J. Biol. Chem.-2001-Waschuk-33561-8

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Cellular Membrane Composition Defines A-Lipid Interactions*

Received for publication, April 23, 2001, and in revised form, June 19, 2001Published, JBC Papers in Press, July 3, 2001, DOI 10.1074/jbc.M103598200

Stephen A. Waschuk, Elyssa A. Elton, Audrey A. Darabie, Paul E. Fraser, and JoAnne

McLaurin

From the Centre for Research in Neurodegenerative Diseases, Departments of Medical Biophysics and LaboratoryMedicine and Pathology, University of Toronto, Toronto, Ontario M5S 3H2, Canada

Alzheimers disease pathology has demonstratedamyloid plaque formation associated with plasmamembranes and the presence of intracellular amyloid-(A) accumulation in specific vesicular compartments.This suggests that lipid composition in different compa-rtments may play a role in A aggregation. To test thishypothesis, we have isolated cellular membranes fromhuman brain to evaluate A40/42-lipid interactions. Pla-sma, endosomal, lysosomal, and Golgi membranes wereisolated using sucrose gradients. Electron microscopydemonstrated that A fibrillogenesis is accelerated in

the presence of plasma and endosomal and lysosomalmembranes with plasma membranes inducing an enha-nced surface organization. Alternatively, interaction ofA with Golgi membranes fails to progress to fibrilformation, suggesting that A-Golgi head group intera-ction stabilizes A. Fluorescence spectroscopy using theenvironment-sensitive probes 1,6-diphenyl-1,3,5-hexat-riene, laurdan,N--dansyl-L-lysine, and merocyanine 540demonstrated variations in the inherent lipid propert-ies at the level of the fatty acyl chains, glycerol bac-kbone, and head groups, respectively. Addition ofA40/42 to the plasma and endosomal and lysosomalmembranes decreases the fluidity not only of the fattyacyl chains but also the head group space, consistentwith A insertion into the bilayer. In contrast, the Golgi

bilayer fluidity is increased by A40/42 binding whichappears to result from lipid head group interactions andthe production of interfacial packing defects.

Alzheimers disease is an age-related disorder that is char-acterized by progressive cognitive decline and neurodegenera-tion (1, 2). Pathological examinations have demonstrated thatone of the key features is the presence of amyloid plaquesassociated with neuritic degeneration. Senile plaques are com-posed predominantly of a 4042-residue peptide, amyloid-(A40/42). The development of Alzheimers disease pathologyhas been proposed to be the result of A1 deposition in associ-

ation with membrane structures. Recent studies have demon-strated that plaque formation may be initiated in a plasmamembrane form (3, 4) and that A deposition in aged dogs isassociated with the extracellular leaflet of the plasma mem-brane (5). Furthermore, the intracellular accumulation of A inlysosomal or late endosomal vesicles in vitro suggest that thesecompartments may be involved in neurotoxicity (69).

A is generated from the proteolytic cleavage of the amyloidprecursor protein in the endoplasmic reticulum to generate

A42 and the trans-Golgi network to generate A40 (1014). It

has also been suggested that A40/42 may also be generated atthe plasma membrane surface. The presence of A in distinctcompartments and the proposal that lipid association is impor-tant for both neurotoxicity and fibrillogenesis suggest that thelipid composition and characteristics of these compartmentsmay play vital roles in the disease process. Previous studies(15, 16) have demonstrated accumulation of A42 in lysosomalcompartments that results in membrane damage as shown byrelease of lysosomal hydrolases and the lysosomal specific dyeacridine orange. Furthermore, A-synaptic plasma membraneinteractions demonstrate that A has a fluidizing effect onmembrane structure as a result of A insertion into the fattyacyl chain region of the bilayer (17). The role of proteins asso-ciated with synaptic plasma membranes could not be distin-guished from A-lipid interactions alone in this study.

Therefore, we undertook the examination of A40 and A42in the presence of bilayers formed from lipids isolated frompost-mortem human cortical gray matter. We chose to evaluatethe membranes involved in both the production of A, Golgiand endosomal, and A pathology, plasma and lysosomal mem-branes. In order to distinguish between A lipid and A proteininteractions in these compartments, we extracted the lipidcomponent and used this as our model membranes. The effectsof A were examined as a consequence of sequence, structure,and concentration, all of which are factors affecting A assem-bly and neurotoxicity. In order to address potential mecha-nisms to help explain the pathological findings, we examinedthe ability of these membranes to facilitate A40/42 assemblyinto amyloid fibers by electron microscopy. Changes in the

membrane physical characteristics as a result of A interac-tions were followed by fluorescence spectroscopy using environ-ment-specific probes.

