The enigma of myelin-associated growth inhibitors in spontaneously regenerating nervous systems

Mitochondrial function in spinal cord injury and regeneration

Many people around the world suffer from some form of paralysis caused by spinal cord injury (SCI), which has an impact on quality and life expectancy. The spinal cord is part of the central nervous system (CNS), which in mammals is unable to regenerate, and to date, there is a lack of full functional recovery therapies for SCI. These injuries start with a rapid and mechanical insult, followed by a secondary phase leading progressively to greater damage. This secondary phase can be potentially modifiable through targeted therapies. The growing literature, derived from mammalian and regenerative model studies, supports a leading role for mitochondria in every cellular response after SCI: mitochondrial dysfunction is the common event of different triggers leading to cell death, cellular metabolism regulates the immune response, mitochondrial number and localization correlate with axon regenerative capacity, while mitochondrial abundance and substrate utilization regulate neural stem progenitor cells self-renewal and differentiation. Herein, we present a comprehensive review of the cellular responses during the secondary phase of SCI, the mitochondrial contribution to each of them, as well as evidence of mitochondrial involvement in spinal cord regeneration, suggesting that a more in-depth study of mitochondrial function and regulation is needed to identify potential targets for SCI therapeutic intervention.

Tenascin-R as a repellent guidance molecule for newly growing and regenerating optic axons in adult zebrafish

In adult fish, in contrast to mammals, new optic axons are continuously added to the optic projection, and optic axons regrow after injury. Thus, pathfinding of optic axons during development, adult growth, and adult regeneration may rely on the same guidance cues. We have shown that tenascin-R, a component of the extracellular matrix, borders the optic pathway in developing zebrafish and acts as a repellent guidance molecule for optic axons. Here we analyze tenascin-R expression patterns along the unlesioned and lesioned optic pathway of adult zebrafish and test the influence of tenascin-R on growing optic axons of adult fish in vitro. Within intraretinal fascicles of optic axons and in the optic nerve, newly added optic axons grow in a tenascin-R immunonegative pathway, which is bordered by tenascin-R immunoreactivity. In the brain, tenascin-R expression domains in the ventral diencephalon, in non-retinorecipient pretectal nuclei and in some tectal layers closely border the optic pathway in unlesioned animals and during axon regrowth. We mimicked these boundary situations with a sharp substrate border of tenascin-R in vitro. Optic axons emanating from adult retinal explants were repelled by tenascin-R substrate borders. This is consistent with a function of tenascin-R as a repellent guidance molecule in boundaries for adult optic axons. Thus, tenascin-R may guide newly added and regenerating optic axons by a contact-repellent mechanism in the optic pathway of adult fish.

Radial glial cells, proliferating periventricular cells, and microglia might contribute to successful structural repair in the cerebral cortex of the lizard Gallotia galloti

Reptiles are the only amniotic vertebrates known to be capable of spontaneous regeneration of the central nervous system (CNS). In this study, we analyzed the reactive changes of glial cells in response to a unilateral physical lesion in the cerebral cortex of the lizard Gallotia galloti, at 1, 3, 15, 30, 120, and 240 days postlesion. The glial cell markers glial fibrillary acidic protein (GFAP), glutamine synthetase (GS), S100 protein, and tomato lectin, as well as proliferating cell nuclear antigen (PCNA) were used to evaluate glial changes occurring because of cortical lesions. A transitory and unilateral upregulation of GFAP and GS in reactive radial glial cells were observed from 15 to 120 days postlesion. In addition, reactive lectin-positive macrophage/microglia were observed from 1 to 120 days postlesion, whereas the expression of S100 protein remained unchanged throughout the examined postlesion period. The matricial zones closest to the lesion site, the sulcus lateralis (SL) and the sulcus septomedialis (SSM), showed significantly increased numbers of dividing cells at 30 days postlesion. At 240 days postlesion, the staining pattern for PCNA, GFAP, GS, and tomato lectin in the lesion site became similar to that observed in unlesioned controls. In addition, ultrastructural data of the lesioned cortex at 240 days postlesion indicated a structural repair process. We conclude that restoration of the glial framework and generation of new neurons and glial cells in the ventricular wall play a key role in the successful structural repair of the cerebral cortex of the adult lizard.

