The pattern of recovery of axons in the nervous system of rats following 2,5-hexanediol intoxication: a question of rheology?

γ-Diketone neuropathy: axon atrophy and the role of cytoskeletal protein adduction

Multifocal giant neurofilamentous axonal swellings and secondary distal degeneration have been historically considered the hallmark features of gamma-diketone neuropathy. Accordingly, research conducted over the past 25 years has been directed toward discerning mechanisms of axonal swelling. However, this neuropathological convention has been challenged by recent observations that swollen axons were an exclusive product of long-term 2.5-hexanedione (HD) intoxication at lower daily dose-rates (e.g., 175 mg/kg/day); that is, higher HD dose-rates (e.g., 400 mg/kg/day) produced neurological deficits in the absence of axonal swellings. The observation that neurological toxicity can be expressed without axonal swelling suggests that this lesion is not an important pathophysiological event. Instead, several research groups have now shown that axon atrophy is prevalent in nervous tissues of laboratory animals intoxicated over a wide range of HD dose-rates. The well-documented nerve conduction defects associated with axon atrophy, in conjunction with the temporal correspondence between this lesion and the onset of neurological deficits, strongly suggest that atrophy has pathophysiological significance. In this commentary, we present evidence that supports a pathognomonic role for axon atrophy in gamma-diketone neuropathy and suggests that the functional consequences of this lesion mediate the corresponding neurological toxicity. Previous research has demonstrated that HD interacts with proteins via formation of pyrrole adducts. We therefore discuss the possibility that this chemical process is essential to the mechanism of atrophy. Evidence presented in this review suggests that "distal axonopathy" is an inaccurate classification and future nosological schemes should be based on the apparent primacy of axon atrophy.

Characterization of carbon disulfide neurotoxicity in C57BL6 mice: behavioral, morphologic, and molecular effects

Female C57BL6 mice were exposed to 0 or 800 ppm carbon disulfide (CS2), 6 h/d, 5 d/wk for 20 weeks. The neurologic function of all mice was assessed once at the end of exposures using a functional observational battery. General health effects included a decrease in body weight gain, piloerection, hunched body posture, and ptosis. Treatment-related effects included altered gait (uncoordinated placement of hind limbs and ataxia) and impaired function on an inverted screen test. In addition, rearing and locomotor movement were decreased in treated mice. Focal to multifocal axonal swelling was seen predominantly in the muscular branch of the posterior tibial nerve, and occasionally giant axonal swelling was detected in the lumbar segment of the spinal cord. Electron microscopic examination revealed swollen axons with massive accumulation of neurofilament proteins within the axoplasm. Covalent cross-linking of erythrocyte spectrin (surrogate protein to neurofilament protein) was demonstrated in mice exposed to CS2 but not in mice receiving filtered air. These data provide supportive evidence that covalent cross-linking of neurofilament proteins is a significant feature of the axonal swellings in mice produced by inhalation exposure to CS2.

In vitro binding of [14C]2,5-hexanedione to rat neuronal cytoskeletal proteins

2,5-Hexanedione (2,5-HD) induces central-peripheral axonpathy characterized by the accumulation of 10-nm neurofilaments proximal to the nodes of Ranvier and a Wallerian-type degeneration. It has been postulated that neurofilament crosslinking may be involved in the production of this axonopathy. A potential initiating event in this neurotoxic process may be the direct binding of 2,5-HD to neurofilament and microtubule proteins. In this study, the in vitro binding of [14C]2,5-HD to neurofilament and microtubule proteins was examined. Neurofilament proteins isolated from rat spinal cord or microtubule proteins isolated from rat brain were incubated in the presence of 2,5-HD at concentrations ranging from 25 to 500 mM. Quantitative analysis of sodium dodecyl sulfate (SDS) polyacrylamide gels revealed a dose- and time-dependent binding of 2,5-HD to both neurofilament proteins and microtubule proteins. Expressed as pmol 2,5-HD bound per microgram protein, the observed relative binding was MAP2 > NF160 > NF200 > > NF68 > tubulin. These data demonstrate the direct binding of 2,5-HD to cytoskeletal proteins including both neurofilaments and microtubules.

Effect of exposure to 2,5-hexanediol in light or darkness on the retina of albino and pigmented rats. I. Morphology

Male albino (Sprague Dawley) and pigmented (Norwegian Brown) rats received 1% 2,5-hexanediol (H) in their drinking water for 5 or 8 weeks, respectively. The rats were housed either in 12 h light (average 30 cd/cm2 inside cage) and 12 h darkness (group LH) or in total darkness (group DH). Two control groups (Light only, LC; Darkness only, DC) were studied in parallel under identical conditions. The animals were sacrificed at the end of H exposure or after an ensuing 13-week period without H but under the same lighting conditions. The retinas of albino rats in the LH group showed a reduction (compared to the LC, DH and DC groups) in the number of nuclei per unit area of the outer nuclear layer (ONL; p < 0.05) and degeneration of the outer segment and the inner segment layers (photoreceptor cells). A less pronounced loss of nuclei was seen in the LC group. No decrease in the number of nuclei, or signs of degeneration, were demonstrated in the albino DH or DC groups. Thirteen weeks after exposure to H, the albino LH rats had lost about 50% of the nuclei in the ONL (p < 0.05) and the outer plexiform layer (OPL) had almost disappeared. At the corresponding time, in the pigmented rats the LH and DH groups differed from the LC and DC groups. The degenerative process resulted in no inflammatory changes in the retina. The results imply an interaction exceeding simple summation after exposure to light and H, in destroying photoreceptors and OPL (p < 0.001) in albino rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Anomalous phosphorylated neurofilament aggregations in central and peripheral axons of hens treated with tri-ortho-cresyl phosphate (TOCP)

