Differential Membrane Toxicity of Amyloid-β Fragments by Pore Forming Mechanisms

A major characteristic of Alzheimer's disease (AD) is the presence of amyloid-β peptide (Aβ) oligomers and aggregates in the brain. It is known that Aβ oligomers interact with the neuronal membrane and induce perforations that cause an influx of calcium ions and enhance the release of synaptic vesicles leading to a delayed synaptic failure by vesicle depletion. To better understand the mechanism by which Aβ exerts its effect on the plasma membrane, we evaluated three Aβ fragments derived from different regions of Aβ(1-42); Aβ(1-28) from the N-terminal region, Aβ(25-35) from the central region, and Aβ(17-42) from the C-terminal region. The neuronal activities of these fragments were examined with patch clamp, immunofluorescence, transmission electron microscopy, aggregation assays, calcium imaging, and MTT reduction assays. The present results indicate that the fragment Aβ(1-28) contributes to aggregation, an increase in intracellular calcium and synaptotoxicity, but is not involved in membrane perforation; Aβ(25-35) is important for membrane perforation, calcium increase, and synaptotoxicity; and Aβ(17-42) induced mitochondrial toxicity similar to the full length Aβ(1-42), but was unable to induce membrane perforation and calcium increase, supporting the idea that it is less toxic in the non-amyloidogenic pathway.

Alzheimer's Aβ interacts with cellular prion protein inducing neuronal membrane damage and synaptotoxicity

A major feature of Alzheimer's disease is the accumulation of β-amyloid (Aβ) peptide in the brain. Recent studies have indicated that Aβ oligomers (Aβo) can interact with the cellular prion protein (PrPc). Therefore, this interaction might be driving some of Aβ toxic effects in the synaptic region. In the present study, we report that Aβo binds to PrPc in the neuronal membrane playing a role on toxic effects induced by Aβ. Phospholipase C-enzymatic cleavage of PrPc from the plasma membrane attenuated the association of Aβo to the neurons. Furthermore, an anti-PrP antibody (6D11) decreased the association of Aβo to hippocampal neurons with a concomitant reduction in Aβo and PrPc co-localization. Interestingly, this antibody blocked the increase in membrane conductance and intracellular calcium induced by Aβo. Thus, the data indicate that PrPc plays a role on the membrane perforations produced by Aβo, the increase in calcium ions and the release of synaptic vesicles that subsequently leads to synaptic failure. Future studies blocking Aβo interaction with PrPc could be important for the discovery of new therapeutic strategies for Alzheimer's disease.

Fibrinogen nitrotyrosination after ischemic stroke impairs thrombolysis and promotes neuronal death

Ischemic stroke is an acute vascular event that compromises neuronal viability, and identification of the pathophysiological mechanisms is critical for its correct management. Ischemia produces increased nitric oxide synthesis to recover blood flow but also induces a free radical burst. Nitric oxide and superoxide anion react to generate peroxynitrite that nitrates tyrosines. We found that fibrinogen nitrotyrosination was detected in plasma after the initiation of ischemic stroke in human patients. Electron microscopy and protein intrinsic fluorescence showed that in vitro nitrotyrosination of fibrinogen affected its structure. Thromboelastography showed that initially fibrinogen nitrotyrosination retarded clot formation but later made the clot more resistant to fibrinolysis. This result was independent of any effect on thrombin production. Immunofluorescence analysis of affected human brain areas also showed that both fibrinogen and nitrotyrosinated fibrinogen spread into the brain parenchyma after ischemic stroke. Therefore, we assayed the toxicity of fibrinogen and nitrotyrosinated fibrinogen in a human neuroblastoma cell line. For that purpose we measured the activity of caspase-3, a key enzyme in the apoptotic pathway, and cell survival. We found that nitrotyrosinated fibrinogen induced higher activation of caspase 3. Accordingly, cell survival assays showed a more neurotoxic effect of nitrotyrosinated fibrinogen at all concentrations tested. In summary, nitrotyrosinated fibrinogen would be of pathophysiological interest in ischemic stroke due to both its impact on hemostasis - it impairs thrombolysis, the main target in stroke treatments - and its neurotoxicity that would contribute to the death of the brain tissue surrounding the infarcted area.

