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Sarcoplasmic reticulum calcium defect in Ras-induced hypertrophic cardiomyopathy heart
Meizi Zheng1, Keith Dilly, Jader Dos Santos Cruz
1Dept. of Physiology, Univ. of Maryland, Baltimore, MD 21201, USA.
Abstract:
The small G protein Ras-mediated signaling pathway has been implicated in the development of hypertrophy and diastolic dysfunction in the heart. Earlier cellular studies have suggested that the Ras pathway is responsible for reduced L-type calcium channel current and sarcoplasmic reticulum (SR) calcium uptake associated with sarcomere disorganization in neonatal cardiomyocytes. In the present study, we investigated the in vivo effects of Ras activation on cellular calcium handling and sarcomere organization in adult ventricular myocytes using a newly established transgenic mouse model with targeted expression of the H-Ras-v12 mutant. The transgenic hearts expressing activated Ras developed significant hypertrophy and postnatal lethal heart failure. In adult ventricular myocytes isolated from the transgenic hearts, the calcium transient was significantly depressed but membrane L-type calcium current was unchanged compared with control littermates. The expressions of sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA)2a and phospholamban (PLB) were significantly reduced at mRNA levels. The amount of SERCA2a protein was also modestly reduced. However, the expression of PLB protein and gross sarcomere organization remained unchanged in the hypertrophic Ras hearts, whereas Ser(16) phosphorylation of PLB was dramatically inhibited in the Ras transgenic hearts compared with controls. Hypophosphorylation of PLB was also associated with a significant induction of protein phosphatase 1 expression. Therefore, our results from this in vivo model system suggest that Ras-induced contractile defects do not involve decreased L-type calcium channel activities or disruption of sarcomere structure. Rather, suppressed SR calcium uptake due to reduced SERCA2a expression and hypophosphorylation of PLB due to changes in protein phosphatase expression may play important roles in the diastolic dysfunction of Ras-mediated hypertrophic cardiomyopathy.
Insights
Ras activation in the heart causes hypertrophy and heart failure by impairing calcium handling. Suppressed calcium uptake and phospholamban hypophosphorylation contribute to diastolic dysfunction in Ras-mediated cardiomyopathy.
Area of Science:
- Cardiovascular Biology
- Molecular Cardiology
- Cellular Signaling
Background:
- The Ras signaling pathway is linked to cardiac hypertrophy and diastolic dysfunction.
- Previous studies suggested Ras affects calcium handling and sarcomere organization in neonatal cardiomyocytes.
Purpose of the Study:
- To investigate the in vivo effects of Ras activation on calcium handling and sarcomere organization in adult ventricular myocytes.
- To elucidate the mechanisms of Ras-mediated hypertrophic cardiomyopathy using a novel transgenic mouse model.
Main Methods:
- Generated a transgenic mouse model expressing activated H-Ras-v12 mutant.
- Isolated adult ventricular myocytes for calcium transient and L-type calcium current measurements.
- Analyzed mRNA and protein levels of SERCA2a, phospholamban (PLB), and protein phosphatase 1.
Main Results:
- Transgenic hearts showed significant hypertrophy and heart failure.
- Adult ventricular myocytes exhibited depressed calcium transients but unchanged L-type calcium current.
- Reduced SERCA2a mRNA and protein, and inhibited PLB phosphorylation were observed.
- PLB protein expression and sarcomere organization were unchanged, but protein phosphatase 1 expression was induced.
Conclusions:
- Ras-induced contractile defects in vivo do not involve altered L-type calcium channel activity or sarcomere disruption.
- Suppressed SR calcium uptake due to reduced SERCA2a expression and PLB hypophosphorylation are key contributors to diastolic dysfunction in Ras-mediated hypertrophic cardiomyopathy.