A pathway for phagosome maturation during engulfment of apoptotic cells

Live-cell imaging in Caenorhabditis elegans reveals the distinct roles of dynamin self-assembly and guanosine triphosphate hydrolysis in the removal of apoptotic cells

Dynamins are large GTPases that oligomerize along membranes. Dynamin's membrane fission activity is believed to underlie many of its physiological functions in membrane trafficking. Previously, we reported that DYN-1 (Caenorhabditis elegans dynamin) drove the engulfment and degradation of apoptotic cells through promoting the recruitment and fusion of intracellular vesicles to phagocytic cups and phagosomes, an activity distinct from dynamin's well-known membrane fission activity. Here, we have detected the oligomerization of DYN-1 in living C. elegans embryos and identified DYN-1 mutations that abolish DYN-1's oligomerization or GTPase activities. Specifically, abolishing self-assembly destroys DYN-1's association with the surfaces of extending pseudopods and maturing phagosomes, whereas inactivating guanosine triphosphate (GTP) binding blocks the dissociation of DYN-1 from these membranes. Abolishing the self-assembly or GTPase activities of DYN-1 leads to common as well as differential phagosomal maturation defects. Whereas both types of mutations cause delays in the transient enrichment of the RAB-5 GTPase to phagosomal surfaces, only the self-assembly mutation but not GTP binding mutation causes failure in recruiting the RAB-7 GTPase to phagosomal surfaces. We propose that during cell corpse removal, dynamin's self-assembly and GTP hydrolysis activities establish a precise dynamic control of DYN-1's transient association to its target membranes and that this control mechanism underlies the dynamic recruitment of downstream effectors to target membranes.

Erythrophagocytosis by angiogenic endothelial cells is enhanced by loss of erythrocyte deformability

OBJECTIVE

Angiogenic endothelial cells can function as phagocytes, and phagocytosis is initiated via the opsonin lactadherin. In this study, we examined the interaction between lactadherin-opsonized erythrocytes with reduced deformability and angiogenic endothelium, as loss of deformability is characteristic for suicidal and aged erythrocytes.

MATERIALS AND METHODS

We used the Arg-Gly-Asp (RGD)-modified erythrocyte model and investigated the deformability parameter by cross-linking erythrocyte membranes through treatment with glutaraldehyde. Association in vitro with primary endothelial cells was detected by flow cytometry and visualized by light, fluorescent, and electron microscopy. Involvement of two crucial factors in phagocytosis, alpha(v)-integrins and Rho guanosine triphosphatase family member Rac1, was studied using small interfering RNA technology. Modified erythrocytes were administered in vivo into tumor-bearing mice to detect phagocytosis by endothelial cells.

RESULTS

Glutaraldehyde-treated (rigid) RGD-modified erythrocytes showed a strongly enhanced endothelial cell association compared to flexible RGD-modified erythrocytes. Knockdown by small interfering RNA lipoplexes of alpha(v)-integrins and Rac1 confirmed classical tethering and internalization of rigid RGD-erythrocytes. Upon in vivo administration, tumor endothelium showed pronounced erythrophagocytosis.

CONCLUSION

The pronounced phagocytosis of opsonized erythrocytes with reduced deformability by angiogenic growth factor-activated endothelial cells evokes new insights in endothelial cell function and suggests a role for these endothelial cells in (hematological) disorders because of their capacity to clear disordered erythrocytes.

PI3K signaling in neutrophils

PI3Ks play important roles in the signaling pathways used by a wide variety of cell surface receptors on neutrophils. Class IB PI3K plays a major role in the initial generation of PtdIns(3,4,5)P₃ by Gi-coupled G-protein coupled receptors (GPCRs) (e.g., receptors for fMLP, C5a, LTB₄). Class IA PI3Ks generate PtdIns(3,4,5)P₃ downstream of receptors which directly or indirectly couple to protein tyrosine kinases such as integrins, FcγRs, cytokine receptors, and GPCRs. The PtdIns(3,4,5)P₃ made by Class I PI3Ks regulates the activity of several different effector proteins, many of which are plasma membrane GEFs or GAPs for small GTPases. Class III PI3K generates PtdIns(3)P in the phagosome membrane and plays an important role in efficient assembly of the NADPH oxidase at this location. Much still remains to be discovered about the molecular details that govern activation of PI3Ks and the mechanisms by which these enzymes regulate complex cellular processes, such as neutrophil spreading, chemotaxis, phagocytosis, and killing of pathogens. However, it is clear from recent use of transgenic mouse models and isoform-selective PI3K inhibitors that these pathways are important in regulating neutrophil recruitment to sites of infection and damage in vivo. Thus, PI3K pathways may present novel opportunities for selective inhibition in some inflammatory pathologies.

