Thin filament length in the cardiac sarcomere varies with sarcomere length but is independent of titin and nebulin

摘要

Thin filament length (TFL) is an important determinant of the force-sarcomere length (SL) relation of cardiac muscle. However, the various mechanisms that control TFL are not well understood. Here we tested the previously proposed hypothesis that the actin-binding protein nebulin contributes to TFL regulation in the heart by using a cardiac-specific nebulin cKO mouse model (ceMHC Cre Neb cKO).Atrial myocytes were studied because nebulin expression has been reported to be most prominent in this cell type. TFL was measured in right and left atrial myocytes using deconvolution optical microscopy and staining for filamentous actin with phalloidin and for the thin filament pointed-end with an antibody to the capping protein Tropomodulin-1 (Tmod1). Results showed that TFLs in Neb cKO and littermate control mice were not different. Thus, deletion of nebulin in the heart does not alter TFL. However, TFL was found to be similar to 0.05 mu m longer in the right than in the left atrium and Tmod1 expression was increased in the right atrium. We also tested the hypothesis that the length of titin's spring region is a factor controlling TFL by studying the Rbm20(Delta RRm) mouse which expresses titins that are similar to 500 kDa (heterozygous mice) and similar to 1000 kDa (homozygous mice) longer than in control mice. Results revealed that TFL was not different in Rbm20(Delta RRM) mice. An unexpected finding in all genotypes studied was that TFL increased as sarcomeres were stretched (similar to 0.1 mu m per 035 mu m of SL increase). This apparent increase in TFL reached a maximum at a SL of similar to 3.0 mu m where TFL was similar to 1.05 mu m. The SL dependence of TFL was independent of chemical fixation or the presence of cardiac myosin-binding protein C (cMyBP-C). In summary, we found that in cardiac myocytes TFL varies with SL in a manner that is independent of the size of titin or the presence of nebulin. (C) 2016 Published by Elsevier Ltd.