Actin-driven protrusions generate rapid long-range membrane tension propagation in cells

 

Background

Cells are surrounded by physical forces, which bend, poke, and stretch the plasma membrane, altering membrane tension^1,2. Forces applied from within the cell by membrane-tethered actin protrusions and cortical contraction also modulate membrane tension². Tension not only governs the shape of cells, but also orchestrates mechanosensitive ion channel opening, receptor trafficking, and endocytosis, which all profoundly affect fundamental cell behaviours like migration, proliferation, and differentiation^3-6.
Tension has been observed to rapidly propagate throughout the plasma membrane upon local mechanical stimulation^6-9. However, there is compelling evidence that suggests the plasma membrane resists tension propagation¹⁰. Currently, it is unclear whether the plasma membrane facilitates or resists tension propagation. Moreover, the relative contribution from the plasma membrane and underlying actin cortex to tension propagation has not been resolved. The authors addressed this by measuring tension propagation upon force inflicted locally on both the membrane and actin cortex using optogenetics and micropipette aspiration, and on the membrane only by optical trap pulling.

 

Key findings

Local actin protrusion elicits cell-wide membrane tension propagation
The authors sought to establish a system that could elicit local membrane stretch while simultaneously measuring tension at a distant site. Thus, they used Opto-PI3K, an optogenetic approach for inducing highly localized and controlled actin protrusion-driven membrane stretch in neutrophil-like HL-60 cells^11,12. Opto-PI3K employs two constructs: (1) membrane-bound iLiD, which is a synthetic protein that undergoes a conformational change upon blue light (488nm) illumination to bind to SspB peptide, and (2) SspB linked to the PI3K iSH2 domain, which constitutively binds endogenous activated PI3K. Upon illumination with blue light, iLiD (1) binds to SspB (2), inducing membrane localization of iSH2 and recruitment of activated PI3K. This drives PIP₃ production, downstream Rac activation, and rapid actin protrusion. To measure tension at a distant site, the authors used optical trapping of lectin-coated beads that bind tightly to the membrane. Using this ingenious system (Figure 1 left), they demonstrated that local actin protrusion-driven membrane stretching results in rapid tension propagation to the opposite side of the cell (Figure 1 right), which is in line with previous observations that support a role for the cell membrane in propagating tension^6-9.

Figure 1. Left: experimental system to simultaneously measure membrane tension while optogenetically inducing actin protrusion. Right: tension increases rapidly upon local actin protrusion on opposite side of the cell.

Forces acting on both plasma membrane and actin cortex drive tension propagation while pulling on membrane alone does not
The authors next aimed to resolve the observed discrepancy of plasma membrane resistance against tension propagation¹⁰. In the study from Shi and colleagues¹⁰, force was applied to the membrane by optical trapping, pulling only the membrane and not the underlying cytoskeleton. To attempt to reproduce the results from this earlier study, the authors utilized a dual tether system, where one optical trap was moved to apply force and the other was kept static to measure tension (Figure 2 left). If propagation would occur, the force measured on the static trap would increase proportionally with movement of the mobile trap. However, consistent with previous observations, movement of the mobile trap did not increase tension measurement on the static trap (Figure 2 right), indicating that force acting on the plasma membrane alone does not drive tension propagation.

Figure 2. Left: dual tether system to assess tension propagation with force applied to the membrane only. Right: tension measurement on the static trap remains constant with force from the mobile trap.

To explain these observations, the authors devised a composite model made up of the elastic plasma membrane, gel-like actin cortex, and adhesive linkers connecting the two. According to this model, tension acting on the plasma membrane alone is resisted by the actin cortex, whereas tension on the cortex and membrane propagates throughout the cell. Simulating membrane tension using this model reproduced the experimental results, where pulling on the membrane alone did not propagate tension and pulling on the cortex generated rapid tension propagation. To further validate this model, the authors performed micropipette aspiration, where a section of the cell is pulled into a micropipette tip by suction, inflicting pulling force on both the membrane and cortex. As expected, they observed rapid membrane tension propagation upon aspiration.

