The core planar cell polarity gene, Vangl2, maintains apical-basal organisation of the corneal epithelium

Van Gogh-like 2 is essential for the architectural patterning of the mammalian biliary tree

BACKGROUND & AIMS

In the developing liver, bipotent epithelial progenitor cells undergo lineage segregation to form hepatocytes, which constitute the bulk of the liver parenchyma, and biliary epithelial cells (cholangiocytes), which comprise the bile duct (a complex tubular network that is critical for normal liver function). Notch and TGFβ signalling promote the formation of a sheet of biliary epithelial cells, the ductal plate, that organises into discontinuous tubular structures. How these structures elongate and connect to form a continuous duct remains undefined. We aimed to define the mechanisms by which the ductal plate transitions from a simple sheet of epithelial cells into a complex and connected bile duct.

METHODS

By combining single-cell RNA sequencing of embryonic mouse livers with genetic tools and organoid models we functionally dissected the role of planar cell polarity in duct patterning.

RESULTS

We show that the planar cell polarity protein VANGL2 is expressed late in intrahepatic bile duct development and patterns the formation of cell-cell contacts between biliary cells. The patterning of these cell contacts regulates the normal polarisation of the actin cytoskeleton within biliary cells and loss of Vangl2 function results in the abnormal distribution of cortical actin remodelling, leading to the failure of bile duct formation.

CONCLUSIONS

Planar cell polarity is a critical step in the post-specification sculpture of the bile duct and is essential for establishing normal tissue architecture.

IMPACT AND IMPLICATIONS

Like other branched tissues, such as the lung and kidney, the bile ducts use planar cell polarity signalling to coordinate cell movements; however, how these biochemical signals are linked to ductular patterning remains unclear. Here we show that the core planar cell polarity protein VANGL2 patterns how cell-cell contacts form in the mammalian bile duct and how ductular cells transmit confluent mechanical changes along the length of a duct. This work sheds light on how biological tubes are patterned across mammalian tissues (including within the liver) and will be important in how we promote ductular growth in patients where the duct is mis-patterned or poorly formed.

Prickle1-driven basement membrane deposition of the iPSC-derived embryoid bodies is separable from the establishment of apicobasal polarity

Basement membrane (BM) component deposition is closely linked to the establishment of cell polarity. Previously, we showed that Prickle1 is crucial for BM deposition and cell polarity events in tear duct elongation. To gain a deeper understanding of the intimate relationship between BM formation and cell polarity, we generated induced pluripotent stem cells (iPSCs)-derived embryoid bodies (EBs) with a basement membrane separating the visceral endoderm (VE) and inner EB cell mass. We found that Prickle1 was highly expressed in VE of the normal EBs, and the Prickle1 mutant EBs displayed severely impaired BM. Notably, the formation of the basement membrane appeared to rely on the proper microtubule network of the VE cells, which was disrupted in the Prickle1 mutant EBs. Moreover, disruption of vesicle trafficking in the VE hindered BM secretion. Furthermore, reintroducing Prickle1 in the mutant EBs completely rescued BM formation but not the apicobasal cell polarity of the VE. Our data, in conjunction with studies by others, highlight the conserved role of Prickle1 in directing the secretion of BM components of the VE cells during embryonic germ layer differentiation, even in the absence of established general polarity machinery. Our study introduces a novel system based on iPSCs-derived EBs for investigating cellular and molecular events associated with cell polarity.

Disrupted endosomal trafficking of the Vangl-Celsr polarity complex underlies congenital anomalies in trachea-esophageal morphogenesis

Disruptions in foregut morphogenesis can result in life-threatening conditions where the trachea and esophagus fail to separate properly, such as esophageal atresia (EA) and tracheoesophageal fistulas (TEF). The developmental basis of these congenital anomalies is poorly understood, but recent genome sequencing reveals that de novo variants in intracellular trafficking genes are enriched in EA/TEF patients. Here we show that mutation of orthologous genes in Xenopus disrupts trachea-esophageal separation similar to EA/TEF patients. We show that the Rab11a recycling endosome pathway is required to localize Vangl-Celsr polarity complexes at the cell surface where opposite sides of the common foregut tube fuse. Partial loss of endosome trafficking or the Vangl/Celsr complex disrupts epithelial polarity and cell division orientation. Mutant cells accumulate at the fusion point, fail to downregulate cadherin, and do not separate into distinct trachea and esophagus. These data provide new insights into the mechanisms of congenital anomalies and general paradigms of tissue fusion during organogenesis.

Corneal epithelial development and homeostasis

The corneal epithelium (CE), the most anterior cellular structure of the eye, is a self-renewing stratified squamous tissue that protects the rest of the eye from external elements. Each cell in this exquisite three-dimensional structure needs to have proper polarity and positional awareness for the CE to serve as a transparent, refractive, and protective tissue. Recent studies have begun to elucidate the molecular and cellular events involved in the embryonic development, post-natal maturation, and homeostasis of the CE, and how they are regulated by a well-coordinated network of transcription factors. This review summarizes the status of related knowledge and aims to provide insight into the pathophysiology of disorders caused by disruption of CE development, and/or homeostasis.

