Short-term statin treatment reduces, and long-term statin treatment abolishes chronic vascular injury by radiation therapy

Background

The incidental use of statins during radiation therapy has been associated with a reduced long-term risk of developing atherosclerotic cardiovascular disease.

Objectives

Determine if irradiation causes chronic vascular injury and whether short-term administration of statins during and after irradiation is sufficient to prevent chronic injury compared to long-term administration.

Methods

C57Bl/6 mice were pretreated with pravastatin for 72 hours and then exposed to 12 Gy x-ray head-and-neck irradiation. Subsequently, they received pravastatin either for one additional day or for one year. Carotid arteries were tested for vascular reactivity and altered gene expression one year after irradiation.

Results

Treatment with pravastatin for 24 hours reduced the loss of endothelium-dependent vasorelaxation and protected against enhanced vasoconstriction after IR. It reduced the expression of some markers associated with inflammation and oxidative stress and modulated that of subunits of the voltage and Ca2+ activated K+ (BK) channel in the carotid artery one year after irradiation. Treatment with pravastatin for one year completely reversed the changes caused by irradiation.

Conclusions

In mice, short-term administration of pravastatin is sufficient to reduce chronic vascular injury after irradiation. Long-term administration eliminates the effects of irradiation. These findings suggest that a prospective treatment strategy involving statins could be effective in patients undergoing radiation therapy. The optimal duration of treatment in humans has yet to be determined.

Regulation of Smooth Muscle Cell Proliferation by Mitochondrial Ca2+ in Type 2 Diabetes

Type 2 diabetes (T2D) is associated with increased risk of atherosclerotic vascular disease due to excessive vascular smooth muscle cell (VSMC) proliferation. Here, we investigated the role of mitochondrial dysfunction and Ca2+ levels in VSMC proliferation in T2D. VSMCs were isolated from normoglycemic and T2D-like mice induced by diet. The effects of mitochondrial Ca2+ uptake were studied using mice with selectively inhibited mitochondrial Ca2+/calmodulin-dependent kinase II (mtCaMKII) in VSMCs. Mitochondrial transition pore (mPTP) was blocked using ER-000444793. VSMCs from T2D compared to normoglycemic mice exhibited increased proliferation and baseline cytosolic Ca2+ levels ([Ca2+]cyto). T2D cells displayed lower endoplasmic reticulum Ca2+ levels, reduced mitochondrial Ca2+ entry, and increased Ca2+ leakage through the mPTP. Mitochondrial and cytosolic Ca2+ transients were diminished in T2D cells upon platelet-derived growth factor (PDGF) administration. Inhibiting mitochondrial Ca2+ uptake or the mPTP reduced VSMC proliferation in T2D, but had contrasting effects on [Ca2+]cyto. In T2D VSMCs, enhanced activation of Erk1/2 and its upstream regulators was observed, driven by elevated [Ca2+]cyto. Inhibiting mtCaMKII worsened the Ca2+ imbalance by blocking mitochondrial Ca2+ entry, leading to further increases in [Ca2+]cyto and Erk1/2 hyperactivation. Under these conditions, PDGF had no effect on VSMC proliferation. Inhibiting Ca2+-dependent signaling in the cytosol reduced excessive Erk1/2 activation and VSMC proliferation. Our findings suggest that altered Ca2+ handling drives enhanced VSMC proliferation in T2D, with mitochondrial dysfunction contributing to this process.

Mechanisms by which statins protect endothelial cells from radiation-induced injury in the carotid artery

Background

The incidental use of statins during radiation therapy has been associated with a reduced long-term risk of developing atherosclerotic cardiovascular disease. However, the mechanisms by which statins protect the vasculature from irradiation injury remain poorly understood.

Objectives

Identify the mechanisms by which the hydrophilic and lipophilic statins pravastatin and atorvastatin preserve endothelial function after irradiation.

Methods

Cultured human coronary and umbilical vein endothelial cells irradiated with 4 Gy and mice subjected to 12 Gy head-and-neck irradiation were pretreated with statins and tested for endothelial dysfunction, nitric oxide production, oxidative stress, and various mitochondrial phenotypes at 24 and 240 h after irradiation.

Results

Both pravastatin (hydrophilic) and atorvastatin (lipophilic) were sufficient to prevent the loss of endothelium-dependent relaxation of arteries after head-and-neck irradiation, preserve the production of nitric oxide by endothelial cells, and suppress the cytosolic reactive oxidative stress associated with irradiation. However, only pravastatin inhibited irradiation-induced production of mitochondrial superoxide; damage to the mitochondrial DNA; loss of electron transport chain activity; and expression of inflammatory markers.

Conclusions

Our findings reveal some mechanistic underpinnings of the vasoprotective effects of statins after irradiation. Whereas both pravastatin and atorvastatin can shield from endothelial dysfunction after irradiation, pravastatin additionally suppresses mitochondrial injury and inflammatory responses involving mitochondria. Clinical follow-up studies will be necessary to determine whether hydrophilic statins are more effective than their lipophilic counterparts in reducing the risk of cardiovascular disease in patients undergoing radiation therapy.

The mitochondrial regulation of smooth muscle cell proliferation in type 2 diabetes

Background

Type 2 diabetes (T2D) is associated with a strongly increased risk for restenosis after angioplasty driven by proliferation of vascular smooth muscle cells (VSMCs). Here, we sought to determine whether and how mitochondrial dysfunction in T2D drives VSMC proliferation with a focus on ROS and intracellular [Ca 2+ ] that both drive cell proliferation, occur in T2D and are regulated by mitochondrial activity.

Methods

Using a diet-induced mouse model of T2D, the inhibition of the mitochondrial Ca 2+ /calmodulin-dependent kinase II (mtCaMKII), a regulator of Ca 2+ entry via the mitochondrial Ca 2+ uniporter selectively in VSMCs, we performed in vivo phenotyping after mechanical injury and established the mechanisms of excessive proliferation in cultured VSMCs.

