Identification and characterization of limbal stem cells

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 extracellular cyclophilin A-integrin β2 complex as a therapeutic target of viral pneumonia

The acute respiratory virus infection can induce uncontrolled inflammatory responses, such as cytokine storm and viral pneumonia, which are the major causes of death in clinical cases. Cyclophilin A (CypA) is mainly distributed in the cytoplasm of resting cells and released into the extracellular space in response to inflammatory stimuli. Extracellular CypA (eCypA) is upregulated and promotes inflammatory response in severe COVID-19 patients. However, how eCypA promotes virus-induced inflammatory response remains elusive. Here, we observe that eCypA is induced by influenza A and B viruses and SARS-CoV-2 in cells, mice, or patients. Anti-CypA mAb reduces pro-inflammatory cytokines production, leukocytes infiltration, and lung injury in virus-infected mice. Mechanistically, eCypA binding to integrin β2 triggers integrin activation, thereby facilitating leukocyte trafficking and cytokines production via the focal adhesion kinase (FAK)/GTPase and FAK/ERK/P65 pathways, respectively. These functions are suppressed by the anti-CypA mAb that specifically blocks eCypA-integrin β2 interaction. Overall, our findings reveal that eCypA-integrin β2 signaling mediates virus-induced inflammatory response, indicating that eCypA is a potential target for antibody therapy against viral pneumonia.

A Biosynthetic Alternative to Human Amniotic Membrane for Use in Ocular Surface Surgery

Purpose

The biosynthetic Symatix membrane (SM) was developed to replace fresh human amniotic membrane (hAM) in ocular surgical applications. The purpose of this study was to test the biocompatibility of the SM with human limbus-derived epithelial cells with regard to their physical and biological properties.

Methods

Different physical properties of SM were tested ex vivo by simulation on human corneas. In vitro, primary limbal epithelial cells from limbal explants were used to test biological properties such as cell migration, proliferation, metabolic activity, and limbal epithelial cell markers on the SM, hAM, and freeze-dried amniotic membrane (FDAM).

Results

The surgical handleability of the SM was equivalent to that of the hAM. Ultrastructural and histological studies demonstrated that epithelial cells on the SM had the typical tightly apposed, polygonal, corneal epithelial cell morphology. The epithelial cells were well stratified on the SM, unlike on the hAM and FDAM. Rapid wound healing occurred on the SM within 3 days. Immunofluorescence studies showed positive expression of CK-19, Col-1, laminin, ZO-1, FN, and p-63 on the SM, plastic, and FDAM compared to positive expression of ZO-1, Col-1, laminin, FN, and p63 and negative expression of CK-19 in the hAM.

Conclusions

These results indicate that the SM is a better substrate for limbal epithelial cell migration, proliferation, and tight junction formation. Altogether, the SM can provide a suitable alternative to the hAM for surgical application in sight-restoring operations.

Translational Relevance

The hAM, currently widely used in ocular surface surgery, has numerous variations and limitations. The biocompatibility of corneal epithelial cells with the SM demonstrated in this study suggests that it can be a viable substitute for the hAM.

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.

Single nuclei transcriptomics of the in situ human limbal stem cell niche

The corneal epithelium acts as a barrier to pathogens entering the eye; corneal epithelial cells are continuously renewed by uni-potent, quiescent limbal stem cells (LSCs) located at the limbus, where the cornea transitions to conjunctiva. There has yet to be a consensus on LSC markers and their transcriptome profile is not fully understood, which may be due to using cadaveric tissue without an intact stem cell niche for transcriptomics. In this study, we addressed this problem by using single nuclei RNA sequencing (snRNAseq) on healthy human limbal tissue that was immediately snap-frozen after excision from patients undergoing cataract surgery. We identified the quiescent LSCs as a sub-population of corneal epithelial cells with a low level of total transcript counts. Moreover, TP63, KRT15, CXCL14, and ITGβ4 were found to be highly expressed in LSCs and transiently amplifying cells (TACs), which constitute the corneal epithelial progenitor populations at the limbus. The surface markers SLC6A6 and ITGβ4 could be used to enrich human corneal epithelial cell progenitors, which were also found to specifically express the putative limbal progenitor cell markers MMP10 and AC093496.1.

New characterization and safety evaluation of human limbal stem cells used in clinical application: fidelity of mitotic process and mitotic spindle morphologies

BACKGROUND

Limbal stem cells (LSCs) are crucial for the regeneration of the corneal epithelium in patients with limbal stem cell deficiency (LSCD). Thus, LSCs during cultivation in vitro should be in highly homogeneous amounts, while potency and expression of stemness without tumorigenesis would be desirable. Therefore, further characterization and safety evaluation of engineered limbal grafts is required to provide safe and high-quality therapeutic applications.

METHODS

After in vitro expansion, LSCs undergo laboratory characterization in a single-cell suspension, cell culture, and in limbal grafts before transplantation. Using a clinically applicable protocol, the data collected on LSCs at passage 1 were summarized, including: identity (cell size, morphology); potency (yield, viability, population doubling time, colony-forming efficiency); expression of putative stem cell markers through flow cytometry, immunofluorescence, and immunohistochemistry. Then, mitotic chromosome stability and normal mitotic outcomes were explored by using live-cell imaging. Finally, impurities, bacterial endotoxins and sterility were determined.

RESULTS

Expression of the stemness marker p63 in single-cell suspension and in cell culture showed high values by different methods. Limbal grafts showed p63-positive cells (78.7 ± 9.4%), Ki67 proliferation (41.7 ± 15.9%), while CK3 was negative. Impurity with 3T3 feeder cells and endotoxins was minimized. We presented mitotic spindles with a length of 11.40 ± 0.54 m and a spindle width of 8.05 ± 0.55 m as new characterization in LSC culture. Additionally, live-cell imaging of LSCs (n = 873) was performed, and only a small fraction < 2.5% of aberrant interphase cells was observed; 2.12 ± 2.10% of mitotic spindles exhibited a multipolar phenotype during metaphase, and 3.84 ± 3.77% of anaphase cells had a DNA signal present within the spindle midzone, indicating a chromosome bridge or lagging chromosome phenotype.

CONCLUSION

This manuscript provides, for the first time, detailed characterization of the parameters of fidelity of the mitotic process and mitotic spindle morphologies of LSCs used in a direct clinical application. Our data show that p63-positive CK3-negative LSCs grown in vitro for clinical purposes undergo mitotic processes with extremely high fidelity, suggesting high karyotype stability. This finding confirms LSCs as a high-quality and safe therapy for eye regeneration in humans.

A Review of Contact Lens-Induced Limbal Stem Cell Deficiency

Limbal stem cell deficiency (LSCD) is a pathologic condition caused by the dysfunction and destruction of stem cells, stem cell precursors and limbal cell niche in the corneal epithelium, leading to severe conjunctivalization of the cornea. Etiologies for LSCD span from congenital (aniridia), traumatic (chemical or thermal injuries), autoimmune (Stevens-Johnson syndrome) and iatrogenic disease to contact lens (CL) wear. Of these, CL wear is the least understood and is often a subclinical cause of LSCD. Even with recent advances in LSCD research, limitations persist in establishing the pathogenesis and treatment guidelines for CL-induced LSCD. A literature search was conducted to include original articles containing patients with CL-induced LSCD. This review will critically discuss the complex pathophysiology behind CL-induced LSCD, the underlying risk factors and epidemiology of the disease as well as methods to obtain a diagnosis. Various treatment options will be reviewed based on proposed treatment strategies.

