Characterization of the let-653 gene in Caenorhabditis elegans

A C. elegans Zona Pellucida domain protein functions via its ZPc domain

Zona Pellucida domain (ZP) proteins are critical components of the body's external-most protective layers, apical extracellular matrices (aECMs). Although their loss or dysfunction is associated with many diseases, it remains unclear how ZP proteins assemble in aECMs. Current models suggest that ZP proteins polymerize via their ZPn subdomains, while ZPc subdomains modulate ZPn behavior. Using the model organism C. elegans, we investigated the aECM assembly of one ZP protein, LET-653, which shapes several tubes. Contrary to prevailing models, we find that LET-653 localizes and functions via its ZPc domain. Furthermore, we show that ZPc domain function requires cleavage at the LET-653 C-terminus, likely in part to relieve inhibition of the ZPc by the ZPn domain, but also to promote some other aspect of ZPc domain function. In vitro, the ZPc, but not ZPn, domain bound crystalline aggregates. These data offer a new model for ZP function whereby the ZPc domain is primarily responsible for matrix incorporation and tissue shaping.

Functional Ultrastructure of the Excretory Gland Cell in Zoonotic Anisakids (Anisakidae, Nematoda)

Excretory and secretory products are crucial for parasite infectivity and host immunomodulation, but the functioning and ultrastructure of the excretory gland cell (EC) that produces these products are still scarcely understood and described. In light of growing reports on anisakiasis cases in Europe, we aimed to characterise the EC of larval Anisakispegreffii and adult Pseudoterranovaazarasi. In the latter, EC starts 0.85 mm from the head tip, measuring 1.936 × 0.564 mm. Larval EC shows a long nucleus with thorn-like extravaginations toward the cytoplasm, numerous electron-dense and -lucent secretory granules spanning from the perinuclear to subplasmalemmal space, an elevated number of free ribosomes, small, spherical mitochondria with few cristae and a laminated matrix, small and few Golgi apparatuses, and few endoplasmic reticula, with wide cisternae complexes. Ultrastructure suggests that anaerobic glycolysis is the main metabolic pathway, obtained through nutrient endocytosis across the pseudocoelomic surface of the EC plasmalemma and its endocytic canaliculi. Thorn-like extravaginations of EC karyotheca likely mediate specific processes (Ca2+ signaling, gene expression, transport, nuclear lipid metabolism) into the extremely wide EC cytosol, enabling focal delivery of a signal to specific sites in a short time. These functional annotations of parasitic EC should help to clarify anisakiasis pathogenesis.

Tubular Excretory Canal Structure Depends on Intermediate Filaments EXC-2 and IFA-4 in Caenorhabditis elegans

The excretory canals of Caenorhabditis elegans are a model for understanding the maintenance of apical morphology in narrow single-celled tubes. Light and electron microscopy shows that mutants in exc-2 start to form canals normally, but these swell to develop large fluid-filled cysts that lack a complete terminal web at the apical surface, and accumulate filamentous material in the canal lumen. Here, whole-genome sequencing and gene rescue show that exc-2 encodes intermediate filament protein IFC-2 EXC-2/IFC-2 protein, fluorescently tagged via clustered regularly interspaced short palindromic repeats/Cas9, is located at the apical surface of the canals independently of other intermediate filament proteins. EXC-2 is also located in several other tissues, though the tagged isoforms are not seen in the larger intestinal tube. Tagged EXC-2 binds via pulldown to intermediate filament protein IFA-4, which is also shown to line the canal apical surface. Overexpression of either protein results in narrow but shortened canals. These results are consistent with a model whereby three intermediate filaments in the canals-EXC-2, IFA-4, and IFB-1-restrain swelling of narrow tubules in concert with actin filaments that guide the extension and direction of tubule outgrowth, while allowing the tube to bend as the animal moves.

