Effects of a nerve-growth factor, embryo age and metabolic inhibitors on growth of fibres and on synthesis of ribonucleic acid and protein in embryonic sympathetic ganglia

Preparation of pure neuronal and non-neuronal cultures from embryonic chick sympathetic ganglia: a new method based on both differential cell adhesiveness and the formation of homotypic neuronal aggregates

A new method has been developed for the preparation of essentially pure primary cultures of neurons and non-neuronal cells from 11-day embryonic chick sympathetic ganglia. This method utilizes (1) differences in cell-to-substrate adhesiveness between neurons and non-neuronal cells and (2) the capacity of neurons to form homotypic aggragates. The maximum difference in adhesiveness between neuronal and non-neuronal cells occurred when the ganglia were dissociated with trypsin following collection in a salt solution lacking divalent cations. This difference allowed the preparation of highly purified non-neuronal cultures and 85-90% pure neuronal cultures. Intermittent agitation during the period of cell separation markedly increased the purity of the neuronal cultures by (1) inhibiting neuronal but not non-neuronal cell attachment and (2) facilitating the formation of homotypic neuronal aggregates in the supernatant. Neuronal and non-neuronal cultures prepared under these conditions were more than 99% pure on the basis of both morphological and biochemical analyses. Both cell types exhibited attachment efficiencies greater than 95% and have been maintained for several weeks in vitro. Thus, completely isolated neuronal and non-neuronal cultures can be prepared and maintained for prolonged periods in the absence of cells of the other type.

The role of nerve-growth factor (NGF) in the central nervous system

NGF is a protein that stimulates growth and differentiation of sympathetic and sensory components of the peripheral nervous system. The purpose of this review is to examine the evidence that NGF has similar activity in the central nervous system. First, the primary mode of interaction of NGF with the nerve cell will be discussed, and the possibility that such an interaction takes place in the brain will be examined. Recent studies have demonstrated that NGF promotes regenerative sprouting of damaged catecholamine-containing neurons in the brain. The next part of the paper reviews this literature, and other findings that indicate or contraindicate a role of NGF in brain maturation of maintenance. The final part of this paper suggests specific avenues for future research in this area, and presents conclusions regarding the literatureon brain activity of NGF to date.

Properties of the beta-nerve growth factor receptor in development

The cell surface receptor for beta-nerve growth factor was used as a probe to study the development of embryonic chick sensory ganglia. The ganglia were shown to lose their responsiveness to nerve growth factor in vitro between 14 and 16 days of embryonic age. This loss occurred by a decrease in the magnitude of the maximum biological response, not by a shifting of the response to higher concentrations. Binding assays for the beta-nerve growth factor receptor, using 125I-radiolabelled beta-nerve growth factor, were performed with cells from sensory ganglia 8, 12, 14, 16, 18, and 21 days of age. The assays revealed a twofold increase in the number of receptor sites per ganglion between 8 and 14 days and a sixfold drop between 14 and 16 days of embryonic life. Neither increase nor decrease was accompanied by a large change in the affinity of the receptor for the protein. Together with the results of the bioassay, the data show that the loss of biological responsiveness is correlated with and may be due to a loss of the cells' ability to bind beta-nerve growth factor. Correlation of the results of the binding assays with the known ontogeny of the chick embryo provides a hint at the role of nerve growth factor in normal development.

Actinomycin D-sensitive periods in the differentiation of Drosophila neurons and muscle cells in vitro

Observations were made of neuroblasts differentiating into neurons, and myoblasts differentiating into myocytes in cultures of embryonic Drosophila cells. Axons greater than 50 mum long appeared in vitro between 7.5 and 16.5 h, and pulsating myocytes appeared between 12.5 and 23.5 h. Actinomycin D treatment prevented neuroblasts and myoblasts from proceeding in differentiation. Neurons became resistant to actinomycin D when they reached the stage of axon initiation, and axon elongation was not actinomycin D-sensitive. Myocytes were sensitive to actinomycin D until they attained the ability to pulsate, and ongoing pulsations were not halted by the drug. Autoradiographs and controls indicated that actinomycin D prevented uridine incorporation by about 90% but prevented leucine and thymidine incorporation by 6% or less in these cells. The result favour the interpretation that transcription is necessary in neuron differentation up to the point of axon initiation and in myocytes up to the point where pulsations can begin. Since ribosomal RNA synthesis is unnecessary for these differentiations, necessary RNA synthesis would be messenger or transfer RNA, or RNA of unknown identity.

Short- and long-term effects of nerve growth factor on the sympathetic nervous system in the adult mouse

Nerve growth factor (NGF) induced a marked increase in the adrenergic nerve terminal networks of several peripheral organs in intact adult and young adult mice. The animals were investigated at 3 days and at 2 months after 6 daily subcutaneous injections of the 7S species of NGF (1 mug/g body weight each dose). At 3 days after the end of the treatment an increased fluorescence intensity, as well as an increased density of the adrenergic terminal plexuses was observed by fluorescence histochemical techniques in iris, salivary glands, heart, intestine, spleen and pancreas. These changes were paralleled by significant increases (up to 55%) in the endogenous NA levels. Increased NA levels were also detected in the brain of the NGF-treated animals. At 2 months after treatment the effects had almost totally disappeared, demonstrating that the NGF-induced overgrowth of the sympathetic axons was only temporary. The increased adrenergic innervation probably resulted from a stimulatory effect of NGF on collateral sprouting from the intact adrenergic axons. In line with the idea that the terminals of peripheral axons are continuously renewed, it is suggested that endogenous NGF could play a regulatory role in such a continuous growth process of the fully developed, adult sympathetic nervous system, directed at the maintenance of an adequate adrenergic terminal network.

