Calreticulin Serves as an "Eat Me" Signal

Apoptotic cells are removed by phagocytosis upon recognition of the dying cells. The exact signals for phagocytosis of apoptotic cells remain to be determined. Gardai et al. suggest that on apoptotic cells, cell surface calreticulin (Crt), although it is also present on healthy cells, becomes recognized by the engulfing cells, thereby contributing to the phagocytosis signal. Phagocytosis of apoptotic cells was decreased if the cells were first exposed to an antibody against Crt. Furthermore, Crt-deficient apoptotic cells were not phagocytosed efficiently in vitro or in vivo. In vitro phagocytosis of the Crt-deficient apoptotic cells was restored if the Crt was added exogenously. Both professional (macrophages) and nonprofessional (fibroblasts) phagocytic cells were dependent on cell surface Crt for recognition of apoptotic cells. Crt is known to bind LRP [low-density lipoprotein (LDL) receptor-related protein], and exposure of macrophages to an antibody against the extracellular, but not the intracellular, domain of LRP inhibited phagocytosis of apoptotic neutrophils. Crt also stimulated macropinocytosis and membrane ruffling through activation of Rac in macrophages, and these responses were absent in LRP –/– mouse embryo fibroblasts (MEFs), which also did not phagocytose apoptotic cells and did not bind exogenously added Crt. Receptor-associated protein (RAP) is a known antagonist of LRP, and treatment of macrophages with exogenous RAP inhibited engulfment of apoptotic cells. Furthermore, RAP and Crt reciprocally displaced each other from the surface of macrophages, which suggests competitive binding for the LRP. Because Crt is present on the surface of nonapoptotic cells, a mechanism must exist to allow the Crt "eat me" signal to be recognized at the proper time. One difference is that Crt was more abundant on the surface and redistributed into patches that were also abundant in phosphatidylserine and the ganglioside GM1 on the surface of apoptotic cells. Also, CD47 (also known as integrin-associated protein), which interacts with SIRPα (signal regulatory protein α) to inhibit phagocytosis, was either decreased at the surface of apoptotic cells or redistributed to patches away from the patches containing Crt. Thus, the "eat me" and "don't eat me" signals appear to segregate in apoptotic cells, which may allow the "eat me" signals to prevail. S. J. Gardai, K. A. McPhillips, S. C. Frasch, W. J. Janssen, A. Starefeldt, J. E. Murphy-Ullrich, D. L. Bratton, P.-A. Oldenborg, M. Michalak, P. M. Henson, Cell-surface calreticulin initiates clearance of viable or apoptotic cells through trans -activation of LRP on the phagocyte. Cell 123 , 321-334 (2005). [PubMed]