Development, structure, and bioengineering of the human corneal stroma: A review of collagen-based implants

Bio-engineering technologies are currently used to produce biomimetic artificial corneas that should present structural, chemical, optical, and biomechanical properties close to the native tissue. These properties are mainly supported by the corneal stroma which accounts for 90% of corneal thickness and is mainly made of collagen type I. The stromal collagen fibrils are arranged in lamellae that have a plywood-like organization. The fibril diameter is between 25 and 35 nm and the interfibrillar space about 57 nm. The number of lamellae in the central stroma is estimated to be 300. In the anterior part, their size is 10-40 μm. They appear to be larger in the posterior part of the stroma with a size of 60-120 μm. Their thicknesses also vary from 0.2 to 2.5 μm. During development, the acellular corneal stroma, which features a complex pattern of organization, serves as a scaffold for mesenchymal cells that invade and further produce the cellular stroma. Several pathways including Bmp4, Wnt/β-catenin, Notch, retinoic acid, and TGF-β, in addition to EFTFs including the mastering gene Pax-6, are involved in corneal development. Besides, retinoic acid and TGF- β seem to have a crucial role in the neural crest cell migration in the stroma. Several technologies can be used to produce artificial stroma. Taking advantage of the liquid-crystal properties of acid-soluble collagen, it is possible to produce transparent stroma-like matrices with native-like collagen I fibrils and plywood-like organization, where epithelial cells can adhere and proliferate. Other approaches include the use of recombinant collagen, cross-linkers, vitrification, plastically compressed collagen or magnetically aligned collagen, providing interesting optical and mechanical properties. These technologies can be classified according to collagen type and origin, presence of telopeptides and native-like fibrils, structure, and transparency. Collagen matrices feature transparency >80% for the appropriate 500-μm thickness. Non-collagenous matrices made of biopolymers including gelatin, silk, or fish scale have been developed which feature interesting properties but are less biomimetic. These bioengineered matrices still need to be colonized by stromal cells to fully reproduce the native stroma.

KLF4 Coordinates Corneal Epithelial Apical-Basal Polarity and Plane of Cell Division and Is Downregulated in Ocular Surface Squamous Neoplasia

Purpose

Previously, we demonstrated that Krüppel-like factor 4 (KLF4) promotes corneal epithelial (CE) homeostasis by suppressing epithelial-mesenchymal transition (EMT) and TGF-β signaling. As TGF-β affects epithelial apicobasal polarity (ABP) and plane of division, we investigated the role of KLF4 in these processes.

Methods

Klf4 was ablated in adult ternary transgenic Klf4^Δ/ΔCE (Klf4^LoxP/LoxP/Krt12^rtTA/rtTA/Tet-O-Cre) mouse CE using doxycycline chow. ABP and plane of division markers’ expression in Klf4^Δ/ΔCE and human ocular surface squamous neoplasia (OSSN) tissues relative to controls was evaluated by quantitative PCR, immunoblots, and/or immunofluorescent staining.

Results

Klf4^Δ/ΔCE CE cells displayed downregulation of apical Pals1 and Crumbs1, apicolateral Par3, and basolateral Scribble, as well as upregulation of Rho family GTPase Cdc42, suggesting disruption of ABP. Phalloidin staining revealed that the Klf4^Δ/ΔCE CE actin cytoskeleton is disrupted. Klf4^Δ/ΔCE cells favored vertical plane of division within 67.5° to 90° of the CE basement membrane (39% and 47% of the dividing cells relative to 23% and 26% in the control based on phospho-histone-H3 and survivin, respectively), resulting in more dividing cells within the Klf4^Δ/ΔCE CE as reported previously. KLF4 was downregulated in human OSSN tissues that displayed EMT and downregulation of PAR3, PALS1, and SCRIB, consistent with a protective role for KLF4.

Conclusions

By demonstrating that Klf4 ablation affects CE expression of ABP markers and Cdc42, cytoskeletal actin organization, and the plane of cell division and that KLF4 is downregulated in OSSN tissues that display EMT and lack ABP, these results elucidate the key integrative role of KLF4 in coordinating CE cell polarity and plane of division, loss of which results in OSSN.

Transformation of the Transcriptomic Profile of Mouse Periocular Mesenchyme During Formation of the Embryonic Cornea

Purpose

Defects in neural crest development are a major contributing factor in corneal dysgenesis, but little is known about the genetic landscape during corneal development. The purpose of this study was to provide a detailed transcriptome profile and evaluate changes in gene expression during mouse corneal development.

Methods

RNA sequencing was used to uncover the transcriptomic profile of periocular mesenchyme (pNC) isolated at embryonic day (E) 10.5 and corneas isolated at E14.5 and E16.5. The spatiotemporal expression of several differentially expressed genes was validated by in situ hybridization.

Results

Analysis of the whole-transcriptome profile between pNC and embryonic corneas identified 3815 unique differentially expressed genes. Pathway analysis revealed an enrichment of differentially expressed genes involved in signal transduction (retinoic acid, transforming growth factor-β, and Wnt pathways) and transcriptional regulation.

Conclusions

Our analyses, for the first time, identify a large number of differentially expressed genes during progressive stages of mouse corneal development. Our data provide a comprehensive transcriptomic profile of the developing cornea. Combined, these data serve as a valuable resource for the identification of novel regulatory networks crucial for the advancement of studies in congenital defects, stem cell therapy, bioengineering, and adult corneal diseases.