Cellular Recycling Reworked: Autophagy Follows Surprising Sex- and Cell-Type Rules in Human Aging

Autophagy, the cellular cleanup system that digests damaged proteins and worn-out organelles, has long been cast as a simple casualty of aging: the older the organism, the weaker the recycling. A new study published in Aging Cell upends that tidy narrative. By directly measuring autophagic activity, rather than merely inferring it from gene expression, researchers at the San Diego Nathan Shock Center and collaborating institutions found that autophagy is remodeled during human aging in ways that depend sharply on both cell type and sex. In some cells the process declines with age; in others it holds steady or even rises. Most strikingly, in adults over seventy, higher autophagy flux across every cell type examined was associated with worse physical function, and a twelve-week program of mild exercise appeared to bring elevated flux in immune cells back down toward more youthful levels.

The technical challenge at the heart of the study is that autophagy is notoriously difficult to measure in living humans. Macroautophagy is a multi-step process in which cellular cargo is sequestered into double-membraned vesicles called autophagosomes and delivered to lysosomes for degradation. Common laboratory readouts, such as the abundance of autophagosomes or levels of the autophagosome-associated protein LC3/Atg8-II, are static snapshots that cannot distinguish a cell cranking out autophagosomes from one whose degradation machinery has stalled. The gold-standard approach, pharmacologically blocking lysosomal degradation and watching cargo accumulate, is not feasible in vivo in people. The research team therefore built a multi-pronged strategy using accessible human cells that retain molecular signatures of their donors’ ages.

The team worked with three complementary cell systems drawn from the well-characterized SHOCK healthy aging cohort, whose participants self-report freedom from chronic disease. Low-passage primary dermal fibroblasts from the skin preserve transcriptional signatures of aging. From those same fibroblasts, the researchers generated induced neurons, or iNs, by directly converting the cells with the neuronal transcription factors NGN2 and ASCL1, creating subject-matched pairs of skin and nerve cells. Finally, they isolated peripheral blood mononuclear cells, or PBMCs, freshly drawn from participants, a minimally invasive window into autophagy in circulating T, B, and NK cells. Measuring all three cell types from overlapping groups of donors allowed the investigators to ask whether autophagy is coordinated across a single individual’s body or regulated independently in each tissue.

The first surprise came from transcription. In fibroblasts from older donors, bulk RNA sequencing revealed broad upregulation of autophagy-related gene programs: modules for macroautophagy, autophagosome assembly, and autophagosome maturation were significantly enriched with age, alongside classic senescence-associated signals such as TNFα, IL6-JAK-STAT3, and p53 signaling. Individual genes behaved heterogeneously, with some, including ATG5 and ATG7, decreasing while most macroautophagy genes rose or peaked in mid-to-late adulthood. In iNs, the pattern was weaker and more scattered; autophagy-related pathways were enriched at the pathway level, but curated gene modules were not significantly changed. If transcription were destiny, one would expect both cell types to show boosted autophagic activity. They did not.

Functional measurements told a very different story. Using chloroquine to block lysosomal degradation for two hours and quantifying the resulting accumulation of LC3B-positive structures, the researchers calculated autophagy flux, the true throughput of the pathway. A critical methodological finding emerged immediately: static LC3B-positive foci did not correlate with flux in any cell type, confirming that autophagosome abundance alone is an unreliable proxy for autophagic activity. Flux, not abundance, is what matters, and flux behaved in ways the transcriptomic data could not have predicted.

In fibroblasts, autophagy flux was sex-specific. Young male donors’ fibroblasts trended toward higher flux than young female donors’, but with age the pattern reversed: flux declined significantly in cells from males, while remaining stable in cells from females, so that older donors of both sexes converged. A multiple linear regression with an age-by-sex interaction confirmed that age was significantly associated with declining flux in males, with the interaction term trending toward significance. In induced neurons, the pattern flipped again. Neuronal flux was stable with age in cells from males but increased with age in cells from females, a significant age-by-sex interaction. When the researchers compared subject-matched fibroblasts and iNs from the same donors, flux in one cell type did not predict flux in the other, demonstrating that autophagy is not coordinately regulated across tissues within an individual.

