Aberrant glial activation and synaptic defects in CaMKII alpha-iCre and nestin-Cre transgenic mouse models

Current scientific research is driven by the ability to manipulate gene expression by utilizing the Cre/loxP system in transgenic mouse models. However, artifacts in Cre-driver mouse lines that introduce undesired effects and confound results are increasingly being reported. Here, we show aberrant neuroinflammation and synaptic changes in two widely used Cre-driver mouse models. Neuroinflammation in CaMKII alpha-iCre mice was characterized by the activation and proliferation of microglia and astrocytes in synaptic layers of the hippocampus. Increased GFAP and Iba1 levels were observed in hippocampal brain regions of 4-, 8- and 22-month-old CaMKII alpha-iCre mice compared to WT littermates. Synaptic changes in NMDAR, AMPAR, PSD95 and phosphorylated CaMKII alpha became apparent in 8-month-old CaMKII alpha-iCre mice but were not observed in 4-month-old CaMKII alpha-iCre mice. Synaptophysin and synaptoporin were unchanged in CaMKII alpha-iCre compared to WT mice, suggesting that synaptic alterations may occur in excitatory postsynaptic regions in which iCre is predominantly expressed. Finally, hippocampal volume was reduced in 22-month-old CaMKII alpha-iCre mice compared to WT mice. We tested the brains of mice of additional common Cre-driver mouse models for neuroinflammation; the nestin-Cre mouse model showed synaptic changes and astrocytosis marked by increased GFAP+astrocytes in cortical and hippocampal regions, while the original CaMKII alpha-Cre T29-1 strain was comparable to WT mice. The mechanisms underlying abnormal neuroinflammation in nestin-Cre and CaMKII alpha-iCre are unknown but may be associated with high levels of Cre expression. Our findings are critical to the scientific community and demonstrate that the correct Cre-driver controls must be included in all studies using these mice.