The liquorice root derivative glycyrrhetinic acid can ameliorate ionoregulatory disturbance in rainbow trout ( Oncorhynchus mykiss ) abruptly exposed to ion-poor water

C-type natriuretic peptide regulates the molecular components of the rainbow trout gill epithelium tight junction complex

Freshwater (FW) fish experience passive paracellular loss of ions into the surrounding environment across water-exposed epithelia such as the gill. The mitigation of paracellular ion loss is thought to be regulated by proteins of the tight junction (TJ) complex and in particular, the large superfamily of claudin (cldn) TJ proteins plays an important role. Transcript and protein levels of TJ proteins in teleosts are known to be under endocrine control of several important osmoregulatory hormones and the current study was aimed at determining whether the osmoregulatory hormone, C-type natriuretic peptide (CNP), can alter paracellular permeability and TJ protein abundance in a primary cultured gill epithelium derived from rainbow trout. Natriuretic peptide receptors were detected in the cultured trout gill epithelium. It was found that (i) developing cultured gill epithelia "grown" in the presence of 10 nM CNP, and (ii) mature cultured gill epithelia exposed to 10 nM CNP for 48 h, exhibited augmented barrier properties. This occurred in association with reduced flux rates of a paracellular permeability marker (polyethylene glycol, molecular mass 400; PEG-400) and, reduced ion efflux (i.e. ion loss) when preparations were exposed to apical FW. Exposure to CNP altered mRNA abundance of cldn-3a, -5a, -6, - 8c, -20a, -25b, -28a, -32a and cgn, but differences in the transcriptional response were observed between chronic and acute CNP exposure. In contrast, chronic and acute exposure to CNP resulted in reduced cldn-10e/Cldn-10e abundance. Data suggest that CNP may play a role in regulating the molecular physiology of the TJ complex in the fish gill epithelium and contribute to the regulation of salt and water balance by influencing the paracellular permeability properties of this tissue.

The mineralocorticoid receptor contributes to barrier function of a model fish gill epithelium

Cortisol-induced epithelial tightening of a primary cultured rainbow trout gill epithelium model occurs in association with reduced paracellular permeability and increased abundance of select barrier-forming tight junction (TJ) proteins. Corticosteroid receptor (CR) pharmacological blocker studies have suggested that to produce this tightening effect, cortisol acts on the mineralocorticoid receptor (MR) as well as glucocorticoid receptors (GRs). This study considered how cortisol influences model gill epithelium permeability and TJ properties by transcriptional knockdown of the gene encoding the MR (mr-KD) using double-stranded RNA. Following mr-KD, a significant reduction in MR protein abundance was observed in the epithelium. The mr-KD epithelium demonstrated reduced transepithelial resistance (TER) and an increase in the paracellular flux of [³H]polyethylene glycol (MW 400 kDa, PEG-400). Concurrently, mRNA abundance of gr2 and 11βhsd increased, indicating a possible compensatory response to mr-KD. Transcript abundance of claudin (cldn)-6, -8d, -23a and -28b decreased while that of cldn-20a increased in mr-KD preparations. Cortisol-induced epithelial tightening was enhanced in mr-KD preparations, suggesting that alterations in CRs and TJ composition augmented model epithelium barrier function in response to lowered MR abundance. Cortisol treatment significantly increased the transcript and protein abundance of TJ proteins such as Cldn-8d and -28b. However, in mr-KD preparations, Cldn-28b protein abundance did not significantly alter in response to cortisol treatment, while Cldn-8d abundance was significantly elevated. Data suggest that mr-KD compromises normal barrier function of a primary cultured rainbow trout gill epithelium in both the presence and absence of cortisol and that Cldn-28b protein abundance may be modulated by cortisol via the MR only.

