Retinal neurite growth on astrocytes is not modified by extracellular matrix, anti-L1 antibody, or oligodendrocytes

A cortical astrocyte subpopulation inhibits axon growth in vitro and in vivo

Astrocytes are the most heterogeneous and predominant glial cell type in the central nervous system. However, the functional significance of this heterogeneity remains to be elucidated. Following injury, damaged astrocytes inhibit axonal regeneration in vivo and in vitro. Cultured primary astrocytes are commonly considered good supportive substrates for neuron attachment and axon regeneration. However, it is not known whether different populations of cells in the heterogeneous astrocyte culture affect neuron behavior in the same way. In the present study, the effect of astrocyte heterogeneity on neuronal attachment and neurite outgrowth was examined using an in vitro and in vivo co-culture system. In vitro, neonatal cortical astrocytes were co-cultured with purified dorsal root ganglia (DRG) neurons and astrocyte growth morphology, neuron attachment and neurite growth were evaluated. The results demonstrated that the heterogeneous astrocyte cells showed two different types of growth pattern, typical and atypical. Typical astrocytes were supportive to neuron attachment and neurite growth, which was consistent with previous studies, whereas atypical astrocytes inhibited neuron attachment and neurite growth. These inhibitory astrocytes exhibited a special growth pattern with various shapes and sizes, a high cell density, few oligodendrocytes on the top layer and occupied a smaller growth area compared with typical astrocytes. Neurites extended freely on typical supportive astrocyte populations, however, moved away when they reached atypical astrocyte growth pattern. Neurons growing on the atypical astrocyte pattern demonstrated minimal neurite outgrowth and these neurites had a dystrophic appearance, however, neuronal survival was unaffected. Immunocytochemistry studies demonstrated that these atypical inhibitory astrocytes were glial fibrillary acidic protein (GFAP) positive cells. The existence of inhibitory astrocyte subpopulations in normal astrocytes reflects the complexity of the function of astrocyte populations. In vivo, DRG neurons in grey matter did not show neurite growth, while DRG neurons survived and showed robust axon outgrowth along the corpus callosum. In conclusion, further studies on this new type of inhibitory astrocyte subpopulation may deepen our understanding of the complex biology of astrocytes.

Schwann cell chondroitin sulfate proteoglycan inhibits dorsal root ganglion neuron neurite outgrowth and substrate specificity via a soma and not a growth cone mechanism

Sensory axons do not regenerate into or within the spinal cord because of the presence of the axon regeneration inhibitor chondroitin sulfate proteoglycan (CSPG) on activated astrocytes. In the peripheral nervous system, CSPG associated with denervated Schwann cells retards axon regeneration, but regeneration occurs because the balance of regenerating, inhibiting, and promoting factors favors regeneration. The present experiments were aimed at determining the mechanism by which Schwann cells inhibit adult human dorsal root ganglia (H-DRG) neuron growth cone elongation and substrate specificity, restricting the growth cones to Schwann cell membranes and inhibiting their growth onto a poly-l-lysine/laminin substrate. Neurites of H-DRG neurons free of soma contact with Schwann cells, or after the Schwann cell membranes' CSPG had been digested, were 11.1-fold longer than those of neurons in soma contact with untreated Schwann cells. Growth cones of DRG neuron somas without Schwann cell CSPG showed no outgrowth inhibition or substrate specificity. These results indicate that the Schwann cell CSPG influences act via contact with neuron somas but not growth cones. These results suggest that eliminating CSPG associated with Schwann cells within DRG in vivo will make the neurons' growth cones insensitive to the regeneration inhibitory influences of CSPG, allowing them to regenerate through the dorsal root entry zone and into and within the spinal cord, where they can establish appropriate and functional synaptic connections.

An in vitro model of the inhibition of axon growth in the lesion scar formed after central nervous system injury

After central nervous system (CNS) injury, meningeal fibroblasts migrate in the lesion center to form a fibrotic scar which is surrounded by end feet of reactive astrocytes. The fibrotic scar expresses various axonal growth-inhibitory molecules and creates a major impediment for axonal regeneration. We developed an in vitro model of the scar using coculture of cerebral astrocytes and meningeal fibroblasts by adding transforming growth factor-beta1 (TGF-beta1), a potent fibrogenic factor. Addition of TGF-beta1 to this coculture resulted in enhanced proliferation of fibroblasts and the formation of cell clusters which consisted of fibroblasts inside and surrounded by astrocytes. The cell cluster in culture densely accumulated the extracellular matrix molecules and axonal growth-inhibitory molecules similar to the fibrotic scar, and remarkably inhibited the neurite outgrowth of cerebellar neurons. Therefore, this culture system can be available to analyze the inhibitory property in the lesion site of CNS.

