Mutations affecting embryonic cell migrations in Caenorhabditis elegans

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.

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.

The Enigmatic Canal-Associated Neurons Regulate Caenorhabditis elegans Larval Development Through a cAMP Signaling Pathway

Caenorhabditis elegans larval development requires the function of the two Canal-Associated Neurons (CANs): killing the CANs by laser microsurgery or disrupting their development by mutating the gene ceh-10 results in early larval arrest. How these cells promote larval development, however, remains a mystery. In screens for mutations that bypass CAN function, we identified the gene kin-29, which encodes a member of the Salt-Inducible Kinase (SIK) family and a component of a conserved pathway that regulates various C. elegans phenotypes. Like kin-29 loss, gain-of-function mutations in genes that may act upstream of kin-29 or growth in cyclic-AMP analogs bypassed ceh-10 larval arrest, suggesting that a conserved adenylyl cyclase/PKA pathway inhibits KIN-29 to promote larval development, and that loss of CAN function results in dysregulation of KIN-29 and larval arrest. The adenylyl cyclase ACY-2 mediates CAN-dependent larval development: acy-2 mutant larvae arrested development with a similar phenotype to ceh-10 mutants, and the arrest phenotype was suppressed by mutations in kin-29 ACY-2 is expressed predominantly in the CANs, and we provide evidence that the acy-2 functions in the CANs to promote larval development. By contrast, cell-specific expression experiments suggest that kin-29 acts in both the hypodermis and neurons, but not in the CANs. Based on our findings, we propose two models for how ACY-2 activity in the CANs regulates KIN-29 in target cells.

Grb7, a Critical Mediator of EGFR/ErbB Signaling, in Cancer Development and as a Potential Therapeutic Target

The partner of activated epidermal growth factor receptor (EGFR), growth factor receptor bound protein-7 (Grb7), a functionally multidomain adaptor protein, has been demonstrated to be a pivotal regulator for varied physiological and pathological processes by interacting with phospho-tyrosine-related signaling molecules to affect the transmission through a number of signaling pathways. In particular, critical roles of Grb7 in erythroblastic leukemia viral oncogene homolog (ERBB) family-mediated cancer development and malignancy have been intensively evaluated. The overexpression of Grb7 or the coamplification/cooverexpression of Grb7 and members of the ERBB family play essential roles in advanced human cancers and are associated with decreased survival and recurrence of cancers, emphasizing Grb7's value as a prognostic marker and a therapeutic target. Peptide inhibitors of Grb7 are being tested in preclinical trials for their possible therapeutic effects. Here, we review the molecular, functional, and clinical aspects of Grb7 in ERBB family-mediated cancer development and malignancy with the aim to reveal alternative and effective therapeutic strategies.

Novel exc Genes Involved in Formation of the Tubular Excretory Canals of Caenorhabditis elegans

Regulation of luminal diameter is critical to the function of small single-celled tubes, of which the seamless tubular excretory canals of Caenorhabditis elegans provide a tractable genetic model. Mutations in several sets of genes exhibit the Exc phenotype, in which canal luminal growth is visibly altered. Here, a focused reverse genomic screen of genes highly expressed in the canals found 18 genes that significantly affect luminal outgrowth or diameter. These genes encode novel proteins as well as highly conserved proteins involved in processes including gene expression, cytoskeletal regulation, and vesicular and transmembrane transport. In addition, two genes act as suppressors on a pathway of conserved genes whose products mediate vesicle movement from early to recycling endosomes. The results provide new tools for understanding the integration of cytoplasmic structure and physiology in forming and maintaining the narrow diameter of single-cell tubules.

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.

Brain specific Lamellipodin knockout results in hyperactivity and increased anxiety of mice

Lamellipodin (Lpd) functions as an important signalling integrator downstream of growth factor and axon guidance receptors. Mechanistically, Lpd promotes actin polymerization by interacting with F-actin and the actin effectors Ena/VASP proteins and the SCAR/WAVE complex. Thereby, Lpd supports lamellipodia protrusion, cell migration and endocytosis. In the mammalian central nervous system, Lpd contributes to neuronal morphogenesis, neuronal migration during development and its C. elegans orthologue MIG-10 also supports synaptogenesis. However, the consequences of loss of Lpd in the CNS on behaviour are unknown. In our current study, we crossed our Lpd conditional knockout mice with a mouse line expressing Cre under the CNS specific Nestin promoter to restrict the genetic ablation of Lpd to the central nervous system. Detailed behavioural analysis of the resulting Nestin-Cre-Lpd knockout mouse line revealed a specific behavioural phenotype characterised by hyperactivity and increased anxiety.

The adaptors Grb10 and Grb14 are calmodulin-binding proteins

We identified the Grb7 family members, Grb10 and Grb14, as Ca²⁺ -dependent CaM-binding proteins using Ca²⁺ -dependent CaM-affinity chromatography as we previously did with Grb7. The potential CaM-binding sites were identified and experimentally tested using fluorescent-labeled peptides corresponding to these sites. The apparent affinity constant of these peptides for CaM, and the minimum number of calcium ions bound to CaM that are required for effective binding to these peptides were also determined. We prepared deletion mutants of the three adaptor proteins lacking the identified sites and determined that they lost or strongly diminished their CaM-binding capacity following the sequence Grb7 > > Grb14 > Grb10. More than one CaM-binding site and/or accessory CaM-binding sites appear to exist in Grb10 and Grb14, as compared to a single one present in Grb7.

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

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

Neuroendocrine modulation sustains the C. elegans forward motor state

Neuromodulators shape neural circuit dynamics. Combining electron microscopy, genetics, transcriptome profiling, calcium imaging, and optogenetics, we discovered a peptidergic neuron that modulates C. elegans motor circuit dynamics. The Six/SO-family homeobox transcription factor UNC-39 governs lineage-specific neurogenesis to give rise to a neuron RID. RID bears the anatomic hallmarks of a specialized endocrine neuron: it harbors near-exclusive dense core vesicles that cluster periodically along the axon, and expresses multiple neuropeptides, including the FMRF-amide-related FLP-14. RID activity increases during forward movement. Ablating RID reduces the sustainability of forward movement, a phenotype partially recapitulated by removing FLP-14. Optogenetic depolarization of RID prolongs forward movement, an effect reduced in the absence of FLP-14. Together, these results establish the role of a neuroendocrine cell RID in sustaining a specific behavioral state in C. elegans.