The Caenorhabditis elegans Excretory System: A Model for Tubulogenesis, Cell Fate Specification, and Plasticity

The excretory system of the nematode Caenorhabditis elegans is a superb model of tubular organogenesis involving a minimum of cells. The system consists of just three unicellular tubes (canal, duct, and pore), a secretory gland, and two associated neurons. Just as in more complex organs, cells of the excretory system must first adopt specific identities and then coordinate diverse processes to form tubes of appropriate topology, shape, connectivity, and physiological function. The unicellular topology of excretory tubes, their varied and sometimes complex shapes, and the dynamic reprogramming of cell identity and remodeling of tube connectivity that occur during larval development are particularly fascinating features of this organ. The physiological roles of the excretory system in osmoregulation and other aspects of the animal's life cycle are only beginning to be explored. The cellular mechanisms and molecular pathways used to build and shape excretory tubes appear similar to those used in both unicellular and multicellular tubes in more complex organs, such as the vertebrate vascular system and kidney, making this simple organ system a useful model for understanding disease processes.

Integrity of Narrow Epithelial Tubes in the C. elegans Excretory System Requires a Transient Luminal Matrix

Most epithelial cells secrete a glycoprotein-rich apical extracellular matrix that can have diverse but still poorly understood roles in development and physiology. Zona Pellucida (ZP) domain glycoproteins are common constituents of these matrices, and their loss in humans is associated with a number of diseases. Understanding of the functions, organization and regulation of apical matrices has been hampered by difficulties in imaging them both in vivo and ex vivo. We identified the PAN-Apple, mucin and ZP domain glycoprotein LET-653 as an early and transient apical matrix component that shapes developing epithelia in C. elegans. LET-653 has modest effects on shaping of the vulva and epidermis, but is essential to prevent lumen fragmentation in the very narrow, unicellular excretory duct tube. We were able to image the transient LET-653 matrix by both live confocal imaging and transmission electron microscopy. Structure/function and fluorescence recovery after photobleaching studies revealed that LET-653 exists in two separate luminal matrix pools, a loose fibrillar matrix in the central core of the lumen, to which it binds dynamically via its PAN domains, and an apical-membrane-associated matrix, to which it binds stably via its ZP domain. The PAN domains are both necessary and sufficient to confer a cyclic pattern of duct lumen localization that precedes each molt, while the ZP domain is required for lumen integrity. Ectopic expression of full-length LET-653, but not the PAN domains alone, could expand lumen diameter in the developing gut tube, where LET-653 is not normally expressed. Together, these data support a model in which the PAN domains regulate the ability of the LET-653 ZP domain to interact with other factors at the apical membrane, and this ZP domain interaction promotes expansion and maintenance of lumen diameter. These data identify a transient apical matrix component present prior to cuticle secretion in C. elegans, demonstrate critical roles for this matrix component in supporting lumen integrity within narrow bore tubes such as those found in the mammalian microvasculature, and reveal functional importance of the evolutionarily conserved ZP domain in this tube protecting activity.

Most organs in the body are made up of networks of tubes that transport fluids or gases. These tubes come in many different sizes and shapes, with some narrow capillaries being only one cell in diameter. As tubes develop and take their final shapes, they secrete various glycoproteins into their hollow interior or lumen. The functions of these luminal proteins are not well understood, but there is increasing evidence that they are important for lumen shaping and that their loss can contribute to diseases such as cardiovascular disease and chronic kidney disease. Through studies of the nematode C. elegans, we identified a luminal glycoprotein, LET-653, that is transiently expressed in multiple developing tube types but is particularly critical to maintain integrity of the narrowest, unicellular tubes. We identified protein domains that direct LET-653 to specific apical matrix compartments and mediate its oscillatory pattern of lumen localization. Furthermore, we showed that the LET-653 tube-protecting activity depends on a Zona Pellucida (ZP) domain similar to that found in the mammalian egg-coat and in many other luminal or sensory matrix proteins involved in human disease.

