Cellular MTT reduction distinguishes the mechanism of action of beta-amyloid from that of tachykinin receptor peptides

Oligomer Formation of Toxic and Functional Amyloid Peptides Studied with Atomistic Simulations

Amyloids are associated with diseases, including Alzheimer's, as well as functional roles such as storage of peptide hormones. It is still unclear what differences exist between aberrant and functional amyloids. However, it is known that soluble oligomers formed during amyloid aggregation are more toxic than the final fibrils. Here, we perform molecular dynamics simulations to study the aggregation of the amyloid-β peptide Aβ25-35, associated with Alzheimer's disease, and two functional amyloid-forming tachykinin peptides: kassinin and neuromedin K. Although the three peptides have similar primary sequences, tachykinin peptides, in contrast to Aβ25-35, form nontoxic amyloids. Our simulations reveal that the charge of the C-terminus is essential to controlling the aggregation process. In particular, when the kassinin C-terminus is not amidated, the aggregation kinetics decreases considerably. In addition, we observe that the monomeric peptides in extended conformations aggregate faster than those in collapsed hairpin-like conformations.

SP protects cerebellar granule cells against beta-amyloid-induced apoptosis by down-regulation and reduced activity of Kv4 potassium channels

The tachykinin endecapeptide substance P (SP) has been demonstrated to exert a functional role in neurodegenerative disorders, including Alzheimer's disease (AD). Aim of the present study was to evaluate the SP neuroprotective potential against apoptosis induced by the neurotoxic beta-amyloid peptide (A beta) in cultured rat cerebellar granule cells (CGCs). We found that SP protects CGCs against both A beta(25-35)- and A beta(1-42)-induced apoptotic CGCs death as revealed by live/dead cell assay, Hoechst staining and caspase(s)-induced PARP-1 cleavage, through an Akt-dependent mechanism. Since in CGCs the fast inactivating or A-type K(+) current (I(KA)) was potentiated by A beta treatment through up-regulation of Kv4 subunits, we investigated whether I(KA) and the related potassium channel subunits could be involved in the SP anti-apoptotic activity. Patch-clamp experiments showed that the A beta-induced increase of I(KA) current amplitude was reversed by SP treatment. In addition, as revealed by Western blot analysis and immunofluorescence studies, SP prevented the up-regulation of Kv4.2 and Kv4.3 channel subunits expression. These results indicate that SP plays a role in the regulation of voltage-gated potassium channels in A beta-mediated neuronal death and may represent a new approach in the understanding and treatment of AD.

The cell-selective neurotoxicity of the Alzheimer's Abeta peptide is determined by surface phosphatidylserine and cytosolic ATP levels. Membrane binding is required for Abeta toxicity

Measurement of Abeta toxicity of cells in culture exposes a subpopulation of cells with resistance to Abeta, even at high concentrations and after long periods of treatment. The cell-selective toxicity of Abeta resembles the selective damage observed in cells of specific regions of the Alzheimer's disease (AD) brain and suggests that there must be particular characteristics or stages of these cells that make them exceptionally sensitive or resistant to the effect of Abeta. Using flow cytometry and cell sorting, we efficiently separated and analyzed the Abeta-sensitive and the Abeta-resistant subpopulations within a variety of neuronal cell lines (PC12, GT1-7) and primary cultured neurons (hippocampal, cortex). We found that this distinctive sensitivity to Abeta was essentially associated with cell membrane Abeta binding. This selective Abeta binding was correlated to distinctive cell characteristics, such as cell membrane exposure of the apoptotic signal molecule phosphatidyl serine, larger cell size, the G1 cell cycle stage, and a lower than normal cytosolic ATP level. The response to Abeta by the cells with high Abeta binding affinity was characterized by a larger calcium response and increased mortality, lactate dehydrogenase release, caspase activation, and DNA fragmentation. The distinctive sensitivity or resistance to Abeta of the different subpopulations was maintained even after multiple cell divisions. We believe that these distinctive cell characteristics are the determining factors for the selective attack of Abeta on cells in culture and in the AD brain.