MATERIALS AND METHODS

PeptidesA40/42, A-(128) were synthesized by solid phase Fmoc(N-(9-fluorenyl)methoxycarbonyl) chemistry by the Hospital for SickChildrens Biotechnology Center (Toronto, Ontario, Canada). They werepurified by reverse phase high pressure liquid chromatography on aC18 Bondapak column. A peptides were initially dissolved in 0.5 mlof 100% trifluoroacetic acid (Aldrich), to ensure that the peptide re-

* This work was supported in part by the Medical Research Council ofCanada (to J. M.), the Natural Sciences and Engineering ResearchCouncil of Canada (to J. M.), the Ontario Mental Health Foundation (toP. E. F.), the Scottish Rite Charitable Foundation (to P. E. F.),and theOntario Alzheimers Association. The costs of publication of this articlewere defrayed in part by the payment of page charges. This article musttherefore be hereby marked advertisement in accordance with 18U.S.C. Section 1734 solely to indicate this fact.

Supported by the Bickell Foundation, the Canadian Foundation forInnovation, and Ontario Innovation Trust. Recipient of the Year 2000Young Investigator Fund Scholarship. To whom correspondence shouldbe addressed: Centre for Research in Neurodegenerative Diseases,Tanz Neuroscience Bldg., 6 Queens Park Crescent West, Toronto, On-tario M5S 3H2, Canada. Tel.: 416-978-1035; Fax: 416-978-1878; E-mail:[email protected].

1 The abbreviations used are: A, amyloid-; DL, N--dansyl-L-lysine; dansyl, 5-dimethylaminonaphthalene-1-sulfonyl; DPH, 1,6-di-phenyl-1,3,5-hexatriene.

THE JOURNAL OF BIOLOGICAL CHEMISTRY Vol. 276, No. 36, Issue of September 7, pp. 3356133568, 2001 2001 by The American Society for Biochemistry and Molecular Biology, Inc. Printed in U.S.A.

This paper is available on line at http://www.jbc.org 33561


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mained monomeric and free of fibril seeds, diluted in distilled H2O, andimmediately lyophilized (18). A peptides were then dissolved at 1mg/ml in 40% trifluoroethanol (Aldrich) in distilled H 2O and stored at

20 C until use. Bee venom mellitin was used as a control peptide(Sigma).

Cellular Membrane IsolationAll cellular membranes were isolated

from post-mortem human gray matter of five male control subjects withpost-mortem intervals of less than 15 h. The male subjects ranged inage from 76 to 80 years without documented signs of clinical dementia.The cause of death in all cases was heart failure. Plasma membranes

were isolated using the method of Hubbard et al. (19), endosomes usingthe method of Gorvelet al. (20), andGolgi membranesisolated using themethod of Duden et al. (21), all of which rely upon the separation of thespecific fraction by differential migration in sucrose density gradients.Lysosomal membranes were isolated using the procedure of Storrie andMadden (22) using flotation on a metrizamide density gradient. Lipidswere extracted from each membrane fraction using chloroform:metha-

nol (2:1) extraction and subsequent concentration under a stream of N2.The samples were stored at 20 C until use. Phospholipid concentra-tion in all samples was determined using the Bartlett assay (23), andcholesterol concentration was determined using the Amplex red assay(Molecular Probes, Eugene, OR).

Electron MicroscopyA40/42 peptides were incubated in the pres-ence and absence of total brain lipid extract bilayers at a final peptideconcentration of 100 g/ml. The A to lipid ratio was maintained at 1:20(by weight). For negative stain electron microscopy, carbon-coated pi-oloform grids were floated on aqueous solutions of peptides. After the

grids were blotted and air-dried, the samples were stained with 1%(w/v) phosphotungstic acid and examined on a Hitachi 7000 electronmicroscope operated at 75 kV (29, 30).

Steady State Fluorescence AnisotropyAnisotropy experiments wereperformed on a PTI fluorimeter equipped with manual polarizers asdescribed previously (24). Excitation and emission wavelengths wereset at 360 and 425 nm with a slit width of 1 and 4 nm, respectively. Oursystem was initially calibrated using 1,6-diphenyl-1,3,5-hexatriene(DPH; Molecular Probes, Eugene, OR) in mineral oil, which should givean anisotropy equal to 1. The g factor was calculated using horizontallypolarized excitation and subsequent comparison of the horizontal and

vertical emissions, which for our machine is 0.883. Lipid vesicles were

diluted to 250 g/ml in phosphate-buffered saline, incubated for 20 30min in the presence and absence of A, and then subsequently incu-bated for a further 30 min with DPH at a 1:500 probe:lipid ratio.Fluorescence intensity was measured with the excitation polarizer inthe vertical position and the analyzing emission polarizer in the vertical

(IVV) and horizontal (IVH) positions; and anisotropy, r, was calculatedusing Equation 1,

rIVVgIVH

IVV 2gIVH(Eq. 1)

Lipid vesicles in theabsence of DPHwere measured in order to evaluate

the effect of light scattering on our measurements. Poly- L-lysine andbovine serum albumin were used as negative controls for the anisotropystudies.

