Discrete limbal epithelial stem cell populations mediate corneal homeostasis and wound healing

IFITM1/OVOL1 Axis Is a Novel Regulator of the Expansion of the Limbal Epithelial Stem/Early Transient Amplifying Cell Population

Limbal epithelial stem cells (LESCs), located in the basal layer of the limbal epithelium, rarely proliferate under normal conditions. Upon proliferation, LESCs give rise to early transient amplifying (eTA) cells, which are thought to be morphologically and phenotypically indistinguishable from LESCs. Following corneal epithelial wounding, LESCs are activated to repair the corneal epithelium via expansion of eTA cells, a process crucial for maintaining corneal epithelial homeostasis and tissue transparency as well as essential for clear vision. To understand how this process is regulated, we conducted a single cell RNA sequencing assay of mouse corneal rims with and without injury and observed an expansion of the stem/eTA cell cluster after corneal injury. Interestingly, we found that Interferon Induced Transmembrane Protein 1 (IFITM1) was predominantly expressed in stem/eTA cells and was positively associated with such stem/eTA cell expansion after corneal wounding. In vivo knockdown of IFITM1 using an AAV (adeno-associated virus) vector significantly attenuated stem/eTA cell expansion and activation of stem/eTA cells to proliferate after mouse corneal wounding. In human limbal epithelial cell cultures, IFITM1 positively impacted the proliferation of stem/eTA cell-enriched limbal epithelial cells, contributing to expansion of the stem/eTA cell population. Such expansion was due, in part, to inhibition of OVOL1 (Ovo like zinc finger 1), a negative regulator of epithelial cell proliferation. These results provide key molecular insights into how stem cell activation and eTA cell expansion are regulated. Elucidating the IFITM1/OVOL1 pathway that governs stem/eTA cell proliferation not only deepens our knowledge of tissue homeostasis but also opens avenues for developing novel regenerative therapies.

COMPARTMENTALIZED 3D BIOPRINTING OF THE LIMBAL NICHE WITH DISTINCT HPSC-LSC SUBPOPULATIONS FOR CORNEAL DISEASE MODELING

Limbal epithelial stem cells (LSCs) are essential for corneal epithelium regeneration and visual acuity. The limbal niche's physicochemical properties regulate LSC function, but their role is not fully understood. Developing in vitro models that mimic the native niche can enhance our understanding of niche functions, despite the challenges of niche complexity. In this study, we created a 3D bioprinted limbal niche model using a hybrid approach that combines two human pluripotent stem cell-derived LSC (hPSC-LSC) subpopulations (p63+ and ABCG2+ cells) within hyaluronic acid (HA)-based bioinks and a stiff polyacrylamide (PA) gel scaffold produced by conventional gel casting. We analyzed the mechanical properties of the bioinks and assessed cell viability, morphology, and protein expression after one week of culture. Finally, we conducted a proof-of-concept wound healing assay using an alkali burn injury model to assess the functionality of the model for research purposes. The results show that this 3D model effectively replicated the mechanical environment of native tissue, maintains stability for one-week post-printing, and supports LSC viability and normal in vitro phenotype. In addition, the wound healing assay showed a cellular response, indicated by non-simultaneous caspase-3 activation of hPSC-LSC subpopulations for 48 hours post-wounding. This model provides a valuable platform for investigating the limbal niche and advancing cellular therapies applicable to other tissue niches throughout the body. STATEMENT OF SIGNIFICANCE: The corneal limbal niche is crucial for corneal regeneration, creating a high demand for in vitro models. However, current models are not sufficiently replicating the complexity of native tissue and importantly, lack the element of recently demostrated limbal stem cell (LSC) heterogeneity. In this study, we combine three key features of the limbus, including stiffness, architecture and compartmentalization, to create limbal niche-mimicking structures using 3D bioprinting with two human pluripotent stem cell derived LSC (hPSC-LSC) subpopulations. We demonstrate structural stability, native tissue-like mechanical properties, sustained cellular viability, stable hPSC-LSC phenotype post-printing, and a tissue-mimicking response to wounding. This approach offers an innovative strategy to model complex niches and advance the understanding of limbal niche functions.

Spatiotemporal single-cell analysis elucidates the cellular and molecular dynamics of human cornea aging

BACKGROUND

The human cornea is a transparent and uniquely ordered optical-biological system. Precise coordination of its cellular mechanisms is essential to maintain its transparency and functionality. However, the spatial, cellular and molecular architecture of the human cornea and its intercellular interactions during aging have not been elucidated.

METHODS

We performed single-cell RNA sequencing (scRNA-seq) and single-cell SpaTial Enhanced REsolution Omics-sequencing (scStereo-seq) analysis in corneal tissue from eight eyes of donors aged 33-88 years to elucidate the spatiotemporal cellular and molecular dynamics of human cornea aging. Immunofluorescence staining and Western blotting were performed to validate the findings.

RESULTS

Spatiotemporal single-cell analysis revealed the complex cellular landscape, spatial organization and intercellular communication within the human cornea. The subpopulations of major cell types of the cornea were elucidated with precise spatial positions. In particular, we identified novel subpopulations, mapped the spatial positioning of limbal stem cells within the limbal niche, and delineated the interactions between major cell types. We observed that three basal cell subsets migrate centripetally from the peripheral to the central cornea with age, suggesting the "spatiotemporal centripetal pattern" as a novel paradigm for the age-related migration of corneal epithelial cells. Furthermore, we elucidated the age-related, region-specific molecular and functional characteristics of the corneal endothelium, demonstrating differential metabolic capacities and functional properties between the peripheral and central regions.

CONCLUSIONS

As the first comprehensive spatiotemporal atlas, our work provides a valuable resource for understanding tissue homeostasis in the human cornea and advances research on corneal pathology, transplantation, senescence and regenerative medicine in the context of corneal aging.

Therapeutic potential of regulatory T cells for stem cell regulation: Insights from Treg-mediated enhancement of limbal stem cell functions

Regulatory T cells (Tregs) play a key role in immunomodulation and tissue regeneration. Limbal stem cells (LSCs) maintain corneal epithelial homeostasis, and LSC deficiency (LSCD) leads to visual impairment. Current LSCD treatments face donor shortages and graft rejection risks. The present study explored Tregs' therapeutic potential for LSCD in a mouse model of graded LSCD and further explored the direct effect of Tregs on LSCs function by in vitro coculturing human-derived cells. Subconjunctival Tregs injection effectively treated mild and moderate LSCD in mouse models. Coculturing human LSCs with human Tregs promoted LSCs migration, proliferation, and stemness maintenance. Furthermore, amphiregulin (AREG), secreted by Tregs, was crucial to their therapeutic effects, as AREG-/- Tregs resulted in diminished efficacy on LSCD mice compared to wild-type Tregs. These findings highlight Tregs as a promising treatment for LSCD, enhancing LSC function partially via AREG production.

HDAC6 deacetylates ENKD1 to regulate mitotic spindle behavior and corneal epithelial homeostasis

Corneal diseases can cause severe visual impairment and even blindness, which have been linked to the interruption of corneal epithelial homeostasis. However, the underlying molecular mechanisms are largely unknown. In this study, by comparing the transcriptomes of keratoconus, bacterial keratitis, viral keratitis, and healthy corneas, we found a steady upregulation of histone deacetylase 6 (HDAC6) in corneal diseases. Consistently, a significant increase in HDAC6 was observed in mouse corneas with bacterial keratitis. Overexpression of HDAC6 in mice results in a significant thickening of the corneal epithelium. Mechanistic studies reveal that HDAC6 overexpression disrupts mitotic spindle orientation and positioning in corneal epithelial cells. Our data further show that HDAC6 deacetylates enkurin domain-containing protein 1 (ENKD1) at lysine 98 and thereby impedes its interaction with γ-tubulin, restraining the centrosomal localization of ENKD1 and its proper function in regulating mitotic spindle behavior. These findings uncover a pivotal role for HDAC6-mediated deacetylation of ENKD1 in the control of corneal epithelial homeostasis, providing potential therapeutic targets for treating corneal diseases.

