RESUMEN
Exposure of developing rats to a diet containing elemental tellurium systemically inhibits cholesterol synthesis at the level of squalene epoxidase. At high tellurium exposure levels (> 0.1% in the diet), there is an associated segmental demyelination of the PNS. Low levels of dietary tellurium (0.0001%) led to in vivo inhibition of squalene epoxidase activity in sciatic nerve, and inhibition increased with increasing exposure levels. With increasing dose and increasing exposure times, there was an increasing degree of demyelination and increasing down-regulation of mRNA levels for myelin P0 protein, ceramide galactosyltransferase (rate-limiting enzyme in cerebroside synthesis), and HMG-CoA reductase (rate-limiting enzyme in cholesterol synthesis). Because these were all down-regulated in parallel, we conclude there is coordinate regulation of the entire program for myelin synthesis in Schwann cells. An anomaly was that at early time points and low tellurium levels, mRNA levels for HMG-CoA reductase were slightly elevated, presumably in response to tellurium-induced sterol deficits. We suggest the eventual down-regulation relates to a separate mechanism by which Schwann cells regulate cholesterol synthesis, related to the need for coordinate synthesis of myelin components. Levels of mRNA for the low-affinity nerve growth factor receptor (indicator of alterations in axon-Schwann cell interactions) and for lysozyme (marker for phagocytic macrophages) were both up-regulated in a dose- and time-dependent manner which correlated with the presence of segmental demyelination. Levels of mRNA coding for myelin-related proteins were down-regulated at low tellurium exposure levels, without demyelination or up-regulation of nerve growth factor receptor. This suggests the down-regulation is related to the tellurium-induced cholesterol deficit, and not to the loss of axonal contact associated with early stages of demyelination or to the entry of activated macrophages.
Asunto(s)
Regulación de la Expresión Génica/efectos de los fármacos , Proteína P0 de la Mielina/biosíntesis , Proteínas de la Mielina/biosíntesis , Células de Schwann/metabolismo , Nervio Ciático/metabolismo , Telurio/farmacología , Acetatos/metabolismo , Alimentación Animal , Animales , Colesterol/biosíntesis , Hidroximetilglutaril-CoA Reductasas/biosíntesis , Cinética , Masculino , Oxigenasas/antagonistas & inhibidores , ARN Mensajero/biosíntesis , Ratas , Ratas Sprague-Dawley , Células de Schwann/efectos de los fármacos , Nervio Ciático/efectos de los fármacos , Escualeno/metabolismo , Escualeno-Monooxigenasa , Telurio/administración & dosificación , Factores de Tiempo , Transcripción Genética/efectos de los fármacosRESUMEN
Following nerve crush, cholesterol from degenerating myelin is conserved and reutilized for new myelin synthesis during nerve regeneration. The possibility that other myelin lipids are salvaged and reutilized has not been investigated previously. We examined the fate of myelin phospholipids and their fatty acyl moieties following nerve crush by electron microscopic autoradiography of myelin lipids prelabeled with [3H]oleate or [2-3H]-glycerol. Both precursors were incorporated predominantly (> 90%) into phospholipids; > 85% of the [3H]-oleate was incorporated as oleate, with the remainder in longer-chain fatty acids. Before nerve crush, both labels were restricted to myelin sheaths. Following nerve crush and subsequent regeneration, over half the label from [3H]oleate, but little from [2-3H]glycerol, remained in nerve. The oleate label was present as fatty acyl moieties in phospholipids and was localized to newly formed myelin sheaths. Among the extracellular soluble lipids within the degenerating nerve, the bulk of the labeled phospholipids floated at the same density as lipoprotein particles. These data indicate that myelin phospholipids are completely hydrolyzed during nerve degeneration, that at least half the resultant free fatty acids are salvaged and reutilized for new myelin synthesis, and that these salvaged fatty acids are transported by a lipoprotein-mediated mechanism similar to that functioning in cholesterol reutilization.
