Reversal of axonal transport in regenerating nerves.

Abstract— Orthograde and retrograde axonal transport were studied in rat sciatic nerves which had been crushed and either allowed to regenerate, or prevented from doing so by tightly ligaturing the nerve. At various intervals after crushing the nerve. L-[³H]leucine was injected into the lumbosacral spinal cord. and the subsequent transport of labeled protein in motoneuron axons was quantitated by measuring the accumulation of labeled protein at collection crushes made proximal to the original nerve crush. Accumulations proximal to the collection crushes (orthograde transport) 9-11 h after injection (p.i.). decreased within I day of nerve injury, but returned to normal values as regeneration proceeded. In non-regenerating nerves accumulations remained depressed for at least 30 days. Accumulations distal to the collection crushes (retrograde transport) 9-11 h pi. increased over the first 5 days following injury but returned to normal values as regeneration proceeded. In non-regenerating nerves accumulations remained elevated. The time-course of retrograde transport of newly-synthesized protein also returned to normal during nerve regeneration. It is suggested that changes in retrograde transport during regeneration may inform the neuron cell body of the progress of regeneration and elicit appropriate metabolic responses. among which may be the changes in orthograde transport that follow axotomy.     

Fast axonal transport in motor nerve regeneration.

This report describes the fast transport of [3H]-leucine-labeled proteins in regenerating rat sciatic motor nerves. A normal rate of fast transport (383 +/- 33 mm/day) was present in the regenerating sprouts, as well as in the central stumps. The rapidly transported proteins passed the level of axotomy without impediment, and accumulated in the endings of the regenerating sprouts, as shown by electron microscope autoradiography. In addition, transported proteins accumulated in terminal neuromas. The relative amount of protein-incorporated radioactivity in the crest of transport in the regenerating nerves was increased compared to control nerves. These results are interpreted to suggest that the mechanism of fast transport is the same in regenerating nerves was increased compared to control nerves. These results are interpreted to suggest that the mechanism of fast transport is the same in regenerating sprouts as in normal axons; during regeneration fast transport appears to add newly synthesized materials to the growing tip.     

Absence of 'superfast' axonal transport in rat sciatic nerve.

Axonal transport of labelled protein was studied in rat sciatic nerve by analyzing nerve segments at intervals after injection of L-[3H]leucine into the lumbar spinal cord. Some nerves were sectioned before injection so that material in transit accumulated proximal to the section. The segments distal to the section served as controls for incorporation into the nerve of blood-borne label. An analysis of TCA-soluble and TCA-insoluble activity in cut and intact nerve segments was also made. No evidence was found for the existence of a 'superfast' component of axonal transport (velocity 2000 mm/day). Results showed that the most rapidly transported protein derived from the neuron soma had a conventional 'fast' velocity of 350-420 mm/day. There was no transport of TCA-soluble material. It is suggested that 'superfast' transport, detected in mice by other investigators, is an artefact resulting from failure to control for incorporation of circulating label into the sciatic nerve.     

Fast axonal transport in motor nerve regeneration

This report describes the fast axonal transport of [³H]-leucine-labeled proteins in regenerating rat sciatic motor nerves. A normal rate of fast transport (383 ± 33 mm/day) was present in the regenerating sprouts, as well as in the central stumps. The rapidly transported proteins passed the level of axotomy without impediment, and accumulated in the endings of the regenerating sprouts, as shown by electron microscope autoradiography. In addition, transported proteins accumulated in terminal neuromas. The relative amount of protein-incorporated radioactivity in the crest of fast transport in the regenerating nerves was increased compared to control nerves. These results are interpreted to suggest that the mechanism of fast transport is the same in regenerating sprouts as in normal axons; during regeneration fast transport appears to add newly synthesized materials to the growing tip.     

Slow axonal transport.

New studies provide further evidence that the neuronal cytoskeleton is the product of a dynamic interplay between axonal transport processes and locally regulated assembly mechanisms. These data confirm that the axonal cytoskeleton in mammalian systems is largely stationary and is maintained by a smaller pool of moving subunits or polymers. Slow axonal transport in certain lower species, however, may exhibit quite different features.     

Does nerve impulse activity modulate fast axonal transport?

