Generation of Rx+/Pax6+ neural retinal precursors from embryonic stem cells

An essential role for Rax in retina and neuroendocrine system development

In vertebrates, the central nervous system (CNS) develops as a highly hierarchical, patterned organ with a vast diversity of neuronal and glial cell types. The vertebrate retina is developmentally a part of the CNS. Establishment of the vertebrate retina requires a series of developmental steps including specification of the anterior neural plate, evagination of the optic vesicles from the ventral forebrain, and differentiation of cells. The transcription factor RAX is a paired-type homeoprotein that plays a critical role in the eye and forebrain development of vertebrate species. Rax is initially expressed in the anterior neural region of developing mouse embryos, and later in the retina, pituitary gland, hypothalamus, and pineal gland. The targeted deletion of Rax in the mouse results in no eye formation and abnormal forebrain formation. In humans, mutations in the RAX gene lead to anophthalmia and microphthalmia. These observations indicate that RAX plays a pivotal role in the establishment of the retina. In addition, recent studies have reported that retina and pituitary gland tissues can be induced in a culture system from embryonic stem cells, using RAX expression as an indicator of neuronal progenitor cells in the induced tissue, and suggesting that the Rax gene is a key factor in neuronal regeneration. This review highlights the biological functions and molecular mechanisms of RAX in retina, pituitary, hypothalamus, and pineal gland development.

Shaping the eye from embryonic stem cells: Biological and medical implications

Organogenesis is regulated by a complex network of intrinsic cues, diffusible signals and cell/cell or cell/matrix interactions that drive the cells of a prospective organ to differentiate and collectively organize in three dimensions. Generating organs in vitro from embryonic stem (ES) cells may provide a simplified system to decipher how these processes are orchestrated in time and space within particular and between neighboring tissues. Recently, this field of stem cell research has also gained considerable interest for its potential applications in regenerative medicine. Among human pathologies for which stem cell-based therapy is foreseen as a promising therapeutic strategy are many retinal degenerative diseases, like retinitis pigmentosa and age-related macular degeneration. Over the last decade, progress has been made in producing ES-derived retinal cells in vitro, but engineering entire synthetic retinas was considered beyond reach. Recently however, major breakthroughs have been achieved with pioneer works describing the extraordinary self-organization of murine and human ES cells into a three dimensional structure highly resembling a retina. ES-derived retinal cells indeed assemble to form a cohesive neuroepithelial sheet that is endowed with the intrinsic capacity to recapitulate, outside an embryonic environment, the main steps of retinal morphogenesis as observed in vivo. This represents a tremendous advance that should help resolving fundamental questions related to retinogenesis. Here, we will discuss these studies, and the potential applications of such stem cell-based systems for regenerative medicine.

Efficient stage-specific differentiation of human pluripotent stem cells toward retinal photoreceptor cells

Recent successes in the stem cell field have identified some of the key chemical and biological cues which drive photoreceptor derivation from human embryonic stem cells (hESC) and human induced pluripotent stem cells (hiPSC); however, the efficiency of this process is variable. We have designed a three-step photoreceptor differentiation protocol combining previously published methods that direct the differentiation of hESC and hiPSC toward a retinal lineage, which we further modified with additional supplements selected on the basis of reports from the eye field and retinal development. We report that hESC and hiPSC differentiating under our regimen over a 60 day period sequentially acquire markers associated with neural, retinal field, retinal pigmented epithelium and photoreceptor cells, including mature photoreceptor markers OPN1SW and RHODOPSIN with a higher efficiency than previously reported. In addition, we report the ability of hESC and hiPSC cultures to generate neural and retinal phenotypes under minimal culture conditions, which may be linked to their ability to endogenously upregulate the expression of a range of factors important for retinal cell type specification. However, cultures that were differentiated with full supplementation under our photoreceptor-induction regimen achieve this within a significantly shorter time frame and show a substantial increase in the expression of photoreceptor-specific markers in comparison to cultures differentiated under minimal conditions. Interestingly, cultures supplemented only with B27 and/or N2 displayed comparable differentiation efficiency to those under full supplementation, indicating a key role for B27 and N2 during the differentiation process. Furthermore, our data highlight an important role for Dkk1 and Noggin in enhancing the differentiation of hESC and hiPSC toward retinal progenitor cells and photoreceptor precursors during the early stages of differentiation, while suggesting that further maturation of these cells into photoreceptors may not require additional factors and can ensue under minimal culture conditions.

Defining the integration capacity of embryonic stem cell-derived photoreceptor precursors

Retinal degeneration is a leading cause of irreversible blindness in the developed world. Differentiation of retinal cells, including photoreceptors, from both mouse and human embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs), potentially provide a renewable source of cells for retinal transplantation. Previously, we have shown both the functional integration of transplanted rod photoreceptor precursors, isolated from the postnatal retina, in the adult murine retina, and photoreceptor cell generation by stepwise treatment of ESCs with defined factors. In this study, we assessed the extent to which this protocol recapitulates retinal development and also evaluated differentiation and integration of ESC-derived retinal cells following transplantation using our established procedures. Optimized retinal differentiation via isolation of Rax.GFP retinal progenitors recreated a retinal niche and increased the yield of Crx(+) and Rhodopsin(+) photoreceptors. Rod birth peaked at day 20 of culture and expression of the early photoreceptor markers Crx and Nrl increased until day 28. Nrl levels were low in ESC-derived populations compared with developing retinae. Transplantation of early stage retinal cultures produced large tumors, which were avoided by prolonged retinal differentiation (up to day 28) prior to transplantation. Integrated mature photoreceptors were not observed in the adult retina, even when more than 60% of transplanted ESC-derived cells expressed Crx. We conclude that exclusion of proliferative cells from ESC-derived cultures is essential for effective transplantation. Despite showing expression profiles characteristic of immature photoreceptors, the ESC-derived precursors generated using this protocol did not display transplantation competence equivalent to precursors from the postnatal retina.

