Effect of Excision of Titin's PEVK Exons 219-225 on Skeletal Muscle Structure and Function

Abstract

We recently published a PEVK KO mouse model in which all exons that constitute the PEVK region of cardiac titin (N2B cardiac isoform), exons 219-225, had been excised, and reported cardiac hypertrophy and increased passive stress (Granzier et al., 2009 Circ Res., 11, 557). Here we investigated the phenotype of EDL (fast twitch) and soleus (slow twitch) skeletal muscle of wildtype and homozygous PEVK KO mice. Muscle mass was significantly increased in both soleus (51±5%) and EDL (21±6%) muscles; we are currently studying whether the underlying hypertrophy mechanisms are similar to those previously found in the heart. Because the excised exons make up a small portion of the PEVK segment of skeletal muscle titins we expected modest differences, if any at all, in passive stress. Unexpectedly, passive stress was significantly increased in soleus and EDL muscles, both when measurements (SL 3.0μm) were made at the whole muscle level (40±7% and 67±16%, respectively) and at the fiber bundle level (50±14% and 81±1%). Gel electrophoresis revealed, in both EDL and soleus, expression of a single titin isoform in wildtype muscle, but surprisingly, co-expression of two isoforms in the KO muscles. The larger isoform co-migrated with the isoform expressed in wt muscle and thus is likely to represent KO titin (exons 219-225 represent ∼30kDa, which is insufficient to cause a mobility difference). The smaller isoform, found in both EDL and soleus KO muscle, is likely to represent a splicing adaption that correlates with increased passive tension. We are currently investigating the identity of these exons using titin exon microarray analysis. In summary, excision of titin exons 219-225 in skeletal muscle results in elevated passive stress with the underlying cause likely being due to an adaption in titin splicing.