Complement control protein explained

Complement control proteins are proteins that interact with components of the complement system.

The complement system is tightly regulated by a network of proteins known as "regulators of complement activation (RCA)" that help distinguish target cells as "self" or "non-self." A subset of this family of proteins, complement control proteins (CCP), are characterized by domains of conserved repeats that direct interaction with components of the complement system.[1] These "Sushi" domains have been used to identify other putative members of the CCP family. There are many other RCA proteins that do not fall into this family.

Most CCPs prevent activation of the complement system on the surface of host cells and protect host tissues against damage caused by autoimmunity. Because of this, these proteins play important roles in autoimmune disorders and cancers.[2]

Members

Most of the well-studied proteins within this family can be categorized in two classes:

Membrane-bound complement regulators

Soluble complement regulators

Other proteins with characteristic CCP domains have been identified including members of the sushi domain containing (SUSD) protein family and Human CUB and sushi multiple domains family (CSMD).[3]

Mechanisms of protection

Every cell in the human body is protected by one or more of the membrane-associated RCA proteins, CR1, DAF or MCP. Factor H and C4BP circulate in the plasma and are recruited to self-surfaces through binding to host-specific polysaccharides such as the glycosaminoglycans.[4]

Most CCPs function by preventing convertase activity. Convertases, specifically the C3 convertases C3b.Bb and C4b.2a, are the enzymes that drive complement activation by activating C3b, a central component of the complement system. Some CCPs, such as CD46, recruit other RCAs to proteolytically inactivate developing convertases. CD55 and other CCPs promote the rapid dissociation of active enzymes. Other CCPs prevent the activity of terminal effectors of the complement system, CD59 for example blocks oligomerization of the complement peptide C9 stalling the formation of the Membrane Attack Complex (MAC).[5]

For example, C3b.Bb is an important convertase that is part of the alternative pathway, and it is formed when factor B binds C3b and is subsequently cleaved. To prevent this from happening, factor H competes with factor B to bind C3b; if it manages to bind, then the convertase is not formed. Factor H can bind C3b much more easily in the presence of sialic acid, which is a component of most cells in the human body; conversely, in the absence of sialic acid, factor B can bind C3b more easily. This means that if C3b is bound to a "self" cell, the presence of sialic acid and the binding of factor H will prevent the complement cascade from activating; if C3b is bound to a bacterium, factor B will bind and the cascade will be set off as normal. This mechanism of immune regulation using Factor H has been exploited by several bacterial pathogens.[6]

Structure

RCA proteins typically possess CCP domains, also termed Sushi domains or Short Consensus Repeats (SCR). Such beta-sandwich domains contain about 60 amino acid residues, each with 4 conserved cysteines arranged in two conserved disulfide bonds (oxidized in 'abab' manner), and a conserved tryptophan, but otherwise can vary greatly in sequence. Recently, it has been demonstrated that the order, spatial relationship, and structure of these domains is essential for determining function.[7]

The first CCP structure determined was a solution structure of the 16th module of factor H (pdb:1hcc).[8] Since then, other CCP domains have been solved either by NMR-spectroscopy (also relaxation studies, e.g. module 2 and 3 from CD55 (pdb:1nwv))[9] or by X-ray diffraction (also with co-crystallized partner, e.g. CR2 CCP modules complexed with C3d (pdb:1ghq)).[10]

Clinical significance

Complement has been implicated in many diseases associated with inflammation and autoimmunity.[11] Efforts to develop therapeutics that target the interactions between the RCA network, CCPs, and components of the complement system have led to the development of successful drugs including Eculizumab.

There are two primary mechanisms by which dysfunction of complement can contribute to tissue damage:[12]

  1. Decreased protection of host tissues from complement activation due to the absence or lack of function of CCPs
  2. Exhaustion of CRAs due to exposure of host cells that activate complement (either through direct damage or dysfunction) or prolonged attack by a potential pathogen such as during sepsis

The importance of complement regulation for good health is highlighted by recent work that seems to imply that individuals carrying point mutations or single nucleotide polymorphisms in their genes for factor H may be more susceptible to diseases including atypical hemolytic uremic syndrome,[13] dense deposit diseases (or membranoproliferative glomerulonephritis type 2) and - most notably because of its prevalence in the elderly - age-related macular degeneration.[14] Transgenic pigs that express human complement regulation factors were some of the first transgenic pigs used for xenotransplantation.[15] [16]

Complement control proteins also play a role in malignancy. Complement proteins protect against malignant cells- both by direct complement attack and through initiation of Complement-dependent cytotoxicity, which synergises with specific monoclonal antibody therapies. However, some malignant cells have been shown to have increased expression of membrane-bound complement control proteins, especially CD46, DAF and CD59.[17] This mechanism allows some tumours to evade complement action.

