Retinal Pigment Epithelial Cell Behavior is Modulated by Alterations in Focal Cell–Substrate Contacts

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.

Effects of fibrin glue as a three-dimensional scaffold in cultivated adult human retinal pigment epithelial cells

This study was conducted to examine morphological, genotypic, and phenotypic alterations occurring in cultured adult human retinal pigment epithelial cells when encapsulated with different concentrations of fibrin glue. Cultivated adult human retinal pigment epithelial cells were encapsulated with different concentrations of fibrin glue, namely FG1 (42 mg/dl), FG2 (84 mg/dl), FG3 (124 mg/dl), FG4 (210 mg/dl), followed by the evaluation of genetic and cytomorphological changes and protein expression. Cultured adult human retinal pigment epithelial cells showed dendritiform morphology during the early days of encapsulation with fibrin glue. Moreover, an increasing inhibitory effect on cell growth was observed with increasing concentrations of fibrin glue. At the transcriptional level, the expression of MMP2, PAX6, and ITGB1 in FG1-encapsulated cells was significantly higher than that in other treated groups; however, the expression of ACTA2 was lower in all fibrin glue-encapsulated groups compared to that in the controls. Immunocytochemistry showed that FG2-encapsulated cells expressed cytokeratin 8/18, RPE65, and ZO-1 proteins, but not PAX6. In conclusion, fibrin glue at a concentration of 84 mg/dl allows proper encapsulation of adult human retinal pigment epithelial cells, while preserving the morphometric, genotypic, and phenotypic features of the cells. This three-dimensional biopolymer can be considered a reliable vehicle for retinal pigment epithelium cell transplantation in cell-based therapies.

Foetal bovine serum can reduce toxicity of indocyanine green, brilliant blue G and trypan blue in ARPE-19 cellular model that suggests new surgical staining protocols for internal limiting membrane peeling procedure

BACKGROUND

To investigate and compare the cytotoxicity of indocyanine green (ICG), brilliant blue G (BBG) and trypan blue (TB) using ARPE-19 cells that have been pre-treated/post-treated with balanced salt solution (BSS) or foetal bovine serum (FBS).

METHODS

The cultured human retina pigment epithelium ARPE-19 cells were pre-treated/post-treated with BSS or FBS (represent the autologous serum in clinic) in parallel with cells being soaked with various concentrations of ICG, BBG and TB. The cells were then assessed for viability, growth rate, reactive oxygen species (ROS) level, mitochondrial membrane potential (Δψ) and mitochondrial mass as cytotoxic indices. For the FBS pre-treated cells, only ROS was examined.

RESULTS

Using the MTT assay, cytotoxicity seemed to appear when the dye concentration was above 2.5 mg/mL for ICG but no cytotoxicity for BBG and TB at the concentrations used. Cell growth was arrested at a concentration 1 mg/mL when ICG or BBG were present but no arrest at any of the tested concentrations was found for TB with the cell-growth curve was slowest for ICG. Cellular ROS levels increased at all concentrations of all dyes, but the increasing slopes were decreased after FBS post-treatment washout.

CONCLUSIONS

As a rinse buffer FBS performs much better than BSS in terms of cell rescue, which agrees with a clinical report when autologous whole blood was applied to macular hole surgery. However, FBS pre-treatment seems to be much better than FBS use as washout buffer in post-treatment.

A computational study of VEGF production by patterned retinal epithelial cell colonies as a model for neovascular macular degeneration

Background

The configuration of necrotic areas within the retinal pigmented epithelium is an important element in the progression of age-related macular degeneration (AMD). In the exudative (wet) and non-exudative (dry) forms of the disease, retinal pigment epithelial (RPE) cells respond to adjacent atrophied regions by secreting vascular endothelial growth factor (VEGF) that in turn recruits new blood vessels which lead to a further reduction in retinal function and vision. In vitro models exist for studying VEGF expression in wet AMD (Vargis et al., Biomaterials 35(13):3999–4004, 2014), but are limited in the patterns of necrotic and intact RPE epithelium they can produce and in their ability to finely resolve VEGF expression dynamics.

