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.

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

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

A common genetic variant of a mitochondrial RNA processing enzyme predisposes to insulin resistance

Mitochondrial energy metabolism plays an important role in the pathophysiology of insulin resistance. Recently, a missense N437S variant was identified in the MRPP3 gene, which encodes a mitochondrial RNA processing enzyme within the RNase P complex, with predicted impact on metabolism. We used CRISPR-Cas9 genome editing to introduce this variant into the mouse Mrpp3 gene and show that the variant causes insulin resistance on a high-fat diet. The variant did not influence mitochondrial gene expression markedly, but instead, it reduced mitochondrial calcium that lowered insulin release from the pancreatic islet β cells of the Mrpp3 variant mice. Reduced insulin secretion resulted in lower insulin levels that contributed to imbalanced metabolism and liver steatosis in the Mrpp3 variant mice on a high-fat diet. Our findings reveal that the MRPP3 variant may be a predisposing factor to insulin resistance and metabolic disease in the human population.

Mitochondrial mistranslation modulated by metabolic stress causes cardiovascular disease and reduced lifespan

Changes in the rate and fidelity of mitochondrial protein synthesis impact the metabolic and physiological roles of mitochondria. Here we explored how environmental stress in the form of a high-fat diet modulates mitochondrial translation and affects lifespan in mutant mice with error-prone (Mrps12^{ep / ep} ) or hyper-accurate (Mrps12^{ha / ha} ) mitochondrial ribosomes. Intriguingly, although both mutations are metabolically beneficial in reducing body weight, decreasing circulating insulin and increasing glucose tolerance during a high-fat diet, they manifest divergent (either deleterious or beneficial) outcomes in a tissue-specific manner. In two distinct organs that are commonly affected by the metabolic disease, the heart and the liver, Mrps12^{ep / ep} mice were protected against heart defects but sensitive towards lipid accumulation in the liver, activating genes involved in steroid and amino acid metabolism. In contrast, enhanced translational accuracy in Mrps12^{ha / ha} mice protected the liver from a high-fat diet through activation of liver proliferation programs, but enhanced the development of severe hypertrophic cardiomyopathy and led to reduced lifespan. These findings reflect the complex transcriptional and cell signalling responses that differ between post-mitotic (heart) and highly proliferative (liver) tissues. We show trade-offs between the rate and fidelity of mitochondrial protein synthesis dictate tissue-specific outcomes due to commonly encountered stressful environmental conditions or aging.

Lack of Strategic Funding and Long-Term Job Security Threaten to Have Profound Effects on Cardiovascular Researcher Retention in Australia

Background

Cardiovascular disease is the leading cause of death in Australia. Investment in research solutions has been demonstrated to yield health and a 9.8-fold return economic benefit. The sector, however, is severely challenged with success rates of traditional peer-reviewed funding in decline. Here, we aimed to understand the perceived challenges faced by the cardiovascular workforce in Australia prior to the COVID-19 pandemic.

Methods

We used an online survey distributed across Australian cardiovascular societies/councils, universities and research institutes over a period of 6 months during 2019, with 548 completed responses. Inclusion criteria included being an Australian resident or an Australian citizen who lived overseas, and a current or past student or employee in the field of cardiovascular research.

Results

The mean age of respondents was 42±13 years, 47% were male, 85% had a full-time position, and 40% were a group leader or laboratory head. Twenty-three per cent (23%) had permanent employment, and 82% of full-time workers regularly worked >40 hours/week. Sixty-eight per cent (68%) said they had previously considered leaving the cardiovascular research sector. If their position could not be funded in the next few years, a staggering 91% of respondents would leave the sector. Compared to PhD- and age-matched men, women were less likely to be a laboratory head and to feel they had a long-term career path as a cardiovascular researcher, while more women were unsure about future employment and had considered leaving the sector (all p<0.05). Greater job security (76%) and government and philanthropic investment in cardiovascular research (72%) were highlighted by responders as the main changes to current practices that would encourage them to stay.

Conclusion

Strategic solutions, such as diversification of career pathways and funding sources, and moving from a competitive to a collaborative culture, need to be a priority to decrease reliance on government funding and allow cardiovascular researchers to thrive.

Fidelity and coordination of mitochondrial protein synthesis in health and disease

The evolutionary acquisition of mitochondria has given rise to the diversity of eukaryotic life. Mitochondria have retained their ancestral α-proteobacterial traits through the maintenance of double membranes and their own circular genome. Their genome varies in size from very large in plants to the smallest in animals and their parasites. The mitochondrial genome encodes essential genes for protein synthesis and has to coordinate its expression with the nuclear genome from which it sources most of the proteins required for mitochondrial biogenesis and function. The mitochondrial protein synthesis machinery is unique because it is encoded by both the nuclear and mitochondrial genomes thereby requiring tight regulation to produce the respiratory complexes that drive oxidative phosphorylation for energy production. The fidelity and coordination of mitochondrial protein synthesis are essential for ATP production. Here we compare and contrast the mitochondrial translation mechanisms in mammals and fungi to bacteria and reveal that their diverse regulation can have unusual impacts on the health and disease of these organisms. We highlight that in mammals the rate of protein synthesis is more important than the fidelity of translation, enabling coordinated biogenesis of the mitochondrial respiratory chain with respiratory chain proteins synthesised by cytoplasmic ribosomes. Changes in mitochondrial protein fidelity can trigger the activation of the diverse cellular signalling networks in fungi and mammals to combat dysfunction in energy conservation. The physiological consequences of altered fidelity of protein synthesis can range from liver regeneration to the onset and development of cardiomyopathy.