The possibility that the amount of newly synthesized material made available for fast axonal transport is regulated by nerve impulse activity was examined in an in vitro preparation of bullfrog dorsal root ganglia (DRG) and sciatic nerve. Under conditions that precluded effects of impulse activity on either uptake or incorporation of precursor, patterned stimulation of the sciatic nerve (1 out of every 2 s) produced a frequency- and time-dependent decrease in the amount of radiolabeled protein accumulating at a nerve ligature. The response to patterned stimulation was significantly greater than that to continuous stimulation when the same number of stimuli were delivered. In unligated nerve preparations, patterned stimulation decreased the amplitude of the transport profile with no concomitant change in the wave front distance. Nerve stimulation produced no observable ultrastructural alterations within neuronal cell bodies of the DRG. We propose that the physiological significance of these results is not that nerve impulse activity decreases fast axonal transport, but that the amount of transport increases during periods of electrical quiescence. According to this hypothesis, activity-dependent macromolecules of the axolemma and nerve terminals are replenished during periods when the neuron is firing less frequently. These findings are discussed in light of reports that chronic in vivo stimulation increases the amount of fast-transported, radiolabeled protein (Chan et al., 1989) and that TTX-blockade of neuronal activity has no effect on protein transport (Edwards and Grafstein, 1984; Riccio and Matthews, 1985).     

Neuritin 1 promotes retinal ganglion cell survival and axonal regeneration following optic nerve crush

