Leukocyte infiltration, neuronal degeneration, and neurite outgrowth after ablation of scar-forming, reactive astrocytes in adult transgenic mice

IL-1-regulated responses in astrocytes: Relevance to injury and recovery

In the central nervous system (CNS), the cellular processes of astrocytes make intimate contact with essentially all areas of the brain. They have also been shown to be functionally coupled to neurons, oligodendrocytes, and other astrocytes via both contact-dependent and non-contact-dependent pathways. These observations have led to the suggestion that a major function of astrocytes in the CNS is to maintain the homeostatic environment, thus promoting the proper functioning of the neuronal network. Inflammation in the CNS disrupts this process either transiently or permanently and, as such, is thought to be tightly regulated by both astrocytes and microglia. The remarkable role that single cytokines, such as TNF and IL-1, may play in this process has now been well accepted, but the extent of the reprogramming of the transcriptional machinery initiated by these factors remains to be fully appreciated. With the advent of microarray technology, a more comprehensive analysis of this process is now available. In this report we review data obtained with this technology to provide an overview of the extent of changes induced in astrocytes by the cytokine IL-1.

Development of novel selective cell ablation in the mammary gland and brain to study cell-cell interactions and chemoprevention

We have generated transgenic mice which express the gene encoding Escherichia coli nitroreductase (NTR) specifically in the luminal epithelial cells of the mammary gland and the glial cells of the brain. The enzyme activates an antitumour drug CB 1954, to produce a cross-linking agent that kills all cells expressing the enzyme. We have shown that administration of the antitumour drug CB 1954 rapidly and selectively kills these cells. Original experiments demonstrated the ability to ablate the luminal cells in the mammary gland with no apparent bystander effect. Subsequently, astrocytes expressing nitroreductase under the targeting of the GFAP promoter were selectively ablated following administration of the prodrug CB 1954 produces a degeneration of granular neurones due to changes in glutamate levels. Recent experiments demonstrated inhibition of myc-dependent mammary tumours using the same enzyme (nitroreductase)-prodrug (CB 1954), combination. Owing to the ease of control of NTR-mediated cell ablation, we anticipate that this system will supersede herpes simplex virus type 1 thymidine kinase. There are widespread potential applications for this approach in the dissection of complex cellular interactions during development and in the adult organism. The present transgenic models also have important applications for the study in vivo of novel prodrugs that can be selected for variable degrees of bystander effects. Such studies will have particular significance for those groups advocating the use of NTR as an appropriate enzyme for gene-directed enzyme prodrug therapy by providing models of a wide range of human disease for mechanistic and therapeutic experimentation. The results clearly demonstrate that the model has potential to study chemoprevention and fundamental questions on cell-cell interactions in cell biology.

Blood-spinal cord barrier after spinal cord injury: relation to revascularization and wound healing

Spinal cord injury produces prominent disruption of the blood-spinal cord barrier. We have defined the blood-spinal cord barrier breakdown to the protein luciferase (61 kDa) in the acutely injured murine spinal cord and during revascularization. We show that newly formed and regenerating blood vessels that have abnormal permeability exhibit differential expression of the glucose-1 transporter (Glut-1), and that its expression is dependent on astrocytes. There was overt extravasation of luciferase within the first hour after injury, a period that coincided with marked tissue disruption within the epicenter of the lesion. Although there was a significant reduction in the number of blood vessels relative to controls by 24 hr after injury, abnormal barrier permeability remained significantly elevated. A second peak of abnormal barrier permeability at 3-7 days postinjury coincided with prominent revascularization of the epicenter. The barrier to luciferase was restored by 21 days postinjury and vascularity was similar to that of controls. During wound-healing process, the cord was reorganized into distinct domains. Between 14 and 21 days postinjury, each domain consisted primarily of nonneuronal cells, including macrophages. Astrocytes were limited characteristically to the perimeter of each domain. Only blood vessels affiliated closely with astrocytes in the perimeter expressed Glut-1, whereas blood vessels within each domain of the repairing cord did not express it. Together, these data demonstrate that both injured and regenerating vessels exhibit abnormal permeability and suggest that Glut-1 expression during revascularization is dependent on the presence of astrocytes.

Mechanism of stem cells in the central nervous system

Injury to the central nervous system (CNS) can result in severe functional impairment. The brain and spinal cord, which constitute the CNS, have been viewed for decades as having a very limited capacity for regeneration. However, over the last several years, the body of evidence supporting the concept of regeneration and continuous renewal of neurons in specific regions of the CNS has increased. This evidence has significantly altered our perception of the CNS and has offered new hope for possible cell therapy strategies to repair lost function. Transplantation of stem cells or the recruitment of endogenous stem cells to repair specific regions of the brain or spinal cord is the next exciting research challenge. However, our understanding of the existing stem cell pool in the adult CNS remains limited. This review will discuss the identification and characterization of CNS stem cells in the adult brain and spinal cord.

Erasure of kinase phosphorylation in astrocytes during oxygen-glucose deprivation is controlled by ATP levels and activation of phosphatases

We have examined the relationship between adenosine triphosphate (ATP) concentration and loss of maintenance of kinase-signalling cascades in primary cortical astrocytes during oxygen-glucose deprivation (OGD) as this may constitute an irreversible step that commits astrocytes to cell death. We report that the phosphorylation of Akt, ERK, JNK and p38 kinases, whose activities depend on serine, threonine and tyrosine phosphorylation, were all increased during OGD. All these phosphorylations were reduced to below detection limits when ATP levels were less than 10% of normal levels. Using ERK and Akt as representative examples, we show that this erasure is not irreversible as both ERK and Akt phosphorylations can be partially restored by addition of glucose under anoxic conditions. We further investigated whether OGD caused any change in phosphatase activity. The PP1/PP2A phosphatase inhibitors okadaic acid and caliculyn A, but not cyclosporine A, delayed the removal of ERK and Akt phosphorylation under OGD. By comparing the extent of phosphorylation increase under OGD and normoxic conditions, we calculate that phosphatase activity was increased by approximately 3.6-fold during OGD. These data show that ATP levels control an important checkpoint in kinase function, and that ATP levels may need to be considered when studies of kinase function in relation to OGD are conducted.

