The transmembrane receptor Uncoordinated5 (Unc5) is essential for heart lumen formation in Drosophila melanogaster

Transport of liquids or gases in biological tubes is fundamental for many physiological processes. Our knowledge on how tubular organs are formed during organogenesis and tissue remodeling has increased dramatically during the last decade. Studies on different animal systems have helped to unravel some of the molecular mechanisms underlying tubulogenesis. Tube architecture varies dramatically in different organs and different species, ranging from tubes formed by several cells constituting the cross section, tubes formed by single cells wrapping an internal luminal space or tubes that are formed within a cell. Some tubes display branching whereas others remain linear without intersections. The modes of shaping, growing and pre-patterning a tube are also different and it is still not known whether these diverse architectures and modes of differentiation are realized by sharing common signaling pathways or regulatory networks. However, several recent investigations provide evidence for the attractive hypothesis that the Drosophila cardiogenesis and heart tube formation shares many similarities with primary angiogenesis in vertebrates. Additionally, another important step to unravel the complex system of lumen formation has been the outcome of recent studies that junctional proteins, matrix components as well as proteins acting as attractant and repellent cues play a role in the formation of the Drosophila heart lumen. In this study we show the requirement for the repulsively active Unc5 transmembrane receptor to facilitate tubulogenesis in the dorsal vessel of Drosophila. Unc5 is localized in the luminal membrane compartment of cardiomyocytes and animals lacking Unc5 fail to form a heart lumen. Our findings support the idea that Unc5 is crucial for lumen formation and thereby represents a repulsive cue acting during Drosophila heart tube formation.     

Lipocalin signaling controls unicellular tube development in the Caenorhabditis elegans excretory system

Unicellular tubes or capillaries composed of individual cells with a hollow lumen perform important physiological functions including fluid or gas transport and exchange. These tubes are thought to build intracellular lumina by polarized trafficking of apical membrane components, but the molecular signals that promote luminal growth and luminal connectivity between cells are poorly understood. Here we show that the lipocalin LPR-1 is required for luminal connectivity between two unicellular tubes in the Caenorhabditis elegans excretory (renal) system, the excretory duct cell and pore cell. Lipocalins are a large family of secreted proteins that transport lipophilic cargos and participate in intercellular signaling. lpr-1 is required at a time of rapid luminal growth, it is expressed by the duct, pore and surrounding cells, and it can function cell non-autonomously. These results reveal a novel signaling mechanism that controls unicellular tube formation, and provide a genetic model system for dissecting lipocalin signaling pathways.     

Mass transfer to fluids flowing through rotating nonaligned straight tubes

Relatively inefficient heat/mass transfer is characteristic of tubular devices if the Reynolds number is low. One method of improving the heat/mass transfer efficiency of such devices is by inducing transverse laminar secondary circulations that are superimposed on the primary flow field; the resulting transverse velocity components lead to fluid mixing and hence augmented mass transfer in the tube lumen. The present work is a theoretical and experimental investigation of the enhanced transport in rotating, nonaligned, straight tubes, a method of transport enhancement that utilizes Coriolis acceleration to create transverse fluid mixing. This technique couples the transport advantages of coiled tubes with the design advantages of straight tubes. The overall mass balance equation is numerically solved for transfer into fluids flowing steadily through rotating nonaligned straight tubes. This solution, for small Coriolis disturbances, incorporates a third order perturbation solution for the primary and secondary flow fields. For sufficiently small Coriolis disturbances the bulk concentration increase is found to be uniquely determined by the value of a single similarity parameter. As the Coriolis disturbance is increased, however, two additional parameters are required to accurately characterize the mass transfer. In general, increasing the Coriolis accelerations results in an increase in mass transfer. There are solution regimens, however, in which increasing this acceleration can lead to a decrease in mass transfer efficiency. This interesting phenomena, which has important design implications, appears to result from velocity-weighting effects on the exiting sample. Experiments, involving the measurement of oxygen transferred into water and blood, produced data that agree with the theoretical predictions.     

