Activation of p38alpha/beta MAPK in myogenesis via binding of the scaffold protein JLP to the cell surface protein Cdo

The p38 mitogen-activated protein kinase (MAPK) pathway plays an important role in cell differentiation, but the signaling mechanisms by which it is activated during this process are largely unknown. Cdo is an immunoglobulin superfamily member that functions as a component of multiprotein cell surface complexes to promote myogenesis. In this study, we report that the Cdo intracellular region interacts with JLP, a scaffold protein for the p38alpha/beta MAPK pathway. Cdo, JLP, and p38alpha/beta form complexes in differentiating myoblasts, and Cdo and JLP cooperate to enhance levels of active p38alpha/beta in transfectants. Primary myoblasts from Cdo(-/-) mice, which display a defective differentiation program, are deficient in p38alpha/beta activity, and the expression of an activated form of MKK6 (an immediate upstream activator of p38) rescues the ability of Cdo(-/-) cells to differentiate. These results document a novel mechanism of signaling during cell differentiation: the interaction of a MAPK scaffold protein with a cell surface receptor.     

A Cdo-Bnip-2-Cdc42 signaling pathway regulates p38alpha/beta MAPK activity and myogenic differentiation

The p38alpha/beta mitogen-activated protein kinase (MAPK) pathway promotes skeletal myogenesis, but the mechanisms by which it is activated during this process are unclear. During myoblast differentiation, the promyogenic cell surface receptor Cdo binds to the p38alpha/beta pathway scaffold protein JLP and, via JLP, p38alpha/beta itself. We report that Cdo also interacts with Bnip-2, a protein that binds the small guanosine triphosphatase (GTPase) Cdc42 and a negative regulator of Cdc42, Cdc42 GTPase-activating protein (GAP). Moreover, Bnip-2 and JLP are brought together through mutual interaction with Cdo. Gain- and loss-of-function experiments with myoblasts indicate that the Cdo-Bnip-2 interaction stimulates Cdc42 activity, which in turn promotes p38alpha/beta activity and cell differentiation. These results reveal a previously unknown linkage between a cell surface receptor and downstream modulation of Cdc42 activity. Furthermore, interaction with multiple scaffold-type proteins is a distinctive mode of cell surface receptor signaling and provides one mechanism for specificity of p38alpha/beta activation during cell differentiation.     

Prediction of the pathogens that are the cause of pneumonia by the battlefield hypothesis

Commensal organisms are frequent causes of pneumonia. However, the detection of these organisms in the airway does not mean that they are the causative pathogens; they may exist merely as colonizers. In up to 50% cases of pneumonia, the causative pathogens remain unidentified, thereby hampering targeting therapies. In speculating on the role of a commensal organism in pneumonia, we devised the battlefield hypothesis. In the "pneumonia battlefield," the organism-to-human cell number ratio may be an index for the pathogenic role of the organism. Using real-time PCR reactions for sputum samples, we tested whether the hypothesis predicts the results of bacteriological clinical tests for 4 representative commensal organisms: Streptococcus pneumoniae, Haemophilus influenzae, Pseudomonas spp., and Moraxella catarrhalis. The cutoff value for the organism-to-human cell number ratio, above which the pathogenic role of the organism was suspected, was set up for each organism using 224 sputum samples. The validity of the cutoff value was then tested in a prospective study that included 153 samples; the samples were classified into 3 groups, and each group contained 93%, 7%, and 0% of the samples from pneumonia, in which the pathogenic role of Streptococcus pneumoniae was suggested by the clinical tests. The results for Haemophilus influenzae, Pseudomonas spp., and Moraxella catarrhalis were 100%, 0%, and 0%, respectively. The battlefield hypothesis enabled legitimate interpretation of the PCR results and predicted pneumonia in which the pathogenic role of the organism was suggested by the clinical test. The PCR reactions based on the battlefield hypothesis may help to promote targeted therapies for pneumonia. The prospective observatory study described in the current report had been registered to the University Hospital Medical Information Network (UMIN) registry before its initiation, where the UMIN is a registry approved by the International Committee of Medical Journal Editors (ICMJE). The UMIN registry number was UMIN000001118: A prospective study for the investigation of the validity of cutoff values established for the HIRA-TAN system (April 9, 2008).     

Suppression of SOCS3 expression in the pancreatic beta-cell leads to resistance to type 1 diabetes

Type 1 diabetes results from the selective destruction of insulin-producing pancreatic beta-cells during islet inflammation, which involves inflammatory cytokines and free radicals. However, mechanisms for protecting beta-cells from destruction have not been clarified. In this study, we define the role of SOCS3 on beta-cell destruction using beta-cell-specific SOCS3-conditional knockout (cKO) mice. The beta-cell-specific SOCS3-deficient mice were resistant to the development of diabetes caused by streptozotocin (STZ), a genotoxic methylating agent, which has been used to trigger beta-cell destruction. The islets from cKO mice demonstrated hyperactivation of STAT3 and higher induction of Bcl-xL than did islets from WT mice, and SOCS3-deficient beta-cells were more resistant to apoptosis induced by STZ in vitro than were WT beta-cells. These results suggest that enhanced STAT3 signaling protects beta-cells from destruction induced by a genotoxic stress and that STAT3/SOCS3 can be a potential therapeutic target for the treatment of type 1 diabetes.     

Role of the TAB2-related protein TAB3 in IL-1 and TNF signaling

The cytokines IL-1 and TNF induce expression of a series of genes that regulate inflammation through activation of NF-kappaB signal transduction pathways. TAK1, a MAPKKK, is critical for both IL-1- and TNF-induced activation of the NF-kappaB pathway. TAB2, a TAK1-binding protein, is involved in IL-1-induced NF-kappaB activation by physically linking TAK1 to TRAF6. However, IL-1-induced activation of NF-kappaB is not impaired in TAB2-deficient embryonic fibroblasts. Here we report the identification and characterization of a novel protein designated TAB3, a TAB2-like molecule that associates with TAK1 and can activate NF-kappaB similar to TAB2. Endogenous TAB3 interacts with TRAF6 and TRAF2 in an IL-1- and a TNF-dependent manner, respectively. Further more, IL-1 signaling leads to the ubiquitination of TAB2 and TAB3 through TRAF6. Cotransfection of siRNAs directed against both TAB2 and TAB3 inhibit both IL-1- and TNF-induced activation of TAK1 and NF-kappaB. These results suggest that TAB2 and TAB3 function redundantly as mediators of TAK1 activation in IL-1 and TNF signal transduction.     

Structural insights into E2–E3 interaction for LC3 lipidation

The members of the LC3/Atg8 family of proteins are covalently attached to phagophore and autophagosomal membranes. At the last step of the LC3 lipidation cascade, LC3 is transferred from the E2 enzyme ATG3 to phosphatidylethanolamine (PE). This transfer is stimulated by the ATG12–ATG5-ATG16L1 E3 complex, but the mechanism is not fully understood. We recently found that ATG12 of the E3 binds to a short sequence in the flexible region (FR) of ATG3 with high affinity, and that this interaction is critical for E2–E3 complex formation. These findings, together with detailed structural analyses of this interaction, define the properties of ATG12 and provide new insights of how LC3 transfer begins with ATG3 recruitment by ATG12.