Sergio T. Ferreira

Aβ Oligomers Induce Neuronal Oxidative Stress through an N-Methyl-D-aspartate Receptor-dependent Mechanism That Is Blocked by the Alzheimer Drug Memantine

Oxidative stress is a major aspect of Alzheimer disease (AD) pathology. We have investigated the relationship between oxidative stress and neuronal binding of Aβ oligomers (also known as ADDLs). ADDLs are known to accumulate in brain tissue of AD patients and are considered centrally related to pathogenesis. Using hippocampal neuronal cultures, we found that ADDLs stimulated excessive formation of reactive oxygen species (ROS) through a mechanism requiring N-methyl-d-aspartate receptor (NMDA-R) activation. ADDL binding to neurons was reduced and ROS formation was completely blocked by an antibody to the extracellular domain of the NR1 subunit of NMDA-Rs. In harmony with a steric inhibition of ADDL binding by NR1 antibodies, ADDLs that were bound to detergent-extracted synaptosomal membranes co-immunoprecipitated with NMDA-R subunits. The NR1 antibody did not affect ROS formation induced by NMDA, showing that NMDA-Rs themselves remained functional. Memantine, an open channel NMDA-R antagonist prescribed as a memory-preserving drug for AD patients, completely protected against ADDL-induced ROS formation, as did other NMDA-R antagonists. Memantine and the anti-NR1 antibody also attenuated a rapid ADDL-induced increase in intraneuronal calcium, which was essential for stimulated ROS formation. These results show that ADDLs bind to or in close proximity to NMDA-Rs, triggering neuronal damage through NMDA-R-dependent calcium flux. This response provides a pathologically specific mechanism for the therapeutic action of memantine, indicates a role for ROS dysregulation in ADDL-induced cognitive impairment, and supports the unifying hypothesis that ADDLs play a central role in AD pathogenesis.

Protection of synapses against Alzheimer's-linked toxins: Insulin signaling prevents the pathogenic binding of Aβ oligomers

Synapse deterioration underlying severe memory loss in early Alzheimers disease (AD) is thought to be caused by soluble amyloid beta (Aβ) oligomers. Mechanistically, soluble Aβ oligomers, also referred to as Aβ-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.

An anti-diabetes agent protects the mouse brain from defective insulin signaling caused by Alzheimer’s disease–associated Aβ oligomers

Defective brain insulin signaling has been suggested to contribute to the cognitive deficits in patients with Alzheimers 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-β peptide (Aβ) 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 Aβ 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.

Soluble protein oligomers as emerging toxins in alzheimer's and other amyloid diseases

Amyloid diseases are a group of degenerative disorders characterized by cell/tissue damage caused by toxic protein aggregates. Abnormal production, processing and/or clearance of misfolded proteins or peptides may lead to their accumulation and to the formation of amyloid aggregates. Early histopathological investigation of affected organs in different amyloid diseases revealed the ubiquitous presence of fibrillar protein aggregates forming large deposits known as amyloid plaques. Further in vitro biochemical and cell biology studies, as well as studies using transgenic animal models, provided strong support to what initially seemed to be a solid concept, namely that amyloid fibrils played crucial roles in amyloid pathogenesis. However, recent studies describing tissue‐specific accumulation of soluble protein oligomers and their strong impact on cell function have challenged the fibril hypothesis and led to the emergence of a new view: Fibrils are not the only toxins derived from amyloidogenic proteins and, quite possibly, not the most important ones with respect to disease etiology. Here, we review some of the recent findings and concepts in this rapidly developing field, with emphasis on the involvement of soluble oligomers of the amyloid‐β peptide in the pathogenesis of Alzheimers disease. Recent studies suggesting that soluble oligomers from different proteins may share common mechanisms of cytotoxicity are also discussed. Increased understanding of the cellular toxic mechanisms triggered by protein oligomers may lead to the development of rational, effective treatments for amyloid disorders. IUBMB Life, 59: 332‐345, 2007

The Aβ oligomer hypothesis for synapse failure and memory loss in Alzheimer’s disease

