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.

Rationale Junctional membrane complexes (JMC) 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 ‘striated muscle preferentially expressed gene’ (SPEG) remains unclear. Objective To determine the role of SPEG in healthy and failing adult hearts. Methods and Results Proteomic analysis of immunoprecipatated JMC-proteins ryanodine receptor type-2 (RyR2) 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 PCR revealed 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. Calcium (Ca2+) and transverse-tubule (TT) imaging of ventricular myocytes from MCM-Spegfl/fl mice post heart failure revealed both increased SR Ca2+ spark frequency and disrupted JMC integrity. Additional studies revealed that TT disruption precedes the development of heart failure 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 TT disruption, a precursor of heart failure development in SPEG deficient mice. Conclusion The novel JMC protein SPEG is downregulated in human failing hearts. Acute loss of SPEG in mouse hearts causes JPH2 dephosphorylation and TT loss associated with downstream Ca2+ mishandling leading to heart failure. Our study suggests that SPEG could be a novel target for the treatment of heart failure.

Background Junctophilin-2 (JPH2) is the primary structural protein for coupling of transverse (T)-tubule associated cardiac L-type Ca channels and type-2 ryanodine receptors on the sarcoplasmic reticulum within junctional membrane complexes 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.

Together, these results indicate that reducing SIT work-interval duration from 30 to 15 s had no impact on training-induced increases in aerobic or anaerobic power, or on increases in lactate threshold (absolute) and critical power.

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.