To characterize the function and mechanisms of cdc42 and sec10 in eye development in zebrafish.
Knockdown of zebrafish cdc42 and sec10 was carried out using antisense morpholino injection. The phenotype of morphants was characterized by histology, immunohistology, and transmission electron microscopy (TEM). To investigate a synergistic genetic interaction between cdc42 and sec10, we titrated suboptimal doses of cdc42 and sec10 morpholinos, and coinjected both morpholinos. To study trafficking, a melanosome transport assay was performed using epinephrine.
Cdc42 and sec10 knockdown in zebrafish resulted in both abnormal eye development and increased retinal cell death. Cdc42 morphants had a relatively normal retinal structure, aside from the absence of most connecting cilia and outer segments, whereas in sec10 morphants, much of the outer nuclear layer, which is composed of the photoreceptor nuclei, was missing and RPE cell thickness was markedly irregular. Knockdown of cdc42 and sec10 also resulted in an intracellular transport defect affecting retrograde melanosome transport. Furthermore, there was a synergistic genetic interaction between zebrafish cdc42 and sec10, suggesting that cdc42 and sec10 act in the same pathway in retinal development.
We propose a model whereby sec10 and cdc42 play a central role in development of the outer segment of the retinal photoreceptor cell by trafficking proteins necessary for ciliogenesis.
Cdc42 and sec10 knockdown in zebrafish leads to increased retinal cell death, as well as photoreceptor and transport defects. There was a genetic interaction between cdc42 and sec10, suggesting they act in the same pathway. These data show that sec10 and cdc42 are necessary for retinal development.
zebrafish; connecting cilium; cdc42; sec10; retinal degeneration; protein trafficking
Wnt5a is important for the development of various organs and postnatal cellular function. Little is known, however, about the role of Wnt5a in kidney development, although WNT5A mutations were identified in patients with Robinow syndrome, a genetic disease which includes developmental defects in kidneys. Our goal in this study was to determine the role of Wnt5a in kidney development.
Whole-mount in situ hybridization was used to establish the expression pattern of Wnt5a during kidney development. Zebrafish with wnt5a knockdown and Wnt5a global knockout mice were used to identify kidney phenotypes.
In zebrafish, wnt5a knockdown resulted in glomerular cyst formation and dilated renal tubules. In mice, Wnt5a global knockout resulted in pleiotropic, but severe, kidney phenotypes, including agenesis, fused kidney, hydronephrosis and duplex kidney/ureter.
Our data demonstrated the important role of Wnt5a in kidney development. Disrupted Wnt5a resulted in kidney cysts in zebrafish and pleiotropic abnormal kidney development in mice.
Wnt5a; Kidney development; Pronephros; Mesonephros; Metanephros
The Drosophila forkhead (Dfkh) family of transcription factors has over 40 family members. One Dfkh family member, BF2 (aka FoxD1), has been shown, by targeted disruption, to be essential for kidney development. In order to determine if other Dfkh family members were involved in kidney development and to search for new members of this family, reverse transcriptase polymerase chain reaction (RT-PCR) was performed using degenerate primers of the consensus sequence of the DNA binding domain of this family and developing rat kidney RNA. The RT-PCR product was used to probe RNA from a developing rat kidney (neonatal), from a 20-day old kidney, and from an adult kidney. The RT-PCR product hybridized only to a developing kidney RNA transcript of ~2.3 kb (the size of BF2). A lambda gt10 mouse neonatal kidney library was then screened, using the above-described RT-PCR product as a probe. Three lambda phage clones were isolated that strongly hybridized to the RT-PCR probe. Sequencing of the RT-PCR product and the lambda phage clones isolated from the developing kidney library revealed Dfkh BF2. In summary, only Dfkh family member BF2, which has already been shown to be essential for nephrogenesis, was identified in our screen and no other candidate Dfkh family members were identified.
Drosophila Forkhead; transcription factor; nephrogenesis
A novel Arl13b effector is identified. Sec8 and Sec5 exocyst subunits are shown to interact with Arl13b in a GTP-dependent manner. This interaction is synergistic in phenotypes caused by impaired ciliogenesis during zebrafish development. Similar to Sec10, Arl13b depletion in mouse kidneys inhibits ciliogenesis and leads to the formation of cysts.
