Adipocytes are the only site of glutamine synthetase expression within the lactating mouse mammary gland

Background Glutamine in milk is believed to play an important role in neonatal intestinal maturation and immune function. For lactating mothers, glutamine utilization is increased to meet the demands of the enlarged intestine and milk production. However, the source of such glutamine during lactation has not been studied. Objective We aimed to assess the effects of lactation on expression of glutamine synthetase (GS) in the mammary gland and other tissues of lactating mice. Methods Mouse tissues were sampled at four time points: 8-week-old (Virgin, control), post-delivery day 5 (PD5, early lactation), PD15 (peak lactation), and involution (4 days after weaning at PD21). We examined the gene expression and protein levels of GS and the first two enzymes of branched-chain amino acid catabolism: branched-chain aminotransferase 2 (BCAT2) and branched-chain ketoacid dehydrogenase subunit E1α (BCKDHA). Results The mRNA expression and protein levels of GS in mammary glands were significantly lower at PD5 and PD15 compared with the control but were restored at involution. Within the mammary gland GS protein was only detected in adipocytes with no evidence of presence in mammary epithelial cells. Compared with the control, mRNA and protein levels of BCAT2 and BCKDHA in mammary glands significantly decreased during lactation and involution. No changes of GS protein levels during lactation were found in liver, skeletal muscle, and lung. In non-mammary adipose tissue, GS protein abundance was higher during lactation compared with the Virgin. Conclusions This work shows that, within the mouse mammary gland, GS is only expressed in adipocytes and that the relative GS abundance in mammary gland sections is lower during lactation. This suggests that mammary adipocytes may be a site of glutamine synthesis in the lactating mouse. Identifying the sources of glutamine production during lactation is important for optimizing milk glutamine concentration to enhance neonatal and maternal health.