Cell therapy in the cornea: The emerging role of microenvironment

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

Towards the Identification and Characterization of Putative Adult Human Lens Epithelial Stem Cells

The anterior lens epithelium has the ability to differentiate into lens fibres throughout its life. The present study aims to identify and functionally characterize the adult stem cells in the human lens epithelium. Whole mounts of lens epithelium from donor eyes (normal/cataract) were immunostained for SOX2, gap junction protein alpha 1 (GJA1), PAX6, α, β and γ-crystallins, followed by a confocal analysis. The functional property of adult stem cells was analysed by their sphere forming ability using cultured lens epithelial cells from different zones. Based on marker expression, the lens epithelium was divided into four zones: the central zone, characterized by a small population of PAX6+, GJA1-, β-crystallin- and γ-crystallin- cells; the germinative zone, characterized by PAX6+, GJA1+, β-crystallin- and γ-crystallin-; the transitional zone, characterized by PAX6+, GJA1+, β-crystallin+ and γ-crystallin-; and the equatorial zone, characterized by PAX6+/-, GJA1+, β-crystallin+, and γ-crystallin+ cells. The putative lens epithelial stem cells identified as SOX2+ and GJA1 membrane expression negative cells were located only in the central zone (1.89 ± 0.84%). Compared to the other zones, a significant percentage of spheres were identified in the central zone (1.68 ± 1.04%), consistent with the location of the putative adult lens epithelial stem cells. In the cataractous lens, an absence of SOX2 expression and a significant reduction in sphere forming ability (0.33 ± 0.11%) were observed in the central zone. The above findings confirmed the presence of putative stem cells in the central zone of the adult human lens epithelium and indicated their probable association with cataract development.

Reduction in trabecular meshwork stem cell content in donor eyes with primary open angle glaucoma

We previously identified and characterized human trabecular meshwork stem cells (TMSCs) based on high expression of ABCG2/p75 positivity and high nucleus to cytoplasmic ratio. These TMSCs expressing high ABCG2 and p75 were located to the insert region of the human TM. Additionally, we demonstrated an age-related reduction in the TMSC content which was significantly associated with TM cell loss. In continuation, this study was aimed to determine the TMSC content in glaucomatous donor eyes wherein a drastic reduction in TM cellularity has already been reported. Anterior segments from known glaucomatous (n = 6) and age-matched normal (n = 8) donors were dissected into four quadrants. A minimum of three sections from each quadrant were used for histopathological analysis as well as immunostaining. Analysis of hematoxylin and eosin-stained sections from glaucomatous tissues revealed a decrease in total TM cellularity, thickening of trabecular beams, fusion of trabeculae, absence of patent Schlemm's canal compared to age-matched controls. In addition, the TM thickness at various positions of the meshwork and the coronal as well as the meridional diameters of the Schlemm's canal were observed to be significantly reduced in glaucomatous eyes. Further, sections from both the groups were immunostained for universal stem cell marker ABCG2 and neural crest derived stem cell marker p75. The images were acquired using Leica SP8 confocal microscope. Quantification of total TM cellularity based on nuclear counterstain (mean ± SD) using ImageJ identified 69.33 ± 12.77 cells/section in control eyes. In glaucomatous donors, the TM cellularity was found to be reduced significantly to 41.83 ± 9.0 (p = 0.0007). In addition, a reduction in the percentage of TMSCs (cells with high ABCG2 expression and p75 positivity) was evident in glaucomatous donors (0.14 ± 0.17%) compared to age-matched controls (4.73 ± 5.46%) (p = 0.064). Thus, the present study confirmed the significant decline in TM cellularity and a reducing trend in the TMSC content, though this reduction was non-significant in glaucomatous donor eyes. Further studies are essential to elucidate the role of TMSCs in the pathogenesis of primary open angle glaucoma.

