The Stimulatory Effects of Intracellular α-Synuclein on Synaptic Transmission Are Attenuated by 2-Octahydroisoquinolin-2(1H)-ylethanamine.

α-Synuclein (αSyn) species can be detected in synaptic boutons, where they play a crucial role in the pathogenesis of Parkinson's Disease (PD). However, the effects of intracellular αSyn species on synaptic transmission have not been thoroughly studied. Here, using patch-clamp recordings in hippocampal neurons, we report that αSyn oligomers (αSynO), intracellularly delivered through the patch electrode, produced a fast and potent effect on synaptic transmission, causing a substantial increase in the frequency, amplitude and transferred charge of spontaneous synaptic currents. We also found an increase in the frequency of miniature synaptic currents, suggesting an effect located at the presynaptic site of the synapsis. Furthermore, our in silico approximation using docking analysis and molecular dynamics simulations showed an interaction between a previously described small anti-amyloid beta (Aß) molecule, termed M30 (2-octahydroisoquinolin-2(1H)-ylethanamine), with a central hydrophobic region of αSyn. In line with this finding, our empirical data aimed to obtain oligomerization states with thioflavin T (ThT) and Western blot (WB) indicated that M30 interfered with αSyn aggregation and decreased the formation of higher-molecular-weight species. Furthermore, the effect of αSynO on synaptic physiology was also antagonized by M30, resulting in a decrease in the frequency, amplitude, and charge transferred of synaptic currents. Overall, the present results show an excitatory effect of intracellular αSyn low molecular-weight species, not previously described, that are able to affect synaptic transmission, and the potential of a small neuroactive molecule to interfere with the aggregation process and the synaptic effect of αSyn, suggesting that M30 could be a potential therapeutic strategy for synucleinopathies.

Synaptic dysregulation and hyperexcitability induced by intracellular amyloid beta oligomers.

Intracellular amyloid beta oligomer (iAßo) accumulation and neuronal hyperexcitability are two crucial events at early stages of Alzheimer's disease (AD). However, to date, no mechanism linking iAßo with an increase in neuronal excitability has been reported. Here, the effects of human AD brain-derived (h-iAßo) and synthetic (iAßo) peptides on synaptic currents and action potential firing were investigated in hippocampal neurons. Starting from 500 pM, iAßo rapidly increased the frequency of synaptic currents and higher concentrations potentiated the AMPA receptor-mediated current. Both effects were PKC-dependent. Parallel recordings of synaptic currents and nitric oxide (NO)-associated fluorescence showed that the increased frequency, related to pre-synaptic release, was dependent on a NO-mediated retrograde signaling. Moreover, increased synchronization in NO production was also observed in neurons neighboring those dialyzed with iAßo, indicating that iAßo can increase network excitability at a distance. Current-clamp recordings suggested that iAßo increased neuronal excitability via AMPA-driven synaptic activity without altering membrane intrinsic properties. These results strongly indicate that iAßo causes functional spreading of hyperexcitability through a synaptic-driven mechanism and offers an important neuropathological significance to intracellular species in the initial stages of AD, which include brain hyperexcitability and seizures.

Boldine Attenuates Synaptic Failure and Mitochondrial Deregulation in Cellular Models of Alzheimer's Disease.

