Julia O. Reynolds

Junctophilin-2 is necessary for T-tubule maturation during mouse heart development

AIMS Transverse tubules (TTs) provide the basic subcellular structures that facilitate excitation-contraction (EC) coupling, the essential process that underlies normal cardiac contractility. Previous studies have shown that TTs develop within the first few weeks of life in mammals but the molecular determinants of this development have remained elusive. This study aims to elucidate the role of junctophilin-2 (JPH2), a junctional membrane complex protein, in the maturation of TTs in cardiomyocytes. METHODS AND RESULTS Using a novel cardiac-specific short-hairpin-RNA-mediated JPH2 knockdown mouse model (Mus musculus; αMHC-shJPH2), we assessed the effects of the loss of JPH2 on the maturation of the ventricular TT structure. Between embryonic day (E) 10.5 and postnatal day (P) 10, JPH2 mRNA and protein levels were reduced by >70% in αMHC-shJPH2 mice. At P8 and P10, knockdown of JPH2 significantly inhibited the maturation of TTs, while expression levels of other genes implicated in TT development remained mostly unchanged. At the same time, intracellular Ca(2+) handling was disrupted in ventricular myocytes from αMHC- shJPH2 mice, which developed heart failure by P10 marked by reduced ejection fraction, ventricular dilation, and premature death. In contrast, JPH2 transgenic mice exhibited accelerated TT maturation by P8. CONCLUSION Our findings suggest that JPH2 is necessary for TT maturation during postnatal cardiac development in mice. In particular, JPH2 may be critical in anchoring the invaginating sarcolemma to the sarcoplasmic reticulum, thereby enabling the maturation of the TT network.

Impaired local regulation of ryanodine receptor type 2 by protein phosphatase 1 promotes atrial fibrillation

AIMS Altered Ca(2+) handling in atrial fibrillation (AF) has been associated with dysregulated protein phosphatase 1 (PP1) and subcellular heterogeneities in protein phosphorylation, but the underlying mechanisms remain unclear. This is due to a lack of investigation into the local, rather than global, regulation of PP1 on different subcellular targets such as ryanodine receptor type 2 (RyR2), especially in AF. METHODS AND RESULTS We tested the hypothesis that impaired local regulation of PP1 causes RyR2 hyperphosphorylation thereby promoting AF susceptibility. To specifically disrupt PP1s local regulation of RyR2, we used the spinophilin knockout (Sp(-/-)) mice (Mus musculus) since PP1 is targeted to RyR2 via spinophilin. Without spinophilin, the interaction between PP1 and RyR2 was reduced by 64%, while RyR2 phosphorylation was increased by 43% at serine (S)2814 but unchanged at S2808. Lipid bilayer experiments revealed that single RyR2 channels isolated from Sp(-/-) hearts had an increased open probability. Likewise, Ca(2+) spark frequency normalized to sarcoplasmic reticulum Ca(2+) content was also enhanced in Sp(-/-) atrial myocytes, but normalized by Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) inhibitors KN-93 and AIP and also by genetic inhibition of RyR2 S2814 phosphorylation. Finally, Sp(-/-) mice exhibited increased atrial ectopy and susceptibility to pacing-induced AF, both of which were also prevented by the RyR2 S2814A mutation. CONCLUSION PP1 regulates RyR2 locally by counteracting CaMKII phosphorylation of RyR2. Decreased local PP1 regulation of RyR2 contributes to RyR2 hyperactivity and promotes AF susceptibility. This represents a novel mechanism for subcellular modulation of calcium channels and may represent a potential drug target of AF.

Junctophilin-2 gene therapy rescues heart failure by normalizing RyR2-mediated Ca2+ release

BACKGROUND Junctophilin-2 (JPH2) is the primary structural protein for the coupling of transverse (T)-tubule associated cardiac L-type Ca channels and type-2 ryanodine receptors on the sarcoplasmic reticulum within junctional membrane complexes (JMCs) in cardiomyocytes. Effective signaling between these channels ensures adequate Ca-induced Ca release required for normal cardiac contractility. Disruption of JMC subcellular domains, a common feature of failing hearts, has been attributed to JPH2 downregulation. Here, we tested the hypothesis that adeno-associated virus type 9 (AAV9) mediated overexpression of JPH2 could halt the development of heart failure in a mouse model of transverse aortic constriction (TAC). METHODS AND RESULTS Following TAC, a progressive decrease in ejection fraction was paralleled by a progressive decrease of cardiac JPH2 levels. AAV9-mediated expression of JPH2 rescued cardiac contractility in mice subjected to TAC. AAV9-JPH2 also preserved T-tubule structure. Moreover, the Ca2+ spark frequency was reduced and the Ca2+ transient amplitude was increased in AAV9-JPH2 mice following TAC, consistent with JPH2-mediated normalization of SR Ca2+ handling. CONCLUSIONS This study demonstrates that AAV9-mediated JPH2 gene therapy maintained cardiac function in mice with early stage heart failure. Moreover, restoration of JPH2 levels prevented loss of T-tubules and suppressed abnormal SR Ca2+ leak associated with contractile failure following TAC. These findings suggest that targeting JPH2 might be an attractive therapeutic approach for treating pathological cardiac remodeling during heart failure.