PSD95 suppresses dendritic arbor development in mature hippocampal neurons by occluding the clustering of NR2B-NMDA receptors

Considerable evidence indicates that the NMDA receptor (NMDAR) subunits NR2A and NR2B are critical mediators of synaptic plasticity and dendritogenesis; however, how they differentially regulate these processes is unclear. Here we investigate the roles of the NR2A and NR2B subunits, and of their scaffolding proteins PSD-95 and SAP102, in remodeling the dendritic architecture of developing hippocampal neurons (2–25 DIV). Analysis of the dendritic architecture and the temporal and spatial expression patterns of the NMDARs and anchoring proteins in immature cultures revealed a strong positive correlation between synaptic expression of the NR2B subunit and dendritogenesis. With maturation, the pruning of dendritic branches was paralleled by a strong reduction in overall and synaptic expression of NR2B, and a significant elevation in synaptic expression of NR2A and PSD95. Using constructs that alter the synaptic composition, we found that either over-expression of NR2B or knock-down of PSD95 by shRNA-PSD95 augmented dendritogenesis in immature neurons. Reactivation of dendritogenesis could also be achieved in mature cultured neurons, but required both manipulations simultaneously, and was accompanied by increased dendritic clustering of NR2B. Our results indicate that the developmental increase in synaptic expression of PSD95 obstructs the synaptic clustering of NR2B-NMDARs, and thereby restricts reactivation of dendritic branching. Experiments with shRNA-PSD95 and chimeric NR2A/NR2B constructs further revealed that C-terminus of the NR2B subunit (tail) was sufficient to induce robust dendritic branching in mature hippocampal neurons, and suggest that the NR2B tail is important in recruiting calcium-dependent signaling proteins and scaffolding proteins necessary for dendritogenesis.

Ibuprofen inhibits the synaptic failure induced by the amyloid-β peptide in hippocampal neurons

Epidemiological studies have reported a decrease in the prevalence of Alzheimer's disease in individuals who chronically use non-steroidal anti-inflammatory drugs (NSAIDs). Clinical trials, on the other hand, have been less positive. Nevertheless, it has been proposed that NSAIDs exert part of their effects by reducing long-term cerebral neuroinflammation, although this mechanism has not been proven. In this study, we report that ibuprofen, one of the more widely used non-steroidal anti-inflammatory drugs, was able to alter the ultrastructure of amyloid-β peptide (Aβ) and significantly decrease its association to neuronal membranes, and consequently, its synaptotoxic effect in rat primary hippocampal and cortical cultures at 24 h incubation. In agreement with these results, we found that the decrease in the frequency of calcium transients with Aβ was partly recovered by addition of ibuprofen (8.0 × 10-2 Hz in control; 3.4 × 10-2 Hz in 5 μM Aβ, and 5.9 × 10-2 Hz in the presence of Aβ and 200 μM ibuprofen). Additionally, this effect correlated well with the increment and recovery of miniature spontaneous currents (47 ± 5% of control in 1 μM Aβ alone and 104 ± 14% in the presence of Aβ and ibuprofen). Our results suggest that ibuprofen could be exerting its neuroprotective effect by directly interacting with Aβ and altering its toxic aggregated forms. We postulate that other ibuprofen analogs with better pharmacological properties might have a higher efficacy in AD.

Nature of the neurotoxic membrane actions of amyloid-β on hippocampal neurons in Alzheimer's disease

The mechanism by which amyloid-β (Aβ) produces brain dysfunction in patients with Alzheimer's disease is largely unknown. According to previous studies, Aβ might share perforating properties with gramicidin, a well-accepted membrane-disrupting peptide. Therefore, we hypothesize that the key steps leading to synaptotoxicity by Aβ and gramicidin involve peptide aggregation, pore formation, and calcium dysregulation. Here, we show that Aβ and gramicidin form aggregates enriched in β-sheet structures using electron microscopy, and Thioflavin and Congo Red staining techniques. Also, we found that Aβ and gramicidin display fairly similar actions in hippocampal cell membranes, i.e. inducing Ca(2+) entry and synaptoxicity characterized by the loss of synaptic proteins and a decrease in neuronal viability. These effects were not observed in a Ca(2+) free solution, indicating that both Aβ and gramicidin induce neurotoxicity by a Ca(2+)-dependent mechanism. Using combined perforated patch clamp and imaging recordings, we found that only Aβ produced a perforation that progressed from a small (Cl(-)-selective pore) to a larger perforation that allowed the entry of fluorescent molecules. Therefore, based on these results, we propose that the perforation at the plasma membrane by Aβ is a dynamic process that is critical in producing neurotoxicity similar to that found in the brains of AD patients.

