Role of a new Rho family member in cell migration and axon guidance in C. elegans

Two C. elegans DM domain proteins, DMD-3 and MAB-3, function in late stages of male somatic gonad development

To bring about sexual dimorphism in form, information from the sex determination pathway must trigger sex-specific modifications in developmental programs. DM-domain encoding genes have been found to be involved in sex determination in a multitude of animals, often at the level of male somatic gonad formation. Here we report our findings that the DM-domain transcription factors MAB-3 and DMD-3 function together in multiple steps during the late stages of C. elegans male somatic gonad development. Both mab-3 and dmd-3 are expressed in the linker cell and hindgut of L4 males and dmd-3 is also expressed in presumptive vas deferens cells. Furthermore, dmd-3, but not mab-3, expression in the linker cell is downstream of nhr-67, a nuclear hormone receptor that was previously shown to control late stages of linker cell migration. In mab-3; dmd-3 double mutant males, the last stage of linker cell migration is partially defective, resulting in aberrant linker cell shapes and often a failure of the linker cell to complete its migration to the hindgut. When mab-3; dmd-3 double mutant linker cells do complete their migration, they fail to be engulfed by the hindgut, indicating that dmd-3 and mab-3 activity are essential for this process. Furthermore, linker cell death and clearance are delayed in mab-3; dmd-3 double mutants, resulting in the linker cell persisting into adulthood. Finally, DMD-3 and MAB-3 function to activate expression of the bZIP transcription factor encoding gene zip-5 and downregulate the expression of the zinc metalloprotease ZMP-1 in the linker cell. Taken together, these results demonstrate a requirement for DM-domain transcription factors in controlling C. elegans male gonad formation, supporting the notion that the earliest DM-domain genes were involved in male somatic gonad development in the last common ancestor of the bilaterians.

The Rac pathway prevents cell fragmentation in a nonprotrusively migrating leader cell during C. elegans gonad organogenesis

The C. elegans hermaphrodite distal tip cell (DTC) leads gonadogenesis. Loss-of-function mutations in a C. elegans ortholog of the Rac1 GTPase (ced-10) and its GEF complex (ced-5/DOCK180, ced-2/CrkII, ced-12/ELMO) cause gonad migration defects related to directional sensing; we discovered an additional defect class of gonad bifurcation in these mutants. Using genetic approaches, tissue-specific and whole-body RNAi, and in vivo imaging of endogenously tagged proteins and marked cells, we find that loss of Rac1 or its regulators causes the DTC to fragment as it migrates. Both products of fragmentation-the now-smaller DTC and the membranous patch of cellular material-localize important stem cell niche signaling (LAG-2 ligand) and migration (INA-1/integrin subunit alpha) factors to their membranes, but only one retains the DTC nucleus and therefore the ability to maintain gene expression over time. The enucleate patch can lead a bifurcating branch off the gonad arm that grows through germ cell proliferation. Germ cells in this branch differentiate as the patch loses LAG-2 expression. While the nucleus is surprisingly dispensable for aspects of leader cell function, it is required for stem cell niche activity long term. Prior work found that Rac1-/-;Rac2-/- mouse erythrocytes fragment; in this context, our new findings support the conclusion that maintaining a cohesive but deformable cell is a conserved function of this important cytoskeletal regulator.

Getting there in one piece: The Rac pathway prevents cell fragmentation in a nonprotrusively migrating leader cell during organogenesis

The C. elegans hermaphrodite distal tip cell (DTC) leads gonadogenesis. Loss-of-function mutations in a C. elegans ortholog of the Rac1 GTPase (ced-10) and its GEF complex (ced-5/DOCK180, ced-2/CrkII, ced-12/ELMO) cause gonad migration defects related to directional sensing; we discovered an additional defect class of gonad bifurcation in these mutants. Using genetic approaches, tissue-specific and whole-body RNAi, and in vivo imaging of endogenously tagged proteins and marked cells, we find that loss of Rac1 or its regulators causes the DTC to fragment as it migrates. Both products of fragmentation-the now-smaller DTC and the membranous patch of cellular material-localize important stem cell niche signaling (LAG-2/DSL ligand) and migration (INA-1/integrin subunit alpha) factors to their membranes, but only one retains the DTC nucleus and therefore the ability to maintain gene expression over time. The enucleate patch can lead a bifurcating branch off the gonad arm that grows through germ cell proliferation. Germ cells in this branch differentiate as the patch loses LAG-2 expression. While the nucleus is surprisingly dispensable for aspects of leader cell function, it is required for stem cell niche activity long-term. Prior work found that Rac1-/-;Rac2-/- mouse erythrocytes fragment; in this context, our new findings support the conclusion that maintaining a cohesive but deformable cell is a conserved function of this important cytoskeletal regulator.

PFTK1 kinase regulates axogenesis during development via RhoA activation

BACKGROUND

PFTK1/Eip63E is a member of the cyclin-dependent kinases (CDKs) family and plays an important role in normal cell cycle progression. Eip63E expresses primarily in postnatal and adult nervous system in Drosophila melanogaster but its role in CNS development remains unknown. We sought to understand the function of Eip63E in the CNS by studying the fly ventral nerve cord during development.

RESULTS

Our results demonstrate that Eip63E regulates axogenesis in neurons and its deficiency leads to neuronal defects. Functional interaction studies performed using the same system identify an interaction between Eip63E and the small GTPase Rho1. Furthermore, deficiency of Eip63E homolog in mice, PFTK1, in a newly generated PFTK1 knockout mice results in increased axonal outgrowth confirming that the developmental defects observed in the fly model are due to defects in axogenesis. Importantly, RhoA phosphorylation and activity are affected by PFTK1 in primary neuronal cultures. We report that GDP-bound inactive RhoA is a substrate of PFTK1 and PFTK1 phosphorylation is required for RhoA activity.

CONCLUSIONS

In conclusion, our work establishes an unreported neuronal role of PFTK1 in axon development mediated by phosphorylation and activation of GDP-bound RhoA. The results presented add to our understanding of the role of Cdks in the maintenance of RhoA-mediated axon growth and its impact on CNS development and axonal regeneration.

Excitable dynamics in a molecularly-explicit model of cell motility: Mixed-mode oscillations and beyond

Mesenchymal cell motility is mainly regulated by two members of the Rho-family of GTPases, called Rac and Rho. The mutual inhibition exerted by these two proteins on each other's activation and the promotion of Rac activation by an adaptor protein called paxillin have been implicated in driving cellular polarization comprised of front (high active Rac) and back (high active Rho) during cell migration. Mathematical modeling of this regulatory network has previously shown that bistability is responsible for generating a spatiotemporal pattern underscoring cellular polarity called wave-pinning when diffusion is included. We previously developed a 6V reaction-diffusion model of this network to decipher the role of Rac, Rho and paxillin (along with other auxiliary proteins) in generating wave-pinning. In this study, we simplify this model through a series of steps into an excitable 3V ODE model comprised of one fast variable (the scaled concentration of active Rac), one slow variable (the maximum paxillin phosphorylation rate - turned into a variable) and a very slow variable (a recovery rate - also turned into a variable). We then explore, through slow-fast analysis, how excitability is manifested by showing that the model can exhibit relaxation oscillations (ROs) as well as mixed-mode oscillations (MMOs) whose underlying dynamics are consistent with a delayed Hopf bifurcation with a canard explosion. By reintroducing diffusion and the scaled concentration of inactive Rac into the model, we obtain a 4V PDE model that generates several unique spatiotemporal patterns that are relevant to cell motility. These patterns are then characterized and their impact on cell motility are explored by employing the cellular potts model (CPM). Our results reveal that wave pinning produces purely very directed motion in CPM, while MMOs allow for meandering and non-motile behaviors to occur. This highlights the role of MMOs as a potential mechanism for mesenchymal cell motility.