Previous biochemical studies demonstrated a dramatic increase in phosphorylation of cytoskeletal proteins that occurs early in organophosphorus ester-induced delayed neurotoxicity (OPIDN). In this report we present immunohistochemical evidence that there is anomalous aggregation of phosphorylated neurofilaments within central and peripheral axons following organophosphate exposure. The morphology, location, and time of appearance of these aggregations are consistent with the hypothesis that the aberrant phosphorylation of cytoskeletal elements is an antecedent to the focal axonal swelling and degeneration characteristic of OPIDN.

Induction of cytochrome P450 isozymes by simultaneous inhalation exposure of hens to n-hexane and methyl iso-butyl ketone (MiBK)

Chickens were exposed simultaneously to the industrial hexacarbon solvents n-hexane and methyl iso-butyl ketone (MiBK). n-Hexane has been shown to be neurotoxic in both humans and other vertebrates. While MiBK is not neurotoxic, it has been shown to greatly synergize the clinical appearance of neurotoxicity in animals exposed to both of these solvents. Groups of hens were exposed for 29 days in inhalation chambers to 1000 ppm n-hexane in combination with 10, 100, 250, 500, or 1000 ppm MiBK. Other groups received either 1000 ppm n-hexane, 1000 ppm MiBK, or ambient air and served as controls. A dose-dependent decrease in body weight and an increase in clinical effects were noted for the highest exposure groups (1000 ppm n-hexane combined with 1000, 500 or 250 ppm MiBK). There was an MiBK dose-dependent increase in cytochrome P450 content and benzphetamine N-demethylase activity, but there was no distinct pattern for ethoxyresorufin O-deethylase or cytochrome c reductase activities. Mixed-function oxidase levels and activities (cytochrome P450 content and benzphetamine N-demethylase) were elevated significantly (P less than 0.05) over controls even in the lowest MiBK group (10 ppm), although there were no clinical signs of neurotoxicity. Four different isozymes of cytochrome P450 were measured immunologically. There was a dose-dependent increase in three of the isozymes, two of which were phenobarbital inducible and one of which was induced by beta-napthoflavone. Quantitatively, the largest increase was in the PB-A isozyme, a phenobarbital-inducible isozyme which accounted for approximately 70% of the cytochrome P450 present in animals treated with MiBK. The results suggest that MiBK selectively induces cytochrome P450 isozymes leading to the metabolic activation of the weak neurotoxicant n-hexane to the potent neurotoxicant 2,5-hexanedione (2,5-HD).

Cholinergic fiber perturbations and neuritic outgrowth produced by intrafimbrial infusion of the neurofilament-disrupting agent 2,5-hexanedione

The compound 2,5-hexanedione (HD) produces axonopathies in peripheral nerves characterized by selective accumulation of neurofilaments. Its direct actions on neurotransmitter-specific neurons in the brain are unknown. In an attempt to address this latter issue, we infused HD into the fimbria and evaluated histochemically and immunohistochemically possible structural alterations in cholinergic neurons projecting from the basal nuclear complex to the hippocampus. Putative cholinergic fibers expressing nerve growth factor receptor and acetylcholinesterase showed increases in caliber and perturbations in trajectories 2-4 days following HD treatment. Similar morphologic changes were observed in neuronal elements processed for the 68 kDa neurofilament protein. At 7 days, short collateral ramifications appeared in many cholinergic axons that were suggestive of neurite outgrowth. Correlated with these fiber alterations was a transient reduction in the number of medial septal and diagonal band somata expressing choline acetyltransferase, which returned to control levels within 6 weeks following HD treatment. These data support the view that neurofilaments play an important, perhaps cytoarchitecturally stabilizing, role in regulating axonal morphology in certain populations of cholinergic neurons.

