Neuronal stem/progenitor cells in the vertebrate eye

We acquire information from the outside world through our eyes which contain the retina, the photosensitive component of the central nervous system. Once the adult mammalian retina is damaged, the retinal neuronal death causes a severe loss of visual function. It has been believed that the adult mammalian retina had no regenerative capacity. However, the identification of neuronal progenitor cells in the retina sheds some light on cellular therapies for damaged retinal regeneration. In this review, we highlight three potential stem/progenitor cells in the eye, the ciliary body epithelium cells, the iris pigmented epithelium cells, and Müller glia. In order to make them prime candidates for the possible treatment of retinal diseases, it is important to understand their basic characters. In addition, we discuss the key signaling molecules that function extracellularly and determine whether neuronal progenitors remain quiescent, proliferate, or differentiate. Finally, we introduce a secreted protein, Tsukushi, which is a possible candidate as a niche molecule for retinal stem/progenitor cells.     

Neural stem cells in the mammalian eye: types and regulation

Neural stem cells/progenitors that give rise to neurons and glia have been identified in different regions of the brain, including the embryonic retina. Recently, such cells have been reported to be present, in a mitotically quiescent state, in the ciliary epithelium of the adult mammalian eye. The retinal and ciliary epithelium stem cells/progenitors appear to share similar signaling pathways that are emerging as important regulators of stem cells in general. Yet, they are different in certain respects, such as in the potential to self-renew. These two neural stem cell/progenitor populations not only will serve as models for investigating stem cell biology but also will help explain the relationships between embryonic and adult neural stem cells/progenitors.     

Pre-induced adult human peripheral blood mononuclear cells migrate widely into the degenerative retinas of rd1 mice

Background aimsRecent advances in stem cell research have raised the possibility of stem cells repairing or replacing retinal photoreceptor cells that are either dysfunctional or lost in many retinal diseases. Various types of stem cells have been used to replace retinal photoreceptor cells. Recently, peripheral blood stem cells, a small proportion of pluripotent stem cells, have been reported to mainly exist in the peripheral blood mononuclear cells (PBMCs).MethodsIn this study, the effects of pre-induced adult human PBMCs (hPBMCs) on the degenerative retinas of rd1 mice were investigated. Freshly isolated adult hPBMCs were pre-induced with the use of the conditioned medium of rat retinas for 4 days and were then labeled with chloromethyl-benzamidodialkylcarbocyanine (CM-DiI) and then transplanted into the subretinal space of the right eye of rd1 mice through a trans-scleral approach. The right eyes were collected 30 days after transplantation. The survival and migration of the transplanted cells in host retinas were investigated by whole-mount retinas, retinal frozen sections and immunofluorescent staining.ResultsAfter subretinal transplantation, pre-induced hPBMCs were able to survive and widely migrate into the retinas of rd1 mice. A few CM-DiI–labeled cells migrated into the inner nuclear layer and the retinal ganglion cell layer. Some transplanted cells in the subretinal space of rd1 host mice expressed the human photoreceptor–specific marker rhodopsin.ConclusionsThis study suggests that pre-induced hPBMCs may be a potential cell source of cell replacement therapy for retinal degenerative diseases.     

Progenitor cells as remote "bioreactors": neuroprotection via modulation of the systemic inflammatory response

Acute central nervous system(CNS)injuries such as spinal cord injury,traumatic brain injury,autoimmune encephalomyelitis,and ischemic stroke are associ- ated with significant morbidity,mortality,and health care costs worldwide.Preliminary research has shown potential neuroprotection associated with adult tissue derived stem/progenitor cell based therapies.While initial research indicated that engraftment and transdif- ferentiation into neural cells could explain the observed benefit,the exact mechanism remains controversial.A second hypothesis details localized stem/progenitor cell engraftment with alteration of the loco-regional milieu;however,the limited rate of cell engraftment makes this theory less likely.There is a growing amount of pre-clinical data supporting the idea that,after intravenous injection,stem/progenitor cells interact with immuno- logic cells located in organ systems distant to the CNS,thereby altering the systemic immunologic/inflammatory response.Such distant cell"bioreactors"could modulate the observed post-injury pro-inflammatory environment and lead to neuroprotection.In this review,we discuss the current literature detailing the above mechanisms of action for adult stem/progenitor cell based therapies in the CNS.     

Control of proliferation in the retina: temporal changes in responsiveness to FGF and TGF alpha.

