Regulation of oligodendrocyte differentiation by fibroblast growth factors

Fibroblast Growth Factor Signalling in the Diseased Nervous System

Fibroblast growth factors (FGFs) act as key signalling molecules in brain development, maintenance, and repair. They influence the intricate relationship between myelinating cells and axons as well as the association of astrocytic and microglial processes with neuronal perikarya and synapses. Advances in molecular genetics and imaging techniques have allowed novel insights into FGF signalling in recent years. Conditional mouse mutants have revealed the functional significance of neuronal and glial FGF receptors, not only in tissue protection, axon regeneration, and glial proliferation but also in instant behavioural changes. This review provides a summary of recent findings regarding the role of FGFs and their receptors in the nervous system and in the pathogenesis of major neurological and psychiatric disorders.

Mature oligodendrocytes bordering lesions limit demyelination and favor myelin repair via heparan sulfate production

Myelin destruction is followed by resident glia activation and mobilization of endogenous progenitors (OPC) which participate in myelin repair. Here we show that in response to demyelination, mature oligodendrocytes (OLG) bordering the lesion express Ndst1, a key enzyme for heparan sulfates (HS) synthesis. Ndst1+ OLG form a belt that demarcates lesioned from intact white matter. Mice with selective inactivation of Ndst1 in the OLG lineage display increased lesion size, sustained microglia and OPC reactivity. HS production around the lesion allows Sonic hedgehog (Shh) binding and favors the local enrichment of this morphogen involved in myelin regeneration. In MS patients, Ndst1 is also found overexpressed in oligodendroglia and the number of Ndst1-expressing oligodendroglia is inversely correlated with lesion size and positively correlated with remyelination potential. Our study suggests that mature OLG surrounding demyelinated lesions are not passive witnesses but contribute to protection and regeneration by producing HS.

EGF Enhances Oligodendrogenesis from Glial Progenitor Cells

Emerging evidence indicates that epidermal growth factor (EGF) signaling plays a positive role in myelin development and repair, but little is known about its biological effects on the early generation and differentiation of oligodendrocyte (OL) lineage cells. In this study, we investigated the role of EGF in early OL development with isolated glial restricted precursor (GRP) cells. It was found that EGF collaborated with Platelet Derived Growth Factor-AA (PDGFaa) to promote the survival and self-renewal of GRP cells, but predisposed GRP cells to develop into O4^- early-stage oligodendrocyte precursor cells (OPCs) in the absence of or PDGFaa. In OPCs, EGF synergized with PDGFaa to maintain their O4 negative antigenic phenotype. Upon PDGFaa withdrawal, EGF promoted the terminal differentiation of OPCs by reducing apoptosis and increasing the number of mature OLs. Together, these data revealed that EGF is an important mitogen to enhance oligodendroglial development.

Transcript profiling of different types of multiple sclerosis lesions yields FGF1 as a promoter of remyelination

Chronic demyelination is a pathological hallmark of multiple sclerosis (MS). Only a minority of MS lesions remyelinates completely. Enhancing remyelination is, therefore, a major aim of future MS therapies. Here we took a novel approach to identify factors that may inhibit or support endogenous remyelination in MS. We dissected remyelinated, demyelinated active, and demyelinated inactive white matter MS lesions, and compared transcript levels of myelination and inflammation-related genes using quantitative PCR on customized TaqMan Low Density Arrays. In remyelinated lesions, fibroblast growth factor (FGF) 1 was the most abundant of all analyzed myelination-regulating factors, showed a trend towards higher expression as compared to demyelinated lesions and was significantly higher than in control white matter. Two MS tissue blocks comprised lesions with adjacent de- and remyelinated areas and FGF1 expression was higher in the remyelinated rim compared to the demyelinated lesion core. In functional experiments, FGF1 accelerated developmental myelination in dissociated mixed cultures and promoted remyelination in slice cultures, whereas it decelerated differentiation of purified primary oligodendrocytes, suggesting that promotion of remyelination by FGF1 is based on an indirect mechanism. The analysis of human astrocyte responses to FGF1 by genome wide expression profiling showed that FGF1 induced the expression of the chemokine CXCL8 and leukemia inhibitory factor, two factors implicated in recruitment of oligodendrocytes and promotion of remyelination. Together, this study presents a transcript profiling of remyelinated MS lesions and identified FGF1 as a promoter of remyelination. Modulation of FGF family members might improve myelin repair in MS.

