The kidney and planar cell polarity

Deficiency of the Planar Cell Polarity Protein Intu Delays Kidney Repair and Suppresses Renal Fibrosis after Acute Kidney Injury

Planar cell polarity (PCP), a process of coordinated alignment of cell polarity across the tissue plane, may contribute to the repair of renal tubules after kidney injury. Intu is a key effector protein of PCP. Herein, conditional knockout (KO) mouse models that ablate Intu specifically from kidney tubules (Intu KO) were established. Intu KO mice and wild-type littermates were subjected to unilateral renal ischemia/reperfusion injury (IRI) or unilateral ureteral obstruction. Kidney repair was evaluated by histologic, biochemical, and immunohistochemical analyses. In vitro, scratch wound healing was examined in Intu-knockdown and control renal tubular cells. Ablation of Intu in renal tubules delayed kidney repair and ameliorated renal fibrosis after renal IRI. Intu KO mice had less renal fibrosis during unilateral ureteral obstruction. Mechanistically, Intu KO kidneys had less senescence but higher levels of cell proliferation and apoptosis during kidney repair after renal IRI. In vitro, Intu knockdown suppressed scratch wound healing in renal tubular cells, accompanied by the abnormality of centrosome orientation. Together, the results provide the first evidence for the involvement of PCP in tubular repair after kidney injury, shedding light on new strategies for improving kidney repair and recovery.

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.

Wnt5a-Vangl1/2 signaling regulates the position and direction of lung branching through the cytoskeleton and focal adhesions

Lung branching morphogenesis requires reciprocal interactions between the epithelium and mesenchyme. How the lung branches are generated at a defined location and projected toward a specific direction remains a major unresolved issue. In this study, we investigated the function of Wnt signaling in lung branching in mice. We discovered that Wnt5a in both the epithelium and the mesenchyme plays an essential role in controlling the position and direction of lung branching. The Wnt5a signal is mediated by Vangl1/2 to trigger a cascade of noncanonical or planar cell polarity (PCP) signaling. In response to noncanonical Wnt signaling, lung cells undergo cytoskeletal reorganization and change focal adhesions. Perturbed focal adhesions in lung explants are associated with defective branching. Moreover, we observed changes in the shape and orientation of the epithelial sheet and the underlying mesenchymal layer in regions of defective branching in the mutant lungs. Thus, PCP signaling helps define the position and orientation of the lung branches. We propose that mechanical force induced by noncanonical Wnt signaling mediates a coordinated alteration in the shape and orientation of a group of epithelial and mesenchymal cells. These results provide a new framework for understanding the molecular mechanisms by which a stereotypic branching pattern is generated.

New mouse models for high resolution and live imaging of planar cell polarity proteins in vivo

The collective polarization of cellular structures and behaviors across a tissue plane is a near universal feature of epithelia known as planar cell polarity (PCP). This property is controlled by the core PCP pathway, which consists of highly conserved membrane-associated protein complexes that localize asymmetrically at cell junctions. Here, we introduce three new mouse models for investigating the localization and dynamics of transmembrane PCP proteins: Celsr1, Fz6 and Vangl2. Using the skin epidermis as a model, we characterize and verify the expression, localization and function of endogenously tagged Celsr1-3xGFP, Fz6-3xGFP and tdTomato-Vangl2 fusion proteins. Live imaging of Fz6-3xGFP in basal epidermal progenitors reveals that the polarity of the tissue is not fixed through time. Rather, asymmetry dynamically shifts during cell rearrangements and divisions, while global, average polarity of the tissue is preserved. We show using super-resolution STED imaging that Fz6-3xGFP and tdTomato-Vangl2 can be resolved, enabling us to observe their complex localization along junctions. We further explore PCP fusion protein localization in the trachea and neural tube, and discover new patterns of PCP expression and localization throughout the mouse embryo.

