The focus of infection is surrounded by epithelial cells that express-defensins

Courtesy of Lide Liu (Univ. of California, Los Angeles)

Antimicrobial Peptides

• Antimicrobial peptides are evolutionarily ancient weapons. Their widespread distribution and abundance (> 800 eukaryotic peptides described) throughout the animal and plant kingdoms suggests that antimicrobial peptides have served a fundamental role in the successful evolution of complex multicellular organisms.

• The fundamental structural principle underlying all classes is the ability of the molecule to adopt a shape in which clusters of hydrophobic and cationic amino acids are spatially organized in discrete sectors of the molecule ('amphipathic' design).

• All antimicrobial peptides are derived from larger precursors, including a signal sequence. Post-translational modifications include proteolytic processing, and in some cases glycosylation, carboxy-terminal amidation and amino-acid isomerization and halogenation.

• Some peptides are derived by proteolysis from larger proteins, such as buforin II from histone 2A and lactoferricin from lactoferrin.

• Most multicellular organisms express a cocktail comprising multiple peptides from several of these structural classes within their 'defensive' tissues.

Antimicrobial peptides from plants and invertebrates

Schroder-CMLS-99

Mammalian Defensins

Yang et al, Trends Immunol, 02

In human, Paneth Cells in Crypts of Leiberkuhn secrete -defensins or cryptins.

proHD5 is proteolytically cleaved extrcellularly by trypsin. It also highlights that this enzyme has other nondigestive function.

Unmodified proHD5 also has antimicrobial activity, especially when tested against Gram-positive L. monocytogenes.

The differential antibacterial activity between the HD5 forms that exist in vivo suggests that proteolytic processing could be a mechanism that diversifies the spectrum of antibiotic activity from a single antimicrobial gene product.

Basis of Specificity of Antimicrobial Peptides

The Shai–Matsuzaki–Huang (SMH) model for AMP activity

How do antimicrobial peptides actually kill microbes?

• The creation of physical holes that cause cellular contents to leak out [BioPhys. J. 79, 2002 (2000)].

• Fatal depolarization of the normally energized bacterial membrane [PNAS 86, 6597 (1989)].

• The activation of deadly processes, such as autolysis, by the induction of hydrolases that degrade the cell wall [Arch. Microbiol. 141, 249 (1985)].

• The scrambling of the usual distribution of lipids between the leaflets of the bilayer, resulting in disturbance of membrane functions [BBA 1462, 1 (1999)].

• The damaging of critical intracellular targets after internalization of the peptide. For example, the peptide pyrrhocoricin, inhibits ATPase actions of DnaK and prevent chaperone-assisted protein folding [Biochem. 40, 3016 (2001)].

Cathelicidins

• Processed by neutrophil elastase in the extracellular milieu.• Extracellular proteolytic activation of AMPs is also conserved in lower

vertebrates and insects.• Antimicrobial activity is synergistic with lysozyme and lactoferrin, PNAS 95(1988),9541

• Activity is dependent on the concentration of NaCl (activity diminishes 4- to 5-fold as the total concentration of NaCl is elevated from 60 to 155 mM), PNAS 95(1988),9541.

Mouse Cathelicidin, CRAMP, Protects the Skin from Group A Streptococcus

Nizet et al, Nature 2001

Important cytokines secreted by macrophages in response to bacterial products