Fresh and globular amyloid beta protein (1-42) induces rapid cellular degeneration: evidence for AbetaP channel-mediated cellular toxicity

Protective effect of trehalose sugar on amyloid-membrane interactions using BLM electrophysiology

Alzheimer's disease (AD) is a neurodegenerative disease characterized by dementia and memory loss in the elderly population. The amyloid-β peptide (Aβ) is one of the main pathogenic factors in AD and is known to cause damage to neuronal cellular membranes. There is no cure currently available for AD, and new approaches, including preventive strategies, are highly desirable. In this work, we explore the possibility of protecting neuronal membranes from amyloid-induced damage with naturally existing sugar trehalose. Trehalose has been shown to protect plant cellular membranes in extreme conditions and modify Aβ misfolding. We hypothesize that trehalose can protect the neuronal membrane from amyloid toxicity. In this work, we studied the protective effect of trehalose against Aβ1-42-induced damage in model lipid membranes (DPPC/POPC/cholesterol) using atomic force microscopy and black lipid membrane electrophysiology. Our results demonstrate that Aβ1-42 damaged membranes and led to ionic current leakage across these membranes due to the formation of various defects and pores. The presence of trehalose reduced the ion current across membranes caused by Aβ1-42 peptide damage, thus efficiently protecting the membranes. These findings suggest that the trehalose sugar can potentially be useful in protecting neuronal membranes against amyloid toxicity in AD.

The amyloid cascade hypothesis: an updated critical review

Results from recent clinical trials of antibodies that target amyloid-β (Aβ) for Alzheimer's disease have created excitement and have been heralded as corroboration of the amyloid cascade hypothesis. However, while Aβ may contribute to disease, genetic, clinical, imaging and biochemical data suggest a more complex aetiology. Here we review the history and weaknesses of the amyloid cascade hypothesis in view of the new evidence obtained from clinical trials of anti-amyloid antibodies. These trials indicate that the treatments have either no or uncertain clinical effect on cognition. Despite the importance of amyloid in the definition of Alzheimer's disease, we argue that the data point to Aβ playing a minor aetiological role. We also discuss data suggesting that the concerted activity of many pathogenic factors contribute to Alzheimer's disease and propose that evolving multi-factor disease models will better underpin the search for more effective strategies to treat the disease.

Amyloid beta peptides (Aβ) from Alzheimer's disease neuronal secretome induce endothelial activation in a human cerebral microvessel model

In Alzheimer's disease (AD), secretion and deposition of amyloid beta peptides (Aβ) have been associated with blood-brain barrier dysfunction. However, the role of Aβ in endothelial cell (EC) dysfunction remains elusive. Here we investigated AD mediated EC activation by studying the effect of Aβ secreted from human induced pluripotent stem cell-derived cortical neurons (hiPSC-CN) harboring a familial AD mutation (Swe^+/+) on human brain microvascular endothelial cells (HBMECs) in 2D and 3D perfusable microvessels. We demonstrated that increased Aβ levels in Swe^+/+ conditioned media (CM) led to stress fiber formation and upregulation of genes associated with endothelial inflammation and immune-adhesion. Perfusion of Aβ-rich Swe^+/+ CM induced acute formation of von Willebrand factor (VWF) fibers in the vessel lumen, which was attenuated by reducing Aβ levels in CM. Our findings suggest that Aβ peptides can trigger rapid inflammatory and thrombogenic responses within cerebral microvessels, which may exacerbate AD pathology.

Insights in Cell Biomechanics through Atomic Force Microscopy

We review the advances obtained by using Atomic Force Microscopy (AFM)-based approaches in the field of cell/tissue mechanics and adhesion, comparing the solutions proposed and critically discussing them. AFM offers a wide range of detectable forces with a high force sensitivity, thus allowing a broad class of biological issues to be addressed. Furthermore, it allows for the accurate control of the probe position during the experiments, providing spatially resolved mechanical maps of the biological samples with subcellular resolution. Nowadays, mechanobiology is recognized as a subject of great relevance in biotechnological and biomedical fields. Focusing on the past decade, we discuss the intriguing issues of cellular mechanosensing, i.e., how cells sense and adapt to their mechanical environment. Next, we examine the relationship between cell mechanical properties and pathological states, focusing on cancer and neurodegenerative diseases. We show how AFM has contributed to the characterization of pathological mechanisms and discuss its role in the development of a new class of diagnostic tools that consider cell mechanics as new tumor biomarkers. Finally, we describe the unique ability of AFM to study cell adhesion, working quantitatively and at the single-cell level. Again, we relate cell adhesion experiments to the study of mechanisms directly or secondarily involved in pathologies.

Current understanding of metal-dependent amyloid-β aggregation and toxicity

The discovery of effective therapeutics targeting amyloid-β (Aβ) aggregates for Alzheimer's disease (AD) has been very challenging, which suggests its complicated etiology associated with multiple pathogenic elements. In AD-affected brains, highly concentrated metals, such as copper and zinc, are found in senile plaques mainly composed of Aβ aggregates. These metal ions are coordinated to Aβ and affect its aggregation and toxicity profiles. In this review, we illustrate the current view on molecular insights into the assembly of Aβ peptides in the absence and presence of metal ions as well as the effect of metal ions on their toxicity.

The Interplay Between Cholesterol and Amyloid-β on HT22 Cell Viability, Morphology, and Receptor Tyrosine Kinase Signaling

BACKGROUND

There is a lack of understanding in the molecular and cellular mechanisms of Alzheimer's disease that has hindered progress on therapeutic development. The focus has been on targeting toxic amyloid-β (Aβ) pathology, but these therapeutics have generally failed in clinical trials. Aβ is an aggregation-prone protein that has been shown to disrupt cell membrane structure in molecular biophysics studies and interfere with membrane receptor signaling in cell and animal studies. Whether the lipid membrane or specific receptors are the primary target of attack has not been determined.

OBJECTIVE

This work elucidates some of the interplay between membrane cholesterol and Aβ42 on HT22 neuronal cell viability, morphology, and platelet-derived growth factor (PDGF) signaling pathways.

METHODS

The effects of cholesterol depletion by methyl-β-cyclodextrin followed by treatment with Aβ and/or PDGF-AA were assessed by MTT cell viability assays, western blot, optical and AFM microscopy.

RESULTS

Cell viability studies show that cholesterol depletion was mildly protective against Aβ toxicity. Together cholesterol reduction and Aβ42 treatment compounded the disruption of the PDGFα receptor activation. Phase contrast optical microscopy and live cell atomic force microscopy imaging revealed that cytotoxic levels of Aβ42 caused morphological changes including cell membrane damage, cytoskeletal disruption, and impaired cell adhesion; cell damage was ameliorated by cellular cholesterol depletion.

CONCLUSIONS

Cholesterol depletion impacted the effects of Aβ42 on HT22 cell viability, morphology, and receptor tyrosine kinase signaling.

