Gene dosage determines the behavioural phenotype in the 5xFAD mouse model of Alzheimer’s disease: analysis of the contribution of brain insulin signalling and gut microbiota

Background: There are many hypotheses about the neuropathological origin of sporadic Alzheimer’s disease (AD). The amyloid cascade hypothesis remains the most widely accepted by the scientific community, although the role of amyloid-β (Aβ) as the sole cause of neuropathological features of the non-inherited form of AD is questionable. Methods: The 5xFAD mouse model shows amyloid plaques formation and cognitive impairment from 4 months of age. However, no attention is paid to the effect of genetic load, heterozygous (HTZ) versus homozygous (HZ) condition, on the behaviour, histology, metabolic physiology, and composition of gut microbiota. We studied the phenotype of HTZ and HZ mice of both sexes at 4-6 months (early disease onset) and 10-12 months of age (late disease onset). Results: We described that from 4-6 months old, both HTZ and HZ mice exhibited hippocampal impairment caused by accumulation of Aβ 40 and Aβ 42 , increased neuroinflammatory markers and altered PI3K-Akt pathway. However, only HZ mice showed cognitive impairment in the Y-maze test and the Morris water maze. In addition, dysregulation of insulin-linked metabolic pathways was detected at the peripheral level between HTZ and HZ mice. This striking difference between behavioural, immunopathological and physiological phenotypes extends to the microbiota in terms of Firmicutes / Bacteroidetes ratio, being most pronounced in HZ mice at 10-12 months old. Conclusions: Taken together, our results suggest that Aβ aggregation may not be the sole cause of worsening cognitive impairment, but that other factors such as PI3K-Akt, insulin-linked metabolic pathways, and microbiota composition could be influencing disease progression, allowing for alternative disease-modifying interventions.