Dishevelled genes mediate a conserved mammalian PCP pathway to regulate convergent extension during neurulation

Planar Cell Polarity Signaling: Coordinated Crosstalk for Cell Orientation

The planar cell polarity (PCP) system is essential for positioning cells in 3D networks to establish the proper morphogenesis, structure, and function of organs during embryonic development. The PCP system uses inter- and intracellular feedback interactions between components of the core PCP, characterized by coordinated planar polarization and asymmetric distribution of cell populations inside the cells. PCP signaling connects the anterior-posterior to left-right embryonic plane polarity through the polarization of cilia in the Kupffer's vesicle/node in vertebrates. Experimental investigations on various genetic ablation-based models demonstrated the functions of PCP in planar polarization and associated genetic disorders. This review paper aims to provide a comprehensive overview of PCP signaling history, core components of the PCP signaling pathway, molecular mechanisms underlying PCP signaling, interactions with other signaling pathways, and the role of PCP in organ and embryonic development. Moreover, we will delve into the negative feedback regulation of PCP to maintain polarity, human genetic disorders associated with PCP defects, as well as challenges associated with PCP.

SCRIB controls apical contractility during epithelial differentiation

Although mutations in the SCRIB gene lead to multiple morphological organ defects in vertebrates, the molecular pathway linking SCRIB to organ shape anomalies remains elusive. Here, we study the impact of SCRIB-targeted gene mutations during the formation of the gut epithelium in an organ-on-chip model. We show that SCRIB KO gut-like epithelia are flatter with reduced exposed surface area. Cell differentiation on filters further shows that SCRIB plays a critical role in the control of apical cell shape, as well as in the basoapical polarization of myosin light chain localization and activity. Finally, we show that SCRIB serves as a molecular scaffold for SHROOM2/4 and ROCK1 and identify an evolutionary conserved SHROOM binding site in the SCRIB carboxy-terminal that is required for SCRIB function in the control of apical cell shape. Our results demonstrate that SCRIB plays a key role in epithelial morphogenesis by controlling the epithelial apical contractility during cell differentiation.

Vangl2 deficient zebrafish exhibit hallmarks of neural tube closure defects

Neural tube defects (NTDs) are among the most devastating and common congenital anomalies worldwide, and the ability to model these conditions in vivo is essential for identifying causative genetic and environmental factors. Although zebrafish are ideal for rapid candidate testing, their neural tubes develop primarily via a solid neural keel rather that the fold-and-fuse method employed by mammals, raising questions about their suitability as an NTD model. Here, we demonstrate that despite outward differences, zebrafish anterior neurulation closely resembles that of mammals. For the first time, we directly observe fusion of the bilateral neural folds to enclose a lumen in zebrafish embryos. The neural folds fuse by zippering between multiple distinct but contiguous closure sites. Embryos lacking vangl2, a core planar cell polarity and NTD risk gene, exhibit delayed neural fold fusion and abnormal neural groove formation, yielding distinct openings and midline bifurcations in the developing neural tube. These data provide direct evidence for fold-and-fuse neurulation in zebrafish and its disruption upon loss of an NTD risk gene, highlighting conservation of vertebrate neurulation and the utility of zebrafish for modeling NTDs.

The complex relationship of Wnt-signaling pathways and cilia

Wnt family proteins are secreted glycolipoproteins that signal through multitude of signal transduction pathways. The Wnt-pathways are conserved and critical in all metazoans. They are essential for embryonic development, organogenesis and homeostasis, and associated with many diseases when defective or deregulated. Wnt signaling pathways comprise the canonical Wnt pathway, best known for its stabilization of β-catenin and associated nuclear β-catenin activity in gene regulation, and several non-canonical signaling branches. Wnt-Planar Cell Polarity (PCP) signaling has received the most attention among the non-canonical Wnt pathways. The relationship of cilia to Wnt-signaling is complex. While it was suggested that canonical Wnt signaling requires cilia this notion was always challenged by results suggesting the opposite. Recent developments provide insight and clarification to the relationship of Wnt signaling pathways and cilia. First, it has been now demonstrated that while ciliary proteins, in particular the IFT-A complex, are required for canonical Wnt/β-catenin signaling, the cilium as a structure is not. In contrast, recent work has defined a diverged canonical signaling branch (not affecting β-catenin) to be required for ciliary biogenesis and cilia function. Furthermore, the non-canonical Wnt-PCP pathway does not affect cilia biogenesis per se, but it regulates the position of cilia within cells in many cell types, possibly in all cells where it is active, with cilia being placed near the side of the cell that has the Frizzled-Dishevelled complex. This Wnt/PCP feature is conserved with both centrioles and basal bodies/cilia being positioned accordingly, and it is also used to align mitotic spindles within the Wnt-PCP polarization axis. It also coordinates the alignment of cilia in multiciliated cells. This article addresses these new insights and different links and relationships between cilia and Wnt signaling.

Prickle and Ror modulate Dishevelled-Vangl interaction to regulate non-canonical Wnt signaling during convergent extension

Convergent extension (CE) is a fundamental morphogenetic process where oriented cell behaviors lead to polarized extension of diverse tissues. In vertebrates, regulation of CE requires both non-canonical Wnt, its co-receptor Ror, and "core members" of the planar cell polarity (PCP) pathway. PCP was originally identified as a mechanism to coordinate the cellular polarity in the plane of static epithelium, where core proteins Frizzled (Fz)/ Dishevelled (Dvl) and Van Gogh-like (Vangl)/ Prickel (Pk) partition to opposing cell cortex. But how core PCP proteins interact with each other to mediate non-canonical Wnt/ Ror signaling during CE is not clear. We found previously that during CE, Vangl cell-autonomously recruits Dvl to the plasma membrane but simultaneously keeps Dvl inactive. In this study, we show that non-canonical Wnt induces Dvl to transition from Vangl to Fz. PK inhibits the transition, and functionally synergize with Vangl to suppress Dvl during CE. Conversely, Ror is required for the transition, and functionally antagonizes Vangl. Biochemically, Vangl interacts directly with both Ror and Dvl. Ror and Dvl do not bind directly, but can be cofractionated with Vangl. We propose that Pk assists Vangl to function as an unconventional adaptor that brings Dvl and Ror into a complex to serves two functions: 1) simultaneously preventing both Dvl and Ror from ectopically activating non-canonical Wnt signaling; and 2) relaying Dvl to Fz for signaling activation upon non-canonical Wnt induced dimerization of Fz and Ror.

The Helicobacter pylori CagA oncoprotein disrupts Wnt/PCP signaling and promotes hyperproliferation of pyloric gland base cells

Helicobacter pylori strains that deliver the oncoprotein CagA into gastric epithelial cells are the major etiologic agents of upper gastric diseases including gastric cancer. CagA promotes gastric carcinogenesis through interactions with multiple host proteins. Here, we show that CagA also disrupts Wnt-dependent planar cell polarity (Wnt/PCP), which orients cells within the plane of an epithelium and coordinates collective cell behaviors such as convergent extension to enable epithelial elongation during development. Ectopic expression of CagA in Xenopus laevis embryos impaired gastrulation, neural tube formation, and axis elongation, processes driven by convergent extension movements that depend on the Wnt/PCP pathway. Mice specifically expressing CagA in the stomach epithelium had longer pyloric glands and mislocalization of the tetraspanin proteins VANGL1 and VANGL2 (VANGL1/2), which are critical components of Wnt/PCP signaling. The increased pyloric gland length was due to hyperproliferation of cells at the gland base, where Lgr5⁺ stem and progenitor cells reside, and was associated with fewer differentiated enteroendocrine cells. In cultured human gastric epithelial cells, the N terminus of CagA interacted with the C-terminal cytoplasmic tails of VANGL1/2, which impaired Wnt/PCP signaling by inducing the mislocalization of VANGL1/2 from the plasma membrane to the cytoplasm. Thus, CagA may contribute to the development of gastric cancer by subverting a Wnt/PCP-dependent mechanism that restrains pyloric gland stem cell proliferation and promotes enteroendocrine differentiation.