Proliferation in the rat retina, as in other parts of the nervous system, occurs during a restricted period of development. In addition to regulating cell number, the mechanisms that control proliferation influence the patterning of tissues, and may affect the determination of cell type. To begin to determine how proliferation is controlled, several growth factors found in the retina were tested for effects on progenitor cell division in culture. Proliferation was enhanced by TGF alpha, bFGF and aFGF, and many of the dividing cells later differentiated into cells with the antigenic phenotypes of retinal neurons and glial cells. The mitotic response of retinal cells to these factors changed during development: progenitor cells from younger retinas (embryonic day 15 to 18; E15-E18) were more responsive to FGF's, while progenitor cells from older retinas (greater than E20) were more responsive to TGF alpha. Progenitor cells stopped dividing in vitro, even when treated with excess mitogen. These observations suggest that proliferation in the retina may be stimulated by multiple mitogenic signals provided by TGF alpha, FGF, or related factors, and that proliferation is not controlled by limiting concentrations of mitogen alone. Rather, these data demonstrate that retinal cells change during development in their responsiveness to mitogenic signals. Such changes may contribute to the regulation of proliferation.     

Cellular interactions determine neuronal phenotypes in rodent retinal cultures.

Progenitor cells isolated from early rat embryo retinas differentiate into phenotypes normally generated early in retinal development (e.g., ganglion cells), whereas progenitors isolated from postnatal retinas differentiate into later-generated retinal cell types (e.g., rod photoreceptors; Reh and Kljavin, J. Neurosci. 9:4179-4189; 1989; Adler and Hatlee, 1989; Science 243:391-393; Sparrow, Hicks, and Barnstable, 1990, Dev. Brain Res. 51:69-84). To determine whether this change in committment is intrinsic to the progenitor cells, or alternatively can be modified by interactions with their developing environment, I co-cultured mouse and rat retinal cells, from different developmental stages, and identified the resulting phenotypes with species-specific and cell class-specific antibodies. I found that the phenotypes into which mouse neuroepithelial cells differentiate depends on the phenotypes of the rat cells that surround them. Retinal precursor cells from embryonic day (E) 10-12 will adopt the rod photoreceptor phenotype only when close to cells expressing this phenotype. By contrast, when the E10-12 retinal progenitor cells are cultured with cells from the cerebral cortex, they differentiate primarily into large multipolar neurons, similar in their morphology and antigen expression to retinal ganglion cells. These results indicate that interactions among the cells of the developing retina are important in the determination of cell fate.     

Biochemistry and biology: Heart-to-heart to investigate cardiac progenitor cells

Background

Cardiac regenerative medicine is a rapidly evolving field, with promising future developments for effective personalized treatments. Several stem/progenitor cells are candidates for cardiac cell therapy, and emerging evidence suggests how multiple metabolic and biochemical pathways strictly regulate their fate and renewal.

Scope of review

In this review, we will explore a selection of areas of common interest for biology and biochemistry concerning stem/progenitor cells, and in particular cardiac progenitor cells. Numerous regulatory mechanisms have been identified that link stem cell signaling and functions to the modulation of metabolic pathways, and vice versa. Pharmacological treatments and culture requirements may be exploited to modulate stem cell pluripotency and self-renewal, possibly boosting their regenerative potential for cell therapy.

Major conclusions

Mitochondria and their many related metabolites and messengers, such as oxygen, ROS, calcium and glucose, have a crucial role in regulating stem cell fate and the balance of their functions, together with many metabolic enzymes. Furthermore, protein biochemistry and proteomics can provide precious clues on the definition of different progenitor cell populations, their physiology and their autocrine/paracrine regulatory/signaling networks.

General significance

Interdisciplinary approaches between biology and biochemistry can provide productive insights on stem/progenitor cells, allowing the development of novel strategies and protocols for effective cardiac cell therapy clinical translation. This article is part of a Special Issue entitled Biochemistry of Stem Cells.     

