Construction and characterization of human salivary histatin-5 multimers

Human antimicrobial peptides and proteins

As the key components of innate immunity, human host defense antimicrobial peptides and proteins (AMPs) play a critical role in warding off invading microbial pathogens. In addition, AMPs can possess other biological functions such as apoptosis, wound healing, and immune modulation. This article provides an overview on the identification, activity, 3D structure, and mechanism of action of human AMPs selected from the antimicrobial peptide database. Over 100 such peptides have been identified from a variety of tissues and epithelial surfaces, including skin, eyes, ears, mouths, gut, immune, nervous and urinary systems. These peptides vary from 10 to 150 amino acids with a net charge between -3 and +20 and a hydrophobic content below 60%. The sequence diversity enables human AMPs to adopt various 3D structures and to attack pathogens by different mechanisms. While α-defensin HD-6 can self-assemble on the bacterial surface into nanonets to entangle bacteria, both HNP-1 and β-defensin hBD-3 are able to block cell wall biosynthesis by binding to lipid II. Lysozyme is well-characterized to cleave bacterial cell wall polysaccharides but can also kill bacteria by a non-catalytic mechanism. The two hydrophobic domains in the long amphipathic α-helix of human cathelicidin LL-37 lays the basis for binding and disrupting the curved anionic bacterial membrane surfaces by forming pores or via the carpet model. Furthermore, dermcidin may serve as ion channel by forming a long helix-bundle structure. In addition, the C-type lectin RegIIIα can initially recognize bacterial peptidoglycans followed by pore formation in the membrane. Finally, histatin 5 and GAPDH(2-32) can enter microbial cells to exert their effects. It appears that granulysin enters cells and kills intracellular pathogens with the aid of pore-forming perforin. This arsenal of human defense proteins not only keeps us healthy but also inspires the development of a new generation of personalized medicine to combat drug-resistant superbugs, fungi, viruses, parasites, or cancer. Alternatively, multiple factors (e.g., albumin, arginine, butyrate, calcium, cyclic AMP, isoleucine, short-chain fatty acids, UV B light, vitamin D, and zinc) are able to induce the expression of antimicrobial peptides, opening new avenues to the development of anti-infectious drugs.

Histatin-derived monomeric and dimeric synthetic peptides show strong bactericidal activity towards multidrug-resistant Staphylococcus aureus in vivo

Homodimerization of histatin-derived peptides generally led to improved bactericidal activity against Staphylococcus aureus in vitro. In vivo, monomers and dimers were equally active in killing bacteria in mice with a soft tissue infection. Altogether, these peptides are promising compounds for the development of novel therapeutics against infections with drug-resistant bacteria.

Histatin-derived peptides: potential agents to treat localised infections

Histatins are a family of histidine-rich, cationic peptides composed of up to 38 amino acids. They are secreted by the salivary glands of humans and some subhuman primates and are thought to be part of the host defence system in the oral cavity. Histatins exhibit in vitro activity against both bacteria and yeast, common to other antimicrobial peptides. Because of these activities, histatin-based peptides could play an important role in the treatment and prevention of infectious diseases. A 12 amino acid amidated fragment of histatin 5, designated P-113, has been identified as the smallest fragment that retains antimicrobial activity comparable to the parent compound. Animal studies and human clinical trials showed that P-113 has potential in preventing the development of gingivitis, with no adverse side effects. Histatin peptides also could be used for other therapeutic applications in which the infection is localised and accessible via topical delivery, such as treatment of candidiasis (thrush) and mucositis in the oral cavity, skin infections and treatment of lung infections afflicting cystic fibrosis patients.

Candida albicans Ssa1/2p is the cell envelope binding protein for human salivary histatin 5

Salivary histatins are a family of small histidine-rich peptides with potent antifungal activity. We previously identified a 70-kDa cell envelope protein in Candida albicans and Saccharomyces cerevisiae that mediates binding of histatin (Hst) 5. Isolation of Hst 5-binding protein followed by matrix-assisted laser desorption ionization mass spectrometry analysis identified this protein as the heat shock protein Ssa1p. Ssa protein and Hst 5-binding protein were found to be co-localized on immunoblots of yeast beta-mercaptoethanol cell wall extracts and cytosolic fractions. Yeast two-hybrid analysis showed strong interactions between Ssa1p and both Hst 3 and Hst 5. To assess functional roles of Ssa proteins in the Hst 5 antifungal mechanism in vivo, both binding and fungicidal assays were carried out using S. cerevisiae isogenic SSA1/SSA2 mutants. 125I-Hst 5 binding assays showed saturable binding (Kd = 2.57 x 10(-6) m) with the wild-type SSA1/SSA2 strain; however, Hst 5 binding with the Deltassa1ssa2 double mutant was reduced (Kd = 1.25 x 10(-6) m). Cell wall HSP70 proteins were also diminished, but still detectable, in S. cerevisiae Deltassa1ssa2 cells and are likely to be Ssa3p or Ssa4p. Hst 5 (31 microm) killed 80% of the wild-type cells in fungicidal assays at room temperature. However, only 50-60% killing of the single mutants (Deltassa1 and Deltassa2) was observed, and fungicidal activity was further reduced to 20-30% in the Deltassa1ssa2 double mutant. Incubation of cells under heat shock conditions increased the sensitivity of cells to Hst 5, which correlated with increased Hst 5-binding activity in Deltassa1ssa2 cells, but not in wild-type cells. This study provides evidence for a novel function for yeast Ssa1/2 proteins as cell envelope binding receptors for Hst 5 that mediate fungicidal activity.

