Abstract
BackgroundSecretion of histotroph during the prolonged pre-implantation phase in mares is crucial to pregnancy maintenance, manifested as increased embryonic loss in mares with age-related endometrial degeneration. Glycogen content of uterine histotroph is higher during the progesterone-dominated phase of the estrous cycle in mares, but regulatory mechanisms are not well understood.MethodsmRNA expression of glycogen-metabolizing enzymes (HK1, HK2, GSK3B, GYS1, PEPCK, PKM, PYGM) in endometrial samples were compared among mares in anestrus, estrus, and at Day 12 of diestrus and pregnancy. In addition, hexokinase 2 (HK2) activity was assessed using a colorimetric assay.ResultsHK2 was the key regulator of glycogen accumulation during diestrus and pregnancy; hexokinase transcript abundance and enzyme activity were significantly higher during diestrus and pregnancy than estrus and anestrus. In addition, despite similar relative transcript abundance, hexokinase activity was significantly greater in the pregnant versus diestrous endometrium. Therefore, we inferred there was regulation of hexokinase activity through phosphorylation, in addition to its regulation at the transcriptional level during early pregnancy. Based on immunohistochemistry, HK2 was localized primarily in luminal and glandular epithelial cells, with weaker staining in stromal cells.ConclusionAmong glycogen metabolizing enzymes identified, expression of HK2 was significantly greater during the progesterone-dominated phase of the cycle.
Highlights
Secretion of histotroph during the prolonged pre-implantation phase in mares is crucial to pregnancy maintenance, manifested as increased embryonic loss in mares with age-related endometrial degeneration
Real-time RT-PCR Relative transcript abundance of hexokinase 2 (HK2) was highest (P < 0.01) in samples collected during diestrus and pregnancy, with no difference between samples collected during anestrus versus estrus
Expression of glycogen synthase 1 (GYS1) in the endometrium was greater during pregnancy than anestrus (P < 0.01)
Summary
Secretion of histotroph during the prolonged pre-implantation phase in mares is crucial to pregnancy maintenance, manifested as increased embryonic loss in mares with age-related endometrial degeneration. Glycogen content of uterine histotroph is higher during the progesterone-dominated phase of the estrous cycle in mares, but regulatory mechanisms are not well understood. Regardless, given the Glycogen, a storage form of glucose molecules, is one of the endometrial secretions that comprise histotroph and contribute to a successful pregnancy in a variety of species, including mink and human [6, 19]. Glycogen content is highest during diestrus, lower during estrus, but almost absent during anestrus. This cyclic variation in glycogen content is likely controlled by variations in progesterone concentrations, characteristic of various phases of the estrous cycle [9, 13].
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