Cationic Antimicrobial Peptides in Humans

Cationic Antimicrobial Peptides in Humans Antimicrobial peptides Introduction Cationic antimicrobial peptides (AMPs) are gene-encoded peptides of the host defence system made up of 12-50 amino acids, with at least 2 positive charges conferred by lysine and arginine residues and about 50% hydrophobic amino acids (Hancock and Scott 2000). They are produced from gene transcription and ribosomal translation and often, further proteolytically processed (Zhoa 2003). The peptides are folded so that non-polar amino acid side-chains form a hydrophobic face and polar, positively charged residues form a hydrophilic face (Robert and Hancock 1997). Expression of antimicrobial peptides can be constitutive or inducible by infectious or inflammatory stimuli like cytokines, bacteria and lipopolysaccharides (LPS) (Cunliffe and Mahida 2004). They have diverse structures to effectively kill a wide range of microbes at prone sites e. g the skin and lungs, and in secretions such as sweat and saliva (Yeaman and Yount 2004; Santamaria 2005). Many mammalian antimicrobial peptides rouse the host’s innate immune system (Jenssen et al 2006) instead of directly killing the host. Peptides which are found in living organisms from bacteria to plants, insects, fish, amphibians to mammals including humans (Kamysz 2005) are recorded in numerous existing databases e. g. AMSDb (Eukaryotic peptides) (Tossi and Sandri 2002), BAPDb (bacterial peptides), ANTIMIC (natural antimicrobial peptides) (Brahmachary et al 2004) and APPDb. Currently, 1831 peptides are hosted by the Antimicrobial peptide database with 99 antiviral, 453 antifungal, 100 anticancer and 1179 antibacterial peptides (The Antimicrobial Peptide database 2010). In humans, antimicrobial peptides are produced by granulocytes, macrophages and most epithelial and endothelial cells. They boost the immune system, have anti-neoplastic properties and help in regulating cell signalling and multiplication. Amphibian AMPs have been discovered from the skin of frogs from families ra nging from Iomedusa, Pipidae, Hyperoliidae, Ranidae, Hylidae, Discoglossidae, Agalychnis and Litoria. The structure of these peptides as unravelled by CD spectroscopy, NMR spectroscopy and molecular modeling (Suh et al 1996) have been found to be generally 10-46 amino acid residues long (Rollins-Smith et al 2005), mostly linear and simple-structured, (Conlon et al 2004) the majority being hydrophobic, cationic and possessing an amphipathic a-helix in nature. Following production, they are stored in the granular glands (poison glands) of skin dermal layer to be secreted in response to injury (Bovbjerg 1963), or as defence against pathogenic bacteria, fungi, viruses and parasites. Biologically active molecules including antimicrobial peptides are produced as large proteins harbouring a signal and an acidic propiece which get cut off to give an active peptide prior to or at secretion from the poison glands (Amiche et al 1999). Cationic peptides are also expressed in the gastric mucosa cells and in the intestinal tract (Kamysz 2005). The best-known peptides isolated from frogs are caeruleins, tachykinins, bradykinins, thyrotropin- releasing hormone (Barra and Simmaco 1995), brevinins, esculentins, magainins, ranatuerins and temporins (Conlon et al 2004). In the past, peptides were extracted using solvents like methanol or acid from the skins of amphibians after sun-drying but with concomitant dwindling of many frog species, other alternative techniques have emerged, one of which comprises stimulating the frog using mild electricity and collecting the skin secretion; 2-4 weeks after, the secretion can be re-collected after replenishment of the glands (Barra and Simmaco 1995). Large amounts of small peptides and their analogues which are resistant to protease cleavage and contain D-amino acids can be chemically synthesised while larger peptides can be expressed in a prokaryotic host from cloned cDNAs coding for a fusion protein (Piers et al 1993). An efficient means of producing therapeutic peptides in transgenic mice red blood cells has been explained by Sharma et al (1994) whereby the required peptide is collected from proteolytic cleavage from the fusion protein where the peptide is at the C-terminal end of human a-globin.