Arl13b belongs to the ADP-ribosylation factor family within the Ras superfamily of regulatory GTPases. Mutations in Arl13b cause Joubert syndrome, which is characterized by congenital cerebellar ataxia, hypotonia, oculomotor apraxia, and mental retardation. Arl13b is highly enriched in cilia and is required for ciliogenesis in multiple organs. Nevertheless, the precise role of Arl13b remains elusive. Here we report that the exocyst subunits Sec8, Exo70, and Sec5 bind preferentially to the GTP-bound form of Arl13b, consistent with the exocyst being an effector of Arl13b. Moreover, we show that Arl13b binds directly to Sec8 and Sec5. In zebrafish, depletion of arl13b or the exocyst subunit sec10 causes phenotypes characteristic of defective cilia, such as curly tail up, edema, and abnormal pronephric kidney development. We explored this further and found a synergistic genetic interaction between arl13b and sec10 morphants in cilia-dependent phenotypes. Through conditional deletion of Arl13b or Sec10 in mice, we found kidney cysts and decreased ciliogenesis in cells surrounding the cysts. Moreover, we observed a decrease in Arl13b expression in the kidneys from Sec10 conditional knockout mice. Taken together, our results indicate that Arl13b and the exocyst function together in the same pathway leading to functional cilia.
Cilia, organelles that function as cellular antennae, are central to the pathogenesis of “ciliopathies”, including various forms of polycystic kidney disease (PKD). To date, however, the molecular mechanisms controlling ciliogenesis and ciliary function remain incompletely understood. A recently proposed model of cell–cell communication, called “urocrine signaling”, hypothesizes that a subset of membrane bound vesicles that are secreted into the urinary stream (termed exosome‐like vesicles, or ELVs), carry cilia‐specific proteins as cargo, interact with primary cilia, and affect downstream cellular functions. This study was undertaken to determine the role of the exocyst, a highly conserved eight‐protein trafficking complex, in the secretion and/or retrieval of ELVs. We used Madin–Darby canine kidney (MDCK) cells expressing either Sec10‐myc (a central component of the exocyst complex) or Smoothened‐YFP (a ciliary protein found in ELVs) in experiments utilizing electron gold microscopy and live fluorescent microscopy, respectively. Additionally, human urinary exosomes were isolated via ultracentrifugation and subjected to mass‐spectrometry‐based proteomics analysis to determine the composition of ELVs. We found, as determined by EM, that the exocyst localizes to primary cilia, and is present in vesicles attached to the cilium. Furthermore, the entire exocyst complex, as well as most of its known regulatory GTPases, are present in human urinary ELVs. Finally, in living MDCK cells, ELVs appear to interact with primary cilia using spinning disc confocal microscopy. These data suggest that the exocyst complex, in addition to its role in ciliogenesis, is centrally involved in the secretion and/or retrieval of urinary ELVs.
Our data suggest that the exocyst complex, in addition to its role in ciliogenesis that we previously described, is centrally involved in the secretion and/or retrieval of urinary exosomes. These results could have important implications for PKD, other renal diseases, as well as normal kidney homeostasis.
Cilia; ciliopathies; ELV; exocyst; polycystic kidney disease; urinary exosomes
The length of primary cilia is associated with normal cell and organ function. In the kidney, the change of functional cilia length/mass is associated with various diseases such as ischemia/reperfusion injury, polycystic kidney disease, and congenital solitary kidney. Here, we investigate whether renal mass reduction affects primary cilia length and function. To induce renal mass reduction, mice were subjected to unilateral nephrectomy (UNx). UNx increased kidney weight and superoxide formation in the remaining kidney. Primary cilia were elongated in proximal tubule cells, collecting duct cells and parietal cells of the remaining kidney. Mn(III) Tetrakis (1-methyl-4-pyridyl) porphyrin (MnTMPyP), an antioxidant, reduced superoxide formation in UNx-mice and prevented the elongation of primary cilia. UNx increased the expression of phosphorylated ERK, p21, and exocyst complex members Sec8 and Sec10, in the remaining kidney, and these increases were prevented by MnTMPyP. In MDCK, a kidney tubular epithelial cell line, cells, low concentrations of H2O2 treatment elongated primary cilia. This H2O2-induced elongation of primary cilia was also prevented by MnTMPyP treatment. Taken together, these data demonstrate that kidney compensation, induced by a reduction of renal mass, results in primary cilia elongation, and this elongation is associated with an increased production of reactive oxygen species (ROS).