Polarity and mixed-mode oscillations may underlie different patterns of cellular migration

In mesenchymal cell motility, several migration patterns have been observed, including directional, exploratory and stationary. Two key members of the Rho-family of GTPases, Rac and Rho, along with an adaptor protein called paxillin, have been particularly implicated in the formation of such migration patterns and in regulating adhesion dynamics. Together, they form a key regulatory network that involves the mutual inhibition exerted by Rac and Rho on each other and the promotion of Rac activation by phosphorylated paxillin. Although this interaction is sufficient in generating wave-pinning that underscores cellular polarization comprised of cellular front (high active Rac) and back (high active Rho), it remains unclear how they interact collectively to induce other modes of migration detected in Chinese hamster Ovary (CHO-K1) cells. We previously developed a six-variable (6V) reaction-diffusion model describing the interactions of these three proteins (in their active/phosphorylated and inactive/unphosphorylated forms) along with other auxiliary proteins, to decipher their role in generating wave-pinning. In this study, we explored, through computational modeling and image analysis, how differences in timescales within this molecular network can potentially produce the migration patterns in CHO-K1 cells and how switching between migration modes could occur. To do so, the 6V model was reduced to an excitable 4V spatiotemporal model possessing three different timescales. The model produced not only wave-pinning in the presence of diffusion, but also mixed-mode oscillations (MMOs) and relaxation oscillations (ROs). Implementing the model using the Cellular Potts Model (CPM) produced outcomes in which protrusions in the cell membrane changed Rac-Rho localization, resulting in membrane oscillations and fast directionality variations similar to those observed experimentally in CHO-K1 cells. The latter was assessed by comparing the migration patterns of experimental with CPM cells using four metrics: instantaneous cell speed, exponent of mean-square displacement ([Formula: see text]-value), directionality ratio and protrusion rate. Variations in migration patterns induced by mutating paxillin's serine 273 residue were also captured by the model and detected by a machine classifier, revealing that this mutation alters the dynamics of the system from MMOs to ROs or nonoscillatory behaviour through variation in the scaled concentration of an active form of an adhesion protein called p21-Activated Kinase 1 (PAK). These results thus suggest that MMOs and adhesion dynamics are the key mechanisms regulating CHO-K1 cell motility.

A metazoan-specific C-terminal motif in EXC-4 and Gα-Rho/Rac signaling regulate cell outgrowth during tubulogenesis in C. elegans

Chloride intracellular channels (CLICs) are conserved proteins for which the cellular and molecular functions remain mysterious. An important insight into CLIC function came from the discovery that Caenorhabditis elegans EXC-4/CLIC regulates morphogenesis of the excretory canal (ExCa) cell, a single-cell tube. Subsequent work showed that mammalian CLICs regulate vascular development and angiogenesis, and human CLIC1 can rescue exc-4 mutants, suggesting conserved function in biological tube formation (tubulogenesis) and maintenance. However, the cell behaviors and signaling pathways regulated by EXC-4/CLICs during tubulogenesis in vivo remain largely unknown. We report a new exc-4 mutation, affecting a C-terminal residue conserved in virtually all metazoan CLICs, that reveals a specific role for EXC-4 in ExCa outgrowth. Cell culture studies suggest a function for CLICs in heterotrimeric G protein (Gα/β/γ)-Rho/Rac signaling, and Rho-family GTPases are common regulators of cell outgrowth. Using our new exc-4 mutant, we describe a previously unknown function for Gα-encoding genes (gpa-12/Gα12/13, gpa-7/Gαi, egl-30/Gαq and gsa-1/Gαs), ced-10/Rac and mig-2/RhoG in EXC-4-mediated ExCa outgrowth. Our results demonstrate that EXC-4/CLICs are primordial players in Gα-Rho/Rac-signaling, a pathway that is crucial for tubulogenesis in C. elegans and in vascular development.

Modulation of Serotonin Receptors in Neurodevelopmental Disorders: Focus on 5-HT7 Receptor

Since neurodevelopmental disorders (NDDs) influence more than 3% of children worldwide, there has been intense investigation to understand the etiology of disorders and develop treatments. Although there are drugs such as aripiprazole, risperidone, and lurasidone, these medications are not cures for the disorders and can only help people feel better or alleviate their symptoms. Thus, it is required to discover therapeutic targets in order to find the ultimate treatments of neurodevelopmental disorders. It is suggested that abnormal neuronal morphology in the neurodevelopment process is a main cause of NDDs, in which the serotonergic system is emerging as playing a crucial role. From this point of view, we noticed the correlation between serotonin receptor subtype 7 (5-HT₇R) and NDDs including autism spectrum disorder (ASD), fragile X syndrome (FXS), and Rett syndrome (RTT). 5-HT₇R modulators improved altered behaviors in animal models and also affected neuronal morphology via the 5-HT₇R/G₁₂ signaling pathway. Through the investigation of recent studies, it is suggested that 5-HT₇R could be a potential therapeutic target for the treatment of NDDs.

Parallel Rap1>RalGEF>Ral and Ras signals sculpt the C. elegans nervous system

Ras is the most commonly mutated oncogene in humans and uses three oncogenic effectors: Raf, PI3K, and RalGEF activation of Ral. Understanding the importance of RalGEF>Ral signaling in cancer is hampered by the paucity of knowledge about their function in animal development, particularly in cell movements. We found that mutations that disrupt function of RalGEF or Ral enhance migration phenotypes of mutants for genes with established roles in cell migration. We used as a model the migration of the canal associated neurons (CANs), and validated our results in HSN cell migration, neurite guidance, and general animal locomotion. These functions of RalGEF and Ral are specific to their control of Ral signaling output rather than other published functions of these proteins. In this capacity Ral functions cell autonomously as a permissive developmental signal. In contrast, we observed Ras, the canonical activator of RalGEF>Ral signaling in cancer, to function as an instructive signal. Furthermore, we unexpectedly identified a function for the close Ras relative, Rap1, consistent with activation of RalGEF>Ral. These studies define functions of RalGEF>Ral, Rap1 and Ras signaling in morphogenetic processes that fashion the nervous system. We have also defined a model for studying how small GTPases partner with downstream effectors. Taken together, this analysis defines novel molecules and relationships in signaling networks that control cell movements during development of the nervous system.

