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Background and purpose Cannabidiol (CBD) has been shown to differentially regulate the mechanistic target of rapamycin complex 1 (mTORC1) in preclinical models of disease, where it reduces activity in models of epilepsies and cancer and increases it in models of multiple sclerosis (MS) and psychosis. Here we investigate the effects of phytocannabinoids on mTORC1 and define a molecular mechanism. Experimental approach A novel mechanism for phytocannabinoids was identified using the tractable model system, Dictyostelium discoideum. Using mouse embryonic fibroblasts, we further validate this new mechanism of action. We demonstrate clinical relevance using cells derived from healthy individuals and from people with MS (pwMS). Key results Both CBD and the more abundant cannabigerol (CBG) enhance mTORC1 activity in D. discoideum. We identify a mechanism for this effect involving inositol polyphosphate multikinase (IPMK), where elevated IPMK expression inverses the response to phytocannabinoids, decreasing mTORC1 activity upon treatment, providing new insight on phytocannabinoids' actions. We further validated this mechanism using mouse embryonic fibroblasts. Clinical relevance of this effect was shown in primary human peripheral blood mononuclear cells, where CBD and CBG treatment increased mTORC1 activity in cells derived from healthy individuals and decreased mTORC1 activity in cells derived from pwMS. Conclusion and implications Our findings suggest that both CBD and the abundant CBG differentially regulate mTORC1 signalling through a mechanism dependent on the activity of the upstream IPMK signalling pathway, with potential relevance to the treatment of mTOR-related disorders, including MS.

Ketogenic diets have been utilized for many years to improve health, and as a dietary approach for the treatment of a range of diseases, where the mechanism of these low carbohydrate and high fat diets is widely considered to be through the production of metabolic products of fat breakdown, called ketones. One of these diets, the medium chain triglyceride ketogenic diet, involves high fat dietary intake in the form of medium chain fatty acids (MCFAs), decanoic and octanoic acid, and is commonly used in endurance and high intensity exercises but has also demonstrated beneficial effects in the treatment of numerous pathologies including drug resistant epilepsy, cancer, and diabetes. Recent advances, using Dictyostelium discoideum as a model, have controversially proposed several direct molecular mechanisms for decanoic acid in this diet, independent of ketone generation. Studies in this model have identified that decanoic acid reduces phosphoinositide turnover, diacylglycerol kinase (DGK) activity, and also inhibits the mechanistic target of rapamycin complex 1 (mTORC1). These discoveries could potentially impact the treatment of a range of disorders including epilepsy, cancer and bipolar disorder. In this review, we summarize the newly proposed mechanisms for decanoic acid, identified using D. discoideum, and highlight potential roles in health and disease treatment.

Single-cell metabolic investigations are hampered by the absence of flexible tools to measure local partial pressure of O(2) (pO(2)) at high spatial-temporal resolution. To this end, we developed an optical sensor capable of measuring local pericellular pO(2) for subcellular resolution measurements with confocal imaging while simultaneously carrying out electrophysiological and/or chemo-mechanical single cell experiments. Here we present the OxySplot optrode, a ratiometric fluorescent O(2)-micro-sensor created by adsorbing O(2)-sensitive and O(2)-insensitive fluorophores onto micro-particles of silica. To protect the OxySplot optrode from the components and reactants of liquid environment without compromising access to O(2), the microparticles are coated with an optically clear silicone polymer (PDMS, polydimethylsiloxane). The PDMS coated OxySplot micro-particles are used alone or in a thin (~50 micron) PDMS layer of arbitrary shape referred to as the OxyMat. Additional top coatings on the OxyMat (e.g., fibronectin, laminin, polylysine, special photoactivatable surfaces etc.) facilitate adherence of cells. The OxySplots report the cellular pO(2) and micro-gradients of pO(2) without disrupting the flow of extracellular solutions or interfering with patch-clamp pipettes, mechanical attachments, and micro-superfusion. Since OxySplots and a cell can be imaged and spatially resolved, calibrated changes of pO(2) and intracellular events can be imaged simultaneously. In addition, the response-time (t(0.5) = 0.7 s, 0 - 160 mm Hg) of OxySplots is ~100 times faster than amperometric Clark-type polarization microelectrodes. Two usage example of OxySplots with cardiomyocytes show (1) OxySplots measuring pericellular pO(2) while tetramethylrhodamine methyl-ester (TMRM) was used to measure mitochondrial membrane potential (ΔΨ(m)); and (2) OxySplots measuring pO(2) during ischemia and reperfusion while rhod-2 was used to measure cytosolic [Ca(2+)](i) levels simultaneously. The OxySplot/OxyMat optrode system provides an affordable and highly adaptable optical sensor system for monitoring pO(2) with a diverse array of imaging systems, including high-speed, high-resolution confocal microscopes while physiological features are measured simultaneously.

Significance The mTORC1 complex provides a critical role in cell function, regulating a variety of processes including growth and autophagy. mTORC1 signaling is hyperactivated in a range of common diseases including cancer, epilepsy, and neurodegenerative disorders. Hence, mTORC1 signaling provides an important target for regulation in many contexts. Here, we show that decanoic acid, a key component of a widely used medicinal diet, reduces mTORC1 activity. We identify this in a tractable model system, and validate it in ex vivo rat brain tissue and in human iPSC-derived astrocytes from patients with a clinically relevant disease. Thus, we provide insight into an easily accessible therapeutic approach for a range of diseases., Low-glucose and -insulin conditions, associated with ketogenic diets, can reduce the activity of the mechanistic target of rapamycin complex 1 (mTORC1) signaling pathway, potentially leading to a range of positive medical and health-related effects. Here, we determined whether mTORC1 signaling is also a target for decanoic acid, a key component of the medium-chain triglyceride (MCT) ketogenic diet. Using a tractable model system, Dictyostelium, we show that decanoic acid can decrease mTORC1 activity, under conditions of constant glucose and in the absence of insulin, measured by phosphorylation of eukaryotic translation initiation factor 4E-binding protein 1 (4E-BP1). We determine that this effect of decanoic acid is dependent on a ubiquitin regulatory X domain-containing protein, mediating inhibition of a conserved Dictyostelium AAA ATPase, p97, a homolog of the human transitional endoplasmic reticulum ATPase (VCP/p97) protein. We then demonstrate that decanoic acid decreases mTORC1 activity in the absence of insulin and under high-glucose conditions in ex vivo rat hippocampus and in tuberous sclerosis complex (TSC) patient-derived astrocytes. Our data therefore indicate that dietary decanoic acid may provide a new therapeutic approach to down-regulate mTORC1 signaling.

AMPA receptors, which shape excitatory postsynaptic currents and are directly involved in overactivation of synaptic function during seizures, represent a well-accepted target for anti-epileptic drugs. Trans-4-butylcyclohexane carboxylic acid (4-BCCA) has emerged as a new promising anti-epileptic drug in several in vitro and in vivo seizure models, but the mechanism of its action remained unknown. The purpose of this study is to characterize structure and dynamics of 4-BCCA interaction with AMPA receptors.We studied the molecular mechanism of AMPA receptor inhibition by 4-BCCA using a combination of X-ray crystallography, mutagenesis, electrophysiological assays, and molecular dynamics simulations.We identified 4-BCCA binding sites in the transmembrane domain (TMD) of AMPA receptor, at the lateral portals formed by transmembrane segments M1-M4. At this binding site, 4-BCCA is very dynamic, assumes multiple poses, and can enter the ion channel pore.4-BCCA represents a low-affinity inhibitor of AMPA receptors that acts at the TMD sites distinct from non-competitive inhibitors, such as the anti-epileptic drug perampanel and the ion channel blockers. Further studies might examine the possibsility of synergistic use of these inhibitors in treatment of epilepsy and a wide range of neurological disorders and gliomas.This article is part of a themed issue on Structure Guided Pharmacology of Membrane Proteins (BJP 75th Anniversary). To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v179.14/issuetoc.

