RNA-seq analysis identifies age-dependent changes in expression of mRNAs - encoding N-glycosylation pathway enzymes in mouse gonadotropes

Follicle-stimulating hormone (FSH) is a glycoprotein that is assembled as a heterodimer of α/β subunits in gonadotropes. Each subunit contains two N-glycan chains. Our previous in vivo genetic studies identified that at least one N-glycan chain must be present on the FSHβ subunit for efficient FSH dimer assembly and secretion. Moreover, macroheterogeneity observed uniquely on human FSHβ results in ratiometric changes in age-specific FSH glycoforms, particularly during menopausal transition. Despite the recognition of many prominent roles of sugars on FSH including dimer assembly and secretion, serum half-life, receptor binding and signal transduction, the N-glycosylation machinery in gonadotropes has never been defined. Here, we used a mouse model in which gonadotropes are GFP-labeled in vivo and achieved rapid purification of GFP⁺ gonadotropes from pituitaries of female mice at reproductively young, middle, and old ages. We identified by RNA-seq analysis 52 mRNAs encoding N-glycosylation pathway enzymes expressed in 3- and 8-10-month-old mouse gonadotropes. We hierarchically mapped and localized the enzymes to distinct subcellular organelles within the N-glycosylation biosynthetic pathway. Of the 52 mRNAs, we found 27 mRNAs are differentially expressed between the 3- and 8-10-month old mice. We subsequently selected 8 mRNAs which showed varying changes in expression for confirmation of abundance in vivo via qPCR analysis, using more expanded aging time points with distinct 8-month and 14-month age groups. Real time qPCR analysis indicated dynamic changes in expression of N-glycosylation pathway enzyme-encoding mRNAs across the life span. Notably, computational analysis predicted the promoters of genes encoding these 8 mRNAs contain multiple high probability binding sites for estrogen receptor-1 and progesterone receptor. Collectively, our studies define the N-glycome and identify age-specific dynamic changes in mRNAs encoding N-glycosylation pathway enzymes in mouse gonadotropes. Our studies suggest the age-related decline in ovarian steroids may regulate expression of N-glycosylation enzymes in mouse gonadotropes and explain the age-related N-glycosylation shift previously observed on human FSHβ subunit in pituitaries of women.