Class I PI3K Biology.

This chapter is an introduction to phosphoinositide 3-kinases (PI3K), with class I PI3Ks as the central focus. First, the various PI3K isoforms in class I are presented with emphasis on their overall structure, subunits, subunit constitutive domains, domain-domain interactions, and functional relevance. This structural analysis is followed by a comprehensive history of seminal investigations into PI3K activity. Next, we highlight the divergent roles of the isoforms: PI3Kα, PI3Kß, PI3Kδ, and PI3Kγ. This section details signaling pathways in which these PI3K isoforms are involved, including the key upstream regulators of PI3K activity and some downstream cellular effects. Nodes of the PI3K pathway are also presented. Inhibitors of some isoforms are discussed to give an overview of the basis of some immunotherapies that are being used to target cell signaling. Finally, the chapter ends with a discussion of the dysregulation of PI3Ks in diseases including APDS, asthma, arthritis, and oncogenic mutations.

Genetic inhibition of nuclear factor of activated T-cell c2 prevents atrial fibrillation in CREM transgenic mice.

AIMS: Abnormal intracellular calcium (Ca2+) handling contributes to the progressive nature of atrial fibrillation (AF), the most common sustained cardiac arrhythmia. Evidence in mouse models suggests that activation of the nuclear factor of activated T-cell (NFAT) signalling pathway contributes to atrial remodelling. Our aim was to determine the role of NFATc2 in AF in humans and mouse models. METHODS AND RESULTS: Expression levels of NFATc1-c4 isoforms were assessed by quantitative reverse transcription-polymerase chain reaction in right atrial appendages from patients with chronic AF (cAF). NFATc1 and NFATc2 mRNA levels were elevated in cAF patients compared with those in normal sinus rhythm (NSR). Western blotting revealed increased cytosolic and nuclear levels of NFATc2 in AF patients. Similar findings were obtained in CREM-IbΔC-X transgenic (CREM) mice, a model of progressive AF. Telemetry ECG recordings revealed age-dependent spontaneous AF in CREM mice, which was prevented by NFATc2 knockout in CREM:NFATc2-/- mice. Programmed electrical stimulation revealed that CREM:NFATc2-/- mice lacked an AF substrate. Morphometric analysis and histology revealed increased atrial weight and atrial fibrosis in CREM mice compared with wild-type controls, which was reversed in CREM:NFATc2-/- mice. Confocal microscopy showed an increased Ca2+ spark frequency despite a reduced sarcoplasmic reticulum (SR) Ca2+ load in CREM mice compared with controls, whereas these abnormalities were normalized in CREM:NFATc2-/- mice. Western blotting revealed that genetic inhibition of Ca2+/calmodulin-dependent protein kinase II-mediated phosphorylation of S2814 on ryanodine receptor type 2 (RyR2) in CREM:RyR2-S2814A mice suppressed NFATc2 activation observed in CREM mice, suggesting that NFATc2 is activated by excessive SR Ca2+ leak via RyR2. Finally, chromatin immunoprecipitation sequencing from AF patients identified Ras and EF-hand domain-containing protein (Rasef) as a direct target of NFATc2-mediated transcription. CONCLUSION: Our findings reveal activation of the NFAT signalling pathway in patients of Chinese and European descent. NFATc2 knockout prevents the progression of AF in the CREM mouse model.

Reversible cardiac disease features in an inducible CUG repeat RNA-expressing mouse model of myotonic dystrophy.

Myotonic dystrophy type 1 (DM1) is caused by a CTG repeat expansion in the DMPK gene. Expression of pathogenic expanded CUG repeat (CUGexp) RNA causes multisystemic disease by perturbing the functions of RNA-binding proteins, resulting in expression of fetal protein isoforms in adult tissues. Cardiac involvement affects 50% of individuals with DM1 and causes 25% of disease-related deaths. We developed a transgenic mouse model for tetracycline-inducible and heart-specific expression of human DMPK mRNA containing 960 CUG repeats. CUGexp RNA is expressed in atria and ventricles and induced mice exhibit electrophysiological and molecular features of DM1 disease, including cardiac conduction delays, supraventricular arrhythmias, nuclear RNA foci with Muscleblind protein colocalization, and alternative splicing defects. Importantly, these phenotypes were rescued upon loss of CUGexp RNA expression. Transcriptome analysis revealed gene expression and alternative splicing changes in ion transport genes that are associated with inherited cardiac conduction diseases, including a subset of genes involved in calcium handling. Consistent with RNA-Seq results, calcium-handling defects were identified in atrial cardiomyocytes isolated from mice expressing CUGexp RNA. These results identify potential tissue-specific mechanisms contributing to cardiac pathogenesis in DM1 and demonstrate the utility of reversible phenotypes in our model to facilitate development of targeted therapeutic approaches.

Loss of SPEG Inhibitory Phosphorylation of Ryanodine Receptor Type-2 Promotes Atrial Fibrillation.

