Left-right Myosin-Is, Myosin1C, and Myosin1D exhibit distinct single molecule behaviors on the plasma membrane of Drosophila macrophages.

Left-right (LR) asymmetry is crucial for animal development, particularly in Drosophila where LR-asymmetric morphogenesis of organs hinges on cellular-level chirality, termed cell chirality. In this species, two class I myosins, Myosin1D (Myo1D), and Myosin1C (Myo1C), respectively determine dextral (wild type) and sinistral (mirror image) cell chirality. Previous studies demonstrated Myo1D's ability to propel F-actin in leftward circles during in vitro gliding assays, suggesting its mechanochemical role in defining dextral chirality. Conversely, Myo1C propels F-actin without exhibiting LR-directional preference in this assay, suggesting at other properties governing sinistral chirality. Given the interaction of Myo1D and Myo1C with the membrane, we hypothesized that differences in their membrane behaviors might be critical in dictating their dextral or sinistral activities. In this study, employing single-molecule imaging analyses, we investigated the dynamic behaviors of Myo1D and Myo1C on the plasma membrane. Our findings revealed that Myo1C exhibits a significantly greater proportion of slow-diffusing population compared to Myo1D. Importantly, this characteristic was contingent upon both head and tail domains of Myo1C. The distinct diffusion patterns of Myo1D and Myo1C did not exert mutual influence on each other. This divergence in membrane diffusion between Myo1D and Myo1C may be crucial for dictating cell and organ chirality.

The Drosophila AWP1 ortholog Doctor No regulates JAK/STAT signaling for left-right asymmetry in the gut by promoting receptor endocytosis.

Many organs of Drosophila show stereotypical left-right (LR) asymmetry; however, the underlying mechanisms remain elusive. Here, we have identified an evolutionarily conserved ubiquitin-binding protein, AWP1/Doctor No (Drn), as a factor required for LR asymmetry in the embryonic anterior gut. We found that drn is essential in the circular visceral muscle cells of the midgut for JAK/STAT signaling, which contributes to the first known cue for anterior gut lateralization via LR asymmetric nuclear rearrangement. Embryos homozygous for drn and lacking its maternal contribution showed phenotypes similar to those with depleted JAK/STAT signaling, suggesting that Drn is a general component of JAK/STAT signaling. Absence of Drn resulted in specific accumulation of Domeless (Dome), the receptor for ligands in the JAK/STAT signaling pathway, in intracellular compartments, including ubiquitylated cargos. Dome colocalized with Drn in wild-type Drosophila. These results suggest that Drn is required for the endocytic trafficking of Dome, which is a crucial step for activation of JAK/STAT signaling and the subsequent degradation of Dome. The roles of AWP1/Drn in activating JAK/STAT signaling and in LR asymmetric development may be conserved in various organisms.

Notch Missense Mutations in Drosophila Reveal Functions of Specific EGF-like Repeats in Notch Folding, Trafficking, and Signaling.

Notch signaling plays various roles in cell-fate specification through direct cell-cell interactions. Notch receptors are evolutionarily conserved transmembrane proteins with multiple epidermal growth factor (EGF)-like repeats. Drosophila Notch has 36 EGF-like repeats, and while some play a role in Notch signaling, the specific functions of most remain unclear. To investigate the role of each EGF-like repeat, we used 19 previously identified missense mutations of Notch with unique amino acid substitutions in various EGF-like repeats and a transmembrane domain; 17 of these were identified through a single genetic screen. We assessed these mutants' phenotypes in the nervous system and hindgut during embryogenesis, and found that 10 of the 19 Notch mutants had defects in both lateral inhibition and inductive Notch signaling, showing context dependency. Of these 10 mutants, six accumulated Notch in the endoplasmic reticulum (ER), and these six were located in EGF-like repeats 8-10 or 25. Mutations with cysteine substitutions were not always coupled with ER accumulation. This suggests that certain EGF-like repeats may be particularly susceptible to structural perturbation, resulting in a misfolded and inactive Notch product that accumulates in the ER. Thus, we propose that these EGF-like repeats may be integral to Notch folding.