Results

In this work, an in silico hybrid agent-based model was developed and validated using the results of this cell culture model of VEGF expression in AMD. The computational model was used to extend the cell culture investigation to explore the dynamics of VEGF expression in different sized patches of RPE cells and the role of negative feedback in VEGF expression. Results of the simulation and the cell culture studies were in excellent qualitative agreement, and close quantitative agreement.

Conclusions

The model indicated that the configuration of necrotic and RPE cell-containing regions have a major impact on VEGF expression dynamics and made precise predictions of VEGF expression dynamics by groups of RPE cells of various sizes and configurations. Coupled with biological studies, this model may give insights into key molecular mechanisms of AMD progression and open routes to more effective treatments.

The formation of a functional retinal pigment epithelium occurs on porous polytetrafluoroethylene substrates independently of the surface chemistry

Subretinal transplantation of functioning retinal pigment epithelial (RPE) cells may have the potential to preserve or restore vision in patients affected by blinding diseases such as age-related macular degeneration (AMD). One of the critical steps in achieving this is the ability to grow a functioning retinal pigment epithelium, which may need a substrate on which to grow and to aid transplantation. Tailoring the physical and chemical properties of the substrate should help the engineered tissue to function in the long term. The purpose of the study was to determine whether a functioning monolayer of RPE cells could be produced on expanded polytetrafluoroethylene substrates modified by either an ammonia plasma treatment or an n-Heptylamine coating, and whether the difference in surface chemistries altered the extracellular matrix the cells produced. Primary human RPE cells were able to form a functional, cobblestone monolayer on both substrates, but the formation of an extracellular matrix to exhibit a network structure took months, whereas on non-porous substrates with the same surface chemistry, a similar appearance was observed after a few weeks. This study suggests that the surface chemistry of these materials may not be the most critical factor in the development of growth of a functional monolayer of RPE cells as long as the cells can attach and proliferate on the surface. This has important implications in the design of strategies to optimise the clinical outcomes of subretinal transplant procedures.

Two-photon polymerization for production of human iPSC-derived retinal cell grafts

Recent advances in induced pluripotent stem cell (iPSC) technology have paved the way for the production of patient-specific neurons that are ideal for autologous cell replacement for treatment of neurodegenerative diseases. In the case of retinal degeneration and associated photoreceptor cell therapy, polymer scaffolds are critical for cellular survival and integration; however, prior attempts to materialize this concept have been unsuccessful in part due to the materials' inability to guide cell alignment. In this work, we used two-photon polymerization to create 180μm wide non-degradable prototype photoreceptor scaffolds with varying pore sizes, slicing distances, hatching distances and hatching types. Hatching distance and hatching type were significant factors for the error of vertical pore diameter, while slicing distance and hatching type most affected the integrity and geometry of horizontal pores. We optimized printing parameters in terms of structural integrity and printing time in order to create 1mm wide scaffolds for cell loading studies. We fabricated these larger structures directly on a porous membrane with 3µm diameter pores and seeded them with human iPSC-derived retinal progenitor cells. After two days in culture, cells nested in and extended neuronal processes parallel to the vertical pores of the scaffolds, with maximum cell loading occurring in 25μm diameter pores. These results highlight the feasibility of using this technique as part of an autologous stem cell strategy for restoring vision to patients affected with retinal degenerative diseases.

STATEMENT OF SIGNIFICANCE

Cell replacement therapy is an important goal for investigators aiming to restore neural function to those suffering from neurodegenerative disease. Cell delivery scaffolds are frequently necessary for the success of such treatments, but traditional biomaterials often fail to facilitate the neuronal orientation and close packing needed to recapitulate the in vivo environment. Here, we use two-photon polymerization to create prototype cell scaffolds with densely packed vertical pores for photoreceptor cell loading and small, interconnected horizontal pores for nutrient diffusion. This study offers a thorough characterization of how two-photon polymerization parameters affect final structural outcomes and printing time. Our findings demonstrate the feasibility of using two-photon polymerization to create scaffolds that can align neuronal cells in 3D and are large enough to be used for transplantation. In future work, these scaffolds could comprise biodegradable materials with tunable microstructure, elastic modulus and degradation time; a significant step towards a promising treatment option for those suffering from late-stage neurodegeneration, including retinal degenerative blindness.

