Effective cell membrane tension is independent of polyacrylamide substrate stiffness.

Most animal cells are surrounded by a cell membrane and an underlying actomyosin cortex. Both structures are linked, and they are under tension. In-plane membrane tension and cortical tension both influence many cellular processes, including cell migration, division, and endocytosis. However, while actomyosin tension is regulated by substrate stiffness, how membrane tension responds to mechanical substrate properties is currently poorly understood. Here, we probed the effective membrane tension of neurons and fibroblasts cultured on glass and polyacrylamide substrates of varying stiffness using optical tweezers. In contrast to actomyosin-based traction forces, both peak forces and steady-state tether forces of cells cultured on hydrogels were independent of substrate stiffness and did not change after blocking myosin II activity using blebbistatin, indicating that tether and traction forces are not directly linked. Peak forces in fibroblasts on hydrogels were about twice as high as those in neurons, indicating stronger membrane-cortex adhesion in fibroblasts. Steady-state tether forces were generally higher in cells cultured on hydrogels than on glass, which we explain by a mechanical model. Our results provide new insights into the complex regulation of effective membrane tension and pave the way for a deeper understanding of the biological processes it instructs.

Noninvasive measurement of the refractive index of cell organelles using surface plasmon resonance microscopy.

The health of a eukaryotic cell depends on the proper functioning of its cell organelles. Characterizing these nanometer- to micrometer-scaled specialized subunits without disturbing the cell is challenging but can also provide valuable insights regarding the state of a cell. We show that objective-based scanning surface plasmon resonance microscopy can be used to analyze the refractive index of cell organelles quantitatively in a noninvasive and label-free manner with a lateral resolution at the diffraction limit.

Nanometer-Resolved Mapping of Cell-Substrate Distances of Contracting Cardiomyocytes Using Surface Plasmon Resonance Microscopy.

It has been shown that quantitative measurements of the cell-substrate distance of steady cells are possible with scanning surface plasmon resonance microscopy setups in combination with an angle resolved analysis. However, the accuracy of the determined cell-substrate distances as well as the capabilities for the investigation of cell dynamics remained limited due to the assumption of a homogeneous refractive index of the cytosol. Strong spatial or temporal deviations between the local refractive index and the average value can result in errors in the calculated cell-substrate distance of around 100 nm, while the average accuracy was determined to 37 nm. Here, we present a combination of acquisition and analysis techniques that enables the measurement of the cell-substrate distance of contractile cells as well as the study of intracellular processes through changes in the refractive index at the diffraction limit. By decoupling the measurement of the cell-substrate distance and the refractive index of the cytoplasm, we could increase the accuracy of the distance measurement on average by a factor of 25 reaching 1.5 nm under ideal conditions. We show a temporal and spatial mapping of changes in the refractive index and the cell-substrate distance which strongly correlate with the action potentials and reconstruct the three-dimensional profile of the basal cell membrane and its dynamics, while we reached an actual measurement accuracy of 2.3 nm.