Functional characterization of adult human trabecular meshwork stem cells

We earlier identified native human trabecular meshwork stem cells (TMSCs) based on two-parameters- high ABCG2 expression and high nucleus to cytoplasmic ratio. The TMSCs also expressed p75 and AnkyrinG. Based on the high expression of ABCG2 and p75, the TMSCs were identified to be located in the Schwalbe's line region of the TM. In continuation, the current study aimed at elucidating the functional characteristics of human TMSCs. Upon culturing, only a small proportion of TM cells (0.96 ± 0.21% in <30 years) expressing stem cell markers ABCG2 and p75 adhered to the culture dish. This proportion significantly reduced with ageing (0.32 ± 0.23% in 30-60 years and 0.35 ± 0.04% in >60 years). Characterization of the primary TM cultures identified 7.00 ± 1.80% of stem cells with label retaining property. Further, cultured cells had the ability to form TM spheres (0.82 ± 0.23%) which consisted of high ABCG2 and p75 positive cells. Upon dexamethasone induction, 86.00 ± 14.87% and 64.60 ± 7.24% of the cells derived from the TM spheres expressed myocilin and exhibited cross linked actin networks respectively, indicating differentiation of the TMSCs in the sphere to TM cells. In addition, the sphere derived TM cells also possessed phagocytic potential (13.28 ± 3.30%) equivalent to primary TM cells (16.33 ± 4.04%) which was evident upon internalization of zymosan particles. In conclusion, this study has established that a proportion of cultured TM cells had the label retaining property as well as sphere forming ability of adult stem cells. Thus, these results confirm the presence of adult stem cells in the human TM that might be responsible for the maintenance of tissue homeostasis.

Identification, quantification and age-related changes of human trabecular meshwork stem cells

Background

Loss of cells in the human trabecular meshwork (TM) has been reported with ageing and in glaucoma. This study aims to identify, quantify and determine the age-related changes of human TM stem cells (TMSCs).

Methods

Isolation of TM cells/ paraffin sectioning was carried out using human corneoscleral rings and whole globes. The TM cells/ sections were immunostained for the stem cell markers ATP-binding cassette protein G2 (ABCG2), nerve growth factor receptor p75 and AnkyrinG (AnkG). Images were acquired using Leica SP8 confocal microscope. The isolated cells were analyzed for two parameters- ABCG2 expression and nucleus to cytoplasmic ratio (N/C ratio). The total number of TM cells and those positive for ABCG2 and p75 in each section were quantified. Spearman rank order correlation was used to determine the association between age and the cell counts.

Results

The TMSCs were identified based on two parameters- high ABCG2 expression and high N/C ratio > 0.7. These stem cells were also positive for p75 and AnkG. The TMSC content based on the two parameters was 21.0 ± 1.4% in < 30 years age group, 12.6 ± 6.6% in 30-60 years and 4.0 ± 3.5% in > 60 years. The stem cells with high ABCG2 and p75 expression were restricted to the Schwalbe's line region of the TM. A significant correlation was observed between the reduction in TMSC content and TM cell count during ageing.

Conclusion

The human TMSCs were identified and quantified based on two parameter analysis. This study established a significant association between age-related reduction in TMSC content and TM cell loss.