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

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

An extracellular mechanism that can explain the neurotoxic effects of α-synuclein aggregates in the brain

Neurodegenerative diseases, such as Parkinson's disease (PD), Alzheimer's disease (AD), and Dementia with Lewy bodies (DLB), display an accumulation of proteins including α-synuclein aggregates in cortical and subcortical regions of the brain. PD is a complex, progressive disease which involves damage of motor and cognitive brain regions, as well as autonomic and sensory areas. Since α-synuclein is a neuronal cytosolic protein, it is assumed that pathogenic changes induced by α-synuclein aggregates occur only at the cytoplasmic level. However, recent studies have identified the presence of extracellular α-synuclein, suggesting that the pathogenic action of this protein may also occur in the extracellular milieu through an unknown mechanism. One of the hypotheses is that extracellular α-synuclein aggregates or oligomers may directly disrupt the neuronal membrane by the formation of a pore reminiscent to the ones formed by β-amyloid aggregates. Here, we will review some evidence that support this mechanism, analyzing the interactions of α-synuclein with components of the plasma membrane, the formation of pore/perforated structures, and the implications on ionic dyshomeostasis. Furthermore, we will also discuss how this mechanism can be integrated into a general phenomenon that may explain the synaptotoxicity and neurotoxicity observed in different neurodegenerative diseases.

Biphasic effects of copper on neurotransmission in rat hippocampal neurons

The importance of copper in the CNS is well documented, but the mechanisms related to its brain functions are poorly understood. Copper is released at the synaptic cleft, where it may modulate neurotransmission. To understand the functional impact of copper on the neuronal network, we have analyzed the synaptic activity of primary rat hippocampal neurons by using different approaches including whole cell patch clamp, recording of calcium transients, immunofluorescence and western blot. Here, we show that copper produces biphasic changes in neurotransmission. When copper is acutely applied to the plate it blocks neurotransmission. Interestingly, when it is applied for 3 h to hippocampal neurons it mainly increases the frequency and amplitude of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)ergic currents (control: 0.21 ± 0.05 Hz/22.9 ± 1.3 pA; copper: 0.68 ± 0.16 Hz/30.5 ± 2.5 pA), intracellular calcium transients (control: 0.05 ± 0.013 Hz; copper: 0.11 ± 0.02 Hz) and evoked AMPA currents (control: EC50 8.3 ± 0.5 μM; copper: EC50 2.9 ± 0.2 μM). Moreover, our results suggest that copper increases GluA1 subunit levels of the AMPA receptor through the anchorage of AMPA receptors to the plasma membrane as a result of PSD-95 accumulation. We also found that copper-treated neurons displayed an undistinguishable neurotransmission to control neurons after 24 h of treatment, indicating that changes in neurotransmission induced by copper at 3 h of incubation are homeostatically regulated after long-term exposure to the metal. Together, our data reveal an unexpected biphasic effect of copper on neurotransmission, which may be relevant to understand the effects of this ion in brain diseases that display copper dyshomeostasis such as that observed in Alzheimer's disease (AD).

Canonical Wnt3a modulates intracellular calcium and enhances excitatory neurotransmission in hippocampal neurons

A role for Wnt signal transduction in the development and maintenance of brain structures is widely acknowledged. Recent studies have suggested that Wnt signaling may be essential for synaptic plasticity and neurotransmission. However, the direct effect of a Wnt protein on synaptic transmission had not been demonstrated. Here we show that nanomolar concentrations of purified Wnt3a protein rapidly increase the frequency of miniature excitatory synaptic currents in embryonic rat hippocampal neurons through a mechanism involving a fast influx of calcium from the extracellular space, induction of post-translational modifications on the machinery involved in vesicle exocytosis in the presynaptic terminal leading to spontaneous Ca(2+) transients. Our results identify the Wnt3a protein and a member of its complex receptor at the membrane, the low density lipoprotein receptor-related protein 6 (LRP6) coreceptor, as key molecules in neurotransmission modulation and suggest cross-talk between canonical and Wnt/Ca(2+) signaling in central neurons.

