An optimized protocol for assessment of sputum macrorheology in health and muco-obstructive lung disease

Optimized Artificial Colonic Mucus Enabling Physiologically Relevant Diffusion Studies of Drugs, Particles, and Delivery Systems

Development of oral drug delivery systems that penetrate the colonic mucus remains challenging. Artificial models of porcine colonic mucus have been developed that mimic the rheology and viscosity of the native mucus and its contents of mucins, protein, and lipids. However, they are less representative with regard to the zeta potential, a factor of importance for charged molecules and particles. This study therefore aimed to improve the existing porcine artificial colonic mucus model by exchanging the polymer backbone (used for viscosity) to more closely mimic the charge of porcine native colonic mucus. Polymers studied were poly(acrylic acid), hydroxyethylcellulose, sodium hyaluronate, sodium alginate, and pectin. The resulting porcine artificial colonic mucus was assayed for apparent viscosity, storage modulus, pH, water content, zeta potential, and pore size. The two best-performing polymers (poly(acrylic acid) and hydroxyethylcellulose) were then assayed with diffusion of FITC-dextran, particle tracking of nanoparticles, and binding of FITC-dextran and contrasted to data generated in porcine native colonic mucus (PNCM). Of the two polymers, PACM based on HEC generated zeta potential and binding kinetics similar to those of PNCM. We conclude that the choice of polymer in PACMs is critical for improving their use in drug development. The extensive characterization of the PACMs further points toward the importance of complementary techniques to determine rheological characteristics, mesh, and pore size.

Photo-regulated disulfide crosslinking: a versatile approach to construct mucus-inspired hydrogels

The remarkable defensive ability of native mucus against pathogens has encouraged scientists to map its structure--property correlation and its influence on immune defense mechanisms. However, its poorly defined structure, source-dependent composition, and low availability limit the usefulness of native mucus in the laboratory. This gap creates a strong demand for the development of synthetic mucus-mimetic materials. Here, we report a straightforward strategy for constructing mucus-mimetic hydrogels through photo-regulated disulfide crosslinking. Light-responsive 1,2-dithiolane attached to a linear polyglycerol sulfate (lPGS) backbone allows the macromolecular building blocks to crosslink and form the hydrogel, which mirrors the chemistry of native mucus hydrogel formation with its disulfide-linked mucin chains. The viscoelastic properties of the hydrogel can be easily tuned by controlling both the light exposure time and the number of 1,2-dithiolane units within the polymer backbone. Furthermore, localized UV irradiation allows for spatially resolved hydrogel formation. Importantly, this synthetic polymer can directly crosslink with native mucin, bovine submaxillary mucin (BSM), to convert it into a hydrogel at physiological pH. The versatility of this approach - hydrogel formation via photo-regulated disulfide crosslinking - can be used to develop a synthetic mucus model.

Modeling Cystic Fibrosis Chronic Infection Using Engineered Mucus-like Hydrogels

The airway mucus of patients with cystic fibrosis has altered properties, which create a microenvironment primed for chronic infections that are difficult to treat. These complex polymicrobial airway infections and corresponding mammalian-microbe interactions are challenging to model in vitro. Here, we report the development of mucus-like hydrogels with varied compositions and viscoelastic properties reflecting differences between healthy and cystic fibrosis airway mucus. Models of cystic fibrosis and healthy airway microenvironments were created by combining the hydrogels with relevant pathogens, human bronchial epithelial cells, and an antibiotic. Notably, pathogen antibiotic resistance was not solely dependent on the altered properties of the mucus-like hydrogels but was also influenced by culture conditions including microbe species, monomicrobial or polymicrobial culture, and the presence of epithelial cells. Additionally, the cystic fibrosis airway model showed the ability to mimic features characteristic of chronic cystic fibrosis airway infections including sustained polymicrobial growth and increased antibiotic tolerance.

A head-to-head comparison of polymer interaction with mucin from porcine stomach and bovine submaxillary glands

Native mucus is heterogeneous, displays high inter-individual variation and is prone to changes during harvesting and storage. To overcome the lack of reproducibility and availability of native mucus, commercially available purified mucins, porcine gastric mucin (PGM) and mucin from bovine submaxillary gland (BSM), have been widely used. However, the question is to which extent the choice of mucin matters in studies of their interaction with polymers as their composition, structure and hence physicochemical properties differ. Accordingly, the interactions between PGM or BSM with two widely used polymers in drug delivery, polyethylene oxide and chitosan, was studied with orthogonal methods: turbidity, dynamic light scattering, and quartz crystal microbalance with dissipation monitoring. Polymer binding and adsorption to the two commercially available and purified mucins, PGM and BSM, is different depending on the mucin type. PEO, known to interact weakly with mucin, only displayed limited interaction with both mucins as confirmed by all employed methods. In contrast, chitosan was able to bind to both PGM and BSM. Interestingly, the results suggest that chitosan interacts with BSM to a greater extent than with PGM indicating that the choice of mucin, PGM or BSM, can affect the outcome of studies of mucin interactions with polymers.

