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Axonal transport in neurological disease.

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Axonal Transport in Neurological Disease
John W. Griffin, MD, and Danny F. Watson, MD, PhD
The axonal transport systems have a wide variety of primary roles and secondary responses in neurological disease
processes. Recent advances in understanding these roles have built on the increasingly detailed insights into the cell
biology of the axon and its supporting cells. Fast transport is a microtubule-based system of bidirectional movement of
membranous organelles; the mechanism of translocation of these organelles involves novel proteins, including the
recently described protein of fast anterograde transport, kinesin. Slow transport conveys the major cytoskeletal elements, microtubules, and neurofilaments. Several types of structural changes in diseased nerve fibers are understood in
terms of underlying transport abnormalities. Altered slow transport of neurofilaments produces changes in axonal
caliber (swelling or atrophy) and is involved in some types of perikaryal neurofibrillary abnormality. Secondary
changes in slow axonal transport-for example, the reordered synthesis and delivery of cytoskeletal proteins after
axotomy-also can produce changes in axonal caliber. Secondary demyelination can be a prominent late consequence
of a sustained alteration of neurofilament transport. Impaired fast transport is found in experimental models of distal
axonal degeneration (dying back). Retrograde axonal transport provides access to &e central nervous system for agents
such as polio virus and tetanus toxin, as well as access for known and hypothetical trophic factors. Correlative studies of
axonal transport, axonal morphometry, cytoskeletal ultrastructure, and molecular biology of cytoskeletal proteins are
providing extremely detailed reconstructions of the pathogenesis of experimental models of neurological disorders. A
major challenge lies in the extension of these approaches to clinical studies.
Griffin JW,
Watson DF. Axonal transport in neurological disease. Ann Neurol 1988;23:3-13
Neurologists deal daily with the consequences of axonal disorders affecting both the peripheral and the
central nervous systems. Acute axonal degeneration
(wallerian degeneration) occurs when axons are interrupted by infarction or trauma in the brain, spinal cord,
and peripheral nerves. Chronic axonal degeneration
occurs in a wide variety of metabolic disorders, hereditary defects, and toxic insults. Clinical experience with
axonal disease led early neurologists and neuropathologists to the concept that there is something special about the design of the neuron that makes its axon
vulnerable to such a broad spectrum of insults. One
classical hypothesis to explain this vulnerability has
been borne out by modern experiments: the axon depends upon the continuous delivery of materials from
the cell body via active intraaxonal transport mechanisms. Within the last few years there has developed a
firm experimental basis for the concept that defects in
the transport of materials within nerve fibers correlate
with, and in some instances underlie, disease processes. Here we summarize the current understanding
of the transport systems and focus on the role of transport abnormalities in a variety of neuropathological
processes, including disorders of axonal caliber, distal
From the Johns Hopkins University School of Medicine, Department of Neurology, Neuromuscular Division, Baltimore, MD.
Received Dec 17, 1986, and in revised form July 27, 1987. Accepted for publication July 30, 1987.
axonal degeneration or “dying back,” and nerve compression and ischemia.
Axonal Renewal by Active Transport
An understanding of the biology of the axon depended
on the demonstration by Waller 1801in 1852 that transection of a peripheral nerve resulted in degeneration
of the distal stump but survival of the proximal stump.
The basic principles of axonal design are now so widely
cited as to be axiomatic: First, the axon can be relatively large in relation to the size of the nerve cell
body. In the long fibers of the sciatic nerve the volume
of axoplasm may exceed that of the cell body by more
than a thousandfold. Second, with rare exceptions, the
axon lacks ribosomes and hence is incapable of synthesizing substantial amounts of protein. Third, while
some transfer of macromolecules from the ensheathing
cells to the axon can occur [23}, the materials transferred appear to be specialized and limited in amount,
so they are insufficient to sustain survival of the axon
distal to a transection. The consequence of these principles is that the axon is dependent on the continuous
delivery of essential materials from the nerve cell
body; complete interruption of the delivery of these
Address correspondence to Dr Griffin, The Johns Hopkins University School of Medicine, Department of Neurology, Neuromuscular
Division, 600 North Wolfe St, Meyer 5-119, Baltimore, MD
Copyright 0 1988 by the American Neurological Association 3
Fig 1. Architecture of normal axoplasm. Cytoskektal elements
include longitudinally oriented microtubules (MT) and
neurofilaments (NF). These structures, together with associated
proteins (seen as adherent ‘tfuzz”in this image), are renewed by
slow transport. Vesichs (V)are conveyed Sy fast transport.