Targeting and therapeutic peptide-based strategies for polycystic kidney disease

Polycystic kidney disease (PKD) is characterized by progressive cyst growth and is a leading cause of renal failure worldwide. Currently, there are limited therapeutic options available to PKD patients, and only one drug, tolvaptan, has been FDA-approved to slow cyst progression. Similar to other small molecule drugs, however, tolvaptan is costly, only moderately effective, and causes adverse events leading to high patient dropout rates. Peptides may mitigate many drawbacks of small molecule drugs, as they can be highly tissue-specific, biocompatible, and economically scaled-up. Peptides can function as targeting ligands that direct therapies to diseased renal tissue, or be potent as therapeutic agents themselves. This review discusses various aberrant signaling pathways in PKD and renal receptors that can be potential targets of peptide-mediated strategies. Additionally, peptides utilized in other kidney applications, but may prove useful in the context of PKD, are highlighted. Insights into novel peptide-based solutions that have potential to improve clinical management of PKD are provided.

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.

In vivo analysis of renal epithelial cells in zebrafish

The zebrafish kidney has been used effectively for studying kidney development, repair and disease. New gene editing capability makes it a more versatile in vivo vertebrate model system to investigate renal epithelial cells in their native environment. In this chapter we focus on dissecting gene function in basic cellular biology of renal epithelial cells, including lumen formation and cell polarity, in intact zebrafish embryos.

Linking Cell Polarity to Cortical Development and Malformations

Cell polarity refers to the asymmetric distribution of signaling molecules, cellular organelles, and cytoskeleton in a cell. Neural progenitors and neurons are highly polarized cells in which the cell membrane and cytoplasmic components are compartmentalized into distinct functional domains in response to internal and external cues that coordinate polarity and behavior during development and disease. In neural progenitor cells, polarity has a prominent impact on cell shape and coordinate several processes such as adhesion, division, and fate determination. Polarity also accompanies a neuron from the beginning until the end of its life. It is essential for development and later functionality of neuronal circuitries. During development, polarity governs transitions between multipolar and bipolar during migration of postmitotic neurons, and directs the specification and directional growth of axons. Once reaching final positions in cortical layers, neurons form dendrites which become compartmentalized to ensure proper establishment of neuronal connections and signaling. Changes in neuronal polarity induce signaling cascades that regulate cytoskeletal changes, as well as mRNA, protein, and vesicle trafficking, required for synapses to form and function. Hence, defects in establishing and maintaining cell polarity are associated with several neural disorders such as microcephaly, lissencephaly, schizophrenia, autism, and epilepsy. In this review we summarize the role of polarity genes in cortical development and emphasize the relationship between polarity dysfunctions and cortical malformations.

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.

The CPLANE protein Intu protects kidneys from ischemia-reperfusion injury by targeting STAT1 for degradation

Intu is known as a ciliogenesis and planar polarity effector (CPLANE) protein. Although roles for Intu have been reported during embryonic development and in the context of developmental disorders, its function and regulation in adult tissues remain poorly understood. Here we show that ablation of Intu specifically in kidney proximal tubules aggravates renal ischemia-reperfusion injury, and leads to defective post-injury ciliogenesis. We identify signal transducer and activator of transcription 1 (STAT1) as a novel interacting partner of Intu. In vitro, Intu and STAT1 colocalize at the centriole/basal body area, and Intu promotes proteasomal degradation of STAT1. During cell stress, Intu expression preserves cilia length and cell viability, and these actions are antagonized by STAT1 expression. Thus, we propose a role for Intu in protecting cells and tissues after injury by targeting STAT1 for degradation and maintaining primary cilia.