Neuritin 1 (Nrn1) is an extracellular glycophosphatidylinositol-linked protein that stimulates axonal plasticity, dendritic arborization and synapse maturation in the central nervous system (CNS). The purpose of this study was to evaluate the neuroprotective and axogenic properties of Nrn1 on axotomized retinal ganglion cells (RGCs) in vitro and on the in vivo optic nerve crush (ONC) mouse model. Axotomized cultured RGCs treated with recombinant hNRN1 significantly increased survival of RGCs by 21% (n=6–7, P<0.01) and neurite outgrowth in RGCs by 141% compared to controls (n=15, P<0.05). RGC transduction with AAV2-CAG–hNRN1 prior to ONC promoted RGC survival (450%, n=3–7, P<0.05) and significantly preserved RGC function by 70% until 28 days post crush (dpc) (n=6, P<0.05) compared with the control AAV2-CAG–green fluorescent protein transduction group. Significantly elevated levels of RGC marker, RNA binding protein with multiple splicing (Rbpms; 73%, n=5–8, P<0.001) and growth cone marker, growth-associated protein 43 (Gap43; 36%, n=3, P<0.01) were observed 28 dpc in the retinas of the treatment group compared with the control group. Significant increase in Gap43 (100%, n=5–6, P<0.05) expression was observed within the optic nerves of the AAV2–hNRN1 group compared to controls. In conclusion, Nrn1 exhibited neuroprotective, regenerative effects and preserved RGC function on axotomized RGCs in vitro and after axonal injury in vivo. Nrn1 is a potential therapeutic target for CNS neurodegenerative diseases.Central nervous system (CNS) trauma and neurodegenerative disorders trigger a cascade of intrinsic and extrinsic cellular events resulting in regenerative failure and subsequent damage to neurons.^{1, 2, 3, 4, 5} The intrinsic factors include deregulation in growth-promoting factors, apoptotic factors, intracellular signaling molecules and trophic factors.⁶ Similarly, the extrinsic factors correlate to growth inhibition due to inhibitory cues^{3, 7, 8, 9, 10, 11, 12, 13} that include myelin and myelin associated inhibitors, glial scarring,^{5, 14} slow clearance of axonal debris,⁷ incorrect development of neuronal projections⁶ and CNS inflammation.^{15, 16} Progressive degeneration of mature retinal ganglion cells (RGCs) has been associated with loss of trophic support,^{8, 9} detrimental inflammatory processes/immune regulation^{10, 11} and apoptotic effectors.^{9, 12, 13, 15, 17}After injury, mammalian RGC axons show only a short-lived sprouting response but no long-distance regeneration through the optic nerve (ON).¹⁶ Glial responses around the affected area are initiated by injured CNS axons.¹⁸ Axons undergoing Wallerian degeneration are surrounded by astrocytes that upregulate glial fibrillary acidic protein (Gfap) expression and these reactive astrocytes contribute to trauma-induced neurodegeneration.¹⁹ Glial scarring inhibits axonal transport after ON crush (ONC)^{5, 14} decreasing transport of proteins involved in neuroprotection and synaptic plasticity. Regenerative failure is a critical endpoint of these destructive triggers culminating in neuronal apoptosis^{3, 20, 21} and inhibition of functional recovery. Intrinsic factors affecting axonal regeneration after CNS injury are crucial for recovery and thus, dysregulation of genes involved in axonal plasticity and outgrowth can prove detrimental to the neuronal recovery.^{22, 23, 24}Current neuroprotection approaches include promoting survival of RGCs by intraocular injections of recombinant factors like ciliary neurotrophic factor (CNTF) and peripheral nerve (PN) transplantations in vitro²⁵ and in vivo after injury.²⁶ Studies performed with glial cell-line-derived neurotrophic factor and neurturin protect RGCs from axotomy-induced apoptosis.²⁷ Further, in the ON injury model, RGC survival was promoted after deletion of CCAAT/enhancer binding protein homologous protein²⁸ and enhanced regeneration observed with co-deletion of kruppel-like factor 4 (Klf4) and suppressor of cytokine signaling 3 (Socs3).²⁹ Intraocular administration of neurotrophin-4 (NT-4) and brain-derived neurotrophic factor (BDNF) after ON transection has also exerted neuroprotective effects on axotomized RGCs. In addition, PNs transplanted adjacent to ONs, ex vivo PN grafts with lenti-viral transduced Schwann cells, and stimulation of inflammatory processes have strong pro-regenerative effects on injured RGCs.^{26, 30, 31, 32, 33}In addition, using adeno-associated-virus (AAV) therapy, AAV mediated expression of CNTF in bcl2 overexpressing transgenic mice increases cell viability and axonal regeneration,³⁴ whereas BDNF promotes survival of RGCs.³⁵ Likewise, experiments with AAV–BDNF, –CNTF and –growth-associated protein 43 (GAP43) have shown that AAV–CNTF was the most crucial for promoting both long-term survival and regeneration.³⁶ The positive effects of CNTF are observed mainly through simultaneous deletion of both PTEN and SOCS3³⁷ and the concurrent activation of mTOR and STAT3 pathways.³⁸ Although CNTF shows robust increase and sustained axon regeneration in injured ONs of rodents, it causes axonal misguidance and aberrant growth.³⁹ Furthermore, it has been shown that CNTF acts as a chemoattractant. CNTF administration onto autologous PN grafts transplanted within transected ON increased regeneration, but these effects were significantly reduced after removal of macrophages from this site.⁴⁰ In addition, the effects of CNTF using PN grafts at ON transection sites are further subject to debate, as previously it has been shown that Ad-CNTF injections preserved RGC axons but did not induce regeneration of axotomized RGCs.⁴¹ Thus, other studies have addressed RGC survivability and axonal regeneration with CNTF and other growth factors,^{35, 36} but most trophic factors affect neuronal survival and regeneration differentially.Previous studies targeting neuronal apoptosis by overexpressing intrinsic growth factors, inhibiting apoptosis and enhancing regeneration in CNS trauma models have established that a multifactorial approach is required for successful and long-lasting therapeutic outcomes.^{6, 36} Current gaps still exist for a key gene that could effectively target neuroprotection, enhance neuron regeneration and sustain neuronal function.One key gene implicated in neuronal plasticity is Neuritin 1 (Nrn1), also known as candidate plasticity gene 15. It has multiple functions and was first identified and characterized when screening for candidate plasticity genes in the rat hippocampal dentate gyrus activated by kainate.^{42, 43, 44} Nrn1 is highly conserved across species⁴⁵ and translates to an extracellular, glycophosphatidylinositol-linked protein (GPI-linked protein), which can be secreted as a soluble form. Nrn1 stimulates axonal plasticity, dendritic arborization and synapse maturation in the CNS.⁴⁶ During early embryonic development, Nrn1 promotes the survival of neural progenitors and differentiated neurons,⁴⁷ while later in development it promotes axonal and dendritic growth and stabilization, allowing maturation and formation of synapses.^{43, 46, 48} In the adult brain, Nrn1 has been correlated with activity-dependent functional plasticity^{45, 49} and is expressed in post mitotic neurons.Nrn1 may be a crucial gene for neuroprotection and regeneration because growth factors such as nerve growth factor (NGF), BDNF and NT-3 as well as neuronal activity can potentiate the expression of Nrn1.^{44, 50} In addition, we reported that Nrn1 mRNA expression appears to be biphasic after ON axonal trauma, indicating a transient attempt by RGCs at neuroprotection/neuroregeneration in response to ONC injury.⁵¹ The dynamic regulation of Nrn1 coupled with neurotrophic effects may promote axonal regeneration in the CNS. To overcome CNS trauma, a new therapy geared towards neuroprotection and effective axonal regeneration is required to enhance a future multifactorial approach. The purpose of this study is to evaluate the therapeutic effects of Nrn1 in mouse RGC cultures as well as in the mouse ONC model. We have identified a distinct neuroprotective and regenerative strategy that prevents neurodegeneration after ON injury. AAV2–hNRN1 expression vectors partially rescued RGCs from apoptosis, maintained RGC function, and initiated regeneration of injured axons.     

