Highlights d Determining the polarity of individual actin filaments inside cells d Reconstruction of actin networks from cryo-tomograms d The polarity of actin changes from mixed to uniform along focal adhesions

The mechanisms underlying the cellular response to extracellular matrices (ECM), consisting of multiple adhesive ligands, are still poorly understood. Here we address this topic by monitoring the differential cellular response to two different extracellular adhesion molecules – fibronectin, a major integrin ligand, and galectin-8, a lectin, that binds β-galactoside residues, as well as to mixtures of the two proteins. Cell spreading on galectin-8 coated substrates results in a larger projected cell area, a more extensive extension of filopodia, yet an inability to form focal adhesions and stress fibers, compared to cell spreading on fibronectin. These differences can be partially reversed by experimental manipulations of small G-proteins of the Rho family and their downstream targets, such as formins, Arp2/3 complex, and Rho kinase. We also show that the physical adhesion of cells to galectin-8 is stronger than the adhesion to fibronectin. Notably, galectin-8 and fibronectin differentially regulate cell spreading and focal adhesion formation, yet they act synergistically to upregulate the number and length of filopodia. The physiological significance of the coherent cellular response to a molecularly complex matrix is discussed.

Supplementary Figure 1 | Missing wedge and projection of subtomograms. (a) Structure of an actin filament (EMD-6179 [1]), that was oriented parallel to the tilt-axis (y-axis), and then distorted by the missing wedge. (b) However, if the filament was oriented parallel to the x-axis (orthogonal to the tilt-axis), the anisotropic distortion caused by the missing wedge in z-direction is substantially more pronounced. (c) Left to right: projection of filament (a) in z-direction, projection of filament (b) in z-direction, and difference image between the two projections. The difference image is featureless, which indicates that the missing wedge induced anisotropy vanishes in the projection images. (d) However, if a mask in z-direction is applied before projection (in this case the height of the mask was 11 nm), the difference image is not featureless anymore. The influence of this mask on the precision of APT is part of the validation of the method. Scale bars 10 nm.

Adherent filopodia are elongated finger-like membrane protrusions, extending from the edges of diverse cell types and participating in cell adhesion, spreading, migration and environmental sensing. The formation and elongation of filopodia are driven by the polymerization of parallel actin filaments, comprising the filopodia cytoskeletal core. Here, we report that adherent filopodia, formed during the spreading of cultured cells on galectin-8-coated substrates, tend to change the direction of their extension in a chiral fashion, acquiring a left-bent shape. Cryo-electron tomography examination indicated that turning of the filopodia tip to the left is accompanied by the displacement of the actin core bundle to the right of the filopodia midline. Reduction of the adhesion to galectin-8 by treatment with thiodigalactoside abolished the filopodia chirality. By modulating the expression of a variety of actin-associated filopodia proteins, we identified myosin-X and formin DAAM1 as major filopodia chirality promoting factors. Formin mDia1, actin filament elongation factor VASP, and actin filament crosslinker fascin were also shown to be involved. Thus, the simple actin cytoskeleton of filopodia, together with a small number of associated proteins are sufficient to drive a complex navigation process, manifested by the development of left-right asymmetry in these cellular protrusions.

Physical interactions of cells with the underlying extracellular matrix (ECM) play key roles in multiple cellular processes. The actin cytoskeleton is a central driver and regulator of cellular dynamics, that produces membrane-protrusions such as lamellipodia and filopodia. Here, we examined actin organization in expanding lamellipodia during early stages of cell spreading. To gain insight into the 3D actin organization, we plated fibroblasts on galectin-8 coated EM grids, an ECM protein presents in disease states. We then combined cryo-electron tomography with advanced image processing tools for reconstructing the structure of F-actin in the lamellipodia. This approach enabled us to resolve the polarity and orientation of filaments, and the structure of the Arp2/3 complexes associated with F-actin branches. We show that F-actin in lamellipodial protrusions forms a dense network with three distinct sub-domains. One consists primarily of radial filaments, with their barbed ends pointing towards the membrane, the other is enriched with parallel filaments that run between the radial fibers, in addition to an intermediate sub-domain. Surprisingly, a minor, yet significant (~10%) population of actin filaments, are oriented with their barbed-ends towards the cell center. Our results provide structural insights into F-actin assembly and dynamic reorganization in the leading edge of spreading cells.