Thiolated polyglycerol sulfate as potential mucolytic for muco-obstructive lung diseases

Increased disulfide crosslinking of secreted mucins causes elevated viscoelasticity of mucus and is a key determinant of mucus dysfunction in patients with cystic fibrosis (CF) and other muco-obstructive lung diseases. In this study, we describe the synthesis of a novel thiol-containing, sulfated dendritic polyglycerol (dPGS-SH), designed to chemically reduce these abnormal crosslinks, which we demonstrate with mucolytic activity assays in sputum from patients with CF. This mucolytic polymer, which is based on a reportedly anti-inflammatory polysulfate scaffold, additionally carries multiple thiol groups for mucolytic activity and can be produced on a gram-scale. After a physicochemical compound characterization, we compare the mucolytic activity of dPGS-SH to the clinically approved N-acetylcysteine (NAC) using western blot studies and investigate the effect of dPGS-SH on the viscoelastic properties of sputum samples from CF patients by oscillatory rheology. We show that dPGS-SH is more effective than NAC in reducing multimer intensity of the secreted mucins MUC5B and MUC5AC and demonstrate significant mucolytic activity by rheology. In addition, we provide data for dPGS-SH demonstrating a high compound stability, low cytotoxicity, and superior reaction kinetics over NAC at different pH levels. Our data support further development of the novel reducing polymer system dPGS-SH as a potential mucolytic to improve mucus function and clearance in patients with CF as well as other muco-obstructive lung diseases.

Mucus-Inspired Self-Healing Hydrogels: A Protective Barrier for Cells against Viral Infection

Mucus is a dynamic biological hydrogel, composed primarily of the glycoprotein mucin, exhibits unique biophysical properties and forms a barrier protecting cells against a broad-spectrum of viruses. Here, this work develops a polyglycerol sulfate-based dendronized mucin-inspired copolymer (MICP-1) with ≈10% repeating units of activated disulfide as cross-linking sites. Cryo-electron microscopy (Cryo-EM) analysis of MICP-1 reveals an elongated single-chain fiber morphology. MICP-1 shows potential inhibitory activity against many viruses such as herpes simplex virus 1 (HSV-1) and SARS-CoV-2 (including variants such as Delta and Omicron). MICP-1 produces hydrogels with viscoelastic properties similar to healthy human sputum and with tuneable microstructures using linear and branched polyethylene glycol-thiol (PEG-thiol) as cross-linkers. Single particle tracking microrheology, electron paramagnetic resonance (EPR) and cryo-scanning electron microscopy (Cryo-SEM) are used to characterize the network structures. The synthesized hydrogels exhibit self-healing properties, along with viscoelastic properties that are tuneable through reduction. A transwell assay is used to investigate the hydrogel's protective properties against viral infection against HSV-1. Live-cell microscopy confirms that these hydrogels can protect underlying cells from infection by trapping the virus, due to both network morphology and anionic multivalent effects. Overall, this novel mucin-inspired copolymer generates mucus-mimetic hydrogels on a multi-gram scale. These hydrogels can be used as models for disulfide-rich airway mucus research, and as biomaterials.

Ileal mucus viscoelastic properties differ in Crohn's disease

Crohn's disease (CD) is an inflammatory bowel disease that can affect any part of the gastrointestinal tract, frequently involving the terminal ileum. While colonic mucus alterations in CD patients have been described, terminal ileal mucus and its mechanobiological properties have been neglected. Our study is the first of its kind to decipher the viscoelastic and network properties of ileal mucus. With that aim, oscillatory rheological shear measurements based on an airway mucus protocol that was thoroughly validated for ileal mucus were performed. Our pilot study analyzed terminal ileum mucus from controls (n = 14) and CD patients (n = 14). Mucus network structure was visualized by scanning electron microscopy. Interestingly, a statistically significant increase in viscoelasticity as well as a decrease in mesh size was observed in ileal mucus from CD patients compared to controls. Furthermore, rheological data were analyzed in relation to study participants' clinical characteristics, revealing a noteworthy trend between non-smokers and smokers. In conclusion, this study provides the first data on the viscoelastic properties and structure of human ileal mucus in the healthy state and Crohn's disease, demonstrating significant alterations between groups and highlighting the need for further research on mucus and its effect on the underlying epithelial barrier.

Comprehensive Characterization of the Viscoelastic Properties of Bovine Submaxillary Mucin (BSM) Hydrogels and the Effect of Additives

This study presents a comprehensive characterization of the viscoelastic and structural properties of bovine submaxillary mucin (BSM), which is widely used as a commercial source to conduct mucus-related research. We conducted concentration studies of BSM and examined the effects of various additives, NaCl, CaCl2, MgCl2, lysozyme, and DNA, on its rheological behavior. A notable connection between BSM concentration and viscoelastic properties was observed, particularly under varying ionic conditions. The rheological spectra could be well described by a fractional Kelvin-Voigt model with a minimum of model parameters. A detailed proteomics analysis provided insight into the protein, especially mucin composition within BSM, showing MUC19 as the main component. Cryo-scanning electron microscopy enabled the visualization of the porous BSM network structure. These investigations give us a more profound comprehension of the BSM properties, especially those pertaining to viscoelasticity, and how they are influenced by concentration and environmental conditions, aspects relevant to the field of mucus research.