Use of magnetization transfer contrast MRI to detect early molecular pathology in Alzheimer's disease.

The purpose of this study was to determine if magnetization transfer contrast (MTC) imaging could be used to detect early macromolecular accumulation in a mouse model of early Alzheimers disease.

Treatment of catecholaminergic polymorphic ventricular tachycardia in mice using novel RyR2-modifying drugs

RATIONALE Catecholaminergic polymorphic ventricular tachycardia (CPVT) is a potentially lethal arrhythmic disorder caused by mutations in the type-2 ryanodine receptor (RyR2). Mutant RyR2 cause abnormal Ca2+ leak from the sarcoplasmic reticulum (SR), which is associated with the development of arrhythmias. OBJECTIVE To determine whether derivatives of tetracaine, a local anesthetic drug with known RyR2 inhibiting action, could prevent CPVT induction by suppression of RyR2-mediated SR Ca2+ leak. METHODS AND RESULTS Confocal microscopy was used to assess the effects of tetracaine and 9 derivatives (EL1-EL9) on spontaneous Ca2+ sparks in ventricular myocytes isolated from RyR2-R176Q/+ mice with CPVT. Whereas each derivative suppressed the Ca2+ spark frequency, derivative EL9 was most effective at the screening dose of 500nmol/L. At this high dose, the Ca2+ transient amplitude was not affected in myocytes from WT or R176Q/+ mice. The IC50 of EL9 was determined to be 13nmol/L, which is about 400× time lower than known RyR2 stabilizer K201. EL9 prevented the induction of ventricular tachycardia observed in placebo-treated R176Q/+ mice, without affecting heart rate or cardiac contractility. CONCLUSIONS Tetracaine derivatives represent a novel class of RyR2 stabilizing drugs that could be used for the treatment of the potentially fatal disorder catecholaminergic polymorphic ventricular tachycardia.

SPEG (Striated Muscle Preferentially Expressed Protein Kinase) Is Essential for Cardiac Function by Regulating Junctional Membrane Complex Activity

Rationale: Junctional membrane complexes (JMCs) in myocytes are critical microdomains, in which excitation–contraction coupling occurs. Structural and functional disruption of JMCs underlies contractile dysfunction in failing hearts. However, the role of newly identified JMC protein SPEG (striated muscle preferentially expressed protein kinase) remains unclear. Objective: To determine the role of SPEG in healthy and failing adult hearts. Methods and Results: Proteomic analysis of immunoprecipitated JMC proteins ryanodine receptor type 2 and junctophilin-2 (JPH2) followed by mass spectrometry identified the serine–threonine kinase SPEG as the only novel binding partner for both proteins. Real-time polymerase chain reaction revealed the downregulation of SPEG mRNA levels in failing human hearts. A novel cardiac myocyte-specific Speg conditional knockout (MCM-Spegfl/fl) model revealed that adult-onset SPEG deficiency results in heart failure (HF). Calcium (Ca2+) and transverse-tubule imaging of ventricular myocytes from MCM-Spegfl/fl mice post HF revealed both increased sarcoplasmic reticulum Ca2+ spark frequency and disrupted JMC integrity. Additional studies revealed that transverse-tubule disruption precedes the development of HF development in MCM-Spegfl/fl mice. Although total JPH2 levels were unaltered, JPH2 phosphorylation levels were found to be reduced in MCM-Spegfl/fl mice, suggesting that loss of SPEG phosphorylation of JPH2 led to transverse-tubule disruption, a precursor of HF development in SPEG-deficient mice. Conclusions: The novel JMC protein SPEG is downregulated in human failing hearts. Acute loss of SPEG in mouse hearts causes JPH2 dephosphorylation and transverse-tubule loss associated with downstream Ca2+ mishandling leading to HF. Our study suggests that SPEG could be a novel target for the treatment of HF.

Striated Muscle Preferentially Expressed Protein Kinase (SPEG) Is Essential for Cardiac Function by Regulating Junctional Membrane Complex Activity