A dendronized polymer variant that facilitates safe delivery of a calcium channel antagonist to the heart

Therapeutic approaches for myocardial ischemia-reperfusion injury (MI) have been ineffective due to limited bioavailability and poor specificity. We have previously shown that a peptide that targets the α-interaction domain of the cardiac L-type calcium channel (AID-peptide) attenuates MI when tethered to transactivator of transcription sequence (TAT) or spherical nanoparticles. However some reservations remain regarding use of these delivery platforms due to the relationship with human immunodeficiency virus, off-target effects and toxicity. Here we investigate the use of linear dendronized polymers (denpols) to deliver AID-peptide as a potential MI therapy using in vitro, ex vivo and in vivo models. Optimized denpol-complexed AID-peptide facilitated in vitro cardiac uptake of AID-peptide, and reduced MI. Maximal in vivo cardiac uptake was achieved within the 2 h therapeutic time window for acute myocardial infarction. Importantly, optimized denpol-complexed AID-peptide was not toxic. This platform may represent an alternative therapeutic approach for the prevention of MI.

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

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

Fidelity of translation initiation is required for coordinated respiratory complex assembly

Mammalian mitochondrial ribosomes are unique molecular machines that translate 11 leaderless mRNAs; however, it is not clear how mitoribosomes initiate translation, since mitochondrial mRNAs lack untranslated regions. Mitochondrial translation initiation shares similarities with prokaryotes, such as the formation of a ternary complex of fMet-tRNA^Met, mRNA and the 28S subunit, but differs in the requirements for initiation factors. Mitochondria have two initiation factors: MTIF2, which closes the decoding center and stabilizes the binding of the fMet-tRNA^Met to the leaderless mRNAs, and MTIF3, whose role is not clear. We show that MTIF3 is essential for survival and that heart- and skeletal muscle-specific loss of MTIF3 causes cardiomyopathy. We identify increased but uncoordinated mitochondrial protein synthesis in mice lacking MTIF3, resulting in loss of specific respiratory complexes. Ribosome profiling shows that MTIF3 is required for recognition and regulation of translation initiation of mitochondrial mRNAs and for coordinated assembly of OXPHOS complexes in vivo.

Stress signaling and cellular proliferation reverse the effects of mitochondrial mistranslation

Translation fidelity is crucial for prokaryotes and eukaryotic nuclear-encoded proteins; however, little is known about the role of mistranslation in mitochondria and its potential effects on metabolism. We generated yeast and mouse models with error-prone and hyper-accurate mitochondrial translation, and found that translation rate is more important than translational accuracy for cell function in mammals. Specifically, we found that mitochondrial mistranslation causes reduced overall mitochondrial translation and respiratory complex assembly rates. In mammals, this effect is compensated for by increased mitochondrial protein stability and upregulation of the citric acid cycle. Moreover, this induced mitochondrial stress signaling, which enables the recovery of mitochondrial translation via mitochondrial biogenesis, telomerase expression, and cell proliferation, and thereby normalizes metabolism. Conversely, we show that increased fidelity of mitochondrial translation reduces the rate of protein synthesis without eliciting a mitochondrial stress response. Consequently, the rate of translation cannot be recovered and this leads to dilated cardiomyopathy in mice. In summary, our findings reveal mammalian-specific signaling pathways that respond to changes in the fidelity of mitochondrial protein synthesis and affect metabolism.

PTCD1 Is Required for 16S rRNA Maturation Complex Stability and Mitochondrial Ribosome Assembly

The regulation of mitochondrial RNA life cycles and their roles in ribosome biogenesis and energy metabolism are not fully understood. We used CRISPR/Cas9 to generate heart- and skeletal-muscle-specific knockout mice of the pentatricopeptide repeat domain protein 1, PTCD1, and show that its loss leads to severe cardiomyopathy and premature death. Our detailed transcriptome-wide and functional analyses of these mice enabled us to identify the molecular role of PTCD1 as a 16S rRNA-binding protein essential for its stability, pseudouridylation, and correct biogenesis of the mitochondrial large ribosomal subunit. We show that impaired mitoribosome biogenesis can have retrograde signaling effects on nuclear gene expression through the transcriptional activation of the mTOR pathway and upregulation of cytoplasmic protein synthesis and pro-survival factors in the absence of mitochondrial translation. Taken together, our data show that impaired assembly of the mitoribosome exerts its consequences via differential regulation of mitochondrial and cytoplasmic protein synthesis.

Impaired calcium handling and mitochondrial metabolic dysfunction as early markers of hypertrophic cardiomyopathy

Hypertrophic cardiomyopathy (HCM) is a primary myocardial disorder, characterised by myocyte remodeling, disorganisation of sarcomeric proteins, impaired energy metabolism and altered cardiac contractility. Gene mutations encoding cardiac contractile proteins account for 60% of HCM aetiology. Current drug therapy including L-type calcium channel antagonists, are used to manage symptoms in patients with overt HCM, but no treatment exists that can reverse or prevent the cardiomyopathy. Design of effective drug therapy will require a clear understanding of the early pathophysiological mechanisms of the disease. Numerous studies have investigated specific aspects of HCM pathophysiology. This review brings these findings together, in order to develop a holistic understanding of the early pathophysiological mechanisms of the disease. We focus on gene mutations in cardiac myosin binding protein-C, β-cardiac myosin heavy chain, cardiac troponin I, and cardiac troponin T, that comprise the majority of all HCM sarcomeric gene mutations. We find that although some similarities exist, each mutation leads to mutation-specific alterations in calcium handling, myofilament calcium sensitivity and mitochondrial metabolic function. This may contribute to the observed clinical phenotypic variability in sarcomeric-related HCM. An understanding of early mutation-specific mechanisms of the disease may provide useful markers of disease progression, and inform therapeutic design.

