Ahmad Ahmado

Protective Effects of Human iPS-Derived Retinal Pigment Epithelium Cell Transplantation in the Retinal Dystrophic Rat

Transformation of somatic cells with a set of embryonic transcription factors produces cells with the pluripotent properties of embryonic stem cells (ESCs). These induced pluripotent stem (iPS) cells have the potential to differentiate into any cell type, making them a potential source from which to produce cells as a therapeutic platform for the treatment of a wide range of diseases. In many forms of human retinal disease, including age-related macular degeneration (AMD), the underlying pathogenesis resides within the support cells of the retina, the retinal pigment epithelium (RPE). As a monolayer of cells critical to photoreceptor function and survival, the RPE is an ideally accessible target for cellular therapy. Here we report the differentiation of human iPS cells into RPE. We found that differentiated iPS-RPE cells were morphologically similar to, and expressed numerous markers of developing and mature RPE cells. iPS-RPE are capable of phagocytosing photoreceptor material, in vitro and in vivo following transplantation into the Royal College of Surgeons (RCS) dystrophic rat. Our results demonstrate that iPS cells can be differentiated into functional iPS-RPE and that transplantation of these cells can facilitate the short-term maintenance of photoreceptors through phagocytosis of photoreceptor outer segments. Long-term visual function is maintained in this model of retinal disease even though the xenografted cells are eventually lost, suggesting a secondary protective host cellular response. These findings have identified an alternative source of replacement tissue for use in human retinal cellular therapies, and provide a new in vitro cellular model system in which to study RPE diseases affecting human patients.

Elucidating the phenomenon of HESC-derived RPE: Anatomy of cell genesis, expansion and retinal transplantation

Healthy Retinal Pigment Epithelium (RPE) cells are required for proper visual function and the phenomenon of RPE derivation from Human Embryonic Stem Cells (HESC) holds great potential for the treatment of retinal diseases. However, little is known about formation, expansion and expression profile of RPE-like cells derived from HESC (HESC-RPE). By studying the genesis of pigmented foci we identified OTX1/2-positive cell types as potential HESC-RPE precursors. When pigmented foci were excised from culture, HESC-RPE expanded to form extensive monolayers, with pigmented cells at the leading edge assuming a precursor role: de-pigmenting, proliferating, expressing keratin 8 and subsequently re-differentiating. As they expanded and differentiated in vitro, HESC-RPE expressed markers of both developing and mature RPE cells which included OTX1/2, Pax6, PMEL17 and at low levels, RPE65. In vitro, without signals from a developing retinal environment, HESC-RPE could produce regular, polarised monolayers with developmentally important apical and basal features. Following transplantation of HESC-RPE into the degenerating retinal environment of Royal College of Surgeons (RCS) dystrophic rats, the cells survived in the subretinal space, where they maintained low levels of RPE65 expression and remained out of the cell cycle. The HESC-RPE cells responded to the in vivo environment by downregulating Pax6, while maintaining expression of other markers. The presence of rhodopsin-positive material within grafted HESC-RPE indicates that in the future, homogenous transplants of this cell type may be capable of supporting visual function following retinal dystrophy.

RPE transplantation and its role in retinal disease.

Retinal pigment epithelial (RPE) transplantation aims to restore the subretinal anatomy and re-establish the critical interaction between the RPE and the photoreceptor, which is fundamental to sight. The field has developed over the past 20 years with advances coming from a large body of animal work and more recently a considerable number of human trials. Enormous progress has been made with the potential for at least partial restoration of visual function in both animal and human clinical work. Diseases that have been treated with RPE transplantation demonstrating partial reversal of vision loss include primary RPE dystrophies such as the merTK dystrophy in the Royal College of Surgeons (RCS) rat and in humans, photoreceptor dystrophies as well as complex retinal diseases such as atrophic and neovascular age-related macular degeneration (AMD). Unfortunately, in the human trials the visual recovery has been limited at best and full visual recovery has not been demonstrated. Autologous full-thickness transplants have been used most commonly and effectively in human disease but the search for a cell source to replace autologous RPE such as embryonic stem cells, marrow-derived stem cells, umbilical cord-derived cells as well as immortalised cell lines continues. The combination of cell transplantation with other modalities of treatment such as gene transfer remains an exciting future prospect. RPE transplantation has already been shown to be capable of restoring the subretinal anatomy and improving photoreceptor function in a variety of retinal diseases. In the near future, refinements of current techniques are likely to allow RPE transplantation to enter the mainstream of retinal therapy at a time when the treatment of previously blinding retinal diseases is finally becoming a reality.

