Hydrophobic Modification of Carboxyl-Terminated PAMAM Dendrimer

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Biological and Environmental Phenomena at the Interface

Hydrophobic Modification of Carboxyl-Terminated PAMAM Dendrimer Surface Creates a Potent Inhibitor of Amyloid-# Fibrillation Ziyuan Wang, Xiaoyan Dong, and Yan Sun Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b02890 • Publication Date (Web): 02 Nov 2018 Downloaded from http://pubs.acs.org on November 4, 2018

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Langmuir

1

Hydrophobic

Modification

of

Carboxyl-Terminated

PAMAM

2

Dendrimer Surface Creates a Potent Inhibitor of Amyloid-β

3

Fibrillation

4 5

Ziyuan Wang, Xiaoyan Dong, Yan Sun*

6 7

Department of Biochemical Engineering and Key Laboratory of Systems

8

Bioengineering of the Ministry of Education, School of Chemical Engineering and

9

Technology, Tianjin University, Tianjin 300354, China

1

ACS Paragon Plus Environment

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11

ABSTRACT:

12

Amyloid β-peptides (Aβ) fibrillogenesis is a major hallmark of Alzheimer’s disease

13

(AD), inhibition of Aβ fibrillation is thus considered as a promising strategy for AD

14

prevention and treatment. Our group has previously proposed the hydrophobic

15

binding-electrostatic repulsion (HyBER) hypothesis, which provides guidance for

16

design of new amyloid inhibitors. Inspired by the HyBER hypothesis, we have herein

17

proposed to synthesize hydrophobic modified generation 5 carboxyl-terminated

18

polyamidoamine (PAMC) dendrimer, denoted as PAMPs, to create a potent inhibitor

19

with a negatively charged hydrophobic surface. Results indicate that the PAMP with a

20

proper degree of phenyl substitution (30% – 42%) alters the conformation of Aβ42

21

through both hydrophobic binding and electrostatic repulsive forces on its surface.

22

With these well-balanced interactions the inhibitor can even completely inhibit the

23

formation of β-sheet structure of the peptide, accompanied by changes at the level of

24

the fibrillary architecture. Moreover, the results also indicate that changes of Aβ42

25

aggregation pathway influenced by the PAMP occur at the very early stage, so the

26

PAMP can significantly avoid the formation of toxic intermediates of Aβ42

27

aggregation.

28 29

KEYWORDS: amyloid beta-peptide; aggregation; inhibitor; dendrimer surface

30

modification; hydrophobic interaction; electrostatic repulsion

2


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INTRODUCTION

33

The aggregation and deposition of amyloid proteins in human body, known as

34

amyloidosis, are pathologically associated with more than 50 human diseases,1

35

including Alzheimer's disease, diabetes mellitus type 2, Parkinson's disease and

36

Huntington's disease.2 Alzheimer’s disease (AD), the most prevalent form of

37

dementia, is pathologically characterized by intracellular neurofibrillary tangles and

38

extracellular senile plaques.3 Although some treatments may temporarily control AD

39

symptoms, no one can stop or reverse its progression.4 The complex molecular

40

mechanism of AD pathogenesis is still not clear, but current knowledge suggests that

41

the aggregation of amyloid-β (Aβ) is closely associated with the progression of AD.5

42

Aβ is composed of 39 to 43-residue peptides produced by the cleavage of amyloid

43

precursor protein (APP).6 Aβ40 is recognized as the most abundant form and Aβ42 is

44

the most toxic one.7 Soluble Aβ monomers can spontaneously aggregate into

45

oligomers and then finally assemble into amyloid fibrils, which result in nerve cell

46

damage and apoptosis.8-10 Moreover, it is generally accepted that soluble Aβ

47

oligomers or protofibrils are the most toxic species and responsible for neuron

48

dysfunction and death.11 Therefore, modulation of Aβ aggregation at the very early

49

stage could be a promising treatment for preventing or delaying the onset of AD.12, 13

50

To date, significant efforts have been made to develop different kinds of amyloid

51

inhibitors,

52

nanoparticles.14-18 Unfortunately, none of them succeeded in clinical trials and AD is

53

still regarded as one of the incurable diseases.19 Thus, development of more effective

54

inhibitors is vital for the cure and prevention of AD.

including

small

organic

compounds,

peptides,

antibodies,

and

55

Our group recently showed that the inhibitory effects of bovine/human serum

56

albumin (BSA/HSA) on Aβ42 aggregation were significantly improved after the

57

amino groups of the protein were converted into carboxyl groups by modification

58

with

59

binding-electrostatic

60

hydrophobically bound onto the protein surface through its hydrophobic patches at the

61

central hydrophobic core or the C terminal, and simultaneously, the bound peptide,

diglycolic

anhydride.20, repulsion

21

Based

(HyBER)

on

the

hypothesis

3


findings, was

a

hydrophobic

proposed:

