Disruption of Core Planar Cell Polarity Signaling Regulates Renal Tubule Morphogenesis but Is Not Cystogenic

Next generation sequencing identifies WNT signalling as a significant pathway in Autosomal Recessive Polycystic Kidney Disease (ARPKD) manifestation and may be linked to disease severity

INTRODUCTION

Autosomal Recessive Polycystic Kidney Disease (ARPKD) is a rare paediatric disease primarily caused by sequence variants in PKHD1. ARPKD presents with considerable clinical variability relating to the type of PKHD1 sequence variant, but not its position. Animal models of Polycystic Kidney Disease (PKD) suggest a complex genetic landscape, with genetic modifiers as a potential cause of disease variability.

METHODS

To investigate in an unbiased manner the molecular mechanisms of ARPKD and identify potential indicators of disease severity, Whole Exome Sequencing (WES) and RNA-Sequencing (RNA-Seq) were employed on human ARPKD kidneys and age-matched healthy controls.

RESULTS

WES confirmed the clinical diagnosis of ARPKD in our patient cohort consisting of ten ARPKD kidneys. Sequence variant type, nor position of PKHD1 sequence variants, was linked to disease severity. Sequence variants in genes associated with other ciliopathies were detected in the ARPKD cohort, but only PKD1 could be linked to disease severity. Transcriptomic analysis on a subset of four ARPKD kidneys representing severe and moderate ARPKD, identified a significant number of genes relating to WNT signalling, cellular metabolism and development. Increased expression of WNT signalling-related genes was validated by RT-qPCR in severe and moderate ARPKD kidneys. Two individuals in our cohort with the same PKHD1 sequence variants but different rates of kidney disease progression, with displayed transcriptomic differences in the expression of WNT signalling genes.

CONCLUSION

ARPKD kidney transcriptomics highlights changes in WNT signalling as potentially significant in ARPKD manifestation and severity, providing indicators for slowing down the progression of ARPKD.

Exploring the principles of embryonic mammary gland branching morphogenesis

Branching morphogenesis is a characteristic feature of many essential organs, such as the lung and kidney, and most glands, and is the net result of two tissue behaviors: branch point initiation and elongation. Each branched organ has a distinct architecture customized to its physiological function, but how patterning occurs in these ramified tubular structures is a fundamental problem of development. Here, we use quantitative 3D morphometrics, time-lapse imaging, manipulation of ex vivo cultured mouse embryonic organs and mice deficient in the planar cell polarity component Vangl2 to address this question in the developing mammary gland. Our results show that the embryonic epithelial trees are highly complex in topology owing to the flexible use of two distinct modes of branch point initiation: lateral branching and tip bifurcation. This non-stereotypy was contrasted by the remarkably constant average branch frequency, indicating a ductal growth invariant, yet stochastic, propensity to branch. The probability of branching was malleable and could be tuned by manipulating the Fgf10 and Tgfβ1 pathways. Finally, our in vivo data and ex vivo time-lapse imaging suggest the involvement of tissue rearrangements in mammary branch elongation.

Planar Cell Polarity in the Multiciliated Epithelial Lining of the Mouse Eustachian Tube

OBJECTIVES

Planar cell polarity (PCP) signaling, essential for uniform alignment and directional beating of motile cilia, has been investigated in multiciliated epithelia. As a complex structure connecting the middle ear to the nasopharynx, the eustachian tube (ET) is important in the onset of ear-nose-throat diseases. However, PCP signaling, including the orientation that is important for ciliary motility and clearance function in the ET, has not been studied. We evaluated PCP in the ET epithelium.

STUDY DESIGN

Morphometric examination of the mouse ET.

METHODS

We performed electron microscopy to assess ciliary polarity in the mouse ET, along with immunohistochemical analysis of PCP protein localization in the ET epithelium.

RESULTS

We discovered PCP in the ET epithelium. Motile cilia were aligned in the same direction in individual and neighboring cells; this alignment manifested as ciliary polarity in multiciliated cells. Additionally, PCP proteins were asymmetrically localized between adjacent cells in the plane of the ET.

CONCLUSIONS

The multiciliated ET epithelium exhibits polarization, suggesting novel structural features that may be critical for ET function.

LEVEL OF EVIDENCE

NA Laryngoscope, 134:3795-3801, 2024.

Vangl-dependent mesenchymal thinning shapes the distal lung during murine sacculation

The planar cell polarity (PCP) complex is speculated to function in murine lung development, where branching morphogenesis generates an epithelial tree whose distal tips expand dramatically during sacculation. Here, we show that PCP is dispensable in the airway epithelium for sacculation. Rather, we find a Celsr1-independent role for the PCP component Vangl in the pulmonary mesenchyme: loss of Vangl1/2 inhibits mesenchymal thinning and expansion of the saccular epithelium. Further, loss of mesenchymal Wnt5a mimics sacculation defects observed in Vangl2-mutant lungs, implicating mesenchymal Wnt5a/Vangl signaling as a key regulator of late lung morphogenesis. A computational model predicts that sacculation requires a fluid mesenchymal compartment. Lineage-tracing and cell-shape analyses are consistent with the mesenchyme acting as a fluid tissue, suggesting that loss of Vangl1/2 impacts the ability of mesenchymal cells to exchange neighbors. Our data thus identify an explicit function for Vangl and the pulmonary mesenchyme in actively shaping the saccular epithelium.

Adhesion G Protein-Coupled Receptor Gpr126 (Adgrg6) Expression Profiling in Diseased Mouse, Rat, and Human Kidneys

Uncovering the function of understudied G protein-coupled receptors (GPCRs) provides a wealth of untapped therapeutic potential. The poorly understood adhesion GPCR Gpr126 (Adgrg6) is widely expressed in developing kidneys. In adulthood, Gpr126 expression is enriched in parietal epithelial cells (PECs) and epithelial cells of the collecting duct and urothelium. Whether Gpr126 plays a role in kidney disease remains unclear. Here, we characterized Gpr126 expression in diseased kidneys in mice, rats, and humans. RT-PCR data show that Gpr126 expression is altered in kidney disease. A quantitative RNAscope® analysis utilizing cell type-specific markers revealed that Gpr126 expression upon tubular damage is mainly increased in cell types expressing Gpr126 under healthy conditions as well as in cells of the distal and proximal tubules. Upon glomerular damage, an increase was mainly detected in PECs. Notably, Gpr126 expression was upregulated in an ischemia/reperfusion model within hours, while upregulation in a glomerular damage model was only detected after weeks. An analysis of kidney microarray data from patients with lupus nephritis, IgA nephropathy, focal segmental glomerulosclerosis (FSGS), hypertension, and diabetes as well as single-cell RNA-seq data from kidneys of patients with acute kidney injury and chronic kidney disease indicates that GPR126 expression is also altered in human kidney disease. In patients with FSGS, an RNAscope® analysis showed that GPR126 mRNA is upregulated in PECs belonging to FSGS lesions and proximal tubules. Collectively, we provide detailed insights into Gpr126 expression in kidney disease, indicating that GPR126 is a potential therapeutic target.

