ALK3 is involved in cell adhesion and migration. (A) Representative images and quantification of the C2C12 cell area after depletion of ALK3 or BMPRII. Deletion of ALK3 but not BMPRII reduced the spreading of C2C12 cells in soft PLL/HA films presenting matrix-bound-BMP2 (bBMP2). At least 100 cells were analyzed per well and plate. Three independent experiments (biological replicates) were performed with two technical replicates per experiment, making up a total of at least 400 cells per condition. Scale bar, 50 μm. (B) Individual migration assay on PLL/HA films with bBMP2, showing that the deletion of ALK3 in C2C12 cells negatively affects the velocity and Euclidean distance traveled. Representative results from three independent experiments are presented as means ± SEM. N ≥ 15 cells per condition. (C) Proximity between opto-CAAX/opto-β3 integrin, opto-ALK3/opto-β3 integrin and opto–BMPRII/opto-β3 integrin induced by blue laser stimulation in MEFsv40 cells seeded on soft films presenting or not bBMP2. Only ALK3/β3 integrin proximity stimulated by blue light was able to induce cell spreading independently of bBMP2. Scale bar, 100 μm. (D) Quantification of the cell area for the experiment in C. Of note, opto-ALK3/opto-β3 integrin proximity optimized cell spreading when bBMP2 was presented. N ≥ 100 cells per condition. Unpaired t test: *0.05 > P > 0.01, ****0.00001 > P.