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, requiresHCO 3 − / CO 3 2 − 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) use 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 elevation in intracellular[ HCO 3 − ] mediated by theN a + / HCO 3 − cotransporter Sp_Slc4a10. However, PMCs lower their pHi regulatory capacities associated with a reduction in NHE activity. Live-cell imaging using green fluorescent protein reporter constructs in combination with intravesicular pH measurements demonstrated alkaline and acidic populations of vesicles in PMCs and extensive trafficking of largeV -type H+ -ATPase (VHA)-rich acidic vesicles in blastocoelar filopodial cells. Pharmacological and gene expression analyses underline a central role of the VHA isoformsSp_ATP6V0a1 ,Sp_ATP6V01_1 andSp_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.