Novel Potential Therapeutic Targets in Autosomal Dominant Polycystic Kidney Disease from the Perspective of Cell Polarity and Fibrosis

Autosomal dominant polycystic kidney disease (ADPKD), a congenital genetic disorder, is a notable contributor to the prevalence of chronic kidney disease worldwide. Despite the absence of a complete cure, ongoing research aims for early diagnosis and treatment. Although agents such as tolvaptan and mTOR inhibitors have been utilized, their effectiveness in managing the disease during its initial phase has certain limitations. This review aimed to explore new targets for the early diagnosis and treatment of ADPKD, considering ongoing developments. We particularly focus on cell polarity, which is a key factor that influences the process and pace of cyst formation. In addition, we aimed to identify agents or treatments that can prevent or impede the progression of renal fibrosis, ultimately slowing its trajectory toward end-stage renal disease. Recent advances in slowing ADPKD progression have been examined, and potential therapeutic approaches targeting multiple pathways have been introduced. This comprehensive review discusses innovative strategies to address the challenges of ADPKD and provides valuable insights into potential avenues for its prevention and treatment.

The CPLANE protein Fuzzy regulates ciliogenesis by suppressing actin polymerization at the base of the primary cilium via p190A RhoGAP

The primary cilium decorates most eukaryotic cells and regulates tissue morphogenesis and maintenance. Structural or functional defects of primary cilium result in ciliopathies, congenital human disorders affecting multiple organs. Pathogenic variants in the ciliogenesis and planar cell polarity effectors (CPLANE) genes FUZZY, INTU and WDPCP disturb ciliogenesis, causing severe ciliopathies in humans and mice. Here, we show that the loss of Fuzzy in mice results in defects of primary cilia, accompanied by increased RhoA activity and excessive actin polymerization at the basal body. We discovered that, mechanistically, Fuzzy interacts with and recruits the negative actin regulator ARHGAP35 (also known as p190A RhoGAP) to the basal body. We identified genetic interactions between the two genes and found that a mutant ArhGAP35 allele increases the severity of phenotypic defects observed in Fuzzy-/- mice. Based on our findings, we propose that Fuzzy regulates ciliogenesis by recruiting ARHGAP35 to the basal body, where the latter likely restricts actin polymerization and modifies the actin network. Our study identifies a mechanism whereby CPLANE proteins control both actin polymerization and primary cilium formation.

The role of cilia during organogenesis in zebrafish

Cilia are hair-like organelles that protrude from the surface of eukaryotic cells and are present on the surface of nearly all human cells. Cilia play a crucial role in signal transduction, organ development and tissue homeostasis. Abnormalities in the structure and function of cilia can lead to a group of human diseases known as ciliopathies. Currently, zebrafish serves as an ideal model for studying ciliary function and ciliopathies due to its relatively conserved structure and function of cilia compared to humans. In this review, we will summarize the different types of cilia that present in embryonic and adult zebrafish, and provide an overview of the advantages of using zebrafish as a vertebrate model for cilia research. We will specifically focus on the roles of cilia during zebrafish organogenesis based on recent studies. Additionally, we will highlight future prospects for ciliary research in zebrafish.

Functions of the primary cilium in the kidney and its connection with renal diseases

The nonmotile primary cilium is a sensory structure found on most mammalian cell types that integrates multiple signaling pathways involved in tissue development and postnatal function. As such, mutations disrupting cilia activities cause a group of disorders referred to as ciliopathies. These disorders exhibit a wide spectrum of phenotypes impacting nearly every tissue. In the kidney, primary cilia dysfunction caused by mutations in polycystin 1 (Pkd1), polycystin 2 (Pkd2), or polycystic kidney and hepatic disease 1 (Pkhd1), result in polycystic kidney disease (PKD), a progressive disorder causing renal functional decline and end-stage renal disease. PKD affects nearly 1 in 1000 individuals and as there is no cure for PKD, patients frequently require dialysis or renal transplantation. Pkd1, Pkd2, and Pkhd1 encode membrane proteins that all localize in the cilium. Pkd1 and Pkd2 function as a nonselective cation channel complex while Pkhd1 protein function remains uncertain. Data indicate that the cilium may act as a mechanosensor to detect fluid movement through renal tubules. Other functions proposed for the cilium and PKD proteins in cyst development involve regulation of cell cycle and oriented division, regulation of renal inflammation and repair processes, maintenance of epithelial cell differentiation, and regulation of mitochondrial structure and metabolism. However, how loss of cilia or cilia function leads to cyst development remains elusive. Studies directed at understanding the roles of Pkd1, Pkd2, and Pkhd1 in the cilium and other locations within the cell will be important for developing therapeutic strategies to slow cyst progression.

Adhesion GPCR Gpr126 (Adgrg6) Expression Profiling in Zebrafish, Mouse, and Human Kidney

Adhesion G protein-coupled receptors (aGPCRs) comprise the second-largest class of GPCRs, the most common target for approved pharmacological therapies. aGPCRs play an important role in development and disease and have recently been associated with the kidney. Several aGPCRs are expressed in the kidney and some aGPCRs are either required for kidney development or their expression level is altered in diseased kidneys. Yet, general aGPCR function and their physiological role in the kidney are poorly understood. Here, we characterize in detail Gpr126 (Adgrg6) expression based on RNAscope® technology in zebrafish, mice, and humans during kidney development in adults. Gpr126 expression is enriched in the epithelial linage during nephrogenesis and persists in the adult kidney in parietal epithelial cells, collecting ducts, and urothelium. Single-cell RNAseq analysis shows that gpr126 expression is detected in zebrafish in a distinct ionocyte sub-population. It is co-detected selectively with slc9a3.2, slc4a4a, and trpv6, known to be involved in apical acid secretion, buffering blood or intracellular pH, and to maintain high cytoplasmic Ca2+ concentration, respectively. Furthermore, gpr126-expressing cells were enriched in the expression of potassium transporter kcnj1a.1 and gcm2, which regulate the expression of a calcium sensor receptor. Notably, the expression patterns of Trpv6, Kcnj1a.1, and Gpr126 in mouse kidneys are highly similar. Collectively, our approach permits a detailed insight into the spatio-temporal expression of Gpr126 and provides a basis to elucidate a possible role of Gpr126 in kidney physiology.

