Axonal regrowth in the amyelinated optic nerve of the myelin-deficient rat: ultrastructural observations and effects of ganglioside administration

Magnetic Resonance Imaging and Spectroscopy Analysis in a Pelizaeus-Merzbacher Disease Rat Model

Pelizaeus−Merzbacher disease (PMD) is an X-linked recessive disorder of the central nervous system. We performed 7 Tesla magnetic resonance imaging of the brain in Tama rats, a rodent PMD model, and control rats, as well as evaluated the diagnostic values. In the white matter of the Tama rats, the T2 values were prolonged, which is similar to that observed in patients with PMD (60.7 ± 1.8 ms vs. 51.6 ± 1.3 ms, p < 0.0001). The apparent diffusion coefficient values in the white matter of the Tama rats were higher than those of the control rats (0.68 ± 0.03 × 10−3 mm2/s vs. 0.64 ± 0.03 × 10−3 mm2/s, p < 0.05). In proton magnetic resonance spectroscopy, the N-acetylaspartate (6.97 ± 0.12 mM vs. 5.98 ± 0.25 mM, p < 0.01) and N-acetylaspartate + N-acetylaspartylglutamate values of the Tama rats were higher (8.22 ± 0.17 mM vs. 7.14 ± 0.35 mM, p < 0.01) than those of the control rats. The glycerophosphocholine + phosphocholine values of the Tama rats were lower than those of the control rats (1.04 ± 0.09 mM vs. 1.45 ± 0.04 mM, p < 0.001). By using Luxol fast blue staining, we confirmed dysmyelination in the Tama rats. These results are similar to those of patients with PMD and other PMD animal models.

Characterization of the optic nerve and retinal ganglion cell layer in the dysmyelinated adult Long Evans Shaker rat: evidence for axonal sprouting

Myelin in the central nervous system (CNS) is hypothesized to help guide the growth of developing axons by inhibiting sprouting of aberrant neurites. Previous studies using animal models lacking CNS myelin have reported that increasing capacity for sprouting axons is negatively correlated with the degree of myelination. In the present study, we investigated the optic nerves of the recently identified Long Evans Shaker (LES) rat with prolonged dysmyelination of adult axons to determine whether the lack of myelin basic protein (MBP) in adult LES rats could manifest as increases in the population of CNS axons. We observed numerous small, unmyelinated axon profiles (<0.3 microm in diameter) clustered in bundles alongside normal caliber axons in dysmyelinated LES rats but not in normal myelinated Long Evans (LE) rats. These putative axon profiles resembled sprouting axons previously described in the CNS. Moreover, the high number of small putative axon profiles could not be accounted for by any significant increases in the number of ganglion cells and displaced amacrine cells in the ganglion cell layer when compared with normal rats as evaluated by using a variety of techniques. This finding suggests that the observed clusters of putative axon profiles were not due to developmental abnormalities in the retina but to the lack of myelin in the optic nerves of LES rats. The adult LES rat, therefore, may serve as a useful model to study the role of myelin in regulating axon development or axon regeneration after CNS injury in the adult mammalian system.

Optic nerve elongation: does it exist?

The length of the optic nerves is a reflection of normal postnatal cranio-orbital development. Unilateral elongation of an optic nerve has been observed in two patients with orbital and skull base neoplasms. In the first case as compared to the patient's opposite, normal optic nerve, an elongated length of the involved optic nerve of 45 mm was present. The involved optic nerve in the second patient was 10 mm longer than the normal opposite optic nerve. The visual and extraocular function was preserved in the second patient. The first patient had only light perception in the affected eye. In this paper, the embryology, anatomy, and physiology of the optic nerve and its mechanisms of stretch and repair are discussed.