Exploring Mechanisms of Lipid Nanoparticle-Mucus Interactions in Healthy and Cystic Fibrosis Conditions

Mucus forms the first defense line of human lungs, and as such hampers the efficient delivery of therapeutics to the underlying epithelium. This holds particularly true for genetic cargo such as CRISPR-based gene editing tools which cannot readily surmount the mucosal barrier. While lipid nanoparticles (LNPs) emerge as versatile non-viral gene delivery systems that can help overcome the delivery challenge, many knowledge gaps remain, especially for diseased states such as cystic fibrosis (CF). This study provides fundamental insights into Cas9 mRNA or ribonucleoprotein-loaded LNP-mucus interactions in healthy and diseased states by assessing the impact of the genetic cargo, mucin sialylation, mucin concentration, ionic strength, pH, and polyethylene glycol (PEG) concentration and nature on LNP diffusivity leveraging experimental approaches and Brownian dynamics (BD) simulations. Taken together, this study identifies key mucus and LNP characteristics that are critical to enabling a rational LNP design for transmucosal delivery.

Longitudinal effects of elexacaftor/tezacaftor/ivacaftor on sputum viscoelastic properties, airway infection and inflammation in patients with cystic fibrosis

BACKGROUND

Recent studies demonstrated that the triple combination cystic fibrosis transmembrane conductance regulator (CFTR) modulator therapy elexacaftor/tezacaftor/ivacaftor (ETI) improves lung function and reduces pulmonary exacerbations in cystic fibrosis (CF) patients with at least one F508del allele. However, effects of ETI on downstream consequences of CFTR dysfunction, i.e. abnormal viscoelastic properties of airway mucus, chronic airway infection and inflammation have not been studied. The aim of this study was to determine the longitudinal effects of ETI on airway mucus rheology, microbiome and inflammation in CF patients with one or two F508del alleles aged ≥12 years throughout the first 12 months of therapy.

METHODS

In this prospective observational study, we assessed sputum rheology, the microbiome, inflammation markers and proteome before and 1, 3 and 12 months after initiation of ETI.

RESULTS

In total, 79 patients with CF and at least one F508del allele and 10 healthy controls were enrolled in this study. ETI improved the elastic modulus and viscous modulus of CF sputum at 3 and 12 months after initiation (all p<0.01). Furthermore, ETI decreased the relative abundance of Pseudomonas aeruginosa in CF sputum at 3 months and increased the microbiome α-diversity at all time points. In addition, ETI reduced interleukin-8 at 3 months (p<0.05) and free neutrophil elastase activity at all time points (all p<0.001), and shifted the CF sputum proteome towards healthy.

CONCLUSIONS

Our data demonstrate that restoration of CFTR function by ETI improves sputum viscoelastic properties, chronic airway infection and inflammation in CF patients with at least one F508del allele over the first 12 months of therapy; however, levels close to healthy were not reached.

Sputum handling for rheology

The rheology of sputum is viewed as a powerful emerging biophysical marker for monitoring muco-obstructive pulmonary diseases such as cystic fibrosis (CF) and non-CF bronchiectasis (NCFB). However, there is no unified practice to process sputa from collection to analysis, which can lead to highly variable, and sometimes inconsistent results. The main objective of this study is to bring light into the handling of sputum samples to establish a standardised and robust protocol before rheological measurements. Sputum collected from 22 CF and 10 NCFB adults, was divided into control (vortexed and fresh: non-heated and non-frozen) and three treated conditions (either non-vortexed, heated or frozen). In addition, 6 CF expectorations were used to study the dynamics of ageing over 24 h. Sputum's mechanical properties were measured with a rotational rheometer to obtain their properties at rest, elastic ([Formula: see text]) and viscous moduli ([Formula: see text]), and at the onset of flow, critical deformation ([Formula: see text]) and critical stress ([Formula: see text]). We demonstrate that heating sputum is completely destructive while freezing sputa at [Formula: see text] has no discernible effect on their rheology. We also show that the variability of rheological measurements largely resulted from the sample's macroscopic heterogeneity, and can be greatly reduced by non-destructive vortex homogenisation. Finally, we observed contrasted ageing effects as a fonction of purulence: while the viscoelasticity of purulent samples reduced by half within 6 h after collection, semi-purulent samples did not evolve. These results guide towards a robust unified protocol for simple sputum handling in rheometry. We therefore suggest to vortex and snap freeze sputum samples immediately after collection when direct testing is not possible.