Alzheimer's disease (AD) is the most common cause of senile dementia worldwide, characterized by both cognitive and behavioral deficits. Amyloid beta peptide (Aß) oligomers (AßO) have been found to be responsible for several pathological mechanisms during the development of AD, including altered cellular homeostasis and synaptic function, inevitably leading to cell death. Such AßO deleterious effects provide a way for identifying new molecules with potential anti-AD properties. Available treatments minimally improve AD symptoms and do not extensively target intracellular pathways affected by AßO. Naturally-derived compounds have been proposed as potential modifiers of Aß-induced neurodysfunction and cytotoxicity based on their availability and chemical diversity. Thus, the aim of this study was to evaluate boldine, an alkaloid derived from the bark and leaves of the Chilean tree Peumus boldus, and its capacity to block some dysfunctional processes caused by AßO. We examined the protective effect of boldine (1-10 µM) in primary hippocampal neurons and HT22 hippocampal-derived cell line treated with AßO (24-48 h). We found that boldine interacts with Aß in silico affecting its aggregation and protecting hippocampal neurons from synaptic failure induced by AßO. Boldine also normalized changes in intracellular Ca2+ levels associated to mitochondria or endoplasmic reticulum in HT22 cells treated with AßO. In addition, boldine completely rescued the decrease in mitochondrial membrane potential (ΔΨm) and the increase in mitochondrial reactive oxygen species, and attenuated AßO-induced decrease in mitochondrial respiration in HT22 hippocampal cells. We conclude that boldine provides neuroprotection in AD models by both direct interactions with Aß and by preventing oxidative stress and mitochondrial dysfunction. Additional studies are required to evaluate the effect of boldine on cognitive and behavioral deficits induced by Aß in vivo.

Gabapentin Inhibits Multiple Steps in the Amyloid Beta Toxicity Cascade.

Oligomeric ß-amyloid peptide (Aß) is one of the main neurotoxic agents of Alzheimer's disease (AD). Oligomers associate to neuronal membranes, forming "pore-like" structures that cause intracellular calcium and neurotransmitter dyshomeostasis, leading to synaptic failure and death. Through molecular screening targeting the C terminal region of Aß, a region involved in the toxic properties of the peptide, we detected an FDA approved compound, gabapentin (GBP), with neuroprotective effects against Aß toxicity. At micromolar concentrations, GBP antagonized peptide aggregation over time and reduced the Aß absorbance plateau to 28% of control. In addition, GBP decreased Aß association to membranes by almost half, and the effects of Aß on intracellular calcium in hippocampal neurons were antagonized without causing effects on its own. Finally, we found that GBP was able to block the synaptotoxicity induced by Aß in hippocampal neurons, increasing post-synaptic currents from 1.7 ± 0.9 to 4.2 ± 0.7 fC and mean relative fluorescence intensity values of SV2, a synaptic protein, from 0.7 ± 0.09 to 1.00 ± 0.08. The results show that GBP can interfere with Aß-induced toxicity by blocking multiple steps, resulting in neuroprotection, which justifies advancing toward additional animal and human studies.

Characterization of a new molecule capable of inhibiting several steps of the amyloid cascade in Alzheimer's disease.

INTRODUCTION: Alzheimer's disease (AD) is the most prevalent neurodegenerative disorder in elderly people. Existent therapies are directed at alleviating some symptoms, but are not effective in altering the course of the disease. METHODS: Based on our previous study that showed that an Aß-interacting small peptide protected against the toxic effects of amyloid-beta peptide (Aß), we carried out an array of in silico, in vitro, and in vivo assays to identify a molecule having neuroprotective properties. RESULTS: In silico studies showed that the molecule, referred to as M30 (2-Octahydroisoquinolin-2(1H)-ylethanamine), was able to interact with the Aß peptide. Additionally, in vitro assays showed that M30 blocked Aß aggregation, association to the plasma membrane, synaptotoxicity, intracellular calcium, and cellular toxicity, while in vivo experiments demonstrated that M30 induced a neuroprotective effect by decreasing the toxicity of Aß in the dentate gyrus of the hippocampus and improving the alteration in spatial memory in behavior assays. DISCUSSION: Therefore, we propose that this new small molecule could be a useful candidate for the additional development of a treatment against AD since it appears to block multiple steps in the amyloid cascade. Overall, since there are no drugs that effectively block the progression of AD, this approach represents an innovative strategy. SIGNIFICANCE: Currently, there is no effective treatment for AD and the expectations to develop an effective therapy are low. Using in silico, in vitro, and in vivo experiments, we identified a new compound that is able to inhibit Aß-induced neurotoxicity, specifically aggregation, association to neurons, synaptic toxicity, calcium dyshomeostasis and memory impairment induced by Aß. Because Aß toxicity is central to AD progression, the inhibition mediated by this new molecule might be useful as a therapeutic tool.

