Approaches for corneal endothelium regenerative medicine

Evolution of therapeutic strategy based on oxidant-antioxidant balance for fuchs endothelial corneal dystrophy

Fuchs endothelial corneal dystrophy (FECD) stands as the most prevalent primary corneal endothelial dystrophy worldwide, posing a significant risk to corneal homeostasis and clarity. Corneal endothelial cells exhibit susceptibility to oxidative stress, suggesting a nuanced relationship between oxidant-antioxidant imbalance and FECD pathogenesis, irrespective of FECD genotype. Given the constrained availability of corneal transplants, exploration into non-surgical interventions becomes crucial. This encompasses traditional antioxidants, small molecule compounds, biologics, and diverse non-drug therapies, such as gene-related therapy, hydrogen therapy and near infrared light therapy. This review concentrates on elucidating the mechanisms behind oxidant-antioxidant imbalance and the evolution of strategies to restore oxidant-antioxidant balance in FECD. It provides a comprehensive overview of both conventional and emerging therapeutic approaches, offering valuable insights for the advancement of non-surgical treatment modalities. The findings herein might establish a robust foundation for future research and the therapeutic strategy of FECD.

An Osteoimmunomodulatory Biopatch Potentiates Stem Cell Therapies for Bone Regeneration by Simultaneously Regulating IL-17/Ferroptosis Signaling Pathways

Currently, there are still great challenges in promoting bone defect healing, a common health problem affecting millions of people. Herein an osteoimmunity-regulating biopatch capable of promoting stem cell-based therapies for bone regeneration is developed. A totally biodegradable conjugate is first synthesized, which can self-assemble into bioactive nano micelles (PPT NMs). This nanotherapy effectively improves the osteogenesis of periodontal ligament stem cells (PDLSCs) under pathological conditions, by simultaneously regulating IL-17 signaling and ferroptosis pathways. Incorporation of PPT NMs into biodegradable electrospun nanofibers affords a bioactive patch, which notably improves bone formation in two rat bone defect models. A Janus bio patch is then engineered by integrating the bioactive patch with a stem cell sheet of PDLSCs. The obtained biopatch shows additionally potentiated bone regeneration capacity, by synergistically regulating osteoimmune microenvironment and facilitating stem cell differentiation. Further surface functionalization of the biopatch with tannic acid considerably increases its adhesion to the bone defect, prolongs local retention, and sustains bioactivities, thereby offering much better repair effects in rats with mandibular or cranial bone defects. Moreover, the engineered bioactive patches display good safety. Besides bone defects, this osteoimmunity-regulating biopatch strategy can be applied to promote stem cell therapies for spinal cord injury, wound healing, and skin burns.

Bioprinting of human pluripotent stem cell derived corneal endothelial cells with hydrazone crosslinked hyaluronic acid bioink

BACKGROUND

Human corneal endothelial cells lack regenerative capacity through cell division in vivo. Consequently, in the case of trauma or dystrophy, the only available treatment modality is corneal tissue or primary corneal endothelial cell transplantation from cadaveric donor which faces a high global shortage. Our ultimate goal is to use the state-of-the-art 3D-bioprint technology for automated production of human partial and full-thickness corneal tissues using human stem cells and functional bioinks. In this study, we explore the feasibility of bioprinting the corneal endothelium using human pluripotent stem cell derived corneal endothelial cells and hydrazone crosslinked hyaluronic acid bioink.

METHODS

Corneal endothelial cells differentiated from human pluripotent stem cells were bioprinted using optimized hydrazone crosslinked hyaluronic acid based bioink. Before the bioprinting process, the biocompatibility of the bioink with cells was first analyzed with transplantation on ex vivo denuded rat and porcine corneas as well as on denuded human Descemet membrane. Subsequently, the bioprinting was proceeded and the viability of human pluripotent stem cell derived corneal endothelial cells were verified with live/dead stainings. Histological and immunofluorescence stainings involving ZO1, Na+/K+-ATPase and CD166 were used to confirm corneal endothelial cell phenotype in all experiments. Additionally, STEM121 marker was used to identify human cells from the ex vivo rat and porcine corneas.

