Modulation of Mannose and Asialoglycoprotein Receptor Expression Determines Glycoprotein Hormone Half-life at Critical Points in the Reproductive Cycle

Background: Unique glycan structures on glycoprotein hormones are recognized by glycan-specific receptors that control hormone half-life in the blood. Results: Clearance rates for recognized glycoproteins are determined by the level of receptor expression in the liver. Conclusion: Expression of glycan-specific receptors is modulated at critical points in the reproductive cycle. Significance: Glycans and glycan-specific receptors play critical roles in reproduction.The rate at which glycoproteins are cleared from the circulation has a critical impact on their biologic activity in vivo. We have shown that clearance rates for glycoproteins such as luteinizing hormone (LH) that undergo regulated release into the circulation determine their potency. Two highly abundant, carbohydrate-specific, endocytic receptors, the asialoglycoprotein receptor (ASGR) and the mannose receptor (ManR) are expressed in the liver by parenchymal and sinusoidal endothelial cells, respectively. We demonstrate that the ManR mediates the clearance of glycoproteins such as LH that bear N-linked glycans terminating with 1,4-linked GalNAc-4-SO4, as well as glycoproteins bearing glycans that terminate with Man. Steady state levels of mRNA encoding the ASGR and the ManR are regulated by progesterone in pregnant mice, reaching maximal levels on day 12.5 of pregnancy. Protein expression and glycan-specific binding activity also increase in the livers of pregnant mice. In contrast, ManR mRNA, but not ASGR mRNA, decreases in male mice at the time of sexual maturation. We show that levels of ManR and ASGR expression control the clearance rate for glycoproteins bearing recognized glycans. Thus, reduced expression of the ManR at the time of sexual maturation will increase the potency of LH in vivo, whereas increased expression during pregnancy will reduce LH potency until progesterone and receptor levels fall prior to parturition.