Cellular and nuclear degradation during apoptosis

Apoptosis ensures quick death and quiet clearance of unwanted or damaged cells, without inducing much, if any, immunological responses from the organism. In metazoan organisms, apoptotic cells are swiftly engulfed by other cells. The degradation of cellular content is initiated in apoptotic cells and completed within engulfing cells. In apoptotic cells, caspase-mediated proteolysis cleaves protein substrates into fragments; nuclear DNA is partially degraded into nucleosomal units; and autophagy potentially contributes to apoptotic cell removal. In engulfing cells, specific signaling pathways promote the sequential fusion of intracellular vesicles with phagosomes and lead to the complete degradation of apoptotic cells in an acidic environment. Phagocytic receptors that initiate the engulfment of apoptotic cells play an additional and crucial role in initiating phagosome maturation through activating these signaling pathways. Here we highlight recent discoveries made in invertebrate models and mammalian systems, focusing on the molecular mechanisms that regulate the efficient degradation of apoptotic cells.

Caenorhabditis elegans myotubularin MTM-1 negatively regulates the engulfment of apoptotic cells

During programmed cell death, apoptotic cells are recognized and rapidly engulfed by phagocytes. Although a number of genes have been identified that promote cell corpse engulfment, it is not well understood how phagocytosis of apoptotic cells is negatively regulated. Here we have identified Caenorhabditis elegans myotubularin MTM-1 as a negative regulator of cell corpse engulfment. Myotubularins (MTMs) constitute a large, highly conserved family of lipid phosphatases. MTM gene mutations are associated with various human diseases, but the cellular functions of MTM proteins are not clearly defined. We found that inactivation of MTM-1 caused significant reduction in cell corpses in strong loss-of-function mutants of ced-1, ced-6, ced-7, and ced-2, but not in animals deficient in the ced-5, ced-12, or ced-10 genes. In contrast, overexpression of MTM-1 resulted in accumulation of cell corpses. This effect is dependent on the lipid phosphatase activity of MTM-1. We show that loss of mtm-1 function accelerates the clearance of cell corpses by promoting their internalization. Importantly, the reduction of cell corpses caused by mtm-1 RNAi not only requires the activities of CED-5, CED-12, and CED-10, but also needs the functions of the phosphatidylinositol 3-kinases (PI3Ks) VPS-34 and PIKI-1. We found that MTM-1 localizes to the plasma membrane in several known engulfing cell types and may modulate the level of phosphatidylinositol 3-phosphate (PtdIns(3)P) in vivo. We propose that MTM-1 negatively regulates cell corpse engulfment through the CED-5/CED-12/CED-10 module by dephosphorylating PtdIns(3)P on the plasma membrane.

Clearance of dead cells is crucial for normal animal development. Cell corpses are recognized, engulfed, and removed by phagocytic cells. However, the mechanisms that regulate phagocytosis are still not well understood, especially the ways in which the process is inhibited (negatively regulated). We screened the nematode worm Caenorhabditis elegans for negative regulators of cell corpse engulfment and identified myotubularin MTM-1. Myotubularins (MTMs) are a family of highly conserved enzymes that remove phosphate groups from membrane lipids. Mutations in human MTM genes are associated with various severe diseases including X-linked myotubular myopathy and Charcot-Marie-Tooth disease, but the normal functions of MTMs are unknown. In this study, we found that MTM-1 inhibits cell corpse engulfment through a series of evolutionarily conserved signaling molecules (the bipartite GEF (CED-5/DOCK180-CED-12/ELMO) and the GTPase CED-10/Rac). The negative regulatory effect of MTM-1 requires both its lipid phosphatase activity and the function of another group of enzymes called PI3-kinases. We propose that MTM-1 acts through Rac GTPase CED-10 by dephosphorylating the lipid PtdIns(3)P on the plasma membrane. We have identified a negative regulatory mechanism of cell corpse engulfment and a previously unknown cellular function of MTM-1, which may provide further insights into the basis of human MTM-related diseases.