The microglial cell. A historical review

Effectively, modern research has confirmed Hortega's view of the origin of the microgliacyte from circulating monocytes of the monocyte-macrophage series that invade the brain during embryonic and early postnatal life. Their phagocytic capacity is exercised during the brain remodelling that marks brain maturation. They then convert to the ramified resting microglial cell visualized in the silver carbonate staining technique of Hortega and by modern lectin-binding methods. In response to injury, reactive microglia exhibit hypertrophy and hyperplasia, and may or may not go on to form typical lipid-laden phagocytes. Activated microglia show upregulation of the many marker antigens they share with circulating monocytes, including the major histocompatibility class (MHC) class II antigens that bespeak their immunocompetent nature. However, MHC class I and II expression and development of immunohistochemical positivity for cytoplasmic and plasma membrane antigens that characterize the monocyte-macrophage do not necessarily indicate an immunological response though there is ample evidence that microglia can serve as antigen-presenting cells. Rather, microglia are extraordinarily sensitive to changes in the brain microenvironment, whatever the nature of the exciting mechanism or substance. They may be considered to serve an ever alert, protective and supportive function that can be assembled rapidly to deal with infections, physical injuries, physiologic changes and systemic influences. In addition to elaboration and secretion of cytokines with varied actions, e.g., suppression of astrogliosis, they secrete factors, including nerve growth factor, which are supportive of neurons. They have an important role in iron metabolism and the storage of iron and ferritin. They may promote central nervous system regeneration. They are prominently involved in such pathologic processes as the acquired immunodeficiency syndrome, multiple sclerosis, prion diseases and the degenerative disorders, e.g., Alzheimer's disease and Parkinson's disease. With aging, they grow more numerous, become richer in iron and ferritin and exhibit phenotypic alteration, e.g., the expression of MHC class II antigens that are not ordinarily demonstrable immunohistochemically in the resting state. The rate of growth of our knowledge of microglia during the last decade has been exponential and continues.

An analysis of astrocytic cell lines with different abilities to promote axon growth

The adult mammalian central nervous system (CNS) lacks the capacity to support axonal regeneration. There is increasing evidence to suggest that astrocytes, the major glial population in the CNS, may possess both axon-growth promoting and axon-growth inhibitory properties and the latter may contribute to the poor regenerative capacity of the CNS. In order to examine the molecular differences between axon-growth permissive and axon-growth inhibitory astrocytes, a panel of astrocyte cell lines exhibiting a range of axon-growth promoting properties was generated and analysed. No clear correlation was found between the axon-growth promoting properties of these astrocyte cell lines with: (i) the expression of known neurite-outgrowth promoting molecules such as laminin, fibronectin and N-cadherin; (ii) the expression of known inhibitory molecules such tenascin and chondroitin sulphate proteoglycan; (iii) plasminogen activator and plasminogen activator inhibitor activity; and (iv) growth cone collapsing activity. EM studies on aggregates formed from astrocyte cell lines, however, revealed the presence of an abundance of extracellular matrix material associated with the more inhibitory astrocyte cell lines. When matrix deposited by astrocyte cell lines was assessed for axon-growth promoting activity, matrix from permissive lines was found to be a good substrate, whereas matrix from the inhibitory astrocyte lines was a poor substrate for neuritic growth. Our findings, taken together, suggest that the functional differences between the permissive and the inhibitory astrocyte cell lines reside largely with the ECM.

Astrocytes from adult rat optic nerves are nonpermissive for regenerating retinal ganglion cell axons

We have directly compared the abilities of astrocytes from newborn and adult rats to support or inhibit the growth of regenerating axons in vitro. Astrocytes prepared from newborn rats were able to promote retinal ganglion cell (RGC) axon growth from embryonic and adult rat and from adult fish retinal explants. Retinal axons from E16 rat retinae grew significantly faster on astrocytes from neonatal rats than those from E18 or adult rat retinae with growth rates comparable to RGC axons from adult fish retinae. RGC regeneration from adult rat retinae was almost completely inhibited on adult rat optic nerve astrocytes. Only axons from adult fish retinae were able to extend onto monolayers from these reactive astrocytes, although their growth rates were significantly reduced. We conclude that the failure of mammalian RGC axons to regrow within the lesioned optic nerve environment is, at least in part, due to nonpermissive aspects of adult "reactive" optic nerve astrocytes. However, the cell intrinsic growth potential of RGCs also appears to influence their ability to extend axons on cellular substrates.

