, hearing and balance, which rely on mechanosensation, are the fastest sensory signals transmitted to the central nervous system. The mechanoelectrical transducer (MET) channel in hair cells is the entryway for the sound-balance-brain interface, but the channel's composition has eluded biologists due to its complexity. Here, we report that the mouse utilizes Piezo1 (Pz1) and Piezo2 (Pz2) isoforms as central components of the MET complex. The Pz channel subunits are expressed in hair-cell stereocilia, are co-localized and co-assembled, and are essential components of the MET complex in vitro and in situ, including integration with the transmembrane channel (Tmc1/2) protein. Mice expressing non-functional Pz1 and Pz2, but not functional Pz1 at the ROSA26 locus under the control of hair-cell promoters, have impaired auditory and vestibular traits that can only be explained if Pz channel multimers are integral to the MET complex. We affirm that Pz protein subunits constitute MET channels and that functional interactions with components of the MET complex yield current properties resembling hair-cell MET currents. Our results demonstrate Pz is a MET channel component central to interacting with MET complex proteins. Results account for the MET channel pore and complex.

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.

Acute phosphatidylinositol 4,5 bisphosphate depletion destabilizes sarcolemmal expression of cardiac L-type Ca2+ channel CaV1.2

Ca_V1.2 channels are critical players in cardiac excitation-contraction coupling, yet we do not understand how they are affected by an important therapeutic target of heart failure drugs and regulator of blood pressure, angiotensin II. Signaling through G_q-coupled AT1 receptors, angiotensin II triggers a decrease in PIP₂, a phosphoinositide component of the plasma membrane (PM) and known regulator of many ion channels. PIP₂ depletion suppresses Ca_V1.2 currents in heterologous expression systems but the mechanism of this regulation and whether a similar phenomenon occurs in cardiomyocytes is unknown. Previous studies have shown that Ca_V1.2 currents are also suppressed by angiotensin II. We hypothesized that these two observations are linked and that PIP₂ stabilizes Ca_V1.2 expression at the PM and angiotensin II depresses cardiac excitability by stimulating PIP₂ depletion and destabilization of Ca_V1.2 expression. We tested this hypothesis and report that Ca_V1.2 channels in tsA201 cells are destabilized after AT1 receptor-triggered PIP₂ depletion, leading to their dynamin-dependent endocytosis. Likewise, in cardiomyocytes, angiotensin II decreased t-tubular Ca_V1.2 expression and cluster size by inducing their dynamic removal from the sarcolemma. These effects were abrogated by PIP₂ supplementation. Functional data revealed acute angiotensin II reduced Ca_V1.2 currents and Ca²⁺ transient amplitudes thus diminishing excitation-contraction coupling. Finally, mass spectrometry results indicated whole-heart levels of PIP₂ are decreased by acute angiotensin II treatment. Based on these observations, we propose a model wherein PIP₂ stabilizes Ca_V1.2 membrane lifetimes, and angiotensin II-induced PIP₂ depletion destabilizes sarcolemmal Ca_V1.2, triggering their removal, and the acute reduction of Ca_V1.2 currents and contractility.

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.

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.

IP3R-driven increases in mitochondrial Ca2+ promote neuronal death in NPC disease

Ca2+ is the most ubiquitous second messenger in neurons whose spatial and temporal elevations are tightly controlled to initiate and orchestrate diverse intracellular signaling cascades. Numerous neuropathologies result from mutations or alterations in Ca2+ handling proteins; thus, elucidating molecular pathways that shape Ca2+ signaling is imperative. Here, we report that loss-of-function, knockout, or neurodegenerative disease-causing mutations in the lysosomal cholesterol transporter, Niemann-Pick Type C1 (NPC1), initiate a damaging signaling cascade that alters the expression and nanoscale distribution of IP3R type 1 (IP3R1) in endoplasmic reticulum membranes. These alterations detrimentally increase Gq-protein coupled receptor-stimulated Ca2+ release and spontaneous IP3R1 Ca2+ activity, leading to mitochondrial Ca2+ cytotoxicity. Mechanistically, we find that SREBP-dependent increases in Presenilin 1 (PS1) underlie functional and expressional changes in IP3R1. Accordingly, expression of PS1 mutants recapitulate, while PS1 knockout abrogates Ca2+ phenotypes. These data present a signaling axis that links the NPC1 lysosomal cholesterol transporter to the damaging redistribution and activity of IP3R1 that precipitates cell death in NPC1 disease and suggests that NPC1 is a nanostructural disease.

