Small GTPase CDC-42 promotes apoptotic cell corpse clearance in response to PAT-2 and CED-1 in C. elegans

SRGP-1/srGAP and AFD-1/afadin stabilize HMP-1/⍺-catenin at rosettes to seal internalization sites following gastrulation in C. elegans

A hallmark of gastrulation is the establishment of germ layers by internalization of cells initially on the exterior. In C. elegans the end of gastrulation is marked by the closure of the ventral cleft, a structure formed as cells internalize during gastrulation, and the subsequent rearrangement of adjacent neuroblasts that remain on the surface. We found that a nonsense allele of srgp-1/srGAP leads to 10-15% cleft closure failure. Deletion of the SRGP-1/srGAP C-terminal domain led to a comparable rate of cleft closure failure, whereas deletion of the N-terminal F-BAR region resulted in milder defects. Loss of the SRGP-1/srGAP C-terminus or F-BAR domain results in defects in rosette formation and defective clustering of HMP-1/⍺-catenin in surface cells during cleft closure. A mutant form of HMP-1/⍺-catenin with an open M domain can suppress cleft closure defects in srgp-1 mutant backgrounds, suggesting that this mutation acts as a gain-of-function allele. Since SRGP-1 binding to HMP-1/⍺-catenin is not favored in this case, we sought another HMP-1 interactor that might be recruited when HMP-1/⍺-catenin is constitutively open. A good candidate is AFD-1/afadin, which genetically interacts with cadherin-based adhesion later during embryonic elongation. AFD-1/afadin is prominently expressed at the vertex of neuroblast rosettes in wildtype, and depletion of AFD-1/afadin increases cleft closure defects in srgp-1/srGAP and hmp-1R551/554A/⍺-catenin backgrounds. We propose that SRGP-1/srGAP promotes nascent junction formation in rosettes; as junctions mature and sustain higher levels of tension, the M domain of HMP-1/⍺-catenin opens, allowing maturing junctions to transition from recruitment of SRGP-1/srGAP to AFD-1/afadin. Our work identifies new roles for ⍺-catenin interactors during a process crucial to metazoan 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.

Network analysis in aged C. elegans reveals candidate regulatory genes of ageing

Ageing is a biological process guided by genetic and environmental factors that ultimately lead to adverse outcomes for organismal lifespan and healthspan. Determination of molecular pathways that are affected with age and increase disease susceptibility is crucial. The gene expression profile of the ideal ageing model, namely the nematode Caenorhabditis elegans mapped with the microarray technology initially led to the identification of age-dependent gene expression alterations that characterize the nematode's ageing process. The list of differentially expressed genes was then utilized to construct a network of molecular interactions with their first neighbors/interactors using the interactions listed in the WormBase database. The subsequent network analysis resulted in the unbiased selection of 110 candidate genes, among which well-known ageing regulators appeared. More importantly, our approach revealed candidates that have never been linked to ageing before, thus suggesting promising potential targets/ageing regulators.

Response of G protein-coupled receptor CED-1 in germline to polystyrene nanoparticles in Caenorhabditis elegans

The deposition of a certain amount of nanopolystyrene (NPS) can be observed in the gonad of Caenorhabditis elegans. However, we still know little about the response of germline towards NPS exposure. In the germline of C. elegans, NPS (1-1000 μg L^-1) increased the expression levels of two G protein-coupled receptors (GPCRs), namely PAQR-2 and CED-1. Moreover, susceptibility to NPS toxicity was observed in ced-1(RNAi) worms, which suggested that the protective response of germline was mediated by GPCR CED-1. In the germline, five proteins (CED-10, VPS-34, SNX-1, RAB-7, and RAB-14) functioned as downstream targets of GPCR CED-1 in controlling NPS toxicity. Furthermore, these five targets in the germline regulated NPS toxicity by affecting the activities of p38 MAPK and insulin signaling pathways in intestinal cells. Therefore, we raised a GPCR CED-1-mediated signaling cascade in the germline in response to NPS exposure, which is helpful for understanding the molecular basis of the germline in response to NPS exposure.

