Cellular bicarbonate accumulation and vesicular proton transport promote calcification in the sea urchin larva

The sea urchin embryo develops a calcitic endoskeleton through intracellular formation of amorphous calcium carbonate (ACC). Intracellular precipitation of ACC, requires HCO3-/CO32- concentrating as well as proton export mechanisms to promote calcification. These processes are of fundamental importance in biological mineralization, but remain largely unexplored. Here we demonstrate that the calcifying primary mesenchyme cells (PMCs) utilize Na+/H+- exchange (NHE) mechanisms to control cellular pH homeostasis during maintenance of the skeleton. During skeleton re-calcification, pHi of PMCs is increased accompanied by substantial elevations in intracellular [HCO3-] mediated by the Na+/HCO3-cotransporter Sp-Slc4a10. However, PMCs lower their pHi regulatory capacities associated with a reduction in NHE activity. Live-cell imaging using GFP reporter constructs in combination with intra-vesicular pH measurements demonstrated alkaline and acidic populations of vesicles in PMCs and extensive trafficking of large V-type H+-ATPase (VHA)-rich acidic vesicles in blastocoelar filopodial cells (BFCs). Pharmacological and gene expression analyses underline a central role of the VHA isoforms Sp-ATP6V0a1, Sp-ATP6V01_1 and Sp-ATPa1-4 for the process of skeleton re-calcification. These results highlight novel pH regulatory strategies in calcifying cells of a marine species with important implications for our understanding of the mineralization process in times of rapid changes in oceanic pH.Funding provided by: Deutsche ForschungsgemeinschaftCrossref Funder Registry ID: http://dx.doi.org/10.13039/501100001659Award Number