Biology subjects

Rebbeck, R. T.

Publications and source records attributed to Rebbeck, R. T..

8 recordsLinked to original sources

bioRxiv2026

Dynamic-Structure Redesign of Calmodulin Reveals Mechanistic Constraints on Ryr2 Regulation

2.Calmodulin (CaM) is a highly conserved Ca2+ sensor that regulates hundreds of cellular targets through Ca2+ -dependent conformational dynamics. Despite its central role in Ca2+ signaling and disease, its evolutionary conservation and structural flexibility have suggested that CaM is resistant to rational redesign. Here, using the cardiac Ca2+ release channel Ryanodine receptor 2 (RyR2) as a model system, we tested whether incorporating conformational dynamics into computational protein design enables functional reengineering of CaM. We first applied a static structure-based redesign to increase CaM-RyR2 affinity. Although the resulting variant bound more tightly to both the RyR2 peptide and the intact channel in vitro, it distorted peptide geometry and worsened Ca2+ leak in cardiomyocytes ex vivo. Guided by molecular dynamics simulations, we then developed a dynamic-structure redesign strategy that preserves conformational integrity while strengthening binding. The resulting CaM variant exhibited increased RyR2 affinity and reduced pathological Ca2+ leak in a disease-relevant model. These findings show that improved binding affinity alone is insufficient to enhance physiological regulation and that successful CaM redesign requires preservation of conformational dynamics. More broadly, they demonstrate that integrating conformational dynamics into protein redesign can enable functionally predictive engineering of flexible regulatory protein-protein interactions.

biophysics↗

bioRxiv2023

Kinetics and Mapping of Ca-driven Calmodulin conformations on Skeletal and Cardiac Muscle Ryanodine Receptors

Calmodulin (CaM) transduces [Ca2+] information regulating the rhythmic Ca2+ cycling between the sarcoplasmic reticulum and cytoplasm during relaxation and contraction in cardiac and skeletal muscle. However, the structural dynamics by which CaM modulates the SR Ca2+ release channel (ryanodine receptor, RyR) at physiologically relevant [Ca2+] is unknown. Using fluorescence lifetime detection of FRET between RyR-bound FKBP and CaM, we resolved different structural states of CaM and Ca-driven shifts in the conformation of CaM bound to RyR. We found that CaM becomes more compact in contracting high-Ca2+ vs. relaxing low-Ca2+. Skeletal and cardiac RyR isoforms show different CaM-RyR conformations, and binding and structural kinetics reflect functional roles. Furthermore, our FRET methods provide insight representative of physiological CaM-RyR structure, in synergy and comparison with cryo-EM models, which result from more disrupted samples. This technology will drive future studies focusing on pathologic CaM-RyR interactions, and on RyR dynamics with other important modulators.

biophysics↗

bioRxiv2023

Molecular Mechanism of a FRET Biosensor for the Cardiac Ryanodine Receptor Pathologically Leaky State

Ca2+ leak from cardiomyocyte sarcoplasmic reticulum (SR) via hyperactive resting cardiac ryanodine receptor channels (RyR2) is pro-arrhythmic. An exogenous peptide, (DPc10) detects leaky RyR2 in cardiomyocytes. Conversely, calmodulin (CaM) inhibits RyR2 leak. These observations have led to designing a FRET biosensor for drug discovery targeting RyR2. Here we used FRET to understand the molecular mechanism driving the DPc10-CaM interdependence when binding RyR2 in SR vesicles. We used donor-FKBP12.6 (D-FKBP) to resolve RyR2 binding of acceptor-CaM (A-CaM). In low nanomolar Ca2+, DPc10 decreased both FRETmax (under saturating [A-CaM]) and the CaM/RyR2 binding affinity. In micromolar Ca2+, DPc10 decreased FRETmax without affecting CaM/RyR2 binding affinity. This correlates with analysis of fluorescence-lifetime-detected FRET indicating that DPc10 lowers occupancy of the RyR2 CaM-binding sites in nanomolar (not micromolar) Ca2+ and lengthens D-FKBP/A-CaM distances independent of [Ca2+]. To observe DPc10/RyR2 binding, we used acceptor-DPc10 (A-DPc10). CaM weakens A-DPc10/RyR2 binding, this effect being larger in micromolar vs. nanomolar Ca2+. Moreover, A-DPc10/RyR2 binding is cooperative in CaM- and FKBP-dependent manner, suggesting that both endogenous modulators promote concerted structural changes between RyR2 protomers for channel regulation. Aided by analysis of cryo-EM structures, these insights inform further development of the DPc10-CaM paradigm for therapeutic discovery targeting RyR2.

