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

Sodium dependence of the nerve growth factor--regulated hexose uptake in chick embryo ganglionic cells

Embryonic dorsal root ganglionic cells, when incubated in vitro in the absence of nerve growth factor (NGF) undergo a general metabolic degeneration which is preceded by certain changes in permeation properties. Previous studies demonstrated that NGF can rapidly modulate permeation properties which regulate the availability to the cell of an important energy source, glucose. Hexose uptake was determined by measuring the ability of the cells to accumulate [3H]labeled 2-deoxy-D-glucose. The work reported here shows that the NGF-dependent portion (about one-third) of the total specific hexose uptake was also dependent on the presence of Na+, with the apparent uptake constant (Kt) for deoxyglucose varying inversely with an external Na+ concentration of 70-140 mM; Vmax was unaffected in this range. Preincubation of ganglionic cells with 10 mM ouabain for 15-60 min, followed by a pulse with [3H]-deoxyglucose, also resulted in 50-95% reduction of the NGF-sensitive uptake. A similar pretreatment of cells with veratridine gave a 25-50% reduction in uptake. The NGF-controlled hexose uptake was also energy dependent, being diminished 50-95% after a 30-90 min preincubation with 2 mM 2,4-dinitrophenol. Uptake activities for other substrates (alpha-aminoisobutyric acid, uridine) which exhibited NGF regulation were likewise Na+-sensitive. These results indicate that availability of major energy substrates to NGF-dependent dorsal root ganglionic neurons is controlled by sodium gradients across their membranes. It is conceivable that NGF provides for maintenance and development of its target neurons by acting on such sodium gradients and, consequently, regulating the intake of essential nutrients.

Nerve growth factor action on 2-deoxy-D-glucose transport in dorsal root ganglionic dissociates from chick embryo

Dorsal root ganglionic cells, when incubated in vitro in the absence of nerve growth factor (NGF), undergo a general metabolic degeneration which is preceded by loss of certain permeation properties. To determine in which ways an absence of NGF can also affect the capacity of these cells to take up an important energy source, namely glucose, experiments were carried out in which cells were incubated with or without NGF for varying times, and then presented with the factor and tested for the ability to take up 3H-labeled 2-deoxy-D-glucose. As with exogenous uridine, hexose transport in DRG cells was reduced by NGF deprivation and restored by delayed NGF administration (up to 6 h). Both the initial rate and equilibrium level were affected in an NGF dose-dependent fashion. Calculation of apparent Kt and V max in NGF-deprived and NGF-supported cells showed about two-fold differences between NGF-controlled and NGF-independent hexose transports, suggesting corresponding differences between NGF-dependent and other ganglionic cells. Restoration of hexose transport by delayed NGF administration took place within minutes of presentation of the factor. The delay before onset of restoration and the speed with which restoration was achieved have been found also to be dependent on the NGF concentration, suggesting that they reflect equilibration kinetics between NGF and its binding sites rather then the development of the response within cells. Thus, NGF can rapidly modulate permeation properties which regulate the availability of major energy substrates for the cell. This effect of NGF is discussed in the content of current views on the mode of action of the factor.

Nerve growth factor action on membrane permeation to exogenous substrates in dorsal root ganglionic dissociates from the chick embryo

An experimental system has been described, in previous studies, where the ability of chick embryo dorsal root ganglionic cells to incorporate radiouridine into RNA declines in the absence of nerve growth factor (NGF), and is promptly restored by delayed supply of the factor. Following these early and fully reversible events, further NGF deprivation causes progressive irreversible damage. The early decline in RNA labeling and its reversion by NGF are accompanied by similar changes in the accumulation of acid-soluble radioactivity from the exogenous radiouridine substrate. In the present study, it is shown that the NGF-dependent accumulation of "soluble" radiomaterials is independent from, and responsible for, the NGF-dependent alterations in RNA labeling. Both changes are measurable with labeled cytidine and guanosine, as well as uridine, and in all cases accumulation of acid-precipitable and acid-soluble radioactivities are strictly proportional to each other. The acid-soluble responses to NGF are not prevented by actinomycin D or cycloheximide treatments, demonstrating that they require neither ongoing syntheses of RNA or protein nor prior effects of NGF on them. Chromatography of acid-soluble radiopools showed that the NGF-dependent increase was not due to a distortion in the intracellular phosphorylation of uridine; but involved corresponding increases in all the radiouridine derivatives including UTP. The time patterns of the acid-soluble response were comparable to those of the RNA labeling response, and maximal NGF effects occurred within minutes of its presentation. Finally, 2-deoxyglucose and a-aminoisobutyric acid, but not leucine, showed NGF-dependent accumulation patterns similar to those of the 3 nucleosides. It is proposed that regulation of selected membrane permeation properties could be the primary process through which NGF exerts its trophic role.