Drosophila Atg16 promotes enteroendocrine cell differentiation via regulation of intestinal Slit/Robo signaling.

Genetic variations of Atg16l1, Slit2 and Rab19 predispose to the development of inflammatory bowel disease (IBD), but the relationship between these mutations is unclear. Here we show that in Drosophila guts lacking the WD40 domain of Atg16, pre-enteroendocrine (pre-EE) cells accumulate that fail to differentiate into properly functioning secretory EE cells. Mechanistically, loss of Atg16 or its binding partner Rab19 impairs Slit production, which normally inhibits EE cell generation by activating Robo signaling in stem cells. Importantly, loss of Atg16 or decreased Slit/Robo signaling triggers an intestinal inflammatory response. Surprisingly, analysis of Rab19 and domain-specific Atg16 mutants indicates that their stem cell niche regulatory function is independent of autophagy. Our study reveals how mutations in these different genes may contribute to IBD.

Retromer Ensures the Degradation of Autophagic Cargo by Maintaining Lysosome Function in Drosophila.

The retromer is an evolutionarily conserved coat complex that consists of Vps26, Vps29, Vps35 and a heterodimer of sorting nexin (Snx) proteins in yeast. Retromer mediates the recycling of transmembrane proteins from endosomes to the trans-Golgi network, including receptors that are essential for the delivery of hydrolytic enzymes to lysosomes. Besides its function in lysosomal enzyme receptor recycling, involvement of retromer has also been proposed in a variety of vesicular trafficking events, including early steps of autophagy and endocytosis. Here we show that the late stages of autophagy and endocytosis are impaired in Vps26 and Vps35 deficient Drosophila larval fat body cells, but formation of autophagosomes and endosomes is not compromised. Accumulation of aberrant autolysosomes and amphisomes in the absence of retromer function appears to be the consequence of decreased degradative capacity, as they contain undigested cytoplasmic material. Accordingly, we show that retromer is required for proper cathepsin L trafficking mainly independent of LERP, the Drosophila homolog of the cation-independent mannose 6-phosphate receptor. Finally, we find that Snx3 and Snx6 are also required for proper autolysosomal degradation in Drosophila larval fat body cells.

Autophagy-from molecular mechanisms to clinical relevance.

Autophagy is a lysosomal degradation pathway of eukaryotic cells that is highly conserved from yeast to mammals. During this process, cooperating protein complexes are recruited in a hierarchic order to the phagophore assembly site (PAS) to mediate the elongation and closure of double-membrane vesicles called autophagosomes, which sequester cytosolic components and deliver their content to the endolysosomal system for degradation. As a major cytoprotective mechanism, autophagy plays a key role in the stress response against nutrient starvation, hypoxia, and infections. Although numerous studies reported that impaired function of core autophagy proteins also contributes to the development and progression of various human diseases such as neurodegenerative disorders, cardiovascular and muscle diseases, infections, and different types of cancer, the function of this process in human diseases remains unclear. Evidence often suggests a controversial role for autophagy in the pathomechanisms of these severe disorders. Here, we provide an overview of the molecular mechanisms of autophagy and summarize the recent advances on its function in human health and disease.

[Role of the stroma in the initiation and progression of tumors]. / A stroma szerepe a tumorok kialakulásában és progressziójában.

In the last decade, growing attention was paid to the observation that tumors did not only consist of cancer cells, they are rather a complex tissue-like mixture of tumor and stromal cells, which are playing an important role in the course of the malignant disease. Their contribution is so essential that without them, tumors are not even able to grow. This short review summarizes how stromal cells can promote the cancerous transformation and early development of tumors, how chronic inflammation contributes to the progression of cancer and how the stroma takes part in the induction of angiogenesis. The main mechanisms by which tumors can escape the immune surveillance will be demonstrated as well as the complex contributions of stroma to the invasion, intravasation and metastasis of cancer cells. Finally, possible and promising therapies will be presented that aim at the stroma and its main effects on the progression of tumors.

Impaired proteasomal degradation enhances autophagy via hypoxia signaling in Drosophila.