biophysics↗

bioRxiv2023

RyR2 Binding of an Antiarrhythmic Cyclic Depsipeptide Mapped Using Confocal Fluorescence Lifetime Detection of FRET

Hyperactivity of cardiac sarcoplasmic reticulum (SR) ryanodine receptor (RyR2) Ca2+-release channels contributes to heart failure and arrhythmias. Reducing RyR2 activity, particularly during cardiac relaxation (diastole), is a desirable therapeutic goal. We previously reported that the unnatural enantiomer (ent) of an insect-RyR activator, verticilide, inhibits porcine and mouse RyR2 at diastolic (nanomolar) Ca2+ and has in vivo efficacy against atrial and ventricular arrhythmia. To determine the ent-verticilide structural mode of action on RyR2 and guide its further development via medicinal chemistry structure-activity relationship studies, here we used fluorescence lifetime (FLT)-measurements of Forster resonance energy transfer (FRET) in HEK293 cells expressing human RyR2. For these studies, we used an RyR-specific FRET molecular-toolkit and computational methods for trilateration (i.e., using distances to locate a point of interest). Multi-exponential analysis of FLT-FRET measurements between four donor-labeled FKBP12.6 variants and acceptor-labeled ent-verticilide, yielded distance relationships placing the acceptor probe at two candidate loci within the RyR2 cryo-EM map. One locus is within the Ry12 domain (at the corner periphery of the RyR2 tetrameric complex). The other locus is sandwiched at the interface between helical domain 1 and the SPRY3 domain. These findings document RyR2-target engagement by ent-verticilide, reveal new insight into the mechanism of action of this new class of RyR2-targeting drug candidate, and can serve as input in future computational determinations of the ent-verticilide binding site on RyR2 that will inform structure-activity studies for lead optimization.

biophysics↗

bioRxiv2023

Cardiomyopathy-Associated Variants Alter the Structure and Function of the α-Actinin-2 Actin-Binding Domain

Hypertrophic cardiomyopathy (HCM), dilated cardiomyopathy (DCM), and restrictive cardiomyopathy (RCM) are characterized by thickening, thinning, or stiffening, respectively, of the ventricular myocardium, resulting in diastolic or systolic dysfunction that can lead to heart failure and sudden cardiac death. Recently, variants in the ACTN2 gene, encoding the protein -actinin-2, have been reported in HCM, DCM, and RCM patients. However, functional data supporting the pathogenicity of these variants is limited, and potential mechanisms by which these variants cause disease are largely unexplored. Currently, NIH ClinVar lists 34 ACTN2 missense variants, identified in cardiomyopathy patients, which we predict are likely to disrupt actin binding, based on their localization to specific substructures in the -actinin-2 actin binding domain (ABD). We investigated the molecular consequences of three ABD localized, HCM-associated variants: A119T, M228T and T247M. Using circular dichroism, we demonstrate that the mutant ABD proteins can attain a well-folded state. However, thermal denaturation studies show that all three mutations are destabilizing, suggesting a structural disruption. Importantly, A119T decreased actin binding, and M228T and T247M cause increased actin binding. We suggest that altered actin binding underlies pathogenesis for cardiomyopathy mutations localizing to the ABD of -actinin-2.

