The Plant Defensin Ppdef1 Is a Novel Topical Treatment for Onychomycosis.

Onychomycosis, or fungal nail infection, causes not only pain and discomfort but can also have psychological and social consequences for the patient. Treatment of onychomycosis is complicated by the location of the infection under the nail plate, meaning that antifungal molecules must either penetrate the nail or be applied systemically. Currently, available treatments are limited by their poor nail penetration for topical products or their potential toxicity for systemic products. Plant defensins with potent antifungal activity have the potential to be safe and effective treatments for fungal infections in humans. The cystine-stabilized structure of plant defensins makes them stable to the extremes of pH and temperature as well as digestion by proteases. Here, we describe a novel plant defensin, Ppdef1, as a peptide for the treatment of fungal nail infections. Ppdef1 has potent, fungicidal activity against a range of human fungal pathogens, including Candida spp., Cryptococcus spp., dermatophytes, and non-dermatophytic moulds. In particular, Ppdef1 has excellent activity against dermatophytes that infect skin and nails, including the major etiological agent of onychomycosis Trichophyton rubrum. Ppdef1 also penetrates human nails rapidly and efficiently, making it an excellent candidate for a novel topical treatment of onychomycosis.

Salt-Tolerant Antifungal and Antibacterial Activities of the Corn Defensin ZmD32.

Pathogenic microbes are developing resistance to established antibiotics, making the development of novel antimicrobial molecules paramount. One major resource for discovery of antimicrobials is the arsenal of innate immunity molecules that are part of the first line of pathogen defense in many organisms. Gene encoded cationic antimicrobial peptides are a major constituent of innate immune arsenals. Many of these peptides exhibit potent antimicrobial activity in vitro. However, a major hurdle that has impeded their development for use in the clinic is the loss of activity at physiological salt concentrations, attributed to weakening of the electrostatic interactions between the cationic peptide and anionic surfaces of the microbial cells in the presence of salt. Using plant defensins we have investigated the relationship between the charge of an antimicrobial peptide and its activity in media with elevated salt concentrations. Plant defensins are a large class of antifungal peptides that have remarkable stability at extremes of pH and temperature as well as resistance to protease digestion. A search of a database of over 1200 plant defensins identified ZmD32, a defensin from Zea mays, with a predicted charge of +10.1 at pH 7, the highest of any defensin in the database. Recombinant ZmD32 retained activity against a range of fungal species in media containing elevated concentrations of salt. In addition, ZmD32 was active against Candida albicans biofilms as well as both Gram negative and Gram-positive bacteria. This broad spectrum antimicrobial activity, combined with a low toxicity on human cells make ZmD32 an attractive lead for development of future antimicrobial molecules.

Structural and functional characterization of the membrane-permeabilizing activity of Nicotiana occidentalis defensin NoD173 and protein engineering to enhance oncolysis.

Defensins are an extensive family of host defense peptides found ubiquitously across plant and animal species. In addition to protecting against infection by pathogenic microorganisms, some defensins are selectively cytotoxic toward tumor cells. As such, defensins have attracted interest as potential antimicrobial and anticancer therapeutics. The mechanism of defensin action against microbes and tumor cells appears to be conserved and involves the targeting and disruption of cellular membranes. This has been best defined for plant defensins, which upon binding specific phospholipids, such as phosphatidylinositol 4,5-bisphosphate (PIP2) and phosphatidic acid, form defensin-lipid oligomeric complexes that destabilize membranes, leading to cell lysis. In this study, to further define the anticancer and therapeutic properties of plant defensins, we have characterized a novel plant defensin, Nicotiana occidentalis defensin 173 (NoD173), from N. occidentalis. NoD173 at low micromolar concentrations selectively killed a panel of tumor cell lines over normal primary cells. To improve the anticancer activity of NoD173, we explored increasing cationicity by mutation, with NoD173 with the substitution of Q22 with lysine [NoD173(Q22K)], increasing the antitumor cell activity by 2-fold. NoD173 and the NoD173(Q22K) mutant exhibited only low levels of hemolytic activity, and both maintained activity against tumor cells in serum. The ability of NoD173 to inhibit solid tumor growth in vivo was tested in a mouse B16-F1 model, whereby injection of NoD173 into established subcutaneous tumors significantly inhibited tumor growth. Finally, we showed that NoD173 specifically targets PIP2 and determined by X-ray crystallography that a high-resolution structure of NoD173, which forms a conserved family-defining cysteine-stabilized-αß motif with a dimeric lipid-binding conformation, configured into an arch-shaped oligomer of 4 dimers. These data provide insights into the mechanism of how defensins target membranes to kill tumor cells and provide proof of concept that defensins are able to inhibit tumor growth in vivo.-Lay, F. T., Ryan, G. F., Caria, S., Phan, T. K., Veneer, P. K., White, J. A., Kvansakul, M., Hulett M. D. Structural and functional characterization of the membrane-permeabilizing activity of Nicotiana occidentalis defensin NoD173 and protein engineering to enhance oncolysis.

