Localization and activation of the Drosophila protease easter require the ER-resident saposin-like protein seele

Isolation of secreted proteins from Drosophila ovaries and embryos through in vivo BirA-mediated biotinylation

The extraordinarily strong non-covalent interaction between biotin and avidin (kD = 10-14-10-16) has permitted this interaction to be used in a wide variety of experimental contexts. The Biotin Acceptor Peptide (BAP), a 15 amino acid motif that can be biotinylated by the E. coli BirA protein, has been fused to proteins-of-interest, making them substrates for in vivo biotinylation. Here we report on the construction and characterization of a modified BirA bearing signals for secretion and endoplasmic reticulum (ER) retention, for use in experimental contexts requiring biotinylation of secreted proteins. When expressed in the Drosophila female germline or ovarian follicle cells under Gal4-mediated transcriptional control, the modified BirA protein could be detected and shown to be enzymatically active in ovaries and progeny embryos. Surprisingly, however, it was not efficiently retained in the ER, and instead appeared to be secreted. To determine whether this secreted protein, now designated secBirA, could biotinylate secreted proteins, we generated BAP-tagged versions of two secreted Drosophila proteins, Torsolike (Tsl) and Gastrulation Defective (GD), which are normally expressed maternally and participate in embryonic pattern formation. Both Tsl-BAP and GD-BAP were shown to exhibit normal patterning activity. Co-expression of Tsl-BAP together with secBirA in ovarian follicle cells resulted in its biotinylation, which permitted its isolation from both ovaries and progeny embryos using Avidin-coupled affinity matrix. In contrast, co-expression with secBirA in the female germline did not result in detectable biotinylation of GD-BAP, possibly because the C-terminal location of the BAP tag made it inaccessible to BirA in vivo. Our results indicate that secBirA directs biotinylation of proteins bound for secretion in vivo, providing access to powerful experimental approaches for secreted proteins-of-interest. However, efficient biotinylation of target proteins may vary depending upon the location of the BAP tag or other structural features of the protein.

Proteolytic cleavage is required for functional neuroligin 2 maturation and trafficking in Drosophila

Neuroligins (Nlgs) are transmembrane cell adhesion molecules playing essential roles in synapse development and function. Genetic mutations in neuroligin genes have been linked with some neurodevelopmental disorders such as autism. These mutated Nlgs are mostly retained in the endoplasmic reticulum (ER). However, the mechanisms underlying normal Nlg maturation and trafficking have remained largely unknown. Here, we found that Drosophila neuroligin 2 (DNlg2) undergoes proteolytic cleavage in the ER in a variety of Drosophila tissues throughout developmental stages. A region encompassing Y642-T698 is required for this process. The immature non-cleavable DNlg2 is retained in the ER and non-functional. The C-terminal fragment of DNlg2 instead of the full-length or non-cleavable DNlg2 is able to rescue neuromuscular junction defects and GluRIIB reduction induced by dnlg2 deletion. Intriguingly, the autism-associated R598C mutation in DNlg2 leads to similar marked defects in DNlg2 proteolytic process and ER export, revealing a potential role of the improper Nlg cleavage in autism pathogenesis. Collectively, our findings uncover a specific mechanism that controls DNlg2 maturation and trafficking via proteolytic cleavage in the ER, suggesting that the perturbed proteolytic cleavage of Nlgs likely contributes to autism disorder.

dOCRL maintains immune cell quiescence by regulating endosomal traffic

Lowe Syndrome is a developmental disorder characterized by eye, kidney, and neurological pathologies, and is caused by mutations in the phosphatidylinositol-5-phosphatase OCRL. OCRL plays diverse roles in endocytic and endolysosomal trafficking, cytokinesis, and ciliogenesis, but it is unclear which of these cellular functions underlie specific patient symptoms. Here, we show that mutation of Drosophila OCRL causes cell-autonomous activation of hemocytes, which are macrophage-like cells of the innate immune system. Among many cell biological defects that we identified in docrl mutant hemocytes, we pinpointed the cause of innate immune cell activation to reduced Rab11-dependent recycling traffic and concomitantly increased Rab7-dependent late endosome traffic. Loss of docrl amplifies multiple immune-relevant signals, including Toll, Jun kinase, and STAT, and leads to Rab11-sensitive mis-sorting and excessive secretion of the Toll ligand Spåtzle. Thus, docrl regulation of endosomal traffic maintains hemocytes in a poised, but quiescent state, suggesting mechanisms by which endosomal misregulation of signaling may contribute to symptoms of Lowe syndrome.

Lowe syndrome is a developmental disorder characterized by severe kidney, eye, and neurological symptoms, and is caused by mutations in the gene OCRL. OCRL has been shown to control many steps of packaging and transport of materials within cells, though it remains unclear which of these disrupted transport steps cause each of the many symptoms in Lowe syndrome patients. We found that in fruit flies, loss of OCRL caused transport defects at specific internal compartments in innate immune cells, resulting in amplification of multiple critical inflammatory signals. Similar inflammatory signals have been implicated in forms of epilepsy, which is a primary symptom in Lowe syndrome patients. Thus, our work uncovers a new function for OCRL in animals, and opens an exciting new avenue of investigation into how loss of OCRL causes the symptoms of Lowe syndrome.

