Design and solution structure of functional peptide mimetics of nerve growth factor.

The C-D loop in nerve growth factor (NGF) is involved in binding to the NGF receptor, TrkA. It is flexible and adopts several different types conformations in different NGF crystal forms. We have previously shown that a small cyclic peptide derived from the C-D loop of NGF binds to the TrkA receptor by mimicking the structure of this loop. To understand structure-function relationships in NGF C-D loop mimetics, we have produced a series of peptides predicted to form different types of beta-turns. The peptides were tested for their ability to promote cell survival in serum-free medium and to induce TrkA tyrosine phosphorylation. NMR structural studies were used to determined the backbone conformation and the spatial orientation of side chains involved in binding to the TrkA receptor. Peptides that form type I or type gammaL-alphaR beta-turns were the most active. The variety of active loop conformations suggests that the mimetics (and NGF) accommodate the binding site on TrkA by an 'induced fit' mechanism. In agreement with this hypothesis, NMR relaxation measurements detected both fast and slow motion in the peptides. We also characterized a retro-inverso peptide derived from the NGF C-D loop. This D-amino acid cyclic peptide did not adopt a conformation homologous to the NGF C-D loop and was inactive. This may be representative of difficulties in producing structural and functional mimetics by retro-inverso schemes.

Genuine monovalent ligands of TrkA nerve growth factor receptors reveal a novel pharmacological mechanism of action.

Developing small molecule agonistic ligands for tyrosine kinase receptors has been difficult, and it is generally thought that such ligands require bivalency. Moreover, multisubunit receptors are difficult to target, because each subunit contributes to ligand affinity, and each subunit may have distinct and sometimes opposing functions. Here, the nerve growth factor receptor subunits p75 and the tyrosine kinase TrkA were studied using artificial ligands that bind specifically to their extracellular domain. Bivalent TrkA ligands afford robust signals. However, genuine monomeric and monovalent TrkA ligands afford partial agonism, activate the tyrosine kinase activity, cause receptor internalization, and induce survival and differentiation in cell lines and primary neurons. Monomeric and monovalent TrkA ligands can synergize with ligands that bind the p75 subunit. However, the p75 ligands used in this study must be bivalent, and monovalent p75 ligands have no effect. These findings will be useful in designing and developing screens of small molecules selective for tyrosine kinase receptors and indicate that strategies for designing agonists of multisubunit receptors require consideration of the role of each subunit. Last, the strategy of using anti-receptor mAbs and small molecule hormone mimics as receptor ligands could be applied to the study of many other heteromeric cell surface receptors.

A designed peptidomimetic agonistic ligand of TrkA nerve growth factor receptors.

A proteolytically stable small molecule beta-turn peptidomimetic, termed D3, was identified as an agonist of the TrkA neurotrophin receptor. D3 binds the Ig-like C2 region of the extracellular domain of TrkA, competes the binding of another TrkA agonist, affords selective trophic protection to TrkA-expressing cell lines and neuronal primary cultures, and induces the differentiation of primary neuronal cultures. These results indicate that a small beta-turn peptidomimetic can activate a tyrosine kinase neurotrophin receptor that normally binds a relatively large protein ligand. Agents such as D3 that bind the extracellular domain of Trk receptors will be useful pharmacological agents to address disorders where Trk receptors play a role, by targeting populations selectively.

A TrkA-selective, fast internalizing nerve growth factor-antibody complex induces trophic but not neuritogenic signals.

Nerve growth factor (NGF) is a neurotrophin that induces neuritogenic and trophic signals by binding to TrkA and/or p75 receptors. We report a comparative study of the binding, internalization, and biological activity of NGF versus that of NGF in association with an anti-NGF monoclonal antibody (mAb NGF30), directed against the C termini of NGF. NGF.mAb complexes do not bind p75 effectively but bind TrkA with high affinity. After binding, NGF. mAb complexes stimulate internalization faster and to a larger degree than NGF. NGF.mAb-induced activation of TrkA, Shc, and MAPK is transient compared with NGF-induced activation; yet NGF and NGF. mAb afford identical trophic responses. In contrast, NGF induces Suc-1-associated neurotrophic activating protein phosphorylation and neuritogenic differentiation, but NGF.mAb does not. Thus, an absolute separation of trophic and neuritogenic function is seen for NGF.mAb, suggesting that biological response modifiers of neurotrophins can afford ligands with selected activities.

Optimal nerve growth factor trophic signals mediated by synergy of TrkA and p75 receptor-specific ligands.

Nerve growth factor (NGF) receptor-mediated signaling was studied using specific monoclonal antibodies (mAbs) as ligands that discriminate between the receptors TrkA and p75. mAb-induced trophic signals were compared with the signals of the natural ligand NGF. In cells expressing TrkA but no p75 receptors (TrkA+ p75(-)), binding of TrkA with mAb 5C3 leads to optimal signals. In cells expressing both TrkA and p75 (TrkA+ p75(+)), binding of TrkA with mAb 5C3 leads to significant but suboptimal signals, and optimal trophic signals are obtained by concomitant binding of TrkA and p75 with mAbs 5C3 and MC192. In TrkA+ p75(+) cells, binding of anti-p75 mAb MC192 also enhances the trophic effect of suboptimal concentrations of NGF. In contrast, in cells expressing p75 receptors singly (TrkA- p75(+)), binding with mAb MC192 or NGF causes very limited or no trophic effects. Thus, the data support the hypothesis that unbound p75 may modulate TrkA trophic signals. Importantly, the data also demonstrate for the first time that in multireceptor systems appropriate combinations of anti-receptor mAbs can fully mimic the signals of a polypeptide growth factor.

Potent human p140-TrkA agonists derived from an anti-receptor monoclonal antibody.

Monoclonal antibody (mAb) 5C3 directed against human p140 TrkA is a structural and functional mimic of nerve growth factor (NGF) and an artificial receptor agonist. mAb 5C3 binds in the NGF-docking site and, like NGF, it promotes TrkA internalization, TrkA and phosphatidylinositol-3 kinase tyrosine phosphorylation, and increased transformation of TrkA-expressing fibroblasts. More important, mAb 5C3 protects human TrkA-expressing cells from apoptotic death in serum-free media. Interestingly, agonistic activity is observed with monomeric F(ab) 5C3 fragments. mAb 5C3 (Kd approximately 2 nM) was used to study features of ligand binding by TrkA and the distribution of TrkA protein in normal human brain.