Aβ

is

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62

which is negatively charged, is electrostatically repulsed by the carboxyl groups of the

63

acidulated serum albumin. The two opposite forces would make Aβ stretch into

64

extended conformations distinctly different from the β-sheet structures, leading to the

65

off-pathway aggregation and/or the decrease of on-pathway aggregation, thus

66

significantly reducing the toxicity of Aβ aggregates. However, the studies of

67

acidulated BSA and HSA on Aβ aggregation are yet insufficient to support the

68

HyBER hypothesis, because of the heterogeneous distribution of negative, positive

69

charged residues and hydrophobic patches on the surface of BSA/HSA makes it

70

difficult to verify their contribution in the inhibition of Aβ aggregation. Therefore,

71

towards a better understanding and further application of the HyBER hypothesis, we

72

propose to use generation 5 (G5) carboxyl-terminated polyamidoamine (PAMC), a

73

biocompatible dendrimer with only carboxyl groups on its surface,22-24 as a base

74

material to create a neat surface with only negative charges and hydrophobic patches.

75

As illustrated in Scheme S1, G5 PAMC owns a highly branched 3D globular and

76

nanostructure with 128 carboxyl groups on the surface and empty hydrophobic

77

cavities inside.25,

78

introduce phenyl groups, the synthesized phenyl-derivatized PAMC (PAMP) surface

79

is distributed with only carboxyl groups and phenyl groups (Scheme 1), and the

80

negatively charged hydrophobic surface is expected to function as an amyloid

81

inhibitor via the HyBER hypothesis, if the hypothesis is true. We have extensively

82

characterized PAMPs of different degrees of substitution (DS) of hydrophobic groups

83

on inhibiting Aβ42 fibrillation and the amyloid cytotoxicity, and the results

84

demonstrated the design.

26

Thus, by surface modification with phenethylamine (PEA) to

85 86

EXPERIMENTAL SECTION

87

Materials. Aβ42 (>95%) was purchased from GL Biochem (Shanghai, China) as

88

lyophilized powder. Generation 5 carboxyl-terminated PAMAM (PAMC) was from

89

Weihai ChenYuan Molecule of New Materials Co. (Weihai, China). Phenethylamine

90

(PEA), Hexafluoro-2-propanol (HFIP), thioflavin T (ThT) and 3-(4,5-dimethy-

91

lthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT), dimethyl sulfoxide (DMSO) 4


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and ethanolamine were purchased from Sigma (St. Louis, MO, USA). Dulbecco’s

93

modified Eagle’s medium (DMEM) and fetal bovine serum (FBS) were from GIBCO

94

(Grand Island, NY, USA). Human neuroblastoma SH-SY5Y cells were obtained from

95

the Cell Bank of the Chinese Academy of Sciences (Shanghai, China). Other

96

chemicals were all of the highest purity available from local sources.

97

Synthesis and Characterization of PAMPs. PEA was grafted to PAMC through

98

an amide bond formed between the amino group of PEA and the carboxyl groups of

99

PAMC. Briefly, 122 mg of PAMC was dissolved in 30.0 mL deionized water. PAMC

100

was activated in the presence of EDC at 25 °C for 15 min with stirring at 170 rpm.

101

Subsequently, PEA was added to the solution and the mixture was mixed by stirring

102

at 170 rpm overnight. To remove excess reactants and precipitates, the resulting

103

PAMP were dialyzed for 7 days by a dialysis bag of 7 kDa. PAMP was collected and

104

freeze-dried under vacuum for 48 h. The lyophilized PAMP was stored at −20 °C.

105

For PAMP3-OH, 122 mg of PAMP3 was dissolved in 30.0 mL deionized water.

106

PAMP3 was activated in the presence of EDC (13.3 mg/mL) at 25 °C for 15 min with

107

stirring at 170 rpm. Subsequently, 0.42 v/v% ethanolamine was added to the solution

108

and the mixture was mixed by stirring at 170 rpm overnight. Then PAMP3-OH was

109

collected by the same method described above.

110

UV absorption spectra of the dendrimers and PEA from 340 nm to 240 nm was

111

measured by the UV/VIS Spectrometer (Lambda 35, PerkinElmer, USA) at a scan

112

speed of 100 nm/min. The 1 H and 13C NMR experiments were performed in D2O

113

with a 500 MHz Varian Inova spectrometer. The dendrimers were diluted in Tris-HCl

114

buffer (10 mM Tris, pH 7.4) and then the samples were put into the cuvette holder of

115

Nano-sizer (Nano-ZS, Malvern, USA) to measure zeta potential. Three measurements

116

were performed and the data were averaged. The structures of PAMC, PAMP3 and

117

PAMP3-OH were studied by Fourier transform infrared spectroscopy (FTIR).