Rationale: Junctional membrane complexes (JMCs) in myocytes are critical microdomains, in which excitation–contraction coupling occurs. Structural and functional disruption of JMCs underlies contractile dysfunction in failing hearts. However, the role of newly identified JMC protein SPEG (striated muscle preferentially expressed protein kinase) remains unclear. Objective: To determine the role of SPEG in healthy and failing adult hearts. Methods and Results: Proteomic analysis of immunoprecipitated JMC proteins ryanodine receptor type 2 and junctophilin-2 (JPH2) followed by mass spectrometry identified the serine–threonine kinase SPEG as the only novel binding partner for both proteins. Real-time polymerase chain reaction revealed the downregulation of SPEG mRNA levels in failing human hearts. A novel cardiac myocyte-specific Speg conditional knockout (MCM-Spegfl/fl) model revealed that adult-onset SPEG deficiency results in heart failure (HF). Calcium (Ca2+) and transverse-tubule imaging of ventricular myocytes from MCM-Spegfl/fl mice post HF revealed both increased sarcoplasmic reticulum Ca2+ spark frequency and disrupted JMC integrity. Additional studies revealed that transverse-tubule disruption precedes the development of HF development in MCM-Spegfl/fl mice. Although total JPH2 levels were unaltered, JPH2 phosphorylation levels were found to be reduced in MCM-Spegfl/fl mice, suggesting that loss of SPEG phosphorylation of JPH2 led to transverse-tubule disruption, a precursor of HF development in SPEG-deficient mice. Conclusions: The novel JMC protein SPEG is downregulated in human failing hearts. Acute loss of SPEG in mouse hearts causes JPH2 dephosphorylation and transverse-tubule loss associated with downstream Ca2+ mishandling leading to HF. Our study suggests that SPEG could be a novel target for the treatment of HF.

Long-Term Simulated Microgravity Causes Cardiac RyR2 Phosphorylation and Arrhythmias in Mice

BACKGROUND Long-term exposure to microgravity during space flight may lead to cardiac remodeling and rhythm disturbances. In mice, hindlimb unloading (HU) mimics the effects of microgravity and stimulates physiological adaptations, including cardiovascular deconditioning. Recent studies have demonstrated an important role played by changes in intracellular Ca handling in the pathogenesis of heart failure and arrhythmia. In this study, we tested the hypothesis that cardiac remodeling following HU in mice involves abnormal intracellular Ca regulation through the cardiac ryanodine receptor (RyR2). METHODS AND RESULTS Mice were subjected to HU by tail suspension for 28 to 56 days in order to induce cardiac remodeling (n=15). Control mice (n=19) were treated equally, with the exception of tail suspension. Echocardiography revealed cardiac enlargement and depressed contractility starting at 28 days post-HU versus control. Moreover, mice were more susceptible to pacing-induced ventricular arrhythmias after HU. Ventricular myocytes isolated from HU mice exhibited an increased frequency of spontaneous sarcoplasmic reticulum (SR) Ca release events and enhanced SR Ca leak via RyR2. Western blotting revealed increased RyR2 phosphorylation at S2814, and increased CaMKII auto-phosphorylation at T287, suggesting that CaMKII activation of RyR2 might underlie enhanced SR Ca release in HU mice. CONCLUSION These data suggest that abnormal intracellular Ca handling, likely due to increased CaMKII phosphorylation of RyR2, plays a role in cardiac remodeling following simulated microgravity in mice.

Novel Junctophilin-2 Mutation A405S Is Associated With Basal Septal Hypertrophy and Diastolic Dysfunction

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Rearrangement of the Protein Phosphatase 1 Interactome During Heart Failure Progression

Background: Heart failure (HF) is a complex disease with a rising prevalence despite advances in treatment. Protein phosphatase 1 (PP1) has long been implicated in HF pathogenesis, but its exact role is both unclear and controversial. Most previous studies measured only the PP1 catalytic subunit (PP1c) without investigating its diverse set of interactors, which confer localization and substrate specificity to the holoenzyme. In this study, we define the PP1 interactome in cardiac tissue and test the hypothesis that this interactome becomes rearranged during HF progression at the level of specific PP1c interactors. Methods: Mice were subjected to transverse aortic constriction and grouped on the basis of ejection fraction into sham, hypertrophy, moderate HF (ejection fraction, 30%–40%), and severe HF (ejection fraction <30%). Cardiac lysates were subjected to affinity purification with anti-PP1c antibodies followed by high-resolution mass spectrometry. PP1 regulatory subunit 7 (Ppp1r7) was knocked down in mouse cardiomyocytes and HeLa cells with adeno-associated virus serotype 9 and siRNA, respectively. Calcium imaging was performed on isolated ventricular myocytes. Results: Seventy-one and 98 PP1c interactors were quantified from mouse cardiac and HeLa lysates, respectively, including many novel interactors and protein complexes. This represents the largest reproducible PP1 interactome data set ever captured from any tissue, including both primary and secondary/tertiary interactors. Nine PP1c interactors with changes in their binding to PP1c were strongly associated with HF progression, including 2 known (Ppp1r7 and Ppp1r18) and 7 novel interactors. Within the entire cardiac PP1 interactome, Ppp1r7 had the highest binding to PP1c. Cardiac-specific knockdown in mice led to cardiac dysfunction and disruption of calcium release from the sarcoplasmic reticulum. Conclusions: PP1 is best studied at the level of its interactome, which undergoes significant rearrangement during HF progression. The 9 key interactors that are associated with HF progression may represent potential targets in HF therapy. In particular, Ppp1r7 may play a central role in regulating the PP1 interactome by acting as a competitive molecular “sponge” of PP1c.