Membrane-bound CSPG mediates growth cone outgrowth and substrate specificity by Schwann cell contact with the DRG neuron cell body and not via growth cone contact

The central nervous system and peripheral nervous system (CNS/PNS) contain factors that inhibit axon regeneration, including myelin-associated glycoprotein (MAG), the Nogo protein, and chondroitin sulfate proteoglycan (CSPG). They also contain factors that promote axon regeneration, such as nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF). Axon regeneration into and within the CNS fails because the balance of factor favors inhibiting regeneration, while in the PNS, the balance of factor favors promoting regeneration. The balance of influences in the CNS can be shifted toward promoting axon regeneration by eliminating the regeneration-inhibiting factors, overwhelming them with regeneration-promoting factors, or making axon growth cones non-receptive to regeneration-inhibiting factors. The present in vitro experiments, using adult rat dorsal root ganglion (DRG) neurons, were designed to determine whether the regeneration-inhibiting influences of Schwann cell CSPG are mediated via Schwann cell membrane contact with the DRG neuron cell body or their growth cones. The average longest neurite of neurons in cell body contact with Schwann cells was 7.4-fold shorter than those of neurons without Schwann cell-neuron cell body contact (naked neurons), and the neurites showed substrate specificity, growing only on the Schwann cell membranes and not extending onto the laminin substrate. The neurites of naked neurons showed no substrate specificity and extended over the laminin substrate, as well as onto and off the Schwann cells. After digesting the Schwann cell CSPG with the enzyme C-ABC, neurons in cell body contact with Schwann cells extended neurites the same length as those of naked neurons, and their neurites showed no substrate selectivity. Further, the neurites of naked neurons were not longer than those of naked neurons not exposed to C-ABC. These data indicate that the extent of neurite outgrowth from adult rat DRG neurons and substrate specificity of their growth cone is mediated via contact between the Schwann cell membrane-bound CSPG and the DRG neuron cell body and not with their growth cones. Further, there was no apparent influence of diffusible or substrate-bound CSPG on neurite outgrowth. These results show that eliminating the CSPG of Schwann cells in contact with the cell body of DRG neurons eliminates the sensitivity of their growth cones to the CSPG-induced outgrowth inhibition. This may in turn allow the axons of these neurons to regenerate through the dorsal roots and into the spinal cord.

Astrocyte-associated fibronectin is critical for axonal regeneration in adult white matter

Although it has been suggested that astroglia guide pioneering axons during development, the cellular and molecular substrates that direct axon regeneration in adult white matter have not been elucidated. We show that although adult cortical neurons were only able to elaborate very short, highly branched, dendritic-like processes when seeded onto organotypic slice cultures of postnatal day 35 (P35) rat brain containing the corpus callosum, adult dorsal root ganglion (DRG) neurons were able to regenerate lengthy axons within the reactive glial environment of this degenerating white matter tract. The callosum in both P35 slices and adult rat brain was rich in fibronectin, but not laminin. Furthermore, the fibronectin was intimately associated with the intratract astrocytes. Blockade of fibronectin function in situ with an anti-fibronectin antibody dramatically decreased outgrowth of DRG neurites, suggesting that fibronectin plays an important role in axon regeneration in mature white matter. The critical interaction between regrowing axons and astroglial-associated fibronectin in white matter may be an additional factor to consider when trying to understand regeneration failure and devising strategies to promote regeneration.