Beta-amyloid causes depletion of synaptic vesicles leading to neurotransmission failure

Alzheimer disease is a progressive neurodegenerative brain disorder that leads to major debilitating cognitive deficits. It is believed that the alterations capable of causing brain circuitry dysfunctions have a slow onset and that the full blown disease may take several years to develop. Therefore, it is important to understand the early, asymptomatic, and possible reversible states of the disease with the aim of proposing preventive and disease-modifying therapeutic strategies. It is largely unknown how amyloid beta-peptide (A beta), a principal agent in Alzheimer disease, affects synapses in brain neurons. In this study, we found that similar to other pore-forming neurotoxins, A beta induced a rapid increase in intracellular calcium and miniature currents, indicating an enhancement in vesicular transmitter release. Significantly, blockade of these effects by low extracellular calcium and a peptide known to act as an inhibitor of the A beta-induced pore prevented the delayed failure, indicating that A beta blocks neurotransmission by causing vesicular depletion. This new mechanism for A beta synaptic toxicity should provide an alternative pathway to search for small molecules that can antagonize these effects of A beta.

Alzheimer beta-amyloid blocks epileptiform activity in hippocampal neurons

Several studies showed that hippocampal neurons respond with an increase in synaptic transmission after chronic blockade of GABA(A) receptors with bicuculline, a neuroplastic phenomenon likely associated to epileptiform states. Here, we tested the effect of Abeta(1-40) oligomers/aggregates, believed to be involved in Alzheimer's Disease (AD) genesis, on this type of synaptic plasticity. In the presence of bicuculline, the frequency of miniature currents increased from 1.2+/-0.4 Hz to 3.1+/-0.6 Hz (n=6, p<0.05). Similarly, current amplitude increased from 45+/-3 pA to 81+/-11 pA (n=5, p<0.05). These effects were completely inhibited in the presence of Abeta(1-40) aggregates. Data suggest that Abeta aggregates exert their influence principally by blocking synaptic transmission and altering the transcriptional pathway associated with CREB-p. In conclusion, neurons exposed to aggregated Abeta(1-40) showed a reduced level of neuronal plasticity and this suggests that they might be acting as anti-epileptiform modulators.

Intracellular amyloid formation in muscle cells of Abeta-transgenic Caenorhabditis elegans: determinants and physiological role in copper detoxification

BACKGROUND

The amyloid beta-peptide is a ubiquitous peptide, which is prone to aggregate forming soluble toxic oligomers and insoluble less-toxic aggregates. The intrinsic and external/environmental factors that determine Abeta aggregation in vivo are poorly understood, as well as the cellular meaning of this process itself. Genetic data as well as cell biological and biochemical evidence strongly support the hypothesis that Abeta is a major player in the onset and development of Alzheimer's disease. In addition, it is also known that Abeta is involved in Inclusion Body Myositis, a common myopathy of the elderly in which the peptide accumulates intracellularly.

RESULTS

In the present work, we found that intracellular Abeta aggregation in muscle cells of Caenorhabditis elegans overexpressing Abeta peptide is affected by two single amino acid substitutions, E22G (Arctic) and V18A (NIC). Both variations show decrease intracellular amyloidogenesis compared to wild type Abeta. We show that intracellular amyloid aggregation of wild type Abeta is accelerated by Cu2+ and diminished by copper chelators. Moreover, we demonstrate through toxicity and behavioral assays that Abeta-transgenic worms display a higher tolerance to Cu2+ toxic effects and that this resistance may be linked to the formation of amyloid aggregates.

CONCLUSION

Our data show that intracellular Abeta amyloid aggregates may trap excess of free Cu2+ buffering its cytotoxic effects and that accelerated intracellular Abeta aggregation may be part of a cell protective mechanism.