Mitochondria (MITO) undergo occasional saltatory movements,
with net transport in either direction.
materials, by mechanical or pharmacological means, results in prompt wallerian degeneration. This dependent axon should not be regarded as a defective cellular design, but rather as an adaptive design that allows
integrated responses of the entire neuron in development and regeneration.
The axon is cytologically simple (Fig 1). The organelles that must be renewed by axonal transport are
consequently limited, and include small numbers of
particulate organelles (mitochondria and smooth membrane-bound tubules and vesicles) and the axolemma.
Most of the cross-sectional area of an axon is occupied
by the longitudinally oriented cytoskeletal elements,
the microtubules (MT) and neurofilaments (NF).
There are abundant crosslinks between these cytoskeletal elements. The recent advent of rotaryshadowed, freeze-etched preparations has provided
dramatic three-dimensional images of the axonal cytoskeleton and its innumerable crosslinkers 135, 65, 773.
The Axonal Transport Systems
Phases of Transport
Nearly 100 years after Waller, Paul Weiss and Helen
Hiscoe 182) demonstrated that gentle nerve compression produced an accumulation of axoplasm proximal
to the compressed segment, and concluded that this
accumulation resulted from interruption of an ongoing
axoplasmic transport. Rapid progress in analyzing axonal transport began with the use of radioisotopic
markers to follow the movement of materials within
4 Annals of Neurology Vol 23 No 1 January 1988
Fig 2. Phases of axonal transport. Proteins synthesized in the
cell bodies at time zem (as might be kzbeled by a pulse of radioactive amino acids) appear as successive waves in peripheral axons.
At time A, fast anterograde axonal transport accounts for the
jirst wave and intraaxonal vesicles are prominently labeied. By
time 8,the main wave Dfanterograde transport bas passed, and
retrograde transport of larger vesicles accounts for a large part of
the label. At time C, there is little movement of labeled material,
andproteins deposited in the axokmma by earlier phases o f
transport are the chief labeled structures. S l w transport awives
lp time D, with heavy labeling of the microtubules and
nerves, as pioneered by Sidney Ochs (for review, see
154)). These tracer experiments soon indicated that
intraaxonal transport is divided into slow and fast
phases. In the anterograde direction (away from the
cell body), the slow transport system conveys the bulk
of the axoplasmic constituents, including the cytoskeletal elements. In contrast, the fast transport system
carries small vesicles and other particulate organelles
[20, 761. Fast transport is bidirectional; a substantial
fraction of the membranes delivered to the nerve terminal is returned to the cell body 141. This retrograde
transport carries primarily prelysosomal organelles
1761 at maximum rates approaching that of fast anterograde transport. Figure 2 summarizes the rates, directions, and constituents of these phases of axonal
Fast Transport
landmark advance in the understanding of axonal
transport is the recent identification of the molecular
mechanisms that propel fast anterograde transport.
Vale and associates 178, 791, and Schnapp and colleagues [661 have isolated a protein, kinesin, from
neural tissues that is capable of translocating endogenous axoplasmic vesicles, or even exogenous latex
beads, along single MTs. The property of kinesin that
permitted its isolation was its propensity to form a
tight complex with MTs when exposed to a nonhydrolytable analogue of ATP. Removal of the analogue
and restoration of ATP released kinesin from the MTs
and restored translocation of vesicles. A protein with
many properties similar to squid kinesin has been
identified in mammalian brain (781, strongly supporting the hypothesis that kinesin represents a generally
occurring family of proteins involved in translocation
of vesicles within cells. There must be more than one
translocator molecule in axons, as kinesin observes
strict polarity of translocation along MTs from the
biochemically defined “minus” end to the “plus” end
There has been controversy as to which part of the
smooth membrane system of the axon undergoes rapid
transport. Using focal cooling of nerve segments,
Tsukita and Ishikawa [76} and Ellisman and Lindsey
{201 have recently shown that the principal transport
vehicles are 50- to 80-nm vesicles. The cisternae and
elongated tubular elements of smooth endoplasmic reticulum in axom appear not to be rapidly transported.
