The Wnt pathway controls cell death engulfment, spindle orientation, and migration through CED-10/Rac

The Rac1 homolog CED-10 is a component of the MES-1/SRC-1 pathway for asymmetric division of the Caenorhabditis elegans EMS blastomere

Asymmetric cell division is essential for the creation of cell types with different identities and functions. The endomesodermal precursor cell (EMS) of the 4-cell Caenorhabditis elegans embryo undergoes an asymmetric division in response to partially redundant signaling pathways. One pathway involves a Wnt signal from the neighboring P2 cell, while the other pathway is defined by the receptor-like MES-1 transmembrane protein localized at the EMS-P2 cell contact and the cytoplasmic kinase SRC-1. In response to these signals, the EMS nuclear-centrosome complex rotates, so that the spindle forms on the anterior-posterior axis; after division, the daughter cell contacting P2 becomes the endodermal precursor cell. Here, we identify the Rac1 homolog CED-10 as a new component of the MES-1/SRC-1 pathway. Loss of CED-10 affects both spindle positioning and endoderm specification in the EMS cell. SRC-1 dependent phosphorylation at the EMS-P2 contact is reduced. However, the asymmetric division of the P2 cell, which is also MES-1 and SRC-1 dependent, appears normal in ced-10 mutants. These and other results suggest that CED-10 acts upstream of, or at the level of, SRC-1 activity in the EMS cell. In addition, we find that the branched actin regulator ARX-2 is enriched at the EMS-P2 cell contact site, in a CED-10-dependent manner. Loss of ARX-2 results in EMS spindle orientation defects, suggesting that CED-10 acts through branched actin to promote spindle orientation in the EMS cell.

Apoptotic and Nonapoptotic Cell Death in Caenorhabditis elegans Development

Programmed cell death (PCD) is an essential component of animal development, and aberrant cell death underlies many disorders. Understanding mechanisms that govern PCD during development can provide insight into cell death programs that are disrupted in disease. Key steps mediating apoptosis, a highly conserved cell death program employing caspase proteases, were first uncovered in the nematode Caenorhabditis elegans, a powerful model system for PCD research. Recent studies in C. elegans also unearthed conserved nonapoptotic caspase-independent cell death programs that function during development. Here, we discuss recent advances in understanding cell death during C. elegans development. We review insights expanding the molecular palette behind the execution of apoptotic and nonapoptotic cell death, as well as new discoveries revealing the mechanistic underpinnings of dying cell engulfment and clearance. A number of open questions are also discussed that will continue to propel the field over the coming years.

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.

The Rac1 homolog CED-10 is a component of the MES-1/SRC-1 pathway for asymmetric division of the C. elegans EMS blastomere

Asymmetric cell division is essential for the creation of cell types with different identities and functions. The EMS blastomere of the four-cell Caenorhabditis elegans embryo undergoes an asymmetric division in response to partially redundant signaling pathways. One pathway involves a Wnt signal emanating from the neighboring P2 cell, while the other pathway is defined by the receptor-like MES-1 protein localized at the EMS/P2 cell contact, and the cytoplasmic kinase SRC-1. In response to these pathways, the EMS nuclear-centrosome complex rotates so that the spindle forms on the anterior-posterior axis; after division, the daughter cell contacting P2 becomes the endodermal precursor cell. Here we identify the Rac1 homolog, CED-10, as a new component of the MES-1/SRC-1 pathway. Loss of CED-10 affects both spindle positioning and endoderm specification. Although MES-1 is still present at the EMS/P2 contact in ced-10 embryos, SRC-1 dependent phosphorylation is reduced. These and other results suggest that CED-10 acts downstream of MES-1 and upstream of, or at the level of, SRC-1 activity. In addition, we find that the branched actin regulator ARX-2 is enriched at the EMS/P2 cell contact site, in a CED-10 dependent manner. Loss of ARX-2 results in spindle positioning defects, suggesting that CED-10 acts through branched actin to promote the asymmetric division of the EMS cell.

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.

A natural transdifferentiation event involving mitosis is empowered by integrating signaling inputs with conserved plasticity factors

Transdifferentiation, or direct cell reprogramming, is the conversion of one fully differentiated cell type into another. Whether core mechanisms are shared between natural transdifferentiation events when occurring with or without cell division is unclear. We have previously characterized the Y-to-PDA natural transdifferentiation in Caenorhabditis elegans, which occurs without cell division and requires orthologs of vertebrate reprogramming factors. Here, we identify a rectal-to-GABAergic transdifferentiation and show that cell division is required but not sufficient for conversion. We find shared mechanisms, including erasure of the initial identity, which requires the conserved reprogramming factors SEM-4/SALL, SOX-2, CEH-6/OCT, and EGL-5/HOX. We also find three additional and parallel roles of the Wnt signaling pathway: selection of a specific daughter, removal of the initial identity, and imposition of the precise final subtype identity. Our results support a model in which levels and antagonistic activities of SOX-2 and Wnt signaling provide a timer for the acquisition of final identity.

Repurposing the Killing Machine: Non-canonical Roles of the Cell Death Apparatus in Caenorhabditis elegans Neurons

Here we highlight the increasingly divergent functions of the Caenorhabditis elegans cell elimination genes in the nervous system, beyond their well-documented roles in cell dismantling and removal. We describe relevant background on the C. elegans nervous system together with the apoptotic cell death and engulfment pathways, highlighting pioneering work in C. elegans. We discuss in detail the unexpected, atypical roles of cell elimination genes in various aspects of neuronal development, response and function. This includes the regulation of cell division, pruning, axon regeneration, and behavioral outputs. We share our outlook on expanding our thinking as to what cell elimination genes can do and noting their versatility. We speculate on the existence of novel genes downstream and upstream of the canonical cell death pathways relevant to neuronal biology. We also propose future directions emphasizing the exploration of the roles of cell death genes in pruning and guidance during embryonic development.

