Establishment of left/right asymmetry in neuroblast migration by UNC-40/DCC, UNC-73/Trio and DPY-19 proteins in C. elegans

The Pax transcription factor EGL-38 links EGFR signaling to assembly of a cell type-specific apical extracellular matrix in the Caenorhabditis elegans vulva

The surface of epithelial tissues is covered by an apical extracellular matrix (aECM). The aECMs of different tissues have distinct compositions to serve distinct functions, yet how a particular cell type assembles the proper aECM is not well understood. We used the cell type-specific matrix of the C. elegans vulva to investigate the connection between cell identity and matrix assembly. The vulva is an epithelial tube composed of seven cell types descending from EGFR/Ras-dependent (1°) and Notch-dependent (2°) lineages. Vulva aECM contains multiple Zona Pellucida domain (ZP) proteins, which are a common component of aECMs across life. ZP proteins LET-653 and CUTL-18 assemble on 1° cell surfaces, while NOAH-1 assembles on a subset of 2° surfaces. All three ZP genes are broadly transcribed, indicating that cell type-specific ZP assembly must be determined by features of the destination cell surface. The paired box (Pax) transcription factor EGL-38 promotes assembly of 1° matrix and prevents inappropriate assembly of 2° matrix, suggesting that EGL-38 promotes expression of one or more ZP matrix organizers. Our results connect the known signaling pathways and various downstream effectors to EGL-38/Pax expression and the ZP matrix component of vulva cell fate execution. We propose that dedicated transcriptional networks may contribute to cell-appropriate assembly of aECM in many epithelial organs.

The Pax transcription factor EGL-38 links EGFR signaling to assembly of a cell-type specific apical extracellular matrix in the Caenorhabditis elegans vulva

The surface of epithelial tissues is covered by an apical extracellular matrix (aECM). The aECMs of different tissues have distinct compositions to serve distinct functions, yet how a particular cell type assembles the proper aECM is not well understood. We used the cell-type specific matrix of the C. elegans vulva to investigate the connection between cell identity and matrix assembly. The vulva is an epithelial tube composed of seven cell types descending from EGFR/Ras-dependent (1°) and Notch-dependent (2°) lineages. Vulva aECM contains multiple Zona Pellucida domain (ZP) proteins, which are a common component of aECMs across life. ZP proteins LET-653 and CUTL-18 assemble on 1° cell surfaces, while NOAH-1 assembles on a subset of 2° surfaces. All three ZP genes are broadly transcribed, indicating that cell-type specific ZP assembly must be determined by features of the destination cell surface. The paired box (Pax) transcription factor EGL-38 promotes assembly of 1° matrix and prevents inappropriate assembly of 2° matrix, suggesting that EGL-38 promotes expression of one or more ZP matrix organizers. Our results connect the known signaling pathways and various downstream effectors to EGL-38/Pax expression and the ZP matrix component of vulva cell fate execution. We propose that dedicated transcriptional networks may contribute to cell-appropriate assembly of aECM in many epithelial organs.

A Bacterial Mannose Binding Lectin as a Tool for the Enrichment of C- and O-Mannosylated Peptides

Mass spectrometry (MS) easily detects C-mannosylated peptides from purified proteins but not from complex biological samples. Enrichment of specific glycopeptides by lectin affinity prior to MS analysis has been widely applied to support glycopeptide identification but was until now not available for C-mannosylated peptides. Here, we used the α-mannose-specific Burkholderia cenocepacia lectin A (BC2L-A) and show that, in addition to its previously demonstrated high-mannose N-glycan binding capability, this lectin is able to retain C- and O-mannosylated peptides. Besides testing binding abilities to standard peptides, we applied BC2L-A affinity to enrich C-mannosylated peptides from complex samples of tryptic digests of HEK293 and MCF10A whole cell extracts, which led to the identification of novel C-mannosylation sites. In conclusion, BC2L-A enabled specific enrichment of C- and O-mannosylated peptides and might have superior properties over other mannose binding lectins for this purpose.

Caenorhabditis elegans ETR-1/CELF has broad effects on the muscle cell transcriptome, including genes that regulate translation and neuroblast migration

Migration of neuroblasts and neurons from their birthplace is central to the formation of neural circuits and networks. ETR-1 is the Caenorhabditis elegans homolog of the CELF1 (CUGBP, ELAV-like family 1) RNA-processing factor involved in neuromuscular disorders. etr-1 regulates body wall muscle differentiation. Our previous work showed that etr-1 in muscle has a non-autonomous role in neuronal migration, suggesting that ETR-1 is involved in the production of a signal emanating from body wall muscle that controls neuroblast migration and that interacts with Wnt signaling. etr-1 is extensively alternatively-spliced, and we identified the viable etr-1(lq61) mutant, caused by a stop codon in alternatively-spliced exon 8 and only affecting etr-1 isoforms containing exon 8. We took advantage of viable etr-1(lq61) to identify potential RNA targets of ETR-1 in body wall muscle using a combination of fluorescence activated cell sorting (FACS) of body wall muscles from wild-type and etr-1(lq61) and subsequent RNA-seq. This analysis revealed genes whose splicing and transcript levels were controlled by ETR-1 exon 8 isoforms, and represented a broad spectrum of genes involved in muscle differentiation, myofilament lattice structure, and physiology. Genes with transcripts underrepresented in etr-1(lq61) included those involved in ribosome function and translation, similar to potential CELF1 targets identified in chick cardiomyocytes. This suggests that at least some targets of ETR-1 might be conserved in vertebrates, and that ETR-1 might generally stimulate translation in muscles. As proof-of-principle, a functional analysis of a subset of ETR-1 targets revealed genes involved in AQR and PQR neuronal migration. One such gene, lev-11/tropomyosin, requires ETR-1 for alternative splicing, and another, unc-52/perlecan, requires ETR-1 for the production of long isoforms containing 3' exons. In sum, these studies identified gene targets of ETR-1/CELF1 in muscles, which included genes involved in muscle development and physiology, and genes with novel roles in neuronal migration.

Protein C-Mannosylation and C-Mannosyl Tryptophan in Chemical Biology and Medicine

C-Mannosylation is a post-translational modification of proteins in the endoplasmic reticulum. Monomeric α-mannose is attached to specific Trp residues at the first Trp in the Trp-x-x-Trp/Cys (W-x-x-W/C) motif of substrate proteins, by the action of C-mannosyltransferases, DPY19-related gene products. The acceptor substrate proteins are included in the thrombospondin type I repeat (TSR) superfamily, cytokine receptor type I family, and others. Previous studies demonstrated that C-mannosylation plays critical roles in the folding, sorting, and/or secretion of substrate proteins. A C-mannosylation-defective gene mutation was identified in humans as the disease-associated variant affecting a C-mannosylation motif of W-x-x-W of ADAMTSL1, which suggests the involvement of defects in protein C-mannosylation in human diseases such as developmental glaucoma, myopia, and/or retinal defects. On the other hand, monomeric C-mannosyl Trp (C-Man-Trp), a deduced degradation product of C-mannosylated proteins, occurs in cells and extracellular fluids. Several studies showed that the level of C-Man-Trp is upregulated in blood of patients with renal dysfunction, suggesting that the metabolism of C-Man-Trp may be involved in human kidney diseases. Together, protein C-mannosylation is considered to play important roles in the biosynthesis and functions of substrate proteins, and the altered regulation of protein C-manosylation may be involved in the pathophysiology of human diseases. In this review, we consider the biochemical and biomedical knowledge of protein C-mannosylation and C-Man-Trp, and introduce recent studies concerning their significance in biology and medicine.