Peripheral olfactory ensheathing cells reduce scar and cavity formation and promote regeneration after spinal cord injury

Bridging of a lesion site and minimizing local damage to create an environment permissive for regeneration are both primary components of a successful strategy to repair spinal cord injury (SCI). Olfactory ensheathing cells (OECs) are prime candidates for autologous transplantation to bridge this gap, but little is known currently about their mechanism of action. In addition, OECs from the accessible lamina propria (LP) of the olfactory mucosa are a more viable source in humans but have yet to be tested for their ability to promote regeneration in established SCI models. Here, mouse LP-OECs expressing green fluorescent protein (GFP) transplanted directly into both rat and mouse dorsolateral spinal cord lesion sites demonstrate limited migration but interact with host astrocytes to develop a new transitional zone at the lesion border. LP-OECs also promote extensive migration of host Schwann cells into the central nervous system repair zone and stimulate angiogenesis to provide a biological scaffold for repair. This novel environment created by transplanted and host glia within the spinal cord inhibits cavity and scar formation and promotes extensive sprouting of multiple sensory and motor axons into and through the lesion site. Sixty days after rat SCI, serotonin- and tyrosine hydroxylase-positive axons sprouted across the lesion into the distal cord, although axotomized rubrospinal axons did not. Thus, even in a xenotransplant paradigm, LP-OECs work collaboratively with host glial cells to create an environment to ameliorate local damage and simultaneously promote a regenerative response in multiple axonal populations.

Restoration of the retinofugal pathway

In a relatively short period of time covering the last 2 decades, regeneration of retinofugal axons has become one of most prominent experimental models in restorative neurobiology. There is now a significant knowledge both on the mechanisms governing retinal ganglion cell responses to transection of the optic nerve, and the subsequent cell-cell interactions accumulating in death of the neurons. In addition, retinofugal axons served as an excellent model to examine whether, and to conclude that these axons have remarkable abilities for re-growth. This last issue was of invaluable importance, because axons could regenerate in vivo, into peripheral nerve grafts, and last but not least within the white matter of the cut optic nerve. As it stands to date, the extremely complex aspects of axonal regeneration will probably be understood within the retinofugal pathway. Final elucidation of this delicate system will essentially lead to some revision of our knowledge concerning neurotraumatology and CNS-repair.

Robust regeneration of adult sensory axons in degenerating white matter of the adult rat spinal cord

We have recently reported that minimally disturbed adult CNS white matter can support regeneration of adult axons by using a novel microtransplantation technique to inject minute volumes of dissociated adult rat dorsal root ganglion neurons directly into adult rat CNS pathways (Davies et al., 1997). This atraumatic injection procedure minimized scarring and allowed considerable numbers of regenerating adult axons immediate access to the adult CNS glial terrain where they rapidly extended for long distances. A critical question remained as to whether degenerating white matter at acute and chronic stages (up to 3 months) after injury could still support regeneration. To investigate this, we have microtransplanted adult sensory neurons into degenerating white matter of the adult rat spinal cord several millimeters rostral to a severe lesion of the dorsal columns. Regeneration of donor sensory axons in both directions away from the site of transplantation was robust even within white matter undergoing fulminant Wallerian degeneration despite intimate contact with myelin. Along their route, the regrowing axons extended large numbers of collaterals into the adjacent dorsal horn. However, after entering the lesion, the rapidly extending growth cones stopped and became dystrophic within high concentrations of reactive glial matrix. Our results offer compelling evidence that the major environmental impediment to regeneration in the adult CNS is the molecular barrier that forms directly at the lesion site, and that degenerating white matter beyond the glial scar has a far greater intrinsic ability to support axon regeneration than previously thought possible.

Expression of polysialylated NCAM but not L1 or N-cadherin by regenerating adult mouse optic fibers in vitro

This study asks if there might be irreversible maturational changes in adult neurons that limit their capacity to regenerate. Retina from adult and embryonic mouse were placed in culture on laminin substrates so that regenerating adult optic fibers could be compared to growing embryonic fibers. Several cell adhesion molecules (CAMs) known to mediate the growth of embryonic neurites on astrocytes were assayed by immunocytochemistry: L1, N-cadherin, and NCAM. Thy 1.2, a potential CAM with inhibitory activity, was also examined. As in vivo, embryonic fibers were found to express both L1 and N-cadherin. In contrast, regenerating adult fibers had no detectable amounts of either of these CAMs. N-Cadherin is normally down regulated during development so its absence in adult fibers suggests it can not be reexpressed during regeneration. L1 is normally found in the proximal regions of adult optic fibers so its absence indicates it is not expressed or transported in regenerating fibers. Adult regenerating fibers expressed high levels of Thy 1.2, which was undetectable in embryonic optic fibers. Thy 1.2 is normally found in mature fibers, indicating this phenotypic feature is preserved during regeneration. Both adult and embryonic fibers showed strong reactivity for NCAM, which in vivo is normally found in embryonic and at lower levels in adult fibers. Surprisingly, both embryonic and regenerating adult fibers expressed high levels of polysialic acid, which is normally absent in adult fibers. NCAM may be one of few CAMs available to adult optic fibers for regeneration on astrocytes.

