Tissue-specific degradation of essential centrosome components reveals distinct microtubule populations at microtubule organizing centers

Somatic polyploidy supports biosynthesis and tissue function by increasing transcriptional output

Cell size and biosynthetic capacity generally increase with increased DNA content. Somatic polyploidy has therefore been proposed to be an adaptive strategy to increase cell size in specialized tissues with high biosynthetic demands. However, if and how DNA concentration limits cellular biosynthesis in vivo is not well understood. Here, we show that polyploidy in the Caenorhabditis elegans intestine is critical for cell growth and yolk biosynthesis, a central role of this organ. Artificially lowering the DNA/cytoplasm ratio by reducing polyploidization in the intestine gave rise to smaller cells with dilute mRNA. Highly expressed transcripts were more sensitive to this mRNA dilution, whereas lowly expressed genes were partially compensated-in part by loading more RNA Polymerase II on the remaining genomes. Polyploidy-deficient animals produced fewer and slower-growing offspring, consistent with reduced synthesis of highly expressed yolk proteins. DNA-dilute cells had normal total protein concentration, which we propose is achieved by increasing the expression of translational machinery at the expense of specialized, cell-type-specific proteins.

Diversity of microtubule arrays in animal cells at a glance

Microtubules are cytoskeletal filaments important for various cellular processes such as intracellular transport, cell division, polarization and migration. Microtubule organization goes hand in hand with cellular function. Motile cells, such as immune cells or fibroblasts, contain microtubule asters attached to the centrosome and the Golgi complex, whereas in many other differentiated cells, microtubules form linear arrays or meshworks anchored at membrane-bound organelles or the cell cortex. Over the past decade, new developments in cell culture, genome editing and microscopy have greatly advanced our understanding of complex microtubule arrays. In this Cell Science at a Glance article and the accompanying poster, we review the diversity of microtubule arrays in interphase animal cells. We describe microtubule network geometries present in various differentiated cells, explore the variety in microtubule-organizing centers responsible for these geometries, and discuss examples of microtubule reorganization in response to functional changes and their interplay with cell motility and tissue development.

Tubulin sequence divergence is associated with the use of distinct microtubule regulators

Diverse eukaryotic cells assemble microtubule networks that vary in structure and composition. While we understand how cells build microtubule networks with specialized functions, we do not know how microtubule networks diversify across deep evolutionary timescales. This problem has remained unresolved because most organisms use shared pools of tubulins for multiple networks, making it difficult to trace the evolution of any single network. In contrast, the amoeboflagellate Naegleria expresses distinct tubulin genes to build distinct microtubule networks: while Naegleria builds flagella from conserved tubulins during differentiation, it uses divergent tubulins to build its mitotic spindle. This genetic separation makes for an internally controlled system to study independent microtubule networks in a single organismal and genomic context. To explore the evolution of these microtubule networks, we identified conserved microtubule-binding proteins and used transcriptional profiling of mitosis and differentiation to determine which are upregulated during the assembly of each network. Surprisingly, most microtubule-binding proteins are upregulated during only one process, suggesting that Naegleria uses distinct component pools to specialize its microtubule networks. Furthermore, the divergent residues of mitotic tubulins tend to fall within the binding sites of differentiation-specific microtubule regulators, suggesting that interactions between microtubules and their binding proteins constrain tubulin sequence diversification. We therefore propose a model for cytoskeletal evolution in which pools of microtubule network components constrain and guide the diversification of the entire network, so that the evolution of tubulin is inextricably linked to that of its binding partners.

Structural insights into the interplay between microtubule polymerases, γ-tubulin complexes and their receptors

The γ-tubulin ring complex (γ-TuRC) is a structural template for controlled nucleation of microtubules from α/β-tubulin heterodimers. At the cytoplasmic side of the yeast spindle pole body, the CM1-containing receptor protein Spc72 promotes γ-TuRC assembly from seven γ-tubulin small complexes (γ-TuSCs) and recruits the microtubule polymerase Stu2, yet their molecular interplay remains unclear. Here, we determine the cryo-EM structure of the Candida albicans cytoplasmic nucleation unit at 3.6 Å resolution, revealing how the γ-TuRC is assembled and conformationally primed for microtubule nucleation by the dimerised Spc72 CM1 motif. Two coiled-coil regions of Spc72 interact with the conserved C-terminal α-helix of Stu2 and thereby position the α/β-tubulin-binding TOG domains of Stu2 in the vicinity of the microtubule assembly site. Collectively, we reveal the function of CM1 motifs in γ-TuSC oligomerisation and the recruitment of microtubule polymerases to the γ-TuRC.

Molecular architectures of centrosomes in C. elegans embryos visualized by cryo-electron tomography

Centrosomes organize microtubules that are essential for mitotic divisions in animal cells. They consist of centrioles surrounded by pericentriolar material (PCM). Questions related to mechanisms of centriole assembly, PCM organization, and spindle microtubule formation remain unanswered, partly due to limited availability of molecular-resolution structural data inside cells. Here, we use cryo-electron tomography to visualize centrosomes across the cell cycle in cells isolated from C. elegans embryos. We describe a pseudo-timeline of centriole assembly and identify distinct structural features in both mother and daughter centrioles. We find that centrioles and PCM microtubules differ in protofilament number (13 versus 11), which could be explained by atypical γ-tubulin ring complexes with 11-fold symmetry identified at the minus ends of short PCM microtubule segments. We further characterize a porous and disordered network that forms the interconnected PCM. Thus, our work builds a three-dimensional structural atlas that helps explain how centrosomes assemble, grow, and achieve function.

MRCK-1 activates non-muscle myosin for outgrowth of a unicellular tube in Caenorhabditis elegans

The formation and patterning of unicellular biological tubes is essential for metazoan development. It is well established that vascular tubes and neurons use similar guidance cues to direct their development, but the downstream mechanisms that promote the outgrowth of biological tubes are not well characterized. We show that the conserved kinase MRCK-1 and its substrate the regulatory light chain of non-muscle myosin, MLC-4, are required for outgrowth of the unicellular excretory canal in C. elegans. Ablation of MRCK-1 or MLC-4 in the canal causes severe truncations with unlumenized projections of the basal membrane. Structure-function analysis of MRCK-1 indicates that the kinase domain, but not the small GTPase-binding CRIB domain, is required for canal outgrowth. Expression of a phosphomimetic form of MLC-4 rescues canal truncations in mrck-1 mutants and shows enrichment at the growing canal tip. Moreover, our work reveals a previously unreported function for non-muscle myosin downstream of MRCK-1 in excretory canal outgrowth that may be conserved in the development of seamless tubes in other organisms.

