Calprotectin

Calprotectin is a complex of the mammalian proteins S100A8 and S100A9.[1][2] In the presence of calcium, calprotectin is capable of sequestering the essential nutrients manganese and zinc[2][3] via chelation.[4] This metal sequestration affords the complex antimicrobial properties.[2][3] Calprotectin is the only known antimicrobial manganese sequestration protein complex.[5] Calprotectin comprises as much as 60% of the soluble protein content of the cytosol of a neutrophil,[2][6][7] and it is secreted by an unknown mechanism during inflammation.[8] Faecal calprotectin has been used to detect intestinal inflammation (colitis or enteritis) and can serve as a biomarker for inflammatory bowel diseases.[6][9] Other names for calprotectin include MRP8-MRP14, calgranulin A and B, cystic fibrosis antigen, L1, 60BB antigen, and 27E10 antigen.[2][8]

Structure

Crystal structure of Mn2+ and Ca2+ loaded calprotectin, showing two S100A8-S100A9 dimers. The grey and blue chains represent S100A8 and S100A9, respectively. Purple spheres represent Mn2+ and green spheres represent Ca2+. Only one manganese ion can bind per calprotectin dimer.

The human homologue of calprotectin is a 24 kDa dimer,[5] and is formed by the protein monomers S100A8 (10,835 Da) and S100A9 (13,242 Da).[10][11] The primary structure of calprotectin can vary between species. For instance, the mouse homologue of S100A8 is 10,295 Da,[12] while the S100A9 homologue is 13,049 Da.[13] Early size exclusion chromatography experiments incorrectly indicated that calprotectin had a molecular mass of 36.5 kDa;[2][7] occasionally this value is used in contemporary literature. Calprotectin S100A8-S100A9 dimers can non-covalently pair with one another to form 48 kDa tetramers.

Metal binding

Calprotectin has a high affinity for calcium, zinc and manganese.[6][7][14] Each of S100A8 and S100A9 contain two EF-hand type Ca2+ binding sites,[5][8] and calprotectin is able to bind a total of four calcium ions per dimer or eight calcium ions per tetramer.[15] Calcium binding induces a conformational change in the complex that improves its affinity for transition metals, and promotes tetramer formation.[2][5] A maximum of two transition metal ions may bind to each calprotectin S100A8-S100A9 dimer.[5]

A calprotectin dimer can bind only one manganese ion with high affinity, and it can do this only in the presence of calcium.[5][16] Zinc can bind at two sites within the calprotectin dimer, and this can occur in the absence of calcium.[2] Calcium, however, improves calprotectin's affinity for zinc.[5] While calprotectin metal binding occurs at the interface of S100A9 and S100A8 monomers, the independent monomers have some capacity for zinc binding, and may contribute to zinc homeostasis within mammals.[2][10][11]

His6 coordination of Mn2+ in calprotectin. S100A8 histidine residues are coloured grey, S100A9 histidine residues are coloured purple.

The first of the two calprotectin metal binding sites consists of a His3Asp motif, with S100A8 contributing two histidine ligands (His83 and His87), and S100A9 contributing a histidine and an aspartic acid ligand (His20 and Asp30).[5] The second site can coordinate metals through a tetra-histidine (His4) or a hexa-histidine (His6) binding motif. In the case of His4 binding, S100A8 coordinates through both His17 and His27 while S100A9 coordinates through His91 and His95.[5] In hexa-histidine binding two further histidine residues, His103 and His105, are recruited from the C-terminal end of S100A9 to enable octahedral coordination of the transition metal.[5] Manganese is bound by the calprotectin dimer at this His6 site.[5] Zinc can be bound to either of the sites that form at the interface between S100A8 and S100A9 monomers.[5][16]

Antimicrobial properties

Transition metals are essential to the survival of all organisms.[17] Mammals strictly limit metal availability as a part of the innate immune system, and this helps prevent infection by microbes and fungi.[17] Calprotectin was first described in the 1980s as a mammalian antimicrobial protein that acts through the sequestration of zinc.[1][2][5] It is now known that calprotectin also has antibacterial and antifungal properties that arise from its ability to sequester manganese.[3][5] Calprotectin is the only known antimicrobial agent that acts through manganese sequestration.[5]