NPC1 regulates the distribution of phosphatidylinositol 4-kinases at Golgi and lysosomal membranes

Cholesterol and phosphoinositides (PI) are two critically important lipids that are found in cellular membranes and dysregulated in many disorders. Therefore, uncovering molecular pathways connecting these essential lipids may offer new therapeutic insights. We report that loss of function of lysosomal Niemann-Pick Type C1 (NPC1) cholesterol transporter, which leads to neurodegenerative NPC disease, initiates a signaling cascade that alters the cholesterol/phosphatidylinositol 4-phosphate (PtdIns4P) countertransport cycle between Golgi-endoplasmic reticulum (ER), as well as lysosome-ER membrane contact sites (MCS). Central to these disruptions is increased recruitment of phosphatidylinositol 4-kinases-PI4KIIα and PI4KIIIβ-which boosts PtdIns4P metabolism at Golgi and lysosomal membranes. Aberrantly increased PtdIns4P levels elevate constitutive anterograde secretion from the Golgi complex, and mTORC1 recruitment to lysosomes. NPC1 disease mutations phenocopy the transporter loss of function and can be rescued by inhibition or knockdown of either key phosphoinositide enzymes or their recruiting partners. In summary, we show that the lysosomal NPC1 cholesterol transporter tunes the molecular content of Golgi and lysosome MCS to regulate intracellular trafficking and growth signaling in health and disease.

β-Adrenergic control of sarcolemmal CaV1.2 abundance by small GTPase Rab proteins

The number and activity of Ca_v1.2 channels in the cardiomyocyte sarcolemma tunes the magnitude of Ca²⁺-induced Ca²⁺ release and myocardial contraction. β-Adrenergic receptor (βAR) activation stimulates sarcolemmal insertion of Ca_V1.2. This supplements the preexisting sarcolemmal Ca_V1.2 population, forming large "superclusters" wherein neighboring channels undergo enhanced cooperative-gating behavior, amplifying Ca²⁺ influx and myocardial contractility. Here, we determine this stimulated insertion is fueled by an internal reserve of early and recycling endosome-localized, presynthesized Ca_V1.2 channels. βAR-activation decreased Ca_V1.2/endosome colocalization in ventricular myocytes, as it triggered "emptying" of endosomal Ca_V1.2 cargo into the t-tubule sarcolemma. We examined the rapid dynamics of this stimulated insertion process with live-myocyte imaging of channel trafficking, and discovered that Ca_V1.2 are often inserted into the sarcolemma as preformed, multichannel clusters. Similarly, entire clusters were removed from the sarcolemma during endocytosis, while in other cases, a more incremental process suggested removal of individual channels. The amplitude of the stimulated insertion response was doubled by coexpression of constitutively active Rab4a, halved by coexpression of dominant-negative Rab11a, and abolished by coexpression of dominant-negative mutant Rab4a. In ventricular myocytes, βAR-stimulated recycling of Ca_V1.2 was diminished by both nocodazole and latrunculin-A, suggesting an essential role of the cytoskeleton in this process. Functionally, cytoskeletal disruptors prevented βAR-activated Ca²⁺ current augmentation. Moreover, βAR-regulation of Ca_V1.2 was abolished when recycling was halted by coapplication of nocodazole and latrunculin-A. These findings reveal that βAR-stimulation triggers an on-demand boost in sarcolemmal Ca_V1.2 abundance via targeted Rab4a- and Rab11a-dependent insertion of channels that is essential for βAR-regulation of cardiac Ca_V1.2.

β-Adrenergic control of sarcolemmal CaV1.2 abundance by small GTPase Rab proteins

The number and activity of Cav1.2 channels in the cardiomyocyte sarcolemma tunes the magnitude of Ca2+-induced Ca2+ release and myocardial contraction. β-Adrenergic receptor (βAR) activation stimulates sarcolemmal insertion of CaV1.2. This supplements the preexisting sarcolemmal CaV1.2 population, forming large "superclusters" wherein neighboring channels undergo enhanced cooperative-gating behavior, amplifying Ca2+ influx and myocardial contractility. Here, we determine this stimulated insertion is fueled by an internal reserve of early and recycling endosome-localized, presynthesized CaV1.2 channels. βAR-activation decreased CaV1.2/endosome colocalization in ventricular myocytes, as it triggered "emptying" of endosomal CaV1.2 cargo into the t-tubule sarcolemma. We examined the rapid dynamics of this stimulated insertion process with live-myocyte imaging of channel trafficking, and discovered that CaV1.2 are often inserted into the sarcolemma as preformed, multichannel clusters. Similarly, entire clusters were removed from the sarcolemma during endocytosis, while in other cases, a more incremental process suggested removal of individual channels. The amplitude of the stimulated insertion response was doubled by coexpression of constitutively active Rab4a, halved by coexpression of dominant-negative Rab11a, and abolished by coexpression of dominant-negative mutant Rab4a. In ventricular myocytes, βAR-stimulated recycling of CaV1.2 was diminished by both nocodazole and latrunculin-A, suggesting an essential role of the cytoskeleton in this process. Functionally, cytoskeletal disruptors prevented βAR-activated Ca2+ current augmentation. Moreover, βAR-regulation of CaV1.2 was abolished when recycling was halted by coapplication of nocodazole and latrunculin-A. These findings reveal that βAR-stimulation triggers an on-demand boost in sarcolemmal CaV1.2 abundance via targeted Rab4a- and Rab11a-dependent insertion of channels that is essential for βAR-regulation of cardiac CaV1.2.

