Experimentally Increasing Titin Compliance in a Novel Mouse Model Attenuates the Frank-Starling Mechanism But Has a Beneficial Effect on Diastole

Abstract

Background Experimentally upregulating compliant titins has been suggested as a therapeutic for lowering pathological diastolic stiffness levels. However, how increasing titin compliance impacts global cardiac function requires in-depth study. We investigate the effect of upregulating compliant titins in a novel mouse model with a genetically altered titin splicing factor; integrative approaches were used from intact cardiomyocyte mechanics to pressure(P)-volume(V) analysis and Doppler echocardiography. Methods and Results Compliant titins were upregulated through deletion of the RNA Recognition Motif of the splicing factor RBM20 (Rbm20ΔRRM mice). A genome-wide exon expression analysis and a candidate approach revealed that the phenotype is likely to be dominated by greatly increased lengths of titin’s spring-elements. At both cardiomyocyte and left ventricular (LV)chamber levels diastolic stiffness was reduced in heterozygous (+/−) Rbm20ΔRRM mice with a further reduction in homozygous (−/−) mice at only the intact myocyte level. Fibrosis was present in only −/− Rbm20ΔRRM hearts. The Frank-Starling Mechanism was reduced in a graded fashion in Rbm20ΔRRM mice, at both the cardiomyocyte and LV chamber levels. Exercise tests revealed an increase in exercise capacity in +/− mice. Conclusions Titin is not only important in diastolic but also in systolic cardiac function. Upregulating compliant titins reduces diastolic chamber stiffness due to increased compliance of myocytes but depresses end-systolic elastance; under conditions of exercise the beneficial effects on diastolic function dominate. Therapeutic manipulation of the RBM20-based splicing system might be able to minimize effects on fibrosis and systolic function while improving diastolic function of heart failure patients.

Titin‐based mechanosensing modulates muscle hypertrophy

“…However, these studies were performed in intact, contracting muscle. Consequently, the results might be confounded by the presence of calcium cycling: calcium binds to the PEVK spring‐domain of titin, changing its stiffness,15, 16, 17 and titin‐based stiffness modulates contractility20, 41, 42, 43, 44 which in turn might affect muscle growth. The importance of the current findings is that they show that the effects of titin‐based mechanosensing occur independent of contractile activity, and can be ascribed to titin's elastic properties.…”

“…Both RBM20 ΔRRM and Ttn ΔIAjxn mouse models have previously been described 8, 20. Briefly, the RBM20 ΔRRM model deletes the RNA recognition motif (exons 6 & 7) of the Rbm20 gene, disabling its splicing function, leading to a longer more compliant titin isoform.…”

RNA Splicing

“…80,81 In mice, the loss of Rbm20 results in a giant isoform of titin (N2BA-G), an increase in titin-based elasticity, and an impaired Frank-Starling mechanism (ie, the ability to increase contractile force with increased sarcomere length). 95 Apart from alternative splicing events in titin, it is now known that RBM20 also regulates alternative splicing events in CamkIIδ, Ryr2, and Cacna1c. 82 Loss of Rbm20 induces a CamkIIδ switch from CamKIIδ-B and CamkIIδ-C to 2 bigger isoforms (CamkIIδ-A and CamkIIδ-9).…”

Sarcomeric protein isoform transitions in cardiac muscle: A journey to heart failure

“…Recently, a muscle-specific splicing factor, RNA binding motif 20 (Rbm20), has been found to regulate titin alternative splicing [75]. The Rbm20 deficient rats express largest titin isoform (~3.83MDa), a most compliant titin isoform, and develop dilated cardiomyopathy [75-77], which has been confirmed by a most recent study with the Rbm20 knockout mice [78]. RBM20 mutations in patients with severe dilated cardiomyopathy also lead to the expression of the larger, compliant fetal cardiac titin isoform [75].…”