A systematic review of in vivo brain insulin resistance biomarkers in humans

Abstract

Type 2 diabetes mellitus (T2DM) is associated with an elevated risk of dementia, prompting interest into the concept of brain-specific insulin resistance. However, the brain's reliance on insulin-independent glucose transporters complicates attempts to measure in vivo brain insulin resistance using the definition of system-wide insulin resistance, which is based on glucose-insulin interactions. In this review, we explore three available biomarkers for evaluating in vivo brain-specific insulin resistance in humans: (1) correlating systemic insulin resistance with brain function, (2) examining functional brain changes after the administration of intranasal insulin, and (3) quantifying insulin signalling proteins in neuronally enriched blood-derived extracellular vesicles. Integrating evidence from these three approaches tentatively suggests for the first time that a comprehensive assessment of the brain's default mode network (DMN), combining these methodologies within a single study, may offer a useful biomarker to quantify in vivo brain-specific insulin resistance in humans. Correlating DMN responses to concentrations of pY-IRS-1 in blood-derived extracellular vesicles would corroborate evidence for a brain-specific biomarker and provide a scalable approach to detecting brain-specific insulin resistance in humans. This advancement would enable in vivo evaluations of insulin resistance in the central nervous system, akin to the precise measurements of systemic insulin resistance seen in T2DM. An established and clearly defined biomarker of in vivo brain insulin resistance in humans would permit further investigation into the links between diabetes and dementia, ultimately bolstering support for secondary dementia prevention by identifying those at higher risk for cognitive decline.