Aggregation State of Metal-Based Nanomaterials at the Pulmonary Surfactant Film Determines Biophysical Inhibition

Biophysical impact of lubricating base oil aerosols on natural pulmonary surfactant film

Lubricating base oils have been extensively employed for producing various industrial and consumer products. Therefore, their environmental and health impacts should be carefully evaluated. Although there have been many reports on pulmonary cytotoxicity and inflammatory responses of inhaled lubricating base oils, their potential influences on pulmonary surfactant (PS) films that play an essential role in maintaining respiratory mechanics and pulmonary immunity remains largely unknown. Here a systematic study on the interactions between an animal-derived natural PS and aerosols of water and representative mineral and vegetable base oils is performed using a novel biophysical assessing technique called constrained drop surfactometry capable of providing in vitro simulations of normal tidal breathing and physiologically relevant temperature and humidity in the lung. It was found that the mineral oil aerosols can impose strong inhibitions to the biophysical property of PS film, while the airborne vegetable oils and water show negligible adverse effects within the studied concentration range. The inhibitory effect is originated from the strong hydrophobicity of mineral oil, which makes it able to disrupt the interfacial molecular ordering of both phospholipid and protein compositions and consequently suppress the formation of condensed phase and multilayer scaffolds in a PS film. ENVIRONMENTAL IMPLICATION: Understanding the biophysical influence of airborne lubricating base oils on pulmonary surfactant (PS) films can provide new insights into the environmental impacts and health concerns of various industrial lubricant products. Here a comparative study on interactions between an animal-derived natural PS film and the aerosols of water and representative mineral and vegetable base oils under the true physiological conditions was conducted in situ using constrained drop surfactometry. We show that the most frequently used mineral base oil can cause strong inhibitions to the PS film by disrupting the molecular ordering of saturated phospholipids and surfactant-associated proteins at the interface.

Recent Advances in Photodynamic Therapy: Metal-Based Nanoparticles as Tools to Improve Cancer Therapy

Cancer remains a significant global health challenge, with traditional therapies like surgery, chemotherapy, and radiation often accompanied by systemic toxicity and damage to healthy tissues. Despite progress in treatment, these approaches have limitations such as non-specific targeting, systemic toxicity, and resistance development in cancer cells. In recent years, nanotechnology has emerged as a revolutionary frontier in cancer therapy, offering potential solutions to these challenges. Nanoparticles, due to their unique physical and chemical properties, can carry therapeutic payloads, navigate biological barriers, and selectively target cancer cells. Metal-based nanoparticles, in particular, offer unique properties suitable for various therapeutic applications. Recent advancements have focused on the integration of metal-based nanoparticles to enhance the efficacy and precision of photodynamic therapy. Integrating nanotechnology into cancer therapy represents a paradigm shift, enabling the development of strategies with enhanced specificity and reduced off-target effects. This review aims to provide a comprehensive understanding of the pivotal role of metal-based nanoparticles in photodynamic therapy. We explore the mechanisms, biocompatibility, and applications of metal-based nanoparticles in photodynamic therapy, highlighting the challenges and the limitations in their use, as well as the combining of metal-based nanoparticles/photodynamic therapy with other strategies as a synergistic therapeutic approach for cancer treatment.

Microplastics and Nanoplastics Impair the Biophysical Function of Pulmonary Surfactant by Forming Heteroaggregates at the Alveolar-Capillary Interface

Microplastics (MPs) are ubiquitous environmental pollutants produced through the degradation of plastic products. Nanoplastics (NPs), commonly coexisting with MPs in the environment, are submicrometer debris incidentally produced from fragmentation of MPs. We studied the biophysical impacts of MPs/NPs derived from commonly used commercial plastic products on a natural pulmonary surfactant extracted from calf lung lavage. It was found that in comparison to MPs/NPs derived from lunch boxes made of polypropylene or from drinking water bottles made of poly(ethylene terephthalate), the MP/NP derived from foam packaging boxes made of polystyrene showed the highest adverse impact on the biophysical function of the pulmonary surfactant. Accordingly, intranasal exposure of MP/NP derived from the foam boxes also induced the most serious proinflammatory responses and lung injury in mice. Atomic force microscopy revealed that NP particles were adsorbed on the air-water surface and heteroaggregated with the pulmonary surfactant film. These results indicate that although the incidentally formed NPs only make up a small mass fraction, they likely play a predominant role in determining the nano-bio interactions and the lung toxicity of MPs/NPs by forming heteroaggregates at the alveolar-capillary interface. These findings may provide novel insights into understanding the health impact of MPs and NPs on the respiratory system.

Pulmonary Surfactant: A Mighty Thin Film

Pulmonary surfactant is a critical component of lung function in healthy individuals. It functions in part by lowering surface tension in the alveoli, thereby allowing for breathing with minimal effort. The prevailing thinking is that low surface tension is attained by a compression-driven squeeze-out of unsaturated phospholipids during exhalation, forming a film enriched in saturated phospholipids that achieves surface tensions close to zero. A thorough review of past and recent literature suggests that the compression-driven squeeze-out mechanism may be erroneous. Here, we posit that a surfactant film enriched in saturated lipids is formed shortly after birth by an adsorption-driven sorting process and that its composition does not change during normal breathing. We provide biophysical evidence for the rapid formation of an enriched film at high surfactant concentrations, facilitated by adsorption structures containing hydrophobic surfactant proteins. We examine biophysical evidence for and against the compression-driven squeeze-out mechanism and propose a new model for surfactant function. The proposed model is tested against existing physiological and pathophysiological evidence in neonatal and adult lungs, leading to ideas for biophysical research, that should be addressed to establish the physiological relevance of this new perspective on the function of the mighty thin film that surfactant provides.