Calcium Signaling Regulated by Cellular Membrane Systems and Calcium Homeostasis Perturbed in Alzheimer’s Disease

Although anything that changes spatiotemporally could be a signal, cells, particularly neurons, precisely manipulate calcium ion (Ca²⁺) to transmit information. Ca²⁺ homeostasis is indispensable for neuronal functions and survival. The cytosolic Ca²⁺ concentration ([Ca²⁺]_CYT) is regulated by channels, pumps, and exchangers on cellular membrane systems. Under physiological conditions, both endoplasmic reticulum (ER) and mitochondria function as intracellular Ca²⁺ buffers. Furthermore, efficient and effective Ca²⁺ flux is observed at the ER-mitochondria membrane contact site (ERMCS), an intracellular membrane juxtaposition, where Ca²⁺ is released from the ER followed by mitochondrial Ca²⁺ uptake in sequence. Hence, the ER intraluminal Ca²⁺ concentration ([Ca²⁺]_ER), the mitochondrial matrix Ca²⁺ concentration ([Ca²⁺]_MT), and the [Ca²⁺]_CYT are related to each other. Ca²⁺ signaling dysregulation and Ca²⁺ dyshomeostasis are associated with Alzheimer’s disease (AD), an irreversible neurodegenerative disease. The present review summarizes the cellular and molecular mechanism underlying Ca²⁺ signaling regulation and Ca²⁺ homeostasis maintenance at ER and mitochondria levels, focusing on AD. Integrating the amyloid hypothesis and the calcium hypothesis of AD may further our understanding of pathogenesis in neurodegeneration, provide therapeutic targets for chronic neurodegenerative disease in the central nervous system.

Intracellular ion changes induced by the exposure to beta-amyloid can be explained by the formation of channels in the lysosomal membranes

In this manuscript, we reassess the data on beta-amyloid-induced changes of intracellular ions concentrations published previously by Abramov et al. (2003, 2004). Their observations made using high-resolution confocal microscopy with fast temporal resolution of images formed by fluorescent ion-sensitive fluorescent probes in living cells represent an unequivocal support for the amyloid channel theory. However, closer look reveals multiple facts which cannot be explained by channel formation in plasma membrane. Recently proposed amyloid degradation toxicity hypothesis provides the interpretation to these facts by considering that channels are formed in the lysosomal membranes.

Synaptic plasticity in Alzheimer's disease and healthy aging

The strength and efficiency of synaptic connections are affected by the environment or the experience of the individual. This property, called synaptic plasticity, is directly related to memory and learning processes and has been modeled at the cellular level. These types of cellular memory and learning models include specific stimulation protocols that generate a long-term strengthening of the synapses, called long-term potentiation, or a weakening of the said long-term synapses, called long-term depression. Although, for decades, researchers have believed that the main cause of the cognitive deficit that characterizes Alzheimer's disease (AD) and aging was the loss of neurons, the hypothesis of an imbalance in the cellular and molecular mechanisms of synaptic plasticity underlying this deficit is currently widely accepted. An understanding of the molecular and cellular changes underlying the process of synaptic plasticity during the development of AD and aging will direct future studies to specific targets, resulting in the development of much more efficient and specific therapeutic strategies. In this review, we classify, discuss, and describe the main findings related to changes in the neurophysiological mechanisms of synaptic plasticity in excitatory synapses underlying AD and aging. In addition, we suggest possible mechanisms in which aging can become a high-risk factor for the development of AD and how its development could be prevented or slowed.

An Unbalanced Synaptic Transmission: Cause or Consequence of the Amyloid Oligomers Neurotoxicity?

Amyloid-β (Aβ) 1-40 and 1-42 peptides are key mediators of synaptic and cognitive dysfunction in Alzheimer's disease (AD). Whereas in AD, Aβ is found to act as a pro-epileptogenic factor even before plaque formation, amyloid pathology has been detected among patients with epilepsy with increased risk of developing AD. Among Aβ aggregated species, soluble oligomers are suggested to be responsible for most of Aβ's toxic effects. Aβ oligomers exert extracellular and intracellular toxicity through different mechanisms, including interaction with membrane receptors and the formation of ion-permeable channels in cellular membranes. These damages, linked to an unbalance between excitatory and inhibitory neurotransmission, often result in neuronal hyperexcitability and neural circuit dysfunction, which in turn increase Aβ deposition and facilitate neurodegeneration, resulting in an Aβ-driven vicious loop. In this review, we summarize the most representative literature on the effects that oligomeric Aβ induces on synaptic dysfunction and network disorganization.

Calcium Dyshomeostasis in Alzheimer's Disease Pathogenesis

Alzheimer’s disease (AD) is the most common age-related neurodegenerative disorder that is characterized by amyloid β-protein deposition in senile plaques, neurofibrillary tangles consisting of abnormally phosphorylated tau protein, and neuronal loss leading to cognitive decline and dementia. Despite extensive research, the exact mechanisms underlying AD remain unknown and effective treatment is not available. Many hypotheses have been proposed to explain AD pathophysiology; however, there is general consensus that the abnormal aggregation of the amyloid β peptide (Aβ) is the initial event triggering a pathogenic cascade of degenerating events in cholinergic neurons. The dysregulation of calcium homeostasis has been studied considerably to clarify the mechanisms of neurodegeneration induced by Aβ. Intracellular calcium acts as a second messenger and plays a key role in the regulation of neuronal functions, such as neural growth and differentiation, action potential, and synaptic plasticity. The calcium hypothesis of AD posits that activation of the amyloidogenic pathway affects neuronal Ca²⁺ homeostasis and the mechanisms responsible for learning and memory. Aβ can disrupt Ca²⁺ signaling through several mechanisms, by increasing the influx of Ca²⁺ from the extracellular space and by activating its release from intracellular stores. Here, we review the different molecular mechanisms and receptors involved in calcium dysregulation in AD and possible therapeutic strategies for improving the treatment.

Ion channel formation by N-terminally truncated Aβ (4-42): relevance for the pathogenesis of Alzheimer's disease

Aβ deposition is a pathological hallmark of Alzheimer's disease (AD). Besides the full-length amyloid forming peptides (Aβ_1-40 and Aβ_1-42), biochemical analyses of brain deposits have identified a variety of N- and C-terminally truncated Aβ variants in sporadic and familial AD patients. However, their relevance for AD pathogenesis remains largely understudied. We demonstrate that Aβ_4-42 exhibits a high tendency to form β-sheet structures leading to fast self-aggregation and formation of oligomeric assemblies. Atomic force microscopy and electrophysiological studies reveal that Aβ_4-42 forms highly stable ion channels in lipid membranes. These channels that are blocked by monoclonal antibodies specifically recognizing the N-terminus of Aβ_4-42. An Aβ variant with a double truncation at phenylalanine-4 and leucine 34, (Aβ_4-34), exhibits unstable channel formation capability. Taken together the results presented herein highlight the potential benefit of C-terminal proteolytic cleavage and further support an important pathogenic role for N-truncated Aβ species in AD pathophysiology.