CNS Myelin: Does a Stabilizing Role in Neurodevelopment Result in Inhibition of Neuronal Repair after Adult Injury?

The inhibitory properties of mature oligodendrocytes and CNS myelin for neurite outgrowth were clearly documented more than a decade ago in studies involving co-cultures of dissociated glial cells and neurons. Since then, in vitro and in vivo studies have begun to characterize some of the CNS myelin-associated inhibitors of neurite growth. Furthermore, experimental techniques for neutralizing or suppressing these inhibitory effects have been developed. The results of several experiments, involving the suppression of myelination in the developing or adult CNS, suggest that the relatively late appearance of CNS myelin during neural development may serve to stabilize and restrict axonal outgrowth (e.g., collateral sprouting) after appropriate axonal connections have been established. This suggested developmental role of myelin may consolidate and limit the degree of axonal plasticity within the adult CNS; consequently, however, it might also limit axonal regeneration after injury.

Regeneration of adult axons in white matter tracts of the central nervous system

It is widely accepted that the adult mammalian central nervous system (CNS) is unable to regenerate axons. In addition to physical or molecular barriers presented by glial scarring at the lesion site, it has been suggested that the normal myelinated CNS environment contains potent growth inhibitors or lacks growth-promoting molecules. Here we investigate whether adult CNS white matter can support long-distance regeneration of adult axons in the absence of glial scarring, by using a microtransplantation technique that minimizes scarring to inject minute volumes of dissociated adult rat dorsal root ganglia directly into adult rat CNS pathways. This atraumatic injection procedure allowed considerable numbers of regenerating adult axons immediate access to the host glial terrain, where we found that they rapidly extended for long distances in white matter, eventually invading grey matter. Abortive regeneration correlated precisely with increased levels of proteoglycans within the extracellular matrix at the transplant interface, whereas successfully regenerating transplants were associated with minimal upregulation of these molecules. Our results demonstrate, to our knowledge for the first time, that reactive glial extracellular matrix at the lesion site is directly associated with failure of axon regrowth in vivo, and that adult myelinated white matter tracts beyond the glial scar can be highly permissive for regeneration.

Activated macrophages and the blood-brain barrier: inflammation after CNS injury leads to increases in putative inhibitory molecules

The cellular responses to spinal cord or brain injury include the production of molecules that modulate wound healing. This study examined the upregulation of chondroitin sulfate proteoglycans, a family of molecules present in the wound healing matrix that may inhibit axon regeneration in the central nervous system (CNS) after trauma. We have demonstrated increases in these putative inhibitory molecules in brain and spinal cord injury models, and we observed a close correlation between the tissue distribution of their upregulation and the presence of inflammation and a compromised blood-brain barrier. We determined that the presence of degenerating and dying axons injured by direct trauma does not provide a sufficient signal to induce the increases in proteoglycans observed after injury. Activated macrophages, their products, or other serum components that cross a compromised blood-brain barrier may provide a stimulus for changes in extracellular matrix molecules after CNS injury. While gliosis is associated with increased levels of proteoglycans, not all reactive astrocytes are associated with augmented amounts of these extracellular matrix molecules, which suggests a heterogeneity among glial cells that exhibit a reactive phenotype. Chondroitin sulfate also demarcates developing cavities of secondary necrosis, implicating these types of boundary molecules in the protective response of the CNS to trauma.

