A protein that was supposed to kill cells turns out to quietly age them instead. That is the finding from a team at the University of Tokyo and St. Jude Childrens Research Hospital, published in Nature Communications on April 6, 2026. The protein is called MLKL, short for mixed lineage kinase domain-like, and it has been understood for years as the executioner of necroptosis, a form of programmed cell death. The new study shows that MLKL has a second job: under stress, it briefly migrates to the mitochondria, collapses the membrane potential, and drives aging in blood stem cells — without killing them.
The discovery matters because it changes what we thought we knew about a protein that sits at the intersection of cell death, inflammation, and stem cell biology. It also matters because drugs that inhibit MLKL already exist, designed to block necroptosis. Whether they could be repurposed to target the non-death mitochondrial function is a question nobody has answered yet.
How the researchers found it
The study began with an accidental observation. Dr. Masayuki Yamashita and his colleagues were running experiments in mice with MLKL knocked out, repeatedly treating them with a chemotherapy agent called 5-fluorouracil. They expected to see the usual pattern of stem cell decline. Instead, the stem cells in the knockout mice looked youthful. The aging-associated functional changes were markedly attenuated — but there was no detectable difference in cell death between the knockout and control animals, according to ScienceDaily.
This prompted a pivot. We discovered an unexpected phenotype, Yamashita explains, 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.
The team used several types of genetically engineered mice, including reporter mice fitted with a FRET-based biosensor that tracks MLKL activation in real time. Under inflammatory stress — induced by polyinosinic-polycytidylic acid, lipopolysaccharide, or tumor necrosis factor alpha — MLKL activated briefly in hematopoietic stem cells, peaked around day one, and resolved by day seven. The effect was specific to stem cells and multipotent progenitors, not to myeloid or lymphoid progenitors.
The critical finding was what MLKL did when it activated. Rather than punching holes in the cell membrane to execute death, it moved to the mitochondria. There it lowered membrane potential, altered mitochondrial structure, and reduced energy production. These changes did not kill the stem cells. But they left them older: less able to renew themselves, biased toward myeloid output over lymphoid output, and depleted of the cells that drive a robust immune response.
In the mice where MLKL was removed or inactivated, the picture was different. Hematopoietic stem cells retained their regenerative capacity, produced healthier immune cells, showed less DNA damage, and maintained better mitochondrial function even under stress. These benefits were observed in older animals and under conditions that normally accelerate aging.
A known protein, a new trick
MLKL is not a new discovery. It emerged as the executor of necroptosis roughly two decades ago, and the broader scientific literature already contains hints that MLKL does things besides kill cells. A 2021 review in Cell Death and Differentiation catalogued its non-necroptotic roles: receptor internalization, endosomal trafficking, extracellular vesicle formation, autophagy, axon repair, and inflammasome regulation. What the Tokyo and St. Jude team added is the specific demonstration that in hematopoietic stem cells, under stress, MLKLs non-death activity drives the functional decline that characterizes aging.
The finding also connects to a broader pattern in aging research. Hematopoietic stem cells are the factories that produce all blood and immune cells. With age, they accumulate DNA damage, lose regenerative potential, and shift their output toward myeloid cells at the expense of lymphoid cells. That shift is implicated in everything from vaccine inefficacy to increased cancer risk to the weakened immune response seen in older adults. The mechanism connecting stress, mitochondrial damage, and that lineage bias has not been fully worked out. This study identifies MLKL as a central node in that chain.
The drug problem
Necrosulfonamide, also known as NSA, is an MLKL inhibitor that was developed to block necroptosis. It has been used in animal studies of neuroinflammation, multiple sclerosis, and various inflammatory conditions. GW806742X is another compound that targets the nucleotide-binding site of MLKL, described in a 2025 Frontiers in Pharmacology review. Both were designed to prevent the death pathway from completing. Neither has been tested specifically for the non-death mitochondrial function that Yamashita and colleagues identified.
This is the central question the study leaves open: whether existing MLKL inhibitors could be used to block the non-death mitochondrial activity, or whether they would need to be reengineered to act selectively. Necroptosis and the non-death mitochondrial damage are mechanistically distinct even though they involve the same protein. Blocking one may not reliably block the other. And blocking both may carry a cost.
The trade-off nobody is talking about
MLKLs role in necroptosis is part of the immune systems toolkit for detecting and eliminating cells that are damaged or infected. The protein also mediates aspects of inflammation and, according to some literature, immune surveillance. A therapy that broadly suppresses MLKL could theoretically impair the bodys ability to clear early tumor cells or control persistent infections.
The study did not address cancer surveillance directly, and the mice used in the experiments were young and otherwise healthy. No lifespan data or tumor incidence data are reported. The trade-off between preserving stem cell function and maintaining immune vigilance is the question that will need to be answered before MLKL inhibitors can be considered as longevity interventions.
What it means for longevity research
The longevity field has invested heavily in a few overlapping approaches: clearing senescent cells with senolytics, restoring NAD+ levels, boosting mitochondrial function with metabolites, and in some cases reprogramming cells toward younger states. Each approach has generated promising mouse data. Each has struggled to translate.
The MLKL finding points to a mechanism that none of these platforms directly target. It is not about accumulating damage that needs to be cleared, nor about energy levels that need to be restored. It is about a single protein that the cell itself uses to manage stress responses — and that, when it misfires, quietly degrades stem cell function from the inside.
Whether that makes MLKL a better drug target than the approaches currently in clinical development is a question the data cannot yet answer. What the data do show is that the protein was hiding in plain sight. MLKL has death in its name. Nobody thought to look past it.