Promoting return of function in multiple sclerosis: An integrated approach

Multiple sclerosis is a disease characterized by inflammatory demyelination, axonal degeneration and progressive brain atrophy. Most of the currently available disease modifying agents proved to be very effective in managing the relapse rate, however progressive neuronal damage continues to occur and leads to progressive accumulation of irreversible disability. For this reason, any therapeutic strategy aimed at restoration of function must take into account not only immunomodulation, but also axonal protection and new myelin formation. We further highlight the importance of an holistic approach, which considers the variability of therapeutic responsiveness as the result of the interplay between genetic differences and the epigenome, which is in turn affected by gender, age and differences in life style including diet, exercise, smoking and social interaction.

The control of branching morphogenesis

Many organs of higher organisms are heavily branched structures and arise by an apparently similar process of branching morphogenesis. Yet the regulatory components and local interactions that have been identified differ greatly in these organs. It is an open question whether the regulatory processes work according to a common principle and how far physical and geometrical constraints determine the branching process. Here, we review the known regulatory factors and physical constraints in lung, kidney, pancreas, prostate, mammary gland and salivary gland branching morphogenesis, and describe the models that have been formulated to analyse their impacts.

PDGF is required for remyelination-promoting IgM stimulation of oligodendrocyte progenitor cell proliferation

Background

Promotion of remyelination is a major goal in treating demyelinating diseases such as multiple sclerosis (MS). The recombinant human monoclonal IgM, rHIgM22, targets myelin and oligodendrocytes (OLs) and promotes remyelination in animal models of MS. It is unclear whether rHIgM22-mediated stimulation of lesion repair is due to promotion of oligodendrocyte progenitor cell (OPC) proliferation and survival, OPC differentiation into myelinating OLs or protection of mature OLs. It is also unknown whether astrocytes or microglia play a functional role in IgM-mediated lesion repair.

Methods

We assessed the effect of rHIgM22 on cell proliferation in mixed CNS glial and OPC cultures by tritiated-thymidine uptake and by double-label immunocytochemistry using the proliferation marker, Ki-67. Antibody-mediated signaling events, OPC differentiation and OPC survival were investigated and quantified by Western blots.

Results

rHIgM22 stimulates OPC proliferation in mixed glial cultures but not in purified OPCs. There is no proliferative response in astrocytes or microglia. rHIgM22 activates PDGFαR in OPCs in mixed glial cultures. Blocking PDGFR-kinase inhibits rHIgM22-mediated OPC proliferation in mixed glia. We confirm in isolated OPCs that rHIgM22-mediated anti-apoptotic signaling and inhibition of OPC differentiation requires PDGF and FGF-2. We observed no IgM-mediated effect in mature OLs in the absence of PDGF and FGF-2.

Conclusion

Stimulation of OPC proliferation by rHIgM22 depends on co-stimulatory astrocytic and/or microglial factors. We demonstrate that rHIgM22-mediated activation of PDGFαR is required for stimulation of OPC proliferation. We propose that rHIgM22 lowers the PDGF threshold required for OPC proliferation and protection, which can result in remyelination of CNS lesions.

