Calbindin-D28K expression induced by glial cell line-derived neurotrophic factor in substantia nigra neurons dependent on PI3K/Akt/NF-κB signaling pathway

Calbindin-D28K is a calcium-binding protein in neuronal cytoplasm, which has the capability to protect neurons from degeneration. It was reported that glial cell line-derived neurotrophic factor (GDNF) increased calbindin-D28K expression in dopaminergic neurons in vitro. It was observed in our research that GDNF also enhanced the expression of calbindin-D28K in adult rat substantia nigra neurons in vivo. To investigate the intracellular signaling pathways underlying the calbindin-D28K expression induced by GDNF, immunoblot and immunoprecipitation analyses were performed in our present study. Our results showed that injection of GDNF alone into substantia nigra of an adult rat brain increased the calbindin-D28K expression; meanwhile, the phosphorylation level of protein kinase B (Akt) and extracellular signal-regulated kinase 1/2 (ERK1/2) increased. However, the calbindin-D28K expression induced by GDNF was specifically blocked by the inhibitor of phosphatidylinositol 3-kinase (PI3K), but the inhibitor of ERK1/2 did not block the calbindin-D28K expression. Furthermore, GDNF administration also caused the nuclear factor κB (NF-κB/p65), to translocate from cytoplasm into the nucleus, and the inhibitor of PI3K effectively blocked the translocation. Immunoprecipitation assay results further demonstrated that it was the p65/p52 complex of NF-κB, rather than the p65/p50 complex that translocated into the neuronal nucleus. The calbindin-D28K expression induced by GDNF was also inhibited when the NF-κB signaling pathway was blocked by Helenalin. These results described a novel mechanism by which the activation of PI3K/Akt→NF-κB (p65/p52) signaling pathway could play a role in the calbindin-D28K expression induced by GDNF.