Microplastics Reduce Lipid Digestion in Simulated Human Gastrointestinal System.

Microplastics (MPs) are unavoidably ingested by humans, and their gastrointestinal processes and impact on lipid digestion are unknown. In the present work, all five MP types used, including polystyrene (PS), polyethylene terephthalate, polyethylene, polyvinyl chloride, and poly(lactic-co-glycolic acid) (80 mg/L in small intestine), significantly reduced lipid digestion in the in vitro gastrointestinal system. PS MPs exhibited the highest inhibition (12.7%) among the five MPs. Lipid digestion decreased with increasing PS concentration, but independent of PS size (50 nm, 1 µm, 10 µm). PS MPs after photoaging by simulated sunlight also significantly decreased lipid digestion. Confocal imaging shows that PS MPs could interact with both lipid droplets and lipases. Two mechanisms underlying the PS-induced digestion inhibition were revealed using both experimental and molecular dynamics simulation approaches: (1) PS MPs decreased the bioavailability of lipid droplets via forming large lipid-MPs heteroaggregates due to the high MP hydrophobicity; and (2) PS MPs adsorbed lipase, and reduced its activity by changing the secondary structure and disturbing the essential open conformation. The first mechanism (MP-lipid interaction) played a more important role in lipid digestion reduction based on interaction energy calculation. These findings reveal potential risk of MPs to human digestion health and nutrient assimilation.

Membrane nanotube pearling restricted by confined polymers.

Increasing evidence showed that membrane nanotubes readily undergo pearling in response to external stimuli, while long tubular membrane structures have been observed connecting cells and functioning as channels for intercellular transport, raising a fundamental question of how the stability of membrane nanotubes is maintained in the cellular environment. Here, combining dissipative particle dynamics simulations, free energy calculations, and a force analysis, we propose and demonstrate that nanotube pearling can be restricted by confined polymers, which can be DNA and protein chains transported through the nanotubes, or actin filaments participating in tube formation and elongation. Thermodynamically, nanotube pearling releases the membrane surface energy, but costs bending energies of both the membrane and the confined polymers. Following the mechanism, the pearling of nanotubes confining longer and stiffer polymers is more difficult as it costs larger polymer bending energies. In dynamics, nanotube pearling occurs by repelling polymers from the region of nanotube shrinking to that of swelling. Shorter polymers can be readily repelled owing to the unbalanced force exerted by the shrinking tube region, whereas longer polymers tend to be trapped at the shrinking region to retard the nanotube pearling. Besides the low surface tension maintained by lipid reservoirs kept in living cells, our results supplement the explanation for the stability of membrane nanotubes, and open up a new avenue to manipulate the shape deformation of tubular membrane structures for study of many biological processes.

Microplastic fragmentation by rotifers in aquatic ecosystems contributes to global nanoplastic pollution.

The role of aquatic organisms in the biological fragmentation of microplastics and their contribution to global nanoplastic pollution are poorly understood. Here we present a biological fragmentation pathway that generates nanoplastics during the ingestion of microplastics by rotifers, a commonly found and globally distributed surface water zooplankton relevant for nutrient recycling. Both marine and freshwater rotifers could rapidly grind polystyrene, polyethylene and photo-aged microplastics, thus releasing smaller particulates during ingestion. Nanoindentation studies of the trophi of the rotifer chitinous mastax revealed a Young's modulus of 1.46 GPa, which was higher than the 0.79 GPa for polystyrene microparticles, suggesting a fragmentation mechanism through grinding the edges of microplastics. Marine and freshwater rotifers generated over 3.48 × 105 and 3.66 × 105 submicrometre particles per rotifer in a day, respectively, from photo-aged microplastics. Our data suggest the ubiquitous occurrence of microplastic fragmentation by different rotifer species in natural aquatic environments of both primary and secondary microplastics of various polymer compositions and provide previously unidentified insights into the fate of microplastics and the source of nanoplastics in global surface waters.

Molecular assembly of extracellular polymeric substances regulating aggregation of differently charged nanoplastics and subsequent interactions with bacterial membrane.

