Developing models of impaired insulin signalling in astrocytes and implications for neuronal support

Abstract: The insulin/insulin-like growth factor-1 signalling (IIS) pathways are implicated in longetivity and in Alzheimer’s disease (AD). We have previously reported that IIS is dysregulated in astrocytes with progression of AD neuropathology. To investigate the impact of altered IIS we have developed models to impair these signalling pathways (Garwood et al., 2015) and, using a novel human astrocyte-neuron co-culture system, have assessed whether loss of insulin or IGF1 signalling affects astrocyte function, specifically neuronal support.
Human primary astrocytes were treated with (1) an IGF1R-specific monoclonal antibody [MAB391] or (2) insulin and fructose in combination [IF] to impair IGF1 or insulin signalling respectively. MAB391- or IF-treated human astrocytes were co-cultured with Lund Human Mesencephalic cells (LUHMES) and neurite outgrowth assays were performed. Astrocytes provided contact-mediated support for neurite outgrowth which was not affected by impaired IGF1 or insulin signalling. Co-cultures were then further challenged to model other changes that occur in AD brain; IGF1-impaired astrocytes were treated with hydrogen peroxide (H2O2) to model oxidative stress; insulin-impaired astrocytes were treated with beta-amyloid (Aβ).
IGF1-impaired astrocytes were less able to protect neurons under conditions of oxidative stress and microarray analysis of these astrocytes identified changes in transcripts involved in astrocyte energy metabolism, particularly those related to complex I assembly. Loss of complex I activity in MAB391-treated astrocytes validated these findings. Experiments looking at the impact of impaired insulin signalling and Aβ on astrocytes are ongoing.

In summary, reduced IGF1 signalling in astrocytes impairs their support for neurons under conditions of stress and is associated with defects in the mitochondrial respiratory chain. These results further our understanding of the involvement of these pathways in disease progression and suggest that restoring IGF1 signalling to optimum levels in astrocytes may preserve their energy metabolism and maintain neuronal support during ageing and neurodegeneration.