Radioiodinated clioquinol as a biomarker for beta-amyloid: Zn complexes in Alzheimer's disease

Neocortical beta-amyloid (Abeta) aggregates in Alzheimer's disease (AD) are enriched in transition metals that mediate assembly. Clioquinol (CQ) targets metal interaction with Abeta and inhibits amyloid pathology in transgenic mice. Here, we investigated the binding properties of radioiodinated CQ ([(125)I]CQ) to different in vitro and in vivo Alzheimer models. We observed saturable binding of [(125)I]CQ to synthetic Abeta precipitated by Zn(2+) (K(d)=0.45 and 1.40 nm for Abeta(1-42) and Abeta(1-40), respectively), which was fully displaced by free Zn(2+), Cu(2+), the chelator DTPA (diethylene triamine pentaacetic acid) and partially by Congo red. Sucrose density gradient of post-mortem AD brain indicated that [(125)I]CQ concentrated in a fraction enriched for both Abeta and Zn, which was modulated by exogenous addition of Zn(2+) or DTPA. APP transgenic (Tg2576) mice injected with [(125)I]CQ exhibited higher brain retention of tracer compared to non-Tg mice. Autoradiography of brain sections of these animals confirmed selective [(125)I]CQ enrichment in the neocortex. Histologically, both thioflavine-S (ThS)-positive and negative structures were labeled by [(125)I]CQ. A pilot SPECT study of [(123)I]CQ showed limited uptake of the tracer into the brain, which did however, appear to be more rapid in AD patients compared to age-matched controls. These data support metallated Abeta species as the neuropharmacological target of CQ and indicate that this drug class may have potential as in vivo imaging agents for Alzheimer neuropathology.

Alzheimer disease beta-amyloid activity mimics cholesterol oxidase

The abnormal accumulation of amyloid beta-peptide (Abeta) in the form of senile (or amyloid) plaques is one of the main characteristics of Alzheimer disease (AD). Both cholesterol and Cu2+ have been implicated in AD pathogenesis and plaque formation. Abeta binds Cu2+ with very high affinity, forming a redox-active complex that catalyzes H2O2 production from O2 and cholesterol. Here we show that Abeta:Cu2+ complexes oxidize cholesterol selectively at the C-3 hydroxyl group, catalytically producing 4-cholesten-3-one and therefore mimicking the activity of cholesterol oxidase, which is implicated in cardiovascular disease. Abeta toxicity in neuronal cultures correlated with this activity, which was inhibited by Cu2+ chelators including clioquinol. Cell death induced by staurosporine or H2O2 did not elevate 4-cholesten-3-one levels. Brain tissue from AD subjects had 98% more 4-cholesten-3-one than tissue from age-matched control subjects. We observed a similar increase in the brains of Tg2576 transgenic mice compared with nontransgenic littermates; the increase was inhibited by in vivo treatment with clioquinol, which suggests that brain Abeta accumulation elevates 4-cholesten-3-one levels in AD. Cu2+-mediated oxidation of cholesterol may be a pathogenic mechanism common to atherosclerosis and AD.

The N‐terminal copper‐binding domain of the amyloid precursor protein protects against Cu2+neurotoxicity in vivo

The amyloid precursor protein (APP) contains a Cu binding domain (CuBD) localized between amino acids 135 and 156 (APP135-156), which can reduce Cu2+ to Cu1+ in vitro. The physiological function of this APP domain has not yet being established; nevertheless several studies support the notion that the CuBD of APP is involved in Cu homeostasis. We used APP synthetic peptides to evaluate their protective properties against Cu2+ neurotoxicity in a bilateral intra-hippocampal injection model. We found that human APP135-156 protects against Cu2+-induced neurotoxic effects, such as, impairment of spatial memory, neuronal cell loss, and astrogliosis. APP135-156 lacking two histidine residues showed protection against Cu2+; however, APP135-156 mutated in cysteine 144, a key residue in the reduction of Cu2+ to Cu1+, did not protect against Cu2+ neurotoxicity. In accordance with recent reports, the CuBD of the Caenorhabditis elegans, APL-1, protected against Cu2+ neurotoxicity in vivo. We also found that Cu2+ neurotoxicity is associated with an increase in nitrotyrosine immunofluorescence as well as with a decrease in Cu2+ uptake. The CuBD of APP therefore may play a role in the detoxification of brain Cu.

