Neurite branching is associated with mixed microtubule polarity in sea anemone neurons

Bilaterian animals can make polarized neurons with functionally distinct dendrites and axons. A central aspect of this polarity is different arrangements of microtubules; axons have plus-end-out microtubules, whereas dendrites contain minus-end-out microtubules, allowing different sets of proteins and organelles to be trafficked to each. In cnidarians, neurons with multiple plus-end-out axon-like neurites have been described. To determine whether neurons with axo-dendritic polarity might exist in cnidarians, we surveyed neurons in the model sea anemone Nematostella vectensis. Microtubule polarity was assessed in mosaic animals expressing EB1-GFP, which binds to growing microtubule plus ends. Neurons were separated into general groups based on morphology. Neurons without any branching had predominantly plus-end-out microtubule polarity. Neurons with at least one neurite branch had significantly more minus-end-out microtubules, and neurons with more than one branch had over 15% minus-end-out microtubules. To identify a population of neurons enriched for branching, we performed a promoter screen. We found that the Shal1 promoter labeled cnidocytes and neurons with branched neurites. In these cells, approximately 30% of microtubules were minus-end-out, which is in the range described for vertebrate dendrites. Finally, we re-examined neurons broadly to identify cells that had both branched and unbranched neurites. When these cells had neurites with different polarities, it was typically the branched one that had mixed microtubules. Thus, in N. vectensis, neurite branching is associated with more mixed microtubule polarity, and our results also suggest that classically polarized neurons may exist in cnidarian animals.