Metal Sequestration and Antimicrobial Activity of Human Calprotectin Are pH-Dependent

Abstract

Human calprotectin (CP, S100A8/S100A9 oligomer) is an abundant innate immune protein that sequesters transition metal ions in the extracellular space to limit nutrient availability and the growth of invading microbial pathogens. Our current understanding of the metal-sequestering ability of CP is based on biochemical and functional studies performed at neutral or near-neutral pH. Nevertheless, CP can be present throughout the human body and is expressed at infection and inflammation sites that tend to be acidic. Here, we evaluate the metal binding and antimicrobial properties of CP in the pH range of 5.0–7.0. We show that Ca­(II)-induced tetramerization, an important process for the extracellular functions of CP, is perturbed by acidic conditions. Moreover, a low pH impairs the antimicrobial activity of CP against some bacterial pathogens, including Staphylococcus aureus and Salmonella enterica serovar Typhimurium. At a mildly acidic pH, CP loses the ability to deplete Mn from microbial growth medium, indicating that Mn­(II) sequestration is attenuated under acidic conditions. Evaluation of the Mn­(II) binding properties of CP at pH 5.0–7.0 indicates that mildly acidic conditions decrease the Mn­(II) binding affinity of the His6 site. Lastly, CP is less effective at preventing capture of Mn­(II) by the bacterial solute-binding proteins MntC and PsaA at low pH. These results indicate that acidic conditions compromise the ability of CP to sequester Mn­(II) and starve microbial pathogens of this nutrient. This work highlights the importance of considering the local pH of biological sites when describing the interplay between CP and microbes in host–pathogen interactions.

Zinc import mediated by AdcABC is critical for colonization of the dental biofilm by Streptococcus mutans in an animal model

“…However, the differences in the levels of metallothionein between the two groups were not significant suggesting that metallothionein levels do not increase in response to caries. Given the very low levels of calprotectin that we found in saliva and recent observations that the manganese‐binding affinity of calprotectin is compromised at low pH (Rosen & Nolan, 2020), we speculate that zinc restriction in the oral cavity is primarily mediated by metallothioneins.…”

“…Although calprotectin can be present in oral fluids and tissues, particularly during inflammatory processes such as periodontitis, oral cancer, and oropharyngeal candidiasis (Holmstrom et al., 2019; Sweet et al., 2001), there is no evidence of increases in salivary calprotectin levels in dental caries. Moreover, recent studies indicated that mildly acidic conditions compromise the ability of calprotectin to chelate manganese but not zinc (Rosen & Nolan, 2020). In addition, salivary glands synthesize metallothioneins, a family of low‐molecular‐weight cysteine‐rich proteins that scavenge free radicals and can also chelate zinc and copper in tissues (Rahman & Karim, 2018).…”

Calprotectin-mediated zinc chelation inhibitsPseudomonas aeruginosaprotease activity in cystic fibrosis sputum

“…Despite the reportedly high concentrations of CP in the serum, sputum, and BALF of CF patients (2,(21)(22)(23)(24), P. aeruginosa appears to be able to access enough zinc to persist. Various environmental factors may influence CP zinc binding such as calcium concentrations (53), pH (54), or the presence of oxidants (55,56). Additionally, while CP in its tetrameric state is resistant to proteolytic degradation, CP is susceptible to oxidation which in turn makes it susceptible to proteolytic degradation by both host and bacterial proteases (55,56).…”

Zinc Import Mediated by AdcABC is Critical for Colonization of the Dental Biofilm byStreptococcus mutansin an Animal Model

“…However, the differences in the levels of metallothionenin between the two groups were not significant suggesting that metallothionein levels do not increase in response to caries. Given the very low levels of calprotectin that we found in saliva and recent observations that the manganese-binding affinity of calprotectin is compromised at low pH (Rosen & Nolan, 2020), we speculate that zinc restriction in the oral cavity is primarily mediated by metallothioneins.…”

“…While calprotectin can be present in oral fluids and tissues, particularly during inflammatory processes such as periodontitis, oral cancer and oropharyngeal candidiasis (Holmstrom et al, 2019;Sweet, Denbury, & Challacombe, 2001), there is no evidence of increases in salivary calprotectin levels in dental caries. Moreover, recent studies indicated that mildly acidic conditions compromise the ability of calprotectin to chelate manganese but not zinc (Rosen & Nolan, 2020). In addition, salivary glands synthesize metallothioneins, a family of low molecular weight cysteine-rich proteins that scavenge free radicals and can also chelate zinc and copper in tissues (Rahman & Karim, 2018).…”