Notch gain of function in mouse periocular mesenchyme downregulates FoxL2 and impairs eyelid levator muscle formation, leading to congenital blepharophimosis

β-Catenin gain of function mutant in mouse periocular neural crest-derived mesenchymal cells impairs embryonic eyelid morphogenesis and leads to blepharophimosis syndrome in mice

PURPOSE

The aberrant canonical Wnt-β-catenin signaling can cause devastating outcomes of tissue morphogenesis and tumor formation. In this study, we examined the impact of overexpression of constitutive active β-catenin in mouse periocular neural crest-derived mesenchymal cells during embryonic eyelid morphogenesis.

METHODS

We expressed a stabilized β-catenin in which the exon 3 of the Ctnnb1 gene was deleted in periocular neural crest (PONC)-derived eyelid stromal cells (Ctnnb1Δex3-PONC). Histopathological examinations were performed to examine the eyelid morphogenetic alterations in Ctnnb1Δex3-PONC mice. Immunohistochemical investigations for cell proliferation, apoptosis, and differentiation were also assessed.

RESULTS

We discovered that nuclear accumulation of β-catenin resulted in a reduction of nuclear Ki-67 and phospho-Erk1/2 expression levels and elevation of apoptosis in PONC cells during embryonic eyelid closure morphogenesis. Interestingly, however, the eyelid epithelial migration was not affected, which resulted in only eyelid epidermal closure but lacked underneath dermal formation at embryonic (E) day 16.5. The sequelae of Ctnnb1Δex3-PONC revealed the malformation of the eyelid margin and Meibomian gland and deficiency of Muller's smooth muscle fibers formation. Consequently, Ctnnb1Δex3-PONC mice manifested blepharophimosis syndrome at P21.

CONCLUSION

Our data suggested that aberrant expression of β-catenin gain of function in PONC interrupts the interplay between epithelium and stroma for the morphogenesis of eyelid closure during embryonic development.

Advances in the genetics of congenital ptosis

Congenital ptosis, a birth defects presents at birth or by 1 year of age, is characterized by the drooping of the upper eyelid. either in isolation (non-syndromic) or with many different systemic disorders (syndromic). The estimated prevalence of ptosis (congenital & acquired) ranges from 0.79 to 1.99 per 10,000 people in different populations, and it is more prevalent in males. The underlying pathogenesis of congenital ptosis are myogenic and neurogenic, related to the development of muscles and nerves. Although most cases are sporadic, there are familial transmission characteristics, including autosomal dominant, recessive mode and X-linkage inheritance patterns. Moreover, some forms are due to chromosomal aberrations and mutations and deletions in mitochondrial DNA. Genes involved in simple congenital ptosis are ZFHX4 and COL25A1. The clinical aspects of various syndromes involving congenital ptosis are partly caused by single gene mutations. However, the pathogenesis of congenital ptosis is not fully understood. We review the reported epidemiology, genetics and clinical features of congenital ptosis and associated syndromes here.

Keratin expression by corneal and limbal stem cells during development

Keratins are the forming units of intermediate filaments (IF) that provide mechanical support, and formation of desmosomes between cells and hemi desmosomes with basement membranes for epithelium integrity. Keratin IF are polymers of obligate heterodimer consisting one type I keratin and one type II keratin molecules. There are 54 functional keratin genes in human genome, which are classified into three major groups, i.e., epithelial keratins, hair follicle cell-specific epithelial keratins and hair keratins. Their expression is cell type-specific and developmentally regulated. Corneal epithelium expresses a subgroup of keratins similar to those of epidermal epithelium. Limbal basal stem cells express K5/K14, and K8/K18 and K8/K19 IF suggesting that there probably are two populations of limbal stem cells (LSCs). In human, LSCs at limbal basal layer can directly stratify and differentiate to limbal suprabasal cells that express K3/K12 IF, or centripetally migrate then differentiate to corneal basal transient amplifying cells (TAC) that co-express both K3/K12 and K5/K14 prior to moving upward and assuming suprabasal cells phenotype of only K3/K12 expression that signifies corneal type epithelium differentiation. In rodent, the differentiated cornea epithelial cells express K5/K12 in lieu of K3/K12, because K3 allele exists as a pseudogene and does not encode a functional K3 protein. The basal corneal cells of new-born mice originate from surface ectoderm during embryonic development slowly commit to differentiation of becoming TAC co-expressing K5/K12 and K5/K14 IF. However, the centripetal migration may still occur at a slower rate in young mice, which is accelerated during wound healing. In this review, we will discuss and compare the cornea-specific keratins expression patterns between corneal and epidermal epithelial cells during mouse development, and between human and mouse during development and homeostasis in adult, and pathology caused by a mutation of keratins.