The sympathetic nervous system is essential for maintenance of cardiac function via activation of post-junctional adrenergic receptors. Prolonged adrenergic receptor activation, however, has deleterious long-term effects leading to hypertrophy and the development of heart failure. Here we investigate the effect of chronic adrenergic receptors activation on excitation-contraction coupling (ECC) in ventricular cardiomyocytes from a previously characterized mouse model of chronic sympathetic hyperactivity, which are genetically deficient in the adrenoceptor α2A and α2C genes (ARDKO). When compared to wild-type (WT) cardiomyocytes, ARDKO displayed reduced fractional shortening (~33%) and slower relaxation (~20%). Furthermore, ARDKO cells exhibited several electrophysiological changes such as action potential (AP) prolongation (~50%), reduced L-type calcium channel (LCC) current (~33%), reduced outward potassium (K+) currents (~30%), and increased sodium/calcium exchanger (NCX) activity (~52%). Consistent with reduced contractility and calcium (Ca2+) currents, the cytosolic Ca2+ ([Ca2+]i) transient from ARDKO animals was smaller and decayed slower. Importantly, no changes were observed in membrane resting potential, AP amplitude, or the inward K+ current. Finally, we modified our existing cardiac ECC computational model to account for changes in the ARDKO heart. Simulations suggest that cellular changes in the ARDKO heart resulted in variable and dyssynchronous Ca2+-induced Ca2+ release therefore altering [Ca2+]i transient dynamics and reducing force generation. In conclusion, chronic sympathetic hyperactivity impairs ECC by changing the density of several ionic currents (and thus AP repolarization) causing altered Ca2+ dynamics and contractile activity. This demonstrates the important role of ECC remodeling in the cardiac dysfunction secondary to chronic sympathetic activity.

Key points Our group previously discovered and characterized the microtubule mechanotransduction pathway linking diastolic stretch to NADPH oxidase 2-derived reactive oxygen species signals that regulate calcium sparks and calcium influx pathways. Here we used focused experimental tests to constrain and expand our existing computational models of calcium signalling in heart. Mechanistic and quantitative modelling revealed new insights in disease including: changes in microtubule network density and properties, elevated NOX2 expression, altered calcium release dynamics, how NADPH oxidase 2 is activated by and responds to stretch, and finally the degree to which normalizing mechano-activated reactive oxygen species signals can prevent calcium-dependent arrhythmias. Abstract Microtubule (MT) mechanotransduction links diastolic stretch to generation of NADPH oxidase 2 (NOX2)-dependent reactive oxygen species (ROS), signals we term X-ROS. While stretch-elicited X-ROS primes intracellular calcium (Ca2+ ) channels for synchronized activation in the healthy heart, the dysregulated excess in this pathway underscores asynchronous Ca2+ release and arrhythmia. Here, we expanded our existing computational models of Ca2+ signalling in heart to include MT-dependent mechanotransduction through X-ROS. Informed by new focused experimental tests to properly constrain our model, we quantify the role of X-ROS on excitation-contraction coupling in healthy and pathological conditions. This approach allowed for a mechanistic investigation that revealed new insights into X-ROS signalling in disease including changes in MT network density and post-translational modifications (PTMs), elevated NOX2 expression, altered Ca2+ release dynamics (i.e. Ca2+ sparks and Ca2+ waves), how NOX2 is activated by and responds to stretch, and finally the degree to which normalizing X-ROS can prevent Ca2+ -dependent arrhythmias.

Visualizing mitochondria in living Dictyostelium discoideum cells using fluorescent dyes is often problematic due to variability in staining, metabolism of the dyes, and unknown potential effects of the dyes on mitochondrial function. We show that fluorescent labelling of mitochondria, using an N-terminal mitochondrial localization sequence derived from the D. discoideum protein GcvH1 (glycine cleavage system H1) attached to a red fluorescent protein enables clear mitochondrial imaging. We also show that this labelling has no effect upon mitochondria load or respiratory function., Highlights • Live-cell visualization of mitochondria dynamics is currently difficult in Dictyostelium discoideum. • We provide an easy and rapid procedure for visualizing mitochondria. • This technology employs a plasmid, REMIT-mRFPmars, expressing a mitochondrial targeting sequence linked to mRFPmars. • Expression of REMIT-mRFPmars does not alter mitochondrial load or function, providing a suitable tag for live-cell studies.

High-fat, low-carbohydrate diets, known as ketogenic diets, have been used as a non-pharmacological treatment for refractory epilepsy. A key mechanism of this treatment is thought to be the generation of ketones, which provide brain cells (neurons and astrocytes) with an energy source that is more efficient than glucose, resulting in beneficial downstream metabolic changes, such as increasing adenosine levels, which might have effects on seizure control. However, some studies have challenged the central role of ketones because medium-chain fatty acids, which are part of a commonly used variation of the diet (the medium-chain triglyceride ketogenic diet), have been shown to directly inhibit AMPA receptors (glutamate receptors), and to change cell energetics through mitochondrial biogenesis. Through these mechanisms, medium-chain fatty acids rather than ketones are likely to block seizure onset and raise seizure threshold. The mechanisms underlying the ketogenic diet might also have roles in other disorders, such as preventing neurodegeneration in Alzheimer's disease, the proliferation and spread of cancer, and insulin resistance in type 2 diabetes. Analysing medium-chain fatty acids in future ketogenic diet studies will provide further insights into their importance in modified forms of the diet. Moreover, the results of these studies could facilitate the development of new pharmacological and dietary therapies for epilepsy and other disorders.

Ketogenic diets, used in epilepsy treatment, are considered to work through reduced glucose and ketone generation to regulate a range of cellular process including autophagy induction. Recent studies into the medium-chain triglyceride (MCT) ketogenic diet have suggested that medium-chain fatty acids (MCFAs) provided in the diet, decanoic acid and octanoic acid, cause specific therapeutic effects independent of glucose reduction, although a role in autophagy has not been investigated. Both autophagy and MCFAs have been widely studied in Dictyostelium, with findings providing important advances in the study of autophagy-related pathologies such as neurodegenerative diseases. Here, we utilize this model to analyze a role for MCFAs in regulating autophagy. We show that treatment with decanoic acid but not octanoic acid induces autophagosome formation and modulates autophagic flux in high glucose conditions. To investigate this effect, decanoic acid, but not octanoic acid, was found to induce the expression of autophagy-inducing proteins (Atg1 and Atg8), providing a mechanism for this effect. Finally, we demonstrate a range of related fatty acid derivatives with seizure control activity, 4BCCA, 4EOA, and Epilim (valproic acid), also function to induce autophagosome formation in this model. Thus, our data suggest that decanoic acid and related compounds may provide a less-restrictive therapeutic approach to activate autophagy.

Alzheimer's disease is the most prevalent form of dementia in the elderly but the precise causal mechanisms are still not fully understood. Growing evidence supports a significant role for Aβ42 oligomers in the development and progression of Alzheimer's. For example, intracellular soluble Aβ oligomers are thought to contribute to the early synaptic dysfunction associated with Alzheimer's disease, but the molecular mechanisms underlying this effect are still unclear. Here, we identify a novel mechanism that contributes to our understanding of the reported synaptic dysfunction. Using primary rat hippocampal neurons exposed for a short period of time to Aβ42 oligomers, we show a disruption in the activity-dependent phosphorylation cycle of SynapsinI at Ser9. SynapsinI is a pre-synaptic protein that responds to neuronal activity and regulates the availability of synaptic vesicles to participate in neurotransmitter release. Phosphorylation of SynapsinI at Ser9, modulates its distribution and interaction with synaptic vesicles. Our results show that in neurons exposed to Aβ42 oligomers, the levels of phosphorylated Ser9 of SynapsinI remain elevated during the recovery period following neuronal activity. We then investigated if this effect could be targeted by a putative therapeutic regime using valproic acid (a short branch-chained fatty acid) that has been proposed as a treatment for Alzheimer's disease. Exposure of Aβ42 treated neurons to valproic acid, showed that it restores the physiological regulation of SynapsinI after depolarisation. Our data provide a new insight on Aβ42-mediated pathology in Alzheimer's disease and supports the use of Valproic acid as a possible pharmaceutical intervention for the treatment of Alzheimer's disease.

