The effect of constitutive over-expression of insulin-like growth factor 1 on the cognitive function in aged mice

The neurotrophic factor insulin-like growth factor (IGF)-1 promotes neurogenesis in the mammalian brain and provides protection against brain injury. However, studies regarding the effects of IGF-1 on cognitive function in aged mice remain limited. We investigated the effects of overexpression of IGF-1 specifically in neural stem cells of the hippocampal dentate gyrus on the recognitive function in 18-month-old transgenic mice. Immunohistocytochemistry and Nissl staining revealed the increased population of BrdU-positive cells as well as the upregulated expression of Nestin and neuronal nuclei (NeuN), respective markers for neural progenitors and neurons, in the hippocampus of the aged IGF-1 transgenic mice versus the wild-type, suggesting that IGF-1 overexpression promotes neurogenesis. In addition, the IGF-1 receptor (IGF-1R), the phosphorylation of Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) and extracellular signal-regulated kinase (ERK) were enhanced in the transgenic mice than in the wild-type. Transgenic mice also showed superior performance in the Morris water maze and step-down memory tests to their wild-type counterparts. Moreover, the learning and memory abilities of transgenic mice were significantly undermined with the blockage of CaMKII and ERK signaling pathway. Accordingly, our findings indicated that IGF-1 may mitigate the aged-associated cognitive decline via promoting neurogenesis in the hippocampus and activating CaMKII and ERK signaling by binding with IGF-1R.