Synaptotagmin-like proteins control the formation of a single apical membrane domain in epithelial cells

The formation of epithelial tissues requires both the generation of apical-basal polarity and the coordination of this polarity between neighbouring cells to form a central lumen. During de novo lumen formation, vectorial membrane transport contributes to the formation of a singular apical membrane, resulting in the contribution of each cell to only a single lumen. Here, from a functional screen for genes required for three-dimensional epithelial architecture, we identify key roles for synaptotagmin-like proteins 2-a and 4-a (Slp2-a/4-a) in the generation of a single apical surface per cell. Slp2-a localizes to the luminal membrane in a PtdIns(4,5)P(2)-dependent manner, where it targets Rab27-loaded vesicles to initiate a single lumen. Vesicle tethering and fusion is controlled by Slp4-a, in conjunction with Rab27/Rab3/Rab8 and the SNARE syntaxin-3. Together, Slp2-a/4-a coordinate the spatiotemporal organization of vectorial apical transport to ensure that only a single apical surface, and thus the formation of a single lumen, occurs per cell.     

FIP5 phosphorylation during mitosis regulates apical trafficking and lumenogenesis

Apical lumen formation is a key step during epithelial morphogenesis. The establishment of the apical lumen is a complex process that involves coordinated changes in plasma membrane composition, endocytic transport, and cytoskeleton organization. These changes are accomplished, at least in part, by the targeting and fusion of Rab11/FIP5‐containing apical endosomes with the apical membrane initiation site (AMIS). Although AMIS formation and polarized transport of Rab11/FIP5‐containing endosomes are crucial for the formation of a single apical lumen, the spatiotemporal regulation of this process remains poorly understood. Here, we demonstrate that the formation of the midbody during cytokinesis is a symmetry‐breaking event that establishes the location of the AMIS. The interaction of FIP5 with SNX18, which is required for the formation of apical endocytic carriers, is inhibited by GSK‐3 phosphorylation at FIP5‐T276. Importantly, we show that FIP5‐T276 phosphorylation occurs specifically during metaphase and anaphase, to ensure the fidelity and timing of FIP5‐endosome targeting to the AMIS during apical lumen formation.     

Enterocyte chloride and water secretion into the small intestine after enterotoxin challenge: Unifying hypothesis or intellectual dead end?

Many forms of diarrhoeal disease, particularly so called “secretory” diarrhoeal disease are thought to arise by the active secretion of chloride ion from the enterocytes, creating an osmotic gradient for fluid movement into the small intestinal lumen. This model implies that normally occurring intestinal secretion is catastrophically enhanced by bacterial enterotoxins. This review advocates that neither normal nor abnormal intestinal secretion from the enterocytes occurs and that no competent proof for chloride secretion exists. Prior to 1970, the physiological evidence failed to support the concept of the formation of intestinal juice as a normal intestinal event. support the concept of the formation of intestinal juice as a normal intestinal event. The concept was later revived to explain the high rate of fluid entry into the lumen after exposure to cholera toxin. Much evidence has been advanced for the chloride secretion hypothesis, the dominant secretory paradigm after 1974, but is the evidence sufficiently compelling for it to be regarded as proving the chloride secretory model? The evidence falls into four categories and a fifth conjectural argument that proposes that an abnormal chloride ion channel in cystic fibrotic sufferers confers a natural selective advantage by preventing diarrhoeal disease. Secretion is putatively demonstrated by 1) showing that mass transfer of fluid is into the lumen (secretion) and not merely a failure to transport out of the lumen (failed absorption). Support is offered by 2) chloride ion flux measurementsin vitro in Ussing chambers and by 3) shortcircuit current measurements that are consistent with and purport to show chloride ion movement into the lumen. In addition, 4) pharmacological agents are identified that affect short-circuit current and these are assumed to be anti-secretory, consistent with the biochemical mechanism for secretion, confirmed wherever possible by mouse knock-out models. Finally, the proxy methods used to study water movement such as elevated short-circuit current measurements show these to be absent in cystic fibrotic patients. The enterocyte secretion hypothesis is challenged here on the basis of an examination of the methods used to show secretion, particularly after exposing the small intestine to heat stable enterotoxin (STa) fromE. coli. STa is thought to be secretory because fluid entry into the lumen is claimed, enhanced isotopic flux of chloride ion towards the lumen occurs, an increase in short-circuit current is found, preventable by various drugs that are deemed likely to be anti-secretory and also because the short-circuit current changes after STa are not seen in cystic fibrotic patients. Using volume recoveryin vivo, STa is found not to be secretory but only anti-absorptive. Hence, other techniques used to show secretion are not fit for that purpose. If STa is identified as secretory and yet no secretion occurs, how reliable is the evidence for other toxins being secretory when these methods are used? This review concludes that chloride ion secretion is unproven. A review of the literature indicates that secretion occurs not because epithelial cells actively pump water but by interdiction of fluid absorption, increased conductivity through tight junctions and an increased hydrostatic driving force through elevated capillary pressure. The exclusive focus on chloride secretion may explain the failure to develop antisecretory drugs over the last three decades.     

