Direct visualization of the movement of the monomeric axonal transport motor UNC-104 along neuronal processes in living Caenorhabditis elegans

The formation and function of axons depends on the microtubule-based transp ort of cellular components from their sites of synthesis in the neuronal ce ll body to their sites of utilization at the axon terminus. To directly vis ualize this axonal transport in a living organism, we constructed transgeni c lines of Caenorhabditis elegans that express green fluorescent protein fu sed to the monomeric synaptic vesicle transport motor, UNC-104. This UNC-10 4::GFP construct rescued the Unc-104 mutant phenotype and was expressed thr oughout the nervous system. Using time-lapse confocal fluorescence microsco py, we were able to visualize fluorescent motor proteins moving in both dir ections along neuronal processes, some of which were identified definitely as axons and others as dendrites. Using kymograph analysis, we followed the movement of >900 particles. Most of them moved in one direction, but not n ecessarily at the same velocity. Ten percent of the observed particles reve rsed direction of movement during the period of observation, and 10% exhibi ted periods of movement interspersed with pauses. During episodes of persis tent movement, particles moved at an average velocity of 1.02 mum/sec, whic h is close to the in vitro velocity of microtubule gliding driven by purifi ed monomeric kinesin at high motor density. To our knowledge, this is the f irst direct visualization and analysis of the movement of specifically labe led microtubule motor proteins along axons in vivo.