Conserved and Distinct Elements of Phagocytosis in Human and C. elegans

Endocytosis provides the cellular nutrition and homeostasis of organisms, but pathogens often take advantage of this entry point to infect host cells. This is counteracted by phagocytosis that plays a key role in the protection against invading microbes both during the initial engulfment of pathogens and in the clearance of infected cells. Phagocytic cells balance two vital functions: preventing the accumulation of cell corpses to avoid pathological inflammation and autoimmunity, whilst maintaining host defence. In this review, we compare elements of phagocytosis in mammals and the nematode Caenorhabditis elegans. Initial recognition of infection requires different mechanisms. In mammals, pattern recognition receptors bind pathogens directly, whereas activation of the innate immune response in the nematode rather relies on the detection of cellular damage. In contrast, molecules involved in efferocytosis—the engulfment and elimination of dying cells and cell debris—are highly conserved between the two species. Therefore, C. elegans is a powerful model to research mechanisms of the phagocytic machinery. Finally, we show that both mammalian and worm studies help to understand how the two phagocytic functions are interconnected: emerging data suggest the activation of innate immunity as a consequence of defective apoptotic cell clearance.

Genes implicated in Caenorhabditis elegans and human health regulate stress resistance and physical abilities in aged Caenorhabditis elegans

Recently, nine Caenorhabditis elegans genes, grouped into two pathways/clusters, were found to be implicated in healthspan in C. elegans and their homologues in humans, based on literature curation, WormBase data mining and bioinformatics analyses. Here, we further validated these genes experimentally in C. elegans. We downregulated the nine genes via RNA interference (RNAi), and their effects on physical function (locomotion in a swim assay) and on physiological function (survival after heat stress) were analysed in aged nematodes. Swim performance was negatively affected by the downregulation of acox-1.1, pept-1, pak-2, gsk-3 and C25G6.3 in worms with advanced age (twelfth day of adulthood) and heat stress resistance was decreased by RNAi targeting of acox-1.1, daf-22, cat-4, pig-1, pak-2, gsk-3 and C25G6.3 in moderately (seventh day of adulthood) or advanced aged nematodes. Only one gene, sad-1, could not be linked to a health-related function in C. elegans with the bioassays we selected. Thus, most of the healthspan genes could be re-confirmed by health measurements in old worms.

The developmental and physiological roles of phagocytosis in Caenorhabditis elegans

Phagocytosis is an essential process by which cellular debris and pathogens are cleared from the environment. Cells extend their plasma membrane to engulf objects and contain them within a limiting membrane for isolation from the cytosol or for intracellular degradation in phagolysosomes. The basic mechanisms of phagocytosis and intracellular clearance are well conserved between animals. Indeed, much of our understanding is derived from studies on the nematode worm, Caenorhabditis elegans. Here, we review the latest progress in understanding the mechanisms and functions of phagocytic clearance from C. elegans studies. In particular, we highlight new insights into phagocytic signaling pathways, phagosome formation and phagolysosome resolution, as well as the challenges in studying these cyclic processes.

Molecular cloning and characterization of DjRac1, a novel small G protein gene from planarian Dugesia japonica

Rac proteins are classified as a subfamily of the Rho family of small G proteins. They are important molecular switches which act as key signal transducers regulating a wide variety of processes in the cell. DjRac1, a novel Rac gene from planarian Dugesia japonica was cloned by RACE method and characterized. This cDNA contains 851 bp with a putative open reading frame of 190 amino acids. It has a predicted molecular mass of 21.12 kDa and an isoelectric point of 8.42. Whole-mount in situ hybridization and relative quantitative real-time PCR were used to study the spatial and temporal expression pattern of DjRac1 from 1 to 7 days in the regenerating planarians. Results showed that the expression of DjRac1 was concentrated in the blastema and the transcription level of DjRac1 was significantly upregulated after amputation within three days, suggesting DjRac1 might participate in the process of regeneration in planarian.

Wnt ligands regulate the asymmetric divisions of neuronal progenitors in C. elegans embryos

Wnt/β-catenin signalling has been implicated in the terminal asymmetric divisions of neuronal progenitors in vertebrates and invertebrates. However, the role of Wnt ligands in this process remains poorly characterized. Here, we used the terminal divisions of the embryonic neuronal progenitors in C. elegans to characterize the role of Wnt ligands during this process, focusing on a lineage that produces the cholinergic interneuron AIY. We observed that, during interphase, the neuronal progenitor is elongated along the anteroposterior axis, then divides along its major axis, generating an anterior and a posterior daughter with different fates. Using time-controlled perturbations, we show that three Wnt ligands, which are transcribed at higher levels at the posterior of the embryo, regulate the orientation of the neuronal progenitor and its asymmetric division. We also identify a role for a Wnt receptor (MOM-5) and a cortical transducer APC (APR-1), which are, respectively, enriched at the posterior and anterior poles of the neuronal progenitor. Our study establishes a role for Wnt ligands in the regulation of the shape and terminal asymmetric divisions of neuronal progenitors, and identifies downstream components.

Tissue polarity and PCP protein function: C. elegans as an emerging model

Polarity is the basis for the generation of cell diversity, as well as the organization, morphogenesis, and functioning of tissues. Studies in Caenorhabditis elegans have provided much insight into PAR-protein mediated polarity; however, the molecules and mechanisms critical for cell polarization within the plane of epithelia have been identified in other systems. Tissue polarity in C. elegans is organized by Wnt-signaling with some resemblance to the Wnt/planar cell polarity (PCP) pathway, but lacking core PCP protein functions. Nonetheless, recent studies revealed that conserved PCP proteins regulate directed cell migratory events in C. elegans, such as convergent extension movements and neurite formation and guidance. Here, we discuss the latest insights and use of C. elegans as a PCP model.

Planar Asymmetries in the C. elegans Embryo Emerge by Differential Retention of aPARs at Cell-Cell Contacts

Formation of the anteroposterior and dorsoventral body axis in Caenorhabditis elegans depends on cortical flows and advection of polarity determinants. The role of this patterning mechanism in tissue polarization after formation of cell-cell contacts is not fully understood. Here, we demonstrate that planar asymmetries are established during left-right symmetry breaking: Centripetal cortical flows asymmetrically and differentially advect anterior polarity determinants (aPARs) from contacts to the medial cortex, resulting in their unmixing from apical myosin. Contact localization and advection of PAR-6 requires balanced CDC-42 activation, while asymmetric retention and advection of PAR-3 can occur independently of PAR-6. Concurrent asymmetric retention of PAR-3, E-cadherin/HMR-1 and opposing retention of antagonistic CDC-42 and Wnt pathway components leads to planar asymmetries. The most obvious mark of planar asymmetry, retention of PAR-3 at a single cell-cell contact, is required for proper cytokinetic cell intercalation. Hence, our data uncover how planar polarity is established in a system without the canonical planar cell polarity pathway through planar asymmetric retention of aPARs.