The microtubule regulator EFA-6 forms cortical foci dependent on its intrinsically disordered region and interactions with tubulins

The EFA6 protein family, originally identified as Sec7 guanine nucleotide exchange factors, has also been found to regulate cortical microtubule (MT) dynamics. Here, we find that in the mature C. elegans epidermal epithelium, EFA-6 forms punctate foci in specific regions of the apical cortex, dependent on its intrinsically disordered region (IDR). The EFA-6 IDR can form biomolecular condensates in vitro. In genetic screens for mutants with altered GFP::EFA-6 localization, we identified a gain-of-function (gf) mutation in α-tubulin tba-1 that induces ectopic EFA-6 foci in multiple cell types. Lethality of tba-1(gf) is partially suppressed by loss of function in efa-6. The ability of TBA-1(gf) to trigger ectopic EFA-6 foci requires β-tubulin TBB-2 and the chaperon EVL-20/Arl2. tba-1(gf)-induced EFA-6 foci display slower turnover, contain the MT-associated protein TAC-1/TACC, and require the EFA-6 MT elimination domain (MTED). Our results reveal functionally important crosstalk between cellular tubulins and cortical MT regulators in vivo.

Germ fate determinants protect germ precursor cell division by reducing septin and anillin levels at the cell division plane

Animal cell cytokinesis, or the physical division of one cell into two, is thought to be driven by constriction of an actomyosin contractile ring at the division plane. The mechanisms underlying cell type-specific differences in cytokinesis remain unknown. Germ cells are totipotent cells that pass genetic information to the next generation. Previously, using formincyk-1(ts) mutant Caenorhabditis elegans 4-cell embryos, we found that the P2 germ precursor cell is protected from cytokinesis failure and can divide with greatly reduced F-actin levels at the cell division plane. Here, we identified two canonical germ fate determinants required for P2-specific cytokinetic protection: PIE-1 and POS-1. Neither has been implicated previously in cytokinesis. These germ fate determinants protect P2 cytokinesis by reducing the accumulation of septinUNC-59 and anillinANI-1 at the division plane, which here act as negative regulators of cytokinesis. These findings may provide insight into the regulation of cytokinesis in other cell types, especially in stem cells with high potency.

The outer kinetochore components KNL-1 and Ndc80 complex regulate axon and neuronal cell body positioning in the C. elegans nervous system

The KMN (Knl1/Mis12/Ndc80) network at the kinetochore, primarily known for its role in chromosome segregation, has been shown to be repurposed during neurodevelopment. Here, we investigate the underlying neuronal mechanism and show that the KMN network promotes the proper axonal organization within the C. elegans head nervous system. Postmitotic degradation of KNL-1, which acts as a scaffold for signaling and has microtubule-binding activities at the kinetochore, led to disorganized ganglia and aberrant placement and organization of axons in the nerve ring - an interconnected axonal network. Through gene-replacement approaches, we demonstrate that the signaling motifs within KNL-1, responsible for recruiting protein phosphatase 1, and activating the spindle assembly checkpoint are required for neurodevelopment. Interestingly, while the microtubule-binding activity is crucial to KMN's neuronal function, microtubule dynamics and organization were unaffected in the absence of KNL-1. Instead, the NDC-80 microtubule-binding mutant displayed notable defects in axon bundling during nerve ring formation, indicating its role in facilitating axon-axon contacts. Overall, these findings provide evidence for a noncanonical role for the KMN network in shaping the structure and connectivity of the nervous system in C. elegans during brain development.

Native molecular architectures of centrosomes in C. elegans embryos

Centrosomes organize microtubules that are essential for mitotic divisions in animal cells. They consist of centrioles surrounded by Pericentriolar Material (PCM). Questions related to mechanisms of centriole assembly, PCM organization, and microtubule formation remain unanswered, in part due to limited availability of molecular-resolution structural analyses in situ. Here, we use cryo-electron tomography to visualize centrosomes across the cell cycle in cells isolated from C. elegans embryos. We describe a pseudo-timeline of centriole assembly and identify distinct structural features including a cartwheel in daughter centrioles, and incomplete microtubule doublets surrounded by a star-shaped density in mother centrioles. We find that centriole and PCM microtubules differ in protofilament number (13 versus 11) indicating distinct nucleation mechanisms. This difference could be explained by atypical γ-tubulin ring complexes with 11-fold symmetry identified at the minus ends of short PCM microtubules. We further characterize a porous and disordered network that forms the interconnected PCM. Thus, our work builds a three-dimensional structural atlas that helps explain how centrosomes assemble, grow, and achieve function.

Somatic polyploidy supports biosynthesis and tissue function by increasing transcriptional output

Cell size and biosynthetic capacity generally increase with increased DNA content. Polyploidy has therefore been proposed to be an adaptive strategy to increase cell size in specialized tissues with high biosynthetic demands. However, if and how DNA concentration limits cellular biosynthesis in vivo is not well understood, and the impacts of polyploidy in non-disease states is not well studied. Here, we show that polyploidy in the C. elegans intestine is critical for cell growth and yolk biosynthesis, a central role of this organ. Artificially lowering the DNA/cytoplasm ratio by reducing polyploidization in the intestine gave rise to smaller cells with more dilute mRNA. Highly-expressed transcripts were more sensitive to this mRNA dilution, whereas lowly-expressed genes were partially compensated - in part by loading more RNA Polymerase II on the remaining genomes. DNA-dilute cells had normal total protein concentration, which we propose is achieved by increasing production of translational machinery at the expense of specialized, cell-type specific proteins.

The protein tyrosine phosphatase PPH-7 is required for fertility and embryonic development in C. elegans at elevated temperatures

Post-translational modifications are key in the regulation of activity, structure, localization, and stability of most proteins in eukaryotes. Phosphorylation is potentially the most studied post-translational modification, also due to its reversibility and thereby the regulatory role this modification often plays. While most research attention was focused on kinases in the past, phosphatases remain understudied, most probably because the addition and presence of the modification is more easily studied than its removal and absence. Here, we report the identification of an uncharacterized protein tyrosine phosphatase PPH-7 in C. elegans, a member of the evolutionary conserved PTPN family of phosphatases. Lack of PPH-7 function led to reduction of fertility and embryonic lethality at elevated temperatures. Proteomics revealed changes in the regulation of targets of the von Hippel-Lindau (VHL) E3 ligase, suggesting a potential role for PPH-7 in the regulation of VHL.

MAP9/MAPH-9 supports axonemal microtubule doublets and modulates motor movement

Microtubule doublets (MTDs) comprise an incomplete microtubule (B-tubule) attached to the side of a complete cylindrical microtubule. These compound microtubules are conserved in cilia across the tree of life; however, the mechanisms by which MTDs form and are maintained in vivo remain poorly understood. Here, we identify microtubule-associated protein 9 (MAP9) as an MTD-associated protein. We demonstrate that C. elegans MAPH-9, a MAP9 homolog, is present during MTD assembly and localizes exclusively to MTDs, a preference that is in part mediated by tubulin polyglutamylation. We find that loss of MAPH-9 causes ultrastructural MTD defects, including shortened and/or squashed B-tubules with reduced numbers of protofilaments, dysregulated axonemal motor velocity, and perturbed cilia function. Because we find that the mammalian ortholog MAP9 localizes to axonemes in cultured mammalian cells and mouse tissues, we propose that MAP9/MAPH-9 plays a conserved role in regulating ciliary motors and supporting the structure of axonemal MTDs.

