Three proteins involved in Caenorhabditis elegans vulval invagination are similar to components of a glycosylation pathway

Chondroitin sulfate in invertebrate development

Chondroitin sulfate (CS) is one of the most evolutionarily conserved glycosaminoglycans (GAGs). Although CS's function in skeletal development is well established in vertebrates, CS exists in more primitive animal species with no cartilage or bone, such as C. elegans and Drosophila, indicating that the original role of CS was not in the skeletal system. In this review, we focus on the roles of CS and the mechanisms of action during development of two genetically trackable model organisms, C. elegans and Drosophila.

The logistics of Wnt production and delivery

Wnts are secreted proteins that control stem cell maintenance, cell fate decisions, and growth during development and adult homeostasis. Wnts carry a post-translational modification not seen in any other secreted protein: during biosynthesis, they are appended with a palmitoleoyl moiety that is required for signaling but also impairs solubility and hence diffusion in the extracellular space. In some contexts, Wnts act only in a juxtacrine manner but there are also instances of long range action. Several proteins and processes ensure that active Wnts reach the appropriate target cells. Some, like Porcupine, Wntless, and Notum are dedicated to Wnt function; we describe their activities in molecular detail. We also outline how the cell infrastructure (secretory, endocytic, and retromer pathways) contribute to the progression of Wnts from production to delivery. We then address how Wnts spread in the extracellular space and form a signaling gradient despite carrying a hydrophobic moiety. We highlight particularly the role of lipid-binding Wnt interactors and heparan sulfate proteoglycans. Finally, we briefly discuss how evolution might have led to the emergence of this unusual signaling pathway.

Fast genetic mapping using insertion-deletion polymorphisms in Caenorhabditis elegans

Genetic mapping is used in forward genetics to narrow the list of candidate mutations and genes corresponding to the mutant phenotype of interest. Even with modern advances in biology such as efficient identification of candidate mutations by whole-genome sequencing, mapping remains critical in pinpointing the responsible mutation. Here we describe a simple, fast, and affordable mapping toolkit that is particularly suitable for mapping in Caenorhabditis elegans. This mapping method uses insertion-deletion polymorphisms or indels that could be easily detected instead of single nucleotide polymorphisms in commonly used Hawaiian CB4856 mapping strain. The materials and methods were optimized so that mapping could be performed using tiny amount of genetic material without growing many large populations of mutants for DNA purification. We performed mapping of previously known and unknown mutations to show strengths and weaknesses of this method and to present examples of completed mapping. For situations where Hawaiian CB4856 is unsuitable, we provide an annotated list of indels as a basis for fast and easy mapping using other wild isolates. Finally, we provide rationale for using this mapping method over other alternatives as a part of a comprehensive strategy also involving whole-genome sequencing and other methods.

Characterization of C. elegans Chondroitin Proteoglycans and Their Large Functional and Structural Heterogeneity; Evolutionary Aspects on Structural Differences Between Humans and the Nematode

Proteoglycans regulate important cellular pathways in essentially all metazoan organisms. While considerable effort has been devoted to study structural and functional aspects of proteoglycans in vertebrates, the knowledge of the core proteins and proteoglycan-related functions in invertebrates is relatively scarce, even for C.elegans. This nematode produces a large amount of non-sulfated chondroitin in addition to small amount of low-sulfated chondroitin chains (Chn and CS chains, respectively). Until recently, 9 chondroitin core proteins (CPGs) had been identified in C.elegans, none of which showed any homology to vertebrate counterparts or to other invertebrate core proteins. By using a glycoproteomic approach, we recently characterized the chondroitin glycoproteome of C.elegans, resulting in the identification of 15 novel CPG core proteins in addition to the 9 previously established. Three of the novel core proteins displayed homology to human proteins, indicating that CPG and CSPG core proteins may be more conserved throughout evolution than previously perceived. Bioinformatic analysis of the primary amino acid sequences revealed that the core proteins contained a broad range of functional domains, indicating that specialization of proteoglycan-mediated functions may have evolved early in metazoan evolution. This review specifically discusses our recent data in relation to previous knowledge of core proteins and GAG-attachment sites in Chn and CS proteoglycans of C.elegans and humans, and point out both converging and diverging aspects of proteoglycan evolution.

N-Glycosylation of the Discoidin Domain Receptor Is Required for Axon Regeneration in Caenorhabditis elegans

Axon regeneration following neuronal injury is an important repair mechanism that is not well understood at present. In Caenorhabditis elegans, axon regeneration is regulated by DDR-2, a receptor tyrosine kinase (RTK) that contains a discoidin domain and modulates the Met-like SVH-2 RTK-JNK MAP kinase signaling pathway. Here, we describe the svh-10/sqv-3 and svh-11 genes, which encode components of a conserved glycosylation pathway, and show that they modulate axon regeneration in C. elegans Overexpression of svh-2, but not of ddr-2, can suppress the axon regeneration defect observed in svh-11 mutants, suggesting that SVH-11 functions between DDR-2 and SVH-2 in this glycosylation pathway. Furthermore, we found that DDR-2 is N-glycosylated at the Asn-141 residue located in its discoidin domain, and mutation of this residue caused an axon regeneration defect. These findings indicate that N-linked glycosylation plays an important role in axon regeneration in C. elegans.

The C. elegans eggshell

In all animals, oocytes are surrounded by an extracellular matrix upon fertilization. This matrix serves similar purposes in each animal. It functions to mediate sperm binding, to prevent polyspermy, to control the chemical environment of the embryo, and to provide physical protection to the embryo as it developes. The synthesis of the C. elegans matrix, or eggshell, begins when the oocyte enters the spermatheca and is fertilized by a single sperm. The process of eggshell synthesis is thought to take place during the completion of the maternal meiotic divisions such that the multi-layered eggshell is completed by anaphase II. The synthesis of the eggshell occurs in a hierarchical pattern in which the outermost layers are synthesized first in order to capture and retain the innermost layers as they form. Recent studies have revealed that the lipid-rich permeability barrier is distinct from the outer trilaminar eggshell. These new findings alter our previous understanding of the eggshell. This chapter aims to define each of the eggshell layers and the molecules that are known to play significant roles in their formation.

Functional Requirements for Heparan Sulfate Biosynthesis in Morphogenesis and Nervous System Development in C. elegans

The regulation of cell migration is essential to animal development and physiology. Heparan sulfate proteoglycans shape the interactions of morphogens and guidance cues with their respective receptors to elicit appropriate cellular responses. Heparan sulfate proteoglycans consist of a protein core with attached heparan sulfate glycosaminoglycan chains, which are synthesized by glycosyltransferases of the exostosin (EXT) family. Abnormal HS chain synthesis results in pleiotropic consequences, including abnormal development and tumor formation. In humans, mutations in either of the exostosin genes EXT1 and EXT2 lead to osteosarcomas or multiple exostoses. Complete loss of any of the exostosin glycosyltransferases in mouse, fish, flies and worms leads to drastic morphogenetic defects and embryonic lethality. Here we identify and study previously unavailable viable hypomorphic mutations in the two C. elegans exostosin glycosyltransferases genes, rib-1 and rib-2. These partial loss-of-function mutations lead to a severe reduction of HS levels and result in profound but specific developmental defects, including abnormal cell and axonal migrations. We find that the expression pattern of the HS copolymerase is dynamic during embryonic and larval morphogenesis, and is sustained throughout life in specific cell types, consistent with HSPGs playing both developmental and post-developmental roles. Cell-type specific expression of the HS copolymerase shows that HS elongation is required in both the migrating neuron and neighboring cells to coordinate migration guidance. Our findings provide insights into general principles underlying HSPG function in development.