Asunto(s)
Ácidos Grasos/metabolismo , Metabolismo de los Lípidos , Vaina de Mielina/fisiología , Regeneración Nerviosa/fisiología , Nervios Periféricos/fisiología , Animales , Autorradiografía , Glicerol/metabolismo , Masculino , Degeneración Nerviosa/fisiología , Ácido Oléico , Ácidos Oléicos/metabolismo , Ratas , Ratas Endogámicas , SolubilidadRESUMEN
Expression of the low-affinity nerve growth factor receptor (NGFR) in the sciatic nerve (particularly Schwann cells) is high during development but is downregulated upon establishment of the mature axon-Schwann cell relationship. NGFR is re-expressed by Schwann cells if this relationship is altered by degeneration of axons (axotomy) or myelin (tellurium intoxication). To determine the sensitivity of NGFR expression to axonal injury, we have assayed NGFR-mRNA levels in proximal and distal regions of nerves exposed to the axonopathic agents acrylamide and isoniazid, as well as in proximal and distal stumps of axotomized nerves. NGFR-mRNA was elevated in all three models and correlated regionally with sites of axonal perturbation. In distal regions of acrylamide- and isoniazid-intoxicated nerves, NGFR-mRNA was elevated at least 2 days prior to visible signs of axonal degeneration as assayed by morphological techniques utilizing light microscopy. NGFR-mRNA was also elevated in proximal regions of axotomized and acrylamide-intoxicated nerves prior to signs of axonal degeneration. In these models, increased mRNA expression correlated with alterations in the size distribution of axonal cross sections. The common response in all of these situations indicates that NGFR expression, in addition to being a marker for axonal degeneration, is also a sensitive indicator of less profound perturbations in normal axon-Schwann cell interactions, including early stages of axonopathy. We suggest that assay for NGFR-mRNA may be utilized as a rapid and simple method (relative to more labor-intensive morphological methods) to screen for peripheral neurotoxicity.(ABSTRACT TRUNCATED AT 250 WORDS)
Asunto(s)
Axones/patología , ARN Mensajero/genética , Receptores de Factor de Crecimiento Nervioso/genética , Nervio Ciático/patología , Acrilamida , Acrilamidas/farmacología , Animales , Northern Blotting , Expresión Génica , Ratas , Células de SchwannRESUMEN
We investigated the temporal course of blood-nerve barrier (BNB) breakdown during the evolution of tellurium neuropathy, ricin neuropathy, and Wallerian degeneration following nerve transection or nerve crush. Blood-nerve barrier permeability was assessed with a 4,000-molecular weight fluoresceinated dextran from three days to 19 weeks after onset of neuropathy. Blood-nerve barrier breakdown was present during the first two weeks in all four models of neuropathy. Restoration of the BNB to the dextran began within four weeks and was complete by 14 weeks in tellurium neuropathy, a model of demyelinating neuropathy characterized by rapid remyelination, and after nerve crush, a model of Wallerian degeneration characterized by rapid axonal regeneration into distal stump. In contrast, there was persistence of BNB breakdown beyond 14 weeks in ricin neuropathy, a model of neuropathy with no axonal regeneration or remyelination, and after nerve transection, a model of Wallerian degeneration characterized by minimal axonal regeneration into distal stump. We conclude from these data that alterations in the BNB over the course of neuropathy differ among various types of neuropathy, and that these alterations are dependent on the form of nerve fiber injury. The lack of regenerating or remyelinating axons in ricin neuropathy and after nerve transection may be responsible for the persistent BNB breakdown found in these neuropathies.
Asunto(s)
Axones/patología , Fenómenos Fisiológicos Sanguíneos , Vaina de Mielina/patología , Enfermedades del Sistema Nervioso/fisiopatología , Sistema Nervioso/fisiopatología , Nervio Ciático/fisiopatología , Animales , Axones/fisiología , Vaina de Mielina/fisiología , Regeneración Nerviosa , Enfermedades del Sistema Nervioso/inducido químicamente , Permeabilidad , Ratas , Ricina , TelurioRESUMEN
Exposure of weanling rats to a diet containing elemental tellurium results in a peripheral neuropathy characterized by segmental demyelination and minimal axonal degeneration. One of the earliest ultrastructural abnormalities in tellurium neuropathy is an increased number of cytoplasmic lipid droplets in myelinating Schwann cells. The pathogenesis of these lipid droplets was investigated using light and electron microscopic autoradiography. Nerve lipids were either "prelabeled" with [3H]acetate via in vivo intraneural injection 3 days before a 2-day exposure to tellurium, or "postlabeled" via in vivo intraneural injection or in vitro incubation with [3H]acetate following a 2-day exposure to tellurium. In the prelabeled nerves, myelin became heavily labeled, but the tellurium-induced cytoplasmic lipid droplets were rarely labeled. In the postlabeled nerves, the tellurium-induced cytoplasmic lipid droplets were the most heavily labeled structures within the nerve. These data indicate that the tellurium-induced lipid droplets in Schwann cells are derived from newly synthesized lipid rather than from the early breakdown and internalization of myelin lipids. The earliest biochemical abnormality observed in tellurium neuropathy is an inhibition of cholesterol synthesis at the squalene epoxidase step. This leads to an accumulation of squalene within the nerve. We conclude that the cytoplasmic lipid droplets in Schwann cells contain this accumulated lipid.
