Defective brain insulin signaling has been suggested to contribute to the cognitive deficits in patients with

Alzheimer's disease (AD) and type 2 diabetes appear to share similar pathogenic mechanisms. dsRNA-dependent protein kinase (PKR) underlies peripheral insulin resistance in metabolic disorders. PKR phosphorylates eukaryotic translation initiation factor 2α (eIF2α-P), and AD brains exhibit elevated phospho-PKR and eIF2α-P levels. Whether and how PKR and eIF2α-P participate in defective brain insulin signaling and cognitive impairment in AD are unknown. We report that β-amyloid oligomers, AD-associated toxins, activate PKR in a tumor necrosis factor α (TNF-α)-dependent manner, resulting in eIF2α-P, neuronal insulin receptor substrate (IRS-1) inhibition, synapse loss, and memory impairment. Brain phospho-PKR and eIF2α-P were elevated in AD animal models, including monkeys given intracerebroventricular oligomer infusions. Oligomers failed to trigger eIF2α-P and cognitive impairment in PKR(-/-) and TNFR1(-/-) mice. Bolstering insulin signaling rescued phospho-PKR and eIF2α-P. Results reveal pathogenic mechanisms shared by AD and diabetes and establish that proinflammatory signaling mediates oligomer-induced IRS-1 inhibition and PKR-dependent synapse and memory loss.

Alzheimer's disease (AD) is a devastating neurodegenerative disorder and a major medical problem. Here, we have investigated the impact of amyloid-␤ (A␤) oligomers, AD-related neurotoxins, in the brains of rats and adult nonhuman primates (cynomolgus macaques). Soluble A␤ oligomers are known to accumulate in the brains of AD patients and correlate with disease-associated cognitive dysfunction. When injected into the lateral ventricle of rats and macaques, A␤ oligomers diffused into the brain and accumulated in several regions associated with memory and cognitive functions. Cardinal features of AD pathology, including synapse loss, tau hyperphosphorylation, astrocyte and microglial activation, were observed in regions of the macaque brain where A␤ oligomers were abundantly detected. Most importantly, oligomer injections induced AD-type neurofibrillary tangle formation in the macaque brain. These outcomes were specifically associated with A␤ oligomers, as fibrillar amyloid deposits were not detected in oligomer-injected brains. Human and macaque brains share significant similarities in terms of overall architecture and functional networks. Thus, generation of a macaque model of AD that links A␤ oligomers to tau and synaptic pathology has the potential to greatly advance our understanding of mechanisms centrally implicated in AD pathogenesis. Furthermore, development of disease-modifying therapeutics for AD has been hampered by the difficulty in translating therapies that work in rodents to humans. This new approach may be a highly relevant nonhuman primate model for testing therapeutic interventions for AD.

Alzheimer's disease (AD) is a devastating neurological disorder that still lacks an effective treatment, and this has stimulated an intense pursuit of disease‐modifying therapeutics. Given the increasingly recognized link between AD and defective brain insulin signaling, we investigated the actions of liraglutide, a glucagon‐like peptide‐1 (GLP‐1) analog marketed for treatment of type 2 diabetes, in experimental models of AD. Insulin receptor pathology is an important feature of AD brains that impairs the neuroprotective actions of central insulin signaling. Here, we show that liraglutide prevented the loss of brain insulin receptors and synapses, and reversed memory impairment induced by AD‐linked amyloid‐β oligomers (AβOs) in mice. Using hippocampal neuronal cultures, we determined that the mechanism of neuroprotection by liraglutide involves activation of the PKA signaling pathway. Infusion of AβOs into the lateral cerebral ventricle of non‐human primates (NHPs) led to marked loss of insulin receptors and synapses in brain regions related to memory. Systemic treatment of NHPs with liraglutide provided partial protection, decreasing AD‐related insulin receptor, synaptic, and tau pathology in specific brain regions. Synapse damage and elimination are amongst the earliest known pathological changes and the best correlates of memory impairment in AD. The results illuminate mechanisms of neuroprotection by liraglutide, and indicate that GLP‐1 receptor activation may be harnessed to protect brain insulin receptors and synapses in AD. © 2018 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.

J. Neurochem. (2010) 115, 1520–1529. Abstract Soluble amyloid‐β peptide (Aβ) oligomers, known to accumulate in Alzheimer’s disease brains, target excitatory post‐synaptic terminals. This is thought to trigger synapse deterioration, a mechanism possibly underlying memory loss in early stage Alzheimer’s disease. A major unknown is the identity of the receptor(s) targeted by oligomers at synapses. Because oligomers have been shown to interfere with N‐methyl‐d‐aspartate receptor (NMDAR) function and trafficking, we hypothesized that NMDARs might be required for oligomer binding to synapses. An amplicon vector was used to knock‐down NMDARs in mature hippocampal neurons in culture, yielding 90% reduction in dendritic NMDAR expression and blocking neuronal oxidative stress induced by Aβ oligomers, a pathological response that has been shown to be mediated by NMDARs. Remarkably, NMDAR knock‐down abolished oligomer binding to dendrites, indicating that NMDARs are required for synaptic targeting of oligomers. Nevertheless, oligomers do not appear to bind directly to NMDARs as indicated by the fact that both oligomer‐attacked and non‐attacked neurons exhibit similar surface levels of NMDARs. Furthermore, pre‐treatment of neurons with insulin down‐regulates oligomer‐binding sites in the absence of a parallel reduction in surface levels of NMDARs. Establishing that NMDARs are key components of the synaptic oligomer binding complex may illuminate the development of novel approaches to prevent synapse failure triggered by Aβ oligomers.