Sarma, Vidur

Background: Age is the most common risk factor for Alzheimer’s disease (AD), a neurodegenerative disorder characterized by the hallmarks of toxic amyloid-β (Aβ) plaques and hyperphosphorylated tau tangles. Moreover, sub-physiological brain insulin levels have emerged as a pathological manifestation of AD. Objective: Identify age-related changes in the plasma disposition and blood-brain barrier (BBB) trafficking of Aβ peptides and insulin in mice. Methods: Upon systemic injection of 125I-Aβ40, 125I-Aβ42, or 125I-insulin, the plasma pharmacokinetics and brain influx were assessed in wild-type (WT) or AD transgenic (APP/PS1) mice at various ages. Additionally, publicly available single-cell RNA-Seq data [GSE129788] was employed to investigate pathways regulating BBB transport in WT mice at different ages. Results: The brain influx of 125I-Aβ40, estimated as the permeability-surface area product, decreased with age, accompanied by an increase in plasma AUC. In contrast, the brain influx of 125I-Aβ42 increased with age, accompanied by a decrease in plasma AUC. The age-dependent changes observed in WT mice were accelerated in APP/PS1 mice. As seen with 125I-Aβ40, the brain influx of 125I-insulin decreased with age in WT mice, accompanied by an increase in plasma AUC. This finding was further supported by dynamic single-photon emission computed tomography (SPECT/CT) imaging studies. RAGE and PI3K/AKT signaling pathways at the BBB, which are implicated in Aβ and insulin transcytosis, respectively, were upregulated with age in WT mice, indicating BBB insulin resistance. Conclusion: Aging differentially affects the plasma pharmacokinetics and brain influx of Aβ isoforms and insulin in a manner that could potentially augment AD risk.

AbstractBackgroundDecreased brain insulin levels exacerbate cognitive decline in AD. Insulin in the brain is derived from systemic circulation via the blood‐brain barrier (BBB). We hypothesize that type II diabetes (T2D) sequelae and Aβ peptide exposure disrupt insulin signaling at the BBB and inhibit insulin delivery to brain. Further, we propose insulin signaling defects at the BBB contribute to Aβ accumulation in AD brain.MethodsThe following studies were performed in wild‐type (WT) mice on regular chow (RC) diet, WT mice on high fat (HF) diet (manifest insulin resistance), APP/PS1 transgenic mice on RC diet (overexpress Aβ), and APP/PS1 mice on HF diet (overexpress Aβ + insulin resistance). After femoral injection of 125I‐insulin or 125I‐Aβ42, the brain accumulation was monitored between 0‐40 min by dynamic SPECT/CT imaging. The brain influx clearance was estimated by the slope obtained from Gjedde‐Patlak graphical analysis. Cerebral microvessels were harvested, and reverse phase protein array (RPPA) was performed to examine insulin signaling changes. Differentially expressed targets were subsequently confirmed by western blot. The brain influx of 125I‐insulin and 125I‐Aβ42 were further assessed in WT‐RC mice after internal carotid infusion with AG1024, a kinase inhibitor of the insulin receptor (IR) and insulin‐like growth factor receptor (IGF‐1R).ResultsCompared to WT‐RC, 125I‐insulin influx was decreased in WT‐HF and APP/PS1‐RC, and was further decreased in APP/PS1‐HF (4.1, 3.4, 3.5 and 2.6*10‐4 mL/min, respectively). Compared to WT‐RC, 125I‐Aβ42 influx was increased in WT‐HF and APP/PS1‐RC (8.6, 28 and 23*10‐4 mL/min, respectively). RPPA analysis revealed global disruptions in BBB insulin signaling pathways. Western blots confirmed reduced expression of IR‐β, p‐AKT and p‐GSK3β in WT‐HF and APP/PS1‐RC compared to WT‐RC. Moreover, the largest decreases were observed in APP/PS1‐HF. Infusion with AG1024 was shown to decrease 125I‐insulin influx, but increase 125I‐Aβ42 influx.ConclusionsBoth T2D and AD mice exhibited decreased brain influx of insulin and increased influx of Aβ42. This was associated with altered expression/activity of insulin signaling kinases at the BBB. Further, IR and/or IGF‐1R kinase activity were shown to differentially regulate BBB trafficking of insulin and Aβ42. Thus, BBB insulin signaling is important for delivering insulin to brain and restricting pathological uptake of Aβ.

Impaired brain clearance of amyloid-beta peptides (Aβ) 40 and 42 across the blood–brain barrier (BBB) is believed to be one of the pathways responsible for Alzheimer’s disease (AD) pathogenesis. Hyperinsulinemia prevalent in type II diabetes was shown to damage cerebral vasculature and increase Aβ accumulation in AD brain. However, there is no clarity on how aberrations in peripheral insulin levels affect Aβ accumulation in the brain. This study describes, for the first time, an intricate relation between plasma insulin and Aβ transport at the BBB. Upon peripheral insulin administration in wild-type mice: the plasma clearance of Aβ40 increased, but Aβ42 clearance reduced; the plasma-to-brain influx of Aβ40 increased, and that of Aβ42 reduced; and the clearance of intracerebrally injected Aβ40 decreased, whereas Aβ42 clearance increased. In hCMEC/D3 monolayers (in vitro BBB model) exposed to insulin, the luminal uptake and luminal-to-abluminal permeability of Aβ40 increased and that of Aβ42 reduced; the abluminal-to-luminal permeability of Aβ40 decreased, whereas Aβ42 permeability increased. Moreover, Aβ cellular trafficking machinery was altered. In summary, Aβ40 and Aβ42 demonstrated distinct distribution kinetics in plasma and brain compartments, and insulin differentially modulated their distribution. Cerebrovascular disease and metabolic disorders may disrupt this intricate homeostasis and aggravate AD pathology.