PDGF and intracellular signaling in the timing of oligodendrocyte differentiation

Neuronal activity triggers calcium waves in hippocampal astrocyte networks

The recent discovery that the neurotransmitter glutamate can trigger actively propagating Ca2+ waves in the cytoplasm of cultured astrocytes suggests the possibility that synaptically released glutamate may trigger similar Ca2+ waves in brain astrocytes in situ. To explore this possibility, we used confocal microscopy and the Ca2+ indicator fluo-3 to study organotypically cultured slices of rat hippocampus, where astrocytic and neuronal networks are intermingled in their normal tissue relationships. We find that astrocytic Ca2+ waves are present under these circumstances and that these waves can be triggered by the firing of glutamatergic neuronal afferents with latencies as short as 2 s. The Ca2+ waves closely resemble those previously observed in cultured astrocytes: they propagate both within and between astrocytes at velocities of 7-27 microns/s at 21 degrees C. The ability of tissue astrocyte networks to respond to neuronal network activity suggests that astrocytes may have a much more dynamic and active role in brain function than has been generally recognized.

Cellular organisation of the optic nerve and the implications for optic neuritis

Opportunities for studying growth, degeneration and repair in the central nervous system have altered over the last decade with the development of techniques for culturing neurones and glia and the availability of immunological or molecular markers that identify separate lineages and their progeny. Much pioneering work has been carried out in the rodent optic nerve but the principles that emerge are representative for other parts of the nervous system; development of neurones and glia may differ substantially in rats and man, so that assumptions must be made in extrapolating from properties of the rat optic nerve to diseases of the human central nervous system.

Phosphorylation of the MARCKS protein (P87), a major protein kinase C substrate, is not an obligatory step in the mitogenic signaling pathway of basic fibroblast growth factor in rat oligodendrocytes

Basic fibroblast growth factor (bFGF) is a well-characterized peptide hormone that has mitogenic activity for various cell types and elicits a characteristic set of responses on the cell types investigated. In this report we confirmed that bFGF is a potent mitogen for rat brain-derived oligodendrocyte (OL) precursor cells as well as for differentiated OL in secondary culture. bFGF was shown to induce expression of the protooncogene c-fos in OL. The role of protein kinase C (PKC) in mediating bFGF-stimulated proliferation as well as c-fos expression in OL was investigated. The PKC activator phorbol 12-myristate 13-acetate (PMA) stimulated c-fos expression but did not trigger cell proliferation. When PKC was down-regulated by pretreatment of OL with PMA for 20 h, the bFGF-mediated stimulations of OL proliferation and c-fos mRNA expression were still observed, whereas the induction of c-fos mRNA by PMA was totally inhibited. These data demonstrate that the bFGF mitogenic signaling pathway in OLs does not require PKC. On the other hand, bFGF was found to stimulate specifically the phosphorylation of a limited number of PKC substrates in oligodendroglial cells, including the MARCKS protein. The bFGF-dependent phosphorylation of MARCKS protein was totally inhibited when PKC was first down-regulated, indicating that the phosphorylation of this protein is PKC dependent. Tryptic digestion of the phosphorylated MARCKS protein revealed that bFGF stimulated specifically the phosphorylation of the MARCKS protein on a single phosphopeptide. We provide evidence that bFGF also stimulated fatty acylation of the MARCKS protein, which might explain the observed specific bFGF-dependent phosphorylation of this protein in OL. We propose that bFGF-dependent fatty acylation and phosphorylation of the MARCKS protein are not essential for the transduction of the bFGF mitogenic signal but are probably linked to differentiation processes elicited by bFGF on OL.

Autocrine inhibition of mitotic activity in cultured oligodendrocyte-type-2 astrocyte (O-2A) precursor cells

During development, oligodendrocytes are generated from a bipotential glial stem cell, the oligodendrocyte-type-2 astrocyte precursor (O-2A). O-2A cells are under the mitogenic influence of the platelet-derived growth factor (PDGF) released from type-1 astrocytes. In vitro experiments have shown that O-2A cells stimulated by PDGF are limited to a set number of divisions and then differentiate to oligodendrocytes by becoming unresponsive to the growth factor. In the healthy adult central nervous system, oligodendrocyte proliferation remains generally quiescent and is possibly under negative growth control. The view that O-2A lineage cells are capable of negatively regulating their own proliferation is supported by the demonstration that conditioned medium obtained from O-2A cultures inhibits their DNA synthesis. In addition to O-2A cells, the newly established CG4 cell line, a derivative of O-2A cells, was found to inhibit O-2A lineage cell proliferation. The antiproliferative activity was present in the media conditioned by CG4 cells that were expanded as undifferentiated O-2A precursors, as well as by CG4 cells induced to differentiate to nonproliferating oligodendrocytes. Moreover, the inhibitory activity was produced by CG4 cells (source cells) propagated by various mitogens. The inhibition of mitotic activity was nearly complete, dose-dependent, fully reversible, and exhibited when CG4 cells (test cells) were stimulated to divide by various mitogens, such as PDGF, basic fibroblast growth factor, or medium conditioned by the neuronal B104 cell line. The inhibition of proliferation was accompanied by the conversion of the phenotype of CG4 cells, from A2B5+/O4- precursors to A2B5-/O4+ pro-oligodendrocytes.(ABSTRACT TRUNCATED AT 250 WORDS)

