Lamin is essential for nuclear localization of the GPI synthesis enzyme PIG-B and GPI-anchored protein production in Drosophila.

Membrane lipid biosynthesis is a complex process that occurs in various intracellular compartments. In Drosophila, phosphatidylinositol glycan-B (PIG-B), which catalyzes addition of the third mannose in glycosylphosphatidylinositol (GPI), localizes to the nuclear envelope (NE). Although this NE localization is essential for Drosophila development, the underlying molecular mechanism remains unknown. To elucidate this mechanism, we identified PIG-B-interacting proteins by performing immunoprecipitation followed by proteomic analysis. We then examined which of these proteins are required for the NE localization of PIG-B. Knockdown of Lamin Dm0, a B-type lamin, led to mislocalization of PIG-B from the NE to the endoplasmic reticulum. Lamin Dm0 associated with PIG-B at the inner nuclear membrane, a process that required the tail domain of Lamin Dm0. Furthermore, GPI moieties were distributed abnormally in the Lamin Dm0 mutant. These data indicate that Lamin Dm0 is involved in the NE localization of PIG-B and is required for proper GPI-anchor modification of proteins.

Hrd1-dependent Degradation of the Unassembled PIGK Subunit of the GPI Transamidase Complex.

Glycosylphosphatidylinositol (GPI)-anchored proteins are post-transcriptionally modified with GPI and anchored to the plasma membrane. GPI is attached to nascent proteins in the endoplasmic reticulum by the GPI transamidase complex, which consists of PIGT, PIGK, GPAA1, PIGU, and PIGS. Of these, PIGK is a catalytic subunit that is unstable without PIGT. This study investigated the pathway by which unassembled PIGK not incorporated into the complex is degraded. We showed that unassembled PIGK was degraded via the proteasome-dependent pathway and that Hrd1 (also known as SYVN1), a ubiquitin ligase involved in the endoplasmic reticulum-associated degradation pathway, was responsible for degradation of unassembled PIGK.Key words: Glycosylphosphatidylinositol, GPI transamidase complex, protein stability, transamidation, ERAD.

SPPL3-dependent downregulation of the synthesis of (neo)lacto-series glycosphingolipid is required for the staining of cell surface CD59.

CD59 is a small glycoprotein modified with a glycophosphatidylinositol (GPI) anchor that prevents the formation of the membrane attack complex, thereby protecting host cells from lysis. A previous study identified that cell surface CD59 staining required the intramembrane protease signal peptide peptidase-like 3 (SPPL3). However, the effect of SPPL3 on the staining of CD59 remains unknown. This study shows that SPPL3 is essential for the surface labeling of CD59 but not of major GPI-anchored proteins. Surface CD59 staining requires the intramembrane protease activity of SPPL3 and SPPL3-mediated suppression of the (neo)lacto-series glycosphingolipids (nsGSLs)-but not N-glycan-synthesis pathway. The abundance of nsGSLs may affect complement-dependent cytotoxicity by altering the abundance or accessibility of cell surface CD59.

Nuclear envelope localization of PIG-B is essential for GPI-anchor synthesis in Drosophila.

Membrane lipid biosynthesis is a complex process that takes place in various intracellular compartments. Glycosylphosphatidylinositol (GPI), a lipid involved in membrane anchoring of some proteins, is synthesized by the PIG enzymes. Most PIGs are localized to the endoplasmic reticulum (ER), but Drosophila PIG-B (DmPIG-B) is localized to the nuclear envelope (NE). To determine whether the NE localization of DmPIG-B is functionally important, we defined the determinants of localization and generated an ER-localized form, denoted DmPIG-B[ER]. The enzymatic activity of DmPIG-B[ER] was comparable to that of NE-localized DmPIG-B[NE]. Expression of DmPIG-B[ER] inefficiently rescued the lethality of the PIG-B mutant, whereas DmPIG-B[NE] rescued this lethality fully. DmPIG-B[ER] was preferentially degraded by lysosomes, suggesting that the NE localization is essential for function and stability of the protein. In addition, we found that the region of the ER proximal to the NE is the site of translation of GPI-anchored proteins and addition of GPI. Thus, the NE and proximal ER may provide a platform for efficient GPI anchoring.

Stability of the transamidase complex catalyzing GPI anchoring of proteins.

