Changes in neurofilament transport coincide temporally with alterations in the caliber of axons in regenerating motor fibers

Neurofilament elongation into regenerating facial nerve axons

Immunocytochemistry was used to show that neurofilaments advance into regenerating facial nerve axons at 2.5 mm/day, which is less than the rate of axonal elongation (4.3 mm/day), measured from the transport of radiolabeled protein into the axons. Thus, the distal region of the newly-regenerated axons is deficient in neurofilaments, and this was confirmed by electron microscopy. These neurofilament-free regenerating axons could also be detected by immunocytochemistry using antibody to protein B50 (GAP43), a component of growth-cones. Immunoblots of nerve segments, incubated with monoclonal antibodies against the three neurofilament proteins, showed that all three proteins were present in the neurofilaments elongating into the regenerating axons, and confirmed the more distal extensions of B50 immunoreactivity. These results show that neurofilament immunocytochemistry underestimates the extent of axonal regeneration, and it is suggested that this technique should be employed with caution in regeneration studies. When the facial nerve received a conditioning lesion 7 days prior to a test lesion, axonal regeneration rate increased to 6.0 mm/day, and there was a proportional increase in neurofilament elongation rate to 4.4 mm/day. This occurred in spite of the reduction in cell body neurofilament protein synthesis induced by the lesions. It is concluded that the rate of neurofilament extension into regenerating axons is not governed by cell body synthesis but by local interactions with other cytoskeletal materials which support the increased regeneration rate of conditioned axons.

Immunocytochemical localization of the intermediate filament protein peripherin in adult mouse adrenal chromaffin cells in culture

Peripherin is the main intermediate filament protein in sympathetic neurons. Immunoreactivity to peripherin was studied in mouse adrenal chromaffin cells after 6 days in culture, and compared to immunoreactivity to tyrosine hydroxylase used as a general marker of chromaffin cells in culture. Most of the cells immunoreactive to tyrosine hydroxylase were rounded, with a glandular phenotype and a few of them had processes. The cells reactive to peripherin only constituted a small proportion of the chromaffin cells (2%), and most of them sent out processes. However, not all the cells with processes were reactive for peripherin. These results did not change in the presence of nerve growth factor. The discussion focuses on the significance of the sub-population of cells reactive to peripherin. We suggest that these cells resemble the small granule chromaffin cells, regarded as an intermediate cell type between glandular cells and neurons. The cells that expressed peripherin here are compared to those selected to form the PC12 clone. The presence of peripherin in only a few of the cells sending out neurite-like processes is discussed in relation to the expression of other neurofilament proteins in developing cells and to the influence of non-chromaffin cells.

Neurofilament gene expression following beta,beta'-iminodipropionitrile (IDPN) intoxication

beta,beta'-Iminodipropionitrile (IDPN) is an agent that produces a disorganization of the axonal cytoskeleton with massive accumulation of neurofilaments in the proximal axon. Abnormalities in axonal transport of neurofilament proteins and in their phosphorylation occur in this model. In this study we evaluated the gene expression of neurofilament and other cytoskeletal components at an early, intermediate and late stage of intoxication to determine whether this neuropathy is directly due to or secondarily affects the expression of these components. Specific cytoskeletal mRNA expression was evaluated in the spinal cords of rats treated with IDPN for varying durations using Northern analysis and in situ hybridization. Our results show no qualitative or quantitative alteration in the mRNA expression of the neurofilament triplet, alpha-tubulin, alpha-actin or glial fibrillary acidic protein. We conclude that abnormalities at various stages of cytoskeletal processing such as the early disorganization of the cytoskeleton, the impairment of neurofilament transport, and the long-term redistribution of neurofilaments along the axon are not directly due to, nor do they affect the gene expression of cytoskeletal components in IDPN neuropathy.

