Involvement of p27KIP1 in the proliferation of the developing corneal endothelium

Corneal endothelial wound healing: understanding the regenerative capacity of the innermost layer of the cornea

Currently, there are very few well-established treatments to stimulate corneal endothelial cell regeneration in vivo as a cure for corneal endothelial dysfunctions. The most frequently performed intervention for a damaged or dysfunctional corneal endothelium nowadays is corneal endothelial keratoplasty, also known as lamellar corneal transplantation surgery. Newer medical therapies are emerging and are targeting the regeneration of the corneal endothelium, helping the patients regain their vision without the need for donor tissue. Alternatives to donor tissues are needed as the aging population requiring transplants, has further exacerbated the pressure on the corneal eye banking system. Significant ongoing research efforts in the field of corneal regenerative medicine have been made to elucidate the underlying pathways and effector proteins involved in corneal endothelial regeneration. However, the literature offers little guidance and selective attention to the question of how to fully exploit these pathways. The purpose of this paper is to provide an overview of wound healing characteristics from a biochemical level in the lab to the regenerative features seen in the clinic. Studying the pathways involved in corneal wound healing together with their key effector proteins, can help explain the effect on the proliferation and migration capacity of the corneal endothelial cells.

Corneal endothelial regeneration in human eyes using endothelium-free grafts

Background

To report on corneal endothelial regeneration, graft clarity, and vision recovery when using endothelium-free grafts.

Methods

We evaluated the donor’s cell viability using trypan blue staining and dual staining with calcein acetoxy methyl ester and ethidium homodimer-1. To preserve eyeball integrity, we performed therapeutic penetrating keratoplasty using cryopreserved donor tissue without endothelium on 195 consecutive patients who suffered from corneal perforation due to progressive primary corneal disease such as herpes simplex keratitis, fungal keratitis, ocular thermal burns, keratoconus, and phlyctenular keratoconjunctivitis. Of these, 18 eyes recovered corneal graft clarity and underwent periodic slit-lamp microscopy, A-scan pachymetry, and in vivo confocal microscopy to observe the clinical manifestations, variations in corneal thickness, and repopulation of the corneal endothelial cells on the donor grafts.

Results

No viable cells were detected in the cryopreserved corneas. After the therapeutic penetrating keratoplasty, notable corneal graft edema was observed in all 18 eyes for 1–4 months, and no corneal endothelial cells were detected on the grafts during this period. Thereafter, we observed gradual and progressive regression and final resolution of the stromal edema, with complete recovery of corneal graft clarity. Through periodic confocal microscopy, we observed the corneal endothelium’s regenerating process, along with single cells bearing multiple nuclei and cell division-like morphology. The regenerated endothelium on the grafts reached a mean cell density of 991 cells/mm². Remarkable vision rehabilitation was achieved in all 18 patients.

Conclusions

We obtained conclusive evidence that host-derived endothelial cells can regenerate a new endothelium over the endothelium-free graft, which possesses normal functions for corneal clarity and vision recovery.

Revisiting Existing Evidence of Corneal Endothelial Progenitors and Their Potential Therapeutic Applications in Corneal Endothelial Dysfunction

PURPOSE

A recent successful clinical trial demonstrated that a less invasive cell-injection procedure is a viable medical modality for treating corneal endothelial dystrophy. This medical advance still relies on human corneal endothelial cell (HCEC) sources derived from rare cornea donations. The progenitor of the corneal endothelium, which has the characteristics of active proliferation and lineage restriction, will be an ideal cell source for expansion ex vivo. However, the distribution of progenitor-like cells in the corneal endothelial sheet has been under debate for more than a decade.

METHODS

This paper re-examines the scientific evidence of the existence of human corneal endothelial progenitors (HCEPs) from the aspects of (1) the origin of cornea formation during ocular development, (2) manifestations from clinical studies, and (3) the distinctive properties of ex vivo-cultured subpopulations.

RESULTS

The discrepancies regarding different types of progenitor-like cells in various locations of the cornea are based on the fact that the corneal endothelium is derived from different cell types with multiple origins during corneal formation.

