Dimeric dipeptide mimetics of the nerve growth factor Loop 4 and Loop 1 activate TRKA with different patterns of intracellular signal transduction

BACKGROUND: This study aimed at developing nerve growth factor (NGF) mimetics that selectively activate specific biological signals and, as a result, lack the side effects of the full-length protein. Two dimeric dipeptides, bis-(N-aminocaproyl-glycyl-L-lysine) hexamethylenediamide (GK-6) and bis(N-succinyl-L-glutamyl-L-lysine) hexamethylenediamide (GK-2), were designed based on the most exposed outside fragments of NGF, namely, the loop 1 and loop 4 β-turn sequences, respectively. These dipeptides exhibited neuroprotective activity in vitro at micro-nanomolar concentrations. RESULTS: Studies on the mechanism of action revealed that both compounds elevate the level of tyrosine kinase A (TrkA) receptor phosphorylation and that they each have different postreceptor signaling patterns. GK-6 increases the levels of extracellular signal-regulated kinase (ERK) and AKT kinase phosphorylation, whereas GK-2 only increases the level of AKT phosphorylation. Apart from the neuroprotective activity, GK-6 promoted differentiation in PC12 cells, whereas GK-2 did not. Furthermore, it was established that the neuroprotective activity of GK-2 was completely abolished by a selective inhibitor of phosphatidylinositol 3-kinase (LY294002) but not by a specific inhibitor of mitogen-activated protein kinases MEK1 and MEK2 (PD98059). In vivo experiments demonstrated that GK-2 did not induce hyperalgesia, which is one of the primary adverse effects of NGF. By contrast, GK-6 produced a significant decrease in the pain threshold of rats as determined by the tail flick test. CONCLUSION: The data obtained suggest that dimeric dipeptide NGF mimetics are promising candidates in the development of pharmacological agents with NGF-like activity that are free of the main side effect of NGF.