LRP2 contributes to planar cell polarity-dependent coordination of motile cilia function

Motile cilia are protruding organelles on specialized epithelia that beat in a synchronous fashion to propel extracellular fluids. Coordination and orientation of cilia beating on individual cells and across tissues is a complex process dependent on planar cell polarity (PCP) signaling. Asymmetric sorting of PCP pathway components, essential to establish planar polarity, involves trafficking along the endocytic path, but the underlying regulatory processes remain incompletely understood. Here, we identified the endocytic receptor LRP2 as regulator of PCP component trafficking in ependyma, a multi-ciliated cell type that is involved in facilitating flow of the cerebrospinal fluid in the brain ventricular system. Lack of receptor expression in gene-targeted mice results in a failure to sort PCP core proteins to the anterior or posterior cell side and, consequently, in the inability to coordinate cilia arrangement and to aligned beating (loss of rotational and translational polarity). LRP2 deficiency coincides with a failure to sort NHERF1, a cytoplasmic LRP2 adaptor to the anterior cell side. As NHERF1 is essential to translocate PCP core protein Vangl2 to the plasma membrane, these data suggest a molecular mechanism whereby LRP2 interacts with PCP components through NHERF1 to control their asymmetric sorting along the endocytic path. Taken together, our findings identified the endocytic receptor LRP2 as a novel regulator of endosomal trafficking of PCP proteins, ensuring their asymmetric partition and establishment of translational and rotational planar cell polarity in the ependyma.

Got WNTS? Insight into bone health from a WNT perspective

WNT signaling, essential for many aspects of development, is among the most commonly altered pathways associated with human disease. While initially studied in cancer, dysregulation of WNT signaling has been determined to be essential for skeletal development and the maintenance of bone health throughout life. In this review, we discuss the role of Wnt signaling in bone development and disease with a particular focus on two areas. First, we discuss the roles of WNT signaling pathways in skeletal development, with an emphasis on congenital and idiopathic skeletal syndromes and diseases that are associated with genetic variations in WNT signaling components. Next, we cover a topic that has long been an interest of our laboratory, how high and low levels of WNT signaling affects the establishment and maintenance of healthy bone mass. We conclude with a discussion of the status of WNT-based therapeutics in the treatment of skeletal disease.

CIC missense variants contribute to susceptibility for spina bifida

Neural tube defects (NTDs) are congenital malformations resulting from abnormal embryonic development of the brain, spine, or spinal column. The genetic etiology of human NTDs remains poorly understood despite intensive investigation. CIC, homolog of the Capicua transcription repressor, has been reported to interact with ataxin-1 (ATXN1) and participate in the pathogenesis of spinocerebellar ataxia type 1. Our previous study demonstrated that CIC loss of function (LoF) variants contributed to the cerebral folate deficiency syndrome by downregulating folate receptor 1 (FOLR1) expression. Given the importance of folate transport in neural tube formation, we hypothesized that CIC variants could contribute to increased risk for NTDs by depressing embryonic folate concentrations. In this study, we examined CIC variants from whole-genome sequencing (WGS) data of 140 isolated spina bifida cases and identified eight missense variants of CIC gene. We tested the pathogenicity of the observed variants through multiple in vitro experiments. We determined that CIC variants decreased the FOLR1 protein level and planar cell polarity (PCP) pathway signaling in a human cell line (HeLa). In a murine cell line (NIH3T3), CIC loss of function variants downregulated PCP signaling. Taken together, this study provides evidence supporting CIC as a risk gene for human NTD.

Celsr1 suppresses Wnt5a-mediated chemoattraction to prevent incorrect rostral migration of facial branchiomotor neurons

In the developing hindbrain, facial branchiomotor (FBM) neurons migrate caudally from rhombomere 4 (r4) to r6 to establish the circuit that drives jaw movements. Although the mechanisms regulating initiation of FBM neuron migration are well defined, those regulating directionality are not. In mutants lacking the Wnt/planar cell polarity (PCP) component Celsr1, many FBM neurons inappropriately migrate rostrally into r3. We hypothesized that Celsr1 normally blocks inappropriate rostral migration of FBM neurons by suppressing chemoattraction towards Wnt5a in r3 and successfully tested this model. First, FBM neurons in Celsr1; Wnt5a double mutant embryos never migrated rostrally, indicating that inappropriate rostral migration in Celsr1 mutants results from Wnt5a-mediated chemoattraction, which is suppressed in wild-type embryos. Second, FBM neurons migrated rostrally toward Wnt5a-coated beads placed in r3 of wild-type hindbrain explants, suggesting that excess Wnt5a chemoattractant can overcome endogenous Celsr1-mediated suppression. Third, rostral migration of FBM neurons was greatly enhanced in Celsr1 mutants overexpressing Wnt5a in r3. These results reveal a novel role for a Wnt/PCP component in regulating neuronal migration through suppression of chemoattraction.

Wnt/planar cell polarity signaling controls morphogenetic movements of gastrulation and neural tube closure

Gastrulation and neurulation are successive morphogenetic processes that play key roles in shaping the basic embryonic body plan. Importantly, they operate through common cellular and molecular mechanisms to set up the three spatially organized germ layers and to close the neural tube. During gastrulation and neurulation, convergent extension movements driven by cell intercalation and oriented cell division generate major forces to narrow the germ layers along the mediolateral axis and elongate the embryo in the anteroposterior direction. Apical constriction also makes an important contribution to promote the formation of the blastopore and the bending of the neural plate. Planar cell polarity proteins are major regulators of asymmetric cell behaviors and critically involved in a wide variety of developmental processes, from gastrulation and neurulation to organogenesis. Mutations of planar cell polarity genes can lead to general defects in the morphogenesis of different organs and the co-existence of distinct congenital diseases, such as spina bifida, hearing deficits, kidney diseases, and limb elongation defects. This review outlines our current understanding of non-canonical Wnt signaling, commonly known as Wnt/planar cell polarity signaling, in regulating morphogenetic movements of gastrulation and neural tube closure during development and disease. It also attempts to identify unanswered questions that deserve further investigations.

DVL1 and DVL3 require nuclear localisation to regulate proliferation in human myoblasts

WNT signalling is essential for regulating a diverse range of cellular processes. In skeletal muscle, the WNT pathway plays crucial roles in maintenance of the stem cell pool and myogenic differentiation. Focus is usually directed at examining the function of central components of the WNT pathway, including β-CATENIN and the GSK3β complex and TCF/LEF transcription factors, in tissue homeostasis and cancer. Other core components of the WNT pathway though, are three dishevelled (DVL) proteins: membrane associated proteins that propagate WNT signalling from membrane to nucleus. Here we examined DVL function in human myogenesis and the muscle-related cancer alveolar rhabdomyosarcoma. We demonstrate that DVL1 and DVL3 are necessary for efficient proliferation in human myoblasts and are important for timely myogenic differentiation. DVL1 and DVL3 also contribute to regulation of proliferation in rhabdomyosarcoma. DVL1 or DVL3 must be present in the nucleus to regulate proliferation, but they operate through different protein domains: DVL3 requires the DIX and PDZ domains, while DVL1 does not. Importantly, DVL1 and DVL3 activity is independent of markedly increased translocation of β-CATENIN to the nucleus, normally a hallmark of active canonical WNT signalling.

