Basic fibroblast growth factor improves recovery after chemically induced breakdown of myelin-like membranes in pure oligodendrocyte cultures

Pathology and current treatment of neurodegenerative sphingolipidoses

Sphingolipidoses constitute a large subgroup of lysosomal storage disorders (LSDs). Many of them are associated with a progressive neurodegeneration. As is the case for LSDs in general, most sphingolipidoses are caused by deficiencies in lysosomal hydrolases. However, accumulation of sphingolipids can also result from deficiencies in proteins involved in the transport or posttranslational modification of lysosomal enzymes, transport of lipids, or lysosomal membrane proteins required for transport of lysosomal degradation end products. The accumulation of sphingolipids in the lysosome together with secondary changes in the concentration and localization of other lipids may cause trafficking defects of membrane lipids and proteins, affect calcium homeostasis, induce the unfolded protein response, activate apoptotic cascades, and affect various signal transduction pathways. To what extent, however, these changes contribute to the pathogenesis of the diseases is not fully understood. Currently, there is no cure for sphingolipidoses. Therapies like enzyme replacement, pharmacological chaperone, and substrate reduction therapy, which have been shown to be efficient in non-neuronopathic LSDs, are currently evaluated in clinical trials of neuronopathic sphingolipidoses. In the future, neural stem cell therapy and gene therapy may become an option for these disorders.

Up-regulation of platelet-derived growth factor by peripheral-blood leukocytes during experimental allergic encephalomyelitis

In multiple sclerosis (MS) and its animal model, experimental allergic encephalomyelitis (EAE), clinical disease is associated with infiltration of the central nervous system (CNS) by immune cells. Subsequent remission with remyelination has been linked to an increased occurrence of oligodendrocyte progenitor (O2A) cells. Platelet-derived growth factor (PDGF) and fibroblast growth factor-2 (FGF-2) are key growth factors for O2A cells, yet little is known about their relevance in EAE and MS. We analyzed the expression of PDGF, FGF-2, and their receptors by peripheral-blood leukocytes (PBLs) and lymphocyte subsets during MBP-induced EAE. Strong up-regulation of PDGF, but not FGF-2, was observed in PBLs, with the highest expression after the disease maximum. T, NK, and NKT cells expressed PDGF, which is a novel observation because thus far only monocytes/macrophages have been reported to express PDGF. These results extend the idea that growth factors may contribute to improved CNS tissue repair, including PDGF, which is secreted by lesion-homing immune cells. The production of PDGF by lymphocytes may have potential therapeutic value when activating or modulating T-cell responses in demyelinating diseases.

Krabbe disease: psychosine-mediated activation of phospholipase A2 in oligodendrocyte cell death

Globoid cell leukodystrophy (Krabbe disease) is an inherited neurological disorder caused by the pathogenomic accumulation of psychosine (galactosylsphingosine), a substrate for the deficient enzyme galactocerebroside beta-galactosidase. This study underscores the mechanism of action of psychosine in the regulation of oligodendrocyte cell death via the generation of lysophosphatidylcholine (LPC) and arachidonic acid (AA) by the activation of secretory phospholipase A2 (sPLA2). There was a significant increase in the level of LPC, indicating a phospholipase A2 (PLA2)-dependent pathobiology, in the brains of Krabbe disease patients and those of twitcher mice, an animal model of Krabbe disease. In vitro studies of the treatment of primary oligodendrocytes and the oligodendrocyte MO3.13 cell line with psychosine also showed the generation of LPC and the release of AA in a dose- and time-dependent manner, indicating psychosine-induced activation of PLA2. Studies with various pharmacological inhibitors of cytosolic phospholipase A2 and sPLA2 and psychosine-mediated induction of sPLA2 enzymatic activity in media supernatant suggest that psychosine-induced release of AA and generation of LPC is mainly contributed by sPLA2. An inhibitor of sPLA2, 7,7-dimethyl eicosadienoic acid, completely attenuated the psychosine-mediated accumulation of LPC levels, release of AA, and generation of reactive oxygen species, and blocked oligodendroyte cell death, as evident from cell survival, DNA fragmentation, and caspase 3 activity assays. This study documents for the first time that psychosine-induced cell death is mediated via the sPLA2 signaling pathway and that inhibitors of sPLA2 may hold a therapeutic potential for protection against oligodendrocyte cell death and resulting demyelination in Krabbe disease.