Molecular markers for corneal epithelial cells in larval vs. adult Xenopus frogs

Corneal Epithelial Stem Cells (CESCs) and their proliferative progeny, the Transit Amplifying Cells (TACs), are responsible for maintaining the integrity and transparency of the cornea. These stem cells (SCs) are widely used in corneal transplants and ocular surface reconstruction. Molecular markers are essential to identify, isolate and enrich for these cells, yet no definitive CESC marker has been established. An extensive literature survey shows variability in the expression of putative CESC markers among vertebrates; being attributed to species-specific variations, or other differences in developmental stages of these animals, approaches used in these studies and marker specificity. Here, we expanded the search for CESC markers using the amphibian model Xenopus laevis. In previous studies we found that long-term label retaining cells (suggestive of CESCs and TACs) are present throughout the larval basal corneal epithelium. In adult frogs, these cells become concentrated in the peripheral cornea (limbal region). Here, we used immunofluorescence to characterize the expression of nine proteins in the corneas of both Xenopus larvae and adults (post-metamorphic). We found that localization of some markers change between larval and adult stages. Markers such as p63, Keratin 19, and β1-integrin are restricted to basal corneal epithelial cells of the larvae. After metamorphosis their expression is found in basal and intermediate layer cells of the adult frog corneal epithelium. Another protein, Pax6 was expressed in the larval corneas, but surprisingly it was not detected in the adult corneal epithelium. For the first time we report that Tcf7l2 can be used as a marker to differentiate cornea vs. skin in frogs. Tcf7l2 is present only in the frog skin, which differs from reports indicating that the protein is expressed in the human cornea. Furthermore, we identified the transition between the inner, and the outer surface of the adult frog eyelid as a key boundary in terms of marker expression. Although these markers are useful to identify different regions and cellular layers of the frog corneal epithelium, none is unique to CESCs or TACs. Our results confirm that there is no single conserved CESC marker in vertebrates. This molecular characterization of the Xenopus cornea facilitates its use as a vertebrate model to understand the functions of key proteins in corneal homeostasis and wound repair.

An Insight into the Difficulties in the Discovery of Specific Biomarkers of Limbal Stem Cells

Keeping the integrity and transparency of the cornea is the most important issue to ensure normal vision. There are more than 10 million patients going blind due to the cornea diseases worldwide. One of the effective ways to cure corneal diseases is corneal transplantation. Currently, donations are the main source of corneas for transplantation, but immune rejection and a shortage of donor corneas are still serious problems. Graft rejection could cause transplanted cornea opacity to fail. Therefore, bioengineer-based corneas become a new source for corneal transplantation. Limbal stem cells (LSCs) are located at the basal layer in the epithelial palisades of Vogt, which serve a homeostatic function for the cornea epithelium and repair the damaged cornea. LSC-based transplantation is one of the hot topics currently. Clinical data showed that the ratio of LSCs to total candidate cells for a transplantation has a significant impact on the effectiveness of the transplantation. It indicates that it is very important to accurately identify the LSCs. To date, several putative biomarkers of LSCs have been widely reported, whereas their specificity is controversial. As reported, the identification of LSCs is based on the characteristics of stem cells, such as a nuclear-to-cytoplasm ratio (N/C) ≥ 0.7, label-retaining, and side population (SP) phenotype. Here, we review recently published data to provide an insight into the circumstances in the study of LSC biomarkers. The particularities of limbus anatomy and histochemistry, the limits of the current technology level for LSC isolation, the heterogeneity of LSCs and the influence of enzyme digestion are discussed. Practical approaches are proposed in order to overcome the difficulties in basic and applied research for LSC-specific biomarkers.

Emerging Therapeutic Strategies for Limbal Stem Cell Deficiency

Identification and characterization of the limbal epithelial stem cells (LESCs) has proven to be a major accomplishment in anterior ocular surface biology. These cells have been shown to be a subpopulation of limbal epithelial basal cells, which serve as the progenitor population of the corneal epithelium. LESCs have been demonstrated to play an important role in maintaining corneal epithelium homeostasis. Many ocular surface diseases, including intrinsic (e.g., Sjogren's syndrome) or extrinsic (e.g., alkali or thermal burns) insults, which impair LESCs, can lead to limbal stem cell deficiency (LSCD). LSCD is characterized by an overgrowth of conjunctival-derived epithelial cells, corneal neovascularization, and chronic inflammation, eventually leading to blindness. Treatment of LSCD has been challenging, especially in bilateral total LSCD. Recently, advances in LESC research have led to novel therapeutic approaches for treating LSCD, such as transplantation of the cultured limbal epithelium. These novel therapeutic approaches have demonstrated efficacy for ocular surface reconstruction and restoration of vision in patients with LSCD. However, they all have their own limitations. Here, we describe the current status of LSCD treatment and discuss the advantages and disadvantages of the available therapeutic modalities.