Centriole elimination during Caenorhabditis elegans oogenesis initiates with loss of the central tube protein SAS-1

In most metazoans, centrioles are lost during oogenesis, ensuring that the zygote is endowed with the correct number of two centrioles, which are paternally contributed. How centriole architecture is dismantled during oogenesis is not understood. Here, we analyze with unprecedent detail the ultrastructural and molecular changes during oogenesis centriole elimination in Caenorhabditis elegans. Centriole elimination begins with loss of the so-called central tube and organelle widening, followed by microtubule disassembly. The resulting cluster of centriolar proteins then disappears gradually, usually moving in a microtubule- and dynein-dependent manner to the plasma membrane. Our analysis indicates that neither Polo-like kinases nor the PCM, which modulate oogenesis centriole elimination in Drosophila, do so in C. elegans. Furthermore, we demonstrate that the central tube protein SAS-1 normally departs initially from the organelle, which loses integrity earlier in sas-1 mutants. Overall, our work provides novel mechanistic insights regarding the fundamental process of oogenesis centriole elimination.

Germ fate determinants protect germ precursor cell division by restricting septin and anillin levels at the division plane

Animal cell cytokinesis, or the physical division of one cell into two, is thought to be driven by constriction of an actomyosin contractile ring at the division plane. The mechanisms underlying cell type-specific differences in cytokinesis remain unknown. Germ cells are totipotent cells that pass genetic information to the next generation. Previously, using formin cyk-1 (ts) mutant C. elegans embryos, we found that the P2 germ precursor cell is protected from cytokinesis failure and can divide without detectable F-actin at the division plane. Here, we identified two canonical germ fate determinants required for P2-specific cytokinetic protection: PIE-1 and POS-1. Neither has been implicated previously in cytokinesis. These germ fate determinants protect P2 cytokinesis by reducing the accumulation of septin UNC-59 and anillin ANI-1 at the division plane, which here act as negative regulators of cytokinesis. These findings may provide insight into cytokinetic regulation in other cell types, especially in stem cells with high potency.

ZIF-1-mediated degradation of zinc finger proteins in the Caenorhabditis elegans germ line

Rapid and conditional protein depletion is the gold standard genetic tool for deciphering the molecular basis of developmental processes. Previously, we showed that by conditionally expressing the E3 ligase substrate adaptor ZIF-1 in Caenorhabditis elegans somatic cells, proteins tagged with the first CCCH Zn finger 1 (ZF1) domain from the germline regulator PIE-1 degrade rapidly, resulting in loss-of-function phenotypes. The described role of ZIF-1 is to clear PIE-1 and several other CCCH Zn finger proteins from early somatic cells, helping to enrich them in germline precursor cells. Here, we show that proteins tagged with the PIE-1 ZF1 domain are subsequently cleared from primordial germ cells (PGCs) in embryos and from undifferentiated germ cells in larvae and adults by ZIF-1. We harness germline ZIF-1 activity to degrade a ZF1-tagged fusion protein from PGCs and show that its depletion produces phenotypes equivalent to those of a null mutation. Our findings reveal that ZIF-1 transitions from degrading CCCH Zn finger proteins in somatic cells to clearing them from undifferentiated germ cells, and that ZIF-1 activity can be harnessed as a new genetic tool to study the early germline.

Persistent cell contacts enable E-cadherin/HMR-1- and PAR-3-based symmetry breaking within a developing C. elegans epithelium

Tissue-wide patterning is essential to multicellular development, requiring cells to individually generate polarity axes and coordinate them in space and time with neighbors. Using the C. elegans intestinal epithelium, we identified a patterning mechanism that is informed by cell contact lifetime asymmetry and executed via the scaffolding protein PAR-3 and the transmembrane protein E-cadherin/HMR-1. Intestinal cells break symmetry as PAR-3 and HMR-1 recruit apical determinants into punctate "local polarity complexes" (LPCs) at homotypic contacts. LPCs undergo an HMR-1-based migration to a common midline, thereby establishing tissue-wide polarity. Thus, symmetry breaking results from PAR-3-dependent intracellular polarization coupled to HMR-1-based tissue-level communication, which occurs through a non-adhesive signaling role for HMR-1. Differential lifetimes between homotypic and heterotypic cell contacts are created by neighbor exchanges and oriented divisions, patterning where LPCs perdure and thereby breaking symmetry. These cues offer a logical and likely conserved framework for how epithelia without obvious molecular asymmetries can polarize.

Multifaceted modes of γ-tubulin complex recruitment and microtubule nucleation at mitotic centrosomes

Microtubule nucleation is mediated by γ-tubulin ring complexes (γ-TuRCs). In most eukaryotes, a GCP4/5/4/6 "core" complex promotes γ-tubulin small complex (γ-TuSC) association to generate cytosolic γ-TuRCs. Unlike γ-TuSCs, however, this core complex is non-essential in various species and absent from budding yeasts. In Drosophila, Spindle defective-2 (Spd-2) and Centrosomin (Cnn) redundantly recruit γ-tubulin complexes to mitotic centrosomes. Here, we show that Spd-2 recruits γ-TuRCs formed via the GCP4/5/4/6 core, but Cnn can recruit γ-TuSCs directly via its well-conserved CM1 domain, similar to its homologs in budding yeast. When centrosomes fail to recruit γ-tubulin complexes, they still nucleate microtubules via the TOG domain protein Mini-spindles (Msps), but these microtubules have different dynamic properties. Our data, therefore, help explain the dispensability of the GCP4/5/4/6 core and highlight the robustness of centrosomes as microtubule organizing centers. They also suggest that the dynamic properties of microtubules are influenced by how they are nucleated.

Mapping of centriolar proteins onto the post-embryonic lineage of C. elegans

Centrioles, together with the surrounding peri-centriolar material (PCM), constitute the centrosome, a major microtubule-organizing center of animal cells. Despite being critical in many cells for signaling, motility and division, centrioles can be eliminated in some systems, including in the vast majority of differentiating cells during embryogenesis in Caenorhabditis elegans. Whether the cells retaining centrioles in the resulting L1 larvae do so because they lack an activity that eliminates centrioles in the other cells is not known. Moreover, the extent to which centrioles and PCM remain present in later stages of worm development, when all cells but those of the germ line are terminally differentiated, is not known. Here, by fusing cells that lack centrioles with cells that retain them, we established that L1 larvae do not possess a diffusible elimination activity sufficient to remove centrioles. Moreover, analyzing PCM core proteins in L1 larval cells that retain centrioles, we found that some such proteins, but not all, are present as well. Furthermore, we uncovered that foci of centriolar proteins remain present in specific terminally differentiated cells of adult hermaphrodites and males, in particular in the somatic gonad. Correlating the time at which cells were born with the fate of their centrioles revealed that it is not cell age, but instead cell fate, that determines whether and when centrioles are eliminated. Overall, our work maps the localization of centriolar and PCM core proteins in the post-embryonic C. elegans lineage, thereby providing an essential blueprint for uncovering mechanisms modulating their presence and function.

