NCBI Bookshelf. A service of the National Library of Medicine, National Institutes of Health.

Siegel GJ, Agranoff BW, Albers RW, et al., editors. Basic Neurochemistry: Molecular, Cellular and Medical Aspects. 6th edition. Philadelphia: Lippincott-Raven; 1999.

  • By agreement with the publisher, this book is accessible by the search feature, but cannot be browsed.
Cover of Basic Neurochemistry

Basic Neurochemistry: Molecular, Cellular and Medical Aspects. 6th edition.

Show details

Fast Axonal Transport

and .

Correspondence to Scott T. Brady, Department of Cell Biology and Neuroscience, University of Texas Southwestern Medical Center, 5323 Harry Hines Boulevard, Dallas, Texas 75235-9111.

Newly synthesized membrane and secretory proteins destined for the axon travel by fast anterograde transport

However, not all membrane-bounded organelles (MBOs) are destined for the axon. As a result, the first stage of transport must be synthesis, sorting and packaging of organelles. Once assembled, the organelle must be committed to the transport machinery and moved down the axon. Finally, organelles must be targeted and delivered to specific domains in the axon, such as presynaptic terminals, axolemma and nodes of Ranvier. Axonal constituents include integral membrane proteins, secretory products, membrane phospholipids, cholesterol and gangliosides. As predicted by the structural hypothesis and apparent in video microscopy, rapid transport is achieved by packaging materials into MBOs (Figs. 28-4 and 28-6). Clearly, an understanding of how MBOs are formed in the cell body and routed to the fast-transport system in axons is essential [7].

Figure 28-6. Summary of pharmacological evidence indicating that newly synthesized membrane and secretory proteins in neurons reach the axons by a pathway similar to that utilized for intracellular transport in non-neuronal cells.

Figure 28-6

Summary of pharmacological evidence indicating that newly synthesized membrane and secretory proteins in neurons reach the axons by a pathway similar to that utilized for intracellular transport in non-neuronal cells. Incorporation sites are indicated (more...)

Passage through the Golgi apparatus is obligatory for most proteins destined for fast transport

In all cell types, secretory and integral membrane proteins are synthesized on polysomes bound to the endoplasmic reticulum. Secretory proteins enter the lumen of the reticulum, whereas membrane proteins become oriented within the bilayer of the membranes (Fig. 28-6). In contrast, components of the cytoskeleton and enzymes of intermediary metabolism are synthesized on so-called free polysomes, which are actually associated with the cytoskeleton. Newly formed membrane-associated proteins must be transferred to the Golgi apparatus for processing and post-translational modification, including glycosylation, sulfation and proteolytic cleavage, as well as for sorting. Pathways for transfer between Golgi and endoplasmic reticulum involve both an MT-dependent step and sorting events mediated by G proteins [8]. After progressing through the Golgi, coated vesicles bud off from the trans-Golgi membrane. Transport to their various destinations is typically mediated by MTs and motor molecules. Membrane and secretory proteins become associated with membranes either during or immediately following their synthesis, then maintain this association throughout their lifetime in the cell. For example, inhibiting synthesis of either protein or phospholipid leads to a proportional decrease in the amount of both protein and phospholipid in fast transport, whereas application of these inhibitors to axons has no effect on transport. This suggests that fast axonal transport depends on de novo synthesis and assembly of membrane components.

Fast-transport proteins leave the endoplasmic reticulum in association with transfer vesicles that bud off and undergo Ca2+-dependent fusion with the Golgi apparatus. Drug studies have demonstrated a requirement that most proteins destined for fast axonal transport traverse the Golgi stacks, where membrane proteins are post-translationally modified, sorted and packaged [9] (see Fig. 2-9). This suggests that proteins in fast axonal transport must either pass through the Golgi complex or associate with proteins that do. Transfer from the Golgi apparatus to the fast-transport system appears to be mediated by clathrin-coated vesicles [8] (see Fig. 2-8). Coated vesicles, however, are rarely observed in axons, and clathrin, the major coat protein, is primarily a slow-transport protein [3]. Thus, Golgi-derived coated vesicles shed their coats prior to undergoing fast transport and travel down the axon as either individual uncoated vesicles or other membrane-bound structures (Fig. 28-6).

