Neuronal corridors

10.1083/jcb.1733r r 1jcb.1733r r1Nicole Le Brasseurlebrasn@roc kefeller. eduMigrating interphase DNA C hien-Hui Chuang (University of Illinois, Urbana, IL) sees chromosomes moving long distances—and not during mitosis. Chuang, Andrew Belmont, and colleagues, now unveil the inducible long-range migration of an interphase chromosome site. An hour or two after mitosis, chromosomes appear to be locked down, as only short-range, tethered diffusion-like movements had been seen during interphase. Yet several groups had noted that certain genes localize at the nuclear periphery when repressed but in the interior when active. Chuang and colleagues visualized the movements accompanying this repositioning. They used an engineered locus that they could turn on through inducible targeting of a transcriptional activator. Upon transcription factor targeting, the group observed rapid, directed movements of the locus from the nuclear envelope toward the interior. The linear paths taken, often approximately perpendicular to the envelope, discount simple diffusion. During this motion, the locus moved at speeds approaching anaphase chromosome separation. Although the existence of a nuclear actin network is still controversial, the group found a dependence of this long-range motion on actin and myosin. If the DNA is indeed transported on actin fi laments, the preferential inward movements suggest that the intranuclear actin network is polarized. The authors are now testing whether the locus returns as directly to the periphery when the transcriptional activator is removed. Reference: Chuang, C.-H., et al. 2006. Curr. Biol. 16:825–831. 10.1083/jcb.1733rr3jcb.1733rr3Nicole [email protected] euronal corridors N eurons migrating from afar do more than create diversity in their new neighborhood. They also build bridges for synaptic connections that would otherwise be impossible, according to new fi ndings from Guillermina López-Bendito, Aline Cautinat, Oscar Marín (Universidad Miguel Hernández, Sant Joan d’Alacant, Spain), Sonia Garel (École Normale Supérieure, Paris, France), and colleagues. Most brain neurons migrate in a radial pattern during development, moving almost directly outward from their origin. But a few classes of neurons migrate in more complex patterns. Interneurons of the cortex, for example, are generated by ventral progenitors that then migrate dorsally, thus introducing a different neuronal subtype in the region. A novel set of such “tangentially” migrating cells, which the authors call corridor cells, are now shown to express a membrane-bound form of neuregulin-1. In combination with soluble neuregulin-1 isoforms produced in the cortex, these molecules allowed the growth of thalamic axons toward the cortex. In the absence of corridor cells, their section of the brain was nonpermissive for thalamic axons, which thus failed to extend toward the cortex. The creation of these mammal-specifi c axonal connections was probably an evolutionary breakthrough. They allow sensory and motor information to be relayed to our center of conscious information processing. This big breakthrough apparently required small changes. “The guidance cues can be generated,” says Garel, “by putting just a few new cells there that will be permissive in a restrictive environment.”