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. In the viviparous teleost Xenotoca eiseni belonging to the family Goodeidae, the intraovarian embryo intakes the maternal component secreted into the ovarian fluid via the trophotaenia. Our previous study reported that the epithelial layer cells of the trophotaenia incorporate a maternal protein via vesicle trafficking. However, the molecules responsible for the absorption were still elusive. Here, we focused on Cubam (Cubilin-Amnionless) as a receptor involved in the absorption, and cathepsin L as a functional protease in the vesicles. Our results indicated that the Cubam receptor is distributed in the apical surface of the trophotaenia epithelium and then is taken into the intracellular vesicles. The trophotaenia possesses acidic organelles in epithelial layer cells and cathepsin L-dependent proteolysis activity. This evidence does not conflict with our hypothesis that receptor-mediated endocytosis and proteolysis play roles in maternal macromolecule absorption via the trohotaenia in viviparous teleosts. Such nutrient absorption involving endocytosis is not a specific trait in 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 from a modification of the intestinal absorption system common in vertebrates. This is a fundamental study to understand the strategic variation of the reproductive system in vertebrates.

Endocytosis-mediated vitellogenin absorption and lipid metabolism in the hindgut-derived pseudoplacenta of the viviparous teleost Xenotoca eiseni

“…Endocytosis-mediated intracellular vesicles in cells were visualized by immunohistochemistry against cubilin, an endocytic receptor expressed in trophotaenia (Fig. 1B) [19]. To trace Vtg absorption, embryos were extracted from pregnant female fish and incubated in PBS containing Vtg-FITC for 1 h. In the case of clathrin inhibition, embryos were exposed to Pitstop-2 for 30 min prior to Vtg addition (Fig.…”

“…We have previously reported that trophotaenia in intraovarian embryos possesses cathepsin L-dependent protease activity [19]. This activity may be related to the degradation of maternal proteins, including Vtg.…”

“…The transcriptome datasets used in this study were obtained from the DNA Databank of Japan (DDBJ: DRA011209 and DRA011388). Detailed experimental information has been described in our previous publications [19, 23]. To calculate transcript per million values, transcript sequences of Poecilia reticulata (National Center for Biotechnology Information [NCBI] Genome, ID: 23338) were used as a reference.…”

“…2B, Table S6). Scavenger receptor class b member 1 (scarb1) is a scavenger receptor for various lipoproteins, similar to cubilin, as previously described (Iida et al, 2021). Lysosomal acid lipase (lipa) is a hydrolase for triglycerides or cholesterol esters that is optimal under acidic conditions, such as the environment in lysosomal vesicles.…”

“…Cubilin-amnionless RME and cathepsin L-dependent proteolysis are active in the epithelial layer cells of trophotaenia [19]. However, the relationship between Vtg and RME has not been demonstrated experimentally.…”