Absence Of Myelin Research Articles

The neurobiology of Schwann cells.

This selective review of Schwann cell biology focuses on questions relating to the origins, development and differentiation of Schwann cells and the signals that control these processes. The importance of neuregulins and their receptors in controlling Schwann cell precursor survival and generation of Schwann cells, and the role of these molecules in Schwann cell biology is addressed. The reciprocal signalling between peripheral glial cells and neurons in development and adult life revealed in recent years is highlighted, and the profound change in survival regulation from neuron-dependent Schwann cell precursors to adult Schwann cells that depend on autocrine survival signals is discussed. Besides providing neuronal and autocrine signals, Schwann cells signal to mesenchymal cells and influence the development of the connective tissue sheaths of peripheral nerves. The importance of Desert Hedgehog in this process is described. The control of gene expression during Schwann cell development and differentiation by transcription factors is reviewed. Knockout of Oct-6 and Krox-20 leads to delay or absence of myelination, and these results are related to morphological or physiological observations on knockout or mutation of myelin-related genes. Finally, the relationship between selected extracellular matrix components, integrins and the cytoskeleton is explored and related to disease.

STRUCTURAL PLASTICITY OF THE ADULT CNS. NEGATIVE CONTROL BY NEURITE GROWTH INHIBITORY SIGNALS

The neuronal network of the adult central nervous system (CNS) retains a limited capacity for growth and structural change. This structural plasticity has been best studied in the context of lesion-induced growth and repair. More recently, structural changes underlying functional plasticity occurring under specific physiological conditions have also been documented, in particular in the cortex and the hippocampus. Areas known for their adult plastic potential retain high levels of the growth associated protein GAP-43, suggesting a persistence of important components of the intracellular growth machinery throughout life. Interestingly, a pronounced negative correlation exists between the levels of GAP-43 and myelination in the adult CNS. Because CNS myelin contains potent neurite growth inhibitory membrane proteins, neurite growth, sprouting and plasticity were investigated in the spinal cord and brain in areas where oligodendrocyte development and myelin formation was experimentally prevented, or in the presence of an inhibitor neutralizing antibody (mAB-IN-1). In all areas, lesion-induced or spontaneous sprouting was enhanced, in parallel with persistent high levels of GAP-43. Thus, spontaneous sprouting of side branches occurred from retinal axons in the optic nerve in the absence of myelin, and target-deprived retinal axons showed increased sprouting and innervation of the contralateral optic tectum in the presence of mAB IN-1. In experimentally myelin-free spinal cords collaterals from intact dorsal roots grew over long distances to innervate deafferented target regions following the section of three dorsal roots. Similarly, the corticospinal tract sprouted across the the midline and re-established a dense plexus of fibres on the contralateral side of the spinal cord following section of one corticospinal tract in juvenile rats. Following bilateral dorsal hemisection of the spinal cord including both corticospinal tracts in young and adult rats, long distance regeneration of corticospinal fibres leading to significant functional improvements of locomotion and certain reflexes was induced by the neurite growth inhibitor neutralizing antibody IN-1.

Oligodendroglia Regulate the Regional Expansion of Axon Caliber and Local Accumulation of Neurofilaments during Development Independently of Myelin Formation

Axon caliber may be influenced by intrinsic neuronal factors and extrinsic factors related to myelination. To understand these extrinsic influences, we studied how axon-caliber expansion is related to changes in neurofilament and microtubule organization as axons of retinal ganglion cells interact with oligodendroglia and become myelinated during normal mouse brain development. Caliber expanded and neurofilaments accumulated only along regions of the axon invested with oligodendroglia. Very proximal portions of axons within a region of the optic nerve from which oligodendrocytes are excluded remained unchanged. More distally, these axons rapidly expanded an average of fourfold as soon as they were recruited to become myelinated between postnatal days 9 and 120. Unmyelinated axons remained unchanged. Axons ensheathed by oligodendroglial processes, but not yet myelinated, were intermediate in caliber and neurofilament number. That oligodendrocytes can trigger regional caliber expansion in the absence of myelin was confirmed using three strains of mice with different mutations that prevent myelin formation but allow wrapping of some axons by oligodendroglial processes. Unmyelinated axons persistently wrapped by oligodendrocytes showed full axon caliber expansion, neurofilament accumulation, and appropriately increased lateral spacing between neurofilaments. Thus, signals from oligodendrocytes, independent of myelin formation, are sufficient to induce full axon radial growth primarily by triggering local accumulation and reorganization of the neurofilament network.