Excess Transforming Growth Factor-α Changed the Cell Properties of Corneal Epithelium and Stroma

Purpose

This study is to investigate the corneal anomaly caused by excess transforming growth factor-α (TGF-α) during mouse development.

Methods

Bitransgenic KeraRT/TGF-α mice, generated via cross-mating tetO-TGF-α and KeraRT mice, were induced to overexpress TGF-α by doxycycline commencing at embryonic day 0 or postnatal day 0 to different developmental stages. Bitransgenic mice with doxycycline induction were defined as TGF-αECK mice (TGF-α excess expression by corneal keratocytes). Mouse eyes were examined by hematoxylin and eosin staining, immunofluorescent staining and transmission electron microscopy. Protein and RNA from mouse cornea were subjected to western blotting and real-time quantitative polymerase chain reaction.

Results

In TGF-αECK mice, TGF-α overexpression resulted in corneal opacity. Excess TGF-α initially caused corneal epithelial hyperplasia and subsequent epithelium degeneration as the mouse developed, which was accompanied by gradually diminished K12 expression from the periphery of corneal epithelium and increased K13 expression toward the corneal center. Interestingly, K14 was detected in all layers of corneal epithelium of TGF-αECK mice, whereas it was limited at basal layer of controls. Transmission electron microscopy showed desmosome loss between corneal epithelial cells of TGF-αECK mice. In TGF-αECK mice, keratocan expression was abolished; α-SMA expression was increased while expression of Col1a1, Col1a2, and Col5a1 was diminished. Cell proliferation increased in the corneal epithelium and stroma, but not in the endothelium of TGF-αECK mice.

Conclusions

Excess TGF-α had detrimental effects on corneal morphogenesis during mouse development in that it changed the cell fate of corneal epithelial cells to assume conjunctival phenotypic expression of K13, and keratocytes to myofibroblast phenotype.

Aberrant expression of a stabilized β-catenin mutant in keratocytes inhibits mouse corneal epithelial stratification

We previously reported that genetic deletion of β-catenin in mouse corneal keratocytes resulted in precocious corneal epithelial stratification. In this study, to strengthen the notion that corneal keratocyte-derived Wnt/β-catenin signaling regulates corneal epithelial stratification during mouse development, we examined the consequence of conditional overexpression of a stabilized β-catenin mutant (Ctnnb1ΔE3) in corneal keratocytes via a doxycycline (Dox)-inducible compound transgenic mouse strain. Histological analysis showed that conditional overexpression of Ctnnb1ΔE3 in keratocytes inhibited corneal epithelial stratification during postnatal development. Unlike the corneal epithelium of the littermate controls, which consisted of 5-6 cell layers at postnatal day 21 (P21), the mutant corneal epithelium contained 1-2 or 2-3 cell layers after Dox induction from embryonic day 0 (E0) to P21 and from E9 to P21, respectively. X-gal staining revealed that Wnt/β-catenin signaling activity was significantly elevated in the corneal keratocytes of the Dox-induced mutant mice, compared to the littermate controls. Furthermore, RT-qPCR and immunostaining data indicated that the expression of Bmp4 and ΔNp63 was downregulated in the mutant corneas, which was associated with reduced corneal epithelial proliferation in mutant epithelium, as revealed by immunofluorescent staining. However, the expression of Krt12, Krt14 and Pax6 in the mutant corneas was not altered after overexpression of Ctnnb1ΔE3 mutant protein in corneal keratocytes. Overall, mutant β-catenin accumulation in the corneal keratocytes inhibited corneal epithelial stratification probably through downregulation of Bmp4 and ΔNp63 in the corneal epithelium.