Effects of axotomy on the expression of NADPH-diaphorase in the visual pathway of the tench

The distribution of NADPH-diaphorase (ND) positive elements was analyzed throughout the visual pathway of the tench in normal conditions and after optic nerve transection. In the control retina, ND-labeled elements were observed in the photoreceptor, inner nuclear, outer nuclear and ganglion cell layers. In the optic nerve of control animals, small and numerous ND-positive glial cells that were identified as presumably astrocyte-like cells were observed. In the optic tracts and optic tectum, a different type of ND-positive glial cell was detected. Axotomy induced severe changes in the ND staining pattern in the visual pathway. A decrease in the number of ND-stained cells was detected in the retina. In the optic nerve of lesioned animals, the number of small cells gradually decreased, whereas the number of large cells did not change. Two new ND-positive cell populations were observed after the lesion: microglial-like cells appeared close to the lesioned area from 24 h to 7 days after transection, and astrocyte-like cells were found throughout the optic nerve from 14 days up to at least 120 days. The total number of ND-stained glial cells increased at 30 and 60 days and returned to control parameters at 120 days. In addition, the number of ND-positive cells increased at the same survival times in the optic tracts and in the retinorecipient strata of the optic tectum with respect to control animals. Thus, degenerative/regenerative processes in the fish visual pathway are accompanied by an increase in the number of ND-positive cells. Synthesis of nitric oxide is elicited in microglial-like cells as a response to axon injury, whereas the expression in astrocyte-like cells seems to be associated with both normal processes under physiological conditions and with the regenerative phase after the lesion.

Immunological approaches to the treatment of spinal cord injury

The innate and adaptive arms of the immune system, represented principally by macrophages and by T and B cells, respectively, provide body tissues with mechanisms of defence, protection and repair. In the central nervous system (CNS), probably because of its status of 'immune privilege', any immune activity has long been viewed as detrimental. Recent studies have provided evidence, however, that immune activity after traumatic CNS injury may have a beneficial effect, manifested by promotion of regeneration and reduction in the secondary degeneration of neurons that escaped direct injury. Rigorous regulation of immune system activity allows the individual to derive the benefit of such neuroprotection without the risk of detrimental side effects. Recently, our research group found a way to boost the T-cell-mediated autoimmune protection while avoiding the risk of autoimmune disease.

Regenerating descending axons preferentially reroute to the gray matter in the presence of a general macrophage/microglial reaction caudal to a spinal transection in adult zebrafish

We analyzed pathway choices of regenerating, mostly supraspinal, descending axons in the spinal cord of adult zebrafish and the cellular changes in the spinal cord caudal to a lesion site after complete spinal transection. Anterograde tracing (by application of the tracer rostral to the spinal lesion site) showed that significantly more descending axons (74%) regenerated in the spinal gray matter of the caudal spinal cord than would be expected from random growth. Retrograde tracing (by application of the tracer caudal to the spinal lesion site) showed that, rostral to the lesion, most of these axons (80%) extended into the major white matter tracts. Thus, ventral descending tracts often were devoid of labeled axons caudal to a spinal lesion but contained many axons rostral to the lesion in the same animals, indicating a pathway switch of descending axons from the white matter to the gray matter. Ascending axons of spinal neurons were not observed regrowing to the rostral tracer application site; therefore, they most likely did not contribute to the axonal populations analyzed. A macrophage/microglia response within 2 days of spinal cord transection, along with phagocytosis of myelin, was observed caudal to the transection by immunohistochemistry and electron microscopy. Nevertheless, caudal to the lesion, descending tracts in the white matter were filled with myelin debris during the time of axonal regrowth, at least up to 6 weeks postlesion. We suggest that the spontaneous regeneration of axons of supraspinal origin after spinal cord transection in adult zebrafish may be due in part to the axons' ability to negotiate novel pathways in the spinal cord gray matter.

The yin and yang of tenascin-R in CNS development and pathology

An important biological consequence of the initial interactions between the cell surface and its extracellular environment is the diversity of cellular responses ranging from overt repulsion or avoidance reaction to stable adhesion or final positioning. It is now evident that positive and negative guiding mechanisms are equally relevant to normal pattern formation during development and decisive for the outcome of a regenerative process. In this context, the present review summarizes the knowledge about the extracellular matrix glycoprotein tenascin-R, a member of the tenascin gene family. In contrast to all other known family members, tenascin-R is exclusively expressed in the central nervous system of vertebrates by oligodendrocytes and neuronal subsets at later developmental stages and in adulthood. We focus on the glycoprotein's structure, tissue distribution and functional implications in the molecular control of axon targeting, neural cell adhesion, migration and differentiation during nervous system morphogenesis and pathology.

