Basic fibroblast growth factor (bFGF), a heparin-binding mitogen for mesoderm-derived cells, also acts as a mitogen, differentiation inducing and maintenance factor for many neuroectodermal cells including glial cells, neurons, paraneurons, and their tumor counterparts. The molecule is expressed in several types of neuroectodermal cells in vitro and in vivo. Furthermore, bFGF occurs in many neuronal target tissues, and can prevent ontogenetic as well as lesion-induced neuron death. Thus, in terms of its wide range of functions, bFGF is apparently more than a ‘classical’ neurotrophic factor. Some of its essential features, such as regulation of expression, local availability and transport in the nervous system remain to be studied.

During a distinct phase of nervous system development, 20-70% (depending on the location) of the neurons originally established are eliminated. This naturally occurring neuron death is thought to be regulated by target-derived proteins called neurotrophic factors (NTFs, recently reviewed by Davies, 1988; Barde, 1989; Hefti et al. 1989, Oppenheim, 1989). The biological features of NTFs have been deduced from those that can be attributed to nerve growth factor (NGF, for recent review see Levi-Montalcini, 1987), which, for a long time, was the one and only NTF. Several definitions for NTFs have been proposed, but we shall focus on a very stringent one. Barde (1988) defined an NTF as a protein that is (1) able to prevent the ontogenetic neuron death (to be shown by in vivo administration of the NTF and inhibiting antibodies), (2) synthesized in the respective neuronal target tissue, (3) present in limiting amounts and (4) taken up by receptor-mediated retrograde axonal transport. So far, only NGF completely fulfills these critera (Barde, 1988, 1989). However, since the neuronal specificity of NGF is restricted to sympathetic and neural crest-derived sensory neurons in the peripheral nervous system (PNS) and magnocellular cholinergic neurons of the forebrain in the central nervous system (CNS) (Barde, 1989; Hefti et al. 1989), it is safe to postulate the existence of additional NTFs.

Recently, two further proteins (brain-derived neurotrophic factor, BDNF, and neurotrophin-3, NT-3) with 50–60% sequence homology to NGF have been described (Barde et al. 1982; Leibrock et al. 1989; Hohn et al. 1990; Maisonpierre et al. 1990).

Several other proteins (recently reviewed by Barde, 1989; Hefti et al. 1989; Walicke, 1989) can also promote survival, induce fiber outgrowth and influence transmitter metabolism of various neurons. Whether these proteins match NGF in terms of the criteria defining an NTF remains to be investigated. Even for BDNF it still has to be shown that (1) anti-BDNF antibodies are able to enhance the amount of dying neurons during ontogenetic cell death, (2) BDNF is present in the target organs of all neuron populations being affected and (3) BDNF is retrogradely transported.

Recently, bFGF has emerged as a protein that is synthesized in the nervous system and exerts a variety of effects on neural cells in vivo and in vitro. The aim of this article is to summarize our present knowledge on neural functions of bFGF, compare them with those of NGF and discuss the pros and cons of bFGF as a neurotrophic factor.

The molecular properties, tissue distribution and functions of bFGF outside the nervous system have been extensively studied and reviewed (e.g. by Gospodarowicz et al. 1986a; Thomas, 1987; Lobb, 1988; Boehlen, 1989; Rifkin and Moscatelli, 1989; Baird and Boehlen, 1990).

The FGF-family

The FGF-family at present consists of seven members, which share approximately 40–60% sequence homology. Best characterized are acidic FGF (aFGF) and basic FGF (bFGF), which will be discussed in more detail. The five other members are the oncogene products kFGF, also designated hst and ksFGF, (Taira et al. 1987; Delli-Bovi et al. 1987; Paterno et al. 1989), int-2 (Dickson and Peters, 1987; Paterno et al. 1989), FGF-5 (Zhan et al. 1988), FGF-6 (Maries et al. 1989) and KGF (Rubin et al. 1989), which were identified as members of the FGF family by their sequence. They also share some functions (mitogenic activity, mesoderm induction, see below) with the well-characterized aFGF and bFGF.