RESULTS

The bioink, modified for human pluripotent stem cell derived corneal endothelial cells successfully supported both the viability and printability of the cells. Following up to 10 days of ex vivo transplantations, STEM121 positive cells were confirmed on the Descemet membrane of rat and porcine cornea demonstrating the biocompatibility of the bioink. Furthermore, biocompatibility was validated on denuded human Descemet membrane showing corneal endothelial -like characteristics. Seven days post bioprinting, the corneal endothelial -like cells were viable and showed polygonal morphology with expression and native-like localization of ZO-1, Na+/K+-ATPase and CD166. However, mesenchymal-like cells were observed in certain areas of the cultures, spreading beneath the corneal endothelial-like cell layer.

CONCLUSIONS

Our results demonstrate the successful printing of human pluripotent stem cell derived corneal endothelial cells using covalently crosslinked hyaluronic acid bioink. This approach not only holds promise for a corneal endothelium transplants but also presents potential applications in the broader mission of bioprinting the full-thickness human cornea.

Femtosecond Laser Cutting of Human Crystalline Lens Capsule and Decellularization for Corneal Endothelial Bioengineering

The bioengineering of corneal endothelial grafts consists of seeding in vitro cultured corneal endothelial cells onto a thin, transparent, biocompatible, and sufficiently robust carrier which can withstand surgical manipulations. This is one of the most realistic alternatives to donor corneas, which are in chronic global shortage. The anterior capsule of the crystalline lens has already been identified as one of the best possible carriers, but its challenging manual preparation has limited its use. In this study, we describe a femtosecond laser cutting process of the anterior capsule of whole lenses in order to obtain capsule discs of 8 mm diameter, similar to conventional endothelial grafts. Circular marks made on the periphery of the disc indicate its orientation. Immersion in water for 3 days is sufficient to completely remove the lens epithelial cells and to enable the seeding of corneal endothelial cells, which remain viable after 27 days of culture. Therefore, this method provides a transparent, decellularized disc ready to form viable tissue engineered endothelial grafts.

Deciphering the molecular symphony: Unraveling endothelial-to-mesenchymal transition in corneal endothelial cells

Understanding the molecular complexity of this phenomenon provides innovative targets for maintaining phenotypic integrity during in vitro expansion, thereby advancing corneal endothelial tissue engineering. In this study, we established an in vitro model to simulate endothelial-to-mesenchymal transition (EndMT) in corneal endothelial cells. Through RNA sequencing, we identified 452 upregulated and 163 downregulated genes, resulting in a total of 615 differentially expressed genes. Key pathways enriched by GO and KEGG analysis include extracellular matrix (ECM) regulation and the PI3K-Akt signaling pathway. Potential hub proteins such as THBS1, ITGA5, COL1A1, and SNAI1/2 were also identified, and their dynamic changes at different time points (0, 2, 12, 24 h) were monitored. Uncovering these key pathways and genes may deepen our understanding of the mechanisms underlying EndMT in corneal endothelial cells, providing valuable insights for optimizing in vitro cultivation strategies.

Impact of culture media on primary human corneal endothelial cells derived from old donors

Corneal endothelial dysfunction is a major indication for corneal transplantation. However, a global shortage of donor corneal tissues and risks associated with corneal surgeries have prompted exploration of alternative options, including tissue-engineered grafts or cell injection therapy. Nonetheless, these approaches require a controlled culture of primary human corneal endothelial cells (HCEnCs). Although HCEnCs established from young donors are generally more proliferative and maintain a better phenotype, corneas from old donors are more frequently accessible from eye banks due to a lower corneal endothelial cell count than the necessary threshold required for transplantation. In this study, we investigated various culture media to evaluate which one is the most appropriate for stimulating the proliferation while maintaining cell morphology and function of HCEnCs derived from old donors (age >65 years). All experiments were performed on paired research-grade donor corneas, divided for the conditions under investigation in order to minimize the inter-donor variability. Cell morphology as well as expression of specific markers were assessed at both mRNA (CD166, SLC4A11, ATP1A1, COL8A1, α-SMA, CD44, COL1A1, CDKN2A, LAP2A and LAP2B) and protein (ZO-1, α-SMA, Ki67 and LAP2) levels. Results obtained showed how the Dual Media formulation maintained the hexagonal phenotype more efficiently than Single Medium, but cell size gradually increased with passages. In contrast, the Single Medium provided a higher proliferation rate and a prolonged in vitro expansion but acquired an elongated morphology. To summarize, Single medium and Dual media preserve morphology and functional phenotype of HCEnCs from old donor corneas at low passages while maintenance of the same cell features at high passages remains an active area of research. The new insights revealed within this work become particularly relevant considering that the elderly population a) is the main target of corneal endothelial therapy, b) represents the majority of corneal donors. Therefore, the proper expansion of HCEnCs from old donors is essential to develop novel personalised therapeutic strategies and reduce requirement of human corneal tissues globally.

