A holey pursuit: lumen formation in the developing kidney

Characterization of Hippo Signaling Components in the Early Dorsal Pancreatic Bud

All pancreatic lineages originate from a transitory structure known as the multipotent progenitor epithelium (MPE), which is a placode formed via epithelial stratification. Cells within the MPE undergo de novo lumenogenesis to give rise to an epithelial plexus, which serves as a progenitor niche for subsequent development of endocrine, ductal and acinar cell types. Recent evidence suggests that Hippo signaling is required for pancreatic cell differentiation, but little is known about the function of Hippo signaling in the development of the MPE. Here, we characterize the expression of YAP1, TAZ, and the Hippo regulators LATS1/2 kinases and MERLIN in early murine pancreatic epithelium, during epithelial stratification, plexus development and emergence of endocrine cells. We find that YAP1 expression is relatively low in the pancreas bud during stratification, but increases by E11.5. Intriguingly, we find that TAZ, but not YAP1, is expressed in early endocrine cells. We further find that MERLIN and LATS1/2 kinases are robustly expressed during the period of rapid stratification and become markedly apical at nascent lumens. To gain a better understanding of how Hippo signaling and lumen formation are connected, we analyzed the expression of Hippo signaling components in an in vitro model of lumen formation and found that they are dynamically regulated during lumenogenesis. Together, our results point to a relationship between Hippo signaling and lumen formation during pancreatic development.

HIGHLIGHTS

YAP1 expression in the early mouse pancreatic anlagen is low until approximately E11.5, when it becomes localized to cell nuclei in multipotent progenitor cells. At E14.5, we find nuclear YAP1 in ductal cells.YAP1 is not expressed in early and midgestation endocrine cells. By contrast, TAZ is expressed in first transition endocrine cells.Hippo regulators MERLIN and LATS1/2 kinases are robustly expressed in the early pancreatic bud by E10.5. Both MERLIN and LATS1/2 exhibit strong apical localization in epithelial cells at nascent microlumens. Using in vitro models of de novo pancreas lumen formation, we show that YAP1 nuclear localization is high in early phases of lumen formation and gradually decreases as lumens matures.

4D pathology: translating dynamic epithelial tubulogenesis to prostate cancer pathology

The Gleason score is the gold standard for grading of prostate cancer (PCa) and is assessed by assigning specific grades to different microscopical growth patterns. Aside from the Gleason grades, individual growth patterns such as cribriform architecture were recently shown to have independent prognostic value for disease outcome. PCa grading is performed on static tissue samples collected at one point in time, whereas in vivo epithelial tumour structures are dynamically invading, branching and expanding into the surrounding stroma. Due to the lack of models that are able to track human PCa microscopical developments over time, our understanding of underlying tissue dynamics is sparse. We postulate that human PCa expansion utilizes embryonic and developmental tubulogenetic pathways. The aim of this study is to provide a comprehensive overview of developmental pathways of normal epithelial tubule formation, elongation, and branching, and relate those to the static microscopical PCa growth patterns observed in daily clinical practise. This study could provide a rationale for the discerned pathological interobserver variability and the clinical outcome differences between PCa growth patterns.

Epithelial tubule interconnection driven by HGF-Met signaling in the kidney

The formation of functional epithelial tubules is a central feature of many organ systems. Although the process of tubule formation by epithelial cells is well-studied, the way in which tubules connect with each other (i.e. anastomose) to form functional networks both in vivo and in vitro is not well understood. A key, unanswered question in the kidney is how the renal vesicles of the embryonic kidney connect with the nascent collecting ducts to form a continuous urinary system. We performed a ligand-receptor pair analysis on single cell RNA-seq data from embryonic mouse kidney tubules undergoing anastomosis to select candidates that might mediate this process in vivo. This analysis identified hepatocyte growth factor (HGF), which has known roles in cell proliferation, migration, and tubulogenesis, as one of several possible candidates. To test this possibility, we designed a novel assay to quantitatively examine epithelial tubule anastomosis in vitro using epithelial spheroids with fluorescently-tagged apical surfaces to enable direct visualization of anastomosis. This revealed that HGF is a potent inducer of tubule anastomosis. Tubule anastomosis occurs through a proliferation-independent mechanism that acts through the MAPK signaling cascade and matrix metalloproteinases (MMPs), the latter suggestive of a role in extracellular matrix turnover. Accordingly, treatment of explanted embryonic mouse kidneys with HGF and collagenase was sufficient to induce kidney tubule anastomosis. These results lay the groundwork for investigating how to promote functional interconnections between tubular epithelia, which have important clinical implications for utilizing in vitro grown kidney tissue in transplant medicine.