RhoG deficiency abrogates cytotoxicity of human lymphocytes and causes hemophagocytic lymphohistiocytosis

Exocytosis of cytotoxic granules (CG) by lymphocytes is required for the elimination of infected and malignant cells. Impairments in this process underly a group of diseases with dramatic hyperferritinemic inflammation termed hemophagocytic lymphohistiocytosis (HLH). Although genetic and functional studies of HLH have identified proteins controlling distinct steps of CG exocytosis, the molecular mechanisms that spatiotemporally coordinate CG release remain partially elusive. We studied a patient exhibiting characteristic clinical features of HLH associated with markedly impaired cytotoxic T lymphocyte (CTL) and natural killer (NK) cell exocytosis functions, who beared biallelic deleterious mutations in the gene encoding the small GTPase RhoG. Experimental ablation of RHOG in a model cell line and primary CTLs from healthy individuals uncovered a hitherto unappreciated role of RhoG in retaining CGs in the vicinity of the plasma membrane (PM), a fundamental prerequisite for CG exocytotic release. We discovered that RhoG engages in a protein-protein interaction with Munc13-4, an exocytosis protein essential for CG fusion with the PM. We show that this interaction is critical for docking of Munc13-4+ CGs to the PM and subsequent membrane fusion and release of CG content. Thus, our study illuminates RhoG as a novel essential regulator of human lymphocyte cytotoxicity and provides the molecular pathomechanism behind the identified here and previously unreported genetically determined form of HLH.

Trio family proteins as regulators of cell migration and morphogenesis in development and disease - mechanisms and cellular contexts

The well-studied members of the Trio family of proteins are Trio and kalirin in vertebrates, UNC-73 in Caenorhabditis elegans and Trio in Drosophila Trio proteins are key regulators of cell morphogenesis and migration, tissue organization, and secretion and protein trafficking in many biological contexts. Recent discoveries have linked Trio and kalirin to human disease, including neurological disorders and cancer. The genes for Trio family proteins encode a series of large multidomain proteins with up to three catalytic activities and multiple scaffolding and protein-protein interaction domains. As such, Trio family proteins engage a wide array of cell surface receptors, substrates and interaction partners to coordinate changes in cytoskeletal regulatory and protein trafficking pathways. We provide a comprehensive review of the specific mechanisms by which Trio family proteins carry out their functions in cells, highlight the biological and cellular contexts in which they occur, and relate how alterations in these functions contribute to human disease.

Reduction of mRNA export unmasks different tissue sensitivities to low mRNA levels during Caenorhabditis elegans development

Animal development requires the execution of specific transcriptional programs in different sets of cells to build tissues and functional organs. Transcripts are exported from the nucleus to the cytoplasm where they are translated into proteins that, ultimately, carry out the cellular functions. Here we show that in Caenorhabditis elegans, reduction of mRNA export strongly affects epithelial morphogenesis and germline proliferation while other tissues remain relatively unaffected. Epithelialization and gamete formation demand a large number of transcripts in the cytoplasm for the duration of these processes. In addition, our findings highlight the existence of a regulatory feedback mechanism that activates gene expression in response to low levels of cytoplasmic mRNA. We expand the genetic characterization of nuclear export factor NXF-1 to other members of the mRNA export pathway to model mRNA export and recycling of NXF-1 back to the nucleus. Our model explains how mutations in genes involved in general processes, such as mRNA export, may result in tissue-specific developmental phenotypes.

Invading, Leading and Navigating Cells in Caenorhabditis elegans: Insights into Cell Movement in Vivo

Highly regulated cell migration events are crucial during animal tissue formation and the trafficking of cells to sites of infection and injury. Misregulation of cell movement underlies numerous human diseases, including cancer. Although originally studied primarily in two-dimensional in vitro assays, most cell migrations in vivo occur in complex three-dimensional tissue environments that are difficult to recapitulate in cell culture or ex vivo Further, it is now known that cells can mobilize a diverse repertoire of migration modes and subcellular structures to move through and around tissues. This review provides an overview of three distinct cellular movement events in Caenorhabditis elegans-cell invasion through basement membrane, leader cell migration during organ formation, and individual cell migration around tissues-which together illustrate powerful experimental models of diverse modes of movement in vivo We discuss new insights into migration that are emerging from these in vivo studies and important future directions toward understanding the remarkable and assorted ways that cells move in animals.

The unc-53 gene negatively regulates rac GTPases to inhibit unc-5 activity during Distal tip cell migrations in C. elegans

The unc-53/NAV2 gene encodes for an adaptor protein required for cell migrations along the anteroposterior (AP) axes of C. elegans. This study identifies unc-53 as a novel component of signaling pathways regulating Distal tip cell (DTC) migrations along the AP and dorsoventral (DV) axes. unc-53 negatively regulates and functions downstream of ced-10/Rac pathway genes; ced-10/Rac and mig-2/RhoG, which are required for proper DTC migration. Moreover, unc-53 exhibits genetic interaction with abl-1 and unc-5, the 2 known negative regulators of ced-10/Rac signaling. Our genetic analysis supports the model, where abl-1 negatively regulates unc-53 during DTC migrations and requirement of unc-53 function during both AP and DV DTC migrations could be due to unc-53 mediated regulation of unc-5 activity.

Functional Coordination of WAVE and WASP in C. elegans Neuroblast Migration

Directional cell migration is critical for metazoan development. We define two molecular pathways that activate the Arp2/3 complex during neuroblast migration in Caenorhabditis elegans. The transmembrane protein MIG-13/Lrp12 is linked to the Arp2/3 nucleation-promoting factors WAVE or WASP through direct interactions with ABL-1 or SEM-5/Grb2, respectively. WAVE mutations partially impaired F-actin organization and decelerated cell migration, and WASP mutations did not inhibit cell migration but enhanced migration defects in WAVE-deficient cells. Purified SEM-5 and MIG-2 synergistically stimulated the F-actin branching activity of WASP-Arp2/3 in vitro. In GFP knockin animals, WAVE and WASP were largely organized into separate clusters at the leading edge, and the amount of WASP was less than WAVE but could be elevated by WAVE mutations. Our results indicate that the MIG-13-WAVE pathway provides the major force for directional cell motility, whereas MIG-13-WASP partially compensates for its loss, underscoring their coordinated activities in facilitating robust cell migration.

A Wnt-planar polarity pathway instructs neurite branching by restricting F-actin assembly through endosomal signaling

Spatial arrangement of neurite branching is instructed by both attractive and repulsive cues. Here we show that in C. elegans, the Wnt family of secreted glycoproteins specify neurite branching sites in the PLM mechanosensory neurons. Wnts function through MIG-1/Frizzled and the planar cell polarity protein (PCP) VANG-1/Strabismus/Vangl2 to restrict the formation of F-actin patches, which mark branching sites in nascent neurites. We find that VANG-1 promotes Wnt signaling by facilitating Frizzled endocytosis and genetically acts in a common pathway with arr-1/β-arrestin, whose mutation results in defective PLM branching and F-actin patterns similar to those in the Wnt, mig-1 or vang-1 mutants. On the other hand, the UNC-6/Netrin pathway intersects orthogonally with Wnt-PCP signaling to guide PLM branch growth along the dorsal-ventral axis. Our study provides insights for how attractive and repulsive signals coordinate to sculpt neurite branching patterns, which are critical for circuit connectivity.