All-trans retinoic acid suppresses the adhering ability of ARPE-19 cells via mitogen-activated protein kinase and focal adhesion kinase

This study investigated the signaling mechanism underlying the anti-adhesive effect of all-trans retinoic acid (ATRA) on retinal pigment epithelial ARPE-19 cells. Adhesion kinetics with or without ATRA treatment were profiled by adhesion assay. Surface coating with type IV collagen, fibronectin, laminin, but not type I collagen, significantly enhanced adhesion and spreading of ARPE-19 cells, while ATRA at subtoxic doses (ranging from 10^-7 to 10^-6 M) profoundly suppressed the extracellular matrix-enhanced adhesion ability. Cell attachment on FN activated PI3K/Akt and MAPK cascades, whereas ATRA pretreatment blunted the early phosphorylation of Akt and MAPK signaling mediators including p38 MAPK, JNK1/2, and ERK1/2. Mechanistically, signaling blockade with selective kinase inhibitors demonstrated that all MAPK pathways were involved in the anti-adhesive effect of ATRA, whereas the PI3K inhibitor treatment significantly potentiated the ATRA-suppressed RPE cell adhesion. Moreover, ATRA treatment did not affect intracellular F-actin distribution, but remarkably reduced focal adhesion kinase (FAK) expression and its nuclear localization during ARPE-19 cell attachment. In conclusion, ATRA suppresses the adhering ability of ARPE-19 cells at least in part through MAPK and FAK pathways. Signaling blockade with PI3K inhibitor could be regarded as an alternative modality for treating proliferative vitreoretinopathy.

Differentiation of Induced Pluripotent Stem Cells to Neural Retinal Precursor Cells on Porous Poly-Lactic-co-Glycolic Acid Scaffolds

PURPOSE

Cell replacement therapy for the treatment of retinal degeneration is an increasingly feasible approach, but one that still requires optimization of the transplantation strategy. To this end, various polymer substrates can increase cell survival and integration, although the effect of their pore size on cell behavior, particularly differentiation, has yet to be explored.

METHODS

Salt crystals of varying known size were used to impart structure to poly(lactic-co-glycolic acid) (PLGA) scaffolds by a salt leaching/solvent evaporation process. Mouse induced pluripotent stem cells (miPSCs) were seeded to the polymer scaffolds and supplemented with retinal differentiation media for up to 2 weeks. Proliferation was measured during the course of 2 weeks, while differentiation was evaluated using cell morphology and expression of early retinal development markers.

RESULTS

The salt leaching method of porous PLGA fabrication resulted in amorphous smooth pores. Cells attached to these scaffolds and proliferated, reaching a maximum cell number at 10 days postseeding that was 5 times higher on porous PLGA than on nonporous controls. The morphology of many of these cells, including their formation of neurites, was suggestive of neural phenotypes, while their expression of Sox2, Pax6, and Otx2 indicates early retinal development.

CONCLUSIONS

The use of porous PLGA scaffolds to differentiate iPSCs to retinal phenotypes is a feasible pretransplantation approach. This adds to an important knowledge base; understanding how developing retinal cells interact with polymer substrates with varying structure is a crucial component of optimizing cell therapy strategies.

Mechanical properties of murine and porcine ocular tissues in compression

Sub-retinal implantation of foreign materials is becoming an increasingly common feature of novel therapies for retinal dysfunction. The ultimate compatibility of implants depends not only on their in vitro chemical compatibility, but also on how well the mechanical properties of the material match those of the native tissue. In order to optimize the mechanical properties of retinal implants, the mechanical properties of the mammalian retina itself must be carefully characterized. In this study, the compressive moduli of eye tissues, especially the retina, were probed using a dynamic mechanical analysis instrument in static mode. The retinal compressive modulus was lower than that of the sclera or cornea, but higher than that of the RPE and choroid. Compressive modulus remained relatively stable with age. Conversely, apparent retinal softening occurred at an early age in mice with inherited retinal degeneration. Compressive modulus is an important consideration for the design of retinal implants. Polymer scaffolds with moduli that are substantially different than that of the native tissue in which they will ultimately reside will be less likely to aid in the differentiation and development of the appropriate cell types in vitro and will have reduced biocompatibility in vivo.

