The finding matters because mTORC1, the mechanistic target of rapamycin complex 1, sits at the very hub of how cells interpret the nutritional environment. When amino acids are abundant, mTORC1 switches on growth programs; when nutrients are scarce, it falls silent and cells shift toward maintenance and repair. Decades of work, from yeast to mice, have established that dampening this pathway, whether by drugs such as rapamycin or by dietary restriction, reliably extends lifespan. Yet mTORC1 is not a single lever but a control panel, and different outputs of the pathway can be engaged or disengaged independently. Understanding which specific downstream branch actually drives aging, rather than growth, has been a stubborn problem in the field.

One of mTORC1’s most enigmatic outputs is the preferential translation of a special class of messenger RNAs known as TOP mRNAs, so named for the 5′ terminal oligopyrimidine motif that caps them. These transcripts overwhelmingly encode ribosomal proteins, the structural components of the protein-synthesizing machinery itself. Because TOP mRNAs are extraordinarily abundant, they impose a heavy translational burden on the cell, and their regulation is notoriously resistant to rapamycin, the canonical mTORC1 inhibitor. In other words, even when rapamycin shuts down most mTORC1 outputs, TOP mRNA translation marches largely on. How this branch is physiologically tuned, and whether it connects to lifespan at all, was unknown.

The new study began with a systems-level observation. By comparing transcriptomic datasets from flies subjected to different diets, ages, and genetic perturbations, the researchers noticed that Lsp2 expression tracked nutrient status with striking fidelity. Feeding flies yeast, a rich source of essential amino acids, potently induced Lsp2, and subsequent dissection showed that this induction ran through mTORC1 itself. Essential amino acids, particularly the branched-chain amino acids leucine, isoleucine and valine, drove Lsp2 expression in the fly’s fat body, an organ that combines the functions of liver and adipose tissue. But the induction was not automatic: it was gated by additional layers of nutrient and hormonal signaling, including insulin signaling, ecdysone, and stress-responsive factors such as FoxO and ATF4, which suppressed the response. Lsp2, in effect, acts as an integration point that only fires when multiple independent signals agree that conditions are truly plentiful.

Then came the surprise. Rather than being a passive storage protein, Lsp2 feeds back into the mTORC1 pathway as an activator, sustaining the phosphorylation of the translational repressor 4E-BP even under conditions where other mTORC1 outputs have been curtailed. 4E-BP is a well-studied brake on translation initiation: when unphosphorylated, it binds the cap-binding protein eIF4E and blocks the assembly of translation initiation complexes. mTORC1 phosphorylation disables 4E-BP, freeing the translational machinery. The team showed that Lsp2 specifically promotes the phosphorylation of 4E-BP in mature adult flies, thereby keeping TOP mRNA translation running at full tilt, while leaving earlier-life mTORC1 activity essentially untouched. This adult-specific, rapamycin-resistant mode of regulation suggested that Lsp2 occupies a unique position in the aging-relevant arm of the pathway.

The functional consequences were dramatic. Flies engineered to lack Lsp2, either through a transposon-derived mutant allele or through adult-specific RNA interference, lived substantially longer than controls across multiple diets and genetic backgrounds. Crucially, the long-lived mutants did not show the trade-offs that often accompany lifespan extension. Egg laying, climbing ability, spontaneous locomotion, and resistance to starvation, oxidative stress, and bacterial infection were all preserved. Developmental timing was normal. The authors even found that Lsp2 acts as an adipokine that limits the release of insulin-like peptide 2 from the brain, yet systemic insulin signaling was not detectably altered. The lifespan effect appeared to be channeled through a specific molecular route rather than a general slowing of metabolism.

That route was revealed by translatomic profiling, a suite of techniques including polysome profiling and ribosome-associated sequencing that measure which mRNAs are actually being translated, not merely transcribed. In Lsp2-deficient flies, the translation of TOP mRNAs, above all the mRNAs encoding cytoplasmic ribosomal proteins, dropped selectively, while the bulk of the transcriptome was barely affected. Polysome profiles revealed a shift in the balance of ribosomal subunits, with an increase in free 60S subunits and a decrease in 40S subunits, consistent with impaired production of the small-subunit components encoded by TOP mRNAs. Genetic epistasis nailed down the mediator: when the gene encoding 4E-BP, called Thor, was deleted, the lifespan extension and the translational changes of Lsp2 loss were both abolished. Lsp2 therefore extends lifespan by relieving an Lsp2-dependent, rapamycin-resistant drive on 4E-BP phosphorylation, which in turn restrains TOP mRNA translation.

The study also resolves a long-standing question about TOP motifs themselves. In mammals, 5′ TOP motifs are the textbook hallmark of ribosomal protein mRNAs, but whether fruit flies possessed functional TOP motifs had been uncertain. By mining a large collection of CAGE-seq datasets, which precisely map transcription start sites across seven species, the researchers showed that TOP motifs are present in nearly all Drosophila ribosomal protein mRNAs and that they co-emerged evolutionarily with 4E-BP itself. This co-evolution makes elegant mechanistic sense: a translational control element is only meaningful in an organism that also carries the repressor that reads it. The conservation of this regulatory logic across hundreds of millions of years underscores how central the governance of ribosome biogenesis is to life history.

The broader implications reach well beyond flies. Rapamycin extends lifespan across species, yet its clinical translation has been complicated by side effects stemming from broad pathway suppression. The Lsp2 study suggests a cleaner target: if a specific, drug-resistant branch of mTORC1 signaling, channeled through 4E-BP and TOP mRNA translation, is what actually drives aging, then interventions aimed selectively at that branch might capture longevity benefits while sparing the pathway’s necessary growth and immune functions. Notably, the researchers found that long-lived Lsp2 mutants were still further responsive to rapamycin, implying that the two interventions act through partly distinct mechanisms and could in principle be combined. Whether mammals harbor an equivalent adipose-derived amplifier of mTORC1 output is now the obvious question, and the answer could reshape how the field thinks about the connection between diet, protein synthesis, and the biology of growing old.

Subject of Research: The role of the Lsp2 protein in linking mTORC1 signaling, TOP mRNA translation, and lifespan regulation in Drosophila.

Article Title: Lsp2 links mTORC1 to TOP mRNA translation and lifespan in Drosophila

Article References: Wang, J., Cai, Z., Gu, J., Xiong, S., Yi, J., Yang, M., Chang, K., Ning, X., Wen, Y., Yan, Y., Lu, J., Wang, Y., & Zhai, Z. (2026). Lsp2 links mTORC1 to TOP mRNA translation and lifespan in Drosophila. Nature. https://doi.org/10.1038/s41586-026-11029-x

Image Credits: AI Generated

DOI: 10.1038/s41586-026-11029-x

Keywords: Lsp2, mTORC1, TOP mRNAs, 4E-BP, Drosophila, lifespan, aging, translation, ribosomal proteins, nutrient sensing, rapamycin, fat body