Hippo kinases maintain polarity during directional cell migration in Caenorhabditis elegans

Precise positioning of cells is crucial for metazoan development. Despite immense progress in the elucidation of the attractive cues of cell migration, the repulsive mechanisms that prevent the formation of secondary leading edges remain less investigated. Here, we demonstrate that Caenorhabditis elegans Hippo kinases promote cell migration along the anterior-posterior body axis via the inhibition of dorsal-ventral (DV) migration. Ectopic DV polarization was also demonstrated in gain-of-function mutant animals for C. elegans RhoG MIG-2. We identified serine 139 of MIG-2 as a novel conserved Hippo kinase phosphorylation site and demonstrated that purified Hippo kinases directly phosphorylate MIG-2^S139 Live imaging analysis of genome-edited animals indicates that MIG-2^S139 phosphorylation impedes actin assembly in migrating cells. Intriguingly, Hippo kinases are excluded from the leading edge in wild-type cells, while MIG-2 loss induces uniform distribution of Hippo kinases. We provide evidence that Hippo kinases inhibit RhoG activity locally and are in turn restricted to the cell body by RhoG-mediated polarization. Therefore, we propose that the Hippo-RhoG feedback regulation maintains cell polarity during directional cell motility.

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.

The Rho GTPase Family Genes in Bivalvia Genomes: Sequence, Evolution and Expression Analysis

Background

Rho GTPases are important members of the Ras superfamily, which represents the largest signaling protein family in eukaryotes, and function as key molecular switches in converting and amplifying external signals into cellular responses. Although numerous analyses of Rho family genes have been reported, including their functions and evolution, a systematic analysis of this family has not been performed in Mollusca or in Bivalvia, one of the most important classes of Mollusca.

Results

In this study, we systematically identified and characterized a total set (Rho, Rac, Mig, Cdc42, Tc10, Rnd, RhoU, RhoBTB and Miro) of thirty Rho GTPase genes in three bivalve species, including nine in the Yesso scallop Patinopecten yessoensis, nine in the Zhikong scallop Chlamys farreri, and twelve in the Pacific oyster Crassostrea gigas. Phylogenetic analysis and interspecies comparison indicated that bivalves might possess the most complete types of Rho genes in invertebrates. A multiple RNA-seq dataset was used to investigate the expression profiles of bivalve Rho genes, revealing that the examined scallops share more similar Rho expression patterns than the oyster, whereas more Rho mRNAs are expressed in C. farreri and C. gigas than in P. yessoensis. Additionally, Rho, Rac and Cdc42 were found to be duplicated in the oyster but not in the scallops. Among the expanded Rho genes of C. gigas, duplication pairs with high synonymous substitution rates (Ks) displayed greater differences in expression.

Conclusion

A comprehensive analysis of bivalve Rho GTPase family genes was performed in scallop and oyster species, and Rho genes in bivalves exhibit greater conservation than those in any other invertebrate. This is the first study focusing on a genome-wide characterization of Rho GTPase genes in bivalves, and the findings will provide a valuable resource for a better understanding of Rho evolution and Rho GTPase function in Bivalvia.

Polarized Rac-dependent protrusions drive epithelial intercalation in the embryonic epidermis of C. elegans

Cell intercalation is a fundamental, coordinated cell rearrangement process that shapes tissues throughout animal development. Studies of intercalation within epithelia have focused almost exclusively on the localized constriction of specific apical junctions. Another widely deployed yet poorly understood alternative mechanism of epithelial intercalation relies on basolateral protrusive activity. Using the dorsal embryonic epidermis of Caenorhabditis elegans, we have investigated this alternative mechanism using high-resolution live cell microscopy and genetic analysis. We find that as dorsal epidermal cells migrate past one another they produce F-actin-rich protrusions polarized at their extending (medial) edges. These protrusions are controlled by the C. elegans Rac and RhoG orthologs CED-10 and MIG-2, which function redundantly to polarize actin polymerization upstream of the WAVE complex and WASP, respectively. We also identify UNC-73, the C. elegans ortholog of Trio, as a guanine nucleotide exchange factor (GEF) upstream of both CED-10 and MIG-2. Further, we identify a novel polarizing cue, CRML-1, which is the ortholog of human capping Arp2/3 myosin I linker (CARMIL), that localizes to the nonprotrusive lateral edges of dorsal cells. CRML-1 genetically suppresses UNC-73 function and, indirectly, actin polymerization. This network identifies a novel, molecularly conserved cassette that regulates epithelial intercalation via basolateral protrusive activity.

The Wnt Frizzled Receptor MOM-5 Regulates the UNC-5 Netrin Receptor through Small GTPase-Dependent Signaling to Determine the Polarity of Migrating Cells

Wnt and Netrin signaling regulate diverse essential functions. Using a genetic approach combined with temporal gene expression analysis, we found a regulatory link between the Wnt receptor MOM-5/Frizzled and the UNC-6/Netrin receptor UNC-5. These two receptors play key roles in guiding cell and axon migrations, including the migration of the C. elegans Distal Tip Cells (DTCs). DTCs migrate post-embryonically in three sequential phases: in the first phase along the Antero-Posterior (A/P) axis, in the second, along the Dorso-Ventral (D/V) axis, and in the third, along the A/P axis. Loss of MOM-5/Frizzled function causes third phase A/P polarity reversals of the migrating DTCs. We show that an over-expression of UNC-5 causes similar DTC A/P polarity reversals and that unc-5 deficits markedly suppress the A/P polarity reversals caused by mutations in mom-5/frizzled. This implicates MOM-5/Frizzled as a negative regulator of unc-5. We provide further evidence that small GTPases mediate MOM-5's regulation of unc-5 such that one outcome of impaired function of small GTPases like CED-10/Rac and MIG-2/RhoG is an increase in unc-5 function. The work presented here demonstrates the existence of cross talk between components of the Netrin and Wnt signaling pathways and provides further insights into the way guidance signaling mechanisms are integrated to orchestrate directed cell migration.

The RhoGAP activity of CYK-4/MgcRacGAP functions non-canonically by promoting RhoA activation during cytokinesis

Cytokinesis requires activation of the GTPase RhoA. ECT-2, the exchange factor responsible for RhoA activation, is regulated to ensure spatiotemporal control of contractile ring assembly. Centralspindlin, composed of the Rho family GTPase-activating protein (RhoGAP) MgcRacGAP/CYK-4 and the kinesin MKLP1/ZEN-4, is known to activate ECT-2, but the underlying mechanism is not understood. We report that ECT-2-mediated RhoA activation depends on the ability of CYK-4 to localize to the plasma membrane, bind RhoA, and promote GTP hydrolysis by RhoA. Defects resulting from loss of CYK-4 RhoGAP activity can be rescued by activating mutations in ECT-2 or depletion of RGA-3/4, which functions as a conventional RhoGAP for RhoA. Consistent with CYK-4 RhoGAP activity contributing to GEF activation, the catalytic domains of CYK-4 and ECT-2 directly interact. Thus, counterintuitively, CYK-4 RhoGAP activity promotes RhoA activation. We propose that the most active form of the cytokinetic RhoGEF involves complex formation between ECT-2, centralspindlin and RhoA.

