Parcas is the predominant Rab11-GEF for rhodopsin transport in Drosophila photoreceptors

The Osiris family genes function as novel regulators of the tube maturation process in the Drosophila trachea

Drosophila trachea is a premier model to study tube morphogenesis. After the formation of continuous tubes, tube maturation follows. Tracheal tube maturation starts with an apical secretion pulse that deposits extracellular matrix components to form a chitin-based apical luminal matrix (aECM). This aECM is then cleared and followed by the maturation of taenidial folds. Finally, air fills the tubes. Meanwhile, the cellular junctions are maintained to ensure tube integrity. Previous research has identified several key components (ER, Golgi, several endosomes) of protein trafficking pathways that regulate the secretion and clearance of aECM, and the maintenance of cellular junctions. The Osiris (Osi) gene family is located at the Triplo-lethal (Tpl) locus on chromosome 3R 83D4-E3 and exhibits dosage sensitivity. Here, we show that three Osi genes (Osi9, Osi15, Osi19), function redundantly to regulate adherens junction (AJ) maintenance, luminal clearance, taenidial fold formation, tube morphology, and air filling during tube maturation. The localization of Osi proteins in endosomes (Rab7-containing late endosomes, Rab11-containing recycling endosomes, Lamp-containing lysosomes) and the reduction of these endosomes in Osi mutants suggest the possible role of Osi genes in tube maturation through endosome-mediated trafficking. We analyzed tube maturation in zygotic rab11 and rab7 mutants, respectively, to determine whether endosome-mediated trafficking is required. Interestingly, similar tube maturation defects were observed in rab11 but not in rab7 mutants, suggesting the involvement of Rab11-mediated trafficking, but not Rab7-mediated trafficking, in this process. To investigate whether Osi genes regulate tube maturation primarily through the maintenance of Rab11-containing endosomes, we overexpressed rab11 in Osi mutant trachea. Surprisingly, no obvious rescue was observed. Thus, increasing endosome numbers is not sufficient to rescue tube maturation defects in Osi mutants. These results suggest that Osi genes regulate other aspects of endosome-mediated trafficking, or regulate an unknown mechanism that converges or acts in parallel with Rab11-mediated trafficking during tube maturation.

Rab11 and Its Role in Neurodegenerative Diseases

Vesicles mediate the trafficking of membranes/proteins in the endocytic and secretory pathways. These pathways are regulated by small GTPases of the Rab family. Rab proteins belong to the Ras superfamily of GTPases, which are significantly involved in various intracellular trafficking and signaling processes in the nervous system. Rab11 is known to play a key role especially in recycling many proteins, including receptors important for signal transduction and preservation of functional activities of nerve cells. Rab11 activity is controlled by GEFs (guanine exchange factors) and GAPs (GTPase activating proteins), which regulate its function through modulating GTP/GDP exchange and the intrinsic GTPase activity, respectively. Rab11 is involved in the transport of several growth factor molecules important for the development and repair of neurons. Overexpression of Rab11 has been shown to significantly enhance vesicle trafficking. On the other hand, a reduced expression of Rab11 was observed in several neurodegenerative diseases. Current evidence appears to support the notion that Rab11 and its cognate proteins may be potential targets for therapeutic intervention. In this review, we briefly discuss the function of Rab11 and its related interaction partners in intracellular pathways that may be involved in neurodegenerative processes.

Stratum is required for both apical and basolateral transport through stable expression of Rab10 and Rab35 in Drosophila photoreceptors

Post-Golgi transport for specific membrane domains, also termed polarized transport, is essential for the construction and maintenance of polarized cells. Highly polarized Drosophila photoreceptors serve as a good model system for studying the mechanisms underlying polarized transport. The Mss4 Drosophila ortholog, Stratum (Strat), controls basal restriction of basement membrane proteins in follicle cells, and Rab8 acts downstream of Strat. We investigated the function of Strat in fly photoreceptors and found that polarized transport in both the basolateral and the rhabdomere membrane domains was inhibited in Strat-deficient photoreceptors. We also observed 79 and 55% reductions in Rab10 and Rab35 levels, respectively, but no reduction in Rab11 levels in whole-eye homozygous clones of Strat^null. Moreover, Rab35 was localized in the rhabdomere, and loss of Rab35 resulted in impaired Rh1 transport to the rhabdomere. These results indicate that Strat is essential for the stable expression of Rab10 and Rab35, which regulate basolateral and rhabdomere transport, respectively, in fly photoreceptors.

