Bacterial Magnetosomes as Novel Platform for the Presentation of Immunostimulatory, Membrane-Bound Ligands in Cellular Biotechnology

Comparing the Colloidal Stabilities of Commercial and Biogenic Iron Oxide Nanoparticles That Have Potential In Vitro/In Vivo Applications

For the potential in vitro/in vivo applications of magnetic iron oxide nanoparticles, their stability in different physiological fluids has to be ensured. This important prerequisite includes the preservation of the particles' stability during the envisaged application and, consequently, their invariance with respect to the transfer from storage conditions to cell culture media or even bodily fluids. Here, we investigate the colloidal stabilities of commercial nanoparticles with different coatings as a model system for biogenic iron oxide nanoparticles (magnetosomes) isolated from magnetotactic bacteria. We demonstrate that the stability can be evaluated and quantified by determining the intensity-weighted average of the particle sizes (Z-value) obtained from dynamic light scattering experiments as a simple quality criterion, which can also be used as an indicator for protein corona formation.

The transcriptomic landscape of Magnetospirillum gryphiswaldense during magnetosome biomineralization

BACKGROUND

One of the most complex prokaryotic organelles are magnetosomes, which are formed by magnetotactic bacteria as sensors for navigation in the Earth's magnetic field. In the alphaproteobacterium Magnetospirillum gryphiswaldense magnetosomes consist of chains of magnetite crystals (Fe₃O₄) that under microoxic to anoxic conditions are biomineralized within membrane vesicles. To form such an intricate structure, the transcription of > 30 specific structural genes clustered within the genomic magnetosome island (MAI) has to be coordinated with the expression of an as-yet unknown number of auxiliary genes encoding several generic metabolic functions. However, their global regulation and transcriptional organization in response to anoxic conditions most favorable for magnetite biomineralization are still unclear.

RESULTS

Here, we compared transcriptional profiles of anaerobically grown magnetosome forming cells with those in which magnetosome biosynthesis has been suppressed by aerobic condition. Using whole transcriptome shotgun sequencing, we found that transcription of about 300 of the > 4300 genes was significantly enhanced during magnetosome formation. About 40 of the top upregulated genes are directly or indirectly linked to aerobic and anaerobic respiration (denitrification) or unknown functions. The mam and mms gene clusters, specifically controlling magnetosome biosynthesis, were highly transcribed, but constitutively expressed irrespective of the growth condition. By Cappable-sequencing, we show that the transcriptional complexity of both the MAI and the entire genome decreased under anaerobic conditions optimal for magnetosome formation. In addition, predominant promoter structures were highly similar to sigma factor σ⁷⁰ dependent promoters in other Alphaproteobacteria.

CONCLUSIONS

Our transcriptome-wide analysis revealed that magnetite biomineralization relies on a complex interplay between generic metabolic processes such as aerobic and anaerobic respiration, cellular redox control, and the biosynthesis of specific magnetosome structures. In addition, we provide insights into global regulatory features that have remained uncharacterized in the widely studied model organism M. gryphiswaldense, including a comprehensive dataset of newly annotated transcription start sites and genome-wide operon detection as a community resource (GEO Series accession number GSE197098).

A Magnetosome-Based Platform for Flow Biocatalysis

Biocatalysis in flow reactor systems is of increasing importance for the transformation of the chemical industry. However, the necessary immobilization of biocatalysts remains a challenge. We here demonstrate that biogenic magnetic nanoparticles, so-called magnetosomes, represent an attractive alternative for the development of nanoscale particle formulations to enable high and stable conversion rates in biocatalytic flow processes. In addition to their intriguing material characteristics, such as high crystallinity, stable magnetic moments, and narrow particle size distribution, magnetosomes offer the unbeatable advantage over chemically synthesized nanoparticles that foreign protein "cargo" can be immobilized on the enveloping membrane via genetic engineering and thus, stably presented on the particle surface. To exploit these advantages, we develop a modular connector system in which abundant magnetosome membrane anchors are genetically fused with SpyCatcher coupling groups, allowing efficient covalent coupling with complementary SpyTag-functionalized proteins. The versatility of this approach is demonstrated by immobilizing a dimeric phenolic acid decarboxylase to SpyCatcher magnetosomes. The functionalized magnetosomes outperform similarly functionalized commercial particles by exhibiting stable substrate conversion during a 60 h period, with an average space-time yield of 49.2 mmol L^-1 h^-1. Overall, our results demonstrate that SpyCatcher magnetosomes significantly expand the genetic toolbox for particle surface functionalization and increase their application potential as nano-biocatalysts.