Results

In T2D, the inhibition of mtCaMKII reduced both neointima formation after mechanical injury and the proliferation of cultured VSMCs. VSMCs from T2D mice displayed accelerated proliferation, reduced mitochondrial Ca 2+ entry and membrane potential with elevated baseline [Ca 2+ ] cyto compared to cells from normoglycemic mice. Accelerated proliferation after PDGF treatment was driven by activation of Erk1/2 and its upstream regulators. Hyperactivation of Erk1/2 was Ca 2+ -dependent rather than mitochondrial ROS-driven Ca 2+ -dependent and included the activation of CaMKII in the cytosol. The inhibition of mtCaMKII exaggerated the Ca 2+ imbalance by lowering mitochondrial Ca 2+ entry and increasing baseline [Ca 2+ ] cyto , further enhancing baseline Erk1/2 activation. With inhibition of mtCaMKII, PDGF treatment had no additional effect on cell proliferation. Inhibition of activated CaMKII in the cytosol decreased excessive Erk1/2 activation and reduced VSMC proliferation.

Conclusions

Collectively, our results provide evidence for the molecular mechanisms of enhanced VSMC proliferation after mechanical injury by mitochondrial Ca 2+ entry in T2D.

Retraction notice to Corrigendum to "Inhibition of CaMKII in mitochondria preserves endothelial barrier function after irradiation" [Free Radical Biol. Med. 146, January (2020) 287-298]

This article has been retracted: please see Elsevier Policy on Article Withdrawal (http://www.elsevier.com/locate/withdrawalpolicy). This article has been retracted at the request of the Authors and Editor-in-Chief. Some of the data presented in Figure 6C, F and G of the paper to which this corrigendum relates were reported incorrectly in the published article. After being contacted by the Journal, the authors discovered an unintentional error in how the original data were analyzed that could affect the accuracy of the subsequent analysis. The raw data were incorrectly grouped in the analysis software, thereby altering the comparisons. Therefore the authors wish to retract the paper and corrigendum and will recollect and reanalyze the data appropriately. The authors apologize for any inconvenience.

Mitochondrial Ca2+ Uptake Drives Endothelial Injury By Radiation Therapy

Radiation therapy strongly increases the risk of atherosclerotic vascular disease, such as carotid stenosis. Radiation induces DNA damage, in particular in mitochondria, but the upstream and downstream signaling events are poorly understood. The objective of this study was to define such mechanisms.

Retraction notice to "Inhibition of CaMKII in mitochondria preserves endothelial barrier function after irradiation" [Free Radic. Biol. Med. 146C (2020) 287-298]

This article has been retracted: please see Elsevier Policy on Article Withdrawal (http://www.elsevier.com/locate/withdrawalpolicy). This article has been retracted at the request of the Authors and Editor-in-Chief. Some of the data presented in Figure 6C, F and G of the above-titled paper were reported incorrectly in the published article. After being contacted by the Journal, the authors discovered an unintentional error in how the original data were analyzed that could affect the accuracy of the subsequent analysis. The raw data were incorrectly grouped in the analysis software, thereby altering the comparisons. Therefore the authors wish to retract the paper and will recollect and reanalyze the data appropriately. The authors apologize for any inconvenience.

Sex‐Specific Differences in Endothelial Function Are Driven by Divergent Mitochondrial Ca 2+ Handling

Background

Sex‐specific differences in vasodilation are mediated in part by differences in cytosolic Ca 2+ handling, but how variations in mitochondrial Ca 2+ contributes to this effect remains unknown. Here, we investigated the extent to which mitochondrial Ca 2+ entry via the MCU (mitochondrial Ca 2+ uniporter) drives sex differences in vasoreactivity in resistance arteries.

Methods and Results

Enhanced vasodilation of mesenteric resistance arteries to acetylcholine (ACh) was reduced to larger extent in female compared with male mice in 2 genetic models of endothelial MCU ablation. Ex vivo Ca 2+ imaging of mesenteric arteries with Fura‐2AM confirmed higher cytosolic Ca 2+ transients triggered by ACh in arteries from female mice versus male mice. MCU inhibition both strongly reduced cytosolic Ca 2+ transients and blocked mitochondrial Ca 2+ entry. In cultured human aortic endothelial cells, treatment with physiological concentrations of estradiol enhanced cytosolic Ca 2+ transients, Ca 2+ buffering capacity, and mitochondrial Ca 2+ entry in response to ATP or repeat Ca 2+ boluses. Further experiments to establish the mechanisms underlying these effects did not reveal significant differences in the expression of MCU subunits, at either the mRNA or protein level. However, estradiol treatment was associated with an increase in mitochondrial mass, mitochondrial fusion, and the mitochondrial membrane potential and reduced mitochondrial superoxide production.

Conclusions

Our data confirm that mitochondrial function in endothelial cells differs by sex, with female mice having enhanced Ca 2+ uptake capacity, and that these differences are attributable to the presence of more mitochondria and a higher mitochondrial membrane potential in female mice rather than differences in composition of the MCU complex.

Knockout of Sorbin And SH3 Domain Containing 2 (Sorbs2) in Cardiomyocytes Leads to Dilated Cardiomyopathy in Mice

Background

Sorbin and SH3 domain containing 2 (Sorbs2) protein is a cytoskeletal adaptor with an emerging role in cardiac biology and disease; yet, its potential relevance to adult‐onset cardiomyopathies remains underexplored. Sorbs2 global knockout mice display lethal arrhythmogenic cardiomyopathy; however, the causative mechanisms remain unclear. Herein, we examine Sorbs2 dysregulation in heart failure, characterize novel Sorbs2 cardiomyocyte‐specific knockout mice (Sorbs2‐cKO), and explore associations between Sorbs2 genetic variations and human cardiovascular disease.

