The β-defensins are a family of small, cationic antimicrobial peptides. They are conserved in a variety of species ranging from lower vertebrates to mammals and plants, and were first identified for their broad-spectrum antimicrobial activity. Since their initial discovery, this role has broadened to include a number of diverse additional functions including the chemotaxis of immune cells (immature dendritic cells, macrophages and CD4+ T cells), the determination of coat colour in dogs and seed maturation in tomatoes.
The aim of this thesis is to investigate the function of vertebrate beta-defensins. 1 address this question by assessing how mouse β-defensin structure affects its bactericidal activity and also by carrying out P-defensin knockdown studies in zebrafish.
In this work, 1 address the question of how β-defensin structure affects its bactericidal activity by examining the effects of sequentially removing amino acids from the N-terminal of murine Defbl4. I show that these deletions reduce bactericidal activity however the effects are much more striking in gram positive species than gram negative. Through the comparison of monomeric and dimeric species as well as analysis of peptide charge and hydrophobicity, this work indicates that a combination of primary sequence and structure is responsible for the bactericidal properties of this peptide.
In addition, this thesis describes the characterisation of three β-defensin-like peptides (Defbll, Defbl2 and DefbB) previously described in zebrafish. 1 utilise antimicrobial assays to determine the bactericidal properties of synthetic Defbl2 and DefbB against a panel of microbes and also show that Defbll is antimicrobially inactive. Furthermore, this work identifies the expression of defbll during zebrafish gastrulation and utilises a combination of wholemount in situ hybridisation, morpholino knockdown and rescue and microarray analysis to ascertain a novel essential role for this peptide in early development.
This work is the first report of a β-defensin being involved in vertebrate development and presents a further widening of the influence of the defensin family.