Adverse Biophysical Impact of e-Cigarette Flavors on Pulmonary Surfactant

The attractiveness and abundance of flavors are primary factors eliciting youth to use e-cigarettes. Emerging studies in recent years revealed the adverse health impact of e-cigarette flavoring chemicals, including disruption of the biophysical function of pulmonary surfactants in the lung. Nevertheless, a comprehensive understanding of the biophysical impact of various flavoring chemicals is still lacking. We used constrained drop surfactometry as a new alternative method to study the biophysical impact of flavored e-cigarette aerosols on an animal-derived natural pulmonary surfactant. The dose of exposure to e-cigarette aerosols was quantified with a quartz crystal microbalance, and alterations to the ultrastructure of the surfactant film were visualized using atomic force microscopy. We have systematically studied eight representative flavoring chemicals (benzyl alcohol, menthol, maltol, ethyl maltol, vanillin, ethyl vanillin, ethyl acetate, and ethyl butyrate) and six popular recombinant flavors (coffee, vanilla, tobacco, cotton candy, menthol/mint, and chocolate). Our results suggested a flavor-dependent inhibitory effect of e-cigarette aerosols on the biophysical properties of the pulmonary surfactant. A qualitative phase diagram was proposed to predict the hazardous potential of various flavoring chemicals. These results provide novel implications in understanding the environmental, health, and safety impacts of e-cigarette aerosols and may contribute to better regulation of e-cigarette products.

Heterogeneous aggregation of carbon and silicon nanoparticles with benzo[a]pyrene modulates their impacts on the pulmonary surfactant film

Inhaled nanoparticles (NPs) can deposit in alveoli where they interact with the pulmonary surfactant (PS) and potentially induce toxicity. Although nano-bio interactions are influenced by the physicochemical properties of NPs, isolated NPs used in previous studies cannot accurately represent those found in atmosphere. Here we used molecular dynamics simulations to investigate the interplay between two types of NPs associated with benzo[a]pyrene (BaP) at the PS film. Silicon NPs (SiNPs), regardless of aggregation and adsorption, directly penetrated through the PS film with minimal disturbance. Meanwhile, BaPs adsorbed on SiNPs were rapidly solubilized by PS, increasing the BaP's bioaccessibility in alveoli. Carbon NPs (CNPs) showed aggregation and adsorption-dependent effects on the PS film. Compared to isolated CNPs, which extracted PS to form biomolecular coronas, aggregated CNPs caused more pronounced PS disruption, especially around irregularly shaped edges. SiNPs in mixture exacerbated the PS perturbation by piercing PS film around the site of CNP interactions. BaPs adsorbed on CNPs were less solubilized and suppressed PS extraction, but aggravated biophysical inhibition by prompting film collapse under compression. These results suggest that for proper assessment of inhalation toxicity of airborne NPs, it is imperative to consider their heterogeneous aggregation and adsorption of pollutants under atmospheric conditions.

Constrained drop surfactometry for studying adsorbed pulmonary surfactant at physiologically relevant high concentrations

Exogenous surfactant therapy has been used as a standard clinical intervention for treating premature newborns with respiratory distress syndrome. The phospholipid concentrations of exogenous surfactants used in clinical practice are consistently higher than 25 mg/mL; while it was estimated that the phospholipid concentration of endogenous surfactant is approximately in the range between 15 and 50 mg/mL. However, most in vitro biophysical simulations of pulmonary surfactants were only capable of studying surfactant concentrations up to 3 mg/mL, one order of magnitude lower than the physiologically relevant concentration. Using a new in vitro biophysical model, called constrained drop surfactometry, in conjunction with atomic force microscopy and other technological advances, we have investigated the biophysical properties, ultrastructure, and topography of the pulmonary surfactant film adsorbed from the subphase at physiologically relevant high surfactant concentrations of 10-35 mg/mL. It was found that the effect of surfactant concentration on the dynamic surface activity of the surfactant film was only important when the surface area of the surfactant film varied no more than 15%, mimicking normal tidal breathing. The adsorbed surfactant film depicts a multilayer conformation consisting of a layer-by-layer assembly of stacked bilayers with the height of the multilayers proportional to the surfactant concentration. Our experimental data suggest that the biophysical function of these multilayer structures formed after de novo adsorption is to act as a buffer zone to store surface-active materials ejected from the interfacial monolayer under extreme conditions such as deep breathing.NEW & NOTEWORTHY An in vitro biophysical model, called constrained drop surfactometry, was developed to study the biophysical properties, ultrastructure, and topography of the pulmonary surfactant film adsorbed from the subphase at physiologically relevant high surfactant concentrations of 10-35 mg/mL. These results suggest that the biophysical function of multilayers formed after de novo adsorption is to act as a buffer zone to store surface-active materials ejected from the interfacial monolayer under extreme conditions such as deep breathing.

