Facile isolation and the characterization of human retinal stem cells

Does the adult human ciliary body epithelium contain "true" retinal stem cells?

Recent reports of retinal stem cells being present in several locations of the adult eye have sparked great hopes that they may be used to treat the millions of people worldwide who suffer from blindness as a result of retinal disease or injury. A population of proliferative cells derived from the ciliary body epithelium (CE) has been considered one of the prime stem cell candidates, and as such they have received much attention in recent years. However, the true nature of these cells in the adult human eye has still not been fully elucidated, and the stem cell claim has become increasingly controversial in light of new and conflicting reports. In this paper, we will try to answer the question of whether the available evidence is strong enough for the research community to conclude that the adult human CE indeed harbors stem cells.

Regenerative therapy for neuronal diseases with transplantation of somatic stem cells

Pluripotent stem cells, which are capable of differentiating in various species of cells, are hoped to be donor cells in transplantation in regenerative medicine. Embryonic stem (ES) cells and induced pluripotent stem cells have the potential to differentiate in approximately all species of cells. However, the proliferating ability of these cells is high and the cancer formation ability is also recognized. In addition, ethical problems exist in using ES cells. Somatic stem cells with the ability to differentiate in various species of cells have been used as donor cells for neuronal diseases, such as amyotrophic lateral sclerosis, spinal cord injury, Alzheimer disease, cerebral infarction and congenital neuronal diseases. Human mesenchymal stem cells derived from bone marrow, adipose tissue, dermal tissue, umbilical cord blood and placenta are usually used for intractable neuronal diseases as somatic stem cells, while neural progenitor/stem cells and retinal progenitor/stem cells are used for a few congenital neuronal diseases and retinal degenerative disease, respectively. However, non-treated somatic stem cells seldom differentiate to neural cells in recipient neural tissue. Therefore, the contribution to neuronal regeneration using non-treated somatic stem cells has been poor and various differential trials, such as the addition of neurotrophic factors, gene transfer, peptide transfer for neuronal differentiation of somatic stem cells, have been performed. Here, the recent progress of regenerative therapies using various somatic stem cells is described.

Adult ciliary epithelial stem cells generate functional neurons and differentiate into both early and late born retinal neurons under non-cell autonomous influences

Background

The neural stem cells discovered in the adult ciliary epithelium (CE) in higher vertebrates have emerged as an accessible source of retinal progenitors; these cells can self-renew and possess retinal potential. However, recent studies have cast doubt as to whether these cells could generate functional neurons and differentiate along the retinal lineage. Here, we have systematically examined the pan neural and retinal potential of CE stem cells.

Results

Molecular and cellular analysis was carried out to examine the plasticity of CE stem cells, obtained from mice expressing green fluorescent protein (GFP) under the influence of the promoter of the rod photoreceptor-specific gene, Nrl, using the neurospheres assay. Differentiation was induced by specific culture conditions and evaluated by both transcripts and protein levels of lineage-specific regulators and markers. Temporal pattern of their levels were examined to determine the expression of genes and proteins underlying the regulatory hierarchy of cells specific differentiation in vitro. Functional attributes of differentiation were examined by the presence of current profiles and pharmacological mobilization of intracellular calcium using whole cell recordings and Fura-based calcium imaging, respectively. We demonstrate that stem cells in adult CE not only have the capacity to generate functional neurons, acquiring the expression of sodium and potassium channels, but also respond to specific cues in culture and preferentially differentiate along the lineages of retinal ganglion cells (RGCs) and rod photoreceptors, the early and late born retinal neurons, respectively. The retinal differentiation of CE stem cells was characterized by the temporal acquisition of the expression of the regulators of RGCs and rod photoreceptors, followed by the display of cell type-specific mature markers and mobilization of intracellular calcium.

Conclusions

Our study demonstrates the bonafide retinal potential of adult CE stem cells and suggests that their plasticity could be harnessed for clinical purposes once barriers associated with any lineage conversion, i.e., low efficiency and fidelity is overcome through the identification of conducive culture conditions.

Gene expression is dynamically regulated in retinal progenitor cells prior to and during overt cellular differentiation

The retina is comprised of one glial and six neuronal populations that are generated from a multipotent pool of retinal progenitor cells (RPCs) during development. To give rise to these different cell types, RPCs undergo temporal identity transitions, displaying distinct gene expression profiles at different stages of differentiation. Little, however, is known about temporal differences in RPC identities prior to the onset of overt cellular differentiation, during the period when a retinal identity is gradually acquired. Here we examined the sequential onset of expression of regional markers (i.e., homeodomain transcription factors) and cell fate determinants (i.e., basic-helix-loop-helix transcription factors and neurogenic genes) in RPCs from the earliest appearance of a morphologically-distinct retina. By performing a comparative analysis of the expression of a panel of 27 homeodomain, basic-helix-loop-helix and Notch pathway genes between embryonic day (E) 8.75 and postnatal day (P) 9, we identified six distinct RPC molecular profiles. At E8.75, the earliest stage assayed, murine RPCs expressed five homeodomain genes and a single neurogenic gene (Pax6, Six3, Six6, Rx, Otx2, Hes1). This early gene expression profile was remarkably similar to that of 'early' RPCs in the amphibian ciliary marginal zone (CMZ), where RPCs are compartmentalised according to developmental stage, and homologs of Pax6, Six3 and Rx are expressed in the 'early' stem cell zone. As development proceeds, expression of additional homeodomain, bHLH and neurogenic genes was gradually initiated in murine RPCs, allowing distinct genetic profiles to also be defined at E9.5, E10.5, E12.5, E15.5 and P0. In addition, RPCs in the postnatal ciliary margin, where retinal stem cells are retained throughout life, displayed a unique molecular signature, expressing all of the early-onset genes as well as several late-onset markers, indicative of a 'mixed' temporal identity. Taken together, the identification of temporal differences in gene expression in mammalian RPCs during pre-neurogenic developmental stages leads to new insights into how regional identities are progressively acquired during development, while comparisons at later stages highlight the dynamic nature of gene expression in temporally distinct RPC pools.

