title: Investigations on the C1q-calreticulin-phosphatidylserine interactions yield new insights into apoptotic cell recognition.
abstract: Both C1q and calreticulin (CRT) are involved in the recognition of apoptotic cells. CRT was initially characterized as a receptor for the C1q collagen-like fragment (CLF), whereas C1q was shown to bind apoptotic cells through its globular region (GR). Using purified CRT and recombinant CRT domains, we now provide unambiguous experimental evidence that, in addition to its CLF, the C1q GR also binds CRT and that both types of interactions are mediated by the CRT globular domain. Surface plasmon resonance analyses revealed that the C1q CLF and GR domains each bind individually to immobilized CRT and its globular domain with K(D) values of (2.6-8.3) × 10(-7) M. Further evidence that CRT binds to the C1q GR was obtained by electron microscopy. The role of CRT in the recognition of apoptotic HeLa cells by C1q was analyzed. The C1q GR partially colocalized with CRT on the surface of early apoptotic cells, and siRNA (small interfering RNA)-induced CRT deficiency resulted in increased apoptotic cell binding to C1q. The interaction between CRT and phosphatidylserine (PS), a known C1q ligand on apoptotic cells, was also investigated. The polar head of PS was shown to bind to CRT with a 10-fold higher affinity (K(D)=1.5 × 10(-5) M) than that determined for C1q, and, accordingly, the C1q GR-PS interaction was impaired in the presence of CRT. Together, these observations indicate that CRT, C1q, and PS are all closely involved in the uptake of apoptotic cells and strongly suggest a combinatorial role of these three molecules in the recognition step.
authors.affiliation: Université Joseph Fourier Grenoble 1, Institut de Biologie Structurale Jean-Pierre Ebel, 38027 Grenoble, France.
authors.forename: Helena
authors.initials: H
authors.lastname: Païdassi
authors.forename: Pascale
authors.initials: P
authors.lastname: Tacnet-Delorme
authors.forename: Mélanie
authors.initials: M
authors.lastname: Verneret
authors.forename: Christine
authors.initials: C
authors.lastname: Gaboriaud
authors.forename: Gunnar
authors.initials: G
authors.lastname: Houen
authors.forename: Karen
authors.initials: K
authors.lastname: Duus
authors.forename: Wai Li
authors.initials: WL
authors.lastname: Ling
authors.forename: Gérard J
authors.initials: GJ
authors.lastname: Arlaud
authors.forename: Philippe
authors.initials: P
authors.lastname: Frachet
mesh_terms: D017209:Apoptosis; D015153:Blotting, Western; D037282:Calreticulin / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism*; D002450:Cell Communication; D015922:Complement C1q / Q000378:metabolism*; D005434:Flow Cytometry; D006367:HeLa Cells; D006801:Humans; D008565:Membrane Proteins / Q000378:metabolism; D010587:Phagocytosis; D010718:Phosphatidylserines / Q000378:metabolism*; D011485:Protein Binding; D034741:RNA, Small Interfering / Q000235:genetics; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D020349:Surface Plasmon Resonance
chemical_list: D037282:Calreticulin; D008565:Membrane Proteins; D010718:Phosphatidylserines; D034741:RNA, Small Interfering; D011994:Recombinant Proteins; D015922:Complement C1q