Superoxide inhibits guanine nucleotide exchange factor (GEF) action on Ras, but not on Rho, through desensitization of Ras to GEF.

Ras and Rho GTPases are molecular switches for various vital cellular signaling pathways. Overactivation of these GTPases often causes development of cancer. Guanine nucleotide exchange factors (GEFs) and oxidants function to upregulate these GTPases through facilitation of guanine nucleotide exchange (GNE) of these GTPases. However, the effect of oxidants on GEF functions, or vice versa, has not been known. We show that, via targeting Ras Cys(51), an oxidant inhibits the catalytic action of Cdc25-the catalytic domain of RasGEFs-on Ras. However, the enhancement of Ras GNE by an oxidant continues regardless of the presence of Cdc25. Limiting RasGEF action by an oxidant may function to prevent the pathophysiological overactivation of Ras in the presence of both RasGEFs and oxidants. The continuous exposure of Ras to nitric oxide and its derivatives can form S-nitrosated Ras (Ras-SNO). This study also shows that an oxidant not only inhibits the catalytic action of Cdc25 on Ras-SNO but also fails to enhance Ras-SNO GNE. This lack of enhancement then populates the biologically inactive Ras-SNO in cells, which may function to prevent the continued redox signaling of the Ras pathophysiological response. Finally, this study also demonstrates that, unlike the case with RasGEFs, an oxidant does not inhibit the catalytic action of RhoGEF-Vav or Dbs-on Rho GTPases such as Rac1, RhoA, RhoC, and Cdc42. This result explains the results of the previous study in which, despite the presence of an oxidant, the catalytic action of Dbs in cells continued to enhance RhoC GNE.

Age-dependent role for Ras-GRF1 in the late stages of adult neurogenesis in the dentate gyrus.

The dentate gyrus of the hippocampus plays a pivotal role in pattern separation, a process required for the behavioral task of contextual discrimination. One unique feature of the dentate gyrus that contributes to pattern separation is adult neurogenesis, where newly born neurons play a distinct role in neuronal circuitry. Moreover,the function of neurogenesis in this brain region differs in adolescent and adult mice. The signaling mechanisms that differentially regulate the distinct steps of adult neurogenesis in adolescence and adulthood remain poorly understood. We used mice lacking RASGRF1(GRF1), a calcium-dependent exchange factor that regulates synaptic plasticity and participates in contextual discrimination performed by mice, to test whether GRF1 plays a role in adult neurogenesis.We show Grf1 knockout mice begin to display a defect in neurogenesis at the onset of adulthood (~2 months of age), when wild-type mice first acquire the ability to distinguish between closely related contexts. At this age, young hippocampal neurons in Grf1 knockout mice display severely reduced dendritic arborization. By 3 months of age, new neuron survival is also impaired. BrdU labeling of new neurons in 2-month-old Grf1 knockout mice shows they begin to display reduced survival between 2 and 3 weeks after birth, just as new neurons begin to develop complex dendritic morphology and transition into using glutamatergic excitatory input. Interestingly, GRF1 expression appears in new neurons at the developmental stage when GRF1 loss begins to effect neuronal function. In addition, we induced a similar loss of new hippocampal neurons by knocking down expression of GRF1 solely in new neurons by injecting retrovirus that express shRNA against GRF1 into the dentate gyrus. Together, these findings show that GRF1 expressed in new neurons promotes late stages of adult neurogenesis. Overall our findings show GRF1 to be an age-dependent regulator of adult hippocampal neurogenesis, which contributes to ability of mice to distinguish closely related contexts.

Mih1/Cdc25 is negatively regulated by Pkc1 in Saccharomyces cerevisiae.

Mitotic cyclin-dependent kinase (CDK) is activated by Cdc25 phosphatase through dephosphorylation at the Wee1-mediated phosphorylation site. In Saccharomyces cerevisiae, regulation of Mih1 (Cdc25 homologue) remains unclear because inactivation/degradation of Swe1 (Wee1 homologue) is the main trigger for G2/M transition. By deleting all mitotic cyclins except Clb2, a strain was created where Mih1 became essential for mitotic entry at high temperatures. Using this novel assay, the essential domain of Mih1 was identified and Mih1 regulation was characterized. Mih1(3E1D) with phosphomimetic substitutions of four putative PKC target residues in Mih1 had a reduced complementation activity, whereas Mih1(4A) with those nonphosphorylatable substitutions was active. The band pattern of Mih1 by SDS-PAGE was similar to that of Mih1(4A) only after inactivation of Pkc1 in a pkc1(ts) mutant. Over-expression of GFP-tagged Mih1 or GFP-Mih1(4A) accumulated as dot-like structures in the nucleus, whereas GFP-Mih1(3E1D) was localized in the cytoplasm. Over-expression of an active form of Pkc1 excluded GFP-Mih1 from the nucleus, but had minimal effect on GFP-Mih1(4A) localization. Furthermore, addition of ectopic nuclear localization signal to the Mih1(3E1D) sequence recovered complementation activity and nuclear localization. These results suggest that Mih1 is negatively regulated by Pkc1-mediated phosphorylation, which excludes it from the nucleus under certain conditions.

Selective cytotoxicity of a bicyclic Ras inhibitor in cancer cells expressing K-Ras(G13D).

Mutation of RAS genes is a critical event in the pathogenesis of different human tumors and in some developmental disorders. Here we present an arabinose-derived bicyclic compound displaying selective cytotoxicity in human colorectal cancer cells expressing K-Ras(G13D), that shows high intrinsic nucleotide exchange rate. We characterize binding of bicyclic compounds by docking and NMR experiments and their inhibitory activity on GEF-mediated nucleotide exchange on wild-type and mutant Ras proteins. We demonstrate that the in vitro inhibition of Ras nucleotide exchange depends on the molar ratio between Ras and its GEF activator, suggesting that the observed in vivo selective effect may depend on biochemical parameters and actual intracellular concentration of the Ras protein and its regulators.

