Intracellular lumen extension requires ERM-1-dependent apical membrane expansion and AQP-8-mediated flux

Glucose stockpile in the intestinal apical brush border in C. elegans

Revealing the mechanisms of glucose transport is crucial for studying pathological diseases caused by glucose toxicities. Numerous studies have revealed molecular functions involved in glucose transport in the nematode Caenorhabditis elegans, a commonly used model organism. However, the behavior of glucose in the intestinal lumen-to-cell remains elusive. To address that, we evaluated the diffusion coefficient of glucose in the intestinal apical brush border of C. elegans by using fluorescent glucose and fluorescence recovery after photobleaching. Fluorescent glucose taken in the intestine of worms accumulates in the apical brush border, and its diffusion coefficient of ∼10-8 cm2/s is two orders of magnitude slower than that in bulk. This result indicates that the intestinal brush border is a viscous layer. ERM-1 point mutations at the phosphorylation site, which shorten the microvilli length, did not significantly affect the diffusion coefficient of fluorescent glucose in the brush border. Our findings imply that glucose enrichment is dominantly maintained by the viscous layer composed of the glycocalyx and molecular complexes on the apical surface.

Chloride intracellular channel (CLIC) proteins function as fusogens

Chloride Intracellular Channel (CLIC) family members uniquely transition between soluble and membrane-associated conformations. Despite decades of extensive functional and structural studies, CLICs' function as ion channels remains debated, rendering our understanding of their physiological role incomplete. Here, we expose the function of CLIC5 as a fusogen. We demonstrate that purified CLIC5 directly interacts with the membrane and induces fusion, as reflected by increased liposomal diameter and lipid and content mixing between liposomes. Moreover, we show that this activity is facilitated by acidic pH, a known trigger for CLICs' transition to a membrane-associated conformation, and that increased exposure of the hydrophobic inter-domain interface is crucial for this process. Finally, mutation of a conserved hydrophobic interfacial residue diminishes the fusogenic activity of CLIC5 in vitro and impairs excretory canal extension in C. elegans in vivo. Together, our results unravel the long-sought physiological role of these enigmatic proteins.

Pulsed stimulated Brillouin microscopy enables high-sensitivity mechanical imaging of live and fragile biological specimens

Brillouin microscopy is an emerging optical elastography technique capable of assessing mechanical properties of biological samples in a three-dimensional, all-optical and noncontact fashion. The typically weak Brillouin scattering signal can be substantially enhanced via a stimulated Brillouin scattering (SBS) process; however, current implementations require high pump powers, which prohibit applications to photosensitive or live imaging of biological samples. Here we present a pulsed SBS scheme that takes advantage of the nonlinearity of the pump-probe interaction. In particular, we show that the required pump laser power can be decreased ~20-fold without affecting the signal levels or spectral precision. We demonstrate the low phototoxicity and high specificity of our pulsed SBS approach by imaging, with subcellular detail, sensitive single cells, zebrafish larvae, mouse embryos and adult Caenorhabditis elegans. Furthermore, our method permits observing the mechanics of organoids and C. elegans embryos over time, opening up further possibilities for the field of mechanobiology.

The biosynthetic-secretory pathway, supplemented by recycling routes, determines epithelial membrane polarity

In prevailing epithelial polarity models, membrane-based polarity cues (e.g., the partitioning-defective PARs) position apicobasal cellular membrane domains. Intracellular vesicular trafficking expands these domains by sorting polarized cargo toward them. How the polarity cues themselves are polarized in epithelia and how sorting confers long-range apicobasal directionality to vesicles is still unclear. Here, a systems-based approach using two-tiered C. elegans genomics-genetics screens identifies trafficking molecules that are not implicated in apical sorting yet polarize apical membrane and PAR complex components. Live tracking of polarized membrane biogenesis indicates that the biosynthetic-secretory pathway, linked to recycling routes, is asymmetrically oriented toward the apical domain during this domain's biosynthesis, and that this directionality is regulated upstream of PARs and independent of polarized target membrane domains. This alternative mode of membrane polarization could offer solutions to open questions in current models of epithelial polarity and polarized trafficking.

Branched-chain actin dynamics polarizes vesicle trajectories and partitions apicobasal epithelial membrane domains

In prevailing epithelial polarity models, membrane- and junction-based polarity cues such as the partitioning-defective PARs specify the positions of apicobasal membrane domains. Recent findings indicate, however, that intracellular vesicular trafficking can determine the position of the apical domain, upstream of membrane-based polarity cues. These findings raise the question of how vesicular trafficking becomes polarized independent of apicobasal target membrane domains. Here, we show that the apical directionality of vesicle trajectories depends on actin dynamics during de novo polarized membrane biogenesis in the C. elegans intestine. We find that actin, powered by branched-chain actin modulators, determines the polarized distribution of apical membrane components, PARs, and itself. Using photomodulation, we demonstrate that F-actin travels through the cytoplasm and along the cortex toward the future apical domain. Our findings support an alternative polarity model where actin-directed trafficking asymmetrically inserts the nascent apical domain into the growing epithelial membrane to partition apicobasal membrane domains.

Methylthiosulfonate-Based Cysteine Modifiers as Alternative Inhibitors of Mercurial-Sensitive Aquaporins

(1) Background: Several members of the ubiquitous aquaporin family, AQP, of water and neutral solute channels carry a cysteine residue in the selectivity filter region. Traditionally, toxic mercury-containing compounds are used to bind to the cysteine as covalent AQP inhibitors for physiological studies or analysis of structure-function relationships. (2) Methods: We tested thiol-reactive methylthiosulfonate reagents, MTS, as alternative Cys modifiers for AQP inhibition. Three MTS reagents transferring S-alkyl moieties of increasing size, i.e., S-methyl, S-n-propyl, and S-benzyl, were used with yeast-expressed water-selective AQP1 and the aquaglyceroporin AQP9. Respective Cys-to-Ala variants and mouse erythrocytes that naturally express AQP1 and AQP9 served as controls. (3) Results: Both wildtype AQP isoforms were inhibited by the Cys modifiers in a size-dependent manner, whereas the Cys-to-Ala-variants exhibited resistance. Sub-millimolar concentrations and incubation times in the minute range were sufficient. The modifications were reversible by treatment with the thiol reagents acetylcysteine, ACC, and dithiothreitol, DTT. (4) Conclusions: MTS reagents represent a valid alternative of low toxicity for the inhibition of mercurial-sensitive AQPs.

