Control of patterns of corneal innervation by Pax6

Examination of Subbasal Nerve Plexus and Central Corneal Stromal Microstructure in Subjects With Congenital Aniridia, Using in Vivo Confocal Laser Scanning Microscopy

PURPOSE

During life up to 70% of aniridia subjects develop aniridia-associated keratopathy (AAK). AAK is characterized by limbal stem cell insufficiency, impaired corneal epithelial cell differentiation and abnormal cell adhesion, which leads to centripetal spreading vascularization, conjunctivalization, and thickening of the cornea. Our aim was to examine the subbasal nerve plexus and central corneal stromal microstructure in subjects with congenital aniridia, using in vivo confocal laser scanning microscopy CLSM.

METHODS

31 eyes of 18 patients (55.6% males, mean age: 25.22 ± 16.35 years) with congenital aniridia and 46 eyes of 29 healthy subjects (41.4% males, mean age 30 ± 14.82 years) were examined using the Rostock Cornea Module of Heidelberg Retina Tomograph-III. At the subbasal nerve plexus, corneal nerve fiber density (CNFD), corneal nerve fiber length (CNFL), corneal total branch density (CTBD), and corneal nerve fiber width (CNFW) were analyzed using ACCMetrics software. Keratocyte density in the anterior, middle and posterior stroma was assessed manually.

RESULTS

The CNFD (2.02 ± 4.08 vs 13.99 ± 6.34/mm2), CNFL (5.78 ± 2.68 vs 10.56 ± 2.82 mm/mm2) and CTBD (15.08 ± 15.62 vs 27.44 ± 15.05/mm2) were significantly lower in congenital aniridia subjects than in controls (p < 0.001 for all). CNFW was significantly higher in aniridia subjects than in controls (0.03 ± 0.004 vs 0.02 ± 0.003 mm/mm2) (p = 0.003). Keratocyte density was significantly lower in all stromal layers of aniridia subjects than in controls (p < 0.001 for all). Stromal alterations included confluent keratocytes, keratocytes with long extensions and hyperreflective dots between keratocytes in aniridia.

CONCLUSIONS

Decrease in CNFD, CNFL, and CTBD, as well as increase in CNFW well refer to the congenital aniridia-associated neuropathy. The decreased keratocyte density and the stromal alterations may be related to an increased cell death in congenital aniridia, nevertheless, stromal changes in different stages of AAK have to be further analyzed in detail.

Cell therapy in the cornea: The emerging role of microenvironment

The cornea is an ideal testing field for cell therapies. Its highly ordered structure, where specific cell populations are sequestered in different layers, together with its accessibility, has allowed the development of the first stem cell-based therapy approved by the European Medicine Agency. Today, different techniques have been proposed for autologous and allogeneic limbal and non-limbal cell transplantation. Cell replacement has also been attempted in cases of endothelial cell decompensation as it occurs in Fuchs dystrophy: injection of cultivated allogeneic endothelial cells is now in advanced phases of clinical development. Recently, stromal substitutes have been developed with excellent integration capability and transparency. Finally, cell-derived products, such as exosomes obtained from different sources, have been investigated for the treatment of severe corneal diseases with encouraging results. Optimization of the success rate of cell therapies obviously requires high-quality cultured cells/products, but the role of the surrounding microenvironment is equally important to allow engraftment of transplanted cells, to preserve their functions and, ultimately, lead to restoration of tissue integrity and transparency of the cornea.

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.

Wiring the ocular surface: A focus on the comparative anatomy and molecular regulation of sensory innervation of the cornea

The cornea is richly innervated with sensory nerves that function to detect and clear harmful debris from the surface of the eye, promote growth and survival of the corneal epithelium and hasten wound healing following ocular disease or trauma. Given their importance to eye health, the neuroanatomy of the cornea has for many years been a source of intense investigation. Resultantly, complete nerve architecture maps exist for adult human and many animal models and these maps reveal few major differences across species. Interestingly, recent work has revealed considerable variation across species in how sensory nerves are acquired during developmental innervation of the cornea. Highlighting such species-distinct key differences, but also similarities, this review provides a full, comparative anatomy analysis of sensory innervation of the cornea for all species studied to date. Further, this article comprehensively describes the molecules that have been shown to guide and direct nerves toward, into and through developing corneal tissue as the final architectural pattern of the cornea's neuroanatomy is established. Such knowledge is useful for researchers and clinicians seeking to better understand the anatomical and molecular basis of corneal nerve pathologies and to hasten neuro-regeneration following infection, trauma or surgery that damage the ocular surface and its corneal nerves.

Multiple roles of Pax6 in postnatal cornea development

Mammalian corneal development is a multistep process, including formation of the corneal epithelium (CE), endothelium and stroma during embryogenesis, followed by postnatal stratification of the epithelial layers and continuous renewal of the epithelium to replace the outermost corneal cells. Here, we employed the Cre-loxP system to conditionally deplete Pax6 proteins in two domains of ocular cells, i.e., the ocular surface epithelium (cornea, limbus and conjunctiva) (OSE) or postnatal CE via K14-cre or Aldh3-cre, respectively. Earlier and broader inactivation of Pax6 in the OSE resulted in thickened OSE with CE and limbal cells adopting the conjunctival keratin expression pattern. More restricted depletion of Pax6 in postnatal CE resulted in an abnormal cornea marked by reduced epithelial thickness despite increased epithelial cell proliferation. Immunofluorescence studies revealed loss of intermediate filament Cytokeratin 12 and diffused expression of adherens junction components, together with reduced tight junction protein, Zonula occludens-1. Furthermore, the expression of Cytokeratin 14, a basal cell marker in apical layers, indicates impaired differentiation of CE cells. Collectively, our data demonstrate that Pax6 is essential for maintaining proper differentiation and strong intercellular adhesion in postnatal CE cells, whereas limbal Pax6 is required to prevent the outgrowth of conjunctival cells to the cornea.