Copper reduction by copper binding proteins and its relation to neurodegenerative diseases

Increasing evidence supports an important role for metals in neurobiology. In fact, copper binding proteins that form bioinorganic complexes are able to display oxidant or anti-oxidant properties, which would impact on neuronal function or in the triggering of neurodegenerative process. Two proteins related to neurodegenerative diseases have been described as copper binding proteins: the amyloid precursor protein (APP), a protein related to Alzheimer's disease, and the Prion protein (PrP), related to Creutzfeldt-Jakob disease. We used different synthetic peptides from APP and PrP sequences in order to evaluate the ability to reduce copper. We observed that APP(135-156), amyloid-beta-peptide (A beta(1-40)), and PrP(59-91) all have copper reducing ability, with the APP(135-156) peptide being more potent than the other fragments. Moreover, we identify His, Cys and Trp residues as key amino acids involved in the copper reduction of A beta, APP and PrP, respectively. We postulated, that in a cellular context, the interaction of these proteins with copper could be necessary to reduce copper on plasma membrane, possibly presenting Cu(I) to the copper transporter, driving the delivery of this metal to antioxidant enzymes. Moreover, protein-metal complexes could be the catalytic centers for the formation of reactive oxygen species involved in the oxidative damage present both in Alzheimer's and Prion disease.

Metalloenzyme-like activity of Alzheimer's disease beta-amyloid. Cu-dependent catalytic conversion of dopamine, cholesterol, and biological reducing agents to neurotoxic H(2)O(2)

Beta-amyloid (Abeta) 1-42, implicated in the pathogenesis of Alzheimer's disease, forms an oligomeric complex that binds copper at a CuZn superoxide dismutase-like binding site. Abeta.Cu complexes generate neurotoxic H(2)O(2) from O(2) through Cu(2+) reduction, but the reaction mechanism has been unclear. We now report that Abeta1-42, when binding up to 2 eq of Cu(2+), generates the H(2)O(2) catalytically by recruiting biological reducing agents as substrates under conditions where the Cu(2+) or reducing agents will not form H(2)O(2) themselves. Cholesterol is an important substrate for this activity, as are vitamin C, L-DOPA, and dopamine (V(max) for dopamine = 34.5 nm/min, K(m) = 8.9 microm). The activity was inhibited by anti-Abeta antibodies, Cu(2+) chelators, and Zn(2+). Toxicity of Abeta in neuronal culture was consistent with catalytic H(2)O(2) production. Abeta was not toxic in cell cultures in the absence of Cu(2+), and dopamine (5 microm) markedly exaggerated the neurotoxicity of 200 nm Abeta1-42.Cu. Therefore, microregional catalytic H(2)O(2) production, combined with the exhaustion of reducing agents, may mediate the neurotoxicity of Abeta in Alzheimer's disease, and inhibitors of this novel activity may be of therapeutic value.

The role of oxidative stress in the toxicity induced by amyloid beta-peptide in Alzheimer's disease

One of the theories involved in the etiology of Alzheimer's disease (AD) is the oxidative stress hypothesis. The amyloid beta-peptide (A beta), a hallmark in the pathogenesis of AD and the main component of senile plaques, generates free radicals in a metal-catalyzed reaction inducing neuronal cell death by a reactive oxygen species mediated process which damage neuronal membrane lipids, proteins and nucleic acids. Therefore, the interest in the protective role of different antioxidants in AD such as vitamin E, melatonin and estrogens is growing up. In this review we summarize data that support the involvement of oxidative stress as an active factor in A beta-mediated neuropathology, by triggering or facilitating neurodegeneration, through a wide range of molecular events that disturb neuronal cell homeostasis.