Collective nuclear behavior shapes bilateral nuclear symmetry for subsequent left-right asymmetric morphogenesis in Drosophila.

Proper organ development often requires nuclei to move to a specific position within the cell. To determine how nuclear positioning affects left-right (LR) development in the Drosophila anterior midgut (AMG), we developed a surface-modeling method to measure and describe nuclear behavior at stages 13-14, captured in three-dimensional time-lapse movies. We describe the distinctive positioning and a novel collective nuclear behavior by which nuclei align LR symmetrically along the anterior-posterior axis in the visceral muscles that overlie the midgut and are responsible for the LR-asymmetric development of this organ. Wnt4 signaling is crucial for the collective behavior and proper positioning of the nuclei, as are myosin II and the LINC complex, without which the nuclei fail to align LR symmetrically. The LR-symmetric positioning of the nuclei is important for the subsequent LR-asymmetric development of the AMG. We propose that the bilaterally symmetrical positioning of these nuclei may be mechanically coupled with subsequent LR-asymmetric morphogenesis.

Maternal almondex, a neurogenic gene, is required for proper subcellular Notch distribution in early Drosophila embryogenesis.

Notch signaling plays crucial roles in the control of cell fate and physiology through local cell-cell interactions. The core processes of Notch signal transduction are well established, but the mechanisms that fine-tune the pathway in various developmental and post-developmental contexts are less clear. Drosophila almondex, which encodes an evolutionarily conserved double-pass transmembrane protein, was identified in the 1970s as a maternal-effect gene that regulates Notch signaling in certain contexts, but its mechanistic function remains obscure. In this study, we examined the role of almondex in Notch signaling during early Drosophila embryogenesis. We found that in addition to being required for lateral inhibition in the neuroectoderm, almondex is also partially required for Notch signaling-dependent single-minded expression in the mesectoderm. Furthermore, we found that almondex is required for proper subcellular Notch receptor distribution in the neuroectoderm, specifically during mid-stage 5 development. The absence of maternal almondex during this critical window of time caused Notch to accumulate abnormally in cells in a mesh-like pattern. This phenotype did not include any obvious change in subcellular Delta ligand distribution, suggesting that it does not result from a general vesicular-trafficking defect. Considering that dynamic Notch trafficking regulates signal output to fit the specific context, we speculate that almondex may facilitate Notch activation by regulating intracellular Notch receptor distribution during early embryogenesis.

Cell Chirality Drives Left-Right Asymmetric Morphogenesis.

Most macromolecules found in cells are chiral, meaning that they cannot be superimposed onto their mirror image. However, cells themselves can also be chiral, a subject that has received little attention until very recently. In our studies on the mechanisms of left-right (LR) asymmetric development in Drosophila, we discovered that cells can have an intrinsic chirality to their structure, and that this "cell chirality" is generally responsible for the LR asymmetric development of certain organs in this species. The actin cytoskeleton plays important roles in the formation of cell chirality. In addition, Myosin31DF (Myo31DF), which encodes Drosophila Myosin ID, was identified as a molecular switch for cell chirality. In other invertebrate species, including snails and Caenorhabditis elegans, chirality of the blastomeres, another type of cell chirality, determines the LR asymmetry of structures in the body. Thus, chirality at the cellular level may broadly contribute to LR asymmetric development in various invertebrate species. Recently, cell chirality was also reported for various vertebrate cultured cells, and studies suggested that cell chirality is evolutionarily conserved, including the essential role of the actin cytoskeleton. Although the biological roles of cell chirality in vertebrates remain unknown, it may control LR asymmetric development or other morphogenetic events. The investigation of cell chirality has just begun, and this new field should provide valuable new insights in biology and medicine.

Dual Roles of O-Glucose Glycans Redundant with Monosaccharide O-Fucose on Notch in Notch Trafficking.

