RESEARCH

 

 

Cationic Antimicrobial Peptides

One of the major research interests in the McDermott lab is the expression and role(s) of cationic antimicrobial peptides at the ocular surface.

Cationic antimicrobial peptides are small peptides (12-50 amino acids long) which have an overall positive charge (hence the cationic part of the name) and which have a broad spectrum of antimicrobial activity. To date over 700 different peptides have been identified in eukaryotic species. These peptides can be divided in to 4 groups : b-sheet peptides which have 2-4 disulphide bonds; a-helix peptides which form an a-helix on contact with membranes; extended peptides which are rich in certain amino acids such as proline or tryptophan; loop peptides which have a single disulphide bond.

Although there are many different peptides they appear to use very similar mechanisms to kill micro-organisms. The most popular model is that the cationic antimicrobial peptide interacts electrostatically with the negatively charged microbial cell membrane. Several molecules of the peptide then come together and form a pore in the membrane. The presence of the pores then allows some small molecules inside of the microorganism to flow out. This disturbs the normal metabolic pathways going on inside the organism, eventually DNA synthesis, protein synthesis and respiration stop and the organism dies. Some times so many pores form that it disturbs the osmotic balance and the organism simply bursts ! Some researchers have described the effect of cationic antimicrobial peptides on a microorganism like a fly being hit by a brick!

Cationic antimicrobial peptides are part of the innate immune system. This system is our first line of defence against pathogenic microorganisms. In humans cationic antimicrobial peptides are chiefly found in two places: neutrophils and epithelial tissue. Neutrophils are white blood cells that travel to sites of injury and infection and there they ingest any microorganism they come in to contact with. Once the organism is ingested it is then killed by the combined action of enzymes and cationic antimicrobial peptides (such as a-defensins and LL-37) that are present within special granules in the neutrophil. Epithelial tissue covers the internal and external surfaces of the body and is the first tissue that a microorganism must breach to gain entry in to the body. The epithelial cells secrete cationic antimicrobial peptides (such as b-defensins and LL-37) in to the surrounding environment to help defend against invasion by microorganisms.

In addition to directly killing microorganisms, cationic antimicrobial peptides have been shown to have a number of effects on human cells. In particular, evidence is mounting to suggest that cationic antimicrobial peptides are important regulators in the immune system. For example some peptides are able to attract specialized white blood cells called T-cells to a site of infection. Some have also been implicated in wound healing processes where they stimulate cell proliferation and formation of new blood vessels.

Cationic Antimicrobial Peptides at the Ocular Surface

As noted earlier, one of the major research interests in our lab is the expression and role(s) of cationic antimicrobial peptides at the ocular surface. In particular we are interested in cationic antimicrobial peptides called b-defensins and LL-37 (the only known human member of the cathelicidin group of antimicrobial peptides).

There are many human b-defensins, but we have found that only a few are expressed by the corneal and conjunctival epithelia. These are human b-defensin (hBD)-1, hBD-2, and hBD-3. We find that two of these, namely hBD-1 and hBD-3 are normally expressed by the epithelial cells. hBD-2 is not usually present in any significant amount but we have found that it is expressed by corneal epithelial cells as they regenerate after an injury and that exposure to inflammatory cytokines such as IL-1 can upregulate hBD-2 expression.

We have also found that while LL-37 is expressed by the normal ocular surface epithelia, it too is upregulated after injury. We have shown that b-defensins and LL-37 have antimicrobial activity against common ocular pathogens such as Pseudomonas aeruginosa and Staphylococcus aureus and also viruses such as adenovirus.

Furthermore we have found that cationic antimicrobial peptides can modulate the behaviour of corneal epithelial cells during wound healing e.g. LL-37 stimulates the cells to release various cytokines.

So we think that the cationic antimicrobial peptides hBD-1, hBD-3 and LL-37provide “base line” antimicrobial protection to the ocular surface, then after injury there is additional expression of hBD-2 and LL-37. This could have two benefits: 1. Gives additional antimicrobial protection to the cornea during a time when it is particularly vulnerable to infection by widening the spectrum of antibacterial activity; 2. May actually help in the wound healing process itself by stimulating the corneal epithelial cells to migrate, proliferate and secrete important cytokines.