Glycosylphosphatidylinositol (GPI) is a glycolipid that anchors some proteins to the plasma membrane. This anchoring is catalyzed by a transamidase complex (TAC) composed of five subunits: PIG-K, GAA1, PIG-U, PIG-T, and PIG-S (Fig. 1A). PIG-K and GAA1 are predicted to catalyze the first and second steps during attachment of proproteins of GPI-anchored proteins (GPI-APs) to GPI. GPI may be delivered by PIG-U, and PIG-T is required for stability of all TAC subunits when overexpressed in cultured cells. However, protein stability of TAC has not been analyzed using loss-of-function mutants for each subunit. Herein, we analyzed the stability of TAC in knockout and/or knockdown mutants for each subunit. PIG-T and PIG-U, or PIG-T and GAA1, were mutually required for stability, and all three subunits were stable without PIG-S or PIG-K. However, these three subunits were essential for the stability of both PIG-S and PIG-K. By contrast, loss of PIG-S reduced the stability of PIG-K and left the other subunits unaffected. Reduction of PIG-K did not impact any of the other subunits. Thus, PIG-T, PIG-U, and GAA1 may form a core complex associated by PIG-S, and these four subunits may stabilize PIG-K, triggering GPI anchoring reactions. Instability of PIG-K in the absence of the other four subunits may ensure that GPI anchoring is catalyzed only by the completely assembled complex.

Dynamic regulation of innate immune responses in Drosophila by Senju-mediated glycosylation.

The innate immune system is the first line of defense encountered by invading pathogens. Delayed and/or inadequate innate immune responses can result in failure to combat pathogens, whereas excessive and/or inappropriate responses cause runaway inflammation. Therefore, immune responses are tightly regulated from initiation to resolution and are repressed during the steady state. It is well known that glycans presented on pathogens play important roles in pathogen recognition and the interactions between host molecules and microbes; however, the function of glycans of host organisms in innate immune responses is less well known. Here, we show that innate immune quiescence and strength of the immune response are controlled by host glycosylation involving a novel UDP-galactose transporter called Senju. In senju mutants, reduced expression of galactose-containing glycans resulted in hyperactivation of the Toll signaling pathway in the absence of immune challenges. Genetic epistasis and biochemical analyses revealed that Senju regulates the Toll signaling pathway at a step that converts Toll ligand Spatzle to its active form. Interestingly, Toll activation in immune-challenged wild type (WT) flies reduced the expression of galactose-containing glycans. Suppression of the degalactosylation by senju overexpression resulted in reduced induction of Toll-dependent expression of an antimicrobial peptide, Drosomycin, and increased susceptibility to infection with Gram-positive bacteria. These data suggest that Senju-mediated galactosylation suppresses undesirable Toll signaling activation during the steady state; however, Toll activation in response to infection leads to degalactosylation, which raises the immune response to an adequate level and contributes to the prompt elimination of pathogens.

Spätzle-Processing Enzyme-independent Activation of the Toll Pathway in Drosophila Innate Immunity.

The Toll pathway regulates innate immunity in insects and vertebrates. The Drosophila Toll receptor is activated by a processed form of a ligand, Spätzle. Spätzle-processing enzyme (SPE) is the only enzyme identified to date that functions in converting Spätzle to an active form during the immune response. In the present study, Toll activation induced by immune challenge was almost suppressed in spätzle mutant larvae and adults, whereas it was present in SPE mutant larvae challenged with Micrococcus luteus and adults challenged with Bacillus subtilis. Our data suggest that an unidentified protease besides SPE processes Spätzle under conditions of microbial challenge.

Phenotype-based clustering of glycosylation-related genes by RNAi-mediated gene silencing.

Glycan structures are synthesized by a series of reactions conducted by glycosylation-related (GR) proteins such as glycosyltransferases, glycan-modifying enzymes, and nucleotide-sugar transporters. For example, the common core region of glycosaminoglycans (GAGs) is sequentially synthesized by peptide-O-xylosyltransferase, ß1,4-galactosyltransferase I, ß1,3-galactosyltransferase II, and ß1,3-glucuronyltransferase. This raises the possibility that functional impairment of GR proteins involved in synthesis of the same glycan might result in the same phenotypic abnormality. To examine this possibility, comprehensive silencing of genes encoding GR and proteoglycan core proteins was conducted in Drosophila. Drosophila GR candidate genes (125) were classified into five functional groups for synthesis of GAGs, N-linked, O-linked, Notch-related, and unknown glycans. Spatiotemporally regulated silencing caused a range of malformed phenotypes that fell into three types: extra veins, thick veins, and depigmentation. The clustered phenotypes reflected the biosynthetic pathways of GAGs, Fringe-dependent glycan on Notch, and glycans placed at or near nonreducing ends (herein termed terminal domains of glycans). Based on the phenotypic clustering, CG33145 was predicted to be involved in formation of terminal domains. Our further analysis showed that CG33145 exhibited galactosyltransferase activity in synthesis of terminal N-linked glycans. Phenotypic clustering, therefore, has potential for the functional prediction of novel GR genes.

