ABSTRACT
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.
Subject(s)
Amyloid beta-Peptides/toxicity , Brain/drug effects , Insulin Receptor Substrate Proteins/metabolism , Polymers/toxicity , Tumor Necrosis Factor-alpha/metabolism , eIF-2 Kinase/metabolism , Alzheimer Disease/metabolism , Alzheimer Disease/pathology , Amyloid beta-Peptides/chemistry , Animals , Brain/metabolism , Disease Models, Animal , Haplorhini/metabolism , Hypoglycemic Agents/pharmacology , Insulin Receptor Substrate Proteins/antagonists & inhibitors , Memory Disorders/metabolism , Memory Disorders/pathology , Mice , Mice, Knockout , Neurons/drug effects , Neurons/metabolism , Phosphorylation/drug effects , Polymers/chemistry , Receptors, Tumor Necrosis Factor, Type I/deficiency , Receptors, Tumor Necrosis Factor, Type I/genetics , Receptors, Tumor Necrosis Factor, Type I/metabolism , Signal Transduction/drug effects , Synapses/drug effects , Synapses/metabolism , Tumor Necrosis Factor-alpha/antagonists & inhibitors , eIF-2 Kinase/deficiency , eIF-2 Kinase/geneticsABSTRACT
Alzheimer's disease (AD) is characterized by brain accumulation of the neurotoxic amyloid-ß peptide (Aß) and by loss of cholinergic neurons and nicotinic acetylcholine receptors (nAChRs). Recent evidence indicates that memory loss and cognitive decline in AD correlate better with the amount of soluble Aß than with the extent of amyloid plaque deposits in affected brains. Inhibition of nAChRs by soluble Aß40 is suggested to contribute to early cholinergic dysfunction in AD. Using phage display screening, we have previously identified a heptapeptide, termed IQ, homologous to most nAChR subtypes, binding with nanomolar affinity to soluble Aß40 and blocking Aß-induced inhibition of carbamylcholine-induced currents in PC12 cells expressing α7 nAChRs. Using alanine scanning mutagenesis and whole-cell current recording, we have now defined the amino acids in IQ essential for reversal of Aß40 inhibition of carbamylcholine-induced responses in PC12 cells, mediated by α7 subtypes and other endogenously expressed nAChRs. We further investigated the effects of soluble Aß, IQ and analogues of IQ on α3ß4 nAChRs recombinantly expressed in HEK293 cells. Results show that nanomolar concentrations of soluble Aß40 potently inhibit the function of α3ß4 nAChRs, and that subsequent addition of IQ or its analogues does not reverse this effect. However, co-application of IQ makes the inhibition of α3ß4 nAChRs by Aß40 reversible. These findings indicate that Aß40 inhibits different subtypes of nAChRs by interacting with specific receptor domains homologous to the IQ peptide, suggesting that IQ may be a lead for novel drugs to block the inhibition of cholinergic function in AD.
Subject(s)
Amyloid beta-Peptides/pharmacology , Nicotine/metabolism , Nicotinic Antagonists/pharmacology , Oligopeptides/chemistry , Oligopeptides/pharmacology , Peptide Fragments/pharmacology , alpha7 Nicotinic Acetylcholine Receptor/chemistry , Amino Acid Sequence , Animals , Gene Expression Regulation/drug effects , HEK293 Cells , Humans , PC12 Cells , Protein Structure, Tertiary , Rats , alpha7 Nicotinic Acetylcholine Receptor/metabolismABSTRACT
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.