Generation and Characterization of a Novel Mouse Line, Keratocan-rtTA (KeraRT), for Corneal Stroma and Tendon Research

Purpose

We created a novel inducible mouse line Keratocan-rtTA (KeraRT) that allows specific genetic modification in corneal keratocytes and tenocytes during development and in adults.

Methods

A gene-targeting vector (Kera- IRES2-rtTA3) was constructed and inserted right after the termination codon of the mouse Kera allele via gene targeting techniques. The resulting KeraRT mouse was crossed to tet-O-Hist1H2B-EGFP (TH2B-EGFP) to obtain KeraRT/TH2B-EGFP compound transgenic mice, in which cells expressing Kera are labeled with green fluorescence protein (GFP) by doxycycline (Dox) induction. The expression patterns of GFP and endogenous Kera were examined in KeraRT/TH2B-EGFP. Moreover, KeraRT was bred with tet-O-TGF-α to generate a double transgenic mouse, KeraRT/tet-O-TGF-α, to overexpress TGF-α in corneal keratocytes upon Dox induction.

Results

Strong GFP-labeled cells were detected in corneal stroma, limbs, and tail when KeraRT/TH2B-EGFP mice were fed Dox chow. There was no GFP in any single transgenic KeraRT or TH2B-EGFP mouse. Histological analysis showed that GFP in the cornea was limited to stromal keratocytes of KeraRT/TH2B-EGFP, which is consistent with Kera expression. Induction of GFP occurred in 24 hours and reached a plateau by 7 days after Dox induction. GFP could be detected 3-months after induction of KeraRT/TH2B-EGFP. Ectopic expression of TGF-α in corneal keratocytes caused hyperplasia in the corneal epithelium and stroma.

Conclusions

The novel Dox inducible KeraRT driver mouse line is a useful genetic tool for gene manipulation and elucidating gene functions in corneal stroma and tendons of limbs and tail during embryonic development, homeostasis and pathogenesis.

Wnt/β-catenin signaling modulates corneal epithelium stratification via inhibition of Bmp4 during mouse development

The development of organs with an epithelial parenchyma relies on reciprocal mesenchymal-epithelial communication. Mouse corneal epithelium stratification is the consequence of a coordinated developmental process based on mesenchymal-epithelial interactions. The molecular mechanism underlying these interactions remains unclear. The Wnt/β-catenin signaling pathway is involved in fundamental aspects of development through the regulation of various growth factors. Here, we show that conditional ablation of either β-catenin (Ctnnb1(cKO)) or co-receptors Lrp5/6 (Lrp5/6(cKO)) in corneal stromal cells results in precocious stratification of the corneal epithelium. By contrast, ectopic expression of a murine Ctnnb1 gain-of-function mutant (Ctnnb1(cGOF)) retards corneal epithelium stratification. We also discovered that Bmp4 is upregulated in the absence of β-catenin in keratocytes, which further triggers ERK1/2 (Mapk3/1) and Smad1/5 phosphorylation and enhances transcription factor p63 (Trp63) expression in mouse corneal basal epithelial cells and in a human corneal epithelial cell line (HTCE). Interestingly, mouse neonates given a subconjunctival BMP4 injection displayed a phenotype resembling that of Ctnnb1(cKO). Conditional ablation of Bmp4 eradicates the phenotype produced in Ctnnb1(cKO) mice. Furthermore, ChIP and promoter-luciferase assays show that β-catenin binds to and suppresses Bmp4 promoter activity. These data support the concept that cross-talk between the Wnt/β-catenin/Bmp4 axis (in the stromal mesenchyme) and Bmp4/p63 signaling (in the epithelium) plays a pivotal role in epithelial stratification during corneal morphogenesis.