Mesenchymal Vangl facilitates airway elongation and widening independently of the planar cell polarity complex

A hallmark of mammalian lungs is the fractal nature of the bronchial tree. In the adult, each successive generation of airways is a fraction of the size of the parental branch. This fractal structure is physiologically beneficial, as it minimizes the energy needed for breathing. Achieving this pattern likely requires precise control of airway length and diameter, as the branches of the embryonic airways initially lack the fractal scaling observed in those of the adult lung. In epithelial monolayers and tubes, directional growth can be regulated by the planar cell polarity (PCP) complex. Here, we comprehensively characterized the roles of PCP-complex components in airway initiation, elongation, and widening during branching morphogenesis of the murine lung. Using tissue-specific knockout mice, we surprisingly found that branching morphogenesis proceeds independently of PCP-component expression in the developing airway epithelium. Instead, we found a novel, Celsr1-independent role for the PCP component Vangl in the pulmonary mesenchyme. Specifically, mesenchymal loss of Vangl1/2 leads to defects in branch initiation, elongation, and widening. At the cellular level, we observe changes in the shape of smooth muscle cells that indicate a potential defect in collective mesenchymal rearrangements, which we hypothesize are necessary for lung morphogenesis. Our data thus reveal an explicit function for Vangl that is independent of the core PCP complex, suggesting a functional diversification of PCP components in vertebrate development. These data also reveal an essential role for the embryonic mesenchyme in generating the fractal structure of airways of the mature lung.

Planar cell polarity regulators in asymmetric organogenesis during development and disease

The phenomenon of planar cell polarity is critically required for a myriad of morphogenetic processes in metazoan and is accurately controlled by several conserved modules. Six "core" proteins, including Frizzled, Flamingo (Celsr), Van Gogh (Vangl), Dishevelled, Prickle, and Diego (Ankrd6), are major components of the Wnt/planar cell polarity pathway. The Fat/Dchs protocadherins and the Scrib polarity complex also function to instruct cellular polarization. In vertebrates, all these pathways are essential for tissue and organ morphogenesis, such as neural tube closure, left-right symmetry breaking, heart and gut morphogenesis, lung and kidney branching, stereociliary bundle orientation, and proximal-distal limb elongation. Mutations in planar polarity genes are closely linked to various congenital diseases. Striking advances have been made in deciphering their contribution to the establishment of spatially oriented pattern in developing organs and the maintenance of tissue homeostasis. The challenge remains to clarify the complex interplay of different polarity pathways in organogenesis and the link of cell polarity to cell fate specification. Interdisciplinary approaches are also important to understand the roles of mechanical forces in coupling cellular polarization and differentiation. This review outlines current advances on planar polarity regulators in asymmetric organ formation, with the aim to identify questions that deserve further investigation.

Mechanism of cystogenesis by Cd79a-driven, conditional mTOR activation in developing mouse nephrons

Polycystic kidney disease (PKD) is a common genetic disorder arising from developmental and postnatal processes. Defects in primary cilia and their signaling (eg, mTOR) underlie the pathogenesis. However, how mTOR regulates tubular integrity remains unclear. The paucity of faithful models has limited our understanding of pathogenesis and, therefore, the refinement of therapeutic targets. To understand the role of mTOR in early cystogenesis, we studied an in-house mouse model, Cd79a-Cre;Tsc1ff. (Cd79a-Tsc1 KO hereafter), recapitulating human autosomal-dominant PKD histology. Cre-mediated Tsc1 depletion driven by the promoter for Cd79a, a known B-cell receptor, activated mTORC1 exclusively along the distal nephron from embryonic day 16 onward. Cysts appeared in the distal nephron at 1 weeks of age and mice developed definite PKD by 4 weeks. Cd79a-Tsc1 KO tubule cells proliferated at a rate comparable to controls after birth but continued to divide even after postnatal day 14 when tubulogenesis is normally completed. Apoptosis occurred only after 9 weeks. During postnatal days 7-11, pre-cystic Cd79a-Tsc1 KO tubule cells showed cilia elongation, aberrant cell intercalation, and mitotic division, suggesting that defective cell planar polarity (PCP) may underlie cystogenesis. mTORC1 was activated in a portion of cyst-lining cells and occasionally even when Tsc1 was not depleted, implying a non-autonomous mechanism. Our results indicate that mTORC1 overactivation in developing distal tubules impairs their postnatal narrowing by disrupting morphogenesis, which orients an actively proliferating cell toward the elongating axis. The interplay between mTOR and cilium signaling, which coordinate cell proliferation with PCP, may be essential for cystogenesis.

How to build an epithelial tree

Nature has evolved a variety of mechanisms to build epithelial trees of diverse architectures within different organs and across species. Epithelial trees are elaborated through branch initiation and extension, and their morphogenesis ends with branch termination. Each of these steps of the branching process can be driven by the actions of epithelial cells themselves (epithelial-intrinsic mechanisms) or by the cells of their surrounding tissues (epithelial-extrinsic mechanisms). Here, we describe examples of how these mechanisms drive each stage of branching morphogenesis, drawing primarily from studies of the lung, kidney, salivary gland, mammary gland, and pancreas, all of which contain epithelial trees that form through collective cell behaviors. Much of our understanding of epithelial branching comes from experiments using mice, but we also include examples here from avian and reptilian models. Throughout, we highlight how distinct mechanisms are employed in different organs and species to build epithelial trees. We also highlight how similar morphogenetic motifs are used to carry out conserved developmental programs or repurposed to support novel ones. Understanding the unique strategies used by nature to build branched epithelia from across the tree of life can help to inspire creative solutions to problems in tissue engineering and regenerative medicine.

Single Gene Mutations in Pkd1 or Tsc2 Alter Extracellular Vesicle Production and Trafficking

Simple Summary

Extracellular vesicles shed from primary cilia may be involved in renal cystogenesis. The disruption of the Pkd1 gene in our cell culture system increased the production of EVs in a similar way that occurs when the Tsc2 gene is disrupted. Disruption of the primary cilia depresses EV production, and this may be the reason that the combined Kif3A/Pkd1 mutant mouse has a less severe phenotype than the Pkd1 mutant alone. We initiated studies aimed at understanding the renal trafficking of renally-derived EVs and found that single gene disruptions can alter the EV kinetics based on dye tracking studies. These results raise the possibility that EV features, such as cargo, dose, tissue half-life, and targeting, may be involved in the disease process, and these features may also be fertile targets for diagnostic, prognostic, and therapeutic investigation.

Abstract

Patients with autosomal dominant polycystic kidney disease (ADPKD) and tuberous sclerosis complex (TSC) are born with normal or near-normal kidneys that later develop cysts and prematurely lose function. Both renal cystic diseases appear to be mediated, at least in part, by disease-promoting extracellular vesicles (EVs) that induce genetically intact cells to participate in the renal disease process. We used centrifugation and size exclusion chromatography to isolate the EVs for study. We characterized the EVs using tunable resistive pulse sensing, dynamic light scattering, transmission electron microscopy, and Western blot analysis. We performed EV trafficking studies using a dye approach in both tissue culture and in vivo studies. We have previously reported that loss of the Tsc2 gene significantly increased EV production and here demonstrate that the loss of the Pkd1 gene also significantly increases EV production. Using a cell culture system, we also show that loss of either the Tsc2 or Pkd1 gene results in EVs that exhibit an enhanced uptake by renal epithelial cells and a prolonged half-life. Loss of the primary cilia significantly reduces EV production in renal collecting duct cells. Cells that have a disrupted Pkd1 gene produce EVs that have altered kinetics and a prolonged half-life, possibly impacting the duration of the EV cargo effect on the recipient cell. These results demonstrate the interplay between primary cilia and EVs and support a role for EVs in polycystic kidney disease pathogenesis.

