Hui Xiao

Apoptosis, a genetically programmed cell death, allows for homeostasis and tissue remodelling during development of all multi-cellular organisms. Phagocytes swiftly recognize, engulf and digest apoptotic cells. Yet, to date the molecular mechanisms underlying this phagocytic process are still poorly understood. To delineate the molecular mechanisms of apoptotic cell clearance in Drosophila, we have carried out a deficiency screen and have identified three overlapping phagocytosis-defective mutants, which all delete the fly homologue of the ced-12 gene, known as Dmel\ced12. As anticipated, we have found that Dmel\ced-12 is required for apoptotic cell clearance, as for its C. elegans and mammalian homologues, ced-12 and elmo, respectively. However, the loss of Dmel\ced-12 did not solely account for the phenotypes of all three deficiencies, as zygotic mutations and germ line clones of Dmel\ced-12 exhibited weaker phenotypes. Using a nearby genetically interacting deficiency, we have found that the polycystic kidney disease 2 gene, pkd2, which encodes a member of the TRPP channel family, is also required for phagocytosis of apoptotic cells, thereby demonstrating a novel role for PKD2 in this process. We have also observed genetic interactions between pkd2, simu, drpr, rya-r44F, and retinophilin (rtp), also known as undertaker (uta), a gene encoding a MORN-repeat containing molecule, which we have recently found to be implicated in calcium homeostasis during phagocytosis. However, we have not found any genetic interaction between Dmel\ced-12 and simu. Based on these genetic interactions and recent reports demonstrating a role for the mammalian pkd-2 gene product in ER calcium release during store-operated calcium entry, we propose that PKD2 functions in the DRPR/RTP pathway to regulate calcium homeostasis during this process. Similarly to its C. elegans homologue, Dmel\Ced-12 appears to function in a genetically distinct pathway.

Corpse-Sorting Machinery: Phagocytosis of apoptotic cells is an integral part of the cell death program and plays critical roles in tissue remodeling, suppression of inflammation, and regulation of immune responses. The clearance of cell corpses requires their engulfment and subsequent degradation by phagocytic cells. During this process, receptors of the CED-1 family play a central role in recognizing cell corpses, transducing engulfment signals, and initiating the maturation of phagosomes containing apoptotic cell corpses. Retromer is a multisubunit protein complex conserved from yeast to mammals that mediates retrograde transport of transmembrane cargo from the endosome to the trans-Golgi network. Failure in recycling these proteins leads to their delivery to lysosomes where they are degraded. Chen et al. (p. 1261 , published online 4 February) report that the Caenorhabditis elegans retromer complex is essential for the phagocytosis of CED-1 and thus for the clearance of apoptotic cells.

Background: Oxidative stress plays a crucial role in chronic complications of diabetes such as diabetic nephropathy (DN). This study investigated the role of peroxynitrite (ONOO^-), generated from inducible nitric oxide synthase (iNOS) induced by tumor necrosis factor alpha (TNF-α), in the pathogenesis of glomerular lesions in diabetic rats. Methods:Diabetes was induced in Sprague Dawley rats by an intraperitoneal injection of streptozocin, and aminoguanidine was used as selective inhibitor of iNOS. The iNOS transcription and protein distribution and content in rat glomeruli were detected. Nitrotyrosine (NT), a specific marker of ONOO-, was measured to represent the distribution and content of ONOO- in rat glomeruli. TNF-α level and nitric oxide (NO) content were evaluated, and the pathological changes in the rat glomeruli were observed. Biochemical indicators of renal function were also measured. Results: TNF-α level and NO content, iNOS expression and its protein content, and NT content increased significantly, in accordance with the pathological changes of glomerulus and renal dysfunction in the diabetes group. Aminoguanidine was found to inhibit iNOS and then reduce ONOO- overformation, attenuating the pathological alterations. Conclusion: This study clarified clearly that exaggerated ONOO- formation, generated from induced iNOS may play a key role in glomerular lesions in diabetic rats.

In Caenorhabditis elegans, the central cell-killing process is essentially controlled by the interplay of four apoptotic factors: EGL-1/BH3-only protein, CED-9/Bcl2, CED-4/Apaf1, and CED-3/caspase. In cells destined to die, EGL-1 binds to CED-9 and results in the release of CED-4 from the mitochondrion-tethered CED-9-CED-4 complex to the perinucleus, which facilitates processing of the CED-3 caspase to cause apoptosis. However, whether additional factors exist to regulate the cell-killing process remains largely unknown. We have identified here WAN-1, the C. elegans ortholog of mammalian adenine nucleotide translocator, as an important cell death regulator. Genetic inactivation of wan-1 significantly suppressed both somatic and germ line cell deaths in C. elegans. Consistently, chemical inhibition of WAN-1 activity also caused strong reduction of germ line apoptosis. WAN-1 localizes to mitochondria and can form complex with both CED-4 and CED-9. Importantly, the cell death initiator EGL-1 can disrupt the interaction between CED-9 and WAN-1. In addition, overexpression of WAN-1 induced ectopic cell killing dependently on the core cell death pathway. These findings suggest that WAN-1 is involved in the central cell-killing process and cooperates with the core cell death machinery to promote programmed cell death in C. 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.