Bone marrow mesenchymal stem cells stimulate proliferation and neuronal differentiation of retinal progenitor cells

During retina development, retinal progenitor cell (RPC) proliferation and differentiation are regulated by complex inter- and intracellular interactions. Bone marrow mesenchymal stem cells (BMSCs) are reported to express a variety of cytokines and neurotrophic factors, which have powerful trophic and protective functions for neural tissue-derived cells. Here, we show that the expanded RPC cultures treated with BMSC-derived conditioned medium (CM) which was substantially enriched for bFGF and CNTF, expressed clearly increased levels of nuclear receptor TLX, an essential regulator of neural stem cell (NSC) self-renewal, as well as betacellulin (BTC), an EGF-like protein described as supporting NSC expansion. The BMSC CM- or bFGF-treated RPCs also displayed an obviously enhanced proliferation capability, while BMSC CM-derived bFGF knocked down by anti-bFGF, the effect of BMSC CM on enhancing RPC proliferation was partly reversed. Under differentiation conditions, treatment with BMSC CM or CNTF markedly favoured RPC differentiation towards retinal neurons, including Brn3a-positive retinal ganglion cells (RGCs) and rhodopsin-positive photoreceptors, and clearly diminished retinal glial cell differentiation. These findings demonstrate that BMSCs supported RPC proliferation and neuronal differentiation which may be partly mediated by BMSC CM-derived bFGF and CNTF, reveal potential limitations of RPC culture systems, and suggest a means for optimizing RPC cell fate determination in vitro.

Acquisition of RGC phenotype in human Müller glia with stem cell characteristics is accompanied by upregulation of functional nicotinic acetylcholine receptors

PURPOSE

Human Müller glia with stem cell characteristics (hMGSCs) can be induced to express genes and proteins of retinal ganglion cells (RGCs) upon in vitro inhibition of Notch-1 activity. However, it is not known whether expression of these markers is accompanied by acquisition of RGC function. This study investigated whether hMGSCs that express RGC markers also display neural functionality, as measured by their intracellular calcium concentration ([Ca(2+)]i) responsiveness following neurotransmitter stimulation in vitro.

METHODS

Changes in mRNA expression of RGC markers and neurotransmitter receptors were assessed either by conventional or quantitative reverse transcription PCR (RT-PCR), while changes in protein levels were confirmed by immunocytochemistry. The [Ca(2+)]i levels were estimated by fluorescence microscopy.

RESULTS

We showed that while undifferentiated hMGSCs displayed a profound elevation of [Ca(2+)]i after stimulation with N-methyl-D-aspartate (NMDA), this was lost following Notch-1 inhibition. Conversely, untreated hMGSCs did not respond to muscarinic receptor stimulation, whereas [Ca(2+)]i was increased in differentiated hMGSCs that expressed RGC precursor markers. Differentiated hMGSC-derived RGCs, but not undifferentiated hMGSCs, responded to stimulation by nicotine with a substantial rise in [Ca(2+)]i, which was inhibited by the α4β2 and α6β2 nicotinic receptor antagonist methyllycaconitine. Notch-1 attenuation not only caused a decrease in the gene expression of the Notch effector HES1 and increased expression of RGC markers, but also an increase in the gene and protein expression of α4 and α6 nicotinic receptor subunits.

CONCLUSIONS

These observations suggest that in response to Notch-1 inhibition, hMGSCs differentiate into a population of RGCs that exhibit some of the functionality observed in differentiated RGCs.

Adult neural stem cells from the subventricular zone: a review of the neurosphere assay

The possibility of obtaining large numbers of cells with potential to become functional neurons implies a great advance in regenerative medicine. A source of cells for therapy is the subventricular zone (SVZ) where adult neural stem cells (NSCs) retain the ability to proliferate, self-renew, and differentiate into several mature cell types. The neurosphere assay, a method to isolate, maintain, and expand these cells has been extensively utilized by research groups to analyze the biological properties of aNSCs and to graft into injured brains from animal models. In this review we briefly describe the neurosphere assay and its limitations, the methods to optimize culture conditions, the identity and the morphology of aNSC-derived neurospheres (including new ultrastructural data). The controversy regarding the identity and "stemness" of cells within the neurosphere is revised. The fine morphology of neurospheres, described thoroughly, allows for phenotypical characterization of cells in the neurospheres and may reveal slight changes that indirectly inform about cell integrity, cell damage, or oncogenic transformation. Along this review we largely highlight the critical points that researchers have to keep in mind before extrapolating results or translating experimental transplantation of neurosphere-derived cells to the clinical setting.

Ephrin-A3 suppresses Wnt signaling to control retinal stem cell potency

The ciliary epithelium (CE) of adult mammals has been reported to provide a source of retinal stem cells (RSCs) that can give rise to all retinal cell types in vitro. A recent study, however, suggests that CE-derived cells possess properties of pigmented ciliary epithelial cells and display little neurogenic potential. Here we show that the neurogenic potential of CE-derived cells is negatively regulated by ephrin-A3, which is upregulated in the CE of postnatal mice and presents a strong prohibitory niche for adult RSCs. Addition of ephrin-A3 inhibits proliferation of CE-derived RSCs and increases pigment 349 cell 359. In contrast, absence of ephrin-A3 promotes proliferation and increases expression of neural progenitor cell markers and photoreceptor progeny. The negative effects of ephrin-A3 on CE-derived RSCs are mediated through activation of an EphA4 receptor and suppression of Wnt3a/β-catenin signaling. Together, our data suggest that CE-derived RSCs contain the intrinsic machinery to generate photoreceptors and other retinal neurons, while the CE of adult mice expresses negative regulators that prohibit the proliferation and neural differentiation of RSCs. Manipulating ephrin and Wnt/β-catenin signaling may, thus, represent a viable approach in activating the endogenous neurogenic potential of CE-derived RSCs for treating photoreceptor damage and retinal degenerative disorders.

