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

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.