C. elegans Dynamin mediates the signaling of phagocytic receptor CED-1 for the engulfment and degradation of apoptotic cells

Phagocytic receptor CED-1 initiates a signaling pathway for degrading engulfed apoptotic cells

Apoptotic cells in animals are engulfed by phagocytic cells and subsequently degraded inside phagosomes. To study the mechanisms controlling the degradation of apoptotic cells, we developed time-lapse imaging protocols in developing Caenorhabditis elegans embryos and established the temporal order of multiple events during engulfment and phagosome maturation. These include sequential enrichment on phagocytic membranes of phagocytic receptor cell death abnormal 1 (CED-1), large GTPase dynamin (DYN-1), phosphatidylinositol 3-phosphate (PI(3)P), and the small GTPase RAB-7, as well as the incorporation of endosomes and lysosomes to phagosomes. Two parallel genetic pathways are known to control the engulfment of apoptotic cells in C. elegans. We found that null mutations in each pathway not only delay or block engulfment, but also delay the degradation of engulfed apoptotic cells. One of the pathways, composed of CED-1, the adaptor protein CED-6, and DYN-1, controls the rate of enrichment of PI(3)P and RAB-7 on phagosomal surfaces and the formation of phagolysosomes. We further identified an essential role of RAB-7 in promoting the recruitment and fusion of lysosomes to phagosomes. We propose that RAB-7 functions as a downstream effector of the CED-1 pathway to mediate phagolysosome formation. Our work suggests that phagocytic receptors, which were thought to act specifically in initiating engulfment, also control phagosome maturation through the sequential activation of multiple effectors such as dynamin, PI(3)P, and Rab GTPases.

Cells undergoing programmed cell death, or apoptosis, within an animal are swiftly engulfed by phagocytes and degraded inside phagosomes, vesicles in which the apoptotic cell is bounded by the engulfing cell's membrane. Little is known about how the degradation process is triggered and controlled. We studied the degradation of apoptotic cells during the development of the nematode Caenorhabditis elegans. Aided by a newly developed live-cell imaging technique, we identified multiple cellular events occurring on phagosomal surfaces and tracked the initiation signal to CED-1, a phagocytic receptor known to recognize apoptotic cells and to initiate their engulfment. CED-1 activates DYN-1, a large GTPase, which further activates downstream events, leading intracellular organelles such as endosomes and lysosomes to deliver to phagosomes various molecules essential for the degradation of apoptotic cells. As well as establishing a temporal order of events that lead to the degradation of apoptotic cells, the results suggest that phagocytic receptors, in addition to initiating phagocytosis, promote phagosome maturation through the sequential activation of multiple effector molecules.

The authors have identified multiple cellular events leading to the degradation of engulfed apoptotic cells in the nematodeC. elegans, and found that CED-1, a phagocytic receptor thought to specifically control apoptotic-cell engulfment, activates a signaling pathway that initiates phagosome maturation.

Six-microns-under acts upstream of Draper in the glial phagocytosis of apoptotic neurons

The removal of apoptotic cells by phagocytic neighbors is essential for metazoan development but remains poorly characterized. Here we report the discovery of a Drosophila phagocytosis receptor, Six-microns-under (SIMU), which is expressed in highly phagocytic cell types during development and required for efficient apoptotic cell clearance by glia in the nervous system and by macrophages elsewhere. SIMU is part of a conserved family of proteins that includes CED-1 and Draper (DRPR). Phenotypic analysis reveals that simu acts upstream of drpr in the same pathway and affects the recognition and engulfment of apoptotic cells, while drpr affects their subsequent degradation. SIMU strongly binds to apoptotic cells, presumably through its EMILIN-like domain, but requires no membrane anchoring, suggesting that it can function as a bridging molecule. Our study introduces an important factor in tissue-resident apoptotic clearance and underscores the prominent role of glia as "semiprofessional" phagocytes in the nervous system.

A pathway for phagosome maturation during engulfment of apoptotic cells

Removal of apoptotic cells is critical for the physiological well-being of the organism and defects in corpse removal have been linked to disease states. Genes regulating corpse recognition and internalization have been identified, 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 Rab5(GDP), thus providing a mechanism for Rab5 recruitment to the nascent phagosome.