Aniridia and the ocular surface: Medical and surgical problems and solutions

Congenital aniridia is a multisystemic genetic disease due to a mutation in PAX6 gene which severely affects the development and functionality of the human eyes. In patients affected by the mutation, aside from the absence or defects of iris tissue formation, abnormalities in position or opacities of the crystalline lens, macular hypoplasia, ocular surface disease is the main cause of visual loss and the deterioration of the quality of life of most patients. Limbal stem cell deficiency combined with tear film instability and secondary dry eye cause aniridic keratopathy which, in advanced stages, ends up in corneal opacification. In this paper, the actual knowledge about congenital aniridia keratopathy physiopathology and medical and surgical treatment options and their efficacy are discussed. Indications and results of topical treatments with artificial tears and blood-derivatives in its initial stages, and different surgical techniques as limbal stem cell transplantation, keratoplasty and keratoprostheses are reviewed. Finally, recent advances and results in regenerative medicine techniques with ex vivo stem cell cultivation or other types of cultivated cells are presented.

Pathophysiology of aniridia-associated keratopathy: Developmental aspects and unanswered questions

Aniridia, a rare congenital disease, is often characterized by a progressive, pronounced limbal insufficiency and ocular surface pathology termed aniridia-associated keratopathy (AAK). Due to the characteristics of AAK and its bilateral nature, clinical management is challenging and complicated by the multiple coexisting ocular and systemic morbidities in aniridia. Although it is primarily assumed that AAK originates from a congenital limbal stem cell deficiency, in recent years AAK and its pathogenesis has been questioned in the light of new evidence and a refined understanding of ocular development and the biology of limbal stem cells (LSCs) and their niche. Here, by consolidating and comparing the latest clinical and preclinical evidence, we discuss key unanswered questions regarding ocular developmental aspects crucial to AAK. We also highlight hypotheses on the potential role of LSCs and the ocular surface microenvironment in AAK. The insights thus gained lead to a greater appreciation for the role of developmental and cellular processes in the emergence of AAK. They also highlight areas for future research to enable a deeper understanding of aniridia, and thereby the potential to develop new treatments for this rare but blinding ocular surface disease.

The pattern of the inferocentral whorl region of the corneal subbasal nerve plexus is altered with age

PURPOSE

To describe the pattern of the nerves in the inferocentral whorl region of the human corneal subbasal nerve plexus (SBNP) in health and diseases known to affect the subbasal nerves.

METHODS

Laser-scanning in vivo confocal microscopy (IVCM) was used to image the SBNP bilaterally in 91 healthy subjects, 39 subjects with type 2 diabetes mellitus (T2DM), and 43 subjects with Parkinson's disease (PD). Whorl regions were classified according to nerve orientation relative to age and health/disease status.

RESULTS

Of 346 examined eyes, 300 (86.7%) had an identifiable whorl pattern. In healthy subjects, a clockwise nerve orientation of the whorl was most common (67.9%), followed by non-rotatory or 'seam' morphology (21.4%), and counterclockwise (10.7%). The clockwise orientation was more prevalent in healthy subjects than in T2DM or PD (P < 0.001). Healthy individuals below 50 years of age had a predominantly clockwise orientation (93.8%) which was reduced to 51.9% in those over 50 years (P < 0.001). Age but not disease status explained whorl orientation in T2DM and PD groups. Moreover, whorl orientation is bilaterally clockwise in the young, but adopts other orientations and becomes asymmetric across eyes with age. Finally, we report reflective 'dot-like' features confined to the whorl region of the subbasal plexus, sometimes appearing in close association with subbasal nerves and present in 84-93% of examined eyes regardless of disease status, eye or sex.

CONCLUSION

Subbasal nerves in the inferocentral whorl region are predominantly clockwise in young, healthy corneas. With aging and conditions of T2DM and PD, counterclockwise and non-rotatory configurations increase in prevalence, and bilateral symmetry is lost. Mechanisms regulating these changes warrant further investigation.

Neuronal-epithelial cell alignment: A determinant of health and disease status of the cornea

PURPOSE

How sensory neurons and epithelial cells interact with one another, and whether this association can be considered an indicator of health or disease is yet to be elucidated.

METHODS

Herein, we used the cornea, Confetti mice, a novel image segmentation algorithm for intraepithelial corneal nerves which was compared to and validated against several other analytical platforms, and three mouse models to delineate this paradigm. For aging, eyes were collected from 2 to 52 week-old normal C57BL/6 mice (n ≥ 4/time-point). For wound-healing and limbal stem cell deficiency, 7 week-old mice received a limbal-sparing or limbal-to-limbal epithelial debridement to their right cornea, respectively. Eyes were collected 2-16 weeks post-injury (n=4/group/time-point), corneas procured, immunolabelled with βIII-tubulin, flat-mounted, imaged by scanning confocal microscopy and analyzed for nerve and epithelial-specific parameters.

RESULTS

Our data indicate that nerve features are dynamic during aging and their curvilinear arrangement align with corneal epithelial migratory tracks. Moderate corneal injury prompted axonal regeneration and recovery of nerve fiber features. Limbal stem cell deficient corneas displayed abnormal nerve morphology, and fibers no longer aligned with corneal epithelial migratory tracks. Mechanistically, we discovered that nerve pattern restoration relies on the number and distribution of stromal-epithelial nerve penetration sites.

CONCLUSIONS

Microstructural changes to innervation may explain corneal complications related to aging and/or disease and facilitate development of new assays for diagnosis and/or classification of ocular and systemic diseases.

Comparative Anatomy of the Mammalian Corneal Subbasal Nerve Plexus

Purpose

The subbasal nerve plexus (SNP) is the densest and most recognizable component of the mammalian corneal innervation; however, the anatomical configuration of the SNP in most animal models remains incompletely described. The purpose of the current study is to describe in detail the SNP architecture in eight different mammals, including several popular animal models used in cornea research.

Methods

Corneal nerves in mouse, rat, guinea pig, rabbit, dog, macaque, domestic pig, and cow eyes were stained immunohistochemically with antiserum directed against neurotubulin. SNP architecture was documented by digital photomicrography and large-scale reconstructions, that is, corneal nerve maps, using a drawing tube attached to a light microscope.

Results

Subbasal nerve fibers (SNFs) in mice, rats, guinea pigs, dogs, and macaques radiated centrally from the corneoscleral limbus toward the corneal apex in a whorl-like or spiraling pattern. SNFs in rabbit and bovine corneas swept horizontally across the ocular surface in a temporal-to-nasal direction and converged on the inferonasal limbus without forming a spiral. SNFs in the pig cornea radiated centrifugally in all directions, like a starburst, from a focal point located equidistant between the corneal apex and the superior pole.