Axonal sprouting after botulinum toxin does not elicit a histological axon reaction

In an attempt to determine which elements of the axon reaction are essential for early axonal outgrowth, axonal sprouting was induced with botulinum toxin (BoTx) and the nerve cell body changes compared with those accompanying axonal growth after nerve trauma. Anterior horn cells of mice were examined histologically at times ranging from 3 days to 3 weeks after either BoTx hindlimb injection or sciatic nerve crush. After sciatic nerve crush there was dispersion of Nissl substance, increase in cell body size, and an increase in neurofilament protein staining. None of these changes were found after BoTx-induced terminal axonal sprouting, suggesting that these morphological features of the axon reaction are not essential for early axonal outgrowth.

Neurofilament and tubulin expression recapitulates the developmental program during axonal regeneration: induction of a specific beta-tubulin isotype

We examined the differential expression of genes encoding three beta-tubulin isotypes (classes I, II, and IV) and the 68-kDa neurofilament protein (NF68) in rat sensory neurons during development, maturation, and axonal regeneration. Expression of the specific beta-tubulin gene encoding the class II isotype was induced to high levels during development and axonal regeneration, whereas the expression of genes encoding the two other isotypes (classes I and IV) remained comparable to mature levels. Conversely, expression of the NF68 gene was relatively low during development and regeneration. Thus, the developmental program for cytoskeletal gene expression is recapitulated during axonal regeneration. The high level of class II beta-tubulin expression found in developing and regenerating neurons occurs during the longitudinal growth of axons. In contrast, induction of NF68 gene expression is associated with the radial growth of axons in maturing neurons.

mRNA levels of all three neurofilament proteins decline following nerve transection

The control of neurofilament (NF) protein gene expression was studied by determining and comparing the levels of mRNA to the heavy (NF-H), mid-sized (NF-M) and light (NF-L) NF protein subunits in rat dorsal root ganglia (DRG) following sciatic nerve transection. mRNA to NF-H (4.5 kb), to NF-M (3.4 kb) and to NF-L (2.5 and 4.0 kb) were identified in Northern blots and quantitated in dot blot analyses, using specific cDNA probes for each NF protein. Following transection and continuing for at least 28 days. The early and co-terminal fall in mRNAs suggests that the 3 NF genes are regulated by common factor(s) and that the function of these factor(s) is influenced by the state of axonal continuity with the target organ.

Quantitative evaluation of axonal regeneration by immunochemical assay for neurofilament protein

In experiments on nerve regeneration requiring assessment of the rate and extent of axonal outgrowth, the availability of a simple and accurate method of quantification would be extremely useful. We approached this issue by modifying the conventional ELISA procedure so as to provide a sensitive, specific, and quantitative biochemical assay of the phosphorylated neurofilament content of homogenates or sections of nerve tissue. The technique involves four sequential steps: (i) adhesion of fixed or fresh homogenates or tissue sections to wells of microtiter plates, (ii) binding of a monoclonal antibody against phosphorylated neurofilament to the tissue, (iii) secondary binding to the anti-phosphorylated neurofilament of a phosphatase-labeled second antibody (antimouse IgG), and (iv) enzymatic assay of alkaline phosphatase activity using a fluorescent substrate (4-methylumbelliferyl phosphate). The technique is sufficiently sensitive to measure the phosphorylated neurofilament content of a 1:100,000 (w/v) homogenate of brain, spinal cord, or peripheral nerve and of single 10-microns paraffin sections of Bouin-fixed rat spinal cord. To validate the applicability of the procedure to the study of nerve regeneration, the sciatic nerve of adult rats was either crushed (to permit regeneration) or transected and ligated (to preclude regeneration). The animals were autopsied 1 to 16 weeks later, when four segments 3-mm in length taken from regions proximal and distal to the lesion site were assayed for phosphorylated filament content. The temporal course of its disappearance during degeneration and its reappearance during regeneration coincided with the known histologic changes in crushed and transected nerves. These findings demonstrate the validity of using the immunochemical assay for PNF in studies of nerve regeneration in the peripheral nervous system and the potential applicability of this procedure to studies on regeneration in the central nervous system.