A review and update on the current status of stem cell therapy and the retina

INTRODUCTION OR BACKGROUND

Many diseases of the retina result in irreversible visual loss. Stem cell (SC) therapy is a rapidly developing field and represents a novel approach to replace non-functioning neuro-retinal cells.

SOURCES OF DATA

A systematic computerized literature search was conducted on PubMed (http://www.ncbi.nlm.nih.gov/pubmed/).

AREAS OF AGREEMENT

The use of stem cells (SCs) in animal models of retinal diseases has resulted in improvement in visual function and performance. SC therapy represents an exciting prospect in restoring vision. Areas of controversy The use of human embryonic SCs raises ethical concerns.

GROWING POINTS

Human trials using SCs in retinal diseases have recently been approved.

AREAS TIMELY FOR DEVELOPING RESEARCH

The success of SCs in retinal therapy depends not only on implanted cell survival, but also on how well SCs migrate, integrate and form synapses. Further research will be needed to overcome these hurdles.

Integration-free induced pluripotent stem cells derived from retinitis pigmentosa patient for disease modeling

We investigated retinitis pigmentosa (RP) caused by a mutation in the gene rhodopsin (RHO) with a patient-specific rod cell model generated from induced pluripotent stem cells (iPSCs) derived from an RP patient. To generate the iPSCs and to avoid the unpredictable side effects associated with retrovirus integration at random loci in the host genome, a nonintegrating Sendai-virus vector was installed with four key reprogramming gene factors (POU5F1, SOX2, KLF4, and c-MYC) in skin cells from an RP patient. Subsequent selection of the iPSC lines was on the basis of karyotype analysis as well as in vitro and in vivo pluripotency tests. Using a serum-free, chemically defined, and stepwise differentiation method, the expressions of specific markers were sequentially induced in a neural retinal progenitor, a retinal pigment epithelial (RPE) progenitor, a photoreceptor precursor, RPE cells, and photoreceptor cells. In the differentiated rod cells, diffused distribution of RHO protein in cytoplasm and expressions of endoplasmic reticulum (ER) stress markers strongly indicated the involvement of ER stress. Furthermore, the rod cell numbers decreased significantly after successive culture, suggesting an in vitro model of rod degeneration. Thus, from integration-free patient-specific iPSCs, RP patient-specific rod cells were generated in vitro that recapitulated the disease feature and revealed evidence of ER stress in this patient, demonstrating its utility for disease modeling in vitro.

Generation and clonal isolation of retinal stem cells from human embryonic stem cells

Retinal stem cells (RSCs) are present within the pigmented ciliary epithelium (CE) of the adult human eye and produce progeny that differentiate in vitro into all neural retinal subtypes and retinal pigmented epithelium (RPE). We hypothesized that a RSC population, similar to the adult CE-derived RSC, is contained within pigmented colonies that arise in long-term cultures of human embryonic stem cells (hESCs) suggested to recapitulate retinal development in vitro. Single pigmented hESC-derived cells were isolated and plated in serum-free media containing growth factors and, after 2 weeks, clonal sphere colonies containing both pigmented and non-pigmented cells were observed. These colonies expressed the early retinal transcription factors Rx, Chx10 and Pax6, and could be dissociated and replated as single cells to form secondary clonal colonies. When allowed to differentiate, expression of markers for both RPE and neurons was observed. Rhodopsin expression was detected after explant co-culture and transplantation into the developing mouse eye as well as following treatment with soluble factors in vitro. We show that RSCs emerge in an in vitro model of retinal development and are a potential source of human photoreceptors for use in transplantation.

Consequences of oxidative stress in age-related macular degeneration

The retina resides in an environment that is primed for the generation of reactive oxygen species (ROS) and resultant oxidative damage. The retina is one of the highest oxygen-consuming tissues in the human body. The highest oxygen levels are found in the choroid, but this falls dramatically across the outermost retina, creating a large gradient of oxygen towards the retina and inner segments of the photoreceptors which contain high levels of polyunsaturated fatty acids. This micro-environment together with abundant photosensitizers, visible light exposure and a high energy demand supports a highly oxidative milieu. However, oxidative damage is normally minimized by the presence of a range of antioxidant and efficient repair systems. Unfortunately, as we age oxidative damage increases, antioxidant capacity decreases and the efficiency of reparative systems become impaired. The result is retinal dysfunction and cell loss leading to visual impairment. It appears that these age-related oxidative changes are a hallmark of early age-related macular degeneration (AMD) which, in combination with hereditary susceptibility and other retinal modifiers, can progress to the pathology and visual morbidity associated with advanced AMD. This review reassesses the consequences of oxidative stress in AMD and strategies for preventing or reversing oxidative damage in retinal tissues.

Feeder cells support the culture of induced pluripotent stem cells even after chemical fixation

Chemically fixed mouse embryonic fibroblasts (MEFs), instead of live feeder cells, were applied to the maintenance of mouse induced pluripotent stem (miPS) cells. Formaldehyde and glutaraldehyde were used for chemical fixation. The chemically fixed MEF feeders maintained the pluripotency of miPS cells, as well as their undifferentiated state. Furthermore, the chemically fixed MEF feeders were reused several times without affecting their functions. These results indicate that chemical fixation can be applied to modify biological feeders chemically, without losing their original functions. Chemically fixed MEF feeders will be applicable to other stem cell cultures as a reusable extracellular matrix candidate that can be preserved on a long-term basis.

