Brain insulin, insulin-like growth factor 1 and glucagon-like peptide 1 signalling in Alzheimer's disease.

Although the brain was once considered an insulin-independent organ, insulin signalling is now recognised as being central to neuronal health and to the function of synapses and brain circuits. Defective brain insulin signalling, as well as related signalling by insulin-like growth factor 1 (IGF-1), is associated with neurological disorders, including Alzheimer's disease, suggesting that cognitive impairment could be related to a state of brain insulin resistance. Here, I briefly review key epidemiological/clinical evidence of the association between diabetes, cognitive decline and AD, as well as findings of reduced components of insulin signalling in AD brains, which led to the initial suggestion that AD could be a type of brain diabetes. Particular attention is given to recent studies illuminating mechanisms leading to neuronal insulin resistance as a key driver of cognitive impairment in AD. Evidence of impaired IGF-1 signalling in AD is also examined. Finally, we discuss potentials and possible limitations of recent and on-going therapeutic approaches based on our increased understanding of the roles of brain signalling by insulin, IGF-1 and glucagon-like peptide 1 in AD.

Interleukin-1ß mediates alterations in mitochondrial fusion/fission proteins and memory impairment induced by amyloid-ß oligomers.

BACKGROUND: The lack of effective treatments for Alzheimer's disease (AD) reflects an incomplete understanding of disease mechanisms. Alterations in proteins involved in mitochondrial dynamics, an essential process for mitochondrial integrity and function, have been reported in AD brains. Impaired mitochondrial dynamics causes mitochondrial dysfunction and has been associated with cognitive impairment in AD. Here, we investigated a possible link between pro-inflammatory interleukin-1 (IL-1), mitochondrial dysfunction, and cognitive impairment in AD models. METHODS: We exposed primary hippocampal cell cultures to amyloid-ß oligomers (AßOs) and carried out AßO infusions into the lateral cerebral ventricle of cynomolgus macaques to assess the impact of AßOs on proteins that regulate mitochondrial dynamics. Where indicated, primary cultures were pre-treated with mitochondrial division inhibitor 1 (mdivi-1), or with anakinra, a recombinant interleukin-1 receptor (IL-1R) antagonist used in the treatment of rheumatoid arthritis. Cognitive impairment was investigated in C57BL/6 mice that received an intracerebroventricular (i.c.v.) infusion of AßOs in the presence or absence of mdivi-1. To assess the role of interleukin-1 beta (IL-1ß) in AßO-induced alterations in mitochondrial proteins and memory impairment, interleukin receptor-1 knockout (Il1r1-/-) mice received an i.c.v. infusion of AßOs. RESULTS: We report that anakinra prevented AßO-induced alteration in mitochondrial dynamics proteins in primary hippocampal cultures. Altered levels of proteins involved in mitochondrial fusion and fission were observed in the brains of cynomolgus macaques that received i.c.v. infusions of AßOs. The mitochondrial fission inhibitor, mdivi-1, alleviated synapse loss and cognitive impairment induced by AßOs in mice. In addition, AßOs failed to cause alterations in expression of mitochondrial dynamics proteins or memory impairment in Il1r1-/- mice. CONCLUSION: These findings indicate that IL-1ß mediates the impact of AßOs on proteins involved in mitochondrial dynamics and that strategies aimed to prevent pathological alterations in those proteins may counteract synapse loss and cognitive impairment in AD.

Hormetic-Like Effects of L-Homocysteine on Synaptic Structure, Function, and Aß Aggregation.

Alzheimer's Disease (AD) is the primary cause of dementia among the elderly population. Elevated plasma levels of homocysteine (HCy), an amino acid derived from methionine metabolism, are considered a risk factor and biomarker of AD and other types of dementia. An increase in HCy is mostly a consequence of high methionine and/or low vitamin B intake in the diet. Here, we studied the effects of physiological and pathophysiological HCy concentrations on oxidative stress, synaptic protein levels, and synaptic activity in mice hippocampal slices. We also studied the in vitro effects of HCy on the aggregation kinetics of Aß40. We found that physiological cerebrospinal concentrations of HCy (0.5 µM) induce an increase in synaptic proteins, whereas higher doses of HCy (30-100 µM) decrease their levels, thereby increasing oxidative stress and causing excitatory transmission hyperactivity, which are all considered to be neurotoxic effects. We also observed that normal cerebrospinal concentrations of HCy slow the aggregation kinetic of Aß40, whereas high concentrations accelerate its aggregation. Finally, we studied the effects of HCy and HCy + Aß42 over long-term potentiation. Altogether, by studying an ample range of effects under different HCy concentrations, we report, for the first time, that HCy can exert beneficial or toxic effects over neurons, evidencing a hormetic-like effect. Therefore, we further encourage the use of HCy as a biomarker and modifiable risk factor with therapeutic use against AD and other types of dementia.

The Involvement of NLRP3 on the Effects of Minocycline in an AD-Like Pathology Induced by ß-Amyloid Oligomers Administered to Mice.

