Anisotropy vs isotropy in living cell indentation with AFM

Area of Science:

• Cellular mechanics
• Biophysics
• Materials science

Background:

• Nanoindentation assumes isotropic cell deformation for mechanical property measurement.
• Cell membrane and cytoskeleton are expected to deform axisymmetrically under spherical indentation.
• Physiological forces can alter cellular mechanical properties.

Purpose of the Study:

• To directly observe and quantify the local deformation geometry of living cells during nanoindentation.
• To investigate the effect of the perinuclear actin cap on cellular mechanical anisotropy.
• To evaluate the suitability of anisotropic models for describing cell deformation.

Main Methods:

• Integrated Atomic Force Microscopy (AFM) and spinning disk confocal (SDC) microscopy for live cell imaging.
• Nanoindentation experiments on various adherent cell types.
• Finite element computations and numerical analysis of deformation data.
• Quantification of the impact of fluorescent stains on mechanical measurements.

Main Results:

• Cells with an actin cap exhibited strongly non-axisymmetric membrane deformation, indicating local mechanical anisotropy.
• Cells lacking an actin cap (MDA-MB-231, latrunculin A-treated NIH 3T3) showed axisymmetric deformation, consistent with isotropy.
• A transverse isotropy model effectively captured the observed non-axisymmetric deformation.

Conclusions:

• The perinuclear actin cap induces local mechanical anisotropy in living cells.
• Standard nanoindentation assumptions of isotropy may not apply to all cell types.
• Accurate mechanical property estimation requires considering cellular anisotropy and employing appropriate modeling techniques.