Glia actively sculpt sensory neurons by controlled phagocytosis to tune animal behavior

Glia in the central nervous system engulf neuron fragments to remodel synapses and recycle photoreceptor outer segments. Whether glia passively clear shed neuronal debris or actively prune neuron fragments is unknown. How pruning of single-neuron endings impacts animal behavior is also unclear. Here, we report our discovery of glia-directed neuron pruning in Caenorhabditis elegans. Adult C. elegans AMsh glia engulf sensory endings of the AFD thermosensory neuron by repurposing components of the conserved apoptotic corpse phagocytosis machinery. The phosphatidylserine (PS) flippase TAT-1/ATP8A functions with glial PS-receptor PSR-1/PSR and PAT-2/α-integrin to initiate engulfment. This activates glial CED-10/Rac1 GTPase through the ternary GEF complex of CED-2/CrkII, CED-5/DOCK180, CED-12/ELMO. Execution of phagocytosis uses the actin-remodeler WSP-1/nWASp. This process dynamically tracks AFD activity and is regulated by temperature, the AFD sensory input. Importantly, glial CED-10 levels regulate engulfment rates downstream of neuron activity, and engulfment-defective mutants exhibit altered AFD-ending shape and thermosensory behavior. Our findings reveal a molecular pathway underlying glia-dependent engulfment in a peripheral sense-organ and demonstrate that glia actively engulf neuron fragments, with profound consequences on neuron shape and animal sensory behavior.

Neurons are tree-shaped cells that receive information through endings connected to neighbouring cells or the environment. Controlling the size, number and location of these endings is necessary to ensure that circuits of neurons get precisely the right amount of input from their surroundings.

Glial cells form a large portion of the nervous system, and they are tasked with supporting, cleaning and protecting neurons. In humans, part of their duties is to ‘eat’ (or prune) unnecessary neuron endings. In fact, this role is so important that defects in glial pruning are associated with conditions such as Alzheimer’s disease. Yet it is still unknown how pruning takes place, and in particular whether it is the neuron or the glial cell that initiates the process.

To investigate this question, Raiders et al. enlisted the common laboratory animal Caenorhabditis elegans, a tiny worm with a simple nervous system where each neuron has been meticulously mapped out. First, the experiments showed that glial cells in C. elegans actually prune the endings of sensory neurons. Focusing on a single glia-neuron pair then revealed that the glial cell could trim the endings of a living neuron by redeploying the same molecular machinery it uses to clear dead cell debris. Compared to this debris-clearing activity, however, the glial cell takes a more nuanced approach to pruning: specifically, it can adjust the amount of trimming based on the activity load of the neuron.

When Raiders et al. disrupted the glial pruning for a single temperature-sensing neuron, the worm lost its normal temperature preferences; this demonstrated how the pruning activity of a single glial cell can be linked to behavior.

Taken together the experiments showcase how C. elegans can be used to study glial pruning. Further work using this model could help to understand how disease emerges when glial cells cannot perform their role, and to spot the genetic factors that put certain individuals at increased risk for neurological and sensory disorders.