Abeta ion channels. Prospects for treating Alzheimer's disease with Abeta channel blockers

The main pathological features in the Alzheimer's brain are progressive depositions of amyloid protein plaques among nerve cells, and neurofibrillary tangles within the nerve cells. The major components of plaques are Abeta peptides. Numerous reports have provided evidence that Abeta peptides are cytotoxic and may play a role in the pathogenesis of AD. An increasing number of research reports support the concept that the Abeta-membrane interaction event may be followed by the insertion of Abeta into the membrane in a structural configuration which forms an ion channel. This review summarizes experimental procedures which have been designed to test the hypothesis that the interaction of Abeta with a variety of membranes, both artificial and natural, results in the subsequent formation of Abeta ion channels We describe experiments, by ourselves and others, that support the view that Abeta is cytotoxic largely due to the action of Abeta channels in the cell membrane. The interaction of Abeta with the surface of the cell membrane may results in the activation of a chain of processes that, when large enough, become cytotoxic and induce cell death by apoptosis. Remarkably, the blockage of Abeta ion channels at the surface of the cell absolutely prevents the activation of these processes at different intracellular levels, thereby preserving the life of the cells. As a prospect for therapy for Alzheimer's disease, our findings at cellular level may be testable on AD animal models to elucidate the potential role and the magnitude of the contribution of the Abeta channels for induction of the disease.

Microtransponders, the miniature RFID electronic chips, as platforms for cell growth in cytotoxicity assays

BACKGROUND

An electronic radio frequency (RF) microchip, the microtransponder (MTP), has been developed as a platform for assays in the fields of genomics and proteomics. Upon activation by light, each MTP provides a unique RF identification (ID) signal that matches a chip to the specific biological material attached to it. The MTP is powered by a photocell and has an antenna that transmits the signal. The aim of the present study was to explore utility of MTPs as a platform for cell growth in cytotoxicity assays.

METHODS

The MCF-7, MCF-116, A549, or T-24 cells growing on MTPs placed in petri dishes or slide chambers were cultured untreated or exposed to antitumor drugs topotecan, mitoxantrone, or onconase for up to 4 days. Their attachment to- and growth on- MTPs was assessed by fluorescence microscopy and laser scanning cytometry (LSC) and compared with growth on the dish surface in the MTP neighborhood. The MTPs were fixed in ethanol, stained with propidium iodide (PI), and interrogated in flow in the instrument capable to rapidly (up to 103 MTPs/s) identify their ID signal and measure fluorescence.

RESULTS

The cells plated on MTPs exhibited similar attachment properties to those plated in culture dishes. When measured by LSC, they had similar mitotic activity, growth rate, and cell cycle distributions as the cells adhering to the culture dish in the neighborhood of MTPs. The fluorescence intensity of MTPs provided information about the cell number per MTP, which made it possible to assess cell growth rate and monitor the cytostatic/cytotoxic effects of the tested drugs.

CONCLUSIONS

The MTP-based system holds promise for the multiplexed cell assays in which numerous different cell lines can be screened for their growth rate or sensitivity while exposed to particular agents in the same vessel. Other advantages of the system are the rapidity of the screening and a very large number of ID codes. Because many cell lines/types can be assayed in a single dish, the system also offers cost savings on tissue culture reagents.

Simple, semiautomatic assay of cytostatic and cytotoxic effects of antitumor drugs by laser scanning cytometry: effects of the bis-intercalator WP631 on growth and cell cycle of T-24 cells

BACKGROUND

Common assays of drug-induced cytotoxicity on adherent cells rely on cell trypsinization followed by count of live and dead cells. To estimate the cell cycle effects, cellular DNA content is analyzed by flow cytometry. This procedure is laborious and time consuming. The alternative viability assays, e.g., based on reduction of 3-(4,5-dimethylthiazol-2-yl) 2,5-diphenyl tetrazolium bromide, although rapid and convenient, do not provide information about individual cells or cell cycle effects and may be biased by growth imbalance.

METHODS

The bladder carcinoma T-24 cells were seeded onto eight-chamber microscope slide-based tissue culture vessels. The novel antitumor drug, the bis-intercalating anthracycline WP631, was administered at various concentrations to different chamber cultures on the same slide; the control cultures were left untreated. After 24, 48, and 72 h, the cultures were fixed, and cellular DNA was stained with 4,6-diamidino-2-phenyl indole (DAPI). The slides were scanned by laser scanning cytometry (LSC) to obtain the number of attached cells per culture chamber and reveal their cell cycle distribution.

