Blebs-Formation, Regulation, Positioning, and Role in Amoeboid Cell Migration

Abstract

In the context of development, tissue homeostasis, immune surveillance, and pathological conditions such as cancer metastasis and inflammation, migrating amoeboid cells commonly form protrusions called blebs. For these spherical protrusions to inflate, the force for pushing the membrane forward depends on actomyosin contraction rather than active actin assembly. Accordingly, blebs exhibit distinct dynamics and regulation. In this review, we first examine the mechanisms that control the inflation of blebs and bias their formation in the direction of the cell's leading edge and present current views concerning the role blebs play in promoting cell locomotion. While certain motile amoeboid cells exclusively form blebs, others form blebs as well as other protrusion types. We describe factors in the environment and cell-intrinsic activities that determine the proportion of the different forms of protrusions cells produce.

Keywords: actin; amoeboid migration; bleb; cell migration; cell polarity; myosin; retrograde flow.

FIGURE 1

Protrusion formation mechanisms. (A) Arrays of polymerizing actin filaments pushing against the plasma membrane generate force to drive membrane protrusion forward. (B) Actomyosin contraction generates hydrostatic pressure that powers the inflation of a spherical membrane bleb.

FIGURE 2

Factors controlling the position of bleb formation. (A) Global hydrostatic pressure induces bleb formation at locations where the actin cortex is disrupted. (B) Preferential initiation of blebs at locations with reduced levels of membrane linker molecules. (C) Locally increased contractility could induce bleb formation at these loci if poroelastic properties of the cytosol prevent rapid pressure equilibration. (D) Depending on membrane curvature, the force resulting from membrane tension is either directed inward in the case of positive membrane curvature (left) or outward for negative membrane curvature (right). Therefore, membrane delamination occurs more readily at locations of negative membrane curvature.

FIGURE 3

Models for directing bleb formation to the front of migrating cells. (A) Actin polymerization at the future leading edge provides a platform for the recruitment of myosin motors. Actomyosin polymerization and contraction result in actin retrograde flow, advecting membrane linker molecules toward the opposite aspect of the cell, thus defining the rear. Accumulation of membrane linker molecules at the rear prevents blebbing at this aspect of the cell. Increased actomyosin contractility at the front could introduce cortical breaks, which, in addition to the reduced levels of membrane linkers, could favor the formation of blebs at the leading edge. The polarized contractile activity could also result in local pressure elevation, favoring bleb formation if the hydrostatic pressure does not rapidly equilibrate throughout the cytosol. (B) Local fluctuations in contractility can cause the flow of actin and myosin toward the contractile region, thus dictating the position of the future rear. High retrograde flow speeds of polymerized actin from the front maintain polarization by establishing a cortical density gradient in the direction of the rear, thus favoring positioning of the bleb at the opposing side. Likewise, membrane linkers accumulate at the rear, potentially further inhibiting bleb formation at this aspect of the cell.

FIGURE 4

Factors determining the degree of bleb formation in migrating cells. While mesenchymal motility is associated with high levels of substrate adhesion (upper left), polymerization-driven amoeboid motility occurs at lower levels of substrate adhesion (upper right). As the contractility level increases, amoeboid-motile cells form more blebs than polymerization-driven protrusions (bottom left). Cells displaying non-persistent bleb-based motility can switch to stable bleb migration at very high contractility levels (bottom right). Confinement can also promote blebbing activity.