Distinct cellular mechanisms of blood vessel fusion in the zebrafish embryo

Although many of the cellular and molecular mechanisms of angiogenesis have been intensely studied [1], little is known about the processes that underlie vascular anastomosis. We have generated transgenic fish lines expressing an EGFP-tagged version of the junctional protein zona occludens 1 (ZO1) to visualize individual cell behaviors that occur during vessel fusion and lumen formation in vivo. These life observations show that endothelial cells (ECs) use two distinct morphogenetic mechanisms, cell membrane invagination and cord hollowing to generate different types of vascular tubes. During initial steps of anastomosis, cell junctions that have formed at the initial site of cell contacts expand into rings, generating a cellular interface of apical membrane compartments, as defined by the localization of the apical marker podocalyxin-2 (Pdxl2). During the cord hollowing process, these apical membrane compartments are brought together via cell rearrangements and extensive junctional remodeling, resulting in lumen coalescence and formation of a multicellular tube. Vessel fusion by membrane invagination occurs adjacent to a preexisting lumen in a proximal to distal direction and is blood-flow dependent. Here, the invaginating inner cell membrane undergoes concomitant apicobasal polarization and the vascular lumen is formed by the extension of a transcellular lumen through the EC, which forms a unicellular or seamless tube.     

Phloem ultrastructure and pressure flow: Sieve-Element-Occlusion-Related agglomerations do not affect translocation

Since the first ultrastructural investigations of sieve tubes in the early 1960s, their structure has been a matter of debate. Because sieve tube structure defines frictional interactions in the tube system, the presence of P protein obstructions shown in many transmission electron micrographs led to a discussion about the mode of phloem transport. At present, it is generally agreed that P protein agglomerations are preparation artifacts due to injury, the lumen of sieve tubes is free of obstructions, and phloem flow is driven by an osmotically generated pressure differential according to Münch's classical hypothesis. Here, we show that the phloem contains a distinctive network of protein filaments. Stable transgenic lines expressing Arabidopsis thaliana Sieve-Element-Occlusion-Related1 (SEOR1)-yellow fluorescent protein fusions show that At SEOR1 meshworks at the margins and clots in the lumen are a general feature of living sieve tubes. Live imaging of phloem flow and flow velocity measurements in individual tubes indicate that At SEOR1 agglomerations do not markedly affect or alter flow. A transmission electron microscopy preparation protocol has been generated showing sieve tube ultrastructure of unprecedented quality. A reconstruction of sieve tube ultrastructure served as basis for tube resistance calculations. The impact of agglomerations on phloem flow is discussed.     

Inorganic and organic anion transport by insect renal epithelia

Insect renal organs typically exhibit high rates of transport of inorganic and organic anions, and therefore provide useful models for the study of epithelial anion transport and its control. Isolated Malpighian tubules of some species secrete a volume of iso-osmotic fluid equal to their own volume in 10-15 s, which means that cellular Cl(-) content is exchanged every 3-5 s. Anion transport can also be achieved against extreme thermodynamic gradients. The concentration of K(+) and Cl(-) in the lumen of the Malpighian tubules of some desert beetles approaches or exceeds saturation. A basolateral Na(+):K(+):2Cl(-) cotransporter plays an important role in vectorial ion transport in Malpighian tubules of many species, but there is also evidence for coupling of Cl(-) transport to the movement of a single cationic species (Na(+) or K(+)). Although an apical vacuolar H(+)-ATPase plays a primary role in energizing transepithelial secretion of chloride via channels or cotransporters in the secretory segment of the Malpighian tubule, several different ATPases have been implicated in reabsorption of Cl(-) by the lower Malpighian tubule or hindgut. Chloride transport is known to be controlled by several neuropeptides, amines and intracellular second messengers. Insect renal epithelia are also important in excretion of potentially toxic organic anions, and the transporters involved may play a role in resistance to insecticides of natural or anthropogenic origin.     

