Barrier Properties and Molecular Physiology of Fish Gill Epithelia and Gill Epithelium Models

The fish gill epithelium interfaces directly with surrounding water and participates in numerous physiological processes. However, the barrier properties of the fish gill epithelium have been largely overlooked until recent studies have suggested that a dynamic repertoire of tight junction (TJ) associated elements control gill epithelium paracellular permeability and dictate barrier function of this tissue. Primary cultured gill epithelium models have become valuable tools to examine gill epithelium permeability, but models have been developed using few species. Furthermore, continued refinement of existing models provides enhanced insight. My thesis work focused on refining an existing primary cultured gill epithelium model derived from a freshwater (FW) bony fish (rainbow trout) by considering how epithelium TJs and permeability might be influenced by native serum as a growth supplement and heparin (an anti-coagulant used to collect extracellular fluid). A FW trout gill model was also used to examine the effect of the thyroid hormone 3,5,3-triiodo-L-thyronine (T3) on gill epithelium TJs and barrier function. Data indicate that (1) native serum enhances the barrier properties of the trout gill model; (2) heparin compromises epithelium integrity; and (3) T3 may induce changes that prepare a diadromous fish for seawater (SW) entry. In addition, I examined a role for putative pore-forming claudin-10 TJ proteins in the mummichog gill and opercular epithelium and provide data supporting an integral role for Cldn-10c in Na+ secretion across the SW fish gill. Finally, I examined the molecular physiology of the lamprey gill epithelium TJ complex through metamorphosis and I developed the first primary cultured gill epithelium model for this basal vertebrate. Peri-metamorphic observations of the lamprey gill TJ complex indicate that SW entry has a much greater impact on TJs than metamorphosis alone, and a cultured lamprey gill epithelium model, derived from FW residing larval lamprey, reveals a comparatively tight epithelium with a TJ profile that mimics the intact gill epithelium. Together, data provide new insight into the barrier function of fish gill epithelia as well as new and improved tools to study this function