Induction of Axonal Outgrowth in Mouse Hippocampal Neurons via Bacterial Magnetosomes

Magnetosomes are membrane-enclosed iron oxide crystals biosynthesized by magnetotactic bacteria. As the biomineralization of bacterial magnetosomes can be genetically controlled, they have become promising nanomaterials for bionanotechnological applications. In the present paper, we explore a novel application of magnetosomes as nanotool for manipulating axonal outgrowth via stretch-growth (SG). SG refers to the process of stimulation of axonal outgrowth through the application of mechanical forces. Thanks to their superior magnetic properties, magnetosomes have been used to magnetize mouse hippocampal neurons in order to stretch axons under the application of magnetic fields. We found that magnetosomes are avidly internalized by cells. They adhere to the cell membrane, are quickly internalized, and slowly degrade after a few days from the internalization process. Our data show that bacterial magnetosomes are more efficient than synthetic iron oxide nanoparticles in stimulating axonal outgrowth via SG.

Identification and elimination of genomic regions irrelevant for magnetosome biosynthesis by large-scale deletion in Magnetospirillum gryphiswaldense

BACKGROUND

Magnetosome formation in the alphaproteobacterium Magnetospirillum gryphiswaldense is controlled by more than 30 known mam and mms genes clustered within a large genomic region, the 'magnetosome island' (MAI), which also harbors numerous mobile genetic elements, repeats, and genetic junk. Because of the inherent genetic instability of the MAI caused by neighboring gene content, the elimination of these regions and their substitution by a compact, minimal magnetosome expression cassette would be important for future analysis and engineering. In addition, the role of the MAI boundaries and adjacent regions are still unclear, and recent studies indicated that further auxiliary determinants for magnetosome biosynthesis are encoded outside the MAI. However, techniques for large-scale genome editing of magnetic bacteria are still limited, and the full complement of genes controlling magnetosome formation has remained uncertain.

RESULTS

Here we demonstrate that an allelic replacement method based on homologous recombination can be applied for large-scale genome editing in M. gryphiswaldense. By analysis of 24 deletion mutants covering about 167 kb of non-redundant genome content, we identified genes and regions inside and outside the MAI irrelevant for magnetosome biosynthesis. A contiguous stretch of ~ 100 kb, including the scattered mam and mms6 operons, could be functionally substituted by a compact and contiguous ~ 38 kb cassette comprising all essential biosynthetic gene clusters, but devoid of interspersing irrelevant or problematic gene content.

CONCLUSIONS

Our results further delineate the genetic complement for magnetosome biosynthesis and will be useful for future large-scale genome editing and genetic engineering of magnetosome biosynthesis.

Towards a 'chassis' for bacterial magnetosome biosynthesis: genome streamlining of Magnetospirillum gryphiswaldense by multiple deletions

BACKGROUND

Because of its tractability and straightforward cultivation, the magnetic bacterium Magnetospirillum gryphiswaldense has emerged as a model for the analysis of magnetosome biosynthesis and bioproduction. However, its future use as platform for synthetic biology and biotechnology will require methods for large-scale genome editing and streamlining.

RESULTS

We established an approach for combinatory genome reduction and generated a library of strains in which up to 16 regions including large gene clusters, mobile genetic elements and phage-related genes were sequentially removed, equivalent to ~ 227.6 kb and nearly 5.5% of the genome. Finally, the fragmented genomic magnetosome island was replaced by a compact cassette comprising all key magnetosome biosynthetic gene clusters. The prospective 'chassis' revealed wild type-like cell growth and magnetosome biosynthesis under optimal conditions, as well as slightly improved resilience and increased genetic stability.

CONCLUSION

We provide first proof-of-principle for the feasibility of multiple genome reduction and large-scale engineering of magnetotactic bacteria. The library of deletions will be valuable for turning M. gryphiswaldense into a microbial cell factory for synthetic biology and production of magnetic nanoparticles.