The CATP-8/P5A-type ATPase functions in multiple pathways during neuronal patterning

The assembly of neuronal circuits involves the migrations of neurons from their place of birth to their final location in the nervous system, as well as the coordinated growth and patterning of axons and dendrites. In screens for genes required for patterning of the nervous system, we identified the catp-8/P5A-ATPase as an important regulator of neural patterning. P5A-ATPases are part of the P-type ATPases, a family of proteins known to serve a conserved function as transporters of ions, lipids and polyamines in unicellular eukaryotes, plants, and humans. While the function of many P-type ATPases is relatively well understood, the function of P5A-ATPases in metazoans remained elusive. We show here, that the Caenorhabditis elegans ortholog catp-8/P5A-ATPase is required for defined aspects of nervous system development. Specifically, the catp-8/P5A-ATPase serves functions in shaping the elaborately sculpted dendritic trees of somatosensory PVD neurons. Moreover, catp-8/P5A-ATPase is required for axonal guidance and repulsion at the midline, as well as embryonic and postembryonic neuronal migrations. Interestingly, not all axons at the midline require catp-8/P5A-ATPase, although the axons run in the same fascicles and navigate the same space. Similarly, not all neuronal migrations require catp-8/P5A-ATPase. A CATP-8/P5A-ATPase reporter is localized to the ER in most, if not all, tissues and catp-8/P5A-ATPase can function both cell-autonomously and non-autonomously to regulate neuronal development. Genetic analyses establish that catp-8/P5A-ATPase can function in multiple pathways, including the Menorin pathway, previously shown to control dendritic patterning in PVD, and Wnt signaling, which functions to control neuronal migrations. Lastly, we show that catp-8/P5A-ATPase is required for localizing select transmembrane proteins necessary for dendrite morphogenesis. Collectively, our studies suggest that catp-8/P5A-ATPase serves diverse, yet specific, roles in different genetic pathways and may be involved in the regulation or localization of transmembrane and secreted proteins to specific subcellular compartments.

Roles of glycoconjugates in neural patterning in C. elegans

Establishment of neural circuits requires reproducible and precise interactions between growing axons, dendrites and their tissue environment. Cell adhesion molecules and guidance factors are involved in the process, but how specificity is achieved remains poorly understood. Glycans are the third major class of biopolymers besides nucleic acids and proteins, and are usually covalently linked to proteins to form glycoconjugates. Common to most glycans is an extraordinary level of molecular diversity, making them attractive candidates to contribute specificity during neural development. Indeed, many genes important for neural development encode glycoproteins, or enzymes involved in synthesizing or modifying glycans. Glycoconjugates are classified based on both the types of glycans and type of attachment that link them to proteins. Here I discuss progress in understanding the function of glycans, glycan modifications and glycoconjugates during neural development in Caenorhabditis elegans. I will also highlight relevance to human disease and known roles of glycoconjugates in regeneration.

A switch from noncanonical to canonical Wnt signaling stops neuroblast migration through a Slt-Robo and RGA-9b/ARHGAP-dependent mechanism

Members of the Wnt family of secreted glycoproteins regulate cell migration through distinct canonical and noncanonical signaling pathways. Studies of vertebrate development and disease have shown that these pathways can have opposing effects on cell migration, but the mechanism of this functional interplay is not known. In the nematode Caenorhabditis elegans, a switch from noncanonical to canonical Wnt signaling terminates the long-range migration of the QR neuroblast descendants, providing a tractable system to study this mechanism in vivo. Here, we show that noncanonical Wnt signaling acts through PIX-1/RhoGEF, while canonical signaling directly activates the Slt-Robo pathway component EVA-1/EVA1C and the Rho GTPase-activating protein RGA-9b/ARHGAP, which are required for migration inhibition. Our results support a model in which cross-talk between noncanonical and canonical Wnt signaling occurs through antagonistic regulation of the Rho GTPases that drive cell migration.

The planar cell polarity protein VANG-1/Vangl negatively regulates Wnt/β-catenin signaling through a Dvl dependent mechanism

Van Gogh-like (Vangl) and Prickle (Pk) are core components of the non-canonical Wnt planar cell polarity pathway that controls epithelial polarity and cell migration. Studies in vertebrate model systems have suggested that Vangl and Pk may also inhibit signaling through the canonical Wnt/β-catenin pathway, but the functional significance of this potential cross-talk is unclear. In the nematode C. elegans, the Q neuroblasts and their descendants migrate in opposite directions along the anteroposterior body axis. The direction of these migrations is specified by Wnt signaling, with activation of canonical Wnt signaling driving posterior migration, and non-canonical Wnt signaling anterior migration. Here, we show that the Vangl ortholog VANG-1 influences the Wnt signaling response of the Q neuroblasts by negatively regulating canonical Wnt signaling. This inhibitory activity depends on a carboxy-terminal PDZ binding motif in VANG-1 and the Dishevelled ortholog MIG-5, but is independent of the Pk ortholog PRKL-1. Moreover, using Vangl1 and Vangl2 double mutant cells, we show that a similar mechanism acts in mammalian cells. We conclude that cross-talk between VANG-1/Vangl and the canonical Wnt pathway is an evolutionarily conserved mechanism that ensures robust specification of Wnt signaling responses.

Wnt proteins are signaling molecules with a wide range of functions in embryonic development and the maintenance of adult tissues. Wnt proteins can trigger several different signaling pathways that are grouped in β-catenin dependent (canonical) and independent (non-canonical) signaling mechanisms. Here, we have investigated cross-talk between these different Wnt signaling pathways. We show that VANG-1/Vangl, a component of the non-canonical planar cell polarity pathway, negatively regulates canonical Wnt signaling. We propose that this cross-talk mechanism ensures that Wnt stimulated cells always activate the proper downstream signaling response.

Apicomplexan C-Mannosyltransferases Modify Thrombospondin Type I-containing Adhesins of the TRAP Family

In many metazoan species, an unusual type of protein glycosylation, called C-mannosylation, occurs on adhesive thrombospondin type 1 repeats (TSRs) and type I cytokine receptors. This modification has been shown to be catalyzed by the Caenorhabditis elegans DPY-19 protein and orthologues of the encoding gene were found in the genome of apicomplexan parasites. Lately, the micronemal adhesin thrombospondin-related anonymous protein (TRAP) was shown to be C-hexosylated in Plasmodium falciparum sporozoites. Here, we demonstrate that also the micronemal protein MIC2 secreted by Toxoplasma gondii tachyzoites is C-hexosylated. When expressed in a mammalian cell line deficient in C-mannosylation, P. falciparum and T. gondii Dpy19 homologs were able to modify TSR domains of the micronemal adhesins TRAP/MIC2 family involved in parasite motility and invasion. In vitro, the apicomplexan enzymes can transfer mannose to a WXXWXXC peptide but, in contrast to C. elegans or mammalian C-mannosyltransferases, are inactive on a short WXXW peptide. Since TSR domains are commonly found in apicomplexan surface proteins, C-mannosylation may be a common modification in this phylum.

Migration of Q Cells in Caenorhabditis elegans

During C. elegans larval development, the Q neuroblasts produce their lineage by three rounds of divisions along with continuous cell migrations. Their neuronal progeny is dispersed from the pharynx to the anus. This in vivo system to study cell migration is appealing for several reasons. The lineage development is stereotyped; functional analysis and genomic screens are rendered easy and powerful thanks to powerful tools; transgenic manipulations and genome engineering are efficient and can be conveniently combined with live-cell imaging. Here we describe a series of protocols in Q cell migration studies, including quantifications of progeny position, genetic screening strategies, preparation of migration mutants or transgenic worms expressing related fluorescent proteins, multipositional time-lapse tracking of Q cell migration using confocal microscopy and image analyses of single cell movements and dynamics.