Prickle isoform participation in distinct polarization events in the Drosophila eye

Planar cell polarity (PCP) signaling regulates several polarization events during development of ommatidia in the Drosophila eye, including directing chirality by polarizing a cell fate choice and determining the direction and extent of ommatidial rotation. The pksple isoform of the PCP protein Prickle is known to participate in the R3/R4 cell fate decision, but the control of other polarization events and the potential contributions of the three Pk isoforms have not been clarified. Here, by characterizing expression and subcellular localization of individual isoforms together with re-analyzing isoform specific phenotypes, we show that the R3/R4 fate decision, its coordination with rotation direction, and completion of rotation to a final ±90° rotation angle are separable polarization decisions with distinct Pk isoform requirements and contributions. Both pksple and pkpk can enforce robust R3/R4 fate decisions, but only pksple can correctly orient them along the dorsal-ventral axis. In contrast, pksple and pkpk can fully and interchangeably sustain coordination of rotation direction and rotation to completion. We propose that expression dynamics and competitive interactions determine isoform participation in these processes. We propose that the selective requirement for pksple to orient the R3/R4 decision and their interchangeability for coordination and completion of rotation reflects their previously described differential interaction with the Fat/Dachsous system which is known to be required for orientation of R3/R4 decisions but not for coordination or completion of rotation.

Mechanical Feedback Control for Multicellular Tissue Size Maintenance: A Minireview

All living tissues and organs have their respective sizes, critical to various biological functions, such as development, growth, and homeostasis. As tissues and organs generally converge to a certain size, intrinsic regulatory mechanisms may be involved in the maintenance of size regulation. In recent years, important findings regarding size regulation have been obtained from diverse disciplines at the molecular and cellular levels. Here, I briefly review the size regulation of biological tissues from the perspective of control systems. This minireview focuses on how feedback systems engage in tissue size maintenance through the mechanical interactions of constituent cell collectives through intracellular signaling. I introduce a general framework of a feedback control system for tissue size regulation, followed by two examples: maintenance of epithelial tissue volume and epithelial tube diameter. The examples deliver the idea of how cellular mechano-response works for maintaining tissue size.

Loss of Planar Cell Polarity Effector Fuzzy Causes Renal Hypoplasia by Disrupting Several Signaling Pathways

In vertebrates, the planar cell polarity (PCP) pathway regulates tissue morphogenesis during organogenesis, including the kidney. Mutations in human PCP effector proteins have been associated with severe syndromic ciliopathies. Importantly, renal hypoplasia has been reported in some patients. However, the developmental disturbance that causes renal hypoplasia is unknown. Here, we describe the early onset of profound renal hypoplasia in mice homozygous for null mutation of the PCP effector gene, Fuzzy. We found that this phenotype is caused by defective branching morphogenesis of the ureteric bud (UB) in the absence of defects in nephron progenitor specification or in early steps of nephrogenesis. By using various experimental approaches, we show that the loss of Fuzzy affects multiple signaling pathways. Specifically, we found mild involvement of GDNF/c-Ret pathway that drives UB branching. We noted the deficient expression of molecules belonging to the Bmp, Fgf and Shh pathways. Analysis of the primary cilia in the UB structures revealed a significant decrease in ciliary length. We conclude that renal hypoplasia in the mouse Fuzzy mutants is caused by defective UB branching associated with dysregulation of ciliary and non-ciliary signaling pathways. Our work suggests a PCP effector-dependent pathogenetic mechanism that contributes to renal hypoplasia in mice and humans.

Frizzled3 inhibits Vangl2-Prickle3 association to establish planar cell polarity in the vertebrate neural plate

The orientation of epithelial cells in the plane of the tissue, known as planar cell polarity (PCP), is regulated by interactions of asymmetrically localized PCP protein complexes. In the Xenopus neural plate, Van Gogh-like2 (Vangl2) and Prickle3 (Pk3) proteins form a complex at the anterior cell boundaries, but how this complex is regulated in vivo remains largely unknown. Here, we use proximity biotinylation and crosslinking approaches to show that Vangl2-Pk3 association is inhibited by Frizzled3 (Fz3, also known as Fzd3), a core PCP protein that is specifically expressed in the neuroectoderm and is essential for the establishment of PCP in this tissue. This inhibition required Fz3-dependent Vangl2 phosphorylaton. Consistent with our observations, the complex of Pk3 with nonphosphorylatable Vangl2 did not polarize in the neural plate. These findings provide evidence for in vivo regulation of Vangl2-Pk3 complex formation and localization by a Frizzled receptor.

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.

Nephronophthisis-Pathobiology and Molecular Pathogenesis of a Rare Kidney Genetic Disease

The exponential rise in our understanding of the aetiology and pathophysiology of genetic cystic kidney diseases can be attributed to the identification of cystogenic genes over the last three decades. The foundation of this was laid by positional cloning strategies which gradually shifted towards next-generation sequencing (NGS) based screenings. This shift has enabled the discovery of novel cystogenic genes at an accelerated pace unlike ever before and, most notably, the past decade has seen the largest increase in identification of the genes which cause nephronophthisis (NPHP). NPHP is a monogenic autosomal recessive cystic kidney disease caused by mutations in a diverse clade of over 26 identified genes and is the most common genetic cause of renal failure in children. NPHP gene types present with some common pathophysiological features alongside a diverse range of extra-renal phenotypes associated with specific syndromic presentations. This review provides a timely update on our knowledge of this disease, including epidemiology, pathophysiology, anatomical and molecular features. We delve into the diversity of the NPHP causing genes and discuss known molecular mechanisms and biochemical pathways that may have possible points of intersection with polycystic kidney disease (the most studied renal cystic pathology). We delineate the pathologies arising from extra-renal complications and co-morbidities and their impact on quality of life. Finally, we discuss the current diagnostic and therapeutic modalities available for disease management, outlining possible avenues of research to improve the prognosis for NPHP patients.

The Wnt/PCP formin Daam1 drives cell-cell adhesion during nephron development

E-cadherin junctions facilitate assembly and disassembly of cell contacts that drive development and homeostasis of epithelial tissues. In this study, using Xenopus embryonic kidney and Madin-Darby canine kidney (MDCK) cells, we investigate the role of the Wnt/planar cell polarity (PCP) formin Daam1 (Dishevelled-associated activator of morphogenesis 1) in regulating E-cadherin-based intercellular adhesion. Using live imaging, we show that Daam1 localizes to newly formed cell contacts in the developing nephron. Furthermore, analyses of junctional filamentous actin (F-actin) upon Daam1 depletion indicate decreased microfilament localization and slowed turnover. We also show that Daam1 is necessary for efficient and timely localization of junctional E-cadherin, mediated by Daam1’s formin homology domain 2 (FH2). Finally, we establish that Daam1 signaling promotes organized movement of renal cells. This study demonstrates that Daam1 formin junctional activity is critical for epithelial tissue organization.