Embryonic stem cells and retinal repair

In this review we examine the potential of embryonic stem cells (ESCs) for use in the treatment of retinal diseases involving photoreceptors and retinal pigment epithelium (RPE). We outline the ontogenesis of target retinal cell types (RPE, rods and cones) and discuss how an understanding of developmental processes can inform our manipulation of ESCs in vitro. Due to their potential for cellular therapy, special emphasis is placed upon the derivation and culture of human embryonic stem cells (HESCs) and their differentiation towards a retinal phenotype. In terms of achieving this goal, we suggest that much of the success to date reflects permissive in vitro environments provided by established protocols for HESC derivation, propagation and neural differentiation. In addition, we summarise key factors that may be important for enhancing efficiency of retinal cell-type derivation from HESCs. The retina is an amenable component of the central nervous system (CNS) and as such, diseases of this structure provide a realistic target for the application of HESC-derived cellular therapy to the CNS. In order to further this goal, the second component of our review focuses on the cellular and molecular cues within retinal environments that may influence the survival and behaviour of transplanted cells. Our analysis considers both the potential barriers to transplant integration in the retina itself together with the remodelling in host visual centres that is known to accompany retinal dystrophy.

Dissecting a role for melanopsin in behavioural light aversion reveals a response independent of conventional photoreception.

Melanopsin photoreception plays a vital role in irradiance detection for non-image forming responses to light. However, little is known about the involvement of melanopsin in emotional processing of luminance. When confronted with a gradient in light, organisms exhibit spatial movements relative to this stimulus. In rodents, behavioural light aversion (BLA) is a well-documented but poorly understood phenomenon during which animals attribute salience to light and remove themselves from it. Here, using genetically modified mice and an open field behavioural paradigm, we investigate the role of melanopsin in BLA. While wildtype (WT), melanopsin knockout (Opn4−/−) and rd/rd cl (melanopsin only (MO)) mice all exhibit BLA, our novel methodology reveals that isolated melanopsin photoreception produces a slow, potentiating response to light. In order to control for the involvement of pupillary constriction in BLA we eliminated this variable with topical atropine application. This manipulation enhanced BLA in WT and MO mice, but most remarkably, revealed light aversion in triple knockout (TKO) mice, lacking three elements deemed essential for conventional photoreception (Opn4−/− Gnat1−/− Cnga3−/−). Using a number of complementary strategies, we determined this response to be generated at the level of the retina. Our findings have significant implications for the understanding of how melanopsin signalling may modulate aversive responses to light in mice and humans. In addition, we also reveal a clear potential for light perception in TKO mice.

Induction of Differentiation by Pyruvate and DMEM in the Human Retinal Pigment Epithelium Cell Line ARPE-19

PURPOSE Cultured retinal pigment epithelium (RPE) may become a therapeutic option for transplantation in retinal disease. However maintaining a native RPE phenotype in vitro has proven challenging. The human RPE cell-line ARPE-19 is used widely as an alternative to primary RPE. It is grown in DMEM/F12 medium as standard, but its phenotype is dependent on culture conditions, and many differentiation markers are usually absent. The purpose of this study was to examine how this sensitive phenotype of ARPE-19 can be modulated by growth media with or without the metabolite pyruvate to elucidate better RPE growth conditions. METHODS ARPE-19 cells at passages p22 to p28 were cultured on filters for up to 3 months in DMEM/F12 or DMEM media with or without pyruvate and 1% fetal calf serum. Assessment of differentiation was performed using pigmentation, immunocytochemistry, protein/mRNA expression, transepithelial resistance, VEGF secretion, and ultrastructure. RESULTS Pyruvate, in combination with DMEM, induced dark pigmentation and promoted differentiation markers such as CRALBP and MerTK. Importantly, RPE65 protein was detected by Western blotting and was enhanced by pyruvate, high glucose, and DMEM. ARPE-19 cells maintained in this medium could also phagocytose human photoreceptor outer segments (POS). VEGF secretion was greater in DMEM cultures and was affected by glucose but not by pyruvate. Pigmentation never occurred in DMEM/F12. CONCLUSIONS This study demonstrated important differentiation markers, including pigmentation and Western blots of RPE65 protein, and showed human POS phagocytosis in ARPE-19 cultures using a simple differentiation protocol. The results favor the use of high-glucose DMEM with pyruvate for future RPE differentiation studies.

Phase 1 clinical study of an embryonic stem cell–derived retinal pigment epithelium patch in age-related macular degeneration

Age-related macular degeneration (AMD) remains a major cause of blindness, with dysfunction and loss of retinal pigment epithelium (RPE) central to disease progression. We engineered an RPE patch comprising a fully differentiated, human embryonic stem cell (hESC)–derived RPE monolayer on a coated, synthetic basement membrane. We delivered the patch, using a purpose-designed microsurgical tool, into the subretinal space of one eye in each of two patients with severe exudative AMD. Primary endpoints were incidence and severity of adverse events and proportion of subjects with improved best-corrected visual acuity of 15 letters or more. We report successful delivery and survival of the RPE patch by biomicroscopy and optical coherence tomography, and a visual acuity gain of 29 and 21 letters in the two patients, respectively, over 12 months. Only local immunosuppression was used long-term. We also present the preclinical surgical, cell safety and tumorigenicity studies leading to trial approval. This work supports the feasibility and safety of hESC-RPE patch transplantation as a regenerative strategy for AMD.