Noncanonical EF-hand motif strategically delays Ca2+ buffering to enhance cardiac performance

Myofilament-based physiological regulatory compensation preserves diastolic function in failing hearts with severe Ca2+ handling deficits

Severe dysfunction in cardiac muscle intracellular Ca2+ handling is a common pathway underlying heart failure. Here we used an inducible genetic model of severe Ca2+ cycling dysfunction by the targeted temporal gene ablation of the cardiac Ca2+ ATPase, SERCA2, in otherwise normal adult mice. In this model, in vivo heart performance was minimally affected initially, even though Serca2a protein was markedly reduced. The mechanism underlying the sustained in vivo heart performance in the weeks prior to complete heart pump failure and death is not clear and is important to understand. Studies were primarily focused on understanding how in vivo diastolic function could be relatively normal under conditions of marked Serca2a deficiency. Interestingly, data show increased cardiac troponin I (cTnI) serine 23/24 phosphorylation content in hearts upon Serca2a ablation in vivo. We report that hearts isolated from the Serca2-deficient mice retained near normal heart pump functional responses to β-adrenergic stimulation. Unexpectedly, using genetic complementation models, in concert with inducible Serca2 ablation, data show that Serca2a-deficient hearts that also lacked the central β-adrenergic signaling-dependent Serca2a negative regulator, phospholamban (PLN), had severe diastolic dysfunction that could still be corrected by β-adrenergic stimulation. Notably, integrating a serines 23/24-to-alanine PKA-refractory sarcomere incorporated cTnI molecular switch complex in mice deficient in Serca2 showed blunting of β-adrenergic stimulation-mediated enhanced diastolic heart performance. Taken together, these data provide evidence of a compensatory regulatory role of the myofilaments as a critical physiological bridging mechanism to aid in preserving heart diastolic performance in failing hearts with severe Ca2+ handling deficits.

Nucleus Mechanosensing in Cardiomyocytes

Cardiac muscle contraction is distinct from the contraction of other muscle types. The heart continuously undergoes contraction-relaxation cycles throughout an animal's lifespan. It must respond to constantly varying physical and energetic burdens over the short term on a beat-to-beat basis and relies on different mechanisms over the long term. Muscle contractility is based on actin and myosin interactions that are regulated by cytoplasmic calcium ions. Genetic variants of sarcomeric proteins can lead to the pathophysiological development of cardiac dysfunction. The sarcomere is physically connected to other cytoskeletal components. Actin filaments, microtubules and desmin proteins are responsible for these interactions. Therefore, mechanical as well as biochemical signals from sarcomeric contractions are transmitted to and sensed by other parts of the cardiomyocyte, particularly the nucleus which can respond to these stimuli. Proteins anchored to the nuclear envelope display a broad response which remodels the structure of the nucleus. In this review, we examine the central aspects of mechanotransduction in the cardiomyocyte where the transmission of mechanical signals to the nucleus can result in changes in gene expression and nucleus morphology. The correlation of nucleus sensing and dysfunction of sarcomeric proteins may assist the understanding of a wide range of functional responses in the progress of cardiomyopathic diseases.

A perinuclear calcium compartment regulates cardiac myocyte hypertrophy

The pleiotropic Ca2+/calmodulin-dependent phosphatase calcineurin is a key regulator of pathological cardiac myocyte hypertrophy. The selective activation of hypertrophic calcineurin signaling under stress conditions has been attributed to compartmentation of Ca2+ signaling in cardiac myocytes. Here, perinuclear signalosomes organized by the scaffold protein muscle A-Kinase Anchoring Protein β (mAKAPβ/AKAP6β) are shown to orchestrate local Ca2+ transients, inducing calcineurin-dependent NFATc nuclear localization and myocyte hypertrophy in response to β-adrenergic receptor activation. Fluorescent biosensors for Ca2+ and calcineurin and protein kinase A (PKA) activity, both diffusely expressed and localized by nesprin-1α to the nuclear envelope, are used to define an autonomous mAKAPβ signaling compartment in adult and neonatal rat ventricular myocytes. Notably, β-adrenergic-stimulated perinuclear Ca2+ and PKA and CaN activity transients depended upon mAKAPβ expression, while Ca2+ elevation and PKA and CaN activity in the cytosol were mAKAPβ independent. Buffering perinuclear cAMP and Ca2+ prevented calcineurin-dependent NFATc nuclear translocation and myocyte hypertrophy, without affecting cardiac myocyte contractility. Additional findings suggest that the perinuclear Ca2+ transients were mediated by signalosome-associated ryanodine receptors regulated by local PKA phosphorylation. These results demonstrate the existence of a functionally independent Ca2+ signaling compartment in the cardiac myocyte regulating hypertrophy and provide a premise for targeting mAKAPβ signalosomes to prevent selectively cardiac hypertrophy in disease.

Targeting parvalbumin promotes M2 macrophage polarization and energy expenditure in mice

Exercise benefits M2 macrophage polarization, energy homeostasis and protects against obesity partially through exercise-induced circulating factors. Here, by unbiased quantitative proteomics on serum samples from sedentary and exercised mice, we identify parvalbumin as a circulating factor suppressed by exercise. Parvalbumin functions as a non-competitive CSF1R antagonist to inhibit M2 macrophage activation and energy expenditure in adipose tissue. More importantly, serum concentrations of parvalbumin positively correlate with obesity in mouse and human, while treating mice with a recombinant parvalbumin blocker prevents its interaction with CSF1R and promotes M2 macrophage polarization and ameliorates diet-induced obesity. Thus, although further studies are required to assess the significance of parvalbumin in mediating the effects of exercise, our results implicate parvalbumin as a potential therapeutic strategy against obesity in mice.

