Effects of dynactin disruption and dynein depletion on axonal microtubules

We investigated potential roles of cytoplasmic dynein in organizing axonal microtubules either by depleting dynein heavy chain from cultured neurons or by experimentally disrupting dynactin. The former was accomplished by siRNA while the latter was accomplished by overexpressing P50-dynamitin. Both methods resulted in a persistent reduction in the frequency of transport of short microtubules. To determine if the long microtubules in the axon also undergo dynein-dependent transport, we ascertained the rates of EGFP-EB3 "comets" observed at the tips of microtubules during assembly. The rates of the comets, in theory, should reflect a combination of the assembly rate and any potential transport of the microtubule. Comets were initially slowed during P50-dynamitin overexpression, but this effect did not persist beyond the first day and was never observed in dynein-depleted axons. In fact, the rates of the comets were slightly faster in dynein-depleted axons. We conclude that the transient effect of P50-dynamitin overexpression reflects a reduction in microtubule polymerization rates. Interestingly, after prolonged dynein depletion, the long microtubules were noticeably misaligned in the distal regions of axons and failed to enter the filopodia of growth cones. These results suggest that the forces generated by cytoplasmic dynein do not transport long microtubules, but may serve to align them with one another and also permit them to invade filopodia.