CONCLUSIONS

Resolving this long-standing issue in corneal biology will enable various types of progenitors to be isolated and their potencies regarding the formation of functional endothelial cells to be examined. Additionally, an effective niche system for quantitatively producing therapeutic cells can be formulated to satisfy the burning need associated with corneal endothelial dystrophy in the future.

A non-canonical role for p27Kip1 in restricting proliferation of corneal endothelial cells during development

The cell cycle regulator p27^Kip1 is a critical factor controlling cell number in many lineages. While its anti-proliferative effects are well-established, the extent to which this is a result of its function as a cyclin-dependent kinase (CDK) inhibitor or through other known molecular interactions is not clear. To genetically dissect its role in the developing corneal endothelium, we examined mice harboring two loss-of-function alleles, a null allele (p27⁻) that abrogates all protein function and a knockin allele (p27^CK−) that targets only its interaction with cyclins and CDKs. Whole-animal mutants, in which all cells are either homozygous knockout or knockin, exhibit identical proliferative increases (~0.6-fold) compared with wild-type tissues. On the other hand, use of mosaic analysis with double markers (MADM) to produce infrequently-occurring clones of wild-type and mutant cells within the same tissue environment uncovers a roughly three- and six-fold expansion of individual p27^CK−/CK− and p27^−/− cells, respectively. Mosaicism also reveals distinct migration phenotypes, with p27^−/− cells being highly restricted to their site of production and p27^CK−/CK− cells more widely scattered within the endothelium. Using a density-based clustering algorithm to quantify dispersal of MADM-generated clones, a four-fold difference in aggregation is seen between the two types of mutant cells. Overall, our analysis reveals that, in developing mouse corneal endothelium, p27 regulates cell number by acting cell autonomously, both through its interactions with cyclins and CDKs and through a cyclin-CDK-independent mechanism(s). Combined with its parallel influence on cell motility, it constitutes a potent multi-functional effector mechanism with major impact on tissue organization.

In Vitro Expansion of Corneal Endothelial Cells for Transplantation

The corneal endothelium forms a leaky barrier between the corneal stroma and the aqueous humor of the anterior chamber. This cell monolayer maintains the corneal stroma in a state of relative dehydration, a process called deturgescence, which is required in order to obtain corneal stromal transparency. Endothelial dysfunctions lead to visual impairment that ultimately can only be treated surgically via the corneal transplantation of a functional endothelium. Shortages of corneas suitable for transplantation has motivated research toward new alternatives involving in vitro corneal endothelial cell (CEC) expansion.This chapter describes current methods that allow isolate and culture CECs. In brief, Descemet membrane is peeled out of the cornea and digested in order to obtain CECs. Cells are then seeded and cultured.

A review of the evidence for in vivo corneal endothelial regeneration

Human corneal endothelium has long been thought to be a nonmitotic cell layer with no endogenous reparative potential. Pathologies that damage endothelial function result in corneal decompensation and, if untreated, blindness. The mainstay of treatment involves partial or complete corneal replacement, amounting to 40% of all corneal transplants performed worldwide. We summarize the case reports describing complications postoperatively in the form of (sub)total graft detachment and those resulting in postoperative bare stroma. Complications during cataract and glaucoma surgeries leading to an uncovered posterior cornea are also included. We discuss the newer treatment strategies that are alternatives for current Descemet membrane endothelial keratoplasty and Descemet stripping automated endothelial keratoplasty, including partial grafts and stripping of the diseased cell layer. In more than half of the cases reviewed, corneal transparency returned despite incomplete or no corneal endothelial cell transplantation. We question the existing paradigm concerning corneal endothelial wound healing in vivo. The data support further clinical study to determine the safety of simple descemethorexis in central endothelial pathologies, such as Fuchs endothelial corneal dystrophy, where presence of healthy peripheral cells may allow successful corneal recompensation without the need for donor cells.