Cell-replacement therapy and neural repair in the retina

Visual impairment severely affects the quality of life of patients and their families and is also associated with a deep economic impact. The most common pathologies responsible for visual impairment and legally defined blindness in developed countries include age-related macular degeneration, glaucoma and diabetic retinopathy. These conditions share common pathophysiological features: dysfunction and loss of retinal neurons. To date, two main approaches are being taken to develop putative therapeutic strategies: neuroprotection and cell replacement. Cell replacement is a novel therapeutic approach to restore visual capabilities to the degenerated adult neural retina and represents an emerging field of regenerative neurotherapy. The discovery of a population of proliferative cells in the mammalian retina has raised the possibility of harnessing endogenous retinal stem cells to elicit retinal repair. Furthermore, the development of suitable protocols for the reprogramming of differentiated somatic cells to a pluripotent state further increases the therapeutic potential of stem-cell-based technologies for the treatment of major retinal diseases. Stem-cell transplantation in animal models has been most effectively used for the replacement of photoreceptors, although this therapeutic approach is also being used for inner retinal pathologies. In this review, we discuss recent advances in the development of cell-replacement approaches for the treatment of currently incurable degenerative retinal diseases.

Relaxation-expansion model for self-driven retinal morphogenesis: a hypothesis from the perspective of biosystems dynamics at the multi-cellular level

The generation of complex organ structures such as the eye requires the intricate orchestration of multiple cellular interactions. In this paper, early retinal development is discussed with respect to the structure formation of the optic cup. Although recent studies have elucidated molecular mechanisms of retinal differentiation, little is known about how the unique shape of the optic cup is determined. A recent report has demonstrated that optic-cup morphogenesis spontaneously occurs in three-dimensional stem-cell culture without external forces, indicating a latent intrinsic order to generate the structure. Based on this self-organizing phenomenon, we introduce the "relaxation-expansion" model to mechanically interpret the tissue dynamics that enable the spontaneous invagination of the neural retina. This model involves three consecutive local rules (relaxation, apical constriction, and expansion), and its computer simulation recapitulates the optic-cup morphogenesis in silico.

Generation of retinal cells from pluripotent stem cells

Retinal degeneration is a leading cause of incurable low vision and blindness worldwide. Most retinal degenerative diseases are caused by irreversible apoptosis of retinal neural cells or adjacent supporting tissue. Because there is no radical treatment for retinal degeneration, most therapies are aimed at specific situations, such as drug or surgical intervention for late complications. Retinal cell replacement would be valuable for regenerating functional retinas, and therefore it is being examined as a next-generation treatment for retinal degeneration. With advances in stem cell biology, considerable progress has been made in recent years on generation of retinal cells. Both sensory retinal neural cells and retinal pigment epithelial cells can be induced in vitro from pluripotent stem cells, including embryonic stem cells and induced pluripotent stem cells. Here, we review the stepwise differentiation of retinal cells from pluripotent stem cells, with emphases on the methodology and application potential.

Mouse embryonic stem cell culture for generation of three-dimensional retinal and cortical tissues

Generation of compound tissues with complex structures is a major challenge in cell biology. In this article, we describe a protocol for mouse embryonic stem cell (ESC) culture for in vitro generation of three-dimensional retinal tissue, comparing it with the culture protocol for cortical tissue generation. Dissociated ESCs are reaggregated in a 96-well plate with reduced cell-plate adhesion and cultured as floating aggregates. Retinal epithelium is efficiently generated when ESC aggregates are cultured in serum-free medium containing extracellular matrix proteins, spontaneously forming hemispherical vesicles and then progressively transforming into a shape reminiscent of the embryonic optic cup in 9-10 d. In long-term culture, the ESC-derived optic cup generates a fully stratified retinal tissue consisting of all major neural retinal components. In contrast, the cortical differentiation culture can be started without exogenous extracellular matrix proteins, and it generates stratified cortical epithelia consisting of four distinct layers in 13 d.

Induced pluripotent stem cell technology for generating photoreceptors

The discovery of methods to produce pluripotent stem cells from human skin cells and other adult tissues has created a new era in stem cell research. In this article, we discuss the generation and use of pluripotent stem cells for the study of retinal disorders and the development of cell-based therapies. We describe advances in protocols for differentiating pluripotent cells into photoreceptor precursors that might be suitable for transplantation and discuss the use of human induced pluripotent stem cell-derived photoreceptors for disease modeling and drug screening.

Pluripotent stem cells for the study of CNS development

The mammalian central nervous system is a complex neuronal network consisting of a diverse array of cellular subtypes generated in a precise spatial and temporal pattern throughout development. Achieving a greater understanding of the molecular and genetic mechanisms that direct a relatively uniform population of neuroepithelial progenitors into diverse neuronal subtypes remains a significant challenge. The advent of pluripotent stem cell (PSC) technology allows researchers to generate diverse neural populations in vitro. Although the primary focus of PSC-derived neural cells has been their therapeutic potential, utilizing PSCs to study neurodevelopment is another frequently overlooked and equally important application. In this review, we explore the potential for utilizing PSCs to study neural development. We introduce the types of neurodevelopmental questions that PSCs can help to address, and we discuss the different strategies and technologies that researchers use to generate diverse subtypes of PSC-derived neurons. Additionally, we highlight the derivation of several thoroughly characterized neural subtypes; spinal motoneurons, midbrain dopaminergic neurons and cortical neurons. We hope that this review encourages researchers to develop innovative strategies for using PSCs for the study of mammalian, and specifically human, neurodevelopment.