Methods and Results

Bioinformatic analyses show myocardial Sorbs2 mRNA is consistently upregulated in humans with adult‐onset cardiomyopathies and in heart failure models. We generated Sorbs2‐cKO mice and report that they develop progressive systolic dysfunction and enlarged cardiac chambers, and they die with congestive heart failure at about 1 year old. After 3 months, Sorbs2‐cKO mice begin to show atrial enlargement and P‐wave anomalies, without dysregulation of action potential–associated ion channel and gap junction protein expressions. After 6 months, Sorbs2‐cKO mice exhibit impaired contractility in dobutamine‐treated hearts and skinned myofibers, without dysregulation of contractile protein expressions. From our comprehensive survey of potential mechanisms, we found that within 4 months, Sorbs2‐cKO hearts have defective microtubule polymerization and compensatory upregulation of structural cytoskeletal and adapter proteins, suggesting that this early intracellular structural remodeling is responsible for contractile dysfunction. Finally, we identified genetic variants that associate with decreased Sorbs2 expression and human cardiac phenotypes, including conduction abnormalities, atrial enlargement, and dilated cardiomyopathy, consistent with Sorbs2‐cKO mice phenotypes.

Conclusions

Our studies show that Sorbs2 is essential for maintaining structural integrity in cardiomyocytes, likely through strengthening the interactions between microtubules and other cytoskeletal proteins at cross‐link sites.

Inhibition of CaMKII in mitochondria preserves endothelial barrier function after irradiation

Damage to the microvascular endothelium is an important part of normal tissue injury after radiation exposure and driven by the production of pro-oxidants. The Ca2+/calmodulin-dependent protein kinase II is present in the mitochondrial matrix (mitoCaMKII) where it regulates Ca2+ uptake via the mitochondrial Ca2+ uniporter (MCU) and pro-oxidant production. Here, we demonstrate that radiation exposure disrupts endothelial cell barrier integrity in vitro, but can be abrogated by inhibition of mitoCaMKII, MCU, or opening of the mitochondrial transition pore. Scavenging of mitochondrial pro-oxidants with mitoTEMPO before, but not after irradiation, protected barrier function. Furthermore, markers of apoptosis and mitochondrial pro-oxidant production were elevated at 24 h following irradiation and abolished by mitoCaMKII inhibition. Endothelial barrier dysfunction was detected as early as 2 h after irradiation. Despite only mildly impaired mitochondrial respiration, the intracellular ATP levels were significantly reduced 4 h after irradiation and correlated with barrier function. MitoCaMKII inhibition improved intracellular ATP concentrations by increasing glycolysis. Finally, DNA double strand break repair and non-homologous end joining, two major drivers of ATP consumption after irradiation, were greatly increased but not significantly affected by mitoCaMKII inhibition. These findings support the hypothesis that mitoCaMKII activity is linked to mitochondrial pro-oxidant production, reduced ATP production, and loss of endothelial barrier function following irradiation. The inhibition of mitoCaMKII is a promising approach to limiting radiation-induced endothelial injury.

Loss of MCU prevents mitochondrial fusion in G1-S phase and blocks cell cycle progression and proliferation

The role of the mitochondrial Ca²⁺ uniporter (MCU) in physiologic cell proliferation remains to be defined. Here, we demonstrated that the MCU was required to match mitochondrial function to metabolic demands during the cell cycle. During the G₁-S transition (the cycle phase with the highest mitochondrial ATP output), mitochondrial fusion, oxygen consumption, and Ca²⁺ uptake increased in wild-type cells but not in cells lacking MCU. In proliferating wild-type control cells, the addition of the growth factors promoted the activation of the Ca²⁺/calmodulin-dependent kinase II (CaMKII) and the phosphorylation of the mitochondrial fission factor Drp1 at Ser⁶¹⁶ The lack of the MCU was associated with baseline activation of CaMKII, mitochondrial fragmentation due to increased Drp1 phosphorylation, and impaired mitochondrial respiration and glycolysis. The mitochondrial fission/fusion ratio and proliferation in MCU-deficient cells recovered after MCU restoration or inhibition of mitochondrial fragmentation or of CaMKII in the cytosol. Our data highlight a key function for the MCU in mitochondrial adaptation to the metabolic demands during cell cycle progression. Cytosolic CaMKII and the MCU participate in a regulatory circuit, whereby mitochondrial Ca²⁺ uptake affects cell proliferation through Drp1.

Defective protein repair under methionine sulfoxide A deletion drives autophagy and ARE-dependent gene transcription

OBJECTIVE

Reduction of oxidized methionines is emerging as a major protein repair pathway. The lack of methionine sulfoxide reductase A (MsrA) exacerbates cardiovascular disease phenotypes driven by increased oxidative stress. However, the role of MsrA on maintaining cellular homeostasis in the absence of excessive oxidative stress is less well understood.

METHODS AND RESULTS

Constitutive genetic deletion of MsrA increased formation of p62-containing protein aggregates, activated autophagy, and decreased a marker of apoptosis in vascular smooth muscle cells (VSMC). The association of Keap1 with p62 was augmented in MsrA-/- VSMC. Keap1 targets the transcription factor Nrf2, which regulates antioxidant genes, for proteasomal degradation. However, in MsrA-/- VSMC, the association of Nrf2 with Keap1 was diminished. Whereas Nrf2 mRNA levels were not decreased in MsrA-/- VSMC, we detected decreased ubiquitination of Nrf2 and a corresponding increase in total Nrf2 protein in the absence of biochemical markers of oxidative stress. Moreover, nuclear-localized Nrf2 was increased under MsrA deficiency, resulting in upregulation of Nrf2-dependent transcriptional activity. Consequently, transcription, protein levels and enzymatic activity of glutamate-cysteine ligase and glutathione reductase were greatly augmented in MsrA-/- VSMC.

SUMMARY

Our findings demonstrate that reversal of methionine oxidation is required for maintenance of cellular homeostasis in the absence of increased oxidative stress. These data provide the first link between autophagy and activation of Nrf2 in the setting of MsrA deletion.