Regenerative potential of the zebrafish corneal endothelium

Corneal transparency, critical for clear vision, is maintained in part by the pump function of the corneal endothelial cells that are arrested in G(1) phase of the cell cycle in adult humans. Thus loss of endothelial cells leads to a decrease in endothelial cell density. A decrease below a critical threshold results in corneal edema and subsequent vision loss. Corneal edema due to endothelial dysfunction is a common indication for transplantation in developed countries. The zebrafish has emerged as a model for vertebrate regeneration due to its ease of genetic manipulation and remarkable regenerative capacity. The purpose of this study was to investigate the response and regenerative potential of the zebrafish corneal endothelium to pharmacological and mechanical injury. Similar to the human cornea, Na(+)/K(+) ATPase activity is necessary to maintain the pump function as intracameral injection of ouabain resulted in an increase in central corneal thickness. Surgical removal of the majority of the central corneal endothelium resulted in a similar increase in corneal thickness. Remarkably, by just one week post-injury the central corneal endothelium had largely re-formed. Immunofluorescence of phosphorylated histone H3 indicated that this recovery correlated with corneal endothelial cells re-entering the cell cycle. In conclusion, our results establish zebrafish as a useful model of corneal injury and repair that may offer insights into the mechanism of cell cycle arrest in human corneal endothelial cells.

Research progress on the negative factors of corneal endothelial cells proliferation

The human corneal endothelium forms a boundary layer between anterior chamber and corneal stoma. The corneal endothelial cells are responsible for maintaining cornea transparency, which is very vital for our visual acuity, via its pump and barrier functions. The adult corneal endothelial cells in vivo lack proliferation in response to the cell loss caused by outer damages and diseases. As a result, in order to compensate for cell loss, corneal endothelial cells migrate and enlarge while not via dividing to increase the endothelial cell density. Therefore, it is not capable for corneal endothelium to restore the corneal clarity. Some researches have proved that in vitro the corneal endothelial maintained proliferation ability. This review describes the current research progress regarding the negative factors that inhibit proliferation of the corneal endothelial cells. This review will mainly present several genes and proteins that inhibit the proliferation of the corneal endothelial cells, of course including some other factors like enzymes and position.

Human corneal endothelial cells employ phosphorylation of p27(Kip1) at both Ser10 and Thr187 sites for FGF-2-mediated cell proliferation via PI 3-kinase

PURPOSE

FGF-2 stimulates cell proliferation of rabbit corneal endothelial cells (rCECs) by degrading the cyclin-dependent kinase inhibitor p27(Kip1) (p27) through its phosphorylation mechanism. The authors investigated whether the cell proliferation of human CECs (hCECs) is also induced by FGF-2 stimulation through the p27 phosphorylation pathway.

METHODS

Expression and activation of protein were analyzed by immunoblotting. Cell proliferation was measured by 3-(4,5-dimethylthiazolyl-2)-2,5-diphenyltetrazolium bromide (MTT) assay. Transfection of hCECs with small interference RNA (siRNA) was performed using a transfection reagent.

RESULTS

FGF-2 stimulated cell proliferation in hCECs; the FGF-2 action was completely blocked by pathway-specific inhibitors for PI 3-kinase (LY294002) and MEK1/2 (U0126), respectively. Using immunoblotting, the authors showed that FGF-2 induced phosphorylation of p27 at both serine 10 (Ser10) and threonine 187 (Thr187) sites. These effects were also completely blocked by LY294002 or U0126. The authors then determined cross-talk between PI 3-kinase and extracellular signal-regulated kinase (ERK)1/2; blocking of ERK1/2 activation by LY294002 indicated that in hCECs ERK1/2 works as a downstream effector to PI 3-kinase for cell proliferation induced by FGF-2, whereas the ERK1/2 pathway in rCECs is parallel to the PI 3-kinase pathway. However, the downstream mechanism involved in cell cycle progression in hCECs is identical to that of rCECs: phosphorylation of p27 at Ser10 was mediated by kinase-interacting stathmin (KIS), confirmed with siRNA to KIS, and phosphorylation of p27 at Thr187 was mediated by cell division cycle 25A (Cdc25A), confirmed using Cdc25A inhibitor. CONCLUSIONS; FGF-2 stimulates proliferation of hCECs through PI 3-kinase and its downstream target ERK1/2 pathways. This linear signal transduction significantly downregulates p27 through its phosphorylation at both Ser10 and Thr187 sites mediated by KIS and Cdc25A, respectively.

