Functional Characterization of Neuroendocrine Regulation of Branchial Carbonic Anhydrase Induction in the Euryhaline Crab Callinectes sapidus

Jan 10, 2015The Biological bulletin

How hormones control the increase of breathing-related enzymes in the blue crab when adapting to different water salinities

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Abstract

Eyestalk ligation in blue crabs increased carbonic anhydrase activity by 1.8-fold and mRNA levels by 100-fold.

Carbonic anhydrase (CA) is important for ion exchange during salinity adaptation in crabs.
In blue crabs, removal of the eyestalk leads to significant increases in CA activity and gene expression.
Re-injection of eyestalk homogenates can reverse the increases in CA activity and reduce gene expression.
Eyestalk ligation enhances CA induction in crabs transferred to lower salinity.
Medullary tissue and sinus gland injections reduced CA activity stimulated by low salinity, indicating localized hormonal regulation.
The presence of a repressor hormone in the eyestalk is suggested, which is down-regulated at low salinity.

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Carbonic anhydrase (CA) plays an essential role as a provider of counterions for Na(+)/H(+) and Cl(-)/HCO3 (-) exchange in branchial ionic uptake processes in euryhaline crustaceans. CA activity and gene expression are low in crabs acclimated to full-strength seawater, with transfer to low salinity resulting in large-scale inductions of mRNA and subsequent enzyme activity in the posterior ion-regulating gills (e.g., G7). In the green crab Carcinus maenas, CA has been shown to be under inhibitory neuroendocrine control by a putative hormone in the x-organ-sinus gland complex (XOSG), located in the eyestalk. This study characterizes the neuroendocrine regulation of CA induction in the blue crab Callinectes sapidus, a commonly used experimental organism for crustacean osmoregulation. In crabs acclimated to full-strength seawater, eyestalk ligation (ESL) triggered a 1.8- and 100-fold increase in CA activity and mRNA, respectively. Re-injection with eyestalk homogenates abolished increases in CA activity and fractionally reduced CA gene expression. ESL also enhanced CA induction by 33% after 96 h in crabs transferred to 15 ppt salinity. Injection of eyestalk homogenates into intact crabs transferred from 35 to 15 ppt diminished by 43% the CA induction stimulated by low salinity. These results point to the presence of a repressor hormone in the eyestalk. Separate injections of medullary tissue (MT) and sinus gland (SG), two components of the eyestalk, reduced salinity-stimulated CA activity by 22% and 49%, suggesting that the putative repressor is localized to the SG. Crabs injected with SG extract harvested from crabs acclimated to 5 ppt showed no decrease in CA activity, demonstrating that the hormone is down-regulated at low salinity. Our results show the presence in the XOSG of an inhibitory compound that regulates salinity-stimulated CA induction.

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Functional Characterization of Neuroendocrine Regulation of Branchial Carbonic Anhydrase Induction in the Euryhaline Crab Callinectes sapidus

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