Rapidly transported constituents are inserted into the
axolemma, both at the synaptic terminal and along the
course of the axon (32, 72).
proportion of the membrane delivered to nerve terminals is returned by retrograde transport to the nerve
cell body, perhaps after undergoing cycles of insertion
into the plasmalemma and subsequent endocytosis.
We consider that one of the important roles of the fast
anterograde transport systems is to provide sufficient
flux of membrane into the nerve terminal to allow for
the existence of retrograde transport. Retrograde
transport normally conveys back to the cell body samples of the synaptic milieu. Some of the nonneuronal
materials carried back in this fashion are of biological
importance; for example, nerve growth factor reaches
developing sympathetic nerve cell bodies via retrograde transport 1341. Other retrogradely transported
materials are of clinical importance: Polio 1411, herpes
simplex 1451, and rabies 1741 viruses reach the central
nervous system in this fashion. Similarly, tetanus toxin
reaches the central nervous system by binding to
specific ganglioside receptors in nerve terminals, then
undergoing endocytosis and retrograde transport in
vesicles {60].Within the nerve cell body it avoids degradation and is passed transynaptically to the inhibitory
terminals surrounding the cell body and proximal dendrites (60, 671, where it prevents release of inhibitory
Slow Transport
Slow transport moves materials exclusively in the anterograde direction. In studies following the transport
of pulse-labeled materials from the nerve cell bodies
down the axon, a wave of radioactivity is found. This
wave moves at a rate of 0.2 to 3 m d d ay , depending
on the nerve and age of the animal 1393. Slow transport conveys two major groups of proteins, termed
SC, and SCb by Lasek and Hoffman {38, 461. The
propulsive mechanisms remain unknown.
Axonal Pathology and Axonal Transport
A decade ago, as simple ligature and isotopic kinetic
data were first generated in animal models of neurological disease, there was widespread expectation
that severe defects or blockade of axonal transport
might explain a number of disorders. In retrospect, it is
clear that sustained blockade of axonal transport
should result in prompt axonal degeneration, and that
in slowly progressive disorders selective or partial defects should be expected. In the past five years there
has been a resurgence of interest in axonal transport, as
appropriate techniques have shown selective transport
defects in a variety of experimental models. Most studies to date have examined the peripheral nervous system, but the resulting concepts almost certainly apply
to diseases of the central nervous system.
Slow Transport and Disorders of Axonal Caliber:
The Cytoskehton as a Determinant of Axonal Caliber
The pathogenetic roles of transport abnormalities and
the transport alterations produced by neuronal disease
are better understood for slow transport than for fast
transport, either anterograde or retrograde. This is at
first surprising, in view of the current ignorance of the
slow transport mechanism and the recent insights into
the mechanisms of fast transport. However, in studies
of pathogenesis this disadvantage is counterbalanced
by the simplicity of composition of the slow component, and the ease in relating changes in the kinetics or
composition of slow transport to consequent structural
changes in the axon. Because the major components of
slow transport are cytoskeletal organelles, alterations
in slow transport produce cytoskeletal abnormalities,
often reflected in changes in caliber.
The axonal organelle that most directly influences
axonal caliber is the NF. Morphometric studies of myelinated axons in rats have shown that NF numbers
correlate closely with cross-sectional areas of myelinated axons (22,273. In contrast, there is relatively little
increment in MT numbers with increasing axonal area
(except in very small NF-poor axons (561). Such evidence suggests that a major function of NFs is to occupy space. This is not a trivial role; by occupying and
organizing axoplasmic space, NF content is the primary determinant of axonal caliber. Axonal caliber in
turn influences basic physiological properties of nerve
fibers, including conduction velocity, excitability, and
extent of myelin formation. It is not entirely flippant to
suggest that in normally myelinated nerves conduction
velocity is determined by, and is a measure of, NF
content in the largest nerve fibers.
Neurological Progress: Griffin and Watson: Axonal Transport 5
From the foregoing, it is apparent that local accumulations of NFs will produce axonal swelling, whereas
depletion will result in axonal atrophy. NF content in a
given segment of an axon can be altered by changes in
the abundance of N F within the slow component, as a
result of altered perikaryal synthesis, or by alterations
in the transport kinetics of NFs.