Regeneration of lesioned rat optic nerve fibers is improved after neutralization of myelin-associated neurite growth inhibitors

Optic nerve axons do not regenerate after lesions in postnatal mammals, except if peripheral nerve transplants are offered as a substrate. In the present study, regeneration was assessed after intracranial freeze-crush lesions of the optic nerve, which completely interrupted all axons. In rats lesioned at the age of 8-9 days and surviving for additional 5-6 days, regenerating retinal fibers were seen to enter and partially cross the lesion site, reaching a maximum distance of 0.8 mm (mean +/- S.E.M. = 0.62 +/- 0.07 mm) in the presence of brain-derived neurotrophic factor (BDNF). Without BDNF, almost all the axons were lost due to axonal die-back. This regeneration distance could be significantly enhanced, up to 1.9 mm, by application of a monoclonal antibody (mAB-IN-1) directed against oligodendrocyte- and myelin-associated neurite growth inhibitory proteins. Similar results were obtained in rats lesioned at 16-18 days and surviving for 2 weeks: whereas fibroblast growth factor (FGF) stimulated sprouting did not exceed distances of 0.5 mm (mean = 0.38 +/- 0.07 mm), FGF and IN-1 antibody treated rats showed regenerations up to 2.3 mm (mean = 1.53 +/- 0.15 mm). The specificity of this effect was confirmed by lesions of myelin- and oligodendrocyte-free optic nerves: optic nerves were locally X-irradiated at birth, day 2, 4 and 6, a procedure which kills the dividing oligodendrocyte precursor cells. When these myelin-free nerves were lesioned at 3 weeks of age, regeneration distances between 2.5 and 3.2 mm were observed 3 weeks later.(ABSTRACT TRUNCATED AT 250 WORDS)

Microenvironmental changes during axonal regrowth in the optic nerve of the myelin deficient rat. Immunocytochemical and ultrastructural observations

Lesion-induced regenerative sprouting of CNS axons is accompanied by reactions of the supporting glia and vascular and connective tissue which may influence the extent of regeneration. In a previous report, it was shown that after crush injury, the amyelinated optic nerve of the myelin deficient (md) mutant rat contains greater numbers of regrowing axons proximal to the site of crush than that of normally myelinated littermates. The present study was designed to compare the response of the microenvironment, i.e. glial cells and vascular and connective tissue, in md and normally myelinated optic nerves 2, 4 and 6 days after crush injury. In unoperated normal optic nerves monoclonal antibodies to the HNK-1 carbohydrate labelled astrocytic processes at the ultrastructural level whereas in unoperated md mutants HNK-1 staining was restricted to axonal surfaces. Immunoreactivity with monoclonal antibodies to stage-specific embryonic antigen-1 (SSEA-1) was confined to astrocytic surfaces in both md and wildtype animals. After axotomy of md optic nerves regrowing axons were more numerous in the proximal site of the crush and extended further into the lesion than in wildtype animals. In both md and wildtype rats regrowing axons were HNK-1-positive. In md rats strong reaction with antibodies to laminin and fibronectin was only seen in 6-day-old lesions of md rats whereas immunoreactivity was less distinct in operated littermate controls. Immunolabelling was obviously associated with blood vessels, since crush lesions in both md and wildtype rats were Schwann cell-free as assessed by electron microscopy and immunocytochemistry. In both operated md and normal littermates crush lesions contained degenerating astrocytes as well as reactive astrocytes in which the intermediate filaments of the perikarya failed to stain immunocytochemically for GFAP, vimentin, desmin, and a common determinant of intermediate filaments. In contrast, reactive astrocytes in the lesion site of normally myelinated rats expressed the SSEA-1 antigen intracytoplasmically whereas in md mutants astrocytes were completely SSEA-1-negative. Infiltration of crush lesions by macrophages was less extensive in md rats than in normal littermates. However the overall content of macrophages in the peritoneal cavity was also reduced. The present study demonstrates that (1) md optic nerves lack HNK-1-reactive astrocytes; (2) in the axotomized wildtype optic nerve impaired axonal regrowth may be associated with distinct immuno-phenotypes of the supporting glial cells, i.e. SSEA-1-positive astrocytes; (3) laminin and fibronectin seem not to be essential for improved axonal regrowth in md rats.