PDGF increases the expression of Fos and Jun in newly formed oligodendrocytes that have become resistant to the mitogenic effect of PDGF

Bipotential glial (O-2A) progenitor cells give rise to oligodendrocytes on a predictable schedule in the developing rat optic nerve. The loss of mitotic responsiveness to platelet-derived growth factor (PDGF) that is seen when O-2A progenitor cells initially differentiate into oligodendrocytes seems to reflect blocks or deficiencies downstream to some of the early intracellular signalling events induced by PDGF. Here we show that PDGF increases the expression of the Fos and Jun nuclear proteins in newly formed oligodendrocytes in vitro, suggesting that at least one intracellular signalling pathway to the nucleus is activated by PDGF in these cells even though they do not synthesize DNA in response to PDGF.

The cellular measurement of time

This review describes three biological processes in which there is evidence for single cells being able to measure elapsed time. We describe the work that has led to this view, and review more recent work that has provided new insights into possible mechanisms for the measurement of time.

The origin of remyelinating cells in the adult central nervous system: the role of the mature oligodendrocyte

The sequence of events by which new oligodendrocytes are generated in the adult mammalian central nervous system has not been clearly defined. Here we review old evidence that remyelinating cells can arise from the division of mature oligodendrocytes. In addition, we report the results of a tissue culture study comparing oligodendrocytes and immature progenitors with regard to their capacity for proliferation, for the generation of new oligodendrocytes and for myelination. Monoclonal antibodies 04 and 01 were used to distinguish oligodendrocytes (04+01+) from progenitors (04+01-). Dissociated cell suspensions from adult rat spinal cord were separated by flow cytometry into 01+ and 01- cell fractions, at greater than 93% purity. The 01+ fraction contained approximately 0.7% 04+01- cells while the 01- fraction contained approximately 4.4% 04+01- cells. Cells from these fractions were plated onto cultures of purified dissociated dorsal root ganglion neurons. The cultures that received 01+ cells developed numerous expanding colonies of cells expressing both 01 and 04, or 04 only, by 8 days and were essentially covered by oligodendrocytes by 16 days. In marked contrast, oligodendrocyte colonies were rare in cultures receiving 01- cells. By 24 days, myelination was extensive in cultures receiving 01+ cells; in contrast, only a few myelin segments were observed in cultures receiving the 01- fraction. Thus, oligodendrocytes (01+ cells) appear more capable than progenitors (04+01-) of generating new myelinating oligodendrocytes in this culture system.

Neurons and astrocytes influence the development of purified O-2A progenitor cells

To investigate the possible role of neurons and astrocytes for oligodendrocyte development we prepared a pure population of precursor cells positive for the precursor marker GD3 with the help of fluorescence-activated cell sorting (FACS). Large numbers of highly purified cells were obtained from postnatal day 1 rat brainstems and cultured in different media and sera, and in conditioned media. As described in the literature for optic nerve O-2A progenitors, GD3-sorted brainstem cells cultured in medium containing 10% fetal calf serum (FCS) acquired a star-shaped morphology and differentiated into GD3- and GFAP-positive type-2 astrocytes. On the other hand, in serum-free medium, most of the cells differentiated into oligodendrocytes (O1-/galactocerebroside-positive). Sensory neuron conditioned media promoted survival and proliferation of the precursor cells. The spontaneous differentiation of progenitor cells into oligodendrocytes was retarded by the mitogen. Antibodies against platelet-derived growth factor (PDGF) completely blocked the mitotic effect and allowed spontaneous oligodendrocyte differentiation to occur. Cultured astrocytes also secreted PDGF as a mitogen. However, postnatal astrocytes also released a potent signal promoting oligodendrocyte differentiation. The type of factor(s) released depended on the age of the astrocytes, since only conditioned medium of postnatal but not of embryonic astrocytes promoted oligodendrocyte differentiation, suggesting that astrocyte maturation directly influences oligodendrocyte differentiation. Different concentrations of PDGF could not reproduce this differentiation-inducing effect. This study suggests that interactions between O-2A progenitor cells, neurons, and astrocytes could be required to regulate and complete the oligodendrocyte developmental pathway. Astrocytes, themselves possibly under neuronal influences, might regulate first the proliferation of the precursor cells, and, later in development, the differentiation into mature oligodendrocytes or type-2 astrocytes.