Spatial and temporal profiles of growth factor expression during CNS demyelination reveal the dynamics of repair priming

Demyelination is the cause of disability in various neurological disorders. It is therefore crucial to understand the molecular regulation of oligodendrocytes, the myelin forming cells in the CNS. Growth factors are known to be essential for the development and maintenance of oligodendrocytes and are involved in the regulation of glial responses in various pathological conditions. We employed the well established murine cuprizone model of toxic demyelination to analyze the expression of 13 growth factors in the CNS during de- and remyelination. The temporal mRNA expression profile during demyelination and the subsequent remyelination were analyzed separately in the corpus callosum and cerebral cortex using laser microdissection and real-time PCR techniques. During demyelination a similar pattern of growth factor mRNA expression was observed in both areas with a strong up-regulation of NRG1 and GDNF and a slight increase of CNTF in the first week of cuprizone treatment. HGF, FGF-2, LIF, IGF-I, and TGF-ß1 were up-regulated mainly during peak demyelination. In contrast, during remyelination there were regional differences in growth factor mRNA expression levels. GDNF, CNTF, HGF, FGF-2, and BDNF were elevated in the corpus callosum but not in the cortex, suggesting tissue differences in the molecular regulation of remyelination in the white and grey matter. To clarify the cellular source we isolated microglia from the cuprizone lesions. GDNF, IGF-1, and FGF mRNA were detected in the microglial fraction with a temporal pattern corresponding to that from whole tissue PCR. In addition, immunohistochemical analysis revealed IGF-1 protein expression also in the reactive astrocytes. CNTF was located in astrocytes. This study identified seven different temporal expression patterns for growth factors in white and grey matter and demonstrated the importance of early tissue priming and exact orchestration of different steps during callosal and cortical de- and remyelination.

Maturation-dependent sensitivity of oligodendrocyte lineage cells to apoptosis: implications for normal development and disease

Apoptosis plays a crucial role in brain development by ensuring that only appropriately growing, migrating, and synapse-forming neurons and their associated glial cells survive. This process involves an intimate relationship between cell-cell interactions and developmental cues and is further impacted by environmental stress during neurogenesis and disease. Oligodendrocytes (OLs), the major myelin-forming cells in the central nervous system, largely form after this wave of neurogenesis but also show a selective vulnerability to cell death stimuli depending on their stage of development. This can affect not only embryonic and early postnatal brain formation but also the response to demyelinating pathologies. In the present review, we discuss the stage-specific sensitivity of OL lineage cells to damage-induced death and how this might impact myelin survival and regeneration during injury or disease.

Delayed IGF-1 administration rescues oligodendrocyte progenitors from glutamate-induced cell death and hypoxic-ischemic brain damage

We previously demonstrated that IGF-1 blocks glutamate-mediated death of late oligodendrocyte progenitors (OPs) by preventing Bax translocation, mitochondrial cytochrome c release and cleavage of caspases 9 and 3. Here, we demonstrate that IGF-1 prevents caspase 3 activation in late OPs when administered up to 16 h following exposure to glutamate. Moreover, late addition of IGF-1 to OPs previously exposed to toxic levels of glutamate promotes oligodendrocyte maturation as measured by myelin basic protein expression. We also demonstrate that intraventricularly administered IGF-1 retains OPs in the perinatal white matter after hypoxia-ischemia when given after insult. These results suggest that delayed administration of IGF-1 will rescue OPs in the immature white matter and promote myelination following hypoxia-ischemia.

Long-term impact of radiation on the stem cell and oligodendrocyte precursors in the brain

Background

The cellular basis of long term radiation damage in the brain is not fully understood.

Methods and Findings

We administered a dose of 25Gy to adult rat brains while shielding the olfactory bulbs. Quantitative analyses were serially performed on different brain regions over 15 months. Our data reveal an immediate and permanent suppression of SVZ proliferation and neurogenesis. The olfactory bulb demonstrates a transient but remarkable SVZ-independent ability for compensation and maintenance of the calretinin interneuron population. The oligodendrocyte compartment exhibits a complex pattern of limited proliferation of NG2 progenitors but steady loss of the oligodendroglial antigen O4. As of nine months post radiation, diffuse demyelination starts in all irradiated brains. Counts of capillary segments and length demonstrate significant loss one day post radiation but swift and persistent recovery of the vasculature up to 15 months post XRT. MRI imaging confirms loss of volume of the corpus callosum and early signs of demyelination at 12 months. Ultrastructural analysis demonstrates progressive degradation of myelin sheaths with axonal preservation. Areas of focal necrosis appear beyond 15 months and are preceded by widespread demyelination. Human white matter specimens obtained post-radiation confirm early loss of oligodendrocyte progenitors and delayed onset of myelin sheath fragmentation with preserved capillaries.

