CED-10/Rac1 mediates axon guidance by regulating the asymmetric distribution of MIG-10/lamellipodin

Adaptor protein Abelson interactor 1 in homeostasis and disease

Dysregulation of Abelson interactor 1 (ABI1) is associated with various states of disease including developmental defects, pathogen infections, and cancer. ABI1 is an adaptor protein predominantly known to regulate actin cytoskeleton organization processes such as those involved in cell adhesion, migration, and shape determination. Linked to cytoskeleton via vasodilator-stimulated phosphoprotein (VASP), Wiskott-Aldrich syndrome protein family (WAVE), and neural-Wiskott-Aldrich syndrome protein (N-WASP)-associated protein complexes, ABI1 coordinates regulation of various cytoplasmic protein signaling complexes dysregulated in disease states. The roles of ABI1 beyond actin cytoskeleton regulation are much less understood. This comprehensive, protein-centric review describes molecular roles of ABI1 as an adaptor molecule in the context of its dysregulation and associated disease outcomes to better understand disease state-specific protein signaling and affected interconnected biological processes.

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.

Muscleblind-1 interacts with tubulin mRNAs to regulate the microtubule cytoskeleton in C. elegans mechanosensory neurons

Regulation of the microtubule cytoskeleton is crucial for the development and maintenance of neuronal architecture, and recent studies have highlighted the significance of regulated RNA processing in the establishment and maintenance of neural circuits. In a genetic screen conducted using mechanosensory neurons of C. elegans, we identified a mutation in muscleblind-1/mbl-1 as a suppressor of loss of kinesin-13 family microtubule destabilizing factor klp-7. Muscleblind-1(MBL-1) is an RNA-binding protein that regulates the splicing, localization, and stability of RNA. Our findings demonstrate that mbl-1 is required cell-autonomously for axon growth and proper synapse positioning in the posterior lateral microtubule (PLM) neuron. Loss of mbl-1 leads to increased microtubule dynamics and mixed orientation of microtubules in the anterior neurite of PLM. These defects are also accompanied by abnormal axonal transport of the synaptic protein RAB-3 and reduction of gentle touch sensation in mbl-1 mutant. Our data also revealed that mbl-1 is genetically epistatic to mec-7 (β tubulin) and mec-12 (α tubulin) in regulating axon growth. Furthermore, mbl-1 is epistatic to sad-1, an ortholog of BRSK/Brain specific-serine/threonine kinase and a known regulator of synaptic machinery, for synapse formation at the correct location of the PLM neurite. Notably, the immunoprecipitation of MBL-1 resulted in the co-purification of mec-7, mec-12, and sad-1 mRNAs, suggesting a direct interaction between MBL-1 and these transcripts. Additionally, mbl-1 mutants exhibited reduced levels and stability of mec-7 and mec-12 transcripts. Our study establishes a previously unknown link between RNA-binding proteins and cytoskeletal machinery, highlighting their crucial roles in the development and maintenance of the nervous system.

Signal amplification in growth cone gradient sensing by a double negative feedback loop among PTEN, PI(3,4,5)P3 and actomyosin

Axon guidance during neural wiring involves a series of precisely controlled chemotactic events by the motile axonal tip, the growth cone. A fundamental question is how neuronal growth cones make directional decisions in response to extremely shallow gradients of guidance cues with exquisite sensitivity. Here we report that nerve growth cones possess a signal amplification mechanism during gradient sensing process. In neuronal growth cones of Xenopus spinal neurons, phosphatidylinositol-3,4,5-trisphosphate (PIP3), an important signaling molecule in chemotaxis, was actively recruited to the up-gradient side in response to an external gradient of brain-derived neurotrophic factor (BDNF), resulting in an intracellular gradient with approximate 30-fold amplification of the input. Furthermore, a reverse gradient of phosphatase and tensin homolog (PTEN) was induced by BDNF within the growth cone and the increased PTEN activity at the down-gradient side is required for the amplification of PIP3 signals. Mechanistically, the establishment of both positive PIP3 and reverse PTEN gradients depends on the filamentous actin network. Together with computational modeling, our results revealed a double negative feedback loop among PTEN, PIP3 and actomyosin for signal amplification, which is essential for gradient sensing of neuronal growth cones in response to diffusible cues.

The ANC-1 (Nesprin-1/2) organelle-anchoring protein functions through mitochondria to polarize axon growth in response to SLT-1

A family of giant KASH proteins, including C. elegans ANC-1 and mammalian Nesprin-1 and -2, are involved in organelle anchoring and are associated with multiple neurodevelopmental disorders including autism, bipolar disorder, and schizophrenia. However, little is known about how these proteins function in neurons. Moreover, the role of organelle anchoring in axon development is poorly understood. Here, we report that ANC-1 functions with the SLT-1 extracellular guidance cue to polarize ALM axon growth. This role for ANC-1 is specific to its longer ANC-1A and ANC-1C isoforms, suggesting that it is mechanistically distinct from previously described roles for ANC-1. We find that ANC-1 is required for the localization of a cluster of mitochondria to the base of the proximal axon. Furthermore, genetic and pharmacological studies indicate that ANC-1 functions with mitochondria to promote polarization of ALM axon growth. These observations reveal a mechanism whereby ANC-1 functions through mitochondria to polarize axon growth in response to SLT-1.

Dendrite regeneration in C. elegans is controlled by the RAC GTPase CED-10 and the RhoGEF TIAM-1

Neurons are vulnerable to physical insults, which compromise the integrity of both dendrites and axons. Although several molecular pathways of axon regeneration are identified, our knowledge of dendrite regeneration is limited. To understand the mechanisms of dendrite regeneration, we used the PVD neurons in C. elegans with stereotyped branched dendrites. Using femtosecond laser, we severed the primary dendrites and axon of this neuron. After severing the primary dendrites near the cell body, we observed sprouting of new branches from the proximal site within 6 hours, which regrew further with time in an unstereotyped manner. This was accompanied by reconnection between the proximal and distal dendrites, and fusion among the higher-order branches as reported before. We quantified the regeneration pattern into three aspects–territory length, number of branches, and fusion phenomena. Axonal injury causes a retraction of the severed end followed by a Dual leucine zipper kinase-1 (DLK-1) dependent regrowth from the severed end. We tested the roles of the major axon regeneration signalling hubs such as DLK-1-RPM-1, cAMP elevation, let-7 miRNA, AKT-1, Phosphatidylserine (PS) exposure/PS in dendrite regeneration. We found that neither dendrite regrowth nor fusion was affected by the axon injury pathway molecules. Surprisingly, we found that the RAC GTPase, CED-10 and its upstream GEF, TIAM-1 play a cell-autonomous role in dendrite regeneration. Additionally, the function of CED-10 in epidermal cell is critical for post-dendrotomy fusion phenomena. This work describes a novel regulatory mechanism of dendrite regeneration and provides a framework for understanding the cellular mechanism of dendrite regeneration using PVD neuron as a model system.

