GRIM-19 Deficiency Promotes Macrophage Polarization to M1 Phenotype Partly Through AKT/mTOR Pathway and Glycolysis in Unexplained Recurrent Spontaneous Abortion

Background: The occurrence of unexplained recurrent spontaneous abortion (URSA) is closely related to the disorder of the immune system, but the mechanisms remain unclear. The purpose of this study was to investigate the expression of GRIM-19 in URSA and its possible pathogenesis according to macrophage polarization. Methods: The correlation of macrophage and pregnancy outcome was determined using GRIM-19+/- mice. GRIM-19 expression in decidua of recurrent abortion patients was analysed using clinical specimens, while THP-1-derived macrophages after downregulation of GRIM-19 was used to explore GRIM-19 roles in regulating macrophage polarization and glycolysis. The mechanism of GRIM-19 regulation was studied using molecular biology and biochemical methods. Findings: First, we found that clodronate-liposome mediated macrophage depletion can reduce the embryo resorption rate in GRIM-19+/- mice, which demonstrated that macrophages play a role in the early embryo loss in GRIM-19+/- mice. We further confirmed that GRIM-19 deficiency promotes macrophage polarization from M2 to M1 phenotype in GRIM-19+/- mice uterus. Moreover, we found that the GRIM-19 deficiency enhances M1 polarization in THP-1-derived macrophages. We also showed that protein levels of p-mTOR and p-AKT decreased significantly after downregulation of GRIM-19 in THP-1-derived macrophages. In parallel to this, the upregulation of the amount of M1 macrophages resulted from GRIM-19 loss was significantly reversed in the cells treated with IGF-1 which is an activator of AKT/mTOR pathway. To provide more direct evidence, we showed that GRIM-19 was downregulated in the uterine decidual macrophages of patients with URSA, and GRIM-19 downregulation is accompanied by increased M1 macrophage polarization. Furthermore, we found that expression of glycolytic enzymes was substantially enhanced in the uterine decidual macrophages of URSA patients, and glycolysis in macrophages can be further enhanced with downregulation of GRIM-19. Interpretation: Our study provides evidence that GRIM-19 deficiency may play a role in regulating macrophage polarization partly through AKT/mTOR signaling axis in URSA and glycolysis may participate in this complicated process. Funding Statement: This work was supported by the National Natural Science Foundation of China (grant numbers 81571511, 81701528, 81370711 and 81601315), the Shandong Provincial Key Research and Development Project (grant number 2019GSF107004), the Shandong Provincial Natural Science Foundation (grant number ZR2017PH052). Declaration of Interests: The authors stated: "None declared." Ethics Approval Statement: All of the procedures of the animal experiments were approved by the Ethical Committee of Shandong University. All efforts were made to minimize suffering of mice.