RhoGAP RGA-8 supports morphogenesis in C. elegans by polarizing epithelia

CDC-42 regulation of non-muscle myosin/NMY-2 is required for polarity maintenance in the one-cell embryo of Caenorhabditis elegans. CDC-42 and NMY-2 regulate polarity throughout embryogenesis, but their contribution to later events of morphogenesis are less understood. We have shown that epidermal enclosure requires the GTPase CED-10/Rac1 and WAVE/Scar complex, its effector, to promote protrusions that drive enclosure through the branch actin regulator Arp2/3. Our analysis here of RGA-8, a homolog of SH3BP1/Rich1/ARHGAP17/Nadrin, with BAR and RhoGAP motifs, suggests it regulates CDC-42, so that actin and myosin/NMY-2 promote ventral enclosure during embryonic morphogenesis. Genetic and molecular data suggest RGA-8 regulates CDC-42, and phenocopies the CDC-42 pathway regulators WASP-1/WSP-1 and the F-BAR proteins TOCA-1 and TOCA-2. Live imaging shows RGA-8 and WSP-1 enrich myosin and regulate F-actin in migrating epidermal cells during ventral enclosure. Loss of RGA-8 alters membrane recruitment of active CDC-42. We propose TOCA proteins and RGA-8 use BAR domains to localize and regenerate CDC-42 activity, thus regulating F-actin levels, through the branched actin regulator WSP-1, and myosin enrichment. RhoGAP RGA-8 thus polarizes epithelia, to promote cell migrations and cell shape changes of embryonic morphogenesis.

Summary: RGA-8, a protein with membrane binding and actin regulatory motifs, promotes embryonic morphogenesis by localizing active CDC-42 in developing epithelia, and controlling actin and actin motors during cell movements.

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.

An EHBP-1-SID-3-DYN-1 axis promotes membranous tubule fission during endocytic recycling

The ACK family tyrosine kinase SID-3 is involved in the endocytic uptake of double-stranded RNA. Here we identified SID-3 as a previously unappreciated recycling regulator in the Caenorhabditis elegans intestine. The RAB-10 effector EHBP-1 is required for the endosomal localization of SID-3. Accordingly, animals with loss of SID-3 phenocopied the recycling defects observed in ehbp-1 and rab-10 single mutants. Moreover, we detected sequential protein interactions between EHBP-1, SID-3, NCK-1, and DYN-1. In the absence of SID-3, DYN-1 failed to localize at tubular recycling endosomes, and membrane tubules breaking away from endosomes were mostly absent, suggesting that SID-3 acts synergistically with the downstream DYN-1 to promote endosomal tubule fission. In agreement with these observations, overexpression of DYN-1 significantly increased recycling transport in SID-3-deficient cells. Finally, we noticed that loss of RAB-10 or EHBP-1 compromised feeding RNAi efficiency in multiple tissues, implicating basolateral recycling in the transport of RNA silencing signals. Taken together, our study demonstrated that in C. elegans intestinal epithelia, SID-3 acts downstream of EHBP-1 to direct fission of recycling endosomal tubules in concert with NCK-1 and DYN-1.

After endocytic uptake, a recycling transport system is deployed to deliver endocytosed macromolecules, fluid, membranes, and membrane proteins back to the cell surface. This process is essential for a series of biological processes such as cytokinesis, cell migration, maintenance of cell polarity, and synaptic plasticity. Recycling endosomes mainly consist of membrane tubules and often undergo membrane fission to generate vesicular carriers, which mediates the delivery of cargo proteins back to the plasma membrane. Previous studies suggested that RAB-10 and its effector protein EHBP-1 function jointly to generate and maintain recycling endosomal tubules. However, the mechanism coupling recycling endosomal tubulation and membrane fission remains elusive. Here, we identified SID-3 as a new interactor of EHBP-1. EHBP-1 is required for the endosomal localization of SID-3 and initiates a protein interaction cascade involving SID-3, NCK-1, and DYN-1/dynamin. We also found that SID-3 functions downstream of EHBP-1 to encourage membrane scission, and that ectopic expression of DYN-1 improves recycling transport in SID-3-depleted cells. Our findings revealed EHBP-1 as a point of convergence between RAB-10-mediated endosomal tubulation and SID-3-assisted membrane tubule fission during endocytic recycling.