U-Net Convolutional Neural Network for Real-Time Prediction of the Number of Cultured Corneal Endothelial Cells for Cellular Therapy

Corneal endothelial decompensation is treated by the corneal transplantation of donor corneas, but donor shortages and other problems associated with corneal transplantation have prompted investigations into tissue engineering therapies. For clinical use, cells used in tissue engineering must undergo strict quality control to ensure their safety and efficacy. In addition, efficient cell manufacturing processes are needed to make cell therapy a sustainable standard procedure with an acceptable economic burden. In this study, we obtained 3098 phase contrast images of cultured human corneal endothelial cells (HCECs). We labeled the images using semi-supervised learning and then trained a model that predicted the cell centers with a precision of 95.1%, a recall of 92.3%, and an F-value of 93.4%. The cell density calculated by the model showed a very strong correlation with the ground truth (Pearson's correlation coefficient = 0.97, p value = 8.10 × 10-52). The total cell numbers calculated by our model based on phase contrast images were close to the numbers calculated using a hemocytometer through passages 1 to 4. Our findings confirm the feasibility of using artificial intelligence-assisted quality control assessments in the field of regenerative medicine.

Development of Thermoplastic Polyurethane Films for the Replacement of Corneal Endothelial Function of Transparency Maintenance

Endothelial keratoplasty is the main surgical procedure for treating corneal endothelial dysfunction (CED), which is limited by the global shortage of donor corneas. Herein, we developed and evaluated the modified thermoplastic polyurethane (M-TPU) films with gelatin-glycidyl methacrylate to replace the corneal endothelial function and maintain corneal transparency. The films displayed comparable light transmission characteristics with normal corneas and clinically favorable mechanical properties for surgical manipulation. After surface modification, the hydrophilicity and biocompatibility of M-TPU films were significantly improved. In the rabbit CED model, the M-TPU implants exhibited firm adhesion to the exposed stromal surface. The rabbit corneal transparency and thickness could be restored completely within 1 week of M-TPU film implantation. There was no significant inflammatory reaction and immune rejection during the follow-up of 1 month. Proteomic analysis suggested that the complement inhibition, the increase of mineral absorption, and the decrease of P53 apoptosis signaling pathway and lysine degradation might be beneficial in maintaining the corneal transparency. Overall, our study demonstrated the potential of M-TPU films as artificial implants for the replacement of corneal endothelial function to restore corneal thickness and transparency.

An effective method for culturing functional human corneal endothelial cells using a xenogeneic free culture medium

Endothelial dysfunction is a leading cause of corneal blindness in developed countries and the only available treatment is the endothelial transplantation. However, the limited availability of suitable donors remains a significant challenge, driving the exploration of alternative regenerative therapies. Advanced Therapy Medicinal Products show promise but must adhere to strict regulations that prohibit the use of animal-derived substances. This study investigates a novel culture methodology using Plasma Rich in Growth Factors (PRGF) as the only source of growth factors for primary cultures of human corneal endothelial cells (CECs). CECs were obtained from discarded corneas or endothelial rings and cultured in two different media: one supplemented with xenogeneic factors and other xenogeneic-free, using PRGF. Comprehensive characterization through immunofluorescence, morphological analyses, trans-endothelial electrical resistance measurements, RNA-seq, and qPCR was conducted on the two groups. Results demonstrate that CECs cultured in the xenogeneic-free medium exhibit comparable gene expression, morphology, and functionality to those cultured in the xenogeneic medium. Notably, PRGF-expanded CECs share 46.9% of the gene expression profile with native endothelium and express all studied endothelial markers. In conclusion, PRGF provides an effective source of xenogeneic-free growth factors for the culture of CECs from discarded corneal tissue. Further studies will be necessary to demonstrate the applicability of these cultures to cell therapies that make clinical translation possible.