Gene therapy involves delivery of exogenous DNA to provide a therapeutic protein. Ideally, a gene therapy vector should be non-toxic, non-immunogenic, easy to produce, and efficient in protecting and delivering DNA into target cells.
Adeno-associated virus (AAV) offers these advantages and few, if any, disadvantages, and over 100 isolates exist. We previously showed that AAV-mediated gene therapy can be used to restore vision to patients with Leber's congenital amaurosis, a disease of childhood blindness.
Here we show that novel recombinant AAV2/8 and AAV2/9 transduce kidney tubule cells with high efficiency both in vitroin cell culture and in vivoin mice. In addition, we adapted and modified a retrograde approach to allow for optimal transgene delivery to renal tubular cells that further minimizes the risk of an immunogenic reaction.
We believe that recombinant AAV2, especially AAV2/8, gene delivery to renal tubule cells via a retrograde approach represents a viable method for gene therapy for a multitude of renal disorders ranging from autosomal dominant polycystic kidney disease to acute kidney injury.
Adeno-associated virus; Gene therapy; Kidney injury
Most cases of congenital obstructive nephropathy are the result of ureteropelvic junction obstructions, and despite their high prevalence, we have a poor understanding of their etiology and scarcity of genetic models. The eight-protein exocyst complex regulates polarized exocytosis of intracellular vesicles in a large variety of cell types. Here we report generation of a conditional knockout mouse for Sec10, a central component of the exocyst, which is the first conditional allele for any exocyst gene. Inactivation of Sec10 in ureteric bud-derived cells using Ksp1.3-Cre mice resulted in severe bilateral hydronephrosis and complete anuria in newborns, with death occurring 6–14 hours after birth. Sec10FL/FL;Ksp-Cre embryos developed ureteropelvic junction obstructions between E17.5 and E18.5 as a result of degeneration of the urothelium and subsequent overgrowth by surrounding mesenchymal cells. The urothelial cell layer that lines the urinary tract must maintain a hydrophobic luminal barrier again urine while remaining highly stretchable. This barrier is largely established by production of uroplakin proteins that are transported to the apical surface to establish large plaques. By E16.5, Sec10FL/FL;Ksp-Cre ureter and pelvic urothelium showed decreased uroplakin-3 protein at the luminal surface, and complete absence of uroplakin-3 by E17.5. Affected urothelium at the UPJ showed irregular barriers that exposed the smooth muscle layer to urine, suggesting this may trigger the surrounding mesenchymal cells to overgrow the lumen. Findings from this novel mouse model show Sec10 is critical for the development of the urothelium in ureters, and provides experimental evidence that failure of this urothelial barrier may contribute to human congenital urinary tract obstructions.
Nonalcoholic fatty liver disease (NAFLD) is one of the most common causes of chronic liver disease such as simple steatosis, nonalcoholic steatohepatitis (NASH), cirrhosis and fibrosis. However, the molecular pathogenesis and genetic variations causing NAFLD are poorly understood. The high prevalence and incidence of NAFLD suggests that genetic variations on a large number of genes might be involved in NAFLD. To identify genetic variants causing inherited liver disease, we used zebrafish as a model system for a large-scale mutant screen, and adopted a whole genome sequencing approach for rapid identification of mutated genes found in our screen. Here, we report on a forward genetic screen of ENU mutagenized zebrafish. From 250 F2 lines of ENU mutagenized zebrafish during post-developmental stages (5 to 8 days post fertilization), we identified 19 unique mutant zebrafish lines displaying visual evidence of hepatomegaly and/or steatosis with no developmental defects. Histological analysis of mutants revealed several specific phenotypes, including common steatosis, micro/macrovesicular steatosis, hepatomegaly, ballooning, and acute hepatocellular necrosis. This work has identified multiple post-developmental mutants and establishes zebrafish as a novel animal model for post-developmental inherited liver disease.
