Axonal transport versus dendritic transport

Neurons have polarized processes for information output and input, axons, and dendrites. This polarized architecture is essential for the neuronal function. An increasing number of molecular components that mediate neuronal polarity establishment have been characterized over the past few years. The vast majority of these molecules include proteins that act in scaffolding protein complexes to sustain the polarized anchoring of molecules. In addition, more signaling and cytoskeleton-associated proteins have been proposed for establishment of polarity. It has become evident that dendritic and axonal transport of molecules depends on scaffolding/adaptor proteins that are recognized by molecular motors. Current and future research in the neuronal cell polarity will be focused on how different cargo molecules transmit their signals to the cytoskeleton and change its dynamic properties to affect the rate and direction of vesicular movement. In this review, we discuss recent evidence that scaffolding proteins can regulate motor motility and guidance by a mechanism of substrate-cytoskeletal coupling and amino acid modifications during polarized transport.     

Axonal protein synthesis and transport

Recent evidence has challenged our ideas about the nature of axonal protein synthesis and transport. Previous metabolic labeling evidence supported the idea that all axonal proteins were synthesized in the cell body and then transported as formed cytoplasmic structures into the axon. Recent evidence suggests that neither the synthesis nor the transport of axonal proteins is that simple. Though most axonal proteins do appear to be synthesized in the neuronal cell body, a small amount of protein appears to be synthesized intra-axonally in some axons. Though small in amount, intra-axonal protein synthesis may be important functionally in some axons. Recent experiments have also begun to identify the presence of a rich array of transport motors in axons, including many members of the kinesin, dynein and myosin families. Progress is being made in identifying which cargoes are being transported by which of these motors. Finally, recent experiments have addressed an old question about whether axoplasmic proteins are transported as filamentous polymers or as soluble components in axons. The answer is that both mechanism can be used in axons. For example, neurofilament protein can move in its particulate or polymeric state, while tubulin can move in its soluble or unpolymerized state.     

Axonal transport and neurodegenerative disease

Neurons have extensive processes and communication between those processes and the cell body is crucial to neuronal function and survival. Thus, neurons are uniquely dependent on microtubule based transport. Growing evidence supports the idea that deficits in axonal transport contribute to pathogenesis in multiple neurodegenerative diseases. We describe the motor, cytoskeletal, and adaptor proteins involved in axonal transport and their interactions. Data linking disruption of axonal transport to diseases such as ALS are discussed. Finally, we explore the pathways that may cause neuronal dysfunction and death.     

Axonal transport—I. Diffusion

The phenomenon of axonal transport of material has been well documented (Ochs, 1971; Lasek, 1970; and Grafstein, 1967). This report seeks to establish the role of diffusion—if any—in such a transport process. We report that diffusion cannot account for the observed build-up of material as reported in the literature.     

Axonal transport—II. Convection

The phenomenon of axonal transport has been well documented (Ochs, 971; Lasek, 1970; and Grafstein, 1967). In a previous paper, we showed how diffusion alone could not account for this process. In this report we show that convection or convection with diffusion can account for the observed build-up of material. By including a first-order catabolic sequestration term, we are able to offer an understanding of the several apparent rates of transport with the same underlying velocity and variable sequestration.     

Axonal transport of tubulin and actin

Brain Cell Biology - Axonal transport is responsible for supplying the axonal processes with proteins that are synthesized in the cell body. Among the proteins that are moved by this mechanism are...     

