Properties of the Specific Binding of 125I-Nerve Growth Factor to Responsive Peripheral Neurons

Binding of alpha-bungarotoxin to chick sympathetic ganglia: properties of the receptor and its rate of appearance during developement

Studies were carried out on the binding of [125I]alpha-bungarotoxin (alphaBT) to membrane fragments of chick sympathetic ganglia. Specific binding of toxin was saturable with a KD of 1.1 nM. The rates of association and dissociation of the toxin from ganglionic membranes were 4.3 X 10(4) M-1 sec-1 and 4.6 X 10(-5) sec-1 (t 1/2 = 4.2 h). respectively. Binding was inhibited (by up to 95%) by low concentrations of nicotinic, but not by a muscarinic cholinergic ligand. The properties of the ganglionic binding site for alphaBT were consistent with its being a nicotinic acetylcholine receptor. The development of toxin receptors in chick ganglia was also studied. From days 7 to 11 in ovo, few receptors were present; from days 12 to 20 in ovo, there was a 10-fold increase in receptor number per ganglion; from hatching to maturity, the receptor number per ganglion slowly increased and reached a maximum of 14 fmoles. The ontogeny of receptors for alphaBT in sympathetic ganglia appears to correlate with the cytological maturation and innervation of the principal neurons.

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.

The mobile receptor hypothesis and "cooperativity" of hormone binding. Application to insulin

The mobile receptor hypothesis has been proposed to describe the process by which hormone receptor binding initiates a biological response; it states that receptors, which can diffuse independently in the plane of the membrane, reversibly associate with effectors to regulate their activity. The affinity for effector is greater when the receptor is occupied by hormone. A mathematical expression of the mobile receptor hypothesis is used to show that: (1) The predicted kinetics of hormone receptor binding may be indistinguishable from "negative cooperativity." (2) Receptor occupancy and biological response may be coupled in a non-linear fashion. By choosing specific parameters, most of the existing data on insulin binding and biological responses can be explained in terms of the mobile receptor hypothesis. Thus, the following are easily explained: (1) A single homogeneous receptor may appear kinetically to be composed of two classes (of high and low affinity) of receptors. (2) Occupancy of the apparent class of high affinity receptors is related linearly to the biological response. (3) The same receptor in different tissues may appear to have different affinity. (4) The binding of different biologically active insulin analogues may exhibit different degrees of "cooperativity." These considerations may also be pertinent to interpretations of other hormone-receptor systems and of various ligand-macromolecule interactions.

Evidence for a single class of thrombin-binding sites of human platelets

We previously demonstrated that human platelets bind a small number of thrombin molecules with high affinity and a larger number with relatively lower affinity (Tollefsen, D. M., Feagler, J. R., and Majerus, P. W. (1974), J. Biol. Chem. 249, 2646). In the present report, equilibrium binding studies using [125I]DIP-thrombin (diisopropylphosphoryl-thrombin) over the range of 0.0002-10 U/ml yield a Hill coefficient of 0.775. Measurements of the rate of dissociation of [125I]DIP-thrombin bound to platelets at relatively high affinity (0.04 U/ml added) indicate a much faster dissociation in the presence of 14 U/ml unlabeled DIP-thrombin (T 1/2 = 1.0 min) than in its absence (T 1/2 - 140 min). [125I]DIP-thrombin bound at low affinity (1.0U/ml added) dissociates from platelets with a T 1/2 = 1.7 min in the absence of unlabeled DIP-thrombin. These results suggest a negative cooperative interaction among receptor sites for thrombin; i.e., as thrombin binds to unoccupied sites, high-affinity receptors are apparently converted to low-affinity receptors. In an attempt to detect whether there is heterogeneity of thrombin receptors, [125I]DIP-thrombin was covalently cross-linked to intact platelets using 1 mM glutaraldehyde. A single complex (apparent molecular weight, 200 000) containing [125I]DIP-thrombin was formed throughout a range of thrombin concentrations in which both high- and low-affinity binding was observed. Since incorporation of [125I]DIP-thrombin into this complex did not occur in the absence of platelets and was inhibited by unlabeled thrombin, the complex may represent thrombin cross-linked to its receptor. We conclude that a single class of receptor sites can account for both high- and low-affinity binding of thrombin to platelets, although interaction between nonidentical sites cannot be excluded.

Cooperative properties of hormone receptors in cell membranes

The binding of many polypeptide hormones to cell surface receptors does not appear to follow the law of mass action. While steady-state binding data are consistent in many cases with either heterogeneous populations of binding sites or interactions of the type known as negative cooperativity, study of the kinetics of dissociation of the type known as negative cooperativity, study of the kinetics of dissociation of the hormone receptor complex allows an unambiguous demonstration of cooperative interactions. Negative cooperativity, which seems to be wide-spread among hormone receptors, provides exquisite sensitivity of the cell at low hormone concentrations while buffering against acutely elevated hormone levels. The molecular mechanisms underlying the cooperativity are still largely unknown. Cooperativity may stem from a conformational transition in individual receptors or involve receptor aggregation in the fluid membrane (clustering) or more extensive membrane phenomena. Thus, new models of hormone action must be considered which integrate the progress in our knowledge of both the complex mechanisms regulating hormone binding to their surface receptors, and the dynamic properties of the cell membrane.

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.