Margarita C. Dinamarca

Amyloid–cholinesterase interactions

Acetylcholinesterase is an enzyme associated with senile plaques. Biochemical studies have indicated that acetylcholinesterase induces amyloid fibril formation by interaction throughout the peripherical anionic site of the enzyme forming highly toxic acetylcholinesterase–amyloid‐β peptide (Aβ) complexes. The pro‐aggregating acetylcholinesterase effect is associated with the intrinsic amyloidogenic properties of the corresponding Aβ peptide. The neurotoxicity induced by acetylcholinesterase–Aβ complexes is higher than the that induced by the Aβ peptide alone, both in vitro and in vivo. The fact that acetylcholinesterase accelerates amyloid formation and the effect is sensitive to peripherical anionic site blockers of the enzyme, suggests that specific and new acetylcholinesterase inhibitors may well provide an attractive possibility for treating Alzheimer’s disease. Recent studies also indicate that acetylcholinesterase induces the aggregation of prion protein with a similar dependence on the peripherical anionic site.

Peroxisomal Proliferation Protects from β-Amyloid Neurodegeneration

Alzheimer disease is a neurodegenerative process that leads to severe cognitive impairment as a consequence of selective death of neuronal populations. The molecular pathogenesis of Alzheimer disease involves the participation of the β-amyloid peptide (Aβ) and oxidative stress. We report here that peroxisomal proliferation attenuated Aβ-dependent toxicity in hippocampal neurons. Pretreatment with Wy-14.463 (Wy), a peroxisome proliferator, prevent the neuronal cell death and neuritic network loss induced by the Aβ peptide. Moreover, the hippocampal neurons treated with this compound, showed an increase in the number of peroxisomes, with a concomitant increase in catalase activity. Additionally, we evaluate the Wy protective effect on β-catenin levels, production of intracellular reactive oxygen species, cytoplasmic calcium uptake, and mitochondrial potential in hippocampal neurons exposed to H2 O2 and Aβ peptide. Results show that the peroxisomal proliferation prevents β-catenin degradation, reactive oxygen species production, cytoplasmic calcium increase, and changes in mitochondrial viability. Our data suggest, for the first time, a direct link between peroxisomal proliferation and neuroprotection from Aβ-induced degenerative changes.

Acetylcholinesterase-Aβ Complexes Are More Toxic than Aβ Fibrils in Rat Hippocampus: Effect on Rat β-Amyloid Aggregation, Laminin Expression, Reactive Astrocytosis, and Neuronal Cell Loss

Neuropathological changes generated by human amyloid-β peptide (Aβ) fibrils and Aβ-acetylcholinesterase (Aβ-AChE) complexes were compared in rat hippocampus in vivo. Results showed that Aβ-AChE complexes trigger a more dramatic response in situ than Aβ fibrils alone as characterized by the following features observed 8 weeks after treatment: 1) amyloid deposits were larger than those produced in the absence of AChE. In fact, AChE strongly stimulates rat Aβ aggregation in vitro as shown by turbidity measurements, Congo Red binding, as well as electron microscopy, suggesting that Aβ-AChE deposits observed in vivo probably recruited endogenous Aβ peptide; 2) the appearance of laminin expressing neurons surrounding Aβ-AChE deposits (such deposits are resistant to disaggregation by laminin in vitro); 3) an extensive astrocytosis revealed by both glial fibrillary acidic protein immunoreactivity and number counting of reactive hypertrophic astrocytes; and 4) a stronger neuronal cell loss in comparison with Aβ-injected animals. We conclude that the hippocampal injection of Aβ-AChE complexes results in the appearance of some features reminiscent of Alzheimer-like lesions in rat brain. Our studies are consistent with the notion that Aβ-AChE complexes are more toxic than Aβ fibrils and that AChE triggered some of the neurodegenerative changes observed in Alzheimers disease brains.

