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Visualization of the dynamics of synaptic vesicle and plasma membrane proteins in living axons.

Nakata T, Terada S, Hirokawa N - J. Cell Biol. (1998)

Bottom Line: Newly synthesized membrane proteins are transported by fast axonal flow to their targets such as the plasma membrane and synaptic vesicles.We found that all of these proteins are transported by tubulovesicular organelles of various sizes and shapes that circulate within axons from branch to branch and switch the direction of movement.These organelles are distinct from the endosomal compartments and constitute a new entity of membrane organelles that mediate the transport of newly synthesized proteins from the trans-Golgi network to the plasma membrane.

View Article: PubMed Central - PubMed

Affiliation: Department of Cell Biology and Anatomy, Graduate School of Medicine, University of Tokyo, Hongo, Tokyo, Japan, 113.

ABSTRACT
Newly synthesized membrane proteins are transported by fast axonal flow to their targets such as the plasma membrane and synaptic vesicles. However, their transporting vesicles have not yet been identified. We have successfully visualized the transporting vesicles of plasma membrane proteins, synaptic vesicle proteins, and the trans-Golgi network residual proteins in living axons at high resolution using laser scan microscopy of green fluorescent protein-tagged proteins after photobleaching. We found that all of these proteins are transported by tubulovesicular organelles of various sizes and shapes that circulate within axons from branch to branch and switch the direction of movement. These organelles are distinct from the endosomal compartments and constitute a new entity of membrane organelles that mediate the transport of newly synthesized proteins from the trans-Golgi network to the plasma membrane.

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Circulation model of axonal transport of plasma membrane and synaptic vesicle proteins. (1) Vesicles protrude from  the TGN in the cell body at the speed of fast axonal transport,  separate from the network, and proceed into axons. (2) In axons,  vesicles are tubular or spherical, and move at an average speed of  ∼0.8 μm/s. Net membrane flow was as high as 50 μm2/min. (3)  The vesicles break down into smaller vesicles while moving down  the axon. (4) Some vesicles switch direction and move retrogradely in axons. (5) Vesicles move from one branch to another  branch of an axon at a branching point. (6) Endosomes also move  bidirectionally. Tubulovesicular organelles circulate within axons  to deliver newly synthesized plasma membrane and synaptic vesicle proteins to the area where they are required.
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Figure 17: Circulation model of axonal transport of plasma membrane and synaptic vesicle proteins. (1) Vesicles protrude from the TGN in the cell body at the speed of fast axonal transport, separate from the network, and proceed into axons. (2) In axons, vesicles are tubular or spherical, and move at an average speed of ∼0.8 μm/s. Net membrane flow was as high as 50 μm2/min. (3) The vesicles break down into smaller vesicles while moving down the axon. (4) Some vesicles switch direction and move retrogradely in axons. (5) Vesicles move from one branch to another branch of an axon at a branching point. (6) Endosomes also move bidirectionally. Tubulovesicular organelles circulate within axons to deliver newly synthesized plasma membrane and synaptic vesicle proteins to the area where they are required.

Mentions: We detected the direct transport of vesicles from one distal branch of an axon to the other at a branching point. We also found that tubulovesicular organelles move bidirectionally in axons, and often switch the direction of movement. This shows that newly synthesized proteins are not transported unidirectionally, but are circulated within axons. These movements appear to be an inefficient way of transporting proteins, but in fact, they are an efficient and simple way to make use of the newly synthesized proteins in branching axons. If the transport of the plasma membrane precursors were rate limiting and unidirectional, when one branch of the axon grows and the others do not, selective transport of the precursors to the growing branch should occur in the axon. If this were the case, we should hypothesize that a signal to preferentially provide the growing branch with precursors would be delivered from the growing tips to the cell body or proximal axons, and according to the signal, the plasma membrane precursors should be sorted at every branching point. This would be a complex and difficult task for neurons. If transport of the plasma membrane is not rate limiting but the transport is unidirectional, it is not necessary to hypothesize the existence of a signal regulating axonal transport. However, in this case, because the transport is assumed to be nonselective at branching points, all the plasma membrane precursors that are transported into nongrowing branches are wasted. Thus, newly synthesized proteins are not efficiently targeted. In our circulation model based on this study (Fig. 17), unused tubulovesicular organelles in nongrowing branches will switch direction and be transported to the growing branch of the axon where the plasma membrane precursors are used efficiently. The growing tips of neurites do not have to transmit a signal to inform the cell body or proximal axon that they are growing, all they have to do is use the precursors that arrive at the growing tips. The cell body does not have to know the position of the neurites that are growing, it need only synthesize a sufficient amount of the precursors for the total neurite outgrowth. Thus, axonal growth can be controlled locally at the growing tip of the axon (Campenot, 1977) by the rate at which the growing tips incorporate the precursors into the plasma membrane.