CaMKII (Ca2+/Calmodulin-Dependent Kinase II) in Mitochondria of Smooth Muscle Cells Controls Mitochondrial Mobility, Migration, and Neointima Formation

OBJECTIVE

The main objective of this study is to define the mechanisms by which mitochondria control vascular smooth muscle cell (VSMC) migration and impact neointimal hyperplasia.

APPROACH AND RESULTS

The multifunctional CaMKII (Ca²⁺/calmodulin-dependent kinase II) in the mitochondrial matrix of VSMC drove a feed-forward circuit with the mitochondrial Ca²⁺ uniporter (MCU) to promote matrix Ca²⁺ influx. MCU was necessary for the activation of mitochondrial CaMKII (mtCaMKII), whereas mtCaMKII phosphorylated MCU at the regulatory site S92 that promotes Ca²⁺ entry. mtCaMKII was necessary and sufficient for platelet-derived growth factor-induced mitochondrial Ca²⁺ uptake. This effect was dependent on MCU. mtCaMKII and MCU inhibition abrogated VSMC migration and mitochondrial translocation to the leading edge. Overexpression of wild-type MCU, but not MCU S92A, mutant in MCU^-/- VSMC rescued migration and mitochondrial mobility. Inhibition of microtubule, but not of actin assembly, blocked mitochondrial mobility. The outer mitochondrial membrane GTPase Miro-1 promotes mitochondrial mobility via microtubule transport but arrests it in subcellular domains of high Ca²⁺ concentrations. In Miro-1^-/- VSMC, mitochondrial mobility and VSMC migration were abolished, and overexpression of mtCaMKII or a CaMKII inhibitory peptide in mitochondria (mtCaMKIIN) had no effect. Consistently, inhibition of mtCaMKII increased and prolonged cytosolic Ca²⁺ transients. mtCaMKII inhibition diminished phosphorylation of focal adhesion kinase and myosin light chain, leading to reduced focal adhesion turnover and cytoskeletal remodeling. In a transgenic model of selective mitochondrial CaMKII inhibition in VSMC, neointimal hyperplasia was significantly reduced after vascular injury.

CONCLUSIONS

These findings identify mitochondrial CaMKII as a key regulator of mitochondrial Ca²⁺ uptake via MCU, thereby controlling mitochondrial translocation and VSMC migration after vascular injury.

Inhibition of the mitochondrial calcium uniporter prevents IL-13 and allergen-mediated airway epithelial apoptosis and loss of barrier function

Mitochondria are increasingly recognized as key mediators of acute cellular stress responses in asthma. However, the distinct roles of regulators of mitochondrial physiology on allergic asthma phenotypes are currently unknown. The mitochondrial Ca²⁺ uniporter (MCU) resides in the inner mitochondrial membrane and controls mitochondrial Ca²⁺ uptake into the mitochondrial matrix. To understand the function of MCU in models of allergic asthma, in vitro and in vivo studies were performed using models of functional deficiency or knockout of MCU. In primary human respiratory epithelial cells, MCU inhibition abrogated mitochondrial Ca²⁺ uptake and reactive oxygen species (ROS) production, preserved the mitochondrial membrane potential and protected from apoptosis in response to the pleiotropic Th2 cytokine IL-13. Consequently, epithelial barrier function was maintained with MCU inhibition. Similarly, the endothelial barrier was preserved in respiratory epithelium isolated from MCU-/- mice after exposure to IL-13. In the ovalbumin-model of allergic airway disease, MCU deficiency resulted in decreased apoptosis within the large airway epithelial cells. Concordantly, expression of the tight junction protein ZO-1 was preserved, indicative of maintenance of epithelial barrier function. These data implicate mitochondrial Ca²⁺ uptake through MCU as a key controller of epithelial cell viability in acute allergic asthma.

Endothelial CaMKII as a regulator of eNOS activity and NO-mediated vasoreactivity

The multifunctional Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) is a serine/threonine kinase important in transducing intracellular Ca²⁺ signals. While in vitro data regarding the role of CaMKII in the regulation of endothelial nitric oxide synthase (eNOS) are contradictory, its role in endothelial function in vivo remains unknown. Using two novel transgenic models to express CaMKII inhibitor peptides selectively in endothelium, we examined the effect of CaMKII on eNOS activation, NO production, vasomotor tone and blood pressure. Under baseline conditions, CaMKII activation was low in the aortic wall. Consistently, systolic and diastolic blood pressure, heart rate and plasma NO levels were unaltered by endothelial CaMKII inhibition. Moreover, endothelial CaMKII inhibition had no significant effect on NO-dependent vasodilation. These results were confirmed in studies of aortic rings transduced with adenovirus expressing a CaMKII inhibitor peptide. In cultured endothelial cells, bradykinin treatment produced the anticipated rapid influx of Ca²⁺ and transient CaMKII and eNOS activation, whereas CaMKII inhibition blocked eNOS phosphorylation on Ser-1179 and dephosphorylation at Thr-497. Ca²⁺/CaM binding to eNOS and resultant NO production in vitro were decreased under CaMKII inhibition. Our results demonstrate that CaMKII plays an important role in transient bradykinin-driven eNOS activation in vitro, but does not regulate NO production, vasorelaxation or blood pressure in vivo under baseline conditions.

Mitochondrial CaMKII inhibition in airway epithelium protects against allergic asthma

Excessive ROS promote allergic asthma, a condition characterized by airway inflammation, eosinophilic inflammation, and increased airway hyperreactivity (AHR). The mechanisms by which airway ROS are increased and the relationship between increased airway ROS and disease phenotypes are incompletely defined. Mitochondria are an important source of cellular ROS production, and our group discovered that Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) is present in mitochondria and activated by oxidation. Furthermore, mitochondrial-targeted antioxidant therapy reduced the severity of allergic asthma in a mouse model. Based on these findings, we developed a mouse model of CaMKII inhibition targeted to mitochondria in airway epithelium. We challenged these mice with OVA or Aspergillus fumigatus. Mitochondrial CaMKII inhibition abrogated AHR, inflammation, and eosinophilia following OVA and A. fumigatus challenge. Mitochondrial ROS were decreased after agonist stimulation in the presence of mitochondrial CaMKII inhibition. This correlated with blunted induction of NF-κB, the NLRP3 inflammasome, and eosinophilia in transgenic mice. These findings demonstrate a pivotal role for mitochondrial CaMKII in airway epithelium in mitochondrial ROS generation, eosinophilic inflammation, and AHR, providing insights into how mitochondrial ROS mediate features of allergic asthma.

