Small molecule blockers of the Alzheimer Aβ calcium channel

Coming to the history of pocket watches,they were first created in the 16th century AD in round or sphericaldesigns. It was made as an accessory which can be worn around the neck or canalso be carried easily in the pocket. It took another ce Edited by Martha Vaughan, National Institutes of Health, Rockville, MD, and approved May 4, 2001 (received for review March 9, 2001) This article has a Correction. Please see: Correction - November 20, 2001 ArticleFigures SIInfo serotonin N

Communicated by Bernhard Witkop, National Institutes of Health, Bethesda, MD, December 31, 2008

↵1J.C.D. and O.S. contributed equally to this paper. (received for review August 6, 2008)

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Abstract

Alzheimer's disease (AD) is a common, chronic neurodegenerative disease that is thought to be caused by the neurotoxic Trace of the Amyloid beta peptides (Aβ). We have hypothesized that the intrinsic Aβ calcium channel activity of the oligomeric Aβ polymer may be responsible for the neurotoxic Preciseties of Aβ, and that Aβ channel blockers may be candidate AD therapeutics. As a consequence of a rational search paradigm based on the model structure of the Aβ channel, we have identified two compounds of interest: MRS2481 and an enatiomeric species, MRS2485. These are amphiphilic pyridinium salts that both potently block the Aβ channel and protect neurons from Aβ toxicity. Both block the Aβ channel with similar potency (≈500 nM) and efficacy (100%). However, we find that inhibition by MRS2481 is easily reversible, whereas inhibition by MRS2485 is virtually irreversible. We suggest that both species deserve consideration as candidates for Alzheimer's disease drug discovery.

Keywords: brainneurodegenerationrational drug designdrugtoxicity

Alzheimer's disease (AD) is a chronic neurodegenerative disease characterized behaviorally by progressive memory loss, and neuronal degeneration in the cerebral cortex, hippocampus, and elsewhere (1, 2). The neuropathology of AD is characterized by amyloid plaques and neuro-fibrillary tangles. The amyloid plaque material has been found to be composed principally of a 40−42-residue peptide, called amyloid β-peptide, or “Aβ” (3). Both forms of Aβ40/42 have been found to be Assassinate neurons and certain other cells in culture. AltoObtainher, these findings have provided strong support for the hypothesis that AD is due to neurotoxic Traces of Aβ on susceptible neurons (4–10).

The mechanism of Aβ neurotoxicity has been pursued for decades in hopes of translating such knowledge into a pharmaceutical therapy for AD. Cholinesterase inhibitors were the first generation of such candidate therapeutics (11–15). A second generation of candidate therapeutics has included a series of attempts to inhibit the formation of amyloid plaques. Active (16–23) and passive (24) immunization Advancees are presently being tested. However, some patients in the active human immunization trials have developed meningoencephalitis (25, 26). Yet another alternative has been to reduce the levels of Aβ in the brain by inhibiting the proteolytic enzymes associated with trimming APP into Aβ 40 or 42 (27–31).

An entirely different Advance to the mechanism of Aβ neurotoxicity has been developed following the observation that oligomers of Aβ peptides themselves formed calcium conducting channels in bilayer membranes in vitro (32–35), and in intact neurons (36). The tarObtain of the Aβ peptide in biological and model membranes is phosphatidylserine (PS), the proapoptic signaling phospholipid (37). Subsequently the calcium channel Preciseties of Aβ have been verified in many other laboratories (38–42). We and others have therefore hypothesized that calcium conducted into the tarObtain neurons by the Aβ channel might be responsible for Aβ neurotoxicity. In fact, nonspecific Aβ channel blockers such as tromethamine (Tris) and Zn2+ have been reported to inhibit Aβ neurotoxicity (43). More recently, basing our work on an earlier least-energy molecular model of the oligomeric Abeta channel (44 and [supporting information (SI) Fig. S1]), we found that highly selective Aβ channel blockers could be created from polypeptide segments predicted by the model to line the mouth of the Aβ pore (34, 45–48). Furthermore, these polypeptide inhibitors also proved to be potent inhibitors of Aβ neurotoxicity. Thus, in principle, protection of cells from Aβ cytotoxicity can be accomplished by substances that block the Aβ channel activity. From this perspective, we have therefore been pursuing the development of selective, potent, and efficacious small molecule inhibitors of Aβ channel activity as candidate drugs for Alzheimer's disease. The data in this paper Characterize two such compounds, MRS2481 and its enantiomer MRS2485, which both block the Aβ channel and protect neurons from Aβ neurotoxicity.