G Proteins and GPCRs in C. elegans Development: A Story of Mutual Infidelity

Many vital processes during C. elegans development, especially the establishment and maintenance of cell polarity in embryogenesis, are controlled by complex signaling pathways. G protein-coupled receptors (GPCRs), such as the four Frizzled family Wnt receptors, are linchpins in regulating and orchestrating several of these mechanisms. However, despite being GPCRs, which usually couple to G proteins, these receptors do not seem to activate classical heterotrimeric G protein-mediated signaling cascades. The view on signaling during embryogenesis is further complicated by the fact that heterotrimeric G proteins do play essential roles in cell polarity during embryogenesis, but their activity is modulated in a predominantly GPCR-independent manner via G protein regulators such as GEFs GAPs and GDIs. Further, the triggered downstream effectors are not typical. Only very few GPCR-dependent and G protein-mediated signaling pathways have been unambiguously defined in this context. This unusual and highly intriguing concept of separating GPCR function and G-protein activity, which is not restricted to embryogenesis in C. elegans but can also be found in other organisms, allows for essential and multi-faceted ways of regulating cellular communication and response. Although its relevance cannot be debated, its impact is still poorly discussed, and C. elegans is an ideal model to understand the underlying principles.

Engulfing cells promote neuronal regeneration and remove neuronal debris through distinct biochemical functions of CED-1

Two important biological events happen coincidently soon after nerve injury in the peripheral nervous system in C. elegans: removal of axon debris and initiation of axon regeneration. But, it is not known how these two events are co-regulated. Mutants of ced-1, a homolog of Draper and MEGF10, display defects in both events. One model is that those events could be related. But our data suggest that they are actually separable. CED-1 functions in the muscle-type engulfing cells in both events and is enriched in muscle protrusions in close contact with axon debris and regenerating axons. Its two functions occur through distinct biochemical mechanisms; extracellular domain-mediated adhesion for regeneration and extracellular domain binding-induced intracellular domain signaling for debris removal. These studies identify CED-1 in engulfing cells as a receptor in debris removal but as an adhesion molecule in neuronal regeneration, and have important implications for understanding neural circuit repair after injury.

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

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

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

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

The directed migration of gonadal distal tip cells in Caenorhabditis elegans requires NGAT-1, a ß1,4-N-acetylgalactosaminyltransferase enzyme

Glycoproteins such as growth factor receptors and extracellular matrix have well-known functions in development and cancer progression, however, the glycans at sites of modification are often heterogeneous molecular populations which makes their functional characterization challenging. Here we provide evidence for a specific, discrete, well-defined glycan modification and regulation of a stage-specific cell migration in Caenorhabditis elegans. We show that a chain-terminating, putative null mutation in the gene encoding a predicted β1,4-N-acetylgalactosaminyltransferase, named ngat-1, causes a maternally rescued temperature sensitive (ts) defect in the second phase of the three phase migration pattern of the posterior, but not the anterior, hermaphrodite Distal Tip Cell (DTC). An amino-terminal partial deletion of ngat-1 causes a similar but lower penetrance ts phenotype. The existence of multiple ts alleles with distinctly different molecular DNA lesions, neither of which is likely to encode a ts protein, indicates that NGAT-1 normally prevents innate temperature sensitivity for phase 2 DTC pathfinding. Temperature shift analyses indicate that the ts period for the ngat-1 mutant defect ends by the beginning of post-embryonic development-nearly 3 full larval stages prior to the defective phase 2 migration affected by ngat-1 mutations. NGAT-1 homologs generate glycan-terminal GalNAc-β1-4GlcNAc, referred to as LacdiNAc modifications, on glycoproteins and glycolipids. We also found that the absence of the GnT1/Mgat1 activity [UDP-N-acetyl-D-glucosamine:α-3-D-mannoside β-1,2-N-acetylglucosaminyltransferase 1 (encoded by C. elegans gly-12, gly-13, and gly-14 and homologous to vertebrate GnT1/Mgat1)], causes a similar spectrum of DTC phenotypes as ngat-1 mutations-primarily affecting posterior DTC phase 2 migration and preventing manifestation of the same innate ts period as ngat-1. GnT1/Mgat1 is a medial Golgi enzyme known to modify mannose residues and initiate N-glycan branching, an essential step in the biosynthesis of hybrid, paucimannose and complex-type N-glycans. Quadruple mutant animals bearing putative null mutations in ngat-1 and the three GnT genes (gly-12, gly-13, gly-14) were not enhanced for DTC migration defects, suggesting NGAT-1 and GnT1 act in the same pathway. These findings suggest that GnTI generates an N-glycan substrate for NGAT-1 modification, which is required at restrictive temperature (25°C) to prevent, stabilize, reverse or compensate a perinatal thermo-labile process (or structure) causing late larval stage DTC phase 2 migration errors.

Diverse mitotic functions of the cytoskeletal cross-linking protein Shortstop suggest a role in Dynein/Dynactin activity

Shortstop (Shot), an actin–microtubule cross-linking protein, interacts with the Dynactin component Arp-1 to control mitotic spindle assembly and positioning in Drosophila. Shot is important for proper chromosome congression and segregation. Loss of Shot in epithelial tissue leads to significant apoptosis, which when blocked leads to epithelial–mesenchymal transition-like changes.

Proper assembly and orientation of the bipolar mitotic spindle is critical to the fidelity of cell division. Mitotic precision fundamentally contributes to cell fate specification, tissue development and homeostasis, and chromosome distribution within daughter cells. Defects in these events are thought to contribute to several human diseases. The underlying mechanisms that function in spindle morphogenesis and positioning remain incompletely defined, however. Here we describe diverse roles for the actin-microtubule cross-linker Shortstop (Shot) in mitotic spindle function in Drosophila. Shot localizes to mitotic spindle poles, and its knockdown results in an unfocused spindle pole morphology and a disruption of proper spindle orientation. Loss of Shot also leads to chromosome congression defects, cell cycle progression delay, and defective chromosome segregation during anaphase. These mitotic errors trigger apoptosis in Drosophila epithelial tissue, and blocking this apoptotic response results in a marked induction of the epithelial–mesenchymal transition marker MMP-1. The actin-binding domain of Shot directly interacts with Actin-related protein-1 (Arp-1), a key component of the Dynein/Dynactin complex. Knockdown of Arp-1 phenocopies Shot loss universally, whereas chemical disruption of F-actin does so selectively. Our work highlights novel roles for Shot in mitosis and suggests a mechanism involving Dynein/Dynactin activation.

