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 RhoGAP RRC-1 is required for the assembly or stability of integrin adhesion complexes and is a member of the PIX pathway in muscle

GTPases cycle between active GTP bound and inactive GDP bound forms. Exchange of GDP for GTP is catalyzed by guanine nucleotide exchange factors (GEFs). GTPase activating proteins (GAPs) accelerate GTP hydrolysis, to promote the GDP bound form. We reported that the RacGEF, PIX-1, is required for assembly of integrin adhesion complexes (IAC) in striated muscle of Caenorhabditis elegans. In C. elegans, IACs are found at the muscle cell boundaries (MCBs), and bases of sarcomeric M-lines and dense bodies (Z-disks). Screening C. elegans mutants in proteins containing RhoGAP domains revealed that loss of function of rrc-1 results in loss of IAC components at MCBs, disorganization of M-lines and dense bodies, and reduced whole animal locomotion. RRC-1 localizes to MCBs, like PIX-1. The localization of RRC-1 at MCBs requires PIX-1, and the localization of PIX-1 requires RRC-1. Loss of function of CED-10 (Rac) shows lack of PIX-1 and RRC-1 at MCBs. RRC-1 exists in a complex with PIX-1. Transgenic rescue of rrc-1 was achieved with wild type RRC-1 but not RRC-1 with a missense mutation in a highly conserved residue of the RhoGAP domain. Our results are consistent with RRC-1 being a RhoGAP for the PIX pathway in muscle.

Caenorhabditis elegans LET-413 Scribble is essential in the epidermis for growth, viability, and directional outgrowth of epithelial seam cells

The conserved adapter protein Scribble (Scrib) plays essential roles in a variety of cellular processes, including polarity establishment, proliferation, and directed cell migration. While the mechanisms through which Scrib promotes epithelial polarity are beginning to be unraveled, its roles in other cellular processes including cell migration remain enigmatic. In C. elegans, the Scrib ortholog LET-413 is essential for apical–basal polarization and junction formation in embryonic epithelia. However, whether LET-413 is required for postembryonic development or plays a role in migratory events is not known. Here, we use inducible protein degradation to investigate the functioning of LET-413 in larval epithelia. We find that LET-413 is essential in the epidermal epithelium for growth, viability, and junction maintenance. In addition, we identify a novel role for LET-413 in the polarized outgrowth of the epidermal seam cells. These stem cell-like epithelial cells extend anterior and posterior directed apical protrusions in each larval stage to reconnect to their neighbors. We show that the role of LET-413 in seam cell outgrowth is likely mediated largely by the junctional component DLG-1 discs large, which we demonstrate is also essential for directed outgrowth of the seam cells. Our data uncover multiple essential functions for LET-413 in larval development and show that the polarized outgrowth of the epithelial seam cells is controlled by LET-413 Scribble and DLG-1 Discs large.

Most cells in multicellular organisms are organized along a directional axis of cell polarity. One protein that is important for this polarized organization is the conserved polarity regulator Scribble. This protein has several functions, including forming the basolateral domains of cells, promoting the formation of cell junctions, and promoting cell migration. How Scribble performs these functions is not fully understood. In this paper we study the role of Scribble during larval development of the small nematode Caenorhabditis elegans using an inducible protein degradation system. We show that Scribble, called LET-413 in C. elegans, is essential in the epidermal epithelium for animal development, as depletion of LET-413 in only this tissue blocks growth. We also demonstrate that LET-413 is required for the polarized outgrowth of an epithelial cell type called the seam cells, a process resembling cell migration. Finally, we show that one major function of LET-413 in seam cell outgrowth is the localization of the junctional component Discs large (DLG-1), which we demonstrate is also essential for this process. Our data thus uncover multiple essential functions for LET-413 in larval development and provide new insights into how the directional outgrowth of epithelial seam cells is controlled.

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.

Microcephaly with altered cortical layering in GIT1 deficiency revealed by quantitative neuroimaging