Role of CaMKII in Ang-II-dependent small artery remodeling

Angiotensin-II (Ang-II) is a well-established mediator of vascular remodeling. The multifunctional calcium-calmodulin-dependent kinase II (CaMKII) is activated by Ang-II and regulates Erk1/2 and Akt-dependent signaling in cultured smooth muscle cells in vitro. Its role in Ang-II-dependent vascular remodeling in vivo is far less defined. Using a model of transgenic CaMKII inhibition selectively in smooth muscle cells, we found that CaMKII inhibition exaggerated remodeling after chronic Ang-II treatment and agonist-dependent vasoconstriction in second-order mesenteric arteries. These findings were associated with increased mRNA and protein expression of smooth muscle structural proteins. As a potential mechanism, CaMKII reduced serum response factor-dependent transcriptional activity. In summary, our findings identify CaMKII as an important regulator of smooth muscle function in Ang-II hypertension in vivo.

The mitochondrial uniporter controls fight or flight heart rate increases

Heart rate increases are a fundamental adaptation to physiological stress, while inappropriate heart rate increases are resistant to current therapies. However, the metabolic mechanisms driving heart rate acceleration in cardiac pacemaker cells remain incompletely understood. The mitochondrial calcium uniporter (MCU) facilitates calcium entry into the mitochondrial matrix to stimulate metabolism. We developed mice with myocardial MCU inhibition by transgenic expression of a dominant negative (DN) MCU. Here we show that DN-MCU mice had normal resting heart rates but were incapable of physiological fight or flight heart rate acceleration. We found MCU function was essential for rapidly increasing mitochondrial calcium in pacemaker cells and that MCU enhanced oxidative phoshorylation was required to accelerate reloading of an intracellular calcium compartment prior to each heartbeat. Our findings show the MCU is necessary for complete physiological heart rate acceleration and suggest MCU inhibition could reduce inappropriate heart rate increases without affecting resting heart rate.

CaMKII and stress mix it up in mitochondria

CaMKII is a newly discovered resident of mitochondria in the heart. Mitochondrial CaMKII promotes poor outcomes after heart injury from a number of pathological conditions, including myocardial infarction (MI), ischemia reperfusion (IR), and stress from catecholamine stimulation. A study using the inhibitor of CaMKII, CaMKIIN, with expression delimited to myocardial mitochondria, indicates that an underlying cause of heart disease results from the opening of the mitochondrial permeability transition pore (mPTP). Evidence from electrophysiological and other experiments show that CaMKII inhibition likely suppresses mPTP opening by reducing Ca²⁺ entry into mitochondria. However, we expect other proteins involved in Ca²⁺ signaling in the mitochondria are affected with CaMKII inhibition. Several outstanding questions remain for CaMKII signaling in heart mitochondria. Most importantly, how does CaMKII, without the recognized N-terminal mitochondrial targeting sequence transfer to mitochondria?

CaMKII is essential for the proasthmatic effects of oxidation

Increased reactive oxygen species (ROS) contribute to asthma, but little is known about the molecular mechanisms connecting increased ROS with characteristic features of asthma. We show that enhanced oxidative activation of the Ca(2+)/calmodulin-dependent protein kinase (ox-CaMKII) in bronchial epithelium positively correlates with asthma severity and that epithelial ox-CaMKII increases in response to inhaled allergens in patients. We used mouse models of allergic airway disease induced by ovalbumin (OVA) or Aspergillus fumigatus (Asp) and found that bronchial epithelial ox-CaMKII was required to increase a ROS- and picrotoxin-sensitive Cl(-) current (ICl) and MUC5AC expression, upstream events in asthma progression. Allergen challenge increased epithelial ROS by activating NADPH oxidases. Mice lacking functional NADPH oxidases due to knockout of p47 and mice with epithelial-targeted transgenic expression of a CaMKII inhibitory peptide or wild-type mice treated with inhaled KN-93, an experimental small-molecule CaMKII antagonist, were protected against increases in ICl, MUC5AC expression, and airway hyperreactivity to inhaled methacholine. Our findings support the view that CaMKII is a ROS-responsive, pluripotent proasthmatic signal and provide proof-of-concept evidence that CaMKII is a therapeutic target in asthma.

β(IV)-Spectrin regulates TREK-1 membrane targeting in the heart

AIMS

Cardiac function depends on the highly regulated and co-ordinate activity of a large ensemble of potassium channels that control myocyte repolarization. While voltage-gated K(+) channels have been well characterized in the heart, much less is known about regulation and/or targeting of two-pore K(+) channel (K(2P)) family members, despite their potential importance in modulation of heart function.

METHODS AND RESULTS

Here, we report a novel molecular pathway for membrane targeting of TREK-1, a mechano-sensitive K(2P) channel regulated by environmental and physical factors including membrane stretch, pH, and polyunsaturated fatty acids (e.g. arachidonic acid). We demonstrate that β(IV)-spectrin, an actin-associated protein, is co-localized with TREK-1 at the myocyte intercalated disc, associates with TREK-1 in the heart, and is required for TREK-1 membrane targeting. Mice expressing β(IV)-spectrin lacking TREK-1 binding (qv(4J)) display aberrant TREK-1 membrane localization, decreased TREK-1 activity, delayed action potential repolarization, and arrhythmia without apparent defects in localization/function of other cardiac potassium channel subunits. Finally, we report abnormal β(IV)-spectrin levels in human heart failure.

