In all organisms, antimicrobial peptides are a major component of the innate immune defense system providing an immediate and rapid response to invading microorganisms (Zasloff 2002). These peptides generally act by forming pores in microbial membranes or otherwise disrupting membrane integrity, which is facilitated by their amphiphilic structure (see Schroder, this volume: Antimicrobial peptides as first-line effector molecules of the human innate immune system).
Much of the work on antimicrobial peptides from the Urochordata has been performed on hemocytes of ascidians of the family Styelidae (for a review, see Tincu and Taylor 2004). Briefly, from the hemocytes of Styela clava, the clavanins (a family of four-helical, amphipathic, histidine-rich antimicrobial peptides that contain 23 amino acids and exhibit C-terminal amidation) were purified (Lee et al., 1997a, b; Menzel et al. 2002). Clavanins A to D resemble the magainins, well characterized antimicrobial peptides from the skin of Xenopus laevis. Synthetic clavanin A displays antimicrobial activity comparable with that of magainins and cecropins (Lee et al. 1997a, b). In addition to Escherichia coli, Listeria monocy-togenes, and Candida albicans, clavanins are broadly effective against gram-positive bacteria, including Staphylococcus aureus (Menzel et al. 2002).
Two phenyalanine-rich antimicrobial peptides from Styela clava, styelin A and B, are effective against gram-negative and gram-positive bacterial pathogens of humans (Lee et al. 1997c). The styelins are highly basic polypeptides, encoded as prepropeptides, with a signal sequence and with cationic sequences in the mature protein counterbalanced by a polyanionic C-terminal extension in its precursor. Styelins also kill the marine bacteria Psychrobacter immobilis and Planococcus citreus in 0.4 M NaCl which approximates seawater salt concentrations (Lee et al. 1997c).
Plicatamide is a potently antimicrobial octapeptide from the blood cells of Styela plicata (Tincu et al. 2000, 2003). Wild-type and methicillin-resistant Staphylococcus aureus respond to plicatamide exposure with a massive potassium efflux that begins within seconds. Soon thereafter, treated bacteria largely cease consuming oxygen, and most become non-viable. Plicatamide forms cation-selective channels in model lipid bilayers composed of bacterial lipids.
Halocyamine A and B are two antimicrobial tetrapeptides isolated from the hemocytes of the ascidian Halocynthia roretzi (Azumi et al. 1990a). Halocyamine A was reported to inhibit the growth of yeast, Escherichia coli (Azumi et al. 1990a), and the marine bacteria Achromobacter aquamarinus and Pseudomonas perfectomarinus (Azumi et al. 1990b, c).
In addition to the halocyamines, antimicrobial peptides of 6.2 kDa and 3.4 kDa have been isolated from Halocynthia roretzi. The first, dicynthaurin, is composed of two 30-residue monomers without any sequence homology to previously identified peptides. Dicynthaurin's broad-spectrum activity includes Micrococcus luteus, Staphylococcus aureus, Listeria monocytogenes, Escherichia coli, and Pseudomonas aeruginosa but not Candida albicans (Lee et al. 2001a, b). The second, halocidin, has a mass of 3443 Da and is composed of two subunits containing 18 and 15 amino acid residues that are linked by a single disulfide bond (Jang et al. 2002). In antimicrobial assays halocidin was found to be active against methicillin-resistant Staphylococcus aureus and multidrug-resistant Pseudomonas aeruginosa (Jang et al. 2002).
Little is known about the signal transduction cascades involved in activating these antimicrobial peptides. Urochordates such as Ciona intestinalis (Azumi et al. 2003) and Boltenia villosa (Davidson and Swalla 2002), however, possess Tolllike receptors (TLRs) and share many components of the corresponding signal transduction cascades with vertebrates. Thus, similar to immune cells in higher organisms, cells in urochordates respond to microbes by the production and secretion of antimicrobial peptides and appear to use conserved signal transduction pathways.
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