Engineered Polyploid Stem Cells Generate Heart Muscle With Heightened Contractile Force

Instead of trying to mature heart cells after they have already formed, researchers have found success by doubling the chromosomes of stem cells before they ever begin to differentiate. By fusing pairs of diploid induced pluripotent stem (iPS) cells, scientists created tetraploid cells carrying four sets of chromosomes.[1][2]

This early intervention is the critical step that allows the resulting heart muscle to mature, bypassing a developmental roadblock that has stalled cardiac regenerative medicine for years. The strategy reproduces in a culture dish a chromosome-doubling process that normally occurs as heart muscle cells mature inside the human body.[2]

Most human cells are diploid, meaning they possess 23 pairs of chromosomes for a total of 46. However, as cardiomyocytes mature in the fetal and neonatal stages, they stop dividing and roughly 80 percent of them become tetraploid, acquiring four copies of each chromosome.[2][5]

Conventional iPS cell-derived cardiomyocytes rarely undergo this natural doubling process. As a result, they remain stuck in an immature, fetal-like state, which has severely restricted their application in regenerative medicine and drug testing.[1][5]

The resulting immaturity has been a stubborn obstacle that has limited the medical usefulness of lab-grown heart tissue. Grafting these fetal-like cells onto an adult heart has been considered a primary cause of arrhythmia risks and stagnant therapeutic effects in clinical trials.[2]

Fusing cells to force maturation

The research team, led by the Regenerative Medicine Project at the Tokyo Metropolitan Institute of Medical Science, realized that existing technologies could not directly tetraploidize cardiomyocytes after they had already differentiated from stem cells.[1][2]

To overcome this limitation, the researchers executed a conceptual reversal. "Instead of trying to double the chromosomes of the differentiated heart cells, the researchers chose to tetraploidize the iPS cells themselves first," the team explained, using Sendai virus particles to fuse pairs of diploid stem cells together.[2]

The resulting fused cells passed several critical quality checks. They were slightly larger than conventional diploid iPS cells, stably maintained their four sets of chromosomes, and retained their pluripotency, which is the defining capacity to differentiate into various cell types.[2]

When these tetraploid iPS cells were subsequently directed to become cardiomyocytes, they successfully replicated the in vivo chromosome doubling process. The engineered cells were visibly larger than their diploid counterparts, mirroring the physical growth of adult heart tissue.[1][2]

Enhanced contractile force and mitochondria

Beyond physical size, the tetraploid-derived cells exhibited significant functional improvements. Analysis of their beating patterns via electrical impedance measurement showed higher peaks and a faster rise to those peaks.[1]

This distinct electrical profile indicates that the tetraploid iPS cell-derived cardiomyocytes contract more strongly and rapidly than conventional cells. The heightened contractile force is a crucial requirement for any tissue intended to support a failing adult heart.[1][5]

The engineered cells also contained a higher mass of mitochondria, the cellular power plants responsible for energy production. Because the heart must beat continuously without rest, this enhanced mitochondrial function is directly linked to the tissue's endurance and overall performance.[1][2]

Furthermore, RNA sequencing revealed that the expression of cell cycle-related genes was significantly downregulated in the tetraploid cells. This reduction confirms that the cells have stopped proliferating, a hallmark of mature, post-mitotic adult heart tissue.[2][5]

Implications for regenerative medicine

Overcoming the immaturity of iPS-derived cardiomyocytes opens new avenues for treating severe heart failure. Previously, grafting fetal-like cells onto an adult heart posed significant risks, as the mismatched tissue could disrupt the organ's highly coordinated electrical rhythm.[2]

The mature tetraploid cells are expected to integrate more safely and effectively into adult cardiac tissue. This improved compatibility could eventually offer a viable transplantation therapy for patients who currently rely on mechanical assist devices or await donor organs.[1][2]

The technology also provides a more accurate platform for drug discovery and safety testing. Because these cells closely mimic adult human heart tissue, pharmaceutical companies can use them to screen novel drug candidates for cardiac toxicity with greater reliability.[1]

The researchers successfully generated these tetraploid cardiomyocytes from two different individuals, mixing two distinct genetic backgrounds. This capability demonstrates the robustness of the technique and its potential applicability across diverse patient populations.[5]

Because polyploidization is a natural feature of maturation in several other cell types, this chromosome-doubling strategy could be adapted to advance regenerative applications across multiple organ systems.[2]

Hepatocytes in the liver, megakaryocytes that produce platelets, and osteoclasts that resorb bone all undergo similar chromosome doubling as they mature. Applying this fusion technique to those lineages could yield similarly mature tissues for research and therapy.[1][2]

The foundational study, published in the journal Communications Biology, was supported by funding from Japan's Ministry of Education, Culture, Sports, Science and Technology. The Regenerative Medicine Project plans to continue developing therapeutic approaches by applying these genetic modifications to human iPS cells.[1][2][5]