Perturbed meibomian gland and tarsal plate morphogenesis by excess TGFα in eyelid stroma

Transforming growth factor alpha (TGFα) belongs to the epidermal growth factor (EGF) family and is known to play an important role during eyelid morphogenesis. In this study, we showed that ectopic expression of TGFα in the stroma of Kera-rtTA/tet-O-TGFα bitransgenic mice results in precocious eye opening, abnormal morphogenesis of the meibomian gland, tendon and tarsal plate malformation and epithelium hyperplasia. TGFα did not change proliferation and differentiation of meibocytes, but promoted proliferation and inhibited differentiation of the tarsal plate tenocytes. These results suggest that proper formation of the tendon and tarsal plate in the mouse eyelid is required for normal morphogenesis of the meibomian gland.

FOXL2 modulates cartilage, skeletal development and IGF1-dependent growth in mice

BACKGROUND

Haploinsufficiency of the FOXL2 transcription factor in humans causes Blepharophimosis/Ptosis/Epicanthus Inversus syndrome (BPES), characterized by eyelid anomalies and premature ovarian failure. Mice lacking Foxl2 recapitulate human eyelid/forehead defects and undergo female gonadal dysgenesis. We report here that mice lacking Foxl2 also show defects in postnatal growth and embryonic bone and cartilage formation.

METHODS

Foxl2 (-/-) male mice at different stages of development have been characterized and compared to wild type. Body length and weight were measured and growth curves were created. Skeletons were stained with alcian blue and/or alizarin red. Bone and cartilage formation was analyzed by Von Kossa staining and immunofluorescence using anti-FOXL2 and anti-SOX9 antibodies followed by confocal microscopy. Genes differentially expressed in skull vaults were evaluated by microarray analysis. Analysis of the GH/IGF1 pathway was done evaluating the expression of several hypothalamic-pituitary-bone axis markers by RT-qPCR.

RESULTS

Compared to wild-type, Foxl2 null mice are smaller and show skeletal abnormalities and defects in cartilage and bone mineralization, with down-regulation of the GH/IGF1 axis. Consistent with these effects, we find FOXL2 expressed in embryos at 9.5 dpc in neural tube epithelium, in head mesenchyme near the neural tube, and within the first branchial arch; then, starting at 12.5 dpc, expressed in cartilaginous tissue; and at PO and P7, in hypothalamus.

CONCLUSIONS

Our results support FOXL2 as a master transcription factor in a spectrum of developmental processes, including growth, cartilage and bone formation. Its action overlaps that of SOX9, though they are antagonistic in female vs male gonadal sex determination but conjoint in cartilage and skeletal development.

Etiology of craniofacial malformations in mouse models of blepharophimosis, ptosis and epicanthus inversus syndrome

Blepharophimosis, ptosis, epicanthus-inversus syndrome (BPES) is an autosomal dominant genetic disorder characterized by narrow palpebral fissures and eyelid levator muscle defects. BPES is often associated to premature ovarian insufficiency (BPES type I). FOXL2, a member of the forkhead transcription factor family, is the only gene known to be mutated in BPES. Foxl2 is essential for maintenance of ovarian identity, but the developmental origin of the facial malformations of BPES remains, so far, unexplained. In this study, we provide the first detailed account of the developmental processes leading to the craniofacial malformations associated to Foxl2. We show that, during development, Foxl2 is expressed both by Cranial Neural Crest Cells (CNCCs) and by Cranial Mesodermal Cells (CMCs), which give rise to skeletal (CNCCs and CMCs) and muscular (CMCs) components of the head. Using mice in which Foxl2 is selectively inactivated in either CNCCs or CMCs, we reveal that expression of Foxl2 in CNCCs is essential for the development of extraocular muscles. Indeed, inactivation of Foxl2 in CMCs has only minor effects on muscle development, whereas its inactivation in CNCCs provokes a severe hypoplasia of the levator palpabrae superioris and of the superior and inferior oblique muscles. We further show that Foxl2 deletion in either CNCCs or CMCs prevents eyelid closure and induces subtle skeletal developmental defects. Our results provide new insights in the complex developmental origin of human BPES and could help to understand the origin of other ocular anomalies associated to this syndrome.

