Deletion of GPR21 improves glucose homeostasis and inhibits the CCL2-CCR2 axis by divergent mechanisms

Cryo-EM structure of orphan G protein-coupled receptor GPR21

GPR21 belongs to class A orphan G protein-coupled receptor (GPCR). The endogenous ligands for human GPR21 remain unidentified. GPR21 expression is associated with developing type 2 diabetes (T2DM), a multifactorial metabolic disease caused by pancreatic β-cell dysfunction, decreasing insulin production, insulin resistance, and obesity. Animal studies suggested that GPR21 is a potential therapeutic target for T2DM treatment. The underlying mechanisms leading to GPR21 self-activation remain unknown. In our co-expression analysis, we noted that GPR21 could also form a stable complex with an unreported Gα protein subtype, Gαs. To gain further insights into the structural mechanisms of GPR21 activation, we employed cryo-electron microscopy (cryo-EM) and single-particle analysis to resolve the high-resolution structure of GPR21-Gαs complexes. The clear electron density map of the GPR21-Gαs provided direct evidence that GPR21 could couple to Gαs protein at physiological conditions. Thus, GPR21 might mediate previously unexplored pathways in normal or pathological conditions, which warrants further investigation. Structure-guided mutagenesis and biochemical analysis revealed that extracellular loop 2 (ECL2) of GPR21 is essential for the receptor transducing intracellular signal via cAMP. Together, the new structure data reveal a novel signaling cascade of human GPR21 mediated by ECL2 and provide fundamental information for future structure-based drug development.

Identification of FGF13 as a Potential Biomarker and Target for Diagnosis of Impaired Glucose Tolerance

Early identification of pre-diabetes provides an opportunity for intervention and treatment to delay its progression to type 2 diabetes mellitus (T2DM). We aimed to identify the biomarkers of impaired glucose tolerance (IGT) through bioinformatics analysis. The GSE76896 dataset, including non-diabetic (ND), IGT, and T2DM clinical samples, was deeply analyzed to identify 309 Co-DEGs for IGT and T2DM. Gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analyses indicated that inflammatory responses and the PI3K-AKT signaling pathway are important patho-physiological features of IGT and T2DM. Protein-protein interaction (PPI) network analysis and cytoHubba technolgy identified seven hub genes: namely, CCL2, CXCL1, CXCL8, EDN1, FGF13, MMP1, and NGF. The expression and ROC curves of these hub genes were validated using the GSE38642 dataset. Through an immunofluorescence assay, we found that the expression of FGF13 in islets of mice in the HFD and T2DM groups was significantly lower than in the control group. Similarly, the level of FGF13 in the sera of IGT and T2DM patients was lower than that in the healthy group. Together, these results suggest that FGF13 can be treated as a novel biomarker of IGT, which may provide new targets for the diagnosis and treatment of pre-diabetes and T2DM.

The role of neuroplastin65 in macrophage against E. coli infection in mice

BACKGROUND

Innate immune response constitutes the first line of defense against pathogens. Inflammatory responses involve close contact between different populations of cells. These adhesive interactions mediate migration of cells to sites of infection leading the effective action of cells within the lesions. Cell adhesion molecules are critical to controlling immune response mediating cell adhesion or chemotaxis, as well as coordinating actin-based cell motility during phagocytosis and chemotaxis. Recently, a newly discovered neuroplastin (Np) adhesion molecule is found to play an important role in the nervous system. However, there is limited information on Np functions in immune response. To understand how Np is involved in innate immune response, a mouse model of intraperitoneal infection was established to investigate the effect of Np on macrophage-mediated clearance of E. coli infection and its possible molecular mechanisms.

METHODS

Specific deficiency mice with Nptn gene controlling Np65 isoform were employed in this study. The expression levels of mRNA and proteins were detected by qPCR and western blot, or evaluated by flow cytometry. The expression level of NO and ROS were measured with their specific indicators. Cell cycle and apoptosis were detected by specific detection kits. Acid phosphatase activity was measured by flow cytometry after labelling with LysoRed fluorescent probe. Bone marrow derived macrophages (BMDMs) were isolated from bone marrow of mice hind legs. Cell proliferation was detected by CCK8 assay. Cell migration was measured by wound healing assay or transwell assay.

RESULTS

The lethal dose of E. coli infection in Np65^-/- mice dropped to the half of lethal dose in WT mice. The bacterial load in the spleen, kidney and liver from Np65^-/- mice were significantly higher than that from WT mice, which were due to the dramatic reduction of NO and ROS production in phagocytes from Np65^-/- mice. Np65 gene deficiency remarkably impaired phagocytosis and function of lysosome in macrophage. Furthermore, Np65 molecule was involved in maturation and proliferation, even in migration and chemotaxis of BMDM in vitro.

CONCLUSION

This study for the first time demonstrates that Np is involved in multi-function of phagocytes during bacterial infection, proposing that Np adhesion molecule plays a critical role in clearing pathogen infection in innate immunity.