CONCLUSIONS

These data provide new insight into membrane targeting of TREK-1 in the heart and establish a broader role for β(IV)-spectrin in organizing functional membrane domains critical for normal heart function.

Oxidized Ca(2+)/calmodulin-dependent protein kinase II triggers atrial fibrillation

BACKGROUND

Atrial fibrillation (AF) is a growing public health problem without adequate therapies. Angiotensin II and reactive oxygen species are validated risk factors for AF in patients, but the molecular pathways connecting reactive oxygen species and AF are unknown. The Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) has recently emerged as a reactive oxygen species-activated proarrhythmic signal, so we hypothesized that oxidized CaMKIIδ could contribute to AF.

METHODS AND RESULTS

We found that oxidized CaMKII was increased in atria from AF patients compared with patients in sinus rhythm and from mice infused with angiotensin II compared with mice infused with saline. Angiotensin II-treated mice had increased susceptibility to AF compared with saline-treated wild-type mice, establishing angiotensin II as a risk factor for AF in mice. Knock-in mice lacking critical oxidation sites in CaMKIIδ (MM-VV) and mice with myocardium-restricted transgenic overexpression of methionine sulfoxide reductase A, an enzyme that reduces oxidized CaMKII, were resistant to AF induction after angiotensin II infusion.

CONCLUSIONS

Our studies suggest that CaMKII is a molecular signal that couples increased reactive oxygen species with AF and that therapeutic strategies to decrease oxidized CaMKII may prevent or reduce AF.

Differential control of calcium homeostasis and vascular reactivity by Ca2+/calmodulin-dependent kinase II

The multifunctional Ca(2+)/calmodulin-dependent kinase II (CaMKII) is activated by vasoconstrictors in vascular smooth muscle cells (VSMC), but its impact on vasoconstriction remains unknown. We hypothesized that CaMKII inhibition in VSMC decreases vasoconstriction. Using novel transgenic mice that express the inhibitor peptide CaMKIIN in smooth muscle (TG SM-CaMKIIN), we investigated the effect of CaMKII inhibition on L-type Ca(2+) channel current (ICa), cytoplasmic and sarcoplasmic reticulum Ca(2+), and vasoconstriction in mesenteric arteries. In mesenteric VSMC, CaMKII inhibition significantly reduced action potential duration and the residual ICa 50 ms after peak amplitude, indicative of loss of L-type Ca(2+) channel-dependent ICa facilitation. Treatment with angiotensin II or phenylephrine increased the intracellular Ca(2+) concentration in wild-type but not TG SM-CaMKIIN VSMC. The difference in intracellular Ca(2+) concentration was abolished by pretreatment with nifedipine, an L-type Ca(2+) channel antagonist. In TG SM-CaMKIIN VSMC, the total sarcoplasmic reticulum Ca(2+) content was reduced as a result of diminished sarcoplasmic reticulum Ca(2+) ATPase activity via impaired derepression of the sarcoplasmic reticulum Ca(2+) ATPase inhibitor phospholamban. Despite the differences in intracellular Ca(2+) concentration, CaMKII inhibition did not alter myogenic tone or vasoconstriction of mesenteric arteries in response to KCl, angiotensin II, and phenylephrine. However, it increased myosin light chain kinase activity. These data suggest that CaMKII activity maintains intracellular calcium homeostasis but is not required for vasoconstriction of mesenteric arteries.

Genetic inhibition of Na+-Ca2+ exchanger current disables fight or flight sinoatrial node activity without affecting resting heart rate

RATIONALE

The sodium-calcium exchanger 1 (NCX1) is predominantly expressed in the heart and is implicated in controlling automaticity in isolated sinoatrial node (SAN) pacemaker cells, but the potential role of NCX1 in determining heart rate in vivo is unknown.

OBJECTIVE

To determine the role of Ncx1 in heart rate.

METHODS AND RESULTS

We used global myocardial and SAN-targeted conditional Ncx1 knockout (Ncx1(-/-)) mice to measure the effect of the NCX current on pacemaking activity in vivo, ex vivo, and in isolated SAN cells. We induced conditional Ncx1(-/-) using a Cre/loxP system. Unexpectedly, in vivo and ex vivo hearts and isolated SAN cells showed that basal rates in Ncx1(-/-) (retaining ≈20% of control level NCX current) and control mice were similar, suggesting that physiological NCX1 expression is not required for determining resting heart rate. However, increases in heart rate and SAN cell automaticity in response to isoproterenol or the dihydropyridine Ca(2+) channel agonist BayK8644 were significantly blunted or eliminated in Ncx1(-/-) mice, indicating that NCX1 is important for fight or flight heart rate responses. In contrast, the pacemaker current and L-type Ca(2+) currents were equivalent in control and Ncx1(-/-) SAN cells under resting and isoproterenol-stimulated conditions. Ivabradine, a pacemaker current antagonist with clinical efficacy, reduced basal SAN cell automaticity similarly in control and Ncx1(-/-) mice. However, ivabradine decreased automaticity in SAN cells isolated from Ncx1(-/-) mice more effectively than in control SAN cells after isoproterenol, suggesting that the importance of NCX current in fight or flight rate increases is enhanced after pacemaker current inhibition.

CONCLUSIONS

Physiological Ncx1 expression is required for increasing sinus rates in vivo, ex vivo, and in isolated SAN cells, but not for maintaining resting heart rate.

Ca2+/calmodulin-dependent protein kinase II-based regulation of voltage-gated Na+ channel in cardiac disease

BACKGROUND

Human gene variants affecting ion channel biophysical activity and/or membrane localization are linked to potentially fatal cardiac arrhythmias. However, the mechanism for many human arrhythmia variants remains undefined despite more than a decade of investigation. Posttranslational modulation of membrane proteins is essential for normal cardiac function. Importantly, aberrant myocyte signaling has been linked to defects in cardiac ion channel posttranslational modifications and disease. We recently identified a novel pathway for posttranslational regulation of the primary cardiac voltage-gated Na(+) channel (Na(v)1.5) by Ca(2+)/calmodulin-dependent protein kinase II (CaMKII). However, a role for this pathway in cardiac disease has not been evaluated.