Concerted regulation of mitochondrial and nuclear non-coding RNAs by a dual-targeted RNase Z

The molecular roles of the dually targeted ElaC domain protein 2 (ELAC2) during nuclear and mitochondrial RNA processing in vivo have not been distinguished. We generated conditional knockout mice of ELAC2 to identify that it is essential for life and its activity is non-redundant. Heart and skeletal muscle-specific loss of ELAC2 causes dilated cardiomyopathy and premature death at 4 weeks. Transcriptome-wide analyses of total RNAs, small RNAs, mitochondrial RNAs, and miRNAs identified the molecular targets of ELAC2 in vivo We show that ELAC2 is required for processing of tRNAs and for the balanced maintenance of C/D box snoRNAs, miRNAs, and a new class of tRNA fragments. We identify that correct biogenesis of regulatory non-coding RNAs is essential for both cytoplasmic and mitochondrial protein synthesis and the assembly of mitochondrial ribosomes and cytoplasmic polysomes. We show that nuclear tRNA processing is required for the balanced production of snoRNAs and miRNAs for gene expression and that 3' tRNA processing is an essential step in the production of all mature mitochondrial RNAs and the majority of nuclear tRNAs.

Frequency-specific effects of repetitive magnetic stimulation on primary astrocyte cultures

BACKGROUND

Repetitive transcranial magnetic stimulation (rTMS) is a non-invasive technique that uses magnetic pulses over the cranium to induce electrical currents in underlying cortical tissue. Although rTMS has shown clinical utility for a number of neurological conditions, we have only limited understanding of how rTMS influences cellular function and cell-cell interactions.

OBJECTIVE

In this study, we sought to investigate whether repeated magnetic stimulation (rMS) can influence astrocyte biology in vitro.

METHOD

We tested four different rMS frequencies and measured the calcium response in primary neonatal astrocyte cultures. We also tested the effect of rMS on astrocyte migration and proliferation in vitro. We tested 3 to 4 culture replicates and 17 to 34 cells for each rMS frequency (sham, 1 Hz, cTBS, 10 Hz and biomemetic high frequency stimulation - BHFS).

RESULTS

Of all frequencies tested, 1 Hz stimulation resulted in a statistically significant rise in intracellular calcium in the cytoplasmic and nuclear compartments of the cultured astrocytes. This calcium rise did not affect migration or proliferation in the scratch assay, though astrocyte hypertrophy was reduced in response to 1 Hz rMS, 24 hours post scratch injury.

CONCLUSION

Our results provide preliminary evidence that rMS can influence astrocyte physiology, indicating the potential for a novel mechanism by which rTMS can influence brain activity.

A Morpholino Oligomer Therapy Regime That Restores Mitochondrial Function and Prevents mdx Cardiomyopathy

Current clinical trials demonstrate Duchenne muscular dystrophy (DMD) patients receiving phosphorodiamidate morpholino oligomer (PMO) therapy exhibit improved ambulation and stable pulmonary function; however, cardiac abnormalities remain. Utilizing the same PMO chemistry as current clinical trials, we have identified a non-toxic PMO treatment regimen that restores metabolic activity and prevents DMD cardiomyopathy. We propose that a treatment regimen of this nature may have the potential to significantly improve morbidity and mortality from DMD by improving ambulation, stabilizing pulmonary function, and preventing the development of cardiomyopathy.

Identification of a novel cAMP dependent protein kinase A phosphorylation site on the human cardiac calcium channel

The "Fight or Flight" response is elicited by extrinsic stress and is necessary in many species for survival. The response involves activation of the β-adrenergic signalling pathway. Surprisingly the mechanisms have remained unresolved. Calcium influx through the cardiac L-type Ca²⁺ channel (Ca_v1.2) is absolutely required. Here we identify the functionally relevant site for PKA phosphorylation on the human cardiac L-type Ca²⁺ channel pore forming α1 subunit using a novel approach. We used a cell free system where we could assess direct effects of PKA on human purified channel protein function reconstituted in proteoliposomes. In addition to assessing open probability of channel protein we used semi-quantitative fluorescent phosphoprotein detection and MS/MS mass spectrometry analysis to demonstrate the PKA specificity of the site. Robust increases in frequency of channel openings were recorded after phosphorylation of the long and short N terminal isoforms and the channel protein with C terminus truncated at aa1504. A protein kinase A anchoring protein (AKAP) was not required. We find the novel PKA phosphorylation site at Ser1458 is in close proximity to the Repeat IV S6 region and induces a conformational change in the channel protein that is necessary and sufficient for increased calcium influx through the channel.