Molecular properties

Basic FGF, like aFGF, varies in size (15.6–17.8 ×103Mr) due to N-terminal truncation. This, possibly tissue-specific, heterogeneity does not seem to affect its functions (see reviews). In its full form, bFGF is a peptide composed of 154 amino acid residues (Ueno et al. 1986).

Sequence analysis has revealed a 5 ′-extended open reading frame (Abraham et al. 1986a,6), suggesting the existence of precursor molecules. Indeed, several bFGF forms of higher molecular mass have been isolated or tentatively identified using immunological techniques (see reviews and below).

Levels of bFGF mRNA are very low in most cell types. This may be a consequence of mRNA instability (Jaye et al. 1986; Abraham et al. 1986a). Protein levels, however, are relatively high in several cell types, especially neuroectodermal tissues such as brain, pituitary and adrenal gland (Gospodarowicz, 1975; Gospodarowicz et al. 1984, 1986b).

Sequence data for the 18 ×103Mr bFGF have revealed the absence of a typical signal sequence (Abraham et al. 1986a), and, accordingly, the issue as to whether or not bFGF is released from cells is controversial (see reviews and below).

The three-dimensional structure of bFGF has not yet been established, but it seems that disulfide bonds may not be essential for activity (Seno et al. 1988). From results obtained with synthetic peptides it has been concluded that the N-terminal as well as C-terminal domains of bFGF are necessary for binding to heparin and to the bFGF receptor (Baird et al. 1988).

Receptors for bFGF have been described, and signal transduction events are very similar to those known for several other growth factors, e.g. oncogene induction, protein kinase C activation, Ca2+ influx, ribosomal protein S6 phosphorylation (see reviews).

Cells and tissues expressing bFGF

Many types of cells and tissues contain heparin-binding mitogenic factors, and in most cases, these factors have been identified as FGFs (see reviews). It was therefore concluded that bFGF is present in most, if not all tissues, and that bFGF can be expressed by most cell types (for a list of tissues and cells containing bFGF, see Lobb, 1988).

The expression of bFGF seems to be regulated differentially depending on cell type and developmental age (Munaim et al. 1988; Grothe and Unsicker, 1990; Grothe et al. 1990b).

Functions outside the nervous system

Basic FGF has been shown to exert a variety of effects on many mesenchymal and neuroectodermal cells, the most prominent being an induction or increase in proliferation. Other effects include survival, morphological changes, anchorage-independent growth, differentiation and delayed senescence (see reviews). These effects may be responsible for the putative participation of bFGF in many essential processes such as mesoderm-induction (Slack et al. 1987 ; Knoechel and Tiedemann, 1989), angiogenesis and neovascularization, tissue differentiation and regulation of tissue-specific functions, wound healing and other regenerative processes. In addition, bFGF may also affect tumor-growth and vascularization as well as diseases associated with abnormal cell proliferation and differentiation (see reviews). The precise roles of bFGF in these events are only poorly understood, but its essential participation is clear and possible clinical applications are beginning to be explored (for reviews see Lobb, 1988; Westphal and Herrmann, 1989; Hefti et al. 1989).

Basic FGF has been purified from bovine pituitary (Gospodarowicz, 1975), brain (bovine, rat, guinea pig, human) (Gospodarowicz et al. 1984; Boehlen et al. 1985; Moscatelli et al. 1987; Presta et al. 19886), as well as eye and retina (Baird et al. 1985; Courty et al. 1987). Sequence data are available, and demonstrate that these organs contain the 18 ×103Mr bFGF and N-terminal truncated forms (Boehlen et al. 1985; Esch et al. 1985; Abraham et al. 1986a, Gimenez-Gallego et al. 1986; Gospodarowicz et al. 19866, Klagsbrun et al. 1987; Ho et al. 1988) similar to those from other tissues. In addition, bFGFs of higher molecular mass have been identified. Biologically active bFGF-like peptides of 21–25 ×103Mr have been isolated from rat and guinea pig brain, respectively (Presta et al. 19886, Moscatelli et al. 1987). Such bFGF forms may have been translated by unusual initiation codons (i.e. CUG instead of AUG) (Florkiewicz and Sommer, 1989; Prats et al. 1989). Anti-bFGF antibodies recognize additional bFGF-like peptides of up to 55 × 103Mr in neuroectoderm-derived tissues (Presta et al. 1988a, Grothe et al. 1990a, Westermann et al. 1990). Whether these immunoreactive proteins are bFGF-forms or immunologically crossreactive proteins different from bFGF has not yet been resolved. An interesting feature, however, is that as well as the 18 ×103Mr bFGF, these larger proteins are expressed in a species and tissue specific pattern. In the rat, Presta et al. (1988a) found that in different regions of the brain a 29 ×103Mr peptide was more abundant than the 18 ×103Mr bFGF, while in the pituitary approximately equal amounts of 18, 27 and 29xlO3Mr proteins were detected. In contrast, bovine pituitary contained 18, 24, 29-33 and 46 ×103Mr immunoreactive proteins (Grothe et al. 1990a). For the adrenal gland, different patterns were found not only in different species (bovine, pig and rat), but also in whole glands as compared to medullary chromaffin cells (Westermann et al. 1990), which are of neuroectodermal origin (Unsicker et al. 1989).