Dpp/BMP transport mechanism is required for wing venation in the sawfly Athalia rosae

The pattern of wing venation varies considerably among different groups of insects and has been used as a means of species-specific identification. However, little is known about how wing venation is established and diversified among insects. The decapentaplegic (Dpp)/bone morphogenetic protein (BMP) signaling pathway plays a critical role in wing vein formation during the pupal stages in Drosophila melanogaster. A key mechanism is BMP transport from the longitudinal veins (LVs) to the posterior crossvein (PCV) by the BMP-binding proteins, short gastrulation (Sog) and twisted gastrulation2/crossveinless (Tsg2/Cv). To investigate whether the BMP transport mechanism is utilized to specify insect wing vein patterns in other than Drosophila, we used the sawfly Athalia rosae as a model, which has distinct venation patterns in the fore- and hindwings. Here, we show that Ar-dpp is ubiquitously expressed in both the fore- and hindwings, but is required for localized BMP signaling that reflects distinct wing vein patterns between the fore- and hindwings. By isolating Ar-tsg/cv in the sawfly, we found that Ar-Tsg/Cv is also required for BMP signaling in wing vein formation and retains the ability to transport Dpp. These data suggest that the BMP transport system is widely used to redistribute Dpp to specify wing venation and may be a basal mechanism underlying diversified wing vein patterns among insects.     

Identification of a Ca2+/H+ antiport in the plant chloroplast thylakoid membrane

To assess the availability of Ca2+ in the lumen of the thylakoid membrane that is required to support the assembly of the oxygen-evolving complex of photosystem II, we have investigated the mechanism of 45Ca2+ transport into the lumen of pea (Pisum sativum) thylakoid membranes using silicone-oil centrifugation. Trans-thylakoid Ca2+ transport is dependent on light or, in the dark, on exogenously added ATP. Both light and ATP hydrolysis are coupled to Ca2+ transport through the formation of a transthylakoid pH gradient. The H+-transporting ionophores nigericin/K+ and carbonyl cyanide 3-chlorophenylhydrazone inhibit the transport of Ca2+. Thylakoid membranes are capable of accumulating up to 30 nmol Ca2+ mg-1 chlorophyll from external concentrations of 15 μM over the course of a 15-min reaction. These results are consistent with the presence of an active Ca2+/H+ antiport in the thylakoid membrane. Ca2+ transport across the thylakoid membrane has significant implications for chloroplast and plant Ca2+ homeostasis. We propose a model of chloroplast Ca2+ regulation whereby the activity of the Ca2+/H+ antiporter facilitates the light-dependent uptake of Ca2+ by chloroplasts and reduces stromal Ca2+ levels.     

Tube morphogenesis: making and shaping biological tubes

Many organs are composed of epithelial tubes that transport vital fluids. Such tubular organs develop in many different ways and generate tubes of widely varying sizes and structures, but always with the apical epithelial surface lining the lumen. We describe recent progress in several diverse cell culture and genetic models of tube morphogenesis, which suggest apical membrane biogenesis, vesicle fusion, and secretion play central roles in tube formation and growth. We propose a unifying mechanism of tube morphogenesis that has been modified to create tube diversity and describe how defects in the tube size-sensing step can lead to polycystic kidney disease.     

Interaction between FIP5 and SNX18 regulates epithelial lumen formation

During the morphogenesis of the epithelial lumen, apical proteins are thought to be transported via endocytic compartments to the site of the forming lumen, although the machinery mediating this transport remains to be elucidated. Rab11 GTPase and its binding protein, FIP5, are important regulators of polarized endocytic transport. In this study, we identify sorting nexin 18 as a novel FIP5-interacting protein and characterize the role of FIP5 and SNX18 in epithelial lumen morphogenesis. We show that FIP5 mediates the transport of apical proteins from apical endosomes to the apical plasma membrane and, along with SNX18, is required for the early stages of apical lumen formation. Furthermore, both proteins bind lipids, and FIP5 promotes the capacity of SNX18 to tubulate membranes, which implies a role for FIP5 and SNX18 in endocytic carrier formation and/or scission. In summary, the present findings support the hypothesis that this FIP5-SNX18 complex plays a pivotal role in the polarized transport of apical proteins during apical lumen initiation in epithelial cells.     

