The semaphorins are a large family of secreted and transmembrane axon guidance molecules that are conserved across phyla. In vertebrates, most attention has focused on semaphorin-3A (SEMA-3A), which mediates the chemorepulsion of a variety of axons, including those of sympathetic, sensory and olfactory neurons. Recent studies have shown that semaphorins signal via a receptor complex that includes neuropilins and plexins; neuropilin-1 (NP-1) in particular, binds SEMA-3A and is required for its function, but it also binds other semaphorins. Therefore, a promising strategy is to mis-express dominant-negative neuropilins in developing axons, and thus to explore the effect of blocking the function of one or more semaphorins. Previous in vitro studies had already shown that a truncated neuropilin-1 (dnNP-1) can act in a dominant-negative fashion, blocking the effects of SEMA-3A and SEMA-3C on sympathetic neurons.Now, Michael Renzi et al. 1xOlfactory sensory axons expressing a dominant-negative semaphorin receptor enter the CNS early and overshoot their target. Renzi, M.J et al. Neuron. 2000; 28: 437–447Abstract | Full Text | Full Text PDF | PubMedSee all References1 from Jonathan Raper's laboratory have used this dominant-negative approach to investigate the projection of olfactory sensory axon projections in the chick embryo. Olfactory projections extend towards the telencephalon, but pause for several days before they contact their target, the olfactory bulb. SEMA-3A expression was observed from embryonic days (E) 5–7 when axons first reach the telencephalon and undergo their waiting period, but expression declined at E9, coinciding with the timing of axonal invasion of the bulb. Because olfactory axons express NP-1 and are sensitive to SEMA-3A in vitro, this data implies that SEMA-3A mediates repulsion to control the duration of this olfactory ‘waiting period’.Renzi et al. electroporated constructs encoding dnNP-1, full-length neuropilin-1 (NP-1), and a control construct, into the olfactory placode of chick embryos in ovo, leading to expression of the construct by olfactory axons. In each case a construct encoding a tag was co-transfected in order to visualize transfected cells. Whereas axons that express the control or NP-1 molecules observed the normal waiting period within the telencephalon, a proportion of axons expressing dnNP-1 overshot their normal stopping point and projected onto the developing olfactory bulb and along the medial forebrain. Aberrant trajectories into the forebrain were also detected at later stages of development, during which dnNP-1-expressing axons were detected on the surface of the olfactory bulb, with some trajectories overshooting the border between the bulb and the forebrain to invade this latter territory.These data provide a nice example of the precise action of repellent axon guidance cues; the authors also speculate that the olfactory–axon waiting period might represent a sort of paralysis or growth-cone collapse, paralleling the in vitro phenomenon that can be induced by addition of semaphorins to responsive axons. This study also adds to the growing body of data that uses electroporation in the chick embryo to mis-express genes of interest, providing an alternative or complementary approach to transgenic mouse studies and to retrovirus-mediated gene transfer.