Repeated injuries dramatically affect cells of the oligodendrocyte lineage: Effects of PDGF and NT-3 in vitro

In multiple sclerosis, relapses occur and remyelination is incomplete, whereas one demyelinating lesion induced by lysophosphatidyl choline (LPC) in rats is completely remyelinated; this process is accelerated by platelet-derived growth factor (PDGF) (Allamargot et al.: Brain Res 918:28-39,2001) and neurotrophin-3 (NT-3) (Jean et al.: Brain Res 972:110-118,2003). Similarly, oligodendrocyte (OL) progenitors might not be depleted by two to three episodes of toxic demyelination (Penderis et al.: Brain 126:1382-1391,2003); nevertheless this does not allow conclusions about the fate of resident cells (mature OL). As myelinated fibers per OL are constantly decreased in chronic MS plaques (Fressinaud and Jean: J Neurochem 85(suppl):100, 2003), this suggests that OL decreased capability to synthesize new myelin membranes could result from successive relapses, impairing thereby remyelination. Thus, we have determined the consequences of multiple versus unique (Fressinaud and Vallat: J Neurosci Res 38:202-213, 1994) LPC treatments on newborn rat brain pure OL cultures, as well as the putative pro-remyelinating effects of PDGF and of NT-3 in these conditions. Split (0.5. 0(-5) M, 6 h x 4) and multiple (2.10(-5) M, 24 h x 2) LPC doses induced more cell loss than a unique treatment (2 x 10(-5) M, 24 h) and there was no recovery. OL progenitors (A2B5+ cells) and differentiated (CNP+) OL were drastically decreased. Moreover, mature (MBP+) OL disappeared from these cultures, indicating that mature OL are also vulnerable to multiple insults. PDGF, as well as NT-3, induced at least partial recovery, and enhanced OL progenitor proliferation. In cultures treated with either of these growth factors, mature OL represented one-fourth of cells and extended numerous ramified processes and putative myelin balls.

Basic Fibroblast Growth Factor Promotes Neuronal Survival but Not Behavioral Recovery in the Transected and Schwann Cell Implanted Rat Thoracic Spinal Cord

It was investigated whether the addition of basic fibroblast growth factor (FGF-2) enhances the efficacy of a Schwann cell (SC) bridge to repair the transected spinal cord by assessing tissue sparing and neuronal survival near the graft-cord interfaces, axonal regeneration and myelination in the graft, and behavioral recovery up to 12 weeks post-grafting. Experimental animals received a bridge of SCs within fibrin containing 1 microg of FGF-2; control animals received a SC implant without FGF-2. Sparing of tissue in a 2.5-mm-long segment near the graft-cord borders was 69% in the rostral and 52% in the caudal cord at 6 weeks post-grafting, not significantly different from the control group. With FGF-2, survival of NeuN-positive cells was increased in the rostral cord: 24.4%, 20.4%, and 17.2% of the number of positive cells in the uninjured cord compared to 13.5%, 9.1%, and 8.9% in controls at 3, 6, and 12 weeks post-grafting, respectively. Similarly, in the caudal cord, survival of NeuN-positive cells was increased with FGF-2: 19.3%, 16.8%, and 14.5% compared to 10.8%, 5.6%, and 6.1% in controls. The staining intensity of glial fibrillary acidic protein was significantly higher at the interfaces of both cord stumps at 3 weeks with SC/FGF-2 grafts; chondroitin sulfate proteoglycan (CS-56) staining was more intense in the rostral cord but only at 6 weeks. Blood vessels in the FGF-2 grafts were larger and less regular in shape than those in control grafts. Axonal growth into the bridge was not improved by the addition of FGF-2. Retrogradely traced neurons were not found rostral to the implant, indicating that axons had not grown a few mm into the caudal spinal tissue. Recovery of hind limb function was similar in both groups. Despite the neuroprotective effects of FGF-2, improved effects on axonal regeneration and functional recovery were not observed.