Proliferative capacity of corneal endothelial cells

The corneal endothelial monolayer helps maintain corneal transparency through its barrier and ionic "pump" functions. This transparency function can become compromised, resulting in a critical loss in endothelial cell density (ECD), corneal edema, bullous keratopathy, and loss of visual acuity. Although penetrating keratoplasty and various forms of endothelial keratoplasty are capable of restoring corneal clarity, they can also have complications requiring re-grafting or other treatments. With the increasing worldwide shortage of donor corneas to be used for keratoplasty, there is a greater need to find new therapies to restore corneal clarity that is lost due to endothelial dysfunction. As a result, researchers have been exploring alternative approaches that could result in the in vivo induction of transient corneal endothelial cell division or the in vitro expansion of healthy endothelial cells for corneal bioengineering as treatments to increase ECD and restore visual acuity. This review presents current information regarding the ability of human corneal endothelial cells (HCEC) to divide as a basis for the development of new therapies. Information will be presented on the positive and negative regulation of the cell cycle as background for the studies to be discussed. Results of studies exploring the proliferative capacity of HCEC will be presented and specific conditions that affect the ability of HCEC to divide will be discussed. Methods that have been tested to induce transient proliferation of HCEC will also be presented. This review will discuss the effect of donor age and endothelial topography on relative proliferative capacity of HCEC, as well as explore the role of nuclear oxidative DNA damage in decreasing the relative proliferative capacity of HCEC. Finally, potential new research directions will be discussed that could take advantage of and/or improve the proliferative capacity of these physiologically important cells in order to develop new treatments to restore corneal clarity.

Endothelial mesenchymal transformation mediated by IL-1β-induced FGF-2 in corneal endothelial cells

This review describes the molecular mechanism of endothelial mesenchymal transformation (EMT) mediated by fibroblast growth factor-2 (FGF-2) in corneal endothelial cells (CECs). Corneal fibrosis is not frequently observed in corneal endothelium/Descemet's membrane complex; but when this pathologic tissue is produced, it causes a loss of vision by physically blocking light transmittance. Herein, we will address the cellular activities of FGF-2 and its signaling pathways during the EMT process. Furthermore, we will discuss the role of inflammation on FGF-2-mediated EMT. Interleukin-1β (IL-1β) greatly upregulates FGF-2 production in CECs, thus leading to FGF-2-mediated EMT; the whole spectrum of the injury-mediated inflammation (IL-1β pathway) and the subsequent EMT process (FGF-2 pathway) will be briefly discussed. Intervention in the two pathways will provide the means to block EMT before inflammation causes an irreversible change, such as the production of retrocorneal fibrous membrane observed in human eyes.

In vitro culture of human fetal corneal endothelial cells

PURPOSE

To explore and optimize a proper culture system for human fetal corneal endothelial cells (hFCECs), including the methods of primary culture, passage and cryopreservation.

METHODS

Fresh fetal corneas were explanted to propagate primary corneal endothelial cells. The cells were cultured in DMEM/F-12 supplemented with 10% fetal bovine serum (FBS) in the absence or presence of the extracts from bovine corneal endothelium cells (bCECs), and the cells were identified with immunocytochemical staining. The passage and cryopreservation of hFCECs were optimized according to previous reports on adult corneal endothelial cells.

RESULTS

Using the explant culture method, hFCECs migrated successfully within 3 days and assumed polygonal-shaped corneal endothelial morphology. The optimizing methods were 0.125% trypsin + 0.02% EDTA for passage and 10% DMSO + 90% FBS for cryopreservation. Recovered hFCECs from cryopreservation remained typical morphology and immunological markers of corneal endothelial cells, including positive staining of NSE, Nestin, Ki67 and ZO-1, and negative staining of CK3/12, which demonstrated that they retained the characterizations of corneal endothelial cells and proliferative capacity. Moreover, the extracts from bCECs can promote the proliferative capacity of hFCECs significantly, while maintaining their typical endothelial morphology.