Epithelial spindle orientation diversities and uncertainties: recent developments and lingering questions

Mitotic spindle orientation is a conserved, dynamic, and highly complex process that plays a key role in dictating the cleavage plane, fate, and positioning of cells within a tissue, therefore laying the blueprint for tissue structure and function. While the spindle-positioning pathway has been extensively studied in lower-model organisms, research over the past several years has highlighted its relevance to mammalian epithelial tissues. Although we continue to gain critical insights into the mechanisms underlying spindle positioning, many uncertainties persist. In this commentary, we will review the protein interactions that modulate spindle orientation and we will present important recent findings that underscore epithelial tissue-specific requirements and variations in this important pathway, as well as its potential relevance to cancer.

Programmed Cell Death During Caenorhabditis elegans Development

Programmed cell death is an integral component of Caenorhabditis elegans development. Genetic and reverse genetic studies in C. elegans have led to the identification of many genes and conserved cell death pathways that are important for the specification of which cells should live or die, the activation of the suicide program, and the dismantling and removal of dying cells. Molecular, cell biological, and biochemical studies have revealed the underlying mechanisms that control these three phases of programmed cell death. In particular, the interplay of transcriptional regulatory cascades and networks involving multiple transcriptional regulators is crucial in activating the expression of the key death-inducing gene egl-1 and, in some cases, the ced-3 gene in cells destined to die. A protein interaction cascade involving EGL-1, CED-9, CED-4, and CED-3 results in the activation of the key cell death protease CED-3, which is tightly controlled by multiple positive and negative regulators. The activation of the CED-3 caspase then initiates the cell disassembly process by cleaving and activating or inactivating crucial CED-3 substrates; leading to activation of multiple cell death execution events, including nuclear DNA fragmentation, mitochondrial elimination, phosphatidylserine externalization, inactivation of survival signals, and clearance of apoptotic cells. Further studies of programmed cell death in C. elegans will continue to advance our understanding of how programmed cell death is regulated, activated, and executed in general.

A role for core planar polarity proteins in cell contact-mediated orientation of planar cell division across the mammalian embryonic skin

The question of how cell division orientation is determined is fundamentally important for understanding tissue and organ shape in both healthy or disease conditions. Here we provide evidence for cell contact-dependent orientation of planar cell division in the mammalian embryonic skin. We propose a model where the core planar polarity proteins Celsr1 and Frizzled-6 (Fz6) communicate the long axis orientation of interphase basal cells to neighbouring basal mitoses so that they align their horizontal division plane along the same axis. The underlying mechanism requires a direct, cell surface, planar polarised cue, which we posit depends upon variant post-translational forms of Celsr1 protein coupled to Fz6. Our hypothesis has parallels with contact-mediated division orientation in early C. elegans embryos suggesting functional conservation between the adhesion-GPCRs Celsr1 and Latrophilin-1. We propose that linking planar cell division plane with interphase neighbour long axis geometry reinforces axial bias in skin spreading around the mouse embryo body.

Brain and muscle aryl hydrocarbon receptor nuclear translocator-like protein-1 cooperates with glycogen synthase kinase-3β to regulate osteogenesis of bone-marrow mesenchymal stem cells in type 2 diabetes

Type 2 diabetes mellitus (T2DM) is associated with inhibited osteogenesis of bone marrow mesenchymal stem cells (BMSCs). Brain and muscle ARNT-like protein 1 (BMAL1) has been linked to the T2DM-related bone remodeling, however, the specific mechanism is still unclear. Herein, we aimed to determine the role of BMAL1 in T2DM-induced suppression of BMSCs osteogenesis. Inhibited osteogenesis and BMAL1 expression were showed in diabetic BMSCs. And while β-catenin and T cell factor (TCF) expression were decreased, the glycogen synthase kinase-3β (GSK-3β) and nemo-like kinase (NLK) expression were increased in diabetic BMSCs. Moreover, over-expression of BMAL1 led to recovered osteogenesis ability and activation of Wnt/β-catenin pathway, which was partially due to inhibition of GSK-3β caused by over-expression of BMAL1. Taken together, our findings provide new insights into the role of BMAL1 in T2DM-induced suppression of BMSCs osteogenesis. Over-expressed BMAL1 could recover BMSCs osteogenesis in T2DM partially by decreasing GSK-3β expression to activate Wnt/β-catenin pathway. BMAL1 may have a potential use in repairing diabetic bone metabolic disorders.

Dissecting Phagocytic Removal of Apoptotic Cells in Caenorhabditis elegans

The unique features of programmed cell death during C. elegans development provide an outstanding system to decipher the mechanisms governing phagocytic removal of apoptotic cells. Like in many other organisms, phagocytosis in C. elegans involves several essential events, including exposure of eat-me signals on the cell corpse surface, cell corpse recognition and engulfment by phagocytes, and maturation of phagosomes for cell corpse destruction. Forward or reverse genetic approaches, microscopy-based cell biological methods, and biochemical assays have successfully been employed to identify key factors that control different steps of phagocytosis and to understand their functions in these cellular events. In this chapter, we mainly describe how to apply genetic and cell biological approaches to dissect cell corpse removal by phagocytosis in C. elegans.

Control of developmental networks by Rac/Rho small GTPases: How cytoskeletal changes during embryogenesis are orchestrated

Small GTPases in the Rho family act as major nodes with functions beyond cytoskeletal rearrangements shaping the Caenorhabditis elegans embryo during development. These small GTPases are key signal transducers that integrate diverse developmental signals to produce a coordinated response in the cell. In C. elegans, the best studied members of these highly conserved Rho family small GTPases, RHO-1/RhoA, CED-10/Rac, and CDC-42, are crucial in several cellular processes dealing with cytoskeletal reorganization. In this review, we update the functions described for the Rho family small GTPases in spindle orientation and cell division, engulfment, and cellular movements during C. elegans embryogenesis, focusing on the Rho subfamily Rac. Please also see the video abstract here.