Inducible expression of FGF2 by a rat oligodendrocyte precursor cell line promotes CNS myelination in vitro

Transplantation of glial cells into the central nervous system (CNS) may be a promising approach for the treatment of myelin disorders such as multiple sclerosis (MS). Myelination by transplantation of oligodendrocyte precursors has been obtained in different animal models of demyelination. A strategy to favor CNS remyelination is to enrich the lesioned areas in growth factors to stimulate the quiescent population of oligodendrocyte precursors. In this context, we have developed a genetically modified CG4 cell line (CG4-FGF2), which are able to release significant amounts of fibroblast growth factor 2 (FGF2) in a controlable fashion in vitro. The data presented here demonstrate that upon induction with Dox, CG4-FGF2 cells retain their capacity to differentiate in vitro. Additionally, we provide evidence that FGF2 release by engineered cells enhance proliferation and migration of cells of the oligodendrocyte lineage without preventing them to differentiate and myelinate axons in vitro.

Transient exposure to FGF2 enhances myelination in embryonic brain cell cocultures

The amount of myelination in vivo and in vitro depends on the number of oligodendrocyte progenitors, their differentiation, and on the neuron function. It has been shown that continuous administration of FGF2, a mitotic and neuroprotective factor, allows oligodendrocyte progenitors to proliferate, but prevents them from differentiating and myelinating. This study was designed to test the effect of transient exposure to FGF2 on myelination in an oligodendrocyte/neuron coculture system. At 2 days in vitro, cultures were treated with a single dose of 20 ng/ml FGF2. Cell proliferation was determined by BrdU uptake. The number of cells of the oligodendrocyte lineage was determined by immunocytology of 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNPase). The maturation of oligodendrocytes and myelination was followed by immunocytological analysis of MBP (myelin basic protein). Electron microscopy was used to study the ultrastructure of myelin. BrdU uptake procedure showed an increase in cell proliferation in FGF2-treated cultures after 48 h of treatment. At 15-18 days in vitro, CNPase(+) and MBP(+) cells were much more abundant in cultures treated with FGF2 than in control cultures. We observed differentiation and maturation of oligodendrocytes and a higher degree of myelination in FGF2-treated cultures compared to controls. Electron microscopy showed the presence of myelin structures in FGF2-treated cultures that did not differ morphologically from those observed in control cultures. Transient exposure of cultured brain cells to FGF2 increased myelination in vitro. Administration of FGF2 over a short period might thus enhance remyelination in demyelinating diseases in vivo.

A single intracerebral microinjection of platelet-derived growth factor (PDGF) accelerates the rate of remyelination in vivo

We had demonstrated that platelet-derived growth factor (PDGF) enhanced the reconstruction of myelin-like membranes after their disruption by lysophosphatidylcholine (LPC) in vitro. To investigate its role in vivo, a demyelinating lesion of the corpus callosum was induced in adult Wistar rats by a stereotaxic microinjection of 1 microl LPC, then 63 pairs of rats received either 1 microg PDGF, or its vehicle buffer which were injected above LPC. The effects of PDGF were significant after 2 weeks: the number of oligodendrocytes (OL) expressing 2',3'-cyclic nucleotide 3'-phosphodiesterase in the lesion increased by 49%, mature OL labelled by in situ hybridization for myelin basic protein-mRNA increased by 27% (P<10(-2)), and the total volume of demyelination decreased by 60% compared to controls. The proliferation of cells of the OL lineage was also enhanced up to 67% by PDGF compared to LPC controls (P<2.5 x 10(-2)). Ultrastructural studies confirmed this dramatic improvement, and the ratio of remyelinated to demyelinated axons, determined at the maximal demyelination site, in the centre of the lesion, increased by 10-fold (P<2.5 x 10(-3)) in animals treated with PDGF. Remyelination was complete after 3 months for both treatments. Neither exacerbation of gliosis nor glial tumoural transformation were observed. Mechanisms through which PDGF improves remyelination could involve proliferation of OL progenitors, and/or of already differentiated surviving OLs, and a chemotactic effect, which had been identified in vitro.