Endoplasmic Reticulum-Localized Transmembrane Protein Dpy19L1 Is Required for Neurite Outgrowth

The endoplasmic reticulum (ER), including the nuclear envelope, is a continuous and intricate membrane-bound organelle responsible for various cellular functions. In neurons, the ER network is found in cell bodies, axons, and dendrites. Recent studies indicate the involvement of the ER network in neuronal development, such as neuronal migration and axonal outgrowth. However, the regulation of neural development by ER-localized proteins is not fully understood. We previously reported that the multi-transmembrane protein Dpy19L1 is required for neuronal migration in the developing mouse cerebral cortex. A Dpy19L family member, Dpy19L2, which is a causative gene for human Globozoospermia, is suggested to act as an anchor of the acrosome to the nuclear envelope. In this study, we found that the patterns of exogenous Dpy19L1 were partially coincident with the ER, including the nuclear envelope in COS-7 cells at the level of the light microscope. The reticular distribution of Dpy19L1 was disrupted by microtubule depolymerization that induces retraction of the ER. Furthermore, Dpy19L1 showed a similar distribution pattern with a ER marker protein in embryonic mouse cortical neurons. Finally, we showed that Dpy19L1 knockdown mediated by siRNA resulted in decreased neurite outgrowth in cultured neurons. These results indicate that transmembrane protein Dpy19L1 is localized to the ER membrane and regulates neurite extension during development.

The Caenorhabditis elegans NF2/Merlin Molecule NFM-1 Nonautonomously Regulates Neuroblast Migration and Interacts Genetically with the Guidance Cue SLT-1/Slit

During nervous system development, neurons and their progenitors migrate to their final destinations. In Caenorhabditis elegans, the bilateral Q neuroblasts and their descendants migrate long distances in opposite directions, despite being born in the same posterior region. QR on the right migrates anteriorly and generates the AQR neuron positioned near the head, and QL on the left migrates posteriorly, giving rise to the PQR neuron positioned near the tail. In a screen for genes required for AQR and PQR migration, we identified an allele of nfm-1, which encodes a molecule similar to vertebrate NF2/Merlin, an important tumor suppressor in humans. Mutations in NF2 lead to neurofibromatosis type II, characterized by benign tumors of glial tissues. Here we demonstrate that in C. elegans, nfm-1 is required for the ability of Q cells and their descendants to extend protrusions and to migrate, but is not required for direction of migration. Using a combination of mosaic analysis and cell-specific expression, we show that NFM-1 is required nonautonomously, possibly in muscles, to promote Q lineage migrations. We also show a genetic interaction between nfm-1 and the C. elegans Slit homolog slt-1, which encodes a conserved secreted guidance cue. Our results suggest that NFM-1 might be involved in the generation of an extracellular cue that promotes Q neuroblast protrusion and migration that acts with or in parallel to SLT-1 In vertebrates, NF2 and Slit2 interact in axon pathfinding, suggesting a conserved interaction of NF2 and Slit2 in regulating migratory events.

The Genetics of Axon Guidance and Axon Regeneration in Caenorhabditis elegans

The correct wiring of neuronal circuits depends on outgrowth and guidance of neuronal processes during development. In the past two decades, great progress has been made in understanding the molecular basis of axon outgrowth and guidance. Genetic analysis in Caenorhabditis elegans has played a key role in elucidating conserved pathways regulating axon guidance, including Netrin signaling, the slit Slit/Robo pathway, Wnt signaling, and others. Axon guidance factors were first identified by screens for mutations affecting animal behavior, and by direct visual screens for axon guidance defects. Genetic analysis of these pathways has revealed the complex and combinatorial nature of guidance cues, and has delineated how cues guide growth cones via receptor activity and cytoskeletal rearrangement. Several axon guidance pathways also affect directed migrations of non-neuronal cells in C. elegans, with implications for normal and pathological cell migrations in situations such as tumor metastasis. The small number of neurons and highly stereotyped axonal architecture of the C. elegans nervous system allow analysis of axon guidance at the level of single identified axons, and permit in vivo tests of prevailing models of axon guidance. C. elegans axons also have a robust capacity to undergo regenerative regrowth after precise laser injury (axotomy). Although such axon regrowth shares some similarities with developmental axon outgrowth, screens for regrowth mutants have revealed regeneration-specific pathways and factors that were not identified in developmental screens. Several areas remain poorly understood, including how major axon tracts are formed in the embryo, and the function of axon regeneration in the natural environment.

Nonautonomous Roles of MAB-5/Hox and the Secreted Basement Membrane Molecule SPON-1/F-Spondin in Caenorhabditis elegans Neuronal Migration

Nervous system development and circuit formation requires neurons to migrate from their birthplaces to specific destinations.Migrating neurons detect extracellular cues that provide guidance information. In Caenorhabditis elegans, the Q right (QR) and Q left (QL) neuroblast descendants migrate long distances in opposite directions. The Hox gene lin-39 cell autonomously promotes anterior QR descendant migration, and mab-5/Hox cell autonomously promotes posterior QL descendant migration. Here we describe a nonautonomous role of mab-5 in regulating both QR and QL descendant migrations, a role masked by redundancy with lin-39 A third Hox gene, egl-5/Abdominal-B, also likely nonautonomously regulates Q descendant migrations. In the lin-39 mab-5 egl-5 triple mutant, little if any QR and QL descendant migration occurs. In addition to well-described roles of lin-39 and mab-5 in the Q descendants, our results suggest that lin-39, mab-5, and egl-5 might also pattern the posterior region of the animal for Q descendant migration. Previous studies showed that the spon-1 gene might be a target of MAB-5 in Q descendant migration. spon-1 encodes a secreted basement membrane molecule similar to vertebrate F-spondin. Here we show that spon-1 acts nonautonomously to control Q descendant migration, and might function as a permissive rather than instructive signal for cell migration. We find that increased levels of MAB-5 in body wall muscle (BWM) can drive the spon-1 promoter adjacent to the Q cells, and loss of spon-1 suppresses mab-5 gain of function. Thus, MAB-5 might nonautonomously control Q descendant migrations by patterning the posterior region of the animal to which Q cells respond. spon-1 expression from BWMs might be part of the posterior patterning necessary for directed Q descendant migration.

The Caenorhabditis elegans Q neuroblasts: A powerful system to study cell migration at single-cell resolution in vivo

During development, cell migration plays a central role in the formation of tissues and organs. Understanding the molecular mechanisms that drive and control these migrations is a key challenge in developmental biology that will provide important insights into disease processes, including cancer cell metastasis. In this article, we discuss the Caenorhabditis elegans Q neuroblasts and their descendants as a tool to study cell migration at single-cell resolution in vivo. The highly stereotypical migration of these cells provides a powerful system to study the dynamic cytoskeletal processes that drive migration as well as the evolutionarily conserved signaling pathways (including different Wnt signaling cascades) that guide the cells along their specific trajectories. Here, we provide an overview of what is currently known about Q neuroblast migration and highlight the live-cell imaging, genome editing, and quantitative gene expression techniques that have been developed to study this process.

EGL-20/Wnt and MAB-5/Hox Act Sequentially to Inhibit Anterior Migration of Neuroblasts in C. elegans

Directed neuroblast and neuronal migration is important in the proper development of nervous systems. In C. elegans the bilateral Q neuroblasts QR (on the right) and QL (on the left) undergo an identical pattern of cell division and differentiation but migrate in opposite directions (QR and descendants anteriorly and QL and descendants posteriorly). EGL-20/Wnt, via canonical Wnt signaling, drives the expression of MAB-5/Hox in QL but not QR. MAB-5 acts as a determinant of posterior migration, and mab-5 and egl-20 mutants display anterior QL descendant migrations. Here we analyze the behaviors of QR and QL descendants as they begin their anterior and posterior migrations, and the effects of EGL-20 and MAB-5 on these behaviors. The anterior and posterior daughters of QR (QR.a/p) after the first division immediately polarize and begin anterior migration, whereas QL.a/p remain rounded and non-migratory. After ~1 hour, QL.a migrates posteriorly over QL.p. We find that in egl-20/Wnt, bar-1/β-catenin, and mab-5/Hox mutants, QL.a/p polarize and migrate anteriorly, indicating that these molecules normally inhibit anterior migration of QL.a/p. In egl-20/Wnt mutants, QL.a/p immediately polarize and begin migration, whereas in bar-1/β-catenin and mab-5/Hox, the cells transiently retain a rounded, non-migratory morphology before anterior migration. Thus, EGL-20/Wnt mediates an acute inhibition of anterior migration independently of BAR-1/β-catenin and MAB-5/Hox, and a later, possible transcriptional response mediated by BAR-1/β-catenin and MAB-5/Hox. In addition to inhibiting anterior migration, MAB-5/Hox also cell-autonomously promotes posterior migration of QL.a (and QR.a in a mab-5 gain-of-function).

