Role of CrkII in Fcγ Receptor-mediated Phagocytosis

Abstract

Phagocytosis of IgG-opsonized pathogens by Fc␥ receptors requires extensive remodeling of the actin cytoskeleton, a process regulated by the small GTPase Rac. Vav was thought to be the guanine nucleotide exchange factor responsible for the activation of Rac, but recent evidence indicates that Fc␥ receptormediated phagocytosis is unaffected in macrophages lacking all three isoforms of Vav. We therefore tested whether another GEF, DOCK180, participates in Fc␥ receptor-initiated phagocytosis. DOCK180 associates with the adaptor protein Crk, which mediates recruitment of the GEF to sites of tyrosine phosphorylation. CrkII and DOCK180 were found to accumulate at the phagocytic cup. Knockdown of Crk or DOCK180 in murine macrophages using small interfering RNA inhibited phagocytosis of IgG-opsonized particles. Moreover, transfection of dominant negative CrkII prevented both recruitment of DOCK180 and the activation of Rac at the phagocytic cup. This is the first report of a role for either Crk or DOCK180 in Fc␥ receptormediated phagocytosis. The Crk-DOCK180 complex is involved in the clearance of apoptotic cells, which unlike the ingestion of IgG-opsonized particles, is an anti-inflammatory process. The finding that CrkII-DOCK180 is also responsible, at least in part, for the effects of Fc␥ receptors implies that additional, parallel pathways must account for the associated pro-inflammatory effect.

Molecular mechanisms of phagocytic uptake in mammalian cells

“…Nevertheless, it is worth mentioning that the notion of two modes of uptake, Type I (Cdc42/Rac-dependent) and Type II (Rho-dependent) is also receiving support from the elucidation of the downstream pathways triggered by phagocytic receptors (see below) and from studies on other receptors, cell types and organisms. Indeed, Lee et al [32] have recently established a role for the GEF DOCK-180 and adapter molecule CrkII in recruiting and activating Rac downstream of FcgR. As indicated above, the best studied of the Rho-family G proteins are RhoA, Rac1 and Cdc42.…”

Adaptor Protein SLAT Modulates Fcγ Receptor-mediated Phagocytosis in Murine Macrophages

“…Mammalian cells lacking CrkII or DOCK180 are deficient in recruitment of Rac-GTP to the Fc␥R-induced phagocytic cup (20). Both CrkII and DOCK180 are recruited to the phagocytic cup, and short interfering RNA-mediated knockdown of CrkII protein expression blocks recruitment of Rac to the phagosome (20). Thus, one possible interpretation of these earlier findings is that inactive Rac is recruited to the phagocytic cup, where it encounters DOCK180 complexed with Elmo and CrkII.…”

“…The involvement of DOCK180/Elmo and CrkII in Rac activation during phagocytosis was originally identified in a Caenorhabditis elegans model (45). Mammalian cells lacking CrkII or DOCK180 are deficient in recruitment of Rac-GTP to the Fc␥R-induced phagocytic cup (20). Both CrkII and DOCK180 are recruited to the phagocytic cup, and short interfering RNA-mediated knockdown of CrkII protein expression blocks recruitment of Rac to the phagosome (20).…”

“…Experiments have established that the Rac GEF DOCK180 (19,45) acts together in a signaling cascade with the adaptor protein CrkII (20) to induce Rac-GTP in Fc-dependent phagocytosis. DOCK180 is constitutively bound to a scaffold protein called Elmo, which supports the GEF activity of DOCK180 toward Rac (19,45).…”

“…Among these effectors are the guanine nucleotide exchange factors (GEFs), which activate the members of Rho GTPase family. Recent experiments indicate a role for a complex containing CrkII-DOCK180 as the Rac GEF and essential for phagocytosis (19,20).…”