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

During central nervous system (CNS) development, glial precursors proliferate in subventricular zones and then migrate throughout the CNS to adopt their final destinations and differentiate into various types of mature glial cells. Although several growth factors promoting the proliferation and/or differentiation of glial precursors have been identified, very little is known about the nature of signals that guide glial cell migration in the CNS. Therefore, we have investigated whether polypeptide growth factors and/or extracellular matrix molecules may mediate the migration of two major glial cell types, type 1 astrocytes and oligodendrocyte-type 2 astrocyte (O-2A) progenitor cells. We show that, in a microchemotaxis chamber assay, type 1 astrocytes move toward laminin and complement-derived C5a. Astrocyte migration toward laminin is inhibited by a laminin-specific pentapeptide, YIGSR-NH2. In contrast, O-2A progenitors migrate toward platelet-derived growth factor (PDGF), which also functions as a mitogen for these cells. Using a new method to simultaneously assay migration and DNA synthesis, we also demonstrate that O-2A progenitors can migrate toward PDGF even when DNA replication is inhibited with an antimitotic agent. Thus, migration of different types of glial cells can be induced in vitro by specific signaling molecules, which are present in the developing brain and may stimulate migration of glial cells prior to CNS myelination.

Regulation of proliferation and differentiation of myoblasts derived from adult mouse skeletal muscle by specific isoforms of PDGF

The expression of receptors and the mitogenic response to PDGF by C2 myoblasts, derived from adult mouse skeletal muscle, was investigated. Employing 125I-PDGF binding assays, we showed that the cells exhibit high level binding of PDGF-BB (approximately 165 x 10(3) molecules/cell at saturation) and much lower binding of the PDGF-AA and PDGF-AB (6-12 x 10(3) molecules/cell at saturation). This indicates that the C2 myoblasts express high levels of PDGF receptor beta-subunits and low levels of alpha-subunits. PDGF-BB enhances the proliferation of C2 cells maintained in 2% FCS by about fivefold. PDGF-AB had a moderate effect on cell proliferation (less than twofold) and PDGF-AA had no effect. Inverse effects of PDGF isoforms on the frequency of differentiated myoblasts were observed; the frequency of myosin-positive cells was reduced in the presence of PDGF-BB while PDGF-AA and PDGF-AB had no effect. PDGF may thus act to increase the number of myoblasts that participate in muscle regeneration following muscle trauma by stimulating the proliferation and by inhibiting the differentiation of myogenic cells.

Extracellular matrix-associated molecules collaborate with ciliary neurotrophic factor to induce type-2 astrocyte development

O-2A progenitor cells give rise to both oligodendrocytes and type-2 astrocytes in vitro. Whereas oligodendrocyte differentiation occurs constitutively, type-2 astrocyte differentiation requires extracellular signals, one of which is thought to be ciliary neurotrophic factor (CNTF). CNTF, however, is insufficient by itself to induce the development of stable type-2 astrocytes. In this report we show the following: (a) that molecules associated with the extracellular matrix (ECM) cooperate with CNTF to induce stable type-2 astrocyte differentiation in serum-free cultures. The combination of CNTF and the ECM-associated molecules thus mimics the effect of FCS, which has been shown previously to induce stable type-2 astrocyte differentiation in vitro. (b) Both the ECM-associated molecules and CNTF act directly on O-2A progenitor cells and can induce them to differentiate prematurely into type-2 astrocytes. (c) ECM-associated molecules also inhibit oligodendrocyte differentiation, even in the absence of CNTF, but this inhibition is not sufficient on its own to induce type-2 astrocyte differentiation. (d) Whereas the effect of ECM on oligodendrocyte differentiation is mimicked by basic fibroblast growth factor (bFGF), the effect of ECM on type-2 astrocyte differentiation is not. (e) The ECM-associated molecules that are responsible for inhibiting oligodendrocyte differentiation and for cooperating with CNTF to induce type-2 astrocyte differentiation are made by non-glial cells in vitro. (f) Molecules that have these activities and bind to ECM are present in the optic nerve at the time type-2 astrocytes are thought to be developing.