Regeneration of axons in the optic nerve of the adult Browman-Wyse (BW) mutant rat

We have studied the regeneration of axons in the optic nerves of the BW rat in which both oligodendrocytes and CNS myelin are absent from a variable length of the proximal (retinal) end of the nerve. In the optic nerves of some of these animals, Schwann cells are present. Axons failed to regenerate in the exclusively astrocytic environment of the unmyelinated segment of BW optic nerves but readily regrew in the presence of Schwann cells even across the junctional zone and into the myelin debris filled distal segment. In the latter animals, the essential condition for regeneration was that the lesion was sited in a region of the nerve in which Schwann cells were resident. Regenerating fibres appeared to be sequestered within Schwann cell tubes although fibres traversed the neuropil intervening between the ends of discontinuous bundles of Schwann cell tubes, in both the proximal unmyelinated and myelin debris laden distal segments of the BW optic nerve. Regenerating axons never grew beyond the distal point of termination of the tubes. These observations demonstrate that central myelin is not an absolute requirement for regenerative failure, and that important contributing factors might include inhibition of astrocytes and/or absence of trophic factors. Regeneration presumably occurs in the BW optic nerve because trophic molecules are provided by resident Schwann cells, even in the presence of central myelin, oligodendrocytes and astrocytes. All the above experimental BW animals also have Schwann cells in their retinae which myelinate retinal ganglion cell axons in the fibre layer. Control animals comprised normal Long Evans Hooded rats, BW rats in which both retina and optic nerve were normal, and BW rats with Schwann cells in the retina but with normal, i.e. CNS myelinated, optic nerves. Regeneration was not observed in any of the control groups, demonstrating that, although the presence of Schwann cells in the retina may enhance the survival of retinal ganglion cells after crush, concomitant regrowth of axons cut in the optic nerve does not take place.

Reactive astrocytes in myelin-deficient rat optic nerve reveal an altered distribution of orthogonal arrays of particles (OAP)

Reactive astrocytes are a common feature of various pathological conditions within the CNS. Morphological changes of reactive astrocytes include an altered nucleus-cytoplasm relationship, nuclear indentations, an increased amount of intermediate filaments, and an immunologically immature phenotype. Additionally, the number of orthogonal arrays of particles (OAP) was found to be increased within parenchymal membranes of reactive astrocytes. This observation prompted us to investigate the distribution of astroglial OAP in the amyelinated CNS of the myelin-deficient (md) rat in which reactive astrocytes prevail. In the present freeze-fracture study, astroglial OAP were determined within endfoot and nonendfoot (parenchymal) membranes in the developing optic nerve of md rats and normal littermates at the age of 12, 23, 40, and 64 days postnatally (dpn). The endfoot OAP density in md astrocytes remained constant during the entire period of investigation. In myelinated littermates, OAP densities continuously increased up to adult values. In contrast, the parenchymal OAP density in md astrocytes increased during the entire period of investigation. In normal littermates, OAP densities remained constant during the first 40 dpn and thereafter increased rapidly. These observations suggest that the absence of myelinogenesis in the md mutant may be a stimulus for parenchymal membranes of reactive astrocytes to insert OAP or to assemble OAP subunits into complete arrays.

Factors influencing the regeneration of axons in the central nervous system

Damage to the central nervous system (CNS) causes damage to neurons. This damage can result in the complete death of neurons, or in them becoming disconnected from their inputs or target structures due to disruption of axons. The main reason why damage to the human CNS is so disastrous and disabling is that axons will not in general regenerate in the mammalian brain, and neurons once lost are not replaced. In order, therefore, to repair the CNS, techniques will have to be developed to replace dead neurons, and induce axon regrowth. Central to the technologies necessary for brain repair is the ability to induce and control the growth of axons, since in a damaged brain both surviving and newly implanted neurons must grow axons to make or remake appropriate synaptic connections. Worthwhile treatments, however, do not necessarily require the repair of all the damaged circuits in the CNS, it may be possible to substantially improve the function of patients with relatively few reconnected axons, if those axons are ones which mediate particularly important behaviours, such as respiration, bladder control, or hand and arm movements.