The role of an ancestral hyperpolarization activated cyclic nucleotide-gated K+-channel in branchial acid-base regulation in the green crab,Carcinus maenas(L.)

Abstract

Numerous electrophysiological studies on branchial K + transport in brachyuran crabs have established an important role for potassium channels in osmoregulatory ion uptake and ammonia excretion in the gill epithelium of decapod crustaceans. However, hardly anything is known of the actual nature of these channels in crustaceans. In the present study, the identification of a hyperpolarization-activated cyclic nucleotide-gated potassium channel (HCN) in the transcriptome of the green crab Carcinus maenas and subsequent performance of quantitative real-time PCR revealed the ubiquitous expression of this channel in this species. Even though mRNA expression levels in the cerebral ganglion were found to be approximately 10 times higher compared with all other tissues, posterior gills still expressed significant levels of HCN, indicating an important role for this transporter in branchial ion regulation. The relatively unspecific K + -channel inhibitor Ba 2+ , as well as the HCN-specific blocker ZD7288, as applied in gill perfusion experiments and electrophysiological studies employing the split gill lamellae revealed the presence of at least two different K + /NH 4 + -transporting structures in the branchial epithelium of C. maenas. Furthermore, HCN mRNA levels in posterior gill 7 decreased significantly in response to the respiratory or metabolic acidosis that was induced by acclimation of green crabs to high environmental P CO2 and ammonia, respectively. Consequently, the present study provides first evidence that HCNpromoted NH 4 + epithelial transport is involved in both branchial acidbase and ammonia regulation in an invertebrate.

Ammonia excretion in aquatic invertebrates: new insights and questions

“…Studies employing ion-selective microelectrodes have indicated that the three-cell excretory system of the nematode Caenorhabditis elegans excretes H + and K + (Adlimoghaddam et al, 2014), which suggests that ammonia might also be excreted by this internal method through an acid-trapping mechanism. In this system, NH 4 + likely enters the excretory cells via basolateral K + transporters, such as Na + /K + -ATPase (NKA) (Adlimoghaddam et al, 2015;Leone et al, 2016) and K + channels (Choe et al, 2000;Fehsenfeld and Weihrauch, 2016b). NH 4 + could then be excreted across the apical membrane in a similar fashion to K + while simultaneous apical H + excretion occurs, creating an acidic boundary layer that traps ammonia as NH 4 + , preventing its back-flow into the excretory system.…”

“…2B). An additional, highly conserved transporter amongst invertebrates, the ancestral hyperpolarizationactivated cyclic nucleotide-gated K + channel (HCN), has recently been found to be involved in transport of hemolymph ammonia into the cytoplasm of branchial epithelial cells of C. maenas (Fehsenfeld and Weihrauch, 2016b). Previously, mammalian HCN2 was shown to be capable of NH 4 + transport (Carrisoza-Gaytán et al, 2011), although there is a lack of evidence supporting a novel transepithelial ammonia transport via HCN.…”

Ammonia excretion and acid-base regulation in the American horseshoe crab,Limulus polyphemus

“…The importance of the ventral halflamella in ammonia transport and acid-base regulation was also evident from the high transcript levels of LpRh-1 and HCN ( Fig. 3A; Carrisoza-Gaytán et al, 2011;Fehsenfeld and Weihrauch, 2016b), and of CA-2, one of the general key players in transepithelial CO 2 transport and ammonia excretion (Fehsenfeld and Weihrauch, 2016a;Gilmour, 2012;Wright and Wood, 2009).…”

Salinity-induced changes in gene expression from anterior and posterior gills of Callinectes sapidus (Crustacea: Portunidae) with implications for crustacean ecological genomics

“…These included upregulation of 1) a chloride channel (7-fold in posterior gills); 2) a degenerin-like protein, which is a type of sodium channel, (34-fold in posterior gills), and; 3) a potassium channel (6-fold in the anterior gills). While such channels have been proposed in models of ion regulation (Fehsenfeld and Weihrauch, 2016; Henry et al , 2012; McNamara and Faria, 2012; Onken et al , 2003; Towle and Weihrauch, 2001), salinity-induced DE of such genes has not been shown previously for many crustaceans. Additional ion transporters of interest include those being DE in other crustaceans but not identified as such in our study (Fig.…”