Titin Isoform Transitions and Passive Stiffness During Skeletal Muscle Development

Abstract

During postnatal striated muscle development, multiple changes in active and passive properties occur, reflecting an altered mechanical demand. For instance, during postnatal cardiac development titin isoform expression switches from large isoforms to small stiffer isoforms, likely affecting diastolic filling behavior. In the present study, we investigated whether titin isoform transitions also take place during skeletal muscle development. We used gel-electrophoresis to determine changes in titin isoform size in mice and rabbits of various ages. A titin exon microarray was used to evaluate transcript expression of all of titin's exons. To investigate the mechanical effect of titin isoform transitions, passive properties of neonatal and adult skeletal muscles were determined.Neonatal mice were found to express large titin isoforms, which are gradually replaced by smaller isoforms during skeletal muscle development. The half transition time of the isoform for tibialis cranialis (TC), soleus, extensor digitorum longus, gastrocnemius and diaphragm were 6, 17, 17, 12, and 10 days, respectively. Essentially similar findings were obtained from NZW rabbits, with the exception that the half-life of the isoform transitions was slightly longer compared to murine skeletal muscles. Titin exon analysis in neonatal murine gastrocnemius muscle revealed increased expression of a large group of exons when compared to adult muscle transcripts, with all upregulated exons coding for exons of the elastic PEVK region of titin. In line with these observations, we found ∼50% lower titin-based passive stiffness of murine neonatal soleus and TC when compared to adult muscle.These data demonstrate that during skeletal muscle development titin isoform transitions occur from large compliant isoforms to smaller and stiffer isoforms in adult muscle, likely due to changed expression of PEVK exons.