During animal development, cells and neurons navigate long distances to reach their final target destinations. Migrating cells are guided by extracellular molecular cues, and cellular responses to these cues are regulated by heparan sulfate proteoglycans. Heparan sulfate proteoglycans are proteins with long heparan sulfate polysaccharide chains attached. Here we identify and study previously unavailable viable mutants that disrupt the elongation of the heparan sulfate chains in the nematode C. elegans. Our analysis shows that these HS-chain-elongation mutations affect the development of the nervous system as they result in misguided migrations of neurons and axons. Furthermore, we find that heparan sulfate chain elongation occurs in numerous cell types during development and that the coordinated production of heparan sulfate proteoglycans, in both the migrating cell and neighboring tissues, ensures proper migration. Our findings highlight the critical roles of heparan sulfate proteoglycans in nervous system development and the evolutionary conservation of the molecular mechanisms driving guided migrations.

Overview of Nucleotide Sugar Transporter Gene Family Functions Across Multiple Species

Glycoproteins and glycolipids are crucial in a number of cellular processes, such as growth, development, and responses to external cues, among others. Polysaccharides, another class of sugar-containing molecules, also play important structural and signaling roles in the extracellular matrix. The additions of glycans to proteins and lipids, as well as polysaccharide synthesis, are processes that primarily occur in the Golgi apparatus, and the substrates used in this biosynthetic process are nucleotide sugars. These proteins, lipids, and polysaccharides are also modified by the addition of sulfate groups in the Golgi apparatus in a series of reactions where nucleotide sulfate is needed. The required nucleotide sugar substrates are mainly synthesized in the cytosol and transported into the Golgi apparatus by nucleotide sugar transporters (NSTs), which can additionally transport nucleotide sulfate. Due to the critical role of NSTs in eukaryotic organisms, any malfunction of these could change glycan and polysaccharide structures, thus affecting function and altering organism physiology. For example, mutations or deletion on NST genes lead to pathological conditions in humans or alter cell walls in plants. In recent years, many NSTs have been identified and functionally characterized, but several remain unanalyzed. This study examined existing information on functionally characterized NSTs and conducted a phylogenetic analysis of 257 NSTs predicted from nine animal and plant model species, as well as from protists and fungi. From this analysis, relationships between substrate specificity and the primary NST structure can be inferred, thereby advancing understandings of nucleotide sugar gene family functions across multiple species.

High-Throughput Cloning of Temperature-Sensitive Caenorhabditis elegans Mutants with Adult Syncytial Germline Membrane Architecture Defects

The adult Caenorhabditis elegans hermaphrodite gonad consists of two mirror-symmetric U-shaped arms, with germline nuclei located peripherally in the distal regions of each arm. The nuclei are housed within membrane cubicles that are open to the center, forming a syncytium with a shared cytoplasmic core called the rachis. As the distal germline nuclei progress through meiotic prophase, they move proximally and eventually cellularize as their compartments grow in size. The development and maintenance of this complex and dynamic germline membrane architecture are relatively unexplored, and we have used a forward genetic screen to identify 20 temperature-sensitive mutations in 19 essential genes that cause defects in the germline membrane architecture. Using a combined genome-wide SNP mapping and whole genome sequencing strategy, we have identified the causal mutations in 10 of these mutants. Four of the genes we have identified are conserved, with orthologs known to be involved in membrane biology, and are required for proper development or maintenance of the adult germline membrane architecture. This work provides a starting point for further investigation of the mechanisms that control the dynamics of syncytial membrane architecture during adult oogenesis.

RAD-6: pyrimidine synthesis and radiation sensitivity in Caenorhabditis elegans

The Caenorhabditis elegans rad-6 (radiation-sensitive-6) mutant was isolated over 25 years ago in a genetic screen that identified mutants with enhanced sensitivity to DNA damaging agents. In the present paper we describe the molecular identification of the rad-6 gene and reveal that it encodes the bifunctional UMP synthase protein, which carries catalytic activities for OPRTase (orotate phosphoribosyltransferase) and ODCase (orotate monophosphate decarboxylase), key enzymes in the de novo pathway of pyrimidine synthesis. Mutations in genes encoding de novo pathway enzymes cause varying degrees of lethality and pleiotropic phenotypes in many organisms, including humans. We have examined how the absence of rad-6 activity leads to both UV-C hypersensitivity and a decline in both metabolic rate and lifespan. We discuss how rad-6 mutants adapt to the loss of the de novo pathway through a dependency on pyrimidine salvage. We establish further that rad-6(mn160) mutants lack ODCase activity because they are resistant to the cytotoxic effects of 5-FOA (5-fluoroorotic acid). Our results have also led to the identification of a metabolic sensor affecting survival and metabolism, which is dependent on the maternal rad-6 genotype.

Structural elucidation of the neutral glycosphingolipids, mono-, di-, tri- and tetraglycosylceramides from the marine crab Erimacrus isenbeckii

The neutral glycosphingolipids, mono-, di-, tri- and tetraglycosylceramides (GL-1, GL-2, GL-3, GL-4a and GL-4b), were identified from whole tissues of the marine crab Erimacrus isenbeckii by successive column chromatography with ion exchange Sephadex (QAE-Sephadex), magnesium silicate (Florisil) and silicic acid (Iatrobeads) resins. Through component analysis, sugar analysis, methylation studies, exoglycosidase cleavage, and various chromatographic and spectrometric techniques, their structures were proposed to be as follows: GL-1, Glcβ1-1Cer; GL-2, Manβ1-4Glcβ1-1Cer; GL-3, Galβ1-3Manβ1-4Glcβ1-1Cer; and GL-4a and GL-4b, Gal3Meα1-4Galβ1-3Manβ1-4Glcβ1-1Cer. The main molecular species of the aliphatic moiety in each purified glycolipid were 18:0, 22:0, 22:1-d14:1 (fatty acid-sphingoid) and 18:0-d16:1 for GL-1; 18:0-d16:1 and 22:1-d14:1, d16:1 for GL-2; 22:1, 24:1-d16:1 for GL-3; 22:1, 24:1-d16:1 for GL-4a; and h22:1, h24:1-d16:1 for GL-4b, respectively. By immunological detection, an arthro-series glycosphingolipid (At3Cer; GlcNAcβ1-3Manβ1-4Glcβ1-1Cer) was also detected as a minor component. The characteristic arthro-series glycosphingolipid has been observed in most animals belonging to the phylum Arthropoda.

Morphogenesis of the caenorhabditis elegans vulva

Understanding how cells move, change shape, and alter cellular behaviors to form organs, a process termed morphogenesis, is one of the great challenges of developmental biology. Formation of the Caenorhabditis elegans vulva is a powerful, simple, and experimentally accessible model for elucidating how morphogenetic processes produce an organ. In the first step of vulval development, three epithelial precursor cells divide and differentiate to generate 22 cells of 7 different vulval subtypes. The 22 vulval cells then rearrange from a linear array into a tube, with each of the seven cell types undergoing characteristic morphogenetic behaviors that construct the vulva. Vulval morphogenesis entails many of the same cellular activities that underlie organogenesis and tissue formation across species, including invagination, lumen formation, oriented cell divisions, cell–cell adhesion, cell migration, cell fusion, extracellular matrix remodeling, and cell invasion. Studies of vulval development have led to pioneering discoveries in a number of these processes and are beginning to bridge the gap between the pathways that specify cells and their connections to morphogenetic behaviors. The simplicity of the vulva and the experimental tools available in C. elegans will continue to make vulval morphogenesis a powerful paradigm to further our understanding of the largely mysterious mechanisms that build tissues and organs.

Coordinated lumen contraction and expansion during vulval tube morphogenesis in Caenorhabditis elegans

Morphogenesis is a developmental phase during which cell fates are executed. Mechanical forces shaping individual cells play a key role during tissue morphogenesis. By investigating morphogenesis of the Caenorhabditis elegans hermaphrodite vulva, we show that the force-generating actomyosin network is differentially regulated by NOTCH and EGFR/RAS/MAPK signaling to shape the vulval tube. NOTCH signaling activates expression of the RHO kinase LET-502 in the secondary cell lineage through the ETS-family transcription factor LIN-1. LET-502 induces actomyosin-mediated contraction of the apical lumen in the secondary toroids, thereby generating a dorsal pushing force. In contrast, MAPK signaling in the primary lineage downregulates LET-502 RHO kinase expression to prevent toroid contraction and allow the gonadal anchor cell to expand the dorsal lumen of the primary toroids. The antagonistic action of the MAPK and NOTCH pathways thus controls vulval tube morphogenesis linking cell fate specification to morphogenesis.