Conclusions

The results of the present study have demonstrated for the first time substantial interspecies differences in the architectural organization of the mammalian SNP. The physiological significance of these different patterns and the mechanisms that regulate SNP pattern formation in the mammalian cornea remain incompletely understood and warrant additional investigation.

Update on corneal neurotisation

Corneal neurotisation describes surgical restoration of nerve growth into the cornea to restore corneal sensation and trophic function. It represents an exciting and effective emerging treatment for neurotrophic keratopathy. Techniques described to date involve either direct nerve transfer or an interpositional nerve graft coapted to a healthy donor nerve. We review the experience to date with particular emphasis on a detailed review of techniques, outcomes and current thoughts.

Evaluation of artificial tears on cornea epithelium healing

AIM

To observe the efficacy of different artificial eye drops on corneal epithelium healing in rabbit.

METHODS

Thirty-five rabbits with 6 mm diameter central corneal epithelium removed were randomly assigned to six groups: 0.9% normal saline (NS) group, 0.1% hyaluronate (HA) group, 0.3% HA group, Tears Naturale Free^® (TNF) group, 0.4% polyethylene glycol (PEG) group, 0.5% carboxymethyl cellulose (CMC) group and blank control group. Treatments were administered topically four times daily. Corneal epithelium healing was evaluated by the percentage reduction in wound area at 24, 36, 48, 60, and 72h after removal of the corneal epithelium. Cornea re-epithelialization was also assessed by histological analysis and electron microscopy.

RESULTS

All corneal wounds completely re-epithelialized in less than 72h. The average re-epithelialization time was 47.61±4.25h in the 0.3% HA group and 49.72±1.05h in the 0.9% NS group, followed by 0.1% HA, TNF, 0.4% PEG, 0.5% CMC, and lastly by the control group. Compared to the control group, there were significant differences among 0.3% HA, 0.9% NS, PEG, and TNF (P<0.05) groups. At the first 24h, re-epithelialization at the 0.3% HA, TNF, and 0.9% NS treatment groups were significantly faster than the other groups. At 48h post-wounding, corneal epithelium is nearly completing re-epithelialization at 0.3% HA and 0.9% NS treatment groups. Electron microscopy revealed that there were a large number of vacuoles in the cells of the 0.9% NS group at 72h.

CONCLUSION

Artificial tears promote corneal re-epithelium varied in the efficacy. Obviously, all artificial eye drops better than blank group. In the process of corneal healing, corneal epithelium cells suffered from hypoxia caused by NS.

In Vivo Confocal Microscopy of Corneal Nerves in Health and Disease

In vivo confocal microscopy (IVCM) is becoming an indispensable tool for studying corneal physiology and disease. Enabling the dissection of corneal architecture at a cellular level, this technique offers fast and noninvasive in vivo imaging of the cornea with images comparable to those of ex vivo histochemical techniques. Corneal nerves bear substantial relevance to clinicians and scientists alike, given their pivotal roles in regulation of corneal sensation, maintenance of epithelial integrity, as well as proliferation and promotion of wound healing. Thus, IVCM offers a unique method to study corneal nerve alterations in a myriad of conditions, such as ocular and systemic diseases and following corneal surgery, without altering the tissue microenvironment. Of particular interest has been the correlation of corneal subbasal nerves to their function, which has been studied in normal eyes, contact lens wearers, and patients with keratoconus, infectious keratitis, corneal dystrophies, and neurotrophic keratopathy. Longitudinal studies have applied IVCM to investigate the effects of corneal surgery on nerves, demonstrating their regenerative capacity. IVCM is increasingly important in the diagnosis and management of systemic conditions such as peripheral diabetic neuropathy and, more recently, in ocular diseases. In this review, we outline the principles and applications of IVCM in the study of corneal nerves in various ocular and systemic diseases.

Neuroanatomy and Neurochemistry of Mouse Cornea

Purpose

To investigate the entire nerve architecture and content of the two main sensory neuropeptides in mouse cornea to determine if it is a good model with similarities to human corneal innervation.

Methods

Mice aged 1 to 24 weeks were used. The corneas were stained with neuronal-class βIII-tubulin, calcitonin gene–related peptide (CGRP), and substance P (SP) antibodies; whole-mount images were acquired to build an entire view of corneal innervation. To test the origin of CGRP and SP, trigeminal ganglia (TG) were processed for immunofluorescence. Relative corneal nerve fiber densities or neuron numbers were assessed by computer-assisted analysis.

Results

Between 1 and 3 weeks after birth, mouse cornea was mainly composed of a stromal nerve network. At 4 weeks, a whorl-like structure (or vortex) appeared that gradually became more defined. By 8 weeks, anatomy of corneal nerves had reached maturity. Epithelial bundles converged into the central area to form the vortex. The number and pattern of whorl-like structures were different. Subbasal nerve density and nerve terminals were greater in the center than the periphery. Nerve fibers and terminals that were CGRP-positive were more abundant than SP-positive nerves and terminals. In trigeminal ganglia, the number of CGRP-positive neurons significantly outnumbered those positive for SP.

Conclusions

This is the first study to show a complete map of the entire corneal nerves and CGRP and SP sensory neuropeptide distribution in the mouse cornea. This finding shows mouse corneal innervation has many similarities to human cornea and makes the mouse an appropriate model to study pathologies involving corneal nerves.

Inactivation of the miR-183/96/182 Cluster Decreases the Severity of Pseudomonas aeruginosa-Induced Keratitis

PURPOSE

The microRNA-183/96/182 cluster (miR-183/96/182) plays important roles in sensory organs. Because the cornea is replete with sensory innervation, we hypothesized that miR-183/96/182 modulates the corneal response to bacterial infection through regulation of neuroimmune interactions.

METHODS

Eight-week-old miR-183/96/182 knockout (ko) mice and their wild-type littermates (wt) were used. The central cornea of anesthetized mice was scarred and infected with Pseudomonas aeruginosa (PA), strain 19660. Corneal disease was graded at 1, 3, and 5 days postinfection (dpi). Corneal RNA was harvested for quantitative RT-PCR. Polymorphonuclear neutrophils (PMN) were enumerated by myeloperoxidase assays; the number of viable bacteria was determined by plate counts, and ELISA assays were performed to determine cytokine protein levels. A macrophage (Mϕ) cell line and elicited peritoneal PMN were used for in vitro functional assays.

