The biology of mucus: Composition, synthesis and organization

Effect of dietary isoleucine on skin mucus barrier and epithelial physical barrier functions of hybrid bagrid catfish Pelteobagrus vachelli × Leiocassis longirostris

The study investigated the effects of dietary isoleucine (Ile) on skin mucus barrier and epithelial physical barrier functions of hybrid bagrid catfish Pelteobagrus vachelli × Leiocassis longirostris. A total of 630 fish (33.11 ± 0.09 g) were fed semi-purified isonitrogenous diets containing 5.0 (control), 7.5, 10.0, 12.5, 15.0, 17.5, and 20.0 g Ile kg ^-1 diet for 8 weeks. The results indicated that dietary Ile increased (P < 0.05) in skin (1) mucus protein content and antimicrobial activity against three gram-negative bacteria (Aeromonas hydrophila, Escherichia coli, and Yersinia ruckeri) and two gram-positive bacteria (Streptococcus agalactiae and Staphylococcus aureus), (2) mucus lysofew information is available about the influencezyme (LZM), acid phosphatase (ACP), and alkaline phosphatase (AKP) activities, and complement 3 and 4 (C3 and C4) and immunoglobulin M (IgM) contents, (3) intelectin 1 (intl1), intelectin 2 (intl2), c-type-lysozyme (c-LZM), g-type-lysozyme (g-LZM), and β-defensin mRNA levels. Dietary Ile decreased (P < 0.05) reactive oxygen species (ROS), malondialdehyde (MDA), and protein carbonyl (PC) contents, and up-regulated (P < 0.05) CuZnSOD, GST, GPX1a, muc5ac, muc5b, zonula occludens-1 (ZO-1), zonula occludens-2 (ZO-2), occludin, and claudin 3 mRNA levels in skin. These results indicated that Ile improved skin mucus barrier function via increasing mucus protein, C3 and C4, and IgM contents and antibacterial factors activities, and promoted epithelial physical barrier function via decreasing skin antioxidant damage and improving tight junction structure in hybrid bagrid catfish.

Soft matter science and the COVID-19 pandemic

Much of the science underpinning the global response to the COVID-19 pandemic lies in the soft matter domain. Coronaviruses are composite particles with a core of nucleic acids complexed to proteins surrounded by a protein-studded lipid bilayer shell. A dominant route for transmission is via air-borne aerosols and droplets. Viral interaction with polymeric body fluids, particularly mucus, and cell membranes controls their infectivity, while their interaction with skin and artificial surfaces underpins cleaning and disinfection and the efficacy of masks and other personal protective equipment. The global response to COVID-19 has highlighted gaps in the soft matter knowledge base. We survey these gaps, especially as pertaining to the transmission of the disease, and suggest questions that can (and need to) be tackled, both in response to COVID-19 and to better prepare for future viral pandemics.

Lactobacillus plantarum KSFY06 and geniposide counteract montmorillonite-induced constipation in Kunming mice

Constipation is a common clinical manifestation of digestive system disorders and occurs worldwide. This study investigated the ability of Lactobacillus plantarum KSFY06 (LP-KSFY06) to promote the action of geniposide in preventing montmorillonite-induced constipation in Kunming mice, with the aim of providing a successful solution. The effects of LP-KSFY06 and geniposide on constipation were measured, and the results showed that the protective effect of geniposide on constipation was enhanced by LP-KSFY06 and that the combination resulted in increased weight, moisture content, and particle number of feces. The first black stool defecation time was decreased from 182 min to 87 min, which clearly indicates that defecating difficulty was alleviated in constipated mice. The synergic intervention of LP-KSFY06 and geniposide (LP + G) assisted in maintaining the body weight of constipated mice. The LP + G intervention significantly increased serum levels of motilin (MTL, 167.8 pg/ml), acetylcholinesterase (AChE, 45.3 pg/ml), substance P (SP, 61.0 pg/ml), vasoactive intestinal peptide (VIP, 70.5 pg/ml), endothelin-1 (ET-1, 16.1 pg/ml), and gastrin (73.0 pg/ml) and remarkably decreased somatostatin (SS, 35.2 pg/ml) when compared to those indexes in the LP-KSFY06 group and geniposide group. The LP + G treatment also significantly increased the mRNA expression of cluster of differentiation 117 (c-Kit), stem cell factor (SCF), glial cell-derived neurotrophic factor (GDNF), and remarkably downregulated the expression of inducible nitric oxide synthase (iNOS), transient receptor potential vanilloid-1 (TRPV1), and cyclooxygenase-2 (COX-2). The experimental results showed that the combination treatment has the strongest prevention effect against constipation, and LP-KSFY06 promotes the ability of geniposide to prevent constipation. Therefore, LP-KSFY06 is a potential probiotic strain with the capacity to prevent montmorillonite-induced constipation.

Heterogeneous antimicrobial activity in broncho-alveolar aspirates from mechanically ventilated intensive care unit patients

Pneumonia is an infection of the lungs, where the alveoli in the affected area are filled with pus and fluid. Although ventilated patients are at risk, not all ventilated patients develop pneumonia. This suggests that the sputum environment may possess antimicrobial activities. Despite the generally acknowledged importance of antimicrobial activity in protecting the human lung against infections, this has not been systematically assessed to date. Therefore, the objective of the present study was to measure antimicrobial activity in broncho-alveolar aspirate (‘sputum”) samples from patients in an intensive care unit (ICU) and to correlate the detected antimicrobial activity with antibiotic levels, the sputum microbiome, and the respective patients’ characteristics. To this end, clinical metadata and sputum were collected from 53 mechanically ventilated ICU patients. The antimicrobial activity of sputum samples was tested against Streptococcus pneumoniae, Staphylococcus aureus and Streptococcus anginosus. Here we show that sputa collected from different patients presented a high degree of variation in antimicrobial activity, which can be partially attributed to antibiotic therapy. The sputum microbiome, although potentially capable of producing antimicrobial agents, seemed to contribute in a minor way, if any, to the antimicrobial activity of sputum. Remarkably, despite its potentially protective effect, the level of antimicrobial activity in the investigated sputa correlated inversely with patient outcome, most likely because disease severity outweighed the beneficial antimicrobial activities.