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

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

The UIG-1/CDC-42 guanine nucleotide exchange factor acts in parallel to CED-10/Rac1 during axon outgrowth in Caenorhabditis elegans

During development of the brain, neuronal circuits are formed through the projection of axons and dendrites in response to guidance signals. Rho GTPases (Rac1/RhoA/Cdc42) are major regulators of axo-dendritic outgrowth and guidance due to their role in controlling actin cytoskeletal dynamics, cell adhesion and motility. Functional redundancy of Rho GTPase-regulated pathways in neuronal development can mask the roles of specific GTPases. To examine potential Rho GTPase redundancy, we utilized a recently isolated hypomorphic mutation in a Caenorhabditis elegans Rac1 protein - CED-10(G30E) - which reduces the GTP binding and inhibits axon outgrowth of the PVQ interneurons. Here, we show that the CDC-42-specific guanine nucleotide exchange factor UIG-1 acts in parallel to CED-10/Rac1 to control PVQ axon outgrowth. UIG-1 performs this function in a cell-autonomous manner. Further, we found that transgenic expression of CDC-42 can compensate for aberrant CED-10(G30E)-regulated signalling during PVQ axon outgrowth. Together, our study reveals a previously unappreciated function for CDC-42 in PVQ axon outgrowth in C. elegans.

The RhoGAP SPV-1 regulates calcium signaling to control the contractility of the Caenorhabditis elegans spermatheca during embryo transits

Contractility of the nonmuscle and smooth muscle cells that comprise biological tubing is regulated by the Rho-ROCK (Rho-associated protein kinase) and calcium signaling pathways. Although many molecular details about these signaling pathways are known, less is known about how they are coordinated spatiotemporally in biological tubes. The spermatheca of the Caenorhabditis elegans reproductive system enables study of the signaling pathways regulating actomyosin contractility in live adult animals. The RhoGAP (GTPase--activating protein toward Rho family small GTPases) SPV-1 was previously identified as a negative regulator of RHO-1/Rho and spermathecal contractility. Here, we uncover a role for SPV-1 as a key regulator of calcium signaling. spv-1 mutants expressing the calcium indicator GCaMP in the spermatheca exhibit premature calcium release, elevated calcium levels, and disrupted spatial regulation of calcium signaling during spermathecal contraction. Although RHO-1 is required for spermathecal contractility, RHO-1 does not play a significant role in regulating calcium. In contrast, activation of CDC-42 recapitulates many aspects of spv-1 mutant calcium signaling. Depletion of cdc-42 by RNA interference does not suppress the premature or elevated calcium signal seen in spv-1 mutants, suggesting other targets remain to be identified. Our results suggest that SPV-1 works through both the Rho-ROCK and calcium signaling pathways to coordinate cellular contractility.

Pathogenetic basis of Takenouchi-Kosaki syndrome: Electron microscopy study using platelets in patients and functional studies in a Caenorhabditis elegans model

The combined phenotype of thrombocytopenia accompanied by intellectual disability in patients with a de novo heterozygous mutation, i.e., p.Tyr64Cys in CDC42, signifies a clinically recognizable novel syndrome that has been eponymized as “Takenouchi-Kosaki syndrome” (OMIM #616737). In the present study, a detailed phenotypic analysis performed for a total of five patients with Takenouchi-Kosaki syndrome revealed that intellectual disability, macrothrombocytopenia, camptodactyly, structural brain abnormalities with sensorineural deafness, hypothyroidism, and frequent infections comprise the cardinal features of this condition. A morphologic analysis of platelets derived from three affected individuals was performed using electron microscopy. The platelets of the three patients were large and spherical in shape. Furthermore, platelet α-granules were decreased, while vacuoles were increased. We further performed a functional analysis of p.Tyr64Cys in CDC42 through CRISPR/Cas9-mediated gene editing in a Caenorhabditis elegans model. This functional analysis suggested that the mutant allele has hypomorphic effects. Takenouchi-Kosaki syndrome is clinically recognizable by the combined phenotype of intellectual disability, macrothrombocytopenia, camptodactyly, structural brain abnormalities with sensorineural deafness, hypothyroidism, and frequent infections as well as the identification of a heterozygous de novo mutation in CDC42, i.e., p.Tyr64Cys.