Amyloid-ß-peptide reduces copper(II) to copper(I) independent of its aggregation state

Alzheimer's disease (AD) is characterized by the deposition of amyloid beta-peptide (A beta) and neuronal degeneration in brain regions involved in learning and memory. One of the leading etiologic hypotheses regarding AD is the involvement of free radical-mediated oxidative stress in neuronal degeneration. Recent evidence suggests that metals concentrated in amyloid deposits may contribute to the oxidative insults observed in AD-affected brains. We hypothesized that A beta peptide in the presence of copper enhances its neurotoxicity generating free radicals via copper reduction. In the present study, we have examined the effect of the aggregation state of amyloid-beta-peptide on copper reduction. In independent experiments we measured the copper-reducing ability of soluble and fibrillar A beta(1-40) forms by bathocuproine assays. As it was previously observed for the amyloid precursor protein (APP), the A beta peptide showed copper-reducing ability. The capacity of A beta to reduce copper was independent of the aggregation state. Finally, the A beta peptide derived from the human sequence has a greater effect than the A beta peptide derived from the rat sequence, suggesting that histidine 13 may play a role in copper reduction. In agreement with this possibility, the A beta peptide reduces less copper in the presence of exogenous histidine.

Cysteine 144 is a key residue in the copper reduction by the beta-amyloid precursor protein

The beta-amyloid precursor protein (beta-APP) contains a copper-binding site localized between amino acids 135 and 156 (beta-APP(135-156)). We have employed synthetic beta-APP peptides to characterize their capacities to reduce Cu(II) to Cu(I). Analogues of the wild-type beta-APP(135-156) peptide, containing specific amino acid substitutions, were used to establish which residues are specifically involved in the reduction of copper by beta-APP(135-156). We report here that beta-APP's copper-binding domain reduced Cu(II) to Cu(I). The single-mutant beta-APP(His147-->Ala) and the double-mutant beta-APP(His147-->Ala/His149-->Ala) showed a small decrease in copper reduction in relation to the wild-type peptide and the beta-APP(Cys144-->Ser) mutation abolished it, suggesting that Cys144 is the key amino acid in the oxidoreduction reaction. Our results confirm that soluble beta-APP is involved in the reduction of Cu(II) to Cu(I).

Intracellular Ca2+ homeostasis in rat round spermatids

Intracellular calcium, [Ca2+]i, can regulate meiotic progression of mammalian oocytes. However, the role of [Ca2+]i in the regulation of the spermatogenic process and its cellular homeostatic mechanisms in spermatogenic cells has not been elucidated. Using intracellular fluorescent probes for Ca2+ and immunodetection of plasma membrane (PM) Ca(2+)-ATPases, we report that: a) rat round spermatids maintain [Ca2+]i levels of 60 +/- 5 nM (SEM), as estimated with fluo-3 in single cells or fura-2 in cells in suspension; b) these cells regulate [Ca2+]i by actively extruding it using a PM Ca(2+)-ATPase; c) rat spermatids also actively transport Ca2+ by sarco-endoplasmic reticulum type ATPases (SERCA); d) rat spermatids possess non-mitochondrial intracellular Ca2+i stores insensitive to thapsigargin but releasable by ionomycin; and e) rat spermatids do not activate Ca2+ entry mechanisms by the release of Ca2+ from SERCA-regulated stores. These results demonstrate that rat round spermatids can generate modulated intracellular Ca2+ signals upon activation of Ca2+ channels or Ca2+ release from intracellular stores.

Stable complexes involving acetylcholinesterase and amyloid-beta peptide change the biochemical properties of the enzyme and increase the neurotoxicity of Alzheimer's fibrils

Brain acetylcholinesterase (AChE) forms stable complexes with amyloid-beta peptide (Abeta) during its assembly into filaments, in agreement with its colocalization with the Abeta deposits of Alzheimer's brain. The association of the enzyme with nascent Abeta aggregates occurs as early as after 30 min of incubation. Analysis of the catalytic activity of the AChE incorporated into these complexes shows an anomalous behavior reminiscent of the AChE associated with senile plaques, which includes a resistance to low pH, high substrate concentrations, and lower sensitivity to AChE inhibitors. Furthermore, the toxicity of the AChE-amyloid complexes is higher than that of the Abeta aggregates alone. Thus, in addition to its possible role as a heterogeneous nucleator during amyloid formation, AChE, by forming such stable complexes, may increase the neurotoxicity of Abeta fibrils and thus may determine the selective neuronal loss observed in Alzheimer's brain.