Metal particles in mucus and hypertrophic tissue of the inferior nasal turbinates from the human upper respiratory tract

Mucosal surfaces are the first mechanical barrier preventing the entry of foreign particles into the organism. The study addresses the detection and analysis of metal-based solid particles in cytological mucus samples from the surface of human hypertrophic tissue in the inferior nasal turbinates in patients diagnosed with chronic rhinitis. Solid particles were characterized by scanning electron microscopy and Raman microspectroscopy; all the biological samples were also subjected to vibration magnetometry. Since the upper airways are the first part of the respiratory tract, which is exposed to inhaled particles, it can be assumed that inhaled particles may be partially deposited in this region. Scanning electron microscopy revealed the presence of metal-based solid particles/clusters in the majority of the analysed cytological mucus samples and also in hypertrophic tissues; in all groups, the particles were of submicron size. Raman microspectroscopy detected the presence of particles/clusters based on amorphous carbon, graphite, calcium carbonate, anatase and barite only in the hypertrophic tissue. The obtained results show that the composition of some of the solid particles (i.e. Ba, Zn, Fe and Ti) detected in the mucus from the surface of the hypertrophic tissues resembled the particles found in the hypertrophic tissue itself. It can be assumed that after the capture of the inhaled particles by the mucus, they penetrate into the deeper layers of tissue.

Exploring the attachment of the Mediterranean medicinal leech (Hirudo verbana) to porous substrates

Haematophagous ectoparasites must ensure a reliable hold to their host during blood meals and, therefore, have evolved a broad spectrum of versatile and effective attachment mechanisms. The Mediterranean medicinal leech (Hirudo verbana), for example, uses suction on both smooth and textured air-tight substrates. However, preliminary studies showed that H. verbana is also capable of attaching itself to air-permeable substrates, where suction does not work. Using high-speed videography and mechanical tests, we comparatively investigated the attachment of H. verbana on both smooth and textured air-tight as well as on porous artificial substrates, also considering the influence of mucus on sucker surfaces. In general, the leech-specific locomotion cycle did not differ between the tested surfaces, and the leeches were able to reliably attach to both air-tight and porous substrates. From our results, we conclude that suction is presumably the primary attachment mechanism of H. verbana. However, secondary mechanisms such as mechanical interlocking with surface asperities and pores or capillary forces occurring at the interface between the mucus-covered suckers and the substratum are also employed. In any case, the rich repertoire of applicable attachment principles renders the organs of H. verbana functionally highly resilient.

Mucus-Penetrating Particles and the Role of Ocular Mucus as a Barrier to Micro- and Nanosuspensions

The ocular surface is naturally covered with a layer of mucus. Along with other functions, this mucus layer serves to trap and eliminate foreign substances, such as allergens, pathogens, and debris. In playing this pivotal role, mucus can also hinder topical delivery of therapeutics to the eye. Recent studies provide evidence that drugs formulated as traditional micro- or nanoparticles are susceptible to entrapment and rapid clearance by ocular mucus. Mucus-penetrating particles (MPPs) is a nanoparticle technology that emerged over the past decade. With a muco-inert surface and a particle size smaller than the mucus mesh size, MPPs can diffuse in ex vivo mucus essentially freely. Preclinical studies have shown that, compared with particles lacking the mucus-penetrating attributes, MPPs can improve the uniformity of drug particle distribution on mucosal surfaces and enhance drug delivery to ocular tissues.

Medical micro/nanorobots in complex media

Medical micro/nanorobots have received tremendous attention over the past decades owing to their potential to be navigated into hard-to-reach tissues for a number of biomedical applications ranging from targeted drug/gene delivery, bio-isolation, detoxification, to nanosurgery. Despite the great promise, the majority of the past demonstrations are primarily under benchtop or in vitro conditions. Many developed micro/nanoscale propulsion mechanisms are based on the assumption of a homogeneous, Newtonian environment, while realistic biological environments are substantially more complex. Moving toward practical medical use, the field of micro/nanorobotics must overcome several major challenges including propulsion through complex media (such as blood, mucus, and vitreous) as well as deep tissue imaging and control in vivo. In this review article, we summarize the recent research efforts on investigating how various complexities in biological environments impact the propulsion of micro/nanoswimmers. We also highlight the emerging technological approaches to enhance the locomotion of micro/nanorobots in complex environments. The recent demonstrations of in vivo imaging, control and therapeutic medical applications of such micro/nanorobots are introduced. We envision that continuing materials and technological innovations through interdisciplinary collaborative efforts can bring us steps closer to the fantasy of "swallowing a surgeon".

Prolonged Infectivity of SARS-CoV-2 in Fomites

We spotted severe acute respiratory syndrome coronavirus 2 on polystyrene plastic, aluminum, and glass for 96 hours with and without bovine serum albumin (3 g/L). We observed a steady infectivity (<1 log₁₀ drop) on plastic, a 3.5 log₁₀ decrease on glass, and a 6 log₁₀ drop on aluminum. The presence of proteins noticeably prolonged infectivity.

S-allylmercaptocysteine inhibits mucin overexpression and inflammation via MAPKs and PI3K-Akt signaling pathways in acute respiratory distress syndrome

Cytokine storm is an important cause of acute respiratory distress syndrome and multiple organ failure. Excessive secretion and accumulation of mucins on the surface of airway cause airway obstruction and exacerbate lung infections. MUC5AC and MUC5B are the main secreted mucins and overexpressed in various inflammatory responses. S-allylmercaptocysteine, a water-soluble organic sulfur compound extracted from garlic, has anti-inflammatory and anti-oxidative effects for various pulmonary diseases. The aim of this work was to investigate the therapeutic effects of SAMC on mucin overproduction and inflammation in 16HBE cells and LPS-induced ARDS mice. Results show that SAMC treatment ameliorated inflammatory cell infiltration and lung histopathological changes in the LPS-induced ARDS mice. SAMC also inhibited the expressions of MUC5AC and MUC5B, decreased the production of pro-inflammatory markers (IL-6, TNF-α, CD86 and IL-12) and increased the production of anti-inflammatory markers (IL-10, CD206 and TGF-β). These results confirm that SAMC had potential beneficial effects on suppressed hyperinflammation and mucin overexpression. Furthermore, SAMC exerted the therapeutic effects through the inhibition of phosphorylation of MAPKs and PI3K-Akt signaling pathways in the 16HBE cells and mice. Overall, our results demonstrate the effects of SAMC on the LPS-induced mucin overproduction and inflammation both in the 16HBE cells and mice.