Stem cells for retinal replacement therapy

Retinal degenerative disease has limited therapeutic options and the possibility of stem cell-mediated regenerative treatments is being actively explored for these blinding retinal conditions. The relative accessibility of this central nervous system tissue and the ability to visually monitor changes after transplantation make the retina and adjacent retinal pigment epithelium prime targets for pioneering stem cell therapeutics. Prior work conducted for several decades indicated the promise of cell transplantation for retinal disease, and new strategies that combine these established surgical approaches with stem cell-derived donor cells is ongoing. A variety of tissue-specific and pluripotent-derived donor cells are being advanced to replace lost or damaged retinal cells and/or to slow the disease processes by providing neuroprotective factors, with the ultimate aim of long-term improvement in visual function. Clinical trials are in the early stages, and data on safety and efficacy are widely anticipated. Positive outcomes from these stem cell-based clinical studies would radically change the way that blinding disorders are approached in the clinic.

Stem cell-based therapeutic applications in retinal degenerative diseases

Retinal degenerative diseases that target photoreceptors or the adjacent retinal pigment epithelium (RPE) affect millions of people worldwide. Retinal degeneration (RD) is found in many different forms of retinal diseases including retinitis pigmentosa (RP), age-related macular degeneration (AMD), diabetic retinopathy, cataracts, and glaucoma. Effective treatment for retinal degeneration has been widely investigated. Gene-replacement therapy has been shown to improve visual function in inherited retinal disease. However, this treatment was less effective with advanced disease. Stem cell-based therapy is being pursued as a potential alternative approach in the treatment of retinal degenerative diseases. In this review, we will focus on stem cell-based therapies in the pipeline and summarize progress in treatment of retinal degenerative disease.

Effective transplantation of photoreceptor precursor cells selected via cell surface antigen expression

Retinal degenerative diseases are a major cause of untreatable blindness. Stem cell therapy to replace lost photoreceptors represents a feasible future treatment. We previously demonstrated that postmitotic photoreceptor precursors expressing an NrlGFP transgene integrate into the diseased retina and restore some light sensitivity. As genetic modification of precursor cells derived from stem cell cultures is not desirable for therapy, we have tested cell selection strategies using fluorochrome-conjugated antibodies recognizing cell surface antigens to sort photoreceptor precursors. Microarray analysis of postnatal NrlGFP-expressing precursors identified four candidate genes encoding cell surface antigens (Nt5e, Prom1, Podxl, and Cd24a). To test the feasibility of using donor cells isolated using cell surface markers for retinal therapy, cells selected from developing retinae by fluorescence-activated cell sorting based on Cd24a expression (using CD24 antibody) and/or Nt5e expression (using CD73 antibody) were transplanted into the wild-type or Crb1(rd8/rd8) or Prph2(rd2/rd2) mouse eye. The CD73/CD24-sorted cells migrated into the outer nuclear layer, acquired the morphology of mature photoreceptors and expressed outer segment markers. They showed an 18-fold higher integration efficiency than that of unsorted cells and 2.3-fold higher than cells sorted based on a single genetic marker, NrlGFP, expression. These proof-of-principle studies show that transplantation competent photoreceptor precursor cells can be efficiently isolated from a heterogeneous mix of cells using cell surface antigens without loss of viability for the purpose of retinal stem cell therapy. Refinement of the selection of donorphotoreceptor precursor cells can increase the number of integrated photoreceptor cells,which is a prerequisite for the restoration of sight.

Simultaneous DNA and RNA isolation from brain punches for epigenetics

BACKGROUND

Epigenetic modifications such as DNA methylation play an important role for gene expression and are regulated by developmental and environmental signals. DNA methylation typically occurs in a highly tissue- and cell-specific manner. This raises a severe challenge when studying discrete, small regions of the brain where cellular heterogeneity is high and tissue quantity limited. Because gene expression and methylation are often tightly linked it appears of interest to compare both parameters in the same sample.

FINDINGS

We present a refined method for the simultaneous extraction of DNA for bisulfite sequencing and RNA for expression analysis from small mouse brain tissue punches. This method can also be easily adapted for other small tissues or cell populations.

CONCLUSIONS

The method described herein results in DNA and RNA of a quantity and quality permitting highly reliable bisulfite analysis and quantitative RT-PCR measurements, respectively.

Induced pluripotent stem cell therapies for geographic atrophy of age-related macular degeneration

There is currently no FDA-approved therapy for treating patients with geographic atrophy (GA), a late stage of age-related macular degeneration (AMD). Cell transplantation has the potential to restore vision in these patients. This review discusses how recent advancement in induced pluripotent stem (iPS) cells provides a promising therapy for GA treatment. Recent advances in stem cell biology have demonstrated that it is possible to derive iPS cells from human somatic cells by introducing reprogramming factors. Human retinal pigment epithelium (RPE) cells and photoreceptors can be derived from iPS cells by defined factors. Studies show that transplanting these cells can stabilize or recover vision in animal models. However, cell derivation protocols and transplantation procedures still need to be optimized. Much validation has to be done before clinical-grade, patient-derived iPS can be applied for human therapy. For now, RPE cells and photoreceptors derived from patient-specific iPS cells can serve as a valuable tool in elucidating the mechanism of pathogenesis and drug discovery for GA.

Stem cells as a therapeutic tool for the blind: biology and future prospects

Retinal degeneration due to genetic, diabetic and age-related disease is the most common cause of blindness in the developed world. Blindness occurs through the loss of the light-sensing photoreceptors; to restore vision, it would be necessary to introduce alternative photosensitive components into the eye. The recent development of an electronic prosthesis placed beneath the severely diseased retina has shown that subretinal stimulation may restore some visual function in blind patients. This proves that residual retinal circuits can be reawakened after photoreceptor loss and defines a goal for stem-cell-based therapy to replace photoreceptors. Advances in reprogramming adult cells have shown how it may be possible to generate autologous stem cells for transplantation without the need for an embryo donor. The recent success in culturing a whole optic cup in vitro has shown how large numbers of photoreceptors might be generated from embryonic stem cells. Taken together, these threads of discovery provide the basis for optimism for the development of a stem-cell-based strategy for the treatment of retinal blindness.