Introduction In all types of muscle cells, contraction is initiated by an increase in intracellular Ca2+. Another similarity among skeletal, cardiac, and smooth muscle cells is that much of the Ca2+ responsible for contraction is released from the SR. Beyond these common features, however

Background and Purpose: Epidiolex, a form of highly purified cannabidiol (CBD) derived from Cannabis plants has demonstrated seizure control activity in patients with Dravet syndrome, without a fully elucidated mechanism of action. We have employed an unbiased approach to investigate this mechanism at a cellular level.\ud Experimental Approach: We use a tractable biomedical model organism, Dictyostelium, to identify protein controlling the effect of CBD and characterize this mechanism. We then translate these results to a Dravet Syndrome mouse model and an acute in vitro seizure model.\ud Key Results: CBD activity is partially dependent upon the mitochondrial glycine cleavage system component, GcvH1 in Dictyostelium, orthologous to the human GCSH protein, which is functionally linked to folate one-carbon metabolism (FOCM). Analysis of FOCM components identified a mechanism for CBD in directly inhibiting methionine synthesis. Analysis of brain tissue from a Dravet syndrome mouse model also showed drastically altered levels of one-carbon components including methionine, and an in vitro rat seizure model showed an elevated level of methionine that is attenuated following CBD treatment. \ud Conclusions and Implications: Our results suggest a novel mechanism for CBD in the regulating methionine levels, and identify altered one-carbon metabolism in Dravet syndrome and seizure activity.\ud voltage-dependent anion selective channel proteins, VDAC1.

Valproic acid (VPA) provides a common treatment for both epilepsy and bipolar disorder; however, common cellular mechanisms relating to both disorders have yet to be proposed. Here, we explore the possibility of a diacylglycerol kinase (DGK) playing a role in regulating the effect of VPA relating to the treatment of both disorders, using the biomedical model Dictyostelium discoideum. DGK enzymes provide the first step in the phosphoinositide recycling pathway, implicated in seizure activity. They also regulate levels of diacylglycerol (DAG), thereby regulating the protein kinase C (PKC) activity that is linked to bipolar disorder-related signalling. Here, we show that ablation of the single Dictyostelium dgkA gene results in reduced sensitivity to the acute effects of VPA on cell behaviour. Loss of dgkA also provides reduced sensitivity to VPA in extended exposure during development. To differentiate a potential role for this DGKA-dependent mechanism in epilepsy and bipolar disorder treatment, we further show that the dgkA null mutant is resistant to the developmental effects of a range of structurally distinct branched medium-chain fatty acids with seizure control activity and to the bipolar disorder treatment lithium. Finally, we show that VPA, lithium and novel epilepsy treatments function through DAG regulation, and the presence of DGKA is necessary for compound-specific increases in DAG levels following treatment. Thus, these experiments suggest that, in Dictyostelium, loss of DGKA attenuates a common cellular effect of VPA relating to both epilepsy and bipolar disorder treatments, and that a range of new compounds with this effect should be investigated as alternative therapeutic agents. This article has an associated First Person interview with the first author of the paper.

Perampanel is an adjunctive treatment for epilepsy that works through the direct inhibition of AMPA receptors. The same molecular mechanism has recently been shown for a fatty acid, decanoic acid, prescribed in the medium chain triglyceride ketogenic diet for the treatment of patients with drug-resistant epilepsy. Because each compound has been proposed to act through a distinct AMPA receptor binding site, we predicted that perampanel and decanoic acid would act synergistically against AMPA receptors and, consequently, seizures. Here, we show a synergistic interaction between perampanel and decanoic acid in direct AMPA receptor inhibition, in an ex vivo model of seizure activity, and against seizure-induced activity in human brain slices. These data support a potential role for combination treatment using perampanel and dietary decanoic acid to provide enhanced seizure control.

Since the original report of seizure control through starvation in the 1920s, the ketogenic diet has been considered an energy-related therapy. The diet was assumed to be functioning through the effect of reduced carbohydrate intake regulating cellular energy state, thus giving rise to seizure control. From this assumption, the generation of ketones during starvation provided an attractive mechanism for this altered energy state; however, many years of research has sought and largely failed to correlate seizure control and ketone levels. Due to this focus on ketones, few studies have examined a role for free fatty acids, as metabolic intermediates between the triglycerides provided in the diet and ketones, in seizure control. Recent discoveries have now suggested that the medium-chain fats, delivered through the medium-chain triglyceride (MCT) ketogenic diet, may provide a key therapeutic mechanism of the diet in seizure control. Here we describe an unusual pathway leading to this discovery, beginning with the use of a tractable non-animal model—Dictyostelium, through to the demonstration that medium-chain fats play a direct role in seizure control, and finally the identification of a mechanism of action of these fats and related congeners leading to reduced neural excitability and seizure control.

Natural compounds often have complex molecular structures and unknown\ud molecular targets. These characteristics make them difficult to analyse using a classical\ud pharmacological approach. Curcumin, the main curcuminoid of turmeric, is a complex\ud molecule possessing wide-ranging biological activities, cellular mechanisms and roles in\ud potential therapeutic treatment including Alzheimer’s disease and cancer. Here, we\ud investigate the physiological effects and molecular targets of curcumin in Dictyostelium\ud discoideum. We show curcumin causes acute effects on cell behaviour, reduces cell growth,\ud and slows multicellular development. We then employ a range of structurally related\ud compounds to show the distinct role of different structural groups cell behaviour, growth, and\ud development, highlighting active moieties in cell function, and showing that these cellular\ud effects are unrelated to the well-known antioxidant activity of curcumin. Molecular\ud mechanisms underlying the effect of curcumin and one synthetic analogue (EF24) were then\ud investigated to identify a curcumin-resistant mutant lacking the protein phosphatase 2A\ud regulatory subunit (PsrA) and an EF24-resistant mutant lacking the presenilin 1 orthologue\ud (PsenB). Using in-silico docking analysis, we then show that curcumin may function through\ud direct binding to a key regulatory region of PsrA. These findings reveal novel cellular and\ud molecular mechanisms for the function of curcumin and related compounds.

The amyloid hypothesis of Alzheimer's disease suggests that synaptic degeneration and pathology is caused by the accumulation of amyloid-β (Aβ) peptides derived from the amyloid precursor protein (APP). Subsequently, soluble Aβ oligomers cause the loss of synaptic proteins from neurons, a histopathological feature of Alzheimer's disease that correlates with the degree of dementia. In this study, the production of toxic forms of Aβ was examined in vitro using 7PA2 cells stably transfected with human APP. We show that conditioned media from 7PA2 cells containing Aβ oligomers caused synapse degeneration as measured by the loss of synaptic proteins, including synaptophysin and cysteine-string protein, from cultured neurons. Critically, conditioned media from 7PA2 cells treated with valproic acid (2-propylpentanoic acid (VPA)) or propylisopropylacetic acid (PIA) did not cause synapse damage. Treatment with VPA or PIA did not significantly affect total Aβ42 concentrations; rather these drugs selectively reduced the concentrations of Aβ42 oligomers in conditioned media. In contrast, treatment significantly increased the concentrations of Aβ42 monomers in conditioned media. VPA or PIA treatment reduced the concentrations of APP within lipid rafts, membrane compartments associated with Aβ production. These effects of VPA and PIA were reversed by the addition of platelet-activating factor, a bioactive phospholipid produced following activation of phospholipase A2, an enzyme sensitive to VPA and PIA. Collectively these data suggest that VPA and PIA reduce Aβ oligomers through inhibition of phospholipase A2 and suggest a novel therapeutic approach to Alzheimer's treatment.