Evidence for astrocyte heterogeneity: a distinct subpopulation of protoplasmic-like glial cells is detected in transgenic mice expressing Lmo1-lacZ

The adult mammalian central nervous system (CNS) contains a large number of different cell types, which arise from the ventricular (VZ) and subventricular zones during embryonic development. In this study, we used a transgenic mouse expressing Lmo1-LacZ from a randomly inserted promoter/reporter gene construct to identify a glial subpopulation. LMO1 is an LIM domain-containing protein, thought to act in protein-protein interactions. We found first that in the adult transgenic CNS, beta-galactosidase (beta-gal) was expressed in a specific subpopulation of protoplasmic-like cells, which did not express detectable levels of glial fibrilary acidic protein unless a lesion was produced. Secondly, during development, beta-gal(+) cells were found arising from discrete regions of the VZ. Taken together, these results identify a subpopulation of protoplasmic glial cells in the adult CNS and suggest that they arise from a restricted VZ region during CNS development.

Tumor necrosis factor is required for RANTES-induced astrocyte monocyte chemoattractant protein-1 production

Astrocytes respond to stimulation with the chemokine RANTES (regulated on activation, normal T cell expressed) by production of a series of cytokines and chemokines, including tumor necrosis factor-alpha (TNF-alpha) and monocyte chemoattractant protein-1 (MCP-1). In the present study we demonstrate that RANTES induces TNF, which in turn stimulates subsequent production of MCP-1. TNF-R1 (p55) serves as the principal receptor responsible for MCP-1 synthesis. The results define an astrocyte proinflammatory cascade that amplifies synthesis of proinflammatory mediators. The implications of these findings to inflammatory diseases of the central nervous system are discussed.

Activation of extracellular signal-regulated kinases (ERK) during reperfusion of ischemic spinal cord

The extracellular signal-regulated kinases (ERK) participate in numerous signaling pathways and are abundantly expressed in the CNS. It has been proposed that ERK activation promotes survival in models of neuronal injury. Inhibition of MEK, the upstream kinase that activates ERK, however, leads to neuroprotection in models of cerebral ischemia and trauma, suggesting that in this context ERK activation contributes to cellular damage. The effect of ischemia and reperfusion on activity and expression of ERK was investigated using a reversible model of rabbit spinal cord ischemia. Active ERK was observed in nai;ve animals, which decreased during 15 to 60 min of ischemia. Upon reperfusion, a robust activation of ERK was observed in animals occluded for 60 min that remained permanently paraplegic. Immunohistochemical analyses revealed increased staining of phosphorylated ERK (pERK) in glial cells and faint nuclear staining in motor neurons of animals occluded for 60 min and reperfused for 18 h. In contrast ERK activity did not increase in animals occluded for 15 min that regained motor function. No evidence of increased pERK immunoreactivity in motor neurons or nuclear translocation was noted in these animals. ERK1 was demonstrated to be identical to a p46 c-Jun/ATF-2 kinase previously shown to be activated by reperfusion after a 60-min occlusion. The results suggest that activation of ERK during reperfusion of ischemic spinal cord participates in the cellular pathways leading to neuronal damage.

The ablation of glial fibrillary acidic protein-positive cells from the adult central nervous system results in the loss of forebrain neural stem cells but not retinal stem cells

The adult mammalian forebrain subependyma contains neural stem cells (NSCs) capable of self-renewal and multilineage differentiation. The in vivo identification of NSCs has not been definitively addressed using a loss of function approach. Using a transgenic mouse expressing herpes-simplex virus thymidine kinase from the glial fibrillary acidic protein (GFAP) promotor, we have selectively killed dividing GFAP-positive cells in the presence of ganciclovir (GCV) and shown a > 95% loss in the numbers of NSCs, as assayed by the formation of clonally derived neurospheres in vitro. This loss is seen following 3 days of GCV exposure in vivo or in vitro only and cannot be rescued by coculturing with pure astrocyte populations or control (green fluorescent protein-expressing) subependymal cells. Exposure to GCV in vitro has no effect on adult retinal stem cells hence, we conclude that adult forebrain NSCs comprise a subpopulation of the GFAP-positive cells within the subependyma.

GFAP knockout mice have increased levels of GDNF that protect striatal neurons from metabolic and excitotoxic insults

In response to injury and degeneration, astrocytes hypertrophy, extend processes, and increase production of glial fibrillary acidic protein (GFAP), an intermediate filament protein located within their cytoplasm. The present study tested the hypothesis that GFAP expression alters the vulnerability of neurons to excitotoxic and metabolic insult induced by 3-nitroproprionic acid (3-NP), an irreversible inhibitor of mitochondrial complex II activity or the excitotoxin quinolinic acid (QA). In this respect, adult GFAP knockout mice (KO) and wild-type control mice (WT) received unilateral intrastriatal injections of 3-NP (200 nmol/microl) or QA (100 nmol/microl) and were killed 1, 2, or 4 weeks later. Lesion volume and neuronal counts were quantified using unbiased stereologic principles. For both QA and 3-NP lesions, a significant decrease in lesion volume and an increase in striatal projection neurons were seen in GFAP KO mice compared with WT mice. Enzyme-linked immunoassay analysis revealed increased basal levels of glial cell derived neurotrophic factor (GDNF) relative to WT mice. In contrast, no differences were observed in the expression of ciliary neurotrophic factor or nerve growth factor. These data strongly suggest that the expression of GFAP is implicated with the production of GDNF to a degree that confers neuroprotection after an excitotoxic or metabolic insult.

Increased adenine nucleotide translocator 1 in reactive astrocytes facilitates glutamate transport

A hallmark of central nervous system (CNS) pathology is reactive astrocyte production of the chronic glial scar that is inhibitory to neuronal regeneration. The reactive astrocyte response is complex; these cells also produce neurotrophic factors and are responsible for removal of extracellular glutamate, the excitatory neurotransmitter that rises to neurotoxic levels in injury and disease. To identify genes expressed by reactive astrocytes, we employed an in vivo model of the glial scar and differential display PCR and found an increase in the level of Ant1, a mitochondrial ATP/ADP exchanger that facilitates the flux of ATP out of the mitochondria. Ant1 expression in reactive astrocytes is regulated by transforming growth factor-beta1, a pluripotent CNS injury-induced cytokine. The significance of increased Ant1 is evident from the observation that glutamate uptake is significantly decreased in astrocytes from Ant1 null mutant mice while a specific Ant inhibitor reduces glutamate uptake in wild-type astrocytes. Thus, the astrocytic response to CNS injury includes an apparent increase in energy mobilization capacity by Ant1 that contributes to neuroprotective, energy-dependent glutamate uptake.