Identification and In Vitro Expansion of Buccal Epithelial Cells

Ex vivo-expanded buccal mucosal epithelial (BME) cell transplantation has been used to reconstruct the ocular surface. Methods for enrichment and maintenance of BME progenitor cells in ex vivo cultures may improve the outcome of BME cell transplantation. However, the parameter of cell seeding density in this context has largely been neglected. This study investigates how varying cell seeding density influences BME cell proliferation and differentiation on tissue culture polystyrene (TCPS). The highest cell proliferation activity was seen when cells were seeded at 5×10⁴ cells/cm². Both below and above this density, the cell proliferation rate decreased sharply. Differential immunofluorescence analysis of surface markers associated with the BME progenitor cell population (p63, CK19, and ABCG2), the differentiated cell marker CK10 and connexin 50 (Cx50) revealed that the initial cell seeding density also significantly affected the progenitor cell marker expression profile. Hence, this study demonstrates that seeding density has a profound effect on the proliferation and differentiation of BME stem cells in vitro, and this is relevant to downstream cell therapy applications.

Differentiation of human embryonic stem cells into corneal epithelial progenitor cells under defined conditions

The development of cell-based therapies using stem cells represents a significant breakthrough in the treatment of limbal stem cell deficiency (LSCD). The aim of this study was to develop a novel protocol to differentiate human embryonic stem cells (hESCs) into corneal epithelial progenitor cells (CEPCs), with similar features to primary cultured human limbal stem cells (LSCs), using a medium composed of DMEM/F12 and defined keratinocyte serum-free medium (KSFM) (1:1) under different carbon dioxide (CO₂) levels in culture. The differentiated cells exhibited a similar morphology to limbal stem cells under 5%, 7%, and 9% CO₂ and expressed the LSC markers ABCG-2 and p63; however, CK14 was only expressed in the cells cultured under 7% and 9% CO₂. Quantitative reverse-transcription polymerase chain reaction (RT-PCR) analysis indicated that the ABCG2, p63, and CK14 levels in the 7% CO₂ and 9% CO₂ groups were higher than those in the 5% CO₂ group and in undifferentiated hESCs (p<0.05). The highest expression of ABCG2 and p63 was exhibited in the cells cultured under 7% CO₂ at day 6 of differentiation. Western blotting indicated that the ABCG2 and p63 levels were higher at day 6 than the other time points in the 7% CO₂ and 9% CO₂ groups. The highest protein expression of ABCG2 and p63 was identified in the 7% CO₂ group. The neural cell-specific marker tubulin β3 and the epidermal marker K1/10 were also detected in the differentiated cells via immunofluorescent staining; thus, cell sorting was performed via fluorescence-activated cell sorting (FACS), and ABCG2-positive cells were isolated as CEPCs. The sorted cells formed three to four layers of epithelioid cells by airlifting culture and expressed ABCG2, p63, CK14, and CK3. In conclusion, the novel induction system conditioned by 7% CO₂ in this study may be an effective and feasible method for CEPC differentiation.

Construction of tissue-engineered full-thickness cornea substitute using limbal epithelial cell-like and corneal endothelial cell-like cells derived from human embryonic stem cells

The aim of this study was to construct a full-thickness artificial cornea substitute in vitro by coculturing limbal epithelial cell-like (LEC-like) cells and corneal endothelial cell-like (CEC-like) cells derived from human embryonic stem cells (hESCs) on APCM scaffold. A 400 μm thickness, 11 mm diameter APCM lamella containing Bowman's membrane was prepared as the scaffold using trephine and a special apparatus made by ourselves. LEC-like cells and CEC-like cells, derived from hESCs as our previously described, were cocultured on the scaffold using a special insert of 24-well plates that enabled seeding both sides of the scaffold. Three or four layers of epithelium-like cells and a uniform monolayer of CEC-like cells could be observed by H&E staining. The thickness, endothelial cell density, and mechanical properties of the construct were similar to that of native rabbit corneas. Immunofluorescence analysis showed expression of ABCG2 and CK3 in the epithelium-like cell layers and expression of N-cadherin, ZO-1 and Na+/K + ATPase in the CEC-like cells. The corneal substitutes were well integrated within the host corneas, and the transparency increased gradually in 8-week follow-up after transplantation in the rabbits. These results suggest that the strategy we developed is feasible and effective for construction of tissue-engineered full-thickness cornea substitute with critical properties of native cornea.