RESULTS

MicroRNA-183/96/182 is expressed in the cornea, and in Mϕ and PMN of both mice and humans. Inactivation of miR-183/96/182 resulted in decreased corneal nerve density compared with wt mice. Overexpression of miR-183/96/182 in Mϕ decreased, whereas knockdown or inactivation of miR-183/96/182 in Mϕ and PMN increased their capacity for phagocytosis and intracellular killing of PA. In PA-infected corneas, ko mice showed decreased proinflammatory neuropeptides such as substance P and chemoattractant molecules, MIP-2, MCP1, and ICAM1; decreased number of PMN at 1 and 5 dpi; increased viable bacterial load at 1 dpi, but decreased at 5 dpi; and markedly decreased corneal disease.

CONCLUSIONS

MicroRNA-183/96/182 modulates the corneal response to bacterial infection through its regulation of corneal innervation and innate immunity.

Congenital Aniridia and the Ocular Surface

Aniridia is a congenital pan-ocular disorder caused by haplo-insufficiency of Pax6, a crucial gene for proper development of the eye. Aniridia affects a range of eye structures, including the cornea, iris, anterior chamber angle, lens, and fovea. The ocular surface, in particular, can be severely affected by a progressive pathology termed aniridia-associated keratopathy (AAK), markedly contributing to impaired vision. The purpose of this review is to provide an update of the current knowledge of the genetic, clinical, micro-morphological, and molecular aspects of AAK. We draw upon material presented in the literature and from our own observations in large aniridia cohorts. We summarize signs and symptoms of AAK, describe current options for management, and discuss the latest research findings that may lead to better diagnosis and new treatment or prevention strategies for this debilitating ocular surface condition.

Corneal Development: Different Cells from a Common Progenitor

Development of the vertebrate cornea is a multistep process that involves cellular interactions between various ectodermal-derived tissues. Bilateral interactions between the neural ectoderm-derived optic vesicles and the cranial ectoderm give rise to the presumptive corneal epithelium and other epithelia of the ocular surface. Interactions between the neural tube and the adjacent ectoderm give rise to the neural crest cells, a highly migratory and multipotent cell population. Neural crest cells migrate between the lens and presumptive corneal epithelium to form the corneal endothelium and the stromal keratocytes. The sensory nerves that abundantly innervate the corneal stroma and epithelium originate from the neural crest- and ectodermal placode-derived trigeminal ganglion. Concomitant with corneal innervation is the formation of the limbal vascular plexus and the establishment of corneal avascularity. This review summarizes historical and current research to provide an overview of the genesis of the cellular layers of the cornea, corneal innervation, and avascularity.

Promoting convergence: the Phi spiral in abduction of mouse corneal behaviors

Why do mouse corneal epithelial cells display spiraling patterns? We want to provide an explanation for this phenomenon by applying an idealized problem solving process. Specifically, we applied complementary line-fitting methods to measure transgenic epithelial reporter expression arrangements displayed on three mature, live enucleated globes to clarify the problem. Two prominent logarithmic curves were discovered, one of which displayed the ϕ ratio, an indicator of the optimal configuration in phyllotactic systems. We then utilized two different computational approaches to expose our current understanding of the behavior. In one procedure, which involved an isotropic mechanics-based finite element method, we successfully produced logarithmic spiral curves of maximum shear strain based pathlines but computed dimensions displayed pitch angles of 35° (ϕ spiral is ~17°), which was altered when we fitted the model with published measurements of coarse collagen orientations. We then used model-based reasoning in context of Peircean abduction to select a working hypothesis. Our work serves as a concise example of applying a scientific habit of mind and illustrates nuances of executing a common method to doing integrative science.

Corneal nerves in health and disease

Corneal nerves are responsible for the sensations of touch, pain, and temperature and play an important role in the blink reflex, wound healing, and tear production and secretion. Corneal nerve dysfunction is a frequent feature of diseases that cause opacities and result in corneal blindness. Corneal opacities rank as the second most frequent cause of blindness. Technological advances in in vivo corneal nerve imaging, such as optical coherence tomography and confocal scanning, have generated new knowledge regarding the phenomenological events that occur during reinnervation of the cornea following disease, injury, or surgery. The recent availability of transgenic neurofluorescent murine models has stimulated the search for molecular modulators of corneal nerve regeneration. New evidence suggests that neuroregenerative and inflammatory pathways in the cornea are intertwined. Evidence-based treatment of neurotrophic corneal diseases includes using neuroregenerative (blood component-based and neurotrophic factors), neuroprotective, and ensconcing (bandage contact lens and amniotic membrane) strategies and avoiding anti-inflammatory therapies, such as cyclosporine and corticosteroids.

Analysis of protein composition and protein expression in the tear fluid of patients with congenital aniridia

Aniridia is a rare congenital genetic disorder caused by haploinsuffiency of the PAX6 gene, the master gene for development of the eye. The expression of tear proteins in aniridia is unknown. To screen for proteins involved in the aniridia pathophysiology, the tear fluid of patients with diagnosed congenital aniridia was examined using two-dimensional electrophoresis (2-DE) and liquid chromatography-tandem mass spectrometry (LC-MS/MS). Two-dimensional map of tear proteins in aniridia has been established and 7 proteins were differentially expressed with P<0.01 between aniridia patients and control subjects. Five of them were more abundant in healthy subjects, particularly α-enolase, peroxiredoxin 6, cystatin S, gelsolin, apolipoprotein A-1 and two other proteins, zinc-α2-glycoprotein and lactoferrin were more expressed in the tears of aniridia patients. Moreover, immunoblot analysis revealed elevated levels of vascular endothelial growth factor (VEGF) in aniridia tears which is in concordance with clinical finding of pathological blood and lymph vessels in the central and peripheral cornea of aniridia patients. The proteins with different expression in patients' tears may be new candidate molecules involved in the pathophysiology of aniridia and thus may be helpful for development of novel treatment strategies for the symptomatic therapy of this vision threatening condition.