DOI:http://dx.doi.org/10.7554/eLife.19887.001

The tubulin repertoire of C. elegans sensory neurons and its context-dependent role in process outgrowth

Microtubules contribute to many cellular processes, including transport, signaling, and chromosome separation during cell division (Kapitein and Hoogenraad, 2015). They are comprised of αβ-tubulin heterodimers arranged into linear protofilaments and assembled into tubes. Eukaryotes express multiple tubulin isoforms (Gogonea et al., 1999), and there has been a longstanding debate as to whether the isoforms are redundant or perform specialized roles as part of a tubulin code (Fulton and Simpson, 1976). Here, we use the well-characterized touch receptor neurons (TRNs) of Caenorhabditis elegans to investigate this question, through genetic dissection of process outgrowth both in vivo and in vitro With single-cell RNA-seq, we compare transcription profiles for TRNs with those of two other sensory neurons, and present evidence that each sensory neuron expresses a distinct palette of tubulin genes. In the TRNs, we analyze process outgrowth and show that four tubulins (tba-1, tba-2, tbb-1, and tbb-2) function partially or fully redundantly, while two others (mec-7 and mec-12) perform specialized, context-dependent roles. Our findings support a model in which sensory neurons express overlapping subsets of tubulin genes whose functional redundancy varies between cell types and in vivo and in vitro contexts.

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.

The many faces of calmodulin in cell proliferation, programmed cell death, autophagy, and cancer

Calmodulin (CaM) is a ubiquitous Ca(2+) receptor protein mediating a large number of signaling processes in all eukaryotic cells. CaM plays a central role in regulating a myriad of cellular functions via interaction with multiple target proteins. This review focuses on the action of CaM and CaM-dependent signaling systems in the control of vertebrate cell proliferation, programmed cell death and autophagy. The significance of CaM and interconnected CaM-regulated systems for the physiology of cancer cells including tumor stem cells, and processes required for tumor progression such as growth, tumor-associated angiogenesis and metastasis are highlighted. Furthermore, the potential targeting of CaM-dependent signaling processes for therapeutic use is discussed.

Nonautonomous regulation of neuronal migration by insulin signaling, DAF-16/FOXO, and PAK-1

Neuronal migration is essential for nervous system development in all organisms and is regulated in the nematode, C. elegans, by signaling pathways that are conserved in humans. Here, we demonstrate that the insulin/IGF-1-PI3K signaling pathway modulates the activity of the DAF-16/FOXO transcription factor to regulate the anterior migrations of the hermaphrodite-specific neurons (HSNs) during embryogenesis of C. elegans. When signaling is reduced, DAF-16 is activated and promotes migration; conversely, when signaling is enhanced, DAF-16 is inactivated, and migration is inhibited. We show that DAF-16 acts nonautonomously in the hypodermis to promote HSN migration. Furthermore, we identify PAK-1, a p21-activated kinase, as a downstream mediator of insulin/IGF-1-DAF-16 signaling in the nonautonomous control of HSN migration. Because a FOXO-Pak1 pathway was recently shown to regulate mammalian neuronal polarity, our findings indicate that the roles of FOXO and Pak1 in neuronal migration are most likely conserved from C. elegans to higher organisms.

Neurons refine the Caenorhabditis elegans body plan by directing axial patterning by Wnts

Metazoans display remarkable conservation of gene families, including growth factors, yet somehow these genes are used in different ways to generate tremendous morphological diversity. While variations in the magnitude and spatio-temporal aspects of signaling by a growth factor can generate different body patterns, how these signaling variations are organized and coordinated during development is unclear. Basic body plans are organized by the end of gastrulation and are refined as limbs, organs, and nervous systems co-develop. Despite their proximity to developing tissues, neurons are primarily thought to act after development, on behavior. Here, we show that in Caenorhabditis elegans, the axonal projections of neurons regulate tissue progenitor responses to Wnts so that certain organs develop with the correct morphology at the right axial positions. We find that foreshortening of the posteriorly directed axons of the two canal-associated neurons (CANs) disrupts mid-body vulval morphology, and produces ectopic vulval tissue in the posterior epidermis, in a Wnt-dependent manner. We also provide evidence that suggests that the posterior CAN axons modulate the location and strength of Wnt signaling along the anterior-posterior axis by employing a Ror family Wnt receptor to bind posteriorly derived Wnts, and hence, refine their distributions. Surprisingly, despite high levels of Ror expression in many other cells, these cells cannot substitute for the CAN axons in patterning the epidermis, nor can cells expressing a secreted Wnt inhibitor, SFRP-1. Thus, unmyelinated axon tracts are critical for patterning the C. elegans body. Our findings suggest that the evolution of neurons not only improved metazoans by increasing behavioral complexity, but also by expanding the diversity of developmental patterns generated by growth factors such as Wnts.

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.

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.

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.

Inverted selective plane illumination microscopy (iSPIM) enables coupled cell identity lineaging and neurodevelopmental imaging in Caenorhabditis elegans

The Caenorhabditis elegans embryo is a powerful model for studying neural development, but conventional imaging methods are either too slow or phototoxic to take full advantage of this system. To solve these problems, we developed an inverted selective plane illumination microscopy (iSPIM) module for noninvasive high-speed volumetric imaging of living samples. iSPIM is designed as a straightforward add-on to an inverted microscope, permitting conventional mounting of specimens and facilitating SPIM use by development and neurobiology laboratories. iSPIM offers a volumetric imaging rate 30× faster than currently used technologies, such as spinning-disk confocal microscopy, at comparable signal-to-noise ratio. This increased imaging speed allows us to continuously monitor the development of C, elegans embryos, scanning volumes every 2 s for the 14-h period of embryogenesis with no detectable phototoxicity. Collecting ∼25,000 volumes over the entirety of embryogenesis enabled in toto visualization of positions and identities of cell nuclei. By merging two-color iSPIM with automated lineaging techniques we realized two goals: (i) identification of neurons expressing the transcription factor CEH-10/Chx10 and (ii) visualization of their neurodevelopmental dynamics. We found that canal-associated neurons use somal translocation and amoeboid movement as they migrate to their final position in the embryo. We also visualized axon guidance and growth cone dynamics as neurons circumnavigate the nerve ring and reach their targets in the embryo. The high-speed volumetric imaging rate of iSPIM effectively eliminates motion blur from embryo movement inside the egg case, allowing characterization of dynamic neurodevelopmental events that were previously inaccessible.