Mitotic Spindle Positioning in the EMS Cell of Caenorhabditis elegans Requires LET-99 and LIN-5/NuMA

Asymmetric divisions produce daughter cells with different fates, and thus are critical for animal development. During asymmetric divisions, the mitotic spindle must be positioned on a polarized axis to ensure the differential segregation of cell fate determinants into the daughter cells. In many cell types, a cortically localized complex consisting of Gα, GPR-1/2, and LIN-5 (Gαi/Pins/Mud, Gαi/LGN/NuMA) mediates the recruitment of dynactin/dynein, which exerts pulling forces on astral microtubules to physically position the spindle. The conserved PAR polarity proteins are known to regulate both cytoplasmic asymmetry and spindle positioning in many cases. However, spindle positioning also occurs in response to cell signaling cues that appear to be PAR-independent. In the four-cell Caenorhabditis elegans embryo, Wnt and Mes-1/Src-1 signaling pathways act partially redundantly to align the spindle on the anterior/posterior axis of the endomesodermal (EMS) precursor cell. It is unclear how those extrinsic signals individually contribute to spindle positioning and whether either pathway acts via conserved spindle positioning regulators. Here, we genetically test the involvement of Gα, LIN-5, and their negative regulator LET-99, in transducing EMS spindle positioning polarity cues. We also examined whether the C. elegans ortholog of another spindle positioning regulator, DLG-1, is required. We show that LET-99 acts in the Mes-1/Src-1 pathway for spindle positioning. LIN-5 is also required for EMS spindle positioning, possibly through a Gα- and DLG-1-independent mechanism.

Programmed cell death and clearance of cell corpses in Caenorhabditis elegans

Programmed cell death is critical to the development of diverse animal species from C. elegans to humans. In C. elegans, the cell death program has three genetically distinguishable phases. During the cell suicide phase, the core cell death machinery is activated through a protein interaction cascade. This activates the caspase CED-3, which promotes numerous pro-apoptotic activities including DNA degradation and exposure of the phosphatidylserine "eat me" signal on the cell corpse surface. Specification of the cell death fate involves transcriptional activation of the cell death initiator EGL-1 or the caspase CED-3 by coordinated actions of specific transcription factors in distinct cell types. In the cell corpse clearance stage, recognition of cell corpses by phagocytes triggers several signaling pathways to induce phagocytosis of apoptotic cell corpses. Cell corpse-enclosing phagosomes ultimately fuse with lysosomes for digestion of phagosomal contents. This article summarizes our current knowledge about programmed cell death and clearance of cell corpses in C. elegans.

WNT-SHH Antagonism Specifies and Expands Stem Cells prior to Niche Formation

Adult stem cell (SC) maintenance and differentiation are known to depend on signals received from the niche. Here, however, we demonstrate a mechanism for SC specification and regulation that is niche independent. Using immunofluorescence, live imaging, genetics, cell-cycle analyses, in utero lentiviral transduction, and lineage-tracing, we show that in developing hair buds, SCs are born from asymmetric divisions that differentially display WNT and SHH signaling. Displaced WNT(lo) suprabasal daughters become SCs that respond to paracrine SHH and symmetrically expand. By contrast, basal daughters remain WNT(hi). They express but do not respond to SHH and hence maintain slow-cycling, asymmetric divisions. Over time, they become short-lived progenitors, generating differentiating daughters rather than SCs. Thus, in contrast to an established niche that harbors a fixed SC pool whose expelled progeny differentiate, asymmetric divisions first specify and displace early SCs into an environment conducive to expansion and later restrict their numbers by switching asymmetric fates.

The adhesion GPCR latrophilin - a novel signaling cascade in oriented cell division and anterior-posterior polarity

Although several signaling pathways in oriented cell division have been well characterized such as delta/notch inductions or wnt/frizzled-based anterior-posterior polarity, there is strong evidence for additional signal pathways controlling early anterior-posterior polarity decisions. The homolog of the adhesion G protein-coupled receptor latrophilin, LAT-1 has been identified as a receptor essential for oriented cell division in an anterior-posterior direction of specific blastomeres in the early C. elegans embryo. We recently conducted a study aiming at clarifying the signals involved in LAT-1 function. We identified a Gs protein/adenylyl cyclase/cAMP pathway in vitro and demonstrated its physiological relevance in oriented cell division. By interaction with a Gs protein LAT-1 elevates cAMP levels. These data indicate that G-protein signaling in oriented cell division is not solely GPCR-independent. This commentary will discuss our findings in the context of the current knowledge of mechanisms controlling oriented cell division and anterior-posterior polarity. Further, we identify open questions which need to be addressed in the future.

Functional Interplay of Two Paralogs Encoding SWI/SNF Chromatin-Remodeling Accessory Subunits During Caenorhabditis elegans Development

SWI/SNF ATP-dependent chromatin-remodeling complexes have been related to several cellular processes such as transcription, regulation of chromosomal stability, and DNA repair. The Caenorhabditis elegans gene ham-3 (also known as swsn-2.1) and its paralog swsn-2.2 encode accessory subunits of SWI/SNF complexes. Using RNA interference (RNAi) assays and diverse alleles we investigated whether ham-3 and swsn-2.2 have different functions during C. elegans development since they encode proteins that are probably mutually exclusive in a given SWI/SNF complex. We found that ham-3 and swsn-2.2 display similar functions in vulva specification, germline development, and intestinal cell proliferation, but have distinct roles in embryonic development. Accordingly, we detected functional redundancy in some developmental processes and demonstrated by RNA sequencing of RNAi-treated L4 animals that ham-3 and swsn-2.2 regulate the expression of a common subset of genes but also have specific targets. Cell lineage analyses in the embryo revealed hyper-proliferation of intestinal cells in ham-3 null mutants whereas swsn-2.2 is required for proper cell divisions. Using a proteomic approach, we identified SWSN-2.2-interacting proteins needed for early cell divisions, such as SAO-1 and ATX-2, and also nuclear envelope proteins such as MEL-28. swsn-2.2 mutants phenocopy mel-28 loss-of-function, and we observed that SWSN-2.2 and MEL-28 colocalize in mitotic and meiotic chromosomes. Moreover, we demonstrated that SWSN-2.2 is required for correct chromosome segregation and nuclear reassembly after mitosis including recruitment of MEL-28 to the nuclear periphery.