DOI:http://dx.doi.org/10.7554/eLife.08898.001

Cell division is a process in which a cell splits to form two daughter cells. In most cases, the cell first duplicates its genetic material and then the two copies are pulled to opposite ends of the cell. A ring of protein filaments—called the contractile ring—then assembles to form a band around the cell at the site of the division. This ring contracts and the force generated separates the cells in a step known as cytokinesis.

A protein belonging to the Rho family, called RhoA, is essential for cytokinesis because it controls the formation of the contractile ring. Rho proteins are switched on by the activities of other proteins called guanine nucleotide exchange factors. Another group of proteins known as ‘GTPase activating proteins’ (or GAPs for short) generally act to promote the ability of Rho proteins to turn themselves off.

In animals and other multicellular organisms, a GAP called CYK-4 largely concentrates on the spindle midzone, but some of the protein also moves to part of the cell membrane near the future site of cell division. It binds to a guanine nucleotide exchange factor called ECT-2 to switch RhoA on, which in turn promotes the formation of the contractile ring. However, it is not clear why a protein that activates RhoA is also able to trigger its inactivation.

In this study, Zhang and Glotzer studied cell division in a roundworm called Caenorhabditis elegans. The experiments show that cells that lacked the GAP activity of CYK-4 were unable to complete cytokinesis because RhoA was not fully switched on. This requirement could be bypassed in cells with mutant forms of ECT-2 that were overactive. Therefore, an activity that was thought to inactivate RhoA actually promotes its activation. Further experiments show that the section (or ‘domain’) of CYK-4 that has GAP activity interacts directly with the guanine nucleotide exchange domain of ECT2. Zhang and Glotzer suggest that this interaction stimulates ECT2 and thereby promotes the activation of RhoA.

Further experiments will reveal how CYK-4 stimulates ECT-2. In addition, it will be important to determine whether other proteins with GAP domains also work in this unconventional way.

DOI:http://dx.doi.org/10.7554/eLife.08898.002

Neuron migration: anillin protects leading edge actin

Establishment of a neuronal system requires proper regulation of the F-actin-rich leading edges of migrating neurons and neurite growth cones. A new study shows that RhoG signals through the multi-domain protein anillin to stabilize F-actin in these structures.

Anillin Regulates Neuronal Migration and Neurite Growth by Linking RhoG to the Actin Cytoskeleton

Neuronal migration and neurite growth are essential events in neural development, but it remains unclear how guidance cues are transduced through receptors to the actin cytoskeleton, which powers these processes. We report that a cytokinetic scaffold protein, Anillin, is redistributed to the leading edge of the C. elegans Q neuroblast during cell migration and neurite growth. To bypass the requirement for Anillin in cytokinesis, we used the somatic CRISPR-Cas9 technique to generate conditional mutations in Anillin. We demonstrate that Anillin regulates cell migration and growth cone extension by stabilizing the F-actin network at the leading edge. Our biochemical analysis shows that the actin-binding domain of Anillin is sufficient to stabilize F-actin by antagonizing the F-actin severing activity of Cofilin. We further uncover that the active form of RhoG/MIG-2 directly binds to Anillin and recruits it to the leading edge. Our results reveal a novel pathway in which Anillin transduces the RhoG signal to the actin cytoskeleton during neuronal migration and neurite growth.

SYD-1C, UNC-40 (DCC) and SAX-3 (Robo) function interdependently to promote axon guidance by regulating the MIG-2 GTPase

During development, axons must integrate directional information encoded by multiple guidance cues and their receptors. Axon guidance receptors, such as UNC-40 (DCC) and SAX-3 (Robo), can function individually or combinatorially with other guidance receptors to regulate downstream effectors. However, little is known about the molecular mechanisms that mediate combinatorial guidance receptor signaling. Here, we show that UNC-40, SAX-3 and the SYD-1 RhoGAP-like protein function interdependently to regulate the MIG-2 (Rac) GTPase in the HSN axon of C. elegans. We find that SYD-1 mediates an UNC-6 (netrin) independent UNC-40 activity to promote ventral axon guidance. Genetic analysis suggests that SYD-1 function in axon guidance requires both UNC-40 and SAX-3 activity. Moreover, the cytoplasmic domains of UNC-40 and SAX-3 bind to SYD-1 and SYD-1 binds to and negatively regulates the MIG-2 (Rac) GTPase. We also find that the function of SYD-1 in axon guidance is mediated by its phylogenetically conserved C isoform, indicating that the role of SYD-1 in guidance is distinct from its previously described roles in synaptogenesis and axonal specification. Our observations reveal a molecular mechanism that can allow two guidance receptors to function interdependently to regulate a common downstream effector, providing a potential means for the integration of guidance signals.

The UNC-6/Netrin receptors UNC-40/DCC and UNC-5 inhibit growth cone filopodial protrusion via UNC-73/Trio, Rac-like GTPases and UNC-33/CRMP

UNC-6/Netrin is a conserved axon guidance cue that can mediate both attraction and repulsion. We previously discovered that attractive UNC-40/DCC receptor signaling stimulates growth cone filopodial protrusion and that repulsive UNC-40–UNC-5 heterodimers inhibit filopodial protrusion in C. elegans. Here, we identify cytoplasmic signaling molecules required for UNC-6-mediated inhibition of filopodial protrusion involved in axon repulsion. We show that the Rac-like GTPases CED-10 and MIG-2, the Rac GTP exchange factor UNC-73/Trio, UNC-44/Ankyrin and UNC-33/CRMP act in inhibitory UNC-6 signaling. These molecules were required for the normal limitation of filopodial protrusion in developing growth cones and for inhibition of growth cone filopodial protrusion caused by activated MYR::UNC-40 and MYR::UNC-5 receptor signaling. Epistasis studies using activated CED-10 and MIG-2 indicated that UNC-44 and UNC-33 act downstream of the Rac-like GTPases in filopodial inhibition. UNC-73, UNC-33 and UNC-44 did not affect the accumulation of full-length UNC-5::GFP and UNC-40::GFP in growth cones, consistent with a model in which UNC-73, UNC-33 and UNC-44 influence cytoskeletal function during growth cone filopodial inhibition.

VAV-1 acts in a single interneuron to inhibit motor circuit activity in Caenorhabditis elegans

The complex molecular and cellular mechanisms underlying neuronal control of animal movement are not well understood. Locomotion of Caenorhabditis elegans is mediated by a neuronal circuit that produces coordinated sinusoidal movement. Here we utilize this simple, yet elegant, behaviour to show that VAV-1, a conserved guanine nucleotide exchange factor for Rho-family GTPases, negatively regulates motor circuit activity and the rate of locomotion. While vav-1 is expressed in a small subset of neurons, we find that VAV-1 function is required in a single interneuron, ALA, to regulate motor neuron circuit activity. Furthermore, we show by genetic and optogenetic manipulation of ALA that VAV-1 is required for the excitation and activation of this neuron. We find that ALA signalling inhibits command interneuron activity by abrogating excitatory signalling in the command interneurons, which is responsible for promoting motor neuron circuit activity. Together, our data describe a novel neuromodulatory role for VAV-1-dependent signalling in the regulation of motor circuit activity and locomotion.

