Ellen J. Collarini

PDGF A chain homodimers drive proliferation of bipotential (O-2A) glial progenitor cells in the developing rat optic nerve.

The bipotential glial progenitor cells (O‐2A progenitors), which during development of the rat optic nerve give rise to oligodendrocytes and type 2 astrocytes, are stimulated to divide in culture by platelet‐derived growth factor (PDGF), and there is evidence that PDGF is important for development of the O‐2A cell lineage in vivo. We have visualized PDGF mRNA in the rat optic nerve by in situ hybridization, and its spatial distribution is compatible with the idea that type 1 astrocytes are the major source of PDGF in the nerve. We can detect mRNA encoding the A chain, but not the B chain of PDGF in the brain and optic nerve, suggesting that the major form of PDGF in the central nervous system is a homodimer of A chains (PDGF‐AA). PDGF‐AA is a more potent mitogen for O‐2A progenitor cells than is PDGF‐BB, while the reverse is true for human or rat fibroblasts. Fibroblasts display two types of PDGF receptors, type A receptors which bind to all three dimeric isoforms of PDGF, and type B receptors which bind PDGF‐BB and PDGF‐AB, but have low affinity for PDGF‐AA. Our results suggest that O‐2A progenitor cells possess predominantly type A receptors, and proliferate during development in response to PDGF‐AA secreted by type 1 astrocytes.

Schwann Cells Secrete a PDGF-like Factor: Evidence for an Autocrine Growth Mechanism involving PDGF

We have investigated the influence of platelet‐derived growth factor (PDGF) in peripheral nervous system gliogenesis using two types of Schwann cell cultures. Short‐term Schwann cell cultures grow very slowly, but when maintained in culture for several months the division rate of some cells increases, and cell lines can be established. We show that Schwann cells in both short‐ and long‐term culture possess PDGF receptors and synthesize DNA in response to PDGF. Competitive binding experiments show that Schwann cells express mainly PDGF β‐receptors and respond better to PDGF‐BB than to PDGF‐AA. Conditioned media from short‐and long‐term Schwann cell cultures contain PDGF‐like mitogenic activity, and anti‐PDGF immunoglobin partially inhibits DNA synthesis in long‐term Schwann cell cultures. Antibody neutralization experiments and Northern blot analyses both indicate that the predominant PDGF isoform in these cultures is PDGF‐BB. PDGF‐like activity is also detected in extracts of rat sciatic nerve. Taken together, these results suggest that PDGF‐BB may stimulate Schwann cell proliferation in an autocrine manner during normal development.

Growth factors and transcription factors in oligodendrocyte development

Summary O-2A progenitor cells, the precursors of oligodendrocytes in the central nervous system (CNS), probably originate in the subventricular germinal zones of the developing CNS, and subsequently migrate away from there to populate the rest of the CNS with oligodendrocytes. We are trying to understand how the O-2A progenitor cells interact with their changing environment as they migrate, and how this influences each stage of their development into mature, myelinating oligodendrocytes. In this article we summarize evidence that platelet-derived growth factor (PDGF) is important for stimulating O-2A progenitor cell proliferation in vivo, and describe our efforts to map the distribution of PDGF and its receptors in the developing rat CNS by in situ hybridization and immunohistochemistry. These studies suggest that, in the CNS, PDGF α-receptor subunits may be restricted to O-2A lineage cells that have started to migrate away from the subventricular zones towards their final destinations. Many neurons express the A and/or B chains of PDGF, and astrocytes express the A chain, but it is not yet clear which of these cell types might be the major source of PDGF for O-2A lineage cells in vivo. O-2A progenitor cells can be purified and maintained in a proliferating state in vitro by culturing in the presence of PDGF and bFGF. Under these conditions, the POU transcription factor SCIP/Tst-1 is expressed at a high level; when oligodendrocyte differentiation is initiated by withdrawing the growth factors, SCIP/Tst-1 mRNA is rapidly down-regulated, followed by a decline in SCIP/Tst-1 protein and sequential activation of myelin-specific genes. These observations suggest that SCIP/Tst-1 may be mechanistically involved in the transition from proliferation to differentiation in the O-2A lineage. By in situ hybridization, SCIP/Tst-1 appears also to be expressed in developing neurons, so perhaps it fulfils a similar function in several different cell lineages in the CNS.