Structural Changes beyond the EF-Hand Contribute to Apparent Calcium Binding Affinities: Insights from Parvalbumins

Members of the parvalbumin (PV) family of calcium (Ca²⁺) binding proteins (CBPs) share a relatively high level of sequence similarity. However, their Ca²⁺ affinities and selectivities against competing ions like Mg²⁺ can widely vary. We conducted molecular dynamics simulations of several α-parvalbumin (αPV) constructs with micromolar to nanomolar Ca²⁺ affinities to identify structural and dynamic features that contribute to their binding of ions. Specifically, we examined a D94S/G98E construct with a lower Ca²⁺ affinity (≈-18 kcal/mol) relative to the wild type (WT) (≈-22 kcal/mol) and an S55D/E59D variant with enhanced affinity (≈-24 kcal/mol). Additionally, we also examined the binding of Mg²⁺ to these isoforms, which is much weaker than Ca²⁺. We used mean spherical approximation (MSA) theory to evaluate ion binding thermodynamics within the proteins' EF-hand domains to account for the impact of ions' finite sizes and the surrounding electrolyte composition. While the MSA scores differentiated Mg²⁺ from Ca²⁺, they did not indicate that Ca²⁺ binding affinities at the binding loop differed between the PV isoforms. Instead, molecular mechanics generalized Born surface area (MM/GBSA) approximation energies, which we used to quantify the thermodynamic cost of structural rearrangement of the proteins upon binding ions, indicated that S55D/E59D αPV favored Ca²⁺ binding by -20 kcal/mol relative to WT versus 30 kcal/mol for D94S/G98E αPV. Meanwhile, Mg²⁺ binding was favored for the S55D/E59D αPV and D94S/G98E αPV variants by -18.32 and -1.65 kcal/mol, respectively. These energies implicate significant contributions to ion binding beyond oxygen coordination at the binding loop, which stemmed from changes in α-helicity, β-sheet character, and hydrogen bonding. Hence, Ca²⁺ affinity and selectivity against Mg²⁺ are emergent properties stemming from both local effects within the proteins' ion binding sites as well as non-local contributions elsewhere. Our findings broaden our understanding of the molecular bases governing αPV ion binding that are likely shared by members of the broad family of CBPs.

SpiCee: A Genetic Tool for Subcellular and Cell-Specific Calcium Manipulation

Calcium is a second messenger crucial to a myriad of cellular processes ranging from regulation of metabolism and cell survival to vesicle release and motility. Current strategies to directly manipulate endogenous calcium signals lack cellular and subcellular specificity. We introduce SpiCee, a versatile and genetically encoded chelator combining low- and high-affinity sites for calcium. This scavenger enables altering endogenous calcium signaling and functions in single cells in vitro and in vivo with biochemically controlled subcellular resolution. SpiCee paves the way to investigate local calcium signaling in vivo and directly manipulate this second messenger for therapeutic use.

cAMP at Perinuclear mAKAPα Signalosomes Is Regulated by Local Ca2+ Signaling in Primary Hippocampal Neurons

The second messenger cyclic adenosine monophosphate (cAMP) is important for the regulation of neuronal structure and function, including neurite extension. A perinuclear cAMP compartment organized by the scaffold protein muscle A-kinase anchoring protein α (mAKAPα/AKAP6α) is sufficient and necessary for axon growth by rat hippocampal neurons in vitro. Here, we report that cAMP at mAKAPα signalosomes is regulated by local Ca²⁺ signaling that mediates activity-dependent cAMP elevation within that compartment. Simultaneous Forster resonance energy transfer (FRET) imaging using the protein kinase A (PKA) activity reporter AKAR4 and intensiometric imaging using the RCaMP1h fluorescent Ca²⁺ sensor revealed that membrane depolarization by KCl selectively induced activation of perinuclear PKA activity. Activity-dependent perinuclear PKA activity was dependent on expression of the mAKAPα scaffold, while both perinuclear Ca²⁺ elevation and PKA activation were dependent on voltage-dependent L-type Ca²⁺ channel activity. Importantly, chelation of Ca²⁺ by a nuclear envelope-localized parvalbumin fusion protein inhibited both activity-induced perinuclear PKA activity and axon elongation. Together, this study provides evidence for a model in which a neuronal perinuclear cAMP compartment is locally regulated by activity-dependent Ca²⁺ influx, providing local control for the enhancement of neurite extension.

Advancing physiological maturation in human induced pluripotent stem cell-derived cardiac muscle by gene editing an inducible adult troponin isoform switch

Advancing maturation of stem cell-derived cardiac muscle represents a major barrier to progress in cardiac regenerative medicine. Cardiac muscle maturation involves a myriad of gene, protein, and cell-based transitions, spanning across all aspects of cardiac muscle form and function. We focused here on a key developmentally controlled transition in the cardiac sarcomere, the functional unit of the heart. Using a gene-editing platform, human induced pluripotent stem cell (hiPSCs) were engineered with a drug-inducible expression cassette driving the adult cardiac troponin I (cTnI) regulatory isoform, a transition shown to be a rate-limiting step in advancing sarcomeric maturation of hiPSC cardiac muscle (hiPSC-CM) toward the adult state. Findings show that induction of the adult cTnI isoform resulted in the physiological acquisition of adult-like cardiac contractile function in hiPSC-CMs in vitro. Specifically, cTnI induction accelerated relaxation kinetics at baseline conditions, a result independent of alterations in the kinetics of the intracellular Ca²⁺ transient. In comparison, isogenic unedited hiPSC-CMs had no cTnI induction and no change in relaxation function. Temporal control of adult cTnI isoform induction did not alter other developmentally regulated sarcomere transitions, including myosin heavy chain isoform expression, nor did it affect expression of SERCA2a or phospholamban. Taken together, precision genetic targeting of sarcomere maturation via inducible TnI isoform switching enables physiologically relevant adult myocardium-like contractile adaptations that are essential for beat-to-beat modulation of adult human heart performance. These findings have relevance to hiPSC-CM structure-function and drug-discovery studies in vitro, as well as for potential future clinical applications of physiologically optimized hiPSC-CM in cardiac regeneration/repair.