Laurdan Generalized PolarizationSteady state excitation and emis-sion spectra were collected on the PTI fluorimeter. Laurdan (MolecularProbes) was added to preformed lipid vesicles in the presence and absenceof A at a 500:1 lipid:probe ratio. The laurdan generalized polarization(GP) parameter as developed by Parasassi et al. (25) is calculated asfollows. The emission GP parameter is given by Equation 2.

GPemI400nmI340nm

I400nmI340nm(Eq. 2)

where I400 nm and I340 nm are the fluorescence intensities measured atall emission wavelengths within 420 and 520 nm. By using fixed exci-tation wavelength of 400 nm and 340 nm, respectively. The excitationGP is given by Equation 3,

GPexI440nmI490nm

I440nmI490nm(Eq. 3)

where I440 nm and I490 nm are the fluorescence intensities at each exci-

tation wavelength from 320 to 420 nm, measured at fixed emissionwavelengths of 440 and 490 nm, respectively.

N--Dansyl-L-lysine Fluorescence SpectroscopyN--Dansyl-L-lysine

(DL, Molecular Probes) was incorporated into lipid vesicles in the pres-ence and absence of A. The fluorescence spectra of DL were evaluatedafter 30 min of incubation at room temperature with an excitationwavelength of 335 nm and emission scan monitored between 380 and

580 nm inclusive. The DL to lipid ratio was maintained at 1:500 (26). Merocyanine 540 Absorption SpectroscopyMerocyanine 540

(MC540, Molecular Probes) absorption spectra were obtained at roomtemperature on a Beckman spectra DU530. The dye was added topreformed vesicles at a probe:lipid ratio of 1:500 (27). Final MC540molar concentration in the cuvette was 21.3 106 M. Absorptionspectra were obtained between 400 and 600 nm with 1-nm steps. Thelipid-alone base line in the absence of MC540 was subtracted from allspectra, and the corresponding spectra are shown in Equation 4,

monomerA DC/2

m

D/2dimer

C monomer

2(Eq. 4)

and were then corrected by referring the absorbances at 600 nm to 0. After this correction, the absorbance values at 569 nm were used tocalculate the dimerization constant (K

d(app)) as by Bernik and Disalvo(28), see Equation 5.

Kappdimer]

[monomer]2(Eq. 5)

where A is the absorbance at 569 nm, is the constant for MC540 dimer

or monomer at the given wavelength, m 1.511 105 and D 5400,and C is the final MC540 concentration.

RESULTS

A Morphological CharacteristicsLipid bilayers have been

shown to affect the assembly of A peptides into amyloid fibers

(2931). In order to determine if A interactions with differentcellular membranes affects fibrillogenesis, we examined A

structural characteristics in the presence of vesicles formed

from Golgi, plasma, lysosomal and endosomal lipids by nega-

tive stain electron microscopy. In the absence of lipid, A as-

sembles into long fibers of varying length, 350 430 , with acharacteristic helical twist of 100 (Fig. 1A). These fibersdemonstrated varying extent of lateral aggregation of fibers

into larger bundles, from 50 representing single fibers to200- diameter bundles. In the presence of plasma lipid vesi-cles, A assembled into fiber bundles along the surface of the

bilayer (Fig. 1B). A fibers were not found on the surface of the

FIG. 1. Negative stain electron microscopy of A42 in the pres-ence of plasma, endosomal, and Golgi membranes. A42 incu-bated in buffer alone (A) demonstrates many long intertwined fibers.When incubated in the presence of plasma membrane (B), a similarstructure of the fibrils to A42 alone could be detected but with in-creased organization along the vesicle surface. Only minor lateral ag-gregation was apparent in the fibrils formed in the presence of lysoso-

mal membranes (C). In the presence of Golgi membranes only a fewprotofibrils of A42 were detected (D). Scale bar is 50 nm.

Cellular Membrane-associated A Fibrillogenesis33562


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bilayers nor in areas devoid of lipid vesicles, suggesting that A

assembly was driven as a result of interaction with the lipid

surface. These results are similar to those that we have re-

ported previously for A interaction with phosphatidylinositol/

brain phosphatidylcholine and total brain lipid vesicles (32,

33). Both endosomal (data not shown) and lysosomal (Fig. 1C)

vesicles demonstrated A fibers associated with both the edges

and surface of the vesicles. Although no fibers were detected in

the absence of lipids, the level of A lateral aggregation andorganization was less than that detected for the plasma mem-

brane lipids. A incubated in the presence of Golgi lipid vesi-

cles was almost devoid of A fibers (Fig. 6D). The odd fiber

could be found across the grid but was not intimately associ-

ated with lipid vesicles, and many areas of lipid vesicles could

be found devoid of fibers. The odd fiber detected in the presence

of Golgi vesicles had morphological characteristics of A pro-

tofibrils. The Golgi lipid vesicles are reminiscent of our previ-

ous results in which we were unable to identify A fibrils in the

presence of ganglioside/phosphatidylcholine membranes when

A was added as a randomly structured peptide (29).