The RhoGAP HUM-7/Myo9 integrates signals to modulate RHO-1/RhoA during embryonic morphogenesis in Caenorhabditis elegans

During embryonic morphogenesis, cells and tissues undergo dramatic movements under the control of F-actin regulators. Our studies of epidermal cell migrations in developing Caenorhabditis elegans embryos have identified multiple plasma membrane signals that regulate the Rac GTPase, thus regulating WAVE and Arp2/3 complexes, to promote branched F-actin formation and polarized enrichment. Here, we describe a pathway that acts in parallel to Rac to transduce membrane signals to control epidermal F-actin through the GTPase RHO-1/RhoA. RHO-1 contributes to epidermal migration through effects on underlying neuroblasts. We identify signals to regulate RHO-1-dependent events in the epidermis. HUM-7, the C. elegans homolog of human MYO9A and MYO9B, regulates F-actin dynamics during epidermal migration. Genetics and biochemistry support that HUM-7 behaves as a GTPase-activating protein (GAP) for the RHO-1/RhoA and CDC-42 GTPases. Loss of HUM-7 enhances RHO-1-dependent epidermal cell behaviors. We identify SAX-3/ROBO as an upstream signal that contributes to attenuated RHO-1 activation through its regulation of HUM-7/Myo9. These studies identify a new role for RHO-1 during epidermal cell migration, and suggest that RHO-1 activity is regulated by SAX-3/ROBO acting on the RhoGAP HUM-7.

The small GTPase RAB-35 defines a third pathway that is required for the recognition and degradation of apoptotic cells

In metazoans, apoptotic cells are swiftly engulfed by phagocytes and degraded inside phagosomes. Multiple small GTPases in the Rab family are known to function in phagosome maturation by regulating vesicle trafficking. We discovered rab-35 as a new gene important for apoptotic cell clearance from a genetic screen targeting putative Rab GTPases in Caenorhabditis elegans. We further identified TBC-10 as a putative GTPase-activating protein (GAP), and FLCN-1 and RME-4 as two putative Guanine Nucleotide Exchange Factors (GEFs), for RAB-35. We found that RAB-35 was required for the efficient incorporation of early endosomes to phagosomes and for the timely degradation of apoptotic cell corpses. More specifically, RAB-35 promotes two essential events that initiate phagosome maturation: the switch of phagosomal membrane phosphatidylinositol species from PtdIns(4,5)P2 to PtdIns(3)P, and the recruitment of the small GTPase RAB-5 to phagosomal surfaces. These functions of RAB-35 were previously unknown. Remarkably, although the phagocytic receptor CED-1 regulates these same events, RAB-35 and CED-1 appear to function independently. Upstream of degradation, RAB-35 also facilitates the recognition of apoptotic cells independently of the known CED-1 and CED-5 pathways. RAB-35 localizes to extending pseudopods and is further enriched on nascent phagosomes, consistent with its dual roles in regulating apoptotic cell-recognition and phagosome maturation. Epistasis analyses indicate that rab-35 acts in parallel to both of the canonical ced-1/6/7 and ced-2/5/10/12 clearance pathways. We propose that RAB-35 acts as a robustness factor, defining a novel pathway that aids these canonical pathways in both the recognition and degradation of apoptotic cells.

Actin organization and endocytic trafficking are controlled by a network linking NIMA-related kinases to the CDC-42-SID-3/ACK1 pathway

