BPS2026 – Toxic competitors of phospholamban contribute to dysregulation of SERCA in dilated cardiomyopathy

Sarco(endo)plasmic reticulum calcium-ATPase (SERCA) activity is reduced in heart failure, leading to impaired cardiac relaxation and poor contractility. Previously, it was thought that this was due to a decrease in SERCA expression, but more recent studies suggested SERCA protein levels were unchanged in heart failure. Here, we explored other possible mechanisms for the observed decrease in SERCA activity, focusing on SERCA regulatory interactions. The promiscuity of the SERCA regulatory binding site led us to hypothesize that undiscovered toxic species may mis-regulate SERCA in heart failure. In particular, we noted that proteolytic degradation is greatly elevated in heart failure, producing an abundance of hydrophobic transmembrane peptides. We hypothesized that some of these degradation products may adversely affect SERCA function or disrupt native regulation. To test this hypothesis, membrane fractions were isolated from hearts of patients with dilated cardiomyopathy (DCM) and compared to non-failing controls. SDS-PAGE and mass spectrometry (MS) revealed candidate transmembrane fragments which were screened for SERCA-binding and inhibition using FRET microscopy and activity assays. Fragments of four membrane proteins (SEC61β, SLMAP, VAMP3, and VAMP8) showed avid interactions with SERCA, reduced ER calcium, and decreased ATPase activity, suggesting they could be a cause of decreased SERCA activity observed in heart failure. Notably, SEC61β and SLMAP displayed an even stronger inhibitory effect than the native regulatory partner PLB. Quantitative MS showed SEC61β and SLMAP are present in comparable levels to PLB, suggesting they may compete for SERCA’s binding site and disrupt native SERCA regulation. Overall, the results suggest a novel pathophysiological mechanism that may account for the impaired calcium handling that is observed in heart failure.