Image based modeling of bleb site selection

Abstract

Cells often employ fast, pressure-driven blebs to move through tissues or against mechanical resistance, but how bleb sites are selected and directed to the cell front remains an open question. Previously, we found that chemotaxing Dictyostelium cells preferentially bleb from concave regions, where membrane tension facilitates membrane-cortex detachment. Now, through a novel modeling approach based on actual cell contours, we use cell geometry to predict where blebs will form in migrating cells. We find that cell geometry alone, and by implication, physical forces in the membrane, is sufficient to predict the location of blebs in rounded cells moving in a highly resistive environment. The model is less successful with more polarized cells moving against less resistance, but can be greatly improved by positing a front-to-back gradient in membrane-cortex adhesion. In accord with this prediction, we find that Talin, which links membrane and cortex, forms such a front-to-back gradient. Thus our model provides a means of dissecting out the role of physical forces in controlling where blebs form, and shows that in certain circumstances they could be the major determining factor.

A bleb initiation model for chemotaxis suggests a role for myosin II clusters and cortex rupture

“…Since linker detachment via cortex rupture may not require linker proteins 50 to be maximally stretched, the use of locations of maximal linker stretching as a locator 51 of nucleation sites may be insufficient. This may very well explain the poor performance 52 of the predictive model in [15] when cell are under low compression force. 53 In this study, we introduce a model of bleb nucleation that unifies the two existing 54 points of view and provides a mechansim for cortex rupture.…”

“…What differs among the theories of nucleation is the 30 mechanism by which the plasma membrane detaches. One school of thought attributes 31 the cortex-membrane detachment directly to hydrostatic pressure differential between 32 the anterior and posterior ends of the cell [15,17,23,24]. The excess pressure detaches 33 membrane to cortex binding proteins (linker proteins) and dislodges the membrane.…”

“…Blebs have been studied from several viewpoints. They may be described as 17 geometric objects (Euclidean) [11], the result of forces on a smooth manifold 18 (differential geometry) [12][13][14][15], the result of physical force (pressure) [16][17][18][19], the result 19 of cortical tension (related to pressure) [6] and the result of fluid dynamics [20,21]. In 20 spite of the substantial progress made in the study of bleb-based motility, the precise 21 mechanism by which cytoskeletal proteins interact with biophysical and geometric forces 22 to generate blebs and coordinate cell movement using these protrusions is still unclear.…”

“…Most researchers agree that blebs initiate when a small patch of membrane is 29 detached from the cortex [1,8,15]. What differs among the theories of nucleation is the 30 mechanism by which the plasma membrane detaches.…”

“…A mathematical model which 62 represents the membrane energy as a Helfrich free energy functional [30,31] is developed 63 and used to test this hypothesis. The model extends the energy functional described 64 in [15] by including a spatially varying hydrostatic pressure term. This term accounts (Sigma-Aldrich).…”