Disease-associated mutations in Niemann-Pick type C1 alter ER calcium signaling and neuronal plasticity

Niemann-Pick type C1 (NPC1) protein is essential for the transport of externally derived cholesterol from lysosomes to other organelles. Deficiency of NPC1 underlies the progressive NPC1 neurodegenerative disorder. Currently, there are no curative therapies for this fatal disease. Given the Ca2+ hypothesis of neurodegeneration, which posits that altered Ca2+ dynamics contribute to neuropathology, we tested if disease mutations in NPC1 alter Ca2+ signaling and neuronal plasticity. We determine that NPC1 inhibition or disease mutations potentiate store-operated Ca2+ entry (SOCE) due to a presenilin 1 (PSEN1)-dependent reduction in ER Ca2+ levels alongside elevated expression of the molecular SOCE components ORAI1 and STIM1. Associated with this dysfunctional Ca2+ signaling is destabilization of neuronal dendritic spines. Knockdown of PSEN1 or inhibition of the SREBP pathway restores Ca2+ homeostasis, corrects differential protein expression, reduces cholesterol accumulation, and rescues spine density. These findings highlight lysosomes as a crucial signaling platform responsible for tuning ER Ca2+ signaling, SOCE, and synaptic architecture in health and disease.

A stochastic model of ion channel cluster formation in the plasma membrane

Ion channels are often found arranged into dense clusters in the plasma membranes of excitable cells, but the mechanisms underlying the formation and maintenance of these functional aggregates are unknown. Here, we tested the hypothesis that channel clustering is the consequence of a stochastic self-assembly process and propose a model by which channel clusters are formed and regulated in size. Our hypothesis is based on statistical analyses of the size distributions of the channel clusters we measured in neurons, ventricular myocytes, arterial smooth muscle, and heterologous cells, which in all cases were described by exponential functions, indicative of a Poisson process (i.e., clusters form in a continuous, independent, and memory-less fashion). We were able to reproduce the observed cluster distributions of five different types of channels in the membrane of excitable and tsA-201 cells in simulations using a computer model in which channels are "delivered" to the membrane at randomly assigned locations. The model's three parameters represent channel cluster nucleation, growth, and removal probabilities, the values of which were estimated based on our experimental measurements. We also determined the time course of cluster formation and membrane dwell time for CaV1.2 and TRPV4 channels expressed in tsA-201 cells to constrain our model. In addition, we elaborated a more complex version of our model that incorporated a self-regulating feedback mechanism to shape channel cluster formation. The strong inference we make from our results is that CaV1.2, CaV1.3, BK, and TRPV4 proteins are all randomly inserted into the plasma membranes of excitable cells and that they form homogeneous clusters that increase in size until they reach a steady state. Further, it appears likely that cluster size for a diverse set of membrane-bound proteins and a wide range of cell types is regulated by a common feedback mechanism.

Niemann-Pick Type C Disease Reveals a Link between Lysosomal Cholesterol and PtdIns(4,5)P2 That Regulates Neuronal Excitability

There is increasing evidence that the lysosome is involved in the pathogenesis of a variety of neurodegenerative disorders. Thus, mechanisms that link lysosome dysfunction to the disruption of neuronal homeostasis offer opportunities to understand the molecular underpinnings of neurodegeneration and potentially identify specific therapeutic targets. Here, using a monogenic neurodegenerative disorder, NPC1 disease, we demonstrate that reduced cholesterol efflux from lysosomes aberrantly modifies neuronal firing patterns. The molecular mechanism linking alterations in lysosomal cholesterol egress to intrinsic tuning of neuronal excitability is a transcriptionally mediated upregulation of the ABCA1 transporter, whose PtdIns(4,5)P₂-floppase activity decreases plasma membrane PtdIns(4,5)P₂. The consequence of reduced PtdIns(4,5)P₂ is a parallel decrease in a key regulator of neuronal excitability, the voltage-gated KCNQ2/3 potassium channel, which leads to hyperexcitability in NPC1 disease neurons. Thus, cholesterol efflux from lysosomes regulates PtdIns(4,5)P₂ to shape the electrical and functional identity of the plasma membrane of neurons in health and disease.

β-adrenergic-mediated dynamic augmentation of sarcolemmal CaV 1.2 clustering and co-operativity in ventricular myocytes

Key points

Prevailing dogma holds that activation of the β‐adrenergic receptor/cAMP/protein kinase A signalling pathway leads to enhanced L‐type Ca_V1.2 channel activity, resulting in increased Ca²⁺ influx into ventricular myocytes and a positive inotropic response. However, the full mechanistic and molecular details underlying this phenomenon are incompletely understood.