 

   
 

References for recent review articles about cationic antimicrobial peptides:

Ganz T. Defensins and other antimicrobial peptides: a historical perspective and an update. Comb Chem High Throughput Screening 2005, 8, 209-17. Lehrer RI. Primate defensins. Nature Revs Micro 2004, 2, 727-738. Yang D,

Biragyn A, Hoover DM et al. Multiple roles of antimicrobial defensins, cathelicidins and eosinophil-derived neurotoxin in host defence. Ann Rev Immunol, 2004, 22,181-215.

Boman HG. Anteribacterial peptides: basic facts and emerging concepts. J Internal Med 2003, 254, 197-215. Zaiou M and GalloRL. Cathelicidins, essential gene-encoded mammalian antibiotics. J Mol Med 2002, 80, 549-561.

Recent publications about cationic antimicrobial peptides at the ocular surface:

Narayanan S, Manning J, Proske R, McDermott AM. (2006) The effect of hyperosmolality on b-defensin gene expression by human corneal epithelial cells. In Press Cornea

Huang LC, Petkova TD, Reins RY, Proske RJ and McDermott AM. Multi-functional roles of human cathelicidin (LL-37) at the ocular surface. Invest Ophthalmol Vis Sci. 2006, 47, 2369-2380.

McDermott AM, RichD, Cullor J, Mannis MJ, Smith W, Reid T, Murphy CJ. The in vitro activity of selected defensins against an isolate of Pseudomonas in the presence of human tears Br. J. Ophthalmol2006, 90, 609-611.

Gordon YJ, Romanoswki EG, McDermottAM. A review of antimicrobial peptides and their therapeutic potential as anti-infective drugs. Curr Eye Res 2005, 30, 505-515.

Gordon YJ, HuangLH, Romanowski EG, Yates KA, Proske RJ, McDermott AM. Human cathelicidin (LL-37), a multifunctional peptide, is expressed by ocular surface epithelia and has potent antibacterial and antiviral activity. Curr Eye Res 2005, 30,385-394.

Huang LC, Jean D,McDermott AM. Effect of preservative free artificial tears on the antimicrobial activity of human b-defensin-2 and Cathelicidin LL-37 in vitro. Eye Contact Lens, 2005, 31, 34-38.

McDermott AM. Defensins and other antimicrobial peptides at the ocular surface. The OcularSurface, 2004, 2, 229-247. NarayananS, Miller WL and McDermottAM. Expression of humanb-defensins in conjunctival epithelium: relevance to dry eye disease. Invest. Ophthalmol.Vis. Sci., 2003, 44, 3795-3801.

McDermottAM, Redfern RL, Zhang B, Pei Y, Huang L and Proske RJ. Defensin expression by the cornea: multiple signaling pathways mediate IL-1b stimulation of hBD-2 expression by human corneal epithelial cells. Invest. Ophthalmol. Vis.Sci., 2003, 44, 1859-1865.

McDermott AM,Redfern RL, Zhang B. Human b-defensin 2 is upregulated during re-epithelialization of the cornea. Curr Eye Res. 2001;22:64-67.

Recent publications by other researchers interested in cationic antimicrobial peptides at the ocular surface:

Rodriguez-MartinezS, Cancino-Diaz ME, Cancino-Diaz JC. Expression of CRAMP via PGN-TLR-2 and of alpha-defensin-3via CpG-ODN-TLR-9 in corneal fibroblasts. Br J Ophthalmol. 2006, 90, 378-82.

Ikeda A, NakanishiY, Sakimoto T, Shoji J, Sawa M, Nemoto N. Expression of beta defensins in ocular surface tissue of experimentally developed allergic conjunctivitis mouse model. Jpn J Ophthalmol. 2006, 50, 1-6.

Kumar A, Zhang J,Yu FS. Toll-likereceptor 2-mediated expression of beta-defensin-2 in human corneal epithelial cells. Microbes Infect. 2006, 8, 380-9.