Orchestrating A/P and D/V guidance - A Wnt/Netrin tale

While ample information was gathered in identifying guidance cues and their downstream mediators, very little is known about how the information from multiple extracellular cues is intracellularly to generate normal patterning. Netrin and Wnt signaling pathways play key roles in normal development as well as in malignancies. In C. elegans, as in vertebrates, dorso-ventral (D/V) graded distributions of UNC-6/Netrin and antero-posterior (A/P) graded distributions of Wnts provide instructive polarity information to guide cells and axons along their respective gradients. In this commentary, I will discuss recent findings demonstrating that these 2 signaling pathways also function redundantly to regulate polarity orthogonal to the axis of their gradation. Thus, Wnt signaling components contribute to D/V polarity, while Netrin signaling components contribute to A/P polarity and their joint action collaboratively governs migratory transitions from one axis to the other. These findings pave the way to unraveling broader roles of Wnt and Netrin signaling pathways, roles that are masked due to their redundant nature, and provide a conceptually novel view of how antero-posterior and dorso-ventral guidance mechanisms are orchestrated to establish polarity in multiple biological processes.

SDN-1/Syndecan Acts in Parallel to the Transmembrane Molecule MIG-13 to Promote Anterior Neuroblast Migration

The Q neuroblasts in Caenorhabditis elegans display left-right asymmetry in their migration, with QR and descendants on the right migrating anteriorly, and QL and descendants on the left migrating posteriorly. Initial QR and QL migration is controlled by the transmembrane receptors UNC-40/DCC, PTP-3/LAR, and the Fat-like cadherin CDH-4. After initial migration, QL responds to an EGL-20/Wnt signal that drives continued posterior migration by activating MAB-5/Hox activity in QL but not QR. QR expresses the transmembrane protein MIG-13, which is repressed by MAB-5 in QL and which drives anterior migration of QR descendants. A screen for new Q descendant AQR and PQR migration mutations identified mig-13 as well as hse-5, the gene encoding the glucuronyl C5-epimerase enzyme, which catalyzes epimerization of glucuronic acid to iduronic acid in the heparan sulfate side chains of heparan sulfate proteoglycans (HSPGs). Of five C. elegans HSPGs, we found that only SDN-1/Syndecan affected Q migrations. sdn-1 mutants showed QR descendant AQR anterior migration defects, and weaker QL descendant PQR migration defects. hse-5 affected initial Q migration, whereas sdn-1 did not. sdn-1 and hse-5 acted redundantly in AQR and PQR migration, but not initial Q migration, suggesting the involvement of other HSPGs in Q migration. Cell-specific expression studies indicated that SDN-1 can act in QR to promote anterior migration. Genetic interactions between sdn-1, mig-13, and mab-5 suggest that MIG-13 and SDN-1 act in parallel to promote anterior AQR migration and that SDN-1 also controls posterior migration. Together, our results indicate previously unappreciated complexity in the role of multiple signaling pathways and inherent left-right asymmetry in the control of Q neuroblast descendant migration.

Teratozoospermia: spotlight on the main genetic actors in the human

BACKGROUND

Male infertility affects >20 million men worldwide and represents a major health concern. Although multifactorial, male infertility has a strong genetic basis which has so far not been extensively studied. Recent studies of consanguineous families and of small cohorts of phenotypically homogeneous patients have however allowed the identification of a number of autosomal recessive causes of teratozoospermia. Homozygous mutations of aurora kinase C (AURKC) were first described to be responsible for most cases of macrozoospermia. Other genes defects have later been identified in spermatogenesis associated 16 (SPATA16) and dpy-19-like 2 (DPY19L2) in patients with globozoospermia and more recently in dynein, axonemal, heavy chain 1 (DNAH1) in a heterogeneous group of patients presenting with flagellar abnormalities previously described as dysplasia of the fibrous sheath or short/stump tail syndromes, which we propose to call multiple morphological abnormalities of the flagella (MMAF).

METHODS

A comprehensive review of the scientific literature available in PubMed/Medline was conducted for studies on human genetics, experimental models and physiopathology related to teratozoospermia in particular globozoospermia, large headed spermatozoa and flagellar abnormalities. The search included all articles with an English abstract available online before September 2014.

RESULTS

Molecular studies of numerous unrelated patients with globozoospermia and large-headed spermatozoa confirmed that mutations in DPY19L2 and AURKC are mainly responsible for their respective pathological phenotype. In globozoospermia, the deletion of the totality of the DPY19L2 gene represents ∼ 81% of the pathological alleles but point mutations affecting the protein function have also been described. In macrozoospermia only two recurrent mutations were identified in AURKC, accounting for almost all the pathological alleles, raising the possibility of a putative positive selection of heterozygous individuals. The recent identification of DNAH1 mutations in a proportion of patients with MMAF is promising but emphasizes that this phenotype is genetically heterogeneous. Moreover, the identification of mutations in a dynein strengthens the emerging point of view that MMAF may be a phenotypic variation of the classical forms of primary ciliary dyskinesia. Based on data from human and animal models, the MMAF phenotype seems to be favored by defects directly or indirectly affecting the central pair of axonemal microtubules of the sperm flagella.

CONCLUSIONS

The studies described here provide valuable information regarding the genetic and molecular defects causing infertility, to improve our understanding of the physiopathology of teratozoospermia while giving a detailed characterization of specific features of spermatogenesis. Furthermore, these findings have a significant influence on the diagnostic strategy for teratozoospermic patients allowing the clinician to provide the patient with informed genetic counseling, to adopt the best course of treatment and to develop personalized medicine directly targeting the defective gene products.

Genetic aspects of monomorphic teratozoospermia: a review

Teratozoospermia is characterized by the presence of spermatozoa with abnormal morphology over 85 % in sperm. When all the spermatozoa display a unique abnormality, teratozoospermia is said to be monomorphic. Two forms of monomorphic teratozoospermia, representing less than 1 % of male infertility, are recognized: macrozoospermia (also called macrocephalic sperm head syndrome) and globozoospermia (also called round-headed sperm syndrome). Macrozoospermia is defined as the presence of a very high percentage of spermatozoa with enlarged head and multiple flagella. Meiotic segregation studies in 30 males revealed that over 90 % of spermatozoa were aneuploid, mainly diploid. Sperm DNA fragmentation studies performed in a few patients showed an increase in DNA fragmentation index compared to fertile men. Four mutations in the AURKC gene, a key player in meiosis and more particularly in spermatogenesis, have been found to be responsible for macrozoospermia. Globozoospermia is characterized by round-headed spermatozoa with an absent acrosome, an aberrant nuclear membrane and midpiece defects. The rate of aneuploidy of various chromosomes in spermatozoa from 26 globozoospermic men was slightly increased compared to fertile men. However, this increase was of the same order as that commonly found in infertile men with altered sperm parameters. The majority of the studies found that globozoospermic males had a sperm DNA fragmentation index higher than in fertile men. Mutations or deletions in three genes, SPATA16, PICK1 and DPY19L2, have been shown to be responsible for globozoospermia. Identification of the genetic causes of macrozoospermia and globozoospermia should help refine diagnosis and treatment of these patients, avoiding long and painful treatments. Elucidating the molecular causes of these defects is of utmost importance as intracytoplasmic sperm injection (ICSI) is very disappointing in these two pathologies.