Effect of Sublethal Concentrations of Metal Nanomaterials on Cell Energy Metabolism

Metallic nanomaterials (MNMs) are widely used in the medical field because of their photocatalytic, optical, electrical, electronic, antibacterial, and bactericidal properties. Despite the advantages of MNMs, there is a lack of complete understanding of their toxicological behavior and their interactions with cellular mechanisms that determine cell fate. Most of the existing studies are acute toxicity studies with high doses, which is not conducive to understanding the toxic effects and mechanisms of homeostasis-dependent organelles, such as mitochondria, which are involved in many cellular functions. In this study, four types of MNMs were used to investigate the effects of metallic nanomaterials on mitochondrial function and structure. We first characterized the four MNMs and selected the appropriate sublethal concentration for application in cells. Mitochondrial characterization, energy metabolism, mitochondrial damage, mitochondrial complex activity, and expression levels were evaluated using various biological methods. The results showed that the four types of MNMs greatly inhibited mitochondrial function and cell energy metabolism and that the material entering the mitochondria damaged the mitochondrial structure. Additionally, the complex activity of mitochondrial electron transport chains is critical for assessing the mitochondrial toxicity of MNMs, which may serve as an early warning of MNM-induced mitochondrial dysfunction and cytotoxicity.

Lessons from the history of inorganic nanoparticles for inhalable diagnostics and therapeutics

The respiratory tract is one of the most accessible ones to exogenous nanoparticles, yet drug delivery by their means to it is made extraordinarily challenging because of the plexus of aerodynamic, hemodynamic and biomolecular factors at cellular and extracellular levels that synergistically define the safety and efficacy of this process. Here, the use of inorganic nanoparticles (INPs) for inhalable diagnostics and therapies of the lung is viewed through the prism of the history of studies on the interaction of INPs with the lower respiratory tract. The most conceptually and methodologically innovative and illuminative studies are referred to in the chronological order, as they were reported in the literature, and the trends in the progress of understanding this interaction of immense therapeutic and toxicological significance are being deduced from it. The most outstanding actual trends delineated include the diminishment of toxicity via surface functionalization, cell targeting, tagging and tracking via controlled binding and uptake, hybrid INP treatments, magnetic guidance, combined drug and gene delivery, use as adjuvants in inhalable vaccines, and other. Many of the understudied research directions, which have been accomplished by the nanostructured organic polymers in the pulmonary niche, are discussed. The progress in the use of INPs as inhalable diagnostics or therapeutics has been hampered by their well-recognized inflammatory potential and toxicity in the respiratory tract. However, the annual numbers of methodologically innovative studies have been on the rise throughout the past two decades, suggesting that this is a prolific direction of research, its comparatively poor commercial takings notwithstanding. Still, the lack of consensus on the effects of many INP compositions at low but therapeutically effective doses, the plethora of contradictory reports on ostensibly identical chemical compositions and NP properties, and the many cases of antagonism in combinatorial NP treatments imply that the rational design of inhalable medical devices based on INPs must rely on qualitative principles for the most part and embrace a partially stochastic approach as well. At the same time, the fact that the most studied INPs for pulmonary applications have been those with some of the thickest records of pulmonary toxicity, e.g., carbon, silver, gold, silica and iron oxide, is a silent call for the expansion of the search for new inorganic compositions for use in inhalable therapies to new territories.

Lung surfactant as a biophysical assay for inhalation toxicology

Lung surfactant (LS) is a mixture of lipids and proteins that forms a thin film at the gas-exchange surfaces of the alveoli. The components and ultrastructure of LS contribute to its biophysical and biochemical functions in the respiratory system, most notably the lowering of surface tension to facilitate breathing mechanics. LS inhibition can be caused by metabolic deficiencies or the intrusion of endogenous or exogenous substances. While LS has been sourced from animals or synthesized for clinical therapeutics, the biofluid mixture has also gained recent interest as a biophysical model for inhalation toxicity. Various methods can be used to evaluate LS function quantitatively or qualitatively after exposure to potential toxicants. A narrative review of the recent literature was conducted. Studies focused whether LS was inhibited by various environmental contaminants, nanoparticles, or manufactured products. A review is also conducted on synthetic lung surfactants (SLS), which have emerged as a promising alternative to conventional animal-sourced LS. The intrinsic advantages and recent advances of SLS make a strong case for more widespread usage in LS-based toxicological assays.

Menthol in Electronic Cigarettes Causes Biophysical Inhibition of Pulmonary Surfactant

With an increasing prevalence of electronic cigarette (e-cigarette) use, especially among youth, there is an urgent need to better understand the biological risks and pathophysiology of health conditions related to e-cigarettes. A majority of e-cigarette aerosols are in the submicron size and would deposit in the alveolar region of the lung, where they must first interact with the endogenous pulmonary surfactant. To date, little is known whether e-cigarette aerosols have an adverse impact on the pulmonary surfactant. We have systematically studied the effect of individual e-cigarette ingredients on an animal-derived clinical surfactant preparation, bovine lipid extract surfactant, using a combination of biophysical and analytical techniques, including in vitro biophysical simulations using constrained drop surfactometry, molecular imaging with atomic force microscopy, chemical assays using carbon nuclear magnetic resonance and circular dichroism, and in silico molecular dynamics simulations. All data collectively suggest that flavorings used in e-cigarettes, especially menthol, play a predominant role in inhibiting the biophysical function of the surfactant. The mechanism of biophysical inhibition appears to involve menthol interactions with both phospholipids and hydrophobic proteins of the natural surfactant. These results provide novel insights into the understanding of the health impact of e-cigarettes and may contribute to a better regulation of e-cigarette products.

