How instructive cues present on the cell surface have their precise effects on the actin cytoskeleton is poorly understood. Semaphorins are one of the largest families of these instructive cues and are widely studied for their effects on cell movement, navigation, angiogenesis, immunology and cancer1. Semaphorins/collapsins were characterized in part on the basis of their ability to drastically alter actin cytoskeletal dynamics in neuronal processes2, but despite considerable progress in the identification of semaphorin receptors and their signalling pathways3, the molecules linking them to the precise control of cytoskeletal elements remain unknown. Recently, highly unusual proteins of the Mical family of enzymes have been found to associate with the cytoplasmic portion of plexins, which are large cell-surface semaphorin receptors, and to mediate axon guidance, synaptogenesis, dendritic pruning and other cell morphological changes4–7. Mical enzymes perform reduction–oxidation (redox) enzymatic reactions4,5,8–10 and also contain domains found in proteins that regulate cell morphology4,11. However, nothing is known of the role of Mical or its redox activity in mediating morphological changes. Here we report that Mical directly links semaphorins and their plexin receptors to the precise control of actin filament (F-actin) dynamics. We found that Mical is both necessary and sufficient for semaphorin–plexin-mediated F-actin reorganization in vivo. Likewise, we purified Mical protein and found that it directly binds F-actin and disassembles both individual and bundled actin filaments. We also found that Mical utilizes its redox activity to alter F-actin dynamics in vivo and in vitro, indicating a previously unknown role for specific redox signalling events in actin cytoskeletal regulation. Mical therefore is a novel F-actin-disassembly factor that provides a molecular conduit through which actin reorganization—a hallmark of cell morphological changes including axon navigation—can be precisely achieved spatiotemporally in response to semaphorins.

Members of the Cas family of Src homology 3 (SH3)-domain-containing cytosolic signaling proteins are crucial regulators of actin cytoskeletal dynamics in non-neuronal cells; however, their neuronal functions are poorly understood. Here, we identify a Drosophila Cas (DCas), find that Cas proteins are highly expressed in neurons and show that DCas is required for correct axon guidance during development. Functional analyses reveal that Cas specifies axon guidance by regulating the degree of fasciculation among axons. These guidance defects are similar to those observed in integrin mutants, and genetic analysis shows that integrins function together with Cas to facilitate axonal defasciculation. These results strongly support Cas proteins working together with integrins in vivo to direct axon guidance events.

Many of the transcription factors and target genes that pattern the developing adult remain unknown. In the present study, we find that an ortholog of the poorly understood transcription factor, glucose transporter (GLUT4) enhancer factor (Glut4EF, GEF) ½also known as the Huntington's disease gene regulatory region-binding protein (HDBP) 1, plays a critical role in specifying normal wing positioning in adult Drosophila. Glut4EF proteins are zinc-finger transcription factors named for their ability to regulate expression of GLUT4 but nothing is known of Glut4EF's in vivo physiological functions. Here, we identify a family of Glut4EF proteins that are well conserved from Drosophila to humans and find that mutations in Drosophila Glut4EF underlie the wing-positioning defects seen in stretch mutants. In addition, our results indicate that previously uncharacterized mutations in Glut4EF are present in at least 11 publicly available fly lines and on the widely used TM3 balancer chromosome. These results indicate that previous observations utilizing these common stocks may be complicated by the presence of Glut4EF mutations. For example, our results indicate that Glut4EF mutations are also present on the same chromosome as two gain-of-function mutations of the homeobox transcription factor Antennapedia (Antp) and underlie defects previously attributed to Antp. In fact, our results support a role for Glut4EF in the modulation of morphogenetic processes mediated by Antp, further highlighting the importance of Glut4EF transcription factors in patterning and morphogenesis.