118

In the stability experiments, different concentrations of dendrimers were dissolved

119

in PBS (100 mM sodium phosphate, 10 mM NaCl, pH 7.4) or PBS (D2O). Then they

120

were incubated by continuous orbital shaking at 150 rpm and 37 °C. At different time

121

points, samples were taken and tested by UV-visible spectroscopy and 1H NMR 5


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122

spectroscopy.

123

Aβ42 Preparation. Aβ42 preparation was described previously.27 Aβ42 was first

124

dissolved in HFIP to 1.0-1.5 mg/mL. The solution was put at 4 °C for at least 2 h and

125

then sonicated for 30 min to destroy the pre-existing aggregates. Next, the solution

126

was centrifuged (16000g) at 4 °C for 30 min to remove the existing Aβ aggregates.

127

The upper 75% of the supernatant was collected and HFIP was removed by vacuum

128

freeze-drying for 24 h. The dried Aβ42 was immediately stored at −20 °C. Before use,

129

Aβ42 was dissolved in 20 mM NaOH and centrifuged for 30 min (16000g) at 4 °C to

130

remove the aggregates, and then diluted with buffer solution containing various

131

concentrations of inhibitors, leading to the final peptide concentration of 25 μM. All

132

the buffer solution used was phosphate buffered saline (100 mM sodium phosphate,

133

10 mM NaCl, pH 7.4), unless otherwise indicated.

134

Thioflavin T Fluorescent Assay. In ex situ ThT assays, Aβ42 samples with

135

different concentrations of dendrimers were incubated by continuous orbital shaking

136

at 150 rpm and 37 °C. At different time points, 180 μL samples were taken and 1.8

137

mL of ThT buffer (25 μM ThT in 25 mM sodium phosphate, pH 6.0) was added into

138

the sample and mixed uniformly. ThT fluorescence intensities were measured by a

139

fluorescence spectrometer (LS-55, Perking Elmer, USA) with a slit width of 5 nm at

140

25 °C with excitation and emission at 440 and 480 nm, respectively. The fluorescence

141

intensity of solution without Aβ42 was subtracted as background from each read with

142

Aβ42.11 Three measurements were performed and the data were averaged.

143

In situ ThT assays, 25 μM Aβ42 with different concentrations of dendrimers were

144

mixed and added in a 96-well plate. The fluorescence intensities were measured by a

145

multimode reader platform (Infinite series, Tecan, Switzerland) at 37 °C with

146

excitation and emission at 440 and 480 nm, respectively. The fluorescence intensity of

147

sample without Aβ42 was subtracted as background from each read with Aβ42.

148

Atomic Force Microscope. Samples for AFM studies were taken directly from the

149

ThT assays, and 10 μL of each sample solution was loaded on freshly cleaved mica

150

surface and then rinsed with deionized water. The mica surface was dried under

151

nitrogen. AFM images were obtained in a multimode atomic force microscope 6


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(CSPM5500, Benyuan, China) in tapping mode. All of the images were collected at a

153

scan rate of 1 Hz and scan lines of 512.

154

CD Spectroscopy. CD spectra of 25 μM Aβ42 monomer or fibril in the absence and

155

presence of dendrimers were recorded using a J-810 spectrometer (Jasco, Japan) at

156

room temperature. A quartz cell with 1 mm path length was used for far-UV (190–

157

260 nm) measurements with 1 nm bandwidth at a scan speed of 100 nm/min. The CD

158

spectra of solutions without Aβ42 were subtracted as background from the CD signals.

159

All spectra were the average of three consecutive scans for each sample.

160

Cell Viability Assay. The SH-SY5Y cells were maintained in high glucose DMEM

161

supplemented with 10% FBS, 100 U/mL penicillin and 100 U/mL streptomycin at

162

37 °C under 5% CO2 in a CO2 cell culture box (HEPA class 100, Thermo Scientific,

163

USA). A total of 5 × 103 cells (90 μL) were seeded for 24 h in a polystyrene 96-well

164

plate. Then, the cells were treated with Aβ42 and dendrimers-modified Aβ42 (10 μL,

165

Aβ42 monomers was co-incubated with the dendrimers at 37 °C for 24 h). The cells

166

were incubated for an additional 24 h, and then 10 μL of MTT solution at the

167

concentration of 5.5 mg/mL in PBS was added into each well and incubated for

168

another 4 h. After centrifuged for 10 min (1000g) at room temperature, the medium

169

was discarded, and 100 μL of DMSO was added to dissolve the purple crystals. The

170

absorbance at 570 nm was measured by a plate reader (Spectra Max, Molecular

171

Devices, USA). The cell viability was calculated using the signals at 570 nm. The

172

wells containing medium only were subtracted as the background from each reading.