Molting is an essential process in the nematode Caenorhabditis elegans during which the epidermal apical extracellular matrix, termed the cuticle, is detached and replaced at each larval stage. The conserved NIMA-related kinases NEKL-2/NEK8/NEK9 and NEKL-3/NEK6/NEK7, together with their ankyrin repeat partners, MLT-2/ANKS6, MLT-3/ANKS3, and MLT-4/INVS, are essential for normal molting. In nekl and mlt mutants, the old larval cuticle fails to be completely shed, leading to entrapment and growth arrest. To better understand the molecular and cellular functions of NEKLs during molting, we isolated genetic suppressors of nekl molting-defective mutants. Using two independent approaches, we identified CDC-42, a conserved Rho-family GTPase, and its effector protein kinase, SID-3/ACK1. Notably, CDC42 and ACK1 regulate actin dynamics in mammals, and actin reorganization within the worm epidermis has been proposed to be important for the molting process. Inhibition of NEKL-MLT activities led to strong defects in the distribution of actin and failure to form molting-specific apical actin bundles. Importantly, this phenotype was reverted following cdc-42 or sid-3 inhibition. In addition, repression of CDC-42 or SID-3 also suppressed nekl-associated defects in trafficking, a process that requires actin assembly and disassembly. Expression analyses indicated that components of the NEKL-MLT network colocalize with both actin and CDC-42 in specific regions of the epidermis. Moreover, NEKL-MLT components were required for the normal subcellular localization of CDC-42 in the epidermis as well as wild-type levels of CDC-42 activation. Taken together, our findings indicate that the NEKL-MLT network regulates actin through CDC-42 and its effector SID-3. Interestingly, we also observed that downregulation of CDC-42 in a wild-type background leads to molting defects, suggesting that there is a fine balance between NEKL-MLT and CDC-42-SID-3 activities in the epidermis.

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

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

Scrambled Eggs: Apoptotic Cell Clearance by Non-Professional Phagocytes in the Drosophila Ovary

For half of a century, it has been known that non-professional phagocytes, such as fibroblasts, endothelial, and epithelial cells, are capable of efferocytosis (engulfment of apoptotic cells). Non-professional phagocytes differ from professional phagocytes in the range and efficiency of engulfment. Much of the recognition and underlying signaling machinery between non-professional and professional phagocytes is the same, but it is not known how the engulfment capacity of non-professional phagocytes is controlled. Moreover, the signaling networks involved in cell corpse recognition, engulfment, and phagosome maturation are only partially understood. The Drosophila ovary provides an excellent system to investigate the regulation of phagocytic activity by epithelial cells, a major class of non-professional phagocytes. During Drosophila oogenesis, mid-stage egg chambers undergo apoptosis of the germline in response to nutrient deprivation. Epithelial follicle cells then undergo major cell shape changes and concomitantly engulf the germline material. Our previous work has established that Draper and the integrin α-PS3/β-PS heterodimer are required in follicle cells for germline cell clearance. In addition, we have characterized phagosome maturation pathways, and found that the JNK pathway amplifies the engulfment response. In this review, we discuss recent advances on the interplay between engulfment pathways in the follicular epithelium for cell clearance in the Drosophila ovary. We also provide a comparison to apoptotic cell clearance mechanisms in C. elegans and mammals, illustrating strong conservation of efferocytosis mechanisms by non-professional phagocytes.

Cdc42 regulates junctional actin but not cell polarization in the Caenorhabditis elegans epidermis

During morphogenesis, adherens junctions (AJs) remodel to allow changes in cell shape and position while preserving adhesion. Here, we examine the function of Rho guanosine triphosphatase CDC-42 in AJ formation and regulation during Caenorhabditis elegans embryo elongation, a process driven by asymmetric epidermal cell shape changes. cdc-42 mutant embryos arrest during elongation with epidermal ruptures. Unexpectedly, we find using time-lapse fluorescence imaging that cdc-42 is not required for epidermal cell polarization or junction assembly, but rather is needed for proper junctional actin regulation during elongation. We show that the RhoGAP PAC-1/ARHGAP21 inhibits CDC-42 activity at AJs, and loss of PAC-1 or the interacting linker protein PICC-1/CCDC85A-C blocks elongation in embryos with compromised AJ function. pac-1 embryos exhibit dynamic accumulations of junctional F-actin and an increase in AJ protein levels. Our findings identify a previously unrecognized molecular mechanism for inhibiting junctional CDC-42 to control actin organization and AJ protein levels during epithelial morphogenesis.

