Differential effects of growth factors on oligodendrocyte progenitor migration

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Abstract

Oligodendrocytes are myelinating cells of the CNS that originate as progenitor cells (OP) in discrete areas of the developing brain. During brain development, OP migrate significant distances prior to proliferating and myelinating the axons of the putative white matter tracts. Growth factors play a major regulatory role in the behavior of OP. Specifically, platelet-derived growth factor A (PDGF-A) and fibroblast growth factor 2 (FGF2) are two of the most well characterized regulators of OP development. Both growth factors interact with tyrosine kinase receptors, activating various intracellular signaling pathways. The current study advances our earlier research by comparing the effects of both PDGF-A and FGF2 on OP migration. Our results show that activation of ERK is required for OP migration. These findings correlate well with our previous demonstration of the ERK pathway mediating PDGF-A induced OP migration. We also demonstrate the significance of threshold levels of growth factors and temporal regulation for OP migration. In addition, ERK activation alone is not sufficient to induce OP migration. The current research supports the involvement of the non-ERK mediated signaling pathway in OP migration.

Introduction

Oligodendrocytes (OLs) are the myelinating cells of the central nervous system (CNS). In the brain, OLs originate as pre-progenitors in the embryonic telencephalon (Rakic and Zecevic, 2003). The pre-progenitors subsequently migrate to the germinal matrix of the subventricular zone (SVZ) via the ganglionic eminences (Rakic and Zecevic, 2003). The OL progenitor cells (OPs) migrate away from the SVZ to populate the white matter tracts of the brain, including the corpus callosum and the cortex (Levison and Goldman, 1993). Thus, OP migration is a pre-requisite for myelination in the brain. Aberrant migration patterns can result in delayed onset or the complete absence of myelination (Back et al., 2001, Volpe, 2001). Unusual OP migration can be a result of lack, or failure, of transiently and temporally regulated functional regulatory signals (Finzsch et al., 2008, Fruttiger et al., 1999, Spassky et al., 2002, Tsai et al., 2002, Tsai et al., 2003). In addition, a large number of locally and transiently expressed environmental cues have been reported to be involved in regulating OP migration, apart from modulating phenotypic plasticity of differentiated OLs. These signals include soluble signaling proteins and extracellular matrix proteins (Canoll et al., 1996, Ellison et al., 1996, Frost et al., 1996, Milner et al., 1997, Tsai et al., 2002).

One such signaling molecule, the platelet derived growth factor A (PDGF-A), is known to be essential for the development of an intact myelin system (Calver et al., 1998, Fruttiger et al., 1999, Richardson et al., 1988). Transgenic animal models show that ablation of PDGF expression, leads to significant developmental defects and death in utero (Fruttiger et al., 1999, Soriano, 1997). A mouse model that is heterozygous for PDGF receptor-alpha (PDGFRα) shows significantly reduced numbers of OPs in the developing CNS (Li et al., 1996, Smith et al., 1991). Further, over-expression of PDGF leads to significantly increased numbers of OPs in the developing CNS (Soriano, 1997). PDGF is known to modulate several different OP behaviors, including proliferation (Baron et al., 2000, Baron et al., 2002, Ebner et al., 2000, Frost et al., 2003), migration (Armstrong et al., 1991, Frost et al., 1996, Frost et al., 2009), and survival (Barres et al., 1992, Ebner et al., 2000). PDGF regulates OP behavior via the PDGFRα, a tyrosine kinase receptor (RTK) which, upon phosphorylation, activates numerous different signaling pathways. Previous studies have shown that OP proliferation is regulated via the PI3K pathway (Ebner et al., 2000), while differentiation is regulated via p38MAPK (Haines et al., 2008). We have previously shown that OP migration is regulated by ERK1/2 (also known as p44/p42MAPK) signaling (Frost et al., 2009). Our earlier study also showed that transient activation of the PDGFRα receptor, for less than 30 min, is sufficient to induce OP migration for at least 72 h. Further, this ERK mediated pathway is maintained, in part, by a positive feedback loop regulated by cytoplasmic phospholipase C (cPLC) (Frost et al., 2009).