G Protein-Coupled Receptor Kinase-Interacting Protein-1 (GIT1) regulates neuronal functions, including cell and axon migration and synapse formation and maintenance, and GIT1 knockout (KO) mice exhibit learning and memory deficits. We noted that male and female GIT1-KO mice exhibit neuroimaging phenotypes including microcephaly, and altered cortical layering, with a decrease in neuron density in cortical layer V. Micro-CT and magnetic resonance microscopy (MRM) were used to identify morphometric phenotypes for the skulls and throughout the GIT1-KO brains. High field MRM of actively-stained mouse brains from GIT1-KO and wild type (WT) controls (n = 6 per group) allowed segmenting 37 regions, based on co-registration to the Waxholm Space atlas. Overall brain size in GIT1-KO mice was ~32% smaller compared to WT controls. After correcting for brain size, several regions were significantly different in GIT1-KO mice relative to WT, including the gray matter of the ventral thalamic nuclei and the rest of the thalamus, the inferior colliculus, and pontine nuclei. GIT1-KO mice had reduced volume of white matter tracts, most notably in the anterior commissure (~26% smaller), but also in the cerebral peduncle, fornix, and spinal trigeminal tract. On the other hand, the basal ganglia appeared enlarged in GIT1-KO mice, including the globus pallidus, caudate putamen, and particularly the accumbens - supporting a possible vulnerability to addiction. Volume based morphometry based on high-resolution MRM (21.5 μm isotropic voxels) was effective in detecting overall, and local differences in brain volumes in GIT1-KO mice, including in white matter tracts. The reduced relative volume of specific brain regions suggests a critical, but not uniform, role for GIT1 in brain development, conducive to brain microcephaly, and aberrant connectivity.

The Rho-GEF PIX-1 directs assembly or stability of lateral attachment structures between muscle cells

PIX proteins are guanine nucleotide exchange factors (GEFs) that activate Rac and Cdc42, and are known to have numerous functions in various cell types. Here, we show that a PIX protein has an important function in muscle. From a genetic screen in C. elegans, we found that pix-1 is required for the assembly of integrin adhesion complexes (IACs) at borders between muscle cells, and is required for locomotion of the animal. A pix-1 null mutant has a reduced level of activated Rac in muscle. PIX-1 localizes to IACs at muscle cell boundaries, M-lines and dense bodies. Mutations in genes encoding proteins at known steps of the PIX signaling pathway show defects at muscle cell boundaries. A missense mutation in a highly conserved residue in the RacGEF domain results in normal levels of PIX-1 protein, but a reduced level of activated Rac in muscle, and abnormal IACs at muscle cell boundaries.

The Predicted RNA-Binding Protein ETR-1/CELF1 Acts in Muscles To Regulate Neuroblast Migration in Caenorhabditis elegans

Neuroblast migration is a critical aspect of nervous system development (e.g., neural crest migration). In an unbiased forward genetic screen, we identified a novel player in neuroblast migration, the ETR-1/CELF1 RNA binding protein. CELF1 RNA binding proteins are involved in multiple aspects of RNA processing including alternative splicing, stability, and translation. We find that a specific mutation in alternatively-spliced exon 8 results in migration defects of the AQR and PQR neurons, and not the embryonic lethality and body wall muscle defects of complete knockdown of the locus. Surprisingly, ETR-1 was required in body wall muscle cells for AQR/PQR migration (i.e., it acts cell non-autonomously). Genetic interactions indicate that ETR-1 acts with Wnt signaling, either in the Wnt pathway or in a parallel pathway. Possibly, ETR-1 is involved in the production of a Wnt signal or a parallel signal by the body wall muscles that controls AQR and PQR neuronal migration. In humans, CELF1 is involved in a number of neuromuscular disorders. If the role of ETR-1/CELF1 is conserved, these disorders might also involve cell or neuronal migration. Finally, we describe a technique of amplicon sequencing to detect rare, cell-specific genome edits by CRISPR/Cas9 in vivo (CRISPR-seq) as an alternative to the T7E1 assay.

Glia Promote Synaptogenesis through an IQGAP PES-7 in C. elegans

Synapses are fundamental to the normal function of the nervous system. Glia play a pivotal role in regulating synaptic formation. However, how presynaptic neurons assemble synaptic structure in response to the glial signals remains largely unexplored. To address this question, we use cima-1 mutant C. elegans as an in vivo model, in which the astrocyte-like VCSC glial processes ectopically reach an asynaptic neurite region and promote presynaptic formation there. Through an RNAi screen, we find that the Rho GTPase CDC-42 and IQGAP PES-7 are required in presynaptic neurons for VCSC glia-induced presynaptic formation. In addition, we find that cdc-42 and pes-7 are also required for normal synaptogenesis during postembryonic developmental stages. PES-7 activated by CDC-42 promotes presynaptic formation, most likely through regulating F-actin assembly. Given the evolutionary conservation of CDC-42 and IQGAPs, we speculate that our findings in C. elegans apply to vertebrates.

The combined effect of oleonuezhenide and wedelolactone on proliferation and osteoblastogenesis of bone marrow mesenchymal stem cells

BACKGROUND

Regulation of the survival and differentiation of bone marrow mesenchymal stem cells is an essential consideration in the development of targeted drugs for treatment of osteoporosis.