Insights into RGS protein function from studies in Caenorhabditis elegans

The nematode worm, Caenorhabditis elegans, contains orthologs of most regulator of G protein signaling (RGS) protein subfamilies and all four G protein α-subunit subfamilies found in mammals. Every C. elegans RGS and Gα gene has been knocked out, and the in vivo functions and Gα targets of a number of RGS proteins have been characterized in detail. This has revealed a complex relationship between the RGS and Gα proteins, in which multiple RGS proteins can regulate the same Gα protein, either by acting redundantly or by exerting control over signaling under different circumstances or in different cells. RGS proteins that are coexpressed can also show specificity for distinct Gα targets in vivo, and the determinants of such specificity can reside outside of the RGS domain. This review will discuss how analysis in C. elegans may aid us in achieving a full understanding of the physiological functions of RGS proteins.

C. elegans Rab GTPase activating protein TBC-2 promotes cell corpse degradation by regulating the small GTPase RAB-5

During apoptosis, dying cells are quickly internalized by neighboring cells or phagocytes, and are enclosed in phagosomes that undergo a maturation process to generate the phagoslysosome, in which cell corpses are eventually degraded. It is not well understood how apoptotic cell degradation is regulated. Here we report the identification and characterization of the C. elegans tbc-2 gene, which is required for the efficient degradation of cell corpses. tbc-2 encodes a Rab GTPase activating protein (GAP) and its loss of function affects several events of phagosome maturation, including RAB-5 release, phosphatidylinositol 3-phosphate dynamics, phagosomal acidification, RAB-7 recruitment and lysosome incorporation, which leads to many persistent cell corpses at various developmental stages. Intriguingly, the persistent cell corpse phenotype of tbc-2 mutants can be suppressed by reducing gene expression of rab-5, and overexpression of a GTP-locked RAB-5 caused similar defects in phagosome maturation and cell corpse degradation. We propose that TBC-2 functions as a GAP to cycle RAB-5 from an active GTP-bound to an inactive GDP-bound state, which is required for maintaining RAB-5 dynamics on phagosomes and serves as a switch for the progression of phagosome maturation.

Rab7: roles in membrane trafficking and disease

The endocytosis pathway controls multiple cellular and physiological events. The lysosome is the destination of newly synthesized lysosomal hydrolytic enzymes. Internalized molecules or particles are delivered to the lysosome for degradation through sequential transport along the endocytic pathway. The endocytic pathway is also emerging as a signalling platform, in addition to the well-known role of the plasma membrane for signalling. Rab7 is a late endosome-/lysosome-associated small GTPase, perhaps the only lysosomal Rab protein identified to date. Rab7 plays critical roles in the endocytic processes. Through interaction with its partners (including upstream regulators and downstream effectors), Rab7 participates in multiple regulation mechanisms in endosomal sorting, biogenesis of lysosome [or LRO (lysosome-related organelle)] and phagocytosis. These processes are closely related to substrates degradation, antigen presentation, cell signalling, cell survival and microbial pathogen infection. Consistently, mutations or dysfunctions of Rab7 result in traffic disorders, which cause various diseases, such as neuropathy, cancer and lipid metabolism disease. Rab7 also plays important roles in microbial pathogen infection and survival, as well as in participating in the life cycle of viruses. Here, we give a brief review on the central role of Rab7 in endosomal traffic and summarize the studies focusing on the participation of Rab7 in disease pathogenesis. The underlying mechanism governed by Rab7 and its partners will also be discussed.

Abl kinase inhibits the engulfment of apoptotic [corrected] cells in Caenorhabditis elegans

The engulfment of apoptotic cells is required for normal metazoan development and tissue remodeling. In Caenorhabditis elegans, two parallel and partially redundant conserved pathways act in cell-corpse engulfment. One pathway includes the adaptor protein CED-2 CrkII and the small GTPase CED-10 Rac, and acts to rearrange the cytoskeleton of the engulfing cell. The other pathway includes the receptor tyrosine kinase CED-1 and might recruit membranes to extend the surface of the engulfing cell. Although many components required for engulfment have been identified, little is known about inhibition of engulfment. The tyrosine kinase Abl regulates the actin cytoskeleton in mammals and Drosophila in multiple ways. For example, Abl inhibits cell migration via phosphorylation of CrkII. We tested whether ABL-1, the C. elegans ortholog of Abl, inhibits the CED-2 CrkII-dependent engulfment of apoptotic cells. Our genetic studies indicate that ABL-1 inhibits apoptotic cell engulfment, but not through CED-2 CrkII, and instead acts in parallel to the two known engulfment pathways. The CED-10 Rac pathway is also required for proper migration of the distal tip cells (DTCs) during the development of the C. elegans gonad. The loss of ABL-1 function partially restores normal DTC migration in the CED-10 Rac pathway mutants. We found that ABI-1 the C. elegans homolog of mammalian Abi (Abl interactor) proteins, is required for engulfment of apoptotic cells and proper DTC migration. Like Abl, Abi proteins are cytoskeletal regulators. ABI-1 acts in parallel to the two known engulfment pathways, likely downstream of ABL-1. ABL-1 and ABI-1 interact physically in vitro. We propose that ABL-1 opposes the engulfment of apoptotic cells by inhibiting ABI-1 via a pathway that is distinct from the two known engulfment pathways.

