Divergent solid-phase synthesis and candidacidal activity of MUC7 D1, a 51-residue histidine-rich N-terminal domain of human salivary mucin MUC7

The Potential Antimicrobial Action of Human Mucin 7 15-Mer Peptide and Its Metal Complexes

Mucin 7 (encoded byMUC7) is a human salivary protein that has a role in the natural immune system. Fragments of mucin 7 exhibit antimicrobial activity against bacteria and yeast. Although the antimicrobial properties of peptides have been known and studied for decades, the exact mechanism of action of antimicrobial peptides (AMPs) is still unclear. It is known that some AMPs require divalent metal ions to activate their activity. Herein, we investigated three 15-mer MUC7 peptides, one of which (mother peptide, sequence, L3) is a synthetic analog of a fragment naturally excised from MUC7 (with His3, His8, and His 14) and its two structural analogs, containing only two histidine residues, His3, His13 and His8, His13 (L2 and L1, respectively). Since there is a correlation between lipophilicity, the presence of metal ions (such as Cu(II) and Zn(II)) and antimicrobial activity of AMP, antimicrobial properties of the studied peptides, as well as their complexes with Cu(II) and Zn(II) ions, were tested for activity against Gram-positive (Enterococcus faecalis, Staphylococcus epidermidis) and Gram-negative (Escherichia coli, Pseudomonas aeruginosa) bacteria and fungi (Candida albicans). The results were correlated with their lipophilicity. Coordination and thermodynamic studies (potentiometry, UV-Vis, CD) revealed the formation of mainly mononuclear complexes in solution for all studied systems with different stability in the physiological pH range.

Elucidating the role of hyposalivation and autoimmunity in oral candidiasis

INTRODUCTION

Oral candidiasis (OC) is a potential oral complication in Sjögren's syndrome (SS). Some studies indicate that the low stimulated salivary flow and not low unstimulated salivary flow is associated with OC in SS, while others report that the underlying autoimmune disorders contribute to OC, based solely on correlation coefficients. Given the conflicting and limited existing evidence, we purposed to ascertain the role of both salivary gland dysfunction (hyposalivation based on unstimulated and stimulated flow rates) and autoimmunity (SS, other autoimmune disorders) in OC among those with SS, other salivary gland dysfunction, and non-salivary gland dysfunction controls (NSGD).

METHODS

A nested case-control study was designed within a larger NIH/NIDCR cohort. Descriptive analyses, nonparametric tests, comparative analyses, and multivariate logistic regression analyses were undertaken.

RESULTS

Data on 1526 subjects (701 SS, 247 ISS, 355 Sicca, and 223 NSGD) were obtained from the source cohort of 2046 and analyzed for this study. The median whole unstimulated salivary flow rate (WUS, ml 15 min^-1 ) was lower in SS (0.8, interquartile range (IQR) 1.8) compared to ISS (5.5, IQR: 5.2, P < 0.001) and NSGD (3.8, IQR: 3.8, P < 0.001) but comparable with that of Sicca (1.0, IQR: 1.5, P = 0.777) participants. The median total stimulated salivary flow rate (TSS, ml 15 min^-1 ) was lowest in SS (7.0, IQR: 12.4, P < 0.001) compared to other groups. Of the 45 OC cases in this cohort, 71.1% (n = 32) were from the SS group. The prevalence of OC was highest in the SS group (4.6%, P = 0.008). SS group had twice the risk of OC than NSGD (OR = 2.2, 95%CI: 1.1-4.2, P = 0.02) and Sicca (OR = 2.2, 95% CI: 1.0-4.8, P = 0.03), adjusting for confounders; hyposalivation [WUS (OR = 5.1, 95%CI: 2.5-10.4, P < 0.001), TSS (OR = 1.9, 95%CI: 1.0-3.5, P = 0.04)], history of other autoimmune disorders (OR = 4.4, 95%CI: 1.7-11.3, P = 0.002), medications for extraglandular manifestations (OR = 2.3, 95%CI: 1.1-4.9, P = 0.03), and diabetes mellitus (4.2, 95%CI: 1.2-15.2, P = 0.02) were independent predictors of OC; females had a lower risk than males (OR = 0.29, 95%CI: 0.13-0.67, P = 0.004). Age, race, anti-SSA/SSB autoantibodies, focus score, other medications, anxiety, fatigue, cigarette smoking, alcohol, and caffeine use were not associated with oral candidiasis.