aPKC Cycles between Functionally Distinct PAR Protein Assemblies to Drive Cell Polarity

The conserved polarity effector proteins PAR-3, PAR-6, CDC-42, and atypical protein kinase C (aPKC) form a core unit of the PAR protein network, which plays a central role in polarizing a broad range of animal cell types. To functionally polarize cells, these proteins must activate aPKC within a spatially defined membrane domain on one side of the cell in response to symmetry-breaking cues. Using the Caenorhabditis elegans zygote as a model, we find that the localization and activation of aPKC involve distinct, specialized aPKC-containing assemblies: a PAR-3-dependent assembly that responds to polarity cues and promotes efficient segregation of aPKC toward the anterior but holds aPKC in an inactive state, and a CDC-42-dependent assembly in which aPKC is active but poorly segregated. Cycling of aPKC between these distinct functional assemblies, which appears to depend on aPKC activity, effectively links cue-sensing and effector roles within the PAR network to ensure robust establishment of polarity.

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.

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.

CDC-42 Orients Cell Migration during Epithelial Intercalation in the Caenorhabditis elegans Epidermis

Cell intercalation is a highly directed cell rearrangement that is essential for animal morphogenesis. As such, intercalation requires orchestration of cell polarity across the plane of the tissue. CDC-42 is a Rho family GTPase with key functions in cell polarity, yet its role during epithelial intercalation has not been established because its roles early in embryogenesis have historically made it difficult to study. To circumvent these early requirements, in this paper we use tissue-specific and conditional loss-of-function approaches to identify a role for CDC-42 during intercalation of the Caenorhabditis elegans dorsal embryonic epidermis. CDC-42 activity is enriched in the medial tips of intercalating cells, which extend as cells migrate past one another. Moreover, CDC-42 is involved in both the efficient formation and orientation of cell tips during cell rearrangement. Using conditional loss-of-function we also show that the PAR complex functions in tip formation and orientation. Additionally, we find that the sole C. elegans Eph receptor, VAB-1, functions during this process in an Ephrin-independent manner. Using epistasis analysis, we find that vab-1 lies in the same genetic pathway as cdc-42 and is responsible for polarizing CDC-42 activity to the medial tip. Together, these data establish a previously uncharacterized role for polarized CDC-42, in conjunction with PAR-6, PAR-3 and an Eph receptor, during epithelial intercalation.

As embryos develop, tissues must change shape to establish an animal’s form. One key form-shaping movement, cell intercalation, often occurs when a tissue elongates in a preferred direction. How cells in epithelial sheets can intercalate while maintaining tissue integrity is not well understood. Here we use the dorsal epidermis in embryos of the nematode worm, C. elegans, to study cell intercalation. As cells begin to intercalate, they form highly polarized tips that lead their migration. While some mechanisms that polarize intercalating cells have been established in other systems, our work identifies a new role for CDC-42—a highly conserved, highly regulated protein that controls the actin cytoskeleton. We previously established that a related protein, Rac, is involved in tip extension during dorsal intercalation. CDC-42 also contributes to this process in addition to helping orient the extending tip. CDC-42 appears to work in conjunction with two other known cell polarity proteins, PAR-3 and PAR-6, and the cell surface receptor, VAB-1. Our work identifies a novel pathway involving proteins conserved from worms to humans that regulates a ubiquitous process during animal development.