Giant axonopathy characterized by intermediate location of axonal enlargements and acceleration of neurofilament transport

It has previously been shown that 2,5-hexanedione (2,5-HD) and its 3,4-dimethyl derivative (3,4-DMHD) induce neurofilamentous accumulations at prenodal sites in distal and proximal, respectively, regions of peripheral axons. For 2,5-HD, neurofilament (NF) transport is accelerated and this is thought to be directly related to the appearance of the axonal enlargements. For 3,4-DMHD, however, the rate of NF transport cannot be assessed owing to the very proximal position of NF accumulation. In the present study, it is shown that administration to rats of 3-methyl-2,5-hexanedione, the structural 'average' of 2,5-HD and 3,4-DMHD, induces NF accumulations at midway axonal positions of the sciatic and optic systems, and results in acceleration of NF in the sections of optic axons proximal to the enlargements. These results suggest that a common mechanism underlies all gamma-diketone neuropathies, and that the proximodistal pattern of axonal enlargements represents pharmacokinetic variables rather than differences in mode of action. The neurotoxicity of gamma-diketones probably arises from pyrrolation of lysine epsilon-amino groups in crucial regions of NF or related proteins responsible for maintaining the proper supramolecular organization of the cytoskeleton. Acceleration of NF transport appears to be a common characteristic of chemically induced axonopathies, regardless of location, and this is contrary to the theory that gamma-diketone-induced NF accumulation results primarily from a progressive cross-linking of NF occurring subsequent to pyrrole formation.

A model for slow axonal transport and its application to neurofilamentous neuropathies

A model for slow axonal transport is developed in which the essential features are reversible binding of cytoskeletal elements and of soluble cytosolic proteins to each other and to motile elements such as actin microfilaments. Computer simulation of the equations of the model demonstrate that the model can account for many of the features of the SCa and SCb waves observed in pulse experiments. The model also provides a unified explanation for the increase and decrease of neurofilament transport rates observed in various toxicant-induced neuropathies.

Cytoskeletal effects of acrylamide and 2,5-hexanedione: selective aggregation of vimentin filaments

Several neurotoxic compounds cause aggregation of neurofilaments in peripheral axons. This may represent a primary action of these chemicals or a secondary response to other cellular damage. To distinguish between these possibilities, the effects of acrylamide and 2,5-hexanedione (2,5-HD) on vimentin were examined in PtK2 cultured cells. Vimentin intermediate filaments were chosen because they are closely related, in structure, to neurofilaments. Effects on other components of the cytoskeleton (cytokeratin filaments, microtubules, and microfilaments) were also determined. Both acrylamide and 2,5-HD caused aggregation of vimentin filaments in a concentration-dependent fashion; these effects occurred at a lower concentration than alterations in other cytoskeletal filaments. The effects of both acrylamide and 2,5-HD were reversible, except at high concentrations of 2,5-HD. Crosslinking of cytoskeletal proteins was also examined. High-molecular-weight proteins with vimentin-like immunoreactivity were detected on blots from cells exposed to high concentrations of 2,5-HD. No crosslinked protein was detected after acrylamide treatment. These results suggest that both acrylamide and 2,5-HD cause a primary collapse of vimentin intermediate filaments in cultured cells. The initial redistribution of vimentin filaments occurred without apparent crosslinking of cytoskeletal proteins. The aggregation of vimentin filaments in cultured cells and of neurofilaments in vivo may share a common molecular mechanism.

Evidence for multiple mechanisms responsible for 2,5-hexanedione-induced neuropathy

The present studies were carried out to investigate the comparative roles of protein cross-linking and alteration in protein phosphorylation in the accumulation of neurofilaments due to aliphatic hexacarbons. In these studies, rats were given 2,5-hexanedione (0, 0.1, 0.25 and 1.0%) for 70 days in their drinking water. In a separate study of in vitro protein phosphorylation rats were given 1% 2,5-hexanedione for 14 days in their drinking water. Spinal cord neurofilaments were isolated and analyzed using sodium dodecyl sulfate-polyacrylamide gel electrophoresis, immunoblotting using anti-neurofilament antibodies, radioimmunoassays (RIAs) of phosphorylated epitopes on neurofilament proteins and protein phosphorylation. Protein cross-linking of neurofilaments was found in all animals treated with 2,5-hexanedione including the lowest dose (0.1%) which did not produce clinical signs of intoxication. Protein phosphorylation of neurofilament proteins, as well as MAP-2 was significantly decreased upon treatment. Protein staining revealed a decreased amount of neurofilament protein and immunoblotting demonstrated neurofilament protein cross-linking in these animals. Protein staining of glial fibrillary acidic protein (GFAP) was unaltered by this treatment. RIAs of phosphorylated and non-phosphorylated epitopes of neurofilament proteins indicated that in vivo phosphorylation of these proteins was also decreased. Two-dimensional gel electrophoresis indicated a shift of the neurofilament proteins to a basic pI, indicating a dephosphorylation of neurofilament proteins. Cross-linked neurofilament proteins also exhibited a pI which was more basic than any of the individual neurofilament proteins. This report demonstrates differential effects of 2,5-hexanedione on neurofilament proteins and indicates that several mechanisms may be responsible for their accumulation.

Mechanisms of neurotoxicity related to selective disruption of microtubules and intermediate filaments

The neuronal response to several neurotoxic chemicals includes disruption of the cytoskeleton such as interactions with microtubules and altered distribution of neurofilaments. Methylmercury (microtubule disrupting) and acrylamide and 2,5-hexanedione (neurofilament disrupting) have been used in a cell culture (PtK2) system to distinguish the cytoskeletal targets of these compounds. Methylmercury caused disassembly of microtubules with secondary collapse of vimentin filaments (epithelial cell equivalent of neurofilaments) at higher concentrations; actin filaments were unaltered. This confirms that disruption of actin does not contribute to methylmercury-induced interference with mitosis. In contrast, both acrylamide and 2,5-hexanedione caused a perinuclear redistribution of vimentin filaments with sparing of microtubules. Biochemical studies revealed that 2,5-hexanedione treatment resulted in high molecular weight vimentin-immunoreactive species, presumably by cross-linking of proteins. Selective action of both acrylamide and 2,5-hexanedione on vimentin filaments and the similarity of effects suggest that a common mechanism of damage may occur whereby these compounds act directly on both vimentin and neurofilaments.