Ca_V1.2 channel clusters decorate T‐tubule sarcolemmas of ventricular myocytes. Within clusters, nanometer proximity between channels permits Ca²⁺‐dependent co‐operative gating behaviour mediated by physical interactions between adjacent channel C‐terminal tails.

We report that stimulation of cardiomyocytes with isoproterenol, evokes dynamic, protein kinase A‐dependent augmentation of Ca_V1.2 channel abundance along cardiomyocyte T‐tubules, resulting in the appearance of channel ‘super‐clusters’, and enhanced channel co‐operativity that amplifies Ca²⁺ influx.

On the basis of these data, we suggest a new model in which a sub‐sarcolemmal pool of pre‐synthesized Ca_V1.2 channels resides in cardiomyocytes and can be mobilized to the membrane in times of high haemodynamic or metabolic demand, to tune excitation–contraction coupling.

Abstract

Voltage‐dependent L‐type Ca_V1.2 channels play an indispensable role in cardiac excitation–contraction coupling. Activation of the β‐adrenergic receptor (βAR)/cAMP/protein kinase A (PKA) signalling pathway leads to enhanced Ca_V1.2 activity, resulting in increased Ca²⁺ influx into ventricular myocytes and a positive inotropic response. Ca_V1.2 channels exhibit a clustered distribution along the T‐tubule sarcolemma of ventricular myocytes where nanometer proximity between channels permits Ca²⁺‐dependent co‐operative gating behaviour mediated by dynamic, physical, allosteric interactions between adjacent channel C‐terminal tails. This amplifies Ca²⁺ influx and augments myocyte Ca²⁺ transient and contraction amplitudes. We investigated whether βAR signalling could alter Ca_V1.2 channel clustering to facilitate co‐operative channel interactions and elevate Ca²⁺ influx in ventricular myocytes. Bimolecular fluorescence complementation experiments reveal that the βAR agonist, isoproterenol (ISO), promotes enhanced Ca_V1.2–Ca_V1.2 physical interactions. Super‐resolution nanoscopy and dynamic channel tracking indicate that these interactions are expedited by enhanced spatial proximity between channels, resulting in the appearance of Ca_V1.2 ‘super‐clusters’ along the z‐lines of ISO‐stimulated cardiomyocytes. The mechanism that leads to super‐cluster formation involves rapid, dynamic augmentation of sarcolemmal Ca_V1.2 channel abundance after ISO application. Optical and electrophysiological single channel recordings confirm that these newly inserted channels are functional and contribute to overt co‐operative gating behaviour of Ca_V1.2 channels in ISO stimulated myocytes. The results of the present study reveal a new facet of βAR‐mediated regulation of Ca_V1.2 channels in the heart and support the novel concept that a pre‐synthesized pool of sub‐sarcolemmal Ca_V1.2 channel‐containing vesicles/endosomes resides in cardiomyocytes and can be mobilized to the sarcolemma to tune excitation–contraction coupling to meet metabolic and/or haemodynamic demands.

Prevailing dogma holds that activation of the β‐adrenergic receptor/cAMP/protein kinase A signalling pathway leads to enhanced L‐type Ca_V1.2 channel activity, resulting in increased Ca²⁺ influx into ventricular myocytes and a positive inotropic response. However, the full mechanistic and molecular details underlying this phenomenon are incompletely understood.

Ca_V1.2 channel clusters decorate T‐tubule sarcolemmas of ventricular myocytes. Within clusters, nanometer proximity between channels permits Ca²⁺‐dependent co‐operative gating behaviour mediated by physical interactions between adjacent channel C‐terminal tails.

We report that stimulation of cardiomyocytes with isoproterenol, evokes dynamic, protein kinase A‐dependent augmentation of Ca_V1.2 channel abundance along cardiomyocyte T‐tubules, resulting in the appearance of channel ‘super‐clusters’, and enhanced channel co‐operativity that amplifies Ca²⁺ influx.

On the basis of these data, we suggest a new model in which a sub‐sarcolemmal pool of pre‐synthesized Ca_V1.2 channels resides in cardiomyocytes and can be mobilized to the membrane in times of high haemodynamic or metabolic demand, to tune excitation–contraction coupling.