Pediatric limbal stem cell deficiency: An overview of a rarely studied pathology

The cornea is the outermost transparent layer of the eye, and the continuous renewal of the corneal epithelium is important for its transparency. This process is primarily facilitated by corneal stem cells, most of which are found at the limbus. However, any deterioration or damage in this region leads to corneal conjunctivalization and consequent limbal stem cell deficiency (LSCD), which compromises corneal transparency. LSCD is an important condition, especially in the pediatric population, as it can lead to corneal vascularization, opacity and ultimately loss of vision and subsequent amblyopia, unlike adults. Although pediatric LSCD is often due to chemical injuries, as in adults, it is also caused by conditions such as congenital aniridia, Steven Johnson Syndrome and various other immunological disorders. Appropriate and timely treatment in pediatric LSCD is of particular importance in preventing progression to amblyopia, unlike adults. Accurate staging of the disease is essential for the necessary medical and surgical treatment decision. While medical approaches are at the forefront to eliminate the causative agent and improve the ocular surface in reversible cases, it is essential to replace the limbal stem cells lost in advanced disease. For these replacement procedures, it is noteworthy that there is a tendency for minimally invasive methods compared to adults to avoid possible complications due to long life expectancy in children. In conclusion, although there are various reviews on limbal stem cell deficiency (LSCD) in adults, we believe that this review for childhood LSCD will make an important contribution to the literature, since the relevant literature currently reported for the pediatric population is limited.

CD4+CD25- T-Cell-Secreted IFN-γ Promotes Corneal Nerve Degeneration in Diabetic Mice

Purpose

This study aimed to explore the relationship between corneal nerve degeneration and elevated dendritic cells (DCs) in diabetic keratopathy.

Methods

Corneas from diabetic and healthy mice were analyzed using single-cell RNA sequencing. Corneal nerve density and DC and T-cell infiltration were quantified through whole-mount corneal staining. Freshly isolated mouse trigeminal ganglion (TG) neurons were co-cultured with immature DCs, mature DCs, activated CD8+ T cells, and CD4+CD25- T cells. TG neurite outgrowth was assessed to identify potential effector cells driving corneal nerve degeneration. In addition, interferon-gamma (IFN-γ) and blocking antibodies were used to evaluate their effects on TG neurite outgrowth and corneal nerve degeneration in mice.

Results

Compared with age-matched healthy mice, diabetic mice exhibited a significant reduction in corneal nerve density and sensitivity, along with increased infiltration of DCs, CD4+ T cells, and CD8+ T cells. In vitro co-culture experiments revealed that CD4+CD25- T cells, rather than DCs and CD8+ T cells, significantly inhibited TG neurite outgrowth. Among cytokines, elevated IFN-γ in diabetic corneas impaired TG neurite outgrowth and induced corneal nerve degeneration, whereas IL-4 and IL-17 had no such effect. Blocking IFN-γ alleviated CD4+CD25- T-cell-induced inhibition of TG neurite outgrowth and corneal nerve degeneration in diabetic mice.

Conclusions

CD4+CD25- T cells, but not DCs or CD8+ T cells, contribute to corneal nerve degeneration in diabetic mice, a process partially mediated by IFN-γ.

Niche-derived Semaphorin 4A safeguards functional identity of myeloid-biased hematopoietic stem cells

Somatic stem cell pools comprise diverse, highly specialized subsets whose individual contribution is critical for the overall regenerative function. In the bone marrow, myeloid-biased hematopoietic stem cells (myHSCs) are indispensable for replenishment of myeloid cells and platelets during inflammatory response but, at the same time, become irreversibly damaged during inflammation and aging. Here we identify an extrinsic factor, Semaphorin 4A (Sema4A), which non-cell-autonomously confers myHSC resilience to inflammatory stress. We show that, in the absence of Sema4A, myHSC inflammatory hyper-responsiveness in young mice drives excessive myHSC expansion, myeloid bias and profound loss of regenerative function with age. Mechanistically, Sema4A is mainly produced by neutrophils, signals via a cell surface receptor, Plexin D1, and safeguards the myHSC epigenetic state. Our study shows that, by selectively protecting a distinct stem cell subset, an extrinsic factor preserves functional diversity of somatic stem cell pool throughout organismal lifespan.

Expression Distribution of Keratins in Normal and Pathological Corneas and the Regulatory Role of Krt17 on Limbal Stem Cells

Purpose

This study aimed to compare variations in keratins (KRTs), particularly stress KRTs, under normal and pathological conditions, with a specific focus on investigating the role of KRT17.

Methods

This research examined changes in KRT and limbal stem cell (LSC) markers in normal and various pathological corneas using mRNA and proteomic sequencing data alongside single-cell sequencing data from normal mouse corneas. The effects of the Krt17 recombinant protein and specific small interfering RNA on the clonal formation and proliferation of human limbal epithelial cells (HLECs) were investigated. mRNA sequencing was conducted on Krt17 knockdown HLECs, and the findings were validated using qPCR, immunofluorescence staining, neutrophil chemotaxis, and herpes simplex virus 1 infection assay. The STRING database was used to predict Krt17's interacting proteins.

Results

Various KRTs in the corneal epithelia exhibited differences in expression levels and patterns. Under pathological conditions, stress KRTs Krt17 and Krt16 were upregulated, while differentiation-related Krt12 was downregulated, and the expression of the LSC markers Krt17, Krt14, and IFITM3 were commonly increased. Supplementation and intervention experiments confirmed that Krt17 promotes clonal formation and proliferation in HLECs. Krt17 knockdown resulted in the upregulation of genes related to inflammation and defense responses, while downregulating molecules associated with differentiation pathways. Krt17 knockdown promoted neutrophil chemotaxis and alleviated herpes simplex virus 1 infection in HLECs.

Conclusions

KRTs play a crucial role in the homeostasis and pathological regulation of the corneal epithelium. The stress Krt17 located in LSCs is involved in regulating the stemness, proliferation, and differentiation of LSCs, as well as immune and defense responses.

Prox1 Protein in Corneal Limbal Lymphatic Vessels Maintains Limbal Stem Cell Stemness and Regulates Corneal Injury Repair

Purpose

The purpose of this study was to elucidate the role of the corneal lymphatic Prox1 gene in modulating limbal stem cell stemness and facilitating corneal injury repair.

Methods

The limbal Prox1 gene was knocked down by adeno-associated virus (AAV). The alkali burn model was induced in the naive group, the AAV-sham group, and the AAV-shProx1 group. Anterior segment photography, fluorescein sodium staining, and hematoxylin and eosin (H&E) staining were conducted immediately on days 1, 3, 7, 14, and 21 post-injury. Immunofluorescent (IF) staining was used to assess Ki67, ΔNp63, and K14. Additionally, seq-mRNA technology facilitated a comparative transcriptomic analysis between the AAV-sham and the AAV-shProx1 groups 7 days post-injury. Key regulated genes were verified by protein level. Furthermore, a co-culture model of lymphatic endothelial cells (LECs) and limbal stem cells (LSCs) was used to investigate the proliferation capacity and stemness expression of LSCs.

Results

Fluorescein sodium staining revealed that the epithelial defect area was significantly larger in the AAV-shProx1 group than in the AAV-sham group on days 1 and 3 post-injury (P < 0.05). Ki67, ΔNp63, and K14 expressions were consistently lower in the AAV-shProx1 group than in the AAV-sham group at distinct time points. Additionally, seq-mRNA results demonstrated that genes (Prox1 and Lyve1) were downregulated while inflammatory factors (Ccl2, Ccl7, IL16, IL1R, and TNFsf11) were upregulated in the AAV-shProx1 group compared with the AAV-sham group. When Prox1 was silenced in LECs, the proliferation and stemness of LSCs were markedly downregulated.

Conclusions

The Prox1 and Lyve1 proteins in lymphatic vessels served as pivotal regulated proteins in corneal injury repair. The draining role of lymphatic vessels during corneal injury was indispensable.

Epigenetic ALYREF/UHRF1/RHOB Axis in Corneal Wound Healing and Implications for Epithelial Tumorigenesis

Purpose

Corneal epithelial wound healing (CEWH) is a complex process influenced by epigenetic regulation. Ubiquitin-like with plant homeodomain (PHD) and ring finger domains 1 (UHRF1), it plays a key role in integrating epigenetic signals. However, its precise function in modulating CEWH remains poorly understood. We describe here the functional mechanisms of UHRF1 in modulating CEWH.

Methods

Quantitative Reverse Transcription PCR (RT-qPCR), immunofluorescence, and gene manipulation were used to investigate UHRF1 expression patterns and functions during CEWH. Integrated multi-omics and targeted bisulfite sequencing (TBS) were performed to reveal the downstream target of UHRF1. Mutation assay was used to examine whether Aly/REF export factor (ALYREF) can recognize and bind RNA m5C-UHRF1. Gene expression profiling interactive analysis (GEPIA) was utilized to validate the correlation of UHRF1 with its upstream and downstream targets.

Results

In this study, we demonstrate that UHRF1 enhances CEWH and sustains DNA methylation during CEWH, which is essential for this effect. A multi-omics analysis identified Ras homolog family member B (RHOB) as a downstream target of UHRF1. Our findings further revealed that UHRF1 epigenetically downregulates RHOB, thereby facilitating CEWH. Moreover, we showed that ALYREF binds to m5C sites on UHRF1 mRNA and enhances its translation. Finally, our analysis of molecular alterations and the clinical significance of ALYREF, UHRF1, and RHOB expression suggests that this epigenetic axis is also relevant in epithelial-derived tumors, which represent approximately 90% of all tumors.