Annexin A5 prevents amyloid-β-induced toxicity in choroid plexus: implication for Alzheimer's disease

In Alzheimer’s disease (AD) amyloid-β (Aβ) deposits may cause impairments in choroid plexus, a specialised brain structure which forms the blood–cerebrospinal fluid (CSF) barrier. We previously carried out a mass proteomic-based study in choroid plexus from AD patients and we found several differentially regulated proteins compared with healthy subjects. One of these proteins, annexin A5, was previously demonstrated implicated in blocking Aβ-induced cytotoxicity in neuronal cell cultures. Here, we investigated the effects of annexin A5 on Aβ toxicity in choroid plexus. We used choroid plexus tissue samples and CSF from mild cognitive impairment (MCI) and AD patients to analyse Aβ accumulation, cell death and annexin A5 levels compared with control subjects. Choroid plexus cell cultures from rats were used to analyse annexin A5 effects on Aβ-induced cytotoxicity. AD choroid plexus exhibited progressive reduction of annexin A5 levels along with progressive increased Aβ accumulation and cell death as disease stage was higher. On the other hand, annexin A5 levels in CSF from patients were found progressively increased as the disease stage increased in severity. In choroid plexus primary cultures, Aβ administration reduced endogenous annexin A5 levels in a time-course dependent manner and simultaneously increased annexin A5 levels in extracellular medium. Annexin A5 addition to choroid plexus cell cultures restored the Aβ-induced impairments on autophagy flux and apoptosis in a calcium-dependent manner. We propose that annexin A5 would exert a protective role in choroid plexus and this protection is lost as Aβ accumulates with the disease progression. Then, brain protection against further toxic insults would be jeopardised.

Medin Oligomer Membrane Pore Formation: A Potential Mechanism of Vascular Dysfunction

Medin, a 50-amino-acid cleavage product of the milk fat globule-EGF factor 8 protein, is one of the most common forms of localized amyloid found in the vasculature of individuals older than 50 years. Medin induces endothelial dysfunction and vascular inflammation, yet despite its prevalence in the human aorta and multiple arterial beds, little is known about the nature of its pathology. Medin oligomers have been implicated in the pathology of aortic aneurysm, aortic dissection, and more recently, vascular dementia. Recent in vitro biomechanical measurements found increased oligomer levels in aneurysm patients with altered aortic wall integrity. Our results suggest an oligomer-mediated toxicity mechanism for medin pathology. Using lipid bilayer electrophysiology, we show that medin oligomers induce ionic membrane permeability by pore formation. Pore activity was primarily observed for preaggregated medin species from the growth-phase and rarely for lag-phase species. Atomic force microscopy (AFM) imaging of medin aggregates at different stages of aggregation revealed the gradual formation of flat domains resembling the morphology of supported lipid bilayers. Transmission electron microscopy images showed the coexistence of compact oligomers, largely consistent with the AFM data, and larger protofibrillar structures. Circular dichroism spectroscopy revealed the presence of largely disordered species and suggested the presence of β-sheets. This observation and the significantly lower thioflavin T fluorescence emitted by medin aggregates compared to amyloid-β fibrils, along with the absence of amyloid fibers in the AFM and transmission electron microscopy images, suggest that medin aggregation into pores follows a nonamyloidogenic pathway. In silico modeling by molecular dynamics simulations provides atomic-level structural detail of medin pores with the CNpNC barrel topology and diameters comparable to values estimated from experimental pore conductances.

The Na+/Ca2+exchanger in Alzheimer's disease

As a pivotal player in regulating sodium (Na⁺) and calcium (Ca²⁺) homeostasis and signalling in excitable cells, the Na⁺/Ca²⁺ exchanger (NCX) is involved in many neurodegenerative disorders in which an imbalance of intracellular Ca²⁺ and/or Na⁺ concentrations occurs, including Alzheimer's disease (AD). Although NCX has been mainly implicated in neuroprotective mechanisms counteracting Ca²⁺ dysregulation, several studies highlighted its role in the neuronal responses to intracellular Na⁺ elevation occurring in several pathophysiological conditions. Since the alteration of Na⁺ and Ca²⁺ homeostasis significantly contributes to synaptic dysfunction and neuronal loss in AD, it is of crucial importance to analyze the contribution of NCX isoforms in the homeostatic responses at neuronal and synaptic levels. Some studies found that an increase of NCX activity in brains of AD patients was correlated with neuronal survival, while other research groups found that protein levels of two NCX subtypes, NCX2 and NCX3, were modulated in parietal cortex of late stage AD brains. In particular, NCX2 positive synaptic terminals were increased in AD cohort while the number of NCX3 positive terminals were reduced. In addition, NCX1, NCX2 and NCX3 isoforms were up-regulated in those synaptic terminals accumulating amyloid-beta (Aβ), the neurotoxic peptide responsible for AD neurodegeneration. More recently, the hyperfunction of a specific NCX subtype, NCX3, has been shown to delay endoplasmic reticulum stress and apoptotic neuronal death in hippocampal neurons exposed to Aβ insult. Despite some issues about the functional role of NCX in synaptic failure and neuronal loss require further studies, these findings highlight the putative neuroprotective role of NCX in AD and open new strategies to develop new druggable targets for AD therapy.

Cholesterol Modulates the Formation of the Aβ Ion Channel in Lipid Bilayers

The misfolding of amyloid beta (Aβ) is one of the predominant hallmarks in the pathology of Alzheimer's disease (AD). In this study, we showed that the formation of the Aβ ion channel on the membrane depended on the cholesterol concentration. From a mechanical aspect, we found that cholesterol levels affected the stability and assembly of lipid bilayers. Measurements on planar lipid bilayers indicated that a small amount of cholesterol interacted with Aβ proteins and promoted the insertion process. Conversely, high cholesterol integrated the lipid bilayer and exerted an opposite effect on Aβ insertion. The Aβ ion channel was then detected by graphene-based field-effect transistors. Results demonstrated that the Aβ ion channel promoted a Ca²⁺ flux in the presence of 15% cholesterol but prevented a Ca²⁺ flux in high cholesterol. Thus, cholesterol had a complex impact on the Aβ ion channel that can be described as two different effects. First, a small amount of cholesterol interacted with Aβ and facilitated the Aβ ion channel formation in the membrane. Second, a large amount of cholesterol did not induce the ion flux in the membrane, which can be explained by the cholesterol damage to the regular distribution of the lipid bilayer. Overall, this study suggested a possible approach to consider cholesterol levels for the treatment of AD patients.

Spatial high resolution of actin filament organization by PeakForce atomic force microscopy

Objectives

To investigate the heterogeneous feature of actin filaments (ACFs) associated with the cellular membrane in HeLa and HCT‐116 cells at the nanoscale level.