The free Ca(2+) concentration within the mitochondrial matrix ([Ca(2+)]m) regulates the rate of ATP production and other [Ca(2+)]m sensitive processes. It is set by the balance between total Ca(2+) influx (through the mitochondrial Ca(2+) uniporter (MCU) and any other influx pathways) and the total Ca(2+) efflux (by the mitochondrial Na(+)/Ca(2+) exchanger and any other efflux pathways). Here we review and analyze the experimental evidence reported over the past 40years which suggest that in the heart and many other mammalian tissues a putative Na(+)/Ca(2+) exchanger is the major pathway for Ca(2+) efflux from the mitochondrial matrix. We discuss those reports with respect to a recent discovery that the protein product of the human FLJ22233 gene mediates such Na(+)/Ca(2+) exchange across the mitochondrial inner membrane. Among its many functional similarities to other Na(+)/Ca(2+) exchanger proteins is a unique feature: it efficiently mediates Li(+)/Ca(2+) exchange (as well as Na(+)/Ca(2+) exchange) and was therefore named NCLX. The discovery of NCLX provides both the identity of a novel protein and new molecular means of studying various unresolved quantitative aspects of mitochondrial Ca(2+) movement out of the matrix. Quantitative and qualitative features of NCLX are discussed as is the controversy regarding the stoichiometry of the NCLX Na(+)/Ca(2+) exchange, the electrogenicity of NCLX, the [Na(+)]i dependency of NCLX and the magnitude of NCLX Ca(2+) efflux. Metabolic features attributable to NCLX and the physiological implication of the Ca(2+) efflux rate via NCLX during systole and diastole are also briefly discussed.

The medium chain triglyceride (MCT) ketogenic diet is used extensively for treating refractory childhood epilepsy. This diet increases the plasma levels of medium straight chain fatty acids. A role for these and related fatty acids in seizure control has not been established. We compared the potency of an established epilepsy treatment, Valproate (VPA), with a range of MCT diet-associated fatty acids (and related branched compounds), using in vitro seizure and in vivo epilepsy models, and assessed side effect potential in vitro for one aspect of teratogenicity, for liver toxicology and in vivo for sedation, and for a neuroprotective effect. We identify specific medium chain fatty acids (both prescribed in the MCT diet, and related compounds branched on the fourth carbon) that provide significantly enhanced in vitro seizure control compared to VPA. The activity of these compounds on seizure control is independent of histone deacetylase inhibitory activity (associated with the teratogenicity of VPA), and does not correlate with liver cell toxicity. In vivo, these compounds were more potent in epilepsy control (perforant pathway stimulation induced status epilepticus), showed less sedation and enhanced neuroprotection compared to VPA. Our data therefore implicates medium chain fatty acids in the mechanism of the MCT ketogenic diet, and highlights a related new family of compounds that are more potent than VPA in seizure control with a reduced potential for side effects. This article is part of the Special Issue entitled ‘New Targets and Approaches to the Treatment of Epilepsy’., Highlights ► Medium chain straight and branched fatty acids were examined for seizure control in vitro. ► Structural specificity was shown for these compounds. ► Side effects were examined for active seizure-control compounds. ► In vivo seizure control, sedation and neuroprotection were shown for these compounds. ► Medium chain fatty acids relating to the MCT ketogenic diet may be active in seizure control.

Rationale: Transverse tubules (TTs) couple electric surface signals to remote intracellular Ca 2+ release units (CRUs). Diffraction-limited imaging studies have proposed loss of TT components as disease mechanism in heart failure (HF). Objectives: Objectives were to develop quantitative super-resolution strategies for live-cell imaging of TT membranes in intact cardiomyocytes and to show that TT structures are progressively remodeled during HF development, causing early CRU dysfunction. Methods and Results: Using stimulated emission depletion (STED) microscopy, we characterized individual TTs with nanometric resolution as direct readout of local membrane morphology 4 and 8 weeks after myocardial infarction (4pMI and 8pMI). Both individual and network TT properties were investigated by quantitative image analysis. The mean area of TT cross sections increased progressively from 4pMI to 8pMI. Unexpectedly, intact TT networks showed differential changes. Longitudinal and oblique TTs were significantly increased at 4pMI, whereas transversal components appeared decreased. Expression of TT-associated proteins junctophilin-2 and caveolin-3 was significantly changed, correlating with network component remodeling. Computational modeling of spatial changes in HF through heterogeneous TT reorganization and RyR2 orphaning (5000 of 20 000 CRUs) uncovered a local mechanism of delayed subcellular Ca 2+ release and action potential prolongation. Conclusions: This study introduces STED nanoscopy for live mapping of TT membrane structures. During early HF development, the local TT morphology and associated proteins were significantly altered, leading to differential network remodeling and Ca 2+ release dyssynchrony. Our data suggest that TT remodeling during HF development involves proliferative membrane changes, early excitation-contraction uncoupling, and network fracturing.

P>Purpose:Valproic acid (VPA) is widely used clinically in epilepsy, bipolar disorder, and migraine. In experimental models, it has also been shown to have neuroprotective and antiepileptogenic effects. Its mechanisms of action in these diverse conditions are, however, unclear, but there is some evidence indicating an effect of VPA upon protein kinase A (PKA) activity. We, therefore, asked whether VPA modulates cyclic adenosine monophosphate (cAMP)/PKA-dependent synaptic plasticity and whether this mode of action could explain its anticonvulsant effect.Methods:We first tested the effects of VPA on PKA-dependent synaptic plasticity at mossy fiber to CA3 synapses in rat hippocampus slices following very high-frequency stimulation or application of the adenylyl cyclase activator forskolin. Using biochemical assays, we then tested whether VPA had a direct effect on PKA activity or an indirect effect through modulating cAMP production. Lastly, VPA and inhibitors of adenylyl cyclase (SQ22536) and PKA (H89) were tested in in vitro models of epileptiform activity induced in hippocampal-entorhinal cortex slices using either pentylenetetrazol (2 mM) or low magnesium.Results:VPA (1 mm) inhibited PKA-dependent long-term potentiation of mossy fiber to CA3 pyramidal cell transmission. However, VPA did not directly modulate PKA activity but rather inhibited the accumulation of cAMP. In acute in vitro seizure models, the anticonvulsant activity of VPA is not mediated through modulation of adenylyl cyclase or PKA.Conclusions:These results indicate that VPA through an action on cAMP accumulation can inhibit synaptic plasticity, but this cannot fully explain its anticonvulsant effect.

The anticonvulsant properties of VPA (valproic acid), a branched short-chain fatty acid, were serendipitously discovered in 1963. Since then, therapeutic roles of VPA have increased to include bipolar disorder and migraine prophylaxis, and have more recently been proposed in cancer, Alzheimer's disease and HIV treatment. These numerous therapeutic roles elevate VPA to near ‘panacea’ level. Surprisingly, the mechanisms of action of VPA in the treatment of many of these disorders remain unclear, although it has been shown to alter a wide variety of signalling pathways and a small number of direct targets. To analyse the mechanism of action of VPA, a number of studies have defined the structural characteristics of VPA-related compounds giving rise to distinct therapeutic and cellular effects, including adverse effects such as teratogenicity and hepatotoxicity. These studies raise the possibility of identifying target-specific novel compounds, providing better therapeutic action or reduced side effects. This short review will describe potential therapeutic pathways targeted by VPA, and highlight studies showing structural constraints necessary for these effects.

SUMMARY Bipolar mood disorder (manic depression) is a major psychiatric disorder whose molecular origins are unknown. Mood stabilisers offer patients both acute and prophylactic treatment, and experimentally, they provide a means to probe the underlying biology of the disorder. Lithium and other mood stabilisers deplete intracellular inositol and it has been proposed that bipolar mood disorder arises from aberrant inositol (1,4,5)-trisphosphate [IP3, also known as Ins(1,4,5)P3] signalling. However, there is no definitive evidence to support this or any other proposed target; a problem exacerbated by a lack of good cellular models. Phosphatidylinositol (3,4,5)-trisphosphate [PIP3, also known as PtdIns(3,4,5)P3] is a prominent intracellular signal molecule within the central nervous system (CNS) that regulates neuronal survival, connectivity and synaptic function. By using the genetically tractable organism Dictyostelium, we show that lithium suppresses PIP3-mediated signalling. These effects extend to the human neutrophil cell line HL60. Mechanistically, we show that lithium attenuates phosphoinositide synthesis and that its effects can be reversed by overexpression of inositol monophosphatase (IMPase), consistent with the inositol-depletion hypothesis. These results demonstrate a lithium target that is compatible with our current knowledge of the genetic predisposition for bipolar disorder. They also suggest that lithium therapy might be beneficial for other diseases caused by elevated PIP3 signalling.