Cytoskeletal proteins as targets for organophosphorus compound and aliphatic hexacarbon-induced neurotoxicity

Concurrent exposures to organophosphorus insecticide leptophos and the industrial solvents n-hexane and toluene were implicated in causing an outbreak of neuropathy in workers. Although both leptophos and n-hexane produce central-peripheral distal axonopathy, the morphology and distribution of neuropathic lesions are distinct, reflecting different modes of action. The molecular mechanisms of organophosphorus compound-induced delayed neurotoxicity (OPIDN) and aliphatic hexacarbon-induced neurotoxicity have been investigated utilizing various biochemical techniques, (i.e. one- and two-dimensional gel electrophoresis, immunoblotting, peptide mapping). Oral administration of tri-o-cresyl phosphate (TOCP) produced delayed neurotoxicity and increased in vitro Ca2+ and calmodulin-dependent kinase protein phosphorylation of cytoskeletal proteins in brain, spinal cord, and sciatic nerve of chickens. This enhanced protein phosphorylation correlated well with the following characteristics of OPIDN: test chemical, whether an OPIDN-producing or not; dose-dependence and time course of the effect; and the animal sex sensitivity, age selectivity, and species susceptibility. The proteins that showed an increased phosphorylation were identified to be; alpha- and beta-tubulin, microtubule-associated protein-2 (MAP-2), and the 3 neurofilament proteins 70 kDa, 160 kDa, and 210 kDa. Further studies suggested that the increased protein phosphorylation is not related to an effect on protein phosphatase or ATPase activity, but rather to altered Ca2+-calmodulin kinase II activity. Aliphatic hexacarbon-induced neurotoxicity is characterized by an accumulation of 10 nm neurofilaments above the nodes of Ranvier in the spinal cord and peripheral nerve. Treatment of rats with 2,5-hexanedione, the active neurotoxic metabolite of n-hexane, produced protein crosslinking in a dose-dependent manner. This treatment also decreased protein phosphorylation of neurofilament proteins as well as MAP-2. These studies demonstrate the involvement of cytoskeletal proteins in the molecular pathogenesis of chemical-induced neurotoxicity.

Axonal transport in neurological disease

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 the 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.

The pathogenesis of equine laryngeal hemiplegia — a review

Recent research on the muscular and nervous changes which occur in idiopathic equine laryngeal hemiplegia has indicated that many of the traditional concepts of the aetiology of this disease are erroneous. In light of the new knowledge gained, the various predispositions and possible causes of laryngeal hemiplegia are discussed, and it is suggested that the underlying mechanism of axonal damage in this neuropathy of horses may be related to abnormal energy metabolism in the axon.

Axonal transport through nodes of Ranvier

Axonally transported glycoproteins are shown to accumulate at nodes of Ranvier. We hypothesize that the increased labeling in nodal regions results from the rheological effects of axonal constriction as well as from selective deposition of some transported labeled molecules.

Cross-linking of neurofilament proteins of rat spinal cord in vivo after administration of 2,5-hexanedione

The aliphatic hexacarbons n-hexane, methyl-n-butyl ketone, and 2,5-hexanedione are known to produce a peripheral neuropathy that involves an accumulation of 10-nm neurofilaments above the nodes of Ranvier in the spinal cord and peripheral nerve. In this study, rats were treated with 0.5% 2,5-hexanedione in drinking water for 180 days, and their spinal cord neurofilaments were isolated after development of the neuropathy. Visualization by sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed a significant reduction in content of the neurofilament triplet proteins in treated animals and the presence of bands migrating at 138K and 260K that were not present in control animals. Analysis of the lanes using immunoblotting procedures and anti-70K, anti-160K, and anti-210K neurofilament antibodies revealed many cross-linked peptides. The 138K band cross-reacted with the anti-160K neurofilament antibody. This suggests that the 138K band is an intramolecular cross-link of the 160K neurofilament subunit. In addition to this peptide, there were numerous high-molecular-weight peptides immunoreactive with all three neurofilament protein antibodies. In addition to cross-linking, there was also a diminished amount of immunoreactive breakdown product of all three neurofilament proteins. This report demonstrates direct evidence of 2,5-hexanedione-induced cross-linking of neurofilament proteins in vivo, which maybe responsible for the accumulation of neurofilament proteins pathognomic of this neuropathy.

