Calcium-vesicles perform active diffusion in the sea urchin embryo during larval biomineralization

ABSTRACT Biomineralization is the process by which organisms use minerals to harden their tissues and provide them with physical support. Biomineralizing cells concentrate the mineral in vesicles that they secret into a dedicated compartment where crystallization occurs. The dynamics of mineral-vesicle motion and the molecular mechanisms that regulate it, are not well understood. Sea urchin larval skeletogenesis provides an excellent platform for the analyses of vesicle kinetics. Here we used calcein labeling and lattice light-sheet microscopy to investigate the three-dimensional (3D) vesicle dynamics in control sea urchin embryos and in Vascular Endothelial Growth Factor Receptor (VEGFR) inhibited embryos, where skeletogenesis is blocked. We developed computational tools for displaying 3D-volumetric movies and for automatically quantifying vesicle dynamics in the different embryonic tissues. Our findings imply that calcium vesicles perform an active diffusion motion in all the cells of the embryo. This mode of diffusion is defined by the mechanical properties of the cells and the dynamic rearrangements of the cytoskeletal network. The diffusion coefficient is larger in the mesenchymal skeletogenic cells compared to the epithelial ectodermal cells, possibly due to the distinct mechanical properties of the two tissues. Vesicle motion is not directed toward the biomineralization compartment, but the vesicles slow down when they approach it, and probably bind for mineral deposition. Under VEGFR inhibition, vesicle volume increases and vesicle speed is reduced but the vesicles continue in their diffusive motion. Overall, our studies provide an unprecedented view of calcium vesicle 3D-dynamics and illuminate possible molecular mechanisms that control vesicle dynamics and deposition. Authors summary Biomineralization is a widespread, fundamental process by which organisms use minerals to harden their tissues. Mineral bearing vesicles were observed in biomineralizing cells and believed to play an essential role in biomineralization, yet little is known about their three-dimensional (3D) dynamics. Here we quantify 3D-vesicle-dynamics during skeleton formation in sea urchin larvae, using lattice-light-sheet microscopy. We discover that calcium vesicles perform an active diffusive motion in both calcifying and non-calcifying cells of the embryo. The motion of the vesicles in the calcifying skeletogenic cells, is not directed toward the biomineralization compartment and has a diffusion coefficient of ~0.01μm2/sec and average speed of ~0.09μm/sec. The inhibition of Vascular Endothelial Growth Factor Receptor (VEGFR) that blocks skeletogenesis, increases vesicle volume and decreases vesicle speed but doesn’t change the diffusion mode in the embryo cells. Our studies reveal the diffusive motion of mineral bearing vesicles and have implications on basic and translational research.