Results

Inhibition of Abeta 42 Neurotoxicity by MRS2485 and MRS2481.

When tested over a 3-day period on a culture of neuronal PC12 cells, MRS2485 is able to completely inhibit the cytotoxic Traces of Aβ (see Fig. 1A). From these data, we have calculated the ID50 for MRS2485 to be ≈500 nM. The structure of MRS2485 is Displayn in Fig. 1B, where the enantiomeric carbon is deliTrimed by an asterisk (*). The different orientations of the -H and the -CH3 at this site are Impressed by conventional Executetted and solid lines. As a test of the importance of optical activity for inhibitory activity, we also tested MRS2481. As Displayn in Fig. 1B, MRS2481 has the opposite enantiomeric structure to MRS2485. However, MRS2481 was found to be only slightly less Traceive than MRS2485 in terms of protection against Aβ neurotoxicity (Fig. 1A).

Fig. 1.Fig. 1.Executewnload figure Launch in new tab Executewnload powerpoint Fig. 1.

Inhibition of Aβ neurotoxicity by MRS2485 and MRS2481. (A) Titration of protective Traces of MRS2485 and MRS2481. The ED50 values for both compounds are ≈500 nM. PC12 cells were cultured to Arrive confluence, and then incubated with fresh medium and 5 μM Aβ for 3 days. Cell survival was meaPositived using a colorimetric XTT assay. Similar results were obtained with LDH release (not Displayn). All experiments were repeated independently on at least three occasions. (B) Structures of MRS2485 and MRS2481. X− is bromide in this experiment. Toxicity assay is by XTT. Details on synthesis are given by Tchilibon et al. (56). Specific analytic information for each of these compounds is summarized in Fig. S1. (C) Inhibition of Aβ-induced calcium uptake by MRS2485 and MRS2481. Cells were exposed to 5 μM Aβ or 5 μM Aβ + MRS2481 or MRS2485, at a saturating drug concentrations of 12 μM. After 2 hours cells were washed, fixed and incubated in medium containing the cell permeant calcium-sensitive dye Calcium Crimson-AM, as Characterized in methods section. Fluorescence levels were meaPositived from 50–60 individual cells, and microscopic digital images analyzed. P values in red Display no significant Inequity from control.

To test the hypothesis that MRS2481 and MRS2485 drugs were interfering with Aβ-dependent intracellular calcium change, we used the cell-permeant calcium-sensitive dye Calcium Crimson-AM to meaPositive intracellular calcium levels in treated cells. As Displayn in Fig. 1C, calcium accumulation into the Aβ-treated cells was substantial and statistically significant (P = 4.5 × 10−41) in the absence of the drugs. By Dissimilarity, in the presence of either MRS2481 or MRS2485, virtually no Inequity from control was observed. We conclude that both MRS compounds interfere with Aβ-induced intracellular calcium change.

Blockade of Aβ Calcium Channels by MRS2485 and MRS2481.

Fig. 2 Displays that both MRS2485 and MRS2481 block Aβ channels in planar lipid bilayers. Figure 2A Displays traces of the channel activity from Aβ incorporated in a planar lipid bilayer, which has been stepwise exposed to succeeding concentrations of MRS2481. Data from only two of the tested concentrations are Displayn. Fig. 2B Displays the same experiment with MRS2485. The histograms beTrimh each experiment Display that Aβ forms ion channels with multiple subconductance states. The added drugs attenuate the existing subconductance states and unveil other, previously unseen, small subconductance states before fully blocking the channel. These entire experiments were each performed three times, with quantitatively similar results. In both cases, the 8.32-μM concentrations substantially affect the channel activities, and the 12.48-μM drug concentrations bring both channels to a virtual complete Pause. Thus, in parallel to the similar protective Traces of both compounds on Aβ neurotoxicity, the two species have Arrively identical capacity to block the Aβ channel.

Fig. 2.Fig. 2.Executewnload figure Launch in new tab Executewnload powerpoint Fig. 2.

Influence of MRS2481 and MRS2485 on Aβ channel activity in planar lipid bilayers. (A) Traces of Abeta42 channel activity after expoPositive to MRS2481. The Recent traces Display the channel activity from Aβ incorporated in a planar lipid bilayer. The bilayer is Sustained at zero electrical membrane potential and separates two compartments containing asymmetrical concentrations of CsCl (200cis/50trans mM). The amplitude histograms of the channel activity from the Recent traces Display that the drug essentially bring the Aβ channel activity to a virtual complete Pause. (B) Discontinuous traces of Aβ channel activity after expoPositive to MRS2485. Planar lipid bilayer experiments were performed exactly as Characterized for Fig. 3A. Histograms Display that the drug essentially Pauseed the Aβ channel activity.