Vangl2 regulates spermatid planar cell polarity through microtubule (MT)-based cytoskeleton in the rat testis

During spermatogenesis, developing elongating/elongated spermatids are highly polarized cells, displaying unique apico-basal polarity. For instance, the heads of spermatids align perpendicular to the basement membrane with their tails pointing to the tubule lumen. Thus, the maximal number of spermatids are packed within the limited space of the seminiferous epithelium to support spermatogenesis.  Herein, we reported findings that  elongating/elongated spermatids displayed planar cell polarity (PCP) in adult rat testes in which the proximal end of polarized spermatid heads were aligned uniformly across the plane of the seminiferous epithelium based on studies  using confocal microscopy and 3-dimensional (D) reconstruction of the seminiferous tubules.  We also discovered  that spermatid PCP was regulated by PCP protein Vangl2 (Van Gogh-like protein 2) since Vangl2 knockdown by RNAi was found to perturb spermatid PCP. More important, Vangl2 exerted its regulatory effects through changes in the organization of the microtubule (MT)-based cytoskeleton in the seminiferous epithelium. These changes were mediated via the downstream signaling proteins atypical protein kinase C ξ (PKCζ) and MT-associated protein (MAP)/microtubule affinity-regulating kinase 2 (MARK2). These findings thus provide new insights regarding the biology of spermatid PCP during spermiogenesis.

Cross talk between the Crumbs complex and Hippo signaling in renal epithelial cells

Cell polarity has a crucial role in organizing cells into tissues and in mediating transport processes and cell-cell communication. Especially the cells of the nephron require apicobasal polarity to establish and maintain their barrier function. The Crumbs complex including the integral membrane protein Crumbs, as well as Pals1 and Patj, is essential for the establishment of apicobasal polarity. The interactions of the core proteins and the interplay with other processes have been characterized in various epithelial cell lines in detail. Notably, Crb2 and Crb3 are expressed within the kidney and play an important role in the proper function of podocytes and tubules, respectively. The interaction of polarity proteins and components of the Hippo pathway-an evolutionarily highly conserved kinase cascade regulating cell proliferation, organ size, and tissue regeneration-has been discovered recently. Here, we discuss potential molecular and physiological links between the Crumbs complex and the Hippo pathway in renal cells.

Planar cell polarity (PCP) proteins and spermatogenesis

In adult mammalian testes, spermatogenesis is comprised of several discrete cellular events that work in tandem to support the transformation and differentiation of diploid spermatogonia to haploid spermatids in the seminiferous epithelium during the seminiferous epithelial cycle. These include: self-renewal of spermatogonial stem cells via mitosis and their transformation into differentiated spermatogonia, meiosis I/II, spermiogenesis and the release of sperms at spermiation. Studies have shown that these cellular events are under precise and coordinated controls of multiple proteins and signaling pathways. These events are also regulated by polarity proteins that are known to confer classical apico-basal (A/B) polarity in other epithelia. Furthermore, spermatid development is likely supported by planar cell polarity (PCP) proteins since polarized spermatids are aligned across the plane of seminiferous epithelium in an orderly fashion, analogous to hair cells in the cochlea of the inner ear. Thus, the maximal number of spermatids can be packed and supported by a fixed population of differentiated Sertoli cells in the limited space of the seminiferous epithelium in adult testes. In this review, we briefly summarize recent findings regarding the role of PCP proteins in the testis. This information should be helpful in future studies to better understand the role of PCP proteins in spermatogenesis.

Does cell polarity matter during spermatogenesis?

Cell polarity is crucial to development since apico-basal polarity conferred by the 3 polarity protein modules (or complexes) is essential during embryogenesis, namely the Par (partition defective)-, the CRB (Crumbs)-, and the Scribble-based polarity protein modules. While these protein complexes and their component proteins have been extensively studied in Drosophila and C. elegans and also other mammalian tissues and/or cells, their presence and physiological significance in the testis remain unexplored until the first paper on the Par-based protein published in 2008. Since then, the Par-, the Scribble- and the CRB-based protein complexes and their component proteins in the testis have been studied. These proteins are known to confer Sertoli and spermatid polarity in the seminiferous epithelium, and they are also integrated components of the tight junction (TJ) and the basal ectoplasmic specialization (ES) at the Sertoli cell-cell interface near the basement membrane, which in turn constitute the blood-testis barrier (BTB). These proteins are also found at the apical ES at the Sertoli-spermatid interface. Thus, these polarity proteins also play a significant role in regulating Sertoli and spermatid adhesion in the testis through their actions on actin-based cytoskeletal function. Recent studies have shown that these polarity proteins are having antagonistic effects on the BTB integrity in which the Par6- and CRB3-based polarity complexes promotes the integrity of the Sertoli cell TJ-permeability barrier, whereas the Scribble-based complex promotes restructuring/remodeling of the Sertoli TJ-barrier function. Herein, we carefully evaluate these findings and provide a hypothetic model regarding their role in the testis in the context of the functions of these polarity proteins in other epithelia, so that better experiments can be designed in future studies to explore their significance in spermatogenesis.

