RIKEN researchers in Japan have uncovered an unexpected way that fruit-fly gut cells respond to nutrition: they appear to sense how “full” they are by measuring the physical thickness of their cytoplasm. When nutrients are scarce, intestinal cells become more watery and fluid, triggering a recently identified form of cell death called erebosis. The discovery challenges the conventional view that cells respond to hunger mainly through biochemical nutrient-sensing pathways and introduces a new concept the researchers call “viscosatiety.”
The study, led by Sa Kan Yoo at the RIKEN Center for Biosystems Dynamics Research, examined how the epithelial cells lining the gut of Drosophila melanogaster are continuously replaced. The intestinal epithelium is exposed directly to food and must constantly renew itself to maintain the gut barrier. Yoo’s group previously reported that this rapid turnover involves erebosis, a distinctive cell-death process that differs from better-known mechanisms such as apoptosis and autophagy. The new work investigated whether dietary nutrients control erebosis through specific molecular signals or through a more general physical property of the cell.
The initial experiments produced results that seemed to fit established ideas in nutritional biology. Flies fed a diet containing little yeast and a high proportion of sugar showed increased erebosis in their gut epithelial cells. However, changing the sugar concentration alone had little effect. The strongest response occurred when the researchers reduced the amino acid content of the food by approximately 90 percent. This suggested that amino acids might act as the critical nutritional signal, possibly through pathways that regulate protein synthesis, growth, and survival.
In many animal cells, amino acids activate signaling networks such as the mechanistic target of rapamycin, or mTOR, which promotes cell growth when nutrients are abundant. The absence of particular amino acids, especially leucine, can suppress mTOR activity and stimulate autophagy, a self-digestion process that helps cells recycle internal components. The RIKEN team therefore expected that blocking these canonical pathways would prevent the intestinal cells from entering erebosis. Instead, the cells continued to undergo erebosis even when the relevant biochemical signaling routes were disrupted.
The researchers then tested individual amino acids at concentrations found in the nutrient-rich diet that normally protected the gut. None of the amino acids was able to suppress erebosis on its own. Only when the concentration of each amino acid was increased substantially did it consistently prevent the cell-death process. This pattern argued against the idea that one particular amino acid was functioning as a specialized signal. It also suggested that the cells were responding to the total quantity of dissolved material rather than to the identity of a specific nutrient.
Further experiments made the mystery more pronounced. The investigators examined common chemical components of amino acids and tested whether their metabolic products, including compounds such as ammonia, urate, and uric acid, might be responsible. They also interfered with amino acid metabolism. These manipulations failed to eliminate the effect. The results indicated that amino acids did not need to be broken down or converted into signaling molecules to influence erebosis. Their physical presence inside the cell appeared to be sufficient.
The breakthrough came when the team used a non-metabolizable amino acid analog. Although the compound could not participate in normal metabolic reactions, it still suppressed erebosis. This finding led the researchers to propose that amino acids alter a biophysical feature of the cytoplasm, the dense fluid interior of the cell. When amino acids accumulate, they increase the concentration of material dissolved in the cytoplasm, making it more viscous. When amino acids are scarce, the cytoplasm becomes less viscous and more watery, potentially changing how proteins, organelles, and cellular structures interact.
To test this idea directly, the researchers identified two chemically distinct molecules that could enter fly cells, increase cytoplasmic viscosity, and remain metabolically inactive. Feeding either compound to the flies produced the same protective effect as a nutrient-rich diet: erebosis was reduced. Because the molecules worked despite having different biochemical structures and no meaningful role in metabolism, their shared physical effect was likely the key factor. The experiments provided evidence that intestinal cells can use cytoplasmic viscosity as a measure of nutritional state.
Yoo and his colleagues describe this proposed sensing mechanism as “viscosatiety,” a term combining viscosity with satiety. In this model, a well-nourished gut cell becomes physically denser and more viscous, receiving a signal that resources are sufficient for survival. A nutrient-deprived cell becomes more dilute and fluid, which may destabilize its internal organization and activate erebosis. The mechanism could allow cells to integrate many nutrients at once without relying on a separate receptor or signaling pathway for each molecule. It may also explain why the total concentration of amino acids, rather than their individual identities, was so important in the experiments.
The findings raise broader questions about how physical properties of cells influence health and disease. Cytoplasmic viscosity can affect the movement of molecules, the speed of chemical reactions, the assembly of protein complexes, and the behavior of organelles. Changes in cellular crowding and fluidity have been linked to processes ranging from development to aging and neurodegeneration, but their role in nutrient sensing has received far less attention. The RIKEN team is now investigating whether erebosis and viscosatiety also occur in the intestinal cells of mice and humans. If a comparable mechanism exists in mammals, it could reveal a previously unrecognized connection between diet, cell mechanics, and the maintenance of the gut barrier.
Subject of Research: Nutrient sensing, cytoplasmic viscosity, intestinal epithelial-cell turnover, and erebosis in fruit flies.
Web References: https://doi.org/10.1073/pnas.2602724123
References: Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.2602724123
Image Credits: RIKEN
Keywords: RIKEN, Sa Kan Yoo, fruit flies, Drosophila, gut epithelium, intestinal cells, erebosis, cell death, amino acids, nutrient sensing, cytoplasmic viscosity, viscosatiety, biophysics, cell biology, metabolism, gut health, cellular physiology
