A solution to Nature's haemoglobin knockout: a plasma-accessible carbonic anhydrase catalyses CO2 excretion in Antarctic icefish gills

In all vertebrates studied to date, CO 2 excretion depends on the enzyme carbonic anhydrase (CA) that catalyses the rapid conversion of HCO 3 − to CO 2 at the gas-exchange organs. The largest pool of CA is present within red blood cells (RBCs) and, in some vertebrates, plasma-accessible CA (paCA) is...

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Bibliographic Details
Published in:Journal of Experimental Biology
Main Authors: Harter, Till S., Sackville, Michael A., Wilson, Jonathan M., Metzger, David C. H., Egginton, Stuart, Esbaugh, Andrew J., Farrell, Anthony P., Brauner, Colin J.
Format: Text
Language:English
Published: The Company of Biologists Ltd 2018
Subjects:
RESEARCH ARTICLE
Antarctic
Antarc*
Icefish
Notothenia rossii
Online Access:http://jeb.biologists.org/cgi/content/short/221/22/jeb190918
https://doi.org/10.1242/jeb.190918
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Summary:In all vertebrates studied to date, CO 2 excretion depends on the enzyme carbonic anhydrase (CA) that catalyses the rapid conversion of HCO 3 − to CO 2 at the gas-exchange organs. The largest pool of CA is present within red blood cells (RBCs) and, in some vertebrates, plasma-accessible CA (paCA) isoforms participate in CO 2 excretion. However, teleost fishes typically do not have paCA at the gills and CO 2 excretion is reliant entirely on RBC CA – a strategy that is not possible in icefishes. As the result of a natural knockout, Antarctic icefishes (Channichthyidae) are the only known vertebrates that do not express haemoglobin (Hb) as adults, and largely lack RBCs in the circulation (haematocrit <1%). Previous work has indicated the presence of high levels of membrane-bound CA activity in the gills of icefishes, but without determining its cellular orientation. Thus, we hypothesised that icefishes express a membrane-bound CA isoform at the gill that is accessible to the blood plasma. The CA distribution was compared in the gills of two closely related notothenioid species, one with Hb and RBCs ( Notothenia rossii ) and one without ( Champsocephalus gunnari ). Molecular, biochemical and immunohistochemical markers indicate high levels of a Ca4 isoform in the gills of the icefish (but not the red-blooded N. rossii ), in a plasma-accessible location that is consistent with a role in CO 2 excretion. Thus, in the absence of RBC CA, the icefish gill could exclusively provide the catalytic activity necessary for CO 2 excretion – a pathway that is unlike that of any other vertebrate.

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