Morphology,fine structure,and functional aspects of the labial gland reservoirs of the subterranean termite Reticulitermes santonensis de Feytaud (Isoptera: Rhinotermitidae)

The morphology and fine structure of the labial gland reservoirs in the subterranean termite Reticulitermes santonensis (Isoptera: Rhinotermitidae) was studied by light and transmission electron microscopy. The reservoir wall consists of a single epithelial cell layer and a cuticular intima. The reservoir ducts are formed by a flat epithelial matrix with cuticular ridges lining the duct lumen. Measurements of the ionic concentrations of reservoir fluids and haemolymph show that the osmolality of reservoir fluid ranges from 7 to 28 mosmol kg⁻¹; the haemolymph osmotic pressure was 201 ± 31 mosmol kg⁻. The reservoir lumen is effectively separated from the haemolymph compartment; a net water flow through the reservoir wall could not be induced in physiological experiments. Moreover, typical epithelial structures associated with a fluid transport against an osmotic gradient are lacking. Thus, our fine structural and physiological data support the view that a water transfer from the haemolymph through the reservoir wall into the reservoir lumen does not occur.     

The Na+/K+ ATPase is required for septate junction function and epithelial tube-size control in the Drosophila tracheal system

Although the correct architecture of epithelial tubes is crucial for the function of organs such as the lung, kidney and vascular system, little is known about the molecular mechanisms that control tube size. We show that mutations in the ATPalpha alpha and nrv2 beta subunits of the Na+/K+ ATPase cause Drosophila tracheal tubes to have increased lengths and expanded diameters. ATPalpha and nrv2 mutations also disrupt stable formation of septate junctions, structures with some functional and molecular similarities to vertebrate tight junctions. The Nrv2 beta subunit isoforms have unique tube size and junctional functions because Nrv2, but not other Drosophila Na+/K+ ATPase beta subunits, can rescue nrv2 mutant phenotypes. Mutations in known septate junctions genes cause the same tracheal tube-size defects as ATPalpha and nrv2 mutations, indicating that septate junctions have a previously unidentified role in epithelial tube-size control. Double mutant analyses suggest that tube-size control by septate junctions is mediated by at least two discernable pathways, although the paracellular diffusion barrier function does not appear to involved because tube-size control and diffusion barrier function are genetically separable. Together, our results demonstrate that specific isoforms of the Na+/K+ ATPase play a crucial role in septate junction function and that septate junctions have multiple distinct functions that regulate paracellular transport and epithelial tube size.     

Carbonic anhydrase in the gastrointestinal mucus of mammals--possible protective role against carbon dioxide

We show here that luminal mucus from the colon and the stomach of guinea pigs, mice and humans exhibits substantial carbonic anhydrase (CA) activity, by which the velocity of the CO(2) hydration reaction is accelerated 1000-2000-fold, approximately 1/10 of what is found in the red cell. Although this CA shares several properties with CA II, studies with CA II-deficient mice show that gastrointestinal mucus CA is not affected in these animals and thus does not appear to be CA II. We speculate that the mucus layer covering the luminal surface of gastrointestinal epithelium can, due to the presence of CA, maintain a normal tissue pCO(2) in the epithelium, even when the pCO(2) values in the lumen are much higher, as is known for stomach and colon. To test this hypothesis, we have developed a mathematical model which describes (a) diffusion of CO(2) and HCO(3)(-) across the mucus layer and (b) H(+) transport mediated by continuous secretion of mucus, which due to its high H(+) buffer capacity transports H(+) by convection towards the lumen. The model predicts that continuous transport of the reaction products of CO(2) towards the lumen, by diffusion and convection, protects the epithelium against high CO(2) partial pressures in the lumen.     

C. elegans daf-6 encodes a patched-related protein required for lumen formation

Sensory organs are often composed of neuronal sensory endings accommodated in a lumen formed by ensheathing epithelia or glia. Here we show that lumen formation in the C. elegans amphid sensory organ requires the gene daf-6. daf-6 encodes a Patched-related protein that localizes to the luminal surfaces of the amphid channel and other C. elegans tubes. While daf-6 mutants display only amphid lumen defects, animals defective for both daf-6 and the Dispatched gene che-14 exhibit defects in all tubular structures that express daf-6. Furthermore, DAF-6 protein is mislocalized, and lumen morphogenesis is abnormal, in mutants with defective sensory neuron endings. We propose that amphid lumen morphogenesis is coordinated by neuron-derived cues and a DAF-6/CHE-14 system that regulates vesicle dynamics during tubulogenesis.     