173

The cell viability data were normalized as a percentage of the control group without

174

Aβ42 and dendrimers. Six replicates were performed, and the data were averaged.28

175

Stopped-Flow Fluorescence Measurement. SX 20 stopped-flow fluorescence

176

instrument (SX20, Applied Photophysics, UK) was used to study the interaction

177

kinetics of Aβ42 with and without the dendrimers on a time scale of milliseconds. The

178

change of Rayleigh light scattering intensity at 37 °C over time was detected with

179

excitation at 435 nm to reflect Aβ42 aggregation states.29 The final concentrations of

180

Aβ42 and the dendrimers for the experiments were both 5μM. The dead time of the

181

experiments was 2 ms. Pro-Data Software was used to collect and analyze the data. 7


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182

Isothermal Titration Calorimetry. A VP isothermal titration calorimeter

183

(VP-ITC, Malvern, UK) was used in PBS at 37 °C to perform the ITC experiments.

184

The solution of dendrimer was loaded in the injection syringe, and after an initial

185

delay of 800 s, a 10 μL aliquot was continuously injected over 20 s for 25 times at a

186

constant interval of 600 s via a 200 rpm rotating stirrer-syringe into the sample cell.

187

The molar ratio of Aβ42 and dendrimers in all ITC experiments was 1:10. The

188

concentration of Aβ42 for experiments was 25 μM. All sample solutions were

189

degassed at 37 °C before the measurements. The data treatment was carried out using

190

single-site binding model in Microcal Origin 7.0 software.

191 192

RESULT AND DISCUSSION

193

Characteristics of PAMPs. Four PAMPs of different DS values of phenyl groups

194

were synthesized. The UV absorption spectra of PAMPs show typical absorbance at

195

260 nm, which is in accordance with the UV absorption of PEA (Figure S1). Thus the

196

DS of PAMP, defined as the percentage of phenyl groups to the original carboxyl

197

groups (128), was determined from the calibration curve of PEA. PAMPs are denoted

198

as PAMP1-4 in the order of the DS values as listed in Table S1. The 1H and 13C

199

NMR spectra of the four PAMPs confirm the PEA modification onto PAMC (Figures

200

S2 and S3). In the 1H NMR spectra, the signal ranging from 7.05 to 7.25 ppm

201

represents the hydrogen of the phenyl ring, which increases accordingly with

202

increasing the DS value. In the 13C NMR spectra, the signals at 140, 128 and 126

203

ppm represent the carbon of the phenyl ring and the signals at 43 and 41 ppm

204

represent the carbon of the methylene nearby the phenyl ring. The zeta potential

205

value, ranging from -22.1 to -17.3 mV, increases with increasing DS because of the

206

decrease of carboxyl groups on the surface after PEA modification (Figure S4).

207

Inhibition on Aβ42 Fibrillation. Thioflavin T (ThT) fluorescence assay was used

208

to examine the effect of PAMC and PAMPs on Aβ42 fibrillation. In principle, low

209

fluorescence intensity implies less β-sheet structure of the aggregates.30 As shown in

210

Figure 1A, PAMC has little effect on Aβ42 aggregation at concentrations up to

211

Aβ42/PAMC=1:10; it starts to show minor effect at a concentration as high as 25 times 8


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of Aβ42. This indicates that PAMC is not an inhibitor of the amyloid peptide. In

213

contrast, the ThT intensity of Aβ42 significantly decreases in a dose-dependent manner

214

when it is co-incubated with PAMPs; an obvious decrease occurred even at a

215

concentration as low as Aβ42/PAMP=1:0.01, particularly for PAMPs of higher DS

216

values (Figure 1B). Both PAMP3 and PAMP4 show strong effects: about 70%

217

reduction of the ThT intensity is observed at an equimolar concentration of Aβ42. As a

218

negative control, PEA is demonstrated not to reduce the ThT fluorescence (Figure

219

S5). Hence, the results indicate that the introduction of hydrophobic groups onto the

220

surface of PAMC creates potent inhibitors of Aβ42 fibrillation. Because ThT

221

fluorescence represents the content of β-sheet structure in the aggregates,30 the results

222

also indicate that PAMPs can effectively reduce the formation of β-sheet structure

223

during Aβ42 aggregation.

224

Atomic force microscope (AFM) was used to detect the morphology of Aβ42

225

aggregates (Figure 2). After 48-h incubation, dense serried and rod-like fibrils are

226

observed in the Aβ42-only group. PAMC does not change the morphology of the

227

aggregates and only a high concentration (625 μM) of PAMC slightly decreases the

228

amount of fibrils. This is in agreement with the above ThT assay (Figure 1A). In the

229

samples with PAMPs, it is clear that Aβ42 fibrils become less at 2.5 μM, and few

230

fibrils but irregular aggregates are observed with 25 μM PAMP3 or PAMP4. In

231

general,

232

PAMP4≥PAMP3>PAMP2>PAMP1, the same as that observed in the ThT assays

233

(Figure 1B). The AFM observations confirm that PAMPs significantly inhibit Aβ42

234

aggregation and alter the ultrastructure of Aβ42 aggregates.

the

potencies

of

PAMPs

are

also

in

the

order

of

235

Structural transitions of Aβ42 during its aggregation were examined by circular

236

dichroism (CD) spectroscopy of the peptide incubated with and without the

237

dendrimers. As shown in Figure 3, in the absence of the agents, the initial secondary

238

structure of Aβ42 is random coil with a negative peak below 200 nm (Figure 3A), and

239

after incubation for 48 h, a positive peak around 195 nm and a negative peak around

240

217 nm appear (Figure 3B), which represents the formation of β-sheet structure.