Evaluating the Impact of Hydrophobic Silicon Dioxide in the Interfacial Properties of Lung Surfactant Films

The interaction of hydrophobic silicon dioxide particles (fumed silicon dioxide), as model air pollutants, and Langmuir monolayers of a porcine lung surfactant extract has been studied in order to try to shed light on the physicochemical bases underlying the potential adverse effects associated with pollutant inhalation. The surface pressure-area isotherms of lung surfactant (LS) films including increasing amounts of particles revealed that particle incorporation into LS monolayers modifies the organization of the molecules at the water/vapor interface, which alters the mechanical resistance of the interfacial films, hindering the ability of LS layers for reducing the surface tension, and reestablishing the interface upon compression. This influences the normal physiological function of LS as is inferred from the analysis of the response of the Langmuir films upon the incorporation of particles against harmonic changes of the interfacial area (successive compression-expansion cycles). These experiments evidenced that particles alter the relaxation mechanisms of LS films, which may be correlated to a modification of the transport of material within the interface and between the interface and the adjacent fluid during the respiratory cycle.

Impact of Polymer Nanoparticles on DPPC Monolayer Properties

The application of surface rheology and Brewster angle microscopy on mixed monolayers of DPPC and polymeric nanoparticles (cationic and anionic) showed that the sign of the particle charge affects the dynamic properties of the monolayers less than the nanoparticles’ ability to aggregate. Under almost physiological conditions, the effect of nanoparticles on the elasticity of DPPC monolayer is insignificant. However, the particles prevent the surface tension from decreasing to extremely low values. This effect could affect the functionality of pulmonary surfactants.

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.

Potential hazards associated with interactions between diesel exhaust particulate matter and pulmonary surfactant

Long term exposure to diesel exhaust particulate matter (DEPM) can induce numerous adverse health effects to the respiratory system. Understanding the interaction between DEPM and pulmonary surfactant (PS) can be an essential step toward preliminary evaluation of the impact of DEPM on pulmonary health. Herein, DEPM was explored for its interaction with 1,2-dipalmitoyl-sn-glycerol-3-phosphocholine (DPPC), the major component of PS. The results indicated that the surface pressure-area (π-A) isotherms of DPPC monolayers shifted toward lower molecular areas and the compression modulus (C_S^-1) reduced in the presence of DEPM. Atomic force microscopy image showed that DEPM can disrupt the ultrastructure of DPPC monolayers along with the direction of lateral compression. In addition, DPPC can in turn condition the surface properties of DEPM, permitting its agglomeration in aqueous media, which was attributed to the adsorption of DEPM to DPPC. Furthermore, the particle-bound polycyclic aromatic hydrocarbons (PAHs) could be desorbed from DEPM by the solubilization of DPPC and it was positively correlated with the hydrophobicity of PAHs. These findings revealed the toxicity of DEPM-associated PAHs and the role of DPPC in facilitating the removal of the inhaled particles, which can provide a new insight into the potential hazards of airborne particles on lung health.

Biophysical properties of tear film lipid layer I. Surface tension and surface rheology

Tear film lipid layer (TFLL) is the outmost layer of the tear film. It plays a crucial role in stabilizing the tear film by reducing surface tension and retarding evaporation of the aqueous layer. Dysfunction of the TFLL leads to dysfunctional tear syndrome, with dry eye disease (DED) being the most prevalent eye disease, affecting 10%-30% of the world population. To date, except for treatments alleviating dry eye symptoms, effective therapeutic interventions in treating DED are still lacking. Therefore, there is an urgent need to understand the biophysical properties of the TFLL with the long-term goal to develop translational solutions in effectively managing DED. Here, we studied the composition-function correlations of an artificial TFLL, under physiologically relevant conditions, using a novel experimental methodology called constrained drop surfactometry. This artificial TFLL was composed of 40% behenyl oleate and 40% cholesteryl oleate, representing the most abundant wax ester and cholesteryl ester in the natural TFLL, respectively, and 15% phosphatidylcholine and 5% palmitic-acid-9-hydroxy-stearic-acid (PAHSA), which represent the two predominant polar lipid classes in the natural TFLL. Our study suggests that the major biophysical function of phospholipids in the TFLL is to reduce the surface tension, whereas the primary function of PAHSA is to optimize the rheological properties of the TFLL. These findings have novel implications in better understanding the physiological and biophysical functions of the TFLL and may offer new translational insight to the treatment of DED.

Competitive and/or cooperative interactions of graphene-family materials and benzo[a]pyrene with pulmonary surfactant: a computational and experimental study

BACKGROUND

Airborne nanoparticles can be inhaled and deposit in human alveoli, where pulmonary surfactant (PS) molecules lining at the alveolar air-water interface act as the first barrier against inhaled nanoparticles entering the body. Although considerable efforts have been devoted to elucidate the mechanisms underlying nanoparticle-PS interactions, our understanding on this important issue is limited due to the high complexity of the atmosphere, in which nanoparticles are believed to experience transformations that remarkably change the nanoparticles' surface properties and states. By contrast with bare nanoparticles that have been extensively studied, relatively little is known about the interactions between PS and inhaled nanoparticles which already adsorb contaminants. In this combined experimental and computational effort, we investigate the joint interactions between PS and graphene-family materials (GFMs) with coexisting benzo[a]pyrene (BaP).

