Inflammation, defective insulin signaling, and neuronal dysfunction in Alzheimer's disease.

A link between Alzheimer's disease (AD) and metabolic disorders has been established, with patients with type 2 diabetes at increased risk of developing AD and vice versa. The incidence of metabolic disorders, including insulin resistance and type 2 diabetes is increasing at alarming rates worldwide, primarily as a result of poor lifestyle habits. In parallel, as the world population ages, the prevalence of AD, the most common form of dementia in the elderly, also increases. In addition to their epidemiologic and clinical association, mounting recent evidence indicates shared mechanisms of pathogenesis between metabolic disorders and AD. We discuss the concept that peripheral and central nervous system inflammation link the pathogenesis of AD and metabolic diseases. We also explore the contribution of brain inflammation to defective insulin signaling and neuronal dysfunction. Last, we review recent evidence indicating that targeting neuroinflammation may provide novel therapeutic avenues for AD.

Activation of D1/D5 dopamine receptors protects neurons from synapse dysfunction induced by amyloid-beta oligomers.

Soluble oligomers of the amyloid-ß peptide (AßOs) accumulate in the brains of Alzheimer disease (AD) patients and are implicated in synapse failure and early memory loss in AD. AßOs have been shown to impact synapse function by inhibiting long term potentiation, facilitating the induction of long term depression and inducing internalization of both AMPA and NMDA glutamate receptors, critical players in plasticity mechanisms. Because activation of dopamine D1/D5 receptors plays important roles in memory circuits by increasing the insertion of AMPA and NMDA receptors at synapses, we hypothesized that selective activation of D1/D5 receptors could protect synapses from the deleterious action of AßOs. We show that SKF81297, a selective D1/D5 receptor agonist, prevented the reduction in surface levels of AMPA and NMDA receptors induced by AßOs in hippocampal neurons in culture. Protection by SKF81297 was abrogated by the specific D1/D5 antagonist, SCH23390. Levels of AMPA receptor subunit GluR1 phosphorylated at Ser(845), which regulates AMPA receptor association with the plasma membrane, were reduced in a calcineurin-dependent manner in the presence of AßOs, and treatment with SKF81297 prevented this reduction. Establishing the functional relevance of these findings, SKF81297 blocked the impairment of long term potentiation induced by AßOs in hippocampal slices. Results suggest that D1/D5 receptors may be relevant targets for development of novel pharmacological approaches to prevent synapse failure in AD.

Protection of synapses against Alzheimer's-linked toxins: insulin signaling prevents the pathogenic binding of Abeta oligomers.

Synapse deterioration underlying severe memory loss in early Alzheimer's disease (AD) is thought to be caused by soluble amyloid beta (Abeta) oligomers. Mechanistically, soluble Abeta oligomers, also referred to as Abeta-derived diffusible ligands (ADDLs), act as highly specific pathogenic ligands, binding to sites localized at particular synapses. This binding triggers oxidative stress, loss of synaptic spines, and ectopic redistribution of receptors critical to plasticity and memory. We report here the existence of a protective mechanism that naturally shields synapses against ADDL-induced deterioration. Synapse pathology was investigated in mature cultures of hippocampal neurons. Before spine loss, ADDLs caused major downregulation of plasma membrane insulin receptors (IRs), via a mechanism sensitive to calcium calmodulin-dependent kinase II (CaMKII) and casein kinase II (CK2) inhibition. Most significantly, this loss of surface IRs, and ADDL-induced oxidative stress and synaptic spine deterioration, could be completely prevented by insulin. At submaximal insulin doses, protection was potentiated by rosiglitazone, an insulin-sensitizing drug used to treat type 2 diabetes. The mechanism of insulin protection entailed a marked reduction in pathogenic ADDL binding. Surprisingly, insulin failed to block ADDL binding when IR tyrosine kinase activity was inhibited; in fact, a significant increase in binding was caused by IR inhibition. The protective role of insulin thus derives from IR signaling-dependent downregulation of ADDL binding sites rather than ligand competition. The finding that synapse vulnerability to ADDLs can be mitigated by insulin suggests that bolstering brain insulin signaling, which can decline with aging and diabetes, could have significant potential to slow or deter AD pathogenesis.

2,4-Dinitrophenol reduces the reactivity of Lys553 in the lower 50-kDa region of myosin subfragment 1.

2,4-Dinitrophenol (DNP) increases the affinity of myosin for actin and accelerates its Mg(2+)ATPase activity, suggesting that it acts on a region of the myosin head that transmits conformational changes to actin- and ATP-binding sites. The binding site/s for DNP are unknown; however similar hydrophobic compounds bind to the 50-kDa subfragment of the myosin head, near the actin-binding interface. In this region, a helix-loop-helix motif contains Lys553, which is specifically labeled with the fluorescent probe 6-[fluorescein-5(and 6)-carboxamido] hexanoic acid succinimidyl ester (FHS). This reaction is sensitive to conformational changes in the helix-loop-helix and the labeling efficiency was reduced when S1 was bound to actin, DNP or nucleotide analogs. The nucleotide analogs had a range of effects (PPi>ADP·AlF(4)(-)>ADP) irrespective of the open-closed state of switch 2. The greatest reduction in labeling was in the presence of actin or DNP. When we measured the effect of each ligand on the fluorescence of FHS previously attached to S1, only DNP quenched the emission. Together, the results suggest that the helix-loop-helix region is flexible, it is part of the communication pathway between the ATP- and actin-binding sites of myosin and it is proximal to the region of myosin where DNP binds.

