UC Santa Cruz

Microtubule motor proteins are known to drive long distance organelle transport in neurons, but there are many motor species and many organelle types, so specific transport mechanisms remain largely undefined. To gain insight into the transport mechanism for dense core vesicles (DCV) that carry neuropeptides, we studied three motors in axons. Our prior work showed that inhibition of Unc-104 (kinesin-3) greatly reduced anterograde and retrograde DCV flux in motor axons, and caused defects in both anterograde and retrograde run parameters (duty cycles, run velocities, and run lengths). Here, using time-lapse imaging of whole, live Drosophila larvae, we report that inhibition of Khc (kinesin-1), well known as a mitochondrial motor, reduces DCV flux in both directions, but inhibits just anterograde run parameters. Specific inhibition of Khc-driven mitochondrial transport by Milton RNAi had little influence on DCV transport, and sedimentation tests showed that both kinesin-1 and -3 co-fractionate with DCVs. These findings suggest that both kinesin-1 and -3 contribute directly to DCV transport, that their functions on anterograde DCVs are interdependent, and that their influences on the retrograde DCV motor are distinct. Additional tests identified cytoplasmic dynein as a third motor and showed that it is needed for anterograde as well as retrograde transport. Overall, our data suggest a mechanism in which both kinesins have interdependent but distinct roles in DCV distribution -- kinesin-3 has a major role in moving neuropeptide DCVs from the cell body into axons. Then kinesin-1 and -3 work together in a dynein-dependent manner to drive highly processive DCV transport toward the axon terminal. Dynein, activated by one or both kinesins, carries excess DCVs that have not been secreted back to the cell body.