Notch is a transmembrane receptor that mediates cell-cell interactions and controls various cell-fate specifications in metazoans. The extracellular domain of Notch contains multiple epidermal growth factor (EGF)-like repeats. At least five different glycans are found in distinct sites within these EGF-like repeats. The function of these individual glycans in Notch signaling has been investigated, primarily by disrupting their individual glycosyltransferases. However, we are just beginning to understand the potential functional interactions between these glycans. Monosaccharide O-fucose and O-glucose trisaccharide (O-glucose-xylose-xylose) are added to many of the Notch EGF-like repeats. In Drosophila, Shams adds a xylose specifically to the monosaccharide O-glucose. We found that loss of the terminal dixylose of O-glucose-linked saccharides had little effect on Notch signaling. However, our analyses of double mutants of shams and other genes required for glycan modifications revealed that both the monosaccharide O-glucose and the terminal dixylose of O-glucose-linked saccharides function redundantly with the monosaccharide O-fucose in Notch activation and trafficking. The terminal dixylose of O-glucose-linked saccharides and the monosaccharide O-glucose were required in distinct Notch trafficking processes: Notch transport from the apical plasma membrane to adherens junctions, and Notch export from the endoplasmic reticulum, respectively. Therefore, the monosaccharide O-glucose and terminal dixylose of O-glucose-linked saccharides have distinct activities in Notch trafficking, although a loss of these activities is compensated for by the presence of monosaccharide O-fucose. Given that various glycans attached to a protein motif may have redundant functions, our results suggest that these potential redundancies may lead to a serious underestimation of glycan functions.

Class I myosins have overlapping and specialized functions in left-right asymmetric development in Drosophila.

The class I myosin genes are conserved in diverse organisms, and their gene products are involved in actin dynamics, endocytosis, and signal transduction. Drosophila melanogaster has three class I myosin genes, Myosin 31DF (Myo31DF), Myosin 61F (Myo61F), and Myosin 95E (Myo95E). Myo31DF, Myo61F, and Myo95E belong to the Myosin ID, Myosin IC, and Myosin IB families, respectively. Previous loss-of-function analyses of Myo31DF and Myo61F revealed important roles in left-right (LR) asymmetric development and enterocyte maintenance, respectively. However, it was difficult to elucidate their roles in vivo, because of potential redundant activities. Here we generated class I myosin double and triple mutants to address this issue. We found that the triple mutant was viable and fertile, indicating that all three class I myosins were dispensable for survival. A loss-of-function analysis revealed further that Myo31DF and Myo61F, but not Myo95E, had redundant functions in promoting the dextral LR asymmetric development of the male genitalia. Myo61F overexpression is known to antagonize the dextral activity of Myo31DF in various Drosophila organs. Thus, the LR-reversing activity of overexpressed Myo61F may not reflect its physiological function. The endogenous activity of Myo61F in promoting dextral LR asymmetric development was observed in the male genitalia, but not the embryonic gut, another LR asymmetric organ. Thus, Myo61F and Myo31DF, but not Myo95E, play tissue-specific, redundant roles in LR asymmetric development. Our studies also revealed differential colocalization of the class I myosins with filamentous (F)-actin in the brush border of intestinal enterocytes.

O-fucose monosaccharide of Drosophila Notch has a temperature-sensitive function and cooperates with O-glucose glycan in Notch transport and Notch signaling activation.

Notch (N) is a transmembrane receptor that mediates the cell-cell interactions necessary for many cell fate decisions. N has many epidermal growth factor-like repeats that are O-fucosylated by the protein O-fucosyltransferase 1 (O-Fut1), and the O-fut1 gene is essential for N signaling. However, the role of the monosaccharide O-fucose on N is unclear, because O-Fut1 also appears to have O-fucosyltransferase activity-independent functions, including as an N-specific chaperon. Such an enzymatic activity-independent function could account for the essential role of O-fut1 in N signaling. To evaluate the role of the monosaccharide O-fucose modification in N signaling, here we generated a knock-in mutant of O-fut1 (O-fut1(R245A knock-in)), which expresses a mutant protein that lacks O-fucosyltransferase activity but maintains the N-specific chaperon activity. Using O-fut1(R245A knock-in) and other gene mutations that abolish the O-fucosylation of N, we found that the monosaccharide O-fucose modification of N has a temperature-sensitive function that is essential for N signaling. The O-fucose monosaccharide and O-glucose glycan modification, catalyzed by Rumi, function redundantly in the activation of N signaling. We also showed that the redundant function of these two modifications is responsible for the presence of N at the cell surface. Our findings elucidate how different forms of glycosylation on a protein can influence the protein's functions.