The isolation, differentiation, and survival in vivo of multipotent cells from the postnatal rat filum terminale

Neural stem cells (NSCs) are undifferentiated cells in the central nervous system (CNS) that are capable of self-renewal and can be induced to differentiate into neurons and glia. Current sources of mammalian NSCs are confined to regions of the CNS that are critical to normal function and surgically difficult to access, which limits their therapeutic potential in human disease. We have found that the filum terminale (FT), a previously unexplored, expendable, and easily accessible tissue at the caudal end of the spinal cord, is a source of multipotent cells in postnatal rats and humans. In this study, we used a rat model to isolate and characterize the potential of these cells. Neurospheres derived from the rat FT are amenable to in vitro expansion in the presence of a combination of growth factors. These proliferating, FT-derived cells formed neurospheres that could be induced to differentiate into neural progenitor cells, neurons, astrocytes, and oligodendrocytes by exposure to serum and/or adhesive substrates. Through directed differentiation using sonic hedgehog and retinoic acid in combination with various neurotrophic factors, FT-derived neurospheres generated motor neurons that were capable of forming neuromuscular junctions in vitro. In addition, FT-derived progenitors that were injected into chick embryos survived and could differentiate into both neurons and glia in vivo.

Multipotent stem cells isolated from the adult mouse retina are capable of producing functional photoreceptor cells

Various stem cell types have been tested for their potential application in treating photoreceptor degenerative diseases, such as retinitis pigmentosa (RP) and age-related macular degeneration (AMD). Only embryonic stem cells (ESCs) have so far been shown to generate functional photoreceptor cells restoring light response of photoreceptor-deficient mice, but there is still some concern of tumor formation. In this study, we have successfully cultured Nestin(+)Sox2(+)Pax6(+) multipotent retinal stem cells (RSCs) from the adult mouse retina, which are capable of producing functional photoreceptor cells that restore the light response of photoreceptor-deficient rd1 mutant mice following transplantation. After they have been expanded for over 35 passages in the presence of FGF and EGF, the cultured RSCs still maintain stable proliferation and differentiation potential. Under proper differentiation conditions, they can differentiate into all the major retinal cell types found in the adult retina. More importantly, they can efficiently differentiate into photoreceptor cells under optimized differentiation conditions. Following transplantation into the subretinal space of slowly degenerating rd7 mutant eyes, RSC-derived photoreceptor cells integrate into the retina, morphologically resembling endogenous photoreceptors and forming synapases with resident retinal neurons. When transplanted into eyes of photoreceptor-deficient rd1 mutant mice, a RP model, RSC-derived photoreceptors can partially restore light response, indicating that those RSC-derived photoreceptors are functional. Finally, there is no evidence for tumor formation in the photoreceptor-transplanted eyes. Therefore, this study has demonstrated that RSCs isolated from the adult retina have the potential of producing functional photoreceptor cells that can potentially restore lost vision caused by loss of photoreceptor cells in RP and AMD.

Advances in retinal stem cell biology

Tremendous progress has been made in recent years to generate retinal cells from pluripotent cell sources. These advances provide hope for those suffering from blindness due to lost retinal cells. Understanding the intrinsic genetic network in model organisms, like fly and frog, has led to a better understanding of the extrinsic signaling pathways necessary for retinal progenitor cell formation in mouse and human cell cultures. This review focuses on the culture methods used by different groups, which has culminated in the generation of laminated retinal tissue from both embryonic and induced pluripotent cells. The review also briefly describes advances made in transplantation studies using donor retinal progenitor and cultured retinal cells.

Stem Cells and Ocular Tissue Regeneration

PURPOSE

Millions worldwide have visual impairments caused by dysfunctional eye components, including cornea, lens, retina, and optic nerve, or the visual cortex in the brain. Insufficient cornea donation and inherent artificial lens problems demand alternative treatment strategies for cornea diseases and cataracts, whereas retinal degenerative diseases, including glaucoma, macular degeneration, and retinitis pigmentosa, still lack effective treatments. Stem cells have been investigated for their potential in various eye-specific pathologies to replace lost retinal ganglion cells and photoreceptors in retinal degenerative diseases and toward engineering transplantable patient-specific cornea or lens.

DESIGN

Many stem cell types, including putative resident eye stem cells, mesenchymal stem cells, embryonic stem cells, and induced pluripotent stem cells, have been investigated for their potential to generate specific cell types in the eye in culture and after transplantation and to engineer eye tissues in combination with structural scaffolds.

METHOD

Cultured stem cells and in vitro differentiated eye-specific cells are transplanted into different locations of the eye to test their ability to produce functional cells for supporting eye functions. In addition, stem cells have been directly tested in vitro for their capacity to engineer eye-specific tissues.

RESULTS

Different stem cell types have been shown to have distinct capacities to produce eye-specific cells or even the entire retina.

CONCLUSIONS

Stem cells offer great hope for treating various eye pathologies. Despite recent progress, many challenges must still be overcome before the era of stem cell-based therapy in the eye truly arrives.

