Regulation of FGF receptors in the oligodendrocyte lineage.

Fibroblast growth factors (FGFs) affect a broad spectrum of developmentally regulated cellular responses involved in the control of growth and differentiation. To identify specific FGF receptor forms involved in these responses, we have characterized FGF receptor transcript expression, and its modulation by FGF-2, as enriched populations of oligodendrocyte progenitors differentiate into mature oligodendrocytes. The data demonstrate that the levels of mRNA expression for FGF high-affinity receptors-1, -2, and -3 are differentially regulated during lineage progression: FGF receptor-1 expression increases with lineage progression, FGF receptor-2 is predominantly expressed by terminally differentiated oligodendrocytes, and FGF receptor-3 reaches a peak level of expression in late progenitors and then declines upon further differentiation; FGF receptor-4 expression was not detected in oligodendrocytes. Distinct patterns of alternatively spliced variants of FGF receptor-1 and -2 transcripts are expressed: the predominant FGF receptor-1 transcripts contain three Ig-like domains (FGF receptor-1 alpha), whereas the FGF receptor-2 transcripts contain two Ig-like domains (FGF receptor-2 beta 2) and this form is up-regulated as oligodendrocytes differentiate. In addition, the expression of these receptors is differentially regulated by the ligand, FGF-2: FGF receptor-1 mRNA expression is up-regulated in early progenitors, and FGF receptor-2 mRNA expression is down-regulated in mature oligodendrocytes. Finally, astrocytes express FGF receptor-1, -2, and -3 transcripts, but at different levels and with different exon utilization (FGF receptor-1 beta, FGF receptor-2 beta 1/beta 2) compared to oligodendrocytes. To our knowledge this is the first report that demonstrates that the mRNA expression of these three FGF receptor types is differentially regulated in primary cells as they differentiate along a lineage from progenitors to terminally differentiated cells. We propose that this pattern of expression provides a molecular basis for the developmentally varying response of cells to a common ligand. For example, according to this hypothesis, in response to FGF-2, FGF receptor-1 transduces signals that stimulate the prolonged proliferation and migration of early progenitors, FGF receptor-3 promotes the proliferation and arrest of differentiation of late progenitors, and FGF receptor-2 transduces signals for terminal differentiation, but not proliferation, in mature oligodendrocytes.