RESULTS

The cell growth and viability plots in the absence and presence of WP621 were constructed from the frequency of the attached cells per chamber. A 50% reduction in cell number was observed at the 75 nM concentration of WP321. Mitotic and postmitotic cells were identified based on high intensity of maximal pixel of DAPI fluorescence. An increase in proportion of cells in G2 was seen at 75-300 nM of WP631. Relatively few (<12%) apoptotic cells, identified by the presence of DNA strand breaks, remained attached in the WP631-treated cultures.

CONCLUSIONS

Because late apoptotic cells detach during culturing, the cells that remain attached in the multi-chamber cultures represent predominantly live cells; the deficit in their number compared with the untreated cultures, recorded by LSC during scanning, provides information about the degree of cytostatic and cytotoxic effects of the studied drug. The possibility to demonstrate the cell cycle distribution, including a distinction between G2 and M cells, provides an additional advantage of this assay. Other parameters that may be associated with the cell cycle perturbation or with induction of apoptosis also can be measured in the same cultures by using the multiparameter capabilities of LSC. Each measured cell can be relocated for imaging or measurement after subsequent staining with other probes.

Inaccuracies in MTS assays: major distorting effects of medium, serum albumin, and fatty acids

Soluble formazan assays are widely used for cell number assessment. However, in our hands, we observed frequent occasions in which the actual cell number was at odds with the assay reading. In this study, we have determined that (i) a large proportion of the reading obtained in commonly used culture media can be caused by media component amplification of formazan production in a way that cannot be corrected for by media-only controls; (ii) the albumin present in 10% serum can reduce the assay absorbance by 50% so that an actual doubling of cell number can be obscured; and (iii) this latter effect is dependent on the concentration of fatty acids. To counter these problems, we have developed a protocol that gives consistent readings that are fully representative of cell number while retaining some of the original advantages of soluble formazan assays.

Backscattered light confocal imaging of intracellular MTT-formazan crystals

Metabolically active animal and plant cells reduce MTT tetrazolium salt to a corresponding nonfluorescent formazan. Reduction of MTT by viable cells is exploited in a number of tests widely used in biological research. The aim of this study was to optimize a microscopy method of detecting small crystals of MTT-formazan formed in intact cells maintained in in vitro cultures. We examined scattering properties of small intracellular crystals of MTT formazan and found that the efficiency of light scattering was dependent on wavelength. Small (<3 microm) crystals of MTT-formazan, formed in viable cells, scattered red, but not blue, light. Large crystals, which are formed later at a stage when cells begin to lose viability, scattered both red and blue light. We conclude that optimal detection of early stages of crystallization of MTT-formazan in living cells is possible using confocal microscopy of red, but not blue, scattered light. High contrast and resolution of images can be achieved by filtering out interference effects in the frequency domain.

Annexin 5 and apolipoprotein E2 protect against Alzheimer's amyloid-beta-peptide cytotoxicity by competitive inhibition at a common phosphatidylserine interaction site

Amyloid-beta-protein (betaA/4, AbetaP) accumulates in the brains of patients with Alzheimer's disease (AD), regardless of genetic etiology, and is thought to be the toxic principle responsible for neuronal cell death. The varepsilon4 allele of apoE has been linked closely to earlier onset of AD and increased deposition of the amyloid peptide, regardless of the clinical status of AD, while the apoE varepsilon2 allele is generally protective. We have previously hypothesized that the cell target for amyloid peptide might be the apoptotic signal molecule phosphatidylserine (PS). We report here that annexin 5, a specific ligand for PS, not only blocks amyloid peptide AbetaP[1-40] cytotoxicity, but competitively inhibits AbetaP[1-40]-dependent aggregation of PS liposomes. In addition, we find that apoE2, but not apoE4, can not only perform the same protective effect on cells exposed to AbetaP[1-40], but can also competitively inhibit PS liposome aggregation and fusion by the amyloid peptide. Altogether, the in vivo and in vitro results reported here provide fundamental insight to the process by which amyloid targets specific neurons for destruction, and suggest that PS may be a surface "receptor" site for AbetaP binding. These results also provide a biochemical mechanism by which the apoE varepsilon2 allele, but not apoE varepsilon4, can be protective towards the incidence and progression of Alzheimer's disease.

Mitochondrial and nonmitochondrial reduction of MTT: interaction of MTT with TMRE, JC-1, and NAO mitochondrial fluorescent probes

BACKGROUND

Bioreduction of water-soluble tetrazolium salts (e.g., MTS, XTT, and MTT) to their respective formazans is generally regarded as an indicator of cell "redox activity." The reaction is attributed mainly to mitochondrial enzymes and electron carriers. However, MTT reduction may also be catalyzed by a number of other nonmitochondrial enzymes. The goal of this work was to establish the sites of MTT reduction in intact HepG2 human hepatoma cells in culture.