BIN1 Induces the Formation of T-Tubules and Adult-Like Ca2+ Release Units in Developing Cardiomyocytes

Human embryonic stem cell-derived cardiomyocytes (hESC-CMs) are at the center of new cell-based therapies for cardiac disease, but may also serve as a useful in vitro model for cardiac cell development. An intriguing feature of hESC-CMs is that although they express contractile proteins and have sarcomeres, they do not develop transverse-tubules (T-tubules) with adult-like Ca²⁺ release units (CRUs). We tested the hypothesis that expression of the protein BIN1 in hESC-CMs promotes T-tubules formation, facilitates Ca_V 1.2 channel clustering along the tubules, and results in the development of stable CRUs. Using electrophysiology, [Ca²⁺ ]_i imaging, and super resolution microscopy, we found that BIN1 expression induced T-tubule development in hESC-CMs, while increasing differentiation toward a more ventricular-like phenotype. Voltage-gated Ca_V 1.2 channels clustered along the surface sarcolemma and T-tubules of hESC-CM. The length and width of the T-tubules as well as the expression and size of Ca_V 1.2 clusters grew, as BIN1 expression increased and cells matured. BIN1 expression increased Ca_V 1.2 channel activity and the probability of coupled gating within channel clusters. Interestingly, BIN1 clusters also served as sites for sarcoplasmic reticulum (SR) anchoring and stabilization. Accordingly, BIN1-expressing cells had more Ca_V 1.2-ryanodine receptor junctions than control cells. This was associated with larger [Ca²⁺ ]_i transients during excitation-contraction coupling. Our data support the view that BIN1 is a key regulator of T-tubule formation and Ca_V 1.2 channel delivery. By studying the role of BIN1 during the differentiation of hESC-CMs, we show that BIN1 is also important for Ca_V 1.2 channel clustering, junctional SR organization, and the establishment of excitation-contraction coupling. Stem Cells 2019;37:54-64.

Abstract 105: Tuning EC-Coupling: β -Adrenergic Receptor Activation Stimulates Dynamic Augmentation of Ca V 1.2 Channel Sarcolemmal Abundance and Cooperativity

Voltage-gated L-type Ca V 1.2 channels play an essential role in cardiac excitation-contraction coupling. These channels decorate the z-lines of ventricular myocytes where they form discrete clusters on the t-tubule membrane. We have previously reported that groups of channels within these clusters are capable of influencing each other’s gating behavior and undergo dynamic, cooperative interactions. This occurs via Ca 2+ -calmodulin-dependent, physical interactions between the C-terminal tails of adjacent channels. The interactions increase the open probability ( P o ) of adjoined channels, and thus amplify Ca 2+ influx into the myocytes. It has long been known that activation of β -adrenergic receptors during the ‘fight or flight’ response, leads to increased mean open time and P o of Ca V 1.2 channels, and thus enhanced Ca 2+ influx. Given our previous finding that Ca V 1.2 channel cooperativity is triggered by Ca 2+ -influx, we predicted that β -AR stimulation could affect Ca V 1.2 channel cooperativity in ventricular myocytes. To test this hypothesis, we investigated the effect of the β -agonist isoproterenol (ISO) on Ca V 1.2 channel clustering and cooperativity using a combination of optical and electrophysiological approaches. Ground State Depletion super-resolution nanoscopy revealed ‘super-clustering’ of Ca V 1.2 channels in ISO-stimulated adult mouse ventricular myocytes. Channels within these super-clusters display enhanced physical interactions compared to control unstimulated cells as revealed by Bimolecular fluorescence complementation. To visualize this process, we performed live cardiomyocyte imaging and observed a robust, dynamic increase in sarcolemmal Ca V 1.2 channel abundance in response to ISO. Functionally, optical and electrophysiologicaI recordings of Ca V 1.2 channel activity revealed that ISO-stimulated cells display enhanced Ca V 1.2 channel cooperativity, and amplified Ca 2+ influx. On the basis of these data, we propose that stimulation of β ARs initiates dynamic augmentation of Ca V 1.2 channel abundance, and enhanced cooperative gating of Ca V 1.2 channels, in the sarcolemma of ventricular myocytes, to amplify Ca 2+ influx into these cells and tune EC-coupling in times of high metabolic or hemodynamic demand.

Dynamic L-type CaV1.2 channel trafficking facilitates CaV1.2 clustering and cooperative gating

L-type Ca_V1.2 channels are key regulators of gene expression, cell excitability and muscle contraction. Ca_V1.2 channels organize in clusters throughout the plasma membrane. This channel organization has been suggested to contribute to the concerted activation of adjacent Ca_V1.2 channels (e.g. cooperative gating). Here, we tested the hypothesis that dynamic intracellular and perimembrane trafficking of Ca_V1.2 channels is critical for formation and dissolution of functional channel clusters mediating cooperative gating. We found that Ca_V1.2 moves in vesicular structures of circular and tubular shape with diverse intracellular and submembrane trafficking patterns. Both microtubules and actin filaments are required for dynamic movement of Ca_V1.2 vesicles. These vesicles undergo constitutive homotypic fusion and fission events that sustain Ca_V1.2 clustering, channel activity and cooperative gating. Our study suggests that Ca_V1.2 clusters and activity can be modulated by diverse and unique intracellular and perimembrane vesicular dynamics to fine-tune Ca²⁺ signals.

