Replicating neuroblastoma cells in different cell cycle phases display different vulnerability to amyloid toxicity

Effects of Acetylcholine on β-Amyloid-Induced cPLA2 Activation in the TB Neuroectodermal Cell Line: Implications for the Pathogenesis of Alzheimer's Disease

The role of β-amyloid (Aβ) in the pathogenesis of Alzheimer's disease (AD) is still considered crucial. The state of Aβ aggregation is critical in promoting neuronal loss and neuronal function impairment. Recently, we demonstrated that Acetylcholine (ACh) is neuroprotective against the toxic effects of Aβ in the cholinergic LAN-2 cells. In biophysical experiments, ACh promotes the soluble Aβ peptide conformation rather than the aggregation-prone β-sheet conformation. In order to better understand the biological role of ACh in AD, we studied the effect of Aβ on the phosphorylation of the cytosolic phospholipase A2 (cPLA2) in the TB neuroectodermal cell line, which differentiates toward a neuronal phenotype when cultured in the presence of retinoic acid (RA). We chose the phosphorylated form of cPLA2 (Ser505, Phospho-cPLA2) as a biomarker to test the influence of ACh on the effects of Aβ in both undifferentiated and RA-differentiated TB cells. Our results show that TB cells are responsive to Aβ. Moreover, in undifferentiated cells 1 h treatment with Aβ induces a 2.5-fold increase of the Phospho-cPLA2 level compared to the control after 24 h in vitro, while no significant difference is observed between Aβ-treated and non-treated cells after 4 and 7 days in vitro. The RA-differentiated cells are not sensitive to Aβ. In TB cell line ACh is able to blunt the effects of Aβ. The ability of ACh to protect non-cholinergic cells against Aβ reinforces the hypothesis that, in addition to its role in cholinergic transmission, ACh could also act as a neuroprotective agent.

Binding affinity of amyloid oligomers to cellular membranes is a generic indicator of cellular dysfunction in protein misfolding diseases

The conversion of peptides or proteins from their soluble native states into intractable amyloid deposits is associated with a wide range of human disorders. Misfolded protein oligomers formed during the process of aggregation have been identified as the primary pathogenic agents in many such conditions. Here, we show the existence of a quantitative relationship between the degree of binding to neuronal cells of different types of oligomers formed from a model protein, HypF-N, and the GM1 content of the plasma membranes. In addition, remarkably similar behavior is observed for oligomers of the Aβ₄₂ peptide associated with Alzheimer’s disease. Further analysis has revealed the existence of a linear correlation between the level of the influx of Ca²⁺ across neuronal membranes that triggers cellular damage, and the fraction of oligomeric species bound to the membrane. Our findings indicate that the susceptibility of neuronal cells to different types of misfolded oligomeric assemblies is directly related to the extent of binding of such oligomers to the cellular membrane.

The nuclear accumulation of alpha-synuclein is mediated by importin alpha and promotes neurotoxicity by accelerating the cell cycle

α-Synuclein (α-syn), a 14 kDa pre-synaptic protein, is widely involved in the Parkinson's disease (PD) pathogenesis. Recent studies have shown that the nuclear accumulation of α-syn might have a toxic effect. The main purpose of the present study was to explore which amino acid residues in α-syn are associated with its nuclear accumulation, the molecule(s) mediated the nuclear import of α-syn, and the role of α-syn accumulated in the nucleus. It has been noted that the nuclear import of α-syn may be mediated by importin α and that both the amino acid residues 1-60 and 103-140 of α-syn were indispensable for its nuclear import. After imported into the nucleus, the accumulated α-syn played a toxic role in both the PC12 cells and the C57 mice. Furthermore, α-syn-nuclear localization signal-injected mice showed behavioral symptoms associated with PD. Further studies performed in vitro showed that the toxicity of α-syn in the nucleus might be due to an interference of the cell cycle. Thus, it can be concluded that α-syn can accumulate in nucleus, which is mediated by importin α, and promote neurotoxicity by accelerating the cell cycle.