The Ror-Family Receptors in Development, Tissue Regeneration and Age-Related Disease

The Ror-family proteins, Ror1 and Ror2, act as receptors or co-receptors for Wnt5a and its related Wnt proteins to activate non-canonical Wnt signaling. Ror1 and/or Ror2-mediated signaling plays essential roles in regulating cell polarity, migration, proliferation and differentiation during developmental morphogenesis, tissue-/organo-genesis and regeneration of adult tissues following injury. Ror1 and Ror2 are expressed abundantly in developing tissues in an overlapping, yet distinct manner, and their expression in adult tissues is restricted to specific cell types such as tissue stem/progenitor cells. Expression levels of Ror1 and/or Ror2 in the adult tissues are increased following injury, thereby promoting regeneration or repair of these injured tissues. On the other hand, disruption of Wnt5a-Ror2 signaling is implicated in senescence of tissue stem/progenitor cells that is related to the impaired regeneration capacity of aged tissues. In fact, Ror1 and Ror2 are implicated in age-related diseases, including tissue fibrosis, atherosclerosis (or arteriosclerosis), neurodegenerative diseases, and cancers. In these diseases, enhanced and/or sustained (chronic) expression of Ror1 and/or Ror2 is observed, and they might contribute to the progression of these diseases through Wnt5a-dependent and -independent manners. In this article, we overview recent advances in our understanding of the roles of Ror1 and Ror2-mediated signaling in the development, tissue regeneration and age-related diseases, and discuss their potential to be therapeutic targets for chronic inflammatory diseases and cancers.

Global analysis of cell behavior and protein localization dynamics reveals region-specific functions for Shroom3 and N-cadherin during neural tube closure

Failures of neural tube closure are common and serious birth defects, yet we have a poor understanding of the interaction of genetics and cell biology during neural tube closure. Additionally, mutations that cause neural tube defects (NTDs) tend to affect anterior or posterior regions of the neural tube but rarely both, indicating a regional specificity to NTD genetics. To better understand the regional specificity of cell behaviors during neural tube closure, we analyzed the dynamic localization of actin and N-cadherin via high-resolution tissue-level time-lapse microscopy during Xenopus neural tube closure. To investigate the regionality of gene function, we generated mosaic mutations in shroom3, a key regulator or neural tube closure. This new analytical approach elucidates several differences between cell behaviors during cranial/anterior and spinal/posterior neural tube closure, provides mechanistic insight into the function of shroom3, and demonstrates the ability of tissue-level imaging and analysis to generate cell biological mechanistic insights into neural tube closure.

Wnt-frizzled planar cell polarity signaling in the regulation of cell motility

The molecular complexes underlying planar cell polarity (PCP) were first identified in Drosophila through analysis of mutant phenotypes in the adult cuticle and the orientation of associated polarized protrusions such as wing hairs and sensory bristles. The same molecules are conserved in vertebrates and are required for the localization of polarized protrusions such as primary or sensory cilia and the orientation of hair follicles. Not only is PCP signaling required to align cellular structures across a tissue, it is also required to coordinate movement during embryonic development and adult homeostasis. PCP signaling allows cells to interpret positional cues within a tissue to move in the appropriate direction and to coordinate this movement with their neighbors. In this review we outline the molecular basis of the core Wnt-Frizzled/PCP pathway, and describe how this signaling orchestrates collective motility in Drosophila and vertebrates. Here we cover the paradigms of ommatidial rotation and border cell migration in Drosophila, and convergent extension in vertebrates. The downstream cell biological processes that underlie polarized motility include cytoskeletal reorganization, and adherens junctional and extracellular matrix remodeling. We discuss the contributions of these processes in the respective cell motility contexts. Finally, we address examples of individual cell motility guided by PCP factors during nervous system development and in cancer disease contexts.

BAC Recombineering and Transgenesis to Study Cell Polarity and Polarized Tissue Morphogenesis in Mice

Planar cell polarity (PCP) signaling plays a critical role in coordinating cell polarity during various organogenesis processes in mammals, and its disruption is causal to numerous congenital disorders in humans. To elucidate its actions in mammals, mouse genetics is an indispensable approach. Given that both gain- and loss-of-function of many PCP genes often cause similar defects, the standard mouse transgenic approach may not always be ideal for studying PCP genes in their wild-type and mutant forms. Here we describe using BAC (bacterial artificial chromosomes) transgenes as a versatile and effective alternative. Transgenes made from BACs, which are genomic clones 100-200 kb in size, can more faithful recapitulate endogenous gene expression levels and patterns. Bacterial based recombination system can be used to efficiently introduce mutations, fluorescent protein tags, and LoxP sites for conditional expressions. Cre can also be inserted into BACs to map the contribution of cells expressing any PCP gene of interest, and study PCP mediated tissue morphogenesis.

Vangl2-environment interaction causes severe neural tube defects, without abnormal neuroepithelial convergent extension

Planar cell polarity (PCP) signalling is vital for initiation of mouse neurulation, with diminished convergent extension (CE) cell movements leading to craniorachischisis, a severe neural tube defect (NTD). Some humans with NTDs also have PCP gene mutations but these are heterozygous, not homozygous as in mice. Other genetic or environmental factors may interact with partial loss of PCP function in human NTDs. We found that reduced sulfation of glycosaminoglycans interacts with heterozygosity for the Lp allele of Vangl2 (a core PCP gene), to cause craniorachischisis in cultured mouse embryos, with rescue by exogenous sulphate. We hypothesised this glycosaminoglycan-PCP interaction may regulate CE but, surprisingly, DiO labeling of the embryonic node demonstrates no abnormality of midline axial extension in sulfation-depleted Lp/+ embryos. Positive-control Lp/Lp embryos show severe CE defects. Abnormalities were detected in the size and shape of somites that flank the closing neural tube in sulfation-depleted Lp/+ embryos. We conclude that failure of closure initiation can arise by a mechanism other than faulty neuroepithelial CE, with possible involvement of matrix-mediated somite expansion, adjacent to the closing neural tube.

Conserved and Divergent Principles of Planar Polarity Revealed by Hair Cell Development and Function

Planar polarity describes the organization and orientation of polarized cells or cellular structures within the plane of an epithelium. The sensory receptor hair cells of the vertebrate inner ear have been recognized as a preeminent vertebrate model system for studying planar polarity and its development. This is principally because planar polarity in the inner ear is structurally and molecularly apparent and therefore easy to visualize. Inner ear planar polarity is also functionally significant because hair cells are mechanosensors stimulated by sound or motion and planar polarity underlies the mechanosensory mechanism, thereby facilitating the auditory and vestibular functions of the ear. Structurally, hair cell planar polarity is evident in the organization of a polarized bundle of actin-based protrusions from the apical surface called stereocilia that is necessary for mechanosensation and when stereociliary bundle is disrupted auditory and vestibular behavioral deficits emerge. Hair cells are distributed between six sensory epithelia within the inner ear that have evolved unique patterns of planar polarity that facilitate auditory or vestibular function. Thus, specialized adaptations of planar polarity have occurred that distinguish auditory and vestibular hair cells and will be described throughout this review. There are also three levels of planar polarity organization that can be visualized within the vertebrate inner ear. These are the intrinsic polarity of individual hair cells, the planar cell polarity or coordinated orientation of cells within the epithelia, and planar bipolarity; an organization unique to a subset of vestibular hair cells in which the stereociliary bundles are oriented in opposite directions but remain aligned along a common polarity axis. The inner ear with its complement of auditory and vestibular sensory epithelia allows these levels, and the inter-relationships between them, to be studied using a single model organism. The purpose of this review is to introduce the functional significance of planar polarity in the auditory and vestibular systems and our contemporary understanding of the developmental mechanisms associated with organizing planar polarity at these three cellular levels.