Effect of Cholesterol on Membrane Fluidity and Association of Aß Oligomers and Subsequent Neuronal Damage: A Double-Edged Sword.

Background: The beta-amyloid peptide (Aß) involved in Alzheimer's disease (AD) has been described to associate/aggregate on the cell surface disrupting the membrane through pore formation and breakage. However, molecular determinants involved for this interaction (e.g., some physicochemical properties of the cell membrane) are largely unknown. Since cholesterol is an important molecule for membrane structure and fluidity, we examined the effect of varying cholesterol content with the association and membrane perforation by Aß in cultured hippocampal neurons. Methods: To decrease or increase the levels of cholesterol in the membrane we used methyl-ß-cyclodextrin (MßCD) and MßCD/cholesterol, respectively. We analyzed if membrane fluidity was affected using generalized polarization (GP) imaging and the fluorescent dye di-4-ANEPPDHQ. Additionally membrane association and perforation was assessed using immunocytochemistry and electrophysiological techniques, respectively. Results: The results showed that cholesterol removal decreased the macroscopic association of Aß to neuronal membranes (fluorescent-puncta/20 µm: control = 18 ± 2 vs. MßCD = 10 ± 1, p < 0.05) and induced a facilitation of the membrane perforation by Aß with respect to control cells (half-time for maximal charge transferred: control = 7.2 vs. MßCD = 4.4). Under this condition, we found an increase in membrane fluidity (46 ± 3.3% decrease in GP value, p < 0.001). On the contrary, increasing cholesterol levels incremented membrane rigidity (38 ± 2.7% increase in GP value, p < 0.001) and enhanced the association and clustering of Aß (fluorescent-puncta/20 µm: control = 18 ± 2 vs. MßCD = 10 ± 1, p < 0.01), but inhibited membrane disruption. Conclusion: Our results strongly support the significance of plasma membrane organization in the toxic effects of Aß in hippocampal neurons, since fluidity can regulate distribution and insertion of the Aß peptide in the neuronal membrane.

The Level of NMDA Receptor in the Membrane Modulates Amyloid-ß Association and Perforation.

Alzheimer's disease is a neurodegenerative disorder that affects mostly the elderly. The main histopathological markers are the senile plaques formed by amyloid-ß peptide (Aß) aggregates that can perforate the plasma membrane of cells, increasing the intracellular calcium levels and releasing synaptic vesicles that finally lead to a delayed synaptic failure. Several membrane proteins and lipids interact with Aß affecting its toxicity in neurons. Here, we focus on NMDA receptors (NMDARs) as proteins that could be modulating the association and neurotoxic perforation induced by Aß on the plasma membrane. In fact, our results showed that decreasing NMDARs, using enzymatic or siRNA approaches, increased the association of Aß to the neurons. Furthermore, overexpression of NMDARs also resulted in an enhanced association between NMDA and Aß. Functionally, the reduction in membrane NMDARs augmented the process of membrane perforation. On the other hand, overexpressing NMDARs had a protective effect because Aß was now unable to cause membrane perforation, suggesting a complex relationship between Aß and NMDARs. Because previous studies have recognized that Aß oligomers are able to increase membrane permeability and produce amyloid pores, the present study supports the conclusion that NMDARs play a critical protective role on Aß actions in hippocampal neurons. These results could explain the lack of correlation between brain Aß burden and clinically observed dementia.

Membrane Damage Induced by Amyloid Beta and a Potential Link with Neuroinflammation.