Another problem still unresolved is the release of bFGF. While the missing signal peptide (Abraham et al. 1986a) argues against a release, there is sufficient evidence to indicate that bFGF may be released from intact cells: (1) bFGF is released by cultured cells (Sato et al. 1989; van Zoelen et al. 1989; Werner et al. 1989; Maier et al. 1990), (2) anti-bFGF antibodies inhibit autocrine growth of endothelial cells (Schweigerer et al. Vè&lb), (3) bFGF is present in secretory organelles of adrenal chromaffin cells (Westermann et al. 1990), (4) many effects of bFGF seem to be mediated by specific, membrane-bound surface receptors (see reviews), (5) bFGF is located in the extracellular matrix in vitro and in vivo (Baird and Ling, 1987; Jeanny et al. 1987; Vlodavski et al. 1987; Moscatelli, 1988; Bashkin et al. 1989; DiMario et al. 1989; Ingber and Folkmann, 1989), and, (6) bFGF is found in body fluids (Chodak et al. 1988; Smith et al. 1989). Thus, bFGF resembles interleukins and some other proteins that can be released from cells despite the lack of a signal peptide (Muesch et al. 1990). Basic FGF may therefore be secreted by a mechanism other than the regulated pathway (Muesch et al. 1990), or via a so far unknown precursor containing a signal peptide. In this context, it is of interest that transfection with a bFGF-signal peptide construct results in the transformation of these cells (Thomas, 1988; Rogelj et al. 1988, 1989).

By its molecular and biological properties, bFGF from the nervous system appears to be identical with bFGF from non-neuronal sources.

Expression in neurons and/or glial cells and their tumor counterparts

Only few and controversial data are available on bFGF expression by neurons and glial cells in vitro. In primary cultures of brain cells and peripheral ganglia, Pettmann et al. (1987) and Janet et al. (1988) found bFGF immunoreactivity exclusively in neurons, but never in glial cells. Ferrara et al. (1988) reported that cultured astrocytes from adult bovine corpus callosum contain bFGF mRNA and protein. Gonzales et al. (1989) showed that in vitro only astrocytes express bFGF mRNA, while in situ neurons of several brain regions contain the mRNA. A neuronal rather than glial location of bFGF in situ was also shown by other authors (Pettmann et al. 1986; Janet et al. 1987; Grothe et al. unpublished). Nonetheless, the satellite glial cells of rat dorsal root ganglia also contain anti-bFGF immunoreactivity in situ (C. Grothe, unpublished data).

In the adult rat brain, bFGF-like immunoreactivity is restricted to neurons of discrete loci, e.g. the hippocampus (Gonzales et al. 1989), parietal cortex (P. Walicke, personal communication) and several, but not all, brain stem nuclei (Grothe et al. 1990b). Furthermore, developmental alterations to this pattern have been observed (Grothe et al. 1990b).

In the retina, bFGF is expressed in photoreceptor cells (Noji et al. 1990). From its specific, possibly light-dependent location in/on rod outer segments, an essential role in phototransduction has been postulated (Mascarelli et al. 1989).