A proteomic approach to investigate the effects of cadmium and lead on human primary renal cells

Cadmium and lead affect the viability of primary human renal cells, inducing alterations in the cellular proteome.

Cannabinoid effects on β amyloid fibril and aggregate formation, neuronal and microglial-activated neurotoxicity in vitro

Cannabinoid (CB) ligands have demonstrated neuroprotective properties. In this study we compared the effects of a diverse set of CB ligands against β amyloid-mediated neuronal toxicity and activated microglial-conditioned media-based neurotoxicity in vitro, and compared this with a capacity to directly alter β amyloid (Aβ) fibril or aggregate formation. Neuroblastoma (SH-SY5Y) cells were exposed to Aβ1-42 directly or microglial (BV-2 cells) conditioned media activated with lipopolysaccharide (LPS) in the presence of the CB1 receptor-selective agonist ACEA, CB2 receptor-selective agonist JWH-015, phytocannabinoids Δ(9)-THC and cannabidiol (CBD), the endocannabinoids 2-arachidonoyl glycerol (2-AG) and anandamide or putative GPR18/GPR55 ligands O-1602 and abnormal-cannabidiol (Abn-CBD). TNF-α and nitrite production was measured in BV-2 cells to compare activation via LPS or albumin with Aβ1-42. Aβ1-42 evoked a concentration-dependent loss of cell viability in SH-SY5Y cells but negligible TNF-α and nitrite production in BV-2 cells compared to albumin or LPS. Both albumin and LPS-activated BV-2 conditioned media significantly reduced neuronal cell viability but were directly innocuous to SH-SY5Y cells. Of those CB ligands tested, only 2-AG and CBD were directly protective against Aβ-evoked SH-SY5Y cell viability, whereas JWH-015, THC, CBD, Abn-CBD and O-1602 all protected SH-SY5Y cells from BV-2 conditioned media activated via LPS. While CB ligands variably altered the morphology of Aβ fibrils and aggregates, there was no clear correlation between effects on Aβ morphology and neuroprotective actions. These findings indicate a neuroprotective action of CB ligands via actions at microglial and neuronal cells.

The amyloid-cell membrane system. The interplay between the biophysical features of oligomers/fibrils and cell membrane defines amyloid toxicity

Amyloid cytotoxicity, structure and polymorphisms are themes of increasing importance. Present knowledge considers any peptide/protein able to undergo misfolding and aggregation generating intrinsically cytotoxic amyloids. It also describes growth and structure of amyloid fibrils and their possible disassembly, whereas reduced information is available on oligomer structure. Recent research has highlighted the importance of the environmental conditions as determinants of the amyloid polymorphisms and cytotoxicity. Another body of evidence describes chemical or biological surfaces as key sites of protein misfolding and aggregation or of interaction with amyloids and the resulting biochemical modifications inducing cell functional/viability impairment. In particular, the membrane lipid composition appears to modulate cell response to toxic amyloids, thus contributing to explain the variable vulnerability to the same amyloids of different cell types. Finally, a recent view describes amyloid toxicity as an emerging property dependent on a complex interplay between the biophysical features of early aggregates and the interacting cell membranes taken as a whole system.

Molecular mechanisms used by chaperones to reduce the toxicity of aberrant protein oligomers

Chaperones are the primary regulators of the proteostasis network and are known to facilitate protein folding, inhibit protein aggregation, and promote disaggregation and clearance of misfolded aggregates inside cells. We have tested the effects of five chaperones on the toxicity of misfolded oligomers preformed from three different proteins added extracellularly to cultured cells. All the chaperones were found to decrease oligomer toxicity significantly, even at very low chaperone/protein molar ratios, provided that they were added extracellularly rather than being overexpressed in the cytosol. Infrared spectroscopy and site-directed labeling experiments using pyrene ruled out structural reorganizations within the discrete oligomers. Rather, confocal microscopy, SDS-PAGE, and intrinsic fluorescence measurements indicated tight binding between oligomers and chaperones. Moreover, atomic force microscopy imaging indicated that larger assemblies of oligomers are formed in the presence of the chaperones. This suggests that the chaperones bind to the oligomers and promote their assembly into larger species, with consequent shielding of the reactive surfaces and a decrease in their diffusional mobility. Overall, the data indicate a generic ability of chaperones to neutralize extracellular misfolded oligomers efficiently and reveal that further assembly of protein oligomers into larger species can be an effective strategy to neutralize such extracellular species.