A novel method for a high enrichment of human corneal epithelial stem cells for genomic analysis

Understanding the molecular mechanisms that regulate the corneal epithelial stem cells (CESCs) in maintaining corneal homeostasis remains elusive largely due to the lack of a specific marker for their isolation. This study aims to enrich CESCs from human donor limbal epithelium and to evaluate the level of enrichment based on expression of ΔNp63α, a putative CESC marker. A two-stage enrichment of CESCs was carried out. (a) The limbal basal epithelial cells were isolated by differential enzymatic treatment and five-fold enrichment was achieved from 2% of CESCs present in the total limbal epithelium. The CESCs were quantified on the basis of two parameters-high expression of p63/ABCG2 and nucleus to cytoplasmic (N/C) ratio ≥0.7. (b) Cytospin smears of isolated basal cells were Giemsa stained and cells with N/C ratio ≥0.7 were separated by laser capture microdissection. This strategy resulted in an enrichment of CESCs to 78.57% based on two-parameter analysis using p63 and 76.66% using ABCG2. RT-PCR was carried out for ΔNp63 isoforms (α, β, and γ) and connexin-43, with GAPDH for normalization. The expression of ΔNp63α was restricted to the enriched population of CESCs in contrast to its absence in limbal basal cells with N/C ratio <0.7 and CCECs. The unique expression of ΔNp63α and 5.9-fold reduced connexin-43 expression in the enriched population of CESCs indicates its high purity. Further analysis of these cells will help in elucidating the molecular mechanisms associated with stemness and also in identifying a specific marker for CESCs.

Contact Lenses: A Delivery Device for Stem Cells to Treat Corneal Blindness

: Worldwide, 45 million people are blind. Corneal blindness is a major cause of visual loss, estimated to affect 10 million. For the most difficult to treat patients, including those with a disease called limbal stem cell deficiency, a donor corneal graft is not a viable option; thus, patients are treated with specialized stem cell grafts, which fail in a significant proportion (30 to 50%) of subjects. This unacceptable failure rate means there is a pressing need to develop minimally invasive, long-lasting, cost-effective therapies to improve patient quality of life and lessen the economic burden. Restoring vision in patients with severe corneal disease is the main focus of our research program; however, to achieve our goals and deliver the best quality stem cell therapy, we must first understand the basic biology of these cells, including their residence, the factors that support their long-term existence, markers to identify and isolate them, and carriers that facilitate expansion, delivery, and protection during engraftment. We recently achieved some of these goals through the discovery of stem cell markers and the development of a novel and innovative contact lens-based cell transfer technique that has been successfully trialed on patients with corneal blindness. Although several popular methodologies are currently available to nurture and transfer stem cells to the patients' ocular surface, contact lenses provide many advantages that will be discussed in this review article. The job for clinician-researchers will be to map precisely how these cells contribute to restoring ocular health and whether improvements in the quality of cells and the cell delivery system can be developed to reduce disease burden.