Axonal transport of phosphatidylcholine and two synthetic analogs

Sonicated emulsions of egg phosphatidylcholine containing either [¹⁴C]-dipalmitoyl phosphatidylcholine (diester-PC) or two metabolically inert analogs. $\text{[}{}^{14}\text{C]-1-}\text{octadecyl}\text{-2-}\text{hexadecyl}\text{-sn-}\text{glycero}\text{-3-}\text{phosphocholine}$ (diether-PC) and [³H]-2-tetradecyloctadecano-(1)-phosphocholine (dialkyl-PC) were injected into the vitreous of the eye of adult rabbits. After 1–40 days, radioactivities were measured in the stations of the optical pathway, and the identities of the labelled lipids arrived at the superior colliculus were ascertained by thin-layer chromatography and treatment with phospholipase A₂, with the following results: (1) phosphatidylcholine and its analogs were taken up from the vitreous by the retina at similar rates: (2) all three lipids were transported in the optic nerve axons at similar rates (‘fast’). They reached maximal concentration in the superior colliculus some 20 days after injection: (3) phosphatidylcholine travelled from vitreous to superior colliculus as the intact molecule: (4) maximal accumulation of the two analogs in the superior colliculus reached only about 1 per cent of that of phosphatidylcholine. The results suggest that the vehicles of fast axonal transport can pick up intact phospholipid molecules originating in the ganglionic cell plasma membrane (and, likely, from other cellular compartments). The packaging process is promoted by the presence of carboxyl ester groups in the phospholipid; this fact suggests the involvement of ganglionic phospholipid transfer protein with specificity for these groups.     

Axonal transport of lipid in goldfish optic axons

After injection of labeled glycerol, choline, or serine into the eye of goldfish, labeled lipids were axonally transported along the optic nerve to the optic tectum. although the different precursors were presumably incorporated into somewhat different lipid populations, all three were approximately equally effective in labeling the lipids transported to the tectum, but the amount of transported material remaining in the nerve was different, being highest with choline and lowest with serine. The labeled lipids appeared in the tectum within 6 hr of the injection, indicating a fast rate of transport, but continued to accumulate over a period of 1–2 weeks, which presumably reflects the time course of their release from the cell body. Since there was a gradual increase in the proportion of labeled lipid in the tectum during this period, some other process in addition to fast axonal transport may have affected the distribution of the lipids along the optic axons. When [³H]choline was used as precursor, the transported material included a small amount of TCA-soluble material, which was probably mainly phosphorylcholine, with labeled acetylcholine appearing in only insignificant amounts. With serine, which gave rise to a large amount of axonally transported protein in addition to lipid, a late increase in the amount of labeled lipid in the tectum was seen, accompanied by a decrease in labeling of the protein fraction.     

Axonal transport and the delivery of pre-synaptic components

The mechanisms for delivering components to nerve terminals are diverse and highly regulated. The diversity of kinesin motors alone is insufficient to account for the specificity of delivery. Additional specificity and control are contributed by adaptor proteins and associated regulatory molecules. The interaction of cargos with these complexes can confer distinct behaviors on the transport of synaptic organelles. The rich regulatory mechanisms of transport that are only now emerging as the cargo-motor complexes are defined and subsequent local events that regulate their dynamic relationship are examined. Here we review recent studies of kinesin-related axonal transport of three crucial synaptic components, Piccolo-bassoon Transport Vesicles (PTVs), Synaptic Vesicle Precursors (SVPs), and mitochondria, and the mechanisms that modulate their transport.     

Axonal transport of phospholipids in rabbit optic pathway

The uptake of different labeled precursors, their incorporation into lipids, and transport along the rabbit optic pathway [ipsilateral retina and optic nerve (ON), and contralateral optic tract (OT), lateral geniculate body (LGB), and superior colliculus (SC)] were investigated. Albino rabbits were used. The following radioactive precursors, either combined or separately, dissolved in 50 l of saline containing 15% BSA, were injected into vitreous body: [2-³H]glycerol (50 Ci), [1-¹⁴C]palmitate (15 Ci), and [1-¹⁴C]linoleate (7.5 Ci). Animals were killed at different time intervals from 1 hr up to 24 days. The radioactivity of total lipids and of different phospholipid classes from total tissue was measured. One hour after the administration of precursors, the radioactivity into the retina was high and the incorporation of [³H]glycerol and [¹⁴C]palmitate increased until 12 hr and 24 hr, respectively. The incorporation of [¹⁴C]linoleate reached a maximum on the second day. The phospholipids of LGB and SC were intensively labeled after 4–8 hr, and their radioactivity increased up to the 10th day after injection, independent of the precursor employed. The results obtained indicate that the labeled hydrophilic and hydrophobic precursors used were actively incorporated into the retina. The phospholipids were later transported at a rapid rate along the optic pathway.A preliminary report of this study has been presented at the Satellite ISN Meeting, Istanbul, September 8–10, 1979.     