Peptides as surface coatings of nanoparticles that penetrate human cystic fibrosis sputum and uniformly distribute in vivo following pulmonary delivery

Therapeutic delivery of drug and gene delivery systems have to traverse multiple biological barriers to achieve efficacy. Mucosal administration, such as pulmonary delivery in cystic fibrosis (CF) disease, remains a significant challenge due to concentrated viscoelastic mucus, which prevents drugs and particles from penetrating the mucus barrier. To address this problem, we used combinatorial peptide-presenting phage libraries and next-generation sequencing (NGS) to identify hydrophilic, net-neutral charged peptide coatings that enable penetration through human CF mucus ex vivo with ~600-fold better penetration than control, improve uptake into lung epithelial cells compared to uncoated or PEGylated-nanoparticles, and exhibit enhanced uniform distribution and retention in the mouse lung airways. These peptide coatings address multiple delivery barriers and effectively serve as excellent alternatives to standard PEG surface chemistries to achieve mucus penetration and address some of the challenges encountered using these chemistries. This biomolecule-based strategy can address multiple delivery barriers and hold promise to advance efficacy of therapeutics for diseases like CF.

Aspiration of superabsorbent polymer beads resulting in focal lung damage: a case report

Background

Concerns have recently been raised about reported incidents of intestinal obstruction following ingestion of Superabsorbent polymer (SAP) beads. Texas Poison Centers reported 110 cases of superabsorbent polymer bead ingestions between 2011 and 2016 (Pediatr Emerg Care 35:426-7, 2019). Furthermore, cases of related auditory complications following the placement of SAP beads into the external auditory canal have also been reported. Here, we report the first case of significant airway damage secondary to the ingestion of a SAP bead (Orbeez), which was aspirated and then overlooked. Further, we hypothesized that the capability of the bead to expand in size once exposed to water from the respiratory mucous may contribute to airway damage.

Case presentation

A 3-year-old boy presented to our hospital with persistent cough and recurrent hospitalizations to the general ward and intensive care unit. The boy was diagnosed with focal lung bronchiectasis in the left lower lobe, which occurred after the patient aspirated an Orbeez bead before a year. The bead was removed using flexible bronchoscopy and a retrieval basket.

Conclusion

Orbeez beads are commonly ingested by young children resulting in gastrointestinal obstruction. The beads can easily be aspirated by children and overlooked by their caregivers and physicians for long periods of time due to their small size. The bead can cause significant airway damage after multiplying in size when coming into contact with respiratory mucus which consists of 95% water.

Chemical and Microstructural Characterization of pH and [Ca2+] Dependent Sol-Gel Transitions in Mucin Biopolymer

Mucus is responsible for controlling transport and barrier function in biological systems, and its properties can be significantly affected by compositional and environmental changes. In this study, the impacts of pH and CaCl₂ were examined on the solution-to-gel transition of mucin, the primary structural component of mucus. Microscale structural changes were correlated with macroscale viscoelastic behavior as a function of pH and calcium addition using rheology, dynamic light scattering, zeta potential, surface tension, and FTIR spectroscopic characterization. Mucin solutions transitioned from solution to gel behavior between pH 4–5 and correspondingly displayed a more than ten-fold increase in viscoelastic moduli. Addition of CaCl₂ increased the sol-gel transition pH value to ca. 6, with a twofold increase in loss moduli at low frequencies and ten-fold increase in storage modulus. Changing the ionic conditions—specifically [H⁺] and [Ca²⁺] —modulated the sol-gel transition pH, isoelectric point, and viscoelastic properties due to reversible conformational changes with mucin forming a network structure via  non-covalent cross-links between mucin chains.

Iota-Carrageenan as an Antiviral Treatment for the Common Cold

Introduction:

The common cold syndrome of acute upper respiratory tract viral infection is the most common disease among mankind and is an extremely common illness in children. There is a great need for a safe and effective antiviral treatment with minimal side effects. The challenge in developing a treatment is the numerous and varied respiratory viruses that cause this common illness and the need for a treatment with good tolerability and safety.

Explanation:

All respiratory viruses must reach the cell surface by passing through respiratory fluid and mucus, and this common feature may allow for the development of antivirals that capture viruses during this transit.

This article discusses how large polyanionic molecules such as iota-carrageenan may trap positively charged respiratory viruses. Iota-carrageenan is a large polysaccharide molecule which is neither absorbed from the respiratory tract nor metabolised. It, therefore, does not have any pharmacological properties. Iota-carrageenan nasal spray has been shown to reduce the titres of respiratory viruses and to reduce the severity of symptoms in placebo-controlled clinical trials, including children and adults. The results of four clinical trials are presented.

Conclusion:

Iota-carrageenan is a good candidate as a safe and effective non-specific antiviral treatment for common cold, and more research is justified on polyanionic molecules like carrageenans as antivirals.

Antibody-mediated trapping in biological hydrogels is governed by sugar-sugar hydrogen bonds

N-glycans on IgG and IgM antibodies (Ab) facilitate Ab-mediated crosslinking of viruses and nanoparticles to the major structural elements of mucus and basement membranes. Nevertheless, the chemical moieties in these biological hydrogel matrices to which Ab can bind remain poorly understood. To gain insights into the chemistries that support Ab-matrix interactions, we systematically evaluated IgG- and IgM-mediated trapping of nanoparticles in different polysaccharide-based biogels with unique chemical features. In agarose, composed of alternating d-galactose and 3,6-anhydro-l-galactopyranose (i.e. hydroxyl groups only), anti-PEG IgM but not anti-PEG IgG trapped PEGylated nanoparticles. In alginate, comprised of homopolymeric blocks of mannuronate and guluronate (i.e. both hydroxyl and carboxyl groups), both IgG and IgM trapped PEGylated nanoparticles. In contrast, chitosan, comprised primarily of glucosamine (i.e. both hydroxyl and primary amine groups), did not facilitate either IgG- or IgM-mediated trapping. IgG-mediated trapping in alginate was abrogated upon removal of IgG N-glycans, whereas IgM-mediated trapping was eliminated in agarose but not alginate upon desialylation. These results led us to propose a model in which hydrogen bonding between carboxyl and hydroxyl groups of glycans on both Ab and matrix facilitates Ab-mediated trapping of pathogens in biogels. Our work here offers a blueprint for designing de novo hydrogels that could harness Ab-matrix interactions for various biomedical and biological applications. STATEMENT OF SIGNIFICANCE: Here, we interrogated the molecular mechanism of antibody-mediated trapping to address what are the chemical moieties on biogels that are essential for facilitating trapping in biogels. We systematically evaluated the potencies of IgG and IgM to trap nanoparticles in different polysaccharide-based biogels with unique and highly defined chemical moieties: hydroxyl groups (agarose), amine groups (chitosan), and carboxyl groups (alginate). We discovered that only hydroxyl/carboxyl hydrogen bonds (and stronger) are sufficiently strong enough to facilitate antibody-mediated trapping; weaker hydroxyl/hydroxyl bonds or hydroxyl/amine bonds fail to adequately slow particles. Our findings presents the first blueprint for how to engineer de novo biogels that are capable of harnessing antibodies to immobilize foreign entities in the biogels, for applications ranging from infectious disease to contraception to purification processes.