Slow Transport and Altered Synthesis of NFs:
Somatofugal Axonal Atrophy
That the proximal stump of a transected axon undergoes reduction in caliber has been known for nearly
100 years. The mechanism of this type of axonal atrophy has been demonstrated in a series of studies by
Paul Hoffman and co-workers 136, 371. Axotomy of
rat sciatic nerves produces a prompt reordering of
perikaryal transcription, with reduction in NF messenger RNA (&A)
and increases in mRNA for tubulin and actin. The alterations in transcription result in
parallel changes in the synthesis of cytoskeletal proteins. The slow component of axonal transport after
axotomy contains a reduced proportion of NF proteins
and an increased relative abundance of tubulin, actin,
and the SCb proteins. These alterations may reflect the
importance of tubulin and actin in axonal sprout outgrowth. The rate of transport of these constituents appears to place an upper limit on the rate of sprout
elongation [831.
The structural consequence of this reordered cytoskeletal synthesis is delivery to the axon of a slow
component containing reduced numbers of NFs that
are arranged with a normal density. The cross-sectional
axonal area decreases in proportion to the reduced N F
content, and the atrophic segment extends distally at
the rate of slow transport as the NF-poor slow component passes down the axon. This process, termed
somatofagal axonal atrophy, appears to be a very common neuropathological change in axonal disorders. A
similar sequence of somatofugal atrophy has been documented early in the course of the distal axonal degenerations produced by acrylamide 1251 (see below). The
signal from the periphery that initiates this sequence is
unknown. Interruption of axonal connection to its normal target is a sufficient stimulus; with reinnervation
normal N F synthesis is restored and axonal caliber returns toward normal. However, interruption of the
axon is not a necessary antecedent of somatofugal atrophy; atrophy can be induced in sensory fibers of
acrylamide-treated animals well before distal axonal
degeneration is present (251. In motor nerves, administration of botulinum toxin is sufficient to induce
somatofugal atrophy (Gold BG, et al: Unpublished
data). Botulinum prevents release of acetylcholine
quanta and thereby induces muscle disuse, but it does
not lead to degeneration or loss of synaptic terminals.
6 Annals of Neurology Vol 2 3 No 1 January 1988
Defects in Slow Transport: Maldistribution of NFs
AXONAL SWELLING. Even when perikaryal synthesis
of NFs remains normal, alterantions in intraaxonal
transport of NFs can cause changes in axonal caliber as
a result of abnormal distribution of NFs along the
axon. In principle, a segment of axon with impaired
NF transport should experience a net increase in NFs,
as more NFs enter the segment than leave it in any
given period. Such N F accumulation has been produced in several experimental settings.
Many recent studies of slow transport have focused
on neurofilamentous disorders of the peripheral nervous system, particularly the giant axonal neuropathies. Giant axonal neuropathies, characterized by focal or multifocal accumulations of NFs somewhere
along the axon, occur in a hereditary form {2, 421 and
after exposure to several toxins, including carbon
disulfide (681 and 2,5-hexanedione (HD), the toxic
metabolite responsible for glue-sniffing neuropathy
170). A particularly dramatic type of giant axonal
neuropathy, p,p'-iminodipropionitrile (IDPN) neurotoxicity, was identified 20 years ago by Chou and Hartmann {I 1, 12). This agent is now known to impair NF
transport severely all along the axon [30, 311. Nearly
identical transport changes have recently been produced by 3,4-dimethyl-2,5-hexanedione (DMHD)
(261, a potent analogue of HD. Other slow component
constituents and fast transport are much less severely
affected by these agents, and recently truly selective
defects in N F transport have been achieved with
IDPN 1571.
The biochemical abnormahty responsible for the NF
transport defect in these disorders is a matter of speculation, but a possible structural basis has been described. IDPN, administered either systemically 129,
551 or locally along the nerve 1281, produces a reorganization of the cytoskeleton in which MTs cluster in
the center of the axon and NFs are segregated into a
subaxolemmal ring (Fig 3). Is is likely that this reorganization of the cytoskeleton reflects loss of interaction of NFs with MTs and other axonal organelles, and
underlies the failure of normal translocation of NFs.
HD produces a similar reorganization of the axoplasm
[28,841. Of particular interest are the recent observations that analogous defects in integrating intermediate
filaments into the cytoskeleton can be identified in
fibroblasts and other nonneural cells in heritable giant
axonal neuropathy 117, 591 and in cells cultured in the
presence of HD 1171. These studies suggest that the
abnormalities in cytoskeletal organization are generalized cellular changes.