Netrins and Frazzled/DCC promote the migration and mesenchymal to epithelial transition of Drosophila midgut cells

Mesenchymal-epithelial transitions (METs) are important in both development and the growth of secondary tumours. Although the molecular basis for epithelial polarity is well studied, less is known about the cues that induce MET. Here we show that Netrins, well known as chemotropic guidance factors, provide a basal polarising cue during the Drosophila midgut MET. Both netrinA and netrinB are expressed in the visceral mesoderm, the substrate upon which midgut cells migrate, while their receptor frazzled (fra) is expressed in midgut cells. Netrins are required to polarise Fra to the basal surface, and Netrins and Fra undergo mutually-dependent endocytosis, with Fra subsequently trafficking to late endosomes. Mutations to fra and netrins affect both migration and MET but to different degrees. Loss of fra strongly delays migration, midgut cells fail to extend protrusions, and apico-basal polarisation of proteins and epithelium formation is inhibited. In netrin mutants, the migration phenotype is weaker and cells still extend protrusions. However, apico-basal polarisation of proteins, including Fra, and FActin is greatly disrupted and a monolayer fails to form. Delocalised accumulations of FActin are prevalent in netrin mutants but not fra mutants suggesting delocalised Fra may disrupt the MET. βPS localisation is also affected in netrin mutants in that a basal gradient is reduced while localisation to the midgut/VM interface is increased. Since a similar effect is seen when endocytosis is inhibited, Netrin and Fra may regulate Integrin turnover. The results suggest Netrin-dependent basal polarisation of Fra is critical for the formation of an epithelium.

The heparan sulfate-modifying enzyme glucuronyl C5-epimerase HSE-5 controls Caenorhabditis elegans Q neuroblast polarization during migration

Directional cell migration is fundamental for neural development, and extracellular factors are pivotal for this process. Heparan sulfate proteoglycans (HSPGs) that carry long chains of differentially modified sugar residues contribute to extracellular matrix; however, the functions of HSPG in guiding cell migration remain elusive. Here, we used the Caenorhabditis elegans mutant pool from the Million Mutation Project and isolated a mutant allele of the heparan sulfate-modifying enzyme glucuronyl C5-epimerase HSE-5. Loss-of-function of this enzyme resulted in defective Q neuroblast migration. We showed that hse-5 controlled Q cell migration in a cell non-autonomous manner. By performing live cell imaging in hse-5 mutant animals, we found that hse-5 controlled initial polarization during Q neuroblast migration. Furthermore, our genetic epistasis analysis demonstrated that lon-2 might act downstream of hse-5. Finally, rescue of the hse-5 mutant phenotype by expression of human and mouse hse-5 homologs suggested a conserved function for this gene in neural development. Taken together, our results indicated that proper HSPG modification in the extracellular matrix by HSE-5 is essential for neuroblast polarity during migration.

Rho-guanine nucleotide exchange factors during development: Force is nothing without control

The development of multicellular organisms is associated with extensive rearrangements of tissues and cell sheets. The driving force for these rearrangements is generated mostly by the actin cytoskeleton. In order to permit the reproducible development of a specific body plan, dynamic reorganization of the actin cytoskeleton must be precisely coordinated in space and time. GTP-exchange factors that activate small GTPases of the Rho family play an important role in this process. Here we review the role of this class of cytoskeletal regulators during important developmental processes such as epithelial morphogenesis, cytokinesis, cell migration, cell polarity, neuronal growth cone extension and phagocytosis in different model systems.

Distinct roles of Rac GTPases and the UNC-73/Trio and PIX-1 Rac GTP exchange factors in neuroblast protrusion and migration in C. elegans

The Rac and Cdc42 GTPases as well as the multiple GTP exchange factors that regulate their activity have been implicated in the pathways that drive actin cytoskeleton reorganization, but the individual contributions of these molecules to cell migration remain unknown. Studies shown here examine the roles of CED-10/Rac, MIG-2/RhoG and CDC-42 in the migration of the QL and QR neuroblasts in C. elegans. CED-10/Rac was found to normally limit protrusion and migration, whereas MIG-2/RhoG was required for protrusion and migration. CED-10/Rac and MIG-2/RhoG also had redundant roles in Q protrusion and migration. Surprisingly, CDC-42 was found to have only weak effects on the protrusion and the migration. We found that a mutation of unc-73/Trio, which encodes a GEF for CED-10/Rac and MIG-2/RhoG, caused protrusions that were thin and filopodia-like, suggesting that UNC-73/Trio is required for robust lamellipodia-like protrusion. A screen of the 19 C. elegans Dbl homology Rho GEF genes revealed that PIX-1 was required for proper Q neuroblast protrusion and migration. Genetic analysis indicated that PIX-1 might act in the CED-10/Rac pathway in parallel to MIG-2/RhoG and that PIX-1 has redundant function with UNC-73/Trio in Q neuroblast protrusion and migration. These results indicate that Rho GTPases and GEFs have both unique and overlapping roles in neuronal migration.

The fat-like cadherin CDH-4 acts cell-non-autonomously in anterior-posterior neuroblast migration

Directed migration of neurons is critical in the normal and pathological development of the brain and central nervous system. In Caenorhabditis elegans, the bilateral Q neuroblasts, QR on the right and QL on the left, migrate anteriorly and posteriorly, respectively. Initial protrusion and migration of the Q neuroblasts is autonomously controlled by the transmembrane proteins UNC-40/DCC, PTP-3/LAR, and MIG-21. As QL migrates posteriorly, it encounters and EGL-20/Wnt signal that induces MAB-5/Hox expression that drives QL descendant posterior migration. QR migrates anteriorly away from EGL-20/Wnt and does not activate MAB-5/Hox, resulting in anterior QR descendant migration. A forward genetic screen for new mutations affecting initial Q migrations identified alleles of cdh-4, which caused defects in both QL and QR directional migration similar to unc-40, ptp-3, and mig-21. Previous studies showed that in QL, PTP-3/LAR and MIG-21 act in a pathway in parallel to UNC-40/DCC to drive posterior QL migration. Here we show genetic evidence that CDH-4 acts in the PTP-3/MIG-21 pathway in parallel to UNC-40/DCC to direct posterior QL migration. In QR, the PTP-3/MIG-21 and UNC-40/DCC pathways mutually inhibit each other, allowing anterior QR migration. We report here that CDH-4 acts in both the PTP-3/MIG-21 and UNC-40/DCC pathways in mutual inhibition in QR, and that CDH-4 acts cell-non-autonomously. Interaction of CDH-4 with UNC-40/DCC in QR but not QL represents an inherent left-right asymmetry in the Q cells, the nature of which is not understood. We conclude that CDH-4 might act as a permissive signal for each Q neuroblast to respond differently to anterior-posterior guidance information based upon inherent left-right asymmetries in the Q neuroblasts.

The novel Rac effector RIN-1 regulates neuronal cell migration and axon pathfinding in C. elegans

Cell migration and axon guidance require proper regulation of the actin cytoskeleton in response to extracellular guidance cues. Rho/Rac small GTPases are essential regulators of actin remodeling. Caenorhabditis elegans CED-10 is a Rac1 homolog that is required for various cellular morphological changes and migration events and is under the control of several guidance signaling pathways. There is still considerable uncertainty regarding events following the activation of guidance receptors by extracellular signals and the regulation of actin dynamics based on spatiotemporally restricted Rac activity. Here we show that the VPS9 domain protein RIN-1 acts as a novel effector for CED-10 in C. elegans. The orthologous mammalian Rin1 protein has previously been identified as an effector for Ras GTPase and is now known to function as a guanine nucleotide exchange factor for Rab5 GTPase. We found that RIN-1 specifically binds to the GTP-bound form of CED-10 and that mutations in rin-1 cause significant defects in migration and axon guidance of restricted neuronal cell types including AVM and HSN neurons, in contrast to the various defects observed in ced-10 mutants. Our analyses place RIN-1 in the Slit-Robo genetic pathway that regulates repulsive signaling for dorsoventral axon guidance. In rin-1 mutants, actin accumulated on both the ventral and dorsal sides of the developing HSN neuron, in contrast to its ventral accumulation in wild type. These results strongly suggest that RIN-1 acts as an effector for CED-10/Rac1 and regulates actin remodeling in response to restricted guidance cues.

