The mechanism of vitamin D uptake: Implications for placental transfer and offspring development

Abstract

Fetal development and lifelong health are influenced by the uterine environment, with an adequate supply of maternal nutrients being crucial. As the interface between maternal and fetal circulations, the placenta is responsible for mediating nutrient delivery to the fetus through the regulation of metabolism and transport pathways. Vitamin D is an important micronutrient during pregnancy, particularly for supporting offspring bone development, but how the placenta regulates vitamin D transfer remains unclear. In this thesis, the mechanisms of cellular vitamin D uptake were explored in vitro, and the role of these mechanisms in the transfer of vitamin D across the placenta and the subsequent effects of fetal growth were explored using human cohort studies.
The role of endocytosis pathways for vitamin D uptake was investigated in vitro in human embryonic kidney (HEK293) epithelial cells using siRNA-mediated knockdown to target specific genes. Knockdown of the clathrin (CLTC) gene, an essential component of the ubiquitous clathrin dependent receptor-mediated endocytosis pathway, significantly increased vitamin D induced reporter gene expression, indicating increased vitamin D uptake and intracellular bioavailability. This may be due to upregulated clathrin-independent endocytosis and intracellular signalling pathways, as observed by mRNA sequencing of CLTC knockdown cells. Knockdown of the PDIA3 gene (required for non-canonical vitamin D signalling) similarly increased vitamin D-induced reporter gene expression. mRNA sequencing analysis indicated that similar pathways to the CLTC gene knockdown were affected, suggesting that PDIA3 signalling regulates intracellular vitamin D bioavailability and that the uptake and signalling pathways may be interconnected.
The expression of the clathrin (CLTC) and PDIA3 genes was explored in term placentas from the Southampton Women’s Survey (SWS) cohort. Negative associations were observed between placental CLTC mRNA expression and maternal and umbilical cord blood 25(OH)D levels, suggesting a role for clathrin-mediated endocytosis in regulating placental vitamin D transfer. Expression of CLTC mRNA positively associated with measures of fetal growth and neonatal size suggesting the pathway is a key mediator of nutrient delivery for the fetus. Expression of PDIA3 mRNA positively associated with fetal bone size suggesting the pathway has a role in mediating the beneficial vitamin D effects on bone development, possibly through regulation of placental calcium handling.
The role of the albumin, a protein that binds and transports vitamin D in the serum, in regulating vitamin D uptake was explored in HEK293 cells in vitro. Increasing albumin concentrations significantly increased vitamin D-induced reporter gene expression, suggesting that cellular vitamin D uptake was facilitated by the binding protein, possibly through interactions with cell surface receptors. The role of the vitamin D binding protein DBP was explored in the SWS and MAVIDOS (Maternal Vitamin D Osteoporosis Study) cohorts. A genetic variant in the maternal and offspring DBP gene (GC) which is associated with greater plasma DBP concentrations was positively associated with umbilical cord blood 25(OH)D concentration, supporting a role for vitamin D binding proteins in facilitating placental vitamin D transfer.
This thesis demonstrates that the regulation of vitamin D uptake and intracellular activity are likely to be more complex than once thought, with potential co-regulation between pathways. The data suggests that clathrin-mediated endocytosis may contribute to the regulation of cellular vitamin D uptake and transfer across the placenta and that these processes could be facilitated by vitamin D binding proteins. This work helps to improve our understanding of how the effects of vitamin D on fetal development are mediated and has implications for the efficacy and design of vitamin D intervention studies during pregnancy.

More information

Identifiers

ORCID for Laura Diana Frances Cooke: orcid.org/0000-0002-8099-9437

ORCID for Rohan Lewis: orcid.org/0000-0003-4044-9104

ORCID for Jane Cleal: orcid.org/0000-0001-7978-4327

ORCID for Jaswinder Sethi: orcid.org/0000-0003-4157-0475

ORCID for Nicholas Harvey: orcid.org/0000-0002-8194-2512

Catalogue record

Export record