A model for cooperative gating of L-type Ca2+ channels and its effects on cardiac alternans dynamics

In ventricular myocytes, membrane depolarization during the action potential (AP) causes synchronous activation of multiple L-type CaV1.2 channels (LTCCs), which trigger the release of calcium (Ca2+) from the sarcoplasmic reticulum (SR). This results in an increase in intracellular Ca2+ (Cai) that initiates contraction. During pulsus alternans, cardiac contraction is unstable, going from weak to strong in successive beats despite a constant heart rate. These cardiac alternans can be caused by the instability of membrane potential (Vm) due to steep AP duration (APD) restitution (Vm-driven alternans), instability of Cai cycling (Ca2+-driven alternans), or both, and may be modulated by functional coupling between clustered CaV1.2 (e.g. cooperative gating). Here, mathematical analysis and computational models were used to determine how changes in the strength of cooperative gating between LTCCs may impact membrane voltage and intracellular Ca2+ dynamics in the heart. We found that increasing the degree of coupling between LTCCs increases the amplitude of Ca2+ currents (ICaL) and prolongs AP duration (APD). Increased AP duration is known to promote cardiac alternans, a potentially arrhythmogenic substrate. In addition, our analysis shows that increasing the strength of cooperative activation of LTCCs makes the coupling of Ca2+ on the membrane voltage (Cai→Vm coupling) more positive and destabilizes the Vm-Cai dynamics for Vm-driven alternans and Cai-driven alternans, but not for quasiperiodic oscillation. These results suggest that cooperative gating of LTCCs may have a major impact on cardiac excitation-contraction coupling, not only by prolonging APD, but also by altering Cai→Vm coupling and potentially promoting cardiac arrhythmias.

Ground State Depletion Super-resolution Imaging in Mammalian Cells

Advances in fluorescent microscopy and cell biology are intimately correlated, with the enhanced ability to visualize cellular events often leading to dramatic leaps in our understanding of how cells function. The development and availability of super-resolution microscopy has considerably extended the limits of optical resolution from ~250-20 nm. Biologists are no longer limited to describing molecular interactions in terms of colocalization within a diffraction limited area, rather it is now possible to visualize the dynamic interactions of individual molecules. Here, we outline a protocol for the visualization and quantification of cellular proteins by ground-state depletion microscopy for fixed cell imaging. We provide examples from two different membrane proteins, an element of the endoplasmic reticulum translocon, sec61β, and a plasma membrane-localized voltage-gated L-type Ca²⁺ channel (CaV1.2). Discussed are the specific microscope parameters, fixation methods, photo-switching buffer formulation, and pitfalls and challenges of image processing.

Ca(2+) entry into neurons is facilitated by cooperative gating of clustered CaV1.3 channels

CaV1.3 channels regulate excitability in many neurons. As is the case for all voltage-gated channels, it is widely assumed that individual CaV1.3 channels behave independently with respect to voltage-activation, open probability, and facilitation. Here, we report the results of super-resolution imaging, optogenetic, and electrophysiological measurements that refute this long-held view. We found that the short channel isoform (CaV1.3S), but not the long (CaV1.3L), associates in functional clusters of two or more channels that open cooperatively, facilitating Ca(2+) influx. CaV1.3S channels are coupled via a C-terminus-to-C-terminus interaction that requires binding of the incoming Ca(2+) to calmodulin (CaM) and subsequent binding of CaM to the pre-IQ domain of the channels. Physically-coupled channels facilitate Ca(2+) currents as a consequence of their higher open probabilities, leading to increased firing rates in rat hippocampal neurons. We propose that cooperative gating of CaV1.3S channels represents a mechanism for the regulation of Ca(2+) signaling and electrical activity.

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

In the heart, reliable activation of Ca²⁺ release from the sarcoplasmic reticulum during the plateau of the ventricular action potential requires synchronous opening of multiple Ca_V1.2 channels. Yet the mechanisms that coordinate this simultaneous opening during every heartbeat are unclear. Here, we demonstrate that Ca_V1.2 channels form clusters that undergo dynamic, reciprocal, allosteric interactions. This ‘functional coupling’ facilitates Ca²⁺ influx by increasing activation of adjoined channels and occurs through C-terminal-to-C-terminal interactions. These interactions are initiated by binding of incoming Ca²⁺ to calmodulin (CaM) and proceed through Ca²⁺/CaM binding to the Ca_V1.2 pre-IQ domain. Coupling fades as [Ca²⁺]_i decreases, but persists longer than the current that evoked it, providing evidence for ‘molecular memory’. Our findings suggest a model for Ca_V1.2 channel gating and Ca²⁺-influx amplification that unifies diverse observations about Ca²⁺ signaling in the heart, and challenges the long-held view that voltage-gated channels open and close independently.

