A comparative ultrastructural study of ?glue? production and secretion of the salivary glands in different species of theDrosophila melanogaster group

Developmental program-independent secretory granule degradation in larval salivary gland cells of Drosophila

Both constitutive and regulated secretion require cell organelles that are able to store and release the secretory cargo. During development, the larval salivary gland of Drosophila initially produces high amount of glue-containing small immature secretory granules, which then fuse with each other and reach their normal 3-3.5 μm in size. Following the burst of secretion, obsolete glue granules directly fuse with late endosomes or lysosomes by a process called crinophagy, which leads to fast degradation and recycling of the secretory cargo. However, hindering of endosome-to-TGN retrograde transport in these cells causes abnormally small glue granules which are not able to fuse with each other. Here, we show that loss of function of the SNARE genes Syntaxin 16 (Syx16) and Synaptobrevin (Syb), the small GTPase Rab6 and the GARP tethering complex members Vps53 and Scattered (Vps54) all involved in retrograde transport cause intense early degradation of immature glue granules via crinophagy independently of the developmental program. Moreover, silencing of these genes also provokes secretory failure and accelerated crinophagy during larval development. Our results provide a better understanding of the relations among secretion, secretory granule maturation and degradation and paves the way for further investigation of these connections in other metazoans.

Drosophila Glue: A Promising Model for Bioadhesion

The glue produced by Drosophila larvae to attach themselves to a substrate for several days and resist predation until the end of metamorphosis represents an attractive model to develop new adhesives for dry environments. The adhesive properties of this interesting material have been investigated recently, and it was found that it binds as well as strongly adhesive commercial tapes to various types of substrates. This glue hardens rapidly after excretion and is made of several proteins. In D. melanogaster, eight glue proteins have been identified: four are long glycosylated mucoproteins containing repeats rich in prolines, serines and threonines, and four others are shorter proteins rich in cysteines. This protein mix is produced by the salivary glands through a complex packaging process that is starting to be elucidated. Drosophila species have adapted to stick to various substrates in diverse environmental conditions and glue genes appear to evolve rapidly in terms of gene number, number of repeats and sequence of the repeat motifs. Interestingly, besides its adhesive properties, the glue may also have antimicrobial activities. We discuss future perspectives and avenues of research for the development of new bioadhesives mimicking Drosophila fly glue.

Maturing secretory granules: Where secretory and endocytic pathways converge

Secretory granules (SGs) are specialized organelles responsible for the storage and regulated release of various biologically active molecules from the endocrine and exocrine systems. Thus, proper SG biogenesis is critical to normal animal physiology. Biogenesis of SGs starts at the trans-Golgi network (TGN), where immature SGs (iSGs) bud off and undergo maturation before fusing with the plasma membrane (PM). How iSGs mature is unclear, but emerging studies have suggested an important role for the endocytic pathway. The requirement for endocytic machinery in SG maturation blurs the line between SGs and another class of secretory organelles called lysosome-related organelles (LROs). Therefore, it is important to re-evaluate the differences and similarities between SGs and LROs.

Type II phosphatidylinositol 4-kinase regulates trafficking of secretory granule proteins in Drosophila

Type II phosphatidylinositol 4-kinase (PI4KII) produces the lipid phosphatidylinositol 4-phosphate (PI4P), a key regulator of membrane trafficking. Here, we generated genetic models of the sole Drosophila melanogaster PI4KII gene. A specific requirement for PI4KII emerged in larval salivary glands. In PI4KII mutants, mucin-containing glue granules failed to reach normal size, with glue protein aberrantly accumulating in enlarged Rab7-positive late endosomes. Presence of PI4KII at the Golgi and on dynamic tubular endosomes indicated two distinct foci for its function. First, consistent with the established role of PI4P in the Golgi, PI4KII is required for sorting of glue granule cargo and the granule-associated SNARE Snap24. Second, PI4KII also has an unforeseen function in late endosomes, where it is required for normal retromer dynamics and for formation of tubular endosomes that are likely to be involved in retrieving Snap24 and Lysosomal enzyme receptor protein (Lerp) from late endosomes to the trans-Golgi network. Our genetic analysis of PI4KII in flies thus reveals a novel role for PI4KII in regulating the fidelity of granule protein trafficking in secretory tissues.

