Abstract
When euryhaline fish move between fresh water (FW) and seawater (SW), the intestine undergoes functional changes to handle imbibed SW. In Japanese medaka, the potential transcellular aquaporin-mediated conduits for water are paradoxically downregulated during SW acclimation, suggesting paracellular transport to be of principal importance in hyperosmotic conditions. In mammals, intestinal claudin-15 (CLDN15) forms paracellular channels for small cations and water, which may participate in water transport. Since two cldn15 paralogs, cldn15a and cldn15b, have previously been identified in medaka, we examined the salinity effects on their mRNA expression and immunolocalization in the intestine. In addition, we analyzed the drinking rate and intestinal water handling by adding non-absorbable radiotracers, 51-Cr-EDTA or 99-Tc-DTPA, to the water. The drinking rate was >2-fold higher in SW than FW-acclimated fish, and radiotracer experiments showed anterior accumulation in FW and posterior buildup in SW intestines. Salinity had no effect on expression of cldn15a, while cldn15b was approximately 100-fold higher in FW than SW. Despite differences in transcript dynamics, Cldn15a and Cldn15b proteins were both similarly localized in the apical tight junctions of enterocytes, co-localizing with occludin and with no apparent difference in localization and abundance between FW and SW. The stability of the Cldn15 protein suggests a physiological role in water transport in the medaka intestine.
Highlights
In fresh water (FW) fishes, the intestinal epithelium must limit excessive fluid absorption while securing dietary ion uptake [1]; in seawater (SW), imbibed water is absorbed in a solute-linked process [1,2]
In preliminary experiments using both FW- and SW-acclimated fish, it was assured that the intestinal accumulation of radioactivity in fish continued linearly in excess of 3 h, and gut-passage time
The drinking ra3teo3fow1f8a19s relatively high in FW-acclimated medaka (5 μL/g/h) but was doubled in fish acclimated to SW (Figure waftera1atieess)nmltr.adibDemdtaiarrsvaaiie1nefntdtilkkeohyiroinnnhrndiggiwngratsrihahnia3snuitknhegbsinamFipwngsWeeecaruatds-ihsoabwuuocdacnsreteililniowlmamnicna3ealisnetneheatxwsd5nci1newemw-lscClesuearirdbtone-eEaafrbtekD3.iaxaohTscn(eAe5(dsdiμsncaooLton5na/f1gtcn-a3o/Cohiunthr)ni-sbnEt(huigdDnotawgwTwtaAarnaat)nsed.crodoiT.otonhTautsehcahbrteieoilnvefdwiodinrtrniygein)n,.ikwntfiThinasthehgtheederrarra.eGcitfcnTeoIlkhirtwmireena,aagdcsttthrerriaadeenftlktetdaeoitrerniiSsvgnitWneikmrlciyaun(atFbhgetiaiiggowrtuiahnoartnees in1F).WD-raicnckliinmgarteadtemmeedaasukare(m5 eμnLt/sgw/he)rbeubtawseads odnoucobulendtiinngfirsahdiaocacclitmivaittyedintothSeWGI(Ftriagcutraef1te).r Dinrciunbkaintigon raatnedmaea1shurreinmsienngtspwereioredbinasceldeaonnwcaotuenr.ting radioactivity in the GI tract after incubation and a 1 h rinsing period in clean water
Summary
In fresh water (FW) fishes, the intestinal epithelium must limit excessive fluid absorption while securing dietary ion uptake [1]; in seawater (SW), imbibed water is absorbed in a solute-linked process [1,2]. The functional plasticity of the enterocytic epithelium is a critical factor in euryhaline fish that are capable of going through salinity transitions. Elevated intestinal aquaporin (Aqp/aqp) abundance in eel [3,4,5] and salmonids [6] in response to SW transfer have led to propose a transcellular water path in these species [7]; in medaka, a consistent downregulation of several intestinal Aqp/aqp isoforms after SW transfer has challenged this model and suggests a major involvement of a paracellular pathway [8]. Taking species differences into account, it appears that the medaka intestine may be a choice comparative model to study paracellular fluid transport because a tight junction defined path seems central, as suggested by Madsen et al [8]
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