Retrocyclins neutralize bacterial toxins by potentiating their unfolding

Abstract

Defensins are a class of immune peptides with a broad range of activities against bacterial, fungal and viral pathogens. Besides exerting direct anti-microbial activity via dis-organization of bacterial membranes, defensins are also able to neutralize various unrelated bacterial toxins. Recently, we have demonstrated that in the case of human α- and β-defensins, this later ability is achieved through exploiting toxins’ marginal thermodynamic stability, i.e. defensins act as molecular anti-chaperones unfolding toxin molecules and exposing their hydrophobic regions and thus promoting toxin precipitation and inactivation [Kudryashova et al. (2014) Immunity 41, 709–721]. Retrocyclins (RCs) are humanized synthetic θ-defensin peptides that possess unique cyclic structure, differentiating them from α- and β-defensins. Importantly, RCs are more potent against some bacterial and viral pathogens and more stable than their linear counterparts. However, the mechanism of bacterial toxin inactivation by RCs is not known. In the present study, we demonstrate that RCs facilitate unfolding of bacterial toxins. Using differential scanning fluorimetry (DSF), limited proteolysis and collisional quenching of internal tryptophan fluorescence, we show that hydrophobic regions of toxins normally buried in the molecule interior become more exposed to solvents and accessible to proteolytic cleavage in the presence of RCs. The RC-induced unfolding of toxins led to their precipitation and abrogated activity. Toxin inactivation by RCs was strongly diminished under reducing conditions, but preserved at physiological salt and serum concentrations. Therefore, despite significant structural diversity, α-, β- and θ-defensins employ similar mechanisms of toxin inactivation, which may be shared by anti-microbial peptides from other families.

Thermodynamic instability of viral proteins is a pathogen-associated molecular pattern targeted by human defensins

“…Thus, it has been demonstrated that a conserved Trp-26 residue is essential for HNP-1 activity as a contributor to hydrophobicity for interaction with target molecules and for formation of dimers15. However, retrocyclins, which are similarly active against viral (current study) and bacterial effectors12, do not have tryptophans, suggesting that the role of this residues is fulfilled by other structural elements. In the absence of a detailed and quantifiable model, we can provide only speculative explanation to the observed differences in activities of the peptides.…”

Full Sequence Amino Acid Scanning of θ-Defensin RTD-1 Yields a Potent Anthrax Lethal Factor Protease Inhibitor

“…41 Precipitation of bacterial toxins in the presence of defensins has been also described as a mechanism the inactivation of some bacterial toxins by these types of peptides. 42 In our study, however, we did not see any evidence of protein precipitation under the conditions used in our assays.…”

Targeting and inactivation of bacterial toxins by human defensins

“…The toxin is comprised of several well-defined components: i) a variable number (from one to five) of effector domains with distinct toxicities found in different combinations; ii) a cysteine protease domain (CPD) always positioned C-terminally to the effector domains and responsible for domain liberation upon delivery into a host cell (Prochazkova et al, 2009; Shen et al, 2009); and iii) the N- and C-terminal glycine-rich Ca 2+ -binding repeats that serve for membrane integration and translocation across the membrane (Kim et al, 2015). The specific enzymatic activity of one of the MARTX effector domains, actin cross-linking domain (ACD; (Heisler et al, 2015; Kudryashova et al, 2016a)) produced by V. cholerae and A. hydrophila , was tested in vitro and found to be potently inhibited by α- (HNP1, HD5; (Kudryashova et al, 2014b)) and θ-defensins (RC1 and RC101; (Kudryashova et al, 2015)). Cytotoxicity of both ACD and Rho inhibitory effector domain (RID; (Ahrens et al, 2013)) was prevented by HNP1 in cell culture experiments (Kudryashova et al, 2014b).…”

“…Just over a decade ago, a group of AMPs called defensins was reported to efficiently neutralize anthrax toxin both in vitro and in an animal model (Kim et al, 2005). In the following years, it became increasingly clear that a small set of defensins are universal effector molecules capable of neutralizing numerous toxins, many of which are unrelated to each other (Giesemann et al, 2008; Hooven et al, 2012; Kim et al, 2006; Kudryashova et al, 2014b; Kudryashova et al, 2015; Lehrer et al, 2009; Wang et al, 2006; Welkos et al, 2011). …”

“…They demonstrate antimicrobial activity against Gram-negative and Gram-positive bacteria and against enveloped and non-enveloped viruses; next, all defensins are signaling molecules that modulate inflammatory response, wound healing, and angiogenesis in peptide- and tissue-specific manner (recently reviewed in (Mattar et al, 2016; Zhao and Lu, 2014)). Finally, defensins can selectively interfere with functional activity of different classes of pathogen effector proteins: various families of pore-forming, non-enzymatic, and enzymatic toxins (Giesemann et al, 2008; Hooven et al, 2012; Kim et al, 2005; Kim et al, 2006; Kudryashova et al, 2014b; Kudryashova et al, 2015; Lehrer et al, 2009; Wang et al, 2006; Welkos et al, 2011) and various viral proteins (Flatt et al, 2013; Gounder et al, 2012; Kudryashova et al, 2016b; Smith et al, 2010). The combination of the antitoxin, microbicidal, and immunomodulatory activities of α- and β-defensins is believed to play an important protective role during infection (Arnett and Seveau, 2011).…”