Priori, S. G. | Search bioRxiv

bioRxiv2026

Cardiac Calsequestrin is a Physiological Dimer that Polymerizes through a Ca2+-Triggered Cooperative Switch

Cardiac Calsequestrin (CASQ2) polymerizes within the junctional sarcoplasmic reticulum to buffer Ca{superscript 2} and regulate ryanodine receptor 2 (RyR2) gating, yet the molecular mechanism governing this process remains poorly understood. Using an integrated set of complementary approaches spanning single-particle biophysics, bulk solution measurements, and polymer chemistry, we demonstrate that CASQ2 is an intrinsic dimer at nanomolar concentrations and under physiological ionic conditions, independently of Ca{superscript 2}. In addition, Ca{superscript 2}-dependent polymerization operates as a highly cooperative switch between a stable oligomeric phase and a high-order polymeric state. Physiological amounts of K ions modulate this switch through a biphasic electrostatic mechanism, supporting polymerization at low concentrations and inhibiting it beyond charge neutralization ([~]194 mM). These findings redefine CASQ2 as an intrinsic dimer with polymerization-switch properties, and provide a mechanistic framework for understanding how catecholaminergic polymorphic ventricular tachycardia type 2 mutations, distributed evenly across the CASQ2 surface, cause disease through two distinct pathological trajectories.

biochemistry↗

bioRxiv2024

Molecular Insights into the Rescue Mechanism of an hERG Activator Against Severe LQT2 Mutations.

BackgroundMutations in the HERG potassium channel are a major cause of long QT syndrome type 2 (LQT2), which can lead to sudden cardiac death. The HERG channel plays a critical role in the repolarization of the myocardial action potential, and loss-of-function mutations prolong cardiac repolarization. MethodsIn this study, we investigated the efficacy and underlying molecular mechanism of ICA-105574, an HERG activator, in shortening the duration of cardiac repolarization in severe LQT2 variants. We characterized the efficacy of ICA-105574 in vivo, using an animal model to assess its ability to shorten the QT interval and in vitro, in cellular models mimicking severe HERG channel mutations (A561V, G628S, and L779P) to evaluate its impact in enhancing IKr current. Additionally, molecular dynamics simulations were used to investigate the molecular mechanism of ICA-105574 action. ResultsIn vivo, ICA-105574 significantly shortened the QT interval. LQT2 mutations drastically reduced IKr amplitude and suppressed tail currents in cellular models. ICA-105574 restored IKr in A561V and G628S. Finally, in silico data showed that ICA-105574 stabilizes a pattern of interactions similar to gain-of-function SQT1 mutations and can reverse the G628S modifications, through an allosteric network linking the binding site to the selectivity filter and the S5P turret helix, thereby restoring its K+ ion permeability. ConclusionsOur results support the development of HERG activators like ICA-105574 as promising pharmacological molecules against some severe LQT2 mutations and suggest that molecular dynamics simulations can be used to test the ability of molecules to modulate HERG function in silico, paving the way for the rational design of new HERG activators.

biophysics↗