Understanding the mammalian TRAP complex function(s)

In eukaryotic cells, about one-third of the synthesized proteins are translocated into the endoplasmic reticulum; they are membrane or lumen resident proteins and proteins direct to the Golgi apparatus. The co-translational translocation takes place through the heterotrimeric protein-conducting channel Sec61 which is associated with the ribosome and many accessory components, such as the heterotetrameric translocon-associated protein (TRAP) complex. Recently, microscopic techniques, such as cryo-electron microscopy and cryo-electron tomography, have enabled the determination of the translocation machinery structure. However, at present, there is a lack of understanding regarding the roles of some of its components; indeed, the TRAP complex function during co-translational translocation needs to be established. In addition, TRAP may play a role during unfolded protein response, endoplasmic-reticulum-associated protein degradation and congenital disorder of glycosylation (ssr4 CDG). In this article, I describe the current understanding of the TRAP complex in the light of its possible function(s).

Dysregulation of Calcium Handling in Duchenne Muscular Dystrophy-Associated Dilated Cardiomyopathy: Mechanisms and Experimental Therapeutic Strategies

: Duchenne muscular dystrophy (DMD) is an X-linked recessive disease resulting in the loss of dystrophin, a key cytoskeletal protein in the dystrophin-glycoprotein complex. Dystrophin connects the extracellular matrix with the cytoskeleton and stabilizes the sarcolemma. Cardiomyopathy is prominent in adolescents and young adults with DMD, manifesting as dilated cardiomyopathy (DCM) in the later stages of disease. Sarcolemmal instability, leading to calcium mishandling and overload in the cardiac myocyte, is a key mechanistic contributor to muscle cell death, fibrosis, and diminished cardiac contractile function in DMD patients. Current therapies for DMD cardiomyopathy can slow disease progression, but they do not directly target aberrant calcium handling and calcium overload. Experimental therapeutic targets that address calcium mishandling and overload include membrane stabilization, inhibition of stretch-activated channels, ryanodine receptor stabilization, and augmentation of calcium cycling via modulation of the Serca2a/phospholamban (PLN) complex or cytosolic calcium buffering. This paper addresses what is known about the mechanistic basis of calcium mishandling in DCM, with a focus on DMD cardiomyopathy. Additionally, we discuss currently utilized therapies for DMD cardiomyopathy, and review experimental therapeutic strategies targeting the calcium handling defects in DCM and DMD cardiomyopathy.

Force and Calcium Transients Analysis in Human Engineered Heart Tissues Reveals Positive Force-Frequency Relation at Physiological Frequency

Force measurements in ex vivo and engineered heart tissues are well established. Analysis of calcium transients (CaT) is complementary to force, and the combined analysis is meaningful to the study of cardiomyocyte biology and disease. This article describes a model of human induced pluripotent stem cell cardiomyocyte-derived engineered heart tissues (hiPSC-CM EHTs) transduced with the calcium sensor GCaMP6f followed by sequential analysis of force and CaT. Average peak analysis demonstrated the temporal sequence of the CaT preceding the contraction twitch. The pharmacological relevance of the test system was demonstrated with inotropic indicator compounds. Force-frequency relationship was analyzed in the presence of ivabradine (300 nM), which reduced spontaneous frequency and unmasked a positive correlation of force and CaT at physiological human heart beating frequency with stimulation frequency between 0.75 and 2.5 Hz (force +96%; CaT +102%). This work demonstrates the usefulness of combined force/CaT analysis and demonstrates a positive force-frequency relationship in hiPSC-CM EHTs.

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Analysis of calcium transients and force in engineered heart tissues

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Accurate replications of drug effects on calcium transients and force analysis

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Positive force- and calcium transients-frequency relationship

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Reverse correlation between omecamtiv mecarbil's inotropic effect and frequency

Cytosolic Ca2+ Buffers Are Inherently Ca2+ Signal Modulators

For precisely regulating intracellular Ca²⁺ signals in a time- and space-dependent manner, cells make use of various components of the "Ca²⁺ signaling toolkit," including Ca²⁺ entry and Ca²⁺ extrusion systems. A class of cytosolic Ca²⁺-binding proteins termed Ca²⁺ buffers serves as modulators of such, mostly short-lived Ca²⁺ signals. Prototypical Ca²⁺ buffers include parvalbumins (α and β isoforms), calbindin-D9k, calbindin-D28k, and calretinin. Although initially considered to function as pure Ca²⁺ buffers, that is, as intracellular Ca²⁺ signal modulators controlling the shape (amplitude, decay, spread) of Ca²⁺ signals, evidence has accumulated that calbindin-D28k and calretinin have additional Ca²⁺ sensor functions. These other functions are brought about by direct interactions with target proteins, thereby modulating their targets' function/activity. Dysregulation of Ca²⁺ buffer expression is associated with several neurologic/neurodevelopmental disorders including autism spectrum disorder (ASD) and schizophrenia. In some cases, the presence of these proteins is presumed to confer a neuroprotective effect, as evidenced in animal models of Parkinson's or Alzheimer's disease.

Ca2+-Binding Proteins of the EF-Hand Superfamily: Diagnostic and Prognostic Biomarkers and Novel Therapeutic Targets

A multitude of Ca²⁺-sensor proteins containing the specific Ca²⁺-binding motif (helix-loop-helix, called EF-hand) are of major clinical relevance in a many human diseases. Measurements of troponin, the first intracellular Ca-sensor protein to be discovered, is nowadays the "gold standard" in the diagnosis of patients with acute coronary syndrome (ACS). Mutations have been identified in calmodulin and linked to inherited ventricular tachycardia and in patients affected by severe cardiac arrhythmias. Parvalbumin, when introduced into the diseased heart by gene therapy to increase contraction and relaxation speed, is considered to be a novel therapeutic strategy to combat heart failure. S100 proteins, the largest subgroup with the EF-hand protein family, are closely associated with cardiovascular diseases, various types of cancer, inflammation, and autoimmune pathologies. The intention of this review is to summarize the clinical importance of this protein family and their use as biomarkers and potential drug targets, which could help to improve the diagnosis of human diseases and identification of more selective therapeutic interventions.