Extracellular polymeric substances (EPS) represent an interface between microbial cells and aquatic environment, where nanoplastics acquire coatings to alter their fate and toxicity. However, little is known about molecular interactions governing modification of nanoplastics at biological interfaces. Molecular dynamics simulations combining experiments were conducted to investigate assembly of EPS and its regulatory roles in the aggregation of differently charged nanoplastics and interactions with bacterial membrane. Driven by hydrophobic and electrostatic interactions, EPS formed micelle-like supramolecular structures with hydrophobic core and amphiphilic exterior. Different components, depending on their hydrophobicity and charge, were found to promote or suppress EPS assembly. Neutral and hydrophobic nanoplastics showed unbiased adsorption of EPS species, while cationic and anionic nanoplastics were distinct and attracted specific molecules of opposite charges. Compared with isolated EPS, assembled EPS concealed hydrophobic groups to be less adsorbed by nanoplastics. Aggregation of nanoplastics was alleviated by EPS due to electrostatic repulsion plus steric hindrance. ESP suppressed binding of cationic nanoplastics to the bacterial membrane through reducing the surface charge. Neutral and anionic nanoplastics showed weak membrane association, but their binding interactions were promoted by EPS. The structural details revealed here provided molecular level insights into modifications of nanoplastics at the eco-environment interface.

Benzo[a]pyrene and heavy metal ion adsorption on nanoplastics regulated by humic acid: Cooperation/competition mechanisms revealed by molecular dynamics simulations.

Nanoplastics adsorb pollutants and organic matter to aggravate or alleviate impact to the eco-environment and human health. However, the interaction mechanisms remain unclear and difficult to study using current experimental techniques. By means of molecular dynamics simulation, here we investigate adsorption of benzo[a]pyrene (BaP) and heavy metal ions (Cu2+) on nanoplastics of different materials and surface charges regulated by humic acid (HA). Among considered materials, polystyrene shows the highest capacity of adsorbing BaPs via forming sandwiched π-stacking structures with benzene rings. Driven by hydrophobic, electrostatic and hydrogen bonding interactions, HAs spontaneously aggregate into micelle-like structures with hydrophobic core and charged exterior accessible to BaPs and Cu2+, respectively. Cationic and neutral nanoplastics adsorb more HAs to form eco-coronas, which modulate BaP and Cu2+ adsorption via following cooperation/competition mechanisms. On one hand, the direct binding of BaPs to nanoplastics is hindered by HAs through BaP encapsulation plus competitive adsorption. On the other hand, adsorbed HAs expose carboxyl groups to offer rich binding sites to promote Cu2+ adsorption on neutral and cationic nanoplastics, while unbound HAs compete with anionic nanoplastics to inhibit Cu2+ adsorption. These results provide molecular level insights into transport, transformation and accessibility of nanoplastics with coexisting contaminants in the aqueous environment.

Molecular modeling of nanoplastic transformations in alveolar fluid and impacts on the lung surfactant film.

Airborne nanoplastics can be inhaled to threaten human health, but research on the inhaled nanoplastic toxicity is in its infancy, and interaction mechanisms are largely unknown. By means of molecular dynamics simulation, we employed spherical nanoplastics of different materials and aging properties to predict and elucidate nanoplastic transformations in alveolar fluid and impacts on the lung surfactant (LS) film at the alveolar air-water interface. Results showed spontaneous adsorption of LS molecules on nanoplastics of 10 nm in diameter, and the adsorption layer can be defined as coronas, which increased the particle size, reduced and equalized the surface hydrophobicity, and endowed nanoplastics with negative surface charges. Nanoplastics of polypropylene and polyvinylchloride materials were dissolved by LS, which could increase bioavailability of polymers and toxic additives. Aging properties represented by the nanoplastic size, polymer's molecular weight and surface chemistry altered nanoplastic transformations through modulating competition between polymer-LS and polymer-polymer interactions. Upon transferred to the alveolar air-water interface through vesicle fusion, nanoplastics could interfere with the normal biophysical function of LS through disrupting the LS ultrastructure and fluidity, and prompting collapse of the LS film. These results provide new molecular level insights into fate and toxicity of airborne nanoplastics in human respiratory system.

Curvature-mediated cooperative wrapping of multiple nanoparticles at the same and opposite membrane sides.