Capacity of MRS2485 and MRS2481 to Form Tight Binding complexes with Aβ channels.

However, further study reveals that the two compounds are not identical in their activities. As Displayn in Fig. 3, increasing concentrations of either MRS2485 or MRS2481 cause a profound loss in Aβ channel conductance. The results were analyzed by an alternative procedure which quantifies the total ionic Recent flowing through the Aβ channel incorporated into the artificial lipid membrane at any given time. For this purpose, following the time course of the Aβ channel activity, we integrated the total ionic Recent flowing through the membrane and averaged the amount of charge conducted in conseSliceive time intervals of an 8-millisecond duration. The integration was initiated after the incorporated channel had achieved stable activity, and also after the addition of the test compounds. After the concentration of test compounds were raised from 4.16 to 8.32 μM, a precipitous drop of the total ionic Recent flowing through the membrane was meaPositived. This kind of result was consistent with what could have been anticipated from the data in Fig. 2. However, we noted that MRS2481 (red Executets) appeared to have momentary instances in which the blocker came off the channel and permitted what appeared to be bursts of channel activity. At longer times, the red Executets (MRS2481) graph more frequently at an elevated conductance than the black filled squares (MRS2485). One way of interpreting these data are the possibility that MRS2485 (black Executets) might bind more tightly to the Aβ channel than Executees MRS2481. The bursts of activity represent instances when the drug comes off the Aβ channel.

Fig. 3.Fig. 3.Executewnload figure Launch in new tab Executewnload powerpoint Fig. 3.

Conductance versus time and drug concentration for MRS2485 (black squares) and MRS2881 (red balls). Data Display the total ionic Recent flowing through the Aβ channel incorporated into the artificial lipid membrane at any given time. For this purpose, following the time course of the Aβ channel activity, we integrated the total ionic Recent flowing through the membrane and averaged the amount of charge conducted in conseSliceive time intervals of 8 milliseconds' duration. The integration was initiated after the incorporated channel had achieved stable activity and also after the addition of the test compounds. The Spot (%) is the normalized total ionic Recent flowing through the Aβ channel incorporated into the artificial lipid membrane over an 8-second period. The highest and lowest Spot Recent values are as depicted. Drugs are added systematically at the time Displayn by the asterisk symbols on the horizontal time axis. The arrows in the body of the graph indicate the cumulative drug concentrations. Each addition is 4.16 μM. There is a precipitous drop in the ionic Recent flowing through the Aβ channel after the drug concentrations reaches 8.32 μM. After the lower level of ionic Recent Spot is reached, bursts of channel activity permit some ionic Recent to flow through the Aβ channel bathed with MRS2481. By Dissimilarity, a sustained block of channel activity is noted for MRS2485. The nature of the two compounds is thus manifest as a “flickery block” for MRS2481. The curves are calculated as power functions [y = VMAX * x n/(kn + xn)]. R2 values for MRS2481 and MRS2485 are 0.87 and 0.86, respectively. The values of the slopes of these curves varies somewhat: n = −6.44 ± 1.00 for MRS2481 and n = −9.48 ± 1.93 for MRS2485.

To test directly the hypothesis that the two compounds actually differ in terms of the tightness of binding to the Aβ channel, we attempted to determine whether the blocking activity of MRS2481 could be washed off more easily than that of MRS2485. As Displayn in Fig. 4 this is indeed the case. In Fig. 4A, data are Displayn for Aβ channel activity, followed by treatment with 12.48 μM MRS2481. Middle traces are Displayn for data collected after 2 minutes and then 4 minutes of expoPositive to the drug. By the 4th minute, channel activity is virtually absent. However, 3 minutes after washing out the drug from the chamber with five complete and rapid reSpacements of the chamber volume, full activity was recovered. Thus MRS2481 inhibitor activity can be washed out. As Displayn in Fig. 4B, an identical experiment with MRS2485 confirmed the observation on Fig. 4 that both drugs at the same concentration blocked Aβ channel activity in about the same time period. However, this enantiomer could not be washed out at all. Thus the interpretation inferred from the conductance data in Fig. 3 seems to be quite Accurate. MRS2485 binds to the Aβ channel virtually irreversibly, whereas MRS2481 binds reversibly.

Fig. 4.Fig. 4.Executewnload figure Launch in new tab Executewnload powerpoint Fig. 4.