Arabidopsis thaliana AtUTr7 encodes a golgi-localized UDP-glucose/UDP-galactose transporter that affects lateral root emergence

Nucleotide sugar transporters (NSTs) are antiporters comprising a gene family that plays a fundamental role in the biosynthesis of complex cell wall polysaccharides and glycoproteins in plants. However, due to the limited number of related mutants that have observable phenotypes, the biological function(s) of most NSTs in cell wall biosynthesis and assembly have remained elusive. Here, we report the characterization of AtUTr7 from Arabidopsis (Arabidopsis thaliana (L.) Heynh.), which is homologous to multi-specific UDP-sugar transporters from Drosophila melanogaster, humans, and Caenorhabditis elegans. We show that AtUTr7 possesses the common structural characteristics conserved among NSTs. Using a green fluorescent protein (GFP) tagged version, we demonstrate that AtUTr7 is localized in the Golgi apparatus. We also show that AtUTr7 is widely expressed, especially in the roots and in specific floral organs. Additionally, the results of an in vitro nucleotide sugar transport assay carried out with a tobacco and a yeast expression system suggest that AtUTr7 is capable of transferring UDP-Gal and UDP-Glc, but not a range of other UDP- and GDP-sugars, into the Golgi lumen. Mutants lacking expression of AtUTr7 exhibited an early proliferation of lateral roots as well as distorted root hairs when cultivated at high sucrose concentrations. Furthermore, the distribution of homogalacturonan with a low degree of methyl esterification differed in lateral root tips of the mutant compared to wild-type plants, although additional analytical procedures revealed no further differences in the composition of the root cell walls. This evidence suggests that the transport of UDP-Gal and UDP-Glc into the Golgi under conditions of high root biomass production plays a role in lateral root and root hair development.

Glycosylation genes expressed in seam cells determine complex surface properties and bacterial adhesion to the cuticle of Caenorhabditis elegans

The surface of the nematode Caenorhabditis elegans is poorly understood but critical for its interactions with the environment and with pathogens. We show here that six genes (bus-2, bus-4, and bus-12, together with the previously cloned srf-3, bus-8, and bus-17) encode proteins predicted to act in surface glycosylation, thereby affecting disease susceptibility, locomotory competence, and sexual recognition. Mutations in all six genes cause resistance to the bacterial pathogen Microbacterium nematophilum, and most of these mutations also affect bacterial adhesion and biofilm formation by Yersinia species, demonstrating that both infection and biofilm formation depend on interaction with complex surface carbohydrates. A new bacterial interaction, involving locomotory inhibition by a strain of Bacillus pumilus, reveals diversity in the surface properties of these mutants. Another biological property--contact recognition of hermaphrodites by males during mating--was also found to be impaired in mutants of all six genes. An important common feature is that all are expressed most strongly in seam cells, rather than in the main hypodermal syncytium, indicating that seam cells play the major role in secreting surface coat and consequently in determining environmental interactions. To test for possible redundancies in gene action, the 15 double mutants for this set of genes were constructed and examined, but no synthetic phenotypes were observed. Comparison of the six genes shows that each has distinctive properties, suggesting that they do not act in a linear pathway.

Biochemical and thermodynamic characterization of mutated β1,4-galactosyltransferase 7 involved in the progeroid form of the Ehlers-Danlos syndrome

Three mutations of the B4GALT7 gene [encoding β1,4-GalT7 (β1,4-galactosyltransferase 7)], corresponding to A186D, L206P and R270C, have been identified in patients with the progeroid form of the Ehlers-Danlos syndrome and are described as being associated with the reduction or loss of β1,4-GalT7 activity. However, the molecular basis of the reduction or loss of activity remained to be determined. In the present study, wild-type, A186D, L206P and R270C β1,4-GalT7 were expressed in CHO618 cells as membrane proteins and in Escherichia coli as soluble proteins fused to MBP (maltose-binding protein). The ability of the expressed proteins to transfer galactose from donor to acceptor substrates was systematically characterized by kinetic analysis. The physicochemical properties of soluble proteins were explored by isothermal titration calorimetry, which is a method of choice when determining the thermodynamic parameters of the binding of substrates. Together, the results showed that: (i) the L206P mutation abolished the activity when L206P β1,4GalT7 was either inserted in the membrane or expressed as a soluble MBP-full-length fusion protein; (ii) the A186D mutation weakly impaired the binding of the donor substrate; and (iii) the R270C mutation strongly impaired the binding of the acceptor substrate. Moreover, the ex vivo consequences of the mutations were investigated by evaluating the priming efficiency of xylosides on GAG (glycosaminoglycan) chain initiation. The results demonstrate a quantitative effect on GAG biosynthesis, depending on the mutation; GAG biosynthesis was fully inhibited by the L206P mutation and decreased by the R270C mutation, whereas the A186D mutation did not affect GAG biosynthesis severely.

Structure-based evolutionary relationship of glycosyltransferases: a case study of vertebrate β1,4-galactosyltransferase, invertebrate β1,4-N-acetylgalactosaminyltransferase and α-polypeptidyl-N-acetylgalactosaminyltransferase

Cell surface glycans play important cellular functions and are synthesized by glycosyltransferases. Structure and function studies show that the donor sugar specificity of the invertebrate β1,4-N-acetyl-glactosaminyltransferase (β4GalNAc-T) and the vertebrate β1,4-galactosyltransferase I (β4Gal-T1) are related by a single amino acid residue change. Comparison of the catalytic domain crystal structures of the β4Gal-T1 and the α-polypeptidyl-GalNAc-T (αppGalNAc-T) shows that their protein structure and sequences are similar. Therefore, it seems that the invertebrate β4GalNAc-T and the catalytic domain of αppGalNAc-T might have emerged from a common primordial gene. When vertebrates emerged from invertebrates, the amino acid that determines the donor sugar specificity of the invertebrate β4GalNAc-T might have mutated, thus converting the enzyme to a β4Gal-T1 in vertebrates.

Phylogenetic conservation of the cell-type-specific Lan3-2 glycoepitope in Caenorhabditis elegans

The biological function of a cell-type-specific glycosylation of an adhesion molecule belonging to the L1CAM immunoglobulin superfamily was previously determined in the nervous system of the embryonic leech, Hirudo medicinalis. The Lan3-2 glycoepitope is a surface marker of sensory afferent neurons and is required for their appropriate developmental collateral branching and synaptogenesis in the CNS. The chemical structure of the Lan3-2 glycoepitope consists of beta-(1,4)-linked mannopyranose. Here, we show the conservation of the cell-type-specific expression of this mannose polymer in Caenorhabditis elegans. The Lan3-2 glycoepitope is expressed on the cell surface of a subset of dissociated embryonic neurons and, in the adult worm, by the pharyngeal motor neuron, M5, and the chemosensory afferents, the amphids. Additionally, the vulval epithelium expresses the Lan3-2 glycoepitope in late L4 larvae and in adult hermaphrodites. To investigate proteins carrying this restrictively expressed glycoepitope, worm extract was immunoaffinity purified with Lan3-2 monoclonal antibody and Western blotted. A polyclonal antibody reactive with the cytoplasmic tail of LAD-1/SAX-7, a C. elegans member of the L1CAM family, recognizes a 270 kDa protein band while Lan3-2 antibody also recognizes a 190 kDa glycoform, its putative Lan3-2 ectodomain. Thus, in C. elegans, as in leech, the Lan3-2 epitope is located on a L1CAM homologue. The cell-type-specific expression of the Lan3-2 glycoepitope shared by leech and C. elegans will be useful for understanding how cell-type-specific glycoepitopes mediate cell-cell interactions during development.