Unraveling the complex genetics of neural tube defects: From biological models to human genomics and back

Neural tube defects (NTDs) are a classic example of preventable birth defects for which there is a proven-effective intervention, folic acid (FA); however, further methods of prevention remain unrealized. In the decades following implementation of FA nutritional fortification programs throughout at least 87 nations, it has become apparent that not all NTDs can be prevented by FA. In the United States, FA fortification only reduced NTD rates by 28-35% (Williams et al., 2015). As such, it is imperative that further work is performed to understand the risk factors associated with NTDs and their underlying mechanisms so that alternative prevention strategies can be developed. However, this is complicated by the sheer number of genes associated with neural tube development, the heterogeneity of observable phenotypes in human cases, the rareness of the disease, and the myriad of environmental factors associated with NTD risk. Given the complex genetic architecture underlying NTD pathology and the way in which that architecture interacts dynamically with environmental factors, further prevention initiatives will undoubtedly require precision medicine strategies that utilize the power of human genomics and modern tools for assessing genetic risk factors. Herein, we review recent advances in genomic strategies for discovering genetic variants associated with these defects, and new ways in which biological models, such as mice and cell culture-derived organoids, are leveraged to assess mechanistic functionality, the way these variants interact with other genetic or environmental factors, and their ultimate contribution to human NTD risk.

Daple deficiency causes hearing loss in adult mice by inducing defects in cochlear stereocilia and apical microtubules

The V-shaped arrangement of hair bundles on cochlear hair cells is critical for auditory sensing. However, regulation of hair bundle arrangements has not been fully understood. Recently, defects in hair bundle arrangement were reported in postnatal Dishevelled-associating protein (ccdc88c, alias Daple)-deficient mice. In the present study, we found that adult Daple^−/− mice exhibited hearing disturbances over a broad frequency range through auditory brainstem response testing. Consistently, distorted patterns of hair bundles were detected in almost all regions, more typically in the basal region of the cochlear duct. In adult Daple^−/− mice, apical microtubules were irregularly aggregated, and the number of microtubules attached to plasma membranes was decreased. Similar phenotypes were manifested upon nocodazole treatment in a wild type cochlea culture without affecting the microtubule structure of the kinocilium. These results indicate critical role of Daple in hair bundle arrangement through the orchestration of apical microtubule distribution, and thereby in hearing, especially at high frequencies.

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.

Organoids as a new model system to study neural tube defects

The neural tube is the first critically important structure that develops in the embryo. It serves as the primordium of the central nervous system; therefore, the proper formation of the neural tube is essential to the developing organism. Neural tube defects (NTDs) are severe congenital defects caused by failed neural tube closure during early embryogenesis. The pathogenesis of NTDs is complicated and still not fully understood even after decades of research. While it is an ethically impossible proposition to investigate the in vivo formation process of the neural tube in human embryos, a newly developed technology involving the creation of neural tube organoids serves as an excellent model system with which to study human neural tube formation and the occurrence of NTDs. Herein we reviewed the recent literature on the process of neural tube formation, the progress of NTDs investigations, and particularly the exciting potential to use neural tube organoids to model the cellular and molecular mechanisms underlying the etiology of NTDs.

Regulation of Dishevelled DEP domain swapping by conserved phosphorylation sites

Wnt signals bind to Frizzled receptors to trigger canonical and noncanonical signaling responses that control cell fates during animal development and tissue homeostasis. All Wnt signals are relayed by the hub protein Dishevelled. During canonical (β-catenin-dependent) signaling, Dishevelled assembles signalosomes via dynamic head-to-tail polymerization of its Dishevelled and Axin (DIX) domain, which are cross-linked by its Dishevelled, Egl-10, and Pleckstrin (DEP) domain through a conformational switch from monomer to domain-swapped dimer. The domain-swapped conformation of DEP masks the site through which Dishevelled binds to Frizzled, implying that DEP domain swapping results in the detachment of Dishevelled from Frizzled. This would be incompatible with noncanonical Wnt signaling, which relies on long-term association between Dishevelled and Frizzled. It is therefore likely that DEP domain swapping is differentially regulated during canonical and noncanonical Wnt signaling. Here, we use NMR spectroscopy and cell-based assays to uncover intermolecular contacts in the DEP dimer that are essential for its stability and for Dishevelled function in relaying canonical Wnt signals. These contacts are mediated by an intrinsically structured sequence spanning a conserved phosphorylation site upstream of the DEP domain that serves to clamp down the swapped N-terminal α-helix onto the structural core of a reciprocal DEP molecule in the domain-swapped configuration. Mutations of this phosphorylation site and its cognate surface on the reciprocal DEP core attenuate DEP-dependent dimerization of Dishevelled and its canonical signaling activity in cells without impeding its binding to Frizzled. We propose that phosphorylation of this crucial residue could be employed to switch off canonical Wnt signaling.

Scribble mutation disrupts convergent extension and apical constriction during mammalian neural tube closure

Morphogenesis of the vertebrate neural tube occurs by elongation and bending of the neural plate, tissue shape changes that are driven at the cellular level by polarized cell intercalation and cell shape changes, notably apical constriction and cell wedging. Coordinated cell intercalation, apical constriction, and wedging undoubtedly require complex underlying cytoskeletal dynamics and remodeling of adhesions. Mutations of the gene encoding Scribble result in neural tube defects in mice, however the cellular and molecular mechanisms by which Scrib regulates neural cell behavior remain unknown. Analysis of Scribble mutants revealed defects in neural tissue shape changes, and live cell imaging of mouse embryos showed that the Scrib mutation results in defects in polarized cell intercalation, particularly in rosette resolution, and failure of both cell apical constriction and cell wedging. Scrib mutant embryos displayed aberrant expression of the junctional proteins ZO-1, Par3, Par6, E- and N-cadherins, and the cytoskeletal proteins actin and myosin. These findings show that Scribble has a central role in organizing the molecular complexes regulating the morphomechanical neural cell behaviors underlying vertebrate neurulation, and they advance our understanding of the molecular mechanisms involved in mammalian neural tube closure.

Chromatin remodeler CHD7 is critical for cochlear morphogenesis and neurosensory patterning

Epigenetic regulation of gene transcription by chromatin remodeling proteins has recently emerged as an important contributing factor in inner ear development. Pathogenic variants in CHD7, the gene encoding Chromodomain Helicase DNA binding protein 7, cause CHARGE syndrome, which presents with malformations in the developing ear. Chd7 is broadly expressed in the developing mouse otocyst and mature auditory epithelium, yet the pathogenic effects of Chd7 loss in the cochlea are not well understood. Here we characterized cochlear epithelial phenotypes in mice with deletion of Chd7 throughout the otocyst (using Foxg1^Cre/+ and Pax2Cre), in the otic mesenchyme (using TCre), in hair cells (using Atoh1Cre), in developing neuroblasts (using NgnCre), or in spiral ganglion neurons (using Shh^Cre/+). Pan-otic deletion of Chd7 resulted in shortened cochleae with aberrant projections and axonal looping, disorganized, supernumerary hair cells at the apical turn and a narrowed epithelium with missing hair cells in the middle region. Deletion of Chd7 in the otic mesenchyme had no effect on overall cochlear morphology. Loss of Chd7 in hair cells did not disrupt their formation or organization of the auditory epithelium. Similarly, absence of Chd7 in spiral ganglion neurons had no effect on axonal projections. In contrast, deletion of Chd7 in developing neuroblasts led to smaller spiral ganglia and disorganized cochlear neurites. Together, these observations reveal dosage-, tissue-, and time-sensitive cell autonomous roles for Chd7 in cochlear elongation and cochlear neuron organization, with minimal functions for Chd7 in hair cells. These studies provide novel information about roles for Chd7 in development of auditory neurons.