Cell-based cardiovascular repair and regeneration in acute myocardial infarction and chronic ischemic cardiomyopathy-current status and future developments

Ischemic heart disease is the main cause of death and morbidity in most industrialized countries. Stem- and progenitor cell-based treatment approaches for ischemic heart disease are therefore an important frontier in cardiovascular and regenerative medicine. Experimental studies have shown that bone-marrow-derived stem cells and endothelial progenitor cells can improve cardiac function after myocardial infarction, clinical phase I and II studies were rapidly initiated to translate this concept into the clinical setting. However, as of now the effects of stem/progenitor cell administration on cardiac function in the clinical setting have not met expectations. Thus, a better understanding of causes of the current limitations of cell-based therapies is urgently required. Importantly, the number and function of endothelial progenitor cells is reduced in patients with cardiovascular risk factors and/or coronary artery disease. These observations may provide opportunities for an optimization of cell-based treatment approaches. This review provides a summary of current evidence for the role and potential of stem and progenitor cells in the pathophysiology and treatment of ischemic heart disease, including the properties, and repair and regenerative capacities of various stem and progenitor cell populations. In addition, we describe modes of stem/progenitor cell delivery, modulation of their homing as well as potential approaches to "prime" stem/progenitor cells for cardiovascular cell-based therapies.     

Adult neurogenesis and brain regeneration in zebrafish

Adult neurogenesis is a widespread trait of vertebrates; however, the degree of this ability and the underlying activity of the adult neural stem cells differ vastly among species. In contrast to mammals that have limited neurogenesis in their adult brains,zebrafish can constitutively produce new neurons along the whole rostrocaudal brain axis throughout its life.This feature of adult zebrafish brain relies on the presence of stem/progenitor cells that continuously proliferate,and the permissive environment of zebrafish brain for neurogenesis. Zebrafish has also an extensive regenerative capacity, which manifests itself in responding to central nervous system injuries by producing new neurons to replenish the lost ones. This ability makes zebrafish a useful model organism for understanding the stem cell activity in the brain, and the molecular programs required for central nervous system regeneration.In this review, we will discuss the current knowledge on the stem cell niches, the characteristics of the stem/progenitor cells, how they are regulated and their involvement in the regeneration response of the adult zebrafish brain. We will also emphasize the open questions that may help guide the future research.     

c-kit marks late retinal progenitor cells and regulates their differentiation in developing mouse retina

Retinal progenitor cells are believed to display altered proliferation and differentiation during retinal development, suggesting that retinal progenitor cell populations are not homogeneous. However, the composition of progenitor cell populations is not known, due in part to the lack of known surface markers identifying distinct stages of retinal progenitor cells. We found a dramatic change in the expression profile of the cell surface antigens c-kit and stage-specific embryonic antigen-1 (SSEA-1) in retinal progenitor cells during development. While SSEA-1 was expressed early in development, c-kit expression peaked in late stage progenitor cells. The identification of these developmental markers enabled us to characterize distinct sub-populations of retinal progenitor cells. Progenitor cell subpopulations expressing either SSEA-1, c-kit, or both showed different proliferation and differentiation abilities. Although SSEA-1-positive cells were augmented by beta-catenin signaling, c-kit-positive cells were positively regulated by Notch signaling. Taken together, our data suggest that c-kit and SSEA-1 can be used to spatiotemporally differentiate retinal progenitor populations that have intrinsically distinct characteristics. Prolonged expression of c-kit by a retrovirus resulted in the promotion of proliferation and the appearance of nestin-positive cells in the presence of the c-kit ligand, stem cell factor (SCF). This suggests a role for c-kit, Notch, and the beta-catenin signaling network in retinal 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.     

Skin-derived multipotent stromal cells – an archrival for mesenchymal stem cells

Progenitor stem cells have been identified, isolated and characterized in numerous tissues and organs. However, their therapeutic potential and the use of these stem cells remain elusive except for a few progenitor cells from bone marrow, umbilical cord blood, eyes and dental pulp. The use of bone marrow-derived hematopoietic stem cells (HSC) or mesenchymal stem cells (MSCs) is restricted due to their extreme invasive procedures, low differentiation potential with age and rejection. Thus, we need a clinical grade alternative to progenitor stem cells with a high potential to differentiate, na?ve and is relatively easy in in vitro propagation. In this review, we summarize cell populations of adherent and floating spheres derived from different origins of skin, or correctly foreskin, by enzymatic digestion compared with established MSCs. The morphology, phenotype, differentiation capability and immunosuppressive property of the adherent cell populations are comparable with MSCs. Serum-free cultured floating spheres have limited mesodermal but higher neurogenic differentation potential, analogous to neural crest stem cells. Both the populations confirmed their plethora potential in in vitro. Together, it may be noted that the skin-derived adherent cell populations and floating cells can be good alternative sources of progenitor cells especially in cosmetic, plastic and sports regenerative medicine.     