The knowledge of the repair of injured neural circuits comes from the study of the regeneration of injured axons. The information receiving neurites, namely dendrites, are also vulnerable to physical insult during stroke and trauma. However, little knowledge is available on the mechanism of dendrite regeneration since the study of Cajal. In order to get insight into this process, we severed both axon and dendrites of PVD neuron in C. elegans using laser. By comparing the roles of axon regeneration pathways in both dendrite and axon regeneration in this neuron, we found that dendrite regeneration is independent of molecular mechanisms involving axon regrowth. We discovered that dendrite regeneration is dependent on the RAC GTPase CED-10 and GEF TIAM-1. Moreover, we found that CED-10 plays roles within both neuron and in the surrounding epithelia for mounting regeneration response to dendrite injury. This work provides mechanistic insight into the process of dendrite repair after physical injury.

The Lamellipodin homologue MIG-10 is not essential for dorsal intercalation in the embryonic epidermis of the C. elegans embryo

Dorsal intercalation of the embryonic epidermis in the Caenorhabditis elegans embryo is a promising system for genetic analysis of convergent extension, a conserved process in animal embryos. We sought to identify functionally important actin regulators in dorsal epidermal cells. A promising candidate is MIG-10, the single MIG-10/RIAM/Lamellipodin (MRL) family member in C. elegans. We endogenously tagged all mig-10 isoforms with mNeonGreen and analyzed mig-10 mutants using 4-dimensional microscopy. MIG-10::mNG is expressed prominently in muscle progenitors but is not detectable in the dorsal epidermis. mig-10(ct41) homozygotes complete dorsal intercalation in a manner indistinguishable from wildtype, indicating MIG-10 is not essential during dorsal intercalation.

From whole organism to ultrastructure: progress in axonal imaging for decoding circuit development

Since the pioneering work of Ramón y Cajal, scientists have sought to unravel the complexities of axon development underlying neural circuit formation. Micrometer-scale axonal growth cones navigate to targets that are often centimeters away. To reach their targets, growth cones react to dynamic environmental cues that change in the order of seconds to days. Proper axon growth and guidance are essential to circuit formation, and progress in imaging has been integral to studying these processes. In particular, advances in high- and super-resolution microscopy provide the spatial and temporal resolution required for studying developing axons. In this Review, we describe how improved microscopy has revolutionized our understanding of axonal development. We discuss how novel technologies, specifically light-sheet and super-resolution microscopy, led to new discoveries at the cellular scale by imaging axon outgrowth and circuit wiring with extreme precision. We next examine how advanced microscopy broadened our understanding of the subcellular dynamics driving axon growth and guidance. We finally assess the current challenges that the field of axonal biology still faces for imaging axons, and examine how future technology could meet these needs.

Mechanosensitive expression of lamellipodin promotes intracellular stiffness, cyclin expression and cell proliferation

Cell cycle control is a key aspect of numerous physiological and pathological processes. The contribution of biophysical cues, such as stiffness or elasticity of the underlying extracellular matrix (ECM), is critically important in regulating cell cycle progression and proliferation. Indeed, increased ECM stiffness causes aberrant cell cycle progression and proliferation. However, the molecular mechanisms that control these stiffness-mediated cellular responses remain unclear. Here, we address this gap and show good evidence that lamellipodin (symbol RAPH1), previously known as a critical regulator of cell migration, stimulates ECM stiffness-mediated cyclin expression and intracellular stiffening in mouse embryonic fibroblasts. We observed that increased ECM stiffness upregulates lamellipodin expression. This is mediated by an integrin-dependent FAK-Cas-Rac signaling module and supports stiffness-mediated lamellipodin induction. Mechanistically, we find that lamellipodin overexpression increased, and lamellipodin knockdown reduced, stiffness-induced cell cyclin expression and cell proliferation, and intracellular stiffness. Overall, these results suggest that lamellipodin levels may be critical for regulating cell proliferation. This article has an associated First Person interview with the first author of the paper.

Coordination between Rac1 and Rab Proteins: Functional Implications in Health and Disease

The small GTPases of the Rho family regulate many aspects of actin dynamics, but are functionally connected to many other cellular processes. Rac1, a member of this family, besides its known function in the regulation of actin cytoskeleton, plays a key role in the production of reactive oxygen species, in gene transcription, in DNA repair, and also has been proven to have specific roles in neurons. This review focuses on the cooperation between Rac1 and Rab proteins, analyzing how the coordination between these GTPases impact on cells and how alterations of their functions lead to disease.

Distinct CED-10/Rac1 domains confer context-specific functions in development

Rac GTPases act as master switches to coordinate multiple interweaved signaling pathways. A major function for Rac GTPases is to control neurite development by influencing downstream effector molecules and pathways. In Caenorhabditis elegans, the Rac proteins CED-10, RAC-2 and MIG-2 act in parallel to control axon outgrowth and guidance. Here, we have identified a single glycine residue in the CED-10/Rac1 Switch 1 region that confers a non-redundant function in axon outgrowth but not guidance. Mutation of this glycine to glutamic acid (G30E) reduces GTP binding and inhibits axon outgrowth but does not affect other canonical CED-10 functions. This demonstrates previously unappreciated domain-specific functions within the CED-10 protein. Further, we reveal that when CED-10 function is diminished, the adaptor protein NAB-1 (Neurabin) and its interacting partner SYD-1 (Rho-GAP-like protein) can act as inhibitors of axon outgrowth. Together, we reveal that specific domains and residues within Rac GTPases can confer context-dependent functions during animal development.

Brain development requires that neurite outgrowth and guidance are precisely regulated. Previous studies have shown that molecular switch proteins called Rac GTPases perform redundant functions in controlling neurite development. Using a pair of bilateral neurons in the nematode Caenorhabditis elegans to model neurite development, we found that a single amino acid in a conserved domain of the Rac GTPase CED-10 is crucial for controlling neurite outgrowth in a partially non-redundant manner. Further, we revealed that lesions in discrete domains in the CED-10 protein lead to distinct developmental defects. Therefore, our in vivo study proposes that regulation of distinct signalling pathways through Rac GTPase protein domains can drive different developmental outcomes.

