General

The activation of the complement cascade produces a number of small (74-77 amino acid) fragments that are bioactive: potent chemoattractants and secretagogues that act on immune and non-immune cells [21]. Very similar peptides can also be released by the actions of non-complement proteases on the parent molecules, for instance during clotting [1]. Initially termed anaphylatoxins because of their effects on mast cell histamine release, peptides C5a, C3a are now known to act at all levels of the immune response [24] and are also involved in neural development and organ regeneration [6,16]. A peptide with similar structure, C4a, appears to be largely inactive in humans [18]. All three peptides are substrates for highly active carboxypeptidases that cleave the C-terminal arginine residue, forming the more stable des-Arg peptides. The primary function of complement peptides is in inflammation and so they are important targets for the development of anti-inflammatory therapies [16]. Only three receptors for complement peptides have been found in mammals but there are no satisfactory antagonists as yet.

Receptors

The small complement peptide receptor family has not been systematically named until now; the receptor for C3a has been known as C3aR [3,10], whereas the two receptors for C5a have been termed CD88, C5aR, C5R1 for the first to be discovered [12], and C5L2, GPR77 and C5R2 for the second [5]. The subcommittee has decided to re-name these receptors according to IUPHAR guidelines as C3a, C5a₁ and C5a₂ receptors, respectively (human gene names C3AR, C5AR1 and C5AR2). For the latest IUPHAR review on complement peptide receptor nomenclature and pharmacology see [17]. Phylogenetically, complement peptide receptors are close to the formyl peptide receptor family and more distantly related to the chemokine receptors [13].

Both C3a and C5a₁ receptors are classical G protein-coupled receptors, signalling primarily via Gai2 although C5a₁ has also been shown to couple to Ga16 [15]. C5a₂ appears to be permanently uncoupled from G proteins but can associate with β-arrestin [14]. However, there are several reports that activation of both C5a₁ and C5a₂ is required for a full pro-inflammatory response, particularly in mice [8]. Signaling through β-arrestin can modulate responses to C3a receptor ligation [30] but has not so far been shown to have much importance for C5a1. Dimerization of C5a₁ and C5a₂ has recently been demonstrated [25], which may be relevant to the role of C5a₂ in the response to C5a.

Pharmacology

The C3a receptor binds only C3a, with no detectable affinity for the metabolite, C3a des-Arg [29]. In contrast, both C5a₁ and C5a₂ bind both C5a and C5a des-Arg [5]. In most studies, C5a₁ has been reported to have a lower affinity for C5a des-Arg, suggesting that des-argination was an inactivation step. However, recent reports have demonstrated that, for some responses, C5a and C5a des-Arg are equipotent, especially when native glycosylated C5a des-Arg is used in preference to recombinant peptide [26].

For all the complement peptides, agonist activity is vested in the C-terminal residues and so short peptide analogs of the C-termini (typically a decapeptide or shorter) have full agonist activity, albeit with a much-reduced potency [9]. These short peptides bind to a receptor site located within the transmembrane (or juxta-membrane) domain of the receptors [20] and there appears to be significant similarity between this domain in C5a₁ and C3a receptors, resulting in cross-reactivity of many peptides [4]. Receptor selectivity of the full length peptides is maintained by binding to a second site, located either at the N-terminus (C5a₁) or the second extracellular loop (C3a) [7,28]. In contrast few of the peptide analogs binds to C5a₂, suggesting that ligand binding is distinctly different for this receptor [27].

Only a single antagonist (SB290157) has been reported for human C3a receptor [2], although this appears to have partial agonist activity in some circumstances [19]. Several C5a₁ antagonists have been reported [21], although none have so far been shown to be effective in humans. C5a₁ antagonists are typically inactive on rodent receptors, making testing in different disease settings difficult. Only one of these antagonists (A8), derived from the C-terminus of C5a, has been shown to have significant activity at C5a₂ [22], making the analysis of the function of this receptor very difficult.

Non-receptor-mediated effects

C4a and C3a (and their des-Arg forms) have potent anti-microbial activity that is independent of receptor binding [23]. C3a is also able to activate certain cell types (e.g. mast cells) by a receptor-independent mechanism, possibly due to the amphipathic nature of this peptide [11].