FGF2 is also known to play a significant role in the regulation of OP behavior, including migration (Fortin et al., 2005, Milner et al., 1997), proliferation (Bogler et al., 1990, Frost et al., 2003, Wolswijk and Noble, 1992), differentiation (Bansal et al., 1996, Baron et al., 2000, Fortin et al., 2005), and survival (Fortin et al., 2005, Murtie et al., 2005). Like PDGF receptors, FGF receptors are also RTKs. There are several different FGF receptors that are expressed at different stages of the OL lineage, and that regulate different behavioral processes of the cell (Bansal et al., 1996, Fortin et al., 2005, McKinnon et al., 1990). In contrast to PDGF-A, FGF2 knockout mice are viable and fertile (Fortin et al., 2005, Milner et al., 1997). Though the white matter of the FGF2−/− mouse appears to be normal (Murtie et al., 2005), the FGF2−/− brain is smaller in size and has reduced cell numbers (Raballo et al., 2000, Vaccarino et al., 1999). However, similar to PDGF-A, FGF2 promotes OP migration (Eccleston and Silberberg, 1985, Milner et al., 1997), proliferation, and prevents their differentiation (Bogler et al., 1990, Eccleston and Silberberg, 1985, Gard and Pfeiffer, 1993).

In this study we used OPs isolated from neonatal rat pups to examine the role of exposure to PDGF-A and FGF2 on ERK activation and OP migration in vitro. We investigated the short term exposure of the growth factors at different concentrations on OP migration. In addition, we examined the significance of the ERK signaling pathway in PDGF-A and FGF2 exposed OPs, using U0126, which is a pharmacological inhibitor of MEK1/2, the upstream activator of ERK1/2. Further, we also compared OP migration elicited by the combinatorial effects of both the growth factors following continuous exposure. We show the existence of a non-ERK signaling pathway involved in growth factor mediated OP migration.

Section snippets

Cell isolation and culture

Oligodendrocyte progenitor cells were isolated from neonatal rat pups (P0-P1), by a previously described method (Armstrong, 1998). The cells are cultured in complete medium (DFG) DMEM (Sigma–Aldrich, Oakville, ON, Canada) supplemented with 5000U penicillin and streptomycin (Sigma–Aldrich), 4 mM l-glutamine (Sigma–Aldrich) and 10% FBS (Hyclone, Nepean, ON, Canada). The medium is changed every third day.

Approximately 7–10 days after plating, the OPs and microglia are dislodged by shaking the

PDGF-A induced dose-dependent response on OP migration for up to 72 h

The dose-dependent effect of PDGF-A on OP migration was studied using the agarose drop assay (Frost et al., 2000, Frost et al., 2009). We used the 72-h time point to assess migration as longer times would require the media to be refreshed, which could significantly affect the outcome of the study).

Continuous exposure to 1 ng/ml PDGF-A resulted in 666.67 ± 56.8 μm migration compared to the control (75.00 ± 26.3 μm). However, there was no significant increase in OP migration when the PDGF concentration

Discussion

Migration of OP is an essential process that precedes proliferation and differentiation of the cells to form myelinating OL. However, the mechanisms that regulate the temporal and spatial dispersal of OP remain to be fully clarified. PDGF-A and FGF2 are both well characterized as potent mitogens for OPs (Ebner et al., 2000, Frost et al., 2003, Richardson et al., 1988, Wolswijk and Noble, 1992). Both growth factors have also been shown to be potent motogens for OPs in vitro (Armstrong et al.,

Acknowledgements

We thank Farhana Begum, Kelvin Au, Cory Kowal and Walter Kim for discussions and critical reading of the manuscript.

References (75)

  • P.A. Eccleston et al.

    Fibroblast growth factor is a mitogen for oligodendrocytes in vitro

    Brain Res.

    (1985)
  • V.P. Eswarakumar et al.

    Cellular signaling by fibroblast growth factor receptors

    Cytokine Growth Factor Rev.

    (2005)
  • M.F. Favata et al.

    Identification of a novel inhibitor of mitogen-activated protein kinase kinase

    J. Biol. Chem.

    (1998)
  • A.L. Gard et al.

    Glial cell mitogens bFGF and PDGF differentially regulate development of O4+GalC-oligodendrocyte progenitors

    Dev. Biol.

    (1993)
  • S. Kanda et al.

    Phosphatidylinositol 3′-kinase-independent p70 S6 kinase activation by fibroblast growth factor receptor-1 is important for proliferation but not differentiation of endothelial cells

    J. Biol. Chem.

    (1997)
  • F. Lachapelle et al.

