Discovery and characterization of an epithelial-specific galectin in the endometrium that forms crystals in the trophectoderm

Abstract

Secretions of the uterus support survival and growth of the conceptus (embryo/fetus and associated membranes) during pregnancy. Galectin-15, also known as OVGAL11 and a previously uncharacterized member of the galectin family of secreted beta-galactoside lectins containing a conserved carbohydrate recognition domain and a separate putative integrin binding domain, was discovered in the uterus of sheep. In endometria of cyclic and pregnant sheep, galectin-15 mRNA was expressed specifically in the endometrial luminal epithelium but not in the conceptus. In pregnant sheep, galectin-15 mRNA expression appeared in the epithelia between days 10 and 12 and increased between days 12 and 16. Progesterone induced and IFN-tau stimulated galectin-15 mRNA in the endometrial epithelium. Galectin-15 protein was concentrated near and on the apical surface of the endometrial luminal epithelia and localized within discrete cytoplasmic crystalline structures of conceptus trophectoderm (Tr). In the uterine lumen, secreted galectin-15 protein increased between days 14 and 16 of pregnancy. Galectin-15 protein was functional in binding lactose and mannose sugars and immunologically identical to the unnamed Mr 14,000 (14K) protein from the ovine uterus that forms crystalline inclusion bodies in endometrial epithelia and conceptus Tr. Based on the functional studies of other galectins, galectin-15 is hypothesized to function extracellularly to regulate Tr migration and adhesion to the endometrial epithelium and intracellularly to regulate Tr cell survival, growth, and differentiation. Galectins may be useful as cellular and molecular markers for endometrial function and receptivity, to enhance conceptus survival and development, and to evaluate and enhance fertility.

Fig. 1.

clustalw alignment of the amino acid sequences of ovine galectin-15 with human galectin-10 and -13. The asterisks denote the conserved residues forming the CRD found in prototypical galectin family members. The underlined amino acids indicate a predicted cell attachment sequence that is a conserved integrin binding site.

Fig. 2.

Galectin-15 mRNA expression in the uterus (study 1). (A) Steady-state levels of galectin-15 mRNA in the endometrium as determined by slot-blot hybridization analysis. (B) In situ hybridization analysis of galectin-15 mRNA in the uterus. C, cyclic; D, day postestrus/mating; P, pregnant; S, stroma. (Bar = 10 μm.)

Fig. 3.

Galectin-15 protein in the endometrium and lumen of the uterus (study 1). (A) Immunohistochemical localization of galectin-15 protein in the endometrium. Sections were not counterstained. (B) Western blot analysis of proteins in the lumen of the uterus. C, cyclic; D, day postestrus/mating; P, pregnant; S, stroma. (Bar = 10 μm.)

Fig. 4.

Effects of progesterone and IFN-τ on galectin-15 mRNA in the uterus (study 2). (A) Steady-state levels of galectin-15 mRNA in the endometrium as determined by slot-blot hybridization analysis. (B) In situ hybridization localization of galectin-15 mRNA in the endometrium. S, stroma. (Bar = 10 μm.)

Fig. 5.

Biochemical characterization of ovine endometrial galectin-15. (A) Carbohydrate-binding analysis of galectin-15 from the uterine lumen of day-16 pregnant ewes by using lactose-agarose and competition with lactose, mannose, or fucose. (B) Immunological identification of galectin-15 from the uterine lumen of day-16 pregnant ewes (study 1) and as the previously uncharacterized 14K protein. (C) Immunological identification of galectin-15 from uterine milk (study 3) as the 14K protein. The anti-oTP-1/oIFN-τ antibody crossreacts with the 14K protein (22), which is ovine endometrial galectin-15.