Materials and Methods

Fluorescence microscopy coupled with atomic force microscopy (AFM) was used to identify and characterize ACFs of cells. The distribution of ACFs was detected by Fluor‐488‐phalloidin–labelled actin. The morphology of the ACFs was probed by AFM images. The spatial correlation of the microvilli and ACFs was explored with different forces of AFM loading on cells.

Results

Intricate but ordered structures of the actin cytoskeletons associated with cellular membrane were characterized and revealed. Two different layers of ACFs with distinct structural organizations were directly observed in HCT‐116 and HeLa cells. Bundle‐shaped ACFs protruding the cellular membrane forming the microvilli, and the network ACFs underneath the cellular membrane were resolved with high resolution under near‐physiological conditions. Approximately 14 nm lateral resolution was achieved when imaging single ACF beneath the cellular membrane. On the basis of the observed spatial distribution of the ultrastructure of the ACF organization, a model for this organization of ACFs was proposed.

Conclusions

We revealed the two layers of the ACF organization in Hela and HCT‐116 cells. The resolved heterogeneous structures at the nanoscale level provide a spatial view of the ACFs, which would contribute to the understanding of the essential biological functions of the actin cytoskeleton.

Metal binding to the amyloid-β peptides in the presence of biomembranes: potential mechanisms of cell toxicity

The amyloid-β (Aβ) peptides are key molecules in Alzheimer's disease (AD) pathology. They interact with cellular membranes, and can bind metal ions outside the membrane. Certain oligomeric Aβ aggregates are known to induce membrane perturbations and the structure of these oligomers-and their membrane-perturbing effects-can be modulated by metal ion binding. If the bound metal ions are redox active, as e.g., Cu and Fe ions are, they will generate harmful reactive oxygen species (ROS) just outside the membrane surface. Thus, the membrane damage incurred by toxic Aβ oligomers is likely aggravated when redox-active metal ions are present. The combined interactions between Aβ oligomers, metal ions, and biomembranes may be responsible for at least some of the neuronal death in AD patients.

Amyloid Peptide β1-42 Induces Integrin αIIbβ3 Activation, Platelet Adhesion, and Thrombus Formation in a NADPH Oxidase-Dependent Manner

The progression of Alzheimer's dementia is associated with neurovasculature impairment, which includes inflammation, microthromboses, and reduced cerebral blood flow. Here, we investigate the effects of β amyloid peptides on the function of platelets, the cells driving haemostasis. Amyloid peptide β1-42 (Aβ1-42), Aβ1-40, and Aβ25-35 were tested in static adhesion experiments, and it was found that platelets preferentially adhere to Aβ1-42 compared to other Aβ peptides. In addition, significant platelet spreading was observed over Aβ1-42, while Aβ1-40, Aβ25-35, and the scAβ1-42 control did not seem to induce any platelet spreading, which suggested that only Aβ1-42 activates platelet signalling in our experimental conditions. Aβ1-42 also induced significant platelet adhesion and thrombus formation in whole blood under venous flow condition, while other Aβ peptides did not. The molecular mechanism of Aβ1-42 was investigated by flow cytometry, which revealed that this peptide induces a significant activation of integrin αIIbβ3, but does not induce platelet degranulation (as measured by P-selectin membrane translocation). Finally, Aβ1-42 treatment of human platelets led to detectable levels of protein kinase C (PKC) activation and tyrosine phosphorylation, which are hallmarks of platelet signalling. Interestingly, the NADPH oxidase (NOX) inhibitor VAS2870 completely abolished Aβ1-42-dependent platelet adhesion in static conditions, thrombus formation in physiological flow conditions, integrin αIIbβ3 activation, and tyrosine- and PKC-dependent platelet signalling. In summary, this study highlights the importance of NOXs in the activation of platelets in response to amyloid peptide β1-42. The molecular mechanisms described in this manuscript may play an important role in the neurovascular impairment observed in Alzheimer's patients.

Calcium signaling in Alzheimer's disease & therapies

Alzheimer's disease (AD) is the most common type of dementia and is characterized by the accumulation of amyloid (Aβ) plaques and neurofibrillary tangles in the brain. Much attention has been given to develop AD treatments based on the amyloid cascade hypothesis; however, none of these drugs had good efficacy at improving cognitive functions in AD patients suggesting that Aβ might not be the disease origin. Thus, there are urgent needs for the development of new therapies that target on the proximal cause of AD. Cellular calcium (Ca²⁺) signals regulate important facets of neuronal physiology. An increasing body of evidence suggests that age-related dysregulation of neuronal Ca²⁺ homeostasis may play a proximal role in the pathogenesis of AD as disrupted Ca²⁺ could induce synaptic deficits and promote the accumulation of Aβ plaques and neurofibrillary tangles. Given that Ca²⁺ disruption is ubiquitously involved in all AD pathologies, it is likely that using chemical agents or small molecules specific to Ca²⁺ channels or handling proteins on the plasma membrane and membranes of intracellular organelles to correct neuronal Ca²⁺ dysregulation could open up a new approach to AD prevention and treatment. This review summarizes current knowledge on the molecular mechanisms linking Ca²⁺ dysregulation with AD pathologies and discusses the possibility of correcting neuronal Ca²⁺ disruption as a therapeutic approach for AD.

Blood-Brain Barrier Disruption and Perivascular Beta-Amyloid Accumulation in the Brain of Aged Rats with Spontaneous Hypertension: Evaluation with Dynamic Contrast-Enhanced Magnetic Resonance Imaging

Objective

Whether blood-brain barrier (BBB) disruption induced by chronic spontaneous hypertension is associated with beta-amyloid (Aβ) accumulation in the brain remains poorly understood. The purpose of this study was to investigate the relationship between BBB disruption and Aβ influx and accumulation in the brain of aged rats with chronic spontaneous hypertension.

Materials and Methods

Five aged spontaneously hypertensive rats (SHRs) and five age-matched normotensive Wistar-Kyoto (WKY) rats were studied. The volume transfer constant (K^trans) obtained from dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) was used to evaluate BBB permeability in the hippocampus and cortex in vivo. The BBB tight junctions, immunoglobulin G (IgG), Aβ, and amyloid precursor protein (APP) in the hippocampus and cortex were examined with immunohistochemistry.

Results

As compared with WKY rats, the K^trans values in the hippocampus and cortex of the SHRs increased remarkably (0.316 ± 0.027 min^-1 vs. 0.084 ± 0.017 min^-1, p < 0.001 for hippocampus; 0.302 ± 0.072 min^-1 vs. 0.052 ± 0.047 min^-1, p < 0.001 for cortex). Dramatic occludin and zonula occludens-1 losses were detected in the hippocampus and cortex of SHRs, and obvious IgG exudation was found there. Dramatic Aβ accumulation was found and limited to the area surrounding the BBB, without extension to other parenchyma regions in the hippocampus and cortex of aged SHRs. Alternatively, differences in APP expression in the hippocampus and cortex were not significant.

