Nanopore sequencing reveals full-length Tropomyosin 1 isoforms and their regulation by RNA binding proteins during rat heart development

Alternative splicing (AS) contributes to the diversity of the proteome by producing multiple isoforms from a single gene. Although short-read RNA sequencing methods have been the gold standard for determining AS patterns of genes, they have a difficulty in defining full length mRNA isoforms assembled using different exon combinations. Tropomyosin 1 (TPM1) is an actin binding protein required for cytoskeletal functions in non-muscle cells and for contraction in muscle cells. Tpm1 undergoes AS regulation to generate muscle versus non-muscle TPM1 protein isoforms with distinct physiological functions. It is unclear which full length Tpm1 isoforms are produced via AS and how they are regulated during heart development. To address these, we utilized nanopore long-read cDNA sequencing without gene-specific PCR amplification. In rat hearts, we identified full length Tpm1 isoforms composed of distinct exons with specific exon linkages. We showed that Tpm1 undergoes AS transitions during embryonic heart development such that muscle-specific exons are connected together generating predominantly muscle specific Tpm1 isoforms in adult hearts. We found that the RNA binding protein RBFOX2 controls AS of rat Tpm1 exon 6a, which is important for cooperative actin binding. Furthermore, RBFOX2 regulates Tpm1 AS of exon 6a antagonistically to the RNA binding protein PTBP1. In sum, we defined full length Tpm1 isoforms with different exon combinations that are tightly regulated during cardiac development and provided insights into regulation of Tpm1 AS by RNA binding proteins. Our results demonstrate that nanopore sequencing is an excellent tool to determine fulllength AS variants of muscle enriched genes.