Conclusions

Our study identifies a novel epigenetic ALYREF/UHRF1/RHOB axis that enhances CEWH. Importantly, this axis appears to be conserved across various epithelial-derived tumors, suggesting its broader biological significance.

The Spatial Transcriptomic Atlas of Human Limbus and Vital Niche Microenvironment Regulating the Fate of Limbal Epithelial Stem Cells

Purpose

This study aimed to generate the spatial atlas of the human limbus using spatial transcriptomic technology and reveal the deep interaction among the niche microenvironment.

Methods

The spatial transcriptomic atlas of human limbus was performed using 10× Genomics Space Ranger software platform. Single-cell RNA sequencing data of human limbal epithelial stem cells (LESCs) were downloaded for integrating analysis.

Results

We profiled more than 400 spots within each sample and spatially located major cell types within the limbus area. LESCs were localized mainly in the basement membrane, and limbal niche cells were situated predominantly within the stromal area. Next, the limbus was divided into four regions based on histological structure, and the differential expressed genes among the four regions were analyzed. Notably, GPHB5 was highly expressed in the epithelium of the middle region and co-staining with deltaNp63 suggested it might be a novel potential biomarker of LESCs. Subsequently, limbal mesenchymal stem cells were found to exhibit the greatest amounts of ligands associated with LESCs. The widespread activity of COL6A2/CD44 signaling among limbal mesenchymal stem cells, melanocytes, immune cells, and LESCs indicate its essential role in mediating bidirectional communication via the collagen pathway.

Conclusions

This research mapped the spatial positioning of key cells within the limbal niche and detailed interactions between major cell types. These findings provide a foundation for further LESC research and enhance our understanding of corneal biology.

Single-Cell Multiomics Analysis of Early Wound Response Programs in the Mouse Corneal Epithelium

Purpose

Wound healing is crucial for restoring homeostasis in living organisms. Although wound response mechanisms have been studied extensively, the gene regulatory programs involved remain to be elucidated. Here, we used single-cell RNA sequencing (RNA-seq) and ATAC sequencing (ATAC-seq) analysis to profile the regulatory landscape of mouse corneal epithelium in early wound response.

Methods

We used our previously published single-cell data sets of homeostatic adult mouse corneal epithelium as the unwounded group. The wounded group data sets were obtained by sequencing the epithelium after an annular epithelial wound. Following the integration of the relevant data sets, the Seurat and ArchR packages were employed for single-cell RNA-seq and single-cell ATAC-seq data processing and downstream analysis, respectively. The Monocle 2 was used for pseudo-time analysis, CellChat for intercellular communication analysis, and pySCENIC for analyzing transcription factors. The expression of key genes was validated via immunofluorescence staining and quantitative real-time PCR.

Results

Our data show that the number of cell type-specific genes decreases and the number of common transcriptional responses increases in early wound response. Concurrently, we find that the chromatin accessibility landscape undergoes significant changes across all epithelial cell types and that the wound-induced open regions are similarly distributed across the genome. Motif enrichment analysis shows that Fosl1/AP-1 binding site is highly enriched among the opened regions. However, by assessing the correlation between changes in chromatin accessibility and gene expression, we observe that only a small subset of wound-induced genes shows a high correlation with the accessibility of nearby chromatin.

Conclusions

Our study provides a detailed single-cell landscape for transcriptomic and epigenetic changes in mouse corneal epithelium during early wound response, which improved our understanding of the mechanisms of wound healing.

Transdifferentiation of rat keratinocyte progenitors to corneal epithelial cells by limbal niche via the STAT3/PI3K/AKT signaling pathway

PURPOSE

To develop a method for enriching keratinocyte progenitor cells (KPCs) and establish a limbal niche (LN)-mediated transdifferentiation protocol of KPCs into corneal epithelial cells.

METHODS

Limbal niche cells (LNCs) were isolated from limbal tissues through enzymatic digestion and characterized. Conditioned medium from LNCs cultures was collected. KPCs were enriched by rapid adhesion of Matrigel and subsequently cultured in either an LNCs-conditioned medium supplemented with KSFM (LN-KS) or SHEM (LN-SH) for 14 days. Corneal-specific marker expression was assessed to evaluate transdifferentiation efficiency. Key transcription factors and signaling pathways involved in the transdifferentiation process were identified through single-cell and RNA sequencing, and were validated by western blot and quantitative real-time PCR.

RESULTS

Both LN-KS and LN-SH protocols successfully induced corneal epithelial cell transdifferentiation from KPCs, with LN-KS demonstrating higher efficiency in generating CK12 + and p63 + cells (p < 0.001). RNA sequencing analysis and western blot have revealed significant activation of STAT3 and PI3K/AKT signaling pathways. Inhibition of STAT3 blocked the activation of PI3K/AKT signaling pathway and impaired corneal epithelial cell transdifferentiation.

CONCLUSIONS

This study demonstrates the ability of LN to promote KPCs transdifferentiation into corneal epithelial cells in vitro, and this process is partially mediated by the STAT3/PI3K/AKT signaling pathway.

Stem Cell-Associated Proteins and Extracellular Matrix Composition of the Human Atrioventricular Junction

The human heart regenerates slowly through life, but how new cells are generated is mostly unknown. The atrioventricular junction (AVj) has been indicated as a potential stem cell niche region. Little is known about the protein composition of the human AVj. To map the extracellular matrix (ECM) and expression of stem cell-related biomarkers, this study compares protein and gene expression patterns in AVj and Left Ventricular (LV) tissues. Biopsies were collected from 15 human hearts. Global quantitative proteomics and mRNA sequencing were used to identify differentially expressed proteins and altered genes. Of the total 4904 identified proteins, 1138 were differently expressed between the AVj and LV. While the top proteins in LV were involved in cardiac motor function and energy regulation, the AVj displayed proteins associated with early cardiomyocyte development, differentiation, proliferation, migration, and hypoxia. Furthermore, several developmental signalling pathways, including TGF-β, TNF, WNT, Notch, and FGF, were represented. RNA-seq data verified that the expressed genes were involved with differentiation, cell growth, proliferation, or ECM organization. Immunohistochemistry confirmed the expression of the stem cell-related biomarkers NPPA and POSTN in the AVj, further strengthening the hypothesis of the AVj as a specialized microenvironment conducive to stem cell niche activity.

A human-like model of aniridia-associated keratopathy for mechanistic and therapeutic studies

Aniridia is a rare congenital condition of abnormal eye development arising principally from heterozygous mutation of the PAX6 gene. Among the multiple complications arising in the eye, aniridia-associated keratopathy (AAK) is a severe vision-impairing condition of the cornea associated with a progressive limbal stem cell deficiency that lacks suitable treatment options. Current mouse models of aniridia do not accurately represent the onset and progression dynamics of human AAK, hindering therapy development. Here, we performed deep phenotyping of a haploinsufficient Pax6+/- small-eye (Sey) mouse model on the 129S1/SvImJ background, which exhibits key features of mild presentation at birth and progressive AAK with aging, mimicking human disease. The model exhibits a slowly progressing AAK phenotype and provides insights into the disease, including disturbed basal epithelial cell organization, function, and marker expression; persistent postnatal lymphangiogenesis; disrupted corneal innervation patterns; and persisting yet altered limbal stem cell marker expression with age. The model recapitulates many of the known features of human disease, enabling investigation of underlying disease mechanisms and, importantly, access to a well-defined temporal window for evaluating future therapeutics.

Foxp3+ regulatory T cells reside within the corneal epithelium and co-localize with limbal stem cells

In this study we investigated the presence of resident Foxp3+ regulatory T cells (Tregs) within the cornea and assessed the role of resident Tregs in corneal epithelial wound healing. Using a mouse model, we showed that in the steady state Foxp3+Tregs are either in close proximity or co-localize with ABCG2+ limbal stem cells. We also showed that these Tregs reside within the epithelial layer and not the corneal stroma. In addition, using a mouse model of mechanical injury, we demonstrated that depletion of Tregs from the cornea prior to corneal mechanical injury, using subconjunctival injection of anti-CD25, was associated with delayed epithelial healing. These results suggest a role for cornea resident Tregs in corneal epithelial cell function and wound healing and opens doors for further exploration of the role of Tregs in limbal stem cell function and survival.

