G-protein-coupled receptor kinase-5 mediates inflammation but does not regulate cellular infiltration or bacterial load in a polymicrobial sepsis model in mice.

NFκB-dependent signaling is an important modulator of inflammation in several diseases including sepsis. G-protein-coupled receptor kinase-5 (GRK5) is an evolutionarily conserved regulator of the NFκB pathway. We hypothesized that GRK5 via NFκB regulation plays an important role in the pathogenesis of sepsis. To test this we utilized a clinically relevant polymicrobial sepsis model in mice that were deficient in GRK5. We subjected wild-type (WT) and GRK5 knockout (KO) mice to cecal ligation and puncture (CLP)-induced polymicrobial sepsis and assessed the various events in sepsis pathogenesis. CLP induced a significant inflammatory response in the WT and this was markedly attenuated in the KO mice. To determine the signaling mechanisms and the role of NFκB activation in sepsis-induced inflammation, we assessed the levels of IκBα phosphorylation and expression of NFκB-dependent genes in the liver in the two genotypes. Both IκBα phosphorylation and gene expression were significantly inhibited in the GRK5 KO compared to the WT mice. Interestingly, however, GRK5 did not modulate either immune cell infiltration (to the primary site of infection) or local/systemic bacterial load subsequent to sepsis induction. In contrast GRK5 deficiency significantly inhibited sepsis-induced plasma corticosterone levels and the consequent thymocyte apoptosis in vivo. Associated with these outcomes, CLP-induced mortality was significantly prevented in the GRK5 KO mice in the presence of antibiotics. Together, our studies demonstrate that GRK5 is an important regulator of inflammation and thymic apoptosis in polymicrobial sepsis and implicate GRK5 as a potential molecular target in sepsis.

Genome-wide RNA interference in Drosophila cells identifies G protein-coupled receptor kinase 2 as a conserved regulator of NF-kappaB signaling.

Because NF-kappaB signaling pathways are highly conserved in evolution, the fruit fly Drosophila melanogaster provides a good model to study these cascades. We carried out an RNA interference (RNAi)-based genome-wide in vitro reporter assay screen in Drosophila for components of NF-kappaB pathways. We analyzed 16,025 dsRNA-treatments and identified 10 novel NF-kappaB regulators. Of these, nine dsRNA-treatments affect primarily the Toll pathway. G protein-coupled receptor kinase (Gprk)2, CG15737/Toll pathway activation mediating protein, and u-shaped were required for normal Drosomycin response in vivo. Interaction studies revealed that Gprk2 interacts with the Drosophila IkappaB homolog Cactus, but is not required in Cactus degradation, indicating a novel mechanism for NF-kappaB regulation. Morpholino silencing of the zebrafish ortholog of Gprk2 in fish embryos caused impaired cytokine expression after Escherichia coli infection, indicating a conserved role in NF-kappaB signaling. Moreover, small interfering RNA silencing of the human ortholog GRK5 in HeLa cells impaired NF-kappaB reporter activity. Gprk2 RNAi flies are susceptible to infection with Enterococcus faecalis and Gprk2 RNAi rescues Toll(10b)-induced blood cell activation in Drosophila larvae in vivo. We conclude that Gprk2/GRK5 has an evolutionarily conserved role in regulating NF-kappaB signaling.