Potential repair of rat spinal cord injuries using stimulated homologous macrophages

The failure of the adult mammalian central nervous system (CNS) to regenerate after injury has long been viewed as a unique phenomenon resulting from the specific nature of this system. The finding that some CNS axons could be induced to regrow if provided with a permissive environment suggested that this failure is a result, at least in part, of the nature of the postinjury neuronal environment. It was further shown that the involvement of inflammatory cells, particularly macrophages, in postinjury processes in the CNS is limited. We have suggested that, to achieve recovery after injury, the adult mammalian CNS may require the assistance of the same postinjury factors as those involved in the recovery of spontaneously regenerating systems but that its accessibility to such assistance is restricted. Accordingly, we proposed that it might be possible to circumvent the restriction, allowing regeneration to occur. We showed that the implantation of autologous macrophages, which had been prestimulated by exposure to a regenerative (sciatic) nerve, into completely transected spinal cords of adult rats led to partial motor recovery. This treatment intervenes in the postinjury process by simulating in the axotomized CNS the events that occur naturally in spontaneously regenerating systems.

Tenascin-R inhibits the growth of optic fibers in vitro but is rapidly eliminated during nerve regeneration in the salamander Pleurodeles waltl

Tenascin-R is a multidomain molecule of the extracellular matrix in the CNS with neurite outgrowth inhibitory functions. Despite the fact that in amphibians spontaneous axonal regeneration of the optic nerve occurs, we show here that the molecule appears concomitantly with myelination during metamorphosis and is present in the adult optic nerve of the salamander Pleurodeles waltl by immunoblots and immunohistochemistry. In vitro, adult retinal ganglion cell axons were not able to grow from retinal explants on a tenascin-R substrate or to cross a sharp substrate border of tenascin-R in the presence of laminin, indicating that tenascin-R inhibits regrowth of retinal ganglion cell axons. After an optic nerve crush, immunoreactivity for tenascin-R was reduced to undetectable levels within 8 d. Immunoreactivity for the myelin-associated glycoprotein (MAG) was also diminished by that time. Myelin was removed by phagocytosing cells at 8-14 d after the lesion, as demonstrated by electron microscopy. Tenascin-R immunoreactivity was again detectable at 6 months after the lesion, correlated with remyelination as indicated by MAG immunohistochemistry. Regenerating axons began to repopulate the distal lesioned nerve at 9 d after a crush and grew in close contact with putative astrocytic processes in the periphery of the nerve, close to the pia, as demonstrated by anterograde tracing. Thus, the onset of axonal regrowth over the lesion site was correlated with the removal of inhibitory molecules in the optic nerve, which may be necessary for successful axonal regeneration in the CNS of amphibians.

Restricted inflammatory reaction in the CNS: a key impediment to axonal regeneration?

Axons in the central nervous system (CNS) of adult mammals do not regenerate after injury. Mammalian CNS differs in this respect from other mammalian tissues, including the peripheral nervous system (PNS), and from the CNS of lower vertebrates. In most parts of the body, including the nervous system, injury triggers an inflammatory reaction involving macrophages. This reaction is needed for tissue healing; when it is delayed or insufficient, healing is incomplete. The CNS, although needing an efficient inflammatory reaction resembling that in the periphery for tissue healing, appears to have lost the ability to supply it. We suggest that restricted CNS recruitment and activation of macrophages are linked to regeneration failure and might reflect the immune privilege that characterizes the mammalian CNS. As macrophages play a critical role in tissue restoration, and because their recruitment and activation are among the most upstream of the events leading to tissue healing, overcoming the deficiencies in these steps might trigger a self-repair processing leading to recovery after CNS injury.

Olfactory ensheathing glia: properties and function

The failure of regenerating axons to grow within the adult mammalian central nervous system (CNS) does not apply to the olfactory bulb (OB). In this structure, normal and transected olfactory axons are able to enter, regenerate, and reestablish lost synaptic contacts with their targets, throughout the lifetime of the organism. A remarkable difference between an axonal growth-permissive structure such as the OB and the remaining CNS resides in the presence of ensheathing glia in the former. These cells exhibit phenotypic and functional properties known to be involved in the process of axonal elongation that may explain the permissibility of the OB to axonal growth. In addition, transplants of ensheathing glia were successfully used to promote axonal regeneration within the injured adult CNS. The axonal growth-promoting properties of ensheathing glia make the study of this cell type interesting to provide an insight into the mechanisms underlying the process of axonal regeneration. Therefore, in this article we review the developmental, morphologic, immunocytochemical, and functional properties presented by this unique glial cell type, and correlate them with the axonal growth-promoting ability of ensheathing glia. In addition, we provide some evidence of the potentiality that ensheathing glia might have as a promoter of axonal regeneration within the injured nervous system.