241

PAMC has no effect on the secondary structure of Aβ42 (Figure 3A and 3B). In the 9


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242

presence of PAMPs, at beginning blue shifts of the spectral peak position are

243

observed along with an increase in the signal intensity (Figure 3C). The signals with

244

PAMP1 and PAMP2 after 48-h incubations show two broad negative peaks around

245

210 and 225 nm (Figure 3D), corresponding to a mixture of α-helix and β-sheet

246

structures,31 which are obviously different from the Aβ42-only sample. Remarkably,

247

there is only a negative peak below 200 nm for the spectrum of Aβ42 incubated with

248

PAMP3 or PAMP4 for 48 h (Figure 3D), which is quite similar to that at the

249

beginning of the incubation (Figure 3C). This indicates that at this concentration

250

PAMP3 and PAMP4 completely inhibit the conformational transition to β-sheet

251

structure.

252

Hence, it can be concluded that PAMPs interfere with the structural transition of

253

Aβ42 during its aggregation. PAMP1/2 of lower DS values mediate Aβ42 to form

254

multiple secondary structures, while PAMP3/4 of higher DS values completely inhibit

255

the conformational transition to β-sheet structure in the aggregates.

256

Then, PAMP3 was selected to investigate the concentration effect on the secondary

257

structure of Aβ42 aggregates (Figure 3E and 3F). It is seen that with increasing

258

PAMP3 concentration more pronounced peak blue shifts and increased amplitudes of

259

the signals are observed at 0 h, and accordingly, after 48-h incubation the signal of

260

β-sheet structure progressively decreases and vanishes till increasing the

261

concentration to 25 μM. These results indicate that PAMP3 modulates the secondary

262

structure of Aβ42 aggregates in a dose-dependent manner. The same conclusion holds

263

for other three PAMPs (data not shown).

264

Inhibition on Amyloid Toxicity. MTT reduction assay using SH-SY5Y cells were

265

performed to assess the effects of the dendrimers on the cytotoxicity of Aβ42 fibrils.

266

The cytotoxicity of the dendrimers towards SH-SY5Y cells was first evaluated. Figure

267

S6 shows that PAMC has almost no cytotoxicity at concentrations up to 62.5 μM,

268

indicating that PAMC is rather biocompatible, consistent with previous reports.24, 32, 33

269

PAMP1-3 also show non-toxicity to the cells, while PAMP4 at 2.5 μM presents a cell

270

survival decrease of about 6% (Figure S7), suggesting a weak toxicity to the

271

SH-SY5Y cells. By contrast, Aβ42 aggregates are toxic to the cells, causing about 10


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39% cell death and PAMC does not affect the toxicity of Aβ42 aggregates (Figure

273

4A). In contrast, Aβ42 incubated with PAMPs exhibits higher cell viability, and the

274

cell viability increases with increasing the molar ratio of PAMP to Aβ42 (Figure 4B).

275

At an equimolar concentration of Aβ42, the cell viability increases by 34% to 57% for

276

the four PAMPs, and the incubation with PAMP3 displays the highest cell viability

277

(96%). PAMP3 presents to be the most effective among the four PAMPs at all the

278

concentrations tested.

279

Stabilities of PAMC and PAMP3. One issue that needs to be addressed is the

280

stabilities of the dendrimers under our experimental conditions.34 Because PAMP3

281

showed the best performance according to the above results, the stabilities of PAMP3

282

and PAMC were evaluated by UV/Vis (Figure S8) and 1H NMR spectroscopies

283

(Figure S9). It is seen that there are almost no changes in the spectra in 72-h

284

incubations. The results indicate that PAMC and PAMP3 remained intact under our

285

experimental conditions (72 h incubation).

286

Mechanistic Discussion. The above results have demonstrated that introduction of

287

hydrophobic groups onto the surface of PAMC creates potent inhibitors of Aβ42

288

fibrillation and the amyloid toxicity, and there is an optimum DS value at which the

289

PAMP (PAMP3) shows the best performance. This indicates that a dendrimer surface

290

with only carboxyl groups does not work, but the carboxyl and phenyl groups on

291

PAMPs work together to present the inhibitory effects, and their density ratio is also a

292

crucial factor influencing the inhibition effects. In order to explore if a dendrimer

293

surface with only hydrophobic groups could work on inhibiting Aβ42 fibrillation,

294

PAMP3 was modified with ethanolamine to derivatize the carboxyl groups to

295

hydroxyl groups. First of all, the product PAMP3-OH was analyzed by FTIR (Figure

296

S10), and 1H and 13C NMR spectroscopies (Figure S11) to identify its structure. In

