G protein-coupled receptor kinase 2 (GRK2) participates in the phosphorylation and desensitization of G protein-coupled receptor (GPCR), impacting various biological processes such as inflammation and cell proliferation. Dysregulated expression and activity of GRK2 have been reported in multiple cells in rheumatoid arthritis (RA). However, whether and how GRK2 regulates synovial hyperplasia and fibroblast-like synoviocytes (FLSs) proliferation is poorly understood. In this study, we investigated the regulation of GRK2 and its biological function in RA. We found that GRK2 transmembrane activity was increased in FLSs of RA patients and collagen-induced arthritis (CIA) rats. Additionally, we noted a positive correlation between high GRK2 expression on the cell membrane and serological markers associated with RA and CIA. Immunoprecipitation-mass spectrometry and pull-down analyses revealed tumor necrosis factor receptor-associated factor 2 (TRAF2) as a novel substrate of GRK2. Furthermore, surface plasmon resonance (SPR) and molecular docking assays determined that the C-terminus of GRK2 binds to the C-terminus of TRAF2 at the Gln340 residue. GRK2 knockdown and the GRK2 inhibitor CP-25 attenuated synovial hyperplasia and FLS proliferation in CIA both in vitro and in vivo by decreasing GRK2 membrane expression and activity. Mechanistically, increased GRK2 transmembrane activity contributed to the recruitment of TRAF2 on the cell membrane, promoting GRK2-TRAF2 interactions that facilitate the recruitment of the E3 ubiquitin ligase TRIM47 to TRAF2. This enhanced TRAF2 Lys63 polyubiquitylation and induced nuclear factor (NF)-κB activation, leading to synovial hyperplasia and abnormal proliferation of FLSs. Our study provides a mechanistic and preclinical rationale for further evaluation of GRK2 as a therapeutic target for RA.

ObjectiveTo explore the mechanism of Danggui Niantongtang （DGNTT） against adjuvant-induced arthritis （AA） in rats with wind-dampness-heat arthralgia （FSR） based on the variation of intestinal flora. MethodA total of 60 SD rats were randomized into normal （control） group， FSR group， low-， medium-， and high-dose DGNTT （5.67， 11.34， 22.68 g·kg^-1） groups， and methotrexate （MTX） group （1.35 mg·kg^-1）， with 10 rats in each group. The rats， except the control group， were injected with Mtb adjuvant and then exposed to artificial climatic chamber （hot and humid with wind） for 64 h for modeling. The rats were treated with water， DGNTT or MTX for 28 days from the day of injection. Arthritis index （AI） of rats was measured and paw volume was determined with a volume meter. The morphology of synovial tissues of the knees was observed based on hematoxylin-eosin （HE） staining and the changes of intestinal flora were analyzed based on 16S rRNA sequencing. ResultDGNTT can alleviate the hyperplasia of synovial tissue and inflammation of AA rats with FSR and inhibit the formation of pannus. The results of 16S rRNA sequencing showed that the relative abundance of Firmicutes， Lactobacillus， Prevotella 9， and Alloprevotella decreased （P<0.05， P<0.01） and the relative abundance of Bacteroidetes and Bacteroides increased （P<0.01） in FSR group compared those in the control group. Compared with the FSR group， all DGNTT groups and MTX group had high relative abundance of Lactobacillus （P<0.05， P<0.01） and low relative abundance of Bacteroidetes （P<0.01） and medium-dose and high-dose DGNTT groups and MTX group showed high abundance of Firmicutes， Prevotella 9， and Alloprevotella and low abundance of Bacteroides （P<0.05， P<0.01）. Spearman's correlation analysis suggested that the abundance of Bacteroides and Helicobacter was in positive correlation with AI （P<0.05）， while the abundance of Prevotella 9 and Candidatus Saccharimonas was in negative correlation with AI （P<0.01， P<0.05）. There was a negative correlation between the abundance of Prevotella 9 and paw volume （P<0.01）， and the abundance of Ruminococcaceae NK4A214 group， Christensenellaceae R-7 group， and Bacteroides was in negative correlation with spleen index （P<0.05）. The abundance of Prevotella 9 was in negative correlation with spleen index （P<0.01）. ConclusionDGNTT is effective for arthritis with FSR， as it can regulate the composition of intestinal flora in AA rats by increasing the abundance of probiotics and inhibiting the growth of pathogenic bacteria. The mechanism is the likelihood that it improves intestinal immune metabolism to ensure intestinal homeostasis.

Activation of the heart normally begins in the sinoatrial node (SAN). Electrical impulses spontaneously released by SAN pacemaker cells (SANPCs) trigger the contraction of the heart. However, the cellular nature of SANPCs remains controversial. Here, we report that SANPCs exhibit glutamatergic neuron-like properties. By comparing the single-cell transcriptome of SANPCs with that of cells from primary visual cortex in mouse, we found that SANPCs co-clustered with cortical neurons. Tissue and cellular imaging confirmed that SANPCs contained key elements of glutamatergic neurotransmitter system, expressing genes encoding glutamate synthesis pathway (Gls), ionotropic and metabotropic glutamate receptors (Grina, Gria3, Grm1 and Grm5), and glutamate transporters (Slc17a7). SANPCs highly expressed cell markers of glutamatergic neurons (Snap25 and Slc17a7), whereas Gad1, a marker of GABAergic neurons, was negative. Functional studies revealed that inhibition of glutamate receptors or transporters reduced spontaneous pacing frequency of isolated SAN tissues and spontaneous Ca