Polydimethylsiloxane as a substrate for retinal pigment epithelial cell growth

Advances in the study of tissue-engineered retinal pigment epithelial cell sheets

Regarded as the most promising treatment modality for retinal degenerative diseases, retinal pigment epithelium cell replacement therapy holds significant potential. Common retinal degenerative diseases, including Age-related Macular Degeneration, are frequently characterized by damage to the unit comprising photoreceptors, retinal pigment epithelium, and Bruch's membrane. The selection of appropriate tissue engineering materials, in conjunction with retinal pigment epithelial cells, for graft preparation, can offer an effective treatment for retinal degenerative diseases. This article presents an overview of the research conducted on retinal pigment epithelial cell tissue engineering, outlining the challenges and future prospects.

Advances in the engineering of the outer blood-retina barrier: From in-vitro modelling to cellular therapy

The outer blood-retina barrier (oBRB), crucial for the survival and the proper functioning of the overlying retinal layers, is disrupted in numerous diseases affecting the retina, leading to the loss of the photoreceptors and ultimately of vision. To study the oBRB and/or its degeneration, many in vitro oBRB models have been developed, notably to investigate potential therapeutic strategies against retinal diseases. Indeed, to this day, most of these pathologies are untreatable, especially once the first signs of degeneration are observed. To cure those patients, a current strategy is to cultivate in vitro a mature oBRB epithelium on a custom membrane that is further implanted to replace the damaged native tissue. After a description of the oBRB and the related diseases, this review presents an overview of the oBRB models, from the simplest to the most complex. Then, we propose a discussion over the used cell types, for their relevance to study or treat the oBRB. Models designed for in vitro applications are then examined, by paying particular attention to the design evolution in the last years, the development of pathological models and the benefits of co-culture models, including both the retinal pigment epithelium and the choroid. Lastly, this review focuses on the models developed for in vivo implantation, with special emphasis on the choice of the material, its processing and its characterization, before discussing the reported pre-clinical and clinical trials.

Engineering a Biomimetic In Vitro Model of Bruch's Membrane Using Hagfish Slime Intermediate Filament Proteins

Bruch's membrane resides in the subretinal tissue and regulates the flow of nutrients and waste between the retinal pigment epithelial (RPE) and vascular layers of the eye. With age, Bruch's membrane becomes thicker, stiffer, and less permeable, which impedes its function as a boundary layer in the subretina. These changes contribute to pathologies such as age-related macular degeneration (AMD). To better understand how aging in Bruch's membrane affects surrounding tissues and to determine the relationship between aging and disease, an in vitro model of Bruch's membrane is needed. An accurate model of Bruch's membrane must be a proteinaceous, semipermeable, and nonporous biomaterial with similar mechanical properties to in vivo conditions. Additionally, this model must support RPE cell growth. While models of subretinal tissue exist, they typically differ from in vivo Bruch's membrane in one or more of these properties. This study evaluates the capability of membranes created from recombinant hagfish intermediate filament (rHIF) proteins to accurately replicate Bruch's membrane in an in vitro model of the subretinal tissue. The physical characteristics of these rHIF membranes were evaluated using mechanical testing, permeability assays, brightfield microscopy, and scanning electron microscopy. The capacity of the membranes to support RPE cell culture was determined using brightfield and fluorescent microscopy, as well as immunocytochemical staining. This study demonstrates that rHIF protein membranes are an appropriate biomaterial to accurately mimic both healthy and aged Bruch's membrane for in vitro modeling of the subretinal tissue.

Potential therapeutic strategies for photoreceptor degeneration: the path to restore vision

Photoreceptors (PRs), as the most abundant and light-sensing cells of the neuroretina, are responsible for converting light into electrical signals that can be interpreted by the brain. PR degeneration, including morphological and functional impairment of these cells, causes significant diminution of the retina's ability to detect light, with consequent loss of vision. Recent findings in ocular regenerative medicine have opened promising avenues to apply neuroprotective therapy, gene therapy, cell replacement therapy, and visual prostheses to the challenge of restoring vision. However, successful visual restoration in the clinical setting requires application of these therapeutic approaches at the appropriate stage of the retinal degeneration. In this review, firstly, we discuss the mechanisms of PR degeneration by focusing on the molecular mechanisms underlying cell death. Subsequently, innovations, recent developments, and promising treatments based on the stage of disorder progression are further explored. Then, the challenges to be addressed before implementation of these therapies in clinical practice are considered. Finally, potential solutions to overcome the current limitations of this growing research area are suggested. Overall, the majority of current treatment modalities are still at an early stage of development and require extensive additional studies, both pre-clinical and clinical, before full restoration of visual function in PR degeneration diseases can be realized.

