Caspase Activity Is Required for Engulfment of Apoptotic Cells

Abstract

Clearance of apoptotic cells by phagocytic neighbors is crucial for normal development of multicellular organisms. However, how phagocytes discriminate between healthy and dying cells remains poorly understood. We focus on glial phagocytosis of apoptotic neurons during development of the Drosophila central nervous system. We identified phosphatidylserine (PS) as a ligand on apoptotic cells for the phagocytic receptor Six Microns Under (SIMU) and report that PS alone is not sufficient for engulfment. Our data reveal that, additionally to PS exposure, caspase activity is required for clearance of apoptotic cells by phagocytes. Here we demonstrate that SIMU recognizes and binds PS on apoptotic cells through its N-terminal EMILIN (EMI), Nimrod 1 (NIM1), and NIM2 repeats, whereas the C-terminal NIM3 and NIM4 repeats control SIMU affinity to PS. Based on the structure-function analysis of SIMU, we discovered a novel mechanism of internal inhibition responsible for differential affinities of SIMU to its ligand which might prevent elimination of living cells exposing PS on their surfaces.

SUPPLEMENTAL MATERIAL

Supplemental material for this article may be found at http://dx.doi.org/10.1128/MCB.00233-13.

ACKNOWLEDGMENTS

We thank V. Auld, O. Shuldiner, H. Steller, E. Arama, and the Bloomington Stock Center for generously providing fly strains. We thank A. Salzberg and T. Schultheiss for comments on the manuscript and the Kurant laboratory members for support and constructive criticism.

This work was supported by grants from IRG (grant no. 2498084), the Israel Ministry of Health (grant no. 3-00000-6162), and the Israel Science Foundation (grant no. 427/11).