Structural plasticity of the adult CNS. Negative control by neurite growth inhibitory signals

The neuronal network of the adult central nervous system (CNS) retains a limited capacity for growth and structural change. This structural plasticity has been best studied in the context of lesion-induced growth and repair. More recently, structural changes underlying functional plasticity occurring under specific physiological conditions have also been documented, in particular in the cortex and the hippocampus. Areas known for their adult plastic potential retain high levels of the growth associated protein GAP-43, suggesting a persistence of important components of the intracellular growth machinery throughout life. Interestingly, a pronounced negative correlation exists between the levels of GAP-43 and myelination in the adult CNS. Because CNS myelin contains potent neurite growth inhibitory membrane proteins, neurite growth, sprouting and plasticity were investigated in the spinal cord and brain in areas where oligodendrocyte development and myelin formation was experimentally prevented, or in the presence of an inhibitor neutralizing antibody (mAB-IN-1). In all areas, lesion-induced or spontaneous sprouting was enhanced, in parallel with persistent high levels of GAP-43. Thus, spontaneous sprouting of side branches occurred from retinal axons in the optic nerve in the absence of myelin, and target-deprived retinal axons showed increased sprouting and innervation of the contralateral optic tectum in the presence of mAB IN-1. In experimentally myelin-free spinal cords collaterals from intact dorsal roots grew over long distances to innervate deafferented target regions following the section of three dorsal roots. Similarly, the corticospinal tract sprouted across the the midline and re-established a dense plexus of fibres on the contralateral side of the spinal cord following section of one corticospinal tract in juvenile rats. Following bilateral dorsal hemisection of the spinal cord including both corticospinal tracts in young and adult rats, long distance regeneration of corticospinal fibres leading to significant functional improvements of locomotion and certain reflexes was induced by the neurite growth inhibitor neutralizing antibody IN-1.

Molecular compartmentation expressed in cerebellar cultures in the absence of neuronal activity and neuron-glia interactions.

The purpose of the study was to determine if zebrin compartmentation developed in permanently isolated cerebellar cultures, in the presence of agents that block neuronal activity and in the absence of myelination and astrocytic ensheathment of Purkinje cells. Parasagittally oriented organotypic cultures derived from newborn mice and carefully undercut at explantation to exclude extracerebellar afferents were subjected to three conditions: 1) Some were maintained in standard nutrient medium; 2) some were chronically exposed to tetrodotoxin and elevated levels of magnesium to block neuronal activity; and 3) some were exposed to cytosine arabinoside for the first 5 days in vitro (DIV) to destroy granule cells and oligodendrocytes and functionally compromise astrocytes, so that the astrocytic survivors did not ensheath Purkinje cells. Cultures fixed as whole-mount preparations were reacted with antibody to zebrin II. Cultures that were cryostat sectioned were dually reacted with antibody to zebrin II and calbindin. Groups of zebrin+ and zebrin- Purkinje cells were evident after 14 DIV in all of the experimental conditions, indicating that zebrin compartmentation developed 1) in isolated cerebellar explants, 2) in the absence of neuronal activity, and 3) in the absence of neuron-glia interactions such as myelination and glial ensheathment of Purkinje cell somata and dendrites. These results are consistent with the concept that expression of the zebrin+ and zebrin- phenotypes is an intrinsic property of Purkinje cells. The fact that zebrin expression seems to depend on an intrinsic program of differentiation in Purkinje cells suggests some role for zebrin compartmentation in cerebellar function.

NMR intensity of corpus callosum differs with age but not with diagnosis of autism.

NMR signal intensities in five regions of the midsagittal corpus callosum were measured in autism patients and normal controls. An age-related increase in signal was observed in the anterior regions in both groups. No significant differences in intensity were detected between the groups. The finding of normal myelination supports the attribution of callosal narrowing to absence of axons rather than absence of myelin.