Systemic administration of appropriate doses of
JDPN or D M H D [11 produces neurofilamentous
swellings in the most proximal axon. This location
reflects the continued delivery of newly synthesized
NFs from the perikarya into axons unable to carry
Fig 3 . CytoskeietaL reorganization after @,p’-iminodipropionit d e (IDPN) administration.Endoneuriai injection of IDPN
near this axon has caused dissociation of microtubulesfrom
neurofilaments.Labeled fast-transport material demonstrated by
autoradiography is associated oniy with the microtubie domain.
The loss of interaction of the newoflaments with the microtubies
may account for the d&ct in newoflament transport. (Modzj5ed
from GrifFnN,
et al {29}, with permissionfrom Journal of
them distally. In principle, axonal swellings may develop at any location where a local impairment in
transport produces a mismatch between delivery and
transport from the segment. The fate of these swellings
when more normal transport resumes is intriguing {9].
For example, after a transient block of N F transport
produced by a single injection of IDPN, the proximal
axonal swellings begin migrating distally down the
nerve at a rate approaching 1 d d a y (in young rats).
As noted above, these results recall the observations of
Weiss and Hiscoe 1821 after the release of mechanical
constriction of nerves, and suggest that their pioneering studies in fact demonstrated axoplasmic transport
and not simply regeneration, as recent reassessments
have concluded.
Monaco and co-workers 1521 have recently described a distinctive alteration in axonal transport
within the optic nerves of rats intoxicated with HD
and carbon disulfide. N F transport was substantially
increased in velocity. This change appears to reflect
another type of direct cytoskeletal toxicity that can
contribute to proximal axonal atrophy and distal neurofilamentous swelling.
Two experimental neuropathies provide an instructive contrast with these predominantly neurofilamentous neuropathies. Medori and associates f49, 501 have
found a mild defect in slow transport in experimental
diabetes in the B-B Wistar rat and in streptozotocininduced diabetes. This defect is more severe for SCb
than SC,, but there is nonetheless a retardation of NF
transport. The structural correlate is enlargement of
the proximal axon and decrease in caliber of the distal
axon. The second experimental model is acrylamide
toxicity, the prototype of a distal axonal degeneration.
As discussed below, acrylamide has profound effects
on fast bidirectional transport in the distal axon.
Neurofilamentous axonal swellings are a variable feature. Gold and co-workers 124, 253 have recently
shown that high single doses of acrylamide can produce early proximal neurofilamentous swellings. These
swellings appear to result from a modest impairment
of slow transport. Unlike the other models discussed,
the defect in slow transport is nonselective, with tubulin and other constituents as severely retarded as the
N F proteins. As described above, this phase of proximal swelling is supplanted by somatofugal axonal atrophy, reflecting a reordering of perikaryal cytoskeletal protein synthesis 1251.
In addition to the neurofilamentous neuropathies,
neurofibrillary changes are prominent in a variety of
degenerative diseases, including Alzheimer’s disease
and at least some types of motor neuron disease {7,
151. Recent studies have asked whether defects in axonal transport might be present in models of perikaryal
degeneration with neurofibrillary changes. For example, aluminum produces N F accumulations in neuronal perikarya and proximal axons. Administration of
aluminum can retard N F transport {5, 737 and reduce
the relative amount of N F protein in the slow component, although modest NF accumulations can be induced without detectable changes in transport 1431.
The details of the transport changes, the distribution of
the lesions, and the fact that cytoskeletal reorganization of the IDPN type cannot be reproduced by aluminum all indicate that aluminum differs from the group
of agents described above. Thus it appears that aluminum exerts a distinctive toxic effect on the neuronal
cytoskeleton, but that it invokes the common pathway
of impaired N F transport. Perhaps the prominence of
neurofibrillary masses (compared to, for example, MT
accumulations) in a wide variety of disorders reflects
the unique inability of NFs to disaggregate into soluble
subunits; hence, NFs may accumulate as a consequence of selective defects in N F transport or nonspecific effects on the axonal transport of all cytoskeletal
ATROPHY. Atrophy is a feature of many
chronic neuropathies, including Friedreich’s ataxia,
Charcot-Marie-Tooth disease (hereditary motor-sensory neuropathy I and II), and uremic neuropathy
118). Atrophy has important implications for myelinforming cells as well as axons. From a pathogenetic
standpoint, axonal atrophy appears to represent the
obverse face of neurofilamentous axonal swelling.
