Axonal and Dendritic Transport

Introduction of Signal Transmission in Cells

In recent years, cell biology research claimed that static cellular states are not suitable for complex patterns of cellular morphology. In contrast, there are multiple factors that mediate signal transmission in cells. In general, the motor proteins are major machines to convert chemical energy to physical force in cells. The molecular motors can be classified as myosins, dyneins, and kinesins. Coding by a large molecular gene family, the kinesin superfamily proteins (KIFs) are important long-distance transporters to the periphery in neurons.

Transport of Axons and Dendrites

In axons, KIF5 (the member of the kinesin-1 family) and KIF1A/KIF1B (member of the kinesin-3 family) drive forward transport from the cell body to the distal end of the axon, while dynein drives the reverse transport towards the cell body.

In the dendritic shaft, the same motor protein can drive bidirectional transport as the microtubules are arranged in mixed polarities. At the distal end of the dendrites, the positive ends of most microtubules face outwards, and the cargo is transported from the cell body to the dendrites mainly by KIF17 (member of kinesin-2 family) and KIF16B (member of kinesin-3 family). At the end of the dendrites, dynein is responsible for the reverse transport of cargo towards the cell body.

Fig.1 Molecular motors responsible for axonal and dendritic transport, arrows indicate the directionality of motor-driven movement.

It is worth noting that in axons and dendrites, microtubules are not only the orbits for molecular motors to drive long-distance transport, the post-translational modification of tubulin also plays an important role in selective transport of cargo to axon and dendrites by molecular motors. Recent studies have found that most acetylated microtubules in dendrites point to the cell body with the positive end. Therefore, kinesin-1, which recognizes acetylated microtubules, returns to the cell body along the microtubules rather than dendritic-driven transport. At the same time, the positive end of the tyrosinated microtubules in the dendrites points to the distal end of the dendrites, enabling the kinesin-2 family members to drive the transport from the cell body to the dendrites. Members of the Myosin family use microfilament fibers as tracks, which are mainly responsible for driving local and short-distance transportation of intracellular cargo. For example, Myosin Va transports the endoplasmic reticulum from the dendritic trunk to the dendritic spines. In long-term potentiation, Myosin Vb activated by calcium signals transports the circulating endosomes carrying the transmitter AMPAR from the base of the dendritic spine to the dendritic spine, facilitating its insertion.

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Reference

1. Setou, M.; et al. Axonal transport versus dendritic transport. Journal of neurobiology. 2004, 58(2): 201-206.