Biochemical composition and dynamics of the axonal cytoskeleton in the corticospinal system of the adult hamster

The corticospinal system is an important central nervous system (CNS) pathway that is implicated in debilitating diseases such as amyotrophic lateral sclerosis and in traumatic injuries to the spinal cord. This study characterizes some of the fundamental biochemical and kinetic properties of normal corticospinal axons, establishing an important reference for studies that aim to elucidate the cellular modifications that result during pathological conditions of these axons. Slow axonal transport which conveys the axonal cytoskeleton as well as cytomatrix constituents, such as many of the metabolic enzymes and regulatory proteins, has been examined. For these studies, [35S]methionine was injected into the sensorimotor cortex of adult male Golden hamsters, and labeled, transported proteins present in corticospinal axons at 1-42 days after injection were assessed using one- and two-dimensional gel electrophoresis/fluorography. The complex group of slow component b (SCb) proteins (including clathrin, actin, enolase, creatine phosphokinase, and many others) was observed to move at a rate of approximately 2 mm/day in adult corticospinal axons. The slow component a (SCa) proteins (tubulins, neurofilament proteins, and actin) were transported at a substantially slower rate of approximately 0.4 mm/day. The biochemical and kinetic properties of slow transport in corticospinal axons were very similar to those previously described in another CNS pathway, axons of retinal ganglion cells, and substantially different from those documented in large, peripheral sensory or motor axons. These findings suggest that some of the basic properties of axonal transport which determine many of the structural and functional properties of axons may be different in the CNS compared to the peripheral nervous system.

Identification of the major multiphosphorylation site in mammalian neurofilaments

The sequence Lys-Ser-Pro-Val-Pro-Lys-Ser-Pro-Val-Glu-Glu-Lys-Gly repeats six times serially in the human midsized neurofilament (NF) protein (NF-M). To establish whether Lys-Ser-Pro-Val(Ala) is the major site for in vivo NF phosphorylation, peptides based on the human NF-M repeat were synthesized and chemically phosphorylated. These synthetic peptides were probed with 515 monoclonal antibodies (mAbs) that were raised to, and distinguished, several differentially phosphorylated forms of NF proteins. Studies with 95 of those mAbs that recognized the peptides before and after chemical phosphorylation demonstrated that a highly immunogenic epitope shared by the peptides is present in NFs from all species tested, including invertebrates. This suggests the phylogenetic conservation of a major NF phosphorylation site. Lastly, a cross-reactive antigenic determinant shared by the peptides and the major NF phosphorylation site was shown to exist in neurofibrillary tangles of patients with Alzheimer disease as well as in two neuron-specific microtubule-associated proteins (MAPs)--i.e., MAP2 and tau.

Early structural changes in the axoplasmic cytoskeleton after axotomy studied by cryofixation

Alterations in the cytoskeleton were studied in the axoplasm of neurites at the tips of proximal stumps of transected chicken sciatic nerves. The studies were carried out using cryofixation with a nitrogen-cooled propane jet. The most immediate effect is the almost complete disassembly of axoplasmic microtubules. This consequently causes the axonal transport of membrane-bounded organelles to cease and results in an accumulation of mitochondria and vesicles of the smooth endoplasmic reticulum. The neurofilament network is partially disorganized. Neurofilaments become shorter and fragmented, and are linked by a large number of anastomosed cross-linkers. The neurofilaments become newly aligned to the axis of the axoplasm and are of normal length 48-72 h after the transsection. At this stage the newly formed neurofilament bundles are in close proximity to the anastomosed cisternae and profiles of the smooth endoplasmic reticulum. The axonal sprouts always show a normally organized cytoskeletal network. These studies support the idea that the rapid remodelling of the neurofilament network is apparently a local event, not dependent on the slow transport of cytoskeletal materials to the tip of the proximal stump. The repair of the degraded cytoskeleton may be in accordance with the function of the endoplasmic reticulum as Ca2+-sequestering membrane system, which may be involved in restoring the physiological conditions of the axoplasm.

