Front-to-Rear Membrane Tension Gradient in Rapidly Moving Cells

Abstract

Membrane tension is becoming recognized as an important mechanical regulator of motile cell behavior. Although membrane-tension measurements have been performed in various cell types, the tension distribution along the plasma membrane of motile cells has been largely unexplored. Here, we present an experimental study of the distribution of tension in the plasma membrane of rapidly moving fish epithelial keratocytes. We find that during steady movement the apparent membrane tension is ∼30% higher at the leading edge than at the trailing edge. Similar tension differences between the front and the rear of the cell are found in keratocyte fragments that lack a cell body. This front-to-rear tension variation likely reflects a tension gradient developed in the plasma membrane along the direction of movement due to viscous friction between the membrane and the cytoskeleton-attached protein anchors embedded in the membrane matrix. Theoretical modeling allows us to estimate the area density of these membrane anchors. Overall, our results indicate that even though membrane tension equilibrates rapidly and mechanically couples local boundary dynamics over cellular scales, steady-state variations in tension can exist in the plasma membranes of moving cells.

Cell Membranes Resist Flow

“…Fluorescently tagged transmembrane proteins typically diffuse freely in both artificial bilayers and in intact cells, albeit with a 10-100 fold lower diffusion coefficient in cells (Kusumi et al, 2005). Together these results, each consistent with the fluid mosaic model (Singer and Nicolson, 1972), led to the widespread belief that two-dimensional (2D) flow of lipids in cells mediates rapid intracellular equilibration of membrane tension (Basu et al, 2016; Diz-Muñoz et al, 2013; Fogelson and Mogilner, 2014; Gauthier et al, 2012; Gauthier et al, 2011; Houk et al, 2012; Huse, 2017; Keren et al, 2008; Keren, 2011; Kozlov and Mogilner, 2007; Lieber et al, 2015; Morris and Homann, 2001; Mueller et al, 2017; Mueller et al, 2017; Ofer et al, 2011; Pontes et al, 2017; Saha et al, 2018; Schweitzer et al, 2014; Sens and Plastino, 2015; Watanabe et al, 2013; Winkler et al, 2016), providing a long-range signaling mechanism analogous to the rapid propagation of electrical signals in neurons (Keren, 2011). Some studies have contemplated the possibility of tension gradients in rapidly migrating cells (Basu et al, 2016; Fogelson and Mogilner, 2014; Lieber et al, 2015; Schweitzer et al, 2014), but in these studies the role of membrane-cytoskeleton friction was assumed to be a modest perturbation on the essentially fluid nature of the membrane.…”

“…Most studies have assumed that membrane tension is homogeneous across a cell (Basu et al, 2016; Diz-Muñoz et al, 2013; Fogelson and Mogilner, 2014; Gauthier et al, 2012; Gauthier et al, 2011; Houk et al, 2012; Huse, 2017; Keren et al, 2008; Keren, 2011; Kozlov and Mogilner, 2007; Lieber et al, 2015; Morris and Homann, 2001; Mueller et al, 2017; Mueller et al, 2017; Ofer et al, 2011; Pontes et al, 2017; Saha et al, 2018; Schweitzer et al, 2014; Sens and Plastino, 2015; Watanabe et al, 2013; Winkler et al, 2016). This assumption was justified either by analogy to isolated lipid bilayers (Keren et al, 2008; Watanabe et al, 2013), or by reference to experiments where membrane tension was globally perturbed via osmotic shocks or drug addition (Gauthier et al, 2011; Houk et al, 2012; Mueller et al, 2017; Raucher and Sheetz, 2000).…”

“…Thus it is plausible that cell membranes, too, could exist in a state that behaves as a 2D fluid on the nanoscale but that is closer to a semi-solid gel on the cellular scale. The 2D-gel hypothesis is incompatible with the many conjectures in the literature that rapid propagation of membrane tension can mediate long-range intracellular signaling (Basu et al, 2016; Diz-Muñoz et al, 2013; Fogelson and Mogilner, 2014; Gauthier et al, 2012; Gauthier et al, 2011; Houk et al, 2012; Huse, 2017; Keren et al, 2008; Keren, 2011; Kozlov and Mogilner, 2007; Lieber et al, 2015; Morris and Homann, 2001; Mueller et al, 2017; Mueller et al, 2017; Ofer et al, 2011; Pontes et al, 2017; Saha et al, 2018; Schweitzer et al, 2014; Sens and Plastino, 2015; Watanabe et al, 2013; Winkler et al, 2016). …”

Shape and Area of Keratocytes Are Related to the Distribution and Magnitude of Their Traction Forces

“…How the distribution of traction forces regulates the shape of the leading edge is another interesting question. The plasma membrane tension of keratocytes is not uniform throughout the cell (Lieber et al, 2013(Lieber et al, , 2015. It is thus possible that the membrane tension around the rear left and right ends, where high traction forces are exerted, is A.…”

Actin Turnover in Lamellipodial Fragments

“…The membrane tension at the leading edge of motile fragments is T ~ 300 pN/µm [55]. We estimate the number of filaments abutting the membrane at the leading edge, based on the filament labeling experiments (Fig.…”