Alternative culture conditions for isolation and expansion of retinal progenitor cells

The combination of bFGF and CHIR99021 maintains stable self-renewal of mouse adult retinal progenitor cells

BACKGROUND

Millions of people are affected with retinal diseases that eventually cause blindness, and retinal progenitor cell (RPC) transplantation is a promising therapeutic avenue. However, RPC expansion and the underlying regulation mechanisms remain elusive.

METHODS

Adult mouse neural RPCs (mNRPCs) were isolated and amplified with the combination of basic fibroblast growth factor (bFGF) and glycogen synthase kinase 3 (GSK3) inhibitor CHIR99021. The progenitor characteristics were evaluated with RT-PCR, immunocytochemistry (ICC), western blot, flow cytometry, and transcriptome analysis prior to transplantation. By treating cells with or without bFGF and CHIR99021 at different time points, the mechanism for mNRPCs' self-renewal was investigated by transcriptome analysis and western blot assay.

RESULTS

mNRPCs were self-renewing in the presence of bFGF and CHIR99021 and showed prominent RPC characteristics. bFGF was essential in promoting cell cycle by facilitating G1/S and G2/M transitions. bFGF combined with CHIR99021 activated the non-canonical Wnt5A/Ca²⁺ pathway and form a calcium homeostasis. In addition, the self-renewing mNRPCs could differentiate into rod photoreceptor-like cells and retinal pigment epithelium (RPE)-like cells by in vitro induction. When green fluorescent protein (GFP)-labeled cells were transplanted into the subretinal space (SRS) of Pde6b (rd1) mice (also known as RD1 mice, or rodless mice), the cells survived for more than 12 weeks and migrated into the retina. Parts of the recipient retina showed positive expression of photoreceptor marker rhodopsin. Transplanted cells can migrate into the retina, mainly into the inner cell layer (INL) and ganglion cell layer (GCL). Some cells can differentiate into astrocytes and amacrine cells. Cultured mNRPCs did not form tumors after transplanted into NOD/SCID mice for 6 months.

CONCLUSIONS

Present study developed an approach to maintain long-term self-renewal of RPCs from adult retinal tissues and revealed that activation of the non-canonical Wnt5A/Ca²⁺ pathway may participate in regulating RPC self-renewal in vitro. This study presents a very promising platform to expand RPCs for future therapeutic application.

Cell replacement and visual restoration by retinal sheet transplants

Retinal diseases such as age-related macular degeneration (ARMD) and retinitis pigmentosa (RP) affect millions of people. Replacing lost cells with new cells that connect with the still functional part of the host retina might repair a degenerating retina and restore eyesight to an unknown extent. A unique model, subretinal transplantation of freshly dissected sheets of fetal-derived retinal progenitor cells, combined with its retinal pigment epithelium (RPE), has demonstrated successful results in both animals and humans. Most other approaches are restricted to rescue endogenous retinal cells of the recipient in earlier disease stages by a 'nursing' role of the implanted cells and are not aimed at neural retinal cell replacement. Sheet transplants restore lost visual responses in several retinal degeneration models in the superior colliculus (SC) corresponding to the location of the transplant in the retina. They do not simply preserve visual performance - they increase visual responsiveness to light. Restoration of visual responses in the SC can be directly traced to neural cells in the transplant, demonstrating that synaptic connections between transplant and host contribute to the visual improvement. Transplant processes invade the inner plexiform layer of the host retina and form synapses with presumable host cells. In a Phase II trial of RP and ARMD patients, transplants of retina together with its RPE improved visual acuity. In summary, retinal progenitor sheet transplantation provides an excellent model to answer questions about how to repair and restore function of a degenerating retina. Supply of fetal donor tissue will always be limited but the model can set a standard and provide an informative base for optimal cell replacement therapies such as embryonic stem cell (ESC)-derived therapy.

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.