297

the FTIR spectrum of PAMP3-OH product, a vibrational stretching band of phenyl

298

group at 1431 cm−1 and further enhancement of secondary amino group stretching

299

band at 3300 cm−1 are observed (Figure S10). In addition, the signal at 3.6 ppm in the

300

1H NMR spectrum represents the hydrogen of -CH2OH (Figure S11A). In the 13C

301

NMR spectrum (Figure S11B), the signals at 60 and 36 ppm represent the carbon of 11


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302

-CH2OH and the one of methylene next to -CH2OH, respectively. In the 13C NMR

303

spectrum of PAMP3 shown in Figure S3, the signals at 180 and 175 ppm represent the

304

carbon of –COOH and the one of acylamino next to –COOH, respectively. There is

305

no signal at 180 or 175 ppm in the 13C NMR spectrum of PAMP3-OH shown in

306

Figure S11B, indicating that there is almost no carboxyl groups in PAMP3-OH.

307

Therefore, the absence of residual carboxyl groups confirmed the correct structure of

308

PAMP3-OH. Furthermore, the zeta potential of PAMP3-OH was determined be to

309

+5.53 mV, much higher than that of PAMP3 (-19.5 mV, Figure S4) due to the

310

reduction of the terminal carboxyl groups, which is also evidence of correct

311

PAMP3-OH structure.

312

Interestingly, ThT fluorescence assay and CD spectroscopy reveal that PAMP3-OH

313

does not reduce the ThT fluorescence and change the secondary structure of Aβ42

314

aggregates (Figure 5). The results indicate that the inhibitor, PAMP3, is destroyed by

315

conversion of the carboxyl groups to hydroxyl groups. This further confirms that a

316

dendrimer surface with both anionic and hydrophobic groups is necessary for the

317

dendrimer to function as an amyloid inhibitor. This implies our design of the amyloid

318

inhibitor based on the HyBER hypothesis is successful. Namely, in the inhibition,

319

Aβ42 is bound to the hydrophobic (phenyl) groups and the bound Aβ42 molecules

320

suffer from electrostatic repulsion by the anionic (carboxyl) groups around the

321

hydrophobic groups. The two opposite forces make Aβ stretch into extended

322

conformations distinctly different from the β-sheet structure,20,

323

formation of off-pathway aggregates (Figure 2) with little β-sheet structure (Figure

324

3D). Scheme 2 represents the on-pathway fibrillation of Aβ42 and the inhibition effect

325

of PAMP by the HyBER effect.

21

leading to the

326

To further investigate the inhibitory mechanism of PAMPs, Rayleigh light

327

scattering (RLS) intensity over time was detected by stopped-flow fluorescence

328

measurement (dead time, 2 ms) to reveal the aggregation states of Aβ42 in a time scale

329

of 100 seconds.29 Figure 6 shows that the RLS signals of Aβ42 and Aβ42 incubated

330

with PAMC, PAMP1, PAMP2 or PAMP3-OH remain almost unchanged over the

331

time range, but the signals of Aβ42 incubated with PAMP3 and PAMP4 increase and 12


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332

reach steady states in about 40 s. This clearly indicates that PAMP3/4 of higher DS

333

values, different from PAMP1/2 of lower DS values, can change the pathway of Aβ42

334

aggregation from the very beginning of the peptide aggregation. This is because at the

335

same dendrimer concentration, PAMP3/4 offers higher concentration of hydrophobic

336

(phenyl) groups to make the HyBER effect happen in more sites.35,

337

pathway changes occur at the very beginning of Aβ42 aggregation, leading to the

338

formation of off-pathway aggregates of little β-sheet structure (Figure 3D) and low

339

toxicity (Figure 4B). Because PAMP3 is more biocompatible than PAMP4 (Figure

340

S7), and has a proper DS value to cause a favorable HyBER effect to affect the

341

fibrillation (Figures 1 and 2), it presents the best performance in the cell viability

342

assays (Figure 4B).

36

Thus, the

343

Finally, ITC was used to investigate the thermodynamic interactions between Aβ42

344

and the dendrimers (Figure S12). Thermodynamic parameters obtained from the ITC

345

experiments are listed in Table 1. The dissociation constants (Kd) for Aβ42 with the

346

dendrimers are all in the micromolar range, and the value of Kd decreases with

347

increasing DS of phenyl groups. For instance, the Kd values for PAMP3 and PAMP4

348

are about 30% smaller than those for PAMC and PAMP1. Moreover, PAMP3-OH

349

shows similar Kd value with PAMP3. The results indicate that PAMP3/4 bind more

350

tightly to Aβ42 than PAMP1, and the conversion of carboxyl to hydroxyl groups does

351

not affect the binding affinity for Aβ42. Moreover, the values of ΔH and TΔS are all

352

positive, and the values of TΔS are larger than those of ΔH. This implies that the

353

interactions between Aβ42 and the dendrimers are entropically favorable, namely

354

hydrophobic interactions are responsible for the binding while electrostatic

355

interactions are unfavorable. This is reasonable because Aβ42, with an isoelectric point

356

of 5.5, carries a net charge of −3.2 at the physiological condition (pH 7.4).37 Namely,

357

both Aβ42 and the dendrimers are negatively charged at pH 7.4 and they are

358

electrostatically repulsive from each other, which is the reason for the positive

359

enthalpy changes (endothermic reactions).