Therapeutic implications of fibroblast growth factors in traumatic spinal cord injury

Neurological damage after acute traumatic spinal cord injury (SCI) results from both the primary mechanical injury as well as the subsequent activation of cell death cascades mediating delayed tissue damage. Since secondary injury following traumatic SCI is a tightly regulated process in which several neurotrophic factors seem to be implicated, administration of these proteins has a clinical interest. Fibroblast growth factor may be one of the agents to be used for the treatment of traumatic SCI.

Basic fibroblast growth factor (bFGF) enhances functional recovery following severe spinal cord injury to the rat

We have recently demonstrated that following a moderate contusion spinal cord injury (SCI) to rats, subsequent administration of basic fibroblast growth factor (bFGF) significantly enhances functional recovery and tissue sparing. To further characterize the effects of bFGF, we evaluated its efficacy after a more severe contusion injury at T(10) using the NYU impactor. Immediately after SCI, osmotic minipumps were implanted into the lateral ventricle and lumbar thecal sac to deliver bFGF at 3 or 6 microg per day versus control vehicle for 1 week. Animals were behaviorally tested for 6 weeks before histological assessment of tissue sparing through the injured segment and glial reactivity distal to the lesion. Compared to moderate SCI, all rats had more prolonged and sustained functional deficits 6 weeks after severe contusion. Subjects treated with bFGF had pronounced recovery of hindlimb movements from 2 to 6 weeks compared to controls, manifested in significantly higher behavioral scores. Only marginal tissue sparing was seen rostral to the injury in bFGF-treated spinal cords versus controls. Optical density measurements of astrocyte and microglial cell immunoreactivity in bFGF-treated spinal cords showed that after 6 weeks they approximated controls, although astrocyte immunoreactivity remained higher in controls rostrally. In summary, intrathecal infusion of bFGF following severe SCI significantly restores gross hindlimb motor function that is not correlated with significant tissue sparing. In light of previous evidence that pharmacological intervention with bFGF after moderate SCI enhances tissue preservation, the current findings indicate that yet undefined mechanisms contribute to the enhanced functional recovery following bFGF treatment.

Temporal analysis of growth factor mRNA expression in myelinating rat brain aggregate cultures: increments in CNTF, FGF-2, IGF-I, and PDGF-AA mRNA are induced by antibody-mediated demyelination

Myelinogenesis in rat brain aggregate cultures is associated with a pattern of growth factor mRNA expression comparable to that of the developing brain. The rate of increase in platelet-derived growth factor-AA (PDGF-AA) expression was greatest just before the detection of myelin basic protein (MBP) mRNA in the cultures and remained high thereafter, consistent with in vivo observations. Levels of fibroblast growth factor-2 (FGF-2) and of ciliary neurotrophic factor (CNTF) mRNA increased continuously over the period of MBP accumulation. High rates of transforming growth factor beta1 (TGF-beta1), insulin-like growth factor-I (IGF-I), and neurotrophin-3 (NT-3) expression at early time points during the culture gradually decreased over time, indicative of a key regulatory role during oligodendrocyte development. The addition of demyelinative anti-myelin oligodendrocyte glycoprotein (anti-MOG) antibody resulted in a significant increase in MBP peptide fragments with a C-terminus at phenylalanine 89 indicating proteolytic breakdown of MBP after myelin phagocytosis. Immediately after antibody treatment the expression of CNTF mRNA was significantly increased, compared with controls, while that of FGF-2 and IGF-I, and of PDGF-AA peaked during the early and later stages of recovery respectively. Thus, specific growth factors combine to regulate myelination and remyelination in the aggregates; these data have implications for demyelinating disease in which protective growth factor secretion may be central to regeneration.

Brain damage in preterm newborns: might enhancement of developmentally regulated endogenous protection open a door for prevention?