BACKGROUND: Enhanced diastolic calcium (Ca2+) release through ryanodine receptor type-2 (RyR2) has been implicated in atrial fibrillation (AF) promotion. Diastolic sarcoplasmic reticulum Ca2+ leak is caused by increased RyR2 phosphorylation by PKA (protein kinase A) or CaMKII (Ca2+/calmodulin-dependent kinase-II) phosphorylation, or less dephosphorylation by protein phosphatases. However, considerable controversy remains regarding the molecular mechanisms underlying altered RyR2 function in AF. We thus aimed to determine the role of SPEG (striated muscle preferentially expressed protein kinase), a novel regulator of RyR2 phosphorylation, in AF pathogenesis. METHODS: Western blotting was performed with right atrial biopsies from patients with paroxysmal AF. SPEG atrial knockout mice were generated using adeno-associated virus 9. In mice, AF inducibility was determined using intracardiac programmed electric stimulation, and diastolic Ca2+ leak in atrial cardiomyocytes was assessed using confocal Ca2+ imaging. Phosphoproteomics studies and Western blotting were used to measure RyR2 phosphorylation. To test the effects of RyR2-S2367 phosphorylation, knockin mice with an inactivated S2367 phosphorylation site (S2367A) and a constitutively activated S2367 residue (S2367D) were generated by using CRISPR-Cas9. RESULTS: Western blotting revealed decreased SPEG protein levels in atrial biopsies from patients with paroxysmal AF in comparison with patients in sinus rhythm. SPEG atrial-specific knockout mice exhibited increased susceptibility to pacing-induced AF by programmed electric stimulation and enhanced Ca2+ spark frequency in atrial cardiomyocytes with Ca2+ imaging, establishing a causal role for decreased SPEG in AF pathogenesis. Phosphoproteomics in hearts from SPEG cardiomyocyte knockout mice identified RyR2-S2367 as a novel kinase substrate of SPEG. Western blotting demonstrated that RyR2-S2367 phosphorylation was also decreased in patients with paroxysmal AF. RyR2-S2367A mice exhibited an increased susceptibility to pacing-induced AF, and aberrant atrial sarcoplasmic reticulum Ca2+ leak, as well. In contrast, RyR2-S2367D mice were resistant to pacing-induced AF. CONCLUSIONS: Unlike other kinases (PKA, CaMKII) that increase RyR2 activity, SPEG phosphorylation reduces RyR2-mediated sarcoplasmic reticulum Ca2+ release. Reduced SPEG levels and RyR2-S2367 phosphorylation typified patients with paroxysmal AF. Studies in S2367 knockin mouse models showed a causal relationship between reduced S2367 phosphorylation and AF susceptibility. Thus, modulating SPEG activity and phosphorylation levels of the novel S2367 site on RyR2 may represent a novel target for AF treatment.

Atrial-Specific Gene Delivery Using an Adeno-Associated Viral Vector.

RATIONALE: Somatic overexpression in mice using an adeno-associated virus (AAV) as gene transfer vectors has become a valuable tool to analyze the roles of specific genes in cardiac diseases. The lack of atrial-specific AAV vector has been a major obstacle for studies into the pathogenesis of atrial diseases. Moreover, gene therapy studies for atrial fibrillation would benefit from atrial-specific vectors. Atrial natriuretic factor (ANF) promoter drives gene expression specifically in atrial cardiomyocytes. OBJECTIVE: To establish the platform of atrial specific in vivo gene delivery by AAV-ANF. METHODS AND RESULTS: We constructed AAV vectors based on serotype 9 (AAV9) that are driven by the atrial-specific ANF promoter. Hearts from mice injected with AAV9-ANF-GFP (green fluorescent protein) exhibited strong and atrial-specific GFP expression without notable GFP in ventricular tissue. In contrast, similar vectors containing a cardiac troponin T promoter (AAV9-TNT4-GFP) showed GFP expression in all 4 chambers of the heart, while AAV9 with an enhanced chicken ß-actin promoter (AAV-enCB-GFP) caused ubiquitous GFP expression. Next, we used Rosa26mT/mG (membrane-targeted tandem dimer Tomato/membrane-targeted GFP), a double-fluorescent Cre reporter mouse that expresses membrane-targeted tandem dimer Tomato before Cre-mediated excision, and membrane-targeted GFP after excision. AAV9-ANF-Cre led to highly efficient LoxP recombination in membrane-targeted tandem dimer Tomato/membrane-targeted green fluorescent protein mice with high specificity for the atria. We measured the frequency of transduced cardiomyocytes in atria by detecting Cre-dependent GFP expression from the Rosa26mT/mG allele. AAV9 dose was positively correlated with the number of GFP-positive atrial cardiomyocytes. Finally, we assessed whether the AAV9-ANF-Cre vector could be used to induce atrial-specific gene knockdown in proof-of-principle experiments using conditional JPH2 (junctophilin-2) knockdown mice. Four weeks after AAV9-ANF-Cre injection, a strong reduction in atrial expression of JPH2 protein was observed. Furthermore, there was evidence for abnormal Ca2+ handling in atrial myocytes isolated from mice with atrial-restricted JPH2 deficiency. CONCLUSIONS: AAV9-ANF vectors produce efficient, dose-dependent, and atrial-specific gene expression following a single-dose systemic delivery in mice. This vector is a novel reagent for both mechanistic and gene therapy studies on atrial diseases.

Genetics of atrial fibrillation: an update.

PURPOSE OF REVIEW: Atrial fibrillation is a common cardiac arrhythmia with a high morbidity and mortality affecting 34 million worldwide. Current therapies are inadequate and often fail to directly address molecular mechanisms of the disease. In this review, we will provide an overview of recent advances in our understanding of the genetic underpinnings of atrial fibrillation. RECENT FINDINGS: Large-scale genetic association studies have more than doubled the number of genetic loci associated with atrial fibrillation during the last year. Studies examining how genes at or near these loci can affect the pathogenesis of atrial fibrillation are ongoing in cellular, animal, and computational models. In addition, several recent clinical studies have also demonstrated that variants at these loci can aid in risk stratification of patients. SUMMARY: There are now over 30 genetic loci associated with atrial fibrillation. A better understanding of how these loci relate to disease pathogenesis may provide insight into novel therapeutic targets and ultimately lead to improved clinical care.

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.