Planar Cell Polarity (PCP) Protein Vangl2 Regulates Ectoplasmic Specialization Dynamics via Its Effects on Actin Microfilaments in the Testes of Male Rats

Planar cell polarity (PCP) proteins confer polarization of a field of cells (eg, elongating/elongated spermatids) within the plane of an epithelium such as the seminiferous epithelium of the tubule during spermatogenesis. In adult rat testes, Sertoli and germ cells were found to express PCP core proteins (eg, Van Gogh-like 2 [Vangl2]), effectors, ligands, and signaling proteins. Vangl2 expressed predominantly by Sertoli cells was localized at the testis-specific, actin-rich ectoplasmic specialization (ES) at the Sertoli-spermatid interface in the adluminal compartment and also Sertoli-Sertoli interface at the blood-testis barrier (BTB) and structurally interacted with actin, N-cadherin, and another PCP/polarity protein Scribble. Vangl2 knockdown (KD) by RNA interference in Sertoli cells cultured in vitro with an established tight junction-permeability barrier led to BTB tightening, whereas its overexpression using a full-length cDNA construct perturbed the barrier function. These changes were mediated through an alteration on the organization actin microfilaments at the ES in Sertoli cells, involving actin-regulatory proteins, epidermal growth factor receptor pathway substrate 8, actin-related protein 3, and Scribble, which in turn affected the function of adhesion protein complexes at the ES during the epithelial cycle of spermatogenesis. Using Polyplus in vivo-jetPEI reagent as a transfection medium to silence Vangl2 in the testis in vivo by RNA interference with high efficacy, Vangl2 KD led to changes in F-actin organization at the ES in the epithelium, impeding spermatid and phagosome transport and spermatid polarity, meiosis, and BTB dynamics. For instance, step 19 spermatids remained embedded in the epithelium alongside with step 9 and 10 spermatids in stages IX-X tubules. In summary, the PCP protein Vangl2 is an ES regulator through its effects on actin microfilaments in the testis.

Protein tyrosine kinase 7 is essential for tubular morphogenesis of the Wolffian duct

The Wolffian duct, the proximal end of the mesonephric duct, undergoes non-branching morphogenesis to achieve an optimal length and size for sperm maturation. It is important to examine the mechanisms by which the developing mouse Wolffian duct elongates and coils for without proper morphogenesis, male infertility will result. Here we show that highly proliferative epithelial cells divide in a random orientation relative to the elongation axis in the developing Wolffian duct. Convergent extension (CE)-like of cell rearrangements is required for elongating the duct while maintaining a relatively unchanged duct diameter. The Wolffian duct epithelium is planar polarized, which is characterized by oriented cell elongation, oriented cell rearrangements, and polarized activity of regulatory light chain of myosin II. Conditional deletion of protein tyrosine kinase 7 (PTK7), a regulator of planar cell polarity (PCP), from mesoderm results in loss of the PCP characteristics in the Wolffian duct epithelium. Although loss of Ptk7 does not alter cell proliferation or division orientation, it affects CE and leads to the duct with significantly shortened length, increased diameter, and reduced coiling, which eventually results in loss of sperm motility, a key component of sperm maturation. In vitro experiments utilizing inhibitors of myosin II results in reduced elongation and coiling, similar to the phenotype of Ptk7 knockout. This data suggest that PTK7 signaling through myosin II regulates PCP, which in turn ensures CE-like of cell rearrangements to drive elongation and coiling of the Wolffian duct. Therefore, PTK7 is essential for Wolffian duct morphogenesis and male fertility.

