A conditional Pax6 depletion study with no morphological effect on the adult mouse corneal epithelium

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.

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.

Gene dosage manipulation alleviates manifestations of hereditary PAX6 haploinsufficiency in mice

In autosomal dominant conditions with haploinsufficiency, a single functional allele cannot maintain sufficient dosage for normal function. We hypothesized that pharmacologic induction of the wild-type allele could lead to gene dosage compensation and mitigation of the disease manifestations. The paired box 6 (PAX6) gene is crucial in tissue development and maintenance particularly in eye, brain, and pancreas. Aniridia is a panocular condition with impaired eye development and limited vision due to PAX6 haploinsufficiency. To test our hypothesis, we performed a chemical screen and found mitogen-activated protein kinase kinase (MEK) inhibitors to induce PAX6 expression in normal and mutant corneal cells. Treatment of newborn Pax6-deficient mice (Pax6Sey-Neu/+ ) with topical or systemic MEK inhibitor PD0325901 led to increased corneal PAX6 expression, improved corneal morphology, reduced corneal opacity, and enhanced ocular function. These results suggest that induction of the wild-type allele by drug repurposing is a potential therapeutic strategy for haploinsufficiencies, which is not limited to specific mutations.

Effect of Ionizing Radiation from Computed Tomography on Differentiation of Human Embryonic Stem Cells into Neural Precursors

We studied the effect of radiation from computed tomography (CT) scans on differentiation of human embryonic stem cells (hESCs) into neuronal lineage. hESCs were divided into three radiation exposure groups: 0-dose, low-dose, or high-dose exposure. Low dose was accomplished with a single 15 mGy CT dose index (CTDI) CT scan that approximated the dose for abdominal/pelvic CT examinations in adults while the high dose was achieved with several consecutive CT scans yielding a cumulative dose of 500 mGy CTDI. The neural induction was characterized by immunocytochemistry. Quantitative polymerase chain reaction (qPCR) and Western blots were used to measure expression of the neuronal markers PAX6 and NES and pluripotency marker OCT4. We did not find any visible morphological differences between neural precursors from irradiated and non-irradiated cells. However, quantitative analyses of neuronal markers showed that PAX6 expression was reduced following exposure to the high dose compared to 0-dose controls, while no such decrease in PAX6 expression was observed following exposure to the low dose. Similarly, a statistically significant reduction in expression of NES was observed following high-dose exposure, while after low-dose exposure, a modest but statistically significant reduction in NES expression was only observed on Day 8 of differentiation. Further studies are warranted to elucidate how lower or delayed expression of PAX6 and NES can impact human fetal brain development.