Retrograde axonal transport of specific macromolecules as a tool for characterizing nerve terminal membranes

The uptake of macromolecules by nerve terminals which is followed by retrograde axonal transport seems to occur by two different mechanisms, a specific and a nonspecific one. The nonspecific uptake depends on the presence of macromolecules (e.g., horseradish peroxidase) in the vicinity of the nerve terminals at very high concentrations and is enhanced by neuronal activity. In contrast, the specific uptake and subsequent retrograde axonal transport becomes apparent at much lower concentrations of the appropriate macromolecules, depends on the affinity of these ligands for specific binding sites on the surface of the neuronal membrane, and is independent of neuronal activity. The fact that lectins and some bacterial toxins bind to specific membrane glycoproteins or glycolipids allows conclusions to be drawn regarding qualitative and even quantitative aspects of the composition of the plasma membrane of the nerve terminals. ¹²⁵I-labelled nerve growth factor (NGF), tetanus toxin, cholera toxin, wheat germ agglutinin (WGA), ricin II, phytohemagglutinin (PHA), and concanavalin A (ConA) were injected into the anterior eye chamber of rats where they were taken up by adrenergic nerve terminals and transported retrogradely to the superior cervical ganglion. The saturation of the uptake-transport found for NGF, WGA, choleragenoid and an atoxic binding-fragment of tetanus toxin indicates that limited numbers of binding sites, which showed also different affinites, are present for each ligand on the membrane of the nerve terminals. Competition experiments showed that the binding sites for the ligands investigated are largely independent. Two different classes of binding sites (high affinity-low capacity and intermediate affinity-intermediate capacity) seem to be involved in the saturable retrograde axonal transport of NGF. In contrast, WGA seems to have only a single class of binding-uptake sites with high capacity and relatively low affinity. Strong evidence for positive cooperativity was obtained for the uptake and subsequent transport of the tetanus toxin fragment.     

Dynamics of fast axonal transport.

A phenomenological model of the process of fast axoplasmic transport is presented. The process was conceived of as occurring in two parts: (a) synthesis and storage of material in a cytoplasmic pool; (b) release from the pool and transport distally along the axon. Considering the fate of labeled proteins, the activity at points along the axon relfects events occurring earlier within the pool through the relationship: g(x,t) = const f(t - x/v); where g(x,t) represents axonal activity, f(t) the pool's activity, and v is the transport speed. Using the idea that when there is no further input of radioactivity into the pool its activity declines exponentially due to export of material to the axon. I generalized this concept to the case where activity enters and leaves the pool simultaneously. The model contains two parameters: the relative turnover rate of the pool, alpha, and T, an interval characteristic of the time of synthesis. From this model, the experimental data is unfolded and yields values for these parameters of alpha = 0.004 min-1 and T approximately 60 min.     

The genetics of axonal transport and axonal transport disorders

Neurons are specialized cells with a complex architecture that includes elaborate dendritic branches and a long, narrow axon that extends from the cell body to the synaptic terminal. The organized transport of essential biological materials throughout the neuron is required to support its growth, function, and viability. In this review, we focus on insights that have emerged from the genetic analysis of long-distance axonal transport between the cell body and the synaptic terminal. We also discuss recent genetic evidence that supports the hypothesis that disruptions in axonal transport may cause or dramatically contribute to neurodegenerative diseases.     