Lpd depletion reveals that SRF specifies radial versus tangential migration of pyramidal neurons

During corticogenesis, pyramidal neurons (~80% of cortical neurons) arise from the ventricular zone (VZ) pass through a multipolar stage to become bipolar and attach to radial glia^{1, 3}, and then migrate to their proper position within the cortex^{1, 2}. As pyramidal neurons migrate radially, they remain attached to their glial substrate as they pass through the subventricular (SVZ) and intermediate (IZ) zones, regions rich in tangentially migrating interneurons and axon fiber tracts. We examined the role of Lamellipodin (Lpd), a homolog of a key regulator of neuronal migration and polarization in C. elegans, in corticogenesis. Lpd depletion caused bipolar pyramidal neurons to adopt a tangential, rather than radial-glial, migration mode without affecting cell fate. Mechanistically, Lpd depletion reduced the activity of SRF, a transcription factor regulated by changes in the ratio of polymerized to unpolymerized actin. Therefore, Lpd depletion exposes a role for SRF in directing pyramidal neurons to select a radial migration pathway along glia rather than a tangential migration mode.

The adiponectin receptor homologs in C. elegans promote energy utilization and homeostasis

Adiponectin is an adipokine with insulin-sensitising actions in vertebrates. Its receptors, AdipoR1 and AdipoR2, are PAQR-type proteins with 7-transmembrane domains and topologies reversed that of GPCR's, i.e. their C-termini are extracellular. We identified three adiponectin receptor homologs in the nematode C. elegans, named paqr-1, paqr-2 and paqr-3. These are differently expressed in the intestine (the main fat-storing tissue), hypodermis, muscles, neurons and secretory tissues, from which they could exert systemic effects. Analysis of mutants revealed that paqr-1 and -2 are novel metabolic regulators in C. elegans and that they act redundantly but independently from paqr-3. paqr-2 is the most important of the three paqr genes: mutants grow poorly, fail to adapt to growth at low temperature, and have a very high fat content with an abnormal enrichment in long (C20) poly-unsaturated fatty acids when combined with the paqr-1 mutation. paqr-2 mutants are also synthetic lethal with mutations in nhr-49, sbp-1 and fat-6, which are C. elegans homologs of nuclear hormone receptors, SREBP and FAT-6 (a Δ9 desaturase), respectively. Like paqr-2, paqr-1 is also synthetic lethal with sbp-1. Mutations in aak-2, the C. elegans homolog of AMPK, or nhr-80, another nuclear hormone receptor gene, suppress the growth phenotype of paqr-2 mutants, probably because they restore the balance between energy expenditure and storage. We conclude that paqr-1 and paqr-2 are receptors that regulate fatty acid metabolism and cold adaptation in C. elegans, that their main function is to promote energy utilization rather than storage, and that PAQR class proteins have regulated metabolism in metazoans for at least 700 million years.

The role of C. elegans Ena/VASP homolog UNC-34 in neuronal polarity and motility

Ena/VASP proteins mediate the effects of guidance cues on the actin cytoskeleton. The single C. elegans homolog of the Ena/VASP family of proteins, UNC-34, is required for the migrations of cells and growth cones. Here we show that unc-34 mutant alleles also interact genetically with Wnt mutants to reveal a role for unc-34 in the establishment of neuronal polarity along the C. elegans anterior-posterior axis. Our mutant analysis shows that eliminating UNC-34 function results in neuronal migration and polarity phenotypes that are enhanced at higher temperatures, revealing a heat-sensitive process that is normally masked by the presence of UNC-34. Finally, we show that the UNC-34 protein is expressed broadly and accumulates in axons and at the apical junctions of epithelial cells. While most mutants lacked detectable UNC-34, three unc-34 mutants that contained missense mutations in the EVH1 domain produced full-length UNC-34 that failed to localize to apical junctions and axons, supporting the role for the EVH1 domain in localizing Ena/VASP family members.

Regulation of T-cell antigen receptor-mediated inside-out signaling by cytosolic adapter proteins and Rap1 effector molecules

Integrins are critical for the migration of T cells to lymphoid organs and to sites of inflammation and are also necessary for productive interactions between T cells and antigen-presenting cells. Integrin activation is enhanced following T-cell receptor (TCR) engagement, as signals initiated by the TCR increase affinity and avidity of integrins for their ligands. This process, known as inside-out signaling, has been shown to require several molecular components including the cytosolic adapter proteins adhesion and degranulation-promoting adapter protein and Src homology 2 domain-containing adapter protein of 55 kDa, the low molecular weight guanosine triphosphatase Rap1, and the Rap1 effector proteins Rap1 guanosine triphosphate-interacting adapter molecule, regulator of adhesion and cell polarization enriched in lymphoid tissues, and protein kinase D1. Herein, we review recent findings about how the TCR is linked to integrin activation through inside-out signaling.

Role of Growth Factor Receptor–Bound Protein 7 in Hepatocellular Carcinoma

The human growth factor receptor-bound protein 7 (Grb7) is an adaptor molecule and is related to cell invasion. In this present study, we investigated the clinical and biological significance of Grb7 expression in human hepatocellular carcinoma (HCC). We reviewed 64 consecutive patients who had undergone liver resection for HCC, and we investigated the correlation between Grb7 expression and clinical outcome. To analyze the biological behavior of Grb7 in vitro and in vivo, we established Grb7 stable knockdown HCC cells using RNA interference technology. The positive staining of Grb7 protein was correlated with portal venous invasion (P < 0.01), hepatic venous invasion (P < 0.01), and intrahepatic metastasis (P < 0.05). Positive expression of Grb7 was significantly correlated with focal adhesion kinase (FAK) protein levels in HCC (P < 0.01). The Grb7- and FAK-positive group showed a significantly poorer prognosis as compared with the Grb7- and FAK-negative group (P < 0.05). Grb7 knockdown HCC cells exhibited significantly lower levels of invasion potential (P < 0.05) and motility (P < 0.05) than the control cells in vitro; moreover, Grb7 knockdown HCC cells showed delayed onset of the tumors compared with the control cells in vivo. Grb7 expression can modulate the invasive phenotype of HCC. Grb7 plays an important role in HCC progression and is strongly associated with expression of FAK. Grb7 could be a therapeutic target in HCC.