Time to make the doughnuts: Building and shaping seamless tubes

A seamless tube is a very narrow-bore tube that is composed of a single cell with an intracellular lumen and no adherens or tight junctions along its length. Many capillaries in the vertebrate vascular system are seamless tubes. Seamless tubes also are found in invertebrate organs, including the Drosophila trachea and the Caenorhabditis elegans excretory system. Seamless tube cells can be less than a micron in diameter, and they can adopt very simple "doughnut-like" shapes or very complex, branched shapes comparable to those of neurons. The unusual topology and varied shapes of seamless tubes raise many basic cell biological questions about how cells form and maintain such structures. The prevalence of seamless tubes in the vascular system means that answering such questions has significant relevance to human health. In this review, we describe selected examples of seamless tubes in animals and discuss current models for how seamless tubes develop and are shaped, focusing particularly on insights that have come from recent studies in Drosophila and C. elegans.

Intracellular lumen extension requires ERM-1-dependent apical membrane expansion and AQP-8-mediated flux

Many unicellular tubes such as capillaries form lumens intracellularly, a process that is not well understood. Here we show that the cortical membrane organizer ERM-1 is required to expand the intracellular apical/lumenal membrane and its actin undercoat during single-cell C.elegans excretory canal morphogenesis. We characterize AQP-8, identified in an ERM-1 overexpression (ERM-1[++]) suppressor screen, as a canalicular aquaporin that interacts with ERM-1 in lumen extension in a mercury-sensitive manner, implicating water-channel activity. AQP-8 is transiently recruited to the lumen by ERM-1, co-localizing in peri-lumenal cuffs interspaced along expanding canals. An ERM-1[++]-mediated increase in the number of lumen-associated canaliculi is reversed by AQP-8 depletion. We propose that the ERM-1-AQP-8 interaction propels lumen extension by translumenal flux, suggesting a direct morphogenetic effect of water-channel-regulated fluid pressure.

A luminal glycoprotein drives dose-dependent diameter expansion of the Drosophila melanogaster hindgut tube

An important step in epithelial organ development is size maturation of the organ lumen to attain correct dimensions. Here we show that the regulated expression of Tenectin (Tnc) is critical to shape the Drosophila melanogaster hindgut tube. Tnc is a secreted protein that fills the embryonic hindgut lumen during tube diameter expansion. Inside the lumen, Tnc contributes to detectable O-Glycans and forms a dense striated matrix. Loss of tnc causes a narrow hindgut tube, while Tnc over-expression drives tube dilation in a dose-dependent manner. Cellular analyses show that luminal accumulation of Tnc causes an increase in inner and outer tube diameter, and cell flattening within the tube wall, similar to the effects of a hydrostatic pressure in other systems. When Tnc expression is induced only in cells at one side of the tube wall, Tnc fills the lumen and equally affects all cells at the lumen perimeter, arguing that Tnc acts non-cell-autonomously. Moreover, when Tnc expression is directed to a segment of a tube, its luminal accumulation is restricted to this segment and affects the surrounding cells to promote a corresponding local diameter expansion. These findings suggest that deposition of Tnc into the lumen might contribute to expansion of the lumen volume, and thereby to stretching of the tube wall. Consistent with such an idea, ectopic expression of Tnc in different developing epithelial tubes is sufficient to cause dilation, while epidermal Tnc expression has no effect on morphology. Together, the results show that epithelial tube diameter can be modelled by regulating the levels and pattern of expression of a single luminal glycoprotein.

Epithelial tubes constitute the functional units of vital organs, and they undergo highly regulated changes in size and shape during development to accommodate the three-dimensional configurations optimal for organ physiology. Through studies of Drosophila melanogaster, we show that epithelial tube diameter can be modelled simply by regulating the levels and pattern of expression of a single glycoprotein. The protein is secreted into the tubular lumen, where it forms a dense matrix and acts in a dose-dependent manner to drive diameter growth. We suggest that deposition of the protein into the lumen promotes local expansion of the lumen volume, and thereby stretching of the surrounding tube wall. Such a mechanism could represent a general means to adjust tube diameter during epithelial organ development.

