Renoguanylin stimulates apical CFTR translocation and decreases HCO3- secretion through PKA activity in the Gulf toadfish (Opsanus beta).

Ilan M Ruhr,Martin Grosell,Kevin L Schauer,Yoshio Takei

doi:10.1242/jeb.173948

Abstract

The guanylin peptides - guanylin, uroguanylin and renoguanylin (RGN) - are endogenously produced hormones in teleost fish enterocytes that are activators of guanylyl cyclase-C (GC-C) and are potent modulators of intestinal physiology, particularly in seawater teleosts. Most notably, they reverse normal net ion-absorbing mechanisms that are vital to water absorption, an important process for seawater teleost survival. The role of guanylin-peptide stimulation of the intestine remains unclear, but it is hypothesized to facilitate the removal of solids from the intestine by providing fluid to enable their removal by peristalsis. The present study used one member of this group of peptides - RGN - to provide evidence for the prominent role that protein kinase A (PKA) plays in mediating the effects of guanylin-peptide stimulation in the posterior intestine of the Gulf toadfish (Opsanus beta). Protein kinase G was found to not mediate the intracellular effects of RGN, despite previous evidence showing that GC-C activation leads to higher cyclic guanosine monophosphate formation. RGN reversed the absorptive short-circuit current and increased conductance in the Gulf toadfish intestine. These effects are correlated to increased trafficking of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel to the apical membrane, which is negated by PKA inhibition. Moreover, RGN decreased HCO3- secretion, likely by limiting apical HCO3-/Cl- exchange (possibly by reducing SLC26a6 activity), a reduction that was enhanced by PKA inhibition. RGN seems to alter PKA activity in the posterior intestine to recruit CFTR to the apical membrane and reduce HCO3- secretion.

Highlights

Seawater presents a major osmoregulatory challenge for teleost fish, because of the direct interaction of their thin respiratory epithelia – the gills – with the environment, which causes unavoidable branchial fluid loss (Evans et al, 2005)
The present study does not definitively show CFTR in the apical membrane of the Gulf toadfish intestine, we have previously shown that blocking CFTR with an inhibitor prevents the reversal of the absorptive ISC by RGN when applied to the apical membrane (Ruhr et al, 2014), while its basolateral application has no effect (I.M.R., personal observations)
The present study revealed that RGN stimulation of the posterior intestine appears to elevate protein kinase A (PKA) activity (Fig. 7)

Summary

Introduction

Seawater presents a major osmoregulatory challenge for teleost fish, because of the direct interaction of their thin respiratory epithelia – the gills – with the environment, which causes unavoidable branchial fluid loss (Evans et al, 2005). The seawater teleost constantly drinks water and absorbs it across the. Imbibed seawater undergoes rapid desalination by the esophagus, so, by the time it enters the intestine, its osmolality is roughly that of blood plasma. The combined effect of these processes, in addition to other ion-absorbing pathways, renders the fluid osmolality in the intestine similar to that of the extracellular fluids and allows for efficient water absorption that is coupled to Na+ and Cl− absorption (Skadhauge, 1969, 1974). Even though net water absorption and the mechanisms that facilitate it are vital to the survival of teleosts living in seawater, recent studies reveal regionspecific areas of the intestine that display net secretory functions, at least periodically (Ruhr et al, 2014, 2016)

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Discussion

Conclusion