The role of the ELAV homologue EXC-7 in the development of the Caenorhabditis elegans excretory canals

The exc mutations of Caenorhabditis elegans alter the position and shape of the apical cytoskeleton in polarized epithelial cells. Mutants in exc-7 form small cysts throughout the tubular excretory canals that regulate organismal osmolarity. We have cloned the exc-7 gene, the closest nematode homologue to the neural RNA-binding protein ELAV. EXC-7 is expressed in the canal for a short time midway through embryogenesis. Cysts in exc-7 mutants do not develop until several hours later, beginning at the time of hatching. We find that the first larval period is when the canal completes the majority of its outgrowth, and adds new apical cytoskeleton at a rapid rate. Ultrastructural studies show that exc-7 mutant defects resemble loss of beta(H)-spectrin (encoded by sma-1) at the distal ends of the excretory canals. In addition, exc-7 mutants exhibit synergistic excretory canal defects with mutations in sma-1, and EXC-7 binds sma-1 mRNA. These data imply that EXC-7 protein may affect expression of sma-1 and other genes to effect proper development of the excretory canals.     

A putative GDP–GTP exchange factor is required for development of the excretory cell in Caenorhabditis elegans

The Caenorhabditis elegans excretory cell extends tubular processes, called canals, along the basolateral surface of the epidermis. Mutations in the exc-5 gene cause tubulocystic defects in this canal. Ultrastructural analysis suggests that exc-5 is required for the proper placement of cytoskeletal elements at the apical epithelial surface. exc-5 encodes a protein homologous to guanine nucleotide exchange factors and contains motif architecture similar to that of FGD1, which is responsible for faciogenital dysplasia. exc-5 interacts genetically with mig-2, which encodes Rho GTPase. These results suggest that EXC-5 controls the structural organization of the excretory canal by regulating Rho family GTPase activities.     

The Caenorhabditis elegans HNF4α Homolog, NHR-31, Mediates Excretory Tube Growth and Function through Coordinate Regulation of the Vacuolar ATPase

Nuclear receptors of the Hepatocyte Nuclear Factor-4 (HNF4) subtype have been linked to a host of developmental and metabolic functions in animals ranging from worms to humans; however, the full spectrum of physiological activities carried out by this nuclear receptor subfamily is far from established. We have found that the Caenorhabditis elegans nuclear receptor NHR-31, a homolog of mammalian HNF4 receptors, is required for controlling the growth and function of the nematode excretory cell, a multi-branched tubular cell that acts as the C. elegans renal system. Larval specific RNAi knockdown of nhr-31 led to significant structural abnormalities along the length of the excretory cell canal, including numerous regions of uncontrolled growth at sites near to and distant from the cell nucleus. nhr-31 RNAi animals were sensitive to acute challenge with ionic stress, implying that the osmoregulatory function of the excretory cell was also compromised. Gene expression profiling revealed a surprisingly specific role for nhr-31 in the control of multiple genes that encode subunits of the vacuolar ATPase (vATPase). RNAi of these vATPase genes resulted in excretory cell defects similar to those observed in nhr-31 RNAi animals, demonstrating that the influence of nhr-31 on excretory cell growth is mediated, at least in part, through coordinate regulation of the vATPase. Sequence analysis revealed a stunning enrichment of HNF4α type binding sites in the promoters of both C. elegans and mouse vATPase genes, arguing that coordinate regulation of the vATPase by HNF4 receptors is likely to be conserved in mammals. Our study establishes a new pathway for regulation of excretory cell growth and reveals a novel role for HNF4-type nuclear receptors in the development and function of a renal system.     

The Caenorhabditis elegans HNF4α Homolog,NHR-31, Mediates Excretory Tube Growth and Function through Coordinate Regulation of the Vacuolar ATPase