Conclusion

Blood-brain barrier disruption is associated with Aβ influx and accumulation in the brain of aged rats with chronic spontaneous hypertension. DCE-MRI can be used as an effective method to investigated BBB damage.

Multiple mechanisms of dimethyl fumarate in amyloid β-induced neurotoxicity in human neuronal cells

Alzheimer disease (AD) is characterized by a complex heterogeneity of pathological changes, and any therapeutic approach categorically requires a multi-targeted way. It has been demonstrated that together with the hallmarks of the disease such as neurofibrillary tangles and senile plaques, oxidative and inflammatory stress covered an important role. Dimethyl fumarate (DMF) is an orally bioavailable methyl ester of fumaric acid and activator of Nrf2 with potential neuroprotective and immunomodulating activities. Therefore, the aim of the present work was to evaluate the potential beneficial effects of DMF, compared with its active metabolite monomethyl fumarate (MMF) (both at 30 μM) in an in vitro Alzheimer's model using SH-SY5Y human neuroblastoma cell lines stimulated with amyloid-beta (Aβ). Moreover, the effect of DMF, compared with MMF, was evaluate by an ex vivo model using organotypic hippocampal slice cultures stimulated with Aβ1-42 (1 μg/ml), to better understand its action in a pathological setting. In both models, DMF pre-treatment (30 μM) preserved cellular viability from Aβ stimulation, reducing tau hyper-phosphorylation, much more efficiently then MMF (30 μM). Moreover, DMF was able to induce an activation of manganese superoxide dismutase (MnSOD) and heme-oxygenase-1 (HO-1), decreasing the severity of oxidative stress. Our results showed important multi-protective effects of DMF pre-treatment from Aβ stimulation both in in vitro and ex vivo models, highlighting an Nrf2/NF-κB-dependent mechanism, which could provide a valuable support to the therapies for neurodegenerative diseases today.

Methods for the Specific Detection and Quantitation of Amyloid-β Oligomers in Cerebrospinal Fluid

Protein misfolding and aggregation are fundamental features of the majority of neurodegenerative diseases, like Alzheimer's disease (AD), Parkinson's disease, frontotemporal dementia, and prion diseases. Proteinaceous deposits in the brain of the patient, e.g., amyloid plaques consisting of the amyloid-β (Aβ) peptide and tangles composed of tau protein, are the hallmarks of AD. Soluble oligomers of Aβ and tau play a fundamental role in disease progression, and specific detection and quantification of the respective oligomeric proteins in cerebrospinal fluid may provide presymptomatically detectable biomarkers, paving the way for early diagnosis or even prognosis. Several studies on the development of techniques for the specific detection of Aβ oligomers were published, but some of the existing tools do not yet seem to be satisfactory, and the study results are contradicting. The detection of oligomers is challenging due to their polymorphous and unstable nature, their low concentration, and the presence of competing proteins and Aβ monomers in body fluids. Here, we present an overview of the current state of the development of methods for Aβ oligomer specific detection and quantitation. The methods are divided in the three subgroups: (i) enzyme linked immunosorbent assays (ELISA), (ii) methods for single oligomer detection, and (iii) others, which are mainly biosensor based methods.

Expression and Processing of Amyloid Precursor Protein in Vascular Endothelium

Amyloid precursor protein (APP) is evolutionary conserved protein expressed in endothelial cells of cerebral and peripheral arteries. In this review, we discuss mechanisms responsible for expression and proteolytic cleavage of APP in endothelial cells. We focus on physiological and pathological implications of APP expression in vascular endothelium.

Amyloid and membrane complexity: The toxic interplay revealed by AFM

Lipid membranes play a fundamental role in the pathological development of protein misfolding diseases. Several pieces of evidence suggest that the lipid membrane could act as a catalytic surface for protein aggregation. Furthermore, a leading theory indicates the interaction between the cell membrane and misfolded oligomer species as the responsible for cytotoxicity, hence, for neurodegeneration in disorders such as Alzheimer's and Parkinson's disease. The definition of the mechanisms that drive the interaction between pathological protein aggregates and plasma membrane is fundamental for the development of effective therapies for a large class of diseases. Atomic force microscopy (AFM) has been employed to study how amyloid aggregates affect the cell physiological properties. Considerable efforts were spent to characterize the interaction with model systems, i.e., planar supported lipid bilayers, but some works also addressed the problem directly on living cells. Here, an overview of the main works involving the use of the AFM on both model system and living cells will be provided. Different kind of approaches will be presented, as well as the main results derived from the AFM analysis.

Amyloid β Ion Channels in a Membrane Comprising Brain Total Lipid Extracts

Amyloid β (Aβ) oligomers are the predominant toxic species in the pathology of Alzheimer's disease. The prevailing mechanism for toxicity by Aβ oligomers includes ionic homeostasis destabilization in neuronal cells by forming ion channels. These channel structures have been previously studied in model lipid bilayers. In order to gain further insight into the interaction of Aβ oligomers with natural membrane compositions, we have examined the structures and conductivities of Aβ oligomers in a membrane composed of brain total lipid extract (BTLE). We utilized two complementary techniques: atomic force microscopy (AFM) and black lipid membrane (BLM) electrical recording. Our results indicate that Aβ1-42 forms ion channel structures in BTLE membranes, accompanied by a heterogeneous population of ionic current fluctuations. Notably, the observed current events generated by Aβ1-42 peptides in BTLE membranes possess different characteristics compared to current events generated by the presence of Aβ1-42 in model membranes comprising a 1:1 mixture of DOPS and POPE lipids. Oligomers of the truncated Aβ fragment Aβ17-42 (p3) exhibited similar ion conductivity behavior as Aβ1-42 in BTLE membranes. However, the observed macroscopic ion flux across the BTLE membranes induced by Aβ1-42 pores was larger than for p3 pores. Our analysis of structure and conductance of oligomeric Aβ pores in a natural lipid membrane closely mimics the in vivo cellular environment suggesting that Aβ pores could potentially accelerate the loss of ionic homeostasis and cellular abnormalities. Hence, these pore structures may serve as a target for drug development and therapeutic strategies for AD treatment.

Investigation of the Subcellular Neurotoxicity of Amyloid-β Using a Device Integrating Microfluidic Perfusion and Chemotactic Guidance

Alzheimer's disease (AD) is a neurodegenerative disorder with the histopathological hallmark of extracellular accumulation of amyloid-β (Aβ) peptide in brain senile plaques. Though many studies have shown the neural toxicity from various forms of Aβ peptides, the subcellular mechanisms of Aβ peptide are still not well understood, partially due to the technical challenges of isolating axons or dendrites from the cell body for localized investigation. In this study, the subcellular toxicity and localization of Aβ peptides are investigated by utilizing a microfluidic compartmentalized device, which combines physical restriction and chemotactic guidance to enable the isolation of axons and dendrites for localized pharmacological studies. It is found that Aβ peptides induced neuronal death is mostly resulted from Aβ treatment at cell body or axonal processes, but not at dendritic neurites. Simply applying Aβ to axons alone induces significant hyperactive spiking activity. Dynamic transport of Aβ aggregates is only observed between axon terminal and cell body. In addition to differential cellular uptake, more Aβ-peptide secretion is detected significantly from axons than from dendritic side. These results clearly demonstrate the existence of a localized mechanism in Aβ-induced neurotoxicity, and can potentially benefit the development of new therapeutic strategies for AD.

