Single-cell monitoring of dry mass and dry mass density reveals exocytosis of cellular dry contents in mitosis

Abstract

Cell mass and composition change with cell cycle progression. Our previous work characterized buoyant mass dynamics in mitosis (Miettinen et al., 2019), but how dry mass and cell composition change in mitosis has remained unclear. To better understand mitotic cell growth and compositional changes, we develop a single-cell approach for monitoring dry mass and the density of that dry mass every ~75 s with 1.3% and 0.3% measurement precision, respectively. We find that suspension grown mammalian cells lose dry mass and increase dry mass density following mitotic entry. These changes display large, non-genetic cell-to-cell variability, and the changes are reversed at metaphase-anaphase transition, after which dry mass continues accumulating. The change in dry mass density causes buoyant and dry mass to differ specifically in early mitosis, thus reconciling existing literature on mitotic cell growth. Mechanistically, cells in early mitosis increase lysosomal exocytosis, and inhibition of lysosomal exocytosis decreases the dry mass loss and dry mass density increase in mitosis. Overall, our work provides a new approach for monitoring single-cell dry mass and dry mass density, and reveals that mitosis is coupled to extensive exocytosis-mediated secretion of cellular contents.

The uniformity and stability of cellular mass density in mammalian cell culture

“…It was surprising to us that lipid changed in the opposite direction from proteins, which would argue that the effects we observed were not just due to changes in volume. Although quantitative regulation of cellular lipid of non-adipocyte cells is not well understood, a recent publication reported a rapid loss of lipid during mitosis ( Miettinen et al, 2022 ), suggesting that exocytosis can have a sizable effect on the lipid mass. An understanding of the nature of lipid mass changes during perturbation of cell shape and cell volume will require future studies.…”

“…Only a few methods can measure mass density directly. The Suspended Microchannel Resonator (SMR) can derive mass density by measuring cell buoyant mass in two different media of different densities, providing sensitivity of 1–6 mg/ml ( Grover et al, 2011 ; Bryan et al, 2014 ; Miettinen et al, 2022 ). However, this method is limited to cells grown in suspension and requires medium replacement.…”

Temporal segregation of biosynthetic processes is responsible for metabolic oscillations during the budding yeast cell cycle

“…The changes in protein synthesis rate during the cell cycle that we report here could thus be well hidden in the noise of the cell mass/ growth rate data from the other work 57 . Moreover, cellular composition changes during the cell cycle could potentially confound a direct comparison of the buoyant-mass data 57 with our dry-mass-related data (as shown in formula 1 in recent work 58 ). The authors of the paper 57 have cautiously not made any conclusion on cell-cycle dynamics of cell growth rate in yeast.…”

Scaling of biosynthesis and metabolism with cell size

“…The consequences of cytoplasm dilution are an active area of research, with recent evidence demonstrating effects on reaction rates ( Jin et al., 2022 ; Molines et al., 2022 ) and phase separation ( Delarue et al., 2018 ) that could negatively impact cell function. New techniques enabling cellular density measurement with unprecedented precision ( Miettinen et al., 2022 ; Oh et al., 2022 ), as well as theoretical approaches applying principles of colloidal physics to study cytoplasmic crowding ( Maheshwari et al., 2019 ), are likely to provide exciting new insights.…”