Critical period-dependent reduction of the permissiveness of cat visual cortex tissue for neuronal adhesion and neurite growth

During postnatal development, the visual cortex undergoes an experience-dependent refinement of its circuitry. This process includes synapse formation, as well as synapse elimination. Both mechanisms appear to be restricted to a limited 'critical period' which lasts for approximately 2 months in cats. We tested whether the termination of the critical period for cortical malleability is paralleled by changes in the growth permissiveness of the tissue. These changes may inhibit progressive reorganization of functional circuitries mediated by axon growth. Embryonic cortical neurons were cultured on unfixed cryostat sections of the visual cortex obtained from cats aged 2-50 weeks. After 2-3 days in vitro the distribution of viable cells and the percentage of neurite-bearing cells were determined and analysed with respect to the developmental age and subdivisions of the underlying tissue substrate. It was shown that cell adhesion and neurite formation are correlated with the developmental age of the substrate tissue and the time period of myelination. While embryonic neurons adhered and survived on grey and white matter tissue from 2- and 4-week-old kittens, there was a significant reduction in cell adhesion on the myelinated white matter regions of the tissue sections of older animals. Quantitative analyses showed that neurite formation by cultured neurons also became successively impaired on grey and white matter areas of tissue substrates, corresponding to the time course of the critical period for cortical malleability. On grey matter tissue this effect was most pronounced between the second and sixth postnatal weeks. The effects were not antagonized by coating the substrate sections with the growth-promoting molecule laminin. It is therefore proposed that neurite growth-inhibiting factors, most probably associated with central nervous system myelin, are gradually expressed postnatally and may contribute to the termination of the critical period in the visual cortex of cats.

Immunohistochemical localization of cell adhesion molecules and cell-cell contact proteins during regeneration of the rat optic nerve induced by sciatic nerve autotransplantation

BACKGROUND

The central nervous system neurons of adult mammals are known to regenerate into peripheral nerve autograft. The localization of cell adhesion molecules and cell-cell contact proteins were studied during axonal regeneration induced by sciatic nerve autotransplantation.

METHODS

A sciatic nerve autograft was anastomosed to the proximal stump of the transected rat optic nerve. Immunofluorescence microscopy, thin sectioning, and immunoelectron microscopy with the preembedding method and ultrathin cryosections were used to localize cell adhesion molecules (L1; neural cell adhesion molecule, NCAM; myelin-associated glycoprotein, MAG) and cell-cell contact proteins (connexins 32, 43, ZO-1) at 3 days to 4 weeks postoperation.

RESULTS

Most regenerating axons contacted astrocytes in the optic nerve and Schwann cells in the graft. Immunoreactivity of NCAM was widely distributed along the surface of axons, astrocytes, Schwann cells, and perineurial cells. The L1 immunoreactivity was confined to the interface of axon-astrocyte and of axon-Schwann cell. MAG immunoreactivity was seen at the interface of axon and myelin within the graft. Connexins 32, 43, and ZO-1 immunoreactivities were observed at contact sites between axons and Schwann cells within the graft.

CONCLUSIONS

Cell adhesion molecules (L1, NCAM, MAG) are localized at the cell surface of regenerating axons, astrocytes, and Schwann cells during optic nerve regeneration elicited by peripheral nerve graft. Cell-cell contact proteins (connexins 32, 43, ZO-1) are present at the interface between axons and Schwann cells in the graft. Our results suggest that these molecules are involved in cell adhesion events during optic nerve regeneration.

Sprouting adult CNS cholinergic axons express NILE and associate with astrocytic surfaces expressing neural cell adhesion molecule

To assess the cellular and molecular substrates for cholinergic axon growth in the adult central nervous system (CNS), we implanted grafts of control and nerve growth factor (NGF)-producing genetically modified fibroblasts within the striatum of rats. Sprouting cholinergic axonal processes that grew into grafts of NGF-producing fibroblasts were fasciculated and followed the surface of astrocytic processes for long distances within the grafts. The close and long distance anatomical relationship between the sprouted axons and the astrocytes supported previous ultrastructural evidence that astrocytes may serve as a cellular substrate for sprouting cholinergic axons in vivo. The sprouted axon processes were associated with the expression of nerve growth factor-inducible large external (NILE) glycoprotein on their surfaces. NILE expression was not seen in control grafts where there was an absence of cholinergic ingrowth. NILE has been demonstrated to play a role in axon fasciculation in a number of other neural systems. The astrocytic processes in both control and NGF-producing fibroblast grafts expressed neural cell adhesion molecule (NCAM), suggesting that NCAM-mediated adhesion may be responsible for the close relationship between the axons and astrocytes within the grafts. NGF-induced heterotypic interactions between neuronal NILE and astroglial NCAM may also be required for adult cholinergic axonal sprouting.