Prickle isoforms control the direction of tissue polarity by microtubule independent and dependent mechanisms

Planar cell polarity signaling directs the polarization of cells within the plane of many epithelia. While these tissues exhibit asymmetric localization of a set of core module proteins, in Drosophila, more than one mechanism links the direction of core module polarization to the tissue axes. One signaling system establishes a polarity bias in the parallel, apical microtubules upon which vesicles containing core proteins traffic. Swapping expression of the differentially expressed Prickle isoforms, Prickle and Spiny-legs, reverses the direction of core module polarization. Studies in the proximal wing and the anterior abdomen indicated that this results from their differential control of microtubule polarity. Prickle and Spiny-legs also control the direction of polarization in the distal wing (D-wing) and the posterior abdomen (P-abd). We report here that this occurs without affecting microtubule polarity in these tissues. The direction of polarity in the D-wing is therefore likely determined by a novel mechanism independent of microtubule polarity. In the P-abd, Prickle and Spiny-legs interpret at least two directional cues through a microtubule-polarity-independent mechanism.

TALPID3 controls centrosome and cell polarity and the human ortholog KIAA0586 is mutated in Joubert syndrome (JBTS23)

Joubert syndrome (JBTS) is a severe recessive neurodevelopmental ciliopathy which can affect several organ systems. Mutations in known JBTS genes account for approximately half of the cases. By homozygosity mapping and whole-exome sequencing, we identified a novel locus, JBTS23, with a homozygous splice site mutation in KIAA0586 (alias TALPID3), a known lethal ciliopathy locus in model organisms. Truncating KIAA0586 mutations were identified in two additional patients with JBTS. One mutation, c.428delG (p.Arg143Lysfs*4), is unexpectedly common in the general population and may be a major contributor to JBTS. We demonstrate KIAA0586 protein localization at the basal body in human and mouse photoreceptors, as is common for JBTS proteins, and also in pericentriolar locations. We show that loss of TALPID3 (KIAA0586) function in animal models causes abnormal tissue polarity, centrosome length and orientation, and centriolar satellites. We propose that JBTS and other ciliopathies may in part result from cell polarity defects.

DOI:http://dx.doi.org/10.7554/eLife.08077.001

Joubert syndrome is a rare and severe neurodevelopmental disease in which two parts of the brain called the cerebellar vermis and brainstem do not develop properly. The disease is caused by defects in the formation of small projections from the surface of cells, called cilia, which are essential for signalling processes inside cells. Mutations in at least 25 genes are known to cause Joubert syndrome, and all encode proteins that create or maintain cilia. However, these mutations account for only half of the cases that have been studied, which indicates that mutations in other genes may also cause Joubert syndrome.

Here, Stephen et al. used genetic techniques called ‘homozygosity mapping’ and ‘whole-exome sequencing’ to search for other mutations that might cause the disease. They found that mutations in a gene encoding a protein called KIAA0586 also cause Joubert syndrome in humans. One of these mutations (c.428delG) is unexpectedly common in the healthy human population. It might be a major contributor to Joubert syndrome, and the manifestation of Joubert syndrome in individuals with this mutation might depend on the presence and nature of other mutations in KIAA0586 and in other genes.

The TALPID3 protein in chickens and other ‘model’ animals is the equivalent of human KIAA0586. A loss of TALPID3 protein in animals has been shown to stop cilia from forming. This protein is found in a structure called the basal body, which is part of a larger structure called the centrosome that anchors cilia to the cell. Here, Stephen et al. show that this is also true in mouse and human eye cells.

Further experiments using chicken embryos show that a loss of the TALPID3 protein alters the location of centrosomes inside cells. TALPID3 is also required for cells and organs to develop the correct polarity, that is, directional differences in their structure and shape. The centrosomes of chicken brain cells that lacked TALPID3 were poorly positioned at the cell surface and abnormally long, which is likely responsible for the cilia failing to form.