Corneal endothelial regeneration and tissue engineering

Human corneal endothelial cells (HCECs) have a limited proliferative capacity. Descemet stripping with automated endothelial keratoplasty (DSAEK) has become the preferred method for the treatment of corneal endothelial deficiency, but it requires a donor cornea. To overcome the shortage of donor corneas, transplantation of cultured HCEC sheets has been attempted in experimental studies. This review summarizes current knowledge about the mechanisms of corneal endothelial wound healing and about tissue engineering for the corneal endothelium. We also discuss recent work on tissue engineering for DSAEK grafts using cultured HCECs and HCEC precursor cell isolation method (the sphere-forming assay). DSAEK grafts (HCEC sheets) were constructed by seeding cultured HCECs on human amniotic membrane, thin human corneal stroma, and collagen sheets. The pump function of the HCEC sheets thus obtained was approximately 75%-95% of that for human donor corneas. HCEC sheets were transplanted onto rabbit corneas after DSAEK. While the untransplanted control group displayed severe stromal edema, the transplanted group had clear corneas throughout the observation period. The sphere-forming assay using donor human corneal endothelium or cultured HCECs can achieved mass production of human corneal endothelial precursors. These findings indicate that cultured HCECs transplanted after DSAEK can perform effective corneal dehydration in vivo and suggest the feasibility of employing the transplantation of cultured HCECs to treat endothelial dysfunction. Additionally, corneal endothelial precursors may be an effective strategy for corneal endothelial regeneration.

Eye development and retinogenesis

Three embryonic tissue sources-the neural ectoderm, the surface ectoderm, and the periocular mesenchyme-contribute to the formation of the mammalian eye. For this reason, the developing eye has presented an invaluable system for studying the interactions among cells and, more recently, genes, in specifying cell fate. This article describes how the eye primordium is specified in the anterior neural plate by four eye field transcription factors and how the optic vesicle becomes regionalized into three distinct tissue types. Specific attention is given to how cross talk between the optic vesicle and surface ectoderm contributes to lens and optic cup formation. This article also describes how signaling networks and cell movements set up axes in the optic cup and establish the multiple cell fates important for vision. How multipotent retinal progenitor cells give rise to the six neuronal and one glial cell type in the mature retina is also explained. Finally, the history and progress of cellular therapeutics for the treatment of degenerative eye disease is outlined. Throughout this article, special attention is given to how disruption of gene function causes ocular malformation in humans. Indeed, the accessibility of the eye has contributed much to our understanding of the basic processes involved in mammalian development.

An in vitro comparison study: the effects of fetal bovine serum concentration on retinal progenitor cell multipotentiality

Retinal progenitor cells (RPCs) are an excellent resource for retinal replacement therapy, because they show enormous potential to differentiate into retinal-specific cell types. While the differentiating influence of serum has long been appreciated, the effects of serum concentration on RPC differentiation into specified retinal neural cells have not been investigated. Using cultured murine RPCs, this study compared the effects of different levels of fetal bovine serum (FBS) (1%, 5%, 10% and 20%) on RPC differentiation in vitro. RPC multipotentiality was assessed by using quantitative polymerase chain reaction (qPCR) to determine the relative expression levels of 10 genes involved in retinal development. In addition, analyses of cell morphology and retinal development-related protein expression were performed using microscopy and immunocytochemistry. The data revealed that 1% FBS-induced cultures preferentially generated rhodopsin- and PKC-α-positive cells. Calbindin and AP2α expression levels were greater in 5% FBS-induced cultures. Brn3a was expressed at similar levels in 1%, 5% and 10% FBS treatment conditions but diminished in 20% FBS conditions. Twenty percent FBS induced more glial fibrillary acid protein (GFAP)-immunoreactive cells corresponding to glia populations. These findings suggest that the concentration of FBS plays an important role in RPC differentiation in vitro. Treatment with low levels of FBS favors differentiation of rhodopsin-positive photoreceptors, interneurons and retinal ganglion cells (RGCs), while high FBS concentrations preferentially induce differentiation of glia cells. These results are expected to facilitate research in the treatment of neurodegenerative retinal diseases.

Using myc genes to search for stem cells in the ciliary margin of the Xenopus retina

The ciliary marginal zone (CMZ) of fish and frog retinas contains cells that proliferate throughout postembryonic development as the retina grows with increasing body size, indicating the presence of stem cells in this region. However, neither the location nor the molecular identity of retinal stem cells has been identified. Here, we show in Xenopus that c-myc and n-myc are sequentially expressed both during development and in the post-embryonic retina. The c-myc+/n-myc- cells near the extreme periphery of the CMZ cycle more slowly and preferentially retain DNA label compared to their more central cmyc+/n-myc+ neighbors which cycle rapidly and preferentially dilute DNA label. During retinal development c-myc is functionally required earlier than n-myc, and n-myc expression depends on earlier c-myc expression. The expression of c-myc but not n-myc in the CMZ depends on growth factor signaling. Our results suggest that c-myc+/n-myc- cells in the far peripheral CMZ are candidates for a niche-dependent population of retinal stem cells that give rise to more centrally located and rapidly dividing n-myc+ progenitors of more limited proliferative potential. Analysis of homologues of these genes in the zebrafish CMZ suggests that the transition from c-myc to n-myc expression might be conserved in other lower vertebrates whose retinas growth throughout life.