Rab11 is essential to pancreas morphogenesis, lumen formation and endocrine mass

The molecular links between tissue-level morphogenesis and the differentiation of cell lineages in the pancreas remain elusive despite a decade of studies. We previously showed that in pancreas both processes depend on proper lumenogenesis. The Rab GTPase Rab11 is essential for epithelial lumen formation in vitro, however few studies have addressed its functions in vivo and none have tested its requirement in pancreas. Here, we show that Rab11 is critical for proper pancreas development. Co-deletion of the Rab11 isoforms Rab11A and Rab11B in the developing pancreatic epithelium (Rab11pancDKO) results in ∼50% neonatal lethality and surviving adult Rab11^pancDKO mice exhibit defective endocrine function. Loss of both Rab11A and Rab11B in the embryonic pancreas results in morphogenetic defects of the epithelium, including defective lumen formation and lumen interconnection. In contrast to wildtype cells, Rab11pancDKO cells initiate the formation of multiple ectopic lumens, resulting in a failure to coordinate a single apical membrane initiation site (AMIS) between groups of cells. This results in a failure to form ducts with continuous lumens. Here, we show that these defects are due to failures in vesicle trafficking, as apical and junctional components remain trapped within Rab11^pancDKO cells. Together, these observations suggest that Rab11 directly regulates epithelial lumen formation and morphogenesis. Our report links intracellular trafficking to organ morphogenesis in vivo and presents a novel framework for decoding pancreatic development.

Shaping subcellular tubes through vesicle trafficking: Common and distinct pathways

Cells with subcellular lumens form some of the most miniature tubes in the tubular organs of animals. These are often crucial components of the system, executing functions at remote body locations. Unlike tubes formed by intercellular or autocellular junctions, the cells with junctionless subcellular lumens face unique challenges in modifying the cell shape and plasma membrane organization to incorporate a membrane-bound tube within, often associated with dramatic cellular growth and extensions. Results in the recent years have shown that membrane dynamics, including both the primary delivery and recycling, is crucial in providing the cell with the flexibility to face these challenges. A significant portion of this information has come from two in vivo invertebrate models; the Drosophila tracheal terminal cells and the C. elegans excretory cell. This review focuses on the data obtained from these systems in the recent past about how trafficking pathways influence subcellular tube and branching morphogenesis. Given that such tubes occur in vertebrate vasculature, these insights are relevant to human health, and we contrast our conclusions with the less understood subcellular tubes of angiogenesis.

Nuclear Factor of Activated T Cells-5 Regulates Notochord Lumenogenesis in Chordate Larval Development

Osmoregulation is essential for organisms to adapt to the exterior environment and plays an important role in embryonic organogenesis. Tubular organ formation usually involves a hyperosmotic lumen environment. The mechanisms of how the cells respond and regulate lumen formation remain largely unknown. Here, we reported that the nuclear factor of activated T cells-5 (NFAT5), the only transcription factor in the NFAT family involved in the cellular responses to hypertonic stress, regulated notochord lumen formation in chordate Ciona. Ciona NFAT5 (Ci-NFAT5) was expressed in notochord, and its expression level increased during notochord lumen formation and expansion. Knockout and expression of the dominant negative of NFAT5 in Ciona embryos resulted in the failure of notochord lumen expansion. We further demonstrated that the Ci-NFAT5 transferred from the cytoplasm into nuclei in HeLa cells under the hyperosmotic medium, indicating Ci-NFAT5 can respond the hypertonicity. To reveal the underly mechanisms, we predicted potential downstream genes of Ci-NFAT5 and further validated Ci-NFAT5-interacted genes by the luciferase assay. The results showed that Ci-NFAT5 promoted SLC26A6 expression. Furthermore, expression of a transport inactivity mutant of SLC26A6 (L421P) in notochord led to the failure of lumen expansion, phenocopying that of Ci-NFAT5 knockout. These results suggest that Ci-NFAT5 regulates notochord lumen expansion via the SLC26A6 axis. Taken together, our results reveal that the chordate NFAT5 responds to hypertonic stress and regulates lumen osmotic pressure via an ion channel pathway on luminal organ formation.

Morphogenetic Roles of Hydrostatic Pressure in Animal Development

During organismal development, organs and systems are built following a genetic blueprint that produces structures capable of performing specific physiological functions. Interestingly, we have learned that the physiological activities of developing tissues also contribute to their own morphogenesis. Specifically, physiological activities such as fluid secretion and cell contractility generate hydrostatic pressure that can act as a morphogenetic force. Here, we first review the role of hydrostatic pressure in tube formation during animal development and discuss mathematical models of lumen formation. We then illustrate specific roles of the notochord as a hydrostatic scaffold in anterior-posterior axis development in chordates. Finally, we cover some examples of how fluid flows influence morphogenetic processes in other developmental contexts. Understanding how fluid forces act during development will be key for uncovering the self-organizing principles that control morphogenesis.

Regulators of the secretory pathway have distinct inputs into single-celled branching morphogenesis and seamless tube formation in the Drosophila trachea

Biological tubes serve as conduits through which gas, nutrients and other important fluids are delivered to tissues. Most biological tubes consist of multiple cells connected by epithelial junctions. Unlike these multicellular tubes, seamless tubes are unicellular and lack junctions. Seamless tubes are present in various organ systems, including the vertebrate vasculature, C.elegans excretory system, and Drosophila tracheal system. The Drosophila tracheal system is a network of air-filled tubes that delivers oxygen to all tissues. Specialized cells within the tracheal system, called terminal cells, branch extensively and form seamless tubes. Terminal tracheal tubes are polarized; the lumenal membrane has apical identity whereas the outer membrane exhibits basal characteristics. Although various aspects of membrane trafficking have been implicated in terminal cell morphogenesis, the precise secretory pathway requirements for basal and apical membrane growth have yet to be elucidated. In the present study, we demonstrate that anterograde trafficking, retrograde trafficking and Golgi-to-plasma membrane vesicle fusion are each required for the complex branched architecture of the terminal cell, but their inputs during seamless lumen formation are more varied. The COPII subunit, Sec31, and ER exit site protein, Sec16, are critical for subcellular tube architecture, whereas the SNARE proteins Syntaxin 5, Syntaxin 1 and Syntaxin 18 are more generally required for seamless tube growth and maintenance. These data suggest that distinct components of the secretory pathway have differential contributions to basal and apical membrane growth and maintenance during terminal cell morphogenesis.