Nuclear receptors of the Hepatocyte Nuclear Factor-4 (HNF4) subtype have been linked to a host of developmental and metabolic functions in animals ranging from worms to humans; however, the full spectrum of physiological activities carried out by this nuclear receptor subfamily is far from established. We have found that the Caenorhabditis elegans nuclear receptor NHR-31, a homolog of mammalian HNF4 receptors, is required for controlling the growth and function of the nematode excretory cell, a multi-branched tubular cell that acts as the C. elegans renal system. Larval specific RNAi knockdown of nhr-31 led to significant structural abnormalities along the length of the excretory cell canal, including numerous regions of uncontrolled growth at sites near to and distant from the cell nucleus. nhr-31 RNAi animals were sensitive to acute challenge with ionic stress, implying that the osmoregulatory function of the excretory cell was also compromised. Gene expression profiling revealed a surprisingly specific role for nhr-31 in the control of multiple genes that encode subunits of the vacuolar ATPase (vATPase). RNAi of these vATPase genes resulted in excretory cell defects similar to those observed in nhr-31 RNAi animals, demonstrating that the influence of nhr-31 on excretory cell growth is mediated, at least in part, through coordinate regulation of the vATPase. Sequence analysis revealed a stunning enrichment of HNF4α type binding sites in the promoters of both C. elegans and mouse vATPase genes, arguing that coordinate regulation of the vATPase by HNF4 receptors is likely to be conserved in mammals. Our study establishes a new pathway for regulation of excretory cell growth and reveals a novel role for HNF4-type nuclear receptors in the development and function of a renal system.     

oko meduzy and related crumbs genes are determinants of apical cell features in the vertebrate embryo

BACKGROUND: Polarity is an essential attribute of most eukaryotic cells. One of the most prominent features of cell polarity in many tissues is the subdivision of cell membrane into apical and basolateral compartments by a belt of cell junctions. The proper formation of this subdivision is of key importance. In sensory cells, for example, the apical membrane compartment differentiates specialized structures responsible for the detection of visual, auditory, and olfactory stimuli. In other tissues, apical specializations are responsible for the propagation of fluid flow. Despite its importance, the role of genetic determinants of apico-basal polarity in vertebrate embryogenesis remains poorly investigated. RESULTS: We show that zebrafish oko meduzy (ome) locus encodes a crumbs gene homolog, essential for the proper apico-basal polarity of neural tube epithelia. Two ome paralogs, crb2b and crb3a, promote the formation of apical cell features: photoreceptor inner segments and cilia in renal and auditory systems. The motility of cilia is defective following the impairment of crb2b function. Apical surface defects in ome- and crb2b-deficient animals are associated with profound disorganization of neuronal architecture and with the formation of pronephric cysts, respectively. Unexpectedly, despite differences in their structure and expression patterns, crumbs genes are, at least partially, functionally interchangeable. CONCLUSIONS: ome and related crumbs genes are necessary for the formation of gross morphological features in several organs, including the CNS and the renal system. On the cellular level, crumbs genes regulate the formation of both ciliary and nonciliary apical membrane compartment.     

Cell Surface Rescue of Kidney Anion Exchanger 1 Mutants by Disruption of Chaperone Interactions

Mutations in the human kidney anion exchanger 1 (kAE1) membrane glycoprotein cause impaired urine acidification resulting in distal renal tubular acidosis (dRTA). Dominant and recessive dRTA kAE1 mutants exhibit distinct trafficking defects with retention in the endoplasmic reticulum (ER), Golgi, or mislocalization to the apical membrane in polarized epithelial cells. We examined the interaction of kAE1 with the quality control system responsible for the folding of membrane glycoproteins and the retention and degradation of misfolded mutants. Using small molecule inhibitors to disrupt chaperone interactions, two functional, dominant kAE1 mutants (R589H and R901stop), retained in the ER and targeted to the proteasome for degradation by ubiquitination, were rescued to the basolateral membrane of Madin-Darby canine kidney cells. In contrast, the Golgi-localized, recessive G701D and the severely misfolded, ER-retained dominant Southeast Asian ovalocytosis (SAO) mutants were not rescued. These results show that functional dRTA mutants are retained in the ER due to their interaction with molecular chaperones, particularly calnexin, and that disruption of these interactions can promote their escape from the ER and cell surface rescue.     

The FGD homologue EXC-5 regulates apical trafficking in C. elegans tubules

Maintenance of the shape of biological tubules is critical for development and physiology of metazoan organisms. Loss of function of the Caenorhabditis elegans FGD protein EXC-5 allows large fluid-filled cysts to form in the lumen of the single-cell excretory canal tubules, while overexpression of exc-5 causes defects at the tubule's basolateral surface. We have examined the effects of altering expression levels of exc-5 on the distribution of fluorescently-marked subcellular organelles. In exc-5 mutants, early endosomes build up in the cell, especially in areas close to cysts, while recycling endosomes are depleted. Endosome morphology changes prior to cyst formation. Conversely, when exc-5 is overexpressed, recycling endosomes are enriched. Since FGD proteins activate the small GTPases CDC42 and Rac, these results support the hypothesis that EXC-5 acts through small GTPases to move material from apical early endosomes to recycling endosomes, and that loss of such movement is likely the cause of tubule deformation both in nematodes and in tissues affected by FGD dysfunction such as Charcot–Marie–Tooth Syndrome type 4H.     

