Is the inside of a living cell a glass stirred by metabolism?
The cell interior stiffens into a glass-like solid when its energy supply is cut, but in living cells metabolic activity keeps it fluidized near a jamming point, largely independent of the actin skeleton.
Source
Activity-dependent glassy cell mechanics Ⅰ: Mechanical properties measured with active microrheology
Study at a glance
- Design
- Other — Lab experiment: feedback-tracking optical-trap active microrheology on PEG-coated beads inside cultured HeLa cells, comparing untreated, actin-disrupted, cell-cycle-labelled and ATP-depleted cells.
- N
- N=21 · 21 probe measurements in untreated HeLa cells; 9 for the cytochalasin D actin-disruption comparison and 9 in ATP-depleted cells.
- Population
- HeLa cells grown as a confluent epithelial-like monolayer
- Outcome
- Frequency-dependent complex shear modulus G(ω) of the cytoplasm
Structured fields used in claim comparison tables when every cited study has a complete layer.
What they did
The authors pushed tiny non-sticky beads inside HeLa cells with an oscillating optical trap and measured how the beads moved, giving the cytoplasm's elastic and viscous response across a wide frequency range. A feedback-controlled stage kept the vigorously jiggling bead in the laser focus. They compared untreated cells with cells whose actin was disrupted by drugs, cells in different interphase stages of the cell cycle, and cells starved of ATP.
What they found
In untreated cells the shear modulus rose with the square root of frequency, the signature predicted for dense disordered colloids near jamming rather than the pattern expected for a cytoskeletal polymer network. Disrupting actin made no significant difference, and moving from G1 to S/G2 raised the moduli only modestly (about 40% and 30%). Depleting ATP added a low-frequency elastic plateau about ten times larger than in untreated cells, meaning the cytoplasm solidified without metabolic activity.
The limits
What it doesn't show
The work uses one cancer cell line in confluent monolayers and probes only the middle of the cell, so it says little about the actin-rich cell cortex or other cell types. Sample sizes are small (21, 9 and 9 measurements, and only 3 paired before-after ATP measurements), and cells in mitosis could not be measured at all. ATP depletion with metabolic poisons may also start cell death, so the stiffening may not be caused by loss of activity alone. The link to critical jamming is an interpretation consistent with the data rather than a direct test.
Key terms
- Active microrheology
- Measuring a material's viscoelasticity by applying a known oscillating force to an embedded probe bead and recording its displacement.
- Complex shear modulus G(ω)
- A frequency-dependent quantity whose real part (G′) measures elastic storage and imaginary part (G″) measures viscous loss.
- Glassy cytoplasm
- The idea that the crowded cell interior behaves like a colloidal glass that is nearly solid unless something drives rearrangements.
- Jamming transition
- The point at which a dense disordered packing of particles changes from flowing to rigid.
- Elastic plateau
- A frequency-independent elastic modulus at low frequencies, indicating solid-like behaviour.
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Quiz yourself
What scaling of the shear modulus with frequency did untreated cells show?
Common questions
Why doesn't disrupting actin change the result if the cytoskeleton is supposed to make cells stiff?
The beads were PEG-coated so they did not bind filaments and were placed deep in cells where actin is sparse; the stiffness measured is that of the crowded cytoplasm, while earlier techniques like AFM measured the actin-rich surface.
What does it mean that activity fluidizes the cell?
Motor proteins and other ATP-powered enzymes stir the crowded interior, letting it rearrange and flow; when ATP is removed that stirring stops and the cytoplasm develops a solid-like elastic plateau.
Why is the square-root frequency dependence interesting?
It differs from the 3/4 exponent typical of semiflexible polymer networks and matches theoretical predictions for disordered colloidal systems at the edge of jamming.
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