It is well accepted that cortical and hippocampal synaptic densities are reduced in Alzheimer's disease (AD). These alterations in neuronal networking occur at the very onset of AD and may lead to the neuronal loss displayed in later stages of the disease, which is characterized by severe cognitive and behavioral impairments. Many studies suggest that amyloid-ß (Aß) oligomers are responsible for synaptic disconnections and neuronal death. The effects of Aß in different brain regions are pleotropic, thus suggesting a common mechanism for toxicity. One potential site for this mechanism of toxicity is the neuronal membrane. It is recognized that Aß can associate to the plasma membrane and induce the formation of pores after the interaction with lipids like GM1 and cholesterol, and proteins such as APP and NMDA receptors. After this early event, the membrane increases its permeability allowing the influx of small ions and larger molecules. Thus, one of the main toxic consequences of Aß oligomer interaction with neurons is an increase in intracellular Ca(2+) concentration that causes alterations in ionic homeostasis. It has been proposed that Aß perforates the membrane similarly to pore-forming toxins producing a series of effects that include synaptic failure and cell death. These actions of Aß appear to be potentiated by neuroinflammation, which results in a series of effects that, when prolonged, will affect membrane integrity, pore formation and cellular homeostasis. Here, we will review the most recent data on Aß actions at the membrane level and how its relationship with neuroinflammation could further potentiate brain impairment in AD. The notion of having drugs acting with dual inhibitory actions, inhibition of membrane damage and inflammation, could serve as a starting conceptual point for the development of new therapies for the disease.

Effects of ethanol on glycinergic synaptic currents in mouse spinal cord neurons.

Ethanol increased the frequency of miniature glycinergic currents [miniature inhibitory postsynaptic currents (mIPSCs)] in cultured spinal neurons. This effect was dependent on intracellular calcium augmentation, since preincubation with BAPTA (an intracellular calcium chelator) or thapsigargin [a sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) pump inhibitor] significantly attenuated this effect. Similarly, U73122 (a phospholipase C inhibitor) or 2-aminoethoxydiphenyl borate [2-APB, an inositol 1,4,5-trisphosphate (IP3) receptor (IP3R) inhibitor] reduced this effect. Block of ethanol action was also achieved after preincubation with Rp-cAMPS, inhibitor of the adenylate cyclase (AC)/PKA signaling pathway. These data suggest that there is a convergence at the level of IP3R that accounts for presynaptic ethanol effects. At the postsynaptic level, ethanol increased the decay time constant of mIPSCs in a group of neurons (30 ± 10% above control, n = 13/26 cells). On the other hand, the currents activated by exogenously applied glycine were consistently potentiated (55 ± 10% above control, n = 11/12 cells), which suggests that ethanol modulates synaptic and nonsynaptic glycine receptors (GlyRs) in a different fashion. Supporting the role of G protein modulation on ethanol responses, we found that a nonhydrolyzable GTP analog [guanosine 5'-O-(3-thiotriphosphate) (GTPγS)] increased the decay time constant in ∼50% of the neurons (28 ± 12%, n = 11/19 cells) but potentiated the glycine-activated Cl(-) current in most of the neurons examined (83 ± 29%, n = 7/9 cells). In addition, confocal microscopy showed that α1-containing GlyRs colocalized with Gß and Piccolo (a presynaptic cytomatrix protein) in ∼40% of synaptic receptor clusters, suggesting that colocalization of Gßγ and GlyRs might account for the difference in ethanol sensitivity at the postsynaptic level.

Inhibition of amyloid beta-induced synaptotoxicity by a pentapeptide derived from the glycine zipper region of the neurotoxic peptide.

A major characteristic of Alzheimer's disease is the presence of amyloid beta (Aß) oligomers and aggregates in the brain. Aß oligomers interact with the neuronal membrane inducing perforations, causing an influx of calcium ions and increasing the release of synaptic vesicles that leads to a delayed synaptic failure by vesicle depletion. Here, we identified a neuroprotective pentapeptide anti-Aß compound having the sequence of the glycine zipper region of the C-terminal of Aß (G33LMVG37). Docking and Förster resonance energy transfer experiments showed that G33LMVG37 interacts with Aß at the C-terminal region, which is important for Aß association and insertion into the lipid membrane. Furthermore, this pentapeptide interfered with Aß aggregation, association, and perforation of the plasma membrane. The synaptotoxicity induced by Aß after acute and chronic applications were abolished by G33LMVG37. These results provide a novel rationale for drug development against Alzheimer's disease.