Primary cilia are found on many epithelial cell types, including renal tubular epithelial cells, in which they are felt to participate in flow sensing and have been linked to the pathogenesis of cystic renal disorders such as autosomal dominant polycystic kidney disease. We previously localized the exocyst, an eight-protein complex involved in membrane trafficking, to the primary cilium of Madin-Darby canine kidney cells and showed that it was involved in cystogenesis. Here, using short hairpin RNA (shRNA) to knockdown exocyst expression and stable transfection to induce exocyst overexpression, we show that the exocyst protein Sec10 regulates primary ciliogenesis. Using immunofluorescence, scanning, and transmission electron microscopy, primary cilia containing only basal bodies are seen in the Sec10 knockdown cells, and increased ciliogenesis is seen in Sec10-overexpressing cells. These phenotypes do not seem to be because of gross changes in cell polarity, as apical, basolateral, and tight junction proteins remain properly localized. Sec10 knockdown prevents normal cyst morphogenesis when the cells are grown in a collagen matrix, whereas Sec10 overexpression results in increased cystogenesis. Transfection with human Sec10 resistant to the canine shRNA rescues the phenotype, demonstrating specificity. Finally, Par3 was recently shown to regulate primary cilia biogenesis. Par3 and the exocyst colocalized by immunofluorescence and coimmunoprecipitation, consistent with a role for the exocyst in targeting and docking vesicles carrying proteins necessary for primary ciliogenesis.
Polycystic kidney disease (PKD) is one of the most common causes of end-stage kidney disease, a devastating disease for which there is no cure. The molecular mechanisms leading to cyst formation in PKD remain somewhat unclear, but many genes are thought to be involved. Wnt5a is a non-canonical glycoprotein that regulates a wide range of developmental processes. Wnt5a works through the planar cell polarity (PCP) pathway that regulates oriented cell division during renal tubular cell elongation. Defects of the PCP pathway have been found to cause kidney cyst formation. Our paper describes a method for developing a zebrafish cystic kidney disease model by knockdown of the wnt5a gene with wnt5a antisense morpholino (MO) oligonucleotides. Tg(wt1b:GFP) transgenic zebrafish were used to visualize kidney structure and kidney cysts following wnt5a knockdown. Two distinct antisense MOs (AUG - and splice-site) were used and both resulted in curly tail down phenotype and cyst formation after wnt5a knockdown. Injection of mouse Wnt5a mRNA, resistant to the MOs due to a difference in primary base pair structure, rescued the abnormal phenotype, demonstrating that the phenotype was not due to “off-target” effects of the morpholino. This work supports the validity of using a zebrafish model to study wnt5a function in the kidney.
Medicine; Issue 94; Wnt5a; polycystic kidney disease; morpholino; microinjection; zebrafish; pronephros
In the past decade, cilia have been found to play important roles in renal cystogenesis. Many genes, such as PKD1 and PKD2 which, when mutated, cause autosomal dominant polycystic kidney disease (ADPKD), have been found to localize to primary cilia. The cilium functions as a sensor to transmit extracellular signals into the cell. Abnormal cilia structure and function are associated with the development of polyscystic kidney disease (PKD). Cilia assembly includes centriole migration to the apical surface of the cell, ciliary vesicle docking and fusion with the cell membrane at the intended site of cilium outgrowth, and microtubule growth from the basal body. This review summarizes the most recent advances in cilia and PKD research, with special emphasis on the mechanisms of cytoplasmic and intraciliary protein transport during ciliogenesis.
polycystic kidney disease; cilia; planar cell polarity; exocyst
A classic in vitro model of branching morphogenesis utilizes the Madin-Darby canine kidney (MDCK) cell line. MDCK Strain II cells form hollow monoclonal cysts in a three-dimensional collagen matrix over the course of ten days and tubulate in response to hepatocyte growth factor (HGF). We and our colleagues previously showed that activation of the extracellular-signal regulated kinase (ERK, aka MAPK) pathway is necessary and sufficient to induce tubulogenesis in MDCK cells. We also showed in a microarray study that one of the genes upregulated by HGF was the known tubulogene fibronectin. Given that HGF activates a multitude of signaling pathways, including ERK/MAPK, to test the intracellular regulatory pathway, we used two distinct inhibitors of ERK activation (U0126 and PD098059). Following induction of MDCK Type II cells with HGF, tubulogenic fibronectin mRNA was upregulated 4-fold by real time PCR, and minimal or no change in fibronectin expression was seen when HGF was added with either U0126 or PD098059. We confirmed these results using an MDCK cell line inducible for Raf, which is upstream of ERK. Following activation of Raf, fibronectin mRNA and protein expression were increased to a similar degree as was seen following HGF induction. Furthermore, MDCK Strain I cells, which originate from collecting ducts and have constitutively active ERK, spontaneously initiate tubulogenesis. We show here that MDCK Strain I cells have high levels of fibronectin mRNA and protein compared to MDCK Strain II cells. When U0126 and PD098059 were added to MDCK Strain I cells, fibronectin mRNA and protein levels were decreased to levels seen in MDCK Strain II cells. These data allow us to complete what we believe is the first description of a tubulogenic pathway from receptor/ligand (HGF/CMET), through an intracellular signaling pathway (ERK/MAPK), to transcription and, finally, secretion of a critical tubuloprotein (fibronectin).