CCPs have been exploited extensively by pathogenic microbes.[18] Neisseria gonorhoeae and Neisseria meningitidis, the bacteria responsible for gonorrhea and meningitis have many well-studied evasion strategies involving CCPs, including binding soluble regulators like Factor H and C4bp. Many viruses, such as Vaccinia incorporate mimics of CCPs into their envelope for the purposes of evading the complement system. Still other microbes such as the measles virus use CCPs as receptors to gain entry to cells during infection. Each of these strategies may provide targets for the development of vaccines, as with the case of N. meningitidis.

Certain forms of schizophrenia are characterised by an underlying biological mechanism of excessive synaptic pruning, mediated by a dysregulated complement system in the brain.[19] Accordingly, genetic variants of a brain-specific complement inhibitor, CSMD1, are associated with the risk of developing schizophrenia.[20] [21]

Further reading

Notes and References

  1. McLure CA, Dawkins RL, Williamson JF, Davies RA, Berry J, Natalie LJ, Laird R, Gaudieri S . 25038346 . 6 . Amino acid patterns within short consensus repeats define conserved duplicons shared by genes of the RCA complex . Journal of Molecular Evolution . 59 . 2 . 143–57 . August 2004 . 15486690 . 10.1007/s00239-004-2609-8 . 2004JMolE..59..143M .
  2. Pangburn MK, Ferreira VP, Cortes C . Discrimination between host and pathogens by the complement system . Vaccine . 26 . I15-21 . December 2008 . Suppl 8 . 19388159 . 2673523 . 10.1016/j.vaccine.2008.11.023 .
  3. Gialeli C, Gungor B, Blom AM . Novel potential inhibitors of complement system and their roles in complement regulation and beyond . Molecular Immunology . 102 . 73–83 . October 2018 . 30217334 . 10.1016/j.molimm.2018.05.023 . Special Issue: 2018 International Complement Workshop . 52278070 .
  4. Langford-Smith A, Day AJ, Bishop PN, Clark SJ . Complementing the Sugar Code: Role of GAGs and Sialic Acid in Complement Regulation . Frontiers in Immunology . 6 . 25 . 2015-02-02 . 25699044 . 4313701 . 10.3389/fimmu.2015.00025 . free .
  5. Zipfel PF, Skerka C . 1723316 . Complement regulators and inhibitory proteins . Nature Reviews. Immunology . 9 . 10 . 729–40 . October 2009 . 19730437 . 10.1038/nri2620 .
  6. Józsi M . Factor H Family Proteins in Complement Evasion of Microorganisms . Frontiers in Immunology . 8 . 571 . 2017-05-18 . 28572805 . 5435753 . 10.3389/fimmu.2017.00571 . free .
  7. Ojha H, Ghosh P, Singh Panwar H, Shende R, Gondane A, Mande SC, Sahu A . Spatially conserved motifs in complement control protein domains determine functionality in regulators of complement activation-family proteins . Communications Biology . 2 . 1 . 290 . December 2019 . 31396570 . 6683126 . 10.1038/s42003-019-0529-9 .
  8. Norman DG, Barlow PN, Baron M, Day AJ, Sim RB, Campbell ID . Three-dimensional structure of a complement control protein module in solution . Journal of Molecular Biology . 219 . 4 . 717–25 . June 1991 . 1829116 . 10.1016/0022-2836(91)90666-t .
  9. Uhrinova S, Lin F, Ball G, Bromek K, Uhrin D, Medof ME, Barlow PN . Solution structure of a functionally active fragment of decay-accelerating factor . Proceedings of the National Academy of Sciences of the United States of America . 100 . 8 . 4718–23 . April 2003 . 12672958 . 153622 . 10.1073/pnas.0730844100 . 2003PNAS..100.4718U . free .