Progressive axonopathy: an inherited neuropathy of boxer dogs. 3. The peripheral axon lesion with special reference to the nerve roots

Progressive axonopathy is an autosomal recessive inherited neuropathy of Boxer dogs with lesions in the CNS and PNS. This paper describes the axonal changes in the lumbar and cervical nerve roots and tibial nerve. By 2 months of age the proximal paranodal areas of many larger diameter fibres show small axonal swellings, sometimes with attenuation or loss of the associated myelin sheath. Axoplasmic changes within swollen and non-swollen fibres include disorganization of the peripheral neurofilaments and small accumulations of vesicles and vesiculo-tubular profiles, particularly in the sub-axolemmal area. Occasional fibres, more often in the cervical roots, are massively distended with disorganized neurofilaments. The frequency of the membranous accumulations decreases with progression of the disease. Many axons show a markedly irregular or corrugated outline and are surrounded by an attenuated sheath. The peripheral axonal cytoskeleton is disorganized and misaligned, whereas the central structures maintain a more normal arrangement. Regenerating axonal clusters are common in the cervical ventral roots but occur infrequently in the lumbar roots. Similar axonal changes occur in the peripheral nerves but at a much lower frequency. Any membranous accumulations or cytoskeletal disorganization are more probable in the proximal tibial nerves, while the frequency of axonal degeneration and regeneration increases distally. The morphological appearances indicate gross disturbances in axon-sheath cell relationships and suggest that abnormalities in the transport of various axoplasmic organelles may be involved in the pathogenesis of the axonal lesion.

Progressive axonopathy: an inherited neuropathy of boxer dogs. 2. The nature and distribution of the pathological changes

This report describes the neuropathology of progressive axonopathy (PA), an autosomal recessive inherited neuropathy of Boxer dogs, which affects CNS and PNS. The nerve roots contain numerous myelin bubbles and proximal paranodal axonal swellings containing vesicles, vesiculo-tubular profiles and disorganized neurofilaments. The myelin sheath overlying such swellings is often attenuated. As the disease develops there are progressive changes in the myelin sheath with thinning at paranodal and internodal locations, loss of myelin from lengths of axon and the formation of short internodes with disproportionately thin sheaths. The abnormalities show a very definite selectivity for nerve roots and proximal nerves. Conversely, the frequency of degeneration and regeneration is greater distally except in the cervical ventral roots which contain numerous regenerating clusters. In the CNS numerous axonal spheroids are found in the lateral and ventral columns of the spinal cord and in various brain stem nuclei, particularly the superior olives, accessory cuneate nuclei and lateral lemniscus and its nucleus. Axonal degeneration which occurs mainly in the cord shows no obvious tract or proximal/distal selectivity. The optic pathways are also involved, predominantly adjacent to the chiasma. The autonomic nervous system is affected and distal limb muscles show varying, but usually minor, degrees of neurogenic atrophy. The condition, which has no obvious direct parallel in human or veterinary medicine, shows gross disturbances of axon-glial inter-relationships in both CNS and PNS.

Pathogenesis of experimental giant neurofilamentous axonopathies: a unified hypothesis based on chemical modification of neurofilaments

This review summarizes current evidence suggesting that the pathogenetic basis of giant axonal neuropathies induced by neurotoxic chemicals involves a direct chemical modification of neurofilaments (NF) and/or related cytoskeletal proteins. Chemical modification of NF is believed to disrupt the normal cytoskeletal organization, which results in an alteration in NF transport rate and accumulation of NF at prenodal sites along the axon. The exact location at which axonal enlargements occur appears to be a continuous function, dependent on both the structure and dosage schedule of the chemical toxin. A unified hypothesis for the neuropathologic effect of the diverse spectrum of toxic chemicals known to induce giant axonopathies is presented, based on recently published data on the structure of NF protein. Neurotoxic chemicals are believed to alter the charge balance of highly ionic domains of NF proteins which are thought to enter into intermolecular coulombic interactions in forming the supramolecular cytoskeletal framework.

Molecular mechanisms of diketone neurotoxicity

The important industrial and commercial solvents n-hexane and methyl n-butyl ketone undergo metabolic conversion in experimental animals and man to the neurotoxic gamma-diketone 2,5-hexanedione. Several molecular mechanisms of action have been proposed to explain the pathogenesis of gamma-diketone neuropathy. Such a mechanism must account for the target organ specificity, neurofilament accumulation, structure/activity relationships, in vivo covalent binding, and apparent direct axonal toxicity encountered in this syndrome. It has been proposed that the gamma-diketones exert their effects by reaction with sulfhydryl moieties of energy-producing axonal glycolytic enzymes, with resultant disruption of axoplasmic transport. Others have suggested that reaction instead occurs with lysine moieties of axonal cytoskeletal proteins to form alkyl pyrrole adducts, leading to damaging physicochemical changes in these proteins. Additional hypotheses involve inhibition of axonal sterologenesis, alterations in nerve membrane properties, and reduced neurofilament proteolysis within the nerve terminal. Although a comprehensive mechanism of action for the gamma-diketones remains to be demonstrated, much progress has been made toward this goal. Ultimate success awaits elucidation of the interactions of the neurotoxic diketones with axonal components at the molecular level. Previous reviews have addressed the historical, pharmacokinetic, and neuropathological aspects of this neuropathy. The present critique will examine proposed molecular mechanisms for the gamma-diketones with regard to theoretical considerations and experimental evidence.