Vascular lumen formation: negativity will tear us apart

Functional blood vessels are essential for vertebrate development, but how endothelial cells initiate lumen formation during vasculogenesis is not known. A new study now reveals that electrostatic repulsion is key.

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.

Electrostatic cell-surface repulsion initiates lumen formation in developing blood vessels

Blood vessels function in the uptake, transport, and delivery of gases and nutrients within the body. A key question is how the central lumen of blood vessels develops within a cord of vascular endothelial cells. Here, we demonstrate that sialic acids of apical glycoproteins localize to apposing endothelial cell surfaces and generate repelling electrostatic fields within an endothelial cell cord. Both in vitro and in vivo experiments show that the negative charge of sialic acids is required for the separation of endothelial cell surfaces and subsequent lumen formation. We also demonstrate that sulfate residues can substitute for sialic acids during lumen initiation. These results therefore reveal a key step in the creation of blood vessels, the most abundant conduits in the vertebrate body. Because negatively charged mucins and proteoglycans are often found on luminal cell surfaces, it is possible that electrostatic repulsion is a general principle also used to initiate lumen formation in other organs.

Cellular and molecular mechanisms underlying the formation of biological tubes

Biological tubes are integral components of many organs. Based on their cellular organization, tubes can be divided into three types: multicellular, unicellular, and intracellular. The mechanisms by which these tubes form during development vary significantly, in many cases even for those sharing a similar final architecture. Here, we present recent advances in studying cellular and molecular aspects of tubulogenesis in different organisms.

CRIP homologues maintain apical cytoskeleton to regulate tubule size in C. elegans

Maintenance of the shape and diameter of biological tubules is a critical task in the development and physiology of all metazoan organisms. We have cloned the exc-9 gene of Caenorhabditis elegans, which regulates the diameter of the single-cell excretory canal tubules. exc-9 encodes a homologue of the highly expressed mammalian intestinal LIM-domain protein CRIP, whose function has not previously been determined. A second well-conserved CRIP homologue functions in multiple valves of C. elegans. EXC-9 shows genetic interactions with other EXC proteins, including the EXC-5 guanine exchange factor that regulates CDC-42 activity. EXC-9 and its nematode homologue act in polarized epithelial cells that must maintain great flexibility at their apical surface; our results suggest that CRIPs function to maintain cytoskeletal flexibility at the apical surface.

Transcriptional regulation of AQP-8, a Caenorhabditis elegans aquaporin exclusively expressed in the excretory system, by the POU homeobox transcription factor CEH-6

Due to the ever changing environmental conditions in soil, regulation of osmotic homeostasis in the soil-dwelling nematode Caenorhabditis elegans is critical. AQP-8 is a C. elegans aquaporin that is expressed in the excretory cell, a renal equivalent tissue, where the protein participates in maintaining water balance. To better understand the regulation of AQP-8, we undertook a promoter analysis to identify the aqp-8 cis-regulatory elements. Using progressive 5' deletions of upstream sequence, we have mapped an essential regulatory region to roughly 300 bp upstream of the translational start site of aqp-8. Analysis of this region revealed a sequence corresponding to a known DNA functional element (octamer motif), which interacts with POU homeobox transcription factors. Phylogenetic footprinting showed that this site is perfectly conserved in four nematode species. The octamer site's function was further confirmed by deletion analyses, mutagenesis, functional studies, and electrophoretic mobility shift assays. Of the three POU homeobox proteins encoded in the C. elegans genome, CEH-6 is the only member that is expressed in the excretory cell. We show that expression of AQP-8 is regulated by CEH-6 by performing RNA interference experiments. CEH-6's mammalian ortholog, Brn1, is expressed both in the kidney and the central nervous system and binds to the same octamer consensus binding site to drive gene expression. These parallels in transcriptional control between Brn1 and CEH-6 suggest that C. elegans may well be an appropriate model for determining gene-regulatory networks in the developing vertebrate kidney.