Asunto(s)
Metabolismo de los Lípidos , Enfermedades del Sistema Nervioso/metabolismo , Células de Schwann/metabolismo , Telurio , Animales , Autorradiografía , Dieta , Lípidos/biosíntesis , Masculino , Microscopía Electrónica , Enfermedades del Sistema Nervioso/inducido químicamente , Enfermedades del Sistema Nervioso/patología , Ratas , Células de Schwann/patología , Telurio/administración & dosificaciónRESUMEN
We examined the sequential changes in the permeability of the BNB to a 4000-molecular-weight fluoresceinated dextran over an 18-week course of ricin neuronopathy. Neuronopathy was produced by injecting ricin into the tibial nerve of 18-day-old Long-Evans rats; permeability of the BNB in proximal sciatic nerve was evaluated by fluorescence microscopy 12 hours to 18 weeks post injection. BNB breakdown occurred two days after ricin injection and continued to be present over the 18-week course of the experiment. Ultrastructural studies revealed that Wallerian-type axonal degeneration was present in proximal sciatic nerve one day post injection. The degeneration involved only a portion of the fibers and was unaccompanied by subsequent axonal regeneration or ricin-induced injury of Schwann cells, endothelial cells or perineurial cells. Immunocytochemical studies revealed that an increased number of endoneurial macrophages did not appear until after the BNB breakdown. We conclude that (1) breakdown of the BNB occurs early and is longlasting in ricin-induced neuronopathy; (2) the BNB breakdown is a consequence of ricin-induced axonal degeneration and is independent of regenerating axons, a systemic toxic effect of ricin on the BNB, or an influx of macrophages; and (3) toxicant-induced degeneration of less than half of the fibers in a nerve is sufficient to cause a localized, persistent breakdown of the BNB.
Asunto(s)
Permeabilidad Capilar/efectos de los fármacos , Fluoresceína-5-Isotiocianato/análogos & derivados , Enfermedades del Sistema Nervioso/inducido químicamente , Neuronas/metabolismo , Ricina/toxicidad , Animales , Dextranos , Fluoresceínas , Colorantes Fluorescentes , Inmunohistoquímica , Microscopía Fluorescente , Enfermedades del Sistema Nervioso/metabolismo , Enfermedades del Sistema Nervioso/fisiopatología , Neuronas/efectos de los fármacos , Ratas , Nervio Ciático/patología , Nervio Ciático/ultraestructura , Factores de TiempoRESUMEN
The appearance of endoneurial edema early in the evolution of tellurium neuropathy raises the possibility that a breakdown of the blood-nerve barrier (BNB) plays a role in the pathogenesis of the tellurium-induced demyelination. To investigate this possibility, we correlated the temporal onset of breakdown of the BNB with inhibition of cholesterol synthesis and ultrastructural abnormalities in sciatic nerve of weanling Long-Evans rats fed a diet containing 1.1% elemental tellurium. Permeability of the BNB was assessed with [125I]-albumin and horseradish peroxidase (HRP); cholesterol synthesis was assessed by incubating segments of sciatic nerve in vitro with [1-14C]acetate. Cholesterol synthesis was severely inhibited and labeled squalene was accumulating in sciatic nerve at 12 hr of tellurium exposure. The permeability of the BNB progressively increased between 24 hr and 72 hr of tellurium exposure. Membrane-delimited vacuoles, lipid droplets and cytoplasmic excrescences appeared in myelinating Schwann cells at 24 hr; demyelinating axons appeared at 48 hr of tellurium exposure. These observations suggest that factors other than BNB breakdown and vasogenic endoneurial edema are responsible for the initial Schwann-cell injury in tellurium neuropathy. However, the early onset of BNB breakdown may have a synergistic role in the pathogenesis of tellurium-induced demyelination.