Ultrastructure of excretory system in <Emphasis Type="Italic">Bothrioplana semperi</Emphasis> (Platyhelminthes,Turbellaria)

The ultrastructure of flame bulbs and epithelium of excretory canals in Bothrioplana semperi (Turbellaria, Seriata) have been studied. The flame bulbs consist of two cells, the terminal cell and the proximal canal cell. The weir is formed by two rows of longitudinal ribs. The ribs of the internal row originate from the flame cell, external ribs are formed by the proximal canal cell. Each external rib has a remarkable bundle of microfilaments, originating in the cytoplasm of the first canal cell distally to the bases of external ribs. Membrane of internal ribs is marked by small electrondense granules, separate or fused to an electron-dense layer, continuous to dense “membrane,” connecting both external and internal ribs. Sparse internal leptotrichs originate from the bottom of the flame bulb cavity. External leptotrichs are lacking. Septate junction is present only in proximal canal cell at the level of tips of cilia. The apical surface of the canal cell bears rare short microvilli. The basal membrane of canal cells forms long invaginations that may reach nearly the apical membrane. The epithelium of excretory canals lacks the cilia. The ultrastructure of flame bulbs and epithelium of the excretory canals in B. semperi shares representatives of suborder Proseriata (Seriata). The contradiction exists in interpretation of the structure of flame bulbs in Proseriata. Ehlers and Sopott-Ehlers assumed that the external ribs are derivatives of the proximal canal cell and internal ones are outgrowths of the terminal cell, while Rohde has found conversely: the external ribs are outgrowths of the terminal cell, the internal ones are outgrowths of the proximal canal cell. However, the illustrations provided by Rohde do not enable to ascertain what cells the internal and external ribs derive from, while illustrations provided by Ehlers justify his interpretation. The order of weir formation in B. semperi confirms the viewpoint of Ehlers. The implication of ultrastructure of flame bulbs in Proseriata, especially of the order of flame bulb formation, in the Platyhelminthes phylogeny has been discussed.     

Tubes and the single C. elegans excretory cell

The formation and regulation of tubule shape and size is fundamental to the development and function of many tissues and organs in metazoan organisms. The excretory canals of the nematode Caenorhabditis elegans are a fascinating example of cell morphogenesis, as the tiny worm manages to create a complicated set of tubular epithelia within a single cell. In addition to the inherent attraction of studying this cytoengineering feat, the excretory cell provides a simple genetically tractable model for studying tubule formation and regulation of tubule diameter. Mutations in the exc genes alter the diameter of the lumenal surface of these tubules. Cloning of these genes reveals a set of proteins that both control tubule diameter and regulate the comparative growth of the apical and basal tubular surfaces.     

Glycosyl Phosphatidylinositol Anchor Biosynthesis Is Essential for Maintaining Epithelial Integrity during Caenorhabditis elegans Embryogenesis

Glycosylphosphatidylinositol (GPI) is a post-translational modification resulting in the attachment of modified proteins to the outer leaflet of the plasma membrane. Tissue culture experiments have shown GPI-anchored proteins (GPI-APs) to be targeted to the apical membrane of epithelial cells. However, the in vivo importance of this targeting has not been investigated since null mutations in GPI biosynthesis enzymes in mice result in very early embryonic lethality. Missense mutations in the human GPI biosynthesis enzyme pigv are associated with a multiple congenital malformation syndrome with a high frequency of Hirschsprung disease and renal anomalies. However, it is currently unknown how these phenotypes are linked to PIGV function. Here, we identify a temperature-sensitive hypomorphic allele of PIGV in Caenorhabditis elegans, pigv-1(qm34), enabling us to study the role of GPI-APs in development. At the restrictive temperature we found a 75% reduction in GPI-APs at the surface of embryonic cells. Consequently, ~80% of pigv-1(qm34) embryos arrested development during the elongation phase of morphogenesis, exhibiting internal cysts and/or surface ruptures. Closer examination of the defects revealed them all to be the result of breaches in epithelial tissues: cysts formed in the intestine and excretory canal, and ruptures occurred through epidermal cells, suggesting weakening of the epithelial membrane or membrane-cortex connection. Knockdown of piga-1, another GPI biosynthesis enzymes resulted in similar phenotypes. Importantly, fortifying the link between the apical membrane and actin cortex by overexpression of the ezrin/radixin/moesin ortholog ERM-1, significantly rescued cyst formation and ruptures in the pigv-1(qm34) mutant. In conclusion, we discovered GPI-APs play a critical role in maintaining the integrity of the epithelial tissues, allowing them to withstand the pressure and stresses of morphogenesis. Our findings may help to explain some of the phenotypes observed in human syndromes associated with pigv mutations.     