Recombinant PBD-1 (porcine beta-defensin 1) expressed in the milk by transplanting transgenic mES-like-derived cells into mouse mammary gland

ES (embryonic stem)-derived cells have been investigated in many animal models of severe injury and degenerative disease. However, few studies have examined the ability of ES-derived cells to improve functional outcome following partially damaged breast and also the modification of mammary tissue to produce costly proteins. This study investigates the feasibility of implanting mES-dK (mouse ES-derived keratinocytes-like) cells stably transfected with a mammary gland special expression vector for the PBD-1 (porcine beta-defensin 1) in developing mammary glands. Our aim was to assess the ability of cell grafting to improve functional outcome following partial damage of the breast, also on the breast modification mammary tissue in mice for the production of PBD-1 protein secreted in the milk. Our results showed that the ratios of the surviving cells labelled with the myoepithelial or luminal cell markers, EMA (epithelial membrane antigen) and CALLA, were 41.7 +/- 15.2% and 28.4 +/- 9.6%, respectively, which revealed that transplanted mES-dK cells survived, integrated in vivo and differentiated into myoepithelial or luminal cells. In addition, Western blot analysis showed that 37.5% (3 out of 8) female transplanted mice had PBD-1 expression in their milk and reached 0.4998, 0.5229 and 0.5195 microg/ml, respectively.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Stem Cell Therapy for Retinal Degeneration: Retinal Neurons from Heterologous Sources

Over the past few years a great deal of interest has been generated in using stem cells/progenitors to treat degenerative diseases that afflict different tissues, including retina. This interest is due to the defining properties of stem cells/progenitors, the ability of these cells to self-renew and generate all the basic cell types of the particular tissue to which they belong. In addition, the recent reports of plasticity of the adult tissue-specific stem cells/progenitors and directed differentiation of the embryonic cells (ES) has fueled the hope for cell and gene therapy using stem cells from heterologous sources. Will this approach work for treating retinal degeneration? Here, we review the current state of knowledge about obtaining retinal cells from heterologous sources, including ES cells.

Regulation of prenatal human retinal neurosphere growth and cell fate potential by retinal pigment epithelium and Mash1

During development of the central nervous system, stem and progenitor cell proliferation and differentiation are controlled by complex inter- and intracellular interactions that orchestrate the precise spatiotemporal production of particular cell types. Within the embryonic retina, progenitor cells are located adjacent to the retinal pigment epithelium (RPE), which differentiates prior to the neurosensory retina and has the capacity to secrete a multitude of growth factors. We found that secreted proteinaceous factors in human prenatal RPE conditioned medium (RPE CM) prolonged and enhanced the growth of human prenatal retinal neurospheres. The growth-promoting activity of RPE CM was mitogen-dependent and associated with an acute increase in transcription factor phosphorylation. Expanded populations of RPE CM-treated retinal neurospheres expressed numerous neurodevelopmental and eye specification genes and markers characteristic of neural and retinal progenitor cells, but gradually lost the potential to generate neurons upon differentiation. Misexpression of Mash1 restored the neurogenic potential of long-term cultures, yielding neurons with phenotypic characteristics of multiple inner retinal cell types. Thus, a novel combination of extrinsic and intrinsic factors was required to promote both progenitor cell proliferation and neuronal multipotency in human retinal neurosphere cultures. These results support a pro-proliferative and antiapoptotic role for RPE in human retinal development, reveal potential limitations of human retinal progenitor culture systems, and suggest a means for overcoming cell fate restriction in vitro.

Retinal stem cells: promising candidates for retina transplantation

Stem cell transplantation is widely considered as a promising therapeutic approach for photoreceptor degeneration, one of the major causes of blindness. In this review, we focus on the biology of retinal stem cells (RSCs) and progenitor cells (RPCs) isolated from fetal, postnatal, and adult animals, with emphasis on those from rodents and humans. We discuss the origin of RSCs/RPCs, the markers expressed by these cells and the conditions for the isolation, culture, and differentiation of these cells in vitro or in vivo by induction with exogenous stimulation.

Revisiting nestin expression in retinal progenitor cells in vitro and after transplantation in vivo

The purpose of this study is to characterize the co-expression of nestin--a neuroectodermal stem cell and a reactive glial marker-with various mature retinal cell markers in retinal progenitor cells (RPCs) expanded in vitro, followed either by in vitro induction or subretinal transplantation. Rat RPCs derived from embryonic day (E) 17 rat retina were expanded in serum free defined culture, and induced to differentiate by all-trans retinoic acid (RA). Following induction, cells were stained for nestin in combination with retinal neuronal and glial markers. Cultured cells were collected for quantitative RT-PCR gene expression analysis prior to and after induction. In a second series, passage 2 RPCs were transplanted into the subretinal space of S334ter-3 retinal degeneration rats at postnatal day 28. After 1-4 weeks, sections through the transplant were double immunostained for nestin and various retinal specific neuronal markers. The cultured RPCs treated with RA exhibited nestin co-expression with various retinal specific markers, including protein kinase C alpha (PKC), neurofilament 200 (NF200), cellular retinaldehyde binding protein (CRALBP), and rhodopsin. Following RA induction, quantitative RT-PCR analysis demonstrated downregulation of nestin, PAX-6, thy1.1, and PKCalpha, and upregulation of rhodopsin, glial fibrillary acidic protein (GFAP), and CrX. No nestin coexpression was observed with any of the retinal specific neuronal markers in RPC transplants in vivo except for some nestin-immunoreactivity overlapping with GFAP positive cells in the host retina. The role of nestin as a unique neural stem/progenitor cell marker should be reconsidered. Nestin expression during RPC maturation appears to be different in vitro versus in vivo.

