Avian antimicrobial host defense peptides: from biology to therapeutic applications

Abstract

Host defense peptides (HDPs) are an important first line of defense with antimicrobial and immunomoduatory properties. Because they act on the microbial membranes or host immune cells, HDPs pose a low risk of triggering microbial resistance and therefore, are being actively investigated as a novel class of antimicrobials and vaccine adjuvants. Cathelicidins and β-defensins are two major families of HDPs in avian species. More than a dozen HDPs exist in birds, with the genes in each HDP family clustered in a single chromosomal segment, apparently as a result of gene duplication and diversification. In contrast to their mammalian counterparts that adopt various spatial conformations, mature avian cathelicidins are mostly α-helical. Avian β-defensins, on the other hand, adopt triple-stranded β-sheet structures similar to their mammalian relatives. Besides classical β-defensins, a group of avian-specific β-defensin-related peptides, namely ovodefensins, exist with a different six-cysteine motif. Like their mammalian counterparts, avian cathelicidins and defensins are derived from either myeloid or epithelial origin expressed in a majority of tissues with broad-spectrum antibacterial and immune regulatory activities. Structure-function relationship studies with several avian HDPs have led to identification of the peptide analogs with potential for use as antimicrobials and vaccine adjuvants. Dietary modulation of endogenous HDP synthesis has also emerged as a promising alternative approach to disease control and prevention in chickens.

Figure 1

Schematic drawing of vertebrate cathelicidin and defensin precursor peptides. (A) Cathelicidins and cathelicidin-related peptides known as NGPs are highly conserved in the cathelin-like domain that contains two disulfide bridges. Unlike cathelicidins whose C-terminal segments are highly variable across species and proteolytically cleaved from the cathelicidin-like domain to become biologically active, NGPs are conserved throughout the entire sequence and functionally active without being processed. (B) The defensin family includes classical α-, β-, and θ-defensins with indicated disulfide bonds as well as four subfamilies of defensin-related peptides with unknown disulfide bonding patterns. Avian-specific ovodefensins contain six cysteines but with a different spacing pattern from that of classical defensins. Rodent-specific CRS1C, CRS4C, and rattusin also exist with 11, 9, and 5 cysteine residues, respectively, that presumably form intermolecular disulfide bonds. Positively and negatively charged amino acids are indicated in red and green, respectively.

Figure 2

Amino acid sequence alignment of four chicken cathelicidins. Conserved sequences are shaded and identical residues are in red. Dashes are created to maximize the alignment. Each cathelicidin precursor consists of a conserved signal peptide sequence, a cathelin-like domain, and a variable C-terminal mature peptide sequence. Four cysteines in the cathelin-like domain are highlighted in yellow. Note that an N-terminal, 117-amino acid segment of CATH-B1 was omitted for clarity.

Figure 3

Amino acid sequence alignment of chicken β-defensins and ovodefensins. Conserved sequences are shaded and identical residues are in red. Dashes are created to maximize the alignment. Each β-defensin precursor is comprised of a conserved signal peptide, an optional short prosequence, and a C-terminal mature sequence consisting of six cysteines. Note that the cysteine spacing patterns are different between chicken ovodefensins (known as gallin 1–3) and classical β-defensins. Additional C-terminal tail sequences of AvBD3, 11, and 13 were omitted for simplicity.

Figure 4

Phylogenetic analysis of avian cathelicidins. The phylogenetic tree was constructed with the full-length amino acid sequences using the neighbor-joining method, and the reliability of each branch was assessed by using 1,000 bootstrap replications. Numbers on the branches indicate the percentage of 1,000 bootstrap samples supporting the branch. The species and GenBank accession number of each sequence are indicated.

Figure 5

Phylogenetic analysis of avian defensins (see Figure 4 legend for details).

Figure 6

Structures of avian cathelicidins and β-defensins. (A) Secondary structural features of cathelicidins and β-defensins. (B) Tertiary ribbon structures of cathelicidins and β-defensins. Polar residues are indicated in blue and nonpolar residues in red. Disulfide bonds of β-defensins are shown in yellow. Protein Data Bank identification number for each molecule is indicated in parenthesis.

Figure 7

Potential therapeutic applications of host defense peptides (HDPs). Synthetic HDPs can be directly administered exogenously as antimicrobials, vaccine adjuvants or infertility drugs. Alternatively, endogenous HDPs can modulated by dietary compounds for antimicrobial therapies.