Atomic force microscopy analysis of cell volume regulation

Proc. Natl. Acad. Sci. U.S.A.

et al. 2014

Self Cite

103

Cell mechanics plays a role in stem cell reprogramming and differentiation. To understand this process better, we created a genetically encoded optical probe, named actin-cpstFRET-actin (AcpA), to report forces in actin in living cells in real time. We showed that stemness was associated with increased force in actin. We reprogrammed HEK-293 cells into stem-like cells using no transcription factors but simply by softening the substrate. However, Madin-Darby canine kidney (MDCK) cell reprogramming required, in addition to a soft substrate, Harvey rat sarcoma viral oncogene homolog expression. Replating the stem-like cells on glass led to redifferentiation and reduced force in actin. The actin force probe was a FRET sensor, called cpstFRET (circularly permuted stretch sensitive FRET), flanked by g-actin subunits. The labeled actin expressed efficiently in HEK, MDCK, 3T3, and bovine aortic endothelial cells and in multiple stable cell lines created from those cells. The viability of the cell lines demonstrated that labeled actin did not significantly affect cell physiology. The labeled actin distribution was similar to that observed with GFPtagged actin. We also examined the stress in the actin cross-linker actinin. Actinin force was not always correlated with actin force, emphasizing the need for addressing protein specificity when discussing forces. Because actin is a primary structural protein in animal cells, understanding its force distribution is central to understanding animal cell physiology and the many linked reactions such as stress-induced gene expression. This new probe permits measuring actin forces in a wide range of experiments on preparations ranging from isolated proteins to transgenic animals.actin | force probe | stem cell | reprogramming | cell mechanics C ells have only three sources of free energy-chemical, electrical, and mechanical potential (1)-and life is driven by the flow of energy between them. The flow of mechanical energy has not been well studied in living cells outside of muscle, yet all cells are subject to endogenous and exogenous mechanical forces. The lack of progress in measuring these forces (stress) has primarily been due to a lack of suitable probes, but that has now been remedied with the development of genetically coded FRETbased force sensors (2). The literature shows many macroscopic effects of mechanical stress on cellular processes including cell motility, embryogenesis, stem cell replication and differentiation (3, 4), bone and muscle homeostasis, gene expression, protein folding (5), and membrane potential (6). However, these studies lacked the ability to measure stress in specific structural proteins, and the analyses have tended to treat cytoskeletal stresses as uniform, which we now know is not true (2, 7). We, and other groups (8), have shown that the distribution of forces is nonuniform in both time and space and protein specificity (9).The force sensors consist of a FRET pair (typically CFP/ YFP) connected by a protein linker, a biological analog of a mechanic...

Section: Resultssupporting

confidence: 74%

Actin stress in cell reprogramming

Guo

Wang

Proc. Natl. Acad. Sci. U.S.A.

et al. 2014

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103

“…We applied anisotonic stress to HEK and BAEC cells transfected with sstFRET-actinin. The stress increased with swelling as predicted by atomic force microscopy (AFM) (Spagnoli et al, 2008), graphically illustrating that osmotic stress is distributed in three dimensions and not confined to the two dimensions of the membrane. We also observed a novel transient (~2 minute) decrease in stress (increase of FRET) with hypotonic challenge, and an inverse increase in stress with a hypertonic challenge.…”

Section: Introductionmentioning

confidence: 95%

“…AFM experiments suggest that osmotic stress is not confined to the cell membrane but is supported mostly by the cytoskeleton (Spagnoli et al, 2008). We tested the prediction by measuring actinin stress in cells subjected to anisotonic solutions.…”

Section: Force Loading On Actinin During An Osmotic Challengementioning

confidence: 99%

See 1 more Smart Citation

Visualizing dynamic cytoplasmic forces with a compliance-matched FRET sensor

Meng

Journal of Cell Science

2011

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127

181

SummaryMechanical forces are ubiquitous modulators of cell activity but little is known about the mechanical stresses in the cell. Genetically encoded FRET-based force sensors now allow the measurement of local stress in specific host proteins in vivo in real time. For a minimally invasive probe, we designed one with a mechanical compliance matching that of many common cytoskeleton proteins. sstFRET is a cassette composed of Venus and Cerulean linked by a spectrin repeat. The stress sensitivity of the probe was measured in solution using DNA springs to push the donor and acceptor apart with 5-7 pN and this produced large changes in FRET. To measure cytoskeletal stress in vivo we inserted sstFRET into -actinin and expressed it in HEK and BAEC cells. Time-lapse imaging showed the presence of stress gradients in time and space, often uncorrelated with obvious changes in cell shape. The gradients could be rapidly relaxed by thrombin-induced cell contraction associated with inhibition of myosin II. The tension in actinin fluctuated rapidly (scale of seconds) illustrating a cytoskeleton in dynamic equilibrium. Stress in the cytoskeleton can be driven by macroscopic stresses applied to the cell. Using sstFRET as a tool to measure internal stress, we tested the prediction that osmotic pressure increases cytoskeletal stress. As predicted, hypotonic swelling increased the tension in actinin, confirming the model derived from AFM. Anisotonic stress also produced a novel transient (~2 minutes) decrease in stress upon exposure to a hypotonic challenge, matched by a transient increase with hypertonic stress. This suggests that, at rest, the stress axis of actinin is not parallel to the stress axis of actin and that swelling can reorient actinin to lie more parallel where it can absorb a larger fraction of the total stress. Protein stress sensors are opening new perspectives in cell biology.

“…By contrast to studies of voltage-or ligand-gated channels, it is difficult to apply a uniform mechanical stimulus to membrane-embedded channels unless they are in spherical vesicles. In principle, an ion channel can respond to increases in membrane tension as a result of cell swelling (Hua et al, 2010), but the literature contains many examples in which osmotic stress is referred to as a mechanical stress, which might not always be correct (Spagnoli et al, 2008). Cells are not spherical shells and large forces are exerted by the cytoskeleton, which lies normal to the membrane.…”

Section: The Forcesmentioning

confidence: 99%

Molecular force transduction by ion channels – diversity and unifying principles

Sukharev

Journal of Cell Science

2012

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149

140

SummaryCells perceive force through a variety of molecular sensors, of which the mechanosensitive ion channels are the most efficient and act the fastest. These channels apparently evolved to prevent osmotic lysis of the cell as a result of metabolite accumulation and/or external changes in osmolarity. From this simple beginning, nature developed specific mechanosensitive enzymes that allow us to hear, maintain balance, feel touch and regulate many systemic variables, such as blood pressure. For a channel to be mechanosensitive it needs to respond to mechanical stresses by changing its shape between the closed and open states. In that way, forces within the lipid bilayer or within a protein link can do work on the channel and stabilize its state. Ion channels have the highest turnover rates of all enzymes, and they can act as both sensors and effectors, providing the necessary fluxes to relieve osmotic pressure, shift the membrane potential or initiate chemical signaling. In this Commentary, we focus on the common mechanisms by which mechanical forces and the local environment can regulate membrane protein structure, and more specifically, mechanosensitive ion channels.