DOI:http://dx.doi.org/10.7554/eLife.05608.001

To pump blood around the body, the muscle cells within the heart must contract and relax together with a regular rhythm. A contraction begins when proteins called Ca_V1.2 channels embedded in the cell membranes of heart cells open to allow calcium ions to enter the cells. The calcium ions that enter through these Ca_V1.2 channels trigger the release of calcium ions from storage compartments within the cells, which leads to the heart contracting. However, to trigger this release of calcium ions, many Ca_V1.2 channels have to open at the same time and we do not yet know how this is co-ordinated.

Dixon et al. studied Ca_V1.2 channels in heart muscle cells from mice. The experiments show that these proteins are arranged in clusters of eight, on average, in the cell membrane. When calcium ions enter the cell they bind to a protein called calmodulin, which in turn binds to a Ca_V1.2 channel. This allows the Ca_V1.2 channels within a cluster to interact with each other. The physical association between Ca_V1.2 channels within clusters enables them to work cooperatively; they open at the same time to allow more calcium ions to enter and then close together to allow the cell to relax.

Dixon et al. found that even when levels of calcium ions in the cells declined, the Ca_V1.2 channels within clusters remained open for a little while longer before they closed. This suggests that the interactions between the Ca_V1.2 channels act as a type of ‘molecular memory’ that may alter how the cells respond to future activity.

These results challenge the previously held view that the Ca_V1.2 channels open and close independently of one another. Future studies will seek to understand the molecular details of how these channels cluster together, and how this clustering affects changes in heart rate and heart abnormalities like long QT syndrome.

DOI:http://dx.doi.org/10.7554/eLife.05608.002

Cellular mechanisms of ventricular arrhythmias in a mouse model of Timothy syndrome (long QT syndrome 8)

Ca(2+) flux through l-type CaV1.2 channels shapes the waveform of the ventricular action potential (AP) and is essential for excitation-contraction (EC) coupling. Timothy syndrome (TS) is a disease caused by a gain-of-function mutation in the CaV1.2 channel (CaV1.2-TS) that decreases inactivation of the channel, which increases Ca(2+) influx, prolongs APs, and causes lethal arrhythmias. Although many details of the CaV1.2-TS channels are known, the cellular mechanisms by which they induce arrhythmogenic changes in intracellular Ca(2+) remain unclear. We found that expression of CaV1.2-TS channels increased sarcolemmal Ca(2+) "leak" in resting TS ventricular myocytes. This resulted in higher diastolic [Ca(2+)]i in TS ventricular myocytes compared to WT. Accordingly, TS myocytes had higher sarcoplasmic reticulum (SR) Ca(2+) load and Ca(2+) spark activity, larger amplitude [Ca(2+)]i transients, and augmented frequency of Ca(2+) waves. The large SR Ca(2+) release in TS myocytes had a profound effect on the kinetics of CaV1.2 current in these cells, increasing the rate of inactivation to a high, persistent level. This limited the amount of influx during EC coupling in TS myocytes. The relationship between the level of expression of CaV1.2-TS channels and the probability of Ca(2+) wave occurrence was non-linear, suggesting that even low levels of these channels were sufficient to induce maximal changes in [Ca(2+)]i. Depolarization of WT cardiomyocytes with a TS AP waveform increased, but did not equalize [Ca(2+)]i, compared to depolarization of TS myocytes with the same waveform. We propose that CaV1.2-TS channels increase [Ca(2+)] in the cytosol and the SR, creating a Ca(2+)overloaded state that increases the probability of arrhythmogenic spontaneous SR Ca(2+) release.

L-type Ca2+ channel function during Timothy syndrome

Voltage-gated, dihydropyridine-sensitive L-type Ca(2+) channels are multimeric proteins composed of a pore-forming transmembrane α(1) subunit (Ca(v)1.2) and accessory β, α(2)δ, and γ subunits. Ca(2+) entry via Ca(v)1.2 channels shapes the action potential (AP) of cardiac myocytes and is required for excitation-contraction coupling. Two de novo point mutations of Ca(v)1.2 glycine residues, G406R and G402S, cause a rare multisystem disorder called Timothy syndrome (TS). Here, we discuss recent work on the mechanisms by which Ca(v)1.2 channels bearing TS mutations display slowed inactivation that leads to increased Ca(2+) influx, prolonging the cardiac AP and promoting lethal arrhythmias. Based on these studies, we propose a model in which the scaffolding protein AKAP79/150 stabilizes the open conformation of Ca(v)1.2-TS channels and facilitates physical interactions among adjacent channels via their C-tails, increasing the activity of adjoining channels and amplifying Ca(2+) influx.