METHODS AND RESULTS

We evaluated the role of CaMKII-dependent phosphorylation in human genetic and acquired disease. We report an unexpected link between a short motif in the Na(v)1.5 DI-DII loop, recently shown to be critical for CaMKII-dependent phosphorylation, and Na(v)1.5 function in monogenic arrhythmia and common heart disease. Experiments in heterologous cells and primary ventricular cardiomyocytes demonstrate that the human arrhythmia susceptibility variants (A572D and Q573E) alter CaMKII-dependent regulation of Na(v)1.5, resulting in abnormal channel activity and cell excitability. In silico analysis reveals that these variants functionally mimic the phosphorylated channel, resulting in increased susceptibility to arrhythmia-triggering afterdepolarizations. Finally, we report that this same motif is aberrantly regulated in a large-animal model of acquired heart disease and in failing human myocardium.

CONCLUSIONS

We identify the mechanism for 2 human arrhythmia variants that affect Na(v)1.5 channel activity through direct effects on channel posttranslational modification. We propose that the CaMKII phosphorylation motif in the Na(v)1.5 DI-DII cytoplasmic loop is a critical nodal point for proarrhythmic changes to Na(v)1.5 in congenital and acquired cardiac disease.

β2-Adrenergic receptor supports prolonged theta tetanus-induced LTP

The widespread noradrenergic innervation in the brain promotes arousal and learning by molecular mechanisms that remain largely undefined. Recent work shows that the β(2)-adrenergic receptor (β(2)AR) is linked to the AMPA-type glutamate receptor subunit GluA1 via stargazin and PSD-95 (Joiner ML, Lise MF, Yuen EY, Kam AY, Zhang M, Hall DD, Malik ZA, Qian H, Chen Y, Ulrich JD, Burette AC, Weinberg RJ, Law PY, El-Husseini A, Yan Z, Hell JW. EMBO J 29: 482-495, 2010). We now demonstrate that the β(2)AR plays a prominent role in long-term potentiation (LTP) induced by a train of 900 stimuli at 5 Hz (prolonged theta-tetanus-LTP, or PTT-LTP) in the hippocampal CA1 region in mice, which requires simultaneous β-adrenergic stimulation. Although PTT-LTP was impaired in hippocampal slices from β(1)AR and β(2)AR knockout (KO) mice, only β(2)AR-selective stimulation with salbutamol supported this PTT-LTP in wild-type (WT) slices, whereas β(1)AR-selective stimulation with dobutamine (+ prazosin) did not. Furthermore, only the β(2)AR-selective antagonist ICI-118551 and not the β(1)AR-selective antagonist CGP-20712 inhibited PTT-LTP and phosphorylation of GluA1 on its PKA site S845 in WT slices. Our analysis of S845A knockin (KI) mice indicates that this phosphorylation is relevant for PTT-LTP. These results identify the β(2)AR-S845 signaling pathway as a prominent regulator of synaptic plasticity.

Catecholamine-independent heart rate increases require Ca2+/calmodulin-dependent protein kinase II

BACKGROUND

Catecholamines increase heart rate by augmenting the cAMP-responsive hyperpolarization-activated cyclic nucleotide-gated channel 4 pacemaker current (I(f)) and by promoting inward Na(+)/Ca(2+) exchanger current (I(NCX)) by a "Ca(2+) clock" mechanism in sinoatrial nodal cells (SANCs). The importance, identity, and function of signals that connect I(f) and Ca(2+) clock mechanisms are uncertain and controversial, but the multifunctional Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) is required for physiological heart rate responses to β-adrenergic receptor (β-AR) stimulation. The aim of this study was to measure the contribution of the Ca(2+) clock and CaMKII to cardiac pacing independent of β-AR agonist stimulation.

METHODS AND RESULTS

We used the L-type Ca(2+) channel agonist Bay K8644 (BayK) to activate the SANC Ca(2+) clock. BayK and isoproterenol were similarly effective in increasing rates in SANCs and Langendorff-perfused hearts from wild-type control mice. In contrast, SANCs and isolated hearts from mice with CaMKII inhibition by transgenic expression of an inhibitory peptide (AC3-I) were resistant to rate increases by BayK. BayK only activated CaMKII in control SANCs but increased L-type Ca(2+) current (I(Ca)) equally in all SANCs, indicating that increasing I(Ca) was insufficient and suggesting that CaMKII activation was required for heart rate increases by BayK. BayK did not increase I(f) or protein kinase A-dependent phosphorylation of phospholamban (at Ser16), indicating that increased SANC Ca(2+) by BayK did not augment cAMP/protein kinase A signaling at these targets. Late-diastolic intracellular Ca(2+) release and I(NCX) were significantly reduced in AC3-I SANCs, and the response to BayK was eliminated by ryanodine in all groups.

CONCLUSIONS

The Ca(2+) clock is capable of supporting physiological fight-or-flight responses, independent of β-AR stimulation or I(f) increases. Complete Ca(2+) clock and β-AR stimulation responses require CaMKII.

A β(IV)-spectrin/CaMKII signaling complex is essential for membrane excitability in mice

Ion channel function is fundamental to the existence of life. In metazoans, the coordinate activities of voltage-gated Na(+) channels underlie cellular excitability and control neuronal communication, cardiac excitation-contraction coupling, and skeletal muscle function. However, despite decades of research and linkage of Na(+) channel dysfunction with arrhythmia, epilepsy, and myotonia, little progress has been made toward understanding the fundamental processes that regulate this family of proteins. Here, we have identified β(IV)-spectrin as a multifunctional regulatory platform for Na(+) channels in mice. We found that β(IV)-spectrin targeted critical structural and regulatory proteins to excitable membranes in the heart and brain. Animal models harboring mutant β(IV)-spectrin alleles displayed aberrant cellular excitability and whole animal physiology. Moreover, we identified a regulatory mechanism for Na(+) channels, via direct phosphorylation by β(IV)-spectrin-targeted calcium/calmodulin-dependent kinase II (CaMKII). Collectively, our data define an unexpected but indispensable molecular platform that determines membrane excitability in the mouse heart and brain.