Structure and cell composition of spheres cultured from human fetal retina

The structure and cell composition of spheres obtained by culturing human fetal retinal cells after 15, 18, 22-23, and 24 weeks of gestation were studied. The cells were cultured as neurospheres: in serum-free medium with growth factors, in which they formed floating spheres. Immunocytochemical analysis showed that cell proliferation in the spheres decreased with increasing fetal age. Stem/progenitor cells, neuroblasts, and photoreceptors were detected in the spheres. Glial cells were detected only in spheres originating from 22- and 24-week fetuses. All spheres, irrespective of age and duration of culturing, consisted of numerous cell rosettes, each histotypically similar to the neuroblastic layer of the developing retina.

Mouse retinal progenitor cell (RPC) cocultivation with retinal pigment epithelial cell culture affects features of RPC differentiation

We provide evidence that coculturing of retinal progenitor cells (RPC) with retinal pigment epithelial cells significantly biases the standard in vitro RPC differentiation patterns. In particular, in cocultivation experiments RPCs lost the ability to differentiate spontaneously and displayed approximately 2.1-2.4-fold increase in immunoreactivity to the neural stem cell marker nestin and approximately 1.6-1.7-fold increase in rod photoreceptor cell rhodopsin marker immunoreactivity. The data suggest the influence of the intercellular interaction networks on RPC differentiation.

Nanobiolistic delivery of indicators to the living mouse retina

The development of a technique to load functional indicators into living neurons is an ongoing challenge in retinal neurophysiology. In a number of live-cell preparations, fluorescence-based indicators have been of particular importance for investigating ionic concentrations, protein localization, and other physiological parameters. In the present study, we demonstrate a novel technique that uses a modified gene gun to propel silver nanoparticles coated with indicators into live retinal neurons, and we highlight the advantages of using this technique to deliver these functional indicators.

In search of a stem cell hierarchy in the human breast and its relevance to breast cancer evolution

By deliberate analogy with the well-established concept of hematopoiesis, the term "mammopoiesis" is occasionally used to describe the development of the different cellular lineages and functional units in the mammary gland. The use of this term signifies a strong bias towards the idea that tissue homeostasis during mammary development, pregnancy, lactation and involution is brought about by the action of somatic stem cells characterized by longevity and multipotency. The progenies hereof eventually differentiate into structurally and functionally well-defined ductal-lobular units. During the past two decades evidence of such a notion in the mouse has developed from being largely circumstantial based on non-clonal in vivo experiments to a quite elaborate characterization of individual candidate stem cells by a number of different properties. Within tumor biology this has led to a renaissance of the concept of tumors as caricatures of tissue renewal. Thus, recent molecular classification of breast cancer based on genome wide expression analysis operates with different subtypes with specific reference to the normal luminal epithelial and myoepithelial/basal lineages in the breast. Apparently some tumors are lineage restricted and others differentiate more broadly as if they have preserved some stem-like properties. This holds promise for the existence of a stem cell hierarchy, the understanding of which may prove to be instrumental in further dissecting the histogenesis of breast cancer evolution. Most attention has been devoted to the question of different cellular origins of cancer subtypes and different susceptibilities of possible stem cells to gain or loss of oncogenes and tumor suppressor genes, respectively. Invaluable progress has been made over the past two decades in culture technology not only in terms of population doubling and clonal growth, but also the availability of lineage specific markers, cell sorting, and three-dimensional functional assays for tissue specific morphogenesis. Transcriptional profiling of stem cell zones has unraveled a hitherto unknown preservation of signaling pathways for maintenance of stem cell properties across tissue boundaries and species. Somatic stem cells have therefore been narrowed down to specific anatomic locations not only in rapidly renewing tissues such as skin and skin derivatives, but also in tissues with slower turnover times, such as lung, kidney and prostate. It is therefore now possible to integrate this information in a search for similar cells within the breast. Even if cell turnover after birth is provided exclusively by dividing lineage-restricted cells, more information about the robustness of breast differentiation programs during tumor progression is still very much required. Complete knowledge of the primary cell of origin of breast cancer and the mechanisms that influence differentiation programs during tumor initiation, promotion and progression may be crucial for the development of novel non-toxic therapies that influence tumor cell behaviour. The scope of this review is to discuss reports that have begun to elucidate the topographic location, key cellular type and lineage fidelity in culture and xenograft models of candidate human breast stem cells and their differentiated progenies with particular emphasis on comparison with the differentiation programs of tumor subtypes.