Fatty Acyl Chain MobilityIn order to characterize differ-

ences in the cellular bilayer properties and determine which is

most influential in determining A fibrillogenesis, we exam-

ined the influence of A40/42 on the physical properties of

these bilayers. Many fluorescent dyes are available that pene-

trate to varying levels into the lipid bilayer and exhibit fluo-

rescent properties indicative of their local environment. We can

utilize these dyes to address the effects of A-lipid interactions

within various cellular membranes. Previous studies using

synthetic lipid bilayers and synaptic plasma membranes have

demonstrated a disordering of the fatty acyl chains after inter-

action with A40 and A42 (17, 34 36). In order to determinethe effect of A40/42 on the mobility of the fatty acyl chains

within bilayers formed from Golgi, endosomal, lysosomal and

plasma membrane lipids, we have examined the steady state

fluorescence anisotropy using the dye, DPH (24). The relative

motion of the DPH dye molecule within the lipid bilayer is

determined by polarized fluorescence and expressed as r, theanisotropy constant. This constant is inversely proportional to

the degree of membrane fluidity.

The relative fluidity of the lipid membranes was found to

vary considerable with Golgi lipid bilayers having the most

rigid structure, whereas plasma membrane, endosomal, and

lysosomal lipid bilayers were more fluid (Table I). Previous

studies have shown that synthetic lipid bilayer fluidity is reg-

ulated by the amount of cholesterol, which exhibits a bimodal

effect on fluidity with increasing levels of cholesterol (34, 3739). The cholesterol to phospholipid ratios of the bilayers were

in the range of that reported previously in the literature (Table

II). The differences in fluidity detected in this study are not

solely related to the cholesterol content of these bilayers as the

plasma and lysosomal membranes have a significant choles-

terol level yet still exhibited a fluid membrane structure.

Therefore, the properties of the different bilayers represent

differences in the total lipid composition rather than choles-

terol per se.

Previous reports (17, 34 36, 39) have shown that the addi-tion of A40 or A2535 to synthetic membrane preparationsor membranes isolated from red blood cells results in a reduc-

tion in membrane fluidity. Many A properties have been

linked to the conformation and aggregation state of the peptide.

In order to investigate the interactions of A40 and A42 with

lipid bilayers, we chose to examine initially soluble, random

structured peptide with bilayers. To ensure that A peptides

meet these criteria and are free of fibril nucleation seeds,

A40/42 peptides were treated with 100% trifluoroacetic acid

followed by lyophilization (18). The lyophilized peptide was

immediately solubilized in 40% trifluoroethanol in order to

make a 1 mg/ml stock solution. As reported previously (40),

A40/42 is partly -helical in 40% trifluoroethanol and upon

dilution into phosphate-buffered saline, pH 7.4, A40/42 ini-

tially adopted a random structure. Our results demonstrate

that addition of randomly structured A40 and A42 decreased

the membrane fluidity of the plasma membrane, endosomal

and lysosomal membranes in a concentration-dependent man-

ner as illustrated by an increase in the anisotropy constant

(Table I and Fig. 2). We could not detect a significant differencein the effect of A40 and A42 on these membranes. In con-

trast, both A40 and A42 induced a significant increase in

fluidity of the Golgi lipid bilayers as demonstrated by the

decrease in the anisotropy constant (Table I). The disordering

effect of A on Golgi lipid membranes is enhanced by increas-

ing A concentration (Fig. 2).

To determine the specificity of A40/42-lipid interactions, we

examined A-(128), which lacks the N-terminal hydrophobicregion, and bee venom mellitin, a pore-forming peptide. Nei-

ther A-(128) nor mellitin altered the fluidity of the Golgimembranes, whereas both decreased the fluidity of plasma

membranes (Table I). These results suggest that the preferen-

tial increase in Golgi membrane fluidity as a result of A40/42

interaction is peptide- and sequence-specific. It is not surpris-

TABLE ISteady state anisotropy measurements of various cellular membranes in the presence and absence of A40/42

Anisotropy was measured using DPH fluorescence at a probe:lipid ratio of 1:500. Peptide was added to lipid vesicles at a 1:20 ratio with a finalpeptide concentration of 10 M. ND indicates not determined.

SampleAnisotropy

Golgi Plasma membrane Endosomal Lysosomal

Control 0.295 0.004 0.221 0.000 0.186 0.002 0.232 0.0002A40 0.284 0.002a 0.259 0.001a 0.196 0.001b 0.268 0.0002a

A42 0.284 0.004a 0.263 0.008b 0.196 0.0002 0.267 0.019b

Seeded A40 0.314 0.011b

0.273 0.009b

ND NDSeeded A42 0.303 0.008b 0.276 0.040b ND ND

A-(128) 0.297 0.002 0.231 0.004 ND NDMellitin 0.294 0.003 0.228 0.008 ND ND

a p 0.001.b p 0.01 by Students t test.

TABLE IICholesterol and phospholipid analysis

Cholesterol content in all membranes was determined using theAmplex red assay, and phospholipid content was determined using theBartlett assay.