A Statistically-Oriented Asymmetric Localization (SOAL) Model for Neuronal Outgrowth Patterning by Caenorhabditis elegans UNC-5 (UNC5) and UNC-40 (DCC) Netrin Receptors

Neurons extend processes that vary in number, length, and direction of "outgrowth". Extracellular cues help determine outgrowth patterns. In Caenorhabditis elegans, neurons respond to the extracellular UNC-6 (netrin) cue via UNC-40 (DCC) and UNC-5 (UNC5) receptors. Previously, we presented evidence that UNC-40 asymmetric localization at the plasma membrane is self-organizing, and that UNC-40 can localize and mediate outgrowth at randomly selected sites. Here, we provide further evidence for a statistically-oriented asymmetric localization (SOAL) model in which UNC-5 receptor activity affects patterns of axon outgrowth by regulating UNC-40 asymmetric localization. According to the SOAL model, the direction of outgrowth activity fluctuates across the membrane over time. Random walk modeling predicts that increasing the degree to which the direction of outgrowth fluctuates will decrease the outward displacement of the membrane. By differentially affecting the degree to which the direction of outgrowth activity fluctuates over time, extracellular cues can produce different rates of outgrowth along the surface and create patterns of "extension". Consistent with the SOAL model, we show that unc-5 mutations alter UNC-40 asymmetric localization, increase the degree to which the direction of outgrowth fluctuates, and reduce the extent of outgrowth in multiple directions relative to the source of UNC-6 These results are inconsistent with current models, which predict that UNC-5 mediates a "repulsive" response to UNC-6 Genetic interactions suggest that UNC-5 acts through the UNC-53 (NAV2) cytoplasmic protein to regulate UNC-40 asymmetric localization in response to both the UNC-6 and EGL-20 (Wnt) extracellular cues.

Mind the (sr)GAP - roles of Slit-Robo GAPs in neurons, brains and beyond

The Slit-Robo GTPase-activating proteins (srGAPs) were first identified as potential Slit-Robo effectors that influence growth cone guidance. Given their N-terminal F-BAR, central GAP and C-terminal SH3 domains, srGAPs have the potential to affect membrane dynamics, Rho family GTPase activity and other binding partners. Recent research has clarified how srGAP family members act in distinct ways at the cell membrane, and has expanded our understanding of the roles of srGAPs in neuronal and non-neuronal cells. Gene duplication of the human-specific paralog of srGAP2 has resulted in srGAP2 family proteins that may have increased the density of dendritic spines and promoted neoteny of the human brain during crucial periods of human evolution, underscoring the importance of srGAPs in the unique sculpting of the human brain. Importantly, srGAPs also play roles outside of the nervous system, including during contact inhibition of cell movement and in establishing and maintaining cell adhesions in epithelia. Changes in srGAP expression may contribute to neurodevelopmental disorders, cancer metastasis and inflammation. As discussed in this Review, much remains to be discovered about how this interesting family of proteins functions in a diverse set of processes in metazoans and the functional roles srGAPs play in human disease.

The mTOR Substrate S6 Kinase 1 (S6K1) Is a Negative Regulator of Axon Regeneration and a Potential Drug Target for Central Nervous System Injury

The mammalian target of rapamycin (mTOR) positively regulates axon growth in the mammalian central nervous system (CNS). Although axon regeneration and functional recovery from CNS injuries are typically limited, knockdown or deletion of PTEN, a negative regulator of mTOR, increases mTOR activity and induces robust axon growth and regeneration. It has been suggested that inhibition of S6 kinase 1 (S6K1, gene symbol: RPS6KB1), a prominent mTOR target, would blunt mTOR's positive effect on axon growth. In contrast to this expectation, we demonstrate that inhibition of S6K1 in CNS neurons promotes neurite outgrowth in vitro by twofold to threefold. Biochemical analysis revealed that an mTOR-dependent induction of PI3K signaling is involved in mediating this effect of S6K1 inhibition. Importantly, treating female mice in vivo with PF-4708671, a selective S6K1 inhibitor, stimulated corticospinal tract regeneration across a dorsal spinal hemisection between the cervical 5 and 6 cord segments (C5/C6), increasing axon counts for at least 3 mm beyond the injury site at 8 weeks after injury. Concomitantly, treatment with PF-4708671 produced significant locomotor recovery. Pharmacological targeting of S6K1 may therefore constitute an attractive strategy for promoting axon regeneration following CNS injury, especially given that S6K1 inhibitors are being assessed in clinical trials for nononcological indications.SIGNIFICANCE STATEMENT Despite mTOR's well-established function in promoting axon regeneration, the role of its downstream target, S6 kinase 1 (S6K1), has been unclear. We used cellular assays with primary neurons to demonstrate that S6K1 is a negative regulator of neurite outgrowth, and a spinal cord injury model to show that it is a viable pharmacological target for inducing axon regeneration. We provide mechanistic evidence that S6K1's negative feedback to PI3K signaling is involved in axon growth inhibition, and show that phosphorylation of S6K1 is a more appropriate regeneration indicator than is S6 phosphorylation.

Rho GTPase-activating proteins: Regulators of Rho GTPase activity in neuronal development and CNS diseases

The Rho family of small GTPases was considered as molecular switches in regulating multiple cellular events, including cytoskeleton reorganization. The Rho GTPase-activating proteins (RhoGAPs) are one of the major families of Rho GTPase regulators. RhoGAPs were initially considered negative mediators of Rho signaling pathways via their GAP domain. Recent studies have demonstrated that RhoGAPs also regulate numerous aspects of neuronal development and are related to various neurodegenerative diseases in GAP-dependent and GAP-independent manners. Moreover, RhoGAPs are regulated through various mechanisms, such as phosphorylation. To date, approximately 70 RhoGAPs have been identified; however, only a small portion has been thoroughly investigated. Thus, the characterization of important RhoGAPs in the central nervous system is crucial to understand their spatiotemporal role during different stages of neuronal development. In this review, we summarize the current knowledge of RhoGAPs in the brain with an emphasis on their molecular function, regulation mechanism and disease implications in the central nervous system.

