Active contractions in single suspended epithelial cells

Investigations of active contractions in tissue cells to date have been focused on cells that exert forces via adhesion sites to substrates or to other cells. In this study we show that also suspended epithelial cells exhibit contractility, revealing that contractions can occur independently of focal adhesions. We employ the Optical Stretcher to measure adhesion-independent mechanical properties of an epithelial cell line transfected with a heat-sensitive cation channel. During stretching the heat transferred to the ion channel causes a pronounced Ca²⁺ influx through the plasma membrane that can be blocked by adequate drugs. This way the contractile forces in suspended cells are shown to be partially triggered by Ca²⁺ signaling. A phenomenological mathematical model is presented, incorporating a term accounting for the active stress exerted by the cell, which is both necessary and sufficient to describe the observed increase in strain when the Ca²⁺ influx is blocked. The median and the shape of the strain distributions depend on the activity of the cells. Hence, it is unlikely that they can be described by a simple Gaussian or log normal distribution, but depend on specific cellular properties such as active contractions. Our results underline the importance of considering activity when measuring cellular mechanical properties even in the absence of measurable contractions. Thus, the presented method to quantify active contractions of suspended cells offers new perspectives for a better understanding of cellular force generation with possible implications for medical diagnosis and therapy.