Molar ratio cholesterol:phospholipid

Experimentalvalues

Literature citationvalues

Golgi membrane 0.27 0.450.5Plasma membrane 0.49 0.41.0Endosomal 0.08 0.10.2Lysosomal 0.47 0.5

Cellular Membrane-associated A Fibrillogenesis 33563


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ing that mellitin decreases the fluidity of plasma membranes

as it inserts into membranes to create pores.

The structural dependence of A effects on membrane fluid-

ity of plasma membrane and Golgi lipid bilayers was examined

by comparing the random structured peptide with A40 and

A42 which exhibit -structure. In contrast to the random

structured peptides, seeded A40 decreased the membrane

fluidity of both plasma and Golgi membranes (Table I). Similar

results were detected for A42. This result suggested to us that

A interactions with lipid bilayers is not only dependent on the

composition of the lipid bilayer but also on the structural char-

acteristics of the peptide.

Dynamics of Lipid Head Groups and InterfaceBesides the

packing of the lipid acyl chains, the dynamics of the polar head

groups and the polarity of the lipid interface are relevant to the

interaction of molecules, i.e. A, with the membrane surface. In

order to obtain an insight into these properties, laurdan and

N--dansyl-L-lysine probes were used. Laurdan naphthalenering is located at the glycerol backbone and is anchored in the

bilayer by the lauroyl moiety, thereby imparting fluorescence

characteristics that are dependent on the polarity of its envi-

ronment (25, 41). The advantages of laurdan are that it is

completely non-fluorescent in aqueous environments, is inde-

pendent of pH between 4 and 10, and independent of lipid

polar head group; therefore fluorescence readings reflect only

the polarity of the probe associated with the bilayer. The

spectral properties of laurdan have been described by the

general polarization equation for both excitation and emis-

sion spectra, which render information about the lipid phase,

polarity, and co-existence of multiple lipid phases within a

single bilayer (25, 41).

Laurdan excitation spectra in the presence of plasma and

Golgi lipid bilayers demonstrate the characteristic red excita-

tion at 340 nm and blue excitation at 380 nm, whereas the

emission spectra indicates a single maximum at 430 nm indic-

ative of blue emission (Fig. 3). The red excitation band intensity

increases in polar solvents, and in hydrogen-bonding solvents,

the red excitation corresponds to the blue emission population

and is especially intense in gel phase lipid bilayers where little

relaxation occurs. The addition of A to laurdan containing

membranes does not change the shape of either the excitation

or emission spectra but affects the intensity of laurdan fluores-

cence in both plasma and Golgi membranes (Fig. 3). The ratio

of the blue to red components in the excitation reflects the

polarity of the probe. Addition of A40 to plasma membrane

bilayers results in an increase in the blue/red excitation ratio

from 1.03 to 2.2, indicating that the environment sensed by

laurdan becomes more hydrophobic after interaction of A with

membranes (Fig. 2). These results suggest that A produces a

displacement of water molecules from the hydration shell of themembrane, as a result of the promotion of lateral phase sepa-

ration and a higher degree of plasma membrane organization.

Increasing the concentration of A does not further alter the

blue/red excitation ratio suggesting that the bilayer has a finite

ability to accommodate A. The generalized polarization emis-

sion (GPem) for plasma membranes was calculated to be 0.48,

which increases to 0.57 in the presence of initially random

structured A40 and A42. The interaction of seeded A40/42

demonstrates the same shift of the GPem to 0.56 and 0.54,

respectively, indicating that in the presence of A the mem-

brane becomes more structured at the head group-fatty acyl

chain interface.

In contrast, both Golgi (Fig. 3B) and endosomal (data not

shown) bilayer blue/red excitation intensities do not change

FIG. 2. The effect of A modulation on membrane fluidity ofcellular lipid bilayers was determined by DPH anisotropy. Theaddition of A40 and A42 to plasma (A) and Golgi membrane (B)

vesicles resulted in contrasting effects on membrane fluidity. Additionof monomeric, randomly structured A decreased the membrane fluid-ity of plasma membranes in a concentration-dependent manner,whereas Golgi membrane fluidity was increased. Data represent themean of at least three separate experiments and are the mean S.D.Students t test indicates the following: *, p 0.01; , p 0.001 whencompared with lipid alone.

FIG. 3. Laurdan emission and excitation spectra of plasma (A)and Golgi (B) membranes in the presence of increasing concen-trations of A40. Vesicles alone (solid line) and in the presence of 5(dotted line) and 10 g (dashed line) o f A40 demonstrate similaroverall spectral characteristics. The intensity of the spectral maximaare affected by the addition of A; these results demonstrate the in-crease (Golgi membranes) and decrease (plasma membranes) in polar-ity of the head group-fatty acyl chain interface.