 

Why I chose this study

Membrane tension regulates fundamental cell processes like migration, proliferation, and differentiation. Accurately measuring and manipulating membrane tension is indispensable for furthering our understanding of cell biology. Since the plasma membrane is tightly tethered to the underlying actin cortex by linker proteins¹³, both membrane and cortex contribute to tension. However, the contribution from each of these elements has not been clear. I believe this study is impactful because it provides a simple and widely applicable model of how tension propagates that integrates both the plasma membrane and actin cortex. In my fields of cancer mechanobiology and cell biology, this model will help inform interpretations of membrane tension observations to not only consider membrane rigidity/fluidity, but the underlying actin cortex as well. This model will help elucidate how in dynamic processes like tumour invasion, the plasma membrane and actin cortex cooperate to navigate the cell through a mechanically heterogenous environment.

 

Questions for the authors

(1) Will pharmacological or genetic perturbation of linker proteins (ezrin, radixin, moesin) slow down tension propagation when force is applied on the membrane and cortex? Will it increase tension propagation upon pulling on the membrane only?

(2) What do you think could be the molecular drivers of tension propagation? Will regulators of cortical contraction (Rho/ROCK) or membrane fluidity (Ex. fatty acid elongases) govern tension propagation?

 

References

1. Pontes, B., Monzo, P., and Gauthier, N.C. (2017). Membrane tension: A challenging but universal physical parameter in Cell Biology. Seminars in Cell & Developmental Biology 71, 30–41.
2. Sitarska, E., and Diz-Muñoz, A. (2020). Pay attention to membrane tension: Mechanobiology of the cell surface. Current Opinion in Cell Biology 66, 11–18.
3. Ranade, S.S., Syeda, R., and Patapoutian, A. (2015). Mechanically activated Ion Channels. Neuron 88, 433.
4. Stewart, M.P., Helenius, J., Toyoda, Y., Ramanathan, S.P., Muller, D.J., and Hyman, A.A. (2011). Hydrostatic pressure and the actomyosin cortex drive mitotic cell rounding. Nature 469, 226–230.
5. De Belly, H., Stubb, A., Yanagida, A., Labouesse, C., Jones, P.H., Paluch, E.K., and Chalut, K.J. (2021). Membrane tension gates ERK-mediated regulation of Pluripotent Cell Fate. Cell Stem Cell 28.
6. Yanagida, A., Corujo-Simon, E., Revell, C.K., Sahu, P., Stirparo, G.G., Aspalter, I.M., Winkel, A.K., Peters, R., De Belly, H., Cassani, D.A.D., et al. (2022). Cell surface fluctuations regulate early embryonic lineage sorting. Cell 185, 1258.
7. A. Diz-Muñoz, K. Thurley, S. Chintamen, S. J. Altschuler, L. F. Wu, D. A. Fletcher, O. D. Weiner, Membrane Tension Acts Through PLD2 and mTORC2 to Limit Actin Network Assembly During Neutrophil Migration. PLOS Biology. 14, e1002474 (2016).
8. A. D. Lieber, S. Yehudai-Resheff, E. L. Barnhart, J. A. Theriot, K. Keren, Membrane Tension in Rapidly Moving Cells Is Determined by Cytoskeletal Forces. Current Biology. 23, 1409–1417 (2013).
9. K. Keren, Z. Pincus, G. M. Allen, E. L. Barnhart, G. Marriott, A. Mogilner, J. A. Theriot, Mechanism of shape determination in motile cells. Nature 453, 475–480 (2008).
10. Z. Shi, Z. T. Graber, T. Baumgart, H. A. Stone, A. E. Cohen, Cell Membranes Resist Flow. Cell. 175, 1769-1779.e13 (2018).
11. Guntas, G., Hallett, R. A., Zimmerman, S. P., Williams, T., Yumerefendi, H., Bear, J. E. & Kuhlman, B. Engineering an improved light-induced dimer (iLiD) for controlling the localization and activity of signaling proteins. Proceedings of the National Academy of Sciences 112, 112–117 (2014).
12. Graziano, B. R., Gong, D., Anderson, K. E., Pipathsouk, A., Goldberg, A. R. & Weiner, O. D. A module for RAC Temporal Signal Integration revealed with Optogenetics. Journal of Cell Biology 216, 2515–2531 (2017).
13. Fehon, R. G., McClatchey, A. I. & Bretscher, A. Organizing the cell cortex: The role of erm proteins. Nature Reviews Molecular Cell Biology 11, 276–287 (2010).

Tags: cell biology, mechanobiology, optogenetics

doi: https://doi.org/10.1242/prelights.33165

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