How blebs and pseudopods cooperate during chemotaxis

Abstract

Two motors can drive extension of the leading edge of motile cells: actin polymerization and myosin-driven contraction of the cortex, producing fluid pressure and the formation of blebs. Dictyostelium cells can move with both blebs and actin-driven pseudopods at the same time, and blebs, like pseudopods, can be orientated by chemotactic gradients. Here we ask how bleb sites are selected and how the two forms of projection cooperate. We show that membrane curvature is an important, yet overlooked, factor. Dictyostelium cells were observed moving under agarose, which efficiently induces blebbing, and the dynamics of membrane deformations were analyzed. Blebs preferentially originate from negatively curved regions, generated on the flanks of either extending pseudopods or blebs themselves. This is true of cells at different developmental stages, chemotaxing to either folate or cyclic AMP and moving with both blebs and pseudopods or with blebs only. A physical model of blebbing suggests that detachment of the cell membrane is facilitated in concave areas of the cell, where membrane tension produces an outward directed force, as opposed to pulling inward in convex regions. Our findings assign a role to membrane tension in spatially coupling blebs and pseudopods, thus contributing to clustering protrusions to the cell front.C rawling cells must restrict protrusions to a limited part of their periphery if they are to move efficiently, and when these cells chemotax, the location of projections must be further controlled by the chemotactic gradient (1-3). Cellular protrusions are of two main types: those driven by actin polymerization, such as pseudopods or lamellipods, and those driven by fluid pressure, which are usually called blebs. Blebs form when the cell membrane locally detaches from the underlying cortex and is driven outward by hydrostatic pressure, created by myosin-IIdriven contraction of the cortex (4, 5). When blebs form, the cortex is left behind as an "F-actin scar," which depolymerizes, while a new actin cortex forms at the freshly exposed membrane.Blebbing is important in cells migrating in three-dimensional environments, such as during tumor invasion (6, 7), zebrafish primordial germ cell migration (8, 9), or migration of the pathogen Entamoeba histolytica in the liver (10). Dictyostelium amoebae can also move with blebs (11). In standard conditions on a 2D surface under buffer, they move mainly with F-actin-driven pseudopods, but switch progressively to bleb-driven motility when faced with mechanical resistance to their movement (12). This can be conveniently applied by inducing the cells to migrate under an elastic overlay, such as agarose, which they must deform to progress (13). Blebbing is stimulated by acute treatment with the chemoattractant cyclic AMP (14), and blebs can be chemotactically orientated by cyclic-AMP gradients (11,12).Actin-driven pseudopods are preferentially formed up-gradient by chemotaxing cells, and they can be induced on the flanks of cells by applying a steep gradient of ch...

Volume Changes During Active Shape Fluctuations in Cells

“…The same mechanism could also explain the experimental observation that blebs are nucleated preferentially in regions of negative membrane curvature [43]. Changes in osmotic pressure gradients could also contribute to the process, as suggested previously [13,20,44], but are not explicitly included in our model.…”

Contractility as a global regulator of cellular morphology, velocity, and directionality in low-adhesive fibrillary micro-environments

“…Beside cortical tension [26,34], loss of cellular adhesion has been discussed as a potential reason for membrane blebbing [31]. Moreover, the impact of membrane curvature has been shown to influence bleb formation in dictyostelium [35]. All these mentioned parameters could potentially play a role in the experiments of this study, as a reduced spreading area on 1D tracks was observed (Fig.…”

A RhoA and Rnd3 cycle regulates actin reassembly during membrane blebbing

“…Similarly, actively migrating PGCs exhibit membrane blebbing in Drosophila melanogaster embryos (2). Dictyostelium use membrane blebbing during chemotaxis (3,4). Thus, membrane blebbing is widely used as a driving force of motility across species (4,5).…”

“…Dictyostelium use membrane blebbing during chemotaxis (3,4). Thus, membrane blebbing is widely used as a driving force of motility across species (4,5). In addition, cancer cells use a membrane blebbing-associated mode of motility in metastasis (6).…”