By Pritam Roy November 21, 2022 Reviewed by Susha Cheriyedath, M.Sc.
In a paper published in PNAS, the researchers introduced Mms6 proteins from magnetotactic bacteria (MTB) into a reverse micelle structure to create a nanoreactor that resembled a magnetosome. This magnetosome-inspired nanoscale chamber synthesized single-domain magnetic nanoparticles (MNPs).
Study: Magnetosome-inspired synthesis of soft ferrimagnetic nanoparticles for magnetic tumor targeting. Image credit: peterschreiber.media/Shutterstock.com
Subsequently, their magnetic characteristics and morphology were analyzed and contrasted with those of natural magnetosomes created by AMB-1 MTB.
One of the most effective methods to improve the targeting efficiency of magnetic nanodrug carriers that use an external magnetic field to reach their targets is magnetic targeting. The magnetosome, a unique “organelle” created by biomineralization in MTB, is a naturally occurring biologically derived MNP. It is crucial that MTB magnetic navigation reacts to geomagnetic fields.
The magnetism and crystal morphology of the MNPs formed via thermal decomposition were identical to those of natural magnetosomes, but the particles were smaller. Thus, the synthesized magnetosome-like MNPs penetrated the lesion area of a tumor mouse model with good monodispersion. This monodispersion increased tumor penetration by orders of magnitude due to its powerful magnetic targeting ability created by soft ferromagnetism. Therefore, the enhancement of tumor penetration resulted in a positive contrast in the tumor stains.
Fig. 1. Nanoscale reaction chambers for biomineralization. (A) MTB magnetosome, where the Mms6 protein assembles into a mineralization template on the inner surface of the magnetosome membrane. The hydrophilic acid-rich region of Mms6 is highlighted in light orange and the N-terminal hydrophobic membrane-associated region in green, respectively; the red dotted frame highlights the lipid bilayer of the magnetosome membrane. (B) Schematic illustration of the synthesis process of magnetosome-like MNPs within the nanoreactor of a reverse micelle containing Mms6. The red dotted frame highlights the surfactant monolayer in the magnetosome-like MNPs.
New targeted nanodrug delivery techniques and the future of tumor therapy
Targeted nanodrug delivery has become one of the promising approaches to nanodrug therapy and tumor imaging with the continued advancement of nanotechnology. Numerous studies in recent years have revealed that although nanomedicines can reach target tumor locations, the average accuracy of tumor targeting is less than one percent.
As a result, enriching nanomedicines in tumor tissues is still a challenge, mainly to increase their effective penetration efficiency. A magnetic targeting technique adds an external magnetic field to a given region after intravenous injection, enriching and collecting MNPs as they move through local blood arteries to reduce the near miss effect of systemic drug delivery.
Due to the dense extracellular matrix (ECM) and high interstitial fluid pressure in tumor tissues, MNPs exhibit tissue penetration and low targeting to solid tumor tissues, like other nanomedicines. The following attributes of MNPs must be present for a magnetic targeting system to be effective in overcoming these biological barriers: (i) adequate saturation magnetization (Ms) and magnetic moment to achieve a fast and effective response to a field external magnetic, (ii) ) superparamagnetic properties to avoid agglomeration of MNPs and (iii) small size to improve penetration ability.
The magnetosome, a unique “organelle” created by biomineralization in MTB, is a naturally occurring biologically derived MNP. It is crucial for MTB magnetic navigation in response to geomagnetic fields. For the AMB-1 MTB, the magnetosome, a nanoscale mineralization chamber, creates magnetic nanocrystals with a cubo-octahedral shape. The AMB-1 MTB responds quickly to an external magnetic field thanks to its special magnetic characteristics. In addition, magnetosome is a wonderful choice for magnetically targeted nanodrug carriers and for magnetic resonance imaging (MRI) operations due to its small size, high Ms, superior stability, and minimal toxicity.
However, in biological fluid and water environments, the high magnetic interaction between natural magnetosome particles leads to precipitation and aggregation, substantially impairing their ability to penetrate tumor cells. Therefore, creating magnetosome-like MNPs that have the benefits of natural magnetosome MNPs without the drawbacks may be an exciting research topic.
In this study, the authors constructed a bio-inspired nanoreactor by integrating Mms6 proteins into a reverse micelle structure. The magnetic characteristics, morphology, and MR relaxation characteristics of the obtained MNPs were investigated and contrasted with natural magnetosomes made by AMB-1 MTB.