Effects of IDPN-induced axonal swellings on conduction in motor nerve fibers

Paranodal demyelination produces a reduction of conduction velocity and conduction block. The relative proportions of these changes appear to vary among different demyelinating disorders. In this study we have examined the effects on conduction of paranodal demyelination produced by giant axonal swellings. The axonal swellings were induced in rats by administration of beta, beta'-iminodipropionitrile (IDPN). In this experimental model synchronous axonal swellings occur in the proximal region of virtually every alpha-motorneuron without evidence of segmental demyelination or fiber loss. Conduction across the motor neuron was evaluated by two methods: a monosynaptic reflex pathway and intracellular recording from single motor neurons. Increases in the delay across the central region of the monosynaptic reflex pathway began between 2 and 4 days after toxin administration. Intracellular studies confirmed that the slowing occurred across the proximal regions of the motor axons; more distal regions of the motor axons were unaffected. The substantial reduction in conduction velocity over the swollen segment occurs with only moderate evidence of conduction block, as assayed by a reduction in the H-reflex/M-response amplitude ratio. Parallel morphological studies showed that in the enlarged fibers the myelin terminal loops maintained contact with the axon but were displaced from the paranodal region into the internode. The appearance of this "passive" paranodal demyelination correlated closely with the increase in conduction delay. We suggest that the contact maintained by the displaced myelin terminal loops with the axolemma allows saltatory conduction to continue, and explains the paucity of conduction block in this model despite the prominent conduction slowing.

The joint neurotoxic action of inhaled methyl butyl ketone vapor and dermally applied O-ethyl O-4-nitrophenyl phenylphosphonothioate in hens: potentiating effect

The neurotoxic action of inhaled technical grade methyl butyl ketone and dermally applied (O-ethyl O-4-nitrophenyl phenylphosphonothioate (EPN) was studied. Three groups of five hens each were treated 5 days/week for 90 days with a dermal dose of 1.0 mg/kg of EPN (85%) on the unprotected back of the neck. These groups were exposed simultaneously to 10, 50, or 100 ppm of technical methyl butyl ketone (MBK; methyl n-butyl ketone:methyl isobutyl ketone, 7:3) in inhalation chambers. A fourth group was treated only with the dose of EPN and a fifth group with only 100 ppm MBK. The control consisted of a group of five hens treated with a dose of 0.1 ml acetone. Treatment was followed by a 30-day observation period. Simultaneous exposure to EPN and MBK greatly enhanced the neurotoxicity produced when compared to the neurotoxicity produced by either chemical when applied alone. Continued exposure to EPN and MBK resulted in earlier onset and more severe signs of neurotoxicity than exposure to either individual compound. The severity and characteristics of histopathologic lesions in hens given the same daily dermal dose of EPN in combination with inhaled MBK depended on the MBK concentration. Histopathologic changes were more severe and prevalent in the 100 ppm MBK:1 mg/kg EPN group than in the others. In this group, Wallerian-type degeneration was seen along with paranodal axonal swellings. The morphology and distribution of these lesions were characteristic of those induced by MBK. In the 50 ppm MBK:1 mg/kg EPN group axonal swelling was evident but not clearly identifiable as paranodal. Hens treated with 10 ppm MBK:1 mg/kg EPN had minimal lesions with low incidence of axonal swellings. These were not as large as those seen in MBK neurotoxicity, but instead resembled the histopathologic lesions caused by EPN. The results indicate that the combined treatment gave a value for neurotoxicity coefficient which was two times the additive neurotoxic effect of each treatment alone. Pretreatment with three daily ip doses of 5 mmol/kg technical grade MBK or methyl n-butyl ketone (MnBK), equally increased chicken hepatic microsomal cytochrome P-450 content. Also, hepatic microsomes from MBK-treated hens metabolized [14C]EPN in vitro to [14C]EPN oxon to a much greater extent than those from control hens. These results suggest that MBK potentiates the neurotoxic effect of EPN, at least in part, by increasing the metabolic activation of EPN to the more neurotoxic metabolite EPN oxon.(ABSTRACT TRUNCATED AT 400 WORDS)

Axonal degeneration distal to the site of accumulation of vesicular profiles in the myelinated fiber axon in experimental isoniazid neuropathy

Morphometric sequential studies of pathologic changes were carried out on myelinated fibers in the lumbar ventral root of Sprague-Dawley rats administered with isoniazid, 1,500 mg/kg body weight, in a single dose. Accumulation of axoplasmic organelles with secondary paranodal retraction of myelin sheath occurred in the middle part of the ventral root as early as day 2 after the administration. On day 3, axonal degeneration started to occur, distal to the middle part, where the accumulation of axoplasmic organelles is prominent. Such accumulation with the possible blockade of the fast axoplasmic transport in the proximal axon may be directly responsible for the distal axonal degeneration. Alternatively such accumulation may be secondary to the distal axonal degeneration. The morphological sequential findings described clearly reflects the pathological events in isoniazid neuropathy.