Cytoplasmic nucleic acid-based XNAs directly enhance live cardiac cell function by a Ca2+ cycling-independent mechanism via the sarcomere

Nucleic acid - protein interactions are critical for regulating gene activation in the nucleus. In the cytoplasm, however, potential nucleic acid-protein functional interactions are less clear. The emergence of a large and expanding number of non-coding RNAs and DNA fragments raises the possibility that the cytoplasmic nucleic acids may interact with cytoplasmic cellular components to directly alter key biological processes within the cell. We now show that both natural and synthetic nucleic acids, collectively XNAs, when introduced to the cytoplasm of live cell cardiac myocytes, markedly enhance contractile function via a mechanism that is independent of new translation, activation of the TLR-9 pathway or by altered intracellular Ca²⁺ cycling. Findings show a steep XNA oligo length-dependence, but not sequence dependence or nucleic acid moiety dependence, for cytoplasmic XNAs to hasten myocyte relaxation. XNAs localized to the sarcomere in a striated pattern and bound the cardiac troponin regulatory complex with high affinity in an electrostatic-dependent manner. Mechanistically, XNAs phenocopy PKA-based modified troponin to cause faster relaxation. Collectively, these data support a new role for cytoplasmic nucleic acids in directly modulating live cell cardiac performance and raise the possibility that cytoplasmic nucleic acid - protein interactions may alter functionally relevant pathways in other cell types.

Gene Transfer of Calcium-Binding Proteins into Adult Cardiac Myocytes

Heart failure is the leading cause of combined morbidity and mortality in the USA with 50% of cases being diastolic heart failure. Diastolic heart failure results from poor myocardial relaxation and inadequate filling of the left ventricular chamber caused in part by calcium-handling dysregulation. In this chapter we describe methods to investigate new approaches of novel human Ca²⁺ binding protein motifs to restore normal Ca²⁺ handling function to diseased myocardium. Gene transfer of parvalbumin into adult cardiac myocytes has been studied as a potential therapeutic, specifically as a strategic Ca²⁺ buffer to correct cardiac mechanical dysfunction in disease. This chapter provides protocols for studying wild-type parvalbumin isoforms and parvalbumins with strategically designed EF-hand motifs in adult cardiac myocytes via acute adenoviral gene transfer. These protocols have been used extensively to optimize parvalbumin function as a potential therapeutic for failing heart muscle.

Effects of Modified Parvalbumin EF-Hand Motifs on Cardiac Myocyte Contractile Function

Cardiac gene delivery of parvalbumin (Parv), an EF-hand Ca(2+) buffer, has been studied as a therapeutic strategy for diastolic heart failure, in which slow Ca(2+) reuptake is an important contributor. A limitation of wild-type (WT) Parv is the significant trade-off between faster relaxation and blunted contraction amplitude, occurring because WT-Parv sequesters Ca(2+) too early in the cardiac cycle and prematurely truncates sarcomere shortening in the facilitation of rapid relaxation. We recently demonstrated that an E → Q substitution (ParvE101Q) at amino acid 12 of the EF-hand Ca(2+)/Mg(2+) binding loop disrupts bidentate Ca(2+) binding, reducing Ca(2+) affinity by 99-fold and increasing Mg(2+) affinity twofold. ParvE101Q caused faster relaxation and not only preserved contractility, but unexpectedly increased it above untreated myocytes. To gain mechanistic insight into the increased contractility, we focused here on amino acid 12 of the EF-hand motif. We introduced an E → D substitution (ParvE101D) at this site, which converts bidentate Ca(2+) coordination to monodentate coordination. ParvE101D decreased Ca(2+) affinity by 114-fold and increased Mg(2+) affinity 28-fold compared to WT-Parv. ParvE101D increased contraction amplitude compared to both untreated myocytes and myocytes with ParvE101Q, with limited improvement in relaxation. Additionally, ParvE101D increased spontaneous contractions after pacing stress. ParvE101D also increased Ca(2+) transient peak height and was diffusely localized around the Z-line of the sarcomere, suggesting a Ca(2+)-dependent mechanism of enhanced contractility. Sarcoplasmic reticulum Ca(2+) load was not changed with ParvE101D, but postpacing Ca(2+) waves were increased. Together, these data show that inverted Ca(2+)/Mg(2+) binding affinities of ParvE101D increase myocyte contractility through a Ca(2+)-dependent mechanism without altering sarcoplasmic reticulum Ca(2+) load and by increasing unstimulated contractions and Ca(2+) waves. ParvE101D provides mechanistic insight into how changes in the Ca(2+)/Mg(2+) binding affinities of parvalbumin's EF-hand motif alter function of cardiac myocytes. These data are informative in developing new Ca(2+) buffering strategies for the failing heart.

Colchicine Depolymerizes Microtubules, Increases Junctophilin-2, and Improves Right Ventricular Function in Experimental Pulmonary Arterial Hypertension

BACKGROUND

Pulmonary arterial hypertension (PAH) is a lethal disease characterized by obstructive pulmonary vascular remodeling and right ventricular (RV) dysfunction. Although RV function predicts outcomes in PAH, mechanisms of RV dysfunction are poorly understood, and RV-targeted therapies are lacking. We hypothesized that in PAH, abnormal microtubular structure in RV cardiomyocytes impairs RV function by reducing junctophilin-2 (JPH2) expression, resulting in t-tubule derangements. Conversely, we assessed whether colchicine, a microtubule-depolymerizing agent, could increase JPH2 expression and enhance RV function in monocrotaline-induced PAH.