CONCLUSION

Salivary gland dysfunction (hyposalivation with WUS being a stronger predictor than TSS) and autoimmunity (SS, other autoimmune disorders, medications, i.e., DMARDS) are both independent predictors of OC. Diabetes mellitus is an independent predictor of OC among those with salivary gland dysfunction. Our findings suggest that these independent predictors should be considered in the prevention and management of OC in this population.

Investigation of the N-terminal coding region of MUC7 alterations in dentistry students with and without caries

Human low-molecular weight salivary mucin (MUC7) is a small, secreted glycoprotein coded by MUC7. In the oral cavity, they inhibit the colonization of oral bacteria, including cariogenic ones, by masking their surface adhesions, thus helping saliva to avoid dental caries. The N-terminal domain is important for low-molecular weight (MG2) mucins to contact with oral microorganisms. In this study, we aimed to identify the N-terminal coding region of the MUC7 gene between individuals with and without caries. Forty-four healthy dental students were enrolled in this study; 24 of them were classified to have caries [decayed, missing, filled-teeth (DMFT) = 5.6] according to the World Health Organization (WHO) criteria, and 20 of them were caries-free (DMFT = 0). Simplified oral hygiene index (OHI-S) and gingival index (GI) were used to determine the oral hygiene and gingival conditions. Total protein levels and salivary total protein levels and salivary buffer capacity (SBC) were determined by Lowry and Ericsson methods. DNA was extracted from peripheral blood cells of all the participants and genotyping was carried out by a polymerase chain reaction (PCR)-sequencing method. No statistical differences were found between two groups in the terms of salivary parameters, oral hygiene and gingival conditions. We detected one common single nucleotide polymorphism (SNP) that leads to a change of asparagine to lysine at codon 80. This substitution was found in 29.0 and 40.0%, respectively, of the groups with and without caries. No other sequence variations were detected. The SNP found in this study may be a specific polymorphism affecting the Turkish population. Further studies with extended numbers are necessary in order to clarify this finding.

Salivary mucins in host defense and disease prevention

Mucus forms a protective coating on wet epithelial surfaces throughout the body that houses the microbiota and plays a key role in host defense. Mucins, the primary structural components of mucus that creates its viscoelastic properties, are critical components of the gel layer that protect against invading pathogens. Altered mucin production has been implicated in diseases such as ulcerative colitis, asthma, and cystic fibrosis, which highlights the importance of mucins in maintaining homeostasis. Different types of mucins exist throughout the body in various locations such as the gastrointestinal tract, lungs, and female genital tract, but this review will focus on mucins in the oral cavity. Salivary mucin structure, localization within the oral cavity, and defense mechanisms will be discussed. These concepts will then be applied to present what is known about the protective function of mucins in oral diseases such as HIV/AIDS, oral candidiasis, and dental caries.

Histidine phosphotransfer proteins in fungal two-component signal transduction pathways

The histidine phosphotransfer (HPt) protein Ypd1 is an important participant in the Saccharomyces cerevisiae multistep two-component signal transduction pathway and, unlike the expanded histidine kinase gene family, is encoded by a single gene in nearly all model and pathogenic fungi. Ypd1 is essential for viability in both S. cerevisiae and in Cryptococcus neoformans. These and other aspects of Ypd1 biology, combined with the availability of structural and mutational data in S. cerevisiae, suggest that the essential interactions between Ypd1 and response regulator domains would be a good target for antifungal drug development. The goal of this minireview is to summarize the wealth of data on S. cerevisiae Ypd1 and to consider the potential benefits of conducting related studies in pathogenic fungi.

Antimicrobial peptide MUC7 12-mer activates the calcium/calcineurin pathway in Candida albicans

MUC7 12-mer is a cationic antimicrobial peptide derived from the N-terminal region of human low-molecular-weight salivary mucin. In order to gain new insights into the modes of action of the 12-mer against opportunistic fungal pathogen Candida albicans, we examined changes in the gene expression profile of C. albicans upon exposure to this peptide. Cells at an early logarithmic phase were exposed to 6 muM peptide and grown until an OD(600 nm) of approximately 0.4 was reached. Changes in gene expression were determined by microarray analysis and showed that 19 out of the total of 531 genes, whose expression was elevated in response to the peptide, are regulated by the calcium/calcineurin signalling pathway. Inactivation of this pathway by deletions, or by FK506, caused hypersensitivity to the peptide, demonstrating the importance of this pathway to the defense of C. albicans against the MUC7 peptide. Other differentially expressed genes that were detected include those encoding subunits of proteasome, and genes involved in cell stress, iron metabolism, cell wall maintenance and small-molecule transport. The presented results suggest that the calcium/calcineurin signalling pathway plays a role in the adaptation of C. albicans to the MUC7 antimicrobial peptide.

