Drinking and Water Handling in the Medaka Intestine: A Possible Role of Claudin-15 in Paracellular Absorption?

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.

The digestive tract as an essential organ for water acquisition in marine teleosts: lessons from euryhaline eels

“…As the aqp genes are downregulated in the medaka intestine after SW transfer [ 139 ], the decreased cldn15 expression in SW may be related to increased water permeability of the paracellular route (see 4.2.1). Recently, Tipsmark et al [ 234 ] suggested that CLDN15 in the medaka intestine may be water-permeable because the density of immunoreactive CLDN15 at TJs did not change in SW despite downregulation of cldn15 transcripts.…”

Special Issue on “The Tight Junction and Its Proteins: More than Just a Barrier”

“…In euryhaline fish, the change between fresh and saltwater conditions leads to adaptive changes to allow the handling of the imbibed seawater [ 42 ]. In the fish intestine, transcellular aquaporin-dependent water transport is downregulated, and paracellular transport via the two paralogs of claudin-15 present in fish, cldn15a and cldn15b, plays a major role.…”