| Hydromineral balance is critical to survival of all organisms. Euryhaline teleosts are capable of surviving wide fluctuations in salinity by rapidly altering the abundance and direction of ion transport within the gill epithelium to achieve a balance of osmotic forces. While the Mitochondrion-Rich Cell (MRC) or cloride cell, is a well-studied system, supporting elements critical to proper MRC function and restoration to homeostasis remain to be identified. Here we describe research into two novel areas governing osmoregulatory physiology in teleost fishes: the claudin gene family, involved in the paracellular movement of water and ions, and leptin, a 16kD cytokine that mobilizes carbohydrate energy during acute periods of osmotic stress.;Claudins, a family of tight junction proteins, are the principal determinants of ion and water permeability between cells via paracellular transport. We investigated three claudins in the euryhaline tilapia (Oreochromis mossambicus) during both acute and long-term salinity transfer: Claudin 3c (Omcldn3c), Claudin 28a (Omcldn28a) and Claudin 30 (Omcldn30). In both short and long-term salinity acclimation, gill Omcldn3c mRNA expression was significantly higher in FW-acclimated fish. Gill Omcldn28a and Omcldn30 were significantly higher in FW-acclimated fish after 7 days, but not in long-term acclimated fish. Tilapia claudin mRNA expression was localized to pavement cells of the gill epithelium using in situ hybridization, but expression was not prevalent in MRC-rich regions. These studies are the first to show gill claudin mRNA is regulated by environmental salinity, and may act to decrease water and ion permeability during FW adaptation.;Claudin gene diversity remains poorly described in teleost fishes, and comparative studies are hindered by poor descriptions of homology. Despite the prominence of the zebrafish (Danio rerio) as a vertebrate developmental model, the full complement of claudins for this species remains unknown. Using the current genomic assembly along with predicted gene sequences (NCBI), we describe 54 zebrafish claudins, of which 24 were previously uncharacterized. Seven zebrafish claudins appear unique, possibly the result of gene duplication or loss in other lineages. Using large-scale phylogenetic analysis, supported with evidence of genomic synteny, we inferred the homology of these claudins to those of other vertebrate groups. The mRNA of these claudins was examined in cDNA from 8 tissues, with nearly all claudin paralogs possessing unique expression profiles.;Leptin is a 16kD cytokine hormone inducing anorexigenic effects on feeding in mammals. As an adipostat, leptin acts with insulin to regulate glucose and triglyceride metabolism in mammals. Here, we describe the molecular sequence of a leptin ortholog (lepa) and its putative receptor (lepr) in the Mozambique tilapia, and examine mRNA expression (in the gill and liver) during salinity transfer. Gill lepa mRNA expression appears consistent with pleitropic actions as a local growth factor. Hepatic mRNA expression of lepa and lepr were significantly upregulated within 4 hours of transfer to 2/3 seawater (25 ppt), but were not responsive to freshwater challenge. We hypothesized leptin may function to mobilize energy reserves during periods of acute hyperosmotic stress.;We then examined the effects of acute SW transfer, along with exogenous administration of recombinant leptin(s) or cortisol, upon key blood metabolites and hepatic energy reserves in tilapia. Our combined findings suggest two mechanisms of glucose mobilization occur during acute hyperosmotic stress: (1) hepatic glycogenolysis induced by the actions of leptin, and (2) cortisol-mediated gluconeogenesis of amino acids in the liver. We proposed a unified model of energy mobilization linking both new and historical observations of carbohydrate and lipid metabolism during salinity adaptation. These studies were the first to show leptin is sensitive to osmotic perturbations, and may act to mobilize glucose during periods of acute hyperosmotic stress. |