CONCLUSIONS

The culture conditions of human fetal corneal endothelial cells were firstly optimized, including the primary culture, passage and cryopreservation. Meanwhile, we confirmed that the extracts from bovine corneal endothelium promoted the proliferative capacity while maintaining the morphology of hFCECs in vitro.

Enhanced survival in vitro of human corneal endothelial cells using mouse embryonic stem cell conditioned medium

Purpose

To determine whether mouse embryonic stem cell conditioned medium (ESC-CM) increases the proliferative capacity of human corneal endothelial cells (HCECs) in vitro.

Methods

Primary cultures of HCECs were established from explants of the endothelial cell layer, including the Descemet’s membrane. Cells were cultured in human corneal endothelium medium (CEM) containing 25% ESC-CM for the experimental group and CEM alone for the control group. Phase-contrast microscopy and reverse-transcription polymerase chain reaction (RT–PCR) were used to identify HCECs. The eruption time and HCEC morphology were observed under phase-contrast microscopy. We detected the protein expression of zona occludens protein-1 (ZO-1; a tight junction protein) and the Na⁺-K⁺-ATPase by western blot analysis and immunocytochemistry. The mRNA expression of the Na⁺-K⁺-ATPase, voltage-dependent anion channel 3 (VDAC3), solute carrier family 4, sodium bicarbonate cotransporter member 4 (SLC4A4), and chloride channel protein 3 (CLCN3) were detected by RT–PCR. To explore the proliferation capacity of HCECs, the colony forming efficiency (CFE) was determined by Giemsa staining and the cellular proliferation marker of Ki-67 protein (Ki-67) positive cells were detected by immunocytochemistry and flow cytometry. Progression of the cell cycle and apoptosis were analyzed by flow cytometry. Negative regulation of the cell cycle, as measured by cyclin-dependent kinase inhibitor p21 (p21) levels, was detected by western blot analysis and immunocytochemistry.

Results

In primary culture, HCECs in the 25%ESC-CM group erupted with polygonal appearance on day 2, while those in the CEM group erupted with slightly larger cells on day 3–4. HCECs in the 25%ESC-CM group could be subcultured until passage 6 without enlargement of cell volume, while those in the CEM group were enlarged and lost their polygonal appearance by passage 2. HCECs in both the 25%ESC-CM and CEM groups expressed ZO-1, Na⁺-K⁺-ATPase, VDAC3, SLC4A4, and CLCN3. The number of Ki67 positive cells, CFE, and percentage of cells entering the S and G₂ phases were higher in the 25%ESC-CM group than in the CEM group. The number of apoptotic cells and p21 protein expression both decreased in the 25%ESC-CM group.

Conclusions

Use of 25%ESC-CM significantly increased the number of proliferating cells. These effects may be achieved through inhibition of p21 expression and apoptosis. These results suggested that 25%ESC-CM may be a new tool for cultivating HCECs for transplantation.

Phosphorylation of p27(KIP1) in the Mitotic Cells of the Corneal Epithelium

PURPOSE

The mechanism in regulation of the cell cycle and proliferation of corneal epithelium in the homeostatic ocular surface remains unclear. The aim of this study is to examine the expression of p27(KIP1) and its phosphorylation in corneal epithelium.

METHODS

The eyes of C57BL/6 mice (7 weeks old) were enucleated. Formalin-fixed and paraffin-embedded tissue sections were examined using immunohistochemistry with anti-p27(KIP1), threonine 187 phosphorylated p27(KIP1) (T187-phospho-p27), and phosphorylated Histon H3 (pHiston H3) antibodies. Anti-T187-phospho-p27 and anti-pHiston H3 polyclonal antibodies were used for parallel immunofluorescent staining.