Influence of Organ Culture on the Characteristics of the Human Limbal Stem Cell Niche

Organ culture storage techniques for corneoscleral limbal (CSL) tissue have improved the quality of corneas for transplantation and allow for longer storage times. Cultured limbal tissue has been used for stem cell transplantation to treat limbal stem cell deficiency (LSCD) as well as for research purposes to assess homeostasis mechanisms in the limbal stem cell niche. However, the effects of organ culture storage conditions on the quality of limbal niche components are less well described. Therefore, in this study, the morphological and immunohistochemical characteristics of organ-cultured limbal tissue are investigated and compared to fresh limbal tissues by means of light and electron microscopy. Organ-cultured limbal tissues showed signs of deterioration, such as edema, less pronounced basement membranes, and loss of the most superficial layers of the epithelium. In comparison to the fresh limbal epithelium, organ-cultured limbal epithelium showed signs of ongoing proliferative activity (more Ki-67+ cells) and exhibited an altered limbal epithelial phenotype with a loss of N-cadherin and desmoglein expression as well as a lack of precise staining patterns for cytokeratin ((CK)14, CK17/19, CK15). The analyzed extracellular matrix composition was mainly intact (collagen IV, fibronectin, laminin chains) except for Tenascin-C, whose expression was increased in organ-cultured limbal tissue. Nonetheless, the expression patterns of cell-matrix adhesion proteins varied in organ-cultured limbal tissue compared to fresh limbal tissue. A decrease in the number of melanocytes (Melan-A+ cells) and Langerhans cells (HLA-DR+, CD1a+, CD18+) was observed in the organ-cultured limbal tissue. The organ culture-induced alterations of the limbal epithelial stem cell niche might hamper its use in the treatment of LSCD as well as in research studies. In contrast, reduced numbers of donor-derived Langerhans cells seem associated with better clinical outcomes. However, there is a need to consider the preferential use of fresh CSL for limbal transplants and to look at ways of improving the limbal stem cell properties of stored CSL tissue.

Limbal Stem Cell Dysfunction Induced by Severe Dry Eye via Activation of the p38 MAPK Signaling Pathway

Severe dry eye (SDE) can cause grievous damage to the ocular surface and result in vision impairment and even blindness. To investigate the fate of limbal stem cells in SDE and the underlying mechanism, the current study established an SDE rat model by removing the extraorbital and infraorbital lacrimal glands and maintaining them in a low-humidity environment. One month after the surgery, aqueous tear secretion was reduced dramatically, blood vessels invaded into the central cornea, and inflammatory cells infiltrated into the limbal stroma. The expressions of keratin 12 and paired box gene 6 were down-regulated dramatically, while those of keratin 10, small proline-rich protein 1b, and mucin 5AC were up-regulated in the corneal epithelium of the SDE rats. Cell proliferation in the limbal epithelium was up-regulated, while the stem/progenitor marker adenosine 5'-triphosphate-binding cassette member 2 and the limbal epithelial colony-forming efficiency were decreased in the SDE condition. Furthermore, the p38 mitogen-activated protein kinase signaling pathway was activated in the limbal corneal epithelium of SDE rats. The abnormal differentiation and stemness loss in the corneal epithelium could be reversed upon treatment with a p38 inhibitor in a SDE in vivo model and in vitro hyperosmolar corneal epithelial culture conditions. These data suggest that SDE can lead to limbal stem cell dysfunction, and p38 mitogen-activated protein kinase signaling pathway activation plays an essential role in this process.

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.

Culturing Limbal Epithelial Cells of Long-term Stored Corneal Donors (Organ Culture) In Vitro - A Stepwise Linear Regression Algorithm

PURPOSE

To assess various potential factors on human limbal epithelial cell (LEC) outgrowth in vitro using corneal donor tissue following long-term storage (organ culture) and a stepwise linear regression algorithm.

METHODS

Of 215 donors, 304 corneoscleral rings were used for our experiments. For digestion of the limbal tissue and isolation of the limbal epithelial cells, the tissue pieces were incubated with 4.0 mg/mL collagenase A at 37 °C with 95% relative humidity and a 5% CO2 atmosphere overnight. Thereafter, limbal epithelial cells were separated from limbal keratocytes using a 20-µm CellTricks filter. The separated human LECs were cultured in keratinocyte serum-free medium medium, 1% penicillin/streptomycin (P/S), 0.02% epidermal growth factor (EGF), and 0.3% bovine pituitary extract (BPE). The potential effect of donor age (covariate), postmortem time (covariate), medium time (covariate), size of the used corneoscleral ring (360°, 270°180°, 120°, 90°, less than 90°) (covariate), endothelial cell density (ECD) (covariate), gender (factor), number of culture medium changes during organ culture (factor), and origin of the donor (donating institution and storing institution, factor) on the limbal epithelial cell outgrowth was analyzed with a stepwise linear regression algorithm.

RESULTS

The rate of successful human LEC outgrowth was 37.5%. From the stepwise linear regression algorithm, we found out that the relevant influencing parameters on the LEC growth were intercept (p < 0.001), donor age (p = 0.002), number of culture medium changes during organ culture (p < 0.001), total medium time (p = 0.181), and size of the used corneoscleral ring (p = 0.007), as well as medium time × size of the corneoscleral ring (p = 0.007).

CONCLUSIONS

The success of LEC outgrowth increases with lower donor age, lower number of organ culture medium changes during storage, shorter medium time in organ culture, and smaller corneoscleral ring size. Our stepwise linear regression algorithm may help us in optimizing LEC cultures in vitro.

The Role of Sensory Innervation in Homeostatic and Injury-Induced Corneal Epithelial Renewal

The cornea is the window through which we see the world. Corneal clarity is required for vision, and blindness occurs when the cornea becomes opaque. The cornea is covered by unique transparent epithelial cells that serve as an outermost cellular barrier bordering between the cornea and the external environment. Corneal sensory nerves protect the cornea from injury by triggering tearing and blink reflexes, and are also thought to regulate corneal epithelial renewal via unknown mechanism(s). When protective corneal sensory innervation is absent due to infection, trauma, intracranial tumors, surgery, or congenital causes, permanent blindness results from repetitive epithelial microtraumas and failure to heal. The condition is termed neurotrophic keratopathy (NK), with an incidence of 5:10,000 people worldwide. In this report, we review the currently available therapeutic solutions for NK and discuss the progress in our understanding of how the sensory nerves induce corneal epithelial renewal.

Tenascins and osteopontin in biological response in cornea

The structural composition, integrity and regular curvature of the cornea contribute to the maintenance of its transparency and vision. Disruption of its integrity caused by injury results in scarring, inflammation and neovascularization followed by losses in transparency. These sight compromising effects is caused by dysfunctional corneal resident cell responses induced by the wound healing process. Upregulation of growth factors/cytokines and neuropeptides affect development of aberrant behavior. These factors trigger keratocytes to first transform into activated fibroblasts and then to myofibroblasts. Myofibroblasts express extracellular matrix components for tissue repair and contract the tissue to facilitate wound closure. Proper remodeling following primary repair is critical for restoration of transparency and visual function. Extracellular matrix components contributing to the healing process are divided into two groups; a group of classical tissue structural components and matrix macromolecules that modulate cell behaviors/activities besides being integrated into the matrix structure. The latter components are designated as matricellular proteins. Their functionality is elicited through mechanisms which modulate the scaffold integrity, cell behaviors, activation/inactivation of either growth factors or cytoplasmic signaling regulation. We discuss here the functional roles of matricellular proteins in mediating injury-induced corneal tissue repair. The roles are described of major matricellular proteins, which include tenascin C, tenascin X and osteopontin. Focus is directed towards dealing with their roles in modulating individual activities of wound healing-related growth factors, e. g., transforming growth factor β (TGF β). Modulation of matricellular protein functions could encompass a potential novel strategy to improve the outcome of injury-induced corneal wound healing.

Wnt/β-Catenin Signaling Activation Induces Differentiation in Human Limbal Epithelial Stem Cells Cultured Ex Vivo

Human limbal epithelial stem cells (hLESCs) continuously replenish lost or damaged human corneal epithelial cells. The percentage of stem/progenitor cells in autologous ex vivo expanded tissue is essential for the long-term success of transplantation in patients with limbal epithelial stem cell deficiency. However, the molecular processes governing the stemness and differentiation state of hLESCs remain uncertain. Therefore, we sought to explore the impact of canonical Wnt/β-catenin signaling activation on hLESCs by treating ex vivo expanded hLESC cultures with GSK-3 inhibitor LY2090314. Real-time qRT-PCR and microarray data reveal the downregulation of stemness (TP63), progenitor (SOX9), quiescence (CEBPD), and proliferation (MKI67, PCNA) genes and the upregulation of genes for differentiation (CX43, KRT3) in treated- compared to non-treated samples. The pathway activation was shown by AXIN2 upregulation and enhanced levels of accumulated β-catenin. Immunocytochemistry and Western blot confirmed the findings for most of the above-mentioned markers. The Wnt/β-catenin signaling profile demonstrated an upregulation of WNT1, WNT3, WNT5A, WNT6, and WNT11 gene expression and a downregulation for WNT7A and DKK1 in the treated samples. No significant differences were found for WNT2, WNT16B, WIF1, and DKK2 gene expression. Overall, our results demonstrate that activation of Wnt/β-catenin signaling in ex vivo expanded hLESCs governs the cells towards differentiation and reduces proliferation and stem cell maintenance capability.