Draper-dependent glial phagocytic activity is mediated by Src and Syk family kinase signalling

The cellular machinery promoting phagocytosis of corpses of apoptotic cells is well conserved from worms to mammals. An important component is the Caenorhabditis elegans engulfment receptor CED-1 (ref. 1) and its Drosophila orthologue, Draper. The CED-1/Draper signalling pathway is also essential for the phagocytosis of other types of 'modified self' including necrotic cells, developmentally pruned axons and dendrites, and axons undergoing Wallerian degeneration. Here we show that Drosophila Shark, a non-receptor tyrosine kinase similar to mammalian Syk and Zap-70, binds Draper through an immunoreceptor tyrosine-based activation motif (ITAM) in the Draper intracellular domain. We show that Shark activity is essential for Draper-mediated signalling events in vivo, including the recruitment of glial membranes to severed axons and the phagocytosis of axonal debris and neuronal cell corpses by glia. We also show that the Src family kinase (SFK) Src42A can markedly increase Draper phosphorylation and is essential for glial phagocytic activity. We propose that ligand-dependent Draper receptor activation initiates the Src42A-dependent tyrosine phosphorylation of Draper, the association of Shark and the activation of the Draper pathway. These Draper-Src42A-Shark interactions are strikingly similar to mammalian immunoreceptor-SFK-Syk signalling events in mammalian myeloid and lymphoid cells. Thus, Draper seems to be an ancient immunoreceptor with an extracellular domain tuned to modified self, and an intracellular domain promoting phagocytosis through an ITAM-domain-SFK-Syk-mediated signalling cascade.

A novel sorting nexin modulates endocytic trafficking and alpha-secretase cleavage of the amyloid precursor protein

Ectodomain shedding of the amyloid precursor protein (APP) by the two proteases alpha- and beta-secretase is a key regulatory event in the generation of the Alzheimer disease amyloid beta peptide (Abeta). beta-Secretase catalyzes the first step in Abeta generation, whereas alpha-secretase cleaves within the Abeta domain, prevents Abeta generation, and generates a secreted form of APP with neuroprotective properties. At present, little is known about the cellular mechanisms that control APP alpha-secretase cleavage and Abeta generation. To explore the contributory pathways, we carried out an expression cloning screen. We identified a novel member of the sorting nexin (SNX) family of endosomal trafficking proteins, called SNX33, as a new activator of APP alpha-secretase cleavage. SNX33 is a homolog of SNX9 and was found to be a ubiquitously expressed phosphoprotein. Exogenous expression of SNX33 in cultured cells increased APP alpha-secretase cleavage 4-fold but surprisingly had little effect on beta-secretase cleavage. This effect was similar to the expression of the dominant negative dynamin-1 mutant K44A. SNX33 bound the endocytic GTPase dynamin and reduced the rate of APP endocytosis in a dynamin-dependent manner. This led to an increase of APP at the plasma membrane, where alpha-secretase cleavage mostly occurs. In summary, our study identifies SNX33 as a new endocytic protein, which modulates APP endocytosis and APP alpha-secretase cleavage, and demonstrates that the rate of APP endocytosis is a major control factor for APP alpha-secretase cleavage.

The small GTPase Rab2 functions in the removal of apoptotic cells in Caenorhabditis elegans

We identify here a novel class of loss-of-function alleles of uncoordinated locomotion(unc)-108, which encodes the Caenorhabditis elegans homologue of the mammalian small guanosine triphosphatase Rab2. Like the previously isolated dominant-negative mutants, unc-108 loss-of-function mutant animals are defective in locomotion. In addition, they display unique defects in the removal of apoptotic cells, revealing a previously uncharacterized function for Rab2. unc-108 acts in neurons and engulfing cells to control locomotion and cell corpse removal, respectively, indicating that unc-108 has distinct functions in different cell types. Using time-lapse microscopy, we find that unc-108 promotes the degradation of engulfed cell corpses. It is required for the efficient recruitment and fusion of lysosomes to phagosomes and the acidification of the phagosomal lumen. In engulfing cells, UNC-108 is enriched on the surface of phagosomes. We propose that UNC-108 acts on phagosomal surfaces to promote phagosome maturation and suggest that mammalian Rab2 may have a similar function in the degradation of apoptotic cells.

Engulfment of apoptotic cells: signals for a good meal

The clearance of apoptotic cells by phagocytes is an integral component of normal life, and defects in this process can have significant implications for self tolerance and autoimmunity. Recent studies have provided new insights into the engulfment process, including how phagocytes seek apoptotic cells, how they recognize and ingest these targets and how they maintain cellular homeostasis after the 'meal'. Several new factors that regulate engulfment have been identified, whereas the roles of some of the older players require revision. This Review focuses on these recent developments and attempts to highlight some of the important questions in this field.

Ubiquitin-proteasome: pallbearer carries the deceased to the grave

Phagocytosis is a complex process that involves multiple cellular functions. In this issue of Immunity, Silva et al. (2007) report that a protein ubiquitylation complex and the proteasome are required for the clearance of apoptotic cells in Drosophila.

A novel requirement for C. elegans Alix/ALX-1 in RME-1-mediated membrane transport

BACKGROUND

Alix/Bro1p family proteins have recently been identified as important components of multivesicular endosomes (MVEs) and are involved in the sorting of endocytosed integral membrane proteins, interacting with components of the ESCRT complex, the unconventional phospholipid LBPA, and other known endocytosis regulators. During infection, Alix can be co-opted by enveloped retroviruses, including HIV, providing an important function during virus budding from the plasma membrane. In addition, Alix is associated with the actin cytoskeleton and might regulate cytoskeletal dynamics.