Tricellular tight junction-associated angulins in the gill epithelium of rainbow trout

Molecular physiology of the tricellular tight junction (tTJ)-associated proteins lipolysis-stimulated lipoprotein receptor ( lsr, = angulin-1) and an immunoglobulin-like domain-containing receptor ( ildr2, ≈angulin-3) was examined in model trout gill epithelia. Transcripts encoding lsr and ildr2 are broadly expressed in trout organs. A reduction in lsr and ildr2 mRNA abundance was observed during and after confluence in flask-cultured gill cells. In contrast, as high-resistance and low-permeability characteristics developed in a model gill epithelium cultured on permeable polyethylene terephthalate membrane inserts, lsr and ildr2 transcript abundance increased. However, as epithelia entered the developmental plateau phase, lsr abundance returned to initial values, while ildr2 transcript abundance remained elevated. When mitochondrion-rich cells were introduced to model preparations, lsr mRNA abundance was unaltered and ildr2 mRNA abundance significantly increased. Transcript abundance of ildr2 was not altered in association with corticosteroid-induced tightening of the gill epithelium, while lsr mRNA abundance decreased. Transcriptional knockdown of the tTJ protein tricelluin (Tric) reduced Tric abundance, increased gill epithelium permeability, and increased lsr without significantly altering ildr2 transcript abundance. Data suggest that angulins contribute to fish gill epithelium barrier properties but that Lsr and Ildr2 seem likely to play different roles. This is because ildr2 typically exhibited increased abundance in association with decreased model permeability, while lsr abundance changed in a manner that suggested a role in Tric recruitment to the tTJ.

Claudin-31 contributes to corticosteroid-induced alterations in the barrier properties of the gill epithelium

The contribution of Claudin-31 (Cldn-31) to corticosteroid-induced tightening of the trout gill epithelium was examined using a primary cultured model preparation. Cldn-31 is a ∼23 kDa protein that localizes to the periphery of gill epithelial cells and diffusely in select gill cells that are Na⁺-K⁺-ATPase-immunoreactive. Transcriptional knockdown (KD) of cldn-31 reduced Cldn-31 abundance and increased epithelium permeability. Under simulated in vivo conditions (apical freshwater), cldn-31 KD increased net ion flux rates (≡ efflux). Cortisol treatment increased Cldn-31 abundance and decreased epithelium permeability. This tightening effect was diminished, but not eliminated, by cldn-31 KD, most likely due to other cortisol-sensitive TJ proteins that were transcriptionally unperturbed or enhanced in cortisol-treated cldn-31 KD preparations. However, cldn-31 KD abolished a cortisol-induced increase in Cldn-8d abundance, which may contribute to compromised cldn-31 KD epithelium permeability. Data suggest an important barrier function for Cldn-31 and an integral role for Cldn-31 in corticosteroid-induced gill epithelium tightening.

A role for tight junction-associated MARVEL proteins in larval sea lamprey (Petromyzon marinus) osmoregulation

This study reports on tight junction-associated MARVEL proteins of larval sea lamprey (Petromyzon marinus) and their potential role in ammocoete osmoregulation. Two Occludin isoforms (designated Ocln and Ocln-a) and a tricellulin (Tric) were identified. Transcripts encoding ocln, ocln-a, and tric were broadly expressed in larval lamprey, with greatest abundance of ocln in gut, liver and kidney, ocln-a in the gill and skin, and tric in the kidney. Ocln and Ocln-a resolved as ∼63 kDa and ∼35 kDa MW proteins respectively while Tric resolved as a ∼50 kDa protein. Ocln immunolocalized to the gill vasculature and in gill mucous cells while Ocln-a localized to the gill pouch and gill epithelium. Both Ocln and Ocln-a localized in the nephron, the epidermis and the luminal side of the gut. In branchial tissue, Tric exhibited punctate localization, consistent with its presence at regions of tricellular contact. Following ion-poor water (IPW) acclimation of ammocoetes, serum [Na+] and [Cl−] reduced, but not [Ca++], and carcass moisture content increased. In association, Ocln abundance increased in skin and kidney, but reduced in gill of IPW-acclimated ammocoetes while Ocln-a abundance reduced in the kidney only. Tric abundance increased in the gill. Region-specific alterations in ocln, ocln-a and tric mRNA abundance was also observed in the gut. Data support a role for Ocln, Ocln-a and Tric in the osmoregulatory strategies of a basal vertebrate.