A large scale screen for neural stem cell markers in Xenopus retina

Neural stem cell research suffers from a lack of molecular markers to specifically assess stem or progenitor cell properties. The organization of the Xenopus ciliary marginal zone (CMZ) in the retina allows the spatial distinction of these two cell types: stem cells are confined to the most peripheral region, while progenitors are more central. Despite this clear advantage, very few genes specifically expressed in retinal stem cells have been discovered so far in this model. To gain insight into the molecular signature of these cells, we performed a large-scale expression screen in the Xenopus CMZ, establishing it as a model system for stem cell gene profiling. Eighteen genes expressed specifically in the CMZ stem cell compartment were retrieved and are discussed here. These encode various types of proteins, including factors associated with proliferation, mitotic spindle organization, DNA/RNA processing, and cell adhesion. In addition, the publication of this work in a special issue on Xenopus prompted us to give a more general illustration of the value of large-scale screens in this model species. Thus, beyond neural stem cell specific genes, we give a broader highlight of our screen outcome, describing in particular other retinal cell markers that we found. Finally, we present how these can all be easily retrieved through a novel module we developed in the web-based annotation tool XenMARK, and illustrate the potential of this powerful searchable database in the context of the retina.

Outer segment formation of transplanted photoreceptor precursor cells

Transplantation of photoreceptor precursor cells (PPCs) into the retina represents a promising treatment for cell replacement in blinding diseases characterized by photoreceptor loss. In preclinical studies, we and others demonstrated that grafted PPCs integrate into the host outer nuclear layer (ONL) and develop into mature photoreceptors. However, a key feature of light detecting photoreceptors, the outer segment (OS) with natively aligned disc membrane staples, has not been studied in detail following transplantation. Therefore, we used as donor cells PPCs isolated from neonatal double transgenic reporter mice in which OSs are selectively labeled by green fluorescent protein while cell bodies are highlighted by red fluorescent protein. PPCs were enriched using CD73-based magnetic associated cell sorting and subsequently transplanted into either adult wild-type or a model of autosomal-dominant retinal degeneration mice. Three weeks post-transplantation, donor photoreceptors were identified based on fluorescent-reporter expression and OS formation was monitored at light and electron microscopy levels. Donor cells that properly integrated into the host wild-type retina developed OSs with the formation of a connecting cilium and well-aligned disc membrane staples similar to the surrounding native cells of the host. Surprisingly, the majority of not-integrated PPCs that remained in the sub-retinal space also generated native-like OSs in wild-type mice and those affected by retinal degeneration. Moreover, they showed an improved photoreceptor maturation and OS formation by comparison to donor cells located on the vitreous side suggesting that environmental cues influence the PPC differentiation and maturation. We conclude that transplanted PPCs, whether integrated or not into the host ONL, are able to generate the cellular structure for effective light detection, a phenomenon observed in wild-type as well as in degenerated retinas. Given that patients suffering from retinitis pigmentosa lose almost all photoreceptors, our findings are of utmost importance for the development of cell-based therapies.

Characterization and retinal neuron differentiation of WERI-Rb1 cancer stem cells

PURPOSE

The evidence is increasing that cancer stem cells (CSCs) expressing embryonic and neuronal stem cell markers are present in human retinoblastoma (Rb). This study was conducted to determine whether stem-like cancer cells (SLCCs) in Rb express retinal stem cell-related genes and whether SLCCs can directly differentiate into retinal neurons.

METHODS

The cancer stem cell characteristics in WERI-Rb1 cells were determined with Hoechst 33,342 staining, clone formation assay, and CD133 flow cytometry. The expression of embryonic stem cell and retinal stem cell-related genes was analyzed with real-time PCR and immunofluorescence. The SLCCs were induced to differentiate into retinal neurons by the addition of Dickkopf-related protein 1 and Lefty-A.

RESULTS

A small but persistent population of cells excluding Hoechst dye in a verapamil-sensitive manner exhibited a cancer stem cell-like phenotype. The SLCCs displayed highly clonogenic abilities and increased CD133 expression with isolation and expansion in culture in serum-free medium. By comparing the expression of stem cell markers, we found Oct3/4 was more highly expressed in the SLCCs than in human embryonic stem cells. Together with the properties of intrinsic retinal stem cell-related gene expression, we found SLCCs can be induced into neuron-like cells that express glial fibrillary acidic protein and rhodopsin (a photoreceptor cell marker).

CONCLUSIONS

These findings provide new insight into cancer stem cells and used a strategy of an artificial change of cancer stem cell fate with transcription factors.

Genetics of primary intraocular tumors

Primary intraocular neoplasms are tumors that originate within the eye. The most common malignant primary intraocular tumor in adults is uveal melanoma and the second is primary intraocular lymphoma or vitreoretinal (intraocular) lymphoma. The most common malignant intraocular tumor in children is retinoblastoma. Genetics plays a vital role in the diagnosis and detection of ocular tumors. In uveal melanoma, monosomy 3 is the most common genetic alteration and somatic mutations of BAP1, a tumor suppressor gene, have been reported in nearly 50% of primary uveal melanomas. The retinoblastoma gene RB1 is the prototype tumor suppressor gene-mutations in RB1 alleles lead to inactivated RB protein and the development of retinoblastoma. Immunoglobulin heavy chain (IgH) or T-cell receptor (TCR) gene rearrangement is observed in B-cell or T-cell primary vitreoretinal lymphoma, respectively. Other factors related to the genetics of these three common malignancies in the eye are discussed and reviewed.

In vitro expanded stem cells from the developing retina fail to generate photoreceptors but differentiate into myelinating oligodendrocytes

Cell transplantation to treat retinal degenerative diseases represents an option for the replacement of lost photoreceptor cells. In vitro expandable cells isolated from the developing mammalian retina have been suggested as a potential source for the generation of high numbers of donor photoreceptors. In this study we used standardized culture conditions based on the presence of the mitogens FGF-2 and EGF to generate high numbers of cells in vitro from the developing mouse retina. These presumptive ‘retinal stem cells’ (‘RSCs’) can be propagated as monolayer cultures over multiple passages, express markers of undifferentiated neural cells, and generate neuronal and glial cell types upon withdrawal of mitogens in vitro or following transplantation into the adult mouse retina. The proportion of neuronal differentiation can be significantly increased by stepwise removal of mitogens and inhibition of the notch signaling pathway. However, ‘RSCs’, by contrast to their primary counterparts in vivo, i.e. retinal progenitor cells, loose the expression of retina-specific progenitor markers like Rax and Chx10 after passaging and fail to differentiate into photoreceptors both in vitro or after intraretinal transplantation. Notably, ‘RSCs’ can be induced to differentiate into myelinating oligodendrocytes, a cell type not generated by primary retinal progenitor cells. Based on these findings we conclude that ‘RSCs’ expanded in high concentrations of FGF-2 and EGF loose their retinal identity and acquire features of in vitro expandable neural stem-like cells making them an inappropriate cell source for strategies aimed at replacing photoreceptor cells in the degenerated retina.