Depression of fast axonal transport produced by tullidora

The fast axoplasmic transport of labeled proteins was studied in cats showing hindlimb paralysis 4-7 weeks after a single oral dose of tullidora (Karwinskia humboldtiana) toxins. The isotope (3H-leucine) was injected into the spinal ganglion and the contralateral spinal cord of the seventh lumbar segment in order to study transport in sensory and motor fibers. The axoplasmic transport in motor fibers of the sciatic nerve was clearly altered in tullidora-treated cats. The majority of these animals showed a gradual decline of radioactivity from the cord to the periphery instead of the clear-cut wave front always seen in normal cats. An apparent wave was seen in three treated cats but the wave peak was behind the normal position and the slope of the wave front was reduced. While the rate of transport indicated by the farthest extent of the foot of the slope was not in all cases significantly changed, the results all indicated a hindered transport by the reduced slope front in the distal segments of the motor axons. In contrast, the axoplasmic transport appeared normal in the sensory fibers of all but one tullidora-treated cat. Light and electron microscopy of medial gastrocnemius and sural (cutaneous) nerves revealed axonal constrictions and axolemal irregularities associated with organelle retention after tullidora treatment. Also, some mitochondria appeared swollen. These changes were more frequent and intense in the motor nerve fibers than in the cutaneous nerve fibers.

The problems of neurons with long axons

Neurons with long axons are unique among cells in having to maintain a very large area of membrane. In this respect they have problems in common with red cells: the latter are separated from the source of their metabolites in time, the former by distance. In equilibrium, maintenance mechanisms are adequate; but in conditions of energy deprivation or deprivation of antioxidant substances such as glutathione and alpha-tocopherol, or when the transport of materials within the neuron is physically obstructed, the system may break down and the longest fibres will always suffer first. The problems are logistical, just as are those of red cells. The association between red-cell disease and neuropathy is not entirely fortuitous.

Changes in terminal sprout formation in rat sternocostalis muscle during chronic intoxication with 2,5 hexanedione

Qualitative and quantitative morphological studies of the sternocostalis muscle innervation were made on rats chronically intoxicated with 2,5 hexanedione (2,5 HD) using the zinc iodide-osmium (ZIO) technique. Two distinct phases were seen in the events at the motor endplate. First, the number of motor endplates forming spontaneous terminal sprouts was found to increase linearly with time and, from the third week onward, the sprouts appeared to become progressively elongated. This latter change was associated with the appearance of swollen axons within intramuscular nerve bundles. Second, from the sixth week onward, wallerian degeneration of nerve fibers was seen and terminal sprouts began to make new arborizations on muscle fibers. By the eighth week, this occurred in as many as 66% of the rats, and collateral sprouting was also observed at this time. The occurrence of increased spontaneous terminal sprouting due to altered neuromuscular function is discussed in the light of axonal changes resulting from neurofilament accumulation following 2,5 HD intoxication.

Distortions of the nodes of Ranvier from axonal distension by filamentous masses in hexacarbon intoxication

A study has been made of the structural changes of nodal and paranodal regions of the nodes of Ranvier of peripheral nerves of rats in which marked accumulations of neurofilaments have occurred within axons under the influence of 2,5-hexanediol over 10 weeks. The neurofilamentous masses caused distension of the axon at two points of apparent weakness as they attempted to slide through the axonal constriction at the nodes. Principally, a spiral axonal protrusion pushed into the zone of unattached myelin loops in the proximal paranodal spinous bracelet of Nageotte. This led to a conical widening of the paranodal constriction and considerable attenuation of the overlying myelin. No degeneration of the myelin occurred however. Alternatively, or additionally, a protrusion occurred of the axon at the nodal region which increased the nodal gap width and occasionally compressed and displaced the adjacent distal paranodal constriction which could have led to some obstruction of axoplasmic flow. Swelling of distal paranodal regions occurred later and was usually associated with proximal swelling. It was also accompanied by evidence suggesting transnodal passage of filamentous material. Sometimes, however, striking nodal constriction occurred in association with symmetrical paranodal swelling. These observations suggest that the spiral glial-axonal relationships at nodes of Ranvier are capable of marked deformation that might allow the intra-axonal neurofilamentous masses to move distally. These findings are discussed in relation to the structural features of the paranodal constrictions.

Ultrastructural features of the Purkinje cell damage caused by acrylamide in the rat: a new phenomenon in cellular neuropathology

Dosing rats with acrylamide leads to the formation in Purkinje cells of juxtanuclear clusters of tubular and vesicular smooth endoplastic reticulum (SER). A microtubule organizing centre forms in relation to these clusters and together they appear to move to the cell surface, where protrusions of plasmalemma form, often with overlying synaptic attachments, containing densely packed tubular and vesicular SER membranes. Usually the microtubule organizing centre immediately underlies this. Subsequently, appearances suggest that astroglial intrusions occur internal to the protrusions described above to which the tubulo-vesicular material appears to be transferred. During these events the organization of the cytoplasm of the Purkinje cell is grossly disturbed with apparent loss and disarray of rough endoplasmic reticulum (RER) and of polyribosomes. This temporal sequence of events can be followed after a single dose of acrylamide. In chronically intoxicated animals vacuolation and swelling of dendrites takes place and the Purkinje cell may die after all stages of the cellular transformations have been present. These unique events appear to be confined to Purkinje cells and are considered probably to be the result of a primary disturbance to SER synthesis caused by acrylamide. It is argued that the changes taking place in acrylamide intoxication in neurons that lead to degeneration in long axons are probably of the same general kind.