The role of nucleotide sugar transporters in development of eukaryotes

The Golgi apparatus membrane of all eukaryotes has nucleotide sugar transporters which play essential roles in the glycosylation of glycoproteins, proteoglycans and glycolipids. Mutations of these transporters have broad developmental phenotypes across many species including diseases in humans and cattle.

A monoclonal antibody toolkit for C. elegans

Background

Antibodies are critical tools in many avenues of biological research. Though antibodies can be produced in the research laboratory setting, most research labs working with vertebrates avail themselves of the wide array of commercially available reagents. By contrast, few such reagents are available for work with model organisms.

Methodology/Principal Findings

We report the production of monoclonal antibodies directed against a wide range of proteins that label specific subcellular and cellular components, and macromolecular complexes. Antibodies were made to synaptobrevin (SNB-1), a component of synaptic vesicles; to Rim (UNC-10), a protein localized to synaptic active zones; to transforming acidic coiled-coil protein (TAC-1), a component of centrosomes; to CENP-C (HCP-4), which in worms labels the entire length of their holocentric chromosomes; to ORC2 (ORC-2), a subunit of the DNA origin replication complex; to the nucleolar phosphoprotein NOPP140 (DAO-5); to the nuclear envelope protein lamin (LMN-1); to EHD1 (RME-1) a marker for recycling endosomes; to caveolin (CAV-1), a marker for caveolae; to the cytochrome P450 (CYP-33E1), a resident of the endoplasmic reticulum; to β-1,3-glucuronyltransferase (SQV-8) that labels the Golgi; to a chaperonin (HSP-60) targeted to mitochondria; to LAMP (LMP-1), a resident protein of lysosomes; to the alpha subunit of the 20S subcomplex (PAS-7) of the 26S proteasome; to dynamin (DYN-1) and to the α-subunit of the adaptor complex 2 (APA-2) as markers for sites of clathrin-mediated endocytosis; to the MAGUK, protein disks large (DLG-1) and cadherin (HMR-1), both of which label adherens junctions; to a cytoskeletal linker of the ezrin-radixin-moesin family (ERM-1), which localized to apical membranes; to an ERBIN family protein (LET-413) which localizes to the basolateral membrane of epithelial cells and to an adhesion molecule (SAX-7) which localizes to the plasma membrane at cell-cell contacts. In addition to working in whole mount immunocytochemistry, most of these antibodies work on western blots and thus should be of use for biochemical fractionation studies.

Conclusions/Significance

We have produced a set of monoclonal antibodies to subcellular components of the nematode C. elegans for the research community. These reagents are being made available through the Developmental Studies Hybridoma Bank (DSHB).

Systematic analysis of dynamic miRNA-target interactions during C. elegans development

Although microRNA (miRNA)-mediated functions have been implicated in many aspects of animal development, the majority of miRNA::mRNA regulatory interactions remain to be characterized experimentally. We used an AIN/GW182 protein immunoprecipitation approach to systematically analyze miRNA::mRNA interactions during C. elegans development. We characterized the composition of miRNAs in functional miRNA-induced silencing complexes (miRISCs) at each developmental stage and identified three sets of miRNAs with distinct stage-specificity of function. We then identified thousands of miRNA targets in each developmental stage, including a significant portion that is subject to differential miRNA regulation during development. By identifying thousands of miRNA family-mRNA pairs with temporally correlated patterns of AIN-2 association, we gained valuable information on the principles of physiological miRNA::target recognition and predicted 1589 high-confidence miRNA family::mRNA interactions. Our data support the idea that miRNAs preferentially target genes involved in signaling processes and avoid genes with housekeeping functions, and that miRNAs orchestrate temporal developmental programs by coordinately targeting or avoiding genes involved in particular biological functions.

Functional redundancy of two nucleoside transporters of the ENT family (CeENT1, CeENT2) required for development ofCaenorhabditis elegans

The genome of Caenorhabditis elegans encodes multiple homologues of the two major families of mammalian equilibrative and concentrative nucleoside transporters. As part of a programme aimed at understanding the biological rationale underlying the multiplicity of eukaryote nucleoside transporters, we have now demonstrated that the nematode genes ZK809.4 (ent-1) and K09A9.3 (ent-2) encode equilibrative transporters, which we designate CeENT1 and CeENT2 respectively. These transporters resemble their human counterparts hENT1 and hENT2 in exhibiting similar broad permeant specificities for nucleosides, while differing in their permeant selectivities for nucleobases. They are insensitive to the classic inhibitors of mammalian nucleoside transport, nitrobenzylthioinosine, dilazep and draflazine, but are inhibited by the vasoactive drug dipyridamole. Use of green fluorescent protein reporter constructs indicated that the transporters are present in a limited number of locations in the adult, including intestine and pharynx. Their potential roles in these tissues were explored by using RNA interference to disrupt gene expression. Although disruption of ent-1 or ent-2 expression alone had no effect, simultaneous disruption of both genes yielded pronounced developmental defects involving the intestine and vulva.

Modulation of xylosyltransferase I expression provides a mechanism regulating glycosaminoglycan chain synthesis during cartilage destruction and repair

Osteoarthritis and rheumatoid arthritis are characterized by loss of proteoglycans (PGs) and their glycosaminoglycan (GAG) chains that are essential for cartilage function. Here, we investigated the role of glycosyltransferases (GTs) responsible for PG-GAG chain assembly during joint cartilage destruction and repair processes. At various times after antigen-induced arthritis (AIA) and papain-induced cartilage repair in rats, PG synthesis and deposition, expression of GTs, and GAG chain composition were analyzed. Our data showed that expression of the GT xylosyltransferase I (XT-I) gene initiating PG-GAG chain synthesis was significantly reduced in AIA rat cartilage and was associated with a decrease in PG synthesis. Interestingly, interleukin-1beta, the main proinflammatory cytokine incriminated in joint diseases, down-regulated the XT-I gene expression with a concomitant decrease in PG synthesis in rat cartilage explants ex vivo. However, cartilage from papain-injected rat knees showed up-regulation of XT-I gene expression and increased PG synthesis at early stages of cartilage repair, a process associated with up-regulation of TGF-beta1 gene expression and mediated by p38 mitogen-activated protein kinase activation. Consistently, silencing of XT-I expression by intraarticular injection of XT-I shRNA in rat knees prevented cartilage repair by decreasing PG synthesis and content. These findings show that GTs play a key role in the loss of PG-GAGs in joint diseases and identify novel targets for stimulating cartilage repair.

APY-1, a novel Caenorhabditis elegans apyrase involved in unfolded protein response signalling and stress responses

Protein glycosylation modulates a wide variety of intracellular events and dysfunction of the glycosylation pathway has been reported in a variety of human pathologies. Endo-apyrases have been suggested to have critical roles in protein glycosylation and sugar metabolism. However, deciphering the physiological relevance of Endo-apyrases activity has actually proved difficult, owing to their complexity and the functional redundancy within the family. We report here that a UDP/GDPase, homologous to the human apyrase Scan-1, is present in the membranes of Caenorhabditis elegans, encoded by the ORF F08C6.6 and hereinafter-named APY-1. We showed that ER stress induced by tunicamycin or high temperature resulted in increased transcription of apy-1. This increase was not observed in C. elegans mutants defective in ire-1 or atf-6, demonstrating the requirement of both ER stress sensors for up-regulation of apy-1. Depletion of APY-1 resulted in constitutively activated unfolded protein response. Defects in the pharynx and impaired organization of thin fibers in muscle cells were observed in adult worms depleted of APY-1. Some of the apy-1(RNAi) phenotypes are suggestive of premature aging, because these animals also showed accumulation of lipofuscin and reduced lifespan that was not dependent on the functioning of DAF-2, the receptor of the insulin/IGF-1 signaling pathway.