Chronic axonal atrophy correlates with a reduction
both in numbers of NFs and in delivery of NFs by
slow transport. As noted, most types of axonal atrophy
Neurological Progress: Griffin and Watson: Axonal Transport
probably reflect somatatofugal atrophy caused by reduced perikaryal NF synthesis 125, 36, 377 or accelerated NF transport 1521.
In some disorders, delivery of NFs to the distal axon
is reduced as a consequence of an “upstream” impairment in transport. In the partial nerve compression
described by Weiss and Hiscoe [82), axoplasmic renewal in the distal axon is impaired by mechanical factors; N F density remains normal in the distal axon in
such preparations, but N F numbers are reduced 1211.
In the IDPN model, axonal atrophy in the distal axon
results from sustained pharmacological impairment of
NF transport 1141. Bizzi and associates 151 have recently shown that the maldistribution of NFs produced
by aluminum results in atrophic segments of the axon
just distal to the neurofilamentous masses.
common factor in several diseases and experimental
models in which demyelination appears to be a secondary response of the myelin-forming cells to primary
axonal changes. The seminal observations came from
morphometric studies by Dyck 1181. Studying a variety of neuropathies, including Friedreich‘s ataxia,
Charcot-Marie-Tooth disease, and uremic neuropathy,
Dyck identified nonrandom demyelination: multiple
demyelinated internodes were present on some nerve
fibers, and none on others. This pattern suggested an
axonal influence. These clinical studies suggested that
axonal atrophy is a common feature of neuropathies in
which secondary demyelination occurs.
Three experimental models have been reported that
allow detailed examination of the myelin changes during axonal atrophy. Axotomy, as indicated above, reduces the amount of NF protein carried down the
proximal stump and results in a reduction in the caliber
of fibers in the proximal stump 119, 371. The fibers
have a crenated appearance in cross-section and within
twelve months begin a sequence of paranodal demyelination followed by segmental or internodal secondary
demyelination and formation of new shorter internodes {19}. More rapid demyelination was found by
Baba and co-workers 133 in the surviving axons distal
to a vigorous nerve compression. In IDPN neuropathy
similar changes occurred in the distal (atrophic) regions
1271. In addition, in the proximal (swollen) regions
repeated demyelination and remyelination were found,
resulting in onion bulbs or redundant Schwann cell
processes characteristic of hypertrophic neuropathies
1313 (Fig 4). These changes appeared to be the result
of initial axonal enlargement, which stimulated one cycle of Schwann cell division and an increase in the
amount of myelin in the sheaths, followed by a relative
decrease in caliber. At this stage the amount of myelin
was excessive for axonal caliber, even though the axon
was still larger than normal, and remodeling of myelin
8 Annals of Neurology Vol 23 No 1 January 1988
occurred. Over time, repetition of this sequence resulted in major onion bulb formation, comparable to
those found in the heritable hypertrophic neuropathies
of humans. Simple inspection of the proximal roots at
this stage could give the misleading impression of primary demyelinating changes. Such experimental observations support the possibility that even some hypertrophic demyelinating neuropathies might represent
Schwann cell responses to primary axonal disease.
Fast Axonal Transport and Distal Axonal
Degeneration: Toxic lmpaimzent of Fast Transport
Many of the peripheral nerve diseases commonly encountered in clinical practice, including nutritional deficiencies, uremia, most toxic neuropathies, and some
diabetic and heritable neuropathies, are characterized
by selective distal axonal degeneration. Distal axonal
degeneration or dying back is defined by the early
involvement of the longest axons and progression of
the degenerative process from distal to proximal with
time 18, 711.
In the most extensively studied experimental model
of distal axonal degeneration, acrylamide neuropathy,
Schaumburg and associates {64} recognized that the
early ultrastructural changes in the distal axon are multifocal rather than being restricted to the nerve terminal. Within the abnormal axonal regions, collections of
particulate organelles are prominent [lo, 13, 331.
Chretien and co-workers {13} showed that these regions represented collections of rapidly transported organelles, reflecting focal breakdown or retardation of
transport. Kinetic studies showed that acrylamide impairs fast bidirectional transport in the distal axon.
