How does type 2 diabetes modify the risk of Alzheimer's disease?

Type 2 diabetes (T2D) and Alzheimer's disease (AD) are both increasing exponentially worldwide. T2D has also been identified as one of 14 modifiable risk factors for dementia, but the mechanism is unknown. T2D could promote dementia via vascular or AD neuropathological changes, and mechanistic hypotheses include central insulin resistance and T2D's peripheral inflammation promoting central inflammation. Here we examine these different hypotheses by reviewing the recent literature in combination with re-analysis of post mortem brain tissue molecular data. Collectively, recent studies and single-cell transcriptomic data suggest that peripheral lipid anomalies and inflammation seen in T2D act together to reduce brain-blood barrier integrity, facilitating aberrant immune signaling between the periphery and the brain. The subsequent promotion of AD-specific microglial subtypes is the most likely mechanism linking T2D and AD. These AD-specific microglial subtypes may be reduced by therapeutic targeting of triggering receptor expressed on myeloid cells 2-apolipoprotein E signaling pathway.