Topographical control of ocular cell types for tissue engineering

Visual impairment affects over 285 million people worldwide and has a major impact on an individual's quality of life. Tissue engineering has the potential to increase the quality of life for many of these patients by preventing vision loss or restoring vision using cell-based therapies. However, these strategies will require an understanding of the microenvironmental factors that influence cell behavior. The eye is a well-organized organ whose structural complexity is essential for proper function. Interactions between ocular cells and their highly ordered extracellular matrix are necessary for maintaining key tissue properties including corneal transparency and retinal lamination. Therefore, it is not surprising that culturing these cells in vitro on traditional flat substrates result in irregular morphology. Instead, topographically patterned biomaterials better mimic native extracellular matrix and have been shown to elicit in vivo-like morphology and gene expression which is essential for tissue engineering. Herein we review multiple methods for producing well-controlled topography and discuss optimal biomaterial scaffold design for cells of the cornea, retina, and lens.

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.

Materials characterization and mechanobiology of the eye

The eye responds to a great deal of internal and external stimuli throughout its normal function. Due to this, a mechanical or chemical analysis alone is insufficient. A systematic materials characterization is needed. A mechanobiological approach is required for a full understanding of the unique properties and function of the eye. This review compiles the mechanical properties of select eye components, summarizes mechanical and chemical testing platforms, and overviews modeling approaches. Analysis is done across studies, experimental methods, and between species in order to summarize what is known about the mechanobiology of the eye. Several opportunities for future research are identified.

Pluripotent human stem cells for the treatment of retinal disease

Despite advancements made in our understanding of ocular biology, therapeutic options for many debilitating retinal diseases remain limited. Stem cell-based therapies are a potential avenue for treatment of retinal disease, and this mini-review will focus on current research in this area. Cellular therapies to replace retinal pigmented epithelium (RPE) and/or photoreceptors to treat age-related macular degeneration (AMD), Stargardt's macular dystrophy, and retinitis pigmentosa are currently being developed. Over the past decade, significant advancements have been made using different types of human stem cells with varying capacities to differentiate into these target retinal cell types. We review and evaluate pluripotent stem cells, both human embryonic stem cells and human induced pluripotent stem cells, as well as protocols for differentiation of ocular cells, and culture and transplant techniques that might be used to deliver cells to patients.

A nanofibrillar surface promotes superior growth characteristics in cultured human retinal pigment epithelium

BACKGROUND

To evaluate the influence of surface topography on the proliferation of the retinal pigment epithelium (RPE) by comparing nanofibrillar and smooth substrates.

METHODS

Electrospun polyamide nanofibers (EPN) are an engineered surface mimicking native basement membranes. Commonly used plastic (polystyrene, PS) and glass substrates have a smooth topography. All were analyzed by scanning electron microscopy. RPE cultures were established from fetal and adult donors. Growth curves were established on the above substrates. Cell cycle and growth fractions were analyzed with 5-ethynyl-2'-deoxyuridine (EdU) and 4',6-diamidino-2-phenylindole (DAPI).

RESULTS

At a magnification of ×5,000, EPN showed randomly overlapping fibers and pores. The surface of glass was slightly studded yet regular, in contrast to ideally smooth PS. Polygonal cells grew on nanofibers in a colony-like distribution, while randomly spread spindle-shaped cell morphologies were seen on smooth surfaces. This was observed at all donor ages. Initial proliferation rates were higher on EPN, and similar final cell densities were reached in all age groups, compared to an age-related decline on PS. EdU/DAPI revealed faster cell cycles on EPN. Growth fractions were higher and maintained longer on EPN. Observed substrate differences in growth behavior were statistically significant.

CONCLUSION

Surface topography appears to induce distinct RPE proliferation characteristics.