Phenotypic plasticity of retinal pigment epithelial cells from adult human eye in vitro

Phenotypic plasticity of retinal pigment epithelial cells from adult human eye was studied by immunohistochemical methods under different culturing conditions. It was found that retinal pigment epithelium in adult human eye is a heterogeneous population of cells demonstrating different behavior in vitro. Some cells retain epithelial morphology for a long time in culture, while others are rapidly transformed into fibroblast-like cells and synthesize proteins typical of proneural, neural, glial, and photoreceptor cells. However, irrespective of initial morphological features differentiation of retinal pigment cells can be modulated by varying culturing conditions.

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.

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.

Derivation of human differential photoreceptor-like cells from the iris by defined combinations of CRX, RX and NEUROD

Examples of direct differentiation by defined transcription factors have been provided for beta-cells, cardiomyocytes and neurons. In the human visual system, there are four kinds of photoreceptors in the retina. Neural retina and iris-pigmented epithelium (IPE) share a common developmental origin, leading us to test whether human iris cells could differentiate to retinal neurons. We here define the transcription factor combinations that can determine human photoreceptor cell fate. Expression of rhodopsin, blue opsin and green/red opsin in induced photoreceptor cells were dependent on combinations of transcription factors: A combination of CRX and NEUROD induced rhodopsin and blue opsin, but did not induce green opsin; a combination of CRX and RX induced blue opsin and green/red opsin, but did not induce rhodopsin. Phototransduction-related genes as well as opsin genes were up-regulated in those cells. Functional analysis; i.e. patch clamp recordings, clearly revealed that generated photoreceptor cells, induced by CRX, RX and NEUROD, responded to light. The response was an inward current instead of the typical outward current. These data suggest that photosensitive photoreceptor cells can be generated by combinations of transcription factors. The combination of CRX and RX generate immature photoreceptors: and additional NEUROD promotes maturation. These findings contribute substantially to a major advance toward eventual cell-based therapy for retinal degenerative diseases.

Functional and molecular characterization of rod-like cells from retinal stem cells derived from the adult ciliary epithelium

In vitro generation of photoreceptors from stem cells is of great interest for the development of regenerative medicine approaches for patients affected by retinal degeneration and for high throughput drug screens for these diseases. In this study, we show unprecedented high percentages of rod-fated cells from retinal stem cells of the adult ciliary epithelium. Molecular characterization of rod-like cells demonstrates that they lose ciliary epithelial characteristics but acquire photoreceptor features. Rod maturation was evaluated at two levels: gene expression and electrophysiological functionality. Here we present a strong correlation between phototransduction protein expression and functionality of the cells in vitro. We demonstrate that in vitro generated rod-like cells express cGMP-gated channels that are gated by endogenous cGMP. We also identified voltage-gated channels necessary for rod maturation and viability. This level of analysis for the first time provides evidence that adult retinal stem cells can generate highly homogeneous rod-fated cells.

New biomimetic directions in regenerative ophthalmology

One of the most complete and permanent ways of treating many causes of visual impairment and blindness is to replace the entire affected tissue with pre-cultured ocular tissues supported and maintained on biomaterial frameworks. One direction towards enhancing ocular tissue regeneration on biomaterials, in the laboratory is by applying biomimicry. Specifically to engineer biomaterials with important functional elements of the native extracellular matrices, such as topography, that support and organise cells into coherent tissues. Further problems in regenerative ophthalmology can be potentially solved through application of biomimicry. They include, more efficient ways of moving and transplanting cultivated tissues into correct therapeutic locations inside the eye and scar-less, non-destructive healing of surgical incisions and wounds, to repair structural integrity of tissues at the ocular surface. Two examples are given to show this potential for redeveloping an ocular epithelium onto a nanostructured insect wing surface and producing an origami membrane modelled on deployable structures in nature. Efforts to harness natural innovation will eventually provide unique designs and structures that cannot for now be made synthetically, for regeneration of clinically acceptable ocular tissues.

Characterization of Progenitor Cells during Canine Retinal Development

We identify the presence of progenitor cells during retinal development in the dog, as this species represents a natural model for studying several breed-specific degenerative retinal disorders. Antibodies to detected progenitor cells (Pax6, C-kit, and nestin) and ganglion cells (BDNF, Brn3a, and Thy1) were used in combination with H3 for the purpose of identifying proliferating cells. Pax6, nestin, C-kit, and H3 were localized mainly in the neuroblastic layer of the retina during the embryonic stage. During the fetal stage, proteins were expressed in the inner neuroblastic layer (INL) as well as in the outer neuroblastic layer; BDNF, Thy1, and Brn3a were also expressed in the INL. During the neonatal stage only C-kit was not expressed. Proliferating cells were present in both undifferentiated and differentiated retina. These results suggest that, during canine retinogenesis, progenitor cells are distributed along the retina and some of these cells remain as progenitor cells of the ganglion cells during the first postnatal days.

