Titin isoform expression in aortic stenosis

Abstract

Titin is a giant sarcomeric protein that plays a major role in determining passive myocardial stiffness. The shorter N2B isoform results in a higher passive myocardial stiffness than the longer N2BA isoform. We hypothesised that the expression of the short N2B isoform would be increased in patients with aortic stenosis compared with healthy controls in response to pressure overload, in order to act as a modulator for the increased demand placed on the left ventricle during the early stages of the hypertrophic response. Myocardial biopsies were obtained from the left ventricle of 19 patients undergoing aortic valve replacement for aortic stenosis who had no significant co-existing coronary artery disease. Left ventricular biopsies were also obtained from 13 donor hearts for comparison. SDS-agarose gels revealed small N2B and large N2BA cardiac titin isoforms, with a mean N2BA/N2B ratio that was significantly decreased in the 19 aortic stenotic patients compared with the 13 controls (0.66+/-0.04 in the normal donor hearts compared with 0.48+/-0.03 in patients with aortic stenosis; P=0.02). However, total titin remained unchanged (0.28+/-0.02 compared with 0.24+/-0.02 respectively; P=0.29). In conclusion, the expression of less N2BA and more N2B titin in response to pressure overload may result in the generation of higher passive tension upon stretch to a given sarcomere length and this might affect cardiac performance.

Figure 1

A representative gel sample from a donor heart (D) that functions as a control and a patient with aortic stenosis (AS). The left band represents a gel sample from soleus muscle. Soleus titin and nebulin function as molecular weight (MW) size markers (3.7 MDa and 0.8 MDa, respectively).

Figure 2

A–D. Titin expression in donor and aortic stenosis hearts based on analysis of SDS-agarose gels. Total titin and N2BA titin (relative to myosin heavy chain/MHC) expression are indistinguishable between donor and aortic stenosis hearts (2A and B), whereas N2B expression (relative to MHC) is significantly increased in the aortic stenosis group (2C). N2BA:N2B expression ratio is significantly lower in aortic stenosis patients (2D).

Figure 3

A. Predicted force-SL relations of single N2B (red) and N2BA molecule (black). B: Average force per titin molecule in control (black) and aortic stenosis sarcomere (blue) and increase in titin force in aortic stenosis over control (red).