Structure of a TRAPPII-Rab11 activation intermediate reveals GTPase substrate selection mechanisms

Rab1 and Rab11 are essential regulators of the eukaryotic secretory and endocytic recycling pathways. The transport protein particle (TRAPP) complexes activate these guanosine triphosphatases via nucleotide exchange using a shared set of core subunits. The basal specificity of the TRAPP core is toward Rab1, yet the TRAPPII complex is specific for Rab11. A steric gating mechanism has been proposed to explain TRAPPII counterselection against Rab1. Here, we present cryo-electron microscopy structures of the 22-subunit TRAPPII complex from budding yeast, including a TRAPPII-Rab11 nucleotide exchange intermediate. The Trs130 subunit provides a "leg" that positions the active site distal to the membrane surface, and this leg is required for steric gating. The related TRAPPIII complex is unable to activate Rab11 because of a repulsive interaction, which TRAPPII surmounts using the Trs120 subunit as a "lid" to enclose the active site. TRAPPII also adopts an open conformation enabling Rab11 to access and exit from the active site chamber.

Proximity proteomics in a marine diatom reveals a putative cell surface-to-chloroplast iron trafficking pathway

Iron is a biochemically critical metal cofactor in enzymes involved in photosynthesis, cellular respiration, nitrate assimilation, nitrogen fixation, and reactive oxygen species defense. Marine microeukaryotes have evolved a phytotransferrin-based iron uptake system to cope with iron scarcity, a major factor limiting primary productivity in the global ocean. Diatom phytotransferrin is endocytosed; however, proteins downstream of this environmentally ubiquitous iron receptor are unknown. We applied engineered ascorbate peroxidase APEX2-based subcellular proteomics to catalog proximal proteins of phytotransferrin in the model marine diatom Phaeodactylum tricornutum. Proteins encoded by poorly characterized iron-sensitive genes were identified including three that are expressed from a chromosomal gene cluster. Two of them showed unambiguous colocalization with phytotransferrin adjacent to the chloroplast. Further phylogenetic, domain, and biochemical analyses suggest their involvement in intracellular iron processing. Proximity proteomics holds enormous potential to glean new insights into iron acquisition pathways and beyond in these evolutionarily, ecologically, and biotechnologically important microalgae.

Sec71 separates Golgi stacks in Drosophila S2 cells

Golgi stacks are the basic structural units of the Golgi. Golgi stacks are separated from each other and scattered in the cytoplasm of Drosophila cells. Here, we report that the ARF-GEF inhibitor Brefeldin A (BFA) induces the formation of BFA bodies, which are aggregates of Golgi stacks, trans-Golgi networks and recycling endosomes. Recycling endosomes are located in the centers of BFA bodies, while Golgi stacks surround them on their trans sides. Live imaging of S2 cells revealed that Golgi stacks repeatedly merged and separated on their trans sides, and BFA caused successive merger by inhibiting separation, forming BFA bodies. S2 cells carrying genome-edited BFA-resistant mutant Sec71M717L did not form BFA bodies at high concentrations of BFA; S2 cells carrying genome-edited BFA-hypersensitive mutant Sec71F713Y produced BFA bodies at low concentrations of BFA. These results indicate that Sec71 is the sole BFA target for BFA body formation and controls Golgi stack separation. Finally, we showed that impairment of Sec71 in fly photoreceptors induces BFA body formation, with accumulation of both apical and basolateral cargoes, resulting in inhibition of polarized transport.

The Recycling Endosome in Nerve Cell Development: One Rab to Rule Them All?

Endocytic recycling is an intracellular process that returns internalized molecules back to the plasma membrane and plays crucial roles not only in the reuse of receptor molecules but also in the remodeling of the different components of this membrane. This process is required for a diversity of cellular events, including neuronal morphology acquisition and functional regulation, among others. The recycling endosome (RE) is a key vesicular component involved in endocytic recycling. Recycling back to the cell surface may occur with the participation of several different Rab proteins, which are master regulators of membrane/protein trafficking in nerve cells. The RE consists of a network of interconnected and functionally distinct tubular subdomains that originate from sorting endosomes and transport their cargoes along microtubule tracks, by fast or slow recycling pathways. Different populations of REs, particularly those formed by Rab11, Rab35, and Arf6, are associated with a myriad of signaling proteins. In this review, we discuss the cumulative evidence suggesting the existence of heterogeneous domains of REs, controlling different aspects of neurogenesis, with a particular focus on the commonalities and singularities of these REs and their contribution to nerve development and differentiation in several animal models.