RESULTS

Here we demonstrate a novel physical interaction between the only apparent Alix/Bro1p family protein in C. elegans, ALX-1, and a key regulator of receptor recycling from endosomes to the plasma membrane, called RME-1. The analysis of alx-1 mutants indicates that ALX-1 is required for the endocytic recycling of specific basolateral cargo in the C. elegans intestine, a pathway previously defined by the analysis of rme-1 mutants. The expression of truncated human Alix in HeLa cells disrupts the recycling of major histocompatibility complex class I, a known Ehd1/RME-1-dependent transport step, suggesting the phylogenetic conservation of this function. We show that the interaction of ALX-1 with RME-1 in C. elegans, mediated by RME-1/YPSL and ALX-1/NPF motifs, is required for this recycling process. In the C. elegans intestine, ALX-1 localizes to both recycling endosomes and MVEs, but the ALX-1/RME-1 interaction appears to be dispensable for ALX-1 function in MVEs and/or late endosomes.

CONCLUSIONS

This work provides the first demonstration of a requirement for an Alix/Bro1p family member in the endocytic recycling pathway in association with the recycling regulator RME-1.

A protocol describing pharynx counts and a review of other assays of apoptotic cell death in the nematode worm Caenorhabditis elegans

Studies of the nematode worm Caenorhabditis elegans have provided important insights into the genetics of programmed cell death (PCD), and revealed molecular mechanisms conserved from nematodes to humans. The organism continues to offer opportunities to investigate the processes of apoptosis under very well-defined conditions and at single-cell resolution in living animals. Here, a survey of the common methods used to study the process of PCD in C. elegans is described. Detailed instructions are provided for one standard method--the counting of extra cells of the anterior pharynx--a quantitative technique that can be used to detect even very subtle alterations in the progression of apoptotic cell death.

Two alternative mechanisms that regulate the presentation of apoptotic cell engulfment signal in Caenorhabditis elegans

Phosphatidylserine exposed on the surface of apoptotic mammalian cells is considered an "eat-me" signal that attracts phagocytes. The generality of using phosphatidylserine as a clearance signal for apoptotic cells in animals and the regulation of this event remain uncertain. Using ectopically expressed mouse MFG-E8, a secreted phosphatidylserine-binding protein, we detected specific exposure of phosphatidylserine on the surface of apoptotic cells in Caenorhabditis elegans. Masking the surface phosphatidylserine inhibits apoptotic cell engulfment. CED-7, an ATP-binding cassette (ABC) transporter, is necessary for the efficient exposure of phosphatidylserine on apoptotic somatic cells, and for the recognition of these cells by phagocytic receptor CED-1. Alternatively, phosphatidylserine exposure on apoptotic germ cells is not CED-7 dependent, but instead requires phospholipid scramblase PLSC-1, a homologue of mammalian phospholipid scramblases. Moreover, deleting plsc-1 results in the accumulation of apoptotic germ cells but not apoptotic somatic cells. These observations suggest that phosphatidylserine might be recognized by CED-1 and act as a conserved eat-me signal from nematodes to mammals. Furthermore, the two different biochemical activities used in somatic cells (ABC transporter) and germ cells (phospholipid scramblase) suggest an increased complexity in the regulation of phosphatidylserine presentation in response to apoptotic signals in different tissues and during different developmental stages.

Cooperation between engulfment receptors: the case of ABCA1 and MEGF10

The engulfment of dying cells is a specialized form of phagocytosis that is extremely conserved across evolution. In the worm, it is genetically controlled by two parallel pathways, which are only partially reconstituted in mammals. We focused on the recapitulation of the CED-1 defined pathway in mammalian systems. We first explored and validated MEGF10, a novel receptor bearing striking structural similarities to CED-1, as a bona fide functional ortholog in mammals and hence progressed toward the analysis of molecular interactions along the corresponding pathway. We ascertained that, in a system of forced expression by transfection, MEGF10 function can be modulated by the ATP binding cassette transporter ABCA1, ortholog to CED-7. Indeed, the coexpression of either a functional or a mutant ABCA1 exerted a transdominant positive or negative modulation on the MEGF10-dependent engulfment. The combined use of biochemical and biophysical approaches indicated that this functional cooperation relies on the alternate association of these receptors with a common partner, endogenously expressed in our cell system. We provide the first working model structuring in mammals the CED-1 dependent pathway.

The Dynami(n)cs of cell corpse engulfment

Engulfment of dying cells plays an important role during animal development and homeostasis, and several proteins involved in this process are known. However, the cell biology underlying phagocyte arm extension and cell corpse degradation is not well understood. A study published in this issue of Developmental Cell (Yu et al., 2006) now demonstrates an important role for the GTPase dynamin in these events.