Transmembrane protein MIG-13 links the Wnt signaling and Hox genes to the cell polarity in neuronal migration

Directional cell migration is a fundamental process in neural development. In Caenorhabditis elegans, Q neuroblasts on the left (QL) and right (QR) sides of the animal generate cells that migrate in opposite directions along the anteroposterior body axis. The homeobox (Hox) gene lin-39 promotes the anterior migration of QR descendants (QR.x), whereas the canonical Wnt signaling pathway activates another Hox gene, mab-5, to ensure the QL descendants' (QL.x) posterior migration. However, the regulatory targets of LIN-39 and MAB-5 remain elusive. Here, we showed that MIG-13, an evolutionarily conserved transmembrane protein, cell-autonomously regulates the asymmetric distribution of the actin cytoskeleton in the leading migratory edge. We identified mig-13 as a cellular target of LIN-39 and MAB-5. LIN-39 establishes QR.x anterior polarity by binding to the mig-13 promoter and promoting mig-13 expression, whereas MAB-5 inhibits QL.x anterior polarity by associating with the lin-39 promoter and downregulating lin-39 and mig-13 expression. Thus, MIG-13 links the Wnt signaling and Hox genes that guide migrations, to the actin cytoskeleton, which executes the motility response in neuronal migration.

Functional transcriptomic analysis of the role of MAB-5/Hox in Q neuroblast migration in Caenorhabditis elegans

Background

Directed cell migration is a fundamental process in normal development and in tumor metastasis. In C. elegans the MAB-5/Hox transcription factor is a determinant of posterior migration of the Q neuroblast descendants. In this work, mab-5 transcriptional targets that control Q descendant migration are identified by comparing RNA-seq profiles in wild type and mab-5 mutant backgrounds.

Results

Transcriptome profiling is a widely-used and potent tool to identify genes involved in developmental and pathological processes, and is most informative when RNA can be isolated from individual cell or tissue types. Cell-specific RNA samples can be difficult to obtain from invertebrate model organisms such as Drosophila and C. elegans. Here we test the utility of combining a whole organism RNA-seq approach with mab-5 loss and gain-of-function mutants and functional validation using RNAi to identify genes regulated by MAB-5 to control Q descendant migration. We identified 22 genes whose expression was controlled by mab-5 and that controlled Q descendant migration. Genes regulated by mab-5 were enriched for secreted and transmembrane molecules involved in basement membrane interaction and modification, and some affected Q descendant migration.

Conclusions

Our results indicate that a whole-organism RNA-seq approach, when combined with mutant analysis and functional validation, can be a powerful method to identify genes involved in a specific developmental process, in this case Q descendant posterior migration. These genes could act either autonomously in the Q cells, or non-autonomously in other cells that express MAB-5. The identities of the genes regulated by MAB-5 indicate that MAB-5 acts by modifying interactions with the basement membrane, resulting in posterior versus anterior migration.

C. elegans DPY-19 is a C-mannosyltransferase glycosylating thrombospondin repeats

Among the different types of protein glycosylation, C-mannosylation of tryptophan residues stands out because of the unique linkage formed between sugar and protein. Instead of the typical O- or N-glycosidic linkage, a C-C bond is used for attachment of a single mannose. C-mannose is characteristically found in thrombospondin type 1 repeats and in the WSXWS motif of type I cytokine receptors. The genetic base of the enzymatic activity catalyzing C-mannosylation was not known. Here we demonstrate that Caenorhabditis elegans DPY-19 is a C-mannosyltransferase. DPY-19 exhibits topological and sequential homology to the N-glycan oligosaccharyltransferase, highlighting an evolutionary link between N- and C-glycosylation. We show that the C. elegans surface receptors MIG-21 and UNC-5 are acceptor substrates of DPY-19 and that C-mannosylation is essential for the secretion of soluble MIG-21. Thereby, our data provide an explanation for the previously described identical Q neuroblast migration phenotypes of dpy-19 and mig-21 mutants.

Association of micropapillary urothelial carcinoma of the bladder and BK viruria in kidney transplant recipients

INTRODUCTION

BK virus (BKV) is an ubiquitous human polyomavirus that establishes latency in urothelium. BKV is known to re-activate in immunosuppressed individuals, and is an increasingly important cause of nephropathy and graft loss in kidney transplant recipients. Animal studies have demonstrated BKV has a potential role as a tumor virus. However, its role in precipitating or facilitating oncogenesis in humans is still debated.

REPORT

We report 2 cases of aggressive micropapillary urothelial carcinoma of the bladder in kidney transplant recipients with persistent BK viruria and preserved graft function.

RESULTS

In both cases, polyomavirus immunohistochemistry performed on the tumor specimens was strongly positive, and limited to the malignant tissue. BKV DNA, viral protein 1, and large T antigen mRNA were detected in the tumor; however, no viral particles were seen on electron microscopy.

CONCLUSION

In one of the cases, BKV integration into the host genome was identified, leading to the truncation of the major viral capsid gene. This finding raises the concern that persisting BK viruria may be a risk factor for this aggressive form of bladder cancer. Further studies to determine screening and management strategies are required.

Live imaging of cellular dynamics during Caenorhabditis elegans postembryonic development

Postembryonic development is an important process of organismal maturation after embryonic growth. Despite key progress in recent years in understanding embryonic development via fluorescence time-lapse microscopy, comparatively less live-cell imaging of postembryonic development has been done. Here we describe a protocol to image larval development in the nematode Caenorhabditis elegans. Our protocol describes the construction of fluorescent transgenic C. elegans, immobilization of worm larvae and time-lapse microscopy analysis. To improve the throughput of imaging, we developed a C. elegans triple-fluorescence imaging approach with a worm-optimized blue fluorescent protein (TagBFP), green fluorescent protein (GFP) and mCherry. This protocol has been previously applied to time-lapse imaging analysis of Q neuroblast asymmetric division, migration and apoptosis, and we show here that it can also be used to image neuritogenesis in the L1 larvae. Other applications are also possible. The protocol can be completed within 3 h and may provide insights into understanding postembryonic development.

Transmembrane proteins UNC-40/DCC, PTP-3/LAR, and MIG-21 control anterior-posterior neuroblast migration with left-right functional asymmetry in Caenorhabditis elegans

Migration of neurons and neural crest cells is of central importance to the development of nervous systems. In Caenorhabditis elegans, the QL neuroblast on the left migrates posteriorly, and QR on the right migrates anteriorly, despite similar lineages and birth positions with regard to the left-right axis. Initial migration is independent of a Wnt signal that controls later anterior-posterior Q descendant migration. Previous studies showed that the transmembrane proteins UNC-40/DCC and MIG-21, a novel thrombospondin type I repeat containing protein, act redundantly in left-side QL posterior migration. Here we show that the LAR receptor protein tyrosine phosphatase PTP-3 acts with MIG-21 in parallel to UNC-40 in QL posterior migration. We also show that in right-side QR, the UNC-40 and PTP-3/MIG-21 pathways mutually inhibit each other's role in posterior migration, allowing anterior QR migration. Finally, we present evidence that these proteins act autonomously in the Q neuroblasts. These studies indicate an inherent left-right asymmetry in the Q neuroblasts with regard to UNC-40, PTP-3, and MIG-21 function that results in posterior vs. anterior migration.