Self-organizing optic-cup morphogenesis in three-dimensional culture

Balanced organogenesis requires the orchestration of multiple cellular interactions to create the collective cell behaviours that progressively shape developing tissues. It is currently unclear how individual, localized parts are able to coordinate with each other to develop a whole organ shape. Here we report the dynamic, autonomous formation of the optic cup (retinal primordium) structure from a three-dimensional culture of mouse embryonic stem cell aggregates. Embryonic-stem-cell-derived retinal epithelium spontaneously formed hemispherical epithelial vesicles that became patterned along their proximal-distal axis. Whereas the proximal portion differentiated into mechanically rigid pigment epithelium, the flexible distal portion progressively folded inward to form a shape reminiscent of the embryonic optic cup, exhibited interkinetic nuclear migration and generated stratified neural retinal tissue, as seen in vivo. We demonstrate that optic-cup morphogenesis in this simple cell culture depends on an intrinsic self-organizing program involving stepwise and domain-specific regulation of local epithelial properties.

Transplantation of adult mouse iPS cell-derived photoreceptor precursors restores retinal structure and function in degenerative mice

This study was designed to determine whether adult mouse induced pluripotent stem cells (iPSCs), could be used to produce retinal precursors and subsequently photoreceptor cells for retinal transplantation to restore retinal function in degenerative hosts. iPSCs were generated using adult dsRed mouse dermal fibroblasts via retroviral induction of the transcription factors Oct4, Sox2, KLF4 and c-Myc. As with normal mouse ES cells, adult dsRed iPSCs expressed the pluripotency genes SSEA1, Oct4, Sox2, KLF4, c-Myc and Nanog. Following transplantation into the eye of immune-compromised retinal degenerative mice these cells proceeded to form teratomas containing tissue comprising all three germ layers. At 33 days post-differentiation a large proportion of the cells expressed the retinal progenitor cell marker Pax6 and went on to express the photoreceptor markers, CRX, recoverin, and rhodopsin. When tested using calcium imaging these cells were shown to exhibit characteristics of normal retinal physiology, responding to delivery of neurotransmitters. Following subretinal transplantation into degenerative hosts differentiated iPSCs took up residence in the retinal outer nuclear layer and gave rise to increased electro retinal function as determined by ERG and functional anatomy. As such, adult fibroblast-derived iPSCs provide a viable source for the production of retinal precursors to be used for transplantation and treatment of retinal degenerative disease.

Differentiation of neuronal cells from NIH/3T3 fibroblasts under defined conditions

We attempted to test whether the differentiated NIH/3T3 fibroblasts could be differentiated into neuronal cells without any epigenetic modification. First, a neurosphere assay was carried out, and we successfully generated neurosphere-like cells by floating cultures of NIH/3T3 fibroblasts in neural stem cell medium. These spheres have the ability to form sub-spheres after three passages, and express the neural progenitor markers Nestin, Sox2, Pax6, and Musashi-1. Second, after shifting to a differentiating medium and culturing for an additional 8 days, cells in these spheres expressed the neuronal markers β-tubulin and neurofilament 200 and the astrocytic marker glial fibrillary acidic protein (GFAP). Finally, after treating the spheres with all-trans retinoic acid and taurine, the expression of β-tubulin was increased and the staining of photoreceptor markers rhodopsin and recoverin was observed. The present study shows that NIH/3T3 fibroblasts can generate neurosphere-like, neuron-like, and even photoreceptor-like cells under defined conditions, suggesting that the differentiated non-neuronal cells NIH/3T3 fibroblasts, but not pluripotent cells such as embryonic stem cells or induced pluripotent stem cells, may have the potential to be transdifferentiated into neuronal cells without adding any epigenetic modifier. This transdifferentiation may be due to the possible neural progenitor potential of NIH/3T3 fibroblasts that remains dormant under normal conditions.

TRPV1 receptors modulate retinal development

We investigated the possible participation of TRPV1 channels in retinal apoptosis and overall development. Retinas from newborn, male albino rats were treated in vitro with capsazepine, a TRPV1 antagonist. The expression of cell cycle markers was not changed after TRPV1 blockade, whereas capsazepine reduced the number of apoptotic cells throughout the retina,increased ERK1/2 and p38 phosphorylation and slightly reduced JNK phosphorylation. The expression of BAD, Bcl-2, as well as integral and cleaved capsase-3 were similar in all experimental conditions. Newborn rats were kept for 2 months after receiving high doses of capsazepine. In their retinas, calbindin and parvalbumin protein levels were upregulated, but only the number of amacrine-like, parvalbumin-positive cells was increased. The numbers of calretinin, calbindin, ChAT, vimentin, PKC-alpha and GABA-positive cells were similar in both conditions. Protein expression of synapsin Ib was also increased in the retinas of capsazepine-treated rats. Calretinin, vimentin, GFAP, synapsin Ia, synaptophysin and light neurofilament protein levels were not changed when compared to control values. Our results indicate that TRPV1 channels play a role in the control of the early apoptosis that occur during retinal development, which might be dependent on MAPK signaling. Moreover, it seems that TRPV1 function might be important for neuronal and synaptic maturation in the retina.