Real-Time Bioimpedance Sensing of Antifibrotic Drug Action in Primary Human Cells

Fibrotic diseases are among the most serious health issues with severe burdens due to their chronic nature and a large number of patients suffering from the debilitating effects and long-term sequelae. Collagenase treatment is a nonsurgical option but has limited results. To date, there is no potent noninvasive solution for fibrosis. Part of the reason for this is the lack of appropriate in vitro live cell screening tools to assess the efficacy of new therapeutical agents. Here, we demonstrate the utility of a cell-based electrochemical impedance biosensor platform to screen the efficacy of potential antifibrotic compounds. The platform employs a label-free and noninvasive strategy to detect the progression of fibrosis and the potency of the antifibrotic molecules in real-time. The fundamental principle that governs this novel system is that dynamic changes in cell shape and adhesion during fibrosis can be measured accurately by monitoring the changes in the impedance. This is achieved by growing the cells on a transparent interdigitated indium tin oxide (ITO) electrodes. It was demonstrated by monitoring the efficacy of a model antifibrotic compound, PXS64, on cells collected from patients with Dupuytren's contracture. We confirmed the validity of the developed biochemical impedance biosensor as an tool for in vitro screening of antifibrotic compounds and provided quantitative information on subcellular influences of the examined chemical molecules using correlative microscopy analyses that monitor the average cell area, cell morphology, and the amount and directionality of the deposited extracellular matrix protein collagen and measurement of cytosolic Ca²⁺ changes.

The cardiac L-type calcium channel alpha subunit is a target for direct redox modification during oxidative stress-the role of cysteine residues in the alpha interacting domain

Cardiovascular disease is the leading cause of death in the Western world. The incidence of cardiovascular disease is predicted to further rise with the increase in obesity and diabetes and with the aging population. Even though the survival rate from ischaemic heart disease has improved over the past 30 years, many patients progress to a chronic pathological condition, known as cardiac hypertrophy that is associated with an increase in morbidity and mortality. Reactive oxygen species (ROS) and calcium play an essential role in mediating cardiac hypertrophy. The L-type calcium channel is the main route for calcium influx into cardiac myocytes. There is now good evidence for a direct role for the L-type calcium channel in the development of cardiac hypertrophy. Cysteines on the channel are targets for redox modification and glutathionylation of the channel can modulate the function of the channel protein leading to the onset of pathology. The cysteine responsible for modification of L-type calcium channel function has now been identified. Detailed understanding of the role of cysteines as possible targets during oxidative stress may assist in designing therapy to prevent the development of hypertrophy and heart failure.

Adult-onset obesity is triggered by impaired mitochondrial gene expression

Mitochondrial gene expression is essential for energy production; however, an understanding of how it can influence physiology and metabolism is lacking. Several proteins from the pentatricopeptide repeat (PPR) family are essential for the regulation of mitochondrial gene expression, but the functions of the remaining members of this family are poorly understood. We created knockout mice to investigate the role of the PPR domain 1 (PTCD1) protein and show that loss of PTCD1 is embryonic lethal, whereas haploinsufficient, heterozygous mice develop age-induced obesity. The molecular defects and metabolic consequences of mitochondrial protein haploinsufficiency in vivo have not been investigated previously. We show that PTCD1 haploinsufficiency results in increased RNA metabolism, in response to decreased protein synthesis and impaired RNA processing that affect the biogenesis of the respiratory chain, causing mild uncoupling and changes in mitochondrial morphology. We demonstrate that with age, these effects lead to adult-onset obesity that results in liver steatosis and cardiac hypertrophy in response to tissue-specific differential regulation of the mammalian target of rapamycin pathways. Our findings indicate that changes in mitochondrial gene expression have long-term consequences on energy metabolism, providing evidence that haploinsufficiency of PTCD1 can be a major predisposing factor for the development of metabolic syndrome.

Auto-regulation in the powerhouse

Mitochondrial flashes have a central role in ensuring that ATP levels remain constant in heart cells.

Dystrophic Cardiomyopathy-Potential Role of Calcium in Pathogenesis, Treatment and Novel Therapies

Duchenne muscular dystrophy (DMD) is caused by defects in the DMD gene and results in progressive wasting of skeletal and cardiac muscle due to an absence of functional dystrophin. Cardiomyopathy is prominent in DMD patients, and contributes significantly to mortality. This is particularly true following respiratory interventions that reduce death rate and increase ambulation and consequently cardiac load. Cardiomyopathy shows an increasing prevalence with age and disease progression, and over 95% of patients exhibit dilated cardiomyopathy by the time they reach adulthood. Development of the myopathy is complex, and elevations in intracellular calcium, functional muscle ischemia, and mitochondrial dysfunction characterise the pathophysiology. Current therapies are limited to treating symptoms of the disease and there is therefore an urgent need to treat the underlying genetic defect. Several novel therapies are outlined here, and the unprecedented success of phosphorodiamidate morpholino oligomers (PMOs) in preclinical and clinical studies is overviewed.

L-type calcium channel: Clarifying the "oxygen sensing hypothesis"

The heart is able to respond acutely to changes in oxygen tension. Since ion channels can respond rapidly to stimuli, the "ion channel oxygen sensing hypothesis" has been proposed to explain acute adaptation of cells to changes in oxygen demand. However the exact mechanism for oxygen sensing continues to be debated. Mitochondria consume the lion's share of oxygen in the heart, fuelling the production of ATP that drives excitation and contraction. Mitochondria also produce reactive oxygen species that are capable of altering the redox state of proteins. The cardiac L-type calcium channel is responsible for maintaining excitation and contraction. Recently, the reactive cysteine on the cardiac L-type calcium channel was identified. These data clarified that the channel does not respond directly to changes in oxygen tension, but rather responds to cellular redox state. This leads to acute alterations in cell signalling responsible for the development of arrhythmias and pathology.