Origins and early development of oligodendrocyte precursor cells

Oligodendrocytes, the myelinating cells of the CNS, develop from glial progenitor cells known as 0-2A progenitors (for reviews see 1,2 and 3). 0-2A progenitor cells are so-called because they can differentiate into either oligodendrocytes or type-2 astrocytes in vitro: in medium containing low (≤ 0.5%) fetal calf serum (FCS) they differentiate into oligodendrocytes whereas in 10% FCS they differentiate into type-2 astrocytes (4). 0-2A progenitors and their differentiated progeny can be distinguished in vitro by morphology and by their characteristic antigenic phenotypes. 0-2A progenitors often have a bipolar morphology and label with monoclonal antibody A2B5 (5), which recognizes a specific set of gangliosides, and with antibodies to the NG2 chondroitin sulphate (6). As they mature, 0-2A progenitors become multi-polar, their proliferative and migratory properties change (7 and 8) and they start to express sulphatide and other related antigens that are recognized by monoclonal antibody 04 (9). Differentiated oligodendrocytes have a complex, process-bearing morphology and label specifically with antibodies against galactocerebroside (GC) (10). Type-2 astrocytes are process-bearing cells in vitro that label with antibodies against the glial fibrillary acidic protein (GFAP). After they differentiate, oligodendrocytes lose the A2B5 antigen whereas type-2 astrocytes retain it. The oligodendrocyte differentiation pathway seems to be the default behaviour for 0-2A progenitors because a single progenitor cell differentiates into an oligodendrocyte if it is cultured on its own in defined, low-serum medium in the absence of other cells (11). It is not known what the active ingredient in FCS is that can induce type-2 astrocyte differentiation in vitro, but the activity can be mimicked by pure ciliary neurotrophic factor (CNTF) in collaboration with uncharacterized extracellular matrix components secreted by cultures of cortical astrocytes (12). Despite careful searching, there is still no definitive evidence for the existence of cells with the antigenic phenotype of type-2 astrocytes in vivo (13). Perhaps type-2 astrocytes will eventually turn up in a restricted region(s) of the CNS or under certain pathological conditions. For the present, however, we regard 0-2A progenitors as dedicated oligodendrocyte progenitors in vivo, but retain the prefix 0-2A as a reminder of their differentiation potential in vitro. Figure 1 shows a diagram of the life history of an oligodendrocyte.

Platelet-Derived Growth Factor and Its Receptors in Central Nervous System Gliogenesis

The optic nerve contains three types of post-mitotic glial cells — oligodendrocytes and two types of astrocytes — but no neuronal cell bodies. These cells can be distinguished in vitro by their morphology and reactivity with certain antibodies. Oligodendrocytes are multipolar cells in vitro which stain with antibodies against galactocerebroside (GC, Ranscht et al., 1982). Both types of astrocytes stain for glial fibrillary acidic protein (GFAP); type-2 astrocytes are process-bearing cells which label in addition with monoclonal antibody A2B5 (Eisenbarth et al., 1979) while type-1 astrocytes are flat fibroblast-like cells which are A2B5-negative. Oligodendrocytes are the myelin-forming cells of the CNS, while type-2 astrocytes apparently extend processes to the gaps (nodes of Ranvier) between adjacent myelinated regions (internodes)(ffrench-Constant and Raff, 1986). The function of type-2 astrocytes is unknown, but it seems probable that they somehow assist saltatory propagation of action potentials. Type-1 astrocytes extend processes to the surface of the nerve and to blood vessels where they induce the capillary endothelial cells to form tight junctions, resulting in the formation of a blood-brain barrier (Janzer and Raff, 1987).