Increased collateral sprouting of primary afferents in the myelin-free spinal cord

After partial lesions, uninjured nerve fibers have been shown to sprout and expand their connections within the CNS of adult mammals. The extent of this anatomical plasticity in adults is rather limited in comparison to embryonic or neonatal animals. Factors that might limit sprouting of nerve fibers and suppress anatomical plasticity in the CNS include myelin-associated neurite growth inhibitory molecules present in the CNS of adult mammals. To examine further the role of these neurite growth inhibitors, we have studied the ability of primary afferent fibers to sprout in the absence of myelin within a partially deafferented spinal cord. Myelination was suppressed in the lower thoracic and lumbar spinal cord of rats using neonatal x-irradiation. Dorsal roots of lumbar segments L2-L4 were cut in myelin-free and normal spinal cords of 8- or 15-d-old rats. Sprouting of primary afferents was measured after 20 d using thiamine monophosphatase (TMP) histochemistry. TMP is a specific marker enzyme for small-diameter primary afferents that terminate in the substantia gelatinosa (lamina II) of the spinal cord. When compared to the control groups, collateral sprouting of TMP-positive afferents was significantly enhanced in the myelin-free spinal cords: in animals deafferented at postnatal day P8, the average volume occupied by sprouting fibers in the upper dorsal horn was 0.103 mm3 +/- 0.008 (mean +/- SEM) in myelin-free spinal cords and 0.044 mm3 +/- 0.011 in control spinal cords. In spinal cords deafferented at P15, this difference was even larger, with 0.106 mm3 +/- 0.010 in the absence of myelin and 0.031 mm3 +/- 0.010 in controls. Our results indicate that myelin and its associated neurite growth inhibitors restrict collateral sprouting. These data provide further evidence that CNS myelin and its associated neurite growth inhibitors are involved in the regulation of anatomical plasticity in the normal CNS.

A role for oligodendrocytes in the stabilization of optic axon numbers

Differentiated oligodendrocytes express neurite growth inhibitory proteins at a time when these cells are involved in the myelination of recently formed fiber pathways. As the process of myelination follows the completion of neurite outgrowth and is concurrent with the stabilization of fiber numbers in a pathway, we set out to determine whether myelination and fiber tract stability could be causally related. Myelin formation was prevented in the rat retinofugal pathway by x-irradiating the optic nerves during oligodendrocyte proliferation. Electron microscopic and immunohistochemical analysis of irradiated optic nerves at P15 showed that oligodendrocytes and myelin were virtually absent. Optic fiber numbers were determined at 2 weeks of age throughout the length of normal and x-irradiated nerves. In some cases, normal or irradiation-treated pups were intraocularly injected with FGF 5 d prior to the fiber count in order to promote neurite outgrowth. Axon counts showed that the total fiber number in a myelin-free optic nerve was 10-30% higher than that of a myelinated nerve. Further, fiber numbers fluctuated by as much as 20% along the length of a myelin-free nerve but were relatively constant throughout the length of a myelinated nerve. Treatment of myelinated nerves with fibroblast growth factor (FGF) had no effect on either total fiber numbers or fiber number fluctuation. Conversely, fiber numbers in myelin-free/FGF-treated optic nerves were as much as 40% higher than in normals. Furthermore, total fiber numbers along the length of these nerves fluctuated by up to 34%. These results indicate that, in the absence of myelination, optic fibers are able to form sprouts.(ABSTRACT TRUNCATED AT 250 WORDS)

Increased expression of the growth-associated protein GAP-43 in the myelin-free rat spinal cord.

