Investigating cell-ECM contact changes in response to hypoosmotic stimulation of hepatocytes in vivo with DW-RICM.

Hepatocyte volume regulation has been shown to play an important role in cellular metabolism, proliferation, viability and especially in hepatic functions such as bile formation and proteolysis. Recent studies on liver explants led to the assumption that cell volume changes present a trigger for outside-in signaling via integrins, a protein family involved in mediating cellular response to binding to the extracellular matrix (ECM). However, it remains elusive how these volume change related signaling events are transducted on a single cell level and how these events are influenced and controlled by ECM interactions. One could speculate that an increase in cell volume leads to an increase in integrin/ECM contacts which causes activation of integrins, which act as mechano-sensors. In order to test this idea, it was an important issue to quantify the cell volume-dependence of the contact areas between the cell and the surrounding ECM. In this study we used two wavelength reflection interference contrast microscopy (DW-RICM) to directly observe the dynamics of cell-substrate contacts, mimicking cell-ECM interactions, in response to a controlled and well-defined volume change induced by hypoosmotic stimulation. This is the first time a non-invasive, label-free method is used to uncover a volume change related response of in vitro hepatocytes in real time. The cell cluster analysis we present here agrees well with previous studies on ex vivo whole liver explants. Moreover, we show that the increase in contact area after cell swelling is a reversible process, while the reorganisation of contacts depends on the type of ECM molecules presented to the cells. As our method complements common whole liver studies providing additional insight on a cell cluster level, we expect this technique to be particular suitable for further detailed studies of osmotic stimulation not only in hepatocytes, but also other cell types.