Absence of Dpy19l2, a new inner nuclear membrane protein, causes globozoospermia in mice by preventing the anchoring of the acrosome to the nucleus

Sperm-head elongation and acrosome formation, which take place during the last stages of spermatogenesis, are essential to produce competent spermatozoa that are able to cross the oocyte zona pellucida and to achieve fertilization. During acrosome biogenesis, acrosome attachment and spreading over the nucleus are still poorly understood and to date no proteins have been described to link the acrosome to the nucleus. We recently demonstrated that a deletion of DPY19L2, a gene coding for an uncharacterized protein, was responsible for a majority of cases of type I globozoospermia, a rare cause of male infertility that is characterized by the exclusive production of round-headed acrosomeless spermatozoa. Here, using Dpy19l2 knockout mice, we describe the cellular function of the Dpy19l2 protein. We demonstrate that the protein is expressed predominantly in spermatids with a very specific localization restricted to the inner nuclear membrane facing the acrosomal vesicle. We show that the absence of Dpy19l2 leads to the destabilization of both the nuclear dense lamina (NDL) and the junction between the acroplaxome and the nuclear envelope. Consequently, the acrosome and the manchette fail to be linked to the nucleus leading to the disruption of vesicular trafficking, failure of sperm nuclear shaping and eventually to the elimination of the unbound acrosomal vesicle. Finally, we show for the first time that Dpy19l3 proteins are also located in the inner nuclear envelope, therefore implying that the Dpy19 proteins constitute a new family of structural transmembrane proteins of the nuclear envelope.

Netrin instructs synaptic vesicle clustering through Rac GTPase, MIG-10, and the actin cytoskeleton

Netrin is a chemotrophic factor known to regulate a number of neurodevelopmental processes, including cell migration, axon guidance, and synaptogenesis. Although the role of Netrin in synaptogenesis is conserved throughout evolution, the mechanisms by which it instructs synapse assembly are not understood. Here we identify a mechanism by which the Netrin receptor UNC-40/DCC instructs synaptic vesicle clustering in vivo. UNC-40 localized to presynaptic regions in response to Netrin. We show that UNC-40 interacted with CED-5/DOCK180 and instructed CED-5 presynaptic localization. CED-5 in turn signaled through CED-10/Rac1 and MIG-10/Lamellipodin to organize the actin cytoskeleton in presynaptic regions. Localization of this signaling pathway to presynaptic regions was necessary for synaptic vesicle clustering during synapse assembly but not for the subcellular localization of active zone proteins. Thus, vesicle clustering and localization of active zone proteins are instructed by separate pathways downstream of Netrin. Our data indicate that signaling modules known to organize the actin cytoskeleton during guidance can be co-opted to instruct synaptic vesicle clustering.

Dpy19l1, a multi-transmembrane protein, regulates the radial migration of glutamatergic neurons in the developing cerebral cortex

During corticogenesis, the regulation of neuronal migration is crucial for the functional organization of the neocortex. Glutamatergic neurons are major excitatory components of the mammalian neocortex. In order to elucidate the specific molecular mechanisms underlying their development, we used single-cell microarray analysis to screen for mouse genes that are highly expressed in developing glutamatergic neurons. We identified dpy-19-like 1 (Dpy19l1), a homolog of C. elegans dpy-19, which encodes a putative multi-transmembrane protein shown to regulate directed migration of Q neuroblasts in C. elegans. At embryonic stages Dpy19l1 is highly expressed in glutamatergic neurons in the mouse cerebral cortex, whereas in the subpallium, where GABAergic neurons are generated, expression was below detectable levels. Downregulation of Dpy19l1 mediated by shRNA resulted in defective radial migration of glutamatergic neurons in vivo, which was restored by the expression of shRNA-insensitive Dpy19l1. Many Dpy19l1-knockdown cells were aberrantly arrested in the intermediate zone and the deep layer and, additionally, some extended single long processes towards the pial surface. Furthermore, we observed defective radial migration of bipolar cells in Dpy19l1-knockdown brains. Despite these migration defects, these cells correctly expressed Cux1, which is a marker for upper layer neurons, suggesting that Dpy19l1 knockdown results in migration defects but does not affect cell type specification. These results indicate that Dpy19l1 is required for the proper radial migration of glutamatergic neurons, and suggest an evolutionarily conserved role for the Dpy19 family in neuronal migration.

MIG-15 and ERM-1 promote growth cone directional migration in parallel to UNC-116 and WVE-1

Neurons require precise targeting of their axons to form a connected network and a functional nervous system. Although many guidance receptors have been identified, much less is known about how these receptors signal to direct growth cone migration. We used Caenorhabditis elegans motoneurons to study growth cone directional migration in response to a repellent UNC-6 (netrin homolog) guidance cue. The evolutionarily conserved kinase MIG-15 [homolog of Nck-interacting kinase (NIK)] regulates motoneuron UNC-6-dependent repulsion through unknown mechanisms. Using genetics and live imaging techniques, we show that motoneuron commissural axon morphology defects in mig-15 mutants result from impaired growth cone motility and subsequent failure to migrate across longitudinal obstacles or retract extra processes. To identify new genes acting with mig-15, we screened for genetic enhancers of the mig-15 commissural phenotype and identified the ezrin/radixin/moesin ortholog ERM-1, the kinesin-1 motor UNC-116 and the actin regulator WVE-1 complex. Genetic analysis indicates that mig-15 and erm-1 act in the same genetic pathway to regulate growth cone migration and that this pathway functions in parallel to the UNC-116/WVE-1 pathway. Further, time-lapse imaging of growth cones in mutants suggests that UNC-116 might be required to stimulate protrusive activity at the leading edge, whereas MIG-15 and ERM-1 maintain low activity at the rear edge. Together, these results support a model in which the MIG-15 kinase and the UNC-116-WVE-1 complex act on opposite sides of the growth cone to promote robust directional migration.

The thrombospondin repeat containing protein MIG-21 controls a left-right asymmetric Wnt signaling response in migrating C. elegans neuroblasts

Wnt proteins are secreted signaling molecules that play a central role in development and adult tissue homeostasis. Although several Wnt signal transduction mechanisms have been described in detail, it is still largely unknown how cells are specified to adopt such different Wnt signaling responses. Here, we have used the stereotypic migration of the C. elegans Q neuroblasts as a model to study how two initially equivalent cells are instructed to activate either β-catenin dependent or independent Wnt signaling pathways to control the migration of their descendants along the anteroposterior axis. We find that the specification of this difference in Wnt signaling response is dependent on the thrombospondin repeat containing protein MIG-21, which acts together with the netrin receptor UNC-40/DCC to control an initial left-right asymmetric polarization of the Q neuroblasts. Furthermore, we show that the direction of this polarization determines the threshold for Wnt/β-catenin signaling, with posterior polarization sensitizing for activation of this pathway. We conclude that MIG-21 and UNC-40 control the asymmetry in Wnt signaling response by restricting posterior polarization to one of the two Q neuroblasts.

Cell migration in multicellular environments

Most experiments observing cell migration use planar plastic or glass surfaces despite these conditions being considerably different from physiological ones. On such planar surfaces, cells take a dorsal-ventral polarity to move two-dimensionally. Cells in tissues, however, interact with surrounding cells and the extracellular matrix such that they transverse three-dimensionally. For this reason, three-dimensional matrices have become more and more popular for cell migration experiments. In addition, recent developments in imaging techniques have enabled high resolution observations of in vivo cell migration. The combination of three-dimensional matrices and such imaging techniques has revealed motile mechanisms in tissues not observable in studies using planar surfaces. Regarding models for such cell migration studies, the cellular slime mould Dictyostelium discoideum is ideal. Single amoeboid cells aggregate into hemispherical mound structures upon starvation to begin a multicellular morphogenesis. These tiny and simple multicellular bodies are suitable for observing the behaviors of individual cells in multicellular structures. Furthermore, the unique life cycle can be exploited to identify which genes are involved in cell migration in multicellular environments. Since mutants lacking such genes are expected to fail to undergo morphogenesis, easy and systematic gene screening is possible by isolating mutants whose developments arrest around the mound stage, which is the case for several mutants lacking specific cytoskeletal proteins. In this article, I discuss the basic elements required for cell migration in multicellular environments and how Dictyostelium can be used to elucidate them.

