Localization of functional receptor epitopes on the structure of ciliary neurotrophic factor indicates a conserved, function-related epitope topography among helical cytokines

By rational mutagenesis, receptor-specific functional analysis, and visualization of complex formation in solution, we identified individual amino acid side chains involved specifically in the interaction of ciliary neurotrophic factor (CNTF) with CNTFR alpha and not with the beta-components, gp130 and LIFR. In the crystal structure, the side chains of these residues, which are located in helix A, the AB loop, helix B, and helix D, are surface accessible and are clustered in space, thus constituting an epitope for CNTFR alpha. By the same analysis, a partial epitope for gp130 was also identified on the surface of helix A that faces away from the alpha-epitope. Superposition of the CNTF and growth hormone structures showed that the location of these epitopes on CNTF is analogous to the location of the first and second receptor epitopes on the surface of growth hormone. Further comparison with proposed binding sites for alpha- and beta-receptors on interleukin-6 and leukemia inhibitory factor indicated that this epitope topology is conserved among helical cytokines. In each case, epitope I is utilized by the specificity-conferring component, whereas epitopes II and III are used by accessory components. Thus, in addition to a common fold, helical cytokines share a conserved order of receptor epitopes that is function related.

A BDNF autocrine loop in adult sensory neurons prevents cell death

During the initial phase of their development, sensory neurons of the dorsal root ganglion (DRG) require target-derived trophic support for their survival, but as they mature they lose this requirement. Because many of these neurons express BDNF (brain-derived neurotrophic factor) messenger RNA, we hypothesized that BDNF might act as an autocrine survival factor in adult DRG neurons, thus explaining their lack of dependence on exogenous growth factors. When cultured adult DRG cells were treated with antisense oligonucleotides to BDNF, expression of BDNF protein was reduced by 80%, and neuronal survival was reduced by 35%. These neurons could be rescued by exogenous BDNF or neurotrophin-3, but not by other growth factors. Similar results were obtained with single-neuron microcultures, whereas microcultures derived from mutant mice lacking BDNF were unaffected by antisense oligonucleotides. Our results strongly support an autocrine role for BDNF in mediating the survival of a subpopulation of adult DRG neurons.

Exchange of a single amino acid interconverts the specific activity and gel mobility of human and rat ciliary neurotrophic factors

Human and rat ciliary neurotrophic factors (CNTF), which share 85% sequence identity, promote the survival of chicken embryo ciliary ganglia neurons in vitro, but display a 4-5-fold difference in specific activity. To explore the origin of this difference and gain insight into the structural organization of CNTF, we created chimeric proteins of these two species. Surprisingly, we found that the differences in two apparently unrelated properties, gel mobility and specific activity, resided in a single amino acid. Substituting arginine residue 63 of rat CNTF into the human sequence created a protein with the properties of rat CNTF. Conversely, substituting the human CNTF glutamine residue 63 into rat CNTF generated a protein with the properties of human CNTF. Binding experiments confirmed that the distinct specific activities of human and rat CNTF and their chimeras reside in structural differences among these ligands rather than species differences in their receptors. Alanine substitution (Q63A) had no effect on the properties of human CNTF, whereas the R63A substitution reduced both the gel mobility and the specific activity of rat CNTF. Finally, a Q95R substitution at a different position of human CNTF had no effect on its properties. These results demonstrate that Arg-63 is both specific and critical in determining the structural differences of human and rat CNTF.