Crosslinking of amyloid-beta peptide to brain acetylcholinesterase

Acetylcholinesterase (AChE) is the enzyme responsible for the hydrolysis of the neurotransmitter acetylcholine in the central nervous system. Recently, we have found that AChE promotes the assembly of amyloid-beta peptides (A beta) into Alzheimer fibrils. The action of AChE on the state of aggregation of the A beta peptide supposes a near neighbor relationship between these two molecules. In the present work, we have studied A beta-AChE interactions using the crosslinker reagent disuccinimidyl suberate (DSS), in the presence of [125I]-A beta peptide. The A beta-AChE complexes formed by crosslinkage were then analyzed by SDS-PAGE and autoradiography. We observed the formation of [125I] A beta-labeled complexes of 70, 160, 250, and 300 kDa corresponding to monomers, dimers, tetramers, and oligomers of AChE, respectively crosslinked with the A beta peptide. Our results suggest that AChE and the A beta peptide may be involved in physiologically relevant interactions, related to the pathogenesis of Alzheimer disease (AD).

Acetylcholinesterase promotes the aggregation of amyloid-beta-peptide fragments by forming a complex with the growing fibrils

Acetylcholinesterase (AChE), an enzyme involved in the hydrolysis of the neurotransmitter acetylcholine, consistently colocalizes with the amyloid deposits characteristic of Alzheimer's disease and may contribute to the generation of amyloid proteins and/or physically affect fibril assembly. In order to identify the structural domains of the amyloid-beta-peptide (Abeta) involved in the aggregation induced by AChE, we have studied the effect of this cholinergic enzyme on Abeta peptide fragments of different sizes. AChE enhanced the aggregation of the Abeta(12-28) and Abeta(25-35) peptides but not of the Abeta(1-16) fragment. The inductive effect of AChE on the aggregation of Abeta(12-28) was abolished by the presence of either Abeta(1-16) or Abeta(9-21). The effect of the enzyme was also analysed using two different mutant fragments, possessing a low and the other a high capacity for fibrillogenesis. The fragments used were Abeta(12-28)Val18-->Ala and Abeta(12-28)Glu22-->Gln, respectively. AChE was able to promote the aggregation of these fragments in a very specific way and both mutant peptides were able to form amyloid fibrils, as revealed by negative staining under the electron microscope. Binding assays indicated that AChE was bound to Abeta(12-28), as well as to the Abeta(1-16) peptide. AChE was seen to form strong complexes with the Abeta(12-28) fibrils as such complexes stained positively for both thioflavine-T and AChE activity, were resistant to high ionic strength treatment, and were partially sensitive to detergents, suggesting that hydrophobic interactions may play a role in the stabilization of the AChE-Abeta complex. Our results suggest that such amyloid-AChE complexes are formed when AChE interacts with the growing amyloid fibrils and accelerates the assembly of Abeta peptides. This is consistent with the fact that AChE is known to be present within Abeta deposits including the pre-amyloid diffuse and mature senile plaques found in Alzheimer's brain.

On stage single cell identification of rat spermatogenic cells

The study of spermatogenic cell physiology has been hindered by the absence of unbiased methods of identification of cells upon which single cell techniques are being applied. In this work, we have used histochemical techniques, digital videoimaging, quantification of chromatin-bound DNA probes, and measurements of cell diameter to identify single spermatogenic cells at different periods of development. Our criteria of identification permit the definition of four developmental stages of spermatogenesis on which to perform single cell analyses: spermatogonia B/preleptotene spermatocytes, leptotene/zygotene spermatocytes, pachytene spermatocytes, and round spermatids. The use of voltage-sensitive dyes and Ca(2+)-sensitive dyes does not interfere with the estimations of DNA content. The estimations of DNA content of spermatogenic cells can be performed both with near-UV excited dyes (H33342) and long wavelength-excited dyes (ethidium bromide), allowing the use of a wide range of physiological and immunocytochemical fluorescent probes to study the spermatogenic process.

[Mandibular reconstruction after hemimandibulectomy. Report of a clinical case]

Reconstruction of the mandible with iliac crest bone graft has been reported since early 1900'S. The series go from one to 160 cases published. This article shows the way we approached de reconstruction of a patient who came with an hemimandibulectomy due to osteosarcoma.