Endogenously generated amyloid-β increases stiffness in human neuroblastoma cells

Amyloid-β (Aβ) is widely recognized as toxic to neuronal cells. Its deposition on plasma and intracellular membranes and aggregation into amyloid plaques can disturb the composition and physiological function of neurons. Whether a physical property of cells, such as stiffness, is altered by endogenously overexpressed Aβ has not yet been investigated. In this study, we used human neuroblastoma cells stably overexpressing amyloid precursor protein (APP) and its Swedish mutant form (APPswe) to measure the changes in cell stiffness. Our results showed that the stiffness of cells overexpressing APP or APPswe was higher than that of control SH-SY5Y cells. Either reducing levels of Aβ with the γ secretase inhibitor DAPT or blocking the membrane calcium channel formed by Aβ with tromethamine decreased cell stiffness to a level close to the control SH-SY5Y cells. Our results suggested that Aβ, not APP, contributed to increased cell stiffness and that closure of calcium channels formed by Aβ can alleviate the effects of Aβ on membrane stiffness.

Alzheimer's disease due to loss of function: A new synthesis of the available data

Alzheimer's Disease (AD) is a highly complex disease involving a broad range of clinical, cellular, and biochemical manifestations that are currently not understood in combination. This has led to many views of AD, e.g. the amyloid, tau, presenilin, oxidative stress, and metal hypotheses. The amyloid hypothesis has dominated the field with its assumption that buildup of pathogenic β-amyloid (Aβ) peptide causes disease. This paradigm has been criticized, yet most data suggest that Aβ plays a key role in the disease. Here, a new loss-of-function hypothesis is synthesized that accounts for the anomalies of the amyloid hypothesis, e.g. the curious pathogenicity of the Aβ42/Aβ40 ratio, the loss of Aβ caused by presenilin mutation, the mixed phenotypes of APP mutations, the poor clinical-biochemical correlations for genetic variant carriers, and the failure of Aβ reducing drugs. The amyloid-loss view accounts for recent findings on the structure and chemical features of Aβ variants and their coupling to human patient data. The lost normal function of APP/Aβ is argued to be metal transport across neuronal membranes, a view with no apparent anomalies and substantially more explanatory power than the gain-of-function amyloid hypothesis. In the loss-of-function scenario, the central event of Aβ aggregation is interpreted as a loss of soluble, functional monomer Aβ rather than toxic overload of oligomers. Accordingly, new research models and treatment strategies should focus on remediation of the functional amyloid balance, rather than strict containment of Aβ, which, for reasons rationalized in this review, has failed clinically.

Effect of amyloids on the vesicular machinery: implications for somatic neurotransmission

Certain neurodegenerative diseases are thought to be initiated by the aggregation of amyloidogenic proteins. However, the mechanism underlying toxicity remains obscure. Most of the suggested mechanisms are generic in nature and do not directly explain the neuron-type specific lesions observed in many of these diseases. Some recent reports suggest that the toxic aggregates impair the synaptic vesicular machinery. This may lead to an understanding of the neuron-type specificity observed in these diseases. A disruption of the vesicular machinery can also be deleterious for extra-synaptic, especially somatic, neurotransmission (common in serotonergic and dopaminergic systems which are specifically affected in Alzheimer's disease (AD) and Parkinson's disease (PD), respectively), though this relationship has remained unexplored. In this review, we discuss amyloid-induced damage to the neurotransmitter vesicular machinery, with an eye on the possible implications for somatic exocytosis. We argue that the larger size of the system, and the availability of multi-photon microscopy techniques for directly visualizing monoamines, make the somatic exocytosis machinery a more tractable model for understanding the effect of amyloids on all types of vesicular neurotransmission. Indeed, exploring this neglected connection may not just be important, it may be a more fruitful route for understanding AD and PD.

Pharmacological modulation of dietary lipid-induced cerebral capillary dysfunction: Considerations for reducing risk for Alzheimer's disease

An increasing body of evidence suggests that cerebrovascular dysfunction and microvessel disease precede the evolution of hallmark pathological features that characterise Alzheimer's disease (AD), consistent with a causal association for onset or progression. Recent studies, principally in genetically unmanipulated animal models, suggest that chronic ingestion of diets enriched in saturated fats and cholesterol may compromise blood-brain barrier (BBB) integrity resulting in inappropriate blood-to-brain extravasation of plasma proteins, including lipid macromolecules that may be enriched in amyloid-β (Aβ). Brain parenchymal retention of blood proteins and lipoprotein bound Aβ is associated with heightened neurovascular inflammation, altered redox homeostasis and nitric oxide (NO) metabolism. Therefore, it is a reasonable proposition that lipid-lowering agents may positively modulate BBB integrity and by extension attenuate risk or progression of AD. In addition to their robust lipid lowering properties, reported beneficial effects of lipid-lowering agents were attributed to their pleiotropic properties via modulation of inflammation, oxidative stress, NO and Aβ metabolism. The review is a contemporary consideration of a complex body of literature intended to synthesise focussed consideration of mechanisms central to regulation of BBB function and integrity. Emphasis is given to dietary fat driven significant epidemiological evidence consistent with heightened risk amongst populations consuming greater amounts of saturated fats and cholesterol. In addition, potential neurovascular benefits associated with the use of hypolipidemic statins, probucol and fenofibrate are also presented in the context of lipid-lowering and pleiotropic properties.

Effect of α-synuclein on membrane permeability and synaptic transmission: a clue to neurodegeneration?

This is an Editorial highlighting the article "Extracellular α-synuclein alters synaptic transmission in rain neurons by perforating the neuronal plasma membrane" by Pacheco and coauthors, in this issue of Journal of Neurochemistry. The authors demonstrate, using a variety of techniques, that alpha-synuclein possesses neurotoxicity toward brain neuronal plasma membranes exposed directly to extracellular alpha-synuclein oligomers. Extracellular oligomeric α-synuclein rapidly associates to hippocampal membranes and induces pore formation in the hippocampal cells. This increases membrane conductance and calcium influx. Oligomeric α-synuclein also induces changes in synaptic current activity in hippocampal neurons. The authors' findings support the pathogenic role of extracellular alpha-synuclein in the brain, and should provide a new strategy for the treatment of Parkinson's disease and other synucleinopathies, neurodegenerative diseases with aberrant accumulation of aggregated alpha-synuclein in neurons, nerve fibers or glial cells. Read the full article 'Extracellular α-synuclein alters synaptic transmission in brain neurons by perforating the neuronal plasma membrane' on page 731.