Turning straw into gold: directing cell fate for regenerative medicine

Regenerative medicine offers the hope that cells for disease research and therapy might be created from readily available sources. To fulfil this promise, the cells available need to be converted into the desired cell types. We review two main approaches to accomplishing this goal: in vitro directed differentiation, which is used to push pluripotent stem cells, including embryonic stem cells or induced pluripotent stem cells, through steps similar to those that occur during embryonic development; and reprogramming (also known as transdifferentiation), in which a differentiated cell is converted directly into the cell of interest without proceeding through a pluripotent intermediate. We analyse the status of progress made using these strategies and highlight challenges that must be overcome to achieve the goal of cell-replacement therapy.

Polarized secretion of PEDF from human embryonic stem cell-derived RPE promotes retinal progenitor cell survival

PURPOSE

Human embryonic stem cell-derived RPE (hES-RPE) transplantation is a promising therapy for atrophic age-related macular degeneration (AMD); however, future therapeutic approaches may consider co-transplantation of hES-RPE with retinal progenitor cells (RPCs) as a replacement source for lost photoreceptors. The purpose of this study was to determine the effect of polarization of hES-RPE monolayers on their ability to promote survival of RPCs.

METHODS

The hES-3 cell line was used for derivation of RPE. Polarization of hES-RPE was achieved by prolonged growth on permeable inserts. RPCs were isolated from 16- to 18-week-gestation human fetal eyes. ELISA was performed to measure pigment epithelium-derived factor (PEDF) levels from conditioned media.

RESULTS

Pigmented RPE-like cells appeared as early as 4 weeks in culture and were subcultured at 8 weeks. Differentiated hES-RPE had a normal chromosomal karyotype. Phenotypically polarized hES-RPE cells showed expression of RPE-specific genes. Polarized hES-RPE showed prominent expression of PEDF in apical cytoplasm and a marked increase in secretion of PEDF into the medium compared with nonpolarized culture. RPCs grown in the presence of supernatants from polarized hES-RPE showed enhanced survival, which was ablated by the presence of anti-PEDF antibody.

CONCLUSIONS

hES-3 cells can be differentiated into functionally polarized hES-RPE cells that exhibit characteristics similar to those of native RPE. On polarization, hES-RPE cells secrete high levels of PEDF that can support RPC survival. These experiments suggest that polarization of hES-RPE would be an important feature for promotion of RPC survival in future cell therapy for atrophic AMD.

Modeling retinal degeneration using patient-specific induced pluripotent stem cells

Retinitis pigmentosa (RP) is the most common inherited human eye disease resulting in night blindness and visual defects. It is well known that the disease is caused by rod photoreceptor degeneration; however, it remains incurable, due to the unavailability of disease-specific human photoreceptor cells for use in mechanistic studies and drug screening. We obtained fibroblast cells from five RP patients with distinct mutations in the RP1, RP9, PRPH2 or RHO gene, and generated patient-specific induced pluripotent stem (iPS) cells by ectopic expression of four key reprogramming factors. We differentiated the iPS cells into rod photoreceptor cells, which had been lost in the patients, and found that they exhibited suitable immunocytochemical features and electrophysiological properties. Interestingly, the number of the patient-derived rod cells with distinct mutations decreased in vitro; cells derived from patients with a specific mutation expressed markers for oxidation or endoplasmic reticulum stress, and exhibited different responses to vitamin E than had been observed in clinical trials. Overall, patient-derived rod cells recapitulated the disease phenotype and expressed markers of cellular stresses. Our results demonstrate that the use of patient-derived iPS cells will help to elucidate the pathogenic mechanisms caused by genetic mutations in RP.

Comparative analysis of the retinal potential of embryonic stem cells and amniotic fluid-derived stem cells

Photoreceptors have recently been generated from mouse and human embryonic stem cells (ESCs), although ethics concerns impede their utilization for cell replacement therapy for retinal disease. Extra-embryonic tissues have received attention as alternative therapeutic sources of stem cells. Human and mouse amniotic fluid-derived stem cells (AFCs) have been reported to be multipotent and express embryonic and adult stem cell markers. Here, in vitro conditions that generate retinal cells from ESCs were used to analyze and compare the retinal potential of murine AFCs and ESCs. We show that AFCs express pluripotency markers (Nanog, Sox2, and Oct3/4) as well as retinal transcription factor genes (Et, Lhx2, Tll1, Six6, Otx2, Pax6, and Fgf15). AFCs from amniotic fluid of Fgf15.gfp, Nrl.gfp, and Crx.gfp embryos cultured in retinal proliferation and differentiation conditions failed to switch on these retinal transgenes. AFCs cultured in retinal-promoting conditions, effective on ESCs, showed reduced expression of retinal markers. Retinal co-cultures activated retinal genes in ESCs but not in AFCs, and migration assays in retinal explants showed limited migration of AFCs compared with ESCs. Unlike ESCs, AFCs do not express the early embryonic ectodermal gene Utf1 and Western analysis of AFCs identified only the B isoform of Oct3/4, rather than the isoform A present in ESCs. We conclude that AFCs have restricted potential and differ considerably from ESCs and retinal progenitor cells. Reprogramming to induce pluripotency or new differentiation protocols will be required to confer retinal potential to AFCs as expression of a subset of pluripotency and retinal markers is not sufficient.

E13.5 retinal progenitors induce mouse bone marrow mesenchymal stromal cells to differentiate into retinal progenitor-like cells

BACKGROUND AIMS

Retinal progenitor cells (RPC) are an excellent resource for retinal replacement therapy but usually unavailable. We attempted to induce bone marrow mesenchymal stromal cells (BMSC) into RPC.