1. Amara U, Rittirsch D, Flierl M, Bruckner U, Klos A, Gebhard F, Lambris JD, Huber-Lang M. (2008) Interaction between the coagulation and complement system. Adv Exp Med Biol, 632: 71-9. [PMID:19025115]

2. Ames RS, Lee D, Foley JJ, Jurewicz AJ, Tornetta MA, Bautsch W, Settmacher B, Klos A, Erhard KF, Cousins RD et al.. (2001) Identification of a selective nonpeptide antagonist of the anaphylatoxin C3a receptor that demonstrates antiinflammatory activity in animal models. J Immunol, 166 (10): 6341-8. [PMID:11342658]

3. Ames RS, Li Y, Sarau HM, Nuthulaganti P, Foley JJ, Ellis C, Zeng Z, Su K, Jurewicz AJ, Hertzberg RP et al.. (1996) Molecular cloning and characterization of the human anaphylatoxin C3a receptor. J Biol Chem, 271 (34): 20231-4. [PMID:8702752]

4. Bautsch W, Kretzschmar T, Stühmer T, Kola A, Emde M, Köhl J, Klos A, Bitter-Suermann D. (1992) A recombinant hybrid anaphylatoxin with dual C3a/C5a activity. Biochem J, 288 ( Pt 1): 261-6. [PMID:1445269]

5. Cain SA, Monk PN. (2002) The orphan receptor C5L2 has high affinity binding sites for complement fragments C5a and C5a des-Arg(74). J Biol Chem, 277 (9): 7165-9. [PMID:11773063]

6. Carmona-Fontaine C, Theveneau E, Tzekou A, Tada M, Woods M, Page KM, Parsons M, Lambris JD, Mayor R. (2011) Complement fragment C3a controls mutual cell attraction during collective cell migration. Dev Cell, 21 (6): 1026-37. [PMID:22118769]

7. Chao TH, Ember JA, Wang M, Bayon Y, Hugli TE, Ye RD. (1999) Role of the second extracellular loop of human C3a receptor in agonist binding and receptor function. J Biol Chem, 274 (14): 9721-8. [PMID:10092660]

8. Chen NJ, Mirtsos C, Suh D, Lu YC, Lin WJ, McKerlie C, Lee T, Baribault H, Tian H, Yeh WC. (2007) C5L2 is critical for the biological activities of the anaphylatoxins C5a and C3a. Nature, 446 (7132): 203-7. [PMID:17322907]

9. Chenoweth DE, Erickson BW, Hugli TE. (1979) Human C5a-related synthetic peptides as neutrophil chemotactic factors. Biochem Biophys Res Commun, 86 (2): 227-34. [PMID:426784]

10. Crass T, Raffetseder U, Martin U, Grove M, Klos A, Köhl J, Bautsch W. (1996) Expression cloning of the human C3a anaphylatoxin receptor (C3aR) from differentiated U-937 cells. Eur J Immunol, 26 (8): 1944-50. [PMID:8765043]

11. Erdei A, Andrásfalvy M, Péterfy H, Tóth G, Pecht I. (2004) Regulation of mast cell activation by complement-derived peptides. Immunol Lett, 92 (1-2): 39-42. [PMID:15081525]

12. Gerard NP, Gerard C. (1991) The chemotactic receptor for human C5a anaphylatoxin. Nature, 349 (6310): 614-7. [PMID:1847994]

13. Joost P, Methner A. (2002) Phylogenetic analysis of 277 human G-protein-coupled receptors as a tool for the prediction of orphan receptor ligands. Genome Biol, 3 (11): RESEARCH0063. [PMID:12429062]

14. Kalant D, MacLaren R, Cui W, Samanta R, Monk PN, Laporte SA, Cianflone K. (2005) C5L2 is a functional receptor for acylation-stimulating protein. J Biol Chem, 280 (25): 23936-44. [PMID:15833747]

15. Klinker JF, Wenzel-Seifert K, Seifert R. (1996) G-protein-coupled receptors in HL-60 human leukemia cells. Gen Pharmacol, 27 (1): 33-54. [PMID:8742493]