A test of double interspecific introgression of nucleoporin genes in Drosophila.

In interspecific hybrids between Drosophila melanogaster and Drosophila simulans, the D. simulans nucleoporin-encoding Nup96(sim) and Nup160(sim) can cause recessive lethality if the hybrid does not also inherit the D. simulans X chromosome. In addition, Nup160(sim) leads to recessive female sterility in the D. melanogaster genetic background. Here, we conducted carefully controlled crosses to better understandthe relationship between Nup96(sim) and Nup160(sim). Nup96(sim) did not lead to female sterility in the D. melanogaster genetic background, and double introgression of Nup96(sim) and Nup160(sim) did not generally lead to lethality when one was heterozygous and the other homozygous (hemizygous). It appears that introgression of additional autosomal D. simulans genes is necessary to cause lethality and that the effect of the introgression is dominant to D. melanogaster alleles. Interestingly, the genetic background affected dominance of Nup96(sim), and double introgression carrying homozygous Nup96(sim) and hemizygous Nup160(sim) resulted in lethality. Thus, Nup96(sim) and Nup160(sim) seem to be two components of the same incompatibility.

Left-right asymmetry is formed in individual cells by intrinsic cell chirality.

Many animals show left-right (LR) asymmetric morphology. The mechanisms of LR asymmetric development are evolutionarily divergent, and they remain elusive in invertebrates. Various organs in Drosophila melanogaster show stereotypic LR asymmetry, including the embryonic gut. The Drosophila embryonic hindgut twists 90° left-handedly, thereby generating directional LR asymmetry. We recently revealed that the hindgut epithelial cell is chiral in shape and other properties; this is termed planar cell chirality (PCC). We previously showed by computer modeling that PCC is sufficient to induce the hindgut rotation. In addition, both the PCC and the direction of hindgut twisting are reversed in Myosin31DF (Myo31DF) mutants. Myo31DF encodes Drosophila MyosinID, an actin-based motor protein, whose molecular functions in LR asymmetric development are largely unknown. Here, to understand how PCC directs the asymmetric cell-shape, we analyzed PCC in genetic mosaics composed of cells homozygous for mutant Myo31DF, some of which also overexpressed wild-type Myo31DF. Wild-type cell-shape chirality only formed in the Myo31DF-overexpressing cells, suggesting that cell-shape chirality was established in each cell and reflects intrinsic PCC. A computer model recapitulating the development of this genetic mosaic suggested that mechanical interactions between cells are required for the cell-shape behavior seen in vivo. Our mosaic analysis also suggested that during hindgut rotation in vivo, wild-type Myo31DF suppresses the elongation of cell boundaries, supporting the idea that cell-shape chirality is an intrinsic property determined in each cell. However, the amount and distribution of F-actin and Myosin II, which are known to help generate the contraction force on cell boundaries, did not show differences between Myo31DF mutant cells and wild-type cells, suggesting that the static amount and distribution of these proteins are not involved in the suppression of cell-boundary elongation. Taken together, our results suggest that cell-shape chirality is intrinsically formed in each cell, and that mechanical force from intercellular interactions contributes to its formation and/or maintenance.

A gain-of-function screen to identify genes that reduce lifespan in the adult of Drosophila melanogaster.