De-Differentiation of Corneal Epithelial Cells Into Functional Limbal Epithelial Stem Cells After the Ablation of Innate Stem Cells

Purpose

Regeneration after tissue injury is often associated with cell fate plasticity, which restores damaged or lost cells. Here, we examined the de-differentiation of corneal epithelial cells (CECs) into functional limbal epithelial stem cells (LESCs) after the ablation of innate stem cells.

Methods

The regeneration of LESCs after the ablation of innate LESCs was identified by a set of markers: ApoE+/Cx43low/CK12-. CK14-CreERT2 or Slc1a3-CreERT mice were crossed with reporter mice to trace the fate of CECs. YAP-TEAD inhibitor verteporfin (VTP) and LATS inhibitor TRULI were used to examine the role of Hippo/YAP pathway in the de-differentiation of CECs.

Results

LESCs-ablation cornea showed to be functionally normal, including the maintenance of corneal transparency, prevention of conjunctivalization, and wound healing rate equivalent to that of normal cornea. ApoE+/Cx43low/CK12- LESCs regenerated at the limbus at 6 days after the ablation of innate stem cells, and maintained for at least 6 months. Corneal epithelial lineage tracing showed that CECs migrated back to the limbus after the ablation of innate stem cells, and de-differentiated into active and quiescent LESCs (aLESCs and qLESCs), which participated in corneal epithelial homeostasis and wound healing, respectively, like their innate counterparts. However, when the limbus niche was destroyed by NaOH (1 M, 5 seconds), CECs that occupied the limbus could not de-differentiate into ApoE+/Cx43low/CK12- LESCs and cornea developed into conjunctivalization. In addition, the protein level and activity of YAP increased at the early stage (1-2 days) after the ablation of limbal epithelium, and decreased when the de-differentiation occurred. The YAP-TEAD inhibitor VTP promoted the de-differentiation, whereas LATS inhibitor TRULI inhibited the de-differentiation of CECs. However, the persistent activation of YAP prevented the de-differentiation of CECs after an additional NaOH burn to the limbal stroma, and VTP could not rescue the capacity of CECs to de-differentiate into LESCs.

Conclusions

These results reveal the de-differentiation of CECs into functional LESCs after the ablation of innate stem cells, and suggest potential role of Hippo/YAP pathway in the de-differentiation of CECs in vivo.

A detailed survey of the murine limbus, its stem cell distribution, and its boundaries with the cornea and conjunctiva

The narrow intersection between the cornea and conjunctiva, otherwise known as the limbus, is purported to harbor stem cells (SCs) that replenish the ocular surface epithelium throughout life. Damage to this site or depletion of its SCs can have dire consequences for eye health and vision. To date, various SC and keratin proteins have been used to identify the limbus, however, none could definitively mark its boundaries. Herein, we use the mouse as a model system to investigate whether structural and phenotypic features can be used to define the limbus and its boundaries with adjacent tissues. We demonstrate that differentially aligned blood and lymphatic vessels, intraepithelial nerves, and basal epithelial cellular and nuclei dimensions can be used as structural landmarks of the limbus. Identification of these features enabled approximation of the limbal expanse, which varied across distinct ocular surface quadrants, with the superior nasal and inferior temporal limbus being the widest and narrowest, respectively. Moreover, label-retaining SCs were unevenly distributed across the ocular circumference, with increased numbers in the superior temporal and inferior temporal moieties. These findings will heighten our current understanding of the SC niche, be beneficial for accurately predicting SC distribution to improve their isolation and devising efficacious cell therapies, and importantly, aid the ongoing search for novel SC markers.

Epidemiology and tumor microenvironment of ocular surface and orbital tumors on growth and malignant transformation

The ocular surface and orbit constitute unique microenvironments in the human body. Current advances in molecular research have deepened our understanding of tumor development in these regions. Tumors exhibit greater heterogeneity compared to normal tissues, as revealed by pathological and histological examinations. The tumor microenvironment (TME) plays a crucial role in the proliferation and progression of cancer cells. Factors from the external environment or the body's own inflammation and microcirculation interact within the TME, maintaining a delicate balance. Disruption of this balance, through uncontrolled signal pathway activation, can transform normal or benign tissues into malignant ones. In recent years, various systemic immunotherapies have been developed for cancer treatment. This study reviews the epidemiology of ocular surface and orbital tumors include squamous cell carcinoma, basal cell carcinoma, sebaceous carcinoma and lymphoma in conjunction with their occurrence, growth, and underlying mechanisms. We propose that by examining clinical histopathological images, we can identify specific and shared microscopic features of tumors. By collecting, classifying, and analyzing data from these clinical histopathological images, we can pinpoint independent diagnostic factors characteristic of tumors. We hope this study provides a basis for future exploration of the mechanisms underlying different ocular diseases.

Development and characterization of a preclinical mouse model of alkali-induced limbal stem cell deficiency

PURPOSE

Limbal stem cell deficiency (LSCD) secondary to ocular surface alkali burn is a blinding condition that features corneal conjunctivalization. Mechanistic insights into its pathophysiology are lacking. Here, we developed a mouse model that recapitulates human disease to comprehensively delineate the clinicopathological features of a conjunctivalized cornea.

METHODS

LSCD was induced in the right eyes of 6-8-week-old C57BL/6 male and female mice (n = 151) by topical administration of 0.25N sodium hydroxide on the cornea. Uninjured left eyes served as controls. Clinical, histological, phenotypic, molecular, and immunological assessments were performed at multiple time-points over 6-months.

RESULTS

Clinically, alkali burn caused persistent corneal opacity (p = 0.0014), increased punctate staining (p = 0.0002), and reduced epithelial thickness (p = 0.0082) compared to controls. Total LSCD was confirmed in corneal whole mounts by loss of K12 protein (p < 0.0001) and mRNA expression (p = 0.0090). Instead, K8+, K13+, K15+ and MUC5AC+ conjunctival epithelia prevailed. 20 % of injured corneas developed islands of K12+ epithelia, suggesting epithelial transdifferentiation. Squamous metaplasia was detected in 50 % of injured corneas. Goblet cell density peaked early post-injury but decreased over time (p = 0.0047). Intraepithelial corneal basal nerve density remained reduced even at 6-months post-injury (p = 0.0487).

CONCLUSIONS

We developed and comprehensively characterized a preclinical mouse model of alkali-induced LSCD. Understanding the pathophysiological processes that transpire on the ocular surface in LSCD is key to discovering, testing, and advancing biological and pharmacological interventions that can be dispensed prior to or in conjunction with stem cell therapy to rehabilitate the cornea and restore vision.

Biologicals and Biomaterials for Corneal Regeneration and Vision Restoration in Limbal Stem Cell Deficiency

The mammalian cornea is decorated with stem cells bestowed with the life-long task of renewing the epithelium, provided they remain healthy, functional, and in sufficient numbers. If not, a debilitating disease known as limbal stem cell deficiency (LSCD) can develop causing blindness. Decades after the first stem cell (SC) therapy is devised to treat this condition, patients continue to suffer unacceptable failures. During this time, improvements to therapeutics have included identifying better markers to isolate robust SC populations and nurturing them on crudely modified biological or biomaterial scaffolds including human amniotic membrane, fibrin, and contact lenses, prior to their delivery. Researchers are now gathering information about the biomolecular and biomechanical properties of the corneal SC niche to decipher what biological and/or synthetic materials can be incorporated into these carriers. Advances in biomedical engineering including electrospinning and 3D bioprinting with surface functionalization and micropatterning, and self-assembly models, have generated a wealth of biocompatible, biodegradable, integrating scaffolds to choose from, some of which are being tested for their SC delivery capacity in the hope of improving clinical outcomes for patients with LSCD.

Restoration of corneal epithelial barrier function: A possible target for corneal neovascularization

Corneal neovascularization (CoNV) is the second leading common cause of vision impairment worldwide and is a blinding pathological alteration brought on by ocular trauma, infection, and other factors. There are some limitations in the treatment of CoNV, hence it's critical to look into novel therapeutic targets. The corneal epithelial barrier, which is the initial barrier of the ocular surface, is an important structure that shields the eye from changes in the internal environment or invasion by the external environment. This study sought to collate evidence on the regulation of corneal epithelial barrier injury on the activation of vascular endothelial cells (VECs), basement membrane (BM) degradation, differentiation, migration, and proliferation of VECs, vascular maturation and stability, and other key processes in CoNV, so as to provide a novel concept for CoNV therapy targeting corneal epithelial barrier repair.