Two Methods for the Isolation and Cultivation of Porcine Primary Corneal Cells

In ophthalmic research, there is a strong need for in vitro corneal cell models. Here, we describe different protocols for the cultivation of primary corneal cells that were isolated from porcine eyes. This primary cell culture can be used to test new therapeutic options for corneal diseases, such as dry eye disease, traumatic injuries, or corneal infections, and to study limbal epithelial stem cell (LESC) expansion. Two different isolation methods were performed: the outgrowth and the collagenase method. To perform the outgrowth protocol, small explants of the corneal limbus were generated and incubated in culture flasks in an incubator for 4-5 weeks. Regarding the collagenase method, to extract corneal cells, porcine corneas were removed, cut into small pieces, and incubated with collagenase. After incubation and centrifugation, the cells were seeded in 6- or 12-well plates and incubated in an incubator for 2-3 weeks. The differences between corneal cell cultivation with fetal bovine serum (FBS) and without it are also discussed. Therefore, the main advantages of the outgrowth method are that it requires fewer porcine eyes, and it takes less time to be performed compared to the collagenase method. On the other hand, with the collagenase method, mature cells are obtained earlier, at about 2 to 3 weeks.

Transcription Factor ATF3 Participates in DeltaNp63-Mediated Proliferation of Corneal Epithelial Cells

Understanding the regulatory mechanisms underlying corneal epithelial cell (CEC) proliferation in vitro may provide the means to boost CEC production in cell therapy for ocular disorders. The transcription factor ΔNp63 plays a crucial role in the proliferation of CECs, but the underlying mechanisms is yet to be elucidated. TP63 and ΔNp63 are encoded by the TP63 gene via alternative promoters. We previously reported that both ΔNp63 and activating transcription factor (ATF3) are substantially expressed in cultured CECs, but the regulatory relationship between ΔNp63 and ATF3 is unknown. In the present study, we found that ΔNp63 increased ATF3 expression and ATF3 promoter activity in cultured CECs. The deletion of the p63 binding core site reduced ATF3 promoter activity. CECs overexpressing ATF3 exhibited significantly greater proliferation than control CECs. ATF3 knockdown suppressed the ΔNp63-induced increase in cell proliferation. Overexpression of ATF3 in CECs significantly elevated protein and mRNA levels of cyclin D. The protein levels of keratin 3/14, integrin β1, and involucrin did not differ between ATF3-overexpressing CECs, ATF3-downregulated CECs, and control cells. In conclusion, our results suggest that ΔNp63 increases CEC proliferation via the ΔNp63/ATF3/CDK pathway.

Characterization of Porcine Ocular Surface Epithelial Microenvironment

The porcine ocular surface is used as a model of the human ocular surface; however, a detailed characterization of the porcine ocular surface has not been documented. This is due, in part, to the scarcity of antibodies produced specifically against the porcine ocular surface cell types or structures. We performed a histological and immunohistochemical investigation on frozen and formalin-fixed, paraffin-embedded ocular surface tissue from domestic pigs using a panel of 41 different antibodies related to epithelial progenitor/differentiation phenotypes, extracellular matrix and associated molecules, and various niche cell types. Our observations suggested that the Bowman's layer is not evident in the cornea; the deep invaginations of the limbal epithelium in the limbal zone are analogous to the limbal interpalisade crypts of human limbal tissue; and the presence of goblet cells in the bulbar conjunctiva. Immunohistochemistry analysis revealed that the epithelial progenitor markers cytokeratin (CK)15, CK14, p63α, and P-cadherin were expressed in both the limbal and conjunctival basal epithelium, whereas the basal cells of the limbal and conjunctival epithelium did not stain for CK3, CK12, E-cadherin, and CK13. Antibodies detecting marker proteins related to the extracellular matrix (collagen IV, Tenascin-C), cell-matrix adhesion (β-dystroglycan, integrin α3 and α6), mesenchymal cells (vimentin, CD90, CD44), neurons (neurofilament), immune cells (HLA-ABC; HLA-DR, CD1, CD4, CD14), vasculature (von Willebrand factor), and melanocytes (SRY-homeobox-10, human melanoma black-45, Tyrosinase) on the normal human ocular surface demonstrated similar immunoreactivity on the normal porcine ocular surface. Only a few antibodies (directed against N-cadherin, fibronectin, agrin, laminin α3 and α5, melan-A) appeared unreactive on porcine tissues. Our findings characterize the main immunohistochemical properties of the porcine ocular surface and provide a morphological and immunohistochemical basis useful to research using porcine models. Furthermore, the analyzed porcine ocular structures are similar to those of humans, confirming the potential usefulness of pig eyes to study ocular surface physiology and pathophysiology.

A Novel Technique of Amniotic Membrane Preparation Mimicking Limbal Epithelial Crypts Enhances the Number of Progenitor Cells upon Expansion

We aimed to investigate whether a novel technique of human amniotic membrane (HAM) preparation that mimics the crypts in the limbus enhances the number of progenitor cells cultured ex vivo. The HAMs were sutured on polyester membrane (1) standardly, to obtain a flat HAM surface, or (2) loosely, achieving the radial folding to mimic crypts in the limbus. Immunohistochemistry was used to demonstrate a higher number of cells positive for progenitor markers p63α (37.56 ± 3.34% vs. 62.53 ± 3.32%, p = 0.01) and SOX9 (35.53 ± 0.96% vs. 43.23 ± 2.32%, p = 0.04), proliferation marker Ki-67 (8.43 ± 0.38 % vs. 22.38 ± 1.95 %, p = 0.002) in the crypt-like HAMs vs. flat HAMs, while no difference was found for the quiescence marker CEBPD (22.99 ± 2.96% vs. 30.49 ± 3.33 %, p = 0.17). Most of the cells stained negative for the corneal epithelial differentiation marker KRT3/12, and some were positive for N-cadherin in the crypt-like structures, but there was no difference in staining for E-cadherin and CX43 in crypt-like HAMs vs. flat HAMs. This novel HAM preparation method enhanced the number of progenitor cells expanded in the crypt-like HAM compared to cultures on the conventional flat HAM.

Detection of Limbal Stem Cells Adhered to Melt Electrospun Silk Fibroin and Gelatin-Modified Polylactic Acid Scaffolds

Limbal stem cells (LSCs) are of paramount importance in corneal epithelial tissue repair. The cornea becomes opaque in case of limbal stem cell deficiency (LSCD), which may cause serious damage to the ocular visual function. There are many techniques to restore damaged epithelium, one of which is the transplantation of healthy cultured LSCs, usually onto a human amniotic membrane or onto bio-based engineered scaffolds in recent years. In this study, melt electrospun polylactic acid (PLA) was modified by silk fibroin or gelatin and further cultured with LSCs originating from three different donors. In terms of physicochemical properties, both modifications slightly increased PLA scaffold porosity (with a significantly larger pore area for the PLA/gelatin) and improved the scaffolds' swelling percentage, as well as their biodegradation rate. In terms of the scaffold application function, the aim was to detect/visualize whether LSCs adhered to the scaffolds and to further determine cell viability (total number), as well as to observe p63 and CK3 expressions in the LSCs. LSCs were attached to the surface of microfibers, showing flattened conformations or 3D spheres in the formation of colonies or agglomerations, respectively. All scaffolds showed the ability to bind the cells onto the surface of individual microfibers (PLA and PLA/gelatin), or in between the microfibers (PLA/silk fibroin), with the latter showing the most intense red fluorescence of the stained cells. All scaffolds proved to be biocompatible, while the PLA/silk fibroin scaffolds showed the highest 98% viability of 2.9 × 10⁶ LSCs, with more than 98% of p63 and less than 20% of CK3 expressions in the LSCs, thus confirming the support of their growth, proliferation and corneal epithelial differentiation. The results show the potential of these bio-engineered scaffolds to be used as an alternative clinical approach.