METHODS

In order to establish the subcellular localization of the sites of reduction of MTT, we imaged the formation of MTT-formazan deposits using backscattered light confocal microscopy. Mitochondria were visualized in viable cells using fluorescent dyes that bind in a manner dependent (JC-1 and TMRE) or independent (NAO) of mitochondrial electric potential.

RESULTS

Only 25-45% of MTT-formazan was associated with mitochondria after 25 min of incubation. No more than 25% of the mitochondrial area on images was occupied by MTT-formazan. Mitochondrial fluorescence of TMRE, NAO, and the monomeric form of JC-1 decreased rapidly in cells incubated with MTT. However, the intensity of fluorescence of JC-1 aggregates dropped by less than 30% at the onset of incubation and remained constant as reduction of MTT proceeded further.

CONCLUSIONS

(1) Most of MTT-formazan deposits are not coincident with mitochondria. (2) Monomeric JC-1, as well as TMRE and NAO, accumulating in mitochondria may be displaced by MTT. Thus, the presence of positively charged organic compounds (like MTT) may distort measurements of mitochondrial transmembrane electric potential, which are based on accumulation of fluorescent dyes.

The role of plasma membrane in bioreduction of two tetrazolium salts, MTT, and CTC

Despite widespread use of various tetrazolium assays, the mechanisms of bioreduction of these compounds have not been fully elucidated. We investigated the capacity of tetrazolium salts to penetrate through intact cell plasma membranes. 5-cyano-2,3-ditolyl tetrazolium chloride (CTC) and 3-(4,5-dimethylthiazol-2-yl)-2,5 diphenyltetrazolium bromide (MTT) tetrazolium salts appear to represent examples of species that are reduced by different mechanisms. We provide evidence suggesting that MTT readily crosses intact plasma membranes and is reduced intracellularly. MTT appears to be reduced by both plasma membrane and intracellular reductases; reducing cells are not damaged and remain metabolically active for at least 45 min. In contrast, CTC remains extracellular with respect to viable cells and thus requires plasma membrane permeable electron carrier to be reduced efficiently. However, reduction of CTC in the presence of an electron carrier inflicts damage on plasma membranes. The intracellular vs extracellular sites of reduction of tetrazolium salts were established on the basis of deposition of formazans. Crystals of formazan were detected using fluorescence or backscattered light confocal laser microscopy. We postulate that the capacity of a tetrazolium salt to cross intact plasma membranes constitutes an important experimental variable which needs to be controlled in order to correctly interpret the outcome of tetrazolium assays designed to measure cellular production of oxygen radicals, activity of mitochondrial, cytosolic, or outer membrane reductases, etc.

Reduction of a tetrazolium salt, CTC, by intact HepG2 human hepatoma cells: subcellular localisation of reducing systems

Cell-mediated reduction of tetrazolium salts, including MTT, XTT, MTS, NBT, NTV, INT, in the presence or absence of intermediate electron carriers is used as a convenient test for animal or bacterial cell viability. Bioreduction of tetrazolium is considered an alternative to a clonogenic assay and a thymidine incorporation assay. However, correlation between clonogenic potential and capacity to reduce tetrazolium has not been demonstrated convincingly. Moreover, despite a wide use of tetrazolium viability assays, the mechanism and subcellular localisation of reducing systems or species in viable intact cells have not been fully elucidated. We report evidence indicating that a tetrazolium salt CTC can be reduced in the presence as well as in the absence of an electron carrier by viable HepG2 human hepatoma cells. CTC-formazan is formed within or at the outer surface of plasma membranes. We hypothesise that in the presence of an electron carrier the electron donors active in the reduction of CTC are located in the intracellular compartment, as well as in plasma membranes. However, in the absence of an electron carrier, the reduction occurs primarily via a plasma membrane-associated enzymatic system or species.