Binding of phosphatidic acid by NsD7 mediates the formation of helical defensin-lipid oligomeric assemblies and membrane permeabilization.

Defensins are cationic antimicrobial peptides that serve as important components of host innate immune defenses, often by targeting cell membranes of pathogens. Oligomerization of defensins has been linked to their antimicrobial activity; however, the molecular basis underpinning this process remains largely unclear. Here we show that the plant defensin NsD7 targets the phospholipid phosphatidic acid (PA) to form oligomeric complexes that permeabilize PA-containing membranes. The crystal structure of the NsD7-PA complex reveals a striking double helix of two right-handed coiled oligomeric defensin fibrils, the assembly of which is dependent upon the interaction with PA at the interface between NsD7 dimers. Using site-directed mutagenesis, we demonstrate that key residues in this PA-binding site are required for PA-mediated NsD7 oligomerization and coil formation, as well as permeabilization of PA-containing liposomes. These data suggest that multiple lipids can be targeted to induce oligomerization of defensins during membrane permeabilization and demonstrate the existence of a "phospholipid code" that identifies target membranes for defensin-mediated attack as part of a first line of defense across multiple species.

The Tomato Defensin TPP3 Binds Phosphatidylinositol (4,5)-Bisphosphate via a Conserved Dimeric Cationic Grip Conformation To Mediate Cell Lysis.

Defensins are a class of ubiquitously expressed cationic antimicrobial peptides (CAPs) that play an important role in innate defense. Plant defensins are active against a broad range of microbial pathogens and act via multiple mechanisms, including cell membrane permeabilization. The cytolytic activity of defensins has been proposed to involve interaction with specific lipid components in the target cell wall or membrane and defensin oligomerization. Indeed, the defensin Nicotiana alata defensin 1 (NaD1) binds to a broad range of membrane phosphatidylinositol phosphates and forms an oligomeric complex with phosphatidylinositol (4,5)-bisphosphate (PIP2) that facilitates membrane lysis of both mammalian tumor and fungal cells. Here, we report that the tomato defensin TPP3 has a unique lipid binding profile that is specific for PIP2 with which it forms an oligomeric complex that is critical for cytolytic activity. Structural characterization of TPP3 by X-ray crystallography and site-directed mutagenesis demonstrated that it forms a dimer in a "cationic grip" conformation that specifically accommodates the head group of PIP2 to mediate cooperative higher-order oligomerization and subsequent membrane permeabilization. These findings suggest that certain plant defensins are innate immune receptors for phospholipids and adopt conserved dimeric configurations to mediate PIP2 binding and membrane permeabilization. This mechanism of innate defense may be conserved across defensins from different species.

Phosphoinositide-mediated oligomerization of a defensin induces cell lysis.

Cationic antimicrobial peptides (CAPs) such as defensins are ubiquitously found innate immune molecules that often exhibit broad activity against microbial pathogens and mammalian tumor cells. Many CAPs act at the plasma membrane of cells leading to membrane destabilization and permeabilization. In this study, we describe a novel cell lysis mechanism for fungal and tumor cells by the plant defensin NaD1 that acts via direct binding to the plasma membrane phospholipid phosphatidylinositol 4,5-bisphosphate (PIP2). We determined the crystal structure of a NaD1:PIP2 complex, revealing a striking oligomeric arrangement comprising seven dimers of NaD1 that cooperatively bind the anionic headgroups of 14 PIP2 molecules through a unique 'cationic grip' configuration. Site-directed mutagenesis of NaD1 confirms that PIP2-mediated oligomerization is important for fungal and tumor cell permeabilization. These observations identify an innate recognition system by NaD1 for direct binding of PIP2 that permeabilizes cells via a novel membrane disrupting mechanism. DOI: http://dx.doi.org/10.7554/eLife.01808.001.

Recombinant expression and purification of the tomato defensin TPP3 and its preliminary X-ray crystallographic analysis.

Class II defensins have been shown to have potent antifungal activity and are being exploited to protect agricultural crops against fungal pathogens. TPP3 is a poorly characterized member of the class II plant defensin family from tomato. To gain structural insight into the function of TPP3, soluble recombinant TPP3 was expressed and purified using the Pichia pastoris expression system, and the crystallization and preliminary X-ray crystallographic analysis of the protein are reported. Crystals of rTPP3 were obtained using the sitting-drop vapour-diffusion method at 293 K. Diffraction data were collected to 1.7 Å resolution. The crystals belonged to the hexagonal space group P6(1)22, with unit-cell parameters a = 64.97, b = 64.97, c = 82.40 Å, α = 90, ß = 90, γ = 120°.