Corneal stem cells niche and homeostasis impacts in regenerative medicine; concise review

The limbal stem cells niche (LSCN) is an optimal microenvironment that provides the limbal epithelial stem cells (LESCs) and strictly regulates their proliferation and differentiation. Disturbing the LSCN homeostasis can lead to limbal stem cell dysfunction (LSCD) and subsequent ocular surface aberrations, such as corneal stromal inflammation, persistent epithelial defects, corneal neovascularisation, lymphangiogenesis, corneal opacification, and conjunctivalization. As ocular surface disorders are considered the second main cause of blindness, it becomes crucial to explore different therapeutic strategies for restoring the functions of the LSCN. A major limitation of corneal transplantation is the current shortage of donor tissue to meet the requirements worldwide. In this context, it becomes mandatory to find an alternative regenerative medicine, such as using cultured limbal epithelial/stromal stem cells, inducing the production of corneal like cells by using other sources of stem cells, and using tissue engineering methods aiming to produce the three-dimensional (3D) printed cornea. Limbal epithelial stem cells have been considered the magic potion for eye treatment. Epithelial and stromal stem cells in the limbal niche hold the responsibility of replenishing the corneal epithelium. These stem cells are being used for transplantation to maintain corneal epithelial integrity and ultimately sustain optimal vision. In this review, we summarised the characteristics of the LSCN and their current and future roles in restoring corneal homeostasis in eyes with LSCD.

Profile of biological characterizations and clinical application of corneal stem/progenitor cells

Corneal stem/progenitor cells are typical adult stem/progenitor cells. The human cornea covers the front of the eyeball, which protects the eye from the outside environment while allowing vision. The location and function demand the cornea to maintain its transparency and to continuously renew its epithelial surface by replacing injured or aged cells through a rapid turnover process in which corneal stem/progenitor cells play an important role. Corneal stem/progenitor cells include mainly corneal epithelial stem cells, corneal endothelial cell progenitors and corneal stromal stem cells. Since the discovery of corneal epithelial stem cells (also known as limbal stem cells) in 1971, an increasing number of markers for corneal stem/progenitor cells have been proposed, but there is no consensus regarding the definitive markers for them. Therefore, the identification, isolation and cultivation of these cells remain challenging without a unified approach. In this review, we systematically introduce the profile of biological characterizations, such as anatomy, characteristics, isolation, cultivation and molecular markers, and clinical applications of the three categories of corneal stem/progenitor cells.

Culture and characterization of canine and feline corneal epithelial organoids: A new tool for the study and treatment of corneal diseases

In this study, we isolated and cultured canine and feline 3D corneal organoids. Samples derived from corneal limbal epithelium from one canine and one feline patient were obtained by enucleation after euthanasia. Stem cell isolation and organoid culture were performed by culturing organoids in Matrigel. Organoids were subsequently embedded in paraffin for further characterization. The expression of key corneal epithelial and stromal cell markers in canine and feline organoids was evaluated at the mRNA level by RNA-ISH and at the protein level by immunofluorescence (IF) and immunohistochemistry (IHC), while histochemical analysis was performed on both tissues and organoids using periodic-acid Schiff (PAS), Sirius Red, Gomori's Trichrome, and Colloidal Iron stains. IF showed consistent expression of AQP1 within canine and feline organoids and tissues. P63 was present in canine tissues, canine organoids, and feline tissues, but not in feline organoids. Results from IHC staining further confirmed the primarily epithelial origin of the organoids. Canine and feline 3D corneal organoids can successfully be cultured and maintained and express epithelial and stem cell progenitor markers typical of the cornea. This novel in vitro model can be used in veterinary ophthalmology disease modeling, corneal drug testing, and regenerative medicine.

Cellular and molecular profiles of larval and adult Xenopus corneal epithelia resolved at the single-cell level

Corneal Epithelial Stem Cells (CESCs) and their proliferative progeny, the Transit Amplifying Cells (TACs), are responsible for homeostasis and maintaining corneal transparency. Owing to our limited knowledge of cell fates and gene activity within the cornea, the search for unique markers to identify and isolate these cells remains crucial for ocular surface reconstruction. We performed single-cell RNA sequencing of corneal cells from larval and adult stages of Xenopus. Our results indicate that as the cornea develops and matures, there is an increase in cellular diversity, which is accompanied by a substantial shift in transcriptional profile, gene regulatory network and cell-cell communication dynamics. Our data also reveals several novel genes expressed in corneal cells and changes in gene expression during corneal differentiation at both developmental time-points. Importantly, we identify specific basal cell clusters in both the larval and adult cornea that comprise a relatively undifferentiated cell type and express distinct stem cell markers, which we propose are the putative larval and adult CESCs, respectively. This study offers a detailed atlas of single-cell transcriptomes in the frog cornea. In the future, this work will be useful to elucidate the function of novel genes in corneal epithelial homeostasis, wound healing and regeneration.

Therapeutic Strategies for Restoring Perturbed Corneal Epithelial Homeostasis in Limbal Stem Cell Deficiency: Current Trends and Future Directions

Limbal stem cells constitute an important cell population required for regeneration of the corneal epithelium. If insults to limbal stem cells or their niche are sufficiently severe, a disease known as limbal stem cell deficiency occurs. In the absence of functioning limbal stem cells, vision-compromising conjunctivalization of the corneal epithelium occurs, leading to opacification, inflammation, neovascularization, and chronic scarring. Limbal stem cell transplantation is the standard treatment for unilateral cases of limbal stem cell deficiency, but bilateral cases require allogeneic transplantation. Herein we review the current therapeutic utilization of limbal stem cells. We also describe several limbal stem cell markers that impact their phenotype and function and discuss the possibility of modulating limbal stem cells and other sources of stem cells to facilitate the development of novel therapeutic interventions. We finally consider several hurdles for widespread adoption of these proposed methodologies and discuss how they can be overcome to realize vision-restoring interventions.

[Regenerative medicine for the corneal epithelium : Cell therapy from bench to bedside]

In the case of thermal or caustic burns of the ocular surface, loss of limbal epithelial stem cells leads to compromised self-renewal of the corneal epithelium. This results in permanent loss of vision. In these situations, transplantation of cultured limbal epithelial cells on an amniotic membrane or fibrin gel as substrate (Holoclar®) can help to regenerate the corneal surface. The required cells are obtained from the healthy partner eye, if available. Adult stem cells from other parts of the body potentially serve as alternative cell sources: hair follicles, oral mucosa, mesenchymal stromal cells, or induced pluripotent stem cells (originally, e.g., skin fibroblasts). The reprogramming of such cells can be achieved with the help of transcription factors. In addition, work is being done on biosynthetic or synthetic matrices, which not only serve as substrate material for the transplantation but also support the functional properties of these cells (self-renewal, corneal epithelial-typical phenotype).

Novel detection of stem cell niche within the stroma of limbus in the rabbit during postnatal development

Identifying and locating stem cell populations in the limbus may lead to developing a cell-based strategy for treating the corneal injury. Therefore, this study was the first to design a follow-up on the microscopical and histomorphometric changes in the rabbit limbus and to localize and demonstrate the limbal stem cell niche during postnatal development. The paraffin sections from the eyes of different postnatal-developmental stages were stained and examined using light microscopy. Furthermore, sections were immunohistologically stained for the epithelial stem cell differentiation marker, cytokeratin-14. Moreover, semithin and ultrathin sections were applied for ultrastructural demonstration of the stem cell niche. This study revealed that the number and thickness of limbal epithelial layers increased with age, whereas the thickness of limbal stroma decreased. Additionally, the immunohistochemical data showed that ck14 staining intensity increased in the limbal region where limbal stem cells reside. The semithin and ultrastructure investigation revealed stem cell clusters within the limbus's underlying stroma close to the blood and nerve supply and surrounded by telocytes. Conclusively, isolated clusters of limbal epithelial stem cells combined with blood vessels, nerve fibers, and telocytes propose a harmonious microenvironment of a stem cell niche.