Mucin-bacterial interactions in the human oral cavity and digestive tract

Mucins are a family of heavily glycosylated proteins that are the major organic components of the mucus layer, the protective layer covering the epithelial cells in many human and animal organs, including the entire gastro-intestinal tract. Microbes that can associate with mucins benefit from this interaction since they can get available nutrients, experience physico-chemical protection and adhere, resulting in increased residence time. Mucin-degrading microorganisms, which often are found in consortia, have not been extensively characterized as mucins are high molecular weight glycoproteins that are hard to study because of their size, complexity and heterogeneity. The purpose of this review is to discuss how advances in mucus and mucin research, and insight in the microbial ecology promoted our understanding of mucin degradation. Recent insight is presented in mucin structure and organization, the microorganisms known to use mucin as growth substrate, with a specific attention on Akkermansia muciniphila, and the molecular basis of microbial mucin degradation owing to availability of genome sequences.

Structure and function of the polymeric mucins in airways mucus

The airways mucus gel performs a critical function in defending the respiratory tract against pathogenic and environmental challenges. In normal physiology, the secreted mucins, in particular the polymeric mucins MUC5AC and MUC5B, provide the organizing framework of the airways mucus gel and are major contributors to its rheological properties. However, overproduction of mucins is an important factor in the morbidity and mortality of chronic airways disease (e.g., asthma, cystic fibrosis, and chronic obstructive pulmonary disease). The roles of these enormous, multifunctional, O-linked glycoproteins in health and disease are discussed.

Factors affecting antimicrobial activity of MUC7 12-mer, a human salivary mucin-derived peptide

Background

MUC7 12-mer (RKSYKCLHKRCR), a cationic antimicrobial peptide derived from the human low-molecular-weight salivary mucin MUC7, possesses potent antimicrobial activity in vitro. In order to evaluate the potential therapeutic application of the MUC7 12-mer, we examined the effects of mono- and divalent cations, EDTA, pH, and temperature on its antimicrobial activity.

Methods

Minimal Inhibitory Concentrations (MICs) were determined using a liquid growth inhibition assay in 96-well microtiter plates. MUC7 12-mer was added at concentrations of 1.56–50 μM. MICs were determined at three endpoints: MIC-0, MIC-1, and MIC-2 (the lowest drug concentration showing 10%, 25% and 50% of growth, respectively). To examine the effect of salts or EDTA, a checkerboard microdilution technique was used. Fractional inhibitory concentration index (FICi) was calculated on the basis of MIC-0. The viability of microbial cells treated with MUC7 12-mer in the presence of sodium or potassium was also determined by killing assay or flow cytometry.

Results

The MICs of MUC7 12-mer against organisms tested ranged from 6.25–50 μM. For C. albicans, antagonism (FICi 4.5) was observed for the combination of MUC7 12-mer and calcium; however, there was synergism (FICi 0.22) between MUC7 12-mer and EDTA, and the synergism was retained in the presence of calcium at its physiological concentration (1–2 mM). No antagonism but additivity or indifference (FICi 0.55–2.5) was observed for the combination of MUC7 12-mer and each K⁺, Na⁺, Mg²⁺, or Zn²⁺. MUC7 12-mer peptide (at 25 μM) also exerted killing activity in the presence of NaCl, (up to 25 mM for C. albicans and up to 150 mM for E. coli, a physiological concentration of sodium in the oral cavity and serum, respectively) and retained candidacidal activity in the presence of KCl (up to 40 mM). The peptide exhibited higher inhibitory activity against C. albicans at pH 7, 8, and 9 than at pH 5 and 6, and temperature up to 60°C did not affect the activity.

Conclusion

MUC7 12-mer peptide is effective anticandidal agent at physiological concentrations of variety of ions in the oral cavity. These results suggest that, especially in combination with EDTA, it could potentially be applied as an alternative therapeutic agent for the treatment of human oral candidiasis.