Nucleotide sugar transporters of the Golgi apparatus

The Golgi apparatus is the major site of protein, lipid and proteoglycan glycosylation. The glycosylation enzymes, as well as kinases and sulfatases that catalyze phosphorylation and sulfation, are localized within the Golgi cisternae in characteristic distributions that frequently reflect their order in a particular pathway (Kornfeld and Kornfeld 1985; Colley 1997). The glycosyl-transferases, sulfotransferases and kinases are “transferases” that require activated donor molecules for the reactions they catalyze. For eukaryotic, fungal and protozoan glycosyltransferases these are the nucleotide sugars UDP-N-acetylglucosamine (UDP-GlcNAc), UDP-galactose (UDP-Gal), GDP-fucose (GDP-Fuc), CMP-sialicacid (CMP-Sia), UDP-glucuronicacid (UDP-GlcA), GDP-mannose (GDP-Man), and UDP-xylose (UDP-Xyl) (Hirschberg et al. 1998). For the kinases, ATP functions as the donor, while for the sulfotransferases, adenosine 3′-phosphate 5′-phosphate (PAPS) acts as the donor (Hirschberg et al. 1998). The active sites of all these enzymes are oriented towards the lumen of the Golgi cisternae. This necessitates the translocation of their donors from the cytosol into the lumenal Golgi compartments. In this chapter we will focus on the structure, function and localization of the Golgi nucleotide sugar transporters (NSTs), and highlight the diseases and developmental defects associated with defective transporters. We direct the reader to several excellent reviews on Golgi transporters for additional details and references (Hirschberg et al. 1998; Berninsone and Hirschberg 2000; Gerardy-Schahn et al. 2001; Handford et al. 2006; Caffaro and Hirschberg 2006).

Molecular cloning and characterization of the Caenorhabditis elegans alpha1,3-fucosyltransferase family

The genome of Caenorhabditis elegans encodes five genes with homology to known alpha1,3 fucosyltransferases (alpha1,3FTs), but their expression and functions are poorly understood. Here we report the molecular cloning and characterization of these C. elegans alpha1,3FTs (CEFT-1 through -5). The open-reading frame for each enzyme predicts a type II transmembrane protein and multiple potential N-glycosylation sites. We prepared recombinant epitope-tagged forms of each CEFT and found that they had unusual acceptor specificity, cation requirements, and temperature sensitivity. CEFT-1 acted on the N-glycan pentasaccharide core acceptor to generate Manalpha1-3(Manalpha1-6)Manbeta1-4GlcNAcbeta1-4(Fucalpha1-3)GlcNAcbeta1-Asn. In contrast, CEFT-2 did not act on the pentasaccharide acceptor, but instead utilized a LacdiNAc acceptor to generate GalNAcbeta1-4(Fucalpha1-3)GlcNAcbeta1-3Galbeta1-4Glc, which is a novel activity. CEFT-3 utilized a LacNAc acceptor to generate Galbeta1-4(Fucalpha1-3)GlcNAcbeta1-3Galbeta1-4Glc without requiring cations. CEFT-4 was similar to CEFT-3, but its activity was enhanced by some divalent cations. Recombinant CEFT-5 was well expressed, but did not act on available acceptors. Each CEFT was optimally active at room temperature and rapidly lost activity at 37 degrees C. Promoter analysis showed that CEFT-1 is expressed in C. elegans eggs and adults, but its expression was restricted to a few neuronal cells at the head and tail. We prepared deletion mutants for each enzyme for phenotypic analysis. While loss of CEFT-1 correlated with loss of pentasaccharide core activity and core alpha1,3-fucosylated glycans in worms, loss of other enzymes did not correlate with any phenotypic changes. These results suggest that each of the alpha1,3FTs in C. elegans has unique specificity and expression patterns.

XT-II, the second isoform of human peptide-O-xylosyltransferase, displays enzymatic activity

Peptide O-xylosyltransferase (EC 2.4.2.26) is the first enzyme required for the generation of chondroitin and heparan sulfate glycosaminoglycan chains of proteoglycans. Cloning of cDNAs has previously shown that, whereas invertebrates generally have a single xylosyltransferase gene, vertebrate genomes encode two similar proteins, xylosyltransferase I and II (XT-I and XT-II). To date, enzymatic activity has only been demonstrated for the human XT-I, Caenorhabditis SQV-6, and Drosophila OXT isoforms. In the present study, we demonstrate that a soluble form of human XT-II expressed in the xylosyltransferase-deficient pgsA-745 (S745) Chinese hamster ovary cell line is indeed capable of catalyzing the transfer of xylose to a variety of peptide substrates; its enzyme activity was also proven using a Pichia-expressed form of XT-II. Its pH, temperature, and cation dependences are similar to those of XT-I expressed in either mammalian cells or yeast. Our data suggest that XT-I and XT-II are, at least in vitro, functionally identical.

lin-35/Rb and the CoREST ortholog spr-1 coordinately regulate vulval morphogenesis and gonad development in C. elegans

Using a genetic screen to identify genes that carry out redundant functions during development with lin-35/Rb, the C. elegans Retinoblastoma family ortholog, we have identified a mutation in spr-1. spr-1 encodes the C. elegans ortholog of human CoREST, a protein containing Myb-like SANT and ELM2 domains, which functions as part of a transcriptional regulatory complex. CoREST recruits mediators of transcriptional repression, including histone deacetylase, and demethylase, and interacts with the tumor suppression protein REST. spr-1/CoREST was previously shown in C. elegans to suppress defects associated with loss of the presenilin sel-12, which functions in the proteolytic processing of LIN-12/Notch. Here we show that lin-35 and spr-1 coordinately regulate several developmental processes in C. elegans including the ingression of vulval cells as well as germline proliferation. We also show that loss of lin-35 and spr-1 hypersensitizes animals to a reduction in LIN-12/Notch activity, leading to the generation of proximal germline tumors. This defect, which is observed in lin-35; spr-1; lin-12(RNAi) and lin-35; spr-1; hop-1(RNAi) triple mutants is likely due to a delay in the entry of germ cells into meiosis.

Role of a single amino acid in the evolution of glycans of invertebrates and vertebrates

Structures of glycoconjugate N-glycans and glycolipids of invertebrates show significant differences from those of vertebrates. These differences are due largely to the vertebrate beta1,4-galactosyltransferase-1 (beta4Gal-T1), which is found as a beta1,4-N-acetylgalactosaminyltransferase (beta4GalNAc-T1) in invertebrates. Mutation of Tyr285 to Ile or Leu in human beta4Gal-T1 converts the enzyme into an equally efficient beta4GalNAc-T1. A comparison of all the human beta4Gal-T1 ortholog enzymes shows that this Tyr285 residue in human beta4Gal-T1 is conserved either as Tyr or Phe in all vertebrate enzymes, while in all invertebrate enzymes it is conserved as an Ile or Leu. We find that mutation of the corresponding Ile residue to Tyr in Drosophila beta4GalNAc-T1 converts the enzyme to a beta4Gal-T1 by reducing its N-acetylgalactosaminyltransferase activity by nearly 1000-fold, while enhancing its galactosyltransferase activity by 80-fold. Furthermore, we find that, similar to the vertebrate/mammalian beta4Gal-T1 enzymes, the wild-type Drosophila beta4GalNAc-T1 enzyme binds to a mammary gland-specific protein, alpha-lactalbumin (alpha-LA). Thus, it would seem that, during the evolution of vertebrates from invertebrates over 500 million years ago, beta4Gal-T1 appeared as a result of the single amino acid substitution of Tyr or Phe for Leu or Ile in the invertebrate beta4GalNAc-T1. Subsequently, the pre-existing alpha-LA-binding site was utilized during mammalian evolution to synthesize lactose in the mammary gland during lactation.