METHODS AND RESULTS

Immunoblots, confocal microscopy, echocardiography, cardiac catheterization, and treadmill testing were used to examine colchicine's (0.5 mg/kg 3 times/week) effects on pulmonary hemodynamics, RV function, and functional capacity. Rats were treated with saline (n=28) or colchicine (n=24) for 3 weeks, beginning 1 week after monocrotaline (60 mg/kg, subcutaneous). In the monocrotaline RV, but not the left ventricle, microtubule density is increased, and JPH2 expression is reduced, with loss of t-tubule localization and t-tubule disarray. Colchicine reduces microtubule density, increases JPH2 expression, and improves t-tubule morphology in RV cardiomyocytes. Colchicine therapy diminishes RV hypertrophy, improves RV function, and enhances RV-pulmonary artery coupling. Colchicine reduces small pulmonary arteriolar thickness and improves pulmonary hemodynamics. Finally, colchicine increases exercise capacity.

CONCLUSIONS

Monocrotaline-induced PAH causes RV-specific derangement of microtubules marked by reduction in JPH2 and t-tubule disarray. Colchicine reduces microtubule density, increases JPH2 expression, and improves both t-tubule architecture and RV function. Colchicine also reduces adverse pulmonary vascular remodeling. These results provide biological plausibility for a clinical trial to repurpose colchicine as a RV-directed therapy for PAH.

Understanding Ion Binding Affinity and Selectivity in β-Parvalbumin Using Molecular Dynamics and Mean Spherical Approximation Theory

Parvalbumin (PV) is a globular calcium (Ca(2+))-selective protein expressed in a variety of biological tissues. Our computational studies of the rat β-parvalbumin (β-PV) isoform seek to elucidate the molecular thermodynamics of Ca(2+) versus magnesium (Mg(2+)) binding at the protein's two EF-hand motifs. Specifically, we have utilized molecular dynamics (MD) simulations and a mean-field electrolyte model (mean spherical approximation (MSA) theory) to delineate how the EF-hand scaffold controls the "local" thermodynamics of Ca(2+) binding selectivity over Mg(2+). Our MD simulations provide the probability density of metal-chelating oxygens within the EF-hand scaffolds for both Ca(2+) and Mg(2+), as well the conformational strain induced by Mg(2+) relative to Ca(2+) binding. MSA theory utilizes the binding domain oxygen and charge distributions to predict the chemical potential of ion binding, as well as their corresponding concentrations within the binding domain. We find that the electrostatic and steric contributions toward ion binding were similar for Mg(2+) and Ca(2+), yet the latter was 5.5 kcal/mol lower in enthalpy when internal strain within the EF hand was considered. We therefore speculate that beyond differences in dehydration energies for the Ca(2+) versus Mg(2+), strain induced in the β-PV EF hand by cation binding significantly contributes to the nearly 10,000-fold difference in binding affinity reported in the literature. We further complemented our analyses of local factors governing cation binding selectivity with whole-protein (global) contributions, such as interhelical residue-residue contacts and solvent exposure of hydrophobic surface. These contributions were found to be comparable for both Ca(2+)- and Mg(2+)-bound β-PV, which may implicate local factors, EF-hand strain, and dehydration, in providing the primary means of selectivity. We anticipate these methods could be used to estimate metal binding thermodynamics across a broad range of PV sequence homologues and EF-hand-containing, Ca(2+) binding proteins.

Inexpensive, serotype-independent protocol for native and bioengineered recombinant adeno-associated virus purification

Recombinant adeno-associated virus (AAV) is a valuable and often used gene therapy vector. With increased demand for highly purified virus comes the need for a standardized purification procedure that is applicable across many serotypes and includes bioengineered viruses. Currently cesium chloride banding or affinity chromatography are the predominate forms of purification. These approaches expose the final purified virus to toxic contaminants or are highly capsid dependent and may require significant optimization to isolate purified AAV. These methods may also limit crude viral lysate processing volume resulting in a significant loss of viral titer. To circumvent these issues, we have developed an AAV purification protocol independent of toxic compounds, supernatant volume and capsid moiety. This purification method standardizes virus purification across native serotype and bioengineered mosaic capsids.

Microtubule-Mediated Misregulation of Junctophilin-2 Underlies T-Tubule Disruptions and Calcium Mishandling in mdx Mice

Cardiac myocytes from the mdx mouse, the mouse model of Duchenne muscular dystrophy, exhibit t-tubule disarray and increased calcium sparks, but a unifying molecular mechanism has not been elucidated. Recently, improper trafficking of junctophilin (JPH)-2 on an altered microtubule network caused t-tubule derangements and calcium mishandling in a pressure-overload heart failure model. Mdx cardiac myocytes have microtubule abnormalities, but how this may affect JPH-2, t-tubules, and calcium handling has not been established. Here, we investigated the hypothesis that an inverse relationship between microtubules and JPH-2 underlies t-tubule disruptions and calcium mishandling in mdx cardiac myocytes. Confocal microscopy revealed t-tubule disorganization in mdx cardiac myocytes. Quantitative Western blot analysis demonstrated JPH-2 was decreased by 75% and showed an inverse hyperbolic relationship with α- and β-tubulin, the individual components of microtubules, in mdx hearts. Colchicine-induced microtubule depolymerization normalized JPH-2 protein levels and localization, corrected t-tubule architecture, and reduced calcium sparks. In summary, these results suggest microtubule-mediated misregulation of JPH-2 causes t-tubule derangements and altered calcium handling in mdx cardiac myocytes.

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Decreased junctophilin-2 levels are associated with cardiac t-tubule derangements in mdx mice, the mouse model of Duchenne muscular dystrophy (DMD).

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Reduced junctophilin-2 protein levels correlate with increases in total microtubule content in mdx hearts.

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Colchicine-mediated microtubule depolymerization increases junctophilin-2 protein levels and improves localization patterns which, in turn, are associated with t-tubule reorganization and reduced calcium sparks.

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This study identifies microtubule-mediated misregulation of junctophilin-2 as a novel molecular mechanism in Duchenne cardiomyopathy.