The role of stem cells in cardiac regeneration

After myocardial infarction, injured cardiomyocytes are replaced by fibrotic tissue promoting the development of heart failure. Cell transplantation has emerged as a potential therapy and stem cells may be an important and powerful cellular source. Embryonic stem cells can differentiate into true cardiomyocytes, making them in principle an unlimited source of transplantable cells for cardiac repair, although immunological and ethical constraints exist. Somatic stem cells are an attractive option to explore for transplantation as they are autologous, but their differentiation potential is more restricted than embryonic stem cells. Currently, the major sources of somatic cells used for basic research and in clinical trials originate from the bone marrow. The differentiation capacity of different populations of bone marrow-derived stem cells into cardiomyocytes has been studied intensively. The results are rather confusing and difficult to compare, since different isolation and identification methods have been used to determine the cell population studied. To date, only mesenchymal stem cells seem to form cardiomyocytes, and only a small percentage of this population will do so in vitro or in vivo. A newly identified cell population isolated from cardiac tissue, called cardiac progenitor cells, holds great potential for cardiac regeneration. Here we discuss the potential of the different cell populations and their usefulness in stem cell based therapy to repair the damaged heart.     

Neural progenitor genes. Germinal zone expression and analysis of genetic overlap in stem cell populations

The identification of the genes regulating neural progenitor cell (NPC) functions is of great importance to developmental neuroscience and neural repair. Previously, we combined genetic subtraction and microarray analysis to identify genes enriched in neural progenitor cultures. Here, we apply a strategy to further stratify the neural progenitor genes. In situ hybridization demonstrates expression in the central nervous system germinal zones of 54 clones so identified, making them highly relevant for study in brain and neural progenitor development. Using microarray analysis we find 73 genes enriched in three neural stem cell (NSC)-containing populations generated under different conditions. We use the custom microarray to identify 38 "stemness" genes, with enriched expression in the three NSC conditions and present in both embryonic stem cells and hematopoietic stem cells. However, comparison of expression profiles from these stem cell populations indicates that while there is shared gene expression, the amount of genetic overlap is no more than what would be expected by chance, indicating that different stem cells have largely different gene expression patterns. Taken together, these studies identify many genes not previously associated with neural progenitor cell biology and also provide a rational scheme for stratification of microarray data for functional analysis.     

Epigenetic Signatures Associated with Different Levels of Differentiation Potential in Human Stem Cells

Background

The therapeutic use of multipotent stem cells depends on their differentiation potential, which has been shown to be variable for different populations. These differences are likely to be the result of key changes in their epigenetic profiles.

Methodology/Principal Findings

to address this issue, we have investigated the levels of epigenetic regulation in well characterized populations of pluripotent embryonic stem cells (ESC) and multipotent adult stem cells (ASC) at the trancriptome, methylome, histone modification and microRNA levels. Differences in gene expression profiles allowed classification of stem cells into three separate populations including ESC, multipotent adult progenitor cells (MAPC) and mesenchymal stromal cells (MSC). The analysis of the PcG repressive marks, histone modifications and gene promoter methylation of differentiation and pluripotency genes demonstrated that stem cell populations with a wider differentiation potential (ESC and MAPC) showed stronger representation of epigenetic repressive marks in differentiation genes and that this epigenetic signature was progressively lost with restriction of stem cell potential. Our analysis of microRNA established specific microRNA signatures suggesting specific microRNAs involved in regulation of pluripotent and differentiation genes.

Conclusions/Significance

Our study leads us to propose a model where the level of epigenetic regulation, as a combination of DNA methylation and histone modification marks, at differentiation genes defines degrees of differentiation potential from progenitor and multipotent stem cells to pluripotent stem cells.     