The predominant neural stem cell isolated from postnatal and adult forebrain but not early embryonic forebrain expresses GFAP

Periventricular germinal zones (GZs) of developing and adult brain contain neural stem cells (NSCs), the cellular identities and origins of which are not defined completely. We used tissue culture techniques and transgenic mice expressing herpes simplex virus thymidine kinase (HSV-TK) from the mouse glial fibrillary acid protein (GFAP) promoter to test the hypothesis that certain NSCs express GFAP. To do so, we determined the relative proportions of multipotent neurospheres that are formed by GFAP-expressing cells derived from GZs at different stages of development. In this transgenic model, dividing GFAP-expressing cells are ablated selectively by treatment with the antiviral agent ganciclovir (GCV). Single-cell analysis showed that transgene-derived HSV-TK was present only in GFAP-expressing cells. GCV applied in vitro eliminated growth of multipotent neurospheres from GZs of postnatal and adult transgenic mice but not early embryonic (embryonic day 12.5) transgenic mice. GCV prevented growth of secondary multipotent neurospheres prepared after passage of primary transgenic neurospheres derived from all three of these developmental stages. In addition, GCV prevented growth of multipotent neurospheres from transgenic astrocyte-enriched cell cultures derived from postnatal GZ, and elaidic acid GCV given for 4 d to adult transgenic mice in vivo abolished the ability to grow multipotent neurospheres from GZ. Extensive control experiments, including clonal analysis, demonstrated that failure of neurosphere growth was not merely secondary to loss of GFAP-expressing support cells or the result of a nonspecific toxic effect. Our findings demonstrate that the predominant multipotent NSCs isolated from postnatal and adult but not early embryonic GZs express GFAP, and that NSCs exhibit heterogeneous expression of intermediate filaments during developmental maturation.

Transient neuronal but persistent astroglial activation of ERK/MAP kinase after focal brain injury in mice

Astrogliosis is a nearly ubiquitous response to a variety of insults to the central nervous system (CNS). This reaction is triggered rapidly, but can persist for years after the initial trauma. Little is known about the signaling mechanisms responsible for this activation and its chronic maintenance. Extracellular signal-regulated kinase/mitogen-activated protein kinase (ERK/MAPK) activation is implicated in several functions important to the reactive glial phenotype such as cellular proliferation and motility. Here we utilize immunohistochemistry with a phosphorylation state-specific antibody (pERK) to characterize the temporal and spatial pattern of ERK/MAPK activation in neurons and glia following a forebrain stab lesion (FSL) in mice. Early activation (1 h) was primarily in perilesional neuronal elements, particularly of the hippocampus. Occasional perilesional glia were also positive for pERK. Additionally, ependymal cells bilaterally stained prominently for pERK. These patterns of pERK immunoreactivity at 1 h were abolished by pretreatment with the selective MEK inhibitor, SL327. ERK/MAPK activation at later time points between 1 day (d) and 30 d was primarily restricted to perilesional astrocytes with maximum labeling at 3 d. However, pERK-positive astrocytes represented only a subset of total GFAP-positive cells and were found more proximal to the lesion suggesting specific functional activation of these cells. Finally, immunostaining for the phosphorylated form of cAMP response element-binding (CREB) protein, a downstream target of the ERK/MAPK cascade, was increased in perilesional glia 7 d after FSL. Sustained activation of the ERK/MAPK signaling pathway in perilesional reactive glia suggests a critical role for this cascade in astrogliosis.

Death receptors on reactive astrocytes: a key role in the fine tuning of brain inflammation?

Immune responses protect the CNS against pathogens. However, the fact that there is little dispensable tissue in the brain makes regulation necessary to avoid disastrous immune-mediated damage. Astrocytes respond vigorously to any brain injury (e.g., tumor, stroke, AD, MS, HIV) and are postulated to play an important role in the fine tuning of brain inflammation. The authors propose that astrocytes use death receptors to modulate pro- and anti-inflammatory effects.

Cytokines: powerful regulators of glial cell activation

It is now clear that cytokines function as powerful regulators of glial cell function in the central nervous system (CNS), either inhibiting or promoting their contribution to CNS pathology. Although these interactions are complex, the availability of animals with targeted deletions of these genes and/or their receptors, as well as transgenic mice in which cytokine expression has been targeted to specific cell types, and the availability of purified populations of glia that can be studied in vitro, has provided a wealth of interesting and frequently surprising data relevant to this activity. A particular feature of many of these studies is that it is the nature of the receptor that is expressed, rather than the cytokine itself, that regulates the functional properties of these cytokines. Because cytokine receptors are themselves modulated by cytokines, it becomes evident that the effects of these cytokines may change dramatically depending upon the cytokine milieu present in the immediate environment. An additional exciting aspect of these studies is the previously underappreciated role of these factors in repair to the CNS. In this review, we focus on current information that has helped to define the role of cytokines in regulating glial cell function as it relates to the properties of microglia and astrocytes.

Astrocytes and brain injury

Astrocytes are the most numerous cell type in the central nervous system. They provide structural, trophic, and metabolic support to neurons and modulate synaptic activity. Accordingly, impairment in these astrocyte functions during brain ischemia and other insults can critically influence neuron survival. Astrocyte functions that are known to influence neuronal survival include glutamate uptake, glutamate release, free radical scavenging, water transport, and the production of cytokines and nitric oxide. Long-term recovery after brain injury, through neurite outgrowth, synaptic plasticity, or neuron regeneration, is influenced by astrocyte surface molecule expression and trophic factor release. In addition, the death or survival of astrocytes themselves may affect the ultimate clinical outcome and rehabilitation through effects on neurogenesis and synaptic reorganization.

Immunosuppressant FK506 affects multiple signaling pathways and modulates gene expression in astrocytes

Brain injury is often associated with proliferation and hypertrophic response of glial cells (reactive gliosis). We have previously reported immunosuppressant effects on survival of glioma cells and adult reactive astrocytes. In the present study, we demonstrate growth-inhibitory effect of FK506 on cortical astrocytes from newborn rats. FK506 inhibits Erk and PI-3K/Akt signaling, two crucial pro-survival pathways. The levels of phosphorylated Akt and p42/44 Erk decline in few hours after FK506 addition. Furthermore, in FK506-treated astrocyte cultures the levels of mRNA encoding PDGF, bFGF, and CNTF decreased. Downregulation of growth factor expression by FK506 may play a role in the inhibition of mitogenic/hypertrophic responses. FasL mRNA level was elevated and interaction of FasL with Fas receptor expressed in astrocytes may trigger cell death. Interestingly, expression of BDNF increased in a dose-dependent manner in FK506-treated astrocytes. Upregulation of BDNF mRNA and protein level in astrocytes exposed to FK506 may underlie neuroprotective action of FK506.