In the normal central nervous system (CNS) the regional expression of the growth-associated protein GAP-43 is complementary to the pattern of myelination. This has led us to suspect that myelin-associated neurite growth inhibitors might contribute to the suppression of GAP-43 expression by suppressing sprouting and plastic changes of synaptic terminals in myelinated CNS areas. In order to study the relationship between myelination and GAP-43 expression more directly, we experimentally prevented myelination of the lumbar spinal cord of rats through neonatal X-irradiation. The GAP-43 protein expression in myelin-free spinal cords was analysed by immunohistochemistry and immunoblotting and compared to age-matched normal spinal cords. We found that in the absence of myelination, GAP-43 expression is strongly increased in the spinal cord of 4-week-old rats. GAP-43 was most strongly expressed in descending fibre tracts, where expression in the normal spinal cord is very low. In grey matter the typical regional pattern of GAP-43 expression did not develop; instead GAP-43 expression was high in all regions of the spinal cord. The overall pattern of myelination and GAP-43 expression in the myelin-free cord resembled that of early postnatal stages. This indicates that the regional down-regulation of GAP-43 expression during normal postnatal development did not occur in the myelin-free areas. Our results support the hypothesis that neurite growth inhibitors from oligodendrocytes and CNS myelin suppress sprouting and plastic changes of synaptic terminals in the normal CNS and are thereby involved in regulating the stability of neural connections.

The corticospinal tract attains a normal configuration in the absence of myelin: Observations in Jimpy mutant mice

The corticospinal projection was examined in dysmyelinated, jimpy mice and in unaffected littermates following cortical injections of either wheat germ agglutinin conjugated to horseradish peroxidase or biocytin. Corticospinal axons in both phenotypes traverse the medulla within a well-defined pyramidal tract, decussate within several fascicles at the spinomedullary junction, and extend down the spinal cord in a compact bundle in the ventral-most part of the dorsal funiculus. Very few labeled fibers are seen separated from the main bundle. This normal configuration of the corticospinal tract is attained despite the virtual absence of CNS myelin in jimpy mice. It seems unlikely then that the myelin normally present in fiber bundles adjacent to this relatively late emerging projection can significantly influence pathway selection during its development.

Myelination requirements of central nervous system glia in vitro: Statistical validation of differences between glia from adult and immature mice

Glia from adult optic nerve, like those from immature nerve, have been shown to colonize and myelinate cerebellar axons in combined cultures. Preliminary experiments suggested, however, that the glia from adults might require the added nutrients and/or chemical signals present in a serum and embryo extract containing medium to produce myelin, whereas glia from immature animals could form myelin in a chemically defined medium described by Hendelman et al. (1985). This observation demanded rigorous confirmation, since it depended on the meaning of a negative result (the absence of myelin), which could be produced by many factors other than the composition of the medium in this complex culture system. Here we report an extensive series of experiments, with controls for all other factors, that provide unquestionable statistical validation of the additional myelination requirements of the glia from adult nerves. The cultures can now be used to identify these required substances, which may be limiting factors for central nervous system myelin regeneration in situ.

Interactions between Schwann cells and CNS axons following a delay in the normal formation of central myelin.

Irradiation of the rat spinal cord during the first postnatal week results in a profound reduction of oligodendrocyte myelin formation in the dorsal funiculi (DF). Despite this absence of myelin, however, axons in the irradiated region in the DF increase in diameter and approximate the size distribution seen in the control spinal cord. By 25 days of age Schwann cells are present in the irradiated DF where they undergo cell division and myelinate the axons. During the early stages of this myelin formation, these intraspinal Schwann cells exhibit a relationship to axons that is somewhat different from that seen in the normal developing peripheral nervous system (PNS). For example, within a given region, intraspinal Schwann cells myelinate axons of large diameter prior to ensheathing bundles of small diameter axons. Additionally, during myelination a Schwann cell will surround a single axon with multiple processes which appear to compete for contact with the axolemma. On axons of larger diameter, the elaboration of these processes is so excessive that it is often difficult to trace them back to the parent Schwann cell. Later, when a single process establishes several spirals about an axon, additional processes are no longer elaborated, and the "extra" processes disappear as myelin formation advances to the stage of compact lamellae. Thereafter, the myelin sheath continues to form in a normal manner. Excess processes have been observed during myelinogenesis in the normal developing PNS, but their frequency in that environment is much less than in the irradiated cord. These observations support the hypothesis that the signal(s) to initiate myelin formation are expressed on the axolemmal surface and are controlled by the neuron. In addition, these observations suggest that the delay in myelination results in an affinity or tropism between axons and Schwann cells which exceeds the level existing at the normal time of myelin formation.