Aquaporin 8ab is required in zebrafish embryonic intestine development

The aquaporin 8 (AQP8) is a small integral membrane protein that selectively transports water and other small uncharged solutes across cell plasma membranes. It has been demonstrated that AQP8 is ubiquitously present in various tissues and organs of mammals, and participates in many physiological and pathological processes. Recent studies showed that AQP8 is highly expressed in the columnar epithelial cells of mammalian colonic mucosa facing lumen, indicating that AQP8 plays potential roles in the physiology and pathophysiology of gastrointestinal tract. However, the role of AQP8 during gastrointestinal tract development is unclear. In the present study, RT-PCR results reveal that the zebrafish genome encodes three kinds of aqp8s ( aqp8aa, aqp8ab, and aqp8b). We use whole mount in situ hybridization to describe aqp8 genes spatiotemporal expression pattern, and the results show that aqp8ab mRNA is detectable mainly in the zebrafish embryonic intestine. To reveal the details of aqp8ab distribution, histological sections are employed. Transverse sections indicate that aqp8ab mRNA expression is more intense in the layer lining the intestinal cavity. Knockout of aqp8ab using the CRISPR/Cas9 system induces intestine development defects and abnormal formation of intestinal lumen. In addition, aqp8ab mRNA significantly rescues the intestine defects in the aqp8ab mutant. These results indicate that aqp8ab is required in the intestine development of zebrafish.

Midbody-Localized Aquaporin Mediates Intercellular Lumen Expansion During Early Cleavage of an Invasive Freshwater Bivalve

Intercellular lumen formation is a crucial aspect of animal development and physiology that involves a complex interplay between the molecular and physical properties of the constituent cells. Embryos of the invasive freshwater mussel Dreissena rostriformis are ideal models for studying this process due to the large intercellular cavities that readily form during blastomere cleavage. Using this system, we show that recruitment of the transmembrane water channel protein aquaporin exclusively to the midbody of intercellular cytokinetic bridges is critical for lumenogenesis. The positioning of aquaporin-positive midbodies thereby influences the direction of cleavage cavity expansion. Notably, disrupting cytokinetic bridge microtubules impairs not only lumenogenesis but also cellular osmoregulation. Our findings reveal a simple mechanism that provides tight spatial and temporal control over the formation of luminal structures and likely plays an important role in water homeostasis during early cleavage stages of a freshwater invertebrate species.

Nematodes as Ghosts of Land Use Past: Elucidating the Roles of Soil Nematode Community Studies as Indicators of Soil Health and Land Management Practices

Soil health is a matter of growing concern because of its degradation due to unsustainable anthropogenic activities over the last few decades. It is maintained by interactions among the components of the soil food web commonly concentrated in the vicinity of the plant roots, called the rhizosphere. The soil food web is dominated by nematodes. They occupy various trophic positions because of their diverse feeding habits. The free-living forms are mainly dependent on soil bacteria and fungi for their nutrition, while the parasitic forms feed on plant roots. The population of these two groups is regulated by the activities of predatory nematodes which can be carnivorous or omnivorous. The soil nematodes thereby partake responsibilities in nutrient cycling, mineralization and decomposition pathways which, in turn, affects the aboveground productivity. This intricately connected food web structure is vulnerable to disturbances like increased soil salinity, acidity, nitrogen enrichment, tillage, crop rotations, fertilizers, pesticides, soil amendment techniques and heavy metal pollution. The effects are reflected by alterations in the abundance and diversity of soil nematodes belonging to various trophic groups. These alterations have been formulated into measurable indices like maturity index (MI), structure index (SI), enrichment index (EI) and channel index (CI). The faunal profile and metabolic footprints of soil nematodes are latest developments in the field of nematode community analyses. Though these indices cannot replace the conventional soil ecotoxicological assays, they can give added information about soil biology which can be utilized to design sustainable land use practices.

Third-Stage Dispersal Juveniles of Bursaphelenchus xylophilus Can Resist Low-Temperature Stress by Entering Cryptobiosis

Simple Summary

Pine wilt disease caused by the nematode Bursaphelenchus xylophilus causes significant harm to China’s forests, but there are currently no effective prevention and control measures. Additionally, this devastating disease is currently spreading northward. We determined that third-stage dispersal juveniles of B. xylophilus can resist low-temperature stress by cryptobiosis, allowing these nematodes to tolerate a greater range of temperatures. These results facilitate the prediction of potential areas at risk for B. xylophilus in the mid-temperature and cold temperature zones of China.

Abstract

Nematodes can enter cryptobiosis by dehydration as an adaptation to low-temperature environments and recover from cryptobiosis by rehydration after environmental improvement. In this work, the survival of Bursaphelenchusxylophilus third-stage dispersal juveniles was studied in response to low-temperature treatment. The average survival rates were 1.7% after −80 °C treatment for 30 d and 82.2% after −20 °C treatment for 30 d. The changes of water content and inorganic salt ions that occur in pine trees during winter gradually alter the osmotic pressure in the liquid environment to dehydrate B. xylophilus juveniles, resulting in improved survival after low-temperature treatment. The survival rate at −20 °C improved to 92.1% when the juveniles entered cryptobiosis by osmotic regulation. The results of this study demonstrate that B. xylophilus third-stage dispersal juveniles can resist low-temperature stress through cryptobiosis, providing the theoretical basis for the identification of areas potentially vulnerable to B. xylophilus in the mid-temperature and cold temperature zones of China.

A polarity pathway for exocyst-dependent intracellular tube extension

Lumen extension in intracellular tubes can occur when vesicles fuse with an invading apical membrane. Within the Caenorhabditis elegans excretory cell, which forms an intracellular tube, the exocyst vesicle-tethering complex is enriched at the lumenal membrane and is required for its outgrowth, suggesting that exocyst-targeted vesicles extend the lumen. Here, we identify a pathway that promotes intracellular tube extension by enriching the exocyst at the lumenal membrane. We show that PAR-6 and PKC-3/aPKC concentrate at the lumenal membrane and promote lumen extension. Using acute protein depletion, we find that PAR-6 is required for exocyst membrane recruitment, whereas PAR-3, which can recruit the exocyst in mammals, appears dispensable for exocyst localization and lumen extension. Finally, we show that CDC-42 and RhoGEF EXC-5/FGD regulate lumen extension by recruiting PAR-6 and PKC-3 to the lumenal membrane. Our findings reveal a pathway that connects CDC-42, PAR proteins, and the exocyst to extend intracellular tubes.

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.

Terminal web and vesicle trafficking proteins mediate nematode single-cell tubulogenesis

Single-celled tubules represent a complicated structure that forms during development, requiring extension of a narrow cytoplasm surrounding a lumen exerting osmotic pressure that can burst the luminal membrane. Genetic studies on the excretory canal cell of Caenorhabditis elegans have revealed many proteins that regulate the cytoskeleton, vesicular transport, and physiology of the narrow canals. Here, we show that βH-spectrin regulates the placement of intermediate filament proteins forming a terminal web around the lumen, and that the terminal web in turn retains a highly conserved protein (EXC-9/CRIP1) that regulates apical endosomal trafficking. EXC-1/IRG, the binding partner of EXC-9, is also localized to the apical membrane and affects apical actin placement and RAB-8-mediated vesicular transport. The results suggest that an intermediate filament protein acts in a novel pathway to direct the traffic of vesicles to locations of lengthening apical surface during single-celled tubule development.