The cell junction protein VAB-9 regulates adhesion and epidermal morphology in C. elegans

Epithelial cell junctions are essential for cell polarity, adhesion and morphogenesis. We have analysed VAB-9, a cell junction protein in Caenorhabditis elegans. VAB-9 is a predicted four-pass integral membrane protein that has greatest similarity to BCMP1 (brain cell membrane protein 1, a member of the PMP22/EMP/Claudin family of cell junction proteins) and localizes to the adherens junction domain of C. elegans apical junctions. Here, we show that VAB-9 requires HMR-1/cadherin for localization to the cell membrane, and both HMP-1/alpha-catenin and HMP-2/beta-catenin for maintaining its distribution at the cell junction. In vab-9 mutants, morphological defects correlate with disorganization of F-actin at the adherens junction; however, localization of the cadherin-catenin complex and epithelial polarity is normal. These results suggest that VAB-9 regulates interactions between the cytoskeleton and the adherens junction downstream of or parallel to alpha-catenin and/or beta-catenin. Mutations in vab-9 enhance adhesion defects through functional loss of the cell junction genes apical junction molecule 1 (ajm-1) and discs large 1 (dlg-1), suggesting that VAB-9 is involved in cell adhesion. Thus, VAB-9 represents the first characterized tetraspan adherens junction protein in C. elegans and defines a new family of such proteins in higher eukaryotes.     

Apico-basal polarity in polycystic kidney disease epithelia

Epithelial cell polarity is essential for the establishment and maintenance of morphological and functional asymmetries that underlie normal renal structure and function and are brought about by the appropriate delivery of growth factor receptors and ion and fluid transporters and channels to apical or basolateral cell membranes. The fundamental process of cellular polarization is established early during development and is controlled by sets of evolutionarily conserved proteins that integrate intrinsic and extrinsic polarity cues. Specialized structural domains between adjacent cells and cells with their matrix, termed adherens junctions (AJ) and focal adhesions (FA), respectively, are formed that contain specific components of multi-molecular complexes acting as sites to recruit proteins and to activate intracellular mechano-transduction pathways. Regulation of these processes results in tight spatio-temporal control of renal tubule growth and lumen diameter. Abnormalities in macromolecular polarization complexes lead to a variety of diseases in different organs, a common example of which is Polycystic Kidney Disease (PKD), where epithelial cysts replace normal renal tubules. Membrane protein polarity defects in Autosomal Dominant (AD) PKD include the mis-polarization of normally basolateral membrane proteins to apical, lumenal membranes, such as epidermal growth factor (EGFR/ErbB) receptors and Na⁺K⁺-ATPase-α1 subunit; mis-polarization of normally apical membrane proteins to basolateral membranes, including the Na⁺K⁺2Cl⁻ (NKCC1) symporter; and the failure to traffic and insert proteins into membranes resulting in their intracellular accumulation, such as E-cadherin and the β1 subunit of Na⁺K⁺-ATPase. Abnormalities in structural AJ, FA and polarity complexes in ADPKD epithelia include loss of E-cadherin, and focal adhesion kinase (FAK), MALS-3, Crb and Dlg complexes as well as disruptions in Rab/sec and syntaxin trafficking and membrane docking pathways. Since proper polarization of epithelial cells lining renal tubules is essential for normal kidney development and differentiation to prevent abnormal cystic dilation, interventions to reverse polarity defects to normal would offer therapeutic opportunities for PKD. This article is part of a Special Issue entitled: Polycystic Kidney Disease.     

hyp loci control cell pattern formation in the vegetative mycelium of Aspergillus nidulans.