ZIF-1-mediated degradation of endogenous and heterologous zinc finger proteins in the C. elegans germ line

Rapid and conditional protein depletion is the gold standard genetic tool for deciphering the molecular basis of developmental processes. Previously, we showed that by conditionally expressing the E3 ligase substrate adaptor ZIF-1 in Caenorhabditis elegans somatic cells, proteins tagged with the first CCCH Zn finger (ZF1) domain from the germline regulator PIE-1 degrade rapidly, resulting in loss-of-function phenotypes. The described role of ZIF-1 is to clear PIE-1 and several other CCCH Zn finger proteins from early somatic cells, helping to enrich them in germline precursor cells. Here, we show that proteins tagged with the PIE-1 ZF1 domain are subsequently cleared from primordial germ cells in embryos and from undifferentiated germ cells in larvae and adults by ZIF-1. We harness germline ZIF-1 activity to degrade a ZF1-tagged heterologous protein from PGCs and show that its depletion produces phenotypes equivalent to those of a null mutation. Our findings reveal that ZIF-1 switches roles from degrading CCCH Zn finger proteins in somatic cells to clearing them from undifferentiated germ cells, and that ZIF-1 activity can be harnessed as a new genetic tool to study the early germ line.

A germline-specific role for unconventional components of the γ-tubulin complex in Caenorhabditis elegans

The γ-tubulin complex (γTuC) is a widely conserved microtubule nucleator, but some of its components, namely GCP4, GCP5 and GCP6 (also known as TUBGCP4, TUBGCP5 and TUBGCP6, respectively), have not been detected in Caenorhabditis elegans. Here, we identified two γTuC-associated proteins in C. elegans, GTAP-1 and GTAP-2, for which apparent orthologs were detected only in the genus Caenorhabditis. GTAP-1 and GTAP-2 were found to localize at centrosomes and the plasma membrane of the germline, and their centrosomal localization was interdependent. In early C. elegans embryos, whereas the conserved γTuC component MZT-1 (also known as MOZART1 and MZT1) was essential for the localization of centrosomal γ-tubulin, depletion of GTAP-1 and/or GTAP-2 caused up to 50% reduction of centrosomal γ-tubulin and precocious disassembly of spindle poles during mitotic telophase. In the adult germline, GTAP-1 and GTAP-2 contributed to efficient recruitment of the γTuC to the plasma membrane. Depletion of GTAP-1, but not of GTAP-2, severely disrupted both the microtubule array and the honeycomb-like structure of the adult germline. We propose that GTAP-1 and GTAP-2 are unconventional components of the γTuC that contribute to the organization of both centrosomal and non-centrosomal microtubules by targeting the γTuC to specific subcellular sites in a tissue-specific manner.

The Caenorhabditis elegans anchor cell transcriptome: ribosome biogenesis drives cell invasion through basement membrane

Cell invasion through basement membrane (BM) barriers is important in development, immune function and cancer progression. As invasion through BM is often stochastic, capturing gene expression profiles of actively invading cells in vivo remains elusive. Using the stereotyped timing of Caenorhabditis elegans anchor cell (AC) invasion, we generated an AC transcriptome during BM breaching. Through a focused RNAi screen of transcriptionally enriched genes, we identified new invasion regulators, including translationally controlled tumor protein (TCTP). We also discovered gene enrichment of ribosomal proteins. AC-specific RNAi, endogenous ribosome labeling and ribosome biogenesis analysis revealed that a burst of ribosome production occurs shortly after AC specification, which drives the translation of proteins mediating BM removal. Ribosomes also enrich near the AC endoplasmic reticulum (ER) Sec61 translocon and the endomembrane system expands before invasion. We show that AC invasion is sensitive to ER stress, indicating a heightened requirement for translation of ER-trafficked proteins. These studies reveal key roles for ribosome biogenesis and endomembrane expansion in cell invasion through BM and establish the AC transcriptome as a resource to identify mechanisms underlying BM transmigration.

CAMSAP2 localizes to the Golgi in islet β-cells and facilitates Golgi-ER trafficking

Glucose stimulation induces the remodeling of microtubules, which potentiates insulin secretion in pancreatic β-cells. CAMSAP2 binds to microtubule minus ends to stabilize microtubules in several cultured clonal cells. Here, we report that the knockdown of CAMSAP2 in primary β-cells reduces total insulin content and attenuates GSIS without affecting the releasability of insulin vesicles. Surprisingly, CAMSAP2 knockdown does not change microtubule stability. Unlike in cultured insulinoma cells, CAMSAP2 in primary β-cells predominantly localizes to the Golgi apparatus instead of microtubule minus ends. This novel localization is specific to primary β- but not α-cells and is independent of microtubule binding. Consistent with its specific localization at the Golgi, CAMSAP2 promotes efficient Golgi-ER trafficking in primary β-cells. Moreover, primary β-cells and insulinoma cells likely express different CAMSAP2 isoforms. We propose that a novel CAMSAP2 isoform in primary β-cells has a non-canonical function, which promotes Golgi-ER trafficking to support efficient production of insulin and secretion.

Context matters: Lessons in epithelial polarity from the Caenorhabditis elegans intestine and other tissues

Epithelia are tissues with diverse morphologies and functions across metazoans, ranging from vast cell sheets encasing internal organs to internal tubes facilitating nutrient uptake, all of which require establishment of apical-basolateral polarity axes. While all epithelia tend to polarize the same components, how these components are deployed to drive polarization is largely context-dependent and likely shaped by tissue-specific differences in development and ultimate functions of polarizing primordia. The nematode Caenorhabditis elegans (C. elegans) offers exceptional imaging and genetic tools and possesses unique epithelia with well-described origins and roles, making it an excellent model to investigate polarity mechanisms. In this review, we highlight the interplay between epithelial polarization, development, and function by describing symmetry breaking and polarity establishment in a particularly well-characterized epithelium, the C. elegans intestine. We compare intestinal polarization to polarity programs in two other C. elegans epithelia, the pharynx and epidermis, correlating divergent mechanisms with tissue-specific differences in geometry, embryonic environment, and function. Together, we emphasize the importance of investigating polarization mechanisms against the backdrop of tissue-specific contexts, while also underscoring the benefits of cross-tissue comparisons of polarity.

Separable mechanisms drive local and global polarity establishment in the Caenorhabditiselegans intestinal epithelium

Apico-basolateral polarization is essential for epithelial cells to function as selective barriers and transporters, and to provide mechanical resilience to organs. Epithelial polarity is established locally, within individual cells to establish distinct apical, junctional and basolateral domains, and globally, within a tissue where cells coordinately orient their apico-basolateral axes. Using live imaging of endogenously tagged proteins and tissue-specific protein depletion in the Caenorhabditiselegans embryonic intestine, we found that local and global polarity establishment are temporally and genetically separable. Local polarity is initiated prior to global polarity and is robust to perturbation. PAR-3 is required for global polarization across the intestine but local polarity can arise in its absence, as small groups of cells eventually established polarized domains in PAR-3-depleted intestines in a HMR-1 (E-cadherin)-dependent manner. Despite the role of PAR-3 in localizing PKC-3 to the apical surface, we additionally found that PAR-3 and PKC-3/aPKC have distinct roles in the establishment and maintenance of local and global polarity. Taken together, our results indicate that different mechanisms are required for local and global polarity establishment in vivo.

