The Secretome of Alginate-Encapsulated Limbal Epithelial Stem Cells Modulates Corneal Epithelial Cell Proliferation

Limbal epithelial stem cells may ameliorate limbal stem cell deficiency through secretion of therapeutic proteins, delivered to the cornea in a controlled manner using hydrogels. In the present study the secretome of alginate-encapsulated limbal epithelial stem cells is investigated. Conditioned medium was generated from limbal epithelial stem cells encapsulated in 1.2% (w/v) calcium alginate gels. Conditioned medium proteins separated by 1-D gel electrophoresis were visualized by silver staining. Proteins of interest including secreted protein acidic and rich in cysteine, profilin-1, and galectin-1 were identified by immunoblotting. The effect of conditioned medium (from alginate-encapsulated limbal epithelial stem cells) on corneal epithelial cell proliferation was quantified and shown to significantly inhibit (P≤0.05) their growth. As secreted protein acidic and rich in cysteine was previously reported to attenuate proliferation of epithelial cells, this protein may be responsible, at least in part, for inhibition of corneal epithelial cell proliferation. We conclude that limbal epithelial stem cells encapsulated in alginate gels may regulate corneal epithelialisation through secretion of inhibitory proteins.     

Y-27632, a ROCK Inhibitor,Promoted Limbal Epithelial Cell Proliferation and Corneal Wound Healing

Transplantation of ex vivo cultured limbal epithelial cells is proven effective in restoring limbal stem cell deficiency. The present study aimed to investigate the promoting effect of Y-27632 on limbal epithelial cell proliferation. Limbal explants isolated from human donor eyes were expanded three weeks on culture dishes and outgrowth of epithelial cells was subsequently subcultured for in vitro experiments. In the presence of Y-27632, the ex vivo limbal outgrowth was accelerated, particularly the cells with epithelial cell-like morphology. Y-27632 dose-dependently promoted the proliferation of in vitro cultured human limbal epithelial cells as examined by phase contrast microscopy and luminescent cell-viability assay 30 hours after the treatment. The colony forming efficacy determined 7 days after the treatment was enhanced by Y-27632 also in a dose-dependent manner. The number of p63- or Ki67-positive cells was dose-dependently increased in Y-27632-treated cultures as detected by immunofluorescent staining and western blotanalysis. Cell cycle analysis by flow cytometric method revealed an increase in S-phase proliferating cells. The epithelial woundclosure rate was shown to be faster in experimental group received topical treatment withY-27632 than the sham control using a rat corneal wounding model. These resultsdemonstrate that Y-27632 can promote both the ex vivo and in vitro proliferation oflimbal epithelial cell proliferation. The in vivo enhanced epithelial wound healingfurther implies that the Y-27632 may act as a new strategy for treating limbal stem cell deficiency.     

Localisation of Epithelial Cells Capable of Holoclone Formation In Vitro and Direct Interaction with Stromal Cells in the Native Human Limbal Crypt

Limbal epithelial stem cells (LESCs) are essential to maintain the transparent ocular surface required for vision. Despite great advances in our understanding of ocular stem cell biology over the last decade, the exact location of the LESC niche remains unclear. In the present study we have used in vitro clonal analysis to confirm that limbal crypts provide a niche for the resident LESCs. We have used high-resolution imaging of the basal epithelial layer at the limbus to identify cells with a morphology consistent with stem cells that were only present within the basal layer of the limbal crypts. These cells are proximal to limbal stromal cells suggesting direct cell-to-cell interaction. Serial block-face scanning electron microscopy (SBFSEM) confirmed that the putative LESCs are indeed in direct contact with cells in the underlying stroma, a contact that is facilitated by focal basement membrane interruptions. Limbal mesenchymal cells previously identified in the human limbus collocate in the crypt-rich limbal stromal area in the vicinity of LESCs and may be involved in the cell-to-cell contact revealed by SBFSEM. We also observed a high population of melanocytes within the basal layer of the limbal crypts. From these observations we present a three dimensional reconstruction of the LESC niche in which the stem cell is closely associated and maintained by both dendritic pigmented limbal melanocytes and elongated limbal stromal cells.     