Rap1 regulates lumen continuity via Afadin in renal epithelia

The continuity of a lumen within an epithelial tubule is critical for its function. We previously found that the F-actin binding protein Afadin is required for timely lumen formation and continuity in renal tubules formed from the nephrogenic mesenchyme in mice. Afadin is a known effector and interactor of the small GTPase Rap1, and in the current study, we examine the role of Rap1 in nephron tubulogenesis. Here, we demonstrate that Rap1 is required for nascent lumen formation and continuity in cultured 3D epithelial spheroids and in vivo in murine renal epithelial tubules derived from the nephrogenic mesenchyme, where its absence ultimately leads to severe morphogenetic defects in the tubules. By contrast, Rap1 is not required for lumen continuity or morphogenesis in renal tubules derived from the ureteric epithelium, which differ in that they form by extension from a pre-existing tubule. We further demonstrate that Rap1 is required for correct localization of Afadin to adherens junctions both in vitro and in vivo. Together, these results suggest a model in which Rap1 localizes Afadin to junctional complexes, which in turn regulates nascent lumen formation and positioning to ensure continuous tubulogenesis.

Computational study of biomechanical drivers of renal cystogenesis

Renal cystogenesis is the pathological hallmark of autosomal dominant polycystic kidney disease, caused by PKD1 and PKD2 mutations. The formation of renal cysts is a common manifestation in ciliopathies, a group of syndromic disorders caused by mutation of proteins involved in the assembly and function of the primary cilium. Cystogenesis is caused by the derailment of the renal tubular architecture and tissue deformation that eventually leads to the impairment of kidney function. However, the biomechanical imbalance of cytoskeletal forces that are altered in cells with Pkd1 mutations has never been investigated, and its nature and extent remain unknown. In this computational study, we explored the feasibility of various biomechanical drivers of renal cystogenesis by examining several hypothetical mechanisms that may promote morphogenetic markers of cystogenesis. Our objective was to provide physics-based guidance for our formulation of hypotheses and our design of experimental studies investigating the role of biomechanical disequilibrium in cystogenesis. We employed the finite element method to explore the role of (1) wild-type versus mutant cell elastic modulus; (2) contractile stress magnitude in mutant cells; (3) localization and orientation of contractile stress in mutant cells; and (4) sequence of cell contraction and cell proliferation. Our objective was to identify the factors that produce the characteristic tubular cystic growth. Results showed that cystogenesis occurred only when mutant cells contracted along the apical-basal axis, followed or accompanied by cell proliferation, as long as mutant cells had comparable or lower elastic modulus than wild-type cells, with their contractile stresses being significantly greater than their modulus. Results of these simulations allow us to focus future in vitro and in vivo experimental studies on these factors, helping us formulate physics-based hypotheses for renal tubule cystogenesis.

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.

Won't You be My Neighbor: How Epithelial Cells Connect Together to Build Global Tissue Polarity

Epithelial tissues form continuous barriers to protect against external environments. Within these tissues, epithelial cells build environment-facing apical membranes, junction complexes that anchor neighbors together, and basolateral surfaces that face other cells. Critically, to form a continuous apical barrier, neighboring epithelial cells must align their apico-basolateral axes to create global polarity along the entire tissue. Here, we will review mechanisms of global tissue-level polarity establishment, with a focus on how neighboring epithelial cells of different origins align their apical surfaces. Epithelial cells with different developmental origins and/or that polarize at different times and places must align their respective apico-basolateral axes. Connecting different epithelial tissues into continuous sheets or tubes, termed epithelial fusion, has been most extensively studied in cases where neighboring cells initially dock at an apical-to-apical interface. However, epithelial cells can also meet basal-to-basal, posing several challenges for apical continuity. Pre-existing basement membrane between the tissues must be remodeled and/or removed, the cells involved in docking are specialized, and new cell-cell adhesions are formed. Each of these challenges can involve changes to apico-basolateral polarity of epithelial cells. This minireview highlights several in vivo examples of basal docking and how apico-basolateral polarity changes during epithelial fusion. Understanding the specific molecular mechanisms of basal docking is an area ripe for further exploration that will shed light on complex morphogenetic events that sculpt developing organisms and on the cellular mechanisms that can go awry during diseases involving the formation of cysts, fistulas, atresias, and metastases.