Conclusions

This study demonstrates that long term radiation injury is associated with irreversible damage to the neural stem cell compartment in the rodent SVZ and loss of oligodendrocyte precursor cells in both rodent and human brain. Delayed onset demyelination precedes focal necrosis and is likely due to the loss of oligodendrocyte precursors and the inability of the stem cell compartment to compensate for this loss.

Defective translation initiation causes vanishing of cerebral white matter

Leukoencephalopathy with vanishing white matter (VWM) is one of the most prevalent inherited white-matter disorders, especially in Caucasian populations. VWM is unusual because of its sensitivity to febrile infections and minor head trauma. The basic defect of this enigmatic brain disease resides in the regulation of initiation of protein synthesis. Recently, undue activation of the unfolded-protein response has emerged as an important factor in the pathophysiology of VWM. Here, we discuss the mechanisms that might be responsible for the selective involvement of the brain white matter in VWM. At present, VWM research is in need of an animal model to study disease mechanisms and therapeutic interventions.

PDGF and FGF2 pathways regulate distinct oligodendrocyte lineage responses in experimental demyelination with spontaneous remyelination

Repair of myelin damage in the adult CNS requires oligodendrocyte progenitor (OP) proliferation and subsequent differentiation into remyelinating oligodendrocytes. Platelet-derived growth factor (PDGF) and fibroblast growth factor-2 (FGF2) have been predicted to act individually and/or cooperatively to generate remyelinating oligodendrocytes. Analysis of PDGF alpha receptor (PDGF alpha R) heterozygous (+/-) mice indicates that PDGF alpha R expression modulates oligodendrocyte density in non-lesioned adult CNS. Analysis of cuprizone demyelination and recovery in PDGF alpha R+/- mice, FGF2 knockout (-/-) mice, and PDGF alpha R+/- FGF2-/- mice demonstrated that: (1) OP proliferation and oligodendrocyte regeneration is impaired in PDGF alpha R heterozygotes, (2) PDGF alpha R+/- and FGF2-/- deletions do not act cooperatively to impair OP amplification, (3) oligodendrocyte differentiation is more frequent in FGF2-/- mice, and (4) FGF2 deletion in combination with the PDGF alpha R+/- genotype rescues impaired oligodendrocyte regeneration of PDGF alpha R heterozygotes. These findings demonstrate distinct roles for PDGF and FGF2 in vivo in the context of a demyelinating disease with spontaneous remyelination.

Integrin-growth factor interactions as regulators of oligodendroglial development and function

Central nervous system (CNS) development requires mechanisms for the regulation of cell number. Although growth factors are essential determinants of the proliferation and apoptosis that determine final numbers, the long-range nature of signals from diffusible growth factors makes them insufficient for the provision of the precise and localized signals required. Integration of integrin and growth factor receptor signaling in controlling cell behavior has been an important theme of research over the past several years. The focus of this review is on the mechanisms by which integrin-growth factor interactions regulate the development of oligodendrocytes and provide a mechanism for controlling, both in space and in time, oligodendrocyte numbers in the developing CNS.

In vivo analysis of oligodendrocyte lineage development in postnatal FGF2 null mice

