IQGAP1 selectively interacts with K-Ras but not with H-Ras and modulates K-Ras function.

K-Ras is frequently mutated and activated especially in pancreatic cancers. To analyze K-Ras function, we have searched for K-Ras interacting proteins and found IQ motif containing GTPase activating protein 1 (IQGAP1) as a novel K-Ras binding protein. IQGAP1 has been known as a scaffold protein for B-Raf, MEK1/2 and ERK1/2. Here we showed that IQGAP1 selectively formed a complex with K-Ras but not with H-Ras, and recruited B-Raf to K-Ras. We found that IQ motif region of IQGAP1 interacted with K-Ras. Both active and inactive K-Ras interacted with IQGAP1, and effector domain mutants of K-Ras also associated with IQGAP1, indicating that IQGAP1 interacts with K-Ras irrespective of Ras-effectors like B-Raf. We also found that overexpression or knock-down of IQGAP1 affected the interaction between K-Ras and B-Raf, and IQGAP1 overexpression increased ERK1/2 phosphorylation in K-Ras dependent manner in PANC1 cells. Our data suggest that IQGAP1 has a novel mechanism to modulate K-Ras pathway.

Structural basis of digoxin that antagonizes RORgamma t receptor activity and suppresses Th17 cell differentiation and interleukin (IL)-17 production.

The retinoic acid-related orphan nuclear receptor γt (RORγt)/RORγ2 is well known as a master regulator of interleukin 17 (IL-17)-producing helper T (Th17) cell development. To develop a therapeutic agent against Th17-mediated autoimmune diseases, we screened chemical compounds and successfully found that digoxin inhibited IL-17 production. Further studies revealed that digoxin bound to the ligand binding domain of RORγt and suppressed Th17 differentiation without affecting Th1 differentiation. To better understand the structural basis for the inhibitory activity of digoxin, we determined the crystal structure of the RORγt ligand-binding domain in complex with digoxin at 2.2 Å resolution. The structure reveals that digoxin binds to the ligand-binding pocket protruding between helices H3 and H11 from the pocket. In addition, digoxin disrupts the key interaction important for the agonistic activity, resulting in preventing the positioning of helix H12 in the active conformation, thus antagonizing coactivator interaction. Functional studies demonstrated that digoxin inhibited RORγt activity and decreased IL-17 production but not RORα activity. Digoxin inhibited IL-17 production in CD4(+) T cells from experimental autoimmune encephalomyelitis mice. Our data indicates that RORγt is a promising therapeutic target for Th17-derived autoimmune diseases and our structural data will help to design novel RORγt antagonists.

Loss of the fragile X mental retardation protein causes aberrant differentiation in human neural progenitor cells.

Fragile X syndrome (FXS) is caused by transcriptional silencing of the FMR1 gene during embryonic development with the consequent loss of the encoded fragile X mental retardation protein (FMRP). The pathological mechanisms of FXS have been extensively studied using the Fmr1-knockout mouse, and the findings suggest important roles for FMRP in synaptic plasticity and proper functioning of neural networks. However, the function of FMRP during early development in the human nervous system remains to be confirmed. Here we describe human neural progenitor cells (NPCs) as a model for studying FMRP functions and FXS pathology. Transcriptome analysis of the NPCs derived from FMR1-knockout human induced pluripotent stem cells (iPSCs) showed altered expression of neural differentiation markers, particularly a marked induction of the astrocyte marker glial fibrillary acidic protein (GFAP). When induced to differentiate, FMRP-deficient neurons continued to express GFAP, and showed less spontaneous calcium bursts than the parental iPSC-derived neurons. Interestingly, the aberrant expression of GFAP and the impaired firing was corrected by treatment with the protein kinase inhibitor LX7101. These findings underscore the modulatory roles of FMRP in human neurogenesis, and further demonstrate that the defective phenotype of FXS could be reversed at least partly by small molecule kinase inhibitors.

Secreted factors from adipose tissue-derived mesenchymal stem cells suppress oxygen/glucose deprivation-induced cardiomyocyte cell death via furin/PCSK-like enzyme activity.

Clinical application of mesenchymal stem cells (MSCs) represents a potential novel therapy for currently intractable deteriorating diseases or traumatic injuries, including myocardial infarction. However, the molecular mechanisms of the therapeutic effects have not been precisely revealed. Herein, we report that conditioned media (CM) from rat adipose tissue-derived MSCs (ASCs) protected adult cardiomyocytes from oxygen/glucose deprivation (OGD)-induced cell death. We focused on furin/PCSK protease activity in ASC-CM because many therapeutic factors of MSCs and soluble cardioprotective factors include the PCSK cleavage site. We found that recombinant furin protected cardiomyocytes from OGD-induced cell death. The ASC-CM had potent furin/PCSK protease activity and the cardioprotective effect of the CM from ASCs in the OGD-assay was abolished by an inhibitor of the furin/PCSK-like enzyme. Microarray analysis and Western blot analysis showed PCSK5A, the secreted type of PCSK5, is the most abundantly secreted PCSK among 7 PCSK family members in ASC. Finally, knockdown of PCSK5A in ASCs decreased both the furin/PCSK protease activity and cardioprotective activity in the CM. These findings indicate an involvement of furin/PCSK-type protease(s) in the anti-ischemic activity of ASCs, and suggest a new mechanism of the therapeutic effect of MSCs.