Bidirectional axonal transport of 16S acetylcholinesterase in rat sciatic nerve

Axonal transport of the 16S Molecular form of acetylcholinesterase (16S-AChE) in doubly ligated rat sciatic nerves was studied by means of velocity sedimentation analysis on sucrose gradients. This form of AChE was selectively confined to motor, and not to sensory, fibers in the sciatic nerve, where it represented 3--4% of total AChE. Its activity increased linearly with time (4--20 hr) in nerve segments (7 mm) proximal to the central ligature (4.5 mU/24hr) and distal to the peripheral ligature (2.0 mU/24 hr). From the linear rates of accumulation of 16S-AChE, we conclude that the enzyme is conveyed by anterograde and retrograde axonal transport at velocities close to those previously defined for the movement of total AChE (410 mm/day, anterograde; 220 mm/day, retrograde). The transport of AChE molecular forms, other than the 16S form, could not be resolved presumably due to their presence in blood as well as at extraaxonal sites. The present findings are consistent with the view that in rat sciatic nerve most, if not all, of the small portion of total AChE (approximately 3%) which is transported may be accounted for by 16S-AChE.     

Enhanced detection and retrograde axonal transport of PrPc in peripheral nerve

Neuroinvasion of the CNS during orally acquired transmissible spongiform encephalopathies (TSEs) may involve the transport of the infectious agent from the periphery to the CNS via the peripheral nerves. If this occurs within axons, the mechanism of axonal transport may be fundamental to the process. In studies of peripheral nerve we observed that the cellular prion protein (PrPc) is highly resistant to detergent extraction. The implication of this is an underestimation of the abundance of PrPc in peripheral nerve. We have developed nerve extraction conditions that enhance the quantification of the protein in nerve 16-fold. Application of these conditions to evaluate the accumulation of PrPc distal to a cut nerve now reveals that PrPc is retrogradely transported from the axon ending. These results provide a potential cellular mechanism for TSE infectivity to gain entry to the CNS from the periphery.     

Wallerian degeneration and axonal regeneration after sciatic nerve crush are altered in ICAM-1-deficient mice

The intercellular cell adhesion molecule-1 (ICAM-1) has been implicated in the recruitment of immune cells during inflammatory processes. Previous studies investigating its involvement in the process of Wallerian degeneration and focusing on its potential role in macrophage recruitement have come to controversial conclusions. To examine whether Wallerian degeneration is altered in the absence of ICAM-1, we have analyzed changes in the expression of axonal and Schwann cell markers following sciatic nerve crush in wildtype and ICAM-1-deficient mice. We report that the lack of ICAM-1 leads to impaired axonal degeneration and regeneration and to alterations in Schwann cell responses following sciatic nerve crush. Degradation of neurofilament protein, the collapse of axonal profiles, and the re-expression of neurofilament proteins are substantially delayed in the distal nerve segment of ICAM-1^-/- mice. In contrast, the degradation of myelin, as determined by immunostaining for myelin protein zero, is unaltered in the mutants. Upregulation of GAP-43 and p75 neurotrophin receptor (p75^NTR) expression, characteristic for Schwann cells dedifferentiating in response to nerve injury, is differentially altered in the mutant animals. These results indicate that ICAM-1 is essential for the normal progression of axonal degeneration and regeneration in distal segments of injured peripheral nerves.     

Melanocortins and fast axonal transport in intact and regenerating sciatic nerve

The effect of ACTH/MSH peptides on fast axonal transport along intact or regenerating sciatic nerve was examined following injection of tritiated leucine into the rat lumbar spinal cord. The rate of fast axonal transport was not significantly changed by treatment with ACTH/MSH(4-10), the ACTH(4-9) analog ORG 2766, hypophysectomy, or adrenalectomy. Fast axonal transport was unchanged in regenerating nerves and in regenerating, ACTH(4-10)-treated nerves. However, treatment with ORG 2766 in dosages of either 1 or 10 micrograms/kg/day IP for seven days significantly reduced (62% and 64%, respectively) the crest height of the fast axonal transport curve of intact sciatic nerve. The results suggest that the reported peptide-induced enhancement of nerve regeneration is not due to changes in the rate of fast axonal transport.     

Motors for fast axonal transport.

In neurons and other animal cells, membrane-bound vesicles course rapidly along cytoskeletal filaments to reach their destinations. Based on a variety of in vivo studies it is becoming clear that the microtubule-based motor, kinesin (and its relatives), drive vesicle movements in axons. Surprisingly, some axonal membranes have the capacity to move on both microtubules and actin filaments.     

