Sarcomeric tropomyosin expression during human iPSC differentiation into cardiomyocytes.

Tropomyosin (TPM) is an essential sarcomeric component, stabilizing the thin filament and facilitating actin's interaction with myosin. In mammals, including humans, there are four TPM genes (TPM1, TPM2, TPM3, and TPM4) each of which generates a multitude of TPM isoforms via alternative splicing and using different promoters. In this study, we have examined the expression of transcripts as well as proteins of various sarcomeric TPM isoforms during human inducible pluripotent stem cell differentiation into cardiomyocytes. During the differentiation time course, we harvested cells on Days 0, 5, 10, 15, and 20 to analyze for various sarcomeric TPM transcripts by qRT-PCR and for sarcomeric TPM proteins using two-dimensional Western blot with sarcomeric TPM-specific CH1 monoclonal antibody followed by mass spectra analyses. Our results show increasing levels of total TPM transcripts and proteins during the period of differentiation, but varying levels of specific TPM isoforms during the same period. By Day 20, the rank order of TPM transcripts was TPM1α > TPM1κ > TPM2α > TPM1μ > TPM3α > TPM4α. TPM1α was the dominant protein produced with some TPM2 and much less TPM1κ and μ. Interestingly, small amounts of two lower molecular weight TPM3 isoforms were detected on Day 15. To the best of our knowledge this is the first demonstration of TPM1μ non-muscle isoform protein expression before and during cardiac differentiation.