SYD-1 Promotes Multiple Developmental Steps Leading to Neuronal Connectivity

The establishment of neuronal connectivity requires precise orchestration of multiple developmental steps, including axon specification, axon guidance, selection of synaptic target sites, and development of synaptic specializations. Although these are separate developmental steps, evidence indicates that some of the signaling molecules that regulate these steps are shared. In this review, we focus on SYD-1, a RhoGAP-like protein that has been implicated in each step of axonal development. We discuss interactions between SYD-1, UNC-40(DCC) and RhoGTPases and highlight both similarities and differences in how SYD-1 functions to regulate the different steps of axonal development. These observations reveal an example of how a signaling protein can be repurposed across sequential developmental steps.

The Genetics of Axon Guidance and Axon Regeneration in Caenorhabditis elegans

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

FLIM FRET Visualization of Cdc42 Activation by Netrin-1 in Embryonic Spinal Commissural Neuron Growth Cones

Netrin-1 is an essential extracellular chemoattractant that signals through its receptor DCC to guide commissural axon extension in the embryonic spinal cord. DCC directs the organization of F-actin in growth cones by activating an intracellular protein complex that includes the Rho GTPase Cdc42, a critical regulator of cell polarity and directional migration. To address the spatial distribution of signaling events downstream of netrin-1, we expressed the FRET biosensor Raichu-Cdc42 in cultured embryonic rat spinal commissural neurons. Using FLIM-FRET imaging we detected rapid activation of Cdc42 in neuronal growth cones following application of netrin-1. Investigating the signaling mechanisms that control Cdc42 activation by netrin-1, we demonstrate that netrin-1 rapidly enriches DCC at the leading edge of commissural neuron growth cones and that netrin-1 induced activation of Cdc42 in the growth cone is blocked by inhibiting src family kinase signaling. These findings reveal the activation of Cdc42 in embryonic spinal commissural axon growth cones and support the conclusion that src family kinase activation downstream of DCC is required for Cdc42 activation by netrin-1.

Impact of next-generation technologies on exploring socioeconomically important parasites and developing new interventions

High-throughput molecular and computer technologies have become instrumental for systems biological explorations of pathogens, including parasites. For instance, investigations of the transcriptomes of different developmental stages of parasitic nematodes give insights into gene expression, regulation and function in a parasite, which is a significant step to understanding their biology, as well as interactions with their host(s) and disease. This chapter (1) gives a background on some key parasitic nematodes of socioeconomic importance, (2) describes sequencing and bioinformatic technologies for large-scale studies of the transcriptomes and genomes of these parasites, (3) provides some recent examples of applications and (4) emphasizes the prospects of fundamental biological explorations of parasites using these technologies for the development of new interventions to combat parasitic diseases.

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.

A novel Netrin-1-sensitive mechanism promotes local SNARE-mediated exocytosis during axon branching

Localized plasma membrane expansion during axon branching mediated by Netrin-1 occurs via TRIM9-dependent regulation of SNARE-mediated vesicle fusion.

Developmental axon branching dramatically increases synaptic capacity and neuronal surface area. Netrin-1 promotes branching and synaptogenesis, but the mechanism by which Netrin-1 stimulates plasma membrane expansion is unknown. We demonstrate that SNARE-mediated exocytosis is a prerequisite for axon branching and identify the E3 ubiquitin ligase TRIM9 as a critical catalytic link between Netrin-1 and exocytic SNARE machinery in murine cortical neurons. TRIM9 ligase activity promotes SNARE-mediated vesicle fusion and axon branching in a Netrin-dependent manner. We identified a direct interaction between TRIM9 and the Netrin-1 receptor DCC as well as a Netrin-1–sensitive interaction between TRIM9 and the SNARE component SNAP25. The interaction with SNAP25 negatively regulates SNARE-mediated exocytosis and axon branching in the absence of Netrin-1. Deletion of TRIM9 elevated exocytosis in vitro and increased axon branching in vitro and in vivo. Our data provide a novel model for the spatial regulation of axon branching by Netrin-1, in which localized plasma membrane expansion occurs via TRIM9-dependent regulation of SNARE-mediated vesicle fusion.

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.

ENU-3 functions in an UNC-6/netrin dependent pathway parallel to UNC-40/DCC/frazzled for outgrowth and guidance of the touch receptor neurons in C. elegans

BACKGROUND

UNC-6 and SLT-1 guide the migrations of the ventrally directed processes of the AVM and PVM touch receptor neurons and UNC-6 guides the axons of the DA and DB classes of motor neurons in C. elegans. The UNC-6 receptors are UNC-5 and UNC-40. The axon outgrowth defects of a subset of the DB motor neurons in the absence of UNC-5 are enhanced by mutations in enu-3.

RESULTS

An enu-3 mutation enhances defects in ventral guidance of the processes of the AVM and PVM touch receptor neurons, the dorsal guidance of the distal tip cell and causes additional architectural defects in axons in unc-40 mutant strains in an UNC-6 dependent manner. These observations suggest that ENU-3 and UNC-40 function in parallel pathways dependent on UNC-6. ENU-3 depends on the presence of UNC-40 for its full effect on motor neuron axon outgrowth.

CONCLUSIONS

ENU-3 works in an UNC-6 dependent pathway parallel to UNC-40 in ventral guidance of AVM and PVM and in dorsal guidance of the distal tip cells. Motor neuron axon outgrowth defects are caused by the presence of UNC-40 and the absence of functional UNC-5 or UNC-6 and defects are enhanced by the absence of functional ENU-3.

Experimental evidence for UNC-6 (netrin) axon guidance by stochastic fluctuations of intracellular UNC-40 (DCC) outgrowth activity

How the direction of axon guidance is determined is not understood. In Caenorhabditis elegans the UNC-40 (DCC) receptor mediates a response to the UNC-6 (netrin) guidance cue that directs HSN axon development. UNC-40 becomes asymmetrically localized within the HSN neuron to the site of axon outgrowth. Here we provide experimental evidence that the direction of guidance can be explained by the stochastic fluctuations of UNC-40 asymmetric outgrowth activity. We find that the UNC-5 (UNC5) receptor and the cytoskeletal binding protein UNC-53 (NAV2) regulate the induction of UNC-40 localization by UNC-6. If UNC-40 localization is induced without UNC-6 by using an unc-53 mutation, the direction of UNC-40 localization undergoes random fluctuations. Random walk models describe the path made by a succession of randomly directed movement. This model was experimentally tested using mutations that affect Wnt/PCP signaling. These mutations inhibit UNC-40 localization in the anterior and posterior directions. As the axon forms in Wnt/PCP mutants, the direction of UNC-40 localization randomly fluctuates; it can localize in either the anterior, posterior, or ventral direction. Consistent with a biased random walk, over time the axon will develop ventrally in response to UNC-6, even though at a discrete time UNC-40 localization and outgrowth can be observed anterior or posterior. Also, axon formation is slower in the mutants than in wild-type animals. This is also consistent with a random walk since this model predicts that the mean square displacement (msd) will increase only linearly with time, whereas the msd increases quadratically with time for straight-line motion.