Systematic Analysis of proteoglycan modification sites in Caenorhabditis elegans by scanning mutagenesis

Proteoglycan modification is essential for development and early cell division in Caenorhabditis elegans. The specification of proteoglycan attachment sites is defined by the Golgi enzyme polypeptide xylosyltransferase. Here we evaluate the substrate specificity of this xylosyltransferase for its downstream targets by using reporter proteins containing proteoglycan modification sites from C. elegans syndecan/SDN-1. The N terminus of the SDN-1 contains a Ser-Gly proteoglycan site at Ser(71), flanked by potential mucin and N-glycosylation sites. However, Ser(71) was exclusively used as a proteoglycan site in vivo, based on mapping studies with a Ser(71) reporter protein, glycosyltransferase RNA interference, and co-expression of worm polypeptide xylosyltransferase. To elucidate the substrate requirements of this enzyme, a library of 42 point mutants of the Ser(71) reporter was expressed in tissue culture. The nematode proteoglycan modification site in SDN-1 required serine (not threonine), two flanking glycine residues (positions -1 and +1), and either one proximal acidic N-terminal amino acid (positions -4, -3, and -2) or a pair of distal N-terminal acidic amino acids (positions -6 and -5). C-terminal acidic amino acids, although present in many proteoglycan modification sites, had minimal impact on xylosylation at Ser(71). Proline inhibited glycosylation when present at -1, +1, or +2. The position of glycine, proline, and acidic amino acids allows the glycosylation machinery to discriminate between mucin and proteoglycan modification sites. The key residues that define proteoglycan modification sites also function with the Drosophila polypeptide xylosyltransferase, indicating that the specificity in the glycosylation process is evolutionarily conserved. Using a neural network method, a preliminary proteoglycan predictor has been developed.

Distinct regulatory elements mediate similar expression patterns in the excretory cell of Caenorhabditis elegans

Identification of cis-regulatory elements and their binding proteins constitutes an important part of understanding gene function and regulation. It is well accepted that co-expressed genes tend to share transcriptional elements. However, recent findings indicate that co-expression data show poor correlation with co-regulation data even in unicellular yeast. This motivates us to experimentally explore whether it is possible that co-expressed genes are subject to differential regulatory control using the excretory cell of Caenorhabditis elegans as an example. Excretory cell is a functional equivalent of human kidney. Transcriptional regulation of gene expression in the cell is largely unknown. We isolated a 10-bp excretory cell-specific cis-element, Ex-1, from a pgp-12 promoter. The significance of the element has been demonstrated by its capacity of converting an intestine-specific promoter into an excretory cell-specific one. We also isolated a cDNA encoding an Ex-1 binding transcription factor, DCP-66, using a yeast one-hybrid screen. Role of the factor in regulation of pgp-12 expression has been demonstrated both in vitro and in vivo. Search for occurrence of Ex-1 reveals that only a small portion of excretory cell-specific promoters contain Ex-1. Two other distinct cis-elements isolated from two different promoters can also dictate the excretory cell-specific expression but are independent of regulation by DCP-66. The results indicate that distinct regulatory elements are able to mediate the similar expression patterns.

A family of genes encoding zona pellucida (ZP) domain proteins is expressed in various epithelial tissues during Drosophila embryogenesis

Zona pellucida (ZP) domain proteins have been identified in various species from worms to humans. Most of the characterized ZP family members are secreted or remain anchored to the plasma membrane where they play a structural role and/or act as receptors. In humans, several ZP proteins attracted attention because of their abundant expression in certain organs and their relation to various diseases. Here, we compare the molecular architecture and embryonic expression pattern of the 18 genes encoding ZP proteins in Drosophila melanogaster. Only five of these genes have been genetically characterized. All ZP genes are expressed in the embryo in epithelial tissues, such as the foregut, the hindgut, the Malpighian tubules, the salivary glands, the tracheal system, sensory organs and epidermis. Five genes are expressed during oogenesis; two of them are transcribed in the follicular epithelium, but not in the germ line cells.

