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The physiological functions of NGF in the peripheral nervous system are already known for a long time (see Levi-Montalcini and Angeletti, 1968; Greene and Shooter, 1980; Thoenen and Barde, 1980). NGF regulates the regionally differential survival of sympathetic and neural crest-derived sensory neurons. This regulatory function comes into play when the axons of NGF-responsive neurons reach their target area (see Davies et al., 1987; Barde, 1989). Furthermore, NGF is essential for the differentiation and maintenance of neuron-specific functions such as the synthesis of enzymes involved in the production of neurotransmitters and neuron-specific peptides e.g. substance P and somatostatin (see Thoenen and Barde, 1980; Otten 1984). NGF acts as a retrograde messenger transferring information from the fields of projection in the periphery to the innervating NGF-responsive neurons (see Davies et al., 1987; Barde 1989). Following Darwinian principles, NGF is produced in the target fields in very small quantities permitting the survival of a limited number of neurons only. ’Injection of exogenous NGF increases the survival of NGF responsive neurons, thus abolishing natural cell death which occurs during normal development (see Hendry, 1980; Thoenen and Barde, 1980).

In order to obtain information on the physiological regulation of NGF-synthesis in the central nervous system (CNS) we investigated the effects of a series of growth factors (known to be present in the CNS) in cultures of purified rat astrocytes and compared these effects with those observed after intraventricular injection of the same molecules. After preliminary experiments had shown that 10% fetal calf serum (FCS) produced a marked increase in NGF-mRNA levels in astrocytes (but neither in microglia nor oligodendrocytes) as demonstrated by Northern blot analysis and in situ hybridization the experiments were performed at low (0.5%) FCS concentrations. Supramaximal concentrations of IL-1 and various growth factors caused a 5- to 7-fold increase in NGF-mRNA after 6 h. By 24 h the NGF-mRNA levels approached control values again, most probably due to inactivation of the added factors since after readdition after 24 h the response was about the same as the initial one. Norepinephrine and 8-bromo-cAMP did not change NGF-mRNA levels. The growth factor-mediated changes in NGF-mRNA levels in astrocyte cultures were not consistently reflected by the changes observed after intraventricular injection. IL-1 produced by far the largest increase in hippocampal NGF-mRNA after intraventricular injection. This large response to IL-1 could result from a positive feedback mechanism, since IL-1beta injection not only increases NGF-mRNA but also IL-1beta-mRNA in the hippocampus. The understanding of the physiological regulation of NGF synthesis in the CNS is the basis for a rational approach to its pharmacological modification. This, in turn, is an attractive alternative to the (long-term) infusion of NGF or the transplantation of NGF-secreting cells with the goal of providing trophic support to the cholinergic neurons of the basal forebrain nuclei. These neurons are consistently affected in the early stages of Alzheimer's disease, their impaired function being essentially responsible for the cognitive deficits.

In newborn rats the levels of nerve growth factor (NGF) mRNA (mRNANGF) and NGF receptor mRNA (mRNA(rec)) in the sciatic nerve were 10 and 120 times higher, respectively, than in adult animals. mRNA(rec) levels decreased steadily from birth, approaching adult levels by the third postnatal week, whereas mRNANGF levels decreased only after the first postnatal week, although also reaching adult levels by the third week. Transection of the adult sciatic nerve resulted in a marked biphasic increase in mRNANGF with time. On the proximal side of the cut, this increase was confined to the area immediately adjacent to the cut; peripherally, a similar biphasic increase was present in all segments. mRNA(rec) levels were also markedly elevated distal to the transection site, in agreement with previous results obtained by immunological methods [Taniuchi, M., Clark, H. B. & Johnson, E. M., Jr. (1986) Proc. Natl. Acad. Sci. USA 83, 4094-4098]. Following a crush lesion (allowing regeneration), the mRNA(rec) levels were rapidly down-regulated as the regenerating nerve fibers passed through the distal segments. Down-regulation of mRNANGF also occurred during regeneration but was slower and not as extensive as that of mRNA(rec) over the time period studied. Changes in mRNANGF and mRNA(rec) occurring in vivo after transection were compared with those observed in pieces of sciatic nerve kept in culture. No difference was found for mRNA(rec). Only the initial rapid increase in mRNANGF occurred in culture, but the in vivo situation could be mimicked by the addition of activated macrophages. This reflects the situation in vivo where, after nerve lesion, macrophages infiltrate the area of the Wallerian degeneration. These results suggest that mRNANGF synthesis in sciatic non-neuronal cells is regulated by macrophages, whereas mRNA(rec) synthesis is determined by axonal contact.

Organ cultures of rat iris show a characteristic change in the levels of both nerve growth factor (NGF) and its mRNA: a rapid but transient initial increase is followed by a smaller but persistently elevated NGF synthesis. This time course may be influenced by release of a factor(s) from degenerating nerve terminals and/or by the lack of some factor(s) repressing NGF synthesis in vivo. We therefore analyzed the influence of biogenic amine transmitter substances and putative neuropeptides on this elevation of NGF synthesis in cultured iris. The marked increase of NGF synthesis seen initially in culture was not completely mimicked by any of the substances tested. A specific increase in NGF production up to 150% of control was observed only with cGMP. We also obtained some evidence that reaction to trauma following the culture procedure could enhance NGF production: cutting of irides into small pieces increased NGF production in culture up to 250% of control and, vice versa, treatment with 1 microM dexamethasone decreased NGF production to about 60% of control. However, the sympathetic neurotransmitter norepinephrine (NE) decreased both NGF and its mRNA levels specifically in a dose-dependent manner (0.01-1 mM) to a minimum of about 25% of control. In situ hybridization with mRNA(NGF)-specific probes showed that in cultures of dissociated iris cells all cells were capable of expressing mRNA(NGF), but that 0.1 mM NE preferentially decreased expression of mRNA(NGF) in smooth muscle cells. Thus, our results indicate that the sympathetic transmitter NE is capable of downregulating NGF synthesis in the target cells of sympathetic neurons.

