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

Insight into the function of voltage-sensing phosphatase in hindgut-derived pseudoplacenta of a viviparous teleost Xenotoca eiseni

Nutrient absorption is essential for animal survival and development. Our previous study on zebrafish reported that nutrient absorption in lysosome-rich enterocytes (LREs) is promoted by the voltage-sensing phosphatase (VSP), which regulates phosphoinositide (PIP) homeostasis via electrical signaling in biological membranes. However, it remains unknown whether this VSP function is shared by different absorptive tissues in other species. Here, we focused on the function of VSP in a viviparous teleost Xenotoca eiseni, whose intraovarian embryos absorb nutrients from the maternal ovarian fluid through a specialized hindgut-derived pseudoplacental structure called trophotaenia. Xenotoca eiseni VSP (Xe-VSP) is expressed in trophotaenia epithelium, an absorptive tissue functionally similar to zebrafish LREs. Notably, the apical distribution of Xe-VSP in trophotaenia epithelial cells closely resembles zebrafish VSP (Dr-VSP) distribution in zebrafish LREs, suggesting a shared role for VSP in absorptive tissues between the two species. Electrophysiological analysis using a heterologous expression system revealed that Xe-VSP preserves functional voltage sensors and phosphatase activity with the leftward shifted voltage sensitivity compared with zebrafish VSP (Dr-VSP). We also identified a single amino acid variation in the S4 helix of Xe-VSP as one of the factors contributing to the leftward shifted voltage sensitivity. This study highlights the biological variation and significance of VSP in various animal species, as well as hinting at the potential role of VSP in nutrient absorption in X. eiseni trophotaenia.NEW & NOTEWORTHY We investigate the voltage-sensing phosphatase (VSP) in Xenotoca eiseni, a viviparous fish whose intraovarian embryos utilize trophotaenia for nutrient absorption. Although X. eiseni VSP (Xe-VSP) shares key features with known VSPs, its distinct voltage sensitivity arises from species-specific amino acid variation. Xe-VSP in trophotaenia epithelium suggests its involvement in nutrient absorption, similar to VSP in zebrafish enterocytes and potentially in species with similar absorptive cells. Our findings highlight the potential role of VSP across species.

Vitellogenin uptake activity in the intestinal ducts of intraovarian embryos in a viviparous teleost Xenotoca eiseni

In the viviparous teleost species belonging to the family Goodeidae, intraovarian embryos absorb maternal supplements while they grow during the gestation period. They take up the components via trophotaeniae, a hindgut-derived placental structure. Our previous study using a goodeid species Xenotoca eiseni revealed that intraovarian embryos absorb the yolk protein vitellogenin (Vtg) via the trophotaenia. However, another group indicated yolk components accumulate in the intestinal lumen of X. eiseni embryos. Here, we investigated whether the intestinal duct is capable of protein uptake, as is the trophotaenia. Immunohistochemical studies indicated that endogenous vitellogenin is detected in the intestinal epithelial cells of the intraovarian embryo. Tracer analysis using FITC-Vtg also indicated that intestinal tissues can take up protein. The endocytosis-related genes expressed in trophotaenia were also detected in the intestinal tissues of the embryo. Lipid transporter genes which are not expressed in the trophotaenia were detected in the embryonic intestine. This evidence suggests that the intraovarian embryo of X. eiseni possesses two distinct sites for uptake of the maternal proteins. However, the presumed functions of the embryonic intestine and trophotaenia might be not identical. The study provides a new perspective on how mother-to-embryo matrotrophic interactions have changed in the evolution of viviparous teleosts.

Voltage-sensing phosphatase (Vsp) regulates endocytosis-dependent nutrient absorption in chordate enterocytes

Voltage-sensing phosphatase (Vsp) is a unique membrane protein that translates membrane electrical activities into the changes of phosphoinositide profiles. Vsp orthologs from various species have been intensively investigated toward their biophysical properties, primarily using a heterologous expression system. In contrast, the physiological role of Vsp in native tissues remains largely unknown. Here we report that zebrafish Vsp (Dr-Vsp), encoded by tpte gene, is functionally expressed on the endomembranes of lysosome-rich enterocytes (LREs) that mediate dietary protein absorption via endocytosis in the zebrafish mid-intestine. Dr-Vsp-deficient LREs were remarkably defective in forming endosomal vacuoles after initial uptake of dextran and mCherry. Dr-Vsp-deficient zebrafish exhibited growth restriction and higher mortality during the critical period when zebrafish larvae rely primarily on exogenous feeding via intestinal absorption. Furthermore, our comparative study on marine invertebrate Ciona intestinalis Vsp (Ci-Vsp) revealed co-expression with endocytosis-associated genes in absorptive epithelial cells of the Ciona digestive tract, corresponding to zebrafish LREs. These findings signify a crucial role of Vsp in regulating endocytosis-dependent nutrient absorption in specialized enterocytes across animal species.

