Abstract
In metazoans, apoptotic cells are swiftly engulfed by phagocytes and degraded inside phagosomes. Multiple small GTPases in the Rab family are known to function in phagosome maturation by regulating vesicle trafficking. We discovered rab-35 as a new gene important for apoptotic cell clearance from a genetic screen targeting putative Rab GTPases in Caenorhabditis elegans. We further identified TBC-10 as a putative GTPase-activating protein (GAP), and FLCN-1 and RME-4 as two putative Guanine Nucleotide Exchange Factors (GEFs), for RAB-35. We found that RAB-35 was required for the efficient incorporation of early endosomes to phagosomes and for the timely degradation of apoptotic cell corpses. More specifically, RAB-35 promotes two essential events that initiate phagosome maturation: the switch of phagosomal membrane phosphatidylinositol species from PtdIns(4,5)P2 to PtdIns(3)P, and the recruitment of the small GTPase RAB-5 to phagosomal surfaces. These functions of RAB-35 were previously unknown. Remarkably, although the phagocytic receptor CED-1 regulates these same events, RAB-35 and CED-1 appear to function independently. Upstream of degradation, RAB-35 also facilitates the recognition of apoptotic cells independently of the known CED-1 and CED-5 pathways. RAB-35 localizes to extending pseudopods and is further enriched on nascent phagosomes, consistent with its dual roles in regulating apoptotic cell-recognition and phagosome maturation. Epistasis analyses indicate that rab-35 acts in parallel to both of the canonical ced-1/6/7 and ced-2/5/10/12 clearance pathways. We propose that RAB-35 acts as a robustness factor, defining a novel pathway that aids these canonical pathways in both the recognition and degradation of apoptotic cells.
Highlights
During the development of metazoans, cells that undergo apoptosis are internalized and degraded by other cells that are referred to as engulfing cells or phagocytes [1,2,3]
We show that the small GTPase RAB35 promotes the recognition of cell corpses necessary for engulfment, as well as the initiation of phagosome maturation after engulfment
We found that RAB-35 aids in the recruitment of RAB-5 and early endosomes to the nascent phagosome
Summary
During the development of metazoans, cells that undergo apoptosis are internalized and degraded by other cells that are referred to as engulfing cells or phagocytes [1,2,3]. The phagocytic removal of apoptotic cells is an evolutionarily conserved event that supports normal tissue turnover and homeostasis, facilitates wound resolution and tissue regeneration, and prevents inflammatory and auto-immune responses induced by the release of dead cell contents [3,4]. Throughout the development of Caenorhabditis elegans hermaphrodites, 300–500 germ cells and 131 somatic cells undergo apoptosis [5,6,7]. Apoptotic cells exhibit a “button-like” and highly refractive morphology under the Differential Interference Contrast (DIC) microscope, and are rapidly engulfed and degraded by multiple types of neighboring cells [5,6,7,8]. Genetic screens and further characterizations of mutations that result in the “cell death abnormal” (Ced) phenotype, characterized by the accumulation of persistent cell corpses, have identified a number of genes that act in the recognition, engulfment, or degradation of cell corpses [9,10]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
