Mutation of Kinesin-6 Kif20b causes defects in cortical neuron polarization and morphogenesis.

Background: How neurons change their cytoskeleton to adopt their complex polarized morphology is still not understood. Growing evidence suggests that proteins that help build microtubule structures during cell division are also involved in building and remodeling the complex cytoskeletons of neurons. Kif20b (previously called MPP1 or Mphosph1) is the most divergent member of the Kinesin-6 family of "mitotic" kinesins that also includes Kif23/MKLP1 and Kif20a/MKLP2. We previously isolated a loss-of-function mouse mutant of Kif20b and showed that it had a thalamocortical axon guidance defect and microcephaly.

Methods: We demonstrate here, using the mouse mutant, that Kif20b is required for neuron morphogenesis in the embryonic neocortex. In vivo and in vitro cortical neurons were labeled and imaged to analyze various aspects of morphogenesis.

Results: Loss of Kif20b disrupts polarization as well as neurite outgrowth, branching and caliber. In vivo, mutant cortical neurons show defects in orientation, and have shorter thinner apical dendrites that branch closer to the cell body. In vitro, without external polarity cues, Kif20b mutant neurons show a strong polarization defect. This may be due in part to loss of the polarity protein Shootin1 from the axonal growth cone. Those mutant neurons that do succeed in polarizing have shorter axons with more branches, and longer minor neurites. These changes in shape are not due to alterations in cell fate or neuron layer type. Surprisingly, both axons and minor neurites of mutant neurons have increased widths and longer growth cone filopodia, which correlate with abnormal microtubule organization. Live analysis of axon extension shows that Kif20b mutant axons display more variable growth with increased retraction.

Conclusions: These results demonstrate that Kif20b is required cell-autonomously for proper morphogenesis of cortical pyramidal neurons. Kif20b regulates neuron polarization, and axon and dendrite branching, outgrowth, and caliber. Kif20b protein may act by bundling microtubules into tight arrays and by localizing effectors such as Shootin1. Thus it may help shape neurites, sustain consistent axon growth, and inhibit branching. This work advances our understanding of how neurons regulate their cytoskeleton to build their elaborate shapes. Finally, it suggests that neuronal connectivity defects may be present in some types of microcephaly.