An investigation of donor and culture parameters which influence epithelial outgrowths from cultured human cadaveric limbal explants

Limbal stem cell deficiency is a blinding disease which affects the cornea at the front of the eye. The definitive cure involves replacing the corneal epithelial (limbal) stem cells, for example by transplanting cultured limbal epithelial cells. One method of performing cultures is to grow a sheet of epithelial cells from a limbal explant on human amniotic membrane. The growth of limbal tissue can be variable. The aim of this study is to investigate how different donor and culture factors influence the ex vivo growth of cadaveric limbal explants. Limbal explant cultures were established from 10 different cadaveric organ cultured corneo‐scleral discs. The growth rate and the time taken for growth to be established were determined. Statistical analysis was performed to assess correlation between these factors and donor variables including donor age, sex, time from donor death to enucleation, time from enucleation to organ culture storage and duration in organ culture. Growth curves consistently showed a lag phase followed by a steeper linear growth phase. Donor age, time between death and enucleation, and time between enucleation and organ culture were not correlated to the lag time or the growth rate. Time in organ culture had a significant correlation with the duration of lag time (P = 0.003), but no relationship with the linear growth rate. This study shows that an important factor correlating with growth variation is the duration of corneo‐scleral tissue in organ culture. Interestingly, donor age was not correlated with limbal explant growth. J. Cell. Physiol. © 2012 Wiley Periodicals, Inc.     

Isoforms of p63 in corneal stem cells cultured on human amniotic membrane

The p63 gene supports stem cell proliferation and regulation in epithelial cells. In this study, corneal epithelial cells were cultured on human amniotic membrane (HAM) and investigated for p63 and its isoform genes. Human limbal biopsies obtained from cadaveric donor eyes were cultivated on intact and denuded HAM. Transactivation (TA) specific domain was positive in the limbal cells cultured over denuded HAM and negative on others. TAp63α,β,γ isoforms are negative in all the limbal cells cultured on intact, denuded and limbal tissues but not in cornel epithelial tissue. p63α isoform is present in all except on denuded HAM. αβ sharing region is not expressed only in cornel epithelial tissue. γ isoform is expressed in all the samples. ΔNp63α region is present in cells cultured over the intact HAM whereas it is negative on the cells cultured over the denuded HAM. The other isoforms such as ΔNp63β and ΔNp63γ are negative in all samples. The limbal cells cultured over the intact HAM were able to maintain high proliferative potential when compared to denuded HAM. Thus, p63 isoforms plays a biological function to retain the proliferative capacity of corneal epithelial cells and maintains the stemness when cultured on intact HAM.     

Limbal niche cells are a potent resource of adult mesenchymal progenitors

Limbal niche cells located in the limbal Palisades of Vogt are mesenchymal stem cells that reside next to limbal basal epithelial cells. Limbal niche cells are progenitors that express embryonic stem cell markers such as Nanog, Nestin, Oct4, Rex1, Sox2 and SSEA4, mesenchymal cell markers such as CD73, CD90 and CD105, and angiogenesis markers such as Flk‐1, CD31, CD34, VWF, PDGFRβ and α‐SMA, but negative for CD45. In addition, the stemness of limbal niche cells can be maintained during their cell culture in a three‐dimension environment. Furthermore, expanded limbal niche cells have the capability to undergo adipogenesis, chondrogenesis, osteogenesis and endogenesis in vitro, indicating that they are in fact a valuable resource of adult progenitors. Furthermore studies on how the limbal niche cells regulate the aforementioned stemness and corneal fate decision are warranted, as those investigations will shed new light on how mesenchymal progenitors reverse limbal stem cell deficiency and lead to new methods for limbal niche cell treatment.     

Ex vivo expansion of limbal stem cells is affected by substrate properties

Limbal epithelial stem cells play a key role in the maintenance and regulation of the corneal surface. Damage or destruction of these cells results in vascularisation and corneal opacity. Subsequent limbal stem cell transplantation requires an ex vivo expansion step and preserving cells in an undifferentiated state remains vital. In this report we seek to control the phenotype of limbal epithelial stem cells by the novel application of compressed collagen substrates. We have characterised the mechanical and surface properties of conventional collagen gels using shear rheology and scanning electron microscopy. In doing so, we provide evidence to show that compressive load can improve the stiffness of collagen substrates. In addition Western blotting and immunohistochemistry display increased cytokeratin 3 (CK3) protein expression relating to limbal epithelial cell differentiation on stiff collagen substrates. Such gels with an elastic modulus of 2900 Pa supported a significantly higher number of cells than less stiff collagen gels (3 Pa). These findings have substantial influence in the development of ocular surface constructs or experimental models particularly in the fields of stem cell research, tissue engineering and regenerative medicine.     

