Abstract and Introduction
Central response to insulin is suspected to be defective in Alzheimer's disease. As most insulin is secreted in the bloodstream by the pancreas, its capacity to regulate brain functions must, at least partly, be mediated through the cerebral vasculature. However, how insulin interacts with the blood–brain barrier and whether alterations of this interaction could contribute to Alzheimer's disease pathophysiology both remain poorly defined.
Here, we show that human and murine cerebral insulin receptors (INSRs), particularly the long isoform INSRα-B, are concentrated in microvessels rather than in the parenchyma. Vascular concentrations of INSRα-B were lower in the parietal cortex of subjects diagnosed with Alzheimer's disease, positively correlating with cognitive scores, leading to a shift towards a higher INSRα-A/B ratio, consistent with cerebrovascular insulin resistance in the Alzheimer's disease brain. Vascular INSRα was inversely correlated with amyloid-β plaques and β-site APP cleaving enzyme 1, but positively correlated with insulin-degrading enzyme, neprilysin and P-glycoprotein. Using brain cerebral intracarotid perfusion, we found that the transport rate of insulin across the blood–brain barrier remained very low (<0.03 μl/g·s) and was not inhibited by an insulin receptor antagonist. However, intracarotid perfusion of insulin induced the phosphorylation of INSRβ that was restricted to microvessels. Such an activation of vascular insulin receptor was blunted in 3xTg-AD mice, suggesting that Alzheimer's disease neuropathology induces insulin resistance at the level of the blood–brain barrier.
Overall, the present data in post-mortem Alzheimer's disease brains and an animal model of Alzheimer's disease indicate that defects in the insulin receptor localized at the blood–brain barrier strongly contribute to brain insulin resistance in Alzheimer's disease, in association with β-amyloid pathology.
The human brain is sensitive to insulin, a hormone essential to life that is also at the heart of the pathophysiology and treatment of diabetes.[1,2] The scientific literature is replete with studies in humans and other species reporting the effects of insulin on memory, cerebral blood flow, eating behaviour and regulation of whole-body metabolism, supporting a therapeutic potential in brain-dependent metabolic disorders.[3–6] Although local synthesis has been detected, most insulin exercising an effect on the brain circulates in the blood after being produced by the pancreas.[7,8] The blood–brain barrier (BBB) is a major interface between the blood and brain, controlling access to cerebral tissues. Therefore, to exert an effect in the CNS, circulating insulin must first interact with its insulin receptor (INSR) located on brain capillary endothelial cells (BCEC) forming the BBB.[8–11]
INSR is a disulphide-linked homodimer (αβ)2 that structurally and genetically belongs to the class II of receptor tyrosine kinases. Alternative splicing produces two isoforms of the α-chain: the short A isoform (INSRα-A) truncated by 12 amino acids (exon 11) and the long B isoform (INSRα-B).[12–15] The regulation of this alternative splicing is not fully elucidated but appears to be tissue-specific. Binding of an agonist to the extracellular α-chain triggers autophosphorylation of the transmembrane β-chain, which contains multiple phosphorylation sites leading to the activation of INSR and downstream signalling pathways.
Beside classical amyloid-β (Aβ) and tau pathologies, Alzheimer's disease is characterized by defective brain uptake of glucose and impaired response to insulin.[4,5,16–19] The most compelling evidence comes from post-mortem indexes of brain insulin resistance, such as changes in INSR or IRS1 phosphorylation status shown to be associated with cognitive impairment.[20,21] Insulin administration in animals is reported to improve memory function, including in mouse models of Alzheimer's disease.[22,23] Several small clinical trials using intranasal delivery to avoid the hypoglycaemic effect of systemically injected insulin have reported benefits to memory scores in cognitively impaired adults.[6,24,25] However, recent larger clinical trials produced unclear results.[6,25–27] Still, increasing insulin sensitivity in the brain remains a promising avenue and a wide range of repurposed diabetes drugs are in ongoing clinical trials for Alzheimer's disease, including metformin, thiazolidinediones and glucagon-like peptide 1 (GLP-1) analogues.[6,28,29]
Whereas cerebral insulin resistance is often assumed to be restricted to neurons in Alzheimer's disease,[30–32] it is supported by limited immunohistochemical (IHC) evidence.[33,34] Most initial studies on the distribution of brain INSR used macroscopic techniques.[35–39] However, recent single-cell transcriptomic analyses indicate that the mRNA transcript encoded by the INSR gene is found in higher concentrations in endothelial cells in the mouse and human brain.[40–42] Accordingly, a growing number of studies are showing that INSR located in the cerebral vasculature plays a key role in the action of insulin on the brain.[11,43,44] Based on the current understanding, INSR at the BBB binds circulating insulin to either act (i) as a classic receptor to trigger cell-signalling pathways within BCEC; or (ii) as a transporter to ferry an insulin molecule into the brain parenchyma.[45–49] Therefore, it remains unclear whether the same INSR protein can exert both roles.[8,50–52]
Given this large set of clinical and preclinical data showing a primary role of insulin on brain function, we aimed to determine how circulating insulin interacts with the BBB INSR and whether the latter is defective in Alzheimer's disease. We first sought to determine INSR levels in microvascular extracts from both mouse and human brain samples relative to brain parenchyma. To probe for changes in Alzheimer's disease, we used microvessel extracts of parietal cortex from participants in the Religious Orders Study who underwent detailed clinical and neuropsychological evaluations.[53–55] Associations with clinical and biochemical data were assessed, including BBB transporters and receptors putatively involved in Aβ transport. To directly investigate the response of brain INSR to circulating insulin, we performed intracarotid insulin perfusion and found that INSR activation is localized at the BBB, where it is impaired by Aβ and tau pathologies in a mouse model of Alzheimer's disease at different ages.
Brain. 2023;146(1):75-90. © 2023 Oxford University Press