The
unc-119 mutant has been one of the favorite injection strains for C. elegans researchers to generate transgenic animals; the mutant severely uncoordinated which can easily be rescued. Although
unc-119 is specifically expressed in neurons and its depletion leads to neuronal morphology defects, the molecular function of UNC-119 in neuron development is completely unknown. Whereas axons and dendrites are characterized by a difference in microtubule polarity enabling selective transport into each neurite, we found that depletion of
unc-119 leads to loss of axon-dendrite microtubule polarity. To study the role of UNC-119 we generated a GFP knock-in line and found that the protein is diffusely localized to axons and dendrites, where it is surprisingly immobile when analyzed by FRAP. Immunoprecipitation experiments performed with the knock-in strain and HEK cells show that UNC-119 binds to both the microtubule binding protein UNC-33 (CRMP) as well as the cortical actin-spectrin organizer UNC-44 (Ankyrin), which were previously found to be important for neuronal microtubule organization (Maniar et al., 2011). This suggests a role for UNC-119 in bridging UNC-33 (CRMP) to UNC-44 (Ankyrin) to form a complex that could anchor microtubules to the cell cortex. Indeed, when analyzing microtubule dynamics using photoactivatable-GFP::TBA-1 (tubulin) the microtubules of wildtype animals are remarkably immobile, whereas in mutants for the above described genes we observed dramatic sliding of microtubules throughout the neuron. In agreement with our hypothesis for UNC-119, we were able to rescue axon-dendrite microtubule organization and the neuronal development defects of the
unc-119 mutant by artificially attaching UNC-33 (CRMP) to the cell cortex. Dendritic microtubule organization has also been shown to depend on the microtubule motor UNC-116 (kinesin-1) (Yan et al., 2013), which suggests that the motor transports microtubules into the correct orientation. The depletion of
unc-116 (kinesin-1) did not lead to microtubule sliding. However, depletion of
unc-116 was able to suppress the microtubule sliding observed in the
unc-33 mutant. This shows that in the absence of the cortical microtubule anchor complex, UNC-116 (kinesin-1) drives excessive microtubule sliding leading to microtubule organization defects. We propose that the proper development of axons and dendrites relies on a fine balance between microtubule transport and cortical microtubule stabilization.