Neuronal migrations along glial fibers provide a primary pathway for the formation of cortical laminae. To examine the mechanisms underlying glial-guided migration, we analyzed the dynamics of cytoskeletal and signaling components in living neurons. Migration involves the coordinated two-stroke movement of a perinuclear tubulin 'cage' and the centrosome, with the centrosome moving forward before nuclear translocation. Overexpression of mPar6alpha disrupts the perinuclear tubulin cage, retargets PKCzeta and gamma-tubulin away from the centrosome, and inhibits centrosomal motion and neuronal migration. Thus, we propose that during neuronal migration the centrosome acts to coordinate cytoskeletal dynamics in response to mPar6alpha-mediated signaling.

During cell division metaphase spindles maintain constant length, whereas spindle microtubules continuously flux polewards, requiring addition of tubulin subunits at microtubule plus-ends, polewards translocation of the microtubule lattice, and removal of tubulin subunits from microtubule minus-ends near spindle poles. How these processes are coordinated is unknown. Here, we show that dynein/dynactin, a multi-subunit microtubule minus-end–directed motor complex, and NuMA, a microtubule cross-linker, regulate spindle length. Fluorescent speckle microscopy reveals that dynactin or NuMA inhibition suppresses microtubule disassembly at spindle poles without affecting polewards microtubule sliding. The observed uncoupling of these two components of flux indicates that microtubule depolymerization is not required for the microtubule transport associated with polewards flux. Inhibition of Kif2a, a KinI kinesin known to depolymerize microtubules in vitro, results in increased spindle microtubule length. We find that dynein/dynactin contribute to the targeting of Kif2a to spindle poles, suggesting a model in which dynein/dynactin regulate spindle length and coordinate flux by maintaining microtubule depolymerizing activities at spindle poles.

Metaphase spindles assemble to a steady state in length by mechanisms that involve microtubule dynamics and motor proteins, but they are incompletely understood. We found that Xenopus extract spindles recapitulate the length of egg meiosis II spindles, by using mechanisms intrinsic to the spindle. To probe these mechanisms, we perturbed microtubule polymerization dynamics and opposed motor proteins and measured effects on spindle morphology and dynamics. Microtubules were stabilized by hexylene glycol and inhibition of the catastrophe factor mitotic centromere-associated kinesin (MCAK) (a kinesin 13, previously called XKCM) and destabilized by depolymerizing drugs. The opposed motors Eg5 and dynein were inhibited separately and together. Our results are consistent with important roles for polymerization dynamics in regulating spindle length, and for opposed motors in regulating the relative stability of bipolar versus monopolar organization. The response to microtubule destabilization suggests that an unidentified tensile element acts in parallel with these conventional factors, generating spindle shortening force.

Structure of the meiotic spindle T he meiotic spindle is made up of shorter microtubules than previously believed, suggest results (Rockefeller University, New York, NY), and colleagues. Current models of the spindle, as a bipolar array of overlapping fi laments extending from opposite spindle poles, will require revision. To get a closer look at the architecture of the meiotic spindle, Yang et al. incorporated labeled tubulin subunits into the spindle in a cell-free system. By refi ning their fl uorescent speckle microscopy techniques, the authors were able for the fi rst time to track individual tubulin subunits (seen as speckles) in a single tubulin polymer. The authors identifi ed pairs of speckles representing subunits on the same fi lament. Speckle separation supplied them with the minimum length of that fi lament. They then fi tted a mathematical model to these observed lengths to predict overall fi lament lengths: most fi laments were only 40% of the total spindle length. The short fi laments were also scattered throughout the spindle. The researchers now propose that the spindle is a tiled array of overlapping short fi laments. The group next examined how spindle-associated proteins might control fi lament and spindle size. By inhibiting microtubule motor proteins, they found that dynein-dynactin limited individual fi ber lengths and thus overall spindle length. Kinesin 5 activity limited the overlap between fi bers by sliding them apart. "Our work suggests the spindle is a self-organizing system, whose stability and functional characteristics are built on these kind of local interactions," says Kapoor. Localized mRNA is the norm L ocation, location, location. It's critical for real estate, proteins, and-according to work by Eric Lécuyer, Henry Krause, and colleagues (University of Toronto, Canada)-mRNAs, too. Several localized mRNAs have been previously studied, but just how many transcripts are localized in the cell, and in what patterns, is unknown. Lécuyer et al. approached this problem by optimizing fl uorescence in situ hybridization (FISH) in a global analysis of developmentally expressed mRNAs. They found that 71% of the mRNAs in early fl y embryos showed specifi c patterns of subcellular localization. In several cases, they found new examples of mRNAs that colocalized with their protein products. Less energy is probably required to transport a few copies of an mRNA than to move around many more copies of the protein. And the proteins will be created where they are needed and possibly prevented from straying where they are not wanted. "We need to revise the textbook image of proteins being made in a centralized location near the nucleus, then traffi cking to their ultimate locations," says Krause. "Our work shows that the mRNAs are an intelligent actor, not just a dumb vehicle for creating proteins." With their new database, the group can now further investigate how and why mRNAs are localized.