Role of enteric glial cells in inflammatory bowel disease

Enteric glial cells (EGCs) represent an extensive but relatively poorly described cell population within the gastrointestinal tract. Accumulating data suggest that EGCs represent the morphological and functional equivalent of CNS astrocytes within the enteric nervous system (ENS). The EGC network has trophic and protective functions toward enteric neurons and is fully implicated in the integration and the modulation of neuronal activities. Moreover, EGCs seem to be active elements of the ENS during intestinal inflammatory and immune responses, sharing with astrocytes the ability to act as antigen-presenting cells and interacting with the mucosal immune system via the expression of cytokines and cytokine receptors. Transgenic mouse systems have demonstrated that specific ablation of EGC by chemical ablation or autoimmune T-cell targeting induces an intestinal pathology that shows similarities to the early intestinal immunopathology of Crohn's disease. EGCs may also share with astrocytes the ability to regulate tissue integrity, thereby postulating that similar interactions to those observed for the blood-brain barrier may also be partly responsible for regulating mucosal and vascular permeability in the gastrointestinal tract. Disruption of the EGC network in Crohn's disease patients may represent one possible cause for the enhanced mucosal permeability state and vascular dysfunction that are thought to favor mucosal inflammation.

Strategies for regeneration and repair in spinal cord traumatic injury

Spinal cord injury is frequently followed by the loss of supraspinal control of sensory, autonomic and motor functions at the sublesional level. In order to enhance recovery in spinal cord-injured patients, we have developed three fundamental strategies in experimental models. These strategies define in turn three chronological levels of postlesional intervention in the spinal cord. Neuroprotection soon after injury using pharmacological tools to reduce the progressive secondary injury processes that follow during the first week after the initial lesion. This strategy was conducted up to clinical trials, showing that a pharmacological therapy can reduce the permanent neurological deficit that usually follows an acute injury of the central nervous system (CNS). A second strategy, which is initiated not long after the lesion, aims at promoting axonal regeneration by acting on the main barrier to regeneration of lesioned axons: the glial scar. Finally a mid-term substitutive strategy is the management of the sublesional spinal cord by sensorimotor stimulation and/or supply of missing key afferents, such as monoaminergic systems. These three strategies are reviewed. Only a combination of these different approaches will be able to provide an optimal basis for potential therapeutic interventions directed to functional recovery after spinal cord injury.

Temporal and spatial profile of phosphorylated mitogen-activated protein kinase pathways after lateral fluid percussion injury in the cortex of the rat brain

Mitogen-activated protein kinases (MAPK) play a crucial role in signal transduction that regulates gene expression through transcriptional factor activity. The purpose of this study was to investigate the temporal expression and topographic distribution of the activated MAPK pathways including extracellular signal-regulated protein kinase (ERK), c-Jun NH(2)-terminal kinase (JNK), and p38 MAPK following traumatic brain injury (TBI) in the cortex of the rat brain. Adult male Sprague-Dawley rats (300-400 g) were subjected to lateral fluid percussion injury of moderate severity (3.5-4.0 atm) using the Dragonfly device model (no. HPD-1700). Phosphorylated-MAPK protein levels were quantified using Western blot analysis. Topographic distribution of immunoreactivity for phosphorylated-MAPK was examined using immunohistochemistry. Our findings showed that TBI significantly increased the phosphorylated-ERK (p-ERK) and -JNK (p-JNK) levels, but not the -p38 (p-p38) protein levels in the cortex surrounding the injury site. The immunoreactivity for p-ERK and p-JNK immediately after TBI were localized in neurons. The immunoreactivity for p-JNK was uniformly but only transiently induced and returned to control levels 1 h after TBI. The immunoreactivity for p-ERK was confirmed up until 30 min after TBI in the superficial neuronal layers. Double immunostaining using a glial-specific marker demonstrated that p-ERK was prominent in astrocytes 6 h after TBI. The current results suggest that the ERK and JNK pathways, but not the p38 MAPK pathways are involved in signal transduction in the cortex following TBI.

Role of laminin bioavailability in the astroglial permissivity for neuritic outgrowth

The mechanisms involved in the failure of an adult brain to regenerate post-lesion remain poorly understood. The reactive gliosis which occurs after an injury to the CNS and leads to the glial scar has been considered as one of the major impediments to neurite outgrowth and axonal regeneration. A glial scar consists mainly of reactive, hypertrophic astrocytes. These reactive cells acquire new properties, leading to A non-permissive support for neurons. Astrogial reactivity is mainly characteriized by a high overexpression of the major component of the gliofilaments, the glial fibrillary acidic protein (GFAP). This GFAP overexpression is related to the astroglial morphological response to injury. We hypothesized that modulation of GFAP synthesis, reversing the hypertrophic phenotype, might also reverse the blockage of neuritic outgrowth observed after a lesion. In this article, we review findings of our group, confirming our hypothesis in a model of lesioned neuron-astrocyte cocultures. We demonstrate that permissivity for neuritic outgrowth is related to phenotypic changes induced in reactive astrocytes transfected by antisense GFAP-mRNA. We also found that this permissivity was related to a neuron-regulated extracellular laminin bioavailability.

Neuroanatomical distribution of CXCR4 in adult rat brain and its localization in cholinergic and dopaminergic neurons

Accumulating evidence supports a role of chemokines and their receptors in brain function. Up to now scarce evidence has been given of the neuroanatomical distribution of chemokine receptors. Although it is widely accepted that chemokine receptors are present on glial cells, especially in pathological conditions, it remains unclear whether they are constitutively present in normal rat brain and whether neurons have the potential to express such chemokine receptors. CXCR4, a G protein-coupled receptor for the chemokine stromal cell-derived factor-1 (SDF-1/CXCL12) was reported to have possible implications in brain development and AIDS-related dementia. By dual immunohistochemistry on brain sections, we clearly demonstrate that CXCR4 is constitutively expressed in adult rat brain, in glial cells (astrocytes, microglia but not oligodendrocytes) as well as in neurons. Neuronal expression of CXCR4 is mainly found in cerebral cortex, caudate putamen, globus pallidus, substantia innominata, supraoptic and paraventricular hypothalamic nuclei, ventromedial thalamic nucleus and substantia nigra. Using confocal microscopy, a differential distribution of CXCR4 in neuronal perikarya and dendrites can be observed according to the brain structure. Furthermore, this work demonstrates for the first time the coexistence of a chemokine receptor with classical neurotransmitters. A localization of CXCR4 is thus observed in neuronal cell bodies expressing choline acetyltransferase-immunoreactivity in the caudate putamen and substantia innominata, as well as in tyrosine hydroxylase-positive neurons in the substantia nigra pars compacta. In conclusion, the constitutive neuronal CXCR4 expression suggests that SDF-1/CXCL12 could be involved in neuronal communication and possibly linked up with cholinergic and dopaminergic neurotransmission and related disorders.

