Passive coupling of membrane tension and cell volume during active response of cells to osmosis
Chloé Roffay,
Guillaume Molinard,
Kyoohyun Kim,
Marta Urbanska,
Virginia Andrade,
Victoria Barbarasa,
Paulina Nowak,
Vincent Mercier,
José García-Calvo, et al.
Proceedings of the National Academy of Sciences of the United States of America
118
(47)
e2103228118
(2021)
| Journal
| PDF
During osmotic changes of their environment, cells actively regulate their volume and plasma membrane tension that can passively change through osmosis. How tension and volume are coupled during osmotic adaptation remains unknown, as their quantitative characterization is lacking. Here, we performed dynamic membrane tension and cell volume measurements during osmotic shocks. During the first few seconds following the shock, cell volume varied to equilibrate osmotic pressures inside and outside the cell, and membrane tension dynamically followed these changes. A theoretical model based on the passive, reversible unfolding of the membrane as it detaches from the actin cortex during volume increase quantitatively describes our data. After the initial response, tension and volume recovered from hypoosmotic shocks but not from hyperosmotic shocks. Using a fluorescent membrane tension probe (fluorescent lipid tension reporter [Flipper-TR]), we investigated the coupling between tension and volume during these asymmetric recoveries. Caveolae depletion and pharmacological inhibition of ion transporters and channels, mTORCs, and the cytoskeleton all affected tension and volume responses. Treatments targeting mTORC2 and specific downstream effectors caused identical changes to both tension and volume responses, their coupling remaining the same. This supports that the coupling of tension and volume responses to osmotic shocks is primarily regulated by mTORC2.
The Xenopus spindle is as dense as the surrounding cytoplasm
Abin Biswas,
Kyoohyun Kim,
Gheorghe Cojoc,
Jochen Guck,
Simone Reber
The mitotic spindle is a self-organizing molecular machine, where hundreds of different molecules continuously interact to maintain a dynamic steady state. While our understanding of key molecular players in spindle assembly is significant, it is still largely unknown how the spindle’s material properties emerge from molecular interactions. Here, we use correlative fluorescence imaging and label-free three-dimensional optical diffraction tomography (ODT) to measure the Xenopus spindle’s mass density distribution. While the spindle has been commonly referred to as a denser phase of the cytoplasm, we find that it has the same density as its surrounding, which makes it neutrally buoyant. Molecular perturbations suggest that spindle mass density can be modulated by tuning microtubule nucleation and dynamics. Together, ODT provides direct, unbiased, and quantitative information of the spindle’s emergent physical properties—essential to advance predictive frameworks of spindle assembly and function.
Contact
Core Facility Microscopy Kyoohyun Kim
Max-Planck-Zentrum für Physik und Medizin Kussmaulallee 2 91054 Erlangen, Germany