Elucidating the Corneal Endothelial Cell Proliferation Capacity through an Interspecies Transcriptome Comparison

The regenerative capacity of corneal endothelial cells (CECs) differs between species; in bigger mammals, CECs are arrested in a non-proliferative state. Damage to these cells can compromise their function causing corneal opacity. Corneal transplantation is the current treatment for the recovery of clear eyesight, but the donor tissue demand is higher than the availability and there is a need to develop novel treatments. Interestingly, rabbit CECs retain a high proliferative profile and can repopulate the endothelium. There is a lack of fundamental knowledge to explain these differences. Gaining information on their transcriptomic variances could allow the identification of CEC proliferation drivers. In this study, human, sheep, and rabbit CECs are analyzed at the transcriptomic level. To understand the differences across each species, a pipeline for the analysis of pathways with different activities is generated. The results reveal that 52 pathways have different activity when comparing species with non-proliferative CECs (human and sheep) to species with proliferative CECs (rabbit). The results show that Notch and TGF-β pathways have increased activity in species with non-proliferative CECs, which might be associated with their low proliferation. Overall, this study illustrates transcriptomic pathway-level differences that can provide leads to develop novel therapies to regenerate the corneal endothelium.

Clinical applications of bioengineered tissue-cellular products for management of corneal diseases

PURPOSE OF REVIEW

To discuss bioengineered tissue-cellular products for treatment of corneal diseases that are currently in clinical use. These include tissue-cellular products that have received regulatory approval, are being used off-label in clinical practice, or are in active use in clinical trials.

RECENT FINDINGS

Due to the global shortage of donor corneal tissue, significant efforts have been made to develop bioengineering tissue-cellular products that can replace or augment the use of cadaveric tissue for corneal transplantation. The development of carrier substrates to support transplantation of cultivated limbal epithelial transplantation (CLET) has been a growing area of research. CLET offers a promising therapeutic alternative to conventional simple limbal epithelial transplantation and keratolimbal allografts for treatment of limbal stem cell deficiency. Engineered tissue matrices and porcine-derived corneas are potential alternatives to human donor tissue in anterior lamellar keratoplasty for corneal ulcers and scars, as well as intrastromal transplants for advanced keratoconus. For endothelial disease, substrate supported cultured endothelial cell grafts, and synthetic barrier devices are promising alternative to traditional endothelial keratoplasties.

SUMMARY

There has been increasing interest in cellular and acellular bioengineered tissue-cellular and synthetic products for treatment of corneal diseases, and many of these products have already seen clinical use. Industry and academia have important roles in advancing these products to later phase clinical trials and comparing them to conventional allograft approaches. Future development of full thickness donor corneas with cultivated epithelium, endothelium, and stromal keratocytes in a biosynthetic matrix will likely be an important next step in tissue alternatives. Continued progress in this field will be critical for addressing the global disease burden from corneal blindness.

A single-cell RNA-seq analysis unravels the heterogeneity of primary cultured human corneal endothelial cells

The cornea is a transparent and avascular tissue located in front of the eye. Its inner surface is lined by a monolayer of corneal endothelial cells (CECs), which maintain the cornea transparency. CECs remain arrested in a non-proliferative state and damage to these cells can compromise their function leading to corneal opacity. The primary culture of donor-derived CECs is a promising cell therapy. It confers the potential to treat multiple patients from a single donor, alleviating the global donor shortage. Nevertheless, this approach has limitations preventing its adoption, particularly culture protocols allow limited expansion of CECs and there is a lack of clear parameters to identify therapy-grade CECs. To address this limitation, a better understanding of the molecular changes arising from the primary culture of CECs is required. Using single-cell RNA sequencing on primary cultured CECs, we identify their variable transcriptomic fingerprint at the single cell level, provide a pseudo-temporal reconstruction of the changes arising from primary culture, and suggest markers to assess the quality of primary CEC cultures. This research depicts a deep transcriptomic understanding of the cellular heterogeneity arising from the primary expansion of CECs and sets the basis for further improvement of culture protocols and therapies.