PURPOSE

The present study aimed to evaluate the combined effect of wedelolactone and oleonuezhenide, two compounds from Chinese formula Er-Zhi-Wan, on osteoblastogenesis and the underlying molecular mechanisms.

METHODS

MTT assay was taken to evaluate cell proliferation. The alkaline phosphatase (ALP) activity assay was used to determine the activity of ALP. Alizarin red S (ARS) staining was taken to indicate the intensity of the calcium deposits. Quantitative real-time PCR and Western blot were performed to the levels of Runx2, Osteocalcin, and Osterix expression in mouse bone marrow mesenchymal stem cells (BMSCs). Ovariectomized mouse model and bone histomorphometric analysis were also used to research the effects of wedelolactone and oleonuezhenide on bone loss caused by ovariectomy.

RESULTS

Wedelolactone combined with oleonuezhenide enhanced osteoblast differentiation and bone mineralization. Osteoblastogenesis-related marker genes including osteocalcin, Runx2, and osteorix were upregulated in the presence of wedelolactone and oleonuezhenide. At the molecular level, oleonuezhenide did not affect GSK-3β phosphorylation induced by wedelolactone, but elevated casein kinase 2-alpha (CK2α) expression, resulting in β-catenin and Runx2 nuclear translocation. In addition, 30 µM wedelolactone-induced cytotoxicity in bone marrow mesenchymal stem cells was relieved by 9 µM oleonuezhenide. These cells were protected by oleonuezhenide and maintained osteoblastic activity. Oleonuezhenide increased Wnt5a and CK2α expression. Wedelolactone-reduced extracellular signal-regulated kinase (ERK) phosphorylation was reversed by oleonuezhenide. In ovariectomized mice, administration of wedelolactone and oleonuezhenide prevented ovariectomy-induced bone loss by enhancing osteoblastic activity.

CONCLUSION

These results suggested that oleonuezhenide enhanced the effects of wedelolactone on osteoblastogenesis. These two compounds could be developed as a combined therapeutic agent for osteoporosis.

Partially overlapping guidance pathways focus the activity of UNC-40/DCC along the anteroposterior axis of polarizing neuroblasts

Directional migration of neurons and neuronal precursor cells is a central process in nervous system development. In the nematode Caenorhabditis elegans, the two Q neuroblasts polarize and migrate in opposite directions along the anteroposterior body axis. Several key regulators of Q cell polarization have been identified, including MIG-21, DPY-19/DPY19L1, the netrin receptor UNC-40/DCC, the Fat-like cadherin CDH-4 and CDH-3/Fat, which we describe in this study. How these different transmembrane proteins act together to direct Q neuroblast polarization and migration is still largely unknown. Here, we demonstrate that MIG-21 and DPY-19, CDH-3 and CDH-4, and UNC-40 define three distinct pathways that have partially redundant roles in protrusion formation, but also separate functions in regulating protrusion direction. Moreover, we show that the MIG-21, DPY-19 and Fat-like cadherin pathways control the localization and clustering of UNC-40 at the leading edge of the polarizing Q neuroblast, and that this is independent of the UNC-40 ligands UNC-6/netrin and MADD-4. Our results provide insight into a novel mechanism for ligand-independent localization of UNC-40 that directs the activity of UNC-40 along the anteroposterior axis.

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.

Expanding functions of GIT Arf GTPase-activating proteins, PIX Rho guanine nucleotide exchange factors and GIT-PIX complexes

The GIT proteins, GIT1 and GIT2, are GTPase-activating proteins (inactivators) for the ADP-ribosylation factor (Arf) small GTP-binding proteins, and function to limit the activity of Arf proteins. The PIX proteins, α-PIX and β-PIX (also known as ARHGEF6 and ARHGEF7, respectively), are guanine nucleotide exchange factors (activators) for the Rho family small GTP-binding protein family members Rac1 and Cdc42. Through their multi-domain structures, GIT and PIX proteins can also function as signaling scaffolds by binding to numerous protein partners. Importantly, the constitutive association of GIT and PIX proteins into oligomeric GIT-PIX complexes allows these two proteins to function together as subunits of a larger structure that coordinates two distinct small GTP-binding protein pathways and serves as multivalent scaffold for the partners of both constituent subunits. Studies have revealed the involvement of GIT and PIX proteins, and of the GIT-PIX complex, in numerous fundamental cellular processes through a wide variety of mechanisms, pathways and signaling partners. In this Commentary, we discuss recent findings in key physiological systems that exemplify current understanding of the function of this important regulatory complex. Further, we draw attention to gaps in crucial information that remain to be filled to allow a better understanding of the many roles of the GIT-PIX complex in health and disease.