Analysis of fibroblast growth factor 2 null (FGF2-/-) and wild-type (FGF2+/+) mice was used to interpret the potential in vivo role of endogenous FGF2 on oligodendrocyte lineage cell (OLC) responses during oligodendrogenesis and myelination. In wild-type mouse spinal cord, FGF2 levels increased approximately threefold between the first and second postnatal weeks, a period corresponding with the peak of oligodendrogenesis. Absence of this developmental FGF2 elevation in FGF2-/- mice eliminated the transient overproduction of oligodendrocytes that is known to occur at the peak of oligodendrogenesis in wild-type mice. Absence of FGF2 did not affect oligodendrocyte progenitor (OP) density or proliferation, based on BrdU incorporation, and also did not alter survival, based on TUNEL analysis. To examine OLC differentiation in vivo, retrovirus encoding-enhanced green fluorescent protein (GFP) was injected into the spinal cord to heritably label endogenous cycling cells in the white matter at postnatal day 7 and then identify the generated cells at postnatal day 28. Phenotypes of cells expressing GFP were identified by morphology and immunolabeling, using CC1 for oligodendrocytes and NG2 combined with platelet-derived growth factor alpha receptor for OPs. Within the population of GFP-labeled cells, the proportion of oligodendrocytes was higher in FGF2-/- mice, indicating that endogenous FGF2 inhibited OLC differentiation in wild-type mice. Furthermore, in FGF2-/- mice fewer cells appeared to be generated from an initial retrovirus-labeled cell, consistent with more frequent differentiation into post-mitotic oligodendrocytes. This in vivo analysis demonstrates that the predominant role of endogenous FGF2 on OLCs in development is inhibition of differentiation.

Coronavirus-Induced Demyelination and Spontaneous Remyelination

MHV-A59 coronavirus infection produces a transient episode of demyelination that is followed by spontaneous remyelination. This paradigm provides a complex lesion environment to examine cellular and molecular mechanisms involved in successful CNS remyelination. Our work in this model has focused on the roles of platelet-derived growth factor and fibroblast growth factor 2 in regulating oligodendrocyte progenitor responses required for remyelination.

Postnatal NG2 proteoglycan-expressing progenitor cells are intrinsically multipotent and generate functional neurons

Neurogenesis is known to persist in the adult mammalian central nervous system (CNS). The identity of the cells that generate new neurons in the postnatal CNS has become a crucial but elusive issue. Using a transgenic mouse, we show that NG2 proteoglycan-positive progenitor cells that express the 2',3'-cyclic nucleotide 3'-phosphodiesterase gene display a multipotent phenotype in vitro and generate electrically excitable neurons, as well as astrocytes and oligodendrocytes. The fast kinetics and the high rate of multipotent fate of these NG2+ progenitors in vitro reflect an intrinsic property, rather than reprogramming. We demonstrate in the hippocampus in vivo that a sizeable fraction of postnatal NG2+ progenitor cells are proliferative precursors whose progeny appears to differentiate into GABAergic neurons capable of propagating action potentials and displaying functional synaptic inputs. These data show that at least a subpopulation of postnatal NG2-expressing cells are CNS multipotent precursors that may underlie adult hippocampal neurogenesis.

Absence of fibroblast growth factor 2 promotes oligodendroglial repopulation of demyelinated white matter

This study takes advantage of fibroblast growth factor 2 (FGF2) knock-out mice to determine the contribution of FGF2 to the regeneration of oligodendrocytes in the adult CNS. The role of FGF2 during spontaneous remyelination was examined using two complementary mouse models of experimental demyelination. The murine hepatitis virus strain A59 (MHV-A59) model produces focal areas of spinal cord demyelination with inflammation. The cuprizone neurotoxicant model causes extensive corpus callosum demyelination without a lymphocytic cell response. In both models, FGF2 expression is upregulated in areas of demyelination in wild-type mice. Surprisingly, in both models, oligodendrocyte repopulation of demyelinated white matter was significantly increased in FGF2 -/- mice compared with wild-type mice and even surpassed the oligodendrocyte density of nonlesioned mice. This dramatic result indicated that the absence of FGF2 promoted oligodendrocyte regeneration, possibly by enhancing oligodendrocyte progenitor proliferation and/or differentiation. FGF2 -/- and +/+ mice had similar oligodendrocyte progenitor densities in normal adult CNS, as well as similar progenitor proliferation and accumulation during demyelination. To directly analyze progenitor differentiation, glial cultures from spinal cords of wild-type mice undergoing remyelination after MHV-A59 demyelination were treated for 3 d with either exogenous FGF2 or an FGF2 neutralizing antibody. Elevating FGF2 favored progenitor proliferation, whereas attenuating endogenous FGF2 activity promoted the differentiation of progenitors into oligodendrocytes. These in vitro results are consistent with enhanced progenitor differentiation in FGF2 -/- mice. These studies demonstrate that the FGF2 genotype regulates oligodendrocyte regeneration and that FGF2 appears to inhibit oligodendrocyte lineage differentiation during remyelination.