How cells remodel their adhesions through cell-surface proteins such as E-cadherin is a central question in epithelial tissue biology. Krneta-Stankic et al. show that the Wnt/PCP formin Daam1 regulates cytoskeletal membrane dynamics and E-cadherin localization within developing nephrons. These findings provide a new framework for studying cell-cell adhesion and nephron morphogenesis.

Planar cell polarity pathway in kidney development, function and disease

Planar cell polarity (PCP) refers to the coordinated orientation of cells in the tissue plane. Originally discovered and studied in Drosophila melanogaster, PCP is now widely recognized in vertebrates, where it is implicated in organogenesis. Specific sets of PCP genes have been identified. The proteins encoded by these genes become asymmetrically distributed to opposite sides of cells within a tissue plane and guide many processes that include changes in cell shape and polarity, collective cell movements or the uniform distribution of cell appendages. A unifying characteristic of these processes is that they often involve rearrangement of actomyosin. Mutations in PCP genes can cause malformations in organs of many animals, including humans. In the past decade, strong evidence has accumulated for a role of the PCP pathway in kidney development including outgrowth and branching morphogenesis of ureteric bud and podocyte development. Defective PCP signalling has been implicated in the pathogenesis of developmental kidney disorders of the congenital anomalies of the kidney and urinary tract spectrum. Understanding the origins, molecular constituents and cellular targets of PCP provides insights into the involvement of PCP molecules in normal kidney development and how dysfunction of PCP components may lead to kidney disease.

Apical Restriction of the Planar Cell Polarity Component VANGL in Pancreatic Ducts Is Required to Maintain Epithelial Integrity

Cell polarity is essential for the architecture and function of numerous epithelial tissues. Here, we show that apical restriction of planar cell polarity (PCP) components is necessary for the maintenance of epithelial integrity. Using the mammalian pancreas as a model, we find that components of the core PCP pathway, such as the transmembrane protein Van Gogh-like (VANGL), become apically restricted over a period of several days. Expansion of VANGL localization to the basolateral membranes of progenitors leads to their death and disruption of the epithelial integrity. VANGL basolateral expansion does not affect apico-basal polarity but acts in the cells where Vangl is mislocalized by reducing Dishevelled and its downstream target ROCK. This reduction in ROCK activity culminates in progenitor cell egression, death, and eventually pancreatic hypoplasia. Thus, precise spatiotemporal modulation of VANGL-dependent PCP signaling is crucial for proper pancreatic morphogenesis.

The Biological Significance and Implications of Planar Cell Polarity for Nephrology

The orientation of cells in two-dimensional and three-dimensional space underpins how the kidney develops and responds to disease. The process by which cells orientate themselves within the plane of a tissue is termed planar cell polarity. In this Review, we discuss how planar cell polarity and the proteins that underpin it govern kidney organogenesis and pathology. The importance of planar cell polarity and its constituent proteins in multiple facets of kidney development is emphasised, including ureteric bud branching, tubular morphogenesis and nephron maturation. An overview is given of the relevance of planar cell polarity and its proteins for inherited human renal diseases, including congenital malformations with unknown aetiology and polycystic kidney disease. Finally, recent work is described outlining the influence of planar cell polarity proteins on glomerular diseases and highlight how this fundamental pathway could yield a new treatment paradigm for nephrology.

Ontogenesis of the tear drainage system requires Prickle1-driven polarized basement membrane deposition

In terrestrial animals, the lacrimal drainage apparatus evolved to serve as conduits for tear flow; however, little is known about the ontogenesis of this system. Here, we define the anatomy of the fully formed tear duct in mice, characterize crucial morphogenetic events for the development of tear duct components and identify the site for primordial tear duct (PTD) initiation. We report that the PTD originates from the orbital lacrimal lamina, a junction formed by the epithelia of the maxillary and lateral nasal processes. We demonstrate that Prickle1, a key component of planar cell polarity signaling, is expressed in progenitors of the PTD and throughout tear duct morphogenesis. Disruption of Prickle1 stalls tear duct elongation; in particular, the loss of basement membrane deposition and aberrant cytoplasmic accumulation of laminin are salient. Altered cell adhesion, cytoskeletal transport systems, vesicular transport systems and cell axis orientation in Prickle1 mutants support the role of Prickle1 in planar cell polarity. Taken together, our results highlight a crucial role of Prickle1-mediated polarized basement membrane secretion and deposition in PTD elongation.

Malformation of Tear Ducts Underlies the Epiphora and Precocious Eyelid Opening in Prickle 1 Mutant Mice: Genetic Implications for Tear Duct Genesis

Purpose

Obstruction of the tear drainage causes a range of ocular surface disorders. Hitherto, the genetics of tear duct development and obstruction has been scarcely explored, and related animal models are lacking. This study aims to study the potential role of the Wnt/PCP pathway mediated by Prickle 1 in tear duct development and diseases.

Methods

A severe hypomorphic Prickle 1 mutant was generated. Histology and immunohistochemistry were performed to compare wild type, Prickle 1 hypomorphic, and null mutant tear ducts. In situ hybridization was conducted to identify the signaling components in the developing tear ducts. Three-dimensional (3D) reconstruction was used to detect the human embryonic tear duct.

Results

Here, we report that a severe Prickle 1 hypomorph mouse line exhibited epiphora. This phenotype was due to the blockage of the tear drainage by incompletely formed nasolacrimal duct (NLD) and lacrimal canaliculi (LC), which also causes precocious eyelid opening. We observed a dose-dependent requirement of Prickle 1 for tear duct outgrowth. An investigation of the expression of Wnt/PCP core genes demonstrated a subset of PCP signaling components expressed in the developing tear duct. Furthermore, Prickle 1 is not required for the expression of Fgfr2/Fgf10 and p63 genes, which are associated with the NLD and LC hypoplasia in humans. Last, we showed that Prickle 1 expression in the developing tear drainage system is conserved between mice and humans.

Conclusions

The study suggests that malformed tear ducts caused by disruption of Prickle 1 underlies the epiphora and precocious eyelid opening.

Ciliopathies and the Kidney: A Review

Primary cilia are specialized sensory organelles that protrude from the apical surface of most cell types. During the past 2 decades, they have been found to play important roles in tissue development and signal transduction, with mutations in ciliary-associated proteins resulting in a group of diseases collectively known as ciliopathies. Many of these mutations manifest as renal ciliopathies, characterized by kidney dysfunction resulting from aberrant cilia or ciliary functions. This group of overlapping and genetically heterogeneous diseases includes polycystic kidney disease, nephronophthisis, and Bardet-Biedl syndrome as the main focus of this review. Renal ciliopathies are characterized by the presence of kidney cysts that develop due to uncontrolled epithelial cell proliferation, growth, and polarity, downstream of dysregulated ciliary-dependent signaling. Due to cystic-associated kidney injury and systemic inflammation, cases result in kidney failure requiring dialysis and transplantation. Of the handful of pharmacologic treatments available, none are curative. It is important to determine the molecular mechanisms that underlie the involvement of the primary cilium in cyst initiation, expansion, and progression for the development of novel and efficacious treatments. This review updates research progress in defining key genes and molecules central to ciliogenesis and renal ciliopathies.