  10. Szakonyi G, Guthridge JM, Li D, Young K, Holers VM, Chen XS . Structure of complement receptor 2 in complex with its C3d ligand . Science . 292 . 5522 . 1725–8 . June 2001 . 11387479 . 10.1126/science.1059118 . 2001Sci...292.1725S . 45893794 .
  11. Wong EK, Kavanagh D . Diseases of complement dysregulation-an overview . Seminars in Immunopathology . 40 . 1 . 49–64 . January 2018 . 29327071 . 5794843 . 10.1007/s00281-017-0663-8 .
  12. Pangburn MK, Ferreira VP, Cortes C . Discrimination between host and pathogens by the complement system . Vaccine . 26 . I15-21 . December 2008 . Suppl 8 . 19388159 . 2673523 . 10.1016/j.vaccine.2008.11.023 .
  13. Buddles MR, Donne RL, Richards A, Goodship J, Goodship TH . Complement factor H gene mutation associated with autosomal recessive atypical hemolytic uremic syndrome . American Journal of Human Genetics . 66 . 5 . 1721–2 . May 2000 . 10762557 . 1378030 . 10.1086/302877 .
  14. Hageman GS, Anderson DH, Johnson LV, Hancox LS, Taiber AJ, Hardisty LI, Hageman JL, Stockman HA, Borchardt JD, Gehrs KM, Smith RJ, Silvestri G, Russell SR, Klaver CC, Barbazetto I, Chang S, Yannuzzi LA, Barile GR, Merriam JC, Smith RT, Olsh AK, Bergeron J, Zernant J, Merriam JE, Gold B, Dean M, Allikmets R . 6 . A common haplotype in the complement regulatory gene factor H (HF1/CFH) predisposes individuals to age-related macular degeneration . Proceedings of the National Academy of Sciences of the United States of America . 102 . 20 . 7227–32 . May 2005 . 15870199 . 1088171 . 10.1073/pnas.0501536102 . free .
  15. Eisenson DL, Hisadome Y, Yamada K . Progress in Xenotransplantation: Immunologic Barriers, Advances in Gene Editing, and Successful Tolerance Induction Strategies in Pig-To-Primate Transplantation . . 13 . 899657 . 2022 . 10.3389/fimmu.2022.899657 . 9157571 . 35663933. free .
  16. Lu T, Yang B, Wang R, Qin C . Xenotransplantation: Current Status in Preclinical Research . . 10 . 3060 . 2020 . 10.3389/fimmu.2019.03060 . 6989439 . 32038617. free .
  17. Ricklin D, Hajishengallis G, Yang K, Lambris JD . Complement: a key system for immune surveillance and homeostasis . Nature Immunology . 11 . 9 . 785–97 . September 2010 . 20720586 . 2924908 . 10.1038/ni.1923 .
  18. Zipfel PF, Hallström T, Riesbeck K . Human complement control and complement evasion by pathogenic microbes--tipping the balance . Molecular Immunology . 56 . 3 . 152–60 . December 2013 . 23810413 . 10.1016/j.molimm.2013.05.222 . 14th European Meeting on Complement in Human Disease, Jena, August 17–21, 2013 . 207007297 .
  19. Baum. Matthew L.. 2018-09-16. The Schizophrenia-Associated Gene, CSMD1, Encodes a Brain-Specific Complement Inhibitor. en.
  20. Liu Y, Fu X, Tang Z, Li C, Xu Y, Zhang F, Zhou D, Zhu C . 6 . Altered expression of the CSMD1 gene in the peripheral blood of schizophrenia patients . BMC Psychiatry . 19 . 1 . 113 . April 2019 . 30987620 . 6466712 . 10.1186/s12888-019-2089-4 . free .
  21. Håvik B, Le Hellard S, Rietschel M, Lybæk H, Djurovic S, Mattheisen M, Mühleisen TW, Degenhardt F, Priebe L, Maier W, Breuer R, Schulze TG, Agartz I, Melle I, Hansen T, Bramham CR, Nöthen MM, Stevens B, Werge T, Andreassen OA, Cichon S, Steen VM . 26368229 . 6 . The complement control-related genes CSMD1 and CSMD2 associate to schizophrenia . English . Biological Psychiatry . 70 . 1 . 35–42 . July 2011 . 21439553 . 10.1016/j.biopsych.2011.01.030 .

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