Induced Pluripotent Stem Cell-Derived Corneal Cells: Current Status and Application

Deficiency and dysfunction of corneal cells leads to the blindness observed in corneal diseases such as limbal stem cell deficiency (LSCD) and bullous keratopathy. Regenerative cell therapies and engineered corneal tissue are promising treatments for these diseases [1]. However, these treatments are not yet clinically feasible due to inadequate cell sources. The discovery of induced pluripotent stem cells (iPSCs) by Shinya Yamanaka has provided a multitude of opportunities in research because iPSCs can be generated from somatic cells, thus providing an autologous and unlimited source for corneal cells. Compared to other stem cell sources such as mesenchymal and embryonic, iPSCs have advantages in differentiation potential and ethical concerns, respectively. Efforts have been made to use iPSCs to model corneal disorders and diseases, drug testing [2], and regenerative medicine [1]. Autologous treatments based on iPSCs can be exorbitantly expensive and time-consuming, but development of stem cell banks with human leukocyte antigen (HLA)- homozygous cell lines can provide cost- and time-efficient allogeneic alternatives. In this review, we discuss the early development of the cornea because protocols differentiating iPSCs toward corneal lineages rely heavily upon recapitulating this development. Differentiation of iPSCs toward corneal cell phenotypes have been analyzed with an emphasis on feeder-free, xeno-free, and well-defined protocols, which have clinical relevance. The application, challenges, and potential of iPSCs in corneal research are also discussed with a focus on hurdles that prevent clinical translation.

Labial Mucosa Stem Cells: Isolation, Characterization, and Their Potential for Corneal Epithelial Reconstruction

Purpose

The purpose of this study was to characterize labial mucosa stem cells (LMSCs) and to investigate their potential for corneal epithelial reconstruction in a rabbit model of total limbal stem cell deficiency (LSCD).

Methods

Rabbit LMSCs (rLMSCs) and human (hLMSCs) LMSCs were derived from labial mucosa and characterized in terms of their proliferation activity by the evaluation of proliferation index (PI) and colony forming efficiency (CFE), cell senescence, and differentiation abilities. The expression of various limbus-specific, stem cell-specific, and epithelial markers was assessed via immunocytochemistry. Flow cytometry was used to evaluate mesenchymal and hematopoietic cell surface markers expression. Chromosomal stability of the derived cells was examined using the conventional GTG-banding technique. To assess the impact of LMSCs on corneal epithelial reconstruction, rLMSCs were seeded onto a decellularized human amniotic membrane (dHAM), thereafter their regeneration potential was examined in the rabbit model of total LSCD.

Results

Both rLMSCs and hLMSCs showed high proliferation and differentiation abilities, entered senescence at later passages, and expressed different stem cell-specific (ABCB5, ALDH3A1, ABCG2, and p63α), mesenchymal (vimentin), and epithelial (CK3/12, CK15) markers. Cell surface antigen expression was similar to other described mesenchymal stem cells. No clonal structural chromosome abnormalities (CSCAs) and the low percentage of non-clonal structural chromosome abnormalities (NSCAs) were observed. Transplantation of rLMSCs promoted corneal epithelial reconstruction and enhanced corneal transparency.

Conclusions

LMSCs have significant proliferation and differentiation abilities, display no detrimental chromosome aberrations, and demonstrate considerable potential for corneal repair.

P-Cadherin Is Expressed by Epithelial Progenitor Cells and Melanocytes in the Human Corneal Limbus

Interactions between limbal epithelial progenitor cells (LEPC) and surrounding niche cells, which include limbal mesenchymal stromal cells (LMSC) and melanocytes (LM), are essential for the maintenance of the limbal stem cell niche required for a transparent corneal surface. P-cadherin (P-cad) is a critical stem cell niche adhesion molecule at various epithelial stem cell niches; however, conflicting observations were reported on the presence of P-cad in the limbal region. To explore this issue, we assessed the location and phenotype of P-cad⁺ cells by confocal microscopy of human corneoscleral tissue. In subsequent fluorescence-activated cell sorting (FACS) experiments, we used antibodies against P-cad along with CD90 and CD117 for the enrichment of LEPC, LMSC and LM, respectively. The sorted cells were characterized by immunophenotyping and the repopulation of decellularized limbal scaffolds was evaluated. Our findings demonstrate that P-cad is expressed by epithelial progenitor cells as well as melanocytes in the human limbal epithelial stem cell niche. The modified flow sorting addressing P-cad as well as CD90 and CD117 yielded enriched LEPC (CD90⁻CD117⁻P-cad⁺) and pure populations of LMSC (CD90⁺CD117⁻P-cad⁻) and LM (CD90⁻CD117⁺P-cad⁺). The enriched LEPC showed the expression of epithelial progenitor markers and better colony-forming ability than their P-cad⁻ counterparts. The cultured LEPC and LM exhibited P-cad expression at intercellular junctions and successfully repopulated decellularized limbal scaffolds. These data suggest that P-cad is a critical cell–cell adhesion molecule, connecting LEPC and LM, which may play an important role in the long-term maintenance of LEPC at the limbal stem cell niche; moreover, these findings led to further improvement of cell enrichment protocols to enhance the yield of LEPC.

Sleep deprivation induces corneal epithelial progenitor cell over-expansion through disruption of redox homeostasis in the tear film

Sleep deficiency, a common public health problem, causes ocular discomfort and affects ocular surface health. However, the underlying mechanism remains unclear. Herein, we identified that short-term sleep deprivation (SD) resulted in hyperproliferation of corneal epithelial progenitor cells (CEPCs) in mice. The expression levels of p63 and Keratin 14, the biomarkers of CEPCs, were upregulated in the corneal epithelium after short-term SD. In addition, SD led to elevated levels of reactive oxygen species (ROS), and subsequent decrease in antioxidant capacity, in the tear film. Exogenous hydrogen peroxide (H₂O₂) could directly stimulate the proliferation of CEPCs in vivo and in vitro. Topical treatment of antioxidant L-glutathione preserved the over-proliferation of CEPCs and attenuated corneal epithelial defects in SD mice. Moreover, the activation of the phosphoinositide 3-kinase (PI3K)/AKT signaling pathway is essential to ROS-stimulated cell proliferation in CEPCs. However, long-term SD ultimately led to early manifestation of limbal stem cell deficiency.

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Sleep deprivation induces the over-expansion of corneal epithelial progenitor cells (CEPCs)

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Sleep deprivation disrupts redox homeostasis in the tear film

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PI3K/AKT signaling pathway activation is essential to ROS-stimulated CEPC over-proliferation

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Topical L-glutathione treatment attenuates CEPC over-proliferation

Signaling Pathways Impact on Induction of Corneal Epithelial-like Cells Derived from Human Wharton’s Jelly Mesenchymal Stem Cells

Corneal epithelium, the outmost layer of the cornea, comprises corneal epithelial cells (CECs) that are continuously renewed by limbal epithelial stem cells (LESCs). Loss or dysfunction of LESCs causes limbal stem cell deficiency (LSCD) which results in corneal epithelial integrity loss and visual impairment. To regenerate the ocular surface, transplantation of stem cell-derived CECs is necessary. Human Wharton’s jelly derived mesenchymal stem cells (WJ-MSCs) are a good candidate for cellular therapies in allogeneic transplantation. This study aimed to test the effects of treatments on three signaling pathways involved in CEC differentiation as well as examine the optimal protocol for inducing corneal epithelial differentiation of human WJ-MSCs. All-trans retinoic acid (RA, 5 or 10 µM) inhibited the Wnt signaling pathway via suppressing the translocation of β-catenin from the cytoplasm into the nucleus. SB505124 downregulated the TGF-β signaling pathway via reducing phosphorylation of Smad2. BMP4 did not increase phosphorylation of Smad1/5/8 that is involved in BMP signaling. The combination of RA, SB505124, BMP4, and EGF for the first 3 days of differentiation followed by supplementing hormonal epidermal medium for an additional 6 days could generate corneal epithelial-like cells that expressed a CEC specific marker CK12. This study reveals that WJ-MSCs have the potential to transdifferentiate into CECs which would be beneficial for further applications in LSCD treatment therapy.