Origins and selection of epidermal progenitors and stem cells: a challenge for tissue engineering

The use of epidermal stem cells and their progeny for tissue engineering and cell therapy represents a source of hope and major interest in view of applications such as replacing the loss of functionality in failing tissues or obtaining physiologic skin equivalents for skin grafting. The use of such cells necessitates the isolation and purification of rare populations of keratinocytes and then increasing their numbers by mass culture. This is not currently possible since part of the specific phenotype of these cells is lost once the cells are placed in culture. Furthermore, few techniques are available to unequivocally detect the presence of skin stem cells and/or their progeny in culture and thus quantify them. Two different sources of stem cells are currently being studied for skin research and clinical applications: skin progenitors either obtained from embryonic stem cells (ESC) or from selection from adult skin tissue. It has been shown that "keratinocyte-like" cells can be derived from ESC; however, the culturing processes must still be optimized to allow for the mass culture of homogeneous populations at a controlled stage of differentiation. The functional characterization of such populations must also be more thoroughly achieved. In order to use stem cells from adult tissues, improvements must be made in order to obtain a satisfactory degree of purification and characterization of this rare population. Distinguishing stem cells from progenitor cells at the molecular level also remains a challenge. Furthermore, stem cell research inevitably requires cultivating these cells outside their physiological environment or niche. It will thus be necessary to better understand the impact of this specific environmental niche on the preservation of the cellular phenotypes of interest.     

Stem cells in the eye

In the adult organism, all tissue renewal and regeneration depends ultimately on somatic stem cells, and the eye is no exception. The importance of limbal stem cells in the maintenance of the corneal epithelium has long been recognised, and such cells are now used clinically for repair of a severely damaged cornea. The slow cycling nature of lens epithelial cells and their ability to terminally differentiate into fiber cells are suggestive of a stem cell lineage. Furthermore, recent studies have identified progenitor cells in the retina and ocular vasculature which may have important implications in health and disease. Although the recent literature has become flooded with articles discussing aspects of stem cells in a variety of tissues our understanding of stem cell biology, especially in the eye, remains limited. For instance, there is no definitive marker for ocular stem cells despite a number of claims in the literature, the patterns of stem cell growth and amplification are poorly understood and the microenvironments important for stem cell regulation and differentiation pathways are only now being elucidated. A greater understanding of ocular stem cell biology is essential if the clinical potential for stem cells is to be realised. For instance; How do we treat stem cell deficiencies? How do we use stem cells to regenerate damaged retinal tissue? How do we prevent stem cell lineages contributing to retinal vascular disease? This review will briefly consider the principal stem cells in the mature eye but will focus in depth on limbal stem cells and corneal epithelium. It will further discuss their role in pathology and their potential for therapeutic intervention.     

Cellular interactions determine neuronal phenotypes in rodent retinal cultures

Progenitor cells isolated from early rat embryo retinas differentiate into phenotypes normally generated early in retinal development (e.g., ganglion cells), whereas progenitors isolated from postnatal retinas differentiate into later-generated retinal cell types (e.g., rod photoreceptors; Reh and Kljavin, J. Neurosci. 9:4179–4189; 1989; Adler and Hatlee, 1989; Science 243:391–393; Sparrow, Hicks, and Barnstable, 1990, Dev. Brain Res. 51:69–84). To determine whether this change in committment is intrinsic to the progenitor cells, or alternatively can be modified by interactions with their developing environment, I co-cultured mouse and rat retinal cells, from different developmental stages, and identified the resulting phenotypes with species-specific and cell class-specific antibodies. I found that the phenotypes into which mouse neuroepithelial cells differentiate depends on the phenotypes of the rat cells that surround them. Retinal precursor cells from embryonic day (E) 10–12 will adopt the rod photoreceptor phenotype only when close to cells expressing this phenotype. By contrast, when the E10–12 retinal progenitor cells are cultured with cells from the cerebral cortex, they differentiate primarily into large multipolar neurons, similar in their morphology and antigen expression to retinal ganglion cells. These results indicate that interactions among the cells of the developing retina are important in the determination of cell fate. © 1992 John Wiley & Sons, Inc.     

The Role of eNSCs in Neurodegenerative Disease

Recent progress in biology has shown that many if not all adult tissues contain a population of stem cells. It is believed that these cells are involved in the regeneration of the tissue or organ in which they reside as a response to the natural turnover of differentiated cells or to injury. In the adult mammalian brain, stem cells in the subventricular zone and the dentate gyrus may also play a role in the replacement of neurons. A positive beneficial response to injury does not necessarily require cell replacement. New findings suggest that some populations of endogenous neural stem cells in the central nervous system may have adopted a function different from cell replacement and are involved in the protection of neurons in diverse paradigms of disease and injury. In this article, we will focus on the immature cell populations of the central nervous system and the signal transduction pathways that regulate them which suggest new possibilities for their manipulation in injury and disease.