Verapamil is Less Effective than Triamcinolone for Prevention of Keloid Scar Recurrence After Excision in a Randomized Controlled Trial

A double-blind randomized controlled trial with a paired split-scar design compared verapamil, an L-type Ca2+ channel antagonist, and triamcinolone for prevention of keloid recurrence after excision. Ca2+ channel blocking activity of verapamil in keloid cells was explored. One keloid was excised per subject and each wound half randomized to receive intralesional injections of triamcinolone (10 mg/ml) or verapamil (2.5 mg/ml) at monthly intervals (4 doses). Interim analysis was performed after 14 subjects were completed. Survival analysis demonstrated significantly higher keloid recurrence with verapamil compared to triamcinolone 12 months post-surgery (log-rank test, p = 0.01) and higher overall risk of recurrence with verapamil (hazard ratio 8.44, 95% CI 1.62-44.05). The study was terminated early according to the stopping guideline (p < 0.05). Verapamil is safe but not as effective as triamcinolone in preventing keloid recurrence after excision. Further study is necessary to determine if clinical response to verapamil is linked to modulation of intracellular Ca2+.

Loss of the RNA-binding protein TACO1 causes late-onset mitochondrial dysfunction in mice

The recognition and translation of mammalian mitochondrial mRNAs are poorly understood. To gain further insights into these processes in vivo, we characterized mice with a missense mutation that causes loss of the translational activator of cytochrome oxidase subunit I (TACO1). We report that TACO1 is not required for embryonic survival, although the mutant mice have substantially reduced COXI protein, causing an isolated complex IV deficiency. We show that TACO1 specifically binds the mt-Co1 mRNA and is required for translation of COXI through its association with the mitochondrial ribosome. We determined the atomic structure of TACO1, revealing three domains in the shape of a hook with a tunnel between domains 1 and 3. Mutations in the positively charged domain 1 reduce RNA binding by TACO1. The Taco1 mutant mice develop a late-onset visual impairment, motor dysfunction and cardiac hypertrophy and thus provide a useful model for future treatment trials for mitochondrial disease.

Mutations in the translational activator of cytochrome c oxidase subunit I (TACO1) causes cytochrome c oxidase deficiency and Leigh Syndrome in patients. Here, the authors characterize mice with a mutation that causes lack of TACO1 expression, identifying a mouse model that could be useful for preclinical trials.

The potential for nanotechnology to improve delivery of therapy to the acute ischemic heart

Treatment of acute cardiac ischemia remains an area in which there are opportunities for therapeutic improvement. Despite significant advances, many patients still progress to cardiac hypertrophy and heart failure. Timely reperfusion is critical in rescuing vulnerable ischemic tissue and is directly related to patient outcome, but reperfusion of the ischemic myocardium also contributes to damage. Overproduction of reactive oxygen species, initiation of an inflammatory response and deregulation of calcium homeostasis all contribute to injury, and difficulties in delivering a sufficient quantity of drug to the affected tissue in a controlled manner is a limitation of current therapies. Nanotechnology may offer significant improvements in this respect. Here, we review recent examples of how nanoparticles can be used to improve delivery to the ischemic myocardium, and suggest some approaches that may lead to improved therapies for acute cardiac ischemia.

The Role of the L-Type Ca2+ Channel in Altered Metabolic Activity in a Murine Model of Hypertrophic Cardiomyopathy

Heterozygous mice (αMHC^403/+) expressing the human disease-causing mutation Arg403Gln exhibit cardinal features of hypertrophic cardiomyopathy (HCM) including hypertrophy, myocyte disarray, and increased myocardial fibrosis. Treatment of αMHC^403/+mice with the L-type calcium channel (I_Ca-L) antagonist diltiazem has been shown to decrease left ventricular anterior wall thickness, cardiac myocyte hypertrophy, disarray, and fibrosis. However, the role of the I_Ca-L in the development of HCM is not known. In addition to maintaining cardiac excitation and contraction in myocytes, the I_Ca-L also regulates mitochondrial function through transmission of movement of I_Ca-L via cytoskeletal proteins to mitochondrial voltage-dependent anion channel. Here, the authors investigated the role of I_Ca-L in regulating mitochondrial function in αMHC^403/+mice. Whole-cell patch clamp studies showed that I_Ca-L current inactivation kinetics were significantly increased in αMHC^403/+cardiac myocytes, but that current density and channel expression were similar to wild-type cardiac myocytes. Activation of I_Ca-L caused a significantly greater increase in mitochondrial membrane potential and metabolic activity in αMHC^403/+. These increases were attenuated with I_Ca-L antagonists and following F-actin or β-tubulin depolymerization. The authors observed increased levels of fibroblast growth factor-21 in αMHC^403/+mice, and altered mitochondrial DNA copy number consistent with altered mitochondrial activity and the development of cardiomyopathy. These studies suggest that the Arg403Gln mutation leads to altered functional communication between I_Ca-L and mitochondria that is associated with increased metabolic activity, which may contribute to the development of cardiomyopathy. I_Ca-L antagonists may be effective in reducing the cardiomyopathy in HCM by altering metabolic activity.

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Heterozygous mice (αMHC^403/+) expressing the human hypertrophic cardiomyopathy (HCM) disease causing mutation Arg403Gln exhibit cardinal features of HCM.

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This study investigated the role of L-type Ca2+ channel (ICa-L) in regulating mitochondrial function in Arg403Gln (αMHC^403/+) mice.

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Activation of ICa-L in αMHC^403/+mice caused a significantly greater increase in mitochondrial membrane potential and metabolic activity when compared to wild-type mice.

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Increases in mitochondrial membrane potential and metabolic activity were attenuated with ICa-L antagonists and when F-actin or β-tubulin were depolymerized.

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ICa-L antagonists may be effective in reducing the cardiomyopathy in HCM by altering metabolic activity.