Cells into tubes: Molecular and physical principles underlying lumen formation in tubular organs

Tubular networks, such as the vascular and respiratory systems, transport liquids and gases in multicellular organisms. The basic units of these organs are tubes formed by single or multiple cells enclosing a luminal cavity. The formation and maintenance of correctly sized and shaped lumina are fundamental steps in organogenesis and are essential for organismal homeostasis. Therefore, understanding how cells generate, shape and maintain lumina is crucial for understanding normal organogenesis as well as the basis of pathological conditions. Lumen formation involves polarized membrane trafficking, cytoskeletal dynamics, and the influence of intracellular as well as extracellular mechanical forces, such as cortical tension, luminal pressure or blood flow. Various tissue culture and in vivo model systems, ranging from MDCK cell spheroids to tubular organs in worms, flies, fish, and mice, have provided many insights into the molecular and cellular mechanisms underlying lumenogenesis and revealed key factors that regulate the size and shape of cellular tubes. Moreover, the development of new experimental and imaging approaches enabled quantitative analyses of intracellular dynamics and allowed to assess the roles of cellular and tissue mechanics during tubulogenesis. However, how intracellular processes are coordinated and regulated across scales of biological organization to generate properly sized and shaped tubes is only beginning to be understood. Here, we review recent insights into the molecular, cellular and physical mechanisms underlying lumen formation during organogenesis. We discuss how these mechanisms control lumen formation in various model systems, with a special focus on the morphogenesis of tubular organs in Drosophila.

A multi-layered and dynamic apical extracellular matrix shapes the vulva lumen in Caenorhabditis elegans

Biological tubes must develop and maintain their proper diameter to transport materials efficiently. These tubes are molded and protected in part by apical extracellular matrices (aECMs) that line their lumens. Despite their importance, aECMs are difficult to image in vivo and therefore poorly understood. The Caenorhabditis elegans vulva has been a paradigm for understanding many aspects of organogenesis. Here we describe the vulva luminal matrix, which contains chondroitin proteoglycans, Zona Pellucida (ZP) domain proteins, and other glycoproteins and lipid transporters related to those in mammals. Confocal and transmission electron microscopy revealed, with unprecedented detail, a complex and dynamic aECM. Different matrix factors assemble on the apical surfaces of each vulva cell type, with clear distinctions seen between Ras-dependent (1°) and Notch-dependent (2°) cell types. Genetic perturbations suggest that chondroitin and other aECM factors together generate a structured scaffold that both expands and constricts lumen shape.

A Series of Tubes: The C. elegans Excretory Canal Cell as a Model for Tubule Development

Formation and regulation of properly sized epithelial tubes is essential for multicellular life. The excretory canal cell of C. elegans provides a powerful model for investigating the integration of the cytoskeleton, intracellular transport, and organismal physiology to regulate the developmental processes of tube extension, lumen formation, and lumen diameter regulation in a narrow single cell. Multiple studies have provided new understanding of actin and intermediate filament cytoskeletal elements, vesicle transport, and the role of vacuolar ATPase in determining tube size. Most of the genes discovered have clear homologues in humans, with implications for understanding these processes in mammalian tissues such as Schwann cells, renal tubules, and brain vasculature. The results of several new genetic screens are described that provide a host of new targets for future studies in this informative structure.

Osmotic gradients induce stable dome morphogenesis on extracellular matrix

One of the fundamental processes in morphogenesis is dome formation, but many of the mechanisms involved are unexplored. Previous in vitro studies showed that an osmotic gradient is the driving factor of dome formation. However, these investigations were performed without extracellular matrix (ECM), which provides structural support to morphogenesis. With the use of ECM, we observed that basal hypertonic stress induced stable domes in vitro that have not been seen in previous studies. These domes developed as a result of ECM swelling via aquaporin water transport activity. Based on computer simulation, uneven swelling, with a positive feedback between cell stretching and enhanced water transport, was a cause of dome formation. These results indicate that osmotic gradients induce dome morphogenesis via both enhanced water transport activity and subsequent ECM swelling.

C-terminal phosphorylation modulates ERM-1 localization and dynamics to control cortical actin organization and support lumen formation during Caenorhabditiselegans development

ERM proteins are conserved regulators of cortical membrane specialization that function as membrane-actin linkers and molecular hubs. The activity of ERM proteins requires a conformational switch from an inactive cytoplasmic form into an active membrane- and actin-bound form, which is thought to be mediated by sequential PIP2 binding and phosphorylation of a conserved C-terminal threonine residue. Here, we use the single Caenorhabditiselegans ERM ortholog, ERM-1, to study the contribution of these regulatory events to ERM activity and tissue formation in vivo Using CRISPR/Cas9-generated erm-1 mutant alleles, we demonstrate that a PIP2-binding site is crucially required for ERM-1 function. By contrast, dynamic regulation of C-terminal T544 phosphorylation is not essential but modulates ERM-1 apical localization and dynamics in a tissue-specific manner, to control cortical actin organization and support lumen formation in epithelial tubes. Our work highlights the dynamic nature of ERM protein regulation during tissue morphogenesis and the importance of C-terminal phosphorylation in fine-tuning ERM activity in a tissue-specific context.

A tensile trilayered cytoskeletal endotube drives capillary-like lumenogenesis

Unicellular tubes are components of internal organs and capillaries. It is unclear how they meet the architectural challenge to extend a centered intracellular lumen of uniform diameter. In an RNAi-based Caenorhabditis elegans screen, we identified three intermediate filaments (IFs)-IFA-4, IFB-1, and IFC-2-as interactors of the lumenal membrane-actin linker ERM-1 in excretory-canal tubulogenesis. We find that IFs, generally thought to affect morphogenesis indirectly by maintaining tissue integrity, directly promote lumenogenesis in this capillary-like single-cell tube. We show that ERM-1, ACT-5/actin, and TBB-2/tubulin recruit membrane-forming endosomal and flux-promoting canalicular vesicles to the lumen, whereas IFs, themselves recruited to the lumen by ERM-1 and TBB-2, restrain lateral vesicle access. IFs thereby prevent cystogenesis, equilibrate the lumen diameter, and promote lumen forward extension. Genetic and imaging analyses suggest that IFB-1/IFA-4 and IFB-1/IFC-2 polymers form a perilumenal triple IF lattice, sandwiched between actin and helical tubulin. Our findings characterize a novel mechanism of capillary-like lumenogenesis, where a tensile trilayered cytoskeletal endotube transforms concentric into directional growth.