Asunto(s)
Enfermedades del Sistema Nervioso Periférico/inducido químicamente , Telurio/toxicidad , Animales , Colesterol/biosíntesis , Enfermedades Desmielinizantes/inducido químicamente , Masculino , Enfermedades del Sistema Nervioso Periférico/metabolismo , Enfermedades del Sistema Nervioso Periférico/patología , Ratas , Ratas Endogámicas , Nervio Ciático/efectos de los fármacos , Nervio Ciático/patologíaRESUMEN
Chronic systemic exposure of rats to the neuronotoxic compound trimethyltin (TMT) results in increased incorporation of radioactive precursors into retinal proteins and glycoproteins. Because this increased metabolic activity is accompanied by minimal subcellular pathological alterations and almost no neuronal necrosis, we suggested that it may represent an early, reactive (compensatory) response (Brain Res. 398, 298-304; 1986). We have now investigated the development of this metabolic response to TMT in more detail. Beginning at 30 d of age, rats received weekly doses of TMT (4 mg/kg body wt) by gavage for up to 7 wk; rates of incorporation of [35S]methionine and [3H]fucose into retinal proteins and glycoproteins, respectively, were then determined using in vitro retinal incubations. The apparent rates of protein synthesis and glycoprotein glycosylation in retinas from TMT-treated animals were normal or slightly decreased after 1-3 wkly doses, but were increased after 4 doses and more markedly increased after 7 doses. Glycoprotein glycosylation was increased to a greater degree (192% of control after 7 wk of dosing) than was protein synthesis (134% of control). The increased incorporation in retinas from TMT-treated animals persisted when retinas were incubated with "flooding" concentrations of precursor (1 mM), suggesting that these increases were not owing to alterations in the size of retinal precursor pools. The preferential increase in glycoprotein glycosylation was partially owing to a selective increase in glycosylation of two molecular species with apparent mol wt of 32 and 45 KDa. Quantitative autoradiographic analysis of newly synthesized proteins and glycoproteins indicated that the TMT-induced increase in metabolic activity was not specific or selective for any retinal layer or cell type. We suggest that the preferential activation of glycoprotein glycosylation, and in particular the increased glycosylation of the 32 and 45 KDa glycoprotein species, may represent part of a compensatory metabolic response of retinal neurons to TMT-induced neuronal injury.
Asunto(s)
Proteínas del Ojo/metabolismo , Neuronas Aferentes/metabolismo , Retina/metabolismo , Compuestos de Trialquiltina/toxicidad , Compuestos de Trimetilestaño/toxicidad , Animales , Autorradiografía , Electroforesis en Gel de Poliacrilamida , Proteínas del Ojo/biosíntesis , Femenino , Glicoproteínas/biosíntesis , Glicoproteínas/metabolismo , Masculino , Fotomicrografía , Ratas , Factores de Tiempo , Compuestos de Trimetilestaño/metabolismoRESUMEN
We studied the synthesis and axonal transport of proteins and glycoproteins in the visual system of adult Long-Evans rats that had received 4 weekly doses of trimethyltin hydroxide (TMT, 4 mg/kg b. wt.) by gastric intubation. One week following the last dose, an in vitro assay was used to study the rate of incorporation of radioactive precursors into various macromolecules of isolated retinas. Retinas from TMT-treated rats showed increased apparent rates of synthesis, relative to retinas from control rats, for proteins [( 35S]methionine precursor) and glycoproteins [( 3H]fucose precursor). Gel electrophoretic analysis of newly synthesized proteins indicated that the increased synthesis was a generalized effect, i.e. it was not restricted to a select subset of proteins. The axonal transport of these macromolecules by retinal ganglion cells to axons (optic tract) and nerve endings (superior colliculus) was examined in vivo following intraocular precursor injection. The amount of material transported, relative to that synthesized in the retina, was not appreciably altered in TMT-treated rats, indicating that TMT did not selectively impair axonal transport. The biochemical changes were accompanied by minimal ultrastructural alterations and little neuronal necrosis in the retina. We suggest that TMT induces increased synthesis of membrane macromolecules in retinal neurons; this may reflect an early reactive (compensatory) response rather than a regressive (degenerative) response of retinal neurons to TMT. Our data do not support the hypothesis that TMT induces a functional impairment of neuronal endoplasmic reticulum or Golgi apparatus.