16. Klos A, Tenner AJ, Johswich KO, Ager RR, Reis ES, Köhl J. (2009) The role of the anaphylatoxins in health and disease. Mol Immunol, 46 (14): 2753-66. [PMID:19477527]

17. Klos A, Wende E, Wareham KJ, Monk PN. (2013) International Union of Pharmacology. LXXXVII. Complement Peptide C5a, C4a, and C3a Receptors. Pharmacol Rev, 65 (1): 500-43. [PMID:23382455]

18. Lienenklaus S, Ames RS, Tornetta MA, Sarau HM, Foley JJ, Crass T, Sohns B, Raffetseder U, Grove M, Hölzer A et al.. (1998) Human anaphylatoxin C4a is a potent agonist of the guinea pig but not the human C3a receptor. J Immunol, 161 (5): 2089-93. [PMID:9725198]

19. Mathieu MC, Sawyer N, Greig GM, Hamel M, Kargman S, Ducharme Y, Lau CK, Friesen RW, O'Neill GP, Gervais FG et al.. (2005) The C3a receptor antagonist SB 290157 has agonist activity. Immunol Lett, 100 (2): 139-45. [PMID:16154494]

20. Monk PN, Barker MD, Partridge LJ, Pease JE. (1995) Mutation of glutamate 199 of the human C5a receptor defines a binding site for ligand distinct from the receptor N terminus. J Biol Chem, 270 (28): 16625-9. [PMID:7622471]

21. Monk PN, Scola AM, Madala P, Fairlie DP. (2007) Function, structure and therapeutic potential of complement C5a receptors. Br J Pharmacol, 152 (4): 429-48. [PMID:17603557]

22. Otto M, Hawlisch H, Monk PN, Müller M, Klos A, Karp CL, Köhl J. (2004) C5a mutants are potent antagonists of the C5a receptor (CD88) and of C5L2: position 69 is the locus that determines agonism or antagonism. J Biol Chem, 279 (1): 142-51. [PMID:14570896]

23. Pasupuleti M, Walse B, Svensson B, Malmsten M, Schmidtchen A. (2008) Rational design of antimicrobial C3a analogues with enhanced effects against Staphylococci using an integrated structure and function-based approach. Biochemistry, 47 (35): 9057-70. [PMID:18690701]

24. Peng Q, Li K, Sacks SH, Zhou W. (2009) The role of anaphylatoxins C3a and C5a in regulating innate and adaptive immune responses. Inflamm Allergy Drug Targets, 8 (3): 236-46. [PMID:19601884]

25. Poursharifi P, Lapointe M, Pétrin D, Devost D, Gauvreau D, Hébert TE, Cianflone K. (2013) C5L2 and C5aR interaction in adipocytes and macrophages: insights into adipoimmunology. Cell Signal, 25 (4): 910-8. [PMID:23268185]

26. Reis ES, Chen H, Sfyroera G, Monk PN, Köhl J, Ricklin D, Lambris JD. (2012) C5a receptor-dependent cell activation by physiological concentrations of desarginated C5a: insights from a novel label-free cellular assay. J Immunol, 189 (10): 4797-805. [PMID:23041570]

27. Scola AM, Higginbottom A, Partridge LJ, Reid RC, Woodruff T, Taylor SM, Fairlie DP, Monk PN. (2007) The role of the N-terminal domain of the complement fragment receptor C5L2 in ligand binding. J Biol Chem, 282 (6): 3664-71. [PMID:17158873]

28. Siciliano SJ, Rollins TE, DeMartino J, Konteatis Z, Malkowitz L, Van Riper G, Bondy S, Rosen H, Springer MS. (1994) Two-site binding of C5a by its receptor: an alternative binding paradigm for G protein-coupled receptors. Proc Natl Acad Sci USA, 91 (4): 1214-8. [PMID:8108389]

29. Wilken HC, Götze O, Werfel T, Zwirner J. (1999) C3a(desArg) does not bind to and signal through the human C3a receptor. Immunol Lett, 67 (2): 141-5. [PMID:10232396]

30. Zaidi AK, Ali H. (2007) C3a receptors signaling in mast cells. Adv Exp Med Biol, 598: 126-40. [PMID:17892209]