Tiam-Rac signaling mediates trans-endocytosis of ephrin receptor EphB2 and is important for cell repulsion

Cell repulsion requires trans-endocytosis of ephrin receptors at cell–cell contact sites, but the mechanisms underlying this process are unclear. Here, Gaitanos et al. show that Tiam–Rac signaling mediates trans-endocytosis of EphB2 and is necessary for cell repulsion.

Ephrin receptors interact with membrane-bound ephrin ligands to regulate contact-mediated attraction or repulsion between opposing cells, thereby influencing tissue morphogenesis. Cell repulsion requires bidirectional trans-endocytosis of clustered Eph–ephrin complexes at cell interfaces, but the mechanisms underlying this process are poorly understood. Here, we identified an actin-regulating pathway allowing ephrinB⁺ cells to trans-endocytose EphB receptors from opposing cells. Live imaging revealed Rac-dependent F-actin enrichment at sites of EphB2 internalization, but not during vesicle trafficking. Systematic depletion of Rho family GTPases and their regulatory proteins identified the Rac subfamily and the Rac-specific guanine nucleotide exchange factor Tiam2 as key components of EphB2 trans-endocytosis, a pathway previously implicated in Eph forward signaling, in which ephrins act as in trans ligands of Eph receptors. However, unlike in Eph signaling, this pathway is not required for uptake of soluble ligands in ephrinB⁺ cells. We also show that this pathway is required for EphB2-stimulated contact repulsion. These results support the existence of a conserved pathway for EphB trans-endocytosis that removes the physical tether between cells, thereby enabling cell repulsion.

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.

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.

PLR-1, a putative E3 ubiquitin ligase, controls cell polarity and axonal extensions in C. elegans

During embryonic development neurons differentiate and extend axons and dendrites that have to reach their appropriate targets. In Caenorhabditis elegans the AVG neuron is the first neuron to extend an axon during the establishment of the ventral nerve cord, the major longitudinal axon tract in the animal. In genetic screens we isolated alleles of plr-1, which caused polarity reversals of the AVG neuron as well as outgrowth and navigation defects of the AVG axon. In addition plr-1 mutants show outgrowth defects in several other classes of neurons as well as the posterior excretory canals. plr-1 is predicted to encode a transmembrane E3 ubiquitin ligase and is widely expressed in the animal including the AVG neuron and the excretory cell. plr-1 has recently been shown to negatively regulate Wnt signalling by removing Wnt receptors from the cell surface. We observed that mutations in a gene reducing Wnt signalling as well as mutations in unc-53/NAV2 and unc-73/Trio suppress the AVG polarity defects in plr-1 mutants, but not the defects seen in other cells. This places plr-1 in a Wnt regulation pathway, but also suggests that plr-1 has Wnt independent functions and interacts with unc-53 and unc-73 to control cell polarity.

Spatial and molecular cues for cell outgrowth during C. elegans uterine development

The Caenorhabditis elegans uterine seam cell (utse) is an H-shaped syncytium that connects the uterus to the body wall. Comprising nine nuclei that move outward in a bidirectional manner, this synctium undergoes remarkable shape change during development. Using cell ablation experiments, we show that three surrounding cell types affect utse development: the uterine toroids, the anchor cell and the sex myoblasts. The presence of the anchor cell (AC) nucleus within the utse is necessary for proper utse development and AC invasion genes fos-1, cdh-3, him-4, egl-43, zmp-1 and mig-10 promote utse cell outgrowth. Two types of uterine lumen epithelial cells, uterine toroid 1 (ut1) and uterine toroid 2 (ut2), mediate proper utse outgrowth and we show roles in utse development for two genes expressed in the uterine toroids: the RASEF ortholog rsef-1 and Trio/unc-73. The SM expressed gene unc-53/NAV regulates utse cell shape; ablation of sex myoblasts (SMs), which generate uterine and vulval muscles, cause defects in utse morphology. Our results clarify the nature of the interactions that exist between utse and surrounding tissue, identify new roles for genes involved in cell outgrowth, and present the utse as a new model system for understanding cell shape change and, putatively, diseases associated with cell shape change.

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.

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.