Several types of neuroectodermal tumor cells can express bFGF, e.g. neuroblastoma cells (Huang et al. 1987; Heymann et al. 1988), glioma cell lines (Rogister et al. 1988; Sato et al. 1989, Okumura et al. 1989; Westermann and Unsicker, 1990) and diverse glial tumors (Paulus et al. 1990) as well as pheochromocytoma cells (see below).

Differences between these tumors have been found with regard to regulation and release of bFGF. For example, bFGF has been reported to be released from human astrocytoma cells (Sato et al. 1989), but not from rat glioma cells (Okumura et al. 1989; Westermann and Unsicker, 1990). Regulation of its expression in C6 glioma cells is controversial. Okumura et al. (1989) have shown that increasing cell density results in an increase of intracellular bFGF protein. In contrast, we have found that bFGF protein decreases with increasing cell density and, furthermore, an external stimulus is essential for initiating bFGF expression (Westermann and Unsicker, 1990). The latter results were also obtained with astrocytoma cells (Sato et al. 1989). Furthermore, FGF-receptor expression also seems to be regulated by cell density (Veomett et al. 1989).

Thus the question as to whether neurons or glial cells, or both, physiologically express bFGF, remains to be answered. Further analyses in this regard should consider that (1) immunological localization of bFGF protein in a cell may reflect uptake from an exogenous source rather than its expression by this cell {in situ hybridizations required), (2) in different regions of the nervous system, bFGF may be expressed by different types of cells and (3) bFGF expression may qualitatively and quantitatively vary during development or different physiological states.

The above data indicate that bFGF expression in vitro is modulated not only by a variety of physiological but also by experimental conditions. Nonetheless, it seems that all neuroectoderm-derived cells have the capacity to express bFGF under distinct circumstances.

Effects on neurons in vitro

The most pronounced effects of bFGF on neurons are the promotion of in vitro survival and neurite outgrowth. Basic FGF maintains neurons of the cerebral cortex (Morrison et al. 1986; Walicke, 1988), hippocampus (Walicke et al. 1986; Mattson et al. 1989), thalamus (Walicke, 1988), striatum (Walicke, 1988), septum (Walicke, 1988; Grothe et al. 1989), mesencephalon (Ferrari et al. 1989) and spinal cord (Unsicker et al. 1987) in culture. The only peripheral nervous system neurons supported by bFGF are those of the chick ciliary ganglion (Unsicker et al. 1987; Giulian et al. 1988).

In explant cultures of rat retinae, bFGF enhanced survival and induced fiber outgrowth of ganglion cells (Baehr et al. 1989; Thanos and van Boxberg, 1989). Highly purified retinal ganglion cells of the chick embryo however did not survive in the presence of bFGF (Lehwalder et al. 1989).

Like NGF, bFGF does not indiscriminately address all neuron populations. Neurons which are obviously not targets for bFGF are located in the subiculum (Walicke et al. 1988), rat nodose and superior cervical ganglia as well as chick sympathetic and dorsal root ganglia (Unsicker et al. 1987).

Besides the survival and neurite outgrowth activities, other effects on neurons have been observed. Basic FGF is a mitogen for neuroblasts (Gensburger et al. 1987) and neuroblastoma cells (Luedecke and Unsicker 1990a,b). In ciliary ganglion neurons (Unsicker et al. 1987; Vaca et al. 1989), septal neurons (Grothe et al. 1989) and spinal cord neurons (McManaman et al. 1989), bFGF increases choline-acetyltransferase (ChAT) activity.

Interestingly, bFGF is able to overcome the toxic activities of glutamate on in vitro survival and neurite outgrowth of cultured hippocampal neurons (Mattson et al. 1989).

Thus, in vitro, bFGF has a variety of effects on neurons, comparable to those of the neurotrophic factor NGF.

Effects on glial cells in vitro

In vitro studies have revealed that bFGF has a variety of effects on glial cells. Several groups have reported on the mitogenic activity of bFGF on astrocytes (Eccleston et al. 1985; Pettmann et al. 1985, 1987; Sensenbrenner et al. 1985; Perraud et al. 1987, 1988a; Delaunoy et al. 1988; Kniss and Burry, 1988; Loret et al. 1989), oligodendrocytes (Eccleston and Silberberg, 1985; Eccleston et al. 1985; Perraud et al. 1987, 1988a; Delaunoy et al. 1988; Besnard et al. 1989) and glioblasts (Delaunoy et al. 1988). Yong et al. (1988a, b) found that bFGF is a mitogen for fetal, but not adult, human astrocytes, and not for oligodendrocytes and Schwann cells.

