Sex chromosome and gonadal regulation of cardiac myofibroblast activation decoupled using mechanically tunable hydrogels and the Four Core Genotypes model

The heart is a muscle that never rests, and like any muscle, it can be injured. When heart tissue is damaged by a heart attack or high blood pressure, the body attempts to repair the wound by laying down scar tissue. This process, known as fibrosis, is carried out by specialized cells called fibroblasts. In a healthy heart, these cells are quiet and help maintain the structure of the organ. But when the heart is under stress, they wake up, transform into a more active state, and begin building a dense network of fibers to hold the heart together. While this scar tissue is necessary to prevent the heart from falling apart, too much of it makes the heart stiff and unable to pump blood effectively. For decades, doctors and scientists have observed that men and women experience heart disease differently. Women often develop a thickening of the heart wall, while men are more prone to the heart chamber stretching out and thinning. These differences suggest that biology plays a deeper role than just the presence of male or female hormones, hinting that the very genetic blueprint inside our cells might be driving these distinct outcomes.

A team of researchers at the University of California, San Diego, set out to untangle this complex mystery. They wanted to know if the differences in heart scarring were caused by the sex hormones produced by the ovaries or testes, or if they were driven directly by the sex chromosomes—the X and Y pairs that determine biological sex. To solve this, they used a special group of mice known as the Four Core Genotypes. In these mice, the researchers could mix and match chromosomes and gonads independently. They created mice that were genetically male (XY) but had female ovaries, and mice that were genetically female (XX) but had male testes, along with the standard combinations. This allowed them to isolate the influence of chromosomes from the influence of hormones.

The researchers then took heart cells from these mice and grew them in a laboratory dish. However, they did not use a standard plastic dish, which is too hard and can trick cells into behaving strangely. Instead, they used a custom-made gel that could be tuned to feel exactly like healthy heart tissue or like the stiff, scarred tissue found in a diseased heart. One version of the gel was soft, mimicking a healthy heart with a stiffness of about 3.86 kilopascals, while the other was much stiffer, mimicking a fibrotic heart at roughly 69.56 kilopascals. By placing the cells on these gels, the team could watch how the cells reacted to the physical environment without the interference of the body's circulating hormones.

What they found was a clear separation of roles. When the cells were grown on the stiff gel, the genetic makeup of the cell itself became the dominant factor. Cells with two X chromosomes, regardless of whether they came from a mouse with male or female reproductive organs, showed a stronger tendency to activate. They built more of the structural fibers that make up scar tissue and turned on specific internal switches that tell the cell to become rigid. This suggests that the number of X chromosomes directly influences how a heart cell responds to the physical stress of a stiffening heart.

On the other hand, the reproductive organs, or gonads, controlled a different set of behaviors. The cells secreted different chemical signals depending on whether they came from a male or female gonad. Cells from male gonads released more inflammatory proteins, which can attract immune cells and drive further damage. Cells from female gonads released more of the proteins that build the scar tissue itself, such as collagen-1 and fibronectin. This means that while the chromosomes dictate how the cell physically hardens, the hormones from the gonads dictate what chemical messages the cell sends to its neighbors.

The study also looked at the specific genes inside these cells. They found that genes on the X chromosome, which are present in double dose in XX cells, were more active on stiff gels. Conversely, genes on the Y chromosome, present only in XY cells, were linked to different pathways. The researchers confirmed that these genetic differences were not just a result of the hormones in the body, but were intrinsic properties of the cells themselves. By using these tunable gels, the team proved that the physical environment of the heart interacts with our genetic sex to determine how heart disease progresses.

This work provides a new way to look at heart disease, showing that the answer is not simply "male" or "female" but a combination of genetic instructions and hormonal signals. The findings suggest that future treatments for heart scarring might need to be tailored not just to a patient's sex, but to the specific genetic and hormonal drivers at play in their heart. By understanding that chromosomes and gonads pull different levers in the heart's repair process, scientists can begin to design therapies that target the right mechanism for the right patient, potentially leading to more effective ways to prevent the heart from becoming too stiff to function.