Corneal optical density: Structural basis, measurements, influencing factors, and roles in refractive surgery

The cornea is the main refractive medium of the human eye, and its clarity is critical to visual acuity. Corneal optical density (COD) is an important index to describe corneal transparency. Intact corneal epithelial and endothelial cells, regular arrangement of collagen fibers in the stroma, and normal substance metabolism are all integral for the cornea to maintain its transparency. In the last two decades, the Pentacam Scheimpflug imaging system has emerged as a breakthrough for the measurement of COD (also called corneal densitometry). It has been found that a wide variety of factors such as age, refractive status, and corneal diseases can affect COD. Different corneal refractive surgery methods also change COD in different corneal regions and layers and affect visual acuity following the surgery. Thus, COD has gradually become a significant indicator to evaluate corneal health, one on which the attention of clinicians has been increasingly focused.

Corneal endothelial cells and acoustic cavitation in phacoemulsification

Postoperative complications of phacoemulsification, such as corneal edema caused by human corneal endothelial cell (CEC) injury, are still a matter of concern. Although several factors are known to cause CEC damage, the influence of ultrasound on the formation of free radicals during surgery should be considered. Ultrasound in aqueous humor induces cavitation and promotes the formation of hydroxyl radicals or reactive oxygen species (ROS). ROS-induced apoptosis and autophagy in phacoemulsification have been suggested to significantly promote CEC injury. CEC cannot regenerate after injury, and measures must be taken to prevent the loss of CEC after phacoemulsification or other CEC injuries. Antioxidants can reduce the oxidative stress injury of CEC during phacoemulsification. Evidence from rabbit eye studies shows that ascorbic acid infusion during operation or local application of ascorbic acid during phacoemulsification has a protective effect by scavenging free radicals or reducing oxidative stress. Both in experiments and clinical practice, hydrogen dissolved in the irrigating solution can also prevent CEC damage during phacoemulsification surgery. Astaxanthin (AST) can inhibit oxidative damage, thereby protecting different cells from most pathological conditions, such as myocardial cells, luteinized granulosa cells of the ovary, umbilical vascular endothelial cells, and human retina pigment epithelium cell line (ARPE-19). However, existing research has not focused on the application of AST to prevent oxidative stress during phacoemulsification, and the related mechanisms need to be studied. The Rho related helical coil kinase inhibitor Y-27632 can inhibit CEC apoptosis after phacoemulsification. Rigorous experiments are required to confirm whether its effect is realized through improving the ROS clearance ability of CEC.

Early and late-onset cell migration from peripheral corneal endothelium

In this study we describe peripheral corneal endothelial cell migration in vitro in the absence and presence of a ROCK-inhibitor. For this study, 21 corneal endothelial graft rims, with attached trabecular meshwork (TM), were prepared from Descemet membrane-endothelial cell sheets by 6.5 mm trepanation. For the initial proof-of-concept, 7 outer graft rims were cultured in a thermo-reversible hydrogel matrix for up to 47 days. To assess the effect of a ROCK-inhibitor, 14 paired outer rims were cultured either with or without ROCK-inhibitor for up to 46 days. At the end of culture, tissue was retrieved from the hydrogel matrix and examined for cell viability and expression of different endothelial cell markers (ZO-1, Na+/K+-ATPase, NCAM, glypican, and vimentin). All cultured rims remained viable and displayed either single regions (n = 5/21) or collective areas (n = 16/21) of cell migration, regardless of the presence or absence of ROCK-inhibition. Migration started after 4±2 days and continued for at least 29 days. The presence of ROCK-inhibitor seemed to contribute to a more regular cell morphology of migrating cells. In addition, 7 outer rims demonstrated a phenotypically distinct late-onset but fast-growing cell population emerging from the area close to the limbus. These cells emerged after 3 weeks of culture and appeared less differentiated compared to other areas of migration. Immunostaining showed that migrated cells maintained the expression patterns of endothelial cell markers. In conclusion, we observed 2 morphologically distinct migrating cell populations with the first type being triggered by a broken physical barrier, which disrupted contact inhibition and the second, late-onset type showing a higher proliferative capacity though appearing less differentiated. This cell subpopulation appeared to be mediated by stimuli other than loss of contact inhibition and ROCK-inhibitor presence. Further exploration of the differences between these cell types may assist in optimizing regenerative treatment options for endothelial diseases.

