Extracellular vesicles, lipids, and lipoproteins in early pregnant sheep
In sheep, pregnancy establishment encompasses conceptus elongation, implantation, and placentation. These events are regulated by factors present within the uterine luminal fluid (ULF) from the endometrial epithelium and the conceptus itself that affect proliferation, migration, attachment, and adhesion of the conceptus trophectoderm. As the peri-implantation period is especially susceptible to pregnancy loss, it is essential to understand the various components and functional roles of substances within the ULF. The central hypothesis of this dissertation is that lipids and lipid associated macromolecules are components of the ULF and mediate endometrial-embryonic crosstalk and regulate conceptus development. This work sought to identify, characterize, and/or determine the roles of: (1) extracellular vesicles (EVs); (2) lipids and metabolites; (3) prostaglandins (PGs); and (4) apolipoproteins present within the ULF of ewes during early gestation. Collectively, the present studies established that: (1) EVs increase within the ULF during the estrous cycle but are depleted in the uterine lumen of pregnant ewes due to uptake by the elongating conceptus; (2) the lipid and protein cargo of uterine EVs is diverse and altered by pregnancy; (3) uterine EVs regulate cellular processes in the conceptus trophectoderm and endometrium including cell proliferation and secretions; (4) various lipids (specifically phospholipids, ceramides, and triglycerides) and metabolites are elevated in the ULF of pregnant ewes; (5) the conceptus lipidome and metabolome is distinct from the ULF and endometrium suggesting selective uptake of ULF substances; (6) the production of PGs by PTGS2 in the conceptus is not required for conceptus elongation; (7) the secretion of APOA1 by the conceptus does not mobilize endometrial lipids into the ULF and is not required for early pregnancy development or survival. Collectively, these studies highlight the complex and dynamic composition of the ULF and support the overall hypothesis that lipids and lipid-associated macromolecules are critical components of the ULF that mediate conceptus-endometrial crosstalk and regulate important developmental processes in the conceptus. Future investigation and expansion of these findings will fill crucial gaps in our knowledge of early pregnancy events and may provide biomarkers or help develop therapies to improve pregnancy outcomes and reproductive efficiency in agricultural species.