Snx3 is important for mammalian neural tube closure via its role in canonical and non-canonical WNT signaling

Disruptions in neural tube (NT) closure result in neural tube defects (NTDs). To understand the molecular processes required for mammalian NT closure, we investigated the role of Snx3, a sorting nexin gene. Snx3^-/- mutant mouse embryos display a fully-penetrant cranial NTD. In vivo, we observed decreased canonical WNT target gene expression in the cranial neural epithelium of the Snx3^-/- embryos and a defect in convergent extension of the neural epithelium. Snx3^-/- cells show decreased WNT secretion, and live cell imaging reveals aberrant recycling of the WNT ligand-binding protein WLS and mis-trafficking to the lysosome for degradation. The importance of SNX3 in WNT signaling regulation is demonstrated by rescue of NT closure in Snx3^-/- embryos with a WNT agonist. The potential for SNX3 to function in human neurulation is revealed by a point mutation identified in an NTD-affected individual that results in functionally impaired SNX3 that does not colocalize with WLS and the degradation of WLS in the lysosome. These data indicate that Snx3 is crucial for NT closure via its role in recycling WLS in order to control levels of WNT signaling.

Decoding Dishevelled-Mediated Wnt Signaling in Vertebrate Early Development

Dishevelled proteins are key players of Wnt signaling pathways. They transduce Wnt signals and perform cellular functions through distinct conserved domains. Due to the presence of multiple paralogs, the abundant accumulation of maternal transcripts, and the activation of distinct Wnt pathways, their regulatory roles during vertebrate early development and the mechanism by which they dictate the pathway specificity have been enigmatic and attracted much attention in the past decades. Extensive studies in different animal models have provided significant insights into the structure-function relationship of conserved Dishevelled domains in Wnt signaling and the implications of Dishevelled isoforms in early developmental processes. Notably, intra- and inter-molecular interactions and Dishevelled dosage may be important in modulating the specificity of Wnt signaling. There are also distinct and redundant functions among Dishevelled isoforms in development and disease, which may result from differential spatiotemporal expression patterns and biochemical properties and post-translational modifications. This review presents the advances and perspectives in understanding Dishevelled-mediated Wnt signaling during gastrulation and neurulation in vertebrate early embryos.

Dual regulation of planar polarization by secreted Wnts and Vangl2 in the developing mouse cochlea

Planar cell polarity (PCP) proteins localize asymmetrically to instruct cell polarity within the tissue plane, with defects leading to deformities of the limbs, neural tube and inner ear. Wnt proteins are evolutionarily conserved polarity cues, yet Wnt mutants display variable PCP defects; thus, how Wnts regulate PCP remains unresolved. Here, we have used the developing cochlea as a model system to show that secreted Wnts regulate PCP through polarizing a specific subset of PCP proteins. Conditional deletion of Wntless or porcupine, both of which are essential for secretion of Wnts, caused misrotated sensory cells and shortened cochlea - both hallmarks of PCP defects. Wntless-deficient cochleae lacked the polarized PCP components dishevelled 1/2 and frizzled 3/6, while other PCP proteins (Vangl1/2, Celsr1 and dishevelled 3) remained localized. We identified seven Wnt paralogues, including the major PCP regulator Wnt5a, which was, surprisingly, dispensable for planar polarization in the cochlea. Finally, Vangl2 haploinsufficiency markedly accentuated sensory cell polarization defects in Wntless-deficient cochlea. Together, our study indicates that secreted Wnts and Vangl2 coordinate to ensure proper tissue polarization during development.

Deletion of the Dishevelled family of genes disrupts anterior-posterior axis specification and selectively prevents mesoderm differentiation

The Dishevelled proteins transduce both canonical Wnt/β-catenin and non-canonical Wnt/planar cell polarity (PCP) signaling pathways to regulate many key developmental processes during embryogenesis. Here, we disrupt both canonical and non-canonical Wnt pathways by targeting the entire Dishevelled family of genes (Dvl1, Dvl2, and Dvl3) to investigate their functional roles in the early embryo. We identified several defects in anterior-posterior axis specification and mesoderm patterning in Dvl1+/-; Dvl2-/-; Dvl3-/- embryos. Homozygous deletions in all three Dvl genes (Dvl TKO) resulted in defects in distal visceral endoderm migration and a complete failure to induce mesoderm formation. To identify potential mechanisms that lead to the defects in the developmental processes preceding gastrulation, we generated Dvl TKO mouse embryonic stem cells (mESCs) and compared the transcriptional profile of these cells with wild-type (WT) mESCs during germ lineage differentiation into 3D embryoid bodies (EBs). While the Dvl TKO mESCs displayed similar morphology, self-renewal properties, and minor transcriptional variation from WT mESCs, we identified major transcriptional dysregulation in the Dvl TKO EBs during differentiation in a number of genes involved in anterior-posterior pattern specification, gastrulation induction, mesenchyme morphogenesis, and mesoderm-derived tissue development. The absence of the Dvls leads to specific down-regulation of BMP signaling genes. Furthermore, exogenous activation of canonical Wnt, BMP, and Nodal signaling all fail to rescue the mesodermal defects in the Dvl TKO EBs. Moreover, endoderm differentiation was promoted in the absence of mesoderm in the Dvl TKO EBs, while the suppression of ectoderm differentiation was delayed. Overall, we demonstrate that the Dvls are dispensable for maintaining self-renewal in mESCs but are critical during differentiation to regulate key developmental signaling pathways to promote proper axis specification and mesoderm formation.

Genetics and signaling mechanisms of orofacial clefts

Craniofacial development involves several complex tissue movements including several fusion processes to form the frontonasal and maxillary structures, including the upper lip and palate. Each of these movements are controlled by many different factors that are tightly regulated by several integral morphogenetic signaling pathways. Subject to both genetic and environmental influences, interruption at nearly any stage can disrupt lip, nasal, or palate fusion and result in a cleft. Here, we discuss many of the genetic risk factors that may contribute to the presentation of orofacial clefts in patients, and several of the key signaling pathways and underlying cellular mechanisms that control lip and palate formation, as identified primarily through investigating equivalent processes in animal models, are examined.

Functional Validation of CLDN Variants Identified in a Neural Tube Defect Cohort Demonstrates Their Contribution to Neural Tube Defects

Neural tube defects (NTDs) are severe malformations of the central nervous system that affect 1–2 individuals per 2,000 births. Their etiology is complex and involves both genetic and environmental factors. Our recent discovery that simultaneous removal of Cldn3, -4, and -8 from tight junctions results in cranial and spinal NTDs in both chick and mouse embryos suggests that claudins play a conserved role in neural tube closure in vertebrates. To determine if claudins were associated with NTDs in humans, we used a Fluidigm next generation sequencing approach to identify genetic variants in CLDN loci in 152 patients with spinal NTDs. We identified eleven rare and four novel missense mutations in ten CLDN genes. In vivo validation of variant pathogenicity using a chick embryo model system revealed that overexpression of four variants caused a significant increase in NTDs: CLDN3 A128T, CLDN8 P216L, CLDN19 I22T, and E209G. Our data implicate rare missense variants in CLDN genes as risk factors for spinal NTDs and suggest a new family of proteins involved in the pathogenesis of these malformations.

