Model systems for studying cell adhesion and biomimetic actin networks

Abstract

Many cellular processes, such as migration, proliferation, wound healing and tumor progression are based on cell adhesion. Amongst different cell adhesion molecules, the integrin receptors play a very significant role. Over the past decades the function and signalling of various such integrins have been studied by incorporating the proteins into lipid membranes. These proteolipid structures lay the foundation for the development of artificial cells, which are able to adhere to substrates. To build biomimetic models for studying cell shape and spreading, actin networks can be incorporated into lipid vesicles, too. We here review the mechanisms of integrin-mediated cell adhesion and recent advances in the field of minimal cells towards synthetic adhesion. We focus on reconstituting integrins into lipid structures for mimicking cell adhesion and on the incorporation of actin networks and talin into model cells.

Keywords: actin network; cell adhesion; giant unilamellar vesicle; integrin; lipid bilayer; protein reconstitution; synthetic cell; talin.

Figure 1

Schematic view of active integrin molecules linking the ECM to the actin cytoskeleton. The heads of the integrin molecules attach directly to their ligand molecules in the ECM; the intracellular tail of integrin binds to proteins like talin and FAK. On the intracellular side, active talin dimers also bind to filamentous actin. Other proteins like FAK form an indirect linkage to the actin cortex together with further cytoplasmic proteins. These intracellular anchor proteins, which include vinculin and α-actinin, help to regulate and reinforce the actin–integrin linkage.

Figure 2

Cryoelectron micrographs of negatively stained DMPG/DMPC vesicles containing integrin α_IIbβ₃. Negatively stained vesicles with (A) high and (B) low surface density of integrin α_IIbβ₃. The scale bar represents 100 nm. (Reprinted with permission from [40]. Copyright (1997) American Chemical Society.)

Figure 3

GUVs containing integrins interacting with a fibrinogen-coated substrate: (A) adhesion is detected with RICM (black patch), (B) Scheme of the minimal cell system. The scale bar represents 10 μm (Reprinted with permission from [54]. Copyright (2009) Elsevier Ltd.)

Figure 4

Dependence of actin/α-actinin network structures on the vesicle size. The 3D reconstructions of networks by confocal fluorescence microscopy (at a temperature of 4 °C). (A) Examples of rings obtained in vesicles of diameter d < 12 μm. (B) Spiderweblike networks formed in vesicles with diameter d > 12 μm (Reprinted with permission from [60]. Copyright (2002) American Physical Society.)

Figure 5

Thin actin protrusions emerge from dendritic actin networks. Phase-contrast (A) and spinning-disc confocal images (B) of lipid membrane (green) and (C) actin (red) show multiple protrusions in the lumen of a GUV. Overlay of the fluorescence images confirms that the membrane protrusions are supported by actin filaments. The scale bar represents 5 μm. (Reprinted with permission from [67]. Copyright (2008) Nature Publishing Group.)

Figure 6

Confocal fluorescence micrographs of giant actin-filled liposomes. Lipid membranes are labelled with rhodamine (red); actin is labelled with AlexaFluor 488 (green). Insets indicate the nature of the actin-membrane interaction. (A) The actin filament solution inside a liposome with an inert membrane (containing PEG-lipids) is homogeneous and displays a depletion zone underneath the membrane. (B) Liposomes, which contain biotinylated lipids, encapsulate networks of biotinylated actin filaments that are coupled to the membrane via biotin–streptavidin bonds. The scale bars represent 10 μm. (Adapted with permission from [72]. Copyright (2011) American Chemical Society.)

Figure 7

Transformed liposomes observed by dark-field microscopy in the presence of talin. Liposomes used were prepared from phosphatidylethanolamine and phosphatidylglycerol (1:1, mol:mol). (A) Closed spherical liposomes obtained in the absence of talin. (B) Cup-shaped liposomes observed in the presence of 0.4 mM talin. Image courtesy of K. Takiguchi (unpublished).