Dietary salt loading and ion-poor water exposure provide insight into the molecular physiology of the rainbow trout gill epithelium tight junction complex

Abstract

This study utilized dietary salt loading and ion-poor water (IPW) exposure of rainbow trout (Oncorhynchus mykiss) to further understand the role of fish gill epithelium tight junction (TJ) physiology in salt and water balance. Gill morphology, biochemistry and molecular physiology were examined, with an emphasis on genes encoding TJ proteins. Fish were either fed a control or salt-enriched diet (~10 % NaCl) for 4 weeks prior to IPW exposure for 24 h. Serum [Na(+)], [Cl(-)] and muscle moisture content were unaltered by salt feeding, but changed in response to IPW irrespective of diet. Dietary salt loading altered the morphology (reduced Na(+)-K(+)-ATPase-immunoreactive cell numbers and surface exposure of mitochondrion-rich cells), biochemistry (decreased vacuolar-type H(+)-ATPase activity) and molecular physiology (decreased nkaα1a and cftrII mRNA abundance) of the gill in a manner indicative of reduced active ion uptake activity. But in control fish and not salt-fed fish, gill mRNA abundance of nkaα1c increased and nbc decreased after IPW exposure. Genes encoding TJ proteins were typically either responsive to salt feeding or IPW, but select genes responded to combined experimental treatment (e.g. IPW responsive but only if fish were salt-fed). Therefore, using salt feeding and IPW exposure, new insights into what factors influence gill TJ proteins and the role that specific TJ proteins might play in regulating the barrier properties of the gill epithelium have been acquired. In particular, evidence suggests that TJ proteins in the gill epithelium, or the regulatory networks that control them, respond independently to external or internal stimuli.

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

“…Both of these problems, as well as the physiological, morphological, cellular and molecular processes that control them, intensify in IPW (e.g. Perry and Laurent, 1989;Chasiotis et al, , 2012bChen et al, 2016;Kolosov and Kelly, 2016). Molecular mechanisms that help to mitigate paracellular ion loss across osmo/ionoregulatory epithelia are only now being uncovered (for reviews, see Chasiotis et al, 2012a;.…”

“…Molecular mechanisms that help to mitigate paracellular ion loss across osmo/ionoregulatory epithelia are only now being uncovered (for reviews, see Chasiotis et al, 2012a;. However, studies on derived fishes have shown that acclimation or exposure to IPW modulates select components of the TJ complex and alters paracellular permeability across ionoregulatory epithelia (Duffy et al, 2011;Chasiotis et al, 2012a;Chen et al, 2016;Kolosov and Kelly, 2016). Because lamprey larvae deal with life in FW in much the same way as derived fishes, acclimation to IPW will likely require modulation of the TJ complex.…”

“…In teleost fishes abruptly exposed or acclimated to IPW, reductions in serum [Na + ] and [Cl − ] and increased muscle moisture content are also typically observed (e.g. Duffy et al, 2011;Kolosov and Kelly, 2016). Reduced serum ion levels in IPWacclimated lamprey larvae suggest that ammocoetes inhabiting ionpoor habitats may have a reduced capacity to deal with ionic imbalance and this should be considered relevant from a lamprey management perspective.…”

Claudins of sea lamprey (Petromyzon marinus) – organ‐specific expression and transcriptional responses to water of varying ion content

“…Nonetheless, the molecular physiology of TJs in the osmoregulatory organs of fishes has only recently come under scrutiny, and knowledge in this area is far behind what we know about transcellular ion transport mechanisms. In the gill and skin of teleosts, the regulated movement of solutes across epithelia has been linked to changes in the transcript and protein abundance of select Cldns and other TJ proteins (Bagherie-Lachidan et al, 2008Chasiotis & Kelly, 2008;Bui & Kelly, 2014;Gauberg et al, 2017;Tacchi et al, 2015;Kolosov and Kelly, 2016). Furthermore, functional knockdown of select cldns (i.e., cldn-8d and -31) as well as ocln and tric in cultured gill epithelium models has been reported to increase paracellular solute permeability, indicating a barrier role for these proteins Kolosov and Kelly, 2018).…”

Malpighian tubules ofTrichoplusia ni: recycling ions via gap junctions and switching between secretion and reabsorption of Na+ and K+ in the distal ileac plexus

“…Total RNA was extracted using previously published protocols (e.g. Kolosov and Kelly, 2016). Briefly, DIP was separated from tracheae and excised from every MT of the fifth instar (control or dietary condition) and kept on ice immersed in RNAlater (Thermo Fisher Scientific).…”