The role of the ELAV homologue EXC-7 in the development of the Caenorhabditis elegans excretory canals

The exc mutations of Caenorhabditis elegans alter the position and shape of the apical cytoskeleton in polarized epithelial cells. Mutants in exc-7 form small cysts throughout the tubular excretory canals that regulate organismal osmolarity. We have cloned the exc-7 gene, the closest nematode homologue to the neural RNA-binding protein ELAV. EXC-7 is expressed in the canal for a short time midway through embryogenesis. Cysts in exc-7 mutants do not develop until several hours later, beginning at the time of hatching. We find that the first larval period is when the canal completes the majority of its outgrowth, and adds new apical cytoskeleton at a rapid rate. Ultrastructural studies show that exc-7 mutant defects resemble loss of beta(H)-spectrin (encoded by sma-1) at the distal ends of the excretory canals. In addition, exc-7 mutants exhibit synergistic excretory canal defects with mutations in sma-1, and EXC-7 binds sma-1 mRNA. These data imply that EXC-7 protein may affect expression of sma-1 and other genes to effect proper development of the excretory canals.

Tube morphogenesis: making and shaping biological tubes

Many organs are composed of epithelial tubes that transport vital fluids. Such tubular organs develop in many different ways and generate tubes of widely varying sizes and structures, but always with the apical epithelial surface lining the lumen. We describe recent progress in several diverse cell culture and genetic models of tube morphogenesis, which suggest apical membrane biogenesis, vesicle fusion, and secretion play central roles in tube formation and growth. We propose a unifying mechanism of tube morphogenesis that has been modified to create tube diversity and describe how defects in the tube size-sensing step can lead to polycystic kidney disease.

Complementary expression patterns of six nonessential Caenorhabditis elegans core 2/I N-acetylglucosaminyltransferase homologues

The Caenorhabditis elegans genome contains 18 sequences related to mammalian core 2/I N-acetylglucosaminyltransferases. The six most closely related genes (gly-1 and gly-15 to gly-19) likely encode active enzymes, because are all transcribed and do not appear to be pseudogenes. Polypeptide divergence and the gene structures are both concordant with a common ancestor at the time of radiation from mammals that underwent three rounds of duplication and, most recently, a tandem duplication. Polypeptide alignments with mammalian homologues do not indicate whether the enzyme specificities are core 2, 4, or I-like or novel, but do clearly demonstrate the secondary structure characteristics of glycosyltransferases. The six homologues have essentially nonoverlapping expression patterns, unrelated by tissue type or cell lineage. The extent varies widely; gly-15 is expressed only in two gland cells, whereas gly-18 is broadly expressed in diverse cell types. gly-1, -15, -18 and -19 are expressed during adulthood; gly-16 and gly-17 appear to be restricted to embryonic or early larval stages. The parsimonious interpretation of the expression pattern and sequence data is that the catalytic activities are similar but with diverged promoters. Null alleles of three of the genes were generated without causing gross abnormality in homozygous animals. RNA-mediated interference experiments also failed to induce defects in the four genes tested. Nevertheless, the nematode has evolved six diverged core 2 GlcNAc-T-like genes, and we postulate that these arose in response to selection pressures to which C. elegans is not ordinarily subjected in the laboratory.