Glial cell types, lineages, and response to injury in rat and fish: implications for regeneration

Axons of the mammalian central nervous system do not regenerate spontaneously after axonal injury, unlike the central nervous system axons of fish and amphibians and the peripheral nervous system of mammals, which possess a good regenerative ability (Grafstein: The Retina: A Model for Cell Biology Studies, Part II, 1986; Kiernan: Biol Rev 54:155-197, 1979; Murray: J Comp Neurol 168:175-196, 1976; Ramón y Cajal: Degeneration and Regeneration of the Nervous System, 1928; Reier and Webster: J Neurocytol 3:591-618, 1974; Sperry: Physiol Zool 23:351-361, 1948). It was previously believed that intrinsic differences between the central nervous system neurons of mammals and fish account for their differences in regenerative ability. The past decade, however, has seen an accumulation of evidence, indicating that mammalian central nervous system neurons are able to regenerate injured axons, at least to some extent. This was first demonstrated by Aguayo and colleagues (David and Aguayo: Science 214:931-933, 1981; Kierstead et al: Science 246:255-257, 1989), who showed that injured mammalian central nervous system axons can grow for a considerable distance into an autograft of a peripheral nerve. It was also demonstrated that injured rabbit optic axons can regenerate into their own environment (i.e., into the distal part of the injured optic nerve), if the injured nerve is treated so as to make it conducive for growth (Lavie et al: J Comp Neurol 298:293-314, 1990; Eitan et al: Science 264:1764-1768, 1994).(ABSTRACT TRUNCATED AT 250 WORDS)

Regrowth of dorsal root axons into a radiation-induced glial-deficient environment in the spinal cord

Exposure of the lumbosacral spinal cord of early postnatal rats to X-rays reduces the glial populations within the irradiated region. The present study examines the ability of axons of a dorsal root subjected to a crush-freeze lesion to grow back into this glial-deficient spinal cord environment, in contrast to the non-irradiated rat. Ultrastructural examination of the dorsal root entry zone (DREZ) 60 days after root injury revealed a well-formed astrocytic scar in this zone and adjacent regions of spinal cord in non-irradiated rats. In contrast, scar formation did not occur in irradiated root-lesioned animals in which the astrocytic response was quite limited. Axons were present in the DREZ and underlying spinal cord in irradiated root-lesioned rats at this time but were absent from these regions in the non-irradiated lesioned controls. These ultrastructural findings are highly suggestive that axons are capable of regrowth into the irradiated spinal cord. Axonal regrowth was assessed further by tracing techniques after application of a combination of peroxidase-labeled wheat germ agglutinin and horseradish peroxidase to the cut end of the root distal to the previously injured site. Labeled axons were readily identified within the spinal gray matter in irradiated lesioned but not in the non-irradiated lesioned rats. These data, together with the ultrastructural observations, are supportive of regrowth of the dorsal root axons into the spinal cord. The radiation-induced changes in the glial populations are discussed with regard to conversion of a normally non-permissive environment into one conducive for axonal regrowth.

Growth and degeneration of axons on astrocyte surfaces: effects on extracellular matrix and on later axonal growth

Cultured astrocytes deposit an extracellular matrix which has been shown by immunocytochemistry to react with antibodies to tenascin, laminin, and fibronectin. Neuronal-glial interaction down-regulates these components of the matrix, causing a reduction in extracellular matrix localized to areas of contact with axons. Axons used for these experiments were from embryonic rat retinal explants. In some experiments explants were removed from the co-cultures and their axons allowed to degenerate. Degeneration of axons did not reverse the local reduction of extracellular matrix brought about by axon outgrowth. The period of axon outgrowth studied was 4-5 days; the period of degeneration was 2-3 days. Astrocytes alone, astrocytes with intact retinal explants, and astrocytes with 2-day degenerated retinal axons were tested for their ability to support neurite outgrowth from embryonic rat cortical neurons. Neurite outgrowth occurred on all astrocyte cultures. Cortical neurite lengths, measured 2 days after plating, were not significantly different between astrocytes alone and astrocytes with degenerated retinal axons. However, there was a tendency for neurites to be shorter on astrocytes with intact retinal axons present. Two conclusions may be drawn from these results. First, the state of differentiation of astrocytes, as marked by their assembly of extracellular matrix, is altered by contact with axons. Second, degeneration of axons alone, in the absence of other cell types, is not a sufficient signal to reestablish assembly of extracellular matrix. However, neither is it a sufficient signal to render astrocytes inhospitable to further axonal outgrowth or regeneration.