Alzheimer's disease (AD) is a neurodegenerative disease which is characterized by progressive memory loss, the accumulation of ß-amyloid peptide (Aß) (mainly Aß1-42), and more recently, by neuroinflammation, which has been highlighted as playing a central role in the development and progress of AD. This study utilized 100-day-old Balb/c mice for the induction of an AD-like dementia model. The animals were administered with Aß1-42 oligomers (400 pmol/site) or artificial cerebrospinal fluid (ACSF) into the left cerebral ventricle. Twenty-four hours after intracerebroventricular administration, the animals were treated with minocycline (50 mg/kg, via oral gavage) for 17 days. The animals' locomotion was evaluated using the open-field test. The spatial memory was tested using the Y-maze, and the aversive memory was evaluated using the inhibitory avoidance task. Treatment with minocycline was shown to improve both spatial and aversive memories in mice that were submitted to the dementia model. In addition, minocycline reduced the levels of Aß and microglial activation in the animals that received the administration of Aß1-42 oligomers. Moreover, the results suggest that the decrease in microglial activation occurred because of a reduction in the levels of toll-like receptors 2 (TLR2) content, and its adapter protein MyD88, as well as a reduction in the levels of the protein NLRP3, which is indispensable in the assembly of inflammasome. These observations were evaluated via immunohistochemistry and confirmed using the Western blot analysis. Treatment with minocycline had no effect in preventing apoptotic morphologic alterations of the neurons. Thus, the anti-inflammatory effect of minocycline involves TLR2 receptors and NLRP3, besides being beneficial by ameliorating memory impairments. Graphical Abstract.

Free-floating adult human brain-derived slice cultures as a model to study the neuronal impact of Alzheimer's disease-associated Aß oligomers.

BACKGROUND: Slice cultures have been prepared from several organs. With respect to the brain, advantages of slice cultures over dissociated cell cultures include maintenance of the cytoarchitecture and neuronal connectivity. Slice cultures from adult human brain have been reported and constitute a promising method to study neurological diseases. Despite this potential, few studies have characterized in detail cell survival and function along time in short-term, free-floating cultures. NEW METHOD: We used tissue from adult human brain cortex from patients undergoing temporal lobectomy to prepare 200 µm-thick slices. Along the period in culture, we evaluated neuronal survival, histological modifications, and neurotransmitter release. The toxicity of Alzheimer's-associated Aß oligomers (AßOs) to cultured slices was also analyzed. RESULTS: Neurons in human brain slices remain viable and neurochemically active for at least four days in vitro, which allowed detection of binding of AßOs. We further found that slices exposed to AßOs presented elevated levels of hyperphosphorylated Tau, a hallmark of Alzheimer's disease. COMPARISON WITH EXISTING METHOD(S): Although slice cultures from adult human brain have been previously prepared, this is the first report to analyze cell viability and neuronal activity in short-term free-floating cultures as a function of days in vitro. CONCLUSIONS: Once surgical tissue is available, the current protocol is easy to perform and produces functional slices from adult human brain. These slice cultures may represent a preferred model for translational studies of neurodegenerative disorders when long term culturing in not required, as in investigations on AßO neurotoxicity.

Amyloid-ß oligomers synaptotoxicity: The emerging role of EphA4/c-Abl signaling in Alzheimer's disease.

Alzheimer's disease (AD) is characterized by progressive memory loss and dementia. The strong correlation between cognitive decline and the loss of synapses supports the idea that synaptic damage is a relevant pathogenic mechanism underlying AD progression. It has been shown that amyloid beta oligomers (AßOs) induce synaptotoxicity ultimately leading to the reduction of dendritic spine density, which underlies cognitive damage. However, the signaling pathways connecting AßOs to synaptic dysfunction have not been completely elucidated. In this review, we have gathered evidence on AßOs receptors and the signaling pathways involved in synaptic damage. We make special emphasis on a new AßOs induced axis that involves the tyrosine kinase ephrin receptor A4 (EphA4) and c-Abl tyrosine kinase activation. EphA4 is a key player in homeostatic plasticity, mediating dendritic spine remodeling and retraction. AßOs aberrantly activate EphA4 leading to dendritic spine elimination. c-Abl is activated in AßOs exposed neurons and in AD patient's brain, and the inhibition of activated c-Abl ameliorates cognitive deficits in AD mouse model. The EphA4 receptor activates c-Abl intracellular signaling. Therefore EphA4 is an emerging AßOs receptor and the activation of the EphA4/c-Abl axis would explain the synaptic spine alterations found in AD.

WASP-1, a canonical Wnt signaling potentiator, rescues hippocampal synaptic impairments induced by Aß oligomers.

Amyloid-ß (Aß) oligomers are a key factor in Alzheimer's disease (AD)-associated synaptic dysfunction. Aß oligomers block the induction of hippocampal long-term potentiation (LTP) in rodents. The activation of Wnt signaling prevents Aß oligomer-induced neurotoxic effects. The compound WASP-1 (Wnt-activating small molecule potentiator-1), has been described as a synergist of the ligand Wnt-3a, enhancing the activation of Wnt/ß-catenin signaling. Herein, we report that WASP-1 administration successfully rescued Aß-induced synaptic impairments both in vitro and in vivo. The activation of canonical Wnt/ß-catenin signaling by WASP-1 increased synaptic transmission and rescued hippocampal LTP impairments induced by Aß oligomers. Additionally, intra-hippocampal administration of WASP-1 to the double transgenic APPswe/PS1dE9 mouse model of AD prevented synaptic protein loss and reduced tau phosphorylation levels. Moreover, we found that WASP-1 blocked Aß aggregation in vitro and reduced pathological tau phosphorylation in vivo. These results indicate that targeting canonical Wnt signaling with WASP-1 could have value for treating AD.

