Biophysical Society Thematic Meeting | Canterbury 2023

Towards a More Perfect Union: Multi-Scale Models of Muscle and Their Experimental Validation

Monday Speaker Abstracts

CA 2+ DYNAMICS IN CARDIAC PATHOLOGICAL MODELS Ana Maria Gomez INSERM, France No Abstract

KBTBD13 R408C -KNOCKIN MOUSE MODEL REVEALS IMPAIRED RELAXATION KINETICS AS NOVEL PATHOMECHANISM FOR NEM6 CARDIOMYOPATHY Rianne J Baelde 1 ; Valentijn J Janssen 1 ; Alexcia Fortes-Monteiro 1 ; Ricardo A Galli 1 ; Sylvia Bogaards 1 ; Mei Methawasin 2 ; Diederik Kuster 1 ; Jolanda van der Velden 1 ; Coen Ottenheijm 1,2 ; Josine M de Winter 1 ; 1 Amsterdam UMC, location VUmc, Physiology, Amsterdam, The Netherlands 2 University of Arizona, Dept. of Cellular and Molecular Medicine, Tucson, AZ, USA A prime example of the impact of impaired relaxation kinetics is nemaline myopathy caused by variants in KBTBD13 (NEM6) encoding kelch repeat and BTB (POZ) domain containing 13. In addition to weakness, NEM6 patients have slowed muscle relaxation, compromising contractility and daily-life activities. The majority of NEM6 patients harbors the Dutch founder variant, c.1222C>T, p.Arg408Cys (KBTBD13 R408C ). Recently, we discovered that the Dutch founder variant not only affects skeletal muscle function, but also the heart. Patients display cardiac abnormalities including systolic dysfunction, diastolic dysfunction, atrial fibrillation, ventricular tachycardia and repolarization disturbances. Our studies on skeletal muscle provided insight in the sarcomere-based pathomechanism in NEM6: KBTBD13 R408C causes structural changes in the actin-based thin filament, increasing thin filament stiffness and impairing relaxation kinetics. To provide insight in the mechanism underlying cardiac dysfunction in NEM6, here we assessed cardiac structure and function in Kbtbd13 R408C -knockin mice, which closely recapitulate the skeletal muscle phenotype. Pressure-volume loop analyses showed that the end-systolic pressure-volume relation was unaffected, but the end-diastolic pressure-volume relation was steeper in Kbtbd13 R408C -knockin mice, indicating diastolic dysfunction. Histological evaluation of cardiac structure revealed no changes in fibrosis in Kbtbd13R408C-knockin mice. Also, no changes in titin isoform composition were found that could account for the increased diastolic stiffness. Next, we studied the contraction and relaxation kinetics at the intact cardiomyocyte level by a high-throughput contractility set-up: Percentage of shortening was not affected, however Time to baseline was increased in Kbtbd13 R408C -knockin mice, indicating impaired relaxation kinetics. In parallel, calcium-handling was assessed by calcium indicator Fura-2AM. These studies reveal that calcium-release is not affected, but calcium-reuptake is impaired in Kbtbd13 R408C -knockin mice, which might contribute to the impaired relaxation kinetics. Current studies focus on how KBTBD13R408C affects calcium-reuptake kinetics and sarcomere kinetics in NEM6 cardiomyocytes. Hence, our studies provide the first insights in the pathomechanism underlying cardiac dysfunction in NEM6.

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