Structure-Function Implications in Alzheimers Disease: Effect of Aβ Oligomers at Central Synapses

Alzheimers disease (AD) is the most prevalent neurodegenerative disease in the growing population of elderly people. A characteristic of AD is the accumulation of plaques in the brain of AD patients, and theses plaques mainly consist of aggregates of amyloid beta-peptide (Abeta). All converging lines of evidence suggest that progressive accumulation of the Abeta plays a central role in the genesis of Alzheimers disease and it was long understood that Abeta had to be assembled into extracellular amyloid fibrils to exert its cytotoxic effects. This process could be modulated by molecular chaperones which inhibit or accelerate the amyloid formation. The enzyme Acetylcholinesterase (AChE) induces Abeta fibrils formation, forming a stable complex highly neurotoxic. On the other hand, laminin inhibit the Abeta fibrils formation and depolymerizate Abeta fibrils also. Over the past decade, data have emerged from the use of several sources of Abeta (synthetic, cell culture, transgenic mice and human brain) to suggest that intermediate species called Abeta oligomers are also injurious. Accumulating evidence suggests that soluble forms of Abeta are indeed the proximate effectors of synapse loss and neuronal injury. On the other hand, the member of the Wnt signaling pathway, beta-catenin was markedly reduced in AD patients carrying autosomal dominant PS-1. Also, neurons incubated with Abeta revealed a significant dose-dependent decrease in the levels of cytosolic beta-catenin an effect which was reversed in cells co-incubated with increasing concentrations of lithium, an activator of Wnt signaling pathway. Wnt signaling blocks the behavioural impairments induced by hippocampal injection of Abeta, therefore the activation of Wnt signaling protects against the Abeta neurotoxicity. Here we review recent progress about Abeta structure and function, from the formation of amyloid fibrils and some molecular chaperones which modulate the amyloidogenesic process to synaptic damage induce by Abeta oligomers.

Hyperforin prevents β -amyloid neurotoxicity and spatial memory impairments by disaggregation of Alzheimer's amyloid- β -deposits

The major protein constituent of amyloid deposits in Alzheimers disease (AD) is the amyloid β-peptide (Aβ). In the present work, we have determined the effect of hyperforin an acylphloroglucinol compound isolated from Hypericum perforatum (St Johns Wort), on Aβ-induced spatial memory impairments and on Aβ neurotoxicity. We report here that hyperforin: (1) decreases amyloid deposit formation in rats injected with amyloid fibrils in the hippocampus; (2) decreases the neuropathological changes and behavioral impairments in a rat model of amyloidosis; (3) prevents Aβ-induced neurotoxicity in hippocampal neurons both from amyloid fibrils and Aβ oligomers, avoiding the increase in reactive oxidative species associated with amyloid toxicity. Both effects could be explained by the capacity of hyperforin to disaggregate amyloid deposits in a dose and time-dependent manner and to decrease Aβ aggregation and amyloid formation. Altogether these evidences suggest that hyperforin may be useful to decrease amyloid burden and toxicity in AD patients, and may be a putative therapeutic agent to fight the disease.

Postsynaptic Receptors for Amyloid-β Oligomers as Mediators of Neuronal Damage in Alzheimer’s Disease

The neurotoxic effect of amyloid-β peptide (Aβ) 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 Aβ receptors in neurons, and is still no clear why the Aβ oligomers only affect the excitatory synapses. Aβ 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 Aβ. In the present review we will document the interaction between Aβ 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 Aβ oligomers receptors exist at the excitatory synapse, which could be the responsible for the neurotoxic effect described for the Aβ oligomers. The characterization of the interaction between Aβ receptors and Aβ oligomers could help to understand the source of the neurologic damage observed in the brain of the Alzheimer’s disease patients.

Neurobiological effects of hyperforin and its potential in Alzheimer's disease therapy

St. Johns Wort (SJW) has been used medicinally for over 5,000 years. Relatively recently, one of its phloroglucinol derivatives, hyperforin, has emerged as a compound of interest. Hyperforin first gained attention as the constituent of SJW responsible for its antidepressant effects. Since then, several of its neurobiological effects have been described, including neurotransmitter re-uptake inhibition, the ability to increase intracellular sodium and calcium levels, canonical transient receptor potential 6 (TRPC6) activation, N-methyl-D-aspartic acid (NMDA) receptor antagonism as well as antioxidant and anti-inflammatory properties. Until recently, its pharmacological actions outside of depression had not been investigated. However, hyperforin has been shown to have cognitive enhancing and memory facilitating properties. Importantly, it has been shown to have neuroprotective effects against Alzheimers disease (AD) neuropathology, including the ability to disassemble amyloid-beta (Abeta) aggregates in vitro, decrease astrogliosis and microglia activation, as well as improve spatial memory in vivo. This review will examine some of the early studies involving hyperforin and its effects in the central nervous system (CNS), with an emphasis on its potential use in AD therapy. With further investigation, hyperforin could emerge to be a likely therapeutical candidate in the treatment of this disease.