BACKGROUND: Two pathways are responsible for the majority of regulated protein catabolism in eukaryotic cells: the ubiquitin-proteasome system (UPS) and lysosomal self-degradation through autophagy. Both processes are necessary for cellular homeostasis by ensuring continuous turnover and quality control of most intracellular proteins. Recent studies established that both UPS and autophagy are capable of selectively eliminating ubiquitinated proteins and that autophagy may partially compensate for the lack of proteasomal degradation, but the molecular links between these pathways are poorly characterized. RESULTS: Here we show that autophagy is enhanced by the silencing of genes encoding various proteasome subunits (α, ß or regulatory) in larval fat body cells. Proteasome inactivation induces canonical autophagy, as it depends on core autophagy genes Atg1, Vps34, Atg9, Atg4 and Atg12. Large-scale accumulation of aggregates containing p62 and ubiquitinated proteins is observed in proteasome RNAi cells. Importantly, overexpressed Atg8a reporters are captured into the cytoplasmic aggregates, but these do not represent autophagosomes. Loss of p62 does not block autophagy upregulation upon proteasome impairment, suggesting that compensatory autophagy is not simply due to the buildup of excess cargo. One of the best characterized substrates of UPS is the α subunit of hypoxia-inducible transcription factor 1 (HIF-1α), which is continuously degraded by the proteasome during normoxic conditions. Hypoxia is a known trigger of autophagy in mammalian cells, and we show that genetic activation of hypoxia signaling also induces autophagy in Drosophila. Moreover, we find that proteasome inactivation-induced autophagy requires sima, the Drosophila ortholog of HIF-1α. CONCLUSIONS: We have characterized proteasome inactivation- and hypoxia signaling-induced autophagy in the commonly used larval Drosophila fat body model. Activation of both autophagy and hypoxia signaling was implicated in various cancers, and mutations affecting genes encoding UPS enzymes have recently been suggested to cause renal cancer. Our studies identify a novel genetic link that may play an important role in that context, as HIF-1α/sima may contribute to upregulation of autophagy by impaired proteasomal activity.

The autophagic roles of Rab small GTPases and their upstream regulators: a review.

Macroautophagy is an evolutionarily conserved degradative process of eukaryotic cells. Double-membrane vesicles called autophagosomes sequester portions of cytoplasm and undergo fusion with the endolysosomal pathway in order to degrade their content. There is growing evidence that members of the small GTPase RAB protein family-the well-known regulators of membrane trafficking and fusion events-play key roles in the regulation of the autophagic process. Despite numerous studies focusing on the functions of RAB proteins in autophagy, the importance of their upstream regulators in this process emerged only in the past few years. In this review, we summarize recent advances on the effects of RABs and their upstream modulators in the regulation of autophagy. Moreover, we discuss how impairment of these proteins alters the autophagic process leading to several generally known human diseases.

Atg6/UVRAG/Vps34-containing lipid kinase complex is required for receptor downregulation through endolysosomal degradation and epithelial polarity during Drosophila wing development.

Atg6 (Beclin 1 in mammals) is a core component of the Vps34 PI3K (III) complex, which promotes multiple vesicle trafficking pathways. Atg6 and Vps34 form two distinct PI3K (III) complexes in yeast and mammalian cells, either with Atg14 or with UVRAG. The functions of these two complexes are not entirely clear, as both Atg14 and UVRAG have been suggested to regulate both endocytosis and autophagy. In this study, we performed a microscopic analysis of UVRAG, Atg14, or Atg6 loss-of-function cells in the developing Drosophila wing. Both autophagy and endocytosis are seriously impaired and defective endolysosomes accumulate upon loss of Atg6. We show that Atg6 is required for the downregulation of Notch and Wingless signaling pathways; thus it is essential for normal wing development. Moreover, the loss of Atg6 impairs cell polarity. Atg14 depletion results in autophagy defects with no effect on endocytosis or cell polarity, while the silencing of UVRAG phenocopies all but the autophagy defect of Atg6 depleted cells. Thus, our results indicate that the UVRAG-containing PI3K (III) complex is required for receptor downregulation through endolysosomal degradation and for the establishment of proper cell polarity in the developing wing, while the Atg14-containing complex is involved in autophagosome formation.

Rab11 facilitates cross-talk between autophagy and endosomal pathway through regulation of Hook localization.

During autophagy, double-membrane autophagosomes deliver sequestered cytoplasmic content to late endosomes and lysosomes for degradation. The molecular mechanism of autophagosome maturation is still poorly characterized. The small GTPase Rab11 regulates endosomal traffic and is thought to function at the level of recycling endosomes. We show that loss of Rab11 leads to accumulation of autophagosomes and late endosomes in Drosophila melanogaster. Rab11 translocates from recycling endosomes to autophagosomes in response to autophagy induction and physically interacts with Hook, a negative regulator of endosome maturation. Hook anchors endosomes to microtubules, and we show that Rab11 facilitates the fusion of endosomes and autophagosomes by removing Hook from mature late endosomes and inhibiting its homodimerization. Thus induction of autophagy appears to promote autophagic flux by increased convergence with the endosomal pathway.