360

There are abundant hydrophobic patches on Aβ42 (Figure S13), which are mainly

361

distributed in the central hydrophobic core (L17-S26) and C-terminal (I31-A42).38 For 13


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362

PAMC, Aβ42 can bind to the internal hydrophobic cavity of the dendrimer. The

363

binding can only reduce the amount of free Aβ42 monomers, but cannot change the

364

conformation of Aβ42. Because the loading of Aβ42 to PAMC is reversible, when Aβ42

365

aggregation causes decrease in Aβ42 concentration in bulk solution, loaded Aβ42

366

monomers will release from the cavities of PAMC. Thus, even at high concentrations,

367

PAMC cannot inhibit the aggregation of Aβ42 effectively. This argument is supported

368

by the aggregation kinetics of Aβ42 shown in Figure S14. As can be seen from Figure

369

S14, although the ThT fluorescence of Aβ42 incubated with PAMC increases at slower

370

rate than the Aβ42-only group, there is no obvious difference between these two

371

groups in the final ThT fluorescence after 48 h incubation.

372

By contrast, hydrophobic binding to the surface phenyl groups of PAMPs leads to

373

the HyBER effect on the bound peptide molecules. The TΔS values are much larger

374

than ΔH, suggesting that Aβ42 is hydrophobically bound on the surface and

375

meanwhile suffers from electrostatic repulsion while keeping the binding state. This

376

makes the HyBER effect work to direct Aβ42 towards off-pathway aggregation.

377

Namely, in the presence of PAMPs, especially of PAMP3 and PAMP4, the HyBER

378

effect on the bound Aβ42 can result in remarkable changes of Aβ42 conformation. This

379

alters the aggregation pathway of Aβ42 from the very beginning, as reflected in

380

Figures 6 and S14, and avoids the formation of toxic intermediates.

381

In addition, compared to the relatively soluble Aβ40, Aβ42 is more prone to

382

aggregation.39, 40 Thus, the aggregation kinetics of Aβ42 in the in situ ThT assay by the

383

multimode reader platform (Infinite series, Tecan) did not show a classical sigmoidal

384

curve (Figures S14).41, 42 The kinetic assay of Aβ42 aggregation was designed only to

385

verify the hypothesis that Aβ42 can bind to the internal hydrophobic cavity of PAMC

386

and then be released. When examining the primary and secondary nucleation

387

processes, Aβ40 is normally used because its aggregation kinetics do show classical

388

sigmoidal curves in the in situ ThT assay by the multimode reader platform.41, 42

389

Therefore, the best PAMP (PAMP3) can significantly modulate the aggregation of

390

Aβ42 and suppress the amyloid cytotoxicity by the HyBER effect as represented in

391

Scheme 2. ITC data show that the binding sites N for Aβ42 binding to the dendrimers 14


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392

are in the range from 1.23 to 1.94 (Table 1).43,

44

393

studies,45 in the case of nonspecific binding, the N value varies according to the ratio

394

of Aβ to an inhibitor used in the ITC assay. Thus, the N value is highly changeable

395

with experimental condition and of limited significance to interpret Aβ42 binding to

396

the dendrimers. Hence, no further discussion is made on it. Moreover, it should be

397

noted that the numbers of Aβ42 binding to one PAMP depicted in Scheme 2 is only for

398

illustration of the binding event, but not for representing the N values listed in Table

399

1.

However, according to previous

400

The different inhibitory effects of these four PAMPs on Aβ42 aggregation indicated

401

that the ratio of hydrophobic groups to anionic groups on the surface of the inhibitors

402

is a key factor. Only inhibitors owning well-balanced hydrophobic binding and

403

electrostatic repulsion forces with Aβ42 can achieve favorable inhibitory effects.

