Myeloid Glutamine Synthetase Protects Against Acute Liver Injury in Mice Independent of Its Enzyme Activity

Glutamine synthetase (GS, encoded by Glul) is a well-known ammonia-detoxifying enzyme, but its function in acute liver injury (ALI) remains unclear.

Lipopolysaccharide (LPS) and D-galactosamine (D-GalN) were utilised to construct the murine ALI model. C57BL/6J-Glul^{em1(flox)Smoc} mice (Glul^f/f) and B6.129-Lyz2^tm1(cre)smoc (Lyz2-Cre) transgenic mice were crossed to generate Lyz2⁺Glul^f/f (Glul^∆Lyz2) mice with a selectively knockout of Glul in myeloid cells. Histological staining experiments were performed to evaluate liver injury. Flow cytometry and RNA sequencing analyses were conducted to investigate the effects of Glul deficiency on liver immunity. Additionally, several strategies were applied to intervene ALI in mice, including administration of CCL2 neutralising antibody or GS inhibitor L-methionine sulfoximine (MSO), as well as adeno-associated virus (AAV)-mediated enhancement of GS expression.

The expression of Glul in myeloid cells was downregulated in wild-type mice after ALI modelling by LPS/D-GalN. Moreover, Glul^∆Lyz2 mice demonstrated aggravated ALI and higher mortality upon LPS/D-GalN challenge, compared to the control Glul^f/f littermates. Notably, Glul deficiency significantly contributed to the activation of monocyte-derived macrophages (MoMFs), secretion of C-C chemokine ligand 2 (CCL2) and the recruitment of C-C chemokine receptor 2-positive monocytes. Treatment with CCL2 neutralising antibody significantly alleviated ALI by inhibiting MoMF activation. Interestingly, although MSO treatment effectively blocked the enzyme activity of GS, it exerted both preventive and therapeutic effects against ALI, which could be attributed to the elevation of GS protein level. Therefore, in vivo global or myeloid GS was overexpressed via AAV delivery system, which demonstrated potent protective efficacy against ALI.

This study provided an updated look at the protective role of GS independent of its enzyme activity in ALI and shed light on the potential therapeutical strategies for ALI intervention.