Stephen et al.'s findings suggest that KIAA0586 is also important for human development through its ability to control the centrosome. Defects in TALPID3 have a more severe effect on animal models than many of the identified KIAA0586 mutations have on humans. Therefore, the next step in this research is to find a more suitable animal in which to study the role of this protein, which may inform efforts to develop treatments for Joubert syndrome.

DOI:http://dx.doi.org/10.7554/eLife.08077.002

Extramacrochaetae functions in dorsal-ventral patterning of Drosophila imaginal discs

One of the seminal events in the history of a tissue is the establishment of the anterior-posterior, dorsal-ventral (D/V) and proximal-distal axes. Axis formation is important for the regional specification of a tissue and allows cells along the different axes to obtain directional and positional information. Within the Drosophila retina, D/V axis formation is essential to ensure that each unit eye first adopts the proper chiral form and then rotates precisely 90° in the correct direction. These two steps are important because the photoreceptor array must be correctly aligned with the neurons of the optic lobe. Defects in chirality and/or ommatidial rotation will lead to disorganization of the photoreceptor array, misalignment of retinal and optic lobe neurons, and loss of visual acuity. Loss of the helix-loop-helix protein Extramacrochaetae (Emc) leads to defects in both ommatidial chirality and rotation. Here, we describe a new role for emc in eye development in patterning the D/V axis. We show that the juxtaposition of dorsal and ventral fated tissue in the eye leads to an enrichment of emc expression at the D/V midline. emc expression at the midline can be eliminated when D/V patterning is disrupted and can be induced in situations in which ectopic boundaries are artificially generated. We also show that emc functions downstream of Notch signaling to maintain the expression of four-jointed along the midline.

Semaphorin-Plexin Signaling Controls Mitotic Spindle Orientation during Epithelial Morphogenesis and Repair

Morphogenesis, homeostasis, and regeneration of epithelial tissues rely on the accurate orientation of cell divisions, which is specified by the mitotic spindle axis. To remain in the epithelial plane, symmetrically dividing epithelial cells align their mitotic spindle axis with the plane. Here, we show that this alignment depends on epithelial cell-cell communication via semaphorin-plexin signaling. During kidney morphogenesis and repair, renal tubular epithelial cells lacking the transmembrane receptor Plexin-B2 or its semaphorin ligands fail to correctly orient the mitotic spindle, leading to severe defects in epithelial architecture and function. Analyses of a series of transgenic and knockout mice indicate that Plexin-B2 controls the cell division axis by signaling through its GTPase-activating protein (GAP) domain and Cdc42. Our data uncover semaphorin-plexin signaling as a central regulatory mechanism of mitotic spindle orientation necessary for the alignment of epithelial cell divisions with the epithelial plane.

Cell-cell interactions driving kidney morphogenesis

The mammalian kidney forms via cell-cell interactions between an epithelial outgrowth of the nephric duct and the surrounding nephrogenic mesenchyme. Initial morphogenetic events include ureteric bud branching to form the collecting duct (CD) tree and mesenchymal-to-epithelial transitions to form the nephrons, requiring reciprocal induction between adjacent mesenchyme and epithelial cells. Within the tips of the branching ureteric epithelium, cells respond to mesenchyme-derived trophic factors by proliferation, migration, and mitosis-associated cell dispersal. Self-inhibition signals from one tip to another play a role in branch patterning. The position, survival, and fate of the nephrogenic mesenchyme are regulated by ECM and secreted signals from adjacent tip and stroma. Signals from the ureteric tip promote mesenchyme self-renewal and trigger nephron formation. Subsequent fusion to the CDs, nephron segmentation and maturation, and formation of a patent glomerular basement membrane also require specialized cell-cell interactions. Differential cadherin, laminin, nectin, and integrin expression, as well as intracellular kinesin and actin-mediated regulation of cell shape and adhesion, underlies these cell-cell interactions. Indeed, the capacity for the kidney to form via self-organization has now been established both via the recapitulation of expected morphogenetic interactions after complete dissociation and reassociation of cellular components during development as well as the in vitro formation of 3D kidney organoids from human pluripotent stem cells. As we understand more about how the many cell-cell interactions required for kidney formation operate, this enables the prospect of bioengineering replacement structures based on these self-organizing properties.