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).

Application of a human mesoderm tissue elongation system in vitro derived from human induced pluripotent stem cells to risk assessment for teratogenic chemicals

Toxic compounds from the mother's diet and medication in addition to genetic factors and infection during pregnancy remain risks for various congenital disorders and misbirth. To ensure the safety of food and drugs for pregnant women, establishment of an in vitro system that morphologically resembles human tissues has been long desired. In this study, we focused on dorsal mesoderm elongation, one of the critical early development events for trunk formation, and we established in vitro autonomous elongating tissues from human induced pluripotent stem cells (hiPSCs). This artificial tissue elongation is regulated by MYOSIN II and FGF signaling, and is diminished by methylmercury or retinoic acid (RA), similar to in vivo human developmental disabilities. Moreover, our method for differentiation of hiPSCs requires only a short culture period, and the elongation is cell number-independent. Therefore, our in vitro human tissue elongation system is a potential tool for risk assessment assays for identification of teratogenic chemicals via human tissue morphogenesis.

mTOR and S6K1 drive polycystic kidney by the control of Afadin-dependent oriented cell division

mTOR activation is essential and sufficient to cause polycystic kidneys in Tuberous Sclerosis Complex (TSC) and other genetic disorders. In disease models, a sharp increase of proliferation and cyst formation correlates with a dramatic loss of oriented cell division (OCD). We find that OCD distortion is intrinsically due to S6 kinase 1 (S6K1) activation. The concomitant loss of S6K1 in Tsc1-mutant mice restores OCD but does not decrease hyperproliferation, leading to non-cystic harmonious hyper growth of kidneys. Mass spectrometry-based phosphoproteomics for S6K1 substrates revealed Afadin, a known component of cell-cell junctions required to couple intercellular adhesions and cortical cues to spindle orientation. Afadin is directly phosphorylated by S6K1 and abnormally decorates the apical surface of Tsc1-mutant cells with E-cadherin and α-catenin. Our data reveal that S6K1 hyperactivity alters centrosome positioning in mitotic cells, affecting oriented cell division and promoting kidney cysts in conditions of mTOR hyperactivity.

Cilia and polycystic kidney disease

Polycystic kidney disease (PKD), comprising autosomal dominant polycystic kidney disease (ADPKD) and autosomal recessive polycystic kidney disease (ARPKD), is characterized by incessant cyst formation in the kidney and liver. ADPKD and ARPKD represent the leading genetic causes of renal disease in adults and children, respectively. ADPKD is caused by mutations in PKD1 encoding polycystin1 (PC1) and PKD2 encoding polycystin 2 (PC2). PC1/2 are multi-pass transmembrane proteins that form a complex localized in the primary cilium. Predominant ARPKD cases are caused by mutations in polycystic kidney and hepatic disease 1 (PKHD1) gene that encodes the Fibrocystin/Polyductin (FPC) protein, whereas a small subset of cases are caused by mutations in DAZ interacting zinc finger protein 1 like (DZIP1L) gene. FPC is a type I transmembrane protein, localizing to the cilium and basal body, in addition to other compartments, and DZIP1L encodes a transition zone/basal body protein. Apparently, PC1/2 and FPC are signaling molecules, while the mechanism that cilia employ to govern renal tubule morphology and prevent cyst formation is unclear. Nonetheless, recent genetic and biochemical studies offer a glimpse of putative physiological malfunctions and the pathomechanisms underlying both disease entities. In this review, I summarize the results of genetic studies that deduced the function of PC1/2 on cilia and of cilia themselves in cyst formation in ADPKD, and I discuss studies regarding regulation of polycystin biogenesis and cilia trafficking. I also summarize the synergistic genetic interactions between Pkd1 and Pkhd1, and the unique tissue patterning event controlled by FPC, but not PC1. Interestingly, while DZIP1L mutations generate compromised PC1/2 cilia expression, FPC deficiency does not affect PC1/2 biogenesis and ciliary localization, indicating that divergent mechanisms could lead to cyst formation in ARPKD. I conclude by outlining promising areas for future PKD research and highlight rationales for potential therapeutic interventions for PKD treatment.

Noncanonical Wnt planar cell polarity signaling in lung development and disease

The planar cell polarity (PCP) signaling pathway is a potent developmental regulator of directional cell behaviors such as migration, asymmetric division and morphological polarization that are critical for shaping the body axis and the complex three-dimensional architecture of tissues and organs. PCP is considered a noncanonical Wnt pathway due to the involvement of Wnt ligands and Frizzled family receptors in the absence of the beta-catenin driven gene expression observed in the canonical Wnt cascade. At the heart of the PCP mechanism are protein complexes capable of generating molecular asymmetries within cells along a tissue-wide axis that are translated into polarized actin and microtubule cytoskeletal dynamics. PCP has emerged as an important regulator of developmental, homeostatic and disease processes in the respiratory system. It acts along other signaling pathways to create the elaborately branched structure of the lung by controlling the directional protrusive movements of cells during branching morphogenesis. PCP operates in the airway epithelium to establish and maintain the orientation of respiratory cilia along the airway axis for anatomically directed mucociliary clearance. It also regulates the establishment of the pulmonary vasculature. In adult tissues, PCP dysfunction has been linked to a variety of chronic lung diseases such as cystic fibrosis, chronic obstructive pulmonary disease, and idiopathic pulmonary arterial hypertension, stemming chiefly from the breakdown of proper tissue structure and function and aberrant cell migration during regenerative wound healing. A better understanding of these (impaired) PCP mechanisms is needed to fully harness the therapeutic opportunities of targeting PCP in chronic lung diseases.

Differential role of planar cell polarity gene Vangl2 in embryonic and adult mammalian kidneys

Planar cell polarity (PCP) pathway is crucial for tissue morphogenesis. Mutations in PCP genes cause multi-organ anomalies including dysplastic kidneys. Defective PCP signaling was postulated to contribute to cystogenesis in polycystic kidney disease. This work was undertaken to elucidate the role of the key PCP gene, Vangl2, in embryonic and postnatal renal tubules and ascertain whether its loss contributes to cyst formation and defective tubular function in mature animals. We generated mice with ubiquitous and collecting duct-restricted excision of Vangl2. We analyzed renal tubules in mutant and control mice at embryonic day E17.5 and postnatal days P1, P7, P30, P90, 6- and 9-month old animals. The collecting duct functions were analyzed in young and adult mutant and control mice. Loss of Vangl2 leads to profound tubular dilatation and microcysts in embryonic kidneys. Mechanistically, these abnormalities are caused by defective convergent extension (larger tubular cross-sectional area) and apical constriction (cuboidal cell shape and a reduction of activated actomyosin at the luminal surface). However, the embryonic tubule defects were rapidly resolved by Vangl2-independent mechanisms after birth. Normal collecting duct architecture and functions were found in young and mature animals. During embryogenesis, Vangl2 controls tubular size via convergent extension and apical constriction. However, rapidly after birth, PCP-dependent control of tubular size is switched to a PCP-independent regulatory mechanism. We conclude that loss of the Vangl2 gene is dispensable for tubular elongation and maintenance postnatally. It does not lead to cyst formation and is unlikely to contribute to polycystic kidney disease.

