Investigating Titin Splicing In Mouse Muscle

Abstract

Titin is a giant protein that is known for its contribution to muscle elasticity and sarcomere assembly in vertebrate striated muscles. There is considerable interest in understanding the expression of the single gene (TTN) that encodes titin because mutations in the coding gene have been implicated in muscular dystrophy. Insights into the regulatory mechanisms involved in titin splicing would contribute to the development of effective treatments for titin associated muscular dystrophy. Therefore, there is a need to expand the current knowledge in titin isoform expression. For this reason, titin expression in skeletal muscles of Mus musculus was investigated. The Mus musculus is a good experimental model for studying the expression and functions of titin in the sarcomere due to the close similarities between the mouse titin orthologue and that of humans. Titin splicing was investigated by sequencing the entire transcriptome of three skeletal muscles, namely Elongus digitorum longus (EDL), psoas and soleus. The experimental procedure involved total RNA extraction, mRNA purification and fragmentation, first and second strand cDNA synthesis, cDNA library preparation, cluster generation, sequencing and computational analysis. Differential exon skipping was observed in the three skeletal tissues. The differential exon skipping led to the expression of different titin isoforms in the EDL, psoas and soleus. Out of 322 titin exons analyzed, 13 exons were skipped in the soleus titin, 14 exons were skipped in the EDL titin and 20 exons were skipped in the psoas titin. Therefore the soleus titin isoform was the largest isoform, followed by the EDL titin and then the psoas titin. The titin isoforms differed from each other in their PEVK domain sequences. The differential expression of the PEVK domain in the three isoforms was speculated to be critical to the role of titin in the tissue in which it is expressed. The pattern of exon splicing in the titin isoforms suggested that there were splicing factors that were ubiquitous and targeted similar splice sites in the three isoforms. These speculated common splicing factors were implicated in the exclusion of the same exons in all three isoforms. On the other hand, the exclusion of specific exons in only one or two of the isoforms pointed to the existence of tissue-specific splicing factors which recognized different splice sites in the three tissues studied and might be responsible for the expression of the different titin isoforms.