Conjunctival epithelial cells can resurface denuded cornea, but do not transdifferentiate to express cornea-specific keratin 12 following removal of limbal epithelium in mouse

Limbal stem cell deficiency contributes to recurrent corneal epithelial defects. We examined whether the conjunctival epithelium can transdifferentiate to corneal epithelium following surgically induced limbal stem cell deficiency. Mice were anesthetized by intraperitoneal injection of sodium pentobarbital. Partial or total epithelial removal was produced with a no. 69 Beaver blade under a dissecting microscope. The wounds were allowed to heal for 0–28 days, and the mice were examined every other day to evaluate re-epithelialization. Corneas were then subjected to histological, immunohistochemical studies and Western blot analysis with epitope-specific anti-keratin 12 antibodies. Partial epithelial defects re-epithelialized within 2 days and were normal in appearance and expressed cornea-specific keratin 12. In eyes with limbal deficiency, re-epithelialization progressed more slowly and was characterized by opacification; epithelial closure usually occurred by the 7th day. This epithelium differed from normal corneal epithelium in basic morphology, cell shape, and the presence of goblet cells at 2 weeks after injury. The epithelium at the center of injured corneas with total defect at 4 weeks had cornealike morphology and was devoid of goblet cells. These epithelial cells derived from conjunctiva did not express the cornea-specific keratin 12, as determined by immunohistochemistry, Western blot analysis and in situ hybridization. As evidenced by differences in morphology and the expression of cornea-specific keratin 12, conjunctival transdifferentiation does not occur in conjunctical overgrowth after the removal of limbal epithelium.     

Culture of limbal stem cells on human amniotic membrane

Limbal stem cells (LSC) have an important role in the maintenance of the corneal surface epithelium, and autologous cultured limbal epithelial cell (HLECs) transplantations have contributed substantially to the treatment of the visually disabling condition known as LSC deficiency. A major challenge is the ability to identify LSC in vitro and in situ, and one of the major controversies in the field relates to reliable LSC markers. This study was carried out to evaluate the culture of a limbal biopsy on human amniotic membrane (HAM): directly on the chorionic side and on intact epithelium, and the expression of the stem cell associated markers: ABCG2, p63. HAM has been extensively used for ocular surface reconstruction and has properties which facilitate the growth of epithelial cells controlling inflammation and scarring.     

Limbal stem cells: Central concepts of corneal epithelial homeostasis

A strong cohort of evidence exists that supports the localisation of corneal stem cells at the limbus. The distinguishing characteristics of limbal cells as stem cells include slow cycling properties, high proliferative potential when required, clonogenicity, absence of differentiation marker expression coupled with positive expression of progenitor markers, multipotency, centripetal migration, requirement for a distinct niche environment and the ability of transplanted limbal cells to regenerate the entire corneal epithelium. The existence of limbal stem cells supports the prevailing theory of corneal homeostasis, known as the XYZ hypothesis where X represents proliferation and stratification of limbal basal cells, Y centripetal migration of basal cells and Z desquamation of superficial cells. To maintain the mass of cornea, the sum of X and Y must equal Z and very elegant cell tracking experiments provide strong evidence in support of this theory. However, several recent stud-ies have suggested the existence of oligopotent stem cells capable of corneal maintenance outside of the limbus. This review presents a summary of data which led to the current concepts of corneal epithelial homeostasis and discusses areas of controversy surrounding the existence of a secondary stem cell reservoir on the corneal surface     

Ophthalmic Applications of Preserved Human Amniotic Membrane: A Review of Current Indications

Preserved human amniotic membrane (AM) is currently being used for a wide spectrum of ocular surface disorders. The AM has a basement membrane, which promotes epithelial cell migration and adhesion. The presence of a unique avascular stromal matrix reduces inflammation, neovascularization and fibrosis. The basic tenets of amniotic membrane transplantation (AMT) are to promote re-epithelialization, to reconstruct the ocular surface and to provide symptomatic relief from surface aberrations. AMT is a useful technique for reconstruction of surface defects resulting from removal of surface tumors and symblephara. AMT has effectively restored a stable corneal epithelium in eyes with, persistent epithelial defects and corneal ulcers. In the setting of acute ocular burns and SJS, AMT has satisfactorily reduced scarring and inflammation. AMT alone may be an effective alternative for partial limbal stem cell deficiency. However remarkable improvements in surface stability have resulted from concurrent AMT and limbal stem cell transplantation, wherein the limbal grafts are obtained from the normal fellow eye, living relative or cadaveric eye. In severe or bilateral cases, well being of the donor eye is a major concern. Currently, the most unique application of preserved human AM in ophthalmology is its use as a substrate for ex-vivo expansion of corneal and conjunctival epithelium. In this novel technique of tissue engineering, epithelial stem cells can be safely harvested and expanded on denuded AM. The resultant composite cultured tissue has been successfully transplanted to restore vision, as well as the structure and function of damaged ocular surfaces.     

