Insulin‐like growth factor 1 gene transfer for sporadic Alzheimer's disease: New evidence for trophic factor mediated hippocampal neuronal and synaptic recovery‐based behavior improvement

Sporadic Alzheimer's disease (sAD) is the most prevalent neurodegenerative disorder with no cure. Patients typically suffer from cognitive impairment imprinted by irreversible neocortex and hippocampal degeneration with overt synaptic and neuron dysfunction. Insulin‐like growth factor 1 (IGF1) has proven to be a potent neuroprotective molecule in animal models of age‐related neurodegeneration. In this regard, adenoviral gene transfer aiming at IGF1 brain overexpression has been hitherto an underexplored approach for the sAD treatment. We postulated enhanced IGF1 signaling in the brain as a restorative means in the diseased brain to revert cognitive deficit and restore hippocampal function. We implemented recombinant adenovirus mediated intracerebroventricular IGF1 gene transfer on the streptozotocin (STZ) induced sAD rat model, using 3‐month‐old male Sprague Dawley rats. This approach enhanced IGF1 signaling in the hippocampus and dampened sAD phosphorylated Tau. We found a remarkable short‐term improvement in species‐typical behavior, recognition memory, spatial memory, and depressive‐like behavior. Histological analysis revealed a significant recovery of immature hippocampal neurons. We additionally recorded an increase in hippocampal microglial cells, which we suggest to exert anti‐inflammatory effects. Finally, we found decreased levels of pre‐ and postsynaptic proteins in the hippocampus of STZ animals. Interestingly, IGF1 gene transfer increased the levels of PSD95 and GAD65/67 synaptic markers, indicating that the treatment enhanced the synaptic plasticity. We conclude that exogenous activation of IGF1 signaling pathway, 1 week after intracerebroventricular STZ administration, protects hippocampal immature neurons, dampens phosphorylated Tau levels, improves synaptic function and therefore performs therapeutically on the sAD STZ model. Hence, this study provides strong evidence for the use of this trophic factor to treat AD and age‐related neurodegeneration.