Impact of Membrane Voltage on Formation and Stability of Human Renal Proximal Tubules in Vitro

More than 15% of adults in the United States suffer from some form of chronic kidney disease (CKD). Current strategies for CKD consist of dialysis or kidney transplant, which, however, can take several years. In this light, tissue engineering and regenerative medicine approaches are the key to improving people's living conditions by advancing previous tissue engineering approaches and seeking new targets as intervention methods for kidney repair or replacement. The membrane voltage (V_m) dynamics of a cell have been associated with cell migration, cell cycle progression, differentiation, and pattern formation. Furthermore, bioelectrical stimuli have been used as a means in the treatment of diseases and wound healing. Here, we investigated the role of V_m as a novel target to guide and manipulate in vitro renal tissue models. Human-immortalized renal proximal tubule epithelial cells (RPTECs-TERT1) were cultured on Matrigel to support the formation of 3D proximal tubular-like structures with the incorporation of a voltage-sensitive dye indicator─bis-(1,3-dibutylbarbituric acid)timethine oxonol (DiBAC). The results demonstrated a correlation between the depolarization and the reorganization of human renal proximal tubule cells, indicating V_m as a candidate variable to control these events. Accordingly, V_m was pharmacologically manipulated using glibenclamide and pinacidil, K_ATP channel modulators, and proximal tubule formation and tubule stability over 21 days were assessed. Chronic manipulation of K_ATP channels induced changes in the tubular network topology without affecting lumen formation. Thus, a relationship was found between the preluminal tubulogenesis phase and K_ATP channels. This relationship may provide future options as a control point during kidney tissue development, treatment, and regeneration goals.

Dissecting nephron morphogenesis using kidney organoids from human pluripotent stem cells

During kidney development the emergence of complex multicellular shapes such as the nephron (the functional unit of the kidney) rely on spatiotemporally coordinated developmental programs. These involve gene regulatory networks, signaling pathways and mechanical forces, that work in concert to shape and form the nephron(s). The generation of kidney organoids from human pluripotent stem cells now represent an unprecedented experimental set up to study these processes. Here we discuss the potential applications of kidney organoids to advance our knowledge of how mechanical forces and cell fate specification spatiotemporally interact to orchestrate nephron patterning and morphogenesis in humans. Progress in innovative techniques for quantifying and perturbing these processes in a controlled manner will be crucial. A mechanistic understanding of the multicellular dynamic processes occurring during nephrogenesis will pave the way to unveil new mechanisms of human kidney disease.

Tissue hydraulics: Physics of lumen formation and interaction

Lumen formation plays an essential role in the morphogenesis of tissues during development. Here we review the physical principles that play a role in the growth and coarsening of lumens. Solute pumping by the cell, hydraulic flows driven by differences of osmotic and hydrostatic pressures, balance of forces between extracellular fluids and cell-generated cytoskeletal forces, and electro-osmotic effects have been implicated in determining the dynamics and steady-state of lumens. We use the framework of linear irreversible thermodynamics to discuss the relevant force, time and length scales involved in these processes. We focus on order of magnitude estimates of physical parameters controlling lumen formation and coarsening.