Another prominent effect of bFGF on astrocytes is to cause a morphological change. Astrocytes grown under different culture conditions mostly appear as flat epithelial cells. In the presence of bFGF the cell bodies become smaller and rounded, and extend several long processes. Simultaneously, a reorganization of intermediate filaments (Weibel et al. 1985) and an increased synthesis of alpha- and beta-tubulins can be observed (Weibel et al. 1987). The intensity of these morphological changes varies between astrocytes from different brain regions (Perraud et al. 1990).

Other effects include quantitative as well as qualitative changes in the synthesis of many different proteins, an increase in S-100 peptide, glutamine synthetase, free ribosomes, Na+ and K+ uptake, S6 protein kinase activity, and plasminogen activator release (Pettmann et al. 1985; Weibel et al. 1985; Latzkovits et al. 1988; Loret et al. 1988; Register et al. 1988; Gavaret et al. 1989). Numbers of Orthogonal array particles (OAPs, possibly a structural equivalent of K+-channels) decrease (Wolburg et al. 1986). Furthermore, bFGF acts as a chemoattractant for astroglial cells (Senior et al. 1986). Most of these events can be correlated with the maturation of astrocytes.

Thus, bFGF appears to be a mitogen and maturation factor for astrocytes. Similar, but not identical, effects have been reported for acidic FGF, cyclic AMP and other growth factors (Weibel et al. 1987; Perraud et al. 1988a,b).

Oligodendrocytes cultured in the presence of bFGF also change their morphology (mostly showing outgrowths of two, long processes) and have increased carboanhydrase activity (Delaunoy et al. 1988), decreased myelin basic protein and inhibition of the enzyme CNP (Besnard et al. 1989). Thus bFGF apparently inhibits the maturation of oligodendrocytes.

Glioblasts cultured in the presence of bFGF proliferate, but fail to express differentiation markers for astro- or oligodendrocytes (Perraud et al. 1988a).

Comparable effects have been reported for glial tumor cells (Register et al. 1988; Westphal et al. 1988, Okumura et al. 1989; Westermann and Unsicker, 1990).

As is known for bFGF activities on cells of mesenchymal origin (for review see Lobb, 1988), heparin and other glucosaminoglycans are potent enhancers of the bFGF effects on glial cells (Perraud et al. 1988a,b).

In summary, bFGF exerts various effects on glial cells, mainly affecting proliferation and differentiation. These effects appear to be different for glioblasts (proliferation, inhibition of differentiation), astrocytes (proliferation and maturation) and oligodendrocytes (possibly proliferation, inhibition of maturation) and further depend upon the site of origin of the cells.

With regard to glial cells, bFGF therefore differs from NGF, which does not influence glial cell morphology and physiology.

To our knowledge, there is only one documentation of an in vivo effect of bFGF on glial cells; this is an increase in number of reactive astrocytes around an axotomy wound, possibly due to the mitogenic activity of bFGF (Barotte et al. 1989).

Chromaffin cells are modified sympathetic neurons which are scattered in peripheral ganglia and particularly prominent in the adrenal medulla (Unsicker et al. 1989).

Expression

Basic FGF is present in adrenal chromaffin cells (Grothe and Unsicker, 1989; Westermann et al. 1990). Immunohistochemistry using rat chromaffin cells has revealed that only a subpopulation, the noradrenergic, and not the adrenergic, cells contain bFGF (Grothe and Unsicker, 1990). Since cultured bovine chromaffin cells also contain anti-bFGF immunoreactive proteins (Blottner et al.1989)it may be inferred that bFGF can be synthezised by chromaffin cells. Basic FGF immunoreactivity is subcellularly localized in the secretory granules of chromaffin cells (Westermann et al. 1990), and may therefore be released together with catecholamines, chromogranins and the other soluble molecules of the vesicle content.

