The Regenerative Medicine Project at our institute is advancing therapeutic strategis for various diseases by introducing a range of modifications to human induced pluripotent stem (iPS) cells.
We are pleased to announce that we have successfully established iPS cells with doubled chromosomes (tetraploid iPS cells) via cell fusion. By differentiating these tetraploid iPS cells into cardiomyocytes, we succeeded in developing a technology that replicates the chromosome doubling that occurs naturally in vivo during cardiomyocyte maturation, thereby enhancing cardiac function.
The research findings were published online in the British scientific journal Communications Biology on Monday, August 17, 2026.
Abstract
Human iPS cells, which can be generated from the cells of our own bodies and possess the ability to differentiate into any cell type, have opened up a wide range of possibilities for diverse medical applications. A research group led by Project Leader Yuichiro Miyaoka of the Regenerative Medicine Project at our institute is developing therapeutic approaches for diseases by applying genetic modifications to these human iPS cells.
In this study, the research group—consisting of Ittetsu Nakajima (then a graduate student at Institute of Science Tokyo), Project Leader Miyaoka (who also serves as an Associate Professor at Graduate School of Medical and Dental Sciences, Institute of Science Tokyo, and as a Visiting Associate Professor at Graduate School of Humanities and Sciences, Ochanomizu University), and Mitsuyoshi Shimane of Nanion Technologies Japan K.K.—successfully established iPS cells with doubled chromosome numbers (tetraploid iPS cells) via cell fusion. By differentiating these tetraploid iPS cells into cardiomyocytes, they successfully replicated the chromosome doubling process that occurs during in vivo cardiomyocyte maturation, thereby developing a technology to enhance myocardial function.
Previously, a major hurdle for clinical and medical applications of iPS cell-derived cardiomyocytes is that they remain immature, resembling an embryonic or fetal state. Addressing this issue, this study focused on the fact that cardiomyocytes double their chromosomes as they mature. By successfully generating tetraploid cardiomyocytes, the team obtained cardiomyocytes possessing functions closer to those of mature cardiac cells. This breakthrough is expected to contribute to diverse medical applications, including transplantation therapies for heart failure, efficacy and safety testing for novel drug candidates, and the construction of disease models to elucidate disease mechanisms.
Figure 1
Normal iPS cells are diploid, and cardiomyocytes derived from these diploid iPS cells mostly remain diploid. Consequently, they cannot replicate the tetraploidization of cardiomyocytes that occurs during maturation in the human body. To address this issue, this study generated cardiomyocytes with doubled chromosomes by first tetraploidizing the iPS cells themselves via cell fusion, and then differentiating them into cardiomyocytes. As expected, the tetraploid iPS cell-derived cardiomyocytes exhibited more mature characteristics.
Research Background
Most cells in the human body are diploid, possessing 23 pairs (a total of 46) of chromosomes. We exist as diploid organisms by inheriting one copy of autosomes 1 through 22, and one sex chromosome (the 23rd chromosome), from each parent. However, in certain cell types such as cardiomyocytes and hepatocytes (liver cells), a phenomenon called "polyploidization" (chromosome doubling) occurs, in which the number of chromosome sets increases as the cells mature. In fact, approximately 80% of adult cardiomyocytes are tetraploid, containing four copies of each of the 23 chromosomes.
Human iPS cells can differentiate into a wide variety of cell types. Among these, cardiomyocytes hold immense promise for clinical application, with clinical trials for transplantation already underway. However, a persistent challenge for medical application has been that conventional iPS cell-derived cardiomyocytes remain immature and exhibit fetal-like properties.
Our team focused on the fact that conventional iPS cell-derived cardiomyocytes rarely undergo the chromosome doubling that naturally accompanies maturation. We hypothesized that if we could generate tetraploid cardiomyocytes from iPS cells, we could significantly improve the functional capabilities of these cardiac cells.