DVL mutations identified from human neural tube defects and Dandy-Walker malformation obstruct the Wnt signaling pathway

Wnt signaling pathways, including the canonical Wnt/β-catenin pathway, planar cell polarity pathway, and Wnt/Ca²⁺ signaling pathway, play important roles in neural development during embryonic stages. The DVL genes encode the hub proteins for Wnt signaling pathways. The mutations in DVL2 and DVL3 were identified from patients with neural tube defects (NTDs), but their functions in the pathogenesis of human neural diseases remain elusive. Here, we sequenced the coding regions of three DVL genes in 176 stillborn or miscarried fetuses with NTDs or Dandy-Walker malformation (DWM) and 480 adult controls from a Han Chinese population. Four rare mutations were identified: DVL1 p.R558H, DVL1 p.R606C, DVL2 p.R633W, and DVL3 p.R222Q. To assess the effect of these mutations on NTDs and DWM, various functional analyses such as luciferase reporter assay, stress fiber formation, and in vivo teratogenic assay were performed. The results showed that the DVL2 p.R633W mutation destabilized DVL2 protein and upregulated activities for all three Wnt signalings (Wnt/β-catenin signaling, Wnt/planar cell polarity signaling, and Wnt/Ca²⁺ signaling) in mammalian cells. In contrast, DVL1 mutants (DVL1 p.R558H and DVL1 p.R606C) decreased canonical Wnt/β-catenin signaling but increased the activity of Wnt/Ca²⁺ signaling, and DVL3 p.R222Q only decreased the activity of Wnt/Ca²⁺ signaling. We also found that only the DVL2 p.R633W mutant displayed more severe teratogenicity in zebrafish embryos than wild-type DVL2. Our study demonstrates that these four rare DVL mutations, especially DVL2 p.R633W, may contribute to human neural diseases such as NTDs and DWM by obstructing Wnt signaling pathways.

Development of the cochlea

The cochlea, a coiled structure located in the ventral region of the inner ear, acts as the primary structure for the perception of sound. Along the length of the cochlear spiral is the organ of Corti, a highly derived and rigorously patterned sensory epithelium that acts to convert auditory stimuli into neural impulses. The development of the organ of Corti requires a series of inductive events that specify unique cellular characteristics and axial identities along its three major axes. Here, we review recent studies of the cellular and molecular processes regulating several aspects of cochlear development, such as axial patterning, cochlear outgrowth and cellular differentiation. We highlight how the precise coordination of multiple signaling pathways is required for the successful formation of a complete organ of Corti.

Pcdh18a regulates endocytosis of E-cadherin during axial mesoderm development in zebrafish

The notochord defines the axial structure of all vertebrates during development. Notogenesis is a result of major cell reorganization in the mesoderm, the convergence and the extension of the axial cells. However, it is currently not fully understood how these processes act together in a coordinated way during notochord formation. The prechordal plate is an actively migrating cell population in the central mesoderm anterior to the trailing notochordal plate cells. We show that prechordal plate cells express Protocadherin 18a (Pcdh18a), a member of the cadherin superfamily. We find that Pcdh18a-mediated recycling of E-cadherin adhesion complexes transforms prechordal plate cells into a cohesive and fast migrating cell group. In turn, the prechordal plate cells subsequently instruct the trailing mesoderm. We simulated cell migration during early mesoderm formation using a lattice-based mathematical framework and predicted that the requirement for an anterior, local motile cell cluster could guide the intercalation and extension of the posterior, axial cells. Indeed, a grafting experiment validated the prediction and local Pcdh18a expression induced an ectopic prechordal plate-like cell group migrating towards the animal pole. Our findings indicate that the Pcdh18a is important for prechordal plate formation, which influences the trailing mesodermal cell sheet by orchestrating the morphogenesis of the notochord.

Dynamic changes in cis-regulatory occupancy by Six1 and its cooperative interactions with distinct cofactors drive lineage-specific gene expression programs during progressive differentiation of the auditory sensory epithelium

The transcription factor Six1 is essential for induction of sensory cell fate and formation of auditory sensory epithelium, but how it activates gene expression programs to generate distinct cell-types remains unknown. Here, we perform genome-wide characterization of Six1 binding at different stages of auditory sensory epithelium development and find that Six1-binding to cis-regulatory elements changes dramatically at cell-state transitions. Intriguingly, Six1 pre-occupies enhancers of cell-type-specific regulators and effectors before their expression. We demonstrate in-vivo cell-type-specific activity of Six1-bound novel enhancers of Pbx1, Fgf8, Dusp6, Vangl2, the hair-cell master regulator Atoh1 and a cascade of Atoh1's downstream factors, including Pou4f3 and Gfi1. A subset of Six1-bound sites carry consensus-sequences for its downstream factors, including Atoh1, Gfi1, Pou4f3, Gata3 and Pbx1, all of which physically interact with Six1. Motif analysis identifies RFX/X-box as one of the most significantly enriched motifs in Six1-bound sites, and we demonstrate that Six1-RFX proteins cooperatively regulate gene expression through binding to SIX:RFX-motifs. Six1 targets a wide range of hair-bundle regulators and late Six1 deletion disrupts hair-bundle polarity. This study provides a mechanistic understanding of how Six1 cooperates with distinct cofactors in feedforward loops to control lineage-specific gene expression programs during progressive differentiation of the auditory sensory epithelium.

Placental defects lead to embryonic lethality in mice lacking the Formin and PCP proteins Daam1 and Daam2

The actin cytoskeleton plays a central role in establishing cell polarity and shape during embryonic morphogenesis. Daam1, a member of the Formin family of actin cytoskeleton regulators, is a Dvl2-binding protein that functions in the Wnt/Planar Cell Polarity (PCP) pathway. To examine the role of the Daam proteins in mammalian development, we generated Daam-deficient mice by gene targeting and found that Daam1, but not Daam2, is necessary for fetal survival. Embryonic development of Daam1 mutants was delayed most likely due to functional defects in the labyrinthine layer of the placenta. Examination of Daam2 and Daam1/2 double mutants revealed that Daam1 and Daam2 are functionally redundant during placental development. Of note, neural tube closure defects (NTD), which are observed in several mammalian PCP mutants, are not observed in Wnt5a or Daam1 single mutants, but arise in Daam1;Wnt5a double mutants. These findings demonstrate a unique function for Daam genes in placental development and are consistent with a role for Daam1 in the Wnt/PCP pathway in mammals.

Mouse Models of Neural Tube Defects

During embryonic development, the central nervous system forms as the neural plate and then rolls into a tube in a complex morphogenetic process known as neurulation. Neural tube defects (NTDs) occur when neurulation fails and are among the most common structural birth defects in humans. The frequency of NTDs varies greatly anywhere from 0.5 to 10 in 1000 live births, depending on the genetic background of the population, as well as a variety of environmental factors. The prognosis varies depending on the size and placement of the lesion and ranges from death to severe or moderate disability, and some NTDs are asymptomatic. This chapter reviews how mouse models have contributed to the elucidation of the genetic, molecular, and cellular basis of neural tube closure, as well as to our understanding of the causes and prevention of this devastating birth defect.