Visualization of the dynamics of synaptic vesicle and plasma membrane proteins in living axons.

Nakata T, Terada S, Hirokawa N - J. Cell Biol. (1998)

Circulation model of axonal transport of plasma membrane and synaptic vesicle proteins. (1) Vesicles protrude from  the TGN in the cell body at the speed of fast axonal transport,  separate from the network, and proceed into axons. (2) In axons,  vesicles are tubular or spherical, and move at an average speed of  ∼0.8 μm/s. Net membrane flow was as high as 50 μm2/min. (3)  The vesicles break down into smaller vesicles while moving down  the axon. (4) Some vesicles switch direction and move retrogradely in axons. (5) Vesicles move from one branch to another  branch of an axon at a branching point. (6) Endosomes also move  bidirectionally. Tubulovesicular organelles circulate within axons  to deliver newly synthesized plasma membrane and synaptic vesicle proteins to the area where they are required.
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Related In: Results  -  Collection

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Figure 17: Circulation model of axonal transport of plasma membrane and synaptic vesicle proteins. (1) Vesicles protrude from the TGN in the cell body at the speed of fast axonal transport, separate from the network, and proceed into axons. (2) In axons, vesicles are tubular or spherical, and move at an average speed of ∼0.8 μm/s. Net membrane flow was as high as 50 μm2/min. (3) The vesicles break down into smaller vesicles while moving down the axon. (4) Some vesicles switch direction and move retrogradely in axons. (5) Vesicles move from one branch to another branch of an axon at a branching point. (6) Endosomes also move bidirectionally. Tubulovesicular organelles circulate within axons to deliver newly synthesized plasma membrane and synaptic vesicle proteins to the area where they are required.
Mentions: We detected the direct transport of vesicles from one distal branch of an axon to the other at a branching point. We also found that tubulovesicular organelles move bidirectionally in axons, and often switch the direction of movement. This shows that newly synthesized proteins are not transported unidirectionally, but are circulated within axons. These movements appear to be an inefficient way of transporting proteins, but in fact, they are an efficient and simple way to make use of the newly synthesized proteins in branching axons. If the transport of the plasma membrane precursors were rate limiting and unidirectional, when one branch of the axon grows and the others do not, selective transport of the precursors to the growing branch should occur in the axon. If this were the case, we should hypothesize that a signal to preferentially provide the growing branch with precursors would be delivered from the growing tips to the cell body or proximal axons, and according to the signal, the plasma membrane precursors should be sorted at every branching point. This would be a complex and difficult task for neurons. If transport of the plasma membrane is not rate limiting but the transport is unidirectional, it is not necessary to hypothesize the existence of a signal regulating axonal transport. However, in this case, because the transport is assumed to be nonselective at branching points, all the plasma membrane precursors that are transported into nongrowing branches are wasted. Thus, newly synthesized proteins are not efficiently targeted. In our circulation model based on this study (Fig. 17), unused tubulovesicular organelles in nongrowing branches will switch direction and be transported to the growing branch of the axon where the plasma membrane precursors are used efficiently. The growing tips of neurites do not have to transmit a signal to inform the cell body or proximal axon that they are growing, all they have to do is use the precursors that arrive at the growing tips. The cell body does not have to know the position of the neurites that are growing, it need only synthesize a sufficient amount of the precursors for the total neurite outgrowth. Thus, axonal growth can be controlled locally at the growing tip of the axon (Campenot, 1977) by the rate at which the growing tips incorporate the precursors into the plasma membrane.

Bottom Line: Newly synthesized membrane proteins are transported by fast axonal flow to their targets such as the plasma membrane and synaptic vesicles.We found that all of these proteins are transported by tubulovesicular organelles of various sizes and shapes that circulate within axons from branch to branch and switch the direction of movement.These organelles are distinct from the endosomal compartments and constitute a new entity of membrane organelles that mediate the transport of newly synthesized proteins from the trans-Golgi network to the plasma membrane.

View Article: PubMed Central - PubMed

Affiliation: Department of Cell Biology and Anatomy, Graduate School of Medicine, University of Tokyo, Hongo, Tokyo, Japan, 113.

ABSTRACT
Newly synthesized membrane proteins are transported by fast axonal flow to their targets such as the plasma membrane and synaptic vesicles. However, their transporting vesicles have not yet been identified. We have successfully visualized the transporting vesicles of plasma membrane proteins, synaptic vesicle proteins, and the trans-Golgi network residual proteins in living axons at high resolution using laser scan microscopy of green fluorescent protein-tagged proteins after photobleaching. We found that all of these proteins are transported by tubulovesicular organelles of various sizes and shapes that circulate within axons from branch to branch and switch the direction of movement. These organelles are distinct from the endosomal compartments and constitute a new entity of membrane organelles that mediate the transport of newly synthesized proteins from the trans-Golgi network to the plasma membrane.

Show MeSH
Related in: MedlinePlus