Effects

Basic FGF induces proliferation, neurite outgrowth and NGF-dependence of adrenal chromaffin precursor cells in vitro (Stemple et al. 1988). A tumor counterpart of chromaffin cells, the PC 12 rat pheochromocytoma cells, respond to bFGF in a manner similar to NGF. Basic FGF induces neurite outgrowth on PC12 cells (Togari et al. 1985; Wagner and D’Amore, 1986; Neufeld et al. 1987; Rydel and Greene, 1987; Schubert et al. 1987; Sigmund et al. 1990). The bFGF effect is different from that of NGF: the neurite network is less dense, protein kinase C inhibition enhances fiber outgrowth and bFGF-induced neurites disappear after about 6 days in culture (Togari et al. 1985; Rydel and Greene, 1987; Sigmund et al.1990). The effects of NGF and bFGF on protein phosphorylation are identical, including reduced phosphorylation of the NGF-sensitive protein Nsp100, and enhanced phosphorylation of a microtubule-associated protein MAP 1.2, tyrosine hydroxylase and a nonhistone protein SMP (Togari et al. 1985; Rydel and Greene, 1987). The expression of the NGF-inducible protein NILE, Thy-1, acetylcholineesterase (Rydel and Greene, 1987) and NGF-receptors (Doherty et al. 1988), as well as c-fos and SCG10, a growth cone membrane protein (Sigmund et al. 1990), are also affected. Differences in the actions of bFGF and NGF include (1) additive effects on the induction of ornithine decarboxylase (Togari et al. 1985), (2) an increase in the release of the β -amyloid precursor proteins NGF (2-fold) and FGF (7-fold) Schubert et al. 1989), (3) inability of bFGF to induce neurite outgrowth and expression of the protease transin in the NGF-responsive PC12/RG-5 subclone (Machida et al. 1989) and (4) involvement of protein kinase C in bFGF-dependent, but not NGF-dependent, c-fos induction (Sigmund et al. 1990). As known for other bFGF-activities, heparin and other glucosaminoglycans are able to enhance the bFGF effects on PC 12 cells (Wagner and D’Amore, 1986; Damon et al. 1988) but probably in a way different from their modulation of the effects of NGF (Neufeld et al. 1987).

Basic FGF also affects the interaction between PC 12 cells. In the presence of bFGF and heparin or chondroitinsulfate as culture matrices, PC 12 cells form ring-like aggregates (Schubert et al. 1987).

In conclusion, these data demonstrate that bFGF and NGF, acting by at least partially different mechanisms, have similar, but not identical effects on chromaffin and pheochromocytoma cells.

Presence of bFGF in neuronal target organs

An important feature of a neurotrophic factor is its expression by the neuronal target tissue. Since bFGF has been shown to be present in almost any tissue and organ (for a review, see Lobb, 1988), this criterion seems to be fulfilled. The relatively large amounts of bFGF present in most tissues, seem to conflict with the NTF-definition of Barde (1988). On the other hand, bFGF in these tissues may have additional functions requiring higher concentrations, and, as discussed by Oppenheim (1989), not ‘limiting amounts’ of factor, but ‘limited access’ (number of axonal branches and synapses) of factor, may control neuron death.

Basic FGF receptors in the nervous system

A further criterion for NTFs is their uptake by specific receptors and a retrograde axonal transport. So far, no retrograde transport of bFGF by neurons has been reported. In the case of embryonic chick ciliary and superior cervical ganglia, bFGF is apparently not retrogradely transported (Hendry and Belford, 1990).

Basic FGF receptors in the nervous system are well established. Courty et al. (1988) have described one type of bFGF receptor in bovine brain; others (Imamura et al. 1988; Ledoux et al. 1989; Mereau et al. 1989) have found two receptor types, differing in molecular mass and affinity for bFGF, in rat, guinea pig and bovine brain. In the chick brain, bFGF receptors are highly expressed during a long developmental period, as compared to other organs (Olwin and Hauschka, 1990).

Up to now, bFGF receptor mapping studies in the nervous system are missing, but from the effects of bFGF on neuroectodermal cells (see above) one may conclude that most of these cells express bFGF receptors.

