Blebs are spherical membrane protrusions often observed during cell migration, cell spreading, cytokinesis, and apoptosis, both in cultured cells and in vivo. Bleb expansion is thought to be driven by the contractile actomyosin cortex, which generates hydrostatic pressure in the cytoplasm and can thus drive herniations of the plasma membrane. However, the role of cortical tension in bleb formation has not been directly tested, and despite the importance of blebbing, little is known about the mechanisms of bleb growth. In order to explore the link between cortical tension and bleb expansion, we induced bleb formation on cells with different tensions. Blebs were nucleated in a controlled manner by laser ablation of the cortex, mimicking endogenous bleb nucleation. Cortical tension was modified by treatments affecting the level of myosin activity or proteins regulating actin turnover. We show that there is a critical tension below which blebs cannot expand. Above this threshold, the maximal size of a bleb strongly depends on tension, and this dependence can be fitted with a model of the cortex as an active elastic material. Together, our observations and model allow us to relate bleb shape parameters to the underlying cellular mechanics and provide insights as to how bleb formation can be biochemically regulated during cell motility.T he cell cortex is a thin meshwork of actin filaments, myosin, and associated proteins that lies beneath the plasma membrane (1). Because of the presence of active myosin motors, which slide filaments with respect to one another in the network, the cortex is under tension. As a result, the cortex exerts pressure on the cytoplasm and can actively contract, driving cell deformations (2).Blebs are spherical membrane protrusions that commonly occur at the cortex during cytokinesis, cell spreading, virus uptake, and apoptosis (3-7). Moreover, increasing evidence points to an essential role for blebs as leading edge protrusions during cell migration in three-dimensional environments, particularly during embryonic development and tumor-cell dissemination (8-11; reviewed in refs. 7, 12). Despite the importance of blebbing, very little is known about the mechanisms of bleb growth.The life cycle of a bleb can be subdivided into three phases (7, 13). First, a bleb is nucleated, either by local detachment of the cortex from the plasma membrane or by local rupture of the cortex. In the subsequent growth phase, a membrane bulge, initially devoid of cortex, expands from the nucleation site. Finally, the cortex gradually reassembles at the bleb membrane, leading to bleb retraction.Bleb formation is often correlated with high myosin II activity, and myosin II inhibition prevents blebbing (6,7,10,14). For that reason, and because of their round shape and rapid expansion, blebs are commonly believed to be a direct mechanical consequence of the hydrostatic pressure exerted on the cytoplasm by the contractile cortex, which would drive bleb growth from places of local cortex weakening without any further r...

Contractile actomyosin rings drive various fundamental morphogenetic processes ranging from cytokinesis to wound healing. Actomyosin rings are generally thought to function by circumferential contraction. Here, we show that the spreading of the enveloping cell layer (EVL) over the yolk cell during zebrafish gastrulation is driven by a contractile actomyosin ring. In contrast to previous suggestions, we find that this ring functions not only by circumferential contraction but also by a flow-friction mechanism. This generates a pulling force through resistance against retrograde actomyosin flow. EVL spreading proceeds normally in situations where circumferential contraction is unproductive, indicating that the flow-friction mechanism is sufficient. Thus, actomyosin rings can function in epithelial morphogenesis through a combination of cable-constriction and flow-friction mechanisms.

The function of many eukaryotic proteins is regulated by highly dynamic changes in their nucleocytoplasmic distribution. The ability to precisely and reversibly control nuclear translocation would, therefore, allow dissecting and engineering cellular networks. Here we develop a genetically encoded, light-inducible nuclear localization signal (LINuS) based on the LOV2 domain of Avena sativa phototropin 1. LINuS is a small, versatile tag, customizable for different proteins and cell types. LINuS-mediated nuclear import is fast and reversible, and can be tuned at different levels, for instance, by introducing mutations that alter AsLOV2 domain photo-caging properties or by selecting nuclear localization signals (NLSs) of various strengths. We demonstrate the utility of LINuS in mammalian cells by controlling gene expression and entry into mitosis with blue light.