OxyR: a molecular code for redox-related signaling

Semaphorins: green light for redox signaling?

How semaphorins stimulate plexins to regulate axon steering is an actively investigated question. Ventura and Pelicci discuss new evidence indicating that MICAL, a putative monoxygenase, transmits the signal from the receptor plexin to the actin cytoskeleton through a redox mechanism. MICAL could act either indirectly, causing a local increase in the concentration of reactive oxygen species (ROS), or directly, inducing redox changes in downstream effectors, such as actin and members of the Rho and Rac family of guanosine triphosphatases. Because semaphorin-plexin signaling plays a role in a number of clinically relevant settings, including recovery from spinal cord trauma, regulation of the immune system, and cancer metastasis, these findings could have important therapeutic implications.

Nanotransducers in cellular redox signaling: modification of thiols by reactive oxygen and nitrogen species

The control of signal transduction involves post-translational modification of proteins at key amino acids. Cysteine residues are important in the control of 'redox' cell-signaling pathways, as thiol chemistry offers the possibility of modification by structurally diverse species, including those derived from oxidized lipids, peroxides or nitric oxide. An important and provocative study of the modification of thiols in the transcription factor OxyR recently extended this hypothesis. The findings offer the enticing possibility that the cell can distinguish between different degrees of oxidant and nitrosative exposure by modification at a single site on a signaling molecule.

Nitrosative stress-induced apoptosis through inhibition of NF-kappa B

Nitrosative stress produced by cytokines predisposes to apoptotic cell death. However, the molecular mechanism by which this occurs is not well understood. We have shown previously that nitric oxide (NO) regulates the activity of the anti-apoptotic transcription factor NF-kappaB. Here we demonstrate that the inhibition of NF-kappaB by NO sensitizes A549 and Jurkat T cells to tumor necrosis factor-alpha (TNFalpha)-induced apoptosis. The molecular basis of NF-kappaB inhibition is different in the two cell types. In A549 cells, NO functions at the nuclear level to inhibit NF-kappaB by S-nitrosylation. In Jurkat cells, NO inhibits the NF-kappaB activating pathway in the cytoplasm at a step proximal to the degradation of IkappaBalpha. The inhibition of NF-kappaB is reflected in the level of intracellular S-nitrosothiols, which are constitutively metabolized. These data suggest that NO can influence cell death by modulating NF-kappaB activity with the sites of inhibition being cell type-specific. The data also show that NO bioactivity regulates tumor necrosis factor-alpha signaling.

MICALs, a family of conserved flavoprotein oxidoreductases, function in plexin-mediated axonal repulsion

Members of the semaphorin family of secreted and transmembrane proteins utilize plexins as neuronal receptors to signal repulsive axon guidance. It remains unknown how plexin proteins are directly linked to the regulation of cytoskeletal dynamics. Here, we show that Drosophila MICAL, a large, multidomain, cytosolic protein expressed in axons, interacts with the neuronal plexin A (PlexA) receptor and is required for Semaphorin 1a (Sema-1a)-PlexA-mediated repulsive axon guidance. In addition to containing several domains known to interact with cytoskeletal components, MICAL has a flavoprotein monooxygenase domain, the integrity of which is required for Sema-1a-PlexA repulsive axon guidance. Vertebrate orthologs of Drosophila MICAL are neuronally expressed and also interact with vertebrate plexins, and monooxygenase inhibitors abrogate semaphorin-mediated axonal repulsion. These results suggest a novel role for oxidoreductases in repulsive neuronal guidance.