Axonal transport of proteoglycans to the goldfish optic tectum

The study addressed the question of whether³⁵SO₄ labeled molecules that the have been delivered to the goldfish optic nerve terminals by rapid axonal transport include soluble proteoglycans. For analysis, tectal homogenates were subfractionated into a souluble fraction (soluble after centrifugation at 105,000g), a lysis fraction (soluble after treatment with hypotonic buffer followed by centrifugation at 105,000g) and a final 105,000g pellet fraction. The soluble fraction contained 25.7% of incorporated radioactivity and upon DEAE chromatographys was resolved into a fraction of sulfated glycoproteins eluting at 0–0.32 M NaCl and containing 39.5% of total soluble label and a fraction eluting at 0.32–0.60 M NaCl containing 53.9% of soluble label. This latter fraction was included on columns of Sepharose CL-6B with or without 4 M guanidine and after pronase digestion was found to have 51% of its radioactivity contained in the glycosaminoglycans (GAGs) heparan sulfate and chondroitin (4 or 6) sulfate in the ratio of 70% to 30%. Mobility of both intact proteoglycans and constituent GAGs on Sepharose CL-6B indicated a size distribution that is smaller than has been observed for proteoglycans and GAGs from cultured neuronal cell lines. Similar analysis of lysis fraction, containing 11.5% of incorporated³⁵SO₄, showed a mixture of heparan sulfate and chondroitin sulfate containing proteoglycans, apparent free heparan sulfate and few, if any, sulfated glycoproteins. Overall, the result support the hypothesis that soluble proteoglycans are among the molecules axonally transported in the visual system.     

Axonal transport of actin in rabbit retinal ganglion cells

We labeled proteins in the cell bodies of rabbit retinal ganglion cells with [35S]methionine and subsequently observed the appearance of radioactive actin in tissues containing the axons and synaptic terminals of these neurons, i.e., the optic nerve (ON), optic tract (OT), lateral geniculate nucleus (LGN) and the superior colliculus (SC). The temporal sequence of appearance of labeled actin (which was identified by its specific binding to DNase I, its electrophoretic mobility, and its peptide map) in these tissues indicated that actin is an axonally transported protein with a maximum transport velocity of 3.4--4.3 mm/d. The kinetics of labeling actin were similar to the kinetics of labeling two proteins (M1 and M2) which resemble myosin; these myosin-like proteins were previously found to be included in the groups of proteins (groups III and IV) transported with the third and fourth most rapid maximum velocities. The similarity in transport between actin and myosin-like proteins supports the idea that a number of proteins in the third and fourth transport groups may be functionally related by virtue of their involvement in a force-generating mechanism and suggests the possibility that these proteins may be axonally transported as a preformed force-generating unit.     

Axonal transport: beyond kinesin and cytoplasmic dynein.

In vitro and in vivo studies of specific neuronal fast and slow transport components are presently reshaping our understanding of how the processes of vesicular and cytoskeletal transport are regulated in axons and dendrites. Evidence suggests that vesicles possess an inherent directionality, possibly the result of their motor receptor proteins responding to intracellular cues, which then allows movement with either kinesin or cytoplasmic dynein.     

Axonal transport of phospholipids in rat visual system.