ERK; MDCK; Tubulogenesis; Fibronectin
Protein translation and translocation at the rough endoplasmic reticulum (RER) are the first steps in the secretory pathway. The translocon through which newly-made proteins are translocated into or across the RER membrane, consists of three main subunits, Sec61α, β, and γ. Sec61β facilitates translocation, and we and others showed that the highly-conserved eight protein exocyst complex interacts with Sec61β. We also showed that the exocyst was involved in basolateral, and not apical, protein synthesis and delivery. Recently, however, exocyst involvement in apical protein delivery was reported. Furthermore, we showed that the exocyst was necessary for formation of primary cilia, organelles found on the apical surface.
GST pulldown was performed on lysate of renal tubule cells to investigate biochemical interactions. Cell-free assays consisting of cell-free extracts from rabbit reticulocytes, pancreatic ER microsomal membranes, transcripts of cDNA from apical and basolateral proteins, ATP/GTP, amino acids, and 35S-methionine for protein detection, were used to investigate the role of the exocyst in synthesis of polarized proteins. P32-orthophosphate and immunoprecipitation with antibody against Sec61β was used to investigate the Sec61β phosphorylation in exocyst Sec10-overexpressing cells.
Sec10 biochemically interacts with Sec61β using GST pulldown. Using cell-free assays, there is enhanced recruitment to ER membranes following exocyst depletion and basolateral VSVG protein translation, compared to apical HA protein translation. Finally, Sec10 overexpression increases Sec61β phosphorylation.
These data confirm that the exocyst is preferentially involved in basolateral protein translation and translocation, and may well act through the phosphorylation of Sec61β.
exocyst; polarity; translation; endoplasmic reticulum
Arteriovenous fistulas (AVFs) are considered superior to arteriovenous grafts and catheters. Never-theless, AVF prevalence in the United States remains under the established target. The complication rates and financial cost of vascular access continue to rise and disproportionately contribute to the burgeoning health care costs. The relationship between financial incentives for a type of vascular access and rate of access placement is unclear. All chronic hemodialysis patients (n=99) receiving care at Philadelphia Veterans Affairs Medical Center as of August 1, 2008 were participants. Demographic characteristics, vascular access type, and nonrelative value unit compensation were assessed as predictors, and the vascular access prevalence rate, operative times, and frequency of access interventions were analyzed. A 73.7% AVF rate was achieved in this cohort of patients with 51.5% diabetes mellitus. The number of access procedures per patient per year remained constant over time. The Philadelphia Veterans Affairs Medical Center, a single payer system, achieved superior AVF prevalence and exceeded the national AVF target. Financial incentives for arteriovenous graft placement currently exist in the United States, as there is similar Medicare reimbursement for arterio-venous graft and basilic vein transposition, despite longer operative times for basilic vein transpositions. The high AVF prevalence at the Philadelphia Veterans Affairs Medical Center may be due to the VA nonrelative value unit-driven system that allows for interdisciplinary care, priority of AVFs, and frequent use of basilic vein transposition surgery, when appropriate. We have identified an important, hypothesis-generating example of a nonrelative value unit-based approach to vascular access yielding superior results with respect to patient care and cost.
Fistula; hemodialysis; health care economics
Autosomal dominant polycystic kidney disease (ADPKD) is characterized by
formation of renal cysts that destroy the kidney. Mutations in PKD1 and PKD2,
encoding polycystins-1 and -2, cause ADPKD. Polycystins are thought to function
in primary cilia, but it is not well understood how these and other proteins are
targeted to cilia. Here, we provide the first genetic and biochemical link
between polycystins and the exocyst, a highly-conserved eight-protein membrane
trafficking complex. We show that knockdown of exocyst component Sec10 yields
cellular phenotypes associated with ADPKD, including loss of flow-generated
calcium increases, hyperproliferation, and abnormal activation of MAPK. Sec10
knockdown in zebrafish phenocopies many aspects of polycystin-2
knockdown—including curly tail up, left-right patterning defects,
glomerular expansion, and MAPK activation—suggesting that the exocyst is
required for pkd2 function in vivo. We observe
a synergistic genetic interaction between zebrafish sec10 and
pkd2 for many of these cilia-related phenotypes.