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after addition of A suggesting that A binding does not alter

the phase of these lipids. Increasing concentrations of A still

did not induce a change in the ratio of excitation intensities

confirming that these bilayers do not undergo a concentration-

dependent phase transition. The endosomal lipid bilayer exhib-

its similar fluidity as the plasma membrane as illustrated by

DPH studies, yet has different laurdan fluorescence character-

istics suggesting that bilayer-specific lipid composition may

alter the resultant A-lipid interactions. In contrast to the

excitation properties of laurdan, the GPem values of Golgi and

endosomal membranes, 0.49 and 0.54, are only affected by the

addition of high concentrations of A demonstrating a decrease

to 0.42 and 0.44 for A40. It is interesting that A42 has a

lesser effect on the GPem than A40 with only modest alter-

ation of the GPem values to 0.47 and 0.51, respectively. These

results suggest that interaction of A42 with both Golgi and

endosomal membranes does not alter the micropolarity or hy-

dration of the interfacial region of the lipid bilayer.

To examine the specificity of the changes in laurdan fluores-

cence properties due to A40/42 interactions with Golgi and

plasma membranes, we examined the effects of A-(128) andmellitin. Addition of A-(128) and mellitin to both bilayersdoes not alter the blue/red excitation ratio. Furthermore, A-

(128) does not shift the GPem of either plasma or Golgi mem-branes. These combined results suggest that A-(128) doesnot alter the micropolarity of the interface and/or insert into

the bilayer. In contrast, mellitin shifts the GPem for Golgi and

plasma membranes from 0.49 and 0.48 to 0.53 and 0.52, re-

spectively. These results suggest that in contrast to A pep-

tides, mellitin binding and pore formation increases the lipid

order after insertion into the bilayer but does not change the

lipid phase.

In order to examine more closely the phase behavior of the

bilayers, the wavelength dependence of both the excitation and

emission spectra was examined (Fig. 4). Lipid bilayers in a pure

gel phase show an independence of generalized polarization

values as a function of wavelength, whereas liquid crystalline

bilayers exhibit a dependence on the excitation wavelength(42). Plasma membrane bilayers exhibit a decrease in the GPexand increase in GPem toward shorter wavelengths, an indica-

tion of the co-existence of lipid phases (Fig. 4A). The addition of

A results in an increase in the slope of the GPem suggesting

that the lipid phase in the bilayer is further altered as a result

of A binding. Alternatively, the GPex and GPem of Golgi lipid

bilayers is independent of wavelength in the presence and

absence of A, confirming that A binding does not alter the

lipid phase (Fig. 4B). These results suggest that binding of A

to Golgi and endosomal membranes can be easily accommo-

dated within the lipid structure, whereas plasma membrane

bilayers undergo a reorganization.

The polarity of the lipid interface can be examined using the

fluorescence ofN--dansyl-L-lysine (26, 43); furthermore, it hasbeen suggested that DL inserts into cholesterol-free phospho-

lipid domains (44, 45). Due to its molecular structure and

location at the interface, DL fluorescence is most sensitive to

the packing constraints and hydration. DL exhibits a strong

fluorescence maximum at 430 nm, which increases in intensity

as a result of A-plasma membrane interactions (data not

shown). These results suggest that A increases the polarity of

the interface which is independent of concentration and struc-

ture. In contrast, both endosomal and Golgi lipid bilayers ex-

hibit DL maxima at 430 and 540 and 520 nm, respectively. The

addition of A40 and A42 to endosomal bilayers results in a

blue shift in the DL maxima; an indication of increased lipid

packing and was independent of peptide structure (data not

shown). DL associated with Golgi membranes demonstrated a

red shift in the fluorescence maxima after addition of A. This

result suggests that A creates more space between the lipid

head groups or causes an increase in the packing defects of

Golgi lipid bilayers.

Lipid Head Group Packing and Surface PropertiesIn order

to examine the lipid head group spacing and surface properties

of these bilayers, merocyanine 540 absorbance spectral proper-

ties were examined. The spectral characteristics of MC540

result from binding of monomeric MC540 and subsequent

dimerization, and both steps are dependent on the packing

properties of the lipid head groups (27, 28). MC540 spectra in

the presence of plasma membrane is characteristic of mostlygel phase lipid head groups, with the characteristic maxima at

500 and 530 nm (Fig. 5A). A small shoulder is present at 570

nm which is characteristic of a small population of monomeric

MC540 insertion into the lipid bilayer. These results suggest

an ordered head group packing in these bilayers as only a small

amount of MC540 is inserted into the head group space. Addi-

tion of A40 to the plasma lipid bilayers decreased the inten-

sity of the MC540 maxima and percent of monomeric MC540

insertion (Fig. 5B). No difference could be detected between

random and -structured A40 suggesting similar effects on

lipid head group rearrangement. A42 did not change the

absorbance spectra of MC540, suggesting that A42 interac-

tion does not affect the head group packing of the plasma

membrane. On the other hand, similar MC540 spectra results

FIG. 4. The wavelength dependence of the excitation and emis-sion generalized polarization of laurdan in plasma (A), endoso-mal (B), and Golgi (C) membranes was determined. GP valueswere calculated from excitation and emission scans before (solid line)and after addition of A42 and 5 (dotted line) and 10 g (dashed line).