In vivo and in vitro proliferative and differentiation activity of human embryonic retinal cells

Differentiation of human embryonic retinal cells (20-22 weeks gestation) was studied using morphological, immunohistochemical, and biomolecular approaches. The retina included several regions differing by the degree of cell differentiation. Mitoses were rarely found in the marginal zone. This zone contained low differentiated cells. The central retinal area consisted of typical layers with differentiated cells. Culturing was accompanied by the formation of aggregates and neurospheres, where mitoses and progenitor or differentiated cells expressing markers of photoreceptors, neurons, and glia were found.

Photoreceptor differentiation and integration of retinal progenitor cells transplanted into transgenic rats

Previous studies evaluating neural stem cells transplanted into the mature retina have demonstrated limited levels of graft-host integration and photoreceptor differentiation. The purpose of this investigation is to enhance photoreceptor cell differentiation and integration of retinal progenitor cells (RPC) following subretinal transplantation into retinal degenerate rats by optimization of isolation, expansion, and transplantation procedures. RPCs were isolated from human placental alkaline phosphatase (hPAP)-positive embryonic day 17 (E17) rat retina and expanded in serum-free defined media. RPCs at passage 2 underwent in vitro induction with all trans retinoic acid or were transplanted into the subretinal space of post-natal day (P) 17 S334ter-3 and S334ter-5 transgenic rats. Animals were examined post-operatively by ophthalmoscopy and optical coherence tomography (OCT) at weeks 1 and 4. Differentiation profiles of RPCs, both in vitro and in vivo were analysed microscopically by immunohistochemistry for various retinal cell specific markers. Our results demonstrated that the majority of passage 2 RPCs differentiated into retina-specific neurons expressing rhodopsin after in vitro induction. Following subretinal transplantation, grafted cells formed a multi-layer cellular sheet in the subretinal space in both S334ter-3 and S334ter-5 rats. Prominent retina-specific neuronal differentiation was observed in both rat lines as evidenced by recoverin or rhodopsin staining in 80% of grafted cells. Less than 5% of the grafted cells expressed glial fibrillary acidic protein. Synapsin-1 (label for nerve terminals) positive neural processes were present at the graft-host interface. Expression profiles of the grafted RPCs were similar to those of RPCs induced to differentiate in vitro using all-trans retinoic acid. In contrast to our previous study, grafted RPCs can demonstrate extensive rhodopsin expression, organize into layers, and show some features of apparent integration with the host retina following subretinal transplantation in slow and fast retinal degenerate rats. The similarity of the in vitro and in vivo RPC differentiation profiles suggests that intrinsic signals may have a significant contribution to RPC cell fate determination.

Maintenance of cell type diversification in the human breast

Recent genome-wide expression analysis of breast cancer has brought new life to the classical idea of tumors as caricatures of the process of tissue renewal as envisioned by Pierce and Speers (Cancer Res 1988;48:1996-2004) more than a decade ago. The search for a cancer founder cell or different cancer founder cells is only possible if a hierarchy of differentiation has been established for the particular tissue in question. In the human breast, the luminal epithelial and myoepithelial lineages have been characterized extensively in situ by increasingly elaborate panel of markers, and methods to isolate, culture, and clone different subpopulations have improved dramatically. Comparisons have been made with the mouse mammary gland in physiological three-dimensional culture assays of morphogenesis, and the plasticity of breast epithelial cells has been challenged by immortalization and transformation. As a result of these efforts, several candidate progenitor cells have been proposed independently of each other, and some of their features have been compared. This research has all been done to better understand breast tissue homeostasis, cell-type diversification in general and breast cancer evolution in particular. The present review discusses the current approaches to address these issues and the measures taken to unravel and maintain cell type diversification for further investigation.