Advances in understanding vertebrate nephrogenesis

The kidney is a complex organ that performs essential functions such as blood filtration and fluid homeostasis, among others. Recent years have heralded significant advancements in our knowledge of the mechanisms that control kidney formation. Here, we provide an overview of vertebrate renal development with a focus on nephrogenesis, the process of generating the epithelialized functional units of the kidney. These steps begin with intermediate mesoderm specification and proceed all the way to the terminally differentiated nephron cell, with many detailed stages in between. The establishment of nephron architecture with proper cellular barriers is vital throughout these processes. Continuously striving to gain further insights into nephrogenesis can ultimately lead to a better understanding and potential treatments for developmental maladies such as Congenital Anomalies of the Kidney and Urinary Tract (CAKUT).

TRPM6 and TRPM7: Novel players in cell intercalation during vertebrate embryonic development

A common theme in organogenesis is how the final structure of organs emerge from epithelial tube structures, with the formation of the neural tube being one of the best examples. Two types of cell movements co-occur during neural tube closure involving the migration of cells toward the midline of the embryo (mediolateral intercalation or convergent extension) as well as the deep movement of cells from inside the embryo to the outside of the lateral side of the neural plate (radial intercalation). Failure of either type of cell movement will prevent neural tube closure, which can produce a range of neural tube defects (NTDs), a common congenital disease in humans. Numerous studies have identified signaling pathways that regulate mediolateral intercalation during neural tube closure. Less understood are the pathways that govern radial intercalation. Using the Xenopus laevis system, our group reported the identification of transient receptor potential (TRP) channels, TRPM6 and TRPM7, and the Mg2+ ion they conduct, as novel and key factors regulating both mediolateral and radial intercalation during neural tube closure. Here we broadly discuss tubulogenesis and cell intercalation from the perspective of neural tube closure and the respective roles of TRPM7 and TRPM6 in this critical embryonic process.

Shaping of the nephron - a complex, vulnerable, and poorly explored backdrop for noxae impairing nephrogenesis in the fetal human kidney

Background

The impairment of nephrogenesis is caused by noxae, all of which are significantly different in molecular composition. These can cause an early termination of nephron development in preterm and low birth weight babies resulting in oligonephropathy. For the fetal human kidney, there was no negative effect reported on the early stages of nephron anlage such as the niche, pretubular aggregate, renal vesicle, or comma-shaped body. In contrast, pathological alterations were identified on subsequently developing S-shaped bodies and glomeruli. While the atypical glomeruli were closely analyzed, the S-shaped bodies and the pre-stages received little attention even though passing the process of nephron shaping. Since micrographs and an explanation about this substantial developmental period were missing, the shaping of the nephron in the fetal human kidney during the phase of late gestation was recorded from a microanatomical point of view.

Results

The nephron shaping starts with the primitive renal vesicle, which is still part of the pretubular aggregate at this point. Then, during extension of the renal vesicle, a complex separation is observed. The medial part of its distal pole is fixed on the collecting duct ampulla, while the lateral part remains connected with the pretubular aggregate via a progenitor cell strand. A final separation occurs, when the extended renal vesicle develops into the comma-shaped body. Henceforth, internal epithelial folding generates the tubule and glomerulus anlagen. Arising clefts at the medial and lateral aspect indicate an asymmetrical expansion of the S-shaped body. This leads to development of the glomerulus at the proximal pole, whereas in the center and at the distal pole, it results in elongation of the tubule segments.

Conclusions

The present investigation deals with the shaping of the nephron in the fetal human kidney. In this important developmental phase, the positioning, orientation, and folding of the nephron occur. The demonstration of previously unknown morphological details supports the search for traces left by the impairment of nephrogenesis, enables to refine the assessment in molecular pathology, and provides input for the design of therapeutic concepts prolonging nephrogenesis.

The atypical mitogen-activated protein kinase ERK3 is essential for establishment of epithelial architecture

Epithelia contribute to physical barriers that protect internal tissues from the external environment and also support organ structure. Accordingly, establishment and maintenance of epithelial architecture are essential for both embryonic development and adult physiology. Here, using gene knockout and knockdown techniques along with gene profiling, we show that extracellular signal-regulated kinase 3 (ERK3), a poorly characterized atypical mitogen-activated protein kinase (MAPK), regulates the epithelial architecture in vertebrates. We found that in Xenopus embryonic epidermal epithelia, ERK3 knockdown impairs adherens and tight-junction protein distribution, as well as tight-junction barrier function, resulting in epidermal breakdown. Moreover, in human epithelial breast cancer cells, inhibition of ERK3 expression induced thickened epithelia with aberrant adherens and tight junctions. Results from microarray analyses suggested that transcription factor AP-2α (TFAP2A), a transcriptional regulator important for epithelial gene expression, is involved in ERK3-dependent changes in gene expression. Of note, TFAP2A knockdown phenocopied ERK3 knockdown in both Xenopus embryos and human cells, and ERK3 was required for full activation of TFAP2A-dependent transcription. Our findings reveal that ERK3 regulates epithelial architecture, possibly together with TFAP2A.

