The effect of the endothelial cell cortex on atomic force microscopy measurements

Area of Science:

• Biophysics
• Cell Biology
• Materials Science

Background:

• Atomic force microscopy (AFM) measurements of cell modulus (E) have shown discrepancies, with sharp tips yielding higher values than spherical tips.
• Endothelial cells, such as human umbilical vein endothelial cells (HUVEC) and Schlemm's canal (SC) cells, possess prominent cell cortices.

Purpose of the Study:

• To investigate if the cell cortex contributes to the higher cell modulus values observed with sharp AFM tips compared to spherical tips.
• To reconcile conflicting results from previous AFM studies on cell mechanical properties.

Main Methods:

• AFM indentation experiments were performed on HUVEC and SC cells using both sharp and spherical tips.
• Finite element modeling was employed to simulate the mechanical response of cells with varying cortex stiffness.
• Experiments involving Latrunculin-A were conducted to assess the role of the actin cytoskeleton.

Main Results:

• AFM measurements confirmed higher apparent modulus with sharp tips (e.g., HUVEC: 3.23 ± 0.54 kPa) compared to spherical tips (e.g., HUVEC: 0.71 ± 0.16 kPa).
• Finite element modeling indicated that a stiff cell cortex could account for the observed tip-dependent modulus.
• Latrunculin-A treatment reduced the measured modulus for both tip types and diminished the difference between sharp and spherical tip measurements.

Conclusions:

• The cell cortex plays a significant role in increasing the apparent endothelial cell modulus when measured with sharp AFM tips.
• Previous explanations like strain hardening or substrate effects are less likely to explain the observed phenomenon.
• Understanding the contribution of the cell cortex is crucial for accurate AFM-based biomechanical characterization of cells.