    Fibroblast growth factor-2 (FGF-2) and platelet-derived growth factor AB (PDGF AB) promote adult SVZ-derived oligodendrogenesis in vivo

    Mol. Cell. Neurosci.

    (2002)
  • S.W. Levison et al.

    Both oligodendrocytes and astrocytes develop from progenitors in the subventricular zone of postnatal rat forebrain

    Neuron

    (1993)
  • L. Li et al.

    Altered development of spinal cord in the mouse mutant (Patch) lacking the PDGF receptor alpha-subunit gene

    Brain Res. Dev. Brain Res.

    (1996)
  • R.D. McKinnon et al.

    FGF modulates the PDGF-driven pathway of oligodendrocyte development

    Neuron

    (1990)
  • W.D. Richardson et al.

    A role for platelet-derived growth factor in normal gliogenesis in the central nervous system

    Cell

    (1988)
  • J. Schlessinger

    Cell signaling by receptor tyrosine kinases

    Cell

    (2000)
  • H.H. Tsai et al.

    The chemokine receptor CXCR2 controls positioning of oligodendrocyte precursors in developing spinal cord by arresting their migration

    Cell

    (2002)
  • P. van Heyningen et al.

    Control of progenitor cell number by mitogen supply and demand

    Curr. Biol.

    (2001)
  • R. Armstrong et al.

    Astrocytes and O-2A progenitors migrate toward distinct molecules in a microchemotaxis chamber

    Ann. N. Y. Acad. Sci.

    (1991)
  • R.C. Armstrong et al.

    Type 1 astrocytes and oligodendrocyte-type 2 astrocyte glial progenitors migrate toward distinct molecules

    J. Neurosci. Res.

    (1990)
  • R.C. Armstrong et al.

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

    J. Neurosci.

    (2002)
  • S.A. Back et al.

    Late oligodendrocyte progenitors coincide with the developmental window of vulnerability for human perinatal white matter injury

    J. Neurosci.

    (2001)
  • R. Bansal et al.

    Expression of FGF receptors 1, 2, 3 in the embryonic and postnatal mouse brain compared with Pdgfralpha, Olig2 and Plp/dm20: implications for oligodendrocyte development

    Dev. Neurosci.

    (2003)
  • R. Bansal et al.

    Specific inhibitor of FGF receptor signaling: FGF-2-mediated effects on proliferation, differentiation, and MAPK activation are inhibited by PD173074 in oligodendrocyte-lineage cells

    J. Neurosci. Res.

    (2003)
  • W. Baron et al.

    The oligodendrocyte precursor mitogen PDGF stimulates proliferation by activation of alphav beta3 integrins

    EMBO J.

    (2002)
  • O. Bogler et al.

    Cooperation between two growth factors promotes extended self-renewal and inhibits differentiation of oligodendrocyte-type-2 astrocyte (O-2A) progenitor cells

    Proc. Natl. Acad. Sci. U. S. A.

    (1990)
  • J.E. Bottenstein et al.

    Growth of a rat neuroblastoma cell line in serum-free supplemented medium

    Proc. Natl. Acad. Sci. U. S. A.

    (1979)
  • S. Ebner et al.

    Distinct roles for PI3K in proliferation and survival of oligodendrocyte progenitor cells

    J. Neurosci. Res.

    (2000)
  • J.A. Ellison et al.

    Evidence for neuronal regulation of oligodendrocyte development: cellular localization of platelet-derived growth factor alpha receptor and A-chain mRNA during cerebral cortex development in the rat

    J. Neurosci. Res.

    (1996)
  • M. Finzsch et al.

    Sox9 and Sox10 influence survival and migration of oligodendrocyte precursors in the spinal cord by regulating PDGF receptor alpha expression

    Development

    (2008)
  • D. Fortin et al.

    Distinct fibroblast growth factor (FGF)/FGF receptor signaling pairs initiate diverse cellular responses in the oligodendrocyte lineage

    J. Neurosci.

    (2005)
  • E. Frost et al.

    Regulation of oligodendrocyte precursor migration by extracellular matrix: evidence for substrate-specific inhibition of migration by tenascin-C

    Dev. Neurosci.

    (1996)
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    This work was funded by The Manitoba Health Research Council (EEF), the Manitoba Institute of Child Health (EEF & MPN), Biogen Idec (MPN & EEF), and the Manitoba Medical Science Foundation (EEF & MPN). P.V. held an MHRC studentship.

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