New Prospects for Retinal Pigment Epithelium Transplantation

ABSTRACT

Retinal pigment epithelium (RPE) transplants rescue photoreceptors in selected animal models of retinal degenerative disease. Early clinical studies of RPE transplants as treatment for age-related macular degeneration (AMD) included autologous and allogeneic transplants of RPE suspensions and RPE sheets for atrophic and neovascular complications of AMD. Subsequent studies explored autologous RPE-Bruch membrane-choroid transplants in patients with neovascular AMD with occasional marked visual benefit, which establishes a rationale for RPE transplants in late-stage AMD. More recent work has involved transplantation of autologous and allogeneic stem cell-derived RPE for patients with AMD and those with Stargardt disease. These early-stage clinical trials have employed RPE suspensions and RPE monolayers on biocompatible scaffolds. Safety has been well documented, but evidence of efficacy is variable. Current research involves development of better scaffolds, improved modulation of immune surveillance, and modification of the extracellular milieu to improve RPE survival and integration with host retina.

Advancement in Nanostructure-Based Tissue-Engineered Biomaterials for Retinal Degenerative Diseases

The review intends to overview a wide range of nanostructured natural, synthetic and biological membrane implants for tissue engineering to help in retinal degenerative diseases. Herein, we discuss the transplantation strategies and the new development of material in combination with cells such as induced pluripotent stem cells (iPSC), mature retinal cells, adult stem cells, retinal progenitors, fetal retinal cells, or retinal pigment epithelial (RPE) sheets, etc. to be delivered into the subretinal space. Retinitis pigmentosa and age-related macular degeneration (AMD) are the most common retinal diseases resulting in vision impairment or blindness by permanent loss in photoreceptor cells. Currently, there are no therapies that can repair permanent vision loss, and the available treatments can only delay the advancement of retinal degeneration. The delivery of cell-based nanostructure scaffolds has been presented to enrich cell survival and direct cell differentiation in a range of retinal degenerative models. In this review, we sum up the research findings on different types of nanostructure scaffolds/substrate or material-based implants, with or without cells, used to deliver into the subretinal space for retinal diseases. Though, clinical and pre-clinical trials are still needed for these transplants to be used as a clinical treatment method for retinal degeneration.

Plasma polymer surface modified expanded polytetrafluoroethylene promotes epithelial monolayer formation in vitro and can be transplanted into the dystrophic rat subretinal space

The aim of this study was to evaluate whether the surface modification of expanded polytetrafluoroethylene (ePTFE) using an n-heptylamine (HA) plasma polymer would allow for functional epithelial monolayer formation suitable for subretinal transplant into a non-dystrophic rat model. Freshly isolated iris pigment epithelial (IPE) cells from two rat strains (Long Evans [LE] and Dark Agouti [DA]) were seeded onto HA, fibronectin-coated n-heptylamine modified (F-HA) and unmodified ePFTE and fibronectin-coated tissue culture (F-TCPS) substrates. Both F-HA ePTFE and F-TCPS substrates enabled functional monolayer formation with both strains of rat. Without fibronectin coating, only LE IPE formed a monolayer on HA-treated ePTFE. Functional assessment of both IPE strains on F-HA ePTFE demonstrated uptake of POS that increased significantly with time that was greater than control F-TCPS. Surgical optimization using Healon GV and mixtures of Healon GV: phosphate buffered saline (PBS) to induce retinal detachment demonstrated that only Healon GV:PBS allowed F-HA ePTFE substrates to be successfully transplanted into the subretinal space of Royal College of Surgeons rats, where they remained flat beneath the neural retina for up to 4 weeks. No apparent substrate-induced inflammatory response was observed by fundus microscopy or immunohistochemical analysis, indicating the potential of this substrate for future clinical applications.