Drosophila melanogaster clip-domain serine proteases: Structure, function and regulation

Mammalian chymotrypsin-like serine proteases (SPs) are one of the best-studied family of enzymes with roles in a wide range of physiological processes, including digestion, blood coagulation, fibrinolysis and humoral immunity. Extracellular SPs can form cascades, in which one protease activates the zymogen of the next protease in the chain, to amplify physiological or pathological signals. These extracellular SPs are generally multi-domain proteins, with pro-domains that are involved in protein-protein interactions critical for the sequential organization of the cascades, the control of their intensity and their proper localization. Far less is known about invertebrate SPs than their mammalian counterparts. In insect genomes, SPs and their proteolytically inactive homologs (SPHs) constitute large protein families. In addition to the chymotrypsin fold, many of these proteins contain additional structural domains, often with conserved mammalian orthologues. However, the largest group of arthropod SP regulatory modules is the clip domains family, which has only been identified in arthropods. The clip-domain SPs are extracellular and have roles in the immune response and embryonic development. The powerful reverse-genetics tools in Drosophila melanogaster have been essential to identify the functions of clip-SPs and their organization in sequential cascades. This review focuses on the current knowledge of Drosophila clip-SPs and presents, when necessary, data obtained in other insect models. We will first cover the biochemical and structural features of clip domain SPs and SPHs. Clip-SPs are implicated in three main biological processes: the control of the dorso-ventral patterning during embryonic development; the activation of the Toll-mediated response to microbial infections and the prophenoloxydase cascade, which triggers melanization. Finally, we review the regulation of SPs and SPHs, from specificity of activation to inhibition by endogenous or pathogen-encoded inhibitors.

Maternal control of the Drosophila dorsal-ventral body axis

The pathway that generates the dorsal-ventral (DV) axis of the Drosophila embryo has been the subject of intense investigation over the previous three decades. The initial asymmetric signal originates during oogenesis by the movement of the oocyte nucleus to an anterior corner of the oocyte, which establishes DV polarity within the follicle through signaling between Gurken, the Drosophila Transforming Growth Factor (TGF)-α homologue secreted from the oocyte, and the Drosophila Epidermal Growth Factor Receptor (EGFR) that is expressed by the follicular epithelium cells that envelop the oocyte. Follicle cells that are not exposed to Gurken follow a ventral fate and express Pipe, a sulfotransferase that enzymatically modifies components of the inner vitelline membrane layer of the eggshell, thereby transferring DV spatial information from the follicle to the egg. These ventrally sulfated eggshell proteins comprise a localized cue that directs the ventrally restricted formation of the active Spätzle ligand within the perivitelline space between the eggshell and the embryonic membrane. Spätzle activates Toll, a transmembrane receptor in the embryonic membrane. Transmission of the Toll signal into the embryo leads to the formation of a ventral-to-dorsal gradient of the transcription factor Dorsal within the nuclei of the syncytial blastoderm stage embryo. Dorsal controls the spatially specific expression of a large constellation of zygotic target genes, the Dorsal gene regulatory network, along the embryonic DV circumference. This article reviews classic studies and integrates them with the details of more recent work that has advanced our understanding of the complex pathway that establishes Drosophila embryo DV polarity. For further resources related to this article, please visit the WIREs website.

CONFLICT OF INTEREST

The authors have declared no conflicts of interest for this article.

Localized serine protease activity and the establishment of Drosophila embryonic dorsoventral polarity

Drosophila embryo dorsoventral polarity is established by a maternally encoded signal transduction pathway in which three sequentially acting serine proteases, Gastrulation Defective, Snake and Easter, generate the ligand that activates the Toll receptor on the ventral side of the embryo. The spatial regulation of this pathway depends upon ventrally restricted expression of the Pipe sulfotransferase in the ovarian follicle during egg formation. Several recent observations have advanced our understanding of the mechanism regulating the spatially restricted activation of Toll. First, several protein components of the vitelline membrane layer of the eggshell have been determined to be targets of Pipe-mediated sulfation. Second, the processing of Easter by Snake has been identified as the first Pipe-dependent, ventrally-restricted processing event in the pathway. Finally, Gastrulation Defective has been shown to undergo Pipe-dependent, ventral localization within the perivitelline space and to facilitate Snake-mediated processing of Easter. Together, these observations suggest that Gastrulation Defective, localized on the interior ventral surface of the eggshell in association with Pipe-sulfated eggshell proteins, recruits and mediates an interaction between Snake and Easter. This event leads to ventrally-restricted processing and activation of Easter and consequently, localized formation of the Toll ligand, and Toll activation.

A ventrally localized protease in the Drosophila egg controls embryo dorsoventral polarity

Drosophila embryo dorsoventral (DV) polarity is defined by serine protease activity in the perivitelline space (PVS) between the embryonic membrane and the inner layer of the eggshell. Gastrulation Defective (GD) cleaves and activates Snake (Snk). Activated Snk cleaves and activates Easter (Ea), exclusively on the ventral side of the embryo. Activated Ea then processes Spätzle (Spz) into the activating ligand for Toll, a transmembrane receptor that is distributed throughout the embryonic plasma membrane. Ventral activation of Toll depends upon the activity of the Pipe sulfotransferase in the ventral region of the follicular epithelium that surrounds the developing oocyte. Pipe transfers sulfate residues to several protein components of the inner vitelline membrane layer of the eggshell. Here we show that GD protein becomes localized in the ventral PVS in a Pipe-dependent process. Moreover, ventrally concentrated GD acts to promote the cleavage of Ea by Snk through an extracatalytic mechanism that is distinct from GD's proteolytic activation of Snk. Together, these observations illuminate the mechanism through which spatially restricted sulfotransferase activity in the developing egg chamber leads to localization of serine protease activity and ultimately to spatially specific activation of the Toll receptor in the Drosophila embryo.