Activity-Dependent Neuroprotective Protein (ADNP)-Derived Peptide (NAP) Counteracts UV-B Radiation-Induced ROS Formation in Corneal Epithelium

The corneal epithelium, the outermost layer of the cornea, acts as a dynamic barrier preventing access to harmful agents into the intraocular space. It is subjected daily to different insults, and ultraviolet B (UV-B) irradiation represents one of the main causes of injury. In our previous study, we demonstrated the beneficial effects of pituitary adenylate cyclase-activating polypeptide (PACAP) against UV-B radiation damage in the human corneal endothelium. Some of its effects are mediated through the activation of the intracellular factor, known as the activity-dependent protein (ADNP). In the present paper, we have investigated the role of ADNP and the small peptide derived from ADNP, known as NAP, in the corneal epithelium. Here, we have demonstrated, for the first time, ADNP expression in human and rabbit corneal epithelium as well as its protective effect by treating the corneal epithelial cells exposed to UV-B radiations with NAP. Our results showed that NAP treatment prevents ROS formation by reducing UV-B-irradiation-induced apoptotic cell death and JNK signalling pathway activation. Further investigations are needed to deeply investigate the possible therapeutic use of NAP to counteract corneal UV-B damage.

Corneal epithelial differentiation of human pluripotent stem cells generates ABCB5+ and ∆Np63α+ cells with limbal cell characteristics and high wound healing capacity

Background

Differentiation of functional limbal stem cells (LSCs) from human pluripotent stem cells (hPSCs) is an important objective which can provide novel treatment solutions for patients suffering from limbal stem cell deficiency (LSCD). Yet, further characterization is needed to better evaluate their immunogenicity and regenerative potential before clinical applications.

Methods

Human PSCs were differentiated towards corneal fate and cryopreserved using a clinically applicable protocol. Resulting hPSC-LSC populations were examined at days 10–11 and 24–25 during differentiation as well as at passage 1 post-thaw. Expression of cornea-associated markers including PAX6, ABCG2, ∆Np63α, CK15, CK14, CK12 and ABCB5 as well as human leukocyte antigens (HLAs) was analyzed using immunofluorescence and flow cytometry. Wound healing properties of the post-thaw hPSC-LSCs were assessed via calcium imaging and scratch assay. Human and porcine tissue-derived cultured LSCs were used as controls for marker expression analysis and scratch assays at passage 1.

Results

The day 24–25 and post-thaw hPSC-LSCs displayed a similar marker profile with the tissue-derived LSCs, showing abundant expression of PAX6, ∆Np63α, CK15, CK14 and ABCB5 and low expression of ABCG2. In contrast, day 10–11 hPSC-LSCs had lower expression of ABCB5 and ∆Np63α, but high expression of ABCG2. A small portion of the day 10–11 cells coexpressed ABCG2 and ABCB5. The expression of class I HLAs increased during hPSC-LSCs differentiation and was uniform in post-thaw hPSC-LSCs, however the intensity was lower in comparison to tissue-derived LSCs. The calcium imaging revealed that the post-thaw hPSC-LSCs generated a robust response towards epithelial wound healing signaling mediator ATP. Further, scratch assay revealed that post-thaw hPSC-LSCs had higher wound healing capacity in comparison to tissue-derived LSCs.

Conclusions

Clinically relevant LSC-like cells can be efficiently differentiated from hPSCs. The post-thaw hPSC-LSCs possess functional potency in calcium responses towards injury associated signals and in wound closure. The developmental trajectory observed during hPSC-LSC differentiation, giving rise to ABCG2⁺ population and further to ABCB5⁺ and ∆Np63α⁺ cells with limbal characteristics, indicates hPSC-derived cells can be utilized as a valuable cell source for the treatment of patients afflicted corneal blindness due to LSCD.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-021-02673-3.

Simplified ex-vivo drug evaluation in ocular surface cells: Culture on cellulose filters of cells obtained by impression cytology

Drug development, resource- and time-intensive, extensively employs cell-based assays to assess the efficacy and safety of candidate drugs. The widely used immortalized cell lines, experimentally convenient, have limited predictive value. In contrast, ex-vivo models more faithfully reproduce diseases but are technically challenging to establish. To address this need, we developed a simplified process for ex-vivo cell culture, demonstrating its feasibility in ocular surface cells. Conjunctival cells were harvested by impression cytology and grown on mixed cellulose ester membrane filters (MCFs). Human and rabbit conjunctival cells cultured on MCFs are 100% viable at 24 h, and 43% viable at 72 h. A gene expression study evaluating 84 genes involved in ocular inflammation demonstrated that ex-vivo culturing maintains intact the expression of two thirds of these genes in human cells. That these cells are suitable for the assessment of ocular drugs was demonstrated by studying the effect of phosphosulindac (PS), a small molecule under development for the treatment of dry eye disease, in both human and rabbit conjunctival cells. PS, for example, suppressed the expression of CXCL10, a cytokine participating in the pathogenesis of dry eye disease, in human and in rabbit conjunctival cells cultured ex-vivo by 32% and 70%, respectively. Conjunctival cells cultured ex-vivo can be transfected to evaluate mechanistic questions. We successfully transfected such cells with a plasmid expressing luciferase under the control of an IFN-γ-responsive promoter or its control plasmid. IFN-γ stimulated luciferase expression by 85% in cells with the responsive plasmid but not in controls; PS significantly suppressed this induction by 37% without affecting the control plasmid. These findings demonstrate that human and rabbit conjunctival cells cultured ex-vivo with our method are viable and maintain their biological integrity; respond to biological and pharmacological agents; and are transfectable with informative plasmids. The unique advantage of this method is to potentially accelerate the development of novel drugs for the treatment of ocular surface diseases, and to advance our understanding of ocular surface pathophysiology.

Single cell transcriptomics reveals the heterogeneity of the human cornea to identify novel markers of the limbus and stroma

The cornea is the clear window that lets light into the eye. It is composed of five layers: epithelium, Bowman's layer, stroma, Descemet's membrane and endothelium. The maintenance of its structure and transparency are determined by the functions of the different cell types populating each layer. Attempts to regenerate corneal tissue and understand disease conditions requires knowledge of how cell profiles vary across this heterogeneous tissue. We performed a single cell transcriptomic profiling of 19,472 cells isolated from eight healthy donor corneas. Our analysis delineates the heterogeneity of the corneal layers by identifying cell populations and revealing cell states that contribute in preserving corneal homeostasis. We identified expression of CAV1, HOMER3 and CPVL in the corneal epithelial limbal stem cell niche, CKS2, STMN1 and UBE2C were exclusively expressed in highly proliferative transit amplifying cells, CXCL14 was expressed exclusively in the suprabasal/superficial limbus, and NNMT was exclusively expressed by stromal keratocytes. Overall, this research provides a basis to improve current primary cell expansion protocols, for future profiling of corneal disease states, to help guide pluripotent stem cells into different corneal lineages, and to understand how engineered substrates affect corneal cells to improve regenerative therapies.

Regulation of Limbal Epithelial Stem Cells: Importance of the Niche

Limbal epithelial stem/progenitor cells (LSCs) reside in a niche that contains finely tuned balances of various signaling pathways including Wnt, Notch, BMP, Shh, YAP, and TGFβ. The activation or inhibition of these pathways is frequently dependent on the interactions of LSCs with various niche cell types and extracellular substrates. In addition to receiving molecular signals from growth factors, cytokines, and other soluble molecules, LSCs also respond to their surrounding physical structure via mechanotransduction, interaction with the ECM, and interactions with other cell types. Damage to LSCs or their niche leads to limbal stem cell deficiency (LSCD). The field of LSCD treatment would greatly benefit from an understanding of the molecular regulation of LSCs in vitro and in vivo. This review synthesizes current literature around the niche factors and signaling pathways that influence LSC function. Future development of LSCD therapies should consider all these niche factors to achieve improved long-term restoration of the LSC population.

Human limbal epithelial stem cell regulation, bioengineering and function

The corneal epithelium is continuously renewed by limbal stem/progenitor cells (LSCs), a cell population harbored in a highly regulated niche located at the limbus. Dysfunction and/or loss of LSCs and their niche cause limbal stem cell deficiency (LSCD), a disease that is marked by invasion of conjunctival epithelium into the cornea and results in failure of epithelial wound healing. Corneal opacity, pain, loss of vision, and blindness are the consequences of LSCD. Successful treatment of LSCD depends on accurate diagnosis and staging of the disease and requires restoration of functional LSCs and their niche. This review highlights the major advances in the identification of potential LSC biomarkers and components of the LSC niche, understanding of LSC regulation, methods and regulatory standards in bioengineering of LSCs, and diagnosis and staging of LSCD. Overall, this review presents key points for researchers and clinicians alike to consider in deepening the understanding of LSC biology and improving LSCD therapies.