Transduced PEP-1-Grb7 fusion protein suppressed LPS-induced COX-2 expression

Although the incidence and severity of atopic dermatitis (AD) is steadily increasing at an alarming rate, its pathogenic mechanisms remain poorly understood yet. Recently, we found that the expression of Grb7 protein was markedly decreased in AD patients using proteomic analysis. In the present study, human Grb7 gene was fused with PEP-1 peptide in a bacterial expression vector to produce a genetic in-frame PEP-1-Grb7 fusion protein. The expressed and purified PEP-1-Grb7 fusion proteins transduced efficiently into skin cells in a time- and dose-dependent manner when added exogenously in culture media. Once inside the cells, the transduced PEP-1-Grb7 protein was stable for 48 h. In addition, transduced PEP-1-Grb7 fusion protein markedly increased cell viability in macrophage RAW 264.7 cells treated with LPS by inhibition of the COX-2 expression level. These results suggest that the PEP-1-Grb7 fusion protein can be used in protein therapy for inflammatory skin disorders, including AD.

VAB-8, UNC-73 and MIG-2 regulate axon polarity and cell migration functions of UNC-40 in C. elegans

One of the most intriguing features of axons is their ability to pioneer precise paths to their targets. How guidance-cue information is interpreted and integrated to form intricate neuronal networks has not been fully deciphered. Using Caenorhabditis elegans, we show that highly conserved receptors that guide pioneer axons along the dorsoventral axis, such as UNC-40 and SAX-3 (receptors for UNC-6 and SLT-1 guidance cues, respectively), can be co-opted to affect axon and cell migrations along the anterior-posterior axis. We further identify the kinesin-related VAB-8 protein as an upstream regulator of UNC-40, illuminating VAB-8's mechanism of action in determining the polarity of cell and axon migration. Finally, we show that UNC-73 and its target MIG-2 function with VAB-8 as upstream regulators of UNC-40 and that MIG-2 activity specifies UNC-40 subcellular localization. These data are indicative of previously unidentified regulatory roles for VAB-8 and small GTPases, which act together to regulate guidance receptor functions.

C. elegans VAB-8 and UNC-73 regulate the SAX-3 receptor to direct cell and growth-cone migrations

During nervous system development, a small number of conserved guidance cues and receptors regulate many axon trajectories. How could a limited number of cues and receptors regulate such complex projection patterns? One way is to modulate receptor function. Here we show that the Caenorhabditis elegans kinesin-related protein VAB-8L, which is necessary and sufficient for posterior cell and growth-cone migrations, directs these migrations by regulating the levels of the guidance receptor SAX-3 (also known as robo). Genetic experiments indicate that VAB-8L and the Rac guanine nucleotide exchange factor activity of UNC-73 (trio) increase the ability of the SLT-1 (slit) and UNC-6 (netrin) guidance pathways to promote posterior guidance. The observations of higher SAX-3 receptor abundance in animals with increasing amounts of VAB-8L, and of physical interactions between UNC-73 and both VAB-8L and the intracellular domain of the SAX-3, support a model whereby VAB-8L directs cell and growth-cone migrations by promoting localization of guidance receptors to the cell surface.

An antagonistic role for theC. elegansSchnurri homolog SMA-9 in modulating TGFβ signaling during mesodermal patterning

In C. elegans, the Sma/Mab TGFβ signaling pathway regulates body size and male tail patterning. SMA-9, the C. elegans homolog of Schnurri, has been shown to function as a downstream component to mediate the Sma/Mab TGFβ signaling pathway in these processes. We have discovered a new role for SMA-9 in dorsoventral patterning of the C. eleganspost-embryonic mesoderm, the M lineage. In addition to a small body size, sma-9 mutant animals exhibit a dorsal-to-ventral fate transformation within the M lineage. This M lineage defect of sma-9 mutants is unique in that animals carrying mutations in all other known components of the TGFβ pathway exhibit no M lineage defects. Surprisingly, mutations in the core components of the Sma/Mab TGFβ signaling pathway suppressed the M lineage defects of sma-9 mutants without suppressing their body size defects. We show that this suppression specifically happens within the M lineage. Our studies have uncovered an unexpected role of SMA-9 in antagonizing the TGFβ signaling pathway during mesodermal patterning,suggesting a novel mode of function for the SMA-9/Schnurri family of proteins.

Multiple Wnts and frizzled receptors regulate anteriorly directed cell and growth cone migrations in Caenorhabditis elegans

A set of conserved molecules guides axons along the metazoan dorsal-ventral axis. Recently, Wnt glycoproteins have been shown to guide axons along the anterior-posterior (A/P) axis of the mammalian spinal cord. Here, we show that, in the nematode Caenorhabditis elegans, multiple Wnts and Frizzled receptors regulate the anterior migrations of neurons and growth cones. Three Wnts are expressed in the tail, and at least one of these, EGL-20, functions as a repellent. We show that the MIG-1 Frizzled receptor acts in the neurons and growth cones to promote their migrations and provide genetic evidence that the Frizzleds MIG-1 and MOM-5 mediate the repulsive effects of EGL-20. While these receptors mediate the effects of EGL-20, we find that the Frizzled receptor LIN-17 can antagonize MIG-1 signaling. Our results indicate that Wnts play a key role in A/P guidance in C. elegans and employ distinct mechanisms to regulate different migrations.

UNC-6/netrin and SLT-1/slit guidance cues orient axon outgrowth mediated by MIG-10/RIAM/lamellipodin

BACKGROUND

Axon migrations are guided by extracellular cues that can act as repellants or attractants. However, the logic underlying the manner through which attractive and repulsive responses are determined is unclear. Many extracellular guidance cues, and the cellular components that mediate their signals, have been implicated in both types of responses.