Research Methods and Results
Conventional technologies were unable to directly tetraploidize cardiomyocytes derived from iPS cells. In this study, instead of attempting to tetraploidize cardiomyocytes after their differentiation from diploid iPS cells, we conceived the idea of tetraploidizing the iPS cells themselves first, and then differentiating them into cardiomyocytes.
To achieve this, we fused diploid iPS cells using Sendai virus particles—which have a well-known cell-fusing effect and are commonly used in cell fusion protocols—to establish tetraploid iPS cells (Figure 2A). The resulting tetraploid iPS cells were slightly larger in size than conventional diploid iPS cells (Figure 2B), stably maintained their four sets of chromosomes (Figure 2C), and retained their pluripotency.
Figure 2: Human iPS Cell Fusion
(A) Cell fusion mediated by Sendai virus particles.
(B) Diploid and tetraploid iPS cells.
(C) Karyotype analysis of the fused tetraploid iPS cells.
When we differentiated these tetraploid iPS cells into cardiomyocytes, they exhibited a larger cell size compared to those derived from diploid iPS cells (Figure 3A). This is consistent with in vivo behavior, where cell size increases during maturation.
Next, we analyzed gene expression profiles using RNA-sequencing (RNA-seq). In tetraploid-derived cardiomyocytes, the expression of cell cycle-related genes was significantly downregulated compared to diploid-derived counterparts. In the human body, cardiomyocytes actively proliferate during the diploid fetal stage, but they stop dividing and become tetraploid as they mature. These results indicate that tetraploid-derived cardiomyocytes possess gene expression profiles that closely resemble those of mature cardiomyocytes.
Furthermore, because cardiomyocytes require a substantial amount of energy to contract, their mitochondria increase and their function improves as they mature. Our analysis of the mitochondria revealed an increase in both mitochondrial quantity and functional capacity (Figure 3B).
Additionally, to conduct electrophysiological analysis, we measured electrical impedance. As cardiomyocytes adhered to the bottom of the culture plate beat, the internal cellular structure shifts, causing an increase in electrical impedance. The measurements demonstrated that the tetraploid-derived cardiomyocytes contracted more strongly and rapidly than the diploid-derived ones (Figure 3C). Strong and fast contraction is another key characteristic of mature cardiomyocytes, and this analysis further supported the trend of enhanced maturity in the tetraploid-derived cardiomyocytes.
Figure 3: Analysis of Tetraploid iPS Cell-Derived Cardiomyocytes
(A) Immunostaining of cTnT, a protein responsible for cardiomyocyte contraction, in diploid- and tetraploid-derived cells. cTnT is expressed in tetraploid iPS cell-derived cardiomyocytes, which are visibly larger than diploid iPS cell-derived cardiomyocytes.
(B) Green staining of mitochondria. An increase in mitochondrial mass is observed in the tetraploid cells, and their mitochondrial function was also enhanced.
(C) Analysis of myocardial beating patterns via electrical impedance measurement. The tetraploid-derived cardiomyocytes (shown in red) exhibit higher peaks and a faster rise to those peaks. This indicates that the tetraploid iPS cell-derived cardiomyocytes contract more strongly and rapidly.
Social Significance
This research has successfully established a technology to produce mature cardiomyocytes from tetraploid iPS cells. By overcoming the limitations associated with the immaturity of conventional diploid iPS cell-derived cardiomyocytes, this technology is expected to significantly contribute to the advancement of regenerative medicine and drug discovery.
Furthermore, this approach is likely to prove highly beneficial for research and regenerative applications involving other cell types that undergo chromosome doubling as part of their natural maturation process, such as hepatocytes (liver cells), skeletal muscle cells, megakaryocytes (platelet-producing cells), and osteoclasts (bone-resorbing cells).
Major Funding and Support for this Study
This research was supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) / Japan Society for the Promotion of Science (JSPS) KAKENHI, the Terumo Life Science Foundation, the Advanced Pharmaceutical Research Promotion Foundation (General Research Grant in Cardiovascular Medicine), and the Cardiovascular Research Promotion Foundation (Bayer Cardiovascular Research Grant).
Communications Biology
Experimental study
Cells
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17-Aug-2026