Tumor penetration was improved by orders of magnitude due to the small size of magnetosome-like MNPs and their powerful magnetic targeting ability created by soft ferromagnetism, revealing beneficial contrast in the tumor area. Thus, magnetosome-like MNPs appeared extremely intriguing for potential nanomedicine applications due to their tumor penetration, magnetic targeting, and MR imaging capabilities.
Fig. 2. Construction of the reverse micelle nanoreactor containing Mms6 and characterization of Mms6 during different reaction stages. (A) DLS profiles of reaction buffer without oleylamine surfactant (gray line), reverse micelle reaction system after reaction at 60 °C for 3 h (red line) and micelle reaction system reverse after reaction at 200 °C for 8 h (blue line). (B) SDS-PAGE analysis of Mms6 after reaction at 60°C and 200°C. Lanes 1 and 4, purified Mms6 protein; lanes 2 and 5, supernatant from control reaction without Mms6 participation; lanes 3 and 6, reaction supernatant at 60 °C (lane 3) and 200 °C (lane 6) with participation of Mms6. Red arrow: Mms6 tetramers; orange arrow: Mms6 dimers; blue arrow: Mms6 degradation fragments. (C) The 1H, 15N-HSQC spectra of Mms6 in the reverse micelle nanoreactor after reaction at 60 °C. Red dotted box: glycine or tryptophan. (D) The 1H, 15N-HSQC spectra of Mms6 in the reverse micelle nanoreactor after reaction at 200 °C. Blue dotted box: glycine. (E) Sequence of Mms 6 with the hydrophilic acid-rich region in blue and the hydrophobic membrane region in pink. Scissors represent the site of degradation.
Experimental assembly
A nanoreactor resembling a magnetosome was constructed and thermal decomposition was used to create iron oxide (Fe3O4) MNPs. In a solution of benzyl ether with 0.1 percent water, Mms6 protein powder and oleylamine were combined to create the Mms6-containing reverse micelle system. Mms6 protein was attached to the surfaces of magnetosome-like MNPs, as shown by the appearance of protein-related FTIR signals. These findings suggested that the assembly of the reverse micelle system and the mineralization process required the Mms6 protein.
The tail vein was used to administer the magnetosome-like MNPs to six-week-old male mammary tumor model mice. For magnetically focused MNP enrichment, a 0.5 T magnet was placed over the tumor region of mice for 120, 60, and 30 minutes. After being exposed to the magnetic field for 30 minutes, the magnetosome-like MNPs showed a markedly increased contrast in the tumor area. In addition, the contrast-to-noise ratio (CNR) variation at the edge of the tumor increased by 132 percent. At the same time, CNR change inside the tumor increased by 110 percent.
Magnetic targeting uses magnetic fields to control the distribution of magnetically responsive nanomedicine carriers or nanoparticles to minimize off-target consequences of systemic administration. Iron oxide (FeO) nanoparticles were crucial for magnetic targeted nanodrug carriers that were biocompatible and offered a viable nanodrug delivery strategy for cancer treatment. Furthermore, due to their large surface area and inherent magnetic and photoelectric capabilities, MNPs could combine various diagnostic procedures, including computed tomography and magnetic resonance imaging.
Tumor-targeting evidence showed that magnetosome-like nanoparticles could enter the inner area of the tumor due to their soft ferromagnetic characteristics and small sizes. Thus, the efficiency of tumor targeting increased dramatically by about three percent. In contrast, magnetic targeting resulted in a 10% reduction in the distribution of magnetosome-like MNPs in the spleen and liver.
These findings demonstrated that magnetosome-like MNPs greatly increased the accuracy of magnetically targeted nanodrug delivery. Due to its remarkable magnetic orientation ability, it was an excellent choice for various biomedical applications.
Fig. 3. HRTEM characterization of natural magnetosomes, magnetosome-like MNPs and magnetic nanocrystals of the control reaction. (A1) and (A2) High-resolution electron micrographs and (A3) three-dimensional crystal morphology of natural magnetosomes of AMB-1. (B1) and (B2) High-resolution electron micrographs and (B3) three-dimensional morphology of magnetosome-like MNPs. (C1) and (C2) High-resolution electron micrographs and (C3) three-dimensional morphology of the MNPs from the control reaction. The insets of (A2), (B2) and (C2) show the corresponding fast Fourier transform patterns of the crystal structure.
Importance of the study
In this study, the authors constructed a magnetosome-like nanoreactor by inserting the amphiphilic Mms6 proteins into self-assembled reverse micelles, recapitulating the two key elements required for MTB biomineralization, including magnetosome regulatory proteins and vesicles of the magnetosome
Magnetosome-like MNPs…