RESULTS

Genetic analyses indicate that MIG-10/RIAM/lamellipodin, a cytoplasmic adaptor protein, functions downstream of the attractive guidance cue UNC-6/netrin and the repulsive guidance cue SLT-1/slit to direct the ventral migration of the AVM and PVM axons in C. elegans. Furthermore, overexpression of MIG-10 in the absence of UNC-6 and SLT-1 induces a multipolar phenotype with undirected outgrowths. Addition of either UNC-6 or SLT-1 causes the neurons to become monopolar. Moreover, the ability of UNC-6 or SLT-1 to direct the axon ventrally is enhanced by the MIG-10 overexpression. We also demonstrate that an interaction between MIG-10 and UNC-34, a protein that promotes actin-filament extension, is important in the response to guidance cues and that MIG-10 colocalizes with actin in cultured cells, where it can induce the formation of lamellipodia.

CONCLUSIONS

We conclude that MIG-10 mediates the guidance of AVM and PVM axons in response to the extracellular UNC-6 and SLT-1 guidance cues. The attractive and repulsive guidance cues orient MIG-10-dependant axon outgrowth to cause a directional response.

Sensitized genetic backgrounds reveal a role for C. elegans FGF EGL-17 as a repellent for migrating CAN neurons

Although many molecules are necessary for neuronal cell migrations in C. elegans, no guidance cues are known to be essential for any of these cells to migrate along the anteroposterior (AP) axis. We demonstrate that the fibroblast growth factor (FGF) EGL-17, an attractant for the migrating sex myoblasts (SMs), repels the CANs, a pair of neurons that migrate posteriorly from the head to the center of the embryo. Although mutations in genes encoding EGL-17/FGF and a specific isoform of its receptor EGL-15/FGFR had little effect on CAN migration, they enhanced the CAN migration defects caused by mutations in other genes. Two cells at the anterior end of the embryo express EGL-17/FGF, raising the possibility that EGL-17/FGF functions as a repellent for migrating CANs. Consistent with this hypothesis, ectopic expression of EGL-17/FGF shifted the final CAN cell positions away from these novel sites of expression. Cell-specific rescue experiments demonstrated that EGL-15/FGFR acts in the CANs to promote their migration. We also found that the tyrosine phosphatase receptor CLR-1 regulates CAN migration by inhibiting EGL-15/FGFR signaling, and that the FGFR adaptor protein SEM-5/GRB2 may mediate EGL-15/FGFR signaling in CAN migration. Thus, EGL-17/FGF signaling through an EGL-15/FGFR isoform and possibly SEM-5/GRB2 mediates both attraction of the SMs and repulsion of the CANs. This study also raises the possibility that several guidance cues regulate cell migrations along the C. elegans AP axis, and their role in these migrations may only be revealed in sensitized genetic backgrounds.

Grb10 and Grb14: enigmatic regulators of insulin action--and more?

The Grb proteins (growth factor receptor-bound proteins) Grb7, Grb10 and Grb14 constitute a family of structurally related multidomain adapters with diverse cellular functions. Grb10 and Grb14, in particular, have been implicated in the regulation of insulin receptor signalling, whereas Grb7 appears predominantly to be involved in focal adhesion kinase-mediated cell migration. However, at least in vitro, these adapters can bind to a variety of growth factor receptors. The highest identity within the Grb7/10/14 family occurs in the C-terminal SH2 (Src homology 2) domain, which mediates binding to activated receptors. A second well-conserved binding domain, BPS [between the PH (pleckstrin homology) and SH2 domains], can act to enhance binding to the IR (insulin receptor). Consistent with a putative adapter function, some non-receptor-binding partners, including protein kinases, have also been identified. Grb10 and Grb14 are widely, but not uniformly, expressed in mammalian tissues, and there are various isoforms of Grb10. Binding of Grb10 or Grb14 to autophosphorylated IR in vitro inhibits tyrosine kinase activity towards other substrates, but studies on cultured cell lines have been conflicting as to whether Grb10 plays a positive or negative role in insulin signalling. Recent gene knockouts in mice have established that Grb10 and Grb14 act as inhibitors of intracellular signalling pathways regulating growth and metabolism, although the phenotypes of the two knockouts are distinct. Ablation of Grb14 enhances insulin action in liver and skeletal muscle and improves whole-body tolerance, with little effect on embryonic growth. Ablation of Grb10 results in disproportionate overgrowth of the embryo and placenta involving unidentified pathways, and also impacts on hepatic glycogen synthesis, and probably on glucose homoeostasis. This review discusses the extent to which previous studies in vitro can account for the observed phenotype of knockout animals, and considers evidence that aberrant function of Grb10 or Grb14 may contribute to disorders of growth and metabolism in humans.

Biochemistry and biomechanics of cell motility

Cell motility is an essential cellular process for a variety of biological events. The process of cell migration requires the integration and coordination of complex biochemical and biomechanical signals. The protrusion force at the leading edge of a cell is generated by the cytoskeleton, and this force generation is controlled by multiple signaling cascades. The formation of new adhesions at the front and the release of adhesions at the rear involve the outside-in and inside-out signaling mediated by integrins and other adhesion receptors. The traction force generated by the cell on the extracellular matrix (ECM) regulates cell-ECM adhesions, and the counter force exerted by ECM on the cell drives the migration. The polarity of cell migration can be amplified and maintained by the feedback loop between the cytoskeleton and cell-ECM adhesions. Cell migration in three-dimensional ECM has characteristics distinct from that on two-dimensional ECM. The migration of cells is initiated and modulated by external chemical and mechanical factors, such as chemoattractants and the mechanical forces acting on the cells and ECM, as well as the surface density, distribution, topography, and rigidity of the ECM.