Canonical Wnt signaling protects hippocampal neurons from Aß oligomers: role of non-canonical Wnt-5a/Ca(2+) in mitochondrial dynamics.

Alzheimer's disease (AD) is the most common type of age-related dementia. The disease is characterized by a progressive loss of cognitive abilities, severe neurodegeneration, synaptic loss and mitochondrial dysfunction. The Wnt signaling pathway participates in the development of the central nervous system and growing evidence indicates that Wnts also regulate the function of the adult nervous system. We report here, that indirect activation of canonical Wnt/ß-catenin signaling using Bromoindirubin-30-Oxime (6-BIO), an inhibitor of glycogen synthase kinase-3ß, protects hippocampal neurons from amyloid-ß (Aß) oligomers with the concomitant blockade of neuronal apoptosis. More importantly, activation with Wnt-5a, a non-canonical Wnt ligand, results in the modulation of mitochondrial dynamics, preventing the changes induced by Aß oligomers (Aßo) in mitochondrial fission-fusion dynamics and modulates Bcl-2 increases induced by oligomers. The canonical Wnt-3a ligand neither the secreted Frizzled-Related Protein (sFRP), a Wnt scavenger, did not prevent these effects. In contrast, some of the Aß oligomer effects were blocked by Ryanodine. We conclude that canonical Wnt/ß-catenin signaling controls neuronal survival, and that non-canonical Wnt/Ca(2+)signaling modulates mitochondrial dysfunction. Since mitochondrial dysfunction is present in neurodegenerative diseases, the therapeutic possibilities of the activation of Wnt signaling are evident.

Deregulation of excitatory neurotransmission underlying synapse failure in Alzheimer's disease.

Alzheimer's disease (AD) is the most common form of dementia in the elderly. Memory loss in AD is increasingly attributed to soluble oligomers of the amyloid-ß peptide (AßOs), toxins that accumulate in AD brains and target particular synapses. Glutamate receptors appear to be centrally involved in synaptic targeting by AßOs. Once bound to neurons, AßOs dysregulate the activity and reduce the surface expression of both N-methyl-D-aspartate (NMDA) and 2-amino-3-(3-hydroxy-5-methyl-isoxazol-4-yl)propanoic acid (AMPA) types of glutamate receptors, impairing signaling pathways involved in synaptic plasticity. In the extracellular milieu, AßOs promote accumulation of the excitatory amino acids, glutamate and D-serine. This leads to overactivation of glutamate receptors, triggering abnormal calcium signals with noxious impacts on neurons. Here, we review key findings linking AßOs to deregulated glutamate neurotransmission and implicating this as a primary mechanism of synapse failure in AD. We also discuss strategies to counteract the impact of AßOs on excitatory neurotransmission. In particular, we review evidence showing that inducing neuronal hyperpolarization via activation of inhibitory GABA(A) receptors prevents AßO-induced excitotoxicity, suggesting that this could comprise a possible therapeutic approach in AD.

Postsynaptic Receptors for Amyloid-ß Oligomers as Mediators of Neuronal Damage in Alzheimer's Disease.

The neurotoxic effect of amyloid-ß peptide (Aß) over the central synapses has been described and is reflected in the decrease of some postsynaptic excitatory proteins, the alteration in the number and morphology of the dendritic spines, and a decrease in long-term potentiation. Many studies has been carried out to identify the putative Aß receptors in neurons, and is still no clear why the Aß oligomers only affect the excitatory synapses. Aß oligomers bind to neurite and preferentially to the postsynaptic region, where the postsynaptic protein-95 (PSD-95) is present in the glutamatergic synapse, and interacts directly with the N-methyl-D-aspartate receptor (NMDAR) and neuroligin (NL). NL is a postsynaptic protein which binds to the presynaptic protein, neurexin to form a heterophilic adhesion complex, the disruption of this interaction affects the integrity of the synaptic contact. Structurally, NL has an extracellular domain homolog to acetylcholinesterase, the first synaptic protein that was found to interact with Aß. In the present review we will document the interaction between Aß and the extracellular domain of NL-1 at the excitatory synapse, as well as the interaction with other postsynaptic components, including the glutamatergic receptors (NMDA and mGluR5), the prion protein, the neurotrophin receptor, and the α7-nicotinic acetylcholine receptor. We conclude that several Aß oligomers receptors exist at the excitatory synapse, which could be the responsible for the neurotoxic effect described for the Aß oligomers. The characterization of the interaction between Aß receptors and Aß oligomers could help to understand the source of the neurologic damage observed in the brain of the Alzheimer's disease patients.