β-Amyloid Oligomers Affect the Structure and Function of the Postsynaptic Region: Role of the Wnt Signaling Pathway

Background: Alzheimer’s disease (AD) is the most prevalent neurodegenerative disease in the growing population of elderly people. Synaptic dysfunction is an early manifestation of AD. The cellular mechanism by which β-amyloid peptide (Aβ) affects synapses remains unclear. Aβ oligomers target synapses in cultured rat hippocampal neurons suggesting that they play a key role in the regulation of synapses. Objective: The aim of this work is to study the effect of Aβ oligomers on the central synapses and the possible role of the Wnt signaling pathway in preventing the Aβ effects. Methods: We used rat hippocampal neurons, immunofluorescence and western blot procedures to detect synaptic proteins. Results: Aβ oligomers induced a reduction of the postsynaptic density protein 95 (PSD-95) and the NMDA glutamate receptors. We found that Wnt-5a, a noncanonical Wnt ligand, prevents the decrease triggered by Aβ oligomers in the glutamate receptor and PSD-95. Conclusion: Altogether, our results suggest that Aβ oligomers decrease the synaptic responses by affecting the postsynaptic region at different levels. The Wnt signaling activation prevents synaptic damage induced by Aβ, which raises the possibility of a new therapeutic intervention for the treatment of synaptic changes observed in AD.

The soluble extracellular fragment of neuroligin-1 targets Aβ oligomers to the postsynaptic region of excitatory synapses

Amyloid-β oligomers (Aβo) play a major role in the synaptic dysfunction of Alzheimers disease (AD). Neuroligins are postsynaptic cell-adhesion molecules, that share an extracellular domain with high degree of similarity to acetylcholinesterase (AChE), one of the first putative Aβo receptors. We recently found that Aβo interact with the soluble N-terminal fragment of neuroligin-1 (NL-1). We report here that Aβo associate with NL-1 at excitatory hippocampal synapses, whereas almost no association was observed with neuroligin-2, an isoform present at inhibitory synapses. Studies using purified hippocampal postsynaptic densities indicate that NL-1 interacts with Aβo in a complex with GluN2B-containing NMDA receptors. Additionally, the soluble fragment of NL-1 was used as a scavenger for Aβo. Field excitatory postsynaptic potentials indicate that fragments of NL-1 protect hippocampal neurons from the impairment induced by Aβo. To our knowledge, this is the first report of the interaction between this extracellular fragment of NL-1 and Aβo, strongly suggest that NL-1 facilitates the targeting of Aβo to the postsynaptic regions of excitatory synapses.

Ring finger protein 10 is a novel synaptonuclear messenger encoding activation of NMDA receptors in hippocampus

Synapses and nuclei are connected by bidirectional communication mechanisms that enable information transfer encoded by macromolecules. Here, we identified RNF10 as a novel synaptonuclear protein messenger. RNF10 is activated by calcium signals at the postsynaptic compartment and elicits discrete changes at the transcriptional level. RNF10 is enriched at the excitatory synapse where it associates with the GluN2A subunit of NMDA receptors (NMDARs). Activation of synaptic GluN2A-containing NMDARs and induction of long term potentiation (LTP) lead to the translocation of RNF10 from dendritic segments and dendritic spines to the nucleus. In particular, we provide evidence for importin-dependent long-distance transport from synapto-dendritic compartments to the nucleus. Notably, RNF10 silencing prevents the maintenance of LTP as well as LTP-dependent structural modifications of dendritic spines. DOI: http://dx.doi.org/10.7554/eLife.12430.001