Lamellipodin and the Scar/WAVE complex cooperate to promote cell migration in vivo

Cell migration is essential for development, but its deregulation causes metastasis. The Scar/WAVE complex is absolutely required for lamellipodia and is a key effector in cell migration, but its regulation in vivo is enigmatic. Lamellipodin (Lpd) controls lamellipodium formation through an unknown mechanism. Here, we report that Lpd directly binds active Rac, which regulates a direct interaction between Lpd and the Scar/WAVE complex via Abi. Consequently, Lpd controls lamellipodium size, cell migration speed, and persistence via Scar/WAVE in vitro. Moreover, Lpd knockout mice display defective pigmentation because fewer migrating neural crest-derived melanoblasts reach their target during development. Consistently, Lpd regulates mesenchymal neural crest cell migration cell autonomously in Xenopus laevis via the Scar/WAVE complex. Further, Lpd's Drosophila melanogaster orthologue Pico binds Scar, and both regulate collective epithelial border cell migration. Pico also controls directed cell protrusions of border cell clusters in a Scar-dependent manner. Taken together, Lpd is an essential, evolutionary conserved regulator of the Scar/WAVE complex during cell migration in vivo.

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.

The netrin receptor DCC focuses invadopodia-driven basement membrane transmigration in vivo

Localized activation of netrin signaling induces focused F-actin formation and the protrusive force necessary for physical displacement of basement membrane during cell transmigration.

Though critical to normal development and cancer metastasis, how cells traverse basement membranes is poorly understood. A central impediment has been the challenge of visualizing invasive cell interactions with basement membrane in vivo. By developing live-cell imaging methods to follow anchor cell (AC) invasion in Caenorhabditis elegans, we identify F-actin–based invadopodia that breach basement membrane. When an invadopodium penetrates basement membrane, it rapidly transitions into a stable invasive process that expands the breach and crosses into the vulval tissue. We find that the netrin receptor UNC-40 (DCC) specifically enriches at the site of basement membrane breach and that activation by UNC-6 (netrin) directs focused F-actin formation, generating the invasive protrusion and the cessation of invadopodia. Using optical highlighting of basement membrane components, we further demonstrate that rather than relying solely on proteolytic dissolution, the AC’s protrusion physically displaces basement membrane. These studies reveal an UNC-40–mediated morphogenetic transition at the cell–basement membrane interface that directs invading cells across basement membrane barriers.

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.

A dominant mutation in mec-7/β-tubulin affects axon development and regeneration in Caenorhabditis elegans neurons

Microtubules are the basic elements of the cytoskeleton. This study demonstrates that a specific mutation in mec-7/β-tubulin is necessary for the correct number of neurites a neuron extends in vivo and the neuron’s capacity for axonal regeneration following injury.

Microtubules have been known for decades to be basic elements of the cytoskeleton. They form long, dynamic, rope-like structures within the cell that are essential for mitosis, maintenance of cell shape, and intracellular transport. More recently, in vitro studies have implicated microtubules as signaling molecules that, through changes in their stability, have the potential to trigger growth of axons and dendrites in developing neurons. In this study, we show that specific mutations in the Caenorhabditis elegans mec-7/β-tubulin gene cause ectopic axon formation in mechanosensory neurons in vivo. In mec-7 mutants, the ALM mechanosensory neuron forms a long ectopic neurite that extends posteriorly, a phenotype that can be mimicked in wild-type worms with a microtubule-stabilizing drug (paclitaxel), and suppressed by mutations in unc-33/CRMP2 and the kinesin-related gene, vab-8. Our results also reveal that these ectopic neurites contain RAB-3, a marker for presynaptic loci, suggesting that they have axon-like properties. Interestingly, in contrast with the excessive axonal growth observed during development, mec-7 mutants are inhibited in axonal regrowth and remodeling following axonal injury. Together our results suggest that MEC-7/β-tubulin integrity is necessary for the correct number of neurites a neuron generates in vivo and for the capacity of an axon to regenerate.

Synaptic vesicle clustering requires a distinct MIG-10/Lamellipodin isoform and ABI-1 downstream from Netrin

The chemotrophic factor Netrin can simultaneously instruct different neurodevelopmental programs in individual neurons in vivo. How neurons correctly interpret the Netrin signal and undergo the appropriate neurodevelopmental response is not understood. Here we identify MIG-10 isoforms as critical determinants of individual cellular responses to Netrin. We determined that distinct MIG-10 isoforms, varying only in their N-terminal motifs, can localize to specific subcellular domains and are differentially required for discrete neurodevelopmental processes in vivo. We identified MIG-10B as an isoform uniquely capable of localizing to presynaptic regions and instructing synaptic vesicle clustering in response to Netrin. MIG-10B interacts with Abl-interacting protein-1 (ABI-1)/Abi1, a component of the WAVE complex, to organize the actin cytoskeleton at presynaptic sites and instruct vesicle clustering through SNN-1/Synapsin. We identified a motif in the MIG-10B N-terminal domain that is required for its function and localization to presynaptic sites. With this motif, we engineered a dominant-negative MIG-10B construct that disrupts vesicle clustering and animal thermotaxis behavior when expressed in a single neuron in vivo. Our findings indicate that the unique N-terminal domains confer distinct MIG-10 isoforms with unique capabilities to localize to distinct subcellular compartments, organize the actin cytoskeleton at these sites, and instruct distinct Netrin-dependent neurodevelopmental programs.