BACKGROUND: Several lines of evidence associate misregulated genetic expression with risk factors for diabetes, Alzheimer's, and other diseases that sporadically develop in healthy adults with no background of hereditary disorders. Thus, we are interested in genes that may be expressed normally through parts of an individual's life, but can cause physiological defects and disease when misexpressed in adulthood. RESULTS: We attempted to identify these genes in a model organism by arbitrarily misexpressing specific genes in adult Drosophila melanogaster, using 14,133 Gene Search lines. We identified 39 "reduced-lifespan genes" that, when misexpressed in adulthood, shortened the flies' lifespan to less than 30% of that of control flies. About half of these genes have human orthologs that are known to be involved in human diseases. For about one-fourth of the reduced-lifespan genes, suppressing apoptosis restored the lifespan shortened by their misexpression. We determined the organs responsible for reduced lifespan when these genes were misexpressed specifically in adulthood, and found that while some genes induced reduced lifespan only when misexpressed in specific adult organs, others could induce reduced lifespan when misexpressed in various organs. This finding suggests that tissue-specific dysfunction may be involved in reduced lifespan related to gene misexpression. Gene ontology analysis showed that reduced-lifespan genes are biased toward genes related to development. CONCLUSIONS: We identified 39 genes that, when misexpressed in adulthood, shortened the lifespan of adult flies. Suppressing apoptosis rescued this shortened lifespan for only a subset of the reduced-lifespan genes. The adult tissues in which gene misexpression caused early death differed among the reduced-lifespan genes. These results suggest that the cause of reduced lifespan upon misexpression differed among the genes.

Disruption of Drosophila melanogaster lipid metabolism genes causes tissue overgrowth associated with altered developmental signaling.

Developmental patterning requires the precise interplay of numerous intercellular signaling pathways to ensure that cells are properly specified during tissue formation and organogenesis. The spatiotemporal function of many developmental pathways is strongly influenced by the biosynthesis and intracellular trafficking of signaling components. Receptors and ligands must be trafficked to the cell surface where they interact, and their subsequent endocytic internalization and endosomal trafficking is critical for both signal propagation and its down-modulation. In a forward genetic screen for mutations that alter intracellular Notch receptor trafficking in Drosophila melanogaster, we recovered mutants that disrupt genes encoding serine palmitoyltransferase and acetyl-CoA carboxylase. Both mutants cause Notch, Wingless, the Epidermal Growth Factor Receptor (EFGR), and Patched to accumulate abnormally in endosomal compartments. In mosaic animals, mutant tissues exhibit an unusual non-cell-autonomous effect whereby mutant cells are functionally rescued by secreted activities emanating from adjacent wildtype tissue. Strikingly, both mutants display prominent tissue overgrowth phenotypes that are partially attributable to altered Notch and Wnt signaling. Our analysis of the mutants demonstrates genetic links between abnormal lipid metabolism, perturbations in developmental signaling, and aberrant cell proliferation.

Protein trafficking abnormalities in Drosophila tissues with impaired activity of the ZIP7 zinc transporter Catsup.

Developmental patterning requires the precise interplay of numerous intercellular signaling pathways to ensure that cells are properly specified during tissue formation and organogenesis. The spatiotemporal function of the Notch signaling pathway is strongly influenced by the biosynthesis and intracellular trafficking of signaling components. Receptors and ligands must be trafficked to the cell surface where they interact, and their subsequent endocytic internalization and endosomal trafficking is crucial for both signal propagation and its down-modulation. In a forward genetic screen for mutations that alter intracellular Notch receptor trafficking in Drosophila epithelial tissues, we recovered mutations that disrupt the Catsup gene, which encodes the Drosophila ortholog of the mammalian ZIP7 zinc transporter. Loss of Catsup function causes Notch to accumulate abnormally in the endoplasmic reticulum (ER) and Golgi compartments, resulting in impaired Notch signaling. In addition, Catsup mutant cells exhibit elevated ER stress, suggesting that impaired zinc homeostasis causes increased levels of misfolded proteins within the secretory compartment.

Loss- and gain-of-function analyses of vacuolar protein sorting 2 in Notch signaling of Drosophila melanogaster.