The nematode Caenorhabditis elegans synthesizes unusual O-linked glycans: identification of glucose-substituted mucin-type O-glycans and short chondroitin-like oligosaccharides

The free-living nematode Caenorhabditis elegans is a relevant model for studies on the role of glycoconjugates during development of multicellular organisms. Several genes coding for glycosyltransferases involved in the synthesis of N- and O-linked glycans have already been isolated, but, apart from repetitive dimers of glycosaminoglycans, no detailed structure of either type of component has been published so far. This study aimed to establish the structures of the major O-glycans synthesized by C. elegans to give an insight into the endogenous glycosyltransferase activities expressed in this organism. By the use of NMR and MS, we have resolved the sequence of seven of these components that present very unusual features. Most of them were characterized by the type-1 core substituted on Gal and/or GalNAc by (beta1-4)Glc and (beta1-6)Glc residues. Another compound exhibited the GalNAc(beta1-4)N-acetylglucosaminitol sequence in the terminal position, to which was attached a tetramer of beta-Gal substituted by both Fuc and 2-O-methyl-fucose residues. Our experimental procedure led also to the isolation of glycosaminoglycan-like components and oligomannosyl-type N-glycans. In particular, the data confirmed that C. elegans synthesizes the ubiquitous linker sequence GlcA(beta1-3)Gal(beta1-3)Gal(beta1-4)Xyl.

nompA encodes a PNS-specific, ZP domain protein required to connect mechanosensory dendrites to sensory structures

Mutations in the no-mechanoreceptor-potential A (nompA) gene, which eliminate transduction in Drosophila mechanosensory organs, disrupt contacts between neuronal sensory endings and cuticular structures. nompA encodes a transmembrane protein with a large, modular extracellular segment that includes a zona pellucida (ZP) domain and several plasminogen N-terminal (PAN) modules. It is specifically expressed in type I sense organs of the peripheral nervous system by the support cells that ensheath the neuronal sensory process. A green fluorescent protein (GFP)-NompA fusion protein is localized to the dendritic cap, an extracellular matrix that covers the ciliary outer segment of the sensory process and that shows organizational defects in nompA mutants. The structure and location of NompA suggest that it forms part of a mechanical linkage required to transmit mechanical stimuli to the transduction apparatus.

A family of secreted mucins from the parasitic nematode Toxocara canis bears diverse mucin domains but shares similar flanking six-cysteine repeat motifs

Infective larvae of the parasitic nematode Toxocara canis secrete a family of mucin-like glycoproteins, which are implicated in parasite immune evasion. Analysis of T. canis expressed sequence tags identified a family of four mRNAs encoding distinct apomucins (Tc-muc-1-4), one of which had been previously identified in the TES-120 family of glycoproteins secreted by this parasite. The protein products of all four cDNAs contain signal peptides, a repetitive serine/threonine-rich tract, and varying numbers of 36-amino acid six-cysteine (SXC) domains. SXC domains are found in many nematode proteins and show similarity to cnidarian (sea anemone) toxins. Antibodies to the SXC domains of Tc-MUC-1 and Tc-MUC-3 recognize differently migrating members of TES-120. TES-120 proteins separated by chromatographic methods showed distinct amino acid composition, mass, and sequence information by both Edman degradation and matrix-assisted laser desorption ionization/time of flight mass spectrometry on peptide fragments. Tc-MUC-1, -2, and -3 were shown to be secreted mucins with real masses of 39.7, 47.8, and 45.0 kDa in contrast to their predicted peptide masses of 15.7, 16.2, and 26.0 kDa, respectively. The presence of SXC domains in all mucin products supports the suggestion that the SXC motif is required for mucin assembly or export. Homology modeling indicates that the six-cysteine domains of the T. canis mucins adopt a similar fold to the sea anemone potassium channel-blocking toxin BgK, forming three disulfide bonds within each subunit.

Conservation and novelty in the evolution of cell adhesion and extracellular matrix genes

New proteins and modules have been invented throughout evolution. Gene "birth dates" in Caenorhabditis elegans range from the origins of cellular life through adaptation to a soil habitat. Possibly half are "metazoan" genes, having arisen sometime between the yeast-metazoan and nematode-chordate separations. These include basement membrane and cell adhesion molecules implicated in tissue organization. By contrast, epithelial surfaces facing the environment have specialized components invented within the nematode lineage. Moreover, interstitial matrices were likely elaborated within the vertebrate lineage. A strategy for concerted evolution of new gene families, as well as conservation of adaptive genes, may underlie the differences between heterochromatin and euchromatin.

