Ca²⁺/calmodulin–dependent kinase II (CaMKII) has been implicated in cardiac hypertrophy and heart failure. We generated mice in which the predominant cardiac isoform, CaMKIIδ, was genetically deleted (KO mice), and found that these mice showed no gross baseline changes in ventricular structure or function. In WT and KO mice, transverse aortic constriction (TAC) induced comparable increases in relative heart weight, cell size, HDAC5 phosphorylation, and hypertrophic gene expression. Strikingly, while KO mice showed preserved hypertrophy after 6-week TAC, CaMKIIδ deficiency significantly ameliorated phenotypic changes associated with the transition to heart failure, such as chamber dilation, ventricular dysfunction, lung edema, cardiac fibrosis, and apoptosis. The ratio of IP₃R2 to ryanodine receptor 2 (RyR2) and the fraction of RyR2 phosphorylated at the CaMKII site increased significantly during development of heart failure in WT mice, but not KO mice, and this was associated with enhanced Ca²⁺ spark frequency only in WT mice. We suggest that CaMKIIδ contributes to cardiac decompensation by enhancing RyR2-mediated sarcoplasmic reticulum Ca²⁺ leak and that attenuating CaMKIIδ activation can limit the progression to heart failure.

In the heart, autophagy has been implicated in cardioprotection and ischemia-reperfusion tolerance, and the dysregulation of autophagy is associated with the development of heart failure. Mitochondrial dynamic proteins are profoundly involved in autophagic processes, especially the initiation and formation of autophagosomes, but it is not clear whether they play any role in cardiac autophagy. We previously reported that mitofusin 2 (MFN2), a mitochondrial outer membrane protein, serves as a major determinant of cardiomyocyte apoptosis mediated by oxidative stress. Here, we reveal a novel and essential role of MFN2 in mediating cardiac autophagy. We found that specific deletion of MFN2 in cardiomyocytes caused extensive accumulation of autophagosomes. In particular, the fusion of autophagosomes with lysosomes, a critical step in autophagic degradation, was markedly retarded without altering the formation of autophagosomes and lysosomes in response to ischemia-reperfusion stress. Importantly, MFN2 co-immunoprecipitated with RAB7 in the heart, and starvation further increased it. Knockdown of MFN2 by shRNA prevented, whereas re-expression of MFN2 restored, the autophagosome-lysosome fusion in neonatal cardiomyocytes. Hearts from cardiac-specific MFN2 knock-out mice had abnormal mitochondrial and cellular metabolism and were vulnerable to ischemia-reperfusion challenge. Our study defined a novel and essential role of MFN2 in the cardiac autophagic process by mediating the maturation of autophagy at the phase of autophagosome-lysosome fusion; deficiency of MFN2 caused multiple molecular and functional defects that undermined cardiac reserve and gradually led to cardiac vulnerability and dysfunction.

Oxidative stress contributes to the pathogenesis of acute lung injury. Protein S-glutathionylation plays an important role in cellular antioxidant defense. Here we report that the expression of deglutathionylation enzyme Grx1 is decreased in the lungs of acute lung injury mice. The acute lung injury induced by hyperoxia or LPS is significantly relieved in Grx1 KO and Grx1^fl/flLysM^cremice, confirming the protective role of Grx1-regulated S-glutathionylation in macrophages. Using a quantitative redox proteomics approach, we show that FABP5 is susceptible to S-glutathionylation under oxidative conditions. S-glutathionylation of Cys127 in FABP5 promotes its fatty acid binding ability and nuclear translocation. Further results indicate S-glutathionylation promotes the interaction of FABP5 and PPARβ/δ, activates PPARβ/δ target genes and suppresses the LPS-induced inflammation in macrophages. Our study reveals a molecular mechanism through which FABP5 S-glutathionylation regulates macrophage inflammation in the pathogenesis of acute lung injury.

Background—: β-Adrenergic receptor (β-AR) activation can provoke cardiac arrhythmias mediated by cAMP-dependent alterations of Ca ²⁺ signaling. However, cAMP can activate both protein kinase A and an exchange protein directly activated by cAMP (Epac), but their functional interaction is unclear. In heart, selective Epac activation can induce potentially arrhythmogenic sarcoplasmic reticulum (SR) Ca ²⁺ release that involves Ca ²⁺ /calmodulin-dependent protein kinase II (CaMKII) effects on the ryanodine receptor (RyR). Methods and Results—: We tested whether physiological β-AR activation causes Epac-mediated SR Ca ²⁺ leak and arrhythmias and whether it requires Epac1 versus Epac2, β ₁ -AR versus β ₂ -AR, and CaMKIIδ-dependent phosphorylation of RyR2-S2814. We used knockout (KO) mice for Epac1, Epac2, or both. All KOs exhibited unaltered basal cardiac function, Ca ²⁺ handling, and hypertrophy in response to pressure overload. However, SR Ca ²⁺ leak induced by the specific Epac activator 8-CPT in wild-type mice was abolished in Epac2-KO and double-KO mice but was unaltered in Epac1-KO mice. β-AR–induced arrhythmias were also less inducible in Epac2-KO versus wild-type mice. β-AR activation with protein kinase A inhibition mimicked 8-CPT effects on SR Ca ²⁺ leak and was prevented by blockade of β ₁ -AR but not β ₂ -AR. CaMKII inhibition (KN93) and genetic ablation of either CaMKIIδ or CaMKII phosphorylation on RyR2-S2814 prevented 8-CPT–induced SR Ca ²⁺ leak. Conclusions—: β ₁ -AR activates Epac2 to induce SR Ca ²⁺ leak via CaMKIIδ-dependent phosphorylation of RyR2-S2814. This pathway contributes to β-AR–induced arrhythmias and reduced cardiac function.

Human mutations in or variants of TBX20 are associated with congenital heart disease, cardiomyopathy, and arrhythmias. To investigate whether cardiac disease in patients with these conditions results from an embryonic or ongoing requirement for Tbx20 in myocardium, we ablated Tbx20 specifically in adult cardiomyocytes in mice. This ablation resulted in the onset of severe cardiomyopathy accompanied by arrhythmias, with death ensuing within 1 to 2 weeks of Tbx20 ablation. Accounting for this dramatic phenotype, we identified molecular signatures that posit Tbx20 as a central integrator of a genetic program that maintains cardiomyocyte function in the adult heart. Expression of a number of genes encoding critical transcription factors, ion channels, and cytoskeletal/myofibrillar proteins was downregulated consequent to loss of Tbx20. Genome-wide ChIP analysis of Tbx20-binding regions in the adult heart revealed that many of these genes were direct downstream targets of Tbx20 and uncovered a previously undescribed DNA-binding site for Tbx20. Bioinformatics and in vivo functional analyses revealed a cohort of transcription factors that, working with Tbx20, integrated multiple environmental signals to maintain ion channel gene expression in the adult heart. Our data provide insight into the mechanisms by which mutations in TBX20 cause adult heart disease in humans.