Deranged sodium to sudden death

In February 2014, a group of scientists convened as part of the University of California Davis Cardiovascular Symposium to bring together experimental and mathematical modelling perspectives and discuss points of consensus and controversy on the topic of sodium in the heart. This paper summarizes the topics of presentation and discussion from the symposium, with a focus on the role of aberrant sodium channels and abnormal sodium homeostasis in cardiac arrhythmias and pharmacotherapy from the subcellular scale to the whole heart. Two following papers focus on Na(+) channel structure, function and regulation, and Na(+)/Ca(2+) exchange and Na(+)/K(+) ATPase. The UC Davis Cardiovascular Symposium is a biannual event that aims to bring together leading experts in subfields of cardiovascular biomedicine to focus on topics of importance to the field. The focus on Na(+) in the 2014 symposium stemmed from the multitude of recent studies that point to the importance of maintaining Na(+) homeostasis in the heart, as disruption of homeostatic processes are increasingly identified in cardiac disease states. Understanding how disruption in cardiac Na(+)-based processes leads to derangement in multiple cardiac components at the level of the cell and to then connect these perturbations to emergent behaviour in the heart to cause disease is a critical area of research. The ubiquity of disruption of Na(+) channels and Na(+) homeostasis in cardiac disorders of excitability and mechanics emphasizes the importance of a fundamental understanding of the associated mechanisms and disease processes to ultimately reveal new targets for human therapy.

Role of the cytoskeleton in communication between L-type Ca(2+) channels and mitochondria

The L-type Ca(2+) channel is the main route for Ca(2+) entry into cardiac myocytes, which is essential for the maintenance of cardiac excitation and contraction. Alterations in L-type Ca(2+) channel activity and Ca(2+) homeostasis have been implicated in the development of cardiomyopathies. Cardiac excitation and contraction is fuelled by ATP, synthesized predominantly by the mitochondria via the Ca(2+)-dependent process oxidative phosphorylation. Mitochondrial reactive oxygen species (ROS) are by-products of oxidative phosphorylation and are associated with the development of cardiac pathology. The cytoskeleton plays a role in the communication of signals from the plasma membrane to intracellular organelles. There is good evidence that both L-type Ca(2+) channel activity and mitochondrial function can be modulated by changes in the cytoskeletal network. Activation of the L-type Ca(2+) channel can regulate mitochondrial function through cytoskeletal proteins as a result of transmission of movement from the β(2)-subunit of the channel that occurs during activation and inactivation of the channel. An association between cytoskeletal proteins and the mitochondrial voltage-dependent anion channel (VDAC) may play a role in this response. The L-type Ca(2+) channel is the initiator of contraction in cardiac muscle and the VDAC is responsible for regulating mitochondrial ATP/ADP trafficking. This article presents evidence that a functional coupling between L-type Ca(2+) channels and mitochondria may assist in meeting myocardial energy demand on a beat-to-beat basis.

How does the heart sense changes in oxygen tension: a role for ion channels?

SIGNIFICANCE

Oxygen plays a key role in cellular metabolism and function. Oxygen delivery to cells is crucial, and a lack of oxygen such as that which occurs during myocardial infarction can be lethal. Cells should, therefore, be able to respond to changes in oxygen tension.

RECENT ADVANCES

Since the first studies examining the acute cellular effect of hypoxia on activation of transmitter release from glomus or type I chemoreceptor cells, it is now known that virtually all cells are able to respond to changes in oxygen tension.

CRITICAL ISSUES

Despite advances made in characterizing hypoxic responses, the identity of the "oxygen sensor" remains debated. Recently, more evidence has evolved as to how cardiac myocytes sense acute changes in oxygen. This review will examine the available evidence in support of acute oxygen-sensing mechanisms providing a brief historical perspective and then more detailed insights into the heart and the role of cardiac ion channels in hypoxic responses.

FUTURE DIRECTIONS

A further understanding of these cellular processes should result in interventions that assist in preventing the deleterious effects of acute changes in oxygen tension such as alterations in contractile function and cardiac arrhythmia.

Nanoparticle-mediated dual delivery of an antioxidant and a peptide against the L-Type Ca2+ channel enables simultaneous reduction of cardiac ischemia-reperfusion injury

Increased reactive oxygen species (ROS) production and elevated intracellular Ca(2+) following cardiac ischemia-reperfusion injury are key mediators of cell death and the development of cardiac hypertrophy. The L-type Ca(2+) channel is the main route for calcium influx in cardiac myocytes. Activation of the L-type Ca(2+) channel leads to a further increase in mitochondrial ROS production and metabolism. We have previously shown that the application of a peptide derived against the alpha-interacting domain of the L-type Ca(2+) channel (AID) decreases myocardial injury post reperfusion. Herein, we examine the efficacy of simultaneous delivery of the AID peptide in combination with the potent antioxidants curcumin or resveratrol using multifunctional poly(glycidyl methacrylate) (PGMA) nanoparticles. We highlight that drug loading and dissolution are important parameters that have to be taken into account when designing novel combinatorial therapies following cardiac ischemia-reperfusion injury. In the case of resveratrol low loading capacity and fast release rates hinder its applicability as an effective candidate for simultaneous therapy. However, in the case of curcumin, high loading capacity and sustained release rates enable its effective simultaneous delivery in combination with the AID peptide. Simultaneous delivery of the AID peptide with curcumin allowed for effective attenuation of the L-type Ca(2+) channel-activated increases in superoxide (assessed as changes in DHE fluorescence; Empty NP = 53.1 ± 7.6%; NP-C-AID = 7.32 ± 3.57%) and mitochondrial membrane potential (assessed as changes in JC-1 fluorescence; Empty NP = 19.8 ± 2.8%; NP-C-AID=13.05 ± 1.78%). We demonstrate in isolated rat hearts exposed to ischemia followed by reperfusion, that curcumin and the AID peptide in combination effectively reduce muscle damage, decrease oxidative stress and superoxide production in cardiac myocytes.