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.

BMP7 dose-dependently stimulates proliferation and cadherin-11 expression via ERK and p38 in a murine metanephric mesenchymal cell line

BMP7 is expressed in ureteric buds and cap mesenchyme of the fetal kidney, mediating branching morphogenesis and survival and priming of metanephric mesenchyme. Although dose‐dependent effects of BMP7 in collecting duct cells have been reported, studies in metanephric mesenchymal cells are lacking. We examined the effects of BMP7 on MAP kinase activation, proliferation, and expression of cadherins in a metanephric mesenchymal cell line MS7 by thymidine incorporation, immunoblot analysis, and quantitative real‐time PCR. The levels of phosphorylated ERK (P‐ERK) and phosphorylated p38 (P‐p38) were not altered at 10 min, 1 h, and 6 h with low‐dose BMP7 (0.25 nmol/L), but were increased at 24 h. At 24 h, P‐ERK was increased with low‐dose BMP7, but not by intermediate‐ (1 nmol/L) or high‐dose (10 nmol/L) BMP7, whereas p38 was activated by intermediate‐dose BMP7. Cell proliferation of MS7 was significantly increased by low‐ and intermediate‐dose BMP7 and decreased by high‐dose BMP7. A p38 inhibitor SB203580 5 μmol/L or a MEK inhibitor PD98059 5 μmol/L abolished BMP7‐stimulated proliferation. Expression of cadherin‐11, an adhesion molecule known to promote cell migration and compaction, was upregulated by intermediate‐dose BMP7. BMP7‐induced cadherin‐11 expression was inhibited by cotreatment with SB203580 and PD98059. Finally, in metanephroi cultured with siRNA for cadherin‐11, the number and thickness of cap mesenchyme were reduced. In conclusion, BMP7 exerts differential effects depending on the concentration; it may expand mesenchymal cells in the stroma where BMP7 concentration is low and may upregulate cadherin‐11 promoting condensation around the tip of ureteric buds.

Afadin and RhoA control pancreatic endocrine mass via lumen morphogenesis

Azizoglu et al. identified mechanisms underlying pancreatic lumen formation and remodeling and show that central lumen network morphogenesis impacts pancreatic endocrine mass. Codepletion of the actomyosin regulator RhoA and Afadin results in defects in the central lumen and arrests lumen remodeling.

Proper lumen morphogenesis during pancreas development is critical to endocrine and exocrine cell fate. Recent studies showed that a central network of lumens (termed core), but not the surrounding terminal branches (termed periphery), produces most islet endocrine cells. To date, it remains unclear how pancreatic lumens form and remodel and which aspects of lumen morphogenesis influence cell fate. Importantly, models testing the function of the central lumen network as an endocrine niche are lacking. Here, we identify mechanisms underlying lumen formation and remodeling and show that central lumen network morphogenesis impacts pancreatic endocrine mass. We show that loss of the scaffolding protein Afadin disrupts de novo lumenogenesis and lumen continuity in the tip epithelium. Codepletion of the actomyosin regulator RhoA and Afadin results in defects in the central lumens and arrests lumen remodeling. This arrest leads to prolonged perdurance of the central lumen network over developmental time and expansion of the endocrine progenitor population and, eventually, endocrine mass. Our study uncovers essential roles of Afadin and RhoA in pancreatic central lumen morphogenesis, which subsequently determines endocrine cell mass.