Roles of early events in the modifications undergone by bovine corneal endothelial cells during wound healing

A mechanical injury in bovine corneal endothelial (BCE) cells in culture induces: (1) a fast calcium wave (FCW); (2) slow increases in cytosolic sodium and calcium, critical for the healing process, and (3) a rise in the apoptotic rate with respect to quiescent cells. In order to investigate the nature of the stimuli that determine the ionic changes and apoptotic response, we performed here studies on a non-injury model of tissue restitution in BCE monolayers. For this, we employed cell cultures grown to confluence in the presence of a Parafilm strip. We observed that, previously to strip removal, most of the border cells had already developed the slow ionic modifications, while in the scratch wounds these changes gradually occur after several hours of healing. This finding suggests that, in BCE cells, the presence of a free edge is sufficient to trigger ionic modifications necessary for wound healing and to elicit an augmented apoptotic response. The apoptotic index of the migrating cells in the Parafilm model (PF) was determined to be approximately two-fold the one of scratch wounds, a result that, in agreement with our previous observations, we attributed to the absence of the FCW in the PF experiments. The findings of this work further contribute to the understanding of epithelial wound healing, a crucial adaptive, and homeostatic response.

The Soluble Guanylate Cyclase Stimulator BAY 41-2272 Attenuates Transforming Growth Factor β1-Induced Myofibroblast Differentiation of Human Corneal Keratocytes

Corneal transparency, necessary for vision and depending on the high organization of stromal extracellular matrix, is maintained by keratocytes. Severe or continuous corneal injuries determine exaggerated healing responses resulting in the formation of irreversible fibrotic scars and vision impairment. Soluble guanylate cyclase (sGC) stimulation demonstrated antifibrotic effects in both experimental fibrosis and human lung and skin fibroblasts. Here, we assessed whether sGC stimulation with BAY 41-2272 could attenuate transforming growth factor β1 (TGFβ1)-induced myofibroblast differentiation of human corneal keratocytes. Cells were challenged with TGFβ1, with/without BAY 41-2272 preincubation, and subsequently assessed for viability, proliferation, migration, chemoinvasion, as well for the expression of myofibroblast/fibroblast activation markers and contractile abilities. Treatment with BAY 41-2272 did not affect keratocyte viability, while preincubation of cells with the sGC stimulator was able to inhibit TGFβ1-induced proliferation, wound healing capacity, and invasiveness. BAY 41-2272 was also able to attenuate TGFβ1-induced myofibroblast-like profibrotic phenotype of keratocytes, as demonstrated by the significant decrease in ACTA2, COL1A1, COL1A2, FN1 and PDPN gene expression, as well as in α-smooth muscle actin, α-1 chain of type I collagen, podoplanin, vimentin and N-cadherin protein expression. Finally, BAY 41-2272 significantly counteracted the TGFβ1-induced myofibroblast-like ability of keratocytes to contract collagen gels, reduced phosphorylated Smad3 protein levels, and attenuated gene expression of proinflammatory cytokines. Collectively, our data show for the first time that BAY 41-2272 is effective in counteracting keratocyte-to-myofibroblast transition, thus providing the rationale for the development of sGC stimulators as novel promising modulators of corneal scarring and fibrosis.

Nanoneedles Induce Targeted siRNA Silencing of p16 in the Human Corneal Endothelium

Nanoneedles can target nucleic acid transfection to primary cells at tissue interfaces with high efficiency and minimal perturbation. The corneal endothelium is an ideal target for nanoneedle-mediated RNA interference therapy aimed at enhancing its proliferative capacity, necessary for tissue regeneration. This work develops a strategy for siRNA nanoninjection to the human corneal endothelium. Nanoneedles can deliver p16-targeting siRNA to primary human corneal endothelial cells in vitro without toxicity. The nanoinjection of siRNA induces p16 silencing and increases cell proliferation, as monitored by ki67 expression. Furthermore, siRNA nanoinjection targeting the human corneal endothelium is nontoxic ex vivo, and silences p16 in transfected cells. These data indicate that nanoinjection can support targeted RNA interference therapy for the treatment of endothelial corneal dysfunction.