Plasma membrane demonstrates a shift in the phase of thelipid bilayer,whereas endosomal and Golgi membrane phases are unaffected byaddition of A42. The laurdan:membrane lipid ratios were 1:500.



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were obtained for endosomal lipid bilayers in the presence ofA (data not shown). The MC540 spectra in the presence of

plasma and endosomal membranes are consistent with varying

levels of A insertion into the bilayers.

These results are contrasted by the MC540 spectra in the

presence of Golgi lipid bilayers, which demonstrate maxima at

530 and 570 nm (Fig. 5A). These spectra are indicative of a

more fluid, liquid-crystalline head group packing and an in-

creased surface potential that allow for increased MC540 mo-

nomeric insertion into the head group space. Addition of A40

and A42 results in an increase in the intensity of both max-

ima, indicating a more fluid environment and increased head

group space or packing defects (Fig. 5C). In contrast, seeded

A40 and A42 decrease the intensity of the 570 nm maxima

suggesting that -structured peptide increases the packing of

the head groups of Golgi membranes. The MC540 absorption

spectra are consistent with A40/42-Golgi interactions occur-

ring predominantly at the head group space.

To investigate the sequence specificity of A40/42 interaction

with Golgi and plasma membrane bilayers, we examined the

interaction of A-(128) under similar conditions. In contrastto A40/42, A-(128) did not affect the shape or intensity ofthe MC540 absorption spectra of Golgi membranes. These re-

sults suggest that A-(128) does not affect head group packingand confirms the DPH and laurdan fluorescent results, which

suggest that A-(128) does not insert into the lipid bilayer(data not shown). Similar to A40/42, mellitin increases the

intensity but not the shape of the MC540 absorption spectra.

Furthermore, MC540 spectra of plasma membranes in the

presence of mellitin are indistinguishable from that of Golgimembranes (Fig. 5D). These results are consistent with mel-

litin insertion into the bilayer and creating increased head

group space or packing defects.

The MC540 monomer-dimer equilibrium is relevant to the

packing properties of the bilayers and can be used as an indi-

cation of lipid head group spacing (27, 28). We have calculated

the apparent dimerization constant for plasma, Golgi, and en-

dosomal lipid bilayers in the presence and absence of A40/42

in both random and -structure (Table III). The most apparent

observation is that the dimerization constant for the various

bilayers differs on the order of 2 magnitudes from each other in

the order Golgi plasma endosomal bilayers. These results

suggest that the head group packing of the Golgi membranes is

less constrained and can accommodate the MC540 dimers.

Furthermore, addition of A40/42 did not significantly alter the

Kd(app) suggesting that the membranes can easily accommodate

A. Our anisotropy studies suggest that both the plasma andendosomal lipid bilayers are both fluid bilayers, whereas the

dimerization constant suggests that the endosomal head group

packing is more rigid than the plasma membrane bilayers.

Addition of A40/42 as a randomly structured peptide did not

alter theKd(app), suggesting that A binding does not alter head

group packing. The dimerization of MC540 in endosomal lipid

bilayers was decreased by A binding as indicated by a 3-fold

increase in the dimerization constant (Table III) after addition

of both random and -structured peptides. These results sug-

gest that A-endosomal interactions further organize the head

group packing and are consistent with our anisotropy studies,

which demonstrate a decrease in the fatty acyl chain fluidity as

a result of A binding to endosomal bilayers.

DISCUSSION

A-lipid interactions have implications not only for A pro-

duction but also for the induction of neurotoxicity and age-

associated pathology. The presence of A aggregates, initiation

of plaque formation, and the dependence of toxicity on the

association with specific lipid compartments suggested that

vesicular lipid composition might be a factor in these processes.

Our fluorescence studies on plasma membrane bilayers suggest

that A inserts into the fatty acyl region of the bilayer. This

result is supported by the anisotropy studies that demonstrate

a dramatic increase in membrane organization as a result of A

interaction, the lipid phase shift associated with A as demon-

strated by the laurdan GP and the lack of head group reorga-

nization as detected by MC540 absorption spectra. Our results

are consistent with previous reports (34, 36) that demonstrated

FIG. 5. The interaction of A40 andA42 with the lipid head groups ofthe various cellular membranes wasexamined using MC540 absorbancespectroscopy. MC540 spectra demon-strate the rigid packing of the plasmamembrane head groups ( A, dashed line),whereas the Golgi membrane headgroups are more fluid ( A, solid line). Ad-dition of A40 (dashed line) an d A42(dotted line) to Golgi membranes (C) re-sulted in an increase in the intensity ofthe MC540 spectra indicative of A-headgroup interactions. In contrast, A42 hadlittle effect on the plasma membrane (B)as indicated by a lack of shift in the spec-tra, whereas A40 decreased the inten-sity of the spectra indicating increasedpacking of the head groups. In contrast,mellitin ( D, dashed line) in the presenceof plasma membrane ( D, solid line)shifted the shape of the MC540 spectra toone that is indistinguishable from Golgimembranes, suggesting creation of lipidhead group packing defects or alteredspacing.