The adaptor Grb7 is a novel calmodulin-binding protein: functional implications of the interaction of calmodulin with Grb7

We demonstrate using Ca2+-dependent calmodulin (CaM)-affinity chromatography and overlay with biotinylated CaM that the adaptor proteins growth factor receptor bound (Grb)7 and Grb7V (a naturally occurring variant lacking the Src homology 2 (SH2) domain) are CaM-binding proteins. Deletion of an amphiphilic basic amino-acid sequence (residues 243-256) predicted to form an alpha-helix located in the proximal region of its pleckstrin homology (PH) domain demonstrates the location of the CaM-binding domain. This site is identical in human and rodents Grb7, and shares great homology with similar regions of Grb10 and Grb14, and the Mig10 protein from Caenorhabditis elegans. We show that Grb7 and Grb7V are present in the cytosol and bound to membranes, while the deletion mutants (Grb7Delta and Grb7VDelta) have less capacity to be associated to membranes. Grb7Delta maintains in part the capacity to bind phosphoinositides, and CaM competes for phosphoinositide binding. Activation of ErbB2 by heregulin beta1 decreases the pool of Grb7 associated to membranes. The cell-permeable CaM antagonist W7 (N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide), but not the CaM-dependent protein kinase II inhibitor KN93, prevents this effect. Highly specific cell-permeable CaM inhibitory peptides decrease the association of Grb7 to membranes. This suggests that CaM regulates the intracellular mobilization of Grb7 in living cells. Direct interaction between enhanced yellow fluorescent protein (EYFP)-Grb7 and enhanced cyan fluorescent protein (ECFP)-CaM chimeras at the plasma membrane of living cells was demonstrated by fluorescence resonance energy transfer (FRET). The FRET signal dramatically decreased in cells loaded with a cell-permeable Ca2+ chelator, and was significantly attenuated when enhanced yellow fluorescent protein-Grb7 chimera (EYFP-Grb7)Delta instead of EYFP-Grb7 was used. Finally, we show that conditioned media from cells transiently transfected with Grb7Delta and Grb7VDelta lost its angiogenic activity, in contrast to those from cells transiently transfected with their wild-type counterparts.

The L-type cyclin CYL-1 and the heat-shock-factor HSF-1 are required for heat-shock-induced protein expression in Caenorhabditis elegans

In a screen for suppressors of activated GOA-1 (Galpha(o)) under the control of the hsp-16.2 heat-shock promoter, we identified three genetic loci that affected heat-shock-induced GOA-1 expression. The cyl-1 mutants are essentially wild type in appearance, while hsf-1 and sup-45 mutants have egg-laying defects. The hsf-1 mutation also causes a temperature-sensitive developmental arrest, and hsf-1 mutants have decreased life span. Western analysis indicated that mutations in all three loci suppressed the activated GOA-1 transgene by decreasing its expression. Heat-shock-induced expression of hsp-16.2 mRNA was reduced in cyl-1 mutants and virtually eliminated in hsf-1 and sup-45 mutants, as compared to wild-type expression. The mutations could also suppress other transgenes under heat-shock control. cyl-1 and sup-45, but not hsf-1, mutations suppressed a defect caused by a transgene not under heat-shock control, suggesting a role in general transcription or a post-transcriptional aspect of gene expression. hsf-1 encodes the C. elegans homolog of the human heat-shock factor HSF1, and cyl-1 encodes a cyclin most similar to cyclin L. We believe HSF-1 acts in heat-shock-inducible transcription and CYL-1 acts more generally in gene expression.

Altered expression and deletion of RMO1 in osteosarcoma

In order to increase our understanding of the molecular events underlying osteosarcoma progression, the expression of approximately 950 genes was examined in 24 primary and metastatic osteosarcoma tumor specimens. A gene, RMO1, was isolated with decreased expression in metastatic samples. Real-Time PCR corroborated this pattern, revealing lower expression in the primary sample in 6 of 7 cases for which both primary and metastatic osteosarcoma samples were available from the same patient (p = 0.034). RMO1 is located at 2q33, a region of frequent loss of heterozygosity in cancer, and exhibited loss of heterozygosity in 6 out of 9 primary osteosarcoma tumor samples (67%). Loss of heterozygosity is evident in primary tumors while the decrease in gene expression is seen in the metastatic samples, indicating that these 2 events are separately implicated in cancer progression. Cloning of RMO1 revealed an open reading frame with multiple splice forms with significant homology to GRB7, 10 and 14 and MIG10 in the region containing a Pleckstrin homology domain and a Ras association domain, suggestive of a role in cell signaling and migration. Northern blot analysis indicated that RMO1 mRNA is ubiquitously expressed in tissues except for peripheral blood leukocytes. These data suggest that RMO1 may be a candidate for a protein involved in inhibiting tumor progression.

PREL1 provides a link from Ras signalling to the actin cytoskeleton via Ena/VASP proteins

Ena/VASP family proteins are important modulators of cell migration and localize to focal adhesions, stress fibres and the very tips of lamellipodia and filopodia. Proline-rich proteins like vinculin and zyxin are well established interaction partners, which mediate Ena/VASP-recruitment via their EVH1-domains to focal adhesions and stress fibres. However, it is still unclear, which binding partners Ena/VASP proteins may have at lamellipodia tips and how their recruitment to these cellular protrusions is regulated. Here, we report the identification of a novel protein with high similarity to the C. elegans MIG-10 protein, which we termed PREL1 (Proline Rich EVH1 Ligand). PREL1 is a 74 kDa protein and shares homology with the Grb7-family of signalling adaptors. We show that PREL1 directly binds to Ena/VASP proteins and co-localizes with them at lamellipodia tips and at focal adhesions in response to Ras activation. Moreover, PREL1 directly binds to activated Ras in a phosphoinositide-dependent manner. Thus, our data pinpoint PREL1 as the first direct link between Ras signalling and cytoskeletal remodelling via Ena/VASP proteins during cell migration and spreading.

RIAM, an Ena/VASP and Profilin ligand, interacts with Rap1-GTP and mediates Rap1-induced adhesion

The small GTPase Rap1 induces integrin-mediated adhesion and changes in the actin cytoskeleton. The mechanisms that mediate these effects of Rap1 are poorly understood. We have identified RIAM as a Rap1-GTP-interacting adaptor molecule. RIAM defines a family of adaptor molecules that contain a RA-like (Ras association) domain, a PH (pleckstrin homology) domain, and various proline-rich motifs. RIAM also interacts with Profilin and Ena/VASP proteins, molecules that regulate actin dynamics. Overexpression of RIAM induced cell spreading and lamellipodia formation, changes that require actin polymerization. In contrast, RIAM knockdown cells had reduced content of polymerized actin. RIAM overexpression also induced integrin activation and cell adhesion. RIAM knockdown displaced Rap1-GTP from the plasma membrane and abrogated Rap1-induced adhesion. Thus, RIAM links Rap1 to integrin activation and plays a role in regulating actin dynamics.