PTRN-1 (CAMSAP) and NOCA-2 (NINEIN) are required for microtubule polarity in Caenorhabditis elegans dendrites

The neuronal microtubule cytoskeleton is key to establish axon-dendrite polarity. Dendrites are characterized by the presence of minus-end out microtubules. However, the mechanisms that organize these microtubules with the correct orientation are still poorly understood. Using Caenorhabditis elegans as a model system for microtubule organization, we characterized the role of 2 microtubule minus-end related proteins in this process, the microtubule minus-end stabilizing protein calmodulin-regulated spectrin-associated protein (CAMSAP/PTRN-1), and the NINEIN homologue, NOCA-2 (noncentrosomal microtubule array). We found that CAMSAP and NINEIN function in parallel to mediate microtubule organization in dendrites. During dendrite outgrowth, RAB-11-positive vesicles localized to the dendrite tip to nucleate microtubules and function as a microtubule organizing center (MTOC). In the absence of either CAMSAP or NINEIN, we observed a low penetrance MTOC vesicles mislocalization to the cell body, and a nearly fully penetrant phenotype in double mutant animals. This suggests that both proteins are important for localizing the MTOC vesicles to the growing dendrite tip to organize microtubules minus-end out. Whereas NINEIN localizes to the MTOC vesicles where it is important for the recruitment of the microtubule nucleator γ-tubulin, CAMSAP localizes around the MTOC vesicles and is cotranslocated forward with the MTOC vesicles upon dendritic growth. Together, these results indicate that microtubule nucleation from the MTOC vesicles and microtubule stabilization are both important to localize the MTOC vesicles distally to organize dendritic microtubules minus-end out.

Molecular architecture of the C. elegans centriole

Uncovering organizing principles of organelle assembly is a fundamental pursuit in the life sciences. Caenorhabditis elegans was key in identifying evolutionary conserved components governing assembly of the centriole organelle. However, localizing these components with high precision has been hampered by the minute size of the worm centriole, thus impeding understanding of underlying assembly mechanisms. Here, we used Ultrastructure Expansion coupled with STimulated Emission Depletion (U-Ex-STED) microscopy, as well as electron microscopy (EM) and electron tomography (ET), to decipher the molecular architecture of the worm centriole. Achieving an effective lateral resolution of approximately 14 nm, we localize centriolar and PeriCentriolar Material (PCM) components in a comprehensive manner with utmost spatial precision. We found that all 12 components analysed exhibit a ring-like distribution with distinct diameters and often with a 9-fold radial symmetry. Moreover, we uncovered that the procentriole assembles at a location on the centriole margin where SPD-2 and ZYG-1 also accumulate. Moreover, SAS-6 and SAS-5 were found to be present in the nascent procentriole, with SAS-4 and microtubules recruited thereafter. We registered U-Ex-STED and EM data using the radial array of microtubules, thus allowing us to map each centriolar and PCM protein to a specific ultrastructural compartment. Importantly, we discovered that SAS-6 and SAS-4 exhibit a radial symmetry that is offset relative to microtubules, leading to a chiral centriole ensemble. Furthermore, we established that the centriole is surrounded by a region from which ribosomes are excluded and to which SAS-7 localizes. Overall, our work uncovers the molecular architecture of the C. elegans centriole in unprecedented detail and establishes a comprehensive framework for understanding mechanisms of organelle biogenesis and function.

Mechanisms of microtubule organization in differentiated animal cells

Microtubules are polarized cytoskeletal filaments that serve as tracks for intracellular transport and form a scaffold that positions organelles and other cellular components and modulates cell shape and mechanics. In animal cells, the geometry, density and directionality of microtubule networks are major determinants of cellular architecture, polarity and proliferation. In dividing cells, microtubules form bipolar spindles that pull chromosomes apart, whereas in interphase cells, microtubules are organized in a cell type-specific fashion, which strongly correlates with cell physiology. In motile cells, such as fibroblasts and immune cells, microtubules are organized as radial asters, whereas in immotile epithelial and neuronal cells and in muscles, microtubules form parallel or antiparallel arrays and cortical meshworks. Here, we review recent work addressing how the formation of such microtubule networks is driven by the plethora of microtubule regulatory proteins. These include proteins that nucleate or anchor microtubule ends at different cellular structures and those that sever or move microtubules, as well as regulators of microtubule elongation, stability, bundling or modifications. The emerging picture, although still very incomplete, shows a remarkable diversity of cell-specific mechanisms that employ conserved building blocks to adjust microtubule organization in order to facilitate different cellular functions.

Microtubule Anchoring: Attaching Dynamic Polymers to Cellular Structures

Microtubules are dynamic, filamentous polymers composed of α- and β-tubulin. Arrays of microtubules that have a specific polarity and distribution mediate essential processes such as intracellular transport and mitotic chromosome segregation. Microtubule arrays are generated with the help of microtubule organizing centers (MTOC). MTOCs typically combine two principal activities, the de novo formation of microtubules, termed nucleation, and the immobilization of one of the two ends of microtubules, termed anchoring. Nucleation is mediated by the γ-tubulin ring complex (γTuRC), which, in cooperation with its recruitment and activation factors, provides a template for α- and β-tubulin assembly, facilitating formation of microtubule polymer. In contrast, the molecules and mechanisms that anchor newly formed microtubules at MTOCs are less well characterized. Here we discuss the mechanistic challenges underlying microtubule anchoring, how this is linked with the molecular activities of known and proposed anchoring factors, and what consequences defective microtubule anchoring has at the cellular and organismal level.

Microtubule-associated proteins promote microtubule generation in the absence of γ-tubulin in human colon cancer cells

The γ-tubulin complex acts as the predominant microtubule (MT) nucleator that initiates MT formation and is therefore an essential factor for cell proliferation. Nonetheless, cellular MTs are formed after experimental depletion of the γ-tubulin complex, suggesting that cells possess other factors that drive MT nucleation. Here, by combining gene knockout, auxin-inducible degron, RNA interference, MT depolymerization/regrowth assay, and live microscopy, we identified four microtubule-associated proteins (MAPs), ch-TOG, CLASP1, CAMSAPs, and TPX2, which are involved in γ-tubulin-independent MT generation in human colon cancer cells. In the mitotic MT regrowth assay, nucleated MTs organized noncentriolar MT organizing centers (ncMTOCs) in the absence of γ-tubulin. Depletion of CLASP1 or TPX2 substantially delayed ncMTOC formation, suggesting that these proteins might promote MT nucleation in the absence of γ-tubulin. In contrast, depletion of ch-TOG or CAMSAPs did not affect the timing of ncMTOC appearance. CLASP1 also accelerates γ-tubulin-independent MT regrowth during interphase. Thus, MT generation can be promoted by MAPs without the γ-tubulin template.