Towards standardized purification of bacterial magnetic nanoparticles for future in vivo applications

Bacterial magnetosomes (MS) are well-defined membrane-enveloped single-domain iron oxide (magnetite) nanoparticles, which are susceptible to genetic and chemical engineering. Additionally, the possibility to manipulate these particles by external magnetic fields facilitates their application in biomedicine and biotechnology, e.g. as magnetic resonance imaging probes or for drug delivery purposes. However, current purification protocols are poorly characterized, thereby hampering standardized and reproducible magnetosome production and thus, reliable testing for in vivo applications. In that context, the establishment of reproducible particle isolation procedures as well as the identification of high quality control parameters and the evaluation of potential cytotoxic effects of purified particles are of major importance. In this study, we characterize a multi-step purification protocol for MS with regard to purity, iron content, size and polydispersity of magnetite particles. In addition, we address potential cytotoxic effects of isolated MS when incubated with mammalian cells. Overall, we provide a detailed overview of the process-structure relationship during the isolation of MS and thus, identify prerequisites for high-yield MS production and their future application in the biomedical and biotechnological field. STATEMENT OF SIGNIFICANCE: Magnetic nanoparticles are of increasing interest for a variety of biomedical and biotechnological applications. Due to their unprecedented material characteristics, bacterial magnetosomes represent a promising alternative to chemically synthesized iron oxide nanoparticles. As applications require well-defined, highly purified and fully characterized nanoparticles, reliable protocols are necessary for efficient and reproducible magnetosome isolation. In our study, we evaluate an improved magnetosome extraction procedure and monitor quality parameters such as particle size distribution, membrane integrity and purity of the suspension by a combination of physicochemical and biochemical methods. Furthermore, the cytotoxicity of the isolated magnetosomes is assessed using different cell lines. In summary, our study helps to establish prerequisites for many real-world applications of magnetosomes in the field of biotechnology and biomedicine.

An automated oxystat fermentation regime for microoxic cultivation of Magnetospirillum gryphiswaldense

BACKGROUND

Magnetosomes produced by magnetotactic bacteria represent magnetic nanoparticles with unprecedented characteristics. However, their use in many biotechnological applications has so far been hampered by their challenging bioproduction at larger scales.

RESULTS

Here, we developed an oxystat batch fermentation regime for microoxic cultivation of Magnetospirillum gryphiswaldense in a 3 L bioreactor. An automated cascade regulation enabled highly reproducible growth over a wide range of precisely controlled oxygen concentrations (1-95% of air saturation). In addition, consumption of lactate as the carbon source and nitrate as alternative electron acceptor were monitored during cultivation. While nitrate became growth limiting during anaerobic growth, lactate was the growth limiting factor during microoxic cultivation. Analysis of microoxic magnetosome biomineralization by cellular iron content, magnetic response, transmission electron microscopy and small-angle X-ray scattering revealed magnetosomal magnetite crystals were highly uniform in size and shape.

CONCLUSION

The fermentation regime established in this study facilitates stable oxygen control during culturing of Magnetospirillum gryphiswaldense. Further scale-up seems feasible by combining the stable oxygen control with feeding strategies employed in previous studies. Results of this study will facilitate the highly reproducible laboratory-scale bioproduction of magnetosomes for a diverse range of future applications in the fields of biotechnology and biomedicine.

A Versatile Toolkit for Controllable and Highly Selective Multifunctionalization of Bacterial Magnetic Nanoparticles

Their unique material characteristics, i.e. high crystallinity, strong magnetization, uniform shape and size, and the ability to engineer the enveloping membrane in vivo make bacterial magnetosomes highly interesting for many biomedical and biotechnological applications. In this study, a versatile toolkit is developed for the multifunctionalization of magnetic nanoparticles in the magnetotactic bacterium Magnetospirillum gryphiswaldense, and the use of several abundant magnetosome membrane proteins as anchors for functional moieties is explored. High-level magnetosome display of cargo proteins enables the generation of engineered nanoparticles with several genetically encoded functionalities, including a core-shell structure, magnetization, two different catalytic activities, fluorescence and the presence of a versatile connector that allows the incorporation into a hydrogel-based matrix by specific coupling reactions. The resulting reusable magnetic composite demonstrates the high potential of synthetic biology for the production of multifunctional nanomaterials, turning the magnetosome surface into a platform for specific versatile display of functional moieties.