IGF-II promotes neuroprotection and neuroplasticity recovery in a long-lasting model of oxidative damage induced by glucocorticoids

Abstract

Insulin-like growth factor-II (IGF-II) is a naturally occurring hormone that exerts neurotrophic and neuroprotective properties in a wide range of neurodegenerative diseases and ageing. Accumulating evidence suggests that the effects of IGF-II in the brain may be explained by its binding to the specific transmembrane receptor, IGFII/M6P receptor (IGF-IIR). However, relatively little is known regarding the role of IGF-II through IGF-IIR in neuroprotection. Here, using adult cortical neuronal cultures, we investigated whether IGF-II exhibits long-term antioxidant effects and neuroprotection at the synaptic level after oxidative damage induced by high and transient levels of corticosterone (CORT). Furthermore, the involvement of the IGF-IIR was also studied to elucidate its role in the neuroprotective actions of IGF-II. We found that neurons treated with IGF-II after CORT incubation showed reduced oxidative stress damage and recovered antioxidant status (normalized total antioxidant status, lipid hydroperoxides and NAD(P) H:quinone oxidoreductase activity). Similar results were obtained when mitochondria function was analysed (cytochrome c oxidase activity, mitochondrial membrane potential and subcellular mitochondrial distribution). Furthermore, neuronal impairment and degeneration were also assessed (synaptophysin and PSD-95 expression, presynaptic function and FluoroJade B® stain). IGF-II was also able to recover the long-lasting neuronal cell damage. Finally, the effects of IGF-II were not blocked by an IGF-IR antagonist, suggesting the involvement of IGF-IIR. Altogether these results suggest that, in or model, IGF-II through IGF-IIR is able to revert the oxidative damage induced by CORT. In accordance with the neuroprotective role of the IGF-II/IGF-IIR reported in our study, pharmacotherapy approaches targeting this pathway may be useful for the treatment of diseases associated with cognitive deficits (i.e., neurodegenerative disorders, depression, etc.).

Keywords: Insulin-like growth factor-II; Insulin-like growth factor-II receptor; Mitochondria; Neuroprotection; Oxidative stress; Synapsis.

Graphical abstract

Fig. 1

IGF-II restores REDOX imbalance produced by CORT treatment. Panel a shows the levels of the antioxidant TAS and panel b shows the level of the oxidative marker LOOH after different treatment conditions. Panel c shows the activity of the antioxidant NQO1. The data represent mean ± SEM from 3 to 4 independent experiments. (*p < 0.05 versus CO; &p < 0.05 versus WCORT-24; #p < 0.05 versus CORT-2; $p < 0.05 versus WCORT-24 + IGF-II).

Fig. 2

IGF-II restores mitochondrial function, including MMP/distribution and COX activity in neurons treated with CORT. Panels a, b, c and d represent the confocal microscopy images corresponding to JC1 staining in CO cells (a), WCORT-24 cells (b), WCORT-24 + IGF-II cells (c) and WCORT-24 + IGF-II + JB1 cells (d); green arrows indicate less polarized mitochondria, red arrows indicate more polarized mitochondria. Panel e represents the MMP in the soma and panel f represents depolarized mitochondria in non soma compartment, valinomycin was used as a control to completely collapse MMP. Panel g represents the COX activity in neuronal cells. Data on panel e, f and g represent mean ± SEM from 3 to 4 independent experiments. (*p < 0.05 versus CO; &p < 0.05 versus WCORT-24; #p < 0.05 versus CORT-2). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).

Fig. 3

IGF-II modulates synaptic protein expression. Panel a shows the modulation of the expression of synaptophysin by IGF-II; quantified data is shown on the left. Panels b, c, d and e represent the confocal microscopy images corresponding to the immunocytochemistry of PSD-95 in CO cells (b), WCORT-24 cells (c), WCORT-24 + IGF-II cells (d) and WCORT-24 + IGF-II + JB1 cells (e). Panel f shows the quantitative analysis of immunocytochemistry of PSD-95. Quantitative data represent the mean ± SEM from 3 to 4 independent experiments. (*p < 0.05 versus CO; &p < 0.05 versus WCORT-24).

Fig. 4

IGF-II modulates synaptic function. Figure a show the time course of FM®1-43 loading/unloading used to study endocytosis and exocytosis. The loading process is monitored by dye uptake and the subsequent increase in neuronal fluorescent intensity (b); conversely exocytosis is examined by dye release and the subsequent decrease in fluorescent intensity (c represents the 1/t½ of unloading process). Quantitative data represent the mean±SEM from 3 to 4 independent experiments. (*p < 0.05 versus CO; &p < 0.05 versus WCORT-24).

Fig. 5

IGF-II improved the neurodegeneration induced by CORT measured by FJ staining. The data represent the mean±SEM from 3 to 4 independent experiments. (*p < 0.05 versus CO; &p < 0.05 versus WCORT-24).