Importantly, we demonstrate a biochemical interaction between Sec10 and the
ciliary proteins polycystin-2, IFT88, and IFT20 and co-localization of the
exocyst and polycystin-2 at the primary cilium. Our work supports a model in
which the exocyst is required for the ciliary localization of polycystin-2, thus
allowing for polycystin-2 function in cellular processes.
ADPKD, the most common potentially lethal monogenetic disorder, is caused by
mutations in PKD1 and PKD2. We are beginning to appreciate the important roles
these gene products, and others, play in cilia, which are thin rod-like
organelles projecting from the cell surface. Defects in cilia function are
associated with a variety of human diseases, including all variants of
polycystic kidney disease. Despite intense study of cilia and how they influence
disease, it is not understood how proteins are targeted and delivered to cilia.
Our work provides the first link between the exocyst, a conserved eight-protein
complex involved in protein localization, and a disease gene, PKD2. Knockdown of
the exocyst protein Sec10 results in a number of cellular- and cilia-related
phenotypes that are also seen upon pkd2 loss—both in
kidney cells and zebrafish. We then demonstrate specific genetic and biochemical
interactions between sec10 and pkd2. We
further show that Sec10 interacts with other ciliary proteins, such as IFT20 and
IFT88. From this work, we propose that the exocyst is involved in bringing
multiple types of ciliary proteins to the cilium. Given that the exocyst is
required for cilia structure and function, the exocyst may play a role in
cilia-related human diseases.
In the fifty years since kidney transplantation was first performed, this procedure has evolved from a highly speculative biomedical endeavor to a medically viable and often standard course of therapy (1). Long-term survival is markedly improved among patients who receive a kidney compared with patients who remain on the waiting list for such an organ (2). As outcomes have improved and more clinical indications have emerged, the number of people awaiting transplantation has grown significantly.
In stark contrast to the robust expansion of the waiting list, the number of available deceased donors has remained relatively constant over the last several years (1). The current mechanism for procuring kidneys relies on voluntary donations by the general public, with the primary motivation being altruism. However, in light of the ever-increasing waiting list, it is the authors’ belief that the current system needs to be revised if supply is ever going to meet demand. In response to this critical organ shortage, different programs have been developed in an attempt to increase organ donation. At present, however, no solution to the problem has emerged. This paper begins by outlining the scope of the problem and current legislation governing the procurement of transplantable organs/tissues in the United States. It continues with an overview of different proposals to increase supply. It concludes by exploring some of the controversy surrounding the proposal to increase donation using financial incentives. Though the following discussion certainly has implications for other transplantable organs, this paper will focus on kidney transplantation because the waiting list for kidneys is by far the longest of all the solid organs; and, as it carries the smallest risk to living donors, is the least ethically problematic.
Bioethics; Transplantation; End Stage Renal Disease
Epithelial cyst and tubule formation are critical processes that
involve transient, highly choreographed changes in cell polarity.
Factors controlling these changes in polarity are largely unknown. One
candidate factor is the highly conserved eight-member protein complex
called the exocyst. We show that during tubulogenesis in an in vitro
model system the exocyst relocalized along growing tubules consistent
with changes in cell polarity. In yeast, the exocyst subunit Sec10p is
a crucial component linking polarized exocytic vesicles with the rest
of the exocyst complex and, ultimately, the plasma membrane. When the
exocyst subunit human Sec10 was exogenously expressed in
epithelial Madin-Darby canine kidney cells, there was a selective
increase in the synthesis and delivery of apical and basolateral
secretory proteins and a basolateral plasma membrane protein, but not
an apical plasma membrane protein. Overexpression of human Sec10
resulted in more efficient and rapid cyst formation and increased
tubule formation upon stimulation with hepatocyte growth factor. We
conclude that the exocyst plays a central role in the development of
epithelial cysts and tubules.