The molecular diversity of glycosaminoglycans shapes animal development

Proteoglycans (PGs), molecules in which glycosaminoglycans (GAGs) are covalently linked to a protein core, are components of the extracellular matrix of all multicellular organisms. Sugar moieties in GAGs are often extensively modified, which make these molecules enormously complex. We discuss here the role of PGs during animal development, emphasizing the in vivo significance of sugar modifications. We explore a model in which the modification patterns of GAG chains may provide a specific code that contributes to the correct development of a multicellular organism.

Chondroitin acts in the guidance of gonadal distal tip cells in C. elegans

In Caenorhabditis elegans hermaphrodites, the U-shaped gonad arms are formed by directed migration of the gonadal distal tip cells (DTCs). The stereotyped pattern of DTC migration is carefully controlled by extracellular and cell surface molecules during larval development. Here we report that two proteins, SQV-5 (chondroitin synthase) and its cofactor MIG-22 (chondroitin polymerizing factor), are required for chondroitin biosynthesis and are essential for the dorsally guided migration of DTCs. We found that MIG-22 is expressed in migrating DTCs, hypodermal seam cells, developing vulva and oocytes. The expression of SQV-5 or MIG-22 in both DTCs and hypodermis rescued the DTC migration defects of the relevant mutants more efficiently than when they were expressed in either single tissue. Furthermore, the expression of SQV-5 by the mig-22 promoter significantly rescued sqv-5 mutants, implying that these two proteins act in the same tissues and that chondroitin proteoglycans produced in both of these tissues are required for DTC migration. The DTC migration defects caused by sqv-5 or mig-22 mutations were partially suppressed in the anterior and enhanced in the posterior DTCs in unc-6, unc-5 or unc-40 mutant backgrounds, suggesting that chondroitin proteoglycans play roles in the UNC-6/netrin-dependent guidance of DTCs.

The CMP-sialic acid transporter is localized in the medial-trans Golgi and possesses two specific endoplasmic reticulum export motifs in its carboxyl-terminal cytoplasmic tail

The addition of sialic acid to glycoproteins and glycolipids requires Golgi sialyltransferases to have access to their glycoconjugate substrates and nucleotide sugar donor, CMP-sialic acid. CMP-sialic acid is transported into the lumen of the Golgi complex through the CMP-sialic acid transporter, an antiporter that also functions to transport CMP into the cytosol. We localized the transporter using immunofluorescence and deconvolution microscopy to test the prediction that it is broadly distributed across the Golgi stack to serve the many sialyltransferases involved in glycoconjugate sialylation. The transporter co-localized with ST6GalI in the medial and trans Golgi, showed partial overlap with a medial Golgi marker and little overlap with early Golgi or trans Golgi network markers. Endoplasmic reticulum-retained forms of sialyltransferases did not redistribute the transporter from the Golgi to the endoplasmic reticulum, suggesting that transporter-sialyltransferase complexes are not involved in transporter localization. Next we evaluated the role of the transporter's N- and C-terminal cytoplasmic tails in its trafficking and localization. The N-tail was not required for either endoplasmic reticulum export or Golgi localization. The C-tail was required for endoplasmic reticulum export and contained di-Ile and terminal Val motifs at its very C terminus that function as independent endoplasmic reticulum export signals. Deletion of the last four amino acids of the C-tail (IIGV) eliminated these export signals and prevented endoplasmic reticulum export of the transporter. This form of the transporter supplied limited amounts of CMP-sialic acid to Golgi sialyltransferases but was unable to completely rescue the transporter defect of Lec2 Chinese hamster ovary cells.

C. elegans pharyngeal morphogenesis requires both de novo synthesis of pyrimidines and synthesis of heparan sulfate proteoglycans

The C. elegans pharynx undergoes elongation and morphogenesis to its characteristic bi-lobed shape between the 2- and 3-fold stages of embryogenesis. During this period, the pharyngeal muscles and marginal cells forming the isthmus between the anterior and posterior pharyngeal bulbs elongate and narrow. We have identified the spontaneous mutant pyr-1(cu8) exhibiting defective pharyngeal isthmus elongation, cytoskeletal organization defects, and maternal effect lethality. pyr-1 encodes CAD, a trifunctional enzyme required for de novo pyrimidine synthesis, and pyr-1(cu8) mutants are rescued by supplying exogenous pyrimidines. Similar pharyngeal defects and maternal effect lethality were found in sqv-1, sqv-8, rib-1 and rib-2 mutants, which affect enzymes involved in heparan sulfate proteoglycan (HSPG) synthesis. rib-1 mutant lethality was enhanced in a pyr-1 mutant background, indicating that HSPG synthesis is very sensitive to decreased pyrimidine pools, and HS disaccharides are moderately decreased in both rib-1 and pyr-1 mutants. We hypothesize that HSPGs are necessary for pharyngeal isthmus elongation, and pyr-1 functions upstream of proteoglycan synthesizing enzymes by providing precursors of UDP-sugars essential for HSPG synthesis.

Dynamic regulation of caveolin-1 trafficking in the germ line and embryo of Caenorhabditis elegans

Caveolin is the major protein component required for the formation of caveolae on the plasma membrane. Here we show that trafficking of Caenorhabditis elegans caveolin-1 (CAV-1) is dynamically regulated during development of the germ line and embryo. In oocytes a CAV-1-green fluorescent protein (GFP) fusion protein is found on the plasma membrane and in large vesicles (CAV-1 bodies). After ovulation and fertilization the CAV-1 bodies fuse with the plasma membrane in a manner reminiscent of cortical granule exocytosis as described in other species. Fusion of CAV-1 bodies with the plasma membrane appears to be regulated by the advancing cell cycle, and not fertilization per se, because fusion can proceed in spe-9 fertilization mutants but is blocked by RNA interference-mediated knockdown of an anaphase-promoting complex component (EMB-27). After exocytosis, most CAV-1-GFP is rapidly endocytosed and degraded within one cell cycle. CAV-1 bodies in oocytes appear to be produced by the Golgi apparatus in an ARF-1-dependent, clathrin-independent, mechanism. Conversely endocytosis and degradation of CAV-1-GFP in embryos requires clathrin, dynamin, and RAB-5. Our results demonstrate that the distribution of CAV-1 is highly dynamic during development and provides new insights into the sorting mechanisms that regulate CAV-1 localization.

AtUTr2 is an Arabidopsis thaliana nucleotide sugar transporter located in the Golgi apparatus capable of transporting UDP-galactose

The synthesis of noncellulosic polysaccharides and glycoproteins in the plant cell Golgi apparatus requires UDP-galactose as a substrate. We have cloned and characterized a nucleotide sugar transporter from Arabidopsis thaliana (L.) Heynh. named AtUTr2. Expression in tobacco and Saccharomyces cerevisiae and subsequent biochemical characterization indicate that AtUTr2 transports UDP-galactose, but not UDP-glucose, UDP-N-acetyl glucosamine, UDP-xylose, UDP-glucuronic acid, GDP-fucose or GDP-mannose. Experiments expressing an AtUTr2-GFP fusion protein in onion epidermal cells suggest that AtUTr2 is located in the Golgi apparatus. Finally, northern analysis indicates that the AtUTr2 transcript was more abundant in roots and calli although it was also present in other Arabidopsis organs but at lower levels. Therefore, AtUTr2 is a nucleotide sugar transporter capable of transporting UDP-galactose that may play an important role in the synthesis of galactose-containing glycoconjugates in Arabidopsis.