Bone Morphogenetic Protein 4 (BMP4) Enhances the Differentiation of Human Induced Pluripotent Stem Cells into Limbal Progenitor Cells

Corneal epithelium maintains visual acuity and is regenerated by the proliferation and differentiation of limbal progenitor cells. Transplantation of human limbal progenitor cells could restore the integrity and functionality of the corneal surface in patients with limbal stem cell deficiency. However, multiple protocols are employed to differentiate human induced pluripotent stem (iPS) cells into corneal epithelium or limbal progenitor cells. The aim of this study was to optimize a protocol that uses bone morphogenetic protein 4 (BMP4) and limbal cell-specific medium. Human dermal fibroblast-derived iPS cells were differentiated into limbal progenitor cells using limbal cell-specific (PI) medium and varying doses (1, 10, and 50 ng/mL) and durations (1, 3, and 10 days) of BMP4 treatment. Differentiated human iPS cells were analyzed by real-time polymerase chain reaction (RT-PCR), Western blotting, and immunocytochemical studies at 2 or 4 weeks after BMP4 treatment. Culturing human dermal fibroblast-derived iPS cells in limbal cell-specific medium and BMP4 gave rise to limbal progenitor and corneal epithelial-like cells. The optimal protocol of 10 ng/mL and three days of BMP4 treatment elicited significantly higher limbal progenitor marker (ABCG2, ∆Np63α) expression and less corneal epithelial cell marker (CK3, CK12) expression than the other combinations of BMP4 dose and duration. In conclusion, this study identified a successful reprogramming strategy to induce limbal progenitor cells from human iPS cells using limbal cell-specific medium and BMP4. Additionally, our experiments indicate that the optimal BMP4 dose and duration favor limbal progenitor cell differentiation over corneal epithelial cells and maintain the phenotype of limbal stem cells. These findings contribute to the development of therapies for limbal stem cell deficiency disorders.     

Use of irradiated human amnion as a matrix for limbal stem cell culture

Several ocular diseases affect the corneal surface; the development of effective technologies for the treatment of corneal lesions has brought about an improvement in the quality of life of affected patients. The aim of this study is to culture and characterize limbal stem cells cultured on gamma (⁶⁰Co) radiosterilized human amnion (RHA). Limbal stem cells were isolated from ten preserved samples of corneal transplant. The cells were cultured since primary culture until expanded cells on RHA and stained with monoclonal antibodies to establish their immunophenotype, after which cytokeratin 12 and Vimentin were positive by immunohistochemistry. The immunophenotype remained constant since primary culture until expanded cells in RHA. The RHA and cells construct were structurally integrated. Immunohistochemistry was cytokeratin 12, Vimentin positive, and cytokeratin 19 negative. In vitro limbal cells maintain a constant epithelial transition immunophenotype in culture up to primary culture until expanded cells on RHA.     

Evaluating alternative stem cell hypotheses for adult corneal epithelial maintenance

In this review we evaluate evidence for three different hypotheses that explain how the corneal epithelium is maintained. The limbal epithelial stem cell(LESC)hypothesis is most widely accepted. This proposes that stem cells in the basal layer of the limbal epithelium, at the periphery of the cornea, maintain themselves and also produce transient(or transit) amplifying cells(TACs). TACs then move centripetally to the centre of the cornea in the basal layer of the corneal epithelium and also replenish cells in the overlying suprabasal layers. The LESCs maintain the corneal epithelium during normal homeostasis and become more active to repair significant wounds. Second, the corneal epithelial stem cell(CESC) hypothesis postulates that, during normal homeostasis, stem cells distributed throughout the basal corneal epithelium, maintain the tissue. According to this hypothesis, LESCs are present in the limbus but are only active during wound healing. We also consider a third possibility, that the corneal epithelium is maintained during normal homeostasis by proliferation of basal corneal epithelial cells without any input from stem cells. After reviewing the published evidence, we conclude that the LESC and CESC hypotheses are consistent with more of the evidence than the third hypothesis, so we do not consider this further. The LESC and CESC hypotheses each have difficulty accounting for one main type of evidence so we evaluate the two key lines of evidence that discriminate between them. Finally, we discuss how lineage-tracing experiments have begun to resolve the debate in favour of the LESC hypothesis. Nevertheless, it also seems likely that some basal corneal epithelial cells can act as long-term progenitors if limbal stem cell function is compromised. Thus, this aspect of the CESC hypothesis may have a lasting impact on our understanding of corneal epithelial maintenance, even if it is eventually shown that stem cells are restricted to the limbus as proposed by the LESC hypothesis.     