Proximity labeling reveals non-centrosomal microtubule-organizing center components required for microtubule growth and localization

Microtubules are polarized intracellular polymers that play key roles in the cell, including in transport, polarity, and cell division. Across eukaryotic cell types, microtubules adopt diverse intracellular organization to accommodate these distinct functions coordinated by specific cellular sites called microtubule-organizing centers (MTOCs). Over 50 years of research on MTOC biology has focused mainly on the centrosome; however, most differentiated cells employ non-centrosomal MTOCs (ncMTOCs) to organize their microtubules into diverse arrays, which are critical to cell function. To identify essential ncMTOC components, we developed the biotin ligase-based, proximity-labeling approach TurboID for use in C. elegans. We identified proteins proximal to the microtubule minus end protein PTRN-1/Patronin at the apical ncMTOC of intestinal epithelial cells, focusing on two conserved proteins: spectraplakin protein VAB-10B/MACF1 and WDR-62, a protein we identify as homologous to vertebrate primary microcephaly disease protein WDR62. VAB-10B and WDR-62 do not associate with the centrosome and instead specifically regulate non-centrosomal microtubules and the apical targeting of microtubule minus-end proteins. Depletion of VAB-10B resulted in microtubule mislocalization and delayed localization of a microtubule nucleation complex ɣ-tubulin ring complex (γ-TuRC), while loss of WDR-62 decreased the number of dynamic microtubules and abolished γ-TuRC localization. This regulation occurs downstream of cell polarity and in conjunction with actin. As this is the first report for non-centrosomal roles of WDR62 family proteins, we expand the basic cell biological roles of this important disease protein. Our studies identify essential ncMTOC components and suggest a division of labor where microtubule growth and localization are distinctly regulated.

Inherited apicobasal polarity defines the key features of axon-dendrite polarity in a sensory neuron

Neurons are highly polarized cells with morphologically and functionally distinct dendritic and axonal processes. The molecular mechanisms that establish axon-dendrite polarity in vivo are poorly understood. Here, we describe the initial polarization of posterior deirid (PDE), a ciliated mechanosensory neuron, during development in vivo through 4D live imaging with endogenously tagged proteins. PDE inherits and maintains apicobasal polarity from its epithelial precursor. Its apical domain is directly transformed into the ciliated dendritic tip through apical constriction, which is followed by axonal outgrowth from the opposite basal side of the cell. The apical Par complex and junctional proteins persistently localize at the developing dendritic domain throughout this transition. Consistent with their instructive role in axon-dendrite polarization, conditional depletion of the Par complex and junctional proteins results in robust defects in dendrite and axon formation. During apical constriction, a microtubule-organizing center (MTOC) containing the microtubule nucleator γ-tubulin ring complex (γ-TuRC) forms along the apical junction between PDE and its sister cell in a manner dependent on the Par complex and junctional proteins. This junctional MTOC patterns neuronal microtubule polarity and facilitate the dynein-dependent recruitment of the basal body for ciliogenesis. When non-ciliated neurons are genetically manipulated to obtain ciliated neuronal fate, inherited apicobasal polarity is required for generating ciliated dendritic tips. We propose that inherited apicobasal polarity, together with apical cell-cell interactions drive the morphological and cytoskeletal polarity in early neuronal differentiation.

Apical PAR complex proteins protect against programmed epithelial assaults to create a continuous and functional intestinal lumen

Sustained polarity and adhesion of epithelial cells is essential for the protection of our organs and bodies, and this epithelial integrity emerges during organ development amidst numerous programmed morphogenetic assaults. Using the developing Caenorhabditis elegans intestine as an in vivo model, we investigated how epithelia maintain their integrity through cell division and elongation to build a functional tube. Live imaging revealed that apical PAR complex proteins PAR-6/Par6 and PKC-3/aPkc remained apical during mitosis while apical microtubules and microtubule-organizing center (MTOC) proteins were transiently removed. Intestine-specific depletion of PAR-6, PKC-3, and the aPkc regulator CDC-42/Cdc42 caused persistent gaps in the apical MTOC as well as in other apical and junctional proteins after cell division and in non-dividing cells that elongated. Upon hatching, gaps coincided with luminal constrictions that blocked food, and larvae arrested and died. Thus, the apical PAR complex maintains apical and junctional continuity to construct a functional intestinal tube.

An acentriolar centrosome at the C. elegans ciliary base

In animal cells, the functions of the microtubule cytoskeleton are coordinated by centriole-based centrosomes via γ-tubulin complexes embedded in the pericentriolar material or PCM.¹ PCM assembly has been best studied in the context of mitosis, where centriolar SPD-2 recruits PLK-1, which in turn phosphorylates key scaffolding components like SPD-5 and CNN to promote expansion of the PCM polymer.^2-4 To what extent these mechanisms apply to centrosomes in interphase or in differentiated cells remains unclear.⁵ Here, we examine a novel type of centrosome found at the ciliary base of C. elegans sensory neurons, which we show plays important roles in neuronal morphogenesis, cellular trafficking, and ciliogenesis. These centrosomes display similar dynamic behavior to canonical, mitotic centrosomes, with a stable PCM scaffold and dynamically localized client proteins. Unusually, however, they are not organized by centrioles, which degenerate early in terminal differentiation.⁶ Yet, PCM not only persists but continues to grow with key scaffolding proteins including SPD-5 expressed under control of the RFX transcription factor DAF-19. This assembly occurs in the absence of the mitotic regulators SPD-2, AIR-1 and PLK-1, but requires tethering by PCMD-1, a protein which also plays a role in the initial, interphase recruitment of PCM in early embryos.⁷ These results argue for distinct mechanisms for mitotic and non-mitotic PCM assembly, with only the former requiring PLK-1 phosphorylation to drive rapid expansion of the scaffold polymer.

MZT Proteins Form Multi-Faceted Structural Modules in the γ-Tubulin Ring Complex

Microtubule organization depends on the γ-Tubulin ring complex (γ-TuRC), a ~2.3-MDa nucleation factor comprising an asymmetric assembly of γ-Tubulin and GCP2-GCP6. However, it is currently unclear how the γ-TuRC-associated microproteins MZT1 and MZT2 contribute to the structure and regulation of the holocomplex. Here, we report cryo-EM structures of MZT1 and MZT2 in the context of the native human γ-TuRC. MZT1 forms two subcomplexes with the N-terminal α-helical domains of GCP3 or GCP6 (GCP-NHDs) within the γ-TuRC “lumenal bridge.” We determine the X-ray structure of recombinant MZT1/GCP6-NHD and find it is similar to that within the native γ-TuRC. We identify two additional MZT/GCP-NHD-like subcomplexes, one of which is located on the outer face of the γ-TuRC and comprises MZT2 and GCP2-NHD in complex with a centrosomin motif 1 (CM1)-containing peptide. Our data reveal how MZT1 and MZT2 establish multi-faceted, structurally mimetic “modules” that can expand structural and regulatory interfaces in the γ-TuRC.

Wieczorek et al. show how the microproteins MZT1 and MZT2 expand binding interfaces across the γ-TuRC—the cell’s microtubule nucleating machinery—by forming similarly shaped, “modular” subcomplexes with the α-helical N-terminal domains of different γ-Tubulin complex proteins (GCPs).