Choline kinase β mutant mice exhibit reduced phosphocholine, elevated osteoclast activity, and low bone mass

The maintenance of bone homeostasis requires tight coupling between bone-forming osteoblasts and bone-resorbing osteoclasts. However, the precise molecular mechanism(s) underlying the differentiation and activities of these specialized cells are still largely unknown. Here, we identify choline kinase β (CHKB), a kinase involved in the biosynthesis of phosphatidylcholine, as a novel regulator of bone homeostasis. Choline kinase β mutant mice (flp/flp) exhibit a systemic low bone mass phenotype. Consistently, osteoclast numbers and activity are elevated in flp/flp mice. Interestingly, osteoclasts derived from flp/flp mice exhibit reduced sensitivity to excessive levels of extracellular calcium, which could account for the increased bone resorption. Conversely, supplementation of cytidine 5'-diphosphocholine in vivo and in vitro, a regimen that bypasses CHKB deficiency, restores osteoclast numbers to physiological levels. Finally, we demonstrate that, in addition to modulating osteoclast formation and function, loss of CHKB corresponds with a reduction in bone formation by osteoblasts. Taken together, these data posit CHKB as a new modulator of bone homeostasis.

Glutathionylation of the L-type Ca2+ channel in oxidative stress-induced pathology of the heart

There is mounting evidence to suggest that protein glutathionylation is a key process contributing to the development of pathology. Glutathionylation occurs as a result of posttranslational modification of a protein and involves the addition of a glutathione moiety at cysteine residues. Such modification can occur on a number of proteins, and exerts a variety of functional consequences. The L-type Ca2+ channel has been identified as a glutathionylation target that participates in the development of cardiac pathology. Ca2+ influx via the L-type Ca2+ channel increases production of mitochondrial reactive oxygen species (ROS) in cardiomyocytes during periods of oxidative stress. This induces a persistent increase in channel open probability, and the resulting constitutive increase in Ca2+ influx amplifies the cross-talk between the mitochondria and the channel. Novel strategies utilising targeted peptide delivery to uncouple mitochondrial ROS and Ca2+ flux via the L-type Ca2+ channel following ischemia-reperfusion have delivered promising results, and have proven capable of restoring appropriate mitochondrial function in myocytes and in vivo.

Decreased myocardial injury and improved contractility after administration of a peptide derived against the alpha-interacting domain of the L-type calcium channel

Background

Myocardial infarction remains the leading cause of morbidity and mortality associated with coronary artery disease. The L‐type calcium channel (I_Ca‐L) is critical to excitation and contraction. Activation of the channel also alters mitochondrial function. Here, we investigated whether application of a alpha‐interacting domain/transactivator of transcription (AID‐TAT) peptide, which immobilizes the auxiliary β₂ subunit of the channel and decreases metabolic demand, could alter mitochondrial function and myocardial injury.

Methods and Results

Treatment with AID‐TAT peptide decreased ischemia‐reperfusion injury in guinea‐pig hearts ex vivo (n=11) and in rats in vivo (n=9) assessed with uptake of nitroblue tetrazolium, release of creatine kinase, and lactate dehydrogenase. Contractility (assessed with catheterization of the left ventricle) was improved after application of AID‐TAT peptide in hearts ex vivo (n=6) and in vivo (n=8) up to 12 weeks before sacrifice. In search of the mechanism for the effect, we found that intracellular calcium ([Ca²⁺]_i, Fura‐2), superoxide production (dihydroethidium fluorescence), mitochondrial membrane potential (Ψ_m, JC‐1 fluorescence), reduced nicotinamide adenine dinucleotide production, and flavoprotein oxidation (autofluorescence) are decreased after application of AID‐TAT peptide.

Conclusions

Application of AID‐TAT peptide significantly decreased infarct size and supported contractility up to 12 weeks postcoronary artery occlusion as a result of a decrease in metabolic demand during reperfusion.

Examining efficacy of "TAT-less" delivery of a peptide against the L-type calcium channel in cardiac ischemia-reperfusion injury

Increased calcium influx through the L-type Ca(2+) channel or overexpression of the alpha subunit of the channel induces cardiac hypertrophy. Cardiac hypertrophy results from increased oxidative stress and alterations in cell calcium levels following ischemia-reperfusion injury and is an independent risk factor for increased morbidity and mortality. We find that decreasing the movement of the auxiliary beta subunit with a peptide derived against the alpha-interacting domain (AID) of the channel attenuates ischemia-reperfusion injury. We compared the efficacy of delivering the AID peptide using a trans-activator of transcription (TAT) sequence with that of the peptide complexed to multifunctional polymeric nanoparticles. The AID-tethered nanoparticles perfused through the myocardium more diffusely and associated with cardiac myocytes more rapidly than the TAT-labeled peptide but had similar effects on intracellular calcium levels. The AID-complexed nanoparticles resulted in a similar reduction in release of creatine kinase and lactate dehydrogenase after ischemia-reperfusion to the TAT-labeled peptide. Since nanoparticle delivery also holds the potential for dual drug delivery, we conclude that AID-complexed nanoparticles may provide an effective platform for peptide delivery in cardiac ischemia-reperfusion injuries.