Single-cell screening of cytosolic [Ca(2+)] reveals cell-selective action by the Alzheimer's Aβ peptide ion channel

Interaction of the Alzheimer's Aβ peptides with the plasma membrane of cells in culture results in chronic increases in cytosolic [Ca(2+)]. Such increases can cause a variety of secondary effects leading to impaired cell growth or cell degeneration. In this investigation, we made a comprehensive study of the changes in cytosolic [Ca(2+)] in single PC12 cells and human neurons stressed by continuous exposure to a medium containing Aβ42 for several days. The differential timing and magnitude of the Aβ42-induced increase in [Ca(2+)] reveal subpopulations of cells with differential sensitivity to Aβ42. These results suggest that the effect produced by Aβ on the level of cytosolic [Ca(2+)] depends on the type of cell being monitored. Moreover, the results obtained of using potent inhibitors of Aβ cation channels such as Zn(2+) and the small peptide NA7 add further proof to the suggestion that the long-term increases in cytosolic [Ca(2+)] in cells stressed by continuous exposure to Aβ is the result of Aβ ion channel activity.

Amyloid beta receptors responsible for neurotoxicity and cellular defects in Alzheimer's disease

Alzheimer's disease (AD) is the most common neurodegenerative disease. Although a major cause of AD is the accumulation of amyloid-β (Aβ) peptide that induces neuronal loss and cognitive impairments, our understanding of its neurotoxic mechanisms is limited. Recent studies have identified putative Aβ-binding receptors that mediate Aβ neurotoxicity in cells and models of AD. Once Aβ interacts with a receptor, a toxic signal is transduced into neurons, resulting in cellular defects including endoplasmic reticulum stress and mitochondrial dysfunction. In addition, Aβ can also be internalized into neurons through unidentified Aβ receptors and induces malfunction of subcellular organelles, which explains some part of Aβ neurotoxicity. Understanding the neurotoxic signaling initiated by Aβ-receptor binding and cellular defects provide insight into new therapeutic windows for AD. In the present review, we summarize the findings on Aβ-binding receptors and the neurotoxicity of oligomeric Aβ.

Molecular understanding of a potential functional link between antimicrobial and amyloid peptides

Antimicrobial and amyloid peptides do not share common sequences, typical secondary structures, or normal biological activity but both the classes of peptides exhibit membrane-disruption ability to induce cell toxicity. Different membrane-disruption mechanisms have been proposed for antimicrobial and amyloid peptides, individually, some of which are not exclusive to either peptide type, implying that certain common principles may govern the folding and functions of different cytolytic peptides and associated membrane disruption mechanisms. Particularly, some antimicrobial and amyloid peptides have been identified to have dual complementary amyloid and antimicrobial properties, suggesting a potential functional link between amyloid and antimicrobial peptides. Given that some similar structural and membrane-disruption characteristics exist between the two classes of peptides, this review summarizes major findings, recent advances, and future challenges related to antimicrobial and amyloid peptides and strives to illustrate the similarities, differences, and relationships in the sequences, structures, and membrane interaction modes between amyloid and antimicrobial peptides, with a special focus on direct interactions of the peptides with the membranes. We hope that this review will stimulate further research at the interface of antimicrobial and amyloid peptides - which has been studied less intensively than either type of peptides - to decipher a possible link between both amyloid pathology and antimicrobial activity, which can guide drug design and peptide engineering to influence peptide-membrane interactions important in human health and diseases.

Disordered amyloidogenic peptides may insert into the membrane and assemble into common cyclic structural motifs

Aggregation of disordered amyloidogenic peptides into oligomers is the causative agent of amyloid-related diseases. In solution, disordered protein states are characterized by heterogeneous ensembles. Among these, β-rich conformers self-assemble via a conformational selection mechanism to form energetically-favored cross-β structures, regardless of their precise sequences. These disordered peptides can also penetrate the membrane, and electrophysiological data indicate that they form ion-conducting channels. Based on these and additional data, including imaging and molecular dynamics simulations of a range of amyloid peptides, Alzheimer's amyloid-β (Aβ) peptide, its disease-related variants with point mutations and N-terminal truncated species, other amyloidogenic peptides, as well as a cytolytic peptide and a synthetic gel-forming peptide, we suggest that disordered amyloidogenic peptides can also present a common motif in the membrane. The motif consists of curved, moon-like β-rich oligomers associated into annular organizations. The motif is favored in the lipid bilayer since it permits hydrophobic side chains to face and interact with the membrane and the charged/polar residues to face the solvated channel pores. Such channels are toxic since their pores allow uncontrolled leakage of ions into/out of the cell, destabilizing cellular ionic homeostasis. Here we detail Aβ, whose aggregation is associated with Alzheimer's disease (AD) and for which there are the most abundant data. AD is a protein misfolding disease characterized by a build-up of Aβ peptide as senile plaques, neurodegeneration, and memory loss. Excessively produced Aβ peptides may directly induce cellular toxicity, even without the involvement of membrane receptors through Aβ peptide-plasma membrane interactions.

Coenzyme Q10 protects human endothelial cells from β-amyloid uptake and oxidative stress-induced injury

Neuropathological symptoms of Alzheimer's disease appear in advances stages, once neuronal damage arises. Nevertheless, recent studies demonstrate that in early asymptomatic stages, ß-amyloid peptide damages the cerebral microvasculature through mechanisms that involve an increase in reactive oxygen species and calcium, which induces necrosis and apoptosis of endothelial cells, leading to cerebrovascular dysfunction. The goal of our work is to study the potential preventive effect of the lipophilic antioxidant coenzyme Q (CoQ) against ß-amyloid-induced damage on human endothelial cells. We analyzed the protective effect of CoQ against Aβ-induced injury in human umbilical vein endothelial cells (HUVECs) using fluorescence and confocal microscopy, biochemical techniques and RMN-based metabolomics. Our results show that CoQ pretreatment of HUVECs delayed Aβ incorporation into the plasma membrane and mitochondria. Moreover, CoQ reduced the influx of extracellular Ca²⁺, and Ca²⁺ release from mitochondria due to opening the mitochondrial transition pore after β-amyloid administration, in addition to decreasing O₂^.− and H₂O₂ levels. Pretreatment with CoQ also prevented ß-amyloid-induced HUVECs necrosis and apoptosis, restored their ability to proliferate, migrate and form tube-like structures in vitro, which is mirrored by a restoration of the cell metabolic profile to control levels. CoQ protected endothelial cells from Aβ-induced injury at physiological concentrations in human plasma after oral CoQ supplementation and thus could be a promising molecule to protect endothelial cells against amyloid angiopathy.