BIOLOGICAL SIGNIFICANCE

This study is first to demonstrate protein composition and protein expression in aniridic tears and identifies proteins with different abundance in tear fluid from patients with congenital aniridia vs. healthy tears.

Ocular surface development and gene expression

The ocular surface-a continuous epithelial surface with regional specializations including the surface and glandular epithelia of the cornea, conjunctiva, and lacrimal and meibomian glands connected by the overlying tear film-plays a central role in vision. Molecular and cellular events involved in embryonic development, postnatal maturation, and maintenance of the ocular surface are precisely regulated at the level of gene expression by a well-coordinated network of transcription factors. A thorough appreciation of the biological characteristics of the ocular surface in terms of its gene expression profiles and their regulation provides us with a valuable insight into the pathophysiology of various blinding disorders that disrupt the normal development, maturation, and/or maintenance of the ocular surface. This paper summarizes the current status of our knowledge related to the ocular surface development and gene expression and the contribution of different transcription factors to this process.

Nerves and neovessels inhibit each other in the cornea

PURPOSE

To evaluate the regulatory cross-talk of the vascular and neural networks in the cornea.

METHODS

b-FGF micropellets (80 ng) were implanted in the temporal side of the cornea of healthy C57Bl/6 mice. On day 7, blood vessels (hemangiogenesis) and nerves were observed by immunofluorescence staining of corneal flat mounts. The next group of mice underwent either trigeminal stereotactic electrolysis (TSE), or sham operation, to ablate the ophthalmic branch of the trigeminal nerve. Blood vessel growth was detected by immunohistochemistry for PECAM-1 (CD31) following surgery. In another set of mice following TSE or sham operation, corneas were harvested for ELISA (VEGFR3 and pigment epithelium-derived factor [PEDF]) and for quantitative RT-PCR (VEGFR3, PEDF, and CD45). PEDF, VEGFR3, beta-3 tubulin, CD45, CD11b, and F4/80 expression in the cornea were evaluated using immunostaining.

RESULTS

No nerves were detected in the areas subject to corneal neovascularization, whereas they persisted in the areas that were neovessel-free. Conversely, 7 days after denervation, significant angiogenesis was detected in the cornea, and this was associated with a significant decrease in VEGFR3 (57.5% reduction, P = 0.001) and PEDF protein expression (64% reduction, P < 0.001). Immunostaining also showed reduced expression of VEGFR3 in the corneal epithelial layer. Finally, an inflammatory cell infiltrate, including macrophages, was observed.

CONCLUSION

Our data suggest that sensory nerves and neovessels inhibit each other in the cornea. When vessel growth is stimulated, nerves disappear and, conversely, denervation induces angiogenesis. This phenomenon, here described in the eye, may have far-reaching implications in understanding angiogenesis.

Critical role of Klf5 in regulating gene expression during post-eyelid opening maturation of mouse corneas

BACKGROUND

Klf5 plays an important role in maturation and maintenance of the mouse ocular surface. Here, we quantify WT and Klf5-conditional null (Klf5CN) corneal gene expression, identify Klf5-target genes and compare them with the previously identified Klf4-target genes to understand the molecular basis for non-redundant functions of Klf4 and Klf5 in the cornea.

METHODOLOGY/PRINCIPAL FINDINGS

Postnatal day-11 (PN11) and PN56 WT and Klf5CN corneal transcriptomes were quantified by microarrays to compare gene expression in maturing WT corneas, identify Klf5-target genes, and compare corneal Klf4- and Klf5-target genes. Whole-mount corneal immunofluorescent staining was employed to examine CD45+ cell influx and neovascularization. Effect of Klf5 on expression of desmosomal components was studied by immunofluorescent staining and transient co-transfection assays. Expression of 714 and 753 genes was increased, and 299 and 210 genes decreased in PN11 and PN56 Klf5CN corneas, respectively, with 366 concordant increases and 72 concordant decreases. PN56 Klf5CN corneas shared 241 increases and 98 decreases with those previously described in Klf4CN corneas. Xenobiotic metabolism related pathways were enriched among genes decreased in Klf5CN corneas. Expression of angiogenesis and immune response-related genes was elevated, consistent with neovascularization and CD45+ cell influx in Klf5CN corneas. Expression of 1574 genes was increased and 1915 genes decreased in WT PN56 compared with PN11 corneas. Expression of ECM-associated genes decreased, while that of solute carrier family members increased in WT PN56 compared with PN11 corneas. Dsg1a, Dsg1b and Dsp were down-regulated in Klf5CN corneas and their corresponding promoter activities were stimulated by Klf5 in transient co-transfection assays.

CONCLUSIONS/SIGNIFICANCE

Differences between PN11 and PN56 corneal Klf5-target genes reveal dynamic changes in functions of Klf5 during corneal maturation. Klf5 contributes to corneal epithelial homeostasis by regulating the expression of desmosomal components. Klf4- and Klf5-target genes are largely distinct, consistent with their non-redundant roles in the mouse cornea.

NK cells modulate the inflammatory response to corneal epithelial abrasion and thereby support wound healing

Natural killer (NK) cells are lymphocytes of the innate immune system that have crucial cytotoxic and regulatory roles in adaptive immunity and inflammation. Herein, we consider a role for these cells in corneal wound healing. After a 2-mm central epithelial abrasion of the mouse cornea, a subset of classic NK cells migrated into the limbus and corneal stroma, peaking at 24 hours with an eightfold increase over baseline. Depletion of γδ T cells significantly reduced NK cell accumulation (>70%; P < 0.01); however, in neutrophil-depleted animals, NK cell influx was normal. Isolated spleen NK cells migrated to the wounded cornea, and this migration was reduced by greater than 60% (P < 0.01) by ex vivo antibody blocking of NK cell CXCR3 or CCR2. Antibody-induced depletion of NK cells significantly altered the inflammatory reaction to corneal wounding, as evidenced by a 114% increase (P < 0.01) in neutrophil influx at a time when acute inflammation is normally waning. Functional blocking of NKG2D, an activating receptor for NK cell cytotoxicity and cytokine secretion, did not inhibit NK cell immigration, but significantly increased neutrophil influx. Consistent with excessive neutrophil accumulation, NK depletion and blocking of NKG2D also inhibited corneal nerve regeneration and epithelial healing (P < 0.01). Findings of this study suggest that NK cells are actively involved in corneal healing by limiting the innate acute inflammatory reaction to corneal wounding.

