A pathway for phagosome maturation during engulfment of apoptotic cells

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Abstract
Removal of apoptotic cells is critical for the physiological well-being of the organism1 and defects in corpse removal have been linked to disease states2,3. Genes regulating corpse recognition and internalization have been identified4, but few molecules involved in the processing of internalized corpses are known. Through a combination of targeted and unbiased reverse genetic screens in Caenorhabditis elegans, and studies in mammalian cells, we have identified genes required for maturation of apoptotic-cell-containing phagosomes. We have further ordered these candidates, which include the GTPases RAB-5 and RAB-7 and the HOPS complex, into a coherent linear pathway for the maturation of apoptotic cells within phagosomes. In depth analysis of two additional candidate genes, the phosphatidylinositol 3 kinase (PI(3)K) vps-34 (A001762) and dyn-1/dynamin, showed an accumulation of internalized, but undegraded, corpses within abnormal Rab5-negative phagosomes. We ordered these candidates in our pathway, with DYN-1 functioning upstream of VPS-34 in the recruitment and/or retention of RAB-5 to the phagosome. Finally, we have also identified a previously undescribed biochemical complex containing Vps34, dynamin and Rab5GDP, thus providing a mechanism for Rab5 recruitment to the nascent phagosome.