Human salivary mucin MUC7 12-mer-L and 12-mer-D peptides: antifungal activity in saliva, enhancement of activity with protease inhibitor cocktail or EDTA, and cytotoxicity to human cells

MUC7 12-mer-L exhibits potent in vitro antifungal activity in low-ionic-strength buffers. In this study, we investigated the anticandidal activity and stability of MUC7 12-mer-L and its all-D-amino-acid isomer, along with Hsn5 12-mer (P113) and magainin-II, in human clarified and unclarified saliva in the absence or presence of protease inhibitor cocktail (PIC, which includes EDTA) or EDTA alone. In the absence of PIC or EDTA in saliva, only MUC7 peptides showed significant candidacidal activity. At a 100 microM concentration in clarified saliva and unclarified saliva, MUC7 12-mer-D demonstrated 94 versus 64% killing, respectively; MUC7 12-mer-L showed 57 versus 32% killing; Hsn5 12-mer showed 16 versus 0% killing; and magainin-II showed no killing. Addition of PIC or EDTA to either saliva caused the enhancement of antifungal activities of all peptides, although to different degrees. Taken together, the results suggest that EDTA (a metal-dependent protease inhibitor and/or divalent cation chelator) enhanced the antifungal activity of all four peptides mainly by chelation of divalent cations present in saliva (known to inhibit peptide antifungal activity), and PIC enhanced the activity of the three L peptides above that achievable by EDTA alone through inhibition of all classes of proteases. Peptide stability in saliva monitored by sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed no degradation of MUC7 12-mer-D and 23, 60, and 75% degradation of MUC7 12-mer-L, Hsn5 12-mer, and magainin-II, respectively. Cytotoxicity assays determined that, at 100 microM peptide concentrations, MUC7 12-mer-D and 12-mer-L caused 3.5 and 4.3% hemolysis in phosphate-buffered saline and no toxicity to the HOK-16B cell line (derived from normal human oral keratinocytes). In summary, MUC7 12-mer peptides appear to be excellent candidates for investigation of antifungal activity in in vivo models of oral candidiasis.

Efficacy of human salivary mucin MUC7-derived peptide and histatin 5 in a murine model of candidiasis

MUC7 16-mer (residues 36-51 of human salivary mucin, MUC7) and histatin 5 possess potent in vitro antifungal activity. In the present study, we have evaluated the efficacy of these peptides in vivo using the experimental model of murine vulvo-vaginal candidiasis. The treatment groups included MUC7 16-mer, histatin 5, clotrimazole (all in pluronic F127 gel), and placebo (gel alone). Mice were treated intravaginally for 7 consecutive days. At the end of the treatment, anticandidal activities were assessed by colony counts and by histological examination. All groups except clotrimazole presented positive cultures; no statistically significant differences were found in fungal burden amongst placebo and any treatment group except clotrimazole. Histopathological findings confirmed the microbiological results; all groups with the exception of clotrimazole showed variable signs of infection.

New Monoclonal Antibodies to Non-Glycosylated Domains of the Secreted Mucins MUC5B and MUC7

The separation and characterization of salivary mucins is not straightforward because of their large size, heterogeneity, and molecular interactions. The MUC5B and MUC7 mucins are major glycoprotein components of saliva that are thought to play a vital role in maintaining oral health. MUC5B is also a major component of respiratory mucus and is produced by the tracheal and bronchial glands, while MUC7 has a more limited pattern of expression in the bronchial tree. MUC5B is a gel-forming mucin and thus confers viscosity, whereas MUC7 is much smaller. MUC7 has anti-fungal activity, and both mucins interact with bacteria. The aim of this work was to produce new monoclonal antibodies that can be used to quantify and characterize these mucins by standard laboratory procedures. Peptide sequences in non-conserved and non-glycosylated regions were selected and monoclonal antibodies produced by an efficient immunization and cloning strategy, and screening against purified mucins. Three new antibodies-EU-MUC5Ba and EU-MUC5Bb (against MUC5B) and EU-MUC7a (against MUC7)-were isolated that do not show cross-reactivity with other gel-forming mucins. All work on immunohistochemistry can be used for semi-quantitative immunoblotting after agarose gel electrophoresis. These reagents are valuable tools to study changes in these mucins in oral and respiratory disease, and unlike other monoclonal antibodies to these mucins they recognize epitopes that are not affected by glycosylation.

