Specialized cells and transporters mediate integument-absorption of environmental peptides in cichlids larvae

Nutrient absorption through the skin and gills into the organisms’ tissues has been documented in several aquatic invertebrates from different phyla. However, the actual absorption mechanism is still unknown. In teleost fish, as in all jawed vertebrates, intestinal absorption is considered as the sole source of nutrients. The proton-dependent peptide transporters (PepT) of the slc15a gene family are the only known mechanism for cellular absorption of di- and tri-peptides within the animal kingdom. In this study, we explored the expression and localization of PepT2 in Mozambique tilapia ( Oreochromis mossambicus ) larvae. Transcript levels of PepT2 in dissected yolk-sacs from larvae showed significant expression during the larval developmental period. Immunofluorescence staining of PepT2 with Na/K-ATPase (NKA) and Na ⁺ /K ⁺ /2Cl ^- co-transporter (NKCC) on the yolk-sac membrane revealed co-staining with NKA and differential-staining with NKCC. While NKA staining was observed on the ionocytes’ basolateral membrane, PepT2 staining was restricted to the apical pit of the ionocytes, facing the surrounding water. In this study, we identified a nutrient transporter located on integument-specific cells, facing the outside aquatic environment. This is the first indication of environmental nutrients absorption in teleosts, and the first evidence of a possible absorption mechanism through PepT2, in specialized yolk-sac ionocytes.