p38 mitogen-activated protein kinases (MAPKs) are activated primarily in response to inflammatory cytokines and cellular stress, and inhibitors which target the p38␣ and p38␤ MAPKs have shown potential for the treatment of inflammatory disease. Here we report the generation and initial characterization of a knockout of the p38␤ (MAPK11) gene. p38␤ ؊/؊ mice were viable and exhibited no apparent health problems. The expression and activation of p38␣, ERK1/2, and JNK in response to cellular stress was normal in embryonic fibroblasts from p38␤ ؊/؊ mice, as was the activation of p38-activated kinases MAPKAP-K2 and MSK1. The transcription of p38-dependent immediate-early genes was also not affected by the knockout of p38␤, suggesting that p38␣ is the predominant isoform involved in these processes. The p38␤ ؊/؊ mice also showed normal T-cell development. Lipopolysaccharide-induced cytokine production was also normal in the p38␤ ؊/؊ mice. As p38 is activated by tumor necrosis factor, the p38␤ ؊/؊ mice were crossed onto a TNF⌬ARE mouse line. These mice overexpress tumor necrosis factor, which results in development symptoms similar to rheumatoid arthritis and inflammatory bowel disease. The progression of these diseases was not however moderated by knockout of p38␤. Together these results suggest that p38␣, and not p38␤, is the major p38 isoform involved in the immune response and that it would not be necessary to retain activity against p38␤ during the development of p38 inhibitors.

Cdc20 is a substrate adaptor and activator of the anaphase-promoting complex/cyclosome (APC/C), the E3 ubiquitin ligase whose activity is required for anaphase onset and exit from mitosis. A green fluorescent protein derivative, Cdc20–GFP, bound to centrosomes throughout the cell cycle and to kinetochores from late prophase to late telophase. We mapped distinct domains of Cdc20 that are required for association with kinetochores and centrosomes. FRAP measurements revealed extremely rapid dynamics at the kinetochores (t 1/2 = 5.1 s) and spindle poles (t 1/2 = 4.7 s). This rapid turnover is independent of microtubules. Rapid transit of Cdc20 through kinetochores may ensure that spindle checkpoint signaling at unattached/relaxed kinetochores can continuously inhibit APC/CCdc20 targeting of anaphase inhibitors (securins) throughout the cell until all the chromosomes are properly attached to the mitotic spindle.

MSK (mitogen- and stress-activated protein kinase) 1 and MSK2 are kinases activated downstream of either the ERK (extracellular-signal-regulated kinase) 1/2 or p38 MAPK (mitogen-activated protein kinase) pathways in vivo and are required for the phosphorylation of CREB (cAMP response element-binding protein) and histone H3. Here we show that the MSKs are involved in regulating the transcription of the immediate early gene Nur77. Stimulation of mouse embryonic fibroblasts with PMA, EGF (epidermal growth factor), TNF (tumour necrosis factor) or anisomycin resulted in induction of the Nur77 mRNA. The induction of Nur77 by TNF and anisomycin was abolished in MSK1/2 double-knockout cells, whereas induction was significantly reduced in response to PMA or EGF. The MSK responsive elements were mapped to two AP (activator protein)-1-like elements in the Nur77 promoter. The induction of Nur77 was also blocked by A-CREB, suggesting that MSKs control Nur77 transcription by phosphorylating CREB bound to the two AP-1-like elements. Consistent with the decrease in Nur77 mRNA levels in the MSK1/2-knockout cells, it was also found that MSKs were required for the induction of Nur77 protein by PMA and TNF. MSKs were also found to be required for the transcription of two genes related to Nur77, Nurr1 and Nor1, which were also transcribed in a CREB- or ATF1 (activating transcription factor-1)-dependent manner. Downstream of anisomycin signalling, a second ERK-dependent pathway, independent of MSK and CREB, was also required for the transcription of Nurr1 and Nor1.