Ophiopogonin D maintains Ca2+ homeostasis in rat cardiomyocytes in vitro by upregulating CYP2J3/EETs and suppressing ER stress

AIM

CYP2J3 in myocardium metabolizes arachidonic acid to 4 regioisomeric epoxyeicosatrienoic acids (EETs), which have diverse biological activities in rat heart. In this study we examined whether CYP2J3 was involved in cardioprotective effects of ophiopogonin D (OPD), a steroidal glycoside isolated from Chinese herb Radix ophiopogonis.

METHODS

Rat cardiomyoblast cell line (H9c2 cells) was tested. Intracellular Ca(2+) concentrations ([Ca(2+)]i) were measured using Fluo-4/AM. The expression of calcium-regulating molecules and ER stress signaling molecules was measured with qRT-PCR and Western blot analyses. Cell apoptosis was quantified with Hoechst 33258 staining and TUNEL assay. The level of 14,15-DHET, a stable metabolite of 14,15-EET, was assessed with ELISA.

RESULTS

Angiotensin II (10(-6) mol/L) significantly decreased the expression of calcium-regulating molecules (SERCA2a, PLB, RyR2 and FKBP12.6), and elevated [Ca(2+)]i in H9c2 cells. Furthermore, angiotensin II markedly increased the expression of ER stress signaling molecules (GRP78, CHOP, p-JNK and cleaved caspase-12) and ER stress-mediated apoptosis. OPD (100, 250 and 500 nmol/L) dose-dependently increased CYP2J3 expression and 14,15-DHET levels in normal H9c2 cells. Pretreatment of H9c2 cells with OPD suppressed angiotensin II-induced abnormalities in Ca(2+) homeostasis, ER stress responses and apoptosis. Overexpression of CYP2J3 or addition of exogenous 14,15-EET also prevented angiotensin II-induced abnormalities in Ca(2+) homeostasis, whereas transfection with CYP2J3 siRNA diminished the effects of OPD on Ca(2+) homeostasis. Furthermore, the intracellular Ca(2+) chelator BAPTA suppressed angiotensin II-induced ER stress responses and apoptosis in H9c2 cells.

CONCLUSION

OPD is a novel CYP2J3 inducer that may offer a therapeutic benefit in treatment of cardiovascular diseases related to disturbance of Ca(2+) homeostasis and ER stress.

Designing proteins to combat disease: Cardiac troponin C as an example

Throughout history, muscle research has led to numerous scientific breakthroughs that have brought insight to a more general understanding of all biological processes. Potentially one of the most influential discoveries was the role of the second messenger calcium and its myriad of handling and sensing systems that mechanistically control muscle contraction. In this review we will briefly discuss the significance of calcium as a universal second messenger along with some of the most common calcium binding motifs in proteins, focusing on the EF-hand. We will also describe some of our approaches to rationally design calcium binding proteins to palliate, or potentially even cure cardiovascular disease. Considering not all failing hearts have the same etiology, genetic background and co-morbidities, personalized therapies will need to be developed. We predict designer proteins will open doors for unprecedented personalized, and potentially, even generalized medicines as gene therapy or protein delivery techniques come to fruition.

In Vivo Analysis of Troponin C Knock-In (A8V) Mice: Evidence that TNNC1 Is a Hypertrophic Cardiomyopathy Susceptibility Gene

BACKGROUND

Mutations in thin-filament proteins have been linked to hypertrophic cardiomyopathy, but it has never been demonstrated that variants identified in the TNNC1 (gene encoding troponin C) can evoke cardiac remodeling in vivo. The goal of this study was to determine whether TNNC1 can be categorized as an hypertrophic cardiomyopathy susceptibility gene, such that a mouse model can recapitulate the clinical presentation of the proband.

METHODS AND RESULTS

The TNNC1-A8V proband diagnosed with severe obstructive hypertrophic cardiomyopathy at 34 years of age exhibited mild-to-moderate thickening in left and right ventricular walls, decreased left ventricular dimensions, left atrial enlargement, and hyperdynamic left ventricular systolic function. Genetically engineered knock-in (KI) mice containing the A8V mutation (heterozygote=KI-TnC-A8V(+/-); homozygote=KI-TnC-A8V(+/+)) were characterized by echocardiography and pressure-volume studies. Three-month-old KI-TnC-A8V(+/+) mice displayed decreased ventricular dimensions, mild diastolic dysfunction, and enhanced systolic function, whereas KI-TnC-A8V(+/-) mice displayed cardiac restriction at 14 months of age. KI hearts exhibited atrial enlargement, papillary muscle hypertrophy, and fibrosis. Liquid chromatography-mass spectroscopy was used to determine incorporation of mutant cardiac troponin C (≈ 21%) into the KI-TnC-A8V(+/-) cardiac myofilament. Reduced diastolic sarcomeric length, increased shortening, and prolonged Ca(2+) and contractile transients were recorded in intact KI-TnC-A8V(+/-) and KI-TnC-A8V(+/+) cardiomyocytes. Ca(2+) sensitivity of contraction in skinned fibers increased with mutant gene dose: KI-TnC-A8V(+/+)>KI-TnC-A8V(+/-)>wild-type, whereas KI-TnC-A8V(+/+) relaxed more slowly on flash photolysis of diazo-2.

CONCLUSIONS

The TNNC1-A8V mutant increases the Ca(2+)-binding affinity of the thin filament and elicits changes in Ca(2+) homeostasis and cellular remodeling, which leads to diastolic dysfunction. These in vivo alterations further implicate the role of TNNC1 mutations in the development of cardiomyopathy.