Bio-inspired human in vitro outer retinal models: Bruch's membrane and its cellular interactions

Retinal degenerative disorders, such as age-related macular degeneration (AMD), are one of the leading causes of blindness worldwide, however, treatments to completely stop the progression of these debilitating conditions are non-existent. Researchers require sophisticated models that can accurately represent the native structure of human retinal tissue to study these disorders. Current in vitro models used to study the retina are limited in their ability to fully recapitulate the structure and function of the retina, Bruch's membrane and the underlying choroid. Recent developments in the field of induced pluripotent stem cell technology has demonstrated the capability of retinal pigment epithelial cells to recapitulate AMD-like pathology. However, such studies utilise unsophisticated, bio-inert membranes to act as Bruch's membrane and support iPSC-derived retinal cells. This review presents a concise summary of the properties and function of the Bruch's membrane-retinal pigment epithelium complex, the initial pathogenic site of AMD as well as the current status for materials and fabrication approaches used to generate in vitro models of this complex tissue. Finally, this review explores required advances in the field of in vitro retinal modelling. STATEMENT OF SIGNIFICANCE: Retinal degenerative disorders such as age-related macular degeneration are worldwide leading causes of blindness. Previous attempts to model the Bruch's membrane-retinal pigment epithelial complex, the initial pathogenic site of age-related macular degeneration, have lacked the sophistication to elucidate valuable insights into disease mechanisms. Here we provide a detailed account of the morphological, physical and chemical properties of Bruch's membrane which may aid the fabrication of more sophisticated and physiologically accurate in vitro models of the retina, as well as various fabrication techniques to recreate this structure. This review also further highlights some recent advances in some additional challenging aspects of retinal tissue modelling including integrated fluid flow and photoreceptor alignment.

Polarized, Cobblestone, Human Retinal Pigment Epithelial Cell Maturation on a Synthetic PEG Matrix

Cell attachment is essential for the growth and polarization of retinal pigment epithelial (RPE) cells. Currently, surface coatings derived from biological proteins are used as the gold standard for cell culture. However, downstream processing and purification of these biological products can be cumbersome and expensive. In this study, we constructed a library of chemically modified nanofibers to mimic the Bruch's membrane of the retinal pigment epithelium. Using atmospheric-pressure plasma-induced graft polymerization with a high-throughput screening platform to modify the nanofibers, we identified three polyethylene glycol (PEG)-grafted nanofiber surfaces (PEG methyl ether methacrylate, n = 4, 8, and 45) from a library of 62 different surfaces as favorable for RPE cell attachment, proliferation, and maturation in vitro with cobblestone morphology. Compared with the biologically derived culture matrices such as vitronectin-based peptide Synthemax, our newly discovered synthetic PEG surfaces exhibit similar growth and polarization of retinal pigment epithelial (RPE) cells. However, they are chemically defined, are easy to synthesize on a large scale, are cost-effective, are stable with long-term storage capability, and provide a more physiologically accurate environment for RPE cell culture. To our knowledge, no one has reported that PEG derivatives directly support attachment and growth of RPE cells with cobblestone morphology. This study offers a unique PEG-modified 3D cell culture system that supports RPE proliferation, differentiation, and maturation with cobblestone morphology, providing a new avenue for RPE cell culture, disease modeling, and cell replacement therapy.

Photocoagulation of human retinal pigment epithelial cells in vitro: evaluation of necrosis, apoptosis, cell migration, cell proliferation and expression of tissue repairing and cytoprotective genes

Aims

Sight-threatening diabetic retinopathy has been treated with photocoagulation for decades but the mechanisms behind the beneficial clinical effects are poorly understood. One target of irradiation and a potential player in this process is the retinal pigment epithelium (RPE). Here we establish an in vitro model for photocoagulation of human RPE cells.

Methods

ARPE-19 cells were exposed to photocoagulation and studied at various time points up to 168h. Lesion morphology, necrosis and apoptosis were investigated by light microscopy; LIVE/DEAD staining and measurements of lactate dehydrogenase activity; and TUNEL- and ELISA-based quantification of DNA fragments, respectively. Cell migration and proliferation were explored using docetaxel and mitomycin C; temporal and spatial changes in proliferation were assessed by confocal immunofluorescence of proliferating cell nuclear antigen. Gene expression was measured by qPCR.

Results

Photocoagulation of ARPE-19 resulted in denaturation of proteins and reproducible lesion formation. A transient peak in necrosis, followed by a peak in apoptosis was observed in cells within the lesions at 6h and 24h, respectively after photocoagulation. Cell proliferation was depressed during the first hours after photocoagulation, back to control levels at 24h and augmented in the following days. These effects were not limited to cells in the lesions, but also evident in neighbouring cells. Changes in cell proliferation during lesion repair were preceded by changes in cell migration. Altered mRNA expression of genes previously implicated in the regulation of cell proliferation (FOS, IL-1β, IL-8, HMGA2), migration and tissue repairing (TGFBR2, ADAMTS6, TIMP3, CTGF) was observed, as well as increased expression of the alarmin IL33 and the cytoprotective gene HSPA6.