N-methyl-D-aspartate receptors are required for synaptic targeting of Alzheimer's toxic amyloid-ß peptide oligomers.

Soluble amyloid-ß peptide (Aß) 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 Aß 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 Aß oligomers.

Alzheimer-associated Aß oligomers impact the central nervous system to induce peripheral metabolic deregulation.

Alzheimer's disease (AD) is associated with peripheral metabolic disorders. Clinical/epidemiological data indicate increased risk of diabetes in AD patients. Here, we show that intracerebroventricular infusion of AD-associated Aß oligomers (AßOs) in mice triggered peripheral glucose intolerance, a phenomenon further verified in two transgenic mouse models of AD. Systemically injected AßOs failed to induce glucose intolerance, suggesting AßOs target brain regions involved in peripheral metabolic control. Accordingly, we show that AßOs affected hypothalamic neurons in culture, inducing eukaryotic translation initiation factor 2α phosphorylation (eIF2α-P). AßOs further induced eIF2α-P and activated pro-inflammatory IKKß/NF-κB signaling in the hypothalamus of mice and macaques. AßOs failed to trigger peripheral glucose intolerance in tumor necrosis factor-α (TNF-α) receptor 1 knockout mice. Pharmacological inhibition of brain inflammation and endoplasmic reticulum stress prevented glucose intolerance in mice, indicating that AßOs act via a central route to affect peripheral glucose homeostasis. While the hypothalamus has been largely ignored in the AD field, our findings indicate that AßOs affect this brain region and reveal novel shared molecular mechanisms between hypothalamic dysfunction in metabolic disorders and AD.

TNF-α mediates PKR-dependent memory impairment and brain IRS-1 inhibition induced by Alzheimer's ß-amyloid oligomers in mice and monkeys.

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 ß-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.

An anti-diabetes agent protects the mouse brain from defective insulin signaling caused by Alzheimer's disease- associated Aß oligomers.

Defective brain insulin signaling has been suggested to contribute to the cognitive deficits in patients with Alzheimer's disease (AD). Although a connection between AD and diabetes has been suggested, a major unknown is the mechanism(s) by which insulin resistance in the brain arises in individuals with AD. Here, we show that serine phosphorylation of IRS-1 (IRS-1pSer) is common to both diseases. Brain tissue from humans with AD had elevated levels of IRS-1pSer and activated JNK, analogous to what occurs in peripheral tissue in patients with diabetes. We found that amyloid-ß peptide (Aß) oligomers, synaptotoxins that accumulate in the brains of AD patients, activated the JNK/TNF-α pathway, induced IRS-1 phosphorylation at multiple serine residues, and inhibited physiological IRS-1pTyr in mature cultured hippocampal neurons. Impaired IRS-1 signaling was also present in the hippocampi of Tg mice with a brain condition that models AD. Importantly, intracerebroventricular injection of Aß oligomers triggered hippocampal IRS-1pSer and JNK activation in cynomolgus monkeys. The oligomer-induced neuronal pathologies observed in vitro, including impaired axonal transport, were prevented by exposure to exendin-4 (exenatide), an anti-diabetes agent. In Tg mice, exendin-4 decreased levels of hippocampal IRS-1pSer and activated JNK and improved behavioral measures of cognition. By establishing molecular links between the dysregulated insulin signaling in AD and diabetes, our results open avenues for the investigation of new therapeutics in AD.

Human apolipoprotein A-I binds amyloid-beta and prevents Abeta-induced neurotoxicity.

Aggregates of the amyloid-beta peptide (Abeta) play a central role in the pathogenesis of Alzheimer's disease (AD). Identification of proteins that physiologically bind Abeta and modulate its aggregation and neurotoxicity could lead to the development of novel disease-modifying approaches in AD. By screening a phage display peptide library for high affinity ligands of aggregated Abeta(1-42), we isolated a peptide homologous to a highly conserved amino acid sequence present in the N-terminus of apolipoprotein A-I (apoA-I). We show that purified human apoA-I and Abeta form non-covalent complexes and that interaction with apoA-I affects the morphology of amyloid aggregates formed by Abeta. Significantly, Abeta/apoA-I complexes were also detected in cerebrospinal fluid from AD patients. Interestingly, apoA-I and apoA-I-containing reconstituted high density lipoprotein particles protect hippocampal neuronal cultures from Abeta-induced oxidative stress and neurodegeneration. These results suggest that human apoA-I modulates Abeta aggregation and Abeta-induced neuronal damage and that the Abeta-binding domain in apoA-I may constitute a novel framework for the design of inhibitors of Abeta toxicity.