L-type Ca(2+) channel contributes to alterations in mitochondrial calcium handling in the mdx ventricular myocyte

The L-type Ca(2+) channel is the main route for calcium entry into cardiac myocytes, and it is essential for contraction. Alterations in whole cell L-type Ca(2+) channel current and Ca(2+) homeostasis have been implicated in the development of cardiomyopathies. Cytoskeletal proteins can influence whole cell L-type Ca(2+) current and mitochondrial function. Duchenne muscular dystrophy is a fatal X-linked disease that leads to progressive muscle weakness due to the absence of cytoskeletal protein dystrophin. This includes dilated cardiomyopathy, but the mechanisms are not well understood. We sought to identify the effect of alterations in whole cell L-type Ca(2+) channel current on mitochondrial function in the murine model of Duchenne muscular dystrophy (mdx). Activation of the L-type Ca(2+) channel with the dihydropyridine agonist BayK(-) caused a significantly larger increase in cytosolic Ca(2+) in mdx vs. wild-type (wt) ventricular myocytes. Consistent with elevated cytosolic Ca(2+), resting mitochondrial Ca(2+), NADH, and mitochondrial superoxide were significantly greater in mdx vs. wt myocytes. Activation of the channel with BayK(-) caused a further increase in mitochondrial Ca(2+), NADH, and superoxide in mdx myocytes. The ratios of the increases were similar to the ratios recorded in wt myocytes. In mitochondria isolated from 8-wk-old mdx hearts, respiration and mitochondrial electron transport chain complex activity were similar to mitochondria isolated from wt hearts. We conclude that mitochondria function at a higher level of resting calcium in the intact mdx myocyte and activation of the L-type Ca(2+) channel contributes to alterations in calcium handling by the mitochondria. This perturbation may contribute to the development of cardiomyopathy.

Regulator of G-protein signaling 5 controls blood pressure homeostasis and vessel wall remodeling

RATIONALE

Regulator of G-protein signaling 5 (RGS5) modulates G-protein-coupled receptor signaling and is prominently expressed in arterial smooth muscle cells. Our group first reported that RGS5 is important in vascular remodeling during tumor angiogenesis. We hypothesized that RGS5 may play an important role in vessel wall remodeling and blood pressure regulation.

OBJECTIVE

To demonstrate that RGS5 has a unique and nonredundant role in the pathogenesis of hypertension and to identify crucial RGS5-regulated signaling pathways.

METHODS AND RESULTS

We observed that arterial RGS5 expression is downregulated with chronically elevated blood pressure after angiotensin II infusion. Using a knockout mouse model, radiotelemetry, and pharmacological inhibition, we subsequently showed that loss of RGS5 results in profound hypertension. RGS5 signaling is linked to the renin-angiotensin system and directly controls vascular resistance, vessel contractility, and remodeling. RGS5 deficiency aggravates pathophysiological features of hypertension, such as medial hypertrophy and fibrosis. Moreover, we demonstrate that protein kinase C, mitogen-activated protein kinase/extracellular signal-regulated kinase, and Rho kinase signaling pathways are major effectors of RGS5-mediated hypertension.

CONCLUSIONS

Loss of RGS5 results in hypertension. Loss of RGS5 signaling also correlates with hyper-responsiveness to vasoconstrictors and vascular stiffening. This establishes a significant, distinct, and causal role of RGS5 in vascular homeostasis. RGS5 modulates signaling through the angiotensin II receptor 1 and major Gαq-coupled downstream pathways, including Rho kinase. So far, activation of RhoA/Rho kinase has not been associated with RGS molecules. Thus, RGS5 is a crucial regulator of blood pressure homeostasis with significant clinical implications for vascular pathologies, such as hypertension.

Myeloperoxidase-derived oxidants inhibit sarco/endoplasmic reticulum Ca2+-ATPase activity and perturb Ca2+ homeostasis in human coronary artery endothelial cells

The sarco/endoplasmic reticulum Ca(2+)-ATPase (SERCA) plays a critical role in Ca(2+) homeostasis via sequestration of this ion in the sarco/endoplasmic reticulum. The activity of this pump is inhibited by oxidants and impaired in aging tissues and cardiovascular disease. We have shown previously that the myeloperoxidase (MPO)-derived oxidants HOCl and HOSCN target thiols and mediate cellular dysfunction. As SERCA contains Cys residues critical to ATPase activity, we hypothesized that HOCl and HOSCN might inhibit SERCA activity, via thiol oxidation, and increase cytosolic Ca(2+) levels in human coronary artery endothelial cells (HCAEC). Exposure of sarcoplasmic reticulum vesicles to preformed or enzymatically generated HOCl and HOSCN resulted in a concentration-dependent decrease in ATPase activity; this was also inhibited by the SERCA inhibitor thapsigargin. Decomposed HOSCN and incomplete MPO enzyme systems did not decrease activity. Loss of ATPase activity occurred concurrent with oxidation of SERCA Cys residues and protein modification. Exposure of HCAEC, with or without external Ca(2+), to HOSCN or HOCl resulted in a time- and concentration-dependent increase in intracellular Ca(2+) under conditions that did not result in immediate loss of cell viability. Thapsigargin, but not inhibitors of plasma membrane or mitochondrial Ca(2+) pumps/channels, completely attenuated the increase in intracellular Ca(2+) consistent with a critical role for SERCA in maintaining endothelial cell Ca(2+) homeostasis. Angiotensin II pretreatment potentiated the effect of HOSCN at low concentrations. MPO-mediated modulation of intracellular Ca(2+) levels may exacerbate endothelial dysfunction, a key early event in atherosclerosis, and be more marked in smokers because of their higher SCN(-) levels.