Intraneuronal Amylin Deposition, Peroxidative Membrane Injury and Increased IL-1β Synthesis in Brains of Alzheimer’s Disease Patients with Type-2 Diabetes and in Diabetic HIP Rats

Abstract

Amylin is a hormone synthesized and co-secreted with insulin by pancreatic β-cells that crosses the blood-brain barrier and regulates satiety. Amylin from humans (but not rodents) has an increased propensity to aggregate into pancreatic islet amyloid deposits that contribute to β-cell mass depletion and development of type-2 diabetes by inducing oxidative stress and inflammation. Recent studies demonstrated that aggregated amylin also accumulates in brains of Alzheimer’s disease (AD) patients, preponderantly those with type-2 diabetes. Here, we report that, in addition to amylin plaques and mixed amylin-Aβ deposits, brains of diabetic patients with AD show amylin immunoreactive deposits inside the neurons. Neuronal amylin formed adducts with 4-hydroxynonenal (4-HNE), a marker of peroxidative membrane injury, and increased synthesis of the proinflammatory cytokine interleukin (IL)-1β. These pathological changes were mirrored in rats expressing human amylin in pancreatic islets (HIP rats) and mice intravenously injected with aggregated human amylin, but not in hyperglycemic rats secreting wild-type non-amyloidogenic rat amylin. In cultured primary hippocampal rat neurons, aggregated amylin increased IL-1β synthesis via membrane destabilization and subsequent generation of 4-HNE. These effects were blocked by membrane stabilizers and lipid peroxidation inhibitors. Thus, elevated circulating levels of aggregated amylin negatively affect the neurons causing peroxidative membrane injury and aberrant inflammatory responses independent of other confounding factors of diabetes. The present results are consistent with the pathological role of aggregated amylin in the pancreas, demonstrate a novel contributing mechanism to neurodegeneration, and suggest a direct, potentially treatable link of type-2 diabetes with AD.

Review: Relationship of type 2 diabetes to human brain pathology

“…Interestingly, IAPP is found in both the parenchyma and blood vessels of patients with AD who do not carry a diagnosis of T2D in about the same concentrations as subjects with only T2D . While IAPP in the aforementioned study was found extracellularly, a follow‐up investigation used immunohistochemistry to demonstrate IAPP within neurones themselves, found in the absence of mRNA, excluding production of IAPP by the neurones . A small study of five persons with ADNC and six controls found IAPP inclusions in the pericytes, cells involved with the BBB (discussed below), around the hippocampus and parahippocampal cortex .…”

Brain microvascular injury and white matter disease provoked by diabetes‐associated hyperamylinemia

“…Anti-mouse or anti-rabbit IgG (1:50, A3562, A3687, Sigma, MO) were secondary antibodies. The specificity of the amylin antibody in both human and rat brain tissues was established in previous studies 6,7,29 .…”

“…Tissues were processed as previously described. 6,29 Antibodies against amylin (1:200, T-4157; Bachem-Peninsula Laboratories, San Carlos, CA), glial fibrillary acidic protein (GFAP; 1:600, MAB360, Millipore), and myelin basic protein (MBP; 1:5,000, AMAB91064, clone CL2829, Sigma) were the primary antibodies. Antimouse or antirabbit IgG (1:50, A3562, A3687, Sigma) were secondary antibodies.…”

“…Similar pathologic changes occur in rodents overexpressing human 27 , but not murine amylin 16 . We previously reported that rats overexpressing human amylin in the pancreas accumulate aggregated amylin in the brain 28 , which correlates with the development of neuroinflammation 28,29 , reduced synthesis (by >50%) of catecholamine precursors (phenylalanine and tyrosine) 30 and neurologic deficits 28 . Furthermore, a recent study demonstrated that transgenic mice overexpressing human amylin in pancreatic islets and a mutant form of the amyloid precursor protein (APP) in neurons have exacerbated Aβ pathology compared to transgenic mice expressing only one or the other protein 31 .…”

Bridging Type 2 Diabetes and Alzheimer's Disease: Assembling the Puzzle Pieces in the Quest for the Molecules With Therapeutic and Preventive Potential

“…In fact, a recent study has shown that brains of type 2 diabetic patients with AD have IAPP deposits inside the neurons promoting the synthesis of proinflammatory cytokine interleukin (IL)‐1β and additionally forming adducts with 4‐hydroxynoneal (4‐HNE), a marker of peroxidative membrane injury. These effects were also observed in mice expressing human IAPP or injected with aggregated human IAPP, thus indicating that elevated circulating levels of IAPP directly correlate with neuronal damage induced by oxidative stress and aberrant inflammatory responses, although the mechanisms underlying intracellular IAPP uptake have not been elucidated to this point …”