Role for TGF‐β1, FGF‐2 and PDGF‐AA in a myelination of CNS aggregate cultures enriched with macrophages

The increase in myelin basic protein (MBP) synthesis observed in brain aggregate cultures supplemented with macrophages is reflected in elevated supernatant protein levels of the key promoters of oligodendrocyte proliferation, fibroblast growth factor‐2 (FGF‐2) and platelet‐derived growth factor‐AA (PDGF‐AA), during the premyelinating phase. Although supernatant levels of transforming growth factor‐β1 (TGF‐β1), the most abundant growth factor produced at the transcriptional and translational levels by phagocytic macrophages, were reduced at this stage, it was the only growth factor for which mRNA expression was increased significantly in macrophage‐enriched cultures. TGF‐β1, which supports oligodendrocyte differentiation, was increased in the supernatant of macrophage‐enriched cultures only after the onset of myelinogenesis. Hence, standard cultures treated with TGF‐β1 during the premyelinating period reproduced effects of macrophage supplementation, inducing an increase in MBP synthesis and in PDGF‐AA and FGF‐2 bioavailability. A similar increase in MBP synthesis in PDGF‐AA treated cultures emphasises its central role in oligodendrocyte progenitor proliferation. In contrast, FGF‐2 blocked MBP synthesis in the cultures. In cultures treated with anti‐TGF‐β1 antibody before or after the first detection of MBP, supernatant levels of TGF‐β1, FGF‐2, and PDGF‐AA were reduced with resultant inhibition of myelination. Paradoxically, supraphysiological TGF‐β1 treatment after the onset of myelination had the same effect on myelin accumulation. These results indicate an enabling and regulatory role for TGF‐β1 in oligodendrocyte development and, as a source of TGF‐β1, macrophages in the inflammatory multiple sclerosis lesion, may have the potential to promote remyelination by modulating the growth factor repertoire in demyelinating disease. © 2003 Wiley‐Liss, Inc.