Hemoglobin‐based oxygen carrier and convection enhanced oxygen transport in a hollow fiber bioreactor

A mathematical model was developed to study O₂ transport in a convection enhanced hepatic hollow fiber (HF) bioreactor, with hemoglobin‐based O₂ carriers (HBOCs) present in the flowing cell culture media stream of the HF lumen. In this study, four HBOCs were evaluated: PEG‐conjugated human hemoglobin (MP4), human hemoglobin (hHb), bovine hemoglobin (BvHb) and polymerized bovine hemoglobin (PolyBvHb). In addition, two types of convective flow in the HF extra capillary space (ECS) were considered in this study. Starling flow naturally occurs when both of the ECS ports are closed. If one of the ECS ports is open, forced convective flow through the ECS will occur due to the imposed pressure difference between the lumen and ECS. This type of flow is referred to as cross‐flow in this work, since some of the fluid entering the HF lumen will pass across the HF membrane and exit via the open ECS port. In this work, we can predict the dissolved O₂ concentration profile as well as the O₂ transport flux in an individual HF of the bioreactor by solving the coupled momentum and mass transport equations. Our results show that supplementation of the cell culture media with HBOCs can dramatically enhance O₂ transport to the ECS (containing hepatocytes) and lead to the formation of an in vivo‐like O₂ spectrum for the optimal culture of hepatocytes. However, both Starling flow and cross‐flow have a very limited effect on O₂ transport in the ECS. Taken together, this work represents a novel predictive tool that can be used to design or analyze HF bioreactors that expose cultured cells to defined overall concentrations and gradients of O₂. Biotechnol. Bioeng. 2009;102: 1603–1612. © 2008 Wiley Periodicals, Inc.     

Lumen morphogenesis in C. elegans requires the membrane-cytoskeleton linker erm-1

Epithelial tubes are basic building blocks of complex organs, but their architectural requirements are not well understood. Here we show that erm-1 is a unique C. elegans ortholog of the ERM family of cytoskeleton-membrane linkers, with an essential role in lumen morphogenesis. ERM-1 localizes to the luminal membranes of those tubular organ epithelia which lack stabilization by cuticle. RNA interference (RNAi), a germline deletion, and overexpression of erm-1 cause cystic luminal phenotypes in these epithelia. Confocal and ultrastructural analyses indicate that erm-1 functions directly in apical membrane morphogenesis, rather than in epithelial polarity and junction assembly as has been previously proposed for ERMs. We also show that act-5/cytoplasmic actin and sma-1/beta-H-spectrin are required for lumen formation and functionally interact with erm-1. Our findings suggest that there are common structural constraints on the architecture of diverse organ lumina.     

Tetraethylammonium and nicotine transport by the Malpighian tubules of insects

We examined transepithelial transport of the prototypical type I organic cation (OC) tetraethylammonium (TEA) and the plant alkaloid nicotine by the isolated Malpighian tubules (MTs) of nine insect species from six orders. Isolated tubules were exposed to radiolabelled forms of either TEA or nicotine in the bathing (basal) fluid. Luminal (apical) secreted fluid was collected and TEA or nicotine concentration was determined. Active net transport of nicotine from bath to lumen was observed by the MTs of all the insects studied. TEA was also transported from bath to lumen in MTs of all species except Rhodnius prolixus and Aedes aegypti. MTs of both of these blood feeders did not show active transport of TEA under normal physiological conditions. Transport of TEA but not nicotine increased during the moult in the MTs of Rhodnius, but the concentrations of TEA in the secreted fluid were still consistent with passive accumulation in response to the lumen-negative transepithelial potential. Nicotine transport by Rhodnius MTs was inhibited by the type II OC quinidine, a known p-glycoprotein inhibitor, but not by the type I OCs N-methylnicotinamide or cimetidine. Taken together, the results suggest that active transport of OCs by the MTs is common among species from different orders and that transepithelial TEA and nicotine transport occur through separate pathways.