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.

P21 activated kinases: structure, regulation, and functions

The p21 activated kinases (Paks) are well known effector proteins for the Rho GTPases Cdc42 and Rac. The Paks contain 6 members, which fall into 2 families of proteins. The first family consists of Paks 1, 2, and 3, and the second consists of Paks 4, 5, and 6. While some of the Paks are ubiquitously expressed, others have more restrictive tissue specificity. All of them are found in the nervous system. Studies using cell culture, transgenic mice, and knockout mice, have revealed important roles for the Paks in cytoskeletal organization and in many aspects of cell growth and development. This review discusses the basic structures of the Paks, and their roles in cell growth, development, and in cancer.

MIG-10 (lamellipodin) has netrin-independent functions and is a FOS-1A transcriptional target during anchor cell invasion in C. elegans

To transmigrate basement membrane, cells must coordinate distinct signaling activities to breach and pass through this dense extracellular matrix barrier. Netrin expression and activity are strongly associated with invasion in developmental and pathological processes, but how netrin signaling is coordinated with other pathways during invasion is poorly understood. Using the model of anchor cell (AC) invasion in C. elegans, we have previously shown that the integrin receptor heterodimer INA-1/PAT-3 promotes netrin receptor UNC-40 (DCC) localization to the invasive cell membrane of the AC. UNC-6 (netrin)/UNC-40 interactions generate an invasive protrusion that crosses the basement membrane. To understand how UNC-40 signals during invasion, we have used genetic, site of action and live-cell imaging studies to examine the roles of known effectors of UNC-40 signaling in axon outgrowth during AC invasion. UNC-34 (Ena/VASP), the Rac GTPases MIG-2 and CED-10 and the actin binding protein UNC-115 (abLIM) are dedicated UNC-40 effectors that are recruited to the invasive membrane by UNC-40 and generate F-actin. MIG-10 (lamellipodin), an effector of UNC-40 in neurons, however, has independent functions from UNC-6/UNC-40. Furthermore, unlike other UNC-40 effectors, its expression is regulated by FOS-1A, a transcription factor that promotes basement membrane breaching. Similar to UNC-40, however, MIG-10 localization to the invasive cell membrane is also dependent on the integrin INA-1/PAT-3. These studies indicate that MIG-10 has distinct functions from UNC-40 signaling in cell invasion, and demonstrate that integrin coordinates invasion by localizing these molecules to the cell-basement membrane interface.

A pre- and co-knockdown of RNAseT enzyme, Eri-1, enhances the efficiency of RNAi induced gene silencing in Caenorhabditis elegans

Background

The approach of RNAi mediated gene knockdown, employing exogenous dsRNA, is being beneficially exploited in various fields of functional genomics. The immense utility of the approach came to fore from studies with model system C. elegans, but quickly became applicable with varied research models ranging from in vitro to various in vivo systems. Previously, there have been reports on the refractoriness of the neuronal cells to RNAi mediated gene silencing following which several modulators like eri-1 and lin-15 were described in C. elegans which, when present, would negatively impact the gene knockdown.

Methodology/Principal Findings

Taking a clue from these findings, we went on to screen hypothesis-driven- methodologies towards exploring the efficiency in the process of RNAi under various experimental conditions, wherein these genes would be knocked down preceding to, or concurrently with, the knocking down of a gene of interest. For determining the efficiency of gene knockdown, we chose to study visually stark phenotypes of uncoordinated movement, dumpy body morphology and blistered cuticle obtained by knocking down of genes unc-73, dpy-9 and bli-3 respectively, employing the RNAi-by-feeding protocol in model system C. elegans.

Conclusions/Significance

Our studies led to a very interesting outcome as the results reveal that amongst various methods tested, pre-incubation with eri-1 dsRNA synthesizing bacteria followed by co-incubation with eri-1 and gene-of-interest dsRNA synthesizing bacteria leads to the most efficient gene silencing as observed by the analysis of marker phenotypes. This provides an approach for effectively employing RNAi induced gene silencing while working with different genetic backgrounds including transgenic and mutant strains.

p21-Activated kinase 5: a pleiotropic kinase

The PAKs (p21-activated kinases) are highly conserved serine/threonine protein kinases which comprise six mammalian PAKs. PAK5 (p21-activated kinase 5) is the least understood member of PAKs that regulate many intracellular processes when they are stimulated by activated forms of the small GTPases Cdc42 and Rac. PAK5 takes an important part in multiple signal pathways in mammalian cells and controls a variety of cellular functions including cytoskeleton organization, cell motility and apoptosis. The main goal of this review is to describe the structure, mechanisms underlying its activity regulation, its role in apoptosis and the likely directions of further research.

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.

Mutationally activated Rho GTPases in cancer

The Rho family of GTPases (members of the Ras superfamily) are best known for their roles in regulating cytoskeletal dynamics. It is also well established that misregulation of Rho proteins contributes to tumorigenesis and metastasis. Unlike Ras proteins, which are frequently mutated in cancer (around 30%), Rho proteins themselves are generally not found to be mutated in cancer. Rather, misregulation of Rho activity in cancer was thought to occur by overexpression of these proteins or by misregulation of molecules that control Rho activity, such as activation or overexpression of GEFs and inactivation or loss of GAPs or GDIs. Recent studies, enabled by next-generation tumor exome sequencing, report activating point mutations in Rho GTPases as driver mutations in melanoma, as well as breast, and head and neck cancers. The Rac1(P29L) mutation identified in these tumor studies was previously identified by our lab as an activating Rac mutation in C. elegans neuronal development, highlighting the conserved nature of this mutation. Furthermore, this finding supports the relevance of studying Rho GTPases in model organisms such as C. elegans to study the mechanisms that underlie carcinogenesis. This review will describe the recent findings that report activating Rho mutations in various cancer types, moving Rho GTPases from molecules misregulated in cancer to mutagenic targets that drive tumorigenesis.

C. elegans PVF-1 inhibits permissive UNC-40 signalling through CED-10 GTPase to position the male ray 1 sensillum

Graded distributions of netrin and semaphorin guidance cues convey instructive polarity information to migrating cells and growth cones, but also have permissive (i.e. non-polarity determining) functions in mammalian development and repair. The permissive functions of these cues are largely uncharacterised at a molecular level. We found previously that UNC-6 (netrin) signals permissively through UNC-40 (DCC) and UNC-5 receptors to prevent anterior displacement of the ray 1 sensillum in the C. elegans male tail. UNC-6/UNC-40 signalling functions in parallel with SMP-1 (semaporin 1)/PLX-1 (plexin) signalling to prevent this defect. Here, we report that a deletion allele of pvf-1, which encodes a VEGF-related protein, causes no ray 1 defects, but enhances ray 1 defects of a plx-1 mutant, and unexpectedly also suppresses unc-6(ev400)-null mutant ray 1 defects. These mutant ray 1 inductive and suppressive effects are mimicked by the ability of unc-40(+) and ced-10(gain-of-function) multi-copy transgene arrays to induce ray 1 defects or suppress unc-6 mutant ray 1 defects, depending on their dosage, suggesting the pvf-1 mutation causes UNC-40 overactivity that interferes with signalling but is partially sensitive to UNC-6. Additional data suggest PVF-1 functions through four VEGF receptor-related proteins and inhibits only CED-10 (a GTPase), but not MIG-2-dependent UNC-40 activity, even though UNC-40 functions through both GTPases to position ray 1. pvf-1 and receptor mutant ray 1 defects are rescued by transgenes expressing mouse VEGF164 and human VEGF receptors, respectively. These data report the first case of VEGF-induced inhibition of the netrin signalling and a molecular conservation of VEGF function from worms to humans.