Pax6: a multi-level regulator of ocular development

Eye development has been a paradigm for the study of organogenesis, from the demonstration of lens induction through epithelial tissue morphogenesis, to neuronal specification and differentiation. The transcription factor Pax6 has been shown to play a key role in each of these processes. Pax6 is required for initiation of developmental pathways, patterning of epithelial tissues, activation of tissue-specific genes and interaction with other regulatory pathways. Herein we examine the data accumulated over the last few decades from extensive analyses of biochemical modules and genetic manipulation of the Pax6 gene. Specifically, we describe the regulation of Pax6's expression pattern, the protein's DNA-binding properties, and its specific roles and mechanisms of action at all stages of lens and retinal development. Pax6 functions at multiple levels to integrate extracellular information and execute cell-intrinsic differentiation programs that culminate in the specification and differentiation of a distinct ocular lineage.

Pathologic epithelial and anterior corneal nerve morphology in early-stage congenital aniridic keratopathy

OBJECTIVE

To document the clinical and morphologic corneal findings in the early stages of congenital aniridic keratopathy in Swedish families.

DESIGN

Prospective, observational, comparative case series.

PARTICIPANTS

A total of 16 eyes of 16 subjects with congenital aniridic keratopathy and a clear central cornea, and 6 eyes from 6 healthy controls (unaffected relatives). Nine of the 16 eyes with aniridia came from 5 families with a documented familial history of aniridia.

METHODS

Detailed ophthalmic examinations included best spectacle-corrected visual acuity (BSCVA), tear film production, tear break-up time (BUT), corneal touch sensitivity, intraocular pressure measurement, ultrasound pachymetry, slit-lamp biomicroscopy, and laser scanning in vivo confocal microscopy (IVCM).

MAIN OUTCOME MEASURES

Confirmed stage of aniridic keratopathy, clinical parameters of cornea and tear film (visual acuity, sensitivity, corneal thickness, tear production, and BUT), and the morphologic status of corneal epithelium, sub-basal nerves, and limbal palisades of Vogt.

RESULTS

In early-stage aniridic keratopathy, BSCVA and tear BUT were reduced relative to controls (P < 0.001 for both), and corneal thickness was increased (P=0.01). Inflammatory dendritic cells were present in the central epithelium in aniridia, with significantly increased density relative to controls (P = 0.001). Discrete focal opacities in the basal epithelial region were present in 5 of 11 aniridia cases with an otherwise clear cornea. Opacities were associated with dendritic cells and harbored structures presumed to be goblet cells. Sub-basal nerves were extremely dense in 3 aniridia cases, and a prominent whorl pattern of nerves and epithelial cells was observed in 1 case. Normal limbal palisade morphology was absent in aniridia but present in controls.

CONCLUSIONS

Early-stage aniridic keratopathy is characterized by the development of focal opacities in the basal epithelium, altered sub-basal nerves, infiltration of the central epithelium by dendritic cells, tear film instability, and increased corneal thickness and degradation of limbal palisade architecture. These findings may help to elucidate the pathogenesis of aniridic keratopathy.

Study of a US cohort supports the role of ZNF644 and high-grade myopia susceptibility

PURPOSE

Myopia, or nearsightedness, is highly prevalent in Asian countries and is considered a serious public health issue globally. High-grade myopia can predispose individuals to myopic maculopathy, premature cataracts, retinal detachment, and glaucoma. A recent study implicated zinc finger protein 644 isoform 1 (ZNF644) variants with non-syndromic high-grade myopia in a Chinese-Asian population. Herein we focused on investigating the role for ZNF644 variants in high-grade myopia in a United States (US) cohort.

METHODS

DNA from a case cohort of 131 subject participants diagnosed with high-grade myopia was screened for ZNF644 variants. Spherical refractive error of -≤-6.00 diopters (D) in at least one eye was defined as affected. All coding, intron/exon boundaries were screened using Sanger sequencing. Single nucleotide allele frequencies were determined by screening 672 ethnically matched controls.

RESULTS

Sequencing analysis did not detect previously reported mutations. However, our analysis identified 2 novel single nucleotide variants (c.725C>T, c.821A>T) in 2 high-grade myopia individuals- one Caucasian and one African American, respectively. These variants were not found in normal controls. A rare variant - dbsSNP132 (rs12117237→c.2119A>G) - with a minor allele frequency of 0.2% was present in 6 additional cases, but was also present in 5 controls.

CONCLUSIONS

Our study has identified two novel variants in ZNF644 associated with high-grade myopia in a US cohort. Our results suggest that ZNF644 may play a role in myopia development.

Three dimensional visualization and fractal analysis of mosaic patches in rat chimeras: cell assortment in liver, adrenal cortex and cornea

The production of organ parenchyma in a rapid and reproducible manner is critical to normal development. In chimeras produced by the combination of genetically distinguishable tissues, mosaic patterns of cells derived from the combined genotypes can be visualized. These patterns comprise patches of contiguously similar genotypes and are different in different organs but similar in a given organ from individual to individual. Thus, the processes that produce the patterns are regulated and conserved. We have previously established that mosaic patches in multiple tissues are fractal, consistent with an iterative, recursive growth model with simple stereotypical division rules. Fractal dimensions of various tissues are consistent with algorithmic models in which changing a single variable (e.g. daughter cell placement after division) switches the mosaic pattern from islands to stripes of cells. Here we show that the spiral pattern previously observed in mouse cornea can also be visualized in rat chimeras. While it is generally held that the pattern is induced by stem cell division dynamics, there is an unexplained discrepancy in the speed of cellular migration and the emergence of the pattern. We demonstrate in chimeric rat corneas both island and striped patterns exist depending on the age of the animal. The patches that comprise the pattern are fractal, and the fractal dimension changes with the age of the animal and indicates the constraint in patch complexity as the spiral pattern emerges. The spiral patterns are consistent with a loxodrome. Such data are likely to be relevant to growth and cell division in organ systems and will help in understanding how organ parenchyma are generated and maintained from multipotent stem cell populations located in specific topographical locations within the organ. Ultimately, understanding algorithmic growth is likely to be essential in achieving organ regeneration in vivo or in vitro from stem cell populations.