Ontogenetic neuron death

The most important feature of NTFs is their ability to prevent the ontogenetic neuron death.

Dreyer et al. (1989) and Hendry et al. (1990) systemically applied bFGF and aFGF, respectively, to chick embryos and analyzed the number of ciliary ganglion neurons (which are known to survive, in vitro, in the presence of bFGF) (Unsicker et al. 1987; Giulian et al. 1988). They reported that neuron death, which occurs between embryonic day 8 (100% neurons) and 14 (56% neurons), is almost completely prevented by bFGF (Dreyer et al. 1989; Hendry et al. 1990).

Studies in other systems will be necessary to further substantiate this in vivo neuronal survival-promoting effect of bFGF; in particular, data on the in vivo effects of blocking anti-bFGF antibodies on nervous system development are required (for aFGF, see Hendry et al. 1990).

Central nervous system lesions

Well established lesions in the CNS are the fimbria-fornix and the optic nerve transection.

Basic FGF has been shown to maintain embryonic septal neurons in vitro (Grothe et al. 1989). Moreover, the protein is present in the hippocampus, an important target tissue of medial septal neurons (Pettmann et al. 1986). In adult rats, unilateral fimbria-fornix transection results in the loss of approximately 87% of the neurons, as visualised by Nissl staining (Otto et al. 1989) or about 60% of the cholinergic neurons in the medial septum (Anderson et al. 1988), and approximately 50% of the cholinergic neurons in the diagonal band of Broca on the ipsilateral side (Anderson et al. 1988). Application of bFGF in gelfoam or by intraventricular infusion significantly reduces this axotomy-induced neuron death. In the medial septum, approximately 20% of the total and 60% of the cholinergic neurons that otherwise would have died are rescued by bFGF. In the diagonal band of Broca about 80% of the cholinergic neurons survive in the presence of bFGF (Anderson et al. 1988). Comparable effects have been obtained with NGF (Otto et al. 1989). Further consequences of bFGF treatment are a reduction of the lesion-induced decrease in ChAT activity in the hippocampus, and, around the wound, an increase in the number of reactive astrocytes (Barotte et al. 1989).

In retinal explant cultures, neurite induction and survival of retinal ganglion cells can be induced by bFGF (Baehr et al. 1989; Thanos and van Boxberg, 1989). The presence of bFGF in the tectum, the target for the optic nerve fibers, however, has not been documented. Following transection of the rat optic nerve, bFGF partially prevents the death of retinal ganglion neurons (approximately 25% of the otherwise dying neurons) (Sievers et al. 1987).

Peripheral nervous system lesions

In the PNS, sciatic nerve transection is a widely used lesion model. While bFGF is present in the skin of embryonic and adult rats (Gonzales et al. 1990; C. Grothe, unpublished data), a target tissue for the sensory nerve fibers, it has no effect on the in vitro survival of dorsal root ganglion neurons of chick embryos (Unsicker et al. 1987; see, however, Watters and Hendry, 1987; Hendry et al. 1990). Even so, local application of bFGF (as well as NGF) at the stump of a transected sciatic nerve rescues approx. 70 – 80% (100% in the case of NGF) of the dorsal root ganglion neurons (Otto et al. 1987). Basic FGF not only maintains these neurons, but also results in regeneration of the nerve fibers over a distance of up to 15 mm (Aebischer et al. 1989).

Neuron death induced by target ablation can also be prevented by bFGF. Blottner et al. (1989) induced the death of approximately 25% of neurons in the intermediolateral column of the spinal cord by destruction of the adrenal medulla, which is known to contain bFGF (Grothe and Unsicker, 1989; Westermann et al. 1990). Substitution with bFGF results in the survival of approximately 85% of the otherwise dying neurons (NGF 0%) (Blottner et al. 1989). Splanchnic nerve transection abolishes these effects of bFGF (Blottner and Unsicker, 1990), suggesting that a retrograde axonal transport of bFGF is necessary for its function. Further evidence for the NTF-like function of bFGF in this system is the developmental correlation between the time of the first appearance of FGF-like neurotrophic activity and immunoreactivity and the onset of the functional innervation of the rat adrenal medulla (Blottner and Unsicker, 1989; Grothe and Unsicker, 1990).

