Abstract
The coordination of metabolic processes to allow increased nutrient uptake and utilization for macromolecular synthesis is central for cell growth. Although studies of bulk cell populations have revealed important metabolic and signaling requirements that impact cell growth on long time scales, whether the same regulation influences short-term cell growth remains an open question. Here we investigate cell growth by monitoring mass accumulation of mammalian cells while rapidly depleting particular nutrients. Within minutes following the depletion of glucose or glutamine, we observe a growth reduction that is larger than the mass accumulation rate of the nutrient. This indicates that if one particular nutrient is depleted, the cell rapidly adjusts the amount that other nutrients are accumulated, which is consistent with cooperative nutrient accumulation. Population measurements of nutrient sensing pathways involving mTOR, AKT, ERK, PKA, MST1, or AMPK, or pro-survival pathways involving autophagy suggest that they do not mediate this growth reduction. Furthermore, the protein synthesis rate does not change proportionally to the mass accumulation rate over these time scales, suggesting that intracellular metabolic pools buffer the growth response. Our findings demonstrate that cell growth can be regulated over much shorter time scales than previously appreciated.
Highlights
Buoyant mass accumulation reflects any change of total cell contents caused by molecules being exchanged with the extracellular environment (Fig. 1a)
There are differences between the bulk culture and suspended microchannel resonator (SMR) environments, cell growth in the SMR system is similar to what is observed in bulk culture in terms of size, inter-division time, and mass accumulation rate[3]
We measured growth of cells for 30 minutes in standard media, followed by 30 minutes in media where total osmolarity was reduced by 20 mM and found that mass accumulation rate (MAR) remained constant while in hypo-osmotic conditions (Fig. 1c), arguing that changes in MAR observed in phosphate buffered saline (PBS) were the result of nutrient depletion
Summary
Buoyant mass accumulation reflects any change of total cell contents caused by molecules being exchanged with the extracellular environment (Fig. 1a). This is a meaningful representation of cell growth for several reasons. A change in buoyant mass reflects the net flux of molecules across the cell membrane regardless of the type of flux–diffusion, active transport, or endo-/exo-cytosis. Combining this knowledge with the SMR’s precision to measure buoyant mass within 0.05% error (Supplementary Fig. 1) enables the direct measurement of single-cell mass accumulation rate (MAR) over a period of 20 minutes
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