Nodal and planar cell polarity signaling cooperate to regulate zebrafish convergence and extension gastrulation movements

During vertebrate gastrulation, convergence and extension (C and E) of the primary anteroposterior (AP) embryonic axis is driven by polarized mediolateral (ML) cell intercalations and is influenced by AP axial patterning. Nodal signaling is essential for patterning of the AP axis while planar cell polarity (PCP) signaling polarizes cells with respect to this axis, but how these two signaling systems interact during C and E is unclear. We find that the neuroectoderm of Nodal-deficient zebrafish gastrulae exhibits reduced C and E cell behaviors, which require Nodal signaling in both cell- and non-autonomous fashions. PCP signaling is partially active in Nodal-deficient embryos and its inhibition exacerbates their C and E defects. Within otherwise naïve zebrafish blastoderm explants, however, Nodal induces C and E in a largely PCP-dependent manner, arguing that Nodal acts both upstream of and in parallel with PCP during gastrulation to regulate embryonic axis extension cooperatively.

Sestd1 Encodes a Developmentally Dynamic Synapse Protein That Complexes With BCR Rac1-GAP to Regulate Forebrain Dendrite, Spine and Synapse Formation

SEC14 and Spectrin domain-1 (Sestd1) is a synapse protein that exhibits a striking shift from the presynaptic to postsynaptic space as neurons mature postnatally in the mouse hippocampus. Hippocampal pyramidal neurons from mice with global genetic deletion of Sestd1 have reduced dendrite arbors, spines, and excitatory synapses. Electrophysiologically this correlates with cell-autonomous reductions in both AMPA- and NMDA-excitatory postsynaptic currents in individual hippocampal neurons from which Sestd1 has been deleted in vivo. These neurodevelopmental and functional deficits are associated with increased activation of the Rho family GTPases Rac1 and RhoA. Co-immunoprecipitation and mass spectrometry reveal that the Breakpoint Cluster Region protein, a Rho GTPase activating protein (GAP), forms complexes with Sestd1 in brain tissue. This complements earlier findings that Sestd1 can also partner with other Rho family GAPs and guanine nucleotide exchange factors. Our findings demonstrate that Sestd1 is a developmentally dynamic synaptic regulator of Rho GTPases that contributes to dendrite and excitatory synapse formation within differentiating pyramidal neurons of the forebrain.

Mutations associated with human neural tube defects display disrupted planar cell polarity in Drosophila

Planar cell polarity (PCP) and neural tube defects (NTDs) are linked, with a subset of NTD patients found to harbor mutations in PCP genes, but there is limited data on whether these mutations disrupt PCP signaling in vivo. The core PCP gene Van Gogh (Vang), Vangl1/2 in mammals, is the most specific for PCP. We thus addressed potential causality of NTD-associated Vangl1/2 mutations, from either mouse or human patients, in Drosophila allowing intricate analysis of the PCP pathway. Introducing the respective mammalian mutations into Drosophila Vang revealed defective phenotypic and functional behaviors, with changes to Vang localization, post-translational modification, and mechanistic function, such as its ability to interact with PCP effectors. Our findings provide mechanistic insight into how different mammalian mutations contribute to developmental disorders and strengthen the link between PCP and NTD. Importantly, analyses of the human mutations revealed that each is a causative factor for the associated NTD.

Prenatal low-dose methyltestosterone, but not dihydrotestosterone, treatment induces penile formation in female mice and guinea pigs†

Genital tubercle has bisexual potential before sex differentiation. Females exposed to androgen during sex differentiation show masculinized external genitalia, but the effects of different androgens on tubular urethral and penile formation in females are mostly unknown. In this study, we compared the masculinization effects of commonly used androgens methyltestosterone, dihydrotestosterone, and testosterone on the induction of penile formation in females. Our results suggested that prenatal treatment with low doses of methyltestosterone, but not same doses of dihydrotestosterone or testosterone, could induce penile formation in female mice. The minimum dose of dihydrotestosterone and testosterone for inducing tubular urethral formation in female mice was, respectively, 50 and 20 times higher than that of methyltestosterone. In vivo methyltestosterone treatment induced more nuclear translocation of androgen receptors in genital tubercles of female mice, affected Wnt signaling gene expressions, and then led to similar patterns of cell proliferation and death in developing genital tubercles to those of control males. We further revealed that low-dose methyltestosterone, but not same dose of dihydrotestosterone or testosterone, treatment induced penile formation in female guinea pigs. Exposure of female mouse genital tubercle organ culture to methyltestosterone, dihydrotestosterone, or testosterone could induce nuclear translocation of androgen receptors, suggesting that the differential effect of the three androgens in vivo might be due to the hormonal profile in mother or fetus, rather than the local genital tissue. To understand the differential role of these androgens in masculinization process involved is fundamental to androgen replacement therapy for diseases related to external genital masculinization.

The physiological role of Wnt pathway in normal development and cancer

Over the decades, many studies have illustrated the critical roles of Wnt signaling pathways in both developmental processes as well as tumorigenesis. Due to the complexity of Wnt signaling regulation, there are still questions to be addressed about ways cells are able to manipulate different types of Wnt pathways in order to fulfill the requirements for normal or cancer development. In this review, we will describe different types of Wnt signaling pathways and their roles in both normal developmental processes and their role in cancer development and progression. Additionally, we will briefly introduce new strategies currently in clinical trials targeting Wnt signaling pathway components for cancer therapy.

Development and Patterning of the Cochlea: From Convergent Extension to Planar Polarity

Within the mammalian cochlea, sensory hair cells and supporting cells are aligned in curvilinear rows that extend along the length of the tonotopic axis. In addition, all of the cells within the epithelium are uniformly polarized across the orthogonal neural-abneural axis. Finally, each hair cell is intrinsically polarized as revealed by the presence of an asymmetrically shaped and apically localized stereociliary bundle. It has been known for some time that many of the developmental processes that regulate these patterning events are mediated, to some extent, by the core planar cell polarity (PCP) pathway. This article will review more recent work demonstrating how components of the PCP pathway interact with cytoskeletal motor proteins to regulate cochlear outgrowth. Finally, a signaling pathway originally identified for its role in asymmetric cell divisions has recently been shown to mediate several aspects of intrinsic hair cell polarity, including kinocilia migration, bundle shape, and elongation.

Convergent extension in mammalian morphogenesis

Convergent extension is a fundamental morphogenetic process that underlies not only the generation of the elongated vertebrate body plan from the initially radially symmetrical embryo, but also the specific shape changes characteristic of many individual tissues. These tissue shape changes are the result of specific cell behaviors, coordinated in time and space, and affected by the physical properties of the tissue. While mediolateral cell intercalation is the classic cellular mechanism for producing tissue convergence and extension, other cell behaviors can also provide similar tissue-scale distortions or can modulate the effects of mediolateral cell intercalation to sculpt a specific shape. Regulation of regional tissue morphogenesis through planar polarization of the variety of underlying cell behaviors is well-recognized, but as yet is not well understood at the molecular level. Here, we review recent advances in understanding the cellular basis for convergence and extension and its regulation.

Adaptor protein-3 complex is required for Vangl2 trafficking and planar cell polarity of the inner ear

Planar cell polarity (PCP) regulates coordinated cellular polarity among neighboring cells to establish a polarity axis parallel to the plane of the tissue. Disruption in PCP results in a range of developmental anomalies and diseases. A key feature of PCP is the polarized and asymmetric localization of several membrane PCP proteins, which is essential to establish the polarity axis to orient cells coordinately. However, the machinery that regulates the asymmetric partition of PCP proteins remains largely unknown. In the present study, we show Van gogh-like 2 (Vangl2) in early and recycling endosomes as made evident by colocalization with diverse endosomal Rab proteins. Vangl2 biochemically interacts with adaptor protein-3 complex (AP-3). Using short hairpin RNA knockdown, we found that Vangl2 subcellular localization was modified in AP-3–depleted cells. Moreover, Vangl2 membrane localization within the cochlea is greatly reduced in AP-3–deficient mocha mice, which exhibit profound hearing loss. In inner ears from AP-3–deficient mocha mice, we observed PCP-dependent phenotypes, such as misorientation and deformation of hair cell stereociliary bundles and disorganization of hair cells characteristic of defects in convergent extension that is driven by PCP. These findings demonstrate a novel role of AP-3–mediated sorting mechanisms in regulating PCP proteins.