Centriole-less pericentriolar material serves as a microtubule organizing center at the base of C. elegans sensory cilia

During mitosis in animal cells, the centrosome acts as a microtubule organizing center (MTOC) to assemble the mitotic spindle. MTOC function at the centrosome is driven by proteins within the pericentriolar material (PCM), however the molecular complexity of the PCM makes it difficult to differentiate the proteins required for MTOC activity from other centrosomal functions. We used the natural spatial separation of PCM proteins during mitotic exit to identify a minimal module of proteins required for centrosomal MTOC function in C. elegans. Using tissue-specific degradation, we show that SPD-5, the functional homolog of CDK5RAP2, is essential for embryonic mitosis, while SPD-2/CEP192 and PCMD-1, which are essential in the one-cell embryo, are dispensable. Surprisingly, although the centriole is known to be degraded in the ciliated sensory neurons in C. elegans,^1-3 we find evidence for "centriole-less PCM" at the base of cilia and use this structure as a minimal testbed to dissect centrosomal MTOC function. Super-resolution imaging revealed that this PCM inserts inside the lumen of the ciliary axoneme and directly nucleates the assembly of dendritic microtubules toward the cell body. Tissue-specific degradation in ciliated sensory neurons revealed a role for SPD-5 and the conserved microtubule nucleator γ-TuRC, but not SPD-2 or PCMD-1, in MTOC function at centriole-less PCM. This MTOC function was in the absence of regulation by mitotic kinases, highlighting the intrinsic ability of these proteins to drive microtubule growth and organization and further supporting a model that SPD-5 is the primary driver of MTOC function at the PCM.

Tuning self-renewal in the Arabidopsis stomatal lineage by hormone and nutrient regulation of asymmetric cell division

Asymmetric and self-renewing divisions build and pattern tissues. In the Arabidopsis stomatal lineage, asymmetric cell divisions, guided by polarly localized cortical proteins, generate most cells on the leaf surface. Systemic and environmental signals modify tissue development, but the mechanisms by which plants incorporate such cues to regulate asymmetric divisions are elusive. In a screen for modulators of cell polarity, we identified CONSTITUTIVE TRIPLE RESPONSE1, a negative regulator of ethylene signaling. We subsequently revealed antagonistic impacts of ethylene and glucose signaling on the self-renewing capacity of stomatal lineage stem cells. Quantitative analysis of cell polarity and fate dynamics showed that developmental information may be encoded in both the spatial and temporal asymmetries of polarity proteins. These results provide a framework for a mechanistic understanding of how nutritional status and environmental factors tune stem-cell behavior in the stomatal lineage, ultimately enabling flexibility in leaf size and cell-type composition.

Epithelial morphogenesis, tubulogenesis and forces in organogenesis

As multi-cellular organisms evolved from small clusters of cells to complex metazoans, biological tubes became essential for life. Tubes are typically thought of as mainly playing a role in transport, with the hollow space (lumen) acting as a conduit to distribute nutrients and waste, or for gas exchange. However, biological tubes also provide a platform for physiological, mechanical, and structural functions. Indeed, tubulogenesis is often a critical aspect of morphogenesis and organogenesis. C. elegans is made up of tubes that provide structural support and protection (the epidermis), perform the mechanical and enzymatic processes of digestion (the buccal cavity, pharynx, intestine, and rectum), transport fluids for osmoregulation (the excretory system), and execute the functions necessary for reproduction (the germline, spermatheca, uterus and vulva). Here we review our current understanding of the genetic regulation, molecular processes, and physical forces involved in tubulogenesis and morphogenesis of the epidermal, digestive and excretory systems in C. elegans.

Methods for Rapid Protein Depletion in C. elegans using Auxin-Inducible Degradation

Numerous methods have been developed in model systems to deplete or inactivate proteins to elucidate their functional roles. In Caenorhabditis elegans, a common method for protein depletion is RNA interference (RNAi), in which mRNA is targeted for degradation. C. elegans is also a powerful genetic organism, amenable to large-scale genetic screens and CRISPR-mediated genome editing. However, these approaches largely lead to constitutive inhibition, which can make it difficult to study proteins essential for development or to dissect dynamic cellular processes. Thus, there have been recent efforts to develop methods to rapidly inactivate or deplete proteins to overcome these barriers. One such method that is proving to be exceptionally powerful is auxin-inducible degradation. In order to apply this approach in C. elegans, a 44-amino acid degron tag is added to the protein of interest, and the Arabidopsis ubiquitin ligase TIR1 is expressed in target tissues. When the plant hormone auxin is added, it mediates an interaction between TIR1 and the degron-tagged protein of interest, which triggers ubiquitination of the protein and its rapid degradation via the proteasome. Here, we have outlined multiple methods for inducing auxin-mediated depletion of target proteins in C. elegans, highlighting the versatility and power of this method. © 2021 Wiley Periodicals LLC. Basic Protocol 1: Long-term auxin-mediated depletion on plates Support Protocol: Preparation of NGM and NGM-auxin plates Basic Protocol 2: Rapid auxin-mediated depletion via soaking Basic Protocol 3: Acute auxin-mediated depletion in isolated embryos Basic Protocol 4: Assessing auxin-mediated depletion.

Polo-like kinase 1 independently controls microtubule-nucleating capacity and size of the centrosome

Centrosomes are composed of a centriolar core surrounded by a pericentriolar material (PCM) matrix that docks microtubule-nucleating γ-tubulin complexes. During mitotic entry, the PCM matrix increases in size and nucleating capacity in a process called centrosome maturation. Polo-like kinase 1 (PLK1) is recruited to centrosomes and phosphorylates PCM matrix proteins to drive their self-assembly, which leads to PCM expansion. Here, we show that in addition to controlling PCM expansion, PLK1 independently controls the generation of binding sites for γ-tubulin complexes on the PCM matrix. Selectively preventing the generation of PLK1-dependent γ-tubulin docking sites led to spindle defects and impaired chromosome segregation without affecting PCM expansion, highlighting the importance of phospho-regulated centrosomal γ-tubulin docking sites in spindle assembly. Inhibiting both γ-tubulin docking and PCM expansion by mutating substrate target sites recapitulated the effects of loss of centrosomal PLK1 on the ability of centrosomes to catalyze spindle assembly.

Visualizing the metazoan proliferation-quiescence decision in vivo

Cell proliferation and quiescence are intimately coordinated during metazoan development. Here, we adapt a cyclin-dependent kinase (CDK) sensor to uncouple these key events of the cell cycle in Caenorhabditis elegans and zebrafish through live-cell imaging. The CDK sensor consists of a fluorescently tagged CDK substrate that steadily translocates from the nucleus to the cytoplasm in response to increasing CDK activity and consequent sensor phosphorylation. We show that the CDK sensor can distinguish cycling cells in G1 from quiescent cells in G0, revealing a possible commitment point and a cryptic stochasticity in an otherwise invariant C. elegans cell lineage. Finally, we derive a predictive model of future proliferation behavior in C. elegans based on a snapshot of CDK activity in newly born cells. Thus, we introduce a live-cell imaging tool to facilitate in vivo studies of cell-cycle control in a wide-range of developmental contexts.