We present a two-component model of brain white matter damage in preterm neonates. The insult component comprises infection and hypoxia-ischemia, which are both associated with inflammation-related abnormalities in the white matter. The developmental component comprises at least three factors, ie, immaturity of the ependymal/endothelial, oligodendroglial, and endogenous protection systems. All three factors are likely contributors to an increased vulnerability of the preterm newborn's white matter. In this article, we focus on recent developments in oligodendrocyte biology that support the view of certain cytokines and growth factors as oligotrophins based on their capability to enhance oligodendrocyte development or survival. We suggest that research into networks of developmentally regulated endogenous protectors (such as oligotrophins) is necessary to broaden our perspectives in brain injury prevention in preterm newborns.

Macrophages in CNS remyelination: friend or foe?

Hematogenous macrophages and resident brain microglia are agents of demyelination in multiple sclerosis (MS) and paradoxically may also participate in remyelination. In vitro studies have shown that macrophage enrichment of aggregate brain cultures promotes myelination per se and enhances the capacity to remyelinate following a demyelinating episode. It has been hypothesized that remyelination in MS is implemented by surviving dedifferentiated oligodendrocytes or by newly recruited progenitors that migrate, proliferate and synthesize myelin in response to signalling molecules in the local environment. We postulate that macrophage-derived cytokines or growth factors may directly or indirectly promote oligodendroglial proliferation and differentiation, contributing to myelin repair in inflammatory demyelinating disease.

Injury stimulates outgrowth and motility of oligodendrocytes grown in vitro

Oligodendrocytes which form myelin within the CNS develop from small, highly motile cells that are largely bipolar into mature cells which extend many processes and which produce myelin membranes around multiple axons. The production of myelin sheaths is thought to anchor mature oligodendrocytes (OLs), limiting their motility. When the brain sustains an injury, OLs do not make a significant effort to remyelinate, a fact attributed to both their lack of proliferation and their inability to migrate or extend processes into areas of injury. To test the motility and growth potential of mature OLs, we have designed an in vitro system in which individual cells can undergo long-term observation. Additionally, cells can be mechanically injured by transection of processes using a low-power laser beam. Both control and injured OLs undergo several types of structural change, including extension and retraction of processes and membranes, as well as changes in process caliber. Some OLs exhibit a high degree of motility, moving several hundred micrometers within days. Rather than interfering with the cells' ability to undergo structural change, injury actually stimulated outgrowth of new processes and motility. Neither injury nor addition of basic fibroblast growth factor (bFGF) increased the rate of OL division. However, bFGF paradoxically caused an increase in uptake of the DNA synthesis marker bromodeoxyuridine and had negative effects on OL survival. The unexpected findings that OLs with a mature phenotype are motile and undergo constant structural modification in vitro and that injury induces certain behaviors suggest that myelin-forming OLs in the brain may be capable of a high degree of plasticity under certain conditions.

In vitro oligodendrogliotrophic properties of cell adhesion molecules in the immunoglobulin superfamily: myelin-associated glycoprotein and N-CAM

To determine if cell recognition molecules interact trophically with oligodendrocytes (OCs), their effect as growth substrates for differentiating oligodendroblasts was studied in primary culture. Oligodendroblasts purified from postnatal rat cerebrum by immunopanning were plated on substratum-bound cell adhesion molecules or extracellular matrix glycoproteins in chemically defined medium in which OCs terminally differentiate but survive poorly. Growth on myelin-associated glycoprotein (MAG) and neural cell adhesion molecule (N-CAM) selectively increased the number of viable cells per culture 2 weeks after plating as much as tenfold and sixfold, respectively, over background survival on an albumin substrate, whereas L1, tenascin-R, tenascin-C, fibronectin, and laminin were ineffective. Neither MAG nor N-CAM stimulated bromodeoxyuridine incorporation into cultures, indicating that enhanced proliferation did not contribute to better survival. Compared to growth on polyornithine alone, oligodendroblast differentiation in the added presence of MAG or N-CAM was qualitatively unchanged; > 90% of surviving cells developed into OCs that matured further by immunocytochemical and morphological criteria. A striking difference, however, was the quantitative effect of MAG and N-CAM substrates on oligodendrite outgrowth, increasing myelin-like membrane formation two- to threefold (> 8 x 10(3) microns2/cell). These findings support the concept that autotypic or heterotypic cell contact-mediated signaling by recognition molecules at the OC surface contributes trophic support of myelinogenesis.