We show that nerve growth factor (NGF) synthesis in developing skin begins with sensory innervation and that sensory neurons do not express NGF receptors until their fibres reach their cutaneous targets. Both cutaneous epithelium and mesenchyme synthesize NGF, the concentration of messenger RNA for NGF being higher in the more densely innervated epithelium.

The importance of Nerve Growth Factor (NGF) for the development and the maintenance of differentiated properties of peripheral sympathetic and neural crest- derived sensory neurons as well as for basal forebrain cholinergic neurons is well established (cf Levi-Montalcini & Angeletti, 1968; Greene & Shooter, 1980; Thoenen & Barde, 1980; Thoenen et al. 1987). NGF interacts with specific neuronal receptors, is internalized and is subsequently retrogradely transported to the perikaryon. During the last few years new methodological approaches allowed the direct analysis of the retrograde messenger function of NGF between target organs and corresponding NGF-responsive neurons. In the adult animal it was demonstrated that the density of sympathetic innervation is correlated with the levels of NGF (Korsching & Thoenen 1983) and mRNANGF (Heumann et al. 1984, Shelton & Reichardt 1984). Thus, the ratio between NGF and mRNANGF in various target tissues of sympathetic or sensory neurons is relatively constant. However, in sympathetic and sensory ganglia containing the cell bodies of NGF-responsive neurons the levels of NGF are (relatively) very high whereas the mRNANGF levels are at the detection limit (Heumann et al. 1984, Davies et al. 1987a). Similarly, in the sciatic nerve (Heumann et al. 1987a) the ratio between NGF and mRNANFG is up to 100–fold higher than in peripheral target tissues of sympathetic and sensory neurons, reflecting the very low contribution of local synthesis to the sciatic NGF content. This is in agreement with the observation that after ligation of the sciatic nerve NGF accumulates distally to the ligature whereas proximaily NGF levels are reduced to the detection limit of the NGF assay (Korsching and Thoenen, 1983b). The importance of the NGF supply from the peripheral target tissues is also demonstrated by the fact that destruction of sympathetic nerve terminals by 6–OH- dopamine results in a rapid decay of NGF levels in the sympathetic ganglia (t1/2 = 4-5 hours) (Korsching and Thoenen, 1985) with a concomitant increase of NGF in the corresponding peripheral target organs.

The intact sciatic nerve contains levels of nerve growth factor (NGF) that are comparable to those of densely innervated peripheral target tissues of NGF-responsive (sympathetic and sensory) neurons. There, the high NGF levels are reflected by correspondingly high mRNA^(NGF) levels. In the intact sciatic nerve, mRNA^(NGF) levels were very low, thus indicating that the contribution of locally synthesized NGF by nonneuronal cells is small. However, after transection an increase of up to 15-fold in mRNA^(NGF) was measured in 4-mm segments collected both proximally and distally to the transection site. Distally to the transection site, augmented mRNA^(NGF) levels occurred in all three 4-mm segments from 6 h to 2 wk after transection, the longest time period investigated. The augmented local NGF synthesis after transection was accompanied by a reexpression of NGF receptors by Schwann cells (NGF receptors normally disappear shortly after birth). Proximal to the transection site, the augmented NGF synthesis was restricted to the very end of the nerve stump that acts as a "substitute target organ" for the regenerating NGF-responsive nerve fibers. While the mRNA^(NGF) levels in the nerve stump correspond to those of a densely innervated peripheral organ, the volume is too small to fully replace the lacking supply from the periphery. This is reflected by the fact that in the more proximal part of the transected sciatic nerve, where mRNA^(NGF) remained unchanged, the NGF levels reached only 40% of control values. In situ hybridization experiments demonstrated that after transection all nonneuronal cells express mRNA^(NGF) and not only those ensheathing the nerve fibers of NGF-responsive neurons.

Background Nogo-66 receptor NgR1 and its structural homologue NgR2 are binding proteins for a number of myelin-associated inhibitory factors. After neuronal injury, these inhibitory factors are responsible for preventing axonal outgrowth via their interactions with NgR1 and NgR2 expressed on neurons. In vitro, cells expressing NgR1/2 are inhibited from adhering to and spreading on a myelin substrate. Neuronal injury also results in the presence of dendritic cells (DCs) in the central nervous system, where they can come into contact with myelin debris. The exact mechanisms of interaction of immune cells with CNS myelin are, however, poorly understood. Methods Human DCs were differentiated from peripheral blood monocytes and mouse DCs were differentiated from wild type and NgR1/NgR2 double knockout bone marrow precursors. NgR1 and NgR2 expression were determined with quantitative real time PCR and immunoblot, and adhesion of cells to myelin was quantified. Results We demonstrate that human immature myeloid DCs express NgR1 and NgR2, which are then down-regulated upon maturation. Human mature DCs also adhere to a much higher extent to a myelin substrate than immature DCs. We observe the same effect when the cells are plated on Nogo-66-His (binding peptide for NgR1), but not on control proteins. Mature DCs taken from Ngr1/2 knockout mice adhere to a much higher extent to myelin compared to wild type mouse DCs. In addition, Ngr1/2 knockout had no effect on in vitro DC differentiation or phenotype. Conclusions These results indicate that a lack of NgR1/2 expression promotes the adhesion of DCs to myelin. This interaction could be important in neuroinflammatory disorders such as multiple sclerosis in which peripheral immune cells come into contact with myelin debris.