AP-1 and clathrin are essential for secretory granule biogenesis in Drosophila

Regulated secretion of hormones, digestive enzymes, and other biologically active molecules requires the formation of secretory granules. Clathrin and the clathrin adaptor protein complex 1 (AP-1) are necessary for maturation of exocrine, endocrine, and neuroendocrine secretory granules. However, the initial steps of secretory granule biogenesis are only minimally understood. Powerful genetic approaches available in the fruit fly Drosophila melanogaster were used to investigate the molecular pathway for biogenesis of the mucin-containing "glue granules" that form within epithelial cells of the third-instar larval salivary gland. Clathrin and AP-1 colocalize at the trans-Golgi network (TGN) and clathrin recruitment requires AP-1. Furthermore, clathrin and AP-1 colocalize with secretory cargo at the TGN and on immature granules. Finally, loss of clathrin or AP-1 leads to a profound block in secretory granule formation. These findings establish a novel role for AP-1- and clathrin-dependent trafficking in the biogenesis of mucin-containing secretory granules.

Larval pupation site preference and its relationship to the glue proteins in a few species of Drosophila

The pupation site preference and the quantity of larval salivary gland secretion proteins (glue proteins) were analysed in 15 species of Drosophila belonging to 3 species groups. The results showed that even under constant environmental conditions, the larvae of different species prefer to pupate at different sites in various proportions. Three patterns of pupation site preference could be recognized on the basis of the preponderance of larvae pupating at different sites. The classification of different species of Drosophila under study into 3 pattern groups does not correspond with their taxonomic classification. The larvae of those species that synthesize large quantities of glue protein tend to pupate in the medium, while those synthesizing half the quantity of glue proteins pupate on the sides of the container (glass wall). The quantity of glue protein is not correlated with the size of the salivary glands.

Spatial organization of microtubules and microfilaments in larval and adult salivary glands of Drosophila melanogaster

We examined the distribution of microtubules and microfilaments by conventional fluorescence microscopy and laser scanning confocal microscopy in larval and adult salivary glands of Drosophila melanogaster. The cells of the larval salivary gland epithelium were characterized by the same spatial distribution of microfilaments, whereas microfilament localization was more complex in adult salivary glands, showing some regional differentiation. Microtubules distributed throughout the cell cytoplasm of the larval salivary glands, whereas in adult glands they were mostly observed in the basal or apical cytoplasm of the cells. These observations were related to the secretory process and the mechanism of saliva discharge.

Rickettsiae-like structures in the larval salivary gland cells of Drosophila auraria

Rickettsiae-like structures were found in the salivary gland cells of Drosophila auraria during different larval and prepupal developmental stages, from the early 3rd instar up to 14 hr after spiracle inversion. These microorganisms are surrounded by a membrane, are constantly intracellular, and occur singly or in groups. Their widespread occurrence in various tissues of other Drosophila species indicates that they can be considered as symbionts, but their actual functional significance (if any) is unknown.

Comparative analysis of glue proteins in the Drosophila nasuta subgroup

The patterns of protein fractions from total salivary glands and from glue plugs were compared in seven members of the Drosophila nasuta subgroup by the use of sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). The glue protein patterns are member specific concerning the numbers and the electrophoretic mobilities of major and minor glue protein fractions. However, the major fractions of all subgroup members could be grouped into five SDS-PAGE domains according to the homologies of their electrophoretic mobilities, prominence of Coomassie blue staining, and PAS reaction. In all subgroup members, major fractions are involved in posttranslational modifications into larger protein molecules of the final glue. Quantitative estimations of the glue proteins in D. n. nasuta and D. n. albomicans reveal that they constitute between 55 and 60% of the total salivary gland proteins, whereas in D. melanogaster and in D. hydei the fraction is only 32 and 35%, respectively.