Acquisition of a quantitative, stoichiometrically conserved ratiometric marker of maturation status in stem cell-derived cardiac myocytes

Abstract

There is no consensus in the stem cell field as to what constitutes the mature cardiac myocyte. Thus, helping formalize a molecular signature for cardiac myocyte maturation would advance the field. In the mammalian heart, inactivation of the "fetal" TNNI gene, TNNI1 (ssTnI), together in temporal concert with its stoichiometric replacement by the adult TNNI gene product, TNNI3 (cTnI), represents a quantifiable ratiometric maturation signature. We examined the TNNI isoform transition in human induced pluripotent stem cell (iPSC) cardiac myocytes (hiPSC-CMs) and found the fetal TNNI signature, even during long-term culture. Rodent stem cell-derived and primary myocytes, however, transitioned to the adult TnI profile. Acute genetic engineering of hiPSC-CMs enabled a rapid conversion toward the mature TnI profile. While there is no single marker to denote the mature cardiac myocyte, we propose that tracking the cTnI:ssTnI protein isoform ratio provides a valuable maturation signature to quantify myocyte maturation status across laboratories.

Graphical abstract

Figure 1

Temporal Expression Pattern of TNNI Protein Isoform Content in Rodent CMs (A) Schematic of TNNI isoform switching time course during development of the mammalian heart. (B and C) Western blot (B) and summary bar graph (C) of ventricles of mouse pups at postnatal days P1–P20. Ventricle from adult mouse is used as a control. (D and E) Western blot (D) and summary bar graph (E) for TnI isoform content in M-NVCM cultured in vitro for D1–D20. (F and G) Western blot (F) and summary bar graph (G) for R-NVCM cultured in vitro for D1–D20. (H and I) Western blot (H) and summary bar graph (I) for Mesp1-mESC-CMs cultured in vitro for D1–D20. See also Figures S1–S3. Data analyzed by two-way ANOVA with Bonferroni post hoc test and expressed as mean ± SEM, n = 3 independent experiments, p < 0.05 for (C), (E), (G), and (I).

Figure 2

Temporal Expression Pattern of TNNI Protein Isoform Content and Sarcomeric Alignment of hiPSC-CMs by Biological Matrix (A) Western blot data using pan TnI antibody showing dominant expression of ssTnI and little detectable cTnI expression even after culture of hiPSC-CMs spontaneously beating for 9.5 months. Adult human ventricular sample was used as positive control (far right lane). (B) Summary bar graph of TnI isoform content in hiPSC-CMs during long-term culture in vitro (taken from western data in part A). (C) Images of rat cardiac thin slice matrix constructs showing decellularization stages of transverse cardiac matrix isolated from whole heart. The whole heart was placed in stainless steel rat heart slicer and cut at 1 mm per thin slice before 1% SDS treatment (I) and 1 hr (II), 2 hr (III), 4 hr (IV), 24 hr (V), and 72 hr (VI) after 1% SDS treatment. Scale bar is 2 mm. (D) Structural organization of hiPSC-CMs during culture with and without biological matrix. The images were taken with 40× objective; the calibration bar is 50 μm. See also Figures S1, S3, and S4.

Figure 3

Temporal Expression Pattern of TNNI Isoform mRNA and Protein in Different hiPSC-CMs Lines (A–C) The temporal developmental transition of ssTnI to cTnI was analyzed at the mRNA level to evaluate the transcriptional regulation in vitro in five hiPSC-CMs lines (A and B). RNA samples were prepared from five hiPSC-CMs lines obtained from different labs employing different differentiation conditions. The analyzed temporal points include day 10, 2 weeks, 1 month, day 80, and 3 and 5 months. A ventricle from an adult heart was used as a control. Quantitative RT-PCR data shows slight increase in the level of cTnI and concomitant reduction in ssTnI for hiPSC-CMs cultured for day 80 and 3 and 5 months in vitro (A and B). Data shown are expressed from average of (two) biological replicates. Western blot data using a pan TnI antibody showing the dominant expression of ssTnI and small yet detectable cTnI expression after culture of hiPSC-CMs for day 80 and 3 and 5 months in vitro (C). (D) Summary of TnI isoform content in hiPSC-CMs obtained from different lines (taken from western data in part C). cTnC is used as cardiac-specific loading control and for normalization of samples for quantification purposes. Data shown are expressed from average of (two) biological replicates.

Figure 4

Accelerated Acquisition of the Mature TNNI Isoform Signature by T3 Supplementation in Rodent CMs, but Not in hiPSC-CMs (A) Schematic of experimental time line for T3 supplementation in hiPSC-CM and rodent CMs. Two experimental time lines were used for hiPSC-CMs. The first was at 2 months postinitiation of beating, and the second time point was shortly after initiation of beating. (B) Representative western blots TnI isoform profile detection after T3 supplementation (10 nM of T3 for 2 weeks in culture of 2 months beating hiPSC-CMs or early time point [1–2 days beating hiPSC-CMs] or 1 day rodent CMs). (C and D) The effect of T3 treatment (T3+) or its absence (T3−) on acquisition of cTnI is quantified in (C) and suppression of ssTnI is quantified in (D). cTnC is used as cardiac-specific loading control and for normalization. Data analyzed by two-tailed t test and expressed as mean ± SEM, n = 3 independent experiments per group, ^∗p < 0.05.

Figure 5

Acquisition of the Mature TNNI Isoform Signature by Gene Transfer in hiPSC-CMs (A) Experimental time line for AdcTnI (flag tagged) gene transfer. (B–D) Western blot probed with pan TnI isoform antibody (B) and summary bar graphs of relative cTnI expression (C) and ssTnI expression (D) in hiPSC-CMs. cTnC is used as cardiac-specific loading control and for normalization of samples for quantification purposes. See also Figure S5. Values are analyzed by one-way ANOVA with Bonferroni post hoc test and expressed as mean ± SEM, n = 3 independent experiments, ^∗p < 0.05.

Figure 6

TNNI Protein Isoform Switch as a Quantitative Maturation Marker for Stem Cell-Derived Cardiac Myocytes (A) Schematic of the cardiac sarcomere thin filament illustrating the stoichiometrically conserved transition from the fetal TnI (ssTnI) to the adult TnI isoform (cTnI) profile. Owing to strict protein content control in the sarcomere, the ssTnI isoform must be removed before the cTnI isoform can occupy the same position in perfect 1:1 synchrony during development. At any time in development, the transition from ssTnI to cTnI is definitive in that it cannot revert (blocked arrow on right). (B) Hypothetical western blot based on published work from Sasse et al. (1993) depicting the TnI isoform profile in cardiac development.