The PAN module: the N-terminal domains of plasminogen and hepatocyte growth factor are homologous with the apple domains of the prekallikrein family and with a novel domain found in numerous nematode proteins

Based on homology search and structure prediction methods we show that (1) the N-terminal N domains of members of the plasminogen/hepatocyte growth factor family, (2) the apple domains of the plasma prekallikrein/coagulation factor XI family, and (3) domains of various nematode proteins belong to the same module superfamily, hereafter referred to as the PAN module. The patterns of conserved residues correspond to secondary structural elements of the known three-dimensional structure of hepatocyte growth factor N domain, therefore we predict a similar fold for all members of this superfamily. Based on available functional informations on apple domains and N domains, it is clear that PAN modules have significant functional versatility, they fulfill diverse biological functions by mediating protein-protein or protein-carbohydrate interactions.

Cystic canal mutants in Caenorhabditis elegans are defective in the apical membrane domain of the renal (excretory) cell

The excretory cell extends a tubular process, or canal, along the basolateral surface of the epidermis to form the nematode renal epithelium. This cell can undergo normal tubulogenesis in isolated cell culture. Mutations in 12 genes cause excretory canal cysts in Caenorhabditis elegans. Genetic interactions, and their similar phenotypes, suggest these genes may encode functionally related proteins. Depending upon genotype and individual canal, defects range from focal cysts, flanked by normal width segments, to regional cysts involving the entire tubule. Oftentimes the enlarged regions are convoluted or partially septated. In mutants with very large cysts, renal function is measurably impaired. Based on histology and ultrastructure, canal cysts likely result from defects of the apical membrane domain. These mutants provide a model of tubulocystic disease without hyperplasia or basement membrane abnormalities. Similar apical mechanisms could regulate tubular morphology of vertebrate nephrons.

Environmental induction and genetic control of surface antigen switching in the nematode Caenorhabditis elegans

Nematodes can alter their surface coat protein compositions at the molts between developmental stages or in response to environmental changes; such surface alterations may enable parasitic nematodes to evade host immune defenses during the course of infection. Surface antigen switching mechanisms are presently unknown. In a genetic study of surface antigen switching, we have used a monoclonal antibody, M37, that recognizes a surface antigen on the first larval stage of the free-living nematode Caenorhabditis elegans. We demonstrate that wild-type C. elegans can be induced to display the M37 antigen on a later larval stage by altering the growth conditions. Mutations that result in nonconditional display of this antigen on all four larval stages fall into two classes. One class defines the new gene srf-6 II. The other mutations are in previously identified dauer-constitutive genes involved in transducing environmental signals that modulate formation of the dauer larva, a developmentally arrested dispersal stage. Although surface antigen switching is affected by some of the genes that control dauer formation, these two process can be blocked separately by specific mutations or induced separately by environmental factors. Based on these results, the mechanisms of nematode surface antigen switching can now be investigated directly.

Prospects for rational approaches to anthelmintic discovery

Rational approaches to anthelmintic discovery include the design of screens for compounds directed at specific proteins in helminths that are pharmacologically distinguishable from their vertebrate homologues. The existence of several anthelmintics that selectively target the neuromusculature of helminths (e.g. levamisole, ivermectin, praziquantel, metrifonate), together with recent basic research in helminth physiology, have contributed to the recognition that neurobiology distinguishes these organisms from their vertebrate hosts. In this survey, we focus on mechanism-based screening and its application to anthelmintic discovery, with particular emphasis on targets in the neuromusculature of helminths. Few of these proteins have been exploited in chemotherapy. However, recent studies in comparative pharmacology and molecular biology, including the C. elegans genome project, have provided insights on potential new targets and, in some cases, molecular probes useful for their incorporation in mechanism-based screens.