GLUL mediates FOXO3 O-GlcNAcylation to regulate the osteogenic differentiation of BMSCs and senile osteoporosis

The abnormal osteogenic differentiation of bone marrow mesenchymal stem cells (BMSCs) is an important cause of senile osteoporosis (SOP). Glutamine synthetase (GLUL) is a key enzyme in glutamine biosynthesis; however, its functional role in SOP remains unclear. Here, we found that GLUL expression was downregulated in the BMSCs of SOP patients. BMSC-conditioned <i>Glul</i>-knockout (KO) mice exhibited dysplasia of the skull and phalanges and osteoporosis due to disordered osteogenic differentiation.Mechanistically, GLUL competitively bound to the Tripartite Motif Containing 25 (TRIM25) SPRY subunit, reduced the ubiquitin-mediated degradation of UDP-N-acetylglucosamine pyrophosphorylase 1 (UAP1) and increased the synthesis of uridine 5-diphosphate N-acetylglucosamine (UDP-GlcNAc), thereby regulating the O-linked β-N-acetylglucosamine modification (O-GlcNAcylation) of serine 296 residues and increasing Forkhead Box O3 (FOXO3) stability to reduce oxidative stress. Moreover, blocking the O-GlcNAcylation of FOXO3 at Ser296 inhibited osteogenic differentiation. Finally, GLUL supplementation specifically in BMSCs slowed bone loss in SOP model mice. Overall, our study suggests that GLUL plays an important role in regulating osteogenic differentiation and bone development, which may have implications for SOP treatment.