Cardiac ischemia (I) caused by coronary artery occlusion or by significantly reduced blood flow may be lethal if not treated. However, ischemia “preconditioning” may provide benefit to future ischemic insults. The treatment for ischemia itself (i.e., reperfusion, R) can have significant adverse effects. For example, reperfusion may underlie myocardial cell metabolic and Ca2+ signaling dysfunction in these robust but non-regenerating contractile cells. This chapter reviews and discusses the experimental evidence that links mitochondrial dysfunction or mitochondrial collapse to cardiomyocyte cell death as primary cause. Pathophysiological changes in the Ca2+ level in the mitochondrial matrix ([Ca2+]m) appear to be pivotal in triggering the collapse. Nevertheless, this and other aspects of the I-R process are not fully understood and are under active investigation as highlighted here. We have divided the presentation into four parts. First, our state-of-the art quantitative understanding of Ca2+ movement across the mitochondrial inner membrane (IMM) is discussed. The presentation focuses specifically on how cytosolic [Ca2+]i signals affect [Ca2+]m and our new understanding of this process that arises in the context of recent molecular characterizations of the transport systems that mediate Ca2+ fluxes across the IMM. Specific emphasis is placed on the mitochondrial Ca2+ uniporter (MCU) and the mitochondrial Na+/Ca2+ exchanger (NCLX). Second, the ionic and energetic imbalances that develop during ischemia are identified and discussed with respect to how these imbalances progressively degrade the ability of cardiomyocytes to regulate [Ca2+]i and [Ca2+]m. Third, the altered regulation of [Ca2+]m during the ischemia is presented and this dysregulation of [Ca2+]m is discussed as a predisposing component of the eventual mitochondrial collapse on reperfusion. Finally, emerging evidence suggests that the mitochondrial collapse is due to a catastrophic activation of mitochondrial permeability transition pores (mPTPs). Controversial molecular aspects of mPTP activation are also highlighted and discussed along with the putative regulation by pathological [Ca2+]m and the possible beneficial consequences of therapeutic control of mPTP behavior.

Objectives: Lithium (Li+) has been suggested to target the enzyme glycogen synthase kinase 3 (GSK-3) as a mechanism of mood stabilization. Inhibition of GSK-3 by a second mood-stabilizer, valproic acid (VPA), has also been reported, but this effect is dependent on cell type. It is currently unknown if carbamazepine (CBZ) inhibits GSK-3 activity. We have sought to compare the inhibitory effect of Li+, VPA and CBZ on GSK-3 activity. Methods: We treated rat primary cultured neurones at three times therapeutic drug concentration with CBZ, VPA and Li+ and examined changes in GSK-3 protein levels, activity and phosphorylation of downstream targets. To eliminate a possible direct effect of these drugs at higher concentrations, we also looked for direct inhibition of both GSK-3 isoforms at a range of concentrations. Results: CBZ, VPA and Li+ did not change the levels of the GSK-3 or produce an irreversible in vivo effect on GSK-3 activity. Only Li+ inhibited the phosphorylation of a cytoskeletal target of GSK-3, tau, whereas CBZ and VPA did not. Surprisingly, none of these drugs altered ?-catenin levels in these cells, a process attenuated by GSK-3 activity. Finally, only Li+ directly inhibits GSK-3 activity (both and ? isoforms) at therapeutic levels in direct biochemical assays. Conclusion: Thus we show that neither GSK-3 nor the altered GSK-3 signalling pathway can provide a common mechanism of action of mood-stabilizing drugs in the mammalian brain.

Many clinical psychiatrists have identified biochemical changes within patients that are common to a particular disorder. Other researchers, using in vitro approaches, have identified effects of psychiatric drugs in cell-based systems and have used these to propose a therapeutic mode of action for these drugs. But few links exist between this patient-based and in vitro research. The enzyme prolyl oligopeptidase provides one such link. This enzyme was initially identified in the cellular slime mould Dictyostelium discoideum as a key controller of sensitivity to mood disorder drugs, and functions to modulate inositol phosphate signalling—in agreement with the ‘inositol depletion’ theory of mood disorder drug action. It has also been found at aberrant levels in various patient populations. In this review I will introduce prolyl oligopeptidase and summarise what is know concerning its possible role in psychiatric disorders.

Calcium-induced calcium (Ca2+) release (CICR) is the fundamental signaling mechanism that enables the voltage-sensitive L-type Ca2+ channels (LCCs) in the sarcolemma to activate the cellular [Ca2+]i transient and thus contraction in the heart. In CICR the LCC Ca2+ influx activates nearby Ca2+ release channels (ryanodine receptors, RyR2s) across a 15 nm “subspace” gap via CICR, just as the Ca2+ released from any open RyR2 can activate other RyR2s located within the same RyR2 cluster. The junctional ensemble of LCCs and the opposing RyR2s constitute the cardiac “Ca2+ release unit” or CRU. When a CRU is activated by any means it produces a Ca2+ spark. Excitation-contraction (EC) coupling is the process by which CRUs are activated quasi-synchronously by the voltage-activated LCCs in a cell during the action potential (AP). Importantly, CICR also occurs independently when the probabilistic opening of RyR2s produce a Ca2+ spark in the absence of an LCC trigger. Ca2+ sparks and individual RyR2 openings constitute the endogenous Ca2+ leak pathway from the sarcoplasmic reticulum (SR), the “cellular Ca2+ store”. However, in pathological conditions the Ca2+ leak can become unstable and produce cell-wide Ca2+ waves and life-threatening Ca2+-dependent arrhythmias. During SR Ca2+ overload, it has been suggested that a putatively novel phenomena dubbed “Store Overload Induced Ca2+ Release” or “SOICR”, that is distinct from CICR, may develop. SOICR's distinctiveness hinges on the molecular ability of SR lumenal Ca2+ to directly activate RyR2 openings without the need for elevated Ca2+ in the subspace to produce CICR. To investigate this hypothesis, we extend our existing model of RyR2 gating to include a formulation for SOICR whereby SR Ca2+ can directly activate a RyR2 channel to open. Using a multi-scale modeling approach that spans from single channel to whole-cell and spatial simulations, we show that both CICR and SOICR gating modes can indeed activate RyR2 channels and modify Ca2+ spark dynamics in a manner consistent with experimental observations. However, detailed comparison of Ca2+ wave generation and CRU-to-CRU Ca2+ wave propagation shows that CICR alone is a sufficient, and necessary, mechanism to explain Ca2+ release in heart under both physiological and pathological conditions.

Consumption of adenosine triphosphate (ATP) by the heart can change dramatically as the energetic demands increase from a period of rest to strenuous activity. Mitochondrial ATP production is central to this metabolic response since the heart relies largely on oxidative phosphorylation as its source of intracellular ATP. Significant evidence has been acquired indicating that Ca(2+) plays a critical role in regulating ATP production by the mitochondria. Here the evidence that the Ca(2+) concentration in the mitochondrial matrix ([Ca(2+)]m) plays a pivotal role in regulating ATP production by the mitochondria is critically reviewed and aspects of this process that are under current active investigation are highlighted. Importantly, current quantitative information on the bidirectional Ca(2+) movement across the inner mitochondrial membrane (IMM) is examined in two parts. First, we review how Ca(2+) influx into the mitochondrial matrix depends on the mitochondrial Ca(2+) channel (i.e., the mitochondrial calcium uniporter or MCU). This discussion includes how the MCU open probability (PO) depends on the cytosolic Ca(2+) concentration ([Ca(2+)]i) and on the mitochondrial membrane potential (ΔΨm). Second, we discuss how steady-state [Ca(2+)]m is determined by the dynamic balance between this MCU-based Ca(2+) influx and mitochondrial Na(+)/Ca(2+) exchanger (NCLX) based Ca(2+) efflux. These steady-state [Ca(2+)]m levels are suggested to regulate the metabolic energy supply due to Ca(2+)-dependent regulation of mitochondrial enzymes of the tricarboxylic acid cycle (TCA), the proteins of the electron transport chain (ETC), and the F1F0 ATP synthase itself. We conclude by discussing the roles played by [Ca(2+)]m in influencing mitochondrial responses under pathological conditions. This article is part of a Special Issue entitled "Mitochondria: From BasicMitochondrial Biology to Cardiovascular Disease."