Aspergillus nidulans grows by apical extension of multinucleate cells called hyphae that are subdivided by the insertion of crosswalls called septa. Apical cells vary in length and number of nuclei, whereas subapical cells are typically 40 microm long with three to four nuclei. Apical cells have active mitotic cycles, whereas subapical cells are arrested for growth and mitosis until branch formation reinitiates tip growth and nuclear divisions. This multicellular growth pattern requires coordination between localized growth, nuclear division, and septation. We searched a temperature-sensitive mutant collection for strains with conditional defects in growth patterning and identified six mutants (designated hyp for hypercellular). The identified hyp mutations are nonlethal, recessive defects in five unlinked genes (hypA-hypE). Phenotypic analyses showed that these hyp mutants have aberrant patterns of septation and show defects in polarity establishment and tip growth, but they have normal nuclear division cycles and can complete the asexual growth cycle at restrictive temperature. Temperature shift analysis revealed that hypD and hypE play general roles in hyphal morphogenesis, since inactivation of these genes resulted in a general widening of apical and subapical cells. Interestingly, loss of hypA or hypB function lead to a cessation of apical cell growth but activated isotropic growth and mitosis in subapical cells. The inferred functions of hypA and hypB suggest a mechanism for coordinating apical growth, subapical cell arrest, and mitosis in A. nidulans.     

Mutations affecting nerve attachment of Caenorhabditis elegans

Using a pan-neuronal GFP marker, a morphological screen was performed to detect Caenorhabditis elegans larval lethal mutants with severely disorganized major nerve cords. We recovered and characterized 21 mutants that displayed displacement or detachment of the ventral nerve cord from the body wall (Ven: ventral cord abnormal). Six mutations defined three novel genetic loci: ven-1, ven-2, and ven-3. Fifteen mutations proved to be alleles of previously identified muscle attachment/positioning genes, mup-4, mua-1, mua-5, and mua-6. All the mutants also displayed muscle attachment/positioning defects characteristic of mua/mup mutants. The pan-neuronal GFP marker also revealed that mutants of other mua/mup loci, such as mup-1, mup-2, and mua-2, exhibited the Ven defect. The hypodermis, the excretory canal, and the gonad were morphologically abnormal in some of the mutants. The pleiotropic nature of the defects indicates that ven and mua/mup genes are required generally for the maintenance of attachment of tissues to the body wall in C. elegans.     

Distinct cell guidance pathways controlled by the Rac and Rho GEF domains of UNC-73/TRIO in Caenorhabditis elegans

The cytoskeleton regulator UNC-53/NAV2 is required for both the anterior and posterior outgrowth of several neurons as well as that of the excretory cell while the kinesin-like motor VAB-8 is essential for most posteriorly directed migrations in Caenorhabditis elegans. Null mutations in either unc-53 or vab-8 result in reduced posterior excretory canal outgrowth, while double null mutants display an enhanced canal extension defect, suggesting the genes act in separate pathways to control this posteriorly directed outgrowth. Genetic analysis of putative interactors of UNC-53 or VAB-8, and cell-specific rescue experiments suggest that VAB-8, SAX-3/ROBO, SLT-1/Slit, and EVA-1 are functioning together in the outgrowth of the excretory canals, while UNC-53 appears to function in a parallel pathway with UNC-71/ADAM. The known VAB-8 interactor, the Rac/Rho GEF UNC-73/TRIO operates in both pathways, as isoform specific alleles exhibit enhancement of the phenotype in double-mutant combination with either unc-53 or vab-8. On the basis of these results, we propose a bipartite model for UNC-73/TRIO activity in excretory canal extension: a cell autonomous function that is mediated by the Rho-specific GEF domain of the UNC-73E isoform in conjunction with UNC-53 and UNC-71 and a cell nonautonomous function that is mediated by the Rac-specific GEF domain of the UNC-73B isoform, through partnering with VAB-8 and the receptors SAX-3 and EVA-1.     