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.

Fish optic nerve oligodendrocytes support axonal regeneration of fish and mammalian retinal ganglion cells

Segments from adult fish and rat retinae were explanted on myelin-marker expressing oligodendrocytes derived from the regenerating goldfish optic nerve. Fish axons grew in high density and even rat retinal axons regenerated to considerable length on the surface of the fish oligodendrocytes, suggesting that this type of fish glia has axon-growth promoting surface components that exert their influence across species boundaries. One interesting surface component of the fish oligodendrocytes as demonstrated here is the E 587 antigen, which is related to the L1 family of cell adhesion molecules. In long term cocultures of oligodendrocytes and retinal axons, the fish glial cells were found to enwrap rat axons. This suggests that the oligodendrocytes of the regenerating goldfish optic nerve/tract may, despite striking differences, represent the equivalent to mammalian optic nerve oligodendrocytes.

Effects of transforming growth factor-beta 1 on the extracellular matrix and cytoskeleton of cultured astrocytes

The present study was performed on primary cultures and subcultures of cerebellar astrocytes in order to investigate the effects of transforming growth factor-beta 1 (TGF beta 1) on proliferation, extracellular matrix (ECM) components, and cytoskeletal structures in relation to morphological changes. The expression and cellular distribution of the ECM components laminin and fibronectin and the cytoskeletal proteins glial fibrillary acidic protein (GFAP) and actin were investigated by immunoblotting, immunocytochemistry, and phalloidin staining. The proliferation of primary cultures was strongly inhibited by TGF beta 1. Treated cells became enlarged and spread onto the substratum. TGF beta 1 promoted the appearance of actin stress fibers and increased the cell actin content. It elicited a slight increase in GFAP expression and induced dispersion of thin filaments of GFAP. TGF beta 1 also stimulated the production of laminin and fibronectin and their incorporation into the ECM of primary cultures grown in medium with or without serum. Astrocytes grown in serum-containing medium for 1 day after subculturing responded strongly to TGF beta 1. Changes promoted by TGF beta 1 in cell shape, cytoskeleton, and ECM production of cultured astrocytes may have relevance for understanding the mechanisms of action of TGF beta 1 during brain development.

Axonal growth on astrocytes is not inhibited by oligodendrocytes

Axon growth in vitro may be inhibited by contact with oligodendrocytes, but most axons grow readily on the surface of astrocyte monolayers. Since both cell types are in close contact with one another in the damaged nervous system, we have examined the growth of axons on cultures which contain both astrocytes and oligodendrocytes. Cultures derived from neonatal rat forebrain develop with a monolayer of large flat astrocytes attached to the culture dish, and with many smaller cells of the oligodendrocyte lineage on their surface. Dorsal root ganglia placed on these cultures grow axons readily, the overall extent of growth being unaffected by the presence or absence of oligodendrocytes, many of which express galactocerebroside and the inhibitory molecule janusin. A previous set of experiments had shown that growth of these axons is inhibited by oligodendrocytes by themselves. Scanning electron microscopy coupled with silver-intensified immunostaining reveals that the axons grow on the surface of the astrocytic layer, underneath the oligodendrocytes, and are therefore in contact with both cell types as they grow. The presence of astrocytes therefore alters the results of axonal contact with oligodendrocytes.

Role of the cell recognition molecule, cognin, in GABAergic differentiation in chick retina