Stable calcium-induced calcium release (CICR) is critical for maintaining normal cellular contraction during cardiac excitation-contraction coupling. The fundamental element of CICR in the heart is the calcium (Ca(2+)) spark, which arises from a cluster of ryanodine receptors (RyR). Opening of these RyR clusters is triggered to produce a local, regenerative release of Ca(2+) from the sarcoplasmic reticulum (SR). The Ca(2+) leak out of the SR is an important process for cellular Ca(2+) management, and it is critically influenced by spark fidelity, i.e., the probability that a spontaneous RyR opening triggers a Ca(2+) spark. Here, we present a detailed, three-dimensional model of a cardiac Ca(2+) release unit that incorporates diffusion, intracellular buffering systems, and stochastically gated ion channels. The model exhibits realistic Ca(2+) sparks and robust Ca(2+) spark termination across a wide range of geometries and conditions. Furthermore, the model captures the details of Ca(2+) spark and nonspark-based SR Ca(2+) leak, and it produces normal excitation-contraction coupling gain. We show that SR luminal Ca(2+)-dependent regulation of the RyR is not critical for spark termination, but it can explain the exponential rise in the SR Ca(2+) leak-load relationship demonstrated in previous experimental work. Perturbations to subspace dimensions, which have been observed in experimental models of disease, strongly alter Ca(2+) spark dynamics. In addition, we find that the structure of RyR clusters also influences Ca(2+) release properties due to variations in inter-RyR coupling via local subspace Ca(2+) concentration ([Ca(2+)]ss). These results are illustrated for RyR clusters based on super-resolution stimulated emission depletion microscopy. Finally, we present a believed-novel approach by which the spark fidelity of a RyR cluster can be predicted from structural information of the cluster using the maximum eigenvalue of its adjacency matrix. These results provide critical insights into CICR dynamics in heart, under normal and pathological conditions.

The therapeutic properties of lithium ions (Li+) are well known; however, the mechanism of their action remains unclear. To investigate this problem, we have isolated Li+-resistant mutants from Dictyostelium. Here, we describe the analysis of one of these mutants. This mutant lacks the Dictyostelium prolyl oligopeptidase gene (dpoA). We have examined the relationship between dpoA and the two major biological targets of lithium: glycogen synthase kinase 3 (GSK-3) and signal transduction via inositol (1,4,5) trisphosphate (IP3). We find no evidence for an interaction with GSK-3, but instead find that loss of dpoA causes an increased concentration of IP3. The same increase in IP3 is induced in wild-type cells by a prolyl oligopeptidase (POase) inhibitor. IP3 concentrations increase via an unconventional mechanism that involves enhanced dephosphorylation of inositol (1,3,4,5,6) pentakisphosphate. Loss of DpoA activity therefore counteracts the reduction in IP3 concentration caused by Li+ treatment. Abnormal POase activity is associated with both unipolar and bipolar depression; however, the function of POase in these conditions is unclear. Our results offer a novel mechanism that links POase activity to IP3 signalling and provides further clues for the action of Li+ in the treatment of depression.

In Santulli et al. (1), RyR2-dependent Ca2+ leak from the sarcoplasmic reticulum (SR) is presented as a cause of mitochondrial dysfunction that contributes to heart failure (HF). The authors provide evidence in support of this provocative and important hypothesis. The strength of this paper comes primarily from the suggestions it provides that associate maladaptive changes of key physiological signaling pathways in the heart with the development of HF. A secondary benefit of the work is the array of questions related to the findings of the authors raised by them and provoked by other recent reports (discussed below). The thinking is that RyR2-based SR Ca2+ leak increases in HF and thereby produces a pathologically elevated mitochondrial matrix Ca2+ ([Ca2+]m) level along with excessive mitochondria reactive oxygen species (ROS) production (1). Together these elements form a causal chain in the development of HF following myocardial infarction (MI). With this argument, the paper throws down the gauntlet to the scientific community, challenging investigators to determine quantitatively how these elements interact to produce HF. Such investigations, armed with a growing arsenal of novel tools, hold an exciting potential for us to better understand cellular, organellar, and molecular dysfunctions that underlie the development of HF. Specifically, we need to understand how the transformation from healthy myocyte to dysfunctional myocyte occurs. The quantitative aspects of this process are critical because under normal conditions only modest increases in [Ca2+]m are required to regulate ATP production (2⇓–4). Similarly, small amounts of ROS generation are a normal part of ATP generation by the electron transport chain (ETC) (5 … [↵][1]2To whom correspondence should be addressed. Email: jlederer{at}umaryland.edu. [1]: #xref-corresp-1-1

Detection of substances tasting bitter to humans occurs in diverse organisms including the social amoeba Dictyostelium discoideum. To establish a molecular mechanism for bitter tastant detection in Dictyostelium, we screened a mutant library for resistance to a commonly used bitter standard, phenylthiourea. This approach identified a G-protein-coupled receptor mutant, grlJ−, which showed a significantly increased tolerance to phenylthiourea in growth, survival and movement. This mutant was not resistant to a structurally dissimilar potent bitter tastant, denatonium benzoate, suggesting it is not a target for at least one other bitter tastant. Analysis of the cell-signalling pathway involved in the detection of phenylthiourea showed dependence upon heterotrimeric G protein and phosphatidylinositol 3-kinase activity, suggesting that this signalling pathway is responsible for the cellular effects of phenylthiourea. This is further supported by a phenylthiourea-dependent block in the transient cAMP-induced production of phosphatidylinositol (3,4,5)-trisphosphate (PIP3) in wild-type but not grlJ− cells. Finally, we have identified an uncharacterized human protein γ-aminobutyric acid (GABA) type B receptor subunit 1 isoform with weak homology to GrlJ that restored grlJ− sensitivity to phenylthiourea in cell movement and PIP3 regulation. Our results thus identify a novel pathway for the detection of the standard bitter tastant phenylthiourea in Dictyostelium and implicate a poorly characterized human protein in phenylthiourea-dependent cell responses.

Mitochondrial calcium (Ca2+) fluxes regulate ATP generation, contribute to the regulation of apoptosis, and when very large can play a role in buffering changes in intracellular Ca2+ concentration ([Ca2+]i). However, fundamental disagreements on the extent and speed of Ca2+ uptake by mitochondria have been reported. The reliability and consistency mitochondrial Ca2+ uptake measurements is consequently critical for our understanding of cell biology and pathology in cells from all tissue including heart, neurons, and kidney. Importantly, measurement of Ca2+ movement across the inner mitochondrial membrane (IMM) has been dramatically enhanced by the molecular identification of the mitochondrial Ca2+ uniporter (MCU) and the mitochondrial Na+/Ca2+ exchanger (NCLX). First, we quantitatively analyze cardiac mitochondrial Ca2+ uptake experiments from the literature. We then interpret the results with respect to measurements conducted on recent MCU candidates which suggest that the conductance of a single MCU is ∼6-7 pS (in 105 mM [Ca2+]). Our quantitative analysis suggests three clear findings: 1. Under physiological conditions, Ca2+ influx into mitochondria is small and is dwarfed by other cytosolic Ca2+ extrusion pathways; 2. MCU-dependent Ca2+ uptake appears to be dependent on [Ca2+]i under physiological conditions; 3. There appear to be hundreds of MCU channels per mitochondrion but they are predominantly closed under physiological conditions. We conclude that under physiological conditions mitochondria do not act as a significant Ca2+ buffers in heart despite being capable of substantial Ca2+ accumulation if non-physiological conditions favor prolonged levels of high extra-mitochondrial [Ca2+].