Mitochondrial flash as a novel biomarker of mitochondrial respiration in the heart

Mitochondrial respiration through electron transport chain (ETC) activity generates ATP and reactive oxygen species in eukaryotic cells. The modulation of mitochondrial respiration in vivo or under physiological conditions remains elusive largely due to the lack of appropriate approach to monitor ETC activity in a real-time manner. Here, we show that ETC-coupled mitochondrial flash is a novel biomarker for monitoring mitochondrial respiration under pathophysiological conditions in cultured adult cardiac myocyte and perfused beating heart. Through real-time confocal imaging, we follow the frequency of a transient bursting fluorescent signal, named mitochondrial flash, from individual mitochondria within intact cells expressing a mitochondrial matrix-targeted probe, mt-cpYFP (mitochondrial-circularly permuted yellow fluorescent protein). This mt-cpYFP recorded mitochondrial flash has been shown to be composed of a major superoxide signal with a minor alkalization signal within the mitochondrial matrix. Through manipulating physiological substrates for mitochondrial respiration, we find a close coupling between flash frequency and the ETC electron flow, as measured by oxygen consumption rate in cardiac myocyte. Stimulating electron flow under physiological conditions increases flash frequency. On the other hand, partially block or slowdown electron flow by inhibiting the F0F1 ATPase, which represents a pathological condition, transiently increases then decreases flash frequency. Limiting electron entrance at complex I by knocking out Ndufs4, an assembling subunit of complex I, suppresses mitochondrial flash activity. These results suggest that mitochondrial electron flow can be monitored by real-time imaging of mitochondrial flash. The mitochondrial flash frequency could be used as a novel biomarker for mitochondrial respiration under physiological and pathological conditions.

β-adrenergic effects on cardiac myofilaments and contraction in an integrated rabbit ventricular myocyte model

A five-state model of myofilament contraction was integrated into a well-established rabbit ventricular myocyte model of ion channels, Ca(2+) transporters and kinase signaling to analyze the relative contribution of different phosphorylation targets to the overall mechanical response driven by β-adrenergic stimulation (β-AS). β-AS effect on sarcoplasmic reticulum Ca(2+) handling, Ca(2+), K(+) and Cl(-) currents, and Na(+)/K(+)-ATPase properties was included based on experimental data. The inotropic effect on the myofilaments was represented as reduced myofilament Ca(2+) sensitivity (XBCa) and titin stiffness, and increased cross-bridge (XB) cycling rate (XBcy). Assuming independent roles of XBCa and XBcy, the model reproduced experimental β-AS responses on action potentials and Ca(2+) transient amplitude and kinetics. It also replicated the behavior of force-Ca(2+), release-restretch, length-step, stiffness-frequency and force-velocity relationships, and increased force and shortening in isometric and isotonic twitch contractions. The β-AS effect was then switched off from individual targets to analyze their relative impact on contractility. Preventing β-AS effects on L-type Ca(2+) channels or phospholamban limited Ca(2+) transients and contractile responses in parallel, while blocking phospholemman and K(+) channel (IKs) effects enhanced Ca(2+) and inotropy. Removal of β-AS effects from XBCa enhanced contractile force while decreasing peak Ca(2+) (due to greater Ca(2+) buffering), but had less effect on shortening. Conversely, preventing β-AS effects on XBcy preserved Ca(2+) transient effects, but blunted inotropy (both isometric force and especially shortening). Removal of titin effects had little impact on contraction. Finally, exclusion of β-AS from XBCa and XBcy while preserving effects on other targets resulted in preserved peak isometric force response (with slower kinetics) but nearly abolished enhanced shortening. β-AS effects on XBCa and XBcy have greater impact on isometric and isotonic contraction, respectively.

Fetal-adult cardiac transcriptome analysis in rats with contrasting left ventricular mass reveals new candidates for cardiac hypertrophy

Reactivation of fetal gene expression patterns has been implicated in common cardiac diseases in adult life including left ventricular (LV) hypertrophy (LVH) in arterial hypertension. Thus, increased wall stress and neurohumoral activation are discussed to induce the return to expression of fetal genes after birth in LVH. We therefore aimed to identify novel potential candidates for LVH by analyzing fetal-adult cardiac gene expression in a genetic rat model of hypertension, i.e. the stroke-prone spontaneously hypertensive rat (SHRSP). To this end we performed genome-wide transcriptome analysis in SHRSP to identify differences in expression patterns between day 20 of fetal development (E20) and adult animals in week 14 in comparison to a normotensive rat strain with contrasting low LV mass, i.e. Fischer (F344). 15232 probes were detected as expressed in LV tissue obtained from rats at E20 and week 14 (p < 0.05) and subsequently screened for differential expression. We identified 24 genes with SHRSP specific up-regulation and 21 genes with down-regulation as compared to F344. Further bioinformatic analysis presented Efcab6 as a new candidate for LVH that showed only in the hypertensive SHRSP rat differential expression during development (logFC = 2.41, p < 0.001) and was significantly higher expressed in adult SHRSP rats compared with adult F344 (+ 76%) and adult normotensive Wistar-Kyoto rats (+ 82%). Thus, it represents an interesting new target for further functional analyses and the elucidation of mechanisms leading to LVH. Here we report a new approach to identify candidate genes for cardiac hypertrophy by combining the analysis of gene expression differences between strains with a contrasting cardiac phenotype with a comparison of fetal-adult cardiac expression patterns.

Gene therapy for heart disease: molecular targets, vectors and modes of delivery to myocardium

Despite the numerous hurdles that gene therapy has encountered along the way, clinical trials over the last few years are showing promising results in many fields of medicine, including cardiology, where many targets are moving toward clinical development. In this review, the authors discuss the current state of the art in terms of clinical and preclinical development. They also examine vector technology and available vector-delivery strategies.

Parvalbumin as a metal-dependent antioxidant

Parvalbumin (PA) is a Ca(2+)-binding protein of vertebrates massively expressed in tissues with high oxygen uptake and respectively elevated level of reactive oxygen species (ROS). To characterize antioxidant properties of PA, antioxidant capacity (AOC) of intact rat α-PA has been explored. ORAC, TEAC and hydrogen peroxide AOC assays evidence conformation-dependent oxidation of the PA. AOC value for the apo-PA 4-11-fold exceeds that for the Ca(2+)-loaded protein. Despite folded conformation of apo-PA, it has AOC equivalent to that of the proteolized protein. The most populated under resting conditions PA form, Mg(2+)-bound PA, has AOC similar to that of apo-PA. ROS-induced changes in absorption spectrum of PA evidence an oxidation of PA's phenylalanines in the ORAC assay. Sensitivity of PA oxidation to its conformation enabled characterization of its metal affinity and pH-dependent behavior: a transition with pKa of 7.6 has been revealed for the Ca(2+)-loaded PA. Since total AOC of PA under in vivo conditions may reach the level of reduced glutathione, we propose that PA might modulate intracellular redox equilibria and/or signaling in a calcium-dependent manner. We speculate that the oxidation-mediated damage of some of PA-GABAergic interneurons observed in schizophrenia is due to a decline in total AOC of the reduced glutathione-PA pair.

