Axon-guidance defects in a Drosophila MICAL loss-of-function (LOF) mutant. Courtesy of Alex Kolodkin, Johns Hopkins School of Medicine, Baltimore, USA.

The semaphorins are important axon-guidance molecules in invertebrates and vertebrates, and they can cause attraction or repulsion. The repulsive response is mediated by the plexin family of receptors, but little is known about how the plexins signal to the cytoskeleton to induce growth cone collapse. Now, a new study implicates a family of oxidoreductases, the MICALs, as a missing link in this pathway.

The identification of the vertebrate MICAL-1 (molecule interacting with CasL) was reported earlier this year, and now Terman et al. describe the isolation of a Drosophila counterpart, and of vertebrate MICAL-2 and -3. MICALs are large cytosolic proteins that consist of several domains. In the Drosophila embryo, MICAL shows a similar expression pattern to Plexin A (PlexA) on central nervous system and motor axons, and the authors showed that it interacts physically with PlexA. MICAL also contains an actin-binding domain and an intermediate-filament-binding region, so it could provide a direct link between the plexins and the cytoskeleton. Vertebrate MICAL-1 interacts with CasL, and as Cas-family proteins interact with signalling proteins that influence neuronal morphology, MICAL could link plexins to cytoskeletal regulators through Cas proteins.

Terman et al. looked for evidence that MICAL acts in the same pathway as Semaphorin 1a (Sema-1a) and PlexA. They found that MICAL loss-of-function caused the same axon-guidance and defasciculation defects as were seen in PlexA and Sema-1a mutants, and overexpression of MICAL had the same effect as overexpression of PlexA. Further evidence for a common pathway came from embryos that were doubly heterozygous for mutations in MICAL and PlexA or Sema-1a, which showed similar axon-guidance defects to Sema-1a;PlexA double heterozygotes.

Which domains in MICAL are important for its function in semaphorin-mediated repulsion? The amino terminus is highly conserved between species, and this region contains a flavoprotein monooxygenase domain, which catalyses an oxidation–reduction (redox) reaction that results in the insertion of an oxygen atom into a substrate. Alterating key amino-acid residues within this domain that are conserved within this enzyme class and are required for their activity compromises MICAL function in motor axons in vivo. In an in vitro rat growth cone assay, Terman et al. showed that semaphorin-mediated axonal repulsion can be inhibited by selective flavoprotein monooxygenase inhibitors.

So, these findings not only tell us more about the signalling pathway that underlies semaphorin-mediated repulsion, but in combination with recent data that implicate 12/15-lipoxygenase in semaphorin signalling, they also provide support for the involvement of redox reactions in this pathway. Oxidation of actin is known to promote its depolymerization, so it will be interesting to see whether redox reactions are a common feature of repulsive neuronal guidance.