Role of PrP(C) Expression in Tau Protein Levels and Phosphorylation in Alzheimer's Disease Evolution

Alzheimer's disease (AD) is characterized by the presence of amyloid plaques mainly consisting of hydrophobic β-amyloid peptide (Aβ) aggregates and neurofibrillary tangles (NFTs) composed principally of hyperphosphorylated tau. Aβ oligomers have been described as the earliest effectors to negatively affect synaptic structure and plasticity in the affected brains, and cellular prion protein (PrP(C)) has been proposed as receptor for these oligomers. The most widely accepted theory holds that the toxic effects of Aβ are upstream of change in tau, a neuronal microtubule-associated protein that promotes the polymerization and stabilization of microtubules. However, tau is considered decisive for the progression of neurodegeneration, and, indeed, tau pathology correlates well with clinical symptoms such as dementia. Different pathways can lead to abnormal phosphorylation, and, as a consequence, tau aggregates into paired helical filaments (PHF) and later on into NFTs. Reported data suggest a regulatory tendency of PrP(C) expression in the development of AD, and a putative relationship between PrP(C) and tau processing is emerging. However, the role of tau/PrP(C) interaction in AD is poorly understood. In this study, we show increased susceptibility to Aβ-derived diffusible ligands (ADDLs) in neuronal primary cultures from PrP(C) knockout mice, compared to wild-type, which correlates with increased tau expression. Moreover, we found increased PrP(C) expression that paralleled with tau at early ages in an AD murine model and in early Braak stages of AD in affected individuals. Taken together, these results suggest a protective role for PrP(C) in AD by downregulating tau expression, and they point to this protein as being crucial in the molecular events that lead to neurodegeneration in AD.

The dual face of connexin-based astroglial Ca(2+) communication: a key player in brain physiology and a prime target in pathology

For decades, studies have been focusing on the neuronal abnormalities that accompany neurodegenerative disorders. Yet, glial cells are emerging as important players in numerous neurological diseases. Astrocytes, the main type of glia in the central nervous system , form extensive networks that physically and functionally connect neuronal synapses with cerebral blood vessels. Normal brain functioning strictly depends on highly specialized cellular cross-talk between these different partners to which Ca(2+), as a signaling ion, largely contributes. Altered intracellular Ca(2+) levels are associated with neurodegenerative disorders and play a crucial role in the glial responses to injury. Intracellular Ca(2+) increases in single astrocytes can be propagated toward neighboring cells as intercellular Ca(2+) waves, thereby recruiting a larger group of cells. Intercellular Ca(2+) wave propagation depends on two, parallel, connexin (Cx) channel-based mechanisms: i) the diffusion of inositol 1,4,5-trisphosphate through gap junction channels that directly connect the cytoplasm of neighboring cells, and ii) the release of paracrine messengers such as glutamate and ATP through hemichannels ('half of a gap junction channel'). This review gives an overview of the current knowledge on Cx-mediated Ca(2+) communication among astrocytes as well as between astrocytes and other brain cell types in physiology and pathology, with a focus on the processes of neurodegeneration and reactive gliosis. Research on Cx-mediated astroglial Ca(2+) communication may ultimately shed light on the development of targeted therapies for neurodegenerative disorders in which astrocytes participate. This article is part of a Special Issue entitled: Calcium signaling in health and disease. Guest Editors: Geert Bultynck, Jacques Haiech, Claus W. Heizmann, Joachim Krebs, and Marc Moreau.

Fitness costs of minimal sequence alterations causing protein instability and toxicity

Destabilization of a protein impairs its metabolic efficiency. It is less clear how often destabilization also results in a gain of toxicity. We derived collections of temperature-sensitive, and thus structurally unstable, mutants of the yeast ADE2 and LYS2 genes by introducing single or very few amino acids substitutions. Overexpression of these mutant proteins led to a common, although unequal, fitness decrease. Interestingly, although the mutant proteins were functionally redundant, higher expression levels were associated with higher fitness. This result suggests that growth was hampered not by the accumulation of damaged chains but by the activities needed to remove them or by the damage caused before they were removed. Our results support the idea that any protein can become toxic when destabilized by a point mutation.

Physicochemical characteristics of soluble oligomeric Aβand their pathologic role in Alzheimer's disease

Extracellular fibrillar amyloid deposits are prominent and universal Alzheimer's disease (AD) features, but senile plaque abundance does not always correlate directly with the degree of dementia exhibited by AD patients. The mechanism(s) and dynamics of Abeta fibril genesis and deposition remain obscure. Enhanced Abeta synthesis rates coupled with decreased degradative enzyme production and accumulating physical modifications that dampen proteolysis may all enhance amyloid deposit formation. Amyloid accumulation may indirectly exert the greatest pathologic effect on the brain vasculature by destroying smooth muscle cells and creating a cascade of negative impacts on cerebral blood flow. The most visible manifestation of amyloid dis-equilibrium could actually be a defense mechanism employed to avoid serious vascular wall degradation while the major toxic effects to the gray and white matter neurons are mediated by soluble oligomeric Abeta peptides with high beta-sheet content. The recognition that dynamic soluble oligomeric Abeta pools exist in AD and are correlated to disease severity led to neurotoxicity and physical conformation studies. It is now recognized that the most basic soluble Abeta peptides are stable dimers with hydrophobic regions sequestered from the aqueous environment and are capable of higher order aggregations. Time course experiments employing a modified ELISA method able to detect Abeta oligomers revealed dynamic intermolecular interactions and additional experiments physically confirmed the presence of stable amyloid multimers. Amyloid peptides that are rich in beta-sheet structure are capable of creating toxic membrane ion channels and a capacity to self-assemble as annular structures was confirmed in vitro using atomic force microscopy. Biochemical studies have established that soluble Abeta peptides perturb metabolic processes, provoke release of deleterious reactive compounds, reduce blood flow, induce mitochondrial apoptotic toxicity and inhibit angiogenesis. While there is no question that gross amyloid deposition does contribute to AD pathology, the destructive potential now associated with soluble Abeta suggests that treatment strategies that target these molecules may be efficacious in preventing some of the devastating effects of AD.

A positive feedback cell signaling nucleation model of astrocyte dynamics

We constructed a model of calcium signaling in astrocyte neural glial cells that incorporates a positive feedback nucleation mechanism, whereby small microdomain increases in local calcium can stochastically produce global cellular and intercellular network scale dynamics. The model is able to simultaneously capture dynamic spatial and temporal heterogeneities associated with intracellular calcium transients in individual cells and intercellular calcium waves (ICW) in spatially realistic networks of astrocytes, i.e., networks where the positions of cells were taken from real in vitro experimental data of spontaneously forming sparse networks, as opposed to artificially constructed grid networks or other non-realistic geometries. This is the first work we are aware of where an intracellular model of calcium signaling that reproduces intracellular dynamics inherently accounts for intercellular network dynamics. These results suggest that a nucleation type mechanism should be further investigated experimentally in order to test its contribution to calcium signaling in astrocytes and in other cells more broadly. It may also be of interest in engineered neuromimetic network systems that attempt to emulate biological signaling and information processing properties in synthetic hardwired neuromorphometric circuits or coded algorithms.