Components of the Engulfment Machinery Have Distinct Roles in Corpse Processing

Billions of cells die in our bodies on a daily basis and are engulfed by phagocytes. Engulfment, or phagocytosis, can be broken down into five basic steps: attraction of the phagocyte, recognition of the dying cell, internalization, phagosome maturation, and acidification. In this study, we focus on the last two steps, which can collectively be considered corpse processing, in which the engulfed material is degraded. We use the Drosophila ovarian follicle cells as a model for engulfment of apoptotic cells by epithelial cells. We show that engulfed material is processed using the canonical corpse processing pathway involving the small GTPases Rab5 and Rab7. The phagocytic receptor Draper is present on the phagocytic cup and on nascent, phosphatidylinositol 3-phosphate (PI(3)P)- and Rab7-positive phagosomes, whereas integrins are maintained on the cell surface during engulfment. Due to the difference in subcellular localization, we investigated the role of Draper, integrins, and downstream signaling components in corpse processing. We found that some proteins were required for internalization only, while others had defects in corpse processing as well. This suggests that several of the core engulfment proteins are required for distinct steps of engulfment. We also performed double mutant analysis and found that combined loss of draper and αPS3 still resulted in a small number of engulfed vesicles. Therefore, we investigated another known engulfment receptor, Crq. We found that loss of all three receptors did not inhibit engulfment any further, suggesting that Crq does not play a role in engulfment by the follicle cells. A more complete understanding of how the engulfment and corpse processing machinery interact may enable better understanding and treatment of diseases associated with defects in engulfment by epithelial cells.

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.

A Sensitized Screen for Genes Promoting Invadopodia Function In Vivo: CDC-42 and Rab GDI-1 Direct Distinct Aspects of Invadopodia Formation

Invadopodia are specialized membrane protrusions composed of F-actin, actin regulators, signaling proteins, and a dynamically trafficked invadopodial membrane that drive cell invasion through basement membrane (BM) barriers in development and cancer. Due to the challenges of studying invasion in vivo, mechanisms controlling invadopodia formation in their native environments remain poorly understood. We performed a sensitized genome-wide RNAi screen and identified 13 potential regulators of invadopodia during anchor cell (AC) invasion into the vulval epithelium in C. elegans. Confirming the specificity of this screen, we identified the Rho GTPase cdc-42, which mediates invadopodia formation in many cancer cell lines. Using live-cell imaging, we show that CDC-42 localizes to the AC-BM interface and is activated by an unidentified vulval signal(s) that induces invasion. CDC-42 is required for the invasive membrane localization of WSP-1 (N-WASP), a CDC-42 effector that promotes polymerization of F-actin. Loss of CDC-42 or WSP-1 resulted in fewer invadopodia and delayed BM breaching. We also characterized a novel invadopodia regulator, gdi-1 (Rab GDP dissociation inhibitor), which mediates membrane trafficking. We show that GDI-1 functions in the AC to promote invadopodia formation. In the absence of GDI-1, the specialized invadopodial membrane was no longer trafficked normally to the invasive membrane, and instead was distributed to plasma membrane throughout the cell. Surprisingly, the pro-invasive signal(s) from the vulval cells also controls GDI-1 activity and invadopodial membrane trafficking. These studies represent the first in vivo screen for genes regulating invadopodia and demonstrate that invadopodia formation requires the integration of distinct cellular processes that are coordinated by an extracellular cue.

During animal development specialized cells acquire the ability move and invade into other tissues to form complex organs and structures. Understanding this cellular behavior is important medically, as cancer cells can hijack the developmental program of invasion to metastasize throughout the body. One of the most formidable barriers invasive cells face is basement membrane–-a thin, dense, sheet-like assembly of proteins and carbohydrates that surrounds most tissues. Cells deploy small, protrusive, membrane associated structures called invadopodia (invasive feet) to breach basement membranes. How invadopodia are formed and controlled during invasion has been challenging to understand, as it is difficult to examine these dynamic structures in live animals. Using the nematode worm Caenorhabditis elegans, we have conducted the first large-scale screen to isolate genes that control invadopodia in live animals. Our screen isolated 13 genes and we confirmed two are key invadopodia regulators: the Rho GTPase CDC-42 that promotes F-actin polymerization at invadopodia to generate the force to breach basement membranes, and the Rab GDI-1 that promotes membrane addition at invadopodia that may allow invadopodia to extend through basement membranes. This work provides new insights into invadopodia construction and identifies potential novel targets for anti-metastasis therapies.