The physiological role of Vsp in zebrafish is assessed, revealing Vsp expression in the mid-intestine for dietary protein absorption. A comparative study on marine invertebrate Ciona intestinalis suggests conservation of Vsp function in the GI tract.

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

Certain viviparous animals possess mechanisms for mother-to-embryo nutrient transport during gestation. Xenotoca eiseni is one such viviparous teleost species in which the mother supplies proteins and other components to the offspring developing in the ovary. The embryo possesses trophotaenia, hindgut-derived placental structure, to receive the maternal supplement. However, research on the molecular mechanisms underlying viviparous species is scarce in non-mammalian vertebrates, including teleosts. Thus, we conducted this study to investigate the mechanism for nutrient absorption and degradation in trophotaeniae of X. eiseni. A tracer assay indicated that a lipid transfer protein, vitellogenin (Vtg), was absorbed into the epithelial layer cells of the trophotaeniae. Vtg uptake was significantly suppressed by Pitstop-2, an inhibitor of clathrin-mediated endocytosis. Gene expression analysis indicated that the genes involved in endocytosis-mediated lipolysis and lysosomal cholesterol transport were expressed in the trophotaeniae. In contrast, plasma membrane transporters expressed in the intestinal tract were not functional in the trophotaeniae. Our results suggested that endocytosis-mediated lysosomal lipolysis is one of the mechanisms underlying maternal component metabolism. Thus, our study demonstrated how viviparous teleost species have acquired a unique developmental system that is based on the hindgut-derived placenta.

Cubilin, the intrinsic factor-vitamin B12 receptor

Cubilin (CUBN), the intrinsic factor-vitamin B12 receptor is a large endocytic protein involved in various physiological functions: vitamin B12 uptake in the gut; reabsorption of albumin and maturation of vitamin D in the kidney; nutrient delivery during embryonic development. Cubilin is an atypical receptor, peripherally associated to the plasma membrane. The transmembrane proteins amnionless (AMN) and Lrp2/Megalin are the currently known molecular partners contributing to plasma membrane transport and internalization of Cubilin. The role of Cubilin/Amn complex in the handling of vitamin B12 in health and disease has extensively been studied and so is the role of the Cubilin-Lrp2 tandem in renal pathophysiology. Accumulating evidence strongly supports a role of Cubilin in some developmental defects including impaired closure of the neural tube. Are these defects primarily caused by the dysfunction of a specific Cubilin ligand or are they secondary to impaired vitamin B12 or protein uptake? We will present the established Cubilin functions, discuss the developmental data and provide an overview of the emerging implications of Cubilin in the field of cardiovascular disease and cancer pathogenesis.

Expression and antimicrobial activity of liver-expressed antimicrobial peptides in the ovaries of the viviparous teleost Xenotoca eiseni

The mechanism via which the mothers of viviparous animals regulate the internal environment of pregnancy-associated organs for maintaining offspring growth is poorly understood. Environmental niches in organs contain fluid components for supporting embryonic growth; however, they may serve as nutrients for microbes. Therefore, microbial control is essential in viviparous animals to reduce the risk of infection in the ovarian lumen. Its importance may be higher than that in the case of oviparous animals. In this study, we investigated the antimicrobial factors in a viviparous teleost, Xenotoca eiseni. Four transcripts of the liver-expressed antimicrobial peptide (LEAP) were identified via RNA-Seq analysis. Some of the genes were expressed in the ovaries or intraovarian embryos of the fish. In particular, high expression of leap1a was detected in the ovaries of both pregnant and non-pregnant fish. Moreover, the ovary extracts from X. eiseni and transformed leap genes exhibited antimicrobial activity against Escherichia coli. Our results suggest that viviparous teleosts utilize antimicrobial peptides to reduce the risk of infection in the ovarian lumen.