PAX Genes in Cancer; Friends or Foes?

PAX genes have been shown to be critically required for the development of specific tissues and organs during embryogenesis. In addition, PAX genes are expressed in a handful of adult tissues where they are thought to play important roles, usually different from those in embryogenesis. A common theme in adult tissues is a requirement for PAX gene expression in adult stem cell maintenance or tissue regeneration. The connections between adult stem cell PAX gene expression and cancer are intriguing, and the literature is replete with examples of PAX gene expression in either situation. Here we systematically review the literature and present an overview of postnatal PAX gene expression in normal and cancerous tissue. We discuss the potential link between PAX gene expression in adult tissue and cancer. In addition, we discuss whether persistent PAX gene expression in cancer is favorable or unfavorable.

Effects of aberrant Pax6 gene dosage on mouse corneal pathophysiology and corneal epithelial homeostasis

Background

Altered dosage of the transcription factor PAX6 causes multiple human eye pathophysiologies. PAX6^+/− heterozygotes suffer from aniridia and aniridia-related keratopathy (ARK), a corneal deterioration that probably involves a limbal epithelial stem cell (LESC) deficiency. Heterozygous Pax6^+/Sey-Neu (Pax6^+/−) mice recapitulate the human disease and are a good model of ARK. Corneal pathologies also occur in other mouse Pax6 mutants and in PAX77^Tg/− transgenics, which over-express Pax6 and model human PAX6 duplication.

Methodology/Principal Findings

We used electron microscopy to investigate ocular defects in Pax6^+/− heterozygotes (low Pax6 levels) and PAX77^Tg/− transgenics (high Pax6 levels). As well as the well-documented epithelial defects, aberrant Pax6 dosage had profound effects on the corneal stroma and endothelium in both genotypes, including cellular vacuolation, similar to that reported for human macular corneal dystrophy. We used mosaic expression of an X-linked LacZ transgene in X-inactivation mosaic female (XLacZ^Tg/−) mice to investigate corneal epithelial maintenance by LESC clones in Pax6^+/− and PAX77^Tg/− mosaic mice. PAX77^Tg/− mosaics, over-expressing Pax6, produced normal corneal epithelial radial striped patterns (despite other corneal defects), suggesting that centripetal cell movement was unaffected. Moderately disrupted patterns in Pax6^+/− mosaics were corrected by introducing the PAX77 transgene (in Pax6^+/−, PAX77^Tg/− mosaics). Pax6^Leca4/+, XLacZ^Tg/− mosaic mice (heterozygous for the Pax6^Leca4 missense mutation) showed more severely disrupted mosaic patterns. Corrected corneal epithelial stripe numbers (an indirect estimate of active LESC clone numbers) declined with age (between 15 and 30 weeks) in wild-type XLacZ^Tg/− mosaics. In contrast, corrected stripe numbers were already low at 15 weeks in Pax6^+/− and PAX77^Tg/− mosaic corneas, suggesting Pax6 under- and over-expression both affect LESC clones.

Conclusions/Significance

Pax6^+/− and PAX77^Tg/− genotypes have only relatively minor effects on LESC clone numbers but cause more severe corneal endothelial and stromal defects. This should prompt further investigations of the pathophysiology underlying human aniridia and ARK.

Removal of the basement membrane enhances corneal wound healing

Recurrent corneal erosions are painful and put patients' vision at risk. Treatment typically begins with debridement of the area around the erosion site followed by more aggressive treatments. An in vivo mouse model has been developed that reproducibly induces recurrent epithelial erosions in wild-type mice spontaneously within two weeks after a single 1.5 mm corneal debridement wound created using a dulled-blade. This study was conducted to determine whether 1) inhibiting MMP9 function during healing after dulled-blade wounding impacts erosion development and 2) wounds made with a rotating-burr heal without erosions. Oral or topical inhibition of MMPs after dulled-blade wounding does not improve healing. Wounds made by rotating-burr heal with significantly fewer erosions than dulled-blade wounds. The localization of MMP9, β4 integrin and basement membrane proteins (LN332 and type VII collagen), immune cell influx, and reinnervation of the corneal nerves were compared after both wound types. Rotating-burr wounds remove the anterior basement membrane centrally but not at the periphery near the wound margin, induce more apoptosis of corneal stromal cells, and damage more stromal nerve fibers. Despite the fact that rotating-burr wounds do more damage to the cornea, fewer immune cells are recruited and significantly more wounds resolve completely.

IL-17 and VEGF are necessary for efficient corneal nerve regeneration

The contribution of acute inflammation to sensory nerve regeneration was investigated in the murine cornea using a model of corneal abrasion that removes the stratified epithelium and subbasal nerve plexus. Abrasion induced accumulation of IL-17(+) CCR6(+) γδ T cells, neutrophils, and platelets in the cornea followed by full restoration of the epithelium and ∼19% regeneration of sensory nerves within 96 hours. Mice deficient in γδ T cells (TCRδ(-/-)) or wild-type mice treated systemically with anti-IL-17 had >50% reduction in leukocyte and platelet infiltration and >50% reduction in nerve regeneration. Strategies used to prevent neutrophil and platelet accumulation (eg, wild-type mice treated with anti-Ly6G or anti-GP1bα antibody to deplete neutrophils or platelets) also resulted in >50% reductions in corneal nerve density. Infiltrating neutrophils and platelets stained positively for VEGF-A, tissue levels of VEGF-A peaked coincidentally with peak tissue levels of neutrophils and platelets, depletion of neutrophils before injury reduced tissue VEGF-A levels by >70%, and wild-type mice treated systemically with anti-VEGF-A antibody exhibited >80% reduction in corneal nerve regeneration. Given the known trophic effects of VEGF-A for neurite growth, the results in this report demonstrate a previously unrecognized beneficial role for the γδ T cell-dependent inflammatory cascade involving IL-17, neutrophils, platelets, and VEGF-A in corneal nerve regeneration.

