Genetic deletion of phosphoinositide 3-kinase (PI3Kγ) in mice (PI3Kγ -/- ) show increased heart rate and ventricular contraction. It is unknown whether PI3Kγ regulates the contraction of isolated cardiomyocytes and calcium recycling machinery underlying this phenotype. Primary adult cardiomyocytes from PI3Kγ -/- mice show altered sarcoendoplasmic reticulum (SR) calcium cycling following caffeine. Unexpectedly, significant reduction in phosphorylation of phospholamban (PLN) at Thr17 was observed in PI3Kγ -/- cardiomyocytes without changes in phosphorylation at Ser16, which is a key regulator of SR calcium re-uptake. Furthermore, loss in PLN phosphorylation in PI3Kγ -/- cardiomyocytes was associated with augmented interaction of PLN with SR calcium ATPase (SERCA). Surprisingly, cardiomyocyte-specific over-expression of kinase-dead PI3Kγ (PI3Kγ inact ) in global PI3Kγ -/- mice (PI3Kγ inact /PI3Kγ -/- ) normalized caffeine-induced calcium re-uptake, PLN phosphorylation at Thr17 and decreased PLN-SERCA interaction. These data suggested kinase-independent function of PI3Kγ in regulation of SR calcium load and PLN phosphorylation. Since phosphorylation of Thr17 of PLN is carried out by Ca 2+ /Calmodulin dependent protein kinase (CamKII), we probed for the role of PI3Kγ in regulation of CamKII in PLN phosphorylation. Mechanistically, PI3Kγ exhibits scaffolding function in recruitment of CamKII to PLN at SR to mediate PLN phosphorylation. Furthermore, we showed that PI3Kγ directly interacts with CamKII. The study unravels a yetto be recognized kinase-independent role of PI3Kγ in regulating PLN with implications in cardiac function.
Mohan et al. (Fri,) studied this question.
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