Experimental measurements suggest that large mitochondrial Ca2+ fluxes are normal and physiological. Here a quantitative investigation of Ca2+ movement through the inner mitochondrial membrane (IMM) has been conducted in isolated rat cardiomyocytes. In these cells, elevated local [Ca2+]i (10 μM) bathes the end microdomains of the thousands of intermyofibrillar mitochondria with every heartbeat. Moreover, the mitochondrial inner membrane potential (ΔΨmito) of ∼ −180 mV provides a strong electrochemical gradient for the movement of Ca2+ from the cytosol into the matrix. The role of ΔΨmito in Ca2+ uptake by mitochondria was tested by examining Ca2+ sparks and [Ca2+]i transients in cells or regions of cells where ΔΨmito is dissipated by photon stress. In addition to the changes in electrochemical potential for Ca2+ entry, depolarization of mitochondria was associated with an increase in cellular reactive oxygen species (ROS) as measured by DCF. Consistent with recent experimental findings, we report a role for localized ROS in altering Ca2+ signaling within the depolarized region. Furthermore, using novel quantitative analysis, we separate the influence of ROS and ΔΨmito on the observed changes in Ca2+ signaling. In doing so, new insights into mitochondria contribution to cytosolic calcium handling are obtained.

Conventional methods used to examine antibody labeled proteins are impeded by the resolution limits of light microscopy. We report new measurements of ryanodine receptor (RyR) clusters using the stimulated emission depletion (STED) microscopy method and correlate RyR morphology with function using a fully stochastic model of Ca2+ release. Sheep atrial myocytes were fixed and RyRs were antibody labeled and fluorescently tagged with Atto-647N. Two pulsed lasers were used for STED microscopy; a 635 nm pulsed at 80 MHz for 80ps to excite and a 780 nm laser, with a donut shaped beam was pulsed at 80MHz for 100fs for depletion. Acquired images showed ∼3.5x increase in resolution compared to conventional confocal methods. After deconvolution, further improvements in both signal-to-noise ratio and resolution were observed. Gaussian fitting of the smallest detectable clusters showed a >4x improvement in resolution, allowing a typical lateral resolution of 40-65 nm. Using the protein size predicted from ultrastructure and as documented in rat ventricular myocytes, calculations of cluster size yielded a mean cluster size of 21 RyRs, with a standard deviation of 33. Over 50% of the clusters measured had a size of

Cardiac ryanodine receptor (RyR2) Ca2+ release channels occur as multi-channel clusters at subcellular Ca2+ release sites in cardiomyocytes. RyR2 clusters are thought to underlie important signal amplification mechanisms including coupled gating of physically associated Ca2+ release channels. The nature of RyR2 channel organization in intact intracellular channel clusters is not known.We have investigated the nanostructure of RyR2 clusters using Stimulated Emission Depletion (STED) nanoscopy with ∼60 nm lateral resolution and indirect immunofluorescence in cardiomyocytes. We compared RyR2 clusters in adult cardiomyocytes from sham hearts with hearts 8 weeks after myocardial infarction (post-MI).Peripheral RyR2 clusters were detected at a density of 1.3 ± 0.2 clusters per μm2 in control cells (2237 clusters) and 1.6 ± 0.1 in diseased cells (1752 clusters). Individual clusters were analyzed by incrementally detecting signal edges which defined intra-cluster substructures and intensity peaks. Within individual clusters, nearest-neighbour distances between intensity peaks showed an asymmetric distribution with a maximum at 100 ± 1 nm in healthy cardiomyocytes. In post-MI cells this distribution showed a left shift with a maximum at 85 ± 3 nm (p < 0.05 Mann-Whitney test; mean distance sham 150.6 ±3.2 nm vs post-MI 129.5 ± 2.7 nm).In conclusion, immuno-labeled cardiac RyR2 clusters showed distinct non-randomly organized sub-structures as evidenced by characteristic intra-cluster peak-to-peak distances. Our data suggest that continuous RyR2 clusters contain substructures which occur repeatedly as densely organized groups of RyR2 channels. In addition, we showed altered RyR2 clusters after myocardial infarction with smaller intra-cluster peak-to-peak distances which implies tighter channel packing. We are preparing a working mathematical model of normal RyR2 cluster function and the role of potential substructures and their changes in heart disease.

Electrical signals in the heart are translated to mechanical contraction by a mechanism called excitation-contraction coupling (ECC) through calcium (Ca2+)-induced-Ca2+-release (CICR). In cardiac myocytes, the L-type Ca2+ channels (LCCs) are located in the sarcolemmal membrane (SL) and open upon sensing an action potential (AP). This Ca2+ signal is translated into the Ca2+ transient (CaT) by triggering further Ca2+release from the sarcoplasmic reticulum (SR) through the ryanodine receptors (RyR) channels. A network of transverse t-tubules (TTs) facilitates the synchronization of the CaT. Imaging studies have shown that in some species, these TTs are sparser in atrial myocytes in comparison to their ventricular counterpart. Furthermore, our laboratory recently found that in atrial cells that underwent rapid atrial pacing (RAP) to simulate pathological effects of atrial fibrillation, the buffering strength was increased. This resulted in smaller central CaT amplitudes and comparable sub-sarcolemmal CaT amplitudes with respect to control cells. In this project we seek to test the effect of buffering in the LCC and its role in the CaT spatiotemporal profile. To elucidate these effects, we performed patch clamp experiments to investigate the differences in the LCC function under different buffering conditions. We incorporated these findings in a minimized spatial computational model to evaluate the impact of buffering and TTs on intracellular Ca2+ signals. This ongoing work seeks to investigate the mechanisms that lead to different spatiotemporal profiles of the CaT and how these differences affect atrial cell function. This work should broaden our understanding of atrial cell function in health and disease.

Existing theory suggests that mitochondria act as significant, dynamic buffers of cytosolic calcium ([Ca(2+)]i) in heart. These buffers can remove up to one-third of the Ca(2+) that enters the cytosol during the [Ca(2+)]i transients that underlie contractions. However, few quantitative experiments have been presented to test this hypothesis. Here, we investigate the influence of Ca(2+) movement across the inner mitochondrial membrane during both subcellular and global cellular cytosolic Ca(2+) signals (i.e., Ca(2+) sparks and [Ca(2+)]i transients, respectively) in isolated rat cardiomyocytes. By rapidly turning off the mitochondria using depolarization of the inner mitochondrial membrane potential (ΔΨm), the role of the mitochondria in buffering cytosolic Ca(2+) signals was investigated. We show here that rapid loss of ΔΨm leads to no significant changes in cytosolic Ca(2+) signals. Second, we make direct measurements of mitochondrial [Ca(2+)] ([Ca(2+)]m) using a mitochondrially targeted Ca(2+) probe (MityCam) and these data suggest that [Ca(2+)]m is near the [Ca(2+)]i level (∼100 nM) under quiescent conditions. These two findings indicate that although the mitochondrial matrix is fully buffer-capable under quiescent conditions, it does not function as a significant dynamic buffer during physiological Ca(2+) signaling. Finally, quantitative analysis using a computational model of mitochondrial Ca(2+) cycling suggests that mitochondrial Ca(2+) uptake would need to be at least ∼100-fold greater than the current estimates of Ca(2+) influx for mitochondria to influence measurably cytosolic [Ca(2+)] signals under physiological conditions. Combined, these experiments and computational investigations show that mitochondrial Ca(2+) uptake does not significantly alter cytosolic Ca(2+) signals under normal conditions and indicates that mitochondria do not act as important dynamic buffers of [Ca(2+)]i under physiological conditions in heart.

SUMMARY Lipid droplet formation and subsequent steatosis (the abnormal retention of lipids within a cell) has been reported to contribute to hepatotoxicity and is an adverse effect of many pharmacological agents including the antiepileptic drug valproic acid (VPA). In this study, we have developed a simple model system (Dictyostelium discoideum) to investigate the effects of VPA and related compounds in lipid droplet formation. In mammalian hepatocytes, VPA increases lipid droplet accumulation over a 24-hour period, giving rise to liver cell damage, and we show a similar effect in Dictyostelium following 30 minutes of VPA treatment. Using 3H-labelled polyunsaturated (arachidonic) or saturated (palmitic) fatty acids, we shown that VPA treatment of Dictyostelium gives rise to an increased accumulation of both types of fatty acids in phosphatidylcholine, phosphatidylethanolamine and non-polar lipids in this time period, with a similar trend observed in human hepatocytes (Huh7 cells) labelled with [3H]arachidonic acid. In addition, pharmacological inhibition of β-oxidation in Dictyostelium phenocopies fatty acid accumulation, in agreement with data reported in mammalian systems. Using Dictyostelium, we then screened a range of VPA-related compounds to identify those with high and low lipid-accumulation potential, and validated these activities for effects on lipid droplet formation by using human hepatocytes. Structure-activity relationships for these VPA-related compounds suggest that lipid accumulation is independent of VPA-catalysed teratogenicity and inositol depletion. These results suggest that Dictyostelium could provide both a novel model system for the analysis of lipid droplet formation in human hepatocytes and a rapid method for identifying VPA-related compounds that show liver toxicology.

