In the intricate world of cultured vertebrate neurons, a fascinating contrast exists in the arrangement of microtubules within axons and dendrites. Axons are characterized by a uniform configuration of microtubules, specifically with their plus-ends oriented distally, away from the cell body – a “plus-end-out” arrangement. However, dendrites present a different picture, showcasing a mixed polarity of microtubules, incorporating both plus-end-out and minus-end-out orientations.
This understanding contrasts with observations in Drosophila and Caenorhabditis elegans neurons, where dendrites are marked by uniparallel minus-end-out microtubules, rather than a non-uniform microtubule organization. To investigate whether this mixed microtubule organization is a conserved feature in vertebrate dendrites, researchers employed live-cell imaging techniques. This systematic analysis focused on microtubule plus-end orientations in various neuronal settings: primary cultures of rat hippocampal and cortical neurons, dentate granule cells within mouse organotypic slices, and layer 2/3 pyramidal neurons in the somatosensory cortex of living mice.
The findings consistently revealed that in both in vitro and in vivo conditions, axons displayed a uniform plus-end-out orientation of microtubules. In stark contrast, dendrites exhibited microtubules with mixed orientations. Further experiments involving laser-based microsurgery to sever dendritic microtubules confirmed this mixed organization by revealing equal numbers of plus- and minus-end-out microtubule orientations throughout the dendritic processes.
Interestingly, the study noted that minus-end-out microtubules in dendrites were generally more stable, exhibiting a stability comparable to the plus-end-out microtubules found in axons. Delving into the developmental aspect, early-stage, nonpolarized neurons displayed a noteworthy characteristic: newly formed neurites already contained microtubules of opposite polarity. This observation suggests that the establishment of the uniform plus-end-out microtubule arrangement is a process specifically associated with axon formation.
Based on these comprehensive findings, the researchers proposed a model highlighting the selective formation of uniform plus-end-out microtubules in axons as a critical mechanism underlying neuronal polarization. This research underscores the fundamental difference in microtubule organization between axons and dendrites in vertebrate neurons, emphasizing the “Opposite To Uniform” nature of dendritic microtubule arrangement and its potential significance in neuronal function and development.
Significance statement: Live-cell imaging analysis across rat hippocampal neurons, mouse dentate granule cells, and layer 2/3 pyramidal neurons in living mice revealed a key difference in microtubule organization. Axons exhibit uniform plus-end-out microtubules, while dendrites display mixed orientations – “opposite to uniform” – both in vitro and in vivo. Early neurites in nonpolarized neurons already show mixed microtubules, suggesting uniform plus-end-out organization is specific to axon formation. This leads to a model where selective formation of uniform axonal microtubules is crucial for neuronal polarization.