Wnt6 plays a complex role in maintaining human limbal stem/progenitor cells

The corneal epithelium is consistently regenerated by limbal stem/progenitor cells (LSCs), a very small population of adult stem cells residing in the limbus. Several Wnt ligands, including Wnt6, are preferentially expressed in the limbus. To investigate the role of Wnt6 in regulating proliferation and maintenance of human LSCs in an in vitro LSC expansion setting, we generated NIH-3T3 feeder cells to overexpress different levels of Wnt6. Characterization of LSCs cultured on Wnt6 expressing 3T3 cells showed that high level of Wnt6 increased proliferation of LSCs. Medium and high levels of Wnt6 also increased the percentage of small cells (diameter ≤ 12 µm), a feature of the stem cell population. Additionally, the percentage of cells expressing the differentiation marker K12 was significantly reduced in the presence of medium and high Wnt6 levels. Although Wnt6 is mostly known as a canonical Wnt ligand, our data showed that canonical and non-canonical Wnt signaling pathways were activated in the Wnt6-supplemented LSC cultures, a finding suggesting that interrelationships between both pathways are required for LSC regulation.

Characterisation of corneas following different time and storage methods for their use as a source of stem-like limbal epithelial cells

The transplantation of expansions of limbal epithelial stem cells (LESC) remains one of the most efficient therapies for the treatment of limbal stem cell deficiency (LSCD) to date. However, the available donor corneas are scarce, and the corneas conserved for long time, under hypothermic conditions (after 7 days) or in culture (more than 28 days), are usually discarded due to poor viability of the endothelial cells. To establish an objective criterion for the utilisation or discarding of corneas as a source of LESC, we characterized, by immunohistochemistry analysis, donor corneas conserved in different conditions and for different periods of time. We also studied the potency of LESCs isolated from these corneas and maintained in culture up to 3 cell passages. We hoped that the study of markers of LESCs present in both the corneoscleral histological sections and the cell cultures would show the adequacy of the methods used for cell isolation and how fit the LESC enrichment of the obtained cell populations to be expanded was. Thus, the expressions of markers of the cells residing in the human limbal and corneal epithelium (cytokeratin CK15 and CK12, vimentin, Collagen VII, p63α, ABCG2, Ki67, Integrin β4, ZO1, and melan A) were analysed in sections of corneoscleral tissues conserved in hypothermic conditions for 2-9 days with post-mortem time (pmt) < 8 h or for 1 day with pmt > 16 h, and in sclerocorneal rims maintained in an organ culture medium for 29 days. Cell populations isolated from donor corneoscleral tissues were also assessed based on these markers to verify the adequacy of isolation methods and the potential of expanding LESCs from these tissues. Positivity for several putative stem cell markers such as CK15 and p63α was detected in all corneoscleral tissues, although a decrease was recorded in the ones conserved for longer times. The barrier function and the ability to adhere to the extracellular matrix were maintained in all the analysed tissues. In limbal epithelial cell cultures, a simultaneous decrease in the melan A melanocyte marker and the putative stem cell markers was detected, suggesting a close relationship between the melanocytes and the limbal stem cells of the niche. Holoclones stained with putative stem cell markers were obtained from long-term, hypothermic, stored sclerocorneal rims. The results showed that the remaining sclerocorneal rims after corneal transplantation, which were conserved under hypothermic conditions for up to 7 days and would have been discarded at a first glance, still maintained their potential as a source of LESC cultures.

Stem Cell Niche Microenvironment: Review

The cornea comprises a pool of self-regenerating epithelial cells that are crucial to preserving clarity and visibility. Limbal epithelial stem cells (LESCs), which live in a specialized stem cell niche (SCN), are crucial for the survival of the human corneal epithelium. They live at the bottom of the limbal crypts, in a physically enclosed microenvironment with a number of neighboring niche cells. Scientists also simplified features of these diverse microenvironments for more analysis in situ by designing and recreating features of different SCNs. Recent methods for regenerating the corneal epithelium after serious trauma, including burns and allergic assaults, focus mainly on regenerating the LESCs. Mesenchymal stem cells, which can transform into self-renewing and skeletal tissues, hold immense interest for tissue engineering and innovative medicinal exploration. This review summarizes all types of LESCs, identity and location of the human epithelial stem cells (HESCs), reconstruction of LSCN and artificial stem cells for self-renewal.

Epithelial Regeneration in Human Corneas Preserved in an Active Storage Machine

Purpose

To characterize the corneal epithelium (CE) and limbal epithelium (LE) of human corneas stored in an innovative active storage machine (ASM) after a period of organ culture (OC).

Methods

Corneas unsuitable for graft and stored in a standard commercial OC medium for 2 to 5 weeks were transferred into our ASM for 14 days. The ASM actively maintained an overpressure on the endothelial side (20 mm Hg) while ensuring medium renewal. We compared three modalities of storage in the ASM's epithelial chamber: (1) alternating exposure to a supplemental hormonal epithelial medium (SHEM) and air (air-lifting), (2) continuous immersion in SHEM, and (3) continuous immersion in OC medium. Passive immersion of the whole cornea in OC medium or of the CE in SHEM with or without airlifting served as controls. Paired corneas were used for better comparability. Histology, differentiation (by immunolabeling), and ultrastructure were analyzed at the end.

Results

The ASM with air-lifting was most effective in regenerating a pluristratified and differentiated CE (apical ZO-1 and MUC16 staining and regeneration of the glycocalyx). In addition, the LE was stratified with preserved expression of ABCB5. The ASM with immersion in SHEM or OC medium gave a less stratified and differentiated CE. In the three control groups, the epithelia, when present, were paucistratified and less differentiated.

Conclusions

In human corneas previously stored in OC, the ASM regenerates a CE with differentiation characteristics close to normal.

Translational Relevance

Regeneration of the epithelium of human corneas discarded by eye banks will increase tissue availability for research.

Canonical NF-κB signaling maintains corneal epithelial integrity and prevents corneal aging via retinoic acid

Disorders of the transparent cornea affect millions of people worldwide. However, how to maintain and/or regenerate this organ remains unclear. Here, we show that Rela (encoding a canonical NF-κB subunit) ablation in K14+ corneal epithelial stem cells not only disrupts corneal regeneration but also results in age-dependent epithelial deterioration, which triggers aberrant wound-healing processes including stromal remodeling, neovascularization, epithelial metaplasia, and plaque formation at the central cornea. These anomalies are largely recapitulated in normal mice that age naturally. Mechanistically, Rela deletion suppresses expression of Aldh1a1, an enzyme required for retinoic acid synthesis from vitamin A. Retinoic acid administration blocks development of ocular anomalies in Krt14-Cre; Relaf/f mice and naturally aged mice. Moreover, epithelial metaplasia and plaque formation are preventable by inhibition of angiogenesis. This study thus uncovers the major mechanisms governing corneal maintenance, regeneration, and aging and identifies the NF-κB-retinoic acid pathway as a therapeutic target for corneal disorders.

A single cell atlas of human cornea that defines its development, limbal progenitor cells and their interactions with the immune cells

Purpose

Single cell (sc) analyses of key embryonic, fetal and adult stages were performed to generate a comprehensive single cell atlas of all the corneal and adjacent conjunctival cell types from development to adulthood.

Methods

Four human adult and seventeen embryonic and fetal corneas from 10 to 21 post conception week (PCW) specimens were dissociated to single cells and subjected to scRNA- and/or ATAC-Seq using the 10x Genomics platform. These were embedded using Uniform Manifold Approximation and Projection (UMAP) and clustered using Seurat graph-based clustering. Cluster identification was performed based on marker gene expression, bioinformatic data mining and immunofluorescence (IF) analysis. RNA interference, IF, colony forming efficiency and clonal assays were performed on cultured limbal epithelial cells (LECs).

