Extracellular carbonic anhydrase activity promotes a carbon concentration mechanism in metazoan calcifying cells

• Main Authors: Matt, Ann-Sophie, Chang, William Weijen, Hu, Marian Y
• Format: Dataset
• Language: English
• Published: PANGAEA 2022
• Subjects: biomineralization; carbon fixation; global carbon cycle; intracellular pH; metabolic CO2; Ocean acidification
• Online Access: https://doi.pangaea.de/10.1594/PANGAEA.947954; https://doi.org/10.1594/PANGAEA.947954

Many calcifying organisms utilize metabolic CO2 to generate CaCO3 minerals to harden their shells and skeletons. Carbonic anhydrases are evolutionary ancient enzymes that were proposed to play a key role in the calcification process with the underlying mechanisms being little understood. Here we used the calcifying primary mesenchyme cells of the sea urchin larva to study the role of cytosolic (iCAs) and extracellular carbonic anhydrases (eCAs) in the cellular carbon concentration mechanism (CCM). Molecular analyses identified iCAs and eCAs in PMCs and highlight the prominent expression of a GPI-anchored membrane-bound CA (Cara7). Intracellular pH recordings in combination with CO2 pulse experiments demonstrated iCA activity in PMCs. iCA activity measurements together with pharmacological approaches revealed an opposing contribution of iCAs and eCAs on the CCM. H+-selective electrodes were used to demonstrate eCA catalyzed CO2 hydration rates at the cell surface. Knock-down of Cara7 reduced extracellular CO2 hydration rates accompanied by impaired formation of specific skeletal segments. Finally, reduced pHi regulatory capacities during inhibition and knock-down of Cara7 underline a role of this eCA in cellular HCO3- uptake. This work revealed the function of carbonic anhydrases in the cellular CCM of a marine calcifying animal. Extracellular hydration of metabolic CO2 by Cara7 coupled to HCO3- uptake mechanisms mitigates the loss of carbon and reduces the cellular proton load during the mineralization process. The findings of this work provide insights into the cellular mechanisms of an ancient biological process that is capable of utilizing CO2 to generate a versatile construction material.