MIG-10 functions with ABI-1 to mediate the UNC-6 and SLT-1 axon guidance signaling pathways

Extracellular guidance cues steer axons towards their targets by eliciting morphological changes in the growth cone. A key part of this process is the asymmetric recruitment of the cytoplasmic scaffolding protein MIG-10 (lamellipodin). MIG-10 is thought to asymmetrically promote outgrowth by inducing actin polymerization. However, the mechanism that links MIG-10 to actin polymerization is not known. We have identified the actin regulatory protein ABI-1 as a partner for MIG-10 that can mediate its outgrowth-promoting activity. The SH3 domain of ABI-1 binds to MIG-10, and loss of function of either of these proteins causes similar axon guidance defects. Like MIG-10, ABI-1 functions in both the attractive UNC-6 (netrin) pathway and the repulsive SLT-1 (slit) pathway. Dosage sensitive genetic interactions indicate that MIG-10 functions with ABI-1 and WVE-1 to mediate axon guidance. Epistasis analysis reveals that ABI-1 and WVE-1 function downstream of MIG-10 to mediate its outgrowth-promoting activity. Moreover, experiments with cultured mammalian cells suggest that the interaction between MIG-10 and ABI-1 mediates a conserved mechanism that promotes formation of lamellipodia. Together, these observations suggest that MIG-10 interacts with ABI-1 and WVE-1 to mediate the UNC-6 and SLT-1 guidance pathways.

To form neural circuits, axons must navigate through the developing nervous system to reach their correct targets. Axon navigation is led by the growth cone, a structure at the tip of the growing axon that responds to extracellular guidance cues. Many of these guidance cues and their receptors have been identified. However, much less is known about the internal signaling events that give rise to the structural changes required for growth cone steering. A key component of the internal response is MIG-10, a protein that becomes asymmetrically localized in response to the extracellular cues. MIG-10 is thought to serve as a scaffold that can spatially control outgrowth-promoting proteins within the growth cone. However, we do not know the identity of the outgrowth-promoting proteins that associate with MIG-10. Here we report that MIG-10 associates physically with the actin regulatory protein ABI-1. We present genetic evidence indicating that ABI-1 functions downstream of MIG-10 to mediate its outgrowth-promoting activity. Additional genetic evidence indicates that these proteins function in both attractive and repulsive guidance signaling pathways. We also present evidence suggesting that the connection between MIG-10 and ABI-1 represents a phylogenetically conserved mechanism for the control of cellular outgrowth.

Abelson interactor-1 (ABI-1) interacts with MRL adaptor protein MIG-10 and is required in guided cell migrations and process outgrowth in C. elegans

Directed cell migration and process outgrowth are vital to proper development of many metazoan tissues. These processes are dependent on reorganization of the actin cytoskeleton in response to external guidance cues. During development of the nervous system, the MIG-10/RIAM/Lamellipodin (MRL) signaling proteins are thought to transmit positional information from surface guidance cues to the actin polymerization machinery, and thus to promote polarized outgrowth of axons. In C. elegans, mutations in the MRL family member gene mig-10 result in animals that have defects in axon guidance, neuronal migration, and the outgrowth of the processes or 'canals' of the excretory cell, which is required for osmoregulation in the worm. In addition, mig-10 mutant animals have recently been shown to have defects in clustering of vesicles at the synapse. To determine additional molecular partners of MIG-10, we conducted a yeast two-hybrid screen using isoform MIG-10A as bait and isolated Abelson-interactor protein-1 (ABI-1). ABI-1, a downstream target of Abl non-receptor tyrosine kinase, is a member of the WAVE regulatory complex (WRC) involved in the initiation of actin polymerization. Further analysis using a co-immunoprecipitation system confirmed the interaction of MIG-10 and ABI-1 and showed that it requires the SH3 domain of ABI-1. Single mutants for mig-10 and abi-1 displayed similar phenotypes of incomplete migration of the ALM neurons and truncated outgrowth of the excretory cell canals, suggesting that the ABI-1/MIG-10 interaction is relevant in vivo. Cell autonomous expression of MIG-10 isoforms rescued both the neuronal migration and the canal outgrowth defects, showing that MIG-10 functions autonomously in the ALM neurons and the excretory cell. These results suggest that MIG-10 and ABI-1 interact physically to promote cell migration and process outgrowth in vivo. In the excretory canal, ABI-1 is thought to act downstream of UNC-53/NAV2, linking this large scaffolding protein to actin polymerization during excretory canal outgrowth. abi-1(RNAi) enhanced the excretory canal truncation observed in mig-10 mutants, while double mutant analysis between unc-53 and mig-10 showed no increased truncation of the posterior canal beyond that observed in mig-10 mutants. Morphological analysis of mig-10 and unc-53 mutants showed that these genes regulate canal diameter as well as its length, suggesting that defective lumen formation may be linked to the ability of the excretory canal to grow out longitudinally. Taken together, our results suggest that MIG-10, UNC-53, and ABI-1 act sequentially to mediate excretory cell process outgrowth.

Semaphorin 4D/Plexin-B1-mediated M-Ras GAP activity regulates actin-based dendrite remodeling through Lamellipodin

Semaphorins have been identified as repulsive guidance molecules in the developing nervous system. We recently reported that the semaphorin 4D (Sema4D) receptor Plexin-B1 induces repulsion in axon and dendrites by functioning as a GTPase-activating protein (GAP) for R-Ras and M-Ras, respectively. In axons, Sema4D stimulation induces growth cone collapse, and downregulation of R-Ras activity by Plexin-B1-mediated GAP activity is required for the action. Axonal R-Ras GAP activity downregulates phosphatidylinositol 3-kinase signaling pathway, and thereby induces inactivation of a microtubule assembly promoter protein, CRMP-2. However, in contrast to the well studied roles of semaphorins and plexins in axonal guidance, signaling molecules linking M-Ras GAP to dendritic cytoskeleton remain obscure. Here we identified an Ena/VASP ligand, Lamellipodin (Lpd), as a novel effector of M-Ras in dendrites. Lpd was expressed in F-actin-rich distal dendritic processes and was required for both basal and M-Ras-mediated dendrite development. Subcellular fractionation showed M-Ras-dependent membrane translocation of Lpd, which was suppressed by Sema4D. Furthermore, the Ena/VASP-binding region within Lpd was required for dendrite development, and its membrane targeting was sufficient to overcome the Sema4D-mediated reduction of dendritic outgrowth and disappearance of F-actin from distal dendrites. Furthermore, in utero electroporation experiments also indicated that regulation of the M-Ras-Lpd system by the GAP activity of Plexin is involved in the normal development of cortical dendrites in vivo. Overall, our study sheds light on how repulsive guidance molecules regulate actin cytoskeleton in dendrites, revealing a novel mechanism that the M-Ras-Lpd system regulates actin-based dendrite remodeling by Sema/Plexin in rats or mice of either sex.