Neurospheres modified to produce glial cell line-derived neurotrophic factor increase the survival of transplanted dopamine neurons

Glial cell line-derived neurotrophic factor (GDNF) has been shown to increase the survival of dopamine neurons in a variety of in vitro and in vivo model systems. Therefore, it constitutes an important therapeutic protein with the potential to ameliorate dopamine neuronal degeneration in Parkinson's disease or to support dopamine neuronal replacement strategies. However, biophysical and practical considerations present obstacles for the direct delivery of the GDNF protein to CNS neurons. Here we show that rodent neural precursor cells isolated and expanded in culture as neurospheres (NS) can be genetically modified to express green fluorescent protein (GFP) or to release GDNF using lentiviral constructs. GDNF-NS increased the fibre outgrowth of primary embryonic dopamine neurons in cocultures, showing that the protein was released in biologically significant quantities. Furthermore, after transplantation into the 6-hydroxydopamine-lesioned rat striatum, GDNF-NS significantly increased the survival of cografted primary dopamine neurons. However, this was not reflected in behavioural recovery in these animals. We found that, by 6 weeks, few cells expressed GDNF or GFP, suggesting either that transgene expression was down-regulated over time or that the cells died. This may explain the initial effects on dopamine neuronal survival within the graft but the lack of long-term effect on subsequent fibre outgrowth and behaviour. Providing sustained levels of neural precursor-mediated transgene expression can be achieved following transplantation in the future; this approach may prove beneficial as an alternative therapeutic strategy in the cell-based management of Parkinson's disease.

Peripheral macrophage recruitment in cuprizone-induced CNS demyelination despite an intact blood-brain barrier

The contribution of peripheral macrophage was assessed in cuprizone intoxication, a model of demyelination and remyelination in which the blood-brain barrier remains intact. Flow cytometry of brain cells isolated from cuprizone-treated mice revealed an increase in the percentage of Mac-1(+)/CD45(hi) peripheral macrophage. To confirm these results in situ, C57BL/6 mice were lethally irradiated, transplanted with bone marrow from GFP-transgenic mice, and exposed to cuprizone. GFP(+) peripheral macrophages were seen in the CNS after 2 weeks of treatment, and infiltration continued through 6 weeks. While the peripheral macrophages were far outnumbered by the resident microglia, their recruitment across the blood-brain barrier alludes to a potentially important role.

RANTES stimulates inflammatory cascades and receptor modulation in murine astrocytes

Cultured mouse astrocytes respond to the CC chemokine RANTES by production of chemokine and cytokine transcripts. Stimulation of astrocytes with 1 nM RANTES or 3-10 nM of the structurally related chemokines (eotaxin, macrophage inflammatory protein-1alpha and -beta [MIP-1alpha, MIP-1beta]) induced transcripts for KC, monocyte chemoattractant protein-1 (MCP-1), tumor necrosis factor-alpha (TNF-alpha), MIP-1alpha, MIP-2, and RANTES in a chemokine and cell-specific fashion. Synthesis of chemokine (KC and MCP-1) and cytokine (TNF-alpha) proteins was also demonstrated. RANTES-mediated chemokine synthesis was specifically inhibited by pertussis toxin, indicating that G-protein-coupled chemokine receptors participated in astrocyte signaling. Astrocytes expressed CCR1 and CCR5 (the redundant RANTES receptors). Astrocytes derived from mice with targeted mutations of either CCR1 or CCR5 respond after RANTES stimulation, suggesting multiple chemokine receptors may separately mediate RANTES responsiveness in astrocytes. Preliminary data suggest activation of the MAP kinase pathway is also critical for RANTES-mediated signaling in astrocytes. Treatment with RANTES specifically modulated astrocyte receptors upregulating intercellular adhesion molecule 1 (ICAM-1) and downregulating CX3CR1 expression. Thus, after chemokine treatment, astrocytes release proinflammatory mediators and reprogram their surface molecules. The combined effects of RANTES may serve to amplify inflammatory responses within the central nervous system.

Changes in astroglial GLT-1 expression after neural transplantation or stab wounds

Uncontrolled release of glutamate from damaged brain initiates events that result in excitotoxic neuronal death. Glutamate uptake by specialized astroglial transporters is essential for control of extracellular glutamate levels. Many studies have demonstrated a reduction in astrocytic GLT-1 expression after different forms of injury. Because extensive neuronal death does not occur after direct cortical stab wounds and viable developing neurons populate fetal CNS grafts, we hypothesized that reactive astroglia associated with these procedures might maintain or up-regulate GLT-1. We examined the temporal and spatial distribution of GLT-1, GFAP and nestin proteins by confocal double-label immunohistochemistry combined with a new methodology in which precise brain areas are microdissected and analyzed for protein content by immunoaffinity chromatography. In stab wounds, GLT-1 protein content did not change compared to normal cortex, as determined by direct protein measurements; GLT-1 colocalized with nestin- and GFAP(+) astroglia adjacent to the lesion. In contrast, host reactive astroglia adjacent to grafts significantly upregulated GLT-1 by 3 days postoperative. The GFAP protein analysis suggests that increased GLT-1 is not the result of greater numbers of activated astroglia around grafts, but that developing graft tissue influences adjacent host astroglia to upregulate GLT-1. GLT-1 protein within grafts was rapidly accelerated to mature levels by just three days, and was expressed by the nestin(+) cell population. These data, which demonstrate immunoexpression of GLT-1 protein combined with a new method for protein measurement in situ indicate that, in contrast to other injury models, astroglial GLT-1 is upregulated or maintained following invasive CNS procedures. (c)2002 Elsevier Science (USA).