Lentiviral Vectors for Ocular Gene Therapy

This review offers the basics of lentiviral vector technologies, their advantages and pitfalls, and an overview of their use in the field of ophthalmology. First, the description of the global challenges encountered to develop safe and efficient lentiviral recombinant vectors for clinical application is provided. The risks and the measures taken to minimize secondary effects as well as new strategies using these vectors are also discussed. This review then focuses on lentiviral vectors specifically designed for ocular therapy and goes over preclinical and clinical studies describing their safety and efficacy. A therapeutic approach using lentiviral vector-mediated gene therapy is currently being developed for many ocular diseases, e.g., aged-related macular degeneration, retinopathy of prematurity, inherited retinal dystrophies (Leber congenital amaurosis type 2, Stargardt disease, Usher syndrome), glaucoma, and corneal fibrosis or engraftment rejection. In summary, this review shows how lentiviral vectors offer an interesting alternative for gene therapy in all ocular compartments.

Human SMILE-Derived Stromal Lenticule Scaffold for Regenerative Therapy: Review and Perspectives

A transparent cornea is paramount for vision. Corneal opacity is one of the leading causes of blindness. Although conventional corneal transplantation has been successful in recovering patients’ vision, the outcomes are challenged by a global lack of donor tissue availability. Bioengineered corneal tissues are gaining momentum as a new source for corneal wound healing and scar management. Extracellular matrix (ECM)-scaffold-based engineering offers a new perspective on corneal regenerative medicine. Ultrathin stromal laminar tissues obtained from lenticule-based refractive correction procedures, such as SMall Incision Lenticule Extraction (SMILE), are an accessible and novel source of collagen-rich ECM scaffolds with high mechanical strength, biocompatibility, and transparency. After customization (including decellularization), these lenticules can serve as an acellular scaffold niche to repopulate cells, including stromal keratocytes and stem cells, with functional phenotypes. The intrastromal transplantation of these cell/tissue composites can regenerate native-like corneal stromal tissue and restore corneal transparency. This review highlights the current status of ECM-scaffold-based engineering with cells, along with the development of drug and growth factor delivery systems, and elucidates the potential uses of stromal lenticule scaffolds in regenerative therapeutics.

Current Advancements in Corneal Cell-Based Therapy

Corneal epithelial stem cells (CEpSCs) mostly reside at the limbal area and are responsible for tissue homeostasis throughout life. Once complete CEpSC deficiency occurs, regenerative medicine cell-based therapy using CEpSCs or their alternatives can provide successful clinical outcomes. Due to an improved understanding of CEpSCs and mucosal epithelial stem cells, major advancements have been made over the past few decades in in vivo and ex vivo cell-based ocular surface reconstruction therapies for the treatment of severe ocular surface diseases. New therapeutic concepts and clinical strategies are emerging for the treatment of corneal endothelial dysfunction. For example, unlike corneal epithelial cells, in vivo corneal endothelial cells (CECs) stop proliferating and are arrested in the G1 phase of the cell cycle due to cell-to-cell contact inhibition and exposure to a high concentration of transforming growth factor-beta in the aqueous humor. Thus, the production of CECs with good functionality in culture has consistently been difficult. To solve this problem, Rho-associated protein kinase inhibition has taken center stage, as it not only makes the production of human CECs in culture closely mimic the functional characteristics of in vivo healthy CECs possible but also helps sustain those biological properties. Thus, cultured human CEC injection therapy is now moving to the forefront for the treatment of corneal endothelial failure. Herein, we summarize key historical discoveries in corneal cell-based regenerative medicine and illustrate the concept of corneal cell therapy for the treatment of refractory corneal epithelial and endothelial diseases.

Fluctuations in Corneal Endothelial LAP2 Expression Levels Correlate with Passage Dependent Declines in Their Cell Proliferative Activity

The corneal endothelium is the inner corneal mono-layered epithelium, fundamental for preserving corneal hydration and transparency. However, molecular mechanisms that regulate corneal endothelial cells (CEnCs), in particular regarding their proliferative capacity, have been only partially elucidated. CEnCs are quiescent in vivo and they easily undergo endothelial to mesenchymal transition (EnMT) in vitro. This study aims to analyze CEnCs behavior and expression in vitro, either in sub-confluent growing (S) or confluent (C) CEnCs cultures. Primary rabbit and human CEnCs were cultured and used for RT-PCR, immunofluorescence or western blot analysis. These methods allowed identifying a novel molecular marker, LAP2, that is upregulated in S while downregulated in C human or rabbit CEnCs. Those results were observed for several subsequent passages in culture and this, together with the correlation between ki67 and LAP2 expression, suggested LAP2 as a novel possible indicator for culture ageing. Finally, treatment with FGF and TGFβ in rCEnCs highlighted how LAP2 can vary as the cells regulate their proliferative state. In conclusion, we have identified a novel marker for CEnCs, LAP2, that regulates its expression depending on the cells sub/confluent state and that correlates with CEnCs proliferation.