Hydroxylated sphingolipid biosynthesis regulates photoreceptor apical domain morphogenesis

Apical domains of epithelial cells often undergo dramatic changes during morphogenesis to form specialized structures, such as microvilli. Here, we addressed the role of lipids during morphogenesis of the rhabdomere, the microvilli-based photosensitive organelle of Drosophila photoreceptor cells. Shotgun lipidomics analysis performed on mutant alleles of the polarity regulator crumbs, exhibiting varying rhabdomeric growth defects, revealed a correlation between increased abundance of hydroxylated sphingolipids and abnormal rhabdomeric growth. This could be attributed to an up-regulation of fatty acid hydroxylase transcription. Indeed, direct genetic perturbation of the hydroxylated sphingolipid metabolism modulated rhabdomere growth in a crumbs mutant background. One of the pathways targeted by sphingolipid metabolism turned out to be the secretory route of newly synthesized Rhodopsin, a major rhabdomeric protein. In particular, altered biosynthesis of hydroxylated sphingolipids impaired apical trafficking via Rab11, and thus apical membrane growth. The intersection of lipid metabolic pathways with apical domain growth provides a new facet to our understanding of apical growth during morphogenesis.

Rab10, Crag and Ehbp1 regulate the basolateral transport of Na+K+ATPase in Drosophila photoreceptors

Cells in situ are often polarized and have multiple plasma membrane domains. To establish and maintain these domains, polarized transport is essential, and its impairment results in genetic disorders. Nevertheless, the underlying mechanisms of polarized transport have not been elucidated. Drosophila photoreceptor offers an excellent model for studying this. We found that Rab10 impairment significantly reduced basolateral levels of Na+K+ATPase, mislocalizing it to the stalk membrane, which is a domain of the apical plasma membrane. Furthermore, the shrunken basolateral and the expanded stalk membranes were accompanied with abnormalities in the Golgi cisternae of Rab10-impaired retinas. The deficiencies of Rab10-GEF Crag or the Rab10 effector Ehbp1 phenocopied Rab10 deficiency, indicating that Crag, Rab10 and Ehbp1 work together for polarized trafficking of membrane proteins to the basolateral membrane. These phenotypes were similar to those seen upon deficiency of AP1 or clathrin, which are known to be involved in the basolateral transport in other systems. Additionally, Crag, Rab10 and Ehbp1 colocalized with AP1 and clathrin on the trans-side of Golgi stacks. Taken together, these results indicate that AP1 and clathrin, and Crag, Rab10 and Ehbp1 collaborate in polarized basolateral transport, presumably in the budding process in the trans-Golgi network.

Recycling endosomes attach to the trans-side of Golgi stacks in Drosophila and mammalian cells

Historically, the trans-Golgi network (TGN) has been recognized as a sorting center of newly synthesized proteins, whereas the recycling endosome (RE) is a compartment where endocytosed materials transit before being recycled to the plasma membrane. However, recent findings revealed that both the TGN and RE connect endocytosis and exocytosis and, thus, are functionally overlapping. Here we report, in both Drosophila and microtubule-disrupted HeLa cells, that REs are interconvertible between two distinct states, namely Golgi-associated REs and free REs. Detachment and reattachment of REs and Golgi stacks are often observed, and newly synthesized glycosylphosphatidylinositol-anchored cargo protein but not vesicular stomatitis virus G protein is transported through these two types of RE. In plants, there are two types of TGN - Golgi-associated TGN and Golgi-independent TGN. We show that dynamics of REs in both Drosophila and mammalian cells are very similar compared with those of plant TGNs. And, together with the similarity on the molecular level, our results indicate that fly and mammalian REs are organelles that are equivalent to TGNs in plants. This suggests that the identities and functional relationships between REs and TGNs should be reconsidered.