Conditionally immortalized mouse embryonic fibroblasts retain proliferative activity without compromising multipotent differentiation potential

Mesenchymal stem cells (MSCs) are multipotent cells which reside in many tissues and can give rise to multiple lineages including bone, cartilage and adipose. Although MSCs have attracted significant attention for basic and translational research, primary MSCs have limited life span in culture which hampers MSCs' broader applications. Here, we investigate if mouse mesenchymal progenitors can be conditionally immortalized with SV40 large T antigen and maintain long-term cell proliferation without compromising their multipotency. Using the system which expresses SV40 large T antigen flanked with Cre/loxP sites, we demonstrate that mouse embryonic fibroblasts (MEFs) can be efficiently immortalized by SV40 large T antigen. The conditionally immortalized MEFs (iMEFs) exhibit an enhanced proliferative activity and maintain long-term cell proliferation, which can be reversed by Cre recombinase. The iMEFs express most MSC markers and retain multipotency as they can differentiate into osteogenic, chondrogenic and adipogenic lineages under appropriate differentiation conditions in vitro and in vivo. The removal of SV40 large T reduces the differentiation potential of iMEFs possibly due to the decreased progenitor expansion. Furthermore, the iMEFs are apparently not tumorigenic when they are subcutaneously injected into athymic nude mice. Thus, the conditionally immortalized iMEFs not only maintain long-term cell proliferation but also retain the ability to differentiate into multiple lineages. Our results suggest that the reversible immortalization strategy using SV40 large T antigen may be an efficient and safe approach to establishing long-term cell culture of primary mesenchymal progenitors for basic and translational research, as well as for potential clinical applications.

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.

[A technique of rhesus monkey neural progenitor cells intravitreal transplant to rats]

To investigate a simple and effective intraocular xenotransplant technique of rhesus monkey neural progenitor cells to rats, mechanical injury was induced in the rat's right retina. And the GFP-labeled rhesus monkey neural progenitor cells suspension was slowly injected into the vitreous space of the right injured and left control eye. Confocal image suggested that the xenografted cells survived in both the injured and control eye, meanwhile the cells integrated in the injured right retina. The results demonstrated that intravitreal xenotransplant could be adopted as a simple and reliable method.

The adult retinal stem cell is a rare cell in the ciliary epithelium whose progeny can differentiate into photoreceptors

Self-renewing, multipotential retinal stem cells (RSCs) reside in the pigmented ciliary epithelium of the peripheral retina in adult mammals. RSCs can give rise to rhodopsin positive-cells, which can integrate into early postnatal retina, and represent a potentially useful option for cellular therapy. The ability to purify a stem cell population and direct the differentiation toward a particular cell lineage is a challenge facing the application of stem cells in regenerative medicine. Here we use cell sorting to prospectively enrich mouse RSCs based on size, granularity and low expression of P-cadherin and demonstrate that only rare cells with defined properties proliferate to form colonies. We show that clonally-derived mouse and human RSC progeny are multipotent and can differentiate into mature rhodopsin-positive cells with high efficiency using combinations of exogenous culture additives known to influence neural retinal development, including taurine and retinoic acid. This directed RSC differentiation follows the temporal sequence of photoreceptor differentiation in vivo, and the cells exhibit morphology, protein and gene expression consistent with primary cultures of rods in vitro. These results demonstrate that the RSC, an adult stem cell, can be enriched and directed to produce photoreceptors as a first step toward a targeted cell replacement strategy to treat retinal degenerative disease.

Analysis of retinoblastoma age incidence data using a fully stochastic cancer model

Retinoblastoma (RB) is an important ocular malignancy of childhood. It has been commonly accepted for some time that knockout of the two alleles of the RB1 gene is the principal molecular target associated with the occurrence of RB. In this article, we examine the validity of the two-hit theory for RB by comparing the fit of a stochastic model with two or more mutational stages. Unlike many such models, our model assumes a fully stochastic stem cell compartment, which is crucial to its behavior. Models are fitted to a population-based dataset comprising 1,553 cases of RB for the period 1962-2000 in Great Britain (England, Scotland and Wales). The population incidence of RB is best described by a fully stochastic model with two stages, although models with a deterministic stem cell compartment yield equivalent fit; models with three or more stages fit much less well. The results strongly suggest that knockout of the two alleles of the RB1 gene is necessary and may be largely sufficient for the development of RB, in support of Knudson's two-hit hypothesis.

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.

Step-wise specification of retinal stem cells during normal embryogenesis

The specification of embryonic cells to produce the retina begins at early embryonic stages as a multi-step process that gradually restricts fate potentials. First, a subset of embryonic cells becomes competent to form retina by their lack of expression of endo-mesoderm-specifying genes. From these cells, a more restricted subset is biased to form retina by virtue of their close proximity to sources of bone morphogenetic protein antagonists during neural induction. During gastrulation, the definitive RSCs (retinal stem cells) are specified as the eye field by interactions with underlying mesoderm and the expression of a network of retina-specifying genes. As the eye field is transformed into the optic vesicle and optic cup, a heterogeneous population of RPCs (retinal progenitor cells) forms to give rise to the different domains of the retina: the optic stalk, retinal pigmented epithelium and neural retina. Further diversity of RPCs appears to occur under the influences of cell-cell interactions, cytokines and combinations of regulatory genes, leading to the differentiation of a multitude of different retinal cell types. This review examines what is known about each sequential step in retinal specification during normal vertebrate development, and how that knowledge will be important to understand how RSCs might be manipulated for regenerative therapies to treat retinal diseases.