Sahenk and Mendell 1631 and Jakobsen and Sidenius
{40} demonstrated delayed turn-around and retrograde transport of anterogradely transported proteins
in the sciatic nerves of rats with acrylamide neuropathy. Acrylamide impairs axonal transport before structural degeneration of the axon occurs; recent studies
by Miller and co-workers {5l} demonstrated that a
single dose of acrylamide retarded retrograde axonal
transport of exogenous markers. The degree of impairment was dose dependent and was found within hours
of administration of acrylamide. These studies indicate
that acrylamide can dramatically impair axonal transport within the distal axon and that the changes are
much more severe than those in the proximal axon,
which include inconsistent abnormalities and modest
retardation of slow transport 125). The transport defect thus appears to be predominantly distal. The
biochemical basis for the transport defect in acrylamide
neuropathy is not yet resolved, but several lines of
evidence have suggested defects in energy metabolism
1691. The transport defect is likely to contribute to (or
may be the cause of)-the subsequent distal axonal degeneration.
Fig 4. Onion bulb formation after p,p’-iminodipropionittrile
(IDPN) administration. Axonal swelling induced IDPN,
followed Sy a return toward normal caliber in some axons, has
led to acute demyelination (A). Arrows indicate intact axons
within demyelinatingfibers. At later stages of demyelination,
proliferated Scbwann cell processes form concentric hyers around
axons (B). The changes in Schwann cells are restricted to regions
of a nerve that undergo swelling and are dependent on this axonal injuence, rather than a direct dfict of IDPN on Schwann
Another agent that produces abrupt changes in axonal transport in the distal axon is N-3-pyridylmethyl
N‘-p-nitrophenylurea (PNU), a rodenticide that was
briefly marketed as Vacor. PNU is a nicotinamide antagonist that produces pancreatic islet cell destruction
and neurotoxicity. Human ingestion of PNU in suicide
attempts produced a dramatic clinical picture characterized by diabetes mellitus and a rapidly evolving sensory and autonomic neuropathy 1471. Studies in our
laboratory have shown that PNU produced a remarkably rapid distal axonal degeneration; rats developed
distal weakness as early as 6 hours after a single dose.
Within 24 to 48 hours wallerian-like degeneration affected the distal regions of sensory and motor fibers.
Axonal transport studies showed a failure of anterograde fast transport, which was restricted to the distal
regions of nerve fibers {8l}. The velocity of fast transport, the amount of label accumulating at ligatures, and
the turnaround of fast transport were all completely
normal when measured in the midthigh region of the
sciatic nerve. By contrast, autoradiography of intramuscular nerve twigs and neuromuscular junctions of
intrinsic muscles of the hind feet showed that virtually
no transported material reached the terminal axons
{Sl]. A more modest impairment of transport between the thigh and foot was found in the posterior
tibial nerve. The explosive abnormdties produced in
the distal axon by PNU could be prevented by administration of nicotinamide, indicating that nicotinamide
plays an important role in axonal maintenance. These
studies suggest the hypothetical sequence of toxic impairment of intermediary metabolism, reduction in
supply of high-energy compounds for the fast axonal
transport system, and degeneration of the distal axon
secondary to failed delivery of the transported material.
Fast Transport: Compression and Iscbemza
Compression of nerve fibers produces reversible physiological dysfunction as a result of ischemia and may
induce more prolonged abnormalities as a result of
nerve fiber distortion {53). More severe acute or sustained pressure results in axonal degeneration. Weiss
and Hiscoe {82} documented the effects of sustained
pressure from nerve compression, and recent studies
by Baba and co-workers 13) and Krarup and associates
{44) have reexamined the effects of vigorous nerve
Neurological Progress: Griffin and Watson: Axonal Transport 9
Fig 5 . Papilledema induced by p,p'-iminodipropionitrile.Axonal swellings occw chiefly as optic nerve (ON) axons approach
the lamina crihsa (arrow). These swellings may elevate the
optic disk (D), causingfrank papillectema. Such axonal swellings ulltvastructurally contain accumulations of neurofilaments,
comparable to the changes in peripheral nerve axons. (Courtesy of
Dr lrma Parhad.)
compression by ligatures. In these studies a large proportion of the axons underwent prompt wallerian degeneration; those fibers that survived showed reduced
cahber and distal axonal degeneration. Dahlin and associates 1161 have recently evaluated the levels of pressure sufficient to induce abnormalities in fast axonal
transport. They found that the relatively modest pressure of 30 mm Hg for 2 hours was sufficient to impair
fast transport; recovery occurred within 24 hours. Indirect arguments suggested that nerve ischemia was
likely to underlie this reversible transport defect. It is
interesting that the pressures found within the carpal
canal of patients with carpal tunnel syndrome were in
the same range {16].
