An Analog of a Dipeptide-Like Structure of FK506 Increases Glial Cell Line-Derived Neurotrophic Factor Expression through cAMP Response Element-Binding Protein Activated by Heat Shock Protein 90/Akt Signaling Pathway

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Abstract
Glial cell line-derived neurotrophic factor (GDNF) is an important neurotrophic factor that has therapeutic implications for neurodegenerative disorders. We previously showed that leucine-isoleucine (Leu-Ile), an analog of a dipeptide-like structure of FK506 (tacrolimus), induces GDNF expression bothinvivoandinvitro. In this investigation, we sought to clarify the cellular mechanisms underlying the GDNF-inducing effect of this dipeptide. Leu-Ile transport was investigated using fluorescein isothiocyanate-Leu-Ile in cultured neurons, and the results showed the transmembrane mobility of this dipeptide. By liquid chromatography-mass spectrometry and quartz crystal microbalance assay, we identified heat shock cognate protein 70 as a protein binding specifically to Leu-Ile, and molecular modeling showed that the ATPase domain is the predicted binding site. Leu-Ile stimulated Akt phosphorylation, which was attenuated significantly by heat shock protein 90 (Hsp90) inhibitor geldanamycin (GA). Moreover, enhanced interaction between phosphorylated Akt and Hsp90 was detected by immunoprecipitation. Leu-Ile elicited an increase in cAMP response element binding protein (CREB) phosphorylation, which was inhibited by GA, indicating that CREB is a downstream target of Hsp90/Akt signaling. Leu-Ile elevated the levels of GDNF mRNA and protein expression, whereas inhibition of CREB blocked such effects. Leu-Ile promoted the binding activity of phosphorylated CREB with cAMP response element. These findings show that CREB plays a key role in transcriptional regulation of GDNF expression induced by Leu-Ile. In conclusion, Leu-Ile activates Hsp90/Akt/CREB signaling, which contributes to the upregulation of GDNF expression. It may represent a novel lead compound for the treatment of dopaminergic neurons or motoneuron diseases.