404 405

CONCLUSIONS

406

In this work, a series of hydrophobically derivatized products of generation 5

407

carboxyl-terminated polyamidoamine (PAMC) dendrimers, denoted as PAMPs with

408

four different degrees of phenyl substitution, were synthesized to create dendrimer

409

surfaces with both negative charges and hydrophobic patches. This is to design an

410

inhibitor against Aβ aggregation and toxicity following the HyBER hypothesis.20, 21 In

411

contrast to PAMC, PAMPs inhibit Aβ42 fibrillogenesis and cytotoxicity, and the effect

412

increases with increasing the DS of phenyl groups. By evaluating the inhibitions on

413

both the amyloid fibrillation and cytotoxicity, PAMP3 (DS, 30.5%) is identified to

414

show the best performance, indicating that a potent inhibitor can be created with the

415

dendrimer at a proper DS value. Extensive biophysical and biological analyses prove

416

that PAMP3 modulates the aggregation pathway of Aβ42 at the very beginning and

417

improves the cell viability from 61% to 96% when incubating with equimolar amount

418

of Aβ42. The results well verify the HyBER hypothesis, namely the hydrophobic

419

binding and electrostatic repulsion from the inhibitor alter Aβ42 conformations,

420

making Aβ42 distinctly different from the β-sheet structure and thus inhibiting the

421

aggregation and toxicity of Aβ42. The effective inhibition of amyloid fibrillation and 15


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422

toxicity by PAMP3 suggests its potentials for development in therapeutic uses in

423

amyloid diseases. The findings also suggest that rational design of inhibitors based on

424

the HyBER theory can be an effective way to the discovery of potent amyloid

425

inhibitors.

426 427

ASSOCIATED CONTENT

428

Supporting Information

429

Chemical structure of PAMC, reaction conditions for the synthesis of the four PAMPs,

430

UV absorption of the dendrimers and PEA, 1H and 13C NMR spectra and zeta

431

potentials of PAMC and PAMPs, ThT fluorescence intensities of Aβ42 incubated with

432

PEA, cytotoxicity assays of PAMC and PAMPs, stabilities of PAMC and PAMP3,

433

FTIR spectra of PAMC, PAMP3 and PAMP3-OH, 1H and 13C NMR spectra of

434

PAMP3-OH, calorimetric titration assays, surface models of Aβ42, aggregation

435

kinetics of Aβ42 incubated with the dendrimers. This material is available free of

436

charge via the Internet at http://pubs.acs.org.

437 438

AUTHOR IMFORMATION

439

Corresponding Author

440

*Tel: +86 22 27403389; Fax: +86 22 27403389; E-mail address: [email protected] (Y.

441

Sun).

442

ORCID

443

Xiaoyan Dong: 0000-0002-8040-5897

444

Yan Sun: 0000-0001-5256-9571

445

Author Contributions

446

Y.S designed the research; Z.W. performed the experiments and analyzed the data;

447

Z.W., X.D., and Y.S wrote or contributed to the writing of the manuscript.

448

Notes

449

The authors declare no competing financial interest.

450 451

ACKNOWLEDGMENTS 16


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452

This work was supported by the National Natural Science Foundation of China (Grant

453

Nos. 21621004 and 91634119).

17


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591 592

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Table 1. Thermodynamic parameters for the interactions between Aβ42 and the dendrimers.

PAMC PAMP1 PAMP3 PAMP4 PAMP3-OH

Kd (μM)

ΔH (kcal/mol)

TΔS (kcal/mol)

ΔG (kcal/mol)

N (Aβ42/Dendrimer)

2.78±0.182 2.75±0.165 1.95±0.180 1.93±0.203 1.91±0.148

3.96±0.157 4.92±0.088 4.88±0.402 3.72±0.097 4.40±0.099

11.86±0.293 12.81±0.132 13.00±0.257 11.83±0.241 12.52±0.636

-7.90±0.139 -7.89±0.044 -8.12±0.164 -8.12±0.144 -8.11±0.035

1.94±0.032 1.23±0.057 1.43±0.057 1.58±0.021 1.92±0.15

593

23


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594 595

Scheme 1. PAMC is converted to PAMP by reaction with phenethylamine.

596

PAMP surface is distributed with carboxyl groups and phenyl groups, so the

597

PAMP with a proper degree of phenyl substitution is expected to function as an

598

amyloid inhibitor via HyBER hypothesis.

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Langmuir

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Figure 1. Normalized ThT fluorescence intensities of Aβ42 (25 μM) fibrillization after

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incubation with the dendrimers at 37 °C for 48 h. ThT fluorescence of Aβ42

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aggregates without the dendrimers was defined as 100%.

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Figure 2. AFM images of 25 μM Aβ42 incubated with different concentrations of

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PAMC or PAMPs at 37 °C for 48 h.

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Figure 3. Far-UV circular dichroism spectra of 25 μM Aβ42 incubated in the absence

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and presence of the dendrimers at 0 h and after incubation for 48 h. (A) and (B) show

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the effect of equimolar PAMC; (C) and (D) show the effect of equimolar PAMPs; (E)

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and (F) show the effect of PAMP3 concentration.

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Figure 4. Viability of SH-SY5Y cells incubated with 2.5 μM Aβ42 together with

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different concentrations of (A) PAMC or (B) PAMPs. Cell viability for treatment with

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PBS buffer alone was set to 100%. ***p

Hydrophobic Modification of Carboxyl-Terminated PAMAM Dendrimer

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