Astrocytes promote process outgrowth by adult human oligodendrocytes in vitro through interaction between bFGF and astrocyte extracellular matrix

Cell-cell interactions regulate many important functions within the central nervous system. In this report, we demonstrate that process outgrowth by adult human oligodendrocytes (OLs) in vitro, an early event of myelinogenesis in vivo, is promoted by astrocytes. To elucidate the mechanisms by which astrocytes might exert this effect, we tested several growth factors known to be produced by astrocytes and found that only basic fibroblast growth factor (bFGF) could enhance process extension by the OL. In correspondence, the treatment of astrocytes with a neutralizing antibody to bFGF decreased their effects in promoting oligodendroglial process outgrowth. The potency of bFGF, however, was only one-third that of astrocytes, and since bFGF did not synergize with other soluble growth factors, we investigated the potential facilitatory role of the extracellular matrix (ECM) deposited by astrocytes. The astrocyte ECM was found to be a promoter of oligodendroglial process extension, and significantly, bFGF synergized with astrocyte ECM to match the potency of live astrocytes. The astrocyte ECM was found in Western blot analyses to contain fibronectin, vitronectin, and laminin. These purified ECM components, as well as heparan sulfate proteoglycan, did not promote oligodendroglial process extension by themselves, although laminin and fibronectin potentiated the effects of bFGF. We conclude that process outgrowth by OLs is guided by astrocytes; the mechanism of the astrocyte effect appears to be due to the combination of bFGF and an unidentified ECM component.

Growth factor stimulation triggers apoptotic cell death in mature oligodendrocytes

Oligodendrocytes are responsible for myelination of axons within the central nervous system and are lost in the lesions of multiple sclerosis. The question of whether mature oligodendrocytes are capable of division, and hence of giving rise to new myelinating cells, remains controversial. We have studied the response in vitro of rat oligodendrocytes to growth factors. Unlike their progenitors which divided vigorously in response to growth factor stimulation, mature oligodendrocytes showed no mitotic response to a number of individual growth factors. However, when exposed to basic fibroblast growth factor (bFGF), mature oligodendrocytes expressed the immediate response gene c-fos and the cell cycle-associated protein proliferating cell nuclear antigen, but failed to divide and ultimately underwent cell death by apoptosis. These findings are consistent with the emerging view that mitosis and apoptosis are closely linked phenomena.

Epigenetic factors up-regulate expression of myelin proteins in the dysmyelinating jimpy mutant mouse

Proteolipid protein (PLP) is a major structural component of central nervous system (CNS) myelin. Evidence exists that PLP or the related splice variant DM-20 protein may also play a role in early development of oligodendrocytes (OLs), the cells that form CNS myelin. There are several naturally occurring mutations of the PLP gene that have been used to study the roles of PLP both in myelination and in OL differentiation. The PLP mutation in the jimpy (jp) mouse has been extensively characterized. These mutants produce no detectable PLP and exhibit an almost total lack of CNS myelin. Additionally, most OLs in affected animals die prematurely, before producing myelin sheaths. We have studied cultures of jp CNS in order to understand whether OL survival and myelin formation require production of normal PLP. When grown in primary cultures, jp OLs mimic the relatively undifferentiated phenotype of jp OLs in vivo. They produce little myelin basic protein (MBP), never immunostain for PLP, and rarely elaborate myelin-like membranes. We report here that jp OLs grown in medium conditioned by normal astrocytes synthesize MBP and incorporate it into membrane expansions. Some jp OLs grown in this way stain with PLP antibodies, including an antibody to a peptide sequence specific for the mutant jp PLP. This study shows that: (1) an absence of PLP does not necessarily lead to dysmyelination or OL death; (2) OLs are capable of translating at least a portion of the predicted jp PLP; (3) the abnormal PLP made in the cultured jp cells is not toxic to OLs. These results also highlight the importance of environmental factors in controlling OL phenotype.