Alzheimers disease (AD) is the 3rd most costly disease and the leading cause of dementia. It can linger for many years, but ultimately is fatal, the 6th leading cause of death. Alzheimers disease (AD) is fatal and affected individuals can sometimes linger many years. Current treatments are palliative and transient, not disease modifying. This article reviews progress in the search to identify the primary AD-causing toxins. We summarize the shift from an initial focus on amyloid plaques to the contemporary concept that AD memory failure is caused by small soluble oligomers of the Aβ peptide, toxins that target and disrupt particular synapses. Evidence is presented that links Aβ oligomers to pathogenesis in animal models and humans, with reference to seminal discoveries from cell biology and new ideas concerning pathogenic mechanisms, including relationships to diabetes and Fragile X. These findings have established the oligomer hypothesis as a new molecular basis for the cause, diagnosis, and treatment of AD.

Inflammation, Defective Insulin Signaling, and Mitochondrial Dysfunction as Common Molecular Denominators Connecting Type 2 Diabetes to Alzheimer Disease

A growing body of evidence supports an intriguing clinical/epidemiological connection between Alzheimer disease (AD) and type 2 diabetes (T2D). T2D patients have significantly increased risk of developing AD and vice versa. Recent studies have begun to reveal common pathogenic mechanisms shared by AD and metabolic disorders, notably obesity and T2D. In T2D and obesity, low-grade chronic inflammation is a key mechanism leading to peripheral insulin resistance, which progressively causes tissue deterioration and overall health decline. In the brain, proinflammatory signaling was recently found to mediate impaired neuronal insulin signaling, synapse deterioration, and memory loss. Here, we review evidence indicating that inflammation, insulin resistance, and mitochondrial dysfunction are common features in AD and T2D. We further propose the hypothesis that dementia and its underlying neuronal dysfunction are exacerbated or driven by peripheral inflammation. Identification of central and peripheral inflammation as potential mediators of brain dysfunction in AD may lead to the development of effective treatments for this devastating disease.

Taurine prevents the neurotoxicity of β-amyloid and glutamate receptor agonists: activation of GABA receptors and possible implications for Alzheimer’s disease and other neurological disorders

Alzheimers disease (AD) and several other neurological disorders have been linked to the overactivation of glutamatergic transmission and exci‐totoxicity as a common pathway of neuronal injury. The β‐amyloid peptide (Aβ) is centrally related to the pathogenesis of AD, and previous reports have demon¬strated that the blockade of glutamate receptors pre¬vents Aβ‐induced neuronal death. We show that tau¬rine, a β‐amino acid found at high concentrations in the brain, protects chick retinal neurons in culture against the neurotoxicity of Aβ and glutamate receptor ago¬nists. The protective effect of taurine is not mediated by interaction with glutamate receptors, as demon¬strated by binding studies using radiolabeled glutamate receptor ligands. The neuroprotective action of taurine is blocked by picrotoxin, an antagonist of GABAA receptors. GABA and the GABAA receptor agonists phenobarbital and melatonin also protect neurons against Aβ ‐induced neurotoxicity. These results suggest that activation of GABA receptors decreases neuronal vulnerability to excitotoxic damage and that pharmaco¬logical manipulation of the excitatory and inhibitory neurotransmitter tonus may protect neurons against a variety of insults. GABAergic transmission may repre¬sent a promising target for the treatment of AD and other neurological disorders in which excitotoxicity plays a relevant role.—Louzada, P. R., Lima, A. C. P., Mendonca‐Silva, D. L., Noël, F., de Mello, F. G., Ferreira, S. T. Taurine prevents the neurotoxicity of β‐amyloid and glutamate receptor agonists: activation of GABA receptors and possible implications for Alz¬heimers disease and other neurological disorders.