Heparan 2-O-sulfotransferase, hst-2, is essential for normal cell migration in Caenorhabditis elegans

The importance of heparan sulfate proteoglycans has been highlighted by a number of human genetic disorders associated with mutations in genes encoding for heparan sulfate proteoglycan protein cores or biosynthetic enzymes required for heparan sulfate (HS) assembly. To study the functional role of HS in Caenorhabditis elegans development cosmid sequence C34F6.4 was identified as the C. elegans ortholog of vertebrate heparan 2-O-sulfotransferase (HS2ST) and the gene named hst-2. HS2ST activity is present in C. elegans and is completely absent in a deletion mutant of hst-2, ok595, and specifically reduced by hst-2 RNA interference. Expression of hst-2 in CHO cells deficient in HS2ST rescues enzyme activity and binding of FGF2 to cell surface HS. hst-2 expression is found in the hypodermis, muscle, distal tip cells (DTCs), and in neurons. A null mutation in hst-2 causes cell migration defects. This work demonstrates sulfotransferase activity in C. elegans and indicates that specific 2-O-sulfate modifications are critical for normal HS functions in controlling cell migration.

Identification and characterization of human Golgi nucleotide sugar transporter SLC35D2, a novel member of the SLC35 nucleotide sugar transporter family

We report the molecular cloning of SLC35D2, a novel member of the SLC35 nucleotide sugar transporter family. The gene SLC35D2 maps to chromosome 9q22.33. SLC35D2 cDNA codes for a hydrophobic protein consisting of 337 amino acid residues with 10 putative transmembrane helices. Northern blot analysis revealed the SLC35D2 mRNA as a single major band corresponding to 2.0 kb in length. SLC35D2 was localized in the Golgi membrane and exhibited around 50% similarity with three nucleotide sugar transporters: human SLC35D1 (UDP-glucuronic acid/UDP-N-acetylgalactosamine transporter), fruitfly fringe connection (frc) transporter, and nematode SQV-7 transporter, the latter two being involved in developmental and organogenetic processes. Heterologous expression of SLC35D2 protein in yeast indicated that UDP-N-acetylglucosamine is a candidate for the substrate(s) of the transporter. The sequence similarity, subcellular localization, and transporting substrate suggest that SLC35D2 is a good candidate for the ortholog of frc transporter, which is involved in the Notch signaling system by providing the fringe N-acetylglucosaminyltransferase with the substrate. We also describe the identification and categorization of the human SLC35 gene family.

Arabidopsis thaliana expresses multiple Golgi-localised nucleotide-sugar transporters related to GONST1

Transport of nucleotide-sugars across the Golgi membrane is required for the lumenal synthesis of a variety of essential cell surface components, and is mediated by nucleotide sugar transporters (NSTs) which are members of the large drug/metabolite superfamily of transporters. Despite the importance of these proteins in plants, so far only two have been described, GONST1 and AtUTr1 from Arabidopsis thaliana. In this work, our aim was to identify further Golgi nucleotide-sugar transporters from Arabidopsis. On the basis of their sequence similarity to GONST1, we found four additional proteins, which we named GONST2, 3, 4 and 5. These putative NSTs were grouped into three clades: GONST2 with GONST1; GONST3 with GONST4; and GONST5 with six further uncharacterized proteins. Transient expression in tobacco cells of a member of each clade, fused to the Green Fluorescent Protein (GFP), suggested that all these putative NSTs are localised in the Golgi. To obtain evidence for nucleotide sugar transport activity, we expressed these proteins, together with the previously characterised GONST1, in a GDP-mannose transport-defective yeast mutant (vrg4-2). We tested the transformants for rescue of two phenotypes associated with this mutation: sensitivity to hygromycin B and reduced glycosylation of extracellular chitinase. GONST1 and GONST2 complemented both phenotypes, indicating that GONST2, like the previously characterized GONST1, is a GDP-mannose transporter. GONST3, 4 and 5 also rescued the antibiotic sensitivity, but not the chitinase glycosylation defect, suggesting that they can also transport GDP-mannose across the yeast Golgi membrane but with a lower efficiency. RT-PCR and analysis of Affymetrix data revealed partially overlapping patterns of expression of GONST1-5 in a variety of organs. Because of the differences in ability to rescue the vrg4 - 2 phenotype, and the different expression patterns in plant organs, we speculate that GONST1 and GONST2 are both GDP-mannose transporters, whereas GONST3, GONST4 and GONST5 may transport other nucleotide-sugars in planta.

Role of glycosylation in development

Researchers have long predicted that complex carbohydrates on cell surfaces would play important roles in developmental processes because of the observation that specific carbohydrate structures appear in specific spatial and temporal patterns throughout development. The astounding number and complexity of carbohydrate structures on cell surfaces added support to the concept that glycoconjugates would function in cellular communication during development. Although the structural complexity inherent in glycoconjugates has slowed advances in our understanding of their functions, the complete sequencing of the genomes of organisms classically used in developmental studies (e.g., mice, Drosophila melanogaster, and Caenorhabditis elegans) has led to demonstration of essential functions for a number of glycoconjugates in developmental processes. Here we present a review of recent studies analyzing function of a variety of glycoconjugates (O-fucose, O-mannose, N-glycans, mucin-type O-glycans, proteoglycans, glycosphingolipids), focusing on lessons learned from human disease and genetic studies in mice, D. melanogaster, and C. elegans.

Identification of 315 genes essential for early zebrafish development

We completed a large insertional mutagenesis screen in zebrafish to identify genes essential for embryonic and early larval development. We isolated 525 mutants, representing lesions in approximately 390 different genes, and we cloned the majority of these. Here we describe 315 mutants and the corresponding genes. Our data suggest that there are roughly 1,400 embryonic-essential genes in the fish. Thus, we have mutations in approximately 25% of these genes and have cloned approximately 22% of them. Re-screens of our collection to identify mutants with specific developmental defects suggest that approximately 50 genes are essential for the development of some individual organs or cell types. Seventy-two percent of the embryonic-essential fish genes have homologues in yeast, 93% have homologues in invertebrates (fly or worm), and 99% have homologues in human. Yeast and worm orthologues of genes that are essential for early zebrafish development have a strong tendency to be essential for viability in yeast and for embryonic development in the worm. Thus, the trait of being a genetically essential gene is conserved in evolution. This mutant collection should be a valuable resource for diverse studies of cell and developmental biology.

Differential sulfations and epimerization define heparan sulfate specificity in nervous system development

Heparan sulfate proteoglycans (HSPG) are components of the extracellular matrix through which axons navigate to reach their targets. The heparan sulfate (HS) side chains of HSPGs show complex and differentially regulated patterns of secondary modifications, including sulfations of distinct hydroxyl groups and epimerization of an asymmetric carbon atom. These modifications endow the HSPG-containing extracellular matrix with the potential to code for an enormous molecular diversity. Attempting to decode this diversity, we analyzed C. elegans animals lacking three HS-modifying enzymes, glucuronyl C5-epimerase, heparan 6O-sulfotransferase, and 2O-sulfotransferase. Each of the mutant animals exhibit distinct as well as overlapping axonal and cellular guidance defects in specific neuron classes. We have linked individual HS modifications to two specific guidance systems, the sax-3/Robo and kal-1/Anosmin-1 systems, whose activity is dependent on different HS modifications in different cellular contexts. Our results demonstrate that the molecular diversity in HS encodes information that is crucial for different aspects of neuronal development.

Molecular physiology and pathology of the nucleotide sugar transporter family (SLC35)

The solute carrier family SLC35 consists of at least 17 molecular species in humans. The family members so far characterized encode nucleotide sugar transporters localizing at the Golgi apparatus and/or the endoplasmic reticulum (ER). These transporters transport nucleotide sugars pooled in the cytosol into the lumen of these organelles, where most glycoconjugate synthesis occurs. Pathological analyses and developmental studies of small, multicellular organisms deficient in nucleotide sugar transporters have shown these transporters to be involved in tumour metastasis, cellular immunity, organogenesis and morphogenesis. Leukocyte adhesion deficiency type II (LAD II) or the congenital disorder of glycosylation type IIc (CDG IIc) are the sole human congenital disorders known to date that are caused by a defect of GDP-fucose transport. Along with LAD II, the possible involvement of nucleotide sugar transporters in disorders of connective tissues and muscles is also discussed.