Guidance for life, cell death, and colorectal neoplasia by netrin dependence receptors

This review is focusing on a critical mediator of embryonic and postnatal development with multiple implications in inflammation, neoplasia, and other pathological situations in brain and peripheral tissues. These morphogenetic guidance and dependence processes are involved in several malignancies targeting the epithelial and immune systems including the progression of human colorectal cancers. We consider the most important findings and their impact on basic, translational, and clinical cancer research. Expected information can bring new cues for innovative, efficient, and safe strategies of personalized medicine based on molecular markers, protagonists, signaling networks, and effectors inherent to the Netrin axis in pathophysiological states.

The lin-4 microRNA targets the LIN-14 transcription factor to inhibit netrin-mediated axon attraction

miR-125 microRNAs, such as lin-4 in Caenorhabditis elegans, were among the first microRNAs discovered, are phylogenetically conserved, and have been implicated in regulating developmental timing. Here, we showed that loss-of-function mutations in lin-4 microRNA increased axon attraction mediated by the netrin homolog UNC-6. The absence of lin-4 microRNA suppressed the axon guidance defects of anterior ventral microtubule (AVM) neurons caused by loss-of-function mutations in slt-1, which encodes a repulsive guidance cue. Selective expression of lin-4 microRNA in AVM neurons of lin-4-null animals indicated that the effect of lin-4 on AVM axon guidance was cell-autonomous. Promoter reporter analysis suggested that lin-4 was likely expressed strongly in AVM neurons during the developmental time frame that the axons are guided to their targets. In contrast, the lin-4 reporter was barely detectable in anterior lateral microtubule (ALM) neurons, axon guidance of which is insensitive to netrin. In AVM neurons, the transcription factor LIN-14, a target of lin-4 microRNA, stimulated UNC-6-mediated ventral guidance of the AVM axon. LIN-14 promoted attraction of the AVM axon through the UNC-6 receptor UNC-40 [the worm homolog of vertebrate Deleted in Colorectal Cancer (DCC)] and its cofactor MADD-2, which signals through both the UNC-34 (Ena) and the CED-10 (Rac1) downstream pathways. LIN-14 stimulated UNC-6-mediated axon attraction in part by increasing UNC-40 abundance. Our study indicated that lin-4 microRNA reduced the activity of LIN-14 to terminate UNC-6-mediated axon guidance of AVM neurons.

The MRL proteins: adapting cell adhesion, migration and growth

MIG-10, RIAM and Lamellipodin (Lpd) are the founding members of the MRL family of multi-adaptor molecules. These proteins have common domain structures but display distinct functions in cell migration and adhesion, signaling, and in cell growth. The binding of RIAM with active Rap1 and with talin provides these MRL molecules with important regulatory roles on integrin-mediated cell adhesion and migration. Furthermore, RIAM and Lpd can regulate actin dynamics through their binding to actin regulatory Ena/VASP proteins. Recent data generated with the Drosophila MRL ortholog called Pico and with RIAM in melanoma cells indicate that these proteins can also regulate cell growth. As MRL proteins represent a relatively new family, many questions on their structure-function relationships remain unanswered, including regulation of their expression, post-translational modifications, new interactions, involvement in signaling and their knockout mice phenotype.

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

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

Netrin (UNC-6) mediates dendritic self-avoidance

Dendrites from a single neuron may be highly branched but typically do not overlap. This self-avoidance behavior has been shown to depend on cell-specific membrane proteins that trigger mutual repulsion. Here we report the surprising discovery that a diffusible cue, the axon guidance protein UNC-6/Netrin, is required for self-avoidance of sister dendrites from the PVD nociceptive neuron in C. elegans. We used time lapse imaging to show that dendrites fail to withdraw upon mutual contact in the absence of UNC-6/Netrin signaling. We propose a model in which the UNC-40/DCC receptor captures UNC-6/Netrin at the tips of growing dendrites for interaction with UNC-5 on the apposing branch to induce mutual repulsion. UNC-40/DCC also responds to dendritic contact through an additional pathway that is independent of UNC-6/Netrin. Our findings offer a new model for how an evolutionarily conserved morphogenic cue and its cognate receptors can pattern a fundamental feature of dendritic architecture.

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.

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

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

TRIM-9 functions in the UNC-6/UNC-40 pathway to regulate ventral guidance

TRIpartite Motif (TRIM) family proteins are ring finger domain-containing, multi-domain proteins implicated in many biological processes. Members of the TRIM-9/C-I subfamily of TRIM proteins, including TRIM-9, MID1 and MID2, have neuronal functions and are associated with neurological diseases. To explore whether the functions of C-I TRIM proteins are conserved in invertebrates, we analyzed Caenorhabditis elegans and Drosophila trim-9 mutants. C. elegans trim-9 mutants exhibit defects in the ventral guidance of hermaphrodite specific neuron (HSN) and the touch neuron AVM. Further genetic analyses indicate that TRIM-9 participates in the UNC-6-UNC-40 attraction pathway. Asymmetric distribution of UNC-40 during HSN development is normal in trim-9 mutants. However, the asymmetric localization of MIG-10, a downstream effector of UNC-40, is abolished in trim-9 mutants. These results suggest that TRIM-9 functions upstream of MIG-10 in the UNC-40 pathway. Moreover, we showed that TRIM-9 exhibits E3 ubiquitin ligase activity in vitro and this activity is important for TRIM-9 function in vivo. Additionally, we found that Drosophila trim-9 is required for the midline attraction of a group of sensory neuron axons. Over-expression of the Netrin/UNC-6 receptor Frazzled suppresses the guidance defects in trim-9 mutants. Our study reveals an evolutionarily conserved function of TRIM-9 in the UNC-40/Frazzled-mediated UNC-6/Netrin attraction pathway.

Neuronal polarity in C. elegans

Neuronal polarity sets the foundation for information processing and signal transmission within neural networks. However, fundamental question of how a neuron develops and maintains structurally and functionally distinct processes, axons and dendrites, is still an unclear. The simplicity and availability of practical genetic tools makes C. elegans as an ideal model to study neuronal polarity in vivo. In recent years, new studies have identified critical polarity molecules that function at different stages of neuronal polarization in C. elegans. This review focuses on how neurons guided by extrinsic cues, break symmetry, and subsequently recruit intracellular molecules to establish and maintain axon-dendrite polarity in vivo.