Asunto(s)
Proteínas del Ojo/biosíntesis , Glicoproteínas/biosíntesis , Retina/metabolismo , Compuestos de Trialquiltina/toxicidad , Compuestos de Trimetilestaño/toxicidad , Animales , Transporte Axonal , Femenino , Fucosa/metabolismo , Masculino , Metionina/metabolismo , Neuronas/metabolismo , Ratas , Células Ganglionares de la Retina/metabolismo , Colículos Superiores/metabolismoRESUMEN
The prevalence of demyelinated fibers in mixed nerve (sciatic) and cutaneous nerve (sural) and the change in lead levels in various tissues over time were assessed in a model of lead neuropathy in the rat. Long-Evans rats were given drinking water containing 4% lead acetate and killed between one and 213 days of exposure. Lead levels in blood, brain, kidney, and femur increased over the 213-day period. Lead levels in sciatic nerve appeared to increase rapidly during the first few weeks of exposure and then decline to a lower plateau. The neuropathy was characterized by segmental demyelination and remyelination; neither axonal degeneration nor a microangiopathy was found. Sciatic nerve had a significantly greater prevalence of demyelinated fibers than sural nerve; the prevalence of demyelinated fibers was similar in proximal and distal sciatic nerve. The variable, brain-lead concentration times days on lead, which is an indicator of cumulative brain exposure, was the best predictor of the prevalence of demyelination. The differential involvement of sciatic and sural nerves in lead neuropathy may either indicate that Schwann cells myelinating different nerve-fiber populations have different susceptibilities to lead toxicity, or that lead preferentially enters sciatic nerve.
Asunto(s)
Intoxicación por Plomo/patología , Nervio Ciático/patología , Nervios Espinales/patología , Nervio Sural/patología , Animales , Femenino , Plomo/metabolismo , Intoxicación por Plomo/metabolismo , Masculino , Modelos Biológicos , Fibras Nerviosas Mielínicas/patología , Enfermedades del Sistema Nervioso Periférico/inducido químicamente , Enfermedades del Sistema Nervioso Periférico/metabolismo , Enfermedades del Sistema Nervioso Periférico/patología , Ratas , Ratas Endogámicas , Nervio Ciático/metabolismo , Nervio Sural/metabolismo , Distribución TisularRESUMEN
Retinal neurons from rats acutely intoxicated with trimethyltin (TMT) were examined by light and electron microscopy to determine if there is a relationship between the subcellular response of a neuron to TMT and its morphologic subspecialization. Subcellular pathologic alterations were present in neurons from all three cellular layers of the sensory retina. However, the type and degree of subcellular response varied among the highly subspecialized neurons of the different retinal layers. Clusters of dense-cored vesicles and tubules were mainly limited to neurons of the ganglion-cell layer, large accumulations of dense bodies were mainly limited to neurons of the inner nuclear layer, and neuronal necrosis was mainly limited to the photoreceptor cells. The inner segment of the photoreceptor cell shared with the perikaryon of more conventional neurons a special vulnerability to TMT cytotoxicity. Our results suggest that the morphologic subspecialization of neurons affects the type and the degree of subcellular response to TMT.
Asunto(s)
Células Fotorreceptoras/ultraestructura , Retina/ultraestructura , Enfermedades de la Retina/inducido químicamente , Células Ganglionares de la Retina/ultraestructura , Compuestos de Trialquiltina/toxicidad , Compuestos de Trimetilestaño/toxicidad , Animales , Masculino , Células Fotorreceptoras/efectos de los fármacos , Ratas , Enfermedades de la Retina/patología , Células Ganglionares de la Retina/efectos de los fármacos , Segmento Externo de la Célula en Bastón/efectos de los fármacos , Segmento Externo de la Célula en Bastón/ultraestructuraRESUMEN
The ultrastructural cytopathologic and cytochemical effects of trimethyltin (TMT) neurotoxicity were delineated in hippocampal and pyriform neurons of acutely intoxicated adult rats. TMT produced neuronal necrosis that preferentially involved hippocampal formation pyriform cortex. The first subcellular alterations were multifocal collection of dense-cored vesicles and tubules and membrane-delimited vacuoles in the cytoplasm of the perikaryon and proximal dendrite. Ultrastructural cytochemical examination revealed that the vesicles and tubules had acid phosphatase activity analagous to Golgi-associated endoplasmic reticulum (GERL). Shortly after the appearance of the GERL-like vesicles and tubules, autophagic vacuoles and polymorphic dense bodies accumulated in the neuronal cytoplasm. Some dense bodies appeared to arise from the dense-cored tubules. Neuronal necrosis was characterized by increased electron density of the cytoplasm and large, electron-dense intranuclear masses. Alterations of mitochondria and other organelles were not observed in the early stages of cell injury. No light- or electron-microscopic alterations were found in liver or kidney. Comparable subcellular alterations were observed in adult and neonatal rats chronically intoxicated with TMT. A series of other trialkyl and tricyclic tins and dimethyltin did not produce similar pathologic findings. The GERL-like accumulations are unique in neuronal cytopathology. These findings suggests that GERL and autophagy play an important role in the pathogenesis of TMT-induced neuronal injury.