RESULTS

Depending on the BaP concentration, molecular agglomeration, and graphene oxidation, different nanocomposite structures are formed via BaPs adsorption on GFMs. Upon deposition of GFMs carrying BaPs at the pulmonary surfactant (PS) layer, competition and cooperation of interactions between different components determines the interfacial processes including BaP solubilization, GFM translocation and PS perturbation. Importantly, BaPs adsorbed on GFMs are solubilized to increase BaP's bioavailability. By contrast with graphene adhering on the PS layer to release part of adsorbed BaPs, more BaPs are released from graphene oxide, which induces a hydrophilic pore in the PS layer and shows adverse effect on the PS biophysical function. Translocation of graphene across the PS layer is facilitated by BaP adsorption through segregating it from contact with PS, while translocation of graphene oxide is suppressed by BaP adsorption due to the increase of surface hydrophobicity. Graphene extracts PS molecules from the layer, and the resultant PS depletion declines with graphene oxidation and BaP adsorption.

CONCLUSION

GFMs showed high adsorption capacity towards BaPs to form nanocomposites. Upon deposition of GFMs carrying BaPs at the alveolar air-water interface covered by a thin PS layer, the interactions of GFM-PS, GFM-BaP and BaP-PS determined the interfacial processes of BaP solubilization, GFM translocation and PS perturbation.

Nanostructured Ceria: Biomolecular Templates and (Bio)applications

Ceria (CeO2) nanostructures are well-known in catalysis for energy and environmental preservation and remediation. Recently, they have also been gaining momentum for biological applications in virtue of their unique redox properties that make them antioxidant or pro-oxidant, depending on the experimental conditions and ceria nanomorphology. In particular, interest has grown in the use of biotemplates to exert control over ceria morphology and reactivity. However, only a handful of reports exist on the use of specific biomolecules to template ceria nucleation and growth into defined nanostructures. This review focusses on the latest advancements in the area of biomolecular templates for ceria nanostructures and existing opportunities for their (bio)applications.

Binding of Benzo[a]pyrene Alters the Bioreactivity of Fine Biochar Particles toward Macrophages Leading to Deregulated Macrophagic Defense and Autophagy

Contaminant-bearing fine biochar particles (FBPs) may exert significantly different toxicity profiles from their contaminant-free counterparts. While the role of FBPs in promoting contaminant uptake has been recognized, it is unclear whether the binding of contaminants can modify the biochemical reactivity and toxicological profiles of FBPs. Here, we show that binding of benzo[a]pyrene (B(a)P, a model polycyclic aromatic hydrocarbon) at environmentally relevant exposure concentrations markedly alters the cytotoxicity of FBPs to macrophages, an important line of innate immune defense against airborne particulate matters (PMs). Specifically, B(a)P-bearing FBPs elicit more severe disruption of the phospholipid membrane, endocytosis, oxidative stress, autophagy, and compromised innate immune defense, as evidenced by blunted proinflammatory effects, compared with B(a)P-free FBPs. Notably, the altered cytotoxicity cannot be attributed to the dissolution of B(a)P from the B(a)P-bearing FBPs, but appears to be related to B(a)P adsorption-induced changes of FBPs bioreactivity toward macrophages. Our findings highlight the significance of environmental chemical transformation in altering the bioreactivity and toxicity of PMs and call for further studies on other types of carbonaceous nanoparticles and additional exposure scenarios.

An adverse outcome pathway for lung surfactant function inhibition leading to decreased lung function

Inhaled substances, such as consumer products, chemicals at the workplace, and nanoparticles, can affect the lung function in several ways. In this paper, we explore the adverse outcome pathway (AOP) that starts when inhaled substances that reach the alveoli inhibit the function of the lung surfactant, and leads to decreased lung function. Lung surfactant covers the inner surface of the alveoli, and regulates the surface tension at the air-liquid interface during breathing. The inhibition of the lung surfactant function leads to alveolar collapse because of the resulting high surface tension at the end of expiration. The collapsed alveoli can be re-opened by inspiration, but this re-opening causes shear stress on cells covering the alveoli. This can damage the alveolar-capillary membrane integrity, allowing blood components to enter the alveolar airspace. Blood components, such as albumin, can interact with the lung surfactant and further inhibit its function. The collapse of the alveoli is responsible for a decrease in the surface area available for blood oxygenation, and it reduces the volume of air that can be inhaled and exhaled. These different key events lead to decreased lung function, characterized by clinical signs of respiratory toxicity and reduced blood oxygenation. Here we present the weight of evidence that supports the AOP, and we give an overview of the methods available in vitro and in vivo to measure each key event of the pathway, and how this AOP can potentially be used in screening for inhalation toxicity.