Human salivary MUC7 mucin peptides: effect of size, charge and cysteine residues on antifungal activity

We have previously shown that MUC7 (human salivary low-molecular-mass mucin) 20-mer: LAHQKPFIRKSYKCLHKRCR (residues 32-51 of the parent MUC7, with a net positive charge of 7) possesses a broad-spectrum antimicrobial activity [Bobek and Situ (2003) Antimicrob. Agents Chemother. 47, 645-652]. The aims of the present study were to determine the minimum peptide chain length and its location within the 20-mer region that retains potent antifungal activity against Candida albicans and Cryptococcus neoformans and to examine the effect of net charge of the peptide as well as the role of cysteine residues on the fungicidal activity. First, several C-terminal truncated MUC7 20-mer fragments (16-mer, 12-mer, 11-mer, 10-mer and 8-mer) and one N-terminal fragment (8-mer-N) were synthesized and tested. The results showed that MUC7 12-mer, located at the C-terminal region of MUC7 20-mer, having a net charge of +6 and exhibiting an amphipathic helical conformation, not only retained but exceeded the antifungal activity of that of 20-mer. Secondly, several variants of the 12-mer peptide containing a lower or no net positive charge, or no cysteine residues were synthesized and tested. A clear correlation between the net positive charge of the 12-mer, its potency and initial interaction of peptide with fungal cells was found by killing assays, fluorescence microscopy and fungal cell-membrane potential measurements. Furthermore, cysteine residues, which play a critical role in bacterial binding, were found to be not important for the fungicidal activity of these peptides. These results identified MUC7 12-mer as a potential candidate for development into a novel antifungal therapeutic agent.

Identification of App1 as a regulator of phagocytosis and virulence of Cryptococcus neoformans

Cryptococcus neoformans is a fungal pathogen that, after inhalation, can disseminate to the brain. Host alveolar macrophages (AMs) represent the first defense against the fungus. Once phagocytosed by AMs, fungal cells are killed by a concerted mechanism, involving the host-cellular response. If the cellular response is impaired, phagocytosis of the fungus may be detrimental for the host, since C. neoformans can grow within macrophages. Here, we identified a novel cryptococcal gene encoding antiphagocytic protein 1 (App1). App1 is a cryptococcal cytoplasmic protein that is secreted extracellularly and found in the serum of infected patients. App1 does not affect melanin production, capsule formation, or growth of C. neoformans. Treatment with recombinant App1 inhibited phagocytosis of fungal cells through a complement-mediated mechanism, and Deltaapp1 mutant is readily phagocytosed by AMs. Interestingly, the Deltaapp1 mutant strain showed a decreased virulence in mice deficient for complement C5 (A/Jcr), but it was hypervirulent in mice deficient for T and NK cells (Tgepsilon26). This study identifies App1 as a novel regulator of virulence for C. neoformans, and it highlights that internalization of fungal cells by AMs increases the dissemination of C. neoformans when the host cellular response is impaired.

MUC7 20-Mer: investigation of antimicrobial activity, secondary structure, and possible mechanism of antifungal action

This study was aimed at examining the spectrum of antimicrobial activity of MUC7 20-mer (N-LAHQKPFIRKSYKCLHKRCR-C; residues 32 to 51 of MUC7, the low-molecular-weight human salivary mucin, comprised of 357 residues) and comparing its antifungal properties to those of salivary histatin 5 (Hsn-5). We also examined the secondary structure of the 20-mer and the possible mechanism of its antifungal action. Our results showed that MUC7 20-mer displays potent killing activity against a variety of fungi and both gram-positive and gram-negative bacteria at micromolar concentrations. Time-dependent killing of Candida albicans and Cryptococcus neoformans by MUC7 20-mer and Hsn-5 indicated differences in killing rates between MUC7 20-mer and Hsn-5. The secondary structure prediction showed that MUC7 20-mer adopts an amphiphilic helix with distinguishable hydrophilic and hydrophobic faces (a characteristic that is associated with antimicrobial activity). In comparison to that of Hsn-5, the fungicidal activity of MUC7 20-mer against C. albicans seems to be independent of fungal cellular metabolic activity, as evidenced by its killing potency at a low temperature (4 degrees C) and in the presence of inhibitors of oxidative phosphorylation in the mitochondrial system. Fluorescence microscopy showed the ability of MUC7 20-mer to cross the fungal cell membrane and to accumulate inside the cells. The internalization of MUC7 20-mer was inhibited by divalent cations. Confocal microscopy of cells doubly labeled with MUC7 20-mer and a mitochondrion-specific dye indicated that mitochondria are not the target of MUC7 20-mer for either C. albicans or C. neoformans.