Atomic force microscope elastography reveals phenotypic differences in alveolar cell stiffness

To understand the connection between alveolar mechanics and key biochemical events such as surfactant secretion, one first needs to characterize the underlying mechanical properties of the lung parenchyma and its cellular constituents. In this study, the mechanics of three major cell types from the neonatal rat lung were studied; primary alveolar type I (AT1) and type II (AT2) epithelial cells and lung fibroblasts were isolated using enzymatic digestion. Atomic force microscopy indentation was used to map the three-dimensional distribution of apparent depth-dependent pointwise elastic modulus. Histograms of apparent modulus data from all three cell types indicated non-Gaussian distributions that were highly skewed and appeared multimodal for AT2 cells and fibroblasts. Nuclear stiffness in all three cell types was similar (2.5 ± 1.0 kPa in AT1 vs. 3.1 ± 1.5 kPa in AT2 vs. 3.3 ± 0.8 kPa in fibroblasts; n = 10 each), whereas cytoplasmic moduli were significantly higher in fibroblasts and AT2 cells (6.0 ± 2.3 and 4.7 ± 2.9 kPa vs. 2.5 ± 1.2 kPa). In both epithelial cell types, actin was arranged in sparse clusters, whereas prominent actin stress fibers were observed in lung fibroblasts. No systematic difference in actin or microtubule organization was noted between AT1 and AT2 cells. Atomic force microscope elastography, combined with live-cell fluorescence imaging, revealed that the stiffer measurements in AT2 cells often colocalized with lamellar bodies. These findings partially explain reported heterogeneity of alveolar cell deformation during in situ lung inflation and provide needed data for better understanding of how mechanical stretch influences surfactant release.