Airborne Transmission of COVID-19: Aerosol Dispersion, Lung Deposition, and Virus-Receptor Interactions

Coronavirus disease 2019 (COVID-19), due to infection by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), is now causing a global pandemic. Aerosol transmission of COVID-19, although plausible, has not been confirmed by the World Health Organization (WHO) as a general transmission route. Considering the rapid spread of SARS-CoV-2, especially nosocomial outbreaks and other superspreading events, there is an urgent need to study the possibility of airborne transmission and its impact on the lung, the primary body organ attacked by the virus. Here, we review the complete pathway of airborne transmission of SARS-CoV-2 from aerosol dispersion in air to subsequent biological uptake after inhalation. In particular, we first review the aerodynamic and colloidal mechanisms by which aerosols disperse and transmit in air and deposit onto surfaces. We then review the fundamental mechanisms that govern regional deposition of micro- and nanoparticles in the lung. Focus is given to biophysical interactions between particles and the pulmonary surfactant film, the initial alveolar-capillary barrier and first-line host defense system against inhaled particles and pathogens. Finally, we summarize the current understanding about the structural dynamics of the SARS-CoV-2 spike protein and its interactions with receptors at the atomistic and molecular scales, primarily as revealed by molecular dynamics simulations. This review provides urgent and multidisciplinary knowledge toward understanding the airborne transmission of SARS-CoV-2 and its health impact on the respiratory system.

Lipid and protein corona of food-grade TiO2 nanoparticles in simulated gastrointestinal digestion

In the presence of biological matrices, engineered nanomaterials, such as TiO₂, develop a biomolecular corona composed of lipids, proteins, etc. In this study, we analyzed the biocorona formed on the food grade TiO₂ (E171) going through an in vitro simulated gastrointestinal digestion system in either a fasting food model (FFM), a standardized food model (SFM), or a high fat food model (HFFM). Lipids and proteins were extracted from the biocorona and underwent untargeted lipidomic and label-free shotgun proteomic analyses. Our results showed that the biocorona composition was different before and after food digestion. After digestion, more diverse lipids were adsorbed compared to proteins, most of which were the enzymes added to the simulated digestion system. The corona lipid profile was distinct from the digested food model they presented in, although similarity in the lipid profiles between the corona and the food matrix increased with the fat content in the food model. The corona formed in the two low-fat environments of FFM and SFM shared a higher degree of similarity while very different from their corresponding matrix, with some lipid species adsorbed with high enrichment factors, indicating specific interaction with the TiO₂ surface outperforming lipid matrix concentration in determination of corona formation. Formation of the biocorona may have contributed to the reduced oxidative stress as well as toxicological impacts observed in cellular studies. The present work is the first to confirm persistent adsorption of biomolecules could occur on ingested nanomaterials in food digestae. More future studies are needed to study the in vivo impacts of the biocorona, and shed lights on how the biocorona affects the biotransformations and fate of the ingested nanomaterials, which may impose impacts on human health.

Acute Inhalation Toxicity After Inhalation of ZnO Nanoparticles: Lung Surfactant Function Inhibition In Vitro Correlates With Reduced Tidal Volume in Mice

People can be exposed to zinc oxide (ZnO) by inhalation of consumer products or during industrial processes. Zinc oxide nanoparticle (NP) exposure can induce acute inhalation toxicity. The toxicological mechanisms underlying the acute effects on the lungs have long focused on the phagolysosomal dissolution of ZnO NPs in macrophages followed by the release of free Zn2+ ions. However, we postulate an alternative mechanism based on the direct interaction of ZnO NPs with the lung surfactant (LS) layer covering the inside of the alveoli. Therefore, we tested the effect of ZnO NPs and Zn2+ ions on the function of LS in vitro using the constrained drop surfactometer. We found that the ZnO NPs inhibited the LS function, whereas Zn2+ ions did not. To examine the role of lung macrophages in the acute toxicity of inhaled ZnO NPs, mice were treated with Clodrosome, a drug that depletes alveolar macrophages, or Encapsome, the empty carrier of the drug. After macrophage depletion, the mice were exposed to an aerosol of ZnO NPs in whole body plethysmographs recording breathing patterns continuously. Mice in both groups developed shallow breathing (reduced tidal volume) shortly after the onset of exposure to ZnO NPs. This suggests a macrophage-independent mechanism of induction. This study shows that acute inhalation toxicity is caused by ZnO NP interaction with LS, independently of NP dissolution in macrophages.

Atomic Force Microscopy Imaging of Adsorbed Pulmonary Surfactant Films

Pulmonary surfactant (PS) is a lipid-protein complex that adsorbs to the air-water surface of the lung as a thin film. Previous studies have suggested that the adsorbed PS film is composed of an interfacial monolayer, plus a functionally attached vesicular complex, called the surface-associated surfactant reservoir. However, direct visualization of the lateral structure and morphology of adsorbed PS films using atomic force microscopy (AFM) has been proven to be technically challenging. To date, all AFM studies of the PS film have relied on the model of Langmuir monolayers. Here, we showed the first, to our knowledge, AFM imaging of adsorbed PS films under physiologically relevant conditions using a novel, to our knowledge, experimental methodology called constrained drop surfactometry. In conjunction with a series of methodological innovations, including subphase replacement, in situ Langmuir-Blodgett transfer, and real-time surface tension control using closed-loop axisymmetric drop shape analysis, constrained drop surfactometry allowed the study of lateral structure and topography of animal-derived natural PS films at physiologically relevant low surface tensions. Our data suggested that a nucleation-growth model is responsible for the adsorption-induced squeeze-out of the PS film, which likely results in an interfacial monolayer enriched in dipalmitoylphosphatidylcholine with the attached multilayered surface-associated surfactant reservoir. These findings were further supported by frequency-dependent measurements of surface dilational rheology. Our study provides novel, to our knowledge, biophysical insights into the understanding of the mechanisms by which the PS film attains low surface tensions and stabilizes the alveolar surface.