Signaling Networks in Epithelial Tube Formation

Epithelial tubes are crucial to the function of organ systems including the excretory, gastrointestinal, cardiovascular, and pulmonary. Studies in the last two decades using in vitro organotypic systems and a variety of animal models have substantiated a large number of the morphogenetic mechanisms required to form epithelial tubes in development and regeneration. Many of these mechanisms modulate the differentiation and proliferation events necessary for generating the cell movements and changes in cell shape to delineate the wide variety of epithelial tube sizes, lengths, and conformations. For instance, when coupled with oriented cell division, proliferation itself plays a role in changes in tube shape and their directed expansion. Most of these processes are regulated in response to signaling inputs from adjacent cells or soluble factors from the environment. Despite the great deal of recent investigation in this direction, the knowledge we have about the signaling pathways associated with all epithelial tubulogenesis in development and regeneration is still very limited.

Fibroblast-derived HGF drives acinar lung cancer cell polarization through integrin-dependent RhoA-ROCK1 inhibition

The formation of lumens in epithelial tissues requires apical-basal polarization of cells, and the co-ordination of this individual polarity collectively around a contiguous lumen. Signals from the Extracellular Matrix (ECM) instruct epithelia as to the orientation of where basal, and thus consequently apical, surfaces should be formed. We report that this pathway is normally absent in Calu-3 human lung adenocarcinoma cells in 3-Dimensional culture, but that paracrine signals from MRC5 lung fibroblasts can induce correct orientation of polarity and acinar morphogenesis. We identify HGF, acting through the c-Met receptor, as the key polarity-inducing morphogen, which acts to activate β1-integrin-dependent adhesion. HGF and ECM-derived integrin signals co-operate via a c-Src-dependent inhibition of the RhoA-ROCK1 signalling pathway via p190A RhoGAP. This occurred via controlling localization of these signalling pathways to the ECM-abutting surface of cells in 3-Dimensional culture. Thus, stromal derived signals can influence morphogenesis in epithelial cells by controlling activation and localization of cell polarity pathways.

Afadin orients cell division to position the tubule lumen in developing renal tubules

In many types of tubules, continuity of the lumen is paramount to tubular function, yet how tubules generate lumen continuity in vivo is not known. We recently found that the F-actin-binding protein afadin is required for lumen continuity in developing renal tubules, though its mechanism of action remains unknown. Here, we demonstrate that afadin is required for lumen continuity by orienting the mitotic spindle during cell division. Using an in vitro 3D cyst model, we find that afadin localizes to the cell cortex adjacent to the spindle poles and orients the mitotic spindle. In tubules, cell division may be oriented relative to two axes: longitudinal and apical-basal. Unexpectedly, in vivo examination of early-stage developing nephron tubules reveals that cell division is not oriented in the longitudinal (or planar-polarized) axis. However, cell division is oriented perpendicular to the apical-basal axis. Absence of afadin in vivo leads to misorientation of apical-basal cell division in nephron tubules. Together, these results support a model whereby afadin determines lumen placement by directing apical-basal spindle orientation, resulting in a continuous lumen and normal tubule morphogenesis.

Role of CD34 family members in lumen formation in the developing kidney

Previous studies have shown CD34 family member Podocalyxin is required for epithelial lumen formation in vitro. We demonstrate that Endoglycan, a CD34 family member with homology to Podocalyxin, is produced prior to lumen formation in developing nephrons. Endoglycan localizes to Rab11-containing vesicles in nephron progenitors, and then relocalizes to the apical surface as progenitors epithelialize. Once an apical/luminal surface is formed, Endoglycan (and the actin-binding protein Ezrin) localize to large, intraluminal structures that may be vesicles/exosomes. We generated mice lacking Endoglycan and found mutants had timely initiation of lumen formation and continuous lumens, similar to controls. Mice with conditional deletion of both Endoglycan and Podocalyxin in developing nephrons also had normal tubular lumens. Despite this, Endoglycan/Podocalyxin is required for apical recruitment of the adaptor protein NHERF1, but not Ezrin, in podocyte precursors, a subset of the epithelia. In summary, while CD34 family members appear dispensable for lumen formation, our data identify Endoglycan as a novel pre-luminal marker and suggest lumen formation occurs via vesicular trafficking of apical cargo that includes Endoglycan.