Disruptions in glucose and amyloid-beta transport in mouse models manifesting metabolic syndrome

Studies in humans and murine models have pointed towards a possible link between metabolic syndrome, which shows insulin resistance and metabolic dysregulation, and Alzheimer’s disease (AD) pathology marked by amyloid-beta (Aβ) accumulation at the BBB and hypometabolism in the brain. Yet, the underlying biological mechanisms by which metabolic syndrome affects these pathological changes in AD brain remain unknown. We hypothesized that insulin resistance is responsible for alterations in blood-brain barrier (BBB) transport of Aβ peptides and glucose. This hypothesis was tested by employing radiolabeled tracers ( ¹²⁵ I-Aβ40, ¹²⁵ I-Aβ42, and ¹⁸ F-FDG) in high-fat diet (HFD)-fed mouse models that manifest metabolic syndrome. Further, we assessed alterations in the expression of various molecular mediators within the brain microcapillaries harvested from both low-fat diet (LFD)-fed and HFD-fed mice. As expected, our studies show that HFD- fed mice developed peripheral insulin resistance and obesity. In addition, HFD-fed mice demonstrated an increase in the influx rate of Aβ peptides and a reduction in ¹⁸ F-FDG (a glucose surrogate) influx rate at the BBB compared to LFD-fed mice. These transport changes are associated with increase in the BBB endothelial expression of RAGE (receptor to traffic Aβ from plasma-to-brain) and a reduction of GLUT1 (glucose transporter) expression in HFD-fed mice compared to LFD-fed mice. Moreover, disruption in insulin signaling, as indicated by reduced pAKT and pERK expression, was observed in HFD-fed mice. Inhibiting AKT or ERK phosphorylation with specific inhibitors resulted in similar changes in Aβ and glucose uptake in polarized BBB endothelial cell monolayers in vitro. These results indicate that HFD induced metabolic syndrome may lead to BBB dysfunction, characterized by increased plasma-to-brain Aβ trafficking and diminished glucose transport at the BBB, thereby aggravating the expression of AD pathological hallmarks.