Novel chemical entities (NCEs) may be investigated for emetic liability in a range of unpleasant experiments involving retching, vomiting or conditioned taste aversion/food avoidance in sentient animals. We have used a range of compounds with known emetic /aversive properties to examine the possibility of using the social amoeba, Dictyostelium discoideum, for research into identifying and understanding emetic liability, and hence reduce adverse animal experimentation in this area. Twenty eight emetic or taste aversive compounds were employed to investigate the acute (10 min) effect of compounds on Dictyostelium cell behaviour (shape, speed and direction of movement) in a shallow chemotaxic gradient (Dunn chamber). Compound concentrations were chosen based on those previously reported to be emetic or aversive in in vivo studies and results were recorded and quantified by automated image analysis. Dictyostelium cell motility was rapidly and strongly inhibited by four structurally distinct tastants (three bitter tasting compounds--denatonium benzoate, quinine hydrochloride, phenylthiourea, and the pungent constituent of chilli peppers--capsaicin). In addition, stomach irritants (copper chloride and copper sulphate), and a phosphodiesterase IV inhibitor also rapidly blocked movement. A concentration-dependant relationship was established for five of these compounds, showing potency of inhibition as capsaicin (IC(50) = 11.9 ± 4.0 µM) > quinine hydrochloride (IC(50) = 44.3 ± 6.8 µM) > denatonium benzoate (IC(50) = 129 ± 4 µM) > phenylthiourea (IC(50) = 366 ± 5 µM) > copper sulphate (IC(50) = 1433 ± 3 µM). In contrast, 21 compounds within the cytotoxic and receptor agonist/antagonist classes did not affect cell behaviour. Further analysis of bitter and pungent compounds showed that the effect on cell behaviour was reversible and not cytotoxic, suggesting an uncharacterised molecular mechanism of action for these compounds. These results therefore demonstrate that Dictyostelium has potential as a non-sentient model in the analysis of the molecular effects of tastants, although it has limited utility in identification of emetic agents in general.

A three dimensional model of calcium dynamics in the rat ventricular myocyte was developed to study the mechanism of calcium homeostasis and pathological calcium dynamics during calcium overload. The model contains 20,000 calcium release units (CRUs) each containing 49 ryanodine receptors. The model simulates calcium sparks with a realistic spontaneous calcium spark rate. It suggests that in addition to the calcium spark-based leak, there is an invisible calcium leak caused by the stochastic opening of a small number of ryanodine receptors in each CRU without triggering a calcium spark. The model also explores the mechanism of calcium wave propagation between release sites under the conditions of calcium overload.

Lithium (Li(+)) is a common treatment for bipolar mood disorder, a major psychiatric illness with a lifetime prevalence of more than 1%. Risk of bipolar disorder is heavily influenced by genetic predisposition, but is a complex genetic trait and, to date, genetic studies have provided little insight into its molecular origins. An alternative approach is to investigate the genetics of Li(+) sensitivity. Using the social amoeba Dictyostelium, we previously identified prolyl oligopeptidase (PO) as a modulator of Li(+) sensitivity. In a link to the clinic, PO enzyme activity is altered in bipolar disorder patients. Further studies demonstrated that PO is a negative regulator of inositol(1,4,5)trisphosphate (IP(3)) synthesis, a Li(+) sensitive intracellular signal. However, it was unclear how PO could influence either Li(+) sensitivity or risk of bipolar disorder. Here we show that in both Dictyostelium and cultured human cells PO acts via Multiple Inositol Polyphosphate Phosphatase (Mipp1) to control gene expression. This reveals a novel, gene regulatory network that modulates inositol metabolism and Li(+) sensitivity. Among its targets is the inositol monophosphatase gene IMPA2, which has also been associated with risk of bipolar disorder in some family studies, and our observations offer a cellular signalling pathway in which PO activity and IMPA2 gene expression converge.

Calcium (Ca2+) signaling in muscle, neuronal, and non-excitable cells has benefited significantly from advances in biological tools and imaging technology, however, the molecular interactions of nanoscopic molecules, structures and compartments has been challenging to study under physiological conditions. Here, we exploit novel computational modeling techniques to examine real-time molecular and cellular physiology in cardiac ventricular myocytes. The model focuses on local and cell-wide Ca2+ signaling phenomena related to calcium induced calcium release from intracellular calcium channels, ryanodine receptors (RyR2s), located on the sarcoplasmic reticulum (SR) membrane. This work is informed by the latest molecular investigations and recent characterizations of channels, transporters, and buffers located in mammalian heart. We have created a detailed, whole-cell model of Ca2+ signaling using a realistic number of calcium release units (CRU) each containing a cluster of stochastically gating RyR2s. During systole the opening of these RyR2s is triggered by Ca2+ entry via voltage gated L-type Ca2+ channels. The synchronized opening of the RyR2 cluster leads to localized elevations of [Ca2+]i known as Ca2+ sparks. During diastole Ca2+ sparks are still observed and are attributed to the finite opening rate of the RyR2. RyR2s are also believed to display unsynchronized or non-spark openings where only a few channels in the CRU open without triggering the remainder of the RyR2 cluster. This non-spark Ca2+ release would be below current experimental detection thresholds and therefore “invisible.” These spark and non-spark openings of RyR2s constitute a molecular basis for Ca2+ leak from the SR. The computational model suggests that a significant fraction of SR Ca2+ leak is due to RyR2s openings that fail to trigger a “visible” Ca2+ spark. Additionally, the fraction of non-spark or “invisible” SR Ca2+ leak increases as SR Ca2+ content declines.

Lithium (Li+) is the mood stabilizer most frequently used in the treatment of bipolar mood disorder; however, its therapeutic mechanism is unknown. In the 1980s, Berridge and colleagues proposed that Li+ treatment acts via inhibition of IMPase (inositol monophosphatase) to deplete the cellular concentration of myo-inositol. Inositol depletion is also seen with the alternative mood stabilizers VPA (valproic acid) and CBZ (carbamazepine), suggesting a common therapeutic action. All three drugs cause changes in neuronal cell morphology and cell chemotaxis; however, it is unclear how reduced cellular inositol modulates these changes in cell behaviour. It is often assumed that reduced inositol suppresses Ins(1,4,5)P3, a major intracellular signal molecule, but there are other important phosphoinostide-based signal molecules in the cell. In the present paper, we discuss evidence that Li+ has a substantial effect on PtdIns(3,4,5)P3, an important signal molecule within the nervous system. As seen for Ins(1,4,5)P3 signalling, suppression of PtdIns(3,4,5)P3 signalling also occurs via an inositol-depletion mechanism. This has implications for the cellular mechanisms controlling phosphoinositide signalling, and offers insight into the genetics underlying risk of bipolar mood disorder.

Phosphoinositide (PI) signalling is one of multiple signalling cascades involved in chemotaxis in Dictyostelium discoideum. PI signalling comprises a complex interaction of multiple enzymes, each with multiple phospholipid substrates and thus products, often relying upon several enzymes in series to produce a signal. PI turnover, controlled by both kinases and phosphatases, is also rapidly triggered and spatially constricted. This complexity makes understanding acute regulation of these signalling components problematic. However, the ubiquitous and extensive roles of phospholipids, including phosphatidylinositol-4,5-diphosphate (PI(4,5)P(2)), in cell signalling and developmental processes make understanding the production of these compounds of great importance. We have shown the acute reduction of PI phosphorylation in response to a widely used bipolar disorder and epilepsy treatment, valproic acid, as a potential therapeutic role for the drug using chemotactically competent Dictyostelium. Here we describe a means for measuring acute in vivo phospholipid labelling in Dictyostelium.