Inhibiting RORγt/Th17 axis for autoimmune disorders.

The recent success reported in late-stage clinical trials for the treatment of psoriasis by antibodies directed against interleukin (IL)-17 or its receptor has validated and strongly supports the development of inhibitors of the IL-17 pathway as a new therapeutic modality in chronic inflammation and autoimmunity. These results also encourage the drug discovery of orally available small molecules that can modulate down the production of IL-17 by Th17 cells (the major IL-17 producers) or the downstream signaling of the IL-17 receptor. Here, we review these strategies with an emphasis on inhibiting the retinoic-acid-related orphan nuclear receptor RORγt, which is the master regulator of Th17 cells and a promising therapeutic target for the treatment of multiple autoimmune disorders.

Proteomic analysis of the SIRT6 interactome: novel links to genome maintenance and cellular stress signaling.

The chromatin regulatory factor SIRT6 plays pivotal roles in metabolism, tumor suppression, and aging biology. Despite the fundamental roles of SIRT6 in physiology and disease, only a handful of molecular and functional interactions of SIRT6 have been reported. Here, we characterize the SIRT6 interactome and identify 80+ novel SIRT6-interacting proteins. The discovery of these SIRT6-associations considerably expands knowledge of the SIRT6 interaction network, and suggests previously unknown functional interactions of SIRT6 in fundamental cellular processes. These include chromatin remodeling, mitotic chromosome segregation, protein homeostasis, and transcriptional elongation. Extended analysis of the SIRT6 interaction with G3BP1, a master stress response factor, uncovers an unexpected role and mechanism of SIRT6 in regulating stress granule assembly and cellular stress resistance.

A novel assay of bacterial peptidoglycan synthesis for natural product screening.

Although a large number of microbial metabolites have been discovered as inhibitors of bacterial peptidoglycan biosynthesis, only a few inhibitors were reported for the pathway of UDP-MurNAc-pentapeptide formation, partly because of the lack of assays appropriate for natural product screening. Among the pathway enzymes, D-Ala racemase (Alr), D-Ala:D-Ala ligase (Ddl) and UDP-MurNAc-tripeptide:D-Ala-D-Ala transferase (MurF) are particularly attractive as antibacterial targets, because these enzymes are essential for growth and utilize low-molecular-weight substrates. Using dansylated UDP-MurNAc-tripeptide and L-Ala as the substrates, we established a cell-free assay to measure the sequential reactions of Alr, Ddl and MurF coupled with translocase I. This assay is sensitive and robust enough to screen mixtures of microbial metabolites, and enables us to distinguish the inhibitors for D-Ala-D-Ala formation, MurF and translocase I. D-cycloserine, the D-Ala-D-Ala pathway inhibitor, was successfully detected by this assay (IC(50)=1.7 microg ml(-1)). In a large-scale screening of natural products, we have identified inhibitors for D-Ala-D-Ala synthesis pathway, MurF and translocase I.

FKS1 mutations responsible for selective resistance of Saccharomyces cerevisiae to the novel 1,3-beta-glucan synthase inhibitor arborcandin C.

Arborcandin C is a novel antibiotic with potent antifungal activity that inhibits 1,3-beta-glucan synthase in fungi. We examined spontaneous Saccharomyces cerevisiae mutants which are selectively resistant to arborcandin C and revealed that a single amino acid replacement in Fks1p of Asn(470) with Lys or of Leu(642) with Ser confers selective resistance on Fks1p mutants.

Deletion of OSH3 gene confers resistance against ISP-1 in Saccharomyces cerevisiae.

Sphingolipids have been reported to regulate the growth and death of mammalian and yeast cells, but their precise mechanisms are unknown. In this paper, it was shown that the deletion of the oxysterol binding protein homologue 3 (OSH3) gene confers hyper resistance against ISP-1, an inhibitor of sphingolipid biosynthesis, in the yeast Saccharomyces cerevisiae. Furthermore, the overexpression of the ROK1 gene, which directly binds to Osh3p, conferred resistance against ISP-1, and the deletion of the KEM1 gene, which regulates microtubule functions, exhibited ISP-1 hypersensitivity. And yet, an ISP-1 treatment caused an abnormal mitotic spindle formation, and the ISP-1-induced cell cycle arrest was rescued by the deletion of the OSH3 gene. Taken together, it is suggested that the expression levels of the OSH3 gene influence the ISP-1 sensitivity of S. cerevisiae, and the sphingolipids are necessary for normal mitotic spindle formation in which the Osh3p may play a pivotal role.