Akkermansia and Microbial Degradation of Mucus in Cats and Dogs: Implications to the Growing Worldwide Epidemic of Pet Obesity

Akkermansia muciniphila is a mucin-degrading bacterium that has shown the potential to provide anti-inflammatory and anti-obesity effects in mouse and man. We here focus on companion animals, specifically cats and dogs, and evaluate the microbial degradation of mucus and its health impact in the context of the worldwide epidemic of pet obesity. A literature survey revealed that the two presently known Akkermansia spp., A. muciniphila and A. glycaniphila, as well as other members of the phylum of Verrucomicrobia seem to be neither very prevalent nor abundant in the digestive tract of cats and dog. While this may be due to methodological aspects, it suggests that bacteria related to Akkermansia are not the major mucus degraders in these pets and hence other mucus-utilizing taxa may deserve attention. Hence, we will discuss the potential of these endogenous mucus utilizers and dietary interventions to boost these as well as the use of Akkermansia spp. related bacteria or their components as strategies to target feline and canine obesity.

Mucus-Inspired Supramolecular Adhesives with Oil-Regulated Molecular Configurations and Long-Lasting Antibacterial Properties

Inspired by mucus, which provides an ideal supramolecular model and whose fluid-like (viscous) and solid-like (elastic) behaviors can be adjusted to meet different physiological requirements, we report oil-regulated supramolecular adhesives by the co-assembly of polyurea oligomers and carvacrol oils. The adhesive is crosslinked by weak but abundant hydrogen bonds, which can be regulated by the incorporated carvacrol oils through the competition of intermolecular hydrogen bonds, presenting a unique set of mucus-mimicking features including oil-regulated mechanics, processability, reusable adhesivity, and extreme longevity in both air and water. Owing to the intrinsic bactericidal effect of the carvacrol oils, the developed adhesives can serve as potent antibacterial coatings with both rapid contact killing (99.9% killing within 15 min) and long-term controlled release abilities (up to 70 days), enabling versatile antibacterial applications in diverse conditions. We envision that these adhesives will be useful in buildings and architectures, community and public facilities, food storage and packaging technologies, functional textiles, and practical biomedical fields.

Physiological status and functional anatomy of zebra fish (Danio rerio) exposed to various levels of Ga3

Gallium (Ga) is one of the intermetallic elements that has been used in cancer treatment for a long time. However, Ga compounds are increasingly being used to make high-speed semiconductors and photoelectric devices. The current work investigated physiological and pathological changes in zebra fish (Danio rerio) exposed to various Ga³⁺ levels (0.55, 1.5, and 3.85 mg/L) over a 14-day test period. Decreases in oxygen consumption were significant (p < 0.05) for groups exposed to 3.85 Ga³⁺ mg/L; this was associated with the fusion of zebra fish gills lamellae. Serum biochemical changes (including aspartate aminotransferase, alanine aminotransferase, and alkaline phosphatase) were consistent with observations of damage to organelles within the hepatocytes at higher Ga³⁺ exposure levels (1.5 and 3.85 mg/L) in zebra fish. We propose <0.55 Ga³⁺ mg/L as a biologically safe concentration that can be used to establish water quality criteria for this teleost model.

Lactobacillus paracasei subsp. paracasei NTU 101 lyophilized powder improves loperamide-induced constipation in rats

Constipation is a condition of the digestive system characterized by formation of hard feces that are difficult to eliminate. It has emerged as a new problem that is commonly encountered by many people and lifestyle changes have been unsuccessful in providing a solution. This study aimed to investigate the effects of Lactobacillus paracasei subsp. paracasei NTU 101 on loperamide-induced constipated rats and on gastrointestinal tract function. Sprague-Dawley rats were administered loperamide (2 mg/kg BW) twice daily as well as 1.3, 2.6, and 13.0 mg/kg BW/rat/d of NTU 101 powder. The control, positive control, and NTU 101 powder groups (0.5, 1, 5×) showed improved intestinal mobility with a statistically significant increase of 12.4%, 14.7%, 12.5%, 13.4%, and 15.1%, respectively (p < 0.05); the fecal water content was also significantly increased by 11.7%, 9.0%, 10.0%, 9.3%, and 11.0%, respectively (p < 0.05), compared to the loperamide group. Furthermore, NTU 101 increased the Bifidobactrium spp. and decreased the Clostridium perfringens content in feces; it increased short-chain fatty acid levels, reduced fecal pH value, enhanced the thickness of the colonic mucosa, and increased the number of mucin-producing goblet cells and interstitial cells of Cajal. Thus, NTU 101 powder was found to alleviate loperamide-induced constipation and improve gastrointestinal tract function.

Peptides as surface coatings of nanoparticles that penetrate human cystic fibrosis sputum and uniformly distribute in vivo following pulmonary delivery

Therapeutic delivery of drug and gene delivery systems have to traverse multiple biological barriers to achieve efficacy. Mucosal administration, such as pulmonary delivery in cystic fibrosis (CF) disease, remains a significant challenge due to concentrated viscoelastic mucus, which prevents drugs and particles from penetrating the mucus barrier. To address this problem, we used combinatorial peptide-presenting phage libraries and next-generation sequencing (NGS) to identify hydrophilic, net-neutral charged peptide coatings that enable penetration through human CF mucus ex vivo with ~600-fold better penetration than control, improve uptake into lung epithelial cells compared to uncoated or PEGylated-nanoparticles, and exhibit enhanced uniform distribution and retention in the mouse lung airways. These peptide coatings address multiple delivery barriers and effectively serve as excellent alternatives to standard PEG surface chemistries to achieve mucus penetration and address some of the challenges encountered using these chemistries. This biomolecule-based strategy can address multiple delivery barriers and hold promise to advance efficacy of therapeutics for diseases like CF.