Notch signaling is an evolutionarily conserved mechanism that controls many cell-fate specifications through local cell-cell interactions. The core mechanisms of Notch activation and its subsequent intracellular signaling are well understood. Various cellular functions are required for the activation and regulation of Notch signaling. Among them, the endocytosis of Notch and its ligands is important for the activation and suppression of Notch signaling. The endosomal sorting complex required for transport (ESCRT) proteins are required to sort ubiquitinated membrane proteins, such as Notch, into early endosomes. A loss-of-function allele of vacuolar protein sorting 2 (vps2), which encodes a component of ESCRT-III, has been reported. However, this vps2 mutant still produces the N-terminal half of the protein, and its phenotypes were studied in only a few organs. Here, we generated the first null mutant allele of Drosophila vps2, designated vps2², to better understand the function of this gene. In Drosophila wing imaginal discs homozygous for the vps2² allele, early endosomes and multivesicular bodies (MVBs) were enlarged, and Notch and Delta accumulated inside them. As reported for the previous vps2 mutant, the epithelium grew excessively under this condition. We further studied the roles of vps2 by RNA interference-knockdown. These experiments revealed that a partial reduction of vps2 attenuated Notch signaling; in contrast, the loss-of-function vps2 mutant is reported to up-regulate the Notch signaling in eye imaginal disc cells. These results suggest that Notch signaling can be up- or down-regulated, depending on the level of vps2 expression. Finally, we found that vps2 overexpression also resulted in early-endosome enlargement and the accumulation of Notch and Delta. In these cells, a portion of the Vps2 protein was detected in MVBs and colocalized with Notch. These data indicate that the expression of vps2 must be precisely regulated to maintain the normal structure of early endosomes.

Dmon1 controls recruitment of Rab7 to maturing endosomes in Drosophila.

The small GTPases Rab5 and Rab7 are important organisers of endosome formation and maturation. In addition, they orchestrate the trafficking of cargo through the endosomal pathway. A crucial event during maturation of endosomes is the replacement of the early organiser Rab5 with the late organiser Rab7 in a process called Rab conversion. Rab conversion is a prerequisite for late events, chief among them the fusion of matured endosomes with the lysosome. Recent work identifies members of the Sand1/Mon1 protein family as crucial factors during this process. Here, we present an analysis of the function of the Drosophila ortholog of mon1/sand1, Dmon1. We found that loss of function of Dmon1 results in an enlargement of maturing endosomes and loss of their association with Rab7. The enlarged endosomes contain Notch and other trans-membrane proteins as cargo. We report the first electron microscopy analysis of Dmon1 cells in a metazoan and extend the analysis of the endosomes in mutant cells. Our results suggest that the phenotype can be explained by the loss of function of Rab7. Moreover, the endosomes of Dmon1 cells mature normally in many aspects, despite the loss of association with Rab7. Surprisingly, we did not observe overactive or ectopic signalling through receptors such as Notch and RTKs in Dmon1 mutant cells, as would have been expected because of the accumulation of receptors in the maturing endosomes of these cells. This was the case even when receptor uptake into intraluminal vesicles was suppressed.

Deficient Notch signaling associated with neurogenic pecanex is compensated for by the unfolded protein response in Drosophila.

The Notch (N) signaling machinery is evolutionarily conserved and regulates a broad spectrum of cell-specification events, through local cell-cell communication. pecanex (pcx) encodes a multi-pass transmembrane protein of unknown function, widely found from Drosophila to humans. The zygotic and maternal loss of pcx in Drosophila causes a neurogenic phenotype (hyperplasia of the embryonic nervous system), suggesting that pcx might be involved in N signaling. Here, we established that Pcx is a component of the N-signaling pathway. Pcx was required upstream of the membrane-tethered and the nuclear forms of activated N, probably in N signal-receiving cells, suggesting that pcx is required prior to or during the activation of N. pcx overexpression revealed that Pcx resides in the endoplasmic reticulum (ER). Disruption of pcx function resulted in enlargement of the ER that was not attributable to the reduced N signaling activity. In addition, hyper-induction of the unfolded protein response (UPR) by the expression of activated Xbp1 or dominant-negative Heat shock protein cognate 3 suppressed the neurogenic phenotype and ER enlargement caused by the absence of pcx. A similar suppression of these phenotypes was induced by overexpression of O-fucosyltransferase 1, an N-specific chaperone. Taking these results together, we speculate that the reduction in N signaling in embryos lacking pcx function might be attributable to defective ER functions, which are compensated for by upregulation of the UPR and possibly by enhancement of N folding. Our results indicate that the ER plays a previously unrecognized role in N signaling and that this ER function depends on pcx activity.