The Caenorhabditis elegans Ror RTK CAM-1 inhibits EGL-20/Wnt signaling in cell migration

During Caenorhabditis elegans development, the HSN neurons and the right Q neuroblast and its descendants undergo long-range anteriorly directed migrations. Both of these migrations require EGL-20, a C. elegans Wnt homolog. Through a canonical Wnt signaling pathway, EGL-20/Wnt transcriptionally activates the Hox gene mab-5 in the left Q neuroblast and its descendants, causing the cells to migrate posteriorly. In this report, we show that CAM-1, a Ror receptor tyrosine kinase (RTK) family member, inhibits EGL-20 signaling. Excess EGL-20, like loss of cam-1, caused the HSNs to migrate too far anteriorly. Excess CAM-1, like loss of egl-20, shifted the final positions of the HSNs posteriorly and caused the left Q neuroblast descendants to migrate anteriorly. The reversal in the migration of the left Q neuroblast and its descendants resulted from a failure to express mab-5, an egl-20 mutant phenotype. Our data suggest that CAM-1 negatively regulates EGL-20.

UNC-39, the C. elegans homolog of the human myotonic dystrophy-associated homeodomain protein Six5, regulates cell motility and differentiation

Mutations in the unc-39 gene of C. elegans lead to migration and differentiation defects in a subset of mesodermal and ectodermal cells, including muscles and neurons. Defects include mesodermal specification and differentiation as well a neuronal migration and axon pathfinding defects. Molecular analysis revealed that unc-39 corresponds to the previously named gene ceh-35 and that the UNC-39 protein belongs to the Six4/5 family of homeodomain transcription factors and is similar to human Six5, a protein implicated in the pathogenesis of type I myotonic dystrophy (DM1). We show that human Six5 and UNC-39 are functional homologs, suggesting that further characterization of the C. elegans unc-39 gene might provide insight into the etiology of DM1.

A cuticle collagen encoded by the lon-3 gene may be a target of TGF-beta signaling in determining Caenorhabditis elegans body shape

The signaling pathway initiated by the TGF-beta family member DBL-1 in Caenorhabditis elegans controls body shape in a dose-dependent manner. Loss-of-function (lf) mutations in the dbl-1 gene cause a short, small body (Sma phenotype), whereas overexpression of dbl-1 causes a long body (Lon phenotype). To understand the cellular mechanisms underlying these phenotypes, we have isolated suppressors of the Sma phenotype resulting from a dbl-1(lf) mutation. Two of these suppressors are mutations in the lon-3 gene, of which four additional alleles are known. We show that lon-3 encodes a collagen that is a component of the C. elegans cuticle. Genetic and reporter-gene expression analyses suggest that lon-3 is involved in determination of body shape and is post-transcriptionally regulated by the dbl-1 pathway. These results support the possibility that TGF-beta signaling controls C. elegans body shape by regulating cuticle composition.

EphB1 associates with Grb7 and regulates cell migration

EphB1 is a member of the Eph family of receptor tyrosine kinases that play important roles in diverse biological processes including nervous system development, angiogenesis, and neural synapsis formation and maturation. Grb7 is an adaptor molecule implicated in the regulation of cell migration. Here we report identification of an interaction between Grb7 and the cytoplasmic domain of EphB1 by using Grb7 as a "bait" in a yeast two-hybrid screening. Co-immunoprecipitation was used to confirm the interaction of Grb7 with the cytoplasmic domain of EphB1 as well as the full-length receptor in intact cells. This interaction is mediated by the SH2 domain of Grb7 and requires tyrosine autophosphorylation of EphB1. Furthermore, Tyr-928 of EphB1 was identified as the primary binding site for Grb7. Stimulation of endogenous EphB1 in embryonal carcinoma P19 cells with its ligand ephrinB1 increased its association with Grb7, which is consistent with a role for the autophosphorylation of EphB1. We also found that EphB1 could phosphorylate Grb7 and mutation of either Tyr-928 or Tyr-594 to Phe decreased this activity. Finally, we show that EphB1 could stimulate fibroblast motility on extracellular matrix in a kinase-dependent manner, which also correlated with its association with Grb7. Consistent with this, co-expression of Grb7 with EphB1 further enhanced cell motility, whereas co-expression of the Grb7 SH2 domain abolished EphB1-stimulated cell migration. Together, our results identified a novel interaction between EphB1 with the adaptor molecule Grb7 and suggested that this interaction may play a role in the regulation of cell migration by EphB1.

Association of Grb7 with phosphoinositides and its role in the regulation of cell migration

Grb7 is the prototype of a family of adaptor molecules that also include Grb10 and Grb14 that share a conserved molecular architecture including Src homology 2 (SH2) and pleckstrin homology (PH) domains. Grb7 has been implicated as a downstream mediator of integrin-FAK signal pathways in the regulation of cell migration, although the molecular mechanisms are still not well understood. In this paper, we investigated the potential role and mechanisms of PH domain in Grb7 in the regulation of cell migration. We found that the PH domain mediated Grb7 binding to phospholipids both in vitro and in intact cells. Furthermore, both Grb7 and its PH domain preferentially interacted with phosphatidylinositol phosphates showing strongest affinity to the D3- and D5-phosphoinositides. The PH domain interaction with phosphoinositides was shown to play a role in the stimulation of cell migration by Grb7. It was also shown to be necessary for Grb7 phosphorylation by FAK, although it was not required for Grb7 interaction with FAK or recruitment to the focal contacts. Last, we found that PI 3-kinase activity played a role in both Grb7 association with phosphoinositides and its stimulation of cell migration. In addition, both FAK binding to PI 3-kinase via its autophosphorylated Tyr(397) and integrin-mediated cell adhesion increased Grb7 association with phosphoinositides. Together, these results identified the Grb7 PH domain interaction with phosphoinositides and suggested a potential mechanism by which several signaling molecules including Grb7, FAK, and PI 3-kinase and their interactions cooperate to mediate signal transduction pathways in integrin-mediated cell migration.