Role of the mucins in pathogenesis of COPD: implications for therapy

Introduction: Evidence accumulated in the last decade has started to reveal the enormous complexity in the expression, interactions and functions of the large number of different mucins present in the different compartments of the human lower airways. This occurs both in normal subjects and in COPD patients in different clinical phases and stages of severity.Areas covered: We review the known physiological mechanisms that regulate mucin production in human lower airways of normal subjects, the changes in mucin synthesis/secretion in COPD patients and the clinical efficacy of drugs that modulate mucin synthesis/secretion.Expert opinion: It is evident that the old simplistic concept that mucus hypersecretion in COPD patients is associated with negative clinical outcomes is not valid and that the therapeutic potential of 'mucolytic drugs' is under-appreciated due to the complexity of the associated molecular network(s). Likewise, our current knowledge of the effects of the drugs already available on the market that target mucin synthesis/secretion/structure in the lower airways is extremely limited and often indirect and more well-controlled clinical trials are needed in this area.

Phosphorylated PEG-emulsifier: Powerful tool for development of zeta potential changing self-emulsifying drug delivery systems (SEDDS)

AIM

It was the aim of this study to synthesize a phosphorylated emulsifier possessing a PEG-linker for establishment of a potent zeta potential changing system in self-emulsifying drug delivery systems (SEDDS).

METHODS

N,N'-Bis(polyoxyethylene)oleylamine (POA) was phosphorylated utilizing pyrophosphoric acid. Successful synthesis of POA bisphosphate (POAP) was confirmed by NMR and HR CS MAS. After incorporation of 1% POAP into SEDDS (Kolliphor RH 40, Capmul PG-8, Labrafac Lipophile WL 1349, Labrafac PG; 30/20/20/30, v/v), according emulsions were incubated with intestinal alkaline phosphatase (IAP) and the zeta potential was measured. Additionally, the amount of released phosphate upon incubation with IAP or on Caco-2 cells was quantified by malachite green assay. Finally, cell viability studies on Caco-2 cells were performed and mucus permeation properties with and without IAP preincubation were assessed.

RESULTS

POAP was synthesized as brown viscous liquid with a yield of 36% and could be incorporated into SEDDS. By incubation with IAP a zeta potential shift from -15.1 to 6.5 mV was observed. A corresponding phosphate release in presence of isolated IAP as well as on Caco-2 cells was found. Assessment of the cytotoxic potential revealed no significant alteration in the safety profile of SEDDS by incorporation of POAP. Mucus permeation studies exposed a 2-fold higher permeation of fluorescein diacetate (FDA) having been embedded in SEDDS loaded with POAP in comparison to blank formulation and 3-fold higher permeability than for emulsions having been preincubated with phosphatase.

CONCLUSION

The novel phosphorylated surfactant exhibiting a PEG-linker facilitated a potent zeta potential change of SEDDS.

Influenza as a molecular walker

The surface of the influenza virus is decorated with the receptor-binding protein hemagglutinin (HA) and the receptor-cleaving enzyme neuraminidase (NA). HA is responsible for host cell recognition, while NA prevents aggregation and entrapment, but the intricate mechanism of how the functions of these glycoproteins cooperate and how they are regulated by mutational responses to environmental pressures remains unclear. Recently, several groups have described the motion of influenza over surfaces and reported that this motion is inhibited by NA inhibitors. We argue that the motion of influenza resembles the motility of artificial receptor-cleaving particles called "molecular spiders". The cleaving of receptors by this type of molecular walkers leads to self-avoiding motion across a surface. When the binding and cleaving rates of molecular spiders are balanced, they move both rapidly and efficiently. The studies of molecular spiders offer new insights into the functional balance of HA and NA, but they do not address the asymmetric distribution of HA and NA on the surface of influenza. We propose that receptor-cleaving molecular walkers could play an important role in the further investigation of the motility of influenza viruses.

Proteomic profiling of salmon skin mucus for the comparison of sampling methods

The epidermal mucus protects fish against harmful environmental factors and the loss of physiological metabolites and water. It is an efficient barrier between the fish and the biosphere. The integrity of the skin mucus is thus of vital importance for the welfare and survival of the fish. Since excreted proteins and small molecules in the mucus can mirror the health status of the fish, it is a valuable matrix for monitoring stress, pathogen exposure, and nutritional effects. Several methods for sampling epidermal mucus from different fish species have previously been described, but information about their efficiency or the comparability of mucus analyses is lacking. In the present study, skin mucus from farmed Atlantic salmon was therefore sampled by three methods, including absorption or wiping with tissue paper, and scraping with a blunt blade, and the mucus proteome was analyzed by ultra-high pressure liquid chromatography high-resolution mass spectrometry. The measured protein contents, numbers, compositions and the observed data quality were compared between sampling methods. Furthermore, functional annotation and classification of the identified proteins was performed. The results showed that the three skin mucus sample types differed qualitatively as well as quantitatively. The absorbed mucus was the least tainted by proteins resulting from damage inflicted to the fish epidermis by the sampling procedure. Wiped mucus showed a better protein yield than absorbed and delivered a larger proteome of identifiable proteins, with less contamination from epithelial proteins than observed for scraped mucus. We recommend that future research of mucus should use the absorption method in cases, where it is important that the mucus is devoid of proteins from the underlying epithelium, and the wiping method, when protein yield is crucial or when the proteome of the outer epithelium is of interest.

Innate Lymphoid Cells: Regulators of Gut Barrier Function and Immune Homeostasis

Innate lymphoid cells (ILCs), identified in the early years of this century as a new class of leukocyte family unlike the B or T lymphocytes, play a unique role bridging the innate and adaptive immune responses in mucosal immunity. Their origin, differentiation, and activation process and functions have caught global interest. Recently, accumulating evidence supports that ILCs are vital regulators for gastrointestinal mucosal homeostasis through interactions with other structural and stromal cells in gut epithelial barriers. This review will explore the functions of ILCs and other cells in maintaining gut homeostasis and feature the crosstalk between ILCs with other cells and potential pharmacotherapy targeting ILCs applicable in intestinal innate immunity.