Platelet-derived growth factor partly prevents chemically induced oligodendrocyte death and improves myelin-like membranes repair in vitro

We have previously shown that pure oligodendrocyte (OL) secondary cultures derived from newborn rat brain, in which cells form myelin-like membranes, can be used as a model to investigate the putative role of growth factors in myelin repair. After disruption of these membranes by lysophosphatidylcholine (LPC), a 3 day treatment with 10 ng/ml basic fibroblast growth factor (bFGF) induced reconstruction of myelin figures, albeit less compacted than in untreated controls. Here we show that in LPC treated cultures: 1) bFGF can not prevent OL from LPC-induced cell death; 2) platelet-derived growth factor (PDGF) pretreatment although preventing some cell death does not improve recovery compared to delayed treatment; 3) PDGF is as potent as bFGF in terms of O-2A progenitor proliferation; 4) PDGF is far more effective than bFGF, inducing the reappearance of more myelin-like structures with a better compaction; 5) there is no potentiation between these growth factors; and 6) after withdrawal of bFGF the compaction of myelin figures partly increases. These results indicate that PDGF, probably by inducing O-2A progenitors to proliferate and then allowing them to differentiate into mature myelinating OL, is a better candidate than bFGF to participate in myelin repair mechanisms in the central nervous system.

A comprehensive analysis of the distribution of FGF-2 and FGFR1 in the rat brain

We have examined the cellular distribution of both FGF-2 and FGFR1 immunoreactivity and their mRNAs throughout the normal adult rat brain in order to reconcile numerous disparate findings in the published literature. The results confirm a widespread distribution of FGF-2 and FGFR1 in the rat brain, and different regions express distinct patterns of FGF-2 and FGFR1 mRNA and protein: neuronal and non-neuronal cells show different subcellular distributions that vary according to the area where they are located. The intensity of the staining and hybridization also varies according to the loci examined and the cell type involved. Astrocytes contain the highest levels of FGF-2 and FGFR1 mRNAs, and characteristically, possess high levels of immunoreactive FGF-2 within the nucleus. Amongst non-neuronal cells, oligodendrocytes do not synthesize or contain significant levels of FGF-2 immunoreactivity however, they do express FGFR1 mRNA. In these cells, immunoreactive FGFR1 is mainly associated with the myelin sheaths of neuronal fibers. In ventricular systems, ependymal cells synthesize and contain immunoreactive FGFR1. In contrast, only cells lining the lateral wall of the IIIrd ventricle express FGF-2 mRNA. Subependymal cells contain high levels of both FGF-2 and FGFR1 immunoreactivity. Neurons express low levels of FGF-2 mRNA and immunoreactive FGF-2 is localized predominantly to the perikaryon. However, selected populations of neurons, such as CA2 field of the hippocampus, show high levels of FGF-2 mRNA, in which the nucleus is strongly immunopositive. Similarly, high levels of FGFR1 mRNA are localized to select populations of neurons (e.g. amygdala). FGFR1 immunoreactivity is mainly associated with myelinated fiber tracts (e.g. striatum), and some neurons show immunoreactivity in the perikaryon (e.g. hippocampus), the nucleus (e.g. mesencephalic trigeminal nucleus), or in axonal projections (e.g. hypothalamus). Remarkably, in many of the areas studied, FGF-2 and FGFR1 mRNA and/or their translated protein do not co-localize in neurons (e.g. neo-cortices) or even in the same regions of the brain (e.g. substantia nigra). In other instances, mRNAs for both FGF-2 and FGFR1 colocalize (e.g. supraoptic nucleus). The brain, in contrast to peripheral tissues, contains high levels of FGF-2 and actively expresses its gene under normal physiological conditions. The highly specific anatomical distribution of immunoreactive FGF-2 in neuronal and non-neuronal brain cells, supports the notion that it plays a multifunctional role in the CNS under normal physiology. By correlating the localization and the synthesis of FGF-2 and one of its high affinity receptors, FGFR1, in the CNS, it should be possible to obtain a better understanding of the roles of FGF-2 in normal and pathological conditions.