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Magnetic nanoparticle–mediated massively parallel mechanical modulation of single-cell behavior

Abstract

We report a technique for generating controllable, time-varying and localizable forces on arrays of cells in a massively parallel fashion. To achieve this, we grow magnetic nanoparticle–dosed cells in defined patterns on micromagnetic substrates. By manipulating and coalescing nanoparticles within cells, we apply localized nanoparticle-mediated forces approaching cellular yield tensions on the cortex of HeLa cells. We observed highly coordinated responses in cellular behavior, including the p21-activated kinase–dependent generation of active, leading edge–type filopodia and biasing of the metaphase plate during mitosis. The large sample size and rapid sample generation inherent to this approach allow the analysis of cells at an unprecedented rate: in a single experiment, potentially tens of thousands of cells can be stimulated for high statistical accuracy in measurements. This technique shows promise as a tool for both cell analysis and control.

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Figure 1: Parallel dynamic localization of magnetic nanoparticle clusters within arrays of cells.
Figure 2: Effects of magnetic field gradient and nanoparticle loading on cell response.
Figure 3: Nanoparticle tension-dependent asymmetry in actin polymerization.
Figure 4: Nanoparticle-mediated mechanical tension generates PAK-dependent filopodia.
Figure 5: Nanoparticle-mediated forces bias mitotic spindle orientation.

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Acknowledgements

This work was partially supported through the US National Institutes of Health Director's New Innovator Award (1DP2OD007113). The authors thank M. Bachman and N. Gunn (University of California, Irvine) for samples of PSR; J. Harrison, M. Glickman and I. Goldberg for assistance with the permalloy electroplating bath; members of the UCLA Advanced Light Microscopy Spectroscopy facility for assistance with confocal microscopy; K. Lin for high-speed imaging assistance; I. Williams for running FACS; and engineers of the UCLA Nanolab for processing assistance.

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Affiliations

Authors

Contributions

P.T. and D.D.C. contributed to the initial concept. P.T. and J.W.J. contributed to the fabrication design. D.D.C. and P.T. designed the integration of magnetic elements and single cells. P.T. developed the final fabrication and cell-patterning protocols. P.T. fabricated the micromagnetic slides and conducted the cell experiments. J.W.J. and P.T. discussed the finite-element simulation. P.T. designed the numerical analysis flow. P.T. and D.D.C. discussed and analyzed the numerical results. All authors wrote the manuscript.

Corresponding author

Correspondence to Dino Di Carlo.

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Competing interests

The authors declare no competing financial interests.

Supplementary information

Supplementary Text and Figures

Supplementary Figures 1–9 (PDF 1355 kb)

Magnetic fluorescent nanoparticles coalesce quickly under high magnetic field stimulation (resin thickness is 0.5 μm).

Nanoparticle assembly occurs over a period of ~30 min at the left edge of the cell. As nanoparticles begin coalescing at the membrane edge, small clusters of nanoparticles enter temporary filopodial protrusions that extend beyond the edge. This effect continues until around the 1-h mark, when the cell membrane begins to yield under the high tension until finally the nanoparticle cluster exhibits a 'pull-in' instability, and the entire nanoparticle cluster protrudes from the edge of the cell membrane. Cell cytoplasm is labeled with calcein AM. (MOV 1472 kb)

High-speed images of magnetic beads manipulated by individual ferromagnetic micromagnets.

Video displays the trajectories of two magnetic beads moving along the substrate surface towards the magnetized elements using resins with thicknesses of 2.5 and 5.3 μm, respectively. (AVI 970 kb)

Confocal microscopy z slices of a single cell under moderate magnetic nanoparticle–mediated tension.

At the z planes where the cell membrane is under magnetic nanoparticle–induced tension, local effects are observed including (i) local deformation of the flanking stress fiber caused by the applied mechanical tension and (ii) flanking actin-rich protrusions emanating from the regions of highest mechanical deformation. Positive myosin-X staining at the tips of protrusions indicate induced active, ECM-attachable filopodia. (AVI 479 kb)

Confocal microscopy z slices showing the rich band of stress fiber–localized phospho-PAK progressing through the cortical regions of high deformation.

This colocalization occurs whether or not the particular cell is expressing a high filopodial asymmetry and is distinct at regions directly above where the nanoparticles are localized. (AVI 1042 kb)

Cell dividing along the axis of force application.

The time-lapse video shows a single cell adhering to an I-shaped fibronectin pattern as it divides under high nanoparticle-induced tension. As the cell undergoes and completes mitosis, the cell divides biased in the direction of the force generated by the magnetic nanoparticles. Upon successful division, both cells adhere and move normally. The nanoparticles remain only in one of the daughter cells. (MOV 936 kb)

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Tseng, P., Judy, J. & Di Carlo, D. Magnetic nanoparticle–mediated massively parallel mechanical modulation of single-cell behavior. Nat Methods 9, 1113–1119 (2012). https://doi.org/10.1038/nmeth.2210

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