Targeting limbal epithelial stem cells: master conductors of corneal epithelial regeneration from the bench to multilevel theranostics

The cornea is the outermost layer of the eye and plays an essential role in our visual system. Limbal epithelial stem cells (LESCs), which are localized to a highly regulated limbal niche, are the master conductors of corneal epithelial regeneration. Damage to LESCs and their niche may result in limbal stem cell deficiency (LSCD), a disease confused ophthalmologists so many years and can lead to corneal conjunctivalization, neovascularization, and even blindness. How to restore the LESCs function is the hot topic for ocular scientists and clinicians around the world. This review introduced LESCs and the niche microenvironment, outlined various techniques for isolating and culturing LESCs used in LSCD research, presented common diseases that cause LSCD, and provided a comprehensive overview of both the diagnosis and multiple treatments for LSCD from basic research to clinical therapies, especially the emerging cell therapies based on various stem cell sources. In addition, we also innovatively concluded the latest strategies in recent years, including exogenous drugs, tissue engineering, nanotechnology, exosome and gene therapy, as well as the ongoing clinical trials for treating LSCD in recent five years. Finally, we highlighted challenges from bench to bedside in LSCD and discussed cutting-edge areas in LSCD therapeutic research. We hope that this review could pave the way for future research and translation on treating LSCD, a crucial step in the field of ocular health.

Activation of limbal epithelial proliferation is partly controlled by the ACE2-LCN2 pathway

In response to corneal injury, an activation of corneal epithelial stem cells and their direct progeny the early transit amplifying (eTA) cells to rapidly proliferate is critical for proper re-epithelialization. Thus, it is important to understand how such stem/eTA cell activation is regulated. Angiotensin-converting enzyme 2 (ACE2) is predominantly expressed in the stem/eTA-enriched limbal epithelium but its role in the limbal epithelium was unclear. Single cell RNA sequencing (scRNA-seq) suggested that Ace2 involved the proliferation of the stem/eTA cells. Ace2 was reduced following corneal injury. Such reduction enhanced limbal epithelial proliferation and downregulated LCN2, a negative regulator of proliferation in a variety of tissues, via upregulating TGFA and consequently activating epidermal growth factor receptor (EGFR). Inhibition of EGFR or overexpression of LCN2 reversed the increased proliferation in limbal epithelial cells lacking ACE2. Our findings demonstrate that after corneal injury, ACE2 is downregulated, which activates limbal epithelial cell proliferation via a TGFA/EGFR/LCN2 pathway.

Limbal graft transplantation: a rare implementation in pediatric limbal stem cell deficiency

PURPOSE

To evaluate limbal graft transplantation success in pediatric patients with chemical injury-induced limbal stem cell deficiency (LSCD) using the 'LSCD Working Group' staging system.

METHODS

Medical records of 11 eyes of 11 children who underwent limbal graft transplantation (limbal autograft/limbal allograft) were included. Surgical success was defined as improvement in the post-operative 1st year LSCD stage.

RESULTS

The mean age was 12 ± 5 (4-17) years. Causative agent was alkaline in 4(36.4%) and acid in 3(27.2%) patients. Limbal autograft was performed in 9 (81.8%) eyes with unilateral LSCD, and allograft transplantation was performed in 2 (18.2%) eyes with bilateral LSCD. The mean follow-up time was 33.89 ± 30.73 (12-102.33) months. The overall limbal graft transplantation success rate was 72.7%. Among 9 patients who receive limbal autograft, 8 had improvement in post-operative LSCD stage, 1 had stable LSCD stage. Of the 2 patients who receive limbal allograft, post-operative LSCD stage remained the same in 1 and worsened in 1 patient. The mean time between injury and the surgery was 30.47 ± 30.08 (7-108.47) months. Penetrating keratoplasty was performed in 3 (27.2%) of 11 patients following limbal graft transplantation.

CONCLUSION

Management of LSCD in children is challenging and appears to be somewhat different from that of adults. Limited data in the literature indicate that cultivated or simple limbal epithelial transplantations (CLET/SLET) are primarily preferred in children. Although the tendency to take small tissue from the healthy eye is noteworthy, conventional limbal allograft and autograft transplantations also show promising results without any further complications in at least 1 year follow-up period.

Diesel exhaust exposure induced squamous metaplasia of corneal epithelium via yes-associated protein activation

Atmospheric pollution has been demonstrated to be associated with ocular surface diseases characterized by corneal epithelial damage, including impaired barrier function and squamous metaplasia. However, the specific mechanisms underlying the impact of atmospheric pollution on corneal damage are still unknow. To address this gap in knowledge, we conducted a study using a whole-body exposure system to investigate the detrimental effects of traffic-related air pollution, specifically diesel exhaust (DE), on corneal epithelium in C57BL/6 mice over a 28-day period. Following DE exposure, the pathological alterations in corneal epithelium, including significant increase in corneal thickness and epithelial stratification, were observed in mice. Additionally, exposure to DE was also shown to disrupt the barrier functions of corneal epithelium, leading to excessive proliferation of basal cells and even causing squamous metaplasia in corneal epithelium. Further studies have found that the activation of yes-associated protein (YAP), characterized by nuclear translocation, may play a significant role in DE-induced corneal squamous metaplasia. In vitro assays confirmed that DE exposure triggered the YAP/β-catenin pathway, resulting in squamous metaplasia and destruction of barrier functions. These findings provide the preliminary evidence that YAP activation is one of the mechanisms of the damage to corneal epithelium caused by traffic-related air pollution. These findings contribute to the knowledge base for promoting eye health in the context of atmospheric pollution.

Corneal Neurotization: Essentials for The Facial Paralysis Surgeon

Deficits in corneal innervation lead to neurotrophic keratopathy (NK). NK is frequently associated with facial palsy, and corneal damage can be accelerated by facial palsy deficits. Corneal nerves are important regulators of limbal stem cells, which play a critical role in epithelial maintenance and healing. Nonsurgical treatments of NK have undergone recent innovation, and growth factors implicated in corneal epithelial renewal are a promising therapeutic avenue. However, surgical intervention with corneal neurotization (CN) remains the only definitive treatment of NK. CN involves the transfer of unaffected sensory donor nerve branches to the affected cornea, and a variety of donor nerves and approaches have been described. CN can be performed in a direct or indirect manner; employ the supraorbital, supratrochlear, infraorbital, or great auricular nerves; and utilize autograft, allograft, or nerve transfer alone. Unfortunately, comparative studies of these factors are limited due to the procedure's novelty and varied recovery timelines after CN. Regardless of the chosen approach, CN has been shown to be a safe and effective procedure to restore corneal sensation and improve visual acuity in patients with NK.

Limbal stem cell therapy

PURPOSE OF REVIEW

To highlight the progress and future direction of limbal stem cell (LSC) therapies for the treatment of limbal stem cell deficiency (LSCD).

RECENT FINDINGS

Direct LSC transplantation have demonstrated good long-term outcomes. Cultivated limbal epithelial transplantation (CLET) has been an alternative to treat severe to total LSCD aiming to improve the safety and efficacy of the LSC transplant. A prospective early-stage uncontrolled clinical trial shows the feasibility and safety of CLET manufactured under xenobiotic free conditions. Other cell sources for repopulating of the corneal epithelium such as mesenchymal stem cells (MSCs) and induced pluripotent stem cells are being investigated. The first clinical trials of using MSCs showed short-term results, but long-term efficacy seems to be disappointing. A better understanding of the niche function and regulation of LSC survival and proliferation will lead to the development of medical therapies to rejuvenate the residual LSCs found in a majority of eyes with LSCD in vivo. Prior efforts have been largely focused on improving LSC transplantation. Additional effort should be placed on improving the accuracy of diagnosis and staging of LSCD, and implementing standardized outcome measures which enable comparison of efficacy of different LSCD treatments for different severity of LSCD. The choice of LSCD treatment will be customized based on the severity of LSCD in the future.

SUMMARY

New approaches for managing different stages of LSCD are being developed. This concise review summarizes the progresses in LSC therapies for LSCD, underlying mechanisms, limitations, and future areas of development.

Clinical and pathologic characterization of a mouse model of graded limbal stem cell deficiency

Limbal stem cell deficiency (LSCD) is a clinically challenging eye disease caused by damage to limbal stem cells (LSCs). Currently, the international consensus classifies LSCD into three clinical stages based on the disease severity. However, no existing animal models attempt to replicate the varying degrees of LSCD observed in clinical cases. The present study demonstrates an easy-to-create, reproducible, and reliable mouse model of graded LSCD. To achieve mild, moderate, or severe LSCD, filter paper rings with a variety of central angles (90°, 180°, or 270°) are utilized to deliver alkali burns to different sizes of the limbal area (1, 2, or 3 quarters). The animal model has successfully resulted in the development of clinical signs and pathological manifestations in escalating severity that are similarly observed in the three clinical stages of LSCD. Our study thus provides new insights into distinct pathological features underlying different grades of LSCD and serves as a new tool for further exploring the disease mechanisms and developing new effective therapeutics for repairing damaged LSCs.