Nucleus Morphometry in Cultured Epithelial Cells Correlates with Phenotype

Phenotype of cultured ocular epithelial transplants has been shown to affect clinical success rates following transplantation to the cornea. The purpose of this study was to evaluate the relationship between cell nucleus morphometry and phenotype in three types of cultured epithelial cells. This study provides knowledge for the development of a non-invasive method of determining the phenotype of cultured epithelium before transplantation. Cultured human conjunctival epithelial cells (HCjE), human epidermal keratinocytes (HEK), and human retinal pigment epithelial cells (HRPE) were analyzed by quantitative immunofluorescence. Assessments of nucleus morphometry and nucleus-to-cytoplasm ratio (N/C ratio) were performed using ImageJ. Spearman's correlation coefficient was employed for statistical analysis. Levels of the proliferation marker PCNA in HCjE, HEK, and HRPE correlated positively with nuclear area. Nuclear area correlated significantly with levels of the undifferentiated cell marker ABCG2 in HCjE. Bmi1 levels, but not p63α levels, correlated significantly with nuclear area in HEK. The N/C ratio did not correlate significantly with any of the immunomarkers in HCjE (ABCG2, CK7, and PCNA) and HRPE (PCNA). In HEK, however, the N/C ratio was negatively correlated with levels of the undifferentiated cell marker CK14 and positively correlated with Bmi1 expression. The size of the nuclear area correlated positively with proliferation markers in all three epithelia. Morphometric indicators of phenotype in cultured epithelia can be identified using ImageJ. Conversely, the N/C ratio did not show a uniform relationship with phenotype in HCjE, HEK, or HRPE. N/C ratio therefore, may not be a useful morphometric marker for in vitro assessment of phenotype in these three epithelia.

From discovery to approval of an advanced therapy medicinal product-containing stem cells, in the EU

In 1997, the human corneal epithelium was reconstructed in vitro and transplanted on patients. Later, it became a routine treatment, before regulations considered advanced therapy medicinal products and drugs on the same lines. Manufacturing, before and after good manufacturing practice setting, was established in different facilities and the clinical application in several hospitals. Advanced therapy medicinal products, including stem cells, are unique products with different challenges than other drugs: some uncertainties, in addition to benefit, cannot be avoided. This review will focus on all recent developments in the stem cell-based corneal therapy.

Long-Term Cultures of Human Cornea Limbal Explants Form 3D Structures Ex Vivo - Implications for Tissue Engineering and Clinical Applications

Long-term cultures of cornea limbal epithelial stem cells (LESCs) were developed and characterized for future tissue engineering and clinical applications. The limbal tissue explants were cultivated and expanded for more than 3 months in medium containing serum as the only growth supplement and without use of scaffolds. Viable 3D cell outgrowth from the explants was observed within 4 weeks of cultivation. The outgrowing cells were examined by immunofluorescent staining for putative markers of stemness (ABCG2, CK15, CK19 and Vimentin), proliferation (p63α, Ki-67), limbal basal epithelial cells (CK8/18) and differentiated cornea epithelial cells (CK3 and CK12). Morphological and immunostaining analyses revealed that long-term culturing can form stratified 3D tissue layers with a clear extracellular matrix deposition and organization (collagen I, IV and V). The LESCs showed robust expression of p63α, ABCG2, and their surface marker fingerprint (CD117/c-kit, CXCR4, CD146/MCAM, CD166/ALCAM) changed over time compared to short-term LESC cultures. Overall, we provide a model for generating stem cell-rich, long-standing 3D cultures from LESCs which can be used for further research purposes and clinical transplantation.

Tualang honey improves human corneal epithelial progenitor cell migration and cellular resistance to oxidative stress in vitro

Stem cells with enhanced resistance to oxidative stress after in vitro expansion have been shown to have improved engraftment and regenerative capacities. Such cells can be generated by preconditioning them with exposure to an antioxidant. In this study we evaluated the effects of Tualang honey (TH), an antioxidant-containing honey, on human corneal epithelial progenitor (HCEP) cells in culture. Cytotoxicity, gene expression, migration, and cellular resistance to oxidative stress were evaluated. Immunofluorescence staining revealed that HCEP cells were holoclonal and expressed epithelial stem cell marker p63 without corneal cytokeratin 3. Cell viability remained unchanged after cells were cultured with 0.004, 0.04, and 0.4% TH in the medium, but it was significantly reduced when the concentration was increased to 3.33%. Cell migration, tested using scratch migration assay, was significantly enhanced when cells were cultured with TH at 0.04% and 0.4%. We also found that TH has hydrogen peroxide (H2O2) scavenging ability, although a trace level of H2O2 was detected in the honey in its native form. Preconditioning HCEP cells with 0.4% TH for 48 h showed better survival following H2O2-induced oxidative stress at 50 µM than untreated group, with a significantly lower number of dead cells (15.3 ± 0.4%) were observed compared to the untreated population (20.5 ± 0.9%, p<0.01). Both TH and ascorbic acid improved HCEP viability following induction of 100 µM H2O2, but the benefit was greater with TH treatment than with ascorbic acid. However, no significant advantage was demonstrated using 5-hydroxymethyl-2-furancarboxaldehyde, a compound that was found abundant in TH using GC/MS analysis. This suggests that the cellular anti-oxidative capacity in HCEP cells was augmented by native TH and was attributed to its antioxidant properties. In conclusion, TH possesses antioxidant properties and can improve cell migration and cellular resistance to oxidative stress in HCEP cells in vitro.