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

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

The small GTPase ARF-1.2 is a regulator of unicellular tube formation in Caenorhabditis elegans

The membrane trafficking events that regulate unicellular tube formation and maintenance are not well understood. Here, using an RNAi screen, we identified the small GTPase ARF1 homolog ARF-1.2 as a regulator of excretory tube formation in Caenorhabditis elegans. RNAi-mediated knockdown and knockout of the arf-1.2 gene resulted in the formation of large intracellular vacuoles at the growth sites (varicosities) of the excretory canals. arf-1.2 mutant animals were sensitive to hyperosmotic conditions. arf-1.2 RNAi affected the localization of the anion transporter SULP-8, which is expressed in the basal plasma membrane of the excretory canals, but did not affect the expression of SULP-4, which is expressed in the apical membrane. The phenotype of arf-1.2 mutants was suppressed by mutation of the small Rho GTPase CDC-42, a regulator of apical/basal traffic balance. These results suggest that ARF-1.2 plays an essential role in basal membrane traffic to regulate the formation of the unicellular excretory tube.

Ezrin enrichment on curved membranes requires a specific conformation or interaction with a curvature-sensitive partner

One challenge in cell biology is to decipher the biophysical mechanisms governing protein enrichment on curved membranes and the resulting membrane deformation. The ERM protein ezrin is abundant and associated with cellular membranes that are flat, positively or negatively curved. Using in vitro and cell biology approaches, we assess mechanisms of ezrin’s enrichment on curved membranes. We evidence that wild-type ezrin (ezrinWT) and its phosphomimetic mutant T567D (ezrinTD) do not deform membranes but self-assemble anti-parallelly, zipping adjacent membranes. EzrinTD’s specific conformation reduces intermolecular interactions, allows binding to actin filaments, which reduces membrane tethering, and promotes ezrin binding to positively-curved membranes. While neither ezrinTD nor ezrinWT senses negative curvature alone, we demonstrate that interacting with curvature-sensing I-BAR-domain proteins facilitates ezrin enrichment in negatively-curved membrane protrusions. Overall, our work demonstrates that ezrin can tether membranes, or be targeted to curved membranes, depending on conformations and interactions with actin and curvature-sensing binding partners.

14-3-3εa directs the pulsatile transport of basal factors toward the apical domain for lumen growth in tubulogenesis

Ascidians have become a powerful model system in which to uncover basic mechanisms that govern body plan specification and elaboration. In particular, the ascidian notochord is a highly tractable model for tubulogenesis. Here, we use chemical genetics to identify roles for 14-3-3εa, and its binding partner ezrin/radixin/moesin (ERM), in tubulogenesis. Combining genetic and chemical perturbations with live cell imaging, we present evidence that 14-3-3εa–ERM interactions are required for tubulogenesis and that they act by promoting a directed cytoplasmic flow, previously uncharacterized, which carries lumen-associated components from the basal domain to the apical domain to feed lumen growth. Because many core components of this system are highly conserved, these results have broad implications for tubulogenesis in many other contexts.

The Ciona notochord has emerged as a simple and tractable in vivo model for tubulogenesis. Here, using a chemical genetics approach, we identified UTKO1 as a selective small molecule inhibitor of notochord tubulogenesis. We identified 14-3-3εa protein as a direct binding partner of UTKO1 and showed that 14-3-3εa knockdown leads to failure of notochord tubulogenesis. We found that UTKO1 prevents 14-3-3εa from interacting with ezrin/radixin/moesin (ERM), which is required for notochord tubulogenesis, suggesting that interactions between 14-3-3εa and ERM play a key role in regulating the early steps of tubulogenesis. Using live imaging, we found that, as lumens begin to open between neighboring cells, 14-3-3εa and ERM are highly colocalized at the basal cortex where they undergo cycles of accumulation and disappearance. Interestingly, the disappearance of 14-3-3εa and ERM during each cycle is tightly correlated with a transient flow of 14-3-3εa, ERM, myosin II, and other cytoplasmic elements from the basal surface toward the lumen-facing apical domain, which is often accompanied by visible changes in lumen architecture. Both pulsatile flow and lumen formation are abolished in larvae treated with UTKO1, in larvae depleted of either 14-3-3εa or ERM, or in larvae expressing a truncated form of 14-3-3εa that lacks the ability to interact with ERM. These results suggest that 14-3-3εa and ERM interact at the basal cortex to direct pulsatile basal accumulation and basal–apical transport of factors that are essential for lumen formation. We propose that similar mechanisms may underlie or may contribute to lumen formation in tubulogenesis in other systems.

Tubular Excretory Canal Structure Depends on Intermediate Filaments EXC-2 and IFA-4 in Caenorhabditis elegans

The excretory canals of Caenorhabditis elegans are a model for understanding the maintenance of apical morphology in narrow single-celled tubes. Light and electron microscopy shows that mutants in exc-2 start to form canals normally, but these swell to develop large fluid-filled cysts that lack a complete terminal web at the apical surface, and accumulate filamentous material in the canal lumen. Here, whole-genome sequencing and gene rescue show that exc-2 encodes intermediate filament protein IFC-2 EXC-2/IFC-2 protein, fluorescently tagged via clustered regularly interspaced short palindromic repeats/Cas9, is located at the apical surface of the canals independently of other intermediate filament proteins. EXC-2 is also located in several other tissues, though the tagged isoforms are not seen in the larger intestinal tube. Tagged EXC-2 binds via pulldown to intermediate filament protein IFA-4, which is also shown to line the canal apical surface. Overexpression of either protein results in narrow but shortened canals. These results are consistent with a model whereby three intermediate filaments in the canals-EXC-2, IFA-4, and IFB-1-restrain swelling of narrow tubules in concert with actin filaments that guide the extension and direction of tubule outgrowth, while allowing the tube to bend as the animal moves.

Insane in the apical membrane: Trafficking events mediating apicobasal epithelial polarity during tube morphogenesis

The creation of cellular tubes is one of the most vital developmental processes, resulting in the formation of most organ types. Cells have co-opted a number of different mechanisms for tube morphogenesis that vary among tissues and organisms; however, generation and maintenance of cell polarity is fundamental for successful lumenogenesis. Polarized membrane transport has emerged as a key driver not only for establishing individual epithelial cell polarity, but also for coordination of epithelial polarization during apical lumen formation and tissue morphogenesis. In recent years, much work has been dedicated to identifying membrane trafficking regulators required for lumenogenesis. In this review we will summarize the findings from the past couple of decades in defining the molecular machinery governing lumenogenesis both in 3D tissue culture models and during organ development in vivo.