Papilledema appears to be a special instance of pressure-related axonal transport abnormalities. Swelling
of the optic nerve head is the funduscopic manifestation of axonal swelling within optic nerve fibers on
the ocular side of the lamina cribrosa. That axonal
swellings can contribute to or underlie papilledema has
been known from classic silver staining studies of human eyes [58]. Recently, experimental models have
been developed to allow investigation of the pathogenesis of the optic axon swelling. In models of both increased intracranial pressure [75] and decreased intraocular pressure {62), accumulations of transported
organelles and papilledema have been induced. Parhad
and co-workers [ 5 7 ) have shown that papilledema of a
special type, related to impairment of slow axonal
transport, develops in animals given IDPN, the
blocker of N F transport described above (Fig 5). The
optic nerve head is the site of preferential accumulation of NFs, perhaps because the lamina cribrosa acts
as a mechanical barrier to more distal passage of
10 Annals of Neurology Vol 23 No 1 January 1988
neurofilamentous masses. In this model the combination of a toxic impairment of transport and mechanical
factors appears to determine the site of NF accumulation.
Another funduscopic finding, cytoid bodies, provides a further opportunity for clinicians to visualize
directly the structural consequences of changes in axonal transport. In the setting of multifocal retinal ischemia, cytoid bodies develop as axonal swellings in the
nerve fiber layer. Under experimental conditions,
these swellings appear within hours after focal retinal
lesions. McLeod {48] has shown that these swellings
contain massive numbers of rapidly transported organelles that accumulate at the severed ends of interrupted optic axons.
A major theme of this report is that selective alterations in axonal transport produce predictable patterns
of axonal abnormality. Experimental models have
shown that impairment of slow transport along the
course of an axon leads to N F accumulation and axonal
enlargement proximal to the site of impairment, and to
downstream distal axonal atrophy. Axonal atrophy
caused by reduced NF content can also be produced
by selective reduction in N F synthesis and delivery to
the initial segment of the axon. Such somatofugal atrophy follows axotomy and is an early indicator of
axonal disease in a variety of other disorders. These
alterations in slow transport produce distortions of axonal size and shape but need not lead to nerve fiber
breakdown and disappearance. In contrast, sustained
impairment of fast axonal transport leads to degeneration of the distal axon. Focal impairment of fast transport leads to local accumulation of vesicular organelles
at the site of impairment.
These patterns are sufficiently consistent to suggest
involvement of specific transport processes in human
diseases in which comparable structural alterations are
found. For example, in reduced NF content and axonal
atrophy found in Charcot-Marie-Tooth disease (hereditary motor-sensory neuropathy I) probably reflects abnormally low synthesis and transport of NF proteins.
Such issues will shortly be addressed in human autopsy
material by the use of cDNA probes to assess perikaryal mRNA content for proteins of interest {36J.
In many chronic human neuropathies, complex axonal abnormalities are found with varying degrees of
distal axonal degeneration, atrophy, focal swelling, and
secondary demyelination. Selective alterations in specific phases of axonal transport are unlikely to explain
such disorders; rather the disease process (for example,
diabetes) may simultaneously impair several nerve
metabolic functions. In this situation, any early and
selective alterations of axonal transport are complicated by layers of secondary changes in axonal struc-
ture, axonal transport, and the associated reactions of
Schwann cells.
More detailed understanding of the alterations in
axonal transport and human neurological diseases will
require studies of transport in humans, or at least in
biopsied human tissue. Brimijoin [GI has pioneered
such studies using accumulation of endogenous transported enzymes in human neuropathies. Observation
of particle movement in nerve biopsies has attained
new impetus from the availabiiity of markedly improved imaging systems and computerized mechanisms for measuring and storing velocities of the particles. Finally, the recent identification of the molecules
that convert metabolic energy into mechanical propulsion for fast transport should permit assays of the
abundance and activity of these molecules in human
diseases. The outlook seems bright for substantially
extending our understanding of the pathogenesis of
axonal disease, from molecular processes to the final
clinical expression.
Supported by NIH Javits Award RO1-14784 and PO1 22849. Dr
Watson is supported by an NINCDS Teacher-Investigator Award
NS00983. We thank our collaborators, Drs Paul Hoffman, Bruce
Trapp, Irma Parhad, and Donald Price.
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