Sarcoplasmic reticulum calcium defect in Ras-induced hypertrophic cardiomyopathy heart

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.

Mutations in JPH2-encoded junctophilin-2 associated with hypertrophic cardiomyopathy in humans

“…There has been significant interest in understanding the molecular and cellular adaptations leading to hypertrophy caused by HCM, hypertension, and valvular disease. Several studies have shown that although the inward calcium current (I Ca ) through the L-type calcium channel is not altered in hypertrophic or failing rat and murine cardiomyocytes, there is a decrease in the intracellular calcium transient, [Ca] I [33,34]. In post-myocardial infarction cardiomyocytes, this normal I Ca and decreased intracellular calcium transient, [Ca] i , was shown to be independent of SR calcium concentration [35].…”

Clinical, pathological, and molecular analyses of cardiovascular abnormalities in Costello syndrome: A Ras/MAPK pathway syndrome

“…The notion that HCM in Costello syndrome is driven by Ras/MAPK activation is supported by the recent description of a transgenic mouse model where targeted expression of an HRas-G12V mutant has Costello syndrome features and develops significant cardiac hypertrophy [Zheng et al, 2004]. A zebrafish model of Costello syndrome expressing oncogenic H-RASV12 also had thicker heart walls [Santoriello et al, 2009], but the observed defects in early heart morphogenesis cannot be directly compared to humans with Costello syndrome (most of whom have the p.G12S mutation).…”

Signaling to Cardiac Hypertrophy: Insights from Human and Mouse RASopathies

“…Recent work has suggested that the selective induction of Gαi in the RAS transgenic heart contributes to impaired sarcoplasmic reticulum calcium cycling (97), and a number of other studies have led to descriptions of RAS-induced alterations in calcium transients (98)(99)(100). Interestingly, abnormalities in delicate calcium homeostasis can trigger cardiac hypertrophy through mechanisms that have not been fully elucidated (101).…”