Mucin adsorbed by E. coli can affect neutrophil activation in vitro

Bacteria colonizing human intestine adhere to the gut mucosa and avoid the innate immune system. We previously demonstrated that Escherichia coli isolates can adsorb mucin from a diluted solution in vitro. Here, we evaluated the effect of mucin adsorption by E. coli cells on neutrophil activation in vitro. Activation was evaluated based on the detection of reactive oxygen species production by a chemiluminescent reaction (ChL), observation of morphological alterations in neutrophils and detection of exocytosis of myeloperoxidase and lactoferrin. We report that mucin adsorbed by cells of SharL1 isolate from Crohn's disease patient's inflamed ileum suppressed the potential for the activation of neutrophils in whole blood. Also, the binding of plasma complement proteins and immunoglobulins to the bacteria was reduced. Desialylated mucin, despite having the same adsorption efficiency to bacteria, had no effect on the blood ChL response. The effect of mucin suggests that it shields epitopes that interact with neutrophils and plasma proteins on the bacterial outer membrane. Potential candidates for these epitopes were identified among the proteins within the bacterial outer membrane fraction by 2D‐PAGE, fluorescent mucin binding on a blot and HPLC‐MS/MS. In vitro, the following proteins demonstrated mucin adsorption: outer membrane porins (OmpA, OmpC, OmpD and OmpF), adhesin OmpX, the membrane assembly factor OmpW, cobalamine transporter, ferrum uptake protein and the elongation factor Ef Tu‐1. In addition to their other functions, these proteins are known to be bacterial surface antigens. Therefore, the shielding of epitopes by mucin may affect the dynamics and intensity of an immune response.

Rhubarb-Evoke Mucus Secretion through Aggregation and Degranulation of Mast Cell in the Colon of Rat: In vivo and ex vivo studies

Rhubarb is commonly used to treat constipation in China for its function of promoting intestinal movement and optimum water content in feces. However, its mechanism of mucus secretion is vague. The aim of the study is to investigate the role of mast cells and enteric neurons in rhubarb extract (RE)-induced mucus secretion in the rat colon. Immunofluorescence was used to detect histamine receptors. Western blotting and 3,3′-diaminobenzidine (DAB) were applied to explore the content changes of mast cells activation. The changes in colonic goblet cells (GCs) were determined by means of PAS/AB staining. An intestinal perfusion system with a Bradford protein assay kit was directly to estimate in vitro secretion. And the cytokines were investigated with ELISA. The longitudinal aspect of this study indicate that the number and water content of faecal pellets were enhanced after the administration of different doses of RE accompanied by mast cells accumulated and increased the content of interferon (IFN) -γ or decreased the levels of interleukin (IL) −10 at doses of 3 and 6 g/kg. Pretreatment with ketotifen, mast cell stabilizer, had partially inhibited on RE-induced mucus secretion. Furthermore, RE induced the release of acetylcholine and mucin-2 in the colonic tissue and the histamine levels from the faeces. The results suggest that RE induced colonic mucus secretion involves mast cell activation and some cytokine.

Secretory Proteins in the Skin Mucus of Nile Tilapia (Oreochromis niloticus) are Modulated Temporally by Photoperiod and Bacterial Endotoxin Cues

Although it is well known that the biological and physical characteristics of skin mucus in fishes are strongly affected by changes in environmental conditions, the influence of photoperiod and time-dependent bacterial endotoxin stimulation is not well documented. In the present study, we determined the diel variations in the basal activities of secretory proteins with known defense functions in the skin mucus of Nile tilapia (Oreochromis niloticus) maintained under two photic environments: equal length of day and night (12L:12D, LD) or total darkness (0L:24D, DD). A second experiment was conducted to determine how time-dependent (i.e., day versus night) lipopolysaccharide (LPS) challenge could influence these skin mucosal defenses. The results revealed that LD signal differentially modulated the activities of mucosal immune molecules. Fish subjected to LD regime showed significantly higher levels of skin mucus lysozyme and protease at nighttime than at daytime. This distinct feature was not observed in fish under DD. There was no general mucosal response patterns to time-dependent LPS challenge. Nonetheless, protease and lysozyme, which were identified to be at elevated levels at night, were significantly modulated when the endotoxin was administered at nighttime. Ceruloplasmin was the only molecule that responded to LPS challenge at daytime, where its activity significantly increased at 8 h post-stimulation. Collectively, the results revealed that photoperiod cues influenced the activities of mucosal defenses and this may play, at least in part, in the temporal sensitivity to bacterial endotoxin.

A rational approach to form disulfide linked mucin hydrogels

We report the design of a mucin hydrogel created using a thiol-based cross-linking strategy. By using a cross-linking reagent capable of forming hydrogen bonds and disulfide linkages within the gel network, we were able to produce mucin-based hydrogels with viscoelastic properties similar to natural mucus as measured by bulk rheology. We confirmed disulfide cross-links strongly contribute to gel formation in our system using chemical treatments to block and reduce cysteines where we found mucin hydrogel network formation was inhibited and disrupted, respectively. Particle tracking microrheology was used to investigate the kinetics and evolution of microstructure and viscoelasticity within the hydrogel as it formed. We found that the rate of gel formation could be tuned by varying the mucin to crosslinker ratio, producing network pore sizes in the range measured previously in human mucus. The results of this work provide a new, simple method for creating mucin hydrogels with physiologically relevant properties using readily available reagents.

Engineering drug delivery systems to overcome mucosal barriers for immunotherapy and vaccination

Mucosal surfaces protect our bodies from pathogens and external irritants using a system of biological barriers. Overcoming these barriers is a significant drug delivery challenge, particularly for immunotherapies that aim to modulate the local immune response. Reaching local lymphoid tissues and draining lymph nodes (LNs) requires crossing the mucus mesh, mucosal epithelium, and either targeting M cells covering lymphoid tissues or utilizing lymphatic transport that shuttles molecules and particulates from the periphery to the LN. We first highlight the barrier properties of mucus and mucosal epithelium, and the function of the mucosal immune system. We then dive into existing drug delivery technologies that have been engineered to overcome each of these barriers. We particularly focus on novel strategies for targeting lymphoid tissues, which has been shown to enhance immunotherapies and vaccinations, via directly targeting LNs, lymphatic vessels, and M cells that transport samples of mucosal content to the lymphoid tissues.

Permeability of the small intestinal mucus for physiologically relevant studies: Impact of mucus location and ex vivo treatment

The small intestinal mucus is a complex colloidal system that coats the intestinal mucosa. It allows passage on nutrients/pharmaceuticals from the gut lumen towards the epithelium, whilst preventing it from direct contact with luminal microorganisms. Mucus collected from intestinal tissue is often used in studies looking at inter-mucosal transport of food particulates, drug carriers, etc. However, detaching the highly hydrated native mucus from the tissue and storing it frozen prior to use may disrupt its physiological microstructure, and thus selective barrier properties. Multiple-particle tracking experiments showed that microstructural organisation of native, jejunal mucus depends on its spatial location in the intestinal mucosa. The inter-villus mucus was less heterogeneous than the mucus covering villi tips in the pig model used. Collecting mucus from tissue and subjecting it to freezing and thawing did not significantly affect (P > 0.05) its permeability to model, sub-micron sized particles, and the microviscosity profile of the mucus reflected the overall profiles recorded for the native mucus in the tissue. This implies the method of collecting and storing mucus is a reliable ex vivo treatment for the convenient planning and performing of mucus-permeability studies that aim to mimic physiological conditions of the transport of molecules/particles in native mucus.