TABLE III

Effect of A on the apparent dimerization constant (Kd(app)) ofmerocyanine 540 in cellular membranes

Peptide was added to lipid vesicles at a 1:20 ratio with a final peptideconcentration of 10 M.

SampleApparent dimerization constant (Kd(app)

Golgi Plasma Endosome

Control 1.1 107 9.1 108 3.7 1010

A40 1.0 107 2.1 109 0A42 1.7 107 9.1 108 1.1 1011

Seeded A40 2.0 107 1.3 109 1.1 1011

Seeded A42 5.8 106 1.0 109 4.2 109



7/8

that addition of A to membranes isolated from cerebellum,

cortex, hippocampus, and striatum or synthetic lipid vesicles

results in a decrease in membrane fluidity. In contrast, Mason

et al. (17) reported that A, both random and aggregated,

increased synaptic plasma membrane fluidity by insertion of

random A into the fatty acyl chain and the presence of aggre-

gated A at the lipid head groups. Our results are in partial

agreement with these results, as we also propose that A exerts

its bilayer effects by inserting into the fatty acyl chains, butdiffer in that our results demonstrate a rigidizing effect. The

discrepancies between these two studies may be accounted for

by the presence of endogenous synaptic membrane proteins

that may compete with lipids for A binding. Our results fur-

ther demonstrate the enhancement and organization of A

fibrillogenesis in the presence of plasma membrane vesicles.

These structural results are consistent with our previous stud-

ies that demonstrated the insertion and fibrillogenesis of A40

on planar bilayers composed of total brain lipid extracts (33)

and the presence of fibers in synthetic phospholipid bilayers as

detected by electron microscopy (32). The ability of A to insert

into the plasma membrane has many implications for both cell

survival and cell surface-driven fibrillogenesis.

Endosomal and lysosomal lipid bilayers have similar prop-erties to plasma membranes except that upon interaction with

A the lipid head groups undergo re-organization. Endosomal

head group organization was initially demonstrated to be rigid

which may limit the level of A insertion into the lipid bilayer.

The endosomal compartment is the site of cholesterol uptake

and recycling within the cell, and the limited A insertion into

the bilayer may result from increased cholesterol to phospho-

lipid ratio. Previous studies have demonstrated that choles-

terol modulates A-lipid interactions by preferential binding,

decreasing the fluidity of the bilayer, and ultimately decreasing

fiber and aggregate formation (34, 35, 39). Furthermore, the

lysosomal and endosomal compartments have been suggested

to be sites of intracellular A accumulation and nucleation. The

increased packing of the endosomal head groups would suggestthat accumulation of A would be near the surface of the

bilayer, a site that would be easily accessible for propagation of

A nucleation and aggregation.

We have demonstrated that the interaction of A with Golgi

membranes is predominantly at the level of the head groups

but also translates into a decreased micropolarity at the head

group-fatty acyl chain interface and decreased order of the

fatty acyl chains. Our studies have demonstrated that Golgi

membranes can easily accommodate A and ultimately inhibit

fiber formation. These results may be due to the high glycolipid

concentration in these bilayers since our previous studies (29)

on isolated brain ganglioside-A interactions demonstrate sim-

ilar inhibition of fibrillogenesis. The mechanism of action was

proposed to be surface binding in an -helical conformationthat prevented conversion to -structure and subsequent fibril-

logenesis. Similarly, the laurdan fluorescence characteristics of

Golgi membranes alone are characteristic of those reported

previously (46, 47) for glycosphingolipid containing aggregates

and vesicles. A40 is generated in the Golgi apparatus, and it

would not be beneficial to cell survival for the Golgi to possess

properties that promote A self-aggregation. Furthermore, it

has been suggested that plasma membrane-generated A oc-

curs in membrane rafts that are rich in glycolipids. These data

may represent a protective mechanism against A toxicity.

Our results demonstrate that the cellular vesicular com-

partments exhibit lipid characteristics that either promote or

inhibit fibril formation by direct interaction with lipid bi-

layer. Although we have examined A-lipid interactions of

various compartments, we have not taken into account the

effect of endogenous cellular proteins. These integral mem-

brane proteins will also have an effect on A-membrane

interactions whether as competitors for A binding, such as

proteoglycans, or as modulators of bilayer properties. Our

results demonstrate differences detected in the A-lipid in-

teractions between the various vesicular compartments,

which may play a role in not only normal cellular processing

and turnover of A but in the progression of disease processes

in Alzheimers disease.

AcknowledgmentsWe thank Dr. N. Wang at the Hospital for SickChildrens Biotechnology Center for the synthesis of all peptides used inthis study, Dr. A. Chakrabartty for use of the PTI fluorescence spec-trometer, the Electron Microscopy Suite at the University of Toronto foruse of Hitachi 7000 electron microscope (CIHR Maintenance Grant),and the Canadian Brain Tissue Bank.

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