Adverse outcome pathways as a tool for the design of testing strategies to support the safety assessment of emerging advanced materials at the nanoscale

Toxicity testing and regulation of advanced materials at the nanoscale, i.e. nanosafety, is challenged by the growing number of nanomaterials and their property variants requiring assessment for potential human health impacts. The existing animal-reliant toxicity testing tools are onerous in terms of time and resources and are less and less in line with the international effort to reduce animal experiments. Thus, there is a need for faster, cheaper, sensitive and effective animal alternatives that are supported by mechanistic evidence. More importantly, there is an urgency for developing alternative testing strategies that help justify the strategic prioritization of testing or targeting the most apparent adverse outcomes, selection of specific endpoints and assays and identifying nanomaterials of high concern. The Adverse Outcome Pathway (AOP) framework is a systematic process that uses the available mechanistic information concerning a toxicological response and describes causal or mechanistic linkages between a molecular initiating event, a series of intermediate key events and the adverse outcome. The AOP framework provides pragmatic insights to promote the development of alternative testing strategies. This review will detail a brief overview of the AOP framework and its application to nanotoxicology, tools for developing AOPs and the role of toxicogenomics, and summarize various AOPs of relevance to inhalation toxicity of nanomaterials that are currently under various stages of development. The review also presents a network of AOPs derived from connecting all AOPs, which shows that several adverse outcomes induced by nanomaterials originate from a molecular initiating event that describes the interaction of nanomaterials with lung cells and involve similar intermediate key events. Finally, using the example of an established AOP for lung fibrosis, the review will discuss various in vitro tests available for assessing lung fibrosis and how the information can be used to support a tiered testing strategy for lung fibrosis. The AOPs and AOP network enable deeper understanding of mechanisms involved in inhalation toxicity of nanomaterials and provide a strategy for the development of alternative test methods for hazard and risk assessment of nanomaterials.

Interaction of industrial smelting soot particles with pulmonary surfactant: Pulmonary toxicity of heavy metal-rich particles

Inhalable particles can influence the interfacial behavior of pulmonary surfactant (PS) resulting in various pulmonary diseases. However, the effects of actually airborne particles on the interfacial behavior of PS and its role in the alteration for soluble metal fraction in particles are entirely unexplored. Herein, we investigated the interaction of PS extracted from porcine lungs with smelting soot fine particles as a model of inhaled heavy metal-rich particles. Our results showed that the phase behavior and foamability of PS were obviously altered in the presence of smelting soot fine particles. In addition, the soluble heavy metals in smelting soot fine particles notably increased in the presence of PS as compared to that of saline solution. Further experiments conducted by adding PS's major components (dipalmitoylphosphatidylcholine, DPPC; bovine serum albumin, BSA) demonstrated that comparison of DPPC, adsorbed BSA is beneficial for the dissolution of heavy metals in smelting soot fine particles. Dynamic light scattering experiments verified that the well dispersion of smelting soot fine particles in the presence of BSA may be responsible for the higher solubility of heavy metals. These findings indicate that PS's interfacial behavior change and PS-enhanced solubilization release of metal components may increase the potentially pulmonary risk in the exposure of airborne fine particles enriched with heavy metals.

Dynamic surface tension of xylem sap lipids

The surface tension of xylem sap has been traditionally assumed to be close to that of the pure water because decreasing surface tension is thought to increase vulnerability to air seeding and embolism. However, xylem sap contains insoluble lipid-based surfactants, which also coat vessel and pit membrane surfaces, where gas bubbles can enter xylem under negative pressure in the process known as air seeding. Because of the insolubility of amphiphilic lipids, the surface tension influencing air seeding in pit pores is not the equilibrium surface tension of extracted bulk sap but the local surface tension at gas-liquid interfaces, which depends dynamically on the local concentration of lipids per surface area. To estimate the dynamic surface tension in lipid layers that line surfaces in the xylem apoplast, we studied the time-dependent and surface area-regulated surface tensions of apoplastic lipids extracted from xylem sap of four woody angiosperm plants using constrained drop surfactometry. Xylem lipids were found to demonstrate potent surface activity, with surface tensions reaching an equilibrium at ~25 mN m-1 and varying between a minimum of 19 mN m-1 and a maximum of 68 mN m-1 when changing the surface area between 50 and 160% around the equilibrium surface area. It is concluded that xylem lipid films in natural conditions most likely range from nonequilibrium metastable conditions of a supersaturated compression state to an undersaturated expansion state, depending on the local surface areas of gas-liquid interfaces. Together with findings that maximum pore constrictions in angiosperm pit membranes are much smaller than previously assumed, low dynamic surface tension in xylem turns out to be entirely compatible with the cohesion-tension and air-seeding theories, as well as with the existence of lipid-coated nanobubbles in xylem sap, and with the range of vulnerabilities to embolism observed in plants.

