UTokyo's Breakthrough Uncovers Non-Lethal Role of MLKL in Stem Cell Decline

The Institute of Medical Science at the University of Tokyo has made a pivotal advance in understanding how hematopoietic stem cells, the foundational cells responsible for generating all blood and immune cells throughout life, deteriorate with age. Researchers led by Masayuki Yamashita, formerly an assistant professor at UTokyo's IMS and now at St. Jude Children's Research Hospital, revealed that the protein mixed lineage kinase domain-like (MLKL) plays a critical role in this process. Traditionally known as the executor of necroptosis—a form of programmed inflammatory cell death—MLKL here exerts its influence without killing cells, instead targeting mitochondria to impair stem cell function.

This discovery, detailed in a study published in Nature Communications on April 6, 2026, challenges long-held views on cellular stress responses. As Japan grapples with one of the world's oldest populations—nearly 30 percent over age 65—such insights are particularly timely, offering potential pathways to bolster blood system resilience in the elderly and improve outcomes for patients undergoing bone marrow transplants or chemotherapy.

Understanding Hematopoietic Stem Cell Aging

Hematopoietic stem cells (HSCs) reside in the bone marrow and self-renew while differentiating into myeloid and lymphoid lineages to sustain lifelong blood production. With advancing age, HSCs exhibit hallmarks of decline: diminished self-renewal capacity, skewed differentiation toward myeloid cells at the expense of lymphocytes, accumulated DNA damage, and metabolic shifts. These changes contribute to immunosenescence, increased susceptibility to infections, and higher risks of blood disorders like myelodysplastic syndromes (MDS).

Prior research at UTokyo and other Japanese institutions, such as Kyoto University's Center for iPS Cell Research and Application (CiRA), has illuminated aspects of HSC aging, including niche interactions and epigenetic reprogramming. However, the converging molecular mechanisms linking everyday stresses—inflammation from infections, replication stress from cell division, or oncogenic pressures—to these functional losses remained elusive until this MLKL study.

The Unexpected Non-Necroptotic Function of MLKL

MLKL forms part of the RIPK3-MLKL signaling axis, activated by receptor-interacting protein kinase 3 (RIPK3) in response to stressors like tumor necrosis factor (TNF). In necroptosis, phosphorylated MLKL oligomerizes, forms plasma membrane pores, and triggers lytic cell death. Yet, in HSCs, the team observed transient MLKL activation—detectable via a FRET biosensor called SMART—without elevated cell death rates.

First author Yuta Yamada, who conducted much of the work as a UTokyo graduate student, noted in the IMSUT press release that MLKL-deficient mice subjected to repeated 5-fluorouracil (5-FU) chemotherapy—a model of replication stress—retained superior HSC regeneration compared to wild-type controls. This prompted deeper investigation into MLKL's 'beyond death' roles.

Mechanisms: Mitochondrial Damage at the Core

Using advanced techniques like transmission electron microscopy (TEM), immunogold labeling, and Seahorse metabolic assays, the researchers pinpointed MLKL's localization to HSC mitochondria. Activated MLKL disrupts mitochondrial membrane potential, induces cristae disorganization, and swells organelles, shifting metabolism from efficient glycolysis to less optimal oxidative phosphorylation.

• In stressed or aged wild-type HSCs, mitochondrial dysfunction correlates with reduced ATP production and glycolytic flux.
• MLKL knockout (KO) HSCs maintain healthy mitochondria, showing higher membrane potential and normalized energy output.
• This damage is cell-intrinsic, independent of bone marrow inflammation, transcriptome alterations, or chromatin accessibility changes.

The study employed competitive bone marrow transplantation assays, where donor HSCs repopulate irradiated recipients. MLKL-KO cells achieved ~60% peripheral blood chimerism versus ~40% for wild-type under stress, with balanced lineage output (e.g., higher B-lymphoid reconstitution).