Nerve growth factor-induced rapid activation of RNA labeling in dorsal root ganglionic dissociates from chick embryos

Dorsal root ganglionic cells, from 8-day chick embryo, undergo anabolic declines when incubated in vitro without Nerve Growth Factor (NGF) or other supportive agents. To determine whether the decline could be opposed by delayed administration of the NGF, cells were incubated without the factor for varying times, then supplies with it and tested periodically with pulses of radiouridine or radioleucine. The decline in RNA labeling was actually reversed by a delayed addition of NGF, and the effect was fully elicited within less than 10 min from the treatment. With delays up to 6 hr, this rapid activation by NGF fully restored the incorporation rate exhibited by fresh cells or by cells continuously incubated with NGF. From the hour 8 on, the NGF-induced activation of RNA labeling fell progressively shorter of restoring maximal performance until, by 18 hr, it was no longer significant. The residual (irreversible) decline in RNA labeling, starting after 6 hr, developed with a time pattern coincidental with that of the irreversible decline in protein labeling also displayed by those untreated cells, and similar to the appearance of RNA degradation and the acceleration of protein degradation. All four such "degenerative" events were fully prevented by NGF when administered with delays shorter than 6 hr, and only interrupted (or delayed) by NGF delivered at later times. Additional experiments revealed that, over the first 6 hr, NGF also prevented or reversed a decline in TCA-soluble radioactivity of the cells, an effect which was not blocked by actinomycin D. This rapid increase of soluble radioactivity could be responsible for the rapid activation of RNA labeling and may also be involved in the prevention by NGF of the later developing degenerative events.

Properties of the beta nerve growth factor receptor of avian dorsal root ganglia

Iodination of the beta nerve growth factor protein up to 4.1 iodines per mole of nerve growth factor was achieved with no loss of biological activity. At this level of incorporation no native or monoiodo nerve growth factor remained, the major species being the tri- and tetraiodo derivatives. An (125)I-labeled beta nerve growth factor of high specific activity containing 0.5 iodine per mole of beta nerve growth factor was used to determine the specific binding of nerve growth factor to dorsal root ganglion cells of 8-day-old embryonic chicks. The specific binding of (125)I-labeled nerve growth factor reached saturation at 30-50 ng/ml and half-saturation at 7-8 ng/ml (0.26 nM). The number of receptors per responsive medio-dorsal cell was calculated to be about 2 x 10(4). The pattern of displacement of bound (125)I-labeled beta nerve growth factor by native beta nerve growth factor showed that the two proteins had identical affinities for the receptor. The specificity of the binding for beta nerve growth factor was demonstrated by the fact that only native beta nerve growth factor displaced the bound (125)I-labeled form. Partially inactivated derivatives of beta nerve growth factor retained the same fraction of their specific-binding capacity as of their biological activity. The specificity of the binding for cell type was shown by the lack of any specific component of (125)I-labeled beta nerve growth factor binding to liver or brain cells. The rate constant for association of beta nerve growth factor with its receptor, k(1), was 1.0 x 10(7) mol(-1) sec(-1) and the rate constant for dissociation, k(-1), 1.2 x 10(-8) sec(-1).

Replacement of nerve-growth factor by ganglionic non-neuronal cells for the survival in vitro of dissociated ganglionic neurons

Nerve-growth factor is known to cause a considerable increase in the number of neurons putting out processes and surviving in cell cultures of dissociated dorsal-root and sympathetic ganglia from embryonic chicks. Similar effects of nerve-growth factor have now been noted with cultures of dissociated dorsal-root ganglia from newborn mice or rats. In all three sensory ganglionic systems, the effects of the nerve-growth factor on fiber production and neuronal survival could be mimicked, in the absence of the factor, by adequate increase of the non-neuronal cells in the cultures. The results suggest a hypothesis that views the role of the nerve-growth factor as subordinate to that of the non-neuronal cells.

Studies on the mechanism of action of nerve growth factor

In this study, explanted chick-embryo sensory ganglia were treated with nerve growth factor, and cellular concentrations of neurotubule protein were measured chemically by a colchicine-binding assay. Even after brief time periods, ganglia treated with growth factor were enriched in neurotubule protein, as compared with untreated (control) ganglia. Furthermore, studies with ganglia treated with both vincristine and growth factor demonstrated that neurotubule protein synthesis can occur even though neurite outgrowth is abolished. Several lines of evidence indicate that the growth factor stimulates de novo synthesis of neurotubule subunit protein, and that this effect precedes neurite extension. Like nerve growth factor, dibutyryl cyclic AMP also stimulates neurite outgrowth from embryonic sensory ganglia, yet it does not increase neurotubule protein levels. Available information suggests that the ability of growth factor to elicit rapid neurite outgrowth is closely related to its ability to increase cellular neurotubule levels. Cyclic AMP appears to stimulate neurite outgrowth by a different mechanism.