Changes in neurofilament gene expression occur after axotomy of dorsal root ganglion neurons: an in situ hybridization study

Neurofilaments (NFs) are predominant elements in large myelinated axons, where they are thought to serve the important function of maintaining axonal caliber. Previous studies have shown that changes in NF synthesis and axonal transport occur after axonal injury in rat dorsal root ganglion (DRG) cells. The resulting reduction in the NF supply to DRG axons is thought to be largely responsible for the observed decrease in axonal diameter in the proximal axonal stump after an injury. In the present study, we test the hypothesis that a change in NF gene expression precedes the changes in synthesis and transport of NF proteins. To address this hypothesis, the levels of mRNA encoding the 68-kilodalton (kd) neurofilament protein (NF68) in adult rat DRG neurons were assessed at different times after peripheral axotomy using in situ hybridization. For these studies we used a 35S-labeled cDNA probe to NF68. The levels of NF68 mRNA in sensory neurons located in ipsilateral fourth and fifth lumbar DRG at 1, 7, and 14 days after sciatic nerve crush were compared to those in normal DRG neurons using quantitative autoradiography. In large DRG neurons (greater than 1000 micron 2), the levels of NF68 mRNA were significantly reduced relative to normal at 1, 7, and 14 days after axotomy. Medium-sized cells (601-1000 micron 2) exhibited a reduction only at 14 days postinjury, and small-sized cells were not significantly affected. These findings indicate that larger DRG neurons which give rise to large myelinated sensory axons exhibit a change in NF gene expression after axonal injury. The observed changes in NF68 mRNA levels temporally precede changes in NF synthesis and transport in injured DRG cells. Thus, a change in NF gene expression may be an important component of an effective regenerative response and a critical step at which axonal caliber is regulated in injured neurons.

Phosphorylation of proteins in normal and regenerating goldfish optic nerve

Within 6 h after radiolabeled phosphate was injected into the eye of goldfish, labeled acid-soluble and acid-precipitable material began to appear in the optic nerve and subsequently also in the lobe of the optic tectum, to which the optic axons project. From the rate of appearance of the acid-precipitable material, a maximal velocity of axonal transport of 13-21 mm/day could be calculated, consistent with fast axonal transport group II. Examination of individual proteins by two-dimensional gel electrophoresis revealed that approximately 20 proteins were phosphorylated in normal and regenerating nerves. These ranged in molecular weight from approximately 18,000 to 180,000 and in pI from 4.4 to 6.9. Among them were several fast transported proteins, including protein 4, which is the equivalent of the growth-associated protein GAP-43. In addition, there was phosphorylation of some recognizable constituents of slow axonal transport, including alpha-tubulin, a neurofilament constituent (NF), and another intermediate filament protein characteristic of goldfish optic axons (ON2). At least some axonal proteins, therefore, may become phosphorylated as a result of the axonal transport of a phosphate carrier. Some of the proteins labeled by intraocular injection of 32P showed changes in phosphorylation during regeneration of the optic axons. By 3-4 weeks after an optic tract lesion, five proteins, including protein 4, showed a significant increase in labeling in the intact segment of nerve between the eye and the lesion, whereas at least four others (including ON2) showed a significant decrease. When local incorporation of radiolabeled phosphate into the nerve was examined by incubating nerve segments in 32P-containing medium, there was little or no labeling of the proteins that showed changes in phosphorylation during regeneration. Segments of either normal or regenerating nerves showed strong labeling of several other proteins, particularly a group ranging in molecular weight from 46,000 to 58,000 and in pI from 4.9 to 6.4. These proteins were presumably primarily of nonneuronal origin. Nevertheless, if degeneration of the axons had been caused by removal of the eye 1 week earlier, most of the labeling of these proteins was abolished. This suggests that phosphorylation of these proteins depends on the integrity of the optic axons.

The pathophysiology of proximal neurofilamentous giant axonal swellings: implications for the pathogenesis of amyotrophic lateral sclerosis

