Researchers have identified a surprising culprit in blood stem cell aging: a protein best known for triggering cell death that actually damages aging cells without killing them. The discovery offers a fresh angle on why our immune systems weaken over time and hints at ways to slow that decline.
Blood stem cells, called hematopoietic stem cells or HSCs, are responsible for producing every type of blood cell in the body. As we age, these cells lose their edge. They regenerate less efficiently, shift toward making certain cell types at the expense of others, and become less capable of mounting a strong immune response. Multiple factors drive this decline, from accumulated cellular damage to chronic inflammation, but scientists have struggled to understand how these stresses work together.
Researchers from The University of Tokyo and St. Jude Children's Research Hospital set out to explore that mystery by focusing on RIPK3 and MLKL, proteins typically associated with necroptosis, a form of programmed cell death. Their work, published in Nature Communications in April 2026, revealed an unexpected twist.
When the team studied mice lacking MLKL and exposed them to aging-mimicking stress, something odd happened. The aging-related damage to HSC function nearly vanished, yet the cells didn't die. This suggested MLKL might be aging stem cells through a mechanism entirely separate from cell death. "We discovered an unexpected phenotype in HSCs of MLKL-knockout mice repeatedly treated with 5-fluorouracil, where aging-associated functional changes were markedly attenuated despite no detectable difference in HSC death, prompting us to investigate whether this pathway might induce functional changes beyond cell death," said Dr. Masayuki Yamashita, who led the research.
To understand how, the team used genetically engineered mice, specialized biosensors to track MLKL activation, and a battery of techniques including RNA sequencing, mitochondrial analysis, and bone marrow transplantation to assess stem cell function.
The results rewrote the textbook on MLKL's role. When activated under stress, MLKL briefly moved to the mitochondria, the cellular power plants. There it caused damage: lowering membrane potential, warping mitochondrial structure, and reducing energy output. These changes triggered the hallmarks of aging in HSCs, including weakened self-renewal, fewer lymphoid cells, and a bias toward myeloid cells.
Removing or blocking MLKL reversed these problems. Stem cells without MLKL retained regenerative capacity, produced healthier immune cells, accumulated less DNA damage, and maintained better mitochondrial health, even in older animals under stress. Remarkably, this protection happened without major shifts in gene expression or DNA accessibility, suggesting MLKL acts at the structural level rather than by controlling genes.
The discovery points toward new therapeutic targets. Rather than targeting the death pathway itself, researchers could develop drugs that protect mitochondria or interfere with MLKL activation specifically in aging stem cells. "In the longer term, this research could lead to therapies that preserve the function of hematopoietic stem cells, ultimately improving recovery and long-term health for patients undergoing chemotherapy, radiation, or transplantation," Yamashita said.
The implications extend beyond basic science. Blood disorders, chemotherapy side effects, and age-related immune decline all stem partly from HSC exhaustion. If MLKL can be safely blocked without triggering broader cell death, it could offer a way to keep blood systems functioning longer.
Author Jessica Williams: "This study cracks open a protein that's been misunderstood for years, and the mechanism is elegant: mitochondrial sabotage without killing the cell, which means the fix might be surprisingly targeted."
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