Development of the cornea of true moles (Talpidae): morphogenesis and expression of PAX6 and cytokeratins

Corneal development and structure were studied in the Iberian mole Talpa occidentalis, which has permanently closed eyelids, and the European mole Talpa europaea, in which the eyes are open. The vertebrate cornea typically maintains a three-layered structure - a stratified epithelium with protective and sensory function, an avascular, hypocellular, collagenous stroma, and an endothelium with both barrier and transport functions that regulates corneal hydration, hence maintaining transparency. Compared to mouse, both mole species had significant corneal specializations, but the Iberian mole had the most divergent phenotype, with no endothelium and a flattened monolayer epithelium. Nevertheless, normal epithelial cell junctions were observed and corneal transparency was maintained. Corneas of European moles have a dysmorphic phenotype that recapitulates the human disorder keratoconus for which no mouse model exists. Mole corneas are vascularized - a situation only previously observed in the manatee Trichechus- and have non-radial patterns of corneal innervation indicative of failure of corneal epithelial cell migration. The transcription factor Pax6 is required for corneal epithelial differentiation in mice, but was found to be dispensable in moles, which had mosaic patterns of PAX6 localization uniquely restricted, in European moles, to the apical epithelial cells. The apparently stalled or abnormal differentiation of corneas in adult moles is supported by their superficial similarity to the corneas of embryonic or neonatal mice, and their abnormal expression of cytokeratin-12 and cytokeratin-5. European moles seem to have maintained some barrier/protective function in their corneas. However, Iberian moles show a more significant corneal regression likely related to the permanent eyelid fusion. In this mole species, adaptation to the arid, harder, Southern European soils could have favoured the transfer of these functions to the permanently sealed eyelids.

Innervation of the mouse cornea during development

PURPOSE

Dense innervation of the cornea is important for maintaining its homeostasis and transparency. Although corneal nerves have been well studied in adults, little is known about mammalian corneal innervation during development. This study provides a detailed profile of nerves at various stages of mouse cornea development.

METHODS

Mouse heads and corneas were collected at various stages of development including embryonic days (E)12.5 to E16.5, postnatal days (P)0, P10, three weeks after birth, and the adult. Corneas were immunostained with an anti-neuron-specific β-tubulin antibody (TUJ1). Fluorescently labeled nerves in whole-mount tissues and sections were imaged and analyzed for their axonal projections during eye development.

RESULTS

The first nerve bundles appear at the periphery of the anterior portion of the eye by E12.5. Initial projection into the stroma occurs at E13.5 without formation of a pericorneal nerve ring. Between E13.5 and E16.5, nerve bundles project directly into the periphery of the presumptive cornea stroma. They branch repeatedly as they extend toward the cornea center and epithelium. Concomitantly, nerve bundles originating from four quadrants of the eye bifurcate into smaller branches that innervate the entire stroma. The first epithelial innervation occurs at E16.5. Epithelial nerves arrange into patterns that project toward the center subsequently forming a swirl at three weeks after birth, which becomes more pronounced in adults.

CONCLUSIONS

Nerve bundles that arise from four quadrants of the eye innervate the mouse cornea. The nerve bundles directly innervate the stroma without forming a pericorneal nerve ring. Radial arrangement of epithelial nerves gradually becomes centrally oriented, subsequently forming a swirl pattern.

Anatomy of the human corneal innervation

The anatomy of the human corneal innervation has been the subject of much investigation; however, a comprehensive description remains elusive. The purpose of the present study was to provide a detailed description of the human corneal innervation using a novel approach involving immunohistochemically stained anterior-cornea whole mounts. Sixteen donor corneas aged 19-78 years were cut with a 6.0 mm trephine into a central plug and two peripheral rims. Each specimen was sectioned serially on a cryostat to produce several 100 microm-thick stromal sections and a 100-140 microm-thick anterior-cornea whole mount that contained the entire corneal epithelium and much of the anterior stroma. The corneal innervation was stained with a primary antibody against beta neurotubulin and subjected to rigorous quantitative and qualitative analyses. The results showed that a mean of 71.3 +/- 14.3, uniformly spaced, main stromal nerve bundles entered the cornea at the corneoscleral limbus. The bundles averaged 20.3 +/- 7.0 microm in diameter, were separated by a mean spacing of 0.49 +/- 0.40 mm, and entered the cornea at a mean distance of 293 +/- 106 microm from the ocular surface. Each stromal bundle gave rise through repetitive branching to a moderately dense midstromal plexus and a dense subepithelial plexus (SEP). The SEP was comprised of modest numbers of straight and curvilinear nerves, most of which penetrated Bowman's membrane to supply the corneal epithelium, and a more abundant and anatomically complex population of tortuous, highly anastomotic nerves that remained largely confined in their distribution to the SEP. SEP density and anatomical complexity varied considerably among corneas and was less dense and patchier in the central cornea. A mean of 204 +/- 58.5 stromal nerves penetrated Bowman's membrane to supply the central 10 mm of corneal epithelium (2.60 nerves/mm(2)). The density of Bowman's membrane penetrations was greater peripherally than centrally. After entering the epithelium, stromal nerves branched into groups of up to twenty subbasal nerve fibers known as epithelial leashes. Leashes in the central and intermediate cornea anastomosed extensively to form a dense, continuous subbasal nerve plexus, while leashes in the peripheral cornea demonstrated fewer anastomoses and were less complex anatomically. Viewed in its entirety, the subbasal nerve plexus formed a gentle, whorl-like assemblage of long curvilinear subbasal fibers, 1.0-8.0 mm in length, that converged on an imaginary seam or gentle spiral (vortex) approximately 2.51 +/- 0.23 mm inferonasal to the corneal apex. Mean subbasal nerve fiber density near the corneal apex was 45.94 +/- 5.20 mm/mm(2) and mean subbasal and interconnecting nerve fiber diameters in the same region were 1.51 +/- 0.74 microm and 0.69 +/- 0.26 microm, respectively. Intraepithelial terminals originated exclusively as branches of subbasal nerves and terminated in all epithelial layers. Nerve terminals in the wing and squamous cell layers were morphologically diverse and ranged in total length from 9 to 780 microm. The suprabasal layers of the central corneal epithelium contained approximately 605.8 terminals/mm(2). The results of this study provide a detailed, comprehensive description of human corneal nerve architecture and density that extends and refines existing accounts. An accurate, detailed model of the normal human corneal innervation may predict or help to understand the consequences of corneal nerve damage during refractive, cataract and other ocular surgeries.