Cells modify their shape in response to the extracellular environment through dynamic remodeling of the actin cytoskeleton by actin-binding proteins (ABPs) 1-4 . The relation between actin dynamics and spreading is well-understood for cells on flat glass coverslips; much less is known about cell morphogenesis in compliant three-dimensional environments, and, in particular, how ABPs contribute to this process 5 .Here, we knocked-out a diverse set of ABPs, and evaluated the effect of each on cell spreading on planar glass surfaces (2D) and in reconstituted collagen gels (3D). Our morphometric analyses identify the Arp2/3 complex and its associated regulatory genes among the ABPs that contribute most strongly to cell spreading in 3D, but marginally in 2D. Cells lacking Arp3 have reduced spreading specifically in 3D, and display stiffness-dependent cell-matrix adhesion defects. Through manipulation of vinculin activity, we determine that the Arp3 knock-out phenotype largely arises from the lack of direct interaction between vinculin and Arp2/3 complex. This interaction is dispensable for cell spreading in 2D. These data highlight that actin architectural features necessary for adhesion formation and cell spreading in 3D are efficiently compensated on flat and stiff surfaces.Cell shape control is impacted by extracellular matrix (ECM) mechanics, composition, and architecture 1,2 . Most of our understanding of the mechanism of cell morphogenesis stems from experiments performed on flat and stiff environments such as glass or plastic. On these planar surfaces (2D), cells are able to spread without restrictions and usually maximally stretch themselves, resulting in a flat morphology with abundant filamentous actin (F-actin) stress fibers (Fig. 1a). When the same cells are embedded in a three-dimensional reconstituted matrix (3D), such as collagen gels, they adopt a multipolar branched morphology with diminished stress fiber formation 6-9 (Fig. 1b,c). These differences are in part manifestation of an altered organization of the F-actin cytoskeleton, which is governed by differential association and activation of ABPs in response to shifts in the cell environment 3,10-14 .To gain insight into the roles of diverse ABPs play in supporting 2D versus 3D cell morphologies, we performed a targeted CRISPR/Cas9-mediated knock-out (KO) screen of formins (mDia1/DIAPH1, mDia2/DIAPH3,

Ehlers-Danlos Syndromes (EDSs) are a group of connective tissue disorders, characterized by skin stretchability, joint hypermobility and instability. Mechanically, various tissues from EDS patients exhibit lowered elastic modulus and lowered ultimate strength. This change in mechanics has been associated with EDS symptoms. However, recent evidence points toward a possibility that the comorbidities of EDS could be also associated with reduced tissue stiffness. In this review, we focus on mast cell activation syndrome and impaired wound healing, comorbidities associated with the classical type (cEDS) and the hypermobile type (hEDS), respectively, and discuss potential mechanobiological pathways involved in the comorbidities.