Activation of Srk1 by the mitogen-activated protein kinase Sty1/Spc1 precedes its dissociation from the kinase and signals its degradation.

Control of cell cycle progression by stress-activated protein kinases (SAPKs) is essential for cell adaptation to extracellular stimuli. The Schizosaccharomyces pombe SAPK Sty1/Spc1 orchestrates general changes in gene expression in response to diverse forms of cytotoxic stress. Here we show that Sty1/Spc1 is bound to its target, the Srk1 kinase, when the signaling pathway is inactive. In response to stress, Sty1/Spc1 phosphorylates Srk1 at threonine 463 of the regulatory domain, inducing both activation of Srk1 kinase, which negatively regulates cell cycle progression by inhibiting Cdc25, and dissociation of Srk1 from the SAPK, which leads to Srk1 degradation by the proteasome.

Fission yeast Clp1p phosphatase affects G2/M transition and mitotic exit through Cdc25p inactivation.

The Cdc14 family of phosphatases specifically reverses proline-directed phosphorylation events. In Saccharomyces cerevisiae, Cdc14p promotes Cdk1p inactivation at mitotic exit by reversing Cdk1p-dependent phosphorylations. Cdk1p is a proline-directed kinase whose activity is required in all eukaryotes for the transit into mitosis. At mitotic commitment, Cdk1p participates in its own regulation by activating the mitotic inducing phosphatase, Cdc25p, and inhibiting the opposing kinase, Wee1p. We have investigated the ability of Schizosaccharomyces pombe Clp1p, a Cdc14p homolog, to disrupt this auto-amplification loop. We show here that Clp1p is required to dephosphorylate, destabilize, and inactivate Cdc25p at the end of mitosis. Clp1p promotes recognition of Cdc25p by the anaphase-promoting complex/cyclosome, an E3 ubiquitin ligase. Failure to inactivate and destabilize Cdc25p in late mitosis delays progression through anaphase, interferes with septation initiation network signaling, and additionally advances the commitment to mitotic entry in the next cycle. This may be a widely conserved mechanism whereby Cdc14 proteins contribute to Cdk1p inactivation.

A fission yeast strain expressing human CDC25A phosphatase: a tool for selectivity studies of pharmacological inhibitors of CDC25.

Fission yeast is a simple eukaryotic model organism in which many aspects of cell cycle control can be explored. We examined by homologous recombination whether the human CDC25A phosphatase could substitute for the function of the fission yeast Cdc25. We first show: (a). that CDC25A efficiently replaces the endogenous Cdc25 mitotic inducer for vegetative growth and (b). that CDC25A is able to partially restore a functional checkpoint in response to both ionising and UV irradiation, but not a DNA replication checkpoint. We then describe a simple assay in which we demonstrate that growth of the humanised CDC25A strain is strongly repressed in a CDC25-dependent manner by BN2003, a potent chemical inhibitor of CDC25 belonging to the benzothiazoledione family. The ease of manipulation of fission yeast humanised CDC25 cells and the simplicity of the above assay offer a powerful tool with which to investigate the specificity of pharmacological inhibitors of CDC25.

The NMDA receptor is coupled to the ERK pathway by a direct interaction between NR2B and RasGRF1.

The NMDA subtype of glutamate receptors (NMDAR) at excitatory neuronal synapses plays a key role in synaptic plasticity. The extracellular signal-regulated kinase (ERK1,2 or ERK) pathway is an essential component of NMDAR signal transduction controlling the neuroplasticity underlying memory processes, neuronal development, and refinement of synaptic connections. Here we show that NR2B, but not NR2A or NR1 subunits of the NMDAR, interacts in vivo and in vitro with RasGRF1, a Ca(2+)/calmodulin-dependent Ras-guanine-nucleotide-releasing factor. Specific disruption of this interaction in living neurons abrogates NMDAR-dependent ERK activation. Thus, RasGRF1 serves as NMDAR-dependent regulator of the ERK kinase pathway. The specific association of RasGRF1 with the NR2B subunit and study of ERK activation in neurons with varied content of NR2B suggests that NR2B-containing channels are the dominant activators of the NMDA-dependent ERK pathway.

PKN delays mitotic timing by inhibition of Cdc25C: possible involvement of PKN in the regulation of cell division.

The role of PKN, a fatty acid- and Rho small GTPase-activated protein kinase, in cell-cycle regulation was analyzed. Microinjection of the active form of PKN into a Xenopus embryo caused cleavage arrest, whereas normal cell division proceeded in the control embryo microinjected with buffer or the inactive form of PKN. Exogenous addition of the active form of PKN delayed mitotic timing in Xenopus egg cycling extracts judging by morphology of sperm nuclei and Cdc2/cyclin B histone H1 kinase activity. The kinase-negative form of PKN did not affect the timing, suggesting that delayed mitotic timing depends on the kinase activity of PKN. The dephosphorylation of Tyr-15 of Cdc2 was also delayed in correlation with Cdc2/cyclin B histone H1 kinase activation in extracts containing active PKN. The Cdc25C activity for the dephosphorylation of Tyr-15 in Cdc2 was suppressed by pretreatment with the active form of PKN. Furthermore, PKN efficiently phosphorylated Cdc25C in vitro, indicating that PKN directly inhibits Cdc25C activity by phosphorylation. These results suggest that PKN plays a significant role in the control of mitotic timing by inhibition of Cdc25C.