Anterograde transport moves synaptic vesicles, axolemmal precursors and mitochondria down the axon

Fast anterograde transport represents movement of MBOs along MTs away from the cell body at rates ranging in mammals from 200 to 400 mm per day or from 2 to 5 μm per second [3,10]. Anterograde transport provides newly synthesized components essential for neuronal membrane function and maintenance. Ultrastructural studies have demonstrated that the material moving in fast anterograde transport includes many small vesicles and tubulovesicular structures as well as mitochondria and dense core vesicles [11,12]. Material in fast anterograde transport is needed for supply and turnover of intracellular membrane compartments (mitochondria and endoplasmic reticulum), secretory products and proteins required for the maintenance of axonal metabolism. The net rate appears to be largely determined by size, with the smallest MBOs in almost constant motion, while mitochondria and larger structures frequently pause, giving a lower average rate [13].

A variety of materials move in fast anterograde transport, including membrane-associated enzymes, neurotransmitters, neuropeptides and membrane lipids. Most are synthesized in the cell body and transported intact, but some processing events occur in transit. For example, neuropeptides may be generated by proteolytic degradation of propeptides (see Chap. 18). This biochemical heterogeneity extends to the MBOs themselves. The small organelles are particularly varied in function and composition: some correspond to synaptic vesicle precursors and contain neurotransmitters and associated proteins, while others may contain channel proteins or other materials destined for the axolemma. Biochemical and morphological studies have provided a description of the materials transported in fast transport but are not as well suited for identifying the underlying molecular mechanisms involved in translocation.

Video microscopy allows study of molecular mechanisms through direct observation of organelle movements while precise control of experimental conditions is maintained. Fast axonal transport continues unabated in isolated axoplasm from giant axons of the squid Loligo pealeii for hours [1,13]. Video microscopy applied to isolated axoplasm permits a more rigorous dissection of the molecular mechanisms for fast axonal transport through biochemical and pharmacological approaches because isolated axoplasm has no plasma membrane or other permeability barriers. Such studies have extended earlier observations on the properties of fast anterograde transport and demonstrated the existence of new motor proteins that are responsible for movement of MBOs in the axon (see below).

Retrograde transport returns trophic factors, exogenous material and old membrane constituents to the cell body

MBOs moving in retrograde transport are structurally heterogeneous and, on average, larger than the structures observed in anterograde transport, which are commonly tubulovesicular [11,12]. Multivesicular or multilamellar bodies are common in retrograde transport, and these are thought to represent materials to be delivered to lysosomes in the cell body. The larger size of these retrograde vectors affects the rate of transport by increasing drag due to interactions with cytoplasmic structures [1,13].

Both morphological and biochemical studies indicate that MBOs returning in retrograde transport differ from those in anterograde transport. Repackaging of membrane components apparently accompanies turnaround, or conversion from anterograde to retrograde transport. The mechanisms of repackaging are incompletely understood, but certain protease inhibitors and neurotoxic agents will inhibit turnaround without affecting either anterograde or retrograde movement. The specificities of effective protease inhibitors implicate a thiol protease, but the identity of the responsible enzyme is unknown [14]. Consistent with this proposal, protease treatment of purified synaptic vesicles affects directionality of their movements in the axoplasm and presynaptic terminals. Fluorescent synaptic vesicles normally move in the anterograde direction, but protease pretreatment of these vesicles results in retrograde transport [10].

Uptake of exogenous materials by endocytosis in distal regions of the axon results in the return of trophic substances and growth factors to the cell body [15]. These factors assure survival of the neuron and modulate neuronal gene expression. Changes in the return of trophic substances play critical roles during development and regeneration of neurites (see Chaps. 27 and 29). Retrograde transport of exogenous substances also provides a pathway for viral agents to enter the CNS. Once retrogradely transported material reaches the cell body, the cargo may be delivered to the lysosomal system for degradation, to nuclear compartments for regulation of gene expression or to the Golgi complex for repackaging.