Nanoparticle translocation across the lung surfactant film regulated by grafting polymers

Nanoparticle-based pulmonary drug delivery has gained significant attention due to its ease of administration, increased bioavailability, and reduced side effects caused by a high systemic dosage. After being delivered into the deep lung, the inhaled nanoparticles first interact with the lung surfactant lining layer composed of phospholipids and surfactant proteins and then potentially cause the dysfunction of the lung surfactant. Conditioning the surface properties of nanoparticles with grafting polymers to avoid these side effects is of crucial importance to the efficiency and safety of pulmonary drug delivery. Herein, we perform coarse-grained molecular simulations to decipher the involved mechanism responsible for the translocation of the polymer-grafted Au nanoparticles across the lung surfactant film. The simulations illustrate that conditioning of the grafting polymers, including their length, terminal charge, and grafting density, can result in different translocation processes. Based on the energy analysis, we find that these discrepancies in translocation stem from the affinity of the nanoparticles with the lipid tails and heads and their contact with the proteins, which can be tuned by the surface polarity and surface charge of the nanoparticles. We further demonstrate that the interaction between the nanoparticles and the lung surfactant is related to the depletion of the lipids and proteins during translocation, which affects the surface tension of the surfactant film. The change in the surface tension in turn affects the nanoparticle translocation and the collapse of the surfactant film. These results can help understand the adverse effects of the nanoparticles on the lung surfactant film and provide guidance to the design of inhaled nanomedicines for improved permeability and targeting.

Adsorption of Phospholipids at the Air-Water Surface

Phospholipids are ubiquitous components of biomembranes and common biomaterials used in many bioengineering applications. Understanding adsorption of phospholipids at the air-water surface plays an important role in the study of pulmonary surfactants and cell membranes. To date, however, the biophysical mechanisms of phospholipid adsorption are still unknown. It is challenging to reveal the molecular structure of adsorbed phospholipid films. Using combined experiments with constrained drop surfactometry and molecular dynamics simulations, here, we studied the biophysical mechanisms of dipalmitoylphosphatidylcholine (DPPC) adsorption at the air-water surface. It was found that the DPPC film adsorbed from vesicles showed distinct equilibrium surface tensions from the DPPC monolayer spread via organic solvents. Our simulations revealed that only the outer leaflet of the DPPC vesicle is capable of unzipping and spreading at the air-water surface, whereas the inner leaflet remains intact and forms an inverted micelle to the interfacial monolayer. This inverted micelle increases the local curvature of the monolayer, thus leading to a loosely packed monolayer at the air-water surface and hence a higher equilibrium surface tension. These findings provide novel insights, to our knowledge, into the mechanism of the phospholipid and pulmonary surfactant adsorption and may help understand the structure-function correlation in biomembranes.

An Optical Method for Quantitatively Determining the Surface Free Energy of Micro- and Nanoparticles

Surface free energy (SFE) of micro- and nanoparticles plays a crucial role in determining the hydrophobicity and wettability of the particles. To date, however, there are no easy-to-use methods for determining the SFE of particles. Here, with the application of several inexpensive, easy-to-use, and commonly available lab procedures and facilities, including particle dispersion, settling/centrifugation, pipetting, and visible-light spectroscopy, we developed a novel technique called the maximum particle dispersion (MPD) method for quantitatively determining the SFE of micro- and nanoparticles. We demonstrated the versatility and robustness of the MPD method by studying nine representative particles of various chemistries, sizes, dimensions, and morphologies. These are triethoxycaprylylsilane-coated zinc oxide nanoparticles, multiwalled carbon nanotubes, graphene nanoplatelets, molybdenum(IV) sulfide flakes, neodymium(III) oxide nanoparticles, two sizes of zeolites, poly(vinylpolypyrrolidone), and polystyrene microparticles. The SFE of these micro- and nanoparticles was found to cover a range from 21 to 36 mJ/m². These SFE values may find applications in a broad spectrum of scientific disciplines including the synthesis of these nanomaterials, such as in liquid-phase exfoliation. The MPD method has the potential to be developed into a standard, low-cost, and easy-to-use method for quantitatively characterizing the SFE and hydrophobicity of particles at the micro- and nanoscale.

Poly(amidoamine) Dendrimer as a Respiratory Nanocarrier: Insights from Experiments and Molecular Dynamics Simulations

Pulmonary drug delivery is superior to the systemic administration in treating lung diseases. An optimal respiratory nanocarrier should be able to efficiently and safely cross the pulmonary surfactant film, which serves as the first biological barrier for respiratory delivery and plays paramount roles in maintaining the proper mechanics of breathing. In this work, we focused on the interactions between poly(amidoamine) (PAMAM) dendrimers and a model pulmonary surfactant. With combined Langmuir monolayer experiments and coarse-grained molecular dynamics simulations, we studied the effect of environmental temperature, size, and surface property of PAMAM dendrimers (G3-OH, G3-NH₂, G5-OH, and G5-NH₂) on the dipalmitoylphosphatidylcholine (DPPC) monolayer. Our simulations indicated that the environmental temperature could significantly affect the influence of PAMAM dendrimers on the DPPC monolayer. Therefore, results obtained at room temperature cannot be directly applied to elucidate interactions at body temperature. Simulations at body temperature found that all tested PAMAM dendrimers can easily penetrate the lipid monolayer during the monolayer expansion process (mimicking "inhalation"), and the cationic PAMAM dendrimers (-NH₂) show promising penetration ability during the monolayer compression process (mimicking "expiration"). Larger PAMAM dendrimers (G5) adsorbed onto the lipid monolayer tend to induce structural collapse and inhibit normal phase transitions of the lipid monolayer. These adverse effects could be mitigated in the subsequent expansion-compression cycle. These findings suggest that the PAMAM dendrimer may be used as a potential respiratory drug nanocarrier.