Calprotectin constitutes up to 60% of soluble protein content in the cytosol of neutrophil granulocytes,[2][6][7] and it can be found at a lower concentration in monocytes, macrophages, and squamous epithelial cells.[2][6][7] Calprotectin enters into pus and abscess fluid during neutrophil cell death, along with other antimicrobial proteins.[2]

Mammalian cells secrete calprotectin during the inflammatory response. For instance, calprotectin is secreted in the mouth during inflammation of the gingiva and during oral candidiasis infection.[18][19] People who have mutations in the calprotectin gene appear susceptible to serious gum infections.[18] Manganese sequestration by calprotectin is likely important during lung inflammation.[3] The exact mechanism by which S100A8 and S100A9 is secreted by mammalian cells during inflammation remains unknown.[8]

Faecal calprotectin

Calprotectin becomes available in the intestinal lumen via leukocyte shedding,[1] active secretion,[2][7] cell disturbance, and cell death.[1][7] This results in elevated faecal calprotectin levels, which can be detected in the stool.[1][7] Elevated faecal calprotectin levels therefore indicate migration of neutrophils into the intestinal mucosa, which occurs during intestinal inflammation.[1][7][14] As people with active inflammatory bowel diseases (IBD) such as ulcerative colitis or Crohn disease have as much as a 10-fold increase in faecal calprotectin levels,[6] the measurement of faecal calprotectin can serve as a biochemical test for these diseases.

Although a relatively new test, faecal calprotectin is regularly used as an indicator for IBD during treatment, and as a diagnostic marker.[9] Faecal calprotectin tests can also function in distinguishing patients with irritable bowel syndrome from those with IBD.[1][7] Calprotectin is useful as a marker, as it is resistant to enzymatic degradation, and can be easily measured in faeces.[20] Although faecal calprotectin correlates significantly with disease activity in people with confirmed IBD,[21] elevated faecal calprotectin can be a false-positive indicator of IBD under some conditions. Importantly, intake of proton pump inhibitor is associated with significantly elevated calprotectin values.[22] Furthermore, positive faecal calprotectin does not help in localizing IBD, or in distinguishing ulcerated colitis from Crohn disease.[1] Faecal calprotectin can also indicate other gastrointestinal conditions such as colorectal cancer, gastroenteritis, and food intolerance.[1] Calprotectin levels vary depending on age, comorbidity, and may vary day-to-day within individuals.[1] Faecal calprotectin could be used as a preliminary screen in otherwise functional patients suspected of having IBD, or as a means of following mucosal healing.[1] The potential for using faecal calprotectin in this way is debated, however, and cut-off levels have not been agreed upon.[1]