Molecular sorting mechanisms ensure delivery of proteins to discrete membrane compartments

While pathways by which selected membrane-associated proteins are delivered to the correct destination have been established in general terms, many intriguing questions regarding the selection process itself remain [16]. How is it that certain membrane proteins remain in the cell body, for example, glycosyltransferases of the Golgi, while others are packaged for delivery to the axon? Among transported proteins, how do some, such as sodium and potassium ion channels, reach the axolemma while others, such as presynaptic receptors, synaptic vesicles or secreted neuropeptides, travel the length of the axon to the nerve terminal or enter the synaptic cleft? Finally, how are organelles such as the synaptic vesicle directed toward axons and presynaptic terminals but not into dendritic arbors? This question becomes particularly compelling for dorsal root ganglion sensory neurons, where the central branch of the single axon has presynaptic terminals while the peripheral branch of that same axon has none.

The answers to these questions remain incomplete, but some mechanisms have begun to emerge [16]. Some information comes from studies on polarized epithelial cells, where the identity of molecular destination signals for delivery of newly synthesized proteins to basolateral or apical membranes can be assayed. These mechanisms are relevant to the neuron because viral proteins which normally go to epithelial basolateral membranes end up in neuronal dendritic compartments, while those targeted to apical compartments may be moved into the axon [17]. However, the underlying mechanisms appear to be complex. Signals may be “added on,” as post-translational modifications including glycosylation, acylation or phosphorylation [18], or “built in,” in the form of discrete amino acid sequences [19]. Both mechanisms appear to operate in cells. For example, addition of mannose-6-phosphate to proteins directs them to lysosomes, while amino acid sequences have been identified that direct proteins into the nucleus or into mitochondria. In general, the targeting signals are likely to direct proteins to specific organelles, whereas other mechanisms direct organelles to appropriate final destinations.

Specific membrane components must be delivered to their sites of utilization and not left at inappropriate sites [3]. A synaptic vesicle should go to presynaptic terminals because they serve no function in an axon or cell body. The problem is compounded because many presynaptic terminals are not at the end of an axon. Often, numerous terminals occur sequentially along a single axon, making en passant contacts with multiple targets. Thus, synaptic vesicles cannot merely move to the end of axonal MTs and targeting of synaptic vesicles becomes a more complex problem. Similar complexities arise with membrane proteins destined for the axolemma or a nodal membrane.

One proposed mechanism for targeting of organelles to terminals may have general implications. The synapsin family of phosphoproteins [20], which is concentrated in the presynaptic terminal, may be involved in targeting synaptic vesicles. Dephosphorylated synapsin binds tightly to both synaptic vesicles and actin microfilaments (MFs), while phosphorylation releases both of them. Dephosphorylated synapsin inhibits axonal transport of MBOs in isolated axoplasm, while phosphorylated synapsin at similar concentrations has no effect [21]. When a synaptic vesicle passes through a region rich in dephosphorylated synapsin, it may be cross-linked to the available MF matrix by synapsin. Such cross-linked vesicles would be removed from fast axonal transport and are effectively targeted to a synapsin- and MF-rich domain, the presynaptic terminal. Calcium-activated kinases subsequently mobilize targeted vesicles for transfer to active zones for neurotransmitter release [20] (see Chap. 9). This suggests a general mechanism that with variations, might target MBOs to other specific domains [3].

Finally, this chapter has focused almost entirely on axonal transport, but dendritic transport also exists. Since dendrites usually include postsynaptic regions while most axons terminate in presynaptic elements, dendritic and axonal transport each receives a number of unique proteins. Evidence for sorting mechanisms comes from studies in cultured hippocampal neurons using two different viral proteins. Basolaterally targeted viral glycoproteins were transported exclusively to the dendritic processes of cultured neurons, whereas glycoproteins of the apically budding virus were found in axons [17]. An added level of complexity for intraneuronal transport phenomena is the intriguing observation that mRNA is routed into dendrites, where it is implicated in local protein synthesis at postsynaptic sites, but that ribosomal components and mRNA are largely excluded from axonal domains [22]. Similar processes of mRNA transport have been described in glial cells [23].

Image ch28f4
Image ch2f9
Image ch2f8

By agreement with the publisher, this book is accessible by the search feature, but cannot be browsed.

Copyright © 1999, American Society for Neurochemistry.
Bookshelf ID: NBK28189