Proliferation-independent role of NF2 (merlin) in limiting biliary morphogenesis

The architecture of individual cells and cell collectives enables functional specification, a prominent example being the formation of epithelial tubes that transport fluid or gas in many organs. The intrahepatic bile ducts (IHBDs) form a tubular network within the liver parenchyma that transports bile to the intestine. Aberrant biliary 'neoductulogenesis' is also a feature of several liver pathologies including tumorigenesis. However, the mechanism of biliary tube morphogenesis in development or disease is not known. Elimination of the neurofibromatosis type 2 protein (NF2; also known as merlin or neurofibromin 2) causes hepatomegaly due to massive biliary neoductulogenesis in the mouse liver. We show that this phenotype reflects unlimited biliary morphogenesis rather than proliferative expansion. Our studies suggest that NF2 normally limits biliary morphogenesis by coordinating lumen expansion and cell architecture. This work provides fundamental insight into how biliary fate and tubulogenesis are coordinated during development and will guide analyses of disease-associated and experimentally induced biliary pathologies.

An Ichor-dependent apical extracellular matrix regulates seamless tube shape and integrity

During sprouting angiogenesis in the vertebrate vascular system, and primary branching in the Drosophila tracheal system, specialized tip cells direct branch outgrowth and network formation. When tip cells lumenize, they form subcellular (seamless) tubes. How these seamless tubes are made, shaped and maintained remains poorly understood. Here we characterize a Drosophila mutant called ichor (ich), and show that ich is essential for the integrity and shape of seamless tubes in tracheal terminal cells. We find that Ich regulates seamless tubulogenesis via its role in promoting the formation of a mature apical extracellular matrix (aECM) lining the lumen of the seamless tubes. We determined that ich encodes a zinc finger protein (CG11966) that acts, as a transcriptional activator required for the expression of multiple aECM factors, including a novel membrane-anchored trypsin protease (CG8213). Thus, the integrity and shape of seamless tubes are regulated by the aECM that lines their lumens.

The C. elegans Excretory Canal as a Model for Intracellular Lumen Morphogenesis and In Vivo Polarized Membrane Biogenesis in a Single Cell: labeling by GFP-fusions, RNAi Interaction Screen and Imaging

The four C. elegans excretory canals are narrow tubes extended through the length of the animal from a single cell, with almost equally far extended intracellular endotubes that build and stabilize the lumen with a membrane and submembraneous cytoskeleton of apical character. The excretory cell expands its length approximately 2,000 times to generate these canals, making this model unique for the in vivo assessment of de novo polarized membrane biogenesis, intracellular lumen morphogenesis and unicellular tubulogenesis. The protocol presented here shows how to combine standard labeling, gain- and loss-of-function genetic or RNA interference (RNAi)-, and microscopic approaches to use this model to visually dissect and functionally analyze these processes on a molecular level. As an example of a labeling approach, the protocol outlines the generation of transgenic animals with fluorescent fusion proteins for live analysis of tubulogenesis. As an example of a genetic approach, it highlights key points of a visual RNAi-based interaction screen designed to modify a gain-of-function cystic canal phenotype. The specific methods described are how to: label and visualize the canals by expressing fluorescent proteins; construct a targeted RNAi library and strategize RNAi screening for the molecular analysis of canal morphogenesis; visually assess modifications of canal phenotypes; score them by dissecting fluorescence microscopy; characterize subcellular canal components at higher resolution by confocal microscopy; and quantify visual parameters. The approach is useful for the investigator who is interested in taking advantage of the C. elegans excretory canal for identifying and characterizing genes involved in the phylogenetically conserved processes of intracellular lumen and unicellular tube morphogenesis.

Neuronal Chemosensation and Osmotic Stress Response Converge in the Regulation of aqp-8 in C. elegans

Aquaporins occupy an essential role in sustaining the salt/water balance in various cells types and tissues. Here, we present new insights into aqp-8 expression and regulation in Caenorhabditis elegans. We show, that upon exposure to osmotic stress, aqp-8 exhibits a distinct expression pattern within the excretory cell compared to other C. elegans aquaporins expressed. This expression is correlated to the osmolarity of the surrounding medium and can be activated physiologically by osmotic stress or genetically in mutants with constitutively active osmotic stress response. In addition, we found aqp-8 expression to be constitutively active in the TRPV channel mutant osm-9(ok1677). In a genome-wide RNAi screen we identified additional regulators of aqp-8. Many of these regulators are connected to chemosensation by the amphid neurons, e.g., odr-10 and gpa-6, and act as suppressors of aqp-8 expression. We postulate from our results, that aqp-8 plays an important role in sustaining the salt/water balance during a secondary response to hyper-osmotic stress. Upon its activation aqp-8 promotes vesicle docking to the lumen of the excretory cell and thereby enhances the ability to secrete water and transport osmotic active substances or waste products caused by protein damage. In summary, aqp-8 expression and function is tightly regulated by a network consisting of the osmotic stress response, neuronal chemosensation as well as the response to protein damage. These new insights in maintaining the salt/water balance in C. elegans will help to reveal the complex homeostasis network preserved throughout species.

Demystification of animal symmetry: symmetry is a response to mechanical forces

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Symmetry is an eye-catching feature of animal body plans, yet its causes are not well enough understood. The evolution of animal form is mainly due to changes in gene regulatory networks (GRNs). Based on theoretical considerations regarding fundamental GRN properties, it has recently been proposed that the animal genome, on large time scales, should be regarded as a system which can construct both the main symmetries – radial and bilateral – simultaneously; and that the expression of any of these depends on functional constraints. Current theories explain biological symmetry as a pattern mostly determined by phylogenetic constraints, and more by chance than by necessity. In contrast to this conception, I suggest that physical effects, which in many cases act as proximate, direct, tissue-shaping factors during ontogenesis, are also the ultimate causes – i.e. the indirect factors which provide a selective advantage – of animal symmetry, from organs to body plan level patterns. In this respect, animal symmetry is a necessary product of evolution. This proposition offers a parsimonious view of symmetry as a basic feature of the animal body plan, suggesting that molecules and physical forces act in a beautiful harmony to create symmetrical structures, but that the concert itself is directed by the latter.

Reviewers

This article was reviewed by Eugene Koonin, Zoltán Varga and Michaël Manuel.