METHODS

BMSC and embryonic day 13.5 (E13.5) RPC derived from wild-type or enhanced green fluorescence protein (EGFP) transgenic (Egfp(+/+)) mice were co-cultured in a transwell or re-aggregation system. Gene and protein expressions were investigated by reverse transcription-polymerase chain reaction (PCR) and immunofluorescence, respectively. Spontaneous cell fusion was evaluated by Chloromethylbenzamido derivative of 1,1'- dioctadecyl-3,3,3',3' - tetramethylindocarbocyanine perchlorate (CM-DiI) labeling together with EGFP tracing.

RESULTS

BMSC from both wild-type and Egfp(+/+) mice displayed similar spindle shapes. The undifferentiated BMSC already expressed immature neural markers but did not express Nfl, Gfap or the retina-related genes Pax6, Math5 and Brn3b. When co-cultured with E13.5 RPC in the transwell system, BMSC displayed transient expression of early retinal development genes, including Pax6, Math5 and Brn3b at 3 days, as well as long-term expression of Nfl (up to 21 days). No expression of the late photoreceptor gene rhodopsin could be detected at any time. In re-aggregation co-culture, E13.5 RPC induced EGFP-positive BMSC to express not only the early retinal development genes but also the late gene rhodopsin. Furthermore, a small fraction of BMSC could be induced to express the synaptophysin protein. Re-aggregation co-culture of CM-DiI-labeled BMSC and EGFP-positive E13.5 RPC displayed minimal co-localization of the two fluorescence signals.

CONCLUSIONS

E13.5 RPC are capable of inducing BMSC towards an RPC fate. The differentiation is independent of cell fusion. Cytokines and cell-cell interactions exert this induction effect, but they have different functions.

Programming embryonic stem cells to neuronal subtypes

Richness of neural circuits and specificity of neuronal connectivity depend on the diversification of nerve cells into functionally and molecularly distinct subtypes. Although efficient methods for directed differentiation of embryonic stem cells (ESCs) into multiple principal neuronal classes have been established, only a few studies systematically examined the subtype diversity of in vitro derived nerve cells. Here we review evidence based on molecular and in vivo transplantation studies that ESC-derived spinal motor neurons and cortical layer V pyramidal neurons acquire subtype specific functional properties. We discuss similarities and differences in the role of cell-intrinsic transcriptional programs, extrinsic signals and cell-cell interactions during subtype diversification of the two classes of nerve cells. We conclude that the high degree of fidelity with which differentiating ESCs recapitulate normal embryonic development provides a unique opportunity to explore developmental processes underlying specification of mammalian neuronal diversity in a simplified and experimentally accessible system.

The new paradigm: retinal pigment epithelium cells generated from embryonic or induced pluripotent stem cells

Compared with neural crest-derived melanocytes, retinal pigment epithelium (RPE) cells in the back of the eye are pigment cells of a different kind. They are a part of the brain, form an epithelial monolayer, respond to distinct extracellular signals, and provide functions that far exceed those of a light-absorbing screen. For instance, they control nutrient and metabolite flow to and from the retina, replenish 11-cis-retinal by re-isomerizing all-trans-retinal generated during photoconversion, phagocytose daily a portion of the photoreceptors' outer segments, and secrete cytokines that locally control the innate and adaptive immune systems. Not surprisingly, RPE cell damage is a major cause of human blindness worldwide, with age-related macular degeneration a prevalent example. RPE replacement therapies using RPE cells generated from embryonic or induced pluripotent stem cells provide a novel approach to a rational treatment of such forms of blindness. In fact, RPE-like cells can be obtained relatively easily when stem cells are subjected to a two-step induction protocol, a first step that leads to a neuroectodermal fate and a second to RPE differentiation. Here, we discuss the characteristics of such cells, propose criteria they should fulfill in order to be considered authentic RPE cells, and point out the challenges one faces when using such cells in attempts to restore vision.

Adult ciliary epithelial cells, previously identified as retinal stem cells with potential for retinal repair, fail to differentiate into new rod photoreceptors

The ciliary margin in lower vertebrates is a site of continual retinal neurogenesis and a stem cell niche. By contrast, the human eye ceases retinal neuron production before birth and loss of photoreceptors during life is permanent and a major cause of blindness. The discovery of a proliferative cell population in the ciliary epithelium (CE) of the adult mammalian eye, designated retinal stem cells, raised the possibility that these cells could help to restore sight by replacing lost photoreceptors. We previously demonstrated the feasibility of photoreceptor transplantation using cells from the developing retina. CE cells could provide a renewable source of photoreceptors for transplantation. Several laboratories reported that these cells generate new photoreceptors, whereas a recent report questioned the existence of retinal stem cells. We used Nrl.gfp transgenic mice that express green fluorescent protein in rod photoreceptors to assess definitively the ability of CE cells to generate new photoreceptors. We report that CE cells expanded in monolayer cultures, lose pigmentation, and express a subset of eye field and retinal progenitor cell markers. Simultaneously, they continue to express some markers characteristic of differentiated CE and typically lack a neuronal morphology. Previously reported photoreceptor differentiation conditions used for CE cells, as well as conditions used to differentiate embryonic retinal progenitor cells (RPCs) and embryonic stem cell-derived RPCs, do not effectively activate the Nrl-regulated photoreceptor differentiation program. Therefore, we conclude that CE cells lack potential for photoreceptor differentiation and would require reprogramming to be useful as a source of new photoreceptors.