Impact of Different Mucoadhesive Polymeric Nanoparticles Loaded in Thermosensitive Hydrogels on Transcorneal Administration of 5-Fluorouracil

In a previous paper a thermosensitive hydrogel formulation based on chitosan or its derivatives (TSOH), containing medicated chitosan nanoparticles (Ch NP) for transcorneal administration of 5-fluorouracil (5-FU) was described. The Ch NP-containing TSOH allowed a time-constant 5-FU concentration in the aqueous for 7 h from instillation. The aim of the present work was to study the impact of the surface characteristics of new NP contained in TSOH on ocular 5-FU bioavailability. The Ch derivatives used to prepare NP were quaternary ammonium-Ch conjugate (QA-Ch), S-protected derivative thereof (QA-Ch-S-pro), and a sulphobutyl chitosan derivative (SB-Ch). All NP types had 300–400 nm size, 16–18% encapsulation efficiency, and retained the entrapped drug for at least 15 h. Drug release from TSOH containing NP based on QA-Ch or QA-Ch-S-pro was virtually equal, whereas with TSOH containing NP based on SB-Ch was significantly slower. Instillation, in rabbit eyes, of NP-containing TSOH based on QA-Ch or SB-Ch led to a plateau in the aqueous concentration vs. time plot in the 1–10 h range with significantly enhanced area under curve (AUC). Negative charges on the NP surface slowed down 5-FU release from TSOH while positive charges increased NP contact with the negatively charged ocular surface. Either results in enhanced ocular bioavailability.

Strategies for measuring airway mucus and mucins

Mucus secretion and mucociliary transport are essential defense mechanisms of the airways. Deviations in mucus composition and secretion can impede mucociliary transport and elicit airway obstruction. As such, mucus abnormalities are hallmark features of many respiratory diseases, including asthma, cystic fibrosis and chronic obstructive pulmonary disease (COPD). Studying mucus composition and its physical properties has therefore been of significant interest both clinically and scientifically. Yet, measuring mucus production, output, composition and transport presents several challenges. Here we summarize and discuss the advantages and limitations of several techniques from five broadly characterized strategies used to measure mucus secretion, composition and mucociliary transport, with an emphasis on the gel-forming mucins. Further, we summarize advances in the field, as well as suggest potential areas of improvement moving forward.

Sialylated Cervical Mucins Inhibit the Activation of Neutrophils to Form Neutrophil Extracellular Traps in Bovine in vitro Model

In order to combat invading pathogens neutrophils can release neutrophil extracellular traps (NETs). However, since NETs can also damage endogenous cells, several control mechanisms for the formation of NETs must work effectively. For instance, neutrophil activation is silenced within blood circulation by the binding of sialylated glycoconjugates to sialic acid binding immunoglobulin-like lectins (Siglecs) on neutrophils. As neutrophils are recruited within the female reproductive tract, after mating, a comparable mechanism may also take place within the bovine cervix to prevent an exaggerated NET formation and thus, infertility. We examined, if the highly glycosylated mucins, which are the major functional fraction of biomolecules in mucus, represent a potential regulator of NET formation. The qPCR data revealed that in polymorphonuclear neutrophils (PMNs) inhibitory Siglecs are the most frequently expressed Siglecs and might be a potential target of sialylated glycans to modulate the activation of PMNs. Remarkably, the addition of bovine cervical mucins significantly inhibited the formation of NET, which had been induced in response to lipopolysaccharides (LPS) or a combination of phorbol myristate acetate (PMA) and ionomycin. The inhibitory effects were independent of the stage of estrous cycle (estrus, luteal, and follicular mucins). PMNs retained their segmented nuclei and membrane perforation was prevented. However, the inhibitory effects were diminished, when sialic acids were released under acidic conditions. Comparable results were achieved, when sialic acids were targeted by neuraminidase digestion, indicating a sialic acid dependent inhibition of NET release. Thus, bovine cervical mucins have an anti-inflammatory capability to modulate NET formation and might be further immunomodulatory biomolecules that support fertility.

Mucus, mucins, and cystic fibrosis

Cystic fibrosis (CF) is both the most common and most lethal genetic disease in the Caucasian population. CF is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene and is characterized by the accumulation of thick, adherent mucus plaques in multiple organs, of which the lungs, gastrointestinal tract and pancreatic ducts are the most commonly affected. A similar pathogenesis cascade is observed in all of these organs: loss of CFTR function leads to altered ion transport, consisting of decreased chloride and bicarbonate secretion via the CFTR channel and increased sodium absorption via epithelial sodium channel upregulation. Mucosa exposed to changes in ionic concentrations sustain severe pathophysiological consequences. Altered mucus biophysical properties and weakened innate defense mechanisms ensue, furthering the progression of the disease. Mucins, the high-molecular-weight glycoproteins responsible for the viscoelastic properties of the mucus, play a key role in the disease but the actual mechanism of mucus accumulation is still undetermined. Multiple hypotheses regarding the impact of CFTR malfunction on mucus have been proposed and are reviewed here. (a) Dehydration increases mucin monomer entanglement, (b) defective Ca²⁺ chelation compromises mucin expansion, (c) ionic changes alter mucin interactions, and (d) reactive oxygen species increase mucin crosslinking. Although one biochemical change may dominate, it is likely that all of these mechanisms play some role in the progression of CF disease. This article discusses recent findings on the initial cause(s) of aberrant mucus properties in CF and examines therapeutic approaches aimed at correcting mucus properties.

Mucins and Pathogenic Mucin-Like Molecules Are Immunomodulators During Infection and Targets for Diagnostics and Vaccines

Mucins and mucin-like molecules are highly O-glycosylated proteins present on the cell surface of mammals and other organisms. These glycoproteins are highly diverse in the apoprotein and glycan cores and play a central role in many biological processes and diseases. Mucins are the most abundant macromolecules in mucus and are responsible for its biochemical and biophysical properties. Mucin-like molecules cover various protozoan parasites, fungi and viruses. In humans, modifications in mucin glycosylation are associated with tumors in epithelial tissue. These modifications allow the distinction between normal and abnormal cell conditions and represent important targets for vaccine development against some cancers. Mucins and mucin-like molecules derived from pathogens are potential diagnostic markers and targets for therapeutic agents. In this review, we summarize the distribution, structure, role as immunomodulators, and the correlation of human mucins with diseases and perform a comparative analysis of mucins with mucin-like molecules present in human pathogens. Furthermore, we review the methods to produce pathogenic and human mucins using chemical synthesis and expression systems. Finally, we present applications of mucin-like molecules in diagnosis and prevention of relevant human diseases.