Corneal stem cells and tissue engineering: Current advances and future perspectives

Major advances are currently being made in regenerative medicine for cornea. Stem cell-based therapies represent a novel strategy that may substitute conventional corneal transplantation, albeit there are many challenges ahead given the singularities of each cellular layer of the cornea. This review recapitulates the current data on corneal epithelial stem cells, corneal stromal stem cells and corneal endothelial cell progenitors. Corneal limbal autografts containing epithelial stem cells have been transplanted in humans for more than 20 years with great successful rates, and researchers now focus on ex vivo cultures and other cell lineages to transplant to the ocular surface. A small population of cells in the corneal endothelium was recently reported to have self-renewal capacity, although they do not proliferate in vivo. Two main obstacles have hindered endothelial cell transplantation to date: culture protocols and cell delivery methods to the posterior cornea in vivo. Human corneal stromal stem cells have been identified shortly after the recognition of precursors of endothelial cells. Stromal stem cells may have the potential to provide a direct cell-based therapeutic approach when injected to corneal scars. Furthermore, they exhibit the ability to deposit organized connective tissue in vitro and may be useful in corneal stroma engineering in the future. Recent advances and future perspectives in the field are discussed.     

Rat limbal epithelial side population cells exhibit a distinct expression of stem cell markers that are lacking in side population cells from the central cornea

The side population (SP) phenotype is shared by stem cells in various tissues and species. Here we demonstrate SP cells with Hoechst dye efflux were surprisingly collected from the epithelia of both the rat limbus and central cornea, unlike in human and rabbit eyes. Our results show that rat limbal SP cells have a significantly higher expression of the stem cell markers ABCG2, nestin, and notch 1, compared to central corneal SP cells. Immunohistochemistry also revealed that ABCG2 and the epithelial stem/progenitor cell marker p63 were expressed only in basal limbal epithelial cells. These results demonstrate that ABCG2 expression is closely linked to the stem cell phenotype of SP cells.     

Niche regulation of corneal epithelial stem cells at the limbus

Among all adult somatic stem cells,those of the corneal epithelium are unique in their exclusive location in a definedlimbai structure termed Palisades of Vogt.As a result,surgical engraftment oflimbal epithelial stem cells with or withoutex vivo expansion has long been practiced to restore sights in patients inflicted with limbal stem cell deficiency.Neverthe-less,compared to other stem cell examples,relatively little is known about the limbal niche,which is believed to play apivotal role in regulating self-renewal and fate decision of limbal epithelial stem cells.This review summarizes relevantliterature and formulates several key questions to guide future research into better understanding of the pathogenesis oflimbal stem cell deficiency and further improvement of the tissue engineering of the corneal epithelium by focusing onthe limbal niche.     

Maintenance and Distribution of Epithelial Stem/Progenitor Cells after Corneal Reconstruction Using Oral Mucosal Epithelial Cell Sheets

We assessed the maintenance and distribution of epithelial stem/progenitor cells after corneal reconstruction using tissue-engineered oral mucosal cell sheets in a rat model. Oral mucosal biopsy specimens were excised from green fluorescent protein (GFP) rats and enzymatically treated with Dispase II. These cells were cultured on inserts with mitomycin C-treated NIH/3T3 cells, and the resulting cell sheets were harvested. These tissue-engineered cell sheets from GFP rats were transplanted onto the eyes of a nude rat limbal stem cell deficiency model. Eight weeks after surgery, ocular surfaces were completely covered by the epithelium with GFP-positive cells. Transplanted corneas expressed p63 in the basal layers and K14 in all epithelial layers. Epithelial cells harvested from the central and peripheral areas of reconstructed corneas were isolated for a colony-forming assay, which showed that the colony-forming efficiency of the peripheral epithelial cells was significantly higher than that of the central epithelial cells 8 weeks after corneal reconstruction. Thus, in this rat model, the peripheral cornea could maintain more stem/progenitor cells than the central cornea after corneal reconstruction using oral mucosal epithelial cell sheets.     

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.