Redundant canonical and noncanonical Caenorhabditis elegans p21-activated kinase signaling governs distal tip cell migrations

p21-activated kinases (Paks) are prominent mediators of Rac/Cdc42-dependent and -independent signaling and regulate signal transduction and cytoskeletal-based cell movements. We used the reproducible migrations of the Caenorhabditis elegans gonadal distal tip cells to show that two of the three nematode Pak proteins, MAX-2 and PAK-1, function redundantly in regulation of cell migration but are regulated by very different mechanisms. First, we suggest that MAX-2 requires CED-10/Rac function and thus functions canonically. Second, PIX-1 and GIT-1 function in the same role as PAK-1, and PAK-1 interaction with PIX-1 is required for PAK-1 activity; thus, PAK-1 functions noncanonically. The human Pak-Pix-Git complex is central to noncanonical Pak signaling and requires only modest Rac/CDC-42 input. Unlike the human complex, our results suggest that the C. elegans Pak-Pix-Git complex requires PAK-1 kinase domain activity. This study delineates signaling network relationships in this cell migration model, thus providing potential further mechanistic insights and an assessment of total Pak contribution to cell migration events.

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.

TIPE2 controls innate immunity to RNA by targeting the phosphatidylinositol 3-kinase-Rac pathway

RNA receptors such as TLR3 and retinoid acid-inducible gene I/melanoma differentiation-associated gene 5 play essential roles in innate immunity to RNA viruses. However, how innate immunity to RNAs is controlled at the molecular level is not well understood. We describe in this study a new regulatory pathway of anti-RNA immunity that is composed of PI3K, its target GTPase Rac, and the newly described immune regulator TNF-α-induced protein 8 like-2 (TIPE2, or TNFAIP8L2). Polyinosinic-polycytidylic acid [Poly (I:C)], a dsRNA receptor ligand, activates Rac via its guanine nucleotide exchange factor Tiam; this leads to the activation of cytokine genes and, paradoxically, downregulation of the Tipe2 gene. TIPE2 is a negative regulator of immunity; its deficiency leads to hyperactivation of the PI3K-Rac pathway as exemplified by enhanced AKT, Rac, P21-activated kinase, and IFN regulatory factor 3 activities. As a consequence, TIPE2 knockout myeloid cells are hyperreactive to Poly (I:C) stimulation, and TIPE2 knockout mice are hypersensitive to Poly (I:C)-induced lethality. These results indicate that TIPE2 controls innate immunity to RNA by targeting the PI3K-Rac pathway. Therefore, manipulating TIPE2 or Rac functions can be effective for controlling RNA viral infections.

Echovirus 30 induced neuronal cell death through TRIO-RhoA signaling activation

BACKGROUND

Echovirus 30 (Echo30) is one of the most frequently identified human enteroviruses (EVs) causing aseptic meningitis and encephalitis. However the mechanism underlying the pathogenesis of Echo30 infection with significant clinical outcomes is not completely understood. The aim of this investigation is to illustrate molecular pathologic alteration in neuronal cells induced by Echo30 infection using clinical isolate from young patient with neurologic involvement.

METHODOLOGY/PRINCIPAL FINDINGS

To characterize the neuronal cellular response to Echo30 infection, we performed a proteomic analysis based on two-dimensional gel electrophoresis (2-DE) and MALDI-TOF/TOF Mass Spectrophotometric (MS) analysis. We identified significant alteration of several protein expression levels in Echo30-infected SK-N-SH cells. Among these proteins, we focused on an outstanding up-regulation of Triple functional domain (TRIO) in Echo30-infected SK-N-SH cells. Generally, TRIO acts as a key component in the regulation of axon guidance and cell migration. In this study, we determined that TRIO plays a role in the novel pathways in Echo30 induced neuronal cell death.

CONCLUSIONS/SIGNIFICANCE

Our finding shows that TRIO plays a critical role in neuronal cell death by Echo30 infection. Echo30 infection activates TRIO-guanine nucleotide exchange factor (GEF) domains (GEFD2) and RhoA signaling in turn. These results suggest that Echo30 infection induced neuronal cell death by activation of the TRIO-RhoA signaling. We expect the regulation of TRIO-RhoA signaling may represent a new therapeutic approach in treating aseptic meningitis and encephalitis induced by Echo30.

Rac GTPase signaling in mechanotransduction during embryonic morphogenesis

How cells sense and respond to mechanical forces is attracting considerable attention. We recently demonstrated that mechanical tension originating from one tissue strongly influences the differentiation and morphogenesis of another tissue during C. elegans embryogenesis (Nature 471:99-103). Specifically, we found that the repeated contractions of muscle cells stimulate a signaling cascade involving the Rac GTPase within the epidermis. This pathway ultimately leads to strengthen hemidesmosome-like junctions and promote embryonic morphogenesis. Our work provides further evidence that mechanical inputs impact on development, much like inputs involving growth factors and morphogens. After briefly outlining the pioneering work that inspired us, I will present the mechanotransduction process underlying the response to tension and the key experiments supporting our conclusions.