Cardiovascular gene therapy for myocardial infarction

INTRODUCTION

Cardiovascular gene therapy is the third most popular application for gene therapy, representing 8.4% of all gene therapy trials as reported in 2012 estimates. Gene therapy in cardiovascular disease is aiming to treat heart failure from ischemic and non-ischemic causes, peripheral artery disease, venous ulcer, pulmonary hypertension, atherosclerosis and monogenic diseases, such as Fabry disease.

AREAS COVERED

In this review, we will focus on elucidating current molecular targets for the treatment of ventricular dysfunction following myocardial infarction (MI). In particular, we will focus on the treatment of i) the clinical consequences of it, such as heart failure and residual myocardial ischemia and ii) etiological causes of MI (coronary vessels atherosclerosis, bypass venous graft disease, in-stent restenosis).

EXPERT OPINION

We summarise the scheme of the review and the molecular targets either already at the gene therapy clinical trial phase or in the pipeline. These targets will be discussed below. Following this, we will focus on what we believe are the 4 prerequisites of success of any gene target therapy: safety, expression, specificity and efficacy (SESE).

MagFRET: the first genetically encoded fluorescent Mg2+ sensor

Magnesium has important structural, catalytic and signaling roles in cells, yet few tools exist to image this metal ion in real time and at subcellular resolution. Here we report the first genetically encoded sensor for Mg²⁺, MagFRET-1. This sensor is based on the high-affinity Mg²⁺ binding domain of human centrin 3 (HsCen3), which undergoes a transition from a molten-globular apo form to a compactly-folded Mg²⁺-bound state. Fusion of Cerulean and Citrine fluorescent domains to the ends of HsCen3, yielded MagFRET-1, which combines a physiologically relevant Mg²⁺ affinity (K_d = 148 µM) with a 50% increase in emission ratio upon Mg²⁺ binding due to a change in FRET efficiency between Cerulean and Citrine. Mutations in the metal binding sites yielded MagFRET variants whose Mg²⁺ affinities were attenuated 2- to 100-fold relative to MagFRET-1, thus covering a broad range of Mg²⁺ concentrations. In situ experiments in HEK293 cells showed that MagFRET-1 can be targeted to the cytosol and the nucleus. Clear responses to changes in extracellular Mg²⁺ concentration were observed for MagFRET-1-expressing HEK293 cells when they were permeabilized with digitonin, whereas similar changes were not observed for intact cells. Although MagFRET-1 is also sensitive to Ca²⁺, this affinity is sufficiently attenuated (K_d of 10 µM) to make the sensor insensitive to known Ca²⁺ stimuli in HEK293 cells. While the potential and limitations of the MagFRET sensors for intracellular Mg²⁺ imaging need to be further established, we expect that these genetically encoded and ratiometric fluorescent Mg²⁺ sensors could prove very useful in understanding intracellular Mg²⁺ homeostasis and signaling.

Direct, differential effects of tamoxifen, 4-hydroxytamoxifen, and raloxifene on cardiac myocyte contractility and calcium handling

Tamoxifen (Tam), a selective estrogen receptor modulator, is in wide clinical use for the treatment and prevention of breast cancer. High Tam doses have been used for treatment of gliomas and cancers with multiple drug resistance, but long QT Syndrome is a side effect. Tam is also used experimentally in mice for inducible gene knockout in numerous tissues, including heart; however, the potential direct effects of Tam on cardiac myocyte mechanical function are not known. The goal of this study was to determine the direct, acute effects of Tam, its active metabolite 4-hydroxytamoxifen (4OHT), and related drug raloxifene (Ral) on isolated rat cardiac myocyte mechanical function and calcium handling. Tam decreased contraction amplitude, slowed relaxation, and decreased Ca²⁺ transient amplitude. Effects were primarily observed at 5 and 10 μM Tam, which is relevant for high dose Tam treatment in cancer patients as well as Tam-mediated gene excision in mice. Myocytes treated with 4OHT responded similarly to Tam-treated cells with regard to both contractility and calcium handling, suggesting an estrogen-receptor independent mechanism is responsible for the effects. In contrast, Ral increased contraction and Ca²⁺ transient amplitudes. At 10 μM, all drugs had a time-dependent effect to abolish cellular contraction. In conclusion, Tam, 4OHT, and Ral adversely and differentially alter cardiac myocyte contractility and Ca²⁺ handling. These findings have important implications for understanding the Tam-induced cardiomyopathy in gene excision studies and may be important for understanding effects on cardiac performance in patients undergoing high-dose Tam therapy.

Intracellular Na⁺ and cardiac metabolism

In heart failure, alterations of excitation-contraction underlie contractile dysfunction. One important defect is an elevation of the intracellular Na(+) concentration in cardiac myocytes ([Na(+)]i), which has an important impact on cytosolic and mitochondrial Ca(2+) homeostasis. While elevated [Na(+)]i is thought to compensate for decreased Ca(2+) load of the sarcoplasmic reticulum (SR), it yet negatively affects energy supply-and-demand matching and can even induce mitochondrial oxidative stress. Here, we review the mechanisms underlying these pathophysiological changes. The chain of events may constitute a vicious cycle of ion dysregulation, oxidative stress and energetic deficit, resembling characteristic cellular deficits that are considered key hallmarks of the failing heart. This article is part of a Special Issue entitled "Na(+) Regulation in Cardiac Myocytes".