[PDF][PDF] Cell-surface calreticulin initiates clearance of viable or apoptotic cells through trans-activation of LRP on the phagocyte

SJ Gardai, KA McPhillips, SC Frasch, WJ Janssen… - Cell, 2005 - cell.com
SJ Gardai, KA McPhillips, SC Frasch, WJ Janssen, A Starefeldt, JE Murphy-Ullrich
Cell, 2005cell.com
Apoptotic-cell removal is critical for development, tissue homeostasis, and resolution of
inflammation. Although many candidate systems exist, only phosphatidylserine has been
identified as a general recognition ligand on apoptotic cells. We demonstrate here that
calreticulin acts as a second general recognition ligand by binding and activating LDL-
receptor-related protein (LRP) on the engulfing cell. Since surface calreticulin is also found
on viable cells, a mechanism preventing inadvertent uptake was sought. Disruption of …
Summary
Apoptotic-cell removal is critical for development, tissue homeostasis, and resolution of inflammation. Although many candidate systems exist, only phosphatidylserine has been identified as a general recognition ligand on apoptotic cells. We demonstrate here that calreticulin acts as a second general recognition ligand by binding and activating LDL-receptor-related protein (LRP) on the engulfing cell. Since surface calreticulin is also found on viable cells, a mechanism preventing inadvertent uptake was sought. Disruption of interactions between CD47 (integrin-associated protein) on the target cell and SIRPα (SHPS-1), a heavily glycosylated transmembrane protein on the engulfing cell, permitted uptake of viable cells in a calreticulin/LRP-dependent manner. On apoptotic cells, CD47 was altered and/or lost and no longer activated SIRPα. These changes on the apoptotic cell create an environment where "don't eat me" signals are rendered inactive and "eat me" signals, including calreticulin and phosphatidylserine, congregate together and signal for removal.
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