Amyloid-β binds to the extracellular cysteine-rich domain of frizzled and inhibits wnt/β-catenin signaling

The amyloid-β peptide (Aβ) plays a major role in neuronal dysfunction and neurotoxicity in Alzheimer disease. However, the signal transduction mechanisms involved in Aβ-induced neuronal dysfunction remain to be fully elucidated. A major current unknown is the identity of the protein receptor(s) involved in neuronal Aβ binding. Using phage display of peptide libraries, we have identified a number of peptides that bind Aβ and are homologous to neuronal receptors putatively involved in Aβ interactions. We report here on a cysteine-linked cyclic heptapeptide (denominated cSP5) that binds Aβ with high affinity and is homologous to the extracellular cysteine-rich domain of several members of the Frizzled (Fz) family of Wnt receptors. Based on this homology, we investigated the interaction between Aβ and Fz. The results show that Aβ binds to the Fz cysteine-rich domain at or in close proximity to the Wnt-binding site and inhibits the canonical Wnt signaling pathway. Interestingly, the cSP5 peptide completely blocks Aβ binding to Fz and prevents inhibition of Wnt signaling. These results indicate that the Aβ-binding site in Fz is homologous to cSP5 and that this is a relevant target for Aβ-instigated neurotoxicity. Furthermore, they suggest that blocking the interaction of Aβ with Fz might lead to novel therapeutic approaches to prevent neuronal dysfunction in Alzheimer disease.

Targeting the neurotoxic species in Alzheimer’s disease: inhibitors of Aβ oligomerization

In the past two decades, a large body of evidence has established a causative role for the β‐amy‐loid peptide (Aβ) in Alzheimers disease (AD). However, recent debate has focused on whether amyloid fibrils or soluble oligomers of Aβ are the main neurotoxic species that contribute to neurodegeneration and dementia. Considerable early evidence has indicated that amyloid fibrils are toxic, but some recent studies support the notion that Aβ oligomers are the primary neurotoxins. While this crucial aspect of AD pathogenesis remains controversial, effective therapeutic strategies should ideally target both oligomeric and fibrillar species of Aβ. Here, we describe the anti‐amyloido‐genic and neuroprotective actions of some di‐ and tri‐substituted aromatic compounds. Inhibition of the formation of soluble Aβ oligomers was monitored using a specific antibody‐based assay that discriminates between Aβ oligomers and monomers. Thioflavin T and electron microscopy were used to screen for inhibitors of fibril formation. Taken together, these results led to the identification of compounds that more effectively block Aβ oligomerization than fibrillization. It is significant that such compounds completely blocked the neurotoxicity of Aβ to rat hippocampal neurons in culture. These findings provide a basis for the development of novel small molecule Aβ inhibitors with potential applications in AD.—De Felice, F. G., Vieira, M. N. N., Saraiva, L. M., Figueroa‐Villar, J. D., Garcia‐Abreu, J., Liu, R., Chang, L., Klein, W. L., Ferreira, S. T. Targeting the neurotoxic species in Alzheimers disease: inhibitors of Aβ oligomerization.

Amyloid-β Peptide Oligomers Disrupt Axonal Transport through an NMDA Receptor-Dependent Mechanism That Is Mediated by Glycogen Synthase Kinase 3β in Primary Cultured Hippocampal Neurons

Disruption of axonal transport is a hallmark of several neurodegenerative diseases, including Alzheimers disease (AD). Even though defective transport is considered an early pathologic event, the mechanisms by which neurodegenerative insults impact transport are poorly understood. We show that soluble oligomers of the amyloid-β peptide (AβOs), increasingly recognized as the proximal neurotoxins in AD pathology, induce disruption of organelle transport in primary hippocampal neurons in culture. Live imaging of fluorescent protein-tagged organelles revealed a marked decrease in axonal trafficking of dense-core vesicles and mitochondria in the presence of 0.5 μm AβOs. NMDA receptor (NMDAR) antagonists, including d-AP5, MK-801, and memantine, prevented the disruption of trafficking, thereby identifying signals for AβO action at the cell membrane. Significantly, both pharmacological inhibition of glycogen synthase kinase-3β (GSK-3β) and transfection of neurons with a kinase-dead form of GSK-3β prevented the transport defect. Finally, we demonstrate by biochemical and immunocytochemical means that AβOs do not affect microtubule stability, indicating that disruption of transport involves a more subtle mechanism than microtubule destabilization, likely the dysregulation of intracellular signaling cascades. Results demonstrate that AβOs negatively impact axonal transport by a mechanism that is initiated by NMDARs and mediated by GSK-3β and establish a new connection between toxic Aβ oligomers and AD pathology.