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Contractile Protein Isoform Switch Prompts iPSC-derived Cardiomyocytes to Mature

Review of “Advancing physiological maturation in human induced pluripotent stem cell‐derived cardiac muscle by gene editing an inducible adult troponin isoform switch” from STEM CELLS by Stuart P. Atkinson

The generation and subsequent differentiation of human induced pluripotent stem cells into beating cardiomyocytes (hiPSC‐CMs) have provided a platform for the study of cardiac physiology and the development of regenerative approaches; however, the immature status of hiPSC‐CMs generated using current approaches has hampered subsequent research [1, 2].

Interestingly, recent studies from the laboratory of Joseph M. Metzger (University of Minnesota, Minneapolis, MN, USA) established that specific myofilament regulatory protein isoform transitions direct the maturation of the cardiac sarcomere, the functional unit of cardiac muscle governing overall heart performance [3, 4].  Specifically, a developmentally directed gene‐regulated mechanism prompts the replacement of the slow skeletal form of the thin myofilament regulatory protein troponin I (ssTnI) in human fetal mammalian cardiac myocytes with an adult cardiac troponin I (cTnI) isoform in the adult heart to allow for developmentally adequate heart pump performance. Unfortunately, the fetal-to-adult switch of these key contractile proteins represents a roadblock to maturity in hiPSC‐CMs [1, 2].

In their new STEM CELLS study, researchers from the Metzger laboratory describe their attempts at promoting hiPSC‐CM maturation by engineering iPSCs through gene-editing technology to carry a drug‐inducible expression cassette controlling the expression of adult cTnI that can be switched on during the in-vitro development of hiPSC‐CMs [5]. Overall, the authors provide evidence that the sarcomeric maturation and induction of adult-like cardiomyocyte function following TnI expression in hiPSC‐CMs may foster the application of these mature cells in the study of cardiac physiology and the development of regenerative approaches.

Wheelwright et al. employed a transcription activator‐like effector nuclease‐based gene-editing platform to insert a doxycycline‐inducible cTnI expression system into a safe harbor site in iPSCs, which they then differentiated into CMs. The induction of cTnI expression in engineered iPSC-CMs led to a significant increase in adult/fetal isoform ratio at the mRNA and protein level and the correct localization of cTnI within the sarcomere.

Encouragingly, this isoform switch prompted the acquisition of physiologically-relevant adult myocardium‐like contraction characteristics (faster cardiac muscle relaxation) that aid normal adult human heart performance [3], suggesting the maturation of fetal-like CMs into adult-like CMs. However, the authors noted that cTnI expression failed to impact other crucial developmentally-regulated maturation events within and outside the context of the sarcomere, including myosin heavy chain isoforms or the expression of calcium handling genes, suggesting that cTnI expression cannot induce CM maturation on a “global” scale alone.

Overall, the authors of this fascinating new study suggest that their findings may represent the first step towards the generation of fully mature iPSC-CMs with relevance to the further understanding of normal human cardiac physiology, drug discovery, disease modeling, and the development of advanced therapeutic strategies for cardiac regeneration/repair.

For more on maturing iPSC-CMs and their enormous potential, stay tuned to the Stem Cells Portal!

References

  1. Bedada Fikru B, Chan Sunny S-K, Metzger Stefania K, et al., Acquisition of a Quantitative, Stoichiometrically Conserved Ratiometric Marker of Maturation Status in Stem Cell-Derived Cardiac Myocytes. Stem Cell Reports 2014;3:594-605.
  2. Bedada FB, Wheelwright M, and Metzger JM, Maturation status of sarcomere structure and function in human iPSC-derived cardiac myocytes. Biochimica et Biophysica Acta (BBA) - Molecular Cell Research 2016;1863:1829-1838.
  3. Davis J, Westfall MV, Townsend D, et al., Designing Heart Performance by Gene Transfer. Physiological Reviews 2008;88:1567-1651.
  4. Yasuda S-i, Coutu P, Sadayappan S, et al., Cardiac Transgenic and Gene Transfer Strategies Converge to Support an Important Role for Troponin I in Regulating Relaxation in Cardiac Myocytes. Circulation Research 2007;101:377-386.
  5. Wheelwright M, Mikkila J, Bedada FB, et al., Advancing physiological maturation in human induced pluripotent stem cell-derived cardiac muscle by gene editing an inducible adult troponin isoform switch. STEM CELLS 2020;38:1254-1266.