A role for Zn2+ in a variety of neurological conditions such as stroke, epilepsy and Alzheimer's disease has been postulated. In many instances, susceptible neurons are located in regions rich in Zn2+ where nerve growth factor (NGF) levels rise as a result of insult. Although the interaction of Zn2+ with this neurotrophin has previously been suggested, the direct actions of the ion on NGF function have not been explored. Molecular modeling studies predict that Zn2+ binding to NGF will induce structural changes within domains of this neurotrophin that participate in the recognition of TrkA and p75NTR. We demonstrate here that Zn2+ alters the conformation of NGF, rendering it unable to bind to p75NTR or TrkA receptors or to activate signal transduction pathways and biological outcomes normally induced by this protein. Similar actions of Zn2+ are also observed with other members of the NGF family, suggesting a modulatory role for this metal ion in neurotrophin function.

Equilibrium binding of 125I-nerve growth factor (125I-NGF) to cells coexpressing the tyrosine kinase receptor A (TrkA) and common neurotrophin receptor (p75NTR), cells coexpressing both receptors where p75NTR is occupied, and cells expressing only p75NTR, revealed reciprocal modulation of receptor affinity states. Analysis of receptor affinity states in PC12 cells, PC12 cells in the presence of brain-derived neurotrophic factor (BDNF), and PC12nnr5 cells suggested that liganded and unliganded p75NTR induce a higher affinity state within TrkA, while TrkA induces a lower affinity state within p75NTR. These data are consistent with receptor allosterism, and prompted a search for TrkA/p75NTR complexes in the absence of NGF. Chemical crosslinking studies revealed high molecular weight receptor complexes that specifically bound 125I-NGF, and were immunoprecipitated by antibodies to both receptors. The heteroreceptor complex of TrkA and p75NTR alters conformation and/or dissociates in the presence of NGF, as indicated by the ability of low concentrations of NGF to prevent heteroreceptor crosslinking. These data suggest a new model of receptor interaction, whereby structural changes within a heteroreceptor complex are induced by ligand binding.

Direct effects of Zn 2+ on the conformation and biological activity of nerve growth factor (NGF) have previously been described. Zn 2+ binds to specific coordination sites within NGF and induces conformational changes within domains that participate in receptor recognition processes. Recent theoretical considerations indicate that other transition metal cations (particularly, Cu2+and Pd 2+) are capable of forming similar complexes with NGF. Inactivation of NGF by transition metal cations is inhibitory to neuronal regeneration and sprouting, and can lead to, cell death under conditions where NGF is required for survival in PC12 cells. In this study we investigated the influence of various metal ions on NGF conformation, geometry of NGF amino terminal peptide and NGF-mediated biological effects in PC12 cells. A number of metal ions (Zn 2+, Cu 2+ and Pd 2+) alter NGF conformation in cell-free assays and inhibit NGF-mediated cell survival. Other metalshave been shown to be either toxic to PC12 cells b~ mechanisms independent of NGF activity (e.g. Ag , Hg 2 § or non-toxic to the cells under conditions tested (e.g. A13+, Cr3+). In conclusion, several metal cations are capable of inhibiting NGF activity, thereby blocking NGF-mediated cell survival and plasticity.

. The importance of His84 of NGF for transition metal cation binding is demonstrated, con®rming the involvement of this residue in metal ion coordination.

No abstract