Glycosaminoglycans (GAGs) suppress the toxicity of HypF-N prefibrillar aggregates

A group of diverse human pathologies is associated with proteins unable to retain their native state and convert into prefibrillar and fibrillar amyloid aggregates that are then deposited in the extracellular space. Glycosaminoglycans (GAGs) have been found to physically associate with these deposits and also to promote their formation in vitro. However, the effect of GAGs on the toxicity of these aggregates has been investigated in only one protein system, the amyloid β peptide associated with Alzheimer's disease. In this study, we investigate whether GAGs affect the toxicity of the N-terminal domain of Escherichia coli HypF (HypF-N) oligomers on Chinese hamster ovarian cells and the mechanism by which such suppression is mediated. The results show that heparin and other GAGs inhibit the toxicity observed by HypF-N oligomers in a dose-dependent manner. GAGs were not found to bind preformed HypF-N oligomers, change their morphological and structural characteristics or disaggregate them. Nevertheless, they were found to bind to the cells' surface and prevent the interaction of the oligomers with the cells. Overall, the results indicate that GAGs have a generic ability to inhibit the toxicity of aberrant protein oligomers and that such toxicity suppression can occur through different mechanisms, such as through binding to the oligomers with consequent loss of interaction of the oligomers to the GAGs present on the cell surface, as proposed previously for amyloid β aggregates, or through mechanisms independent of direct GAG-oligomer binding, as shown here for HypF-N aggregates.

Embryonic stem and haematopoietic progenitor cells resist to Aβ oligomer toxicity and maintain the differentiation potency in culture

Regenerative medicine deals with the possible use of stem cells to repair tissues damaged by aging and related diseases, including amyloidoses. In the latter case, the toxicity of the amyloid deposits can, in principle, question the possibility to graft specific tissues by undifferentiated cells. To assess whether stem cells are vulnerable to amyloid toxicity, we exposed, in culture, murine embryonic stem (ES) cells and haematopoietic progenitor (HP) cells to oligomers of the amyloidogenic peptide Aβ42 at concentrations previously shown to be cytotoxic to several other cell types. These stem cells did not display any sign of apoptosis and their survival, proliferation and differentiation were not affected by the oligomers although the MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay revealed that ES, but not HP, cells displayed some impaired ability to reduce the tetrazole salts possibly as a result of transient oxidative stress. Our results support a remarkable resistance of the investigated stem cells against amyloids and hence their potential use in cell therapy of Alzheimer's disease and, possibly, other amyloid diseases.

G93A SOD1 alters cell cycle in a cellular model of Amyotrophic Lateral Sclerosis

Amyotrophic Lateral Sclerosis (ALS) is a neurodegenerative multifactorial disease characterized, like other diseases such as Alzheimer's disease (AD), Parkinson's disease (PD) or frontotemporal dementia (FTD), by the degeneration of specific neuronal cell populations. Motor neuron loss is distinctive of ALS. However, the causes of onset and progression of motor neuron death are still largely unknown. In about 2% of all cases, mutations in the gene encoding for the Cu/Zn superoxide dismutase (SOD1) are implicated in the disease. Several alterations in the expression or activation of cell cycle proteins have been described in the neurodegenerative diseases and related to cell death. In this work we show that mutant SOD1 can alter cell cycle in a cellular model of ALS. Our findings suggest that modifications in the cell cycle progression could be due to an increased interaction between mutant G93A SOD1 and Bcl-2 through the cyclins regulator p27. As previously described in post mitotic neurons, cell cycle alterations could fatally lead to cell death.