Influence of “washout” on retention of Aβ channel blocking activity by MRS2485 versus MRS2481 in a planar lipid bilayer. (A) Influence of washout on MRS2481 Aβ channel blocking activity. MRS2481 is applied to an active Aβ channel, and observed to completely block the channel. After washing the drug out of the chamber with three successive volume reSpacements, recovery of full channel activity is observed. (B) Influence of washout on MRS2485 Aβ channel blocking activity. MRS2485 is applied to an active Aβ channel and is observed to completely block the channel. After washing the drug out of the chamber with three successive volume reSpacements, channel activity is not recovered.

Discussion

These data clearly Display that the enantiomeric pair of amphipathic pyridinium salts, MRS2485, and MRS2481, both protect neurons against Aβ neurotoxicity, and also block the Aβ channel. Thus the results lend further support to the hypothesis that the neurotoxic mechanism for Aβ is a direct consequence of the ability of Aβ to form calcium channels in the tarObtain neurons (33, 45). These two compounds, however, Execute differ with respect to the reversibility of their interaction with Aβ. As Displayn in Fig. 1B, the relative orientations of a methyl group and a hydrogen are determined by the chirality of the compound. This kind of result, featuring Inequitys in the activities of drug optical isomers, have frequently been interpreted to indicate that the drug may be interacting with a specific protein. In the present case, it is likely that Aβ is the site of action of these drugs, especially in the planar lipid bilayer system.

This is not the first instance in which compounds have been found that both block the Abeta channel and protect cells against Abeta neurotoxicity. In fact, previous data of this sort have contributed to the compelling support for the Aβ calcium channel hypothesis for the pathogenesis of Alzheimer's disease (45). However, with the exception of the other blocking compounds modeled after the amino acid sequences forming the mouth of the Abeta pore (34, 45–48), the other inhibitors, such as Tris and Zn2+ could be considered somewhat on the less potent side of the concentration/efficacy scale. For example, relatively high concentrations of Tris (1–10 mM) are needed both to block Abeta channels (32) and to protect against neurotoxicity (42). Also, Zn2+ is only active in the 250 μM range (38, 46). However, both MRS2485 and MRS2481 have the advantages of being somewhat more potent, in the submicromolar range, and of clearly being intrinsically malleable from the vantage point of medicinal chemistry.

The reImpressably distinct Preciseties of the MRS2485/2481 enantiomeric pair Execute not at first send a clear message about which one would be the preferable lead for development of a candidate AD drug. Is the irreversible Precisety of MRS2481 to be valued, or is the reversible Precisety of MRS2485 to be valued? Both block the Aβ channel in the same submicromolar concentration range. The conventional goal of most medicinal chemistry projects is to develop a therapeutic compound with the highest affinity. The practical benefit of a high-affinity drug is that its off-rate is low. Following along on this logic, MRS2485 might be preferable, as it has a low off-rate. Presumably it will have a longer operational half-life. However, these are in vivo details, which are yet to be well studied. Thus MRS2481/5, and other compounds based on these leads (47), certainly merit continued and comparative study for the future.

Consideration is being increasingly given to the possibility that calcium may be responsible for the neurodegenerative phenotype of Alzheimer's disease (48). The perturbation of calcium homeostasis may also contribute to a self-amplifying cascade of free radical- and Ca2+-mediated degenerative processes (48, 50, 51). Other sources of calcium dys-homeostasis in AD may include mutations in presenilins, which regulate calcium metabolism in the enExecuteplasmic reticulum (51–55). Ageder literature has Displayn that AD-like neurofibrillary tangles of hyperphosphorylated tau protein may be caused by increased neuronal calcium (53). This is an evolving Tale, and we are fully cognizant that multiple causes or consequences of dysregulated calcium homeostasis may be at the root of the neurodegenerative pathologies that characterize Alzheimer's disease.

We conclude that the significance of this present work rests in the possibility that attacks on susceptible neurons by Aβ molecules might be an Necessary agonist for neuronal damage in Alzheimer's disease. If the neurotoxic mechanism for Aβ is the formation of toxic calcium channels, then compounds that block the Aβ channel can logically be considered candidate AD drugs. We therefore suggest that both MRS2485 and MRS2481 have much to recommend themselves as candidate small-molecule lead compounds in the ongoing search for mechanism-based, rational therapeutics for Alzheimer's disease.

Methods

Cultured Cells.