Vangl2 cooperates with Rab11 and Myosin V to regulate apical constriction during vertebrate gastrulation

Core planar cell polarity (PCP) proteins are well known to regulate polarity in Drosophila and vertebrate epithelia; however, their functions in vertebrate morphogenesis remain poorly understood. In this study, we describe a role for PCP signaling in the process of apical constriction during Xenopus gastrulation. The core PCP protein Vangl2 is detected at the apical surfaces of cells at the blastopore lip, and it functions during blastopore formation and closure. Further experiments show that Vangl2, as well as Daam1 and Rho-associated kinase (Rock), regulate apical constriction of bottle cells at the blastopore and ectopic constriction of ectoderm cells triggered by the actin-binding protein Shroom3. At the blastopore lip, Vangl2 is required for the apical accumulation of the recycling endosome marker Rab11. We also show that Rab11 and the associated motor protein Myosin V play essential roles in both endogenous and ectopic apical constriction, and might be involved in Vangl2 trafficking to the cell surface. Overexpression of Rab11 RNA was sufficient to partly restore normal blastopore formation in Vangl2-deficient embryos. These observations suggest that Vangl2 affects Rab11 to regulate apical constriction during blastopore formation.

Planar cell polarity of the kidney

Planar cell polarity (PCP) or tissue polarity refers to the polarization of tissues perpendicular to the apical-basal axis. Most epithelia, including the vertebrate kidney, show signs of planar polarity. In the kidney, defects in planar polarity are attributed to several disease states including multiple forms of cystic kidney disease. Indeed, planar cell polarity has been shown to be essential for several cellular processes that appear to be necessary for establishing and maintaining tubule diameter. However, uncovering the genetic mechanisms underlying PCP in the kidney has been complicated as the roles of many of the main players are not conserved in flies and vice versa. Here, we review a number of cellular and molecular processes that can affect PCP of the kidney with a particular emphasis on the mechanisms that do not appear to be conserved in flies or that are not part of canonical determinants.

Renal branching morphogenesis: morphogenetic and signaling mechanisms

The human kidney is composed of an arborized network of collecting ducts, calyces and urinary pelvis that facilitate urine excretion and regulate urine composition. The renal collecting system is formed in utero, completed by the 34th week of gestation in humans, and dictates final nephron complement. The renal collecting system arises from the ureteric bud, a derivative of the intermediate-mesoderm derived nephric duct that responds to inductive signals from adjacent tissues via a process termed ureteric induction. The ureteric bud subsequently undergoes a series of iterative branching and remodeling events in a process called renal branching morphogenesis. Altered signaling that disrupts patterning of the nephric duct, ureteric induction, or renal branching morphogenesis leads to varied malformations of the renal collecting system collectively known as congenital anomalies of the kidney and urinary tract (CAKUT) and is the most frequently detected congenital renal aberration in infants. Here, we describe critical morphogenetic and cellular events that govern nephric duct specification, ureteric bud induction, renal branching morphogenesis, and cessation of renal branching morphogenesis. We also highlight salient molecular signaling pathways that govern these processes, and the investigative techniques used to interrogate them.