Results

scRNA-Seq analysis of 21,343 cells from four adult human corneas and adjacent conjunctivas revealed the presence of 21 cell clusters, representing the progenitor and differentiated cells in all layers of cornea and conjunctiva as well as immune cells, melanocytes, fibroblasts, and blood/lymphatic vessels. A small cell cluster with high expression of limbal progenitor cell (LPC) markers was identified and shown via pseudotime analysis to give rise to five other cell types representing all the subtypes of differentiated limbal and corneal epithelial cells. A novel putative LPCs surface marker, GPHA2, expressed on the surface of 0.41% ± 0.21 of the cultured LECs, was identified, based on predominant expression in the limbal crypts of adult and developing cornea and RNAi validation in cultured LECs. Combining scRNA- and ATAC-Seq analyses, we identified multiple upstream regulators for LPCs and demonstrated a close interaction between the immune cells and limbal progenitor cells. RNA-Seq analysis indicated the loss of GPHA2 expression and acquisition of proliferative limbal basal epithelial cell markers during ex vivo LEC expansion, independently of the culture method used. Extending the single cell analyses to keratoconus, we were able to reveal activation of collagenase in the corneal stroma and a reduced pool of limbal suprabasal cells as two key changes underlying the disease phenotype. Single cell RNA-Seq of 89,897 cells obtained from embryonic and fetal cornea indicated that during development, the conjunctival epithelium is the first to be specified from the ocular surface epithelium, followed by the corneal epithelium and the establishment of LPCs, which predate the formation of limbal niche by a few weeks.

Conclusions

Our scRNA-and ATAC-Seq data of developing and adult cornea in steady state and disease conditions provide a unique resource for defining genes/pathways that can lead to improvement in ex vivo LPCs expansion, stem cell differentiation methods and better understanding and treatment of ocular surface disorders.

Macrophage-Mediated Tissue Vascularization: Similarities and Differences Between Cornea and Skin

Macrophages are critical mediators of tissue vascularization both in health and disease. In multiple tissues, macrophages have been identified as important regulators of both blood and lymphatic vessel growth, specifically following tissue injury and in pathological inflammatory responses. In development, macrophages have also been implicated in limiting vascular growth. Hence, macrophages provide an important therapeutic target to modulate tissue vascularization in the clinic. However, the molecular mechanisms how macrophages mediate tissue vascularization are still not entirely resolved. Furthermore, mechanisms might also vary among different tissues. Here we review the role of macrophages in tissue vascularization with a focus on their role in blood and lymphatic vessel formation in the barrier tissues cornea and skin. Comparing mechanisms of macrophage-mediated hem- and lymphangiogenesis in the angiogenically privileged cornea and the physiologically vascularized skin provides an opportunity to highlight similarities but also tissue-specific differences, and to understand how macrophage-mediated hem- and lymphangiogenesis can be exploited for the treatment of disease, including corneal wound healing after injury, graft rejection after corneal transplantation or pathological vascularization of the skin.

Timolol induces necroptosis, apoptosis and senescence concentration-dependently in rabbit Limbal stem cells in vitro

Limbal stem cells (LSCs) are crucial for corneal transparency and vision. Any damages to LSCs might lead to limbal stem cell deficiency resulting in corneal opacification and even blindness. Here, we investigated the cytotoxicity of timolol and its underlying mechanisms in rabbit LSCs (rLSCs) in vitro. High concentrations of 0.5% and 0.25% timolol induced necroptosis in rLSCs to upregulate receptor interacting protein kinase (RIPK)1, RIPK3, mixed lineage kinase domain-like (MLKL) and phosphorylated MLKL along with downregulation of caspase-8 and caspase-2 within 4 h. While, median concentrations of 0.125% to 0.0625% timolol induced apoptosis in the rLSCs within 28 h. The apoptotic mechanism in the median-concentration timolol-treated rLSCs is probably via extrinsic apoptosis pathway by activating caspase-2, caspase-8 and caspase-3 and intrinsic apoptosis pathway triggered by excessive generation of ROS and subsequent DNA damage to upregulate Bax and Bad, downregulate Bcl-2 and Bcl-xL, subsequently disrupt mitochondrial membrane potential, cytosolically translocate cytochrome c and apoptosis-inducing factor, and activate caspase-9. In addition, low concentration of 0.03125% timolol induced senescence in the rLSCs by elevating ROS level and increasing number of senescence associated β-galactosidase positive cells at 28 h. Our findings reveal that timolol induces necroptosis, apoptosis and senescence concentration-dependently in rLSCs in vitro.

Process development and safety evaluation of ABCB5+ limbal stem cells as advanced-therapy medicinal product to treat limbal stem cell deficiency

Background

While therapeutic success of the limbal tissue or cell transplantation to treat severe cases of limbal stem cell (LSC) deficiency (LSCD) strongly depends on the percentage of LSCs within the transplanted cells, prospective LSC enrichment has been hampered by the intranuclear localization of the previously reported LSC marker p63. The recent identification of the ATP-binding cassette transporter ABCB5 as a plasma membrane-spanning marker of LSCs that are capable of restoring the cornea and the development of an antibody directed against an extracellular loop of the ABCB5 molecule stimulated us to develop a novel treatment strategy based on the utilization of in vitro expanded allogeneic ABCB5⁺ LSCs derived from human cadaveric limbal tissue.

Methods

We developed and validated a Good Manufacturing Practice- and European Pharmacopeia-conform production and quality-control process, by which ABCB5⁺ LSCs are derived from human corneal rims, expanded ex vivo, isolated as homogenous cell population, and manufactured as an advanced-therapy medicinal product (ATMP). This product was tested in a preclinical study program investigating the cells’ engraftment potential, biodistribution behavior, and safety.

Results

ABCB5⁺ LSCs were reliably expanded and manufactured as an ATMP that contains comparably high percentages of cells expressing transcription factors critical for LSC stemness maintenance (p63) and corneal epithelial differentiation (PAX6). Preclinical studies confirmed local engraftment potential of the cells and gave no signals of toxicity and tumorgenicity. These findings were sufficient for the product to be approved by the German Paul Ehrlich Institute and the U.S. Food & Drug Administration to be tested in an international multicenter phase I/IIa clinical trial (NCT03549299) to evaluate the safety and therapeutic efficacy in patients with LSCD.

Conclusion

Building upon these data in conjunction with the previously shown cornea-restoring capacity of human ABCB5⁺ LSCs in animal models of LSCD, we provide an advanced allogeneic LSC-based treatment strategy that shows promise for replenishment of the patient’s LSC pool, recreation of a functional barrier against invading conjunctival cells and restoration of a transparent, avascular cornea.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-021-02272-2.

Limbal Stem Cells on Bacterial Nanocellulose Carriers for Ocular Surface Regeneration

Limbal stem cells (LSCs) are already used in cell-based treatments for ocular surface disorders. Clinical translation of LSCs-based therapies critically depends on the successful delivery, survival, and retention of these therapeutic cells to the desired region. Such a major bottleneck could be overcome by using an appropriate carrier to provide anchoring sites and structural support to LSC culture and transplantation. Bacterial nanocellulose (BNC) is an appealing, yet unexplored, candidate for this application because of its biocompatibility, animal-free origin and mechanical stability. Here, BNC as a vehicle for human embryonic stem cells-derived LSC (hESC-LSC) are investigated. To enhance cell-biomaterial interactions, a plasma activation followed by a Collagen IV and Laminin coating of the BNC substrates is implemented. This surface functionalization with human extracellular matrix proteins greatly improved the attachment and survival of hESC-LSC without compromising the flexible, robust and semi-transparent nature of the BNC. The surface characteristics of the BNC substrates are described and a preliminary ex vivo test in simulated transplantation scenarios is provided. Importantly, it is shown that hESC-LSC retain their self-renewal and stemness characteristics up to 21 days on BNC substrates. These results open the door for future research on hESC-LSC/BNC constructs to treat severe ocular surface pathologies.

Limbal stem cell diseases

The function of limbal stem/progenitor cells (LSCs) is critical to maintain corneal epithelial homeostasis. Many external insults and intrinsic defects can be deleterious to LSCs and their niche microenvironment, resulting in limbal stem cell dysfunction or deficiency (LSCD). Ocular comorbidities, frequent in eyes with LSCD, can exacerbate the dysfunction of residual LSCs, and limit the survival of transplanted LSCs. Clinical presentation and disease evolution vary among different etiologies of LSCD. New ocular imaging modalities and molecular markers are now available to standardize the diagnosis criteria and stage the severity of the disease. Medical therapies may be sufficient to reverse the disease if residual LSCs are present. A stepwise approach should be followed to optimize the ocular surface, eliminate the causative factors and treat comorbid conditions, before considering surgical interventions. Furthermore, surgical options are selected depending on the severity and laterality of the disease. The standardized diagnostic criteria to stage the disease is necessary to objectively evaluate and compare the efficacy of the emerging customized therapies.