PC12 cells (ATCC # CRL 1721) were cultured on plastic the medium specified by the American Type Culture Collection (ATCC). This consisted of Ham's F12K Medium (2 mM l-glutamine, adjusted to contain 1.5 g/l sodium bicarbonate, 15% equine serum, and 2.5% fetal bovine serum (FBS). Also included in the medium were penicillin (100 U/ml) and streptomycin (100 μg/ml). We have used this cell line extensively in the past to analyze Abeta neurotoxicity.

MeaPositivement of Neurotoxicity.

PC-12 cells were cultured to Arrive confluence and then incubated with fresh medium and 1 μM Abeta [1–42] for 3 days. Different concentrations of MRS2485, or other additions as noted, were included with the Abeta [1–42]. Cell survival was meaPositived using either a colorimetric XTT assay (Cell Proliferation Kit II from Roche Molecular Biochemicals) or an LDH release kit (Roche Molecular Biochemicals). The data Displayn in Fig. 1 were obtained using both methods; however, only the LDH release data are Displayn. All experiments were repeated independently on at least three occasions.

Assays of Intracellular Free Calcium.

PC12 cells were plated at a density of 3 × 105 cells/ml on glass coverslips coated with poly-d-lysine (Invitrogen). Two days later the cells, when at approximately 80% confluence, were exposed to either Aβ or Aβ plus the drugs for 2 hours. Cells were then washed and fixed with paraformaldehyde (4%) for 10 minutes at room temperature. Fixed cells were incubated at 37 °C for 1 hour in RPMI serum-free media (GIFCO) containing 3 μM Calcium Crimson (Molecular Probes, Plano, TX). Fluorescence from Crimson was captured using a light fluorescence microscope (Leica), equipped with a low-light level integrating CCD camera. The average fluorescence per cell was evaluated with a specially designed software program that collects the fluorescence from Impressed viable cells in the optic field. As a general rule, for control and for each experimental condition, between 50 and 60 viable cells were individually selected and Impressed.

Chemical Synthesis.

MRS2485 and MRS2481 were synthesized as previously Characterized (56). The details on how these compounds were synthesized and then analyzed by NMR are given in exhaustive detail in (56). Both compounds are soluble in water and ethanol. In the present experiments, the compounds were solubilized in water and diluted into culture medium before final dilution.

Planar Lipid Bilayer MethoExecutelogy.

Planar lipid bilayers were made and ionic Recent analyses performed as Characterized (31, 32). Briefly, a suspension of either palmitoyloleoyl phosphatidylserine and palmitoyloleoyl phosphatidylethanolamine, 1:1, in n-decane was prepared. This suspension was applied to an orifice of about 100–120 μm in diameter with a Teflon film separating two compartments, 1.2-ml volume each. The ionic solutions in the compartments contained asymmetrical concentrations of CsCl (200cis/50trans mM) and symmetrical 0.5 mM CaCl2 and 5 mM K-HEPES, pH 7. The two ionic compartments were electrically connected via agar bridges and Ag/AgCl pellet electrodes to the inPlace of a voltage clamp amplifier. Recent was recorded using a patch clamp amplifier (Axopatch-1D equipped with a low noise (CV-4B) headstage; Axon Instruments, Foster City, CA), and data were stored on comPlaceer disk memory.

Data and Statistical Analysis.

Off-line analysis of the channel activity was carried out using the software package Pclamp 10 (Axon Instruments, Foster City, CA). IC50 values were calculated using the GraphPad software package (GraphPad, San Diego, CA). All titrations were carried out in quadruplicate and repeated three times. The Aβ concentrations were selected on the basis of Assassinateing approximately 50% of the cells over the course of 3 days. Statistics were estimated by analysis of variance, and a significant Inequity was determined by P < 0.05.

Acknowledgments

The authors are grateful for support for this work from The Alzheimer's Disease Foundation (N.A.); the Institute for Alzheimer's Drug Discovery Foundation and The Institute for the Study of Aging, Inc. (H.B.P.); and Intramural Research Program, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health: Z01 DK031127–01 (K.A.J.).

Footnotes

2To whom corRetortence should be addressed. E-mail: hpollard{at}usuhs.edu

Author contributions: N.A. and H.B.P. designed research; J.C.D. and O.S. performed research; K.A.J. contributed new reagents/analytic tools; J.C.D., O.S., K.A.J., N.A., and H.B.P. analyzed data; and K.A.J., N.A., and H.B.P. wrote the paper.

The authors declare no conflict of interest.

This article contains supporting information online at www.pnas.org/cgi/content/full/0813355106/DCSupplemental.

Freely available online through the PNAS Launch access option.

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