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

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

Adhesion GPCRs Govern Polarity of Epithelia and Cell Migration

In multicellular organisms cells spatially arrange in a highly coordinated manner to form tissues and organs, which is essential for the function of an organism. The component cells and resulting structures are often polarised in one or more axes, and how such polarity is established and maintained correctly has been one of the major biological questions for many decades. Research progress has shown that many adhesion GPCRs (aGPCRs) are involved in several types of polarity. Members of the two evolutionarily oldest groups, Flamingo/Celsr and Latrophilins, are key molecules in planar cell polarity of epithelia or the propagation of cellular polarity in the early embryo, respectively. Other adhesion GPCRs play essential roles in cell migration, indicating that this receptor class includes essential molecules for the control of various levels of cellular organisation.

Oriented Cell Division in the C. elegans Embryo Is Coordinated by G-Protein Signaling Dependent on the Adhesion GPCR LAT-1

Orientation of spindles and cell division planes during development of many species ensures that correct cell-cell contacts are established, which is vital for proper tissue formation. This is a tightly regulated process involving a complex interplay of various signals. The molecular mechanisms underlying several of these pathways are still incompletely understood. Here, we identify the signaling cascade of the C. elegans latrophilin homolog LAT-1, an essential player in the coordination of anterior-posterior spindle orientation during the fourth round of embryonic cell division. We show that the receptor mediates a G protein-signaling pathway revealing that G-protein signaling in oriented cell division is not solely GPCR-independent. Genetic analyses showed that through the interaction with a G_s protein LAT-1 elevates intracellular cyclic AMP (cAMP) levels in the C. elegans embryo. Stimulation of this G-protein cascade in lat-1 null mutant nematodes is sufficient to orient spindles and cell division planes in the embryo in the correct direction. Finally, we demonstrate that LAT-1 is activated by an intramolecular agonist to trigger this cascade. Our data support a model in which a novel, GPCR-dependent G protein-signaling cascade mediated by LAT-1 controls alignment of cell division planes in an anterior-posterior direction via a metabotropic G_s-protein/adenylyl cyclase pathway by regulating intracellular cAMP levels.

During embryogenesis an entire organism develops from a single cell. This process is vital for the formation of life, thus cell division occurs with a very distinct orientation and pattern that is tightly controlled by several signaling pathways. The mechanisms underlying these pathways are complex and not yet fully understood. In the roundworm Caenorhabditis elegans, a common genetic model, the patterns and orientations in which cells divide in the embryo have been well characterized offering an ideal model to study the molecular mechanisms involved. Here, we show that the signal mediated by the adhesion G protein-coupled receptor LAT-1 is based on cAMP. This second messenger is essential for the orientation of distinct cell division planes in the early embryo. Studies based on a lat-1 knockout mutant reveal that LAT-1 signaling affects the levels of the second messenger cAMP in the cells via a specific G protein. Thereby the receptor is activated by an intrinsic sequence. This pathway is the first one clearly shown to involve a G protein-coupled receptor-dependent G-protein signal in orientation of embryonic cell division, offering a novel level of regulation of this process among other described pathways.

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

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

LINKIN, a new transmembrane protein necessary for cell adhesion

In epithelial collective migration, leader and follower cells migrate while maintaining cell–cell adhesion and tissue polarity. We have identified a conserved protein and interactors required for maintaining cell adhesion during a simple collective migration in the developing C. elegans male gonad. LINKIN is a previously uncharacterized, transmembrane protein conserved throughout Metazoa. We identified seven atypical FG–GAP domains in the extracellular domain, which potentially folds into a β-propeller structure resembling the α-integrin ligand-binding domain. C. elegans LNKN-1 localizes to the plasma membrane of all gonadal cells, with apical and lateral bias. We identified the LINKIN interactors RUVBL1, RUVBL2, and α-tubulin by using SILAC mass spectrometry on human HEK 293T cells and testing candidates for lnkn-1-like function in C. elegans male gonad. We propose that LINKIN promotes adhesion between neighboring cells through its extracellular domain and regulates microtubule dynamics through RUVBL proteins at its intracellular domain.

DOI:http://dx.doi.org/10.7554/eLife.04449.001

In animals, cells can move from one place to another to shape tissues, heal wounds, or defend against invading microbes. A cell may move alone or it may be attached to others and move as part of a group. One member of the group leads this ‘collective migration’, but it is not known how the cells are able to stick to each other and move together.

Collective migration takes place in the male gonad—the organ that makes sperm cells—in larvae of the nematode worm C. elegans. As the gonad matures, a group of cells form a simple chain that can move together. Kato et al. found that a protein called LINKIN must be present for this to happen.

LINKIN is found in the membrane that surrounds animal cells. One section of the protein—called the β-propeller—sits on the outside surface of the membrane. The structure of the β-propeller is similar to a section of another protein—called α-integrin—that also allows cells to attach, suggesting LINKIN may work in a similar way.

LINKIN is found in many animals, so Kato et al. searched for proteins that can interact with it in human cells. This search revealed three proteins that can interact with LINKIN and are required for the cells to move together. Two of the proteins control elements of the internal scaffolding of the cell: this scaffolding, which is known as the cytoskeleton, is involved in moving the cells.

The experiments suggest that LINKIN coordinates the process of binding together with the changes in the cytoskeleton that are needed to allow the cells to move as one. The next challenge is to understand how LINKIN changes the internal program of the cells to achieve this.