Conclusions

Using a laser system and experimental settings that comply with standards used in clinical practice, we have established a suitable model for in vitro photocoagulation of human RPE cells to isolate their contribution to the beneficial effects of laser treatment.

Montmorillonite clay based polyurethane nanocomposite as substrate for retinal pigment epithelial cell growth

The subretinal transplantation of retinal pigment epithelial cells (RPE cells) grown on polymeric supports may have interest in retinal diseases affecting RPE cells. In this study, montmorillonite based polyurethane nanocomposite (PU-NC) was investigated as substrate for human RPE cell growth (ARPE-19 cells). The ARPE-19 cells were seeded on the PU-NC, and cell viability, proliferation and differentiation were investigated. The results indicated that ARPE-19 cells attached, proliferated onto the PU-NC, and expressed occludin. The in vivo ocular biocompatibility of the PU-NC was assessed by using the HET-CAM; and through its implantation under the retina. The direct application of the nanocomposite onto the CAM did not compromise the vascular tissue in the CAM surface, suggesting no ocular irritancy of the PU-NC film. The nanocomposite did not elicit any inflammatory response when implanted into the eye of rats. The PU-NC may have potential application as a substrate for RPE cell transplantation.

Biocompatibility of printed paper-based arrays for 2-D cell cultures

The use of paper-based test platforms in cell culture experiments is demonstrated. The arrays used for two-dimensional cell cultures were prepared by printing patterned structures on a paper substrate using a hydrophobic polydimethylsiloxane (PDMS) ink. The non-printed, PDMS-free areas formed the array for the cell growth experiments. Cell imaging was enabled by using a lipophilic staining agent. A set of coated paper substrates was prepared to study the effect of the physicochemical properties of the substrate (topography, roughness and surface energetics) on cell attachment and growth. The studied paper substrates were found to be cell-repellent or cell-supporting. Cell growth was supported by substrates with a large bearing area, low surface area ratio (Sdr), high total surface free energy and an intermediate electron donor surface energy component. The cells were grown to full confluency within 72 h.

Plasma polymer coatings to aid retinal pigment epithelial growth for transplantation in the treatment of age related macular degeneration

Subretinal transplantation of functioning retinal pigment epithelial (RPE) cells grown on a synthetic substrate is a potential treatment for age-related macular degeneration (AMD), a common cause of irreversible vision loss in developed countries. Plasma polymers give the opportunity to tailor the surface chemistry of the artificial substrate whilst maintaining the bulk properties. In this study, plasma polymers with different functionalities were investigated in terms of their effect on RPE attachment and growth. Plasma polymers of acrylic acid (AC), allyl amine (AM) and allyl alcohol (AL) were fabricated and characterised using X-ray photoelectron spectroscopy (XPS) and water contact angle measurements. Octadiene (OD) hydrocarbon films and tissue culture polystyrene were used as controls. Wettability varied from hydrophobic OD to relatively hydrophilic AC. XPS demonstrated four very different surfaces with the expected functionalities. Attachment, proliferation and morphological examination of an RPE cell line and primary RPE cells were investigated. Both cell types grew on all surfaces, with the exception of OD, although the proliferation rate of primary cells was low. Good epithelial morphology was also demonstrated. Plasma polymerised films show potential as cell carrier surfaces for RPE cells in the treatment of AMD.

Developing methacrylate-based copolymers as an artificial Bruch's membrane substitute

Age-related macular degeneration (AMD) is the most common cause of blindness in the developed world. There is currently no treatment for the cellular loss, which is characteristic of AMD. Transplantation of retinal pigment epithelium (RPE) cells represents a potential therapy. Because of AMD-related pathology in the native support, Bruch's membrane, transplanted RPE cells require a scaffold to reside on. We present here the development of an electrospun fibrous scaffold derived from methyl methacrylate and poly(ethylene glycol) (PEG) methacrylate for novel application as an RPE scaffold. Scaffolds were chemically modified to improve cell adhesion by functionalization not previously reported for this type of copolymer system. A human RPE cell line was used to investigate cell-scaffold interactions for up to two weeks in vitro. Scanning electron microscopy was used to characterize the fibrous scaffolds and confirm cell attachment. By day 15, cell area was significantly (p < 0.001) enhanced on scaffolds with chemical modification of the PEG chain terminus. In addition, significantly, less-apoptotic cell death was demonstrable on these modified surfaces.