Eyelid closure in embryogenesis is required for ocular adnexa development

PURPOSE

Mammalian eye development requires temporary fusion of the upper and lower eyelids in embryogenesis. Failure of lid closure in mice leads to an eye open at birth (EOB) phenotype. Many genetic mutant strains develop this phenotype and studies of the mutants lead to a better understanding of the signaling mechanisms of morphogenesis. The present study investigates the roles of lid closure in eye development.

METHODS

Seven mutant mouse strains were generated by different gene ablation strategies that inactivated distinct signaling pathways. These mice, including systemic ablation of Map3k1 and Dkk2, ocular surface epithelium (OSE) knockout of c-Jun and Egfr, conditional knockout of Shp2 in stratified epithelium (SE), as well as the Map3k1/Jnk1 and Map3k1/Rhoa compound mutants, all exhibited defective eyelid closure. The embryonic and postnatal eyes in these mice were characterized by histology and immunohistochemistry.

RESULTS

Some eye abnormalities, such as smaller lens in the Map3k1-null mice and Harderian gland hypoplasia in the Dkk2-null mice, appeared to be mutant strain-specific, whereas other abnormalities were seen in all mutants examined. The common defects included corneal erosion/ulceration, meibomian gland hypoplasia, truncation of the eyelid tarsal muscles, failure of levator palpebrae superioris (LPS) extension into the upper eyelid and misplacement of the inferior oblique (IO) muscle and inferior rectus (IR) muscle. The muscle defects were traced to the prenatal fetuses.

CONCLUSIONS

In addition to providing a protective barrier for the ocular surface, eyelid closure in embryogenesis is required for the development of ocular adnexa, including eyelid and extraocular muscles.

A piggyBac insertion disrupts Foxl2 expression that mimics BPES syndrome in mice

Blepharophimosis, ptosis, epicanthus inversus syndrome (BPES) is an autosomal dominant genetic disorder characterized by small palpebral fissures and other craniofacial malformations, often with (type I) but could also without (type II) premature ovarian failure. While mutations of the forkhead transcription factor FOXL2 are associated with and likely be responsible for many BPES cases, how FOXL2 affects craniofacial development remain to be understood. Through a large-scale piggyBac (PB) insertion mutagenesis, we have identified a mouse mutant carrying a PB insertion ∼160 kb upstream of the transcription start site (TSS) of Foxl2. The insertion reduces, but not eliminates, the expression of Foxl2. This mutant, but not its revertant, displays BPES-like conditions such as midface hypoplasia, eyelid abnormalities and female subfertility. Further analysis indicates that the mutation does not affect mandible, but causes premature fusion of the premaxilla-maxilla suture, smaller premaxilla and malformed maxilla during midface development. We further identified an evolutionarily conserved fragment near the insertion site and observed enhancer activity of this element in tissue culture cells. Analyses using DNase I hypersensitivity assay and chromosome conformation capture assay in developing maxillary and periocular tissues suggest that the DNA region near the insertion site likely interacts with Foxl2 TSS. Therefore, this mutant presents an excellent animal model for mechanistic study of BPES and regulation of Foxl2.