RESULTS

pHiston H3-immunopositive cells were noted in basal cells of the corneal epithelium. At high magnification of DAPI nuclear staining, mitotic and non-mitotic cells were observed in corneal basal layer. p27(KIP1)-positive nuclei were detected in corneal basal cells, where non-mitotic basal cells were located. In contrast, mitotic cells showed under detectable level on p27(KIP1) immunoreactivity. Immunoreactivity for T187-phospho-p27 was detected in basal cells of the corneal epithelium. At high magnification, it was confirmed that the immunopositive cells were mitotic cells. Immunoreactivity of T187-phospho-p27 as well as pHiston H3 was localized in the same corneal basal cells using double-staining immunohistochemistry.

CONCLUSIONS

These results suggested that degradation of p27(KIP1) regulates progression into mitosis in corneal basal cells.

The human ubiquitin conjugating enzyme, UBE2E3, is required for proliferation of retinal pigment epithelial cells

PURPOSE

Cell cycle progression is governed by the coordinated activities of kinases, phosphatases, and the ubiquitin system. The entire complement of ubiquitin pathway components that mediate this process in retinal pigment epithelial (RPE) cells remains to be identified. This study was undertaken to determine whether the human ubiquitin-conjugating enzyme, UBE2E3, is essential for RPE cell proliferation.

METHODS

UBE2E3 expression and localization in telomerase-immortalized, human RPE cells was determined with a UBE2E3-specific antibody. The necessity for UBE2E3 in RPE proliferation was determined using small interfering (si)RNA to target the expression of the enzyme. Cell counts and immunolabeling for the proliferation marker Ki-67 and the cyclin-dependent kinase inhibitor p27(Kip1) were performed to assess the consequences of UBE2E3 depletion. A mouse strain harboring a disrupted allele of UbcM2 (the mouse counterpart of UBE2E3) with the coding sequence for beta-galactosidase was used to track the developmental expression of the enzyme in murine RPE cells.

RESULTS

UBE2E3 localized in the nucleus of the immortalized RPE cells. Depletion of the enzyme by siRNA resulted in a cell-cycle exit accompanied by a loss of Ki-67, an increase in p27(Kip1), and a doubling in cell area. Rescue experiments confirmed the specificity of the RNA interference. In vivo, UbcM2 was transcriptionally downregulated during RPE development in the mouse.

CONCLUSIONS

UBE2E3 is essential for the proliferation of RPE-1 cells and is downregulated during RPE layer maturation in the developing mouse eye. These findings indicate that UBE2E3 is a major enzyme in modulating the balance between RPE cell proliferation and differentiation.

FGF-2-mediated signal transduction during endothelial mesenchymal transformation in corneal endothelial cells

This review describes the molecular mechanism of endothelial mesenchymal transformation (EMT) mediated by fibroblast growth factor 2 (FGF-2) in corneal endothelial cells. Corneal fibrosis is rarely observed in corneal endothelium/Descemet's membrane complex; but when this pathologic tissue occurs, it causes a loss of vision. Herein, we will address the cellular activities of FGF-2 and its signaling pathways during EMT. FGF-2 has 5 isoforms: 4 nuclear high molecular weight isoforms and 1 extracellular matrix (ECM) isoform. The vast majority of studies published in the field to date have described the effect of the ECM isoform that is released into the extracellular space, from which it can access plasma membrane receptors. Our discussion will focus on the ECM isoform and its receptor-mediated signal transduction.

The other pigment cell: specification and development of the pigmented epithelium of the vertebrate eye

Vertebrate retinal pigment epithelium (RPE) cells are derived from the multipotent optic neuroepithelium, develop in close proximity to the retina, and are indispensible for eye organogenesis and vision. Recent advances in our understanding of RPE development provide evidence for how critical signaling factors operating in dorso-ventral and distal-proximal gradients interact with key transcription factors to specify three distinct domains in the budding optic neuroepithelium: the distal future retina, the proximal future optic stalk/optic nerve, and the dorsal future RPE. Concomitantly with domain specification, the eye primordium progresses from a vesicle to a cup, RPE pigmentation extends towards the ventral side, and the future ciliary body and iris form from the margin zone between RPE and retina. While much has been learned about the molecular networks controlling RPE cell specification, key questions concerning the cell proliferative parameters in RPE and the subsequent morphogenetic events still need to be addressed in greater detail.