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.

From particle attachment to space-filling coral skeletons

“…This bilayer surrounds the vesicle, as long as it is intracellular, and then fuses again with the apical membrane when the content of the vesicle is exocytosed and released into the CF. Membrane-bound, intracellular vesicles enriched in calcium and transporting it to the biomineralization site were not only observed in corals and anemones ( 25 ), but also in sea urchin embryos ( 40 ). Vesicles carrying ACC are extracellular in chicken eggshell formation, but they are also membrane-bound ( 41 ).…”