A tissue-engineered approach towards retinal repair: scaffolds for cell transplantation to the subretinal space

BACKGROUND

Several mechanisms of retina degeneration result in the deterioration of the outer retina and can lead to blindness. Currently, with the exception of anti-angiogenic treatments for wet age-related macular degeneration, there are no treatments that can restore lost vision. There is evidence that photoreceptors and embryonic retinal tissue, transplanted to the subretinal space, can form new synapses with surviving host neurons. However, these transplants have yet to result in a clinical treatment for retinal degeneration.

METHODS

This article reviews the current literature on the transplantation of scaffolds with retinal and retinal pigmented epithelial (RPE) cells to the subretinal space. We discuss the types of cells and materials that have been investigated for transplantation to the subretinal space, summarize the current findings, and present opportunities for future research and the next generation of scaffolds for retinal repair.

RESULTS

Challenges to cell transplantation include limited survival upon implantation and the formation of abnormal cell architectures in vivo. Scaffolds have been shown to enhance cell survival and direct cell differentiation and organization in a number of models of retinal degeneration.

CONCLUSIONS

The transplantation of cells within a scaffold represents a possible treatment to repair retinal degeneration and restore vision in effected patients. Materials have been developed for the delivery of retinal and RPE cells separately however, the development of a combined tissue-engineered scaffold targeting both cell populations represents a promising direction for retinal repair.

Cryogenic Etching of Silicon: An Alternative Method For Fabrication of Vertical Microcantilever Master Molds

This paper examines the use of deep reactive ion etching (DRIE) of silicon with fluorine high-density plasmas at cryogenic temperatures to produce silicon master molds for vertical microcantilever arrays used for controlling substrate stiffness for culturing living cells. The resultant profiles achieved depend on the rate of deposition and etching of a SiO _x F _y polymer, which serves as a passivation layer on the sidewalls of the etched structures in relation to areas that have not been passivated with the polymer. We look at how optimal tuning of two parameters, the O₂ flow rate and the capacitively coupled plasma (CCP) power, determine the etch profile. All other pertinent parameters are kept constant. We examine the etch profiles produced using e-beam resist as the main etch mask, with holes having diameters of 750 nm, 1 µm, and 2 µm.

Hexagonal micron scale pillars influence epithelial cell adhesion, morphology, proliferation, migration, and cytoskeletal arrangement

A desirable attribute of implants penetrating epithelium is the inhibition of downward epithelial migration. Simple grooved topographies can inhibit this migration either directly or indirectly by promoting connective tissue attachment, but few studies have focused on the direct effect of geometrically complex topographies on epithelial behavior. Therefore, we examined the influence of novel topographies comprising square floors surrounded by six-sided pillars on periodontal ligament epithelial cell adhesion, morphology, cytoskeletal organization, and migration. Relative to cells on smooth surface, epithelial cells on the pillar substrata adhered closely, exhibited reduced proliferation, had a reduced velocity, but higher persistence. Vinculin staining demonstrated that cells formed mature adhesions on the pillar tops, but smaller punctate adhesion in the gaps and on the pillar walls. Overall more mature adhesions were found on pillars compared to smooth surfaces, which may account for the reduced speed of migration limited on the pillars. F-actin stress fibers were predominantly found on pillar tops within 6 h, whereas microtubules (MTs) had a tendency to form in the gaps between the six-sided pillars. In conclusion, microfabricated pillars altered epithelial migration in ways that could prove useful in inhibition of epithelial downward migration on transmucosal implants.

Retinal prosthesis

Retinal prostheses represent the best near-term hope for individuals with incurable, blinding diseases of the outer retina. On the basis of the electrical activation of nerves, prototype retinal prostheses have been tested in blind humans and have demonstrated the capability to elicit the sensation of light and to give test subjects the ability to detect motion. To improve the visual function in implant recipients, a more sophisticated device is required. Simulations suggest that 600-1000 pixels will be required to provide visual function such as face recognition and reading. State-of-the-art implantable stimulator technology cannot produce such a device, which mandates the advancement of the state of the art in areas such as analog microelectronics, wireless power and data transfer, packaging, and stimulating electrodes.