UNC-6/netrin and its receptors UNC-5 and UNC-40/DCC modulate growth cone protrusion in vivo in C. elegans

The UNC-6/netrin guidance cue functions in axon guidance in vertebrates and invertebrates, mediating attraction via UNC-40/DCC family receptors and repulsion via by UNC-5 family receptors. The growth cone reads guidance cues and extends lamellipodia and filopodia, actin-based structures that sense the extracellular environment and power the forward motion of the growth cone. We show that UNC-6/netrin, UNC-5 and UNC-40/DCC modulated the extent of growth cone protrusion that correlated with attraction versus repulsion. Loss-of-function unc-5 mutants displayed increased protrusion in repelled growth cones, whereas loss-of-function unc-6 or unc-40 mutants caused decreased protrusion. In contrast to previous studies, our work suggests that the severe guidance defects in unc-5 mutants may be due to latent UNC-40 attractive signaling that steers the growth cone back towards the ventral source of UNC-6. UNC-6/Netrin signaling also controlled polarity of growth cone protrusion and F-actin accumulation that correlated with attraction versus repulsion. However, filopodial dynamics were affected independently of polarity of protrusion, indicating that the extent versus polarity of protrusion are at least in part separate mechanisms. In summary, we show here that growth cone guidance in response to UNC-6/netrin involves a combination of polarized growth cone protrusion as well as a balance between stimulation and inhibition of growth cone (e.g. filopodial) protrusion.

The growth cone cytoskeleton in axon outgrowth and guidance

Axon outgrowth and guidance to the proper target requires the coordination of filamentous (F)-actin and microtubules (MTs), the dynamic cytoskeletal polymers that promote shape change and locomotion. Over the past two decades, our knowledge of the many guidance cues, receptors, and downstream signaling cascades involved in neuronal outgrowth and guidance has increased dramatically. Less is known, however, about how those cascades of information converge and direct appropriate remodeling and interaction of cytoskeletal polymers, the ultimate effectors of movement and guidance. During development, much of the communication that occurs between environmental guidance cues and the cytoskeleton takes place at the growing tip of the axon, the neuronal growth cone. Several articles on this topic focus on the "input" to the growth cone, the myriad of receptor types, and their corresponding cognate ligands. Others investigate the signaling cascades initiated by receptors and propagated by second messenger pathways (i.e., kinases, phosphatases, GTPases). Ultimately, this plethora of information converges on proteins that associate directly with the actin and microtubule cytoskeletons. The role of these cytoskeletal-associated proteins, as well as the cytoskeleton itself in axon outgrowth and guidance, is the subject of this article.

ENU-3 is a novel motor axon outgrowth and guidance protein in C. elegans

During the development of the nervous system, the migration of many cells and axons is guided by extracellular molecules. These molecules bind to receptors at the tips of the growth cones of migrating axons and trigger intracellular signaling to steer the axons along the correct trajectories. We have identified a novel mutant, enu-3 (enhancer of Unc), that enhances the motor neuron axon outgrowth defects observed in strains of Caenorhabditis elegans that lack either the UNC-5 receptor or its ligand UNC-6/Netrin. Specifically, the double-mutant strains have enhanced axonal outgrowth defects mainly in DB4, DB5 and DB6 motor neurons. enu-3 single mutants have weak motor neuron axon migration defects. Both outgrowth defects of double mutants and axon migration defects of enu-3 mutants were rescued by expression of the H04D03.1 gene product. ENU-3/H04D03.1 encodes a novel predicted putative trans-membrane protein of 204 amino acids. It is a member of a family of highly homologous proteins of previously unknown function in the C. elegans genome. ENU-3 is expressed in the PVT interneuron and is weakly expressed in many cell bodies along the ventral cord, including those of the DA and DB motor neurons. We conclude that ENU-3 is a novel C. elegans protein that affects both motor axon outgrowth and guidance.

Activation of ADF/cofilin mediates attractive growth cone turning toward nerve growth factor and netrin-1

Proper neural circuitry requires that growth cones, motile tips of extending axons, respond to molecular guidance cues expressed in the developing organism. However, it is unclear how guidance cues modify the cytoskeleton to guide growth cone pathfinding. Here, we show acute treatment with two attractive guidance cues, nerve growth factor (NGF) and netrin-1, for embryonic dorsal root ganglion and temporal retinal neurons, respectively, results in increased growth cone membrane protrusion, actin polymerization, and filamentous actin (F-actin). ADF/cofilin (AC) family proteins facilitate F-actin dynamics, and we found the inactive phosphorylated form of AC is decreased in NGF- or netrin-1-treated growth cones. Directly increasing AC activity mimics addition of NGF or netrin-1 to increase growth cone protrusion and F-actin levels. Extracellular gradients of NGF, netrin-1, and a cell-permeable AC elicit attractive growth cone turning and increased F-actin barbed ends, F-actin accumulation, and active AC in growth cone regions proximal to the gradient source. Reducing AC activity blunts turning responses to NGF and netrin. Our results suggest that gradients of NGF and netrin-1 locally activate AC to promote actin polymerization and subsequent growth cone turning toward the side containing higher AC activity.

Initiating and growing an axon

The ability of neurons to form a single axon and multiple dendrites underlies the directional flow of information transfer in the central nervous system. Dendrites and axons are molecularly and functionally distinct domains. Dendrites integrate synaptic inputs, triggering the generation of action potentials at the level of the soma. Action potentials then propagate along the axon, which makes presynaptic contacts onto target cells. This article reviews what is known about the cellular and molecular mechanisms underlying the ability of neurons to initiate and extend a single axon during development. Remarkably, neurons can polarize to form a single axon, multiple dendrites, and later establish functional synaptic contacts in reductionist in vitro conditions. This approach became, and remains, the dominant model to study axon initiation and growth and has yielded the identification of many molecules that regulate axon formation in vitro (Dotti et al. 1988). At present, only a few of the genes identified using in vitro approaches have been shown to be required for axon initiation and outgrowth in vivo. In vitro, axon initiation and elongation are largely intrinsic properties of neurons that are established in the absence of relevant extracellular cues. However, the importance of extracellular cues to axon initiation and outgrowth in vivo is emerging as a major theme in neural development (Barnes and Polleux 2009). In this article, we focus our attention on the extracellular cues and signaling pathways required in vivo for axon initiation and axon extension.