Expression of signaling components in embryonic eyelid epithelium

Closure of an epithelium opening is a critical morphogenetic event for development. An excellent example for this process is the transient closure of embryonic eyelid. Eyelid closure requires shape change and migration of epithelial cells at the tip of the developing eyelids, and is dictated by numerous signaling pathways. Here we evaluated gene expression in epithelial cells isolated from the tip (leading edge, LE) and inner surface epithelium (IE) of the eyelid from E15.5 mouse fetuses by laser capture microdissection (LCM). We showed that the LE and IE cells are different at E15.5, such that IE had higher expression of muscle specific genes, while LE acquired epithelium identities. Despite their distinct destinies, these cells were overall similar in expression of signaling components for the “eyelid closure pathways”. However, while the LE cells had more abundant expression of Fgfr2, Erbb2, Shh, Ptch1 and 2, Smo and Gli2, and Jag1 and Notch1, the IE cells had more abundant expression of Bmp5 and Bmpr1a. In addition, the LE cells had more abundant expression of adenomatosis polyposis coli down-regulated 1 (Apcdd1), but the IE cells had high expression of Dkk2. Our results suggest that the functionally distinct LE and IE cells have also differential expression of signaling molecules that may contribute to the cell-specific responses to morphogenetic signals. The expression pattern suggests that the EGF, Shh and NOTCH pathways are preferentially active in LE cells, the BMP pathways are effective in IE cells, and the Wnt pathway may be repressed in LE and IE cells via different mechanisms.

Lumican binds ALK5 to promote epithelium wound healing

Lumican (Lum), a small leucine-rich proteoglycan (SLRP) family member, has multiple matricellular functions both as an extracellular matrix component and as a matrikine regulating cell proliferation, gene expression and wound healing. To date, no cell surface receptor has been identified to mediate the matrikine functions of Lum. This study aimed to identify a perspective receptor that mediates Lum effects on promoting wound healing. Transforming growth factor-β receptor 1 (ALK5) was identified as a potential Lum-interacting protein through in silico molecular docking and molecular dynamics. This finding was verified by biochemical pull-down assays. Moreover, the Lum function on wound healing was abrogated by an ALK5-specific chemical inhibitor as well as by ALK5 shRNAi. Finally, we demonstrated that eukaryote-specific post-translational modifications are not required for the wound healing activity of Lum, as recombinant GST-Lum fusion proteins purified from E. coli and a chemically synthesized LumC₁₃ peptide (the last C-terminal 13 amino acids of Lum) have similar effects on wound healing in vitro and in vivo.

Wakayama Symposium: Epithelial-mesenchymal interactions in eyelid development

Various congenital anomalies of the eyelids can result from abnormal tissue proliferation, fusion, and reopening of the eyelids. Therefore, it is important to study the molecular mechanisms underlying eyelid development, focusing on cell behaviors. Mammalian eyelid development occurs in four steps: specification, growth, epithelial fusion, and reopening. Epithelial-mesenchymal interactions are fundamental processes during eyelid formation, and epithelial factors functioning within the eyelid epithelium are also critical. Studies with mutant and genetically modified mice have revealed that various signaling pathways and transcriptional factors are involved in mouse eyelid development. In this review, eyelid morphogenetic factors or pathways are described, as revealed by their mutant phenotype, eye-open at birth (EOB). These include FGFR2b-FGF10, EGFR-ERK, MEKK-JNK, BMP, Shh, Wnt, GPR48, Jun, Forkhead, and Grainyhead.

Wakayama Symposium: Notch-FoxL2-α-SMA axis in eyelid levator muscle development and congenital blepharophimosis

This review summarizes our recent findings regarding the Notch signaling pathway in regulating normal eyelid morphogenesis and its role in the pathogenesis of human congenital blepharophimosis, ptosis, and epicanthus inversus syndrome (BPES). We used genetic and molecular biological approaches to investigate the mechanism by which Notch1 activation controls expression of FoxL2, which in turn activates smooth muscle actin gene expression in periocular mesenchyma to control eyelid levator smooth muscle formation.