Morphogenesis of honeybee hypopharyngeal gland during pupal development

Background

The hypopharyngeal gland of worker bees contributes to the production of the royal jelly fed to queens and larvae. The gland consists of thousands of two-cell units that are composed of a secretory cell and a duct cell and that are arranged in sets of about 12 around a long collecting duct.

Results

By fluorescent staining, we have examined the morphogenesis of the hypopharyngeal gland during pupal life, from a saccule lined by a pseudostratified epithelium to the elaborate organ of adult worker bees. The hypopharyngeal gland develops as follows. (1) Cell proliferation occurs during the first day of pupal life in the hypopharyngeal gland primordium. (2) Subsequently, the epithelium becomes organized into rosette-like units of three cells. Two of these will become the secretory cell and the duct cell of the adult secretory units; the third cell contributes only temporarily to the development of the secretory units and is eliminated by apoptosis in the second half of pupal life. (3) The three-cell units of flask-shaped cells undergo complex changes in cell morphology. Thus, by mid-pupal stage, the gland is structurally similar to the adult hypopharyngeal gland. (4) Concomitantly, the prospective secretory cell attains its characteristic subcellular organization by the invagination of a small patch of apical membrane domain, its extension to a tube of about 100 μm in length (termed a canaliculus), and the expansion of the tube to a diameter of about 3 μm. (6) Finally, the canaliculus-associated F-actin system becomes reorganized into rings of bundled actin filaments that are positioned at regular distances along the membrane tube.

Conclusions

The morphogenesis of the secretory units in the hypopharyngeal gland of the worker bee seems to be based on a developmental program that is conserved, with slight modification, among insects for the production of dermal glands. Elaboration of the secretory cell as a unicellular seamless epithelial tube occurs by invagination of the apical membrane, its extension likely by targeted exocytosis and its expansion, and finally the reorganisation of the membrane-associated F-actin system. Our work is fundamental for future studies of environmental effects on hypopharyngeal gland morphology and development.

Electronic supplementary material

The online version of this article (doi:10.1186/s12983-017-0207-z) contains supplementary material, which is available to authorized users.

Directional Fluid Transport across Organ-Blood Barriers: Physiology and Cell Biology

Directional fluid flow is an essential process for embryo development as well as for organ and organism homeostasis. Here, we review the diverse structure of various organ-blood barriers, the driving forces, transporters, and polarity mechanisms that regulate fluid transport across them, focusing on kidney-, eye-, and brain-blood barriers. We end by discussing how cross talk between barrier epithelial and endothelial cells, perivascular cells, and basement membrane signaling contribute to generate and maintain organ-blood barriers.

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

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

Facilitation of Endosomal Recycling by an IRG Protein Homolog Maintains Apical Tubule Structure in Caenorhabditis elegans

Determination of luminal diameter is critical to the function of small single-celled tubes. A series of EXC proteins, including EXC-1, prevent swelling of the tubular excretory canals in Caenorhabditis elegans In this study, cloning of exc-1 reveals it to encode a homolog of mammalian IRG proteins, which play roles in immune response and autophagy and are associated with Crohn's disease. Mutants in exc-1 accumulate early endosomes, lack recycling endosomes, and exhibit abnormal apical cytoskeletal structure in regions of enlarged tubules. EXC-1 interacts genetically with two other EXC proteins that also affect endosomal trafficking. In yeast two-hybrid assays, wild-type and putative constitutively active EXC-1 binds to the LIM-domain protein EXC-9, whose homolog, cysteine-rich intestinal protein, is enriched in mammalian intestine. These results suggest a model for IRG function in forming and maintaining apical tubule structure via regulation of endosomal recycling.

C. elegans flavin-containing monooxygenase-4 is essential for osmoregulation in hypotonic stress

Studies in Caenorhabditis elegans have revealed osmoregulatory systems engaged when worms experience hypertonic conditions, but less is known about measures employed when faced with hypotonic stress. Inactivation of fmo-4, which encodes flavin-containing monooxygenase-4, results in dramatic hypoosmotic hypersensitivity; worms are unable to prevent overwhelming water influx and swell rapidly, finally rupturing due to high internal hydrostatic pressure. fmo-4 is expressed prominently in hypodermis, duct and pore cells but is excluded from the excretory cell. Thus, FMO-4 plays a crucial osmoregulatory role by promoting clearance of excess water that enters during hypotonicity, perhaps by synthesizing an osmolyte that acts to establish an osmotic gradient from excretory cell to duct and pore cells. C. elegans FMO-4 contains a C-terminal extension conserved in all nematode FMO-4s. The coincidently numbered human FMO4 also contains an extended C-terminus with features similar to those of FMO-4. Although these shared sequence characteristics suggest potential orthology, human FMO4 was unable to rescue the fmo-4 osmoregulatory defect. Intriguingly, however, mammalian FMO4 is expressed predominantly in the kidney - an appropriate site if it too is, or once was, involved in osmoregulation.

Time to make the doughnuts: Building and shaping seamless tubes

A seamless tube is a very narrow-bore tube that is composed of a single cell with an intracellular lumen and no adherens or tight junctions along its length. Many capillaries in the vertebrate vascular system are seamless tubes. Seamless tubes also are found in invertebrate organs, including the Drosophila trachea and the Caenorhabditis elegans excretory system. Seamless tube cells can be less than a micron in diameter, and they can adopt very simple "doughnut-like" shapes or very complex, branched shapes comparable to those of neurons. The unusual topology and varied shapes of seamless tubes raise many basic cell biological questions about how cells form and maintain such structures. The prevalence of seamless tubes in the vascular system means that answering such questions has significant relevance to human health. In this review, we describe selected examples of seamless tubes in animals and discuss current models for how seamless tubes develop and are shaped, focusing particularly on insights that have come from recent studies in Drosophila and C. elegans.

Current advances in the functional studies of fatty acids and fatty acid-derived lipids in C. elegans

Fatty acids and fatty acid-derived lipids (FAs/FADLs) play essential roles in many living organisms, including contributions to membrane structure and signaling transduction. Aberrant metabolism of FAs/FADLs often causes diseases and health problems. However, the detailed mechanistic studies of specific FAs/FADLs in vivo are limited. C. elegans has been an effective model system for FA/ FADL studies due to its powerful genetics and conserved lipid biosynthetic pathways. The recently developed high-throughput analytic tools also enable sophisticated profiling of lipids molecules in C. elegans, which is critical for understanding their specific functions. Here we review a subset of current advances in FA/FADL functional studies in C. elegans.