PBMCs added a third pattern. Across twenty-three participants spanning ages twenty-four to eighty, autophagy flux in fresh immune cells showed no significant association with chronological age and no sex differences. What changed instead was variability: flux became markedly more heterogeneous with advancing age, with a trend toward higher values in individuals around seventy and older. This echoes recent independent reports that autophagy flux in freshly isolated PBMCs can rise with age. The researchers verified that the findings were not artifacts of age-related shifts in immune cell composition, either in whole blood or after isolation.

The most provocative results connected these cellular measurements to whole-body physiology. SHOCK participants underwent standardized fitness assessments, including maximal treadmill testing for VO2max, grip strength, walking speed, blood pressure, and the Short Physical Performance Battery. Across the full cohort, the correlations between flux and fitness varied by cell type: higher flux in fibroblasts and PBMCs sometimes tracked with more youthful cardiovascular measures, such as higher maximum heart rate and lower resting heart rate, while higher flux in iNs correlated with weaker grip strength and slower walking speed. But when the analysis focused on participants aged seventy and older, a consistent picture emerged: higher autophagy flux in all three cell types was associated with reduced physical function. Higher PBMC flux accompanied lower VO2max, slower walking speed, and longer chair-stand times; higher fibroblast flux accompanied reduced resting VO2; and higher iN flux accompanied dramatically reduced maximal grip strength. The authors caution that these subgroup correlations rest on small samples, but the direction was strikingly uniform.

Why would elevated recycling signal decline rather than resilience in the oldest adults? The authors propose that the meaning of high flux depends on context. In younger people, high flux may reflect abundant autophagic capacity, a well-oiled maintenance system. In the very old, elevated flux may instead represent increased autophagic demand, a compensatory response to accumulating cellular stress and damage, or a struggle to keep pace with degradation needs. This reframing carries real consequences for interventions. Strategies aimed at simply boosting autophagy, whether pharmacological or dietary, may need to account for biological age and tissue context, because pushing flux higher in an eighty-year-old could be counterproductive if the elevation already reflects stress rather than capacity.

The study’s final act tested whether the system is modifiable. In a pilot within the STRONG fall-prevention exercise cohort, five participants aged seventy-seven to eighty-eight completed twelve weeks of mild exercise, one-hour weekly sessions focused on balance, strength, and posture. At baseline, four of the five showed PBMC autophagy flux more than one standard deviation above the SHOCK cohort mean, consistent with the link between elevated flux and reduced function. After the program, flux decreased in four of five participants, falling back within one standard deviation of the younger cohort average, while SPPB scores improved. Immune cell composition was unchanged, suggesting the effect was not a shift in cell populations. Intriguingly, the single participant whose flux rose was the youngest and most physically capable at baseline. The sample is tiny and the timepoints limited, but the implication is bold: autophagy flux in human immune cells is not a fixed consequence of aging but a dynamic, responsive parameter, one that mild exercise can nudge back toward a youthful setpoint while physical performance improves. Larger studies with frailty indices, cognitive measures, and single-cell resolution will be needed to confirm when rising flux is beneficial and when it is a warning light, but this work makes clear that the old dogma of universal autophagic decline cannot survive direct measurement.

Subject of Research: Sex- and cell-type-specific remodeling of autophagy flux during human aging and its relationship to physical function

Article Title: Autophagy Flux Is Remodeled Sex‐ and Cell Type‐Specifically During Human Aging, and Is Linked to Reduced Physical Function in Older Adults

Article References: Moreno, T. M., Heimler, S. R., Moran, R. J., Barkai, H. S., Scandalis, L., Traxler, L., Neil, A., Dozier, S., Bergstrom, J., Colas, A. R., Ranade, S. S., Bang, A. G., Mertens, J., Wing, D., Molina, A. J., & Kumsta, C. (2026). Autophagy Flux Is Remodeled Sex‐ and Cell Type‐Specifically During Human Aging, and Is Linked to Reduced Physical Function in Older Adults. Aging Cell, 25(10), Article e70734. https://doi.org/10.1111/acel.70734

Image Credits: AI Generated

DOI: 10.1111/acel.70734

Keywords: autophagy, aging, autophagy flux, induced neurons, fibroblasts, PBMCs, physical function, exercise, chloroquine assay, LC3B, sex differences, healthy aging