FGF signaling regulates Wnt ligand expression to control vulval cell lineage polarity in C. elegans

The interpretation of extracellular cues leading to the polarization of intracellular components and asymmetric cell divisions is a fundamental part of metazoan organogenesis. The Caenorhabditis elegans vulva, with its invariant cell lineage and interaction of multiple cell signaling pathways, provides an excellent model for the study of cell polarity within an organized epithelial tissue. Here, we show that the fibroblast growth factor (FGF) pathway acts in concert with the Frizzled homolog LIN-17 to influence the localization of SYS-1, a component of the Wnt/β-catenin asymmetry pathway, indirectly through the regulation of cwn-1. The source of the FGF ligand is the primary vulval precursor cell (VPC) P6.p, which controls the orientation of the neighboring secondary VPC P7.p by signaling through the sex myoblasts (SMs), activating the FGF pathway. The Wnt CWN-1 is expressed in the posterior body wall muscle of the worm as well as in the SMs, making it the only Wnt expressed on the posterior and anterior sides of P7.p at the time of the polarity decision. Both sources of cwn-1 act instructively to influence P7.p polarity in the direction of the highest Wnt signal. Using single molecule fluorescence in situ hybridization, we show that the FGF pathway regulates the expression of cwn-1 in the SMs. These results demonstrate an interaction between FGF and Wnt in C. elegans development and vulval cell lineage polarity, and highlight the promiscuous nature of Wnts and the importance of Wnt gradient directionality within C. elegans.

Serotonergic neurosecretory synapse targeting is controlled by netrin-releasing guidepost neurons in Caenorhabditis elegans

Neurosecretory release sites lack distinct postsynaptic partners, yet target to specific circuits. This targeting specificity regulates local release of neurotransmitters and modulation of adjacent circuits. How neurosecretory release sites target to specific regions is not understood. Here we identify a molecular mechanism that governs the spatial specificity of extrasynaptic neurosecretory terminal (ENT) formation in the serotonergic neurosecretory-motor (NSM) neurons of Caenorhabditis elegans. We show that postembryonic arborization and neurosecretory terminal targeting of the C. elegans NSM neuron is dependent on the Netrin receptor UNC-40/DCC. We observe that UNC-40 localizes to specific neurosecretory terminals at the time of axon arbor formation. This localization is dependent on UNC-6/Netrin, which is expressed by nerve ring neurons that act as guideposts to instruct local arbor and release site formation. We find that both UNC-34/Enabled and MIG-10/Lamellipodin are required downstream of UNC-40 to link the sites of ENT formation to nascent axon arbor extensions. Our findings provide a molecular link between release site development and axon arborization and introduce a novel mechanism that governs the spatial specificity of serotonergic ENTs in vivo.

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.

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.

Dock3 regulates BDNF-TrkB signaling for neurite outgrowth by forming a ternary complex with Elmo and RhoG

Dock3, a new member of the guanine nucleotide exchange factor family, causes cellular morphological changes by activating the small GTPase Rac1. Overexpression of Dock3 in neural cells promotes neurite outgrowth through the formation of a protein complex with Fyn and WAVE downstream of brain-derived neurotrophic factor (BDNF) signaling. Here, we report a novel Dock3-mediated BDNF pathway for neurite outgrowth. We show that Dock3 forms a complex with Elmo and activated RhoG downstream of BDNF-TrkB signaling and induces neurite outgrowth via Rac1 activation in PC12 cells. We also show the importance of Dock3 phosphorylation in Rac1 activation and show two key events that are necessary for efficient Dock3 phosphorylation: membrane recruitment of Dock3 and interaction of Dock3 with Elmo. These results suggest that Dock3 plays important roles downstream of BDNF signaling in the central nervous system where it stimulates actin polymerization by multiple pathways.

The Rac GTP exchange factor TIAM-1 acts with CDC-42 and the guidance receptor UNC-40/DCC in neuronal protrusion and axon guidance

The mechanisms linking guidance receptors to cytoskeletal dynamics in the growth cone during axon extension remain mysterious. The Rho-family GTPases Rac and CDC-42 are key regulators of growth cone lamellipodia and filopodia formation, yet little is understood about how these molecules interact in growth cone outgrowth or how the activities of these molecules are regulated in distinct contexts. UNC-73/Trio is a well-characterized Rac GTP exchange factor in Caenorhabditis elegans axon pathfinding, yet UNC-73 does not control CED-10/Rac downstream of UNC-6/Netrin in attractive axon guidance. Here we show that C. elegans TIAM-1 is a Rac-specific GEF that links CDC-42 and Rac signaling in lamellipodia and filopodia formation downstream of UNC-40/DCC. We also show that TIAM-1 acts with UNC-40/DCC in axon guidance. Our results indicate that a CDC-42/TIAM-1/Rac GTPase signaling pathway drives lamellipodia and filopodia formation downstream of the UNC-40/DCC guidance receptor, a novel set of interactions between these molecules. Furthermore, we show that TIAM-1 acts with UNC-40/DCC in axon guidance, suggesting that TIAM-1 might regulate growth cone protrusion via Rac GTPases in response to UNC-40/DCC. Our results also suggest that Rac GTPase activity is controlled by different GEFs in distinct axon guidance contexts, explaining how Rac GTPases can specifically control multiple cellular functions.

Axons extend great distances to make precise synaptic connections in the developing nervous system. Axons are guided to their targets by the growth cone, a dynamic structure at the axon distal tip that senses extracellular cues telling the axon where to go. In response to guidance cues, growth cones alter their shape and motility resulting in outgrowth and turning. The cytoskeleton (actin and microtubules) underlies growth cone motility and guidance. The signaling mechanisms linking guidance receptors to cytoskeletal change remain mysterious. Here, we define a new signaling mechanism downstream of the guidance receptor UNC-40/DCC involving the GTPases CDC-42 and Rac, which have long been known to control growth cone protrusion. We show that CDC-42 and Rac act in a linear pathway in axon guidance; CDC-42 acts upstream of the GTPase regulatory molecule TIAM-1, which is a GTP exchange factor specific for Rac and which activates Rac signaling. We also show that TIAM-1 acts with UNC-40/DCC signaling in protrusion and axon guidance. Our results imply that Rac GTPase function in axon guidance is complex and that distinct GEFs (TIAM-1 and UNC-73/Trio) might control Rac GTPases in different aspects of axon guidance.

