(707b) Modifying Histatin 5 for Enhanced Resistance to Candida albicans Secreted Aspartic Proteases

Histatin 5 is a highly cationic antimicrobial peptide found in human saliva and it has potential as an antifungal therapeutic to treat Candida albicans infections. C. albicans is a commensal organism that is part of the normal flora of oral cavity, but it is also an opportunistic fungal pathogen that can cause infections, especially in immunocompromised patients. Although histatin 5 has strong and specific antifungal activity against C. albicans, the pathogen produces a family of ten secreted and cell-wall bound enzymes called secreted aspartic proteases (Saps), some of which can degrade histatin 5. Several of the Saps have been previously shown to inactivate histatin 5 by cutting the peptide at either side of a positively charged lysine residue. To improve the therapeutic potential of histatin 5 and better understand its interaction with various Saps and C. albicans cells, we have designed and tested variants of histatin 5 that have each of the lysine residues substituted with either an arginine or a leucine. All variants were found to have antifungal activity similar to the parent histatin 5, which shows that the peptide is tolerant of arginine and leucine modifications without affecting its fungicidal capabilities. On the other hand, several of the peptide variants showed a significant difference in susceptibility to proteolysis by purified Sap2 and Sap9. Replacing Lys17 with either arginine or leucine led to almost complete resistance to proteolysis by both Saps, while only 45% and 10% of the unmodified histatin 5 remained intact after exposure to Sap2 and Sap9, respectively. Interestingly, substitution at Lys13 to either leucine or arginine made the peptide more prone to proteolysis by Sap9, with Lys13Leu resulting in complete degradation by Sap9. Similar results were obtained after incubation of histatin 5 and the modified peptides with C. albicans cells. Both of the Lys17 substitutions showed less degradation while Lys13Leu mutation showed more degradation compared to histatin 5. These results show that peptide modifications could provide insight into the interaction between histatin 5 and the Saps and aid in designing more robust histatin 5 analogs with enhanced therapeutic potential.