Previous work showed that GABAergic differentiation in developing chick retina depends on insulin and cell interactions. Here, we investigated whether it depended on cell signaling mediated by retina cognin, a 50 kDa cell recognition molecule. Cognin mediates cell adhesion in vitro and occurs on retinal neurons that become both GABAergic and cholinergic. We investigated two markers of GABAergic differentiation: glutamate decarboxylase (GAD) activity and high-affinity GABA uptake. Both increase during differentiation of retinal neurons in culture and can be easily measured. We blocked cognin-mediated cell signaling with cognin antibody and found a reduction of the developmental increase in GAD activity in cultures of retinal neurons from 7 and 11 day chick embryos. There was no reduction of high-affinity GABA uptake. This suggested that cognin-mediated signaling was necessary for the normal developmental increase in GAD but not for high-affinity GABA uptake. These results contrasted with our previous observations on cholinergic differentiation in cultured retinal neurons. We found that cognin antibody blocked the normal developmental increase in choline acetyltransferase (ChAT) only if the cells were exposed before embryonic day 7. Thus, while both GAD and ChAT activity appear to be controlled by cell signaling involving cognin, the periods of developmental sensitivity for the two differentiation markers are different. Antibodies to other adhesion molecules, Ng-CAM, and N-cadherin, did not similarly affect GAD activity. Antibodies to laminin at a 10-fold higher concentration inhibited GAD activity only in early embryonic retina. Tests for protein synthesis and "housekeeping" enzyme activity demonstrated that the cognin antibody effect was selective for neuronal differentiation pathways.(ABSTRACT TRUNCATED AT 250 WORDS)

The cell recognition molecule, cognin, mediates choline acetyltransferase activity in embryonic chick retina

Cell signaling and cell-cell interactions play an important role in neuronal differentiation in the embryonic CNS. Previous work (Hausman, R.E., Vivek Sagar, G.D. and Shah, B.H., Dev. Brain Res., 59 (1991) 31-37) had shown that cholinergic differentiation in the embryonic chick retina depends on insulin and neuron-neuron interactions. Here, we pursued the molecular nature of that dependence on cell interactions. The embryonic chick retina is known to contain several cell adhesion or recognition molecules. We asked if retina cognin, a 50 kDa cell surface-associated protein, played a role in controlling cholinergic differentiation in the developing chick retina. As previously, cholinergic differentiation was measured by two markers: choline acetyltransferase (ChAT) activity and high-affinity choline uptake. We used polyclonal antibody to cognin to determine if blocking cognin-mediated cell interactions would affect the normal embryonic increases in these cholinergic markers. We demonstrated a 40% inhibition of the normal developmental appearance of ChAT activity in retina neuronal cultures from early development, but no effect in cultures from more differentiated retina. The inhibition was selective for retina, since it was not seen in neural tissues like cerebrum and cerebellum that also express ChAT. In contrast to the effect of insulin, choline uptake was not affected by treatment with cognin antibody. Antibodies to two other cell recognition molecules present in the retina (Ng-CAM and N-cadherin) did not block the normal developmental appearance of ChAT. These results suggest that cognin-mediated interactions play a unique role in the control of one aspect of cholinergic differentiation in the developing chick retina.

Glial response to dorsal root lesion in the irradiated spinal cord

Exposure of the rat lumbar spinal cord to X-rays during the early postnatal period results in a marked reduction in the glial populations within the irradiated region. The present study was undertaken to determine what effects this reduction of glia, particularly astrocytes, has on the pattern and characteristics of the scar formation that follows root injury in the normal spinal cord. Morphological assessments 60 days following injury of the right L4 dorsal root revealed a distinct difference in the extent of the astrocyte response between the irradiated and the nonirradiated rats. In the nonirradiated animals, a thick astrocytic scar composed of multiple layers of astrocyte processes formed over the dorsal horn and adjacent portions of the dorsal surface of the cord. This astrocytic response was not confined to the surface of the spinal cord but extended also into the root, i.e., into regions normally considered as PNS. In irradiated rats, the astrocytes did not form a thick scar nor did they extend into the injured root. Instead, they formed a glia limitans and were at most only one or two layers thick over the region of cord comparable to that occupied by the thick astrocytic scar in the nonirradiated rats. Mechanisms involved in the glial response of the irradiated spinal cord to dorsal root injury are discussed, particularly with regard to the possible positive effect that this reduction in scar formation may have on regrowth of injured dorsal root axons into the spinal cord environment.

Intrinsic neuronal determinants of regeneration

Axon growth and axon regeneration are co-operative processes; the speed and extent of axon growth are influenced both by the properties of the environment surrounding the axon growth cone, and the properties of the neuron itself. In recent years, the environmental influences on axon growth have received most of the attention directed towards this area of research, but the properties of the neurons themselves are likely to be just as important. Within both adults and embryos there are differences in the growth potential of different neuronal types, and there is also evidence for an overall decrease in the vigour of axon growth with neuronal age.