Balanced ubiquitylation and deubiquitylation of Frizzled regulate cellular responsiveness to Wg/Wnt.

Wingless (Wg)/Wnt has been proposed to exert various functions as a morphogen depending on the levels of its signalling. Therefore, not just the concentration of Wg/Wnt, but also the responsiveness of Wg/Wnt-target cells to the ligand, must have a crucial function in controlling cellular outputs. Here, we show that a balance of ubiquitylation and deubiquitylation of the Wg/Wnt receptor Frizzled determines the cellular responsiveness to Wg/Wnt both in mammalian cells and in Drosophila, and that the cell surface level of Frizzled is regulated by deubiquitylating enzyme UBPY/ubiquitin-specific protease 8 (USP8). Although ubiquitylated Frizzled underwent lysosomal trafficking and degradation, UBPY/USP8-dependent deubiquitylation led to recycling of Frizzled to the plasma membrane, thereby elevating its surface level. Importantly, a gain and loss of UBPY/USP8 function led to up- and down-regulation, respectively, of canonical Wg/Wnt signalling. These results unveil a novel mechanism that regulates the cellular responsiveness to Wg/Wnt by controlling the cell surface level of Frizzled.

PIGB maintains nuclear lamina organization in skeletal muscle of Drosophila.

The nuclear lamina (NL) plays various roles and participates in nuclear integrity, chromatin organization, and transcriptional regulation. Lamin proteins, the main components of the NL, form a homogeneous meshwork structure under the nuclear envelope. Lamins are essential, but it is unknown whether their homogeneous distribution is important for nuclear function. Here, we found that PIGB, an enzyme involved in glycosylphosphatidylinositol (GPI) synthesis, is responsible for the homogeneous lamin meshwork in Drosophila. Loss of PIGB resulted in heterogeneous distributions of B-type lamin and lamin-binding proteins in larval muscles. These phenotypes were rescued by expression of PIGB lacking GPI synthesis activity. The PIGB mutant exhibited changes in lamina-associated domains that are large heterochromatic genomic regions in the NL, reduction of nuclear stiffness, and deformation of muscle fibers. These results suggest that PIGB maintains the homogeneous meshwork of the NL, which may be essential for chromatin distribution and nuclear mechanical properties.

Dynamic movement of the Golgi unit and its glycosylation enzyme zones.

Knowledge on the distribution and dynamics of glycosylation enzymes in the Golgi is essential for better understanding this modification. Here, using a combination of CRISPR/Cas9 knockin technology and super-resolution microscopy, we show that the Golgi complex is assembled by a number of small 'Golgi units' that have 1-3 µm in diameter. Each Golgi unit contains small domains of glycosylation enzymes which we call 'zones'. The zones of N- and O-glycosylation enzymes are colocalised. However, they are less colocalised with the zones of a glycosaminoglycan synthesizing enzyme. Golgi units change shapes dynamically and the zones of glycosylation enzymes rapidly move near the rim of the unit. Photobleaching analysis indicates that a glycosaminoglycan synthesizing enzyme moves between units. Depletion of giantin dissociates units and prevents the movement of glycosaminoglycan synthesizing enzymes, which leads to insufficient glycosaminoglycan synthesis. Thus, we show the structure-function relationship of the Golgi and its implications in human pathogenesis.

Balanced ubiquitination determines cellular responsiveness to extracellular stimuli.

Signal strength evoked by ligand stimulation is crucial for cellular responses such as fate decision, cell survival/death, secretion, and migration. For example, morphogens are secreted signaling molecules that form concentration gradients within tissues and induce distinct cell fates in a signal strength-dependent manner. In addition to extracellular ligand abundance, the sensitivity of signal-receiving cells to ligands also influences signal strength. Cell sensitivity to ligands is controlled at various levels: receptor presentation at the cell surface, positive/negative regulation of signal transduction, and target gene activation/repression. While the regulation of signal transduction and gene transcription is well studied, receptor presentation is still not fully understood. Recently, it was reported that cellular sensitivity to the Wingless (Wg)/Wnt morphogen is regulated by balanced ubiquitination and deubiquitination of its receptor Frizzled (Fz). In this review, we review how ubiquitination regulates receptor presentation at the cell surface for the detection of extracellular signal strength.

Identification of genes required for neural-specific glycosylation using functional genomics.