Differential activation of mitogen-activated protein kinase pathways after traumatic brain injury in the rat hippocampus

Mitogen-activated protein kinases, which play a crucial role in signal transduction, are activated by phosphorylation in response to a variety of mitogenic signals. In the present study, the authors used Western blot analysis and immunohistochemistry to show that phosphorylated extracellular signal-regulated protein kinase (p-ERK) and c-Jun NH2-terminal kinase (p-JNK), but not p38 mitogen-activated protein kinase, significantly increased in both the neurons and astrocytes after traumatic brain injury in the rat hippocampus. Different immunoreactivities of p-ERK and p-JNK were observed in the pyramidal cell layers and dentate hilar cells immediately after traumatic brain injury. Immunoreactivity for p-JNK was uniformly induced but was only transiently induced throughout all pyramidal cell layers. However, strong immunoreactivity for p-ERK was observed in the dentate hilar cells and the damaged CA3 neurons, along with the appearance of pyknotic morphologic changes. In addition, immunoreactivity for p-ERK was seen in astrocytes surrounding dentate and CA3 pyramidal neurons 6 hours after traumatic brain injury. These findings suggest that ERK and JNK but not p38 cascades may be closely involved in signal transduction in the rat hippocampus after traumatic brain injury.

Tumor necrosis factor inhibits neurite outgrowth and branching of hippocampal neurons by a rho-dependent mechanism

In response to injury and inflammation of the CNS, brain cells including microglia and astrocytes secrete tumor necrosis factor-alpha (TNF). This pro-inflammatory cytokine has been implicated in both neuronal cell death and survival. We now provide evidence that TNF affects the formation of neurites. Neurons cultured on astrocytic glial cells exhibited reduced outgrowth and branching of neurites after addition of recombinant TNF or prestimulation of glial cells to secrete TNF. This effect was absent in neurons of TNF receptor-deficient mice cultured on prestimulated glia of wild-type mice and was reverted by blocking TNF with soluble TNF receptor IgG fusion protein. TNF activated in neurons the small GTPase RhoA. By inactivating Rho with C3 transferase, the inhibitory effect of TNF on neurite outgrowth and branching was abolished. These results suggest that glia-derived TNF, as part of an injury or inflammatory process, can inhibit neurite elongation and branching during development and regeneration.

Astroglial permissivity for neuritic outgrowth in neuron-astrocyte cocultures depends on regulation of laminin bioavailability

The molecular determinants underlying the failure of axons to regenerate in the CNS after injury were studied in an in vitro model of astrogliosis and neuronal coculture. Mechanically lesioned neuron-astrocyte mouse cortical cocultures were treated with antisense glial fibrillary acidic protein (GFAP)-mRNA in order to inhibit the formation of gliofilaments that occurs in response to injury. This inhibition relieves the blockage of neuron migration and neuritic outgrowth observed after lesion, and migrating neurons reappeared, supported by a laminin-labeled extracellular network (permissive conditions). We then questioned the relationship between this permissivity and laminin production. Follow-up studies on the concentration of laminin indicated that, after antisense treatment, the laminin level was increased in the cocultures and was under the control of astrocyte-neuron interactions. The addition of exogenous laminin favored neuronal migration and neurite outgrowth, whereas neutralizing laminin bioavailability with antibodies recognizing the astroglial laminin resulted in an inhibition of both neuronal access to the lesion site and neurite outgrowth, suggesting an active role for laminin in the permissive process. This permissive process could be associated with modulation of extracellular matrix (ECM) molecule degradation by proteinases. Among the latter, matrix metalloproteinases (MMPs) are involved in the breakdown of the ECM component. Our investigation showed a net decrease of the matrix metalloproteinase MMP-2 expression and activity and an increase of its endogenous inhibitor TIMP-2 expression. Both proteins associated with permissivity should be involved in the laminin stabilization and cell-matrix interactions. High levels of laminin and laminin bioavailability, consequent to a reduction in astrogliosis, may be important permissive elements for neuronal migration and neurite outgrowth postlesion.

Enteric glial cells. An upstream target for induction of necrotizing enterocolitis and Crohn's disease?

As a direct consequence of the sophisticated arrangement of its intrinsic neurons, the gastrointestinal tract is unique among peripheral organs, in its ability to mediate its own reflexes. Neurons of the enteric nervous system are intimately associated with enteric glial cells. These supporting cells do not resemble Schwann cells, the glial cell found in all other parts of the peripheral nervous system, but share many similarities with astrocytes of the central nervous system. Ablation of enteric glial cells in adult transgenic mice has demonstrated that these cells are essential to maintain the integrity of the small intestine. Acute loss of enteric glial cells induces massive pathological changes with similarities to necrotizing enterocolitis (NEC) and early Crohn's disease. These human conditions share some mechanistic similarities. Identification of enteric glial cell dysfunction/loss as sufficient to induce necrotic/inflammatory bowel disease may be important to understand the pathogenesis of both NEC and Crohn's disease.

Induction of type IV collagen and other basement-membrane-associated proteins after spinal cord injury of the adult rat may participate in formation of the glial scar

We investigated the spatial and temporal expression of basement-membrane-forming and neurite-outgrowth-supporting matrix proteins after a unilateral dorsal root injury combined with a collagen I/laminin-1 graft and a stab wound lesion to the dorsal horn of the adult rat spinal cord. Ten days after injury, the gamma1 laminin was induced in the reactive glia. At this early stage, the glial cells failed to express type IV collagen and the alpha1 laminin. One month after injury, reactive astrocytes in the dorsal horn of the lesioned side expressed gamma1 laminin, type IV collagen, and the alpha1 laminin whereas astrocytes of the normal spinal cord or the uninjured contralateral dorsal horn were negative. Both astrocytes and neurons of the ipsilateral ventral horn were induced to express laminin-1 and gamma1 laminin. Astrocytes of the ipsilateral ventral horn also expressed type IV collagen. Simultaneously with the changes in expression of the extracellular matrix proteins, the expression pattern of basic fibroblast growth factor (FGF-2) was markedly altered after spinal cord injury. In normal and contralateral spinal cord, FGF-2 was expressed in nerve fibers, but its expression changed from neuronal into glial in the ipsilateral spinal cord within 1 month after injury. Four months after injury, expression of both type IV collagen and the alpha1 laminin had declined, but the astrocytes at the injury site continued expressing the gamma1 laminin. Cultured astrocytes were negative for type IV collagen, but several cytokines, including IL-1beta and TGFbeta1, induced expression of type IV collagen in the astrocytes. These factors also increased deposition of type IV collagen matrix in the glial cultures. These results indicate that type IV collagen and the alpha1 laminin are induced in reactive astrocytes after spinal cord injury in vivo. Induction of type IV collagen in astrocytes in vitro by cytokines indicates that blood-borne or local factors at the injury site may induce the spinal cord glial expression of type IV collagen in vivo. Simultaneous expression of laminin-1 and alpha1 laminin with type IV collagen is known to lead to production of basement membranes. This may hamper the neurite-outgrowth-promoting potential of the gamma1 laminin by initiating formation of the glial scar.