Differences in myelination between spinal cord and corticular tissue transplanted intraocularly in rats

This paper compares the myelination in rat cortex and spinal cord transplanted on the embryonic day (E) 12, 14, 16, 18, 20 and right after birth (P0) into the anterior eye chamber of adult rats. Myelinated fibers were not observed in either cortical or spinal cord transplants of E12. When transplanted on E14, abundant myelinated fibers developed in the spinal cord and gathered at the periphery of the transplants as a "white matter". In the grafted cortex myelinated fibers were found when transplantation occurred on E16 or later. The myelinated fibers, however, remained scarce or formed only narrow bundles. The number and distribution of myelinated fibers did not depend on the donor's age between E16 and P0: even in the latter case transplanted cortex were found without myelination. The differences could be attributed to the different development of the cortical and spinal cord oligodendrocytes and to the different intrinsic organization of the grafted samples.

Capability for reactive gliosis develops prenatally in the diencephalon but not in the cortex of rats

In this study, the glial reactions to stab wounds were investigated on a large population of newborn (P0) and fetal rats, by the immunohistochemical staining of the glial fibrillary acidic protein. The lesions penetrated both the cortex and the diencephalon. The fetuses were lesioned in utero from the 17th embryonic day (E17) and were born on E22 or E23 in the natural way. In the cortex usually no reactive gliosis developed although definitive tissue destructions remained after the lesion. Weak and incomplete glial reactions were observed in a few cases of E20 or P0 lesions only. In the diencephalon, however, the same stabbings provoked massive glial reactions. The timing and the morphology of this reaction were similar to those found in adult animals. At E17 the lesion did not result in reactive gliosis even in the diencephalon. Our study highlights two phenomena: (i) depending on the brain area servere glial reactions can already follow fetal lesions, and (ii) the appearance of the capability for glial reactions may be a stage of the local tissue maturation in every brain area and cannot be considered as a function of brain development in general. Probably, the capability for glial reactions can take place only when certain histogenetic processes (e.g., cell migration, axon growth, apoptosis) have been at least mostly accomplished, but which of the local development events are the determining ones remains to be investigated.

Glial lineages and myelination in the central nervous system

Oligodendrocytes, derived from stem cell precursors which arise in subventricular zones of the developing central nervous system, have as their specialist role the synthesis and maintenance of myelin. Astrocytes contribute to the cellular architecture of the central nervous system and act as a source of growth factors and cytokines; microglia are bone-marrow derived macrophages which function as primary immunocompetent cells in the central nervous system. Myelination depends on the establishment of stable relationships between each differentiated oligodendrocyte and short segments of several neighbouring axons. There is growing evidence, especially from studies of glial cell implantation, that oligodendrocyte precursors persist in the adult nervous system and provide a limited capacity for the restoration of structure and function in myelinated pathways damaged by injury or disease.

Increased expression of specific recognition molecules by retinal ganglion cells and by optic pathway glia accompanies the successful regeneration of retinal axons in adult zebrafish

Retinal ganglion cells (RGCs) in adult zebrafish can regenerate their axons. We show that successful axonal regeneration is accompanied by the re-expression by RGCs of mRNAs encoding specific recognition molecules that are expressed at high levels in the larval retina but are down-regulated in the adult. Message levels for 11.1 and 11.2 (two homologs of mammalian L1), n-cam (homologous to mammalian N-CAM), beta 3 (related to the beta 3 and beta 2 subunits of mammalian Na,K-ATPase), and tn-c (homologous to mammalian tenascin-C) were high in larval RGCs undergoing axonogenesis and low in adult RGCs. After an optic nerve crush, axotomized adult RGCs showed increased levels of 11.1, 11.2 and n-cam mRNA expression, whereas the levels of beta 3 and tn-cmRNA remained unchanged. The optic nerve crush also induced the expression of some of these mRNAs in the optic nerve and tract where they are not normally detectable. This lesion induced up-regulation by presumptive glia was observed for 11.1, 11.2, n-cam and beta 3 but not for tn-c. The combination of a neuronal (intrinsic) response to axotomy with an environmental (extrinsic) response may be an important determinant allowing for the successful axonal regeneration.