Glycosylation plays crucial regulatory roles in various biological processes such as development, immunity, and neural functions. For example, α1,3-fucosylation, the addition of a fucose moiety abundant in Drosophila neural cells, is essential for neural development, function, and behavior. However, it remains largely unknown how neural-specific α1,3-fucosylation is regulated. In the present study, we searched for genes involved in the glycosylation of a neural-specific protein using a Drosophila RNAi library. We obtained 109 genes affecting glycosylation that clustered into nine functional groups. Among them, members of the RNA regulation group were enriched by a secondary screen that identified genes specifically regulating α1,3-fucosylation. Further analyses revealed that an RNA-binding protein, second mitotic wave missing (Swm), upregulates expression of the neural-specific glycosyltransferase FucTA and facilitates its mRNA export from the nucleus. This first large-scale genetic screen for glycosylation-related genes has revealed novel regulation of fucTA mRNA in neural cells.

Cisterna-specific localization of glycosylation-related proteins to the Golgi apparatus.

The Golgi apparatus is an intracellular organelle playing central roles in post-translational modification and in the secretion of membrane and secretory proteins. These proteins are synthesized in the endoplasmic reticulum (ER) and transported to the cis-, medial-and trans-cisternae of the Golgi. While trafficking through the Golgi, proteins are sequentially modified with glycan moieties by different glycosyltransferases. Therefore, it is important to analyze the glycosylation function of the Golgi at the level of cisternae. Markers widely used for cis-, medial- and trans-cisternae/trans Golgi network (TGN) in Drosophila are GM130, 120 kDa and Syntaxin16 (Syx16); however the anti-120 kDa antibody is no longer available. In the present study, Drosophila Golgi complex-localized glycoprotein-1 (dGLG1) was identified as an antigen recognized by the anti-120 kDa antibody. A monoclonal anti-dGLG1 antibody suitable for immunohistochemistry was raised in rat. Using these markers, the localization of glycosyltransferases and nucleotide-sugar transporters (NSTs) was studied at the cisternal level. Results showed that glycosyltransferases and NSTs involved in the same sugar modification are localized to the same cisternae. Furthermore, valuable functional information was obtained on the localization of novel NSTs with as yet incompletely characterized biochemical properties.

Identification of proteasome components required for apical localization of Chaoptin using functional genomics.

Abstract: the distinct localization of membrane proteins with regard to cell polarity is crucial for the structure and function of various organs in multicellular organisms. However, the molecules and mechanisms that regulate protein localization to particular subcellular domains are still largely unknown. To identify the genes involved in regulation of protein localization, the authors performed a large-scale screen using a Drosophila RNA interference (RNAi) library, by which Drosophila genes could be knocked down in a tissue- and stage-specific manner. Drosophila photoreceptor cells have a morphologically distinct apicobasal polarity, along which Chaoptin (Chp), a glycosylphosphatidylinositol (GPI)-anchored membrane protein, and the Na (+) , K(+) -ATPase are localized to the apical and basolateral domains, respectively. By examining the subcellular localization of these proteins, the authors identified 106 genes whose knockdown resulted in mislocalization of Chp and Na(+) , K(+) -ATPase. Gene ontology analysis revealed that the knockdown of proteasome components resulted in mislocalization of Chp to the basolateral plasma membrane. These results suggest that the proteasome is involved, directly or indirectly, in selective localization of Chp to the apical plasma membrane of Drosophila photoreceptor cells.

Autophagy-dependent rhodopsin degradation prevents retinal degeneration in Drosophila.

Recent studies have demonstrated protective roles for autophagy in various neurodegenerative disorders, including the polyglutamine diseases; however, the role of autophagy in retinal degeneration has remained unclear. Accumulation of activated rhodopsin in some Drosophila mutants leads to retinal degeneration, and although it is known that activated rhodopsin is degraded in endosomal pathways in normal photoreceptor cells, the contribution of autophagy to rhodopsin regulation has remained elusive. This study reveals that activated rhodopsin is degraded by autophagy in collaboration with endosomal pathways to prevent retinal degeneration. Light-dependent retinal degeneration in the Drosophila visual system is caused by the knockdown or mutation of autophagy-essential components, such as autophagy-related protein 7 and 8 (atg-7/atg-8), or genes essential for PE (phosphatidylethanolamine) biogenesis and autophagosome formation, including Phosphatidylserine decarboxylase (Psd) and CDP-ethanolamine:diacylglycerol ethanolaminephosphotransferase (Ept). The knockdown of atg-7/8 or Psd/Ept produced an increase in the amount of rhodopsin localized to Rab7-positive late endosomes. This rhodopsin accumulation, followed by retinal degeneration, was suppressed by overexpression of Rab7, which accelerated the endosomal degradation pathway. These results indicate a degree of cross talk between the autophagic and endosomal/lysosomal pathways. Importantly, a reduction in rhodopsin levels rescued Psd knockdown-induced retinal degeneration. Additionally, the Psd knockdown-induced retinal degeneration phenotype was enhanced by Ppt1 inactivation, which causes infantile neuronal ceroid lipofuscinosis, implying that autophagy plays a significant role in its pathogenesis. Collectively, the current data reveal that autophagy suppresses light-dependent retinal degeneration in collaboration with the endosomal degradation pathway and that rhodopsin is a key substrate for autophagic degradation in this context.