Nanoparticles for the regulation of intestinal inflammation: opportunities and challenges

Prevalence of chronic inflammation of the gastrointestinal tract is increasing, emerging as a public health challenge. Conventional drug delivery systems targeting the colon have improved the treatment of inflammatory bowel disease. However, therapy frequently results in inconsistent efficacy and toxicity problems. Novel approaches based on nanoparticles offer several advantages over conventional dosage forms due to their ability to selectively target inflamed tissues. Several formulation efforts have been made in order to obtain increasingly selective nanosized systems, some with promising results in animal models of colitis. Despite all advances, no nanomedicines are yet approved for clinical use in inflammatory bowel disease. This review discusses the most recent efforts made toward the development of nanoparticles for regulating chronic intestinal inflammation.

Intestinal Barrier Function in Gluten-Related Disorders

Gluten-related disorders include distinct disease entities, namely celiac disease, wheat-associated allergy and non-celiac gluten/wheat sensitivity. Despite having in common the contact of the gastrointestinal mucosa with components of wheat and other cereals as a causative factor, these clinical entities have distinct pathophysiological pathways. In celiac disease, a T-cell mediate immune reaction triggered by gluten ingestion is central in the pathogenesis of the enteropathy, while wheat allergy develops as a rapid immunoglobulin E- or non-immunoglobulin E-mediated immune response. In non-celiac wheat sensitivity, classical adaptive immune responses are not involved. Instead, recent research has revealed that an innate immune response to a yet-to-be-defined antigen, as well as the gut microbiota, are pivotal in the development in this disorder. Although impairment of the epithelial barrier has been described in all three clinical conditions, its role as a potential pathogenetic co-factor, specifically in celiac disease and non-celiac wheat sensitivity, is still a matter of investigation. This article gives a short overview of the mucosal barrier of the small intestine, summarizes the aspects of barrier dysfunction observed in all three gluten-related disorders and reviews literature data in favor of a primary involvement of the epithelial barrier in the development of celiac disease and non-celiac wheat sensitivity.

Liquid-liquid phase transitions and amyloid aggregation in proteins related to cancer and neurodegenerative diseases

Liquid-liquid phase separation (LLPS) and phase transition (LLPT) of proteins and nucleic acids have emerged as a new paradigm in cell biology. Here we will describe the recent findings about LLPS and LLPT, including the molecular and physical determinants leading to their formation, the resulting functions and their implications in cell physiology and disease. Amyloid aggregation is implicated in many neurodegenerative diseases and cancer, and LLPS of proteins involved in these diseases appear to be related to their function in different cell contexts. Amyloid formation would correspond to an irreversible liquid-to-solid transition, as clearly observed in the case of PrP, TDP43, FUS/TLS and tau protein in neurodegenerative pathologies as well as with the mutant tumor suppressor p53 in cancer. Nucleic acids play a modulatory effect on both LLPS and amyloid aggregation. Understanding the molecular events regulating how the demixing process advances to solid-like fibril materials is crucial for the development of novel therapeutic strategies against cancer and neurodegenerative maladies.

Docosahexaenoic and Eicosapentaenoic Acids Prevent Altered-Muc2 Secretion Induced by Palmitic Acid by Alleviating Endoplasmic Reticulum Stress in LS174T Goblet Cells

Diets high in saturated fatty acids (FA) represent a risk factor for the development of obesity and associated metabolic disorders, partly through their impact on the epithelial cell barrier integrity. We hypothesized that unsaturated FA could alleviate saturated FA-induced endoplasmic reticulum (ER) stress occurring in intestinal secretory goblet cells, and consequently the reduced synthesis and secretion of mucins that form the protective mucus barrier. To investigate this hypothesis, we treated well-differentiated human colonic LS174T goblet cells with palmitic acid (PAL)—the most commonly used inducer of lipotoxicity in in vitro systems—or n-9, n-6, or n-3 unsaturated fatty acids alone or in co-treatment with PAL, and measured the impact of such treatments on ER stress and Muc2 production. Our results showed that only eicosapentaenoic (EPA) and docosahexaenoic (DHA) acids protect goblet cells against ER stress-mediated altered Muc2 secretion induced by PAL, whereas neither linolenic acid nor n-9 and n-6 FA are able to provide such protection. We conclude that EPA and DHA could represent potential therapeutic nutrients against the detrimental lipotoxicity of saturated fatty acids, associated with type 2 diabetes and obesity or inflammatory bowel disease. These in vitro data remain to be explored in vivo in a context of dietary obesity.

The impact of apical and basolateral albumin on intestinal zinc resorption in the Caco-2/HT-29-MTX co-culture model

The molecular mechanisms of intestinal zinc resorption and its regulation are still topics of ongoing research. To this end, the application of suitable in vitro intestinal models, optimized with regard to their cellular composition and medium constituents, is of crucial importance. As one vital aspect, the impact of cell culture media or buffer compounds, respectively, on the speciation and cellular availability of zinc has to be considered when investigating zinc resorption. Thus, the present study aims to investigate the impact of serum, and in particular its main constituent serum albumin, on zinc uptake and toxicity in the intestinal cell line Caco-2. Furthermore, the impact of serum albumin on zinc resorption is analyzed using a co-culture of Caco-2 cells and the mucin-producing goblet cell line HT-29-MTX. Apically added albumin significantly impaired zinc uptake into enterocytes and buffered its cytotoxicity. Yet, undigested albumin does not occur in the intestinal lumen in vivo and impairment of zinc uptake was abrogated by digestion of albumin. Interestingly, zinc uptake, as well as gene expression studies of mt1a and selected intestinal zinc transporters after zinc incubation for 24 h, did not show significant differences between 0 and 10% serum. Importantly, the basolateral application of serum in a transport study significantly enhanced fractional apical zinc resorption, suggesting that the occurrence of a zinc acceptor in the plasma considerably affects intestinal zinc resorption. This study demonstrates that the apical and basolateral medium composition is crucial when investigating zinc, particularly its intestinal resorption, using in vitro cell culture.