Noncentrosomal microtubules in C. elegans epithelia

The microtubule cytoskeleton has a dual contribution to cell organization. First, microtubules help displace chromosomes and provide tracks for organelle transport. Second, microtubule rigidity confers specific mechanical properties to cells, which are crucial in cilia or mechanosensory structures. Here we review the recently uncovered organization and functions of noncentrosomal microtubules in C. elegans epithelia, focusing on how they contribute to nuclear positioning and protein transport. In addition, we describe recent data illustrating how the microtubule and actin cytoskeletons interact to achieve those functions.

Plasticity of the brush border - the yin and yang of intestinal homeostasis

The brush border on the apical surface of enterocytes is a highly specialized structure well-adapted for efficient digestion and nutrient transport, whilst at the same time providing a protective barrier for the intestinal mucosa. The brush border is constituted of a densely ordered array of microvilli, protrusions of the plasma membrane, which are supported by actin-based microfilaments and interacting proteins and anchored in an apical network of actomyosin and intermediate filaments, the so-called terminal web. The highly dynamic, specialized apical domain is both an essential partner for the gut microbiota and an efficient signalling platform that enables adaptation to physiological stimuli from the external and internal milieu. Nevertheless, genetic alterations or various pathological stresses, such as infection, inflammation, and mechanical or nutritional alterations, can jeopardize this equilibrium and compromise intestinal functions. Long-time neglected, the intestinal brush-border shall be enlightening again as the central actor of the complex but essential intestinal homeostasis. Here, we review the processes and components involved in brush border organization and discuss pathological mechanisms that can induce brush border defects and their physiological consequences.

A combined binary interaction and phenotypic map of C. elegans cell polarity proteins

The establishment of cell polarity is an essential process for the development of multicellular organisms and the functioning of cells and tissues. Here, we combine large-scale protein interaction mapping with systematic phenotypic profiling to study the network of physical interactions that underlies polarity establishment and maintenance in the nematode Caenorhabditis elegans. Using a fragment-based yeast two-hybrid strategy, we identified 439 interactions between 296 proteins, as well as the protein regions that mediate these interactions. Phenotypic profiling of the network resulted in the identification of 100 physically interacting protein pairs for which RNAi-mediated depletion caused a defect in the same polarity-related process. We demonstrate the predictive capabilities of the network by showing that the physical interaction between the RhoGAP PAC-1 and PAR-6 is required for radial polarization of the C. elegans embryo. Our network represents a valuable resource of candidate interactions that can be used to further our insight into cell polarization.

Substrate Availability of Mutant SPT Alters Neuronal Branching and Growth Cone Dynamics in Dorsal Root Ganglia

Serine palmitoyltransferase (SPT) is a key enzyme in the first step of sphingolipid biosynthesis. Mutations in the SPTLC1 gene that encodes for SPT subunits cause hereditary sensory neuropathy type 1. However, little is understood about how mutant SPT regulates mechanisms of sensory neuron and axonal growth. Using transgenic mice overexpressing the C133W SPT mutant, we found that mutant dorsal root ganglia (DRG) during growth in vitro exhibit increased neurite length and branching, coinciding with elevated expression of actin-cross-linking proteins at the neuronal growth cone, namely phosphorylated Ezrin/Radixin/Moesin. In addition, inhibition of SPT was able to reverse the mutant phenotype. Because mutant SPT preferentially uses l-alanine over its canonical substrate l-serine, we also investigated the effects of substrate availability on DRG neurons. Supplementation with l-serine or removal of l-alanine independently restored normal growth patterns in mutant SPTLC1(C133W) DRG. Therefore, we report that substrate availability and selectivity of SPT influence the regulation of neurite growth in DRG neurons.

SIGNIFICANCE STATEMENT

Hereditary sensory neuropathy type 1 is an autosomal-dominant disorder that leads to a sensory neuropathy due to mutations in the serine palmitoyltransferase (SPT) enzyme. We investigated how mutant SPT and substrate levels regulate neurite growth. Because SPT is an important enzyme in the synthesis of sphingolipids, our data are of broader significance to other peripheral and metabolic disorders.

A new paradigm for animal symmetry

My aim in this article is to soften certain rigid concepts concerning the radial and bilateral symmetry of the animal body plan, and to offer a more flexible framework of thinking for them, based on recent understandings of how morphogenesis is regulated by the mosaically acting gene regulatory networks. Based on general principles of the genetic regulation of morphogenesis, it can be seen that the difference between the symmetry of the whole body and that of minor anatomical structures is only a question of a diverse timing during development. I propose that the animal genome, as such, is capable of expressing both radial and bilateral symmetries, and deploys them according to the functional requirements which must be satisfied by both the anatomical structure and body as a whole. Although it may seem paradoxical, this flexible view of symmetry, together with the idea that symmetry is strongly determined by function, bolsters the concept that the presence of the two main symmetries in the animal world is not due to chance: they are necessary biological patterns emerging in evolution.

Guidance of subcellular tubulogenesis by actin under the control of a synaptotagmin-like protein and Moesin

Apical membranes in many polarized epithelial cells show specialized morphological adaptations that fulfil distinct physiological functions. The air-transporting tubules of Drosophila tracheal terminal cells represent an extreme case of membrane specialization. Here we show that Bitesize (Btsz), a synaptotagmin-like protein family member, is needed for luminal membrane morphogenesis. Unlike in multicellular tubes and other epithelia, where it influences apical integrity by affecting adherens junctions, Btsz here acts at a distance from junctions. Localized at the luminal membrane through its tandem C2 domain, it recruits activated Moesin. Both proteins are needed for the integrity of the actin cytoskeleton at the luminal membrane, but not for other pools of F-actin in the cell, nor do actin-dependent processes at the outer membrane, such as filopodial activity or membrane growth depend on Btsz. Btsz and Moesin guide luminal membrane morphogenesis through organizing actin and allowing the incorporation of membrane containing the apical determinant Crumbs.

The terminal branches of the Drosophila tracheal network have intracellular tubules that grow through elongation of membrane invaginations. Here, the authors identify the synaptotagmin-like protein Bitesize as a regulator of actin-dependent luminal membrane morphogenesis.

Polarized protein transport and lumen formation during epithelial tissue morphogenesis

One of the major challenges in biology is to explain how complex tissues and organs arise from the collective action of individual polarized cells. The best-studied model of this process is the cross talk between individual epithelial cells during their polarization to form the multicellular epithelial lumen during tissue morphogenesis. Multiple mechanisms of apical lumen formation have been proposed. Some epithelial lumens form from preexisting polarized epithelial structures. However, de novo lumen formation from nonpolarized cells has recently emerged as an important driver of epithelial tissue morphogenesis, especially during the formation of small epithelial tubule networks. In this review, we discuss the latest findings regarding the mechanisms and regulation of de novo lumen formation in vitro and in vivo.