Cell death or apoptosis is a normal part of animal development, as is the engulfment and removal of dead cells by other cells. In the nematode Caenorhabditis elegans, ten highly conserved proteins have been characterized previously for their roles in engulfment and in cell migration, both of which involve the formation of cellular extensions. Little is known, however, about how engulfment is inhibited. In mammals, the tyrosine kinase Abl, which regulates the actin cytoskeleton and which when misexpressed causes two types of leukemia, prevents the CrkII protein from facilitating cell migration. CrkII functions in engulfment in C. elegans and mammals. We tested whether the C. elegans homolog of Abl, ABL-1, could inhibit engulfment. We found that ABL-1 functions as an inhibitor of apoptotic cell engulfment and cell migration. However, our analysis further showed that ABL-1 does not function by inhibiting other known engulfment proteins, including C. elegans CrkII. Our data indicate that ABL-1 blocks ABI-1, the C. elegans homolog of the mammalian and Drosophila Abl-interactor (Abi) cytoskeletal-regulatory proteins. We propose that ABL-1 acts via ABI-1 to inhibit a newly identified pathway during cell corpse engulfment and cell migration.

We show thatC. elegans Abl (ABL-1) inhibits the engulfment of apoptotic cells via a newly defined pathway that includes theC. elegans homolog of the cytoskeletal regulator Abl-interactor.

An essential role for calcium flux in phagocytes for apoptotic cell engulfment and the anti-inflammatory response

Cells undergo programmed cell death/apoptosis throughout the lifespan of an organism. The subsequent immunologically silent removal of apoptotic cells plays a role in the maintenance of tolerance; defects in corpse clearance have been associated with autoimmune disease. A number of receptors and signaling molecules involved in this process have been identified, but intracellular signaling downstream of corpse recognition is only now being defined. Calcium plays a key role as a second messenger in many cell types, leading to the activation of downstream molecules and eventual transcription of effector genes; however, the role of calcium signaling during apoptotic cell removal is unclear. Here, using studies in cell lines and in the context of a whole organism, we show that apoptotic cell recognition induces both an acute and sustained calcium flux within phagocytes and that the genes required for calcium flux are essential for engulfment. Furthermore, we provide evidence that both the release of calcium from the endoplasmic reticulum and the entry of extracellular calcium through CRAC channels into the phagocytes are important during engulfment. Moreover, knockdown in Caenorhabditis elegans of stim-1 and jph-1, two genes linked to the entry of extracellular calcium into cells, led to increased persistence of apoptotic cells in the nematode. Loss of these genes seemed to affect early signaling events, leading to a decreased enrichment of actin adjacent to the apoptotic cell during corpse removal. We also show that calcium is crucial for the secretion of TGF-beta by the phagocytes during the engulfment of apoptotic cells. Taken together, these data point to an earlier unappreciated and evolutionarily conserved role for calcium flux at two distinguishable steps: the formation of the phagocytic cup and the internalization of the apoptotic cell, and the anti-inflammatory signaling induced in phagocytes by contact with apoptotic cells.

Genetic control of cellular quiescence in S. pombe

Transition from proliferation to quiescence brings about extensive changes in cellular behavior and structure. However, the genes that are crucial for establishing and/or maintaining quiescence are largely unknown. The fission yeast Schizosaccharomyces pombe is an excellent model in which to study this problem, because it becomes quiescent under nitrogen starvation. Here, we characterize 610 temperature-sensitive mutants, and identify 33 genes that are required for entry into and maintenance of quiescence. These genes cover a broad range of cellular functions in the cytoplasm, membrane and nucleus. They encode proteins for stress-responsive and cell-cycle kinase signaling pathways, for actin-bound and osmo-controlling endosome formation, for RNA transcription, splicing and ribosome biogenesis, for chromatin silencing, for biosynthesis of lipids and ATP, for cell-wall and membrane morphogenesis, and for protein trafficking and vesicle fusion. We specifically highlight Fcp1, a CTD phosphatase of RNA polymerase II, which differentially affects the transcription of genes that are involved in quiescence and proliferation. We propose that the transcriptional role of Fcp1 is central in differentiating quiescence from proliferation.