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

Domain 1 of the low-molecular-weight human salivary mucin, designated MUC7 D1, spans the 51 N-terminal amino acid residues. This domain contains a 15-residue basic histidine-rich subdomain (R3-Q17) which has 53% sequence similarity to histatin 5 (Hsn-5), a salivary molecule known to exert potent in vitro cidal activity against Candida albicans and many other medically important fungi. The MUC7 D1-15mer and its derivatives have previously been synthesized in our laboratory and their candidacidal activities have been found to be inferior to that of Hsn-5. We were therefore intrigued to explore the candidacidal potency of the full-length MUC7 D1 (51-mer). Linear solid-phase synthesis of this domain has been accomplished following standard Fmoc chemistry. The problems of partial coupling, owing to the peptide chain length, at several stages of the solid-phase step-by-step synthesis were circumvented either by double-coupling techniques or efficient coupling procedures. The MUC7 D1 peptide was purified to homogeneity by conventional reverse-phase HPLC using two columns connected in series. Secondary structure of the purified peptide was assessed by circular dichroism (CD) spectroscopy in phosphate buffer and trifluoroethanol and compared to that of MUC7 D1-15mer and Hsn-5. The MUC7 D1 candidacidal activity was assessed against azole-sensitive and azole-resistant C. albicans strains and was found, unlike that of the MUC7 D1-15mer, to be comparable with that of Hsn-5, indicating that in addition to Hsn-5, MUC7 D1 could provide an attractive alternative to the classical antifungal agents. The candidacidal potency of MUC7 D1, like that of MUC7 D1-15mer, and of Hsn-5, appears to be largely dependent on peptide charge, irrespective of alpha-helical structure.

In vitro assessment of antifungal therapeutic potential of salivary histatin-5, two variants of histatin-5, and salivary mucin (MUC7) domain 1

Human salivary histatin-5 (Hsn-5) is a 24-residue peptide that possesses potent antifungal activity in vitro. The MUC7 gene encodes human salivary low-molecular-weight mucin (MG2). The candidacidal activity of MUC7 domain 1 (MUC7 D1, the N-terminal 51 amino acid residues of MUC7) in vitro has also been demonstrated. In this study, we have investigated the antifungal therapeutic potential of Hsn-5, its two variants, R12I/K17N and R12I/H21L, and MUC7 D1. First, these peptides were tested for activities against different clinically important fungi. We found them to possess broad-spectrum antifungal activities; specifically, most exhibited excellent in vitro activity against eight clinically important fungal strains tested, including Candida albicans and Candida glabrata and their azole-resistant counterparts and Cryptococcus neoformans and its amphotericin B-resistant counterpart. These findings also suggest that the mechanism of action of both Hsn-5 and MUC7 D1 for these fungi is different from that of amphotericin B or azole antifungal agents. Second, we examined the stability of these peptides in whole human saliva and human serum. In saliva, the Hsn-5 variants R12I/K17N and R12I/H21L and MUC7 D1 degraded at a lower rate than Hsn-5. In human serum, MUC7 D1 was also more stable than Hsn-5; both peptides were more stable in serum than in saliva. Third, we examined the cytotoxicity of these peptides using human erythrocytes and two human cell lines (KB and HSG). No (or very low) hemolytic activity was observed with any of the four peptides, even at the highest protein concentration tested (200 microM), while amphotericin B caused 100% hemolysis at only 12.5 microM. The toxic effects of Hsn-5 and MUC7 D1 toward KB and HSG cells were also much lower than that of amphotericin B as measured by trypan blue exclusion. Together, these findings indicate that the investigated peptides possess high antifungal therapeutic potential, in particular for the treatment of drug-resistant fungal strains associated with immunocompromised (particularly human immunodeficiency virus-infected) patients. The same peptides could also be used as components of artificial saliva for patients with salivary dysfunction.