The primary cilium: guardian of organ development and homeostasis

The primary cilium is an antenna-like organelle that plays a vital role in organ generation and maintenance. It protrudes from the cell surface where it receives signals from the surrounding environment and relays them into the cell. These signals are then integrated to give the required outputs in terms of proliferation, differentiation, migration and polarization that ultimately lead to organ development and homeostasis. Defects in cilia function underlie a wide range of diverse but related human developmental or degenerative diseases. Collectively known as ciliopathies, these disorders present with varying severity and multiple organ involvement. The appreciation of the medical importance of the primary cilium has stimulated a huge effort into studies of the underlying cellular mechanisms. These in turn have revealed that ciliopathies result not only from defective assembly or organization of the primary cilium, but also from impaired ciliary signaling. This special edition of Organogenesis contains a set of review articles that highlight the role of the primary cilium in organ development and homeostasis, much of which has been learnt from studies of the associated human diseases. Here, we provide an introductory overview of our current understanding of the structure and function of the cilium, with a focus on the signaling pathways that are coordinated by primary cilia to ensure proper organ generation and maintenance.

Cell intercalation from top to bottom

Animal development requires a carefully orchestrated cascade of cell fate specification events and cellular movements. A surprisingly small number of choreographed cellular behaviours are used repeatedly to shape the animal body plan. Among these, cell intercalation lengthens or spreads a tissue at the expense of narrowing along an orthogonal axis. Key steps in the polarization of both mediolaterally and radially intercalating cells have now been clarified. In these different contexts, intercalation seems to require a distinct combination of mechanisms, including adhesive changes that allow cells to rearrange, cytoskeletal events through which cells exert the forces needed for cell neighbour exchange, and in some cases the regulation of these processes through planar cell polarity.

Crumbs3 is essential for proper epithelial development and viability

First identified in Drosophila, the Crumbs (Crb) proteins are important in epithelial polarity, apical membrane formation, and tight junction (TJ) assembly. The conserved Crb intracellular region includes a FERM (band 4.1/ezrin/radixin/moesin) binding domain (FBD) whose mammalian binding partners are not well understood and a PDZ binding motif that interacts with mammalian Pals1 (protein associated with lin seven) (also known as MPP5). Pals1 binds Patj (Pals1-associated tight-junction protein), a multi-PDZ-domain protein that associates with many tight junction proteins. The Crb complex also binds the conserved Par3/Par6/atypical protein kinase C (aPKC) polarity cassette that restricts migration of basolateral proteins through phosphorylation. Here, we describe a Crb3 knockout mouse that demonstrates extensive defects in epithelial morphogenesis. The mice die shortly after birth, with cystic kidneys and proteinaceous debris throughout the lungs. The intestines display villus fusion, apical membrane blebs, and disrupted microvilli. These intestinal defects phenocopy those of Ezrin knockout mice, and we demonstrate an interaction between Crumbs3 and ezrin. Taken together, our data indicate that Crumbs3 is crucial for epithelial morphogenesis and plays a role in linking the apical membrane to the underlying ezrin-containing cytoskeleton.

Planar cell polarity pathway regulates nephrin endocytosis in developing podocytes

The noncanonical Wnt/planar cell polarity (PCP) pathway controls a variety of cell behaviors such as polarized protrusive cell activity, directional cell movement, and oriented cell division and is crucial for the normal development of many tissues. Mutations in the PCP genes cause malformation in multiple organs. Recently, the PCP pathway was shown to control endocytosis of PCP and non-PCP proteins necessary for cell shape remodeling and formation of specific junctional protein complexes. During formation of the renal glomerulus, the glomerular capillary becomes enveloped by highly specialized epithelial cells, podocytes, that display unique architecture and are connected via specialized cell-cell junctions (slit diaphragms) that restrict passage of protein into the urine; podocyte differentiation requires active remodeling of cytoskeleton and junctional protein complexes. We report here that in cultured human podocytes, activation of the PCP pathway significantly stimulates endocytosis of the core slit diaphragm protein, nephrin, via a clathrin/β-arrestin-dependent endocytic route. In contrast, depletion of the PCP protein Vangl2 leads to an increase of nephrin at the cell surface; loss of Vangl2 functions in Looptail mice results in disturbed glomerular maturation. We propose that the PCP pathway contributes to podocyte development by regulating nephrin turnover during junctional remodeling as the cells differentiate.