Neurotoxicity and protein binding of 2,5-hexanedione in the hen

Previous studies in this laboratory have demonstrated 2,5-dimethylpyrrole adduct formation during in vitro exposure of protein amino groups to the neurotoxic n-hexane metabolite 2,5-hexanedione (2,5-HD). The present investigation reports in vivo pyrrole adduct formation in neural and nonneural protein from 2,5-HD-treated animals. Adult, White-Leghorn hens were given daily doses of either 200 or 70 mg 2,5-HD/kg, po, for up to 55 or 135 days, respectively. Additional animals were given 70 mg/kg for 63 days and then allowed to recover for 72 more days. Protein separation by gel electrophoresis followed by staining with a pyrrole-specific reagent yielded evidence of widespread adduct formation in protein from serum, liver, kidney, brain, and purified myelin. Binding was particularly strong in serum albumin nd myelin basic protein. Quantitation of the adduct in these tissues revealed that its formation reached peak levels at 20 days in high dose and 30 days in low-dose animals. Levels subsequently declined, suggesting the presence of a clearance mechanism capable of removing altered protein during continuing 2,5-HD exposure. Protein from animals on the recovery regimen contained no detectable pyrrole adduct. Pyrrole adduct formation was also detected in neurofilament protein preparations, although protein yields were too low to allow assessment of clearance. Hens at both dosages displayed clinical signs indicative of CNS and PNS neuropathy. Histologic findings included axonal swelling and degeneration in peripheral nerve and some spinal cord nerve tracts. A hypothesis is proposed involving differential clearance of pyrrole adduct from neural vs nonneural tissue to explain the mechanism of action and target organ specificity of 2,5-hexanedione.

The effects of 2,5-hexanedione on axonal regeneration after nerve crush in the rat

The pattern of recovery of myelinated axons in the posterior tibial nerve after crushing was studied in rats chronically intoxicated with 2,5-hexanedione. It was given for 2 weeks before crushing (200 mg/kg i.p. 5 times a week) or additionally for two further weeks after the nerve crush. Two animals were examined from each group at approximately 1,2,3,4 and 8 weeks later. Return of function in poisoned animals was slower than in the controls. The numbers of regenerating myelinated fibres was severely reduced in poisoned animals up to 4 weeks later, but by 8 weeks the numbers equalled those in the control nerves. Marked impairment of initiation of neurite outgrowth was found, but once begun, axonal growth was comparable to controls and myelination occurred normally. Above the crush for 10 mm, filament-filled axonal swellings were found in poisoned animals accompanied by varying amounts of retrograde axonal degeneration. These findings are discussed in relation to the role of normal neurofilaments in axonal growth and the effects of probably cross-linking of these by 2,5-hexanedione on regenerating neurites.

The early evolution of neurofilamentous accumulations due to 2,5-hexanediol in the optic pathways of the rat

Rats were given 2,5-hexanediol in their water for more than 5 weeks. The changes in the optic pathways were studied both qualitatively and quantitatively. Increase in 10-nm filaments within axons was noticeable from 10 days onwards in the superior colliculus and in the brachium of the superior colliculus. From then onwards there was a steady increase in the number of affected axons, and their gross enlargement occurred. The proportion of affected fibres in the brachia was in fact few, but all fibres were of retinotectal origin. Less than half of those in the superior colliculus that were swollen were of retinal origin. Measurement of axon diameter versus myelin sheath thickness showed gross relative thinning of the latter. Axon degeneration did not occur but there was increasing ultrastructural evidence of impairment of transport of organelles both centrifugally and centripetally as the filamentous masses accumulated.

Recovery from 2,5-hexanediol intoxication of the retinotectal tract of the rat. An ultrastructural study

Rats were given 2,5-hexanediol, a metabolite of n-hexane, in the drinking water until they developed a marked degree of paresis over about 7 weeks and were then allowed to recover naturally. The time course and the manner of removal of the neurofilamentous masses accumulated within axons caused by the intoxication were followed by electron microscopy over the subsequent 8 weeks. The neurofilamentous masses slowly disappeared completely from the axons of this tract, without there being any degeneration, over 6-7 weeks. They disappeared first from the fibres in the brachium of the superior colliculus, perhaps by transport towards the terminals, and later from the axons within the superior colliculus itself. Particularly in preterminal fibres in the superior colliculus the filamentous accumulations became permeated by a network of smooth endoplasmic reticulum which may have played a part in the removal of the filaments. Accumulations of mitochondria and dense bodies in preterminal regions, presumed to be caused by obstruction to retrograde transport, disappeared pari passu with loss of the filaments. The significance of these events in relation to neurofilament metabolism is discussed.