Cumulative Activation of Akt and Consequent Inhibition of Glycogen Synthase Kinase-3 by Brain-Derived Neurotrophic Factor and Insulin-Like Growth Factor-1 in Cultured Hippocampal Neurons

Abstract

Brain-derived neurotrophic factor (BDNF) and insulin-like growth factor-1 (IGF-1) seem to play key roles in mediating neuronal plasticity in the hippocampus. In the current studies, we have used cultured hippocampal neurons to study possible interactions between the two growth factors in modulating neuronal signaling pathways. BDNF and IGF-1 were found to each effectively activate the neuroprotective Akt pathway, with the magnitude of activation being at least additive when cultures were simultaneously treated with supramaximal concentrations of peptides. Likewise, a cumulative inhibitory Akt-dependent phosphorylation of proapoptotic glycogen synthase kinase-3 was observed. Immunofluorescent studies demonstrated that a single population of neurons responded to BDNF and IGF-1. In contrast, the magnitude of BDNF-stimulated extracellular signal-regulated kinase (ERK) activation was found to be much greater than that of IGF-1-stimulated ERK, such that the difference in magnitude stimulated by BDNF in the presence and absence of IGF-1 did not reach statistical significance. Consistent with the observed agonist-stimulated activation of Akt, BDNF and IGF-1 were both found to act as neurotrophins, enhancing neuronal survival under low-insulin culture conditions. Maximal survival was achieved when both growth factors were present. These findings provide insight into the significance of multiple growth factors stimulating activation of ERK and Akt in the central nervous system. In some cases, the magnitude of activation required to elicit biological responses may be achieved only with a combination of compounds.

Interaction of FGF-2 with IGF-1 and BDNF in stimulating Akt, ERK, and neuronal survival in hippocampal cultures

“…Moreover, further increasing Akt by combining FGF-2 with either IGF-1 or BDNF led to no further enhancement of neuroprotection. This is in contrast to our previous findings that IGF-1 and BDNF cumulatively enhance cell survival, in addition to cumulatively activating Akt (Johnson-Farley et al, 2006).…”

“…We therefore sought to determine whether differences in the pattern of activation of these 2 kinases could account for the increased efficacy of FGF-2 vs. IGF-1 and BDNF in inducing neuroprotection. We have previously reported that maximal activation of ERK and Akt by IGF-1 and BDNF is stimulated by 100 ng/ml and 20 ng/ml concentrations, respectively (Johnson-Farley et al, 2006). The magnitude of IGF-1-and FGF-2-stimulated activation of Akt were found to be comparable (figure 4).…”

“…This is consistent with enhancement of neuronal survival and not proliferation of contaminating, non-neuronal cells. Surprisingly, treatment with 100 ng/ml IGF or 20 ng/ml BDNF, concentrations that we have previously found to maximally activate ERK and Akt (Johnson-Farley et al, 2006), stimulated only 17% and 12% increases in neuronal survival (figure 3B). FGF-2 is more similar to BDNF than IGF-1 in stimulating activation of Akt and ERK Akt and ERK have been shown to play major roles in mediating the neuroprotective actions of growth factors.…”

“…al., 2004;Zheng and Quirion, Correspondence should be addressed to Dr. Daniel S. Cowen at the Department of Psychiatry, UMDNJ-Robert Wood Johnson Medical School, 125 Paterson Street, Suite 2200, New Brunswick, NJ 08901;(732) and (732) . Email: cowends@umdnj.edu2004; Johnson-Farley et al, 2006). Similarly, BDNF has been shown to inhibit, apoptosis in hippocampal neurons Johnson-Farley et al, 2006).…”

“…Email: cowends@umdnj.edu2004; Johnson-Farley et al, 2006). Similarly, BDNF has been shown to inhibit, apoptosis in hippocampal neurons Johnson-Farley et al, 2006). Expression of FGF-2, a less well characterized growth factor, has also been reported to be increased by antidepressants in cortex and hippocampus (Mallei et al, 2002;Maragnolli et al, 2004).…”