Neurofilamentous giant axonal swellings are observed in a number of human disorders, although they can manifest at different locations (i.e. proximal or distal) along the axon. Recent advances in understanding the pathogenesis of these changes has resulted from correlations of ultrastructural changes with abnormalities in the axonal transport of neurofilament proteins in experimental models produced by toxic chemicals. Using single, high doses of either acrylamide or 2,5-hexanedione, a reduction in neurofilament transport has been shown in the rat sciatic nerve. In contrast to the distal axonal swellings observed upon repeated exposures to these agents, modest proximal axonal swellings containing increased neurofilament content are found following high dose exposures. Thus, regardless of the location of swelling production, a defect in slow transport appears to underlie swelling formation. beta,beta'-Iminodipropionitrile (IDPN) produces proximal neurofilamentous giant axonal swellings which are indistinguishable from those observed in some patients with amyotrophic lateral sclerosis (ALS). Although not a model for ALS, IDPN provides a means to study the functional consequences of proximal giant axonal swellings. Intracellular recordings from IDPN-intoxicated cats reveal a number of abnormalities which may have electrophysiological counterparts in ALS, suggesting that the swellings may be important in the expression of the disease. Although axonal degeneration is rarely observed in the cat, perikaryal recordings reveal a number of alterations which are strikingly similar to those obtained from chromatolytic motor neurons following nerve transection. A perturbation of "trophic" signals from the periphery may be involved in the generation of axotomy-like changes in IDPN-intoxicated cats.

Postnatal development of microtubules and neurofilaments in the rat optic nerve: a quantitative study

In this paper the postnatal changes in the cytoskeleton of the rat optic nerve fibers are described and quantified. These changes are also compared with other parameters such as myelination and axonal caliber with the aim of describing a general pattern of optic nerve maturation from a morphological point of view. The results showed that during the first postnatal week microtubules outnumbered neurofilaments but between days 8 and 20 the neurofilaments rapidly increased and on day 20 were about twice as numerous as microtubules. This proportion remained almost unaltered from the end of the third week to the 44th postnatal day. The comparison with other parameters suggested that the cytoskeleton, and in particular the proportion between its components, may be a more reliable index for measuring optic nerve maturation than other variables commonly used.

Properties of highly viscous gels formed by neurofilaments in vitro. A possible consequence of a specific inter-filament cross-bridging

Neurofilaments freshly isolated from bovine spinal cord form a reversible gel in vitro, consisting of nearly parallel and interlinked filaments organized in bundles. This phenomenon is obtained above a critical neurofilament concentration and is highly sensitive to denaturation. No gelation occurs with neurofilaments reconstituted from urea-solubilized subunits. The velocity of the gelation kinetics, optimum at a slightly acidic pH, is inhibited by low and high ionic strength and activated by millimolar concentrations of Mg2+ and other bivalent cations. No protein other than the purified neurofilament preparation itself (80-95% neurofilament triplet) is necessary for the formation of a gel. However, purified cytoskeletal proteins from microtubules and neurofilaments influence the viscosity of the native preparation. These observations suggest a reticulation in vitro between neurofilaments, dependent upon a fragile conformation of the polymers and possibly mediated through the high-Mr neurofilament subunits (200 kDa and 150 kDa). The significance of these results is discussed with regard to the inter-neurofilament cross-bridging in situ involving the 200 kDa subunit described by Hirokawa, Glicksman & Willard [(1984) J. Cell Biol. 98, 1523-1536].

Retrograde axonal transport in rat sciatic nerve after nerve crush injury

We investigated the quantitative alterations in retrograde transport of proteins following a nerve crush injury using the 3H N-succinimidyl propionate (3H NSP) method in rat sciatic nerve. After subepineurial injection of 3H NSP into the nerve the amount of radioactively labeled proteins accumulating in the cell bodies of the motor and sensory neurons was determined 1 day or 7 days later in nerves which had been crushed distal to the injection site 1, 3, 5, 7, or 33 days prior to 3H NSP labeling. One day accumulation in the DRG and spinal cord was not altered by nerve crush. Seven day accumulation in the DRG was initially slightly increased, then fell to 73% of control by 7 days, remaining reduced 33 days after crush. Seven day accumulation in the spinal cord was reduced to 25% of control 1 day after crush and remained at that low level except for 5 days post-crush when a normal amount of labeled protein was transported to the spinal cord. The time course of these changes suggests that quantitative alterations in retrograde transport may be involved in the long-term trophic interactions between the cell body and periphery, but are too slow to account for the earliest perikaryal responses to injury. In addition, the difference between the alterations of retrograde transport in motor and sensory neurons may reflect fundamental differences in the composition of retrograde transport in those different systems.