Comparative studies on peptides representing the so-called tachykinin-like region of the Alzheimer Abeta peptide [Abeta(25-35)]

In an attempt to answer the question of whether or not the so-called tachykinin-like region of the Alzheimer beta-amyloid protein [Abeta(25-35)] can act as a tachykinin, the sequences Abeta(25-35), Abeta(25-35)amide and their norleucine-35 and phenylalanine-31 analogues were synthesized. These peptides were examined with ligand binding studies, electron microscopy, CD and NMR. In all cases some differences were found between the Abeta(25-35) analogue and the corresponding Phe31 peptide. In addition, in ligand displacement studies on tachykinin NK1 receptors, only the Phe31 analogue showed activity comparable to that of genuine tachykinins. We conclude that peptides based on Abeta(25-35) but with a Phe residue at position 31 do display properties typical of a tachykinin, but that peptides with Ile at this position do not.

Activation of LA-N-2 cell phospholipase D by amyloid beta protein (25-35)

Amyloid beta protein is the major protein component of neuritic plaques found in the brain of Alzheimer's disease. The activation of phospholipase D by amyloid beta protein (25-35), quisqualate and phorbol 12, 13-dibutyrate was investigated in LA-N-2 cells by measuring phosphatidylethanol formation. The activation of phospholipase D by quisqualate and APP (25-35) was calcium-independent. The AbetaP (25-35) and quisqualate activation of phospholipase D appeared to be mediated through a pertussis toxin-sensitive GTP-binding protein. Phospholipase D activation by AbetaP (25-35), quisqualate and phorbol dibutyrate was not blunted by the protein kinase C inhibitors, staurosporine, H-7 and RO-31-8220. However, it was abolished by overnight exposure to phorbol dibutyrate. This activation of phospholipase D was prevented by the tyrosine kinase inhibitor, genistein but not by tyrophostin A. Several excitatory amino acid antagonists were tested for their ability to prevent the phospholipase D activation by quisqualate and AbetaP (25-35). Only NBQX was effective with an IC50 of 75 microM for AbetaP (25-35) and quisqualate. Activation of phospholipase D by AbetaP or quisqualate was absent in LA-N-2 cells previously desensitized by quisqualate or AbetaP (25-35), but the activation by phorbol dibutyrate was unaltered. The responsiveness to AbetaP and quisqualate in previously desensitized cells reappeared subsequent to a period of resensitization. The observations with the antagonist NBQX, and the desensitization and resensitization experiments, are consistent with a receptor occupancy mediated activation of phospholipase D by quisqualate and by AbetaP (25-35).

The tachykinin NK1 receptor. Part II: Distribution and pathophysiological roles

The tachykinin NK1 receptor is widely distributed in both the central and peripheral nervous system. In the CNS, NK1 receptors have been implicated in various behavioural responses and in regulating neuronal survival and degeneration. Moreover, central NK1 receptors regulate cardiovascular and respiratory function and are involved in activating the emetic reflex. At the spinal cord level, NK1 receptors are activated during the synaptic transmission, especially in response to noxious stimuli applied at the receptive field of primary afferent neurons. Both neurophysiological and behavioural evidences support a role of spinal NK1 receptors in pain transmission. Spinal NK1 receptors also modulate autonomic reflexes, including the micturition reflex. In the peripheral nervous system, tachykinin NK1 receptors are widely expressed in the respiratory, genitourinary and gastrointestinal tracts and are also expressed by several types of inflammatory and immune cells. In the cardiovascular system, NK1 receptors mediate endothelium-dependent vasodilation and plasma protein extravasation. At respiratory level, NK1 receptors mediate neurogenic inflammation which is especially evident upon exposure of the airways to irritants. In the carotid body, NK1 receptors mediate the ventilatory response to hypoxia. In the gastrointestinal system, NK1 receptors mediate smooth muscle contraction, regulate water and ion secretion and mediate neuro-neuronal communication. In the genitourinary tract, NK1 receptors are widely distributed in the renal pelvis, ureter, urinary bladder and urethra and mediate smooth muscle contraction and inflammation in response to noxious stimuli. Based on the knowledge of distribution and pathophysiological roles of NK1 receptors, it has been anticipated that NK1 receptor antagonists may have several therapeutic applications at central and peripheral level. At central level, it is speculated that NK1 receptor antagonists could be used to produce analgesia, as antiemetics and for treatment of certain forms of urinary incontinence due to detrusor hyperreflexia. In the peripheral nervous system, tachykinin NK1 receptor antagonists could be used in several inflammatory diseases including arthritis, inflammatory bowel diseases and cystitis. Several potent tachykinin NK1 receptor antagonists are now under evaluation in the clinical setting, and more information on their usefulness in treatment of human diseases will be available in the next few years.