AAV9-Syn-Cre

Reduced insulin-like growth factor 1 (IGF-1) is associated with cognitive impairment and increased risk of neurodegenerative diseases in advanced age. The researchers hypothesized that IGF-1 continues to promote the structure and function of hippocampal neurons after development and therefore, loss of IGF-1 signaling in adult neurons would result in impaired spatial learning and memory. To test this, the IGF-1 receptor (IGF-1R) was genetically targeted in hippocampal neurons of adult male and female mice. Male mice deficient in neuronal IGF-1R exhibited impairments in spatial learning as evidenced by increased path length and errors in a radial arm water maze. There were no differences in learning and memory in female mice. Golgi-Cox staining revealed a reduction in the number of dendrites in neurons in the CA1 region of the hippocampus of male mice. Decreased MAPK and increased ROCK activity were also observed in these tissues. In vitro studies showed that the impaired neurite outgrowth due to inhibition of IGF-1R signaling could be rescued by pharmacological inhibitors of ROCK. However, ROCK inhibition did not fully rescue learning impairments or synaptic nuclei number in mice deficient in neuronal IGF-1R. Altogether, these results suggest that IGF-1 continues to support spatial learning and memory and neuronal structure in adulthood.

Hippocampal neuron-specific IGF-1R deletion (*IGFR-KO) was induced in male and female igfrf/f mice at 3-4 months of age by stereotaxic injections of Cre recombinase encoding viral vectors (AAV9-Syn-Cre or control AAV9-Syn-GFP) (Figure 1A). IGF-1R was reduced by 39.6% in these neuron-specific *IGFR-KO male and female mice compared with GFP-transduced controls (*IGFR-WT) (Figure 1B). No significant differences in growth hormone receptor (GHR), insulin receptor (InsR), or IGF-1 expression were observed in the hippocampus of *IGFR-KO mice (Figure 1B). Two to three months after knockout, spatial learning/memory was assessed in the radial arm water maze. Both *IGFR-WT and *IGFR-KO mice learned how to find the escape platform over the three-day training phase, as evidenced by a reduction in the total path length to the escape platform (Figure 1C) and a reduction in the total number of errors in finding the platform (Figure 1E). However, *IGFR-KO mice went further and made more errors than controls throughout the learning phase (Figure 1C and E). When memory extinction and relearning were assessed during the reversal phase of the maze, male *IGFR-KO mice again traveled significantly more and made more errors than controls (Figure 1D and F).

Figure 1. Adult IGF-1R Signaling in Neurons Regulates Spatial Learning and Memory in Male Mice. (Hayes C A, et al., 2021)