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

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. M 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- Speg fl/fl ) model revealed that adult-onset SPEG-deficiency results in heart failure. Calcium (Ca 2+ ) and transverse-tubule (TT) imaging of ventricular myocytes from MCM- Speg fl/fl mice post heart failure revealed both increased SR Ca 2+ spark frequency and disrupted JMC integrity. Additional studies revealed that TT disruption precedes the development of heart failure development in MCM- Speg fl/fl mice. Although total JPH2 levels were unaltered, JPH2 phosphorylation levels were found to be reduced in MCM- Speg fl/fl mice, suggesting that loss of SPEG phosphorylation of JPH2 led to TT disruption, a precursor of heart failure 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 TT loss associated with downstream Ca 2+ mishandling leading to heart failure. Our study suggests that SPEG could be a novel target for the treatment of heart failure.