Human corneal epithelial subpopulations: oxygen dependent ex vivo expansion and transcriptional profiling

Corneal epithelium is being regenerated throughout life by limbal epithelial stem cells (LESCs) believed to be located in histologically defined stem cell niches in corneal limbus. Defective or dysfunctional LESCs result in limbal stem cell deficiency (LSCD) causing pain and decreased visual acuity. Since the first successful treatment of LSCD by transplantation of ex vivo expanded LESCs in 1997, many attempts have been carried out to optimize culture conditions to improve the outcome of surgery. To date, progress in this field of bioengineering is substantially hindered by both the lack of specific biomarkers of LESCs and the lack of a precise molecular characterization of in situ epithelial subpopulations. The aim of this dissertation was to optimize culture systems with regard to the environmental oxygen concentration for selective ex vivo expansion of LESCs and to analyse in situ subpopulations in human corneal epithelium using a combination of laser capture microdissection and RNA sequencing for global transcriptomic profiling. We compared dissociation cultures, using either expansion on γ-irradiated NIH/3T3 feeder cells in serum-rich medium or expansion directly on plastic in serum-free EpiLife medium, using a range of physiologically relevant oxygen concentrations (2%, 5%, 10%, 15% and 20%). Using immunocytochemistry and advanced fluorescence microscopy, cells were characterized regarding growth, cell cycle distribution, colony-forming efficiency (CFE), phenotypes and cytomorphometry. Limbal epithelial cells expanded in 2% O2 exhibited slow growth, low fraction of cells in S/G2 , high CFE, high expression of stem cell markers ABCG2 and p63α, and low fraction of differentiation marker CK3 resembling a LESC phenotype. The effect of hypoxia to maintain LESCs in culture was not dependent on the system used for propagation (Bath et al. 2013a). Laser capture microdissection was used to isolate cellular subpopulations in situ from the spatially defined differentiation pathway in human corneal epithelium according to an optimized protocol for maintenance of expression profiles. Isolated total RNA from basal limbal crypts (BLCs), superficial limbal crypts (SLCs), paracentral/central cornea and limbal stroma was amplified and converted to fragmented cDNA libraries for use in deep paired-end next-generation sequencing. Global transcriptional profiling was carried out using bioinformatics. The location of primitive cells in BLCs, migratory and activated cells in SLCs and differentiated cells in paracentral/central cornea was evident from mapping of significantly upregulated genes in each compartment to the gene ontology (GO). Interestingly, many GO terms in BLCs were also involved in neurogenic processes, whereas many GO terms in SLCs were related to vasculature. Mapping upregulated genes in BLCs to pathway annotations in Kyoto Encyclopedia of Genes and Genomes described many active pathways as signalling and cancer-associated pathways. We supply extensive information on possible novel biomarkers, reveal insight into both active pathways and novel regulators of LESCs such as Lrig1 and SOX9 and provide an immense amount of data for future exploration (Bath et al. 2013b). Selective ex vivo expansion of LESCs in hypoxia and the comprehensive molecular characterization of corneal epithelial subpopulations in situ are expected to be beneficial for the future treatment of LSCD by cultured limbal epithelial transplantation.