Corneal injury repair and the potential involvement of ZEB1

The cornea, consisting of three cellular and two non-cellular layers, is the outermost part of the eyeball and frequently injured by external physical, chemical, and microbial insults. The epithelial-to-mesenchymal transition (EMT) plays a crucial role in the repair of corneal injuries. Zinc finger E-box binding homeobox 1 (ZEB1), an important transcription factor involved in EMT, is expressed in the corneal tissues. It regulates cell activities like migration, transformation, and proliferation, and thereby affects tissue inflammation, fibrosis, tumor metastasis, and necrosis by mediating various major signaling pathways, including transforming growth factor (TGF)-β. Dysfunction of ZEB1 would impair corneal tissue repair leading to epithelial healing delay, interstitial fibrosis, neovascularization, and squamous cell metaplasia. Understanding the mechanism underlying ZEB1 regulation of corneal injury repair will help us to formulate a therapeutic approach to enhance corneal injury repair.

Glycoprotein hormone subunit alpha 2 (GPHA2): A pituitary stem cell-expressed gene associated with NOTCH2 signaling

NOTCH2 is expressed in pituitary stem cells and is necessary for stem cell maintenance, proliferation, and differentiation. However, the pathways NOTCH2 engages to affect pituitary development remain unclear. In this study, we hypothesized that glycoprotein hormone subunit A2 (GPHA2), a corneal stem cell factor and ligand for the thyroid stimulating hormone receptor (TSHR), is downstream of NOTCH2 signaling. We found Gpha2 is expressed in quiescent pituitary stem cells by RNAscope in situ hybridization and scRNA seq. In Notch2 conditional knockout pituitaries, Gpha2 mRNA is reduced compared with control littermates. We then investigated the possible functions of GPHA2. Pituitaries treated with a GPHA2 peptide do not have a change in proliferation. However, in dissociated adult pituitary cells, GPHA2 increased pCREB expression and this induction was reversed by co-treatment with a TSHR inhibitor. These data suggest GPHA2 is a NOTCH2 related stem cell factor that activates TSHR signaling, potentially impacting pituitary development.

Sox9 marks limbal stem cells and is required for asymmetric cell fate switch in the corneal epithelium

Adult tissues with high cellular turnover require a balance between stem cell renewal and differentiation, yet the mechanisms underlying this equilibrium are unclear. The cornea exhibits a polarized lateral flow of progenitors from the peripheral stem cell niche to the center; attributed to differences in cellular fate. To identify genes that are critical for regulating the asymmetric fates of limbal stem cells and their transient amplified progeny in the central cornea, we utilized an in vivo cell cycle reporter to isolate proliferating basal cells across the anterior ocular surface epithelium and performed single-cell transcriptional analysis. This strategy greatly increased the resolution and revealed distinct basal cell identities with unique expression profiles of structural genes and transcription factors. We focused on Sox9; a transcription factor implicated in stem cell regulation across various organs. Sox9 was found to be differentially expressed between limbal stem cells and their progeny in the central corneal. Lineage tracing analysis confirmed that Sox9 marks long-lived limbal stem cells and conditional deletion led to abnormal differentiation and squamous metaplasia in the central cornea. These data suggest a requirement for Sox9 for the switch to asymmetric fate and commitment toward differentiation, as transient cells exit the limbal niche. By inhibiting terminal differentiation of corneal progenitors and forcing them into perpetual symmetric divisions, we replicated the Sox9 loss-of-function phenotype. Our findings reveal an essential role for Sox9 for the spatial regulation of asymmetric fate in the corneal epithelium that is required to sustain tissue homeostasis.

The Potential Reversible Transition between Stem Cells and Transient-Amplifying Cells: The Limbal Epithelial Stem Cell Perspective

Stem cells (SCs) undergo asymmetric division, producing transit-amplifying cells (TACs) with increased proliferative potential that move into tissues and ultimately differentiate into a specialized cell type. Thus, TACs represent an intermediary state between stem cells and differentiated cells. In the cornea, a population of stem cells resides in the limbal region, named the limbal epithelial stem cells (LESCs). As LESCs proliferate, they generate TACs that move centripetally into the cornea and differentiate into corneal epithelial cells. Upon limbal injury, research suggests a population of progenitor-like cells that exists within the cornea can move centrifugally into the limbus, where they dedifferentiate into LESCs. Herein, we summarize recent advances made in understanding the mechanism that governs the differentiation of LESCs into TACs, and thereafter, into corneal epithelial cells. We also outline the evidence in support of the existence of progenitor-like cells in the cornea and whether TACs could represent a population of cells with progenitor-like capabilities within the cornea. Furthermore, to gain further insights into the dynamics of TACs in the cornea, we outline the most recent findings in other organ systems that support the hypothesis that TACs can dedifferentiate into SCs.

Squishy matters - Corneal mechanobiology in health and disease

The cornea, as a dynamic and responsive tissue, constantly interacts with mechanical forces in order to maintain its structural integrity, barrier function, transparency and refractive power. Cells within the cornea sense and respond to various mechanical forces that fundamentally regulate their morphology and fate in development, homeostasis and pathophysiology. Corneal cells also dynamically regulate their extracellular matrix (ECM) with ensuing cell-ECM crosstalk as the matrix serves as a dynamic signaling reservoir providing biophysical and biochemical cues to corneal cells. Here we provide an overview of mechanotransduction signaling pathways then delve into the recent advances in corneal mechanobiology, focusing on the interplay between mechanical forces and responses of the corneal epithelial, stromal, and endothelial cells. We also identify species-specific differences in corneal biomechanics and mechanotransduction to facilitate identification of optimal animal models to study corneal wound healing, disease, and novel therapeutic interventions. Finally, we identify key knowledge gaps and therapeutic opportunities in corneal mechanobiology that are pressing for the research community to address especially pertinent within the domains of limbal stem cell deficiency, keratoconus and Fuchs' endothelial corneal dystrophy. By furthering our understanding corneal mechanobiology, we can contextualize discoveries regarding corneal diseases as well as innovative treatments for them.

The Role of Insulin-like Growth Factor (IGF) System in the Corneal Epithelium Homeostasis-From Limbal Epithelial Stem Cells to Therapeutic Applications

The corneal epithelium, comprising three layers of cells, represents the outermost portion of the eye and functions as a vital protective barrier while concurrently serving as a critical refractive structure. Maintaining its homeostasis involves a complex regenerative process facilitated by the functions of the lacrimal gland, tear film, and corneal nerves. Crucially, limbal epithelial stem cells located in the limbus (transitional zone between the cornea and the conjunctiva) are instrumental for the corneal epithelium integrity by replenishing and renewing cells. Re-epithelialization failure results in persistent defects, often associated with various ocular conditions including diabetic keratopathy. The insulin-like growth factor (IGF) system is a sophisticated network of insulin and other proteins essential for numerous physiological processes. This review examines its role in maintaining the corneal epithelium homeostasis, with a special focus on the interplay with corneal limbal stem cells and the potential therapeutic applications of the system components.

The single-cell transcriptomic atlas and RORA-mediated 3D epigenomic remodeling in driving corneal epithelial differentiation

Proper differentiation of corneal epithelial cells (CECs) from limbal stem/progenitor cells (LSCs) is required for maintenance of ocular homeostasis and clear vision. Here, using a single-cell transcriptomic atlas, we delineate the comprehensive and refined molecular regulatory dynamics during human CEC development and differentiation. We find that RORA is a CEC-specific molecular switch that initiates and drives LSCs to differentiate into mature CECs by activating PITX1. RORA dictates CEC differentiation by establishing CEC-specific enhancers and chromatin interactions between CEC gene promoters and distal regulatory elements. Conversely, RORA silences LSC-specific promoters and disrupts promoter-anchored chromatin loops to turn off LSC genes. Collectively, our work provides detailed and comprehensive insights into the transcriptional dynamics and RORA-mediated epigenetic remodeling underlying human corneal epithelial differentiation.

Dopamine Receptor 1 Treatment Promotes Epithelial Repair of Corneal Injury by Inhibiting NOD-Like Receptor Protein 3-Associated Inflammation

Purpose

To elucidate the influence of dopamine receptor 1 (DRD1) on the proliferation of mouse corneal epithelial cells (MCECs) under inflammatory conditions.