FOXF2 is required for cochlear development in humans and mice

Molecular mechanisms governing the development of the human cochlea remain largely unknown. Through genome sequencing, we identified a homozygous FOXF2 variant c.325A>T (p.I109F) in a child with profound sensorineural hearing loss (SNHL) associated with incomplete partition type I anomaly of the cochlea. This variant is not found in public databases or in over 1000 ethnicity-matched control individuals. I109 is a highly conserved residue in the forkhead box (Fox) domain of FOXF2, a member of the Fox protein family of transcription factors that regulate the expression of genes involved in embryogenic development as well as adult life. Our in vitro studies show that the half-life of mutant FOXF2 is reduced compared to that of wild type. Foxf2 is expressed in the cochlea of developing and adult mice. The mouse knockout of Foxf2 shows shortened and malformed cochleae, in addition to altered shape of hair cells with innervation and planar cell polarity defects. Expressions of Eya1 and Pax3, genes essential for cochlear development, are reduced in the cochleae of Foxf2 knockout mice. We conclude that FOXF2 plays a major role in cochlear development and its dysfunction leads to SNHL and developmental anomalies of the cochlea in humans and mice.

PDZ Domains as Drug Targets

Protein-protein interactions within protein networks shape the human interactome, which often is promoted by specialized protein interaction modules, such as the postsynaptic density-95 (PSD-95), discs-large, zona occludens 1 (ZO-1) (PDZ) domains. PDZ domains play a role in several cellular functions, from cell-cell communication and polarization, to regulation of protein transport and protein metabolism. PDZ domain proteins are also crucial in the formation and stability of protein complexes, establishing an important bridge between extracellular stimuli detected by transmembrane receptors and intracellular responses. PDZ domains have been suggested as promising drug targets in several diseases, ranging from neurological and oncological disorders to viral infections. In this review, the authors describe structural and genetic aspects of PDZ-containing proteins and discuss the current status of the development of small-molecule and peptide modulators of PDZ domains. An overview of potential new therapeutic interventions in PDZ-mediated protein networks is also provided.

Grxcr2 is required for stereocilia morphogenesis in the cochlea

Hearing and balance depend upon the precise morphogenesis and mechanosensory function of stereocilia, the specialized structures on the apical surface of sensory hair cells in the inner ear. Previous studies of Grxcr1 mutant mice indicated a critical role for this gene in control of stereocilia dimensions during development. In this study, we analyzed expression of the paralog Grxcr2 in the mouse and evaluated auditory and vestibular function of strains carrying targeted mutations of the gene. Peak expression of Grxcr2 occurs during early postnatal development of the inner ear and GRXCR2 is localized to stereocilia in both the cochlea and in vestibular organs. Homozygous Grxcr2 deletion mutants exhibit significant hearing loss by 3 weeks of age that is associated with developmental defects in stereocilia bundle orientation and organization. Despite these bundle defects, the mechanotransduction apparatus assembles in relatively normal fashion as determined by whole cell electrophysiological evaluation and FM1-43 uptake. Although Grxcr2 mutants do not exhibit overt vestibular dysfunction, evaluation of vestibular evoked potentials revealed subtle defects of the mutants in response to linear accelerations. In addition, reduced Grxcr2 expression in a hypomorphic mutant strain is associated with progressive hearing loss and bundle defects. The stereocilia localization of GRXCR2, together with the bundle pathologies observed in the mutants, indicate that GRXCR2 plays an intrinsic role in bundle orientation, organization, and sensory function in the inner ear during development and at maturity.

Mutational analysis of dishevelled genes in zebrafish reveals distinct functions in embryonic patterning and gastrulation cell movements

Wnt signaling plays critical roles in dorsoventral fate specification and anteroposterior patterning, as well as in morphogenetic cell movements. Dishevelled proteins, or Dvls, mediate the activation of Wnt/ß-catenin and Wnt/planar cell polarity pathways. There are at least three highly conserved Dvl proteins in vertebrates, but the implication of each Dvl in key early developmental processes remains poorly understood. In this study, we use genome-editing approach to generate different combinations of maternal and zygotic dvl mutants in zebrafish, and examine their functions during early development. Maternal transcripts for dvl2 and dvl3a are most abundantly expressed, whereas the transcript levels of other dvl genes are negligible. Phenotypic and molecular analyses show that early dorsal fate specification is not affected in maternal and zygotic dvl2 and dvl3a double mutants, suggesting that the two proteins may be dispensable for the activation of maternal Wnt/ß-catenin signaling. Interestingly, convergence and extension movements and anteroposterior patterning require both maternal and the zygotic functions of Dvl2 and Dvl3a, but these processes are more sensitive to Dvl2 dosage. Zygotic dvl2 and dvl3a double mutants display mild axis extension defect with correct anteroposterior patterning. However, maternal and zygotic double mutants exhibit most strongly impaired convergence and extension movements, severe trunk and posterior deficiencies, and frequent occurrence of cyclopia and craniofacial defects. Our results suggest that Dvl2 and Dvl3a products are required for the activation of zygotic Wnt/ß-catenin signaling and Wnt/planar cell polarity pathway, and regulate zygotic developmental processes in a dosage-dependent manner. This work provides insight into the mechanisms of Dvl-mediated Wnt signaling pathways during early vertebrate development.

The embryogenesis of most animals is first supported by maternal gene products accumulated in the oocyte, and then by the expression of genes from the zygote. In all vertebrates, there are at least three Dishevelled (Dvl) proteins, which play critical roles in normal development and human diseases. They are both maternally and zygotically expressed, and can activate the ß-catenin-dependent Wnt pathway that regulates gene expression and cell fate, and the ß-catenin-independent Wnt pathway that orchestrates cell movements. In zebrafish embryo, Dvl2 and Dvl3a are most abundant, but their functions are not fully understood. We find that maternally and zygotically expressed Dvl2 plays a predominant role in the elongation of the anteroposterior axis, and the expression of genes involved in the development of the posterior region. Dvl3a cooperates with Dvl2 in these processes. Analyses after loss-of-function of these genes indicate that deficiency of maternal and zygotic Dvl2 and Dvl3a results in embryos with cyclopia, craniofacial defects, and severe abnormality in the trunk and posterior regions. Many human birth defects and other diseases, like cancer, are attributed to the dysfunction of the Wnt pathways. Our results help to understand the mechanisms of Dvl-mediated Wnt pathway activation, and the causes of developmental disorders.

A non-autonomous function of the core PCP protein VANGL2 directs peripheral axon turning in the developing cochlea

The cochlea is innervated by neurons that relay sound information from hair cells to central auditory targets. A subset of these are the type II spiral ganglion neurons, which have nociceptive features and contribute to feedback circuits providing neuroprotection in extreme noise. Type II neurons make a distinctive 90° turn towards the cochlear base to synapse with 10-15 outer hair cells. We demonstrate that this axon turning event requires planar cell polarity (PCP) signaling and is disrupted in Vangl2 and Celsr1 knockout mice, and that VANGL2 acts non-autonomously from the cochlea to direct turning. Moreover, VANGL2 is asymmetrically distributed at intercellular junctions between cochlear supporting cells, and in a pattern that could allow it to act directly as an axon guidance cue. Together, these data reveal a non-autonomous function for PCP signaling during axon guidance occurring in the tissue that is innervated, rather than the navigating growth cone.