Evidence from Mouse Models of Aging and Stress

In 18-month-old mice, MLKL activation was elevated in HSCs, particularly males, driving myeloid bias (increased neutrophils) and lymphoid loss. DNA damage markers like γH2AX foci were reduced in KO HSCs, alongside preserved self-renewal in serial transplants.

Stresses mimicking clinical scenarios—poly(I:C) for viral inflammation, lipopolysaccharide (LPS), or RUNX1 mutations for MDS—activated the RIPK3-MLKL axis selectively in HSCs, not progenitors. RIPK3-KO mirrored MLKL-KO protection, confirming the pathway.

Recombinant MLKL protein applied to isolated mitochondria recapitulated damage, while kinase-dead mutants did not, highlighting phosphorylation-dependent pore formation on inner membranes.

UTokyo's Leadership in Stem Cell Aging Research

The Institute of Medical Science (IMSUT) at UTokyo hosts the Division of Stem Cell and Molecular Medicine, where much of this work originated. Headed by experts like Atsushi Iwama, the division integrates genomics, proteomics, and functional assays to dissect stem cell fate. Yamashita's group previously identified inflammation's role in HSC attrition, building to this MLKL insight.

UTokyo's efforts align with Japan's national priorities. The government funds regenerative medicine via AMED (Japan Agency for Medical Research and Development), with billions allocated to iPS cell initiatives at CiRA (Kyoto University) and beyond. Aging research receives priority due to demographic pressures, with programs like UTOPIA supporting stem cell biology.

This study exemplifies international collaboration, linking UTokyo with St. Jude, underscoring Japanese universities' global stature in hematology.

Broader Landscape of HSC Research in Japanese Higher Education

Japan's universities drive stem cell innovation. Kyoto University's CiRA, founded by Nobel laureate Shinya Yamanaka, pioneers iPS-derived therapies for age-related diseases. Tokyo Medical and Dental University (Science Tokyo) explores HSC niche aging, while RIKEN and national centers advance clinical translation.

Funding from MEXT and JSPS supports young investigators like Yamada (ASH-JSH awardee). Challenges include lab-to-clinic gaps, but conditional approvals for iPS therapies (e.g., heart failure, Parkinson's) signal momentum. This MLKL work could integrate with iPS rejuvenation strategies, enhancing transplant viability.

Therapeutic Implications for Japan's Aging Society

With HSCs central to blood cancers and immune decline, targeting MLKL offers promise. Inhibiting its mitochondrial translocation—perhaps via small molecules or gene editing—could preserve HSC quality during aging or therapy. Yamashita envisions 'mitochondrial-protective or necroptosis-modulating drugs' for chemotherapy patients, reducing long-term cytopenias.

In Japan, where elderly comprise a third of the population by 2030, such advances could mitigate MDS incidence and boost transplant success rates. Clinical trials might leverage UTokyo's expertise, aligning with regenerative medicine laws accelerating cell therapies.

Challenges and Future Directions

While compelling in mice, human translation requires validation. HSC heterogeneity complicates targeting, and off-target effects on necroptosis in pathogens must be weighed. Future studies at UTokyo may explore MLKL in other stem cells (e.g., neural, muscle) or combine with Yamanaka factors for reprogramming.

Collaborations with CiRA could test MLKL inhibition in iPS-HSCs. Japan's higher education ecosystem, with competitive grants and tech transfer offices, positions universities to lead these efforts.

Photo by Vitaly Gariev on Unsplash

Expert Perspectives and Ongoing Initiatives

Dr. Iwama highlighted the study's novelty: 'MLKL links stresses to post-transcriptional aging hallmarks.' Yamada emphasized unexpected resilience in KO models. At IMSUT, ongoing proteomics (Yuji Watanabe) and imaging refine mechanisms.

UTokyo recruits globally for stem cell roles, fostering innovation amid Japan's demographic crisis. This research not only advances knowledge but inspires the next generation of Japanese biomedical scientists.