Adult retinal stem cells revisited

Recent advances in retinal stem cell research have raised the possibility that these cells have the potential to be used to repair or regenerate diseased retina. Various cell sources for replacement of retinal neurons have been identified, including embryonic stem cells, the adult ciliary epithelium, adult Müller stem cells and induced pluripotent stem cells (iPS). However, the true stem cell nature of the ciliary epithelium and its possible application in cell therapies has now been questioned, leaving other cell sources to be carefully examined as potential candidates for such therapies. The need for identification of the ontogenetic state of grafted stem cells in order to achieve their successful integration into the murine retina has been recognized. However, it is not known whether the same requirements may apply to achieve transplant cell integration into the adult human eye. In addition, the existence of natural barriers for stem cell transplantation, including microglial accumulation and abnormal extracellular matrix deposition have been demonstrated, suggesting that several obstacles need to be overcome before such therapies may be implemented. This review addresses recent scientific developments in the field and discusses various strategies that may be potentially used to design cell based therapies to treat human retinal disease.

Induced pluripotent stem cells generate both retinal ganglion cells and photoreceptors: therapeutic implications in degenerative changes in glaucoma and age-related macular degeneration

The direct reprogramming of somatic cells to a pluripotent state holds significant implications for treating intractable degenerative diseases by ex vivo cell therapy. In addition, the reprogrammed cells can serve as a model for diseases and the discovery of drugs and genes. Here, we demonstrate that mouse fibroblast induced pluripotent stem cells (iPSCs) represent a renewable and robust source of retinal progenitors, capable of generating a wide range of retinal cell types that includes retinal ganglion cells (RGCs), cone, and rod photoreceptors. They respond to simulated microenvironment of early and late retinal histogenesis by differentiating into stage-specific retinal cell types through the recruitment of normal mechanisms. The depth of the retinal potential of iPSCs suggests that they may be used to formulate stem cell approaches to understand and treat a wide range of retinal degenerative diseases from glaucoma to age-related macular degeneration (AMD).

Cotylenin A inhibits cell proliferation and induces apoptosis and PAX6 mRNA transcripts in retinoblastoma cell lines

PURPOSE

Retinoblastoma, a childhood cancer of the retina, is caused by inactivation of the tumor suppressor gene retinoblastoma (RB). Cotylenin A (CN-A), a novel fusicoccane-diterpene glycoside, accelerates the differentiation of several types of myeloid cell lines and is a candidate for a new type of anticancer therapeutic agent with this effect. However, whether CN-A has the same effect on retinoblastoma cells is unknown. We studied the response of two retinoblastoma cell lines, Y-79 and WERI-Rb-1, to CN-A.

METHODS

We studied the response of two retinoblastoma cell lines to CN-A with respect to cell growth, apoptosis, morphology, mRNA, protein expression analysis of specific genes (N-myc, cyclin-dependent kinase inhibitor 1A [P21], paired box gene 6 [PAX6], and rhodopsin [RHO]), and activity of three PAX6 promoters (P0, P1, and Palpha).

RESULTS

CN-A inhibited cell proliferation and induced apoptosis via caspase activity in the two retinoblastoma cell lines. In addition, CN-A induced mRNA expression of P21, PAX6, and RHO and protein expression of P21. In Y-79 cells, PAX6 P1 promoter was activated by CN-A. In WERI-Rb-1 cells, PAX6 P0, P1, and Palpha promoter were activated by CN-A. CN-A decreased mRNA and protein expression of N-myc in two retinoblastoma cell lines.

CONCLUSIONS

The responses of retinoblastoma cells to CN-A include inhibition of cell growth, induction of apoptosis, and the potential to change neuroblastoma characteristics of retinoblastoma cells.

Mechanosensitive hair cell-like cells from embryonic and induced pluripotent stem cells

Mechanosensitive sensory hair cells are the linchpin of our senses of hearing and balance. The inability of the mammalian inner ear to regenerate lost hair cells is the major reason for the permanence of hearing loss and certain balance disorders. Here, we present a stepwise guidance protocol starting with mouse embryonic stem and induced pluripotent stem cells, which were directed toward becoming ectoderm capable of responding to otic-inducing growth factors. The resulting otic progenitor cells were subjected to varying differentiation conditions, one of which promoted the organization of the cells into epithelial clusters displaying hair cell-like cells with stereociliary bundles. Bundle-bearing cells in these clusters responded to mechanical stimulation with currents that were reminiscent of immature hair cell transduction currents.

Immortalized human fetal retinal cells retain progenitor characteristics and represent a potential source for the treatment of retinal degenerative disease

Human fetal retinal cells have been widely advocated for the development of cellular replacement therapies in patients with retinal dystrophies and age-related macular degeneration. A major limitation, however, is the lack of an abundant and renewable source of cells to meet therapeutic demand, although theoretically this may be addressed through the use of immortalized retinal progenitor cell lines. Here, we have used the temperature-sensitive tsA58 simian virus SV40 T antigen to conditionally immortalize human retinal progenitor cells isolated from retinal tissue at 10-12 weeks of gestation. We show that immortalized human fetal retinal cells retain their progenitor cell properties over many passages, and are comparable with nonimmortalized human fetal retinal cultures from the same gestational period with regard to expression of certain retinal genes. To evaluate the capacity of these cells to integrate into the diseased retina and to screen for potential tumorigenicity, cells were grafted into neonatal hooded Lister rats and RCS dystrophic rats. Both cell lines exhibited scarce integration into the host retina and failed to express markers of mature differentiated retinal cells. Moreover, although immortalized cells showed a greater propensity to survive, the cell lines demonstrated poor long-term survival. All grafts were infiltrated with host macrophage/microglial cells throughout their duration of survival. This study demonstrates that immortalized human fetal retinal progenitor cells retain their progenitor characteristics and may therefore have therapeutic potential in strategies that demand a renewable and consistent supply of donor cells for the treatment of degenerative retinal diseases.