DOI:http://dx.doi.org/10.7554/eLife.04449.002

Signaling by the engulfment receptor draper: a screen in Drosophila melanogaster implicates cytoskeletal regulators, Jun N-terminal Kinase, and Yorkie

Draper, the Drosophila melanogaster homolog of the Ced-1 protein of Caenorhabditis elegans, is a cell-surface receptor required for the recognition and engulfment of apoptotic cells, glial clearance of axon fragments and dendritic pruning, and salivary gland autophagy. To further elucidate mechanisms of Draper signaling, we screened chromosomal deficiencies to identify loci that dominantly modify the phenotype of overexpression of Draper isoform II (suppressed differentiation of the posterior crossvein in the wing). We found evidence for 43 genetic modifiers of Draper II. Twenty-four of the 37 suppressor loci and 3 of the 6 enhancer loci were identified. An additional 5 suppressors and 2 enhancers were identified among mutations in functionally related genes. These studies reveal positive contributions to Drpr signaling for the Jun N-terminal Kinase pathway, supported by genetic interactions with hemipterous, basket, jun, and puckered, and for cytoskeleton regulation as indicated by genetic interactions with rac1, rac2, RhoA, myoblast city, Wiskcott-Aldrich syndrome protein, and the formin CG32138, and for yorkie and expanded. These findings indicate that Jun N-terminal Kinase activation and cytoskeletal remodeling collaborate in Draper signaling. Relationships between Draper signaling and Decapentaplegic signaling, insulin signaling, Salvador/Warts/Hippo signaling, apical-basal cell polarity, and cellular responses to mechanical forces are also discussed.

The BED finger domain protein MIG-39 halts migration of distal tip cells in Caenorhabditis elegans

Organs are often formed by the extension and branching of epithelial tubes. An appropriate termination of epithelial tube extension is important for generating organs of the proper size and morphology. However, the mechanism by which epithelial tubes terminate their extension is mostly unknown. Here we show that the BED-finger domain protein MIG-39 acts to stop epithelial tube extension in Caenorhabditis elegans. The gonadal leader cells, called distal tip cells (DTCs), migrate in a U-shaped pattern during larval development and stop migrating at the young adult stage, generating a gonad with anterior and posterior U-shaped arms. In mig-39 mutants, however, DTCs overshot their normal stopping position. MIG-39 promoted the deceleration of DTCs, leading to the proper timing and positioning of the cessation of DTC migration. Among three Rac GTPase genes, mutations in ced-10 and rac-2 enhanced the overshoot of anterior DTCs, while they suppressed that of posterior DTCs of mig-39 mutants. On the other hand, the mutation in mig-2 suppressed both the anterior and posterior DTC defects of mig-39. Genetic analyses suggested that MIG-39 acts in parallel with Rac GTPases in stopping DTC migration. We propose a model in which the anterior and posterior DTCs respond in an opposite manner to the levels of Rac activities in the cessation of DTC migration.

Syndecan defines precise spindle orientation by modulating Wnt signaling in C. elegans

Wnt signals orient mitotic spindles in development, but it remains unclear how Wnt signaling is spatially controlled to achieve precise spindle orientation. Here, we show that C. elegans syndecan (SDN-1) is required for precise orientation of a mitotic spindle in response to a Wnt cue. We find that SDN-1 is the predominant heparan sulfate (HS) proteoglycan in the early C. elegans embryo, and that loss of HS biosynthesis or of the SDN-1 core protein results in misorientation of the spindle of the ABar blastomere. The ABar and EMS spindles both reorient in response to Wnt signals, but only ABar spindle reorientation is dependent on a new cell contact and on HS and SDN-1. SDN-1 transiently accumulates on the ABar surface as it contacts C, and is required for local concentration of Dishevelled (MIG-5) in the ABar cortex adjacent to C. These findings establish a new role for syndecan in Wnt-dependent spindle orientation.

Diversity and convergence in the mechanisms establishing L/R asymmetry in metazoa

Differentiating left and right hand sides during embryogenesis represents a major event in body patterning. Left-Right (L/R) asymmetry in bilateria is essential for handed positioning, morphogenesis and ultimately the function of organs (including the brain), with defective L/R asymmetry leading to severe pathologies in human. How and when symmetry is initially broken during embryogenesis remains debated and is a major focus in the field. Work done over the past 20 years, in both vertebrate and invertebrate models, has revealed a number of distinct pathways and mechanisms important for establishing L/R asymmetry and for spreading it to tissues and organs. In this review, we summarize our current knowledge and discuss the diversity of L/R patterning from cells to organs during evolution.

Wnt-signaling and planar cell polarity genes regulate axon guidance along the anteroposterior axis in C. elegans

During the development of the nervous system, neurons encounter signals that inform their outgrowth and polarization. Understanding how these signals combinatorially function to pattern the nervous system is of considerable interest to developmental neurobiologists. The Wnt ligands and their receptors have been well characterized in polarizing cells during asymmetric cell division. The planar cell polarity (PCP) pathway is also critical for cell polarization in the plane of an epithelium. The core set of PCP genes include members of the conserved Wnt-signaling pathway, such as Frizzled and Disheveled, but also the cadherin-domain protein Flamingo. In Drosophila, the Fat and Dachsous cadherins also function in PCP, but in parallel to the core PCP components. C. elegans also have two Fat-like and one Dachsous-like cadherins, at least one of which, cdh-4, contributes to neural development. In C. elegans Wnt ligands and the conserved PCP genes have been shown to regulate a number of different events, including embryonic cell polarity, vulval morphogenesis, and cell migration. As is also observed in vertebrates, the Wnt and PCP genes appear to function to primarily provide information about the anterior to posterior axis of development. Here, we review the recent work describing how mutations in the Wnt and core PCP genes affect axon guidance and synaptogenesis in C. elegans.

Netrins and Wnts function redundantly to regulate antero-posterior and dorso-ventral guidance in C. elegans

Guided migrations of cells and developing axons along the dorso-ventral (D/V) and antero-posterior (A/P) body axes govern tissue patterning and neuronal connections. In C. elegans, as in vertebrates, D/V and A/P graded distributions of UNC-6/Netrin and Wnts, respectively, provide instructive polarity information to guide cells and axons migrating along these axes. By means of a comprehensive genetic analysis, we found that simultaneous loss of Wnt and Netrin signaling components reveals previously unknown and unexpected redundant roles for Wnt and Netrin signaling pathways in both D/V and A/P guidance of migrating cells and axons in C. elegans, as well as in processes essential for organ function and viability. Thus, in addition to providing polarity information for migration along the axis of their gradation, Wnts and Netrin are each able to guide migrations orthogonal to the axis of their gradation. Netrin signaling not only functions redundantly with some Wnts, but also counterbalances the effects of others to guide A/P migrations, while the involvement of Wnt signaling in D/V guidance identifies Wnt signaling as one of the long sought mechanisms that functions in parallel to Netrin signaling to promote D/V guidance of cells and axons. These findings provide new avenues for deciphering how A/P and D/V guidance signals are integrated within the cell to establish polarity in multiple biological processes, and implicate broader roles for Netrin and Wnt signaling - roles that are currently masked due to prevalent redundancy.