Subunits of the GPI transamidase complex localize to the endoplasmic reticulum and nuclear envelope in Drosophila.

A total of 10-20% of plasma membrane proteins are anchored by glycosylphosphatidylinositol (GPI). GPI is attached to proteins by GPI transamidase (GPI-T), which contains five subunits named PIGK, PIGS, PIGT, PIGU, and GPAA1. We previously reported that PIGT localizes near the nucleus in Drosophila. However, localizations of the other four subunits remain unknown. Here, we show that a catalytic subunit of GPI-T, PIGK, mainly localizes to the endoplasmic reticulum (ER), while the other four subunits localize to the nuclear envelope (NE) and ER. The NE/ER localization ratio of PIGS differs between cell types and developmental stages. Our results suggest that GPI-T catalyzes GPI attachment in the ER and the other four subunits may have other unknown functions in the NE.

Spatial and temporal regulation of glycosylation during Drosophila eye development.

Glycosylation plays an essential role during development, in processes such as morphogen distribution, cell-to-cell communication, and extracellular matrix formation. Glycosylation is regulated during development in both a spatial and temporal manner. This study presents a detailed description of glycan distribution from late pupal to adult stages in Drosophila ommatidia by using nine different lectins. The lectins ConA, LCA, and DSA, which recognize high-mannose or complex types of N-linked glycans stain both apical and basolateral membranes of photoreceptor cells, whereas SBA, DBA, and ABA lectins, which bind to mucin-type O-glycans, label the inter-rhabdomeral space. The O-linked GlcNAc moiety is strongly labeled by WGA on the nuclear membrane. The localization of these glycans does not change throughout late pupal development. In contrast, the abundance of O-linked glycans, bisected oligosaccharides, and GlcNAc-containing glycans detected by PNA, PHA-E4, and WGA, respectively, is reduced in rhabdomeres and other subcellular domains during late pupal development. Some of these glycans have also been detected in the Golgi and/or putative secretory vesicles, suggesting their dynamic transport during development. These glycans, whose expression is dynamically regulated in a spatial and temporal manner, may therefore play critical roles in ommatidial development.

N-Glycosylation of the Drosophila neural protein Chaoptin is essential for its stability, cell surface transport and adhesive activity.

Glycosylation of proteins can modulate their function in a striking variety of systems, including immune responses, neuronal activities and development. The Drosophila protein, Chaoptin (Chp), is essential for the development and maintenance of photoreceptor cells. This protein is heavily glycosylated, but the possible role of this glycosylation is not well-understood. Here we show that mutations introduced into about 1/3 of 16 potential N-linked glycosylation sites within Chp impaired its cell adhesive activities when expressed in Drosophila S2 cells. Mutation of 2/3 of the glycosylation sites resulted in a marked decrease in Chp protein abundance. These results suggest that N-linked glycosylation of Chp is essential for its stability and activity.

In Vivo RNAi-Based Screens: Studies in Model Organisms.

RNA interference (RNAi) is a technique widely used for gene silencing in organisms and cultured cells, and depends on sequence homology between double-stranded RNA (dsRNA) and target mRNA molecules. Numerous cell-based genome-wide screens have successfully identified novel genes involved in various biological processes, including signal transduction, cell viability/death, and cell morphology. However, cell-based screens cannot address cellular processes such as development, behavior, and immunity. Drosophila and Caenorhabditis elegans are two model organisms whose whole bodies and individual body parts have been subjected to RNAi-based genome-wide screening. Moreover, Drosophila RNAi allows the manipulation of gene function in a spatiotemporal manner when it is implemented using the Gal4/UAS system. Using this inducible RNAi technique, various large-scale screens have been performed in Drosophila, demonstrating that the method is straightforward and valuable. However, accumulated results reveal that the results of RNAi-based screens have relatively high levels of error, such as false positives and negatives. Here, we review in vivo RNAi screens in Drosophila and the methods that could be used to remove ambiguity from screening results.