Complications following pancreatic transplantations: imaging features

Whole organ vascularized pancreatic transplant is a recognized treatment for diabetes and is increasingly being performed worldwide. The procedure itself is complex and is associated with significant mortality and morbidity. Despite improvements in surgical techniques, postoperative complications of pancreatic transplantation are still common and include graft rejection, pancreatitis, peripancreatic fluid collections, exocrine leaks, vascular thrombosis, and hemorrhage. In this pictorial essay, we review clinical presentation and imaging features of these complications. We also briefly discuss technique and complications of islet cell transplants.

Restoration of normal L-type Ca2+ channel function during Timothy syndrome by ablation of an anchoring protein

RATIONALE

L-type Ca(2+) (Ca(V)1.2) channels shape the cardiac action potential waveform and are essential for excitation-contraction coupling in heart. A gain-of-function G406R mutation in a cytoplasmic loop of Ca(V)1.2 channels causes long QT syndrome 8 (LQT8), a disease also known as Timothy syndrome. However, the mechanisms by which this mutation enhances Ca(V)1.2-LQT8 currents and generates lethal arrhythmias are unclear.

OBJECTIVE

To test the hypothesis that the anchoring protein AKAP150 modulates Ca(V)1.2-LQT8 channel gating in ventricular myocytes.

METHODS AND RESULTS

Using a combination of molecular, imaging, and electrophysiological approaches, we discovered that Ca(V)1.2-LQT8 channels are abnormally coupled to AKAP150. A pathophysiological consequence of forming this aberrant ion channel-anchoring protein complex is enhanced Ca(V)1.2-LQT8 currents. This occurs through a mechanism whereby the anchoring protein functions like a subunit of Ca(V)1.2-LQT8 channels that stabilizes the open conformation and augments the probability of coordinated openings of these channels. Ablation of AKAP150 restores normal gating in Ca(V)1.2-LQT8 channels and protects the heart from arrhythmias.

CONCLUSION

We propose that AKAP150-dependent changes in Ca(V)1.2-LQT8 channel gating may constitute a novel general mechanism for Ca(V)1.2-driven arrhythmias.

Disk diffusion antimicrobial susceptibility testing for clinical and epidemiologic purposes

Antimicrobial susceptibility testing is one of the most important and useful tests performed by the clinical microbiology laboratory. The value of disk susceptibility testing in epidemiology is enhanced by its simplicity of performance, reproducibility of results, and ability to indicate whether a single strain is likely to be the cause of many infections. By using a specific set of antimicrobials for either gram-positive or gram-negative bacterial susceptibility testing and by following recommended testing procedures, laboratories can reliably identify similarities and differences in strains. We propose a list of antimicrobial agents chosen specifically for their value in disk susceptibility testing for clinical and epidemiologic purposes and describe the recommended methods of testing.

Economic costs of respiratory tract infections in the United States

To evaluate one aspect of the impact of respiratory tract infections in the United States, national survey data were used to estimate direct and indirect economic costs. Overall, upper and lower respiratory tract infections are estimated to be responsible for approximately $15 billion in direct treatment costs. Physician charges account for about one half and hospital care accounts for approximately one quarter of these costs. An estimated 1.25 million patients are hospitalized yearly for community-acquired respiratory tract infections, and charges for their care are projected to exceed $4 billion. Almost 300 thousand patients acquire nosocomial respiratory infections yearly, and charges for treating these infections are approximately $470 million. Although costs associated with hospital care are substantial, approximately $10 billion (67 percent of the total estimated cost of these infections) results from treatment of patients in ambulatory settings. It is not possible to calculate the full magnitude of the indirect costs of respiratory infections, but losses in income of employed persons who miss work because of infection are calculated to be more than $9 billion per year.

Nosocomial respiratory infections

Nosocomial respiratory tract infections are major causes of excessive morbidity and mortality in US hospitals, affecting an estimated five to ten of every 1,000 patients. Patients with serious underlying diseases have an especially high risk of acquiring these infections, and that risk is magnified by exposure to respiratory therapy. Until recently, contaminated respiratory care devices were a major cause of infection, but procedures for the management of these devices have decreased their role substantially. Now, aspiration of oropharyngeal flora appears to be responsible for most cases of bacterial respiratory infections. Therefore, techniques to alter the flora of the oropharynx and to diminish the risk of aspiration are important priorities for infection control. Exposure to intensive care units (ICUs) is also a major risk factor for nosocomial pulmonary infection, and person-to-person spread of microorganisms within ICUs seems to be responsible for some of these infections. Increasing numbers of pulmonary infections are being caused by newly identified or previously uncommon pathogens, and this shift requires that traditional control techniques be reevaluated to assure that they remain relevant. Few new techniques have been developed recently to alter host susceptibility, prevent acquisition of virulent pathogens, or eradicate infectious organisms from their sources, but effective infection control programs in hospitals appear to reduce the incidence of nosocomial pulmonary infection, and implementation of these programs deserves serious consideration in all hospitals.