Regeneration of immature mammalian spinal cord after injury

In this review we describe the growth of regenerating fibres through lesions in immature mammalian spinal cord. In newborn opossums and foetal rats, repair occurs rapidly and reliably without antibodies, implants or bridges of undamaged spinal cord. In the neonatal opossum one can compare recovery from lesions made to the CNS at various stages of development in the animal and in culture. As the CNS matures, the capacity for regeneration ceases abruptly. In particular, the extracellular matrix and molecules associated with glia have been shown to play a role in promoting and inhibiting regeneration. Major problems concern the precision with which regenerating axons become reconnected to their targets, and the specificity needed for recovery of function.

Axonal sprouting and synaptogenesis in temporal lobe epilepsy: possible pathogenetic and therapeutic roles of neurite growth inhibitory factors

Axonal sprouting and synaptic reorganization within the temporal lobe following neuronal injury have been implicated in the pathogenesis of temporal lobe epilepsy (TLE). The molecular species responsible for these structural changes have yet to be fully defined. The recent characterization of molecules whose normal function within the nervous system is to inhibit neurite outgrowth and synaptogenesis prompts the suggestion that a diminution or loss of such molecules might be of relevance to the pathogenesis of TLE. If so, the possibility of developing a novel therapeutic approach to TLE, distinct from currently available symptomatic therapies, to arrest the pathogenetic processes is also raised.

Isolation of cDNA clones encoding RICH: a protein induced during goldfish optic nerve regeneration with homology to mammalian 2',3'-cyclic-nucleotide 3'-phosphodiesterases

Using data derived from peptide sequencing of p68/70, a protein doublet induced during optic nerve regeneration in goldfish, we have isolated cDNAs that encode RICH (regeneration-induced CNPase homolog) from a goldfish regenerating retina cDNA library. The predicted RICH protein comprises 411 amino acids, possesses a pI of 4.48, and shows significant homology to the mammalian myelin marker enzyme 2',3'-cyclic-nucleotide 3'-phosphodiesterase (CNPase; EC 3.1.4.37). The mRNA encoding RICH was demonstrated, by both Northern blot analysis and RNase protection assays, to be induced as much as 8-fold in regenerating goldfish retinas at 20 days after nerve crush. Analysis of total RNA samples from various tissues showed a broad distribution of RICH mRNA, with the highest levels observed in gravid ovary. The data obtained strongly suggest that RICH is identical or very similar to p68/70. The molecular cloning of RICH provides the means for a more detailed analysis of its function in nerve regeneration. Additionally, the homology of RICH and CNPase suggests that further investigation may provide additional insight into the role of these proteins in the nervous system.

Olfactory bulb ensheathing glia: a unique cell type with axonal growth-promoting properties

The olfactory bulb (OB) is a structure of the central nervous system (CNS) in which axonal growth occurs throughout the lifetime of the organism. A major difference between the OB and the remaining CNS is the presence of ensheathing glia in the first two layers of the OB. Ensheathing glia display properties that might be involved in the process of regeneration and they appear to be responsible for the permissibility of the adult OB to axonal growth. In fact, transplants of ensheathing glia can be used as promoters of axonal regeneration within the adult CNS. The axonal growth-promoting properties of ensheathing glia make the study of this cell type interesting for understanding the mechanisms underlying axonal regeneration. Several groups have studied OB ensheathing cells extensively in an attempt to classify them within any of the known glial groups. However, this cell type does not exhibit the phenotypic features of any glial population described thus far. In this article we review the characteristics that differentiate ensheathing glia from other peripheral and central glial populations as well as the properties that involve them in axonal regeneration. The evidence suggests that ensheathing glia are unique, have their own identity, and do not belong to any previously described glial type.

Perspectives on axonal regeneration in the mammalian CNS

In the CNS of mammals, axonal regeneration is limited by inhibitory influences of the glial and extracellular environment. Myelin-associated inhibitors of neurite growth, as well as some properties of so-called 'reactive astrocytes' which make the environment non-permissive for axonal growth, contribute to the inhibitory nature of the mammalian CNS. Furthermore, mechanisms for effective removal or neutralization of inhibitory components of cell debris are lacking in the mature mammalian CNS. However, in a permissive environment, mammalian CNS axons are able to regrow, to recognize target areas and to re-establish functional synapses with target neurones. Moreover, recent observations suggest that guiding molecules, like those required for axon guidance in the developing CNS, become expressed after lesions. Regenerating CNS axons seem to be able to recognize such guidance cues. Thus, even regenerating CNS axons of mammals might ultimately succeed in re-establishing topographically ordered functional synapses in their target regions.