Interaction between the integrin Mac-1 and signal regulatory protein α (SIRPα) mediates fusion in heterologous cells

Abstract

Macrophage fusion leading to the formation of multinucleated giant cells is a hallmark of chronic inflammation. Several membrane proteins have been implicated in mediating cell-cell attachment during fusion, but their binding partners remain unknown. Recently, we demonstrated that interleukin-4 (IL-4)induced fusion of mouse macrophages depends on the integrin macrophage antigen 1 (Mac-1). Surprisingly, the genetic deficiency of intercellular adhesion molecule 1 (ICAM-1), an established ligand of Mac-1, did not impair macrophage fusion, suggesting the involvement of other counter-receptors. Here, using various approaches, including signal regulatory protein ␣ (SIRP␣) knockdown, recombinant proteins, adhesion and fusion assays, biolayer interferometry, and peptide libraries, we show that SIRP␣, which, similar to ICAM-1, belongs to the Ig superfamily and has previously been implicated in cell fusion, interacts with Mac-1. The following results support the conclusion that SIRP␣ is a ligand of Mac-1: (a) recombinant ectodomain of SIRP␣ supports adhesion of Mac-1-expressing cells; (b) Mac-1-SIRP␣ interaction is mediated through the ligand-binding ␣ M I-domain of Mac-1; (c) recognition of SIRP␣ by the ␣ M Idomain conforms to general principles governing binding of Mac-1 to many of its ligands; (d) SIRP␣ reportedly binds CD47; however, anti-CD47 function-blocking mAb produced only a limited inhibition of macrophage adhesion to SIRP␣; and (e) co-culturing of SIRP␣-and Mac-1-expressing HEK293 cells resulted in the formation of multinucleated cells. Taken together, these results identify SIRP␣ as a counter-receptor for Mac-1 and suggest that the Mac-1-SIRP␣ interaction may be involved in macrophage fusion. Cell-cell fusion is a fundamental biological process that is required for development and homeostasis (1-3). Cellular cro ARTICLE

The Promiscuous Profile of Complement Receptor 3 in Ligand Binding, Immune Modulation, and Pathophysiology

“…SIRPα (MFR), another member of the Ig superfamily, is involved in macrophage fusion, which accompanies chronic inflammatory conditions. The α M I domain mediates the interaction of CR3 with this counterreceptor ( 210 ) and exhibits binding to the soluble Ig1-2-3 ectodomains of SIRPα ( 199 ). Interestingly, the inactive form of α M I can still bind SIRPα, albeit to a lesser extent ( 199 ).…”

“…The α M I domain mediates the interaction of CR3 with this counterreceptor ( 210 ) and exhibits binding to the soluble Ig1-2-3 ectodomains of SIRPα ( 199 ). Interestingly, the inactive form of α M I can still bind SIRPα, albeit to a lesser extent ( 199 ). Basic amino acids flanked by lipophilic residues were identified as binding motifs within SIRPα ( 49 ).…”

Osteoclast Multinucleation: Review of Current Literature

“…Therefore, other unknown factors seem to be involved in MFR-mediated osteoclast multinucleation. Podolnikova et al recently reported that integrin macrophage antigen 1 (Mac-1) expressed on macrophages is a counter-receptor for MFR, and that their interaction is involved in macrophage fusion [ 104 ].…”

Heterogeneity and Actin Cytoskeleton in Osteoclast and Macrophage Multinucleation

“…Macrophages spontaneously fuse with colon carcinoma and melanoma cell lines [ 67 ]. The idea of heterotypic macrophage fusion is supported by the fact that αMβ2 integrin-overexpressed HEK293 cells are able to fuse with the signal regulatory protein α (SIRPα)-overexpressed HEK293 cells in the presence of IL-4 [ 68 ]. Cell fusion does not occur among HEK293 cells, and those overexpressed either αMβ2 integrin or SIRPα alone.…”

“…Migrating cells should recognize other cells and check fusion competency to avoid aberrant fusion. Many molecules are expected to be involved in the recognition process [ 12 ], including CD9 [ 129 ], CD200 [ 130 ], Connexin 43 [ 131 ], E-cadherin [ 132 ], protocadherin-7 [ 133 ], phosphatidylserine [ 134 ], and Mac-1/SIRPα [ 68 ].…”