The influence of extremely low frequency magnetic field and magnetic nanoparticle on Aβ1-40 aggregation in vitro

Summary form only given. The magnetic nanoparticle had been found in the brain of the aging population especially Alzheimer's disease (AD) patients. Base on the existing researches, it has been speculated that the magnetic nanoparticles may coexist with senile plaque, a hallmark of AD which consists of excessive accumulation of β-Amyloid (Aβ). Epidemiological investigation had presented possible correlation between extremely low frequency magnetic field (ELF MF) and AD. Therefore a hypothesis was presented that the magnetic nanoparticles might be taken as a media between ELF MF and AD to have an effect on the organism and affect the process of diseases. In this study we try to investigate the role of magnetic nanoparticle and ELF MF (50 Hz, 400 μT) exposure in the process of Aβ
_1-40
aggregation in vitro with a magnetic nanoparticle (Fe
₃
O
₄
) coated with SiO
₂
(MNP-SiO
₂
). Aβ
_1-40
was treated to be monomers by HFIP, and then mixed with MNP-SiO
₂
followed by incubation at 37 for 48 h with or without MF exposure. Tricine-SDS-PAGE was used to identify the molecular weight (MW) of Aβ
_1-40
aggregates. The results showed that Aβ
_1-40
monomer was ~5 kDa by HFIP treatment (0 h) and a small amount of dimer (~10 kDa) appeared. After incubation at 37 for 48 h, evident dimer bands emerged in the Aβ
_1-40
samples incubation alone (Aβ-sham group and Aβ-MF group) in addition to the monomer band; Meanwhile, the dimer bands and significant high MW bands (~70 kDa and ~90 kDa)appeared in the Aβ
_1-40
co-incubation samples with MNP-SiO
₂
with or without magnetic field exposure (i.e. Aβ-MNP-MF group and Aβ-MNP-sham group respectively). The atomic force microscope (AFM) was used to observe the 3D morphology of Aβ
_1-40
aggregates . In the sample of 0h (Aβ-0h), a loose pseudo spherical structure can be observed . After incubation at 37 for 48 h, Aβ aggregates appeared in both Aβ-sham and Aβ-MF samples . In the sham group, the pseudo spherical structure still exists with a smaller size in the Aβ-sham sample and some short aggregates were visible; in the sample of Aβ-MNP-sham, most pseudo spherical structure disappeared and protofibrils/fibers-like aggregates came into appear with a loosely structure . In MF group, the morphology of Aβ-MF sample was similar to Aβ-MNP-sham sample where short fibrillike aggregates appeared with a loose structure; in the Aβ-MNP-MF sample, the longer fibril-like aggregates formed with a compact structure . In comparison with Aβ
_1-40
aggregation without MF exposure, it seemed Aβ
_1-40
aggregated more significantly under MF exposure; compared with Aβ incubation alone, MNP-SiO
₂
made Aβ aggregation more easily since there were more longer protofibrils/fibrils . Roughness analysis revealed that Z values, Ra and Rq were higher in the Aβ-0h sample . After incubation for 48 h, that in the Aβ-sham and Aβ-MNP-sham samples had the lowest roughness, and the roughness of Aβ-MF and Aβ-MNP-MF samples exceeded that of the Aβ-0h, Aβ-sham and Aβ-MNP-sham samples which showed significances (P<;0 .01) . After incubation for 48 h, the order of the roughness is as follows: Aβ-sham <; Aβ-MNP-sham <; Aβ-MF <; Aβ-MNP-MF . Combined with image analysis, it was presumed that Aβ aggregation began with the larger and uneven pseudo spherical structure followed with the pseudo spherical structure collapsing and unfolding . And then the polypeptide elongated and bundled accompanied with protofibrils/fibrils maturation . In this process, the MF exposure and magnetic particles promoted the aggregation of Aβ, which resulted in the transformation of loose structure to compact structure . It should be noted that although the roughness of Aβ-MNP-MF sample were higher than that of Aβ-MF sample, the result had no statistical significance . Transmission electron microscope observation showed that the pseudo spherical structure with a diameter ranging from 10 to 26 nm was present in Aβ-0h sample, and a few of unconspicuous aggregates were observed . After 48 h of incubation, obvious fibril-like aggregates appeared in all the samples (Aβ-sham, Aβ-MF, Aβ-MNP-sham and Aβ-MNP-MF) with pseudo spherical structure existing; the protofibrils/fibrils structure could be observed with a length of several microns, and the fibrils had crosses and branches . In conclusion, we observed that magnetic nanoparticles co-incubation promoted Aβ
_1-40
aggregation and ELF MF exposure enhanced the effect of the magnetic nanoparticles . Therefore it is possible that the magnetic nanoparticles mediated the effect of ELF MF on Aβ aggregation . The results will contribute to the explanation of the relationship among ELF MF, magnetic nanoparticles and neurodegeneration diseases as AD .