Developmental features of the neonatal brain: MR imaging. Part I. Gray-white matter differentiation and myelination.

To establish the normal appearance of the neonatal brain, 51 neonates, 29-42 weeks postconception, underwent magnetic resonance (MR) imaging with a 0.6-T magnet in a prospective study. T1-weighted images were used to devise stages for the appearance of gray-white matter differentiation and extent of myelination. The results show that from 29 to 42 weeks postconception, changes in gray-white matter differentiation and myelination follow the stages in an orderly and predictable fashion. Changes in white matter intensity appear related to progressive decrease in brain water content. Myelination progresses cephalad from the brain stem at 29 weeks to reach the centrum semiovale by 42 weeks. Delayed myelination, defined as the absence of myelin in the corona radiata by 37 weeks, was seen in nine infants with complicated perinatal courses. Awareness of these developmental features should help to minimize misinterpretation of normal changes in the neonatal brain and lead to earlier detection of pathologic conditions, both with MR imaging and computed tomography.

β-Mannosidosis: Myelin and oligodendrocyte lesions in brain and spinal cord

Caprine β-mannosidosis, an autosomal recessive defect of glycoprotein catabolism, is associated with a deficiency of tissue and plasma -mannosidase and with tissue accumulation and urinary excretion of oligosaccharides, including the trisaccharide Man(β1-4)GlcNAc(βl-4)GlcNAc and the disaccharide Man(β1-4)GlcNAc. This genetic disorder is evident at birth, with severe neurological deficits including a marked intention tremor, pendular nystagmus, ataxia and inability to stand. Major pathological characteristics described in Nubian goats in Michigan and in Anglo-Nubian goats in New South Wales include widespread cytoplasmic vacuolation in the nervous system and viscera, axonal spheroids, and severe myelin paucity in the brain but not spinal cord or peripheral nerves. Light microscopic examination revealed marked regional variation in the severity of central nervous system myelin deficits, with some brain areas showing nearly complete absence of myelin and other regions characterized by the presence of 25-50% of the control number of myelin sheaths.

Saxitoxin binding to central and peripheral nervous tissue of the myelin deficiency (md) mutant rat

Tritium-labeled saxitoxin binding was assayed in the CNS and PNS of normal and of ‘myelin deficiency’ (md) mutant rat pups. This mutant in the Wistar rat is characterized by a virtually complete amyelination of the CNS, the PNS being normal. No significant difference was found in the saxitoxin binding capacity of md and normal brain, spinal cord, or sciatic nerves. This suggests that the presence or absence of myelin does not influence the number of voltage-sensitive sodium channels in the rat.

An experimental study of peripheral nerve regeneration after x-irradiation.

The effects of x-irradiation on peripheral nerve regeneration in the mouse were examined using quantitative and ultrastructural techniques. One hind limb was exposed to 15 or 20 Gy one day before, or 20 Gy three days after the right sciatic nerve was crushed. The distal sciatic and posterior tibial nerves were examined up to 120 days after injury. Irradiation before injury initially reduced the proliferation of endoneurial cells although cell numbers reached control levels after a few weeks. Myelination was delayed but not prevented, and after exposure to 20 Gy the myelin sheaths remained abnormally thin. In nerves irradiated after injury endoneurial cellular proliferation was severely and persistently reduced, and many fibres showed segmental absence of myelin. Internodes abutting the nonmyelinated segments were abnormally long, suggesting that contact with adjacent internodes may limit elongation of internodes during normal regeneration.

Development of the pyramidal tract in the hamster. II. An electron microscopic study.

We undertook a qualitative and quantitative electron microscopic study of the growth and development of the pyramidal tract in the hamster to investigate the mode of growth of the axons, the possibility of fiber degeneration during development, and the process of myelination. By calculating the total fiber number as the product of axon density and tract area for several postnatal ages, we found that the pyramidal tract grows through the medulla as a compact bundle containing nearly twice the number of fibers as the mature tract. During the second postnatal week there is a substantial loss of axons followed in the third and fourth weeks by a more gradual loss such that by 34 days after birth the total number of axons reaches the adult value. Myelination in the hamster pyramidal tract begins at 7 days and continues at a very slow rate until the third postnatal week, when a dramatic increase in myelin formation occurs. By 34 days after birth the number of myelinated axons is approximately 80% that of the adult. as has been reported for other CNS tracts, there does not seem to be a "critical diameter" of an axon that absolutely determines the presence or absence of myelin on a fiber. However, all axons above 0.5 micron in diameter are myelinated at approximately the same rate, while those under this diameter are myelinated much more slowly and even in the adult make up only a small percentage of the total myelinated fibers.