Epidermal PAR-6 and PKC-3 are essential for larval development of C. elegans and organize non-centrosomal microtubules

The cortical polarity regulators PAR-6, PKC-3, and PAR-3 are essential for the polarization of a broad variety of cell types in multicellular animals. In C. elegans, the roles of the PAR proteins in embryonic development have been extensively studied, yet little is known about their functions during larval development. Using inducible protein degradation, we show that PAR-6 and PKC-3, but not PAR-3, are essential for postembryonic development. PAR-6 and PKC-3 are required in the epidermal epithelium for animal growth, molting, and the proper pattern of seam-cell divisions. Finally, we uncovered a novel role for PAR-6 in organizing non-centrosomal microtubule arrays in the epidermis. PAR-6 was required for the localization of the microtubule organizer NOCA-1/Ninein, and defects in a noca-1 mutant are highly similar to those caused by epidermal PAR-6 depletion. As NOCA-1 physically interacts with PAR-6, we propose that PAR-6 promotes non-centrosomal microtubule organization through localization of NOCA-1/Ninein.

An interphase contractile ring reshapes primordial germ cells to allow bulk cytoplasmic remodeling

Some cells discard undesired inherited components in bulk by forming large compartments that are subsequently eliminated. Caenorhabditis elegans primordial germ cells (PGCs) jettison mitochondria and cytoplasm by forming a large lobe that is cannibalized by intestinal cells. Although PGCs are nonmitotic, we find that lobe formation is driven by constriction of a contractile ring and requires the RhoGEF ECT-2, a RhoA activator also essential for cytokinesis. Whereas centralspindlin activates ECT-2 to promote cytokinetic contractile ring formation, we show that the ECT-2 regulator NOP-1, but not centralspindlin, is essential for PGC lobe formation. We propose that lobe contractile ring formation is locally inhibited by the PGC nucleus, which migrates to one side of the cell before the cytokinetic ring assembles on the opposite cortex. Our findings reveal how components of the cytokinetic contractile ring are reemployed during interphase to create compartments used for cellular remodeling, and they reveal differences in the spatial cues that dictate where the contractile ring will form.

Growth cone-localized microtubule organizing center establishes microtubule orientation in dendrites

A polarized arrangement of neuronal microtubule arrays is the foundation of membrane trafficking and subcellular compartmentalization. Conserved among both invertebrates and vertebrates, axons contain exclusively 'plus-end-out' microtubules while dendrites contain a high percentage of 'minus-end-out' microtubules, the origins of which have been a mystery. Here we show that in Caenorhabditis elegans the dendritic growth cone contains a non-centrosomal microtubule organizing center (MTOC), which generates minus-end-out microtubules along outgrowing dendrites and plus-end-out microtubules in the growth cone. RAB-11-positive endosomes accumulate in this region and co-migrate with the microtubule nucleation complex γ-TuRC. The MTOC tracks the extending growth cone by kinesin-1/UNC-116-mediated endosome movements on distal plus-end-out microtubules and dynein clusters this advancing MTOC. Critically, perturbation of the function or localization of the MTOC causes reversed microtubule polarity in dendrites. These findings unveil the endosome-localized dendritic MTOC as a critical organelle for establishing axon-dendrite polarity.

A perinuclear microtubule-organizing centre controls nuclear positioning and basement membrane secretion

Non-centrosomal microtubule-organizing centres (ncMTOCs) have a variety of roles presumed to serve the diverse functions of the range of cell types in which they are found. ncMTOCs are diverse in their composition, subcellular localization, and function. Here we report a perinuclear MTOC in Drosophila fat body cells that is anchored by Msp300/Nesprin at the cytoplasmic surface of the nucleus. Msp300 recruits the MT minus-end protein Patronin/CAMSAP, which functions redundantly with Ninein to further recruit the MT polymerase Msps/XMAP215 to assemble non-centrosomal MTs and does so independently of the widespread MT nucleation factor γ-tubulin. Functionally, the fat body ncMTOC and the radial MT arrays it organizes is essential for nuclear positioning and for secretion of basement membrane components via retrograde dynein-dependent endosomal trafficking that restricts plasma membrane growth. Together, this study identifies a perinuclear ncMTOC with unique architecture and MT regulation properties that serves vital functions.

Microtubule plus-end dynamics link wound repair to the innate immune response

The skin protects animals from infection and physical damage. In Caenorhabditis elegans, wounding the epidermis triggers an immune reaction and a repair response, but it is not clear how these are coordinated. Previous work implicated the microtubule cytoskeleton in the maintenance of epidermal integrity (Chuang et al., 2016). Here, by establishing a simple wounding system, we show that wounding provokes a reorganisation of plasma membrane subdomains. This is followed by recruitment of the microtubule plus end-binding protein EB1/EBP-2 around the wound and actin ring formation, dependent on ARP2/3 branched actin polymerisation. We show that microtubule dynamics are required for the recruitment and closure of the actin ring, and for the trafficking of the key signalling protein SLC6/SNF-12 toward the injury site. Without SNF-12 recruitment, there is an abrogation of the immune response. Our results suggest that microtubule dynamics coordinate the cytoskeletal changes required for wound repair and the concomitant activation of innate immunity.

Auxin-mediated Protein Degradation in Caenorhabditis elegans

The auxin-inducible degron (AID) technology was recently adapted for use in the nematode Caenorhabditis elegans. Rapid degradation of C. elegans proteins tagged with an AID is mediated by a plant-specific F-box protein, transport inhibitor response 1 (TIR1), and occurs only in the presence of the phytohormone auxin. The first iteration of this technology elicited protein degradation in C. elegans through a naturally occurring form of auxin, indole-3-acetic acid (IAA). Here, we present a protocol that uses 1-naphthaleneacetic acid, potassium salt (K-NAA), an indole-free synthetic auxin analog. At equal concentration, K-NAA is as effective as IAA in standard nematode growth media (NGM). K-NAA is also effective in physiological buffer (M9), allowing for high-throughput experimentation. The main advantages of K-NAA are twofold: first, its photostability prevents light-induced compound degradation during storage and the production of toxic indole-derivatives during fluorescence microscopy of live cells; and second, its water solubility eliminates the need of using ethanol to dissolve the auxin compound, a solvent that may confound C. elegans lifespan and behavioral assays. In this protocol, we describe our method of degrading C. elegans proteins using K-NAA on solid and in liquid media, as well as our method of analyzing protein degradation.

Generation and regulation of microtubule network asymmetry to drive cell polarity

Microtubules control cell architecture by serving as a scaffold for intracellular transport, signaling, and organelle positioning. Microtubules are intrinsically polarized, and their orientation, density, and post-translational modifications both respond and contribute to cell polarity. Animal cells that can rapidly reorient their polarity axis, such as fibroblasts, immune cells, and cancer cells, contain radially organized microtubule arrays anchored at the centrosome and the Golgi apparatus, whereas stably polarized cells often acquire non-centrosomal microtubule networks attached to the cell cortex, nucleus, or other structures. Microtubule density, longevity, and post-translational modifications strongly depend on the dynamics of their plus ends. Factors controlling microtubule plus-end dynamics are often part of cortical assemblies that integrate cytoskeletal organization, cell adhesion, and secretion and are subject to microtubule-dependent feedback regulation. Finally, microtubules can mechanically contribute to cell asymmetry by promoting cell elongation, a property that might be important for cells with dense microtubule arrays growing in soft environments.