N ‐glycosylation‐dependent block is a novel mechanism for drug‐induced cardiac arrhythmia

Voltage-gated potassium channels formed with the cardiac subunit HERG and a polymorphic variant of MinK-related peptide 1 (MiRP1) exhibit increased susceptibility to the antibiotic sulfamethoxazole (SMX) compared with channels formed with wild-type (WT) subunits. Here the molecular bases for SMX high-affinity block are investigated. The polymorphism causes a benign T to A amino acid mutation at position 8 (T8A) that destroys an N-glycosylation site of MiRP1. In vitro disruption of glycosylation by mutagenesis or in vivo by treatment with neuraminidase is associated with increased susceptibility to SMX and to other elementary agents such as divalent cations. Defective glycosylation does not affect the ability of T8A to form stable complexes with HERG, but rather it increases drug susceptibility through structural modifications in the channel complex. We conclude that N-glycosylation may play a key role in the etiology of life-threatening arrhythmia.

Caenorhabditis elegans early embryogenesis and vulval morphogenesis require chondroitin biosynthesis

Defects in glycosaminoglycan biosynthesis disrupt animal development and can cause human disease. So far much of the focus on glycosaminoglycans has been on heparan sulphate. Mutations in eight squashed vulva (sqv) genes in Caenorhabditis elegans cause defects in cytokinesis during embryogenesis and in vulval morphogenesis during postembryonic development. Seven of the eight sqv genes have been shown to control the biosynthesis of the glycosaminoglycans chondroitin and heparan sulphate. Here we present the molecular identification and characterization of the eighth gene, sqv-5. This gene encodes a bifunctional glycosyltransferase that is probably localized to the Golgi apparatus and is responsible for the biosynthesis of chondroitin but not heparan sulphate. Our findings show that chondroitin is crucial for both cytokinesis and morphogenesis during C. elegans development.

Identification and characterization of a Drosophila melanogaster ortholog of human beta1,4-galactosyltransferase VII

Drosophila melanogaster is widely considered to be an attractive model organism for studying the functions of the carbohydrate moieties of glycoconjugates produced by higher eukaryotes. However, the pathways of glycoconjugate biosynthesis are not as well defined in insects as they are in higher eukaryotes. One way to address this problem is to identify genes in the Drosophila genome that might encode relevant functions, express them, and determine the functions of the gene products by direct biochemical assays. In this study, we used this approach to identify a putative Drosophila beta4-galactosyltransferase gene and determine the enzymatic activity of its product. Biochemical assays demonstrated that this gene product could transfer galactose from UDP-galactose to a beta-xylosyl acceptor, but not to other acceptors in vitro. The apparent K(m) values for the donor and acceptor substrates indicated that this gene product is a functional galactosyltransferase. Additional assays showed that the enzyme is activated by manganese, has a slightly acidic pH optimum, and is localized in the insect cell Golgi apparatus. These results showed that Drosophila encodes an ortholog of human beta4-galactosyltransferase-VII, also known as galactosyltransferase I, which participates in proteoglycan biosynthesis by transferring the first galactose to xylose in the linkage tetrasaccharide of glycosaminoglycan side chains.

The Caenorhabditis elegans genes sqv-2 and sqv-6, which are required for vulval morphogenesis, encode glycosaminoglycan galactosyltransferase II and xylosyltransferase

In mutants defective in any of eight Caenorhabditis elegans sqv (squashed vulva) genes, the vulval extracellular space fails to expand during vulval morphogenesis. Strong sqv mutations result in maternal-effect lethality, caused in part by the failure of the progeny of homozygous mutants to initiate cytokinesis and associated with the failure to form an extracellular space between the egg and the eggshell. Recent studies have implicated glycosaminoglycans in these processes. Here we report the cloning and characterization of sqv-2 and sqv-6. sqv-6 encodes a protein similar to human xylosyltransferases. Transfection of sqv-6 restored xylosyltransferase activity to and rescued the glycosaminoglycan biosynthesis defect of a xylosyltransferase mutant hamster cell line. sqv-2 encodes a protein similar to human galactosyltransferase II. A recombinant SQV-2 fusion protein had galactosyltransferase II activity with substrate specificity similar to that of human galactosyltransferase II. We conclude that C. elegans SQV-6 and SQV-2 likely act in concert with other SQV proteins to catalyze the stepwise formation of the proteoglycan core protein linkage tetrasaccharide GlcAbeta1,3Galbeta1, 3Galbeta1,4Xylbeta-O-(Ser), which is common to the two major types of glycosaminoglycans in vertebrates, chondroitin and heparan sulfate. Our results strongly support a model in which C. elegans vulval morphogenesis and zygotic cytokinesis depend on the expression of glycosaminoglycans.

Protein glycosylation in disease: new insights into the congenital muscular dystrophies

Glycosylation is the most frequent modification of proteins and is important for many ligand-receptor interactions. Recently, defects in protein glycosylation have been linked to several forms of congenital muscular dystrophy that are frequently associated with brain abnormalities. Muscle-eye-brain disease and Walker-Warburg syndrome are caused by mutations in enzymes involved in O-mannosylation, whereas Fukuyama congenital muscular dystrophy and congenital muscular dystrophy type 1C are caused by mutations in genes that encode putative glycosyltransferases. The common factor in these disorders is defective processing and maturation of a protein called alpha-dystroglycan. This is thought to disrupt the link between alpha-dystroglycan and components of the extracellular matrix, and result in muscle disease and, in many cases, a neuronal-migration disorder.

Identification and characterization of three Drosophila melanogaster glucuronyltransferases responsible for the synthesis of the conserved glycosaminoglycan-protein linkage region of proteoglycans. Two novel homologs exhibit broad specificity toward oligosaccharides from proteoglycans, glycoproteins, and glycosphingolipids

The Drosophila melanogaster genome contains three putative glucuronyltransferases homologous to human GlcAT-I and GlcAT-P. These enzymes are predicted to be beta1,3-glucuronyltransferases involved in the synthesis of the glycosaminoglycan (GAG)-protein linkage region of proteoglycans and the HNK-1 carbohydrate epitope of glycoproteins, respectively. The genes encode active enzymes, which we have designated DmGlcAT-I, DmGlcAT-BSI, and DmGlcAT-BSII (where BS stands for "broad specificity"). Protein A-tagged truncated soluble forms of all three enzymes efficiently transfer GlcUA from UDP-GlcUA to the linkage region trisaccharide Galbeta1-3Galbeta1-4Xyl. Strikingly, DmGlcAT-I has specificity for Galbeta1-3Galbeta1-4Xyl, whereas DmGlcAT-BSI and DmGlcAT-BSII act on a wide array of substrates with non-reducing terminal beta1,3- and beta1,4-linked Gal residues. Their highest activities are obtained with asialoorosomucoid with a terminal Galbeta1-4GlcNAc sequence, indicating their possible involvement in the synthesis of the HNK-1 epitope in addition to the GAG-protein linkage region. Galbeta1-3GlcNAc and Galbeta1-3GalNAc, disaccharide structures widely found in N- and O-glycans of glycoproteins and glycolipids, also serve as acceptors for DmGlcAT-BSI and -BSII. Transcripts of all three enzymes are ubiquitously expressed throughout the developmental stages and in adult tissues of Drosophila. Thus, all three glucuronyltransferases are likely involved in the synthesis of the GAG-protein linkage region in Drosophila, and DmGlcAT-BSI and -BSII appear to be involved in various GlcUA transfer reactions for the synthesis of proteoglycans, glycoproteins, and glycolipids. This activity distinguishes these glucuronyltransferases from their mammalian homologs GlcAT-P and GlcAT-D (or -S). Sequence alignment of the Drosophila glucuronyltransferases with homologs in human, rat, and Caenorhabditis elegans demonstrates the conservation of a majority of the critical amino acid residues in the active sites of the three Drosophila enzymes.