Caspase activation and inflammation observed in the olfactory bulb of APOE4 transgenic mice

Abstract

Background Alzheimer disease (AD) is a chronic neurodegenerative disorder that affects millions of individuals worldwide. Olfactory dysfunction is a common symptom of several neurological disorders including AD. Studying the mechanisms underlying the olfactory dysfunction may lead to the discovery of biomarkers and/or treatments for neurodegenerative diseases. Objectives: To characterize the olfactory system in a murine model expressing a genetic risk factor for AD. We investigated olfactory brain region atrophy as well as caspase expression and tissue inflammation in the APOE4 mouse model of AD. Method We assessed the olfactory system in 12 month old wild type (WT) and APOE4 olfactory bulb (OB) for atrophy, caspase activation, markers of neurogenesis and inflammation. We used Western blotting, RT‐PCR and immunohistochemistry. Result There was a significant decrease in OB bulb weight in APOE4 mice compared to WT (p<0.01). There was a significant increase in the full length (p<0.05) form of caspase‐8 and in the p30 fragment (p<0.05) in the APOE4 OB. Moreover, a trend increase in caspase‐8 mRNA (p=0.08) and significant increase in caspase‐9 mRNA in the APOE4 OB vs. WT. Furthermore, there was a decrease in the expression of the mature neuronal marker NeuN in the OB of the APOE4 mice compared to WT. This was with both the 46kDa (p<0.05) and 48kDa (p<0.05) isoforms of NeuN. As atrophy was observed in the APOE4 OB we then assessed for inflammation. Using the IBA‐1 marker for microglia demonstrated that there was a significant increase in IBA‐1 expression (p<0.01) and a trend increased in IBA‐1 positive cells (p=0.054) in the APOE4 OB compared to WT Conclusion These data suggest that there are alterations in neurogenesis in the OB of human APOE4 transgenic mice in addition to inflammation. This corresponds well with what has been observed in human AD brain tissue. Caspase activities detected may play a role in the inflammation observed. Studying the olfactory system in the aging population may help to discover biomarkers in the early stages of AD