EH domain proteins regulate cardiac membrane protein targeting

RATIONALE

Cardiac membrane excitability is tightly regulated by an integrated network of membrane-associated ion channels, transporters, receptors, and signaling molecules. Membrane protein dynamics in health and disease are maintained by a complex ensemble of intracellular targeting, scaffolding, recycling, and degradation pathways. Surprisingly, despite decades of research linking dysfunction in membrane protein trafficking with human cardiovascular disease, essentially nothing is known regarding the molecular identity or function of these intracellular targeting pathways in excitable cardiomyocytes.

OBJECTIVE

We sought to discover novel pathways for membrane protein targeting in primary cardiomyocytes.

METHODS AND RESULTS

We report the initial characterization of a large family of membrane trafficking proteins in human heart. We used a tissue-wide screen for novel ankyrin-associated trafficking proteins and identified 4 members of a unique Eps15 homology (EH) domain-containing protein family (EHD1, EHD2, EHD3, EHD4) that serve critical roles in endosome-based membrane protein targeting in other cell types. We show that EHD1-4 directly associate with ankyrin, provide the first information on the expression and localization of these molecules in primary cardiomyocytes, and demonstrate that EHD1-4 are coexpressed with ankyrin-B in the myocyte perinuclear region. Notably, the expression of multiple EHD proteins is increased in animal models lacking ankyrin-B, and EHD3-deficient cardiomyocytes display aberrant ankyrin-B localization and selective loss of Na/Ca exchanger expression and function. Finally, we report significant modulation of EHD expression following myocardial infarction, suggesting that these proteins may play a key role in regulating membrane excitability in normal and diseased heart.

CONCLUSIONS

Our findings identify and characterize a new class of cardiac trafficking proteins, define the first group of proteins associated with the ankyrin-based targeting network, and identify potential new targets to modulate membrane excitability in disease. Notably, these data provide the first link between EHD proteins and a human disease model.

I(f) and SR Ca(2+) release both contribute to pacemaker activity in canine sinoatrial node cells

Increasing evidence suggests that cardiac pacemaking is the result of two sinoatrial node (SAN) cell mechanisms: a 'voltage clock' and a Ca(2+) dependent process, or 'Ca(2+) clock.' The voltage clock initiates action potentials (APs) by SAN cell membrane potential depolarization from inward currents, of which the pacemaker current (I(f)) is thought to be particularly important. A Ca(2+) dependent process triggers APs when sarcoplasmic reticulum (SR) Ca(2+) release activates inward current carried by the forward mode of the electrogenic Na(+)/Ca(2+) exchanger (NCX). However, these mechanisms have mostly been defined in rodents or rabbits, but are unexplored in single SAN cells from larger animals. Here, we used patch-clamp and confocal microscope techniques to explore the roles of the voltage and Ca(2+) clock mechanisms in canine SAN pacemaker cells. We found that ZD7288, a selective I(f) antagonist, significantly reduced basal automaticity and induced irregular, arrhythmia-like activity in canine SAN cells. In addition, ZD7288 impaired but did not eliminate the SAN cell rate acceleration by isoproterenol. In contrast, ryanodine significantly reduced the SAN cell acceleration by isoproterenol, while ryanodine reduction of basal automaticity was modest ( approximately 14%) and did not reach statistical significance. Importantly, pretreatment with ryanodine eliminated SR Ca(2+) release, but did not affect basal or isoproterenol-enhanced I(f). Taken together, these results indicate that voltage and Ca(2+) dependent automaticity mechanisms coexist in canine SAN cells, and suggest that I(f) and SR Ca(2+) release cooperate to determine baseline and catecholamine-dependent automaticity in isolated dog SAN cells.

Proarrhythmic defects in Timothy syndrome require calmodulin kinase II

BACKGROUND

Timothy syndrome (TS) is a disease of excessive cellular Ca(2+) entry and life-threatening arrhythmias caused by a mutation in the primary cardiac L-type Ca(2+) channel (Ca(V)1.2). The TS mutation causes loss of normal voltage-dependent inactivation of Ca(V)1.2 current (I(Ca)). During cellular Ca(2+) overload, the calmodulin-dependent protein kinase II (CaMKII) causes arrhythmias. We hypothesized that CaMKII is a part of the proarrhythmic mechanism in TS.

METHODS AND RESULTS

We developed an adult rat ventricular myocyte model of TS (G406R) by lentivirus-mediated transfer of wild-type and TS Ca(V)1.2. The exogenous Ca(V)1.2 contained a mutation (T1066Y) conferring dihydropyridine resistance, so we could silence endogenous Ca(V)1.2 with nifedipine and maintain peak I(Ca) at control levels in infected cells. TS Ca(V)1.2-infected ventricular myocytes exhibited the signature voltage-dependent inactivation loss under Ca(2+) buffering conditions, not permissive for CaMKII activation. In physiological Ca(2+) solutions, TS Ca(V)1.2-expressing ventricular myocytes exhibited increased CaMKII activity and a proarrhythmic phenotype that included action potential prolongation, increased I(Ca) facilitation, and afterdepolarizations. Intracellular dialysis of a CaMKII inhibitory peptide, but not a control peptide, reversed increases in I(Ca) facilitation, normalized the action potential, and prevented afterdepolarizations. We developed a revised mathematical model that accounts for CaMKII-dependent and CaMKII-independent effects of the TS mutation.

CONCLUSIONS

In TS, the loss of voltage-dependent inactivation is an upstream initiating event for arrhythmia phenotypes that are ultimately dependent on CaMKII activation.