Cell membrane interactions with nanoparticles (NPs) are essential to cellular functioning and mostly accompanied by membrane curvature generation and sensing. Multiple NPs inducing curvature from one side of a membrane are believed to be wrapped cooperatively by the membrane through curvature-mediated interactions. However, little is known about another biologically ubiquitous and important case, i.e., NPs binding to opposite membrane sides induce a curved bend of different directions. Combining coarse-grained molecular dynamics and theoretical analysis, here we systematically investigate the cooperative effect in the wrapping of multiple adhesive NPs at the same and opposite membrane sides and demonstrate the importance of the magnitude and direction of the membrane bend in regulating curvature-mediated NP interactions. Effects of the NP size, size difference, initial distance, number, and strength of adhesion with the membrane on the wrapping cooperativity and wrapping states are analyzed. For NPs binding to the same membrane side, rich membrane wrapping and NP aggregation states are observed, and the curvature-mediated interactions could be either attractive or repulsive, depending on the initial NP distance and the competition between the membrane bending, NP binding and membrane protrusion. In sharp contrast, the interaction between two NPs binding to opposite membrane sides is always attractive and the cooperative wrapping of NPs is promoted, as the curved membrane regions induced by the NPs are shared in a manner that the NP-membrane contact is increased and the energy cost of membrane bending is reduced. Owing to the ubiquity and heterogeneity of membrane shaping proteins in biology, our results enrich the cutting-edge knowledge on the curvature-mediated interaction of NPs for better and profound understanding on high-order cooperative assemblies of NPs or proteins in numerous biological processes.

Design of Small Nanoparticles Decorated with Amphiphilic Ligands: Self-Preservation Effect and Translocation into a Plasma Membrane.

Design of nanoparticles (NPs) for biomedical applications requires a thorough understanding of cascades of nano-bio interactions at different interfaces. Here, we take into account the cascading effect of NP functionalization on interactions with target cell membranes by determining coatings of biomolecules in biological media. Cell culture experiments show that NPs with more hydrophobic surfaces are heavily ingested by cells in both the A549 and HEK293 cell lines. However, before reaching the target cell, both the identity and amount of recruited biomolecules can be influenced by the pristine NPs' hydrophobicity. Dissipative particle dynamics (DPD) simulations show that hydrophobic NPs acquire coatings of more biomolecules, which may conceal the properties of the as-engineered NPs and impact the targeting specificity. Based on these results, we propose an amphiphilic ligand coating on NPs. DPD simulations reveal the design principle, following which the amphiphilic ligands first curl in solvent to reduce the surface hydrophobicity, thus suppressing the assemblage of biomolecules. Upon attaching to the membrane, the curled ligands extend and rearrange to gain contacts with lipid tails, thus dragging NPs into the membrane for translocation. Three NP-membrane interaction states are identified that are found to depend on the NP size and membrane surface tension. These results can provide useful guidelines to fabricate ligand-coated NPs for practical use in targeted drug delivery, and motivate further studies of nano-bio-interactions with more consideration of cascading effects.

Self-assembly of semiflexible homopolymers into helical bundles: a brownian dynamics simulation study.

The controllable self-assembly of semiflexible homopolymers into regular bundles has received much attention because of its potential importance in various fields, such as the storage of elastic energy, the fabrication of nanostructures, and the formation of the cytoskeleton in living cells. In this article, using computer simulations, we investigate how semiflexible homopolymers anchored on a substrate self-organize into ordered structures, focusing on both the patterns formed and the dynamics of self-assembly. For the self-assembly pattern, four different patterns, including patterns with unclustered polymers, disordered semispherical clusters, highly ordered helical bundles, and parallel bundles, are observed from our simulations. The formation of stable bundles requires semiflexible homopolymers having a sufficient molecule length and intermediate bending stiffness, whereas the formation of the helical structures depends on the balance between the inter-homopolymer attraction and the bending stiffness of homopolymers. Furthermore, the bundle formation reinforces the bending stiffness, and the stiffness is further enhanced by the helical bundling. For the dynamic aspect, both hierarchical bundling and nonhierarchical bundling are observed from our simulations.