The oligodendrocyte precursor mitogen PDGF stimulates proliferation by activation of alpha(v)beta3 integrins

Central nervous system development requires precise and localized regulation of neural precursor behaviour. Here we show how the interaction between growth factor and integrin signalling pathways provides a mechanism for such precision in oligodendrocyte progenitor (OP) proliferation. While physiological concentrations of platelet-derived growth factor (PDGF) were not in themselves sufficient to promote OP proliferation, they did so on extracellular matrix (ECM) substrates that bind alpha(v)beta3 integrin. Upon PDGF-AA exposure and alpha(v)beta3 engagement, a physical co-association between both receptors was demonstrated, confirming the interaction between these signalling pathways. Furthermore, we found that PDGFalphaR stimulated a protein kinase C-dependent activation of integrin alpha(v)beta3, which in turn induced OP proliferation via a phosphatidylinositol 3-kinase-dependent signalling pathway. These studies establish a mechanism by which OP proliferation is dependent on the availability of both an ECM ligand and a mitogenic growth factor. Growth factor- mediated integrin activation is the critical integrative step in proliferation signalling, and ensures that the response of neural precursor cells to long-range cues can be regulated by their cellular neighbours, allowing precise control of cell behaviour during development.

Macrophage inflammatory protein-1alpha (MIP-1alpha), MIP-1beta, and RANTES mRNA semiquantification and protein expression in active demyelinating multiple sclerosis (MS) lesions

MS is a demyelinating disease characterized by infiltration of monocytes and lymphocytes into the brain parenchyma, destruction of oligodendrocytes and loss of myelin. Since chemokines play a major role in the migration of monocytes and T cells, we here investigated the expression of the CC chemokines MIP-1alpha, MIP-1beta, and RANTES in brain tissue from MS patients using reverse transcriptase-polymerase chain reaction techniques. Both MIP-1beta as well as RANTES were found to be significantly elevated in brain tissue of MS patients. In addition, MIP-1alpha was also increased, although not significantly. Immunohistochemistry revealed that, whereas RANTES was mainly localized in reactive astrocytes, MIP-1alpha and MIP-1beta immunoreactivity was predominantly found in perivascular and parenchymal macrophages, containing myelin degradation products. Thus, chemokines appear to be associated with MS and an increased chemokine expression may further enhance disease progression by attracting more leucocytes into the brain parenchyma and by activation of effector functions of astrocytes and microglial cells.

Oligodendrocyte precursor migration and differentiation: combined effects of PSA residues, growth factors, and substrates

Using the oligosphere strategy (V. Avellana-Adalid et al., 1996, J. Neurosci. Res. 45, 558-570), we compared the migratory behavior of oligodendrocyte preprogenitors (OPP) that expressed the polysialylated form of the neural cell adhesion molecule (PSA-NCAM) and of GD3-positive oligodendrocyte progenitors (OP). To study the role of PSA in OPP migration, we used endoneuraminidase-N, which specifically cleaves PSA from NCAM. Kinetic data showed that (i) migration velocity decreased with time and was favored on polyornithine compared to Matrigel; (ii) cells emerging from spheres enriched in PSA-NCAM+ OPP migrated farther than those from spheres enriched in GD3+ OP, their migration being enhanced by the addition of growth factors; (iii) removal of PSA from NCAM moderately reduced OPP migration and induced their differentiation in GD3+ OP and GFAP+ astrocytes; (iv) blocking integrins reduced their migration, suggesting an alternative mechanism of migration. Altogether these data illustrate that motility and differentiation of OPP involve the combinatorial action of PSA-NCAM, molecules of the ECM and their receptors, and growth factors.