Polarized cellular mechano-response system for maintaining radial size in developing epithelial tubes

Size control in biological tissues involves multicellular communication via mechanical forces during development. Although fundamental cellular behaviours in response to mechanical stimuli underlie size maintenance during morphogenetic processes, the mechanisms underpinning the cellular mechano-response system that maintains size along an axis of a polarized tissue remain elusive. Here, we show how the diameter of an epithelial tube is maintained during murine epididymal development by combining quantitative imaging, mechanical perturbation and mathematical modelling. We found that epithelial cells counteract compressive forces caused by cell division exclusively along the circumferential axis of the tube to produce polarized contractile forces, eventually leading to an oriented cell rearrangement. Moreover, a mathematical model that includes the polarized mechano-responsive regime explains how the diameter of proliferating tubes is maintained. Our findings pave the way for an improved understanding of the cellular response to mechanical forces that involves collective multicellular behaviours for organizing diverse tissue morphologies.

Summary: Polarized cellular constriction responding to mechanical stress controls the diameter of a developing epithelial tube during murine epididymal development.

Four-jointed knock-out delays renal failure in an ADPKD model with kidney injury

Autosomal Dominant Polycystic Kidney Disease is characterised by the development of fluid‐filled cysts in the kidneys which lead to end‐stage renal disease (ESRD). In the majority of cases, the disease is caused by a mutation in the Pkd1 gene. In a previous study, we demonstrated that renal injury can accelerate cyst formation in Pkd1 knock‐out (KO) mice. In that study, we found that after injury four‐jointed (Fjx1), an upstream regulator of planar cell polarity and the Hippo pathway, was aberrantly expressed in Pkd1 KO mice compared to WT. Therefore, we hypothesised a role for Fjx1 in injury/repair and cyst formation. We generated single and double deletion mice for Pkd1 and Fjx1, and we induced toxic renal injury using the nephrotoxic compound 1,2‐dichlorovinyl‐cysteine. We confirmed that nephrotoxic injury can accelerate cyst formation in Pkd1 mutant mice. This caused Pkd1 KO mice to reach ESRD significantly faster; unexpectedly, double KO mice survived significantly longer. Cyst formation was comparable in both models, but we found significantly less fibrosis and macrophage infiltration in double KO mice. Taken together, these data suggest that Fjx1 disruption protects the cystic kidneys against kidney failure by reducing inflammation and fibrosis. Moreover, we describe, for the first time, an interesting (yet unidentified) mechanism that partially discriminates cyst growth from fibrogenesis. © 2019 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.

VANGL2 regulates luminal epithelial organization and cell turnover in the mammary gland

The VANGL family of planar cell polarity proteins is implicated in breast cancer however its function in mammary gland biology is unknown. Here, we utilized a panel of Vang1 and Vangl2 mouse alleles to examine the requirement of VANGL family members in the murine mammary gland. We show that Vang1CKO^Δ/Δ glands display normal branching while Vangl2^flox/flox and Vangl2^Lp/Lp tissue exhibit several phenotypes. In MMTV-Cre;Vangl2^flox/flox glands, cell turnover is reduced and lumens are narrowed. A Vangl2 missense mutation in the Vangl2^Lp/Lp tissue leads to mammary anlage sprouting defects and deficient outgrowth with transplantation of anlage or secondary tissue fragments. In successful Vangl2^Lp/Lp outgrowths, three morphological phenotypes are observed: distended ducts, supernumerary end buds, and ectopic acini. Layer specific defects are observed with loss of Vangl2 selectively in either basal or luminal layers of mammary cysts. Loss in the basal compartment inhibits cyst formation, but has the opposite effect in the luminal compartment. Candidate gene analysis on MMTV-Cre;Vangl2^flox/flox and Vangl2^Lp/Lp tissue reveals a significant reduction in Bmi1 expression, with overexpression of Bmi1 rescuing defects in Vangl2 knockdown cysts. Our results demonstrate that VANGL2 is necessary for normal mammary gland development and indicate differential functional requirements in basal versus luminal mammary compartments.

Biophysics in oviduct: Planar cell polarity, cilia, epithelial fold and tube morphogenesis, egg dynamics

Organs and tissues in multi-cellular organisms exhibit various morphologies. Tubular organs have multi-scale morphological features which are closely related to their functions. Here we discuss morphogenesis and the mechanical functions of the vertebrate oviduct in the female reproductive tract, also known as the fallopian tube. The oviduct functions to convey eggs from the ovary to the uterus. In the luminal side of the oviduct, the epithelium forms multiple folds (or ridges) well-aligned along the longitudinal direction of the tube. In the epithelial cells, cilia are formed orienting toward the downstream of the oviduct. The cilia and the folds are supposed to be involved in egg transportation. Planar cell polarity (PCP) is developed in the epithelium, and the disruption of the Celsr1 gene, a PCP related-gene, causes randomization of both cilia and fold orientations, discontinuity of the tube, inefficient egg transportation, and infertility. In this review article, we briefly introduce various biophysical and biomechanical issues in the oviduct, including physical mechanisms of formation of PCP and organized cilia orientation, epithelial cell shape regulation, fold pattern formation generated by mechanical buckling, tubulogenesis, and egg transportation regulated by fluid flow. We also mention about possible roles of the oviducts in egg shape formation and embryogenesis, sinuous patterns of tubes, and fold and tube patterns observed in other tubular organs such as the gut, airways, etc.

Polycystic kidney disease

Cystic kidneys are common causes of end-stage renal disease, both in children and in adults. Autosomal dominant polycystic kidney disease (ADPKD) and autosomal recessive polycystic kidney disease (ARPKD) are cilia-related disorders and the two main forms of monogenic cystic kidney diseases. ADPKD is a common disease that mostly presents in adults, whereas ARPKD is a rarer and often more severe form of polycystic kidney disease (PKD) that usually presents perinatally or in early childhood. Cell biological and clinical research approaches have expanded our knowledge of the pathogenesis of ADPKD and ARPKD and revealed some mechanistic overlap between them. A reduced 'dosage' of PKD proteins is thought to disturb cell homeostasis and converging signalling pathways, such as Ca²⁺, cAMP, mechanistic target of rapamycin, WNT, vascular endothelial growth factor and Hippo signalling, and could explain the more severe clinical course in some patients with PKD. Genetic diagnosis might benefit families and improve the clinical management of patients, which might be enhanced even further with emerging therapeutic options. However, many important questions about the pathogenesis of PKD remain. In this Primer, we provide an overview of the current knowledge of PKD and its treatment.