Protein folding and misfolding on surfaces

Protein folding, misfolding and aggregation, as well as the way misfolded and aggregated proteins affects cell viability are emerging as key themes in molecular and structural biology and in molecular medicine. Recent advances in the knowledge of the biophysical basis of protein folding have led to propose the energy landscape theory which provides a consistent framework to better understand how a protein folds rapidly and efficiently to the compact, biologically active structure. The increased knowledge on protein folding has highlighted its strict relation to protein misfolding and aggregation, either process being in close competition with the other, both relying on the same physicochemical basis. The theory has also provided information to better understand the structural and environmental factors affecting protein folding resulting in protein misfolding and aggregation into ordered or disordered polymeric assemblies. Among these, particular importance is given to the effects of surfaces. The latter, in some cases make possible rapid and efficient protein folding but most often recruit proteins/peptides increasing their local concentration thus favouring misfolding and accelerating the rate of nucleation. It is also emerging that surfaces can modify the path of protein misfolding and aggregation generating oligomers and polymers structurally different from those arising in the bulk solution and endowed with different physical properties and cytotoxicities.

Differentiation increases the resistance of neuronal cells to amyloid toxicity

A substantial lack of information is recognized on the features underlying the variable susceptibility to amyloid aggregate toxicity of cells with different phenotypes. Recently, we showed that different cell types are variously affected by early aggregates of a prokaryotic hydrogenase domain (HypF-N). In the present study we investigated whether differentiation affects cell susceptibility to amyloid injury using a human neurotypic SH-SY5Y cell differentiation model. We found that retinoic acid-differentiated cells were significantly more resistant against Abeta1-40, Abeta1-42 and HypF-N prefibrillar aggregate toxicity respect to undifferentiated cells treated similarly. Earlier and sharper increases in cytosolic Ca(2+) and ROS with marked lipid peroxidation and mitochondrial dysfunction were also detected in exposed undifferentiated cells resulting in apoptosis activation. The reduced vulnerability of differentiated cells matched a more efficient Ca(2+)-ATPase equipment and a higher total antioxidant capacity. Finally, increasing the content of membrane cholesterol resulted in a remarkable reduction of vulnerability and ability to bind the aggregates in either undifferentiated and differentiated cells.

Seladin-1/DHCR24 protects neuroblastoma cells against Abeta toxicity by increasing membrane cholesterol content

The role of brain cholesterol in Alzheimer's disease (AD) is currently a matter of debate. Experimental evidence suggests that reducing circulating and brain cholesterol protects against AD, however recent data indicate that low membrane cholesterol results in neurode-generation and that the cholesterol synthesis catalyst seladin-1 is down-regulated in AD-affected brain regions. We previously reported a significant correlation between resistance to amyloid toxicity and content of membrane cholesterol in differing cultured cell types. Here we provide evidence that Abeta42 pre-fibrillar aggregates accumulate more slowly and in reduced amount at the plasma membrane of human SH-SY5Y neuroblastoma cells overexpressing seladin-1 or treated with PEG-cholesterol than at the membrane of control cells. The accumulation was significantly increased in cholesterol-depleted cells following treatment with the specific seladin-1 inhibitor 5,22E-cholestadien-3-ol or with methyl-beta-cyclodextrin. The resistance to amyloid toxicity and the early cytosolic Ca2+ rise following exposure to Abeta42 aggregates were increased and prevented, respectively, by increasing membrane cholesterol whereas the opposite effects were found in cholesterol-depleted cells. These results suggest that seladin-1-dependent cholesterol synthesis reduces membrane-aggregate interaction and cell damage associated to amyloid-induced imbalance of cytosolic Ca2+. Our findings extend recently reported data indicating that seladin-1 overexpression directly enhances the resistance to Abeta toxicity featuring seladin-1/DHCR 24 as a possible new susceptibility gene for sporadic AD.