Emerging findings from studies of phospholipase D in model organisms (and a short update on phosphatidic acid effectors)

Phospholipase D (PLD) catalyses the hydrolysis of phosphatidylcholine to generate phosphatidic acid and choline. Historically, much PLD work has been conducted in mammalian settings although genes encoding enzymes of this family have been identified in all eukaryotic organisms. Recently, important insights on PLD function are emerging from work in yeast, but much less is known about PLD in other organisms. In this review we will summarize what is known about phospholipase D in several model organisms, including C. elegans, D. discoideum, D. rerio and D. melanogaster. In the cases where knockouts are available (C. elegans, Dictyostelium and Drosophila) the PLD gene(s) appear not to be essential for viability, but several studies are beginning to identify pathways where this activity has a role. Given that the proteins in model organisms are very similar to their mammalian counterparts, we expect that future studies in model organisms will complement and extend ongoing work in mammalian settings. At the end of this review we will also provide a short update on phosphatidic acid targets, a topic last reviewed in 2006.

Lysosome biogenesis mediated by vps-18 affects apoptotic cell degradation in Caenorhabditis elegans

Appropriate clearance of apoptotic cells (cell corpses) is an important step of programmed cell death. Although genetic and biochemical studies have identified several genes that regulate the engulfment of cell corpses, how these are degraded after being internalized in engulfing cell remains elusive. Here, we show that VPS-18, the Caenorhabditis elegans homologue of yeast Vps18p, is critical to cell corpse degradation. VPS-18 is expressed and functions in engulfing cells. Deletion of vps-18 leads to significant accumulation of cell corpses that are not degraded properly. Furthermore, vps-18 mutation causes strong defects in the biogenesis of endosomes and lysosomes, thus affecting endosomal/lysosomal protein degradation. Importantly, we demonstrate that phagosomes containing internalized cell corpses are unable to fuse with lysosomes in vps-18 mutants. Our findings thus provide direct evidence for the important role of endosomal/lysosomal degradation in proper clearance of apoptotic cells during programmed cell death.

mTOR signaling: RAG GTPases transmit the amino acid signal

mTOR (mammalian target of rapamycin) is a highly conserved nutrient-responsive regulator of cell growth that is found in all eukaryotes. The mechanism by which amino acids signal to mTOR has remained one of the largest outstanding questions in the field. Two recent complimentary studies provide compelling evidence that the Rag family of small GTPases is both necessary and sufficient to transmit a positive signal from amino acids to mTOR.

Phagosome maturation: going through the acid test

Phagosome maturation is the process by which internalized particles (such as bacteria and apoptotic cells) are trafficked into a series of increasingly acidified membrane-bound structures, leading to particle degradation. The characterization of the phagosomal proteome and studies in model organisms and mammals have led to the identification of numerous candidate proteins that cooperate to control the maturation of phagosomes containing different particles. A subset of these candidate proteins makes up the first pathway to be identified for the maturation of apoptotic cell-containing phagosomes. This suggests that a machinery that is distinct from receptor-mediated endocytosis is used in phagosome maturation.

Phagosome maturation during the removal of apoptotic cells: receptors lead the way

In metazoan organisms, cells undergoing apoptosis are rapidly engulfed and degraded by phagocytes. Defects in apoptotic-cell clearance result in inflammatory and autoimmune responses. However, little is known about how apoptotic-cell degradation is initiated and regulated and how different phagocytic targets induce different immune responses from their phagocytes. Recent studies in mammalian systems and invertebrate model organisms have led to major progress in identifying new factors involved in the maturation of phagosomes containing apoptotic cells. These studies have delineated signaling pathways that promote the sequential incorporation of intracellular organelles to phagosomes and have also discovered that phagocytic receptors produce the signals that initiate phagosome maturation. Here, we discuss these exciting new findings, focusing on the mechanisms that regulate the interactions between intracellular organelles and phagosomes.

Phagocytic signaling: you can touch, but you can't eat

The ability of phagocytes to discriminate between viable/healthy and apoptotic/foreign/abnormal cells is of fundamental importance; a recent study provides new molecular insights into the function of CD47-SIRP alpha signaling in this discrimination.