Enterocolitis induced by autoimmune targeting of enteric glial cells: a possible mechanism in Crohn's disease?

Early pathological manifestations of Crohn's disease (CD) include vascular disruption, T cell infiltration of nerve plexi, neuronal degeneration, and induction of T helper 1 cytokine responses. This study demonstrates that disruption of the enteric glial cell network in CD patients represents another early pathological feature that may be modeled after CD8(+) T cell-mediated autoimmune targeting of enteric glia in double transgenic mice. Mice expressing a viral neoself antigen in astrocytes and enteric glia were crossed with specific T cell receptor transgenic mice, resulting in apoptotic depletion of enteric glia to levels comparable in CD patients. Intestinal and mesenteric T cell infiltration, vasculitis, T helper 1 cytokine production, and fulminant bowel inflammation were characteristic hallmarks of disease progression. Immune-mediated damage to enteric glia therefore may participate in the initiation and/or the progression of human inflammatory bowel disease.

Brain remodeling due to neuronal and astrocytic proliferation after controlled cortical injury in mice

The persistence of neural stem cells into adulthood has been an area of intense investigation in recent years. There is limited knowledge about how an acquired brain injury might affect the ability of neural precursor cells to proliferate and repopulate injured areas. In the present study we utilize a controlled cortical impact model of traumatic brain injury in adult mice and subsequent BrdU labeling to demonstrate that there is significant proliferation of neural precursors in response to traumatic brain injury in areas both proximal and distal to the injury site. The fate of the proximal proliferation is almost exclusively astrocytic at 60-days post injury and demonstrates that newly generated cells make up much of the astrogliotic scar. Moreover, in areas more distal from the injury site, neurogenesis occurs within the granular layer of the dentate gyrus at a level more than five-fold greater than in controls. These data demonstrate that neural proliferation plays key roles in the remodeling that occurs after traumatic brain injury and suggests a mechanism as to how functional recovery after traumatic brain injuries continues to occur long after the injury itself.

A review and rationale for the use of genetically engineered animals in the study of traumatic brain injury

The mechanisms underlying secondary cell death after traumatic brain injury (TBI) are poorly understood. Animal models of TBI recapitulate many clinical and pathologic aspects of human head injury, and the development of genetically engineered animals has offered the opportunity to investigate the specific molecular and cellular mechanisms associated with cell dysfunction and death after TBI, allowing for the evaluation of specific cause-effect relations and mechanistic hypotheses. This article represents a compendium of the current literature using genetically engineered mice in studies designed to better understand the posttraumatic inflammatory response, the mechanisms underlying DNA damage, repair, and cell death, and the link between TBI and neurodegenerative diseases.

The regulation of alpha chemokines during HIV-1 infection and leukocyte activation: relevance for HIV-1-associated dementia

Cellular immunity against human immunodeficiency virus type 1 (HIV-1)-infected brain macrophages serves to prevent productive viral replication in the nervous system. Inevitably, during advanced disease, this antiretroviral response breaks down. This could occur through virus-induced dysregulation of lymphocyte trafficking. Thus, we studied the production of non-ELR-containing alpha-chemokines and their receptor (CXCR3) expression in relevant virus target cells. Macrophages, lymphocytes, and astrocytes secreted alpha-chemokines after HIV-1 infection and/or immune activation. Lymphocyte CXCR3-mediated chemotactic responses were operative. In all, alpha-chemokine-mediated T cell migration continued after HIV-1 infection and the neuroinflammatory events operative during productive viral replication in brain.

Immune function of astrocytes

Astrocytes are the major glial cell within the central nervous system (CNS) and have a number of important physiological properties related to CNS homeostasis. The aspect of astrocyte biology addressed in this review article is the astrocyte as an immunocompetent cell within the brain. The capacity of astrocytes to express class II major histocompatibility complex (MHC) antigens and costimulatory molecules (B7 and CD40) that are critical for antigen presentation and T-cell activation are discussed. The functional role of astrocytes as immune effector cells and how this may influence aspects of inflammation and immune reactivity within the brain follows, emphasizing the involvement of astrocytes in promoting Th2 responses. The ability of astrocytes to produce a wide array of chemokines and cytokines is discussed, with an emphasis on the immunological properties of these mediators. The significance of astrocytic antigen presentation and chemokine/cytokine production to neurological diseases with an immunological component is described.

Interleukin-1 beta is required for the early evolution of reactive astrogliosis following CNS lesion

The CNS response to injury is characterized by the rapid activation of astrocytes in a process known as astrogliosis. The function of reactive astrocytes is controversial, in that both beneficial and detrimental properties are postulated. Identification of the molecules involved in regulating astrogliosis is an important step towards understanding astrocyte functions and establishing suitable conditions for CNS regeneration. We previously reported that inflammatory cytokines are regulators of astrogliosis but the key cytokine involved in initiating astrogliosis was unclear. We describe here that the elevation of glial fibrillary acid protein (GFAP) transcripts follows the very early rise of interleukin (IL)-1beta mRNA in a murine corticectomy model of CNS lesion. Furthermore, the injury-induced upregulation of GFAP mRNA and protein did not occur in mice genetically deficient for IL-1beta compared to wild-type animals. This was correlated with an absence of an increase in GFAP-immunoreactivity (GFAP-ir) in IL-1beta-null mice at 2 and 3 days of injury. However, by 5 to 7 days after the lesion, GFAP-ir was not different between cytokine-deficient and wild-type controls. Functionally, mice lacking IL-1beta exhibited a significant impairment in reformation of the blood-brain barrier (BBB) following corticectomy compared to wild-type controls. These findings suggest that the rapid production of IL-1beta following trauma plays a beneficial role in initiating astrogliosis in an attempt to restore the integrity of the BBB and seal off the wound site.

