Genuine Monovalent Ligands of TrkA Nerve Growth Factor Receptors Reveal a Novel Pharmacological Mechanism of Action

Abstract

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.

Small, Nonpeptide p75NTRLigands Induce Survival Signaling and Inhibit proNGF-Induced Death

“…This is consistent with studies showing no effects on survival of monomeric p75 NTR ligands, including monovalent Fabs (Maliartchouk et al, 2000b) and monomeric cyclic peptides (Longo et al, 1997), whereas related bivalent forms in each study promoted cell survival. However, it is unlikely that these monomeric ligands engage the receptor in the same way as the natural ligands.…”

Design of Potent Peptide Mimetics of Brain-derived Neurotrophic Factor

“…Similar experiments using known trkB inhibitors, such as K252a (43), could be used to confirm the involvement of trkB. The tricyclic dimeric peptide described here is the most potent member of a growing class of small molecule neurotrophin mimetics, including a trkA mimetic developed from a small cyclic peptide based on loop 4 of NGF (44), and a series of dimeric ␤-turn mimetics that act as agonists of trkC (45). To date, the neurotrophins have failed to achieve clinical success in the treatment of neurodegenerative diseases.…”

“…1B). The distance between ␣-carbon atoms of Val 44 -Val 44 and Ser 45 -Ser 45 fell within the maximal preferred distance of Cys-to-Cys constraint (6.6 Å as determined by conformational analyses). Incorporation of a Cys-to-Cys dimerizing linkage at these two positions yielded the bicyclic dimers 3 and 4 (Table I), which displayed relatively low r.m.s.…”

Small molecule BDNF mimetics activate TrkB signaling and prevent neuronal degeneration in rodents

“…Hence whether such peptides can function as ligands that activate TrkB remains to be determined. Unlike TrkA and TrkC, for which there is evidence that "monovalent" cyclic peptidomimetics can exhibit small degrees of agonist activity (55,56), for TrkB only "dimeric" peptides have as yet been found to demonstrate trophic activity. This finding is consistent with the classical model in which the BDNF dimer is thought to induce dimerization leading to activation (autophosphorylation) of TrkB.…”