Hyperleucinemia causes hippocampal retromer deficiency linking diabetes to Alzheimer's disease

Abstract

Type 2 diabetes (T2D) is a major risk factor for late-onset Alzheimer's disease (AD). A variety of metabolic changes related to T2D (e.g. hyperinsulinemia, hyperglycemia, and elevated branched-chain amino acids) have been proposed as mechanistic links, but the basis for this association remains unknown. Retromer-dependent trafficking is implicated in the pathogenesis of AD, and two key retromer proteins, VPS35 and VPS26, are deficient in the hippocampal formation of AD patients. We characterized VPS35 levels in five different mouse models of T2D/obesity to identify specific metabolic factors that could affect retromer levels in the brain. Mouse models in which hyperleucinemia was present displayed hippocampus-selective retromer deficiency. Wild-type lean mice fed a high leucine diet also developed hippocampal-selective retromer deficiency, and neuronal-like cells grown in high ambient leucine had reduced retromer complex proteins. Our results suggest that hyperleucinemia may account, in part, for the association of insulin resistance/T2D with AD.

Keywords: Alzheimer's disease; Diabetes; Obesity; Retromer.

Fig. 1

Deficiency of retromer protein levels observed specifically in the hippo-campus of mouse models of T2D/obesity with elevations in plasma leucine. (A, B) VPS35 protein levels in the hippocampus (A) and cerebellum (B) of mouse models of T2D/obesity as determined by Western blot. Data are shown as % of WT/control expression normalized to actin levels (mean ± SEM; n = 5). **P < 0.01, ***P < 0.001. Gly-APN indicates transgenic overexpression of Gly-Adiponectin in both ob/ob and +/+ animals (mean ± SEM; n = 10). (C) Circulating plasma leucine concentrations in leptin signaling-deficient mice compared to controls with normal leptin signaling (mean ± SEM; n = 5). *P < 0.05, **P < 0.01 (vs. paired control).

Fig. 2

Physiological measurements and retromer protein expression in control vs. HLD-fed mice (white vs. black symbols/bars, respectively). (A) Growth curves, (B) glucose tolerance (C) plasma leucine levels, and (D) brain region-specific retromer protein levels are shown for control and HLD-fed mice (mean ± SEM, n = 8 per feeding regimen). *P < 0.05, **P < 0.01, ***P < 0.001.

Fig. 3

Dose-dependent retromer protein deficiency in leucine-supplemented differentiated Neuro2A cells (A–C) Representative Western blots of VPS35, VPS26, VPS34, and tubulin are shown for cells treated with 100% (A), 250% (B), and 500% (C) additional leucine (control = 0.8 mM leucine). Average expression of retromer proteins VPS35 and VPS26 and the PI3KIII Kinase VPS34 in cells supplemented with leucine (black bars) compared to non-supplemented controls (white bars) is summarized (mean ± SEM; n = 6). *P < 0.05, **P < 0.01, ***P < 0.001.