The Grb7 family proteins: structure, interactions with other signaling molecules and potential cellular functions

Grb7 family adaptor molecules consist of Grb7, Grb10 and Grb14, each of which has several splicing variants. Like other adaptor molecules, Grb7 family proteins function to mediate the coupling of multiple cell surface receptors to downstream signaling pathways in the regulation of various cellular functions. They share significant sequence homology with each other and a conserved molecular architecture including an amino-terminal proline-rich region, a central segment termed the GM region (for Grb and Mig) which includes a PH domain and shares sequence homology with the Caenorhabditis elegans protein, Mig-10, involved in embryonic migration, and a carboxyl-terminal SH2 domain. Grb7 family proteins are differentially expressed in a variety of tissues. They are phosphorylated on serine/threonine as well as tyrosine residues, although the kinases responsible have not been well characterized. Grb7 family proteins are mainly localized in the cytoplasm, but have been observed at the plasma membrane, focal contacts, or mitochondria under certain conditions. A large number of receptor tyrosine kinases and other signaling molecules can associate with Grb7 family proteins, mostly through the SH2 domains. Various isoforms of Grb10 have been shown to regulate cell proliferation and apoptosis, whereas Grb7 has been found to regulate cell migration and also implicated in tumor progression. Future studies of interests will include identification of potential downstream effectors of Grb7 family proteins as well as understanding of the mechanisms of specificity of the different family members in signal transduction.

Genes regulating touch cell development in Caenorhabditis elegans

To identify genes regulating the development of the six touch receptor neurons, we screened the F(2) progeny of mutated animals expressing an integrated mec-2::gfp transgene that is expressed mainly in these touch cells. From 2638 mutated haploid genomes, we obtained 11 mutations representing 11 genes that affected the production, migration, or outgrowth of the touch cells. Eight of these mutations were in known genes, and 2 defined new genes (mig-21 and vab-15). The mig-21 mutation is the first known to affect the asymmetry of the migrations of Q neuroblasts, the cells that give rise to two of the six touch cells. vab-15 is a msh-like homeobox gene that appears to be needed for the proper production of touch cell precursors, since vab-15 animals lacked the four more posterior touch cells. The remaining touch cells (the ALM cells) were present but mispositioned. A similar touch cell phenotype is produced by mutations in lin-32. A more severe phenotype; i.e., animals often lacked ALM cells, was seen in lin-32 vab-15 double mutants, suggesting that these genes acted redundantly in ALM differentiation. In addition to the touch cell abnormalities, vab-15 animals variably exhibit embryonic or larval lethality, cell degenerations, malformation of the posterior body, uncoordinated movement, and defective egg laying.

Focal adhesion kinases: interest in immunoendocrinology, developmental biology, and cancer

The research field on focal adhesion-related kinases started a decade ago, but the term focal adhesion was introduced for the first time nearly 20 yr before. Since its identification, many studies have enlightened the role of the first intermediate of focal adhesion-related signals in a large number of biologic and physiologic processes. In this review, we try to integrate the most recent data about the known focal adhesion-related kinases, and we focus on three topics in which they deserve great interest: neuroendocrine-immune interactions, developmental biology, and proliferative diseases.

Role of Grb7 targeting to focal contacts and its phosphorylation by focal adhesion kinase in regulation of cell migration

We have previously described Grb7 association with focal adhesion kinase (FAK) and its possible roles in cell migration. In this paper, we investigated the mechanisms by which Grb7 and its association with FAK regulate cell migration. We found that deletion of the Grb7 SH2 domain eliminated partial Grb7 localization to focal contacts and its ability to stimulate cell migration. Replacement of the SH2 domain with the focal adhesion targeting sequence from FAK resulted in the focal contacts localization of the chimeric molecule and restored its activity to stimulate cell migration. We also found that Grb7 could be phosphorylated by FAK, which was dependent on the FAK kinase activity but not the presence of the Src family kinases. Cell adhesion also enhanced Grb7 phosphorylation in FAK+/+ cells but not FAK-/- cells, suggesting that Grb7 is a physiological substrate of FAK. Furthermore, both Grb7 and the chimeric molecule did not increase migration of FAK-/- cells, although the chimeric molecule was targeted to the focal contacts. Last, we showed that other Grb7 family members could not stimulate cell migration under similar experimental conditions. Together, these results demonstrate a role for Grb7 targeting to focal contacts and its phosphorylation by FAK in the regulation of cell migration.

Control of DAF-7 TGF-(alpha) expression and neuronal process development by a receptor tyrosine kinase KIN-8 in Caenorhabditis elegans

KIN-8 in C. elegans is highly homologous to human ROR-1 and 2 receptor tyrosine kinases of unknown functions. These kinases belong to a new subfamily related to the Trk subfamily. A kin-8 promoter::gfp fusion gene was expressed in ASI and many other neurons as well as in pharyngeal and head muscles. A kin-8 deletion mutant was isolated and showed constitutive dauer larva formation (Daf-c) phenotype: about half of the F(1) progeny became dauer larvae when they were cultivated on an old lawn of E. coli as food. Among the cells expressing kin-8::gfp, only ASI sensory neurons are known to express DAF-7 TGF-(beta), a key molecule preventing dauer larva formation. In the kin-8 deletion mutant, expression of daf-7::gfp in ASI was greatly reduced, dye-filling in ASI was specifically lost and ASI sensory processes did not completely extend into the amphid pore. The Daf-c phenotype was suppressed by daf-7 cDNA expression or a daf-3 null mutation. ASI-directed expression of kin-8 cDNA under the daf-7 promoter or expression by a heat shock promoter rescued the dye-filling defect, but not the Daf-c phenotype, of the kin-8 mutant. These results show that the kin-8 mutation causes the Daf-c phenotype through reduction of the daf-7 gene expression and that KIN-8 function is cell-autonomous for the dye-filling in ASI. KIN-8 is required for the process development of ASI, and also involved in promotion of daf-7 expression through a physiological or developmental function.