Methods

In vitro, immortalized MCECs (iMCECs) were treated with IL-1β, with and without pcDNA3.1_DRD1. Primary MCECs (pMCECs) were exposed to IL-1β, with and without DRD1 agonist (A68930). Cell proliferation was quantified using the Cell Counting Kit-8 (CCK-8) assay and immunofluorescence staining for Ki-67 and p63. Expression levels of NOD-like receptor protein 3 (NLRP3), IL-1β, and IL-6 were assessed. To establish a corneal injury model in mice, a 2-mm superficial keratectomy was performed. Either 0.1% A68930 or PBS was topically administered three times daily to the injured eyes for up to 5 days post-injury. Immunofluorescence analysis was employed to evaluate the expression of Ki-67, p63, and CD45 in mouse corneas. Western blotting and real-time quantitative PCR were utilized for quantitative analysis of DRD1, NLRP3, IL-1β, and IL-6 in mouse corneas. Corneal epithelial regeneration was monitored through fluorescein sodium staining for a duration of up to 5 days following the injury.

Results

Overexpression of DRD1 and A68930 promoted MCEC proliferation and suppressed the expression of NLRP3, IL-1β, and IL-6 in vitro. Topical application of the 0.1% A68930 following mechanical corneal injury in mice led to increased Ki-67 and p63 expression compared to PBS treatment. Furthermore, topical administration of the 0.1% A68930 reduced the expression of CD45, NLRP3, IL-1β, and IL-6. Analysis with fluorescein sodium indicated accelerated corneal epithelial regeneration in the 0.1% A68930 treatment group.

Conclusions

DRD1 treatment counteracts NLRP3-associated inflammation and facilitates epithelial repair of corneal injury.

Single-cell transcriptomic analysis of corneal organoids during development

Corneal organoids are useful tools for disease modeling and tissue transplantation; however, they have not yet been well studied during maturation. We characterized human iPSC-derived corneal organoids at 1, 2, 3, and 4 months of development using single-cell RNA sequencing to determine the cellular heterogeneity at each stage. We found pluripotent cell clusters committed to epithelial cell lineage at 1 month; early corneal epithelial, endothelial, and stromal cell markers at 2 months; keratocytes as the largest cell population at 3 months; and a large epithelial cell population at 4 months. We compared organoid to fetal corneal development at different stages and found that 4-month organoids closely resemble the corneal cellular complexity of the fetal (16 post conception week) and adult cornea. Using RNA velocity trajectory analysis, we found that less differentiated cells appear to give rise to corneal epithelial cells during development.

Eyes open on stem cells

Recently, the murine cornea has reemerged as a robust stem cell (SC) model, allowing individual SC tracing in living animals. The cornea has pioneered seminal discoveries in SC biology and regenerative medicine, from the first corneal transplantation in 1905 to the identification of limbal SCs and their transplantation to successfully restore vision in the early 1990s. Recent experiments have exposed unexpected properties attributed to SCs and progenitors and revealed flexibility in the differentiation program and a key role for the SC niche. Here, we discuss the limbal SC model and its broader relevance to other tissues, disease, and therapy.

Single-cell transcriptomics of the ocular anterior segment: a comprehensive review

Elucidating the cellular and genetic composition of ocular tissues is essential for uncovering the pathophysiology of ocular diseases. Since the introduction of single-cell RNA sequencing (scRNA-seq) in 2009, vision researchers have performed extensive single-cell analyses to better understand transcriptome complexity and heterogeneity of ocular structures. This technology has revolutionized our ability to identify rare cell populations and to make cross-species comparisons of gene expression in both steady state and disease conditions. Importantly, single-cell transcriptomic analyses have enabled the identification of cell-type specific gene markers and signalling pathways between ocular cell populations. While most scRNA-seq studies have been conducted on retinal tissues, large-scale transcriptomic atlases pertaining to the ocular anterior segment have also been constructed in the past three years. This timely review provides vision researchers with an overview of scRNA-seq experimental design, technical limitations, and clinical applications in a variety of anterior segment-related ocular pathologies. We review open-access anterior segment-related scRNA-seq datasets and illustrate how scRNA-seq can be an indispensable tool for the development of targeted therapeutics.

Toward Corneal Limbus In Vitro Model: Regulation of hPSC-LSC Phenotype by Matrix Stiffness and Topography During Cell Differentiation Process

A functional limbal epithelial stem cells (LSC) niche is a vital element in the regular renewal of the corneal epithelium by LSCs and maintenance of good vision. However, little is known about its unique structure and mechanical properties on LSC regulation, creating a significant gap in development of LSC-based therapies. Herein, the effect of mechanical and architectural elements of the niche on human pluripotent derived LSCs (hPSC-LSC) phenotype and growth is investigated in vitro. Specifically, three formulations of polyacrylamide gels with different controlled stiffnesses are used for culture and characterization of hPSC-LSCs from different stages of differentiation. In addition, limbal mimicking topography in polydimethylsiloxane is utilized for culturing hPSC-LSCs at early time point of differentiation. For comparison, the expression of selected key proteins of the corneal cells is analyzed in their native environment through whole mount staining of human donor corneas. The results suggest that mechanical response and substrate preference of the cells is highly dependent on their developmental stage. In addition, data indicate that cells may carry possible mechanical memory from previous culture matrix, both highlighting the importance of mechanical design of a functional in vitro limbus model.

Limbal Epithelial Stem Cells in the Diabetic Cornea

Continuous replenishment of the corneal epithelium is pivotal for maintaining optical transparency and achieving optimal visual perception. This dynamic process is driven by limbal epithelial stem cells (LESCs) located at the junction between the cornea and conjunctiva, which is otherwise known as the limbus. In patients afflicted with diabetes, hyperglycemia-induced impairments in corneal epithelial regeneration results in persistent epithelial and other defects on the ocular surface, termed diabetic keratopathy (DK), which progressively diminish vision and quality of life. Reports of delayed corneal wound healing and the reduced expression of putative stem cell markers in diabetic relative to healthy eyes suggest that the pathogenesis of DK may be associated with the abnormal activity of LESCs. However, the precise role of these cells in diabetic corneal disease is poorly understood and yet to be comprehensively explored. Herein, we review existing literature highlighting aberrant LESC activity in diabetes, focusing on factors that influence their form and function, and emerging therapies to correct these defects. The consequences of malfunctioning or depleted LESC stocks in DK and limbal stem cell deficiency (LSCD) are also discussed. These insights could be exploited to identify novel targets for improving the management of ocular surface complications that manifest in patients with diabetes.

Single-Cell Transcriptomics Reveals Cellular Heterogeneity and Complex Cell-Cell Communication Networks in the Mouse Cornea

Purpose

To generate a single-cell RNA-sequencing (scRNA-seq) map and construct cell-cell communication networks of mouse corneas.

Methods

C57BL/6 mouse corneas were dissociated to single cells and subjected to scRNA-seq. Cell populations were clustered and annotated for bioinformatic analysis using the R package "Seurat." Differential expression patterns were validated and spatially mapped with whole-mount immunofluorescence staining. Global intercellular signaling networks were constructed using CellChat.

Results

Unbiased clustering of scRNA-seq transcriptomes of 14,732 cells from 40 corneas revealed 17 cell clusters of six major cell types: nine epithelial cell, three keratocyte, two corneal endothelial cell, and one each of immune cell, vascular endothelial cell, and fibroblast clusters. The nine epithelial cell subtypes included quiescent limbal stem cells, transit-amplifying cells, and differentiated cells from corneas and two minor conjunctival epithelial clusters. CellChat analysis provided an atlas of the complex intercellular signaling communications among all cell types.

Conclusions

We constructed a complete single-cell transcriptomic map and the complex signaling cross-talk among all cell types of the cornea, which can be used as a foundation atlas for further research on the cornea. This study also deepens the understanding of the cellular heterogeneity and heterotypic cell-cell interaction within corneas.

Single-cell RNA sequencing of retina revealed novel transcriptional landscape in high myopia and underlying cell-type-specific mechanisms

High myopia is a leading cause of blindness worldwide with increasing prevalence. Retina percepts visual information and triggers myopia development, but the underlying etiology is not fully understood because of cellular heterogeneity. In this study, single-cell RNA sequencing analysis was performed on retinas of mouse highly myopic and control eyes to dissect the involvement of each cell type during high myopia progression. For highly myopic photoreceptors, Hk2 inhibition underlying metabolic remodeling from aerobic glycolysis toward oxidative phosphorylation and excessive oxidative stress was identified. Importantly, a novel Apoe + rod subpopulation was specifically identified in highly myopic retina. In retinal neurons of highly myopic eyes, neurodegeneration was generally discovered, and the imbalanced ON/OFF signaling driven by cone-bipolar cells and the downregulated dopamine receptors in amacrine cells were among the most predominant findings, indicating the aberrant light processing in highly myopic eyes. Besides, microglia exhibited elevated expression of cytokines and TGF-β receptors, suggesting enhanced responses to inflammation and the growth-promoting states involved in high myopia progression. Furthermore, cell-cell communication network revealed attenuated neuronal interactions and increased glial/vascular interactions in highly myopic retinas. In conclusion, this study outlines the transcriptional landscape of highly myopic retina, providing novel insights into high myopia development and prevention.