D-JNK signaling in visceral muscle cells controls the laterality of the Drosophila gut.

Although bilateral animals appear to have left-right (LR) symmetry from the outside, their internal organs often show directional and stereotypical LR asymmetry. The mechanisms by which the LR axis is established in vertebrates have been extensively studied. However, how each organ develops its LR asymmetric morphology with respect to the LR axis is still unclear. Here, we showed that Drosophila Jun N-terminal kinase (D-JNK) signaling is involved in the LR asymmetric looping of the anterior-midgut (AMG) in Drosophila. Mutant embryos of puckered (puc), which encodes a D-JNK phosphatase, showed random laterality of the AMG. Directional LR looping of the AMG required D-JNK signaling to be down-regulated by puc in the trunk visceral mesoderm. Not only the down-regulation, but also the activation of D-JNK signaling was required for the LR asymmetric looping. We also found that the LR asymmetric cell rearrangement in the circular visceral muscle (CVM) was regulated by D-JNK signaling and required for the LR asymmetric looping of the AMG. Rac1, a Rho family small GTPase, augmented D-JNK signaling in this process. Our results also suggest that a basic mechanism for eliciting LR asymmetric gut looping may be conserved between vertebrates and invertebrates.

The O-fucosyltransferase O-fut1 is an extracellular component that is essential for the constitutive endocytic trafficking of Notch in Drosophila.

Notch is a transmembrane receptor that mediates the cell-cell interactions necessary for many cell-fate decisions. Endocytic trafficking of Notch plays important roles in the activation and downregulation of this receptor. A Drosophila O-FucT-1 homolog, encoded by O-fut1, catalyzes the O-fucosylation of Notch, a modification essential for Notch signaling and ligand binding. It was recently proposed that O-fut1 acts as a chaperon for Notch in the endoplasmic reticulum and is required for Notch to exit the endoplasmic reticulum. Here, we report that O-fut1 has additional functions in the endocytic transportation of Notch. O-fut1 was indispensable for the constitutive transportation of Notch from the plasma membrane to the early endosome, which we show was independent of the O-fucosyltransferase activity of O-fut1. We also found that O-fut1 promoted the turnover of Notch, which consequently downregulated Notch signaling. O-fut1 formed a stable complex with the extracellular domain of Notch. In addition, O-fut1 protein added to conditioned medium and endocytosed was sufficient to rescue normal Notch transportation to the early endosome in O-fut1 knockdown cells. Thus, an extracellular interaction between Notch and O-fut1 is essential for the normal endocytic transportation of Notch. We propose that O-fut1 is the first example, except for ligands, of a molecule that is required extracellularly for receptor transportation by endocytosis.

Roles of single-minded in the left-right asymmetric development of the Drosophila embryonic gut.

Many animals have genetically determined left-right (LR) asymmetry of their internal organs. The midline structure of vertebrate embryos has important roles in LR asymmetric development both as the signaling center for LR asymmetry and as a barrier to inappropriate LR signaling across the midline. However, in invertebrates, the functions of the midline in LR asymmetric development are unknown. To elucidate these roles, we studied the involvement of single-minded (sim) in the LR asymmetry of the Drosophila embryonic gut, which develops in a stereotypic, asymmetric manner. sim encodes a bHLH/PAS transcription factor that is required for the development of the ventral midline structure. Here we report that sim was expressed in the midline of the foregut and hindgut primordia. The handedness of the embryonic gut was affected in sim mutant embryos and in embryos overexpressing sim. However, midline-derived events, which involve Slit/Robo and EGFr signaling and direct the development of the tissues adjacent to the midline, did not affect the laterality of this organ, suggesting a crucial role for the midline itself in LR asymmetry. In the sim mutants, the midline structures of the embryonic anal pad were deformed. The mis-expression of sim in the anal-pad primordium induced LR defects. We also found that different portions of the embryonic gut require sim functions at different times for normal LR asymmetry. Our results suggest that the midline structures are involved in the LR asymmetric development of the Drosophila embryonic gut.