A Novel Role for Histatin 5 in Combination with Zinc to Promote Commensalism in C. albicans Survivor Cells

Abstract

Candida albicans is maintained as a commensal by immune mechanisms at the oral epithelia. Oral antifungal peptide Histatin 5 (Hst 5) may function in innate immunity, but the specific role Hst 5 plays in C. albicans commensalism is unclear. Since Zn-binding potentiates the candidacidal activity of Hst 5, we hypothesized that Hst 5+Zn would elicit a unique fungal stress response to shape interactions between C. albicans and oral epithelial cells (OECs). We found that Hst 5+Zn but not Hst 5 alone resulted in the activation of cell wall integrity (CWI) signaling, and deletion mutants were then used to determine that CWI-mediated chitin synthesis was protective against killing. Using flow cytometry, we confirmed that Hst 5+Zn-treated cells had significantly elevated levels of cell-wall chitin, mannan and β-1,3 glucan compared to Hst 5-treated cells. We then tested the activation of host signaling components involved in C. albicans cell-wall recognition. The immunoblot assay of C. albicans-exposed oral epithelial cells showed increased activation of EphA2 and NF-κB but not EGFR. Interestingly, C. albicans treated with Hst 5+Zn induced the global suppression of pro-inflammatory cytokine release from OECs, but an increase in negative regulator IL-10. Hst 5+Zn-treated cells were more adherent but ultimately less invasive to OECs than control cells, thus indicating lowered virulence. Therefore, Hst 5+Zn-treated C. albicans cells are discerned by epithelial monolayers, but are less virulent and promote anti-inflammatory signaling, suggesting that Hst 5+Zn in combination could play a role in regulating commensalism of oral C. albicans through cell wall reorganization.

Zinc binding inhibits cellular uptake and antifungal activity of Histatin-5 in Candida albicans

“…It is likely this concentration-dependent effect of Zn 2+ has resulted in the varied reports regarding the effect of Zn 2+ on Hist-5 activity in the literature. 20,25,28,29 While our data reveal clear Zn-dependent effects on Hist-5 activity and uptake, the question that remains is why does Zn 2+ affect Hist-5 in this manner? C. albicans is a commensal organism that inhabits mucosal membranes of the human body, including the oral cavity.…”

“…Here, we add to the growing body of literature suggesting that Hist-5 may participate in interactions with cells that are not solely for antifungal purposes but rather promote and maintain microbial homeostasis for oral microbial health. 29, 53…”

“…Uptake of Hist-5 by C. albicans is widely accepted as a requirement for antifungal activity, as Hist-5 is thought to have intracellular targets. 9,25,29,41 We observed a decrease in Hist-5 activity as a function of increasing Zn 2+ concentration, thus we performed timelapse microscopy with C. albicans exposed to Hist-5* treated with a range of added Zn 2+ concentrations to determine whether the decrease in antifungal activity could be the result of decreased peptide internalization.…”

“…25 The authors subsequently followed up this study by showing that Hist-5+Zn 2+ treatment not only promotes fungicidal activity but also has a role in maintaining commensalism of C. albicans in the oral cavity. 29 While these studies suggest a role for Zn 2+ in modulating the effects of Hist-5 on C. albicans, the overall response remains unclear and contradictory. 20,25,28 Concentrations of Hist-5 and Zn 2+ in saliva are dynamic ranging from 0.7 -30 µM for Hist-5 30,31 and 0.0001 -155 µM for Zn 2+ .…”

Fungicidal Activity of AP10W, a Short Peptide Derived from AP-2 Complex Subunit mu-A, In Vitro and In Vivo

“…The modes of action of antifungal drugs are generally associated with disruption of the fungal cell wall, damage of plasma membrane, disturbance of intracellular components, or induction of ROS production [ 33 , 34 , 35 , 36 ]. Here we show that AP10W interacts with the fungi via their cell wall components laminarin, mannan and chitin, which can lead to disruption or destabilization of the fungal cell wall.…”