Abstract— Seventeen day old rats were injected intraocularly with a phospholipid precursor, [³²P]phosphate, and a glycoprotein precursor, [³H]fucose. Animals were killed between 1 h and 21 days later, and structures of the visual pathway (retina, optic nerve, optic tract, lateral geniculate body, and superior colliculus) were dissected. Radioactivity in phospholipids ([³²P] in solvent-extracted material) and in glycoproteins ([³H] in solvent-extracted residue) was determined. Incorporation of [³H]fucose into retinal glycoproteins peaked at 6–8 h. Labelled glycoproteins were present in superior colliculus by 2h after injection, indicating a rapid rate of transport; maximal labelling was at 8–10 h after injection. Incorporation of [³²P]phosphate into retinal phospholipids peaked at 1 day after injection. Phospholipids were also rapidly transported since label was present in the superior colliculus by 3 h after injection: however, maximal labelling did not occur until 5–6 days. These results indicate that newly synthesized phospholipids enter a preexisting pool, part of which is later committed to transport at a rapid rate. Transported phospholipids were catabolized at the nerve endings with a maximum half-life of several days; there was minimal recycling of precursor label. Lipids were fractionated by thin-layer chromatography, and radioactivity in individual phospholipid classes determined. Choline and ethanolamine phosphoglycerides were the major transported phospholipids, together accounting for approx 85% of the total transported lipid radioactivity. At early time points, the ratio of radioactivity in choline phosphoglycerides to that in ethanolamine phosphoglycerides increased in structures progressively removed from the site of synthesis (retina) but by 2 days approached a constant value. In each structure, choline phosphoglyceride-ethanolamine phosphoglyceride radioactivity ratios decreased with time, rapidly at first, but plateaued by 2 days. These results indicate that choline phosphoglycerides are committed to transport sooner than ethanolamine phosphoglycerides. Some experiments were also conducted using [2-³H]glycerol as a phospholipid precursor. Results concerning incorporation of this precursor into individual phospholipid classes and their subsequent axonal transport were comparable to those obtained using [³²P]phosphate, with the following exceptions: (a) incorporation of [2-³H]glycerol into retinal phospholipids was relatively rapid (near-maximal levels at 1 h after injection) although transport to the superior colliculus showed an extended time course very similar to [³²P]-labelled lipids; (b) [2-³H]glycerol was somewhat less efficient than [³²P]phosphate in labelling lipids committed to transport relative to labelling those which remained in the retina; and (c) [2-³H]glycerol did not label plasmalogens.     

Axonal transport involvement in long-lasting synaptic modifications in Blatta orientalis

To determine whether axonal transport plays a role in the establishment of long-lasting changes in synaptic transmission, the effects of colchicine on transport and on synaptic modifications induced by hyperactivity were studied in the nerve cord of the cockroach Blatta orientalis. Application of a lead weight on the insect's dorsum, and the consequent exaggerated use of antigravity reflexes, facilitated synaptic transmission along a particular nervous pathway in the metathoracic ganglion. Application of colchicine in the prothoracic ganglion reversibly blocked such synaptic facilitation and temporarily interfered with the transport of proteins along the cord. Five components of axonal transport, moving at 2, 10, 25, 75, and 150 mm/day, were altered by colchicine treatment with a temporal course that coincided with the reversible inhibition of synaptic facilitation. These results were brought about by colchicine acting directly on axonal transport at the level of the prothoracic ganglion, rather than on synaptic transmission measured at the metathoracic ganglion. The temporal correlation observed between the effects of colchicine on axonal transport and on synaptic facilitation strongly suggest that the transport process is essential for long-lasting synaptic modifications to take place.     

Axonal transport of microtubules: the long and short of it

Recent studies on cultured neurons have demonstrated that microtubules are transported down the axon in the form of short polymers. The transport of these microtubules is bidirectional, intermittent, asynchronous, and occurs at the fast rate of known motors. The majority of the microtubule mass in the axon exists in the form of longer immobile microtubules. We have proposed a model called 'cut and run', in which the longer microtubules are mobilized by enzymes that sever them into shorter mobile polymers. In this view, the molecular motors that transport microtubules are not selective for short microtubules but rather impinge upon microtubules irrespective of their length. In the case of the longer microtubules, these motor-driven forces do not transport the microtubules in a rapid and concerted fashion but presumably affect them nonetheless. Here, we discuss the mechanisms by which the short microtubules are transported and suggest possibilities for how analogous mechanisms may align and organize the longer microtubules and functionally integrate them with each other and with the actin cytoskeleton.