THE MECHANISM OF THE NEPHRIDIAL APPARATUS OF PARAMECIUM MUL TIMICRONUCLEATUM : I. Expulsion of Water from the Vesicle

Recent analysis of the mechanism of the nephridial apparatus of Paramecium multimicronucleatum by high-speed cinematography (300 fps at x 250) confirms the observations by electron microscopy (Schneider, 1960) that once the pore is opened, the vesicle is invaginated by adjacent cytoplasm and is emptied by collapsing under pressure from that cytoplasm, aided perhaps by pressure of the fibrils which anchor the ampullae to the excretory canal. There is no indication of active contraction of the vesicle or its membrane. There is no permanent pore to the vesicle. The vesicle is closed by a sealing of the ruptured membrane where it is in contact with the pellicular excretory canal. At onset of expulsion of vesicular fluid the membrane across the basal opening of the excretory canal is ripped along one semicircular portion of the excretory pore and is driven up against the opposite wall as a flap while the water rushes out. A constriction of the vesicular and cell membranes at the base of the excretory canal reseals the opening.     

Lysophosphatidic acid is a modulator of cyst growth in autosomal dominant polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is characterized by the slow growth of multiple fluid-filled cysts predominately in the kidney tubules and liver bile ducts. Elucidation of mechanisms that control cyst growth will provide the basis for rational therapeutic intervention. We used electrophysiological methods to identify lysophosphatidic acid (LPA) as a component of cyst fluid and serum that stimulates secretory Cl- transport in the epithelial cell type that lines renal cysts. LPA effects are manifested through receptors located on the basolateral membrane of the epithelial cells resulting in stimulation of channel activity in the apical membrane. Concentrations of LPA measured in human ADPKD cyst fluid and in normal serum are sufficient to maximally stimulate ion transport. Thus, cyst fluid seepage and/or leakage of vascular LPA into the interstitial space are capable of stimulating epithelial cell secretion resulting in cyst enlargement. These observations are particularly relevant to the rapid decline in renal function in late-stage disease and to the "third hit" hypothesis that renal injury exacerbates cyst growth.     

The cooperation of FGF receptor and Klotho is involved in excretory canal development and regulation of metabolic homeostasis in Caenorhabditis elegans

FGFs have traditionally been associated with cell proliferation, morphogenesis, and development; yet, a subfamily of FGFs (FGF19, -21, and -23) functions as hormones to regulate glucose, lipid, phosphate, and vitamin D metabolism with impact on energy balance and aging. In mammals, Klotho and beta-Klotho are type 1 transmembrane proteins that function as obligatory co-factors for endocrine FGFs to bind to their cognate FGF receptors (FGFRs). Mutations in Klotho/beta-Klotho or fgf19, -21, or -23 are associated with a number of human diseases, including autosomal dominant hypophosphatemic rickets, premature aging disorders, and diabetes. The Caenorhabditis elegans genome contains two paralogues of Klotho/beta-Klotho, klo-1, and klo-2. klo-1 is expressed in the C. elegans excretory canal, which is structurally and functionally paralogous to the vertebrate kidney. KLO-1 associates with EGL-15/FGFR, suggesting a role for KLO-1 in the fluid homeostasis phenotype described previously for egl-15/fgfr mutants. Altered levels of EGL-15/FGFR signaling lead to defects in excretory canal development and function in C. elegans. These results suggest an evolutionarily conserved function for the FGFR-Klotho complex in the development of excretory organs such as the mammalian kidney and the worm excretory canal. These results also suggest an evolutionarily conserved function for the FGFR-Klotho axis in metabolic regulation.     

A transient luminal chitinous matrix is required to model epithelial tube diameter in the Drosophila trachea

Epithelial tubes are found in many vital organs and require uniform and correct tube diameters for optimal function. Tube size depends on apical membrane growth and subapical cytoskeletal reorganization, but the cues that coordinate these events to ensure functional tube shape remain elusive. We find that epithelial tubes in the Drosophila trachea require luminal chitin polysaccharides to attain the correct diameter. Tracheal chitin forms a broad transient filament within the tubes during the restricted period of expansion. Loss of chitin causes tubular constrictions and cysts associated with irregular subapical cytoskeletal organization, without affecting epithelial integrity and polarity. Analysis of previously identified tube expansion mutants in genes encoding septate junction proteins further suggests that septate junction components may function in tubulogenesis through their role in luminal matrix assembly. We propose that the transient luminal protein/polysaccharide matrix is sensed by the epithelial cells and coordinates cytoskeletal organization to ensure uniform lumen diameter.