See also

References

  1. 1 2 3 4 5 6 7 8 9 10 11 12 Lehmann, F. S.; Burri, E.; Beglinger, C. (13 October 2014). "The role and utility of faecal markers in inflammatory bowel disease". Therapeutic Advances in Gastroenterology. 8 (1): 23–36. PMC 4265086Freely accessible. PMID 25553077. doi:10.1177/1756283X14553384.
  2. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Striz, I; Trebichavsky, I (2004). "Calprotectin - a pleiotropic molecule in acute and chronic inflammation.". Physiological research / Academia Scientiarum Bohemoslovaca. 53 (3): 245–53. PMID 15209531.
  3. 1 2 3 4 Costa, Lucio G; Aschner, Michael (2014). Manganese in Health and Disease. Royal Society of Chemistry. p. 146. ISBN 1849739439. Retrieved 27 January 2015.
  4. Clark, HL; et al. (2016), "Zinc and manganese chelation by neutrophil s100a8/a9 (calprotectin) limits extracellular Aspergillus fumigatus hyphal growth and corneal infection", J Immunol, 196 (1): 336–344, PMID 26582948, doi:10.4049/jimmunol.1502037.
  5. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Brophy, Megan Brunjes; Nolan, Elizabeth M. (16 January 2015). "Manganese and Microbial Pathogenesis: Sequestration by the Mammalian Immune System and Utilization by Microorganisms". ACS Chemical Biology. 10: 150116125412006. doi:10.1021/cb500792b.
  6. 1 2 3 4 5 6 Marshall, William Marshall,; Lapsley, Marta; Day, Andrew; Ayling, Ruth (2014). Clinical Biochemistry: Metabolic and Clinical Aspects (3 ed.). Elsevier Health Sciences, 2014. ISBN 9780702054785. Retrieved 19 January 2015.
  7. 1 2 3 4 5 6 7 8 9 10 Gupta, Ramesh (2014). Biomarkers in toxicology. San Diego, CA: Academic Press. pp. 272–273. ISBN 9780124046498. Retrieved 19 January 2015.
  8. 1 2 3 4 Celio, Marco R.; Pauls, Thomas; Schwaller, Beat (1996). Guidebook to the calcium-binding proteins. Oxford: Sambrook & Tooze Publication at Oxford University Press. pp. 147–148. ISBN 0198599501.
  9. 1 2 van Rheenen PF, Van de Vijver E, Fidler V (2010). "Faecal calprotectin for screening of patients with suspected inflammatory bowel disease: diagnostic meta-analysis". BMJ. 341: c3369. PMC 2904879Freely accessible. PMID 20634346. doi:10.1136/bmj.c3369. Lay summary MedScape.
  10. 1 2 UniProt Consortium. "P05109- S10A8_HUMAN". http://www.uniprot.org/. UniProt Consortium. Retrieved 21 January 2015. External link in |website= (help)
  11. 1 2 UniProt Consortium. "P06702- S10A9_HUMAN". http://www.uniprot.org/. UniProt Consortium. Retrieved 21 January 2015. External link in |website= (help)
  12. UniProt Consortium. "P27005- S10A8_MOUSE". http://www.uniprot.org/. UniProt Consortium. Retrieved 21 January 2015. External link in |website= (help)
  13. UniProt Consortium. "P31725- S10A9_MOUSE". http://www.uniprot.org/. UniProt Consortium. Retrieved 21 January 2015. External link in |website= (help)
  14. 1 2 Evans, G.O. (2009). Animal Clinical Chemistry: A Practical Handbook for Toxicologists and Biomedical Researchers (2 ed.). Boca Raton: Taylor & Francis. pp. 107–108. ISBN 9781420080124. Retrieved 19 January 2015.
  15. Strupat, K; Rogniaux, H; Van Dorsselaer, A; Roth, J; Vogl, T (September 2000). "Calcium-induced noncovalently linked tetramers of MRP8 and MRP14 are confirmed by electrospray ionization-mass analysis.". Journal of the American Society for Mass Spectrometry. 11 (9): 780–8. PMID 10976885. doi:10.1016/s1044-0305(00)00150-1.
  16. 1 2 Maret, Wolfgang; Wedd, Anthony (2014). Binding, transport and storage of metal ions in biological cells. [S.l.]: Royal Soc Of Chemistry. p. 271. ISBN 9781849735995. Retrieved 27 January 2015.
  17. 1 2 Hood, M; Skaar, E (2013). "Nutritional immunity: transition metals at the pathogen-host interface". Nature Reviews Microbiology. 10 (8): 525–537. PMC 3875331Freely accessible. PMID 22796883. doi:10.1038/nrmicro2836.
  18. 1 2 Schaechter, Moselio (2009). Encyclopedia of microbiology (3 ed.). [S.l.]: Elsevier. p. 570. ISBN 0123739446. Retrieved 27 January 2015.
  19. Vacharaksa, Anjalee (2007). Restricted HIV-1 Infection Increases Susceptibility of Candida Infection in Oral Keratinocytes. ProQuest. p. 20. ISBN 9780549367666. Retrieved 27 January 2015.
  20. Tibble J, Teahon K, Thjodleifsson B, Roseth A, Sigthorsson G, Bridger S, Foster R, Sherwood R, Fagerhol M, Bjarnason I (2000). "A simple method for assessing intestinal inflammation in Crohn's disease". Gut. 47 (4): 506–13. PMC 1728060Freely accessible. PMID 10986210. doi:10.1136/gut.47.4.506.
  21. D'Haens G, Ferrante M, Vermeire S, Baert F, Noman M, Moortgat L, Geens P, Iwens D, Aerden I, Van Assche G, Van Olmen G, Rutgeerts P (2012). "Fecal calprotectin is a surrogate marker for endoscopic lesions in inflammatory bowel disease". Inflamm. Bowel Dis. 18 (12): 2218–24. PMID 22344983. doi:10.1002/ibd.22917.
  22. Poullis A, Foster R, Mendall MA, Shreeve D, Wiener K (2003). "Proton pump inhibitors are associated with elevation of faecal calprotectin and may affect specificity". Eur J Gastroenterol Hepatol. 15 (5): 573–4; author reply 574. PMID 12702920. doi:10.1097/00042737-200305000-00021.
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