*3.1. Transcriptional Changes of Titin Isosforms Alter Passive Tension and Myocardial Sti*ff*ness in Heart Failure*

Due to the critical role of titin in the sarcomere and cardiomyocyte, modifications and changes to titin are seen in many forms of heart failure regardless of the etiology. Specifically, transcriptional regulation of *TTN* and selection of isoforms contribute to adaptation to cardiac changes as well as maladaptation and cardiac dysfunction. Understanding and altering transcriptional regulation of titin represents a therapeutic target for treating systolic and diastolic heart failure. Titin has six named major transcriptional isoforms that impart different structural properties based on their size and extensibility [56]. Several of these isoforms are expressed only in neonatal tissue or in skeletal muscle. The two predominant isoforms in adult cardiac tissue are N2B and N2BA [57]. These isoforms differ in the I-band domain where the N2B isoform contains a short PEVK region and few Ig domains, making

it relatively short and stiff. The N2BA isoform has a longer PEVK region and more spring-like Ig domains making it relatively long and soft [56] (Figure 4). The ratio of titin isoform expression in cardiac tissue is correlated with morphology and global cardiac function. In normal hearts, the ratio of N2BA:N2B is typically 70:30 [58]. In heart tissue from patients with end stage DCM, the predominant titin isoform is the longer, softer N2BA. This correlated with decreased passive tension measured on left ventricular muscle strips and in cardio-myofibrils with estimated decreased stiffness of 10% [18,58]. The predominance of the N2BA isoform in patients with DCM also correlated with echocardiographic findings with increased end diastolic volume: pressure ratio suggesting lower global myocardial stiffness [18]. These results suggest that as heart failure progresses there is a maladaptive response leading to selection of the longer, softer N2BA isoform that decreases sarcomere passive stiffness leading to increased end diastolic volume that correlates with ventricular dilation (Figure 4).

**Figure 4.** Transcriptional selection of titin isoforms affects cardiomyocyte passive stiffness. There are two major transcriptional isoforms of titin that are expressed in adult cardiac tissue. The N2B isoform has a shorter PEVK domain and has fewer Ig-like domains making it a shorter and stiffer protein. The N2BA isoform has a larger PEVK domain with more Ig-like domains and an N2A element making it a longer and softer protein. The increased size and extensibility of the N2BA isoform decreases cardiomyocyte passive stiffness and is correlated with dilated cardiomyopathy. Modified from [59].
