Fibroblast (heparin-binding) growing factors in neuronal development and repair

Nearly thirty growth and trophic factors that have been purified from mammalian tissues in the last 15 yr have been found to share chemical identity. The results of their chemical purification and molecular cloning show that they are two distinct polypeptides (Mr 17,400 and 18,400), each of which gives rise to families of smaller size peptides. These peptides share a common affinity for heparin. In view of this property, a common nomenclature for the two principle peptide growth factors (heparin-binding growth factor classes 1 and 2; HBGF-1 and -2) has been proposed. However, the names acidic and basic Fibroblast Growth Factors (aFGF,bFGF), which were applied to them originally to describe their mitogenic activity, are more commonly in use and will therefore be adopted in this review. Brain tissue is one of the richest sources of FGFs. It has been used as a starting point for their chemical purification and to prepare genomic libraries for molecular cloning of the aFGF and bFGF genes. There is increasing evidence that these growth factors, expressed in neurons and glia throughout the mammalian nervous system, are implicated in neuronal cell proliferation, differentiation, and histogenesis. FGFs have a strong affinity not only for heparin, but also for the related heparan sulphate proteoglycans that are abundant in neural tissues. This fact provides a clue to the importance of tissue-associated proteoglycans in mediating the release, sequestration, and activation of FGFs and the modulation of their receptor binding and bioactivity. The relevance of FGFs to neural development and their mechanisms of action in neurons will be considered in light of the existing literature describing their biological properties and activity in mesodermal cell types. Evidence is reviewed showing that FGFs have in vivo biological activity, ameliorating the degeneration of central and peripheral neurons after axotomy. The presence and implications of high levels of FGFs in adult mammalian brain provides a direction for future research into neural regeneration. The bioactivity of FGFs in neural tissue may not depend on the regulation of their expression per se, but on the subregional modification of their interaction with proteoglycans.