Hyperglycemia modulates extracellular amyloid-β concentrations and neuronal activity in vivo

Abstract

Epidemiological studies show that patients with type 2 diabetes (T2DM) and individuals with a diabetes-independent elevation in blood glucose have an increased risk for developing dementia, specifically dementia due to Alzheimer's disease (AD). These observations suggest that abnormal glucose metabolism likely plays a role in some aspects of AD pathogenesis, leading us to investigate the link between aberrant glucose metabolism, T2DM, and AD in murine models. Here, we combined two techniques – glucose clamps and in vivo microdialysis – as a means to dynamically modulate blood glucose levels in awake, freely moving mice while measuring real-time changes in amyloid-β (Aβ), glucose, and lactate within the hippocampal interstitial fluid (ISF). In a murine model of AD, induction of acute hyperglycemia in young animals increased ISF Aβ production and ISF lactate, which serves as a marker of neuronal activity. These effects were exacerbated in aged AD mice with marked Aβ plaque pathology. Inward rectifying, ATP-sensitive potassium (K(ATP)) channels mediated the response to elevated glucose levels, as pharmacological manipulation of K(ATP) channels in the hippocampus altered both ISF Aβ levels and neuronal activity. Taken together, these results suggest that K(ATP) channel activation mediates the response of hippocampal neurons to hyperglycemia by coupling metabolism with neuronal activity and ISF Aβ levels.

Figure 3. Pharmacological manipulation of K_ATP channels blocks hyperglycemia-induced increases in ISF Aβ.

(A) In 18-month-old APP/PS1 mice, glucose clamps increase ISF Aβ. However, when diazoxide, a K_ATP agonist, is given via reverse microdialysis during hyperglycemia, the increase in ISF Aβ is blocked (38.84% ± 3.5% vs. 9.33% ± 2.1% increase). (B) Treatment with diazoxide decreased hippocampal ISF lactate in 18-month-old APP/PS1 mouse brains during hyperglycemia, suggesting decreased neuronal activity coincides with decreased ISF Aβ (11.8% ± 3.6% vs. –4.043% ± 3.4%). Data represent mean ± SEM. For all analyses, n = 6–7 mice/group, *P < 0.05, ***P < 0.001 using 2-way ANOVA.

Figure 2. Modulation of hippocampal K_ATP channels affects ISF Aβ and lactate in vivo.

(A) Glibenclamide, a K_ATP antagonist, was given via reverse microdialysis and increased ISF Aβ in a dose-dependent manner, with a maximal increase of 36.2% ± 3.2% at 100 μM. The left panel represents a time course of the 100 μM dose, while the right demonstrates the dose-dependent effects of glibenclamide. (B) ISF lactate increased in a dose-dependent manner, with a maximal increase of 73.3-5% ± 19.8%. The left panel represents a time course of the 100 μM dose, while the right illustrates the dose-dependent effects of glibenclamide. (C) Diazoxide, a K_ATP agonist, did not alter ISF Aβ levels. The left panel demonstrates a time course of the 300 μM dose of diazoxide, where the right shows that diazoxide does not affect ISF Aβ at any dose. (D) Diazoxide (300 μM) decreased ISF lactate by 22.5% ± 4.3% after administration. The left panel demonstrates the time course of the 300 μM dose of diazoxide, while the right shows a dose response of diazoxide. Data represent mean ± SEM. For all analyses, n = 5–7 mice/group per glibenclamide dose and n = 4–5 mice/group per diazoxide dose. *P < 0.05, **P < 0.01, ***P < 0.001 using a 1-way ANOVA.

Figure 1. Hyperglycemia increases ISF glucose and Aβ levels in the APP/PS1 hippocampus in vivo.

(A) Blood glucose levels in 3-month-old APP/PS1 mice (n = 6–8 mice/group) during fasted baseline and hyperglycemic clamp. (B) ISF glucose levels increased from 0.158 ± 0.004 to 0.306 ± 0.011 mmol/l during hyperglycemia in 3-month-old APP/PS1 mice (n = 6–8 mice/group). (C) Hyperglycemia increased ISF Aβ levels by 24.5% ± 3.8% in 3-month-old APP/PS1 mice during and after clamp (n = 6–7 mice/group). (D) ISF Aβ correlates with ISF glucose during hyperglycemia (n = 6 mice, Pearson’s r = 0.6032; P < 0.01). (E) ISF glucose levels increased from 0.153 ± 0.003 to 0.397 ± 0.0149 mmol/l during hyperglycemia in 18-month-old APP/PS1 mice (n = 6–7 mice/group). (F) Hyperglycemia increased ISF Aβ levels by 38.8% ± 5.3% in 18-month-old APP/PS1 mice, during and after glucose clamps (n = 6–7 mice/group). (G) Hyperglycemia increases ISF lactate, a marker of neuronal activity (n = 7 mice/group). (H) Compound E decreases ISF Aβ during hyperglycemia and does not alter ISF Aβ half-life, demonstrating hyperglycemia alters Aβ production, not Aβ clearance. Data represent mean ± SEM. Significance denoted *P < 0.05, **P < 0.01, and ***P < 0.001 using 2-way ANOVA (C and F) or Student’s t tests (A, B, E, G, and H).