Functional analysis of retinal microglia and their effects on progenitors

The identification of stem/progenitor cells within the retinal neural environment has opened up the possibility of therapy via cellular replacement and/or reprogramming of resident cell populations. Within the neuro-retinal niche, following injury or in disease states (including inflammation and degeneration), cellular responses affect tissue homeostasis, reduce cell density, disrupt tissue architecture, and produce scar formation. Microglia (resident retinal immune cell tissue macrophage) are key to the maintenance of retinal homeostasis and are implicated in responses that may influence the control and behavior of retinal progenitors. Factors to consider in the generation of a transplantable cell resource with good migratory and integrative capacity include their yield, purity, and functional viability. Utilizing human postmortem retina, we have created a research platform to isolate, culture, and characterize adult retinal microglia as well as analyze their effect on retinal progenitors. Here, we describe techniques using magnetic labeled bead cell separation to isolate pure populations of retinal CD133(+) precursor cells and CD11b(+) microglia from primary adult retinal cell suspensions (RCSs), enabling flow cytometric cell phenotypic and qPCR genotypic analysis, as well as functional analysis by real-time ratiometric calcium imaging.

Neural stem cell niches in health and diseases

Presence of neural stem cells in adult mammalian brains, including human, has been clearly demonstrated by several studies. The functional significance of adult neurogenesis is slowly emerging as new data indicate the sensitivity of this event to several "every day" external stimuli such as physical activity, learning, enriched environment, aging, stress and drugs. In addition, neurogenesis appears to be instrumental for task performance involving complex cognitive functions. Despite the growing body of evidence on the functional significance of NSC and despite the bulk of data concerning the molecular and cellular properties of NSCs and their niches, several critical questions are still open. In this work we review the literature describing i) old and new sites where NSC niche have been found in the CNS; ii) the intrinsic factors regulating the NSC potential; iii) the extrinsic factors that form the niche microenvironment. Moreover, we analyse NSC niche activation in iv) physiological and v) pathological conditions. Given the not static nature of NSCs that continuously change phenotype in response to environmental clues, a unique "identity card" for NSC identification is still lacking. Moreover, the multiple location of NSC niches that increase in diseases, leaves open the question of whether and how these structures communicate throughout long distance. We propose a model where all the NSC niches in the CNS may be connected in a functional network using the threads of the meningeal net as tracks.

Subretinal transplantation of putative retinal pigment epithelial cells derived from human embryonic stem cells in rat retinal degeneration model

OBJECTIVE

To differentiate the human embryonic stem cells (hESCs) into the retinal pigment epithelium (RPE) in the defined culture condition and determine its therapeutic potential for the treatment of retinal degenerative diseases.

METHODS

The embryoid bodies were formed from hESCs and attached on the matrigel coated culture dishes. The neural structures consisting neural precursors were selected and expanded to form rosette structures. The mechanically isolated neural rosettes were differentiated into pigmented cells in the media comprised of N2 and B27. Expression profiles of markers related to RPE development were analyzed by reverse transcription-polymerase chain reaction and immunostaining. Dissociated putative RPE cells (10(5) cells/5 µL) were transplanted into the subretinal space of rat retinal degeneration model induced by intravenous sodium iodate injection. Animals were sacrificed at 1, 2, and 4 weeks after transplantation, and immnohistochemistry study was performed to verify the survival of the transplanted cells.

RESULTS

The putative RPE cells derived from hESC showed characteristics of the human RPE cells morphologically and expressed molecular markers and associated with RPE fate. Grafted RPE cells were found to survive in the subretinal space up to 4 weeks after transplantation, and the expression of RPE markers was confirmed with immunohistochemistry.

CONCLUSION

Transplanted RPE cells derived from hESC in the defined culture condition successfully survived and migrated within subretinal space of rat retinal degeneration model. These results support the feasibility of the hESC derived RPE cells for cell-based therapies for retinal degenerative disease.

An in vitro comparison of two different subpopulations of retinal progenitor cells for self-renewal and multipotentiality

Retinal progenitor cells (RPCs) show enormous potential for the treatment of retinal degenerative diseases. It is well known that in vitro cultures of RPCs comprise suspension spheres and adherent cells, but the differences between the two cell populations are not fully understood. In this study, cultured RPCs were sorted into suspension and adherent cells. Analyses of cell morphology, cell growth and retinal progenitor-related expression markers were performed using quantitative polymerase chain reaction (qPCR) and immunocytochemistry to identify the proliferative and multipotent capacity of the cells in vitro. The data showed that both the suspension and adherent cells were maintained in an undifferentiated state, although the former exhibited a greater proliferative potential than the latter. Immunocytochemistry analysis indicated that the two subsets of RPCs were able to differentiate into different retinal cells in the presence of fetal bovine serum (FBS); the adherent cells were more likely to differentiate toward the β3-tubulin-, AP2α- and Map2-positive neuronal lineage, while the suspension cells were more effective at differentiating into rod photoreceptors, which was consistent with the qPCR results. These findings suggest that adherent RPCs may be a potential candidate for retinal interneuron or ganglion cell substitution therapies, whereas suspension RPCs may be preferred for photoreceptor cell replacement.

Progenitors for the corneal endothelium and trabecular meshwork: a potential source for personalized stem cell therapy in corneal endothelial diseases and glaucoma

Several adult stem cell types have been found in different parts of the eye, including the corneal epithelium, conjunctiva, and retina. In addition to these, there have been accumulating evidence that some stem-like cells reside in the transition area between the peripheral corneal endothelium (CE) and the anterior nonfiltering portion of the trabecular meshwork (TM), which is known as the Schwalbe's Ring region. These stem/progenitor cells may supply new cells for the CE and TM. In fact, the CE and TM share certain similarities in terms of their embryonic origin and proliferative capacity in vivo. In this paper, we discuss the putative stem cell source which has the potential for replacement of lost and nonfunctional cells in CE diseases and glaucoma. The future development of personalized stem cell therapies for the CE and TM may reduce the requirement of corneal grafts and surgical treatments in glaucoma.