Studies on the myelin protein changes and antigenic properties of rabbit sciatic nerves undergoing Wallerian degeneration

Quantitative changes in myelin proteins have been studied during Wallerian degeneration of the rabbit sciatic nerve using polyacrylamide gel electrophoresis in phenol:formic acid:water and radioimmunoassay. The nerves were removed from saline-perfused rabbits at different time points from 2–420 days after unilateral transection. The main loss of the 3 major PNS myelin proteins, P 0 (major PAS-positive protein in peripheral nerve), P 1 (large basic protein, identical to CNS myelin basic protein) and P 2 (small PNS-specific basic protein), occurred between 4 and 18 days after transection, coinciding with the decrease in the amount of myelin and myelin debris seen morphologically. There was no evidence from polyacrylamide gel electrophoresis for an early selective loss of either of the myelin basic proteins. In 2 transected nerves removed from non-perfused animals, an increased amount of 1 protein band (molecular weight approximately 16,000 daltons) was found, but this band was not present in saline-perfused nerves. The results from radioimmunoassay indicated that the concentration of P 1 in nerves at 10 or more days after transection was considerably lower than suggested by the electrophoretic data. The reason for this discrepancy is discussed. In nerves having undergone degeneration for long periods (128 and 420 days) the concentration of P 1 was below the limit of detection of the radioimmunoassay method. Since at this stage the nerve consists of Schwann cells and connective tissue the result suggests that in the absence of myelin and axons Schwann cells contain little or no myelin basic protein, P 1. The ability of the degenerating nerves to induce experimental allergic encephalomyelitis (EAE)/neuritis (EAN) in guinea pigs was studied. The disease-inducing properties of the nerve were qualitatively unaltered at 10 days after transection. After 18 or more days' degeneration, neither the whole transected nerve nor acid extracts of it were able to induce experimental allergic disease.

Degraded proteolipid in multiple sclerosis plaques-histochemical electrophoretic observations

Electrophoretic analysis of multiple sclerosis plaques has disclosed the presence of a protein band with a mobility approximately the same as myelin proteolipid. The presence of this band was unexpected in view of the virtual absence of myelin from plaques. By application of histochemical staining procedures to electrophoretic ‘micro’ gels it has been shown that this plaque protein does not stain with Sudan Black (for lipid) and dimethylaminobenzaldehyde (for tryptophan) and cannot, therefore, be identified as normal proteolipid. The abnormal band is not associated with glial components or residual myelin within the plaque. We consider that this material might be a degradation product of myelin proteolipid formed during the demyelinating process.

Long-term cultivation of dissociated neurons from embryonic chick dorsal root ganglia in the rose chamber

1. 1. A technique is described which enables the cultivation of dissociated neurons and glial cells from embryonic chick dorsal root ganglia in Rose chambers. 2. 2. Cultivated cells rapidly develop a system of communicating fibers. Neuronal perikarya are well differentiated. 3. 3. The mixed neural and non-neural cell populations undergo changes during the 2-month period of cultivation, which is divided into three stages. In the first stage (until 6–8 days), rounded neurons produce new processes. In the second stage (1–4 weeks), there is an extensive proliferation of non-neural elements and Schwann cells coinciding with a reduction of neurons. The last stage (4 weeks and older) is characterized by degeneration of fibroblasts and other elements which previously were dominant and the presence of residual neurons, often connected by nerve processes. 4. 4. Dissociated neurons were maintained in Rose chambers for more than two months. Most neurons were bipolar or multipolar. 5. 5. There was an absence of myelination despite the association of Schwann cells with axons. Peculiar phase-dense bulbs which stain for lipid were detected on axons.