While ample information was gathered in past decades on identifying guidance cues and their downstream mediators, very little is known about how the information from multiple extracellular cues is integrated within the cell to generate normal patterning. Netrin and Wnt signaling pathways are both critical to multiple developmental processes and play key roles in normal development as well as in malignancies. The UNC-6/Netrin guidance cue has a conserved role in guiding cell and growth cone migrations along the dorso-ventral axis, whereas Wnts are critical for determining polarity and guidance along the antero-posterior axis. In this study we show that these two signaling pathways function redundantly in both antero-posterior and dorso-ventral guidance as well as in processes essential for viability. Furthermore, we demonstrate that a fine balance between Wnt and Netrin signaling pathways is critical for proper polarity establishment and identify Wnt signaling as one of the long sought mechanisms that signal in parallel to Netrin to promote dorso-ventral guidance of cells and axons in Caenorhabditis elegans. These findings pave the way to unraveling the broader roles of Wnt and Netrin signaling pathways and provide a conceptually novel view of how antero-posterior and dorso-ventral guidance mechanisms are orchestrated.

The lysosomal cathepsin protease CPL-1 plays a leading role in phagosomal degradation of apoptotic cells in Caenorhabditis elegans

In Caenorhabditis elegans, the lysosomal cathepsin protease CPL-1 is indispensable for clearance of apoptotic cells by playing a leading role in destruction of cell corpses in phagolysosomes.

During programmed cell death, the clearance of apoptotic cells is achieved by their phagocytosis and delivery to lysosomes for destruction in engulfing cells. However, the role of lysosomal proteases in cell corpse destruction is not understood. Here we report the identification of the lysosomal cathepsin CPL-1 as an indispensable protease for apoptotic cell removal in Caenorhabditis elegans. We find that loss of cpl-1 function leads to strong accumulation of germ cell corpses, which results from a failure in degradation rather than engulfment. CPL-1 is expressed in a variety of cell types, including engulfment cells, and its mutation does not affect the maturation of cell corpse–containing phagosomes, including phagosomal recruitment of maturation effectors and phagosome acidification. Of importance, we find that phagosomal recruitment and incorporation of CPL-1 occurs before digestion of cell corpses, which depends on factors required for phagolysosome formation. Using RNA interference, we further examine the role of other candidate lysosomal proteases in cell corpse clearance but find that they do not obviously affect this process. Collectively, these findings establish CPL-1 as the leading lysosomal protease required for elimination of apoptotic cells in C. elegans.

Oriented divisions, fate decisions

During development, the establishment of proper tissue architecture depends upon the coordinated control of cell divisions not only in space and time, but also direction. Execution of an oriented cell division requires establishment of an axis of polarity and alignment of the mitotic spindle along this axis. Frequently, the cleavage plane also segregates fate determinants, either unequally or equally between daughter cells, the outcome of which is either an asymmetric or symmetric division, respectively. The last few years have witnessed tremendous growth in understanding both the extrinsic and intrinsic cues that position the mitotic spindle, the varied mechanisms in which the spindle orientation machinery is controlled in diverse organisms and organ systems, and the manner in which the division axis influences the signaling pathways that direct cell fate choices.

Multiple functions of the noncanonical Wnt pathway

Thirty years after the identification of WNTs, understanding of their signal transduction pathways continues to expand. Here, we review recent advances in characterizing the Wnt-dependent signaling pathways in Caenorhabditis elegans linking polar signals to rearrangements of the cytoskeleton in different developmental processes, such as proper mitotic spindle orientation, cell migration, and engulfment of apoptotic corpses. In addition to the well-described transcriptional outputs of the canonical and noncanonical Wnt pathways, new branches regulating nontranscriptional outputs that control RAC (Ras related GTPase) activity are also discussed. These findings suggest that Wnt signaling is a master regulator not only of development, but also of cell polarization.

Molecular pathways regulating mitotic spindle orientation in animal cells

Orientation of the cell division axis is essential for the correct development and maintenance of tissue morphology, both for symmetric cell divisions and for the asymmetric distribution of fate determinants during, for example, stem cell divisions. Oriented cell division depends on the positioning of the mitotic spindle relative to an axis of polarity. Recent studies have illuminated an expanding list of spindle orientation regulators, and a molecular model for how cells couple cortical polarity with spindle positioning has begun to emerge. Here, we review both the well-established spindle orientation pathways and recently identified regulators, focusing on how communication between the cell cortex and the spindle is achieved, to provide a contemporary view of how positioning of the mitotic spindle occurs.

Clathrin and AP2 are required for phagocytic receptor-mediated apoptotic cell clearance in Caenorhabditis elegans

Clathrin and the multi-subunit adaptor protein complex AP2 are central players in clathrin-mediated endocytosis by which the cell selectively internalizes surface materials. Here, we report the essential role of clathrin and AP2 in phagocytosis of apoptotic cells. In Caenorhabditis elegans, depletion of the clathrin heavy chain CHC-1 and individual components of AP2 led to a significant accumulation of germ cell corpses, which resulted from defects in both cell corpse engulfment and phagosome maturation required for corpse removal. CHC-1 and AP2 components associate with phagosomes in an inter-dependent manner. Importantly, we found that the phagocytic receptor CED-1 interacts with the α subunit of AP2, while the CED-6/Gulp adaptor forms a complex with both CHC-1 and the AP2 complex, which likely mediates the rearrangement of the actin cytoskeleton required for cell corpse engulfment triggered by the CED-1 signaling pathway. In addition, CHC-1 and AP2 promote the phagosomal association of LST-4/Snx9/18/33 and DYN-1/dynamin by forming a complex with them, thereby facilitating the maturation of phagosomes necessary for corpse degradation. These findings reveal a non-classical role of clathrin and AP2 and establish them as indispensable regulators in phagocytic receptor-mediated apoptotic cell clearance.