UNC-73/trio RhoGEF-2 activity modulates Caenorhabditis elegans motility through changes in neurotransmitter signaling upstream of the GSA-1/Galphas pathway

Rho-family GTPases play regulatory roles in many fundamental cellular processes. Caenorhabditis elegans UNC-73 RhoGEF isoforms function in axon guidance, cell migration, muscle arm extension, phagocytosis, and neurotransmission by activating either Rac or Rho GTPase subfamilies. Multiple differentially expressed UNC-73 isoforms contain a Rac-specific RhoGEF-1 domain, a Rho-specific RhoGEF-2 domain, or both domains. The UNC-73E RhoGEF-2 isoform is activated by the G-protein subunit Gαq and is required for normal rates of locomotion; however, mechanisms of UNC-73 and Rho pathway regulation of locomotion are not clear. To better define UNC-73 function in the regulation of motility we used cell-specific and inducible promoters to examine the temporal and spatial requirements of UNC-73 RhoGEF-2 isoform function in mutant rescue experiments. We found that UNC-73E acts within peptidergic neurons of mature animals to regulate locomotion rate. Although unc-73 RhoGEF-2 mutants have grossly normal synaptic morphology and weak resistance to the acetylcholinesterase inhibitor aldicarb, they are significantly hypersensitive to the acetylcholine receptor agonist levamisole, indicating alterations in acetylcholine neurotransmitter signaling. Consistent with peptidergic neuron function, unc-73 RhoGEF-2 mutants exhibit a decreased level of neuropeptide release from motor neuron dense core vesicles (DCVs). The unc-73 locomotory phenotype is similar to those of rab-2 and unc-31, genes with distinct roles in the DCV-mediated secretory pathway. We observed that constitutively active Gαs pathway mutations, which compensate for DCV-mediated signaling defects, rescue unc-73 RhoGEF-2 and rab-2 lethargic movement phenotypes. Together, these data suggest UNC-73 RhoGEF-2 isoforms are required for proper neurotransmitter signaling and may function in the DCV-mediated neuromodulatory regulation of locomotion rate.

DPY19L2 deletion as a major cause of globozoospermia

Globozoospermia, characterized by round-headed spermatozoa, is a rare (< 0.1% in male infertile patients) and severe teratozoospermia consisting primarily of spermatozoa lacking an acrosome. Studying a Jordanian consanguineous family in which five brothers were diagnosed with complete globozoospermia, we showed that the four out of five analyzed infertile brothers carried a homozygous deletion of 200 kb on chromosome 12 encompassing only DPY19L2. Very similar deletions were found in three additional unrelated patients, suggesting that DPY19L2 deletion is a major cause of globozoospermia, given that 19% (4 of 21) of the analyzed patients had such deletion. The deletion is most probably due to a nonallelic homologous recombination (NAHR), because the gene is surrounded by two low copy repeats (LCRs). We found DPY19L2 deletion in patients from three different origins and two different breakpoints, strongly suggesting that the deletion results from recurrent events linked to the specific architectural feature of this locus rather than from a founder effect, without fully excluding a recent founder effect. DPY19L2 is associated with a complete form of globozoospermia, as is the case for the first two genes found to be associated with globozoospermia, SPATA16 or PICK1. However, in contrast to SPATA16, for which no pregnancy was reported, pregnancies were achieved, via intracytoplasmic sperm injection, for two patients with DPY19L2 deletion, who then fathered three children.

A recurrent deletion of DPY19L2 causes infertility in man by blocking sperm head elongation and acrosome formation

An increasing number of couples require medical assistance to achieve a pregnancy, and more than 2% of the births in Western countries now result from assisted reproductive technologies. To identify genetic variants responsible for male infertility, we performed a whole-genome SNP scan on patients presenting with total globozoospermia, a primary infertility phenotype characterized by the presence of 100% round acrosomeless spermatozoa in the ejaculate. This strategy allowed us to identify in most patients (15/20) a 200 kb homozygous deletion encompassing only DPY19L2, which is highly expressed in the testis. Although there was no known function for DPY19L2 in humans, previous work indicated that its ortholog in C. elegans is involved in cell polarity. In man, the DPY19L2 region has been described as a copy-number variant (CNV) found to be duplicated and heterozygously deleted in healthy individuals. We show here that the breakpoints of the deletions are located on a highly homologous 28 kb low copy repeat (LCR) sequence present on each side of DPY19L2, indicating that the identified deletions were probably produced by nonallelic homologous recombination (NAHR) between these two regions. We demonstrate that patients with globozoospermia have a homozygous deletion of DPY19L2, thus indicating that DPY19L2 is necessary in men for sperm head elongation and acrosome formation. A molecular diagnosis can now be proposed to affected men; the presence of the deletion confirms the diagnosis of globozoospermia and assigns a poor prognosis for the success of in vitro fertilization.

Evidence for somatic gene conversion and deletion in bipolar disorder, Crohn's disease, coronary artery disease, hypertension, rheumatoid arthritis, type-1 diabetes, and type-2 diabetes

BACKGROUND

During gene conversion, genetic information is transferred unidirectionally between highly homologous but non-allelic regions of DNA. While germ-line gene conversion has been implicated in the pathogenesis of some diseases, somatic gene conversion has remained technically difficult to investigate on a large scale.

METHODS

A novel analysis technique is proposed for detecting the signature of somatic gene conversion from SNP microarray data. The Wellcome Trust Case Control Consortium has gathered SNP microarray data for two control populations and cohorts for bipolar disorder (BD), cardiovascular disease (CAD), Crohn's disease (CD), hypertension (HT), rheumatoid arthritis (RA), type-1 diabetes (T1D) and type-2 diabetes (T2D). Using the new analysis technique, the seven disease cohorts are analyzed to identify cohort-specific SNPs at which conversion is predicted. The quality of the predictions is assessed by identifying known disease associations for genes in the homologous duplicons, and comparing the frequency of such associations with background rates.

RESULTS

Of 28 disease/locus pairs meeting stringent conditions, 22 show various degrees of disease association, compared with only 8 of 70 in a mock study designed to measure the background association rate (P < 10-9). Additional candidate genes are identified using less stringent filtering conditions. In some cases, somatic deletions appear likely. RA has a distinctive pattern of events relative to other diseases. Similarities in patterns are apparent between BD and HT.

CONCLUSIONS

The associations derived represent the first evidence that somatic gene conversion could be a significant causative factor in each of the seven diseases. The specific genes provide potential insights about disease mechanisms, and are strong candidates for further study.

Analysis of cell migration using Caenorhabditis elegans as a model system

The nematode Caenorhabditis elegans is an excellent model system in which to study long-distance cell migration in vivo. This chapter describes methods used to study a subset of migratory cells in the hermaphrodite nematode, the distal tip cells. These methods take advantage of the organism's transparent body and the expression of green fluorescent protein to observe cell migration and behavior. Additionally, the availability of nematode mutants and gene knockdown techniques that affect cell migration allow the analysis and comparison of wild-type and aberrant migratory paths. Methods for nematode growth and maintenance, strain acquisition, observation and live imaging, gene knockdown, and analysis of cell migration defects are covered.

Roles for netrin signaling outside of axon guidance: a view from the worm

The Netrin family of extracellular ligands and their receptors were the first identified signaling pathway regulating axon guidance. Subsequent work across model systems has begun to reveal the interactions that take place downstream of Netrin reception to facilitate growth cone migration. Though intensely studied, many aspects of this signaling system remain unclear. Even less understood are the growing number of contexts in which Netrin signaling influences cells beyond axon guidance and even outside the nervous system. Genetic and cell-biological studies in C. elegans have played an instrumental role in identifying critical functions for Netrin ligands in setting up specialized and potentially adhesive membrane-associated domains within a broad range of cell types. Here we review recent literature implicating Netrin or its receptors in morphogenetic processes outside of growth cone regulation with a special focus on studies in C. elegans that suggest cell biological mechanisms for Netrin signaling.