biochemistry↗

bioRxiv2023

FRET assay for live-cell high-throughput screening of the cardiac SERCA pump yields multiple classes of small-molecule allosteric modulators

We have used FRET-based biosensors in live cells, in a robust high-throughput screening (HTS) platform, to identify small-molecules that alter the structure and activity of the cardiac sarco/endoplasmic reticulum calcium ATPase (SERCA2a). Our primary aim is to discover drug-like small-molecule activators that improve SERCAs function for the treatment of heart failure. We have previously demonstrated the use of an intramolecular FRET biosensor, based on human SERCA2a, by screening a small validation library using novel microplate readers that can detect the fluorescence lifetime or emission spectrum with high speed, precision, and resolution. Here we report results from a 50,000-compound screen using the same biosensor, with hit compounds functionally evaluated using Ca2+-ATPase and Ca2+-transport assays. We focused on 18 hit compounds, from which we identified eight structurally unique compounds and four compound classes as SERCA modulators, approximately half of which are activators and half are inhibitors. While both activators and inhibitors have therapeutic potential, the activators establish the basis for future testing in heart disease models and lead development, toward pharmaceutical therapy for heart failure.

biophysics↗

bioRxiv2022

Identification of β-III-spectrin actin-binding modulators for treatment of spinocerebellar ataxia

{beta}-III-spectrin is a key cytoskeletal protein that localizes to the soma and dendrites of cerebellar Purkinje cells, and is required for dendritic arborization and signaling. A spinocerebellar ataxia type 5 (SCA5) L253P mutation in the cytoskeletal protein {beta}-III-spectrin causes high-affinity actin binding. Previously we reported a cell-based fluorescence assay for identification of small molecule actin-binding modulators of the L253P mutant {beta}-III-spectrin. Here we describe a complementary, in vitro, fluorescence resonance energy transfer (FRET) assay that uses purified L253P {beta}-III-spectrin actin-binding domain (ABD) and F-actin. To validate the assay, we screened a 2,684-compound library of FDA-approved drugs. Importantly, the screening identified numerous compounds that decreased FRET between fluorescently labeled L253P ABD and F-actin. The activity and target of multiple Hit compounds were confirmed in orthologous co-sedimentation actin-binding assays. Through future medicinal chemistry, the Hit compounds can potentially be developed into a SCA5-specific therapeutic. Furthermore, our validated FRET-based in vitro HTS platform is poised for screening large compound libraries for {beta}-III-spectrin ABD modulators.

biophysics↗

bioRxiv2021

Cardiac RyR N-terminal region biosensors for FRET-based high-throughput screening

The N-terminal region (NTR) of the ryanodine receptor (RyR) calcium channels is critical to the regulation of Ca2+ release during excitation-contraction coupling. NTR hosts numerous mutations linked to skeletal and cardiac myopathies (RyR1 and RyR2, respectively), highlighting its potential as therapeutic target. Here, we labeled the NTR of mouse RyR2 at subdomains A, B, and C with donor and acceptor pairs for fluorescence resonance energy transfer (FRET), obtaining two biosensors. Using fluorescence lifetime (FLT)-detection of intramolecular FRET, we developed high-throughput screening (HTS) assays with the biosensors to identify small-molecule modulators of RyR. We screened a 1280-compound validation library and identified several hits. Hits with saturable FRET dose-response profiles, and previously unreported effects on RyR activity, were further tested using [3H]ryanodine binding to isolated sarcoplasmic reticulum vesicles, to measure their effects on full-length RyR opening in its natural membrane environment. We identified three novel inhibitors of both RyR1 and RyR2, and two RyR1-selective inhibitors at nanomolar Ca2+. These compounds may function as inhibitors of leaky RyRs in muscle. Two of these hits activated RyR1 only at micromolar Ca2+, highlighting them as potential activators of excitation-contraction coupling. These results indicate that large-scale HTS using this platform can lead to compounds with potential for therapeutic development.

biophysics↗