Rapid axonal transport in Xenopus nerve in divalent cation free media

An investigation was made of the effects of bathing media low in divalent cations on rapid axonal transport in the sciatic nerve of the amphibian Xenopus laevis. The anterograde transport of a pulse of [35S]methionine proteins was observed using a multiple proportional counter as the detector. Organelles undergoing anterograde and retrograde transport were detected by light microscopy. The structure of nerve fibers was examined by light and electron microscopy. There was no significant difference in the anterograde transport of proteins in nerves bathed in normal medium (NM) containing millimolar Ca2+ and Mg2+ and in those bathed in calcium-free medium (CaFM) containing Mg2+. The anterograde transport of labelled proteins continued at a normal velocity in nerves bathed in divalent cation free medium (DCFM) for at least 14 h. DCFM did cause some alterations in protein transport: the ratio of the plateau (following pulse passage) to the peak radioactivity was increased, the pulse amplitude decreased more rapidly, and the label continued to arrive at the distal end of the nerve for greater than 16 h. Anterograde and retrograde organelle transport continued normally for periods of greater than or equal to 4 h in fibres bathed in DCFM. All myelinated fibres became distorted within 4 h in DCFM. Similar distortion was rare in fibres bathed in CaFM. The results indicate that axonal transport in Xenopus is largely independent of lowered concentrations of divalent cations in the bathing medium. Those alterations in axonal transport that were produced by DCFM may have been secondary to morphological changes in the nerve fibres.     

The synthesis and transport of lipids for axonal growth and nerve regeneration

Neurons are unique polarized cells in which the growing axon is often located up to a meter or more from the cell body. Consequently, the intracellular movement of membrane lipids and proteins between cell bodies and axons poses a special challenge. The mechanisms of lipid transport within neurons are, for the most part, unknown although lipid transport via vesicles and via cholesterol- and sphingolipid-rich 'rafts' are considered likely mechanisms. Very active anterograde and retrograde transport of lipid-containing vesicles occurs between the cell body and distal axons. However, it is becoming clear that the axon need not obtain all of its membrane constituents from the cell body. For example, the synthesis of phosphatidylcholine, the major membrane phospholipid, occurs in axons, and its synthesis at this location is required for axonal elongation. In contrast, cholesterol synthesis appears to occur only in cell bodies, and cholesterol is efficiently delivered from cell bodies to axons by anterograde transport. Cholesterol that is required for axonal growth can also be exogenously supplied from lipoproteins to axons of cultured neurons. Several studies have suggested a role for apolipoprotein E in lipid delivery for growth and regeneration of axons after a nerve injury. Alternatively, or in addition, apolipoprotein E has been proposed to be a ligand for receptors that mediate signal transduction cascades. Lipids are also transported from axons to myelin, although the importance of this process for myelination is not clear.     

Components of fast and slow axonal transport in the goldfish optic nerve

The composition of the fast and slow components of axonal transport in the goldfish optic nerve was investigated, using specific radioactive precursors injected into the eye. Tritiated glucosamine and fucose label macromolecules, presumably glycoproteins, which are rapidly transported from the eye to the optic tectum. Material labeled with these precursors is not evident in the slowly transported component. Glucosamine and fucose incorporation are blocked when a protein synthesis inhibitor, acetoxycycloheximide, is injected into the eye concurrently with the precursors. As well as labeling macromolecules, ³H-glucosamine and ³H-N-acetylmannosamine ( a precursor of sialic acids) also label rapidly-transported chloroform-methanol-extractable material which may contain transported glycolipids. Two procedures were used to show that the slow component of axonal transport contains tubulin, a protein characteristic of the microtubules:

• (a) Tracer doses of tritiated colchicine injected into the eye label a wave of radioactivity which moves 0.5 mm/day, the rate of slow axonal transport in the goldfish optic nerve. We believe this wave represents the movement of colchicine which is bound to colchicine-binding protein moving in the slow component of axonal transport.
• (b) Tritiated proline labels a slowly transported protein which is precipitated by vinblastine and has a mobility on polyacrylamide gels comparable to authentic tubulin. These results indicate that the fast and slow components of axonal transport each provide specific chemical substances to the nerve endings.