Hematopoietic stem cells (HSCs) are the factory that produces all our blood cells: red blood cells, white blood cells, and platelets. With age, they function less and less, impairing the immune system, energy, and immunity. A new study published in Nature Communications by Professor Yuta Yamada and colleagues identified the surprising culprit: MLKL, a protein primarily known for its role in "killing" cells, turns out to also be the cause of stem cell aging, and in an unexpected way.
Who is MLKL and what does it usually do?
MLKL (Mixed Lineage Kinase domain-Like) is the executor of a process called necroptosis, a type of "programmed cell death" different from apoptosis. When a cell is severely damaged or infected, MLKL is activated, moves to the cell membrane, and tears it apart. This is a protective process, removing damaged cells from the system.
The surprising discovery: MLKL is active even without killing
Yamada's team used genetically engineered mice with a fluorescent sensor that lights up when MLKL is activated. They identified a new phenomenon: in blood stem cells, MLKL is active but does not kill the cell. Instead, it accumulates in the mitochondria, where it:
- Damages mitochondrial membranes
- Reduces energy production (poor glycolysis)
- Decreases the self-renewal capacity of the stem cell
- Impairs the production of lymphoid lineage cells, which generate immune B and T cells
The factors that activate MLKL
The team identified three key factors that activate MLKL in stem cells without killing:
- Chronic inflammation (inflammaging): Low levels of inflammatory cytokines that increase with age
- Replicative stress: High demand for stem cell division, causing DNA damage
- Oncogenic stress: Potential beginnings of cancerous mutations that are activated but suppressed by the system
The connection to longevity
In aged mice, MLKL showed increased activity, and stem cell function significantly deteriorated. When MLKL was genetically removed, those mice maintained a younger, more functional blood and immune system: healthier mitochondria, less DNA damage, preserved stem cell self-renewal capacity, and maintained production of lymphoid cells (B and T) instead of declining. Importantly, the study did not examine lifespan or report differences in mortality, but rather preservation of blood and immune system function. That is, regarding blood and immunity, MLKL is not just a marker of aging—it is one of the causes of part of it.
What are the implications for humans?
Pharmaceutical companies are already developing MLKL inhibitors for use in inflammatory diseases. If this research is confirmed in humans, those same inhibitors could also serve as longevity supplements:
- Stronger immune system in older adults
- Normal blood production over a longer period
- Reduced risk of myelodysplastic syndromes and leukemia
The bottom line
The protein whose role is to "kill dangerous cells" turns out to be one of the key drivers of functional decline with age, even without killing. This is an example of what longevity research is beginning to understand: aging is not a failure of one major system. It is an accumulation of small processes, each operating slightly differently than it should. And now we have another target for treatment.
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