Transcripts and Protein Expression of Different HMW Non‐Myofibrillar Tpm1 Isoforms in Human iPSC ‐ CMs During Differentiation

Mammals have four tropomyosin (TPM) genes (TPM1, TPM2, TPM3, and TPM4) that produce various isoforms through alternative splicing. TPM1 generates both myofibrillar (Tpm1.1, Tpm1.2) and non-myofibrillar isoforms, including those with an exon 9d peptide or with an exon 9a peptide. These non-myofibrillar isoforms are involved in regulating actin-based structures and processes in non-muscle cells. This study focused on the expression of the non-myofibrillar HMW Tpm1 isoforms (Tpm1.3, Tpm1.4, Tpm1.5, Tpm1.6, Tpm1.7, and Tpm1.14) during human inducible pluripotent stem cells (hiPSC) differentiation into cardiomyocytes (CMs) at different time points (Days 0, 5, 10, 15, 20). We have determined the expression of various Tpm1 transcripts by qRT-PCR using isoform-specific primer-pairs. Western blotting with Tpm1-exon 6a and Tpm1-exon 9d antibodies and 2D Western blotting with Tpm1-exon 6a antibody followed by mass spectra analyses were used to evaluate protein expression. Transcripts of non-myofibrillar Tpm1 isoforms peaked at Day 15 and continued at a slightly lower level in mature hiPSC-CM until Day 20. However, no expression of Tpm1.3 or Tpm1.14 has been observed. Protein expression of Tpm1.4, Tpm1.5, Tpm1.6, and Tpm1.7 increases up to Day 15 but practically disappears by Day 20 hiPSC-CM, suggesting their production is decreased, or more likely they are degraded intracellularly. The results suggest that the proteins have the potential to be transiently involved in the early stages of CM differentiation. This study also demonstrates that Tpm1.5, when fused with YFP in an expression construct and transfected into embryonic chicken CM and Day 20 mature hiPSC-CMs, can be organized into cardiac myofibrils despite being previously characterized as a non-muscle isoform. Our observation is further substantiated by the fact that YFP-Tpm1.5 fusion protein can be ectopically expressed and incorporated into the myofibrils of chicken myotube skeletal muscle, which is known to be more stringent than cardiac muscle with regards to myofibril remodeling. Paradoxically, anti-6a antibody fails to recognize the organized YFP-Tpm1.5 fusion protein in embryonic chicken CMs, embryonic chicken skeletal muscle myotubes, or in Day 20 mature hiPSC-CMs. Interestingly, the antibody recognizes the denatured YFP-Tpm1 fusion protein in Western blot analyses. It is well-documented in the literature that an antibody epitope may fail to recognize its antigen when the antigen is in its native state in living cells, often because the epitope is buried, altered conformationally, or inaccessible. Conversely, while the Tpm1.Ex6a antibody stains Tpm1.5 in cell nuclei, the YFP-Tpm1.5 fusion protein does not localize to the cell nuclei. This suggests conformational differences between nuclear and cytoplasmic Tpm1 protein(s) and differential access of the fusion protein to different cellular locations. This also suggests that it is endogenous Tpm1 6a-containing protein identified in the nuclei.