Alternatives to endokeratoplasty: an attempt towards reducing global demand of human donor corneas

The cornea is an anterior transparent tissue of the eye that enables the transmission of surrounding light to the back of the eye, which is essential for maintaining clear vision. Corneal endothelial diseases can lead to partial or total blindness; hence, surgical replacement of the diseased corneal tissue with a healthy cadaveric donor graft becomes necessary when the endothelium is damaged. Keratoplasties face a huge challenge due to a worldwide shortage in the supply of human donor corneas. Hence, alternative solutions such as cell or tissue engineering-based therapies have been investigated for reducing the global demand of donor corneas. This review aims at highlighting studies that have been successful at replacing partial or total endothelial keratoplasty.

Tissue engineered scaffolds for corneal endothelial regeneration: a material's perspective

Currently, the treatment of corneal diseases caused by damage to the corneal endothelium requires a donor cornea. Because of their limited availability (1 donor cornea for 70 patients in need), researchers are investigating alternative approaches that are independent of donor tissue. One of them includes the development of a tissue engineered scaffold onto which corneal endothelial cells are seeded. In order to function as a suitable substrate, some of its essential properties including thickness, permeability, transparency and mechanical strength should meet certain demands. Additionally, the membrane should be biocompatible and allow the formation of a functional endothelium on the surface. Many materials have already been investigated in this regard including natural, semi-synthetic and synthetic polymers. In the current review, we present an overview of their characteristics and provide a critical view on the methods exploited for material characterization. Next, also the suitability of scaffolds to serve their purpose is discussed along with an overview of natural tissues (e.g. amniotic membrane and lens capsule) previously investigated for this application. Eventually, we propose a consistent approach to be exploited ideally for membrane characterization in future research. This will allow a scientifically sound comparison of materials and membranes investigated by different research groups, hence benefitting research towards the creation of a suitable/optimal tissue engineered endothelial graft.

Tissue engineered endothelial keratoplasty in rabbit: tips and tricks

PURPOSE

To report a detailed surgical procedure of tissue engineered endothelial keratoplasty (TEEK) in a rabbit model and its postoperative evaluation.

METHODS

TEEKs were prepared 7 days before transplantation by seeding human or rabbit corneal endothelial cells on either femtosecond laser-cut ultrathin human stromal lamellae (fs-UTSL) or femtosecond laser-cut human anterior lens capsule (fs-HALC). Thirty transplantations were performed on aphakic eyes. Recombinant tissue plasminogen activator (rTPA) was used throughout the surgery. The native endothelium was removed by full-surface scraping and central descemetorhexis. The transplantation was performed as a human Descemet's membrane endothelial keratoplasty. Controls included Descemetorhexis only and transplantation of carrier alone. Postoperative follow-up was performed by slit lamp and optical coherence tomography, followed by histology.

RESULTS

Controls remained oedematous. No fibrin occurred during surgery. All but three TEEKs adhered immediately. One/6 fs-UTSL and 9/16 fs-HALC cleared perfectly (p = 0.161). All failures could be explained by at least one of the following causes intraoperative bleeding, vitreous prolapsus, early partial detachment, postoperative irido corneal synechiea/angle closure. Presumed immune rejection was observed in three rabbits only after 4 weeks. Immunostaining with anti-human CD166 allowed to perfectly differentiate human cells from rabbit cells. In successful TEEK at 3 or 4 weeks, human cells formed a normal endothelium and started migrating outside the carrier.

CONCLUSION

Though the transplantation of a TEEK in rabbits is a complex model with many causes of failure, established procedure including use of rTPA allows reliable preclinical study. In addition, we suggest that fs-HALC might be a potential carrier for TEEK.

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.