Cardiac hypertrophy in mice expressing unphosphorylatable phospholemman

Abstract

AimsElevation of intracellular Na in the failing myocardium contributes to contractile dysfunction, the negative force–frequency relationship, and arrhythmias. Although phospholemman (PLM) is recognized to form the link between signalling pathways and Na/K pump activity, the possibility that defects in its regulation contribute to elevation of intracellular Na has not been investigated. Our aim was to test the hypothesis that the prevention of PLM phosphorylation in a PLM3SA knock-in mouse (in which PLM has been rendered unphosphorylatable) will exacerbate cardiac hypertrophy and cellular Na overload. Testing this hypothesis should determine whether changes in PLM phosphorylation are simply bystander effects or are causally involved in disease progression.Methods and resultsIn wild-type (WT) mice, aortic constriction resulted in hypophosphorylation of PLM with no change in Na/K pump expression. This under-phosphorylation of PLM occurred at 3 days post-banding and was associated with a progressive decline in Na/K pump current and elevation of [Na]i. Echocardiography, morphometry, and pressure-volume (PV) catheterization confirmed remodelling, dilation, and contractile dysfunction, respectively. In PLM3SA mice, expression of Na/K ATPase was increased and PLM decreased such that net Na/K pump current under quiescent conditions was unchanged (cf. WT myocytes); [Na+]i was increased and forward-mode Na/Ca exchanger was reduced in paced PLM3SA myocytes. Cardiac hypertrophy and Na/K pump inhibition were significantly exacerbated in banded PLM3SA mice compared with banded WT.ConclusionsDecreased phosphorylation of PLM reduces Na/K pump activity and exacerbates Na overload, contractile dysfunction, and adverse remodelling following aortic constriction in mice. This suggests a novel therapeutic target for the treatment of heart failure.

Na+/Ca2+ exchange and Na+/K+‐ATPase in the heart

“…Recently, Boguslavskyi et al . () and Correll et al . () have reported a reduction in both PLM expression and phosphorylation in mouse models of hypertrophy that, in the Boguslavskyi study, has been directly related to declining Na + /K + pump function and intracellular Na + elevation.…”

“…, b ; Boguslavskyi et al . ). Specifically in cardiac hypertrophy many studies have shown that Na + /K + pump function, and/or expression, is reduced (Pogwizd et al .…”

“…have used a mutant PLM 3SA knock‐in mouse (in which PLM has been rendered unphosphorylatable) (Boguslavskyi et al . ). They showed that preventing PLM phosphorylation in mice subjected to aortic constriction further inhibits NKA function, increases cellular Na + overload and exacerbates adverse hypertrophic remodelling.…”

“…Adaptive increases in α2 expression are also seen in mice expressing an unphosphorylatable form of PLM (PLM 3SA ), perhaps allowing the normalization of EC coupling despite an increase in the bulk cytoplasmic Na + concentration (Boguslavskyi et al . ).…”

Substrate recognition by the cell surface palmitoyl transferase DHHC5

“…DHHC5 therefore inhibits the pump through both palmitoylation of PLM and the MEND pathway. The convergence of multiple signaling pathways on PLM highlights the key role of this regulatory protein in control of intracellular Na in cardiac myocytes (30). Intracellular Na influences a vast array of cellular processes in the heart (12), including contractility via NCX and mitochondrial energetics (31).…”