For all the above lesion systems, it remains to be shown whether bFGF acts directly on the rescued neurons, or indirectly by inducing glial or other cells to synthesize and secrete a trophic factor.

Thus, bFGF resembles NGF in that it is able to prevent the physiological as well as lesion-induced neuron death of distinct neuronal populations, and to accelerate the regeneration of nerve connections in vivo.

Growth factors in general are of considerable interest Tor their clinical potential (for recent reviews, see Goustin et al. 1986; Deuel, 1987; Foster, 1988; Lobb, 1988; Westphal and Herrmann, 1989). Basic FGF, with its broad spectrum of functions, is a prominent candidate in this regard. Outside the nervous system, bFGF has been postulated to be associated with several pathological processes of enhanced or reduced cell growth (e.g. tumors, range of vascularization). Moreover, prominent roles in soft and hard tissue repair as well as immunomodulation have been observed. In addition, bFGF may be of diagnostic interest, because increased levels in different body fluids have been found in patients with tumors and retinopathies (discussed by Lobb, 1988).

By analogy, one may therefore speculate that bFGF may also be involved in neurological disorders.

Neural trauma

To date, therapy of neuronal trauma is mainly restricted to surgery, and especially in the CNS, only poor or no reconstitution is possible. The results obtained in central and peripheral lesion studies (e.g. axonal regeneration, increase of reactive astrocytes, prevention of neuron death, see above) however, provide evidence that bFGF may be used as a pharmacological tool to initiate or enhance regeneration processes after neuronal trauma.

Neuronal degeneration

Degenerative diseases of the CNS, such as Morbus Alzheimer, M. Parkinson and others, are characterized by neuron losses and connections in which bFGF may have an indirect role.

It has been shown that glioma and pheochromocytoma cells, as a consequence of bFGF-treatment, drastically enhance the synthesis and release of the β-amyloid precursor protein (Schubert et al. 1989; Quon et al. 1990) which is also enhanced and extracellularly deposited in Alzheimer’s disease (Anderton et al. 1988; Glenner, 1988).

In Parkinson’s disease, grafting of chromaffin tissue can result in at least partial reconstitution of the normal physiological status (see e.g. Olson, 1988), which may be due to liberation of growth factors rather than dopamine from the grafted cells. Consistent with the presence and possible release of bFGF by chromaffin cells is the observation that in an animal model of Parkinson’s disease, bFGF is capable of partially reversing the degeneration, inducing nerve fiber growth and restoring dopamine and tyrosine hydroxylase levels (Otto and Unsicker, 1990).

Neuronal tumors

Screening of neuroectodermal tumors has shown that most of them not only contain, but also respond to, bFGF (Westphal et al. 1988, 1989; Paulus et al. 1990). Accordingly, possible participation of bFGF in initiation as well as growth and invasion of brain tumors may be conceived (for the transforming potential of FGF, see e.g. Thomas, 1988). Because of its angiogenic effects, bFGF may also be involved in the strong vascularization of brain tumors (Brem, 1976). Further-more, increased expression of bFGF by brain tumors may result in increased levels of bFGF in the cerebrospinal fluid (CSF). In fact, growth factor activites resembling bFGF have been detected in the CSF of brain tumor patients but not in normal CSF (Lopez-Pousa et al. 1981; Brem et al. 1983).

Facts outlined above provide an indication of the putative functions bFGF may have in the nervous system. It may also have become clear, however, that we are far away from fully understanding the physiological role of bFGF in the nervous system.

Basic FGF shares a variety of functions with the fully established NTF, NGF. According to the NTF-definition of Barde (1988) two points remain to be clarified: (1) in vivo effects of inhibiting antibodies and (2) retrograde axonal transport. The issue of limiting amounts of factor available to nerve endings will be difficult to settle.

In addition to its NTF-like effects, bFGF, unlike NGF, also affects non-neuronal (glial, endothelial, etc.) cells of the nervous system and, furthermore, may be involved in growth and vascularization of brain tumors.

Thus, bFGF appears to be a physiological growth factor for neuroectodermal cells with a broader spectrum of functions than a classical NTF.

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