Cubam receptor-mediated endocytosis in hindgut-derived pseudoplacenta of a viviparous teleost Xenotoca eiseni

Abstract

Nutrient transfer from mother to the embryo is essential for reproduction in viviparous animals. Here, we focused on the molecular mechanism of nutrient absorption in hindgut-derived pseudoplacenta (trophotaenia) of the viviparous teleost Xenotoca eiseni. The candidate genes involved in the process were identified by RNA-Seq, and then protein distribution and functions were investigated using molecular biological methods. Our results suggested that Cubam (Cubilin-Amnionless) receptor-mediated endocytosis and cathepsin L-dependent proteolysis are involved in the maternal macromolecule absorption via the trophotaenia. Such a nutrient absorption mechanism involving endocytosis is not a specific trait to viviparous fish. Similar processes have been reported in the larval stage of oviparous fish or the suckling stage of viviparous mammals. Our findings suggest that the viviparous teleost acquired trophotaenia-based viviparity via a modification of the intestinal absorption system common in vertebrates. This is a fundamental study to understand the strategic variation in the reproductive system of vertebrates.