Planar cell polarity in organ formation

The planar cell polarity (PCP) pathway controls a variety of morphological events across many species. During embryonic development, the PCP pathway regulates coordinated behaviour of groups of cells to direct morphogenetic processes such as convergent extension and collective cell migration. In this review we discuss the increasingly prominent role of the PCP pathway in organogenesis, focusing on the lungs, kidneys and heart. We also highlight emerging evidence that PCP gene mutations are associated with adult diseases.

Theoretical tool bridging cell polarities with development of robust morphologies

Despite continual renewal and damages, a multicellular organism is able to maintain its complex morphology. How is this stability compatible with the complexity and diversity of living forms? Looking for answers at protein level may be limiting as diverging protein sequences can result in similar morphologies. Inspired by the progressive role of apical-basal and planar cell polarity in development, we propose that stability, complexity, and diversity are emergent properties in populations of proliferating polarized cells. We support our hypothesis by a theoretical approach, developed to effectively capture both types of polar cell adhesions. When applied to specific cases of development – gastrulation and the origins of folds and tubes – our theoretical tool suggests experimentally testable predictions pointing to the strength of polar adhesion, restricted directions of cell polarities, and the rate of cell proliferation to be major determinants of morphological diversity and stability.

Cells have the power to organise themselves to form complex and stable structures, whether it is to create a fully shaped baby from a single egg, or to allow adult salamanders to grow a new limb after losing a leg. This ability has been scrutinised at many different levels. For example, researchers have looked at the chemical messages exchanged by cells, or they have recorded the different shapes an embryo goes through during development. However, it is still difficult to reconcile the information from these approaches into a description that makes sense at multiple scales.

When an embryo develops, sheets of cells fold and unfold to create complex 3D shapes, like the tubes that make our lungs. Moulding sheets into tubes relies on interactions between cells that are not the same in all directions. In fact, two types of asymmetry (or polarity) guide these interactions. Apical-basal polarity runs across a sheet of cells, which means that the top surface of the sheet differs from the bottom. Planar cell polarity runs along the sheet and distinguishes one end from the other. For instance, apical-basal polarity marks the inner and outer surfaces of our skin, while planar cell polarity controls the direction in which our hair grows.

Nissen et al. set out to investigate how these polarities help cells in an embryo organise themselves to form complicated folds and tubes. To do this, simple mathematical representations of both apical-basal and planar cell polarities were designed. The representations were then combined to create computer simulations of groups of cells as these divide and interact with each other.

Simulations of ‘cells’ with only apical-basal polarity were able to generate different shapes in the ‘tissues’ produced, including many found in living organisms. External conditions, such as how cells were arranged to start with, determined the resulting shape. With both apical-basal and planar cell polarities, the simulations reproduced an important change that occurs during early development. They also replicated how the tubes that transport nutrients and oxygen form.

These results show that simple properties of individual cells, such as polarities, can produce different shapes in developing tissues and organs, without the need for a complicated overarching program. Abnormal changes in cell polarity are also associated with diseases such as cancer. The mathematical model developed by Nissen et al. could therefore be a useful tool to study these events.

Tubular cell loss in early inv/nphp2 mutant kidneys represents a possible homeostatic mechanism in cortical tubular formation

Inversion of embryonic turning (inv) cystic mice develop multiple renal cysts and are a model for type II nephronophthisis (NPHP2). The defect of planar cell polarity (PCP) by oriented cell division was proposed as the underlying cellular phenotype, while abnormal cell proliferation and apoptosis occur in some polycystic kidney disease models. However, how these cystogenic phenotypes are linked and what is most critical for cystogenesis remain largely unknown. In particular, in early cortical cytogenesis in the inv mutant cystic model, it remains uncertain whether the increased proliferation index results from changes in cell cycle length or cell fate determination. To address tubular cell kinetics, doubling time and total number of tubular cells, as well as amount of genomic DNA (gDNA), were measured in mutant and normal control kidneys. Despite a significantly higher bromodeoxyuridine (BrdU)-proliferation index in the mutant, total tubular cell number and doubling time were unaffected. Unexpectedly, the mutant had tubular cell loss, characterized by a temporal decrease in tubular cells incorporating 5-ethynyl-2´-deoxyuridine (EdU) and significantly increased nuclear debris. Based on current data we established a new multi-population shift model in postnatal renal development, indicating that a few restricted tubular cell populations contribute to cortical tubular formation. As in the inv mutant phenotype, the model simulation revealed a large population of tubular cells with rapid cell cycling and tubular cell loss. The proposed cellular kinetics suggest not only the underlying mechanism of the inv mutant phenotype but also a possible renal homeostatic mechanism for tubule formation.

Adhesion GPCRs in Kidney Development and Disease

Chronic kidney disease (CKD) represents the fastest growing pathology worldwide with a prevalence of >10% in many countries. In addition, kidney cancer represents 5% of all new diagnosed cancers. As currently no effective therapies exist to restore kidney function after CKD- as well as cancer-induced renal damage, it is important to elucidate new regulators of kidney development and disease as new therapeutic targets. G protein-coupled receptors (GPCRs) represent the most successful class of pharmaceutical targets. In recent years adhesion GPCRs (aGPCRs), the second largest GPCR family, gained significant attention as they are present on almost all mammalian cells, are associated to a plethora of diseases and regulate important cellular processes. aGPCRs regulate for example cell polarity, mitotic spindle orientation, cell migration, and cell aggregation; all processes that play important roles in kidney development and/or disease. Moreover, polycystin-1, a major regulator of kidney development and disease, contains a GAIN domain, which is otherwise only found in aGPCRs. In this review, we assess the potential of aGPCRs as therapeutic targets for kidney disease. For this purpose we have summarized the available literature and analyzed data from the databases The Human Protein Atlas, EURExpress, Nephroseq, FireBrowse, cBioPortal for Cancer Genomics and the National Cancer Institute Genomic Data Commons data portal (NCIGDC). Our data indicate that most aGPCRs are expressed in different spatio-temporal patterns during kidney development and that altered aGPCR expression is associated with a variety of kidney diseases including CKD, diabetic nephropathy, lupus nephritis as well as renal cell carcinoma. We conclude that aGPCRs present a promising new class of therapeutic targets and/or might be useful as diagnostic markers in kidney disease.

The Genetic and Cellular Basis of Autosomal Dominant Polycystic Kidney Disease-A Primer for Clinicians

Autosomal dominant polycystic kidney disease (ADPKD) is one of the most common genetic disorders worldwide. In recent decades, the field has undergone a revolution, starting with the identification of causal ADPKD genes, including PKD1, PKD2, and the recently identified GANAB. In addition, advances defining the genetic mechanisms, protein localization and function, and the identification of numerous pathways involved in the disease process, have contributed to a better understanding of this illness. Together, this has led to a better prognosis, diagnosis, and treatment in clinical practice. In this mini review, we summarize and discuss new insights about the molecular mechanisms underlying ADPKD, including its genetics, protein function, and cellular pathways.