Polyurethanes as supports for human retinal pigment epithelium cell growth

PURPOSE

The transplant of retinal pigment epithelium (RPE) cells on supports may well be an effective therapeutic approach to improve the visual results of patients with age-related macular degeneration. In this study, two biodegradable polyurethanes were investigated as supports for human RPE cells (ARPE-19).

METHODS

Polyurethane aqueous dispersions based on poly(caprolactone) and/or poly(ethylene glycol) as soft segments, and isophorone diisocyanate and hydrazine as hard segments were prepared. Polyurethane films were produced by casting the dispersions and allowing them to dry at room temperature for one week. The ARPE-19 cells were seeded onto the polyurethane films and they were investigated as supports for in vitro adhesion, proliferation, and uniform distribution of differentiated ARPE-19 cells. Additionally, the in vivo ocular biocompatibility of the polyurethane films was evaluated.

RESULTS

The RPE adhered to and proliferated onto the polyurethane supports, thus establishing cell-PUD surface interactions. Upon confluence, the cells formed an organized monolayer, exhibited a polygonal appearance, and displayed actin filaments which ran along the upper cytoplasm. At 15 days of seeding, the occluding expression was confirmed between adjacent cells, representing the barrier functionality of epithelial cells on polymeric surfaces and the establishment of cell-cell interactions. Results from the in vivo study indicated that polyurethanes exhibited a high degree of short-term intraocular biocompatibility.

CONCLUSIONS

Biodegradable polyurethane films display the proper mechanical properties for an easy transscleral-driven subretinal implantation and can be considered as biocompatible supports for a functional ARPE-19 monolayer.

Morphological and functional studies of rat hepatocytes on a hydrophobic or hydrophilic polydimethylsiloxane surface

This study describes the morphological and functional behavior of rat hepatocytes on a polydimethylsiloxane (PDMS)-coated surface. Hepatocytes were cultured on hydrophobic or hydrophilic PDMS-coated surfaces in serum-free and serum-containing media. In the serum-free medium, almost all hepatocytes adhered onto the surface irrespective of the wettability, with a cell adhesion ratio of >90% at 24h. In the serum-containing medium, although they strongly adhered onto the hydrophilic surface (cell adhesion ratio >85%), the ratio on the hydrophobic surface was <15%. Furthermore, hepatocytes in the serum-free medium gradually formed spheroids irrespective of the surface characteristics; however, on the hydrophilic surface in the serum-containing medium, they maintained a monolayer configuration for up to 10 days, and their numbers gradually decreased over time. Expression levels of the functional activities (albumin secretion and ammonia removal) and the cell-cell adhesion molecules (cadherin and connexin-32) were higher in the hepatocytes that formed spheroids compared to those which assumed a monolayer configuration, and these levels were maintained for at least 10 days. These results suggest that the wettability of PDMS and the composition of the culture medium together control the cell adhesion, morphology and expression of functional genes in hepatocytes.

Carbodiimide cross-linked hyaluronic acid hydrogels as cell sheet delivery vehicles: characterization and interaction with corneal endothelial cells

It was reported that cell-adhesive gelatin discs have been successfully used as delivery vehicles for intraocular grafting of bioengineered corneal endothelial cell sheets. Development of alternative biomaterials to bovine-based gelatin vehicles can potentially eliminate the risk of bovine spongiform encephalopathy. In the present work, to investigate whether it was appropriate for use as cell sheet delivery vehicles, 1-ethyl-3-(3-dimethyl aminopropyl) carbodiimide (EDC) cross-linked hyaluronic acid (HA) hydrogels were studied by determinations of morphological characteristic, mechanical and thermal property, water content, in vitro degradability and cytocompatibility. Glutaraldehyde (GTA) cross-linked HA samples were used for comparison. It was found that HA discs after cross-linking significantly increased its tensile stress but reduced its tensile strain, water uptake and enzymatic degradability. The results of differential scanning calorimetry demonstrated that cross-linking could lead to the alteration of polymer structure. In addition, the EDC-cross-linked HA discs had a smoother surface structure, a faster degradation rate and a relatively lower cytotoxicity as compared to the GTA cross-linked counterparts. It is concluded that EDC can be successfully applied for HA cross-linking to fabricate structurally stable, mechanically reinforced, readily deformable, transparent and cytocompatible HA hydrogel discs with the potential to be applied as delivery vehicles for corneal endothelial cell therapy.