Nerve growth factor signaling, neuroprotection, and neural repair

Nerve growth factor (NGF) was discovered 50 years ago as a molecule that promoted the survival and differentiation of sensory and sympathetic neurons. Its roles in neural development have been characterized extensively, but recent findings point to an unexpected diversity of NGF actions and indicate that developmental effects are only one aspect of the biology of NGF. This article considers expanded roles for NGF that are associated with the dynamically regulated production of NGF and its receptors that begins in development, extends throughout adult life and aging, and involves a surprising variety of neurons, glia, and nonneural cells. Particular attention is given to a growing body of evidence that suggests that among other roles, endogenous NGF signaling subserves neuroprotective and repair functions. The analysis points to many interesting unanswered questions and to the potential for continuing research on NGF to substantially enhance our understanding of the mechanisms and treatment of neurological disorders.

Human MBP-specific T cells regulate IL-6 gene expression in astrocytes through cell-cell contacts and soluble factors

One of the distinctive features of multiple sclerosis (MS) attacks is homing to the CNS of activated T cells able to orchestrate humoral and cell-based events, resulting in immune-mediated injury to myelin and oligodendrocytes. Of the complex interplay occurring between T cells and CNS constituents, we have examined some aspects of T-cell interactions with astrocytes, the major components of the glial cells. Specifically, we focused on the ability of T cells to regulate the gene expression of interleukin-6 (IL-6) in astrocytes, based on previous evidence showing the involvement of this cytokine in CNS disorders. We found that T-cell adhesion and T-cell soluble factors induce IL-6 gene expression in U251 astrocytes through distinct signaling pathways, respectively, resulting in the activation of NF-kappaB and IRF-1 transcription factors. In a search for effector molecules at the astrocyte surface, we found that alpha3beta1 integrins play a role in NF-kappaB activation induced by T-cell contact, whereas interferon-gamma (IFN-gamma) receptors dominate in IRF-1 induction brought about by T-cell-derived soluble factors. Similar phenomena were observed also in normal fetal astrocyte cultures. We therefore propose that through astrocyte induction, T cells may indirectly regulate the availability of a cytokine which is crucial in modulating fate and behavior of cell populations involved in the pathogenesis of MS inflammatory lesions.

A transgenic mouse model for inducible and reversible dysmyelination

Oligodendrocytes are glial cells devoted to the production of myelin sheaths. Myelination of the CNS occurs essentially after birth. To delineate both the times of oligodendrocyte proliferation and myelination, as well as to study the consequence of dysmyelination in vivo, a model of inducible dysmyelination was developed. To achieve oligodendrocyte ablation, transgenic animals were generated that express the herpes virus 1 thymidine kinase (HSV1-TK) gene under the control of the myelin basic protein (MBP) gene promoter. The expression of the MBP-TK transgene in oligodendrocytes is not toxic on its own; however, toxicity can be selectively induced by the systemic injection of animals with nucleoside analogs, such as FIAU [1-(2-deoxy-2-fluoro-beta-delta-arabinofuranosyl)-5-iodouracil]. This system allows us to control the precise duration of the toxic insult and the degree of ablation of oligodendrocytes in vivo. We show that chronic treatment of MBP-TK mice with FIAU during the first 3 postnatal weeks triggers almost a total depletion of oligodendrocytes in the CNS. These effects are accompanied by a behavioral phenotype characterized by tremors, seizures, retarded growth, and premature animal death. We identify the period of highest oligodendrocytes division in the first 9 postnatal days. Delaying the beginning of FIAU treatments results in different degrees of dysmyelination. Dysmyelination in MBP-TK mice is always accompanied by astrocytosis. Thus, this transgenic line provides a model to study the events occurring during dysmyelination of various intensities. It also represents an invaluable tool to investigate remyelination in vivo.

Conditional ablation of neurones in transgenic mice

Conditional targeted ablation of specific cell populations in living transgenic animals is a very powerful strategy to determine cell functions in vivo. This approach would be of particular value to study the functions of distinct neuronal populations; however, the transgene of choice for conditional cell ablation studies in mice, the herpes simplex virus thymidine kinase gene, cannot be used to ablate neurones as its principal mode of action relies on cell proliferation. Here we report that expression of the E.coli nitroreductase gene (Ntr) and metabolism of the prodrug CB1954 (5-aziridin-1-yl-2-4-dinitrobenzamide) to its cytotoxic derivative can be used to conditionally and acutely ablate specific neuronal populations in vivo. As proof of principal, we have ablated olfactory and vomeronasal receptor neurones by expressing Ntr under the control of the olfactory marker protein (OMP) gene promoter. We demonstrate that following CB1954 administration, olfactory and vomeronasal receptor neurones expressing the transgene were selectively eliminated from the olfactory epithelium (OE), and projections to the olfactory bulb (OB) were lost. The functional efficacy of cell ablation was demonstrated using a highly sensitive behavioural test to show that ablated mice had lost the olfactory ability to discriminate distinct odors and were consequently rendered anosmic. Targeted expression of Ntr to specific neuronal populations using conventional transgenes, as described here, or by "knock-in" gene targeting using embryonic stem cells may be of significant value to address the functions of distinct neuronal populations in vivo.

Differences in Neurotrophic Factor Gene Expression Profiles between Neonate and Adult Rat Spinal Cord after Injury

The capacity of the central nervous system for axonal growth decreases as the age of the animal at the time of injury increases. Changes in the expression of neurotrophic factors within embryonic and early postnatal spinal cord suggest that a lack of trophic support contributes to this restrictive growth environment. We examined neurotrophic factor gene profiles by ribonuclease protection assay in normal neonate and normal adult spinal cord and in neonate and adult spinal cord after injury. Our results show that in the normal developing spinal cord between postnatal days 3 (P3) and P10, compared to the normal adult spinal cord, there are higher levels of nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin 3 (NT-3), and glial-derived neurotrophic factor (GDNF) mRNA expression and a lower level of ciliary neurotrophic factor (CNTF) mRNA expression. Between P10 and P17, there is a significant decrease in the expression of NGF, BDNF, NT-3, and GDNF mRNA and a contrasting steady and significant increase in the level of CNTF mRNA expression. These findings show that there is a critical shift in neurotrophic factor expression in normal developing spinal cord between P10 and P17. In neonate spinal cord after injury, there is a significantly higher level of BDNF mRNA expression and a significantly lower level of CNTF mRNA expression compared to those observed in the adult spinal cord after injury. These findings suggest that high levels of BDNF mRNA expression and low levels of CNTF mRNA expression play important roles in axonal regrowth in early postnatal spinal cord after injury.