Neurofilament gene expression: a major determinant of axonal caliber

Within the wide spectrum of axonal diameters occurring in mammalian nerve fibers, each class of neurons has a relatively restricted range of axonal calibers. The control of caliber has functional significance because diameter is the principal determinant of conduction velocity in myelinated nerve fibers. Previous observations support the hypothesis that neurofilaments (NF) are major intrinsic determinants of axonal caliber in large myelinated nerve fibers. Following interruption of axons (axotomy) by crushing or cutting a peripheral nerve, caliber is reduced in the proximal axonal stumps, which extend from the cell bodies to the site of axotomy. (The distal axonal stumps, which are disconnected from the cell bodies, degenerate and are replaced by the outgrowth of regenerating axonal sprouts arising from the proximal stump). This reduction in axonal caliber in the proximal stumps is associated with a selective diminution in the amount of NF protein undergoing slow axonal transport in these axons, with a decrease in axonal NF content, and with reduced conduction velocity. The present report demonstrates that changes in axonal caliber after axotomy correlate with a selective alteration in NF gene expression. Hybridization with specific cDNAs was used to measure levels of mRNA encoding the 68-kDa neurofilament protein (NF68), beta-tubulin, and actin in lumbar sensory neurons of rat at various times after crushing the sciatic nerve. Between 4 and 42 days after axotomy by nerve crush, the levels of NF68 mRNA were reduced 2- to 3-fold. At the same times, the levels of tubulin and actin mRNAs were increased several-fold. These findings support the hypothesis that the expression of a single set of neuron-specific genes (encoding NF) directly determines axonal caliber, a feature of neuronal morphology with important consequences for physiology and behavior.

Acrylamide neuropathy: changes in the composition of proteins of fast axonal transport resemble those observed in regenerating axons

Proteins conveyed by fast axonal transport along sensory and motor axons of rat sciatic nerve were labelled with L-[35S]methionine and characterized by one- and two-dimensional electrophoresis on polyacrylamide gels, followed by fluorography. Nerves from normal or bis-acrylamide-treated animals were compared with nerves from acrylamide-treated animals and nerves regenerating after a crush axotomy. In both sensory and motor axons significant changes in the pattern of labelled bands on one-dimensional gels occurred after 10 days of acrylamide treatment (50 mg/kg daily, i.p.). These changes resembled those seen in regenerating axons, but were less pronounced. No changes were detectable after shorter periods of treatment, even though the onset of the neuropathy, assessed by a behavioral test, occurred on days 4-6 of treatment. Two-dimensional separations of the labelled proteins revealed increased labelling of growth-associated protein 43 in acrylamide-treated animals, but again this was less pronounced than in regenerating nerves. Acrylamide treatment induces changes in composition of fast-transported protein that are qualitatively similar to those seen after axotomy. Since these changes are not detectable until the neuropathy is advanced, it is unlikely that they are causative factors. Instead, they are most likely a result of the cell body reaction previously observed in acrylamide intoxication, a reaction that resembles that produced by axotomy.

Structural protein transport in elongating motor axons after sciatic nerve crush. Effect of a conditioning lesion

In elongating motor axons of the rat sciatic nerve, the maximum outgrowth rate increased from 4.6 to 5.3 mm/d (5.3-6.1 X 10(-8) m/s) when a testing lesion of spinal nerves L4 and L5 was preceded 2 wk earlier by a conditioning lesion of the sciatic nerve. Axonal outgrowth was examined by measuring the transport of 35[S]methionine-labeled structural proteins (tubulin, actin, and neurofilament triplet) from "parent" axon stumps into "daughter" axon sprouts. Since these proteins are conveyed by the slow component of axonal transport at 1-5 mm/d (1.2-6.0 X 10(-8) m/s), the isotope was injected into the spinal cord 1 wk before the testing lesion. Nerves were removed 8 d after the testing lesion, sectioned into 3-mm segments, and homogenized; soluble proteins were separated by polyacrylamide gel electrophoresis. Fluorographs were used as templates to identify gel segments for removal, solubilization, and liquid scintillation counting. Distributions of mean radioactivity for tubulin, actin, and neurofilament triplet were plotted for animals receiving a conditioning vs sham-conditioning lesion. Greater amounts of tubulin and actin were transported into daughter axons in the conditioned group. Tubulin was mainly increased in axon shafts, whereas actin was mainly increased in axon tips. These findings suggest that the axonal transport of tubulin and actin governs the rate of elongation.

