Molecular motors implicated in the axonal transport of tau and alpha-synuclein.

Tau and alpha-synuclein are both proteins implicated in the pathology of neurodegenerative disease. Here we have investigated the mechanisms of axonal transport of tau and alpha-synuclein, because failure of axonal transport has been implicated in the development of several neurodegenerative disorders. We found that the transport of both of these proteins depend on an intact microtubule- but not actin-cytoskeleton, and that tau and alpha-synuclein both move at overall slow rates of transport. We used time-lapse video microscopy to obtain images of live neurons that had been transfected with plasmids expressing proteins tagged with enhanced green fluorescent protein. We found that particulate structures containing tau or alpha-synuclein travel rapidly when moving along axons but spend the majority of the time paused, and these structures have similar characteristics to those previously observed for neurofilaments. The motile particles containing tau or alpha-synuclein colocalise with the fast-transporting molecular motor kinesin-1 in neurons. Co-immunoprecipitation experiments demonstrate that tau and alpha-synuclein are each associated with complexes containing kinesin-1, whereas only alpha-synuclein appears to interact with dynein-containing complexes. In vitro glutathione S-transferase-binding assays using rat brain homogenate or recombinant protein as bait reveals a direct interaction of kinesin-1 light chains 1 and 2 with tau, but not with alpha-synuclein. Our findings suggest that the axonal transport of tau occurs via a mechanism utilising fast transport motors, including the kinesin family of proteins, and that alpha-synuclein transport in neurons may involve both kinesin and dynein motor proteins.

Parkinson's disease alpha-synuclein mutations exhibit defective axonal transport in cultured neurons.

Alpha-synuclein is a major protein constituent of Lewy bodies and mutations in alpha-synuclein cause familial autosomal dominant Parkinson's disease. One explanation for the formation of perikaryal and neuritic aggregates of alpha-synuclein, which is a presynaptic protein, is that the mutations disrupt alpha-synuclein transport and lead to its proximal accumulation. We found that mutant forms of alpha-synuclein, either associated with Parkinson's disease (A30P or A53T) or mimicking defined serine, but not tyrosine, phosphorylation states exhibit reduced axonal transport following transfection into cultured neurons. Furthermore, transfection of A30P, but not wild-type, alpha-synuclein results in accumulation of the protein proximal to the cell body. We propose that the reduced axonal transport exhibited by the Parkinson's disease-associated alpha-synuclein mutants examined in this study might contribute to perikaryal accumulation of alpha-synuclein and hence Lewy body formation and neuritic abnormalities in diseased brain.

The slow axonal transport of the microtubule-associated protein tau and the transport rates of different isoforms and mutants in cultured neurons.

We demonstrate that the microtubule-associated protein tau, in the form of enhanced green fluorescent protein (EGFP) tau, is transported along axons of neurons in culture in the slow component of axonal transport with a speed comparable with that previously measured in vivo. It was demonstrated that the EGFP tag has no effect on transport characteristics, and the methodology enables slow transport rates of individual tau isoforms and tau mutants to be measured. We also expressed EGFP-tagged tau isoforms containing either three or four C-terminal repeats and zero or two N-terminal inserts in cultured neurons. No significant differences were found in the average rate of slow transport of the wild-type tau isoforms, suggesting that the exon 10 C-terminal repeat or the N-terminal inserts do not contain regions that play a significant regulatory role in axonal transport. Similarly, we found that missense mutations in tau have no noticeable effect on the rate of transport; hence their ability to cause neurodegeneration is by another mechanism other than that affecting the overall slow axonal transport of tau.