The miR-183/96/182 cluster is a checkpoint for resident immune cells and shapes the cellular landscape of the cornea

PURPOSE

The conserved miR-183/96/182 cluster (miR-183C) regulates both corneal sensory innervation and corneal resident immune cells (CRICs). This study is to uncover its role in CRICs and in shaping the corneal cellular landscape at a single-cell (sc) level.

METHODS

Corneas of naïve, young adult [2 and 6 months old (mo)], female miR-183C knockout (KO) mice and wild-type (WT) littermates were harvested and dissociated into single cells. Dead cells were removed using a Dead Cell Removal kit. CD45+ CRICs were enriched by Magnetic Activated Cell Sorting (MACS). scRNA libraries were constructed and sequenced followed by comprehensive bioinformatic analyses.

RESULTS

The composition of major cell types of the cornea stays relatively stable in WT mice from 2 to 6 mo, however the compositions of subtypes of corneal cells shift with age. Inactivation of miR-183C disrupts the stability of the major cell-type composition and age-related transcriptomic shifts of subtypes of corneal cells. The diversity of CRICs is enhanced with age. Naïve mouse cornea contains previously-unrecognized resident fibrocytes and neutrophils. Resident macrophages (ResMφ) adopt cornea-specific function by expressing abundant extracellular matrix (ECM) and ECM organization-related genes. Naïve cornea is endowed with partially-differentiated proliferative ResMφ and contains microglia-like Mφ. Resident lymphocytes, including innate lymphoid cells (ILCs), NKT and γδT cells, are the major source of innate IL-17a. miR-183C limits the diversity and polarity of ResMφ.

CONCLUSION

miR-183C serves as a checkpoint for CRICs and imposes a global regulation of the cellular landscape of the cornea.

Long-term PM2.5 exposure disrupts corneal epithelial homeostasis by impairing limbal stem/progenitor cells in humans and rat models

BACKGROUND

Limbal stem/progenitor cells (LSPCs) play a crucial role in maintaining corneal health by regulating epithelial homeostasis. Although PM2.5 is associated with the occurrence of several corneal diseases, its effects on LSPCs are not clearly understood.

METHODS

In this study, we explored the correlation between PM2.5 exposure and human limbal epithelial thickness measured by Fourier-domain Optical Coherence Tomography in the ophthalmologic clinic. Long- and short-term PM2.5 exposed-rat models were established to investigate the changes in LSPCs and the associated mechanisms.

RESULTS

We found that people living in regions with higher PM2.5 concentrations had thinner limbal epithelium, indicating the loss of LSPCs. In rat models, long-term PM2.5 exposure impairs LSPCs renewal and differentiation, manifesting as corneal epithelial defects and thinner epithelium in the cornea and limbus. However, LSPCs were activated in short-term PM2.5-exposed rat models. RNA sequencing implied that the circadian rhythm in LSPCs was perturbed during PM2.5 exposure. The mRNA level of circadian genes including Per1, Per2, Per3, and Rev-erbα was upregulated in both short- and long-term models, suggesting circadian rhythm was involved in the activation and dysregulation of LSPCs at different stages. PM2.5 also disturbed the limbal microenvironment as evidenced by changes in corneal subbasal nerve fiber density, vascular density and permeability, and immune cell infiltration, which further resulted in the circadian mismatches and dysfunction of LSPCs.

CONCLUSION

This study systematically demonstrates that PM2.5 impairs LSPCs and their microenvironment. Moreover, we show that circadian misalignment of LSPCs may be a new mechanism by which PM2.5 induces corneal diseases. Therapeutic options that target circadian rhythm may be viable options for improving LSPC functions and alleviating various PM2.5-associated corneal diseases.

Mass cytometry reveals the corneal immune cell changes at single cell level in diabetic mice

Introduction

Diabetic ocular complications include sight-threatening consequences and decreased corneal sensitivity, characterized by decreased tear production, corneal sensitivity and delayed corneal epithelial wound healing. The pathogenesis of diabetic corneal disorders remains largely unknown. Growing evidence implies the participation of immune cells in the development of diabetic corneal diseases. Nonetheless, the immunological changes that result in diabetic corneal problems are largely unknown.

Methods

Mass cytometry by time of flight (CyTOF) was used to investigate immune cell cluster alterations associated with diabetic corneal disorders. CyTOF test was performed on corneal cells at a single level from 21-week-old diabetic (db/db) and non-diabetic (db/m) mice. A panel of 41 immune-related markers monitored different immune cell types in diabetic corneas. To investigate the proportion of each immune cell subpopulation, an unsupervised clustering method was employed, and T-distributed stochastic neighbor embedding was used to visualize the distinctions between different immune cell subsets.

Results

Through CyTOF test, we identified 10 immune cell subsets in the corneal tissues. In a novel way, we discovered significant immune alterations in diabetic corneas, including pronounced alterations in T cells and myeloid cell subgroups in diabetic corneas linked to potential biomarkers, including CD103, CCR2, SiglecF, Ly6G, and CD172a. Comprehensive immunological profiling indicated remarkable changes in the immune microenvironment in diabetic corneas, characterized by a notable decrease in CD103+CD8+ tissue-resident memory T (TRM) cells and Tregs, as well as a dramatic increase of γδT cells and subsets of CD11b+Ly6G+ myeloid-derived suppressor cells (MDSCs).

Conclusion

CyTOF analysis revealed significant alterations in the immune microenvironment during the development of diabetic corneal complications. This study mapped the immune microenvironment landscape of type 2 diabetic corneas, providing a fundamental understanding of immune-driven diabetic corneal disorders.

Single-cell transcriptomics implicates the FEZ1-DKK1 axis in the regulation of corneal epithelial cell proliferation and senescence

Limbal stem/progenitor cells (LSC) represent the source of corneal epithelium renewal. LSC proliferation and differentiation are essential for corneal homeostasis, however, the regulatory mechanism remains largely unexplored. Here, we performed single-cell RNA sequencing and discovered proliferation heterogeneity as well as spontaneously differentiated and senescent cell subgroups in multiply passaged primary LSC. Fasciculation and elongation protein zeta 1 (FEZ1) and Dickkopf-1 (DKK1) were identified as two significant regulators of LSC proliferation and senescence. These two factors were mainly expressed in undifferentiated corneal epithelial cells (CECs). Knocking down the expression of either FEZ1 or DKK1 reduced cell division and caused cell cycle arrest. We observed that DKK1 acted as a downstream target of FEZ1 in LSC and that exogenous DKK1 protein partially prevented growth arrest and senescence upon FEZ1 suppression in vitro. In a mouse model of corneal injury, DKK1 also rescued the corneal epithelium after recovery was inhibited by FEZ1 suppression. Hence, the FEZ1-DKK1 axis was required for CEC proliferation and the juvenile state and can potentially be targeted as a therapeutic strategy for promoting recovery after corneal injury.

Sympathetic Nerves Coordinate Corneal Epithelial Wound Healing by Controlling the Mobilization of Ly6Chi Monocytes From the Spleen to the Injured Cornea

Purpose

This study aims to investigate the potential involvement of spleen-derived monocytes in the repair process following corneal epithelial abrasion.

Methods

A corneal epithelial abrasion model was established in male C57BL/6J mice, and the dynamic changes of monocyte subpopulations in the injured cornea were analyzed using flow cytometry. The effects of Ly6Chi monocyte depletion and local adoptive transfer of purified Ly6Chi monocytes on wound closure and neutrophil recruitment to the injured cornea were observed. The effect of sympathetic nerves on the recruitment of spleen-derived Ly6Chi monocytes to the injured cornea was also investigated using multiple methods. The emigration of fluorescence-labeled monocytes to the injured cornea was validated through intravital microscopy. Finally, differential genes between different groups were identified through high-throughput RNA sequencing and analyzed for functional enrichment, followed by verification by quantitative PCR.

Results

Ly6Chi monocytes were present in large numbers in the injured cornea prior to neutrophil recruitment. Predepletion of Ly6Chi monocytes significantly inhibited neutrophil recruitment to the injured cornea. Furthermore, surgical removal of the spleen significantly reduced the number of Ly6Chi monocytes in the injured cornea. Further observations revealed that sympathetic blockade significantly reduced the number of Ly6Chi monocytes recruited to the injured cornea. In contrast, administration of the β2-adrenergic receptor agonist significantly increased the number of Ly6Chi monocytes recruited to the injured cornea in animals treated with sympathectomy and catecholamine synthesis inhibition.

Conclusions

Our results suggest that spleen-derived Ly6Chi monocytes, under the control of the sympathetic nervous system, play a critical role in the inflammatory response following corneal injury.