Progressive axonopathy: an inherited neuropathy of boxer dogs. Quantitative and morphometric analysis of the peripheral nerve lesion

Previous studies have described and illustrated the lesions in the peripheral nerves in progressive axonopathy, an inherited neuropathy of Boxer dogs. The present paper assesses these changes using quantitative techniques. Cervical and lumbar nerve roots and tibial, phrenic and medial cutaneous radial nerves have been studied in affected and age-matched normal dogs aged 2 months to 3 years. The dorsal and ventral nerve roots, and to a lesser extent the proximal nerves, contain a proportion of swollen myelinated axons whereas in the middle and distal nerves the larger diameter fibres fail to develop to their expected maximum calibre. The unmyelinated axons remain the same size as those in normal dogs. Myelin sheath changes, with attenuation or loss of the sheath and/or remyelination, become increasingly prevalent through the course of the disease, always maintaining a proximal to distal decrease in their frequency. Quantification indicates that, particularly in the ventral roots, many axons have disproportionately thin sheaths with shortened internodes. Axonal degeneration and regeneration increase in frequency in the distal nerves as the disease progresses. The cervical ventral roots prove an exception in that they contain large numbers of regenerating clusters at most stages. It is suggested that in progressive axonopathy an axonal transport failure may occur in the roots leading to the axonal swellings, as a result of which a developmental hypoplasia occurs in the more distal, larger diameter fibres. The prominent, but unevenly distributed, myelin sheath changes indicate a severe disturbance in axon-sheath cell inter-relationships.

Multiple fates of newly synthesized neurofilament proteins: evidence for a stationary neurofilament network distributed nonuniformly along axons of retinal ganglion cell neurons

We have studied the fate of neurofilament proteins (NFPs) in mouse retinal ganglion cell (RGC) neurons from 1 to 180 d after synthesis and examined the proximal-to-distal distribution of the newly synthesized 70-, 140-, and 200-kD subunits along RGC axons relative to the distribution of neurofilaments. Improved methodology for intravitreal delivery of [3H]proline enabled us to quantitate changes in the accumulation and subsequent decline of radiolabeled NFP subunits at various postinjection intervals and, for the first time, to estimate the steady state levels of NFPs in different pools within axons. Two pools of newly synthesized triplet NFPs were distinguished based on their kinetics of disappearance from a 9-mm "axonal window" comprising the optic nerve and tract and their temporal-spatial distribution pattern along axons. The first pool disappeared exponentially between 17 and 45 d after injection with a half-life of 20 d. Its radiolabeled wavefront advanced along axons at 0.5-0.7 mm/d before reaching the distal end of the axonal window at 17 d, indicating that this loss represented the exit of neurofilament proteins composing the slowest phase of axoplasmic transport (SCa or group V) from axons. About 32% of the total pool of radiolabeled neurofilament proteins, however, remained in axons after 45 d and disappeared exponentially at a much slower rate (t 1/2 = 55 d). This second NFP pool assumed a nonuniform distribution along axons that was characterized proximally to distally by a 2.5-fold gradient of increasing radioactivity. This distribution pattern did not change between 45 and 180 d indicating that neurofilament proteins in the second pool constitute a relatively stationary structure in axons. Based on the relative radioactivities and residence time (or turnover) of each neurofilament pool in axons, we estimate that, in the steady state, more neurofilament proteins in mouse RGC axons may be stationary than are undergoing continuous slow axoplasmic transport. This conclusion was supported by biochemical analyses of total NFP content and by electron microscopic morphometric studies of neurofilament distribution along RGC axons. The 70-, 140-, and 200-kD subunits displayed a 2.5-fold proximal to distal gradient of increasing content along RGC axons. Neurofilaments were more numerous at distal axonal levels, paralleling the increased content of NFP.(ABSTRACT TRUNCATED AT 400 WORDS)