Molecular Characterization of a Dual Domain Carbonic Anhydrase From the Ctenidium of the Giant Clam, Tridacna squamosa, and Its Expression Levels After Light Exposure, Cellular Localization, and Possible Role in the Uptake of Exogenous Inorganic Carbon

Abstract

A Dual-Domain Carbonic Anhydrase (DDCA) had been sequenced and characterized from the ctenidia (gills) of the giant clam, Tridacna squamosa, which lives in symbiosis with zooxanthellae. DDCA was expressed predominantly in the ctenidium. The complete cDNA coding sequence of DDCA from T. squamosa comprised 1,803 bp, encoding a protein of 601 amino acids and 66.7 kDa. The deduced DDCA sequence contained two distinct α-CA domains, each with a specific catalytic site. It had a high sequence similarity with tgCA from Tridacna gigas. In T. squamosa, the DDCA was localized apically in certain epithelial cells near the base of the ctenidial filament and the epithelial cells surrounding the tertiary water channels. Due to the presence of two transmembrane regions in the DDCA, one of the Zn2+-containing active sites could be located externally and the other one inside the cell. These results denote that the ctenidial DDCA was positioned to dehydrate HCO3- to CO2 in seawater, and to hydrate the CO2 that had permeated the apical membrane back to HCO3- in the cytoplasm. During insolation, the host clam needs to increase the uptake of inorganic carbon from the ambient seawater to benefit the symbiotic zooxanthellae; only then, can the symbionts conduct photosynthesis and share the photosynthates with the host. Indeed, the transcript and protein levels of DDCA/DDCA in the ctenidium of T. squamosa increased significantly after 6 and 12 h of exposure to light, respectively, denoting that DDCA could participate in the light-enhanced uptake and assimilation of exogenous inorganic carbon.

With illumination, the fluted giant clam,Tridacna squamosa, upregulates the protein abundance of an apical Na+: glucose cotransporter 1 homolog in its ctenidium, and increases exogenous glucose absorption that can be impeded by urea

“…Our results indicate that SGLT1-like was regulated principally through translation, as light exposure did not have a significant effect on its transcript level. This differs from Dual Domain Carbonic Anhydrase (Koh et al, 2018), Glutamine Synthetase (Hiong et al, 2017a) and Na + /H + Exchanger 3-like (Hiong et al, 2017b), of which both the transcript level and the protein abundance increase significantly in the ctenidium of T. squamosa during 12 h of light exposure. Overall, these results corroborate the proposition that T. squamosa could perform lightenhanced glucose/urea absorption.…”

“…The surface area is further increased by numerous water channels found below the filaments inside the ctenidium. The ctenidium of T. squamosa is known to express transporters and enzymes (Ip et al, 2015) related to nitrogen transport and assimilation (DUR3-like, Chan et al, 2018;Ammonia Transporter 1, Boo et al, 2018;Glutamine Synthetase, Hiong et al, 2017a), inorganic carbon absorption (Dual Domain Carbonic Anhydrase; Koh et al, 2018) and proton excretion (Na + /H + Exchanger 3-like, Hiong et al, 2017b; Vacuolar-type H + -ATPase subunit A, Ip et al, 2018). In the present study, we demonstrated that SGLT1-like was expressed predominantly in the ctenidium of T. squamosa.…”

“…Exogenous glucose uptake may support the metabolic needs of the ctenidium The ctenidium of T. squamosa is metabolically active owing to its important roles in, for example, inorganic carbon absorption (Koh et al, 2018), excretion of excess H + (Hiong et al, 2017b;Ip et al, 2018), and nitrogen transport and assimilation (Chan et al, 2018;Boo et al, 2018;Hiong et al, 2017a). It contains few zooxanthellae, and is located inside the mantle cavity with no direct exposure to sunlight.…”

“…Nonetheless, giant clams are known to display light-dependent physiological properties, including light-enhanced shell formation (Sano et al, 2012;Ip et al, 2017a) and light-enhanced nitrogen uptake (Wilkerson and Trench, 1986;Chan et al, 2018). In addition, the gene and/or protein expression levels of some of their enzymes and transporters are also light dependent (Hiong et al, 2017a,b;Boo et al, 2017Boo et al, , 2018Ip et al, 2015Ip et al, , 2017aIp et al, ,b, 2018Koh et al, 2018;Chan et al, 2018;Chew et al, 2019). As such, it is highly probable that giant clams can increase the expression of SGLT1-like/SGLT1-like in the ctenidium and the rate of glucose uptake during light exposure.…”

Intracellular pH regulation in mantle epithelial cells of the Pacific oyster, Crassostrea gigas

“…Interestingly, these proteins have also been associated with biomineralization (Medakovic et al 2000 ; Zhang et al 2012 ). Specifically, a suite of novel molecular studies lend support to the role of these carbonic anhydrases in acquisition of inorganic carbon during calcification (Wang et al 2017 ; Koh et al 2018 ; Chew et al 2019 ). Additionally, it has been suggested that NHE proteins promote calcification by aiding in the removal of proton byproducts (Hiong et al 2017 ; Cao-Pham et al 2019 ).…”