Na+/Cl- cotransporter 2 is not fish-specific and is widely found in amphibians, non-avian reptiles, and select mammals

Navigating the multifaceted intricacies of the Na+-Cl- cotransporter, a highly regulated key effector in the control of hydromineral homeostasis

The Na+-Cl- cotransporter (NCC; SLC12A3) is a highly regulated integral membrane protein that is known to exist as three splice variants in primates. Its primary role in the kidney is to mediate the cosymport of Na+ and Cl- across the apical membrane of the distal convoluted tubule. Through this role and the involvement of other ion transport systems, NCC allows the systemic circulation to reclaim a fraction of the ultrafiltered Na+, K+, Cl-, and Mg+ loads in exchange for Ca2+ and [Formula: see text]. The physiological relevance of the Na+-Cl- cotransport mechanism in humans is illustrated by several abnormalities that result from NCC inactivation through the administration of thiazides or in the setting of hereditary disorders. The purpose of the present review is to discuss the molecular mechanisms and overall roles of Na+-Cl- cotransport as the main topics of interest. On reading the narrative proposed, one will realize that the knowledge gained in regard to these themes will continue to progress unrelentingly no matter how refined it has now become.

slc26a12-A novel member of the slc26 family, is located in tandem with slc26a2 in coelacanths, amphibians, reptiles, and birds

Solute carrier family 26 (Slc26) is a family of anion exchangers with 11 members in mammals (named Slc26a1-a11). Here, we identified a novel member of the slc26 family, slc26a12, located in tandem with slc26a2 in the genomes of several vertebrate lineages. BLAST and synteny analyses of various jawed vertebrate genome databases revealed that slc26a12 is present in coelacanths, amphibians, reptiles, and birds but not in cartilaginous fishes, lungfish, mammals, or ray-finned fishes. In some avian and reptilian lineages such as owls, penguins, egrets, and ducks, and most turtles examined, slc26a12 was lost or pseudogenized. Phylogenetic analysis showed that Slc26a12 formed an independent branch with the other Slc26 members and Slc26a12, Slc26a1 and Slc26a2 formed a single branch, suggesting that these three members formed a subfamily in Slc26. In jawless fish, hagfish have two genes homologous to slc26a2 and slc26a12, whereas lamprey has a single gene homologous to slc26a2. African clawed frogs express slc26a12 in larval gills, skin, and fins. These results show that slc26a12 was present at least before the separation of lobe-finned fish and tetrapods; the name slc26a12 is appropriate because the gene duplication occurred in the distant past.

Cellular mechanisms of ion and acid-base regulation in teleost gill ionocytes

The mechanism(s) of sodium, chloride and pH regulation in teleost fishes has been the subject of intense interest for researchers over the past 100 years. The primary organ responsible for ionoregulatory homeostasis is the gill, and more specifically, gill ionocytes. Building on the theoretical and experimental research of the past, recent advances in molecular and cellular techniques in the past two decades have allowed for substantial advances in our understanding of mechanisms involved. With an increased diversity of teleost species and environmental conditions being investigated, it has become apparent that there are multiple strategies and mechanisms employed to achieve ion and acid-base homeostasis. This review will cover the historical developments in our understanding of the teleost fish gill, highlight some of the recent advances and conflicting information in our understanding of ionocyte function, and serve to identify areas that require further investigation to improve our understanding of complex cellular and molecular machineries involved in iono- and acid-base regulation.

Endocrine control of gill ionocyte function in euryhaline fishes

The endocrine system is an essential regulator of the osmoregulatory organs that enable euryhaline fishes to maintain hydromineral balance in a broad range of environmental salinities. Because branchial ionocytes are the primary site for the active exchange of Na+, Cl-, and Ca2+ with the external environment, their functional regulation is inextricably linked with adaptive responses to changes in salinity. Here, we review the molecular-level processes that connect osmoregulatory hormones with branchial ion transport. We focus on how factors such as prolactin, growth hormone, cortisol, and insulin-like growth-factors operate through their cognate receptors to direct the expression of specific ion transporters/channels, Na+/K+-ATPases, tight-junction proteins, and aquaporins in ion-absorptive (freshwater-type) and ion-secretory (seawater-type) ionocytes. While these connections have historically been deduced in teleost models, more recently, increased attention has been given to understanding the nature of these connections in basal lineages. We conclude our review by proposing areas for future investigation that aim to fill gaps in the collective understanding of how hormonal signaling underlies ionocyte-based processes.

Functional Divergence in Solute Permeability between Ray-Finned Fish-Specific Paralogs of aqp10

Aquaporin (Aqp) 10 is a member of the aquaglyceroporin subfamily of water channels, and human Aqp10 is permeable to solutes such as glycerol, urea, and boric acid. Tetrapods have a single aqp10 gene, whereas ray-finned fishes have paralogs of this gene through tandem duplication, whole-genome duplication, and subsequent deletion. A previous study on Aqps in the Japanese pufferfish Takifugu rubripes showed that one pufferfish paralog, Aqp10.2b, was permeable to water and glycerol, but not to urea and boric acid. To understand the functional differences of Aqp10s between humans and pufferfish from an evolutionary perspective, we analyzed Aqp10s from an amphibian (Xenopus laevis) and a lobe-finned fish (Protopterus annectens) and Aqp10.1 and Aqp10.2 from several ray-finned fishes (Polypterus senegalus, Lepisosteus oculatus, Danio rerio, and Clupea pallasii). The expression of tetrapod and lobe-finned fish Aqp10s and Aqp10.1-derived Aqps in ray-finned fishes in Xenopus oocytes increased the membrane permeabilities to water, glycerol, urea, and boric acid. In contrast, Aqp10.2-derived Aqps in ray-finned fishes increased water and glycerol permeabilities, whereas those of urea and boric acid were much weaker than those of Aqp10.1-derived Aqps. These results indicate that water, glycerol, urea, and boric acid permeabilities are plesiomorphic activities of Aqp10s and that the ray-finned fish-specific Aqp10.2 paralogs have secondarily reduced or lost urea and boric acid permeability.