Normal levels of Rac1 are important for dendritic but not axonal development in hippocampal neurons

BACKGROUND INFORMATION

Rho family GTPases are required for cytoskeletal reorganization and are considered important for the maturation of neurons. Among these proteins, Rac1 is known to play a crucial role in the regulation of actin dynamics, and a number of studies indicate the involvement of this protein in different steps of vertebrate neuronal maturation. There are two distinct Rac proteins expressed in neurons, namely the ubiquitous Rac1 and the neuron-specific Rac3. The specific functions of each of these GTPases during early neuronal development are largely unknown.

RESULTS

The combination of the knockout of Rac3 with Rac1 down-regulation by siRNA (small interfering RNA) has been used to show that down-regulation of Rac1 affects dendritic development in mouse hippocampal neurons, without affecting axons. F-actin levels are strongly decreased in neuronal growth cones following down-regulation of Rac1, and time-lapse analysis indicated that the reduction of Rac1 levels decreases growth-cone dynamics.

CONCLUSIONS

These results show that normal levels of endogenous Rac1 activity are critical for early dendritic development, whereas dendritic outgrowth is not affected in hippocampal neurons from Rac3-null mice. On the other hand, early axonal development appears normal after Rac1 down-regulation. Our findings also suggest that the initial establishment of neuronal polarity is not affected by Rac1 down-regulation.

Analysis of Rho GTPase function in axon pathfinding using Caenorhabditis elegans

We provide information and protocols for the analysis of Rho GTPase function in axon pathfinding in Caenorhabditis elegans. The powerful molecular, genetic, imaging, and transgenic tools available in C. elegans make it an excellent system in which to study the in vivo roles of Rho GTPases. Methods for imaging of axon morphology in Rho GTPase single and double mutants are provided, as well as methods for the construction of transgenic C. elegans strains carrying exogenously introduced transgenes that drive the expression of constitutively active and dominant negative mutants.

LIN-39 and the EGFR/RAS/MAPK pathway regulate C. elegans vulval morphogenesis via the VAB-23 zinc finger protein

Morphogenesis represents a phase of development during which cell fates are executed. The conserved hox genes are key cell fate determinants during metazoan development, but their role in controlling organ morphogenesis is less understood. Here, we show that the C. elegans hox gene lin-39 regulates epidermal morphogenesis via its novel target, the essential zinc finger protein VAB-23. During the development of the vulva, the egg-laying organ of the hermaphrodite, the EGFR/RAS/MAPK signaling pathway activates, together with LIN-39 HOX, the expression of VAB-23 in the primary cell lineage to control the formation of the seven vulval toroids. VAB-23 regulates the formation of homotypic contacts between contralateral pairs of cells with the same sub-fates at the vulval midline by inducing smp-1 (semaphorin) transcription. In addition, VAB-23 prevents ectopic vulval cell fusions by negatively regulating expression of the fusogen eff-1. Thus, LIN-39 and the EGFR/RAS/MAPK signaling pathway, which specify cell fates earlier during vulval induction, continue to act during the subsequent phase of cell fate execution by regulating various aspects of epidermal morphogenesis. Vulval cell fate specification and execution are, therefore, tightly coupled processes.

The phosphoinositide phosphatase MTM-1 regulates apoptotic cell corpse clearance through CED-5–CED-12 in C. elegans

Multicellular organisms use programmed cell death to eliminate unwanted or potentially harmful cells. Improper cell corpse removal can lead to autoimmune diseases. The development of interventional therapies that increase engulfment activity could represent an attractive approach to treat such diseases. Here, we describe mtm-1, the Caenorhabditis elegans homolog of human myotubularin 1, as a potential negative regulator of apoptotic cell corpse clearance. Loss of mtm-1 function leads to substantially reduced numbers of persistent cell corpses in engulfment mutants, which is a result of a restoration of engulfment function rather than of impaired or delayed programmed cell death. Epistatic analyses place mtm-1 upstream of the ternary GEF complex, which consists of ced-2, ced-5 and ced-12, and parallel to mig-2. Over-activation of engulfment results in the removal of viable cells that have been brought to the verge of death under limiting caspase activity. In addition, mtm-1 also promotes phagosome maturation in the hermaphrodite gonad, potentially through CED-1 receptor recycling. Finally, we show that the CED-12 PH domain can bind to PtdIns(3,5)P2 (one target of MTM-1 phosphatase activity), suggesting that MTM-1 might regulate CED-12 recruitment to the plasma membrane.

Polarized myosin produces unequal-size daughters during asymmetric cell division

Asymmetric positioning of the mitotic spindle before cytokinesis can produce different-sized daughter cells that have distinct fates. Here, we found an asymmetric division in the Caenorhabditis elegans Q neuroblast lineage that began with a centered spindle but generated different-sized daughters, the smaller (anterior) of which underwent apoptosis. During this division, more myosin II accumulated anteriorly, suggesting that asymmetric contractile forces might produce different-sized daughters. Indeed, partial inactivation of anterior myosin by chromophore-assisted laser inactivation created a more symmetric division and allowed the survival and differentiation of the anterior daughter. Thus, the balance of myosin activity on the two sides of a dividing cell can govern the size and fate of the daughters.

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.

Dock3 induces axonal outgrowth by stimulating membrane recruitment of the WAVE complex

Atypical Rho-guanine nucleotide exchange factors (Rho-GEFs) that contain Dock homology regions (DHR-1 and DHR-2) are expressed in a variety of tissues; however, their functions and mechanisms of action remain unclear. We identify key conserved amino acids in the DHR-2 domain that are critical for the catalytic activity of Dock-GEFs (Dock1-4). We further demonstrate that Dock-GEFs directly associate with WASP family verprolin-homologous (WAVE) proteins through the DHR-1 domain. Brain-derived neurotrophic factor (BDNF)-TrkB signaling recruits the Dock3/WAVE1 complex to the plasma membrane, whereupon Dock3 activates Rac and dissociates from the WAVE complex in a phosphorylation-dependent manner. BDNF induces axonal sprouting through Dock-dependent Rac activation, and adult transgenic mice overexpressing Dock3 exhibit enhanced optic nerve regeneration after injury without affecting WAVE expression levels. Our results highlight a unique mechanism through which Dock-GEFs achieve spatial and temporal restriction of WAVE signaling, and identify Dock-GEF activity as a potential therapeutic target for axonal regeneration.

CACN-1/Cactin interacts genetically with MIG-2 GTPase signaling to control distal tip cell migration in C. elegans

The two specialized C. elegans distal tip cells (DTCs) provide an in vivo model system for the study of developmentally regulated cell migration. We identified cacn-1/cactin, a well-conserved, novel regulator of cell migration in a genome-wide RNAi screen for regulators of DTC migration. RNAi depletion experiments and analysis of the hypomorphic allele cacn-1(tm3126) indicate that CACN-1 is required during DTC migration for proper pathfinding and for cessation of DTC migration at the end of larval morphogenesis. Strong expression of CACN-1 in the DTCs, and data from cell-specific RNAi depletion experiments, suggest that CACN-1 is required cell-autonomously to control DTC migration. Importantly, genetic interaction data with Rac GTPase activators and effectors suggest that CACN-1 acts specifically to inhibit the mig-2/Rac pathway, and in parallel to ced-10/Rac, to control DTC pathfinding.